A decline in skin regenerative capacity, with disturbed barrier function, impaired wound healing and increased carcinogenesis are leading causes of increasing morbidity and mortality in the elderly. Understanding the underlying molecular and cellular mechanisms is important for the development of strategies to intervene in aging- and disease-associated loss of skin function. We hypothesized a critical role of the mammalian target of rapamycin (mTOR) kinase in epidermal barrier formation and homeostasis. mTOR senses and integrates environmental cues from nutrients and growth factors, acting as important nexus for cellular signals to control growth and metabolism.
To dissect the role of mTOR pathway activation in epidermal development and homeostasis we specifically disrupted individual components of this pathway in mice by conditional gene targeting. TOR mediates its activities through the assembly of two structurally distinct multiprotein complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), and we generated multiple mouse lines that specifically inactivated these complexes in the epidermis. We found that mTOR signaling is essential for skin morphogenesis as epidermal-specific Mtor mutants (mTOREKO) are viable but die shortly after birth due to lack of a protective epidermal barrier. As revealed by qRT-PCR analysis, Western blot analysis and immunohistochemistry phosphorylation of downstream targets of both mTORC1 (S6K, 4E-BP1) and mTORC2 (Akt-pS473, PKCa) were significantly attenuated in epidermal tissues of mTOREKO mutants. To determine whether mTOR function in epidermal development is primarily mediated by mTORC1 or mTORC2, in addition we generated mice with epidermal loss specifically for mTORC1 or mTORC2. Interestingly, epidermis-specific loss of Rptor (RapEKO), which encodes an essential component of mTORC1, confers the same skin phenotype as seen in mTOREKO mutants. In contrast, newborns with an epidermal deficiency of Rictor (RicEKO), an essential component of mTORC2, survive despite a hypoplastic epidermis.
Collectively, we provide genetic evidence for a fundamental role of mTOR signaling in the formation and maintenance of a protective epidermal barrier. We discovered distinct functions for mTORC1 and mTORC2 in skin barrier formation, which cannot compensate for each other. Our findings unravel important and novel mechanistic insights in epidermal development, maintenance and disease.