Hsueh-Ning Liu

Protection of p27Kip1 mRNA by quaking RNA binding proteins promotes oligodendrocyte differentiation

The quaking (Qk) locus expresses a family of RNA binding proteins, and the expression of several alternatively spliced isoforms coincides with the development of oligodendrocytes and the onset of myelination. Quaking viable (Qkv) mice harboring an autosomal recessive mutation in this locus have uncompacted myelin in the central nervous system owing to the inability of oligodendrocytes to properly mature. Here we show that the expression of two QKI isoforms, absent from oligodendrocytes of Qkv mice, induces cell cycle arrest of primary rat oligodendrocyte progenitor cells and differentiation into oligodendrocytes. Injection of retroviruses expressing QKI into the telencephalon of mouse embryos induced differentiation and migration of multipotential neural progenitor cells into mature oligodendrocytes localized in the corpus callosum. The mRNA encoding the cyclin-dependent kinase (CDK)-inhibitor p27Kip1 was bound and stabilized by QKI, leading to an increased accumulation of p27Kip1 protein in oligodendrocytes. Our findings demonstrate that QKI is upstream of p27Kip1 during oligodendrocyte differentiation.

AMPA receptor-mediated toxicity in oligodendrocyte progenitors involves free radical generation and activation of JNK, calpain and caspase 3

The molecular mechanisms underlying AMPA (α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate) receptor‐mediated excitotoxicity were characterized in rat oligodendrocyte progenitor cultures. Activation of AMPA receptors, in the presence of cyclothiazide to selectively block desensitization, produced a massive Ca2+ influx and cytotoxicity which were blocked by the antagonists CNQX and GYKI 52466. A role for free radical generation in oligodendrocyte progenitor cell death was deduced from three observations: (i) treatment with AMPA agonists decreased intracellular glutathione; (ii) depletion of intracellular glutathione with buthionine sulfoximine potentiated cell death; and (iii) the antioxidant N‐acetylcysteine replenished intracellular glutathione and protected cultures from AMPA receptor‐mediated toxicity. Cell death displayed some characteristics of apoptosis, including DNA fragmentation, chromatin condensation and activation of caspase‐3 and c‐Jun N‐terminal kinase (JNK). A substrate of calpain and caspase‐3, α‐spectrin, was cleaved into characteristic products following treatment with AMPA agonists. In contrast, inhibition of either caspase‐3 by DEVD‐CHO or calpain by PD 150606 protected cells from excitotoxicity. Our results indicate that overactivation of AMPA receptors causes apoptosis in oligodendrocyte progenitors through mechanisms involving Ca2+ influx, depletion of glutathione, and activation of JNK, calpain, and caspase‐3.

Developmental differences in H2O2-induced oligodendrocyte cell death: role of glutathione, mitogen-activated protein kinases and caspase 3

The molecular mechanisms underlying H2O2‐induced toxicity were characterized in rat oligodendrocyte cultures. While progenitor cells were more sensitive than mature oligodendrocytes to H2O2, the antioxidant, N‐acetyl‐l‐cysteine, blocked toxicity at both stages of development. Differentiated oligodendrocytes contained more glutathione than did progenitors and were less susceptible to decreases in glutathione concentration induced by H2O2 stress. As free radicals have been considered to serve as second messengers, we examined the effect of H2O2 on activation of the mitogen‐activated protein kinases (MAPK), extracellular signal‐regulated kinases (ERK) 1/2 and p38. H2O2 caused a time‐ and concentration‐dependent increase in MAPK phosphorylation, an effect that was totally blocked by N‐acetyl‐l‐cysteine. Further exploration of potential mechanisms involved in oligodendrocyte cell death showed that H2O2 treatment caused DNA condensation and fragmentation at both stages of development, whereas caspase 3 activation and poly (ADP‐ribose) polymerase cleavage were significantly increased only in oligodendrocyte progenitors. The pan‐caspase inhibitor, benzyloxycarbonyl‐Val‐Ala‐Asp fluoromethyl ketone, blocked DNA fragmentation in progenitors and produced a small but significant level of protection from H2O2 toxicity in progenitors and mature oligodendrocytes. In contrast, inhibitors of both p38 and MEK reduced H2O2‐induced death most significantly in oligodendrocytes. The poly (ADP‐ribose) polymerase inhibitor, PJ34, reduced H2O2‐induced toxicity on its own but was most effective when combined with benzyloxycarbonyl‐Val‐Ala‐Asp fluoromethyl ketone or PD169316. The finding that molecular mechanisms conferring resistance to reactive oxygen species toxicity are regulated during oligodendrocyte differentiation may be of importance in designing therapies for certain neurological diseases affecting white matter.

Exposure of developing oligodendrocytes to cadmium causes HSP72 induction, free radical generation, reduction in glutathione levels, and cell death.

Primary cultures of oligodendrocytes were used to study the toxic effects of cadmium chloride. Cell viability was evaluated by the mitochondrial dehydrogenase activity and confirmed by propidium iodide (PI) fluorescence staining. The expression of the 72 kDa stress protein, HSP72, was assayed by Western blot analysis. The results showed that Cd(2+)-induced toxicity was dependent on the time and dose of exposure, as well as on the developmental stage of the cultures. Oligodendrocyte progenitors were more vulnerable to Cd(2+) toxicity than were mature oligodendrocytes. Mature oligodendrocytes accumulated relatively higher levels of Cd(2+) than did progenitors, as determined by (109)CdCl(2) uptake; treatment with the metal ion caused a more pronounced reduction in intracellular glutathione levels and significantly higher free radical accumulation in progenitors. The latter could explain the observed differences in Cd(2+) susceptibility. HSP72 protein expression was increased both in progenitors and in mature cells exposed to Cd(2+). Pretreatment with N-acetylcysteine, a thiocompound with antioxidant activity and a precursor of glutathione, prevented Cd(2+)-induced (i) reduction in glutathione levels and (ii) induction of HSP72 and diminished (i) Cd(2+) uptake and (ii) Cd(2+)-evoked cell death. In contrast, buthionine sulfoximine, an inhibitor of gamma-glutamyl-cysteine synthetase, depleted glutathione, and potentiated the toxic effect of Cd(2+). These results strongly suggest that Cd(2+)-induced cytotoxicity in oligodendrocytes is mediated by reactive oxygen species and is modulated by glutathione levels.

Glutamate Induces c-fos Proto-oncogene Expression and Inhibits Proliferation in Oligodendrocyte Progenitors: Receptor Characterization

The effect of glutamate on c‐fos expression in oligodendrocyte progenitors was investigated by Northern blot analysis. Glutamate caused rapid and transient induction. Both 6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione (CNQX) and 6,7‐dinitroquinoxaline‐2,3‐dione (DNQX), two competitive non‐NMDA ionotropic receptor antagonists, reduced glutamate‐induced c‐fos expression, whereas the NMDA antagonist MK‐801 was ineffective. In addition, the glutamate receptor agonists (±)‐α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid hydrobromide (AMPA) and kainate strongly induced c‐fos. However, the metabotropic receptor agonist trans‐(±)‐1‐amino‐(1S,3R)‐cyclopentanedicarboxylic acid (trans‐(±)‐ACPD) did not increase c‐fos mRNA level and the antagonist L‐(+)‐2‐amino‐3‐phosphonopropionic acid did not block glutamate‐induced c‐fos mRNA. These findings indicate that c‐fos induction in oligodendrocyte progenitors is mediated through the AMPA/kainate receptors, while NMDA and metabotropic receptor subtypes are not involved. Chelation of extracellular calcium by EDTA prevented glutamate‐induced c‐fos expression. Similarly, the protein kinase C inhibitor 1‐(5‐isoquinoline‐sulphonyl)‐2‐methylpiperazine dihydrochloride (H7) and down‐regulation of protein kinase C by prolonged exposure to phorbol‐12‐myristate 13‐acetate blocked c‐fos induction. These results suggest that induction of c‐fos through AMPA/kainate receptors is dependent on extracellular calcium influx and involves downstream activation of phorbol ester‐sensitive protein kinase C. The effect of glutamate on oligodendrocyte progenitor proliferation was assessed by [3H]thymidine incorporation. Glutamate and the agonists kainate and AMPA, but not trans‐(±)‐ACPD, caused a dose‐dependent decrease in [3H]thymidine incorporation. All these pharmacological agents were not toxic to oligodendrocyte progenitors. CNQX reversed the inhibitory effects produced by glutamate and the various agonists. These results suggest that glutamate may modulate the growth and differentiation of oligodendrocytes in the central nervous system.

Cadmium exposure induces mitochondria-dependent apoptosis in oligodendrocytes.

Cadmium toxicity has been associated with learning disabilities and Parkinsonian symptoms in humans. We have previously shown that cultured oligodendrocytes are directly damaged by cadmium exposure. Here, we characterized the molecular mechanisms underlying cadmium-induced cell death in oligodendrocyte progenitors (OLP). Cadmium caused a concentration-dependent decrease in cell viability as assessed by mitochondrial dehydrogenase activity and by the cellular release of lactate dehydrogenase (LDH). A short exposure (1h) to cadmium (25-100 microM), followed by several hours of recovery, produced a predominant apoptotic mechanism of cell death, involving the mitochondrial intrinsic pathway, as evidenced by nuclear condensation, DNA fragmentation, bax integration into the outer mitochondrial membrane, cytochrome c release, and activation of caspases-9 and -3. Pretreatment of OLPs with the pan-caspase inhibitor, zVAD-fmk, prevented caspase-3 activation but only slightly reduced cell death 11h after cadmium exposure and failed to prevent cadmium-induced bax insertion into the mitochondrial membrane. In contrast, the anti-oxidant N-acetyl cysteine blocked caspase-3 activation and significantly protected OLPs from cadmium-induced cell death. Continuous exposure (18-48 h) of OLPs to low micromolar concentrations (0.001-25 microM) of cadmium significantly decreased mitochondrial metabolic activity, increased LDH leakage starting at 5 microM and maximally activated caspase-3. These results suggest that cadmium induces OLP cell death mainly by apoptosis, and at higher concentrations or with prolonged exposure to the heavy metal there is an increase in cytoplasmic membrane damage, an index of necrosis. More importantly, transient exposure to cadmium is sufficient to damage OLPs and could in principle impair myelination in the neonate.

Glutamate-stimulated production of inositol phosphates is mediated by Ca2+ influx in oligodendrocyte progenitors

The effect of glutamate on the accumulation of [3H]inositol phosphates was examined in oligodendrocyte progenitor cultures prepared from rat brains. Glutamate, and the analogues alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, caused a concentration- and time-dependent increase in [3H]inositol trisphosphate (IP3) formation and the effect was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a competitive AMPA and kainate receptor antagonist. Similarly, the more selective, noncompetitive antagonist of AMPA receptors, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), significantly reduced the effect of both AMPA and kainate. In contrast, antagonists of N-methyl-D-aspartate (NMDA) receptor, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5, 10-imine (MK-801) and R(-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), and antagonists of metabotropic receptors, L(+)-2-amino-3-phosphono-propanoic acid (L-AP3) and alpha-methyl-4-carboxyphenylglycine (MCPG), were ineffective. These results suggest that the effect of glutamate on [3H]IP3 accumulation is mediated through ionotropic AMPA receptors. Cyclothiazide, an inhibitor of AMPA receptor desensitization, strongly potentiated the AMPA and kainate-stimulated [3H]IP3 formation as well as the uptake of 45Ca2+ in line with the previous findings. 45Ca2+ uptake evoked by AMPA or kainate, in combination with cyclothiazide, was also prevented by both CNQX and GYKI 52466. Glutamate-stimulated [3H]IP3 accumulation was prevented by EGTA, suggesting a requirement for extracellular calcium. Pre-incubation with the voltage-gated Ca2+ channel blockers, diltiazem, nifedipine and CdCl2, partially prevented the glutamate-induced [3H]IP3 accumulation as well as 45Ca2+ uptake. Similarly, the Na+/Ca2+ exchanger blockers benzamil and 3,4-dichlorobenzamil reduced significantly kainate-stimulated 45Ca2+ uptake. These data indicate that glutamate-induced [3H]IP3 accumulation is triggered by calcium influx via AMPA receptors, voltage-gated calcium channels and the Na+/Ca2+ exchanger operating in reverse mode.

Molecular pathways mediating activation by kainate of mitogen-activated protein kinase in oligodendrocyte progenitors

Oligodendroglial cells express ionotropic glutamate receptors of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid hydrobromide (AMPA) and kainate (KA) subtypes. Recently, we reported that AMPA receptor agonists increased 45Ca2+ uptake and phospholipase C (PLC) activity. To further elucidate the intracellular signaling mechanisms, we examined the effects of AMPA and KA on mitogen-activated protein kinase (MAPK). KA caused a time- and concentration-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) and the effect was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a competitive AMPA/KA receptor antagonist. Furthermore, the noncompetitive antagonists of AMPA receptor GYKI 52466 and LY 303070 prevented the actions of the agonists, indicating that the effect of KA on MAPK activation is mediated through AMPA receptors in oligodendrocyte progenitors. Chelation of extracellular Ca2+ by EDTA or inhibition of PLC with U73122 abolished MAPK activation by KA. In addition, KA-stimulated MAPK activation was reduced by the protein kinase C (PKC) inhibitors, H7 and bisindolylmaleimide, as well as downregulation of PKC by prolonged exposure to phorbol esters. The involvement of PKC in the signal transduction pathways was further supported by the ability of KA to induce translocation of PKC measured by [3H]PDBu binding. Interestingly, a wortmannin-sensitive phosphatidylinositol 3-kinase and a pertussis toxin (PTX)-sensitive G protein form part of the molecular pathways mediating MAPK activation by AMPA receptor. A specific inhibitor of MAPK kinase, PD 098059, blocked MAPK activation and reduced KA-induced c-fos gene expression. All together, these results indicate that MAPK is implicated in the transmission of AMPA signaling to the nucleus and requires extracellular Ca2+, and PLC/PKC activation.

Regulation of muscarinic receptor function in developing oligodendrocytes by agonist exposure

Oligodendrocytes, the myelin forming cells in the CNS, express muscarinic acetylcholine receptors (mAChR), primarily M3, coupled to various signal transduction pathways. In the present study we have investigated whether mAChR undergo functional agonist‐induced regulation in cultured oligodendrocyte progenitors and differentiated oligodendrocytes. The muscarinic agonist, carbachol (CCh) caused a time‐dependent desensitization of phosphoinositide (PI) hydrolysis, and the internalization and down‐regulation of receptors. Short‐time desensitization (5 min) of PI hydrolysis occurred without receptor internalization and reached 54% by 1 h. The same treatment decreased cell surface receptors labelled with the non‐permeable ligand [3H]‐NMS by 47%, while total receptor density ([3H]‐scopolamine binding) decreased by 30%. Longer CCh treatment down‐regulated receptors by 70% and desensitized the PI response by 80%. Although protein kinase C (PKC) activation desensitized mAChR, CCh‐mediated desensitization was independent of PKC. Inhibition of receptor endocytosis by low temperature during the pre‐stimulation period or in the presence of hyperosmotic sucrose (0.5 M) blocked desensitization, receptor internalization and down‐regulation. Recovery of surface mAChR and their functional activity following down‐regulation was slow, returning to control levels by 24 h after agonist removal. In progenitor cells, dose‐response curves for CCh‐mediated PI hydrolysis and c‐fos mRNA expression showed that newly synthesized mAChR were supersensitive after recovery. Overall, the present results provide evidence of functional agonist‐mediated mAChR regulation in brain oligodendroglial cells.

Responses of Oligodendrocytes to Classical Neurotransmitters: Signaling Pathways

In the central nervous system (CNS), the time and region-specific pattern of myelination suggests the existence of neuronal signals which regulate oligodendrocyte proliferation and differentiation as well as myelin formation (15,29). Neurotransmitters are a group of molecules of major importance for the CNS. Their main function is to serve as chemical signals of intercellular communication by activation of specific receptors and second messenger systems in postsynaptic cells. However, there is ample pharmacological evidence suggesting that some of these substances regulate trophic biological processes (see 28 and 31 for review).