Control of nitric oxide production by endogenous TNF-alpha in mouse retinal pigmented epithelial and Muller glial cells

Association between self-reported bupropion use and glaucoma: a population-based study

PURPOSE

To investigate the relationship between self-reported bupropion use and self-reported glaucoma in a nationally representative sample of the US population.

METHODS

This cross-sectional study included 6760 participants in the National Health and Nutrition Examination Survey (NHANES) between 2005 and 2008, age ≥40 years, who responded to a question regarding their glaucoma status. Participants were interviewed regarding the use of prescription medications, and those ascertained as having used bupropion were further divided into groups based on duration of usage. Other relevant information, including demographics, comorbidities and health-related behaviours, was obtained via interview. Multivariate logistic regression was performed to determine the OR and 95% CIs for association between bupropion use and prevalent glaucoma. Covariates in the final multivariate model included parameters associated with glaucoma at p<0.1: age, gender, ethnicity and annual income.

RESULTS

453 participants self-reported a diagnosis of glaucoma, and 108 reported bupropion medication use. Participants who reported using bupropion for more than 1 year had decreased odds of self-reporting a diagnosis of glaucoma (unadjusted OR=0.5, 95% CI 0.01 to 0.52; adjusted OR=0.1, 95% CI 0.01 to 0.81) compared with those not using bupropion or using it for less than a year.

CONCLUSIONS

Bupropion use, particularly for an extended period of time, may be associated with a reduced risk of glaucomatous disease.

Glaucomatous neurodegeneration: an eye on tumor necrosis factor-alpha

Glaucoma, a neurodegenerative disease, is currently being treated by modulation of one of its primary risk factors, the elevated intraocular pressure. Newer therapies that can provide direct neuroprotection to retinal ganglion cells are being extensively investigated. Tumor necrosis factor-α, a cytokine, has been recognized to play an important role in pro and antiapoptotic cellular events. In this paper we review the relevant literature to understand (1) The association of increased expression of tumor necrosis factor-α with glaucomatous neurodegeneraion, (2) Modulation of tumor necrosis factor-α expression by exposure to various risk factors of glaucoma, (3) Downstream cellular signaling mechanisms following interaction of tumor necrosis factor-α with its receptors and (4) Role of tumor necrosis factor-α as a possible target for therapeutic intervention in glaucoma. Literature was reviewed using PubMed search engine with relevant key words and a total of 82 English language papers published from 1990 to 2010 are included in this review.

Reduced nitro-oxidative stress and neural cell death suggests a protective role for microglial cells in TNFalpha-/- mice in ischemic retinopathy

PURPOSE

Neovascularization occurs in response to tissue ischemia and growth factor stimulation. In ischemic retinopathies, however, new vessels fail to restore the hypoxic tissue; instead, they infiltrate the transparent vitreous. In a model of oxygen-induced retinopathy (OIR), TNFalpha and iNOS, upregulated in response to tissue ischemia, are cytotoxic and inhibit vascular repair. The aim of this study was to investigate the mechanism for this effect.

METHODS

Wild-type C57/BL6 (WT) and TNFalpha(-/-) mice were subjected to OIR by exposure to 75% oxygen (postnatal days 7-12). The retinas were removed during the hypoxic phase of the model. Retinal cell death was determined by TUNEL staining, and the microglial cells were quantified after Z-series capture with a confocal microscope. In situ peroxynitrite and superoxide were measured by using the fluorescent dyes DCF and DHE. iNOS, nitrotyrosine, and arginase were analyzed by real-time PCR, Western blot analysis, and activity determined by radiolabeled arginine conversion. Astrocyte coverage was examined after GFAP immunostaining.

RESULTS

The TNFalpha(-/-) animals displayed a significant reduction in TUNEL-positive apoptotic cells in the inner nuclear layer of the avascular retina compared with that in the WT control mice. The reduction coincided with enhanced astrocytic survival and an increase in microglial cells actively engaged in phagocytosing apoptotic debris that displayed low ROS, RNS, and NO production and high arginase activity.

CONCLUSIONS

Collectively, the results suggest that improved vascular recovery in the absence of TNFalpha is associated with enhanced astrocyte survival and that both phenomena are dependent on preservation of microglial cells that display an anti-inflammatory phenotype during the early ischemic phase of OIR.

TNF-alpha signaling in glaucomatous neurodegeneration

Growing evidence supports the role of tumor necrosis factor-alpha (TNF-alpha) as a mediator of neurodegeneration in glaucoma. Glial production of TNF-alpha is increased, and its death receptor is upregulated on retinal ganglion cells (RGCs) and optic nerve axons in glaucomatous eyes. This multifunctional cytokine can induce RGC death through receptor-mediated caspase activation, mitochondrial dysfunction, and oxidative stress. In addition to direct neurotoxicity, potential interplay of TNF-alpha signaling with other cellular events associated with glaucomatous neurodegeneration may also contribute to spreading neuronal damage by secondary degeneration. Opposing these cell death-promoting signals, binding of TNF receptors can also trigger the activation of survival signals. A critical balance between a variety of intracellular signaling pathways determines the predominant in vivo bioactivity of TNF-alpha as best exemplified by differential responses of RGCs and glia. This review focuses on the present evidence supporting the involvement of TNF-alpha signaling in glaucomatous neurodegeneration and possible treatment targets to provide neuroprotection.

MAP kinase phosphatase 1 controls innate immune responses and suppresses endotoxic shock

Septic shock is a leading cause of morbidity and mortality. However, genetic factors predisposing to septic shock are not fully understood. Excessive production of proinflammatory cytokines, particularly tumor necrosis factor (TNF)-α, and the resultant severe hypotension play a central role in the pathophysiological process. Mitogen-activated protein (MAP) kinase cascades are crucial in the biosynthesis of proinflammatory cytokines. MAP kinase phosphatase (MKP)-1 is an archetypal member of the dual specificity protein phosphatase family that dephosphorylates MAP kinase. Thus, we hypothesize that knockout of the Mkp-1 gene results in prolonged MAP kinase activation, augmented cytokine production, and increased susceptibility to endotoxic shock. Here, we show that knockout of Mkp-1 substantially sensitizes mice to endotoxic shock induced by lipopolysaccharide (LPS) challenge. We demonstrate that upon LPS challenge, Mkp-1^−/− cells exhibit prolonged p38 and c-Jun NH₂-terminal kinase activation as well as enhanced TNF-α and interleukin (IL)-6 production compared with wild-type cells. After LPS challenge, Mkp-1 knockout mice produce dramatically more TNF-α, IL-6, and IL-10 than do wild-type mice. Consequently, Mkp-1 knockout mice develop severe hypotension and multiple organ failure, and exhibit a remarkable increase in mortality. Our studies demonstrate that MKP-1 is a pivotal feedback control regulator of the innate immune responses and plays a critical role in suppressing endotoxin shock.

Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Because tumor necrosis factor (TNF)-alpha can upregulate alveolar fluid clearance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-alpha-mediated increase in epithelial fluid transport were studied. In rats, 5 microg of TNF-alpha in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. We also tested a triple-mutant TNF-alpha that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effect; the mutant also has decreased binding affinity to both TNF-alpha receptors. The triple-mutant TNF-alpha did not increase AFC. Perfusion of human A549 cells, patched in the whole cell mode, with TNF-alpha (120 ng/ml) resulted in a sustained increase in Na(+) currents from 82 +/- 9 to 549 +/- 146 pA (P < 0.005; n = 6). The TNF-alpha-elicited Na(+) current was inhibited by amiloride, and there was no change when A549 cells were perfused with the triple-mutant TNF-alpha or after preincubation with blocking antibodies to the two TNF-alpha receptors before perfusion with TNF-alpha. In conclusion, although TNF- alpha can initiate acute inflammation and edema formation in the lung, TNF-alpha can also increase AFC by an amiloride-sensitive, cAMP-independent mechanism that enhances the resolution of alveolar edema in pathological conditions by either binding to its receptors or activating Na(+) channels by means of its lectinlike domain.

Increased production of tumor necrosis factor-alpha by glial cells exposed to simulated ischemia or elevated hydrostatic pressure induces apoptosis in cocultured retinal ganglion cells

Although glial cells in the optic nerve head undergo a reactivation process in glaucoma, the role of glial cells during glaucomatous neurodegeneration of retinal ganglion cells is unknown. Using a coculture system in which retinal ganglion cells and glial cells are grown on different layers but share the same culture medium, we studied the influences of glial cells on survival of retinal ganglion cells after exposure to different stress conditions typified by simulated ischemia and elevated hydrostatic pressure. After the exposure to these stressors, we observed that glial cells secreted tumor necrosis factor-alpha (TNF-alpha) as well as other noxious agents such as nitric oxide into the coculture media and facilitated the apoptotic death of retinal ganglion cells as assessed by morphology, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, and caspase activity. The glial origin of these noxious effects was confirmed by passive transfer experiments. Furthermore, retinal ganglion cell apoptosis was attenuated approximately 66% by a neutralizing antibody against TNF-alpha and 50% by a selective inhibitor of inducible nitric oxide synthase (1400W). Because elevated intraocular pressure and ischemia are two prominent stress factors identified in the eyes of patients with glaucoma, these findings reveal a novel glia-initiated pathogenic mechanism for retinal ganglion cell death in glaucoma. In addition, these findings suggest that the inhibition of TNF-alpha that is released by reactivated glial cells may provide a novel therapeutic target for neuroprotection in the treatment of glaucomatous optic neuropathy.

Retinal pigment epithelium-immune system interactions: cytokine production and cytokine-induced changes

Vision is dependent on proper function of several intraocular structures. Immune responses to eliminate invading pathogens from the eye may threat vision by causing damage to these structures. Therefore, immunological defence of the eye should be carefully balanced between efficacy and maintenance of functional integrity. The eye is equipped with several regulatory mechanisms to prevent certain immune and inflammatory responses and is, therefore, regarded as an immune privileged site. The retinal pigment epithelium (RPE) contributes to the immune privileged status of the eye as part of the blood-eye barrier and by the secretion of immunosuppressive factors inside the eye. RPE cells, however, may also play an important role in the development of immune and inflammatory responses in the posterior part of the eye. During the last decade it has become clear that RPE cells are highly sensitive to a variety of inflammatory cytokines. Under inflammatory conditions, RPE cells produce a myriad of cytokines that may activate the resident ocular cells or attract and activate leukocytes. Cytokine stimulation of RPE cells causes profound effects, including nitric oxide secretion, cell surface expression of MHC class II and adhesion molecules and abrogation of barrier function. This article provides a comprehensive review of the literature concerning RPE cells and cytokines.

Free radical mediated photoreceptor damage in uveitis

Uveitis is a major cause of blindness, with the visual loss that occurs being due primarily to retinal tissue damage. The tissue damage is mediated mainly by phagocytic inflammatory cells, such as macrophages, by the release of various proteolytic enzymes, arachidonic acid metabolites, cytokines and free radicals. The latter are found to be potent cytotoxic agents that readily cause tissue damage by peroxidation of lipid cell membranes. Recent studies of experimental uveitis indicate that other potent oxidants are generated in uveitis by macrophages. One of these is ONOO-, which is formed from *NO and O(-)2. The macrophages generate *NO preferentially in the outer retina following iNOS expression. In these phagocytes, outer retinal proteins, especially arrestin, are found to be potent iNOS inducers. Current studies of RPE show that these cells protect the retina from ONOO- mediated damage in uveitis by releasing a novel protein called retinal pigment epithelial protective protein. This protein is found to suppress O(-)2 and *NO generation by the phagocytes, in both in vitro and in vivo uveitis models. The protective protein expression is restricted to RPE, its suppressive effect is a result of the inhibition of the phosphorylation of cytosolic proteins, p47-phox, required for the assembly of NADPH and activation of NFkappaB, which are required for generation of 0(-)2 and expression of iNOS respectively. Either pharmacologically or chemically, up-regulation of RPP generation could help in preventing retinal degeneration in uveitis or other degenerative dis

Differential expression of inducible nitric oxide synthase gene by alveolar and peritoneal macrophages in lipopolysaccharide-hyporesponsive C3H/HeJ mice

In lipopolysaccharide (LPS)-hyporesponsive C3H/HeJ mice, alveolar macrophages (AMphi) produce much more tumour necrosis factor-alpha than peritoneal macrophages (PMphi) when stimulated with LPS (10 microgram/ml), but the induction of inducible nitric oxide synthase (iNOS) gene expression and production of nitric oxide (NO) in AMphi are not found. In the present study, we determined the induction of iNOS gene expression, using semi-quantitative reverse transcription-polymerase chain reaction, and the release of NO in AMphi and PMphi from C3H/HeJ and C3H/HeN mice. The results showed the induction of iNOS mRNA accumulation in a dose-dependent manner by LPS alone or in combination with interferon-gamma in both macrophages. The effects of the stimuli on iNOS gene expression and NO production were significantly higher in AMphi than in the PMphi of C3H/HeJ mice. The response of macrophages from C3H/HeN mice was similar to those from C3H/HeJ mice, but the difference of iNOS gene expression between AMphi and PMphi in C3H/HeN mice was not as striking as in C3H/HeJ mice. The results show that the iNOS gene expression and NO production were activated differently in AMphi and PMphi and suggest that the functional properties of macrophages isolated from distinct origins are different.

The pathophysiology of multiple sclerosis: the mechanisms underlying the production of symptoms and the natural history of the disease

The pathophysiology of multiple sclerosis is reviewed, with emphasis on the axonal conduction properties underlying the production of symptoms, and the course of the disease. The major cause of the negative symptoms during relapses (e.g. paralysis, blindness and numbness) is conduction block, caused largely by demyelination and inflammation, and possibly by defects in synaptic transmission and putative circulating blocking factors. Recovery from symptoms during remissions is due mainly to the restoration of axonal function, either by remyelination, the resolution of inflammation, or the restoration of conduction to axons which persist in the demyelinated state. Conduction in the latter axons shows a number of deficits, particularly with regard to the conduction of trains of impulses and these contribute to weakness and sensory problems. The mechanisms underlying the sensitivity of symptoms to changes in body temperature (Uhthoff's phenomenon) are discussed. The origin of 'positive' symptoms, such as tingling sensations, are described, including the generation of ectopic trains and bursts of impulses, ephaptic interactions between axons and/or neurons, the triggering of additional, spurious impulses by the transmission of normal impulses, the mechanosensitivity of axons underlying movement-induced sensations (e.g. Lhermitte's phenomenon) and pain. The clinical course of the disease is discussed, together with its relationship to the evolution of lesions as revealed by magnetic resonance imaging and spectroscopy. The earliest detectable event in the development of most new lesions is a breakdown of the blood-brain barrier in association with inflammation. Inflammation resolves after approximately one month, at which time there is an improvement in the symptoms. Demyelination occurs during the inflammatory phase of the lesion. An important mechanism determining persistent neurological deficit is axonal degeneration, although persistent conduction block arising from the failure of repair mechanisms probably also contributes.

Nitric oxide synthase-containing cells in the retina, pigmented epithelium, choroid, and sclera of the chick eye

Nitric oxide is a nonconventional neurotransmitter that is produced as needed by the enzyme nitric oxide synthase (NOS). NOS has been detected in numerous neural structures, including distinct populations of retinal neurons in a variety of vertebrate species. The purpose of this study was to identify NOS-containing cells in the retina and extraretinal ocular tissues of hatched chicks. NOS was detected in frozen sections by using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and antisera to neuronal NOS. In the retina, NADPH-diaphorase and NOS immunolabelling were present in four subtypes of amacrine cells, some ganglion cells, efferent fibers, efferent target cells, and neuronal processes in both plexiform layers, whereas diaphorase alone was detected in photoreceptor ellipsoids and Müller cells. In addition, NADPH-diaphorase and immunoreactive NOS were detected in axon bundles and innervation to vascular smooth muscle in the choroid, whereas stromal and endothelial cells in the choroid, scleral chondrocytes, and the retinal pigmented epithelium contained only NADPH-diaphorase. The excitotoxin quisqualate destroyed all but one subtype of NOS-immunoreactive amacrine cell and caused increased NADPH-diaphorase activity in Müller cells. We conclude that nitric oxide is produced by many different cells in the chick eye, including retinal amacrine and ganglion cells, Müller cells, retinal pigmented epithelium, and cells in the choroid, and likely has a broad range of visual and regulatory functions.