Circadian Rhythms Regulate the NLRP3 Inflammasome in Macrophages

Macrophages are sentinels of the innate immune system distributed throughout the body that are central to the amplification and resolution of tissue inflammation. Macrophages possess intrinsic circadian rhythmicity in physiology and behavior, which are internally regulated by the molecular clock. Molecular clock proteins are emerging as key regulators of the macrophage inflammatory process, and circadian rhythmicity in the pathogenesis of inflammatory disease is gaining attention in medicine and pharmaceuticals. However, we currently lack a full understanding of the molecular mechanisms by which the circadian clock guides and controls macrophage inflammation. IL-1β is a potent pyrogenic cytokine released by macrophages during inflammation through an inflammatory form of programmed cell death, termed pyroptosis. Inflammasomes are intracellular multiprotein complexes that facilitate the activation and release of IL-1β. The NLRP3 inflammasome becomes active in response to acute disruptions in cellular homeostasis. In this study, I have discovered that the circadian clock times the activity of the NLRP3 inflammasome, and that the molecular clock protein BMAL1 negatively impacts NLRP3 inflammasome activation.

I demonstrate that circadian rhythms are prevalent throughout macrophage gene and protein expression, however these rhythms are mutually disjoint. Despite distinct means of circadian regulation, rhythmic genes and proteins are highly associated with regulating cellular metabolism. Circadian genes and proteins in macrophages reach peak expression between 2 predominant and opposing clock phases, designated as “early” and “late” circadian phases. I further identify that oxidative mitochondrial metabolism is rhythmic in synchronized macrophages. Uncoupling mitochondrial metabolism with FCCP inhibits the time of day activation of the NLRP3 inflammasome, leading to reduced IL-1β release and pyroptosis. Therefore, I propose that circadian regulation of macrophage metabolism and mitochondrial homeostasis is important for the regulation of the NLRP3 inflammasome.

In summary, I have identified a novel mechanism by which the molecular clock regulates the NLRP3 inflammasome through BMAL1’s control of mitochondrial function. Furthermore, I found that mitochondrial function is essential for both priming and activation of the NLRP3 inflammasome. This may have significant implications in inflammatory diseases driven by the NLRP3 inflammasome that exhibit circadian rhythms in disease severity.