Genetic and Molecular Analysis of the Central and Peripheral Circadian Clockwork of Mice

Abstract

INTRODUCTIONThe previous decade has witnessed major, indeed astonishing, advances in our understanding of the molecular genetic and cellular bases to circadian timing in mammals (Reppert and Weaver 2002;Lowrey and Takahashi 2004). The SCN of the hypothalamus were first revealed as the "body clock" more than 30 years ago (Weaver 1998). They remain preeminent as the pacemaker responsible for coordinating circadian physiology across the organism and synchronizing it to solar time by retinally mediated entrainment. But in addition to the SCN, we now appreciate that most major organ systems and diverse cell types contain local circadian clockworks and that SCN-dependent synchronization of these local clocks and their dependent transcriptomes orchestrates daily physiology (Akhtar et al. 2002;Panda et al. 2002). The SCN are therefore no longer thought of as coercing a passive periphery into rhythmic behavior; rather, they harness the intrinsic rhythmicity of organs, both sustaining their amplitude and setting their phase. This precise and elaborate temporal coordination of physiology underlies the marked daily prevalence of morbidity in cardiovascular, metabolic, and other diseases (Hastings et al. 2003). Moreover, circadian disturbance, be it environmental or genetic, is linked to malignancy and metabolic and mental illness. Understanding how the SCN operate as a pacemaker tissue and how their output coordinates circadian physiology are outstanding questions with considerable relevance to human health. They also provide a cardinal example of how regulated gene expression can control complex behavior-the Holy Grail of molecular neurobiology. The purpose of this chapter is to review recent studies from our laboratories that have addressed these issues. Our broad strategy has been to use real-time imaging of circadian gene expression to explore molecular timekeeping in genetically modified mice. We offer specific examples illustrating how the period of the central and peripheral clocks is determined by the rate of proteasomal degradation of circadian proteins, how neuropeptidergic signaling synchronizes and sustains cellular circadian pacemaking in the SCN, and how behavioral control by the SCN is mediated via neuropeptides not required for pacemaking but necessary for downstream signaling. Finally, we consider how SCN outputs coordinate metabolically relevant pathways, focusing on liver function. It is likely that the relevance of clocks to medicine will expand far beyond sleep disorders to encompass metabolic disturbances of many kinds. Understanding the cellular and molecular genetic bases of peripheral circadian physiology is therefore a strategic aim of the field. SETTING CELLULAR CIRCADIAN PERIOD BY REGULATED PROTEASOMAL DEGRADATIONThe cellular oscillator of the SCN is viewed as a series of interlocked transcriptional/posttranslational feedback loops (Reppert and Weaver 2002;Lowrey and Takahashi 2004). Expression of the genes encoding the negative regulators Cryptochrome (Cry) and Period (Per) is trans-activated at...

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