A mouse and a fruit fly walk into a bar... This is not the beginning of a terrible joke, but the approach used by researchers to examine the molecular mechanisms involved in alcohol abuse with animal models. People who are less sensitive to the adverse effects of alcohol are more susceptible to its abuse. Rothenfluh et al. and Offenhäuser et al. report that mice and fruit flies with mutations in actin regulators are less sensitive to alcohol-induced sedation in two recent articles in Cell.
Alcohol initially increases, then decreases locomotor activity. In this latter period of sedation, motor coordination declines, as shown by loss of the righting reflex, in which animals placed on their backs return to a standing position.
Rothenfluh et al. identified white rabbit mutants with resistance to the sedative effects of alcohol in a screen of Drosophila P-element insertion lines. Rho small GTPases regulate actin in dendritic spines and axons. Contrary to their name, Rho GTPase-activating proteins (GAPs) quickly terminate Rho GTPase activity and therefore inhibit Rho GTPase signaling. The authors mapped the white rabbit mutation to the RhoGAP gene RhoGAP18B. They identified four GAP domain-containing RhoGAP18B transcripts and named them RA-RD. Drosophila with disruption of the RC transcript were resistant to the sedative effects of alcohol, and expression of RhoGAP18B-RC in adulthood conferred alcohol sensitivity to white rabbit mutants.
RhoGAP18B mediates the sedative effects of alcohol in Drosophila by restraining Rho small GTPase activity. Drosophila with constitutively active Rho or Rac were less sensitive to the sedative effects of alchohol, relative to wild-type Drosophila. White rabbit mutants lacking a Rho or Rac allele showed increased loss of righting.
Offenhäuser et al. found that another actin regulator, epidermal growth factor pathway substrate 8 (Eps8), is involved in the sedative effects of alcohol in the mouse. Following alcohol treatment, Eps8 knockouts had faster righting reflexes and fell less quickly on a rotating rod than did wild-type mice.
Consistent with its role in motor coordination, Eps8 expression was enriched in the cerebellum. Eps8 colocalized with F-actin in post-synaptic densities of cerebellar granule cells, and Eps8 co-immunoprecipitated with NMDA receptor subunits. Cerebellar granule neurons from Eps8 knockouts showed increased NMDA currents relative to wild-type neurons. F-actin immunoreactivity decreased in cerebellar granule neurites treated with NMDA, suggesting that NMDA induces actin depolymerization. In contrast, Eps8 knockout neurons were resistant to NMDA-induced depolymerization. The actin depolymerization drug latrunculin A decreased NMDA currents in Eps8 knockout neurons, suggesting that NMDA currents are elevated in Eps8 knockouts due to enhanced actin stability, according to the authors.
Alcohol causes sedation by depolymerizing actin in the cerebellum. Alcohol inhibited NMDA currents in wild-type neurons, but had no effect on Eps8 knockout neurons. Whereas alcohol treatment reduced F-actin immunoreactivity in wild-type cerebellar granule neurites, Eps8 knockout neurons were resistant to alcohol-induced actin depolymerization.
Eps8 knockouts showed increased preference for alcohol relative to wild-type mice, consistent with increased alcohol abuse among people who are resistant to alcohol's adverse effects. These data suggest that drugs that restrain actin polymerization specifically in the cerebellum might be used to treat alcohol abuse.