Developmental programming of cortial interneurons

AIM OF THE STUDY: The majority of interneurons originate from the MGE, including PV, SST and NPY expressing subgroups. Although the MGE has been defined as the region of origin for these subgroups, three important questions are still open. First, it was unclear if a spatial or temporal distribution exists for the origin of SST and PV expressing interneurons within the MGE. Second, it remained to be determined if specific interneuron types originate from the same progenitors within the MGE. Finally, it was unclear how fate specification within the MGE is controlled. The aim of the study was to address these questions and characterize in more detail the origins of MGE derived PV and SST expressing interneuron subgroups and to unravel molecular mechanisms controlling interneuron fate specification in the MGE progenitor domain. OUTLINE OF THE THESIS: Firstly, to determine if a spatial or temporal distribution was present for PV and SST expressing interneurons within the MGE, multiple transplantation experiments were performed. Initially, GFP labeled ventral MGE and dorsal MGE derived cells were transplanted in neonate cortex and analyzed for their ratio of PV and SST expressing interneurons (see chapter 2). However, additional experiments revealed that these transplantation studies were compromised due to migration of progenitors within the MGE, from ventral to dorsal MGE. Therefore, the transplantation experiment was redesigned using cells that were labeled during Sphase, when MGE cells are not migrating within the MGE, to correct for migration within the MGE (see chapter 4). Two time points in MGE development were analyzed: E13.5 and E15.5 to include the temporal aspect of fate specification to the study. Secondly, to determine if specific interneuron types originate from the same progenitor within the MGE, the PV group was studied in more detail. Within the PV group, two major cell types exist, being the chandelier cells and the basket cells. The chandelier cells are of particular interest as these are master regulators of cortical excitatory circuits. Chapter 3 focuses on the possibility that chandelier and basket cells share a common progenitor in the MGE. To label clonal lineages in the MGE, the Mosaic Analysis with Double Markers (MADM) technology was used. Using a specific driver gene, Nkx2.1, MADM technology was limited to the MGE and labeled cells were analyzed for the ratio of chandelier to basket cells. Finally, the study aimed to unravel molecular mechanisms controlling fate specification of interneurons in the MGE progenitor domain. In chapter 2 it was shown that a dorsal to ventral difference in gene expression exists within the MGE, suggesting a molecular basis underlying the spatial bias in fate specification of interneurons. The role of the Sonic Hedgehog (Shh) signaling pathway was studied in more detail, since Shh pathway members were among the genes that were differentially expressed. Moreover, Shh signaling is elevated specifically in the dorsal MGE, an area that preferably gives rise to SST expressing interneurons. Based on this, it was hypothesized that Shh signaling may play a role in fate specification of SST expressing interneurons. Also, recent work in our laboratory showed that exogenous Shh resulted in a decrease in PV and an increase in SST cells (Xu et al 2010a). In chapter 5, the regulation of Shh signaling in MGE was analyzed. It was tested if the dorsal midline of the telencephalon, more specifically the choroid plexus that rests on the dMGE, signals into the dMGE and causes Shh signaling to be increased in that area. To this end, the EMX2 mouse model that lacks a fully developed midline and choroid plexus was used. In addition, future directions for studying intra-MGE patterning will be discussed