Developmental regulation of zinc homeostasis in differentiating oligodendrocytes

Oligodendrocytes develop through well characterized stages and understanding pathways regulating their differentiation remains an active area of investigation. Zinc is required for the function of many enzymes, proteins and transcription factors, including those important in myelination and mitosis. Our previous studies using the ratiometric zinc sensor chromis-1 demonstrated a reduction in intracellular free zinc concentrations in mature oligodendrocytes compared with earlier stages (Bourassa et al., 2018). We performed a more detailed developmental study to better understand the temporal course of zinc homeostasis across the oligodendrocyte lineage. Using chromis-1, we found a transient increase in free zinc after developing oligodendrocytes were switched into differentiation medium. To gather other evidence for dynamic regulation of free zinc during oligodendrocyte development, qPCR was used to evaluate mRNA expression of the major zinc storage proteins metallothioneins (MTs), and metal regulatory transcription factor 1 (MTF-1) which controls expression of MTs. MT-1, MT-2 and MTF1 mRNAs were all increased several fold in mature oligodendrocytes compared to developing oligodendrocytes. To assess the depth of the zinc buffer, we assayed zinc release from intracellular stores using the oxidizing thiol reagent 2,2'-dithiodipyridine (DTDP). Exposure to DTDP resulted in a ∼100% increase in free zinc in developing oligodendrocytes but, paradoxically more modest ∼60% increase in mature oligodendrocytes despite the increased expression of MTs. These results suggest that zinc homeostasis is regulated during oligodendrocyte development, that oligodendrocytes are a useful model for studying zinc homeostasis in the central nervous system, and that regulation of zinc homeostasis may be important in oligodendrocyte differentiation.