Light can synchronise peripheral clocks autonomously from each other [darkness experiment (DD)]

Organisms have adapted to the changing environmental conditions within the 24h cycle of the day by temporally segregating tissue physiology to the optimal time of the day. On the cellular level temporal segregation of physiological processes is established by the circadian clock, a Bmal1 dependent transcriptional oscillator network. The circadian clocks within individual cells of a tissue are synchronised by environmental signals, mainly light, in order to reach temporally segregated physiology on the tissue level. However, how light mediated synchronisation of peripheral tissue clocks is achieved mechanistically and whether circadian clocks in different organs are autonomous or interact with each other to achieve rhythmicity is unknown. Here we report that light can synchronise core circadian clocks in two peripheral tissues, the epidermis and liver hepatocytes, even in the complete absence of functional clocks in any other tissue within the whole organism. On the other hand, tissue extrinsic circadian clock rhythmicity is necessary to retain rhythmicity of the epidermal clock in the absence of light, proving for the first time that the circadian clockwork acts as a memory of time for the synchronisation of peripheral clocks in the absence of external entrainment signals. Furthermore, we find that tissue intrinsic Bmal1 is an important regulator of the epidermal differentiation process whose deregulation leads to a premature aging like phenotype of the epidermis. Thus, our results establish a new model for the segregation of peripheral tissue physiology whereby the synchronisation of peripheral clocks is acquired by the interaction of a light dependent but circadian clock independent pathway with circadian clockwork dependent cues. Overall design: Determining the epidermal circadian transcriptome in the presence or absence of non-epidermal clocks after 6-7 days in complete darkness (DD).

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Welz PS, Zinna VM, Symeonidi A, Koronowski KB, Kinouchi K, Smith JG, Guillén IM, Castellanos A, Furrow S, Aragón F, Crainiciuc G, Prats N, Caballero JM, Hidalgo A, Sassone-Corsi P, Benitah SA