Crossing the embryonic midline: molecular mechanisms regulating axon responsiveness at an intermediate target

Muscular remodeling and anteroposterior patterning during tapeworm segmentation

BACKGROUND

Tapeworms are parasitic flatworms that independently evolved a segmented body plan, historically confounding comparisons with other animals. Anteroposterior (AP) patterning in free-living flatworms and in tapeworm larvae is associated with canonical Wnt signaling and positional control genes (PCGs) are expressed by their musculature in regionalized domains along the AP axis. Here, we extend investigations of PCG expression to the adult of the mouse bile-duct tapeworm Hymenolepis microstoma, focusing on the growth zone of the neck region and the initial establishment of segmental patterning.

RESULTS

We show that the adult musculature includes new, segmental elements that first appear in the neck and that the spatial patterns of Wnt factors are consistent with expression by muscle cells. Wnt factor expression is highly regionalized and becomes AP-polarized in segments, marking them with axes in agreement with the polarity of the main body axis, while the transition between the neck and strobila is specifically demarcated by the expression domain of a Wnt11 paralog.

CONCLUSION

We suggest that segmentation could originate in the muscular system and participate in patterning the AP axis through regional and polarized expression of PCGs, akin to the gene regulatory networks employed by free-living flatworms and other animals.

Robo2 Receptor Gates the Anatomical Divergence of Neurons Derived From a Common Precursor Origin

Sensory information relayed to the brain is dependent on complex, yet precise spatial organization of neurons. This anatomical complexity is generated during development from a surprisingly small number of neural stem cell domains. This raises the question of how neurons derived from a common precursor domain respond uniquely to their environment to elaborate correct spatial organization and connectivity. We addressed this question by exploiting genetically labeled mouse embryonic dorsal interneuron 1 (dI1) neurons that are derived from a common precursor domain and give rise to spinal projection neurons with distinct organization of cell bodies with axons projecting either commissurally (dI1c) or ipsilaterally (dI1i). In this study, we examined how the guidance receptor, Robo2, which is a canonical Robo receptor, influenced dI1 guidance during embryonic development. Robo2 was enriched in embryonic dI1i neurons, and loss of Robo2 resulted in misguidance of dI1i axons, whereas dI1c axons remained unperturbed within the mantle zone and ventral commissure. Further, Robo2 profoundly influenced dI1 cell body migration, a feature that was partly dependent on Slit2 signaling. These data suggest that dI1 neurons are dependent on Robo2 for their organization. This work integrated with the field support of a model whereby canonical Robo2 vs. non-canonical Robo3 receptor expression facilitates projection neurons derived from a common precursor domain to read out the tissue environment uniquely giving rise to correct anatomical organization.

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

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

Molecular mechanisms regulating axon responsiveness at the midline

During embryonic development in bilaterally symmetric organisms, correct midline crossing is important for the proper formation of functional neural circuits. The aberrant development of neural circuits can result in multiple neurodevelopmental disorders, including horizontal gaze palsy, congenital mirror movement disorder, and autism spectrum disorder. Thus, understanding the molecular mechanisms that regulate proper axon guidance at the midline can provide insights into the pathology of neurological disorders. The signaling mechanisms that regulate midline crossing have been extensively studied in the Drosophila ventral nerve cord and the mouse embryonic spinal cord. In this review, we discuss these axon guidance mechanisms, highlighting the most recent advances in the understanding of how commissural axons switch their responsiveness from attractants to repellents during midline crossing.

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

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

Ndfip Proteins Target Robo Receptors for Degradation and Allow Commissural Axons to Cross the Midline in the Developing Spinal Cord

Commissural axons initially respond to attractive signals at the midline, but once they cross, they become sensitive to repulsive cues. This switch prevents axons from re-entering the midline. In insects and mammals, negative regulation of Roundabout (Robo) receptors prevents premature response to the midline repellant Slit. In Drosophila, the endosomal protein Commissureless (Comm) prevents Robo1 surface expression before midline crossing by diverting Robo1 into late endosomes. Notably, Comm is not conserved in vertebrates. We identified two Nedd-4-interacting proteins, Ndfip1 and Ndfip2, that act analogously to Comm to localize Robo1 to endosomes. Ndfip proteins recruit Nedd4 E3 ubiquitin ligases to promote Robo1 ubiquitylation and degradation. Ndfip proteins are expressed in commissural axons in the developing spinal cord and removal of Ndfip proteins results in increased Robo1 expression and reduced midline crossing. Our results define a conserved Robo1 intracellular sorting mechanism between flies and mammals to avoid premature responsiveness to Slit.

Brain Tumor promotes axon growth across the midline through interactions with the microtubule stabilizing protein Apc2

Commissural axons must cross the midline to establish reciprocal connections between the two sides of the body. This process is highly conserved between invertebrates and vertebrates and depends on guidance cues and their receptors to instruct axon trajectories. The DCC family receptor Frazzled (Fra) signals chemoattraction and promotes midline crossing in response to its ligand Netrin. However, in Netrin or fra mutants, the loss of crossing is incomplete, suggesting the existence of additional pathways. Here, we identify Brain Tumor (Brat), a tripartite motif protein, as a new regulator of midline crossing in the Drosophila CNS. Genetic analysis indicates that Brat acts independently of the Netrin/Fra pathway. In addition, we show that through its B-Box domains, Brat acts cell autonomously to regulate the expression and localization of Adenomatous polyposis coli-2 (Apc2), a key component of the Wnt canonical signaling pathway, to promote axon growth across the midline. Genetic evidence indicates that the role of Brat and Apc2 to promote axon growth across the midline is independent of Wnt and Beta-catenin-mediated transcriptional regulation. Instead, we propose that Brat promotes midline crossing through directing the localization or stability of Apc2 at the plus ends of microtubules in navigating commissural axons. These findings define a new mechanism in the coordination of axon growth and guidance at the midline.

The establishment of neuronal connections that cross the midline of the animal is essential to generate neural circuits that coordinate the left and right sides of the body. Axons that cross the midline to form these connections are called commissural axons and the molecules and mechanisms that control midline axon crossing are remarkably conserved across animal evolution. In this study we have used a genetic screen in the fruit fly in an attempt to uncover additional players in this key developmental process, and have identified a novel role for the Brain Tumor (Brat) protein in promoting commissural axon growth across the midline. Unlike its previous described functions, in the context of midline axon guidance Brat cooperates with the microtubule stabilizing protein Apc2 to coordinate axon growth and guidance. Molecular and genetic analyses point to the conserved B box motifs of the Brat protein as key in promoting the association of Apc2 with the plus ends of microtubules. Brat is highly conserved and future studies will determine whether homologous genes play analogous roles in mammalian neural development.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Protein O-Mannosyltransferases Affect Sensory Axon Wiring and Dynamic Chirality of Body Posture in the Drosophila Embryo

Genetic defects in protein O-mannosyltransferase 1 (POMT1) and POMT2 underlie severe muscular dystrophies. POMT genes are evolutionarily conserved in metazoan organisms. In Drosophila, both male and female POMT mutants show a clockwise rotation of adult abdominal segments, suggesting a chirality of underlying pathogenic mechanisms. Here we described and analyzed a similar phenotype in POMT mutant embryos that shows left-handed body torsion. Our experiments demonstrated that coordinated muscle contraction waves are associated with asymmetric embryo rolling, unveiling a new chirality marker in Drosophila development. Using genetic and live-imaging approaches, we revealed that the torsion phenotype results from differential rolling and aberrant patterning of peristaltic waves of muscle contractions. Our results demonstrated that peripheral sensory neurons are required for normal contractions that prevent the accumulation of torsion. We found that POMT mutants show abnormal axonal connections of sensory neurons. POMT transgenic expression limited to sensory neurons significantly rescued the torsion phenotype, axonal connectivity defects, and abnormal contractions in POMT mutant embryos. Together, our data suggested that protein O-mannosylation is required for normal sensory feedback to control coordinated muscle contractions and body posture. This mechanism may shed light on analogous functions of POMT genes in mammals and help to elucidate the etiology of neurological defects in muscular dystrophies.SIGNIFICANCE STATEMENT Protein O-mannosyltransferases (POMTs) are evolutionarily conserved in metazoans. Mutations in POMTs cause severe muscular dystrophies associated with pronounced neurological defects. However, neurological functions of POMTs remain poorly understood. We demonstrated that POMT mutations in Drosophila result in abnormal muscle contractions and cause embryo torsion. Our experiments uncovered a chirality of embryo movements and a unique POMT-dependent mechanism that maintains symmetry of a developing system affected by chiral forces. Furthermore, POMTs were found to be required for proper axon connectivity of sensory neurons, suggesting that O-mannosylation regulates the sensory feedback controlling muscle contractions. This novel POMT function in the peripheral nervous system may shed light on analogous functions in mammals and help to elucidate pathomechanisms of neurological abnormalities in muscular dystrophies.

Sema-1a Reverse Signaling Promotes Midline Crossing in Response to Secreted Semaphorins

Commissural axons must cross the midline to form functional midline circuits. In the invertebrate nerve cord and vertebrate spinal cord, midline crossing is mediated in part by Netrin-dependent chemoattraction. Loss of crossing, however, is incomplete in mutants for Netrin or its receptor Frazzled/DCC, suggesting the existence of additional pathways. We identified the transmembrane Semaphorin, Sema-1a, as an important regulator of midline crossing in the Drosophila CNS. We show that in response to the secreted Semaphorins Sema-2a and Sema-2b, Sema-1a functions as a receptor to promote crossing independently of Netrin. In contrast to other examples of reverse signaling where Sema1a triggers repulsion through activation of Rho in response to Plexin binding, in commissural neurons Sema-1a acts independently of Plexins to inhibit Rho to promote attraction to the midline. These findings suggest that Sema-1a reverse signaling can elicit distinct axonal responses depending on differential engagement of distinct ligands and signaling effectors.

A versatile genetic tool to study midline glia function in the Drosophila CNS

Neuron-glial interactions are crucial for growth, guidance and ensheathment of axons across species. In the Drosophila CNS midline, neuron-glial interactions underlie ensheathment of commissural axons by midline glial (MG) cells in a manner similar to mammalian oligodendrocytes. Although there has been some advance in the study of neuron-glial interactions and ensheathment of axons in the CNS midline, key aspects of axonal ensheathment are still not fully understood. One of the limitations has been the unavailability of MG membrane markers that could highlight the glial processes wrapping the axons. Previous studies have identified two key molecular players from the neuronal and glial cell types in the CNS midline. These are the neuronal transmembrane protein Neurexin IV (Nrx IV) and the membrane-anchored MG protein Wrapper, both of which interact in trans to mediate neuron-glial interactions and ensheathment of commissural axons. In the current study, we attempt to further our understanding of MG biology and try to overcome some of the technical difficulties posed by the lack of a robust MG driver that will specifically allow expression or knockdown of genes in MG. We report the generation of BAC transgenic flies of wrapper-GAL4 and demonstrate how these flies could be used as a genetic tool to understand MG biology. We have utilized the GAL4/UAS system to drive GFP-reporter lines (membrane-bound mCD8-GFP; microtubule-associated tau-GFP) and nuclear lacZ using wrapper-GAL4 to highlight the MG cells and/or their processes that surround and perform axonal ensheathment functions in the embryonic midline. We also describe the utility of the wrapper-GAL4 driver line to down-regulate known MG genes specifically in Wrapper-positive cells. Finally, we validate the functionality of the wrapper-GAL4 driver by rescue of wrapper mutant phenotypes and lethality. Together, these studies provide us with a versatile genetic tool to investigate MG functions and will aid in future investigations where genetic screens using wrapper-GAL4 could be designed to identify novel molecular players at the Drosophila midline and unravel key aspects of MG biology.

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.

Cooperation and crosstalk in axon guidance cue integration: Additivity, synergy, and fine-tuning in combinatorial signaling

Neural circuit development involves the coordinated growth and guidance of axons to their targets. Following the identification of many guidance cue molecules, recent experiments have focused on the interactions of their signaling cascades, which can be generally classified as additive or non-additive depending on the signal convergence point. While additive (parallel) signaling suggests limited molecular interaction between the pathways, non-additive signaling involves crosstalk between pathways and includes more complex synergistic, hierarchical, and permissive guidance cue relationships. Here the authors have attempted to classify recent studies that describe axon guidance signal integration according to these divisions. They also discuss the mechanistic implications of such interactions, as well as general ideas relating signal integration to the generation of diversity of axon guidance responses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 891-904, 2017.

Commissural axon navigation: Control of midline crossing in the vertebrate spinal cord by the semaphorin 3B signaling

The mechanisms governing the navigation of commissural axons during embryonic development have been extensively investigated in the past years, often using the drosophila ventral nerve cord and the spinal cord as model systems. Similarities but also specificities in the general strategies, the molecular signals as well as in the regulatory pathways controlling the response of commissural axons to the guidance cues have been found between species. Whether the semaphorin signaling contributes to midline crossing in the fly nervous system remains unknown, while in contrast, it does play a prominent contribution in vertebrates. In this review we discuss the functions of the semaphorins during commissural axon guidance in the developing spinal cord, focusing on the family member semaphorin 3B (Sema3B) in the context of midline crossing in the spinal cord.

IMP2 axonal localization, RNA interactome, and function in the development of axon trajectories

RNA-based regulatory mechanisms play important roles in the development and plasticity of neural circuits and neurological disease. Developing axons provide a model well suited to the study of RNA-based regulation, and contain specific subsets of mRNAs that are locally translated and have roles in axon pathfinding. However, the RNA-binding proteins involved in axon pathfinding, and their corresponding mRNA targets, are still largely unknown. Here we find that the RNA-binding protein IMP2 (Igf2bp2) is strikingly enriched in developing axon tracts, including in spinal commissural axons. We used the HITS-CLIP approach to perform a genome-wide identification of RNAs that interact directly with IMP2 in the native context of developing mouse brain. This IMP2 interactome was highly enriched for mRNA targets related to axon guidance. Accordingly, IMP2 knockdown in the developing spinal cord led to strong defects in commissural axon trajectories at the midline intermediate target. These results reveal a highly distinctive axonal enrichment of IMP2, show that it interacts with a network of axon guidance-related mRNAs, and reveal that it is required for normal axon pathfinding during vertebrate development.

The Intracellular Domain of the Frazzled/DCC Receptor Is a Transcription Factor Required for Commissural Axon Guidance

In commissural neurons of Drosophila, the conserved Frazzled (Fra)/Deleted in Colorectal Cancer (DCC) receptor promotes midline axon crossing by signaling locally in response to Netrin and by inducing transcription of commissureless (comm), an antagonist of Slit-Roundabout midline repulsion, through an unknown mechanism. Here, we show that Fra is cleaved to release its intracellular domain (ICD), which shuttles between the cytoplasm and the nucleus, where it functions as a transcriptional activator. Rescue and gain-of-function experiments demonstrate that the Fra ICD is sufficient to regulate comm expression and that both γ-secretase proteolysis of Fra and Fra's function as a transcriptional activator are required for its ability to regulate comm in vivo. Our data uncover an unexpected role for the Fra ICD as a transcription factor whose activity regulates the responsiveness of commissural axons at the midline and raise the possibility that nuclear signaling may be a common output of axon guidance receptors.

Robo2 acts in trans to inhibit Slit-Robo1 repulsion in pre-crossing commissural axons

During nervous system development, commissural axons cross the midline despite the presence of repellant ligands. In Drosophila, commissural axons avoid premature responsiveness to the midline repellant Slit by expressing the endosomal sorting receptor Commissureless, which reduces surface expression of the Slit receptor Roundabout1 (Robo1). In this study, we describe a distinct mechanism to inhibit Robo1 repulsion and promote midline crossing, in which Roundabout2 (Robo2) binds to and prevents Robo1 signaling. Unexpectedly, we find that Robo2 is expressed in midline cells during the early stages of commissural axon guidance, and that over-expression of Robo2 can rescue robo2-dependent midline crossing defects non-cell autonomously. We show that the extracellular domains required for binding to Robo1 are also required for Robo2's ability to promote midline crossing, in both gain-of-function and rescue assays. These findings indicate that at least two independent mechanisms to overcome Slit-Robo1 repulsion in pre-crossing commissural axons have evolved in Drosophila.