Transcription of different exons 1 of the human neuronal nitric oxide synthase gene is dynamically regulated in a cell- and stimulus-specific manner

Expression of neuronal NO synthase α- and β-isoforms in skeletal muscle of mice

Knowledge of the primary structure of neuronal NO synthase (nNOS) in skeletal muscle is still conflicting and needs further clarification. To elucidate the expression patterns of nNOS isoforms at both mRNA and protein level, systematic reverse transcription (RT)-PCR and epitope mapping by qualitative immunoblot analysis on skeletal muscle of C57/BL6 mice were performed. The ability of the nNOS isoforms to form aggregates was characterized by native low-temperature polyacrylamide electrophoresis (LT-PAGE). The molecular analysis was focused on the rectus femoris (RF) muscle, a skeletal muscle with a nearly balanced ratio of nNOS α- and β-isoforms. RT-PCR amplificates from RF muscles showed exclusive exon-1d mRNA expression, either with or without exon-μ. Epitope mapping demonstrated the simultaneous expression of the nNOS splice variants α/μ, α/non-μ, β/μ and β/non-μ. Furthermore, immunoblotting suggests that the transition between nNOS α- and β-isoforms lies within exon-3. In LT-PAGE, three protein nNOS associated aggregates were detected in homogenates of RF muscle and tibialis anterior muscle: a 320 kDa band containing nNOS α-isoforms, while 250 and 300 kDa bands consist of nNOS β-isoforms that form homodimers or heterodimers with non-nNOS proteins.

The Physiological Function of nNOS-Associated CAPON Proteins and the Roles of CAPON in Diseases

In this review, the structure, isoform, and physiological role of the carboxy-terminal PDZ ligand of neuronal nitric oxide synthase (CAPON) are summarized. There are three isoforms of CAPON in humans, including long CAPON protein (CAPON-L), short CAPON protein (CAPON-S), and CAPON-S' protein. CAPON-L includes three functional regions: a C-terminal PDZ-binding motif, carboxypeptidase (CPE)-binding region, and N-terminal phosphotyrosine (PTB) structural domain. Both CAPON-S and CAPON-S' only contain the C-terminal PDZ-binding motif. The C-terminal PDZ-binding motif of CAPON can bind with neuronal nitric oxide synthase (nNOS) and participates in regulating NO production and neuronal development. An overview is given on the relationship between CAPON and heart diseases, diabetes, psychiatric disorders, and tumors. This review will clarify future research directions on the signal pathways related to CAPON, which will be helpful for studying the regulatory mechanism of CAPON. CAPON may be used as a drug target, which will provide new ideas and solutions for treating human diseases.

Synthesis and biological evaluation of clovamide analogues as potent anti-neuroinflammatory agents in vitro and in vivo

A series of clovamide analogues, namely, 1a-13a and 1b-13b, was synthesized and evaluated for their anti-neuroinflammatory activities using BV-2 microglia cells. Among these compounds, six (1b, 4b-8b) showed NO inhibition with no or weak cytotoxicity (CC₅₀ > 100 μM), especially 4b, and showed an IC₅₀ value of 2.67 μM. Enzyme activity and docking assay revealed that the six compounds, especially 4b, target inducible NO synthase (iNOS) and exhibit potent inhibitory effects on iNOS with IC₅₀ values ranging from 1.01 μM to 29.23 μM 4b significantly suppressed the expression of pro-inflammatory cytokines in lipopolysaccharide-stimulated cells. Notably, the oral administration of 4b remarkably improved dyskinesia, reduced the expression of glial fibrillary acidic protein (GFAP)-a marker of neuroinflammation, and increased tyrosine hydroxylase-positive cells in 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine-induced Parkinson's disease (PD) mouse models. These observations demonstrated that 4b is an effective and promising candidate for PD therapy.

Modulated expression of genes associated with NO signal transduction contributes to the cholesterol-lowering effect of electro-acupuncture

Electro-acupuncture (EA) at Fenglong acupoint (ST40) can lower the levels of serum cholesterol and triacylglycerols. To study the hepatic genes responsible for the cholesterol-lowering effect of EA, suppression subtractive hybridization combined with the switch mechanism at the 5'-end of RNA template cDNA synthesis and long-distance PCR were employed using hepatic tissues from hypercholesterolemia and EA-treated mice. 68 % of the identified genes are involved in metabolism, immune response, and signal transduction pathways. Real-time PCR and western blot indicate that EA at ST40 induces the expression of nNOS and Mt1, two genes involved in NO signal transduction. EA treatment for hypercholesterolemia thus involves the modulation of several biological pathways and provides a physiological link between NO signal transduction and the cholesterol-lowering effect of EA.

Truncated IRAG variants modulate cGMP-mediated inhibition of human colonic smooth muscle cell contraction

Nitric oxide (NO) induces relaxation of colonic smooth muscle cells predominantly by cGMP/cGMP-dependent protein kinase I (cGKI)-induced phosphorylation of the inositol 1,4,5-trisphosphate receptor (IP(3)R)-associated cGMP kinase substrate (IRAG), to block store-dependent calcium signaling. In the present study we analyzed the structure and function of the human IRAG/MRVI1 gene. We describe four unique first exon variants transcribed from individual promoters in diverse human tissues. Tissue-specific alternative splicing with exon skipping and alternative splice donor and acceptor site usage further increases diversity of IRAG mRNA variants that encode for NH(2)- and COOH-terminally truncated proteins. At the functional level, COOH-terminally truncated IRAG variants lacking both the cGKI phosphorylation and the IP(3)RI interaction site counteract cGMP-mediated inhibition of calcium transients and relaxation of human colonic smooth muscle cells. Since COOH-terminally truncated IRAG mRNA isoforms are widely expressed in human tissues, our results point to an important role of IRAG variants as negative modulators of nitric oxide/cGKI-dependent signaling. The complexity of alternative splicing of the IRAG gene impressively demonstrates how posttranscriptional processing generates functionally distinct proteins from a single gene.

Research progress on neurobiology of neuronal nitric oxide synthase

Neuronal nitric oxide synthase (nNOS) is mainly expressed in neurons, to some extent in astrocytes and neuronal stem cells. The alternative splicing of nNOS mRNA generates 5 isoforms of nNOS, including nNOS-α, nNOS-β, nNOS-µ, nNOS-γ and nNOS-2. Monomer of nNOS is inactive, and dimer is the active form. Dimerization requires tetrahydrobiopterin (BH4), heme and L-arginine binding. Regulation of nNOS expression relies largely on cAMP response element-binding protein (CREB) activity, and nNOS activity is regulated by heat shock protein 90 (HSP90)/HSP70, calmodulin (CaM), phosphorylation and dephosphorylation at Ser847 and Ser1412, and the protein inhibitor of nNOS (PIN). There are primarily 9 nNOS-interacting proteins, including post-synaptic density protein 95 (PSD95), clathrin assembly lymphoid leukemia (CALM), calcium/calmodulin-dependent protein kinase II alpha (CAMKIIA), Disks large homolog 4 (DLG4), DLG2, 6-phosphofructokinase, muscle type (PFK-M), carboxy-terminal PDZ ligand of nNOS (CAPON) protein, syntrophin and dynein light chain (LC). Among them, PSD95, CAPON and PFK-M are important nNOS adapter proteins in neurons. The interaction of PSD95 with nNOS controls synapse formation and is implicated in N-methyl-D-aspartic acid-induced neuronal death. nNOS-derived NO is implicated in synapse loss-mediated early cognitive/motor deficits in several neuropathological states, and negatively regulates neurogenesis under physiological and pathological conditions.

S-nitrosothiol signals in the enteric nervous system: lessons learnt from big brother

Nitric oxide (NO) is a functionally important neurotransmitter signaling molecule generated by mammalian and bacterial nitric oxide synthases (NOS), and by chemical conversion of dietary nitrite in the gastrointestinal (GI) tract. Neuronal NOS (nNOS) is the most abundant isoenzyme in the enteric nervous system, and targeted deletion in transgenic mice has clearly demonstrated its importance in normal gut function. Enteric neuropathy is also often associated with abnormal NO production, for example in achalasia and diabetic gastroparesis. Not surprisingly therefore, aberrant nNOS activity is widely implicated in enteric disease, and represents a potential molecular target for therapeutic intervention. One physiological signaling mechanism of NO bioactivity is through chemical reaction with the heme center of guanylyl cyclase, resulting in the conversion of cGMP from GTP. This second messenger nucleotide signal activates cGMP-dependent protein kinases, phosphodiesterases, and ion channels, and is implicated in the neuronal control of GI function. However, few studies in the GI tract have fully related NO bioactivity with specific molecular targets of NO-derived signals. In the central nervous system (CNS), it is now increasingly appreciated that NO bioactivity is often actively transduced via S-nitrosothiol (SNO) signals rather than via activation of guanylyl cyclase. Moreover, aberrant S-nitrosylation of specific molecular targets is implicated in CNS pathology. S-nitrosylation refers to the post-translational modification of a protein cysteine thiol by NO, forming an endogenous SNO. Because cysteine residues are often key regulators of protein function, S-nitrosylation represents a physiologically important signaling mechanism analogous to other post-translational modifications, such as O-phosphorylation. This article provides an overview of how neurotransmitter NO is produced by nNOS as this represents the most prominent and well defined source of SNO production in the enteric nervous system. Further, it provides a perspective of how S-nitrosylation signals derived from multiple diverse sources may potentially transduce NO bioactivity in the GI tract. Possible lessons that might be learnt from the CNS, such as SNO mediated auto-inhibition of nNOS activity and modulation of neuronal cell death, are also explored as these may have pathophysiological relevance in enteric neuropathy. Thus, S-nitrosylation may mediate previously underappreciated NO-derived signals in the enteric nervous system that regulate homeostatic gut functions and disease susceptibility.

Genetic analysis of nitric oxide synthase 1 variants in schizophrenia and bipolar disorder

Nitric oxide (NO) is a neurotransmitter that acts as a second messenger of the N-methyl-D-aspartate receptor and interacts with the dopaminergic and the serotonergic systems. NO involvement in pathological processes relevant to neuropsychiatric disorders stems from its ability to modulate certain forms of synaptic plasticity, and from its capacity to be transformed to a highly active free radical. Additionally, multiple links have been reported between the NO-producing enzyme, nitric oxide synthase (NOS) 1, and both schizophrenia and bipolar disorder (BPD). RNA and DNA isolated from dorsolateral-prefrontal cortices of schizophrenia patients, bipolar patients and controls (n = 26, 30 and 29, respectively) were donated by the Stanley Foundation Brain Collection. Gene expression was measured by Real-Time-PCR. Genetic polymorphisms were genotyped by restriction-fragment length-polymorphism analysis, and by product-size determination of PCR products amplified with a fluorescent primer.Expression analysis of pan-NOS1, as well as of 2 of its isoforms, "NOS1_1d" and "NOS1_1f", which differ in their first exons and translational strength, revealed a trend for pan-NOS1 over-expression (P = 0.075) in schizophrenia patients (1.33-fold), and significant over-expression (P < 0.05) of NOS1_1d and NOS1_1f in this group (1.54-fold and 1.61-fold, respectively). No expressional alteration was observed in BPD. Polymorphisms at the promoters of NOS1_1d and NOS1_1f, previously shown to be functional in vitro, revealed no significant allelic or genotypic differences among clinical groups and showed no effect on these transcripts' expression. In conclusion, understanding the molecular mechanisms underlying the over-expression of specific NOS1 isoforms, which is unique to schizophrenia, may assist in identifying targets for new drugs.

Retinoic acid-induced nNOS expression depends on a novel PI3K/Akt/DAX1 pathway in human TGW-nu-I neuroblastoma cells

Neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) acts as a neurotransmitter and intracellular signaling molecule in the central and peripheral nervous system. NO regulates multiple processes like neuronal development, plasticity, and differentiation and is a mediator of neurotoxicity. The nNOS gene is highly complex with 12 alternative first exons, exon 1a-1l, transcribed from distinct promoters, leading to nNOS variants with different 5'-untranslated regions. Transcriptional control of the nNOS gene is not understood in detail. To investigate regulation of nNOS gene expression by retinoic acid (RA), we used the human neuroblastoma cell line TGW-nu-I as a model system. We show that RA induces nNOS transcription in a protein synthesis-dependent fashion. We identify the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway and the atypical orphan nuclear receptor DAX1 (NR0B1) as critical mediators involved in RA-induced nNOS gene transcription. RA treatment increases DAX1 expression via PI3K/Akt signaling. Upregulation of DAX1 expression in turn induces nNOS transcription in response to RA. These results identify nNOS as a target gene of a novel RA/PI3K/Akt/DAX1-dependent pathway in human neuroblastoma cells and stress the functional importance of the transcriptional regulator DAX1 for nNOS gene expression in response to RA treatment.

Cannabinoid regulation of nitric oxide synthase I (nNOS) in neuronal cells

In our previous studies, CB(1) cannabinoid receptor agonists stimulated production of cyclic GMP and translocation of nitric oxide (NO)-sensitive guanylyl cyclase in neuronal cells (Jones et al., Neuropharmacology 54:23-30, 2008). The purpose of these studies was to elucidate the signal transduction of cannabinoid-mediated neuronal nitric oxide synthase (nNOS) activation in neuronal cells. Cannabinoid agonists CP55940 (2-[(1S,2R,5S)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol), WIN55212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate), and the metabolically stable analog of anandamide, (R)-(+)-methanandamide stimulated NO production in N18TG2 cells over a 20-min period. Rimonabant (N-(piperidin-lyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-H-pyrazole-3-carboxamide), a CB(1) receptor antagonist, partially or completely curtailed cannabinoid-mediated NO production. Inhibition of NOS activity (N ( G )-nitro-L: -arginine) or signaling via Gi/o protein (pertussis toxin) significantly limited NO production by cannabinoid agonists. Ca(2+) mobilization was not detected in N18TG2 cells after cannabinoid treatment using Fluo-4 AM fluorescence. Cannabinoid-mediated NO production was attributed to nNOS activation since endothelial NOS and inducible NOS protein and mRNA were not detected in N18TG2 cells. Bands of 160 and 155 kDa were detected on Western blot analysis of cytosolic and membrane fractions of N18TG2 cells, using a nNOS antibody. Chronic treatment of N18TG2 cells with cannabinoid agonists downregulated nNOS protein and mRNA as detected using Western blot analysis and real-time polymerase chain reaction, respectively. Cannabinoid agonists stimulated NO production via signaling through CB(1) receptors, leading to activation of Gi/o protein and enhanced nNOS activity. The findings of these studies provide information related to cannabinoid-mediated NO signal transduction in neuronal cells, which has important implications in the ongoing elucidation of the endocannabinoid system in the nervous system.

The untranslated region of exon 2 of the human neuronal nitric oxide synthase (NOS1) gene exerts regulatory activity

Expressional dysregulation of the human neuronal nitric oxide synthase (NOS1) gene represents an important mechanism in the pathogenesis of certain neuronal disease states. The structure and regulation of the human NOS1 gene is highly complex based on cell type- and stimulus-dependent usage of multiple exon 1 variants. Here we demonstrate that the untranslated region of exon 2 exerts promoter and enhancer functions as well, facilitated in large part by cooperative interaction of two conserved adjacent CREB/AP-1 binding sites. In human neuronal A673 cells, NOS1 expression is stimulated by several compounds which act through these sites, but also stimulate the combined promoter region of exons 1f and 1g. While stimulation of NOS1 expression by dibutyryl-cAMP is mediated by protein kinase A (blocked by H-89), the antiepileptic drug valproic acid is likely to activate phosphatidylinositol-3 kinase (inhibited by LY 294002).

Identification of nuclear factor-kappaB responsive element within the neuronal nitric oxide synthase exon 1f-specific promoter

Transcriptional regulation of the neuronal nitric oxide synthase gene (nNOS) is particularly complex as 12 distinct transcripts derived from different first exons are expressed in a tissue- and cell-specific manner. The exon 1f mRNA is relatively highly expressed in nervous system and relies upon exon 1f-specific promoter activity. Using conventional and real-time reverse transcription-polymerase chain reaction, we found exon 1f mRNA was the major transcript of the nNOS gene in human neuroblastoma SK-N-SH cells. We analyzed a 1090 bp fragment of 1f promoter by TRANSFAC-TESS and Match softwares and luciferase assay, and found an important positive transcriptional regulation region that contained a putative nuclear factor (NF)-kappaB binding site. Subsequently, using electrophoresis mobility shift and chromatin immunoprecipitation assays, we identified this site to be the NF-kappaB responsive element, a crucial positive regulator in the activation of the nNOS 1f promoter. Taken together, our study identified an NF-kappaB responsive element within nNOS 1f promoter and showed that it plays an important role in the transactivation of nNOS 1f mRNA, the major transcript of nNOS in SK-N-SH cells.

Regulation of neuronal nitric oxide synthase exon 1f gene expression by nuclear factor-kappaB acetylation in human neuroblastoma cells

The neuronal nitric oxide synthase (nNOS) is predominantly expressed in nervous tissues and subject to complex transcriptional controls. To determine the effect of acetylation on nNOS expression, human neuroblastoma SK-N-SH cells were treated with trichostatin A (TSA), a histone deacetylase inhibitor. As a consequence, total and exon 1f-specific nNOS mRNA, nNOS protein and nNOS-derived nitric oxide production were increased. Immunoprecipitation and western blot showed both nuclear factor-kappaB (NF-kappaB) subunits p65 and p50 were acetylated in the presence of TSA. The enhancement of the p65 and p50 acetylation was in accordance with their increased binding affinities to the NF-kappaB responsive element, which was identified at position -893 to -884 of the nNOS exon 1f promoter. Luciferase assays revealed that TSA up-regulated the transcriptional activity of the nNOS 1f promoter through NF-kappaB-mediated transactivation. Taken together, we demonstrate that acetylation plays a crucial role in nNOS expression and suggest that acetylation of NF-kappaB p65 and p50 subunits by TSA treatment may augment their DNA-binding affinities, thereby activating the nNOS exon 1f promoter. It may be one of the mechanisms by which acetylation modulates nNOS expression and nitric oxide output in SK-N-SH cells and may be the molecular basis for certain neurological disorders.

Complex splicing control of the human Thrombopoietin gene by intronic G runs

The human thrombopoietin (THPO) gene displays a series of alternative splicing events that provide valuable models for studying splicing mechanisms. The THPO region spanning exon 1–4 presents both alternative splicing of exon 2 and partial intron 2 (IVS2) retention following the activation of a cryptic 3′ splice site 85 nt upstream of the authentic acceptor site. IVS2 is particularly rich in stretches of 3–5 guanosines (namely, G1–G10) and we have characterized the role of these elements in the processing of this intron. In vivo studies show that runs G7–G10 work in a combinatorial way to control the selection of the proper 3′ splice site. In particular, the G7 element behaves as the splicing hub of intron 2 and its interaction with hnRNP H1 is critical for the splicing process. Removal of hnRNP H1 by RNA interference promoted the usage of the cryptic 3′ splice site so providing functional evidence that this factor is involved in the selection of the authentic 3′ splice site of THPO IVS2.

Transcription of human neuronal nitric oxide synthase mRNAs derived from different first exons is partly controlled by exon 1-specific promoter sequences

The human neuronal nitric oxide synthase (NOS1) gene is subject to extensive splicing. A total of 12 NOS1 mRNA species have been identified. They differ in their 5' ends and are derived from 12 different first exons (termed exons 1a to 1l). Various cell lines whose NOS1 first exon expression patterns were representative of human brain, skin, and skeletal muscle were identified. These included A673 neuroepithelioma cells, SK-N-MC neuroblastoma cells, HaCaT keratinocyte-like cells, and C2C12 myocyte-like cells. In these cell lines, correlations were found between the exon 1 variants preferentially expressed and the promoter activities of their cognate 5' flanking sequences. These data demonstrate that expression of the different exon 1-related splice variants of NOS1 mRNA is controlled directly (at least in part) by the associated 5' flanking sequences.

Nitric oxide synthase (NOS)-interacting protein interacts with neuronal NOS and regulates its distribution and activity

Mechanisms governing the activity of neuronal nitric oxide synthase (nNOS), the major source of nitric oxide (NO) in the nervous system, are not completely understood. We report here a protein-protein interaction between nNOS and NOSIP (nitric oxide synthase-interacting protein) in rat brain in vivo. NOSIP and nNOS are concentrated in neuronal synapses and demonstrate significant colocalization in various regions of the central and peripheral nervous systems. NOSIP produces a significant reduction in nNOS activity in a neuroepithelioma cell line stably expressing nNOS. Furthermore, overexpression of NOSIP in cultured primary neurons reduces the availability of nNOS in terminal dendrites. These results thus suggest that the interaction between NOSIP and nNOS is functionally involved in endogenous mechanisms regulating NO synthesis. Furthermore, we found that the subcellular distribution and expression levels of NOSIP are dynamically regulated by neuronal activity in vitro as well as in vivo, suggesting that NOSIP may contribute to a mechanism via which neuronal activity regulates the synaptic availability and activity of nNOS.

The Neuronal Nitric Oxide Synthase Is Upregulated in Mouse Skin Repair and in Response to Epidermal Growth Factor in Human HaCaT Keratinocytes

Expression of nNOS mRNA was found in normal human and mouse skin tissue. Upon wounding, we observed a rapid downregulation of nNOS mRNA and protein in wounds of mice; however, when repair continued, nNOS mRNA was strongly upregulated and nNOS protein expression peaked at late stages of healing. Immunohistochemistry revealed wound keratinocytes as the cellular source of nNOS. In line with the in vivo situation, we found a basal expression of nNOS in the human keratinocyte cell line HaCaT. A marked stimulation of nNOS expression in the cells was achieved with epidermal growth factor receptor (EGFR) ligands such as epidermal growth factor (EGF), heparin-binding EGF, transforming growth factor-alpha and two alternate splicing forms of the neuregulin gene. EGF-induced induction of nNOS was completely inhibited by the specific EGFR antagonist PD153035 and by the EGFR and Janus kinase 2/3 inhibitor AG490. Activation of EGFR might contribute to the observed upregulation of nNOS also in skin repair, as we found a spatial and temporal correlation of phosphorylated EGFR (Y1173) with nNOS expression at the wound site. Thus, in addition to the inducible- and endothelial-type NOS isoforms, also nNOS expression is regulated in the process of cutaneous wound repair.