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

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.

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.

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.

Post-crossing segment of dI1 commissural axons forms collateral branches to motor neurons in the developing spinal cord

The dI1 commissural axons in the developing spinal cord, upon crossing the midline through the floor plate, make a sharp turn to grow rostrally. These post-crossing axons initially just extend adjacent to the floor plate without entering nearby motor columns. However, it remains poorly characterized how these post-crossing dI1 axons behave subsequently to this process. In the present study, to address this issue, we examined in detail the behavior of post-crossing dI1 axons in mice, using the Atoh1 enhancer-based conditional expression system that enables selective and sparse labeling of individual dI1 axons, together with Hb9 and ChAT immunohistochemistry for precise identification of spinal motor neurons (MNs). We found unexpectedly that the post-crossing segment of dI1 axons later gave off collateral branches that extended laterally to invade motor columns. Interestingly, these collateral branches emerged at around the time when their primary growth cones initiated invasion into motor columns. In addition, although the length of the laterally growing collateral branches increased with age, the majority of them remained within motor columns. Strikingly, these collateral branches further gave rise to multiple secondary branches in the region of MNs that innervate muscles close to the body axis. Moreover, these axonal branches formed presynaptic terminals on MNs. These observations demonstrate that dI1 commissural neurons develop axonal projection to spinal MNs via collateral branches arising later from the post-crossing segment of these axons. Our findings thus reveal a previously unrecognized projection of dI1 commissural axons that may contribute directly to generating proper motor output.

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.

The role of TGF-β superfamily signaling in neurological disorders

The TGF-β superfamily signaling is involved in a variety of biological processes during embryogenesis and in adult tissue homeostasis. Faulty regulation of the signaling pathway that transduces the TGF-β superfamily signals accordingly leads to a number of ailments, such as cancer and cardiovascular, metabolic, urinary, intestinal, skeletal, and immune diseases. In recent years, a number of studies have elucidated the essential roles of TGF-βs and BMPs during neuronal development in the maintenance of appropriate innervation and neuronal activity. The new advancement implicates significant roles of the aberrant TGF-β superfamily signaling in the pathogenesis of neurological disorders. In this review, we compile a number of reports implicating the deregulation of TGF-β/BMP signaling pathways in the pathogenesis of cognitive and neurodegenerative disorders in animal models and patients. We apologize in advance that the review falls short of providing details of the role of TGF-β/BMP signaling or mechanisms underlying the pathogenesis of neurological disorders. The goal of this article is to reveal a gap in our knowledge regarding the association between TGF-β/BMP signaling pathways and neuronal tissue homeostasis and development and facilitate the research with a potential to develop new therapies for neurological ailments by modulating the pathways.

Making sense out of spinal cord somatosensory development

The spinal cord integrates and relays somatosensory input, leading to complex motor responses. Research over the past couple of decades has identified transcription factor networks that function during development to define and instruct the generation of diverse neuronal populations within the spinal cord. A number of studies have now started to connect these developmentally defined populations with their roles in somatosensory circuits. Here, we review our current understanding of how neuronal diversity in the dorsal spinal cord is generated and we discuss the logic underlying how these neurons form the basis of somatosensory circuits.

Dendrosomatic Sonic Hedgehog Signaling in Hippocampal Neurons Regulates Axon Elongation

The presence of Sonic Hedgehog (Shh) and its signaling components in the neurons of the hippocampus raises a question about what role the Shh signaling pathway may play in these neurons. We show here that activation of the Shh signaling pathway stimulates axon elongation in rat hippocampal neurons. This Shh-induced effect depends on the pathway transducer Smoothened (Smo) and the transcription factor Gli1. The axon itself does not respond directly to Shh; instead, the Shh signal transduction originates from the somatodendritic region of the neurons and occurs in neurons with and without detectable primary cilia. Upon Shh stimulation, Smo localization to dendrites increases significantly. Shh pathway activation results in increased levels of profilin1 (Pfn1), an actin-binding protein. Mutations in Pfn1's actin-binding sites or reduction of Pfn1 eliminate the Shh-induced axon elongation. These findings indicate that Shh can regulate axon growth, which may be critical for development of hippocampal neurons.

SIGNIFICANCE STATEMENT

Although numerous signaling mechanisms have been identified that act directly on axons to regulate their outgrowth, it is not known whether signals transduced in dendrites may also affect axon outgrowth. We describe here a transcellular signaling pathway in embryonic hippocampal neurons in which activation of Sonic Hedgehog (Shh) receptors in dendrites stimulates axon growth. The pathway involves the dendritic-membrane-associated Shh signal transducer Smoothened (Smo) and the transcription factor Gli, which induces the expression of the gene encoding the actin-binding protein profilin 1. Our findings suggest scenarios in which stimulation of Shh in dendrites results in accelerated outgrowth of the axon, which therefore reaches its presumptive postsynaptic target cell more quickly. By this mechanism, Shh may play critical roles in the development of hippocampal neuronal circuits.

Integration of shallow gradients of Shh and Netrin-1 guides commissural axons

During nervous system development, gradients of Sonic Hedgehog (Shh) and Netrin-1 attract growth cones of commissural axons toward the floor plate of the embryonic spinal cord. Mice defective for either Shh or Netrin-1 signaling have commissural axon guidance defects, suggesting that both Shh and Netrin-1 are required for correct axon guidance. However, how Shh and Netrin-1 collaborate to guide axons is not known. We first quantified the steepness of the Shh gradient in the spinal cord and found that it is mostly very shallow. We then developed an in vitro microfluidic guidance assay to simulate these shallow gradients. We found that axons of dissociated commissural neurons respond to steep but not shallow gradients of Shh or Netrin-1. However, when we presented axons with combined Shh and Netrin-1 gradients, they had heightened sensitivity to the guidance cues, turning in response to shallower gradients that were unable to guide axons when only one cue was present. Furthermore, these shallow gradients polarized growth cone Src-family kinase (SFK) activity only when Shh and Netrin-1 were combined, indicating that SFKs can integrate the two guidance cues. Together, our results indicate that Shh and Netrin-1 synergize to enable growth cones to sense shallow gradients in regions of the spinal cord where the steepness of a single guidance cue is insufficient to guide axons, and we identify a novel type of synergy that occurs when the steepness (and not the concentration) of a guidance cue is limiting.

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.

BMP signaling in telencephalic neural cell specification and maturation

Bone morphogenetic proteins (BMPs) make up a family of morphogens that are critical for patterning, development, and function of the central and peripheral nervous system. Their effects on neural cells are pleiotropic and highly dynamic depending on the stage of development and the local niche. Neural cells display a broad expression profile of BMP ligands, receptors, and transducer molecules. Moreover, interactions of BMP signaling with other incoming morphogens and signaling pathways are crucial for most of these processes. The key role of BMP signaling suggests that it includes many regulatory mechanisms that restrict BMP activity both temporally and spatially. BMPs affect neural cell fate specification in a dynamic fashion. Initially they inhibit proliferation of neural precursors and promote the first steps in neuronal differentiation. Later on, BMP signaling effects switch from neuronal induction to promotion of astroglial identity and inhibition of neuronal or oligodendroglial lineage commitment. Furthermore, in postmitotic cells, BMPs regulate cell survival and death, to modulate neuronal subtype specification, promote dendritic and axonal growth and induce synapse formation and stabilization. In this review, we examine the canonical and non-canonical mechanisms of BMP signal transduction. Moreover, we focus on the specific role of BMPs in the nervous system including their ability to regulate neural stem cell proliferation, self-renewal, lineage specification, and neuronal function.

Building Spinal and Brain Commissures: Axon Guidance at the Midline

Commissural circuits are brain and spinal cord connections which interconnect the two sides of the central nervous system (CNS). They play essential roles in brain and spinal cord processing, ensuring left-right coordination and synchronization of information and commands. During the formation of neuronal circuits, all commissural neurons of the central nervous system must accomplish a common task, which is to project their axon onto the other side of the nervous system, across the midline that delineates the two halves of the CNS. How this task is accomplished has been the topic of extensive studies over the last past 20 years and remains one of the best models to investigate axon guidance mechanisms. In the first part of this review, I will introduce the commissural circuits, their general role in the physiology of the nervous system, and their recognized or suspected pathogenic properties in human diseases. In the second part of the review, I will concentrate on two commissural circuits, the spinal commissures and the corpus callosum, to detail the cellular and molecular mechanisms governing their formation, mostly during their navigation at the midline.

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.

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.