FGF as a Target-Derived Chemoattractant for Developing Motor Axons Genetically Programmed by the LIM Code

Fibroblast growth factor signaling in axons: from development to disease

The fibroblast growth factor (FGF) family regulates various and important aspects of nervous system development, ranging from the well-established roles in neuronal patterning to more recent and exciting functions in axonal growth and synaptogenesis. In addition, FGFs play a critical role in axonal regeneration, particularly after spinal cord injury, confirming their versatile nature in the nervous system. Due to their widespread involvement in neural development, the FGF system also underlies several human neurological disorders. While particular attention has been given to FGFs in a whole-cell context, their effects at the axonal level are in most cases undervalued. Here we discuss the endeavor of the FGF system in axons, we delve into this neuronal subcompartment to provide an original view of this multipurpose family of growth factors in nervous system (dys)function. Video Abstract.

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ VSX2+ optic-disc cells. Single-cell RNA sequencing of these organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in these organoids differentially expressed FGF8 and FGF9; FGFR inhibitions drastically decreased early RGC differentiation and directional axon growth. Through the RGC-specific cell-surface marker CNTN2 identified here, electrophysiologically excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report the self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ optic-disc cells. Single-cell RNA sequencing of CONCEPT organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in CONCEPT organoids differentially expressed FGF8 and FGF9 ; FGFR inhibitions drastically decreased RGC differentiation and directional axon growth. Through the identified RGC-specific cell-surface marker CNTN2, electrophysiologically-excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Impact statement

A human telencephalon-eye organoid model that exhibited axon growth and pathfinding from retinal ganglion cell (RGC) axons is reported; via cell surface marker CNTN2 identified using scRNA-seq, early RGCs were isolated under a native condition.

Roles of Fibroblast Growth Factors in the Axon Guidance

Fibroblast growth factors (FGFs) have been widely studied by virtue of their ability to regulate many essential cellular activities, including proliferation, survival, migration, differentiation and metabolism. Recently, these molecules have emerged as the key components in forming the intricate connections within the nervous system. FGF and FGF receptor (FGFR) signaling pathways play important roles in axon guidance as axons navigate toward their synaptic targets. This review offers a current account of axonal navigation functions performed by FGFs, which operate as chemoattractants and/or chemorepellents in different circumstances. Meanwhile, detailed mechanisms behind the axon guidance process are elaborated, which are related to intracellular signaling integration and cytoskeleton dynamics.

Celsr1 suppresses Wnt5a-mediated chemoattraction to prevent incorrect rostral migration of facial branchiomotor neurons

In the developing hindbrain, facial branchiomotor (FBM) neurons migrate caudally from rhombomere 4 (r4) to r6 to establish the circuit that drives jaw movements. Although the mechanisms regulating initiation of FBM neuron migration are well defined, those regulating directionality are not. In mutants lacking the Wnt/planar cell polarity (PCP) component Celsr1, many FBM neurons inappropriately migrate rostrally into r3. We hypothesized that Celsr1 normally blocks inappropriate rostral migration of FBM neurons by suppressing chemoattraction towards Wnt5a in r3 and successfully tested this model. First, FBM neurons in Celsr1; Wnt5a double mutant embryos never migrated rostrally, indicating that inappropriate rostral migration in Celsr1 mutants results from Wnt5a-mediated chemoattraction, which is suppressed in wild-type embryos. Second, FBM neurons migrated rostrally toward Wnt5a-coated beads placed in r3 of wild-type hindbrain explants, suggesting that excess Wnt5a chemoattractant can overcome endogenous Celsr1-mediated suppression. Third, rostral migration of FBM neurons was greatly enhanced in Celsr1 mutants overexpressing Wnt5a in r3. These results reveal a novel role for a Wnt/PCP component in regulating neuronal migration through suppression of chemoattraction.

Transcriptional Control of Axon Guidance at Midline Structures

The development of the nervous system is a time-ordered and multi-stepped process that includes neurogenesis and neuronal specification, axonal navigation, and circuits assembly. During axonal navigation, the growth cone, a dynamic structure located at the tip of the axon, senses environmental signals that guide axons towards their final targets. The expression of a specific repertoire of receptors on the cell surface of the growth cone together with the activation of a set of intracellular transducing molecules, outlines the response of each axon to specific guidance cues. This collection of axon guidance molecules is defined by the transcriptome of the cell which, in turn, depends on transcriptional and epigenetic regulators that modify the structure and DNA accessibility to determine what genes will be expressed to elicit specific axonal behaviors. Studies focused on understanding how axons navigate intermediate targets, such as the floor plate of vertebrates or the mammalian optic chiasm, have largely contributed to our knowledge of how neurons wire together during development. In fact, investigations on axon navigation at these midline structures led to the identification of many of the currently known families of proteins that act as guidance cues and their corresponding receptors. Although the transcription factors and the regulatory mechanisms that control the expression of these molecules are not well understood, important advances have been made in recent years in this regard. Here we provide an updated overview on the current knowledge about the transcriptional control of axon guidance and the selection of trajectories at midline structures.

Establishing the Molecular and Functional Diversity of Spinal Motoneurons

Spinal motoneurons are a remarkably diverse class of neurons responsible for facilitating a broad range of motor behaviors and autonomic functions. Studies of motoneuron differentiation have provided fundamental insights into the developmental mechanisms of neuronal diversification, and have illuminated principles of neural fate specification that operate throughout the central nervous system. Because of their relative anatomical simplicity and accessibility, motoneurons have provided a tractable model system to address multiple facets of neural development, including early patterning, neuronal migration, axon guidance, and synaptic specificity. Beyond their roles in providing direct communication between central circuits and muscle, recent studies have revealed that motoneuron subtype-specific programs also play important roles in determining the central connectivity and function of motor circuits. Cross-species comparative analyses have provided novel insights into how evolutionary changes in subtype specification programs may have contributed to adaptive changes in locomotor behaviors. This chapter focusses on the gene regulatory networks governing spinal motoneuron specification, and how studies of spinal motoneurons have informed our understanding of the basic mechanisms of neuronal specification and spinal circuit assembly.

Zebrafish Minichromosome Maintenance Protein 5 Gene Regulates the Development and Migration of Facial Motor Neurons via Fibroblast Growth Factor Signaling

Minichromosome maintenance protein 5 (MCM5), a member of the microchromosomal maintenance protein family, plays an important role in the initiation and extension of DNA replication. However, its role in neural development in zebrafish remains unclear. Here, we used morpholino (MO) and CRISPR/Cas9 to knock down mcm5 and investigated the developmental features of facial motor neurons (FMNs) in the hindbrain of zebrafish. We found that knockdown of mcm5 using mcm5 MO resulted in a small head, small eyes, and a blurred midbrain-hindbrain boundary, while MO injection of mcm5 led to decrease in FMNs and their migration disorder. However, the mutant of mcm5 only resulted in the migration defect of FMNs rather than quantity change. We further investigated the underlying mechanism of mcm5 in the development of hindbrain using in situ hybridization (ISH) and fgfr1a mRNA co-injected with mcm5 MO. Results from ISH showed that the fibroblast growth factor (FGF) signaling pathway was changed when the MCM5 function was lost, with the decrease in fgfr1a and the increase in fgf8, while that of pea3 had opposite trend. FMN development defects were rescued by fgfr1a mRNA co-injected with mcm5 MO. Our results demonstrated that FGF signaling pathway is required for FMN development in zebrafish. Specifically, mcm5 regulates FMN development during zebrafish growing.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

Familiar growth factors have diverse roles in neural network assembly

The assembly of neuronal circuits during development depends on guidance of axonal growth cones by molecular cues deposited in their environment. While a number of families of axon guidance molecules have been identified and reviewed, important and diverse activities of traditional growth factors are emerging. Besides clear and well recognized roles in the regulation of cell division, differentiation and survival, new research shows later phase roles for a number of growth factors in promoting neuronal migration, axon guidance and synapse formation throughout the nervous system.

Enhanced FGFR3 activity in postmitotic principal neurons during brain development results in cortical dysplasia and axonal tract abnormality

Abnormal levels of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) have been detected in various neurological disorders. The potent impact of FGF-FGFR in multiple embryonic developmental processes makes it challenging to elucidate their roles in postmitotic neurons. Taking an alternative approach to examine the impact of aberrant FGFR function on glutamatergic neurons, we generated a FGFR gain-of-function (GOF) transgenic mouse, which expresses constitutively activated FGFR3 (FGFR3K650E) in postmitotic glutamatergic neurons. We found that GOF disrupts mitosis of radial-glia neural progenitors (RGCs), inside-out radial migration of post-mitotic glutamatergic neurons, and axonal tract projections. In particular, late-born CUX1-positive neurons are widely dispersed throughout the GOF cortex. Such a cortical migration deficit is likely caused, at least in part, by a significant reduction of the radial processes projecting from RGCs. RNA-sequencing analysis of the GOF embryonic cortex reveals significant alterations in several pathways involved in cell cycle regulation and axonal pathfinding. Collectively, our data suggest that FGFR3 GOF in postmitotic neurons not only alters axonal growth of postmitotic neurons but also impairs RGC neurogenesis and radial glia processes.

TAG-1 Multifunctionality Coordinates Neuronal Migration, Axon Guidance, and Fasciculation

Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges during development because their cell bodies reside within the central nervous system (CNS) and their axons project to various targets in the body periphery. The molecular mechanisms that contain MN somata within the spinal cord while allowing their axons to exit the CNS and navigate to their final destinations remain incompletely understood. We find that the MN cell surface protein TAG-1 anchors MN cell bodies in the spinal cord to prevent their emigration, mediates motor axon fasciculation during CNS exit, and guides motor axons past dorsal root ganglia. TAG-1 executes these varied functions in MN development independently of one another. Our results identify TAG-1 as a key multifunctional regulator of MN wiring that coordinates neuronal migration, axon fasciculation, and axon guidance.

Suter et al. demonstrate that the motor neuron cell surface molecule TAG-1 confines motor neurons to the central nervous system, promotes motor axon fasciculation, and steers motor axons past inappropriate targets. This study highlights how a single cell adhesion molecule coordinates multiple steps in neuronal wiring through partially divergent mechanisms.

Novel concept for the epaxial/hypaxial boundary based on neuronal development

Trunk muscles in vertebrates are classified as either dorsal epaxial or ventral hypaxial muscles. Epaxial and hypaxial muscles are defined as muscles innervated by the dorsal and ventral rami of spinal nerves, respectively. Each cluster of spinal motor neurons passing through dorsal rami innervates epaxial muscles, whereas clusters traveling on the ventral rami innervate hypaxial muscles. Herein, we show that some motor neurons exhibiting molecular profiles for epaxial muscles follow a path in the ventral rami. Dorsal deep-shoulder muscles and some body wall muscles are defined as hypaxial due to innervation via the ventral rami, but a part of these ventral rami has the molecular profile of motor neurons that innervate epaxial muscles. Thus, the epaxial and hypaxial boundary cannot be determined simply by the ramification pattern of spinal nerves. We propose that, although muscle innervation occurs via the ventral rami, dorsal deep-shoulder muscles and some body wall muscles represent an intermediate group that lies between epaxial and hypaxial muscles.

PBX1 acts as terminal selector for olfactory bulb dopaminergic neurons

Neuronal specification is a protracted process that begins with the commitment of progenitor cells and culminates with the generation of mature neurons. Many transcription factors are continuously expressed during this process but it is presently unclear how these factors modify their targets as cells transition through different stages of specification. In olfactory bulb adult neurogenesis, the transcription factor PBX1 controls neurogenesis in progenitor cells and the survival of migrating neuroblasts. Here, we show that, at later differentiation stages, PBX1 also acts as a terminal selector for the dopaminergic neuron fate. PBX1 is also required for the morphological maturation of dopaminergic neurons and to repress alternative interneuron fates, findings that expand the known repertoire of terminal-selector actions. Finally, we reveal that the temporal diversification of PBX1 functions in neuronal specification is achieved, at least in part, through the dynamic regulation of alternative splicing. In Caenorhabditis elegans, PBX/CEH-20 also acts as a dopaminergic neuron terminal selector, which suggests an ancient role for PBX factors in the regulation of terminal differentiation of dopaminergic neurons.

FGF10 regulates thalamocortical axon guidance in the developing thalamus

Thalamocortical axons (TCAs) transmit sensory information to the neocortex by responding to a variety of guidance cues in the environment. Similar to classical guidance cues (ephrins, slits, semaphorins and netrins), morphogens of FGFs can also help axons navigate to their targets. Here, expression analyses reveal that FGF10 is expressed in the chick prethalamus during the navigation of TCAs. Then, using ex vivo analyses in chick explants, we demonstrate a dose-dependent effect of FGF10 on thalamic axons: low concentration of FGF10 attracts thalamic axons, while high concentration FGF10 repels thalamic axons. Moreover, inhibition of FGF10 function indicates that FGF10 exerts a direct effect on thalamic axons. Together, these studies reveal a direct role for the member of FGF7 subfamily, FGF10, in the axonal navigation of TCAs.

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.

Transcriptional mechanisms of motor neuron development in vertebrates and invertebrates

Across phylogeny, motor neurons (MNs) represent a single but often remarkably diverse neuronal class composed of a multitude of subtypes required for vital behaviors, such as eating and locomotion. Over the past decades, seminal studies in multiple model organisms have advanced our molecular understanding of the early steps of MN development, such as progenitor specification and acquisition of MN subtype identity, by revealing key roles for several evolutionarily conserved transcription factors. However, very little is known about the molecular strategies that allow distinct MN subtypes to maintain their identity- and function-defining features during the late steps of development and postnatal life. Here, we provide an overview of invertebrate and vertebrate studies on transcription factor-based strategies that control early and late steps of MN development, aiming to highlight evolutionarily conserved gene regulatory principles necessary for establishment and maintenance of neuronal identity.

p190RhoGAP Filters Competing Signals to Resolve Axon Guidance Conflicts

The rich functional diversity of the nervous system is founded in the specific connectivity of the underlying neural circuitry. Neurons are often preprogrammed to respond to multiple axon guidance signals because they use sequential guideposts along their pathways, but this necessitates a strict spatiotemporal regulation of intracellular signaling to ensure the cues are detected in the correct order. We performed a mouse mutagenesis screen and identified the Rho GTPase antagonist p190RhoGAP as a critical regulator of motor axon guidance. Rather than acting as a compulsory signal relay, p190RhoGAP uses a non-conventional GAP-independent mode to transiently suppress attraction to Netrin-1 while motor axons exit the spinal cord. Once in the periphery, a subset of axons requires p190RhoGAP-mediated inhibition of Rho signaling to target specific muscles. Thus, the multifunctional activity of p190RhoGAP emerges from its modular design. Our findings reveal a cell-intrinsic gate that filters conflicting signals, establishing temporal windows of signal detection.

Glial cells maintain synapses by inhibiting an activity-dependent retrograde protease signal

Glial cells regulate multiple aspects of synaptogenesis. In the absence of Schwann cells, a peripheral glial cell, motor neurons initially innervate muscle but then degenerate. Here, using a genetic approach, we show that neural activity-regulated negative factors produced by muscle drive neurodegeneration in Schwann cell-deficient mice. We find that thrombin, the hepatic serine protease central to the hemostatic coagulation cascade, is one such negative factor. Trancriptomic analysis shows that expression of the antithrombins serpin C1 and D1 is significantly reduced in Schwann cell-deficient mice. In the absence of peripheral neuromuscular activity, neurodegeneration is completely blocked, and expression of prothrombin in muscle is markedly reduced. In the absence of muscle-derived prothrombin, neurodegeneration is also markedly reduced. Together, these results suggest that Schwann cells regulate NMJs by opposing the effects of activity-regulated, muscle-derived negative factors and provide the first genetic evidence that thrombin plays a central role outside of the coagulation system.

DYRK1A and cognition: A lifelong relationship

The dosage of the serine threonine kinase DYRK1A is critical in the central nervous system (CNS) during development and aging. This review analyzes the functions of this kinase by considering its interacting partners and pathways. The role of DYRK1A in controlling the differentiation of prenatal newly formed neurons is presented separately from its role at the pre- and post-synaptic levels in the adult CNS; its effects on synaptic plasticity are also discussed. Because this kinase is positioned at the crossroads of many important processes, genetic dosage errors in this protein produce devastating effects arising from DYRK1A deficiency, such as in MRD7, an autism spectrum disorder, or from DYRK1A excess, such as in Down syndrome. Effects of these errors have been shown in various animal models including Drosophila, zebrafish, and mice. Dysregulation of DYRK1A levels also occurs in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Finally, this review describes inhibitors that have been assessed in vivo. Accurate targeting of DYRK1A levels in the brain, with either inhibitors or activators, is a future research challenge.

Development, functional organization, and evolution of vertebrate axial motor circuits

Neuronal control of muscles associated with the central body axis is an ancient and essential function of the nervous systems of most animal species. Throughout the course of vertebrate evolution, motor circuits dedicated to control of axial muscle have undergone significant changes in their roles within the motor system. In most fish species, axial circuits are critical for coordinating muscle activation sequences essential for locomotion and play important roles in postural correction. In tetrapods, axial circuits have evolved unique functions essential to terrestrial life, including maintaining spinal alignment and breathing. Despite the diverse roles of axial neural circuits in motor behaviors, the genetic programs underlying their assembly are poorly understood. In this review, we describe recent studies that have shed light on the development of axial motor circuits and compare and contrast the strategies used to wire these neural networks in aquatic and terrestrial vertebrate species.

Ephexin1 Is Required for Eph-Mediated Limb Trajectory of Spinal Motor Axons

The precise assembly of a functional nervous system relies on the guided migration of axonal growth cones, which is made possible by signals transmitted to the cytoskeleton by cell surface-expressed guidance receptors. We investigated the function of ephexin1, a Rho guanine nucleotide exchange factor, as an essential growth-cone guidance intermediary in the context of spinal lateral motor column (LMC) motor axon trajectory selection in the limb mesenchyme. Using in situ mRNA detection, we first show that ephexin1 is expressed in LMC neurons of chick and mouse embryos at the time of spinal motor axon extension into the limb. Ephexin1 loss of function and gain of function using in ovo electroporation in chick LMC neurons, of either sex, perturbed LMC axon trajectory selection, demonstrating an essential role of ephexin1 in motor axon guidance. In addition, ephexin1 loss in mice of either sex led to LMC axon trajectory selection errors. We also show that ephexin1 knockdown attenuates the growth preference of LMC neurites against ephrins in vitro and Eph receptor-mediated retargeting of LMC axons in vivo, suggesting that ephexin1 is required in Eph-mediated LMC motor axon guidance. Finally, both ephexin1 knockdown and ectopic expression of nonphosphorylatable ephexin1 mutant attenuated the retargeting of LMC axons caused by Src overexpression, implicating ephexin1 as an Src target in Eph signal relay in this context. In summary, our findings demonstrate that ephexin1 is essential for motor axon guidance and suggest an important role in relaying ephrin:Eph signals that mediate motor axon trajectory selection.SIGNIFICANCE STATEMENT The proper development of functioning neural circuits requires precise nerve connections among neurons or between neurons and their muscle targets. The Eph tyrosine kinase receptors expressed in neurons are important in many contexts during neural-circuit formation, such as axon outgrowth, axon guidance, and synaptic formation, and have been suggested to be involved in neurodegenerative disorders, including amyotrophic lateral sclerosis and Alzheimer's disease. To dissect the mechanism of Eph signal relay, we studied ephexin1 gain of function and loss of function and found ephexin1 essential for the development of limb nerves toward their muscle targets, concluding that it functions as an intermediary to relay Eph signaling in this context. Our work could thus shed new light on the molecular mechanisms controlling neuromuscular connectivity during embryonic development.

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.

Engrailed controls epaxial-hypaxial muscle innervation and the establishment of vertebrate three-dimensional mobility

Chordates are characterised by contractile muscle on either side of the body that promotes movement by side-to-side undulation. In the lineage leading to modern jawed vertebrates (crown group gnathostomes), this system was refined: body muscle became segregated into distinct dorsal (epaxial) and ventral (hypaxial) components that are separately innervated by the medial and hypaxial motors column, respectively, via the dorsal and ventral ramus of the spinal nerves. This allows full three-dimensional mobility, which in turn was a key factor in their evolutionary success. How the new gnathostome system is established during embryogenesis and how it may have evolved in the ancestors of modern vertebrates is not known. Vertebrate Engrailed genes have a peculiar expression pattern as they temporarily demarcate a central domain of the developing musculature at the epaxial-hypaxial boundary. Moreover, they are the only genes known with this particular expression pattern. The aim of this study was to investigate whether Engrailed genes control epaxial-hypaxial muscle development and innervation. Investigating chick, mouse and zebrafish as major gnathostome model organisms, we found that the Engrailed expression domain was associated with the establishment of the epaxial-hypaxial boundary of muscle in all three species. Moreover, the outgrowing epaxial and hypaxial nerves orientated themselves with respect to this Engrailed domain. In the chicken, loss and gain of Engrailed function changed epaxial-hypaxial somite patterning. Importantly, in all animals studied, loss and gain of Engrailed function severely disrupted the pathfinding of the spinal motor axons, suggesting that Engrailed plays an evolutionarily conserved role in the separate innervation of vertebrate epaxial-hypaxial muscle.

Receptor Tyrosine Kinases and Phosphatases in Neuronal Wiring: Insights From Drosophila

Tyrosine phosphorylation is at the crossroads of many signaling pathways. Brain wiring is not an exception, and several receptor tyrosine kinases (RTKs) and tyrosine receptor phosphates (RPTPs) have been involved in this process. Considerable work has been done on RTKs, and for many of them, detailed molecular mechanisms and functions in several systems have been characterized. In contrast, RPTPs have been studied considerably less and little is known about their ligands and substrates. In both families, we find redundancy between different members to accomplish particular wiring patterns. Strikingly, some RTKs and RPTPs have lost their catalytic activity during evolution, but not their importance in biological processes. In this regard, we have to keep in mind that these proteins have multiple domains and some of their functions are independent of tyrosine phosphorylation/dephosphorylation. Since RTKs and RPTPs are enzymes involved not only in early stages of axon and dendrite pathfinding but also in synapse formation and physiology, they have a potential as drug targets. Drosophila has been a key model organism in the search of a better understanding of brain wiring, and its sophisticated toolbox is very suitable for studying the function of genes with pleiotropic functions such as RTKs and RPTPs, from wiring to synaptic formation and function. In these review, we mainly cover findings from this model organism and complement them with discoveries in vertebrate systems.

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.

TDP-43 loss of function inhibits endosomal trafficking and alters trophic signaling in neurons

Nuclear clearance of TDP-43 into cytoplasmic aggregates is a key driver of neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), but the mechanisms are unclear. Here, we show that TDP-43 knockdown specifically reduces the number and motility of RAB11-positive recycling endosomes in dendrites, while TDP-43 overexpression has the opposite effect. This is associated with delayed transferrin recycling in TDP-43-knockdown neurons and decreased β2-transferrin levels in patient CSF Whole proteome quantification identified the upregulation of the ESCRT component VPS4B upon TDP-43 knockdown in neurons. Luciferase reporter assays and chromatin immunoprecipitation suggest that TDP-43 represses VPS4B transcription. Preventing VPS4B upregulation or expression of its functional antagonist ALIX restores trafficking of recycling endosomes. Proteomic analysis revealed the broad reduction in surface expression of key receptors upon TDP-43 knockdown, including ErbB4, the neuregulin 1 receptor. TDP-43 knockdown delays the surface delivery of ErbB4. ErbB4 overexpression, but not neuregulin 1 stimulation, prevents dendrite loss upon TDP-43 knockdown. Thus, impaired recycling of ErbB4 and other receptors to the cell surface may contribute to TDP-43-induced neurodegeneration by blocking trophic signaling.

The atypical cadherin Celsr1 functions non-cell autonomously to block rostral migration of facial branchiomotor neurons in mice

The caudal migration of facial branchiomotor (FBM) neurons from rhombomere (r) 4 to r6 in the hindbrain is an excellent model to study neuronal migration mechanisms. Although several Wnt/Planar Cell Polarity (PCP) components are required for FBM neuron migration, only Celsr1, an atypical cadherin, regulates the direction of migration in mice. In Celsr1 mutants, a subset of FBM neurons migrates rostrally instead of caudally. Interestingly, Celsr1 is not expressed in the migrating FBM neurons, but rather in the adjacent floor plate and adjoining ventricular zone. To evaluate the contribution of different expression domains to neuronal migration, we conditionally inactivated Celsr1 in specific cell types. Intriguingly, inactivation of Celsr1 in the ventricular zone of r3-r5, but not in the floor plate, leads to rostral migration of FBM neurons, greatly resembling the migration defect of Celsr1 mutants. Dye fill experiments indicate that the rostrally-migrated FBM neurons in Celsr1 mutants originate from the anterior margin of r4. These data suggest strongly that Celsr1 ensures that FBM neurons migrate caudally by suppressing molecular cues in the rostral hindbrain that can attract FBM neurons.

Functional Diversification of Motor Neuron-specific Isl1 Enhancers during Evolution

Functional diversification of motor neurons has occurred in order to selectively control the movements of different body parts including head, trunk and limbs. Here we report that transcription of Isl1, a major gene necessary for motor neuron identity, is controlled by two enhancers, CREST1 (E1) and CREST2 (E2) that allow selective gene expression of Isl1 in motor neurons. Introduction of GFP reporters into the chick neural tube revealed that E1 is active in hindbrain motor neurons and spinal cord motor neurons, whereas E2 is active in the lateral motor column (LMC) of the spinal cord, which controls the limb muscles. Genome-wide ChIP-Seq analysis combined with reporter assays showed that Phox2 and the Isl1-Lhx3 complex bind to E1 and drive hindbrain and spinal cord-specific expression of Isl1, respectively. Interestingly, Lhx3 alone was sufficient to activate E1, and this may contribute to the initiation of Isl1 expression when progenitors have just developed into motor neurons. E2 was induced by onecut 1 (OC-1) factor that permits Isl1 expression in LMCm neurons. Interestingly, the core region of E1 has been conserved in evolution, even in the lamprey, a jawless vertebrate with primitive motor neurons. All E1 sequences from lamprey to mouse responded equally well to Phox2a and the Isl1-Lhx3 complex. Conversely, E2, the enhancer for limb-innervating motor neurons, was only found in tetrapod animals. This suggests that evolutionarily-conserved enhancers permit the diversification of motor neurons.

During evolution, motor neurons became specialized to control movements of different body parts including head, trunk and limbs. Here we report that two enhancers of Isl1, E1 and E2, are active together with transcription factors in motor neurons. Surprisingly, E1 and its response to transcription factors has been conserved in evolution from the lamprey to man, whereas E2 is only found in animals with limbs. Our study provides an evolutionary example of how functional diversification of motor neurons is achieved by a dynamic interplay between enhancers and transcription factors.

FGF8 coordinates tissue elongation and cell epithelialization during early kidney tubulogenesis

When a tubular structure forms during early embryogenesis, tubular elongation and lumen formation (epithelialization) proceed simultaneously in a spatiotemporally coordinated manner. We here demonstrate, using the Wolffian duct (WD) of early chicken embryos, that this coordination is regulated by the expression of FGF8, which shifts posteriorly during body axis elongation. FGF8 acts as a chemoattractant on the leader cells of the elongating WD and prevents them from epithelialization, whereas static (‘rear’) cells that receive progressively less FGF8 undergo epithelialization to form a lumen. Thus, FGF8 acts as a binary switch that distinguishes tubular elongation from lumen formation. The posteriorly shifting FGF8 is also known to regulate somite segmentation, suggesting that multiple types of tissue morphogenesis are coordinately regulated by macroscopic changes in body growth.

Highlighted article: Body axis elongation is regulated by posterior FGF8 signals . In chicken, nephric duct extension also requires this FGF8 signal, while low FGF8 anteriorly triggers duct lumen formation.

Assembly and function of spinal circuits for motor control

Control of movement is a fundamental and complex task of the vertebrate nervous system, which relies on communication between circuits distributed throughout the brain and spinal cord. Many of the networks essential for the execution of basic locomotor behaviors are composed of discrete neuronal populations residing within the spinal cord. The organization and connectivity of these circuits is established through programs that generate functionally diverse neuronal subtypes, each contributing to a specific facet of motor output. Significant progress has been made in deciphering how neuronal subtypes are specified and in delineating the guidance and synaptic specificity determinants at the core of motor circuit assembly. Recent studies have shed light on the basic principles linking locomotor circuit connectivity with function, and they are beginning to reveal how more sophisticated motor behaviors are encoded. In this review, we discuss the impact of developmental programs in specifying motor behaviors governed by spinal circuits.

Zebrafish foxc1a drives appendage-specific neural circuit development

Neural connectivity between the spinal cord and paired appendages is key to the superior locomotion of tetrapods and aquatic vertebrates. In contrast to nerves that innervate axial muscles, those innervating appendages converge at a specialized structure, the plexus, where they topographically reorganize before navigating towards their muscle targets. Despite its importance for providing appendage mobility, the genetic program that drives nerve convergence at the plexus, as well as the functional role of this convergence, are not well understood. Here, we show that in zebrafish the transcription factor foxc1a is dispensable for trunk motor nerve guidance but is required to guide spinal nerves innervating the pectoral fins, equivalent to the tetrapod forelimbs. In foxc1a null mutants, instead of converging with other nerves at the plexus, pectoral fin nerves frequently bypass the plexus. We demonstrate that foxc1a expression in muscle cells delineating the nerve path between the spinal cord and the plexus region restores convergence at the plexus. By labeling individual fin nerves, we show that mutant nerves bypassing the plexus enter the fin at ectopic positions, yet innervate their designated target areas, suggesting that motor axons can select their appropriate fin target area independently of their migration through the plexus. Although foxc1a mutants display topographically correct fin innervation, mutant fin muscles exhibit a reduction in the levels of pre- and postsynaptic structures, concomitant with reduced pectoral fin function. Combined, our results reveal foxc1a as a key player in the development of connectivity between the spinal cord and paired appendages, which is crucial for appendage mobility.

Foxp1-mediated programming of limb-innervating motor neurons from mouse and human embryonic stem cells

Spinal motor neurons (MNs) control diverse motor tasks including respiration, posture and locomotion that are disrupted by neurodegenerative diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy. Methods directing MN differentiation from stem cells have been developed to enable disease modelling in vitro. However, most protocols produce only a limited subset of endogenous MN subtypes. Here we demonstrate that limb-innervating lateral motor column (LMC) MNs can be efficiently generated from mouse and human embryonic stem cells through manipulation of the transcription factor Foxp1. Foxp1-programmed MNs exhibit features of medial and lateral LMC MNs including expression of specific motor pool markers and axon guidance receptors. Importantly, they preferentially project axons towards limb muscle explants in vitro and distal limb muscles in vivo upon transplantation-hallmarks of bona fide LMC MNs. These results present an effective approach for generating specific MN populations from stem cells for studying MN development and disease.

Control of axon guidance and neurotransmitter phenotype of dB1 hindbrain interneurons by Lim-HD code

Hindbrain dorsal interneurons (HDIs) are implicated in receiving, processing, integrating, and transmitting sensory inputs from the periphery and spinal cord, including the vestibular, auditory, and proprioceptive systems. During development, multiple molecularly defined HDI types are set in columns along the dorsoventral axis, before migrating along well-defined trajectories to generate various brainstem nuclei. Major brainstem functions rely on the precise assembly of different interneuron groups and higher brain domains into common circuitries. Yet, knowledge regarding interneuron axonal patterns, synaptic targets, and the transcriptional control that govern their connectivity is sparse. The dB1 class of HDIs is formed in a district dorsomedial position along the hindbrain and gives rise to the inferior olive nuclei, dorsal cochlear nuclei, and vestibular nuclei. dB1 interneurons express various transcription factors (TFs): the pancreatic transcription factor 1a (Ptf1a), the homeobox TF-Lbx1 and the Lim-homeodomain (Lim-HD), and TF Lhx1 and Lhx5. To decipher the axonal and synaptic connectivity of dB1 cells, we have used advanced enhancer tools combined with conditional expression systems and the PiggyBac-mediated DNA transposition system in avian embryos. Multiple ipsilateral and contralateral axonal projections were identified ascending toward higher brain centers, where they formed synapses in the Purkinje cerebellar layer as well as at discrete midbrain auditory and vestibular centers. Decoding the mechanisms that instruct dB1 circuit formation revealed a fundamental role for Lim-HD proteins in regulating their axonal patterns, synaptic targets, and neurotransmitter choice. Together, this study provides new insights into the assembly and heterogeneity of HDIs connectivity and its establishment through the central action of Lim-HD governed programs.

Motoneurons derived from induced pluripotent stem cells develop mature phenotypes typical of endogenous spinal motoneurons

Induced pluripotent cell-derived motoneurons (iPSCMNs) are sought for use in cell replacement therapies and treatment strategies for motoneuron diseases such as amyotrophic lateral sclerosis (ALS). However, much remains unknown about the physiological properties of iPSCMNs and how they compare with endogenous spinal motoneurons or embryonic stem cell-derived motoneurons (ESCMNs). In the present study, we first used a proteomic approach and compared protein expression profiles between iPSCMNs and ESCMNs to show that <4% of the proteins identified were differentially regulated. Like ESCs, we found that mouse iPSCs treated with retinoic acid and a smoothened agonist differentiated into motoneurons expressing the LIM homeodomain protein Lhx3. When transplanted into the neural tube of developing chick embryos, iPSCMNs selectively targeted muscles normally innervated by Lhx3 motoneurons. In vitro studies showed that iPSCMNs form anatomically mature and functional neuromuscular junctions (NMJs) when cocultured with chick myofibers for several weeks. Electrophysiologically, iPSCMNs developed passive membrane and firing characteristic typical of postnatal motoneurons after several weeks in culture. Finally, iPSCMNs grafted into transected mouse tibial nerve projected axons to denervated gastrocnemius muscle fibers, where they formed functional NMJs, restored contractile force. and attenuated denervation atrophy. Together, iPSCMNs possess many of the same cellular and physiological characteristics as ESCMNs and endogenous spinal motoneurons. These results further justify using iPSCMNs as a source of motoneurons for cell replacement therapies and to study motoneuron diseases such as ALS.

Transcription factors and effectors that regulate neuronal morphology

Transcription factors establish the tremendous diversity of cell types in the nervous system by regulating the expression of genes that give a cell its morphological and functional properties. Although many studies have identified requirements for specific transcription factors during the different steps of neural circuit assembly, few have identified the downstream effectors by which they control neuronal morphology. In this Review, we highlight recent work that has elucidated the functional relationships between transcription factors and the downstream effectors through which they regulate neural connectivity in multiple model systems, with a focus on axon guidance and dendrite morphogenesis.

Motor neurons and the generation of spinal motor neuron diversity

Motor neurons (MNs) are neuronal cells located in the central nervous system (CNS) controlling a variety of downstream targets. This function infers the existence of MN subtypes matching the identity of the targets they innervate. To illustrate the mechanism involved in the generation of cellular diversity and the acquisition of specific identity, this review will focus on spinal MNs (SpMNs) that have been the core of significant work and discoveries during the last decades. SpMNs are responsible for the contraction of effector muscles in the periphery. Humans possess more than 500 different skeletal muscles capable to work in a precise time and space coordination to generate complex movements such as walking or grasping. To ensure such refined coordination, SpMNs must retain the identity of the muscle they innervate. Within the last two decades, scientists around the world have produced considerable efforts to elucidate several critical steps of SpMNs differentiation. During development, SpMNs emerge from dividing progenitor cells located in the medial portion of the ventral neural tube. MN identities are established by patterning cues working in cooperation with intrinsic sets of transcription factors. As the embryo develop, MNs further differentiate in a stepwise manner to form compact anatomical groups termed pools connecting to a unique muscle target. MN pools are not homogeneous and comprise subtypes according to the muscle fibers they innervate. This article aims to provide a global view of MN classification as well as an up-to-date review of the molecular mechanisms involved in the generation of SpMN diversity. Remaining conundrums will be discussed since a complete understanding of those mechanisms constitutes the foundation required for the elaboration of prospective MN regeneration therapies.

Lhx3 and Lhx4 suppress Kolmer-Agduhr interneuron characteristics within zebrafish axial motoneurons

A central problem in development is how fates of closely related cells are segregated. Lineally related motoneurons (MNs) and interneurons (INs) express many genes in common yet acquire distinct fates. For example, in mouse and chick Lhx3 plays a pivotal role in the development of both cell classes. Here, we utilize the ability to recognize individual zebrafish neurons to examine the roles of Lhx3 and its paralog Lhx4 in the development of MNs and ventral INs. We show that Lhx3 and Lhx4 are expressed by post-mitotic axial MNs derived from the MN progenitor (pMN) domain, p2 domain progenitors and by several types of INs derived from pMN and p2 domains. In the absence of Lhx3 and Lhx4, early-developing primary MNs (PMNs) adopt a hybrid fate, with morphological and molecular features of both PMNs and pMN-derived Kolmer-Agduhr' (KA') INs. In addition, we show that Lhx3 and Lhx4 distinguish the fates of two pMN-derived INs. Finally, we demonstrate that Lhx3 and Lhx4 are necessary for the formation of late-developing V2a and V2b INs. In conjunction with our previous work, these data reveal that distinct transcription factor families are deployed in post-mitotic MNs to unequivocally assign MN fate and suppress the development of alternative pMN-derived IN fates.

Skin as a living coloring book: how epithelial cells create patterns of pigmentation

The pigmentation of mammalian skin and hair develops through the interaction of two basic cell types - pigment donors and recipients. The pigment donors are melanocytes, which produce and distribute melanin through specialized structures. The pigment recipients are epithelial cells, which acquire melanin and put it to use, collectively yielding the pigmentation visible to the eye. This review will focus on the pigment recipients, the historically less understood cell type. These end-users of pigment are now known to exert a specialized control over the patterning of pigmentation, as they identify themselves as melanocyte targets, recruit pigment donors, and stimulate the transfer of melanin. As such, this review will discuss the evidence that the skin is like a coloring book: the pigment recipients create a 'picture,' a blueprint for pigmentation, which is colorless initially but outlines where pigment should be placed. Melanocytes then melanize the recipients and 'color in' the picture.

Development of the prethalamus is crucial for thalamocortical projection formation and is regulated by Olig2

Thalamocortical axons (TCAs) pass through the prethalamus in the first step of their neural circuit formation. Although it has been supposed that the prethalamus is an intermediate target for thalamocortical projection formation, much less is known about the molecular mechanisms of this targeting. Here, we demonstrated the functional implications of the prethalamus in the formation of this neural circuit. We show that Olig2 transcription factor, which is expressed in the ventricular zone (VZ) of prosomere 3, regulates prethalamus formation, and loss of Olig2 results in reduced prethalamus size in early development, which is accompanied by expansion of the thalamic eminence (TE). Extension of TCAs is disorganized in the Olig2-KO dorsal thalamus, and initial elongation of TCAs is retarded in the Olig2-KO forebrain. Microarray analysis demonstrated upregulation of several axon guidance molecules, including Epha3 and Epha5, in the Olig2-KO basal forebrain. In situ hybridization showed that the prethalamus in the wild type excluded the expression of Epha3 and Epha5, whereas loss of Olig2 resulted in reduction of this Ephas-negative area and the corresponding expansion of the Ephas-positive TE. Dissociated cultures of thalamic progenitor cells demonstrated that substrate-bound EphA3 suppresses neurite extension from dorsal thalamic neurons. These results indicate that Olig2 is involved in correct formation of the prethalamus, which leads to exclusion of the EphA3-expressing region and is crucial for proper TCA formation. Our observation is the first report showing the molecular mechanisms underlying how the prethalamus acts on initial thalamocortical projection formation.

The complex morphology of reactive astrocytes controlled by fibroblast growth factor signaling

Astrocytes are the most abundant cell-type of the human brain and play a variety of roles in brain homeostasis and synaptic maturation, under normal conditions. However, astrocytes undergo dramatic pathological changes in response to brain injury, such as reactive gliosis and glial scar formation. Although abnormal hypertrophy and massive proliferation of astrocytes are obvious, the molecular identity and cues that dictate the structural changes in reactive astrocytes remain unclear. This study proposes that fibroblast growth factor (FGF) signaling is responsible for making astrocyte morphology more complex and hypertrophic in response to an inflammatory stimulus such as lipopolysaccharide. Primary astrocytes isolated from perinatal brains developed more branches in the presence of FGF8 or lesser branches in the presence of FGF2. Introduction of the constitutively active form of the FGF receptor 3 (caFGFR3) into the brain increases the structural complexity, with greater glial fibrillary acidic protein level in astrocytes, while overexpression of a dominant-negative form of FGFR3 (dnFGFR3) reduces it. Treatment of FGF8 facilitated the wound-healing process of primary astrocytes in vitro by changing their morphology, indicating that the FGF signal may control the responsiveness of astrocytes in injury conditions. Finally, the blockade of FGF signaling by introducing dnFGFR3 at the site of reactive gliosis reduces astrocyte branch formation and minimizes hypertrophic responses during reactive gliosis. Taken together, these results indicate that FGF8-FGFR3 signaling controls structural changes in astrocytes during reactive gliosis, under pathogenic conditions.

Generating spinal motor neuron diversity: a long quest for neuronal identity

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

A developmental approach to predicting neuronal connectivity from small biological datasets: a gradient-based neuron growth model

Relating structure and function of neuronal circuits is a challenging problem. It requires demonstrating how dynamical patterns of spiking activity lead to functions like cognitive behaviour and identifying the neurons and connections that lead to appropriate activity of a circuit. We apply a “developmental approach” to define the connectome of a simple nervous system, where connections between neurons are not prescribed but appear as a result of neuron growth. A gradient based mathematical model of two-dimensional axon growth from rows of undifferentiated neurons is derived for the different types of neurons in the brainstem and spinal cord of young tadpoles of the frog Xenopus. Model parameters define a two-dimensional CNS growth environment with three gradient cues and the specific responsiveness of the axons of each neuron type to these cues. The model is described by a nonlinear system of three difference equations; it includes a random variable, and takes specific neuron characteristics into account. Anatomical measurements are first used to position cell bodies in rows and define axon origins. Then a generalization procedure allows information on the axons of individual neurons from small anatomical datasets to be used to generate larger artificial datasets. To specify parameters in the axon growth model we use a stochastic optimization procedure, derive a cost function and find the optimal parameters for each type of neuron. Our biologically realistic model of axon growth starts from axon outgrowth from the cell body and generates multiple axons for each different neuron type with statistical properties matching those of real axons. We illustrate how the axon growth model works for neurons with axons which grow to the same and the opposite side of the CNS. We then show how, by adding a simple specification for dendrite morphology, our model “developmental approach” allows us to generate biologically-realistic connectomes.

Dbx1 triggers crucial molecular programs required for midline crossing by midbrain commissural axons

Axon guidance by commissural neurons has been well documented, providing us with a molecular logic of how midline crossing is achieved during development. Despite these advances, knowledge of the intrinsic genetic programs is still limited and it remains obscure whether the expression of a single transcription factor is sufficient to activate transcriptional programs that ultimately enable midline crossing. Here, we show in the mouse that the homeodomain transcription factor Dbx1 is expressed by a subset of progenitor cells that give rise to commissural neurons in the dorsal midbrain. Gain- and loss-of-function analyses indicate that the expression of Dbx1 alone is sufficient and necessary to trigger midline crossing in vivo. We also show that Robo3 controls midline crossing as a crucial downstream effector of the Dbx1-activated molecular programs. Furthermore, Dbx1 suppresses the expression of the transcriptional program for ipsilateral neuron differentiation in parallel. These results suggest that a single transcription factor, Dbx1, has an essential function in assigning midline-crossing identity, thereby contributing crucially to the establishment of the wiring laterality in the developing nervous system.

Transcriptional regulation of guidance at the midline and in motor circuits

Axon navigation through the developing body of an embryo is a challenging and exquisitely precise process. Axonal processes within the nervous system harbor extremely complicated internal regulatory mechanisms that enable each of them to respond to environmental cues in a unique way, so that every single neuron has an exact stereotypical localization and axonal projection pattern. Receptors and adhesion molecules expressed on axonal membranes will determine their guidance properties. Axon guidance is thought to be controlled to a large extent through transcription factor codes. These codes would be responsible for the deployment of specific guidance receptors and adhesion molecules on axonal membranes to allow them to reach their targets. Although families of transcriptional regulators as well as families of guidance molecules have been conserved across evolution, their relationships seem to have developed independently. This review focuses on the midline and the neuromuscular system in both vertebrates and Drosophila in which such relationships have been particularly well studied.

Optogenetic-mediated increases in in vivo spontaneous activity disrupt pool-specific but not dorsal-ventral motoneuron pathfinding

Rhythmic waves of spontaneous electrical activity are widespread in the developing nervous systems of birds and mammals, and although many aspects of neural development are activity-dependent, it has been unclear if rhythmic waves are required for in vivo motor circuit development, including the proper targeting of motoneurons to muscles. We show here that electroporated channelrhodopsin-2 can be activated in ovo with light flashes to drive waves at precise intervals of approximately twice the control frequency in intact chicken embryos. Optical monitoring of associated axial movements ensured that the altered frequency was maintained. In embryos thus stimulated, motor axons correctly executed the binary dorsal-ventral pathfinding decision but failed to make the subsequent pool-specific decision to target to appropriate muscles. This observation, together with the previous demonstration that slowing the frequency by half perturbed dorsal-ventral but not pool-specific pathfinding, shows that modest changes in frequency differentially disrupt these two major pathfinding decisions. Thus, many drugs known to alter early rhythmic activity have the potential to impair normal motor circuit development, and given the conservation between mouse and avian spinal cords, our observations are likely relevant to mammals, where such studies would be difficult to carry out.

The C. elegans CDK8 Mediator module regulates axon guidance decisions in the ventral nerve cord and during dorsal axon navigation

Receptors expressed on the growth cone of outgrowing axons detect cues required for proper navigation. The pathway choices available to an axon are in part defined by the set of guidance receptors present on the growth cone. Regulated expression of receptors and genes controlling the localization and activity of receptors ensures that axons respond only to guidance cues relevant for reaching their targets. In genetic screens for axon guidance mutants, we isolated an allele of let-19/mdt-13, a component of the Mediator, a large ~30 subunit protein complex essential for gene transcription by RNA polymerase II. LET-19/MDT-13 is part of the CDK8 module of the Mediator. By testing other Mediator components, we found that all subunits of the CDK8 module as well as some other Mediator components are required for specific axon navigation decisions in a subset of neurons. Expression profiling demonstrated that let-19/mdt-13 regulates the expression of a large number of genes in interneurons. A mutation in the sax-3 gene, encoding a receptor for the repulsive guidance cue SLT-1, suppresses the commissure navigation defects found in cdk-8 mutants. This suggests that the CDK8 module specifically represses the SAX-3/ROBO pathway to ensure proper commissure navigation.