Semaphorin D acts as a repulsive factor for entorhinal and hippocampal neurons

Functions of Plexins/Neuropilins and Their Ligands during Hippocampal Development and Neurodegeneration

There is emerging evidence that molecules, receptors, and signaling mechanisms involved in vascular development also play crucial roles during the development of the nervous system. Among others, specific semaphorins and their receptors (neuropilins and plexins) have, in recent years, attracted the attention of researchers due to their pleiotropy of functions. Their functions, mainly associated with control of the cellular cytoskeleton, include control of cell migration, cell morphology, and synapse remodeling. Here, we will focus on their roles in the hippocampal formation that plays a crucial role in memory and learning as it is a prime target during neurodegeneration.

The chemorepellent draxin is involved in hippocampal mossy fiber projection

Lamina-specific afferent innervation of the mammalian hippocampus is critical for its function. We investigated the relevance of the chemorepellent draxin to the laminar projections of three principal hippocampal afferents: mossy fibers, entorhinal, and associational/commissural fibers. We observed that draxin deficiency led to abnormal projection of mossy fibers but not other afferents. Immunohistochemical analysis indicated that draxin is expressed in the dentate gyrus and cornu ammonis (CA) 3 at postnatal day 0, when dentate granule cells begin to extend mossy fibers towards CA3. Furthermore, a neurite growth assay using dissociated cells of the neonatal dentate gyrus revealed that draxin inhibited the growth of calbindin-D28k-expressing mossy fibers in vitro. Taken together, we conclude that draxin is a key molecule in the regulation of mossy fiber projections.

New functions of Semaphorin 3E and its receptor PlexinD1 during developing and adult hippocampal formation

The development and maturation of cortical circuits relies on the coordinated actions of long and short range axonal guidance cues. In this regard, the class 3 semaphorins and their receptors have been seen to be involved in the development and maturation of the hippocampal connections. However, although the role of most of their family members have been described, very few data about the participation of Semaphorin 3E (Sema3E) and its receptor PlexinD1 during the development and maturation of the entorhino-hippocampal (EH) connection are available. In the present study, we focused on determining their roles both during development and in adulthood. We determined a relevant role for Sema3E/PlexinD1 in the layer-specific development of the EH connection. Indeed, mice lacking Sema3E/PlexinD1 signalling showed aberrant layering of entorhinal axons in the hippocampus during embryonic and perinatal stages. In addition, absence of Sema3E/PlexinD1 signalling results in further changes in postnatal and adult hippocampal formation, such as numerous misrouted ectopic mossy fibers. More relevantly, we describe how subgranular cells express PlexinD1 and how the absence of Sema3E induces a dysregulation of the proliferation of dentate gyrus progenitors leading to the presence of ectopic cells in the molecular layer. Lastly, Sema3E mutant mice displayed increased network excitability both in the dentate gyrus and the hippocampus proper.

Protein Tyrosine Phosphatase δ Mediates the Sema3A-Induced Cortical Basal Dendritic Arborization through the Activation of Fyn Tyrosine Kinase

Leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) are critical for axonal guidance; however, their relation to specific guidance cues is poorly defined. We here show that PTP-3, a LAR homolog in Caenorhabditis elegans, is involved in axon guidance regulated by Semaphorin-2A-signaling. PTPδ, one of the vertebrate LAR class PTPs, participates in the Semaphorin-3A (Sema3A)-induced growth cone collapse response of primary cultured dorsal root ganglion neurons from Mus musculus embryos. In vivo, however, the contribution of PTPδ in Sema3A-regualted axon guidance was minimal. Instead, PTPδ played a major role in Sema3A-dependent cortical dendritic growth. Ptpδ^-/- and Sema3a^-/- mutant mice exhibited poor arborization of basal dendrites of cortical layer V neurons. This phenotype was observed in both male and female mutants. The double-heterozygous mutants, Ptpδ^+/-; Sema3a^+/-, also showed a similar phenotype, indicating the genetic interaction. In Ptpδ^-/- brains, Fyn and Src kinases were hyperphosphorylated at their C-terminal Tyr527 residues. Sema3A-stimulation induced dephosphorylation of Tyr527 in the dendrites of wild-type cortical neurons but not of Ptpδ^-/- Arborization of cortical basal dendrites was reduced in Fyn^-/- as well as in Ptpδ^+/-; Fyn^+/- double-heterozygous mutants. Collectively, PTPδ mediates Sema3A-signaling through the activation of Fyn by C-terminal dephosphorylation.SIGNIFICANCE STATEMENT The relation of leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) and specific axon guidance cues is poorly defined. We show that PTP-3, a LAR homolog in Caenorhabditis elegans, participates in Sema2A-regulated axon guidance. PTPδ, a member of vertebrate LAR class PTPs, is involved in Sema3A-regulated cortical dendritic growth. In Sema3A signaling, PTPδ activates Fyn and Src kinases by dephosphorylating their C-terminal Tyr residues. This is the first evidence showing that LAR class PTPs participate in Semaphorin signaling in vivo.

Semaphorin3A elevates vascular permeability and contributes to cerebral ischemia-induced brain damage

Semaphorin 3A (Sema3A) increased significantly in mouse brain following cerebral ischemia. However, the role of Sema3A in stroke brain remains unknown. Our aim was to determine wether Sema3A functions as a vascular permeability factor and contributes to ischemic brain damage. Recombinant Sema3A injected intradermally to mouse skin, or stereotactically into the cerebral cortex, caused dose- and time-dependent increases in vascular permeability, with a degree comparable to that caused by injection of a known vascular permeability factor vascular endothelial growth factor receptors (VEGF). Application of Sema3A to cultured endothelial cells caused disorganization of F-actin stress fibre bundles and increased endothelial monolayer permeability, confirming Sema3A as a permeability factor. Sema3A-mediated F-actin changes in endothelial cells were through binding to the neuropilin2/VEGFR1 receptor complex, which in turn directly activates Mical2, a F-actin modulator. Down-regulation of Mical2, using specific siRNA, alleviated Sema3A-induced F-actin disorganization, cellular morphology changes and endothelial permeability. Importantly, ablation of Sema3A expression, cerebrovascular permeability and brain damage were significantly reduced in response to transient middle cerebral artery occlusion (tMCAO) and in a mouse model of cerebral ischemia/haemorrhagic transformation. Together, these studies demonstrated that Sema3A is a key mediator of cerebrovascular permeability and contributes to brain damage caused by cerebral ischemia.

Roles of semaphorin-6B and plexin-A2 in lamina-restricted projection of hippocampal mossy fibers

Hippocampal mossy fibers project preferentially to the proximal-most lamina of the suprapyramidal region of CA3, the stratum lucidum, and proximal-most parts of the infrapyrmidal region of CA3c. Molecular mechanisms that govern the lamina-restricted projection of mossy fibers, however, have not been fully understood. We previously studied functions of neural repellent Semaphorin-6A (Sema6A), a class 6 transmembrane semaphorin, and its receptors, plexin-A2 (PlxnA2) and PlxnA4, in mossy fiber projection and have proposed that PlxnA4-expressing mossy fibers are principally prevented from entering the Sema6A-expressing suprapyramidal and infrapyramidal regions of CA3 but are permitted to grow into proximal parts of the regions, where repulsive activity of Sema6A is competitively suppressed by PlxnA2 (Suto et al., 2007). In the present study we demonstrate that Sema6B, another class 6 transmembrane semaphorin, is expressed in CA3 and repels mossy fibers in a PlxnA4-dependent manner in vitro. In Sema6B-deficient mice several mossy fibers aberrantly project to the stratum radiatum and the stratum oriens. The number of aberrant mossy fibers is increased in Sema6A;Sema6B double knock-out mice, indicating that Sema6A and Sema6B function additively to regulate proper projection of mossy fibers. PlxnA2 does not suppress the Sema6B response, but itself promotes growth of mossy fibers. Based on these results, we propose that the balance between mossy fiber repulsion by Sema6A and Sema6B and attraction by PlxnA2 and unknown molecule(s) prescribes the areas permissive for mossy fibers to innervate.

Plasticity-related gene 5 (PRG5) induces filopodia and neurite growth and impedes lysophosphatidic acid- and nogo-A-mediated axonal retraction

The authors have cloned a novel member of the PRG family that induces filopodia growth in a Cdc42-independent manner. Hence, studies in primary neurons revealed that PRG5 impedes RhoA-mediated axon collapse induced by LPA and Nogo-A. These data reveal a new function of PRG5 with impact on neurite growth in an axonal growth inhibitory environment.

Members of the plasticity-related gene (PRG1-4) family are brain-specific integral membrane proteins and implicated in neuronal plasticity, such as filopodia formation and axon growth after brain lesion. Here we report on the cloning of a novel member of the PRG family, PRG5, with high homologies to PRG3. PRG5 is regulated during brain and spinal cord development and is exclusively allocated within the nervous system. When introduced in neurons, PRG5 is distributed in the plasma membrane and induces filopodia as well as axon elongation and growth. Conversely, siRNA mediated knockdown of PRG5 impedes axon growth and disturbs filopodia formation. Here we show that PRG5 induces filopodia growth independently of Cdc42. Moreover, axon collapse and RhoA activation induced by LPA and myelin-associated neurite inhibitor Nogo-A is attenuated in the presence of PRG5, although direct activation of the RhoA-Rho-PIP5K kinase pathway abolishes PRG5 -formed neurites. Thus, we describe here the identification of a novel member of the PRG family that induces filopodia and axon elongation in a Cdc42-independent manner. In addition, PRG5 impedes brain injury-associated growth inhibitory signals upstream of the RhoA-Rho kinase pathway.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Axonal wiring of guanylate cyclase-D-expressing olfactory neurons is dependent on neuropilin 2 and semaphorin 3F

The olfactory system of the mouse includes several subsystems that project axons from the olfactory epithelium to the olfactory bulb. Among these is a subset of neurons that do not express the canonical pathway of olfactory signal transduction, but express guanylate cyclase-D (GC-D). These GC-D-positive (GC-D+) neurons are not known to express odorant receptors. Axons of GC-D+ neurons project to the necklace glomeruli, which reside between the main and accessory olfactory bulbs. To label the subset of necklace glomeruli that receive axonal input from GC-D+ neurons, we generated two strains of mice with targeted mutations in the GC-D gene (Gucy2d). These mice co-express GC-D with an axonal marker, tau-beta-galactosidase or tauGFP, by virtue of a bicistronic strategy that leaves the coding region of the Gucy2d gene intact. With these strains, the patterns of axonal projections of GC-D+ neurons to necklace glomeruli can be visualized in whole mounts. We show that deficiency of one of the neuropilin 2 ligands of the class III semaphorin family, Sema3f, but not Sema3b, phenocopies the loss of neuropilin 2 (Nrp2) for axonal wiring of GC-D+ neurons. Some glomeruli homogeneously innervated by axons of GC-D+ neurons form ectopically within the glomerular layer, across wide areas of the main olfactory bulb. Similarly, axonal wiring of some vomeronasal sensory neurons is perturbed by a deficiency of Nrp2 or Sema3f, but not Sema3b or Sema3c. Our findings provide genetic evidence for a Nrp2-Sema3f interaction as a determinant of the wiring of axons of GC-D+ neurons into the unusual configuration of necklace glomeruli.

Interactions between Plexin-A2, Plexin-A4, and Semaphorin 6A Control Lamina-Restricted Projection of Hippocampal Mossy Fibers

Hippocampal mossy fibers project preferentially to the stratum lucidum, the proximal-most lamina of the suprapyramidal region of CA3. The molecular mechanisms that govern this lamina-restricted projection are still unknown. We examined the projection pattern of mossy fibers in mutant mice for semaphorin receptors plexin-A2 and plexin-A4, and their ligand, the transmembrane semaphorin Sema6A. We found that plexin-A2 deficiency causes a shift of mossy fibers from the suprapyramidal region to the infra- and intrapyramidal regions, while plexin-A4 deficiency induces inappropriate spreading of mossy fibers within CA3. We also report that the plexin-A2 loss-of-function phenotype is genetically suppressed by Sema6A loss of function. Based on these results, we propose a model for the lamina-restricted projection of mossy fibers: the expression of plexin-A4 on mossy fibers prevents them from entering the Sema6A-expressing suprapyramidal region of CA3 and restricts them to the proximal-most part, where Sema6A repulsive activity is attenuated by plexin-A2.

Microarray analysis of the fetal hippocampus in the Emx2 mutant

Deficiency in the transcription factor Emx2 causes a specific alteration of hippocampal development, which has been well analyzed morphologically. We are currently using microarrays and in situ hybridization to characterize gene expression in the Emx2 mutant hippocampus. In this report on our preliminary results for the fetal stage, we identify a group of genes for most of which a putative relation to Emx2 pathways has not been previously recognized. Some candidates are development genes or are involved in functional maturation, and show expression in the hippocampal plate and/or developing dentate gyrus. A second class of candidates label neuronal, glial or vascular structures in the outer marginal zone, and likely represent markers for cell populations specifically absent in the mutant. Our results point at pathways and processes altered in the mutant, particularly the Notch and chemokine pathways, the processes of cell migration, axonal guidance and angiogenesis, and the relation of pia and Cajal-Retzius cells with hippocampal morphogenesis.

Semaphorins and their receptors in lamprey CNS: Cloning, phylogenetic analysis, and developmental changes during metamorphosis

The large, conserved semaphorin gene family encodes axon guidance molecules in both invertebrates and vertebrates. The primitive vertebrate lamprey diverged near the time of vertebrate origins and is useful for understanding the gene duplication events that led to the increased complexity of the vertebrate genome. We characterized the sequence and expression pattern of semaphorins and their receptors genes in the sea lamprey, Petromyzon marinus. We uncovered two members of the semaphorin family in sea lamprey. The first encodes a diffusible class 3 type semaphorin protein that is most similar to the human and mouse Sema3F (71% amino acid identity). The second encodes a transmembrane class 4 type semaphorin that is most similar to mouse Sema4D and human Sema4G, with 38% amino acid identity within the Sema domain. We also identified in lamprey two members of the semaphorin receptor family, lamprey Plexin A1 and Plexin A2. Phylogenetic analysis indicates that lamprey Sema3 and Sema4 represent precursor genes existing prior to the origin of the vertebrate Sema3A-G and Sema4A-G subfamilies. Therefore, the gene duplication event that gave rise to those subfamilies must have occurred after the divergence of jawed vertebrates from jawless fish. These semaphorins and plexins are expressed in unique and dynamic patterns in lamprey spinal cord and brain during development.

Ephrin-A5 modulates the topographic mapping and connectivity of commissural axons in murine hippocampus

Entorhinal and commissural/associational projections show a non-overlapping distribution in the hippocampus proper and the dentate gyrus. The expression of Ephrins and their Eph receptors in the developing hippocampus indicates that this family of axonal guidance molecules may modulate the formation of these connections. Here we focused on the role of the ephrin-A5 ligand in the development of the main hippocampal afferents. In situ hybridization showed that ephrin-A5 mRNA was detected mainly in the principal cells of the hippocampus proper and in the dentate gyrus throughout postnatal development. Immunocytochemical analyses revealed prominent expression of the EphA3 receptor, a putative receptor for ephrin-A5, in the main cells and the neuropil of the developing hippocampus. Tracing experiments in ephrin-A5(-/-) mice showed that commissural projections were transiently altered in the hippocampus proper at P5, but they were mistargeted throughout the postnatal development in the dentate gyrus. Immunocytochemistry with anti-calbindin antibodies revealed that the dentate mossy fiber projection was not altered in ephrin-A5(-/-) mice. Electron microscopy studies showed alterations in the density of synapses and spines in commissural/associational layers, but not in entorhinal layers, and in the mossy fibers in these animals. Taken together, these findings indicate that ephrin-A5 signaling is involved in the formation and maturation of synapses in the hippocampus.

Role of class 3 semaphorins in the development and maturation of the septohippocampal pathway

In examining the role of Class 3 secreted semaphorins in the prenatal and postnatal development of the septohippocampal pathway, we found that embryonic (E14-E16) septal axons were repelled by the cingulate cortex and the striatum. We also found that the hippocampus exerts chemorepulsion on dorsolateral septal fibers, but not on fibers arising in the medial septum/diagonal band complex, which is the source of septohippocampal axons. These data indicate that endogenous chemorepellents prevent the growth of septal axons in nonappropriate brain areas and direct septohippocampal fibers to the target hippocampus. The embryonic septum expressed np-1 and np-2 mRNAs, and the striatum and cerebral cortex expressed sema 3A and sema 3F. Experiments with recombinant semaphorins showed that Sema 3A and 3F, but not Sema 3C or 3E, induce chemorepulsion of septal axons. Sema 3A and 3F also induce growth cone collapse of septal axons. This indicates that these factors are endogenous cues for the early guidance of septohippocampal fibers, including cholinergic and gamma-aminobutyric acid (GABA)ergic axons, during the embryonic stages. During postnatal stages, when target cell selection and synaptogenesis take place, np-1 and np-2 were expressed by septohippocampal neurons at all ages tested. In the target hippocampus, pyramidal and granule cells expressed sema 3E and sema 3A, whereas most interneurons expressed sema 3C, but few expressed sema 3E or 3A. Combined tracing and expression studies showed that GABAergic septohippocampal fibers terminated preferentially onto sema 3C-positive interneurons. In contrast, cholinergic septohippocampal fibers terminated onto sema 3E and sema 3A-expressing pyramidal and granule cells. The data suggest that Class 3 secreted semaphorins are involved in postnatal development. Moreover, because GABAergic and cholinergic axons terminate onto neurons expressing distinct, but overlapping, patterns of semaphorin expression, semaphorin functions may be regulated by different signaling mechanisms at postnatal stages.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

Mécanismes cellulaires et moléculaires de la croissance axonale

INTRODUCTION

During embryonic and post-natal development, numerous axonal connections are formed establishing a functional nervous system. Knowledge of the underlying molecular and cellular mechanisms controlling this phenomenon is improving.

STATE OF THE ART

In this review, we present the general principles of axon guidance together with the major families of guidance signals. This includes the tyrosine kinase receptors Eph and their ligands Ephrins, the netrins, the semaphorins, the slits and other major components of the extracellular matrix. These types of guidance signals share common functional properties leading to actin cytoskeleton remodelling. The direct or indirect interactions between the receptors of these guidance cues and actin modulators is the final step of the signalling cascade constituting the fundamental mechanism defining the orientation and extension of the axonal growth cone. These factors are involved in the formation of many, if not all, axonal projections for which they act as repulsive (inhibitory) or attractive (promoting) signals.

PERSPECTIVES

the knowledge of these mechanisms is particularly interesting since the inhibition of axonal outgrowth is considered to be one of the major obstacles to nerve regeneration in the central nervous system. Indeed, most of the guidance signals expressed during brain development are up-regulated in lesion sites where they contribute to the lack of nerve re-growth. Here, we present the nature of the mechanical barrier, the so called glial scar, and we describe the major inhibitory molecules preventing axonal extension.

CONCLUSION

the comprehension of the molecular mechanisms involved in axon growth and guidance represents a major advance towards the definition of novel therapeutic strategies improving nerve regeneration. The path to the clinical application of these molecular factors remains long. Nevertheless, the next decade will undoubtedly provide challenging data that will modify the current therapeutic approaches.

Cocaine-induced expression changes of axon guidance molecules in the adult rat brain

Administration of drugs of abuse induces strong molecular adaptations and plasticity within the mesolimbic dopamine (DA) system, a pathway essential for reward-seeking behavior. Little is known about the specific targets involved in this neuroadaptation process, but there are indications that cocaine and other drugs of abuse share the ability to alter the morphology of neuronal dendrites and spines, the primary site of excitatory synapses in the brain. Axon guidance molecules, the very molecular cues that regulate the formation of axon-target connections during development, may mediate these alterations. To test this hypothesis, we investigated mRNA expression changes of 39 axon guidance molecules, including 17 Semaphorins, 12 Ephs, 8 Ephrins, and 2 neuropilins in the mesolimbic dopamine system of cocaine-treated animals under different paradigms by mean of DNA-Microarray and quantitative real-time PCR. In all cases, strong changes in gene expression are observed, yielding to up or downregulation of these axon guidance molecules. Our data suggest that cocaine treatment induces activation of a complex program of synaptic rearrangements, which may partly recapitulate the plastic changes occurring during development, and may underlie the important neuroplastic adaptations that occur in the reward- and memory-related brain centers following drug action. We conclude that in some brain regions, exposure to psychomotor-stimulant drugs produce expression changes in axon guidance molecules, which may contribute to cognitive deficits associated with drug abuse.

A complementary peptide approach applied to the design of novel semaphorin/neuropilin antagonists

Semaphorin 3A can inhibit axonal growth and induce neuronal apoptosis following binding to neuropilin-1, with the membrane proximal MAM (meprin, A5, mu) domain in neuropilin-1 playing a key role in the formation of a higher order receptor complex. If functional motifs on semaphorin 3A and/or the MAM domain can be identified, then small-constrained peptides might be developed as antagonists. We have scored peptide pairs for complementary hydropathy and antisense homology to identify a candidate functional motif in the Ig domain of semaphorin 3A, and in the MAM domain of neuropilin-1. Synthetic peptides corresponding to these sequences fully inhibit growth cone collapse induced by semaphorin 3A. A number of smaller peptides derived from the parental sequence also inhibited the response, particularly after they were constrained by a disulfide bond. Finally, we have used an algorithm to design a peptide that is a near-perfect hydropathic complement of the candidate functional site in the MAM domain; this also inhibits the semaphorin 3A response. Thus, an algorithm-driven methodology has led to the identification of three independent semaphorin 3A antagonists. Semaphorin 3F stimulates growth cone collapse following binding to the closest relative to neuropilin-1 in the genome, neuropilin-2. Where tested, the peptides that antagonise semaphorin 3A failed to inhibit the semaphorin 3F response.

Temporal specific patterns of semaphorin gene expression in rat brain after kainic acid-induced status epilepticus

Mossy fiber sprouting and other forms of synaptic reorganization may form the basis for a recurrent excitatory network in epileptic foci. Four major classes of axon guidance molecules--the ephrins, netrins, slits, and semaphorins--provide targeting information to outgrowing axons along predetermined pathways during development. These molecules may also play a role in synaptic reorganization in the adult brain and thereby promote epileptogenesis. We studied semaphorin gene expression, as assessed by in situ hybridization, using riboprobes generated from rat cDNA in an adult model of synaptic reorganization, kainic acid (KA)-induced status epilepticus (SE). Within the first week after KA-induced SE, semaphorin 3C, a class III semaphorin, mRNA content is decreased in the CA1 area of the hippocampus and is increased in the upper layers of cerebral cortex. Another class III semaphorin, semaphorin 3F, is also decreased in CA1 and CA3 of hippocampus within the first week after KA-SE. These changes in gene expression are principally confined to neurons. By contrast, there was little change in the semaphorin 4C mRNA content of CA1 neurons at this time. No changes in expression of semaphorin 3A and 4C genes were detected 28 days after KA-induced SE. Regulation of semaphorin gene expression after KA-induced SE suggests that neurons may regulate the expression of axonal guidance molecules and thereby contribute to synaptic reorganization after injury of the mature brain. The anatomic locale of the altered semaphorin gene expression may serve as a marker for specific networks undergoing synaptic reorganization in the epileptic brain.

Neuropilin and class 3 semaphorins in nervous system regeneration

Injury to the mature mammalian central nervous system (CNS) is often accompanied by permanent loss of function of the damaged neural circuits. The failure of injured CNS axons to regenerate is thought to be caused, in part, by neurite outgrowth inhibitory factors expressed in and around the lesion. These include several myelin associated inhibitors, proteoglycans, and tenascin-R. Recent studies have documented the presence of class 3 semaphorins in fibroblast-like meningeal cells present in the core of the neural scar formed following CNS injury. Class 3 semaphorins display neurite growth-inhibitory effects on growing axons during embryonic development. The induction of the expression of class 3 semaphorins in the neural scar and the persistent expression of their receptors, the neuropilins and plexins, by injured CNS neurons suggest that they contribute to the regenerative failure of CNS neurons. Neuropilins are also expressed in the neural scar in a subpopulation of meningeal fibroblast and in neurons in the vicinity of the scar. Semaphorin/neuropilin signaling might therefore also be important for cell migration, angiogenis and neuronal cell death in or around neural scars. In contrast to neurons in the CNS, neuropilin/plexin positive neurons in the PNS do display long distance regeneration following injury. Injured PNS neurons do not encounter a semaphorin positive neural scar. Furthermore, Semaphorin 3A is downregulated in the regenerating spinal motor neurons themselves. This was accompanied by a transient upregulation of Semaphorin 3A in the target muscle. These observations suggest that the injury induced regulation of Semaphorin 3A in the PNS contributes to successful regeneration and target reinnervation. Future studies in genetically modified mice should provide more insight into the mechanisms by which neuropilins and semaphorins influence nervous system regeneration and degeneration.

Basal forebrain cholinergic cell attachment and neurite outgrowth on organotypic slice cultures of hippocampal formation

Distributions of somata and neurites of cholinergic neurons were studied after seeding dissociated cells onto organotypic slice cultures. Slice cultures were made from hippocampal formation and adjacent cortical regions from rats or mice. Dissociated cell suspensions of basal forebrain tissue from rat or mouse fetuses were seeded onto the slice cultures. Combined cultures were maintained for 1-21 days in vitro. Cultures processed for acetylcholinesterase (AChE) histochemistry demonstrated non-random patterns of cholinergic cells and their neurites. Labeled cells appeared most frequently in the molecular layer of the dentate gyrus, and in the deeper layers of cortical regions adjacent to the hippocampus. Neurites extending from these labeled cells appeared to target the dentate molecular layer and the cortical subplate layer. By 4 days in vitro, AChE-positive basal forebrain cells display several short and thick neurites that appear to be dendrites, and one long process that appears to be an axon. By 5 days in vitro, dendrites are well developed; by 7 days the presumed axon has extended widely over the cortical target zone. These neurites are maintained through 3 weeks in culture. Distributions of cells varied with the age of the slice. AChE-labeled cells were not seen overlying hippocampal tissue when dissociated cells were seeded on slice cultures made from day 0 rats, but a few labeled cells were seen when seeded on slices from day 2 rats. Clear non-random patterns of labeled cells and neurite outgrowth were seen on slice cultures from day 5 or older pups. The non-random distribution seen with AChE-positive neurons was not seen using other techniques that labeled all cells (non-selective fluorescent labels) or all neurons; these techniques resulted in labeled cells scattered apparently homogenously across the slice culture.These studies demonstrate a non-random pattern of attachment or differentiation of basal forebrain cholinergic neurons when these cells are seeded onto cultured cortical slices; this pattern mimics the normal patterns of basal forebrain cholinergic projections to these cortical regions. These data suggest that the factors that normally guide basal forebrain-derived cholinergic axons to their target cells in vivo are present and detectable in this model system.

Entorhinal axons exhibit sprouting in CA1 subfield of the adult hippocampus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainic acid administration in rat, a model of temporal lobe epilepsy, results in CA3 pyramidal neuron degeneration leading to deafferentation of CA1 pyramidal neurons. Denervation in CA1 shows a near-complete recovery of synaptic density over 2-3 months, but the source of axons participating in the reinnervation is not clear. This study investigated the contribution of the entorhinal cortex in this reinnervation by comparing the distribution of the entorhinal axons in the CA1 subfield between the intact hippocampus and the CA3-lesioned hippocampus at 3 months after administration of kainic acid. Entorhinal axons were visualized by anterograde tracing using injections of the biotinylated dextran amine into the entorhinal cortex. In the CA1 subfield of the intact hippocampus, entorhinal axons were conspicuous in the alveus and the stratum lacunosum moleculare. The distribution in the strata oriens, pyramidale, and radiatum was sparse and was characterized by isolated entorhinal fibers of the alvear pathway crossing these strata to the stratum lacunosum moleculare. However, after kainic acid-induced CA3 lesion, the density of entorhinal axons increased significantly in the CA1 stratum radiatum (375% of the intact hippocampus), as a large number of axons emanating from the entorhinal fiber plexus in the stratum lacunosum moleculare invaded the stratum radiatum. The stratum radiatum also exhibited wavy entorhinal axons filled with boutons and oriented parallel to the stratum pyramidale, suggesting collateral sprouting from entorhinal axons traversing the stratum radiatum. Thus, a significant aberrant sprouting of entorhinal axons occurs into the CA1 stratum radiatum after CA3 lesion. The sprouted fibers appear to come from both entorhinal fiber plexus in the stratum lacunosum moleculare (translaminar sprouting) and entorhinal axons traversing the stratum radiatum (intralaminar sprouting). However, the major contribution appears to be from the entorhinal plexus in the stratum lacunosum moleculare. This aberrant sprouting may lead to altered afferent excitatory connectivity in the CA1 subfield and contribute to the persistent CA1 hyperexcitability that occurs after the CA3 lesion.

The axonal chemorepellant semaphorin 3A is increased in the cerebellum in schizophrenia and may contribute to its synaptic pathology

The neuropathological features of schizophrenia are suggestive of a developmentally induced impairment of synaptic connectivity. Semaphorin 3A (sema3A) might contribute to this process because it is a secreted chemorepellant which regulates axonal guidance. We have investigated sema3A in the cerebellum (an area in which expression persists in adulthood), and measured its abundance in 16 patients with schizophrenia and 16 controls. In adults, sema3A was predominantly localized to the inner part of the molecular layer neuropil, whereas infants and rats showed greater labelling of Purkinje cell bodies. Sema3A was increased in schizophrenia, as shown by enzyme-linked immunosorbent assay (+28%; P<0.05) and immunohistochemistry (+45%; P<0.01). We also measured reelin mRNA, since reelin is involved in related developmental processes and is decreased in other brain regions in schizophrenia. Reelin mRNA showed a trend reduction in the subjects with schizophrenia (-26%; P=0.07) and, notably, was negatively correlated with sema3A. Sema3A also correlated negatively with synaptophysin and complexin II mRNAs. The results show that sema3A is elevated in schizophrenia, and is associated with downregulation of genes involved in synaptic formation and maintenance. In this respect, sema3A appears to contribute to the synaptic pathology of schizophrenia, perhaps via ongoing effects of persistent sema3A elevation on synaptic plasticity. The findings are consistent with an early neurodevelopmental origin for the disorder, and the reciprocal changes in sema3A and reelin may be indicative of a pathogenic mechanism that affects the balance between trophic and inhibitory factors regulating synaptogenesis.

Differential responsiveness to the chemorepellent Semaphorin 3A distinguishes Ipsi- and contralaterally projecting axons in the chick midbrain

In the chick dorsal mesencephalon, the optic tectum, the developing axons must choose between remaining on the same side of the midline or growing across it. The ipsilaterally projecting axons, forming the tectobulbar tract, course circumferentially toward the ventrally situated floor plate but before reaching the basal mesencephalon, the tegmentum, gradually turn caudally. Here, they follow the course of the medial longitudinal fasciculus (MLF), located parallel to the floor plate. By in vivo labeling of tectal axons, we could demonstrate that these axons arise primarily in the dorsal tectum. To test the idea that chemorepellent molecules are involved in guidance of the nondecussating axons, we performed coculture experiments employing tectal explants from various positions along the dorso-ventral axis. Axons emanating from dorsal tectal explants were strongly repelled by diencephalic tissue containing the neurons that give rise to the MLF whereas ventral tectal axons showed only a moderate response. This inhibitory effect was substantially neutralized by the addition of anti-neuropilin-1 antibodies. A similar differential response of axons was observed when tectal explants were cocultured with cell aggregates secreting the chemorepellent Semaphorin 3A (Sema3A). Sema3B and Sema3C, respectively, did not inhibit growth of tectal axons. In addition, neither the floor plate nor Slit2-secreting cell aggregates influenced outgrowth of dorsal fibers. In Sema3A-deficient mice, DiI-labeling revealed that dorsal mesencephalic axons cross the MLF instead of turning posteriorly upon reaching the fiber tract, thus behaving like the ventrally originating contralaterally projecting axons. A differential responsiveness of tectal axons to Sema3A most likely released by the MLF thus contributes to pathfinding in the ventral mesencephalon.

Hyaluronan-associated adhesive cues control fiber segregation in the hippocampus

In various brain regions, particularly in the hippocampus, afferent fiber projections terminate in specific layers. Little is known about the molecular cues governing this laminar specificity. To this end we have recently shown that the innervation pattern of entorhinal fibers to the hippocampus is mimicked by the lamina-specific adhesion of entorhinal cells on living hippocampal slices, suggesting a role of adhesion molecules in the positioning of entorhinal fibers. Here, we have analyzed the role of extracellular matrix components in mediating this lamina-specific adhesion. We show that hyaluronidase treatment of hippocampal slices abolishes lamina-specific adhesion as well as layer-specific growth of entorhinal fibers to the dentate outer molecular layer in organotypic slice cultures. We conclude that hyaluronan-associated molecules play a crucial role in the formation of the lamina-specific entorhinal projection to the hippocampus.

Age-dependent effects of secreted Semaphorins 3A, 3F, and 3E on developing hippocampal axons: in vitro effects and phenotype of Semaphorin 3A (-/-) mice

We studied the role of Semaphorins in the formation of hippocampal connections at embryonic and early postnatal stages. We show that the embryonic entorhinal cortex has a repulsive effect on embryonic hippocampal axons that disappears gradually at postnatal stages. Such chemorepulsion is blocked by Neuropilin-1 and -2 blocking antibodies. However, at perinatal stages, the inner layers of the entorhinal cortex attract CA1 axons. At these stages, Sema3A and Sema3F bind commissural and entorhinal axons. Sema3A and Sema3F repel hippocampal axons at E14-P2, but not at E13. A similar spatiotemporal pattern of chemorepulsion is observed for Sema3A on entorhinal axons, in contrast to Sema3F, which repels these axons only at postnatal ages. Sema3E also repels hippocampal axons but exclusively at E14. We show that Sema3A and Sema3F can induce the collapse of hippocampal growth cones and that membrane-bound Sema3A and Sema3F can guide hippocampal axons in the stripe assay. In sema3A (-/-) mice, the entorhinohippocampal projection is largely normal although single axons innervate aberrantly the stratum radiatum and the hilus. Thus, the chemorepulsion evoked by Sema3A, Sema3E, and Sema3F is dynamically regulated in the developing hippocampal formation.

Differential expression of plexin-A subfamily members in the mouse nervous system

Plexins comprise a family of transmembrane proteins (the plexin family) which are expressed in nervous tissues. Some plexins have been shown to interact directly with secreted or transmembrane semaphorins, while plexins belonging to the A subfamily are suggested to make complexes with other membrane proteins, neuropilins, and propagate chemorepulsive signals of secreted semaphorins of class 3 into cells or neurons. Despite that much information has been gathered on the plexin-semaphorin interaction, the role of plexins in the nervous system is not well understood. To gain insight into the functions of plexins in the nervous system, we analyzed spatial and temporal expression patterns of three members of the plexin-A subfamily (plexin-A1, -A2, and -A3) in the developing mouse nervous system by in situ hybridization analysis in combination with immunohistochemistry. We show that the three plexins are differentially expressed in sensory receptors or neurons in a developmentally regulated manner, suggesting that a particular plexin or set of plexins is shared by neuronal elements and functions as the receptor for semaphorins to regulate neuronal development.

New molecules for hippocampal development

Pathfinding by developing axons towards their proper targets is an essential step in establishing appropriate neuronal connections. Recent work involving cell culture assays and molecular biology strategies, including knockout animals, strongly indicates that a complex network of guidance signals regulates the formation of hippocampal connections during development. Outgrowing axons are routed towards the hippocampal formation by specific expression of long-range cues, which include secreted class 3 semaphorins, netrin 1 and Slit proteins. Local membrane- or substrate-anchored molecules, such as ligands of the ephrin A subclass, provide layer-specific positional information. Understanding the molecular mechanisms that underlie axonal guidance during hippocampal development might be of importance in making therapeutic use of sprouting fibers, which are produced following the loss of afferents in CNS lesion.

Aberrant development of hippocampal circuits and altered neural activity in netrin 1-deficient mice

Diffusible factors, including netrins and semaphorins, are believed to be important cues for the formation of neural circuits in the forebrain. Here we have examined the role of netrin 1 in the development of hippocampal connections. We show that netrin 1 and its receptor, Dcc, are expressed in the developing fimbria and in projection neurons, respectively, and that netrin 1 promotes the outgrowth of hippocampal axons in vitro via DCC receptors. We also show that the hippocampus of netrin 1-deficient mice shows a misorientation of fiber tracts and pathfinding errors, as detected with antibodies against the surface proteins TAG-1, L1 and DCC. DiI injections show that hippocampal commissural axons do not cross the midline in these mutants. Instead, when axons approach the midline, they turn ventrally and form a massive aberrant projection to the ipsilateral septum. In addition, both the ipsilateral entorhino-hippocampal and the CA3-to-CA1 associational projections show an altered pattern of layer-specific termination in netrin 1-deficient mice. Finally, optical recordings with the Ca(2+) indicator Fura 2-AM show that spontaneous neuronal activity is reduced in the septum of netrin 1-mutant mice. We conclude that netrin 1 is required not only for the formation of crossed connections in the forebrain, but also for the appropriate layer-specific targeting of ipsilateral projections and for the control of normal levels of spontaneous neural activity.

Semaphorin function in the developing invertebrate peripheral nervous system

Different members of the semaphorin family of secreted and transmembrane guidance molecules play important and diverse roles during neuronal development. Within the developing grasshopper limb bud, two semaphorins are expressed in relatively non-overlapping and distinct expression patterns. The establishment of the tibial sensory projection within the limb bud relies on the combinatorial action of both semaphorins. In this review, we describe the function of the two semaphorins in axonal guidance and propose that a hierarchy of cues guide sensory neurons in the developing peripheral nervous system.Key words: semaphorin, axon guidance, grasshopper, peripheral nervous system, review.

Expression and putative role of neuropilin-1 in the early scaffold of axon tracts in embryonic Xenopus brain

Although the general principles of axon guidance in vitro are understood, little is known about how axons respond to the myriad of cues in vivo and navigate axon pathways within the complex milieu of the embryonic brain. Although neuropilin-1 is an axon guidance receptor for chemorepulsive ligands in the class 3 subfamily of semaphorins, its role in directing axon growth in vivo is unknown. In the present study, we have examined the expression and role of neuropilin-1 in the embryonic forebrain of Xenopus. Neuropilin-1 was selectively expressed by a subset of axons in the early scaffold of axon tracts. These axons arise from the presumptive telencephalic nucleus, cross the rostral midline by means of the postoptic commissure, and enter the major longitudinal tract of the prosencephalon, the tract of the postoptic commissure. At the level of the mesencephalon, these axons diverge and enter one of two axon tracts: either the ventral longitudinal tract or the ventral commissure. This same population of axons also expresses NOC-2, a novel glycoform of the neural cell adhesion molecule N-CAM. We have previously revealed the presence of a chemorepulsive activity underlying the pathway followed by these axons as they cross the ventral commissure. When neuropilin-1 was overexpressed after blastomere injections of synthetic RNA transcripts, NOC-2 axons entered the ventral commissure but failed to cross the midline. Instead, these axons were inhibited from growing ventrally within the commissural pathway. These results suggest that the level of neuropilin-1 in the NOC-2 subpopulation of axons is critical for determining whether these axons reach the ventral midline. Thus, neuropilin-1 may a specific role in directing the growth of NOC-2 axons across the ventral midline in the early embryonic mesencephalon.

Aberrant trajectory of entorhino-dentate axons in the mutant Shaking Rat Kawasaki: a Dil-labelling study

The Shaking Rat Kawasaki (SRK) is a neurological mutant that exhibits abnormalities of cell migration and lamination, with many similarities to the mouse reeler mutant. We recently used lamina-specific antibody staining to show that despite severe aberrations in the laminar organization of the SRK dentate gyrus, the entorhinal terminal field in the outer dentate molecular layer appeared relatively normal (Woodhams & Terashima, 1999, J. Comp. Neurol. 409 p57). However, neurofilament immunostaining suggested that entorhino-dentate afferents take an abnormal trajectory in reaching their appropriate targets, the granule cells dendrites. In the present study, anterograde tracing with the carbocyanine dye 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) has been used to delineate directly the path that entorhinal axons take to the dentate gyrus, confirming that in SRK entorhinal axons do indeed reach their appropriate terminal fields in the molecular layer, with laminar segregation between projections from the lateral and medial entorhinal cortices. However, these fibres fail to cross the hippocampal fissure between the subiculum and the dentate gyrus, coursing instead parallel to it until they curve round the deepest point of the fissure in field CA3. Similar findings were seen in the murine reeler mutant. Insertion of DiI crystals into the entorhinal cortex of neonatal rats also retrogradely labelled the developmentally transient Cajal-Retzius cells at the hippocampal fissure; these survive for longer in SRK than in normal littermates. The presence of a marked astrogliosis at the SRK hippocampal fissure may play a part in determining the abnormal trajectory taken by entorhino-dentate afferents in this mutant.

Sema3C and netrin-1 differentially affect axon growth in the hippocampal formation

The interaction between outgrowing neurons and their targets is a central element in the development of the afferent and efferent connections of the hippocampal system. This requires that axonal growth cones recognize specific guidance cues in the appropriate target area. At present, little is known about the mechanisms that determine the lamina-specific termination of hippocampal afferents. In order to understand the role of different guidance factors, we analyzed the effects of Sema3C and Netrin-1 on explants from the entorhinal cortex, dentate gyrus, cornu ammonis regions CA1 and CA3 and medial septum in a collagen coculture assay. Our observations suggest that both semaphorins and netrin play important roles in the neuron-target interactions in the hippocampal system. Sema3C is involved in the control of the ingrowth of the septohippocampal projection. We also show that netrin-1 is involved in attracting commissural neurons from dentate gyrus/hilus and CA3 to their target area in the contralateral hippocampus.

Semaphorins and their receptors in vertebrates and invertebrates

The semaphorins are a family of intercellular signaling proteins that has grown to include 19 identified members in higher vertebrates. Several of its members act as axonal guidance molecules. One participates in signaling in the immune system. The majority, however, do not yet have known biological functions. Recent studies have shown that neuropilins and plexins act as receptors for semaphorins. The most important challenge for the future is to define the biological roles of semaphorins in vivo.

Neuropilin-2 is required in vivo for selective axon guidance responses to secreted semaphorins

Neuropilins are receptors for class 3 secreted semaphorins, most of which can function as potent repulsive axon guidance cues. We have generated mice with a targeted deletion in the neuropilin-2 (Npn-2) locus. Many Npn-2 mutant mice are viable into adulthood, allowing us to assess the role of Npn-2 in axon guidance events throughout neural development. Npn-2 is required for the organization and fasciculation of several cranial nerves and spinal nerves. In addition, several major fiber tracts in the brains of adult mutant mice are either severely disorganized or missing. Our results show that Npn-2 is a selective receptor for class 3 semaphorins in vivo and that Npn-1 and Npn-2 are required for development of an overlapping but distinct set of CNS and PNS projections.

Neuropilin-2 regulates the development of selective cranial and sensory nerves and hippocampal mossy fiber projections

Neuropilin-1 and neuropilin-2 bind differentially to different class 3 semaphorins and are thought to provide the ligand-binding moieties in receptor complexes mediating repulsive responses to these semaphorins. Here, we have studied the function of neuropilin-2 through analysis of a neuropilin-2 mutant mouse, which is viable and fertile. Repulsive responses of sympathetic and hippocampal neurons to Sema3F but not to Sema3A are abolished in the mutant. Marked defects are observed in the development of several cranial nerves, in the initial central projections of spinal sensory axons, and in the anterior commissure, habenulo-interpeduncular tract, and the projections of hippocampal mossyfiber axons in the infrapyramidal bundle. Our results show that neuropilin-2 is an essential component of the Sema3F receptor and identify key roles for neuropilin-2 in axon guidance in the PNS and CNS.

Target- and maturation-specific membrane-associated molecules determine the ingrowth of entorhinal fibers into the hippocampus

In this study the role of membrane-associated molecules involved in entorhinohippocampal pathfinding was examined. First outgrowth preferences of entorhinal neurites were analyzed on membrane carpets obtained from their proper target area, the hippocampus, and compared to preferences on control membranes from brain regions which do not receive afferent connections from the entorhinal cortex. On a substrate consisting of alternating lanes of hippocampal and control membranes, entorhinal neurites exhibited a strong tendency to grow on lanes of hippocampal membrane. These tissue-specific outgrowth preferences were maintained even on membrane preparations from adult brain tissue devoid of myelin. To determine the possible maturation dependence of these membranes, we examined guidance preferences of entorhinal neurites on hippocampal membranes of different developmental stages ranging from embryonic to postnatal and adult. Given a choice between alternating lanes of embryonic (E15-E16) and neonatal (P0-P1) hippocampal membranes, entorhinal neurites preferred to extend on neonatal membranes. No outgrowth preferences were observed on membranes obtained between E19 and P10. From P10 onward there was a reoccurrence of a preference for postnatal membrane lanes when neurites were presented with a choice between P15, P30, and adult membranes (>P60). This choice behavior of entorhinal neurites temporally correlates with the ingrowth of the perforant path into the hippocampus and with the stabilization of this brain area in vivo. Experiments in which postnatal and adult hippocampal membranes were heat inactivated or treated to remove molecules sensitive to phosphatidylinositol-specific phospholipase C demonstrated that entorhinal fiber preferences were controlled in this assay by attractive guidance cues and were independent of phosphatidylinositol-sensitive linked molecules. Moreover, entorhinal neurites displayed a positive discrimination for membrane-associated guidance cues of their target field, thus preferring to grow on membranes from the molecular layer of the dentate gyrus compared with CA3 or hilus membranes. Heat-inactivation experiments indicated that preferential growth of entorhinal axons is due to a specific attractivity of the molecular layer substrate. The data presented demonstrate that outgrowth of entorhinal fibers on hippocampal membranes is target and maturation dependent.