Regulation of cortical dendrite development by Slit-Robo interactions

Expression and role of Roundabout-1 in embryonic Xenopus forebrain

The receptor Roundabout-1 (Robo1) and its ligand Slit are known to influence axon guidance and central nervous system (CNS) patterning in both vertebrate and nonvertebrate systems. Although Robo-Slit interactions mediate axon guidance in the Drosophila CNS, their role in establishing the early axon scaffold in the embryonic vertebrate brain remains unclear. We report here the identification and expression of a Xenopus Robo1 orthologue that is highly homologous to mammalian Robo1. By using overexpression studies and immunohistochemical and in situ hybridization techniques, we have investigated the role of Robo1 in the development of a subset of neurons and axon tracts in the Xenopus forebrain. Robo1 is expressed in forebrain nuclei and in neuroepithelial cells underlying the main axon tracts. Misexpression of Robo1 led to aberrant development of axon tracts as well as the ectopic differentiation of forebrain neurons. These results implicate Robo1 in both neuronal differentiation and axon guidance in embryonic vertebrate forebrain.

Slit1 and slit2 proteins control the development of the lateral olfactory tract

The development of olfactory bulb projections that form the lateral olfactory tract (LOT) is still poorly understood. The septum and the olfactory cortex have been shown to secrete diffusible factors repelling olfactory axons in vitro and are likely to cause the axons to avoid the septum region in vivo. Slit2, a member of the Slit gene family, has been proposed to be this septal factor based on its expression in the embryonic septum and its ability to repel and collapse olfactory axons. However, this issue is still controversial, and recent in vitro studies have questioned the role of the septum and Slit proteins in organizing LOT projections. We therefore decided to examine directly the role of Slit proteins in mediating olfactory axon guidance in vivo using mice with targeted deletions in the Slit1 and Slit2 genes. When olfactory bulb explants are cocultured with septum from Slit1- and/or Slit2-deficient mice, the septum repulsive activity for olfactory bulb axons is progressively abolished in a gene dose-dependent manner. Anterograde tracing of olfactory bulb axons showed that the LOT develops normally in Slit1 or Slit2 single-deficient mice but is completely disorganized in Slit1/Slit2 double-deficient embryos, with many axons reaching the midline and entering the septum region. Therefore, our study showed that the septum chemorepellent is a combination of Slit1 and Slit2 and that these molecules play a significant role in olfactory bulb axon guidance in vivo.

Slit2, a branching-arborization factor for sensory axons in the Mammalian CNS

Axons that carry information from the sensory periphery first elongate unbranched and form precisely ordered tracts within the CNS. Later, they begin collateralizing into their proper targets and form terminal arbors. Target-derived factors that govern sensory axon elongation and branching-arborization are not well understood. Here we report that Slit2 is a major player in branching-arborization of central trigeminal axons in the brainstem. Embryonic trigeminal axons initially develop unbranched as they form the trigeminal tract within the lateral brainstem; later, they emit collateral branches into the brainstem trigeminal nuclei and form terminal arbors therein. In whole-mount explant cultures of this pathway, embryonic day 15 (E15) rat central trigeminal axons retain their unbranched growth within the tract, whereas E17 trigeminal axons show branching and arborization in the brainstem trigeminal nuclei, much like that seen in vivo. Similar observations were made in E13 and E15 mouse embryos. We cocultured Slit2-expressing tissues or cells with the whole-mount explant cultures of the central trigeminal pathway derived from embryonic rats or mice. When central trigeminal axons are exposed to ectopic Slit2 during their elongation phase, they show robust and premature branching and arborization. Blocking available Slit2 reverses this effect on axon growth. Spatiotemporal expression of Slit2 and Robo receptor mRNAs within the brainstem trigeminal nuclei and the trigeminal ganglion during elongation and branching-arborization further corroborates our experimental results.

Control of cortical interneuron migration by neurotrophins and PI3-kinase signaling

During telencephalic development, cells from the medial ganglionic eminence (MGE) are thought to migrate to the neocortex to give rise to a majority of cortical GABAergic interneurons. By combining time-lapse video-microscopy, immunofluorescence and pharmacological perturbations in a new in vitro migration assay, we find that MGE-derived cells migrate through the entire extent of the cortex and into the CA fields of the hippocampus, but avoid the dentate gyrus. Migrating neurons initially travel within the marginal zone and intermediate zone, and can enter the cortical plate from either location. Tangential migration is strongly stimulated by BDNF and NT4 and attenuated by the Trk-family inhibitor, K252a, suggesting that migration is regulated by TrkB signaling. Furthermore, TrkB-null mice show a significant decrease in the number of calbindin-positive neurons migrating tangentially in the embryonic cortex. BDNF and NT4 cause rapid activation of PI3-kinase in MGE cells, and inhibition of PI3-kinase (but not of MAP kinase or PLCγ) dramatically attenuates tangential migration. These observations suggest that TrkB signaling, via PI3-kinase activation, plays an important role in controlling interneuron migration in the developing cerebral cortex.

Genetic manipulation of single neurons in vivo reveals specific roles of flamingo in neuronal morphogenesis

To study the roles of intracellular factors in neuronal morphogenesis, we used the mosaic analysis with a repressible cell marker (MARCM) technique to visualize identifiable single multiple dendritic (MD) neurons in living Drosophila larvae. We found that individual neurons in the peripheral nervous system (PNS) developed clear morphological polarity and diverse dendritic branching patterns in larval stages. Each MD neuron in the same dorsal cluster developed a unique dendritic field, suggesting that they have specific physiological functions. Single-neuron analysis revealed that Flamingo did not affect the general dendritic branching patterns in postmitotic neurons. Instead, Flamingo limited the extension of one or more dorsal dendrites without grossly affecting lateral branches. The dendritic overextension phenotype was partially conferred by the precocious initiation of dorsal dendrites in flamingo mutant embryos. In addition, Flamingo is required cell autonomously to promote axonal growth and to prevent premature axonal branching of PNS neurons. Our molecular analysis also indicated that the amino acid sequence near the first EGF motif is important for the proper localization and function of Flamingo. These results demonstrate that Flamingo plays a role in early neuronal differentiation and exerts specific effects on dendrites and axons.

Calcium regulation of dendritic growth via CaM kinase IV and CREB-mediated transcription

We report that CaM kinase IV and CREB play a critical role in mediating calcium-induced dendritic growth in cortical neurons. Calcium-dependent dendritic growth is suppressed by CaM kinase inhibitors, a constitutively active form of CaM kinase IV induces dendritic growth in the absence of extracellular stimulation, and a kinase-dead form of CaM kinase IV suppresses dendritic growth induced by calcium influx. CaM kinase IV activates the transcription factor CREB, and expression of a dominant negative form of CREB blocks calcium- and CaM kinase IV-induced dendritic growth. In cortical slice cultures, dendritic growth is attenuated by inhibitors of voltage-sensitive calcium channels and by dominant negative CREB. These experiments indicate that calcium-induced dendritic growth is regulated by activation of a transcriptional program that involves CaM kinase IV and CREB-mediated signaling to the nucleus.

GAPs in Slit-Robo signaling

Neuronal migration requires the integration of a number of diverse environmental cues and subsequent translation to specific responses such as directed cytoskeletal remodeling. Accurate knowledge of the signal transduction pathways linking activation of surface receptors to actin dynamics is necessary in order to understand the regulation of such processes. Activation of the Roundabout (Robo) receptor mediates a repulsive response in certain pioneering axons and migratory neurons. Recently, Wong et al have described a signaling link between Robo activation and specific GTPase activity that appears to regulate neuronal migration. A novel family of GTPase regulators, responsive to Slit-Robo engagement, has been identified and convincingly shown to alter the migration of neuronal cells. This study not only delineates a specific signaling route from guidance receptors to directed neuronal movement, but also offers clues towards potential regulatory mechanisms that ensure specificity of the Slit-Robo response.

Slit proteins prevent midline crossing and determine the dorsoventral position of major axonal pathways in the mammalian forebrain

We report that Slit proteins, a family of secreted chemorepellents, are crucial for the proper development of several major forebrain tracts. Mice deficient in Slit2 and, even more so, mice deficient in both Slit1 and Slit2 show significant axon guidance errors in a variety of pathways, including corticofugal, callosal, and thalamocortical tracts. Analysis of multiple pathways suggests several generalizations regarding the functions of Slit proteins in the brain, which appear to contribute to (1) the maintenance of dorsal position by prevention of axonal growth into ventral regions, (2) the prevention of axonal extension toward and across the midline, and (3) the channeling of axons toward particular regions.

Conserved cues for axon and dendrite growth in the developing cortex

The cellular and molecular mechanisms that guide axonal and dendritic differentiation in the cerebral cortex are just beginning to be defined. Many of the molecular signals that guide axons also, and sometimes simultaneously, influence dendritic growth. Whitford et al. (2002 [this issue of Neuron]) demonstrate that in addition to their roles in axon guidance and cell migration cue, Slit proteins can also regulate dendritic growth.