p63 identifies keratinocyte stem cells

TLDR

The proliferative compartment of stratified squamous epithelia contains stem and transient amplifying keratinocytes, yet no polypeptide has been identified that is confined exclusively to stem cells. The study demonstrates that the p63 transcription factor, a p53 homologue essential for regenerative proliferation in epithelial development, distinguishes human keratinocyte stem cells from their TA progeny. Human keratinocyte stem and TA cells form holoclones and paraclones in culture, and clonal evolution from stem to TA cells is promoted by the sigma isoform of the 14‑3‑3 family of proteins. p63 is abundantly expressed in epidermal and limbal holoclones, absent in paraclones, and markedly reduced in meroclones; down‑regulation of 14‑3‑3σ keeps cells in the stem compartment with maintained p63, making p63 a practical marker for keratinocyte stem cells in clinical therapy and tumorigenesis research.

Abstract

The proliferative compartment of stratified squamous epithelia consists of stem and transient amplifying (TA) keratinocytes. Some polypeptides are more abundant in putative epidermal stem cells than in TA cells, but no polypeptide confined to the stem cells has yet been identified. Here we show that the p63 transcription factor, a p53 homologue essential for regenerative proliferation in epithelial development, distinguishes human keratinocyte stem cells from their TA progeny. Within the cornea, nuclear p63 is expressed by the basal cells of the limbal epithelium, but not by TA cells covering the corneal surface. Human keratinocyte stem and TA cells when isolated in culture give rise to holoclones and paraclones, respectively. We show by clonal analysis that p63 is abundantly expressed by epidermal and limbal holoclones, but is undetectable in paraclones. TA keratinocytes, immediately after their withdrawal from the stem cell compartment (meroclones), have greatly reduced p63, even though they possess very appreciable proliferative capacity. Clonal evolution (i.e., generation of TA cells from precursor stem cells) is promoted by the sigma isoform of the 14-3-3 family of proteins. Keratinocytes whose 14-3-3final sigma has been down-regulated remain in the stem cell compartment and maintain p63 during serial cultivation. The identification of p63 as a keratinocyte stem cell marker will be of practical importance for the clinical application of epithelial cultures in cell therapy as well as for studies on epithelial tumorigenesis.