PROJECT SUMMARY / ABSTRACT Circadian clocks play fundamental roles in regulating essential cellular and physiological processes. The mammalian clock is comprised of cell-autonomous oscillators orchestrated by the hypothalamic suprachiasmatic nuclei (SCN) to perform tissue and systemic functions. More than a dozen core components of the oscillator have been identified; however, significant knowledge gaps remain regarding regulatory mechanisms/components and tissue-specific functions in the clock system. My previous research has provided important insights into mammalian circadian rhythms. For example, I generated Per2::Luc reporter mice which proved to be a powerful reagent ubiquitously employed in the clock field. We recently reported a second- generation reporter mouse line, Per2::LucSV, and demonstrated a novel miRNA regulation of PER2 accumulation and a positive role of PER2 in its own transcription. Most relevant to the current MIRA application, I have been interested in combining mouse forward genetic screening and mechanistic studies to probe fundamental clock functions. Previously we reported the identification by mouse screening and mechanistic dissection of two antagonistic E3 ligase, FBXL3 and FBXL21 in circadian period regulation. More recently, we uncovered a GSK-3beta-FBXL21 regulatory cascade controlling rhythmic degradation of the sarcomere protein TCAP and skeletal muscle function. Building on these prior studies, the current proposal aims to determine tissue-specific circadian mechanisms of FBXL21 and to identify novel clock components from a streamlined mouse screening. We will examine new targets and functions of FBXL21 in striated muscles including skeletal and cardiac muscles, focusing on proteostasis and myogenic differentiation. Leveraging expertise in mouse forward genetic screening, I recently performed a genetic screening for dominant phenotypes using an efficient breeding/phenotyping scheme. Whole-exome sequencing and variant analysis pinpointed a novel circadian mutant line with a lengthened circadian wheel-running period and age- dependent neurodegeneration. We will identify this new genetic component of the clock and characterize the underlying regulatory mechanisms. Overall, these studies promise to discover important mechanisms and functions of circadian rhythms in mammals. I have established an integrative research capability combining mouse genetics and phenotyping, biochemical/molecular/cellular studies, imaging methodologies, and omics platforms, complemented by expertise from a broad network of collaborators. These together form an excellent foundation for the proposed research. The ultimate goal is to understand how biological timing governs bodily function and what we can do to safeguard our health by optimizing our natural clock.