RESUMO
Netrin1 has been proposed to act from the floor plate (FP) as a long-range diffusible chemoattractant for commissural axons in the embryonic spinal cord. However, netrin1 mRNA and protein are also present in neural progenitors within the ventricular zone (VZ), raising the question of which source of netrin1 promotes ventrally directed axon growth. Here, we use genetic approaches in mice to selectively remove netrin from different regions of the spinal cord. Our analyses show that the FP is not the source of netrin1 directing axons to the ventral midline, while local VZ-supplied netrin1 is required for this step. Furthermore, rather than being present in a gradient, netrin1 protein accumulates on the pial surface adjacent to the path of commissural axon extension. Thus, netrin1 does not act as a long-range secreted chemoattractant for commissural spinal axons but instead promotes ventrally directed axon outgrowth by haptotaxis, i.e., directed growth along an adhesive surface.
Assuntos
Orientação de Axônios/genética , Axônios/metabolismo , Fatores de Crescimento Neural/genética , Células-Tronco Neurais/metabolismo , Medula Espinal/embriologia , Proteínas Supressoras de Tumor/genética , Animais , Axônios/ultraestrutura , Fatores Quimiotáticos/genética , Fatores Quimiotáticos/metabolismo , Imageamento Tridimensional , Imuno-Histoquímica , Hibridização In Situ , Camundongos , Camundongos Knockout , Microscopia Confocal , Fatores de Crescimento Neural/metabolismo , Netrina-1 , Neurogênese/genética , RNA Mensageiro/metabolismo , Medula Espinal/ultraestrutura , Proteínas Supressoras de Tumor/metabolismoRESUMO
During cerebellar development, Sonic hedgehog (Shh) signaling drives the proliferation of granule cell precursors (GCPs). Aberrant activation of Shh signaling causes overproliferation of GCPs, leading to medulloblastoma. Although the Shh-binding protein Boc associates with the Shh receptor Ptch1 to mediate Shh signaling, whether Boc plays a role in medulloblastoma is unknown. Here, we show that BOC is upregulated in medulloblastomas and induces GCP proliferation. Conversely, Boc inactivation reduces proliferation and progression of early medulloblastomas to advanced tumors. Mechanistically, we find that Boc, through elevated Shh signaling, promotes high levels of DNA damage, an effect mediated by CyclinD1. High DNA damage in the presence of Boc increases the incidence of Ptch1 loss of heterozygosity, an important event in the progression from early to advanced medulloblastoma. Together, our results indicate that DNA damage promoted by Boc leads to the demise of its own coreceptor, Ptch1, and consequently medulloblastoma progression.