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Kinases SPAK and OSR1 Are Upregulated by Estradiol and Activate NKCC1 in the Developing Hypothalamus

In immature neurons the amino acid neurotransmitter, GABA provides the dominant mode for neuronal excitation by inducing membrane depolarization due to Cl− efflux through GABAA receptors (GABAARs). The driving force for Cl− is outward because the Na+-K+-2Cl− cotransporter (NKCC1) elevates the Cl− concentration in these cells. GABA-induced membrane depolarization and the resulting activation of voltage-gated Ca2+ channels is fundamental to normal brain development, yet the mechanisms that regulate depolarizing GABA are not well understood. The neurosteroid estradiol potently augments depolarizing GABA action in the immature hypothalamus by enhancing the activity of the NKCC1 cotransporter. Understanding how estradiol controls NKCC1 activity will be essential for a complete understanding of brain development. We now report that estradiol treatment of newborn rat pups significantly increases protein levels of two kinases upstream of the NKCC1 cotransporter, SPAK (STE20/SPS1-related proline alanine rich kinase) and OSR1 (oxidative stress response kinase). The estradiol-induced increase is transcription dependent, and its time course parallels that of estradiol-enhanced phosphorylation of NKCC1. Antisense oligonucleotide-mediated knockdown of SPAK, and to a lesser degree of OSR1, precludes estradiol-mediated enhancement of NKCC1 phosphorylation. Functionally, knockdown of SPAK or OSR1 in embryonic hypothalamic cultures diminishes estradiol-enhanced Ca2+ influx induced by GABAAR activation. Our data suggest that SPAK and OSR1 may be critical factors in the regulation of depolarizing GABA-mediated processes in the developing brain. It will be important to examine these kinases with respect to sex differences and developmental brain anomalies in future studies.