Pax-6 is expressed early in the differentiation of a corneal epithelial model system

Pax-6 is a regulatory gene with a major role during visual system development, but its association with corneal epithelial differentiation is not clearly established. Using the RCE1-(5T5) cell line, which mimics corneal epithelial differentiation, we analyzed Pax-6 biological role. Immunostaining of proliferating colonies and confluent sheets showed that Pax-6-positive cells were also K3 keratin-positive, suggesting that Pax-6 is expressed in differentiating cells. Pax-6 mRNA was barely expressed in early cell cultures; but after confluence, its levels raised up to fivefold as demonstrated by Northern blot and RT-qPCR. The raise in Pax-6 expression preceded for 9 h the increase in LDH-H and LDH-M mRNAs, previously shown as early markers of corneal epithelial cell differentiation. The full-length mRNAs encoding for the two major Pax-6 isoforms were found at very low levels in proliferating cells, and abundantly expressed in the confluent stratified epithelia; Pax-6 mRNA was 2- to 2.5-fold more abundant than Pax-6(5a) mRNA. The ectopic expression of Pax-6 or Pax-6(5a) decreased proliferative ability leading to the formation of abortive, non-proliferative colonies. In contrast, culture conditions that delay or block corneal epithelial cell differentiation reduced or inhibited the expression of Pax-6. Collectively, results show that Pax-6 is the earlier differentiation marker expressed by corneal epithelial cells, and open the possibility for a major role of Pax-6 as the main driver of the differentiation of corneal epithelial cells.