Extracellular Vesicles: Potential Participants in Circadian Rhythm Synchronization.

The circadian rhythm (CR) is a set of autonomous endogenous oscillators. Exposure to the 24-hour day-night cycle synchronizes our CR system, maintaining homeostasis and human health. Several mechanisms for the CR system have been proposed, including those underlying the function (transcriptional-translational negative-feedback loops, or TTFLs), mechanisms regulating the TTFLs, and the mechanism by which the "server clock" is synchronized to environmental time. Several pathways downstream of the "server clock" perform well-characterized biological functions. However, the synchronization between the "server clock" (the endogenous master clock seated in the suprachiasmatic nucleus within the hypothalamus) and the "client clock" (imbedded in nearly every cell in the form of interlocking TTFLs) is difficult to explain with current theories. Extracellular vesicles (EVs), which are involved in intercellular communication and have recently been found to participate in regulation of the "client clock", might be the answer to this question. In this review, we summarize the current knowledge of CRs, TTFLs, and EVs, examine research findings about the functions of EVs in the CR system, and discuss the issues requiring attention in future research.