Regulation of lens volume: implications for lens transparency

Lens transparency is critically dependent on the maintenance of an ordered tissue architecture, and disruption of this order leads to light scatter and eventually lens cataract. Hence the volume of the fiber cells that make up the bulk of the lens needs to be tightly regulated if lens transparency is to be preserved. While it has long been appreciated that the lens can regulate its volume when placed in anisosmotic solutions, recent work suggests that the lens also actively maintains its volume under steady-state conditions. Furthermore, the process of fiber cell elongation necessitates that differentiating fiber cells dramatically increase their volume in response to growth factors. The cellular transport mechanisms that mediate the regulation of fiber cell volume in the lens cortex are only just beginning to be elucidated. In this region, fiber cells are continuously undergoing a process of differentiation that creates an inherent gradient of cells at different stages of elongation. These cells express different complements of transport proteins involved in volume regulation. In addition, transport processes at different depths into the lens are differentially influenced by electrochemical gradients that alter with distance into the lens. Taken together, our work suggests that the lens has spatially distinct ion influx and efflux pathways that interact to control its steady-state volume, its response to hypotonic swelling, and the elongation of differentiating fibers. Based on this work, we present a model which may explain the unique damage phenotype observed in diabetic cataract, in terms of the uncoupling or dysregulation of these ion influx and efflux pathways.