The diverse roles of smads in dorsal spinal cord development

2011-09-19Bone Morphogenetic Proteins (BMPs) have disparate functions establishing neural circuitry in the dorsal spinal cord. BMPs first induce specific neuronal cell fates, including the most dorsal commissural sensory neurons, dorsal interneuron (dI) 1 population. Subsequently, the BMPs act as a guidance cue to direct dI1 axons away from the dorsal midline. Here, we assess how dI1 neurons interpret the BMPs to accomplish these diverse cellular responses. In the traditional signaling cascade, BMPs specify cell fate by binding to a complex of BMP receptors (Bmprs) and thereby activating the Smad family of transcriptional regulators. Our previous studies have shown that the Bmprs also mediate the guidance activities of the BMPs. However, it remains unresolved which intracellular effectors are activated by the Bmprs to control axon dynamics. To further understand the role of the canonical BMP signaling pathway in establishing neural circuitry, we have examined whether the Smad complex, which acts downstream of the Bmprs, also regulates cell fate determination and axonal pathfinding. ❧ The evolutionarily conserved Smad complex is comprised of a combination of the BMP specific receptor-activated (R) Smads, Smad1, Smad5 and Smad8, and common mediator (Co) Smad4. Inhibitory (I) Smads, Smad6 and Smad7, block the signaling of this complex. To elucidate whether these Smads mediate the disparate tasks of BMP signaling in dorsal spinal cord development, we employed various gain and loss-of-function techniques to test the sufficiency and requirement of the Smads in dorsal cell fate specification and axon dynamics. ❧ We first looked at how functionally blocking BMP signaling via the overexpression of I-Smads, Smad6 and Smad7, would affect these processes. Unexpectedly, our experiments in chicken embryos revealed that Smad6 and Smad7 have dissimilar expression patterns in the developing spinal cord and in accordance with their expression distribution, regulate the distinct outcomes of BMP signaling. In particular, Smad7 primarily limits the role of BMPs to specify the dorsal-most neurons, whereas only Smad6 acts to inhibit the outgrowth of dI1 axons. ❧ Next, we investigated the role of the BMP specific R-Smads, Smad1, Smad5 and Smad8, in orchestrating cell fate determination and axon guidance events. Similar to the I-Smads, these R-Smads have strikingly divergent expression patterns in the developing spinal cord suggesting that these Smads also differentially affect BMP mediated cell fate decisions and axonal outgrowth. Supporting this hypothesis, when Smad5, but not Smad1, function is decreased in chicken and mouse embryos, there is a loss of dorsal neural progenitors and neurons. In contrast, only Smad1 is necessary to control dI1 axon outgrowth in our loss-of-function studies. ❧ Taken together, these results suggest that different components of the canonical Smad signaling pathway are regulating the disparate outcomes of BMP signaling. Specifically, Smad5 and Smad7 are primarily involved in dorsal cell specification, whereas Smad1 and Smad6 are mediating axiogenesis. Therefore, not only does this research resolve how a cell can use the BMPs to mediate both cell fate and axon dynamics, but also highlights the previously uncharacterized individual functions of R-Smads and I-Smads, proteins that are principally thought to function interchangeably within their subgroup classification. Thus, the mechanism of how one cell interprets the same extracellular signal for distinctive tasks lies within the division of labor in the second messenger pathway, which is the utilization of unique intracellular effectors