Circadian regulation of endoplasmic reticulum calcium response in mouse cultured astrocytes

The circadian clock, an internal time-keeping system orchestrates 24-hour rhythms in physiology and behavior by governing rhythmic transcription within cells. Astrocyte, the most abundant glial cell type, play crucial roles in central nervous system functions. However, a detailed understanding of how the circadian clock impacts functions of astrocyte remains largely unexplored. In this study, utilizing circadian clock- synchronized mouse cultured cortical astrocytes and RNA sequencing, we identified 412 circadian rhythmic transcripts with a distinct astrocyte-specific expression pattern. A Gene Ontology analysis of these rhythmic transcripts highlighted genes implicated in Ca2+ homeostasis as being under circadian control. Notably, Herpud1 (Herp) exhibited robust circadian rhythmicity at both mRNA and protein levels, a rhythm disrupted in astrocytes lacking the circadian transcription factor, BMAL1. HERP regulated endoplasmic reticulum (ER) Ca2+ release by modulating the degradation of inositol 1,4,5-trisphosphate receptors (ITPRs). Intriguingly, ATP-stimulated ER Ca2+ release varied with the circadian cycle, being more pronounced at subjective night, likely owing to the rhythmic expression of ITPR2. Furthermore, this rhythmic ER Ca2+ response led to day/night variations in the phosphorylation of Cx43 (Ser368) and the gap junctional communication. Given the role of gap junction channel (GJC) in propagating Ca2+ signals, we suggest that this circadian regulation of ER Ca2+ responses could markedly affect astrocytic modulation of synaptic activity according to the time of day. Overall, our study enhances the understanding of how circadian clock influences astrocyte function in the CNS, shedding light on their potential role in daily variations of brain activity and health. ### Competing Interest Statement The authors have declared no competing interest.