Noncanonical Sequences Involving NHERF1 Interaction with NPT2A Govern Hormone-Regulated Phosphate Transport: Binding Outside the Box

Abstract

Na+/H+ exchange factor-1 (NHERF1), a multidomain PDZ scaffolding phosphoprotein, is required for the type II sodium-dependent phosphate cotransporter (NPT2A)-mediated renal phosphate absorption. Both PDZ1 and PDZ2 domains are involved in NPT2A-dependent phosphate uptake. Though harboring identical core-binding motifs, PDZ1 and PDZ2 play entirely different roles in hormone-regulated phosphate transport. PDZ1 is required for the interaction with the C-terminal PDZ-binding sequence of NPT2A (-TRL). Remarkably, phosphocycling at Ser290 distant from PDZ1, the penultimate step for both parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) regulation, controls the association between NHERF1 and NPT2A. PDZ2 interacts with the C-terminal PDZ-recognition motif (-TRL) of G Protein-coupled Receptor Kinase 6A (GRK6A), and that promotes phosphorylation of Ser290. The compelling biological puzzle is how PDZ1 and PDZ2 with identical GYGF core-binding motifs specifically recognize distinct binding partners. Binding determinants distinct from the canonical PDZ-ligand interactions and located “outside the box” explain PDZ domain specificity. Phosphorylation of NHERF1 by diverse kinases and associated conformational changes in NHERF1 add more complexity to PDZ-binding diversity.

Clinical and Functional Assessment of Digenicity in Renal Phosphate Wasting

“…In the absence of this compensation due to digenic or oligogenic defects in both phosphate transporters, phosphate reabsorption activity was significantly reduced, leading to more severe presentations of renal calcification and bone mineralization disorders, as seen in patient 1. [33]; both of these lead to the phosphorylation of NHERF1 to uncouple it from NaPi2a and inhibit phosphate uptake. Therefore, genetic defects in NHERF1 increase PTHinduced cAMP generation but also functional alterations between the direct interaction of NHERF1 and NaPi2a, both resulting in an enhanced inhibition of phosphate uptake and urinary phosphate wasting.…”

“…Although our studies did not demonstrate such an additional effect of wild-type NHERF1, which was possibly due to a dosing effect as oocytes express intrinsic NHERF1, the addition of mutated NHERF1 did result in a significant decrease in phosphate uptake. [33]; both of these lead to the phosphorylation of NHERF1 to uncouple it from NaPi2a and inhibit phosphate uptake. Therefore, genetic defects in NHERF1 increase PTH-induced cAMP generation but also functional alterations between the direct interaction of NHERF1 and NaPi2a, both resulting in an enhanced inhibition of phosphate uptake and urinary phosphate wasting.…”

RGS14 regulates PTH- and FGF23-sensitive NPT2A-mediated renal phosphate uptake via binding to the NHERF1 scaffolding protein

“…The particular RSG14 ligand sequence is -Asp-Ser-Ala-Leu. We expected that Asn or Gly mutations at the obligate -3 position (Asp 563 ) would disrupt binding to NHERF1 ( 60 ). Indeed, Asn substitution abolished binding as anticipated, but Gly replacement was innocuous.…”

Growth hormone therapy in HHRH

“…In patients without any defect in their phosphate transport but with growth hormone deficiency, an increase in serum phosphate was also observed when treated with rhGH ( Ahmad et al, 2003 ). The putative mechanism is through NPT2A regulated phosphate transport which is influenced by insulin-like growth factor 1 and insulin and growth hormone ( Mamonova and Friedman, 2021 ).…”