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

Nitric oxide negatively regulates proliferation and promotes neuronal differentiation through N-Myc downregulation

Nitric oxide (NO) has been found to act as an important negative regulator of cell proliferation in several systems. We report here that NO negatively regulates proliferation of neuronal cell precursors and promotes their differentiation by downregulating the oncogene N-Myc. We have studied this regulatory function of NO in neuroblastoma cell lines (SK-N-BE) and in primary cerebellar granule cell cultures. In a neuronal NO synthase (nNOS) overexpressing neuroblastoma cell line exposed to the differentiative action of retinoic acid, NO slowed down proliferation and accelerated differentiation towards a neuronal phenotype. This effect was accompanied by a parallel decrease of N-Myc expression. Similar results could be obtained in parental SK-N-BE cells by providing an exogenous source of NO. Pharmacological controls demonstrated that NO's regulatory actions on cell proliferation and N-Myc expression were mediated by cGMP as an intermediate messenger. Furthermore, NO was found to modulate the transcriptional activity of N-Myc gene promoter by acting on the E2F regulatory region, possibly through the control of Rb phosphorylation state, that we found to be negatively regulated by NO. In cerebellar granule cell cultures, NOS inhibition increased the division rate of neuronal precursors, in parallel with augmented N-Myc expression. Because a high N-Myc expression level is essential for neuroblastoma progression as well as for proliferation of neuronal precursors, its negative regulation by NO highlights a novel physiopathological function of this important messenger molecule.

Nitric Oxide Is a Regulator of Hematopoietic Stem Cell Activity

Hematopoietic stem cells give rise to various multipotent progenitor populations, which expand in response to cytokines and which ultimately generate all of the elements of the blood. Here we show that it is possible to increase the number of stem and progenitor cells in the bone marrow (BM) by suppressing the activity of NO synthases (NOS). Exposure of mice to NOS inhibitors, either directly or after irradiation and BM transplantation, increases the number of stem cells in the BM. In the transplantation model, this increase is followed by a transient increase in the number of neutrophils in the peripheral blood. Thus, our results indicate that NO is important for the control of hematopoietic stem cells in the BM. They further suggest that suppression of NO synthase activity may allow expansion of the number of hematopoietic stem and progenitor cells or neutrophils for therapeutic purposes.

YY1 binding to a subset of p53 DNA-target sites regulates p53-dependent transcription

The tumor suppressor protein p53 regulates gene transcription through binding to specific DNA-target sites. We here demonstrate that a subset of these sites is targeted by another DNA-binding factor. Binding specificity, reactivity with specific antibodies, and experiments with purified protein identified the factor as the multifunctional transcription regulator YY1. The YY1 core binding sequence ACAT is present in the center of p53-half-binding sites in the p21 and GADD45 genes regulating growth arrest and DNA repair, respectively, but is absent in those of the Bax gene critical for apoptosis. In transfection experiments YY1 inhibits p53-activated transcription from the p53-binding site that contains the ACAT sequence. YY1 and p53 are colocalized around the nucleoli and in discrete nuclear domains in PC12 cells undergoing apoptosis. YY1 might attenuate p53-dependent transcription from a subset of p53-target genes and this may be relevant for directing cells either to growth arrest or apoptosis upon p53 activation.

The localization of neuronal nitric oxide synthase may influence its role in neuronal precursor proliferation and synaptic maintenance

Neuronal nitric oxide synthase (nNOS) is implicated in some developmental processes, including neuronal survival, differentiation, and precursor proliferation. To define the roles of nNOS in neuronal development, we utilized the olfactory system as a model. We hypothesized that the role of nNOS may be influenced by its localization. nNOS expression was developmentally regulated in the olfactory system. During early postnatal development, nNOS was expressed in developing neurons in the olfactory epithelium (OE), while in the adult its expression was restricted to periglomerular (PG) cells in the olfactory bulb (OB). At postnatal week 1 (P1W), loss of nNOS due to targeted gene deletion resulted in a decrease in immature neurons in the OE due to decreased proliferation of neuronal precursors. While the pool of neuronal precursors and neurogenesis normalized in the nNOS null mouse by P6W, there was an overgrowth of mitral or tufted cells dendrites and a decreased number of active synapses in the OB. Cyclic GMP (cGMP) immunostaining was reduced in the OE and in the glomeruli of the OB at early postnatal and adult ages, respectively. Our results suggest that nNOS appears necessary for neurogenesis in the OE during early postnatal development and for glomerular organization in the OB in the adult. Thus, the location of nNOS, either within cell bodies or perisynaptically, may influence its developmental role.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Nitric oxide acts as a slow-down and search signal in developing neurites

Nitric oxide (NO) has been demonstrated to act as a signaling molecule during neuronal development, but its precise function is unclear. Here we investigate whether NO might function at the neuronal growth cone to affect growth cone motility. We have previously demonstrated that growth cones of identified neurons from the snail Helisoma trivolvis show a rapid and transient increase in filopodial length in response to NO, which was regulated by soluble guanylyl cyclase (sGC) [S. Van Wagenen and V. Rehder (1999) J. Neurobiol., 39, 168-185]. Because in vivo studies have demonstrated that growth cones have longer filopodia and advance more slowly in regions where pathfinding decisions are being made, this study aimed to establish whether NO could function as a combined 'slow-down and search signal' for growth cones by decreasing neurite outgrowth. In the presence of the NO donor NOC-7, neurites of B5 neurons showed a concentration-dependent effect on neurite outgrowth, ranging from slowing at low, stopping at intermediate and collapsing at high concentrations. The effects of the NO donor were mimicked by directly activating sGC with YC-1, or by increasing its product with 8-bromo-cGMP. In addition, blocking sGC in the presence of NO with NS2028 blocked the effect of NO, suggesting that NO affected outgrowth via sGC. Ca2+ imaging of growth cones with Fura-2 indicated that [Ca2+]i increased transiently in the presence of NOC-7. These results support the hypothesis that NO can function as a potent slow/stop signal for developing neurites. When coupled with transient filopodia elongation, this phenomenon emulates growth cone searching behavior.

DeltaNp73beta is active in transactivation and growth suppression

p73, a p53 family protein, shares significant sequence homolog and functional similarity with p53. However, unlike p53, p73 has at least seven alternatively spliced isoforms with different carboxyl termini (p73alpha-eta). Moreover, the p73 gene can be transcribed from a cryptic promoter located in intron 3, producing seven more proteins (DeltaNp73alpha-eta). DeltaNp73, which does not contain the N-terminal activation domain in p73, has been thought to be transcriptionally inactive and dominant negative over p53 or p73. To systemically analyze the activity of the DeltaN variant, we generated stable cell lines, which inducibly express DeltaNp73alpha, DeltaNp73beta, and various DeltaNp73beta mutants by using the tetracycline-inducible expression system. Surprisingly, we found that DeltaNp73beta is indeed active in inducing cell cycle arrest and apoptosis. Importantly, we found that, when DeltaNp73beta is expressed at a physiologically relevant level, it is capable of suppressing cell growth. We then demonstrated that these DeltaNp73beta activities are not cell type specific. We showed that the 13 unique residues at the N terminus are required for DeltaNp73beta to suppress cell growth. We also found that, among the 13 residues, residues 6 to 10 are critical to DeltaNp73beta function. Furthermore, we found that DeltaNp73beta is capable of inducing some p53 target genes, albeit to a lesser extent than does p73beta. Finally, we found that the 13 unique residues, together with the N-terminal PXXP motifs, constitute a novel activation domain. Like DeltaNp73beta, DeltaNp73gamma is active in transactivation. However, unlike DeltaNp73beta, DeltaNp73alpha is inactive in suppressing cell growth. Our data, together with others' previous findings, suggest that DeltaNp73beta may have distinct functions under certain cellular circumstances.

Depletion of deoxyribonucleoside triphosphate pools in tumor cells by nitric oxide

Nitric oxide displays pro- and anti-tumor activities, prompting further studies to better understand its precise role. Nitric oxide inhibits ribonucleotide reductase (RnR), the limiting enzyme for de novo dNTP synthesis. We report here the first detailed analysis of dNTP variations induced in tumor cells by NO. NO prodrugs induced a depletion in dNTP pools and an activation of the pyrimidine salvage pathway, as did hydroxyurea, the prototypic RnR inhibitor. In the presence of dipyridamole, which blocked salvaged dNTP synthesis, depletion of dNTP pools was also observed in tumor cells cocultured with macrophages expressing the high-output iNOS activity. This effect was rapid, reversible, blocked by NO scavengers, and cGMP independent. It was quantitatively correlated to iNOS activity. In the absence of dipyridamole, NO still induced a decrease in dATP concentration in tumor cells cocultured with macrophages, whereas surprisingly, concentrations of dCTP and dTTP expanded considerably, resulting in a strong imbalance in dNTP pools. NO prodrugs did not cause such an increase in pyrimidine dNTP, suggesting that pyrimidine nucleosides were released by NO-injured macrophages. Altered dNTP levels have been reported to promote mutagenesis and apoptosis. It is suggested that abnormal changes in dNTP pools in tumors might contribute to NO-dependent toxicity.

The inhibition of nitric oxide synthase enhances PSA-NCAM expression and CREB phosphorylation in the rat hippocampus

It is well known that nitric oxide (NO) acts downstream of NMDA receptor activation, which regulates the neural plasticity in the brain. In the present study, the effect of L-NAME, a non-selective nitric oxide synthase (NOS) inhibitor, on neural plasticity in the hippocampus was investigated. L-NAME increased the expression of PSA-NCAM and pCREB in the adult rat hippocampus. The co-localization of PSA-NCAM and pCREB indicates a possible relationship between the two in the granule cell layer in the dentate gyrus. Our results demonstrate that NO, as a subsignal of NMDA receptors, could be involved in the structural plasticity of the granule cell layer in the dentate gyrus by regulating the expression of PSA-NCAM and pCREB in the hippocampus.

Nitric oxide and cyclic nucleotides are regulators of neuronal migration in an insect embryo

The dynamic regulation of nitric oxide synthase (NOS) activity and cGMP levels suggests a functional role in the development of nervous systems. We report evidence for a key role of the NO/cGMP signalling cascade on migration of postmitotic neurons in the enteric nervous system of the embryonic grasshopper. During embryonic development, a population of enteric neurons migrates several hundred micrometers on the surface of the midgut. These midgut neurons (MG neurons) exhibit nitric oxide-induced cGMP-immunoreactivity coinciding with the migratory phase. Using a histochemical marker for NOS, we identified potential sources of NO in subsets of the midgut cells below the migrating MG neurons. Pharmacological inhibition of endogenous NOS, soluble guanylyl cyclase (sGC) and protein kinase G (PKG) activity in whole embryo culture significantly blocks MG neuron migration. This pharmacological inhibition can be rescued by supplementing with protoporphyrin IX free acid, an activator of sGC, and membrane-permeant cGMP, indicating that NO/cGMP signalling is essential for MG neuron migration. Conversely, the stimulation of the cAMP/protein kinase A signalling cascade results in an inhibition of cell migration. Activation of either the cGMP or the cAMP cascade influences the cellular distribution of F-actin in neuronal somata in a complementary fashion. The cytochemical stainings and experimental manipulations of cyclic nucleotide levels provide clear evidence that NO/cGMP/PKG signalling is permissive for MG neuron migration, whereas the cAMP/PKA cascade may be a negative regulator. These findings reveal an accessible invertebrate model in which the role of the NO and cyclic nucleotide signalling in neuronal migration can be analyzed in a natural setting.

Nitric oxide negatively regulates mammalian adult neurogenesis

Neural progenitor cells are widespread throughout the adult central nervous system but only give rise to neurons in specific loci. Negative regulators of neurogenesis have therefore been postulated, but none have yet been identified as subserving a significant role in the adult brain. Here we report that nitric oxide (NO) acts as an important negative regulator of cell proliferation in the adult mammalian brain. We used two independent approaches to examine the function of NO in adult neurogenesis. In a pharmacological approach, we suppressed NO production in the rat brain by intraventricular infusion of an NO synthase inhibitor. In a genetic approach, we generated a null mutant neuronal NO synthase knockout mouse line by targeting the exon encoding active center of the enzyme. In both models, the number of new cells generated in neurogenic areas of the adult brain, the olfactory subependyma and the dentate gyrus, was strongly augmented, which indicates that division of neural stem cells in the adult brain is controlled by NO and suggests a strategy for enhancing neurogenesis in the adult central nervous system.

Altered neuronal nitric oxide synthase expression in the cerebellum of calcium channel mutant mice

Tottering, rolling Nagoya, and leaner mutant mice all exhibit cerebellar ataxia to varying degrees, from mild (tottering mice) to severe (leaner mice). Collectively, these mice are regarded as tottering locus mutants because each of these mutant mice expresses a different autosomal recessive mutation in the gene coding for the alpha(1A) calcium ion channel protein, which is the pore forming subunit for P/Q-type high voltage activated calcium ion channels. These mutant mice all exhibit varying degrees of cerebellar dysfunction and neuronal cell death. Nitric oxide (NO) is an important messenger molecule in the central nervous system, especially in the cerebellum, and it is produced via the enzyme, nitric oxide synthase (NOS). We investigated expression of neuronal-NOS (n-NOS) in the cerebella of all three mutant mice, as revealed by NADPH-diaphorase (NADPH-d) histochemical staining, quantitation of n-NOS protein using Western blotting and quantitation of n-NOS mRNA using in situ hybridization. The expression of n-NOS mRNA and protein as well as the NADPH-d histochemical reaction were elevated in tottering and rolling Nagoya cerebella. n-NOS mRNA and the NADPH-d histochemical reaction were decreased in the leaner cerebellum, but the leaner mouse n-NOS protein concentration was not significantly different compared to age- and gender-matched controls. These findings suggest that NO may act as an important mediator in the production of the neuropathology observed in these mutant mice.

The chronic inhibition of nitric oxide synthase enhances cell proliferation in the adult rat hippocampus

We investigated the effect of chronic blocking nitric oxide synthase (NOS), an enzyme producing NO from L-arginine, on granule cell proliferation in the dentate gyrus of adult rats under normal conditions. We treated 7-nitroindazole (7-NI) for 5, 15, and 25 days or N-nitro-L-arginine-methyl ester (L-NAME) for 25 days to block NOS activity and subsequently injected 5-bromo-2-deoxyuridine (BrdU) to detect proliferating cells. The BrdU-immunoreactive (IR) cell number was significantly increased in the 7-NI 15 and 25 day treated group, but not in the control or in the 7-NI 5 day treated group. L-NAME treatment for 25 days significantly increased BrdU-IR cells versus the control and 7-NI 25 day treated group. In addition, nissl staining showed no cell death occurred in the dentate gyrus after 7-NI or L-NAME 25 day treatments. Our results demonstrate that chronic inhibition of NOS increases cell proliferation and has no effect on cell death in the dentate gyrus of the rat hippocampus, which suggests that NO may regulate cell proliferation in the dentate gyrus.

Nitric oxide-mediated inhibition of Hdm2-p53 binding

It has become increasingly evident that nitric oxide exerts its effects, in part, by S-nitrosylation of cysteine residues. We tested in vitro whether nitric oxide may indirectly control p53 by S-nitrosylation and inactivation of the p53 negative regulator, Hdm2. Treatment of Hdm2 with a nitric oxide donor inhibits Hdm2-p53 binding, a critical step in Hdm2 regulation of p53. The presence of excess amounts of cysteine or dithiothreitol blocks this inhibition of binding. Moreover, nitric oxide inhibition of Hdm2-p53 binding was found to be reversible. Sulfhydryl sensitivity and reversibility are consistent with nitrosylation. Finally, we have identified a critical cysteine residue that nitric oxide modifies to disrupt Hdm2-p53 binding. This cysteine is proximal to the Hdm2-p53 binding interface and is conserved across species from zebrafish to humans. Mutation of this residue from a cysteine to an alanine does not interfere with binding but rather eliminates the sensitivity of Hdm2 to nitric oxide inactivation.

Sildenafil (Viagra) induces neurogenesis and promotes functional recovery after stroke in rats

BACKGROUND AND PURPOSE

We tested the hypothesis that sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, promotes functional recovery and neurogenesis after stroke.

METHODS

Male Wistar rats were subjected to embolic middle cerebral artery occlusion. Sildenafil (Viagra) was administered orally for 7 consecutive days starting 2 or 24 hours after stroke onset at doses of 2 or 5 mg/kg per day. Ischemic rats administered the same volume of tap water were used as a control group. Functional outcome tests (foot-fault, adhesive removal) were performed. Rats were killed 28 days after stroke for analysis of infarct volume and newly generated cells within the subventricular zone and the dentate gyrus. Brain cGMP levels, expression of PDE5, and localized cerebral blood flow were measured in additional rats.

RESULTS

Treatment with sildenafil significantly (P<0.05) enhanced neurological recovery in all tests performed. There was no significant difference of infarct volume among the experimental groups. Treatment with sildenafil significantly (P<0.05) increased numbers of bromodeoxyuridine-immunoreactive cells in the subventricular zone and the dentate gyrus and numbers of immature neurons, as indicated by betaIII-tubulin (TuJ1) immunoreactivity in the ipsilateral subventricular zone and striatum. The cortical levels of cGMP significantly increased after administration of sildenafil, and PDE5 mRNA was present in both nonischemic and ischemic brain.

CONCLUSIONS

Sildenafil increases brain levels of cGMP, evokes neurogenesis, and reduces neurological deficits when given to rats 2 or 24 hours after stroke. These data suggest that this drug that is presently in the clinic for sexual dysfunction may have a role in promoting recovery from stroke.

Essential role of cAMP-response element-binding protein activation by A2A adenosine receptors in rescuing the nerve growth factor-induced neurite outgrowth impaired by blockage of the MAPK cascade

We found in the present study that stimulation of the A(2A) adenosine receptor (A(2A)-R) using an A(2A)-selective agonist (CGS21680) rescued the blockage of nerve growth factor (NGF)-induced neurite outgrowth when the NGF-evoked MAPK cascade was suppressed by an MEK inhibitor (PD98059) or by a dominant-negative MAPK mutant (dnMAPK). This action of A(2A)-R (designated as the A(2A)-rescue effect) can be blocked by two inhibitors of protein kinase A (PKA) and was absent in a PKA-deficient PC12 variant. Activation of the cAMP/PKA pathway by forskolin exerted the same effect as that by A(2A)-R stimulation. PKA, thus, appears to mediate the A(2A)-rescue effect. Results from cAMP-response element-binding protein (CREB) phosphorylation at serine 133, trans-reporting assays, and overexpression of two dominant-negative CREB mutants revealed that A(2A)-R stimulation led to activation of CREB in a PKA-dependent manner and subsequently reversed the damage of NGF-evoked neurite outgrowth by PD98059 or dnMAPK. Expression of an active mutant of CREB readily rescued the NGF-induced neurite outgrowth impaired by dnMAPK, further strengthening the importance of CREB in the NGF-mediated neurite outgrowth process. Moreover, simultaneous activation of the A(2A)-R/PKA/CREB-mediated and the phosphatidylinositol 3-kinase pathways caused neurite outgrowth that was not suppressed by a selective inhibitor of TrkA, indicating that transactivation of TrkA was not involved. Collectively, CREB functions in conjunction with the phosphatidylinositol 3-kinase pathway to mediate the neurite outgrowth process in PC12 cells.

Hypoxia-induced cell death and changes in hypoxia-inducible factor-1 activity in PC12 cells upon exposure to nerve growth factor

The transcription factor hypoxia-inducible factor-1 (HIF-1) strongly contributes to the expression of adaptive genes under hypoxic conditions. In addition, HIF-1 has been implicated in the regulation of delayed neuronal cell death. Suspension-grown and adherent PC12 cells treated with NGF were used as an experimental model for studying the relationship between hypoxia-induced cell death and activation of HIF-1. Cell damage was assessed by flow cytometry of double-stained (Annexin V and propidiumiodide) cells, and by analysis of the overall death parameters LDH and mitochondrial dehydrogenase. In parallel, cells were transfected with a control and a three-hypoxia-responsive-elements (HRE)-containing vector and HIF-1-driven luciferase activity was determined. Exposure of NGF-treated PC12 cells to hypoxia resulted in a higher cell death rate when compared to untreated controls. PC12 cells exposed for 2 days to NGF exhibited a decrease of HIF-1 activity up to a factor of ten. This decrease may contribute to the enhanced hypoxia-induced cell death via reduced expression of HIF-1alpha-regulated genes responsible for adaptation to hypoxia, like those for glucose transport proteins and enzymes of the glycolytic chain. The decrease in HIF-1 activity and the increase in hypoxia sensitivity may suggest that NGF act as an hierarchically organized signaling molecule.

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.

NOS-II is involved in early differentiation of murine cortical, retinal and ES cell-derived neurons-an immunocytochemical and functional approach

Nitric oxide (NO), a cell-derived highly diffusible and unstable gas is regarded to be involved in inter- and intracellular communication in the nervous system. Based on findings about the expression of the inducible NO synthase (NOS) isoform during development of early mouse olfactory as well as vestibulocochlear receptor neurons, we intended to prove a general role of this isoform for neuronal differentiation. Using immunohistochemical techniques, an exclusive expression of the inducible NOS-II isoform in early post-mitotic neurons of the developing mouse cortex and retina can be detected. In a pharmacological approach using cultures of the mouse cortex as well as embryonic stem cell-derived neural precursor cells, we investigated the functional role of NO on initial neuronal differentiation. Effects of NOS inhibitors and NO donors on the morphological differentiation were correlated with developmentally regulated calcium current densities, focusing on the effects of the specific NOS-II inhibitor GW 274150. Furthermore, involvement of the soluble guanylate cyclase (sGC)/cGMP signaling cascade was pharmacologically investigated. Our data indicate that while a specific block of NOS-II provokes a clear inhibition of neurite outgrowth formation as well as a decrease of calcium current densities, the inverse is true for exogenous NO donation. In line with lacking immunoreactivity for the sGC and cGMP there are only minor effects of compounds manipulating the sGC/cGMP pathway, suggesting the downstream sGC/cGMP pathway not to be essential in these early differentiation steps.

An examination of heme action in gene expression: heme and heme deficiency affect the expression of diverse genes in erythroid k562 and neuronal PC12 cells

The aim of this study was to identify key genes whose expression is altered by heme and heme deficiency in the human erythroleukemia K562 cells and in the NGF-induced rat pheochromocytoma neuronal PC12 cells, respectively. By quantitative RT-PCR, Northern blotting, and Western blotting analyses, we found that the expression of the CDK inhibitors p18 and p21 was upregulated at the early and late stages of heme-induced erythroid differentiation of K562 cells, respectively, while the expression of cyclin D1 was downregulated. Data from succinyl acetone and desferrioxamine treatments suggest that these effects of heme in K562 cells were specific. Further, by microarray expression analysis, we found that inhibition of heme synthesis by succinyl acetone in NGF-induced PC12 cells drastically altered the expression of several groups of important neuronal genes, including the structural genes encoding neurofilament proteins and synaptic vesicle proteins, regulatory genes encoding signaling components beta-arrestin and p38 MAPK, and stress-response genes encoding hsp70. These results show that heme and heme deficiency affect the expression of diverse genes in a cell-type specific manner in mammalian cells, and that heme, although needed at different levels, is critical for both erythropoiesis and neurogenesis. These studies provide insights into how heme may act to control diverse regulatory processes in mammals.

Nitric oxide and cyclic GMP are involved in angiotensin II AT(2) receptor effects on neurite outgrowth in NG108-15 cells

In their undifferentiated state, NG108-15 cells express only the angiotensin II (Ang II) type 2 receptor (AT(2)). We have previously shown that Ang II induced neurite outgrowth of NG108-15 cells, a process involving sustained activation of p42/p44(mapk) activity. We have also shown that Ang II stimulates nitric oxide (NO) production. The aim of the present study was to investigate the role of the NO/cyclic GMP (cGMP) cascade in the signal transduction of the AT(2) receptor-stimulated neurite outgrowth. Three-day treatment of cells with dbcGMP induced neurite outgrowth as did Ang II. Preincubation with an inhibitor of cGMP-dependent protein kinase, KT5823, resulted in the formation of short neurites, while in the presence of LY83583 or methylene blue, two inhibitors of guanylyl cyclase, cells resembled control cells with only one or two thin processes. Western blot analyses indicated that nNOS was present in NG108-15 cells. Immunoprecipitation with antiphosphotyrosine antibodies showed that Ang II induced NOS activity and increased cGMP production through a Gi-dependent pathway. However, neither L-NAME, KT5823, nor LY83583 affected the activation of p42/p44(mapk) induced by Ang II, indicating that the pathway NO/guanylyl cyclase/cGMP was not involved in Ang II-induced activation of MAPK. The present results suggest that the neurite outgrowth induced by Ang II results from at least parallel but complementary pathways, one involved in neurite elongation (through the cooperation of MAPK and PKG) and the other involved in sprouting (through cGMP).

Nitric oxide is an essential negative regulator of cell proliferation in Xenopus brain

Mechanisms controlling the transition of a neural precursor cell from proliferation to differentiation during brain development determine the distinct anatomical features of the brain. Nitric oxide (NO) may mediate such a transition, because it can suppress DNA synthesis and cell proliferation. We cloned the gene encoding the neuronal isoform of Xenopus NO synthase (XNOS) and found that in the developing brain of Xenopus tadpoles, a zone of XNOS-expressing cells lies adjacent to the zone of dividing neuronal precursors. Exogenous NO, supplied to the tadpole brain in vivo, decreased the number of proliferating cells and the total number of cells in the optic tectum. Conversely, inhibition of NOS activity in vivo increased the number of proliferating cells and the total number of cells in the optic tectum. NOS inhibition yielded larger brains with grossly perturbed organization. Our results indicate that NO is an essential negative regulator of neuronal precursor proliferation during vertebrate brain development.

A receptor-mediated mechanism of nicotine toxicity in oral keratinocytes

Smoking and smokeless tobacco cause morbidity that originates from the epithelium lining of the skin and upper digestive tract. Oral keratinocytes (OKC) express nicotinic acetylcholine receptors (nAChRs) that bind nicotine (Nic). We studied the mechanism of the receptor-mediated toxicity of tobacco products on OKC. Preincubation of normal human OKC with Nic altered the ligand-binding kinetics of their nAChRs, suggesting that the nAChRs underwent structural changes. This hypothesis was confirmed by the finding that exposure of OKC to Nic causes transcriptional and translational changes. Through RT-PCR and immunoblotting, we found a 1.5- to 2.9-fold increase in the mRNA and protein levels of alpha3, alpha5, alpha7, beta2, and beta4 nAChR subunits. Exposure of OKC to Nic also changed the mRNA and protein levels of the cell cycle and cell differentiation markers Ki-67, PCNA, p21, cyclin D1, p53, filaggrin, loricrin, and cytokeratins 1 and 10. The nicotinic antagonist mecamylamine prevented these changes, which indicates that the Nic-induced changes in the expression of both the nAChR and the cell cycle and cell differentiation genes resulted from pharmacologic stimulation of nAChRs with Nic. To establish the relevance of these findings to the pathobiologic effects of tobacco products in vivo, we studied the above parameters in the oral tissue of rats and mice after their exposure for 3 weeks to environmental cigarette smoke or drinking water containing equivalent concentrations of Nic that are pathophysiologically relevant. The changes of the nAChRs and the cell cycle and cell differentiation genes were similar to those found in vitro. The results of indirect immunofluorescence assay of tissue specimens validated these findings. Thus, some pathobiologic effects of tobacco products in oral tissues may stem from Nic-induced alterations of the structure and function of keratinocyte nAChRs responsible for the physiologic regulation of the cell cycle by the cytotransmitter acetylcholine.

p38 mitogen-activated protein kinase-independent induction of gadd45 expression in nerve growth factor-induced apoptosis in medulloblastomas

We describe a novel nerve growth factor (NGF)-signaling pathway leading to gadd45 induction that is independent of JNK and p38 MAPK. We used cDNA arrays representing 588 genes to investigate the role of differential gene expression in NGF-mediated pleiotropic responses. We compared the gene expression profiles obtained from MED283-TrkA cells undergoing NGF-induced apoptosis to PC12 cells undergoing NGF-induced differentiation. An early and specific transcriptional target of NGF in MED283-TrkA cells was the DNA-damage-inducible gene gadd45. Its magnitude of induction directly correlated with the magnitude of apoptosis in MED283 clones transfected with mutant TrkA receptors. Although gadd45 has been implicated in stress response signaling, in vitro kinase assays indicated that NGF neither activated c-Jun NH2-terminal kinase (JNK) nor p38 mitogen-activated protein kinase (MAPK). Furthermore, the p38 MAPK inhibitor SB203580 (20 microM) failed to prevent NGF-induced apoptosis and NGF-induced gadd45 expression. These results suggest that differential regulation of gadd45 expression possibly through BRCA1 may be a potential mechanism whereby NGF regulates pleiotropic responses.

Activation of the mitogen-activated protein kinase cascade through nitric oxide synthesis as a mechanism of neuritogenic effect of genipin in PC12h cells

Prominent neurite outgrowth induced by genipin, a plant-derived iridoid, was substantially inhibited by addition of NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase (NOS) inhibitor, and carboxy-PTIO, an NO scavenger, in PC12h cells. Increases of the NADPH-diaphorase activity and neuronal and inducible NOS proteins in cells preceded the neurite outgrowth after addition of genipin to medium. NO donors could induce the neurite outgrowth dose-dependently in the cells. On the other hand, an inhibitor of soluble guanylate cyclase (SGC), which is known to be a stimulatory target of NO, abolished greatly the genipin-induced neurite outgrowth. Addition of extracellular signal-regulated kinase (ERK) kinase inhibitors could almost completely abolish the neurite induction. L-NAME remarkably depressed genipin-stimulated phosphorylation of ERK-1 and -2. A neuritogenic effect of nerve growth factor (NGF) in PC12h cells was also remarkably inhibited by the NOS inhibitor, NO scavenger and SGC inhibitor. These findings suggest that induced NO production followed by cyclic GMP-mediated stimulation of the mitogen-activated protein kinase (MAPK) cascade is implicated in the neuritogenesis by genipin and NGF in PC12h cells.

Cyclin-dependent kinases and stroke

Cyclin-dependent kinases (CDKs) are a group of enzymes predominately known for their role in cell cycle regulation in proliferating cell types. Increasing evidence, however, suggests that CDKs also promote death in neurones. These observations have lead to the notion that CDKs may serve as a therapeutic target for neuropathological conditions such as stroke. Accordingly, in this review, we will examine the evidence which indicates a role for CDKs in neuronal death and evaluate the potential of CDK inhibitors as a therapeutic target for stroke.

Nitric oxide inhibits the proliferation of murine microglial MG5 cells by a mechanism involving p21 but independent of p53 and cyclic guanosine monophosphate

We investigated the effect of nitric oxide (NO) on the proliferation of microglial MG5 cells established from p53-deficient mice. Cells were treated with bacterial lipopolysaccharide and interferon-gamma, and expression of inducible NO synthase (iNOS) and p21/waf1, a cyclin-dependent kinase inhibitor protein which is a critical downstream effector of p53, was investigated by RNA blot and immunoblot analyses. iNOS mRNA was induced 2 h after treatment and increased with time up to 24 h. p21 mRNA was expressed at a low level in untreated cells and increased with a kinetics similar to that for iNOS mRNA. iNOS and p21 proteins were also induced. An NO donor SNAP induced p21 mRNA and protein. SNAP inhibited incorporation of [(3)H]thymidine in MG5 cells in a dose-dependent manner. 8-Bromo-cGMP neither induced p21 mRNA nor inhibited [(3)H]thymidine incorporation. These results suggest that NO inhibits the proliferation of MG5 cells by induction of p21, which occurs independent of p53 and cGMP.

Inhibition of neuronal nitric oxide synthase enhances cell proliferation in the dentate gyrus of the adrenalectomized rat

Recent studies have demonstrated that the elimination of adrenal steroids by an adrenalectomy (ADX) increases the expression of neuronal nitric oxide synthase (NOS), and that it increases cell proliferation in the rat dentate gyrus. However, no evidence has been presented to date which indicates that NO regulates cell proliferation in the dentate gyrus of the adult rats. In this study, the effect of blocking NO production on ADX-induced increase of cell proliferation and serotonergic innervation was examined in the rat dentate gyrus. 7-nitroindazole (7-NI; 30 mg/kg, intraperitoneally), a selective inhibitor of neuronal NOS, was injected 1 day before an ADX and then once every 24 h for 4 days after the ADX subsequently. The proliferating cells were identified with 5-bromo-2-deoxyuridine (BrdU) immunostaining. Long-term inhibition of the neuronal NOS by 7-NI markedly increased the BrdU-labeled cell population density 4-18-fold in the dentate gyrus of the adrenalectomized rats compared to that in the vehicle-injected adrenalectomized rats. Immunoreactivity of serotonin, known as a mediator of granule cell genesis, was detected only in the dentate gyrus of 7-NI-injected adrenalectomized rats. These results indicate that NO may be involved in the cell proliferation in the dentate gyrus of the adrenalecomized rat and that serotonin may mediate the regulatory effect of NO on the cell proliferation in rat dentate gyrus.

The Ras-ERK pathway is required for the induction of neuronal nitric oxide synthase in differentiating PC12 cells

We have studied the role of MAP kinase pathways in neuronal nitric oxide synthase (nNOS) induction during the differentiation of PC12 cells. In nerve growth factor (NGF)-treated PC12 cells, we find nNOS induced at RNA and protein levels, resulting in increased NOS activity. We note that neither nNOS mRNA, nNOS protein nor NOS activity is induced by NGF treatment in cells that have been infected with a dominant negative Ras adenovirus. We have also used drugs that block MAP kinase pathways and assessed their ability to inhibit nNOS induction. Even though U0126 and PD98059 are both MEK inhibitors, we find that U0126, but not PD98059, blocks induction of nNOS protein and NOS activity in NGF-treated PC12 cells. Also, the p38 kinase inhibitor, SB203580, does not block nNOS induction in our clone of PC12 cells. Since the JNK pathway is not activated in NGF-treated PC12 cells, we conclude that the Ras-ERK pathway and not the p38 or JNK pathway is required for nNOS induction in NGF-treated PC12 cells. We find that U0126 is much more effective than PD98059 in blocking the Ras-ERK pathway, thereby explaining the discrepancy in nNOS inhibition. We conclude that the Ras-ERK pathway is required for nNOS induction.

Disturbance of neuronal plasticity is a critical pathogenetic event in Alzheimer's disease

Brain areas affected by AD pathology are primarily those structures that are invovled in the regulation of "higher brain functions". The functions these areas subserve such as learning, memory, perception, self-awareness, and consciousness require a life-long re-fittng of synaptic contacts that allows for the acquistion of new epigenetic information, a process based on a particularly high degree of structural plasticity. Here, we outline a hypothesis that it is the "labile state fo differentiation" of a subset of neurons in the adult brain that allows for ongoing neuroplastic processes after development is completed but at the same time renders these neurons particularly vulnerable. Mechanisms of molecular and cellular control of neuronal differentiation and proliferation might, thus, not only play a role during development but critically involved in the pathogenesis of neurodegeneration.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

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.

Expression and function of inducible nitric oxide synthase in neurons

Enzymatically derived nitric oxide (NO) has been implicated in numerous physiological and pathological processes in the brain. Whereas during development NO participates in developmental and maturation processes, excess NO production in the adult in response to inflammation, injury, or trauma participates in both cell death and repair. The expression and activity of the inducible isoform of NO synthase (iNOS) play a pivotal role in sustained and elevated NO release. Recent evidence suggests that neurons can respond to proinflammatory stimuli and take part in brain inflammation. Neuronal iNOS expression has been described in different experimental settings, including cytokine stimulation of neuronal cell lines and primary neurons in vitro as well as in animal models of stroke and neurodegeneration. This article outlines different conditions leading to iNOS gene transcription and expression in neurons and neuronal cells and highlights the potential impact on human brain inflammation and neurodegeneration.

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.

Specific pattern of p53 phosphorylation during nitric oxide-induced cell cycle arrest

Nitric oxide (NO) is an efficient inhibitor of cell proliferation. Here we show that part of the antiproliferative activity of NO in fibroblasts is mediated through p53 signaling pathway. Cells from p53-/- knockout mice are compromised in their ability to stop dividing in the presence of NO. NO strongly induces expression of genes which are transcriptional targets of p53, and p53 is necessary for some, but not all, of the transcription activation effects of NO. Furthermore, NO strongly increases the cellular level of p53 protein. Since phosphorylation of particular residues of the p53 molecule has been correlated with its functional activity, we determined the phosphorylation pattern of p53 molecule after exposure to NO and compared it with the phosphorylation patterns that develop upon treatment with gamma-irradiation, UV light, and adriamycin. We found that NO induces a specific signature pattern of p53 phosphorylation, distinct from the patterns evoked by other inducers. This study suggests that NO activates specific signaling pathways that may partially overlap, but that do not coincide, with signaling pathways activated by other known inducers of p53 activity.

Mediation of nerve growth factor-driven cell cycle arrest in PC12 cells by p53. Simultaneous differentiation and proliferation subsequent to p53 functional inactivation

Upon stimulation with nerve growth factor (NGF), PC12 cells extend neurites and cease to proliferate by influencing cell cycle proteins. Previous studies have shown that neuritogenesis and a block at the G(1)/S checkpoint correlate with the nuclear translocation of and an increase in the p53 tumor suppressor protein. This study was designed to determine if p53 plays a direct role in mediating NGF-driven G(1) arrest. A retroviral vector that overexpresses a temperature-sensitive p53 mutant protein (p53ts) was used to extinguish the function of endogenous p53 in PC12 cells in a dominant-negative manner at the nonpermissive temperature. NGF treatment led to transactivation of a p53 response element in a luciferase reporter construct in PC12 cells, whereas this response to NGF was absent in PC12(p53ts) cells at the nonpermissive temperature. With p53 functionally inactivated, NGF failed to activate growth arrest, as measured by bromodeoxyuridine incorporation, and also failed to induce p21/WAF1 expression, as measured by Western blotting. Since neurite outgrowth proceeded unharmed, 50% of the cells simultaneously demonstrated neurite morphology and were in S phase. Both PC12 cells expressing SV40 T antigen and PC12 cells treated with p53 antisense oligonucleotides continued through the cell cycle, confirming the dependence of the NGF growth arrest signal on a p53 pathway. Activation of Ras in a dexamethasone-inducible PC12 cell line (GSRas1) also caused p53 nuclear translocation and growth arrest. Therefore, wild-type p53 is indispensable in mediating the NGF antiproliferative signal through the Ras/MAPK pathway that regulates the cell cycle of PC12 cells.

Nitric oxide and cGMP influence axonogenesis of antennal pioneer neurons

The grasshopper embryo has been used as a convenient system with which to investigate mechanisms of axonal navigation and pathway formation at the level of individual nerve cells. Here, we focus on the developing antenna of the grasshopper embryo (Schistocerca gregaria) where two siblings of pioneer neurons establish the first two axonal pathways to the CNS. Using immunocytochemistry we detected nitric oxide (NO)-induced synthesis of cGMP in the pioneer neurons of the embryonic antenna. A potential source of NO are NADPH-diaphorase-stained epithelial cells close to the basal lamina. To investigate the role of the NO/cGMP signaling system during pathfinding, we examined the pattern of outgrowing pioneer neurons in embryo culture. Pharmacological inhibition of soluble guanylyl cyclase (sGC) and of NO synthase (NOS) resulted in an abnormal pattern of pathway formation in the antenna. Axonogenesis of both pairs of pioneers was inhibited when specific NOS or sGC inhibitors were added to the culture medium; the observed effects include the loss axon emergence as well as retardation of outgrowth, such that growth cones do not reach the CNS. The addition of membrane-permeant cGMP or a direct activator of the sGC enzyme to the culture medium completely rescued the phenotype resulting from the block of NO/cGMP signaling. These results indicate that NO/cGMP signaling is involved in axonal elongation of pioneer neurons in the antenna of the grasshopper.

Induction of IW32 erythroleukemia cell differentiation by p53 is dependent on protein tyrosine phosphatase

The biological activity of p53 in IW32 erythroleukemia cells was investigated. IW32 cells had no detectable levels of p53 mRNA and protein expression. By transfecting a temperature-sensitive mutant p53 cDNA, tsp53val135, into the cells, we have established several clones stably expressing the mutant p53 allele. At permissive temperature, these p53 transfectants were arrested in G1 phase and underwent apoptosis. Moreover, differentiation along the erythroid pathway was observed as evidenced by increased benzidine staining and mRNA expression of beta-globin and the erythroid-specific delta-aminolevulinic acid synthase (ALAS-E). Treatment of cells with protein tyrosine phosphatase inhibitor vanadate blocked the p53-induced differentiation, but not that of cell death or growth arrest. Increased protein tyrosine phosphatase activity as well as mRNA levels of PTPbeta2 and PTPepsilon could be observed by wildtype p53 overexpression. These results indicate that p53 induced multiple phenotypic consequences through separate signal pathways in IW32 erythroleukemia cells, and protein tyrosine phosphatase is required for the induced differentiation.

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.

Inhibition of phospholipase A(2) activity by S-nitroso-cysteine in a cyclic GMP-independent manner in PC12 cells

Arachidonic acid and nitric oxide (NO) act as retrograde and intercellular messengers in the nervous system. Regulation of cyclooxygenase is well established, but regulation of phospholipase A(2), the enzyme responsible for the liberation of arachidonic acid, by NO has not been thoroughly investigated. Using the PC12 cell line as a neuronal model, we studied the effects of exogenous NO compounds on arachidonic acid release. Incubation with Ca(2+) ionophores or mastoparan (wasp venom peptide) stimulated [3H]arachidonic acid release from prelabeled PC12 cells. [3H]Arachidonic acid release was inhibited by cytosolic phospholipase A(2) inhibitors, but not by dithiothreitol. A cytosolic phospholipase A(2) protein band with a molecular mass of approximately 100 kDa was detected by immunoblotting. S-Nitroso-cysteine inhibited basal and stimulated [3H]arachidonic acid release in concentration-dependent manners. Other NO compounds such as sodium nitroprusside and S-nitroso-N-acetylpenicillamine did not affect [3H]arachidonic acid release. N-Ethylmaleimide also inhibited [3H]arachidonic acid release. The inhibitory effects of S-nitroso-cysteine and N-ethylmaleimide were irreversible, because [3H]arachidonic acid release from PC12 cells preincubated with S-nitroso-cysteine or N-ethylmaleimide was much lower than that from nontreated cells. These findings suggest (a) cytosolic phospholipase A(2) is activated by Ca(2+) or mastoparan, and inhibited by S-nitroso-cysteine in a cyclic GMP-independent manner, (b) N-ethylmaleimide also inhibits cytosolic phospholipase A(2) and arachidonic acid release in PC12 cells. S-Nitroso-cysteine can regulate the production of other retrograde messenger arachidonic acid.

MAPK-dependent expression of p21(WAF) and p27(kip1) in PMA-induced differentiation of HL60 cells

Treatment of HL60 cells with phorbol 12-myristate 13-acetate (PMA) results in growth arrest and differentiation towards the macrophage lineage. PMA-induced changes are easily monitored by morphological changes while cells in suspension start adhering onto substrate. PMA induces rapid activation of the extracellular signal-regulated kinases (ERKs). Activation of the ERK pathway is essential to PMA-induced differentiation of HL60 cells. PMA also induces the expression of the cyclin-dependent kinase inhibitors p21(WAF) and p27(kip1), which is modulated by the use of an inhibitor of the ERK cascade. This implies that a link exists between ERK activation and p21(WAF) and p27(kip1) induction in the process of terminal differentiation.

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.

A role for nuclear PTEN in neuronal differentiation

Mutations of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a protein and lipid phosphatase, have been associated with gliomas, macrocephaly, and mental deficiencies. We have assessed PTEN's role in the nervous system and find that PTEN is expressed in mouse brain late in development, starting at approximately postnatal day 0. In adult brain, PTEN is preferentially expressed in neurons and is especially evident in Purkinje neurons, olfactory mitral neurons, and large pyramidal neurons. To analyze the function of PTEN in neuronal differentiation, we used two well established model systems-pheochromocytoma cells and cultured CNS stem cells. PTEN is expressed during neurotrophin-induced differentiation and is detected in both the nucleus and cytoplasm. Suppression of PTEN levels with antisense oligonucleotides does not block initiation of neuronal differentiation. Instead, PTEN antisense leads to death of the resulting, immature neurons, probably during neurite extension. In contrast, PTEN is not required for astrocytic differentiation. These observations indicate that PTEN acts at multiple sites in the cell, regulating the transition of differentiating neuroblasts to postmitotic neurons.

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.

The urokinase plasminogen activator receptor (UPAR) is preferentially induced by nerve growth factor in PC12 pheochromocytoma cells and is required for NGF-driven differentiation

Nerve growth factor (NGF)-driven differentiation of PC12 pheochromocytoma cells is a well studied model used both to identify molecular, biochemical, and physiological correlates of neurotrophin-driven neuronal differentiation and to determine the causal nature of specific events in this differentiation process. Although epidermal growth factor (EGF) elicits many of the same early biochemical and molecular changes in PC12 cells observed in response to NGF, EGF does not induce molecular or morphological differentiation of PC12 cells. The identification of genes whose expression is differentially regulated by NGF versus EGF in PC12 cells has, therefore, been considered a source of potential insight into the molecular specificity of neurotrophin-driven neuronal differentiation. A "second generation" representational difference analysis procedure now identifies the urokinase plasminogen activator receptor (UPAR) as a gene that is much more extensively induced by NGF than by EGF in PC12 cells. Both an antisense oligonucleotide for the UPAR mRNA and an antibody directed against UPAR protein block NGF-induced morphological and biochemical differentiation of PC12 cells; NGF-induced UPAR expression is required for subsequent NGF-driven differentiation.

Aberrant expression of nNOS in pyramidal neurons in Alzheimer's disease is highly co-localized with p21ras and p16INK4a

Aberrancies of growth and proliferation-regulating mechanisms might be critically involved in the processes of neurodegeneration in Alzheimer's disease (AD). Expression of p21ras and further downstream signalling elements involved in regulation of proliferation and differentiation as, for example, MEK, ERK1/2, cyclins, cyclin-dependent kinases and their inhibitors such as those of the p16INK4a family, are elevated early during the course of neurodegeneration. Activation of p21ras can also directly be triggered by nitric oxide (NO), synthesized in the brain by various isoforms of nitric oxide synthase (NOS) that might be differentially involved into the pathomechanism of AD. To study the potential link of NO and critical regulators of cellular proliferation and differentiation in the process of neurofibrillary degeneration, we analyzed the expression pattern of NOS-isoforms, p21ras and p16INK4a compared to neurofibrillary degeneration in AD. Additionally to its expression in a subtype of cortical interneurons that contain the nNOS-isoform also in normal brain, nNOS was detected in pyramidal neurons containing neurofibrillary tangles or were even unaffected by neurofibrillary degeneration. Expression of nNOS in these neurons was highly co-localized with p21ras and p16INK4a. Because endogenous NO can activate p21ras in the same cell which in turn leads to cellular activation and stimulation of NOS expression [H.M. Lander, J.S. Ogiste, S.F.A. Pearce, R. Levi, A. Novogrodsky, Nitric oxide-stimulated guanine nucleotide exchange on p21 ras, J. Biol. Chem. 270 (1995) 7017-7020], the high level of co-expression of NOS and p21ras in neurons vulnerable to neurofibrillary degeneration early in the course of AD thus provides the basis for an autocrine feedback mechanism that might exacerbate the progression of neurodegeneration in a self-propagating manner.

Activation of c-Ha-Ras by nitric oxide modulates survival responsiveness in neuronal PC12 cells

p21(c-Ha-Ras) (Ras) can be activated by the guanine nucleotide exchange factor mSOS1 or by S-nitrosylation of cysteine 118 via nitric oxide (NO). To determine whether these two Ras-activating mechanisms modulate distinct biological effects, a NO-nonresponsive Ras mutant (Ras(C118S)) was stably expressed in the PC12 cells, a cell line that generates NO upon nerve growth factor treatment. We report here that Ras(C118S) functions indistinguishably from wild type Ras in activating and maintaining the mSOS1- and Raf-1-dependent mitogen-activated protein kinase cascade necessary for neuronal differentiation. However, continuous (>5 days) exposure to nerve growth factor reveals that, in contrast to parental or wild-type Ras-overexpressing PC12 cells, Ras(C118S)-expressing PC12 cells cannot sustain the basal interaction between Ras and phosphatidylinositol 3-kinase. This results in spontaneous apoptosis of these cells despite the presence of nerve growth factor and serum. Thus unique downstream effector interactions and biological outcomes can be differentially modulated by distinct modes of Ras activation.

Tumor suppressor p53 but not cGMP mediates NO-induced expression of p21(Waf1/Cip1/Sdi1) in vascular smooth muscle cells

Cyclin-dependent kinase inhibitor p21(Waf1/Cip1/Sdi1) has been suggested to be involved in the antiproliferative effect of nitric oxide (NO) in vascular smooth muscle cells (VSMCs). To elucidate the mechanism underlying NO-induced p21 expression, we investigated the roles of tumor suppressor p53 and the guanylate cyclase-cGMP pathway. The induction of p21 by the NO donor S-nitroso-N-acetylpenicillamine (SNAP) seemed to be due to transactivation because SNAP elevated the activity of p21 promoter but did not stabilize p21 mRNA and protein. Because SNAP did not stimulate the deletion mutant of p21 promoter that lacked p53 binding sites, we tested the involvement of p53. The expression level of p53 was down-regulated after mitogenic stimulation, whereas it was sustained in the presence of SNAP. SNAP markedly stimulated DNA binding activity of p53. Furthermore, SNAP failed to induce p21 in VSMCs obtained from p53-knock out mice and in A431 cells that contained mutated p53. The antiproliferative effect of SNAP also was attenuated in these cells. NO stimulates guanylate cyclase and its product cGMP has been shown to inhibit VSMC proliferation. However, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a guanylate cyclase inhibitor, did not prevent SNAP-induced p21 expression. 8-Bromo-cGMP, 3-isobutyl-1-methylxanthine, and their combination did not induce p21. Although 8-bromo-cGMP had a small antiproliferative effect, the elevation of cGMP concentration induced by SNAP was little throughout the G(1) phase. The antiproliferative effect of SNAP was not attenuated by Rp-8-bromoguanosine-3',5'-monophosphorothioate, an inhibitor of cGMP-dependent protein kinase. These results suggested that NO induces p21 through a p53-dependent but cGMP-independent pathway.

The Raf-1/MEK/ERK pathway regulates the expression of the p21(Cip1/Waf1) gene in chondrocytes

The gene encoding the cyclin-dependent kinase inhibitor p21(Cip1/Waf1) is up-regulated in many differentiating cells, including maturing chondrocytes. Since strict control of chondrocyte proliferation is essential for proper bone formation and since p21 is likely involved in this control, we initiated analyses of the mechanisms regulating expression of p21 in chondrocytes. p21 expression and promoter activity was strongly increased during the differentiation of chondrogenic MCT cells. We have identified a 68-base pair fragment conferring transcriptional up-regulation of the p21 gene in chondrocytes. The activity of this fragment required active Raf-1 in MCT cells as well as in primary mouse chondrocytes. Inhibition of downstream factors of Raf-1 (MEK1/2, ERK1/2, and Ets2) also repressed the activity of the 68-base pair fragment in MCT cells. The chemical MEK1/2 inhibitor PD98059 reduced protein levels of p21 in MCTs and primary mouse chondrocytes. These data suggest that signaling through the Raf-1 pathway is necessary for the optimal expression of p21 in chondrocytes and may play an important role in the control of bone formation.