Acute regulation of NHE3 by protein kinase A requires a multiprotein signal complex.

Biochemical and cellular experiments in fibroblasts have established the requirement for a member of the PDZ motif Na(+)/H(+) exchanger regulatory factor family of proteins (NHERF and NHERF2) in cAMP-mediated phosphorylation and inhibition of NHE3 activity. NHERF interacts with the actin cytoskeleton through the scaffolding protein ezrin to target a multiprotein signal complex to the plasma membrane. Recent experiments have focused on elements of this model. First, using specific antibodies, NHERF was identified in the renal proximal tubule, where it colocalized with ezrin and NHE3. NHERF2 was seen in glomeruli, the renal vasculature, and collecting duct cells, where it colocalized with ROMK. This distinct nephron localization suggests different physiologic roles for NHERF and NHERF2. Second, the signal-complex model of protein kinase A regulation of NHE3 developed in fibroblasts has been extended to epithelial cells by the development of a dominant-negative opossum kidney cell line expressing an ezrin binding domain-deficient truncation of NHERF. Preliminary studies indicate that these cells have normal basal Na+/H+ exchanger activity but a blunted inhibitory response to cAMP. Third, biochemical, biophysical, and cell experiments have indicated that NHERF binds to itself in a head-to-head configuration, raising the possibility that dimerization may alter the availability of active NHERF. The potential role of the NHERF proteins in the kidney has been expanded by recent studies indicating their involvement in the membrane targeting, trafficking, sorting, and regulation of a range of other transporters, receptors, and signaling proteins. NHERF and related PDZ-containing proteins may serve as adapters for regulation of renal transporters.