Identification of a neuronal cell surface molecule, plexin, in mice

PlexinA1 deficiency in BALB/cAJ mice leads to excessive self-grooming and reduced prepulse inhibition

PlexinA1 (PlxnA1) is a transmembrane receptor for semaphorins, a large family of proteins that act as axonal guidance cues during nervous system development. However, there are limited studies on PlxnA1 function in neurobehavior. The present study examined if PlxnA1 deficiency leads to behavioral abnormalities in BALB/cAJ mice. PlxnA1 knockout (KO) mice were generated by homologous recombination and compared to wild type (WT) littermates on a comprehensive battery of behavioral tests, including open field assessment of spontaneous ambulation, state anxiety, and grooming, home cage grooming, the wire hang test of muscle strength, motor coordination on the rotarod task, working memory on the Y maze alternation task, cued and contextual fear conditioning, anxiety on the elevated plus maze, sociability to intruders, and sensory processing as measured by prepulse inhibition (PPI). Measures of motor performance, working memory, fear memory, and sociability did not differ significantly between genotypes, while PlxnA1 KO mice displayed excessive self-grooming, impaired PPI, and slightly lower anxiety. These results suggest a crucial role for PlxnA1 in the development and function of brain regions controlling self-grooming and sensory gating. PlxnA1 KO mice may be a valuable model to investigate the repetitive behaviors and information processing deficits characteristic of many neurodevelopmental and psychiatric disorders.

The IκB kinase complex is required for plexin-B-mediated activation of RhoA

Plexins are widely expressed transmembrane proteins that mediate the cellular effects of semaphorins. The molecular mechanisms of plexin-mediated signal transduction are still poorly understood. Here we show that signalling via B-family plexins leading to the activation of the small GTPase RhoA requires activation of the IκB kinase (IKK)-complex. In contrast, plexin-B-dependent regulation of R-Ras activity is not affected by IKK activity. This regulation of plexin signalling depends on the kinase activity of the IKK-complex, but is independent of NF-κB activation. We confirm that the IKK-complex is active in tumour cells and osteoblasts, and we demonstrate that plexin-B-dependent tumour cell invasiveness and regulation of osteoblast differentiation require an active IKK-complex. This study identifies a novel, NF-κB-independent function of the IKK-complex and shows that IKK directs plexin-B signalling to the activation of RhoA.

Spatiotemporal expression pattern of Myt/NZF family zinc finger transcription factors during mouse nervous system development

BACKGROUND

Three members of the Myt/NZF family of transcription factors are involved in many processes of vertebrate development. Several studies have reported that Myt1/NZF-2 has a regulatory function in the development of cultured oligodendrocyte progenitors or in neuronal differentiation during Xenopus primary neurogenesis. However, little is known about the proper function of Myt/NZF family proteins during mammalian nervous system development. To assess the possible function of Myt/NZF transcription factors in mammalian neuronal differentiation, we determined the comparative spatial and temporal expression patterns of all three types of Myt/NZF family genes in the embryonic mouse nervous system using quantitative reverse transcriptase polymerase chain reaction and in situ hybridization.

RESULTS

All three Myt/NZF family genes were extensively expressed in developing mouse nervous tissues, and their expression was transient. NZF-1 was expressed later in post-mitotic neurons. NZF-2 was initially expressed in neuronal cells a little earlier than NZF-3. NZF-3 was initially expressed in neuronal cells, just after proliferation was complete.

CONCLUSION

These expression patterns suggest that the expression of NZF family genes is spatially and temporally regulated, and each Myt/NZF family gene may have a regulatory function in a specific phase during neuronal differentiation.

Myt/NZF family transcription factors regulate neuronal differentiation of P19 cells

During mammalian central nervous system development, neural stem cells differentiate and then mature into various types of neurons. Myelin transcription factor (Myt)/neural zinc finger (NZF) family proteins were first identified as myelin proteolipid protein promoter binding factors and were shown to be involved in oligodendrocyte development. In this study, we found that Myt/NZF family molecules were expressed during neuronal differentiation in vivo and in vitro. Transient over-expression of Myt/NZF family genes could convert undifferentiated P19 cells into neurons without induction by retinoic acid (RA), and the ability of these genes to induce neuronal differentiation was comparable to that of Neurog1 and Neurod1. Additionally, we found that St18 (or NZF-3) was induced by several bHLH transcription factors. When NZF-3 and Neurog1 were co-expressed in P19 cells, the rate of neuronal differentiation was significantly increased. These data suggest not only that NZF-3 works downstream of Neurog1 but also that it plays a crucial role together with Neurog1 in neuronal differentiation.

Diverse functions for the semaphorin receptor PlexinD1 in development and disease

Plexins are a family of single-pass transmembrane proteins that serve as cell surface receptors for Semaphorins during the embryonic development of animals. Semaphorin-Plexin signaling is critical for many cellular aspects of organogenesis, including cell migration, proliferation and survival. Until recently, little was known about the function of PlexinD1, the sole member of the vertebrate-specific PlexinD (PlxnD1) subfamily. Here we review novel findings about PlxnD1's roles in the development of the cardiovascular, nervous and immune systems and salivary gland branching morphogenesis and discuss new insights concerning the molecular mechanisms of PlxnD1 activity.

The proteome of the presynaptic active zone: from docked synaptic vesicles to adhesion molecules and maxi-channels

The presynaptic proteome controls neurotransmitter release and the short and long term structural and functional dynamics of the nerve terminal. Using a monoclonal antibody against synaptic vesicle protein 2 we immunopurified a presynaptic compartment containing the active zone with synaptic vesicles docked to the presynaptic plasma membrane as well as elements of the presynaptic cytomatrix. Individual protein bands separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were subjected to nanoscale-liquid chromatography electrospray ionization-tandem mass spectrometry. Combining this method with 2-dimensional benzyldimethyl-n-hexadecylammonium chloride/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization time of flight and immunodetection we identified 240 proteins comprising synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery, proteins involved in intracellular signal transduction, a large variety of adhesion molecules and proteins potentially involved in regulating the functional and structural dynamics of the pre-synapse. Four maxi-channels, three isoforms of voltage-dependent anion channels and the tweety homolog 1 were co-isolated with the docked synaptic vesicles. As revealed by in situ hybridization, tweety homolog 1 reveals a distinct expression pattern in the rodent brain. Our results add novel information to the proteome of the presynaptic active zone and suggest that in particular proteins potentially involved in the short and long term structural modulation of the mature presynaptic compartment deserve further detailed analysis.

The PLEXIN PLX-2 and the ephrin EFN-4 have distinct roles in MAB-20/Semaphorin 2A signaling in Caenorhabditis elegans morphogenesis

Semaphorins are extracellular proteins that regulate axon guidance and morphogenesis by interacting with a variety of cell surface receptors. Most semaphorins interact with plexin-containing receptor complexes, although some interact with non-plexin receptors. Class 2 semaphorins are secreted molecules that control axon guidance and epidermal morphogenesis in Drosophila and Caenorhabditis elegans. We show that the C. elegans class 2 semaphorin MAB-20 binds the plexin PLX-2. plx-2 mutations enhance the phenotypes of hypomorphic mab-20 alleles but not those of mab-20 null alleles, indicating that plx-2 and mab-20 act in a common pathway. Both mab-20 and plx-2 mutations affect epidermal morphogenesis during embryonic and in postembryonic development. In both contexts, plx-2 null mutant phenotypes are much less severe than mab-20 null phenotypes, indicating that PLX-2 is not essential for MAB-20 signaling. Mutations in the ephrin efn-4 do not synergize with mab-20, indicating that EFN-4 may act in MAB-20 signaling. EFN-4 and PLX-2 are coexpressed in the late embryonic epidermis where they play redundant roles in MAB-20-dependent cell sorting.

Failure to confirm an association between the PLXNA2 gene and schizophrenia in a Japanese population

Plexins are receptors for multiple classes of semaphorins, either alone or in combination with neuropilins. Plexins participate in many cellular events that include axonal repulsion, axonal attraction, cell migration, axon pruning, and synaptic plasticity. PLXNA2 maps to chromosome 1q32. Several linkage studies reported schizophrenia susceptibility loci in the 1q22-42 region. A recent study reported that intronic single nucleotide polymorphisms (SNPs) of PLXNA2 were associated with schizophrenia in a European American population. We attempted to replicate this finding in a Japanese sample of 336 patients with schizophrenia and 304 controls. In addition, we examined 3 non-synonymous SNPs (Arg5Gln, GLn57Arg, and Ala267Thr) in PLXNA2. Genotyping was performed by the TaqMan allelic discrimination assay. There was no significant difference in genotype or allele distribution of either the 4 intronic SNPs or the 3 non-synonymous SNPs between patients and controls. Furthermore, haplotype-based analyses did not provide evidence for an association. These results suggest that PLXNA2 may not play a major role in the development of schizophrenia in our Japanese sample.

Class A plexin expression in axotomized rubrospinal and facial motoneurons

The semaphorin family of guidance molecules plays a role in many aspects of neural development, and more recently semaphorins have been implicated to contribute to the failure of injured CNS neurons to regenerate. While semaphorin expression patterns after neural injury are partially understood, little is known about the expression of their signal transducing transmembrane receptors, the plexins. Therefore, in this study, we compared the expression patterns of all class A plexins (Plxn-A1, A2, A3, A4) in mouse CNS (rubrospinal) and peripheral nervous system (PNS)-projecting (facial) motoneurons for up to two weeks following axonal injury. Using in situ hybridization, immunohistochemistry, and Western blot analysis, in rubrospinal neurons, Plxn-A1 mRNA and protein and Plxn-A4 expression did not change as a result of injury while Plxn-A2 mRNA increased and Plxn-A3 mRNA was undetectable. In facial motoneurons, Plxn-A1, -A3 and -A4 mRNA expression increased, Plxn-A2 mRNA decreased while Plxn-A1 protein expression did not change following injury. We demonstrate that with the exception of the absence of Plxn-A3 mRNA in rubrospinal neurons, both injured rubrospinal (CNS) and facial (PNS) neurons maintain expression of all plexin A family members tested. Hence, there are distinct expression patterns of the individual plexin-A family members suggesting that regenerating rubrospinal and facial motoneurons have a differential ability to transduce semaphorin signals.

A novel role for Sema3A in neuroprotection from injury mediated by activated microglia

Microglia exist under physiological conditions in a resting state but become activated after neuronal injury. Recent studies have highlighted the reciprocal role of neurons in controlling both the number and activity of microglia. In this study, microglia derived from newborn rat cortices were cultured and activated by interferon-gamma (IFNgamma) treatment, then exposed to recombinant Sema3A or conditioned medium derived from stressed embryonic cortical neurons. We found that activation of microglia by IFNgamma induced differential upregulation of the semaphorin receptors Plexin-A1 and Neuropilin-1. This result was confirmed by Northern blotting, reverse transcription-PCR, and Western blotting. Furthermore, recombinant Sema3A induced apoptosis of microglia when added to the in vitro culture, and a similar result was obtained on activated microglia when Sema3A was produced by stressed neurons. Using an in vivo model of microglia activation by striatal injection of lipopolysaccharide demonstrated a corresponding upregulation of Plexin-A1 and Neuropilin-1 in activated microglia and enhanced production of Sema3A by stressed adult neurons. These results suggest a novel semaphorin-mediated mechanism of neuroprotection whereby stressed neurons can protect themselves from further damage by activated microglia.

Plexin-a4 mediates axon-repulsive activities of both secreted and transmembrane semaphorins and plays roles in nerve fiber guidance

It has been proposed that four members of the plexin A subfamily (plexin-As; plexin-A1, -A2, -A3, and -A4) and two neuropilins (neuropilin-1 and neuropilin-2) form complexes and serve as receptors for class 3 secreted semaphorins (Semas), potent neural chemorepellents. The roles of given plexin-As in semaphorin signaling and axon guidance, however, are mostly unknown. Here, to elucidate functions of plexin-A4 in semaphorin signaling and axon guidance events in vivo, we generated plexin-A4 null mutant mice by targeted disruption of the plexin-A4 gene. Plexin-A4 mutant mice were defective in the trajectory and projection of peripheral sensory axons and sympathetic ganglion (SG) axons and the formation of the anterior commissure and the barrels. The defects in peripheral sensory and SG axons were fundamentally related to those of neuropilin-1 or Sema3A mutant embryos reported but were more moderate than the phenotype in these mutants. The growth cone collapse assay showed that dorsal root ganglion axons and SG axons of plexin-A4 mutant embryos partially lost their responsiveness to Sema3A. These results suggest that plexin-A4 plays roles in the propagation of Sema3A activities and regulation of axon guidance and that other members of the plexin-A subfamily are also involved in the propagation of Sema3A activities. Plexin-A4-deficient SG axons did not lose their responsiveness to Sema3F, suggesting that plexin-A4 serves as a Sema3A-specific receptor, at least in SG axons. In addition, the present study showed that plexin-A4 bound class 6 transmembrane semaphorins, Sema6A and Sema6B, and mediated their axon-repulsive activities, independently of neuropilin-1. Our results imply that plexin-A4 mediates multiple semaphorin signals and regulates axon guidance in vivo.

The expression of plexins during mouse embryogenesis

Plexins are large transmembrane proteins that are receptors for semaphorins, either alone or in a complex with neuropilin-1 or -2. Nine different mouse plexins have been found: Plexin-A1-4, -B1-3, -C1 and -D1. The expression and function of plexins in non-neuronal tissues has been poorly characterized, although Plexin-A1 has been shown to have a role during lung and cardiac morphogenesis. We have done an extensive non-radioactive in situ hybridisation survey of Plxna1-a4, Plxnb1 -b3 and Plxnc1 in E14 mouse embryo. At E14, Plxnb3 expression could not be detected by in situ hybridisation. All other plexins studied are widely expressed both in neuronal and non-neuronal tissues. We have also followed the expression patterns of plexins during the development of the kidney, tooth and testis. Plxnb1 and Plxnb2 are expressed in the immature glomeruli and mesenchyme of the developing kidney. In the tooth bud, Plxna1 and Plxnb1 are expressed in the oral epithelium, enamel knot and in both the inner and outer enamel epithelium, whereas the expression of Plxnb2 is more restricted to the inner enamel epithelium. In the testis, Plxna1, Plxnb1 and Plxnc1 are expressed in the developing sex chords. This study shows that during development, plexins are expressed in specific and distinct patterns also in non-neuronal tissues.

Wound healing and inflammation genes revealed by array analysis of 'macrophageless' PU.1 null mice

BACKGROUND

Wound healing is a complex process requiring the collaborative efforts of different tissues and cell lineages, and involving the coordinated interplay of several phases of proliferation, migration, matrix synthesis and contraction. Tissue damage also triggers a robust influx of inflammatory leukocytes to the wound site that play key roles in clearing the wound of invading microbes but also release signals that may be detrimental to repair and lead to fibrosis.

RESULTS

To better define key cellular events pivotal for tissue repair yet independent of inflammation we have used a microarray approach to determine a portfolio of over 1,000 genes expressed across the repair response in a wild-type neonatal mouse versus its PU.1 null sib. The PU.1 null mouse is genetically incapable of raising the standard inflammatory response, because it lacks macrophages and functioning neutrophils, yet repairs skin wounds rapidly and with reduced fibrosis. Conversely, by subtraction, we have determined genes that are either expressed by leukocytes, or upregulated by fibroblasts, endothelial cells, muscle cells and others at the wound site, as a consequence of inflammation. To determine the spatial expression pattern for several genes in each cluster we have also performed in situ hybridization studies.

CONCLUSIONS

Cluster analysis of genes expressed after wounding wild-type mice versus PU.1 null sibs distinguishes between tissue repair genes and genes associated with inflammation and its consequences. Our data reveal and classify several pools of genes, giving insight into their likely functions during repair and hinting at potential therapeutic targets.

Plexin-B1/RhoGEF-mediated RhoA activation involves the receptor tyrosine kinase ErbB-2

Plexins are widely expressed transmembrane proteins that mediate the effects of semaphorins. The molecular mechanisms of plexin-mediated signal transduction are still rather unclear. Plexin-B1 has recently been shown to mediate activation of RhoA through a stable interaction with the Rho guanine nucleotide exchange factors PDZ-RhoGEF and LARG. However, it is unclear how the activity of plexin-B1 and its downstream effectors is regulated by its ligand Sema4D. Here, we show that plexin-B family members stably associate with the receptor tyrosine kinase ErbB-2. Binding of Sema4D to plexin-B1 stimulates the intrinsic tyrosine kinase activity of ErbB-2, resulting in the phosphorylation of both plexin-B1 and ErbB-2. A dominant-negative form of ErbB-2 blocks Sema4D-induced RhoA activation as well as axonal growth cone collapse in primary hippocampal neurons. Our data indicate that ErbB-2 is an important component of the plexin-B receptor system and that ErbB-2-mediated phosphorylation of plexin-B1 is critically involved in Sema4D-induced RhoA activation, which underlies cellular phenomena downstream of plexin-B1, including axonal growth cone collapse.

PlexinA4 is necessary as a downstream target of Islet2 to mediate Slit signaling for promotion of sensory axon branching

Slit is a secreted protein known to repulse the growth cones of commissural neurons. By contrast, Slit also promotes elongation and branching of axons of sensory neurons. The reason why different neurons respond to Slit in different ways is largely unknown. Islet2 is a LIM/homeodomain-type transcription factor that specifically regulates elongation and branching of the peripheral axons of the primary sensory neurons in zebrafish embryos. We found that PlexinA4, a transmembrane protein known to be a co-receptor for class III semaphorins, acts downstream of Islet2 to promote branching of the peripheral axons of the primary sensory neurons. Intriguingly, repression of PlexinA4 function by injection of the antisense morpholino oligonucleotide specific to PlexinA4 or by overexpression of the dominant-negative variant of PlexinA4 counteracted the effects of overexpression of Slit2 to induce branching of the peripheral axons of the primary sensory neurons in zebrafish embryos, suggesting involvement of PlexinA4 in the Slit signaling cascades for promotion of axonal branching of the sensory neurons. Colocalized expression of Robo, a receptor for Slit2, and PlexinA4 is observed not only in the primary sensory neurons of zebrafish embryos but also in the dendrites of the pyramidal neurons of the cortex of the mammals, and may be important for promoting the branching of either axons or dendrites in response to Slit, as opposed to the growth cone collapse.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

Discovery of semaphorin receptors, neuropilin and plexin, and their functions in neural development

The semaphorin receptors neuropilin and plexin were initially identified as antigens for monoclonal antibodies MAb-A5 and MAb-B2, which bind to specific neuropiles and plexiform layers within the Xenopus tadpole nervous systems, several years before the discovery of the first semaphorin. This article provides an overview of how neuropilin and plexin were discovered. In addition, it describes the functions of neuropilin in the signaling of chemorepulsive activities of class 3 semaphorins and roles of neuropilin-mediated semaphorin activities in the directional guidance of the peripheral nervous system (PNS) and integration of the peripheral ganglia.

From the discovery of neuropilin to the determination of its adhesion sites

Neuropilin (NRP) and plexin (Plex) that are now known to be semaphorin receptors were initially identified as antigens for monoclonal antibodies (MAbs) that bound to particular neuropiles and plexiform layers of the Xenopus tadpole optic tectum, several years before the discovery of semaphorin. The extracellular segment of the NRP protein is a mosaic of 3 functionally different protein motifs that are thought to be involved in molecular and/or cellular interactions, suggesting that NRP serves in a various cell-cell interaction by binding a variety of molecules. The first identified function of NRP was the cell adhesion activity; Cell reaggregation study using NRP-expressing cell lines revealed that NRP can mediate cell adhesion via heterophilic molecular interaction. Later, NRP was shown to bind semaphorins and vascular endothelial growth factor (VEGF). It was also shown that NRP makes receptor complexes with Plex to propagate semaphorin signals.

Identification and characterization of a novel mouse plexin, plexin-A4

Plexins belonging to the plexin-A subfamily form complexes with neuropilins and propagate signals of class 3 semaphorins into neurons, even though they do not directly bind the semaphorins. In this study, we identified a new member of the plexin-A subfamily in the mice, plexin-A4, and showed that it was expressed in the developing nervous system with a pattern different to that of other members of the plexin-A subfamily (plexin-A1, plexin-A2 and plexin-A3). COS-7 cells coexpressing plexin-A4 with neuropilin-1 were induced to contract by Sema3A, a member of the class 3 semaphorin. Ectopic expression of plexin-A4 in mitral cells that are originally insensitive to Sema3A resulted in the collapse of growth cones in the presence of Sema3A. These results suggest that plexin-A4 plays a role in the propagation of Sema3A activities.

NZF-2b is a novel predominant form of mouse NZF-2/MyT1, expressed in differentiated neurons especially at higher levels in newly generated ones

NZF-2 (MyT1) is a member of C2HC-type zinc finger transcription factors. A novel form of mouse NZF-2 has been isolated. This novel form, NZF-2b, has an additional C2HC-type zinc finger motif. The expression levels of NZF-2b are by far the more predominant than those of the already known form of NZF-2. In embryonic mouse nervous system, the expression of NZF-2b starts as early as at 9.5 days post-coitum (dpc) in newly differentiated neurons in the central nervous system (CNS) and the peripheral nervous system (PNS).

Caenorhabditis elegansPlexinA, PLX-1, interacts with transmembrane semaphorins and regulates epidermal morphogenesis

The plexin family transmembrane proteins are putative receptors for semaphorins, which are implicated in the morphogenesis of animal embryos, including axonal guidance. We have generated and characterized putative null mutants of the C. elegans plexinA gene, plx-1. plx-1 mutants exhibited morphological defects: displacement of ray 1 and discontinuous alae. The epidermal precursors for the affected organs were aberrantly arranged in the mutants, and a plx-1::gfp transgene was expressed in these epidermal precursor cells as they underwent dynamic morphological changes. Suppression of C. elegans transmembrane semaphorins, Ce-Sema-1a and Ce-Sema-1b, by RNA interference caused a displacement of ray 1 similar to that of plx-1 mutants, whereas mutants for the Ce-Sema-2a/mab-20 gene, which encodes a secreted-type semaphorin, exhibited phenotypes distinct from those of plx-1 mutants. A heterologous expression system showed that Ce-Sema-1a, but not Ce-Sema-2a, physically bound to PLX-1. Our results indicate that PLX-1 functions as a receptor for transmembrane-type semaphorins, and, though Ce-Sema-2a and PLX-1 both play roles in the regulation of cellular morphology during epidermal morphogenesis, they function rather independently.

Semaphorin 3A elicits stage-dependent collapse, turning, and branching in Xenopus retinal growth cones

The semaphorin receptor, neuropilin-1 (NP-1), was first identified in Xenopus as the A5 antigen and is expressed abundantly in developing retinal ganglion cells (RGCs). Here we show that growth cones acquire responsiveness to semaphorin 3A (Sema 3A) with age and that the onset of responsiveness correlates with the appearance of NP-1 immunoreactivity. Growth cones from "old" (stage 35/36) retinal explants collapse rapidly (5-10 min) in response to Sema 3A and turn away from a gradient of Sema 3A, whereas "young" growth cones (stage 24) are insensitive to Sema 3A. Moreover, transfection of full-length NP-1 into young neurons confers premature Sema 3A sensitivity. When young neurons are aged in culture they develop Sema 3A sensitivity in parallel with those in vivo, suggesting that an intrinsic mechanism of NP-1 regulation mediates this age-dependent change. Sema 3A-induced collapse is transient, and after recovery approximately 30% of growth cones extend new branches within 1 hr, implicating Sema 3A as a branching factor. Pharmacological inhibitors were used to investigate whether these three Sema 3A-induced behaviors (collapse, turning, and branching) use distinct second messenger signaling pathways. All three behaviors were found to be mediated via cGMP. In situ hybridization shows that Sema 3A is expressed in the tectum and at the anterior boundary of the optic tract where axons bend caudally, suggesting that Sema 3A/NP-1 interactions play a role in guiding axons in the optic tract and in stimulating terminal branching in the tectum.

Molecular basis of semaphorin-mediated axon guidance

The semaphorin family of proteins constitute one of the major cues for axonal guidance. The prototypic member of this family is Sema3A, previously designated semD/III or collapsin-1. Sema3A acts as a diffusible, repulsive guidance cue in vivo for the peripheral projections of embryonic dorsal root ganglion neurons. Sema3A binds with high affinity to neuropilin-1 on growth cone filopodial tips. Although neuropilin-1 is required for Sema3A action, it is incapable of transmitting a Sema3A signal to the growth cone interior. Instead, the Sema3A/neuropilin-1 complex interacts with another transmembrane protein, plexin, on the surface of growth cones. Certain semaphorins, other than Sema3A, can bind directly to plexins. The intracellular domain of plexin is responsible for initiating the signal transduction cascade leading to growth cone collapse, axon repulsion, or growth cone turning. This intracellular cascade involves the monomeric G-protein, Rac1, and a family of neuronal proteins, the CRMPs. Rac1 is likely to be involved in semaphorin-induced rearrangements of the actin cytoskeleton, but how plexin controls Rac1 activity is not known. Vertebrate CRMPs are homologous to the Caenorhabditis elegans unc-33 protein, which is required for proper axon morphology in worms. CRMPs are essential for Sema3A-induced, neuropilin-plexin-mediated growth cone collapse, but the molecular interactions of growth cone CRMPs are not well defined. Mechanistic aspects of plexin-based signaling for semaphorin guidance cues may have implications for other axon guidance events and for the basis of growth cone motility.

Determination of cell adhesion sites of neuropilin-1

Neuropilin-1 is a type 1 membrane protein with three distinct functions. First, it can mediate cell adhesion via a heterophilic molecular interaction. Second, in neuronal cells, neuropilin-1 binds the class 3 semaphorins, which are neuronal chemorepellents, and plays a role in the directional guidance of axons. Neuropilin-1 is expected to form complexes with the plexinA subfamily members and mediate the semaphorin-elicited inhibitory signals into neurons. Third, in endothelial cells, neuropilin-1 binds a potent endothelial cell mitogen, vascular endothelial growth factor (VEGF)(165), and regulates vessel formation. Though the binding sites in neuropilin-1 for the class 3 semaphorins and VEGF(165) have been analyzed, the sites involved in cell adhesion activity of the molecule have not been identified. In this study, we produced a variety of mutant neuropilin-1s and tested their cell adhesion activity. We showed that the b1 and b2 domains within the extracellular segment of neuropilin-1 were required for the cell adhesion activity, and peptides with an 18-amino acid stretch in the b1 and b2 domains were sufficient to induce the cell adhesion activity. In addition, we demonstrated that the cell adhesion ligands for neuropilin-1 were proteins and distributed in embryonic mesenchymal cells but distinct from the class 3 semaphorins, VEGF, or plexins.

Plexins are a large family of receptors for transmembrane, secreted, and GPI-anchored semaphorins in vertebrates

In Drosophila, plexin A is a functional receptor for semaphorin-1a. Here we show that the human plexin gene family comprises at least nine members in four subfamilies. Plexin-B1 is a receptor for the transmembrane semaphorin Sema4D (CD100), and plexin-C1 is a receptor for the GPI-anchored semaphorin Sema7A (Sema-K1). Secreted (class 3) semaphorins do not bind directly to plexins, but rather plexins associate with neuropilins, coreceptors for these semaphorins. Plexins are widely expressed: in neurons, the expression of a truncated plexin-A1 protein blocks axon repulsion by Sema3A. The cytoplasmic domain of plexins associates with a tyrosine kinase activity. Plexins may also act as ligands mediating repulsion in epithelial cells in vitro. We conclude that plexins are receptors for multiple (and perhaps all) classes of semaphorins, either alone or in combination with neuropilins, and trigger a novel signal transduction pathway controlling cell repulsion.

Analysis of Two Cosmid Clones from Chromosome 4 of Drosophila melanogaster Reveals Two New Genes Amid an Unusual Arrangement of Repeated Sequences

Chromosome 4 from Drosophila melanogaster has several unusual features that distinguish it from the other chromosomes. These include a diffuse appearance in salivary gland polytene chromosomes, an absence of recombination, and the variegated expression of P-element transgenes. As part of a larger project to understand these properties, we are assembling a physical map of this chromosome. Here we report the sequence of two cosmids representing ∼5% of the polytenized region. Both cosmid clones contain numerous repeated DNA sequences, as identified by cross hybridization with labeled genomic DNA, BLAST searches, and dot matrix analysis, which are positioned between and within the transcribed sequences. The repetitive sequences include three copies of the mobile element Hoppel, one copy of the mobile element HB, and 18 DINE repeats. DINE is a novel, short repeated sequence dispersed throughout both cosmid sequences. One cosmid includes the previously described cubitus interruptus(ci) gene and two new genes: that a gene with a predicted amino acid sequence similar to ribosomal protein S3a which is consistent with the Minute(4)101 locus thought to be in the region, and a novel member of the protein family that includes plexin and met–hepatocyte growth factor receptor. The other cosmid contains only the two short 5′-most exons from thezinc-finger-homolog-2 (zfh-2) gene. This is the first extensive sequence analysis of noncoding DNA from chromosome 4. The distribution of the various repeats suggests its organization is similar to the β-heterochromatic regions near the base of the major chromosome arms. Such a pattern may account for the diffuse banding of the polytene chromosome 4 and the variegation of many P-element transgenes on the chromosome.

Receptors for collapsin/semaphorins

Chemorepulsive signals that repel or paralyze neuronal growth cones have been found to play important roles in axon guidance in a stereotyped manner. Recent progress in the identification of neuropilins as the receptors for class III secreted collapsin/semaphorin subfamily members, which are neuronal repellents, and in the analysis of mutant mice lacking neuropilin function has confirmed the importance of these chemorepellents in axon guidance. In addition, characterization of the neuropilin protein has yielded new insights into the functions of this molecule in vascular formation and in axon guidance.

A poxvirus-encoded semaphorin induces cytokine production from monocytes and binds to a novel cellular semaphorin receptor, VESPR

The vaccinia virus A39R protein is a member of the semaphorin family. A39R.Fc protein was used to affinity purify an A39R receptor from a human B cell line. Tandem mass spectrometry of receptor peptides yielded partial amino acid sequences that allowed the identification of corresponding cDNA clones. Sequence analysis of this receptor indicated that it is a novel member of the plexin family and identified a semaphorin-like domain within this family, thus suggesting an evolutionary relationship between receptor and ligand. A39R up-regulated ICAM-1 on, and induced cytokine production from, human monocytes. These data, then, describe a receptor for an immunologically active semaphorin and suggest that it may serve as a prototype for other plexin-semaphorin binding pairs.