Guanylyl cyclase activity in rat brain myelin and white matter

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.

Role of calcium in nitric oxide-induced cytotoxicity: EGTA protects mouse oligodendrocytes

Active nitrogen species are overproduced in inflammatory brain lesions in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). NO has been shown to mediate the death of oligodendrocytes (OLs), a primary target of damage in MS. To develop strategies to protect OLs, we examined the mechanisms of cytotoxicity of two NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and sodium nitroprusside (SNP) on mature mouse OLs. Nitrosonium ion (NO+) rather than NO. mediates damage with both SNAP and SNP, as shown by significant protection with hemoglobin (HbO2), but not with the NO. scavenger PTIO. SNAP and SNP differ in time course and mechanisms of killing OLs. With SNAP, OL death is delayed for at least 6 hr, but with SNP, OL death is continuous over 18 hr with no delay. Relative to NO release, SNP is more toxic than SNAP, due to synergism of NO with cyanide released by SNP. SNAP elicits a Ca2+ influx in over half of the OLs within min. Further, OL death due to NO release from SNAP is Ca2+-dependent, because the Ca2+ chelator EGTA protects OLs from killing by SNAP, and also from killing by the NONOates NOC-9 and NOC-18, which spontaneously release NO. SNP does not elicit a Ca2+ influx, and EGTA is not protective. In comparison to the N20.1 OL cell line (Boullerne et al., [1999] J. Neurochem. 72:1050-1060), mature OLs are (1) more sensitive to SNAP, (2) much more resistant to SNP, (3) sensitive to cyanide, but not iron, and (4) exhibit a Ca2+ influx and EGTA protection in response to NO generated by SNAP.

Cytokines, signal transduction, and inflammatory demyelination: review and hypothesis

The mechanism of focal demyelination in multiple sclerosis has been a long-standing enigma of this disorder. Cytokines, a diverse family of signalling molecules, are viewed as potential mediators of the process based on clinical observations and studies with animal models and tissue/cell culture systems. Myelin and oligodendrocyte (OL) destruction occur in cultured preparations subjected to cytokines such as tumor necrosis factor-alpha (TNF alpha) and lymphotoxin (LT). Many studies have shown these and other cytokines to be elevated at lesion sites and in the CSF of multiple sclerosis (MS) patients, with similar findings in animal models. Some variability in the nature of MS lesion formation has been reported, both OLs and myelin being primary targets. To account for myelin destruction in the presence of apparently functional OLs we hypothesize that cytokines such as TNF alpha and LT alpha contribute to myelin damage through triggering of specific reactions within the myelin sheath. We further propose that neutral sphingomyelinase (SMase) is one such enzyme, two forms of which have been detected in purified myelin. An additional event is accumulation of cholesterol ester, apparently a downstream consequence of cytokine-induced SMase. The resulting lipid changes are viewed as potentially destabilizing to myelin, which may render it more vulnerable to attack by invading and resident phagocytes.

Myelin contains neutral sphingomyelinase activity that is stimulated by tumor necrosis factor-alpha

Purified myelin from mouse brain was found to contain two forms of neutral sphingomyelinase, one Mg2+ dependent and the other Mg2+ independent. The former had a pH optimum of 7.5 and Km of 0.35 mM, whereas the corresponding values for the latter were pH 8.0 and Km 3.03 mM. Specific activity of the Mg(2+)-dependent enzyme showed a rostral-caudal gradient, ranging from 75 nmol/mg protein/hr in myelin from cerebral hemispheres to 21 nmol/mg protein/hr in myelin from spinal cord. Relative specific activity was approximately 20% that of brain stem or cerebral hemisphere homogenate. Treatment of myelin with taurocholate or high salt concentration did not significantly reduce activity of the Mg(2+)-dependent enzyme. The activity of that enzyme did not change with time or in the presence or absence of protease inhibitors; by contrast, that of Mg(2+)-independent enzyme decreased sharply in the absence of protease inhibitors but rose in their presence. To test for the effect of tumor necrosis factor-alpha (TNF alpha) on myelin sphingomyelinase, mouse brain myelin was labeled in vivo by intracerebral injection of [3H]acetate into 18-20-day-old mice. After 40 hr, brain stems were removed, minced, and treated with TNF alpha in Krebs-Ringer solution, after which myelin was immediately isolated. Separation and counting of individual lipids revealed TNF alpha treatment to cause increased labeling of myelin ceramide and cholesterol ester with concomitant decrease in myelin sphingomyelin. Western blotting of myelin proteins using antibodies to the two TNF alpha receptors as probes revealed the presence of the p75 receptor. Implications of these findings in relation to possible mechanisms of autoimmune demyelination are discussed.

Region-specific developmental patterns of atrial natriuretic factor- and nitric oxide-activated guanylyl cyclases in the postnatal frontal rat brain

In the rat central nervous system, cyclic GMP can be produced by two isoforms of guanylyl cyclase: a cytosolic isoform, which is activated by nitric oxide, and a membrane-bound isoform, activated by atrial natriuretic factor. We studied the development of guanylyl cyclase activity upon maturation of the rat forebrain from postnatal days 4 to 24, using a combined immunocytochemical and biochemical approach. Atrial natriuretic factor-activated particulate guanylyl cyclase activity was found to decrease in the frontal cortex, in the lateral septum and in the piriform cortex upon maturation. A transient expression of atrial natriuretic factor-sensitive guanylyl cyclase activity was observed at postnatal day 8 in the caudate putamen complex, whereas an increase was observed in the lateral olfactory tract from postnatal days 8 to 24. Biochemical and immunocytochemical studies using the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester, or the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinaloxin-1-one, indicated high levels of endogenous nitric oxide release at postnatal days 4 and 8. This activity decreased strongly in all brain areas examined. From postnatal day 8 onwards, atrial natriuretic factor-responsive cyclic GMP-immunoreactive cells could be characterized as astrocytes, with the exception of those in the the lateral olfactory tract, where the myelinated fibers became cyclic GMP producing. Furthermore, our results on activation of both guanylyl cyclases at postnatal day 8 leads to the suggestion that both isoforms might be found in the same cells. This study shows that there are pronounced differences between various frontal brain areas in the development of the responsiveness of both the particulate and soluble isoforms of guanylyl cyclase, and lends further support to the hypothesis that natriuretic peptides have a role in neuronal growth and plasticity of the rat brain.

Characterization of guanylyl cyclase in purified myelin

This study was undertaken to characterize the enzymatic properties of the particulate guanylyl cyclase previously shown to be present at a high level of activity in purified rat brain myelin. Significant activation was achieved by both Lubrol-PX and Triton X-100, the latter being somewhat more effective. A pH optimum of 7.8 was observed, compared to 7.4 for microsomes. Employing 1.2 mM GTP with 1% Triton X-100, linearity of response was observed up to 60 min and approximately 1.2 mg of myelin protein. Kinetic analysis revealed Km values of 0.258mM and 0.486mM for myelin and microsomes, respectively, similar values being obtained by Lineweaver-Burke analysis or Direct Linear Plot. Vmax values were 20 and 266 pmol/mg protein/min for myelin and microsomes, respectively. Washing of the myelin with 0.5 M NaCl or 0.1% Na taurocholate did not remove a significant amount of guanylyl cyclase activity, indicating the enzyme to be intrinsic to the myelin sheath.