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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.
November 20, 1998 -- Vol. 2 Number 47
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The pursuit of science has often been compared to the scaling of
mountains, high and not so high. But who amongst us can hope,
even in imagination, to scale the Everest and reach its summit
when the sky is blue and the air is still, and in the stillness
of the air survey the entire Himalayan range in the dazzling
white of the snow stretching to infinity? None of us can hope for
a comparable vision of nature and of the universe around us. But
there is nothing mean or lowly in standing in the valley below
and awaiting the Sun to rise over Kinchinjunga.
-- Subrahmanyan Chandrasekhar (1910-1995)
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Contents of This Issue:
1. Against Programming Future Fundamental Research
2. Star Formation and Galaxy Collisions
3. Earthquake Triggering of Volcanic Eruptions
4. A Scientific Basis for Climate Forecasting
5. A Molecular Dating Test of the Cambrian Explosion Hypothesis
6. Protein Thermostability Above 100 Degrees Celsius
7. Beta-Amyloid Precursor Protein and Memory
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1. AGAINST PROGRAMMING FUTURE FUNDAMENTAL RESEARCH
Given that science is an enterprise essential to the well-being
of society, the perennial questions are how to fund it, what to
fund, and with how much money? Often, the attitude is that there
is all this money to be used, and with clever policy decisions we
ought to be able to choose and spur the right horse to get us
where we want to go as fast as possible. The difficulty is that
choosing the "right" horse is never obvious, and there is often a
danger that a management process may retard rather than enhance
scientific progress. ... ... Paul Berg and Maxine Singer, in an
opinion piece, consider the dangers of managed science in the
context of modern biology. The authors make the following points:
1) Considering the past 150 years of biological research
(especially genetics), it is clear that success has frequently
been contingent on the choice of the experimental system, and
that often what turned out to be an important experimental system
was first developed on the fringes of science and not in the
mainstream. 2) Mendel's 19th century breeding experiments with
*pea plants defined the early science of genetics, and his work
was first rediscovered at the turn of the century by plant
breeders who contributed to the extension and generalization of
mendelian ideas and to the development of the American corn
industry. 3) By 1914, the fruit fly *Drosophila melanogaster had
displaced corn as the more advantageous organism for genetic
investigation, resulting in the work of T.H. Morgan, A.H.
Sturtevant, C.B. Bridges, H.J. Muller, and their students. 4) At
about 1935, Drosophila proved inadequate to pursue the extant
questions in genetics, and the new experimental system was the
common bread mold *Neurospora crassa, adopted as an experimental
tool by B.O. Dodge, C. Lindegren, G.W. Beadle, and E. Tatum. From
this work came the idea that each gene is responsible for one
enzyme required for the synthesis of a particular cellular
constituent. 5) By the late 1940s, Tatum had adopted a new
experimental system for examining the relation between genes and
cellular functions, the common intestinal bacterium *Escherichia
coli. 6) In the 1950s, attention shifted to still another
experimental system, the viruses (*bacteriophages) that attack
the E. coli bacterium, these viruses having a simpler structure
than bacteria but containing organized genomes. It was the study
of bacteriophages that stimulated Watson and Crick's efforts to
determine the structure of DNA. 7) In the 1960s and 1970s, *yeast
became an experimental system essential to biological research,
with certain yeast genes virtually identical to certain human
genes. 8) More recently, the *nematode worm *Caenorhabditis
elegans has revealed unexpected attributes of the developmental
process, especially the programmed cell death (*apoptosis) of
certain *differentiated cells. 9) Current genetic analysis takes
advantage of decades of work on mutant mice, and recently
genetically altered mice have provided special tools for the
study of gene replacement, very early development, and disease
pathology. 10) Plant genetics research has become a focus again
with attention on the plant *Arabidopsis thaliana, an easily
maintained laboratory plant with a small genome and a rapid life
cycle. The authors conclude: "Nothing in the human-made world
rivals the complexity and diversity of living things. There are,
in nature, concepts that no one has yet imagined. Looking back
over the past 150 years... it seems that the fringes, not the
mainstream, are the most promising places to discover
revolutionary advances. Attempts to program the direction and
tools of genetic research could not have foreseen the diverse
sources from which progress resulted. The lesson is that those
who attempt to program future fundamental research, however well
motivated by medical, agricultural, or social needs, are likely
to divert researchers from the fringes where the most promising
discoveries are often made."
-----------
P. Berg and M. Singer (2 installations, US)
Inspired choices.
(Science 30 Oct 98 282:873)
QY: Paul Berg, Stanford University 415-723-3058.
-----------
Text Notes:
... ... *pea plants: The advantage of Mendel's pea plants was the
possibility of controlled pollination and development of highly
inbred varieties with clearly defined traits. Although it is
often not mentioned, the monk and priest Gregor Mendel (1822-
1884) had training in mathematics and science at the University
of Vienna, and he was in fact a science teacher before his work
with peas began in 1857. When Mendel sent his unpublished paper
to Nageli, an eminent but classical biologist, Nageli was
apparently repelled by the mathematics. Mendel finally published
in 1865 in an obscure journal, the *Transactions of the Brno
Natural History Society*. The work remained ignored and unnoticed
until 1900, when the botanist Hugo De Vries came across the paper
and brought the Mendelian laws of inheritance to the attention of
the scientific world.
... ... *Drosophila melanogaster: A major advantage of this
experimental system is the presence of giant chromosomes in the
insect's salivary glands. (In cells with chromosomes, the
chromosomes are the physical structure into which DNA is
organized and on which genes are carried.) Drosophila also has a
short reproductive cycle (approximately 10 days), and it produces
100 to 400 progeny per mating.
... ... *Neurospora crassa: In the wild (i.e., natural) state
this mold will grow on a nutrient medium containing sugar as the
only organic compound except for a small required concentration
of biotin. Induced mutations (e.g., produced by x-rays) can
result in mutants that require other organic substances, and
systematic analysis of the genetics of these mutants and their
new requirements made possible an understanding of the genetics
of a number of biochemical pathways and of the enzymes that
control these pathways.
... ... *Escherichia coli: This is a rather ubiquitous bacterium
present in the intestinal tracts of animals, in soil, and in
water. Its advantage as an experimental system is its simple
genetic machinery and rapid growth characteristics. E. coli was
the first animal organism used for cloning and propagating the
genes of other species.
... ... *bacteriophages: Bacteriophage is a virus that
infects bacteria, the virus consisting essentially of a naked
strand of DNA surrounded by a complex polyhedral shell ("capsid")
composed mainly of glycoproteins.
... ... *yeast: Yeast are unicellular fungi that reproduce by
budding. The most important yeast species in research is the
common bread and beer yeast Saccharomyces cerevisiae.
... ... *nematode: An abundant and ubiquitous phylum of
unsegmented roundworms.
... ... *Caenorhabditis elegans: This is a small (1 mm) nematode
worm. It is transparent, hermaphroditic, free-living, and found
in soil. It has a relatively small genome (approximately 3000
genes), and only a few types of cells in its body. It has a 16-hr
embryogenesis that can be achieved in a petri dish, and is thus
highly suitable for the study of developmental and behavioral
genetics.
... ... *apoptosis: In general, the term "apoptosis" refers to
programmed cell death, whether as a part of normal tissue
differentiation and development, or as a program activated in a
defective cell. In the molecular biology of cancer, apoptosis is
the name given to the programmed cell death provoked by the
proteins expressed by tumor suppressor genes. Thus, malignant
cells are defective cells with a deactivated apoptosis program,
and this allows malignant cells to survive and replicate.
... ... *differentiated cells: Refers to developmental cell
specialization (morphology and biochemistry) resulting from
activation (and/or deactivation) of specific parts of the cell
genome.
... ... *Arabidopsis thaliana: (thale cress) A weed of the
mustard family with a small genome of 120 million base pairs.
Arabidopsis is now an important laboratory species, and it is
presently the model for physiological, biochemical, cell
biological, and developmental studies of over 250,000 plant
species.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 20Nov98
2. STAR FORMATION AND GALAXY COLLISIONS
Galaxies, systems of stars, interstellar gas, and interstellar
dust all bound together by gravity, range from dwarf galaxies of
a million stars to supergiant galaxies with over 10^(12) stars.
During this century, our views concerning galaxies other than our
own have undergone considerable revision. ... ... R. Genzel et al
present a review of recent work on galaxy collisions, and the
authors make the following points: 1) Not until 1920 was it
established that "galaxies" are large concentrations of stars and
interstellar gas like our Milky Way, but located large distances
away. Until the mid 1950s, the accepted view was that galaxies
are "island universes" very much isolated from each other. 2) In
1956, The astronomer Fritz Zwicky called attention to filaments
and jets of stars in adjacent galaxies, and he suggested these
could be large scale "tidal effects" due to the galactic mutual
gravitational interaction. But Zwicky's interpretation met with
widespread skepticism until numerical models indicated the slow
but close passage of two galaxies can create long tails, bridges,
and spiral structures purely as the result of the gravitational
forces and torques between the two colliding galaxies. 3) The
consensus view of galaxies changed after the mid-1980s as a
result of the first space-borne *infrared telescope (IRAS), which
surveyed the entire sky in the 12 to 120 micron band. Although
most of the several tens of thousands of galaxies identified by
IRAS are normal galaxies like our own Milky Way, there are also
now hundreds of identified galaxies whose total energy output is
dominated by emission in the far infrared band, and the most
spectacular of these are the ultra-luminous infrared galaxies,
which have luminosities resembling that of extreme *active
galactic nuclei such as *quasars. Recent observations have
established that most of these ultra-luminous infrared galaxies
are extreme interacting systems with strong signatures of recent
tidal disturbances, in some cases showing that the two galaxies
are actually close to merging into a single system. The key
questions are, a) Why is there such a strong correlation between
infrared luminosity and galaxy interaction? b) What powers these
galaxies (active star formation or *black holes)?, and c) How are
these galaxies evolving with time? The authors summarize: "It is
becoming increasingly clear that collisions between galaxies play
an important role in galaxy evolution. The ultraluminous infrared
galaxies are predominantly powered by enormous star-formation
events that are triggered in the last phases of such collisions.
These bursts occur just before the galaxies merge to form single
*elliptical galaxies."
-----------
R. Genzel et al (Max Planck Institute for Extraterrestrial
Physics Garching, DE)
Star formation triggered by galaxy collisions.
(Nature 29 Oct 98 395:859)
QY: Reinhard Genzel, Max-Planck Institut fur Extraterrestrische
Physik, 85740 Garching, DE.
-----------
Text Notes:
... ... *infrared telescope: Infrared radiation involves
wavelengths between the red end of the visible spectrum and the
radio bands of the spectrum, from about 0.8 microns to about 1000
microns. The submillimeter infrared band lies between the
far-infrared band (at about 0.1 millimeter) and the microwave
band (above 1 millimeter). Submillimeter wavelengths trace
directly the emission from dust that has been warmed by massive
star-formation activity.
... ... *active galactic nuclei: Central regions of galaxies in
which considerable energy is generated by processes other than
those operating in ordinary stars. The energy may result from the
accretion of material into a massive black hole situated at the
core of the galaxy.
... ... *quasars: (quasi-stellar objects). Extremely luminous
sources radiating energy over the entire spectrum from x-rays to
radio waves, and which are apparently the oldest and most distant
objects in the universe. They are believed to involve massive
*black holes
... ... *black holes: If the terminal stages of star death leave
a remnant star mass greater than 3 solar masses, the ultimate
gravitational collapse will produce a "black hole", a
relativistic singularity. A black hole is a localized region of
space from which neither matter nor radiation can escape. The
"trapping" occurs because the requisite escape velocity, which
can be calculated from the relevant equations, exceeds the
velocity of light and is therefore unattainable. Another view of
a black hole is that it is a mass that has collapsed to such a
small volume that its gravity prevents the escape of all
radiation. If quasars indeed involve black holes, the radiation
is from material just outside the black hole, and not from
anything within it. Nothing inside a black hole can get out of
it.
... ... *elliptical galaxies: In 1925, the astronomer Edwin
Hubble introduced a classification scheme for galaxies that is
now widely used. The scheme has 3 main types of galaxy:
elliptical, spiral, and barred spiral, and each is further
divided into morphological subtypes. Elliptical galaxies are
apparently composed of old stars, contain little dust, are the
central dominant galaxies in rich clusters, and many are powerful
sources of radio wavelength radiation. The size of elliptic
galaxies ranges from dwarf ellipticals (10^(8) stars) to
supergiant ellipticals (10^(13) stars). Elliptical galaxies are
believed to be a stage in the evolution of galaxies, and
different cosmological models make specific predictions about the
number of elliptical galaxies as a function of redshift. In
principle, therefore, observations of elliptical galaxies can be
used to discriminate between models.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 20Nov98
-------------------
Related Background:
ON THE EVOLUTION OF GALAXIES IN A CHANGING UNIVERSE
In a detailed review of research in this century concerning the
evolution of galaxies, Alan Dressler (Carnegie Observatories
Pasadena, US) makes the following points: 1) When the century
opened, most astronomers assumed the Universe was eternal and
basically changeless, its general structure immutable. In 1915,
with the publication of Einstein's general theory of relativity,
there were implications of the cosmic role of gravity, but these
implications were for the most part ignored. Indeed, even after
1924 and the proof by *Edwin Hubble (1889-1953) that the spiral
nebulae are other galaxies at vast distances, astronomers were
slow to recognize the implications of the new observations. 2)
The first big step in changing the view of the cosmos was the
construction by *George Ellery Hale (1868-1938) of the 100-inch
Hooker reflector on Mount Wilson (US), a project completed in
1918. This was the telescope used by Hubble and his colleagues to
reveal the large-scale organization of the Universe into
galaxies, the vast size of the Universe, and the expansion of the
Universe. *George Lemaitre (1894-1966) soon proposed that the
expansion of the Universe implied a dense explosive birth of the
Universe at a specific finite time in the past (the event that
came to be called the Big Bang). 3) An even greater telescope was
needed, and again George Hale led the way in the building of the
200-inch reflector on Palomar Mountain (US), that instrument
finally completed in 1948. The 200-inch telescope produced the
first observations of galaxy evolution -- the first evidence that
galaxies observed at high *redshift are unlike galaxies closer to
us in time. An even greater accomplishment of the 200-inch
telescope was a series of observations concerning the spectra of
*quasars, and the evidence for their immense distances and
luminosities. Theorists eventually proposed that quasars were
*black holes of 100 million solar-masses or more. It is now clear
that most galaxies with a central bulge, including our own
Galaxy, harbor massive black holes at their cores. 4) In 1961,
Allan Sandage published a landmark paper outlining the
possibility of testing cosmological models with the 200-inch
telescope, and over the next two decades Sandage devoted himself
to this project. Unfortunately, the underlying premise of the
project -- that the brightest galaxy in every galactic cluster
has about the same true luminosity -- was demonstrated by
*Beatrice Tinsley (1941-1981) to be untenable. 5) In the fall of
1977, at a Yale University (US) conference on the evolution of
galaxies, Harvey Butcher and Augustus Oemler presented their
evidence for relatively young star-forming galaxies. This
evidence, which implied strong galaxy evolution during relatively
recent cosmic time, met with controversy and skepticism. 6) In
the 1980s, observations by various groups proved that Butcher and
Oemler were correct, and it was now understood that these
relatively young galaxies were often producing new stars in huge
bursts. These bursting galaxies are evidently spirals with a more
disheveled appearance than is common today, and in their twisted
and distorted disks huge numbers of stars were recently born. 7)
During the past 2 years, among the most interesting results of
various observations with various instruments is the formulation
of the so-called Madau diagram (popularized by Piero Madau) that
plots the Universe-wide rate of star formation from early times
to today, spanning almost the whole history of the cosmos. The
rate of star formation apparently rose rapidly in the first few
billion years, the peak rate at about 5 or 6 billion years later
at redshifts of 1 to 2. (Our Sun apparently formed at a time
corresponding approximately to redshift = 0.5.) The author
concludes: "Our generations are fortunate to live to see one of
the great mysteries of where we came from in process of being
solved."
QY: Alan Dressler, Carnegie Observatories, Pasadena, Ca. US.
(Sky & Telescope October 1998) (Science-Week 11 Sep 98)
-------------------
Related Background:
... ... *Edwin Hubble (1889-1953): Hubble first studied law
before switching to astronomy at the age of 25. He began his work
at the Mount Wilson Observatory with the 100-inch telescope at
the age of 30. In 1941, at the age of 52, he tried to join the US
Army to fight the Nazis, but he was persuaded that he could do
more in war-related research.
... ... *George Ellery Hale (1868-1938): Hale is best known for
his work building large-telescopes (and for obtaining the funds
for the Yerkes Observatory, named after the street-car magnate
Charles Tyson Yerkes), but already at the age of 21 he invented
the spectroheliograph, a device that made it possible to
photograph the light of a single spectral line of the sun, and he
made several ground-breaking observations with this instrument.
... ... *George Lemaitre (1894-1966): Lemaitre began his
professional life as a civil engineer, then at 21 he switched to
physics and mathematics. He also became a Roman Catholic priest
at the age of 22. After obtaining his PhD at the Massachusetts
Institute of Technology in 1927, he settled in Belgium as a
professor of astrophysics at the University of Louvain. At the
time of his death, he was president of the Pontifical Academy of
Sciences at Rome. Lemaitre's theoretical ideas concerning the
origin of the Universe were published in 1927, when he was 31,
but the paper was largely unnoticed until the astrophysicist
Arthur Eddington (1882-1944) called attention to it much later.
... ... *redshift: Redshift (symbol: z) is a lengthening of the
wavelengths of electromagnetic radiation from a source caused
either by the movement of the source (Doppler effect) or by the
expansion of the universe (cosmological redshift). Redshift is
defined as the change in wavelength of a particular spectral line
divided by the unshifted wavelength of that line. Large redshifts
imply large radial velocities (which imply large distances,
according to current cosmological theory), but at redshifts
greater than about 0.2 there is a relativistic divergence from a
linear relation. A redshift of 4.0 corresponds to an object
receding with a radial velocity 92% that of the velocity of
light. The largest astrophysical redshifts so far observed are of
the order of z = 4.9.
... ... *quasars: (quasi-stellar objects). Extremely luminous
sources radiating energy over the entire spectrum from x-rays to
radio waves, and which are apparently the oldest and most distant
objects in the universe.
... ... *black holes: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation. Space and
time essentially have no meaning in a black hole. The boundary of
the black hole is called the "event horizon", because any event
within the boundary is invisible outside, the invisibility
resulting from the fact that no radiation can escape to be
detected. The radius of the black hole depends upon how much
matter has fallen into the region; it is called the "Schwarzchild
radius", and it is usually a few kilometers. However, massive
black holes are possible and are thought to be the source of
quasars. If quasars indeed involve black holes, the radiation is
from material just outside the black hole, and not from anything
within it. Nothing inside a black hole can get out of it.
... ... *Beatrice Tinsley (1941-1981): During her short life,
Tinsley managed to be a force in astronomy from her first entry
into the field. At the age of 25, an unknown graduate student at
the University of Texas, she rose before an audience about to
hear Allan Sandage and publicly challenged his idea that giant
elliptical galaxies exhibited luminosities constant enough to be
used as "standard candles" to estimate distances. She proved her
point by the age of 36, and the variability of galaxy
luminosities became the consensus view. It was Tinsley who co-
hosted the 1977 Yale conference that set the course of galaxy-
evolution studies. She died 4 years later of cancer. Near the
end, she wrote the following: "Let me be like Bach, creating
fugues; till suddenly the pen will move no more."
-------------------
Related Background:
ON THE FORMATION OF ELLIPTICAL GALAXIES
... The furthest galaxy on record is at a redshift z = 4.92),
which implies a distance of approximately 13 billion light years
(i.e., an object date of 13 billion years ago.) The term "near
infrared" refers to the range 0.8 to 8 microns. Now S. Zepf
(Univ. of California Berkeley, US) reports an analysis of deep
optical and near-infrared images indicates there are fewer
galaxies with very red colors than predicted by models that
propose completion of elliptical galaxy star formation by
approximately z = 5. This suggests that elliptical galaxies must
have significant star formation at z < 5. The author proposes
that either ellipticals form at moderate redshifts, where a large
initial burst of star formation is shrouded by dust, or that they
form through the merging of smaller galaxies.
QY: Stephen A. Zepf
(Nature 27 Nov 97) (Science-Week 19 Dec 97)
-------------------
Related Background:
ASTRONOMERS AGREEING ON THEORY OF GALAXY FORMATION AND EVOLUTION
Among contemporary cosmologists, there are two prevailing models
for the formation of galaxies. One model is hierarchical, in the
sense that small amorphous proto-galaxies are considered to form
first, these evolving into spiral galaxies, and the spiral
galaxies then merging to form elliptical galaxies. The other
model is a completely different picture, considering the various
galaxies to form from the condensation of single massive dust
clouds, with the particular type of galaxy formed dependent on
the nature of the dust cloud collapse. At two recent cosmology
symposia, in view of new red-shift data (shifts to the red end of
the spectrum of light from the galaxies) provided by the Hubble
Space Telescope, cosmologists are apparently forming a consensus
that the first idea, the hierarchical model, is more consistent
with the observed data than the second model. The Hubble Space
Telescope has brought a renaissance to cosmology, and we are only
at the beginning of the new era.
(Nature 26 Jun 97) (Science-Week 3 Jul 97)
3. EARTHQUAKE TRIGGERING OF VOLCANIC ERUPTIONS
Although earthquakes and volcanic eruptions are each
manifestations of large-scale *tectonic plate and mantle motions,
it is apparently usually thought that the occurrences of these
events are not directly related. There have been some studies,
however, in which triggering of volcanic eruptions by earthquakes
remote from the volcano has been proposed. ... ... A.T. Linde and
I.S. Sacks now report an examination of the historical record of
volcanic eruptions and major earthquakes during the past 400
years to determine if there are indeed significantly more
eruptions immediately following large earthquakes. The authors
report that within a day or two of large earthquakes there are
many more eruptions within a range of 750 kilometers than would
otherwise be expected. Additionally, the author point out that it
is well known that volcanoes separated by hundreds of kilometers
frequently erupt in unison. The authors suggest that the
characteristics of such eruption pairs, and their own historical
determinations, are consistent with the hypothesis that the
second eruption is triggered by earthquakes associated with the
first eruption. The authors suggest that their work emphasizes
the need for a program of continuous tectonic deformation
monitoring of active volcanoes on a global scale, with sufficient
sensitivity to be able to detect deformation transients on short
timescales. The authors conclude: "Very few active volcanoes are
currently monitored well enough to allow detection of potentially
important transient deformations."
-----------
A.T. Linde and I.S. Sacks (Carnegie Inst. of Washington, US)
Triggering of volcanic eruptions.
(Nature 29 Oct 98 395:888)
QY: Alan T. Linde
-----------
Text Notes:
... ... *tectonic plate and mantle motions: Seismic studies
indicate the interior of the Earth consists of three parts: a
metallic core, a dense rocky mantle, and a thin low-density
crust. The central part of the core is solid, but the outer part
of the core is evidently liquid. The mantle, the layer of dense
rock and metal oxides between the molten part of the core and the
surface, has plastic properties (i.e., it is a solid capable of
flow under pressure). The term "lithosphere" refers to the outer
layer of the Earth, comprising the crust and upper mantle, and
extending to a depth of 50 to 70 kilometers. The traditional view
of tectonics (changes in the structure of the Earth's crust) is
that the lithosphere consists of a strong brittle layer overlying
a weak ductile layer, the system producing two forms of
deformation, namely, brittle fracture in the upper layer
(accompanied by earthquakes), and aseismic (without earthquakes)
ductile flow in the lower layer. The current consensus is that
this view is generally correct but imprecise, since the
accumulated evidence is now interpreted to indicate that
frictional events along fault lines, rather than new fractures,
are the causes of earthquakes. Plate tectonics is the current
consensus theory that the Earth's lithosphere is broken into
fairly rigid plates, seven major plates and many smaller plates,
and that convection within the underlying less rigid
"asthenosphere" causes the plates (and the associated continents
and crust) to move relative to each other, the movement
manifested in *continental drift and *sea-floor spreading.
... ... *continental drift: The slow movement of the Earth's land
masses, a shifting across the underlying molten material.
... ... *sea-floor spreading: The process whereby sea floor is
continuously created as the crustal plates move apart and
continuously destroyed where the plates push against each other.
4. A SCIENTIFIC BASIS FOR CLIMATE FORECASTING
At the beginning of the 20th century it was believed that it
should be possible to predict weather by solving the mathematical
equations that describe the physical laws that govern the motion
of air. It took several decades to develop an appropriate set of
equations and numerical and computational techniques to solve
these equations, and by 1960, routine weather predictions using
global observations, complex mathematical equations, and fast
computers seemed immediately at hand. The promise, however, was
short lived, because in the early 1960s it was discovered that
the mathematical equations for weather forecasting represent a
forced *dissipative nonlinear dynamic system that exhibits
*chaotic behavior, which means that even an infinitesimally small
uncertainty in the initial conditions will grow exponentially to
make the forecast useless after a finite amount of time.
... ... J. Shukla now presents a study involving analyses of
weather observations coupled with model simulations. The author
reports that although the Earth's atmosphere is generally
considered to be an example of a chaotic system that is
sensitively dependent on initial conditions, certain regions of
the atmosphere are an exception, with wind patterns and rainfall
in certain regions of the tropics so strongly determined by the
temperature of the underlying sea surface that they do not show
sensitive dependence on the initial conditions of the atmosphere.
The author suggests it should therefore be possible to predict
the large-scale tropical circulation and rainfall for as long as
the ocean temperature can be predicted, and that if changes in
tropical Pacific sea-surface temperature are quite large, even
the extratropical circulation over some regions, especially over
the Pacific-North American sector, is predictable. In particular,
the author suggests that for all future major *El Nino events, it
should be possible to predict large-scale changes in the winter
season mean circulation over North America several months in
advance, provided we can predict tropical sea surface
temperatures.
-----------
J. Shukla (George Mason University, US)
Predictability in the midst of chaos: A scientific basis for
climate forecasting.
(Science 23 Oct 98 282:728)
QY: J. Shukla, George Mason University 703-993-2400
-----------
Text Notes:
... ... *dissipative: In general, a dissipative system
is a system that loses energy by conversion of energy into heat.
... ... *chaotic behavior: In the study of physical systems, the
term "chaotic behavior" has a specific meaning: the behavior of a
system is said to be "chaotic" if its final state is so
sensitive to the system's precise initial conditions that the
behavior of the system is in effect unpredictable and cannot be
distinguished from a random process, even though the behavior of
the system is strictly determinate in a mathematical sense. In
other words, a deterministic system characterized by extremely
sensitive instabilities, despite the system being determinate,
can exhibit behavior that is unpredictable, and the system is
then called "chaotic". During the past several decades, the
analysis of such chaotic systems has intrigued both physicists
and mathematicians.
... ... *El Nino: El Nino is an aperiodic intermittent (2 to 10
years) flow of unusually warm surface water along the western
coast of South America, the flow capable of causing abnormally
high rainfall in usually dry areas and severe local ecosystem
dislocations -- what is termed an El Nino "event". El Ninos are
regional phenomena, but they have global consequences. The name
"El Nino" ("The Child") arose because the phenomenon usually
occurs around Christmas. In 1986, M.A. Cane and S.A. Zebiak
proposed a model for making forecasts of El Nino several seasons
ahead by applying Newton's equations of motion and the laws of
thermodynamics to the dynamics of the ocean and atmosphere of the
tropical Pacific.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 20Nov98
5. A MOLECULAR DATING TEST OF THE CAMBRIAN EXPLOSION HYPOTHESIS
The early history of the *Metazoa, whether the Metazoa originated
as part of a *Cambrian "explosion" or with an extended
*Precambrian "phylogenetic fuse", remains controversial in
evolutionary biology. The Cambrian explosion hypothesis -- that
the phyla and even classes of the animal kingdom originated in a
rapid evolutionary radiation at the base of the Cambrian at 545
to 560 million years ago -- rests on the sudden appearance of a
diverse range of animals in the fossil record. Although recent
discoveries of *Ediacaran metazoans have extended the record of
sponges and bilateral animals to 570 million years ago, the
biological affinities of many Ediacaran organisms remains
controversial, and the earliest paleontological evidence of
metazoan life is no more than 600 million years ago. However, the
absence of earlier metazoan fossils could have been caused by
systematic biases in preservation that left the Precambrian
history of recognized phyla unrecorded in fossils. Molecular
studies have the potential to shed light on the origin of the
animal phyla by providing independent estimates of the divergence
times, but molecular studies have been criticized for failing to
account adequately for variations in rate of evolution.
... ... L. Bromham et al now present a study involving a method
of dating divergence times from molecular data (both
*mitochondrial DNA and nuclear DNA) which addresses the
criticisms of earlier studies, and which the authors state
provides more realistic but wider confidence intervals. The
authors report their data are not compatible with the Cambrian
explosion hypothesis as an explanation for the origin of metazoan
phyla, and they suggest their data provide additional support for
an extended period of Precambrian metazoan diversification. The
authors conclude: "Although we cannot provide precise estimates
of the origin of metazoan phyla, we can use our results to
confidently reject the Cambrian explosion hypothesis, which rests
on a literal interpretation of the fossil record and assumes that
special evolutionary phenomena, capable of producing profound
differentiation in a short period, operated in the Cambrian but
not before or since. By contrast, the Precambrian phylogenetic
fuse hypothesis assumes no more than we already know to be
reasonable: that lineages can diverge gradually over time and
that the fossil record contains gaps that can greatly reduce the
chances of finding fossils for certain periods or particular
types of organisms.
-----------
L. Bromham et al (3 installations, UK NZ)
Testing the Cambrian explosion hypothesis by using a molecular
dating technique.
(Proc. Natl. Acad. Sci. US 13 Oct 98 95:12386)
QY: Lindell Bromham
-----------
Text Notes:
... ... *Metazoa: In general, the term "metazoa" refers to all
multicellular animals. Among important distinguishing
characteristics of metazoa are cell differentiation and
intercellular communication. For certain multicellular colonial
entities such as sponges, some biologists prefer the term
"parazoa".
... ... *Cambrian "explosion": The geological period known as the
Cambrian is the time frame from about 505 million years ago to
545 million years ago. Its most outstanding aspect is the rather
sudden appearance of numerous invertebrate fossils, so numerous
that some have termed it an explosion of evolutionary processes.
Many of the life forms that existed during the Cambrian are long
extinct, but their fossils are numerous, and through their
fossils the various Cambrian species have been the subject of
much study by paleobiologists. The Cambrian explosion of life
forms has been a long-standing puzzle for paleobiologists, and at
present there is apparently no single generally accepted
explanation. Among the ideas proposed have been, 1) that the
explosion of new forms resulted from a sudden increase in
atmospheric oxygen; 2) that the explosion is only apparent, and
the Precambrian, the period previous to the Cambrian, lacks
fossils because of heat and pressure associated with important
geological changes; 3) that living forms evolved mostly in
freshwater areas, and are therefore absent in Precambrian
sediments, which are primarily marine; 4) that changes in the
shape and extent of shorelines produced by continental drift
dramatically transformed climate and environment; 5) that the
previous evolution of DNA recombination and regulatory genes
culminated in and sparked the diversity and anatomical complexity
manifested in the explosion; 6) that an exponential increase of
species could become significant only after attaining a threshold
value at the start of the Cambrian; and, 7) that once
multicellular organisms appeared, the intrinsic possibilities for
variation increased enormously with a resultant explosion of
evolved forms. Unfortunately, there is no evidence to suggest a
selection of one of these proposals, although some of them are
less convincing than others. And of course the truth may be that
more than one factor was involved. No matter the origin, the
Cambrian explosion is apparently accepted by most paleobiologists
as a real discontinuity, a period that saw the sudden emergence
of dozens of new orders and phyla, including sponges, annelids,
crustaceans, hemichordates, brachiopods, and mollusks.
... ... *Precambrian "phylogenetic fuse": The term "fuse" here is
a metaphor for a proposed long series of precursor events leading
to the apparent "explosion" in the fossil record. The essential
question is whether the apparent explosion in the fossil record
was the result of relatively sudden changes in preservation
constraints (e.g., hard-body vs. soft-body forms), or whether a
set of special conditions (e.g., climate change or atmospheric
oxygen availability) provoked an actual rapid diversification of
forms.
... ... *Ediacaran metazoans: The term "Ediacaran" refers to an
assemblage (until recently the oldest) of soft-bodied marine
animals, the assemblage first discovered in the Ediacara Hills in
Australia.
... ... *mitochondrial DNA: Mitochondrial DNA (sometimes denoted
as mtDNA), found in the mitochondria of all eukaryotes, is
believed to evolve in parallel with nuclear DNA, but since sperm
lose their mitochondria, it is inherited only in the maternal
lineage in animals.
-------------------
Related Background:
EVOLUTION: 1 BILLION YR OLD FOSSILS OF TRIPLOBLASTIC ANIMALS
There are two contradictory views concerning the early history of
*metazoans. The "*Cambrian explosion" hypothesis is based on
Cambrian shell fossils and *Burgess-type exceptionally well-
preserved fossil deposits ("lagerstatten"). This hypothesis
suggests that animal phyla originated rather suddenly
approximately 580 million years ago during the *Proterozoic-
Phanerozoic transition. The alternative hypothesis (called "slow
burn") suggests that animals developed more slowly, first
appearing, according to some molecular analyses, more than 1
billion years ago. ... ... A. Seilacher et al now report data
from the Mesoproterozoic Chorhat Sandstone site in central India.
The authors report that certain *bedding plane features are
biological and can be interpreted as the burrows of wormlike
undermat miners (i.e., *infaunal animals that excavated tunnels
underneath *microbial mats). The authors suggest these burrows
indicate that *triploblastic animals existed more than a billion
years ago. The authors further suggest their evidence indicates
that the diversification of animal designs proceeded very slowly
before the appearance of organisms with hard skeletons during the
Cambrian evolutionary explosion, and that the diversification
occurred before the ecological changes that accompanied that
explosion.
-----------
A. Seilacher et al (3 installations, DE IN US)
Triploblastic animals more than 1 billion years ago: Trace fossil
evidence from India.
(Science 2 Oct 98 282:80)
QY: Friedrich Pfluger, Yale University, 203-432-4771.
-----------
Text Notes:
... ... *metazoans: In general, the term "metazoa" refers to all
multicellular animals. Among important distinguishing
characteristics of metazoa are cell differentiation and
intercellular communication. For certain multicellular colonial
entities such as sponges, some biologists prefer the term
"parazoa".
... ... *Cambrian: Refers to the approximate general time-frame
500 to 600 million years ago (but often considered as 505 to 545
million years ago).
... ... *Burgess-type: The Burgess Shale fossil deposit site in
British Columbia (CA) is apparently a geological accident of
superb preservation, and one that has revealed to paleontologists
the limitations of their views of the varieties of life in the
*Cambrian period. All the fossils expected for the Cambrian are
found in the Burgess Shale, but studies have demonstrated in
addition a remarkable array of perfectly preserved soft-bodied
animals, including a variety of worms, *arthropods, sponges,
*brachiopods, and some bizarre forms seemingly unrelated to any
known groups. The consequence was a new appreciation among
paleontologists of the tremendous increase in metazoan diversity
that apparently occurred during the Cambrian period, and indeed
this increase is now considered one of the most important events
in the history of life on Earth.
... ... *arthropods: The largest phylum in the Animal Kingdom in
terms of both number of taxa and biomass, but the taxonomy has
undergone revision, and it is now essentially an informal
classification. In general, the arthropods are characterized by a
tough chitinous protective covering (exoskeleton) flexible only
at the joints (e.g., insects).
... ... *brachiopods: A phylum of bivalve coelomate invertebrates
resembling bivalve molluscs. The term "bivalve" refers to the
presence of a hinged shell; the term "coelomate" refers to the
presence of a true body cavity.
... ... *Proterozoic-Phanerozoic transition: The Proterozoic eon
is the time-frame 2600 million years ago to 600 million years
ago. The Phanerozoic eon is the time-frame 600 million years ago
to the present. During the Phanerozoic, atmospheric oxygen
apparently increased by a factor of 10 to its present value.
... ... *bedding plane: A distinct surface separating two
strata, the bedding plane marking a break in the continuity of
sedimentation, a break that can be caused by a period of erosion
or a cessation of sediment supply.
... ... *infaunal: Refers to an organism that burrows into the
substrate.
... ... *microbial mats: A dense microbial layer. The authors
propose this served as a food source for wormlike animals
exploiting its decaying base.
... ... *triploblastic animals: These are animals with a body
organization derived from 3 germ layers (ectoderm, endoderm,
mesoderm). The category includes all metazoans except
*coelenterates, which are diploblastic.
... ... *coelenterates: A phylum of mostly marine multicellular
animals. (e.g., jellyfish).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 30Oct98
-------------------
Related Background:
DISCOVERY OF 3-DIMENSIONAL ALGAE AND ANIMAL EMBRYO FOSSILS
A phosphorite is a sedimentary rock composed chiefly of phosphate
minerals. The Proterozoic era (also called the Algonkian) is the
geologic time between the Archean and the Paleozoic, with the
Archean beginning about 3.9 billion years ago and involving the
first appearance of sedimentary rocks and the first primitive
organisms at the bottom of the oceans. In paleontology, the term
"radiation" refers to a diverging and diversifying spread of
animals or plants into new environments with a resultant
production of new evolutionary forms, and the Ediacaran radiation
refers to an assemblage (until now the oldest) of soft-bodied
marine animals, the assemblage first discovered in the Ediacara
Hills in Australia. The algae comprise a large mixed group of
photosynthetic and essentially single-celled plants, and are
considered ancestral to modern green plants. Thalli are primitive
types of plant bodies not differentiated into stems, leaves, and
roots; the term also refers to the gametophyte generation (the
phase of the plant life cycle producing reproductive cells) of
some ferns and lichens. The term "cleavage stages" refers to the
early stages of embryo formation when the egg cell rapidly
divides into smaller and smaller cells. The "Bilateria" are a
major division of the animal kingdom comprising all forms with
bilateral symmetry, and the term "bilaterians" refers to the
first such forms appearing after the emergence of protozoa. The
term "phylogeny" refers to the evolutionary history of an
organism or group of organisms. ... ... Xiao et al (3 authors at
2 installations, US CN) report the discovery of phosphorites of
the late Neoproterozoic (570 +- 20 million years ago) in the
Doushantuo Formation, southern China, an apparent exceptional
record of multicellular life from just before the Ediacaran
radiation of macroscopic animals. Abundant thalli with cellular
structures preserved in 3-dimensional detail show that late-
Proterozoic algae already possessed many of the anatomical and
reproductive features seen in modern marine flora. Embryos
preserved in early cleavage stages indicate the divergence of
lineages leading to bilaterians may have occurred well before
their macroscopic traces or body fossils appear in the geological
record. The authors suggest that discovery of these fossils shows
that the early evolution of multicellular organisms is amenable
to direct paleontological inquiry, and that paleontological
observations, together with insights from molecular phylogeny and
developmental genetics, can facilitate a modern integration of
phylogeny, development, and paleontology that extends deeply into
evolutionary history to address the early evolution of
multicellular life.
QY: Andrew H. Knoll, Botanical Museum, Harvard University,
617-495-1000
(Nature 5 Feb 98) (Science-Week 20 Feb 98)
-------------------
Related Background:
PRECAMBRIAN SPONGES WITH CELLULAR STRUCTURES
The sponges (Porifera) are a phylum of primitive multi-cellular
animals (Metazoa), always attached at one point to a substrate,
usually without a definite symmetry, and usually marine. The
sponge body is a loose aggregation of cells with little
intercellular coordination, but the cells are specialized into
various types with various functions important to the viability
of the entire organism. There is also in sponges an internal
"skeleton" of chalk, silica, or protein, and "calcareous sponges"
are sponges containing a relatively large amount of calcium
carbonate. In this report, "spicules" are the hard internal
structures in sponges composed primarily of silica or calcium
salts, and a "monoaxonal" spicule is a spicule having essentially
only one dimension (e.g., needle-like). The term "epidermis"
refers to the outermost layer of cells in any multicellular
organism; "porocytes" are tubular cells that constitute the walls
of certain sponges; "amoebocytes" are freely moving cells (sponge
cells in this context) within a metazoan tissue; "sclerocytes"
are cells involved in the formation of the sponge skeleton;
"spongocoel" refers to the branching internal cavity of a sponge,
the cavity having a connection in one place or another to the
external aqueous environment; "flagella" are long and thin
cellular organelles that protrude from the surfaces of cells and
are specialized to produce locomotion. The Cambrian period
extended from 545 to 505 million years ago, and was the time
during which many multicellular organisms first arose, and the
Vendian period is the Precambrian metazoan fossil period. The
term "Cambrian explosion" refers to the apparent relatively
sudden appearance of an enormous number of living forms during
the Cambrian period. ... ... Li et al (3 authors at 2 install-
ations, TW CN) report the identification of sponge remains in the
Early Vendian Doushantuo phosphate deposit (cf. previous report,
this issue) in south China. The skeletons consist of siliceous
monoaxonal spicules, with preserved soft tissues including the
epidermis, porocytes, amoebocytes, sclerocytes, and spongocoel,
and among thousands of metazoan embryos a sponge larva having a
shoe-shaped morphology and dense peripheral flagella. The authors
suggest the data indicate the calcareous sponges may have an
extended history in the Late Precambrian, and that animals lived
40 to 50 million years before the Cambrian explosion. They
further suggest these Doushantuo rocks provide a potentially
inexhaustible resource for understanding the early evolution of
animal life.
QY: Chia-Wei Li, National Tsing Hua Univ., Hsinchu, Taiwan, China
(Science 6 Feb 98) (Science-Week 20 Feb 98)
-------------------
Related Background:
DISCOVERY OF LOWER CAMBRIAN FOSSILIZED METAZOAN EMBRYOS
... Until now, [Cambrian fossil] studies have been of the fossils
of mature organisms, fossils easily recognized in the Cambrian
strata with the naked eye or a hand lens. This week there is a
report by Stefan Bengtson and Yue Zhao (Swedish Museum of Natural
History, SE; Chinese Academy of Geological Sciences, CN) of
analysis of small globular fossils known as Olivooides and
Markuelia from basal Cambrian rocks in China and Siberia. They
show that these microscopic globular fossils contain developing
embryos of metazoans (multi-cellular organisms), the various
fossil embryos exhibiting all stages of development. The fossils
were apparently formed by phosphatization during the conversion
of sediment to limestone rock. In the laboratory, the limestone
rock was dissolved in 10% acetic acid to recover the phosphatized
fossils. The authors suggest this type of fossil formation may be
widespread and of great potential for paleobiologic research.
They also suggest that if metazoan embryos have been rare as
fossils, they may have merely been overlooked because of their
minute size and nondescript morphology.
QY: S. Bengtson, Dept. Paleozoology, Swed. Mus. of Nat. Hist.,
Box 50007, S-104 05 Stockholm SE
(Science 12 Sep) (Science-Week 26 Sep 97)
6. PROTEIN THERMOSTABILITY ABOVE 100 DEGREES CELSIUS
The discovery of hyperthermophilic microorganisms that grow
optimally at or near 100 deg C has provoked a radical revision of
ideas concerning *protein thermostability. Most of the enzymes
from these organisms are stable for many hours at or above 100
deg C, suggesting that these enzymes must embody most of the
mechanisms of thermostability that occur in extremely
thermostable proteins. Detailed study of these enzymes may
therefore identify key structural determinants of thermal
stability at very high temperatures. Trends commonly associated
with elevated thermostability in proteins include: a) relatively
small solvent-exposed surface area; b) increased packing density
that reduces cavities in the *hydrophobic core; c) an increase in
core hydrophobicity; d) decreased length of surface loops; e)
hydrogen bonds between polar residues. Also, a prominent role for
ion pairs in stabilization of proteins at or above 100 deg C,
where the hydrophobic effect is minimal, has been inferred from
recently solved structures of several proteins from extreme
thermophiles. As yet, there is no apparent general rule that
governs amino acid composition of thermostable proteins, and
methods for predicting and designing stabilizing mutations are
not reliable. ... ... C. Vetriani et al now report a study
involving *homology-based modeling and direct structural
comparison of 2 hexameric enzymes (glutamate dehydrogenases) from
the hyperthermophiles Pyrococcus furiosus and Thermococcus
litoralis whose optimal growth temperatures are 100 deg C and 88
deg C respectively, the purpose of the study to determine key
stabilizing features of the two enzymes. The study included
genetic engineering mutations and in vitro measurements of the
thermal stability of the altered enzymes. The two enzymes as
*wild types are 87 percent homologous, but differ 16-fold in
thermal stability at 104 deg C. As wild types, an intersubunit
ion-pair network is substantially reduced in the less stable
enzyme from T. litoralis. The authors altered 2 residues to
increase ion-pair interactions in the T. litoralis enzyme. Single
mutations both had adverse effects on the thermostability of the
protein, but with both mutations in place a 4-fold improvement of
stability at 104 deg C occurred. The catalytic properties of the
enzyme were unaffected by the mutations. The authors suggest
their results indicate that extensive ion-pair networks may
provide a general strategy for manipulating enzyme
thermostability of multisubunit enzymes. They further suggest
their results emphasize the importance of the exact local
environment of a residue in determining its effects on stability.
Finally, the authors suggest their study provides a model for
future alterations of stability of multisubunit enzymes by using
strategies that do not affect the catalytic properties of the
enzymes, and that the indication is that the manipulation of ion
pairs may be an important mechanism to adapt hyperstable enzymes
for use as industrial biocatalysts.
-----------
C. Vetriani et al (3 installations, US ZA UK)
Protein thermostability above 100 deg C: A key role for ionic
interactions.
(Proc. Natl. Acad. Sci. US 13 Oct 98 95:12300)
QY: Frank T. Robb
-----------
Text Notes:
... ... *protein thermostability: In general, protein
"thermostability" refers to endurance of a protein at a high
temperature before irreversible denaturation (complete unfolding
and loss of catalytic activity) occurs.
... ... *hydrophobic core: The hydrophobic "core" of a protein
exists only in the folded state, where the protein has assumed a
configuration that brings hydrophobic residues together in
clusters to form a hydrophobic microphase.
... ... *homology-based modeling: This refers to modeling based
on residue-by-residue comparison in order to identify residues
that may be involved in activity differences between the
proteins.
... ... *wild types: The unmutated form found in the organism.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 20Nov98
-------------------
Related Background:
COMPLETE GENOME OF A HYPERTHERMOPHILIC BACTERIUM
In evolutionary biology, the term "divergence" refers to the
manifestation of new genetic characteristics in a subpopulation
of a species when the subpopulation has become separated from the
larger population by physical barriers -- new genetic character-
istics appearing as the organism adapts to new conditions.
Thermophiles are microorganisms that thrive under conditions of
high temperature such as those in hot springs and deep ocean
vents. "E. coli" refers to the bacterium Escherichia coli, a rod-
shaped intestinal organism that has been used extensively in
research. ... ... Deckert et al (15 authors at 4 installations,
US DE) report the complete genome sequence of Aquifex aeolicus.
The organism is apparently one of the earliest to diverge, and is
one of the most thermophilic bacteria known. It can grow on
hydrogen, oxygen, carbon dioxide, and mineral salts, and at 95
deg C. The complete genome is one-third the size of the E. coli
genome, with 1,551,335 DNA base pairs. The authors suggest that
the large number of diverse genome sequences of various
biological organisms that will become available in the future
will allow more detailed correlation of global genomic properties
with particular physiologies.
QY: Ronald V. Swanson
(Nature 26 Mar 98) (Science-Week 10 Apr 98)
7. BETA-AMYLOID PRECURSOR PROTEIN AND MEMORY
Alzheimer's disease is the most common cause of progressive
cognitive decline and dementia in aged humans, and the deposition
of *beta-amyloid peptide(s) in *extracellular neuritic plaques of
Alzheimer's disease patients is an early and invariant feature of
this neurodegenerative disorder. Beta-amyloid peptides are
derived from a large *membrane-spanning beta-amyloid precursor
protein encoded by a single gene located on *chromosome 21. The
normal physiological functions of the beta-amyloid precursor
protein and its secreted derivatives are still poorly understood.
... ... H. Meziane et al now report a study investigating
whether
beta-amyloid precursor proteins have memory enhancing actions
when directly administered to mice performing various learning
tasks and to mice rendered *amnestic by administering the
*anticholinergic drug *scopolamine. The authors report that when
administered *intracerebroventricularly to mice performing
various learning tasks involving either *short-term or *long-term
memory, secreted forms of beta-amyloid precursor protein have
potent memory-enhancing effects and block learning deficits
induced by scopolamine, and that the memory-enhancing effects of
beta-amyloid precursor protein were observed over a wide range of
extremely low doses (0.05 to 5000 picograms i.c.v.). The authors
suggest their results indicate that beta-amyloid precursor
proteins play an important role in the formation and/or the
consolidation of the *memory trace, and that as memory function
is one of the earliest and most affected functions in Alzheimer's
disease, it is tempting to speculate that altered levels of beta-
amyloid proteins in Alzheimer's disease patients could explain,
at least in part, some of the memory deficits observed in this
disease.
-----------
H. Meziane et al (2 installations, FR US)
Memory-enhancing effects of secreted forms of the beta-amyloid
precursor protein in normal and amnestic mice.
(Proc. Natl. Acad. Sci. US 13 Oct 98 95:12683)
QY: A. Ungerer
-----------
Text Notes:
... ... *beta-amyloid peptide(s): Alzheimer's disease is
characterized by the presence of large numbers of extracellular
agglomerations (plaques) and intracellular *neurofibril tangles
in the cerebral cortex of the brain. There is also a massive
neuronal cell loss. While plaques and tangles are found in normal
aging brains, they are more numerous and widespread in
Alzheimer's disease. The major protein component of the plaques
is a 39 to 43 amino acid peptide called beta-amyloid, which is
now known to be derived from a much larger protein called the
amyloid precursor protein. This latter protein has been found to
be expressed in every tissue studied.
... ... *neurofibril: A filamentous structure seen with the light
microscope and composed of ultramicroscopic tubular and
filamentous protein arrays (neurotubules and microfilaments). The
function of these structures is unknown.
... ... *extracellular neuritic plaques: Deposits that are
outside cells and related to neuropathology.
... ... *membrane-spanning: A membrane-spanning or trans-membrane
protein essentially has 3 domains: extracellular, membrane, and
cytoplasmic (intracellular). In other words, the protein extends
completely through the plasma membrane, protruding from the outer
surface and inner surface of the membrane. The outer portion of
the protein may be a receptor for one or more ligands, the
protein acting to transduce a signal to the interior of the cell.
... ... *chromosome 21: The fact that amyloid precursor protein
is encoded by a gene in human chromosome 21 has been of great
interest. In Down's syndrome there is an extra copy of chromosome
21, and Down's syndrome patients usually develop Alzheimer's
disease in their thirties.
... ... *amnestic: (amnesic) In general, exhibiting an impairment
of memory function.
... ... *anticholinergic: In general, antagonistic to the action
of cholinergic neurons, which are neurons that use acetylcholine
as a *neurotransmitter substance.
... ... *neurotransmitter substance: Neurotransmitters are
chemical substances released at the terminals of nerve axons in
response to the propagation of an impulse to the end of that
axon. The neurotransmitter substance diffuses into the synapse,
the junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *scopolamine: (hyoscine) A potent alkaloid with
anticholinergic action. Scopolamine has been shown to induce
amnesia in mice. It has a long history of therapeutic use in
humans (e.g., prevention of motion sickness) and is a component
of nonprescription proprietary hypnotics (substances that induce
drowsiness and sleep).
... ... *intracerebroventricularly: This refers to direct
injection into the cerebrospinal fluid in a cerebral ventricle,
the purpose to bypass the *blood-brain barrier.
... ... *blood-brain barrier: A selective mechanism opposing the
passage of most ions and large molecular-weight compounds from
the blood to brain tissue, the mechanism operating in a
continuous layer of endothelial cells connected by tight
junctions.
... ... *short-term or *long-term memory: Short-term memory is a
phase of the memory process with a rapid decay (e.g., seconds to
minutes), as opposed to long-term memory, which is essentially
permanent and available for future retrieval.
... ... *memory trace: (engram) In general, the physical basis of
a stored memory.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 20Nov98
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