Kainic acid-mediated upregulation of matrix metalloproteinase-9 promotes retinal degeneration.
Excitotoxicity has been proposed to play a pivotal role in retinal damage, but the mechanisms that underlie retinal damage are not clearly understood. In this study, the role of matrix metalloproteinases in excitotoxin-mediated retinal damage was investigated. KA, CNQX (6-cyano-7-nitroquinoxaline-2,3,-dione), NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline), MK801, or PBS was injected into the vitreous of CD-1 mice. MMP expression in the retina was analyzed by zymography, Western blot, and immunohistochemistry. Retinal ganglion cells (RGCs) were retrogradely labeled with aminostilbamidine methanesulfonate (Molecular Probes, Eugene, OR), and loss of fluorescently labeled RGCs in retinal flatmounts was quantified. Apoptotic cell death was assessed by TUNEL staining. Astrocyte activation was determined by immunohistochemistry, and laminin decrease was determined by immunohistochemistry and Western blot analysis. Intravitreal injection of KA caused time- and dose-related MMP-9 upregulation in the retina. Increased MMP-9 activity and protein levels were associated with activation of astrocytes. Astrocyte-associated MMP-9 correlated with a decrease in laminin immunoreactivity in the ganglion cell layer and significant loss of retinal ganglion cells. KA-mediated upregulation of MMP-9 activity was associated with apoptosis of cells in the ganglion cell layer as early as 6 hours after injection, followed by apoptosis in cells in the inner nuclear layer by day 1. Intravitreal injection of the non-NMDA receptor antagonists, CNQX and NBQX decreased KA-induced MMP-9 activity and protein levels in the retina and attenuated retinal degeneration, whereas the NMDA receptor antagonist MK801 failed to offer protection. Further, a synthetic MMP inhibitor GM6001 decreased KA-mediated MMP-9 activity and offered significant protection against ganglion cell loss in the retina. These results indicate that KA-mediated upregulation of MMP-9 activity promotes retinal degeneration and suggest that inhibition of KA-mediated MMP activity may offer protection against excitotoxin-induced retinal damage.