Circadian Regulation of Cellular Homeostasis

Biological clocks play a key role in how organisms adapt to daily (circadian), monthly (circalunar), and annual (circannual) changes in the environment by regulating rhythmic fluctuations in metabolism, hormone and neurotransmitter release, sensory capabilities and behaviors, including sleep. Disruption of circadian rhythms (e.g. shift work, time zone changes (“jet lag”), or social jet lag), can result in significant physiological consequences including sleep and metabolic disorders, as well as increased risk of stroke and cancer. Recent data indicate that the circadian clock is developmentally regulated and that its time-keeping activity is suspended in differentiating tissues to allow clock components to function in a “developmental clock”, that regulates stem/progenitor cell biology and differentiation, among other processes. However, it is not known how this functional shift from circadian to developmental activities is achieved, nor is it known how clock components control normal or malignant stem/progenitor cell behaviors. The mouse mammary gland is a powerful models system for the study of circadian rhythms, development, and stem/progenitor cell biology. Lineage tracing studies have demonstrated that the mammary gland harbors dynamic cell populations in which self-renewing unipotent basal and luminal stem cell lineages give rise to their respective lineages during ductal elongation, and are responsible for breast cancer cellular heterogeneity. We have shown that PER2, a transcription factor in the circadian clock, is differentially expressed during mammary gland development, and is required for branching morphogenesis, suggesting a circadian clock-independent developmental role. Our recent data using Per2-/- mouse mammary glands suggest that PER2 may play a critical role in mammary epithelial cell lineage commitment and homeostasis, and may do so by regulating the expression of EYA2 as well as influencing Wnt/b-catenin and TGF-b signaling. Moreover, we have found that circadian disruption in mice results in down regulation of Per2 expression in mammary epithelial cells and increased EYA2 expression and activation of Wnt/b-catenin. Based on these new results, we hypothesize that PER2 functions to integrate the circadian clock with the developmental clock to control homeostasis of mammary epithelial cell types during ductal elongation and branching morphogenesis. To address this hypothesis three Specific Aims are proposed. Aim 1 will test circadian and circadian disruption dependent changes in stem and progenitor cells in the developing mammary gland. Aim 2 will focus on the role of PER2 in lineage commitment using genetic reporters to trace mammary cell differentiation and examine the effect of PER2 on stem cell homeostasis. Studies in Aim 3 will determine PER2-dependent regulation of EYA2 gene expression and regulation of b-catenin in mammary gland morphogenesis. Upon successful completion of this aim we will have a better understanding of the molecular mechanisms that govern the interaction of PER2 and EYA2 as well as the effect on WNT/ TGFb signaling in branching morphogenesis.