Axon repulsion from the midline of the Drosophila CNS requires slit function

Neural Regulation of Vascular Development: Molecular Mechanisms and Interactions

As a critical part of the circulatory system, blood vessels transport oxygen and nutrients to every corner of the body, nourishing each cell, and also remove waste and toxins. Defects in vascular development and function are closely associated with many diseases, such as heart disease, stroke, and atherosclerosis. In the nervous system, the nervous and vascular systems are intricately connected in both development and function. First, peripheral blood vessels and nerves exhibit parallel distribution patterns. In the central nervous system (CNS), nerves and blood vessels form a complex interface known as the neurovascular unit. Second, the vascular system employs similar cellular and molecular mechanisms as the nervous system for its development. Third, the development and function of CNS vasculature are tightly regulated by CNS-specific signaling pathways and neural activity. Additionally, vascular endothelial cells within the CNS are tightly connected and interact with pericytes, astrocytes, neurons, and microglia to form the blood-brain barrier (BBB). The BBB strictly controls material exchanges between the blood and brain, maintaining the brain's microenvironmental homeostasis, which is crucial for the normal development and function of the CNS. Here, we comprehensively summarize research on neural regulation of vascular and BBB development and propose directions for future research.

Spatiotemporal changes in Netrin/Dscam1 signaling dictate axonal projection direction in Drosophila small ventral lateral clock neurons

Axon projection is a spatial- and temporal-specific process in which the growth cone receives environmental signals guiding axons to their final destination. However, the mechanisms underlying changes in axonal projection direction without well-defined landmarks remain elusive. Here, we present evidence showcasing the dynamic nature of axonal projections in Drosophila's small ventral lateral clock neurons (s-LNvs). Our findings reveal that these axons undergo an initial vertical projection in the early larval stage, followed by a subsequent transition to a horizontal projection in the early-to-mid third instar larvae. The vertical projection of s-LNv axons correlates with mushroom body calyx expansion, while the s-LNv-expressed Down syndrome cell adhesion molecule (Dscam1) interacts with Netrins to regulate the horizontal projection. During a specific temporal window, locally newborn dorsal clock neurons secrete Netrins, facilitating the transition of axonal projection direction in s-LNvs. Our study establishes a compelling in vivo model to probe the mechanisms of axonal projection direction switching in the absence of clear landmarks. These findings underscore the significance of dynamic local microenvironments in the complementary regulation of axonal projection direction transitions.

Slit-Robo expression in the leech nervous system: insights into eyespot evolution

BACKGROUND

Slit and Robo are evolutionarily conserved ligand and receptor proteins, respectively, but the number of slit and robo gene paralogs varies across recent bilaterian genomes. Previous studies indicate that this ligand-receptor complex is involved in axon guidance. Given the lack of data regarding Slit/Robo in the Lophotrochozoa compared to Ecdysozoa and Deuterostomia, the present study aims to identify and characterize the expression of Slit/Robo orthologs in leech development.

RESULTS

We identified one slit (Hau-slit), and two robo genes (Hau-robo1 and Hau-robo2), and characterized their expression spatiotemporally during the development of the glossiphoniid leech Helobdella austinensis. Throughout segmentation and organogenesis, Hau-slit and Hau-robo1 are broadly expressed in complex and roughly complementary patterns in the ventral and dorsal midline, nerve ganglia, foregut, visceral mesoderm and/or endoderm of the crop, rectum and reproductive organs. Before yolk exhaustion, Hau-robo1 is also expressed where the pigmented eye spots will later develop, and Hau-slit is expressed in the area between these future eye spots. In contrast, Hau-robo2 expression is extremely limited, appearing first in the developing pigmented eye spots, and later in the three additional pairs of cryptic eye spots in head region that never develop pigment. Comparing the expression of robo orthologs between H. austinensis and another glossiphoniid leech, Alboglossiphonia lata allows to that robo1 and robo2 operate combinatorially to differentially specify pigmented and cryptic eyespots within the glossiphoniid leeches.

CONCLUSIONS

Our results support a conserved role in neurogenesis, midline formation and eye spot development for Slit/Robo in the Lophotrochozoa, and provide relevant data for evo-devo studies related to nervous system evolution.

Visceral mesoderm signaling regulates assembly position and function of the Drosophila testis niche

Tissue homeostasis often requires a properly placed niche to support stem cells. Morphogenetic processes that position a niche are just being described. For the Drosophila testis, we recently showed that pro-niche cells, specified at disparate positions during early gonadogenesis, must assemble into one collective at the anterior of the gonad. We now find that Slit and FGF signals emanating from adjacent visceral mesoderm regulate assembly. In response to signaling, niche cells express islet, which we find is also required for niche assembly. Without signaling, niche cells specified furthest from the anterior are unable to migrate, remaining dispersed. The function of such niches is severely disrupted, with niche cells evading cell cycle quiescence, compromised in their ability to signal the incipient stem cell pool, and failing to orient stem cell divisions properly. Our work identifies both extrinsic signaling and intrinsic responses required for proper assembly and placement of the testis niche.

Drosophila Tet Is Required for Maintaining Glial Homeostasis in Developing and Adult Fly Brains

Ten-eleven translocation (TET) proteins are crucial epigenetic regulators highly conserved in multicellular organisms. TETs’ enzymatic function in demethylating 5-methyl cytosine in DNA is required for proper development and TETs are frequently mutated in cancer. Recently, Drosophila melanogaster Tet (dTet) was shown to be highly expressed in developing fly brains and discovered to play an important role in brain and muscle development as well as fly behavior. Furthermore, dTet was shown to have different substrate specificity compared with mammals. However, the exact role dTet plays in glial cells and how ectopic TET expression in glial cells contributes to tumorigenesis and glioma is still not clear. Here, we report a novel role for dTet specifically in glial cell organization and number. We show that loss of dTet affects the organization of a specific glia population in the optic lobe, the “optic chiasm” glia. Additionally, we find irregularities in axon patterns in the ventral nerve cord (VNC) both, in the midline and longitudinal axons. These morphologic glia and axonal defects were accompanied by locomotor defects in developing larvae escalating to immobility in adult flies. Furthermore, glia homeostasis was disturbed in dTet-deficient brains manifesting in gain of glial cell numbers and increased proliferation. Finally, we establish a Drosophila model to understand the impact of human TET3 in glia and find that ectopic expression of hTET3 in dTet-expressing cells causes glia expansion in larval brains and affects sleep/rest behavior and the circadian clock in adult flies.

Proteolytic cleavage of Slit by the Tolkin protease converts an axon repulsion cue to an axon growth cue in vivo

Slit is a secreted protein that has a canonical function of repelling growing axons from the CNS midline. The full-length Slit (Slit-FL) is cleaved into Slit-N and Slit-C fragments, which have potentially distinct functions via different receptors. Here, we report that the BMP-1/Tolloid family metalloprotease Tolkin (Tok) is responsible for Slit proteolysis in vivo and in vitro. In Drosophilatok mutants lacking Slit cleavage, midline repulsion of axons occurs normally, confirming that Slit-FL is sufficient to repel axons. However, longitudinal axon guidance is highly disrupted in tok mutants and can be rescued by midline expression of Slit-N, suggesting that Slit is the primary substrate for Tok in the embryonic CNS. Transgenic restoration of Slit-N or Slit-C does not repel axons in Slit-null flies. Slit-FL and Slit-N are both biologically active cues with distinct axon guidance functions in vivo Slit signaling is used in diverse biological processes; therefore, differentiating between Slit-FL and Slit fragments will be essential for evaluating Slit function in broader contexts.

Drosophila Tet Is Expressed in Midline Glia and Is Required for Proper Axonal Development

Ten-Eleven Translocation (TET) proteins are important epigenetic regulators that play a key role in development and are frequently deregulated in cancer. Drosophila melanogaster has a single homologous Tet gene (dTet) that is highly expressed in the central nervous system during development. Here, we examined the expression pattern of dTet in the third instar larval CNS and discovered its presence in a specific set of glia cells: midline glia (MG). Moreover, dTet knockdown resulted in significant lethality, locomotor dysfunction, and alterations in axon patterning in the larval ventral nerve cord. Molecular analyses on dTet knockdown larvae showed a downregulation in genes involved in axon guidance and reduced expression of the axon guidance cue Slit. Our findings point toward a potential role for dTet in midline glial function, specifically the regulation of axon patterning during neurodevelopment.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Slit-Robo signalling in heart development

The Slit ligands and their Robo receptors are well-known for their roles during axon guidance in the central nervous system but are still relatively unknown in the cardiac field. However, data from different animal models suggest a broad involvement of the pathway in many aspects of heart development, from cardiac cell migration and alignment, lumen formation, chamber formation, to the formation of the ventricular septum, semilunar and atrioventricular valves, caval veins, and pericardium. Absence of one or more of the genes in the pathway results in defects ranging from bicuspid aortic valves to ventricular septal defects and abnormal venous connections to the heart. Congenital heart defects are the most common congenital malformations found in life new-born babies and progress in methods for large scale human genetic testing has significantly enhanced the identification of new causative genes involved in human congenital heart disease. Recently, loss of function variants in ROBO1 have also been linked to ventricular septal defects and tetralogy of Fallot in patients. Here, we will give an overview of the role of the Slit-Robo signalling pathway in Drosophila, zebrafish, and mouse heart development. The extent of these data warrant further attention on the SLIT-ROBO signalling pathway as a candidate for an array of human congenital heart defects.

Effects of neuroactive agents on axonal growth and pathfinding of retinal ganglion cells generated from human stem cells

We recently established a novel method for generating functional human retinal ganglion cells (RGCs) from human induced pluripotent cells (hiPSCs). Here, we confirmed that RGCs can also be generated from human embryonic stem cells (hESCs). We investigated the usefulness of human RGCs with long axons for assessing the effects of chemical agents, such as the neurotrophic factor, nerve growth factor (NGF), and the chemorepellent factors, semaphorin 3 A (SEMA3A) and SLIT1. The effects of direct and local administration of each agent on axonal projection were evaluated by immunohistochemistry, real-time polymerase chain reaction (PCR), and real-time imaging, in which the filopodia of the growth cone served as an excellent marker. A locally sustained agent system showed that the axons elongate towards NGF, but were repelled by SEMA3A and SLIT1. Focally transplanted beads that released SLIT1 bent the pathfinding of axons, imitating normal retinal development. Our innovative system for assessing the effects of chemical compounds using human RGCs may facilitate development of novel drugs for the examination, prophylaxis, and treatment of diseases. It may also be useful for observing the physiology of the optic nerve in vitro, which might lead to significant progress in the science of human RGCs.

Increased Expression of Slit2 and its Robo Receptors During Astroglial Scar Formation After Transient Focal Cerebral Ischemia in Rats

Slit2, a secreted glycoprotein, has recently been implicated in the post-ischemic astroglial reaction. The objective of this study was to investigate the temporal changes and cellular localization of Slit2 and its receptors, Robo1, Robo2, and Robo4, in a rat transient focal ischemia model induced by middle cerebral artery occlusion. We used double- and triple-immunolabeling to determine the cell-specific changes in Slit2 and its receptors during a 10-week post-ischemia period. The expression profiles of Slit2 and the Robo receptors shared overlapping expression patterns in sham-operated and ischemic striatum. Constitutive expression of Slit2 and Robo receptors was observed in striatal neurons with weak intensity, whereas in rats reperfused after ischemic insults, these immunoreactivities were increased in reactive astrocytes. Astroglial induction of Slit2 and Robo in the peri-infarct region was distinct on days 7-14 after reperfusion and thereafter increased progressively throughout the 10-week experimental period. Slit2 and Robo were prominently expressed in the perinuclear cytoplasm and main processes of reactive astrocytes forming the astroglial scar. This observation was confirmed by quantification of the mean fluorescence intensity of Slit2 and Robo receptors over reactive astrocytes localized at the edge of the infarct area. However, activated microglia/macrophages in the peri-infarct area were devoid of any specific labeling for Slit2 and Robo. Thus, our data revealed a selective and sustained induction of Slit2 and Robo in astrocytes localized throughout the astroglial scar after ischemic stroke, suggesting that Slit2/Robo signaling participates in glial scar formation and brain remodeling following ischemic injury.

Increased expression of Slit2 and its receptors Robo1 and Robo4 in reactive astrocytes of the rat hippocampus after transient forebrain ischemia

Slit2 is a secreted glycoprotein that was originally identified as a chemorepulsive factor in the developing brain; however, it was recently reported that Slit2 is associated with adult neuronal function including a variety of pathophysiological processes. To elucidate whether Slit2 is implicated in the pathophysiology of ischemic injury, we investigated the temporal changes and cellular localization of Slit2 and its predominant receptors, Robo1 and Robo4, for 28 days after transient forebrain ischemia. Slit2 and its receptors had similar overall expression patterns in the control and ischemic hippocampi. The ligand and receptors were constitutively expressed in hippocampal neurons in control animals; however, in animals with ischemic injury, their upregulation was detected in reactive astrocytes, but not in neurons or activated microglia, in the CA1 region. Astroglial induction of Slit2 and its receptors occurred by day 3 after reperfusion, and appeared to increase progressively until the final time point on day 28. Their temporal expression patterns overlapped with the time period in which reactive astrocytes undergo dynamic structural changes and appear hypertrophic in the ischemic hippocampus. The immunohistochemical data were consistent with the results of the immunoblot analyses, indicating that the expression of Slit2 and Robo increased progressively over the relatively long period of 28 days examined here. Collectively, these results suggest that Slit2/Robo signaling may be involved in regulating the astroglial reaction via autocrine or paracrine mechanisms in post-ischemic processes. Moreover, this may contribute to the dynamic morphological changes that occur in astrocytes in response to ischemic injury.

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.

A comparison of midline and tracheal gene regulation during Drosophila development

Within the Drosophila embryo, two related bHLH-PAS proteins, Single-minded and Trachealess, control development of the central nervous system midline and the trachea, respectively. These two proteins are bHLH-PAS transcription factors and independently form heterodimers with another bHLH-PAS protein, Tango. During early embryogenesis, expression of Single-minded is restricted to the midline and Trachealess to the trachea and salivary glands, whereas Tango is ubiquitously expressed. Both Single-minded/Tango and Trachealess/Tango heterodimers bind to the same DNA sequence, called the CNS midline element (CME) within cis-regulatory sequences of downstream target genes. While Single-minded/Tango and Trachealess/Tango activate some of the same genes in their respective tissues during embryogenesis, they also activate a number of different genes restricted to only certain tissues. The goal of this research is to understand how these two related heterodimers bind different enhancers to activate different genes, thereby regulating the development of functionally diverse tissues. Existing data indicates that Single-minded and Trachealess may bind to different co-factors restricted to various tissues, causing them to interact with the CME only within certain sequence contexts. This would lead to the activation of different target genes in different cell types. To understand how the context surrounding the CME is recognized by different bHLH-PAS heterodimers and their co-factors, we identified and analyzed novel enhancers that drive midline and/or tracheal expression and compared them to previously characterized enhancers. In addition, we tested expression of synthetic reporter genes containing the CME flanked by different sequences. Taken together, these experiments identify elements overrepresented within midline and tracheal enhancers and suggest that sequences immediately surrounding a CME help dictate whether a gene is expressed in the midline or trachea.

Isoflurane post-conditioning protects primary cultures of cortical neurons against oxygen and glucose deprivation injury via upregulation of Slit2/Robo1

Different mechanisms have been suggested to contribute to isoflurane-mediated neuroprotection. Previous studies have suggested that the protein Slit can abrogate neuronal death in mixed neuronal-glial cultures exposed to oxygen-glucose deprivation (OGD) and reperfusion (OGD/R). We hypothesized that isoflurane increases the expression of Slit and its receptor Robo when cortical neurons are exposed to OGD/R. To test this hypothesis, we exposed primary cortical neurons to OGD for 90 min and reperfusion for 24h and investigated how isoflurane post-conditioning affected cell survival and expression of Slit2 and receptors Robo1 and Robo4. Cell survival increased after administration of isoflurane, as assessed by the lactate dehydrogenase assay, trypan blue analysis, and propidium iodide staining. Western blot analysis showed that cleaved caspase-3 was increased after OGD/R(P<0.01) but reduced by isoflurane post-conditioning. Real-time PCR and Western blot analysis showed that the expression levels of Slit2 and Robo1, but not Robo4, were increased after OGD/R (P<0.5) and increased even further by isoflurane post-conditioning (P<0.01). Our results suggest that isoflurane post-conditioning markedly attenuates apoptosis and necrosis of cortical neurons exposed to OGD/R possibly in part via elevation of Slit2 and Robo1 expression. These findings provide a novel explanation for the pleiotropic effects of isoflurane that could benefit the central nervous system.

RabGDI controls axonal midline crossing by regulating Robo1 surface expression

BACKGROUND

Axons navigate to their future synaptic targets with the help of choice points, intermediate targets that express axon guidance cues. Once they reach a choice point, axons need to switch their response from attraction to repulsion in order to move on with the next stage of their journey. The mechanisms underlying the change in axonal responsiveness are poorly understood. Commissural axons become sensitive to the repulsive activity of Slits when they cross the ventral midline of the CNS. Responsiveness to Slits depends on surface expression of Robo receptors. In Drosophila, Commissureless (Comm) plays a crucial regulatory role in midline crossing by keeping Robo levels low on precommissural axons. Interestingly, to date no vertebrate homolog of comm has been identified. Robo3/Rig1 has been shown to control Slit sensitivity before the midline, but without affecting Robo1 surface expression.

RESULTS

We had identified RabGDI, a gene linked to human mental retardation and an essential component of the vesicle fusion machinery, in a screen for differentially expressed floor-plate genes. Downregulation of RabGDI by in ovo RNAi caused commissural axons to stall in the floor plate, phenocopying the effect observed after downregulation of Robo1. Conversely, premature expression of RabGDI prevented commissural axons from entering the floor plate. Furthermore, RabGDI triggered Robo1 surface expression in cultured commissural neurons. Taken together, our results identify RabGDI as a component of the switching mechanism that is required for commissural axons to change their response from attraction to repulsion at the intermediate target.

CONCLUSION

RabGDI takes over the functional role of fly Comm by regulating the surface expression of Robo1 on commissural axons in vertebrates. This in turn allows commissural axons to switch from attraction to repulsion at the midline of the spinal cord.

Role of leucine-rich repeat proteins in the development and function of neural circuits

The nervous system consists of an ensemble of billions of neurons interconnected in a highly specific pattern that allows proper propagation and integration of neural activities. The organization of these specific connections emerges from sequential developmental events including axon guidance, target selection, and synapse formation. These events critically rely on cell-cell recognition and communication mediated by cell-surface ligands and receptors. Recent studies have uncovered central roles for leucine-rich repeat (LRR) domain-containing proteins, not only in organizing neural connectivity from axon guidance to target selection to synapse formation, but also in various nervous system disorders. Their versatile LRR domains, in particular, serve as key sites for interactions with a wide diversity of binding partners. Here, we focus on a few exquisite examples of secreted or membrane-associated LRR proteins in Drosophila and mammals and review the mechanisms by which they regulate diverse aspects of nervous system development and function.

Functional screening of Alzheimer pathology genome-wide association signals in Drosophila

We have leveraged a Drosophila model relevant to Alzheimer disease (AD) for functional screening of findings from a genome-wide scan for loci associated with a quantitative measure of AD pathology in humans. In six of the 15 genomic regions evaluated, we successfully identified a causal gene for the association, on the basis of in vivo interactions with the neurotoxicity of Tau, which forms neurofibrillary tangles in AD. Among the top results, rs10845990 within SLC2A14, encoding a glucose transporter, showed evidence of replication for association with AD pathology, and gain and loss of function in glut1, the Drosophila ortholog, was associated with suppression and enhancement of Tau toxicity, respectively. Our strategy of coupling genome-wide association in humans with functional screening in a model organism is likely to be a powerful approach for gene discovery in AD and other complex genetic disorders.

Common factors regulating patterning of the nervous and vascular systems

The vascular and the nervous systems of vertebrates share many features with similar and often overlapping anatomy. The parallels between these two systems extend to the molecular level, where recent work has identified ever-increasing similarities between the molecular mechanisms employed in the specification, differentiation, and patterning of both systems. This review discusses some of the most recent literature on this subject, with particular emphasis on the roles that the Ephrin, Semaphorin, Netrin, and Slit signaling pathways play in vascular development.

Assisted morphogenesis: glial control of dendrite shapes

Neurons display a myriad of dendritic architectures, reflecting their diverse roles in information processing and transduction in the nervous system. Recent findings suggest that neuronal signals may not account for all aspects of dendrite morphogenesis. Observations from C. elegans and other organisms suggest that glial cells can affect dendrite length and guidance, as well as localization and shapes of dendritic receptive structures, such as dendritic spines and sensory cilia. Thus, besides direct roles in controlling neuronal activity, glia contribute to neuron function by ensuring that neurons attain their proper shapes.

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.

Interdependence of macrophage migration and ventral nerve cord development in Drosophila embryos

During embryonic development, Drosophila macrophages (haemocytes) undergo a series of stereotypical migrations to disperse throughout the embryo. One major migratory route is along the ventral nerve cord (VNC), where haemocytes are required for the correct development of this tissue. We show, for the first time, that a reciprocal relationship exists between haemocytes and the VNC and that defects in nerve cord development prevent haemocyte migration along this structure. Using live imaging, we demonstrate that the axonal guidance cue Slit and its receptor Robo are both required for haemocyte migration, but signalling is not autonomously required in haemocytes. We show that the failure of haemocyte migration along the VNC in slit mutants is not due to a lack of chemotactic signals within this structure, but rather to a failure in its detachment from the overlying epithelium, creating a physical barrier to haemocyte migration. This block of haemocyte migration in turn disrupts the formation of the dorsoventral channels within the VNC, further highlighting the importance of haemocyte migration for correct neural development. This study illustrates the important role played by the three-dimensional environment in directing cell migration in vivo and reveals an intriguing interplay between the developing nervous system and the blood cells within the fly, demonstrating that their development is both closely coupled and interdependent.

Slits and their receptors

Slit was identified in Drosophila embryo as a gene involved in the patterning of larval cuticle. It was later shown that Slit is synthesized in the fly central nervous system by midline glia cells. Slit homologues have since been found in C. elegans and many vertebrate species, from amphibians, fishes, birds to mammals. A single slit was isolated in invertebrates, whereas there are three slit genes (slit1-slit3) in mammals, that have around 60% homology. All encodes large ECM glycoproteins of about 200 kDa (Fig. 1A), comprising, from their N terminus to their C terminus, a long stretch of four leucine rich repeats (LRR) connected by disulphide bonds, seven to nine EGF repeats, a domain, named ALPS (Agrin, Perlecan, Laminin, Slit) or laminin G-like module (see ref 17), and a cystein knot (Fig. 1A). Alternative spliced transcripts have been reported for Drosophila Slit2, human Slit2 and Slit3, and Slit1. Moreover, two Slit1 isoforms exist in zebrafish as a consequence of gene duplication. Last, in mammals, two Slit2 isoforms can be purified from brain extracts, a long 200 kDa one and a shorter 150 kDa form (Slit2-N) that was shown to result from the proteolytic processing of full-length Slit2. Human Slit and Slit3 and Drosophila Slit are also cleaved by an unknown protease in a large N-terminal fragment and a shorter C-terminal fragment, suggesting conserved mechanisms for Slit cleavage across species. Moreover, Slit fragments have different cell association characteristics in cell culture suggesting that they may also have different extents of diffusion, different binding properties, and, hence, different functional activities in vivo. This conclusion is supported by in vitro data showing that full-length Slit2 functions as an antagonist of Slit2-N in the DRG branching assay, and that Slit2-N, not full-length Slit2, causes collapse of OB growth cones. In addition, Slit1-N and full-length Slit1 can induce branching of cortical neurons (see below), but only full-length Slit1 repels cortical axons. Structure-function analysis in vertebrates and Drosophila demonstrated that the LRRs of Slits are required and sufficient to mediate their repulsive activities in neurons. More recent detailed structure function analysis of the LRR domains of Drosophila Slit, revealed that the active site of Slit (at least regarding its pro-angiogenic activity) is located on the second of the fourth LRR (LRR2), which is highly conserved between Slits. Slit can also dimerize through the LRR4 domain and the cystein knot.However, a Slit1 spliced-variant that lacks the cysteine knot and does not dimerize is still able to repel OB axons.

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.

Robos are required for the correct targeting of retinal ganglion cell axons in the visual pathway of the brain

Axonal projections from the retina to the brain are regulated by molecules including the Slit family of ligands [Thompson, H., Barker, D., Camand, O., Erskine, L., 2006a. Slits contribute to the guidance of retinal ganglion cell axons in the mammalian optic tract. Dev. Biol. 296, 476-484, Thompson, H., Camand, O., Barker, D., Erskine, L., 2006b. Slit proteins regulate distinct aspects of retinal ganglion cell axon guidance within dorsal and ventral retina. J. Neurosci. 26, 8082-8091]. However, the roles of Slit receptors in mammals, (termed Robos), have not been investigated in visual system development. Here we examined Robo1 and 2 mutant mice and found that Robos regulate the correct targeting of retinal ganglion cell (RGC) axons along the entire visual projection. We noted aberrant projections of RGC axons into the cerebral cortex, an area not normally targeted by RGC axons. The optic chiasm was expanded along the rostro-caudal axis (similar to Slit mutant mice, Plump, A.S., Erskine, L., Sabatier, C., Brose, K., Epstein, C.J., Goodman, C.S., Mason, C.A., Tessier-Lavigne, M., 2002. Slit1 and Slit2 cooperate to prevent premature midline crossing of retinal axons in the mouse visual system. Neuron 33, 219-232), with ectopic crossing points, and some axons projecting caudally toward the corticospinal tract. Further, we found that axons exuberantly projected into the diencephalon. These defects were more pronounced in Robo2 than Robo1 knockout animals, implicating Robo2 as the predominant Robo receptor in visual system development.

Robo-Slit interactions regulate longitudinal axon pathfinding in the embryonic vertebrate brain

The early network of axons in the embryonic brain provides connectivity between functionally distinct regions of the nervous system. While many of the molecular interactions driving commissural pathway formation have been deciphered, the mechanisms underlying the development of longitudinal tracts remain unclear. We have identified here a role for the Roundabout (Robo) family of axon guidance receptors in the positioning of longitudinally projecting axons along the dorsoventral axis in the embryonic zebrafish forebrain. Using a loss-of-function approach, we established that Robo family members exhibit complementary functions in the tract of the postoptic commissure (TPOC), the major longitudinal tract in the forebrain. Robo2 acted initially to split the TPOC into discrete fascicles upon entering a broad domain of Slit1a expression in the ventrocaudal diencephalon. In contrast, Robo1 and Robo3 restricted the extent of defasciculation of the TPOC. In this way, the complementary roles of Robo family members balance levels of fasciculation and defasciculation along this trajectory. These results demonstrate a key role for Robo-Slit signaling in vertebrate longitudinal axon guidance and highlight the importance of context-specific guidance cues during navigation within complex pathways.

Leucine-rich repeat C4 protein is involved in nervous tissue development and neurite outgrowth, and induction of glioma cell differentiation

LRRC4, leucine-rich repeat C4 protein, has been identified in human (GenBank accession No. AF196976), mouse (GenBank accession No. DQ177325), rat (GenBank accession No. DQ119102) and bovine (GenBank accession No. DQ164537) with identical domains. In terms of their similarity, the genes encoding LRRC4 in these four mammalian species are orthogs and therefore correspond to the same gene entity. Based on previous research, and using in situ hybridization, we found that LRRC4 had the strongest expression in hippocampal CA1 and CA2, the granule cells of the dentate gyrus region, the mediodoral thalamic nucleus, and cerebella Purkinje cell layers. Using a P19 cell model, we also found that LRRC4 participates in the differentiation of neuron and glia cells. In addition, extracellular proteins containing both an LRR cassette and immunoglobulin domains have been shown to participate in axon guidance. Our data from neurite outgrowth assays indicated that LRRC4 promoted neurite extension of hippocampal neurons, and induced differentiation of glioblastoma U251 cells into astrocyte-like cells, confirmed by morphology observation and glial fibrillary acidic protein expression.

Slit-Robo interactions during cortical development

Interneurons are an integral part of cortical neuronal circuits. During the past decade, numerous studies have shown that these cells, unlike their pyramidal counterparts that are derived from the neuroepithelium along the lumen of the lateral ventricles, are generated in the ganglionic eminences in the subpallium. They use tangential migratory paths to reach the cortex, guided by intrinsic and extrinsic cues. Evidence is now emerging which suggests that the family of Slit proteins, acting through Robo receptors, play a role not only in axon guidance in the developing forebrain, but also as guiding signals in the migration of cortical interneurons. Here we describe the patterns of expression of Slit and Robo at different stages of forebrain development and review the evidence in support of their role in cortical interneuron migration. Slit-Robo signal transduction mechanisms are also important during normal development in a number of systems in the body and in disease states, making them potential therapeutic targets for the treatment of neurological disorders and certain types of cancer.

The Drosophila RNA-binding protein ELAV is required for commissural axon midline crossing via control of commissureless mRNA expression in neurons

Drosophila ELAV is the founding member of an evolutionarily conserved family of RNA-binding proteins considered as key inducers of neuronal differentiation. Although several ELAV-specific targets have been identified, little is known about the role of elav during neural development. Here, we report a detailed characterization of the elav mutant commissural phenotype. The reduced number of commissures in elav mutant embryos is not due to loss or misspecification of neural cells but results from defects in commissural axon projections across the midline. We establish a causal relationship between the elav mutant commissural phenotype and a reduction in the expression of commissureless, a key component of the Robo/Slit growth cone repulsive signalling pathway. In the nerve cord of elav mutant embryos, comm mRNA expression is strongly reduced in neurons, but not in midline glial cells. Furthermore, specific expression of an elav transgene in posterior neurons of each segment of an elav mutant nerve cord restores comm mRNA expression in these cells, as well as the formation of posterior commissures. Finally, forced expression of comm in specific commissural neuron subsets rescues the midline crossing defects of these neurons in elav mutant embryos, further indicating that elav acts cell autonomously on comm expression.

Glial and neuronal functions of the Drosophila homolog of the human SWI/SNF gene ATR-X (DATR-X) and the jing zinc-finger gene specify the lateral positioning of longitudinal glia and axons

Neuronal-glial communication is essential for constructing the orthogonal axon scaffold in the developing Drosophila central nervous system (CNS). Longitudinal glia (LG) guide extending commissural and longitudinal axons while pioneer and commissural neurons maintain glial survival and positioning. However, the transcriptional regulatory mechanisms controlling these processes are not known. Previous studies showed that the midline function of the jing C2H2-type zinc-finger transcription factor was only partially required for axon scaffold formation in the Drosophila CNS. We therefore screened for gain-of-function enhancers of jing gain of function in the eye and identified the Drosophila homolog of the disease gene of human alpha-thalassemia/mental retardation X-linked (ATR-X) as well as other genes with potential roles in gene expression, translation, synaptic transmission, and cell cycle. jing and DATR-X reporter genes are expressed in both CNS neurons and glia, including the LG. Coexpression of jing and DATR-X in embryonic neurons synergistically affects longitudinal connective formation. During embryogenesis, jing and DATR-X have autonomous and nonautonomous roles in the lateral positioning of LG, neurons, and longitudinal axons as shown by cell-specific knockdown of gene expression. jing and DATR-X are also required autonomously for glial survival. jing and DATR-X mutations show synergistic effects during longitudinal axon formation suggesting that they are functionally related. These observations support a model in which downstream gene expression controlled by a potential DATR-X-Jing complex facilitates cellular positioning and axon guidance, ultimately allowing for proper connectivity in the developing Drosophila CNS.

Slit coordinates cardiac morphogenesis in Drosophila

Slit is a secreted guidance cue that conveys repellent or attractive signals from target and guidepost cells. In Drosophila, responsive cells express one or more of three Robo receptors. The cardial cells of the developing heart express both Slit and Robo2. This is the first report of coincident expression of a Robo and its ligand. In slit mutants, cardial cell alignment, polarization and uniform migration are disrupted. The heart phenotype of robo2 mutants is similar, with fewer migration defects. In the guidance of neuronal growth cones in Drosophila, there is a phenotypic interaction between slit and robo heterozygotes, and also with genes required for Robo signaling. In contrast, in the heart, slit has little or no phenotypic interaction with Robo-related genes, including Robo2, Nck2, and Disabled. However, there is a strong phenotypic interaction with Integrin genes and their ligands, including Laminin and Collagen, and intracellular messengers, including Talin and ILK. This indicates that Slit participates in adhesion or adhesion signaling during heart development.

Regulation of commissural axon pathfinding by slit and its Robo receptors

Commissural axons grow along complex pathways toward, across, and beyond the midline of the central nervous system. Taking commissural axons in the vertebrate spinal cord and the Drosophila ventral nerve cord as examples, we examine how commissural axon pathfinding is regulated by the Slit family of guidance cues and their Robo family receptors. We extract several principles that seem likely to apply to other axons and other contexts, such as the reiterative use of the same guidance molecules in distinct pathfinding decisions, the transcriptional specification of a pathway, the posttranscriptional regulation of growth along the pathway, and the possible role of feedback mechanisms to ensure the fidelity of pathfinding choices. Such mechanisms may help explain how a relatively small number of guidance molecules can generate complex and stereotyped wiring patterns. We also highlight the many gaps in our understanding of commissural axon pathfinding and question some widely accepted views. We hope that this review encourages further efforts to tackle these questions, in the expectation that this system will continue to reveal the general principles of axon pathfinding.

Molecular mechanisms of optic axon guidance

Axon guidance is one of the critical processes during vertebrate central nervous system (CNS) development. The optic nerve, which contains the axons of retinal ganglion cells, has been used as a powerful model to elucidate some of the mechanisms underlying axon guidance because it is easily manipulated experimentally, and its function is well understood. Recent molecular biology studies have revealed that numerous guidance molecules control the development of the visual pathway. This review introduces the molecular mechanisms involved in each critical step during optic axon guidance. Axonal projections to the optic disc are thought to depend on adhesion molecules and inhibitory extracellular matrices such as chondroitin sulfate. The formation of the head of the optic nerve and the optic chiasm require ligand-receptor interactions between netrin-1 and the deleted in colorectal cancer receptor, and Slit proteins and Robo receptors, respectively. The gradient distributions of ephrin ligands and Eph receptors are essential for correct ipsilateral projections at the optic chiasm and the topographic mapping of axons in the superior colliculus/optic tectum. The precise gradient is regulated by transcription factors determining the retinal dorso-ventral and nasal-temporal polarities. Moreover, the axon guidance activities by Slit and semaphorin 5A require the existence of heparan sulfate, which binds to numerous guidance molecules. Recent discoveries about the molecular mechanisms underlying optic nerve guidance will facilitate progress in CNS developmental biology and axon-regeneration therapy.

Axon guidance at the midline: from mutants to mechanisms

How axons in the developing nervous system successfully navigate to their correct targets is a fundamental problem in neurobiology. Understanding the mechanisms that mediate axon guidance will give important insight into how the nervous system is correctly wired during development and may have implications for therapeutic approaches to developmental brain disorders and nerve regeneration. Achieving this understanding will require unraveling the molecular logic that ensures the proper expression and localization of axon guidance cues and receptors, and elucidating the signaling events that regulate the growth cone cytoskeleton in response to guidance receptor activation. Studies of axon guidance at the midline of many experimental systems, from the ventral midline of Drosophila to the vertebrate spinal cord, have led to important mechanistic insights into the complex problem of wiring the nervous system. Here we review recent advances in understanding the regulation of midline axon guidance, with a particular emphasis on the contributions made from molecular genetic studies of invertebrate model systems.

Roundabout 2 regulates migration of sensory neurons by signaling in trans

BACKGROUND

Roundabout (Robo) receptors and their ligand Slit are important regulators of axon guidance and cell migration. The development of Drosophila embryonic sense organs provides a neuronal migration paradigm where the in vivo roles of Slit and Robo can be assayed using genetics.

RESULTS

Here we show that Slit-Robo signaling controls migration of Drosophila larval sensory neurons that are part of the Chordotonal (Cho) stretch receptor organs. We used live imaging to show that abdominal Cho organs normally migrate ventrally during development, whereas thoracic Cho organs do not. Robo2 overexpression in cis (in the sensory neurons) or in trans (on neighboring visceral mesoderm) transforms abdominal organs to a thoracic morphology and position by blocking migration, while loss of Slit-Robo signaling produces a reverse transformation in which thoracic organs migrate ectopically. Rescue and tissue-specific knockout experiments indicate that trans signaling by Robo2 contributes to the normal positioning of the thoracic Cho organs. The differential positioning of Cho organs between the thorax and abdomen is known to be regulated by Hox genes, and we show that the essential Hox cofactor Homothorax, represses Robo2 expression in the abdominal visceral mesoderm.

CONCLUSIONS

Our results suggest that segment-specific neuronal migration patterns are directed through a novel signaling complex (the "Slit sandwich") in which Robo2 on the thoracic visceral mesoderm binds to Slit and presents it to Robo receptors on Cho neurons. The differential positioning of Cho organs between thorax and abdomen may be determined by Hox gene-mediated repression of robo2.

Modulation of inflammation by slit protein in vivo in experimental crescentic glomerulonephritis

A basic conservation of cell migration guidance mechanisms in the nervous and immune systems was proposed when Slit, known for its role in axon guidance, was found to inhibit chemokine-induced leukocyte chemotaxis in vitro. These studies examined the role of Slit2 in modulating inflammation in vivo. In a rat model of glomerulonephritis, endogenous glomerular Slit2 expression fell after disease induction, and its inhibition during the early disease period accelerated inflammation. Ex vivo glomerular leukocytes showed decreased chemokine and chemoattractant-induced chemotaxis in response to Slit2, suggesting an anti-inflammatory role for glomerular Slit2. In contrast to the effect of inhibition, glomerulonephritis was ameliorated by systemic Slit2 administration. Slit2 treatment improved disease histologically and also improved renal function when given early in the disease course. Leukocytes harvested from rats receiving Slit2 showed decreased monocyte chemoattractant protein-1 (MCP)-1-mediated migration, consistent with a peripheral Slit2 effect. In keeping with this functional alteration, Slit2-mediated inhibition of RAW264.7 cell chemotaxis was associated with decreased levels of active cdc42 and Rac1, implicating GTPases in leukocyte Slit2 signaling. These findings suggest a role for endogenous Slit2 in the inhibition of chemoattractant-mediated signals, demonstrate a potentially important anti-inflammatory effect for Slit2 in vivo, and provide further evidence for conserved mechanisms guiding the process of migration in distinct cell types.

Congenital diaphragmatic hernia, kidney agenesis and cardiac defects associated with Slit3-deficiency in mice

Slit3 along with Slit1 and Slit2 comprise the Slit family of proteins. The latter two proteins are known to be involved in axon guidance and cell migration during animal development. However, little is know about the functions of Slit3. We created a Slit3-deficient mouse model from an OmniBank ES cell line with a Slit3 allele trapped by insertional mutagenesis to analyze the in vivo functions of this protein. In this model, congenital diaphragmatic hernia is the most obvious phenotype. Herniation was found to be caused by a defective central tendon (CT) of the diaphragm that remained fused with the liver. Electron microscopic analyses of the defective CT revealed disorganized collagen fibrils that failed to form tight collagen bundles. The hearts of Slit3-deficient mice have an enlarged right ventricle. In addition, 20% of homozygous mice also showed a range of kidney defects that include unilateral or bilateral agenesis of the kidney and ureter, or varying degrees of renal hypoplasia. Thus, we concluded that Slit3 is involved in the development of multiple organ systems that include the diaphragm and the kidney. Slit3-deficient mice represent a genetic animal model for physiological and pathological studies of congenital diaphragmatic hernia.

The Drosophila Wnt5 protein mediates selective axon fasciculation in the embryonic central nervous system

The decision of whether and where to cross the midline, an evolutionarily conserved line of bilateral symmetry in the central nervous system, is the first task for many newly extending axons. We show that Wnt5, a member of the conserved Wnt secreted glycoprotein family, is required for the formation of the anterior of the two midline-crossing commissures present in each Drosophila hemisegment. Initial path finding of pioneering neurons across the midline in both commissures is normal in wnt5 mutant embryos; however, the subsequent separation of the early midline-crossing axons into two distinct commissures does not occur. The majority of the follower axons that normally cross the midline in the anterior commissure fail to do so, remaining tightly associated near their cell bodies, or projecting inappropriately across the midline in between the commissures. The lateral and intermediate longitudinal pathways also fail to form correctly, similarly reflecting earlier failures in pathway defasciculation. Panneural expression of Wnt5 in a wnt5 mutant background rescues both the commissural and longitudinal defects. We show that the Wnt5 protein is predominantly present on posterior commissural axons and at a low level on the anterior commissure and longitudinal projections. Finally, we demonstrate that transcriptional repression of wnt5 in AC neurons by the recently described Wnt5 receptor, Derailed, contributes to this largely posterior commissural localization of Wnt5 protein.

Two divergent slit1 genes in zebrafish

Members of the Slit family regulate axon guidance and cell migration. To date, three vertebrate slit1 genes have been identified in mammals and orthologs of two, slit2 and slit3, have been identified in zebrafish. Here, we describe the cloning of full-length cDNAs for two zebrafish slit orthologs, slit1a and slit1b. Both predicted proteins contain the conserved motifs that characterize other vertebrate Slits. slit1a and slit1b are both expressed in the midline, hypochord, telencephalon, and hindbrain. Apart from these shared expression domains, however, their expression patterns largely differ. Whereas slit1a is expressed broadly in the central nervous system (CNS) and in the somites, pectoral fin buds, tail bud, and caudal fin folds, slit1b is expressed in the olfactory system throughout embryonic and larval development, and in the retina during larval stages. Their expression patterns, particularly that of slit1a, suggest that Slit proteins may have roles in tissue morphogenesis in addition to their established roles in axon guidance and cell migration.

Slit-Robo signalling prevents sensory cells from crossing the midline in Drosophila

Maintenance of bilateral symmetry throughout animal development requires that both left and right halves of the body follow nearly identical patterns of cell proliferation, differentiation, death and migration. During formation of the perfectly bilateral Drosophila larval peripheral nervous system (PNS), the sensory precursor cells of the ventral multidendritic neuron vmd1a originating from each hemisegment migrate away from the ventral midline. Our observations indicate that in slit mutant embryos, as well as in robo, robo2 double mutants, sensory precursor cells of the left and right vmd1a neurons aberrantly cluster at the midline and then the pair of vmd1a neurons migrate to their final position on the same side of the embryo. This results in disruption of PNS bilateral symmetry. Expression of slit at the midline rescues the slit mutant vmd1a phenotype, suggesting that midline-secreted Slit activates Robo/Robo2 signalling to control the migration of the vmd1a sensory precursor cells. Our study indicates that midline-secreted Slit prevents vmd1a sensory cells from crossing the midline and thereby maintains PNS bilateral symmetry during development.

Dendritic guidance

Like axons, dendrites need guidance for proper orientation and positioning within the brain. Guidance determines synaptic connectivity as well as the strength of transmission. Recent in vivo studies have demonstrated that several cell-surface receptors, previously known as axon guidance molecules, are also responsible for the directed outgrowth of dendrites. Collectively, these studies reveal that the function of guidance molecules in individual neurons and individual processes is diverse and likely to be specifically regulated. Here, these studies are reviewed and emerging issues and implications are discussed.

Spatially and temporally restricted chemoattractive and chemorepulsive cues direct the formation of the nigro-striatal circuit

Identifying cellular and molecular mechanisms that direct the formation of circuits during development is thought to be the key to reconstructing circuitry lost in adulthood to neurodegenerative disorders or common traumatic injuries. Here we have tested whether brain regions situated in and around the developing nigro-striatal pathway have particular chemoattractive or chemorepulsive effects on mesencephalic dopamine axons, and whether these effects are temporally restricted. Mesencephalic explants from embryonic day (E)12 rats were either cultured alone or with coexplants from the embryonic, postnatal or adult medial forebrain bundle region (MFB), striatum, cortex, brain stem or thalamus. Statistical analysis of axon growth responses revealed a potent chemoattraction to the early embryonic MFB (i.e. E12-15) that diminished (temporally) in concert with the emergence of chemoattraction to the striatum in the late embryonic period (i.e. E19+). Repulsive responses by dopaminergic axons were obvious in cocultures with embryonic brain stem and cortex, however, there was no effect by the thalamus. Such results suggest that the nigro-striatal circuit is formed via spatially and temporally distributed chemoattractive and chemorepulsive elements that: (i) orientate the circuit in a rostral direction (via brain stem repulsion); (ii) initiate outgrowth (via MFB attraction); (iii) prevent growth beyond the target region (via cortical repulsion); and (iv) facilitate target innervation (via striatal chemoattraction). Subsequent studies will focus on identifying genes responsible for these events so that their products may be exploited to increase the integration of neuronal transplants to the mature brain, or provide a means to (re)establish the nigro-striatal circuit in vivo.

Roundabout gene family functions during sensory axon guidance in the drosophila embryo are mediated by both Slit-dependent and Slit-independent mechanisms

roundabout (robo) family genes play key roles in axon guidance in a wide variety of animals. We have investigated the roles of the robo family members, robo, robo2, and robo3, in the guidance of sensory axons in the Drosophila embryo. In robo(-/-), slit(-/-), and robo(-/+) slit(-/+) mutants, lateral cluster sensory neurons misproject to cells and axons in the nearby ventral' (v') cluster. These phenotypes, together with the normal expression pattern of Slit and Robo, suggest that Slit ligand secreted from the epidermis interacts with Robo receptors on lateral cluster sensory growth cones to limit their exploration of nearby attractive substrates. The most common sensory axon phenotype seen in robo2(-/-) mutants was misprojection of dorsal cluster sensory axons away from their normal growth substrate, the transverse connective of the trachea. slit appears to play no role in this aspect of sensory axon growth. Robo2 is expressed, not on the dorsal sensory axons, but on the transverse connective. These results suggest a novel, non-cell-autonomous mechanism for axon guidance by robo family genes: Robo2 expressed on the trachea acts as an attractant for the dorsal sensory growth cones.

Signaling at the growth cone: ligand-receptor complexes and the control of axon growth and guidance

The guidance of axons during the establishment of the nervous system is mediated by a variety of extracellular cues that govern cytoskeletal dynamics in axonal growth cones. A large number of these guidance cues and their cell-surface receptors have now been identified, and the intracellular signaling pathways by which these cues induce cytoskeletal rearrangements are becoming defined. This review summarizes our current understanding of the major families of axon guidance cues and their receptors, with a particular emphasis on receptor signaling mechanisms. We also discuss recent advances in understanding receptor cross talk and how the activities of guidance cues and their receptors are modulated during neural development.