Cystatin C in the Eye and in Specific Model Systems

Abstract: It is a necessity of life to make proteins, and it is an equal necessity to degrade those proteins, to eliminate dysfunctional proteins, and leave room for new ones. The processes of protein synthesis and degradation into peptides - proteolysis - have attracted a great interest during the last decades. The eye contains several highly specialized tissues, in which the protein turnover is of vital interest. Reduced control of proteolysis might, for example, impair the transparency of the lens and the cornea. It may also reduce the capacity to eliminate used photoreceptor elements, a task normally carried out by the specialized retinal pigment epithelial cells, as part of the constant renewal and degradation of our photoreceptors, resulting in accumulation of debris, and leading to visual impairment. We have identified cystatin C, on the mRNA and protein levels, in the mouse, rat and human eye. Furthermore, we have described its precise localization in the ocular tissues, using immunohistochemistry. We show that it is present in the epithelium, endothelium and the stromal cells of the cornea, the epithelial cells lining the ciliary processes, stromal cells of the iris, the lens epithelial cells, the pigment epithelial cells, ganglion cells and occasional other cells in the retina, as well as in the aqueous humor (paper I and II). Our mRNA data suggests that it is locally produced. Cystatin C is the strongest of the few known inhibitors of the lysosomal cysteine protease cathepsin B. We have identified cathepsin B in ocular tissues, both on the protein and mRNA levels (paper III). The immunoreactivity pattern of cathepsin B is strikingly similar to that of cystatin C in all examined ocular tissues apart from the retina, suggesting that cystatin C is a dominating cathepsin B inhibitor in vivo in the cornea, the ciliary body, the iris and the lens. In these cells, cystatin C appears to be localized to the cytoplasm, whereas cathepsin B is localized to the lysosomes. In the retina there are differences in staining pattern, suggesting that another cystatin may be the dominating inhibitor of cathepsin B activity. Since cystatin C is a secreted protein, it is assumed to play its role in the extracellular compartment. There is no previously known specific uptake system for cystatin C. We show that there is an active, high-affinity, and energy-requiring uptake system for cystatin C into macrophages in vitro (paper IV), as well as into several cell types, in the cornea, the ciliary body and the retina, of the eye, in vivo as well as in vitro (paper V). The cell types that take up cystatin C in the eye (paper V) are generally the same types that contain endogenous cystatin C (paper I and II), suggesting that much or all cystatin C seen intracellularly in the normal eye may have been taken up from the surrounding extracellular space. This uptake system may regulate the extracellular levels of cystatin C in the eye, and possibly also in other tissues.