Whole brain spheroid cultures as a model to study the development of nitric oxide synthase-guanylate cyclase signal transduction

The Interplay between cGMP and Calcium Signaling in Alzheimer's Disease

Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger and a key molecule in many important signaling cascades in the body and brain, including phototransduction, olfaction, vasodilation, and functional hyperemia. Additionally, cGMP is involved in long-term potentiation (LTP), a cellular correlate of learning and memory, and recent studies have identified the cGMP-increasing drug Sildenafil as a potential risk modifier in Alzheimer's disease (AD). AD development is accompanied by a net increase in the expression of nitric oxide (NO) synthases but a decreased activity of soluble guanylate cyclases, so the exact sign and extent of AD-mediated imbalance remain unclear. Moreover, human patients and mouse models of the disease present with entangled deregulation of both cGMP and Ca2+ signaling, e.g., causing changes in cGMP-mediated Ca2+ release from the intracellular stores as well as Ca2+-mediated cGMP production. Still, the mechanisms governing such interplay are poorly understood. Here, we review the recent data on mechanisms underlying the brain cGMP signaling and its interconnection with Ca2+ signaling. We also discuss the recent evidence stressing the importance of such interplay for normal brain function as well as in Alzheimer's disease.

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.

Nitric oxide targets oligodendrocytes and promotes their morphological differentiation

In the central nervous system, nitric oxide (NO) transmits signals from one neurone to another, or from neurones to astrocytes or blood vessels, but the possibility of oligodendrocytes being physiological NO targets has been largely ignored. By exploiting immunocytochemistry for cGMP, the second messenger generated on activation of NO receptors, oligodendrocytes were found to respond to both exogenous and endogenous NO in cerebellar slices from rats aged 8 days to adulthood. Atrial natriuretic peptide, which acts on membrane-associated guanylyl cyclase-coupled receptors, also raised oligodendrocyte cGMP in cerebellar slices. The main endogenous source of NO accessing oligodendrocytes appeared to be the neuronal NO synthase isoform, which was active even under basal conditions and in a manner that was independent of glutamate receptors. Oligodendrocytes in brainstem slices were also shown to be potential NO targets. In contrast, in the optic nerve, oligodendrocyte cGMP was raised by natriuretic peptides but not NO. When cultures of cerebral cortex were continuously exposed to low NO concentrations (estimated as 40–90 pM), oligodendrocytes responded with a striking increase in arborization. This stimulation of oligodendrocyte growth could be replicated by low concentrations of 8-bromo-cGMP (maximum effect at 1 µM). It is concluded that oligodendrocytes are probably widespread targets for physiological NO (or natriuretic peptide) signals, with the resulting rise in cGMP serving to enhance their growth and maturation. NO might help coordinate the myelination of axons to the ongoing level of neuronal activity during development and could potentially contribute to adaptive changes in myelination in the adult.

Glial cells as sources and targets of natriuretic peptides

Natriuretic peptides and their receptors are widely expressed in mammalian CNS and increasing evidence implicates them in the regulation of neural development, synaptic transmission and processing of information, and neuroprotection. Although the peptides have been mainly localized in neuronal populations they are also produced in glial cells. Astroglia and microglia also express functional natriuretic peptide receptors that can regulate important physiological responses. In this article we review evidence on the localization of natriuretic peptides and their receptors in astroglial and microglial cells and summarize data supporting the participation of this signalling system in neuron-glia and glia-brain blood vessel communication relevant to CNS function.

Cyclic GMP phosphodiesterase inhibition alters the glial inflammatory response, reduces oxidative stress and cell death and increases angiogenesis following focal brain injury

Recent evidence obtained in cultured glial cells indicates that cGMP-mediated pathways regulate cytoskeleton dynamics, glial fibrillary acidic protein expression and motility in astrocytes, as well as inflammatory gene expression in microglia, suggesting a role in the regulation of the glial reactive phenotype. The aim of this work was to examine if cGMP regulates the glial inflammatory response in vivo following CNS damage caused by a focal cryolesion onto the cortex in rats. Results show that treatment with the cGMP phosphodiesterase inhibitor zaprinast (10 mg/kg i.p.) 2 h before and 24 and 48 h after the lesion results 3 days post-lesion in notably enhanced astrogliosis manifested by increased glial fibrillary acidic protein immunoreactivity and protein levels around the lesion. In contrast, zaprinast decreased the number of round/ameboid lectin-positive cells and the expression of the activated microglia/macrophage markers Iba-1 and CD11b indicating decreased recruitment and activation of these cells. This altered inflammatory response is accompanied by a decrease in protein oxidative stress, apoptotic cell death and neuronal degeneration. In addition, zaprinast enhanced angiogenesis in the lesioned cortex probably as a result of vascular endothelial growth factor expression in reactive astrocytes. These results suggest that regulation of the glial inflammatory response may contribute to the reported neuroprotective effects of cGMP-phosphodiesterase inhibitors in brain injury.

The ANP-cGMP-protein kinase G pathway induces a phagocytic phenotype but decreases inflammatory gene expression in microglial cells

Reactive gliosis is a prominent feature of CNS injury that involves dramatic changes in glial cell morphology together with increased motility, phagocytic activity, and release of inflammatory mediators. We have recently demonstrated that stimulation of the cGMP-protein kinase G (PKG) pathway by NO or atrial natriuretic peptide (ANP) regulates cytoskeleton dynamics and motility in rat astrocytes in culture. In this work, we show that the cGMP-PKG pathway stimulated by ANP, but not by NO, regulates microglial cell morphology by inducing a dramatic reorganization in the actin cytoskeleton. Both ANP (0.01-1.0 microM) and the permeable cGMP analog, dibutyryl-cGMP (1-100 microM), promote a rapid (maximal at 30 min) and concentration-dependent increase in size, rounding, and lamellipodia and filopodia formation in rat brain cultured microglia. These morphological changes involve an augment and redistribution of F-actin and result in increased phagocytic activity. ANP-induced rearrangements in actin cytoskeleton and inert particle phagocytosis are prevented by the PKG inhibitor, Rp-8-Br-PET-cGMPS (0.5 microM), and involve inhibition of RhoA GTPase and activation of Rac1 and Cdc42. However, ANP does not induce NO synthase Type 2 (NOS-2) or tumor necrosis factor-alpha expression and is able to decrease lipopolysaccharide (LPS)-elicited induction of these inflammatory genes. The morphological changes and the decrease of LPS-induced NOS-2 expression produced by ANP in cultured microglia are also observed by immunostaining in organotypic cultures from rat hippocampus. These results suggest that stimulation of the ANP-cGMP-PKG pathway in microglia could play a beneficial role in the resolution of neuroinflammation by removing dead cells and decreasing levels of proinflammatory mediators.

Regulation and function of cyclic GMP-mediated pathways in glial cells

A large body of evidence supports a role for the NO-cGMP-protein kinase G pathway in the regulation of synaptic transmission and plasticity, brain development and neuroprotection. Circumstantial evidence implicates natriuretic peptide-stimulated cGMP formation in the same CNS functions. In addition to neurons, both cGMP-mediated pathways are functional in glial cells and an increasing number of reports indicate that they may control important aspects of glial cell physiology relevant to neuronal function. In this article we briefly review the regulation of cGMP formation in glial cells and summarize recent evidence indicating that cGMP-mediated pathways can play important roles in astroglial and microglial function in normal and diseased brain.

Stirred vessel cultures of rat brain cells aggregates: characterization of major metabolic pathways and cell population dynamics

We report a study on neural metabolism of long-term three-dimensional cultures of rat embryonic brain cells in stirred vessels. Our experimental setup was optimized to keep viable aggregate cultures with neuronal maintenance for up to 44 days. Results show that aggregate size and shape could be hydrodynamically controlled depending on the impeller design, avoiding necrotic centers or significant losses in cell viability. Aggregates were composed mainly of neurons until day 16, whereas an effective growth of the glial population was observed after day 21. Cell metabolic status was evaluated by quantification of several metabolites in the culture medium; amino acid metabolism was used as a marker of metabolic interrelationships between neural cell types. Furthermore, (13)C-NMR spectroscopy was used on day 31 to explore specific metabolic pathways: incubation with [1-(13)C]glucose for 45 hr produced an increase in label incorporation in extracellular alanine, lactate, and glutamine, reflecting mainly astrocytic metabolism. The contribution of anaplerotic vs. oxidative pathways for glutamine synthesis was determined: a 92% reduction in the pyruvate carboxylase flux during the first 41 hr of incubation suggested a decrease in the need for replacing tricarboxylic acid cycle intermediates. We believe that our data corroborate the aggregating cultures as an attractive system to analyze brain cell metabolism being a valuable tool to model metabolic fluxes for in vitro brain diseases.

Neurons exposed to ammonia reproduce the differential alteration in nitric oxide modulation of guanylate cyclase in the cerebellum and cortex of patients with liver cirrhosis

The activation of soluble guanylate cyclase by nitric oxide is increased in the frontal cortex but is reduced in the cerebellum of patients who died with liver cirrhosis. The aims of this work were to assess whether hyperammonemia is responsible for the region-selective alterations in guanylate cyclase modulation in liver cirrhosis and to assess whether the alteration occurs in neurons or in astrocytes. The activation of guanylate cyclase by nitric oxide was lower in cerebellar neurons exposed to ammonia (1.5-fold) than in control neurons (3.3-fold). The activation of guanylate cyclase by nitric oxide was higher in cortical neurons exposed to ammonia (8.7-fold) than in control neurons (5.5-fold). The activation was not affected in cerebellar or cortical astrocytes. These findings indicate that hyperammonemia is responsible for the differential alterations in the modulation of soluble guanylate cyclase in cerebellum and cerebral cortex of cirrhotic patients. Moreover, the alterations occur specifically in neurons and not in astrocytes.

Cerebral expression of the α2-subunit of soluble guanylyl cyclase is linked to cerebral maturation and sensory pathway refinement during postnatal development

Soluble guanylyl cylase (sGC) has been identified for being a receptor for the gaseous transmitters nitric oxide and carbon monoxide. Currently four subunits alpha1, alpha2, beta1, and beta2 have been characterized. Heterodimers of alpha and beta-subunits as well as homodimers of the beta2-subunit are known to constitute functional sGC which use GTP to form cGMP a potent signal molecule in a multitude of second messenger cascades. Since NO-cGMP signaling plays a pivotal role in neuronal development we analyzed the maturational expression pattern of the newly characterized alpha2-subunit of sGC within the brain of Wistar rats by means of RNase protection assay and immunohistochemistry. alpha2-subunit mRNA as well as immunoreactive alpha2-protein increased during postnatal cerebral development. Topographical analysis revealed a selective high expression of the alpha2-subunit in the choroid plexus and within developing sensory systems involving the olfactory and somatosensory system of the forebrain as well as parts of the auditory and visual system within the hindbrain. In cultured cortical neurons the alpha2-subunit was localized to the cell membrane, especially along neuronal processes. During the first 11 days of postnatal development several cerebral regions showed a distinct expression of the alpha2-subunit which was not paralleled by the alpha1/beta1-subunits especially within the developing thalamo-cortical circuitries of the somatosensory system. However, at later developmental stages all three subunits became more homogenously distributed among most cerebral regions, indicating that functional alpha1/beta1 and alpha2/beta1 heterodimers of sGC could be formed. Our findings indicate that the alpha2-subunit is an essential developmentally regulated constituent of cerebral sensory systems during maturation. In addition the alpha2-subunit may serve other functions than forming a functional heterodimer of sGC during the early phases of sensory pathway refinement.

Evaluation of 3-nitrotyrosine as a marker for 3-nitropropionic acid-induced oxidative stress in Lewis and Wistar rats and strain-specific whole brain spheroid cultures

The present study investigated whether 3-nitrotyrosine is an early marker for neurodegenerative processes involving oxidative stress. We characterized the 3-nitrotyrosine formation after 3-nitropropionic acid (3-NP) exposure in the whole brain spheroid culture model and in a rat model, using Lewis and Wistar rats. Increased 3-nitrotyrosine concentration in spheroid cultures from Lewis rats was observed at lower dose of and shorter exposure time to 3-NP as compared to alterations in glial fibrillary acidic protein concentration, decrease in glutamine synthetase activity or cell loss. Five days of exposure to 3-NP (5 mM) resulted in decreased staining of GABAergic processes, while neuronal nitric oxide synthase staining was preserved. In addition, staining of EAAC1, anti-2',3'-cyclic nucleotide 3'-phosphohydrolase and ED1 was diminished after treatment of spheroid cultures with 3-nitropropionic acid (5 mM), while isolectin B4 staining was increased. Dithiothreitol and vitamin E inhibited the increased formation of 3-nitrotyrosine. Interestingly, N(G)-nitro-L-arginine methyl ester increased the 3-nitrotyrosine formation. No increased 3-nitrotyrosine concentration was shown after exposure to 3-nitropropionic acid during 5 days in spheroid cultures obtained from Wistar rats. In the striatum of 3-NP-exposed Lewis and Wistar rats, no change in 3-nitrotyrosine concentration was observed, whereas only in Wistar rats the glial fibrillary acidic protein concentration was increased in addition to activation of microglial cells. It is concluded that 3-nitrotyrosine was a more sensitive marker for oxidative stress-induced neurodegeneration than glial fibrillary acidic protein and glutamine synthase in spheroid cell cultures of Lewis rats. Finally, the similarities between the 3-NP spheroid model and the vivo model indicate that the spheroid cultures provide a good alternative for chronic exposure of animals to neurotoxins.