Isolation of an "early" transit amplifying keratinocyte population in human epidermis: a role for the low affinity neurotrophin receptor CD271.

In the interfollicular epidermis (IFE), stem cells (KSC) generate transit amplifying (TA) cells that, after symmetric divisions, produce differentiating daughters. Here, we isolated and characterized the highly proliferative interfollicular epidermal basal cell population "early" TA (ETA) cells, based on their capacity to adhere to type IV collagen. Proliferation and colony forming efficiency in ETA cells are lower than in KSC, but higher than in "late" TA (LTA). Stemness, proliferation and differentiation markers confirmed that ETA cell display a unique phenotype. Skin reconstructs derived from ETA cells present different features (epidermal thickness, Ki67 and Survivin expression), as compared to skin equivalents generated from either KSC or LTA cells. The low affinity neurotrophin receptor CD271, which regulates the KSC to TA cells transition in the human epidermis through an on/off switch control mechanism, is predominantly expressed in ETA cells. Skin equivalents generated from siRNA CD271 ETA cells display a more proliferative and less differentiated phenotype, as compared to mock-derived reconstructs. Consistently, CD271 overexpression in LTA cells generates a more proliferative skin equivalent than mock LTA cells. Finally, CD271 level declines with cellular senescence, while it induces a delay in p16INK4 expression. We conclude that ETA cells represent the first KSC progenitor with exclusive features. CD271 identifies and modulates ETA cells, thus participating in the early differentiation and regenerative capacity of human epidermis.