Repulsive axon guidance: Abelson and Enabled play opposing roles downstream of the roundabout receptor

How do dendrites take their shape?

Recent technical advances have made possible the visualization and genetic manipulation of individual dendritic trees. These studies have led to the identification and characterization of molecules that are important for different aspects of dendritic development. Although much remains to be learned, the existing knowledge has allowed us to take initial steps toward a comprehensive understanding of how complex dendritic trees are built. In this review, we describe recent advances in our understanding of the molecular mechanisms underlying dendritic morphogenesis, and discuss their cell-biological implications.

Regulating cellular actin assembly

Cellular actin assembly is tightly regulated. The study of pathogen motility has led to the identification of several cellular factors that are critical for controlling this process. Pathogens such as Listeria require Ena/VASP and Arp2/3 proteins to translate actin polymerization into movement. Recent work has extended these observations and uncovered some similarities and surprising differences in the way cells and pathogens utilize the actin cytoskeleton.

Calmodulin and profilin coregulate axon outgrowth in Drosophila

Coordinated regulation of actin cytoskeletal dynamics is critical to growth cone movement. The intracellular molecules calmodulin and profilin actively regulate actin-based motility and participate in the signaling pathways used to steer growth cones. Here we show that in the developing Drosophila embryo, calmodulin and profilin convey complimentary information that is necessary for appropriate growth cone advance. Reducing calmodulin activity by expression of a dominant inhibitor (KA) stalls axon extension of pioneer neurons within the CNS, while a partial loss of profilin function decreases extension of motor axons in the periphery. Yet, surprisingly, when calmodulin and profilin are simultaneously reduced, the ability of both CNS pioneer axons and motor axons to extend beyond the choice points is restored. In the CNS, at the time when growth cones must decide whether to cross or not to cross the midline, a reduction in calmodulin and/or roundabout signaling causes axons to cross the midline inappropriately. These inappropriate crossings are suppressed when profilin activity is simultaneously reduced. Interestingly, the mutual suppression of calmodulin and profilin activity requires a minimal level of profilin. In KA combinations with profilin null alleles, defects in axon extension and midline guidance are synergistically enhanced rather than suppressed. Together, our data indicate that the growth cone must coordinate the activity of both calmodulin and profilin in order to advance past selected choice points, including those dictating midline crossovers.

Actin-based motility: stop and go with Ena/VASP proteins

Proteins of the Ena/VASP (Enabled/vasodilator-stimulated phosphoprotein) family are involved in Abl and/or cyclic nucleotide-dependent protein kinase signaling pathways. These proteins are also crucial factors in regulating actin dynamics and associated processes such as cell-cell adhesion, platelet function and actin-based motility of both cytopathogenic Listeria and their eukaryotic host cells. Although biochemical mechanisms have emerged depicting Ena/VASP proteins as enhancers of actin filament formation, increasing evidence also suggests that these proteins have inhibitory functions in integrin regulation, cell motility and axon guidance.

RETRACTED: Hierarchical organization of guidance receptors: silencing of netrin attraction by slit through a Robo/DCC receptor complex

Axonal growth cones that cross the nervous system midline change their responsiveness to midline guidance cues: They become repelled by the repellent Slit and simultaneously lose responsiveness to the attractant netrin. These mutually reinforcing changes help to expel growth cones from the midline by making a once-attractive environment appear repulsive. Here, we provide evidence that these two changes are causally linked: In the growth cones of embryonic Xenopus spinal axons, activation of the Slit receptor Roundabout (Robo) silences the attractive effect of netrin-1, but not its growth-stimulatory effect, through direct binding of the cytoplasmic domain of Robo to that of the netrin receptor DCC. Biologically, this hierarchical silencing mechanism helps to prevent a tug-of-war between attractive and repulsive signals in the growth cone that might cause confusion. Molecularly, silencing is enabled by a modular and interlocking design of the cytoplasmic domains of these potentially antagonistic receptors that predetermines the outcome of their simultaneous activation.

The cell biology of neuronal navigation

Morphogenesis of the nervous system requires the directed migration of postmitotic neurons to designated locations in the nervous system and the guidance of axon growth cones to their synaptic targets. Evidence suggests that both forms of navigation depend on common guidance molecules, surface receptors and signal transduction pathways that link receptor activation to cytoskeletal reorganization. Future challenges remain not only in identifying all the components of the signalling pathways, but also in understanding how these pathways achieve signal amplification and adaptation-two essential cellular processes for neuronal navigation.

Identification and characterization of roundabout orthologs in zebrafish

The Roundabout (Robo) family of receptors and their extracellular ligands, the Slit protein family, play important roles in repulsive axon guidance. First identified in Drosophila, Robo receptors form an evolutionarily conserved sub-family of the immunoglobulin (Ig) superfamily that are characterized by the presence of five Ig repeats and three fibronectin-type III repeats in the extracellular domain, a transmembrane domain, and a cytoplasmic domain with several conserved motifs that play important roles in Robo-mediated signaling (Cell 92 (1998) 205; Cell 101 (2000) 703). Robo family members have now been identified in C. elegans, Xenopus, rat, mouse, and human (Cell 92 (1998) 205; Cell 92 (1998) 217; Cell 96 (1999) 807; Dev. Biol. 207 (1999) 62). Furthermore, multiple robo genes have been described in Drosophila, rat, mouse and humans, raising the possibility of potential redundancy and diversity in robo gene function. As a first step in elucidating the role of Robo receptors during vertebrate development, we identified and characterized two Robo family members from zebrafish. We named these zebrafish genes robo1 and robo3, reflecting their amino acid sequence similarity to other vertebrate robo genes. Both genes are dynamically expressed in the developing nervous system in distinct patterns. robo3 is expressed during the first day of development in the hindbrain and spinal cord and is later expressed in the tectum and retina. robo1 nervous system expression appears later in development and is more restricted. Moreover, both genes are expressed in non-neuronal tissues consistent with additional roles for these genes during development.

Rho GTPases in growth cone guidance

It is now well established that the small GTPases of the Rho family--Rac, Cdc42 and Rho--regulate growth cone morphology. Less clear is their role in guiding the growth cone. Do they act permissively, providing the dynamic actin structures needed for guidance? Or do they act instructively, transducing specific guidance signals? Recent studies have provided the first strong evidence for an instructive role: extracellular guidance cues can modulate Rho GTPase activities in vitro, and Rho GTPase activators function in growth cone guidance in vivo. The pathways linking Rho GTPases and the actin cytoskeleton are also rapidly coming into view, revealing further points of regulation by extracellular guidance cues. The growth cone is therefore guided by signals transduced both via and independently of Rho GTPases.

Overexpression of a slit homologue impairs convergent extension of the mesoderm and causes cyclopia in embryonic zebrafish

Slit is expressed in the midline of the central nervous system both in vertebrates and invertebrates. In Drosophila, it is the midline repellent acting as a ligand for the Roundabout (Robo) protein, the repulsive receptor which is expressed on the growth cones of the commissural neurons. We have isolated cDNA fragments of the zebrafish slit2 and slit3 homologues and found that both genes start to be expressed by the midgastrula stage well before the axonogenesis begins in the nervous system, both in the axial mesoderm, and slit2 in the anterior margin of the neural plate and slit3 in the polster at the anterior end of the prechordal mesoderm. Later, expression of slit2 mRNA is detected mainly in midline structures such as the floor plate cells and the hypochord, and in the anterior margins of the neural plates in the zebrafish embryo, while slit3 expression is observed in the anterior margin of the prechordal plate, the floorplate cells in the hindbrain, and the motor neurons both in the hindbrain and the spinal cord. To study the role of Slit in early embryos, we overexpressed Slit2 in the whole embryos either by injection of its mRNA into one-cell stage embryos or by heat-shock treatment of the transgenic embryos which carries the slit2 gene under control of the heat-shock promoter. Overexpression of Slit2 in such ways impaired the convergent extension movement of the mesoderm and the rostral migration of the cells in the dorsal diencephalon and resulted in cyclopia. Our results shed light on a novel aspect of Slit function as a regulatory factor of mesodermal cell movement during gastrulation.

Actin dynamics and cell-cell adhesion in epithelia

Recent advances in the field of intercellular adhesion highlight the importance of adherens junction association with the underlying actin cytoskeleton. In skin epithelial cells a dynamic feature of adherens junction formation involves filopodia, which physically project into the membrane of adjacent cells, catalyzing the clustering of adherens junction protein complexes at their tips. In turn, actin polymerization is stimulated at the cytoplasmic interface of these complexes. Although the mechanism remains unclear, the VASP/Mena family of proteins seems to be involved in organizing actin polymerization at these sites. In vivo, adherens junction formation appears to rely upon filopodia in processes where epithelial sheets must be physically moved closer to form stable intercellular connections, for example, in ventral closure in embryonic development or wound healing in the postnatal animal.

Short-range and long-range guidance by Slit and its Robo receptors: a combinatorial code of Robo receptors controls lateral position

Slit is secreted by midline glia in Drosophila and functions as a short-range repellent to control midline crossing. Although most Slit stays near the midline, some diffuses laterally, functioning as a long-range chemorepellent. Here we show that a combinatorial code of Robo receptors controls lateral position in the CNS by responding to this presumptive Slit gradient. Medial axons express only Robo, intermediate axons express Robo3 and Robo, while lateral axons express Robo2, Robo3, and Robo. Removal of robo2 or robo3 causes lateral axons to extend medially; ectopic expression of Robo2 or Robo3 on medial axons drives them laterally. Precise topography of longitudinal pathways appears to be controlled by a combination of long-range guidance (the Robo code determining region) and short-range guidance (discrete local cues determining specific location within a region).

Selecting a longitudinal pathway: Robo receptors specify the lateral position of axons in the Drosophila CNS

On each side of the midline of the Drosophila CNS, axons are organized into a series of parallel pathways. Here we show that the midline repellent Slit, previously identified as a short-range signal that regulates midline crossing, also functions at long range to pattern these longitudinal pathways. In this long-range function, Slit signals through the receptors Robo2 and Robo3. Axons expressing neither, one, or both of these receptors project in one of three discrete lateral zones, each successively further from the midline. Loss of robo2 or robo3 function repositions axons closer to the midline, while gain of robo2 or robo3 function shifts axons further from the midline. Local cues further refine the lateral position. Together, these long- and short-range guidance cues allow growth cones to select with precision a specific longitudinal pathway.

Short-range and long-range guidance by slit and its Robo receptors. Robo and Robo2 play distinct roles in midline guidance

Previous studies showed that Roundabout (Robo) in Drosophila is a repulsive axon guidance receptor that binds to Slit, a repellent secreted by midline glia. In robo mutants, growth cones cross and recross the midline, while, in slit mutants, growth cones enter the midline but fail to leave it. This difference suggests that Slit must have more than one receptor controlling midline guidance. In the absence of Robo, some other Slit receptor ensures that growth cones do not stay at the midline, even though they cross and recross it. Here we show that the Drosophila genome encodes three Robo receptors and that Robo and Robo2 have distinct functions, which together control repulsive axon guidance at the midline. The robo,robo2 double mutant is largely identical to slit.

Negative regulation of fibroblast motility by Ena/VASP proteins

Ena/VASP proteins have been implicated in cell motility through regulation of the actin cytoskeleton and are found at focal adhesions and the leading edge. Using overexpression, loss-of-function, and inhibitory approaches, we find that Ena/VASP proteins negatively regulate fibroblast motility. A dose-dependent decrease in movement is observed when Ena/VASP proteins are overexpressed in fibroblasts. Neutralization or deletion of all Ena/VASP proteins results in increased cell movement. Selective depletion of Ena/VASP proteins from focal adhesions, but not the leading edge, has no effect on motility. Constitutive membrane targeting of Ena/VASP proteins inhibits motility. These results are in marked contrast to current models for Ena/VASP function derived mainly from their role in the actin-driven movement of Listeria monocytogenes.