Glial membranes at the node of Ranvier prevent neurite outgrowth

Nodes of Ranvier are regularly placed, nonmyelinated axon segments along myelinated nerves. Here we show that nodal membranes isolated from the central nervous system (CNS) of mammals restricted neurite outgrowth of cultured neurons. Proteomic analysis of these membranes revealed several inhibitors of neurite outgrowth, including the oligodendrocyte myelin glycoprotein (OMgp). In rat spinal cord, OMgp was not localized to compact myelin, as previously thought, but to oligodendroglia-like cells, whose processes converge to form a ring that completely encircles the nodes. In OMgp-null mice, CNS nodes were abnormally wide and collateral sprouting was observed. Nodal ensheathment in the CNS may stabilize the node and prevent axonal sprouting.     

Neuroblastoma: synchronization of neurite outgrowth in cultures grown on collagen

Neurite outgrowth in cultured neuroblastoma cells is inhibited in cells grown on a collagen substratum as compared to those grown on glass. Rapid, synchronized initiation of neurite outgrowth occurs after hydrolysis of the underlying collagen with collagenase. Axonated cultures exhibit an increased RNA and protein content as compared to cultures grown on collagen.     

A domain in TNF receptors that mediates ligand-independent receptor assembly and signaling

A conserved domain in the extracellular region of the 60- and 80-kilodalton tumor necrosis factor receptors (TNFRs) was identified that mediates specific ligand-independent assembly of receptor trimers. This pre-ligand-binding assembly domain (PLAD) is physically distinct from the domain that forms the major contacts with ligand, but is necessary and sufficient for the assembly of TNFR complexes that bind TNF-alpha and mediate signaling. Other members of the TNFR superfamily, including TRAIL receptor 1 and CD40, show similar homotypic association. Thus, TNFRs and related receptors appear to function as preformed complexes rather than as individual receptor subunits that oligomerize after ligand binding.     

A gain-of-function mutation in a cytokinin receptor triggers spontaneous root nodule organogenesis

Legume root nodules originate from differentiated cortical cells that reenter the cell cycle and form organ primordia. We show that perception of the phytohormone cytokinin is a key element in this switch. Mutation of a Lotus japonicus cytokinin receptor gene leads to spontaneous development of root nodules in the absence of rhizobia or rhizobial signal molecules. The mutant histidine kinase receptor has cytokinin-independent activity and activates an Escherichia coli two-component phosphorelay system in vivo. Mutant analysis shows that cytokinin signaling is required for cell divisions that initiate nodule development and defines an autoregulated process where cytokinin induction of nodule stem cells is controlled by shoots.     

High-throughput mapping of a dynamic signaling network in mammalian cells

Signaling pathways transmit information through protein interaction networks that are dynamically regulated by complex extracellular cues. We developed LUMIER (for luminescence-based mammalian interactome mapping), an automated high-throughput technology, to map protein-protein interaction networks systematically in mammalian cells and applied it to the transforming growth factor-beta (TGFbeta) pathway. Analysis using self-organizing maps and k-means clustering identified links of the TGFbeta pathway to the p21-activated kinase (PAK) network, to the polarity complex, and to Occludin, a structural component of tight junctions. We show that Occludin regulates TGFbeta type I receptor localization for efficient TGFbeta-dependent dissolution of tight junctions during epithelial-to-mesenchymal transitions.     

Use of logic relationships to decipher protein network organization

A major focus of genome research is to decipher the networks of molecular interactions that underlie cellular function. We describe a computational approach for identifying detailed relationships between proteins on the basis of genomic data. Logic analysis of phylogenetic profiles identifies triplets of proteins whose presence or absence obey certain logic relationships. For example, protein C may be present in a genome only if proteins A and B are both present. The method reveals many previously unidentified higher order relationships. These relationships illustrate the complexities that arise in cellular networks because of branching and alternate pathways, and they also facilitate assignment of cellular functions to uncharacterized proteins.     

T cell receptor peptide therapy triggers autoregulation of experimental encephalomyelitis

Encephalitogenic T cells specific for myelin basic protein share common V beta 8 peptide sequences in their T cell receptor (TCR) that can induce autoregulatory T cells and antibodies that prevent clinical signs of experimental autoimmune encephalomyelitis (EAE). It is not known, however, if TCR peptides can treat established disease. To test its therapeutic value, TCR-V beta 8-39-59 peptide was injected into rats with clinical signs of EAE. This treatment reduced disease severity and speeded recovery, apparently by boosting anti-V beta 8 T cells and antibodies raised naturally in response to encephalitogenic V beta 8+ T cells. These results demonstrate that synthetic TCR peptides can be used therapeutically, and implicate the TCR-V beta 8-39-59 sequence as a natural idiotope involved in EAE recovery. Similarly, human TCR peptides may be effective in enhancing natural regulation of autoreactive T cells that share common V genes.     

Maternal oxytocin triggers a transient inhibitory switch in GABA signaling in the fetal brain during delivery

We report a signaling mechanism in rats between mother and fetus aimed at preparing fetal neurons for delivery. In immature neurons, gamma-aminobutyric acid (GABA) is the primary excitatory neurotransmitter. We found that, shortly before delivery, there is a transient reduction in the intracellular chloride concentration and an excitatory-to-inhibitory switch of GABA actions. These events were triggered by oxytocin, an essential maternal hormone for labor. In vivo administration of an oxytocin receptor antagonist before delivery prevented the switch of GABA actions in fetal neurons and aggravated the severity of anoxic episodes. Thus, maternal oxytocin inhibits fetal neurons and increases their resistance to insults during delivery.     

Structural adaptations in a membrane enzyme that terminates endocannabinoid signaling

Cellular communication in the nervous system is mediated by chemical messengers that include amino acids, monoamines, peptide hormones, and lipids. An interesting question is how neurons regulate signals that are transmitted by membrane-embedded lipids. Here, we report the 2.8 angstrom crystal structure of the integral membrane protein fatty acid amide hydrolase (FAAH), an enzyme that degrades members of the endocannabinoid class of signaling lipids and terminates their activity. The structure of FAAH complexed with an arachidonyl inhibitor reveals how a set of discrete structural alterations allows this enzyme, in contrast to soluble hydrolases of the same family, to integrate into cell membranes and establish direct access to the bilayer from its active site.     

Identification of a thyroid hormone receptor that is pituitary-specific

Three cellular homologs of the v-erbA oncogene were previously identified in the rat; two of them encode high affinity receptors for the thyroid hormone triiodothyronine (T3). A rat complementary DNA clone encoding a T3 receptor form of the ErbA protein, called r-ErbA beta-2, was isolated. The r-ErbA beta-2 protein differs at its amino terminus from the previously described rat protein encoded by c-erbA beta and referred to as r-ErbA beta-1. Unlike the other members of the c-erbA proto-oncogene family, which have a wide tissue distribution, r-erbA beta-2 appears to be expressed only in the anterior pituitary gland. In addition, thyroid hormone downregulates r-erbA beta-2 messenger RNA but not r-erbA beta-1 messenger RNA in a pituitary tumor-derived cell line. The presence of a pituitary-specific form of the thyroid hormone receptor that may be selectively regulated by thyroid hormone could be important for the differential regulation of gene expression by T3 in the pituitary gland.     

beta-Arrestin: a protein that regulates beta-adrenergic receptor function

Homologous or agonist-specific desensitization of beta-adrenergic receptors is thought to be mediated by a specific kinase, the beta-adrenergic receptor kinase (beta ARK). However, recent data suggest that a cofactor is required for this kinase to inhibit receptor function. The complementary DNA for such a cofactor was cloned and found to encode a 418-amino acid protein homologous to the retinal protein arrestin. The protein, termed beta-arrestin, was expressed and partially purified. It inhibited the signaling function of beta ARK-phosphorylated beta-adrenergic receptors by more than 75 percent, but not that of rhodopsin. It is proposed that beta-arrestin in concert with beta ARK effects homologous desensitization of beta-adrenergic receptors.     

网络教学平台的设计与开发

计算机和网络技术日新月异的发展,正在迅速地改变我们传统的生活、工作和学习方式.融合影像、语音和数据等多媒体信息网络技术的成熟,以及Internet的不断普及,使得远程教学获得了前所未有的强大技术支持手段.     

MAP kinase phosphatase as a locus of flexibility in a mitogen-activated protein kinase signaling network

Intracellular signaling networks receive and process information to control cellular machines. The mitogen-activated protein kinase (MAPK) 1,2/protein kinase C (PKC) system is one such network that regulates many cellular machines, including the cell cycle machinery and autocrine/paracrine factor synthesizing machinery. We used a combination of computational analysis and experiments in mouse NIH-3T3 fibroblasts to understand the design principles of this controller network. We find that the growth factor-stimulated signaling network containing MAPK 1, 2/PKC can operate with one (monostable) or two (bistable) stable states. At low concentrations of MAPK phosphatase, the system exhibits bistable behavior, such that brief stimulus results in sustained MAPK activation. The MAPK-induced increase in the amounts of MAPK phosphatase eliminates the prolonged response capability and moves the network to a monostable state, in which it behaves as a proportional response system responding acutely to stimulus. Thus, the MAPK 1, 2/PKC controller network is flexibly designed, and MAPK phosphatase may be critical for this flexible response.     

Comment on "Maternal oxytocin triggers a transient inhibitory switch in GABA signaling in the fetal brain during delivery"

Tyzio et al. (Reports, 15 December 2006, p. 1788) reported that maternal oxytocin triggers a transient excitatory-to-inhibitory switch of gamma-aminobutyric acid (GABA) signaling during labor, thus protecting the fetal rat brain from anoxic injury. However, a body of evidence supports the possibility that oxytocin is released from the fetal pituitary during delivery, not only from the mother, particularly under conditions of hypoxic stress.     

Long-distance signaling in nodulation directed by a CLAVATA1-like receptor kinase

Proliferation of legume nodule primordia is controlled by shoot-root signaling known as autoregulation of nodulation (AON). Mutants defective in AON show supernodulation and increased numbers of lateral roots. Here, we demonstrate that AON in soybean is controlled by the receptor-like protein kinase GmNARK (Glycine max nodule autoregulation receptor kinase), similar to Arabidopsis CLAVATA1 (CLV1). Whereas CLV1 functions in a protein complex controlling stem cell proliferation by short-distance signaling in shoot apices, GmNARK expression in the leaf has a major role in long-distance communication with nodule and lateral root primordia.     

Polarization of chemoattractant receptor signaling during neutrophil chemotaxis

Morphologic polarity is necessary for chemotaxis of mammalian cells. As a probe of intracellular signals responsible for this asymmetry, the pleckstrin homology domain of the AKT protein kinase (or protein kinase B), tagged with the green fluorescent protein (PHAKT-GFP), was expressed in neutrophils. Upon exposure of cells to chemoattractant, PHAKT-GFP is recruited selectively to membrane at the cell's leading edge, indicating an internal signaling gradient that is much steeper than that of the chemoattractant. Translocation of PHAKT-GFP is inhibited by toxin-B from Clostridium difficile, indicating that it requires activity of one or more Rho guanosine triphosphatases.     

BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF

B cell homeostasis has been shown to critically depend on BAFF, the B cell activation factor from the tumor necrosis factor (TNF) family. Although BAFF is already known to bind two receptors, BCMA and TACI, we have identified a third receptor for BAFF that we have termed BAFF-R. BAFF-R binding appears to be highly specific for BAFF, suggesting a unique role for this ligand-receptor interaction. Consistent with this, the BAFF-R locus is disrupted in A/WySnJ mice, which display a B cell phenotype qualitatively similar to that of the BAFF-deficient mice. Thus, BAFF-R appears to be the principal receptor for BAFF-mediated mature B cell survival.     

Dual signaling regulated by calcyon, a D1 dopamine receptor interacting protein

The synergistic response of cells to the stimulation of multiple receptors has been ascribed to receptor cross talk; however, the specific molecules that mediate the resultant signal amplification have not been defined. Here a 24-kilodalton single transmembrane protein, designated calcyon, we functionally characterize that interacts with the D1 dopamine receptor. Calcyon localizes to dendritic spines of D1 receptor-expressing pyramidal cells in prefrontal cortex. These studies delineate a mechanism of Gq- and Gs-coupled heterotrimeric GTP-binding protein-coupled receptor cross talk by which D1 receptors can shift effector coupling to stimulate robust intracellular calcium (Ca2+i) release as a result of interaction with calcyon. The role of calcyon in potentiating Ca2+-dependent signaling should provide insight into the D1 receptor-modulated cognitive functions of prefrontal cortex.