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

Sema-3E/PlexinD1 axis modulates dendritic cell phenotypes and functions: Current status and future implications

This comprehensive research review explores the complex interplay between the Sema-3E/PlexinD1 axis and dendritic cells (DCs), highlighting its critical role in immune modulation with implications for clinical application Critical regulators of immune responses Dendritic cells are central to adaptive immunity, and the Sema-3E /PlexinD1 axis emerges as a key modulator affecting their phenotypes and functions Review delineates the impact of this signaling axis on DC maturation, migration, antigen presentation, and cytokine production, unravels its multifaceted role in shaping the immune response. Recognizing the limitations and gaps in current knowledge, the study highlights the need for further studies to condition downstream signaling events and related information experienced by the Sema-3E/PlexinD1 axis emphasizes the clarity of the immune system. The review concludes by identifying opportunities for translation, focusing on therapeutic and diagnostic potential. It highlights the importance of collaborative, interdisciplinary efforts to address the challenges and harness the therapeutic and pathological potential of targeting the Sema-3E/PlexinD1 axis, thus opening the way for transformative advances in immunology and clinical medicine.

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.

Neuronal diversity and reciprocal connectivity between the vertebrate hippocampus and septum

A hallmark of the nervous system is the precision with which myriad cell types are integrated into functional networks that control complex behaviors. The limbic system governs evolutionarily conserved processes essential for survival. The septum and the hippocampus are central to the limbic system, and control not only emotion-related behaviors but also learning and memory. Here, we provide a developmental and evolutionary perspective of the hippocampus and septum and highlight the neuronal diversity and circuitry that connects these two central components of the limbic system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles Comparative Development and Evolution > Regulation of Organ Diversity.

Characterization of plexinA and two distinct semaphorin1a transcripts in the developing and adult cricket Gryllus bimaculatus

Guidance cues act during development to guide growth cones to their proper targets in both the central and peripheral nervous systems. Experiments in many species indicate that guidance molecules also play important roles after development, though less is understood about their functions in the adult. The Semaphorin family of guidance cues, signaling through Plexin receptors, influences the development of both axons and dendrites in invertebrates. Semaphorin functions have been extensively explored in Drosophila melanogaster and some other Dipteran species, but little is known about their function in hemimetabolous insects. Here, we characterize sema1a and plexA in the cricket Gryllus bimaculatus. In fact, we found two distinct predicted Sema1a proteins in this species, Sema1a.1 and Sema1a.2, which shared only 48% identity at the amino acid level. We include a phylogenetic analysis that predicted that many other insect species, both holometabolous and hemimetabolous, express two Sema1a proteins as well. Finally, we used in situ hybridization to show that sema1a.1 and sema1a.2 expression patterns were spatially distinct in the embryo, and both roughly overlap with plexA. All three transcripts were also expressed in the adult brain, mainly in the mushroom bodies, though sema1a.2 was expressed most robustly. sema1a.2 was also expressed strongly in the adult thoracic ganglia while sema1a.1 was only weakly expressed and plexA was undetectable.

Crmp4-KO Mice as an Animal Model for Investigating Certain Phenotypes of Autism Spectrum Disorders

Previous research has demonstrated that the collapsin response mediator protein (CRMP) family is involved in the formation of neural networks. A recent whole-exome sequencing study identified a de novo variant (S541Y) of collapsin response mediator protein 4 (CRMP4) in a male patient with autism spectrum disorder (ASD). In addition, Crmp4-knockout (KO) mice show some phenotypes similar to those observed in human patients with ASD. For example, compared with wild-type mice, Crmp4-KO mice exhibit impaired social interaction, abnormal sensory sensitivities, broader distribution of activated (c-Fos expressing) neurons, altered dendritic formation, and aberrant patterns of neural gene expressions, most of which have sex differences. This review summarizes current knowledge regarding the role of CRMP4 during brain development and discusses the possible contribution of CRMP4 deficiencies or abnormalities to the pathogenesis of ASD. Crmp4-KO mice represent an appropriate animal model for investigating the mechanisms underlying some ASD phenotypes, such as impaired social behavior, abnormal sensory sensitivities, and sex-based differences, and other neurodevelopmental disorders associated with sensory processing disorders.

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.

Large-Scale Cognitive GWAS Meta-Analysis Reveals Tissue-Specific Neural Expression and Potential Nootropic Drug Targets

Here, we present a large (n = 107,207) genome-wide association study (GWAS) of general cognitive ability ("g"), further enhanced by combining results with a large-scale GWAS of educational attainment. We identified 70 independent genomic loci associated with general cognitive ability. Results showed significant enrichment for genes causing Mendelian disorders with an intellectual disability phenotype. Competitive pathway analysis implicated the biological processes of neurogenesis and synaptic regulation, as well as the gene targets of two pharmacologic agents: cinnarizine, a T-type calcium channel blocker, and LY97241, a potassium channel inhibitor. Transcriptome-wide and epigenome-wide analysis revealed that the implicated loci were enriched for genes expressed across all brain regions (most strongly in the cerebellum). Enrichment was exclusive to genes expressed in neurons but not oligodendrocytes or astrocytes. Finally, we report genetic correlations between cognitive ability and disparate phenotypes including psychiatric disorders, several autoimmune disorders, longevity, and maternal age at first birth.

Cornu Ammonis Regions-Antecedents of Cortical Layers?

Studying neocortex and hippocampus in parallel, we are struck by the similarities. All three to four layered allocortices and the six layered mammalian neocortex arise in the pallium. All receive and integrate multiple cortical and subcortical inputs, provide multiple outputs and include an array of neuronal classes. During development, each cell positions itself to sample appropriate local and distant inputs and to innervate appropriate targets. Simpler cortices had already solved the need to transform multiple coincident inputs into serviceable outputs before neocortex appeared in mammals. Why then do phylogenetically more recent cortices need multiple pyramidal cell layers? A simple answer is that more neurones can compute more complex functions. The dentate gyrus and hippocampal CA regions-which might be seen as hippocampal antecedents of neocortical layers-lie side by side, albeit around a tight bend. Were the millions of cells of rat neocortex arranged in like fashion, the surface area of the CA pyramidal cell layers would be some 40 times larger. Even if evolution had managed to fold this immense sheet into the space available, the distances between neurones that needed to be synaptically connected would be huge and to maintain the speed of information transfer, massive, myelinated fiber tracts would be needed. How much more practical to stack the "cells that fire and wire together" into narrow columns, while retaining the mechanisms underlying the extraordinary precision with which circuits form. This demonstrably efficient arrangement presents us with challenges, however, not the least being to categorize the baffling array of neuronal subtypes in each of five "pyramidal layers." If we imagine the puzzle posed by this bewildering jumble of apical dendrites, basal dendrites and axons, from many different pyramidal and interneuronal classes, that is encountered by a late-arriving interneurone insinuating itself into a functional circuit, we can perhaps begin to understand why definitive classification, covering every aspect of each neurone's structure and function, is such a challenge. Here, we summarize and compare the development of these two cortices, the properties of their neurones, the circuits they form and the ordered, unidirectional flow of information from one hippocampal region, or one neocortical layer, to another.

The GABAergic septohippocampal connection is impaired in a mouse model of tauopathy

Alzheimer's disease (AD), the most common cause of dementia nowadays, has been linked to alterations in the septohippocampal pathway (SHP), among other circuits in the brain. In fact, the GABAergic component of the SHP, which controls hippocampal rhythmic activity crucial for learning and memory, is altered in the J20 mouse model of AD-a model that mimics the amyloid pathology of this disease. However, AD is characterized by another pathophysiological hallmark: the hyperphosphorylation and aggregation of the microtubule-associated protein Tau. To evaluate whether tauopathies alter the GABAergic SHP, we analyzed transgenic mice expressing human mutated Tau (mutations G272V, P301L, and R406W, VLW transgenic strain). We show that pyramidal neurons, mossy cells, and some parvalbumin (PARV)-positive hippocampal interneurons in 2- and 8-month-old (mo) VLW mice accumulate phosphorylated forms of Tau (P-Tau). By tract-tracing studies of the GABAergic SHP, we describe early-onset deterioration of GABAergic septohippocampal (SH) innervation on PARV-positive interneurons in 2-mo VLW mice. In 8-mo animals, this alteration was more severe and affected mainly P-Tau-accumulating PARV-positive interneurons. No major loss of GABAergic SHP neurons or PARV-positive hippocampal interneurons was observed, thereby indicating that this decline is not caused by neuronal loss but by the reduced number and complexity of GABAergic SHP axon terminals. The decrease in GABAergic SHP described in this study, targeted onto the PARV-positive/P-Tau-accumulating inhibitory neurons in the hippocampus, establishes a cellular correlation with the dysfunctions in rhythmic neuronal activity and excitation levels in the hippocampus. These dysfunctions are associated with the VLW transgenic strain in particular and with AD human pathology in general. These data, together with our previous results in the J20 mouse model, indicate that the GABAergic SHP is impaired in response to both amyloid-β and P-Tau accumulation. We propose that alterations in the GABAergic SHP, together with a dysfunction of P-Tau-accumulating PARV-positive neurons, contribute to the cognitive deficits and altered patterns of hippocampal activity present in tauopathies, including AD.

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.

Periosteum Metabolism and Nerve Fiber Positioning Depend on Interactions between Osteoblasts and Peripheral Innervation in Rat Mandible

The sympathetic nervous system controls bone remodeling by regulating bone formation and resorption. How nerves and bone cells influence each other remains elusive. Here we modulated the content or activity of the neuropeptide Vasoactive Intestinal Peptide to investigate nerve-bone cell interplays in the mandible periosteum by assessing factors involved in nerve and bone behaviors. Young adult rats were chemically sympathectomized or treated with Vasoactive Intestinal Peptide or Vasoactive Intestinal Peptide_10-28, a receptor antagonist. Sympathectomy depleted the osteogenic layer of the periosteum in neurotrophic proNerve Growth Factor and neurorepulsive semaphorin3a; sensory Calcitonin-Gene Related Peptide-positive fibers invaded this layer physiologically devoid of sensory fibers. In the periosteum non-osteogenic layer, sympathectomy activated mast cells to release mature Nerve Growth Factor while Calcitonin-Gene Related Peptide-positive fibers increased. Vasoactive Intestinal Peptide treatment reversed sympathectomy effects. Treating intact animals with Vasoactive Intestinal Peptide increased proNerve Growth Factor expression and stabilized mast cells. Vasoactive Intestinal Peptide_10-28 treatment mimicked sympathectomy effects. Our data suggest that sympathetic Vasoactive Intestinal Peptide modulate the interactions between nervous fibers and bone cells by tuning expressions by osteogenic cells of factors responsible for mandible periosteum maintenance while osteogenic cells keep nervous fibers at a distance from the bone surface.

Semaphorin 3F and neuropilin-2 control the migration of human T-cell precursors

Neuropilins and semaphorins are known as modulators of axon guidance, angiogenesis, and organogenesis in the developing nervous system, but have been recently evidenced as also playing a role in the immune system. Here we describe the expression and role of semaphorin 3F (SEMA3F) and its receptor neuropilin-2 (NRP2) in human T cell precursors. NRP2 and SEMA3F are expressed in the human thymus, in both lymphoid and non-lymphoid compartments. SEMA3F have a repulsive effect on thymocyte migration and inhibited CXCL12- and sphingosine-1-phosphate (S1P)-induced thymocyte migration by inhibiting cytoskeleton reorganization prior to stimuli. Moreover, NRP2 and SEMA3F are expressed in human T-cell acute lymphoblastic leukemia/lymphoma primary cells. In these tumor cells, SEMA3F also blocks their migration induced by CXCL12 and S1P. Our data show that SEMA3F and NRP2 are further regulators of human thymocyte migration in physiological and pathological conditions.

Accelerated aging of the GABAergic septohippocampal pathway and decreased hippocampal rhythms in a mouse model of Alzheimer's disease

Patients with Alzheimer's disease (AD) display altered functioning of cortical networks, including altered patterns of synchronous activity and a serious deficit in cholinergic septohippocampal (SH) innervation. However, the mechanisms underlying these alterations and the implication of the GABAergic SH component in AD are largely unknown. In addition, the GABAergic septohippocampal pathway (SHP) is believed to regulate synchronous hippocampal activity by controlling the activity of interneurons. Here we show, using well-characterized pathway tracing experiments, that innervation of the GABAergic SHP decreases during normal aging. Furthermore, in an AD mouse model (hAPP(Sw,Ind); J20 mice), the GABAergic SHP shows a dramatic and early onset of this decrease in 8-mo-old mice. This decline is not caused by neuronal loss, but by the reduced number and complexity of GABAergic SH axon terminals. Finally, we demonstrate that hippocampal θ and γ rhythm power spectra are markedly diminished in 8-mo-old behaving mice expressing mutated hAPP. In addition to the well-known loss of cholinergic input to the hippocampus in AD, these data suggest that the altered patterns of synchronous activity seen in patients with AD could be caused by the loss of GABAergic SH axons, which modulate hippocampal network activities.

Semaphorin 3C is not required for the establishment and target specificity of the GABAergic septohippocampal pathway in vitro

The septohippocampal (SH) pathway comprises cholinergic and GABAergic fibers. Whereas the former establish synaptic contacts with all types of hippocampal neurons, the latter form complex baskets specifically on interneurons. The GABAergic SH function is associated with the control of hippocampal synchronous networks. Little is known about the mechanisms involved in the formation of the GABAergic SH pathway. Semaphorin (Sema) 3C is expressed in most hippocampal interneurons targeted by these axons. To ascertain whether Sema 3C influences the formation of the SH pathway, we analyzed the development of this connection in Sema 3C-deficient mice. As these animals die at birth, we developed an in vitro organotypic co-culture model reproducing the postnatal development of the SH pathway. In these SH co-cultures, the GABAergic SH pathway developed with target specificity similar to that present in vivo. SH axons formed incipient baskets on several types of hippocampal interneurons at 7 days in vitro, which increased their complexity by 18-25 days in vitro. These SH fibers formed symmetric synaptic contacts on GABAergic interneurons. This synaptic specificity was not influenced by the absence of entorhinal afferents. Finally, the absence of Sema 3C in target neurons or its blockage by neuropilin-1 and -2 ectodomains in slice co-cultures did not lead to major changes in either the target specificity of the GABAergic SH pathway or its density of innervation. We conclude that the formation and synaptic specificity of the GABAergic SH pathway relies on robust molecular mechanisms, independent of Sema 3C, that are retained in our in vitro co-culture model.

Fragile X syndrome: the GABAergic system and circuit dysfunction

Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.

GWAS findings for human iris patterns: associations with variants in genes that influence normal neuronal pattern development

Human iris patterns are highly variable. The origins of this variation are of interest in the study of iris-related eye diseases and forensics, as well as from an embryological developmental perspective, with regard to their possible relationship to fundamental processes of neurodevelopment. We have performed genome-wide association scans on four iris characteristics (crypt frequency, furrow contractions, presence of peripupillary pigmented ring, and number of nevi) in three Australian samples of European descent. Both the discovery (n = 2121) and replication (n = 499 and 73) samples showed evidence for association between (1) crypt frequency and variants in the axonal guidance gene SEMA3A (p = 6.6 × 10(-11)), (2) furrow contractions and variants within the cytoskeleton gene TRAF3IP1 (p = 2.3 × 10(-12)), and (3) the pigmented ring and variants in the well-known pigmentation gene SLC24A4 (p = 7.6 × 10(-21)). These replicated findings individually accounted for around 1.5%-3% of the variance for these iris characteristics. Because both SEMA3A and TRAFIP1 are implicated in pathways that control neurogenesis, neural migration, and synaptogenesis, we also examined the evidence of enhancement among such genes, finding enrichment for crypts and furrows. These findings suggest that genes involved in normal neuronal pattern development may also influence tissue structures in the human iris.

Semaphorin 3F is a bifunctional guidance cue for dopaminergic axons and controls their fasciculation, channeling, rostral growth, and intracortical targeting

Dopaminergic neurons in the mesodiencephalon (mdDA neurons) make precise synaptic connections with targets in the forebrain via the mesostriatal, mesolimbic, and mesoprefrontal pathways. Because of the functional importance of these remarkably complex ascending axon pathways and their implication in human disease, the mechanisms underlying the development of these connections are of considerable interest. Despite extensive in vitro studies, the molecular determinants that ensure the perfect formation of these pathways in vivo remain mostly unknown. Here, we determine the embryonic origin and ontogeny of the mouse mesoprefrontal pathway and use these data to reveal an unexpected requirement for semaphorin 3F (Sema3F) and its receptor neuropilin-2 (Npn-2) during mdDA pathway development using tissue culture approaches and analysis of sema3F(-/-), npn-2(-/-), and npn-2(-/-);TH-Cre mice. We show that Sema3F is a bifunctional guidance cue for mdDA axons, some of which have the remarkable ability to regulate their responsiveness to Sema3F as they develop. During early developmental stages, Sema3F chemorepulsion controls previously uncharacterized aspects of mdDA pathway development through both Npn-2-dependent (axon fasciculation and channeling) and Npn-2-independent (rostral growth) mechanisms. Later on, chemoattraction mediated by Sema3F and Npn-2 is required to orient mdDA axon projections in the cortical plate of the medial prefrontal cortex. This latter finding demonstrates that regulation of axon orientation in the target field occurs by chemoattractive mechanisms, and this is likely to also apply to other neural systems. In all, this study provides a framework for additional dissection of the molecular basis of mdDA pathway development and disease.

Implication of neuropilin 2/semaphorin 3F in retinocollicular map formation

Neural representations of the environment within the brain take the form of topographic maps whose formation relies on graded expression of axon guidance molecules. Retinocollicular map formation, from retinal ganglion cells (RGCs) to the superior colliculus (SC) in the midbrain, is mainly driven by Eph receptors and their ligands ephrins. However, other guidance molecules participate in the formation of this map. Here we demonstrate that the receptor Neuropilin-2 is expressed in an increasing nasal-temporal gradient in RGCs, whereas one of its ligands, Semaphorin3F, but not other Sema3 molecules, presents a graded low-rostral to high-caudal expression in the SC when mapping is underway. Neuropilin-2 and its coreceptor Plexin A1 are present on RGC growth cones. Collapse assays demonstrate that Semaphorin3F induces significant growth cone collapse of temporal, but not nasal, RGCs expressing high levels of Neuropilin-2. Our results suggest that Neuropilin-2/Semaphorin3F are new candidates involved in retinotopy formation within the SC.

Axon guidance in the dopamine system

Meso-diencephalic dopamine neurons (mdDA) neurons are located in the retrorubral field (RRF), substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) and give rise to prominent ascending axon projections. These so-called mesotelencephalic projections are organized into three main pathways: the mesostriatal, mesocortical and mesolimbic pathways. Mesotelencephalic pathways in the adult nervous system have been studied in much detail as a result of their important physiological functions and their implication in psychiatric, neurological and neurodegenerative disease. In comparison, relatively little is known about the formation of these projection systems during embryonic and postnatal development. However, understanding the formation of mdDA neurons and their projections is essential for the design of effective therapies for mdDA neuron-associated neurological and neurodegenerative disorders. Here we summarize our current knowledge of the ontogeny of mdDA axon projections in subsystems of the developing rodent central nervous system (CNS) and discuss the cellular and molecular mechanisms that mediate mdDA axon guidance in these CNS regions.

Effects of ethanol on axon outgrowth and branching in developing rat cortical neurons

Humans exposed prenatally to ethanol can exhibit brain abnormalities and cognitive impairment similar to those seen in patients expressing mutant forms of the L1 cell adhesion molecule (L1CAM). The resemblance suggests that L1CAM may be a target for ethanol, and consistent with this idea, ethanol can inhibit L1CAM adhesion in cell lines and L1CAM-mediated outgrowth and signaling in cerebellar granule neurons. However, it is not known whether ethanol inhibits L1CAM function in other neuron types known to require L1CAM for appropriate development. Here we asked whether ethanol alters L1CAM function in neurons of the rat cerebral cortex. We find that ethanol does not alter axonal polarization, L1CAM-dependent axon outgrowth or branching, or L1CAM recycling in axonal growth cones. Thus, ethanol inhibition of L1CAM is highly dependent on neuronal context.

Semaphorin-3A and its receptor neuropilin-1 are predominantly expressed in endothelial cells along the rostral migratory stream of young and adult mice

In the adult brain, neuroblasts originating in the subventricular zone migrate through the rostral migratory stream to the olfactory bulb. While migrating, neuroblasts undergo progressive differentiation until reaching their final locations and fates. Because molecules involved in migration may also exert differentiating effects on young neurons, the identification of factors that support migration could also shed light on the processes of adult neuroblast differentiation. This is the case for members of the family of semaphorins and of its cognate receptors, the neuropilins. Here, we have evaluated the presence of semaphorin-3A and of its receptor neuropilin-1 along the rostral migratory stream in young and adult mice by using immunocytochemical, histochemical, and in situ hybridization techniques. Our morphological studies show that semaphorin-3A and neuropilin-1 are both mainly expressed on endothelial cells along the rostral migratory stream during postnatal development. Our results suggest that endothelial cells constitute the primary source and target of semaphorin-3A along the rostral migratory stream. Moreover, the present work outlines the potential role of blood vessels on neuroblast migration in the postnatal rostral migratory stream.

Semaphorins 3A, 3C, and 3F in mesencephalic dopaminergic axon pathfinding

By analyzing the mechanisms that govern dopaminergic axon pathfinding from the midbrain to the striatum in embryonic rat brains, we identified neuroepithelial regions that exert chemotropic effects on mesencephalic dopaminergic axons. Explants from the pretectum and the striatum showed an attractive effect, whereas those from the midhindbrain boundary, the dorsal thalamus, and the ventral thalamus had no effect. Expression of semaphorin (Sema) 3C and Sema3F in the pretectum and of Sema3A in the striatum suggested a role for these axon guidance molecules in dopaminergic axon pathfinding. When expressed in HEK293 cell aggregates, Sema3C had an attractive effect and enhanced axon growth, Sema3A enhanced axon growth, and Sema3F had a repulsive effect on dopaminergic axons. Antineuropilin-1 and antineuropilin-2 antibodies reduced attraction by the pretectum, whereas attraction by the striatum was not affected by the presence of antineuropilin-1 antibodies. Moreover, neuropilin-1- and neuropilin-2-soluble Fc chimeras reduced the attraction by the pretectum. These results suggest that semaphorins may help to establish the dopaminergic projection from the midbrain to the striatum during embryonic development.

Factors defining a pacemaker region for synchrony in the hippocampus

Synchronous activities of neuronal populations are often initiated in a pacemaker region and spread to recruit other regions. Here we examine factors that define a pacemaker site. The CA3a region acts as the pacemaker for disinhibition induced synchrony in guinea pig hippocampal slices and CA3b is a follower region. We found CA3a pyramidal cells were more excitable and fired in bursts more frequently than CA3b cells. CA3a cells had more complex dendritic arbors than CA3b cells especially in zones targetted by recurrent synapses. The product of the density of pyramidal cell axon terminals and dendritic lengths in innervated zones predicted a higher recurrent synaptic connectivity in the CA3a than in the CA3b region. We show that some CA3a cells but few CA3b cells behave as pacemaker cells by firing early during population events and by recruiting follower cells to fire. With a greater excitability and enhanced synaptic connectivity these CA3a cells may also possess initiating functions for other hippocampal ensemble activities initiated in this region.

Developmental and adult expression of semaphorin 2a in the cricketGryllus bimaculatus

Developmental guidance cues act to direct growth cones to their correct targets in the nervous system. Recent experiments also demonstrate that developmental cues are expressed in the adult mammalian nervous system, although their function in the brain is not yet clear. The semaphorin gene family has been implicated in the growth of dendrites and axons in a number of different species. While the expression of semaphorin and its influence on tibial pioneer neurons in the developing limb bud have been well characterized in the grasshopper, the expression of semaphorin 2a (sema2a) has not been explored in the adult insect. In this study we used polymerase chain reaction (PCR) with degenerate and gene-specific primers to clone part of the secreted form of sema2a from Gryllus bimaculatus. Using in situ hybridization and immunohistochemistry, we confirmed that sema2a mRNA and protein expression patterns in the embryonic cricket were similar to that seen in the grasshopper. We also showed that tibial neuron development in crickets was comparable to that described in grasshopper. An examination of both developing and adult cricket brains showed that sema2a mRNA and protein were expressed in the Kenyon cells in mushroom bodies, an area involved in learning and memory. Sema2a expression was most obvious near the apex of the mushroom body in a region surrounding the neurogenic tip, which produces neurons throughout the life of the cricket. We discuss the role of neurogenesis in learning and memory and the potential involvement of semaphorin in this process.

Hippocampal CA3 pyramidal cells selectively innervate aspiny interneurons

The specific connectivity among principal cells and interneurons determines the flow of activity in neuronal networks. To elucidate the connections between hippocampal principal cells and various classes of interneurons, CA3 pyramidal cells were intracellularly labelled with biocytin in anaesthetized rats and the three-dimensional distribution of their axon collaterals was reconstructed. The sections were double-stained for substance P receptor (SPR)- or metabotropic glutamate receptor 1alpha (mGluR-1alpha)-immunoreactivity to investigate interneuron targets of the CA3 pyramidal cells. SPR-containing interneurons represent a large portion of the GABAergic population, including spiny and aspiny classes. Axon terminals of CA3 pyramidal cells contacted SPR-positive interneuron dendrites in the hilus and in all hippocampal strata in both CA3 and CA1 regions (7.16% of all boutons). The majority of axons formed single contacts (87.5%), but multiple contacts (up to six) on single target neurons were also found. CA3 pyramidal cell axon collaterals innervated several types of morphologically different aspiny SPR-positive interneurons. In contrast, spiny SPR-interneurons or mGluR-1alpha-positive interneurons in the hilus, CA3 and CA1 regions were rarely contacted by the filled pyramidal cells. These findings indicate a strong target selection of CA3 pyramidal cells favouring the activation of aspiny classes of interneurons.

Semaphorin 3C regulates endothelial cell function by increasing integrin activity

Class 3 semaphorins (sema 3) are secreted guidance proteins. Sema 3A expressed by endothelial cells controls vascular morphogenesis through integrin inhibition. Sema 3C is required for normal cardiovascular patterning. Here we examined the potential role of sema 3C as regulator of endothelial cell function in vitro using mouse glomerular endothelial cells (MGEC). We determined that MGEC express sema 3C mRNA and protein and its receptors mRNA. Recombinant sema 3C induced MGEC proliferation 18 +/- 2% above control, as assessed by bromodeoxyuridine (BrdU) incorporation, and reduced starvation-induced apoptosis by 46 +/- 3%, as indicated by an in situ marker of activated caspase 3. Sema 3C increased MGEC adhesion to fibronectin 79 +/- 13% and to collagen 55 +/- 12% as compared with control. Sema 3C-induced MGEC adhesion was prevented by integrin blocking antibodies and involved beta1 integrin serine phosphorylation. Sema 3C-induced MGEC adhesion and proliferation were similar to those induced by vascular endothelial growth factor (VEGF)-A. Sema 3C induced a 44 +/- 11% increase in MGEC directional migration and stimulated MGEC capillary-like network formation on collagen I gels. Collectively, our data indicate that sema 3C promotes glomerular endothelial cell proliferation, adhesion, directional migration, and tube formation in vitro by stimulating integrin phosphorylation and VEGF120 secretion, functions that are similar to VEGF-A and opposite to sema 3A.