Ryanodine receptor- and sodium-calcium exchanger-mediated spontaneous calcium activity in immature oligodendrocytes in cultures

Abstract

Myelination in the central nervous system depends on interactions between axons and oligodendrocyte precursor cells (OPCs). Action potentials in an axon can be followed by release of biologically active substances, like glutamate, which can instruct OPCs to start myelination. Myelin Basic Protein (MBP) is an "executive molecule of myelin" required for the formation of compact myelin. As cells of the oligodendrocyte lineage (OLCs) are capable of producing MBP in pure oligodendrocyte cultures, i.e. without neurons, we investigated Ca²⁺ signaling in developing OLCs in cultures. We show that spontaneous Ca²⁺ transients (CTs) occur at very low frequency in both bipolar OPCs and mature oligodendrocytes. In contrast immature OLCs (imOLCs), cells with several thick processes, demonstrate a relatively high frequency of CTs. Moreover, CT frequency in imOLC processes is much higher as compared with the somatic CT frequency. Somatic CTs are almost completely blocked by thapsigargin, an antagonist of sarco-(endo-) plasmic reticulum Ca²⁺ ATPase, and ryanodine, a blocker of ryanodine receptors, indicating an involvement of Ca²⁺ release from the endoplasmic reticulum. Ryanodine strongly reduces CT frequency in imOLC processes. Ouabain, an antagonist of Na⁺, K⁺-ATPase (NKA), applied at low concentration increases CT frequency, while KB-R7943, a blocker of reverse mode of Na⁺, Ca²⁺ exchanger (NCX), decreases CT frequency. We suggest that local RyR-NCX-(NKA?) interaction might underlie the generation of CTs in imOLC in the absence of neurons, and this activity influences oligodendrocyte maturation.

Keywords: calcium transient; myelin basic protein; oligodendrocyte maturation; ryanodine receptor; sodium-calcium exchanger.