Kir4.1-dependent astrocyte-fast motor neuron interactions are required for peak strength

Diversified neurons are essential for sensorimotor function, but whether astrocytes become specialized to optimize circuit performance remains unclear. Large fast α-motor neurons (FαMNs) of spinal cord innervate fast-twitch muscles that generate peak strength. We report that ventral horn astrocytes express the inward rectifying K+ channel Kir4.1 (aka, Kcnj10) around MNs in a VGLUT1-dependent manner. Loss-of astrocyte-encoded Kir4.1 selectively altered FαMN size, function and led to reduced peak strength. Overexpression of Kir4.1 in astrocytes was sufficient to increase MN size through activation of the PI3K/mTOR/pS6 pathway. Kir4.1 was downregulated cell-autonomously in astrocytes derived from amyotrophic lateral sclerosis (ALS) patients with SOD1 mutation. However, astrocyte Kir4.1 was dispensable for FαMN survival even in the mutant SOD1 background. These findings show that astrocyte Kir4.1 is essential for maintenance of peak strength and suggest that Kir4.1 downregulation uncouples symptoms of muscle weakness from MN cell death in ALS. 8 samples total consisting of 4 biological replicates per group (Aldh1l1-cre:Kir4.1-floxed/floxed vs cre-negative) of flow sorted Aldh1l1-GFP+ astrocytes from spinal cord at P12-P14