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Nephrin binds to the COOH terminus of a large-conductance Ca²⁺-activated K⁺ channel isoform and regulates its expression on the cell surface

Department of Biology and Biochemistry, University of Houston, Houston, Texas

Submitted 14 March 2008 ; accepted in final form 12 May 2008

We carried out a yeast two-hybrid screen to identify proteins that interact with large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels encoded by the Slo1 gene. Nephrin, an essential adhesion and scaffolding molecule expressed in podocytes, emerged in this screen. The Slo1-nephrin interaction was confirmed by coimmunoprecipitation from the brain and kidney, from HEK-293T cells expressing both proteins, and by glutathione S-transferase pull-down assays. We detected nephrin binding to the Slo1_VEDEC splice variant, which is typically retained in intracellular stores, and to the β4-subunit. However, we did not detect significant binding of nephrin to the Slo1_QEERL or Slo1_EMVYR splice variants. Coexpression of nephrin with Slo1_VEDEC increased expression of functional BK_Ca channels on the surface of HEK-293T cells but did not affect steady-state surface expression of the other COOH-terminal Slo1 variants. Nephrin did not affect the kinetics or voltage dependence of channel activation in HEK-293T cells expressing Slo1. Stimulation of Slo1_VEDEC surface expression in HEK-293T cells was also observed by coexpressing a small construct encoding only the distal COOH-terminal domains of nephrin that interact with Slo1. Reduction of endogenous nephrin expression by application of small interfering RNA to differentiated cells of an immortalized podocyte cell line markedly reduced the steady-state surface expression of Slo1 as assessed by electrophysiology and cell-surface biotinylation assays. Nephrin therefore plays a role in organizing the surface expression of ion channel proteins in podocytes and may play a role in outside-in signaling to allow podocytes to adapt to mechanical or neurohumoral stimuli originating in neighboring cells.

podocyte; Slo1; calcium; trafficking

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