Absence of nitric oxide synthase in rat oligodendrocytes: a light and electron microscopic study

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.

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.

Citrulline immunohistochemistry may not necessarily identify nitric oxide synthase activity: the pitfall of peptidylarginine deiminase

Nitric oxide synthase (NOS) converts L-arginine as a substrate to form nitric oxide and the "by-product" citrulline. To characterize NOS activity in the nervous tissue at the single-cell level, citrulline immunostaining is considered to be a suitable means of working on the principle that in brain tissue, due to the incomplete urea cycle, citrulline is produced exclusively by NOS. This assumption is correct for free citrulline but it does not consider the conversion of arginine to citrulline residues of proteins by the calcium-dependent peptidylarginine deiminase (PAD). Using a polyclonal antiserum against citrulline we observed in cerebellar cell cultures immunopositivity in a few, mostly NOS-positive, neurons, in activated microglia, and in oligodendroglia (which under control conditions are in doubt to be able to express NOS), but not in astroglia. Treatment with the excitotoxin kainate substantially enhanced the staining intensity for citrulline in neurons and glial cells. To distinguish between free (NOS-related) and protein-bound (PAD-related) citrulline we blocked NOS activity by 7-nitroindazole or L-N5-(1-iminoethyl)lysine. The results provide evidence that citrulline immunolabeling results partly from PAD-mediated protein citrullination, enhanced pathophysiologically under stimulated conditions by exposure to kainate. Our immunocytochemical observations were corroborated by Western blot analysis showing several bands of citrulline-positive proteins, whose number and staining intensity depended on kainate treatment and calcium ions.

Nitric oxide-induced mitochondrial dysfunction: implications for neurodegeneration

Excessive generation of nitric oxide (NO) has been implicated in the pathogenesis of several neurodegenerative disorders. Damage to the mitochondrial electron transport chain has also been implicated in these disorders. NO and its toxic metabolite peroxynitrite (ONOO(-)) can inhibit the mitochondrial respiratory chain, leading to energy failure and ultimately cell death. There appears to be a differential susceptibility of brain cell types to NO/ONOO(-), which may be influenced by factors including cellular antioxidant status and the ability to maintain energy requirements in the face of marked respiratory chain damage. Although formation of NO/ONOO(-) following cytokine exposure does not affect astrocyte survival, these molecules may diffuse out and cause mitochondrial damage to neighboring NO/ONOO(-)-sensitive cells such as neurons. Evidence suggests that NO/ONOO(-) causes release of neuronal glutamate, leading to glutamate-induced activation of neuronal NO synthase and generation of further damaging species. While neurons appear able to recover from short-term exposure to NO/ONOO(-), extending the period of exposure results in persistent damage to the respiratory chain and cell death ensues. These findings have important implications for acute infection vs. chronic neuroinflammatory disease states. The evidence for NO/ONOO(-)-mediated mitochondrial damage in neurodegenerative disorders is reviewed and potential therapeutic strategies are discussed.

TNFalpha downregulates PPARdelta expression in oligodendrocyte progenitor cells: implications for demyelinating diseases

TNFalpha has been implicated in several demyelinating disorders, including multiple sclerosis (MS) and X-adrenoleukodystrophy (X-ALD). TNFalpha abundance is greatly increased in the areas surrounding damaged regions of the central nervous system of patients with MS and X-ALD, but its role in the observed demyelination remains to be elucidated. A class of nuclear receptors, the peroxisome proliferator-activated receptors (PPARs), has been implicated in several physiological and pathological processes. In particular, PPARdelta has been shown to promote oligodendrocyte (OL) survival and differentiation and PPARgamma has been implicated in inflammation. In the present study, we investigate on the effects of TNFalpha on OLs during differentiation in vitro. The results obtained show that TNFalpha treatment impairs PPARdelta expression with concomitant decrease of lignocerolyl-CoA synthase and very-long-chain fatty acid beta-oxidation as well as plasmalogen biosynthesis. We propose a hypothetical model possibly explaining the perturbation effects of proinflammatory cytokines on myelin synthesis, maturation, and turnover.

Effects of axotomy on the expression of NADPH-diaphorase in the visual pathway of the tench

The distribution of NADPH-diaphorase (ND) positive elements was analyzed throughout the visual pathway of the tench in normal conditions and after optic nerve transection. In the control retina, ND-labeled elements were observed in the photoreceptor, inner nuclear, outer nuclear and ganglion cell layers. In the optic nerve of control animals, small and numerous ND-positive glial cells that were identified as presumably astrocyte-like cells were observed. In the optic tracts and optic tectum, a different type of ND-positive glial cell was detected. Axotomy induced severe changes in the ND staining pattern in the visual pathway. A decrease in the number of ND-stained cells was detected in the retina. In the optic nerve of lesioned animals, the number of small cells gradually decreased, whereas the number of large cells did not change. Two new ND-positive cell populations were observed after the lesion: microglial-like cells appeared close to the lesioned area from 24 h to 7 days after transection, and astrocyte-like cells were found throughout the optic nerve from 14 days up to at least 120 days. The total number of ND-stained glial cells increased at 30 and 60 days and returned to control parameters at 120 days. In addition, the number of ND-positive cells increased at the same survival times in the optic tracts and in the retinorecipient strata of the optic tectum with respect to control animals. Thus, degenerative/regenerative processes in the fish visual pathway are accompanied by an increase in the number of ND-positive cells. Synthesis of nitric oxide is elicited in microglial-like cells as a response to axon injury, whereas the expression in astrocyte-like cells seems to be associated with both normal processes under physiological conditions and with the regenerative phase after the lesion.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Modulation of NADPH-diaphorase and glial fibrillary acidic protein by progesterone in astrocytes from normal and injured rat spinal cord

Progesterone (P4) can be synthesized in both central and peripheral nervous system (PNS) and exerts trophic effects in the PNS. To study its potential effects in the spinal cord, we investigated P4 modulation (4 mg/kg/day for 3 days) of two proteins responding to injury: NADPH-diaphorase, an enzyme with nitric oxide synthase activity, and glial fibrillary acidic protein (GFAP), a marker of astrocyte reactivity. The proteins were studied at three levels of the spinal cord from rats with total transection (TRX) at T10: above (T5 level), below (L1 level) and caudal to the lesion (L3 level). Equivalent regions were dissected in controls. The number and area of NADPH-diaphorase active or GFAP immunoreactive astrocytes/0.1 mm(2) in white matter (lateral funiculus) or gray matter (Lamina IX) was measured by computerized image analysis. In controls, P4 increased the number of GFAP-immunoreactive astrocytes in gray and white matter at all levels of the spinal cord, while astrocyte area also increased in white matter throughout and in gray matter at the T5 region. In control rats P4 did not change NADPH-diaphorase activity. In rats with TRX and not receiving hormone, a general up-regulation of the number and area of GFAP-positive astrocytes was found at all levels of the spinal cord. In rats with TRX, P4 did not change the already high GFAP-expression. In the TRX group, instead, P4 increased the number and area of NADPH-diaphorase active astrocytes in white and gray matter immediately above and below, but not caudal to the lesion. Thus, the response of the two proteins to P4 was conditioned by environmental factors, in that NADPH-diaphorase activity was hormonally modulated in astrocytes reacting to trauma, whereas up-regulation of GFAP by P4 was produced in resting astrocytes from non-injured animals.