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ScienceWeek
SCIENCEWEEK
July 18, 2003
Vol. 7 - Number 29B
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CONTENTS:
1. Condensed Matter: Quasi-particles and Cuprate Superconductors 2. Geochemistry: Ancient Oceans and Oxygen 3. Condensed Matter: On the Seebeck Effect 4. Cell Biology: Microtubules 5. Cell Biology: Integrins 6. Medical Biology: Amebiasis
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1. CONDENSED MATTER: QUASIPARTICLES AND CUPRATE SUPERCONDUCTORS
In general, the term "quasiparticle" refers to a propagated perturbation in a medium (or field) that behaves as a particle, with energy (mass) and momentum, and that can be treated as such theoretically.
At temperatures close to absolute zero (-273.15 degrees Celsius), the thermal, electric, and magnetic properties of many substances undergo dramatic changes. One such phenomenon is superconductivity, which occurs below a critical temperature specific for each substance that exhibits the effect.
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes (1853-1926), who was awarded the Nobel Prize for Physics in 1913 for his low temperature research. Kamerlingh Onnes found that the electrical resistance of a mercury wire suddenly disappears when the wire is cooled below a temperature of approximately 4 degrees kelvin. Similar behavior (but at widely varying critical temperatures) has been found in approximately 25 other chemical elements, including lead and tin, and in thousands of alloys and chemical compounds. Apart from these known superconducting materials, all other substances investigated to within fractions of a degree of absolute zero show normal (non-superconducting) resistance to the flow of electric currents.
For almost 50 years after the discovery of superconductivity by Kamerlingh Onnes, there was no successful fundamental theory that could explain the phenomenon. Finally, in 1957, an apparently satisfactory theory of superconductivity was presented by John Bardeen (1908-1991), Leon N. Cooper, and John R. Schrieffer, who all shared the Nobel Prize for Physics in 1972. The theory is now called the BCS theory of superconductivity.
The essential aspect of BCS theory is the grouping of electrons in superconductors in pairs ("Cooper pairs"), with the motions of all the Cooper pairs within a single superconductor correlated, i.e., the population of Cooper-pair electrons constituting a system that functions essentially as a single entity. (In quantum mechanical terms, each Cooper pair consists of electrons of opposite spins, thus forming a spin-zero single boson, and the population of bosons form a Bose-Einstein condensate described by a single wave function.) Application of an electric voltage to the superconductor causes all Cooper pairs to move, the movement constituting a current. When the voltage is removed, current continues to flow indefinitely because the Cooper pairs (as members of a coherent condensate) are not scattered by the atomic lattice. As a superconductor is warmed, its Cooper pairs separate into individual electrons, and the material becomes non-superconducting.
Such was the theory of superconductivity for nearly 30 years, the theory successfully predicting the behavior of superconducting materials with critical temperatures close to absolute zero. In 1986, Karl A. Mueller and J. Georg Bednorz discovered that certain materials exhibit superconductivity at temperatures as high as 35 degrees kelvin, and compounds retaining superconductivity at temperatures as high as 160 degrees kelvin have since been found. Mueller and Bednorz were awarded the Nobel Prize in Physics in 1987 for their work with high-temperature superconductors. Such high-temperature superconductors all contain copper and oxygen atoms that form planes or chains of atoms in the crystal, and it is believed that anisotropy is an important factor in their superconducting behavior. These materials are ceramic oxides, and because they are superconducting at temperatures easily obtainable with liquid nitrogen, great effort has been expended to find applications for these substances. But problems of brittleness, instabilities, and the aggregation of impurities at surfaces have slowed progress. Nevertheless, in contrast to superconducting ceramics, superconducting metals and their alloys must be cooled to near absolute zero with liquid helium, a process much more expensive than cooling with liquid nitrogen. Superconducting ceramics thus remain an important frontier of research in materials science.
In terms of theory, what is significant is that BCS theory apparently cannot provide a complete explanation of the behavior of high-temperature ceramic superconductors. A version of BCS theory may explain how superconductivity occurs in certain ceramic materials, but no complete theory of high-temperature superconductivity in ceramic materials has yet been proposed.
The following points are made by N. Gedik et al (Science 2003 300:1410):
1) Quasiparticles are the elementary excitations of a superconductor, created when a Cooper pair of electrons breaks apart. The dynamic properties of quasiparticles, that is, their rates of diffusion, scattering, trapping, and recombination, are critical for applications of conventional superconductors in x-ray detectors and in the manipulation of superconductor-based qubits. In more exotic superconductors such as the high-transition-temperature (Tc) cuprates, a better understanding of quasiparticle dynamics may help to uncover the mechanism for Cooper pairing.
2) Cuprate superconductors have an unusual quasiparticle spectrum. The minimum energy for the creation of a quasiparticle depends on the direction of its momentum. It is zero for momenta in the "nodal" direction, oriented at 45 degrees relative to the Cu-O bond. The most energetically expensive quasiparticles are the "antinodal" ones, whose momenta are nearly parallel to the bond. The antinodal quasiparticles are the mystery particles of cuprate superconductivity. Because they feel the pairing interaction most strongly, their properties may hold the key to high-Tc superconductivity. Unfortunately, their tendency to form strong pairs makes them difficult to study. In thermal equilibrium, the population of quasiparticles is overwhelmingly dominated by the low-energy nodal ones. As a result, transport measurements performed in equilibrium, such as microwave and thermal conductivity, are insensitive to antinodal quasiparticles. 3) In summary: The authors report a transport study of nonequilibrium quasiparticles in a high-transition-temperature cuprate superconductor using the transient grating technique. Low-intensity laser excitation (at a photon energy of 1.5 electron volts) was used to introduce a spatially periodic density of quasiparticles into a high-quality untwinned single crystal of YBa(sub2)Cu(sub3)O(sub6.5). Probing the evolution of the initial density through space and time yielded the quasiparticle diffusion coefficient and the inelastic and elastic scattering rates. The technique reported here is potentially applicable to precision measurements of quasiparticle dynamics not only in cuprate superconductors but in other electronic systems as well.
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2. GEOCHEMISTRY: ANCIENT OCEANS AND OXYGEN
The following points are made by Matthew T. Hurtgen (Nature 2003 423:592):
1) Discovering how oxygen levels in Earth's atmosphere and oceans have varied over time is a goal of compelling interest -- not least because of the oxygen-dependence of so many life-forms, including ourselves. But how do you estimate an oxygenation state that existed back in deep time? For the oceans at least, sulfate is a crucial indicator, because oxygen largely controls sulfate concentrations. This is because the primary source of seawater sulfate is river input, resulting in part from the oxidative weathering of iron disulfide (pyrite, FeS(sub2)) on land.
2) Over the past 543 million years, Earth's oceans have largely been well oxygenated -- at least well enough to allow the persistence of multicellular life -- and rich in sulfate. Before about 2.5 billion years ago, however, they are believed to have been mostly anoxic (without oxygen) and sulfate poor. In 1998, Donald Canfield proposed that for much of the intervening time, the Proterozoic, the oceans had an intermediate oxidation state -- i.e., the bottom waters were anoxic and sulfidic, a condition known as "euxinic", and the surface waters were oxic and contained modest amounts of sulfate. Until now, there has been no concrete evidence for the existence of two chemically distinct water masses during the Proterozoic.
3) Shen et al (Nature 2003 423:632) have presented two new data sets from sedimentary rocks in Australia's Northern Territory known as the Roper Group. These rocks date to the middle Proterozoic, about 1.5 billion years ago, and their chemistry provides testimony to a continuum of environments that run from onshore to offshore, through near- and far-coastal conditions, to deeper ocean settings. The first set of data, involving iron chemistry, backs up Canfield's ideas. The second set, which uses sulfur isotopes, is consistent with previous hypotheses proposing the existence at that time of a sulfate-minimum zone -- i.e., a region of water along the coast in which sulfate levels were lower than those of the ocean in general -- and low sulfate concentrations in the middle Proterozoic ocean.
Related Material:
ON THE EARLY DEVELOPMENT OF AN OXYGEN-RICH EARTH ATMOSPHERE
It is currently believed that the oxygen concentration in Earth's atmosphere may have remained at 1 percent of its present level until approximately 2 billion years ago, after which the concentration gradually increased to its present value with the increasing success of photosynthetic life forms. Fossil oxygen-generating cyanobacteria have been dated as far back as 3.5 billion years ago, but the rate at which oxygen accumulated in the atmosphere because of photosynthesis is not known.
Although the time-frame of the increase in oxygen concentration of the atmosphere is uncertain, the consensus among researchers is that the initiation of an oxygen atmosphere increased the number and kinds of organisms capable of using aerobic metabolic pathways. By the start of the Cambrian period 570 million years ago, or somewhat earlier, oxygen levels had apparently increased enough to permit rapid evolution of large oxygen-utilizing multicellular organisms.
The following points are made by Norman H. Sleep (Nature 2001 410:317):
1) The author points out that although oxygen now constitutes approximately 20 percent of the gas in the atmosphere, before approximately 2.5 billion years ago it was apparently only a trace constituent. The Earth is unique among the planets in the Solar System in having an oxygenated atmosphere. The atmospheres of the other planets are anoxic because oxygen levels are kept relatively low by an equilibrium system involving chemical processes in crusts, mantles, and volcanic gases. On the Earth, oxygen levels increased over geological time apparently mainly as a result of photosynthesis, which can be expressed as the general reaction carbon dioxide --> carbon + oxygen.
2) Over the eons, a vast amount of organic carbon has become locked up in sedimentary rocks, a small part of it in coal and oil, thus preventing the reverse reaction to equilibrium that would create carbon dioxide and lower atmospheric levels of oxygen. However the advent of oxygen-producing photosynthesis cannot be the entire story of the evolution of Earth's atmosphere, since photosynthesis apparently existed long before the well-documented rise in oxygen levels 2.5 billion years ago (the Archaean-Proterozoic transition).
3) L.R. Kump et al (Geochem. Geophys. Geosyst. 2001) are now proposing that changes in the deep interior of the Earth affected the composition of volcanic gases, and that this led to the rise in atmospheric oxygen levels 2.5 billion years ago. The idea is that although photosynthesis is presently the main source of oxygen in Earth's atmosphere, it may have been geological activity that first allowed an oxygen-rich atmosphere to develop.
4) The author (Sleep) concludes: "It was over two centuries ago that Antoine Lavoisier figured out that we breathe oxygen. But we still don't know how an oxygen-rich atmosphere arose. Clearly, processes at both the Earth's surface and in its bowels were involved. Exactly how and by how much each contributed remain open questions."
Related Background:
EARTH SCIENCE: ATMOSPHERIC OXYGEN OVER PHANEROZOIC TIME
The term "Phanerozoic time" refers to the past 550 million years, the time during which most higher organisms arose and evolved, both in the oceans and on the continents. The evolution of atmospheric oxygen over geologic time is believed to have been both a major cause and a major effect of biological evolution, since oxygen is both consumed by plant and animal respiration and produced by photosynthesis. On a geologic time scale (i.e., millions of years) the global biogeochemical cycles of carbon and sulfur, involving the exchange of reduced carbon and sulfur between rocks and the atmosphere-plus-oceans, constitute the major controls on the levels of oxygen. Therefore, the study of these cycles and how they may have varied in the geological past is important to the history of both the atmosphere and of Earth surface environments.
The following points are made by Robert A. Berner (Proc. Nat. Acad. Sci. 1999 96:10955):
1) The processes that affect the evolution of atmospheric oxygen as it relates to the carbon and sulfur cycles over geologic time include the following:
a) Input to the oceans of carbon dioxide and dissolved carbonate and sulfate derived from oxidation, during chemical weathering on the continents, of ancient organic matter and pyrite [FeS(sub2)] in *sedimentary rocks.
b) Reduction and removal of dissolved inorganic carbon from sea water and fresh water via the synthesis of organic matter, followed by a burial of dead organic remains in bottom sediments.
c) Removal of sulfate from seawater via bacterial reduction to hydrogen sulfide followed by the reaction of hydrogen sulfide to form sedimentary pyrite.
d) The reaction of hot *basalt with sulfate in seawater at mid-oceanic rises.
e) Degassing of reduced carbon- and sulfur-containing gases to the atmosphere-plus-oceans as a result of the thermal decomposition of deeply buried organic matter and pyrite by *diagenesis, *metamorphism, and volcanism. On arriving at the Earth's surface, these reduced gases are rapidly oxidized by oxygen.
2) In addition to the above carbon and sulfur cycles which involve atmospheric oxygen, there are cycles important to carbon and sulfur mass balance, particularly isotope mass balance, but which do not involve atmospheric oxygen. These cycles include a) the weathering of calcium carbonate and calcium sulfate minerals on the continents, and b) the formation of these minerals in the oceans followed by burial in sediments.
3) Calculation of the effect of changes in the global carbon and sulfur cycles over geologic time on atmospheric oxygen has been attempted with various models:
a) Sediment composition models: In these models, the chemical composition of sedimentary rocks of various ages is combined with the abundance of these rocks (corrected for postdepositional erosional/metamorphosic loss) to calculate the original rates of burial of organic carbon and pyrite sulfur in sediments.
b) Nutrient models: In these models the burial rate of organic carbon is assumed to be limited by the availability of the nutrients nitrogen and phosphorus, which leads to consideration of the cycles of these elements, as well as those of carbon and sulfur.
c) Isotope mass balance models: In these models, carbon and sulfur isotopic data for seawater composition over geologic time are used to calculate the values of fluxes.
A major problem with all models is the extreme sensitivity of the mass of atmospheric oxygen to very small imbalances in the burial and weathering fluxes.
4) One study using sediment abundance data, along with assumed rapid recycling of sediments to stabilize oxygen, shows a pronounced and extended rise in atmospheric oxygen over the period 375 to 275 million years ago, spanning the *Carboniferous and Permian periods. What could have brought this about? The modeling indicates that increased oxygen production caused by increased burial of organic carbon is the chief suspect. This increased burial is attributed to the rise and spread of large woody vascular plants on the continents beginning at about 375 million years ago. The plants supplied a new source of organic matter to be buried on land and carried to the oceans via rivers. This "new" carbon was added to that already being buried in the oceans, thus increasing the total global burial flux.
Notes:
Sedimentary rocks are rocks formed by the hardening of accumulated particles (sediments) that were transported by agents such as wind and water. Such rocks are the prime source of fossils.
Basalt is a dark gray to black igneous rock of volcanic origin that cools rapidly. "Igneous rocks" are rocks that have congealed from a molten mass.
In general, the term "diagenesis" refers to all the changes that occur in a sediment at low temperature and pressure after deposition. With increasing temperature and pressure, diagenesis grades into "*metamorphism".
In general, in this context, the term "metamorphism" refers to the process of changing the characteristics of a rock in response to changes in temperature, pressure, or volatile content. Most metamorphic changes do not include bulk chemical changes, but merely the crystallization of new mineral phases. Examples of the transformation of sediments through diagenesis and metamorphism are sand to sandstone and peat to coal.
The "Carboniferous period" is the time-frame 362.5 to 290 million years ago. The "Permian period" is the time-frame 290 to 245 million years ago.
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3. CONDENSED MATTER: ON THE SEEBECK EFFECT
In nature, what are called "thermoelectric effects" occur when both electric and thermal currents are present. (So-called "transverse thermoelectric effects" occur when in addition there is a magnetic field normal to these currents.) There are essentially three ordinary thermoelectric effects:
The "Seebeck effect", discovered in 1822 by Thomas Seebeck (1770-1831), relates to the electromotive force developed in a circuit consisting of different conducting elements, not all of whose contacts are at the same temperature.
The "Peltier effect", discovered in 1834 by Jean Peltier (1785-1845), involves the reversible heating or cooling which occurs at a contact between two dissimilar conductors when electric current flows from one conductor to another.
The "Thomson effect" (Kelvin effect) discovered by Lord Kelvin (William Thomson) (1824-1907) in 1856, refers to the reversible heat absorption which occurs when an electric current flows in a homogeneous conductor in which there is a temperature gradient. All three effects are related by thermodynamics: if one effect is known, the other two can be derived.
The so-called Seebeck and Peltier coefficients are measures of the respective effects: the larger the coefficient, the greater the effect. In the Seebeck effect, for example, the generated voltage difference is proportional to the temperature difference, with the Seebeck coefficient another name for the proportionality factor.
The following points are made by Cronin B. Vining (Nature 2003 423:391):
1) Thermocouples generate a voltage in a temperature gradient, a phenomenon known as "thermopower", or the Seebeck effect, after its discoverer Thomas Johann Seebeck 1770-1831). These devices have found a range of applications, from cooling devices for seats in luxury automobiles to power supplies for spacecraft (including the Voyager missions). Metallic thermocouples generate relatively small voltages, but semiconductor thermocouples produce much larger voltages and can convert heat directly to electricity or generate cooling from an electrical input.
2) Two different groups have reported semiconductor thermoelectric materials that are about twice as efficient as any previously known, achieved by carefully controlling the composition and structure of the materials on the atomic scale. But an entirely different approach to high thermopower uses magnetic cobalt oxides -- layered materials that combine the thermopower of semiconductors with the electrical conductivity of metals.
3) These cobalt oxides (Na(subx)Co(sub2)O(sub4)) were first considered as thermoelectric materials by Terasaki et al (1997) and have a variety of unusual properties. They are ionically bonded (unlike the classic semiconductor thermoelectric materials, which are covalently bonded), and can be doped with varying numbers of sodium atoms to achieve the desired properties. At room temperature, their thermopower is as much as ten times larger than might be expected, much larger than is typical of metals.
4) At the same time, Na(subx)Co(sub2)O(sub4) has some unusual magnetic properties. At low temperatures it is an antiferromagnet, which means that there are "spins" in the material that take on a particular kind of order. Unlike most magnets, the spins in Na(subx)Co(sub2)O(sub4) are not fixed to specific atoms within the lattice but instead are free to move about the crystal. When these spins move, they carry some energy with them -- and by doing so, contribute to the thermopower of the material. This doesn't happen in an ordinary metal, or in an ordinary magnet, because there the spins don't move. The moving spins -- or, more technically, spin entropy -- are thought to be behind the enhanced thermopower of cobalt oxides.
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4. CELL BIOLOGY: MICROTUBULES
The structural framework of eukaryotic cells (biological cells with nuclei and other membrane-bound internal structures), called the "cytoskeleton", consists of an arrangement of macromolecular structures: microtubules, intermediate filaments, and microfilaments. The microtubules are hollow cylinders approximately 24 nanometers in diameter, many microns in length, and consist of heterodimers of alpha- and beta-tubulin proteins plus a variable set of other proteins. They form the scaffolding of the mitotic spindle (an important structure in cell division), organize other cytoplasmic structures, and are the structural core of various organelles involved in cell movement (cilia and flagella).
Beginning in the early 1960s, the work of S. Inouye provided evidence that microtubules exist in equilibrium with free tubulin: microtubule assembly-disassembly is regulated by factors that influence the equilibrium between polymerized and nonpolymerized tubulin. In the early 1970s, R. Weisenberg demonstrated that microtubules assemble spontaneously in cell extracts that have been warmed to 37 degrees centigrade in the absence of calcium ions and in the presence of guanosine triphosphate.
In general, under appropriate conditions, microtubules spontaneously assemble when a solution containing purified tubulin protein and guanosine triphosphate is warmed from 7 degrees centigrade to 36 degrees centigrade. In the laboratory, following the spontaneous assembly of tubulin heterodimers into microtubules, the microtubules further spontaneously organize into superstructures, striped or circular macroscopic patterns that can be detected with suitable polarization optics. These macroscopic patterns are formed as a connected final self-organization following tubulin polymerization into microtubules. According to J. Taboy et al (1990), these macroscopic microtubule patterns are evidence of a self-organizing system behaving according to "reaction-diffusion mechanisms".
The following points are made by Joe Howard and Anthony A. Hyman (Nature 2003 422:753):
1) The textbook functions of microtubules are to act as beams that provide mechanical support for the shape of cells, and as tracks along which molecular motors move organelles from one part of the cell to another. To perform these functions, a cell must control the assembly and orientation of its microtubule cytoskeleton. Microtubules assemble by polymerization of alpha-beta dimers of tubulin. Polymerization is a polar process that reflects the polarity of the tubulin dimer, which in turn dictates the polarity of the microtubule. In vitro, purified tubulin polymerizes more quickly from the "plus" end, which is terminated by the beta-subunit. The other, slow-growing end is known as the "minus" end, and is terminated by the alpha-subunit.
2) In animal cells, minus ends are generally anchored at "centrosomes", which consist of specialized microtubule-based structures called "centrioles", surrounded by pericentriolar proteins. In yeast, the analogous structure is the spindle pole body. An important component of the centrosome is an unusual form of tubulin, gamma-tubulin, which is thought to initiate nucleation by forming rings that act as templates for new microtubule growth. After nucleation, microtubules grow out with their plus ends leading into the cytoplasm. Thus to a first approximation, the distribution of the microtubule cytoskeleton is determined by the location of the centrosome.
3) The first clue as to how cells rearrange the distribution of microtubules came from the discovery that during the polymerization of pure tubulin, plus ends switch between phases of slow growth and rapid shrinkage. The conversion from growing to shrinking is called "catastrophe", whereas the conversion from shrinking to growing is called "rescue". Analysis in tissue culture cells and in cellular extracts soon confirmed that this behavior, termed "dynamic instability", is a feature of microtubules growing under physiological conditions.
4) In summary: An important function of microtubules is to move cellular structures such as chromosomes, mitotic spindles and other organelles around the inside of cells. This is achieved by attaching the ends of microtubules to cellular structures; as the microtubules grow and shrink, the structures are pushed or pulled around the cell. How do the ends of microtubules couple to cellular structures, and how does this coupling regulate the stability and distribution of the microtubules? It is now clear that there are at least three properties of a microtubule end: it has alternate structures; it has a biochemical transition defined by GTP hydrolysis; and it forms a distinct target for the binding of specific proteins. These different properties can be unified by thinking of the microtubule as a molecular machine, which switches between growing and shrinking modes. Each mode is associated with a specific end structure on which end-binding proteins can assemble to modulate dynamics and couple the dynamic properties of microtubules to the movement of cellular structures.
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5. CELL BIOLOGY: INTEGRINS
The following points are made by Richard O. Hynes (Science 2003 300:755):
1) Biologists have developed two different models to describe how plasma membrane proteins called integrins control cell adhesion. In the first model, conformational activation of individual integrins is postulated to increase their affinity for ligands, such as fibronectin or fibrinogen. In the second model, clustering of integrins in the plane of the plasma membrane is proposed to increase the avidity of cell adhesion without affecting ligand affinity. Evidence for both models exists. Given that conformational activation and lateral clustering typically occur together, it is possible that one event leads to the other, although opinions differ as to which occurs first. Li et al (Science 2003 300:795) have presented structural data revealing that conformational activation and lateral clustering of integrins are closely and inextricably coupled.
2) During the past 2 years, integrin research has been fueled by several exciting discoveries. These discoveries include the structure of the extracellular domain of V3 integrin, the details of interactions of integrin cytoplasmic domains, and conformational changes that lead to integrin activation. These results have led to two widely accepted notions. The alpha and beta subunits of the integrin heterodimer are in equilibrium between an inactive state in which the stalks of the heterodimer are bent in the middle, and an active state in which the stalks straighten and separate. Activation of integrins takes place from the outside of the cell through binding of ligand to the extracellular domain, and also from the inside of the cell through binding of the cytoskeletal protein talin to the cytoplasmic domain. Thus, activation of the ligand-binding extracellular domain is coupled with activation of the cytoplasmic domain, which is linked to the cytoskeleton and associated signaling complexes.
3) But what about the two integrin transmembrane segments that traverse the lipid bilayer, connecting the extracellular and cytoplasmic domains? Nuclear magnetic resonance (NMR) imaging studies suggest that the alpha-IIb and beta-3 transmembrane segments of alpha-II-beta-b3 integrin are predominantly in the alpha-helical conformation. If, in the inactive state, the external portion of the stalks and the internal cytoplasmic domains are adjacent to each other, then the two transmembrane segments should also be associated. However, when reconstituted in phospholipid micelles the two isolated alpha and beta transmembrane segments do not readily interact with one another, preferring instead to form homo-oligomers. The alpha-IIb transmembrane-cytoplasmic segment forms homodimers, whereas the beta-3 transmembrane-cytoplasmic segment forms homotrimers. In both cases, these homo-oligomers are in equilibrium with their constituent monomers.
Related Material:
ON INTEGRINS OF THE CELL SURFACE
Membrane proteins are classified into a number of types, depending on the number of transmembrane domains, the orientation of the protein, and the presence of other kinds of attachment to the lipid bilayer. Type 1 membrane proteins are defined as membrane proteins having a single transmembrane domain with the C-terminus on the cytoplasmic side of the cell membrane.
The following points are made by J-P. Xiong et al (Science 2001 294:339):
1) Integrins are large heterodimeric cell surface receptors found in many animal species ranging from sponges to mammals. These receptors are involved in fundamental cellular processes such as attachment, migration, proliferation, differentiation, and survival. Integrins also contribute to the initiation and/or progression of many common diseases, including neoplasia, tumor metastasis, immune dysfunction, ischemia-reperfusion injury, viral infections, osteoporosis, and coagulopathies.
2) An integrin is approximately 280 angstroms long, and consists of one alpha subunit (150 to 180 kilodaltons) and one beta subunit (approximately 90 kilodaltons), both of which are type 1 membrane proteins. 18 alpha and 8 beta mammalian subunits are known, which assemble non-covalently into 24 different heterodimers. Contacts between the alpha and beta subunits primarily involve their NH(sub2)-terminal halves, which together form a globular head. The remaining portions form 2 rod-shaped tails that span the plasma membrane.
3) Like other receptors, integrins transmit signals to the cell interior ("outside-in signaling"), signals that regulate organization of the cytoskeleton, activate kinase signaling cascades, and modulate the cell cycle and gene expression. Unlike other receptors, however, ligand binding with integrins is not generally
constitutive but is regulated to reflect the activation state of the cell. This "inside-out regulation" of integrin affinity protects the host from pathological integrin-mediated adhesion. Inside-out and outside-in signaling are associated with distinct conformational changes in the integrin extracellular segment. These changes vary with cell type and the state and nature of the ligand, and are modulated by divalent cations that are also required for integrin-ligand interaction.
4) The authors report they have solved the crystal structure of the extracellular portion of an integrin (alpha-V-beta-3) at 3.1 angstroms resolution. The 12 domains assemble into an ovoid "head" and two "tails". In the crystal, the integrin is severely bent at a defined region in its tails, reflecting an unusual flexibility that may be linked to integrin regulation.
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6. MEDICAL BIOLOGY: AMEBIASIS
The following points are made by R. Haque et al (New Engl. J. Med. 2003 348:1565):
1) Diarrheal diseases continue to be major causes of morbidity and mortality in children in developing countries. For example, in Bangladesh, 1 in 30 children dies of diarrhea or dysentery by his or her 5th birthday. In developed countries the microorganisms that cause diarrheal disease remain of concern because of their potential use as bioterrorist agents.
2) Bacillary dysentery is most commonly caused by microorganisms belonging to the genus shigella, whereas amebic dysentery is caused by the protozoan parasite Entamoeba histolytica. The annual number of shigella infections throughout the world is believed to be approximately 164 million. Estimates of E. histolytica infections have primarily been based on examinations of stool for ova and parasites, but these tests are insensitive and cannot differentiate E. histolytica from morphologically identical species that are nonpathogenic, such as E. dispar and E. moshkovskii.
3) Specific and sensitive means to detect E. histolytica in stool are now available and include antigen detection and the polymerase chain reaction (PCR). Two recent studies in developing countries used these modern diagnostic tests. A three-year study in Dhaka, Bangladesh, showed that preschool children had a 2.2 percent frequency of amebic dysentery, as compared with a 5.3 percent frequency of shigella dysentery. The annual incidence of amebic liver abscess was reported to be 21 cases per 100,000 inhabitants in Hue City, Vietnam. In the US, where fecal–oral transmission is unusual, amebiasis is most commonly seen in immigrants from and travelers to developing countries. The disease is more severe in the very young and old and in patients receiving corticosteroids.
4) Molecular phylogeny places entamoeba on one of the lowermost branches of the eukaryotic tree, closest to dictyostelium. Although the organism was originally thought to lack mitochondria, nuclear-encoded mitochondrial genes and a remnant organelle have now been identified. Unusual features of entamoeba include polyploid chromosomes that vary in length; multiple origins of DNA replication; abundant, repetitive DNA; closely spaced genes that largely lack introns; a novel GAAC element controlling the expression of messenger RNA; and unique endocytic pathways.
5) Ingestion of the quadrinucleate cyst of E. histolytica from fecally contaminated food or water initiates infection. This is a daily occurrence among the poor in developing countries and is a threat to inhabitants of developed countries, as the epidemic linked to contaminated municipal water supplies in Tbilisi, Republic of Georgia, demonstrates. Excystation in the intestinal lumen produces trophozoites that use the galactose and N-acetyl-D-galactosamine (Gal/GalNAc)–specific lectin to adhere to colonic mucins and thereby colonize the large intestine. The reproduction of trophozoites has no sexual cycle, and the overall population of E. histolytica appears to be clonal. Aggregation of amebae in the mucin layer most likely triggers encystation by means of the Gal/GalNAc-specific lectin. Cysts excreted in stool perpetuate the life cycle by further fecal–oral spread.
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