Transcription of the human neuronal nitric oxide synthase gene in the central nervous system is mediated by multiple promoters

Molecular mechanisms of neuronal nitric oxide synthase in cardiac function and pathophysiology

Neuronal nitric oxide synthase (nNOS or NOS1) is the major endogenous source of myocardial nitric oxide (NO), which facilitates cardiac relaxation and modulates contraction. In the healthy heart it regulates intracellular Ca(2+), signalling pathways and oxidative homeostasis and is upregulated from early phases upon pathogenic insult. nNOS plays pivotal roles in protecting the myocardium from increased oxidative stress, systolic/diastolic dysfunction, adverse structural remodelling and arrhythmias in the failing heart. Here, we show that the downstream target proteins of nNOS and underlying post-transcriptional modifications are shifted during disease progression from Ca(2+)-handling proteins [e.g. PKA-dependent phospholamban phosphorylation (PLN-Ser(16))] in the healthy heart to cGMP/PKG-dependent PLN-Ser(16) with acute angiotensin II (Ang II) treatment. In early hypertension, nNOS-derived NO is involved in increases of cGMP/PKG-dependent troponin I (TnI-Ser(23/24)) and cardiac myosin binding protein C (cMBP-C-Ser(273)). However, nNOS-derived NO is shown to increase S-nitrosylation of various Ca(2+)-handling proteins in failing myocardium. The spatial compartmentation of nNOS and its translocation for diverse binding partners in the diseased heart or various nNOS splicing variants and regulation in response to pathological stress may be responsible for varied underlying mechanisms and functions. In this review, we endeavour to outline recent advances in knowledge of the molecular mechanisms mediating the functions of nNOS in the myocardium in both normal and diseased hearts. Insights into nNOS gene regulation in various tissues are discussed. Overall, nNOS is an important cardiac protector in the diseased heart. The dynamic localization and various mediating mechanisms of nNOS ensure that it is able to regulate functions effectively in the heart under stress.

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.

Hypoxia induces a functionally significant and translationally efficient neuronal NO synthase mRNA variant

We tested the hypothesis that induction of neuronal NO synthase (nNOS) impairs vascular smooth muscle contractility after hypoxia. nNOS protein was increased in aorta, mesenteric arterioles, pulmonary arteries, brain, and diaphragm from rats exposed to 8% O2 for 48 hours and in human aortic SMCs after hypoxic incubation (1% O2). Ca-dependent NO synthase activity was increased in endothelium-denuded aortic segments from hypoxia-exposed rats. N-nitro-L-arginine methyl ester enhanced the contractile responses of endothelium-denuded aortic rings and mesenteric arterioles from hypoxia-exposed but not normoxic rats (P < 0.05). The hypoxia-inducible mRNA transcript expressed by human cells was found to contain a novel 5'-untranslated region, consistent with activation of transcription in the genomic region contiguous with exon 2. Translational efficiency of this transcript is markedly increased compared with previously described human nNOS mRNAs. Transgenic mice possessing a lacZ reporter construct under control of these genomic sequences demonstrated expression of the construct after exposure to hypoxia (8% O2, 48 hours) in the aorta, mesenteric arterioles, renal papilla, and brain. These results reveal a novel human nNOS promoter that confers the ability to rapidly upregulate nNOS expression in response to hypoxia with a functionally significant effect on vascular smooth muscle contraction.

Expression of nitric oxide synthase isoforms is reduced in late-gestation ovine fetal brainstem

Fetal baroreflex responsiveness increases in late gestation. An important modulator of baroreflex activity is the generation of nitric oxide in the brainstem nuclei that integrate afferent and efferent reflex activity. The present study was designed to test the hypothesis that nitric oxide synthase (NOS) isoforms are expressed in the fetal brainstem and that the expression of one or more of these enzymes is reduced in late gestation. Brainstem tissue was rapidly collected from fetal sheep of known gestational ages (80, 100, 120, 130, 145 days gestation and 1 day and 1 wk postnatal). Neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) mRNA was measured using real-time PCR methodology specific for ovine NOS isoforms. The three enzymes were measured at the protein level using Western blot methodology. In tissue prepared for histology separately, the cellular pattern of immunostaining was identified in medullae from late-gestation fetal sheep. Fetal brainstem contained mRNA and protein of all three NOS isoforms, with nNOS the most abundant, followed by iNOS and eNOS, respectively. nNOS and iNOS mRNA abundances were highest at 80 days' gestation, with statistically significant decreases in abundance in more mature fetuses and postnatal animals. nNOS and eNOS protein abundance also decreased as a function of developmental age. nNOS and eNOS were expressed in neurons, iNOS was expressed in glia, and eNOS was expressed in vascular endothelial cells. We conclude that all three isoforms of NOS are constitutively expressed within the fetal brainstem, and the expression of all three forms is reduced with advancing gestation. We speculate that the reduced expression of NOS in this brain region plays a role in the increased fetal baroreflex activity in late gestation.

Mechanisms of M1 muscarinic receptor-mediated up-regulation of neuronal nitric oxide synthase in N1E-115 neuroblastoma cells

The neuronal form of nitric oxide synthase (nNOS) was generally assumed to be constitutively expressed at a constant level. However, it is now becoming recognized that its expression can be modulated by a number of physiological and pathophysiological conditions. Previously, we reported that nNOS expression is up-regulated after prolonged muscarinic M(1) receptor stimulation. In this work, we report that muscarinic receptor activation signals the up-regulation of nNOS via multiple pathways in N1E-115 mouse neuroblastoma cells. These include protein kinase C (PKC) activation, cytosolic calcium mobilization and NO production. Further characterization showed that the half-life of nNOS is slightly, but significantly, increased in agonist-pretreated cells compared with vehicle-treated control cells. Based on these data, it appears that the level of nNOS expression is modulated in a complex manner by a number of mechanisms that include, but might not be limited to, those described here.

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

An extensive screening of the human neuronal nitric oxide synthase (nNOS) mRNAs in various human tissues and cell lines unraveled an extreme complexity in the transcription of this gene. Using 5' rapid amplification of cDNA ends (5'-RACE), ten different exons 1 (named 1a-1l) were identified. They were spliced in a cell-specific manner to a common exon 2, which bears the translational start site. Three first exons (1d, 1g and 1f) were used predominantly for the transcription of the nNOS gene (146 out of 197 5'-RACE clones contained these exons). Exon 1k was found alone, but in many instances was interposed between exons 1b, 1d, 1g, 1i or 1j and the common exon 2. In addition to the cell-specific heterogeneity of human nNOS transcripts, nNOS is highly regulated at the transcriptional level. In resting A673 neuroepithelioma cells, the prevalent nNOS transcript was the exon 1g mRNA (with minor expression of exons 1d+1k and exon 1f mRNAs). When the cells were treated with dibutyryl-cAMP, nNOS mRNA was markedly upregulated. This upregulation was solely due to an increase in exon 1f mRNA, while the expression of the other mRNA species remained unchanged. Human HaCat keratinocyte-like cells expressed the exon 1i+1k and 1i nNOS transcripts under basal conditions. When stimulated with epidermal growth factor, only the exon 1i+1k transcript was upregulated. Although these nNOS transcripts do not differ in their translated region, the various mRNAs may trigger post-transcriptional effects such as changes in mRNA stability and translation efficiency.

Rapid expression of neuronal and inducible nitric oxide synthases during post-ischemic reperfusion in rat brain

OBJECTIVE

To determine whether neuronal and inducible nitric oxide synthase (nNOS and iNOS) isoforms are expressed within cortical neurons during early reperfusion after focal cerebral ischemia.

METHODS

Male spontaneously hypertensive rats underwent occlusion of the left middle cerebral artery for 2 h. Coronal brain sections with normal and ischemic cortex were obtained after 15 min or 1, 6 or 24 h of reperfusion. Immunohistochemical and double-label immunofluorescent techniques were used to confirm cellular identity and localize nNOS and iNOS.

RESULTS

Immunoreactive nNOS was identified within isolated neurons in layer V of normal cortex. However, the number of nNOS-immunoreactive neurons in ischemic cortex rose markedly at 15 min and persisted for 24 h (P< or =0.001 at each time point when compared to normal cortex). Cells that were immunoreactive for nNOS appeared in perivascular clusters within ischemic brain at all sampling times. Immunoreactive iNOS was also expressed within neurons in ischemic cortex, peaking after 15 min of reperfusion (P< or =0.01). Although nNOS-immunoreactive neurons were observed in random numbers within normal tissue throughout reperfusion, iNOS-immunoreactive neurons increased steadily in the same region (P< or =0.05).

CONCLUSIONS

Ischemic neurons become immunoreactive for both nNOS and iNOS during early reperfusion. Expression of iNOS immunoreactivity in unaffected neurons may reflect transcription of immediate early genes in response to stimulatory neurotransmission from ischemic cortex.

The 5'2 promoter of the neuronal nitric oxide synthase dual promoter complex mediates inducibility by nerve growth factor

Neuronal nitric oxide synthase (nNOS) is induced by nerve growth factor (NGF) in pheochromocytoma PC12 cells. Previous studies from our laboratory identified two closely linked promoters (designated 5'1 and 5'2) that mediate transcription of the human nNOS gene in the brain [J. Xie, P. Roddy, T.K. Rife, F. Murad, A.P. Young, Two closely linked but separable promoters for human neuronal nitric oxide synthase gene transcription, Proc. Natl. Acad. Sci. U. S. A. 92 (1995) 1242-1246]. In this report, we demonstrate that luciferase fusion genes under transcriptional control by the 5'1 and 5'2 dual promoter complex are inducible by NGF in stably transformed PC12 cells. In sharp contrast, neither epidermal growth factor (EGF) nor fibroblast growth factor 2 (FGF2) are able to significantly enhance the expression of NOS-luciferase fusion genes. Deletion studies indicate that the 5'2 promoter plays a major role in mediating NGF inducibility. The 5'2 promoter contains six potential Ets binding sites as well as four potential AP1 binding sites. Thus, it is possible that activation of Ets and/or AP1 transcription factors by the Ras-Raf-MAP kinase cascade contributes to the NGF-mediated induction of nNOS.

Neuronal-type NO synthase: transcript diversity and expressional regulation

Of the three established isoforms of NO synthase, the gene for the neuronal-type enzyme (NOS I) is by far the largest and most complicated one. The genomic locus of the human NOS I gene is located on chromosome 12 and distributed over a region greater than 200 kb. The nucleotide sequence corresponding to the major neuronal mRNA transcript is encoded by 29 exons. The full-length open reading frame codes for a protein of 1434 amino acids with a predicted molecular weight of 160.8 kDa. However, both in rodents and in humans, multiple, tissue-specific or developmentally regulated NOS I mRNA transcripts have been reported. They arise from the initiation by different transcriptional units containing alternative promoters (at least eight in the human gene), cassette exon deletions or insertions, and/or the usage of alternate polyadenylation signals. Depending on the insertions and deletions, translation results in functional or nonfunctional proteins. The use of alternative promoters can influence gene expression by various means. Indeed, NOS I is not a static, constitutively expressed enzyme, but subject to expressional regulation by various compounds and conditions. The molecular mechanisms underlying these regulations are currently being studied in several laboratories including our own.