[Exploring for integrative signals guiding intracellular circadian clock synchronization: discovery of the BMAL1-ISR...

Cell-autonomous circadian clocks are essential for organisms to adapt to daily environmental changes and optimize physiological timing. However, the common initial signaling events shared by diverse synchronizing cues-such as light, temperature, and drugs-remain elusive. Here, we investigated the relationship between clock synchronization and subcellular localization dynamics of core clock proteins, BMAL1 and CLOCK, in NIH-3T3 fibroblasts. We identified an immediate, synchronized nuclear accumulation of BMAL1 immediately following various clock-resetting treatments, termed the "Immediate Synchronization Response" (ISR). This was accompanied by nuclear CLOCK accumulation. ISR phenomenon was also observed in other rodent clock cell models like C6 glioma cells. Mechanistically, phosphorylation of BMAL1 at Ser90 by Casein Kinase 2 (CK2) increased immediately upon stimulation. Consistent with the role of this phosphorylation in promoting nuclear BMAL1, pharmacological CK2 inhibition partially suppressed both the acute increase in Per2 expression and the clock phase shifts. Furthermore, mathematical simulation analysis supported the notion that increased phosphorylation levels of BMAL1 and its subsequent nuclear localization could serve as a driving force for phase shifts in the circadian oscillatory system. In conclusion, our findings suggest that BMAL1-ISR acts as a key integrative "switching signal" linking diverse synchronization cues to molecular clock oscillations.