Corneal Epithelium Expresses a Variant of P2X7 Receptor in Health and Disease

Courtney Mankus, Celeste Rich, Martin Minns, Vickery Trinkaus-Randall

Improper wound repair of the corneal epithelium can alter refraction of light resulting in impaired vision. We have shown that ATP is released after injury, activates purinergic receptor signaling pathways and plays a major role in wound closure. In many cells or tissues, ATP activates P2X(7) receptors leading to cation fluxes and cytotoxicity. The corneal epithelium is an excellent model to study the expression of both the full-length P2X(7) form (defined as the canonical receptor) and its truncated forms. When Ca(2+) mobilization is induced by BzATP, a P2X(7) agonist, it is attenuated in the presence of extracellular Mg(2+) or Zn(2+), negligible in the absence of extracellular Ca(2+), and inhibited by the competitive P2X7 receptor inhibitor, A438079. BzATP enhanced phosphorylation of ERK. Together these responses indicate the presence of a canonical or full-length P2X(7) receptor. In addition BzATP enhanced epithelial cell migration, and transfection with siRNA to the P2X(7) receptor reduced cell migration. Furthermore, sustained activation did not induce dye uptake indicating the presence of truncated or variant forms that lack the ability to form large pores. Reverse transcription-polymerase chain reaction and Northern blot analysis revealed a P2X(7) splice variant. Western blots identified a full-length and truncated form, and the expression pattern changed as cultures progressed from monolayer to stratified. Cross-linking gels demonstrated the presence of homo- and heterotrimers. We examined epithelium from age matched diabetic and non-diabetic corneas patients and detected a 4-fold increase in P2X(7) mRNA from diabetic corneal epithelium compared to non-diabetic controls and an increased trend in expression of P2X(7)variant mRNA. Taken together, these data indicate that corneal epithelial cells express full-length and truncated forms of P2X(7), which ultimately allows P2X(7) to function as a multifaceted receptor that can mediate cell proliferation and migration or cell death.