Circadian rhythms are necessary to coordinate the timing of key behavioral and physiological processes in mammals [1-3]. However, while our understanding of the function of circadian clock genes in embryonic development is rapidly advancing [4-6], the molecular mechanisms through which these rhythms emerge during stem cell differentiation remain elusive [7]. Recently, signaling by reactive oxygen species (redox signaling) has emerged as an essential link between cellular metabolism and circadian rhythms in adult function [8, 9]. Signaling from the pentose phosphate pathway through production of redox cofactor NADPH is an important regulator of transcriptional oscillations, influencing the expression of core circadian clock genes through the redox-sensitive transcription factor NRF2 [10]. NRF2 is a crucial regulator of embryonic stem cell pluripotency and self-renewal, but whether redox signaling contributes to the development of circadian rhythms in differentiating stem cells remains completely unexplored [11]. Using fluorescent reporters of the hydrogen peroxide and Per2 expression, we propose to simultaneously visualize reactive oxygen species and circadian rhythms in single cells for the first time. By combining this novel model system with CRISPR/Cas9-mediated genome editing approaches, we will causally test the role of redox signaling in the development of circadian rhythms in human induced pluripotent stem cells undergoing directed differentiation to glutamatergic neurons. Using adult hippocampal neurogenesis as an in vivo model system for neuronal differentiation, we will further explore the function of redox-circadian coupling in coordinating the sequential development and circuit integration of adult-born granule cells. The long-term objective of this proposal is to create a novel model system to explore the mechanisms through which reciprocal interaction between redox and circadian transcription factor networks direct the proper sequential timing of development. While the current proposal seeks to investigate how redox-circadian coupling drives the differentiation of pluripotent and adult stem cells, we aim to describe a general paradigm for the coordination of metabolism, cell division, and stem cell homeostasis in health and disease. Hypothesis: We hypothesize that redox signaling drives the development of circadian rhythms in stem cells following the loss of pluripotency, and that reciprocal regulation between redox signaling and circadian rhythms drives the cellular maturation. We predict that disrupting redox-circadian coupling in adult neural stem cells through acute Nrf2 knockout will induce cell division and differentiation, but hinder the development of circadian rhythms and normal maturation of adult-born granule cells. We will test this hypothesis in the following aims:
Aim 1 : Causally link redox signaling to circadian rhythm development in human induced pluripotent stem cells Aim 2: Determine impact of Nrf2 KO-mediated disruption of redox-circadian coupling on adult neurogenesis
Circadian rhythm dysregulation has been has been linked to several neurodegenerative disorders including Alzheimer's disease [12, 13] and Parkinson's disease [14, 15]. Using novel genetic tools and stem cell-based models to causally assess the role of redox signaling in the development of circadian rhythms, this research may shed light on the mechanism(s) by which oxidative stress driven by circadian dysregulation contributes to neurodegenerative disease.
