Calcium-Induced Disruption of the Lens Cytoskeleton

Abstract

Ionic homeostasis is essential to lens clarity and the lens epithelium plays a large part in homeostasis, through vectorial transport. In most epithelia maintenance of vectorial function depends on the cytoskeleton. Capsule/epithelium preparations from human donor lenses have been used for immunohisto-chemical investigations of both normal epithelial cell cytoskeletal structure, and of structural changes induced by increasing cell calcium. A sustained increase in intracellular calcium, induced by incubation with A23187 and W7, led to a loss of cytoskeletal organisation. Changes included depolymerisation of cytoplasmic actin filaments, disaggregation of microtubules and initial thickening, followed by breakdown, of vimentin intermediate filaments. In addition, spectrin staining, normally confined to the basal-lateral plasma membranes, became diffuse within the cytoplasm. Such disruptions of cytoskeletal structure would decrease ionic control, leading to increasing osmotic stress.

The involvement of calpains in opacification induced by Ca2+‐overload in ovine lens culture

“…The observation of excessive activation of Ca 2+ ‐dependent proteases, calpains 1 and 2, and the considerable degradation of cytoskeletal proteins in our ovine model implicated calpains in the pathology of ionomycin induced opacification. The two cytoskeletal proteins, vimentin and spectrin, studied in this research are excellent substrates for calpain in in vitro assays 25–28 . Ca 2+ ‐induced degradation of cytoskeletal proteins has also been found in other organ‐culture studies with rat, 10,29 bovine, 27 and human 30 lenses.…”

Lens intermediate filaments

“…This residual 51 kDa fragment appears, on the basis of epitope-specific antibody labeling, to retain the predicted central rod domain, and to remain associated with the plasma membrane(FitzGerald 1988; Brunkener and Georgatos 1992). IFs in general are known to be early targets of calcium-activated proteases, and this class of protease has been shown to be capable of mediating the degradation of BF and IF proteins in the lens, under experimental conditions(Ireland and Maisel 1984; Marcantonio and Duncan 1991; Marcantonio 1992; Marcantonio 1996; Sanderson, Marcantonio et al 1996; Sanderson, Marcantonio et al 2000; de Iongh, Lovicu et al 2001; de Iongh, Wederell et al 2005). Proteolysis might at first be considered the undesirable fate of a protein destined for life in a cell incapable of protein turn over and renewal.…”

Evaluation of a novel calpain inhibitor as a treatment for cataract

“…Cytoskeletal proteins are known calpain substrates 27,34 and are degraded in the presence of Ca 2+ in bovine, 35 rodent, 36 ovine 20 and human 26 lens culture systems. The proteolysis of lens cytoskeletal proteins, particularly spectrin and vimentin, correlate with the loss of lens transparency 20 .…”

“…31 Also as a lysosomal protease, cathepsin B would not have access to the cytoplasmic crystallins and cytoskeleton, which are proteolysed during cataractogenesis. 17,32,33 Cytoskeletal proteins are known calpain substrates 27,34 and are degraded in the presence of Ca 2+ in bovine, 35 rodent, 36 ovine 20 and human 26 lens culture systems. The proteolysis of lens cytoskeletal proteins, particularly spectrin and vimentin, correlate with the loss of lens transparency.…”