Comparison Between Rat Brain Calbindin- and Calretinin-Immuno-reactivities

Abstract

Several calcium-binding proteins are present in the central nervous system including the closely related protein calbindin-D 28K (Taylor, 1974; Baimbridge et al., 1982), and calretinin (Rogers, 1987). The existence of calretinin became apparent with the demonstration by immunoblotting that rat cerebral extracts contained two proteins cross-reacting with calbindin antiserum (Pochet et al. 1985). The two proteins differed in size with one being about 2 kDa larger than the other. Subsequently, calretinin was cloned and sequenced (Rogers, 1987) and appeared to be identical to the larger protein recognized by anti-calbindin. Because of the high degree of homology between calbindin and calretinin (Rogers, 1987; Wilson et al., 1988 and Parmentier, 1989), antiserum against either protein may cross-react with the other and therefore the immunohistochemical mapping in brain must be re-assessed. The existence of a single immunoreactive protein band in gels made from some brain extracts does not rule out the possibility that calbindin antiserum used recognized calretinin. Indeed, calretinin-like immunoreactivity is negative in large rat brain areas such as cerebral cortex, most parts of the thalamus and hippocampus (Rogers et al. 1989). A single protein band immunoreactive for calbindin antiserum may thus mean lack of calretinin rather than no cross-immunoreactivity. Cross reactivity between calretinin antiserum and calbindin is easier to check because calbindin-like immunoreactivity is present nearly everywhere in the rat brain.