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

Muller cell reactivation (gliosis) is an early response in glaucomatous retina. Previous studies have demonstrated that activation of P2X(7) receptors results in retinal ganglion cell (RGC) apoptosis. Here, the issues of whether and how activated Muller cells may contribute to RGC apoptosis through P2X(7) 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 Muller cells induced an increase in glial fibrillary acidic protein (GFAP) expression, indicative of Muller cell gliosis. In addition, an increase in adenosine triphosphate (ATP) release from purified cultured Muller 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 Muller cell medium induced an increase in the number of TUNEL-positive cells in cultured RGCs, which was mimicked by benzoylbenzoyl adenosine triphosphate (BzATP), a P2X(7) receptor agonist, but was partially blocked by brilliant blue G (BBG), a P2X(7) 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 Muller cells may release excessive ATP, in turn leading to RGC apoptosis through activating P2X(7) receptors in these cells. (C) 2016 Elsevier Ltd. All rights reserved.