Caspase remodeling of the spectrin membrane skeleton during lens development and aging

Terminal differentiation of lens fiber cells resembles the apoptotic process in that organelles are lost, DNA is fragmented, and changes in membrane morphology occur. However, unlike classically apoptotic cells, which are disintegrated by membrane blebbing and vesiculation, aging lens fiber cells are compressed into the center of the lens, where they undergo cell-cell fusion and the formation of specialized membrane interdigitations. In classically apoptotic cells, caspase cleavage of the cytoskeletal protein alpha-spectrin to approximately 150-kDa fragments is believed to be important for membrane blebbing. We report that caspase(s) cleave alpha-spectrin to approximately 150-kDa fragments and beta-spectrin to approximately 120- and approximately 80-kDa fragments during late embryonic chick lens development. These fragments continue to accumulate with age so that in the oldest fiber cells of the adult lens, most, if not all, of the spectrin is cleaved to discrete fragments. Thus, unlike classical apoptosis, where caspase-cleaved spectrin is short lived, lens fiber cells contain spectrin fragments that appear to be stable for the lifetime of the organism. Moreover, fragmentation of spectrin results in reduced membrane association and thus may lead to permanent remodeling of the membrane skeleton. Partial and specific proteolysis of membrane skeleton components by caspases may be important for age-related membrane changes in the lens.