Progressive loss of a glial potassium channel (KCNJ10) in the spinal cord of the SOD1 (G93A) transgenic mouse model of amyotrophic lateral sclerosis.

Transgenic mice expressing the superoxide dismutase G93A mutation (SOD1(G93A)) were used to investigate the role of glial inwardly rectifying K(+) (Kir)4.1 channels, which buffer extracellular K(+) increases in response to neuronal excitation. A progressive decrease in Kir4.1 immunoreactivity was observed predominantly in the ventral horn of SOD1(G93A) mutants. Immunoblotting of spinal cord extracts mirrored these changes by showing a loss of Kir4.1 channels from presymptomatic stages onwards. Kir4.1 channels were found to be expressed in the spinal cord grey matter, targetting astrocytes and clustering around capillaries, supporting their role in clearance of extracellular K(+). To understand the functional implications of extracellular K(+) increases, we challenged the NSC34 motor neurone cell line with increasing extracellular K(+) concentrations. Exposure to high extracellular K(+) induced progressive motor neurone cell death. We suggest that loss of Kir4.1 impairs perineural K(+) homeostasis and may contribute to motor neurone degeneration in SOD1(G93A) mutants by K(+) excitotoxic mechanisms.

Lack of the Kir4.1 channel subunit abolishes K+ buffering properties of astrocytes in the ventral respiratory group: impact on extracellular K+ regulation.

Ongoing rhythmic neuronal activity in the ventral respiratory group (VRG) of the brain stem results in periodic changes of extracellular K+. To estimate the involvement of the weakly inwardly rectifying K+ channel Kir4.1 (KCNJ10) in extracellular K+ clearance, we examined its functional expression in astrocytes of the respiratory network. Kir4.1 was expressed in astroglial cells of the VRG, predominantly in fine astrocytic processes surrounding capillaries and in close proximity to VRG neurons. Kir4.1 expression was up-regulated during early postnatal development. The physiological role of astrocytic Kir4.1 was studied using mice with a null mutation in the Kir4.1 channel gene that were interbred with transgenic mice expressing the enhanced green fluorescent protein in their astrocytes. The membrane potential was depolarized in astrocytes of Kir4.1-/- mice, and Ba2+-sensitive inward K+ currents were diminished. Brain slices from Kir4.1-/- mice, containing the pre-Bötzinger complex, which generates a respiratory rhythm, did not show any obvious differences in rhythmic bursting activity compared with wild-type controls, indicating that the lack of Kir4.1 channels alone does not impair respiratory network activity. Extracellular K+ measurements revealed that Kir4.1 channels contribute to extracellular K+ regulation. Kir4.1 channels reduce baseline K+ levels, and they compensate for the K+ undershoot. Our data indicate that Kir4.1 channels 1) are expressed in perineuronal processes of astrocytes, 2) constitute the major part of the astrocytic Kir conductance, and 3) contribute to regulation of extracellular K+ in the respiratory network.