Abstract
Gene regulation in the brain, well-described in early development by an extensive literature, continues throughout the lifespan. However, little is currently known about gene regulation during adult brain maturation and even less about cell type-specific mechanisms. The Nelson lab developed a set of genetic tools to label specific cell types in the mouse brain using an enhancer trap based on an mCitrine reporter to study cell type-specific gene regulation. Although mCitrine expression patterns in the enhancer-trap mouse lines are stable over generations, we observe loss of mCitrine in individual mice as they age in some cell types, but not others. The transcripts for mCitrine and tTA, a component of the enhancer trap construct amplifying reporter expression, both decrease with the loss of mCitrine fluorescence. Pivotally, this silencing does not occur outside of the central nervous system and is not explained by cell death. DNA methylation is involved in this silencing event since knocking out DNA methyltransferases 1 and 3a substantially delay silencing. The silencing could reflect either global regulation throughout the genome or a genomic surveillance mechanism acting specifically on the transgene. To help distinguish between these possibilities, we utilized Assay for Transposase-Accessible Chromatin Sequencing (ATAC-seq) to examine whether changes in accessibility of regulatory elements in cells before and after age-dependent silencing is global or localized. Preliminary results show higher accessibility near the insertion site in younger samples but global increase in accessibility in older samples. This suggests a local mechanism and argues for a previously undescribed genomic surveillance mechanism silencing transgenes in the adult brain. However, more careful analysis is needed to confirm this conclusion. Overall, our efforts suggest that silencing is an endogenous regulatory process and epigenetic regulation of neuronal gene expression continues to be refined into adulthood.