Tyrosine phosphorylation modulates cell surface expression of chloride cotransporters NKCC2 and KCC3

Introduction: Cellular chloride transport has a fundamental role in cell volume regulation and membrane potential, both in normal and tumour cells (1,2). Cellular chloride entry or exit are mediated at the plasma membrane by cotransporter proteins of the solute carrier 12 family. For example, NKCC2 resorbs chloride with sodium and potassium ions at the apical membrane of epithelial cells in the kidney, whereas KCC3 releases chloride with potassium ions at the basolateral membrane. Their ion transport activity is regulated by protein phosphorylation in response to signaling pathways. An additional regulatory mechanism concerns the amount of cotransporter molecules inserted into the plasma membrane. Experimental: Co-transporter constructs were transfected into HEK293 cells and the activity of SYK kinase modulated by incubation with SYK inhibitors or by co-transfection with siRNAs, kinase-dead, or constitutively active SYK mutants. Co-transporter abundance in the plasma membrane was analyzed by biotinylation of cell surface proteins. Results: Here we describe that tyrosine phosphorylation of NKCC2 and KCC3 regulates their plasma membrane expression levels. We identified that spleen tyrosine kinase (SYK) phosphorylates a specific N-terminal tyrosine residue in each cotransporter. Experimental depletion of endogenous SYK or pharmacological inhibition of its kinase activity increased the abundance of NKCC2 at the plasma membrane of human embryonic kidney cells. In contrast, overexpression of a constitutively active SYK mutant decreased NKCC2 membrane abundance. Intriguingly, the same experimental approaches revealed the opposite effect on KCC3 abundance at the plasma membrane, compatible with the known antagonistic roles of NKCC and KCC cotransporters in cell volume regulation. Conclusions: We identified a novel pathway modulating the cell surface expression of NKCC2 and KCC3 and show that this same pathway has opposite functional outcomes for these two cotransporters. The findings have several biomedical implications considering the role of these cotransporters in regulating blood pressure and cell volume.Funding: UID/MULTI/04046/2019 (BioISI); PTDC/BIA-CEL/28408/2017; Fellowship SFRH/BD/52488/2014 to CAL.