Immunohistochemical localization of calpains and calpastatin in the rabbit eye

The localization of the two Ca-activated extralysosomal proteases m-calpain and mu-calpain in the eye of the adult rabbit was examined by immunohistochemistry, using poly- and monoclonal antibodies against the corresponding rabbit antigens. Immunoreactivity against the two forms of calpains was observed in the epithelial cells on the external and internal surface of the cornea as well as in the epithelial cells covering the iris and ciliary body. The sclera and choroid layers showed a relatively weak immunoreactivity. Using anti m-calpain antibodies, the pigment epithelium in the retina was heavily labelled as well as the outer and inner plexiform layers. The other and inner borders of the Müller cells were clearly labelled. The outer segments of the receptor cells showed a strong immunoreactivity for both mu-calpain and m-calpain. Labelling was also observed in the retinal ganglion cells and in the nerve fiber layer. The immunohistochemical localization of calpastatin, an endogenous inhibitor of both m- and mu-calpain was also examined. A high level of calpastatin immunoreactivity was observed in the outer segments of the receptor cells. The results may be compatible with a role for calpains, especially m-calpain, in the secretory/phagocytic process and as modulators of the cytoskeleton in cell processes.