Temperature impacts human epidermal stem cell behaviour through thermoTRPs and mTOR signalling

Adult stem cells are characterized by their unique ability to self-renew and give rise to a progeny of specialized/differentiated cells. Therefore, they have a remarkable potential for regenerative medicine. However, the use of adult stem cells is restrained by several obstacles such as limited in vitro amplification and uncontrolled differentiation. Adult stem cells reside in tightly regulated microenvironments termed “niches”. The niche is composed of nursing cells, extracellular matrix and a variety of signalling factors that dynamically regulate stem cell behaviour. Consequently, recapitulating the niche in vitro is fundamental in manipulating adult stem cells. In recent years, the physicochemical microenvironment of the niche (e.g. temperature, pH, oxygen tension) has proven to be as important as classical signalling pathways for the regulation of stem cell fate. Therefore, the aim of this thesis is to understand the influence of temperature variations on human epidermal stem cell behaviour. This effort has been accomplished in three main directions. First, we have contributed to the development of two different telemetric temperature sensors that precisely measure and record temperature in vivo and in vitro. These sensors have allowed us to demonstrate that epidermal keratinocytes are continuously subjected to small physiological temperature variations (3 ± 2oC) in vivo. Then, we have engineered a device that allows a tight and dynamic control of temperature while simultaneously providing real-time imaging of cultured human epidermal stem cells. It is shown that temperature variations of 1oC are not trivial, since they significantly affect human epidermal stem cell growth and proliferation. Second, we have shown that human keratinocytes sense temperature by means of temperature-sensitive calcium channels termed thermoTRPs (Transient Receptor Potential). Consequently, we have monitored intracellular calcium concentrations to evaluate the reaction of epidermal stem cells to thermal or chemical stimulations. The strongest calcium influxes have been observed after temperature drops (from 37 to 32oC) suggesting that human keratinocytes are particularly sensitive to temperature decreases. Furthermore, we have demonstrated the importance of these channels for keratinocyte viability by silencing a thermoTRP channel (i.e. TRPV3) using lentiviral delivery of shRNA. Third, we have demonstrated that thermal signals affecting keratinocytes are integrated through the mTOR (mammalian Target Of Rapamycin) signalling pathway, since cool stimuli (32oC) decreased mTOR complex 1 (mTORC1) kinase activity. Culture at 32oC and 100nm rapamycin treatment affected growth and proliferation of human keratinocyte stem cells similarly, further stressing the inhibitory effects of cool temperatures on mTORC1. We also have observed that inhibition of mTORC1 induced nuclear translocation of mTOR, suggesting a putative transcriptional role for mTOR. To conclude, we have established that prolonged inhibition of mTORC1 favours an apparent maintenance of the stem cell phenotype by reducing clonal conversion, without however stopping stem cell aging. Altogether, our results demonstrate that thermoTRPs are wired to the mTOR signalling in human epidermal stem cells. Furthermore, our work thoroughly supports the hypothesis that temperature is a significant actor of the epidermal stem cell niche as it can directly affect stem cell behaviour.