ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

February 11, 2000 -- Vol. 4 Number 6

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This fright, this night of the mind, must be dispelled
not by the rays of the sun, nor day's bright spears,
but by the face of nature and her laws.
-- Lucretius (95-55 B.C.)

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Contents of This Issue:

1. Cell Biology: Functional Modules in Biological Organization
2. Neurobiology: Atomic Scale Movements in Potassium Channels
3. Immunology: Molecular Mimicry and Autoimmunity
4. Medical Biology: Elimination of Prions by Branched Polyamines
5. Astronomy: Reflected Starlight from a Giant Extrasolar Planet
6. Prospects for the Search for Extraterrestrial Intelligence
 
In Focus: On the Uniqueness of Humankind

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1. CELL BIOLOGY: FUNCTIONAL MODULES IN BIOLOGICAL ORGANIZATION
     The term "phenomenology" has a variety of meanings, but in
this report we are concerned with only one meaning of the term:
we take the term "phenomenology" to refer to a scientific
approach that focuses on explanations based on formal
relationships among observed entities or processes, as opposed to
an approach ("reductionist") that focuses on explanations based
on analysis of the fundamental constituents of such entities or
processes. Using the terms in this way, we have the following
examples: a) Thermodynamics is a phenomenological approach to the
behavior of a gas; statistical mechanics is a reductionist
approach to the behavior of a gas. b) Mendelian genetics is a
phenomenological approach to the inheritance of traits; molecular
genetics is a reductionist approach to the inheritance of traits.
One can think of similar dichotomies in almost every field in
science.
     The term "reductionist" has had an unfortunate history in
biology, where it has been used to characterize the idea that any
biological entity or process can be "explained" in terms of the
laws of physics and chemistry. Certainly, the behavior of every
entity or process in the natural world is ultimately totally
dependent on the laws of physics and chemistry (which leads to
the idea that the behavior can "in principle" be derived
["explained"] from such laws), but the actual practical
possibility of any explanations of the behavior of observable
entities or processes in terms of the laws of physics and
chemistry depends on the current state of our knowledge
concerning both the observables and the fundamental laws. In the
practice of science, it can be argued that it does not matter
much which approach is used, phenomenological or reductionist,
provided the approach produces results that are useful, or which
help in understanding the behavior of the entity or process, or
which suggest new and intriguing questions. Beyond this, the
discussion properly belongs in the domain of philosophy and not
science.
     The above preamble is necessary in the context of the
present report, since the report concerns a recent article in
which a group of authors (2 molecular biologists, a biophysicist,
and a physiologist) call for a more "phenomenological" approach
to cell biology, an interesting idea, since cell biology is not
one of those areas of biology where such appeals are common.
During the last 50 years, in fact, cell biology has experienced a
remarkable flowering based on the application of fundamental
biochemistry, biophysics, and molecular biology to entities and
processes recognizable at the cellular level (i.e., micron-scale
objects).
... ... L.H. Hartwell et al (4 authors at 3 installations, US)
present an essay calling for a transition from molecular to
"modular" cell biology, the authors making the following points:
     1) The authors begin their essay with the following
statement: "Although living systems obey the laws of physics and
chemistry, the notion of function or purpose differentiates
biology from other natural sciences. Organisms exist to
reproduce, whereas, outside religious belief, rocks and stars
have no purpose. Selection for function has produced the living
cell, with a unique set of properties that distinguish it from
inanimate systems of interacting molecules." [Editor's note:
Contrast with this the remarks in the relevant background
material below.]
     2) The authors propose that a major challenge for science in
the 21st century is to develop an integrated understanding of how
biological cells and organisms survive and reproduce. The authors
suggest that cell biology is in transition from a science that
was preoccupied with assigning functions to individual proteins
or genes, to a science that is now attempting to cope with the
complex sets of molecules that interact to form "functional
modules".
     3) The authors define a "functional module" as a discrete
entity whose function is separable from those of other modules.
This separation depends on chemical isolation, which can
originate from spatial localization or from chemical specificity.
For example, a ribosome, the module that synthesizes proteins,
concentrates the reactions involved in making a polypeptide into
a single particle, thus spatially isolating its function. Modules
can be insulated from or connected to each other. The authors
suggest that in the future, the higher-level properties of cells,
such as their ability to integrate information from multiple
sources, will be described by the pattern of connections among
their functional modules.
     4) The authors point out that the number of cellular
functional modules that have been analyzed in detail is very
small, and each of these efforts has required intensive study.
The authors suggest that biologists need to study more functions
at the modular level and develop methods that make it easier to
determine the relationship of inputs to outputs of modules, their
biochemical connectivity, and the states of key intermediates
within them.
     5) The authors suggest that the best test of our
understanding of cells will be to make quantitative predictions
about their behavior and test them. This will require detailed
simulations of the biochemical processes occurring within the
modules. "But making predictions is not synonymous with
understanding. We need to develop simplifying, higher-level
models and find general principles that will allow us to grasp
and manipulate the functions of biological modules."
     6) The authors summarize their essay: "Cellular functions,
such as signal transmission, are carried out by 'modules' made up
of many species of interacting molecules. Understanding how
modules work has depended on combining phenomenological analysis
with molecular studies. General principles that govern the
structure and behavior of modules may be discovered with help
from synthetic sciences such as engineering and computer science,
from stronger interactions between experiment and theory in cell
biology, and from an appreciation of evolutionary constraints."
-----------
Editor's note: The essential idea here can be presented as
follows: Consider a computer, a machine with a "purpose" -- to
compute. A computer operates on its inputs in specific ways to
produce specific outputs. A "flow diagram" of computer dynamics
is a phenomenological description of the behavior of the machine.
A complete "wiring diagram" of electrical entities and events in
the machine is a reductionist description of the behavior of the
machine. (Of course, from the perspective of quantum mechanics,
the wiring diagram is itself phenomenological.) Suppose we are
given a machine and know nothing about it except that it operates
on inputs to produce outputs. If our problem is to predict the
behavior of the machine in response to particular inputs, there
will come a time when a flow diagram, albeit "phenomenological",
will be of immense value in understanding how the machine works.
What the authors propose is that much of the future of cell
biology will lie in the construction of the equivalent of
detailed and predictive flow diagrams for the internal operations
of biological cells.
-----------
L.H. Hartwell: From molecular to modular cell biology.
(Nature 2 Dec 99 402supp:C47)
QY: Leland H. Hartwell, Fred Hutchinson Cancer Center, Seattle,
WA 98109 US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
-------------------
Related Background:
IN FOCUS: ON FUNCTION AND TELEOLOGY IN BIOLOGY
"Biology, and especially evolutionary biology, is rife with
claims concerning what various characteristics are "for". The
heart exists for the purpose of pumping blood. Bears have fur in
order to ward off the cold. Functional claims of this sort have
quite disappeared from physics. Whereas Aristotle thought that
planets, no less than living things, have goals, this
teleological conception of the physical world is now a relic of a
bygone age. Planets move as they do because of the laws of
motion; they do not act as they do for the good of anything.
Darwin is rightly famous for having introduced an important
materialist element into the science of life. But rather than
banishing functional notions from biology, he showed how they can
be domesticated within a materialist framework. Organisms are
goal-directed systems because they have evolved. Their behaviors
are suited to the tasks of survival and reproduction because
natural selection has allowed some traits, but not others, to be
passed from ancestors to descendants. Even if Darwinism
legitimates talk of goal and purpose within biology, the question
of what such talk means remains to be addressed. The heart does
many things. It pumps blood, but it also makes noise and takes up
space in our chests. Why are we inclined to say that pumping
blood is part of the heart's function, but making noise and
taking up space are not?"
-----------
Elliott Sober: _Conceptual Issues in Evolutionary Biology_
(MIT Press, Cambridge 1995, p.x)
(Science-Week 9 Jul 99)


2. NEUROBIOLOGY: ATOMIC SCALE MOVEMENTS IN POTASSIUM CHANNELS
The functional electrical activity of nerve cells is based
essentially on the rapid movements of ions across the membranes
of these cells, especially the movements of sodium and potassium
ions. These ion movements occur through special pores ("ion
channels") in the cell membrane, and one of the important
problems during the past few decades has been to characterize
these ion channels at the molecular level. Most ion channels are
selective, allowing only ions of a certain type to pass, and an
individual cell has ion channels with various ion selectivities.
The selectivity of an ion channel can be "gated", the channel
effectively opened or closed, and ion channels are said to
"voltage-gated" or "ligand-gated", depending on how the change in
selectivity is provoked. The term "voltage-gated" refers to the
opening or closing of an ion channel by changes in the electrical
potential across the membrane, while the term "ligand-gated"
refers to opening and closing of an ion channel by interactions
between ligands and membrane receptors. It has become apparent
that voltage-gated ion channels are transmembrane proteins
consisting of 4 identical subunits, each of which contains 6
transmembrane segments. Studies of the potassium ion channel have
identified two segments that contain several charged protein
residues, and these charged residues apparently sense changes in
the potential difference across the membrane and form part of the
membrane "voltage sensor". Although these regions apparently
undergo conformational changes in response to changes in membrane
potential, little is known about the nature of these changes.
... ... A. Cha et al (4 authors at 2 installations, US) report a
study of molecular movements of the voltage-sensing region in a
potassium channel, the authors making the following points:
     1) The authors used *lanthanide-based fluorescence resonance
energy transfer to measure distances between *Shaker potassium-
channel protein subunits at specific residues. Voltage dependent
distance changes of up to 3.2 angstroms were measured at several
sites near one of the charged protein segments (S4). These
movements directly correlated with electrical measurements of the
voltage sensor, establishing a link between physical changes and
electrical charge movement.
     2) The authors suggest that the measured distance changes
indicate that the region associated with the S4 segment undergoes
a rotation and possible tilt, rather than a large transmembrane
movement, in response to voltage.
     3) The authors conclude: "These results demonstrate the
first in situ measurement of atomic scale movement in a
transmembrane protein."
... ... In a contiguous and related report, K.S. Glauner et al (4
authors at University of California Berkeley, US) present a study
of voltage-sensor movements of a potassium channel, the authors
making the following points:
     1) The authors used fluorescence resonance energy transfer
as a "spectroscopic ruler" to determine distances between S4
subunits in the Shaker potassium channel in different gating
states.
     2) The authors conclude their experimental evidence is
consistent with the S4 subunit being a tilted helix that twists
during activation. The authors propose that helical twist
contributes to the movement of charged side chains across the
membrane electric field and that this movement is involved in
coupling voltage-sensing to gating.
-----------
A. Cha et al: Atomic scale movement of the voltage-sensing region
in a potassium channel measured via spectroscopy.
(Nature 16 Dec 99 402:809)
QY: Francisco Bezanilla [fbesanil@ucla.edu]
-----------
K.S. Glauner et al: Spectroscopic mapping of voltage sensor
movement in the Shaker potassium channel.
(Nature 16 Dec 99 402:813)
QY: E.Y. Isacoff [eisacoff@socrates.berkeley.edu]
-----------
Text Notes:
... ... *lanthanide-based fluorescence resonance energy transfer:
The term "fluorescence resonance energy transfer" (also called
"fluorescence energy transfer") refers to energy transfer between
two fluorophores (chemical groups or molecules capable of
fluorescence). If the two fluorophores are attached to a molecule
at different positions, observations of fluorescence energy
transfer between them can be used to determine the distance
between the two attachment positions. Lanthanide-based resonance
energy transfer is a modification of conventional fluorescence
resonance energy transfer in which a long-lived lanthanide donor
transfers energy in a distance-dependent manner to a conventional
organic fluorescent acceptor. This technique has previously been
used to measure angstrom-scale conformational changes in
proteins.
... ... *Shaker potassium-channel protein: The term "Shaker" here
refers to a mutant of the fruitfly Drosophila, the mutant
exhibiting intense shaking of the legs and body in response to
exposure to a volatile anaesthetic. Genetic analysis of the
mutation some years ago led to the sequencing of a Drosophila
gene expressing a potassium-channel protein, the shaking of the
insect apparently resulting from a mutation in this gene, with
the mutation producing long-lasting potassium ion currents when
nerve fibers are activated. Once the Shaker gene was sequenced, a
conventional procedure was developed in the 1980s to have this
gene expressed in frog egg cells (oocytes) in order to advance
the study of the behavior of potassium ion channels. Thus, the
potassium ion channels in this report are ion channels derived
from the fruitfly Drosophila and expressed in the frog Xenopus
laevis egg cell. The essential aspects of frog oocytes is that
they are large cells (up to 1 millimeter in diameter), and the
introduction of foreign *messenger RNA into the egg cell readily
results in the production of the protein encoded by the
introduced messenger RNA.
... ... *messenger RNA: (mRNA) The ribonucleic acid molecule
transcribed from DNA that carries the coded information
specifying the sequence of amino acids in a protein.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
STRUCTURAL REARRANGEMENTS IN POTASSIUM CHANNEL GATING
Ion channels are protein channels in cell membranes that allow
ions to pass from extracellular solution to intracellular
solution and vice versa. Most ion channels are selective,
allowing only certain ions to pass, and an individual cell has
ion channels with various ion selectivities. The selectivity of
an ion channel can be "gated", the channel effectively opened or
closed, and ion channels are said to voltage-gated or ligand-
gated, depending on how the change in selectivity is provoked.
The opening of a previously closed ion channel produces a sudden
increase in transmembrane conductance for that ion, and the
process is called "activation". The gating of the movements of
ions through ion channels is of considerable importance for
various processes in all living systems, and forms the basis of
the electrical activity of all nervous systems. Recently (see
background material below), an important advance in ion-channel
research occurred with the experimental determination of the
crystal structure of a potassium channel (KcsA) in the bacterial
species Streptomyces lividans. The structure involves a
tetrameric complex with a centrally located pore framed by the
apposition of individual subunits, each subunit with 2
transmembrane helices (TM1 and TM2) flanking a "selectivity
filter". Intensive studies of this potassium channel in *planar
lipid bilayers have been in progress in a number of laboratories.
... ... E. Porozo et al (3 authors at University of Virginia, U)
now report a study of the structural rearrangements underlying
activation gating in this potassium channel, the study using
*spin-labeling methods and *electron paramagnetic resonance
spectroscopy. The authors report that a comparison of the closed
and open conformations of the channel revealed periodic changes
in spin-label mobility and intersubunit *spin-spin interaction
consistent with rigid-body movements of the two transmembrane
helices TM1 and TM2. These changes involve translations and
counterclockwise rotations of both helices relative to the center
of symmetry of the channel. The movement of TM2 apparently
increases the diameter of the permeation pathway along the point
of convergence of the four subunits, thus opening the pore.
Although the extracellular residues flanking the selectivity
filter remained immobile during gating, small movements were
detected at the *C-terminal end of the pore helix, and the
authors suggest this has possible implications for the gating
mechanism.
-----------
E. Perozo et al: Structural rearrangements underlying
K(+)-channel activation gating.
(Science 2 Jul 99 285:73)
QY: Eduardo Perozo [eperozo@virginia.edu]
-----------
Text Notes:
... ... *planar lipid bilayers: The cell membrane consists of a
lipid bilayer and associated proteins, the ensemble approximately
75 to 100 angstroms in thickness. Similar membranes are also
found within a cell surrounding various organelles. Lipid
bilayers are spontaneously forming self-organizing bimolecular
layers of certain molecules (lipids) with long nonpolar chains
terminated by a polar group. In addition to their presence in
cell membranes, such molecules (surfactants) are also found in
soaps. A variety of artificial lipid bilayer membrane systems can
be investigated in the laboratory.
... ... *spin-labeling methods: A "spin-label" is a synthetic
paramagnetic organic free radical incorporated in a macromolecule
or assemblage of macromolecules and used, in particular, in
electron paramagnetic resonance spectroscopy.
... ... *electron paramagnetic resonance spectroscopy: (ESR) This
technique is used to investigate paramagnetic centers in a
molecular system. Only electrons whose spin is not paired with
the oppositely directed spin of another electron give an ESR
signal. With this technique, information can be obtained about
certain transitional ions, free radicals, and free electron
centers. A probe giving an ESR signal can be incorporated into
membrane lipids or attached to proteins to enable otherwise
inaccessible systems to be studied. Through analysis of ESR
spectra, rates of molecular motion and relative orientation of
spin-labeled molecules whose motion is restrained by surrounding
molecules can be determined. Measurements of rates of molecular
motion and molecular orientation have proved to be important in
the study of a variety of biological problems.
... ... *spin-spin interaction: In this context, an interaction
of two neighboring paramagnetic entities, the interaction
producing a change in ESR signal.
... ... *C-terminal end: In general, this refers to the end of
any polypeptide chain at which the 1-carboxy function of a
constituent alpha-amino acid is not attached in peptide linkage
to another amino acid residue.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
-------------------
Related Background:
ANALYSIS OF POTASSIUM ION MEMBRANE CHANNEL STRUCTURE
... The potassium ion channel from the prokaryotic soil bacterium
Streptomyces lividans is an integral membrane protein with
sequence similarity to all known potassium ion channels,
particularly in the pore region. ... ... Doyle et al (8 authors
at Rockefeller University, US) report an x-ray analysis (data to
3.2 angstroms) of the Streptomyces lividans potassium channel
reveals four identical subunits create an inverted cone cradling
the selectivity filter of the pore in its outer end. The narrow
selectivity filter is only 12 angstroms long, whereas the
remainder of the pore is wider and lined with hydrophobic amino
acids. The selectivity filter is apparently held open by
structural constraints to coordinate potassium ions but not
smaller sodium ions. The authors suggest the architecture of the
pore establishes the physical principles underlying selective
potassium ion conduction.
QY: Roderick MacKinnon (mackinn@rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
SIMILAR STRUCTURE OF PROKARYOTIC VS. EUKARYOTIC K(+) CHANNELS
Toxins from scorpion venom are known to interact with potassium
ion channels in eukaryotic cell membranes. Mackinnon et al (5
authors at Rockefeller University, US) report the use of resin-
attached mutant potassium ion channels from the bacterium
Streptomyces lividans to screen scorpion venom, and the toxins
that interact with the channel were identified by mass
spectrometry. The authors suggest their results indicate that the
prokaryotic potassium ion channel, whose structure has now been
revealed, has the same pore structure as eukaryotic potassium ion
channels, and that this structural conservation, through the
application of their techniques, offers a new approach to
potassium ion channel pharmacology.
QY: Roderick MacKinnon (mackinn@rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
A POTASSIUM CHANNEL MUTATION IN NEONATAL HUMAN EPILEPSY
Epilepsy is not a single disease, but a group of nervous system
disorders exhibiting similar symptoms of repeated episodes of
convulsive seizures, sensory disorders, abnormal behavior,
blackouts, etc. Apparently, all types of epilepsy involve
uncontrolled electrical discharge from neurons in the cerebral
cortex. Most epilepsy is of unknown etiology, but there are known
links to head injury, brain infection, brain tumor, blood vessel
disturbances, intoxication, chemical imbalances, etc. The term
"idiopathic" means without a known cause (e.g., idiopathic
epilepsy as opposed to epilepsy produced by brain injury), but
the term is also used for pathology of apparent genetic etiology
when the physiological consequence of the gene mutation is
unknown. In the context of studies of biological cell membranes,
the term "ion selectivity" refers to the ability of all cell
membranes, particularly nerve cell membranes, to distinguish
between various ions such as Na(+), K(+), Ca(2+), Cl(-), etc.
There is much evidence that this ion selectivity involves
specific pores or channels in the cell membrane, with certain
channels specific for certain ions, the channels capable of being
opened or closed (gated) depending on conditions and various
interactions with ligands binding to receptors. Since the
movement of ions is by definition an electric current, ion-
selective channels can be viewed as involved in "conductance"
pathways, and there are methods for measuring individual cellular
ion currents. In most neurons, potassium channel conductance is
involved in the maintenance of membrane polarization and
repolarization following excitation, a critical factor, since
loss of membrane polarization can result in repetitive
uncontrolled firing (production of action potentials) by the
nerve cell. Oocytes are egg cells, and frog oocytes are
convenient systems for genetic manipulation and large enough for
various experimental procedures.
... ... Biervert et al (7 authors at 3 installations, DE AU),
investigating an age-specific human epileptic syndrome of genetic
etiology, report that an associated potassium channel gene, when
expressed in frog oocytes investigated with electrophysiological
methods, indicates the mutant channel does not yield measurable
potassium currents, and that impairment of potassium-dependent
repolarization produced by mutation of the gene is the likely
cause of the human age- specific epileptic syndrome. The authors
point out that no other gene defects have yet been identified in
human idiopathic epilepsies.
QY: Thomas J. Jentsch 
(Science 16 Jan 98) (Science-Week 30 Jan 98)
[For more information: http://scienceweek.com/search/search.htm]


3. IMMUNOLOGY: MOLECULAR MIMICRY AND AUTOIMMUNITY
In general, the term "antigen" refers to any chemical substance
that provokes characteristic responses of the immune system to
invasion by "foreign" molecules. Antigens are usually
macromolecules with only certain chemical moieties (antigen
determinants or "epitopes") responsible for provoking the immune
response. The term "self-antigen" (also: "autoantigen") refers to
a chemical moiety present in the tissues of the host which itself
provokes characteristic responses of the immune system. An
"autoimmune disease" is the consequence of an immune response
against self-antigens that results in the damage and eventual
dysfunction of organs that become targeted by the immune system.
Among known or suspected human autoimmune diseases are rheumatoid
arthritis, systemic lupus erythematosus, rheumatic fever,
hemolytic and pernicious anemias, Addison's disease, insulin-
dependent (type I) diabetes mellitus, myasthenia gravis, multiple
sclerosis, and others. Although the triggering event in most
autoimmune diseases is unknown, an infectious cause has long been
postulated to explain the development of autoimmunity.
... ... L.J. Albert and R.D. Inman (University of Toronto, CA)
present a review of the possible involvement of infection and
"molecular mimicry" in the etiology of autoimmune diseases, the
authors making the following points:
     1) Molecular mimicry is one mechanism by which infectious
agents (or other exogenous substances) may trigger an immune
response against autoantigens. According to this hypothesis, a
susceptible host acquires an infection with a pathogen that has
antigens that are immunologically similar to certain host
antigens but differ sufficiently to induce an immune response
when presented to *T cells. As a result, the tolerance to
autoantigens breaks down, and the pathogen-specific immune
response which is generated cross-reacts with host tissues to
cause tissue damage and disease. This model has persisted for
more than thirty years because it offers an attractive conceptual
link between a physiological response (defense against infection)
and a pathologic process (autoimmunity). 
     2) Molecular mimicry has been linked to the pathogenesis of
several important conditions, such as multiple sclerosis and type
1 diabetes mellitus. In the case of *multiple sclerosis, it has
been hypothesized that the disease is initiated by an infection
early in life by a virus that shares antigenic structures with
central nervous system tissue of the host. The anti-viral immune
response of the host then cross-reacts with central nervous
system self-antigens, such as *myelin basic protein, leading to
the destruction of the myelin of myelinated nerve fibers
(demyelination). Subsequent viral infections are thought to cause
exacerbations of the disease by reactivating the immune response
against viral antigens and autoantigens. Although there is some
evidence that infectious agents play a part in the pathogenesis
of multiple sclerosis and many other autoimmune diseases, it has
not yet been proved that molecular mimicry is the initiating
factor in any of these diseases.
     3) The authors state that the experimental and clinical
models which they have reviewed "fall short of resolving the key
issues in the demonstration of molecular mimicry as a pathogenic
mechanism in autoimmune disease." For researchers in this area,
challenges remain. Infection is common; autoimmunity is not
common. The frequency of shared peptide sequences and the
flexibility inherent in immune recognition suggest that mimicry
may be ubiquitous in biological systems. Defining the default
pathways that normally protect the host from the dangers of an
autoreactive response during or after infection remains an
important area of research.
     4) The authors conclude: "Molecular mimicry has remained an
attractive explanation for autoimmune diseases for three decades,
mainly on the basis of circumstantial evidence. No data
convincingly demonstrate that mimicry is an important mechanism
in the development of autoimmune disease in humans. Nonetheless,
molecular mimicry retains an intrinsic appeal, because it links
current concepts of the role of the immune system in the host
defense with concepts of autoimmunity."
-----------
L.J. Albert and R.D. Inman: Molecular mimicry and autoimmunity.
(New England J. Med. 30 Dec 99 341:2068)
QY: Robert D. Inman [rinman@torhosp.toronto.on.ca]
-----------
Text Notes:
... ... *T cells: (T-cells; T-lymphocytes) Lymphocytes are a type
of leukocyte (white blood cell) involved in the immune response.
There are two classes of lymphocytes: 1) B-cells, which, when
presented with a foreign chemical entity (antigen), change into
antibody producing plasma cells; and, 2) T-cells, which
aggressively interact directly with foreign invaders such as
bacteria and viruses. Certain types of T-cells are also involved
in B-cell production of antibodies. (The notation for these cells
is based on the locus of maturation: T cells mature in the thymus
gland; B cells mature in bone marrow.)
... ... *multiple sclerosis: See background material below.
... ... *myelin basic protein: See background material below.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
MEDICAL BIOLOGY: A POSSIBLE CAUSE OF MULTIPLE SCLEROSIS
In the vertebrate central nervous system, the axons of nerve
cells involved in physiological functions that require rapid
signaling (for example, the neural control of voluntary muscle)
are wrapped in a special sheath called myelin. The myelin sheath
consists of concentric layers of electrically insulating lipid-
protein material, but the sheath is periodically interrupted
along the length of the axon, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. Multiple sclerosis is a human disease
characterized by the progressive loss of the myelin of the brain
and spinal cord, with the physiological disruptions to be
expected from such loss, considering the significance of myelin
in the functioning of nerve cells. The cause of the disease is
unknown, but an immunological abnormality is suspected. It has
also been postulated that the cause is infection by a latent
virus, with viral activation and expression triggering a
secondary immune response. There is some evidence for genetic
susceptibility, and there is also evidence that environment may
be a factor, since the disease is 5 times more common in
temperate climates than in the tropics.
... ... D.C. Shields et al (4 authors at 2 installations, US JP)
now present a study of the mechanism of demyelinization in
multiple sclerosis, the authors reporting the following:
     1) In autoimmune demyelinating diseases such as multiple
sclerosis, the degradation of myelin proteins results in the
destabilization of the myelin sheath. Protein-degrading enzymes
(proteases, proteinases) have been implicated in myelin protein
degradation, and recent studies have demonstrated increased
expression and activity of a calcium-activated neutral proteinase
(calpain) in experimental allergic encephalomyelitis, the
corresponding animal model of multiple sclerosis.
     2) In the present study, calpain activity and expression
were evaluated in white matter from human patients with multiple
sclerosis and Parkinson's and Alzheimer's diseases and compared
with that of white matter from normal controls. Analysis
indicates that the active form of calpain and calpain-specific
degradation products were increased by 90.1 percent and 52.7
percent, respectively, in multiple sclerosis plaques compared
with normal white matter, and that calpain expression increased
by more than a factor of 4 compared with normal white matter.
Calpain activity and expression were not increased significantly
in white matter from patients with Parkinson's or Alzheimer's
diseases compared with that of normal controls.
     3) The authors suggest that because calpain degrades all
major myelin proteins, the increased activity and expression of
this proteinase may play a critical role in the degradation of
myelin in autoimmune demyelinating diseases such as multiple
sclerosis.
-----------
D.C. Shields et al: A putative mechanism of demyelination in
multiple sclerosis by a proteolytic enzyme, calpain.
(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11486)
QY: Naren L. Banik [baniknl@musc.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 19Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
HERPES VIRUS MOLECULAR MIMICRY AND AUTOIMMUNE DISEASE
An autoimmune disease is one of a large group of diseases
characterized by cells of the immune system attacking other cells
of the body, in effect failing to recognize these other cells as
"friendly". Antigens are chemical moieties, often proteins, that
provoke immune responses, and the "epitope" is the small region
of the antigen apparently involved in binding or recognition of
the antigen. T-cells, of which there are various types, are
aggressive immune system cells, first formed in bone marrow, then
maturing in the thymus gland [hence: T(hymus)-cell]. A keratitis
is an inflammation of the cornea, and stromal keratitis is an
inflammation of the connective tissue framework of the cornea.
Invasion of the corneal stroma by herpes simplex virus is one of
the possible consequences of herpes simplex infection. Viral
infection is sometimes associated with the initiation or
exacerbation of autoimmune disease, although the underlying
mechanisms have been unclear. One proposed mechanism is that
viral chemical moieties that mimic host antigens trigger certain
T-cells to destroy host tissue.
... ... Zhao et al (5 authors at 2 installations, US) report that
an epitope of a coat protein of herpes simplex virus type 1 (KOS
strain) is recognized by autoreactive T-cells that target corneal
antigens in a mouse model of autoimmune herpes stromal keratitis.
Mutant viruses that lacked this epitope did not induce autoimmune
disease. The authors suggest that expression of molecular mimics
can influence the development of autoimmune disease after viral
infection. QY: Zi-Shan Zhao, Dept. of Pathology, Harvard Univ.
Medical School 617-432-1550.
(Science 27 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
HERPESVIRUS LINKED TO MULTIPLE SCLEROSIS
In the vertebrate central nervous system, the axons of nerve
cells involved in physiological functions that require rapid
signaling (for example, the neural control of voluntary muscle)
are wrapped in a special sheath called myelin. The myelin sheath
consists of concentric layers of electrically insulating lipid
material, but the sheath is periodically interrupted, and at the
points where the sheath is interrupted so is the electrical
insulation interrupted. The result, predictable from the
classical physics of electrical transmission lines and the
electrical parameters of nerve fibers, is that the propagation of
an electrical pulse along such nerve fibers occurs at a velocity
much higher than that found in unmyelinated fibers. Multiple
sclerosis is a human disease characterized by the progressive
loss of the myelin of the brain and spinal cord, with the
physiological disruptions to be expected from such loss,
considering the significance of myelin in the functioning of
nerve cells. The herpesviruses are a class of viruses producing
the complex of herpes diseases, and HHV-6 is a recently
discovered strain of herpesvirus that apparently causes an infant
and early childhood disease called roseala infantum. Jacobson et
al (National Institutes of Health, US) report that a study of
multiple sclerosis patients (36 patients and 66 controls)
revealed that 70% of these patients were infected with the strain
of herpesvirus HHV-6. They also report that magnetic resonance
imaging detected numerous myelin lesions in the brain of a
deceased multiple sclerosis patient, and an autopsy revealed
HHV-6 in the lesions but not in the adjoining normal tissues.
Some multiple sclerosis specialists are expressing reservations
about the interpretation of these results, stating it is possible
the viral infection is a consequence rather than a cause of
multiple sclerosis.
QY: Steven Jacobson, National Institute of Neurological Disorders
and Stroke, NIH, Bethesda, MD 20892-0148
(Nature Medicine December 1997) (Science-Week 26 Dec 97)
-------------------
Related Background:
EVIDENCE FOR RETROVIRUS INVOLVEMENT IN AUTOIMMUNE DIABETES
Retroviruses store their genetic information in molecules of RNA
rather than in molecules of DNA. When they enter a cell, they
carry an enzyme called reverse transcriptase, which utilizes RNA
as a template for directing DNA synthesis. The end result of a
successful retroviral invasion of the cell is that the virally
produced DNA becomes integrated into the host DNA, where it can
be maintained for an indefinite period of time. In fact, there is
evidence that parts of the human genome are actually pieces of
endogenous ancient retroviral DNA now permanent genetic
residents, and that under specific conditions these endogenous
retroviruses can be activated to begin a pathogenic process. Now
B. Conrad et al report that the trigger for the autoimmune
disease insulin-dependent diabetes mellitus may be a retrovirus.
This idea has been suggested previously, but this new evidence is
apparently stronger, including isolation of the complete genome
of a heretofore unknown endogenous retrovirus in the supernatant
from pancreatic-islet cultures from a diabetic patient. Workers
in the field are calling this an exiting advance in the
understanding of this autoimmune disease.
(Cell 90:303 1997) (Science-Week 5 Sep 97)
[For more information: http://scienceweek.com/search/search.htm]


4. MEDICAL BIOLOGY: ELIMINATION OF PRIONS BY BRANCHED POLYAMINES
     During the past several decades, a number of discoveries
have led to the molecular and genetic characterization of the
transmissible agent causing "*scrapie", a degenerative central
nervous system disease of sheep. Studies have identified a
scrapie-specific protein in preparations from scrapie-infected
brains of sheep, this protein capable of producing the symptoms
of scrapie in previously uninfected sheep. Attempts to identify
additional components of the infectious agent, such as nucleic
acid, have been unsuccessful. To distinguish this agent from
viruses and *viroids, the term "prion" was introduced to
emphasize its combined proteinaceous and infectious nature.
     The prion protein (PrP) is encoded by the host's chromosomal
DNA, and an abnormal isoform of the protein is apparently
associated with disease transmissibility. The abnormal isoform
differs physically from the normal cellular isoform by its
configuration (high *beta-sheet content), its insolubility in
detergents, its propensity to aggregate, and its relative
resistance to breakdown by hydrolysis (proteolysis).
     In addition to sheep scrapie, prions have been found to be
involved in a number of human diseases: *Kuru, *Creutzfeldt-Jakob
disease, *Gerstmann-Straussler-Sheinker disease, and *fatal
familial insomnia. *Bovine spongiform encephalopathy ("mad cow
disease"), which is thought to result from the ingestion of feeds
and bone meal prepared from rendered sheep offal, has been
responsible for deaths of more than 150,000 cattle in the UK
since 1986, and there is evidence that suggests the possibility
of transmission of prion disease from such animals to humans.
     There are two forms of prion disease: a) infectious forms
result from *horizontal transmission of infectious prions, e.g.,
bovine spongiform encephalopathy, non-familial Creutzfeldt-Jakob
disease, and kuru; b) inherited forms of prion disease, e.g.,
Gerstmann-Straussler-Scheinker disease, fatal familial insomnia,
and familial Creutzfeldt-Jakob disease, comprise 10 to 15 percent
of all cases and are associated with mutations in the PrP gene.
... ... S. Supattapone et al (University of California San
Francisco, US) report a discovery of a possible chemotherapy for
prion diseases, the authors making the following points:
     1) The normal cellular form of prion protein contains 3
*alpha-helices and little beta-sheet content. In contrast, the
scrapie prion form is rich in beta-sheet structure. The
accumulation of scrapie prion in the central nervous system
precedes neurologic dysfunction accompanied by neuronal
vacuolization and astrocyte gliosis.
     2) The spectrum of prion diseases now includes a new form of
human prion disease, "new variant Creutzfeldt-Jakob disease"
(nvCJD), which has emerged in the UK and France. Several lines of
evidence have suggested a link between the new variant
Creutzfeldt-Jakob outbreak and a preceding epidemic in the UK of
bovine spongiform encephalopathy. Although it too early to
predict the number of nvCJD cases that might eventually arise in
the UK and elsewhere, it is clear that effective therapeutics for
prion diseases are urgently needed. Unfortunately, although a
number of compounds, including amphoteracins, sulfated
polyanions, Congo red dye, and anthracycline antibiotics, have
been reported as prospective therapeutic agents, all have
demonstrated only modest potential to impede prion propagation,
and none have been shown to effect the removal of preexisting
prions from an infected host.
     3) The authors report that non-cytotoxic concentrations of
*branched polyamines can rapidly eliminate scrapie prions from
cultured chronically infected *neuroblastoma cells (culture type
ScN2a). These compounds appear to act by stimulating normal
cellular mechanisms to destroy scrapie prions. Once purged of
scrapie prions, the treated cells remain free from evidence of
scrapie infection during repeated serial passage in polyamine-
free media. The authors suggest that branched polyamines might be
useful therapeutic agents for treatment of prion diseases and
"other degenerative central nervous system disorders
characterized by deposits of abnormal proteins such as
Alzheimer's disease, Parkinson's disease, amyotrophic lateral
sclerosis, and fronto-temporal dementia."
-----------
S. Supattapone et al: Elimination of prions by branched
polyamines and implications for therapeutics.
(Proc. Natl. Acad. Sci. US 7 Dec 99 96:14529)
QY: Stanley B. Prusiner, University of California San Francisco
415-476-4044.
-----------
Text Notes:
... ... *scrapie: Susceptibility to scrapie varies among
different breeds of sheep, with goats 100 percent susceptible.
The disease is transmissible to laboratory monkeys, mice, and
hamsters.
... ... *viroids: The term "viroids" refers to the pathological
agents of a number of transmissible plant diseases. The pathogens
are small, single-stranded, covalently closed circular RNA
molecules existing as highly base-paired rod-like structures
without protein coats (i.e., without capsids).
... ... *beta-sheet: The "secondary structure" of a protein is
determined by interactions between the sequential units,
particularly hydrogen bonding between particular amino acids and
nonpolar interactions between hydrophobic regions, the
interactions producing, in general, three local or global
secondary structure variants: alpha helix, beta sheet, and tight
turn. An "alpha helix" is a spiral configuration of a polypeptide
chain in which successive turns of the helix are held together by
hydrogen bonds between the amide (peptide) links, the carbonyl
group of any given residue being hydrogen-bonded to the imino
group of the 3rd residue behind it in the chain. The term "beta
sheet" (beta- pleated sheet) refers to an array of two or more
"beta strands", with each beta strand consisting of two
polypeptide chains in a so-called "beta configuration", which in
turn is a stable configuration of a polypeptide chain in which
the chain is almost fully extended and hydrogen-bonded to an
adjacent polypeptide chain. The third secondary structure
variant, "tight turn" (beta bend; beta turn) refers to a bending
of a short stretch of polypeptide chain that allows the main
direction of the chain to change. The turn consists of 4 amino
acid residues in which the CO group of residue n is
hydrogen-bonded to the NH group of residue n + 3.
... ... *Kuru: This disease is similar to Creutzfeldt-Jakob
disease, and is a human spongiform encephalopathy. (The term
"spongiform" refers to the sponge-like appearance of the infected
brain.) Kuru occurs only in the easter highlands of New Guinea,
occurs more frequently in women than in men, which apparently
coincides with the customs surrounding cannibalism in a society
where the remains of dead relatives are handled and eaten
primarily by children and women. After cannibalism was outlawed,
the incidence of the disease decreased, and the current consensus
is that cannibalism was the primary mode of transmission of the
pathological agent.
... ... *Creutzfeldt-Jakob disease: Until 30 years ago,
Creutzfeldt-Jakob disease was an obscure form of dementia unknown
to most physicians. The name is now familiar to the medical
community as the major *transmissible spongiform encephalopathy
(or prion disease) in humans, and familiar to research scientists
because of its strange causative agent (prions) that exhibit
apparently novel modes of replication and transmission.
... ... *transmissible spongiform encephalopathy: In general, an
encephalopathy is any disorder of the brain. In this context, the
term "spongiform" refers to the sponge-like texture or appearance
of the brain upon autopsy. In this context, the term
"transmissible" means capable of being transmitted from one
individual to another.
... ... *Gerstmann-Straussler-Sheinker disease: A familial (i.e.,
inherited) form of Creutzfeldt-Jakob disease. The disease is
transmissible to animals, has an earlier onset than C-J disease
(40 vs. 60 years), and a longer average duration of disease (5
years vs. 9 months).
... ... *fatal familial insomnia: Another familial form of
Creutzfeldt-Jakob disease. This very rare disease is difficult to
transmit to experimental animals. The age of onset varies widely,
the course of the disease averaging 13 months. (Note: All the
human spongiform encephalopathies are invariably fatal.)
... ... *Bovine spongiform encephalopathy: "Mad cow disease" is
similar to scrapie. In 1996, the new variant of Creutzfeldt-Jakob
disease (new-variant CJD) was recognized in the UK population,
primarily in younger people, the new disease with distinctive
pathological characteristics similar to those seen in macaque
monkeys infected with the agent of bovine spongiform
encephalopathy. The major present concern is that the pathogen of
bovine spongiform encephalopathy may have spread to humans in the
UK and continental Europe through ingestion of infected cattle
meat.
... ... *horizontal transmission: In general, in this context,
transmission from one contemporaneous individual to another in a
population, as opposed to "vertical transmission", which is
transmission from parent to offspring.
... ... *alpha-helices: See note on "beta-sheet" above.
... ... *branched polyamines: The branched polyamines involved in
this study ranged in molecular weight from 517 to 800,000
daltons, with primary NH(sub2) groups ranging in number from 4 to
more than 500.
... ... *neuroblastoma cells: "Neuroblastomas" are malignant
neoplasms characterized by only slightly differentiated immature
nerve cells of embryonic type. Such cells are readily maintained
in laboratory culture dishes.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
EVIDENCE FOR TRANSMISSION OF ANIMAL PRION DISEASE TO HUMANS
... M.R. Scott et al (7 authors at 2 installations, US UK)
present transgenetic evidence for transmission of bovine
spongiform encephalopathy prions to humans, the authors making
the following points:
     1) There is concern that bovine spongiform encephalopathy
may have been recently passed from cattle to humans, resulting in
approximately 50 cases of an atypical new variant of Creutzfeldt-
Jakob disease in teenagers and young adults. Epidemiological
findings, *gel electrophoresis of the prion protein, and
transmission to inbred mice and primates have each raised the
possibility of a link between bovine spongiform encephalopathy
and the new variant of Creutzfeldt-Jakob disease.
     2) More than 175,000 cattle, primarily dairy cows, have died
of bovine spongiform encephalopathy over the past decade. How
many more cattle were exposed to BSE prions but slaughtered
before developing clinical signs is uncertain. Given the enormity
of the affected cattle population in the UK, a means of assessing
risks to the human population is paramount, and more sensitive
methods for the detection of prions are urgently needed. The
magnitude of the potential risk to the human population is still
speculative, but the death rates from new-variant-Creutzfeldt-
Jakob disease per year had remained approximately constant until
recently, when a disturbing high number of deaths from the
disease, a total of 9 new cases, was reported in the last quarter
of 1998. Although it is not yet known whether this trend will
continue, the possibility that a large section of the population
is at high risk must be seriously entertained.
     3) The authors report their new studies indicate that
*transgenic mice expressing bovine prion protein *serially
propagate bovine spongiform encephalopathy prions, and that there
is no species barrier for transmission from cattle to mice. These
same mice were also highly susceptible to a new variant of
Creutzfeldt-Jakob disease and natural sheep scrapie. The
incubation times (approximately 250 days), neuropathology, and
disease-causing prion protein isoforms in the experimental mice
inoculated with human new-variant Creutzfeldt-Jakob disease and
bovine spongiform encephalopathy brain extracts were
indistinguishable and differed dramatically from those seen in
these mice with natural scrapie prions.
     4) The authors suggest their findings provide the most
compelling evidence to date that prions from cattle with bovine
spongiform encephalopathy have infected humans and caused fatal
neurodegeneration.
-----------
[Editor's note: Although we do not disagree with the
interpretations and conclusions of the authors, it should be
pointed out that despite the title of their paper, the authors
provide direct evidence for transmission of new-variant
Creutzfeldt-Jakob disease from humans to mice, but evidence only
by implication for transmission from mice (or any other species)
to humans. The import of the work is the apparent experimental
absence of a significant species barrier to transmission of this
disease, at least in experiments involving human to animal
transmission in the reported population of transgenic mice.]
-----------
M.R. Scott et al: Compelling transgenetic evidence for
transmission of bovine spongiform encephalopathy prions to
humans.
(Proc. Natl. Acad. Sci. US 21 Dec 99 96:15137)
QY: Michael R. Scott [abbott@itsa.ucsf.edu]
-----------
Text Notes:
... ... *protease-resistant isoform: A "protease" is any enzyme
that breaks down proteins by hydrolysis. In this context, an
"isoform" is any one of multiple forms of a functional protein
that differ in amino acid sequence and *electrophoretic mobility.
... ... *electrophoretic mobility: In general, electrophoresis is
a laboratory technique used to separate macromolecules on the
basis of electric charge and size, the technique involving
application of an electric field to a population of
macromolecules dispersing according to their electric mobilities.
... ... *gel electrophoresis: Gel electrophoresis is a type of
"zone electrophoresis" in which the supporting medium is a gel of
uniform concentration. In zone electrophoresis, a solution of
protein (or other molecules) is placed at the starting position
as a thin band or spot in an inert supporting medium (paper,
starch gel, polyacrylamide gel, etc.) containing buffer solution.
An electric potential is then applied to the supporting medium,
causing the proteins (or other substances) to migrate to give
distinct bands or zones which can be located in situ by staining,
light absorption, etc., or by analysis after elution of discrete
pieces of the supporting medium.
... ... *transgenic mice: A "transgenic mouse" is a mouse into
which genetic material from another organism has been
transferred, the transferred and incorporated new genes then
being expressed with the resultant production of specific
proteins.
... ... *serially propagate: The general paradigm for "serial
propagation" of a disease is as follows: After infection of
individual (or group) A by the disease, following the incubation
period, infectious material is removed from A and used to infect
B; then, after the incubation period, infectious material is
removed from B and used to infect C; and so on.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 21Jan00
[For more information: http://scienceweek.com/search/search.htm]


5. ASTRONOMY: REFLECTED STARLIGHT FROM A GIANT EXTRASOLAR PLANET
Currently, the identification and study of extrasolar planets
depends for the most part on indirect methods such as those
involving the measurement of perturbations of the observed
brightness or motions of their parent stars. The ideal method
would be direct imaging of extrasolar planets, and this would
considerably enhance the possibilities for understanding their
nature. A major problem in direct imaging of extrasolar planets
is that the bright light from the parent star (more particularly,
its diffracted halo in the imaging apparatus) can easily
overwhelm nearby faint light sources such as orbiting planets.
... ... A.C. Cameron et al (4 authors at 2 installations, UK) now
report detection and measurement of starlight reflected from an
extrasolar planet, the authors making the following points:
     1) In the 4 years following the discovery (by M. Mayor and
D. Queloz [1995]) of a planet orbiting the star 51 Pegasi,
approximately 20 other planets have been detected through their
influence on the radial velocities of lines in the spectra of
their parent stars. The orbital motion of the planet can be
detected by perturbations of the motion of the parent star
("reflex motion"), and these perturbations can be measured using
high-precision spectroscopy. This indirect technique cannot
investigate the radius or composition of the planet, and can
place only a lower limit on the mass of the planet.
     2) The authors report the probable detection of *Doppler-
shifted starlight reflected from the planet known to orbit tau-
Bootis with a period of just 3.3 days. The authors find that the
*orbital inclination of the planet is approximately 29 degrees,
from which the authors infer that the mass is approximately 8
times that of Jupiter, and that the planet has the size and
reflectivity expected for a "*gas-giant planet".
     3) The authors point out that if this is indeed a giant
planet, with a Jupiter size and *albedo, the scattered starlight
will be 10,000 to 20,000 times fainter than the parent star even
under the most favorable planet-illumination conditions.
     4) The authors conclude: "Our candidate detection of
starlight scattered from the atmosphere of an extrasolar planet
strengthens the case for the existence of the giant, close-
orbiting planets whose presence has so far been inferred only
indirectly from the reflex motions of their parent stars... The
close-orbiting planets of other systems, including both 51 Pegasi
and the nu-Andromedae triple-planet system, should be amenable to
similar studies in the near future."
-----------
A.C. Cameron: Probable detection of starlight reflected from the
giant planet orbiting tau-Bootis.
(Nature 16 Dec 99 402:751)
QY: Andrew Collier Cameron [andrew.cameron@st-and.ac.uk]
-----------
Text Notes:
... ... *Doppler-shifted starlight: In general, the term "Doppler
shift" refers to the change in wavelength of electromagnetic
radiation as a result of relative movement between the source and
the observer.
... ... *orbital inclination: In general, the angle between the
orbital plane of a body and a reference plane centered on the
object about which the body is revolving. In this context, the
orbital inclination of the planet is not directly measured, but
is implied by a "best-fitting" orbital velocity.
... ... *gas-giant planet: In general, a planet of much larger
mass and diameter than the Earth, and which consists mostly of
gas. In our own Solar System, Jupiter, Saturn, Uranus, and
Neptune are gas-giant planets.
... ... *albedo: In this context, the fraction of total starlight
falling on a planet that is reflected from it. In general, the
albedo is equal to the amount of light reflected divided by the
amount of light received.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
GIANT PLANETS VS. BROWN DWARFS
Filipe D. Santos (Centro de Fisica da Universidade de Lisboa, PT)
presents a short review of current ideas concerning giant
extrasolar planets and *brown dwarf stars. The author makes the
following points: 1) The recent discoveries of planets orbiting
nearby Sun-like stars have revealed that planetary systems can be
surprisingly diverse. The initial discovery in 1995 of the planet
around the star 51 Pegasi was a surprise because it is apparently
a planet with mass about that of Jupiter (at least 0.44 Jupiter-
mass) and an orbital period of only 4.2 days, which implies that
the planet is 20 times closer to its star than Earth is to the
sun. 2) Seven additional planets around solar-type stars have
since been discovered, with Jupiter-mass values ranging from 0.44
to 6.84. 3) Two critical questions are, a) Where should we set
the dividing line that distinguishes massive planets from brown
dwarfs? and, b) What are the mechanisms leading to the formation
of massive planets and brown dwarfs? 4) Brown dwarfs are expected
to have masses smaller than the hydrogen-burning limit of
approximately 0.075 solar-mass (approximately 75 Jupiter-mass),
but probably larger than the deuterium-burning limit of 0.013
solar-mass (approximately 13 Jupiter-mass). 5) Like the companion
massive planets mentioned, several companion brown dwarfs to
solar-type stars have also been identified. One method of
investigating brown dwarfs involves *astrometric measurements,
and in all cases of brown dwarfs investigated by the astrometric
method, the masses are above or very close to the hydrogen-
burning limit. The extant data thus suggest that the distribution
of mass of brown dwarfs does not extend to masses as small as
giant planets. Also, the new measurements indicate that brown
dwarfs orbiting solar-type stars are very rare. 6) The discovery
of Jupiter-mass planets with orbits very close to their stars
poses a considerable problem, because it is difficult to
understand how such planets could form in place. (Five known
Jupiter-mass planets have orbital radii smaller than the distance
from Mercury to the Sun.) The suggestion has been made that these
planets formed at larger distances and migrated inward, but the
proposed migration mechanisms are not yet empirically
distinguishable. The author concludes: "Clearly the discovery of
planetary systems outside our solar system has opened a Pandora's
box of startling phenomena and new questions."
QY: Filipe D. Santos [fdsantos@milkyway.cii.fc.ul.pt]
(Science 17 Jul 98 281:359) (Science-Week 31 Jul 98)
-------------------
Related Background:
... ... *brown dwarf stars: Brown dwarf stars are formed by the
contraction of a lump of gas with a mass too small for nuclear
reactions to begin in the core. Such a star has a relatively
short-lived luminosity (approximately 100 million years) as the
result of conversion of gravitational energy to radiation. The
surface temperature of a brown dwarf is below 2500 degrees
kelvin. As recently as 1994, brown dwarfs were "theoretical"
stars, with no brown dwarfs considered to be unambiguously
identified.
... ... *astrometric measurements: This method of detection
infers the presence of a companion to a star by measuring the
position of the star as it orbits the center of mass of the
entire system. From the orbital inclination, the real mass of the
companion can be derived.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 31Jul98
[For more information: http://scienceweek.com/search/search.htm]


6. PROSPECTS FOR THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
The conjured image is poignant: intelligent life sprinkled
throughout our Galaxy, each sprinkle separated from the others by
1000 light years, each sprinkle searching for the others with
radio transmitters and receivers, small robotic spacecraft sent
beeping into empty space between the stars, the beeping like a
faint bleating in the dark as the sprinkles search for each
other. Of course, the conjured image may be wrong: there may be
intelligent life dense in the Galaxy; or we may be alone. It does
not matter. For the human species on this planet Earth, the quest
is part of our destiny, part of what we do as a species, and it
will go on as long as we remain civilized.
... ... J.C. Tarter and C.F. Chyba (SETI Institute, US) present a
review of current and future efforts in the search for
extraterrestrial intelligence, the authors making the following
points:
     1) During the past 40 years, researchers have conducted
searches for radio signals from an extraterrestrial technology,
sent spacecraft to all but one of the planets in our Solar
System, and expanded our knowledge of the conditions in which
living systems can survive. The public perception is that we have
looked extensively for signs of life elsewhere. But in reality,
we have hardly begun to search. Assuming our current,
comparatively robust space program continues, by 2050 we may
finally know whether there is, or ever was, life elsewhere in our
Solar System. At a minimum, we will have thoroughly explored the
most likely candidates, a task not yet accomplished. We will have
discovered whether life exists on Jupiter's moon Europa, or on
Mars. And we will have undertaken the systematic exobiological
exploration of planetary systems around other stars, seeking
traces of life in the spectra of planetary atmospheres. These
surveys will be complemented by expanded searches for intelligent
signals.
     2) The authors point out that although the current language
is that of a "search for extraterrestrial intelligence" (SETI),
what is being sought is evidence of extraterrestrial
technologies. Until now, researchers have concentrated on only
one specific technology -- radio transmissions at wavelengths
with weak natural backgrounds and little absorption. No verified
evidence of a distant technology has been found, but the null
result may have more to do with limitations in range and
sensitivity than with actual lack of civilization. The most
distant star probed directly is still less than 1 percent of the
distance across our Galaxy.
     3) The authors conclude: "If by 2050 we have found no
evidence of an extraterrestrial technology, it may be because
technical intelligence almost never evolves, or because technical
civilizations rapidly bring about their own destruction, or
because we have not yet conducted an adequate search using the
right strategy. If humankind is still here in 2050 and still
capable of doing SETI searches, it will mean that our technology
has not yet been our own undoing -- a hopeful sign for life
generally. By then we may begin considering the active
transmission of a signal for someone else to find, at which point
we will have to tackle the difficult questions of who will speak
for Earth and what they will say."
-----------
J.C. Tarter and C.F. Chyba: Is there life elsewhere in the
Universe?
(Scientific American December 1999)
QY: Jill C. Tarter, SETI Institute, Mountain View, Calif. US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 11Feb00
-------------------
Related Background:
ON CARBON IN THE UNIVERSE
Carbon is a major factor in the evolutionary scheme of the
Universe because of its abundance and its ability to form complex
chemical entities. It is apparently also a key element in the
evolution of prebiotic molecules. The different forms of cosmic
carbon range from carbon atoms and carbon-bearing molecules to
complex solid-state carbonaceous structures, and evidence
gathered during the past decade has considerably enhanced our
understanding of the physical and chemical properties of carbon
materials in space. ... ...  Th. Henning and F. Salama (2
installations, DE US) present a detailed review of the subject,
the authors making the following points: 1) More than 75 percent
of the 118 *interstellar and circumstellar molecules identified
to date are carbon-bearing molecules, and one component of
interstellar dust is evidently carbonaceous. The cosmic evolution
of carbon from the interstellar medium into *protoplanetary disks
and *planetesimals, and finally into habitable bodies, is
intrinsic to the study of the origin of life. 2) Carbon plays an
important role in the physical evolution of the interstellar
medium because it is the main supplier of free electrons in
diffuse interstellar clouds, thus contributing to the heating of
interstellar gas. 3) The observation of unidentified ubiquitous
molecular and solid-state features in astronomical spectra, and
the realization that these features are linked to carbonaceous
materials, have resulted in major scientific progress in the past
decade. Laboratory and theoretical studies stimulated by these
astronomical observations have led to a better understanding of
the various forms of cosmic carbon such as polycyclic aromatic
hydrocarbons, carbon-chain molecules, carbon clusters, and
carbonaceous solids. These investigations have also led to the
detection of novel forms of carbon and laid the foundations for
the chemistry of *fullerenes. 4) The authors present the
following categorization of carbon in space:
... a) Carbon-rich circumstellar envelopes around *red giant and
*asymptotic giant branch (AGB) stars: CO, C(sub2)H(sub2), complex
hydrocarbons, gas-phase polycyclic aromatic hydrocarbons.
... b) Diffuse interstellar medium: C+, simple diatomic
molecules, gas-phase polycyclic aromatic hydrocarbons and carbon
chains.
... c) Dense interstellar medium: CO, complex hydrocarbons.
... d) Interstellar material in primitive meteorites: polycyclic
aromatic hydrocarbons.
5) The authors suggest that the widespread distribution of
complex organics in the interstellar medium has profound
implications for our understanding of a) the chemical complexity
of the interstellar medium, b) the evolution of prebiotic
molecules, c) the impact of this evolution on the origin and
evolution of life on early Earth through the exogenous delivery
(by cometary encounters and meteoritic bombardments) of prebiotic
organics.
-----------
Th. Henning and F. Salama: Carbon in the Universe.
(Science 18 Dec 98 282:2204)
QY: Th. Henning, Astrophysikalisches Institut und Universitats-
Sternwarte, Schillergabchen 2-3, D-07745, Jena DE.
-----------
Text Notes:
... ... *interstellar and circumstellar molecules: In this
context, an interstellar molecule is any molecule that occurs
naturally in clouds of gas and dust in space. In general, a
circumstellar molecule is any molecule that occurs in gas and
dust surrounding a star.
... ... *protoplanetary disks: These are dust disks surrounding
young stars; it is from these disks that planets presumably form.
... ... *planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.
... ... *fullerenes: Fullerenes are large molecules composed
entirely of carbon, with the chemical formula C(sub n), where n
is any even number from 32 to over 100. They apparently have the
structure of a hollow spheroidal cage with a surface network of
carbon atoms connected in hexagonal and pentagonal rings.
... ... *red giant: A red giant star is a star in a late
stage of evolution. Having exhausted the hydrogen fuel in its
core, the star is burning elements heavier than hydrogen. It has
a surface temperature of less than 4700 degrees Kelvin and a
diameter 10 to 100 times that of the Sun.
... ... *asymptotic giant branch (AGB) stars: These are stars
that occupy a strip in the *Hertzsprung-Russell diagram that is
almost parallel to and just above what is called the "giant
branch" off the *Main Sequence. Stars evolve from the horizontal
H-R branch to the asymptotic giant branch when they have
exhausted the helium in their cores and are instead burning
helium in a shell.
... ... *Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type, and is perhaps the most useful diagrammatic aid in
astrophysics. It allows the portrayal of the evolution of a star
as occurring along various paths in the diagram.
... ... *Main Sequence: The Main Sequence is a region on the
Hertzsprung-Russell diagram where most stars lie, including our
own Sun. The evolution of a star can be diagrammed as a movement
along the Main Sequence and an eventual branching off the Main
Sequence to regions associated with various types of old stars.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 26Feb99
[For more information: http://scienceweek.com/search/search.htm]


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

IN FOCUS: ON THE UNIQUENESS OF HUMANKIND
"Every creature alive is the product of a unique history. The
statistical probability of its precise reduplication on another
planet is so small as to be meaningless. Life, even cellular
life, may exist out yonder in the dark. But high or low in
nature, it will not wear the shape of man. That shape is the
evolutionary product of a strange, long wandering through the
attics of the forest roof, and so great are the chances of
failure, that no pseudo-human thing is likely ever to come that
way again... In a universe whose size is beyond imagining, where
our world floats like a dust mote in the void of night, men have
grown inconceivably lonely. We scan the time scale and the
mechanisms of life itself for portents and signs of the
invisible. As the only thinking mammals on this planet -- perhaps
the only thinking animals in the entire sidereal universe -- the
burden of consciousness has grown heavy upon us. We watch the
stars, but the signs are uncertain. We uncover the bones of the
past and seek for our origins. There is a path there, but it
appears to wander. The vagaries of the road may have a meaning
however; it is thus we torture ourselves. Lights come and go in
the night sky. Men, troubled at last by the things they build,
may toss in their sleep and dream bad dreams, or lie awake while
the meteors whisper greenly overhead. But nowhere in all space or
on a thousand worlds will there be men to share our loneliness.
There may be wisdom; there may be power; somewhere across space
great instruments, handled by strange, manipulative organs, may
stare vainly at our floating cloud wrack, their owners yearning
as we yearn. Nevertheless, in the nature of life and in the
principles of evolution, we have had our answer. Of men
elsewhere, and beyond, there will be none forever."
-----------
Loren C. Eisley: _The Immense Journey_
(Random House, New York 1953)


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