Tissue biology studies of histone modification, nascent transcription, and post-transcription regulation

Abstract Basic cellular processes essential to mammalian tissue development and adult regeneration, are often controlled by gene expression regulation at the mRNA level. Total mRNA level for each gene depends on two mechanisms: active (or ‘nascent’) transcription and post-transcriptional RNA stability/degradation (e.g. turnover rates). Nascent transcription itself is regulated by multiple steps of recruiting histone-modifying enzymes and opening chromatin, RNA polymerase II (Pol II) initiation and pausing, and the release of Pol II into productive RNA elongation. Diverse mechanisms that regulate total mRNA level may be important in controlling distinct biological processes and pathologies, but this is poorly understood in vivo. Using skin as a model system we will map lineage-specific gene patterns of RNA Pol II activity and nascent transcription in specific cell-types within their natural tissue milieu in the absence of cell isolation. Furthermore, we will investigate changes in these nascent-transcription patterns along an adult tissue stem cell activation and differentiation path, using hair follicle as a lineage prototype. In addition, our work will help dissect relative contributions of active transcription vs post-transcriptional RNA turnover/stability to overall mRNA levels in specific cell types in vivo. Furthermore, we will determine how a known histone repressive mark (H3K9me3), poorly understood in mammalian tissues, acts on nascent transcription and RNA Pol II pausing in vivo. We will uncover for the first time the physiological role of the 3 main H3K9me3 histone methyl transferases in skin development and homeostasis. Finally, we will investigate how H3K9me3 acts on nascent transcription in vivo using our newly developed mouse genetics tools and methodologies. Our work will provide previously unavailable mechanistic insight into gene regulation in mammalian tissue biology using skin as a model system.