The bone morphogenetic protein roof plate chemorepellent regulates the rate of commissural axonal growth

Interplay of canonical and LIMK mediated non-canonical BMP signaling is essential for regulating differential thickness and invagination during chick forebrain roof plate morphogenesis

Telencephalic hemisphere formation is a complex and precisely timed process, which begins in the chick forebrain with an invagination in the middle of the roof plate. However, the factor(s) that determine the position/site of invagination in the roof plate remain to be elucidated. In this study, we have demonstrated that as development proceeds, a region of lower thickness appears in the middle of the roof plate, which marks the position where the invagination begins. Our investigations have implicated an interplay between the canonical (pSMAD 1/5/9 dependent) and the non-canonical (LIMK dependent) arms of BMP signaling in regulating this process. We have demonstrated that LIMK dependent non-canonical BMP signaling induces high levels of phosphorylated Cofilin (pCofilin) in the middle of the roof plate, which in turn alters Actin cytoskeleton dynamics, resulting in this region being thinner than the lateral regions. This study has provided the first mechanistic insight into how forebrain roof plate invagination begins and has thrown light on the role played by BMP signaling in this process.

Retinoic acid, an essential component of the roof plate organizer, promotes the spatiotemporal segregation of dorsal neural fates

Dorsal neural tube-derived retinoic acid promotes the end of neural crest production and transition into a definitive roof plate. Here, we analyze how this impacts the segregation of central and peripheral lineages, a process essential for tissue patterning and function. Localized in ovo inhibition in quail embryos of retinoic acid activity followed by single-cell transcriptomics unraveled a comprehensive list of differentially expressed genes relevant to these processes. Importantly, progenitors co-expressed neural crest, roof plate and dI1 interneuron markers, indicating a failure in proper lineage segregation. Furthermore, separation between roof plate and dI1 interneurons is mediated by Notch activity downstream of retinoic acid, highlighting their crucial role in establishing the roof plate-dI1 boundary. Within the peripheral branch, where absence of retinoic acid resulted in neural crest production and emigration extending into the roof plate stage, sensory progenitors failed to separate from melanocytes, leading to formation of a common glia-melanocyte cell with aberrant migratory patterns. In summary, the implementation of single-cell RNA sequencing facilitated the discovery and characterization of a molecular mechanism responsible for the segregation of dorsal neural fates during development.

BMPRII+ neural precursor cells isolated and characterized from organotypic neurospheres: an in vitro model of human fetal spinal cord development

Roof plate secretion of bone morphogenetic proteins (BMPs) directs the cellular fate of sensory neurons during spinal cord development, including the formation of the ascending sensory columns, though their biology is not well understood. Type-II BMP receptor (BMPRII), the cognate receptor, is expressed by neural precursor cells during embryogenesis; however, an in vitro method of enriching BMPRII+ human neural precursor cells (hNPCs) from the fetal spinal cord is absent. Immunofluorescence was undertaken on intact second-trimester human fetal spinal cord using antibodies to BMPRII and leukemia inhibitory factor (LIF). Regions of highest BMPRII+ immunofluorescence localized to sensory columns. Parenchymal and meningeal-associated BMPRII+ vascular cells were identified in both intact fetal spinal cord and cortex by co-positivity with vascular lineage markers, CD34/CD39. LIF immunostaining identified a population of somas concentrated in dorsal and ventral horn interneurons, mirroring the expression of LIF receptor/CD118. A combination of LIF supplementation and high-density culture maintained culture growth beyond 10 passages, while synergistically increasing the proportion of neurospheres with a stratified, cytoarchitecture. These neurospheres were characterized by BMPRII+/MAP2ab+/-/βIII-tubulin+/nestin-/vimentin-/GFAP-/NeuN- surface hNPCs surrounding a heterogeneous core of βIII-tubulin+/nestin+/vimentin+/GFAP+/MAP2ab-/NeuN- multipotent precursors. Dissociated cultures from tripotential neurospheres contained neuronal (βIII-tubulin+), astrocytic (GFAP+), and oligodendrocytic (O4+) lineage cells. Fluorescence-activated cell sorting-sorted BMPRII+ hNPCs were MAP2ab+/-/βIII-tubulin+/GFAP-/O4- in culture. This is the first isolation of BMPRII+ hNPCs identified and characterized in human fetal spinal cords. Our data show that LIF combines synergistically with high-density reaggregate cultures to support the organotypic reorganization of neurospheres, characterized by surface BMPRII+ hNPCs. Our study has provided a new methodology for an in vitro model capable of amplifying human fetal spinal cord cell numbers for > 10 passages. Investigations of the role BMPRII plays in spinal cord development have primarily relied upon mouse and rat models, with interpolations to human development being derived through inference. Because of significant species differences between murine biology and human, including anatomical dissimilarities in central nervous system (CNS) structure, the findings made in murine models cannot be presumed to apply to human spinal cord development. For these reasons, our human in vitro model offers a novel tool to better understand neurodevelopmental pathways, including BMP signaling, as well as spinal cord injury research and testing drug therapies.

The role of microtubule-associated protein tau in netrin-1 attractive signaling

Direct binding of netrin receptors with dynamic microtubules (MTs) in the neuronal growth cone plays an important role in netrin-mediated axon guidance. However, how netrin-1 (NTN1) regulates MT dynamics in axon turning remains a major unanswered question. Here, we show that the coupling of netrin-1 receptor DCC with tau (MAPT)-regulated MTs is involved in netrin-1-promoted axon attraction. Tau directly interacts with DCC and partially overlaps with DCC in the growth cone of primary neurons. Netrin-1 induces this interaction and the colocalization of DCC and tau in the growth cone. The netrin-1-induced interaction of tau with DCC relies on MT dynamics and TUBB3, a highly dynamic β-tubulin isotype in developing neurons. Netrin-1 increased cosedimentation of DCC with tau and TUBB3 in MTs, and knockdown of either tau or TUBB3 mutually blocked this effect. Downregulation of endogenous tau levels by tau shRNAs inhibited netrin-1-induced axon outgrowth, branching and commissural axon attraction in vitro, and led to defects in spinal commissural axon projection in vivo. These findings suggest that tau is a key MT-associated protein coupling DCC with MT dynamics in netrin-1-promoted axon attraction.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

The Role of LIM Kinases during Development: A Lens to Get a Glimpse of Their Implication in Pathologies

The organization of cell populations within animal tissues is essential for the morphogenesis of organs during development. Cells recognize three-dimensional positions with respect to the whole organism and regulate their cell shape, motility, migration, polarization, growth, differentiation, gene expression and cell death according to extracellular signals. Remodeling of the actin filaments is essential to achieve these cell morphological changes. Cofilin is an important binding protein for these filaments; it increases their elasticity in terms of flexion and torsion and also severs them. The activity of cofilin is spatiotemporally inhibited via phosphorylation by the LIM domain kinases 1 and 2 (LIMK1 and LIMK2). Phylogenetic analysis indicates that the phospho-regulation of cofilin has evolved as a mechanism controlling the reorganization of the actin cytoskeleton during complex multicellular processes, such as those that occur during embryogenesis. In this context, the main objective of this review is to provide an update of the respective role of each of the LIM kinases during embryonic development.

Axon guidance at the spinal cord midline-A live imaging perspective

During neural circuit formation, axons navigate several choice points to reach their final target. At each one of these intermediate targets, growth cones need to switch responsiveness from attraction to repulsion in order to move on. Molecular mechanisms that allow for the precise timing of surface expression of a new set of receptors that support the switch in responsiveness are difficult to study in vivo. Mostly, mechanisms are inferred from the observation of snapshots of many different growth cones analyzed in different preparations of tissue harvested at distinct time points. However, to really understand the behavior of growth cones at choice points, a single growth cone should be followed arriving at and leaving the intermediate target. Existing ex vivo preparations, like cultures of an "open-book" preparation of the spinal cord have been successfully used to study floor plate entry and exit, but artifacts prevent the analysis of growth cone behavior at the floor plate exit site. Here, we describe a novel spinal cord preparation that allows for live imaging of individual axons during navigation in their intact environment. When comparing growth cone behavior in our ex vivo system with snapshots from in vivo navigation, we do not see any differences. The possibility to observe the dynamics of single growth cones navigating their intermediate target allows for measuring growth speed, changes in morphology, or aberrant behavior, like stalling and wrong turning. Moreover, observation of the intermediate target-the floor plate-revealed its active participation and interaction with commissural axons during midline crossing.

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.

Apcdd1 is a dual BMP/Wnt inhibitor in the developing nervous system and skin

Animal development and homeostasis depend on precise temporal and spatial intercellular signaling. Components shared between signaling pathways, generally thought to decrease specificity, paradoxically can also provide a solution to pathway coordination. Here we show that the Bone Morphogenetic Protein (BMP) and Wnt signaling pathways share Apcdd1 as a common inhibitor and that Apcdd1 is a taxon-restricted gene with novel domains and signaling functions. Previously, we showed that Apcdd1 inhibits Wnt signaling (Shimomura et al., 2010), here we find that Apcdd1 potently inhibits BMP signaling in body axis formation and neural differentiation in chicken, frog, zebrafish. Furthermore, we find that Apcdd1 has an evolutionarily novel protein domain. Our results from experiments and modeling suggest that Apcdd1 may coordinate the outputs of two signaling pathways that are central to animal development and human disease.

Overexpression of long non-coding RNA SNHG16 against cerebral ischemia-reperfusion injury through miR-106b-5p/LIMK1 axis

Long non-coding RNA (LncRNA) involved in types of physiological insults and diseases via regulating the responses of complex molecular, including cerebral ischemia-reperfusion (I/R) injury. LncRNA SNHG16 played a potential role in ketamine-induced neurotoxicity. In this study, we utilized an in vitro cell model of I/R to examine the specific function and mechanism of LncRNA SNHG16 in oxygen-glucose deprivation and reperfusion (OGD/R) induced SH-SY5Y cells. After in vitro treatment of OGD/R, the lower the SH-SY5Y cell survival, the higher cell the apoptosis and increased caspase-3 activity was observed. Also, OGD/R induced endoplasmic reticulum stress (ERS) through increasing GRP78 and CHOP expressions and down-regulated LncRNA SNHG16 in SH-SY5Y cells. Conversely, LncRNA SNHG16 overexpression promoted OGD/R induced SH-SY5Y cell survival, suppressed its apoptosis, and caspase-3 activity. GRP78 and CHOP expressions were significantly suppressed in LncRNA SNHG16 overexpressing cells. MiR-106b-5p expression was increased and LIMK1 expression was down-regulated in OGD/R induced SH-SY5Y cells, and these effects were reversed by LncRNA SNHG16 overexpression, respectively. Moreover, LIMK1 is a direct target of MiR-106b-5p, and knockdown of LIMK1 reversed the effects of LncRNA SNHG16 on OGD/R-induced SH-SY5Y cells biology. Altogether, these results confirmed an important neuroprotection role of LncRNA SNHG16 in OGD/R induced SH-SY5Y cells injury, and miR-106b-5p/LIMK1 signal axis was involved in the action of LncRNA SNHG16.

The Cofilin/Limk1 Pathway Controls the Growth Rate of Both Developing and Regenerating Motor Axons

Regenerating axons often have to grow considerable distances to reestablish circuits, making functional recovery a lengthy process. One solution to this problem would be to co-opt the "temporal" guidance mechanisms that control the rate of axon growth during development to accelerate the rate at which nerves regenerate in adults. We have previously found that the loss of Limk1, a negative regulator of cofilin, accelerates the rate of spinal commissural axon growth. Here, we use mouse models to show that spinal motor axon outgrowth is similarly promoted by the loss of Limk1, suggesting that temporal guidance mechanisms are widely used during development. Furthermore, we find that the regulation of cofilin activity is an acute response to nerve injury in the peripheral nervous system. Within hours of a sciatic nerve injury, the level of phosphorylated cofilin dramatically increases at the lesion site, in a Limk1-dependent manner. This response may be a major constraint on the rate of peripheral nerve regeneration. Proof-of-principle experiments show that elevating cofilin activity, through the loss of Limk1, results in faster sciatic nerve growth, and improved recovery of some sensory and motor function.SIGNIFICANCE STATEMENT The studies shed light on an endogenous, shared mechanism that controls the rate at which developing and regenerating axons grow. An understanding of these mechanisms is key for developing therapies to reduce painful recovery times for nerve-injury patients, by accelerating the rate at which damaged nerves reconnect with their synaptic targets.

Commissural axon guidance in the developing spinal cord: from Cajal to the present day

During neuronal development, the formation of neural circuits requires developing axons to traverse a diverse cellular and molecular environment to establish synaptic contacts with the appropriate postsynaptic partners. Essential to this process is the ability of developing axons to navigate guidance molecules presented by specialized populations of cells. These cells partition the distance traveled by growing axons into shorter intervals by serving as intermediate targets, orchestrating the arrival and departure of axons by providing attractive and repulsive guidance cues. The floor plate in the central nervous system (CNS) is a critical intermediate target during neuronal development, required for the extension of commissural axons across the ventral midline. In this review, we begin by giving a historical overview of the ventral commissure and the evolutionary purpose of decussation. We then review the axon guidance studies that have revealed a diverse assortment of midline guidance cues, as well as genetic and molecular regulatory mechanisms required for coordinating the commissural axon response to these cues. Finally, we examine the contribution of dysfunctional axon guidance to neurological diseases.

LIMK1 and LIMK2 regulate cortical development through affecting neural progenitor cell proliferation and migration

LIMK1 and LIMK2 are key downstream targets to mediate the effects of the Rho family small GTPases and p21-activated kinases (PAK) in the regulation of the actin cytoskeleton. LIMKs are also critical for synaptic transmission, plasticity and memory formation. Changes in LIMK signaling are associated with several neurodevelopmental and neurodegenerative diseases, including autism, intellectual disability and Alzheimer's disease. However, the role of LIMK signaling in brain development remains unknown. In this study, we used LIMK1 KO and LIMK2 KO mice to investigate the role of LIMK signaling in the cerebral cortical development. We found that these KO mice are reduced in the number of pyramidal neurons in upper cortical layers and this reduction is accompanied by a smaller pool of neural progenitor cells and impaired neuronal migration. These results are similar to those found in PAK1 KO mice and suggest that LIMK-dependent actin regulation may play a key role in mediating the effects of PAK1 and Rho signaling in the regulation of cortical development.

Understanding axon guidance: are we nearly there yet?

During nervous system development, neurons extend axons to reach their targets and form functional circuits. The faulty assembly or disintegration of such circuits results in disorders of the nervous system. Thus, understanding the molecular mechanisms that guide axons and lead to neural circuit formation is of interest not only to developmental neuroscientists but also for a better comprehension of neural disorders. Recent studies have demonstrated how crosstalk between different families of guidance receptors can regulate axonal navigation at choice points, and how changes in growth cone behaviour at intermediate targets require changes in the surface expression of receptors. These changes can be achieved by a variety of mechanisms, including transcription, translation, protein-protein interactions, and the specific trafficking of proteins and mRNAs. Here, I review these axon guidance mechanisms, highlighting the most recent advances in the field that challenge the textbook model of axon guidance.

The developmental and genetic basis of 'clubfoot' in the peroneal muscular atrophy mutant mouse

Genetic factors underlying the human limb abnormality congenital talipes equinovarus ('clubfoot') remain incompletely understood. The spontaneous autosomal recessive mouse 'peroneal muscular atrophy' mutant (PMA) is a faithful morphological model of human clubfoot. In PMA mice, the dorsal (peroneal) branches of the sciatic nerves are absent. In this study, the primary developmental defect was identified as a reduced growth of sciatic nerve lateral motor column (LMC) neurons leading to failure to project to dorsal (peroneal) lower limb muscle blocks. The pma mutation was mapped and a candidate gene encoding LIM-domain kinase 1 (Limk1) identified, which is upregulated in mutant lateral LMC motor neurons. Genetic and molecular analyses showed that the mutation acts in the EphA4-Limk1-Cfl1/cofilin-actin pathway to modulate growth cone extension/collapse. In the chicken, both experimental upregulation of Limk1 by electroporation and pharmacological inhibition of actin turnover led to defects in hindlimb spinal motor neuron growth and pathfinding, and mimicked the clubfoot phenotype. The data support a neuromuscular aetiology for clubfoot and provide a mechanistic framework to understand clubfoot in humans.

Perturbation of canonical and non-canonical BMP signaling affects migration, polarity and dendritogenesis of mouse cortical neurons

Bone morphogenetic protein (BMP) signaling has been implicated in the regulation of patterning of the forebrain and as a regulator of neurogenesis and gliogenesis in the mammalian cortex. However, its role in other aspects of cortical development in vivo remains unexplored. We hypothesized that BMP signaling might regulate additional processes during the development of cortical neurons after observing active BMP signaling in a spatiotemporally dynamic pattern in the mouse cortex. Our investigation revealed that BMP signaling specifically regulates the migration, polarity and the dendritic morphology of upper layer cortical neurons born at E15.5. On further dissection of the role of canonical and non-canonical BMP signaling in each of these processes, we found that migration of these neurons is regulated by both pathways. Their polarity, however, appears to be affected more strongly by canonical BMP signaling, whereas dendritic branch formation appears to be somewhat more strongly affected by LIMK-mediated non-canonical BMP signaling.

Netrin1 establishes multiple boundaries for axon growth in the developing spinal cord

The canonical model for netrin1 function proposed that it acted as a long-range chemotropic axon guidance cue. In the developing spinal cord, floor-plate (FP)-derived netrin1 was thought to act as a diffusible attractant to draw commissural axons to the ventral midline. However, our recent studies have shown that netrin1 is dispensable in the FP for axon guidance. We have rather found that netrin1 acts locally: netrin1 is produced by neural progenitor cells (NPCs) in the ventricular zone (VZ), and deposited on the pial surface as a haptotactic adhesive substrate that guides Dcc⁺ axon growth. Here, we further demonstrate that this netrin1 pial-substrate has an early role orienting pioneering spinal axons, directing them to extend ventrally. However, as development proceeds, commissural axons choose to grow around a boundary of netrin1 expressing cells in VZ, instead of continuing to extend alongside the netrin1 pial-substrate in the ventral spinal cord. This observation suggests netrin1 may supply a more complex activity than pure adhesion, with netrin1-expressing cells also supplying a growth boundary for axons. Supporting this possibility, we have observed that additional domains of netrin1 expression arise adjacent to the dorsal root entry zone (DREZ) in E12.5 mice that are also required to sculpt axonal growth. Together, our studies suggest that netrin1 provides "hederal" boundaries: a local growth substrate that promotes axon extension, while also preventing local innervation of netrin1-expressing domains.

Netrin1 Produced by Neural Progenitors, Not Floor Plate Cells, Is Required for Axon Guidance in the Spinal Cord

Netrin1 has been proposed to act from the floor plate (FP) as a long-range diffusible chemoattractant for commissural axons in the embryonic spinal cord. However, netrin1 mRNA and protein are also present in neural progenitors within the ventricular zone (VZ), raising the question of which source of netrin1 promotes ventrally directed axon growth. Here, we use genetic approaches in mice to selectively remove netrin from different regions of the spinal cord. Our analyses show that the FP is not the source of netrin1 directing axons to the ventral midline, while local VZ-supplied netrin1 is required for this step. Furthermore, rather than being present in a gradient, netrin1 protein accumulates on the pial surface adjacent to the path of commissural axon extension. Thus, netrin1 does not act as a long-range secreted chemoattractant for commissural spinal axons but instead promotes ventrally directed axon outgrowth by haptotaxis, i.e., directed growth along an adhesive surface.

Where does axon guidance lead us?

During neural circuit formation, axons need to navigate to their target cells in a complex, constantly changing environment. Although we most likely have identified most axon guidance cues and their receptors, we still cannot explain the molecular background of pathfinding for any subpopulation of axons. We lack mechanistic insight into the regulation of interactions between guidance receptors and their ligands. Recent developments in the field of axon guidance suggest that the regulation of surface expression of guidance receptors comprises transcriptional, translational, and post-translational mechanisms, such as trafficking of vesicles with specific cargos, protein-protein interactions, and specific proteolysis of guidance receptors. Not only axon guidance molecules but also the regulatory mechanisms that control their spatial and temporal expression are involved in synaptogenesis and synaptic plasticity. Therefore, it is not surprising that genes associated with axon guidance are frequently found in genetic and genomic studies of neurodevelopmental disorders.

Early Commissural Diencephalic Neurons Control Habenular Axon Extension and Targeting

Most neuronal populations form on both the left and right sides of the brain. Their efferent axons appear to grow synchronously along similar pathways on each side, although the neurons or their environment often differ between the two hemispheres [1-4]. How this coordination is controlled has received little attention. Frequently, neurons establish interhemispheric connections, which can function to integrate information between brain hemispheres (e.g., [5]). Such commissures form very early, suggesting their potential developmental role in coordinating ipsilateral axon navigation during embryonic development [4]. To address the temporal-spatial control of bilateral axon growth, we applied long-term time-lapse imaging to visualize the formation of the conserved left-right asymmetric habenular neural circuit in the developing zebrafish embryo [6]. Although habenular neurons are born at different times across brain hemispheres [7], we found that elongation of habenular axons occurs synchronously. The initiation of axon extension is not controlled within the habenular network itself but through an early developing proximal diencephalic network. The commissural neurons of this network influence habenular axons both ipsilaterally and contralaterally. Their unilateral absence impairs commissure formation and coordinated habenular axon elongation and causes their subsequent arrest on both sides of the brain. Thus, habenular neural circuit formation depends on a second intersecting commissural network, which facilitates the exchange of information between hemispheres required for ipsilaterally projecting habenular axons. This mechanism of network formation may well apply to other circuits, and has only remained undiscovered due to technical limitations.

From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate

During vertebrate development, the central (CNS) and peripheral nervous systems (PNS) arise from the neural plate. Cells at the margin of the neural plate give rise to neural crest cells, which migrate extensively throughout the embryo, contributing to the majority of neurons and all of the glia of the PNS. The rest of the neural plate invaginates to form the neural tube, which expands to form the brain and spinal cord. The emergence of molecular cloning techniques and identification of fluorophores like Green Fluorescent Protein (GFP), together with transgenic and electroporation technologies, have made it possible to easily visualize the cellular and molecular events in play during nervous system formation. These lineage-tracing techniques have precisely demonstrated the migratory pathways followed by neural crest cells and increased knowledge about their differentiation into PNS derivatives. Similarly, in the spinal cord, lineage-tracing techniques have led to a greater understanding of the regional organization of multiple classes of neural progenitor and post-mitotic neurons along the different axes of the spinal cord and how these distinct classes of neurons assemble into the specific neural circuits required to realize their various functions. Here, we review how both classical and modern lineage and marker analyses have expanded our knowledge of early peripheral nervous system and spinal cord development.

Vertebrate spinal commissural neurons: a model system for studying axon guidance beyond the midline

For bilaterally symmetric organisms, the transfer of information between the left and right side of the nervous system is mediated by commissures formed by neurons that project their axons across the body midline to the contralateral side of the central nervous system (CNS). After crossing the midline, many of these axons must travel long distances to reach their targets, including those that extend from spinal commissural neurons. Owing to the highly stereotyped trajectories of spinal commissural neurons that can be divided into several segments as these axons project to their targets, it is an ideal system for investigators to ask fundamental questions related to mechanisms of short- and long-range axon guidance, fasciculation, and choice point decisions at the midline intermediate target. In addition, studies of patterning genes of the nervous system have revealed complex transcription factor codes that function in a combinatorial fashion to specify individual classes of spinal neurons including commissural neurons. Despite these advances and the functional importance of spinal commissural neurons in mediating the transfer of external sensory information from the peripheral nervous system (PNS) to the CNS, only a handful of studies have begun to elucidate the mechanistic logic underlying their long-range pathfinding and the characterization of their synaptic targets. Using in vitro assays, in vivo labeling methodologies, in combination with both loss- and gain-of-function experiments, several studies have revealed that the molecular mechanisms of long-range spinal commissural axon pathfinding involve an interplay between classical axon guidance cues, morphogens and cell adhesion molecules. For further resources related to this article, please visit the WIREs website.

Modifying lipid rafts promotes regeneration and functional recovery

Ideal strategies to ameliorate CNS damage should promote both neuronal survival and axon regeneration. The receptor Neogenin promotes neuronal apoptosis. Its ligand prevents death, but the resulting repulsive guidance molecule a (RGMa)-Neogenin interaction also inhibits axonal growth, countering any prosurvival benefits. Here, we explore strategies to inhibit Neogenin, thus simultaneously enhancing survival and regeneration. We show that bone morphogenetic protein (BMP) and RGMa-dependent recruitment of Neogenin into lipid rafts requires an interaction between RGMa and Neogenin subdomains. RGMa or Neogenin peptides that prevent this interaction, BMP inhibition by Noggin, or reduction of membrane cholesterol all block Neogenin raft localization, promote axon outgrowth, and prevent neuronal apoptosis. Blocking Neogenin raft association influences axonal pathfinding, enhances survival in the developing CNS, and promotes survival and regeneration in the injured adult optic nerve and spinal cord. Moreover, lowering cholesterol disrupts rafts and restores locomotor function after spinal cord injury. These data reveal a unified strategy to promote both survival and regeneration in the CNS.

Canonical BMP-Smad signalling promotes neurite growth in rat midbrain dopaminergic neurons

Ventral midbrain (VM) dopaminergic (DA) neurons project to the dorsal striatum via the nigrostriatal pathway to regulate voluntary movements, and their loss leads to the motor dysfunction seen in Parkinson's disease (PD). Despite recent progress in the understanding of VM DA neurogenesis, the factors regulating nigrostriatal pathway development remain largely unknown. The bone morphogenetic protein (BMP) family regulates neurite growth in the developing nervous system and may contribute to nigrostriatal pathway development. Two related members of this family, BMP2 and growth differentiation factor (GDF)5, have neurotrophic effects, including promotion of neurite growth, on cultured VM DA neurons. However, the molecular mechanisms regulating their effects on DA neurons are unknown. By characterising the temporal expression profiles of endogenous BMP receptors (BMPRs) in the developing and adult rat VM and striatum, this study identified BMP2 and GDF5 as potential regulators of nigrostriatal pathway development. Furthermore, through the use of noggin, dorsomorphin and BMPR/Smad plasmids, this study demonstrated that GDF5- and BMP2-induced neurite outgrowth from cultured VM DA neurons is dependent on BMP type I receptor activation of the Smad 1/5/8 signalling pathway.

The multiple activities of BMPs during spinal cord development

Bone morphogenetic proteins (BMPs) are one of the main classes of multi-faceted secreted factors that drive vertebrate development. A growing body of evidence indicates that BMPs contribute to the formation of the central nervous system throughout its development, from the initial shaping of the neural primordium to the generation and maturation of the different cell types that form the functional adult nervous tissue. In this review, we focus on the multiple activities of BMPs during spinal cord development, paying particular attention to recent results that highlight the complexity of BMP signaling during this process. These findings emphasize the unique capacity of these signals to mediate various functions in the same tissue throughout development, recruiting diverse effectors and strategies to instruct their target cells.

BMP-Smad 1/5/8 signalling in the development of the nervous system

The transcription factors, Smad1, Smad5 and Smad8, are the pivotal intracellular effectors of the bone morphogenetic protein (BMP) family of proteins. BMPs and their receptors are expressed in the nervous system (NS) throughout its development. This review focuses on the actions of Smad 1/5/8 in the developing NS. The mechanisms by which these Smad proteins regulate the induction of the neuroectoderm, the central nervous system (CNS) primordium, and finally the neural crest, which gives rise to the peripheral nervous system (PNS), are reviewed herein. We describe how, following neural tube closure, the most dorsal aspect of the tube becomes a signalling centre for BMPs, which directs the pattern of the development of the dorsal spinal cord (SC), through the action of Smad1, Smad5 and Smad8. The direct effects of Smad 1/5/8 signalling on the development of neuronal and non-neuronal cells from various neural progenitor cell populations are then described. Finally, this review discusses the neurodevelopmental abnormalities associated with the knockdown of Smad 1/5/8.

BMP2 and GDF5 induce neuronal differentiation through a Smad dependant pathway in a model of human midbrain dopaminergic neurons

Parkinson's disease is the second most common neurodegenerative disease, and is characterised by the progressive degeneration of the nigrostriatal dopaminergic (DA) system. Current treatments are symptomatic, and do not protect against the DA neuronal loss. One of the most promising treatment approaches is the application of neurotrophic factors to rescue the remaining population of nigrostriatal DA neurons. Therefore, the identification of new neurotrophic factors for midbrain DA neurons, and the subsequent elucidation of the molecular bases of their effects, are important. Two related members of the bone morphogenetic protein (BMP) family, BMP2 and growth differentiation factor 5 (GDF5), have been shown to have neurotrophic effects on midbrain DA neurons both in vitro and in vivo. However, the molecular (signalling pathway(s)) and cellular (direct neuronal or indirect via glial cells) mechanisms of their effects remain to be elucidated. Using the SH-SH5Y human neuronal cell line, as a model of human midbrain DA neurons, we have shown that GDF5 and BMP2 induce neurite outgrowth via a direct mechanism. Furthermore, we demonstrate that these effects are dependent on BMP type I receptor activation of canonical Smad 1/5/8 signalling.

Bilaterally symmetric populations of chicken dI1 (commissural) axons cross the floor plate independently of each other

Axons use temporal and directional guidance cues at intermediate targets to set the rate and direction of growth towards their synaptic targets. Our recent studies have shown that disrupting the temporal guidance process, by unilaterally accelerating the rate at which spinal dI1 (commissural) axons grow, resulted in turning errors both in the ventral spinal cord and after crossing the floor plate. Here we investigate a mechanistic explanation for these defects: the accelerated dI1 axons arrive in the ventral spinal cord before necessary fasciculation cues from incoming dI1 axons from the opposite side of the spinal cord. The identification of such an interaction would support a model of selective fasciculation whereby the pioneering dI1 axons serve as guides for the processes of the bilaterally symmetrical population of dI1 neurons. To test this model, we first developed the ability to “double” in ovo electroporate the embryonic chicken spinal cord to independently manipulate the rate of growth of the two bilateral populations of dI1 axons. Second, we examined the requirement for a putative bilateral interaction by unilaterally ablating the dI1 population in cultured explants of chicken embryonic spinal cord. Surprisingly, we find no evidence for a bilateral dI1 axon interaction, rather dI1 axons appear to project independently of each other.

Rnf165/Ark2C enhances BMP-Smad signaling to mediate motor axon extension

Little is known about extrinsic signals required for the advancement of motor neuron (MN) axons, which extend over long distances in the periphery to form precise connections with target muscles. Here we present that Rnf165 (Arkadia-like; Arkadia2; Ark2C) is expressed specifically in the nervous system and that its loss in mice causes motor innervation defects that originate during development and lead to wasting and death before weaning. The defects range from severe reduction of motor axon extension as observed in the dorsal forelimb to shortening of presynaptic branches of the phrenic nerve, as observed in the diaphragm. Molecular functional analysis showed that in the context of the spinal cord Ark2C enhances transcriptional responses of the Smad1/5/8 effectors, which are activated (phosphorylated) downstream of Bone Morphogenetic Protein (BMP) signals. Consistent with Ark2C-modulated BMP signaling influencing motor axons, motor pools in the spinal cord were found to harbor phosphorylated Smad1/5/8 (pSmad) and treatment of primary MN with BMP inhibitor diminished axon length. In addition, genetic reduction of BMP-Smad signaling in Ark2C (+/-) mice caused the emergence of Ark2C (-/-) -like dorsal forelimb innervation deficits confirming that enhancement of BMP-Smad responses by Ark2C mediates efficient innervation. Together the above data reveal an involvement of BMP-Smad signaling in motor axon advancement.

Integration of guidance cues: parallel signaling and crosstalk

Growing axons are exposed to various guidance cues en route to their targets. Although many guidance molecules have been identified and their effects on axon behavior extensively studied, how axons react to combinations of signals remains largely unexplored. We review recent studies investigating the combined actions of guidance cues present at the same choice points. Two main scenarios are emerging from these studies: parallel signaling and crosstalk between guidance systems. In the first case, cues act in an additive manner, whereas in the second case the outcome is non-additive and differs from the sum of individual effects, suggesting more complex signal integration in the growth cone. Some of the molecular mechanisms underlying these interactions are beginning to be unraveled.

BMP receptor 1A regulates development of hypothalamic circuits critical for feeding behavior

Hypothalamic neural circuits are known to regulate energy homeostasis and feeding behavior, but how these circuits are established during development is not well understood. Here we report that embryonic neural progenitors that express the transcription factor OLIG1 contribute neurons to the ventral hypothalamus including the arcuate nucleus (ARH), a center that regulates feeding behavior. Ablation of bone morphogenetic protein receptor 1a (BMPR1A) in the OLIG1 lineage resulted in hypophagia, hypoglycemia, and weight loss after the second postnatal week with death by week 4. Differentiation and specification of inhibitory hypothalamic neurons contributing to melanocortin and dopaminergic systems were abnormal in the BMPR1A-deficient ARH. Although the hypophagia promoted expression of the orexigenic neuropeptide agouti related protein (AgRP) in the BMPR1A-deficient ARH, there was a profound decrease of AgRP(+) axonal terminals in the mutant ARH targets including dorsomedial and paraventricular hypothalamic nuclei. Projection of AgRP(+) neurons to these nuclei is known to be regulated by leptin. Leptin injection in neonatal mice increased bone morphogenic protein (BMP) signaling in the ventral hypothalamus, and blocking BMP signaling prevented leptin-induced neurite outgrowth in ARH explant cultures. These findings suggest that BMPR1A signaling is critical for postnatal establishment of leptin-responsive orexigenic fibers from ARH to multiple hypothalamic nuclei. More generally these observations indicate that BMPR1A signaling regulates postnatal establishment of OLIG1 lineage-derived ARH neuronal circuits that are critical for leptin-mediated feeding behavior.

Type Ib BMP receptors mediate the rate of commissural axon extension through inhibition of cofilin activity

Bone morphogenetic proteins (BMPs) have unexpectedly diverse activities establishing different aspects of dorsal neural circuitry in the developing spinal cord. Our recent studies have shown that, in addition to spatially orienting dorsal commissural (dI1) axons, BMPs supply 'temporal' information to commissural axons to specify their rate of growth. This information ensures that commissural axons reach subsequent signals at particular times during development. However, it remains unresolved how commissural neurons specifically decode this activity of BMPs to result in their extending axons at a specific speed through the dorsal spinal cord. We have addressed this question by examining whether either of the type I BMP receptors (Bmpr), BmprIa and BmprIb, have a role controlling the rate of commissural axon growth. BmprIa and BmprIb exhibit a common function specifying the identity of dorsal cell fate in the spinal cord, whereas BmprIb alone mediates the ability of BMPs to orient axons. Here, we show that BmprIb, and not BmprIa, is additionally required to control the rate of commissural axon extension. We have also determined the intracellular effector by which BmprIb regulates commissural axon growth. We show that BmprIb has a novel role modulating the activity of the actin-severing protein cofilin. These studies reveal the mechanistic differences used by distinct components of the canonical Bmpr complex to mediate the diverse activities of the BMPs.

Members of the BMP, Shh, and FGF morphogen families promote chicken statoacoustic ganglion neurite outgrowth and neuron survival in vitro

Mechanosensory hair cells of the chicken inner ear are innervated by the peripheral processes of statoacoustic ganglion (SAG) neurons. Members of several morphogen families are expressed within and surrounding the chick inner ear during stages of SAG axon outgrowth and pathfinding. On the basis of their localized expression patterns, we hypothesized that bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and sonic hedgehog (Shh) may function as guidance cues for growing axons and/or may function as trophic factors once axons have reached their targets. To test this hypothesis, three-dimensional collagen cultures were used to grow Embryonic Day 4 (E4) chick SAG explants for 24 h in the presence of purified proteins or beads soaked in proteins. The density of neurite outgrowth was quantified to determine effects on neurite outgrowth. Explants displayed enhanced neurite outgrowth when cultured in the presence of purified BMP4, BMP7, a low concentration of Shh, FGF8, FGF10, or FGF19. In contrast, SAG neurons appeared unresponsive to FGF2. Collagen gel cultures were labeled with terminal dUTP nick-end labeling and immunostained with anti-phosphohistone H3 to determine effects on neuron survival and proliferation, respectively. Treatments that increased neurite outgrowth also yielded significantly fewer apoptotic cells, with no effect on cell proliferation. When presented as focal sources, BMP4, Shh, and FGFs -8, -10, and -19 promoted asymmetric outgrowth from the ganglion in the direction of the beads. BMP7-soaked beads did not induce this response. These results suggest that a subset of morphogens enhance both survival and axon outgrowth of otic neurons.

BMP receptor-activated Smads confer diverse functions during the development of the dorsal spinal cord

Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.

LAR receptor tyrosine phosphatases and HSPGs guide peripheral sensory axons to the skin

BACKGROUND

Peripheral axons of somatosensory neurons innervate the skin early in development to detect touch stimuli. Embryological experiments had suggested that the skin produces guidance cues that attract sensory axons, but neither the attractants nor their neuronal receptors had previously been identified.

RESULTS

To investigate peripheral axon navigation to the skin, we combined live imaging of developing zebrafish Rohon-Beard (RB) neurons with molecular loss-of-function manipulations. Simultaneously knocking down two members of the leukocyte antigen-related (LAR) family of receptor tyrosine phosphatases expressed in RB neurons, or inhibiting their function with dominant-negative proteins, misrouted peripheral axons to internal tissues. Time-lapse imaging indicated that peripheral axon guidance, rather than outgrowth or maintenance, was defective in LAR-deficient neurons. Peripheral axons displayed a similar misrouting phenotype in mutants defective in heparan sulfate proteoglycan (HSPG) production and avoided regions in which HSPGs were locally degraded.

CONCLUSIONS

HSPGs and LAR family receptors are required for sensory axon guidance to the skin. Together, our results support a model in which peripheral HSPGs are attractive ligands for LAR receptors on RB neurons.

Type III neuregulin 1 regulates pathfinding of sensory axons in the developing spinal cord and periphery

Sensory axons must develop appropriate connections with both central and peripheral targets. Whereas the peripheral cues have provided a classic model for neuron survival and guidance, less is known about the central cues or the coordination of central and peripheral connectivity. Here we find that type III Nrg1, in addition to its known effect on neuron survival, regulates axon pathfinding. In type III Nrg1(-/-) mice, death of TrkA(+) nociceptive/thermoreceptive neurons was increased, and could be rescued by Bax elimination. In the Bax and type III Nrg1 double mutants, axon pathfinding abnormalities were seen for TrkA(+) neurons both in cutaneous peripheral targets and in spinal cord central targets. Axon guidance phenotypes in the spinal cord included penetration of axons into ventral regions from which they would normally be repelled by Sema3A. Accordingly, sensory neurons from type III Nrg1(-/-) mice were unresponsive to the repellent effects of Sema3A in vitro, which might account, at least in part, for the central projection phenotype, and demonstrates an effect of type III Nrg1 on guidance cue responsiveness in neurons. Moreover, stimulation of type III Nrg1 back-signaling in cultured sensory neurons was found to regulate axonal levels of the Sema3A receptor neuropilin 1. These results reveal a molecular mechanism whereby type III Nrg1 signaling can regulate the responsiveness of neurons to a guidance cue, and show that type III Nrg1 is required for normal sensory neuron survival and axon pathfinding in both central and peripheral targets.

Inductive specification and axonal orientation of spinal neurons mediated by divergent bone morphogenetic protein signaling pathways

Background

Bone morphogenetic protein (BMP)7 evokes both inductive and axon orienting responses in dorsal interneurons (dI neurons) in the developing spinal cord. These events occur sequentially during the development of spinal neurons but in these and other cell types such inductive and acute chemotactic responses occur concurrently, highlighting the requirement for divergent intracellular signaling. Both type I and type II BMP receptor subtypes have been implicated selectively in orienting responses but it remains unclear how, in a given cell, divergence occurs. We have examined the mechanisms by which disparate BMP7 activities are generated in dorsal spinal neurons.

Results

We show that widely different threshold concentrations of BMP7 are required to elicit the divergent inductive and axon orienting responses. Type I BMP receptor kinase activity is required for activation of pSmad signaling and induction of dI character by BMP7, a high threshold response. In contrast, neither type I BMP receptor kinase activity nor Smad1/5/8 phosphorylation is involved in the low threshold orienting responses of dI axons to BMP7. Instead, BMP7-evoked axonal repulsion and growth cone collapse are dependent on phosphoinositide-3-kinase (PI3K) activation, plausibly through type II receptor signaling. BMP7 stimulates PI3K-dependent signaling in dI neurons. BMP6, which evokes neural induction but does not have orienting activity, activates Smad signaling but does not stimulate PI3K.

Conclusions

Divergent signaling through pSmad-dependent and PI3K-dependent (Smad-independent) mechanisms mediates the inductive and orienting responses of dI neurons to BMP7. A model is proposed whereby selective engagement of BMP receptor subunits underlies choice of signaling pathway.

Cell type specific, traceable gene silencing for functional gene analysis during vertebrate neural development

Many genes have several, sometimes divergent functions during development. Therefore, timing of gene knockdown for functional analysis during development has to be done with precise temporal control, as loss of a gene's function at early stages prevents its analysis later in development. RNAi, in combination with the accessibility of chicken embryos, is an effective approach for temporally controlled analysis of gene function during neural development. Here, we describe novel plasmid vectors that contain cell type-specific promoters/enhancers to drive the expression of a fluorescent marker, followed directly by a miR30-RNAi transcript for gene silencing. These vectors allow for direct tracing of cells experiencing gene silencing by the bright fluorescence. The level of knockdown is sufficient to reproduce the expected pathfinding defects upon perturbation of genes with known axon guidance functions. Mixing different vectors prior to electroporation enables the simultaneous knockdown of multiple genes in independent regions of the spinal cord. This permits complex cellular and molecular interactions to be examined during development, in a fast and precise manner. The advancements of the in ovo RNAi technique that we describe will not only markedly enhance functional gene analysis in the chicken, but also could be adapted to other organisms in developmental studies.

Neogenin may functionally substitute for Dcc in chicken

Dcc is the key receptor that mediates attractive responses of axonal growth cones to netrins, a family of axon guidance cues used throughout evolution. However, a Dcc homolog has not yet been identified in the chicken genome, raising the possibility that Dcc is not present in avians. Here we show that the closely related family member neogenin may functionally substitute for Dcc in the developing chicken spinal cord. The expression pattern of chicken neogenin in the developing spinal cord is a composite of the distribution patterns of both rodent Dcc and neogenin. Moreover, whereas the loss of mouse neogenin has no effect on the trajectory of commissural axons, removing chicken neogenin by RNA interference results in a phenotype similar to the functional inactivation of Dcc in mouse. Taken together, these data suggest that the chick neogenin is functionally equivalent to rodent Dcc.

Inhibitory Smads differentially regulate cell fate specification and axon dynamics in the dorsal spinal cord

The roof plate resident BMPs have sequential functions in the developing spinal cord, establishing cell fate and orienting axonal trajectories. These activities are, however, restricted to the dI1-dI3 neurons in the most dorsal region of the spinal cord. What limits the extent of the action of the BMPs to these neurons? To address this question, we have examined both the distribution of the inhibitory Smads (I-Smads), Smad6 and Smad7 in the spinal cord and the consequence of ectopically expressing the I-Smads in chicken embryos. Our studies suggest that the I-Smads function in vivo to restrict the action of BMP signaling in the dorsal spinal cord. Moreover, the I-Smads have distinct roles in regulating the diverse activities of the BMPs. Thus, the ectopic expression of Smad7 suppresses the dI1 and dI3 neural fates and concomitantly increases the number of dI4-dI6 spinal neurons. In contrast, Smad6 most potently functions to block dI1 axon outgrowth. Taken together, these experiments suggest that the I-Smads have distinct roles in spatially limiting the response of cells to BMP signaling.