PlexinA1 signaling directs the segregation of proprioceptive sensory axons in the developing spinal cord

Calcium-binding protein parvalbumin in the spinal cord and dorsal root ganglia

Parvalbumin is one of the calcium-binding proteins. In the spinal cord, it is mainly expressed in inhibitory neurons; in the dorsal root ganglia, it is expressed in proprioceptive neurons. In contrast to in the brain, weak systematization of parvalbumin-expressing neurons occurs in the spinal cord. The aim of this paper is to provide a systematic review of parvalbumin-expressing neuronal populations throughout the spinal cord and the dorsal root ganglia of mammals, regarding their mapping, co-expression with some functional markers. The data reviewed are mostly concerning rodentia species because they are predominantly presented in literature.

Semaphorin 6 Family-An Important Yet Overlooked Group of Signaling Proteins Involved in Cancerogenesis

According to recent evidence, some groups of semaphorins (SEMAs) have been associated with cancer progression. These proteins are able to modulate the cellular signaling of particular receptor tyrosine kinases (RTKs) via the stimulation of SEMA-specific coreceptors, namely plexins (plexin-A, -B, -C, -D) and neuropilins (Np1, Np2), which share common domains with RTKs, leading to the coactivation of the latter receptors. MET, ERBB2, VEGFR2, PFGFR, and EGFR, among others, represent acknowledged targets of semaphorins that are often associated with tumor progression or poor prognosis. In particular, higher expression of SEMA6 family proteins in cancer cells and stromal cells of the cancer niche is often associated with enhanced tumor angiogenesis, metastasis, and resistance to anticancer therapy. Notably, high SEMA6 expression in malignant tumor cells such as melanoma, pleural mesothelioma, gastric cancer, lung adenocarcinoma, and glioblastoma may serve as a prognostic biomarker of tumor progression. To date, very few studies have focused on the mechanisms of transmembrane SEMA6-driven tumor progression and its underlying interplay with RTKs within the tumor microenvironment. This review presents the growing evidence in the literature on the complex and shaping role of SEMA6 family proteins in cancer responsiveness to environmental stimuli.

Axon Fasciculation, Mediated by Transmembrane Semaphorins, Is Critical for the Establishment of Segmental Specificity of Corticospinal Circuits

Axon fasciculation is thought to be a critical step in neural circuit formation and function. Recent studies have revealed various molecular mechanisms that underlie axon fasciculation; however, the impacts of axon fasciculation, and its corollary, defasciculation, on neural circuit wiring remain unclear. Corticospinal (CS) neurons in the sensorimotor cortex project axons to the spinal cord to control skilled movements. In rodents, the axons remain tightly fasciculated in the brain and traverse the dorsal funiculus of the spinal cord. Here we show that plexinA1 (PlexA1) and plexinA3 (PlexA3) receptors are expressed by CS neurons, whereas their ligands, semaphorin-5A (Sema5A) and semaphorin-5B (Sema5B) are expressed in the medulla at the decussation site of CS axons to inhibit premature defasciculation of these axons. In the absence of Sema5A/5B-PlexA1/A3 signaling, some CS axons are prematurely defasciculated in the medulla of the brainstem, and those defasciculated CS axons aberrantly transverse in the spinal gray matter instead of the spinal dorsal funiculus. In the absence of Sema5A/Sema5B-PlexA1/A3 signaling, CS axons, which would normally innervate the lumbar spinal cord, are unbundled in the spinal gray matter, and prematurely innervate the cervical gray matter with reduced innervation of the lumbar gray matter. In both Sema5A/5B and PlexA1/A3 mutant mice (both sexes), stimulation of the hindlimb motor cortex aberrantly evokes robust forelimb muscle activation. Finally, Sema5A/5B and PlexA1/A3 mutant mice show deficits in skilled movements. These results suggest that proper fasciculation of CS axons is required for appropriate neural circuit wiring and ultimately affect the ability to perform skilled movements.SIGNIFICANCE STATEMENT Axon fasciculation is believed to be essential for neural circuit formation and function. However, whether and how defects in axon fasciculation affect the formation and function of neural circuits remain unclear. Here we examine whether the transmembrane proteins semaphorin-5A (Sema5A) and semaphorin-5B (Sema5B), and their receptors, plexinA1 (PlexA1) and plexinA3 (PlexA3) play roles in the development of corticospinal circuits. We find that Sema5A/Sema5B and PlexA1/A3 are required for proper axon fasciculation of corticospinal neurons. Furthermore, Sema5A/5B and PlexA1/A3 mutant mice show marked deficits in skilled motor behaviors. Therefore, these results strongly suggest that proper corticospinal axon fasciculation is required for the appropriate formation and functioning of corticospinal circuits in mice.

Semaphorin-6D and Plexin-A1 Act in a Non-Cell-Autonomous Manner to Position and Target Retinal Ganglion Cell Axons

Semaphorins and Plexins form ligand/receptor pairs that are crucial for a wide range of developmental processes from cell proliferation to axon guidance. The ability of semaphorins to act both as signaling receptors and ligands yields a multitude of responses. Here, we describe a novel role for Semaphorin-6D (Sema6D) and Plexin-A1 in the positioning and targeting of retinogeniculate axons. In Plexin-A1 or Sema6D mutant mice of either sex, the optic tract courses through, rather than along, the border of the dorsal lateral geniculate nucleus (dLGN), and some retinal axons ectopically arborize adjacent and lateral to the optic tract rather than defasciculating and entering the target region. We find that Sema6D and Plexin-A1 act together in a dose-dependent manner, as the number of the ectopic retinal projections is altered in proportion to the level of Sema6D or Plexin-A1 expression. Moreover, using retinal in utero electroporation of Sema6D or Plexin-A1 shRNA, we show that Sema6D and Plexin-A1 are both required in retinal ganglion cells for axon positioning and targeting. Strikingly, nonelectroporated retinal ganglion cell axons also mistarget in the tract region, indicating that Sema6D and Plexin-A1 can act non-cell-autonomously, potentially through axon-axon interactions. These data provide novel evidence for a dose-dependent and non-cell-autonomous role for Sema6D and Plexin-A1 in retinal axon organization in the optic tract and dLGN.SIGNIFICANCE STATEMENT Before innervating their central brain targets, retinal ganglion cell axons fasciculate in the optic tract and then branch and arborize in their target areas. Upon deletion of the guidance molecules Plexin-A1 or Semaphorin-6D, the optic tract becomes disorganized near and extends within the dorsal lateral geniculate nucleus. In addition, some retinal axons form ectopic aggregates within the defasciculated tract. Sema6D and Plexin-A1 act together as a receptor-ligand pair in a dose-dependent manner, and non-cell-autonomously, to produce this developmental aberration. Such a phenotype highlights an underappreciated role for axon guidance molecules in tract cohesion and appropriate defasciculation near, and arborization within, targets.

Sema6D Regulates Zebrafish Vascular Patterning and Motor Neuronal Axon Growth in Spinal Cord

Vessels and nerves are closely associated in anatomy as well as functions. Accumulating evidences have demonstrated that axon-guiding signals may affect endothelial cells migration and path finding, which is crucial for the patterning of both the complex vascular network and neural system. However, studies regarding the functional overlap between vascular and neuronal orchestrating are still incomplete. Semaphorin6D (Sema6D) belongs to the Semaphorin family and has been identified as an important regulating factor in diverse biological processes. Its roles in vascular development are still unclear. Here, we confirmed that sema6D is enriched in neural system and blood vessels of zebrafish embryos by in situ hybridization. Then, the deficiency of sema6D caused by specific antisense morpholino-oligonucleotides (MO) led to dramatic path finding defects in both intersegmental vessels (ISVs) and primary motor neurons (PMNs) of spinal cord in zebrafish embryos. Furthermore, these defective phenotypes were confirmed in F0 generation of sema6D knockouts and rescue experiments by overexpression of sema6D mRNA in sema6D morphants. These data collectively indicate that sema6D regulates zebrafish vascular patterning and motor neuronal axon growth in the spinal cord, which might be of great therapeutical use to regulate vessel and nerve guidance in the relevant diseases that affect both systems.

Semaphorin 6C Suppresses Proliferation of Pancreatic Cancer Cells via Inhibition of the AKT/GSK3/β-Catenin/Cyclin D1 Pathway

Semaphorins (SEMAs) are axon guidance factors that participate in axonal connections and nerve system development. However, the functional roles of SEMAs in tumorigenesis are still largely uncovered. By using in silico data analysis, we found that SEMA6C was downregulated in pancreatic cancer, and its reduction was correlated with worse survival rates. RNA sequencing revealed that cell cycle-related genes, especially cyclin D1, were significantly altered after blockage of SEMA6C by neutralizing antibodies or ectopic expressions of SEMA6C. Mechanistic investigation demonstrated that SEMA6C acts as a tumor suppressor in pancreatic cancer by inhibiting the AKT/GSK3 signaling axis, resulting in a decrease in cyclin D1 expression and cellular proliferation. The enhancement of cyclin D1 expression and cyclin-dependent kinase activation in SEMA6C-low cancer created a druggable target of CDK4/6 inhibitors. We also elucidated the mechanism underlying SEMA6C downregulation in pancreatic cancer and demonstrated a novel regulatory role of miR-124-3p in suppressing SEMA6C. This study provides new insights of SEMA6C-mediated anti-cancer action and suggests the treatment of SEMA6C-downregulated cancer by CDK4/6 inhibitors.

The cellular and molecular basis of somatosensory neuron development

Primary somatosensory neurons convey salient information about our external environment and internal state to the CNS, allowing us to detect, perceive, and react to a wide range of innocuous and noxious stimuli. Pseudo-unipolar in shape, and among the largest (longest) cells of most mammals, dorsal root ganglia (DRG) somatosensory neurons have peripheral axons that extend into skin, muscle, viscera, or bone and central axons that innervate the spinal cord and brainstem, where they synaptically engage the central somatosensory circuitry. Here, we review the diversity of mammalian DRG neuron subtypes and the intrinsic and extrinsic mechanisms that control their development. We describe classical and contemporary advances that frame our understanding of DRG neurogenesis, transcriptional specification of DRG neurons, and the establishment of morphological, physiological, and synaptic diversification across somatosensory neuron subtypes.

Biallelic and monoallelic variants in PLXNA1 are implicated in a novel neurodevelopmental disorder with variable cerebral and eye anomalies

PURPOSE

To investigate the effect of PLXNA1 variants on the phenotype of patients with autosomal dominant and recessive inheritance patterns and to functionally characterize the zebrafish homologs plxna1a and plxna1b during development.

METHODS

We assembled ten patients from seven families with biallelic or de novo PLXNA1 variants. We describe genotype-phenotype correlations, investigated the variants by structural modeling, and used Morpholino knockdown experiments in zebrafish to characterize the embryonic role of plxna1a and plxna1b.

RESULTS

Shared phenotypic features among patients include global developmental delay (9/10), brain anomalies (6/10), and eye anomalies (7/10). Notably, seizures were predominantly reported in patients with monoallelic variants. Structural modeling of missense variants in PLXNA1 suggests distortion in the native protein. Our zebrafish studies enforce an embryonic role of plxna1a and plxna1b in the development of the central nervous system and the eye.

CONCLUSION

We propose that different biallelic and monoallelic variants in PLXNA1 result in a novel neurodevelopmental syndrome mainly comprising developmental delay, brain, and eye anomalies. We hypothesize that biallelic variants in the extracellular Plexin-A1 domains lead to impaired dimerization or lack of receptor molecules, whereas monoallelic variants in the intracellular Plexin-A1 domains might impair downstream signaling through a dominant-negative effect.

Development of the Vertebrate Trunk Sensory System: Origins, Specification, Axon Guidance, and Central Connectivity

Crucial to an animal's movement through their environment and to the maintenance of their homeostatic physiology is the integration of sensory information. This is achieved by axons communicating from organs, muscle spindles and skin that connect to the sensory ganglia composing the peripheral nervous system (PNS), enabling organisms to collect an ever-constant flow of sensations and relay it to the spinal cord. The sensory system carries a wide spectrum of sensory modalities - from sharp pain to cool refreshing touch - traveling from the periphery to the spinal cord via the dorsal root ganglia (DRG). This review covers the origins and development of the DRG and the cells that populate it, and focuses on how sensory connectivity to the spinal cord is achieved by the diverse developmental and molecular processes that control axon guidance in the trunk sensory system. We also describe convergences and differences in sensory neuron formation among different vertebrate species to gain insight into underlying developmental mechanisms.

SlitC-PlexinA1 mediates iterative inhibition for orderly passage of spinal commissural axons through the floor plate

Spinal commissural axon navigation across the midline in the floor plate requires repulsive forces from local Slit repellents. The long-held view is that Slits push growth cones forward and prevent them from turning back once they became sensitized to these cues after midline crossing. We analyzed with fluorescent reporters Slits distribution and FP glia morphology. We observed clusters of Slit-N and Slit-C fragments decorating a complex architecture of glial basal process ramifications. We found that PC2 proprotein convertase activity contributes to this pattern of ligands. Next, we studied Slit-C acting via PlexinA1 receptor shared with another FP repellent, the Semaphorin3B, through generation of a mouse model baring PlexinA1Y1815F mutation abrogating SlitC but not Sema3B responsiveness, manipulations in the chicken embryo, and ex vivo live imaging. This revealed a guidance mechanism by which SlitC constantly limits growth cone exploration, imposing ordered and forward-directed progression through aligned corridors formed by FP basal ramifications.

Dorsal commissural axon guidance in the developing spinal cord

Commissural axons have been a key model system for identifying axon guidance signals in vertebrates. This review summarizes the current thinking about the molecular and cellular mechanisms that establish a specific commissural neural circuit: the dI1 neurons in the developing spinal cord. We assess the contribution of long- and short-range signaling while sequentially following the developmental timeline from the birth of dI1 neurons, to the extension of commissural axons first circumferentially and then contralaterally into the ventral funiculus.

Olig2-Induced Semaphorin Expression Drives Corticospinal Axon Retraction After Spinal Cord Injury

Axon regeneration is limited in the central nervous system, which hinders the reconstruction of functional circuits following spinal cord injury (SCI). Although various extrinsic molecules to repel axons following SCI have been identified, the role of semaphorins, a major class of axon guidance molecules, has not been thoroughly explored. Here we show that expression of semaphorins, including Sema5a and Sema6d, is elevated after SCI, and genetic deletion of either molecule or their receptors (neuropilin1 and plexinA1, respectively) suppresses axon retraction or dieback in injured corticospinal neurons. We further show that Olig2+ cells are essential for SCI-induced semaphorin expression, and that Olig2 binds to putative enhancer regions of the semaphorin genes. Finally, conditional deletion of Olig2 in the spinal cord reduces the expression of semaphorins, alleviating the axon retraction. These results demonstrate that semaphorins function as axon repellents following SCI, and reveal a novel transcriptional mechanism for controlling semaphorin levels around injured neurons to create zones hostile to axon regrowth.

Semaphorin-Mediated Corticospinal Axon Elimination Depends on the Activity-Induced Bax/Bak-Caspase Pathway

Axon guidance molecules and neuronal activity have been implicated in the establishment and refinement of neural circuits during development. It is unclear, however, whether these guidance molecule- and activity-dependent mechanisms interact with one another to shape neural circuit formation. The formation of corticospinal (CS) circuits, which are essential for voluntary movements, involves both guidance molecule- and activity-dependent components during development. We previously showed that semaphorin6D (Sema6D)-plexinA1 (PlexA1) signaling eliminates ipsilateral projections of CS neurons in the spinal cord, while other studies demonstrate that CS projections to the spinal cord are eliminated in an activity-dependent manner. Here we show that inhibition of cortical neurons during postnatal development causes defects in elimination of ipsilateral CS projections in mice. We further show that mice that lack the activity-dependent Bax/Bak pathway or caspase-9 similarly exhibit defects in elimination of ipsilateral CS projections, suggesting that the activity-dependent Bax/Bak-caspase-9 pathway is essential for the removal of ipsilateral CS projections. Interestingly, either inhibition of neuronal activity in the cortex or deletion of Bax/Bak in mice causes a reduction in PlexA1 protein expression in corticospinal neurons. Finally, intracortical microstimulation induces activation of only contralateral forelimb muscles in control mice, whereas it induces activation of both contralateral and ipsilateral muscles in mice with cortical inhibition, suggesting that the ipsilaterally projecting CS axons that have been maintained in mice with cortical inhibition form functional connections. Together, these results provide evidence of a potential link between the repellent signaling of Sema6D-PlexA1 and neuronal activity to regulate axon elimination.SIGNIFICANCE STATEMENT Both axon guidance molecules and neuronal activity regulate axon elimination to refine neuronal circuits during development. However, the degree to which these mechanisms operate independently or cooperatively to guide network generation is unclear. Here, we show that neuronal activity-driven Bax/Bak-caspase signaling induces expression of the PlexA1 receptor for the repellent Sema6D molecule in corticospinal neurons (CSNs). This cascade eliminates ipsilateral projections of CSNs in the spinal cord during early postnatal development. The absence of PlexA1, neuronal activity, Bax and Bak, or caspase-9 leads to the maintenance of ipsilateral projections of CSNs, which can form functional connections with spinal neurons. Together, these studies reveal how the Sema6D-PlexA1 signaling and neuronal activity may play a cooperative role in refining CS axonal projections.

Shootin-1 is required for nervous system development in zebrafish

BACKGROUND

Semaphorin6A (Sema6A) and its PlexinA2 (PlxnA2) receptor canonically function as repulsive axon guidance cues. To understand downstream signaling mechanisms, we performed a microarray screen and identified the "clutch molecule" shootin-1 (shtn-1) as a transcriptionally repressed target. Shtn-1 is a key proponent of cell migration and neuronal polarization and must be regulated during nervous system development. The mechanisms of Shtn-1 regulation and the phenotypic consequences of loss of repression are poorly understood.

RESULTS

We demonstrate shtn-1 overexpression results in impaired migration of the optic vesicles, lack of retinal pigmented epithelium, and pathfinding errors of retinotectal projections. We also observed patterning defects in the peripheral nervous system. Importantly, these phenotypes were rescued by overexpressing PlxnA2.

CONCLUSIONS

We demonstrate a functional role for repression of shtn-1 by PlxnA2 in development of the eyes and peripheral nervous system in zebrafish. These results demonstrate that careful regulation of shtn-1 is critical for development of the nervous system.

The Positional Logic of Sensory-Motor Reflex Circuit Assembly

Throughout his scientific career, Tom Jessell pioneered the spinal cord as a model system to study the molecular programs of neural specification, axon guidance, and connection specificity. His contributions to these fields and more broadly to that of developmental neuroscience will continue to inspire and define many generations of researchers. It is challenging to capture all of Tom's findings in one essay, and therefore, here we wish to briefly highlight his contributions to the problem of connection specificity, with a focus on the spinal sensory-motor reflex circuit. In particular, emphasis will be placed on discoveries from his laboratory that revealed a significant role of positional strategies in establishing selective sensory-motor connections. This work introduced novel principles of neuronal connectivity that may apply to how precise circuit wiring occurs throughout the nervous system.

Roles of axon guidance molecules in neuronal wiring in the developing spinal cord

The spinal cord receives, relays and processes sensory information from the periphery and integrates this information with descending inputs from supraspinal centres to elicit precise and appropriate behavioural responses and orchestrate body movements. Understanding how the spinal cord circuits that achieve this integration are wired during development is the focus of much research interest. Several families of proteins have well-established roles in guiding developing spinal cord axons, and recent findings have identified new axon guidance molecules. Nevertheless, an integrated view of spinal cord network development is lacking, and many current models have neglected the cellular and functional diversity of spinal cord circuits. Recent advances challenge the existing spinal cord axon guidance dogmas and have provided a more complex, but more faithful, picture of the ontogenesis of vertebrate spinal cord circuits.

PLXNA1 and PLXNA3 cooperate to pattern the nasal axons that guide gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons regulate puberty onset and sexual reproduction by secreting GnRH to activate and maintain the hypothalamic-pituitary-gonadal axis. During embryonic development, GnRH neurons migrate along olfactory and vomeronasal axons through the nose into the brain, where they project to the median eminence to release GnRH. The secreted glycoprotein SEMA3A binds its receptors neuropilin (NRP) 1 or NRP2 to position these axons for correct GnRH neuron migration, with an additional role for the NRP co-receptor PLXNA1. Accordingly, mutations in SEMA3A, NRP1, NRP2 and PLXNA1 have been linked to defective GnRH neuron development in mice and inherited GnRH deficiency in humans. Here, we show that only the combined loss of PLXNA1 and PLXNA3 phenocopied the full spectrum of nasal axon and GnRH neuron defects of SEMA3A knockout mice. Together with Plxna1, the human orthologue of Plxna3 should therefore be investigated as a candidate gene for inherited GnRH deficiency.

A Spatiotemporal Sequence of Sensitization to Slits and Semaphorins Orchestrates Commissural Axon Navigation

Accurate perception of guidance cues is crucial for cell and axon migration. During initial navigation in the spinal cord, commissural axons are kept insensitive to midline repellents. Upon midline crossing in the floor plate, they switch on responsiveness to Slit and Semaphorin repulsive signals and are thus propelled away and prevented from crossing back. Whether and how the different midline repellents control specific aspects of this navigation remain to be elucidated. We set up a paradigm for live-imaging and super-resolution analysis of PlexinA1, Neuropilin-2, and Robo1/2 receptor dynamics during commissural growth cone navigation in chick and mouse embryos. We uncovered a remarkable program of sensitization to midline cues achieved by unique spatiotemporal sequences of receptor allocation at the growth-cone surface that orchestrates receptor-specific growth-cone behavior changes. This reveals post-translational mechanisms whereby coincident guidance signals are temporally resolved to allow the generation of specific guidance responses.

Skilled Movements in Mice Require Inhibition of Corticospinal Axon Collateral Formation in the Spinal Cord by Semaphorin Signaling

Corticospinal (CS) neurons in layer V of the sensorimotor cortex are essential for voluntary motor control. Those neurons project axons to specific segments along the rostro-caudal axis of the spinal cord, and reach their spinal targets by sending collateral branches interstitially along axon bundles. Currently, little is known how CS axon collaterals are formed in the proper spinal cord regions. Here, we show that the semaphorin3A (Sema3A)-neuropilin-1 (Npn-1) signaling pathway is an essential negative regulator of CS axon collateral formation in the spinal cord from mice of either sex. Sema3A is expressed in the ventral spinal cord, whereas CS neurons express Npn-1, suggesting that Sema3A might prevent CS axons from entering the ventral spinal cord. Indeed, the ectopic expression of Sema3A in the spinal cord in vivo inhibits CS axon collateral formation, whereas Sema3A or Npn-1 mutant mice have ectopic CS axon collateral formation within the ventral spinal cord compared with littermate controls. Finally, Npn-1 mutant mice exhibit impaired skilled movements, likely because of aberrantly formed CS connections in the ventral spinal cord. These genetic findings reveal that Sema3A-Npn-1 signaling-mediated inhibition of CS axon collateral formation is critical for proper CS circuit formation and the ability to perform skilled motor behaviors.SIGNIFICANCE STATEMENT CS neurons project axons to the spinal cord to control skilled movements in mammals. Previous studies revealed some of the molecular mechanisms underlying different phases of CS circuit development such as initial axon guidance in the brain, and midline crossing in the brainstem and spinal cord. However, the molecular mechanisms underlying CS axon collateral formation in the spinal gray matter has remained obscure. In this study, using in vivo gain-of- and loss-of-function experiments, we show that Sema3A-Npn-1 signaling functions to inhibit CS axon collateral formation in the ventral spinal cord, allowing for the development of proper skilled movements in mice.

Semaphorin-5B Controls Spiral Ganglion Neuron Branch Refinement during Development

During nervous system development, axons often undergo elaborate changes in branching patterns before circuits have achieved their mature patterns of innervation. In the auditory system, type I spiral ganglion neurons (SGNs) project their peripheral axons into the cochlear epithelium and then undergo a process of branch refinement before forming synapses with sensory hair cells. Here, we report that Semaphorin-5B (Sema5B) acts as an important mediator of this process. During cochlear development in mouse, immature hair cells express Sema5B, whereas the SGNs express both PlexinA1 and PlexinA3, which are known Sema5B receptors. In these studies, genetic sparse labeling and three-dimensional reconstruction techniques were leveraged to determine the morphologies of individual type I SGNs after manipulations of Sema5B signaling. Treating cultured mouse cochleae with Sema5B-Fc (to activate Plexin-As) led to type I SGNs with less numerous, but longer terminal branches. Conversely, cochleae from Sema5b knock-out mice showed type I SGNs with more numerous, but shorter terminal branches. In addition, conditional loss of Plxna1 in SGNs (using Bhlhb5Cre) led to increased type I SGN branching, suggesting that PlexinA1 normally responds to Sema5B in this process. In these studies, mice of either sex were used. The data presented here suggest that Sema5B-PlexinA1 signaling limits SGN terminal branch numbers without causing axonal repulsion, which is a role that distinguishes Sema5B from other Semaphorins in cochlear development.SIGNIFICANCE STATEMENT The sensorineural components of the cochlea include hair cells, which respond mechanically to sound waves, and afferent spiral ganglion neurons (SGNs), which respond to glutamate released by hair cells and transmit auditory information into the CNS. An important component of synapse formation in the cochlea is a process of SGN "debranching" whereby SGNs lose extraneous branches before developing unramified bouton endings that contact the hair cells. In this work, we have found that the transmembrane ligand Semaphorin-5B and its receptor PlexinA1 regulate the debranching process. The results in this report provide new knowledge regarding the molecular control of cochlear afferent innervation.

The DCBLD receptor family: emerging signaling roles in development, homeostasis and disease

The discoidin, CUB, and LCCL domain-containing (DCBLD) receptor family are composed of the type-I transmembrane proteins DCBLD1 and DCBLD2 (also ESDN and CLCP1). These proteins are highly conserved across vertebrates and possess similar domain structure to that of neuropilins, which act as critical co-receptors in developmental processes. Although DCBLD1 remains largely uncharacterized, the functional and mechanistic roles of DCBLD2 are emerging. This review provides a comprehensive discussion of this presumed receptor family, ranging from structural and signaling aspects to their associations with cancer, physiology, and development.

Genetic Markers of ADHD-Related Variations in Intracranial Volume

OBJECTIVE

Attention deficit hyperactivity disorder (ADHD) is a common and highly heritable neurodevelopmental disorder with a complex pathophysiology. Intracranial volume (ICV) and volumes of the nucleus accumbens, amygdala, caudate nucleus, hippocampus, and putamen are smaller in people with ADHD compared with healthy individuals. The authors investigated the overlap between common genetic variation associated with ADHD risk and these brain volume measures to identify underlying biological processes contributing to the disorder.

METHODS

The authors combined genome-wide association results from the largest available studies of ADHD (N=55,374) and brain volumes (N=11,221-24,704), using a set of complementary methods to investigate overlap at the level of global common variant genetic architecture and at the single variant level.

RESULTS

Analyses revealed a significant negative genetic correlation between ADHD and ICV (r_g=-0.22). Meta-analysis of single variants revealed two significant loci of interest associated with both ADHD risk and ICV; four additional loci were identified for ADHD and volumes of the amygdala, caudate nucleus, and putamen. Exploratory gene-based and gene-set analyses in the ADHD-ICV meta-analytic data showed association with variation in neurite outgrowth-related genes.

CONCLUSIONS

This is the first genome-wide study to show significant genetic overlap between brain volume measures and ADHD, both on the global and the single variant level. Variants linked to smaller ICV were associated with increased ADHD risk. These findings can help us develop new hypotheses about biological mechanisms by which brain structure alterations may be involved in ADHD disease etiology.

Guidance Molecules in Vascular Smooth Muscle

Several highly conserved families of guidance molecules, including ephrins, Semaphorins, Netrins, and Slits, play conserved and distinct roles in tissue remodeling during tissue patterning and disease pathogenesis. Primarily, these guidance molecules function as either secreted or surface-bound ligands that interact with their receptors to activate a variety of downstream effects, including cell contractility, migration, adhesion, proliferation, and inflammation. Vascular smooth muscle cells, contractile cells comprising the medial layer of the vessel wall and deriving from the mural population, regulate vascular tone and blood pressure. While capillaries lack a medial layer of vascular smooth muscle, mural-derived pericytes contribute similarly to capillary tone to regulate blood flow in various tissues. Furthermore, pericyte coverage is critical in vascular development, as perturbations disrupt vascular permeability and viability. During cardiovascular disease, smooth muscle cells play a more dynamic role in which suppression of contractile markers, enhanced proliferation, and migration lead to the progression of aberrant vascular remodeling. Since many types of guidance molecules are expressed in vascular smooth muscle and pericytes, these may contribute to blood vessel formation and aberrant remodeling during vascular disease. While vascular development is a large focus of the existing literature, studies emerged to address post-developmental roles for guidance molecules in pathology and are of interest as novel therapeutic targets. In this review, we will discuss the roles of guidance molecules in vascular smooth muscle and pericyte function in development and disease.

Sensory and descending motor circuitry during development and injury

Proprioceptive sensory input and descending supraspinal projections are two major inputs that feed into and influence spinal circuitry and locomotor behaviors. Here we review their influence on each other during development and after spinal cord injury. We highlight developmental mechanisms of circuit formation as they relate to the sensory-motor circuit and its reciprocal interactions with local spinal interneurons, as well as competitive interactions between proprioceptive and descending supraspinal inputs in the setting of spinal cord injury.

Genes regulated by SATB2 during neurodevelopment contribute to schizophrenia and educational attainment

SATB2 is associated with schizophrenia and is an important transcription factor regulating neocortical organization and circuitry. Rare mutations in SATB2 cause a syndrome that includes developmental delay, and mouse studies identify an important role for SATB2 in learning and memory. Interacting partners BCL11B and GATAD2A are also schizophrenia risk genes indicating that other genes interacting with or are regulated by SATB2 are making a contribution to schizophrenia and cognition. We used data from Satb2 mouse models to generate three gene-sets that contain genes either functionally related to SATB2 or targeted by SATB2 at different stages of development. Each was tested for enrichment using the largest available genome-wide association studies (GWAS) datasets for schizophrenia and educational attainment (EA) and enrichment analysis was also performed for schizophrenia and other neurodevelopmental disorders using data from rare variant sequencing studies. These SATB2 gene-sets were enriched for genes containing common variants associated with schizophrenia and EA, and were enriched for genes containing rare variants reported in studies of schizophrenia, autism and intellectual disability. In the developing cortex, genes targeted by SATB2 based on ChIP-seq data, and functionally affected when SATB2 is not expressed based on differential expression analysis using RNA-seq data, show strong enrichment for genes associated with EA. For genes expressed in the hippocampus or at the synapse, those targeted by SATB2 are more strongly enriched for genes associated EA than gene-sets not targeted by SATB2. This study demonstrates that single gene findings from GWAS can provide important insights to pathobiological processes. In this case we find evidence that genes influenced by SATB2 and involved in synaptic transmission, axon guidance and formation of the corpus callosum are contributing to schizophrenia and cognition.

Schizophrenia is a complex disorder caused by many genes. Using new gene discoveries to understand pathobiology is a foundation for development of new treatments. Current drugs for schizophrenia are only partially effective and do not treat cognitive deficits, which are key factors for explaining disability, leading to unemployment, homelessness and social isolation. Genome-wide association studies (GWAS) of schizophrenia have been effective at identifying individual SNPs and genes that contribute to risk but have struggled to immediately uncover the bigger picture of the underlying biology of the disorder. Here we take an individual gene identified in a schizophrenia GWAS called SATB2, which on its own is a very important regulator of brain development. We use functional genomics data from mouse studies to identify sets of others genes that are influenced by SATB2 during development. We show that these gene sets are enriched for common variants associated with schizophrenia and educational attainment (used as a proxy for cognition), and for rare variants that increase risk of various neurodevelopmental disorders. This study provides evidence that the molecular mechanisms that underpin schizophrenia and cognitive function include disruption of biological processes influenced by SATB2 as the brain is being organized and wired during development.

Identification of genes associated with cortical malformation using a transposon-mediated somatic mutagenesis screen in mice

Mutations in genes involved in the production, migration, or differentiation of cortical neurons often lead to malformations of cortical development (MCDs). However, many genetic mutations involved in MCD pathogenesis remain unidentified. Here we developed a genetic screening paradigm based on transposon-mediated somatic mutagenesis by in utero electroporation and the inability of mutant neuronal precursors to migrate to the cortex and identified 33 candidate MCD genes. Consistent with the screen, several genes have already been implicated in neural development and disorders. Functional disruption of the candidate genes by RNAi or CRISPR/Cas9 causes altered neuronal distributions that resemble human cortical dysplasia. To verify potential clinical relevance of these candidate genes, we analyzed somatic mutations in brain tissue from patients with focal cortical dysplasia and found that mutations are enriched in these candidate genes. These results demonstrate that this approach is able to identify potential mouse genes involved in cortical development and MCD pathogenesis.

Cortical malformations have a variety of causes. Here the authors use transposon mutagenesis to insert mutations into neural stem cells in the developing mouse cortex to screen for new candidate genes for cortical malformation, and validate some targets in human brain tissue.

Cas Adaptor Proteins Coordinate Sensory Axon Fasciculation

Development of complex neural circuits like the peripheral somatosensory system requires intricate mechanisms to ensure axons make proper connections. While much is known about ligand-receptor pairs required for dorsal root ganglion (DRG) axon guidance, very little is known about the cytoplasmic effectors that mediate cellular responses triggered by these guidance cues. Here we show that members of the Cas family of cytoplasmic signaling adaptors are highly phosphorylated in central projections of the DRG as they enter the spinal cord. Furthermore, we provide genetic evidence that Cas proteins regulate fasciculation of DRG sensory projections. These data establish an evolutionarily conserved requirement for Cas adaptor proteins during peripheral nervous system axon pathfinding. They also provide insight into the interplay between axonal fasciculation and adhesion to the substrate.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Molecular mechanisms underlying monosynaptic sensory-motor circuit development in the spinal cord

Motor behaviors are precisely controlled by the integration of sensory and motor systems in the central nervous system (CNS). Proprioceptive sensory neurons, key components of the sensory system, are located in the dorsal root ganglia and project axons both centrally to the spinal cord and peripherally to muscles and tendons, communicating peripheral information about the body to the CNS. Changes in muscle length detected by muscle spindles, and tension variations in tendons conveyed by Golgi tendon organs, are communicated to the CNS through group Ia /II, and Ib proprioceptive sensory afferents, respectively. Group Ib proprioceptive sensory neurons connect with motor neurons indirectly through spinal interneurons, whereas group Ia/II axons form both direct (monosynaptic) and indirect connections with motor neurons. Although monosynaptic sensory-motor circuits between spindle proprioceptive sensory neurons and motor neurons have been extensively studied since 1950s, the molecular mechanisms underlying their formation and upkeep have only recently begun to be understood. We will discuss our current understanding of the molecular foundation of monosynaptic circuit development and maintenance involving proprioceptive sensory neurons and motor neurons in the mammalian spinal cord. Developmental Dynamics 247:581-587, 2018. © 2017 Wiley Periodicals, Inc.

Control of species-dependent cortico-motoneuronal connections underlying manual dexterity

Superior manual dexterity in higher primates emerged together with the appearance of cortico-motoneuronal (CM) connections during the evolution of the mammalian corticospinal (CS) system. Previously thought to be specific to higher primates, we identified transient CM connections in early postnatal mice, which are eventually eliminated by Sema6D-PlexA1 signaling. PlexA1 mutant mice maintain CM connections into adulthood and exhibit superior manual dexterity as compared with that of controls. Last, differing PlexA1 expression in layer 5 of the motor cortex, which is strong in wild-type mice but weak in humans, may be explained by FEZF2-mediated cis-regulatory elements that are found only in higher primates. Thus, species-dependent regulation of PlexA1 expression may have been crucial in the evolution of mammalian CS systems that improved fine motor control in higher primates.

Identification of novel risk loci for restless legs syndrome in genome-wide association studies in individuals of European ancestry: a meta-analysis

BACKGROUND

Restless legs syndrome is a prevalent chronic neurological disorder with potentially severe mental and physical health consequences. Clearer understanding of the underlying pathophysiology is needed to improve treatment options. We did a meta-analysis of genome-wide association studies (GWASs) to identify potential molecular targets.

METHODS

In the discovery stage, we combined three GWAS datasets (EU-RLS GENE, INTERVAL, and 23andMe) with diagnosis data collected from 2003 to 2017, in face-to-face interviews or via questionnaires, and involving 15 126 cases and 95 725 controls of European ancestry. We identified common variants by fixed-effect inverse-variance meta-analysis. Significant genome-wide signals (p≤5 × 10-8) were tested for replication in an independent GWAS of 30 770 cases and 286 913 controls, followed by a joint analysis of the discovery and replication stages. We did gene annotation, pathway, and gene-set-enrichment analyses and studied the genetic correlations between restless legs syndrome and traits of interest.

FINDINGS

We identified and replicated 13 new risk loci for restless legs syndrome and confirmed the previously identified six risk loci. MEIS1 was confirmed as the strongest genetic risk factor for restless legs syndrome (odds ratio 1·92, 95% CI 1·85-1·99). Gene prioritisation, enrichment, and genetic correlation analyses showed that identified pathways were related to neurodevelopment and highlighted genes linked to axon guidance (associated with SEMA6D), synapse formation (NTNG1), and neuronal specification (HOXB cluster family and MYT1).

INTERPRETATION

Identification of new candidate genes and associated pathways will inform future functional research. Advances in understanding of the molecular mechanisms that underlie restless legs syndrome could lead to new treatment options. We focused on common variants; thus, additional studies are needed to dissect the roles of rare and structural variations.

FUNDING

Deutsche Forschungsgemeinschaft, Helmholtz Zentrum München-Deutsches Forschungszentrum für Gesundheit und Umwelt, National Research Institutions, NHS Blood and Transplant, National Institute for Health Research, British Heart Foundation, European Commission, European Research Council, National Institutes of Health, National Institute of Neurological Disorders and Stroke, NIH Research Cambridge Biomedical Research Centre, and UK Medical Research Council.

Synapse Formation in Monosynaptic Sensory-Motor Connections Is Regulated by Presynaptic Rho GTPase Cdc42

Spinal reflex circuit development requires the precise regulation of axon trajectories, synaptic specificity, and synapse formation. Of these three crucial steps, the molecular mechanisms underlying synapse formation between group Ia proprioceptive sensory neurons and motor neurons is the least understood. Here, we show that the Rho GTPase Cdc42 controls synapse formation in monosynaptic sensory-motor connections in presynaptic, but not postsynaptic, neurons. In mice lacking Cdc42 in presynaptic sensory neurons, proprioceptive sensory axons appropriately reach the ventral spinal cord, but significantly fewer synapses are formed with motor neurons compared with wild-type mice. Concordantly, electrophysiological analyses show diminished EPSP amplitudes in monosynaptic sensory-motor circuits in these mutants. Temporally targeted deletion of Cdc42 in sensory neurons after sensory-motor circuit establishment reveals that Cdc42 does not affect synaptic transmission. Furthermore, addition of the synaptic organizers, neuroligins, induces presynaptic differentiation of wild-type, but not Cdc42-deficient, proprioceptive sensory neurons in vitro Together, our findings demonstrate that Cdc42 in presynaptic neurons is required for synapse formation in monosynaptic sensory-motor circuits.

SIGNIFICANCE STATEMENT

Group Ia proprioceptive sensory neurons form direct synapses with motor neurons, but the molecular mechanisms underlying synapse formation in these monosynaptic sensory-motor connections are unknown. We show that deleting Cdc42 in sensory neurons does not affect proprioceptive sensory axon targeting because axons reach the ventral spinal cord appropriately, but these neurons form significantly fewer presynaptic terminals on motor neurons. Electrophysiological analysis further shows that EPSPs are decreased in these mice. Finally, we demonstrate that Cdc42 is involved in neuroligin-dependent presynaptic differentiation of proprioceptive sensory neurons in vitro These data suggest that Cdc42 in presynaptic sensory neurons is essential for proper synapse formation in the development of monosynaptic sensory-motor circuits.

SoxC Transcription Factors Promote Contralateral Retinal Ganglion Cell Differentiation and Axon Guidance in the Mouse Visual System

Transcription factors control cell identity by regulating diverse developmental steps such as differentiation and axon guidance. The mammalian binocular visual circuit is comprised of projections of retinal ganglion cells (RGCs) to ipsilateral and contralateral targets in the brain. A transcriptional code for ipsilateral RGC identity has been identified, but less is known about the transcriptional regulation of contralateral RGC development. Here we demonstrate that SoxC genes (Sox4, 11, and 12) act on the progenitor-to-postmitotic transition to implement contralateral, but not ipsilateral, RGC differentiation, by binding to Hes5 and thus repressing Notch signaling. When SoxC genes are deleted in postmitotic RGCs, contralateral RGC axons grow poorly on chiasm cells in vitro and project ipsilaterally at the chiasm midline in vivo, and Plexin-A1 and Nr-CAM expression in RGCs is downregulated. These data implicate SoxC transcription factors in the regulation of contralateral RGC differentiation and axon guidance.

Contribution of semaphorins to the formation of the peripheral nervous system in higher vertebrates

Semaphorins are a large family of proteins characterized by sema domains and play a key role not only in the formation of neural circuits, but in the immune system, angiogenesis, tumor progression, and bone metabolism. To date, 15 semaphorins have been reported to be involved in the formation of the peripheral nervous system (PNS) in higher vertebrates. A number of experiments have revealed their functions in the PNS, where they act mainly as axonal guidance cues (as repellents or attractants). Semaphorins also play an important role in the migration of neurons and formation of sensory-motor connections in the PNS. This review summarizes recent knowledge regarding the functions of higher vertebrate semaphorins in the formation of the PNS.

Floor plate-derived neuropilin-2 functions as a secreted semaphorin sink to facilitate commissural axon midline crossing

Commissural axon guidance depends on a myriad of cues expressed by intermediate targets. Secreted semaphorins signal through neuropilin-2/plexin-A1 receptor complexes on post-crossing commissural axons to mediate floor plate repulsion in the mouse spinal cord. Here, we show that neuropilin-2/plexin-A1 are also coexpressed on commissural axons prior to midline crossing and can mediate precrossing semaphorin-induced repulsion in vitro. How premature semaphorin-induced repulsion of precrossing axons is suppressed in vivo is not known. We discovered that a novel source of floor plate-derived, but not axon-derived, neuropilin-2 is required for precrossing axon pathfinding. Floor plate-specific deletion of neuropilin-2 significantly reduces the presence of precrossing axons in the ventral spinal cord, which can be rescued by inhibiting plexin-A1 signaling in vivo. Our results show that floor plate-derived neuropilin-2 is developmentally regulated, functioning as a molecular sink to sequester semaphorins, preventing premature repulsion of precrossing axons prior to subsequent down-regulation, and allowing for semaphorin-mediated repulsion of post-crossing axons.

Axon Guidance Molecules and Neural Circuit Remodeling After Spinal Cord Injury

…once the development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. Santiago Ramón y Cajal Cajal's neurotropic theory postulates that the complexity of the nervous system arises from the collaboration of neurotropic signals from neuronal and non-neuronal cells and that once development has ended, a paucity of neurotropic signals means that the pathways of the central nervous system are "fixed, ended, immutable". While the capacity for regeneration and plasticity of the central nervous system may not be quite as paltry as Cajal proposed, regeneration is severely limited in scope as there is no spontaneous regeneration of long-distance projections in mammals and therefore limited opportunity for functional recovery following spinal cord injury. It is not a far stretch from Cajal to hypothesize that reappropriation of the neurotropic programs of development may be an appropriate strategy for reconstitution of injured circuits. It has become clear, however, that a significant number of the molecular cues governing circuit development become re-active after injury and many assume roles that paradoxically obstruct the functional re-wiring of severed neural connections. Therefore, the problem to address is how individual neural circuits respond to specific molecular cues following injury, and what strategies will be necessary for instigating functional repair or remodeling of the injured spinal cord.

Corticospinal axons make direct synaptic connections with spinal motoneurons innervating forearm muscles early during postnatal development in the rat

KEY POINTS

Direct connections between corticospinal (CS) axons and motoneurons (MNs) appear to be present only in higher primates, where they are essential for discrete movement of the digits. Their presence in adult rodents was once claimed but is now questioned. We report that MNs innervating forearm muscles in infant rats receive monosynaptic input from CS axons, but MNs innervating proximal muscles do not, which is a pattern similar to that in primates. Our experiments were carefully designed to show monosynaptic connections. This entailed selective electrical and optogenetic stimulation of CS axons and recording from MNs identified by retrograde labelling from innervated muscles. Morphological evidence was also obtained for rigorous identification of CS axons and MNs. These connections would be transient and would regress later during development. These results shed light on the development and evolution of direct CS-MN connections, which serve as the basis for dexterity in humans. Recent evidence suggests there is no direct connection between corticospinal (CS) axons and spinal motoneurons (MNs) in adult rodents. We previously showed that CS synapses are present throughout the spinal cord for a time, but are eliminated from the ventral horn during development in rodents. This raises the possibility that CS axons transiently make direct connections with MNs located in the ventral horn of the spinal cord. This was tested in the present study. Using cervical cord slices prepared from rats on postnatal days (P) 7-9, CS axons were stimulated and whole cell recordings were made from MNs retrogradely labelled with fluorescent cholera toxin B subunit (CTB) injected into selected groups of muscles. To selectively activate CS axons, electrical stimulation was carefully limited to the CS tract. In addition we employed optogenetic stimulation after injecting an adeno-associated virus vector encoding channelrhodopsin-2 (ChR2) into the sensorimotor cortex on P0. We were then able to record monosynaptic excitatory postsynaptic currents from MNs innervating forearm muscles, but not from those innervating proximal muscles. We also showed close contacts between CTB-labelled MNs and CS axons labelled through introduction of fluorescent protein-conjugated synaptophysin or the ChR2 expression system. We confirmed that some of these contacts colocalized with postsynaptic density protein 95 in their partner dendrites. It is intriguing from both phylogenetic and ontogenetic viewpoints that direct and putatively transient CS-MN connections were found only on MNs innervating the forearm muscles in infant rats, as this is analogous to the connection pattern seen in adult primates.

Class 3 semaphorins negatively regulate dermal lymphatic network formation

The development of a patterned lymphatic vascular network is essential for proper lymphatic functions during organ development and homeostasis. Here we report that class 3 semaphorins (SEMA3s), SEMA3F and SEMA3G negatively regulate lymphatic endothelial cell (LEC) growth and sprouting to control dermal lymphatic network formation. Neuropilin2 (NRP2) functions as a receptor for SEMA3F and SEMA3G, as well as vascular endothelial growth factor C (VEGFC). In culture, Both SEMA3F and SEMA3G inhibit VEGFC-mediated sprouting and proliferation of human dermal LECs. In the developing mouse skin, Sema3f is expressed in the epidermis and Sema3g expression is restricted to arteries, whereas their receptor Nrp2 is preferentially expressed by lymphatic vessels. Both Sema3f;Sema3g double mutants and Nrp2 mutants exhibit increased LEC growth in the skin. In contrast, Sema3f;Sema3g double mutants display increased lymphatic branching, while Nrp2 mutants exhibit reduced lymphatic branching. A targeted mutation in PlexinA1 or PlexinA2, signal transducers forming a receptor complex with NRP2 for SEMA3s, exhibits an increase in LEC growth and lymphatic branching as observed in Sema3f;Sema3g double mutants. Our results provide the first evidence that SEMA3F and SEMA3G function as a negative regulator for dermal lymphangiogenesis in vivo. The reciprocal phenotype in lymphatic branching between Sema3f;Sema3g double mutants and Nrp2 mutants suggest a complex NRP2 function that regulates LEC behavior both positively and negatively, through a binding with VEGFC or SEMA3s.

Sensory and spinal inhibitory dorsal midline crossing is independent of Robo3

Commissural neurons project across the midline at all levels of the central nervous system (CNS), providing bilateral communication critical for the coordination of motor activity and sensory perception. Midline crossing at the spinal ventral midline has been extensively studied and has revealed that multiple developmental lineages contribute to this commissural neuron population. Ventral midline crossing occurs in a manner dependent on Robo3 regulation of Robo/Slit signaling and the ventral commissure is absent in the spinal cord and hindbrain of Robo3 mutants. Midline crossing in the spinal cord is not limited to the ventral midline, however. While prior anatomical studies provide evidence that commissural axons also cross the midline dorsally, little is known of the genetic and molecular properties of dorsally-crossing neurons or of the mechanisms that regulate dorsal midline crossing. In this study, we describe a commissural neuron population that crosses the spinal dorsal midline during the last quarter of embryogenesis in discrete fiber bundles present throughout the rostrocaudal extent of the spinal cord. Using immunohistochemistry, neurotracing, and mouse genetics, we show that this commissural neuron population includes spinal inhibitory neurons and sensory nociceptors. While the floor plate and roof plate are dispensable for dorsal midline crossing, we show that this population depends on Robo/Slit signaling yet crosses the dorsal midline in a Robo3-independent manner. The dorsally-crossing commissural neuron population we describe suggests a substrate circuitry for pain processing in the dorsal spinal cord.

The interplay between DNA methylation and sequence divergence in recent human evolution

Despite the increasing knowledge about DNA methylation, the understanding of human epigenome evolution is in its infancy. Using whole genome bisulfite sequencing we identified hundreds of differentially methylated regions (DMRs) in humans compared to non-human primates and estimated that ∼25% of these regions were detectable throughout several human tissues. Human DMRs were enriched for specific histone modifications and the majority were located distal to transcription start sites, highlighting the importance of regions outside the direct regulatory context. We also found a significant excess of endogenous retrovirus elements in human-specific hypomethylated.

We reported for the first time a close interplay between inter-species genetic and epigenetic variation in regions of incomplete lineage sorting, transcription factor binding sites and human differentially hypermethylated regions. Specifically, we observed an excess of human-specific substitutions in transcription factor binding sites located within human DMRs, suggesting that alteration of regulatory motifs underlies some human-specific methylation patterns. We also found that the acquisition of DNA hypermethylation in the human lineage is frequently coupled with a rapid evolution at nucleotide level in the neighborhood of these CpG sites. Taken together, our results reveal new insights into the mechanistic basis of human-specific DNA methylation patterns and the interpretation of inter-species non-coding variation.

Altered proliferative ability of neuronal progenitors in PlexinA1 mutant mice

Cortical interneurons are generated predominantly in the medial ganglionic eminence (MGE) and migrate through the ventral and dorsal telencephalon before taking their final positions within the developing cortical plate. Previously we demonstrated that interneurons from Robo1 knockout (Robo1(-/-)) mice contain reduced levels of neuropilin 1 (Nrp1) and PlexinA1 receptors, rendering them less responsive to the chemorepulsive actions of semaphorin ligands expressed in the striatum and affecting their course of migration (Hernandez-Miranda et al. [2011] J. Neurosci. 31:6174-6187). Earlier studies have highlighted the importance of Nrp1 and Nrp2 in interneuron migration, and here we assess the role of PlexinA1 in this process. We observed significantly fewer cells expressing the interneuron markers Gad67 and Lhx6 in the cortex of PlexinA1(-/-) mice compared with wild-type littermates at E14.5 and E18.5. Although the level of apoptosis was similar in the mutant and control forebrain, proliferation was significantly reduced in the former. Furthermore, progenitor cells in the MGE of PlexinA1(-/-) mice appeared to be poorly anchored to the ventricular surface and showed reduced adhesive properties, which may account for the observed reduction in proliferation. Together our data uncover a novel role for PlexinA1 in forebrain development.

Expression of the immunoglobulin superfamily cell adhesion molecules in the developing spinal cord and dorsal root ganglion

Cell adhesion molecules belonging to the immunoglobulin superfamily (IgSF) control synaptic specificity through hetero- or homophilic interactions in different regions of the nervous system. In the developing spinal cord, monosynaptic connections of exquisite specificity form between proprioceptive sensory neurons and motor neurons, however, it is not known whether IgSF molecules participate in regulating this process. To determine whether IgSF molecules influence the establishment of synaptic specificity in sensory-motor circuits, we examined the expression of 157 IgSF genes in the developing dorsal root ganglion (DRG) and spinal cord by in situ hybridization assays. We find that many IgSF genes are expressed by sensory and motor neurons in the mouse developing DRG and spinal cord. For instance, Alcam, Mcam, and Ocam are expressed by a subset of motor neurons in the ventral spinal cord. Further analyses show that Ocam is expressed by obturator but not quadriceps motor neurons, suggesting that Ocam may regulate sensory-motor specificity in these sensory-motor reflex arcs. Electrophysiological analysis shows no obvious defects in synaptic specificity of monosynaptic sensory-motor connections involving obturator and quadriceps motor neurons in Ocam mutant mice. Since a subset of Ocam⁺ motor neurons also express Alcam, Alcam or other functionally redundant IgSF molecules may compensate for Ocam in controlling sensory-motor specificity. Taken together, these results reveal that IgSF molecules are broadly expressed by sensory and motor neurons during development, and that Ocam and other IgSF molecules may have redundant functions in controlling the specificity of sensory-motor circuits.

Congenital diseases and semaphorin signaling: overview to date of the evidence linking them

Semaphorins and their receptors, neuropilins and plexins, were initially characterized as a modulator of axonal guidance during development, but are now recognized as a regulator of a wide range of developmental events including morphogenesis and angiogenesis, and activities of the immune system. Owing to the development of next-generation sequencing technologies together with other useful DNA assays, it has also become clear that semaphorin signaling plays a crucial role in many congenital diseases such as retinal degeneration and congenital heart defects. This review summarizes the recent knowledge about the relationship between a variety of congenital diseases and semaphorin signaling.

PlexinA1 is a new Slit receptor and mediates axon guidance function of Slit C-terminal fragments

Robo-Slit and Plexin-Semaphorin signaling participate in various developmental and pathogenic processes. During commissural axon guidance in the spinal cord, chemorepulsion by Semaphorin3B and Slits controls midline crossing. Slit processing generates an N-terminal fragment (SlitN) that binds to Robo1 and Robo2 receptors and mediates Slit repulsive activity, as well as a C-terminal fragment (SlitC) with an unknown receptor and bioactivity. We identified PlexinA1 as a Slit receptor and found that it binds the C-terminal Slit fragment specifically and transduces a SlitC signal independently of the Robos and the Neuropilins. PlexinA1-SlitC complexes are detected in spinal cord extracts, and ex vivo, SlitC binding to PlexinA1 elicits a repulsive commissural response. Analysis of various ligand and receptor knockout mice shows that PlexinA1-Slit and Robo-Slit signaling have complementary roles during commissural axon guidance. Thus, PlexinA1 mediates both Semaphorin and Slit signaling, and Slit processing generates two active fragments, each exerting distinct effects through specific receptors.

Semaphorin 5A inhibits synaptogenesis in early postnatal- and adult-born hippocampal dentate granule cells

Human SEMAPHORIN 5A (SEMA5A) is an autism susceptibility gene; however, its function in brain development is unknown. In this study, we show that mouse Sema5A negatively regulates synaptogenesis in early, developmentally born, hippocampal dentate granule cells (GCs). Sema5A is strongly expressed by GCs and regulates dendritic spine density in a cell-autonomous manner. In the adult mouse brain, newly born Sema5A-/- GCs show an increase in dendritic spine density and increased AMPA-type synaptic responses. Sema5A signals through PlexinA2 co-expressed by GCs, and the PlexinA2-RasGAP activity is necessary to suppress spinogenesis. Like Sema5A-/- mutants, PlexinA2-/- mice show an increase in GC glutamatergic synapses, and we show that Sema5A and PlexinA2 genetically interact with respect to GC spine phenotypes. Sema5A-/- mice display deficits in social interaction, a hallmark of autism-spectrum-disorders. These experiments identify novel intra-dendritic Sema5A/PlexinA2 interactions that inhibit excitatory synapse formation in developmentally born and adult-born GCs, and they provide support for SEMA5A contributions to autism-spectrum-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.