Expression of neuronal-NOS in developing basal forebrain cholinergic neurons: regulation by NGF

(H)IF applicable: promotion of neurogenesis by induced HIF-2 signalling after ischaemia

HIF-2 represents a tissue-specific isoform of the hypoxia-inducible factors (HIFs) which regulate oxygen homeostasis in the cell. In acute oxygen deficiency, HIF transcription factors ensure the timely restoration of adequate oxygen supply. Particularly in medical conditions such as stroke, which have a high mortality risk due to ischaemic brain damage, rapid recovery of oxygen supply is of extraordinary importance. Nevertheless, the endogenous mechanisms are often not sufficient to respond to severe hypoxic stress with restoring oxygenation and fail to protect the tissue. Herein, we analysed murine neurospheres without functioning HIF-2α and found that special importance in the differentiation of neurons can be attributed to HIF-2 in the brain. Other processes, such as cell migration and signal transduction of different signalling pathways, appear to be mediated to some extent via HIF-2 and illustrate the function of HIF-2 in brain remodelling. Without hypoxic stress, HIF-2 in the brain presumably focuses on the fine-tuning of the neural network. However, a therapeutically increase of HIF-2 has the potential to regenerate or replace destroyed brain tissue and help minimize the consequences of an ischaemic stroke.

TNF-α Pretreatment Improves the Survival and Function of Transplanted Human Neural Progenitor Cells Following Hypoxic-Ischemic Brain Injury

Neural progenitor cells (NPCs) therapy offers great promise in hypoxic-ischemic (HI) brain injury. However, the poor survival of implanted NPCs in the HI host environment limits their therapeutic effects. Tumor necrosis factor-alpha (TNF-α) is a pleiotropic cytokine that is induced in response to a variety of pathological processes including inflammation and immunity. On the other hand, TNF-α has protective effects on cell apoptosis and death and affects the differentiation, proliferation, and survival of neural stem/progenitor cells in the brain. The present study investigated whether TNF-α pretreatment on human NPCs (hNPCs) enhances the effectiveness of cell transplantation therapy under ischemic brain. Fetal brain tissue-derived hNPCs were pretreated with TNF-α before being used in vitro experiments or transplantation. TNF-α significantly increased expression of cIAP2, and the use of short hairpin RNA-mediated knockdown of cIAP2 demonstrated that cIAP2 protected hNPCs against HI-induced cytotoxicity. In addition, pretreatment of hNPCs with TNF-α mediated neuroprotection by altering microglia polarization via increased expression of CX3CL1 and by enhancing expression of neurotrophic factors. Furthermore, transplantation of TNF-α-treated hNPCs reduced infarct volume and improved neurological functions in comparison with non-pretreated hNPCs or vehicle. These findings show that TNF-α pretreatment, which protects hNPCs from HI-injured brain-induced apoptosis and increases neuroprotection, is a simple and safe approach to improve the survival of transplanted hNPCs and the therapeutic efficacy of hNPCs in HI brain injury.

Growth factors for the treatment of ischemic brain injury (growth factor treatment)

In recent years, growth factor therapy has emerged as a potential treatment for ischemic brain injury. The efficacy of therapies that either directly introduce or stimulate local production of growth factors and their receptors in damaged brain tissue has been tested in a multitude of models for different Central Nervous System (CNS) diseases. These growth factors include erythropoietin (EPO), vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and insulin-like growth factor (IGF-1), among others. Despite the promise shown in animal models, the particular growth factors that should be used to maximize both brain protection and repair, and the therapeutic critical period, are not well defined. We will review current pre-clinical and clinical evidence for growth factor therapies in treating different causes of brain injury, as well as issues to be addressed prior to application in humans.

Enhanced expression of hypothalamic nitric oxide synthase in rats developmentally exposed to organophosphates

Nitric oxide synthase (NOS) is highly expressed in the hypothalamus, and nitric oxide (NO) specifically contributes to the regulation of neuronal activity within distinct hypothalamic regions. We studied the long-lasting effects of developmental exposure to low doses of organophosphate chlorpyrifos (CPF) and diazinon (DZN) on the expression of NOS in the hypothalamic subnuclei that subserve neuroendocrine, autonomic and cognitive functions. A daily dose of 1 mg/kg of either CPF or DZN was administered to developing rats during gestational days 15-18 or postnatal days (PND) 1-4. Brain sections from PND 60 rats were processed using NADPH-diaphorase (NADPH-d) and neuronal NOS (nNOS) immunohistochemistry. The number of labeled neurons and the optical density (OD) were assessed in the supraoptic (SON), paraventricular (PVN), medial septum, vertical limb, and horizontal limb of the diagonal band. Developmental exposure to organophosphates increased the number of labeled neurons and OD in different subnuclei in the hypothalamus without gender selectivity. The effect on OD was more pronounced and was significant for more cases. Prenatal exposure to CPF and DZN significantly increased the OD in all regions studied with the exception of PVN. Neonatal exposure to DZN also consistently increased OD in all studied subnuclei. For rats that treated with CPF during early postnatal period, this effect was statistically significant only for the SON and PVN. These findings suggest that overexpression of NOS in the hypothalamus may contribute to the mechanisms inducing or compensating for endocrine, autonomic and cognitive abnormalities after developmental exposure to organophosphates.

Detection of nitric oxide synthase (NOS) immunoreactive neurons in the human septal area: a matter of method?

There is a remarkable discrepancy between biochemical and cell morphological findings with regard to the presence of NADPH diaphorase/neuronal nitric oxide synthase (NOS) in the primate septal area. Whereas considerable concentrations of neuronal nitric oxide synthase and high enzyme activities have been measured in postmortem human septal nuclei, histochemical studies were either unable to detect any nitric oxide synthase immunoreactivity in primate septal neurons, or found only a very few nitrergic neurons in this region. This study aimed to investigate the possible presence of nitrergic neurons in human the septal region in greater detail. After having studied a total of 16 postmortem human brains we conclude that the immunohistochemical demonstration of nitric oxide synthase in human septal neurons is largely dependent on the mode of tissue handling: in brain specimens which were fixed en-bloc with paraffin and embedded in paraplast, nitric oxide synthase immunoreactivity is barely detectable, whereas a satisfying immunostaining is obtained on free-floating frozen sections after an immersion-fixation with 4% paraformaldehyde and 0.5% glutaraldehyde, followed by sucrose protection of the specimens. We show herein that there are indeed nitric oxide synthase-containing neurons in the human septum, thus supporting results from previous biochemical studies.

The role of NADPH oxidase, neuronal nitric oxide synthase and poly(ADP ribose) polymerase in oxidative neuronal death induced in cortical cultures by brain-derived neurotrophic factor and neurotrophin-4/5

Certain neurotrophins promote or induce oxidative neuronal death in cortical cultures. However, the effector mechanisms mediating this phenomenon have not been delineated. In this study, we investigated the possibility that NADPH oxidase and nitric oxide synthase (NOS) function as such effectors. Western blot analysis showed that treatment with brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-4/5 increased the levels of NADPH oxidase subunits. Moreover, neurotrophin treatment resulted in membrane translocation of p67phox, a characteristic feature of NADPH oxidase activation. Administration of the specific NADPH oxidase inhibitor, 4-(2-aminoethyl)benzenesulfonylfluoride (AEBSF), attenuated increases in oxygen free radicals thereby suggesting that NADPH oxidase contributes to the oxidative stress induced by neurotrophins. Furthermore, neuronal death induced by BDNF or NT-4/5 was significantly attenuated by AEBSF. Treatment with BDNF has previously been shown to induce neuronal NOS (nNOS). Our data indicated that inhibitors of nNOS attenuated neuronal death induced by BDNF or NT-4/5, consistent with an active role of nNOS in the mediation of neurotrophin neurotoxicity. As in other models of oxidative cell death, BDNF-induced neuronal death was accompanied by poly(ADP ribose) polymerase (PARP) activation. AEBSF or N-nitro-l-arginine (NNA) reduced BDNF-mediated PARP activation. PARP and poly(ADP ribose) glycohydrolase (PARG) are actively involved in mediating neurotrophin neurotoxicity since inhibitors of PARP and PARG significantly reduced levels of cell death. These results suggest that NADPH oxidase and nNOS contribute to increased oxidative stress, subsequent activation of PARP/PARG, and neuronal death induced by prolonged neurotrophin exposure.

Inhibitors of nitric oxide synthase attenuate nerve growth factor-mediated increases in choline acetyltransferase expression in PC12 cells

NGF can regulate nitric oxide synthase (NOS) expression and nitric oxide (NO) can modulate NGF-mediated neurotrophic responses. To investigate the role of NO in NGF-activated expression of cholinergic phenotype, PC12 cells were treated with either the nonselective NOS inhibitor L-NAME (N (omega)-nitro-L-arginine methylester) or the inducible NOS selective inhibitor MIU (s-methylisothiourea), and the effect on NGF-stimulated ChAT mRNA levels and ChAT specific activity was determined. NGF increased steady-state levels of mRNA and protein for both inducible and constitutive isozymes of NOS in PC12 cells, and led to enhanced NOS activity and NO production. MIU and, to a lesser extent, L-NAME blocked neurite outgrowth in nerve growth factor (NGF)-treated PC12 cells. Both L-NAME and MIU attenuated NGF-mediated increases in choline transferase (ChAT)-specific activity and prevented the increase in expression of ChAT mRNA normally produced by NGF treatment of PC12 cells. The present study indicates that NO may be involved in the modulation of signal transduction pathways by which NGF leads to increased ChAT gene expression in PC12 cells.

Structure and projections of white matter neurons in the postnatal rat visual cortex

Transient contributions of subplate neurons to the initial development of the cortex are well-characterized, yet little data are available on a subpopulation of subplate neurons that persist in the white matter (WM) of the cerebral cortex across development. To characterize the WM neurons, differential interference contrast and Nomarski optics were used to visualize individual cells in the WM in slices of rat visual cortex at postnatal ages 9-23. Soma-dendritic morphology and local axonal projection patterns, including probable synaptic innervation sites of their axons, were identified by intracellular filling with biocytin during electrophysiologic recordings. Dendritic branches of all WM neurons, tripartitioned here into upper, middle, and deep divisions, extend throughout the WM and frequently into the overlying cortex. Axonal arborizations from most WM neurons, including apparent boutons, project into adjacent WM with many also innervating overlying cortical layers, whereas some project into the stratum oriens/alveus of the hippocampal formation. Processes of a subset of WM neurons appear to be confined to the WM itself. By using antimicrotubule associated protein (MAP2) immunostaining to quantify the density of WM neurons in rat visual cortex, we find that their overall numbers decrease to approximately 30% of initial levels during postnatal development. During this same developmental period, an increasing percentage of WM neurons contain the synthetic enzyme for nitric oxide, nitric oxide synthase (NOS), as evaluated by immunostaining. Thus, WM neurons that survive the initial perinatal period of cell death are positioned under the laminae of the maturing cortex to potentially modulate the integration of visual signals through either conventional synaptic or nonconventional (diffusible NO signaling) mechanisms.

Cell-cycle arrest in TrkA-expressing NIH3T3 cells involves nitric oxide synthase

We have examined nerve growth factor (NGF)-triggered signaling in two NIH3T3 cell lines exogenously expressing the NGF receptor, TrkA. TRK1 cells cease to proliferate and extend long processes in response to NGF, while E25 cells continue to proliferate in the presence of NGF. These two cell lines express similar levels of TrkA and respond to NGF with rapid elevation of mitogen-activated protein kinase (MAPK) activity. MAPK activation is slightly more sustained for E25 cells than for TRK1 cells, although sustained activation of MAPK has been suggested to cause cell-cycle arrest. As judged by NADPH-diaphorase staining, nitric oxide synthase (NOS) activity is increased in TRK1 cells upon exposure to NGF. In contrast, diaphorase staining in E25 cells is unaffected by NGF treatment. Immunocytochemistry shows that levels of the brain NOS (bNOS) isoform are increased in TRK1, but not E25, cells exposed to NGF. Furthermore, Western blots show that NGF elevated cyclin-dependent kinase inhibitor, p21(WAF1), in TRK1 cells only. NGF-induced p21(WAF1) expression, cell-cycle arrest and process extension are abolished by N-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of NOS. The inactive enantiomer, D-NAME, did not inhibit these responses. Furthermore, even though E25 cells do not respond to NGF or nitric oxide donors, they do undergo a morphological change in response to NGF plus a nitric oxide donor. Therefore, NOS and p21(WAF1) are induced only in the TrkA-expressing NIH3T3 cell line that undergoes cell-cycle arrest and morphological changes in response to NGF. These results demonstrate that sustained activation of MAPK is not the sole determining factor for NGF-induced cell-cycle arrest and implicate NO in the cascade of events leading to NGF-induced morphological changes and cell-cycle arrest.

Overexpression of Akt inhibits NGF-induced growth arrest and neuronal differentiation of PC12 cells

To investigate the role of Akt in nerve growth factor (NGF)-induced neuronal differentiation, PC12 cells ectopically expressing wild-type or dominant-inhibitory forms of Akt were analyzed. NGF-induced neurite outgrowth was greatly accelerated in cells expressing dominant-inhibitory Akt, compared to parental PC12 cells, but was almost completely blocked in cells expressing wild-type Akt. Since neuronal differentiation requires an arrest of cell growth, several aspects of cell growth of the different cell lines were compared. Cells expressing wild-type Akt were not susceptible to the growth-arresting effect of NGF, whereas parental PC12 cells and notably cells expressing mutant Akt were so affected. Accompanying this, the expressions of CDKs and p21(WAF1) were down- and up-regulated, respectively, in both parental PC12 cells and cells expressing mutant Akt. When treated with some growth arrest-inducing agents such as sodium nitroprusside, forskolin and butyrolactone I, cells expressing wild-type Akt regained their responsiveness to the effects of NGF on differentiation. In summary, our results indicate that Akt overrides the growth-arresting effect of NGF and thereby, negatively regulates neuronal differentiation.

NT-4/5 exacerbates free radical-induced neuronal necrosis in vitro and in vivo

Neurotrophins render neurons highly vulnerable to certain injuries. We examined the possibility that NT-4/5 would enhance free radical neurotoxicity in vivo as well as in vitro. Striatal neurons exposed to 10 microM Fe(2+) or 1 mM l-buthionine-[S, R]-sulfoximine (BSO) underwent mild degeneration within 24 h. With concurrent addition of 10-100 ng/ml NT-4/5, neuronal death following exposure to Fe(2+) or BSO was significantly increased and suppressed by addition of 100 microM trolox, an antioxidant. In the adult brain, the intrastriatal injections of 20 nmol Fe(2+) revealed features of neuronal necrosis such as swelling cell body and mitochondria, fenestration of plasma membrane prior to nuclear membrane, and scattering condensation of nuclear chromatin. Cotreatment with 1.8 microg NT-4/5 augmented the striatal damage 24 h following the injections of Fe(2+). This study implies that free radicals produce necrotic degeneration in vivo as well as in vitro that becomes more sensitive in the presence of neurotrophins.

Retinoic acid-mediated enhancement of the cholinergic/neuronal nitric oxide synthase phenotype of the medial septal SN56 clone: establishment of a nitric oxide-sensitive proapoptotic state

It is unclear what mechanisms lead to the degeneration of basal forebrain cholinergic neurons in Alzheimer's or other human brain diseases. Some brain cholinergic neurons express neuronal nitric oxide (NO) synthase (nNOS), which produces a free radical that has been implicated in some forms of neurodegeneration. We investigated nNOS expression and NO toxicity in SN56 cells, a clonal cholinergic model derived from the medial septum of the mouse basal forebrain. We show here that, in addition to expressing choline acetyltransferase (ChAT), SN56 cells express nNOS. Treatment of SN56 cells with retinoic acid (RA; 1 microM) for 48 h increased ChAT mRNA (+126%), protein (+88%), and activity (+215%) and increased nNOS mRNA (+98%), protein (+400%), and activity (+15%). After RA treatment, SN56 cells became vulnerable to NO excess generated with S-nitro-N-acetyl-DL-penicillamine (SNAP) and exhibited increased nuclear DNA fragmentation that was blocked with a caspase-3 inhibitor. Treatment with dexamethasone, which largely blocked the RA-mediated increase in nNOS expression, or inhibition of nNOS activity with methylthiocitrulline strongly potentiated the apoptotic response to SNAP in RA-treated SN56 cells. Caspase-3 activity was reduced when SNAP was incubated with cells or cell lysates, suggesting that NO can directly inhibit the protease. Thus, whereas RA treatment converts SN56 cells to a proapoptotic state sensitive to NO excess, endogenously produced NO appears to be anti-apoptotic, possibly by tonically inhibiting caspase-3.

Partial cloning of constitutive and inducible nitric oxide synthases and detailed neuronal expression of NOS mRNA in the cerebellum and optic tectum of adult Atlantic salmon (Salmo salar)

Studies of different species have implicated nitric oxide (NO) synthase (NOS) in various physiological and pathological events. Three major NOS isoforms are present in the brain of mammals; endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS). Little is known about the significance of the presence of these proteins in the brain. We report the first investigation into the presence of nNOS and iNOS isoforms in a teleost, adult Atlantic salmon (Salmo salar). Complementary DNA was synthesized from cerebellum and thymus mRNA using RT-PCR techniques with primers against conserved regions of NOS. Cloning and sequencing revealed a partial gene sequence of 560 bp corresponding to mammalian nNOS from cerebellum cDNA. The predicted protein sequence of identified salmon nNOS possessed 85% identity to that of mammalian nNOS. Northern blot analysis of different tissues revealed expression in brain and heart, and indicated expression of three different nNOS mRNAs in the brain. In addition, a 389 bp sequence corresponding to iNOS was identified in thymus cDNA. Salmon iNOS is almost identical to rainbow trout iNOS (95%), but shows much less amino acid identity to goldfish (65%) and mammalian (52%) iNOS. Phylogenetically, all vertebrate nNOS and iNOS homologues are clustered separately. Expression studies by means of in situ hybridization revealed abundant nNOS mRNA transcripts in distinct neuronal populations throughout the Purkinje cell layer of the corpus cerebellum and the periventricular layer of the optic tectum. Our data show that adult Atlantic salmon possess a gene encoding an nNOS isoform and putative alternatively spliced forms, which are expressed in distinct neuronal populations in the cerebellum and optic tectum, and in yet unidentified cell types in the heart. The data suggest that the arising of different vertebrate NOS isoforms is an evolutionary old event. The well conserved sequences present in salmon and mammalian nNOS may reflect their importance in protein function, whereas interspecies distributional differences in cellular expression of nNOS and sequence differences of iNOS may reflect variations and specializations in routes of NO action in the vertebrate phylogeny.

Roles of NMDA receptor activity and nitric oxide production in brain development

The concept that neural activity is important for brain maturation has focused much research interest on the developmental role of the NMDA receptor, a key mediator of experience-dependent synaptic plasticity. However, a mechanism able to link spatial and temporal parameters of synaptic activity during development emerged as a necessary condition to explain how axons segregate into a common brain region and make specific synapses on neuronal sub-populations. To comply with this developmental constraint, it was proposed that nitric oxide (NO), or other substances having similar chemical and biological characteristics, could act as short-lived, activity-dependent spatial signals, able to stabilize active synapses by diffusing through a local volume of tissue. The present article addresses this issue, by reviewing the experimental evidence for a correlated role of the activity of the NMDA receptor and the production of NO in key steps of neural development. Evidence for such a functional coupling emerges not only concerning synaptogenesis and formation of neural maps, for which it was originally proposed, but also for some earlier phases of neurogenesis, such as neural cell proliferation and migration. Regarding synaptogenesis and neural map formation in some cases, there is so far no conclusive experimental evidence for a coupled functional role of NMDA receptor activation and NO production. Some technical problems related to the use of inhibitors of NO formation and of gene knockout animals are discussed. It is also suggested that other substances, known to act as spatial signals in adult synaptic plasticity, could have a role in developmental plasticity. Concerning the crucial developmental phase of neuronal survival or elimination through programmed cell death, the well-documented survival role related to NMDA receptor activation also starts to find evidence for a concomitant requirement of downstream NO production. On the basis of the reviewed literature, some of the major controversial issues are addressed and, in some cases, suggestions for possible future experiments are proposed.

Mature and developing visual system of Ceratitis capitata (Diptera, Tephritidae): histochemical evidence of nitric oxide synthase in the wild type and the white eye mutant strains

Nitric oxide (NO) is acknowledged as a messenger molecule in the nervous system. It has a role in the modulation of the chemosensory information and seems implicated also in visual processes and visually guided behaviour of some insects. In the present study, we used two different strains of the medfly Ceratitis capitata (Diptera, Tephritidae), a wild type eye colour and a white eye mutant line, as models to clarify the involvement of NO in the mature and developing visual system. The comparison between the pattern of enzyme histochemical localization of NO synthase (NOS), through NADPH diaphorase (NADPHd) staining, in the optic lobes of the two strains revealed for adults a stronger intensity of reaction in all the neuropiles and the sub-retinic monopolar cell layer of the wild type flies, with respect to the white eye mutant correspondent areas. Anti-NOS immunocytochemistry correlated with these results, underlying reactivity both in fine fibres and varicosities and in cell bodies and supporting the idea of presence of NOS also in the retina of the medfly optic lobes. NADPHd reactivity was present in the first developmental stages of the white eye mutant also, but at lower intensity than wild type, and it decreased in some areas during the transition to adult fly stage both in the wild type and in the white eye mutant. All these observations together indicate that changes in the NO system of C. capitata could be related to the visual information processing, when the visual response or discrimination are altered. Furthermore, NO may be involved in the establishment of the retinal projection pattern and in the control of optic lobes morphogenesis.

Neurotrophin-mediated potentiation of neuronal injury

The neurotrophins are a diverse family of peptides which activate specific tyrosine kinase-linked receptors. Over the past five decades, since the pioneering work of Levi-Montalcini and colleagues, the critical role that neurotrophins play in shaping the developing nervous system has become increasingly established. These molecules, which include the nerve growth factor (NGF)-related peptides, NGF, brain-derived neurotrophic factor (BDNF), NT-4/5 and NT-3, promote differentiation and survival in the developing nervous system, and to a lesser extent in the adult nervous system. As survival-promoting molecules, neurotrophins have been studied as potential neuroprotective agents, and have shown beneficial effects in many model systems. However, a surprising "dark side" to neurotrophin behavior has emerged from some of these studies implying that, under certain pathological conditions, neurotrophins may exacerbate, rather than alleviate, injury. How neurotrophins cause these deleterious consequences is a question which is only beginning to be answered, but initial work supports altered free radical handling or modification of glutamate receptor expression as possible mechanisms underlying these effects. This review will focus on evidence suggesting that neurotrophins may enhance injury under certain circumstances and on the mechanisms behind these injury-promoting aspects.

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.

The generation of nitric oxide by G protein-coupled receptors

The highly reactive free radical gas, nitric oxide, serves a variety of biomodulatory functions and has been implicated in a growing array of physiological and pathophysiological states. The striking differences between this labile substance and other, more conventional, signaling molecules highlight the tight degree of nitric oxide regulation that is required in order to maintain appropriate cellular homeostasis. The generation of nitric oxide represents a common component of the signal transduction pathways of a number of chemical signaling molecules that act via binding to G protein-coupled receptors. This review focuses on the relationship between this receptor superfamily, the generation of nitric oxide via the actions of the nitric oxide synthases and some of the inter- and intracellular roles of nitric oxide.

Analysis of NO synthase expression in neuronal, astroglial and fibroblast-like derivatives differentiating from PCC7-Mz1 embryonic carcinoma cells

We studied the expression of the NO synthase isoforms in an in vitro model of neural development using RT-PCR, Western blot and immunohistochemistry. Murine PCC7-Mz1 cells (Jostock et al., Eur. J. Cell Biol. 76, 63-76, 1998) differentiate in the presence of all-trans retinoic acid and dibutyryl cAMP along the neural pathway into neuron-like, fibroblast-like and astroglia-like cells. Undifferentiated cells showed immunofluorescent staining for neuronal-type NOS I and endothelial-type NOS III. This expression pattern was retained in those cells differentiating into neurofilament- and tau protein-positive neuronal cells. Thymocyte alloantigen (Thy1.2/CD 90.2)-positive fibroblasts, appearing around day 3, and glial fibrillary acidic protein (GFAP)-positive astroglial cells, appearing after day 6 of differentiation, stained negative for any NOS isoform. Starting at day 6 of differentiation, expression of inducible-type NOS II could be stimulated with cytokines in a subset of cells, which may represent activated astrocytes. NOS II was always undetectable in non-induced cultures. These data indicate that the ability of stem cells to express NOS I and NOS III is only retained when the cells differentiate along the neuronal lineage, while a small subpopulation of cells acquires the ability to express NOS II in response to cytokines.

A novel, nerve growth factor-activated pathway involving nitric oxide, p53, and p21WAF1 regulates neuronal differentiation of PC12 cells

During development, neuronal differentiation is closely coupled with cessation of proliferation. We use nerve growth factor (NGF)-induced differentiation of PC12 pheochromocytoma cells as a model and find a novel signal transduction pathway that blocks cell proliferation. Treatment of PC12 cells with NGF leads to induction of nitric oxide synthase (NOS) (Peunova, N., and Enikolopov, G. (1995) Nature 375, 68-73). The resulting nitric oxide (NO) acts as a second messenger, activating the p21(WAF1) promoter and inducing expression of p21(WAF1) cyclin-dependent kinase inhibitor. NO activates the p21(WAF1) promoter by p53-dependent and p53-independent mechanisms. Blocking production of NO with an inhibitor of NOS reduces accumulation of p53, activation of the p21(WAF1) promoter, expression of neuronal markers, and neurite extension. To determine whether p21(WAF1) is required for neurite extension, we prepared a PC12 line with an inducible p21(WAF1) expression vector. Blocking NOS with an inhibitor decreases neurite extension, but induction of p21(WAF1) with isopropyl-1-thio-beta-D-galactopyranoside restored this response. Levels of p21(WAF1) induced by isopropyl-1-thio-beta-D-galactopyranoside were similar to those induced by NGF. Therefore, we have identified a signal transduction pathway that is activated by NGF; proceeds through NOS, p53, and p21(WAF1) to block cell proliferation; and is required for neuronal differentiation by PC12 cells.

Differential susceptibility to neurotoxicity mediated by neurotrophins and neuronal nitric oxide synthase

NMDA neurotoxicity, which is mediated, in part, by formation of nitric oxide (NO) via activation of neuronal NO synthase (nNOS), is modulated by neurotrophins. nNOS expression in rat and mouse primary neuronal cultures grown on a glial feeder layer is significantly less than that of neurons grown on a polyornithine (Poly-O) matrix. Neurotrophins markedly increase the number of nNOS neurons, nNOS protein, and NOS catalytic activity and enhance NMDA neurotoxicity via NO-dependent mechanisms when neurons are grown on glial feeder layers. In contrast, when rat or mouse primary cortical neurons are grown on a Poly-O matrix, neurotrophins have no influence on nNOS neuronal number or NOS catalytic activity and reduce NMDA neurotoxicity. Primary neuronal cultures from mice lacking nNOS grown on a glial feeder layer fail to respond to neurotrophin-mediated enhancement of neurotoxicity. Together, these results indicate that nNOS expression and NMDA NO-mediated neurotoxicity are dependent, in part, on the culture paradigm, and neurotrophins regulate the susceptibility to NMDA neurotoxicity via modulation of nNOS. Furthermore, these results support the idea that NMDA neurotoxicity in culture is critically dependent on the developmental state of the neurons being assessed and suggest that, when cortical neurons are cultured on a glial feeder layer, they do not reach nearly as mature a phenotype as when grown on a Poly-O matrix.

Barbiturate-induced expression of neuronal nitric oxide synthase in the rat cerebellum

Neuronal nitric oxide synthase (nNOS) is known to share some structural and functional similarities with the cytochrome P450 mixed function oxidase system. Unlike P450, it does not require a second enzyme, reductase, to transfer electrons. This characteristic is similar to P450(BM-3) of Bacillus megaterium. P450(BM-3) and certain mammalian subfamilies of P450, such as P4502B, are known to be induced by phenobarbital (PB), and these P450s share a consensus sequence called the Barbie box. Because of the similarities nNOS shares with P450(BM-3) and other mammalian P450s, we have examined whether nNOS also responds to PB treatment. We have used semi-quantitative PCR, Western blot analysis, a functional assay, and immunohistochemistry in order to answer this question. These data show a threefold increase in nNOS mRNA expression, more modest nNOS protein and activity induction, and no discernible changes in localization of nNOS within the cerebellum following PB treatment.