• Energy Research

Brain lesions in alcoholics are multifactorial in origin. Ethanol neurotoxicity, Wernicke's encephalopathy, hepatocerebral degeneration, head trauma, central pontine myelinolysis, Marchiafava‐Bignami syndrome, pellagra, and premorbid pathological conditions, such as fetal alcohol syndrome, may all contribute to cognitive dysfunction in alcoholics. With the exception of ethanol neurotoxicity, all of these conditions are associated with specific neuropathological lesions. Wernicke's encephalopathy, the neurological syndrome of thiamine deficiency, is frequently overlooked during life and may cause global dementia as well as the more familiar Korsakoff's amnestic syndrome. Distinguishing ethanol neurotoxicity from nutritional deficiency can be facilitated by magnetic resonance imaging, which can visualize some of the specific macroscopic lesions of Wernicke's encephalopathy, central pontine myelinolysis, cerebellar degeneration, and Marchiafava‐Bignami syndrome. Computerized morphometric studies of alcoholic brains have revealed ventricular enlargement, selective loss of subcortical white matter, and alterations in neuronal size, number, architecture, and synaptic complexity. These lesions tend to be more severe when there is coexisting nutritional deficiency or liver disease, suggesting that ethanol neurotoxicity may not be the sole cause. A search for similar lesions in nonalcoholic Wernicke's encephalopathy and nonalcoholic liver disease will help determine the specificity of these lesions.

Mental retardation, hydrocephalus, and agenesis of the corpus callosum are observed both in fetal alcohol syndrome (FAS) and in children with mutations in the gene for the cell adhesion molecule L1. We studied the effects of ethanol on cell-cell adhesion in mouse fibroblasts transfected with human L1. L1-transfected fibroblasts exhibited increased cell-cell adhesion compared with wild-type or vector-transfected controls. Ethanol potently and completely inhibited L1-mediated adhesion both in transfected L cells and NIH/3T3 cells. Half-maximal inhibition was observed at 7 mM ethanol, a concentration achieved in blood and brain after ingesting one alcoholic beverage. In contrast, ethanol did not inhibit the adhesion of fibroblasts transfected with vector alone or with N-CAM-140. L1-mediated cell-cell adhesion was inhibited with increasing potency by n-propanol and n-butanol, but was not inhibited at all by n-alcohols of 5 to 8 carbons, acetaldehyde, or acetate, suggesting that ethanol interacts directly with a small hydrophobic pocket within L1. Phenylalanine, teratogenic anticonvulsants, and high concentrations of glucose did not inhibit L1-mediated cell-cell adhesion. Ethanol also inhibited potently the heterotypic adhesion of rat cerebellar granule cells to a monolayer of L1-transfected NIH/3T3 cells, but had no effect on their adhesion to N-CAM-140 or vector-transfected NIH/3T3 cells. Because L1 plays a role in both neural development and learning, ethanol inhibition of L1-mediated cell-cell interactions could contribute to FAS and ethanol-associated memory disorders.

Ethanol may modulate endogenous opioid systems by disrupting opioid receptor signalling. Low concentrations of ethanol slightly potentiate mu-opioid receptor binding by increasing receptor Bmax, and, in some cases, chronic ethanol exposure decreases the density or affinity of the mu-opioid receptors. By contrast, high concentrations of ethanol acutely decrease delta-opioid receptor binding by decreasing receptor affinity, whereas chronic exposure of animals and neuronal cell lines to lower concentrations of ethanol leads to possibly adaptive increases in the density or affinity of the delta-opioid receptors. In the neuronal cell line NG108-15, ethanol does not up-regulate the delta-opioid receptor by blocking receptor degradation or endocytosis, but protein synthesis is required for this response. Up-regulation of the delta-opioid receptor renders ethanol-treated NG108-15 cells 3.5-fold more sensitive to opioid inhibition of adenylyl cyclase. Long-term treatment with ethanol also increases maximal opioid inhibition in NG108-15 cells, possibly by decreasing levels of G alpha s and its mRNA. Ethanol differentially modulates signal transduction proteins in three additional neuronal cell lines, N18TG2, N4TG1, and N1E-115. Ethanol-treated N18TG2 cells show the least up-regulation of the delta-opioid receptor, little heterologous desensitization of adenylyl cyclase, and no changes in G alpha s or G alpha i. By contrast, ethanol-treated N1E-115 cells show the largest up-regulation of the delta-opioid receptor, the most heterologous desensitization of adenylyl cyclase, and concentration-dependent decreases in G alpha s and increases in G alpha i. Further analysis of these related neuronal cell lines may help to identify the molecular elements that endow some, but not all, neuronal cells with the capacity to adapt to ethanol.

The mechanisms by which ethanol damages the developing and adult central nervous system (CNS) remain unclear. Activity-dependent neuroprotective protein (ADNP) is a glial protein that protects the CNS against a wide array of insults and is critical for CNS development. NAPVSIPQ (NAP), a potent active fragment of ADNP, potentiated axon outgrowth in cerebellar granule neurons by activating the sequential tyrosine phosphorylation of Fyn kinase and the scaffold protein Crk-associated substrate (Cas). Pharmacological inhibition of Fyn kinase or expression of a Fyn kinase siRNA abolished NAP-mediated axon outgrowth. Concentrations of ethanol attained after social drinking blocked NAP-mediated axon outgrowth (IC50= 17 mM) by inhibiting NAP activation of Fyn kinase and Cas. These findings identify a mechanism for ADNP regulation of glial–neuronal interactions in developing cerebellum and a pathogenesis of ethanol neurotoxicity.

There is a genetic contribution to fetal alcohol spectrum disorders (FASD), but the identification of candidate genes has been elusive. Ethanol may cause FASD in part by decreasing the adhesion of the developmentally critical L1 cell adhesion molecule through interactions with an alcohol binding pocket on the extracellular domain. Pharmacologic inhibition or genetic knockdown of ERK2 did not alter L1 adhesion, but markedly decreased ethanol inhibition of L1 adhesion in NIH/3T3 cells and NG108-15 cells. Likewise, leucine replacement of S1248, an ERK2 substrate on the L1 cytoplasmic domain, did not decrease L1 adhesion, but abolished ethanol inhibition of L1 adhesion. Stable transfection of NIH/3T3 cells with human L1 resulted in clonal cell lines in which L1 adhesion was consistently sensitive or insensitive to ethanol for more than a decade. ERK2 activity and S1248 phosphorylation were greater in ethanol-sensitive NIH/3T3 clonal cell lines than in their ethanol-insensitive counterparts. Ethanol-insensitive cells became ethanol sensitive after increasing ERK2 activity by transfection with a constitutively active MAP kinase kinase 1. Finally, embryos from two substrains of C57BL mice that differ in susceptibility to ethanol teratogenesis showed corresponding differences in MAPK activity. Our data suggest that ERK2 phosphorylation of S1248 modulates ethanol inhibition of L1 adhesion by inside-out signaling and that differential regulation of ERK2 signaling might contribute to genetic susceptibility to FASD. Moreover, identification of a specific locus that regulates ethanol sensitivity, but not L1 function, might facilitate the rational design of drugs that block ethanol neurotoxicity.

NAPVSIPQ (NAP), an active fragment of the glial-derived activity-dependent neuroprotective protein, is protective at femtomolar concentrations against a wide array of neural insults and prevents ethanol-induced fetal wastage and growth retardation in mice. NAP also antagonizes ethanol inhibition of L1-mediated cell adhesion (ethanol antagonism). We performed an Ala scanning substitution of NAP to determine the role of ethanol antagonism and neuroprotection in NAP prevention of ethanol embryotoxicity. The Ser-Ile-Pro region of NAP was crucial for both ethanol antagonism and protection of cortical neurons from tetrodotoxin toxicity (neuroprotection). Ala replacement of either Ser-5 or Pro-7 (P7A-NAP) abolished NAP neuroprotection but minimally changed the efficacy of NAP ethanol antagonism. In contrast, Ala replacement of Ile-6 (I6A-NAP) caused a decrease in potency (>2 logarithmic orders) with only a small reduction (<10%) in the efficacy of NAP neuroprotection but markedly reduced the efficacy (50%) and the potency (5 logarithmic orders) of NAP ethanol antagonism. Ethanol significantly reduced the number of paired somites in mouse whole-embryo culture; this effect was prevented significantly by 100 pM NAP or by 100 pM P7A-NAP, but not by 100 pM I6A-NAP. The structure–activity relation for NAP prevention of ethanol embryotoxicity was similar to that for NAP ethanol antagonism and different from that for NAP neuroprotection. These findings support the hypothesis that NAP antagonism of ethanol inhibition of L1 adhesion plays a central role in NAP prevention of ethanol embryotoxicity and highlight the potential importance of ethanol effects on L1 in the pathophysiology of fetal alcohol syndrome.

Gestational exposure to ethanol causes defects in neuronal migration, fasciculation, and synaptogenesis, developmental events that depend on the patterned expression and function of cell adhesion molecules (CAMs). Recombinant human osteogenic protein-1 (hOP-1) increases cell-cell adhesion and promotes cell clustering in proliferating neuroblastoma x glioma hybrid NG108-15 cells by strongly inducing N-CAM and L1. Here we show that concentrations of ethanol achieved during social drinking inhibit hOP-1-induced cell clustering without affecting cell proliferation, the induction and cell surface expression of N-CAM and L1, or the alternative splicing and sialylation of N-CAM. This inhibition was reproduced by other alcohols in proportion to their chain length, but not by teratogenic anticonvulsants or phenylalanine. Ethanol inhibition of hOP-1 morphogenesis was inversely proportional to the concentration of hOP-1 and, hence, to the levels of N-CAM and L1. Low concentrations of ethanol (IC50 5-10 mM) inhibited cell-cell adhesion in hOP-1-treated cells, and this action too was reproduced more potently by propanol and butanol. Ethanol may perturb brain and skeletal development by inhibiting CAM-mediated cell-cell interactions.

Increasing evidence suggests that ethanol damages the developing nervous system partly by disrupting the L1 cell adhesion molecule. Ethanol inhibits L1-mediated cell adhesion, and compounds that antagonize this action also prevent ethanol-induced embryotoxicity. Two such compounds are the small peptides NAPVSIPQ (NAP) and SALLRSIPA (SAL). We showed previously that NAP and SAL antagonize ethanol inhibition of L1 adhesion at femtomolar to picomolar concentrations. Here we demonstrate that, despite this extraordinary potency, both NAP and SAL lack stereospecificity. d-NAP, a peptide composed entirely of d-amino acids, was an effective ethanol antagonist in NIH/3T3 cells transfected with human L1 and in the NG108-15 neural cell line. Interestingly, Ala-substituted derivatives of d-NAP demonstrate the same structure-activity relation as the corresponding derivatives of l-NAP. The Ser-Ile-Pro motif was important for the ethanol antagonist activity of d-NAP, l-NAP, and l-SAL, with Ile being the most critical element in all three. Like l-NAP, d-NAP effectively reduced ethanol-induced growth retardation in mouse whole embryo culture. The potential resistance of d-peptides to proteases makes d-NAP a potentially attractive agent for the prevention of fetal alcohol syndrome.