Novel Role of Sorting Nexin 5 in Renal D1 Dopamine Receptor Trafficking and Function: Implications for Hypertension

FASEB J. 2013 May;27(5):1808-19.

Van Anthony M. Villar1,2*, Ines Armando1,2*, Hironobu Sanada3, Lauren C. Frazer2, Christen M. Russo2, Patricia M. Notario2, Hewang Lee1,2, Lauren Comisky2, Holly Ann Russell2, Yu Yang1, Julie A. Jurgens1, Pedro A. Jose1,2,4, and John E. Jones1,2

1Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201 (current)

2Department of Pediatrics, Georgetown University School of Medicine,
Washington, DC  20057

3Division of Health Science Research, Fukushima Welfare Federation of Agricultural Cooperatives, Japan

1Department of Physiology, University of Maryland School of Medicine, Baltimore, MD 21201 (current)



The D1 dopamine receptor (D1R) is widely expressed in the kidney and plays a crucial role in blood pressure regulation. Although much is known about D1R desensitization, especially through G protein-coupled receptor kinase 4 (GRK4), comparatively little is known about other aspects of D1R trafficking and the proteins involved in the process. We now report the discovery of a dynamic interaction between sorting nexin 5 (SNX5), a component of the mammalian retromer, and D1R in human renal epithelial cells. We show that internalization of agonist-activated D1R is regulated by both SNX5 and GRK4, and that SNX5 is critical to the recycling of the receptor to the plasma membrane. SNX5 depletion increases agonist-activated D1R phosphorylation (>50% at basal condition), prevents D1R internalization and cAMP response, and delays receptor recycling compared to mock siRNA-transfected controls. Moreover, renal-restricted subcapsular infusion of Snx5-specific siRNA (vs. mock siRNA) decreases sodium excretion (D=-0.2±0.005 mEq/mg creatinine) and further elevates the systolic blood pressure (D=48±5 mm Hg) in spontaneously hypertensive rats, indicating that Snx5 depletion impairs renal D1R function. These studies demonstrate an essential role for SNX5 in regulating D1R function, which may have important diagnostic, prognostic, and therapeutic implications in the management of essential hypertension.

PMID: 23195037



Dopamine is important for the regulation of blood pressure, sodium balance, and renal function (Jose 2003). During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance through its actions on renal dopamine receptors (Armando 2011). In the kidney, dopamine promotes the excretion of excess salt (natriuresis) by inhibiting both proximal and distal tubule NaCl reabsorption. All of the dopamine receptors belong to the family of G protein-coupled receptors (GPCRs) whose signaling is mediated primarily by its interaction with and activation of the heterotrimeric GTP-binding proteins (G proteins). All of these receptor subtypes are expressed in the kidney, highlighting their important homeostatic role on fluid and electrolyte balance.

An essential requirement for the maintenance of homeostasis in any living organism is the ability of the cells to sense the external environment, respond appropriately, and rapidly adapt to changes in the extracellular milieu. Accordingly, many important physiological processes are governed by the coordinated actions of signaling pathways that are mediated by multiple receptors. Achieving this regulation is highly pertinent for many receptors, including the renal dopamine receptors, which are activated during salt-replete states. As with most GPCRs, the normal function of the dopamine D1 receptor (D1R) starts with ligand occupation of the receptor followed by receptor phosphorylation and internalization (desensitization), endosomal transport, and terminates with either degradation or recycling back to the plasma membrane. The contrasting fates of the receptors are critical in dictating the extent of the response, i.e., signal decay (receptor degradation) or propagation (recycling and resensitization). Any perturbation of these steps in receptor trafficking may lead to receptor dysfunction, impaired homeostatic responses, and disease state.

The regulatory mechanisms or proteins involved in D1R trafficking and signaling are neither completely understood nor identified. A few proteins have been identified that interact with and modulate D1R expression or trafficking, including neurofilament-M, NMDA receptors, calnexin, RanBP, and the N-ethylmaleimide factor; however, most of these interactions have been expounded in context with neuronal function. We have reported that GRK4 is required for the proper orientation of the D1R on the plasma membrane (Gildea 2006, 2009), as well as for D1R desensitization and endocytosis (Watanabe 2002) in human renal proximal tubule cells. In this report, we have identified the sorting nexin 5 (SNX5) as a binding partner of D1R and demonstrated the dynamic interaction among SNX5, GRK4, and D1R. SNX5 belongs to the sorting nexin family of proteins that are involved in various aspects of protein trafficking after receptor endocytosis, and is a component of the mammalian retromer.

Based on our results, we have proposed a model whereby D1R, SNX5, and GRK4 dynamically interact in a cohesive signaling complex in lipid raft membrane microdomains of renal proximal tubule cells. We posited that SNX5 may function as a structural/scaffold or an accessory/chaperone protein for D1R and is crucial for its endocytosis and recycling. SNX5 may also impose steric hindrance to GRK4 to limit its access to the phosphorylation sites of D1R. Loss of SNX5 results in impaired D1R endocytosis and delayed receptor recycling. From a functional standpoint, SNX5 loss also results in the blunting of cAMP production in response to agonist activation of D1R, leading to increased renal tubular sodium transport and ultimately to decreased sodium excretion (anti-natriuresis) and increased systemic blood pressure (Figure 1). Our results do not only illustrate a new concept for D1R trafficking in renal epithelial cells but also demonstrate a novel mechanism for the development of hypertension—a mechanism that involves the loss of a trafficking protein, SNX5.

Van Anthony M. Villar-3

FIGURE 1. Simplified Schema on how SNX5 Loss may Result in Hypertension.



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