This Week in Science

Science  27 Jun 1997:
Vol. 276, Issue 5321, pp. 1945
  1. Seeing single defects in diamonds

    Two reports focus on scanning probe methods for characterizing the structures of technologically important materials. Magnetoresistive materials change their conductivity in the presence of magnetic fields and have potential applications in magnetic recording. Q. Lu et al. (p. 2006) used a low-temperature magnetic force microscope to image magnetic domains of calcium-doped LaMnO3 films. As the films were warmed to the temperature range where the films transform from ferromagnetic to paramagnetic (around 245 kelvin), the domains became less stable and merged and split as they decreased in extent. Ferroelectric materials with domains of different polarizations have attracted interest for applications in nonlinear optics and acoustics, but mapping the domain structures has usually required etching and optical imaging. Y. Lu et al. (p. 2004) have developed a near-field microwave microscope that uses a scanning tip in a noncontact mode. Changes in the dielectric constant and ferroelectric domain boundaries were mapped out with submicrometer resolution on an yttrium-doped LiNbO3 sample.

  2. Scanning materials

    Two reports focus on scanning probe methods for characterizing the structures of technologically important materials. Magnetoresistive materials change their conductivity in the presence of magnetic fields and have potential applications in magnetic recording. Q. Lu et al. (p. 2006) used a low-temperature magnetic force microscope to image magnetic domains of calcium-doped LaMnO3 films. As the films were warmed to the temperature range where the films transform from ferromagnetic to paramagnetic (around 245 kelvin), the domains became less stable and merged and split as they decreased in extent. Ferroelectric materials with domains of different polarizations have attracted interest for applications in nonlinear optics and acoustics, but mapping the domain structures has usually required etching and optical imaging. Y. Lu et al. (p. 2004) have developed a near-field microwave microscope that uses a scanning tip in a noncontact mode. Changes in the dielectric constant and ferroelectric domain boundaries were mapped out with submicrometer resolution on an yttrium-doped LiNbO3 sample.

  3. Stacking colors

    Organic light-emitting devices (OLEDs) have potential uses in flat panel displays. Shen et al. (p. 2009) have taken advantage of transparent electrode materials to create a multilayer device that stacks red, green, and blue OLEDs vertically, an arrangement that simplifies patterning across the display. The intensity of each color can be varied independently.

  4. Assembly under stress

    When molecules adsorb in layers on a surface, the forces between the molecules can induce stresses in the underlying substrate. Thus stress measurements can give insights into the growth mechanism of a molecular layer and the interactions between the molecules and the surface. Berger et al. (p. 2021) developed a sensor for measuring surface stresses and illustrate their approach for the self-assembly of alkanethiols on gold. Self-assembly led to compressive surface stress, and stress was greater for longer alkyl chains.

  5. Armed and ready

    Although susceptible plants may respond to a certain pathogen with the symptoms of disease, that same pathogen may be unable to produce disease in other plant species. Such plants, nonetheless, recognize the presence of the microbe with a system of non-host defense responses. Ligterink et al. (p. 2054) have identified one of the key elements of the non-host defense response. The elicitor-responsive mitogen-activated protein (MAP) kinase is activated after the initial response by ion channels. After activation, it translocates into the cell nucleus where it may affect interactions of transcription factors with the genes that encode other elements of the plant defense armamentarium.

  6. Quantity, not quality?

    What parameters best define the ability of antibodies to protect a host from infection? Bachmann et al. (p. 2024) used a collection of monoclonal antibodies against a common viral antigen (the glycoprotein of vesicular stomatitis virus) to compare neutralization of the virus in vitro and protection against infection in vivo. Surprisingly, as long as antibodies were above a minimum avidity threshold and a minimum concentration threshold, they provided protection against infection in the whole animal. Parameters that correlated closely with neutralization in vitro, such as avidity, neutralization rate constant, and in vitro neutralizing activity, failed to correlate with protection in vivo. These results may prove useful in designing antibody therapies to fight viral infection.

  7. Bacterial blocking

    When several bacterial strains infect a host, they do not cooperate but actually interfere with each other. This effect has been thought to arise from growth inhibition, but Ji et al. (p. 2027) show that in Staphylococcus aureus the virulence factors and other extracellular proteins associated with the agr locus from one strain inhibited agr expression in another strain and appeared to limit its infectivity. The peptide and receptor sequences can vary markedly, which indicates that some mechanism generates hypervariability.

  8. LTP origins

    Long-term potentiation (LTP) is a well-studied model of neural plasticity, but the molecular mechanisms have been elusive. Barria et al. (p. 2042; see the Perspective by Lisman et al., p. 2001) now show that the long-lasting increase in the postsynaptic response in the CA1 area of the hippocampus is due to persistent phosphorylation of Thr286 on the AMPA-type glutamate receptor, catalyzed by calmodulin-dependent protein kinase II (CaM-KII). This result completes the pathway for LTP in these cells: The neurotransmitter, glutamate, binds to the N-methyl-D-aspartate receptor, which, when the postsynaptic cell is depolarized, allows calcium to flow in and activate CaM-KII. CaM-KII can then phosphorylate the AMPA receptor and the cell exhibits a greater response.

  9. Creeping along

    The 1989 Loma Prieta earthquake was the largest earthquake to rupture the San Andreas fault in California since the great 1906 San Francisco earthquake. The earthquake temporarily decreased stress and aseismic slip (creep) across the parallel Hayward fault. Lienkaemper et al. now show that the creep rate on the southern part of the Hayward fault has increased again. The observations also imply that the creep changes are responding to stress changes of about 1 bar.

  10. Polymers under strain

    What happens to polymer chains in solution when flow conditions, such as a region of changing velocity, cause them to stretch out? Theory has suggested that the polymers would remain coiled unless a critical velocity gradient was exceeded, which would then cause a transition to a stretched state. Experimentally, however, it has proven difficult to actually image individual polymers under such flow conditions. Perkins et al. fluorescently labeled lambda-phage DNA, and images of hundreds of individual molecules undergoing elongational flow revealed a variety of stretched conformations, even for molecules of similar length under the same stretching conditions. [See the Perspective by de Gennes.]

  11. Connected chloroplasts

    Chloroplasts, and possibly other plastids, have long been thought to be independent subcellular organelles, perhaps derived from an ancient endosymbiotic event in the evolution of the eukaryotic cell. Köhler et al. have now found that at least some of the plastids are interconnected by tubules. In plants engineered to express the green fluorescent protein in their chloroplasts, the chloroplasts and tubules are lit up. Photobleaching of one or the other of interconnected chloroplasts shows that contents of the chloroplasts can be exchanged through the tubules. Earlier observation technologies hinted at the existence of these tubules, but were not so successful at capturing the existence of the fragile, dynamic interconnections.

  12. EBV infection model

    Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with B cell lymphomas, nasopharyngeal carcinoma, and certain types of Hodgkin's disease. Until now, no animal model has reproduced the key features of the EBV infection cycle in humans. Moghaddam et al. found that a rhesus lymphocryptovirus similar to EBV is also transmitted orally, induces similar immune responses, causes pathology in the lymph nodes and abnormal proliferations of lymphoid cells, shows latent infection in the peripheral blood, and is secreted persistently into the lower part of the pharynx. This model system will be helpful for the study of the mechanisms of viral persistence and latency and in the development of vaccines and therapeutics for EBV-related diseases.

  13. Extra oxygen

    When muscles or brain work hard, their blood flow increases to supply the extra oxygen they require. Stamler et al. suggest that this regulation is accomplished by the allosteric properties of one form of the oxygen carrier hemoglobin, S-nitrosohemoglobin (SNO-Hb). Under low oxygen tension, SNO-Hb causes vascular relaxation and increased blood flow. Protein modeling studies suggest that when oxygen dissociates from SNO-Hb under low oxygen tension, the resulting conformational change also liberates nitric oxide from thiols of SNO-Hb. This nitric oxide then likely causes local vascular relaxation and increased blood flow.

  14. Inside views

    Current medical imaging technologies allow noninvasive visualization of tissue anatomy in the human body at resolutions ranging from 100 micrometers to 1 millimeter. These technologies are generally not sensitive enough to detect early-stage tissue abnormalities associated with diseases such as cancer and atherosclerosis, which require micrometer-scale resolution. Tearney et al. adapted optical coherence tomography (OCT) to allow visualization of tissue in a living animal by means of a catheter-endoscope. They used this new method, called “optical biopsy,” to obtain cross-sectional images of the rabbit gastrointestinal and respiratory tracts at 10-micrometer resolution. [See the cover.]

  15. Requirements for T cell survival

    T cells provide protection from infection. If they have not yet come into contact with the antigen for which they have a specific receptor, they are called “virgin” or “naïve” cells. After they have been exposed to antigen and become activated, they can enter another developmental stage, called “memory” cells. Just how easy or difficult is it for naïve or memory CD8+ T cells to survive? Tanchot et al. have determined that the most stringent conditions [correct major histocompatibility complex (MHC) class I molecule and correct antigen] are necessary for naïve cells to proliferate. If only the correct MHC is present, the naïve cells will survive, but not expand their numbers. Memory cells will proliferate in the presence of the right MHC, but need only a nonspecific class I interaction to survive. Thus, the memory cells, which are postulated to provide the protection afforded by vaccination and booster immunizations, have more relaxed requirements, which explains their faster response time. If these requirements can be generalized to T cells in other systems, it may be possible to develop better vaccines and immunization strategies. [See the Perspective by Benoist and Mathis.]

  16. Gene association with familial Parkinson's disease

    The degenerative disorder Parkinson's disease progresses from tremor and muscle rigidity to total disability and death. Although many cases are sporadic, some show familial inheritance. Polymeropoulos et al. have found that inheritance of Parkinson's disease in one Italian kindred and three unrelated families of Greek origin is associated with a mutation in the gene encoding a-synuclein. This evolutionarily conserved gene is expressed in the brain and may be involved with neuronal plasticity. Further studies of this gene may provide insights into the molecular mechanisms that lead to Parkinson's disease. [See the news story by Vogel.]

  17. Effects of cannabinoids

    Two reports focus on the effects of cannabinoids, the active component in marijuana, in the brain. Corticotropin-releasing factor (CRF) has been associated with the mediation of the stress and negative affective consequences of withdrawal from drugs of abuse, such as alcohol, cocaine, and opiates. De Fonseca et al., studying brain CRF systems, now show that cannabinoids should be added to the above list. Rats were treated daily for 2 weeks with a potent synthetic cannabinoid (HU-210). Withdrawal, induced by a cannabinoid antagonist, increased the brain CRF and also produced a brain pattern (Fos activation) characteristic of withdrawal. The results imply that long-term exposure to cannabinoids are accompanied by neuroadaptive changes that lead to enhanced release of CRF in the central amygdala of the brain as well as activation of the central amygdaloid nuclei during withdrawal. A key brain system for the rewarding properties of drugs (and pleasurable experiences) is the mesolimbic dopamine pathway, which terminates in the nucleus accumbens and uses dopamine as a neurotransmitter. Tanda et al. have now shown that cannabinoids, the active ingredient in marijuana, cause release of dopamine in the outer part of the nucleus accumbens (the shell) but not in the core. This effect is similar to the action of more powerful drugs of abuse such as heroin and nicotine and adds to the evidence for the addictive properties of Cannabis. [See the news story by Wickelgren.]

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