Semaphorins

Semaphorins 567

Semaphorins J Verhaagen, Netherlands Institute for Neuroscience, Amsterdam, The Netherlands R J Pasterkamp, University Medical Center Utrecht, Utrecht, The Netherlands ã 2009 Elsevier Ltd. All rights reserved.

In the nineteenth century, a ‘semaphore line’ was a signaling system composed of rods or flags that was widely used in the maritime world to navigate ships, in rail transportation, and to direct movements of army battalions. In the early 1990s, a family of proteins was discovered that functions as signals involved in growth cone guidance and navigation. The term ‘semaphore’ was adopted to refer to this family of proteins as semaphorins. The first semaphorin, fasciclin IV, was identified in the grasshopper and was later renamed semaphorin-1a. The first vertebrate semaphorin was originally designated collapsin-1, a name referring to its collapsing effect on growth cones of chick sensory ganglia. Collapsin-1 was later renamed semaphorin 3A. In 1999, the Semaphorin Nomenclature Committee published a unified nomenclature for the semaphorins/collapsins based on structural features and phylogenetic tree analysis. To date, more then 25 semaphorin genes have been identified that encode both membraneassociated and secreted molecules subdivided into eight classes. Most semaphorins contain approximately 750 amino acids and share a conserved ‘sema’ domain of approximately 500-amino-acid residues (Figure 1). This article reviews the current knowledge on how semaphorins function in neuronal network formation during development. In addition, it discusses progress in understanding the role of semaphorins during adulthood in synaptic transmission and plasticity, in neural trauma, and in certain neurological and psychiatric diseases. The article does not touch upon the role these proteins play in organogenesis, vascularization, the immune system, and cancer.

The Concept of Chemorepulsion Approximately 20 years ago, it was recognized that developing axons bend away from or can be inhibited by (at that time) hypothetical molecular cues they encounter on their path of growth. The first molecules shown to be able to mediate such effects were subsequently identified in genetic or antibody perturbation screens or by combining biochemical purification with well-defined bioassays as read-out systems for chemorepulsive activity (Figure 2). To date, five families of cellular guidance molecules that can

function as chemorepulsive proteins are known, including (in order of their discovery) netrins, ephrins, semaphorins, slits, and repulsive guidance molecule (RGM). Interestingly, some members of these protein families also influence axon guidance decisions by functioning as neurite attractants. A large body of work has shown that the ability of growth cones to respond to chemorepulsive proteins is critical for the formation of stereotypical neural pathways and maps. Chemorepulsion is therefore an essential element of the total arsenal of coordinated morphogenetic processes that shape the brain’s circuitry. In the mid-1980s, observations on the differential substrate properties of peripheral and central nervous system (CNS) tissue led to the hypothesis that central nervous system myelin contains inhibitory factors for regenerating nerve fibers. Myelin-associated proteins, including Nogo, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp), were subsequently identified as inhibitors of regenerative neurite outgrowth in the CNS. Thus, around 1985 two independent lines of thinking led to the notion that the inhibitory or repulsive regulation of neurite outgrowth during development and the inhibition of regenerative neurite outgrowth are regulated by distinct molecular entities. Recently, however, developmental chemorepulsive guidance cues, including semaphorins, ephrins, and RGMs, have also been linked to regenerative failure. Therefore, the concept of chemorepulsion appears to have implications for our understanding of the molecular mechanisms that underlie the failure of damaged nerve tracts to regenerate as well.

Semaphorins Signal through Multimeric Receptor Complexes To detect semaphorins in the extracellular space, growth cones at the leading tip of extending axons express specialized receptor complexes. A unifying feature of the neuronal semaphorin receptor complexes identified to date is the presence of a member of the plexin family within each complex (Figure 1). The founding member of this homogeneous family of transmembrane proteins, plexinA1 (plexin), was found in a screen for antigens involved in axon guidance in the developing tadpole retinotectal system. Since the identification of plexinA1, ten more plexins have been discovered (two plexins can be found in invertebrates and nine in vertebrates) and categorized into four subclasses on the basis of sequence similarities (Figures 1(a)–1(d)). Like semaphorins, extracellular sema domains are characteristic of plexins and it

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Figure 1 Semaphorins and their neuronal receptors. The semaphorin family is subdivided into eight classes. Classes 1 and 2 contain semaphorins found in invertebrate species, classes 3–7 vertebrate semaphorins, and class V viral semaphorins. Semaphorins exist as secreted (classes 2, 3, and V) and membrane-associated molecules (classes 1 and 4–7). All neuronal semaphorin receptors identified to date contain a member of the plexin family. Semaphorins can either bind directly to plexins (a; classes 1, 2, 4, V, and Sema3E) or require neuropilins as ligand binding subunits (b; class 3 semaphorins). b1 subunit-containing integrins also serve as receptors for class 7 semaphorins in a plexin-independent fashion (c). Modulatory receptor subunits such as L1 and Off-track have also been identified.

is believed that an intramolecular interaction between the plexin sema domain and the rest of the plexin extracellular domain inhibits receptor activity in the absence of semaphorin ligand. The plexin intracellular region shares homology with the GAP domain of p120 RasGAP and is highly conserved between plexins. Dominant-negative forms of plexins lacking their cytoplasmic region fail to mediate semaphorin-mediated repulsion which shows that plexins function as signaltransducing receptor subunits. Although most semaphorins exert their functions by directly interacting with plexins, class 3 semaphorins (Sema3s), except for Sema3E, fail to bind any of the known plexin family members. Instead, Sema3s require neuropilins as essential semaphorin-binding coreceptors to signal through class A plexins (plexinAs) (Figure 1). Neuropilins are transmembrane proteins with short intracellular domains that lack intrinsic enzymatic activity and that are indispensable for Sema3 function. The initial identification of neuropilin-1 (A5, neuropilin (Npn)) in the Xenopus laevis nervous system using hybridoma techniques was followed by the characterization of the only other neuropilin, neuropilin-2, in rodents. Cell culture studies

suggest that all four plexinAs (plexinA1–A4) can form receptor complexes with both neuropilins. The specific responses of different classes of neurons to individual Sema3s can be explained in part by restricted and unique ‘neuropilin’ distribution patterns and by the preferential binding of individual Sema3s to Npn-1 and/or Npn-2. PlexinAs also contribute to the specificity of axonal responses to Sema3s. A large body of work supports the idea that in vivo Npn-1/ plexinA4 complexes act as principal Sema3A receptors, whereas Npn-2/plexinA3 complexes mediate responses to Sema3F. The Npn/plexinA complexes that mediate the in vivo responses of other Sema3s remain to be identified. In addition to Sema3s, neuropilins serve as (co)receptors for structurally unrelated ligands such as VEGFs and PDGFs. Although less well-characterized, semaphorin receptor (components) unrelated to plexins and neuropilins do exist in neurons: (1) off-track, a protein similar to receptor tyrosine kinases but with a catalytically inactive kinase domain, associates with the Drosophila PlexA receptor to mediate semaphorin repulsion; (2) cell adhesion molecules of the immunoglobulin superfamily, including L1 and NrCAM, play a role in

Semaphorins 569 Growth cone steering

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Figure 2 In vitro assays for studying the effects of chemorepellents on growth cone behavior and axon steering. (a) In the growth cone collapse assay, dissociated neurons are grown and a putative chemorepulsive protein is added to the tissue culture medium to assess whether it can cause growth cone collapse. (b) Similarly, in the growth cone steering assay, dissociated neurons are grown but now the chemorepulsive protein is supplied in a small gradient by a micropipette. Chemorepulsive proteins will cause the growth cone to steer away from the gradient. (c) In the repulsion assay, a neuronal explant is cultured next to an aggregate of nonneuronal cells that secretes control or chemorepulsive proteins in a collagen gel. The chemorepulsive gradient that emerges from the aggregate will cause the axons to grow away from the aggregate. (d) Whereas the first three assays (a–c) are normally used to study secreted chemorepellents, the stripe assay is used to study membrane-associated proteins. Alternating stripes of protein or cell membranes are generated and explants are grown on top. If the axons can choose between lanes of control and chemorepulsive proteins, they will grow preferentially on control lanes.

the responsiveness of axons to Sema3A and are thought to be part of the neuropilin–plexinA receptor complex; and (3) b1 subunit-containing integrins act as Sema7A receptors, or components thereof (Figure 1). Whether nonneuronal semaphorin receptors such as CD72 or Tim2 identified in the immune and other organ systems also function in neuronal semaphorin signaling remains to be determined.

Intracellular Signaling On activation by semaphorins, plexins induce a series of intracellular signaling events that regulate the (dis) assembly of the neuronal cytoskeleton to control growth cone morphology and motility. The signaling mechanisms that are utilized by repulsive Sema3s have been studied in most detail and are summarized here. One of the first cytosolic proteins shown to link Sema3 receptor complexes to the neuronal cytoskeleton is collapsin response mediator protein (CRMP)-2, a member of a small family of membrane-associated phosphoproteins. Five different CRMPs have been identified, which in addition to plexinAs bind a plethora of other intracellular proteins, including kinases

and cytoskeleton-associated proteins. Interestingly, sequential phosphorylation of CRMPs by different kinases is essential for Sema3-induced growth cone collapse as well as for semaphorin-independent functions of CRMPs during axonogenesis. In addition to CRMPs, members of the Rho family of monomeric GTPases play a prominent role in repulsive Sema3 signaling. RhoGTPases are critical regulators of cytoskeletal dynamics. Both RhoGTPases and the proteins that regulate their activity (i.e., GEFs and GAPs) associate with the plexin cytoplasmic region. In addition, the plexin cytoplasmic domain contains a RasGAP domain which in the case of plexinAs and plexinBs acts to inactivate R-Ras. This is considered as one of several signaling events by which plexins slow down integrin signaling to reduce growth cone adhesion and allow for changes in growth cone morphology. Cytoplasmic kinases also play a prominent role in Sema3s repulsion. After the initial observation of tyrosine phosphorylation of plexins in culture, several cytoplasmic kinases have been implicated in Sema3-mediated growth cone collapse and steering (e.g., Fyn, Cdk5, and GSK-3). However, their functional roles have not been fully explored. This also

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holds true for cyclic nucleotide signaling. cGMP and cAMP have strong modulatory effects on growth cone turning responses induced by semaphorins. However, our understanding of how cyclic nucleotide levels are regulated under physiological conditions by semaphorins is still rudimentary. In addition to the four classes of signaling molecules discussed previously (CRMPs, RhoGTPases, kinases, and cyclic nucleotides), an ever increasing number of intracellular proteins and signaling mechanisms are implicated in semaphorin function. For example, recent findings support roles for redox reactions, novel protein synthesis, and microdomain signaling in axon responses induced by semaphorins. In addition, a high degree of cross-talk between semaphorin and unrelated signaling cascades (e.g., those mediating integrin or neurotrophic factor functions) exists that integrates different cellular processes during neuronal network formation. Furthermore, signaling cascades downstream of semaphorins other than Sema3s are being unveiled at a rapid pace, revealing both shared and unique signaling mechanisms between different classes of semaphorins. This, together with evidence that different classes of plexins may interact and the possibility for reverse signaling (i.e., plexin acting as a ligand and semaphorins as receptors), indicates that we are just beginning to understand the complexity of the semaphorin signaling network.

Semaphorin Function during Neurodevelopment Loss-of-function studies in mice and invertebrate species such as Drosophila and Caenorhabditis elegans have provided compelling insights into the role of semaphorins during development. Several general themes have emerged. First, deletion of semaphorin or semaphorin receptor genes often results in severe defasciculation of peripheral nerves. Second, the effects on CNS development are usually more subtle. For example, errors in CNS axon guidance as a result of semaphorin or semaphorin receptor deletion in mice are most pronounced in the olfactory system, hippocampus, anterior commissure, and thalamocortical projections. Third, many of the null mutant mouse lines display severe abnormalities in the development of nonneural tissues, particularly the cardiovascular system. This may be an important reason for the perinatal death of many of these mutants. Fourth, the phenotypes of a ligand mutant and its corresponding receptor mutant are usually very similar. For instance, the phenotype of Sema3A and Npn-1 knockouts and of Sema3F and Npn-2 are remarkably similar. This indicates that these two secreted semaphorins indeed signal through Npn-1 and Npn-2,

respectively, in vivo. Knockouts for plexinA3 and plexinA4 also display phenotypes that suggest that these plexins function as receptor components for Sema3F and Sema3A, respectively. Similarly, the neural phenotypes observed in fruit flies lacking Sema-1a resemble those of PlexA-deficient flies. Studies on genetically modified mice have also revealed additional novel functions for semaphorins in the developing nervous system, including their involvement in pruning of axons, synapse development and maturation, and the migration of neural crest and cerebellar granual cells.

Semaphorins in the Adult Intact Nervous System The expression of several secreted and transmembrane semaphorins persists in the adult nervous system. This intriguing observation indicates that these guidance molecules also have functions in the fully wired adult brain. Unlike their role in neural development, research on a function for semaphorins in the adult nervous system is only just beginning. So far, biochemical and cell culture studies have shown that several class 4 semaphorins are localized at synapses and associate with PDZ domains in postsynaptic density protein-95 (PSD-95), suggesting a role in as yet unknown aspects of synaptic functioning. Two Sema3s, Sema3F and Sema3A, have been studied in the context of synaptic transmission. Sema3F modulates synaptic transmission in acute slices of the adult rat hippocampus and Sema3A application to fully differentiated cultured hippocampal neurons decreases the efficacy of synaptic transmission. The expression of some secreted and membrane-associated semaphorins is sensitive to electrical activity, as has been shown in temporal lobe epilepsy and kainate acid-induced status epilepticus. Downregulation of Sema3s in the entorhinal–hippocampal complex of epileptic rats has been associated with pathological mossy fiber sprouting into the molecular layer of the hippocampus. Sema3A induces clustering of PSD-95 and NP-1 at synaptic sites in cultured neurons. In the cortex of adult Sema3A knockout mice, spine density appears to be reduced.

Semaphorins and Neurodegenerative and Psychiatric Disorders Traumatic Injury

Regenerating axons stop growing when they reach the border of the glial–fibrotic scar because they encounter a potent molecular barrier that inhibits growth cone motility. Semaphorins have been implicated as factors

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that contribute to the failure of axon regeneration. Theories on semaphorin function in traumatic nervous system injury are based on the cellular context in which semaphorins, their receptors, and intracellular signaling molecules are (re)expressed after damage. Four possible functions have been postulated. First, semaphorins may act in concert with the classical myelin-associated inhibitors of regeneration: multiple membrane-associated semaphorins are expressed by oligodendrocytes and Sema4D is upregulated in oligodendrocytes located in the vicinity of CNS lesions. Second, Sema3s are expressed by fibroblasts in the neural scar and may act as repulsive signals for injured axons approaching the scar. Evidence has shown that semaphorins can bind to chondroitin sulfate proteoglycans; these molecules are prominent components of the extracellular matrix of the neural scar and influence the activity of axonal guidance cues such as semaphorins. Third, neovascularization of the scar may be affected by semaphorins: blood vessels in the scar express Npn-1 and are surrounded by meningeal fibroblasts that secrete Sema3s. Sema3s may also directly influence the formation of the scar by affecting the migration of cells in the scar. Finally, in the peripheral nervous system semaphorins are regulated following injury in motor neurons, in epineural fibroblasts, and in a subpopulation of terminal Schwann cells at neuromuscular junctions where they may be involved in limiting plasticity of motor axons. The first causal evidence for an inhibitory role of Sema3s in axon regeneration comes from a study in which application of a Sema3A-blocking agent isolated from Penicillum resulted in enhanced regeneration. Additional evidence for a role of Sema3A in injuryassociated changes in neurite outgrowth comes from observations on viral vector-mediated expression of Sema3A in the spinal cord following a dorsal root lesion. Enhanced ectopic Sema3A expression counteracts the nerve growth factor-induced sprouting response of injured sensory neurons, thereby preventing neuropathic pain. Neurodegenerative Disorders: Alzheimer’s and Parkinson’s Disease

Alzheimer’s and Parkinson’s disease have in common the fact that specific populations of neurons and synaptic contacts atrophy and are then gradually lost. Alterations in neuronal connectivity in the mature nervous system appear to depend on a delicate balance between growth-promoting and growth-inhibiting influences. When this balance is disturbed, neurons may lose synaptic contacts and could eventually degenerate. It has been postulated that Sema3A may be involved in the pathogenesis of Alzheimer’s disease. The expression and distribution of Sema3A are altered

in the subiculum and hippocampus during the early stages of Alzheimer’s disease. As described previously, Sema3A can stimulate Cdk-5 and GSK-3beta, which can lead to (hyper)phosphorylation of CRMP-2 and microtubule-associated protein-1B (MAP1B). During the clinical stages of Alzheimer’s disease, a protein complex is formed in neurons that are affected by the disease consisting of Sema3A, plexinA1, plexinA2, and hyperphosphorylated CRMP-2 and MAP1B. It is not known whether Sema3A contributes directly or indirectly to the pathogenesis of Alzheimer’s disease, but it is important to note that Sema3A can act as an apoptotic factor and as an axon retraction inducer during development. Both processes are part of the neuropathological repertoire in Alzheimer’s disease. The possible involvement of Sema3A in dopamine-induced oxidative stress (one possible mechanism responsible for dopaminergic (DA) neuron death in Parkinson’s disease) is so far only based on in vitro evidence. Application of Sema3A to E14 murine ventral mesencephalic neurons induces apoptosis of DA neurons. Antibodies that block Sema3A function can protect neurons from DA-induced apoptosis. Neurodevelopmental Disorder: Schizophrenia

Two observations have linked possible changes in semaphorin signaling to schizophrenia. First, the expression of Sema3A is increased in the cerebellum of schizophrenic patients and elevated expression of Sema3A correlates with a decline in expression of synaptophysin and reelin, proteins involved in synapse formation and synaptic transmission. Second, variants of plexinA2 have been associated with schizophrenia in a genomewide association study. These findings are in line with an early developmental origin of this psychiatric disorder and may point to a disturbed balance between trophic and chemorepulsive influences in this disease. Neuromuscular Disease: Amyotrophic Lateral Sclerosis

In amyotrophic lateral sclerosis (ALS), selective synaptic weakening and denervation of muscle fibers starts long before the actual motor neuron degeneration occurs and takes place prior to the clinical manifestation of the disease. Furthermore, fast-fatiguable muscle fibers are selectively denervated during the earliest phases of the disease. The chemorepellent Sema3A is selectively upregulated in terminal Schwann cell that occupy the neuromuscular junctions of fast-fatiguable muscle fibers in a mouse model of ALS. This observation has led to the hypothesis that Sema3A may act as a motor axon retraction inducer at the neuromuscular synapse in ALS, thereby contributing to the neuropathology of ALS. Alternatively,

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elevated levels of Sema3A at the neuromuscular junction may affect retrograde signaling in motor neurons, a process that is disturbed in ALS.

Future Challenges The current literature underscores that a more rigorous dissection of the function(s) of semaphorins in the adult intact and dysfunctional nervous system is essential. The hypotheses that predict a role for these chemorepulsive proteins in the normal physiology and plasticity of the adult brain and in neuropathological processes are predominantly based on descriptive findings. Many of the null mutant mice that have been so useful to study the role of semaphorins during development die soon after birth. Mouse mutants that do survive into adulthood may not be good models to study semaphorin function during adulthood because compensatory changes may mask functionally relevant phenotypes. Therefore, mice with conditional and cell type-specific semaphorin or semaphorin receptor gene ablation or transgenic animals with targeted and conditional overexpression will have to be generated to study the role of semaphorins in adult neurons. Furthermore, other experimental strategies (genetic modification based on direct viral vector-mediated expression in adult rodent brain and pharmacological intervention) will be instrumental to advance our knowledge of the role of semaphorins in the normal intact adult nervous system and under neuropathological conditions. During the past century, our view of the nervous system as a fixed and immutable structure has changed radically. Most neuroscientists will agree that the nervous system is a highly plastic organ in which the structure and function of connections and synapses are continually modified. Like the formation of neuronal connections during development, the ongoing plasticity in the mature nervous system is regulated by a myriad of molecular interactions. It will be fascinating to witness how future neurobiological research will define the precise function(s) of semaphorins in plasticity of the mature nervous system. Moreover, whether or not the chemorepulsive activity of these molecules contributes to the limited capacity of the nervous system to repair itself after damage is another pivotal issue that will be resolved in the future. See also: Alzheimer’s Disease: Neurodegeneration; Amyotrophic Lateral Sclerosis (ALS): Disease Mechanisms; Axon Guidance: Building Pathways with Molecular Cues in Vertebrate Sensory Systems; Axon Guidance: Guidance Cues and Guidepost Cells; Axonal Pathfinding: Netrins; Axonal Pathfinding: Guidance

Activities of Sonic Hedgehog (Shh); Axonal Pathfinding: Extracellular Matrix Role; Axon Guidance: Morphogens as Chemoattractants and Chemorepellants; Growth Cones; Neurodegeneration in Psychiatric Illness; Parkinson’s Disease: Alpha-Synuclein and Neurodegeneration.

Further Reading Adams RH, Lohrum M, Klostermann A, Betz H, and Puschel AW (2006) The chemorepulsive activity of secreted semaphorins is regulated by furin-dependent proteolytic processing. EMBO Journal 20: 6077–6086. Bagri A, Cheng HJ, Yaron A, Pleasure SJ, and Tessier-Lavigna M (2003) Stereotyped pruning of long hippocampal axon branches triggered by retraction inducers of the semaphoring family. Cell 113: 285–299. Barzilai A, Zilkha-Falb R, Daily D, et al. (2000) The molecular mechanism of dopamine-induced apoptosis: Identification and characterization of genes that mediate dopamine toxicity. Journal of Neural Transmission Supplement 60: 59–76. Castellani V, Che´dotal A, Schachner M, Faivre-Sarrailh C, and Rougon G (2000) Analysis of the L1-deficient mouse phenotype reveals cross-talk between sema3A and L1 signaling pathways in axonal guidance. Neuron 27: 237–249. DeWinter F, Vo T, Stam FJ, et al. (2006) The expression of the chemorepellent Semaphorin 3A is selectively induced in terminal Schwann cells of a subset of neuromuscular synapses that display limited anatomical plasticity and enhanced vulnerability in motor neuron disease. Molecular and Cellular Neuroscience 32: 102–117. Eastwood SL, Law AJ, Everall IP, and Harrison PJ (2003) The axonal chemorepellant semaphorin 3A is increased in the cerebellum in schizophrenia and may contribute to its synaptic pathology. Molecular Psychiatry 8: 1248–1255. Giger RJ, Pasterkamp RJ, Heijnen S, Holtmaat AJGD, and Verhaagen J (1998) Anatomical distribution of the chemorepellent semaphorin III/collapsin-1 in the adult rat and human brain: Predominant expression in structures of the olfactory– hippocampal pathway and the motor system. Journal of Neuroscience Research 52: 27–42. Good PF, Alapat D, Hsu A, et al. (2004) A role for semaphorin 3A signaling in the degeneration of hippocampal neurons during Alzheimer’s disease. Journal of Neurochemistry 91: 716–736. Goshima Y, Kawakami T, Hori H, et al. (1997) A novel action of collapsin: Collapsin-1 increases antero- and retrograde axoplasmic transport independently of growth cone collapse. Journal of Neurobiology 33: 316–328. Kantor DB, Chivatakarn O, Peer KL, et al. (2004) Semaphorin 5A is a bifunctional axon guidance cue regulated by heparin and chondroitin sulfate proteoglycans. Neuron 44: 961–975. Kapfhammer JP, Grunewald BE, and Raper JA (1986) The selective inhibition of growth cone extension by specific neurites in culture. Journal of Neuroscience 6: 2527–2534. Kerjan G, Dolan J, Haumaitre C, et al. (2005) The transmembrane semaphorin Sema6A controls cerebellar granule cell migration. Nature Neuroscience 8: 1516–1524 (doi:10.1038/nn1555). Kikuchi K, Kishino A, Konishi O, et al. (2003) In vitro and in vivo characterization of a novel semaphorin 3A inhibitor, SM-216289 or xanthofulvin. Journal of Biological Chemistry 278: 42985– 42991. Kolodkin AL, Levengood DV, Rowe EG, Tai YT, Giger RJ, and Ginty DD (1997) Neuropilin is a semaphorin III receptor. Cell 90: 753–762.

Semaphorins 573 Kolodkin AL, Matthes DJ, and Goodman CS (1993) The semaphorin genes encode a family of transmembrane and secreted growth cone guidance molecules. Cell 75: 1389–1399. Luo Y, Raible D, and Raper JA (1993) Collapsin: A protein in brain that induces the collapse and paralysis of neuronal growth cones. Cell 75: 217–227. Pasterkamp RJ, Giger RJ, Ruitenberg MJ, et al. (1999) Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissues formed following injuries of adult but not neonatal CNS. Molecular and Cellular Neuroscience 13: 143–166. Pasterkamp RJ, Peschon JJ, Spriggs MK, and Kolodkin AL (2003) Semaphorin 7A promotes axon outgrowth through integrins and MAPks. Nature 424: 398–405. Sahay A, Kim C-H, Sepkuty JP, et al. (2005) Secreted semaphorins modulate synaptic transmission in the adult hippocampus. Journal of Neuroscience 25: 3613–3620. Schwab ME and Thoenen H (1985) Dissociated neurons regenerate into sciatic but not optic nerve explants in culture

irrespective of neurotrophic factors. Journal of Neuroscience 5: 2415–2423. Takahashi T, Fournier A, Nakamura F, et al. (1999) Plexin– neuropilin-1 complexes form functional semaphorin-3A receptors. Cell 99: 59–69. Tamagnone L, Artigiani S, Chen H, et al. (1999) Plexins are a large family of receptors for transmembrane, secreted, and GPIanchored semaphorins in vertebrates. Cell 99(1): 71–80. Taniguchi M, Yuasa S, Fujisawa H, et al. (1997) Disruption of semaphorin III/D gene causes severe abnormality in peripheral nerve projection. Neuron 19: 519–530. Unified, Nomenclature for the Semaphorins/Collapsins(1999) Letter to the editor. Cell 97: 551–552. Yang J, Houk B, Shah J, et al. (2005) Genetic background regulates semaphorin gene expression and epileptogenesis in mouse brain after kainic acid status epilepticus. Neuroscience 131: 853–869. Yaron A, Huang P-H, Cheng H-J, and Tessier-Lavigne M (2005) Differential requirement for plexin-A3 and-A4 in mediating responses of sensory and sympathetic neurons to distinct class 3 semaphorins. Neuron 45: 513–523.