Clock Genes Regulate Ca²⁺ Signaling and Mitochondrial Bioenergetics to Inhibit Sjögren Disease

Although Ca^{2 +} signaling and metabolism have been identified as key determinants for the development of Sjögren disease (SjD), the intricate connection between them and salivary gland physiology remains poorly understood.

Fluorescence-based Ca²⁺ imaging, RNA sequencing, and mitochondrial activity were used to investigate the effects of circadian rhythm and salivary gland dysfunction. Critical findings were confirmed by studying mouse models of SjD and human salivary gland samples.

We identified that Ca²⁺ entry is essential in modulating Clock genes and circadian rhythm, which also modulate salivary gland secretion and the expression of Stim and Orai genes. Mechanistically, our data show that Bmal2 binding in the promoter region of Stim1 modulates its expression, thereby regulating Ca²⁺ entry and mitochondrial bioenergetics and providing cyclic rhythm-mediated regulation of cellular physiology. To assess Ca²⁺-dependent circadian rhythms, we used diet perturbation, in which a Ca²⁺-deficient diet specifically impacts the Clock gene's rhythm, which was reversed by Ca²⁺ supplementation. Circadian rhythm-mediated regulation of fluid secretion, as well as STIM1 genes, was also altered in an SjD mouse model. In addition, human patients with SjD also showed dysregulation of CLOCK and STIM1 genes, which could alter salivary physiology, leading to the development of the SjD phenotype.

Our results provide the first comprehensive evidence of a reciprocal relationship between circadian rhythm, Ca²⁺ signaling, and metabolism, which is critical for cellular physiology and disease development/progression.