Define Interneuron Subpopulations in the Mouse Spinal Cord during Development

Precise organization of neural circuits ensures proper functioning of the central nervous system (CNS). Assembly of these circuits requires correct specification of neuronal populations defined during development. One goal in my research is to understand developmental processes underlying formation of functional groups of spinal neurons. Different progenitor domains in the embryonic spinal cord give rise to the motoneurons (MNs) and different interneuron (IN) classes, each with a unique molecular profile. Distinct types of spinal INs emerge from these progenitor domains, and, as recently revealed, some undergo further subdivision to form heterogeneous populations. As yet, however, little is known about molecular mechanisms that govern such subdivision. My lab focuses on an IN group known as V3s as a model system to study subdivision of spinal INs during development. V3 INs arise from a ventral progenitor domain, p3, and express the Sim1 transcription factor at early postmitotic stages. V3 INs are glutamatergic and commissural. In mice, genetic deletion of the entire V3 population prevents animals from exhibiting robust and stable gait. We recently demonstrated that at least two V3 subpopulations, ventral and dorsal, reside in mature spinal cord, each with distinct physiological and anatomical properties. Nevertheless, molecular markers required to define these subpopulations and their function remain unavailable. Moreover, our most recent studies, supported by NSERC funding, suggest that differing physiological properties displayed by V3 subpopulations are present embryonically and that Sim1 may play different roles in regulating their establishment. Therefore, we propose to define molecular markers of V3 subpopulations and mechanisms underlying Sim1 function during development. To do so, we will use transgenic mice to trace V3 INs in wild-type control and Sim1-deficient mice during embryogenesis. We will combine electrophysiology, immunohistochemistry, in situ hybridization and microarray/RNA sequencing screening to conduct the following studies: Aim I. Define physiological V3 subpopulations at embryonic stages to establish a timeline showing when V3 subpopulations start to display distinctive physiological properties. Aim II. Identify specific molecular markers, especially specific transcription factors, of V3 subpopulations to define molecular profiles for different V3 subpopulations. Aim III. Investigate molecular pathways that regulate the V3 subpopulation formation by Sim1 to determine which downstream molecular factors are regulated by the Sim1 transcription factor during embryonic development. Successful completion of these studies will allow us to distinguish V3 subpopulations and determine their function in spinal circuits. Our work could also pave the way for studies designed to assess formation of other IN subpopulations.