Regulation of Merkel cell development by Pax6

Merkel cells are mechanoreceptors widely distributed in the vertebrate skin. In rodents, Merkel cells within the whisker pads are innervated by free sensory nerve endings derived from the maxillary branch of the trigeminal nerve. This study identified expression of the transcription factor Pax6 in Merkel cells and investigated its role. Immunohistochemistry and Western blot for Pax6 and Merkel cell markers, cytokeratin-8 (K8) and cytokeratin-20 (K20) were performed in wild-type and Pax6 (-/-) fetuses. The subcellular localisation of Pax6 in Merkel cells in vitro was manipulated using hydrogen peroxide. Pax6 was primarily localised within the cytoplasm of the Merkel cells at birth, but postnatally was also detected within the nuclei. In vitro, after 4 days in culture Pax6 protein was completely relocated to the nuclei of fetal-derived Merkel cells, mimicking the in vivo situation, suggesting that Pax6 acts as an active nucleo-cytoplasmic shuttling protein in common with many other homeodomain transcription factors. The subcellular localisation of Pax6 could be modulated in vitro by changing the redox potential of the culture medium for Merkel cells. Differentiation of cultured Pax6 (-/-) Merkel cells was shown to be inhibited. At perinatal stages, it was found that Pax6 is required for maintaining cytokeratin-8 expression, an early Merkel cell marker, whereas cytokeratin-20 was retained by the Pax6 (-/-) mutant cells. Pax6 is expressed in developing Merkel cells as a nucleo-cytoplasmic shuttling protein and its activity is required for normal differentiation, possibly through regulating cell maturation.