Ethanol neurotoxicity in the developing cerebellum: underlying mechanisms and implications

Abstract

Ethanol is the main constituent of alcoholic beverages that exerts toxicity to neuronal development. Ethanol affects synaptogenesis and prevents proper brain development. In humans, synaptogenesis takes place during the third trimester of pregnancy, and in rodents this period corresponds to the initial few weeks of postnatal development. In this period neuronal maturation and differentiation begin and neuronal cells start migrating to their ultimate destinations. Although the neuronal development of all areas of the brain is affected, the cerebellum and cerebellar neurons are more susceptible to the damaging effects of ethanol. Ethanol's harmful effects include neuronal cell death, impaired differentiation, reduction of neuronal numbers, and weakening of neuronal plasticity. Neuronal development requires many hormones and growth factors such as retinoic acid, nerve growth factors, and cytokines. These factors regulate development and differentiation of neurons by acting through various receptors and their signaling pathways. Ethanol exposure during development impairs neuronal signaling mechanisms mediated by the N-methyl-d-aspartate (NMDA) receptors, the retinoic acid receptors, and by growth factors such as brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-I), and basic fibroblast growth factor (bFGF). In combination, these ethanol effects disrupt cellular homeostasis, reduce the survival and migration of neurons, and lead to various developmental defects in the brain. Here we review the signaling mechanisms that are required for proper neuronal development, and how these processes are impaired by ethanol resulting in harmful consequences to brain development.

Figure 1

Cross section of postnatal day seven rat cerebellum (a), showing external granule layer (EGL), Purkinje cells and internal granule layer (IGL) (b). During cerebellar development, granule cell precursors (GCPs) present in EGL proliferate and differentiated into mature granule neurons. Mature neurons start migrating past the Purkinje cell layer to form the internal granule layer (IGL) (c).

Figure 2

In ethanol metabolism, the enzyme alcohol dehydrogenase oxidizes ethanol to acetaldehyde, while cytochrome P450-2E1 enzyme converts ethanol to acetaldehyde and H₂O₂. Acetaldehyde interacts with proteins and forms protein-acetaldehyde adducts (acetaldehyde-hemocyanin adduct). Hydrogen peroxide and acetaldehyde (via transcriptional activation of NADPH oxidase, xanthine oxidase, and iNOS) generate free radicals (reactive oxygen species, ROS/reactive nitrogen species, RNS), which oxidize proteins, lipids, and DNA leading to apoptotic cell death in the developing cerebellum.

Figure 3

Ethanol-induced changes in signaling pathways/components (red colored arrow) in the developing cerebellum. Ethanol exposure inhibits axon outgrowth, cytoskeletal rearrangement, and neuronal differentiation via L1/Src/ERK1/2 pathways and RhoA and Rac1/Cdc42 activation. Ethanol also inhibits the cell proliferation by altering the levels of proteins required for cell cycle progression, e.g., increasing the levels of cyclin D/CDK4/6 and by decreasing the levels of cyclin A. Ethanol impairs NMDA, BDNF/TrkB, IGF/IGFR and IL1β/IL1βR-mediated pathways and their downstream signals which are required for the migration and survival of neurons. Generation of oxidative stress and reduction in the level of enzymes required for the removal of free radicals by ethanol damages mitochondria and initiates cell death in neurons.