REVIEWS Nitric oxide signalling in kidney regulation and cardiometabolic health Mattias Carlström Abstract | The prevalence of cardiovascular and metabolic disease coupled with kidney dysfunction is increasing worldwide. This triad of disorders is associated with considerable morbidity and mortality as well as a substantial economic burden. Further understanding of the underlying pathophysiological mechanisms is important to develop novel preventive or therapeutic approaches. Among the proposed mechanisms, compromised nitric oxide (NO) bioactivity associated with oxidative stress is considered to be important. NO is a short-lived diatomic signalling molecule that exerts numerous effects on the kidneys, heart and vasculature as well as on peripheral metabolically active organs. The enzymatic l-arginine-dependent NO synthase (NOS) pathway is classically viewed as the main source of endogenous NO formation. However, the function of the NOS system is often compromised in various pathologies including kidney, cardiovascular and metabolic diseases. An alternative pathway, the nitrate–nitrite–NO pathway, enables endogenous or dietary-derived inorganic nitrate and nitrite to be recycled via serial reduction to form bioactive nitrogen species, including NO, independent of the NOS system. Signalling via these nitrogen species is linked with cGMP-dependent and independent mechanisms. Novel approaches to restoring NO homeostasis during NOS deficiency and oxidative stress have potential therapeutic applications in kidney, cardiovascular and metabolic disorders. The prevalence of cardiovascular disorders, including a substantial contribution of increased generation of hypertension, and metabolic disorders such as type 2 NADPH oxidase­derived and mitochondria­derived diabetes mellitus (T2DM), is increasing worldwide. reactive oxygen species (ROS) and oxidative stress cou­ These disorders are closely coupled with the develop­ pled with reduced nitric oxide (NO) bioactivity and ment and progression of kidney disease, which signifi­ endothelial dysfunction7–10. NO is a short­lived diatomic cantly increases patient morbidity and mortality1,2. The signalling molecule that exerts multiple effects on kidney, resulting societal economic burden is immense and fur­ cardiovascular and metabolic functions, including mod­ ther understanding of the underlying pathophysiological ulation of renal autoregulation, tubular fluid and electro­ mechanisms is urgently needed to enable the develop­ lyte transport, vascular tone, blood pressure, platelet ment of novel preventive and therapeutic nutritional and aggregation, immune cell activation, insulin­glucose pharmacological strategies2. The kidney, cardio vascular homeostasis and mitochondrial function. The classical and metabolic phenotypes (that is, kidney disease, cardio­ view is that nitric oxide synthase (NOS) systems are the vascular disease and T2DM) are interrelated, suggesting main source of endogenous NO formation. However, an that this triad of disorders share common underlying alternative pathway exists whereby the supposedly inert pathological mechanisms. The exact causes of these dis­ oxidation products of NO, that is, inorganic nitrate and orders, the interactions between organ systems and the nitrite, undergo serial reductions to form NO and other complex pathophysiological mechanism(s) that underlie closely related bioactive nitrogen oxide species11–13. the initiation, maintenance and progression of disease are The important role of NO in the regulation of kid­ complex and not fully understood. ney, cardiovascular and metabolic functions in health Department of Physiology Potential mechanisms that might contribute to the and disease has led to substantial interest in the iden­ and Pharmacology, development of kidney disease, cardiovascular dis­ tification of methods to therapeutically modulate NO Karolinska Institutet, Stockholm, Sweden. ease and T2DM include hyperglycaemia, altered lipid bioactivity. In this Review, I discuss the physiological e-mail: mattias.carlstrom metabolism, low­grade inflammation, overactivity of roles of NO, the direct and indirect effects by which this @ki.se the renin–angiotensin–aldosterone system (RAAS), molecule influences kidney function and its association https://doi.org/10.1038/ increased sympathetic nerve activity and altered micro­ with cardiometabolic complications. I also highlight s41581-021-00429-z biota3–6. In addition, several studies have suggested novel approaches to restoring NO homeostasis during NATURE REVIEWS | NEPHROLOGY VOLUME 17 | SEPTEMBER 2021 | 575 0123456789();: REVIEWS Key points via protein–protein interactions and regulation of sub­ cellular localization, which can increase or decrease • Nitric oxide and other bioactive nitrogen species have pivotal roles in multiple their enzymatic activity31–33. Acute changes in nNOS physiological functions, including modulation of the kidney, cardiovascular and and eNOS activity in the vasculature and the kidney metabolic systems; in the kidney, nitric oxide has a crucial role in autoregulation are mainly modulated via post­translational mecha­ and modulation of tubular transport. • Nitric oxide is classically derived from l-arginine-dependent nitric oxide synthases, nisms, whereas chronic changes in NO synthesis are but can also be formed via serial reduction of inorganic nitrate and nitrite, that is, regulated by altered eNOS or nNOS transcription and 18,34,35 the nitrate–nitrite–nitric oxide pathway. translation . Activation of iNOS is associated with • The nitrate–nitrite–nitric oxide pathway can be boosted via the diet and is of inflammatory processes and leads to significantly higher particular importance in conditions where the activity of the nitric oxide synthase levels of NO than those that are produced by constitutive system is reduced, such as hypoxia, ischaemia or low pH. activation of other NOS isoforms. The resulting acute • Signalling via bioactive nitrogen species is linked with both cGMP-dependent and increases in NO have beneficial antimicrobial effects independent mechanisms. against bacteria, viruses and fungi. However, induc­ • Reduced nitric oxide bioactivity has been associated with ageing and kidney, tion of iNOS is also associated with chronic low­grade cardiovascular and metabolic disorders, which are often coupled with oxidative stress. inflammation in cardiovascular, metabolic and kidney 36 • Novel pharmacological and nutritional strategies that increase nitric oxide bioactivity disorders . and reduce oxidative stress could be potential therapies for preventing and treating The constitutive NOS systems are generally thought kidney disease and associated cardiometabolic complications. to be the main source of endogenous NO production and signalling under normal, healthy conditions, but are often dysfunctional in pathological conditions includ­ NOS deficiency with a focus on the alternative nitrate– ing cardiovascular disorders9 and chronic kidney disease nitrite–NO pathway, which can be boosted via dietary (CKD)37,38. This dysfunction is associated with reduced intake, particularly by eating green leafy vegetables. NO bioactivity. The mechanisms that contribute to reduced NO formation and compromised signalling are The classical NO synthase systems multifactorial and include reduced NOS expression, lim­ NO is endogenously generated by numerous cells ited substrate availability, uncoupling of NOS, elevated throughout the body via three different NOS systems levels of endogenous NOS inhibitors such as asymmetric (Fig. 1)14–16. Neuronal NOS (nNOS; also known as NOS1) dimethylarginine and compromised signalling in states and endothelial NOS (eNOS; also known as NOS3) of oxidative stress due to direct scavenging by ROS or are constitutively expressed, whereas inducible NOS oxidation of the heme group in sGC (Fe2+ to Fe3+), which (iNOS; also known as NOS2) is mainly associated with renders it insensitive to activation by NO9,39. inflammatory conditions16,17. l­Arginine, molecular oxygen, NADPH and tetrahydrobiopterin (BH4) are Kidney expression of NOS isoforms equally important substrates or co­factors that lead Expression of all three NOS isoforms and sGC has been to equimolar generation of NO and l­citrulline18,19. reported in the kidney, although some discrepancies In the endothelium, eNOS­derived NO has a central exist regarding the expression levels along the nephron role in the regulation of blood flow and maintenance when comparing data from human and animal studies. of endothelial integrity. The activity of eNOS and Moreover, splice variants of nNOS and iNOS40,41 can nNOS is regulated by intracellular calcium, which influence their activity and function. In the macula activates calmodulin. In turn, calmodulin binds and densa, splice variants (α, β and γ isoforms) have been increases NOS enzyme activity. This process results in reported to have a substantial impact on the func­ NO­mediated activation of soluble guanylate cyclase tion of nNOS42, thus modulating renin secretion and (sGC) and increased formation of cyclic GMP (cGMP), autoregulatory mechanisms. which activates cGMP­dependent protein kinases. Expression of nNOS in the macula densa and eNOS This NO–sGC–cGMP signalling pathway mediates in the kidney vasculature has been consistently doc­ many of the effects of NO bioactivity on cardiovascu­ umented; some but not all studies have also reported lar, kidney and metabolic functions20. However, other eNOS expression in tubular epithelial cells. In normal, bioactive