WNK1-regulated inhibitory phosphorylation of the KCC2 cotransporter maintains the depolarizing action of GABA in immature neurons

Activation of Cl^--permeable g-aminobutyric acid type A (GABAA) receptors elicits synaptic inhibition in mature neurons but excitation in immature neurons. This developmental "switch" in the GABA function depends on a postnatal decrease in intraneuronal Cl^- concentration mediated by KCC2, a Cl^-- extruding K+-Cl^- cotransporter. We showed that the serine-threonine kinase WNK1 [with no lysine (K)] forms a physical complex with KCC2 in the developing mouse brain. Dominant-negative mutation, genetic depletion, or chemical inhibition of WNK1 in immature neurons triggered a hyperpolarizing shift in GABA activity by enhancing KCC2-mediated Cl^- extrusion. This increase in KCC2 activity resulted from reduced inhibitory phosphorylation of KCC2 at two C-terminal threonines, Thr⁹⁰⁶ and Thr¹⁰⁰⁷. Phosphorylation of both Thr⁹⁰⁶ and Thr¹⁰⁰⁷ was increased in immature versus mature neurons. Together, these data provide insight into the mechanism regulating Cl^- homeostasis in immature neurons, and suggest that WNK1- regulated changes in KCC2 phosphorylation contribute to the developmental excitatory-to-inhibitory GABA sequence.