The inwardly rectifying potassium channel Kir4.1 has been suggested to underlie the principal K⁺conductance of mammalian Müller cells and to participate in the generation of field potentials and regulation of extracellular K⁺in the retina. To further assess the role of Kir4.1 in the retina, we generated a mouse line with targeted disruption of theKir4.1gene (Kir4.1 −/−). Müller cells from Kir4.1 −/− mice were not labeled with an anti-Kir4.1 antibody, although they appeared morphologically normal when stained with an anti-glutamine synthetase antibody. In contrast, in Müller cells from wild-type littermate (Kir4.1 +/+) mice, Kir4.1 was present and localized to the proximal endfeet and perivascular processes.In situwhole-cell patch-clamp recordings showed a 10-fold increase in the input resistance and a large depolarization of Kir4.1 −/− Müller cells compared with Kir4.1 +/+ cells. The slow PIII response of the light-evoked electroretinogram (ERG), which is generated by K⁺fluxes through Müller cells, was totally absent in retinas from Kir4.1 −/− mice. The b-wave of the ERG, in contrast, was spared in the null mice. Overall, these results indicate that Kir4.1 is the principal K⁺channel subunit expressed in mouse Müller glial cells. The highly regulated localization and the functional properties of Kir4.1 in Müller cells suggest the involvement of this channel in the regulation of extracellular K⁺in the mouse retina.

Stria vascularis of the cochlea generates the endocochlear potential and secretes K⁺. K⁺ is the main charge carrier and the endocochlear potential the main driving force for the sensory transduction that leads to hearing. Stria vascularis consists of two barriers, marginal cells that secrete potassium and basal cells that are coupled via gap junctions to intermediate cells. Mice lacking the KCNJ10 (Kir4.1) K⁺ channel in strial intermediate cells did not generate an endocochlear potential. Endolymph volume and K⁺ concentration ([K⁺]) were reduced. These studies establish that the KCNJ10 K⁺ channel provides the molecular mechanism for generation of the endocochlear potential in concert with other transport pathways that establish the [K⁺] difference across the channel. KCNJ10 is also a limiting pathway for K⁺ secretion.

To understand the cellular and in vivo functions of specific K⁺ channels in glia, we have studied mice with a null mutation in the weakly inwardly rectifying K⁺ channel subunit Kir4.1. Kir4.1−/− mice display marked motor impairment, and the cellular basis is hypomyelination in the spinal cord, accompanied by severe spongiform vacuolation, axonal swellings, and degeneration. Immunostaining in the spinal cord of wild-type mice up to postnatal day 18 reveals that Kir4.1 is expressed in myelin-synthesizing oligodendrocytes, but probably not in neurons or glial fibrillary acidic protein-positive (GFAP-positive) astrocytes. Cultured oligodendrocytes from developing spinal cord of Kir4.1−/− mice lack most of the wild-type K⁺ conductance, have depolarized membrane potentials, and display immature morphology. By contrast, cultured neurons from spinal cord of Kir4.1−/− mice have normal physiological characteristics. We conclude that Kir4.1 forms the major K⁺ conductance of oligodendrocytes and is therefore crucial for myelination. The Kir4.1 knock-out mouse is one of the few CNS dysmyelinating or demyelinating phenotypes that does not involve a gene directly involved in the structure, synthesis, degradation, or immune response to myelin. Therefore, this mouse shows how an ion channel mutation could contribute to the polygenic demyelinating diseases.

G protein-gated inward rectifier K⁺(GIRK) channels mediate hyperpolarizing postsynaptic potentials in the nervous system and in the heart during activation of Gα(i/o)-coupled receptors. In neurons and cardiac atrial cells the time course for receptor-mediated GIRK current deactivation is 20–40 times faster than that observed in heterologous systems expressing cloned receptors and GIRK channels, suggesting that an additional component(s) is required to confer the rapid kinetic properties of the native transduction pathway. We report here that heterologous expression of “regulators of G protein signaling” (RGS proteins), along with cloned G protein-coupled receptors and GIRK channels, reconstitutes the temporal properties of the native receptor → GIRK signal transduction pathway. GIRK current waveforms evoked by agonist activation of muscarinic m₂receptors or serotonin 1A receptors were dramatically accelerated by coexpression of either RGS1, RGS3, or RGS4, but not RGS2. For the brain-expressed RGS4 isoform, neither the current amplitude nor the steady-state agonist dose-response relationship was significantly affected by RGS expression, although the agonist-independent “basal” GIRK current was suppressed by ≈40%. Because GIRK activation and deactivation kinetics are the limiting rates for the onset and termination of “slow” postsynaptic inhibitory currents in neurons and atrial cells, RGS proteins may play crucial roles in the timing of information transfer within the brain and to peripheral tissues.