Functional characterization of cryptochromes CRY1 and CRY2 as metabolic sensors in the circadian regulation of mammalian physiology
Final Report Abstract
Circadian clocks coordinate physiology with light-dark cycles through alternating actions of gene activators and repressors. Dysregulation of circadian rhythms contributes to metabolic and cardiovascular diseases suggesting a crosstalk between circadian and metabolic pathways. However, the molecular mechanisms of this crosstalk remain poorly understood. A recently identified mechanism, by which metabolic signals can reset the timing of circadian clocks, is the destabilization of cryptochromes by AMP-activated protein kinase (AMPK)-mediated phosphorylation. Cryptochromes (CRY1 and CRY2) are transcriptional co-repressors that bind and inhibit the core clock transcription factors CLOCK and BMAL1. Interestingly, cryptochromes also interact with a subset of nuclear hormone receptors, among them PPARδ, a key regulator of metabolic gene expression in muscle, which has been linked to increased exercise endurance when constitutively activated. This suggests that cryptochromes may function as metabolic sensors, cross-regulating circadian and metabolic gene expression. The outlined study addresses this hypothesis by investigating the molecular regulation of PPARδ by cryptochromes and their role in skeletal muscle metabolism and exercise physiology. Protein-biochemical analyses revealed that the interaction of cryptochromes and PPARδ is liganddependent which was further supported by the preferential binding of cryptochromes to the ligandbinding domain of PPARδ. Moreover these analyses lead to the striking observation that CRY1 promotes RXRβ association to PPARδ in the absence of ligand. Analyses of PPARδ target gene expression in muscle cells and tissue further showed an impaired ligand-mediated induction of these genes when cryptochromes are absent, suggesting that an initial interaction of PPARδ and cryptochromes is crucial to prepare for efficient target gene transcription once the ligand is present. A role for cryptochromes in exercise physiology is clearly supported by the finding that CRY- deficient mice have increased running capacity and endurance as well as preliminary data showing consistent changes at the level of individual muscle fibers in these mice. The obtained data support the idea of a so far unknown and likely diurnal mechanism of PPARδ regulation. The findings of this study will help to further elucidate the therapeutic potential of PPARδ activation and may lead to new treatment strategies for metabolic diseases by incorporating circadian biology into pharmacological interventions as well as lifestyle changes such as increased physical activity.
