Local circadian clock gates cell cycle progression of transient amplifying cells during regenerative hair cycling

Abstract

Regenerative cycling of hair follicles offers an unique opportunity to explore the role of circadian clock in physiological tissue regeneration. We focused on the role of circadian clock in actively proliferating transient amplifying cells, as opposed to quiescent stem cells. We identified two key sites of peripheral circadian clock activity specific to regenerating anagen hair follicles, namely epithelial matrix and mesenchymal dermal papilla. We showed that peripheral circadian clock in epithelial matrix cells generates prominent daily mitotic rhythm. As a consequence of this mitotic rhythmicity, hairs grow faster in the morning than in the evening. Because cells are the most susceptible to DNA damage during mitosis, this cycle leads to a remarkable time-of-day-dependent sensitivity of growing hair follicles to genotoxic stress. Same doses of γ-radiation caused dramatic hair loss in wild-type mice when administered in the morning, during mitotic peak, compared with the evening, when hair loss is minimal. This diurnal radioprotective effect becomes lost in circadian mutants, consistent with asynchronous mitoses in their hair follicles. Clock coordinates cell cycle progression with genotoxic stress responses by synchronizing Cdc2/Cyclin B-mediated G 2 /M checkpoint. Our results uncover diurnal mitotic gating as the essential protective mechanism in highly proliferative hair follicles and offer strategies for minimizing or maximizing cytotoxicity of radiation therapies.proliferation | radiation therapy | hair cycle M any biological events are rhythmic at different levels of organization, from cellular to behavioral. Diverse clock mechanisms have evolved to endow such rhythmic events with proper periodicity. The circadian clock helps organisms anticipate predictable daily changes in their environment and to prepare for diurnal and seasonal adaptations. Mechanistically, the circadian clock is based on the autoregulatory gene expression feedback loop in its core consisting of Clock/brain and muscle ARNT-like 1 (Bmal1)/neuronal PAS domain-containing protein 2 transcription factors that induce Period (Per)/Cryptochrome (Cry) genes expression, the protein products of which, in turn, inhibit these factors (1). Through its output mechanisms, the circadian clock generates daily fluctuations in various homeostatic processes (2). In mammals, a "master circadian clock" in the suprachiasmatic nucleus (SCN) uses both direct and indirect mechanisms to generate daily rhythms in several systemic signaling factors. In the peripheral tissues, local circadian clock also orchestrates intrinsic rhythms for their respective functions.We wanted to examine the role of circadian rhythms in hair cycle, which is a complex regenerative process consisting of sequential phases of hair production (anagen), followed by hair follicle inactivity (telogen). The timing of anagen initiation and anagen cessation (also known as catagen) largely determine the length of this cycle. Following anagen initiation, proliferation of epithelial cells and termin...

Brain and muscle Arnt-like 1 promotes skeletal muscle regeneration through satellite cell expansion

“…It has been increasingly recognized that the circadian clock regulates stem cell properties in various tissues, such as the hair follicle [39][40][41], intestine [42], the hematopoetic system [43] and neuronal organs [44]. Particularly, in epidermal and intestinal stem cells, the circadian regulation is a critical timing mechanism involved in physiological tissue remodeling [39][40][41][42].…”

Circadian clock-mediated control of stem cell division and differentiation: beyond night and day

“…In contrast to the severe defects observed in circadian clock-deficient adult mice during epidermal wound healing, the skin in newborns is normal in morphology (Kowalska et al, 2013). Furthermore, despite the alterations in hair follicle regeneration observed in Clock mutants, initial hair growth in pups is unchanged (Plikus et al, 2013). Only in some instances has the importance of non-circadian expression of Clock genes during mammalian development been postulated, e.g.…”

“…Instead, these animals lose a time-of-day-dependent genoprotective effect: whereas wildtype mice show time-of-day-dependent radiation toxicity, Clock mutants do not. Thus, a circadian clock has been suggested here to optimize DNA repair and replication cycles, primarily via CDC2/ CYCLIN B-mediated synchronization of the G2/M checkpoint (Plikus et al, 2013).…”

Circadian rhythms synchronize mitosis in Neurospora crassa

“…2C). We also used an established mitosis marker, phospho-histone H3 (pH3) antibody, as an independent measurement of mitosis (36,37). We observed more pH3-positive nuclei at DD25 (CT15) than at DD35 (CT2) (Fig.…”