NIAAA/VA Researchers Identify Possible Site of Alcohol Damage
Alcohol Damage Bethesda, Maryland. Researchers at Harvard Medical School and the Brockton/West Roxbury Veterans Administration Medical Center report in today's issue of The Journal of Cell Biology a molecular action of alcohol that may produce some of the damage seen in fetal alcohol syndrome (FAS) and fetal alcohol effects (FAE). The same mechanism may play a role in such adult memory disorders as alcoholic blackouts and chronic cognitive disorders, the authors suggest.
Grants from the National Institutes of Health's National Institute on Alcohol Abuse and Alcoholism and the U.S. Department of Veterans Affairs' Medical Research Service funded the work by Michael Charness, M.D., and colleagues Ranesh Ramanathan, Michael Wilkemeyer, Ph.D., Bina Mittal, Ph.D., and George Perides, Ph.D. The researchers transfected the human gene for the neural cell adhesion molecule L1 into mouse fibroblasts (connective tissue cells) to observe alcohol's effects. They found that ethanol potently and completely inhibited the adhesiveness mediated by human L1 in mouse fibroblast cells.
The research team then tested ethanol's effects on cerebellar granule cells from rat neonates (equivalent in development to the third trimester of human gestation), which also bear L1 molecules. Again, they found that ethanol potently inhibited L1-mediated cell-cell adhesion--this time in cells of the developing nervous system.
In both experiments, Charness and his associates observed significant inhibition of cell-cell adhesion at blood alcohol concentrations equivalent to those produced in humans by one drink.
"The striking sensitivity of L1 to low alcohol concentrations suggests a possible mechanism whereby light drinking may damage a developing fetus. This important finding strengthens recommendations by the U.S. Surgeon General, the American Medical Association, the Fetal Alcohol Study Group of the Research Society on Alcoholism, and the American College of Obstetrics and Gynecology that women who are pregnant should abstain from alcohol," said NIAAA Director Enoch Gordis, M.D.
The leading preventable cause of mental retardation in the United States, FAS affects about 6 percent of the offspring of alcoholic mothers. Children with FAS exhibit growth retardation and malformations of the brain, face, and heart, while children with less severe FAE exhibit subtle neurobehavioral deficits. Animal studies have shown that growth and development are affected in a dose-dependent manner and that the type of defect depends on the timing of alcohol exposure. Recent epidemiologic studies have indicated that low levels of prenatal alcohol exposure can produce dose-dependent alterations in growth, head circumference, cognitive function, and behavior.
NIAAA supports research into several potential mechanisms whereby alcohol may damage the developing brain and other organs: Ethanol may induce excessive cell loss in cell populations undergoing normal programmed cell death; ethanol also may generate metabolites or the formation of reactive oxygen species that disrupt cell function; and ethanol seems to alter cell responses to molecules that regulate neuronal proliferation, migration, and differentiation.
Within the last category, L1 and other cell adhesion molecules are essential for normal human nervous system development. Molecular tags that protrude from nerve cell membranes and stick to similar molecules on adjacent cells, L1 and other cell adhesion molecules guide neuronal migration and tract formation during nervous system development, and mutations in the gene for L1 result in brain malformations.
Scientists recently recognized that children born with L1 mutations exhibit mental retardation, spastic gait, and a variety of brain malformations including enlarged ventricles (hydrocephalus), and agenesis of the corpus callosum (the bundle of nerve fibers that connect the right and left sides of the brain). Noting that retardation, hydrocephalus, and agenesis of the corpus callosum also occur in FAS, Charness and colleagues asked whether alcohol inhibits the adhesiveness of cells bearing the L1 molecule.
"We found the effects of alcohol on L1-mediated cell-cell adhesion to be surprisingly sensitive and specific, said Charness in a separate interview: "Teratogens other than alcohol, including anticonvulsants, had no effect on L1, and alcohol did not inhibit adhesion mediated by a related neural cell adhesion molecule, N-CAM."
L1 also is present in neurons of the mature nervous system, where it is believed to play a role in the long-term synaptic changes that influence learning and memory. The authors suggest that, since learning and memory are known to be inhibited by L1 antibodies and alcohol, alcohol's effects on L1-mediated cell-cell adhesion may be responsible in part for alcohol-related memory problems.
A new research direction emerging from the study concerns the portion of L1 on which alcohol acts. For years, scientists have believed that alcohol damages cells by dissolving into the fatty portion of the cell membrane and disrupting the function of proteins within that membrane. However, the Harvard/VA experiments indicate that ethanol probably acts at a specific site on the L1 molecule and does so by some action other than permeating the cell membrane. The adhesive properties of L1 have been mapped to regions of the molecule that protrude from the cell membrane, rasing the possibility that ethanol interacts with the extracellular portion of L1. Charness and colleagues plan experiments to mutate regions of the L1 gene to ascertain the precise portion of the molecule disrupted by ethanol.
"The ability to define a molecular target of alcohol will facilitate the rational design of drugs to block some of its damaging effects," said Gordis. "Using advanced molecular biology and molecular genetic techniques, NIAAA researchers will move quickly to explore the hypotheses generated by today's report."