Sergi Simó

MAP1B Is Required for Netrin 1 Signaling in Neuronal Migration and Axonal Guidance

BACKGROUND The signaling cascades governing neuronal migration and axonal guidance link extracellular signals to cytoskeletal components. MAP1B is a neuron-specific microtubule-associated protein implicated in the crosstalk between microtubules and actin filaments. RESULTS Here we show that Netrin 1 regulates, both in vivo and in vitro, mode I MAP1B phosphorylation, which controls MAP1B activity, in a signaling pathway that depends essentially on the kinases GSK3 and CDK5. We also show that map1B-deficient neurons from the lower rhombic lip and other brain regions have reduced chemoattractive responses to Netrin 1 in vitro. Furthermore, map1B mutant mice have severe abnormalities, similar to those described in netrin 1-deficient mice, in axonal tracts and in the pontine nuclei. CONCLUSIONS These data indicate that MAP1B phosphorylation is controlled by Netrin 1 and that the lack of MAP1B impairs Netrin 1-mediated chemoattraction in vitro and in vivo. Thus, MAP1B may be a downstream effector in the Netrin 1-signaling pathway.

Cullin 5 Regulates Cortical Layering by Modulating the Speed and Duration of Dab1-Dependent Neuronal Migration

The multilayered mammalian neocortex develops by the coordinated immigration and differentiation of cells that are produced at distant sites. Correct layering requires an extracellular protein, Reelin (Reln), an intracellular signaling molecule, Disabled-1 (Dab1), and an E3 ubiquitin ligase, Cullin-5 (Cul5). Reln activates Dab1, which is then degraded by Cul5. Here we test whether Cul5 regulates neuron layering by affecting Dab1 stability or other mechanisms. We find that a stabilized mutant Dab1, which resists Cul5-dependent degradation, causes a similar phenotype to Cul5 deficiency. Moreover, Cul5 has no effect when Dab1 is absent. The effects of Cul5 and Dab1 are cell autonomous, and Cul5 regulates movement of early as well as late cortical neurons. Removing Cul5 increases the speed at which neurons migrate through the cortical plate by reducing the time spent stationary and increasing the speed of individual steps. These results show that Cul5 regulates neuron layering by stimulating Dab1 degradation and that Cul5 controls migration speed and stopping point, and they demonstrate the importance of negative feedback in signaling during cortical development.

Reelin and mDab1 regulate the development of hippocampal connections

We analyze in this study the participation of Reelin and mDab1 in the development of hippocampal connections. We show that mDab1 is present in growth cones and axonal tracts of developing hippocampal afferents. mdab1-deficiency produces severe alterations in the entorhino-hippocampal and commissural connections identical to those described in reeler mice, including innervation of ectopic areas, formation of abnormal patches of fiber termination and a delay in the refinement of projections. Organotypic slice cultures combining tissue from mdab1-mutant and control mice demonstrate that the abnormalities observed in the mutant entorhino-hippocampal projection are caused by mdab1-deficiency in both the projecting neurons and target hippocampal cells. Axonal afferents that innervate the hippocampus react to Reelin by reducing axonal growth, and increasing growth cone collapse and axonal branching. Altogether these results indicate that Reelin and mDab1 participate in the development and refinement of hippocampal connections by regulating axonal extension, targeting and branching.

Rbx2 regulates neuronal migration through different cullin 5-RING ligase adaptors.

Morphogenesis requires the proper migration and positioning of different cell types in the embryo. Much more is known about how cells start and guide their migrations than about how they stop when they reach their destinations. Here we provide evidence that Rbx2, a subunit of the Cullin 5-RING E3 ubiquitin ligase (CRL5) complex, stops neocortical projection neurons at their target layers. Rbx2 mutation causes neocortical and cerebellar ectopias dependent on Dab1, a key signaling protein in the Reelin pathway. SOCS7, a CRL5 substrate adaptor protein, is also required for neocortical layering. SOCS7-CRL5 complexes stimulate the ubiquitylation and turnover of Dab1. SOCS7 is upregulated during projection neuron migration, and unscheduled SOCS7 expression stops migration prematurely. Cerebellar development requires Rbx2 but not SOCS7, pointing to the importance of other CRL5 adaptors. Our results suggest that CRL5 adaptor expression is spatiotemporally regulated to modulate Reelin signaling and ensure normal neuron positioning in the developing brain.

Netrin1 exerts a chemorepulsive effect on migrating cerebellar interneurons in a Dcc-independent way

Few studies have addressed the issue of how GABAergic interneurons in the cerebellar cortex migrate or what guidance cues steer them. Recent data show that their development starts at the cerebellar germinal epithelium on top of the fourth ventricle. These interneurons continue to proliferate in the postnatal cerebellar white matter and later migrate to their final position in the cerebellar cortex. Here we report the chemorepulsive action of Netrin1 on postnatal cerebellar interneurons in vitro and also show the expression pattern of Netrin1 and its receptors Dcc and Unc5. Our expression results further suggest that Netrin1 is involved in the migration of GABAergic interneurons in vivo. Moreover, our data point to Bergmann glial fibers as possible tracks for these cells en route to the molecular layer. Finally, experiments using blocking antibodies allow us to conclude that Dcc, although expressed by postnatal cerebellar interneurons, is not involved in the repulsive response triggered by Netrin1 in these cells.

Cullin 5 destabilizes Cas to inhibit Src-dependent cell transformation

ABSTRACT Phosphorylation-dependent protein ubiquitylation and degradation provides an irreversible mechanism to terminate protein kinase signaling. Here, we report that mammary epithelial cells require cullin-5–RING–E3-ubiquitin-ligase complexes (Cul5-CRLs) to prevent transformation by a Src–Cas signaling pathway. Removal of Cul5 stimulates growth-factor-independent growth and migration, membrane dynamics and colony dysmorphogenesis, which are all dependent on the endogenous tyrosine kinase Src. Src is activated in Cul5-deficient cells, but Src activation alone is not sufficient to cause transformation. We found that Cul5 and Src together stimulate degradation of the Src substrate p130Cas (Crk-associated substrate). Phosphorylation stimulates Cas binding to the Cul5-CRL adaptor protein SOCS6 and consequent proteasome-dependent degradation. Cas is necessary for the transformation of Cul5-deficient cells. Either knockdown of SOCS6 or use of a degradation-resistant Cas mutant stimulates membrane ruffling, but not other aspects of transformation. Our results show that endogenous Cul5 suppresses epithelial cell transformation by several pathways, including inhibition of Src–Cas-induced ruffling through SOCS6.

Semaphorin 6C leads to GSK-3-dependent growth cone collapse and redistributes after entorhino-hippocampal axotomy.

We studied the changes in the distribution of a specific variant of Semaphorin Y/6C (Sema6C) in mouse forebrain after axotomy of the entorhino-hippocampal perforant pathway. We found this isoform to be widely expressed during development, remaining in the adult and showing variations in distribution when the perforant pathway was axotomized. These changes were detected in both the hippocampal and entorhinal cortices. Sema6C1 immunoreactivity (IR) was high in the stratum radiatum of the hippocampus proper and the inner molecular layer of the dentate gyrus; the entorhinal cortex showed Sema6C1 IR in both cell bodies and in fibers of the II/III and V/VI layers. In axotomized animals, the IR of the ipsilateral, but not the contralateral, hemisphere showed that IR had moved into the stratum lacunosum-moleculare, the medial molecular layer of the dentate gyrus and the fibers, but not the cell bodies, of the entorhinal cortex. These results were not reproduced after lateral axotomy of the fimbria fornix, indicating a specific role for Sema6C variants in the generation and/or stability of entorhino-hippocampal synapses. Growth cone collapse of entorhinal and pyramidal neurons, as well as activation of glycogen synthase kinase-3 (GSK-3) through depletion of the inactive pool, induced by diffusible Sema6C1 further supports this view.

Regulation of dendritic branching by Cdc42 GAPs

Nerve cells form elaborate, highly branched dendritic trees that are optimized for the receipt of synaptic signals. Recent work published in this issue of Genes & Development by Rosario and colleagues (pp. 1743-1757) shows that a Cdc42-specific GTPase-activating protein (NOMA-GAP) regulates the branching of dendrites by neurons in the top layers of the mouse cortex. The results raise interesting questions regarding the specification of arbors in different cortical layers and the mechanisms of dendrite branching.

Interactions between Dab1, Crk/CrkL and Cullin 5 regulate proper lamination of the cortical plate

been demonstrated in vivo. In sensory neurons Sema3A-induced repellent activity is mediated by PlexinA4, but can be substituted to a large degree by PlexinA3. Here we show that, in contrast, Sema3A-induced death is completely dependent on the PlexinA3 receptor. This PlexinA3-dependent cell death is mediated via BAX activation. In addition to being protected from Sema3A-induced death, PlexinA3 / neurons are also partly protected from death by neurotrophin withdrawal. Consistent with a role for PlexinA3 in cell death, there is decreased apoptosis and an increase in the number of DRG neurons in PlexinA3 / embryos. We found no reduction in cell death in Sema3A / mice, but did find a significant reduction in cell death in Sema3A / mice that are also missing one allele of BAX, when compared to wild-type or BAX+/ only embryos, revealing an important role for the PlexinA3/ Sema3A-signaling pathway in determining neuronal cell numbers in vivo. We thus propose that the widely held view of neuronal cell death being at least partially due to limiting amounts of neurotrophins should be modified to include the idea that a balance between death-inducing signals, such as semaphorins, and survival signals, such as neurotrophins, determines whether a cell dies or survives.