In early postnatal brain development, the maturation of inhibitory neurons (IN) triggers a short period of heightened plasticity in cortical circuits. The transplantation of young INs into adult recipient brain regions has been shown to create a new period of circuit plasticity and enable robust, sustained recovery from cognitive and functional deficits. Efforts to determine the underlying biology have largely focused on the transplanted INs- however, recent findings in mouse visual cortex (V1) indicate that transplant-reactivated plasticity is, in fact, expressed by the endogenous INs. How does young IN transplantation induce their adult counterparts to re-express developmentally-restricted programs for experience-dependent plasticity?

To gain insight into the mechanisms of transplant-induced plasticity, I used bulk sequencing of host INs in adult transplant recipient V1. To isolate transcriptomic changes specific to transplant-reactivated plasticity, I collected various controls to account for baseline age-related expression, effects of cell transplantation, stage of reactivation, and effects of the experimental pipeline itself. As expected, gene ontology (GO) analysis of top differentially-expressed (DE) transcripts in host INs shows upregulation of ion transport, cell-to-cell signaling, and plasticity-related gene expression pathways. Interestingly, GO results also highlight processes important for neuronal development. While synaptic plasticity and cellular development are undoubtedly linked, the reemergence of early signaling pathways, differentiation markers, and chromatin remodeling elements together suggest that transplanted INs induce the cellular rejuvenation of adult host INs.

One DE gene found in these transcriptomic differential comparisons is Calbindin (Calb1), a calcium-binding protein predominantly expressed in INs. To gain further insight, I next utilized immunohistochemistry to validate the transplant-induced upregulation of Calb1 at the protein level. I find that Calb1 is highly expressed during V1 postnatal maturation and drastically reduced with age. Intriguingly, IN transplantation appears to restore low, adult Calb1 expression to the higher levels seen during development. To determine whether this change has importance in vivo, I conducted gain/loss-of-function studies of Calb1 expression in adult INs using viral-mediated, cell-type specific targeting strategies. Using intrinsic signal imaging of V1 cortical responses to assess experience-dependent plasticity, initial cohorts suggest a number of surprising results: 1) Calb1 silencing blocks normal establishment of experience-dependent plasticity during the ocular dominance critical period, 2) Calb1 silencing during IN transplantation blocks establishment of transplant-reactivated plasticity, and 3) Calb1 overexpression in adult INs is sufficient to reactivate plasticity without transplantation. Altogether this thesis offers novel insight into the cell and molecular mechanisms that underlie IN transplantation, and posits Calb1 as a key rejuvenating factor.