Elements of the nitric oxide/cGMP pathway expressed in cerebellar granule cells: biochemical and functional characterisation

Real-Time Imaging Reveals Augmentation of Glutamate-Induced Ca2+ Transients by the NO-cGMP Pathway in Cerebellar Granule Neurons

Dysfunctions of NO-cGMP signaling have been implicated in various neurological disorders. We have studied the potential crosstalk of cGMP and Ca²⁺ signaling in cerebellar granule neurons (CGNs) by simultaneous real-time imaging of these second messengers in living cells. The NO donor DEA/NO evoked cGMP signals in the granule cell layer of acute cerebellar slices from transgenic mice expressing a cGMP sensor protein. cGMP and Ca²⁺ dynamics were visualized in individual CGNs in primary cultures prepared from 7-day-old cGMP sensor mice. DEA/NO increased the intracellular cGMP concentration and augmented glutamate-induced Ca²⁺ transients. These effects of DEA/NO were absent in CGNs isolated from knockout mice lacking NO-sensitive guanylyl cyclase. Furthermore, application of the cGMP analogues 8-Br-cGMP and 8-pCPT-cGMP, which activate cGMP effector proteins such as cyclic nucleotide-gated cation channels and cGMP-dependent protein kinases (cGKs), also potentiated glutamate-induced Ca²⁺ transients. Western blot analysis failed to detect cGK type I or II in our primary CGNs. The addition of phosphodiesterase (PDE) inhibitors during cGMP imaging showed that CGNs degrade cGMP mainly via Zaprinast-sensitive PDEs, most likely PDE5 and/or PDE10, but not via PDE1, 2, or 3. In sum, these data delineate a cGK-independent NO-cGMP signaling cascade that increases glutamate-induced Ca²⁺ signaling in CGNs. This cGMP–Ca²⁺ crosstalk likely affects neurotransmitter-stimulated functions of CGNs.

Altered Synaptic Membrane Retrieval after Strong Stimulation of Cerebellar Granule Neurons in Cyclic GMP-Dependent Protein Kinase II (cGKII) Knockout Mice

The nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (cGK) signaling pathway regulates the clustering and the recruitment of proteins and vesicles to the synapse, thereby adjusting the exoendocytic cycle to the intensity of activity. Accordingly, this pathway can accelerate endocytosis following large-scale exocytosis, and pre-synaptic cGK type II (cGKII) plays a major role in this process, controlling the homeostatic balance of vesicle exocytosis and endocytosis. We have studied synaptic vesicle recycling in cerebellar granule cells from mice lacking cGKII under strong and sustained stimulation, combining imaging techniques and ultrastructural analyses. The ultrastructure of synapses in the adult mouse cerebellar cortex was also examined in these animals. The lack of cGKII provokes structural changes to synapses in cultured cells and in the cerebellar cortex. Moreover, endocytosis is slowed down in a subset of boutons in these cells when they are stimulated strongly. In addition, from the results obtained with the selective inhibitor of cGKs, KT5823, it can be concluded that cGKI also regulates some aspects of vesicle cycling. Overall, these results confirm the importance of the cGMP pathway in the regulation of vesicle cycling following strong stimulation of cerebellar granule cells.

The regulation of synaptic vesicle recycling by cGMP-dependent protein kinase type II in cerebellar granule cells under strong and sustained stimulation

From the early periods of neurogenesis and migration, up until synaptogenesis, both nitric oxide (NO) and its downstream messenger, cGMP, are thought to influence the development of neurons. The NO/cGMP/cGMP-dependent protein kinase (cGK) pathway regulates the clustering and recruitment of synaptic proteins and vesicles to the synapse, adjusting the exoendocytic cycle to the intensity of activity and accelerating endocytosis following large-scale exocytosis. Here, we show that blockage of the N-methyl-D-aspartate receptor impairs the cycling of synaptic vesicles in a subset of boutons on cerebellar granule cells, an effect that was reversed by increasing cGMP. Furthermore, we demonstrate that presynaptic cGK type II (cGKII) plays a major role in this process. Using the FM1-43 dye to track vesicle recycling, we found that knockdown of cGKII and/or the application of a cGK inhibitor reduced the efficiency of synaptic vesicle recycling to a similar extent. Likewise, in cerebellar granule cells transfected with vGlut1-pHluorin to follow the exoendocytotic cycle, application of a cGK inhibitor slowed vesicle endocytosis when exocytosis was accelerated through strong and sustained stimulation. Additionally, ultrastructural analysis showed that cGKII knockdown or inhibition favored the formation of endosomal-like structures after strong and sustained stimulation. We conclude that cGKII controls the homeostatic balance of vesicle exocytosis and endocytosis in synaptic boutons of rat cerebellar granule cells.

Effects of selective inhibitors of neuronal and inducible NO-synthase on ATP content and survival of cultured rat cerebellar neurons during hyperstimulation of glutamate receptors

We studied the effects of selective inhibitors of neuronal and inducible NO-synthase (7-nitroindazole and aminoguanidine) and non-selective NO-synthase inhibitor L-NAME on ATP content and survival of cultured rat cerebellar neurons during hyperstimulation of glutamate receptors with toxic doses of glutamate. Application of 100 μM glutamate reduced ATP content in the primary culture of 7-8- and 14-15-day-old cerebellar granule cells by 66 and 49%, respectively, in comparison with the control. Inhibition of nitric oxide synthesis with 7-nitroindazole during glutamate exposure in the culture of 7-8-day-old neurons and with 7-nitroindazole and aminoguanidine in the culture of 14-15-day-old neurons ensured better protection of cells from ATP level decrease than non-specific inhibition with L-NAME. In addition, inhibition of neuronal and inducible NO-synthase during glutamate exposure decreased death of "young" neurons, whereas death of "old" neurons remained high under these conditions.

The type II cGMP dependent protein kinase regulates GluA1 levels at the plasma membrane of developing cerebellar granule cells

Trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) is regulated by specific interactions with other proteins and by post-translational mechanisms, such as phosphorylation. We have found that the type II cGMP-dependent protein kinase (cGKII) phosphorylates GluA1 (formerly GluR1) at S845, augmenting the surface expression of AMPARs at both synaptic and extrasynaptic sites. Activation of cGKII by 8-Br-cGMP enhances the surface expression of GluA1, whereas its inhibition or suppression effectively diminished the expression of this protein at the cell surface. In granule cells, NMDA receptor activation (NMDAR) stimulates nitric oxide and cGMP production, which in turn activates cGKII and induces the phosphorylation of GluA1, promoting its accumulation in the plasma membrane. GluA1 is mainly incorporated into calcium permeable AMPARs as exposure to 8-Br-cGMP or NMDA activation enhanced AMPA-elicited calcium responses that are sensitive to NASPM inhibition. We summarize evidence for an increase of calcium permeable AMPA receptors downstream of NMDA receptor activation that might be relevant for granule cell development and plasticity.

Functional cGMP-gated channels in cerebellar granule cells

Cyclic nucleotide-gated channels (CNGCs) are important transducers of external signals in sensory processes. These channels are ubiquitously expressed in a variety of neurons, and are necessary to transduce signals for growth cone guidance and plasticity. Here, we demonstrate that the CNGC subunits (CNGA1 and CNGB1, presumably the 1b isoform) are expressed in rat cerebellar granule cells and that they combine to form functional channels. The expression of the mRNAs that encode these proteins is maximal after 7 days in cell culture, when the channels are expressed at synapses and co-localize with the synaptic marker synapsin I. These ligand-gated channels are functional and can be blocked by Mg(2+) or L-cis-diltiazem. Moreover, channel opening in response to increases in intracellular cGMP results in Ca(2+) entry into the cell. Chronic blockade (96 h) of these channels with L-cis-diltiazem significantly decreases the number of functional boutons, as determined by their capacity to load and unload the styryl dye FM1-43 when stimulated. Moreover, the unloading kinetics is modified from a biphasic to a monophasic profile in a subset of synaptic boutons. These channels are also expressed in early developmental stages, both in the soma and in emerging processes, and CNGA1 can be detected in growth cones. Pharmacological blockade of these channels with L-cis-diltiazem causes an overall change in growth cone morphology, impairing the formation of lamellipodia between filopodia and increasing the number of filopodia. J

Nos2 inactivation promotes the development of medulloblastoma in Ptch1(+/-) mice by deregulation of Gap43-dependent granule cell precursor migration

Medulloblastoma is the most common malignant brain tumor in children. A subset of medulloblastoma originates from granule cell precursors (GCPs) of the developing cerebellum and demonstrates aberrant hedgehog signaling, typically due to inactivating mutations in the receptor PTCH1, a pathomechanism recapitulated in Ptch1^+/− mice. As nitric oxide may regulate GCP proliferation and differentiation, we crossed Ptch1^+/− mice with mice lacking inducible nitric oxide synthase (Nos2) to investigate a possible influence on tumorigenesis. We observed a two-fold higher medulloblastoma rate in Ptch1^+/− Nos2^−/− mice compared to Ptch1^+/− Nos2^+/+ mice. To identify the molecular mechanisms underlying this finding, we performed gene expression profiling of medulloblastomas from both genotypes, as well as normal cerebellar tissue samples of different developmental stages and genotypes. Downregulation of hedgehog target genes was observed in postnatal cerebellum from Ptch1^+/+ Nos2^−/− mice but not from Ptch1^+/− Nos2^−/− mice. The most consistent effect of Nos2 deficiency was downregulation of growth-associated protein 43 (Gap43). Functional studies in neuronal progenitor cells demonstrated nitric oxide dependence of Gap43 expression and impaired migration upon Gap43 knock-down. Both effects were confirmed in situ by immunofluorescence analyses on tissue sections of the developing cerebellum. Finally, the number of proliferating GCPs at the cerebellar periphery was decreased in Ptch1^+/+ Nos2^−/− mice but increased in Ptch1^+/− Nos2^−/− mice relative to Ptch1^+/− Nos2^+/+ mice. Taken together, these results indicate that Nos2 deficiency promotes medulloblastoma development in Ptch1^+/− mice through retention of proliferating GCPs in the external granular layer due to reduced Gap43 expression. This study illustrates a new role of nitric oxide signaling in cerebellar development and demonstrates that the localization of pre-neoplastic cells during morphogenesis is crucial for their malignant progression.

Medulloblastoma is a common pediatric brain tumor, a subtype of which is driven by aberrant hedgehog pathway activation in cerebellar granule cell precursors. Although this tumor etiology has been intensively investigated in the well-established Ptch1^+/− mouse model, knowledge is still lacking about the molecular interactions between neoplastic transformation and other developmental processes. Nitric oxide (NO) has been reported to be involved in controlling proliferation and differentiation of these cells. Therefore, inactivation of the NO–producing enzyme Nos2 in combination with the mutated Ptch1 gene should provide insight into how developmental regulation influences pathogenesis. Here, we describe a new role for NO in developing neuronal precursors of the cerebellum facilitating physiologically accurate migration via regulation of Gap43. We further demonstrate that disturbance of these processes leads to retention of granule precursor cells to the cerebellar periphery. Together with the sustained proliferation of these cells in combined Ptch1^+/− Nos2^−/− mice, this effect results in an increased medulloblastoma incidence relative to Ptch1^+/− mice and demonstrates a new disease-promoting mechanism in this tumor entity.

Efficient synaptic vesicle recycling after intense exocytosis concomitant with the accumulation of non-releasable endosomes at early developmental stages

Following the exocytosis of neurotransmitter-containing synaptic vesicles, endocytosis is fundamental to re-establishing conditions for synaptic transmission. As there are distinct endocytotic pathways that each differ in their efficiency to generate releasable synaptic vesicles, we used the dye FM1-43 to track vesicle recycling, and to determine whether nerve terminals use multiple pathways of endocytosis. We identified two types of synaptic boutons in cultured cerebellar granule cells that were characterized by weak or strong FM1-43-unloading profiles. Decreasing the extent of exocytosis dramatically increased the proportion of synaptic boutons that exhibited strong FM1-43-unloading and dramatically reduced the number of endosome-like structures. Hence, we concluded that efficient recycling of synaptic vesicles is concomitant with the formation of non-releasable endosomes in both types of synaptic boutons, although to different extents. Furthermore, cell maturation in culture increased the proportion of synaptic boutons that were capable of an intense release response, whereas the chronic blockage of synaptic activity diminished the capacity of boutons to release dye.

Membrane depolarization regulates AMPA receptor subunit expression in cerebellar granule cells in culture

The physiological responses of AMPA receptors can be modulated through the differential expression of their subunits and by modifying their number at the cell surface. Here we have studied the expression of AMPA receptor subunits (GluR1-4) mRNAs in cerebellar granule cells grown in depolarizing (25mMK(+)) medium, and we have evaluated the effect of decreasing the [K(+)] in the culture medium for 24 h on both GluR1-4 expression (both mRNA and protein) and their presence at the plasma membrane. The expression of the four AMPAR subunits increases as the [K(+)] decreases, although the increase in GluR2 and GluR3 was only observed in the cell soma but not in the dendrites. Calcium entry through L-type calcium channel and CaMKIV activation are responsible for the reduction in the expression of AMPA receptor subunits in cells cultured in depolarizing conditions. Indeed, prolonged reduction of extracellular [K(+)] or blockage of L-type calcium channels enhanced both the surface insertion of the four AMPAR subunits and the AMPA response measured through intracellular calcium increase. These findings reveal a balanced increase in functional AMPA receptors at the surface of cells that can trigger strong increases in calcium in response to the persistent reduction of calcium entry.

Knockdown of the neuronal nitric oxide synthase gene retard the development of the cerebellar granule neurons in vitro

The role of endogenous neuronal nitric oxide synthase (nNOS) gene in the development of cerebellar granule neurons (CGNs) is conflicting. Here, we tested the effect of antisense oligos (AS-ODN) on the endogenous nNOS gene and the development of the CGNs in vitro. The expression of nNOS increased in a development-dependent pattern both in terms of mRNA and protein. AS-ODN down-regulated nNOS gene, but in a posttranscriptional manner. Knockdown of nNOS protein decreased the viability of the CGNs from 7 to 13 days in culture (DIC). This activity of AS-ODN was mimicked by nNOS inhibitor I. The antagonist (nNOSi, MK-801, or ODQ) -induced decrease of cell viability was normalized by the provision of the sodium nitroprusside, an NO donor. This study provides direct evidence that endogenous nNOS, mainly by means of its principal product NO, plays an active role in sustaining the survival of developing CGNs at transition from differentiation to maturation.

Nitric oxide and cGMP signal transduction positively regulates the motility of human neuronal precursor (NT2) cells

Developmental studies in both vertebrates and invertebrates implicate an involvement of nitric oxide (NO) signaling in cell proliferation, neuronal motility, and synaptic maturation. However, it is unknown whether NO plays a role in the development of the human nervous system. We used a model of human neuronal precursor cells from a well-characterized teratocarcinoma cell line (NT2). The precursor cells proliferate during retinoic acid treatment as spherical aggregate culture that stains for nestin and betaIII-tubulin. Cells migrate out of the aggregates to acquire fully differentiated neuronal phenotypes. The cells express neuronal nitric oxide synthase and soluble guanylyl cyclase (sGC), an enzyme that synthesizes cGMP upon activation by NO. The migration of the neuronal precursor cell is blocked by the use of nNOS, sGC, and protein kinase G (PKG) inhibitors. Inhibition of sGC can be rescued by a membrane permeable analog of cGMP. In gain of function experiments the application of a NO donor and cGMP analog facilitate cell migration. Our results from the differentiating NT2 model neurons point towards a vital role of the NO/cGMP/PKG signaling cascade as positive regulator of cell migration in the developing human brain.

Different effect of chronic stress on learning and memory in BALB/c and C57BL/6 inbred mice: Involvement of hippocampal NO production and PKC activity

Nitric oxide (NO) has been involved in many pathophysiological brain processes. Recently, we showed that neuronal nitric oxide synthase (nNOS)-mediated decrease in NO production is involved in memory impairment induced by chronic mild stress (CMS) in BALB/c mice. Two genetically different inbred murine strains, C57BL/6 and BALB/c, show distinct behavioral responses, neurodevelopmental and neurochemical parameters. Here, we perform a comparative study on CMS effects upon learning and memory in both strains, analyzing the role of NO production and its regulation by protein kinase C (PKC). Stressed BALB/c, but not C57Bl/6 mice, showed a poor learning performance in both the open field and passive avoidance inhibitory tasks. Also, CMS induced a diminished NO production by nNOS, associated with an increment in gamma and zeta PKC isoenzymes in BALB/c mice. In C57BL/6 mice, CMS had no effect on NO production, but increased delta and decreased betaI PKC isoforms. In vivo administration of a NOS inhibitor induced behavioral alterations in both strains. These results suggest a differential effect of stress, with BALB/c being more vulnerable to stress than C57BL/6 mice. This effect could be related to a differential regulation of NOS and PKC isoenzymes, pointing to an important role of NO in learning and memory.

Suppression of guanylyl cyclase (beta1 subunit) expression impairs neurite outgrowth and synapse maturation in cultured cerebellar granule cells

The increased expression of different soluble guanylyl cyclase (sGC) subunits during development is consistent with these proteins participating in the formation and establishment of interneuronal contacts. Functional sGC is generated by the dimerization of an alpha-subunit (sGCalpha1/2) with the beta1-subunit (sGCbeta1), and both depletion of the sGCbeta1 subunit and inhibiting sGC activity impair neurite outgrowth. Similarly, impairing sGC activity diminishes the amount of growth-associated protein (GAP-43) and synapsin I, two proteins that participate in axon elongation and synaptogenesis, suggesting a role for sGC in these processes. Indeed, fewer synapses form when sGC is inhibited, as witnessed by FM1-43 imaging and synapsin I immunostaining, and the majority of synapses that do form remain functionally immature. These findings highlight the importance of sGC in the regulation of neurite outgrowth and synapse formation, and in the functional maturation of cerebellar granule cells in vitro.

Cannabinoid receptor-mediated translocation of NO-sensitive guanylyl cyclase and production of cyclic GMP in neuronal cells

Cannabinoid agonists regulate NO and cyclic AMP production in N18TG2 neuroblastoma cells, leading to the hypothesis that neuronal cyclic GMP production could be regulated by CB(1) cannabinoid receptors. NO (nitric oxide)-sensitive guanylyl cyclase (GC) is a heterodimeric cytosolic protein that mediates the down-stream effects of NO. Genes of proteins in the cyclic GMP pathway (alpha(1), alpha(2), and beta(1) subunits of NO-sensitive GC and PKG1, but not PKG2) were expressed in N18TG2 cells, as was the CB(1) but not the CB(2) cannabinoid receptor. Stimulation of N18TG2 cells by cannabinoid agonists CP55940 and WIN55212-2 increased cyclic GMP levels in an ODQ-sensitive manner. GC-beta(1) in membrane fractions was increased after 5 or 20 min stimulation, and was significantly depleted in the cytosol by 1h. The cytosolic pool of GC-beta(1) was replenished after 48 h of continued cannabinoid drug treatment. Translocation of GC-beta(1) from the cytosol was blocked by the CB(1) antagonist rimonabant (SR141716) and by the Gi/o inactivator pertussis toxin, indicating that the CB(1) receptor and Gi/o proteins are required for translocation. Long-term treatment with rimonabant or pertussis toxin reduced the amount of GC-beta(1) in the cytosolic pool. We conclude that CB(1) receptors stimulate cyclic GMP production and that intracellular translocation of GC from cytosol to the membranes is intrinsic to the mechanism and may be a tonically active or endocannabinoid-regulated process.

Nitric oxide from neuronal nitric oxide synthase sensitises neurons to hypoxia-induced death via competitive inhibition of cytochrome oxidase

Hypoxia/ischaemia is known to trigger neuronal death, but the role of neuronal nitric oxide synthase (nNOS) in this process is controversial. Nitric oxide (NO) inhibits cytochrome oxidase in competition with oxygen. We tested whether NO derived from nNOS synergises with hypoxia to induce neuronal death by inhibiting mitochondrial cytochrome oxidase. Sixteen hours of hypoxia (2% oxygen) plus deoxyglucose (an inhibitor of glycolysis) caused extensive, excitotoxic death of neurons in rat cerebellar granule cell cultures. Three different nNOS inhibitors (including the selective inhibitor N-4S-4-amino-5-2-aminoethyl-aminopentyl-N'-nitroguanidine) decreased this neuronal death by half, indicating a contribution of nNOS to hypoxic death. The selective nNOS inhibitor did not, however, block neuronal death induced either by added glutamate or by added azide (an uncompetitive inhibitor of cytochrome oxidase), indicating that nNOS does not act downstream of glutamate or cytochrome oxidase. Hypoxia plus deoxyglucose-induced glutamate release and neuronal depolarisation, and the nNOS inhibitor decreased this. Hypoxia inhibited cytochrome oxidase activity in the cultures, but a selective nNOS inhibitor prevented this inhibition, indicating NO from nNOS was inhibiting cytochrome oxidase in competition with oxygen. These data indicate that hypoxia synergises with NO from nNOS to induce neuronal death via cytochrome oxidase inhibition causing neuronal depolarisation. This mechanism might contribute to ischaemia/stroke-induced neuronal death in vivo.

Nitric oxide has dual opposite roles during early and late phases of apoptosis in cerebellar granule neurons

The involvement and the role of nitric oxide (NO) as a signaling molecule in the course of neuronal apoptosis, whether unique or modulated during the progression of the apoptotic program, has been investigated in a cellular system consisting of cerebellar granule cells (CGCs) where apoptosis can be induced by lowering extracellular potassium. Several parameters involved in NO signaling pathway, such as NO production, neuronal nitric oxide synthase (nNOS) expression, and cyclic GMP (cGMP) production were examined in the presence or absence of different inhibitors. We provide evidence that nitric oxide has dual and opposite effects depending on time after induction of apoptosis. In an early phase, up to 3 h of apoptosis, nitric oxide supports survival of CGCs through a cGMP-dependent mechanism. After 3 h, nNOS expression and activity decreased resulting in shut down of NO and cGMP production. Residual NO then contributes to the apoptotic process by reacting with rising superoxide anions leading to peroxynitrite production and protein inactivation. We conclude that whilst NO over-production protects neurons from death in the early phase of neuronal damage, its subsequent reduction may contribute to neuronal degeneration and ultimate cell death.

Loss of hippocampal neuronal nitric oxide synthase contributes to the stress-related deficit in learning and memory

Nitric oxide (NO) has been involved in many pathophysiological brain processes. However, the exact role of NO in the cognitive deficit associated to chronic stress exposure has not been elucidated. In this study, we investigated the participation of hippocampal NO production and their regulation by protein kinase C (PKC) in the memory impairment induced in mice subjected to chronic mild stress model (CMS). CMS mice showed a poor learning performance in both open field and passive avoidance inhibitory task respect to control mice. Histological studies showed a morphological alteration in the hippocampus of CMS mice. On the other hand, chronic stress induced a diminished NO production by neuronal nitric oxide synthase (nNOS) correlated with an increment in gamma and zeta PKC isoenzymes. Partial restoration of nNOS activity was obtained after PKC activity blockade. NO production by inducible nitric oxide synthase isoform was not detected. The magnitude of oxidative stress, evaluated by reactive oxygen species production, after excitotoxic levels of NMDA was increased in hippocampus of CMS mice. Moreover, ROS formation was higher in the presence of nNOS inhibitor in both control and CMS mice. Finally, treatment of mice with nNOS inhibitors results in behavioural alterations similar to those observed in CMS animals. These findings suggest a novel role for nNOS showing protective activity against insults that trigger tissue toxicity leading to memory impairments.

Cerebellar localization of the NO-receptive soluble guanylyl cyclase subunits-alpha(2)/beta (1) in non-human primates

Nitric-oxide-sensitive guanylyl cyclase (NO-sGC) plays a pivotal role in many second messenger cascades. Neurotransmission- and neuropathology-related changes in NO-sGC have been suggested. However, the cellular localization of NO-sGC in primate brains, including humans, remains unknown. Biochemical evidence has linked the alpha(2)-subunit of NO-sGC directly to neurotransmission in rodents. Here, we have used a recently characterized subunit-specific antibody for the localization of the alpha(2)-subunit on sections from the cerebelli of the common marmoset (Callithrix jacchus; New World monkey) and macaque monkeys (Macaca mulatta, M. fascicularis; Old World monkeys). In contrast to the more ubiquitous cytoplasmic presence of subunit-beta(1), the alpha(2)-subunit is mainly confined to the somato-dendritic membrane including the spines of the Purkinje cells. Only limited colocalization with presynaptically localized synaptophysin has been seen under our staining conditions, indicating a higher abundance of subunit-alpha(2) at the postsynaptic site. This localization indicates that subunit-alpha(2) links NO-sGC to neurotransmission, whereas subunit-beta(1) may act as a cytoplasmic regulator/activator by contributing to active heterodimer formation via translocation from the cytoplasm to the cell membrane. The last-mentioned action may be a prerequisite for generating nitric-oxide-dependent, subcellular, and postsynaptically localized cGMP signals along neuronal processes.

NMDA induces post-transcriptional regulation of alpha2-guanylyl-cyclase-subunit expression in cerebellar granule cells

Activation of N-methyl-D-aspartate (NMDA) glutamate receptors commonly affects gene expression in different neurons. We reported previously that chronic treatment of rat cerebellar granule cells with NMDA (24 hours) upregulates the expression of mRNA encoding the alpha2 subunit of the nitric-oxide-sensitive guanylyl cyclase. However, the molecular mechanisms involved in this process remained to be elucidated. Here, we have performed mRNA-decay experiments using the transcriptional inhibitor actinomycin D, providing evidence that the half-life of alpha2 mRNA is significantly prolonged in cells exposed to NMDA. The role of the 3' untranslated region of the alpha2 transcripts in NMDA-induced mRNA stabilisation was examined and an association between the RNA-binding proteins AUF1 and ELAV-like protein 1 (HuR/HuA), and endogenous alpha2 mRNA was demonstrated in vivo, as revealed by coimmunoprecipitation experiments with specific antibodies against AUF1 and HuR. Further studies indicated that stimulation of the NMDA receptor induces a downregulation in AUF1 levels stabilising the alpha2 mRNA transcripts. These events are triggered through a mechanism that depends on formation of nitric oxide, and on the subsequent activation of guanylyl cyclase and cGMP dependent protein kinases.

Nitric oxide induces phosphodiesterase 4B expression in rat pulmonary artery smooth muscle cells

Phosphodiesterases (PDE) metabolize cyclic nucleotides limiting the effects of vasodilators such as prostacyclin and nitric oxide (NO). In this study, DNA microarray techniques were used to assess the impact of NO on expression of PDE genes in rat pulmonary arterial smooth muscle cells (rPASMC). Incubation of rPASMC with S-nitroso-l-glutathione (GSNO) increased expression of a PDE isoform that specifically metabolizes cAMP (PDE4B) in a dose- and time-dependent manner. GSNO increased PDE4B protein levels, and rolipram-inhibitable PDE activity was 2.3 +/- 1.0-fold greater in GSNO-treated rPASMC than in untreated cells. The soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one, and the cAMP-dependent protein kinase inhibitor, H89, prevented induction of PDE4B gene expression by GSNO, but the protein kinase G (PKG) inhibitors, Rp-8-pCPT-cGMPs and KT-5823, did not. Incubation of rPASMC with IL-1beta and tumor necrosis factor-alpha induced PDE4B gene expression, an effect that was inhibited by l-N(6)-(1-iminoethyl)lysine, an antagonist of NO synthase 2 (NOS2). The GSNO-induced increase in PDE4B mRNA levels was blocked by actinomycin D but augmented by cycloheximide. Infection of rPASMC with an adenovirus specifying a dominant negative cAMP response element binding protein (CREB) mutant inhibited the GSNO-induced increase of PDE4B gene expression. These results suggest that exposure of rPASMC to NO induces expression of PDE4B via a mechanism that requires cGMP synthesis by sGC but not PKG. The GSNO-induced increase of PDE4B gene expression is CREB dependent. These findings demonstrate that NO increases expression of a cAMP-specific PDE and provide evidence for a novel "cross talk" mechanism between cGMP and cAMP signaling pathways.

Expression of cGMP-dependent protein kinases (I and II) and neuronal nitric oxide synthase in the developing rat cerebellum

The expression of neuronal nitric oxide synthase (nNOS) and the cGMP-dependent protein kinases cGKI and cGKII in rat cerebellum was evaluated at different developmental stages by quantitative RT-PCR and Western blotting. mRNAs coding for these proteins were detected in the cerebella of rats aged 7, 14 and 21 days. Expression levels, nevertheless, varied significantly at each of these developmental stages. While nNOS and cGKI mRNA levels steadily increased during development, cGKII mRNA showed a different behaviour pattern, with similar levels observed on postnatal days 7 and 14 and increased levels noted on postnatal day 21. Moreover, protein expression profiles for nNOS and cGKI showed similar patterns to the mRNAs encoding these proteins. Our results reveal the developmental regulation of the expression of these proteins in the cerebellum, giving rise to higher levels as the cerebellum matures.