UCLA

Embryonic development relies on the coordinated action of multiple signaling pathways that guide pluripotent cells toward specific fates. Among these, Bone Morphogenetic Proteins (BMPs), are essential in the development of various tissues and organ systems. This versatility raises a fundamental question: how does a single signaling pathway generate such diverse responses? In this thesis, I investigate how downstream BMP signals are interpreted during dorsal spinal cord development, with a focus on the distinct contributions of Smad1 and Smad5. I summarize work that leverages in vitro differentiation systems to dissect BMP pathway mechanisms and demonstrate how these platforms can be further utilized to study dorsal interneuron (dI) specification.First, we establish a directed differentiation protocol that allows us to generate the full complement of dorsal interneurons from mouse embryonic stem cells. This highly synchronized in vitro model system serves as a platform for probing BMP dependent signaling events that are difficult to isolate during in vivo development of the dorsal spinal cord. Next, I used this in vitro model system to investigate whether Smad1 and Smad5 transmit BMP signals in distinct ways during neural differentiation. We found that Smad5 acts early in lineage specification, preventing bipotent neuromesodermal progenitors from adopting mesodermal fates and instead steering them toward neural derivatives. In addition, Smad5 is required for the specification of dP1–dP3 progenitors and their subsequent differentiation into dI1–dI3 interneurons. In contrast, Smad1 primarily functions to restrict the expansion of the dP1 lineage. Together, these findings demonstrate that R Smads are not functionally interchangeable and instead contribute distinct regulatory activities during dorsal spinal cord development. Finally, we extend these approaches to human systems by developing a neuromesodermal progenitor based differentiation protocol that produces human dIs spanning anterior–posterior identities. Our work illustrates how in vitro model systems can be used to interrogate developmental signaling pathways, highlights that the differential functions of R-Smads contributes to the promiscuity of the BMP signaling pathway and provides new insights into how dorsal spinal cord diversity is established.