Sorting nexin 1 loss results in D5 dopamine receptor dysfunction in human renal proximal tubule cells and hypertension in mice

Renal autocrine neuropeptide FF (NPFF) signaling regulates blood pressure

The kidney and brain play critical roles in the regulation of blood pressure. Neuropeptide FF (NPFF), originally isolated from the bovine brain, has been suggested to contribute to the pathogenesis of hypertension. However, the roles of NPFF and its receptors, NPFF-R1 and NPFF-R2, in the regulation of blood pressure, via the kidney, are not known. In this study, we found that the transcripts and proteins of NPFF and its receptors, NPFF-R1 and NPFF-R2, were expressed in mouse and human renal proximal tubules (RPTs). In mouse RPT cells (RPTCs), NPFF, but not RF-amide-related peptide-2 (RFRP-2), decreased the forskolin-stimulated cAMP production in a concentration- and time-dependent manner. Furthermore, dopamine D1-like receptors colocalized and co-immunoprecipitated with NPFF-R1 and NPFF-R2 in human RPTCs. The increase in cAMP production in human RPTCs caused by fenoldopam, a D1-like receptor agonist, was attenuated by NPFF, indicating an antagonistic interaction between NPFF and D1-like receptors. The renal subcapsular infusion of NPFF in C57BL/6 mice decreased renal sodium excretion and increased blood pressure. The NPFF-mediated increase in blood pressure was prevented by RF-9, an antagonist of NPFF receptors. Taken together, our findings suggest that autocrine NPFF and its receptors in the kidney regulate blood pressure, but the mechanisms remain to be determined.

Insights of Endocytosis Signaling in Health and Disease

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.

Sorting nexins: role in the regulation of blood pressure

Sorting nexins (SNXs) are a family of proteins that regulate cellular cargo sorting and trafficking, maintain intracellular protein homeostasis, and participate in intracellular signaling. SNXs are also important in the regulation of blood pressure via several mechanisms. Aberrant expression and dysfunction of SNXs participate in the dysregulation of blood pressure. Genetic studies show a correlation between SNX gene variants and the response to antihypertensive drugs. In this review, we summarize the progress in SNX-mediated regulation of blood pressure, discuss the potential role of SNXs in the pathophysiology and treatment of hypertension, and propose novel strategies for the medical therapy of hypertension.

Sorting nexins as a promising therapeutic target for cardiovascular disorders: An updated overview

Sorting nexins (SNXs) are involved in sorting the protein cargo within the endolysosomal system. Recently, several studies have shown the role of SNXs in cardiovascular pathology. SNXs exert both physiologic and pathologic functions in the cardiovascular system by regulating protein sorting and trafficking, maintaining protein homeostasis, and participating in multiple signaling pathways. SNX deficiency results in blood pressure response to dopamine 5 receptor [D5R] stimulation. SNX knockout protected against atherosclerosis lesions by suppressing foam cell formation. Moreover, SNXs can act as endogenous anti-arrhythmic agents via maintenance of calcium homeostasis. Overexpression SNXs also can reduce cardiac fibrosis in atrial fibrillation. The SNX-STAT3 interaction in cardiac cells promoted heart failure. SNXs may have the potential to act as a pharmacological target against specific cardiovascular diseases.

Increased AT1 receptor expression mediates vasoconstriction leading to hypertension in Snx1-/- mice

Angiotensin II type 1 receptor (AT1R) is a vital therapeutic target for hypertension. Sorting nexin 1 (SNX1) participates in the sorting and trafficking of the renal dopamine D5 receptor, while angiotensin and dopamine are counterregulatory factors in the regulation of blood pressure. The effect of SNX1 on AT1R is not known. We hypothesized that SNX1, through arterial AT1R sorting and trafficking, is involved in blood pressure regulation. CRISPR/Cas9 system-generated SNX1-/- mice showed dramatic elevations in blood pressure compared to their wild-type littermates. The angiotensin II-mediated contractile reactivity of the mesenteric arteries and AT1R expression in the aortas were also increased. Moreover, immunofluorescence and immunoprecipitation analyses revealed that SNX1 and AT1R were colocalized and interacted in the aortas of wild-type mice. In vitro studies revealed that AT1R protein levels and downstream calcium signaling were upregulated in A10 cells treated with SNX1 siRNA. This may have resulted from decreased AT1R protein degradation since the AT1R mRNA levels showed no changes. AT1R protein was less degraded when SNX1 was downregulated, as reflected by a cycloheximide chase assay. Furthermore, proteasomal rather than lysosomal inhibition increased AT1R protein content, and this effect was accompanied by decayed binding of ubiquitin and AT1R after SNX1 knockdown. Confocal microscopy revealed that AT1R colocalized with PSMD6, a proteasomal marker, and the colocalization was reduced after SNX1 knockdown. These findings suggest that SNX1 sorts AT1R for proteasomal degradation and that SNX1 impairment increases arterial AT1R expression, leading to increased vasoconstriction and blood pressure.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

SNX-PXA-RGS-PXC Subfamily of SNXs in the Regulation of Receptor-Mediated Signaling and Membrane Trafficking

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.

Elucidating the Role of Lipid Rafts on G Protein-Coupled Receptor Function in the Mouse Kidney: An In Vivo Approach

Numerous G protein-coupled receptors (GPCRs) and GPCR-signaling molecules reside in lipid rafts and thus, are inherently regulated in these microdomains. However, the limitations of current methods to investigate lipid raft biology and GPCR activity in situ have hindered the complete understanding of the molecular underpinnings of GPCR trafficking and signaling, especially in the whole organism. This book chapter details an innovative in vivo approach to study the crucial role of lipid rafts on the workings of GPCRs in the mouse kidney. This protocol involves the use of a modified mini osmotic pump to deliver an agent that selectively disrupts the lipid raft in the kidney.

Sorting Nexins in Protein Homeostasis

Protein homeostasis is maintained by removing misfolded, damaged, or excess proteins and damaged organelles from the cell by three major pathways; the ubiquitin-proteasome system, the autophagy-lysosomal pathway, and the endo-lysosomal pathway. The requirement for ubiquitin provides a link between all three pathways. Sorting nexins are a highly conserved and diverse family of membrane-associated proteins that not only traffic proteins throughout the cells but also provide a second common thread between protein homeostasis pathways. In this review, we will discuss the connections between sorting nexins, ubiquitin, and the interconnected roles they play in maintaining protein quality control mechanisms. Underlying their importance, genetic defects in sorting nexins are linked with a variety of human diseases including neurodegenerative, cardiovascular diseases, viral infections, and cancer. This serves to emphasize the critical roles sorting nexins play in many aspects of cellular function.

Lipid Rafts and Dopamine Receptor Signaling

The renal dopaminergic system has been identified as a modulator of sodium balance and blood pressure. According to the Centers for Disease Control and Prevention, in 2018 in the United States, almost half a million deaths included hypertension as a primary or contributing cause. Renal dopamine receptors, members of the G protein-coupled receptor family, are divided in two groups: D1-like receptors that act to keep the blood pressure in the normal range, and D2-like receptors with a variable effect on blood pressure, depending on volume status. The renal dopamine receptor function is regulated, in part, by its expression in microdomains in the plasma membrane. Lipid rafts form platforms within the plasma membrane for the organization and dynamic contact of molecules involved in numerous cellular processes such as ligand binding, membrane sorting, effector specificity, and signal transduction. Understanding all the components of lipid rafts, their interaction with renal dopamine receptors, and their signaling process offers an opportunity to unravel potential treatment targets that could halt the progression of hypertension, chronic kidney disease (CKD), and their complications.

Sorting nexin 1 loss results in increased oxidative stress and hypertension

Acute renal depletion of sorting nexin 1 (SNX1) in mice results in blunted natriuretic response and hypertension due to impaired dopamine D₅ receptor (D₅ R) activity. We elucidated the molecular mechanisms for these phenotypes in Snx1^-/- mice. These mice had increased renal expressions of angiotensin II type 1 receptor (AT₁ R), NADPH oxidase (NOX) subunits, D₅ R, and NaCl cotransporter. Basal reactive oxygen species (ROS), NOX activity, and blood pressure (BP) were also higher in Snx1^-/- mice, which were normalized by apocynin, a drug that prevents NOX assembly. Renal proximal tubule (RPT) cells from hypertensive (HT) Euro-American males had deficient SNX1 activity, impaired D₅ R endocytosis, and increased ROS compared with cells from normotensive (NT) Euro-American males. siRNA-mediated depletion of SNX1 in RPT cells from NT subjects led to a blunting of D₅ R agonist-induced increase in cAMP production and decrease in Na⁺ transport, effects that were normalized by over-expression of SNX1. Among HT African-Americans, three of the 12 single nucleotide polymorphisms interrogated for the SNX1 gene were associated with a decrease in systolic BP in response to hydrochlorothiazide (HCTZ). The results illustrate a new paradigm for the development of hypertension and imply that the trafficking protein SNX1 may be a crucial determinant for hypertension and response to antihypertensive therapy.

Lipid rafts are required for effective renal D1 dopamine receptor function

Effective receptor signaling is anchored on the preferential localization of the receptor in lipid rafts, which are plasma membrane platforms replete with cholesterol and sphingolipids. We hypothesized that the dopamine D₁ receptor (D₁ R) contains structural features that allow it to reside in lipid rafts for its activity. Mutation of C347 palmitoylation site and Y218 of a newly identified Cholesterol Recognition Amino Acid Consensus motif resulted in the exclusion of D₁ R from lipid rafts, blunted cAMP response, impaired sodium transport, and increased oxidative stress in renal proximal tubule cells (RPTCs). Kidney-restricted silencing of Drd1 in C57BL/6J mice increased blood pressure (BP) that was normalized by renal tubule-restricted rescue with D₁ R-wild-type but not the mutant D₁ R 347A that lacks a palmitoylation site. Kidney-restricted disruption of lipid rafts by β-MCD jettisoned the D₁ R from the brush border, decreased sodium excretion, and increased oxidative stress and BP in C57BL/6J mice. Deletion of the PX domain of the novel D₁ R-binding partner sorting nexin 19 (SNX19) resulted in D₁ R partitioning solely to non-raft domains, while silencing of SNX19 impaired D₁ R function in RPTCs. Kidney-restricted silencing of Snx19 resulted in hypertension in C57BL/6J mice. Our results highlight the essential role of lipid rafts for effective D₁ R signaling.

Role of Thioredoxin 1 in Impaired Renal Sodium Excretion of hD 5 R F173L Transgenic Mice

Background Dopamine D5 receptor (D5R) plays an important role in the maintenance of blood pressure by regulating renal sodium transport. Our previous study found that human D5R mutant F173L transgenic ( hD 5 R F173L-TG) mice are hypertensive. In the present study, we aimed to investigate the mechanisms causing this renal D5R dysfunction in hD 5 R F173L-TG mice. Methods and Results Compared with wild-type D5R-TG ( hD 5 R WT-TG) mice, hD 5 R F173L-TG mice have higher blood pressure, lower basal urine flow and sodium excretion, and impaired agonist-mediated natriuresis and diuresis. Enhanced reactive oxygen species production in hD 5 R F173L-TG mice is caused, in part, by decreased expression of antioxidant enzymes, including thioredoxin 1 (Trx1). Na+-K+-ATPase activity is increased in mouse renal proximal tubule cells transfected with hD 5 R F173L, but is normalized by treatment with exogenous recombinant human Trx1 protein. Regulation of Trx1 by D5R occurs by the phospholipase C/ protein kinase C (PKC) pathway because upregulation of Trx1 expression by D5R does not occur in renal proximal tubule cells from D1R knockout mice in the presence of a phospholipase C or PKC inhibitor. Fenoldopam, a D1R and D5R agonist, stimulates PKC activity in primary renal proximal tubule cells of hD5R WT -TG mice, but not in those of hD 5 R F173L-TG mice. Hyperphosphorylation of hD5RF173L and its dissociation from Gαs and Gαq are associated with impairment of D5R-mediated inhibition of Na+-K+-ATPase activity in hD 5 R F173L-TG mice. Conclusions These suggest that hD 5 R F173L increases blood pressure, in part, by decreasing renal Trx1 expression and increasing reactive oxygen species production. Hyperphosphorylation of hD5RF173L, with its dissociation from Gαs and Gαq, is the key factor in impaired D5R function of hD 5 R F173L-TG mice.

The emerging role of sorting nexins in cardiovascular diseases

The sorting nexin (SNX) family consists of a diverse group of cytoplasmic- and membrane-associated phosphoinositide-binding proteins that play pivotal roles in the regulation of protein trafficking. This includes the entire endocytic pathway, such as endocytosis, endosomal sorting, and endosomal signaling. Dysfunctions of SNX pathway are involved in several forms of cardiovascular disease (CVD). Moreover, SNX gene variants are associated with CVDs. In this review, we discuss the current knowledge on SNX-mediated regulatory mechanisms and their roles in the pathogenesis and treatment of CVDs.

G protein-coupled receptor 37L1 regulates renal sodium transport and blood pressure

G protein-coupled receptors (GPCRs) in the kidney regulate the reabsorption of essential nutrients, ions, and water from the glomerular filtrate. Abnormalities in renal epithelial ion transport play important roles in the pathogenesis of essential hypertension. The orphan G protein-coupled receptor 37L1 (GPR37L1), also known as endothelin receptor type B-like protein (ETBR-LP2), is expressed in several regions in the brain, but its expression profile and function in peripheral tissues are poorly understood. We found that GPR37L1 mRNA expression is highest in the brain, followed by the stomach, heart, testis, and ovary, with moderate expression in the kidney, pancreas, skeletal muscle, liver, lung, and spleen. Immunofluorescence analyses revealed the expression of GPR37L1 in specific regions within some organs. In the kidney, GPR37L1 is expressed in the apical membrane of renal proximal tubule cells. In human renal proximal tubule cells, the transient expression of GPR37LI increased intracellular sodium, whereas the silencing of GPR37LI decreased intracellular sodium. Inhibition of Na⁺/H⁺ exchanger isoform 3 (NHE3) activity abrogated the GPR37L1-mediated increase in intracellular sodium. Renal-selective silencing of Gpr37l1 in mice increased urine output and sodium excretion and decreased systolic and diastolic blood pressures. The renal-selective silencing of GPR37L1 decreased the protein expression of NHE3 but not the expression of Na⁺-K⁺-ATPase or sodium-glucose cotransporter 2. Our findings show that in the kidney, GPR37L1 participates in renal proximal tubule luminal sodium transport and regulation of blood pressure by increasing the renal expression and function of NHE3 by decreasing cAMP production. The role of GPR37L1, expressed in specific cell types in organs other than the kidney, remains to be determined.

Sorting nexin-1 is a candidate tumor suppressor and potential prognostic marker in gastric cancer

Sorting nexin-1 (SNX1) is an important functional protein in cell endocytosis, efflux, protein sorting, cell signal transduction, etc; however, the expression, the role and clinical relevance of SNX1 have not been investigated in gastric cancer (GC). In this study, we first performed a bioinformatics investigation using the data obtained from The Cancer Genome Atlas (TCGA) database. The result showed that SNX1 mRNA levels were significantly lower in GC tissues than in paracancerous tissues. In a study of 150 cases of GC, including 60 cases with paired paracancerous and cancer tissues and 90 cases with detailed follow-up information, SNX1 expression was analyzed by immunohistochemistry. Our study on paired paracancerous and cancer tissues showed that SNX1 protein expression remarkably decreased in GC tissues (50/60, 83.33%). A study on 90 patients with detailed follow-up information showed that tumors with higher SNX1 protein level were correlated with better clinicopathologic stages (p = 0.0285), nodal status (p = 0.0286), smaller tumor sizes (p = 0.0294) and a better survival rate in patients with GC (p = 0.0245). Univariate analysis of the 90 patients with GC showed that low-level SNX1 was significantly correlated with decreased overall survival of GC patients (p = 0.008), and associated with a relatively higher cumulative hazard of death. Exogenous expression of SNX1 inhibited the growth, migration, invasion and promoted the apoptosis and enhanced the sensitivity of GC cells to the chemotherapeutic drug 5-Fluorouracil (5-Fu) in vitro, while knockdown of SNX1 by short hairpin RNA (shRNA) significantly promoted the growth, migration, invasion and reduced the apoptosis and the sensitivity of GC cells to 5-Fu. SNX1 also showed to influence the levels of epithelial-mesenchymal transition markers including Vimentin, Snail, and E-cadherin in GC cells in vitro. Taken together, we propose here that SNX1 serves as a tumor suppressor and prognostic marker that reduces tumor cell malignancy for GC.

Loss of renal SNX5 results in impaired IDE activity and insulin resistance in mice

AIMS/HYPOTHESIS

We hypothesised that renal sorting nexin 5 (SNX5) regulates the insulin-degrading enzyme (IDE) and, thus, circulating insulin levels. We therefore studied the dynamic interaction between SNX5 and IDE in human renal proximal tubule cells (hRPTCs), as well as in rat and mouse kidneys.

METHODS

The regulation of IDE by SNX5 expressed in the kidney was studied in vitro and in vivo. Snx5 or mock siRNA was added to immortalised hRPTCs (passage <20) in culture or selectively infused, via osmotic mini-pump, into the remnant kidney of uninephrectomised mice and rats.

RESULTS

SNX5 co-localised with IDE at the plasma membrane and perinuclear area of hRPTCs and in the brush border membrane of proximal tubules of human, rat, and mouse kidneys. Insulin increased the co-localisation and co-immunoprecipitation of SNX5 and IDE in hRPTCs. Silencing SNX5 in hRPTCs decreased IDE expression and activity. Renal-selective silencing of Snx5 (SNX5 protein: 100 ± 25 vs 29 ± 10, p < 0.05 [% of control]) in C57Bl/6J mice decreased IDE protein (100 ± 13 vs 57 ± 6, p < 0.05 [% of control]) and urinary insulin excretion, impaired the responses to insulin and glucose, and increased blood insulin and glucose levels. Spontaneously hypertensive rats (SHRs) had increased blood insulin and glucose levels and decreased renal SNX5 (100 ± 27 vs 29 ± 6, p < 0.05 [% of control]) and IDE (100 ± 5 vs 75 ± 4, p < 0.05 [% of control]) proteins, compared with normotensive Wistar-Kyoto (WKY) rats. Kidney Snx5-depleted WKY rats also had increased blood insulin and glucose levels. The expression of SNX5 and IDE was decreased in RPTCs from SHRs and hypertensive humans compared with cells from normotensive volunteers, indicating a common cause for hyperinsulinaemia and hypertension.

CONCLUSIONS/INTERPRETATION

Renal SNX5 positively regulates IDE expression and function. This study is the first to demonstrate the novel and crucial role of renal SNX5 in insulin and glucose metabolism.

Proteomic profile and toxicity pathway analysis in zebrafish embryos exposed to bisphenol A and di-n-butyl phthalate at environmentally relevant levels

Bisphenol A (BPA) and di-n-butyl phthalate (DBP) are well-known endocrine-disrupting chemicals (EDCs) that have human health risks. Chronic exposure to BPA and DBP increases the occurrence of human disease. Despite the potential for exposure in embryonic development, the mechanism of action of BPA and DBP on vertebrate development and disease still remains unclear. In the present study, we identified proteins and protein networks that are perturbed by BPA and DBP during zebrafish (Danio rerio) development. Zebrafish embryos were exposed to environmentally relevant levels of BPA (10 μg/L) and DBP (50 μg/L) for 96 h. By iTRAQ labeling quantitative proteomics, a set of 26 and 41 differentially expressed proteins were identified in BPA- and DBP-treated zebrafish embryos, respectively. Integrated toxicity analysis predicted that these proteins function in common regulatory networks that are significantly associated with developmental and metabolic disorders. Exposure to low concentrations of BPA and DBP has potential health risks in zebrafish embryos. Our results also show that BPA and DBP significantly up-regulate the expression levels of multiple network proteins, providing valuable information about the molecular actions of BPA and DBP on the developmental systems.

Caspase-mediated proteolysis of the sorting nexin 2 disrupts retromer assembly and potentiates Met/hepatocyte growth factor receptor signaling

The unfolding of apoptosis involves the cleavage of hundreds of proteins by the caspase family of cysteinyl peptidases. Among those substrates are proteins involved in intracellular vesicle trafficking with a net outcome of shutting down the crucial processes governing protein transport to organelles and to the plasma membrane. However, because of the intertwining of receptor trafficking and signaling, cleavage of specific proteins may lead to unintended consequences. Here we show that in apoptosis, sorting nexin 1 and 2 (SNX1 and SNX2), two proteins involved in endosomal sorting, are cleaved by initiator caspases and also by executioner caspase-6 in the case of SNX2. Moreover, SNX1 is cleaved at multiple sites, including following glutamate residues. Cleavage of SNX2 results in a loss of association with the endosome-to-trans-Golgi network transport protein Vps35 and in a delocalization from endosomes of its associated partner Vps26. We also demonstrate that SNX2 depletion causes an increase in hepatocyte growth factor receptor tyrosine phosphorylation and Erk1/2 signaling in cells. Finally, we show that SNX2 mRNA and protein levels are decreased in colorectal carcinoma and that lower SNX2 gene expression correlates with an increase in cancer patient mortality. Our study reveals the importance to characterize the cleavage fragments produced by caspases of specific death substrates given their potential implication in the mechanism of regulation of physiological (signaling/trafficking) pathways or in the dysfunction leading to pathogenesis.

Renal rescue of dopamine D2 receptor function reverses renal injury and high blood pressure

Dopamine D2 receptor (DRD2) deficiency increases renal inflammation and blood pressure in mice. We show here that long-term renal-selective silencing of Drd2 using siRNA increases renal expression of proinflammatory and profibrotic factors and blood pressure in mice. To determine the effects of renal-selective rescue of Drd2 expression in mice, the renal expression of DRD2 was first silenced using siRNA and 14 days later rescued by retrograde renal infusion of adeno-associated virus (AAV) vector with DRD2. Renal Drd2 siRNA treatment decreased the renal expression of DRD2 protein by 55%, and DRD2 AAV treatment increased the renal expression of DRD2 protein by 7.5- to 10-fold. Renal-selective DRD2 rescue reduced the expression of proinflammatory factors and kidney injury, preserved renal function, and normalized systolic and diastolic blood pressure. These results demonstrate that the deleterious effects of renal-selective Drd2 silencing on renal function and blood pressure were rescued by renal-selective overexpression of DRD2. Moreover, the deleterious effects of 45-minute bilateral ischemia/reperfusion on renal function and blood pressure in mice were ameliorated by a renal-selective increase in DRD2 expression by the retrograde ureteral infusion of DRD2 AAV immediately after the induction of ischemia/reperfusion injury. Thus, 14 days after ischemia/reperfusion injury, the renal expression of profibrotic factors, serum creatinine, and blood pressure were lower in mice infused with DRD2 AAV than in those infused with control AAV. These results indicate an important role of renal DRD2 in limiting renal injury and preserving normal renal function and blood pressure.

Human GRK4γ142V Variant Promotes Angiotensin II Type I Receptor-Mediated Hypertension via Renal Histone Deacetylase Type 1 Inhibition

The influence of a single gene on the pathogenesis of essential hypertension may be difficult to ascertain, unless the gene interacts with other genes that are germane to blood pressure regulation. G-protein-coupled receptor kinase type 4 (GRK4) is one such gene. We have reported that the expression of its variant hGRK4γ(142V) in mice results in hypertension because of impaired dopamine D1 receptor. Signaling through dopamine D1 receptor and angiotensin II type I receptor (AT1R) reciprocally modulates renal sodium excretion and blood pressure. Here, we demonstrate the ability of the hGRK4γ(142V) to increase the expression and activity of the AT1R. We show that hGRK4γ(142V) phosphorylates histone deacetylase type 1 and promotes its nuclear export to the cytoplasm, resulting in increased AT1R expression and greater pressor response to angiotensin II. AT1R blockade and the deletion of the Agtr1a gene normalize the hypertension in hGRK4γ(142V) mice. These findings illustrate the unique role of GRK4 by targeting receptors with opposite physiological activity for the same goal of maintaining blood pressure homeostasis, and thus making the GRK4 a relevant therapeutic target to control blood pressure.

Antihypertensive mechanisms of intra-renal dopamine

PURPOSE OF REVIEW

This review will highlight recent findings concerning the regulation and signalling of the intrarenal dopaminergic system and the emerging evidence for its importance in blood pressure regulation.

RECENT FINDINGS

There is an increasing evidence that the intrarenal dopaminergic system plays an important role in the regulation of blood pressure, and defects in dopamine signalling appear to be involved in the development of hypertension. Recent experimental models have definitively demonstrated that abnormalities in intrarenal dopamine production or receptor signalling can predispose to salt-sensitive hypertension and a dysregulated renin-angiotensin system. There are also new results indicating the importance of dopamine receptor mediated regulation of salt and water homeostasis along the nephron, and new studies indicating the role that the intrarenal dopaminergic system plays to mitigate the production of reactive oxygen species and progression of chronic renal disease.

SUMMARY

New studies underscore the importance of the intrarenal dopaminergic system in the regulation of renal function and indicate how alterations in dopamine production or signalling may underlie the development of hypertension and kidney injury.

Sorting nexin 5 and dopamine d1 receptor regulate the expression of the insulin receptor in human renal proximal tubule cells

Sorting nexin 5 (SNX5) belongs to the SNX family, which is composed of a diverse group of proteins that mediate trafficking of plasma membrane proteins, receptors, and transporters. SNX5 is important in the resensitization of the dopamine D1-like receptor (D1R). D1R is uncoupled from its effector proteins in hypertension and diabetes, and treatment of diabetes restores D1R function and insulin receptor (IR) expression. We tested the hypothesis that the D1R and SNX5 regulate IR by studying the expression, distribution, dynamics, and functional consequences of their interaction in human renal proximal tubule cells (hRPTCs). D1R, SNX5, and IR were expressed and colocalized in the brush border of RPTs. Insulin promoted the colocalization of SNX5 and IR at the perinuclear area of hRPTCs. Unlike SNX5, the D1R colocalized and coimmunoprecipitated with IR, and this interaction was enhanced by insulin. To evaluate the role of SNX5 and D1R on IR signaling, we silenced via RNA interference the endogenous expression of SNX5 or the D1R gene DRD1 in hRPTCs. We observed a decrease in IR expression and abundance of phosphorylated IR substrate and phosphorylated protein kinase B, which are crucial components of the IR signal transduction pathway. Our data indicate that SNX5 and D1R are necessary for normal IR expression and activity. It is conceivable that D1R and SNX5 may interact to increase the sensitivity to insulin via a positive regulation of IR and insulin signaling.

Dopamine D₁-like receptors regulate the α₁A-adrenergic receptor in human renal proximal tubule cells and D₁-like dopamine receptor knockout mice

The homeostatic control of blood pressure hinges upon the delicate balance between prohypertensinogenic and antihypertensinogenic systems. D₁-like dopamine receptors [dopamine D₁ and D₅ receptors (D₁Rs and D₅Rs, respectively)] and the α₁A-adrenergic receptor (α₁A-AR) are expressed in the renal proximal tubule and engender opposing effects on Na(+) transport, i.e., natriuresis (via D₁Rs and D5Rs) or antinatriuresis (via α₁A-ARs). We tested the hypothesis that the D₁R/D₅R regulates the α₁A-AR. D₁-like dopamine receptors coimmunoprecipitated, colocalized, and cofractionated with α₁A-ARs in lipid rafts in immortalized human renal proximal tubule cells. Long-term treatment with the D₁R/D₅R agonist fenoldopam resulted in decreased D₁R and D₅R expression but increased α₁A-AR abundance in the plasma membrane. Short-term fenoldopam treatment stimulated the translocation of Na(+)-K(+)-ATPase from the plasma membrane to the cytosol that was partially reversed by an α₁A-AR agonist, which by itself induced Na(+)-K(+)-ATPase translocation from the cytosol to the plasma membrane. The α₁A-AR-specific agonist A610603 also minimized the ability of fenoldopam to inhibit Na(+)-K(+)-ATPase activity. To determine the interaction among D₁Rs, D₅Rs, and α₁A-ARs in vivo, we used phenylephrine and A610603 to decrease Na(+) excretion in several D1-like dopamine receptor knockout mouse strains. Phenylephrine and A61603 treatment resulted in a partial reduction of urinary Na(+) excretion in wild-type mice and its abolition in D1R knockout, D₅R knockout, and D₁R-D₅R double-knockout mice. Our results demonstrate the ability of the D₁-like dopamine receptors to regulate the expression and activity of α₁A-AR. Elucidating the intricacies of the interaction among these receptors is crucial for a better understanding of the crosstalk between anti- and pro-hypertensive systems.

Dopamine D3 receptor inhibits the ubiquitin-specific peptidase 48 to promote NHE3 degradation

The dopamine D3 receptor (D3R) is crucial in the regulation of blood pressure and sodium balance, in that Drd3 gene ablation in mice results in hypertension and failure to excrete a dietary salt load. The mechanism responsible for the renal sodium retention in these mice is largely unknown. We now offer and describe a novel mechanism by which D3R decreases sodium transport in the long term by inhibiting the deubiquitinylating activity of ubiquitin-specific peptidase 48 (USP48), thereby promoting Na(+)-H(+) exchanger (NHE)-3 degradation. We found that stimulation with the D3R-specific agonist PD128907 (1 μM, 30 min) promoted the interaction and colocalization among D3R, NHE3, and USP48; inhibited USP48 activity (-35±6%, vs. vehicle), resulting in increased ubiquitinylated NHE3 (+140±10%); and decreased NHE3 expression (-50±9%) in human renal proximal tubule cells (hRPTCs). USP48 silencing decreased NHE3's half-life (USP48 siRNA t1/2=6.1 h vs. vehicle t1/2=12.9 h), whereas overexpression of USP48 increased NHE3 half-life (t1/2=21.8 h), indicating that USP48 protects NHE3 from degradation via deubiquitinylation. USP48 accounted for ∼30% of the total deubiquitinylating activity in these cells. Extending our studies in vivo, we found that pharmacologic blockade of D3R via the D3R-specific antagonist GR103691 (1 μg/kg/min, 4 d) in C57Bl/6J mice increased renal NHE3 expression (+310±15%, vs. vehicle), whereas an innovative kidney-restricted Usp48 silencing via siRNA (3 μg/d, 7 d) increased ubiquitinylated NHE3 (+250±30%, vs. controls), decreased total NHE3 (-23±2%), and lowered blood pressure (-24±2 mm Hg), compared with that in control mice that received either the vehicle or nonsilencing siRNA. Our data demonstrate a crucial role for the dynamic interaction between D3R and USP48 in the regulation of NHE3 expression and function.