CALRETICULIN EXPRESSION IN SPINAL MOTONEURONS OF THE RAT

We have examined the expression of calreticulin in rat spinal motoneur ons 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 p rotein responsible for intracompartmental Ca2+-storage in a number of different cell types. The results of the present immunohistochemical s tudy show that all spinal motoneurons express calreticulin at approxim ately the same level; no significant differences in cytoplasmic immuno staining intensity were observed between different motoneuron pools or between small and large spinal motoneurons. Immunoelectron microscopy revealed that the intracellular localization of calreticulin within s pinal motoneurons was confined to the endoplasmic reticulum and to sph erical or pleiomorphic, frequently 'coated' vesicles with a diameter r anging between 120 and 150 nm. Some of these vesicles may represent th e so-called calciosomes, the intracellular Ca2+-storage vesicles descr ibed in liver cells and in cerebellar Purkinje cells. The molecular co mponents responsible for the uptake and release of Ca2+ from the Ca2+- storage organelles in spinal motoneurons still remain to be identified .