Gary V. Desir

Renalase is a novel, soluble monoamine oxidase that regulates cardiac function and blood pressure

The kidney not only regulates fluid and electrolyte balance but also functions as an endocrine organ. For instance, it is the major source of circulating erythropoietin and renin. Despite currently available therapies, there is a marked increase in cardiovascular morbidity and mortality among patients suffering from end-stage renal disease. We hypothesized that the current understanding of the endocrine function of the kidney was incomplete and that the organ might secrete additional proteins with important biological roles. Here we report the identification of a novel flavin adenine dinucleotide-dependent amine oxidase (renalase) that is secreted into the blood by the kidney and metabolizes catecholamines in vitro (renalase metabolizes dopamine most efficiently, followed by epinephrine, and then norepinephrine). In humans, renalase gene expression is highest in the kidney but is also detectable in the heart, skeletal muscle, and the small intestine. The plasma concentration of renalase is markedly reduced in patients with end-stage renal disease, as compared with healthy subjects. Renalase infusion in rats caused a decrease in cardiac contractility, heart rate, and blood pressure and prevented a compensatory increase in peripheral vascular tone. These results identify renalase as what we believe to be a novel amine oxidase that is secreted by the kidney, circulates in blood, and modulates cardiac function and systemic blood pressure.

International Union of Pharmacology. XLI. Compendium of Voltage-Gated Ion Channels: Potassium Channels

This summary article presents an overview of the molecular relationships among the voltage-gated potassium channels and a standard nomenclature for them, which is derived from the IUPHAR Compendium of Voltage-Gated Ion Channels.1 The complete Compendium, including data tables for each member of the potassium channel family can be found at http://www.iuphar-db.org/iuphar-ic/.

Catecholamines Regulate the Activity, Secretion, and Synthesis of Renalase

Background— We previously identified renalase, a secreted novel amine oxidase that specifically degrades circulating catecholamines. Parenteral administration of either native or recombinant renalase lowers blood pressure, heart rate, and cardiac contractility by metabolizing circulating catecholamines. Renalase plasma levels are markedly reduced in patients with chronic kidney disease. It is not known whether endogenous renalase contributes to the regulation of catecholamines. Methods and Results— We show here that circulating renalase lacks significant amine oxidase activity under basal conditions (prorenalase) but that a brief surge of epinephrine lasting <2 minutes causes renalase activity to increase from 48±18 to 2246±98 arbitrary units (n=3; P<0.002). Enzyme activation is detectable within 30 seconds and sustained for at least 60 minutes. Analysis of epinephrine-mediated hemodynamic changes in normotensive rats indicates that prorenalase becomes maximally activated when systolic pressure increases by >5 mm Hg. The catecholamine surge also leads to a 2.8-fold increase in plasma renalase concentration. Cultured cells exposed to dopamine upregulate steady-state renalase gene expression by >10-fold. The time course of prorenalase activation is abnormal in rats with chronic kidney disease. Conclusions— These data identify a novel mechanism for the regulation of circulating catecholamines. In the renalase pathway, excess catecholamine facilitates the conversion of prorenalase, an inactive plasma amine oxidase, to renalase, which can degrade catecholamines. Excess catecholamines not only regulate the activation of prorenalase but also promote its secretion and synthesis. Because chronic kidney disease is associated with a number of systemic abnormalities, including activation of the sympathetic nervous system, increased catecholamines levels, cardiac hypertrophy, and hypertension, renalase replacement is an attractive therapeutic modality owing to its role in catecholamine metabolism.

Renalase deficiency aggravates ischemic myocardial damage

Chronic kidney disease (CKD) leads to an 18-fold increase in cardiovascular complications not fully explained by traditional risk factors. Levels of renalase, a recently discovered oxidase that metabolizes catecholamines, are decreased in CKD. Here we show that renalase deficiency in a mouse knockout model causes increased plasma catecholamine levels and hypertension. Plasma blood urea nitrogen, creatinine, and aldosterone were unaffected. However, knockout mice had normal systolic function and mild ventricular hypertrophy but tolerated cardiac ischemia poorly and developed myocardial necrosis threefold more severe than that found in wild-type mice. Treatment with recombinant renalase completely rescued the cardiac phenotype. To gain insight into the mechanisms mediating this cardioprotective effect, we tested if gene deletion affected nitrate and glutathione metabolism, but found no differences between hearts of knockout and wild-type mice. The ratio of oxidized (NAD) to reduced (NADH) nicotinamide adenine dinucleotide in cardiac tissue, however, was significantly decreased in the hearts of renalase knockout mice, as was plasma NADH oxidase activity. In vitro studies confirmed that renalase metabolizes NADH and catecholamines. Thus, renalase plays an important role in cardiovascular pathology and its replacement may reduce cardiac complications in renalase-deficient states such as CKD.

Regulation of blood pressure and cardiovascular function by renalase

The renalase pathway is a previously unrecognized mechanism for regulating cardiac function and blood pressure. In this pathway, renalase, a novel secreted amine oxidase that is inactive at baseline, is rapidly turned on ( ~ 10 fold increase) by either a modest increase in blood pressure or by brief surges in plasma catecholamines. The active enzyme degrades circulating catecholamines, causing a significant fall in blood pressure. Plasma catecholamines not only activate renalase enzymatic activity but also lead to a 3-4 fold stimulation of renalase secretion. The renalase knockout mouse (KO) is hypertensive and exquisitely sensitive to cardiac ischemia. Abnormalities in the renalase pathway are present in animal models of chronic kidney disease (CKD) and hypertension. Two single-nucleotide polymorphisms (SNPs) in the renalase gene were found to be associated with essential hypertension in man. Blood renalase levels are inversely correlated with glomerular filtration rate (GFR) and are markedly reduced in patients with end-stage kidney disease (ESRD). We hypothesize that renalase is secreted into blood by the kidney (although also expressed in heart, skeletal muscle, and small intestine) and plays a key role in regulating blood pressure and cardiovascular function, and that abnormalities in the renalase pathway contribute to the heightened cardiovascular risks observed in patients with CKD.

Renalase Lowers Ambulatory Blood Pressure by Metabolizing Circulating Adrenaline

Background Blood pressure is acutely regulated by the sympathetic nervous system through the action of vasoactive hormones such as epinephrine, norepinephrine, and dopamine. Renalase, a recently described, secreted flavoprotein, acutely decreases systemic pressure when administered in vivo. Single‐nucleotide polymorphisms present in the gene are associated with hypertension, cardiac disease, and diabetes. Although renalases crystal structure was recently solved, its natural substrate(s) remains undefined. Methods and Results Using in vitro enzymatic assays and in vivo administration of recombinant renalase, we show that the protein functions as a flavin adenine dinucleotide– and nicotinamide adenine dinucleotide–dependent oxidase that lowers blood pressure by degrading plasma epinephrine. The enzyme also metabolizes the dopamine precursor l‐3,4‐dihydroxyphenylalanine but has low activity against dopamine and does not metabolize norepinephrine. To test if epinephrine and l‐3,4‐dihydroxyphenylalanine were renalases only substrates, 17 246 unique small molecules were screened. Although the search revealed no additional, naturally occurring compounds, it identified dobutamine, isoproterenol, and α‐methyldopa as substrates of renalase. Mutational analysis was used to test if renalases hypotensive effect correlated with its enzymatic activity. Single–amino acid mutations that decrease its enzymatic activity to varying degrees comparably reduce its hypotensive effect. Conclusions Renalase metabolizes circulating epinephrine and l‐3,4‐dihydroxyphenylalanine, and its capacity to decrease blood pressure is directly correlated to its enzymatic activity. These findings highlight a previously unrecognized mechanism for epinephrine metabolism and blood pressure regulation, expand our understanding of the sympathetic nervous system, and could lead to the development of novel therapeutic modalities for the treatment of hypertension. (J Am Heart Assoc. 2012;1:e002634 doi: 10.1161/JAHA.112.002634.)

A Functional Polymorphism in Renalase (Glu37Asp) Is Associated with Cardiac Hypertrophy, Dysfunction, and Ischemia: Data from the Heart and Soul Study

Background Renalase is a soluble enzyme that metabolizes circulating catecholamines. A common missense polymorphism in the flavin-adenine dinucleotide-binding domain of human renalase (Glu37Asp) has recently been described. The association of this polymorphism with cardiac structure, function, and ischemia has not previously been reported. Methods We genotyped the rs2296545 single-nucleotide polymorphism (Glu37Asp) in 590 Caucasian individuals and performed resting and stress echocardiography. Logistic regression was used to examine the associations of the Glu37Asp polymorphism (C allele) with cardiac hypertrophy (LV mass>100 g/m2), systolic dysfunction (LVEF<50%), diastolic dysfunction, poor treadmill exercise capacity (METS<5) and inducible ischemia. Results Compared with the 406 participants who had GG or CG genotypes, the 184 participants with the CC genotype had increased odds of left ventricular hypertrophy (OR = 1.43; 95% CI 0.99–2.06), systolic dysfunction (OR = 1.72; 95% CI 1.01–2.94), diastolic dysfunction (OR = 1.75; 95% CI 1.05–2.93), poor exercise capacity (OR = 1.61; 95% CI 1.05–2.47), and inducible ischemia (OR = 1.49, 95% CI 0.99–2.24). The Glu37Asp (CC genotype) caused a 24-fold decrease in affinity for NADH and a 2.3-fold reduction in maximal renalase enzymatic activity. Conclusions A functional missense polymorphism in renalase (Glu37Asp) is associated with cardiac hypertrophy, ventricular dysfunction, poor exercise capacity, and inducible ischemia in persons with stable coronary artery disease. Further studies investigating the therapeutic implications of this polymorphism should be considered.

Renalase deficiency in chronic kidney disease, and its contribution to hypertension and cardiovascular disease.

Purpose of reviewRecent experimental data shed light on the regulation of renalase, a secreted amine oxidase, which circulates in an inactive form (prorenalase). Abnormalities in the renalase pathway are evident not only in animal models of chronic kidney disease, but also during the development of hypertension, at a time when kidney function appears normal. Recent findingsProrenalase is rapidly (30–60 s) activated by increased plasma catecholamines and systolic blood pressure. Catecholamine administration promotes the secretion of preformed renalase within 5 min. Plasma renalase is markedly reduced in patients with chronic kidney disease and end-stage renal disease, and in animal models of chronic kidney disease and salt-dependent hypertension. Rats subjected to subtotal nephrectomy develop hypertension and chronic kidney disease, and exhibit low plasma and cardiac renalase, and abnormal renalase activation. SummaryThe renalase pathway is a previously unrecognized mechanism for regulating circulating catecholamines, cardiac function and blood pressure. In this pathway, prorenalase is rapidly activated by increased catecholamines and converted to renalase, which in turn degrades catecholamines. Abnormalities in the renalase pathway are evident in animal models of chronic kidney disease and hypertension. Collectively, these data suggest that renalase plays a key role in the regulation of sympathetic tone, blood pressure and cardiac function.

Molecular cloning of a glibenclamide-sensitive, voltage-gated potassium channel expressed in rabbit kidney.

Shaker genes encode voltage-gated potassium channels (Kv). We have shown previously that genes from Shaker subfamilies Kv1.1, 1.2, 1.4 are expressed in rabbit kidney. Recent functional and molecular evidence indicate that the predominant potassium conductance of the kidney medullary cell line GRB-PAP1 is composed of Shaker-like potassium channels. We now report the molecular cloning and functional expression of a new Shaker-related voltage-gated potassium channel, rabKv1.3, that is expressed in rabbit brain and kidney medulla. The protein, predicted to be 513 amino acids long, is most closely related to the Kv1.3 family although it differs significantly from other members of that family at the amino terminus. In Xenopus oocytes, rabKv1.3 cRNA expresses a voltage activated K current with kinetic characteristics similar to other members of the Kv1.3 family. However, unlike previously described Shaker channels, it is sensitive to glibenclamide and its single channel conductance saturates. This is the first report of the functional expression of a voltage-gated K channel clone expressed in kidney. We conclude that rabKv1.3 is a novel member of the Shaker superfamily that may play an important role in renal potassium transport.

Role of renalase in the regulation of blood pressure and the renal dopamine system.

Purpose of reviewRenalase is a secreted amine oxidase that is synthesized in the kidney, and that metabolizes circulating catecholamines. Tissue and plasma renalase levels are decreased in models of chronic kidney disease. Recent data indicate that renalase deficiency is associated with increased blood pressure and elevated circulating catecholamines. The mechanisms of hypertension in renalase deficiency and the possibility that renalase regulates the renal dopamine system are discussed. Recent findingsCharacterization of the renalase knockout mouse model revealed that renalase deficiency increases SBP and DBP. Renal and cardiac functions are unaffected, but there is evidence of sympathetic activation, with elevation of plasma and urine catecholamines. Renalase is continually excreted in urine, and is enzymatically active and could modulate catecholamines levels in tubular fluid. Renalase expression is modulated by salt intake, and recombinant renalase has a potent and prolonged hypotensive effect on blood pressure in Dahl salt-sensitive rats and rats with chronic kidney disease. Plasma renalase levels are inversely associated with SBP in patients with resistant hypertension. A functional mutation in renalase (Glu37Asp) associated with essential hypertension also predicts more severe cardiac hypertrophy, dysfunction, and ischemia in individuals with stable coronary artery disease, comparable blood pressure and normal renal function. SummaryUrinary renalase metabolizes urinary catecholamines, and perhaps regulates dopamine concentration in luminal fluid, and modulate proximal tubular sodium transport. Renalase deficiency is associated with increased sympathetic tone and resistant hypertension. Recombinant renalase is a potent antihypertensive agent in Dahl salt-sensitive rats and in rats with chronic kidney disease.