Motor neuron cell bodies are actively positioned by Slit/Robo repulsion and Netrin/DCC attraction

Slit-Robo signaling supports motor neuron avoidance of the spinal cord midline through DCC antagonism and other mechanisms

Axon pathfinding and neuronal migration are orchestrated by attractive and repulsive guidance cues. In the mouse spinal cord, repulsion from Slit proteins through Robo family receptors and attraction to Netrin-1, mediated by the receptor DCC, control many aspects of neural circuit formation. This includes motor neuron wiring, where Robos help prevent both motor neuron cell bodies and axons from aberrantly crossing the spinal cord midline. These functions had been ascribed to Robo signaling being required to counter DCC-mediated attraction to Netrin-1 at the midline, either by mediating repulsion from midline-derived Slits or by silencing DCC signaling. However, the role of DCC in promoting motor neuron and axon midline crossing had not been directly tested. Here, we used in vivo mouse genetics and in vitro axon turning assays to further explore the interplay between Slit and Netrin signaling in motor neuron migration and axon guidance relative to the midline. We find that DCC is a major driver of midline crossing by motor axons, but not motor neuron cell bodies, when Robo1 and Robo2 are knocked out. Further, in vitro results indicate that Netrin-1 attracts motor axons and that Slits can modulate the chemotropic response to Netrin-1, converting it from attraction to repulsion. Our findings indicate that Robo signaling allows both motor neuron cell bodies and axons to avoid the midline, but that only motor axons require this pathway to antagonize DCC-dependent midline attraction, which likely involves a combination of mediating Slit repulsion and directly influencing Netrin-DCC signaling output.

Hodological patterning as an organizing principle in vertebrate motor circuitry

Hodological patterning refers to developmental mechanisms that link the location of neurons in the brain or spinal cord to specific axonal trajectories that direct connectivity to synaptic targets either within the central nervous system or in the periphery. In vertebrate motor circuits, hodological patterning has been demonstrated at different levels, from the final motor output of somatic and preganglionic autonomic neurons targeting peripheral motoneurons and ganglion cells, to premotor inputs from spinal and brainstem neuron populations targeting the somatic motoneurons and preganglionic autonomic neurons, to cortical neurons that delegate movement commands to the brainstem and spinal neurons. In many cases molecular profiling reveals potential underlying mechanisms whereby selective gene expression creates the link between location and axon trajectory. At the cortical level, somatotopic organization suggests a potential underlying hodological patterning, but this has not been proven. This review describes examples of hodological patterning in motor circuits and covers current knowledge about how this patterning arises.

Netrin-1 as A neural guidance protein in development and reinnervation of the larynx

Neural guidance proteins participate in motor neuron migration, axonal projection, and muscle fiber innervation during development. One of the guidance proteins that participates in axonal pathfinding is Netrin-1. Despite the well-known role of Netrin-1 in embryogenesis of central nervous tissue, it is still unclear how the expression of this guidance protein contributes to primary innervation of the periphery, as well as reinnervation. This is especially true in the larynx where Netrin-1 is upregulated within the intrinsic laryngeal muscles after nerve injury and where blocking of Netrin-1 alters the pattern of reinnervation of the intrinsic laryngeal muscles. Despite this consistent finding, it is unknown how Netrin-1 expression contributes to guidance of the axons towards the larynx. Improved knowledge of Netrin-1's role in nerve regeneration and reinnervation post-injury in comparison to its role in primary innervation during embryological development, may provide insights in the search for therapeutics to treat nerve injury. This paper reviews the known functions of Netrin-1 during the formation of the central nervous system and during cranial nerve primary innervation. It also describes the role of Netrin-1 in the formation of the larynx and during recurrent laryngeal reinnervation following nerve injury in the adult.

The long noncoding RNA nHOTAIRM1 is necessary for differentiation and activity of iPSC-derived spinal motor neurons

The mammalian nervous system is made up of an extraordinary array of diverse cells that form intricate functional connections. The programs underlying cell lineage specification, identity and function of the neuronal subtypes are managed by regulatory proteins and RNAs, which coordinate the succession of steps in a stereotyped temporal order. In the central nervous system (CNS), motor neurons (MNs) are responsible for controlling essential functions such as movement, breathing, and swallowing by integrating signal transmission from the cortex, brainstem, and spinal cord (SC) towards peripheral muscles. A prime role in guiding the progression of progenitor cells towards the MN fate has been largely attributed to protein factors. More recently, the relevance of a class of regulatory RNAs abundantly expressed in the CNS - the long noncoding RNAs (lncRNAs) - has emerged overwhelmingly. LncRNA-driven gene expression control is key to regulating any step of MN differentiation and function, and its derangement profoundly impacts neuronal pathophysiology. Here, we uncover a novel function for the neuronal isoform of HOTAIRM1 (nHOTAIRM1), a lncRNA specifically expressed in the SC. Using a model system that recapitulates spinal MN (spMN) differentiation, we show that nHOTAIRM1 intervenes in the binary cell fate decision between MNs and interneurons, acting as a pro-MN factor. Furthermore, human iPSC-derived spMNs without nHOTAIRM1 display altered neurite outgrowth, with a significant reduction of both branch and junction numbers. Finally, the expression of genes essential for synaptic connectivity and neurotransmission is also profoundly impaired when nHOTAIRM1 is absent in spMNs. Mechanistically, nHOTAIRM1 establishes both direct and indirect interactions with a number of target genes in the cytoplasm, being a novel post-transcriptional regulator of MN biology. Overall, our results indicate that the lncRNA nHOTAIRM1 is essential for the specification of MN identity and the acquisition of proper morphology and synaptic activity of post-mitotic MNs.

Clinical and Genetic Analysis of A Father-Son Duo with Monomelic Amyotrophy: Case Report

Monomelic Amyotrophy (MMA) is a rare neurological disorder restricted to one upper limb, predominantly affecting young males with an unknown aetiopathogenesis. We report a familial case of father-son duo affected by MMA. Whole exome sequencing identified genetic variations in SLIT1, RYR3 and ARPP21 involved in axon guidance, calcium homeostasis and regulation of calmodulin signaling respectively. This is the first attempt to define genetic modifiers associated with MMA from India and advocates to extend genetic screening to a larger cohort. Deciphering the functional consequences of variations in these genes will be crucial for unravelling the pathogenesis of MMA.

A tug of war between DCC and ROBO1 signaling during commissural axon guidance

Dynamic and coordinated axonal responses to changing environments are critical for establishing neural connections. As commissural axons migrate across the CNS midline, they are suggested to switch from being attracted to being repelled in order to approach and to subsequently leave the midline. A molecular mechanism that is hypothesized to underlie this switch in axonal responses is the silencing of Netrin1/Deleted in Colorectal Carcinoma (DCC)-mediated attraction by the repulsive SLIT/ROBO1 signaling. Using in vivo approaches including CRISPR-Cas9-engineered mouse models of distinct Dcc splice isoforms, we show here that commissural axons maintain responsiveness to both Netrin and SLIT during midline crossing, although likely at quantitatively different levels. In addition, full-length DCC in collaboration with ROBO3 can antagonize ROBO1 repulsion in vivo. We propose that commissural axons integrate and balance the opposing DCC and Roundabout (ROBO) signaling to ensure proper guidance decisions during midline entry and exit.

Axons will not be silenced! A balancing act of attractive receptors

Axons crossing the CNS midline regulate their responsiveness to both attractive and repulsive cues. In this issue, Dailey-Krempel et al. find different modes of action for DCC isoforms and uncover evidence against the silencing model of axon guidance.

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Hop Mice Display Synchronous Hindlimb Locomotion and a Ventrally Fused Lumbar Spinal Cord Caused by a Point Mutation in Ttc26

Identifying the spinal circuits controlling locomotion is critical for unravelling the mechanisms controlling the production of gaits. Development of the circuits governing left-right coordination relies on axon guidance molecules such as ephrins and netrins. To date, no other class of proteins have been shown to play a role during this process. Here, we have analyzed hop mice, which walk with a characteristic hopping gait using their hindlimbs in synchrony. Fictive locomotion experiments suggest that a local defect in the ventral spinal cord contributes to the aberrant locomotor phenotype. Hop mutant spinal cords had severe morphologic defects, including the absence of the ventral midline and a poorly defined border between white and gray matter. The hop mice represent the first model where, exclusively found in the lumbar domain, the left and right components of the central pattern generators (CPGs) are fused with a synchronous hindlimb gait as a functional consequence. These defects were associated with abnormal developmental processes, including a misplaced notochord and reduced induction of ventral progenitor domains. Whereas the underlying mutation in hop mice has been suggested to lie within the Ttc26 gene, other genes in close vicinity have been associated with gait defects. Mouse embryos carrying a CRISPR replicated point mutation within Ttc26 displayed an identical morphologic phenotype. Thus, our data suggest that the assembly of the lumbar CPG network is dependent on fully functional TTC26 protein.

Expression of Slit and Robo during remodeling of corticospinal tract in cervical spinal cord in middle cerebral artery occlusion rats

BACKGROUND

Slits and Robos were associated with the generation of axons of corticospinal tract during the corticospinal tract (CST) remodeling after the cerebral ischemic stroke (CIS). However, little is known about the mechanism of CST remodeling. In this study, we detected the expression of Slits and Robos in middle cerebral artery occlusion (MCAO) rats to investigate the roles of Slits and Robos in the CIS.

METHODS

MCAO model was established using modified Zea Longa method. Beam walking test (BWT) was conducted to evaluate the motor function. The images of the track of cortical spinal cord beam on day 7, 14 and 21 were observed by anterograde CST tracing. Biopinylated dextan amine (BDA) was used to mark CST anterogradely. Expression of GAP-43 mRNA and GAP-43 protein in cervical spinal cord was detected by Real-Time PCR and Western blot analysis, respectively. The expression of Slit1, Slit2 and Robo1 in cervical spinal cord was detected by immunofluorescence staining.

RESULTS

The scores in the model group were significantly reduced compared to sham-operation group on day 7 (P < 0.001), 14 (P < 0.001) and 21 (P < 0.001), respectively. There was no significant difference in the score on day 7, 14 and 21 of the sham-operation groups (P > 0.05). In contrast, significant increase was noticed in the scores in model group, presenting a time-dependent manner. More CST staining fibers could be observed at the degenerative side in the model group compared with that of the sham-operation group on day 21. GAP-43 mRNA expression in the model group showed significant increase compared to that of sham-operation group on day 14 (P = 0.015) and 21 days (P = 0.002). The expression of GAP-43 protein in model group showed significant increase compared to that of sham-operation group on day 14 (P = 0.022) and day 21 (P = 0.008), respectively. The expression of Slit1 and Slit2 showed increase on day 14 and day 21, while the expression of Robo1 showed significant decrease in MCAO rats.

CONCLUSION

Up-regulation of Slit1 and Slit2 and the downregulation of Robo1 may be related to the axons of CST midline crossing in spinal cord of MCAO rat during the spontaneous recovery of impaired motor function.

Smoothened overexpression causes trochlear motoneurons to reroute and innervate ipsilateral eyes

The trochlear projection is unique among the cranial nerves in that it exits the midbrain dorsally to innervate the contralateral superior oblique muscle in all vertebrates. Trochlear as well as oculomotor motoneurons uniquely depend upon Phox2a and Wnt1, both of which are downstream of Lmx1b, though why trochlear motoneurons display such unusual projections is not fully known. We used Pax2-cre to drive expression of ectopically activated Smoothened (SmoM2) dorsally in the midbrain and anterior hindbrain. We documented the expansion of oculomotor and trochlear motoneurons using Phox2a as a specific marker at E9.5. We show that the initial expansion follows a demise of these neurons by E14.5. Furthermore, SmoM2 expression leads to a ventral exit and ipsilateral projection of trochlear motoneurons. We compare that data with Unc5c mutants that shows a variable ipsilateral number of trochlear fibers that exit dorsal. Our data suggest that Shh signaling is involved in trochlear motoneuron projections and that the deflected trochlear projections after SmoM2 expression is likely due to the dorsal expression of Gli1, which impedes the normal dorsal trajectory of these neurons.

Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

Cell migration and axon guidance at the border between central and peripheral nervous system

The central and peripheral nervous system (CNS and PNS, respectively) are composed of distinct neuronal and glial cell types with specialized functional properties. However, a small number of select cells traverse the CNS-PNS boundary and connect these two major subdivisions of the nervous system. This pattern of segregation and selective connectivity is established during embryonic development, when neurons and glia migrate to their destinations and axons project to their targets. Here, we provide an overview of the cellular and molecular mechanisms that control cell migration and axon guidance at the vertebrate CNS-PNS border. We highlight recent advances on how cell bodies and axons are instructed to either cross or respect this boundary, and present open questions concerning the development and plasticity of the CNS-PNS interface.

Slit/Robo signals prevent spinal motor neuron emigration by organizing the spinal cord basement membrane

The developing spinal cord builds a boundary between the CNS and the periphery, in the form of a basement membrane. The spinal cord basement membrane is a barrier that retains CNS neuron cell bodies, while being selectively permeable to specific axon types. Spinal motor neuron cell bodies are located in the ventral neural tube next to the floor plate and project their axons out through the basement membrane to peripheral targets. However, little is known about how spinal motor neuron cell bodies are retained inside the ventral neural tube, while their axons can exit. In previous work, we found that disruption of Slit/Robo signals caused motor neuron emigration outside the spinal cord. In the current study, we investigate how Slit/Robo signals are necessary to keep spinal motor neurons within the neural tube. Our findings show that when Slit/Robo signals were removed from motor neurons, they migrated outside the spinal cord. Furthermore, this emigration was associated with abnormal basement membrane protein expression in the ventral spinal cord. Using Robo2 and Slit2 conditional mutants, we found that motor neuron-derived Slit/Robo signals were required to set up a normal basement membrane in the spinal cord. Together, our results suggest that motor neurons produce Slit signals that are required for the basement membrane assembly to retain motor neuron cell bodies within the spinal cord.

Pou2f2 Regulates the Distribution of Dorsal Interneurons in the Mouse Developing Spinal Cord

Spinal dorsal interneurons, which are generated during embryonic development, relay and process sensory inputs from the periphery to the central nervous system. Proper integration of these cells into neuronal circuitry depends on their correct positioning within the spinal parenchyma. Molecular cues that control neuronal migration have been extensively characterized but the genetic programs that regulate their production remain poorly investigated. Onecut (OC) transcription factors have been shown to control the migration of the dorsal interneurons (dINs) during spinal cord development. Here, we report that the OC factors moderate the expression of Pou2f2, a transcription factor essential for B-cell differentiation, in spinal dINs. Overexpression or inactivation of Pou2f2 leads to alterations in the differentiation of dI2, dI3 and Phox2a-positive dI5 populations and to defects in the distribution of dI2-dI6 interneurons. Thus, an OC-Pou2f2 genetic cascade regulates adequate diversification and distribution of dINs during embryonic development.

Muscle regulates mTOR dependent axonal local translation in motor neurons via CTRP3 secretion: implications for a neuromuscular disorder, spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder, which causes dysfunction/loss of lower motor neurons and muscle weakness as well as atrophy. While SMA is primarily considered as a motor neuron disease, recent data suggests that survival motor neuron (SMN) deficiency in muscle causes intrinsic defects. We systematically profiled secreted proteins from control and SMN deficient muscle cells with two combined metabolic labeling methods and mass spectrometry. From the screening, we found lower levels of C1q/TNF-related protein 3 (CTRP3) in the SMA muscle secretome and confirmed that CTRP3 levels are indeed reduced in muscle tissues and serum of an SMA mouse model. We identified that CTRP3 regulates neuronal protein synthesis including SMN via mTOR pathway. Furthermore, CTRP3 enhances axonal outgrowth and protein synthesis rate, which are well-known impaired processes in SMA motor neurons. Our data revealed a new molecular mechanism by which muscles regulate the physiology of motor neurons via secreted molecules. Dysregulation of this mechanism contributes to the pathophysiology of SMA.

Temporal regulation of axonal repulsion by alternative splicing of a conserved microexon in mammalian Robo1 and Robo2

Proper connectivity of the nervous system requires temporal and spatial control of axon guidance signaling. As commissural axons navigate across the CNS midline, ROBO-mediated repulsion has traditionally been thought to be repressed before crossing, and then to become upregulated after crossing. The regulation of the ROBO receptors involves multiple mechanisms that control protein expression, trafficking, and activity. Here, we report that mammalian ROBO1 and ROBO2 are not uniformly inhibited precrossing and are instead subject to additional temporal control via alternative splicing at a conserved microexon. The NOVA splicing factors regulate the developmental expression of ROBO1 and ROBO2 variants with small sequence differences and distinct guidance activities. As a result, ROBO-mediated axonal repulsion is activated early in development to prevent premature crossing and becomes inhibited later to allow crossing. Postcrossing, the ROBO1 and ROBO2 isoforms are disinhibited to prevent midline reentry and to guide postcrossing commissural axons to distinct mediolateral positions.

Ex Vivo Oculomotor Slice Culture from Embryonic GFP-Expressing Mice for Time-Lapse Imaging of Oculomotor Nerve Outgrowth

Accurate eye movements are crucial for vision, but the development of the ocular motor system, especially the molecular pathways controlling axon guidance, has not been fully elucidated. This is partly due to technical limitations of traditional axon guidance assays. To identify additional axon guidance cues influencing the oculomotor nerve, an ex vivo slice assay to image the oculomotor nerve in real-time as it grows towards the eye was developed. E10.5 Isl^MN-GFP embryos are used to generate ex vivo slices by embedding them in agarose, slicing on a vibratome, then growing them in a microscope stage-top incubator with time-lapse photomicroscopy for 24-72 h. Control slices recapitulate the in vivo timing of outgrowth of axons from the nucleus to the orbit. Small molecule inhibitors or recombinant proteins can be added to the culture media to assess the role of different axon guidance pathways. This method has the advantages of maintaining more of the local microenvironment through which axons traverse, not axotomizing the growing axons, and assessing the axons at multiple points along their trajectory. It can also identify effects on specific subsets of axons. For example, inhibition of CXCR4 causes axons still within the midbrain to grow dorsally rather than ventrally, but axons that have already exited ventrally are not affected.

Developmental Requirement of Homeoprotein Otx2 for Specific Habenulo-Interpeduncular Subcircuits

The habenulo-interpeduncular system (HIPS) is now recognized as a critical circuit modulating aversion, reward, and social behavior. There is evidence that dysfunction of this circuit leads to psychiatric disorders. Because psychiatric diseases may originate in developmental abnormalities, it is crucial to investigate the developmental mechanisms controlling the formation of the HIPS. Thus far, this issue has been the focus of limited studies. Here, we explored the developmental processes underlying the formation of the medial habenula (MHb) and its unique output, the interpeduncular nucleus (IPN), in mice independently of their gender. We report that the Otx2 homeobox gene is essential for the proper development of both structures. We show that MHb and IPN neurons require Otx2 at different developmental stages and, in both cases, Otx2 deletion leads to disruption of HIPS subcircuits. Finally, we show that Otx2⁺ neurons tend to be preferentially interconnected. This study reveals that synaptically connected components of the HIPS, despite radically different developmental strategies, share high sensitivity to Otx2 expression.SIGNIFICANCE STATEMENT Brain reward circuits are highly complex and still poorly understood. In particular, it is important to understand how these circuits form as many psychiatric diseases may arise from their abnormal development. This work shows that Otx2, a critical evolutionary conserved gene implicated in brain development and a predisposing factor for psychiatric diseases, is required for the formation of the habenulo-interpeduncular system (HIPS), an important component of the reward circuit. Otx2 deletion affects multiple processes such as proliferation and migration of HIPS neurons. Furthermore, neurons expressing Otx2 are preferentially interconnected. Therefore, Otx2 expression may represent a code that specifies the connectivity of functional subunits of the HIPS. Importantly, the Otx2 conditional knock-out animals used in this study might represent a new genetic model of psychiatric diseases.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Motor axons are guided to exit points in the spinal cord by Slit and Netrin signals

In the spinal cord, motor axons project out the neural tube at specific exit points, then bundle together to project toward target muscles. The molecular signals that guide motor axons to and out of their exit points remain undefined. Since motor axons and their exit points are located near the floor plate, guidance signals produced by the floor plate and adjacent ventral tissues could influence motor axons as they project toward and out of exit points. The secreted Slit proteins are major floor plate repellents, and motor neurons express two Slit receptors, Robo1 and Robo2. Using mutant mouse embryos at early stages of motor axon exit, we found that motor exit points shifted ventrally in Robo1/2 or Slit1/2 double mutants. Along with the ventral shift, mutant axons had abnormal trajectories both within the neural tube toward the exit point, and after exit into the periphery. In contrast, the absence of the major ventral attractant, Netrin-1, or its receptor, DCC caused motor exit points to shift dorsally. Netrin-1 attraction on spinal motor axons was demonstrated by in vitro explant assays, showing that Netrin-1 increased outgrowth and attracted cultured spinal motor axons. The opposing effects of Slit/Robo and Netrin-1/DCC signals were tested genetically by combining Netrin-1 and Robo1/2 mutations. The location of exit points in the combined mutants was significantly recovered to their normal position compared to Netrin-1 or Robo1/2 mutants. Together, these results suggest that the proper position of motor exit points is determined by a "push-pull" mechanism, pulled ventrally by Netrin-1/DCC attraction and pushed dorsally by Slit/Robo repulsion.

Motor neuron migration and positioning mechanisms: New roles for guidance cues

Motor neurons differentiate from progenitor cells and cluster as motor nuclei, settling next to the floor plate in the brain stem and spinal cord. Although precise positioning of motor neurons is critical for their functional input and output, the molecular mechanisms that guide motor neurons to their proper positions remain poorly understood. Here, we review recent evidence of motor neuron positioning mechanisms, highlighting situations in which motor neuron cell bodies can migrate, and experiments that show that their migration is regulated by axon guidance cues. The view that emerges is that motor neurons are actively trapped or restricted in static positions, as the cells balance a push in the dorsal direction by repulsive Slit/Robo cues and a pull in the ventral direction by attractive Netrin-1/DCC cues. These new functions of guidance cues are necessary fine-tuning to set up patterns of motor neurons at their proper positions in the neural tube during embryogenesis.

Axon pathfinding for locomotion

Motor neurons of the spinal cord are responsible for the assembly of neuromuscular connections indispensable for basic locomotion and skilled movements. A precise spatial relationship exists between the position of motor neuron cell bodies in the spinal cord and the course of their axonal projections to peripheral muscle targets. Motor neuron innervation of the vertebrate limb is a prime example of this topographic organization and by virtue of its accessibility and predictability has provided access to fundamental principles of motor system development and neuronal guidance. The seemingly basic binary map established by genetically defined motor neuron subtypes that target muscles in the limb is directed by a surprisingly large number of directional cues. Rather than being simply redundant, these converging signaling pathways are hierarchically linked and cooperate to increase the fidelity of axon pathfinding decisions. A current priority is to determine how multiple guidance signals are integrated by individual growth cones and how they synergize to delineate class-specific axonal trajectories.

Axon guidance pathways served as common targets for human speech/language evolution and related disorders

Human and several nonhuman species share the rare ability of modifying acoustic and/or syntactic features of sounds produced, i.e. vocal learning, which is the important neurobiological and behavioral substrate of human speech/language. This convergent trait was suggested to be associated with significant genomic convergence and best manifested at the ROBO-SLIT axon guidance pathway. Here we verified the significance of such genomic convergence and assessed its functional relevance to human speech/language using human genetic variation data. In normal human populations, we found the affected amino acid sites were well fixed and accompanied with significantly more associated protein-coding SNPs in the same genes than the rest genes. Diseased individuals with speech/language disorders have significant more low frequency protein coding SNPs but they preferentially occurred outside the affected genes. Such patients' SNPs were enriched in several functional categories including two axon guidance pathways (mediated by netrin and semaphorin) that interact with ROBO-SLITs. Four of the six patients have homozygous missense SNPs on PRAME gene family, one youngest gene family in human lineage, which possibly acts upon retinoic acid receptor signaling, similarly as FOXP2, to modulate axon guidance. Taken together, we suggest the axon guidance pathways (e.g. ROBO-SLIT, PRAME gene family) served as common targets for human speech/language evolution and related disorders.

Gaskell revisited: new insights into spinal autonomics necessitate a revised motor neuron nomenclature

Several concepts developed in the nineteenth century have formed the basis of much of our neuroanatomical teaching today. Not all of these were based on solid evidence nor have withstood the test of time. Recent evidence on the evolution and development of the autonomic nervous system, combined with molecular insights into the development and diversification of motor neurons, challenges some of the ideas held for over 100 years about the organization of autonomic motor outflow. This review provides an overview of the original ideas and quality of supporting data and contrasts this with a more accurate and in depth insight provided by studies using modern techniques. Several lines of data demonstrate that branchial motor neurons are a distinct motor neuron population within the vertebrate brainstem, from which parasympathetic visceral motor neurons of the brainstem evolved. The lack of an autonomic nervous system in jawless vertebrates implies that spinal visceral motor neurons evolved out of spinal somatic motor neurons. Consistent with the evolutionary origin of brainstem parasympathetic motor neurons out of branchial motor neurons and spinal sympathetic motor neurons out of spinal motor neurons is the recent revision of the organization of the autonomic nervous system into a cranial parasympathetic and a spinal sympathetic division (e.g., there is no sacral parasympathetic division). We propose a new nomenclature that takes all of these new insights into account and avoids the conceptual misunderstandings and incorrect interpretation of limited and technically inferior data inherent in the old nomenclature.

Ocular Motor Nerve Development in the Presence and Absence of Extraocular Muscle

Purpose

To spatially and temporally define ocular motor nerve development in the presence and absence of extraocular muscles (EOMs).

Methods

Myf5^cre mice, which in the homozygous state lack EOMs, were crossed to an Isl^MN:GFP reporter line to fluorescently label motor neuron cell bodies and axons. Embryonic day (E) 11.5 to E15.5 wild-type and Myf5^cre/cre:Isl^MN:GFP whole mount embryos and dissected orbits were imaged by confocal microscopy to visualize the developing oculomotor, trochlear, and abducens nerves in the presence and absence of EOMs. E11.5 and E18.5 brainstems were serially sectioned and stained for Islet1 to determine the fate of ocular motor neurons.

Results

At E11.5, all three ocular motor nerves in mutant embryos approached the orbit with a trajectory similar to that of wild-type. Subsequently, while wild-type nerves send terminal branches that contact target EOMs in a stereotypical pattern, the Myf5^cre/cre ocular motor nerves failed to form terminal branches, regressed, and by E18.5 two-thirds of their corresponding motor neurons died. Comparisons between mutant and wild-type embryos revealed novel aspects of trochlear and oculomotor nerve development.

Conclusions

We delineated mouse ocular motor nerve spatial and temporal development in unprecedented detail. Moreover, we found that EOMs are not necessary for initial outgrowth and guidance of ocular motor axons from the brainstem to the orbit but are required for their terminal branching and survival. These data suggest that intermediate targets in the mesenchyme provide cues necessary for appropriate targeting of ocular motor axons to the orbit, while EOM cues are responsible for terminal branching and motor neuron survival.

Spiral Ganglion Neuron Projection Development to the Hindbrain in Mice Lacking Peripheral and/or Central Target Differentiation

We investigate the importance of the degree of peripheral or central target differentiation for mouse auditory afferent navigation to the organ of Corti and auditory nuclei in three different mouse models: first, a mouse in which the differentiation of hair cells, but not central auditory nuclei neurons is compromised (Atoh1-cre; Atoh1f/f ); second, a mouse in which hair cell defects are combined with a delayed defect in central auditory nuclei neurons (Pax2-cre; Atoh1f/f ), and third, a mouse in which both hair cells and central auditory nuclei are absent (Atoh1-/-). Our results show that neither differentiated peripheral nor the central target cells of inner ear afferents are needed (hair cells, cochlear nucleus neurons) for segregation of vestibular and cochlear afferents within the hindbrain and some degree of base to apex segregation of cochlear afferents. These data suggest that inner ear spiral ganglion neuron processes may predominantly rely on temporally and spatially distinct molecular cues in the region of the targets rather than interaction with differentiated target cells for a crude topological organization. These developmental data imply that auditory neuron navigation properties may have evolved before auditory nuclei.

ISL1-based LIM complexes control Slit2 transcription in developing cranial motor neurons

LIM-homeodomain (HD) transcription factors form a multimeric complex and assign neuronal subtype identities, as demonstrated by the hexameric ISL1-LHX3 complex which gives rise to somatic motor (SM) neurons. However, the roles of combinatorial LIM code in motor neuron diversification and their subsequent differentiation is much less well understood. In the present study, we demonstrate that the ISL1 controls postmitotic cranial branchiomotor (BM) neurons including the positioning of the cell bodies and peripheral axon pathfinding. Unlike SM neurons, which transform into interneurons, BM neurons are normal in number and in marker expression in Isl1 mutant mice. Nevertheless, the movement of trigeminal and facial BM somata is stalled, and their peripheral axons are fewer or misrouted, with ectopic branches. Among genes whose expression level changes in previous ChIP-seq and microarray analyses in Isl1-deficient cell lines, we found that Slit2 transcript was almost absent from BM neurons of Isl1 mutants. Both ISL1-LHX3 and ISL1-LHX4 bound to the Slit2 enhancer and drove endogenous Slit2 expression in SM and BM neurons. Our findings suggest that combinations of ISL1 and LHX factors establish cell-type specificity and functional diversity in terms of motor neuron identities and/or axon development.

Netrin1/DCC signaling promotes neuronal migration in the dorsal spinal cord

BACKGROUND

Newborn neurons often migrate before undergoing final differentiation, extending neurites, and forming synaptic connections. Therefore, neuronal migration is crucial for establishing neural circuitry during development. In the developing spinal cord, neuroprogenitors first undergo radial migration within the ventricular zone. Differentiated neurons continue to migrate tangentially before reaching the final positions. The molecular pathways that regulate these migration processes remain largely unknown. Our previous study suggests that the DCC receptor is important for the migration of the dorsal spinal cord progenitors and interneurons. In this study, we determined the involvement of the Netrin1 ligand and the ROBO3 coreceptor in the migration.

RESULTS

By pulse labeling neuroprogenitors with electroporation, we examined their radial migration in Netrin1 (Ntn1), Dcc, and Robo3 knockout mice. We found that all three mutants exhibit delayed migration. Furthermore, using immunohistochemistry of the BARHL2 interneuron marker, we found that the mediolateral and dorsoventral migration of differentiated dorsal interneurons is also delayed. Together, our results suggest that Netrin1/DCC signaling induce neuronal migration in the dorsal spinal cord.

CONCLUSIONS

Netrin1, DCC, and ROBO3 have been extensively studied for their functions in regulating axon guidance in the spinal commissural interneurons. We reveal that during earlier development of dorsal interneurons including commissural neurons, these molecules play an important role in promoting cell migration.

Hyperexpressed Netrin-1 Promoted Neural Stem Cells Migration in Mice after Focal Cerebral Ischemia

Endogenous Netrin-1 (NT-1) protein was significantly increased after cerebral ischemia, which may participate in the repair after transient cerebral ischemic injury. In this work, we explored whether NT-1 can be steadily overexpressed by adeno-associated virus (AAV) and the exogenous NT-1 can promote neural stem cells migration from the subventricular zone (SVZ) region after cerebral ischemia. Adult CD-1 mice were injected stereotacticly with AAV carrying NT-1 gene (AAV-NT-1). Mice underwent 60 min of middle cerebral artery (MCA) occlusion 1 week after injection. We found that NT-1 mainly expressed in neuron and astrocyte, and the expression level of NT-1 significantly increased 1 week after AAV-NT-1 gene transfer and lasted for 28 days, even after transient middle cerebral artery occlusion (tMCAO) as well (p < 0.05). Immunohistochemistry results showed that the number of neural stem cells was greatly increased in the SVZ region of AAV-NT-1-transduced mice compared with control mice. Our study showed that overexpressed NT-1 promoted neural stem cells migration from SVZ. This result suggested that NT-1 is a promising factor for repairing and remodeling after focal cerebral ischemia.

EphrinB2 sharpens lateral motor column division in the developing spinal cord

Background

During sensori-motor circuit development, the somas of motoneurons (MN) are distributed in a topographic manner in the ventral horn of the neural tube. Indeed, their position within the lateral motor columns (LMC) correlates with axonal trajectories and identity of target limb muscles. The mechanisms by which this topographic distribution is established remains poorly understood. To address this issue, we assessed the role of ephrinB2 in MN topographic organization in the developing mouse spinal cord.

Results

First, we used a reporter mouse line to establish the spatio-temporal expression pattern of EfnB2 in the developing LMC. We show that early in LMC development, ephrinB2 is differentially expressed in MN of the lateral versus medial LMC, suggesting a possible role in MN sorting and/or migration. We demonstrate that while MN-specific excision of EfnB2 did not perturb specification or migration of MN, conditional loss of ephrinB2 led to the blurring of the LMC divisional boundary and to errors in the selection of LMC axon trajectory in the limb.

Conclusions

Altogether, our study uncovered a novel cell autonomous role for ephrinB2 in LMC MN thus emphasizing the prevalent role of this ephrin member in maintaining cell population boundaries.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-015-0051-9) contains supplementary material, which is available to authorized users.

Expression of Robo protein in bladder cancer tissues and its effect on the growth of cancer cells by blocking Robo protein

This study aimed to detect the expression of Slit signaling protein ligand Robo protein in human bladder cancer and para-carcinoma tissue, and observe the tumor cell survival and growth by inoculating the bladder cancer cells with the blocked signaling protein into the subcutaneous tissue of nude mice. The expression of Robo protein was detected in T24 cells in human bladder uroepithelium carcinoma and cultivated human bladder uroepithelium carcinoma confirmed by pathological diagnosis. The cultivated T24 cells were coated by the protein antibody and human bladder uroepithelium carcinoma T24 tumor-bearing mice model was established. The tumor cell survival and growth were observed in the antibody coating group and non-coating group. The tumor body size was measured. The immunohistochemical detection showed that Robo protein isoforms Robo1 and Robo 4 were expressed in T24 cells of cancer tissues, paracarcinoma tissues and cultured human uroepithelium carcinoma. The expression of Robo1 was significantly higher than that of Robo4 (P<0.05). The cancer cells could be detected in nodular tumor of mice in each group. The volume of the tumor-bearing mice in the nodular tumor of the non-coating group was larger than that of anti-Robol antibody coating group and the difference was statistically significant (P<0.01). There was no significant difference in tumor volume between anti-Robo4 antibody coating group and non-coating group (P>0.05); The difference was statistically significant compared with the anti-Robo1 antibody coating group (P<0.01). In conclusion, Robo protein isoforms Robo1 and Robo4 were expressed in human bladder cancer T24 cells. To block Robo4 signal protein had little effect on the survival and growth of the transplantation tumor and to block Robo1 signal protein would seriously affect the survival and growth of the transplantation tumor, suggesting that Robo1 might play an important role in the growth and metastasis of bladder cancer, and might become a new target for the treatment of human bladder cancer and drug research.

Slit and Semaphorin signaling governed by Islet transcription factors positions motor neuron somata within the neural tube

Motor neurons send out axons to peripheral muscles while their cell bodies remain in the ventral spinal cord. The unique configuration of motor neurons spanning the border between the CNS and PNS has been explained by structural barriers such as boundary cap (BC) cells, basal lamina and radial glia. However, mechanisms in motor neurons that retain their position have not been addressed yet. Here we demonstrate that the Islet1 (Isl1) and Islet2 (Isl2) transcription factors, which are essential for acquisition of motor neuron identity, also contribute to restrict motor neurons within the neural tube. In mice that lack both Isl1 and Isl2, large numbers of motor neurons exited the neural tube, even prior to the appearance of BC cells at the ventral exit points. Transcriptional profiling of motor neurons derived from Isl1 null embryonic stem cells revealed that transcripts of major genes involved in repulsive mechanisms were misregulated. Particularly, expression of Neuropilin1 (Npr1) and Slit2 mRNA was diminished in Islet mutant mice, and these could be target genes of the Islet proteins. Consistent with this mechanism, Robo and Slit mutations in mice and knockdown of Npr1 and Slit2 in chick embryos caused motor neurons to migrate to the periphery. Together, our study suggests that Islet genes engage Robo-Slit and Neuropilin-Semaphorin signaling in motor neurons to retain motor somata within the CNS.