This Week in Science

Science  24 Nov 1995:
Vol. 270, Issue 5240, pp. 1277
  1. Balancing act in neuronal development

    Proper neuronal development requires that many neurons receive signals that cause them to undergo apoptosis and die. Xia et al. (p. 1326) have uncovered a role for mitogen-activated protein (MAP) kinases, which are activated by numerous extracellular stimuli, in regulating apoptosis. In pheochromocytoma cells that are deprived of nerve growth factor, two members of the MAP kinase family have opposing effects on apoptosis. Activity of ERKs—MAP kinases that are activated by growth factors—inhibits apoptosis, whereas activity of JNKs—MAP kinases that are activated by various forms of environmental stress—promotes apoptosis. The balance of the activities of these two signaling pathways is apparently critical in controlling survival of these cells.

  2. Hopping cations

    Diffusion of cations in crystalline solids can control many processes, including grain boundary growth, ionic conductivity, and phase separation. Xu and Stebbins (p. 1332) have followed the hopping of 6Li+ cations in Li4 SiO4 using two-dimensional nuclear magnetic resonance methods. This approach allowed the rates and activation barriers between particular sites in the crystal to be measured directly.

  3. Barring Earth's halo

    Ultraviolet images of the Earth reveal an auroral halo caused by the interaction of geomagnetic field lines with the solar wind. At times a connecting bar is seen that runs over the magnetic poles in the noon to midnight direction. It has been thought that this barred aurora, or θaurora, occurs when the interplanetary magnetic field (IMF) is pointed north. Newell and Meng (p. 1338) present satellite data which show that θ auroras occur when the IMF flips south after a long period of pointing northward.

  4. Opening the Red Sea

    The Red Sea is an example of continental rifting forming a new ocean basin. Sea floor spreading seems to have initiated in the south and is now migrating toward the north, but knowledge of the early development of the rift is critical for testing models of continental breakup. Fission track dating studies by Omar and Steckler (p. 1341) show that the initial breakup of the crust involved two coherent pulses of uplift, erosion, and extension along the length of the future Red Sea, beginning about 34 million years ago. The plates were not unzipped from south to north, but opened as rigid bodies hinged to the north.

  5. Deactivating decoys

    Cells infected with virus are targeted for destruction by cytotoxic T lymphocytes (CTLs). Meier et al. (p. 1360) show that mutations help HIV-infected cells evade this immune response. Activation of the CTL requires the interaction of the T cell receptor with a human leukocyte antigen class I surface complex on the infected cell that contains a viral peptide. Presentation of complexes with peptides that vary only slightly from the target peptide can inactivate or anergize the CTL, so instead of being destroyed, the infected cell actually inactivates a CTL that could have destroyed other cells containing different HIV variants.

  6. Steroid enhancer

    Steroid receptors form a family of ligand-inducible transcription factors that regulate genes in response to hormones. Onate et al. (p. 1354) have isolated and characterized a protein, SRC-1 (steroid receptor coactivator-1), that enhances the transcriptional activity of multiple steroid receptors in a ligand-dependent fashion. The enhancing activity of SRC-1 seems limited to steroid receptors. A truncated form of SRC-1 acts as a dominant negative repressor and prevents the hormone-induced transcriptional activity of several steroid receptors.

  7. Taking such pains

    Opioid analgesics, such as morphine, can block dull persistent pain but are ineffective against acute sensations like a pinprick. Taddese et al. (p. 1366) performed patch clamp experiments on nociceptive neurons, which evoke pain responses, in the tooth pulp of rats. Activating the m opioid receptors inhibited calcium channels of small, slow-conducting nociceptive neurons that transmit the signals for persistent pain but had no effect on the large, rapidly conducting nociceptive neurons that transmit the initial sharp pain response.

  8. Neuronal activity and adhesion

    Nervous system development is modulated both by neuronal activity and by cellular interactions through adhesion and recognition molecules located on the cell surface. Itoh et al. (p. 1369) found that when neurons taken from the mouse dorsal root ganglion were stimulated at a particular frequency before synaptogenesis, expression of the neural cell adhesion molecule L1 as well as adhesion of these neurons to other cell types was reduced. Stimulation at other frequencies, or after synaptogenesis, left L1 expression and intercellular adhesion unaffected. Certain types of neuronal activity can regulate expression of specific adhesion molecules that help pattern the developing nervous system.

  9. CLIP unclips

    Major histocompatibility complex (MHC) class II molecules present peptides derived from exogenous antigens. Newly synthesized class II molecules are prevented from binding endogenous peptides by a polypeptide known as the invariant chain. In particular, the CLIP fragment (for class II-associated invariant chain peptides) blocks the peptide-binding groove. The mechanism whereby CLIP is removed to allow binding of exogenous peptide is unclear. Recent evidence pointed to a role for the nonclassical MHC molecule HLA-DM, but a fraction of molecules in DM-negative cells are expressed normally. Kropshofer et al. (p. 1357) show that self-release of CLIP occurs under appropriate conditions. Self-release occurs at endosomal pH and is catalyzed by the amino-terminal segment, which lies outside the groove.

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