Transgenic Access to Semilunar Granule of the Dentate Gyrus - PROJECT SUMMARY To inform the origin and treatment of mental health and neurological disorders, there is a need to access specific cell types of the brain and determine how their interactions produce cognition, affect, and behavior. The long- term goal of this project is to understand the biology of an understudied cell type, semilunar granule cells (SLGC), in hippocampus-dependent processes. The dentate gyrus (DG) is a “gate” to the hippocampus. The principal cells of the DG, Granule Cells (GC), broadly excite CA3 pyramidal cells, which in turn excite CA1 pyramidal cells. Therefore, tight control of GC activity prevents hippocampal hyperexcitability that could impair memory and produce seizures. It is thus critical to understand how this gate functions, to determine how GC activity is constrained. In this proposal, the central goal is to develop tools to test the hypothesis that SLGCs function in vivo to limit GC activity. Based upon literature from ex vivo slice work, it has been determined that, in response to excitatory input, SLGCs fire persistently and activate interneurons to inhibit GCs. Thus, SLGCs are poised in the circuit to limit GC activity, but this model has gone untested. Whereas the in vivo roles of GCs are well defined thanks to genetic tools for their specific access, a barrier to progress has been a lack of tools that grant experimental access to SLGCs in vivo. This project pursues methods for recombinase expression in SLGCs of the mouse in vivo and the implementation of these tools to test the hypothesis that SLGCS constrain GC activity. To pursue this objective, 2 Aims are pursued. In Aim 1, strategies for recombinase expression in SLGCs of the mouse will be optimized. Aim 1 will be achieved through two independent strategies. The recombinase Cre is broadly utilized, e.g. Cre-on viral tools or conditional knockout of floxed alleles. In Aim 1, a tamoxifen-Inducible strategy Cre in SLGCs will be optimized. The approach is to characterize at least three promising mouse lines. However, SLGCs and GCs are spatially intermingled and have a high degree of transcriptional similarity, so a single gene approach may not succeed. Therefore, the second strategy of Aim 1 is an Intersectional method to achieve action of the alternative recombinase Flp in SLGCs. The approach is to utilize virus mediated expression of Flp, the expression and activity of which is gated by the SLGC selective expression of two different genes. By either strategy, this project will yield the first methods to selectively access SLGCs in vivo. Aim 2 is to determine if SLGC prevent GC hyperactivity in vivo. The strategy is to utilize recombinase mediated silencing of SLGCs and determine the impacts upon GC physiology and upon hippocampal circuit function using varied ex vivo and in vivo electrophysiology, imaging, and behavioral assays. This project will facilitate studies of the cell and circuit basis of learning and memory and will inform models of hippocampal development and pathologies, serving NIMH Goal 1 to Define the Brain Mechanisms Underlying Complex Behaviors.
