Bmal1 expression is minimal or absent in human and mouse cerebral microglia
Microglia orchestrate immunological responses in the brain and play an important role in maintaining homeostatic brain functions. Several studies have reported clock gene expression in microglia and the circadian rhythm they drive has been linked to the modulation of immune responses and neuronal functions. In the current study, complementary approaches, including immunofluorescence, multiplexed fluorescence in situ hybridization, and liquid chromatography-mass spectrometry proteomics of isolated CD11b+ microglia, were combined with publicly available transcriptomic and epigenomic datasets to investigate the expression of the core clock gene BMAL1 in human post-mortem cortical and limbic areas as well as mouse brain. The majority of BMAL1-expressing cells were found to be neurons, with microglia representing a negligeable proportion. We also identified significantly lower chromatin accessibility or "openness" for BMAL1 gene regulatory regions (such as promoters and enhancers) in microglia compared to neurons. These regulatory regions in microglia were enriched for ETS domain transcription factor (TF) binding sites. Together, this suggests a strong role of chromatin remodeling factors in suppressing BMAL1 gene expression in microglia. Finally, while we observed a very low expression, BMAL1 TF motifs were accessible in open chromatin landscape of microglia, which may lead to downstream gene-regulatory effects upon binding, even if BMAL1 expression is constitutively low. Overall, our results reveal low or absent expression of BMAL1 in microglia and point towards potential epigenetic mechanisms regulating its expression in these cells.