Independent functions of Slit-Robo repulsion and Netrin-Frazzled attraction regulate axon crossing at the midline in Drosophila

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

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

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

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

Construction and reconstruction of brain circuits: normal and pathological axon guidance

Perception of our environment entirely depends on the close interaction between the central and peripheral nervous system. In order to communicate each other, both systems must develop in parallel and in coordination. During development, axonal projections from the CNS as well as the PNS must extend over large distances to reach their appropriate target cells. To do so, they read and follow a series of axon guidance molecules. Interestingly, while these molecules play critical roles in guiding developing axons, they have also been shown to be critical in other major neurodevelopmental processes, such as the migration of cortical progenitors. Currently, a major hurdle for brain repair after injury or neurodegeneration is the absence of axonal regeneration in the mammalian CNS. By contrasts, PNS axons can regenerate. Many hypotheses have been put forward to explain this paradox but recent studies suggest that hacking neurodevelopmental mechanisms may be the key to promote CNS regeneration. Here we provide a seminar report written by trainees attending the second Flagship school held in Alpbach, Austria in September 2018 organized by the International Society for Neurochemistry (ISN) together with the Journal of Neurochemistry (JCN). This advanced school has brought together leaders in the fields of neurodevelopment and regeneration in order to discuss major keystones and future challenges in these respective fields.

Control of nerve cord formation by Engrailed and Gooseberry-Neuro: A multi-step, coordinated process

One way to better understand the molecular mechanisms involved in the construction of a nervous system is to identify the downstream effectors of major regulatory proteins. We previously showed that Engrailed (EN) and Gooseberry-Neuro (GsbN) transcription factors act in partnership to drive the formation of posterior commissures in the central nervous system of Drosophila. In this report, we identified genes regulated by both EN and GsbN through chromatin immunoprecipitation ("ChIP on chip") and transcriptome experiments, combined to a genetic screen relied to the gene dose titration method. The genomic-scale approaches allowed us to define 175 potential targets of EN-GsbN regulation. We chose a subset of these genes to examine ventral nerve cord (VNC) defects and found that half of the mutated targets show clear VNC phenotypes when doubly heterozygous with en or gsbn mutations, or when homozygous. This strategy revealed new groups of genes never described for their implication in the construction of the nerve cord. Their identification suggests that, to construct the nerve cord, EN-GsbN may act at three levels, in: (i) sequential control of the attractive-repulsive signaling that ensures contralateral projection of the commissural axons, (ii) temporal control of the translation of some mRNAs, (iii) regulation of the capability of glial cells to act as commissural guideposts for developing axons. These results illustrate how an early, coordinated transcriptional control may orchestrate the various mechanisms involved in the formation of stereotyped neuronal networks. They also validate the overall strategy to identify genes that play crucial role in axonal pathfinding.

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

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

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

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

Dscam1 Forms a Complex with Robo1 and the N-Terminal Fragment of Slit to Promote the Growth of Longitudinal Axons

The Slit protein is a major midline repellent for central nervous system (CNS) axons. In vivo, Slit is proteolytically cleaved into N- and C-terminal fragments, but the biological significance of this is unknown. Analysis in the Drosophila ventral nerve cord of a slit allele (slit-UC) that cannot be cleaved revealed that midline repulsion is still present but longitudinal axon guidance is disrupted, particularly across segment boundaries. Double mutants for the Slit receptors Dscam1 and robo1 strongly resemble the slit-UC phenotype, suggesting they cooperate in longitudinal axon guidance, and through biochemical approaches, we found that Dscam1 and Robo1 form a complex dependent on Slit-N. In contrast, Robo1 binding alone shows a preference for full-length Slit, whereas Dscam1 only binds Slit-N. Using a variety of transgenes, we demonstrated that Dscam1 appears to modify the output of Robo/Slit complexes so that signaling is no longer repulsive. Our data suggest that the complex is promoting longitudinal axon growth across the segment boundary. The ability of Dscam1 to modify the output of other receptors in a ligand-dependent fashion may be a general principle for Dscam proteins.

Slit-Robo signaling

Slits are secreted proteins that bind to Roundabout (Robo) receptors. Slit-Robo signaling is best known for mediating axon repulsion in the developing nervous system. However, in recent years the functional repertoire of Slits and Robo has expanded tremendously and Slit-Robo signaling has been linked to roles in neurogenesis, angiogenesis and cancer progression among other processes. Likewise, our mechanistic understanding of Slit-Robo signaling has progressed enormously. Here, we summarize new insights into Slit-Robo evolutionary and system-dependent diversity, receptor-ligand interactions, signaling crosstalk and receptor activation.

Mushroom body defect is required in parallel to Netrin for midline axon guidance in Drosophila

The outgrowth of many neurons within the central nervous system is initially directed towards or away from the cells lying at the midline. Recent genetic evidence suggests that a simple model of differential sensitivity to the conserved Netrin attractants and Slit repellents is insufficient to explain the guidance of all axons at the midline. In the Drosophila embryonic ventral nerve cord, many axons still cross the midline in the absence of the Netrin genes (NetA and NetB) or their receptor frazzled. Here we show that mutation of mushroom body defect (mud) dramatically enhances the phenotype of Netrin or frazzled mutants, resulting in many more axons failing to cross the midline, although mutations in mud alone have little effect. This suggests that mud, which encodes a microtubule-binding coiled-coil protein homologous to NuMA and LIN-5, is an essential component of a Netrin-independent pathway that acts in parallel to promote midline crossing. We demonstrate that this novel role of Mud in axon guidance is independent of its previously described role in neural precursor development. These studies identify a parallel pathway controlling midline guidance in Drosophila and highlight a novel role for Mud potentially acting downstream of Frizzled to aid axon guidance.

Netrin1-DCC-Mediated Attraction Guides Post-Crossing Commissural Axons in the Hindbrain

Commissural axons grow along precise trajectories that are guided by several cues secreted from the ventral midline. After initial attraction to the floor plate using Netrin1 activation of its main attractive receptor, DCC (deleted in colorectal cancer), axons cross the ventral midline, and many turn to grow longitudinally on the contralateral side. After crossing the midline, axons are thought to lose their responsiveness to Netrin1 and become sensitive to midline Slit-Robo repulsion. We aimed to address the in vivo significance of Netrin1 in guiding post-crossing axon trajectories in mouse embryos. Surprisingly, in contrast to the spinal cord, Netrin1 and DCC mutants had abundant commissural axons crossing in the hindbrain. In Netrin1 and DCC mutants, many post-crossing axons made normal turns to grow longitudinally, but projected abnormally at angles away from the midline. In addition, exposure of cultured hindbrain explants to ectopic Netrin1 caused attractive deflection of post-crossing axons. Thus, Netrin1-DCC signaling is not required to attract pre-crossing axons toward the hindbrain floor plate, but is active in post-crossing guidance. Also in contrast with spinal cord, analysis of hindbrain post-crossing axons in Robo1/2 mutant embryos showed that Slit-Robo repulsive signaling was not required for post-crossing trajectories. Our findings show that Netrin1-DCC attractive signaling, but not Slit-Robo repulsive signaling, remains active in hindbrain post-crossing commissural axons to guide longitudinal trajectories, suggesting surprising regional diversity in commissural axon guidance mechanisms.

SIGNIFICANCE STATEMENT

The left and right sides of the brainstem and spinal cord are connected primarily by axon fibers that grow across the ventral midline, and then away on the other side to their targets. Based on spinal cord, axons are initially attracted by diffusible attractive protein signals to approach and cross the midline, and then are thought to switch to repulsive cues to grow away on the opposite side. Our results in the hindbrain show that the major midline attractant, Netrin1, is not required for midline crossing. However, the post-crossing axons depend on Netrin1 attraction to set their proper trajectories on the other side. Overall, these findings suggest that commissural axons use distinct mechanisms to navigate in different CNS regions.

Hydroxyurea-mediated neuroblast ablation establishes birth dates of secondary lineages and addresses neuronal interactions in the developing Drosophila brain

The Drosophila brain is comprised of neurons formed by approximately 100 lineages, each of which is derived from a stereotyped, asymmetrically dividing neuroblast. Lineages serve as structural and developmental units of Drosophila brain anatomy and reconstruction of lineage projection patterns represents a suitable map of Drosophila brain circuitry at the level of neuron populations ("macro-circuitry"). Two phases of neuroblast proliferation, the first in the embryo and the second during the larval phase (following a period of mitotic quiescence), produce primary and secondary lineages, respectively. Using temporally controlled pulses of hydroxyurea (HU) to ablate neuroblasts and their corresponding secondary lineages during the larval phase, we analyzed the effect on development of primary and secondary lineages in the late larval and adult brain. Our findings indicate that timing of neuroblast re-activation is highly stereotyped, allowing us to establish "birth dates" for all secondary lineages. Furthermore, our results demonstrate that, whereas the trajectory and projection pattern of primary and secondary lineages is established in a largely independent manner, the final branching pattern of secondary neurons is dependent upon the presence of appropriate neuronal targets. Taken together, our data provide new insights into the degree of neuronal plasticity during Drosophila brain development.

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

Motor neurons differentiate from a ventral column of progenitors and settle in static clusters, the motor nuclei, next to the floor plate. Within these cell clusters, motor neurons receive afferent input and project their axons out to muscle targets. The molecular mechanisms that position motor neurons in the neural tube remain poorly understood. The floor plate produces several types of guidance cues with well-known roles in attracting and repelling axons, including the Slit family of chemorepellents via their Robo receptors, and Netrin1 via its DCC attractive receptor. In the present study we found that Islet1(+) motor neuron cell bodies invaded the floor plate of Robo1/2 double mutant mouse embryos or Slit1/2/3 triple mutants. Misplaced neurons were born in their normal progenitor column, but then migrated tangentially into the ventral midline. Robo1 and 2 receptor expression in motor neurons was confirmed by reporter gene staining and anti-Robo antibody labeling. Mis-positioned motor neurons projected their axons longitudinally within the floor plate, and failed to reach their normal exit points. To test for potential counteracting ventral attractive signals, we examined Netrin-1 and DCC mutants, and found that motor neurons shifted dorsally in the hindbrain and spinal cord, suggesting that Netrin-1/DCC signaling normally attracts motor neurons closer to the floor plate. Our results show that motor neurons are actively migrating cells, and are normally trapped in a static position by Slit/Robo repulsion and Netrin-1/DCC attraction.

Pioneer midbrain longitudinal axons navigate using a balance of Netrin attraction and Slit repulsion

Background

Longitudinal axons grow parallel to the embryonic midline to connect distant regions of the central nervous system. Previous studies suggested that repulsive midline signals guide pioneer longitudinal axons by blocking their entry into the floor plate; however, the role of midline attractants, and whether attractant signals may cooperate with repulsive signals, remains unclear. In this study we investigated the navigation of a set of pioneer longitudinal axons, the medial longitudinal fasciculus, in mouse embryos mutant for the Netrin/Deleted in Colorectal Cancer (DCC) attractants, and for Slit repellents, as well as the responses of explanted longitudinal axons in vitro.

Results

In mutants for Netrin1 chemoattractant or DCC receptor signaling, longitudinal axons shifted away from the ventral midline, suggesting that Netrin1/DCC signals act attractively to pull axons ventrally. Analysis of mutants in the three Slit genes, including Slit1/2/3 triple mutants, suggest that concurrent repulsive Slit/Robo signals push pioneer axons away from the ventral midline. Combinations of mutations between the Netrin and Slit guidance systems provided genetic evidence that the attractive and repulsive signals balance against each other. This balance is demonstrated in vitro using explant culture, finding that the cues can act directly on longitudinal axons. The explants also reveal an unexpected synergy of Netrin1 and Slit2 that promotes outgrowth.

Conclusions

These results support a mechanism in which longitudinal trajectories are positioned by a push-pull balance between opposing Netrin and Slit signals. Our evidence suggests that longitudinal axons respond directly and simultaneously to both attractants and repellents, and that the combined signals constrain axons to grow longitudinally.

unfulfilled interacting genes display branch-specific roles in the development of mushroom body axons in Drosophila melanogaster

The mushroom body (MB) of Drosophila melanogaster is an organized collection of interneurons that is required for learning and memory. Each of the three subtypes of MB neurons, γ, α´/β´, and α/β, branch at some point during their development, providing an excellent model in which to study the genetic regulation of axon branching. Given the sequential birth order and the unique patterning of MB neurons, it is likely that specific gene cascades are required for the different guidance events that form the characteristic lobes of the MB. The nuclear receptor UNFULFILLED (UNF), a transcription factor, is required for the differentiation of all MB neurons. We have developed and used a classical genetic suppressor screen that takes advantage of the fact that ectopic expression of unf causes lethality to identify candidate genes that act downstream of UNF. We hypothesized that reducing the copy number of unf-interacting genes will suppress the unf-induced lethality. We have identified 19 candidate genes that when mutated suppress the unf-induced lethality. To test whether candidate genes impact MB development, we performed a secondary phenotypic screen in which the morphologies of the MBs in animals heterozygous for unf and a specific candidate gene were analyzed. Medial MB lobes were thin, missing, or misguided dorsally in five double heterozygote combinations (;unf/+;axin/+, unf/+;Fps85D/+, ;unf/+;Tsc1/+, ;unf/+;Rheb/+, ;unf/+;msn/+). Dorsal MB lobes were missing in ;unf/+;DopR2/+ or misprojecting beyond the termination point in ;unf/+;Sytβ double heterozygotes. These data suggest that unf and unf-interacting genes play specific roles in axon development in a branch-specific manner.

Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

To develop a functional nervous system, axons must initially navigate through a complex environment, directed by guidance ligands and receptors. These receptors must link to intracellular signaling cascades to direct axon pathfinding decisions. The Abelson tyrosine kinase (Abl) plays a crucial role in multiple Drosophila axon guidance pathways during development, though the mechanism by which Abl elicits a diverse set of guidance outputs is currently unknown. We identified Abl in a genetic screen for genes that contribute to Netrin-dependent axon guidance in midline-crossing (commissural) neurons. We find that Abl interacts both physically and genetically with the Netrin receptor Frazzled, and that disrupting this interaction prevents Abl from promoting midline axon crossing. Moreover, we find that Abl exerts its diverse activities through at least two different mechanisms: (1) a partly kinase-independent, structural function in midline attraction through its C-terminal F-actin binding domain (FABD) and (2) a kinase-dependent inhibition of repulsive guidance pathways that does not require the Abl C terminus. Abl also regulates motor axon pathfinding through a non-overlapping set of functional domains. These results highlight how a multifunctional kinase can trigger diverse axon guidance outcomes through the use of distinct structural motifs.

Frazzled/DCC facilitates cardiac cell outgrowth and attachment during Drosophila dorsal vessel formation

Drosophila embryonic dorsal vessel (DV) morphogenesis is a highly stereotyped process that involves the migration and morphogenesis of 52 pairs of cardioblasts (CBs) in order to form a linear tube. This process requires spatiotemporally-regulated localization of signaling and adhesive proteins in order to coordinate the formation of a central lumen while maintaining simultaneous adhesion between CBs. Previous studies have shown that the Slit/Roundabout and Netrin/Unc5 repulsive signaling pathways facilitate site-specific loss of adhesion between contralateral CBs in order to form a luminal space. However, the concomitant mechanism by which attraction initiates CB outgrowth and discrete localization of adhesive proteins remains poorly understood. Here we provide genetic evidence that Netrin signals through DCC (Deleted in Colorectal Carcinoma)/UNC-40/Frazzled (Fra) to mediate CB outgrowth and attachment and that this function occurs prior to and independently of Netrin/UNC-5 signaling. fra mRNA is expressed in the CBs prior to and during DV morphogenesis. Loss-of-fra-function results in significant defects in cell shape and alignment between contralateral CB rows. In addition, CB outgrowth and attachment is impaired in both fra loss- and gain-of-function mutants. Deletion of both Netrin genes (NetA and NetB) results in CB attachment phenotypes similar to fra mutants. Similar defects are also seen when both fra and unc5 are deleted. Finally we show that Fra accumulates at dorsal and ventral leading edges of paired CBs, and this localization is dependent upon Netrin. We propose that while repulsive guidance mechanisms contribute to lumen formation by preventing luminal domains from coming together, site-specific Netrin/Frazzled signaling mediates CB attachment.

Src inhibits midline axon crossing independent of Frazzled/Deleted in Colorectal Carcinoma (DCC) receptor tyrosine phosphorylation

The phylogenetically conserved Netrin family of chemoattractants signal outgrowth and attractive turning of commissural axons through the Deleted in Colorectal Carcinoma (DCC) family of receptors. Src family kinases are thought to be major signaling effectors of Netrin/DCC. In vertebrates, Src and the closely related Fyn kinases phosphorylate DCC and form a receptor-bound signaling complex leading to activation of downstream effectors. Here we show that, in the Drosophila embryonic CNS, Src kinases are dispensable for midline attraction of commissural axons. Consistent with this observation, tyrosine phosphorylation of the Netrin receptor DCC or its Drosophila ortholog, Frazzled, is not necessary for attraction to Netrin. Moreover, we uncover an unexpected function of Src kinases: inhibition of midline axon crossing through a novel mechanism. We propose that distinct signaling outputs must exist for midline axon crossing independent of Src kinases in commissural neurons.

Emergent growth cone responses to combinations of Slit1 and Netrin 1 in thalamocortical axon topography

How guidance cues are integrated during the formation of complex axonal tracts remains largely unknown. Thalamocortical axons (TCAs), which convey sensory and motor information to the neocortex, have a rostrocaudal topographic organization initially established within the ventral telencephalon [1-3]. Here, we show that this topography is set in a small hub, the corridor, which contains matching rostrocaudal gradients of Slit1 and Netrin 1. Using in vitro and in vivo experiments, we show that Slit1 is a rostral repellent that positions intermediate axons. For rostral axons, although Slit1 is also repulsive and Netrin 1 has no chemotactic activity, the two factors combined generate attraction. These results show that Slit1 has a dual context-dependent role in TCA pathfinding and furthermore reveal that a combination of cues produces an emergent activity that neither of them has alone. Our study thus provides a novel framework to explain how a limited set of guidance cues can generate a vast diversity of axonal responses necessary for proper wiring of the nervous system.

The Adam family metalloprotease Kuzbanian regulates the cleavage of the roundabout receptor to control axon repulsion at the midline

Slits and their Roundabout (Robo) receptors mediate repulsive axon guidance at the Drosophila ventral midline and in the vertebrate spinal cord. Slit is cleaved to produce fragments with distinct signaling properties. In a screen for genes involved in Slit-Robo repulsion, we have identified the Adam family metalloprotease Kuzbanian (Kuz). Kuz does not regulate midline repulsion through cleavage of Slit, nor is Slit cleavage essential for repulsion. Instead, Kuz acts in neurons to regulate repulsion and Kuz can cleave the Robo extracellular domain in Drosophila cells. Genetic rescue experiments using an uncleavable form of Robo show that this receptor does not maintain normal repellent activity. Finally, Kuz activity is required for Robo to recruit its downstream signaling partner, Son of sevenless (Sos). These observations support the model that Kuz-directed cleavage is important for Robo receptor activation.

Netrin-guided accessory cell morphogenesis dictates the dendrite orientation and migration of a Drosophila sensory neuron

Accessory cells, which include glia and other cell types that develop in close association with neurons, have been shown to play key roles in regulating neuron development. However, the underlying molecular and cellular mechanisms remain poorly understood. A particularly intimate association between accessory cells and neurons is found in insect chordotonal organs. We have found that the cap cell, one of two accessory cells of v'ch1, a chordotonal organ in the Drosophila embryo, strongly influences the development of its associated neuron. As it projects a long dorsally directed cellular extension, the cap cell reorients the dendrite of the v'ch1 neuron and tows its cell body dorsally. Cap cell morphogenesis is regulated by Netrin-A, which is produced by epidermal cells at the destination of the cap cell process. In Netrin-A mutant embryos, the cap cell forms an aberrant, ventrally directed process. As the cap cell maintains a close physical connection with the tip of the dendrite, the latter is dragged into an abnormal position and orientation, and the neuron fails to undergo its normal dorsal migration. Misexpression of Netrin-A in oenocytes, secretory cells that lie ventral to the cap cell, leads to aberrant cap cell morphogenesis, suggesting that Netrin-A acts as an instructive cue to direct the growth of the cap cell process. The netrin receptor Frazzled is required for normal cap cell morphogenesis, and mutant rescue experiments indicate that it acts in a cell-autonomous fashion.

Drosophila neurexin IV interacts with Roundabout and is required for repulsive midline axon guidance

Slit/Roundabout (Robo) signaling controls midline repulsive axon guidance. However, proteins that interact with Slit/Robo at the cell surface remain largely uncharacterized. Here, we report that the Drosophila transmembrane septate junction-specific protein Neurexin IV (Nrx IV) functions in midline repulsive axon guidance. Nrx IV is expressed in the neurons of the developing ventral nerve cord, and nrx IV mutants show crossing and circling of ipsilateral axons and fused commissures. Interestingly, the axon guidance defects observed in nrx IV mutants seem independent of its other binding partners, such as Contactin and Neuroglian and the midline glia protein Wrapper, which interacts in trans with Nrx IV. nrx IV mutants show diffuse Robo localization, and dose-dependent genetic interactions between nrx IV/robo and nrx IV/slit indicate that they function in a common pathway. In vivo biochemical studies reveal that Nrx IV associates with Robo, Slit, and Syndecan, and interactions between Robo and Slit, or Nrx IV and Slit, are affected in nrx IV and robo mutants, respectively. Coexpression of Nrx IV and Robo in mammalian cells confirms that these proteins retain the ability to interact in a heterologous system. Furthermore, we demonstrate that the extracellular region of Nrx IV is sufficient to rescue Robo localization and axon guidance phenotypes in nrx IV mutants. Together, our studies establish that Nrx IV is essential for proper Robo localization and identify Nrx IV as a novel interacting partner of the Slit/Robo signaling pathway.

Netrins guide migration of distinct glial cells in the Drosophila embryo

Development of the nervous system and establishment of complex neuronal networks require the concerted activity of different signalling events and guidance cues, which include Netrins and their receptors. In Drosophila, two Netrins are expressed during embryogenesis by cells of the ventral midline and serve as attractant or repellent cues for navigating axons. We asked whether glial cells, which are also motile, are guided by similar cues to axons, and analysed the influence of Netrins and their receptors on glial cell migration during embryonic development. We show that in Netrin mutants, two distinct populations of glial cells are affected: longitudinal glia (LG) fail to migrate medially in the early stages of neurogenesis, whereas distinct embryonic peripheral glia (ePG) do not properly migrate laterally into the periphery. We further show that early Netrin-dependent guidance of LG requires expression of the receptor Frazzled (Fra) already in the precursor cell. At these early stages, Netrins are not yet expressed by cells of the ventral midline and we provide evidence for a novel Netrin source within the neurogenic region that includes neuroblasts. Later in development, most ePG transiently express uncoordinated 5 (unc5) during their migratory phase. In unc5 mutants, however, two of these cells in particular exhibit defective migration and stall in, or close to, the central nervous system. Both phenotypes are reversible in cell-specific rescue experiments, indicating that Netrin-mediated signalling via Fra (in LG) or Unc5 (in ePG) is a cell-autonomous effect.

In the absence of frazzled over-expression of Abelson tyrosine kinase disrupts commissure formation and causes axons to leave the embryonic CNS

BACKGROUND

In the Drosophila embryonic nerve cord, the formation of commissures require both the chemoattractive Netrin receptor Frazzled (Fra) and the Abelson (Abl) cytoplasmic tyrosine kinase. Abl binds to the cytoplasmic domain of Fra and loss-of-function mutations in abl enhance fra-dependent commissural defects. To further test Abl's role in attractive signaling, we over-expressed Abl in Fra mutants anticipating rescue of commissures.

METHODOLOGY/PRINCIPAL FINDINGS

The Gal4-UAS system was used to pan-neurally over-express Abl in homozygous fra embryos. Surprisingly, this led to a significant decrease in both posterior and anterior commissure formation and induced some commissural and longitudinal axons to project beyond the CNS/PNS border. Re-expressing wild-type Fra, or Fra mutants with a P-motif deleted, revert both commissural and exiting phenotypes, indicating that Fra is required but not a specific P-motif. This is supported by S2 cell experiments demonstrating that Abl binds to Fra independent of any specific P-motif and that Fra continues to be phosphorylated when individual P-motifs are removed. Decreasing midline repulsion by reducing Robo signaling had no effect on the Abl phenotype and the phenotypes still occur in a Netrin mutant. Pan-neural over-expression of activated Rac or Cdc42 in a fra mutant also induced a significant loss in commissures, but axons did not exit the CNS.

CONCLUSION/SIGNIFICANCE

Taken together, these data suggest that Fra activity is required to correctly regulate Abl-dependent cytoskeletal dynamics underlying commissure formation. In the absence of Fra, increased Abl activity appears to be incorrectly utilized downstream of other guidance receptors resulting in a loss of commissures and the abnormal projections of some axons beyond the CNS/PNS border.

Axon guidance at the midline: of mice and flies

In bilaterally symmetric organisms, the midline is a critical organizing center for the developing central nervous system. There is a striking conservation of the molecules and mechanisms that control axon path finding at the midline in vertebrate and invertebrate nervous systems. The majority of axons in the CNS cross the midline before projecting to their contralateral synaptic targets and this crossing decision is under exquisite spatial and temporal regulation. Growing commissural axons initially respond to attractive signals, while inhibiting responses to repulsive signals. Once across, repulsion dominates, allowing axons to leave and preventing them from re-entering the midline. Here we review recent advances in flies and mice that illuminate the molecular mechanisms underlying the establishment of precise connectivity at the midline.

Netrin-DCC, Robo-Slit, and heparan sulfate proteoglycans coordinate lateral positioning of longitudinal dopaminergic diencephalospinal axons

Longitudinal axons provide connectivity between remote areas of the nervous system. Although the molecular determinants driving commissural pathway formation have been well characterized, mechanisms specifying the formation of longitudinal axon tracts in the vertebrate nervous system are largely unknown. Here, we study axon guidance mechanisms of the longitudinal dopaminergic (DA) diencephalospinal tract. This tract is established by DA neurons located in the ventral diencephalon and is thought to be involved in modulating locomotor activity. Using mutant analysis as well as gain of function and loss of function experiments, we demonstrate that longitudinal DA axons navigate by integrating long-range signaling of midline-derived cues. Repulsive Robo2/Slit signaling keeps longitudinal DA axons away from the midline. In the absence of repulsive Robo2/Slit function, DA axons are attracted toward the midline by DCC (deleted in colorectal cancer)/Netrin1 signaling. Thus, Slit-based repulsion counteracts Netrin-mediated attraction to specify lateral positions of the DA diencephalospinal tract. We further identified heparan sulfate proteglycans as essential modulators of DA diencephalospinal guidance mechanisms. Our findings provide insight into the complexity of positioning far-projecting longitudinal axons and allow us to provide a model for DA diencephalospinal pathfinding. Simultaneous integrations of repulsive and attractive long-range cues from the midline act in a concerted manner to define lateral positions of DA longitudinal axon tracts.

Midline governs axon pathfinding by coordinating expression of two major guidance systems

Formation of the neural network requires concerted action of multiple axon guidance systems. How neurons orchestrate expression of multiple guidance genes is poorly understood. Here, we show that Drosophila T-box protein Midline controls expression of genes encoding components of two major guidance systems: Frazzled, ROBO, and Slit. In midline mutant, expression of all these molecules are reduced, resulting in severe axon guidance defects, whereas misexpression of Midline induces their expression. Midline is present on the promoter regions of these genes, indicating that Midline controls transcription directly. We propose that Midline controls axon pathfinding through coordinating the two guidance systems.

A frazzled/DCC-dependent transcriptional switch regulates midline axon guidance

Precise wiring of the nervous system depends on coordinating the action of conserved families of proteins that direct axons to their appropriate targets. Slit-roundabout repulsion and netrin-deleted in colorectal cancer (DCC) (frazzled) attraction must be tightly regulated to control midline axon guidance in vertebrates and invertebrates, but the mechanism mediating this regulation is poorly defined. Here, we show that the Fra receptor has two genetically separable functions in regulating midline guidance in Drosophila. First, Fra mediates canonical chemoattraction in response to netrin, and, second, it functions independently of netrin to activate commissureless transcription, allowing attraction to be coupled to the down-regulation of repulsion in precrossing commissural axons.

Pioneer longitudinal axons navigate using floor plate and Slit/Robo signals

Longitudinal axons transmit all signals between the brain and spinal cord. Their axon tracts through the brain stem are established by a simple set of pioneer axons with precise trajectories parallel to the floor plate. To identify longitudinal guidance mechanisms in vivo, the overall role of floor plate tissue and the specific roles of Slit/Robo signals were tested. Ectopic induction or genetic deletion of the floor plate diverted longitudinal axons into abnormal trajectories. The expression patterns of the diffusible cues of the Slit family were altered in the floor plate experiments, suggesting their involvement in longitudinal guidance. Genetic tests of Slit1 and Slit2, and the Slit receptors Robo1 and Robo2 were carried out in mutant mice. Slit1;Slit2 double mutants had severe longitudinal errors, particularly for ventral axons, including midline crossing and wandering longitudinal trajectories. Robo1 and Robo2 were largely genetically redundant, and neither appeared to specify specific tract positions. However, combined Robo1 and Robo2 mutations strongly disrupted each pioneer tract. Thus, pioneer axons depend on long-range floor plate cues, with Slit/Robo signaling required for precise longitudinal trajectories.

Dscam guides embryonic axons by Netrin-dependent and -independent functions

Developing axons are attracted to the CNS midline by Netrin proteins and other as yet unidentified signals. Netrin signals are transduced in part by Frazzled (Fra)/DCC receptors. Genetic analysis in Drosophila indicates that additional unidentified receptors are needed to mediate the attractive response to Netrin. Analysis of Bolwig's nerve reveals that Netrin mutants have a similar phenotype to Down Syndrome Cell Adhesion Molecule (Dscam) mutants. Netrin and Dscam mutants display dose sensitive interactions, suggesting that Dscam could act as a Netrin receptor. We show using cell overlay assays that Netrin binds to fly and vertebrate Dscam, and that Dscam binds Netrin with the same affinity as DCC. At the CNS midline, we find that Dscam and its paralog Dscam3 act redundantly to promote midline crossing. Simultaneous genetic knockout of the two Dscam genes and the Netrin receptor fra produces a midline crossing defect that is stronger than the removal of Netrin proteins, suggesting that Dscam proteins also function in a pathway parallel to Netrins. Additionally, overexpression of Dscam in axons that do not normally cross the midline is able to induce ectopic midline crossing, consistent with an attractive receptor function. Our results support the model that Dscam proteins function as attractive receptors for Netrin and also act in parallel to Frazzled/DCC. Furthermore, the results suggest that Dscam proteins have the ability to respond to multiple ligands and act as receptors for an unidentified midline attractive cue. These functions in axon guidance have implications for the pathogenesis of Down Syndrome.

Netrin, Slit and Wnt receptors allow axons to choose the axis of migration

One of the challenges to understanding nervous system development has been to establish how a fairly limited number of axon guidance cues can set up the patterning of very complex nervous systems. Studies on organisms with relatively simple nervous systems such as Drosophila melanogaster and C. elegans have provided many insights into axon guidance mechanisms. The axons of many neurons migrate along both the dorsal-ventral (DV) and the anterior-posterior (AP) axes at different phases of development, and in addition they may also cross the midline. Axon migration in the dorsal-ventral (DV) direction is mainly controlled by Netrins with their receptors; UNC-40/DCC and UNC-5, and the Slits with their receptors; Robo/SAX-3. Axon guidance in the anterior-posterior (AP) axis is mainly controlled by Wnts with their receptors; the Frizzleds/Fz. An individual axon may be subjected to opposing attractive and repulsive forces coming from opposite sides in the same axis but there may also be opposing cues in the other axis of migration. All the information from the cues has to be integrated within the growth cone at the leading edge of the migrating axon to elicit a response. Recent studies have provided insight into how this is achieved. Evidence suggests that the axis of axon migration is determined by the manner in which Netrin, Slit and Wnt receptors are polarized (localized) within the neuron prior to axon outgrowth. The same molecules are involved in both axon outgrowth and axon guidance, for at least some neurons in C. elegans, whether the cue is the attractive cue UNC-6/Netrin working though UNC-40/DCC or the repulsive cue SLT-1/Slit working though the receptor SAX-3/Robo (Adler et al., 2006, Chang et al., 2006, Quinn et al., 2006, 2008). The molecules involved in cell signaling in this case are polarized within the cell body of the neuron before process outgrowth and direct the axon outgrowth. Expression of the Netrin receptor UNC-40/DCC or the Slit receptor SAX-3/Robo in axons that normally migrate in the AP direction causes neuronal polarity reversal in a Netrin and Slit independent manner (Levy-Strumpf and Culotti 2007, Watari-Goshima et al., 2007). Localization of the receptors in this case is caused by the kinesin-related VAB-8L which appears to govern the site of axon outgrowth in these neurons by causing receptor localization. Therefore, asymmetric localization of axon guidance receptors is followed by axon outgrowth in vivo using the receptor's normal cue, either attractive, repulsive or unknown cues.

Frazzled cytoplasmic P-motifs are differentially required for axon pathway formation in the Drosophila embryonic CNS

Frazzled is a Netrin-dependent chemoattractive receptor required for axon pathway formation in the developing Drosophila embryonic CNS. The cytoplasmic domain is important and contains three conserved P-motifs (P1, P2, and P3) thought to initiate intracellular signaling cascades and to crosstalk with other receptors during axon pathway formation. Here, we rescue homozygous frazzled embryos by pan-neurally expressing a series of mutants lacking either the cytoplasmic domain or one of the conserved P-motifs and assess the ability of these mutants to rescue frazzled defects in commissural, longitudinal and motor axon pathways. Surprisingly, while the cytoplasmic domain is required, removal of an individual P-motif does not prevent gross formation of commissures. However, removal of P3 from Fra does prevent eagle-expressing commissural axons from crossing the midline in the posterior commissure suggesting that some neurons have a stronger requirement for P3-dependent signaling. Indeed, axons within the longitudinal connective as well as a small subset of motor neurons within the ISNb pathway also specifically require P3 to project to their targets correctly. In these latter axon projections, deleting the P1-motif appears to de-regulate the receptor's activity, actually increasing the frequency of motor neuron projection errors and inducing ectopic midline crossing errors. Collectively, these data demonstrate the critical nature of both the P1 and the P3-motifs to Frazzled function in vivo during axon pathway formation.

A concerted action of Engrailed and Gooseberry-Neuro in neuroblast 6-4 is triggering the formation of embryonic posterior commissure bundles

One challenging question in neurogenesis concerns the identification of cues that trigger axonal growth and pathfinding to form stereotypic neuronal networks during the construction of a nervous system. Here, we show that in Drosophila, Engrailed (EN) and Gooseberry-Neuro (GsbN) act together as cofactors to build the posterior commissures (PCs), which shapes the ventral nerve cord. Indeed, we show that these two proteins are acting together in axon growth and midline crossing, and that this concerted action occurs at early development, in neuroblasts. More precisely, we identified that their expressions in NB 6-4 are necessary and sufficient to trigger the formation of the PCs, demonstrating that segmentation genes such as EN and GsbN play a crucial role in the determination of NB 6-4 in a way that will later influence growth and guidance of all the axons that form the PCs. We also demonstrate a more specific function of GsbN in differentiated neurons, leading to fasciculations between axons, which might be required to obtain PC mature axon bundles.

Abelson tyrosine kinase and Calmodulin interact synergistically to transduce midline guidance cues in the Drosophila embryonic CNS

Calmodulin and Abelson tyrosine kinase are key signaling molecules transducing guidance cues at the Drosophila embryonic midline. A reduction in the signaling strength of either pathway alone induces ectopic midline crossing errors in a few segments. When Calmodulin and Abelson signaling levels are simultaneously reduced, the frequency of ectopic crossovers is synergistically enhanced as all segments exhibit crossing errors. But as the level of signaling is further reduced, commissures begin to fuse and large gaps form in the longitudinal connectives. Quantitative analysis suggests that the level of Abelson activity is particularly important. Like Calmodulin, Abelson interacts with son-of-sevenless to increase ectopic crossovers suggesting all three contribute to midline repulsive signaling. Axons cross the midline in almost every segment if Frazzled is co-overexpressed with the Calmodulin inhibitor, but the crossovers induced by the Calmodulin inhibitor itself do not require endogenous Frazzled. Thus, Calmodulin and Abelson tyrosine kinase are key signaling molecules working synergistically to transduce both midline attractive and repulsive cues. While they may function downstream of specific receptors, the emergence of commissural and longitudinal connective defects point to a novel convergence of Calmodulin and Abelson signaling during the regulation of actin and myosin dynamics underlying a guidance decision.

Cytoplasmic domain requirements for Frazzled-mediated attractive axon turning at the Drosophila midline

The conserved DCC ligand-receptor pair Netrin and Frazzled (Fra) has a well-established role in axon guidance. However, the specific sequence motifs required for orchestrating downstream signaling events are not well understood. Evidence from vertebrates suggests that P3 is important for transducing Netrin-mediated turning and outgrowth, whereas in C. elegans it was shown that the P1 and P2 conserved sequence motifs are required for a gain-of-function outgrowth response. Here, we demonstrate that Drosophila fra mutant embryos exhibit guidance defects in a specific subset of commissural axons and these defects can be rescued cell-autonomously by expressing wild-type Fra exclusively in these neurons. Furthermore, structure-function studies indicate that the conserved P3 motif (but not P1 or P2) is required for growth cone attraction at the Drosophila midline. Surprisingly, in contrast to vertebrate DCC, P3 does not mediate receptor self-association, and self-association is not sufficient to promote Fra-dependent attraction. We also show that in contrast to previous findings, the cytoplasmic domain of Fra is not required for axonal localization and that neuronal expression of a truncated Fra receptor lacking the entire cytoplasmic domain (Fra delta C) results in dose-dependent defects in commissural axon guidance. These findings represent the first systematic dissection of the cytoplasmic domains required for Fra-mediated axon attraction in the context of full-length receptors in an intact organism and provide important insights into attractive axon guidance at the midline.

Frazzled regulation of myosin II activity in the Drosophila embryonic CNS

Frazzled (Fra) is a chemoattractive guidance receptor regulating the cytoskeletal dynamics underlying growth cone steering at the Drosophila embryonic midline. Here, by genetically evaluating the role of Rho GTPases in Fra signaling in vivo, we uncover a Rho-dependent pathway apparently regulating conventional myosin II activity. Midline crossing errors induced by expressing activated Cdc42(v12) or Rac(v12) are suppressed by a heterozygous loss of fra(4) signaling but, in a Fra(wt) gain-of-function condition, no interaction is detected. In contrast, the frequency of crossovers is enhanced approximately 5-fold when Fra(wt) is co-expressed with activated Rho(v14) and this interaction specifically requires the cytoplasmic P3 motif of Fra. Expression of Rho(v14) and activated MLCK (ctMLCK) synergistically increase ectopic crossovers and both require phosphorylation of the regulatory light chain (Sqh) of myosin II. Abelson tyrosine kinase may also help regulate myosin II activity. Heterozygous abl(4) abolishes the midline crossing errors induced by ctMLCK alone or in combination with Fra(wt); suppression of Rho(v14) crossovers is not observed. Interestingly, an interaction between Fra and an activated Abl (Bcr-Abl) also specifically requires the P3 motif. Therefore, the P3 motif of Frazzled appears to initiate Rho and Abl dependent signals to directly or indirectly regulate myosin II activity in growth cones.