CALCIUM STORAGE IN NONMUSCLE TISSUES - IS THE RETINA SPECIAL

It is now well established that calreticulin is a high capacity Ca2+-binding protein which is a major Ca2+ storage protein of the lumen of endoplasmic reticulum membranes in a wide variety of tissues with the exception of skeletal and cardiac muscles. However, in nervous tissue, confusion exists regarding the nature of the intracellular Ca2+ stores, as the organelle responsible for Ca2+ storage has been identified as the endoplasmic reticulum by some investigators and as the specialized organelle, calciosome by others. Calreticulin, calsequestrin, and calsequestrin-like proteins have all been, on different occasions, reported to be present in calciosomes. Cerebral and cerebellar tissues, moreover, have been shown to contain somewhat different systems of Ca2+-buffering proteins. In the present paper we discuss evidence that the Ca2+ storage systems of the retina may prove to be more complex than those of other neuronal tissues. Biochemical and immunocytochemical evidence indicates the presence of either an isoform of calreticulin or another protein that is antigenically similar to calreticulin, but of slightly higher molecular weight, in the endoplasmic reticulum of both neurons and Muller glia from rabbit neural retina. However, as retinal neurons express Purkinjc cell markers, one may expect to observe the presence of calsequestrin in these cells as well. Secondly, antibodies against the onchocercal RAL-1 antigen recognize a protein sharing 62-65% amino acid sequence identity with calreticulin. The anti-RAL-1 antibodies show specificity for the retina. Whether or not the RAL-1 antigen is an active part of the Ca2+ storage systems of the retina remains to be verified. Finally, the retinal epithelial cells of the retina have an additional active system for Ca2+ uptake and storage associated with melanin-containing pigment granules. Thus, taking into account the differences in Ca2+ uptake, storage, and release systems noted for neuronal versus nonneuronal nonmuscle tissues, the retina might show yet another diversification of the intracellular Ca2+ management system.