Calreticulin expression in spinal motoneurons of the rat

We have examined the expression of calreticulin in rat spinal motoneurons in order to reveal the occurrence and distribution of Ca2+-storage organelles in these neurons. Calreticulin, the non-muscle equivalent of calsequestrin, is the low-affinity, high-capacity calcium-binding protein responsible for intracompartmental Ca2+-storage in a number of different cell types. The results of the present immunohistochemical study show that all spinal motoneurons express calreticulin at approximately the same level; no significant differences in cytoplasmic immunostaining intensity were observed between different motoneuron pools or between small and large spinal motoneurons. Immunoelectron microscopy revealed that the intracellular localization of calreticulin within spinal motoneurons was confined to the endoplasmic reticulum and to spherical or pleiomorphic, frequently 'coated' vesicles with a diameter ranging between 120 and 150 nm. Some of these vesicles may represent the so-called calciosomes, the intracellular Ca2+-storage vesicles described in liver cells and in cerebellar Purkinje cells. The molecular components responsible for the uptake and release of Ca2+ from the Ca2+-storage organelles in spinal motoneurons still remain to be identified.