Chromatin and Transcriptional Characteristics of Ovine Brown and White Adipose Tissues

Adipose tissue depots can have very different functions in mammals. In a newborn ruminant there are distinct depots of brown adipose tissue primarily involved in regulating nonshivering thermogenesis, and white adipose tissue functioning in energy homeostasis through regulation of triglyceride metabolism and storage. The major ruminant brown fat depots are replaced by white fat within a few weeks of birth, although some brown fat persists into adulthood. As a first stage of a larger study (EpiSCOPE) investigating the impacts of maternal periconceptual over-nutrition on lamb production traits, we are identifying transcriptional, chromatin and DNA methylation differences between white and brown adipose tissues to better understand their functional differences. Genome-wide chromatin immunoprecipitation coupled with next generation sequencing (ChIP-seq) was employed to profile the chromatin marks histone H3 lysine 27 trimethylation (H3K27me3), H3K27ac and H3K4me3. This information was then integrated with transcriptional data generated by RNA-seq. Initial results demonstrate H3K27me3 is a repressive chromatin mark with different patterns of modification. The peaks fell into two major categories, local and regional peaks, with the latter typically encompassing the gene body and often multiple genes.  Genome position and intensity of H3K27me3 modification defined distinct gene functions. Genes marked by H3K27me3 were generally common to both adipose tissue depots although some differences were identified. In contrast, there were substantial transcriptional differences between these tissues reflecting their functional diversity. These initial analyses reveal remarkable modifications of the epigenome that comment on aspects of chromosomal architecture and gene activity in the context of adipose tissue function.