Regulation of mitochondria function by natriuretic peptides

Abstract

Natriuretic peptides (NPs) are well known to promote renal Na⁺ excretion, counteracting the effects of the renin-angiotensin-aldosterone system. Thus, NPs serve as a key component in the maintenance of blood pressure, influencing fluid retention capabilities via osmoregulation. Recently, NPs have been shown to affect lipolysis and enhance lipid oxidation and mitochondrial respiration. Here, we provide an overview of current knowledge about the relationship between NPs and mitochondria-mediated processes such as reactive oxygen species production, Ca²⁺ signaling, and apoptosis. Establishing a clear physiological and mechanistic connection between NPs and mitochondria in the cardiovascular system will open new avenues of research aimed at understanding and potentially using it as a therapeutic target from a completely new angle.

NATRIURETIC PEPTIDES AND THEIR FUNCTIONS

Natriuretic peptides (NPs) are hormones secreted from the heart to promote Na⁺ excretion by the kidneys. Currently, there are three known peptides in the NP family: atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). Binding of these NPs to their respective guanylate cyclase (GC)-coupled receptors results in the conversion of GTP to the secondary messenger cGMP. cGMP activates a plethora of downstream signaling cascades, which leads to various biological responses (40). So far three natriuretic peptide receptors have been discovered: natriuretic peptide receptor A (NPRA; GC coupled), natriuretic peptide receptor B (NPRB; GC coupled), and natriuretic peptide receptor C (NPRC). ANP and BNP have high affinity to NPRA, whereas CNP has a higher affinity to NPRB. NPRC serves as a “clearance receptor” for all three natriuretic peptides (25). Upon binding to NPRC, the NP is internalized and degraded by lysosomal ligand hydrolysis, and NPRC is recycled back in the plasma membrane (38). Degradation of NPs is also modulated by proteolytic enzymes such as neprilysin or the insulin-degrading enzyme in the extracellular space (38). A simplified schematic illustration of the NP receptor binding, downstream signaling, and degradation is shown in Fig. 1.

Fig. 1.

Schematic overview of natriuretic peptide (NP)-mediated regulation of mitochondria function in various tissues. Upon binding to natriuretic peptide receptor A (NPRA) and natriuretic peptide receptor B (NPRB), NPs trigger the receptors’ guanylyl cyclase (GC) activity and production of cGMP. cGMP activates PKG, which can increase the peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α level and therefore stimulate mitochondria biogenesis and ATP synthesis. PGC-1α is also capable of activating mitochondrial uncoupling protein (mUP), which uncouples the electron transport chain (ETC). In addition, PKG is known to activate the mitochondrial ATP-sensitive K⁺ (mK_ATP) channel, which attenuates mitochondrial permeability transition pore (mPTP) formation, mitochondria swelling, and eventually apoptosis. Binding of NPs to natriuretic peptide receptor C (NPRC) leads to internalization of the NP-NPRC complex, lysosomal degradation of NPs, and receptor recycling back to the plasma membrane. Circulating levels of NPs are regulated by neprilysin (Nep), a membrane-bound metalloproteinase capable of degrading NPs.

Well-established physiological effects of NPs include a decrease in blood pressure via vasorelaxation, antagonism of the renin-angiotensin-aldosterone system, and inhibition of cardiac hypertrophy (3). Much of the natriuretic activity of NPs is mainly attributed to an inhibitory effect on various Na⁺-reabsorbing channels and transporters along the nephron. For example, NPs have been shown to inhibit tubular-basolateral Na⁺-K⁺-ATPase, Na⁺-K⁺-Cl⁻ cotransporter, and epithelial Na⁺ channel activity as well as vasopressin-induced renal water reabsorption (50). The cumulative inhibitory effect on these channels and transporters leads to diminished renal Na⁺ reabsorption and thus decreased water retention and blood pressure.

Recently, it has been discovered that NPs are involved in lipolysis, lipid oxidation, and mitochondrial respiration (40). Evidence of a correlation between the activity of the NP system and mitochondrial respiration and biogenesis has been explored to some extent; however, little is known about these pathways in the kidney. Because mitochondria play a major role in programmed cell death and ATP provision for general cell metabolic needs, further exploration of their relationship with the NP system may lead to a better understanding of how NPs affect various cardiorenal and metabolic functions in health and disease and to the discovery of novel pharmacological targets. This review will focus on the effects of NPs on mitochondria metabolism and biogenesis and the potential importance of this interaction for cardiorenal disease.

MITOCHONDRIA IN THE CARDIORENAL SYSTEM

Mitochondria play key roles in a plethora of physiological and pathophysiological processes. First and foremost, they are the source of energy (ATP) in most tissues and organs (8). Additionally, mitochondria mediate Ca²⁺ signaling, reactive oxygen species (ROS) and nitrogen species production, as well as metabolism of endogenous and exogenous compounds. In the cardiovascular system in particular, mitochondria produce nitric oxide, which affects blood pressure via smooth muscle relaxation (43). Muscle cells are rich with mitochondria and rely on them to provide ATP for contraction, and the role of mitochondria as regulators of Ca²⁺ is very important in this case (16). Another critical function of mitochondria is the initiation of cell death (apoptosis and necrosis), which is an essential physiological response that is tightly controlled but, under certain conditions, may cause damage. For example, during ischemia-reperfusion (I/R) injury of the heart, mitochondria are involved in the processes that injure the cardiomyocytes and cardiac vasculature (through activation of mitochondria membrane protein complexes referred to as “death channels,” impairment of the oxidative phosphorylation pathway, and decreased antioxidant defenses; see Refs. 9, 27, and 49).

The structure, density, and function of the mitochondria vary greatly depending on the needs of different tissues and organs. For instance, some cells are more dependent on glycolysis and either do not contain a large number of mitochondria or do not have them at all, like red blood cells (5). In other cell types, with considerable energy needs, mitochondria density is very high, and cell function is therefore directly dependent on mitochondria health (41). For instance, kidney cells have the second highest mitochondrial content and oxygen consumption rate after the heart (35, 37), with considerable energy needs (42). Disruption of mitochondrial homeostasis in the early stages of acute kidney injury (AKI) is an important factor that has been shown to drive tubular injury and renal dysfunction. Hyperglycemia-induced ATP depletion triggers changes in renal mitochondrial morphology that lead to the onset of diabetic nephropathy in diabetes mellitus (2). A recent study by Kruger et al. (22) revealed that normalizing tubular cell mitochondrial function and energy balance could be an important preventative strategy in kidney disease. Therefore, correcting abnormal electron transport chain function directly, and/or by targeting the pathways that regulate mitochondrial biogenesis, may be used to improve renal disease outcomes by restoring mitochondrial function and thus assisting with organ repair (46).

THE NP-MITOCHONDRIA SIGNALING AXIS IN PHYSIOLOGY AND PATHOPHYSIOLOGY

NPs have been shown to have diverse effects on mitochondria in various tissues. It has been established that NPs can promote mitochondrial biogenesis and fat oxidation (28), and ANP has been reported to upregulate mitochondrial fat oxidative capacity and respiration in mouse skeletal muscle (33). In addition, a positive correlation was established in the muscles between gene expression of Npra and Ppargc1a, the master regulator for mitochondrial biogenesis, which encodes for peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α protein (17). In the same cells, BNP has been demonstrated to decrease mitochondrial ROS production and lead to the opening of mitochondrial ATP-sensitive (mK_ATP) K⁺ channels (48). A study (19) involving adipocytes revealed increased intracellular temperature and increased mRNA levels of uncoupling protein-1 after 1 h of incubation with ANP, whereas the intracellular ATP concentration was not changed, implying the presence of uncoupled mitochondrial respiration.

One of the major downstream NP-related cellular events is an increase in cGMP levels that results from activation of the NP receptors’ GC activity (4). cGMP is a common regulator of ion channel conductance, glycogenolysis, and cellular apoptosis (1). In addition, it is a known regulator of smooth muscle cell contractility and, therefore, vascular blood flow (39). The effects of cGMP on mitochondria have been extensively described in the literature; cGMP induces mitochondrial biogenesis in vitro and in vivo and therefore might have beneficial effects in AKI and other pathologies characterized by mitochondrial dysfunction and suppressed mitochondrial biogenesis (51). cGMP has been shown to inhibit the formation of mitochondrial permeability transition pore (mPTP) and reduce cell death from I/R injury in the heart and other tissues (11). Furthermore, it is believed that PGC-1α gene expression, along with the expression of several oxidative phosphorylation pathway genes, is partially induced in a cGMP-dependent manner (17). CNP, acting through NPRB, can stimulate the production of cGMP in neurons (53). Some studies have suggested that NPs, specifically via the GC-cGMP and cGMP-dependent protein kinase pathway, can promote skeletal muscle mitochondrial biogenesis and fat oxidation to prevent obesity and glucose intolerance (28); NPs were shown to induce lipolysis and acutely increase free fatty acid availability in humans (3). At the same time, other publications revealed that cGMP, on the other hand, can decrease the efficiency of mitochondrial ATP synthesis (without impacting mitochondrial content or ultrastructure; see Ref. 29). Despite the abundant evidence of the importance of NP-mediated cGMP effects in various tissues and organs, further research is required on the specific mechanisms mediating these events, especially in disease states.

The known effects of NPs on Ca²⁺ signaling are of particular interest, especially taking into consideration the importance of mitochondria in Ca²⁺ handling; however, the data are limited. BNP exerted protective effects in I/R injury by blocking the mitochondrial Ca²⁺ uniporter (44). In addition, BNP has been shown to modulate overall Ca²⁺ levels in airway smooth muscle cells (36). In addition, in failing cardiomyocytes, ANP was able to normalize aberrant diastolic Ca²⁺ sparks, through suppression of mitochondrial ROS generation, which might be a contributing factor in cardiomyocyte survival during heart failure (34). An increase in plasma NP levels is typically used as a clinical marker of cardiac dysfunction and heart failure (13). Several studies have shown that, in I/R injury, NPs may exhibit protective effects through mitochondria-mediated mechanisms. For instance, intravenous administration of ANP (30 min) inhibited I/R-induced mitochondrial damage in the pig myocardium. Further examination of myocardial tissue using electron microscopy revealed swollen mitochondria with sparse cristae in the control group, as opposed to normal mitochondrial morphology in the ANP-treated group. BNP showed similar protection against I/R in the skeletal muscle of Wistar rats (48). It has also been reported that BNP did not exhibit such protective functions when mitochondrial mK_ATP channels were blocked, suggesting that opening of this channel can contribute to the observed beneficial effects (48). Another study that investigated into mK_ATP channels in rat myocardial mitochondria indicated that cGMP mediates the opening of this channel (11) through the activation of PKG that leads to mitochondrial ROS production (12). In this case, ROS production would serve as a secondary messenger, resulting in the activation of downstream kinases, including PKC, which then blocks the mPTP. In support of this, BNP prevented I/R-induced mPTP opening in cultured cardiomyocytes and exerted protective effects by suppressing the mitochondrial death pathway in a phosphatidylinositol 3-kinase/Akt-dependent manner (45).

ANP is an intracrine signaling molecule, and adrenal cortical cells are among the target cells that have been shown to internalize it. High-affinity binding sites for ANP have been identified on adrenal mitochondrial membranes, and intracellular ANP has been found in association with mitochondria (18, 30, 31). Interestingly, the published data are very sparse as to the regulation of mitochondria function by NPs in the kidney. A study in the early 1980s showed that direct application of recombinant ANP to renal mitochondria inhibited step 3 mitochondrial respiration and decreased the ADP-to-oxygen ratio; ANP also induced dramatic mitochondrial swelling (20). A 2016 study by Moriyama et al. (32) pointed out that oxygen consumption in renal mitochondria was attenuated by ANP in Sprague-Dawley rats. A possibility of a link between NPs and mitochondria in renal tissues has not been explored in enough detail; however, plenty of studies have shown that cGMP (produced because of GC activity of NPRA and NPRB) can play an important role in the kidney (10, 21). Based on the literature, we can hypothesize that the effects of ANP on mitochondria function can be cGMP mediated, since cGMP is known to prevent mitochondria permeability transition, decrease mitochondrial depolarization, and reduce cell death (6, 10, 11, 21, 24, 47). In renal tissue, activation of particulate GC A (pGC-A) in the kidney yields increased Na⁺ and water excretion (7, 14). There are several mechanisms that contribute to this effect, including reduced Na⁺ reabsorption, an increased glomerular membrane ultrafiltration coefficient, and elevated glomerular hydrostatic pressure (10). pGC-A/cGMP activity is also involved in the modulation of renal vascular function and glomerular filtration rate. Specifically, it was found that cGMP induces efferent arteriole constriction and afferent arteriole dilation, thus enhancing the glomerular filtration rate (15, 26). Renal dysfunction is observed in rodents with pGC-A gene knockout, which also exhibit augmented fibrosis, indicative of a cGMP-dependent protective effect against fibrosis (23, 52). Despite these physiologically relevant findings, data linking NPs and their receptor activity via cGMP to mitochondria function or associated cascades are essentially lacking.

CONCLUSIONS

It is becoming clear that NPs play a role in mitochondria functionality, especially in the heart and skeletal muscles. However, the overall effect of NP on mitochondria is not well defined, and further studies are warranted before NPs can be targeted in mitochondria-related therapeutic pathways. Although the effects of NPs on renal function are known, there is little knowledge on how NPs affect renal mitochondria functionality and biogenesis. Elucidation of the precise mechanisms of this NP-mitochondria interaction could potentially lead to novel therapeutic mechanisms for mitochondria-affecting diseases, such as AKI and chronic kidney disease.

GRANTS

This work was supported by National Institutes of Health (NIH) Grant R00-DK-105160, Polycystic Kidney Disease Foundation Grant 221G18a, and American Physiological Society Research Career Enhancement and Lazaro J. Mandel Young Investigator awards (to D. V. Ilatovskaya) and by NIH Grant U54-DA-016511 and Biomedical Laboratory Research and Development Service of the Veterans Affairs Office of Research and Development Award IK2BX003922 (to K. Y. DeLeon-Pennell). This work was also financially supported, in part, by the 2019 American Physiological Society S&R Foundation Ryuji Ueno Award (to K. Y. DeLeon-Pennell).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

M.D., A.B.N., and D.V.I. prepared figures; M.D., A.B.N., K.Y.D.-P., and D.V.I. drafted manuscript; M.D., A.B.N., K.Y.D.-P., and D.V.I. edited and revised manuscript; M.D., A.B.N., K.Y.D.-P., and D.V.I. approved final version of manuscript.