Changes in intracellular [Ca 2 þ ] i levels have been shown to influence developmental processes that accompany the transition of human oligodendrocyte precursor cells (OPCs) into mature myelinating oligodendrocytes and are required for the initiation of the myelination and re-myelination processes. In the present study, we explored whether calcium signals mediated by the selective sodium calcium exchanger (NCX) family members NCX1, NCX2, and NCX3, play a role in oligodendrocyte maturation. Functional studies, as well as mRNA and protein expression analyses, revealed that NCX1 and NCX3, but not NCX2, were divergently modulated during OPC differentiation into oligodendrocyte phenotype. In fact, whereas NCX1 was downregulated, NCX3 was strongly upregulated during oligodendrocyte development. The importance of calcium signaling mediated by NCX3 during oligodendrocyte maturation was supported by several findings. Indeed, whereas knocking down the NCX3 isoform in OPCs prevented the upregulation of the myelin protein markers 2 0 ,3 0 -cyclic nucleotide-3 0 -phosphodiesterase (CNPase) and myelin basic protein (MBP), its overexpression induced an upregulation of CNPase and MBP. Furthermore, NCX3-knockout mice showed not only a reduced size of spinal cord but also marked hypo-myelination, as revealed by decrease in MBP expression and by an accompanying increase in OPC number. Collectively, our findings indicate that calcium signaling mediated by NCX3 has a crucial role in oligodendrocyte maturation and myelin formation. Oligodendrocytes, the myelin-forming cells of the CNS, arise from oligodendrocyte precursor cells (OPCs) that colonize both the gray and the white brain matter during development. 1 Although many OPCs differentiate into mature myelinating oligodendrocytes during the early and much later stages of human brain development, a considerable number of them do persist in the adult brain, and may provide a source of new oligodendrocytes, as well as protoplasmic astrocytes and neurons. 2,3 Because of their apparent stem-like characteristics, adult OPCs have recently gained much attention, for they are regarded as a potential reservoir of cells capable of self-renewal, differentiation, and re-myelination after CNS injury. 4 Thus, understanding how oligodendrocyte development proceeds and what factors govern the differentiation fate of OPCs is crucial to discover new effective therapeutic targets for de-myelinating diseases such as multiple sclerosis (MS).Changes in intracellular calcium levels have a critical role in OPC migration, lineage progression, and differentiation; furthermore, they are required for myelination and remyelination processes. [5][6][7] The Na þ /Ca 2 þ exchanger (NCX), 8 a transmembrane domain protein, which, by operating in a bidirectional way, couples the efflux of Ca 2 þ to the influx of Na þ into the cell or, vice versa, the influx of Ca 2 þ to the efflux of Na þ , is involved in the regulation of diverse neuronal and glial cell functions. 8,9 Furthermore, NCX is involved in regulating int...