Comparison of Lp82- and m-calpain-mediated proteolysis during cataractogenesis in Shumiya cataract rat (SCR)

Purpose. It is well known that m-calpain, a ubiquitous calpain, is involved in cataract formation in rodent lens. Involvement of Lp82, a lens-specific calpain, in the cataract formation is also suggested. However, the exact relationship between Lp82-mediated proteolysis and lens opacification has not yet been established. We therefore compared Lp82- and m-calpain-mediated proteolyses of αA-crystallin during cataractogenesis to clarify whether Lp82 is involved in cataract formation. Methods. In order to analyze the Lp82- and m-calpain-mediated proteolyses, we developed antibodies exclusively specific to the proteolytic products of αA-crystallin produced by Lp82 and m-calpain actions, respectively. The proteolytic profiles of αA-crystallin by Lp82 and m-calpain during cataractogenesis in SCR lenses were analyzed by Western blotting and immunohistochemical staining. Results. While m-calpain-mediated proteolysis was detected predominantly in cataractous lenses, Lp82-mediated proteolysis was detected not only in cataractous but in normal lenses. The m-calpain-mediated proteolysis was observed in restricted areas developing and destined to develop opacification, i.e., the nuclear and perinuclear regions of lens. On the other hand, Lp82-mediated proteolysis was observed not only in the same regions but also in the cortical region where opacity does not develop. Unlike m-calpain-mediated proteolysis, Lp82-mediated proteolysis was not inhibited by the oral administration of aminoguanidine (AG), which acts to prevent lens opacification. Conclusions. From these results, it is shown that there is no direct contribution of Lp82-mediated proteolysis to cataract formation in SCR. Rather, Lp82 may function in fiber cell development and/or fiber cell remodeling during lens maturation under physiological conditions, since Lp82-mediated proteolysis occurs in the cortical region of normal lens.