Effects of transient ethanol exposure on the incorporation of [(3)H]ethanolamine into plasmalogen in the differentiating CG-4 oligodendrocyte cell line

Acetaldehyde, not ethanol, impairs myelin formation and viability in primary mouse oligodendrocytes

BACKGROUND

Excessive ethanol (EtOH) drinking is associated with white matter loss in the brain at all stages of life. Myelin-forming oligodendrocytes (OLs) are a major component of white matter, but their involvement in EtOH-mediated white matter loss is unclear. Myelination continues throughout the life with highest rates during fetal development and adolescence. However, little is known about the effects of EtOH and its principal metabolite acetaldehyde (ACD) on OLs at the cellular level.

METHODS

We compared the responses to different concentrations of EtOH or ACD by primary OLs in culture.

RESULTS

EtOH did not cause significant cell death at concentrations lower than 120 mM, even after 24 hours. In comparison, ACD was highly lethal at doses above 50 μM. High concentrations of EtOH (120 mM) and ACD (500 μM) for 24 hours did not reduce myelin in mature OLs. Myelin production and OL differentiation were significantly impaired by 7 days exposure to 500 or 50 μM ACD but not 120 mM EtOH.

CONCLUSIONS

This study shows that OLs are relatively resistant to EtOH, even at a concentration more than 4 times the typical blood EtOH concentrations associated with social drinking (10 to 30 mM). In contrast, OLs are much more sensitive to ACD than EtOH, particularly with long-term exposure. This suggests that part of white matter loss in response to EtOH, especially during high rates of myelin formation, may be due in part to the effects of its principal metabolite ACD.

Ethanol alters the expressions of c-Fos and myelin basic protein in differentiating oligodendrocytes

Myelination occurs in the central nervous system of the human fetus, adolescents, and young adults. Ethanol interferes with myelination in part by altering the composition of the myelin sheath. Here we show that ethanol also affected the expression of the transcription factor c-Fos in differentiating oligodendrocytes (OLGs). Central glial-4 OLG progenitors were induced to differentiate in the absence and presence of 100 mM ethanol, and ethanol-caused changes in the levels of c-Fos and myelin basic protein (MBP) were determined by Western blot analysis at selected developmental stages. The relatively high c-Fos level in progenitors did not immediately decrease to a low level at the onset of differentiation but displayed a downregulation at a later developmental stage. Ethanol delayed the developmental c-Fos downregulation maintaining c-Fos at a 45% higher level at 2 days of differentiation (DoD). Ethanol also decreased the rate of the burst of MBP expression that occurred between 1 and 2 DoD, reducing the MBP level by 47% at 2 DoD. The ethanol-caused delays of c-Fos downregulation and MBP upregulation were both blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM). Likewise, treatment of OLGs with a low 5-nM concentration of the PKC activator by 12-O-tetradecanoylphorbol-13-acetate mimicked the ethanol effects on the expression of both proteins, effects that were also counteracted by BIM. The results indicate that ethanol-caused delays of the stage-specific c-Fos downregulation and the inhibition of MBP expression both occur through a PKC-mediated mechanism. The ethanol-caused delay in c-Fos downregulation may disrupt normal timing for expression of genes involved in OLG differentiation, and the inhibited MBP expression may alter the myelin sheath composition.

Plasmalogen deficiency in cerebral adrenoleukodystrophy and its modulation by lovastatin

In cerebral adrenoleukodystrophy (cALD), an accumulation of very long chain fatty acids stems from a defect of the peroxisomal ALD protein (ALDP) and results in the loss of myelin/oligodendrocytes, induction of inflammatory disease and mental deterioration. In brain white matter of cALD patients, we observed not only increased levels of very long chain fatty acid but also reduced levels of plasmenylethanolamine (PlsEtn) and increased levels of reactive oxygen species (ROS). The loss of PlsEtn was greatest in the plaque area and lesser but significant at histologically normal-looking areas of the cALD brain. The reduction in PlsEtn was related to oxidative stress, as supported by increased levels of reactive lipid aldehydes (4-hydroxynonenal and acrolein) and deleterious oxidized proteins (protein carbonyl) in all areas of the cALD brain. This inverse relationship between the levels of PlsEtn and reactive oxygen species (ROS) was further supported in an in vitro study using gene-silencing for dihydroxyacetone phosphate-acyl transferase, a key enzyme for PlsEtn biosynthesis. Levels of PlsEtn were also found decreased in vitro following gene-silencing for the ALDP/ALD-related protein. Furthermore, low levels of PlsEtn were detected in brain white matter of ALDP knock out (KO) mice. A treatment of ALDP KO mice with lovastatin increased PlsEtn levels in the brain. Further, in an in vitro study, lovastatin treatment of rat C6 glial cells increased PlsEtn biosynthesis and reduced the cytokine-induced ROS accumulation. In summary, this study reports that altered metabolism of PlsEtn and ROS in cALD may be corrected by lovastatin treatment.

Lipid abnormalities in succinate semialdehyde dehydrogenase (Aldh5a1-/-) deficient mouse brain provide additional evidence for myelin alterations

Earlier work from our laboratory provided evidence for myelin abnormalities (decreased quantities of proteins associated with myelin compaction, decreased sheath thickness) in cortex and hippocampus of Aldh5a1(-/-) mice, which have a complete ablation of the succinate semialdehyde dehydrogenase protein [E.A. Donarum, D.A. Stephan, K. Larkin, E.J. Murphy, M. Gupta, H. Senephansiri, R.C. Switzer, P.L. Pearl, O.C. Snead, C. Jakobs, K.M. Gibson, Expression profiling reveals multiple myelin alterations in murine succinate semialdehyde dehydrogenase deficiency, J. Inher. Metab. Dis. 29 (2006) 143-156]. In the current report, we have extended these findings via comprehensive analysis of brain phospholipid fractions, including quantitation of fatty acids in individual phospholipid subclasses and estimation of hexose-ceramide in Aldh5a1(-/-) brain. In comparison to wild-type littermates (Aldh5a1(+/+)), we detected a 20% reduction in the ethanolamine glycerophospholipid content of Aldh5a1(-/-)mice, while other brain phospholipids (choline glycerophospholipid, phosphatidylserine and phosphatidylinositol) were within normal limits. Analysis of individual fatty acids in each of these fractions revealed consistent alterations in n-3 fatty acids, primarily increased 22:6n-3 levels (docosahexaenoic acid; DHA). In the phosphatidyl serine fraction there were marked increases in the proportions of polyunsaturated fatty acids with corresponding decreases of monounsaturated fatty acids. Interestingly, the levels of hexose-ceramide (glucosyl- and galactosylceramide, principal myelin cerebrosides) were decreased in Aldh5a1(-/-) brain tissue (one-tailed t test, p=0.0449). The current results suggest that lipid and myelin abnormalities in this animal may contribute to the pathophysiology.

Human neural stem cells are more sensitive than astrocytes to ethanol exposure

BACKGROUND

Exposure to ethanol (EtOH) can be deleterious to the developing central nervous system. The mechanisms by which EtOH exposure induces neural pathology in utero remain unclear. However, EtOH-induced increases in protein kinase C (PKC) have been associated with apoptosis in human primary cell cultures. Although the toxic effects of EtOH on differentiated neural cells have been studied in laboratory animal models, the susceptibility of the human neural stem cells (NSCs) that predominate in the central nervous system during embryonic development has not been addressed.

METHODS

For this study, fetal human brain cells, which satisfied the criteria for NSCs by being CD133-positive, nestin-positive, and differentiated glial fibrillary acidic protein-positive human astrocytes, were studied. The cytotoxic potential of EtOH in NSC and astrocyte cultures was studied by using morphological and biochemical methods. In addition, membrane and cytosolic fraction PKC activity for each cell type was assessed.

RESULTS

NSC showed a dose-dependent increase in EtOH-induced toxicity as estimated by terminal transferase-mediated dUTP nick end labeling (TUNEL) stain and viability assays. TUNEL staining indicating DNA degradation consistent with programmed (apoptotic) cell death was detectable in 90% of NSC 16 hr after 2 hr exposure to 10 mM EtOH. NSC also showed a concentration-dependent increase in membrane, but not cytosol, PKC activity over the same EtOH dose range. By contrast, astrocytes showed no cytotoxic effects at any concentrations of EtOH used (0-10 mM). PKC activity of both the membrane and cytosolic fragments from astrocytes also was unaffected by this range of doses.

CONCLUSIONS

This study demonstrates the susceptibility of human NSCs, compared with astrocytes, to EtOH and indicates that alterations in PKC signal transduction in NSC may play a role in EtOH-induced neuropathological processes.

Ethanol specifically decreases peroxisome proliferator activated receptor beta in B12 oligodendrocyte-like cells

Peroxisome proliferator activated receptors (PPARs) are nuclear receptors that control important genes involved in lipid metabolism. Their role in nerve cells is uncertain, although anomalous myelination of the corpus callosum has been described in the PPARbeta-null mouse, and abnormalities of this tissue have been documented in fetal alcohol syndrome in humans. We report here that ethanol treatment of B12 oligodendrocyte-like cells induces a concentration- and time-dependent decrease in the mRNA and protein levels of PPARbeta, with no effect on PPARalpha or PPARgamma. The effect on PPARbeta is seen as an increase in mRNA degradation, as assessed by run-off assays, due to a significant decrease in PPARbeta mRNA half-life, with no observed changes in intracellular localization. Our results suggest a possible link between PPARbeta function and ethanol-induced abnormal myelination in oligodendrocytes.

Protein kinase C inhibitors counteract the ethanol effects on myelin basic protein expression in differentiating CG-4 oligodendrocytes

Abnormal formation of myelin appears to be one defect contributing to the development of the neuropathology associated with the fetal alcohol syndrome. Using the CG-4 cell line we previously showed that 25-75 mM EtOH downregulates the expression of myelin basic protein (MBP) in differentiating oligodendrocytes (OLGs) without affecting morphological development (Dev. Brain Res. 128 (2001) 9). Here we showed that a relatively low concentration of 12-phorbol-13-myristate acetate (PMA) mimicked the EtOH-caused inhibition of MBP expression without affecting morphology. The inhibition of MBP expression by 100 mM EtOH or 1 nM PMA was completely counteracted by three inhibitors of protein kinase C (PKC), bisindolylmaleimide I, chelerythrine chloride, and calphostin C, indicating that EtOH downregulated MBP expression by activating PKC. We investigated whether the EtOH activation resulted, in part, from upregulation of the expression of PKC isozymes. Of 11 PKC isozymes examined, CG-4 OLGs expressed nine; PKCs alpha, beta1, beta2; delta, epsilon, eta; lambda, zeta; mu; while PKC isozymes gamma and theta were not detected. Only five PKC isozymes, alpha, beta1, beta2, eta, and mu, displayed developmental changes in expression. However, EtOH did not upregulate the early expression of any PKC isozyme during the first 2 days of differentiation, the developmental stage when it downregulates MBP expression in CG-4 cells. The similar effects of PMA and EtOH indicate that EtOH delays MBP expression by activating at least one phorbol ester-sensitive PKC isozyme in oligodendrocytes without upregulating its expression.

Ethanol exerts different effects on myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase expression in differentiating CG-4 oligodendrocytes

Evidence suggests that abnormal myelination is one factor contributing to the neuoropathology associated with fetal alcohol syndrome. We investigated the potential teratogenic effects of ethanol (EtOH) on myelin formation by determining its effects on the developmentally regulated increased expression of myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in differentiating CG-4 oligodendrocytes (OLGs). By using CG-4 OLGs in vitro we identified processes altered by ethanol actions exerted directly on OLGs. During the first 8 days of development, EtOH decreased the expression of the major structural 18.5 and 14 kDa MBP isoforms by at least 40% at 4 days of development. EtOH concentrations between 25 and 75 mM inhibited MBP expression in a dose-dependent manner. Adding or withdrawing EtOH on specific days of differentiation reversibly modulated the expression of MBP, and the degree of inhibition was directly related to the length of ethanol exposure. As little as two consecutive days of EtOH exposure either early or late during development caused at least a 20% inhibition, however, no short critical time window of EtOH vulnerability for the inhibition was observed. The ethanol effect was selective for MBP expression, as EtOH did not alter the developmentally-regulated increased expression of CNP isozymes or enzyme activity. The results indicate that one factor contributing to the development of fetal alcohol syndrome may be defective myelination resulting from delayed and decreased MBP expression.