The p75 neurotrophin receptor acts as a "switch on-off" protein in keratinocyte differentiation

p75 neurotrophin receptor (p75NTR) mediates neurotrophin (NT) signals alone or in combination with the high affinity receptor Trk, also in cell settings outside the nervous system. In the present work, we studied the role of p75NTR during keratinocyte differentiation. p75NTR protein was up-regulated after serum treatment in both confluent and post-confluent keratinocytes, together with the expression of cytokeratin 10 (CK10) and involucrin. Moreover, calcium treatment of subconfluent keratinocytes induced the expression of involucrin, CK10, and the up-regulation of p75NTR, both at the mRNA and at the protein level. Western blotting and confocal analysis showed that p75NTR positive (+) keratinocytes isolated by magnetic cell sorting, expressed less CK10 and CK15, as compared to p75NTR negative (-) cells. p75NTR+ keratinocytes formed a lower number of colonies and proliferated to a lesser extent, as compared to p75NTR- cells. p75NTR is only expressed in the basal keratinocyte layer and is confined to a MIB-1 negative transit amplifying (TA) cell population. p75NTR+ keratinocytes, isolated from TA cells, expressed more survivin and CK15, while displayed less CK10 than p75NTR- TA. Colony forming efficiency was higher in p75NTR+ TA than in p75NTR- TA cells. Moreover, long term proliferation analysis revealed that p75NTR+ TA yielded a higher number of cells than p75NTR- TA. p75NTR retroviral infection of stem cells induced a more differentiated phenotype, with the same features of TA cells. On the other hand, when p75NTR was silenced, calcium treatment failed to induce differentiation in subconfluent keratinocytes, as shown by the absence of involucrin expression. These results suggest that p75NTR+ keratinocytes represent a subpopulation of TA cells, directly derived from stem cells that just started the differentiation process. p75NTR may act as a “switch on-off” protein in stem-TA transition initiating keratinocyte differentiation.