Nitric-oxide synthase and neurodegeneration/neuroprotection

Pathophysiology of primary open-angle glaucoma from a neuroinflammatory and neurotoxicity perspective: a review of the literature

PURPOSE

Glaucoma is the leading cause of blindness in humans, affecting 2% of the population. This disorder can be classified into various types including primary, secondary, glaucoma with angle closure and with open angle. The prevalence of distinct types of glaucoma differs for each particular region of the world. One of the most common types of this disease is primary open-angle glaucoma (POAG), which is a complex inherited disorder characterized by progressive retinal ganglion cell death, optic nerve head excavation and visual field loss. Nowadays, POAG is considered an optic neuropathy, while intraocular pressure is proposed to play a fundamental role in its pathophysiology and especially in optic disk damage. However, the exact mechanism of optic nerve head damage remains a topic of debate. This literature review aims to bring together the information on the pathophysiology of primary open-angle glaucoma, particularly focusing on neuroinflammatory mechanisms leading to the death of the retinal ganglion cell.

METHODS

A literature search was done on PubMed using key words including primary open-angle glaucoma, retinal ganglion cells, Müller cells, glutamate, glial cells, ischemia, hypoxia, exitotoxicity, neuroinflammation, axotomy and neurotrophic factors. The literature was reviewed to collect the information published about the pathophysiologic mechanisms of RGC death in the POAG, from a neuroinflammatory and neurotoxicity perspective.

RESULTS

Proposed mechanisms for glaucomatous damage are a result of pressure in RGC followed by ischemia, hypoxia of the ONH, and consequently death due to glutamate-induced excitotoxicity, deprivation of energy and oxygen, increase in levels of inflammatory mediators and alteration of trophic factors flow. These events lead to blockage of anterograde and retrograde axonal transport with ensuing axotomy and eventually blindness.

CONCLUSIONS

The damage to ganglion cells and eventually glaucomatous injury can occur via various mechanisms including baric trauma, ischemia and impact of metabolic toxins, which triggers an inflammatory process and secondary degeneration in the ONH.

Oroxylin A promotes retinal ganglion cell survival in a rat optic nerve crush model

Purpose

To investigate the effect of oroxylin A on the survival of retinal ganglion cells (RGC) and the activation of microglial cells in a rat optic nerve (ON) crush model.

Methods

Oroxylin A (15mg/Kg in 0.2ml phosphate-buffered saline) or phosphate-buffered saline (PBS control) was immediately administered after ON crush once by subcutaneous injection. Rats were euthanized at 2 weeks after the crush injury. The density of RGC was counted by retrograde labeling with FluoroGold and immunostaining of retina flat mounts for Brn3a. Electrophysiological visual function was assessed by flash visual evoked potentials (FVEP). TUNEL assay, immunoblotting analysis of glial fibrillary acidic protein (GFAP), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in the retinas, and immunohistochemistry of GFAP in the retinas and ED1 in the ON were evaluated.

Results

Two weeks after the insult, the oroxylin A-treated group had significantly higher FG labeled cells and Brn3a+ cells suggesting preserved RGC density in the central and mid-peripheral retinas compared with those of the PBS-treated group. FVEP measurements showed a significantly better preserved latency of the P1 wave in the ON-crushed, oroxylin A-treated rats than the ON-crushed, PBS treated rats. TUNEL assays showed fewer TUNEL positive cells in the ON-crushed, oroxylin A-treated rats. The number of ED1 positive cells was reduced at the lesion site of the optic nerve in the ON-crushed, oroxylin A-treated group. Increased GFAP expression in the retina was reduced greatly in ON-crushed, oroxylin A-treated group. Furthermore, administration of oroxylin A significantly attenuated ON crush insult-induced iNOS and COX-2 expression in the retinas.

Conclusions

These results demonstrated that oroxylin A hasss neuroprotective effects on RGC survival with preserved visual function and a decrease in microglial infiltration in the ONs after ON crush injury.

Anti-Inflammatory and Antioxidative Stress Effects of Oryzanol in Glaucomatous Rabbits

Purpose. γ-Oryzanol works by anti-inflammatory and radical scavenging activity as a neuroprotective, anticancer, antiulcer, and immunosuppressive agent. The present study was conducted to investigate effect of oryzanol in acute and chronic experimental glaucoma in rabbits. Methods. Effect of oryzanol was evaluated in 5% dextrose induced acute model of ocular hypertension in rabbit eye. Chronic model of glaucoma was induced with subconjunctival injection of 5% of 0.3 ml phenol. Treatment with oryzanol was given for next two weeks after induction of glaucoma. From anterior chamber of rabbit eye aqueous humor was collected to assess various oxidative stress parameters like malondialdehyde, superoxide dismutase, glutathione peroxidase, catalase, nitric oxide, and inflammatory parameters like TNF-α and IL-6. Structural damage in eye was examined by histopathological studies. Results. In acute model of ocular hypertension oryzanol did not alter raised intraocular pressure. In chronic model of glaucoma oryzanol exhibited significant reduction in oxidative stress followed by reduction in intraocular pressure. Oryzanol treatment reduced level of TNF-α and IL-6. Histopathological studies revealed decreased structural damage of trabecular meshwork, lamina cribrosa, and retina with oryzanol treatment. Conclusions. Oryzanol showed protective effect against glaucoma by its antioxidative stress and anti-inflammatory property. Treatment with oryzanol can reduce optic nerve damage.

Neuroprotective effect of 4-(Phenylsulfanyl)butan-2-one on optic nerve crush model in rats

This study is to investigate the effect of coral-related compound, 4-(phenylsulfanyl)butan-2-one (4-PSB-2) on optic nerves (ON) and retinal ganglion cells (RGC) in a rat model subjected to ON crush. The ONs of adult male Wistar rat (150-180 g) were crushed by a standardized method. The control eyes received a sham operation. 4-PSB-2 (5 mg/kg in 0.2 mL phosphate-buffered saline) or phosphate-buffered saline (PBS control) was immediately administered after ON crush once by subcutaneous injection. Rats were euthanized at 2 weeks after the crush injury. RGC density was counted by retrograde labeling with FluoroGold (FG) application to the superior colliculus, and visual function was assessed by flash visual evoked potentials (FVEP). TUNEL assay, immunoblotting analysis of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX2) in the retinas, and immunohistochemistry of ED1 in the ON were evaluated. Two weeks after the insult, the RGC densities in the central and mid-peripheral retinas in ON-crushed, 4-PSB-2-treated rats were significantly higher than that of the corresponding ON-crushed, PBS-treated rats FVEP measurements showed a significantly better preserved latency of the P1 wave in the ON-crushed, 4-PSB-2-treated rats than the ON-crushed, PBS treated rats. TUNEL assays showed fewer TUNEL positive cells in the ON-crushed, 4-PSB-2-treated rats. The number of ED1 positive cells was reduced at the lesion site of the optic nerve in the ON-crushed, 4-PSB-2-treated group. Furthermore, administration of 4-PSB-2 significantly attenuated ON crush insult-stimulated iNOS and COX2 expression in the retinas. These results demonstrated that 4-PSB-2 protects RGCs and helps preserve the visual function in the rat model of optic nerve crush. 4-PSB-2 may work by being anti-apoptotic and by attenuation of the inflammatory responses involving less ED1 positive cells infiltration in ON as well as suppression of iNOS/COX-2 signaling pathway in the retinas to rescue RGCs after ON crush injury.

Biomarkers of primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is a primary neuronal disease of the optic nerve without a definable cause, and is often associated with increased intraocular pressure. Worldwide, POAG is the second leading cause of blindness; there are 45 million people today with POAG and bilateral blindness is present in 4.5 million of these. In order to elucidate the possible etiologic factors in POAG, we have cataloged all known biomarkers in the aqueous humor, trabecular meshwork, optic nerve and blood into four categories, namely extracellular matrix (ECM), cell signaling molecules, aging/stress and immunity-related changes. We present a theoretical model to show possible signaling pathways of the ECM, cell signaling and innate immune response through activation of Toll-like receptor 4. Our article suggests that ECM and innate immune biomarkers are the lead candidates for developing the 'POAG biomarker signature'. We suggest that current research is critical to pinpoint the causes of the disease so that new treatment modalities can become available for better regulation of the intraocular pressure and neuroprotection of the optic nerve.

Wavelength-dependent change of retinal nerve fiber layer reflectance in glaucomatous retinas

PURPOSE

Retinal nerve fiber layer (RNFL) reflectance is often used in optical methods for RNFL assessment in clinical diagnosis of glaucoma, yet little is known about the reflectance property of the RNFL under the development of glaucoma. This study measured the changes in RNFL reflectance spectra that occurred in retinal nerve fiber bundles with different degrees of glaucomatous damage.

METHODS

A rat model of glaucoma with laser photocoagulation of trabecular meshwork was used. Reflectance of the RNFL in an isolated retina was measured at wavelengths of 400-830 nm. Cytostructural distribution of the bundles measured optically was evaluated by confocal imaging of immunohistochemistry staining of cytoskeletal components, F-actin, microtubules, and neurofilaments. RNFL reflectance spectra were studied in bundles with normal-looking appearance, early F-actin distortion, and apparent damage of all cytoskeletal components. Changes of RNFL reflectance spectra were studied at different radii (0.22, 0.33, and 0.44 mm) from the optic nerve head (ONH).

RESULTS

Bundles in 30 control retinas and 41 glaucomatous retinas were examined. In normal retinas, reflectance spectra were similar along the same bundles. In glaucomatous retinas, reflectance spectra changed along bundles with the spectra becoming flatter as bundle areas approached the ONH.

CONCLUSIONS

Elevation of intraocular pressure (IOP) causes nonuniform changes in RNFL reflectance across wavelengths. Changes of reflectance spectra occur early in bundles near the ONH and prior to apparent cytoskeletal distortion. Using the ratio of RNFL reflectance measured at different wavelengths can provide early and sensitive detection of glaucomatous damage.

β-Amyloid peptide increases levels of iron content and oxidative stress in human cell and Caenorhabditis elegans models of Alzheimer disease

Recent studies indicate that the deposition of β-amyloid peptide (Aβ) is related to the pathogenesis of Alzheimer disease (AD); however, the underlying mechanism is still not clear. The abnormal interactions of Aβ with metal ions such as iron are implicated in the process of Aβ deposition and oxidative stress in AD brains. In this study, we observed that Aβ increased the levels of iron content and oxidative stress in SH-SY5Y cells overexpressing the Swedish mutant form of human β-amyloid precursor protein (APPsw) and in Caenorhabditis elegans Aβ-expressing strain CL2006. Intracellular iron and calcium levels and reactive oxygen species and nitric oxide generation significantly increased in APPsw cells compared to control cells. The activity of superoxide dismutase and the antioxidant levels of APPsw cells were significantly lower than those of control cells. Moreover, iron treatment decreased cell viability and mitochondrial membrane potential and aggravated oxidative stress damage as well as the release of Aβ1-40 from the APPsw cells. The iron homeostasis disruption in APPsw cells is very probably associated with elevated expression of the iron transporter divalent metal transporter 1, but not transferrin receptor. Furthermore, the C. elegans with Aβ-expression had increased iron accumulation. In aggregate, these results demonstrate that Aβ accumulation in neuronal cells correlated with neuronal iron homeostasis disruption and probably contributed to the pathogenesis of AD.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

The Ginkgo biloba extract (EGb 761) provides a neuroprotective effect on retinal ganglion cells in a rat model of chronic glaucoma

PURPOSE

To investigate the effect of Ginkgo biloba extract (EGb 761) against neurotoxicity of retinal ganglion cells of rats with chronic moderately elevated intraocular pressure (IOP).

METHODS

Unilateral chronic moderately elevated IOP was produced in rats by cautery of three episcleral vessels. Secondary degeneration was measured with and without EGb 761 for 5 months. At 5 months, retinal ganglion cells were labeled with a fast blue tracer applied to both superior colliculi. Densities of surviving retinal ganglion cells were estimated by counting fast blue labeled cells in whole mounted retinas.

RESULTS

When compared with their contralateral control eyes with normal IOP, in the peripheral retina, retinal ganglion cell loss in eyes with chronic, moderately elevated IOP was 29.8 +/- 1.5% (n=5) at 5 months in untreated animals and 4.6 +/- 4.5% (n=5) at 5 months in treated animals with EGb 761.

CONCLUSIONS

Pretreatment and early posttreatment with EGb 761 is an effective neuroprotectant in a rat model of chronic glaucoma.

Dynamic contrast-enhanced MRI of ocular biotransport in normal and hypertensive eyes

This study aims to employ in vivo dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) to evaluate the ocular transport following an induction of ocular hypertension in a rat model of chronic glaucoma. Upon systemic administration of Gd-DTPA solution, T1-weighted signal increase was observed in the vitreous body of the glaucomatous eye but not the control eye. This increase occurred earlier in the anterior vitreous body than the preretinal vitreous. Further, there was an earlier Gd-DTPA transport into the anterior chamber in the majority of glaucomatous eyes. Our DCE-MRI findings revealed the leakage of Gd-DTPA at the aqueous-vitreous interface, which was likely resulted from increased permeability of blood-aqueous or aqueous-vitreous barrier. These may explain the sources of changing biochemical compositions in the chamber components, which may implicate the neurodegenerative processes in the glaucomatous visual components.

Proton magnetic resonance spectroscopy revealed choline reduction in the visual cortex in an experimental model of chronic glaucoma

Glaucoma is a neurodegenerative disease of the visual system. While elevated intraocular pressure is considered to be a major risk factor, the primary cause and pathogenesis of this disease are still unclear. This study aims to employ in vivo proton magnetic resonance spectroscopy ((1)H MRS) to evaluate the metabolic changes in the visual cortex in a rat model of chronic glaucoma. Five Sprague-Dawley female rats were prepared to induce ocular hypertension unilaterally in the right eye by photocoagulating the episcleral and limbal veins using an argon laser. Single voxel (1)H MRS was performed on each side of the visual cortex 6 weeks after laser treatment. Relative to the creatine level, the choline level was found to be significantly lower in the left glaucomatous visual cortex than the right control visual cortex in all animals. In addition, a marginally significant increase in glutamate level was observed in the glaucomatous visual cortex. No apparent difference was observed between contralateral sides of the visual cortex in T1-weighted or T2-weighted imaging. The results of this study showed that glaucoma is accompanied with alterations in the metabolism of choline-containing compounds in the visual cortex contralateral to the glaucomatous rat eye. These potentially associated the pathophysiological mechanisms of glaucoma with the dysfunction of the cholinergic system in the visual pathway. (1)H MRS is a potential tool for studying the metabolic changes in glaucoma in vivo in normally appearing brain structures, and may possess direct clinical applications for humans. Measurement of the Cho:Cr reduction in the visual cortex may be a noninvasive biomarker for this disease.

Evaluation of the visual system in a rat model of chronic glaucoma using manganese-enhanced magnetic resonance imaging

This study aims to employ in vivo manganese-enchanced MRI (MEMRI) to evaluate dynamically the Mn(2+) enhancements along the visual pathway following an induction of ocular hypertension in a rat model of chronic glaucoma. Results showed an accumulation of Mn(2+) ions in the vitreous humor of the glaucomatous eye, with no statistical changes in the total retinal thickness but a possible occlusion of the ions at the optic nerve head. Meanwhile, there was a reduction in Mn(2+) transport in the glaucomatous optic nerve in the later stage of our model. Fewer enhancements in the visual cortex projected from the glaucomatous eye were also detectable. These may help understand the disease mechanisms, monitor the effect of drug interventions to glaucoma models, and complement the conventional techniques in examining the visual components.

Fermented papaya preparation attenuates beta-amyloid precursor protein: beta-amyloid-mediated copper neurotoxicity in beta-amyloid precursor protein and beta-amyloid precursor protein Swedish mutation overexpressing SH-SY5Y cells

Recent studies indicate that the deposition of beta-amyloid (Abeta) is related in the pathogenesis of Alzheimer's disease (AD), but the underlying mechanism is still not clear. The abnormal interactions of Abeta with metal ions such as copper are implicated in the process of Abeta deposition and oxidative stress in AD brains. In the present study, we established a new AD model, using which we found that copper triggered the Abeta neurotoxicity in SH-SY5Y cells overexpressing the Swedish mutant form of human APP (APPsw) in a concentration dependent manner. Fermented papaya preparation (FPP) has shown high free radical scavenging ability in vivo and in vitro. FPP post-treatment increased cell viability and decreased the intracellular [Ca2+]i, reactive oxygen species (ROS) generation such as hydroxyl free radical and superoxide anion and nitric oxide (NO) accumulation in the cell. Our results also show that FPP prevents the cell apoptosis through bax/bcl-2 sensitive pathway.

Inhibitory effects of long-term administration of ferulic acid on astrocyte activation induced by intracerebroventricular injection of beta-amyloid peptide (1-42) in mice

Accumulating evidence indicates that glial cells are actively involved in the pathogenesis of Alzheimer's disease. We recently reported protective effects of long-term administration of ferulic acid against learning and memory deficit induced by centrally administered beta-amyloid peptide (Abeta)1-42 in mice. In that report, we found that the Abeta1-42-induced increases in immunoreactivities of glial fibrillary acidic protein, the astrocyte marker, and interleukin(IL)-1beta in the hippocampus are also suppressed by pretreatment with ferulic acid. In the present study, we aimed to further characterize the effect of long-term administration of ferulic acid on the centrally administered Abeta1-42-induced activation of glial cells in mice. Mice were allowed free access to drinking water (control) or water containing ferulic acid (0.006%) for 4 weeks, and then Abeta1-42 (410 pmol) was administered via intracerebroventricular injection. Intracerebroventricularly injected Abeta1-42 induced an increase in immunoreactivities of endothelial nitric oxide synthase (eNOS) and 3-nitrotyrosine (3-NT) in the activated astrocytes in the hippocampus. Pretreatment of ferulic acid for 4 weeks prevented the Abeta1-42-induced increase in eNOS and 3-NT immunoreactivities. Administration of ferulic acid per se induced a transient and slight increase in eNOS immunoreactivity in the hippocampus on day 14, which returned to basal levels on day 28. Intracerebroventricularly injected Abeta1-42 also increased interleukin-1alpha(IL-1alpha) immunoreactivity in the hippocampus, which was also suppressed by pretreatment with ferulic acid. These results demonstrate that long-term administration of ferulic acid induces suppression of the centrallly injected Abeta1-42-induced activation of astrocytes which is suggested to underlie the protective effect of ferulic acid against Abeta1-42 toxicity in vivo.

The intimate relation between nitric oxide and superoxide in apoptosis and cell survival

Intra- and intercellular communication in or between cells allows adaptation to changes in the environment. Formation of reactive oxygen (ROS) and nitrogen (RNS) species in response to external insults gained considerable attention in provoking cell demise along an apoptotic subroute of cell death, thus attributing radical formation to pathologies. In close association, stabilization of the tumor suppressor p53 and activation of caspases convey proapoptotic signaling. Complexity was added with the notion that ROS and RNS signals overlap and/or produce synergistic as well as antagonistic effects. With respect to nitric oxide (NO) signaling, it became clear that the molecule is endowed with pro- or antiapoptotic signaling capabilities, depending to some extend on the concentration and cellular context, i.e., ROS generation. Here, some established concepts are summarized that allow an explanation of p53 accumulation under the impact of NO and an understanding of NO-evoked cell protection at the level of caspase inhibition, cyclic GMP formation, or expression of antiapoptotic proteins. In addition, the overlapping sphere of ROS and RNS signaling is recapitulated to appreciate cell physiology/pathology with the notion that marginal changes in the flux rates of either NO or superoxide may shift vital signals used for communication and cell survival into areas of pathology in close association with apoptosis/necrosis.

Inducible form of nitric oxide synthase expression in rat cortical neuronal cells in vitro

The inducible form of nitric oxide synthase (iNOS) is an essential element of the immune response, which is expressed primarily in microglial cells within the CNS. Exposure of rat cortical neuronal cells to the pro-inflammatory bacterial endotoxin lipopolysaccharide (LPS) resulted in a significant increase in the expression of the cellular iNOS protein expression and NO generation (which serves as an indirect measure of NOS catalytic activity). These effects were potentiated by costimulation with interferon-gamma (IFNgamma) and the increase in NO generation was abolished by the iNOS selective inhibitor 1400W, although this did not attenuate the toxin-induced increase in the enzyme expression. As the cortex is one of the principal areas to be targeted in Alzheimer's disease (AD), the present findings may help to further our understanding of the biochemical events associated with the neurodegenerative process.

Protection against murine leukemia virus-induced spongiform myeloencephalopathy in mice overexpressing Bcl-2 but not in mice deficient for interleukin-6, inducible nitric oxide synthetase, ICE, Fas, Fas ligand, or TNF-R1 genes

Some murine leukemia viruses (MuLVs), among them Cas-Br-E and ts-1 MuLVs, are neurovirulent, inducing spongiform myeloencephalopathy and hind limb paralysis in susceptible mice. It has been shown that the env gene of these viruses harbors the determinant of neurovirulence. It appears that neuronal loss occurs by an indirect mechanism, since the target motor neurons have not been found to be infected. However, the pathogenesis of the disease remains unclear. Several lymphokines, cytokines, and other cellular effectors have been found to be aberrantly expressed in the brains of infected mice, but whether these are required for the development of the neurodegenerative lesions is not known. In an effort to identify the specific effectors which are indeed required for the initiation and/or development of spongiform myeloencephalopathy, we inoculated gene-deficient (knockout [KO]) mice with ts-1 MuLV. We show here that interleukin-6 (IL-6), inducible nitric oxide synthetase (iNOS), ICE, Fas, Fas ligand (FasL), and TNF-R1 KO mice still develop signs of disease. However, transgenic mice overexpressing Bcl-2 in neurons (NSE/Bcl-2) were largely protected from hind limb paralysis and had less-severe spongiform lesions. These results indicate that motor neuron death occurs in this disease at least in part by a Bcl-2-inhibitable pathway not requiring the ICE, iNOS, Fas/FasL, TNF-R1, and IL-6 gene products.

Nitric oxide: NO apoptosis or turning it ON?

Nitric oxide (NO) is known for its diverse activities throughout biology. Among signaling qualities, NO affects cellular decisions of life and death either by turning on apoptotic pathways or by shutting them off. Although copious reports support both notions, the dichotomy of NO actions remains unsolved. Proapoptotic pathways of NO are compatible with established signaling circuits appreciated for mitochondria-dependent roads of death, with some emphasis on the involvement of the tumor suppressor p53 as a target during cell death execution. Antiapoptotic actions of NO are numerous, ranging from an immediate interference with proapoptotic signaling cascades to long-lasting effects based on expression of cell protective proteins with some interest on the ability of NO-redox species to block caspases by S-nitrosylation/S-nitrosation. Summarizing emerging concepts to understand p53 accumulation on the one hand while proposing inhibition of procaspase processing on the other may help to define the pro- versus antiapoptotic roles of NO.

Nitric oxide regulates cGMP-dependent cAMP-responsive element binding protein phosphorylation and Bcl-2 expression in cerebellar neurons: implication for a survival role of nitric oxide

Nitric oxide (NO) is a small, diffusible, highly reactive molecule with a dichotomous regulatory role in the brain: an intra- and intercellular messenger under physiological conditions and a neurodegenerative agent under pathological conditions. We have recently demonstrated that long-lasting exposure to an neuronal nitric oxide synthase (nNOS) inhibitor down-regulated serine/threonine kinase (Akt) survival pathway and caused apoptosis in cerebellar granule cell cultures. The present study further substantiates the role of NO in neuronal survival by demonstrating that blocking its production down-regulates the activity of cAMP-responsive element binding protein (CREB), a transcription factor involved in cell survival and synaptic plasticity. Pharmacological dissection of the pathway linking NO to CREB shows that cGMP and its kinase are intermediate effectors. We also identify Bcl-2 as one of the anti-apoptotic genes down-regulated by NO shortage and decreased CREB phosphorylation. These results not only confirm the role of CREB in neuronal survival but also provide circumstantial evidence for a novel link among NO, CREB activation and survival.

Antiproliferative effect of nitric oxide on rat glomerular mesangial cells via inhibition of mitogen-activated protein kinase

The effect of nitric oxide (NO) donors and lipopolysaccharide (LPS) on the proliferation of rat glomerular mesangial cells was characterized. Exogenous application of a NO donor inhibited serum-induced proliferation in a time- and dose-dependent manner. S-Nitrosoglutathione (GSNO) also increased cGMP generation and arachidonic acid release, but it did not cause any measurable increase in the cytosolic Ca2+ concentration. Chelation of cytosolic Ca2+ or inhibition of mitogen-activated protein kinase (MAPK) kinase had an inhibitory effect on proliferation, but neither enhanced the antiproliferative effect of GSNO. In contrast, inhibition of guanylate cyclase or phospholipase A2 had no effect on proliferation, but partially reversed GSNO-induced antiproliferation by approximately 98 and 65%, respectively. GSNO did not cause cell death. Incubation of cells with LPS induced endogenous NO generation and had an antiproliferative effect. LPS-induced antiproliferation was reversed completely by inhibition of nitric oxide synthase and partially by inhibition of guanylate cyclase or phospholipase A2. GSNO or LPS inhibited serum-induced MAPK activation, and both effects were partially reversed by inhibition of guanylate cyclase or phospholipase A2. Inclusion of 8-bromo-cGMP or arachidonic acid in the growth medium resulted in a similar antiproliferative effect. In conclusion, in rat glomerular mesangial cells, MAPK inhibition and an antiproliferative effect could be induced by either an increase in the cellular concentration of NO or exposure of the cells to LPS. Part of the effect of NO was attributable to the increased cellular cGMP generation and arachidonic acid release.

Expression of endothelial and inducible NOS-isoforms is increased in Alzheimer's disease, in APP23 transgenic mice and after experimental brain lesion in rat: evidence for an induction by amyloid pathology

The nitric oxide-synthesizing enzyme nitric oxide synthase (NOS) is present in the mammalian brain in three different isoforms, two constitutive enzymes (i.e., neuronal, nNOS, and endothelial eNOS) and one inducible enzyme (iNOS). All three isoforms are aberrantly expressed in Alzheimer's disease giving rise to elevated levels of nitric oxide apparently involved in the pathogenesis of this disease by various different mechanisms including oxidative stress and activation of intracellular signalling mechanisms. It still is a matter of debate, however, whether the abnormal expression of NOS isoforms has some primary importance in the pathogenetic chain and might thus be a potential therapeutic target or only reflects a secondary effect that occurs at more advanced stages of the disease process. To tackle this question, we analysed the expression of both eNOS and iNOS in patients with sporadic AD, in transgenic mice expressing human amyloid precursor protein (APP) with the Swedish double mutation under control of the Thy1 promotor (APP23 mice), and after electrolytic cortical lesion in rat, an experimental paradigm associated with elevated expression of APP. In all three conditions, an astrocytosis was induced accompanied by a strong increase of both iNOS and eNOS. Both NOS isoforms were frequently though not always colocalized. Thus, based on the expression pattern of NOS isoforms three types of astrocytes, expressing only one of the two isoforms or both together could be distinguished. In both AD and transgenic mice eNOS-expressing astrocytes exceeded iNOS-expressing astrocytes in number. Astrocytes with elevated levels of iNOS or eNOS were constantly seen in direct association with Abeta-deposits in AD and transgenic mice and were found in the vicinity of the lesion site in the rat cortex. The results of the present study show that expression of both iNOS and eNOS is increased in activated astrocytes under experimental conditions associated with elevated expression of APP (electrolytic brain lesion) or Abeta-deposition (APP23 transgenic mice). Therefore, it is suggested that altered expression of these NOS isoforms being part of AD pathology is secondary to the amyloid pathology and might not be primarily involved in the pathogenetic chain though it might contribute to the maintenance, self-perpetuation and progression of the neurodegenerative process.

Neurodegeneration and glia response in rat hippocampus following nitro-L-arginine methyl ester (L-NAME)

Hippocampal neurodegeneration and glia response was examined following administration of the nitric oxide synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME). Male Long-Evans rats received L-NAME (50 mg/kg, ip) either once or twice a day for 4 days. Both dosing schedules decreased NOS-activity by approximately 90%. At 10 and 30 days following cessation of L-NAME (2x/day), moderate neuronal death was evident in CA1-2 pyramidal cells and dentate granule cells. Neurodegeneration was accompanied by increased astrocyte glial fibrillary acidic protein (GFAP) immunoreactivity yet, minimal astrocyte hypertrophy. Microglia response was limited to an increase in ramified microglia at 10 days, returning to normal by 30 days. As early as 4 days post-dosing (2x/day), GFAP mRNA levels were significantly elevated as were mRNA levels for tumor necrosis factor-alpha (TNFalpha), interleukin-1alpha (IL-1alpha), and interleukin 6 (IL-6). No alterations were seen with L-NAME dosing limited to once a day. The co-administration of a hippocampal neurotoxicant, trimethyltin (TMT), with the last dose of L-NAME (2x/day), produced an additive response pattern of neuronal degeneration including both CA1-2 and CA3-4 pyramidal neurons accompanied by TMT-induced astrocyte hypertrophy and prominent microglia reactivity. This was preceded by elevations in mRNA levels for GFAP, TNFalpha, IL-1alpha, and IL-6 similar to those seen with each substance alone. These data suggest that high levels of L-NAME can produce a pro-inflammatory environment in the brain and that neurodegeneration and neuroglia responses in the hippocampus can be induced by an alteration in the balance and regulation of local nitric oxide levels.

Review: effects of nitric oxide on eye diseases and their treatment

Both underproduction and overproduction of nitric oxide (NO) could lead to various eye diseases. It is known that endothelial NO synthase (eNOS) and neuronal NOS (nNOS) are activated in normal tissues to produce NO for physiological functions. Thus, underproduction of NO results in various eye diseases which could be corrected by providing NOS substrates or NO donors to lower the intraocular pressure, increase ocular blood flow, relax ciliary muscle, etc. On the other hand, immunological NOS (iNOS) is inducible only in pathological conditions by endotoxins, inflammation, and certain cytokines, such as interleukin-1 (IL-1), IL-6, TNF (tumor necrosis factor) and the like. Once induced, iNOS will produce large amounts of NO for long periods of time, so that NO is converted into NO2, nitrite, peroxynitrite and free radicals to induce pathophysiological actions, such as optic nerve degeneration and posterior retinal degeneration lesion, which lead to glaucoma, retinopathy, age-related macular degeneration (AMD), myopia, cataracts and uveitis. To treat/prevent these eye diseases, inhibitors of iNOS activity and/or iNOS induction could be tried.

Exogenous administration of L-arginine protects cholinergic neurons from colchicine neurotoxicity

The effect of L-arginine administration on susceptibility/resistance of NO-producing neurons was investigated in non-ischemic neurodegenerative conditions. ChAT mRNA level was measured by in situ hybridization in the basal forebrain after i.c.v. colchicine administration. Colchicine induced down-regulation of ChAT mRNA level followed by degeneration of cholinergic neurons. L-Arginine treatment increased ChAT mRNA level and prevented/delayed ChAT mRNA decrease by colchicine. These data suggest that L-arginine treatment may protect cholinergic neurons from colchicine-induced degeneration.