Fine modulation of stem cell niche through mTOR pathway : Effects of physical and molecular parameters regulating mTOR pathway on keratinocyte stem cells

Epidermis is the outermost layer of the skin and is in direct contact with the external environment, requiring keratinocyte stem cells to adapt constantly. Recently, our laboratory has put forward the role of mTOR in temperature-mediated keratinocyte stem cell response (Brouard, Nanba et al., unpublished). In this work, I have demonstrated that mTOR (mammalian Target Of Rapamycin) protein levels in the epidermis are higher at 20°C than at 37°C, while active/phosphorylated mTOR (p-mTOR) is downregulated when temperature is decreased from 37°C to 20°C. To determine the relevance of different proteins in the mTOR signalling pathway, RNAi lentiviral constructs able to reduce cellular RNA levels were successfully designed. These include: mTORC1 and mTORC2 associated proteins (i.e. Raptor and Rictor respectively) which correspond to two major branches within the overall signalling pathway; EGF receptor (EGFR), which is one of the "entries" into the pathway; and hypoxia inducible factor (HIF-1α), since hypoxic conditions are a known input of mTOR. Then, I have shown that keratinocytes infected with a viral construct targeting EGFR initiate scattered colonies, composed of large flat cells, instead of circular and compact ones. Furthermore, these cells also have decreased EGFR levels, as well as decreased mTOR levels suggesting a direct correlation between these two members of the pathway. Finally, in order to quantify changes in growth rates and to track protein nuclear translocation upon temperature changes, a device is being developed in collaboration with the Laboratory of Biomedical Optics (LOB, Prof. T. Lasser). Taken together, my results demonstrate that mTOR plays a crucial role in modulating the stem cell niche in response to temperature variations. Furthermore, they provide some very effective tools to study and understand how these pathways regulate the keratinocyte stem cell niche.