The nitric oxide/cyclic GMP system in astroglial cells

Dysfunctional cGMP Signaling Leads to Age-Related Retinal Vascular Alterations and Astrocyte Remodeling in Mice

The nitric oxide-guanylyl cyclase-1-cyclic guanylate monophosphate (NO-GC-1-cGMP) pathway is integral to the control of vascular tone and morphology. Mice lacking the alpha catalytic domain of guanylate cyclase (GC1^-/-) develop retinal ganglion cell (RGC) degeneration with age, with only modest fluctuations in intraocular pressure (IOP). Increasing the bioavailability of cGMP in GC1^-/- mice prevents neurodegeneration independently of IOP, suggesting alternative mechanisms of retinal neurodegeneration. In continuation to these studies, we explored the hypothesis that dysfunctional cGMP signaling leads to changes in the neurovascular unit that may contribute to RGC degeneration. We assessed retinal vasculature and astrocyte morphology in young and aged GC1^-/- and wild type mice. GC1^-/- mice exhibit increased peripheral retinal vessel dilation and shorter retinal vessel branching with increasing age compared to Wt mice. Astrocyte cell morphology is aberrant, and glial fibrillary acidic protein (GFAP) density is increased in young and aged GC1^-/- mice, with areas of dense astrocyte matting around blood vessels. Our results suggest that proper cGMP signaling is essential to retinal vessel morphology with increasing age. Vascular changed are preceded by alterations in astrocyte morphology which may together contribute to retinal neurodegeneration and loss of visual acuity observed in GC1^-/- mice.

Mechanisms Involved in the Remyelinating Effect of Sildenafil

Remyelination occurs in demyelinated lesions in multiple sclerosis (MS) and pharmacological treatments that enhance this process will critically impact the long term functional outcome in the disease. Sildenafil, a cyclic GMP (cGMP)-specific phosphodiesterase 5 inhibitor (PDE5-I), is an oral vasodilator drug extensively used in humans for treatment of erectile dysfunction and pulmonary arterial hypertension. PDE5 is expressed in central nervous system (CNS) neuronal and glial populations and in endothelial cells and numerous studies in rodent models of neurological disease have evidenced the neuroprotective potential of PDE5-Is. Using myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) as a MS model, we previously showed that daily administration of sildenafil starting at peak disease rapidly ameliorates clinical symptoms while administration at symptoms onset prevents disease progression. These beneficial effects of the drug involved down-regulation of adaptive and innate immune responses, protection of axons and oligodendrocytes (OLs) and promotion of remyelination. In this work we have investigated mechanisms involved in the remyelinating effect of sildenafil. Using demyelinated organotypic cerebellar slice cultures we demonstrate that sildenafil stimulates remyelination by direct effects on CNS cells in a nitric oxide (NO)-cGMP-protein kinase G (PKG)-dependent manner. We also show that sildenafil treatment enhances OL maturation and induces expression of the promyelinating factor ciliary neurotrophic factor (CNTF) in spinal cord of EAE mice and in cerebellar slice cultures. Furthermore, we demonstrate that sildenafil promotes a M2 phenotype in bone marrow derived macrophages (BMDM) and increases myelin phagocytosis in these cells and in M2 microglia/macrophages in the spinal cord of EAE mice. Taken together these data indicate that promotion of OL maturation directly or through induction of growth factor expression, regulation of microglia/macrophage inflammatory phenotype and clearance of myelin debris may be relevant mechanisms involved in sildenafil enhancement of remyelination in demyelinated tissue and further support the contention that this well tolerated drug could be useful for ameliorating MS pathology.

New perspectives in cyclic nucleotide-mediated functions in the CNS: the emerging role of cyclic nucleotide-gated (CNG) channels

Cyclic nucleotides play fundamental roles in the central nervous system (CNS) under both physiological and pathological conditions. The impact of cAMP and cGMP signaling on neuronal and glial cell functions has been thoroughly characterized. Most of their effects have been related to cyclic nucleotide-dependent protein kinase activity. However, cyclic nucleotide-gated (CNG) channels, first described as key mediators of sensory transduction in retinal and olfactory receptors, have been receiving increasing attention as possible targets of cyclic nucleotides in the CNS. In the last 15 years, consistent evidence has emerged for their expression in neurons and astrocytes of the rodent brain. Far less is known, however, about the functional role of CNG channels in these cells, although several of their features, such as Ca(2+) permeability and prolonged activation in the presence of cyclic nucleotides, make them ideal candidates for mediators of physiological functions in the CNS. Here, we review literature suggesting the involvement of CNG channels in a number of CNS cellular functions (e.g., regulation of membrane potential, neuronal excitability, and neurotransmitter release) as well as in more complex phenomena, like brain plasticity, adult neurogenesis, and pain sensitivity. The emerging picture is that functional and dysfunctional cyclic nucleotide signaling in the CNS has to be reconsidered including CNG channels among possible targets. However, concerted efforts and multidisciplinary approaches are still needed to get more in-depth knowledge in this field.

Expression of olfactory-type cyclic nucleotide-gated channels in rat cortical astrocytes

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels activated by cyclic AMP (cAMP) or cyclic GMP (cGMP). They were originally identified in retinal and olfactory receptors, but evidence has also emerged for their expression in several mammalian brain areas. Because cGMP and cAMP control important aspects of glial cell physiology, we wondered whether CNG channels are expressed in astrocytes, the most functionally relevant glial cells in the CNS. Immunoblot and immunofluorescence experiments demonstrated expression of the CNG channel olfactory-type A subunit, CNGA2, in cultured rat cortical astrocytes. In patch-clamp experiments, currents elicited in these cells by voltage ramps from -100 to +100 mV in the presence of the cGMP analogue, dB-cGMP, were significantly reduced by the CNG channel blockers, L-cis-diltiazem (LCD) and Cd(2+) . The reversal potentials of the LCD- and Cd(2+) -sensitive currents were more positive than that of K(+) , as expected for a mixed cation current. Noninactivating, voltage-independent currents were also elicited by extracellular application of the membrane permeant cGMP analogue, 8-Br-cGMP. These effects were blocked by LCD and were mimicked by natriuretic peptide receptor activation and inhibition of phosphodiesterase activity. Voltage-independent, LCD-sensitive currents were also elicited by 8-Br-cGMP in astrocytes of hippocampal and neocortical brain slices. Immunohistochemistry confirmed a broad distribution of CNG channels in astrocytes of the rat forebrain, midbrain, and hindbrain. These findings suggest that CNG channels are downstream targets of cyclic nucleotides in astrocytes, and they may be involved in the glial-mediated regulation of CNS functions under physiological and pathological conditions.

Cyclic GMP-dependent pathways protect differentiated oligodendrocytes from multiple types of injury

The cyclic GMP analog 8-bromo-cyclic GMP (8-Br-cGMP) protects differentiated murine oligodendrocytes (OLs) from caspase-mediated death initiated by staurosporine, thapsigargin or kainate. Caspase-independent death caused by high levels of NO is also partially prevented by 8-Br-cGMP. Inhibitors of protein kinase G (cGMP-dependent protein kinase, cGK) reversed protection, supporting involvement of cGK. Since NO stimulates soluble guanylate cyclase, increasing cGMP, we treated OLs with low levels of NO and observed partial protection against thapsigargin, staurosporine and kainate. Two inhibitors of mitochondrial pore transition (MPT), cyclosporin A and bongkrekic acid, were poorly protective, indicating that cGMP is not acting primarily by blocking MPT. 8Br-cGMP was more effective than 8Br-cAMP in protecting against staurosporine or release of intracellular Ca(++) by thapsigargin. The cAMP analog exhibited little or no protection against kainate or high levels of NO. Thus cGK signaling is more effective than protein kinase A or phosphodiesterase 3 signaling in preventing OL death.

Nitric oxide synthase expression and nitric oxide toxicity in oligodendrocytes

Oligodendrocytes (OLG) have more complex interactions with nitric oxide (NO) than initially suspected. Historically, OLG were seen only as targets of high NO levels released from other cells. Expression of nitric oxide synthase type II (NOS-2) in primary cultures of OLGs stimulated by cytokines led to controversy due to the presence of small numbers of microglia, cells also inducible for NOS-2 expression. The present review summarizes the findings that immature OLG express NOS-2, but that they do not in their most mature stage in culture as membrane sheet-bearing cells. This raises questions about the regulation of NOS-2 expression in OLG. Additionally, novel data are presented on NOS-3 expression in cultured OLG. If confirmed in vivo, this finding suggests that constitutive NOS-3 expression may play a key role in OLG injury due to its activation by calcium, in interaction with pathways mediating glutamate toxicity. The authors discuss in vivo NO levels to place in vitro findings in context, and compare OLG sensitivity to NO with that of other brain cells. Lastly, the multiple interactions of NO are considered with regard to glutamate cytotoxicity, the antioxidant glutathione, mitochondrial function, and myelin architecture.

Limited capacity for ammonia removal by brain in chronic liver failure: potential role of nitric oxide

Chronic liver failure leads to hyperammonemia and consequently increased brain ammonia concentrations, resulting in hepatic encephalopathy. When the liver fails to regulate ammonia concentrations, the brain, devoid of a urea cycle, relies solely on the amidation of glutamate to glutamine through glutamine synthetase, to efficiently clear ammonia. Surprisingly, under hyperammonemic conditions, the brain is not capable of increasing its capacity to remove ammonia, which even decreases in some regions of the brain. This non-induction of glutamine synthetase in astrocytes could result from possible limiting substrates or cofactors for the enzyme, or an indirect effect of ammonia on glutamine synthetase expression. In addition, there is evidence that nitration of the enzyme resulting from exposure to nitric oxide could also be implicated. The present review summarizes these possible factors involved in limiting the increase in capacity of glutamine synthetase in brain, in chronic liver failure.

Roles of Ca2+ and the Ca2+-sensing receptor (CASR) in the expression of inducible NOS (nitric oxide synthase)-2 and its BH4 (tetrahydrobiopterin)-dependent activation in cytokine-stimulated adult human astrocytes

Since NO production by NOS-2 made by astrocytes activated by proinflammatory cytokines contributes to the killing of neurons in variously damaged human brains, knowing the mechanisms responsible for NOS-2 expression should contribute to developing effective therapeutics. The expression and activation of NOS-2 in normal adult human cerebral cortical astrocytes treated with three proinflammatory cytokines, IL-1beta, TNF-alpha, and IFN-gamma, are driven by two separable mechanisms. NOS-2 expression requires a burst of p38 MAPK activity, while the activation of the resulting enzyme protein requires MEK/ERK-dependent BH4 (tetrahydrobiopterin) synthesis between 24 and 24.5 h after adding the cytokines to the culture medium. Here we show that NOS-2 expression in the activated astrocytes requires that the culture medium contain 1.8 mM Ca2+, but it is unaffected by inhibiting calcium-sensing receptors (CASRs) with NPS 89636. However, NOS-2 activation is inhibited by NPS 89626 during the MEK/ERK-dependent stage between 24 and 24.5 h after adding the cytokines, and this inhibition can be overridden by exogenous BH4. Therefore, NOS-2 expression and the subsequent BH4-dependent NOS-2-activation in human astrocytes need 1.8 mM Ca2+ to be in the culture medium, while NOS-2 activation also needs functional CASRs between 24 and 24.5 h after cytokine addition. These findings raise the possibility that calcilytic drugs prevent NO-induced damage and death of human neurons.

Circadian rhythm generation and entrainment in astrocytes

In mammals, the master circadian pacemaker is considered the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN consists of a heterogeneous population of neurons and relatively understudied glia. We investigated whether glia, like neurons, rhythmically express circadian genes. We generated pure cultures of cortical astrocytes from Period2::luciferase (Per2::luc) knock-in mice and Period1::luciferase (Per1::luc) transgenic rats and recorded bioluminescence as a real-time reporter of gene activity. We found that rat Per1::luc and mouse Per2::luc astroglia express circadian rhythms with a genetically determined period. These rhythms damped out after several days but were reinitiated by a variety of treatments, including a full volume exchange of the medium. If cultures were treated before damping out, the phase of Per1::luc rhythmicity was shifted, depending on the time of the pulse relative to the peak of Per1 expression. Glial rhythms entrained to daily 1.5 degrees C temperature cycles and were significantly sustained when cocultured with explants of the adult SCN but not with cortical explants. Thus, multiple signals, including a diffusible factor(s) from the SCN, are sufficient to either entrain or restart circadian oscillations in cortical glia.

Functional expression of constitutive nitric oxide synthases regulated by voltage-gated Na+ and Ca2+ channels in cultured human astrocytes

We report the functional characterization of constitutive nitric oxide synthase(s) (NOS) such as neuronal and endothelial NOS in cultured human astrocytes. Exposure of cultured human astrocytes to 1 microM veratridine or 50 mM KCl produced a pronounced increase in a calmodulin-dependent NOS activity estimated from cGMP formation. The functional expression of voltage-gated Na(+) channel, which is estimated by the response to veratridine, appeared to be earlier (at second day in culture) than that of voltage-gated Ca(2+) channels, which are estimated by the response to the KCl stimulation (at fourth day in culture). The KCl-evoked NO synthesis was totally reversed by L-type Ca(2+) channel blockers such as nifedipine and verapamil, but not by omega-conotoxin GVIA, an N-type Ca(2+) channel blocker, or omega-agatoxin IVA, a P/Q-type Ca(2+) channel blocker. In addition, verapamil abolished the KCl-induced increase in the intracellular free Ca(2+) concentration. RT-PCR analysis revealed that mRNA for neuronal and endothelial NOS was expressed in human astrocytes. In addition, Western blot analysis and double labeling of NOS and glial fibrillary acidic protein (GFAP) showed that cultured human astrocytes expressed neuronal NOS and endothelial NOS as well as the alpha(1) subunit of Ca(2+) channel. These results suggest that human astrocytes express constitutive NOS that are regulated by voltage-gated L-type Ca(2+) channel as well as Na(+) channel.

Interleukin-1 beta and lipopolysaccharide decrease soluble guanylyl cyclase in brain cells: NO-independent destabilization of protein and NO-dependent decrease of mRNA

We previously showed that soluble guanylyl cyclase (sGC) is down-regulated in astroglial cells after exposure to LPS. Here, we show that this effect is not mediated by released IL-1beta but that this cytokine is also able to decrease NO-dependent cGMP accumulation in a time- and concentration-dependent manner. The effect of IL-1beta is receptor-mediated, mimicked by tumor necrosis factor-alpha and involves a decrease in sGC activity and protein. IL-1beta and LPS decrease the half-life of the sGC beta1 subunit by a NO-independent but transcription- and translation-dependent mechanism. Additionally, both agents induce a NO-dependent decrease of sGC subunit mRNA. Decreased sGC subunit protein and mRNA levels are also observed in adult rat brain after focal injection of IL-1beta or LPS.

Beta-amyloid peptides decrease soluble guanylyl cyclase expression in astroglial cells

In astroglial cells beta-amyloid peptides (betaA) induce a reactive phenotype and increase expression of NO synthase. Here we show that treatment of rat brain astrocytes with betaA decreases their capacity to accumulate cyclic GMP (cGMP) in response to NO as a result of a decreased expression of soluble guanylyl cyclase (sGC) at the protein and mRNA levels. Potentiation of betaA-induced NO formation by interferon-gamma did not result in a larger decrease in cGMP formation and inhibition of NO synthase failed to reverse down-regulation of sGC, indicating that NO is not involved. The betaA effect was prevented by the protein synthesis inhibitor cycloheximide. Intracerebral betaA injection also decreased sGC beta1 subunit mRNA levels in adult rat hippocampus and cerebellum. A loss of sGC in reactive astrocytes surrounding beta-amyloid plaques could be a mechanism to prevent excess signalling via cGMP at sites of high NO production.