OSTEOPONTIN INHIBITS OSMOTIC SWELLING OF RETINAL GLIAL (MULLER) CELLS BY INDUCING RELEASE OF VEGF

Osmotic swelling of retinal neurons and glial cells is an important pathogenic factor of retinal edema formation. Here, we show that the neuroprotective factor osteopontin (OPN), which is released from retinal glial (Muller) cells after stimulation of the cells with glial cell line-derived neurotrophic factor (Del Rio et al., 2011, Glia 59:821-832), inhibits the swelling of rat Muller cells induced by hypoosmotic exposure of retinal slices in the presence of barium ions and H2O2, respectively, and in slices of postischemic retinas. OPN did not inhibit the hypoosmotic swelling of bipolar cells in slices of control and postischemic retinas. The inhibitory effect of OPN on Muller cell swelling was dose-dependent, with a half-maximal effect at similar to 0.6 ng/ml. The effect of OPN was abrogated in the presence of pharmacological blockers of vascular endothelial growth factor (VEGF) receptor-2, metabotropic glutamate receptors, and purinergic receptors (P2Y(1), adenosine A(1) receptors), as well as of a neutralizing anti-VEGF antibody. The data suggest that OPN induces the release of VEGF, glutamate, ATP, and adenosine from Muller cells. The effect of OPN was also prevented by blockers of voltage-gated sodium channels (tetrodotoxin), T-type voltage-gated calcium channels (kurtoxin), potassium channels (clofilium), and chloride channels 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). The swelling-inhibitory effect of OPN was dependent on intracellular calcium signaling, activation of phospholipase C and protein kinase C, and vesicular exocytosis of glutamate. In retinal slices, Muller glial cells display immunoreactivity of OPN. The data suggest that Muller cell-derived OPN has (in addition to the effects on photoreceptors and retinal neurons) autocrine effects. The neuroprotective effects of OPN may be in part mediated by the prevention of cytotoxic Muller cell swelling and the release of VEGF and adenosine from Muller cells. (C) 2013 IBRO. Published by Elsevier Ltd. All rights reserved.