Use of chimeric forms of neuronal nitric-oxide synthase as dominant negative mutants

Because the functional form of neuronal nitric-oxide synthase (nNOS) is a homodimer, we investigated whether we could disrupt dimer formation with inactive nNOS chimeras acting as dominant negative mutants. To test this hypothesis, we either expressed the heme and reductase regions of rat nNOS as single domains or produced fusion proteins between the rat nNOS heme domain and various other electron-shuttling proteins. A dominant negative potential of these constructs was demonstrated by their ability to reduce NOS activity when transfected into a cell line stably expressing rat nNOS. In the presence of these nNOS mutant proteins, cellular levels of inactive nNOS monomers were significantly increased, indicating that their mechanism of action is through the disruption of nNOS dimer formation. These dominant negative mutants should prove valuable in analyzing the role of nNOS in biological systems.

The synergistic action of ethanol and nerve growth factor in the induction of neuronal nitric oxide synthase

Ethanol alone had no effect on neuronal nitric oxide synthase (nNOS) expression in PC12 cells. However, in the presence of nerve growth factor (NGF), nNOS expression was amplified (threefold, P < 0.05), compared to NGF alone. This increase was eliminated with pretreatment of PC12 cells with staurosporine, suggesting that the effects of ethanol on nNOS expression are mediated by a protein kinase C-dependent pathway.

Chimeric forms of neuronal nitric oxide synthase identify different regions of the reductase domain that are essential for dimerization and activity

Nitric oxide synthase (NOS) is the enzyme responsible for the conversion of L-arginine to L-citrulline and nitric oxide. Dimerization of the enzyme is an absolute requirement for catalytic activity. Each NOS monomer contains an N-terminal heme-binding domain and a C-terminal reductase domain. It is unclear how the reductase domain is involved in controlling dimerization and whether dimer formation alone controls enzyme activity. Our initial studies demonstrated that no dimerization or activity could be detected when the reductase domain of rat neuronal NOS (nNOS) was expressed either separately or in combination with the heme domain. To further evaluate the reductase domain, a set of expression plasmids was created by replacing the reductase domain of nNOS with other electron-transport proteins, thereby creating nNOS chimeric fusion proteins. The rat nNOS heme domain was linked with either cytochrome P450 reductase, adrenodoxin reductase, or the reductase domain from Bacillus megaterium cytochrome P450, BM-3. All the chimeric enzymes retained the ability to dimerize but were unable to metabolize L-arginine (<8% of wildtype activity levels), indicating that dimerization alone is insufficient to produce an active enzyme. Because the greatest regions of homology between electron-transport proteins are in the flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide phosphate (NADPH) binding regions, we produced truncation mutants within the nNOS reductase domain to investigate the role of these sequences in the ability of nNOS to dimerize and to metabolize L-arginine. The results demonstrated that the deletion of the final 56 amino acids or the NADPH-binding region had no effect on dimerization but produced an inactive enzyme. However, when the FAD-binding site (located between amino acids 920 and 1161) was deleted, both activity and dimerization were abolished. These results implicate sequences within the FAD-binding site as essential for nNOS dimerization but sequences within amino acids 1373 to 1429 as essential for activity.

Both neuronal NO synthase and nitric oxide are required for PC12 cell differentiation: a cGMP independent pathway

PC12 cells are used as a model system to study neuronal differentiation. Nerve growth factor (NGF) triggers a differentiation pathway in PC12 cells. Neurite outgrowth (a morphological marker of differentiation) in PC12 cells is significantly reduced in the presence of the NOS inhibitor l-NAME, but not d-NAME, implicating NOS in the differentiation process. Previously we have shown that the neuronal NO synthase (nNOS) isoform is induced in PC12 cells in the presence of NGF. Thus, we wished to further evaluate the role of nNOS and NO in PC12 cell differentiation. When a dominant negative mutant nNOS expression vector was transiently transfected into NGF-treated PC12 cells, it significantly reduced PC12 cell neurite outgrowth. Thus, we concluded that the NO required for PC12 cell differentiation, in response to NGF, is produced by nNOS. NO alone was insufficient to induce differentiation as cells treated with the NO donor, sodium nitroprusside did not produce neurites. Treatment of PC12 cells with oxyhemoglobin (an NO scavenger) was also found to significantly reduce the number of neurites produced by PC12 cells treated with NGF. Thus, NO appears to be necessary, but not sufficient, to induce differentiation, and its mode of action appears to be extracellular. A well documented action of NO is to activate soluble guanylate cyclase. Thus, we determined the role of soluble guanylate cyclase activation as a means by which NO induces PC12 cell differentiation. However, in the presence of NGF (to prime PC12 cells for differentiation) and l-NAME (to specifically remove the NO component), 8Br-cGMP (a cGMP analog) failed to induce PC12 cell differentiation. In addition, blockade of sGC activity with specific inhibitors failed to block NGF-induced PC12 cell differentiation. We conclude that the NO required for PC12 cell differentiation is produced by nNOS and that the NO exerts its effects on surrounding PC12 cells in a sGC/cGMP independent manner.

Coordinated regulation of genes of the nitric oxide and endothelin pathways during the development of pulmonary hypertension in fetal lambs

Ligation of the ductus arteriosus in utero produces fetal and neonatal pulmonary hypertension and alterations in the hemodynamic responses to nitric oxide and endothelin-1 in fetal and newborn lambs. To determine whether fetal pulmonary hypertension alters the expression of the genes of the nitric oxide and endothelin-1 pathways, seven fetal lambs (123-126-d gestation) underwent ligation of the ductus arteriosus. Near-term (138-139-d gestation), total lung RNA, and protein were prepared from control and ductal ligation fetal lambs for RNase protection assays and Western blotting. Ligation of the ductus arteriosus was associated with decreased expression of endothelial nitric oxide synthase mRNA and protein, and the alpha1 and the beta1 subunits of soluble guanylate cyclase protein; and with increased expression of phosphodiesterase V mRNA. Ligation of the ductus arteriosus was also associated with increased expression of preproendothelin-1 mRNA and with decreased expression of endothelin B receptor (ET(B)) mRNA. These results suggest that there is coordinated regulation of genes of the nitric oxide pathway, which would decrease nitric oxide and cGMP concentration, thereby decreasing pulmonary vasodilator activity. There is also coordinated regulation of genes of the endothelin-1 pathway, which would increase endothelin-1 concentration and limit ET(B) receptor activation, thereby increasing pulmonary vasoconstrictor activity. These alterations in gene expression would increase fetal pulmonary vascular resistance, contributing to the development of pulmonary hypertension after birth.

Increased endothelial NOS in lambs with increased pulmonary blood flow and pulmonary hypertension

Altered pulmonary vascular reactivity is a source of morbidity and mortality for children with congenital heart defects and increased pulmonary blood flow. Nitric oxide (NO) is an important mediator of pulmonary vascular reactivity. The objective of this study was to characterize potential early alterations in expression, localization, and activity of endothelial NO synthase (eNOS) induced by increased pulmonary blood flow and pulmonary hypertension. Utilizing aortopulmonary vascular graft placement in the fetal lamb, we have established a unique animal model of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. Ten fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). RNase protection assays and Western blotting were performed on lung tissue prepared from 4-wk-old shunt lambs and age-matched controls. eNOS mRNA (2.4:1, P < 0.05) and protein (2. 08:1, P < 0.05) were increased in lungs of shunt lambs. In situ hybridization and immunohistochemistry revealed that the increase was confined to the endothelium of pulmonary arteries. eNOS protein (1.55:1, P < 0.05) and tissue cGMP concentrations (2.1:1, P < 0.05) were also increased in isolated fifth-generation pulmonary arteries of shunt lambs. In addition, total lung eNOS activity was increased (2.9:1, P < 0.05). Thus we report a previously undescribed, early upregulation of eNOS gene expression and activity in lambs with increased pulmonary blood flow and pulmonary hypertension.

Nitric oxide exposure inhibits endothelial NOS activity but not gene expression: a role for superoxide

Recent studies have characterized a rebound pulmonary vasoconstriction with abrupt withdrawal of inhaled nitric oxide (NO) during therapy for pulmonary hypertension, suggesting that inhaled NO may downregulate basal NO production. However, the exact mechanism of this rebound pulmonary hypertension remains unclear. The objectives of these studies were to determine the effect of NO exposure on endothelial NO synthase (eNOS) gene expression, enzyme activity, and posttranslational modification in cultured pulmonary arterial endothelial cells. Sodium nitroprusside (SNP) treatment had no effect on eNOS mRNA or protein levels but did produce a significant decrease in enzyme activity. Furthermore, although SNP treatment induced protein kinase C (PKC)-dependent eNOS phosphorylation, blockade of PKC activity did not protect against the effects of SNP. When the xanthine oxidase inhibitor allopurinol or the superoxide scavenger 4,5-dihydroxy-1-benzene-disulfonic acid were co-incubated with SNP, the inhibitory effects on eNOS activity could be partially alleviated. Also, the levels of superoxide were found to be elevated 4.5-fold when cultured pulmonary arterial endothelial cells were exposed to the NO donor spermine/NO. This suggests that NO can stimulate xanthine oxidase to cause an increase in cellular superoxide generation. A reaction between NO and superoxide would produce peroxynitrite, which could then react with the eNOS protein, resulting in enzyme inactivation. This mechanism may explain, at least in part, how NO produces NOS inhibition in vivo and may delineate, in part, the mechanism of rebound pulmonary hypertension after withdrawal of inhaled NO.

Regulation of ductus arteriosus patency by nitric oxide in fetal lambs: the role of gestation, oxygen tension, and vasa vasorum

We hypothesized that nitric oxide (NO) production by the fetal ductus arteriosus is limited because of low fetal PO2, but that at neonatal PO2, NO might be an important regulator of ductus arteriosus tone. We exposed isolated rings of fetal lamb ductus arteriosus to elevated PO2. L-NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase (NOS), and methylene blue and 6-anilino-5,8-quinolinedione (LY83583), inhibitors of guanylate cyclase, produced constriction of the ductus arteriosus. When ductus arteriosus rings were exposed to low PO2, L-NAME had no effect, and methylene blue and LY83583 had only a small effect on ductus arteriosus tone. Sodium nitroprusside and calcium ionophore A23187 relaxed ductus arteriosus rings more than aortic rings, and relaxed ductus arteriosus rings from immature fetuses more than those from late gestation fetuses. In contrast, ductus arteriosus rings from both early and late gestation were equally sensitive to 8-bromo-cGMP. By both reverse transcriptase-polymerase chain reaction and immunohistochemistry, endothelial cell NOS and inducible calcium-independent NOS, but not nerve cell NOS, were detected in the ductus arteriosus. Inducible NOS was expressed only by endothelial cells lining the ductus arteriosus lumen; in contrast, endothelial cell NOS was expressed by both luminal and vasa vasorum endothelial cells. The role of inducible NOS in the ductus arteriosus is uncertain because the potency of a specific inducible NOS inhibitor in constricting the ductus arteriosus was negligible compared with that of an endothelial cell NOS inhibitor. We speculate that NO may be an important regulator of ductus arteriosus tone at high but not low PO2. The endothelial cell NOS isoform found in vasa vasorum may be an important source of NO because removal of ductus arteriosus luminal endothelium only partially blocks the effects of L-NAME, methylene blue, and LY83583.

Inducible nitric oxide synthase and the effect of aminoguanidine in experimental neonatal meningitis

This study explored the role of inducible nitric oxide (NO) synthase (iNOS) in an infant rat model of group B streptococcal meningitis. Brain iNOS activity increased during meningitis (P < .001), and iNOS was detected by immunocytochemistry in the walls of meningeal vessels and cells of the cerebrospinal fluid (CSF) inflammation. Animals treated with iNOS inhibitor aminoguanidine (AG; 130 mg/kg every 8 h) had reduced NO production (P < .05), higher CSF bacterial titers (P < .05), and increased incidence of seizures (P < .01) compared with untreated infected animals. AG also increased areas of severe hypoperfusion in the cortex (31% +/- 14% in controls vs. 56% +/- 16% in AG; P < .01) and the extent of cortical neuronal injury, both when administered at the time of infection (P < .05) and in established meningitis (P < .02). Thus, NO produced by iNOS may be beneficial in this model of experimental meningitis by reducing cerebral ischemia.

Growth factor induction of nitric oxide synthase in rat pheochromocytoma cells

Previous work has suggested that nerve growth factor treatment of PC12 cells induces neuronal nitric oxide synthase, and possibly also endothelial nitric oxide synthase (NOS) and inducible NOS. To further analyze this process we exposed rat pheochromocytoma (PC12) cells to increasing concentrations of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), nerve growth factor (NGF), and vascular endothelial cell growth factor (VEGF). Changes in NOS expression were then analyzed by Western blotting, using antisera generated against the three isoforms of NOS. Our results demonstrate that neuronal NOS was induced by growth factors that promote both differentiation (bFGF, NGF) and proliferation (EGF). nNOS levels were unaffected by VEGF treatment. In contrast, the levels of endothelial and inducible NOS were undetectable in these same cells, suggesting that different clonal lines of PC12 cells have different isoform complements.

Ventilation and oxygenation induce endothelial nitric oxide synthase gene expression in the lungs of fetal lambs

At birth, ventilation and oxygenation immediately decrease pulmonary vascular resistance (PVR) and increase pulmonary blood flow (PBF); more gradual changes occur over the next several hours. Nitric oxide, produced by endothelial nitric oxide synthase (eNOS), mediates these gradual changes. To determine how ventilation and oxygenation affect eNOS gene expression, 12 fetal lambs were ventilated for 8 h without changing fetal descending aortic blood gases or pH (rhythmic distension) or with 100% oxygen (O2 ventilation). Vascular pressures and PBF were measured. Total RNA, protein, and tissue sections were prepared from lung tissue for RNase protection assays, Western blotting, and in situ hybridization. O2 ventilation increased PBF and decreased PVR more than rhythmic distension (P < 0.05). Rhythmic distension increased eNOS mRNA expression; O2 ventilation increased eNOS mRNA expression more and increased eNOS protein expression (P < 0.05). To define the mechanisms responsible for these changes, ovine fetal pulmonary arterial endothelial cells were exposed to 1, 21, or 95% O2 or to shear stress. 95% O2 increased eNOS mRNA and protein expression (P < 0.05). Shear stress increased eNOS mRNA and protein expression (P < 0.05). Increased oxygenation but more importantly increased PBF with increased shear stress induce eNOS gene expression and contribute to pulmonary vasodilation after birth.

Increased expression of nitric oxide synthase in the myometrium of the pregnant rat uterus

Nitric oxide (NO) relaxes uterine smooth muscle and is produced by the pregnant uterus. Our previous studies revealed an increase in rat uterine NO synthase (NOS) activity in pregnancy and a decline at term. In the present study, we have examined the distribution of NOS isoform expression to determine whether their regulation is consistent with a role in the inhibition of uterine contractions before term. At day 17-18 of pregnancy, NOS immunohistochemistry revealed expression of two isoforms: endothelial constitutive form of NOS (ecNOS) in vascular endothelium and inducible form of NOS (iNOS) in myometrial and vascular smooth muscle and in decidual epithelium. Immunoblotting revealed that expression of iNOS declined nearly fivefold, whereas ecNOS declined twofold in laboring rats at term. We conclude that iNOS is expressed in myometrium of pregnant rat uterus but not the virgin rat and that iNOS expression declines at term when labor is present. The pattern of changes in myometrial iNOS expression with advancing gestation suggests that NO could act in an autocrine and/or paracrine manner to inhibit uterine contractions before term.

Regulation of neuronal nitric oxide synthase through alternative transcripts

Nitric oxide (NO) participates in diverse physiological processes ranging from neurotransmission to muscle relaxation. Neuronal-derived NO can be either beneficial or detrimental depending on the cellular context. Neuronal NO synthase (nNOS) must therefore be tightly regulated. One level of regulation involves synthesis of numerous nNOS mRNA transcripts. At least six distinct molecular species of nNOS mRNA are expressed in a tissue and developmentally-regulated manner. Alternative splicing allows the creation of nNOS proteins differing in both enzymatic characteristics and structural features. As one example, we find that there are nNOS mRNAs lacking exon 2. One isoform, nNOS beta, retains full enzymatic activity but lacks a major protein-protein interaction domain (PDZ domain) responsible for targeting nNOS to synaptic membranes. This alternative splicing produces a mislocalized but fully active protein which may be relevant to certain pathologies. As evidence of this, we find that many human brain tumors express an alternatively spliced form of nNOS that co-migrates with nNOS beta, and lacks exon 2. Finally, we also find a 2.5-kb testis-specific nNOS mRNA corresponding to the C-terminal reductase domain of nNOS whose function is unclear.

Construction of a P450c27 fusion enzyme: a useful tool for analysis of vitamin D3 25-hydroxylase activity

Liver mitochondrial P450c27, encoded by the CYP27 gene, can catalyse the 25-hydroxylation of vitamin D3 and the 27-hydroxylation of sterols. To facilitate the study of this enzyme in cell culture systems, we engineered a fusion protein consisting of P450c27 coupled to its electron-transport accessory proteins, ferredoxin and ferredoxin reductase, and assessed its enzyme activity by measuring the C-25 and C-27 (side-chain) hydroxylation of 1 alpha-hydroxyvitamin D3 (1 alpha-OH-D3). When incubated with 1 alpha-OH-D3, COS-1 cells transfected with a vector expressing the fusion protein produced 1 alpha,25-(OH)2D2 and 1 alpha,27-(OH)2D3 about four times more efficiently than did cells transfected with three individual components of the fusion. When incubated with the natural substrate, vitamin D3, the efficiency of hydroxylation was lower than that for 1 alpha-OH-D3 but still 1.7-fold higher for the fusion protein than for its individual components. The fusion protein was also able to reproduce qualitatively and quantitatively the activity shown by P450c27 in liver cells in situ. The P450c27-ferredoxin reductase-ferredoxin fusion construct represents a valuable tool for establishing the substrate specificity of this liver cytochrome and for evaluating its potential for activating pro-drug analogues of vitamin D.

Neonatal mice lacking neuronal nitric oxide synthase are less vulnerable to hypoxic-ischemic injury

We hypothesized that elimination of neuronal nitric oxide synthase (nNOS) by targeted disruption of the nNOS gene would result in amelioration of damage seen after hypoxia-ischemia in the developing brain since nitric oxide (NO) has been implicated in glutamate-mediated neurotoxicity after ischemia. Both wildtype and nNOS-deficient pups were subjected to focal ischemia followed by 1.5 h of hypoxia at Postnatal Day 7. Seven days later, brains of surviving animals were analyzed for damage. The nNOS-deficient pups (n = 17) had less histopathologic evidence of injury in both the hippocampus (P = 0.008) and the cortex (P = 0.0008) than the wildtype (n = 30) mice. When injured, the nNOS-deficient mice had damage that was limited to the hippocampus. These results support a role for neuronally produced NO in injury after perinatal hypoxia-ischemia.

Selective destruction of nitric oxide synthase neurons with quisqualate reduces damage after hypoxia-ischemia in the neonatal rat

The vulnerability of the developing CNS to hypoxia-ischemia (H-I) differs from that of the mature brain and is due in part to release of nitric oxide (NO) from parenchymal neurons. If NO is important in the generation of excitotoxic injury after H-I in the developing CNS, then selective destruction of the neuronal nitric oxide synthase (nNOS) cells before H-I should lessen the injury seen after the insult. Using low dose quisqualic acid (QA) injected into neonatal (postnatal d 7) parietal cortex, the nNOS neurons were eliminated while sparing other neuronal and glial populations as ascertained by NADPH diaphorase histochemistry, nNOS immunocytochemistry, and Nissl counterstain. Animals subjected to focal ischemia followed by global hypoxia 24 h after the intracortical injection of QA had more viable cortex remaining than vehicle-injected animals (83.4 +/- 4.3% versus 62.7 +/- 8.3%) and lower injury severity represented by less neuronal loss and gliosis. Intracortical injections of QA without H-I resulted in minimal cell loss at the injection site with elimination of nNOS neurons throughout the parietal cortex. Microglial and astrocytic proliferation was seen in areas damaged by H-I 3 wk after injury and clearly marked infarcted areas. Prevention or elimination of NO production from nNOS cells can prevent much of the delayed neuronal necrosis seen after H-I in the developing CNS.

Characterization of rat neuronal nitric oxide synthase expressed in Saccharomyces cerevisiae

A cDNA encoding rat neuronal nitric oxide synthase (nNOS) was cloned into the yeast expression vector pMA56 to generate pA379. Transformation of Saccharomyces cerevisiae strain BJ2168 with this plasmid resulted in the synthesis of nNOS at levels of 0.5-1.0 mg/liter. The protein is localized in the cytosol and is catalytically active as determined by the conversion of [3H]-L-arginine to [3H]-L-citrulline and NO. The enzyme was purified by calmodulin-Sepharose affinity chromatography and its catalytic activity was found to be both calcium and calmodulin dependent. Overexpression of nNOS in S. cerevisiae and purification of the recombinant protein will facilitate detailed characterization of this important enzyme.

Expression of neuronal nitric oxide synthase corresponds to regions of selective vulnerability to hypoxia-ischaemia in the developing rat brain

Nitric oxide (NO) has been implicated in the pathogenesis of brain injury from hypoxia-ischaemia. In the brain, the enzyme responsible for NO synthesis is neuronal nitric oxide synthase (nNOS). Using in situ hybridization, immunohistochemistry and NADPH diaphorase histochemistry, we examined the spatial and temporal expression of nNOS during development of the rat brain to determine whether the expression of nNOS delineates the areas of the brain that are selectively vulnerable to hypoxic-ischaemia injury. The expression of nNOS was localized to discrete areas of the brain. nNOS could be detected in the developing forebrain in the 10-day-old embryo (E10). From E14 to E18, the highest level of expression was in the cortical plate, where the majority of neurons were positive. However, this expression diminished with time; in the adult there were only a few nNOS-positive neurones in the deep layers of the cortex. Expression of nNOS was not detected prenatally in the basal ganglia. There was transient high-level expression during the first postnatal week. Thereafter, the basal ganglia exhibited the adult pattern of expression. Expression of nNOS could be detected in the hippocampus at E16. This expression remained constant with regional localization in layers CA1 and CA3 in the adult. Similarly, nNOS expression in the developing cerebellum was observed only after birth. From the first day after birth (P1) to P6, expression was limited to the molecular cell layer. As the cerebellum matured, nNOS expression could be detected in the inner granular layer. By P21, the adult distribution of nNOS expression was observed. All regions expressing nNOS mRNA also demonstrated nNOS protein expression and NADPH diaphorase catalytic activity. Our results demonstrate that nNOS expression in the developing brain correlates with regions of selective vulnerability to hypoxic-ischaemic injury, and, therefore, supports a role for NO in hypoxic-ischaemic injury in the developing brain.

Expression and characterization of truncated human heme oxygenase (hHO-1) and a fusion protein of hHO-1 with human cytochrome P450 reductase

A human heme oxygenase (hHO-1) gene without the sequence coding for the last 23 amino acids has been expressed in Escherichia coli behind the pho A promoter. The truncated enzyme is obtained in high yields as a soluble, catalytically-active protein, making it available for the first time for detailed mechanistic studies. The purified, truncated hHO-1/heme complex is spectroscopically indistinguishable from that of the rat enzyme and converts heme to biliverdin when reconstituted with rat liver cytochrome P450 reductase. A self-sufficient heme oxygenase system has been obtained by fusing the truncated hHO-1 gene to the gene for human cytochrome P450 reductase without the sequence coding for the 20 amino acid membrane binding domain. Expression of the fusion protein in pCWori+ yields a protein that only requires NADPH for catalytic turnover. The failure of exogenous cytochrome P450 reductase to stimulate turnover and the insensitivity of the catalytic rate toward changes in ionic strength establish that electrons are transferred intramolecularly between the reductase and heme oxygenase domains of the fusion protein. The Vmax for the fusion protein is 2.5 times higher than that for the reconstituted system. Therefore, either the covalent tether does not interfere with normal docking and electron transfer between the flavin and heme domains or alternative but equally efficient electron transfer pathways are available that do not require specific docking.

Prenatal diagnosis of congenital lipoid adrenal hyperplasia

Congenital lipoid adrenal hyperplasia (lipoid CAH) is a rare genetic disorder of adrenal and gonadal steroidogenesis of unknown cause in which cholesterol cannot be converted to pregnenolone. As a result, affected individuals can make no steroid hormones, so that all affected newborns are phenotypic females, irrespective of karyotype. We studied two pregnancies in a family with two previously affected children by examining fetal karyotype, genital ultrasonography, and amniotic fluid steroid concentrations and by performing ACTH tests on family members. Prenatal diagnosis correctly identified both an unaffected XX fetus and an affected XY fetus. In the affected pregnancy, amniotic fluid concentrations of progesterone and pregnenolone were 30% and 50% of normal, respectively, but concentrations of 17 alpha-hydroxypregnenolone, 17 alpha-hydroxyprogesterone, cortisol, dehydroepiandrosterone, androstenedione, and estriol were either extremely low or undetectable, suggesting that these detected steroids were donated by maternal steroidogenesis. Fetal cord blood obtained at the termination of pregnancy showed very low concentrations of estrogens donated by the mother's circulation. Absent fetal steroidogenesis was confirmed by gas chromatography and mass spectrometry of both fetal and maternal serum. The responses of 10 different steroids to adrenal stimulation with ACTH in the obligately heterozygous parents were normal. Thus, unlike the case with other forms of CAH, heterozygosity cannot be determined by hormonal responses to provocative testing with ACTH. Immunocytochemistry and Western blotting showed that the affected placental tissue contained P450scc protein, confirming that P450scc is intact in these patients.

NOS induction by NGF in basal forebrain cholinergic neurones: evidence for regulation of brain NOS by a neurotrophin

Nerve growth factor (NGF) acts through trkA receptors to serve as a trophic factor for cholinergic neurones in the medial septal nucleus (MSN) and vertical limb of the diagonal band (VDB). Herein, we show that brain nitric oxide synthase (NOS), which synthesizes the neuromodulator nitric oxide, is selectively expressed in a large fraction of trkA-containing neurones in the MSN and VDB. Axotomy of these neurones gave evidence that NOS expressing cholinergic neurones innervate the hippocampus. NGF infusion induced a robust, specific increase in NOS expression in basal forebrain cholinergic neurones. These results indicate that brain NOS can be regulated by a neurotrophic factor and suggest that NGF influences forebrain function by regulating production of nitric oxide as well as acetylcholine.

Nitric oxide synthesized by gonadotropin-releasing hormone neurons is a mediator of N-methyl-D-aspartate (NMDA)-induced GnRH secretion

N-Methyl-D-aspartate (NMDA) directly stimulates gonadotropin-releasing hormone (GnRH) neurons to secrete GnRH. It is not known if this stimulatory effect of NMDA is mediated by NO. Northern blot analysis of the immortalized hypothalamic GnRH neuronal cells (GT1-1) mRNA with a neuronal NOS cDNA revealed this clonal cell line expressed neuronal NOS transcripts as a single 10.5-kb band. Immunoblot analysis of GT1-1 proteins with anti-neuronal NOS serum showed that the GT1-1 cells contain neuronal NOS. GT1-1 cells were used to study the effects of NO and NMDA on GnRH release. L-Arginine (10(-2) M), a precursor of NO enhances basal GnRH secretion. Both oxyhemoglobin (Hb)(10(-6)-10(-4) M), a NO scavenger and N omega-nitro-L-arginine (NNA)(10(-3),10(-2) M), a NOS inhibitor and inactivator block basal as well as NMDA-induced GnRH release. Sodium nitroprusside (SNP) (10(-4), 10(-3) M), a NO donor stimulates GnRH release, an effect inhibited by Hb. Incubation of GT1-1 cells in Ca(2+)-free medium abolished the stimulatory effect of NMDA on GnRH release. In contrast, incubation in medium with increasing concentrations of Ca2+ enhances basal GnRH release as well as augments NMDA-mediated GnRH release. The results demonstrate that L-arginine-NO pathway is functional in the GT1-1 cells and an increase in intracellular Ca2+ [Ca2+]i following NMDA receptor activation activates NOS to generate NO. We conclude that endogenous NO mediates, at least in part, basal as well as NMDA-stimulated GnRH release and may play a role as an intercellular messenger in synchronizing pulsatile GnRH release.

The mitochondrial environment is required for activity of the cholesterol side-chain cleavage enzyme, cytochrome P450scc

Steroidogenesis is initiated by the conversion of cholesterol to pregnenolone by mitochondrial cytochrome P450scc [cholesterol, reduced-adrenal-ferredoxin:oxygen oxidoreductase (side-chain-cleaving); EC 1.14.15.6]. Several subsequent steroidal conversions occur in the endoplasmic reticulum (ER), but the last step in the production of glucocorticoids and mineralocorticoids again occurs in the mitochondria. Although cellular compartmentalization of steroidogenic enzymes appears to be a feature of all steroidogenic pathways, some reports indicate that cholesterol can be converted to pregnenolone outside the mitochondria. To investigate whether P450scc can function outside the mitochondria, we constructed vectors producing P450scc and various fusion enzymes of P450scc with electron-transport proteins and directed their expression to either the ER or the mitochondria. Whether targeted to mitochondria or to the ER, plasmid vectors encoding P450scc and fusion proteins of P450scc with either mitochondrial or microsomal electron-transport proteins produced immunodetectable protein. When expressed in mitochondria, all of these constructions converted 22-hydroxycholesterol to pregnenolone, but when expressed in the ER none of them produced pregnenolone. These results show that P450scc can function only in the mitochondria. Furthermore, it appears to be the mitochondrial environment that is required, rather than the specific mitochondrial electron-transport intermediates.