Semaphorin3A regulates synaptic function of differentiated hippocampal neurons

Perineuronal nets' role in metabolism

Perineuronal nets (PNNs), specialized extracellular matrix (ECM) structures that envelop neurons, have recently been recognized as key players in the regulation of metabolism. This review explores the growing body of knowledge concerning PNNs and their role in metabolic control, drawing insights from recent research and relevant studies. The pivotal role of PNNs in the context of energy balance and whole body blood glucose is examined. This review also highlights novel findings, including the effects of astroglia, microglia, sex and gonadal hormones, nutritional regulation, circadian rhythms, and age on PNNs dynamics. These findings illuminate the complex and multifaceted role of PNNs in metabolic health.

Role of Semaphorin 3A in common psychiatric illnesses such as schizophrenia, depression, and anxiety

With societal development and an ageing population, psychiatric disorders have become a common cause of severe and long-term disability and socioeconomic burdens worldwide. Semaphorin 3A (Sema-3A) is a secreted glycoprotein belonging to the semaphorin family. Sema-3A is well known as an axon guidance factor in the neuronal system and a potent immunoregulator at all stages of the immune response. It is reported to have various biological functions and is involved in many human diseases, including autoimmune diseases, angiocardiopathy, osteoporosis, and tumorigenesis. The signals of sema-3A involved in the pathogenesis of these conditions, are transduced through its cognate receptors and diverse downstream signalling pathways. An increasing number of studies show that sema-3A plays important roles in synaptic and dendritic development, which are closely associated with the pathophysiological mechanisms of psychiatric disorders, including schizophrenia, depression, and autism, suggesting the involvement of sema-3A in the pathogenesis of mental diseases. This indicates that mutations in sema-3A and alterations in its receptors and signalling may compromise neurodevelopment and predispose patients to these disorders. However, the role of sema-3A in psychiatric disorders, particularly in regulating neurodevelopment, remains elusive. In this review, we summarise the recent progress in understanding sema-3A in the pathogenesis of mental diseases and highlight sema-3A as a potential target for the prevention and treatment of these diseases.

Critical Role of VEGF as a Direct Regulator of Photoreceptor Function

Vascular endothelial growth factor (VEGF or VEGF-A), a major pathogenic factor for diabetic and hypoxic blood-retina barrier (BRB) diseases, has been shown to act as a direct functional regulator for neurons in the peripheral and central nerve systems. To determine if VEGF plays a direct role in regulating retinal neuronal function, we established specific experimental procedures and examined the effect of recombinant VEGF (rVEGF) on photoreceptor function with electroretinography (ERG) in mice. In our case, rVEGF caused a significant reduction of scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes in a dose-dependent manner in dark-adapted wild-type (WT) mice, shortly after the intravitreal delivery of rVEGF in dark. However, the effect of rVEGF on photoreceptor function was nullified in adult Akita diabetic mice. Our data strongly suggest that VEGF is a direct regulator of photoreceptor function and VEGF upregulation contributes significantly to the diabetes-induced reduction of photoreceptor function. In this chapter, we will discuss the relevant background, key experimental procedures and results, and clinical significance of our work.

Guidance landscapes unveiled by quantitative proteomics to control reinnervation in adult visual system

In the injured adult central nervous system (CNS), activation of pro-growth molecular pathways in neurons leads to long-distance regeneration. However, most regenerative fibers display guidance defects, which prevent reinnervation and functional recovery. Therefore, the molecular characterization of the proper target regions of regenerative axons is essential to uncover the modalities of adult reinnervation. In this study, we use mass spectrometry (MS)-based quantitative proteomics to address the proteomes of major nuclei of the adult visual system. These analyses reveal that guidance-associated molecules are expressed in adult visual targets. Moreover, we show that bilateral optic nerve injury modulates the expression of specific proteins. In contrast, the expression of guidance molecules remains steady. Finally, we show that regenerative axons are able to respond to guidance cues ex vivo, suggesting that these molecules possibly interfere with brain target reinnervation in adult. Using a long-distance regeneration model, we further demonstrate that the silencing of specific guidance signaling leads to rerouting of regenerative axons in vivo. Altogether, our results suggest ways to modulate axon guidance of regenerative neurons to achieve circuit repair in adult.

VEGF as a Direct Functional Regulator of Photoreceptors and Contributing Factor to Diabetes-Induced Alteration of Photoreceptor Function

Vascular endothelial growth factor (VEGF) is a major therapeutic target for blood-retina barrier (BRB) breakdown in diabetic retinopathy (DR), age-related macular degeneration (AMD), and other hypoxic retinal vascular disorders. To determine whether VEGF is a direct regulator of retinal neuronal function and its potential role in altering vision during the progression of DR, we examined the immediate impact of recombinant VEGF (rVEGF) on photoreceptor function with electroretinography in C57BL6 background wild-type (WT) and Akita spontaneous diabetic mice. Shortly after intravitreal injections, rVEGF caused a significant reduction of scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes in a dose-dependent manner in dark-adapted 1.5-mo-old WT mice. Compared with WT controls, 5-mo-old Akita spontaneous diabetic mice demonstrated a significant reduction in scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes. However, the effect of rVEGF altered photoreceptor function in WT controls was diminished in 5-mo-old Akita spontaneous diabetic mice. In conclusion, our results suggest that VEGF is a direct functional regulator of photoreceptors and VEGF up-regulation in DR is a contributing factor to diabetes-induced alteration of photoreceptor function. This information is critical to the understanding of the therapeutic effect and to the care of anti-VEGF drug-treated patients for BRB breakdown in DR, AMD, and other hypoxic retinal vascular disorders.

Semaphorins in Adult Nervous System Plasticity and Disease

Semaphorins, originally discovered as guidance cues for developing axons, are involved in many processes that shape the nervous system during development, from neuronal proliferation and migration to neuritogenesis and synapse formation. Interestingly, the expression of many Semaphorins persists after development. For instance, Semaphorin 3A is a component of perineuronal nets, the extracellular matrix structures enwrapping certain types of neurons in the adult CNS, which contribute to the closure of the critical period for plasticity. Semaphorin 3G and 4C play a crucial role in the control of adult hippocampal connectivity and memory processes, and Semaphorin 5A and 7A regulate adult neurogenesis. This evidence points to a role of Semaphorins in the regulation of adult neuronal plasticity. In this review, we address the distribution of Semaphorins in the adult nervous system and we discuss their function in physiological and pathological processes.

Semaphorin 3A controls enteric neuron connectivity and is inversely associated with synapsin 1 expression in Hirschsprung disease

Most of the gut functions are controlled by the enteric nervous system (ENS), a complex network of enteric neurons located throughout the wall of the gastrointestinal tract. The formation of ENS connectivity during the perinatal period critically underlies the establishment of gastrointestinal motility, but the factors involved in this maturation process remain poorly characterized. Here, we examined the role of Semaphorin 3A (Sema3A) on ENS maturation and its potential implication in Hirschsprung disease (HSCR), a developmental disorder of the ENS with impaired colonic motility. We found that Sema3A and its receptor Neuropilin 1 (NRP1) are expressed in the rat gut during the early postnatal period. At the cellular level, NRP1 is expressed by enteric neurons, where it is particularly enriched at growth areas of developing axons. Treatment of primary ENS cultures and gut explants with Sema3A restricts axon elongation and synapse formation. Comparison of the ganglionic colon of HSCR patients to the colon of patients with anorectal malformation shows reduced expression of the synaptic molecule synapsin 1 in HSCR, which is inversely correlated with Sema3A expression. Our study identifies Sema3A as a critical regulator of ENS connectivity and provides a link between altered ENS connectivity and HSCR.

Functional analysis of SEMA3A variants identified in Chinese patients with isolated hypogonadotropic hypogonadism

Isolated hypogonadotropic hypogonadism (IHH) is a rare disorder characterized by impaired sexual development and infertility, caused by the deficiency of hypothalamic gonadotropin-releasing hormone neurons. IHH is named Kallmann's syndrome (KS) or normosmic IHH (nIHH) when associated with a defective or normal sense of smell. Variants in SEMA3A have been recently identified in patients with KS. In this study, we screened SEMA3A variants in a cohort of Chinese patients with IHH by whole exome sequencing. Three novel heterozygous SEMA3A variants (R197Q, R617Q and V458I) were identified in two nIHH and one KS patients, respectively. Functional studies indicated that R197Q and R617Q variants were ineffective in activating the phosphorylation of FAK (focal adhesion kinase) in GN11 cells, despite normal production and secretion in HEK293T cells. The V458I SEMA3A had defect in secretion as it was not detected in the conditioned medium from HEK293T cells. Compared with wild type SEMA3A protein, all three SEMA3A mutant proteins were ineffective in inducing the migration of GN11 cells. Our study further showed the contribution of SEMA3A loss-of-function variants to the pathogenesis of IHH.

Inhibition of Semaphorin3A Promotes Ocular Dominance Plasticity in the Adult Rat Visual Cortex

Perineuronal nets (PNNs) are condensed structures in the extracellular matrix that mainly surround GABA-ergic parvalbumin-positive interneurons in the adult brain. Previous studies revealed a parallel between PNN formation and the closure of the critical period. Moreover, ocular dominance plasticity is enhanced in response to PNN manipulations in adult animals. However, the mechanisms through which perineuronal nets modulate plasticity are still poorly understood. Recent work indicated that perineuronal nets may convey molecular signals by binding and storing proteins with important roles in cellular communication. Here we report that semaphorin3A (Sema3A), a chemorepulsive axon guidance cue known to bind to important perineuronal net components, is necessary to dampen ocular dominance plasticity in adult rats. First, we showed that the accumulation of Sema3A in PNNs in the visual cortex correlates with critical period closure, following the same time course of perineuronal nets maturation. Second, the accumulation of Sema3A in perineuronal nets was significantly reduced by rearing animals in the dark in the absence of any visual experience. Finally, we developed and characterized a tool to interfere with Sema3A signaling by means of AAV-mediated expression of receptor bodies, soluble proteins formed by the extracellular domain of the endogenous Sema3A receptor (neuropilin1) fused to a human IgG Fc fragment. By using this tool to antagonize Sema3A signaling in the adult rat visual cortex, we found that the specific inhibition of Sema3A promoted ocular dominance plasticity. Thus, Sema3A accumulates in perineuronal nets in an experience-dependent manner and its presence in the mature visual cortex inhibits plasticity.

Expression of class III Semaphorins and their receptors in the developing chicken (Gallus gallus) inner ear

Class III Semaphorin (Sema) secreted ligands are known to repel neurites expressing Neuropilin (Nrp) and/or Plexin (Plxn) receptors. There is, however, a growing body of literature supporting that Sema signaling also has alternative roles in development such as synaptogenesis, boundary formation, and vasculogenesis. To evaluate these options during inner ear development, we used in situ hybridization or immunohistochemistry to map the expression of Sema3D, Sema3F, Nrp1, Nrp2, and PlxnA1 in the chicken (Gallus gallus) inner ear from embryonic day (E)5-E10. The resulting expression patterns in either the otic epithelium or its surrounding mesenchyme suggest that Sema signaling could be involved in each of the varied functions reported for other tissues. Sema3D expression flanking the sensory tissue in vestibular organs suggests that it may repel Nrp2- and PlxnA1-expressing neurites of the vestibular ganglion away from nonsensory epithelia, thus channeling them into the sensory domains at E5-E8. Expression of Sema signaling genes in the sensory hair cells of both the auditory and vestibular organs on E8-E10 may implicate Sema signaling in synaptogenesis. In the nonsensory regions of the cochlea, Sema3D in the future tegmentum vasculosum opposes Nrp1 and PlxnA1 in the future cuboidal cells; the abutment of ligand and receptors in adjacent domains may enforce or maintain the boundary between them. In the mesenchyme, Nrp1 colocalized with capillary-rich tissue. Sema3D immediately flanks this Nrp1-expressing tissue, suggesting a role in endothelial cell migration towards the inner ear. In summary, Sema signaling may play multiple roles in the developing inner ear.

De novo assembly of a transcriptome for the cricket Gryllus bimaculatus prothoracic ganglion: An invertebrate model for investigating adult central nervous system compensatory plasticity

The auditory system of the cricket, Gryllus bimaculatus, demonstrates an unusual amount of anatomical plasticity in response to injury, even in adults. Unilateral removal of the ear causes deafferented auditory neurons in the prothoracic ganglion to sprout dendrites across the midline, a boundary they typically respect, and become synaptically connected to the auditory afferents of the contralateral ear. The molecular basis of this sprouting and novel synaptogenesis in the adult is not understood. We hypothesize that well-conserved developmental guidance cues may recapitulate their guidance functions in the adult in order to facilitate this compensatory growth. As a first step in testing this hypothesis, we have generated a de novo assembly of a prothoracic ganglion transcriptome derived from control and deafferented adult individuals. We have mined this transcriptome for orthologues of guidance molecules from four well-conserved signaling families: Slit, Netrin, Ephrin, and Semaphorin. Here we report that transcripts encoding putative orthologues of most of the candidate developmental ligands and receptors from these signaling families were present in the assembly, indicating expression in the adult G. bimaculatus prothoracic ganglion.

Retrograde semaphorin-plexin signalling drives homeostatic synaptic plasticity

Homeostatic signaling systems ensure stable, yet flexible neural activity and animal behavior^1–4. Defining the underlying molecular mechanisms of neuronal homeostatic signaling will be essential in order to establish clear connections to the causes and progression of neurological disease. Presynaptic homeostatic plasticity (PHP) is a conserved form of neuronal homeostatic signaling, observed in organisms ranging from Drosophila to human^1,5. Here, we demonstrate that Semaphorin2b (Sema2b) is target-derived signal that acts upon presynaptic PlexinB (PlexB) receptors to mediate the retrograde, homeostatic control of presynaptic neurotransmitter release at the Drosophila neuromuscular junction. Sema2b-PlexB signaling regulates the expression of PHP via the cytoplasmic protein Mical and the oxoreductase-dependent control of presynaptic actin^6,7. During neural development, Semaphorin-Plexin signaling instructs axon guidance and neuronal morphogenesis^8–10. Yet, Semaphorins and Plexins are also expressed in the adult brain^11–16. Here we demonstrate that Semaphorin-Plexin signaling controls presynaptic neurotransmitter release. We propose that Sema2b-PlexB signaling is an essential platform for the stabilization of synaptic transmission throughout life.

Neuroplasticity pathways and protein-interaction networks are modulated by vortioxetine in rodents

Background

The identification of biomarkers that predict susceptibility to major depressive disorder and treatment response to antidepressants is a major challenge. Vortioxetine is a novel multimodal antidepressant that possesses pro-cognitive properties and differentiates from other conventional antidepressants on various cognitive and plasticity measures. The aim of the present study was to identify biological systems rather than single biomarkers that may underlie vortioxetine’s treatment effects.

Results

We show that the biological systems regulated by vortioxetine are overlapping between mouse and rat in response to distinct treatment regimens and in different brain regions. Furthermore, analysis of complexes of physically-interacting proteins reveal that biomarkers involved in transcriptional regulation, neurodevelopment, neuroplasticity, and endocytosis are modulated by vortioxetine. A subsequent qPCR study examining the expression of targets in the protein–protein interactome space in response to chronic vortioxetine treatment over a range of doses provides further biological validation that vortioxetine engages neuroplasticity networks. Thus, the same biology is regulated in different species and sexes, different brain regions, and in response to distinct routes of administration and regimens.

Conclusions

A recurring theme, based on the present study as well as previous findings, is that networks related to synaptic plasticity, synaptic transmission, signal transduction, and neurodevelopment are modulated in response to vortioxetine treatment. Regulation of these signaling pathways by vortioxetine may underlie vortioxetine’s cognitive-enhancing properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0376-x) contains supplementary material, which is available to authorized users.

VEGF production and signaling in Müller glia are critical to modulating vascular function and neuronal integrity in diabetic retinopathy and hypoxic retinal vascular diseases

Müller glia (MG) are major retinal supporting cells that participate in retinal metabolism, function, maintenance, and protection. During the pathogenesis of diabetic retinopathy (DR), a neurovascular disease and a leading cause of blindness, MG modulate vascular function and neuronal integrity by regulating the production of angiogenic and trophic factors. In this article, I will (1) briefly summarize our work on delineating the role and mechanism of MG-modulated vascular function through the production of vascular endothelial growth factor (VEGF) and on investigating VEGF signaling-mediated MG viability and neural protection in diabetic animal models, (2) explore the relationship among VEGF and neurotrophins in protecting Müller cells in in vitro models of diabetes and hypoxia and its potential implication to neuroprotection in DR and hypoxic retinal diseases, and (3) discuss the relevance of our work to the effectiveness and safety of long-term anti-VEGF therapies, a widely used strategy to combat DR, diabetic macular edema, neovascular age-related macular degeneration, retinopathy of prematurity, and other hypoxic retinal vascular disorders.

Chronic vortioxetine treatment in rodents modulates gene expression of neurodevelopmental and plasticity markers

The multimodal antidepressant vortioxetine displays an antidepressant profile distinct from those of conventional selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) and possesses cognitive-enhancing properties in preclinical and clinical studies. Recent studies have begun to investigate molecular mechanisms that may differentiate vortioxetine from other antidepressants. Acute studies in adult rats and chronic studies in a middle-aged mouse model reveal upregulation of several markers that play a central role in synaptic plasticity. However, the effect of chronic vortioxetine treatment on expression of neuroplasticity and neurodevelopmental biomarkers in naïve rats has not been evaluated. In the present study, we demonstrate that vortioxetine at a range of doses regulates expression of genes associated with plasticity in the frontal cortex, hippocampus, region encompassing the amygdala, as well as in blood, and displays similar effects relative to the SSRI fluoxetine in adult naïve rats. These genes encode immediate early genes (IEGs), translational regulators, and the neurodevelopmental marker Sema4g. Similar findings detected in brain regions and in blood provide a potential translational impact, and vortioxetine appears to consistently regulate signaling in these networks of neuroplasticity and developmental markers.

Chondroitin sulfates and their binding molecules in the central nervous system

Chondroitin sulfate (CS) is the most abundant glycosaminoglycan (GAG) in the central nervous system (CNS) matrix. Its sulfation and epimerization patterns give rise to different forms of CS, which enables it to interact specifically and with a significant affinity with various signalling molecules in the matrix including growth factors, receptors and guidance molecules. These interactions control numerous biological and pathological processes, during development and in adulthood. In this review, we describe the specific interactions of different families of proteins involved in various physiological and cognitive mechanisms with CSs in CNS matrix. A better understanding of these interactions could promote a development of inhibitors to treat neurodegenerative diseases.

A critical role for VEGF and VEGFR2 in NMDA receptor synaptic function and fear-related behavior

Vascular endothelial growth factor (VEGF) is known to be required for the action of antidepressant therapies but its impact on brain synaptic function is poorly characterized. Using a combination of electrophysiological, single-molecule imaging and conditional transgenic approaches, we identified the molecular basis of the VEGF effect on synaptic transmission and plasticity. VEGF increases the postsynaptic responses mediated by the N-methyl-D-aspartate type of glutamate receptors (GluNRs) in hippocampal neurons. This is concurrent with the formation of new synapses and with the synaptic recruitment of GluNR expressing the GluN2B subunit (GluNR-2B). VEGF induces a rapid redistribution of GluNR-2B at synaptic sites by increasing the surface dynamics of these receptors within the membrane. Consistently, silencing the expression of the VEGF receptor 2 (VEGFR2) in neural cells impairs hippocampal-dependent synaptic plasticity and consolidation of emotional memory. These findings demonstrated the direct implication of VEGF signaling in neurons via VEGFR2 in proper synaptic function. They highlight the potential of VEGF as a key regulator of GluNR synaptic function and suggest a role for VEGF in new therapeutic approaches targeting GluNR in depression.

Loss of MeCP2 disrupts cell autonomous and autocrine BDNF signaling in mouse glutamatergic neurons

Mutations in the MECP2 gene cause the neurodevelopmental disorder Rett syndrome (RTT). Previous studies have shown that altered MeCP2 levels result in aberrant neurite outgrowth and glutamatergic synapse formation. However, causal molecular mechanisms are not well understood since MeCP2 is known to regulate transcription of a wide range of target genes. Here, we describe a key role for a constitutive BDNF feed forward signaling pathway in regulating synaptic response, general growth and differentiation of glutamatergic neurons. Chronic block of TrkB receptors mimics the MeCP2 deficiency in wildtype glutamatergic neurons, while re-expression of BDNF quantitatively rescues MeCP2 deficiency. We show that BDNF acts cell autonomous and autocrine, as wildtype neurons are not capable of rescuing growth deficits in neighboring MeCP2 deficient neurons in vitro and in vivo. These findings are relevant for understanding RTT pathophysiology, wherein wildtype and mutant neurons are intermixed throughout the nervous system.

DOI:http://dx.doi.org/10.7554/eLife.19374.001

Rett syndrome is a progressive brain disorder. Individuals with the condition (who are typically girls) grow normally until they are 6-18 months old and then developmentally regress, with symptoms including anxiety, impaired coordination, seizures and breathing problems.

Rett syndrome is caused by mutations in the gene that encodes a protein called MeCP2. Researchers know that MeCP2 is vital for “excitatory” neurons in the brain to communicate with (and activate) their neighbors. Neurons that lack MeCP2 tend to make fewer of the connections across which they communicate – called synapses – with others.

Many researchers who study Rett syndrome use male mice that lack the MeCP2 protein. This mouse model mimics the symptoms seen in Rett patients, but at a faster and more severe rate. These studies have shown that restoring normal levels of the protein in neurons prevents the majority of Rett-like symptoms in these mice and reverses the disorder.

MeCP2 controls the activity of a number of other genes. These include the gene that produces a protein called Brain-Derived Neurotrophic Factor (BDNF), which helps neurons to grow. Sampathkumar et al. have now studied neurons from mouse models of Rett syndrome to investigate whether BDNF can overcome the defects seen in neurons that lack MeCP2. Viewed under a high-powered microscope, the Rett-like neurons appear smaller than healthy neurons and form fewer synapses. However, increasing the amount of BDNF in the diseased neurons restores normal growth and enables the cells to form more synapses.

Girls with Rett syndrome tend to have a mixture of healthy neurons and those that do not produce the right amount of MeCP2. To mimic this, Sampathkumar et al. grew a mixture of normal and Rett-like mouse neurons in a culture dish. The healthy neurons did not help the diseased neurons to form the correct number of synapses. However, increasing the levels of BDNF in the Rett-like neurons enhanced their ability to form synapses, and increased their cell size to match their healthy counterparts.

Further work is now required to uncover whether manipulating the gene that encodes BDNF – or other genes that MeCP2 controls the activity of – in the brain can reduce the symptoms and slow the progression of Rett syndrome.

DOI:http://dx.doi.org/10.7554/eLife.19374.002

Regulation of dendritic development by semaphorin 3A through novel intracellular remote signaling

Numerous cell adhesion molecules, extracellular matrix proteins and axon guidance molecules participate in neuronal network formation through local effects at axo-dendritic, axo-axonic or dendro-dendritic contact sites. In contrast, neurotrophins and their receptors play crucial roles in neural wiring by sending retrograde signals to remote cell bodies. Semaphorin 3A (Sema3A), a prototype of secreted type 3 semaphorins, is implicated in axon repulsion, dendritic branching and synapse formation via binding protein neuropilin-1 (NRP1) and the signal transducing protein PlexinAs (PlexAs) complex. This review focuses on Sema3A retrograde signaling that regulates dendritic localization of AMPA-type glutamate receptor GluA2 and dendritic patterning. This signaling is elicited by activation of NRP1 in growth cones and is propagated to cell bodies by dynein-dependent retrograde axonal transport of PlexAs. It also requires interaction between PlexAs and a high-affinity receptor for nerve growth factor, toropomyosin receptor kinase A. We propose a control mechanism by which retrograde Sema3A signaling regulates the glutamate receptor localization through trafficking of cis-interacting PlexAs with GluA2 along dendrites; this remote signaling may be an alternative mechanism to local adhesive contacts for neural network formation.

The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity

During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.

Differential expression of sema3A and sema7A in a murine model of multiple sclerosis: Implications for a therapeutic design

We characterised the expression of semaphorin (sema)3A, sema7A and their receptors in the immune and the central nervous system (CNS) at different stages of experimental autoimmune encephalomyelitis (EAE). We also studied their expression in neonatal and adult oligodendrocyte progenitor cell (OPC) and in mature oligodendrocyte cultures. Our results show that sema3A is increased in the CNS and decreased in the immune system upon EAE induction. However, sema7A expression is increased in both the CNS and the immune system during EAE. We also detected sema3A, sema7A and their receptors in neonatal and adult OPCs and in mature oligodendrocytes. These data suggest that sema3A and sema7A are involved in the pathogenesis of EAE, in the modulation of the immune response and in the neurodegeneration that take place in the CNS. Sema7A may represent an intriguing potential therapeutic target for the treatment of both the neurodegenerative and immune-mediated disease processes in MS.

Temporal patterns of gene expression during calyx of held development

Relating changes in gene expression to discrete developmental events remains an elusive challenge in neuroscience, in part because most neural territories are comprised of multiple cell types that mature over extended periods of time. The medial nucleus of the trapezoid body (MNTB) is an attractive vertebrate model system that contains a nearly homogeneous population of neurons, which are innervated by large glutamatergic nerve terminals called calyces of Held (CH). Key steps in maturation of CHs and MNTB neurons, including CH growth and competition, occur very quickly for most cells between postnatal days (P)2 and P6. Therefore, we characterized genome-wide changes in this system, with dense temporal sampling during the first postnatal week. We identified 541 genes whose expression changed significantly between P0-6 and clustered them into eight groups based on temporal expression profiles. Candidate genes from each of the eight profile groups were validated in separate samples by qPCR. Our tissue sample permitted comparison of known glial and neuronal transcripts and revealed that monotonically increasing or decreasing expression profiles tended to be associated with glia and neurons, respectively. Gene ontology revealed enrichment of genes involved in axon pathfinding, cell differentiation, cell adhesion and extracellular matrix. The latter category included elements of perineuronal nets, a prominent feature of MNTB neurons that is morphologically distinct by P6, when CH growth and competition are resolved onto nearly all MNTB neurons. These results provide a genetic framework for investigation of general mechanisms responsible for nerve terminal growth and maturation.

Plexin-B3 suppresses excitatory and promotes inhibitory synapse formation in rat hippocampal neurons

Molecular mechanisms underlying synaptogenesis and synaptic plasticity have become one of the main topics in neurobiology. Increasing evidence suggests that axon guidance molecules including semaphorins and plexins participate in synapse formation and elimination. Although class B plexins are widely expressed in the brain, their role in the nervous system remains poorly characterized. We previously identified that B-plexins modulate microtubule dynamics and through this impact dendrite growth in rat hippocampal neurons. Here, we demonstrate that Plexin-B2 and Plexin-B3 are present in dendrites, but do not localize in synapses. We find that overexpression of all B-plexins leads to decreased volume of excitatory synapses, and at the same time Plexin-B1 and Plexin-B3 promote inhibitory synapse assembly. Plexin-B3 mutants revealed that these processes use different downstream pathways. While elimination of excitatory synapses is the result of Plexin-B3 binding to microtubule end binding proteins EB1 and EB3, the increase in inhibitory synapses is mediated by regulation of Ras and Rho GTPases. Overall, our findings demonstrate that Plexin-B3 contributes to regulating synapse formation.

Developmental gene expression profile of axon guidance cues in Purkinje cells during cerebellar circuit formation

The establishment of precise neural circuits during development involves a variety of contact-mediated and secreted guidance molecules that are expressed in a complementary fashion by different cell types. To build a functional circuit, each cell type must first trigger an intrinsic genetic program that is led by their environment at a key time point. It is therefore essential to identify the different cell-specific and stage-specific transcriptional profiles expressed by neurons. However, very few studies have been done to address this issue thus far. Herein, we have carried out a large-scale quantitative real-time PCR analysis of all classical axon guidance molecules (i.e., Semaphorins, Netrins, Ephrins, and Slits) and their receptors expressed by Purkinje cells (PCs) at specific stages of postnatal cerebellar development in vivo. Most cerebellar connections are setup in a well-characterized sequential manner during postnatal development and lead to the fine regulation of the PC, the sole output of the structure. Our analysis of the relative expression of these guidance cues has uncovered a dynamic expression pattern corresponding to specific stages of cerebellar development, thus providing a starting point for studying the role of these axon guidance molecules in cerebellar wiring.

Perineuronal net digestion with chondroitinase restores memory in mice with tau pathology

Alzheimer's disease is the most prevalent tauopathy and cause of dementia. We investigate the hypothesis that reactivation of plasticity can restore function in the presence of neuronal damage resulting from tauopathy. We investigated two models with tau hyperphosphorylation, aggregation and neurodegeneration: a transgenic mouse model in which the mutant P301S tau is expressed in neurons (Tg P301S), and a model in which an adeno-associated virus expressing P301S tau (AAV-P301S) was injected in the perirhinal cortex, a region critical for object recognition (OR) memory. Both models show profound loss of OR memory despite only 15% neuronal loss in the Tg P301S and 26% in AAV-P301S-injected mice. Recordings from perirhinal cortex slices of 3month-old P301S transgenic mice showed a diminution in synaptic transmission following temporal stimulation. Chondroitinase ABC (ChABC) can reactivate plasticity and affect memory through actions on perineuronal nets. ChABC was injected into the perirhinal cortex and animals were tested for OR memory 1week later, demonstrating restoration of OR memory to normal levels. Synaptic transmission indicated by fEPSP amplitude was restored to control levels following ChABC treatment. ChABC did not affect the progression of neurodegenerative tauopathy. These findings suggest that increasing plasticity by manipulation of perineuronal nets offers a novel therapeutic approach to the treatment of memory loss in neurodegenerative disorders.

Towards an understanding of semaphorin signalling in the spinal cord

Semaphorins are a large family of proteins that are classically associated with axon guidance. These proteins and their interacting partners, the neuropilins and plexins are now known to be key mediators in a variety of processes throughout the nervous system ranging from synaptic refinement to the correct positioning of neuronal and glial cell bodies. Recently, much attention has been given to the roles semaphorins play in other body tissues including the immune and vascular systems. This review wishes to draw attention back to the nervous system, specifically focusing on the role of semaphorins in the development of the spinal cord and their proposed roles in the adult. In addition, their functions in spinal cord injury at the glial scar are also discussed.

Brain endothelial cells control fertility through ovarian-steroid-dependent release of semaphorin 3A

Endothelial-cell–derived Sema3A promotes axonal outgrowth and plasticity and thereby regulates neurohormone release in the adult rodent brain in response to the ovarian cycle.

Neuropilin-1 (Nrp1) guides the development of the nervous and vascular systems, but its role in the mature brain remains to be explored. Here we report that the expression of the 65 kDa isoform of Sema3A, the ligand of Nrp1, by adult vascular endothelial cells, is regulated during the ovarian cycle and promotes axonal sprouting in hypothalamic neurons secreting gonadotropin-releasing hormone (GnRH), the neuropeptide controlling reproduction. Both the inhibition of Sema3A/Nrp1 signaling and the conditional deletion of Nrp1 in GnRH neurons counteract Sema3A-induced axonal sprouting. Furthermore, the localized intracerebral infusion of Nrp1- or Sema3A-neutralizing antibodies in vivo disrupts the ovarian cycle. Finally, the selective neutralization of endothelial-cell Sema3A signaling in adult Sema3a^loxP/loxP mice by the intravenous injection of the recombinant TAT-Cre protein alters the amplitude of the preovulatory luteinizing hormone surge, likely by perturbing GnRH release into the hypothalamo-hypophyseal portal system. Our results identify a previously unknown function for 65 kDa Sema3A-Nrp1 signaling in the induction of axonal growth, and raise the possibility that endothelial cells actively participate in synaptic plasticity in specific functional domains of the adult central nervous system, thus controlling key physiological functions such as reproduction.

In the developing embryo, endothelial cells release chemotropic signals such as Semaphorin 3A (Sema3A) that, upon activation of its receptor Neuropilin-1 (Nrp1), regulate neuronal migration and axon guidance. However, whether endothelial cells in the adult brain retain the ability to secrete molecules that influence neuronal function is unknown. Here we show in the adult brain of rodents that vascular endothelial cells release Sema3A and that the amount released is regulated by the ovulatory cycle. Sema3A, in turn, promotes the outgrowth of axons of hypothalamic neurons that express Neuropilin-1 towards the endothelial wall of portal blood vessels. These neurons release there the neuropeptide that controls reproduction: gonadotropin-releasing hormone (GnRH). Notably, this endothelial-cell-mediated sprouting of GnRH axons regulates neuropeptide release at a key stage of the estrous cycle, the proestrus, when the surge of GnRH triggers ovulation. Thus, by promoting GnRH axonal growth in the adult brain, Sema3A/Neuropilin-1 plays a pivotal role in orchestrating the central control of reproduction. Our results suggest a model in which vascular endothelial cells are dynamic signaling components that relay peripheral information to the brain to control key physiological functions, including species survival.

Semaphorins and the dynamic regulation of synapse assembly, refinement, and function

Semaphorins are phylogenetically conserved proteins expressed in most organ systems, including the nervous system. Following their description as axon guidance cues, semaphorins have been implicated in multiple aspects of nervous system development. Semaphorins are key regulators of neural circuit assembly, neuronal morphogenesis, assembly of excitatory and inhibitory synapses, and synaptic refinement. Semaphorins contribute to the balance between excitatory and inhibitory synaptic transmission, and electrical activity can modulate semaphorin signaling in neurons. This interplay between guidance cue signaling and electrical activity has the potential to sculpt the wiring of neural circuits and to modulate their function.

Targeting the neural extracellular matrix in neurological disorders

The extracellular matrix (ECM) is known to regulate important processes in neuronal cell development, activity and growth. It is associated with the structural stabilization of neuronal processes and synaptic contacts during the maturation of the central nervous system. The remodeling of the ECM during both development and after central nervous system injury has been shown to affect neuronal guidance, synaptic plasticity and their regenerative responses. Particular interest has focused on the inhibitory role of chondroitin sulfate proteoglycans (CSPGs) and their formation into dense lattice-like structures, termed perineuronal nets (PNNs), which enwrap sub-populations of neurons and restrict plasticity. Recent studies in mammalian systems have implicated CSPGs and PNNs in regulating and restricting structural plasticity. The enzymatic degradation of CSPGs or destabilization of PNNs has been shown to enhance neuronal activity and plasticity after central nervous system injury. This review focuses on the role of the ECM, CSPGs and PNNs; and how developmental and pharmacological manipulation of these structures have enhanced neuronal plasticity and aided functional recovery in regeneration, stroke, and amblyopia. In addition to CSPGs, this review also points to the functions and potential therapeutic value of these and several other key ECM molecules in epileptogenesis and dementia.

Modulation of semaphorin3A in perineuronal nets during structural plasticity in the adult cerebellum

In the adult central nervous system (CNS) subsets of neurons are enwrapped by densely organized extracellular matrix structures, called perineuronal nets (PNNs). PNNs are formed at the end of critical periods and contribute to synapse stabilization. Enzymatic degradation of PNNs or genetic deletion of specific PNN components leads to the prolongation of the plasticity period. PNNs consist of extracellular matrix molecules, including chondroitin sulfate proteoglycans, hyaluronan, tenascins and link proteins. It has been recently shown that the chemorepulsive axon guidance protein semaphorin3A (Sema3A) is also a constituent of PNNs, binding with high affinity to the sugar chains of chondroitin sulfate proteoglycans. To elucidate whether the expression of Sema3A is modified in parallel with structural plasticity in the adult CNS, we examined Sema3A expression in the deep cerebellar nuclei of the adult mouse in a number of conditions associated with structural reorganization of the local connectivity. We found that Sema3A in PNNs is reduced during enhanced neuritic remodeling, in both physiological and injury-induced conditions. Moreover, we provide evidence that Sema3A is tightly associated with Purkinje axons and their terminals and its amount in the PNNs is related to Purkinje cell innervation of DCN neurons, but not to glutamatergic inputs. On the whole these data suggest that Sema3A may contribute to the growth-inhibitory properties of PNNs and Purkinje neurons may directly control their specific connection pattern through the release and capture of this guidance cue in the specialized ECM that surrounds their terminals.

Semaphorin 3A binds to the perineuronal nets via chondroitin sulfate type E motifs in rodent brains

Chondroitin sulfate (CS) and the CS-rich extracellular matrix structures called perineuronal nets (PNNs) restrict plasticity and regeneration in the CNS. Plasticity is enhanced by chondroitinase ABC treatment that removes CS from its core protein in the chondroitin sulfate proteoglycans or by preventing the formation of PNNs, suggesting that chondroitin sulfate proteoglycans in the PNNs control plasticity. Recently, we have shown that semaphorin3A (Sema3A), a repulsive axon guidance molecule, localizes to the PNNs and is removed by chondroitinase ABC treatment (Vo, T., Carulli, D., Ehlert, E. M., Kwok, J. C., Dick, G., Mecollari, V., Moloney, E. B., Neufeld, G., de Winter, F., Fawcett, J. W., and Verhaagen, J. (2013) Mol. Cell. Neurosci. 56C, 186-200). Sema3A is therefore a candidate for a PNN effector in controlling plasticity. Here, we characterize the interaction of Sema3A with CS of the PNNs. Recombinant Sema3A interacts with CS type E (CS-E), and this interaction is involved in the binding of Sema3A to rat brain-derived PNN glycosaminoglycans, as demonstrated by the use of CS-E blocking antibody GD3G7. In addition, we investigate the release of endogenous Sema3A from rat brain by biochemical and enzymatic extractions. Our results confirm the interaction of Sema3A with CS-E containing glycosaminoglycans in the dense extracellular matrix of rat brain. We also demonstrate that the combination of Sema3A and PNN GAGs is a potent inhibitor of axon growth, and this inhibition is reduced by the CS-E blocking antibody. In conclusion, Sema3A binding to CS-E in the PNNs may be a mechanism whereby PNNs restrict growth and plasticity and may represent a possible point of intervention to facilitate neuronal plasticity.

Class 3 semaphorin mediates dendrite growth in adult newborn neurons through Cdk5/FAK pathway

Class 3 semaphorins are well-known axonal guidance cues during the embryonic development of mammalian nervous system. However, their activity on postnatally differentiated neurons in neurogenic regions of adult brains has not been characterized. We found that silencing of semaphorin receptors neuropilins (NRP) 1 or 2 in neural progenitors at the adult mouse dentate gyrus resulted in newly differentiated neurons with shorter dendrites and simpler branching in vivo. Tyrosine phosphorylation (Tyr 397) and serine phosphorylation (Ser 732) of FAK were essential for these effects. Semaphorin 3A and 3F mediate serine phosphorylation of FAK through the activation of Cdk5. Silencing of either Cdk5 or FAK in newborn neurons phenocopied the defects in dendritic development seen upon silencing of NRP1 or NRP2. Furthermore, in vivo overexpression of Cdk5 or FAK rescued the dendritic phenotypes seen in NRP1 and NRP2 deficient neurons. These results point to a novel role for class 3 semaphorins in promoting dendritic growth and branching during adult hippocampal neurogenesis through the activation of Cdk5-FAK signaling pathway.

Semaphorin7A and its receptors: pleiotropic regulators of immune cell function, bone homeostasis, and neural development

Semaphorins form a large, evolutionary conserved family of cellular guidance signals. The semaphorin family contains several secreted and transmembrane proteins, but only one GPI-anchored member, Semaphorin7A (Sema7A). Although originally identified in immune cells, as CDw108, Sema7A displays widespread expression outside the immune system. It is therefore not surprising that accumulating evidence supports roles for this protein in a wide variety of biological processes in different organ systems and in disease. Well-characterized biological effects of Sema7A include those during bone and immune cell regulation, neuron migration and neurite growth. These effects are mediated by two receptors, plexinC1 and integrins. However, most of what is known today about Sema7A signaling concerns Sema7A-integrin interactions. Here, we review our current knowledge of Sema7A function and signaling in different organ systems, highlighting commonalities between the cellular effects and signaling pathways activated by Sema7A in different cell types. Furthermore, we discuss a potential role for Sema7A in disease and provide directions for further research.

Role of microRNAs in Semaphorin function and neural circuit formation

Since the discovery of the first microRNA (miRNA) almost 20 years ago, insight into their functional role has gradually been accumulating. This class of non-coding RNAs has recently been implicated as key molecular regulators in the biology of most eukaryotic cells, contributing to the physiology of various systems including immune, cardiovascular, nervous systems and also to the pathophysiology of cancers. Interestingly, Semaphorins, a class of evolutionarily conserved signalling molecules, are acknowledged to play major roles in these systems also. This, combined with the fact that Semaphorin signalling requires tight spatiotemporal regulation, a hallmark of miRNA expression, suggests that miRNAs could be crucial regulators of Semaphorin function. Here, we review evidence suggesting that Semaphorin signalling is regulated by miRNAs in various systems in health and disease. In particular, we focus on neural circuit formation, including axon guidance, where Semaphorin function was first discovered.

Emerging roles for semaphorins and VEGFs in synaptogenesis and synaptic plasticity

Synapse formation, maintenance and plasticity are critical for the correct function of the nervous system and its target organs. During development, these processes enable the establishment of appropriate neural circuits. During adulthood, they allow adaptation to both physiological and environmental changes. In this review, we discuss emerging roles for two families of classical axon and vascular guidance cues in synaptogenesis and synaptic plasticity, the semaphorins and the vascular endothelial growth factors (VEGFs). Their contribution to synapse formation and function add a new facet to the spectrum of overlapping and complementary roles for these molecules in development, adulthood and disease.

Dopaminergic axon guidance: which makes what?

Mesotelencephalic pathways in the adult central nervous system have been studied in great detail because of their implication in major physiological functions as well as in psychiatric, neurological, and neurodegenerative diseases. However, the ontogeny of these pathways and the molecular mechanisms that guide dopaminergic axons during embryogenesis have been only recently studied. This line of research is of crucial interest for the repair of lesioned circuits in adulthood following neurodegenerative diseases or common traumatic injuries. For instance, in the adult, the anatomic and functional repair of the nigrostriatal pathway following dopaminergic embryonic neuron transplantation suggests that specific guidance cues exist which govern embryonic fibers outgrowth, and suggests that axons from transplanted embryonic cells are able to respond to theses cues, which then guide them to their final targets. In this review, we first synthesize the work that has been performed in the last few years on developing mesotelencephalic pathways, and summarize the current knowledge on the identity of cellular and molecular signals thought to be involved in establishing mesotelencephalic dopaminergic neuronal connectivity during embryogenesis in the central nervous system of rodents. Then, we review the modulation of expression of these molecular signals in the lesioned adult brain and discuss their potential role in remodeling the mesotelencephalic dopaminergic circuitry, with a particular focus on Parkinson's disease (PD). Identifying guidance molecules involved in the connection of grafted cells may be useful for cellular therapy in Parkinsonian patients, as these molecules may help direct axons from grafted cells along the long distance they have to travel from the substantia nigra to the striatum.

Semaphorin signaling in vertebrate neural circuit assembly

Neural circuit formation requires the coordination of many complex developmental processes. First, neurons project axons over long distances to find their final targets and then establish appropriate connectivity essential for the formation of neuronal circuitry. Growth cones, the leading edges of axons, navigate by interacting with a variety of attractive and repulsive axon guidance cues along their trajectories and at final target regions. In addition to guidance of axons, neuronal polarization, neuronal migration, and dendrite development must be precisely regulated during development to establish proper neural circuitry. Semaphorins consist of a large protein family, which includes secreted and cell surface proteins, and they play important roles in many steps of neural circuit formation. The major semaphorin receptors are plexins and neuropilins, however other receptors and co-receptors also mediate signaling by semaphorins. Upon semaphorin binding to their receptors, downstream signaling molecules transduce this event within cells to mediate further events, including alteration of microtubule and actin cytoskeletal dynamics. Here, I review recent studies on semaphorin signaling in vertebrate neural circuit assembly, with the goal of highlighting how this diverse family of cues and receptors imparts exquisite specificity to neural complex connectivity.

Secreted human amyloid precursor protein binds semaphorin 3a and prevents semaphorin-induced growth cone collapse

The amyloid precursor protein (APP) is well known for giving rise to the amyloid-β peptide and for its role in Alzheimer's disease. Much less is known, however, on the physiological roles of APP in the development and plasticity of the central nervous system. We have used phage display of a peptide library to identify high-affinity ligands of purified recombinant human sAPPα₆₉₅ (the soluble, secreted ectodomain from the main neuronal APP isoform). Two peptides thus selected exhibited significant homologies with the conserved extracellular domain of several members of the semaphorin (Sema) family of axon guidance proteins. We show that sAPPα₆₉₅ binds both purified recombinant Sema3A and Sema3A secreted by transfected HEK293 cells. Interestingly, sAPPα₆₉₅ inhibited the collapse of embryonic chicken (Gallus gallus domesticus) dorsal root ganglia growth cones promoted by Sema3A (K_d≤8·10⁻⁹ M). Two Sema3A-derived peptides homologous to the peptides isolated by phage display blocked sAPPα binding and its inhibitory action on Sema3A function. These two peptides are comprised within a domain previously shown to be involved in binding of Sema3A to its cellular receptor, suggesting a competitive mechanism by which sAPPα modulates the biological action of semaphorins.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.

Oligodendrocytes as regulators of neuronal networks during early postnatal development

Oligodendrocytes are the glial cells responsible for myelin formation. Myelination occurs during the first postnatal weeks and, in rodents, is completed during the third week after birth. Myelin ensures the fast conduction of the nerve impulse; in the adult, myelin proteins have an inhibitory role on axon growth and regeneration after injury. During brain development, oligodendrocytes precursors originating in multiple locations along the antero-posterior axis actively proliferate and migrate to colonize the whole brain. Whether the initial interactions between oligodendrocytes and neurons might play a functional role before the onset of myelination is still not completely elucidated. In this article, we addressed this question by transgenically targeted ablation of proliferating oligodendrocytes during cerebellum development. Interestingly, we show that depletion of oligodendrocytes at postnatal day 1 (P1) profoundly affects the establishment of cerebellar circuitries. We observed an impressive deregulation in the expression of molecules involved in axon growth, guidance and synaptic plasticity. These effects were accompanied by an outstanding increase of neurofilament staining observed 4 hours after the beginning of the ablation protocol, likely dependent from sprouting of cerebellar fibers. Oligodendrocyte ablation modifies localization and function of ionotropic glutamate receptors in Purkinje neurons. These results show a novel oligodendrocyte function expressed during early postnatal brain development, where these cells participate in the formation of cerebellar circuitries, and influence its development.

Decorin, erythroblastic leukaemia viral oncogene homologue B4 and signal transducer and activator of transcription 3 regulation of semaphorin 3A in central nervous system scar tissue

Scar tissue at sites of traumatic injury in the adult central nervous system presents a combined physical and molecular impediment to axon regeneration. Of multiple known central nervous system scar associated axon growth inhibitors, semaphorin 3A has been shown to be strongly expressed by invading leptomeningeal fibroblasts. We have previously demonstrated that infusion of the small leucine-rich proteoglycan decorin results in major suppression of several growth inhibitory chondroitin sulphate proteoglycans and growth of adult sensory axons across acute spinal cord injuries. Furthermore, decorin treatment of leptomeningeal fibroblasts significantly increases their ability to support neurite growth of co-cultured adult dorsal root ganglion neurons. In the present study we show that decorin has the ability to suppress semaphorin 3A expression within adult rat cerebral cortex scar tissue and in primary leptomeningeal fibroblasts in vitro. Infusion of decorin core protein for eight days resulted in a significant reduction of semaphorin 3A messenger RNA expression within injury sites compared with saline-treated control animals. Both in situ hybridization and immunostaining confirmed that semaphorin 3A messenger RNA expression and protein levels are significantly reduced in decorin-treated animals. Similarly, decorin treatment decreased the expression of semaphorin 3A messenger RNA in cultured rat leptomeningeal fibroblasts compared with untreated cells. Mechanistic studies revealed that decorin-mediated suppression of semaphorin 3A critically depends on erythroblastic leukaemia viral oncogene homologue B4 and signal transducer and activator of transcription 3 function. Collectively, our studies show that in addition to suppressing the levels of inhibitory chondroitin sulphate proteoglycans, decorin has the ability to suppress semaphorin 3A in the injured central nervous system. Our findings provide further evidence for the use of decorin as a potential therapy for promoting axonal growth and repair in the injured adult mammalian brain and spinal cord.

Animals lacking link protein have attenuated perineuronal nets and persistent plasticity

Chondroitin sulphate proteoglycans in the extracellular matrix restrict plasticity in the adult central nervous system and their digestion with chondroitinase reactivates plasticity. However the structures in the extracellular matrix that restrict plasticity are unknown. There are many changes in the extracellular matrix as critical periods for plasticity close, including changes in chondroitin sulphate proteoglycan core protein levels, changes in glycosaminoglycan sulphation and the appearance of dense chondroitin sulphate proteoglycan-containing perineuronal nets around many neurons. We show that formation of perineuronal nets is triggered by neuronal production of cartilage link protein Crtl1 (Hapln1), which is up-regulated in the visual cortex as perineuronal nets form during development and after dark rearing. Mice lacking Crtl1 have attenuated perineuronal nets, but the overall levels of chondroitin sulphate proteoglycans and their pattern of glycan sulphation are unchanged. Crtl1 knockout animals retain juvenile levels of ocular dominance plasticity and their visual acuity remains sensitive to visual deprivation. In the sensory pathway, axons in knockout animals but not controls sprout into the party denervated cuneate nucleus. The organization of chondroitin sulphate proteoglycan into perineuronal nets is therefore the key event in the control of central nervous system plasticity by the extracellular matrix.

Guidance molecules in synapse formation and plasticity

A major goal of modern neuroscience research is to understand the cellular and molecular processes that control the formation, function, and remodeling of chemical synapses. In this article, we discuss the numerous studies that implicate molecules initially discovered for their functions in axon guidance as critical regulators of synapse formation and plasticity. Insights from these studies have helped elucidate basic principles of synaptogenesis, dendritic spine formation, and structural and functional synapse plasticity. In addition, they have revealed interesting dual roles for proteins and cellular mechanisms involved in both axon guidance and synaptogenesis. Much like the dual involvement of morphogens in early cell fate induction and axon guidance, many guidance-related molecules continue to play active roles in controlling the location, number, shape, and strength of neuronal synapses during development and throughout the lifetime of the organism. This article summarizes key findings that link axon guidance molecules to specific aspects of synapse formation and plasticity and discusses the emerging relationship between the molecular and cellular mechanisms that control both axon guidance and synaptogenesis.

Neuropilin 1 directly interacts with Fer kinase to mediate semaphorin 3A-induced death of cortical neurons

Neuropilins (NRPs) are receptors for the major chemorepulsive axonal guidance cue semaphorins (Sema). The interaction of Sema3A/NRP1 during development leads to the collapse of growth cones. Here we show that Sema3A also induces death of cultured cortical neurons through NRP1. A specific NRP1 inhibitory peptide ameliorated Sema3A-evoked cortical axonal retraction and neuronal death. Moreover, Sema3A was also involved in cerebral ischemia-induced neuronal death. Expression levels of Sema3A and NRP1, but not NRP2, were significantly increased early during brain reperfusion following transient focal cerebral ischemia. NRP1 inhibitory peptide delivered to the ischemic brain was potently neuroprotective and prevented the loss of motor functions in mice. The integrity of the injected NRP1 inhibitory peptide into the brain remained unchanged, and the intact peptide permeated the ischemic hemisphere of the brain as determined using MALDI-MS-based imaging. Mechanistically, NRP1-mediated axonal collapse and neuronal death is through direct and selective interaction with the cytoplasmic tyrosine kinase Fer. Fer RNA interference effectively attenuated Sema3A-induced neurite retraction and neuronal death in cortical neurons. More importantly, down-regulation of Fer expression using Fer-specific RNA interference attenuated cerebral ischemia-induced brain damage. Together, these studies revealed a previously unknown function of NRP1 in signaling Sema3A-evoked neuronal death through Fer in cortical neurons.

Infragranular gene expression disturbances in the prefrontal cortex in schizophrenia: signature of altered neural development?

The development of the human neocortex gives rise to a complex cytoarchitecture, grouping together cells with similar structure, connectivity and function. As a result, the six neocortical laminae show distinct molecular content. In schizophrenia, many anatomical and neurochemical changes appear to be restricted to a subset of lamina and/or cell types. In this study, we hypothesized that supragranular (SG; laminae II-III) and infragranular layers (IG; laminae V-VI) of area 46 in the human prefrontal cortex will show distinct and specific transcriptome alterations between subjects with schizophrenia and matched controls. To enhance sample homogeneity, we compared the gene expression patterns of the SG and IG layers of 8 matched middle-aged male subjects with schizophrenia to 8 pairwise matched controls using two replicate DNA microarrays for each sample. The study revealed strong disease-related laminar expression differences between the SG and IG layers. Expression changes were dominated by an overall underexpression of the IG-enriched genes in the schizophrenia subjects compared to normal control subjects. Furthermore, using a diagnosis-blind, unsupervised clustering of the control-derived SG or IG-enriched transcripts, the IG-enriched markers segregated the subjects with schizophrenia from the matched controls with a high degree of confidence. Importantly, multiple members of the semaphorin gene family reported altered gene expression, suggesting that the IG gene expression disturbances in subjects with schizophrenia may be a result of altered cortical development and disrupted brain connectivity.

Secreted semaphorins control spine distribution and morphogenesis in the postnatal CNS

The majority of excitatory synapses in the mammalian CNS are formed on dendritic spines1, and spine morphology and distribution are critical for synaptic transmission2–6, synaptic integration and plasticity7. Here, we show that a secreted semaphorin, Sema3F, is a negative regulator of spine development and synaptic structure. Mice with null mutations in genes encoding Sema3F, and its holoreceptor components neuropilin-2 (Npn-2) and plexinA3 (PlexA3), exhibit increased dentate gyrus (DG) granule cell (GC) and cortical layer V pyramidal neuron spine number and size, and also aberrant spine distribution. Moreover, Sema3F promotes loss of spines and excitatory synapses in dissociated neurons in vitro, and in Npn-2^−/− brain slices cortical layer V and DG GCs exhibit increased mEPSC frequency. In contrast, a distinct Sema3A–Npn-1/PlexA4 signaling cascade controls basal dendritic arborization in layer V cortical neurons but does not influence spine morphogenesis or distribution. These disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of Npn-2 to apical dendrites and of Npn-1 to all dendrites of cortical pyramidal neurons. Therefore, Sema3F signaling controls spine distribution along select dendritic processes, and distinct secreted semaphorin signaling events orchestrate CNS connectivity through the differential control of spine morphogenesis, synapse formation, and the elaboration of dendritic morphology.

An atypical role for collapsin response mediator protein 2 (CRMP-2) in neurotransmitter release via interaction with presynaptic voltage-gated calcium channels

Collapsin response mediator proteins (CRMPs) specify axon/dendrite fate and axonal growth of neurons through protein-protein interactions. Their functions in presynaptic biology remain unknown. Here, we identify the presynaptic N-type Ca(2+) channel (CaV2.2) as a CRMP-2-interacting protein. CRMP-2 binds directly to CaV2.2 in two regions: the channel domain I-II intracellular loop and the distal C terminus. Both proteins co-localize within presynaptic sites in hippocampal neurons. Overexpression in hippocampal neurons of a CRMP-2 protein fused to enhanced green fluorescent protein caused a significant increase in Ca(2+) channel current density, whereas lentivirus-mediated CRMP-2 knockdown abolished this effect. Interestingly, the increase in Ca(2+) current density was not due to a change in channel gating. Rather, cell surface biotinylation studies showed an increased number of CaV2.2 at the cell surface in CRMP-2-overexpressing neurons. These neurons also exhibited a significant increase in vesicular release in response to a depolarizing stimulus. Depolarization of CRMP-2-enhanced green fluorescent protein-overexpressing neurons elicited a significant increase in release of glutamate compared with control neurons. Toxin block of Ca(2+) entry via CaV2.2 abolished this stimulated release. Thus, the CRMP-2-Ca(2+) channel interaction represents a novel mechanism for modulation of Ca(2+) influx into nerve terminals and, hence, of synaptic strength.

Modulation of synaptic transmission and plasticity by cell adhesion and repulsion molecules

Adhesive and repellent molecular cues guide migrating cells and growing neurites during development. They also contribute to synaptic function, learning and memory in adulthood. Here, we review the roles of cell adhesion molecules of the immunoglobulin superfamily (Ig-CAMs) and semaphorins (some of which also contain Ig-like domains) in regulation of synaptic transmission and plasticity. Interestingly, among the seven studied Ig-CAMs, the neuronal cell adhesion molecule proved to be important for all tested forms of hippocampal plasticity, while its associated unusual glycan polysialic acid is necessary and sufficient part for synaptic plasticity only at CA3-CA1 synapses. In contrast, Thy-1 and L1 specifically regulate long-term potentiation (LTP) at synapses formed by entorhinal axons in the dentate gyrus and cornu ammonis, respectively. Contactin-1 is important for long-term depression but not for LTP at CA3-CA1 synapses. Analysis of CHL1-deficient mice illustrates that at intermediate stages of development a deficit in a cell adhesion molecule is compensated but appears as impaired LTP during early and late postnatal development. The emerging mechanisms by which adhesive Ig-CAMs contribute to synaptic plasticity involve regulation of activities of NMDA receptors and L-type Ca2+ channels, signaling via mitogen-activated protein kinase p38, changes in GABAergic inhibition and motility of synaptic elements. Regarding repellent molecules, available data for semaphorins demonstrate their activity-dependent regulation in normal and pathological conditions, synaptic localization of their receptors and their potential to elevate or inhibit synaptic transmission either directly or indirectly.