Involvement of P2X7 receptors in retinal ganglion cell apoptosis induced by activated Müller cells

Müller cell reactivation (gliosis) is an early response in glaucomatous retina. Previous studies have demonstrated that activation of P2X7 receptors results in retinal ganglion cell (RGC) apoptosis. Here, the issues of whether and how activated Müller cells may contribute to RGC apoptosis through P2X7 receptors were investigated. Either intravitreal injection of (S)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor (mGluR I) agonist, in normal rat retinas, or DHPG treatment of purified cultured rat retinal Müller cells induced an increase in glial fibrillary acidic protein (GFAP) expression, indicative of Müller cell gliosis. In addition, an increase in adenosine triphosphate (ATP) release from purified cultured Müller cells was detected during DHPG treatment (for 10 min to 48 h), which was mediated by the intracellular mGluR5/Gq/PI-PLC/PKC signaling pathway. Intravitreal injection of DHPG mimicked the reduction in the number of fluorogold retrogradely labeled RGCs in chronic ocular hypertension (COH) rats. Treatment with the conditioned culture medium (CM) obtained from the DHPG-activated Müller cell medium induced an increase in the number of TUNEL-positive cells in cultured RGCs, which was mimicked by benzoylbenzoyl adenosine triphosphate (BzATP), a P2X7 receptor agonist, but was partially blocked by brilliant blue G (BBG), a P2X7 receptor antagonist. Moreover, the CM treatment of cultured RGCs significantly increased Bax protein level and decreased Bcl-2 protein level, which was also mimicked by BzATP and partially blocked by BBG, respectively. These results suggest that reactivated Müller cells may release excessive ATP, in turn leading to RGC apoptosis through activating P2X7 receptors in these cells.