Scientists demonstrate a unique fluctuating clock mechanism, which controls the skin epidermis proliferation and behaviour in response to light and dark cues.
Circadian rhythms are well known to have essential roles in regulating body’s activities in response to day and night cycles. The master pacemaker – suprachiasmatic nucleus (SCN) – is contained in the hypothalamus; however, individual clocks have been shown to operate at cellular level too. In fact, recent research has demonstrated different tissues can have different clocks, hence different ages.
A novel study published in Cell Stem Cell reveals an even more unexpected function of the circadian clock of skin epidermis.
Epidermal cells of human skin have a vital role in protecting the body from outside influences, such as UV light, chemical stress and external injuries. Risk of some of these factors peaks at a certain time of the 24-hour cycle. Scientists demonstrate the circadian clock of epidermal cells enables them to “anticipate” the greatest exposure to UV light, thus inducing the mechanisms for DNA protection and repair at the appropriate hours. Such cell activity is limited to afternoons and evenings, since this is when cells proceed to DNA replication and are at their most vulnerable to UV damage.
Moreover, the same clock genes that maintain a 24-hour cycle were shown to influence differentiation of progenitor cells, which is responsible for renewal of skin layers. Cell proliferation and differentiation peak at night, when exposure to UV and free radicals is lowest. This mechanism ensures proliferation in absence of negative external influence, which could otherwise lead to premature aging.
Differential activities throughout the day are thought to rely on the oscillation of circadian proteins, creating distinct waves – afternoon and evening waves are responsible for DNA repair, protection against UV and cell division, whereas late night and morning waves drive proliferation of progenitor cells and renewal of the skin.
Thus, epidermal cells seem to have a life of their own, optimised for the protective function of the skin. By turning appropriate activities on and off in a timely manner protects the cells from stress leading to carcinogenesis and accelerated aging.