Nitric oxide synthases in heart failure

Abstract

Significance: The regulation of myocardial function by constitutive nitric oxide synthases (NOS) is important for the maintenance of myocardial Ca(2+) homeostasis, relaxation and distensibility, and protection from arrhythmia and abnormal stress stimuli. However, sustained insults such as diabetes, hypertension, hemodynamic overload, and atrial fibrillation lead to dysfunctional NOS activity with superoxide produced instead of NO and worse pathophysiology.

Recent advances: Major strides in understanding the role of normal and abnormal constitutive NOS in the heart have revealed molecular targets by which NO modulates myocyte function and morphology, the role and nature of post-translational modifications of NOS, and factors controlling nitroso-redox balance. Localized and differential signaling from NOS1 (neuronal) versus NOS3 (endothelial) isoforms are being identified, as are methods to restore NOS function in heart disease.

Critical issues: Abnormal NOS signaling plays a key role in many cardiac disorders, while targeted modulation may potentially reverse this pathogenic source of oxidative stress.

Future directions: Improvements in the clinical translation of potent modulators of NOS function/dysfunction may ultimately provide a powerful new treatment for many hearts diseases that are fueled by nitroso-redox imbalance.

FIG. 1.

Protein structure of nitric oxide synthase 3 (NOS3) and NOS1 splice variants. All NOSs include an oxygenase and a reductase domain and a calmodulin (CaM) binding site. The oxygenase moiety contains the L-arginine, heme, and tetrahydrobiopterin (BH4)-binding domains; whereas the reductase moiety contains the flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide phosphate (NADPH)- binding domains. An additional PDZ domain is found in the N-terminal of the α,μ-NOS1 and NOS1–2 variants and allows the enzyme to bind other proteins with a similar structure. aa, aminoacids. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 2.

Regulation of myocardial Ca²⁺ fluxes by NOS-derived NO. NOS1-derived NO has been shown to inhibit Ca²⁺ influx via the L-type Ca²⁺ channel (LTCC), increase Ca²⁺ reuptake in the sarcoplasmic reticulum (SR) by increasing phospholamban (PLB) phosphorylation, and regulate the release of Ca²⁺ from the SR through changes in ryanodine receptor Ca²⁺ release channel (RyR) S-nitrosylation. NOS1 activity is, in turn, modulated by the Ca²⁺ flux via the plasmalemma Ca²⁺ ATPase (plasma membrane calcium/CaM dependent ATPase [PMCA]). Endothelial NOS3-derived NO has been reported to reduce myofilament Ca²⁺ sensitivity via PKG-mediated phosphorylation, inhibit myocardial oxygen consumption, and regulate intracellular cAMP levels (and thus, β-adrenergic responses) by modulating phosphodiesterase activity. The stimulation of myocardial NOS3 activity by mechanical stretch has been shown to increase the open probability of the RyR Ca²⁺ release channel. SERCA2a, SR Ca ATPase; NCX, sodium calcium exchanger. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 3.

The role of BH4 and its oxidized products in the regulation of NOS activity. In the absence of BH4, or when BH2 binds to the enzyme, NOS becomes “uncoupled” and produces superoxide instead of NO. NOS-derived superoxide, as well as superoxide from other sources such as NADPH oxidase and xanthine oxidoreductase, may, in turn, oxidize BH4 to BH2 and biopterin. Dihydrofolate reductase (DHFR) can recycle BH2 back to BH4. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 4.

Differential mechanisms of NOS3 uncoupling. When NOS3 is fully coupled, electrons flow from the reductase domain of one subunit into the oxygenase domain of the other, and NO is produced. When NOS3 is uncoupled, L-arginine oxidation becomes uncoupled from electron flow through the enzyme, and superoxide is produced instead of NO. A unique mechanism for the uncoupling of NOS3 has been identified where the modification of C689 and C908 by glutathionylation results in superoxide production from the reductase domain of the enzyme (40). (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 5.

Therapeutic approaches for treating “NOS-opathy.” Schematic showing various components of NOS-related dysfunction and where and how different therapeutic approaches are attempting to ameliorate this pathophysiology. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).