RESUMEN
Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.
Asunto(s)
Receptores de Proteínas Morfogenéticas Óseas/metabolismo , Proteínas Smad Reguladas por Receptores/metabolismo , Médula Espinal/embriología , Médula Espinal/metabolismo , Animales , Proteínas Aviares/antagonistas & inhibidores , Proteínas Aviares/genética , Proteínas Aviares/metabolismo , Axones/metabolismo , Secuencia de Bases , Embrión de Pollo , Regulación del Desarrollo de la Expresión Génica , Inmunohistoquímica , Hibridación in Situ , Interneuronas/citología , Interneuronas/metabolismo , Ratones , Ratones Mutantes , Ratones Transgénicos , Modelos Neurológicos , Neurogénesis , ARN Interferente Pequeño/genética , Ratas , Proteínas Smad Reguladas por Receptores/antagonistas & inhibidores , Proteínas Smad Reguladas por Receptores/genética , Proteína Smad1/antagonistas & inhibidores , Proteína Smad1/genética , Proteína Smad1/metabolismo , Proteína Smad5/antagonistas & inhibidores , Proteína Smad5/genética , Proteína Smad5/metabolismo , Médula Espinal/citologíaRESUMEN
The roof plate resident BMPs have sequential functions in the developing spinal cord, establishing cell fate and orienting axonal trajectories. These activities are, however, restricted to the dI1-dI3 neurons in the most dorsal region of the spinal cord. What limits the extent of the action of the BMPs to these neurons? To address this question, we have examined both the distribution of the inhibitory Smads (I-Smads), Smad6 and Smad7 in the spinal cord and the consequence of ectopically expressing the I-Smads in chicken embryos. Our studies suggest that the I-Smads function in vivo to restrict the action of BMP signaling in the dorsal spinal cord. Moreover, the I-Smads have distinct roles in regulating the diverse activities of the BMPs. Thus, the ectopic expression of Smad7 suppresses the dI1 and dI3 neural fates and concomitantly increases the number of dI4-dI6 spinal neurons. In contrast, Smad6 most potently functions to block dI1 axon outgrowth. Taken together, these experiments suggest that the I-Smads have distinct roles in spatially limiting the response of cells to BMP signaling.
