The expanding roles of neuronal nitric oxide synthase (NOS1)

Abstract

The nitric oxide synthases (NOS; EC 1.14.13.39) use L-arginine as a substrate to produce nitric oxide (NO) as a by-product in the tissue microenvironment. NOS1 represents the predominant NO-producing enzyme highly enriched in the brain and known to mediate multiple functions, ranging from learning and memory development to maintaining synaptic plasticity and neuronal development, Alzheimer's disease (AD), psychiatric disorders and behavioral deficits. However, accumulating evidence indicate both canonical and non-canonical roles of NOS1-derived NO in several other tissues and chronic diseases. A better understanding of NOS1-derived NO signaling, and identification and characterization of NO-metabolites in non-neuronal tissues could become useful in diagnosis and prognosis of diseases associated with NOS1 expression. Continued investigation on the roles of NOS1, therefore, will synthesize new knowledge and aid in the discovery of small molecules which could be used to titrate the activities of NOS1-derived NO signaling and NO-metabolites. Here, we address the significance of NOS1 and its byproduct NO in modifying pathophysiological events, which could be beneficial in understanding both the disease mechanisms and therapeutics.

Keywords: Cancer; Cardiovascular; Diabetes; Epigenetics; Nitric oxide; Nitric oxide signaling; Nitric oxide synthase; Redox; Transcriptional regulation; Vasodilation.

Figure 1. Production of nitric oxide (NO) and functional domains of human NOS1, NOS2, and NOS3.

(A) Production of nitric oxide (NO). NOS converts L-arginine to L-citrulline in presence of nicotinamide adenine dinucleotide phosphate (NADPH) and oxygen to produce highly diffusible NO free radical in the tissue microenvironment. (B) Functional domains of human NOS1, NOS2, and NOS3. NOS1 harbors a PDZ domain at the NH2-terminus. The oxygenase and reductase domains are as shown. The oxygenase domain contains heme and tetrahydrobiopterin (BH4) interacting sites, whereas the reductase domain contains interacting sites for FMN, FAD, and NADPH; the FMN domain connects to the oxygenase domain via a calmodulin-binding (CaM) domain. The NOS1 and NOS3 proteins contain an autoinhibition segment that interrupts the FMN domain, while NOS2 lacks. Myristoylation (Myr), palmitoylation (Palm), zinc-ligating (Zn) positions are as shown.

Figure 2. Transcriptional regulation of NOS1.

(A) Nucleotide sequence of the human NOS1-promoter of -500 bp upstream of the transcription start site (TSS) ATG as shown. The locations of putative binding sites for KLF2/KLF4, Hypoxia Response Element (HRE), OCT4, and SOX2 are indicated. (B) Hypothetical diagram of transcriptional regulation of NOS1 and the potential roles of epigenetic mediators KLF2/KLF4, HRE, OCT4, and SOX2. Epigenetic mediator-induced expression of NOS1 gene could occur prior to the appearance of genomic instability, thereafter mutation and/or deletion of critical genes could drive tumor cell proliferation, resist apoptosis, together alter NOS1 expression. Additionally, SNPs and polymorphic loci in cis-regulating elements (e.g., enhancer) could up-or downregulate NOS1 expression. Expression of NOS1 gene could be measured by RT-PCR, epigenetic modifications by DNA methylation assays or by chromatin immunoprecipitation (ChiP) experiments.

Figure 3. Inflammatory and anti-inflammatory activities of NOS1-derived NO in indicated diseases.

NOS1-mediated production of NO acts as an inflammatory molecule and mediates the progression of disease such as macrophage NOS1-driven atherosclerosis, and in a subset of cancer, obesity and diabetes; while it acts as a protector in conditions such as endothelial NOS1-driven atherosclerosis, infantile hypertrophic pyloric stenosis (IHPS), sepsis and achalasia.