Differential immunocytochemical localization of calretinin in the pineal gland of three mammalian species.

Abstract

Calcium plays an important role for signal transduction in the mammalian pineal organ. The regulation of the intracellular concentration of free calcium probably involves calcium-binding proteins of the calmodulin superfamily. In the present study, we have investigated the expression of calretinin, one member of this superfamily, in the pineal organ of hamsters, gerbils and guinea-pigs by means of immunochemical and immunocytochemical analyses with a calretinin-specific antiserum. In immunoblots this antibody recognized a single protein band of approximately 29 kDa in the brain and pineal organ of all three mammalian species. Immunocytochemical investigations of serial semithin sections of plastic-embedded pineals revealed the constant occurrence of variable numbers of calretinin-positive cells throughout all glands. In order to identify the immunopositive cells precisely, adjacent sections were exposed to antibodies against various marker proteins of pineal cell types, i.e., synaptophysin, neuron-specific enolase, protein gene product 9.5, S-antigen, vimentin and S-100. By this approach, calretinin could be localized to vimentin-positive cells in the gerbil which are generally considered as interstitial glial cells. Likewise, calretinin-positive cells in the guinea-pig probably correspond to interstitial cells, taking into account their morphology and the lack of calretinin immunoreactivity in pinealocytes. The unusual expression of calretinin in astrocyte-like cells further supports the notion that pineal glial cells are endowed with peculiar properties. In contrast to gerbil and guinea-pig, a subpopulation of pinealocytes displayed calretinin immunoreactivity in the hamster. This finding adds to the hypothesis that in pinealocytes of some species calretinin plays a role in calcium-mediated signal transduction which eventually is linked to melatonin synthesis. Our results demonstrate that calretinin is a regular constituent of pineal glands in three mammalian species, but that its cellular localisation shows interspecific variation. This variation suggests that the protein is involved in diverse calcium-mediated functions in the mammalian pineal gland.