Alteration of circadian machinery in monocytes underlies chronic kidney disease-associated cardiac inflammation and fibrosis

Dysfunction of the circadian clock has been implicated in the pathogenesis of cardiovascular disease. The CLOCK protein is a core molecular component of the circadian oscillator, so that mice with a mutated Clock gene (Clk/Clk) exhibit abnormal rhythms in numerous physiological processes. However, here we report that chronic kidney disease (CKD)-induced cardiac inflammation and fibrosis are attenuated in Clk/Clk mice even though they have high blood pressure and increased serum angiotensin II levels. A search for the underlying cause of the attenuation of heart disorder in Clk/Clk mice with 5/6 nephrectomy (5/6Nx) led to identification of the monocytic expression of G protein-coupled receptor 68 (GPR68) as a risk factor of CKD-induced inflammation and fibrosis of heart. 5/6Nx induces the expression of GPR68 in circulating monocytes via altered CLOCK activation by increasing serum levels of retinol and its binding protein (RBP4). The high-GPR68-expressing monocytes have increased potential for producing inflammatory cytokines, and their cardiac infiltration under CKD conditions exacerbates inflammation and fibrosis of heart. Serum retinol and RBP4 levels in CKD patients are also sufficient to induce the expression of GPR68 in human monocytes. Our present study reveals an uncovered role of monocytic clock genes in CKD-induced heart failure. Alteration of circadian rhythms is often observed in patients with chronic kidney disease (CKD). Here, the authors show that CKD-induced dysfunction of the circadian clock increases the expression of G protein-coupled receptor 68 in circulating monocytes and that their cardiac infiltration exacerbates inflammation and fibrosis of heart.

Automated summary

Dysfunction of the circadian clock is linked to cardiovascular disease pathogenesis. Even in the presence of high blood pressure and elevated levels of angiotensin II, Clk/Clk mice with chronic kidney disease show reduced cardiac inflammation and fibrosis. The monocytic expression of GPR68, induced by altered CLOCK activation, exacerbates heart inflammation and fibrosis under CKD conditions.