Deterioration of energy metabolism is a central aging process, contributing to the overall decline in physiology and behavior. The circadian clock, a network of cellular oscillators driving output gene expression with ~24 hr rhythmicity, plays an important role in of energy homeostasis and can cause severe consequences when disrupted. For example, modern lifestyle-related circadian perturbation aggravates the current epidemic of the metabolic syndrome. During aging, the clock and clock-controlled circadian rhythms also display attenuated oscillatory amplitude, concomitant with metabolic decline. Environmental/dietary intervention and genetic studies further support a strong regulatory function of the circadian clock at the interface of aging and energy metabolism. The ROR (Retinoid acid receptor-related Orphan Receptor) subfamily of nuclear receptors are key components of the circadian oscillator that function to sustain robust oscillatory amplitude and drive circadian expression of genes. In a high-throughput chemical screen, we previously identified Nobiletin (NOB), a polymethoxylated flavonoid, as a clock-enhancing compound that activates RORs (specifically the alpha and gamma subtypes, encoded by Rora and Rorc) to elevate circadian amplitude and improve energy homeostasis in obese/diabetic mice. Importantly, in naturally aged mice, we showed that the ROR-NOB axis strengthens metabolic homeostasis and promotes energy expenditure in part via mitochondrial activation in skeletal muscle, ultimately bolstering healthy aging and survival. Based on exciting preliminary results from these studies, we hypothesize that RORs play a pivotal and modifiable role in circadian regulation of energy metabolism during aging.
In Aim 1, building on our strong preliminary studies, we will further define beneficial roles of ROR activation in insulin sensitivity and assess the insulin signaling pathway of naturally aged mice.
In Aim 2, we will determine age- related changes in circadian and metabolic functions of muscle-specific Rora/c KO mice, with a particular focus on respiratory complexes and the mitochondrial signature lipid cardiolipin.
In Aim 3, using a newly developed chromatin immunoprecipitation protocol for muscle, we will determine the transcriptional and epigenetic mechanisms by which RORs integrates circadian regulation of metabolic and mitochondrial genes during aging. Together, the proposed studies will provide key mechanistic insights on the role of circadian clockwork in metabolic aging, and pinpoint RORs as modifiable targets for development of novel therapeutic strategies to activate clocks, delay aging and prolong healthspan. The innovations include a conceptual paradigm to enhance clock function for healthy aging, novel circadian regulation of mitochondrial respiratory complex architecture and cardiolipin synthesis, and therapeutic utility of RORs and NOB against age-related decline. I have assembled a team of co-Investigators and collaborators with excellent track record in respective areas. Given the pressing lifestyle-related health challenges, the proposed study will have both basic and translational impact on healthy aging through vigorous circadian timing.
Relevance to public health: The project will highlight our circadian clock as a regulatory mechanism closely associated with hallmarks of metabolic aging. The study will establish the nuclear receptor RORs as modifiable circadian regulators during aging. Given the prevalent circadian disruption in modern lifestyle, results from the proposed experiments will illuminate an important circadian interventional strategy to delay age-related metabolic decline and promote healthy aging.