The SNX-PXA-RGS-PXC subfamily of sorting nexins (SNXs) belongs to the superfamily of SNX proteins. SNXs are characterized by the presence of a common phox-homology (PX) domain, along with other functional domains that play versatile roles in cellular signaling and membrane trafficking. In addition to the PX domain, the SNX-PXA-RGS-PXC subfamily, except for SNX19, contains a unique RGS (regulators of G protein signaling) domain that serves as GTPase activating proteins (GAPs), which accelerates GTP hydrolysis on the G protein α subunit, resulting in termination of G protein-coupled receptor (GPCR) signaling. Moreover, the PX domain selectively interacts with phosphatidylinositol-3-phosphate and other phosphoinositides found in endosomal membranes, while also associating with various intracellular proteins. Although SNX19 lacks an RGS domain, all members of the SNX-PXA-RGS-PXC subfamily serve as dual regulators of receptor cargo signaling and endosomal trafficking. This review discusses the known and proposed functions of the SNX-PXA-RGS-PXC subfamily and how it participates in receptor signaling (both GPCR and non-GPCR) and endosomal-based membrane trafficking. Furthermore, we discuss the difference of this subfamily of SNXs from other subfamilies, such as SNX-BAR nexins (Bin-Amphiphysin-Rvs) that are associated with retromer or other retrieval complexes for the regulation of receptor signaling and membrane trafficking. Emerging evidence has shown that the dysregulation and malfunction of this subfamily of sorting nexins lead to various pathophysiological processes and disorders, including hypertension.

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Sorting nexin 19 (SNX19), a member of the SNX-PXA-RGS-PXC subfamily of sorting nexins, plays a vital role in lipid raft microdomain localization, but its association with flotillin-1 and caveolin-1, two of the most important lipid raft components, and in particular its linkage to the endocytosis and trafficking of dopamine D1 receptor (D 1 R) have not been determined in the renal proximal tubule (RPT). In the present study, we found that SNX19 was localized with lipid rafts in human and mouse RPT cells (RPTCs), which regulated D 1 R endocytosis. In wild type (WT) mouse RPTCs, SNX19 co-localized with flotillin-1 and caveolin-1, which was increased by treatment with fenoldopam (Fen, 25 nM, 30 min, n=4), a D1-like receptor agonist but not by vehicle (Veh). The targeting of SNX19 in lipid rafts and its colocalization with flotillin-1 and caveolin-1 were attenuated by deletion of a flotillin binding domain ( EEGPGTETETGLPVS ) (ΔFlot1), a caveolin-1 binding domain ( YHTVNRRYREF ) (ΔCav1), or both domains (ΔFlot1/Cav) in mouse RPTCs. The increase in intracellular cAMP production caused by Fen was also impaired or eliminated by the deletion of either flotillin-1 or caveolin-1 or both binding domains (WT-Veh: 100±16.6%, n=3, WT-Fen: 195.3±10.6%, n=4; ΔFlot1-Veh: 93.1±11.8%, n=4, ΔFlot1-Fen: 144.8±9.4%, n=3; ΔCav1-Veh: 90.9±16.3%, n=3, ΔCav1-Fen: 106.9±11.7%, n=4; ΔFlot/Cav-Veh: 63.8±10.3%, n=4, ΔFlot/Cav-Fen: 86.5±8.1%, n=4). Fen (25 nM, 30 min, n=3) decreased the colocalization of SNX19 with microtubules without an effect on actin filaments, which was abrogated by nocodazole (10 μM, 1hr), a microtubule depolymerization inhibitor. Furthermore, the renal-selective silencing of SNX19 in C57BL/6 mice (n=3) by the renal subcapsular infusion of specific SNX19 siRNA decreased renal sodium excretion and increased systolic blood pressure (Mock siRNA: 96.8±0.88 mmHg, n=3; SNX19 siRNA: 117.7±5.77 mmHg, n=3). In conclusion, SNX19 contains both flotillin-1 and caveolin-1 binding domains and its binding with flotillin-1 and caveolin-1 are important in SNX19-mediated D 1 R signaling, renal sodium transport, and blood pressure regulation.

Peroxiredoxin‐4 (PRDX4), an endoplasmic reticulum (ER)‐localized antioxidant enzyme, plays an essential role in cellular redox homeostasis by reducing hydrogen peroxide from thiol‐containing compounds. The dopamine D5 Receptor (D5R) also plays a protective role against oxidative stress. We hypothesize that D5R interacts with the PRDX4 to reduce oxidative stress in the kidney. In D5R‐HEK 293 cells, fenoldopam (FEN, 25 nM/12 hr, n=4), a D1R and D5R receptor agonist, increased PRDX4 protein expression (1.92±0.12‐fold over basal level, n=4), mainly in non‐lipid raft (LR) fractions (LRs: 24.9±11.4%, non‐LRs: 75.1±11.4%, baseline; LRs: 30.9±13.9%, non‐LRs: 174.1±16.7%, FEN). By contrast, fenoldopam did not affect PRDX4 protein expression in D1R‐HEK 293 cells, indicating a D5R‐specific effect on PRDX4 protein expression. In human renal proximal tubule cells (hRPTCs) and D5R HEK293 cells, fenoldopam increased the co‐immunoprecipitation between PRDX4 and D5R and their co‐localization in the ER. SiRNA‐mediated silencing of PRDX4 increased hydrogen peroxide production in both the vehicle (Veh)‐ and fenoldopam‐treated hRPTCs [(scrambled siRNA: 100 ±15.1% and 55.2± 7.2% with Veh and FEN, respectively; PRDX4 siRNA: 161.8±15.3% and 145.1±14.6 % with Veh and FEN, respectively, n=4)]. The D5R protects against inflammation and siRNA silencing of PRDX4 increased the production of the pro‐inflammatory cytokines, interleukin‐1β [26.88±3.8 and 46.40±4.2 pg/mL (n=3, D5R‐HEK 293); 15.87±1.2 and 37.9±1.4 pg/mL (n=3, hRPTCs), and tumor necrosis factors [131.7±6.5 and 271.2±18.1 pg/mL (n=4, D5R‐HEK 293); 108.8±11.8 and 240.1±13.7 pg/mL (n=4, hRPTCs)]. In D5R‐HEK293 and hRPTCs, the fenoldopam‐mediated decrease in ER‐resident caspase‐12 was also impaired by gene silencing of PRDX4. Furthermore, PRDX4 protein expression was reduced in the kidney cortices of Drd5‐/‐ mice, relative to Drd5 WT mice (WT: 1.00±0.12, n=4; Drd5‐/‐: 0.64±0.13, n=4; P<0.05). We conclude that D5R positively interacts with PRDX4 to reduce ER stress in the kidney.