Kir4.1 expression by astrocytes and oligodendrocytes in CNS white matter: a developmental study in the rat optic nerve

Deletion studies in transgenic mice indicate that the potassium inward rectifying channel Kir4.1 is crucial for oligodendrocyte differentiation and has a special role in regulation of extracellular potassium (K+), a major function of astrocytes. However, there are conflicting reports on whether Kir4.1 is expressed by white matter astrocytes and oligodendrocytes, raising doubts over its functions. Here, we have examined Kir4.1 expression in astrocytes and oligodendrocytes of the rat optic nerve, a typical central nervous system white matter tract. Single and double immunofluorescence labelling was performed on frozen sections from optic nerves aged postnatal day (P)5, 10, 15, 20 and adult, using anti-Kir4.1 antibodies and the glia-specific antibodies glial fibrillary acidic protein (GFAP, astrocytes), carbonic anhydrase II (CAII, oligodendrocyte somata and processes) and myelin basic protein (MBP, oligodendrocyte myelin sheaths). The results demonstrate Kir4.1 expression in rows of glial cells as early as P5, and this pattern persisted throughout development and into adulthood, consistent with early expression of Kir4.1 on developing oligodendrocytes. Clear co-expression of Kir4.1 and GFAP is first evident at P10 and increases to adult levels by P15 and P20, which correlates with the development of K+ regulation between P15 and P20. Astrocyte expression of Kir4.1 is localized to perivascular end-feet and fine processes within the fascicles of myelinated axons, consistent with a role in K+ spatial buffering between nodes of Ranvier and blood vessels. By contrast, Kir4.1 is concentrated in the cell bodies of oligodendrocytes, and there is no apparent co-expression with MBP+ myelin sheaths, suggesting oligodendroglial Kir4.1 channels are not involved in K+ regulation. The results support roles for Kir4.1 in both oligodendrocyte differentiation and K+ regulation by astrocytes.