Involvement of Calpain in Retinal Ganglion Cell Damage: Proposed Mechanism for Glaucoma in Rat and Human

Abstract

Abstract: : Purpose: Glaucoma is associated with death of ganglion cells in retina. Influx of calcium and activation of calpains (neutral cysteine proteases) have been implicated in neuronal cell death due to traumatic brain injury, spinal cord injury, and retinal ischemia. The purpose of the present study was to determine if elevated intraocular pressure (IOP) causes ganglion cell death in rat retinas by increasing calcium and activation of calpains. Results were extrapolated by comparing in vitro, calpain–induced proteolysis of proteins from rat and human retinas. Methods: IOP in rats was elevated to 120 or 160 mm Hg for 60 min. Retinal ganglion cells were evaluated after H&E staining. Total calcium was measured by atomic absorption. Activity of calpain and proteolysis of calpain substrates were analyzed by casein zymography and immunoblotting. Soluble proteins from rat and human retinas were incubated with calcium, and the resulting calpain–induced proteolysis was observed by immunoblotting. Results: Elevated IOP caused increased total calcium, activation of calpains, proteolysis of α–spectrin, and loss of ganglion cells in rat retinas. Incubation with calcium led to calpain–induced proteolysis of α–spectrin in both rat and human samples. Calpain inhibitor prevented this in vitro proteolysis of α–spectrin. Conclusions: Our results suggested that increased calcium and subsequent proteolysis by activated calpains were involved in retinal ganglion cell death from glaucoma in the rat model. Calpain inhibitors may be candidate drugs for treatment of retinal degeneration resulting from glaucoma. Dr. Shearer has a significant financial interest (research contract and consulting fee) in Senju Pharmaceutical Co. Ltd., a company which may have commercial interest in the results of this research and technology. This potential conflict of interest has been reviewed and managed by the OHSU Conflict of Interest in Research Committee.

Keywords: calcium • ganglion cells • proteolysis