School of Medicine and Health Sciences Poster Presentations

Title

Molecular mechanisms underlying the GRK4 65L-mediated hypertension in mice

Document Type

Poster

Abstract Category

Basic Biomedical Sciences

Keywords

Kidney, salt sensitivity, dopamine receptor, sodium transporter, GRK4

Publication Date

Spring 5-1-2019

Abstract

The genetic causes of salt sensitivity and hypertension in humans are not completely understood. The kidney plays a preeminent regulatory role in blood pressure (BP) homeostasis and water and electrolyte balance. The renal dopamine receptors, D1R and D3R, engender natriuresis via the inhibition of renal Na+ transport, whereas the angiotensin II type 1 receptor (AT1R) does the opposite. The renal paracrine inhibition of Na+ transport by dopamine is impaired in salt-sensitive rats, mice, and humans. Agonist activation promotes the phosphorylation of D1R and D3R by the G protein-coupled receptor kinase type 4 (GRK4), whose gene variants impair D1R and D3R activity. The global expression of GRK4 65R>L in transgenic mice results in salt-sensitive hypertension, in part, due to increased endogenous GRK4 and AT1R expression. To demonstrate the specific renal causal mechanisms in GRK4 65R>L-mediated hypertension, we heterologously expressed the GRK4 65R>L vs. GRK4 wild-type (WT) transgenes in the kidneys of Grk4 knockout mice on normal salt diet. The transgenes were delivered selectively into the renal tubules by the bilateral retrograde ureteral infusion of AAV-9 vectors. The expression and distribution along the entire nephron of the GRK4 WT and GRK4 65R>L were similar in both groups. However, the renal tubule-restricted expression of GRK4 65R>L increased the BP (117±4 vs. 93±1 mm Hg, P<0.05, n=4), while that of the GRK4 WT only tended to increase the BP (105±6 vs. 96±2 mm Hg, n=5), indicating that the presence of the GRK4 variant in the kidney caused the increase in BP. We next evaluated the renal expression profiles of select genes. We found that the pro-natriuretic D1R (0.81±0.01 vs. 1.28±0.04, P<0.01) and D3R (0.44±0.02 vs. 1.27±0.07, P<0.01) were decreased whereas the anti-natriuretic Na+/K+-ATPase (1.14±0.024 vs. 1.0 ±0.007, P<0.05) and alpha-ENaC (1.4±0.14 vs. 1.0±0.11, P<0.05) were increased, demonstrating the mechanistic changes that underlie the hypertension in these mice. Interestingly, we also observed that the proximal tubule Na+ transporters NaPi2 (0.81±0.02 vs. 1.04±0.02), SGLT2 (0.89±0.03 vs. 1.07±0.05), NBCe2 (0.50±0.07 vs. 1.15±0.03), and the AT1R (0.82±0.02 vs 1.02±0.02) were decreased, which may represent insufficient compensatory mechanisms against the increase in BP. Our results highlight the underlying and compensatory renal mechanisms for the hypertension that developed in mice with either kidney-restricted or globally expressed GRK4 65R>L.

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Presented at Research Days 2019.

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Molecular mechanisms underlying the GRK4 65L-mediated hypertension in mice

The genetic causes of salt sensitivity and hypertension in humans are not completely understood. The kidney plays a preeminent regulatory role in blood pressure (BP) homeostasis and water and electrolyte balance. The renal dopamine receptors, D1R and D3R, engender natriuresis via the inhibition of renal Na+ transport, whereas the angiotensin II type 1 receptor (AT1R) does the opposite. The renal paracrine inhibition of Na+ transport by dopamine is impaired in salt-sensitive rats, mice, and humans. Agonist activation promotes the phosphorylation of D1R and D3R by the G protein-coupled receptor kinase type 4 (GRK4), whose gene variants impair D1R and D3R activity. The global expression of GRK4 65R>L in transgenic mice results in salt-sensitive hypertension, in part, due to increased endogenous GRK4 and AT1R expression. To demonstrate the specific renal causal mechanisms in GRK4 65R>L-mediated hypertension, we heterologously expressed the GRK4 65R>L vs. GRK4 wild-type (WT) transgenes in the kidneys of Grk4 knockout mice on normal salt diet. The transgenes were delivered selectively into the renal tubules by the bilateral retrograde ureteral infusion of AAV-9 vectors. The expression and distribution along the entire nephron of the GRK4 WT and GRK4 65R>L were similar in both groups. However, the renal tubule-restricted expression of GRK4 65R>L increased the BP (117±4 vs. 93±1 mm Hg, P<0.05, n=4), while that of the GRK4 WT only tended to increase the BP (105±6 vs. 96±2 mm Hg, n=5), indicating that the presence of the GRK4 variant in the kidney caused the increase in BP. We next evaluated the renal expression profiles of select genes. We found that the pro-natriuretic D1R (0.81±0.01 vs. 1.28±0.04, P<0.01) and D3R (0.44±0.02 vs. 1.27±0.07, P<0.01) were decreased whereas the anti-natriuretic Na+/K+-ATPase (1.14±0.024 vs. 1.0 ±0.007, P<0.05) and alpha-ENaC (1.4±0.14 vs. 1.0±0.11, P<0.05) were increased, demonstrating the mechanistic changes that underlie the hypertension in these mice. Interestingly, we also observed that the proximal tubule Na+ transporters NaPi2 (0.81±0.02 vs. 1.04±0.02), SGLT2 (0.89±0.03 vs. 1.07±0.05), NBCe2 (0.50±0.07 vs. 1.15±0.03), and the AT1R (0.82±0.02 vs 1.02±0.02) were decreased, which may represent insufficient compensatory mechanisms against the increase in BP. Our results highlight the underlying and compensatory renal mechanisms for the hypertension that developed in mice with either kidney-restricted or globally expressed GRK4 65R>L.