Role of central nervous system insulin resistance in fetal alcohol spectrum disorders

Abstract

Fetal alcohol spectrum disorder (FASD) is the most common preventable cause of mental retardation in the USA. Ethanol impairs neuronal survival and function by two major mechanisms: 1) it inhibits insulin signaling required for viability, metabolism, synapse formation, and acetylcholine production; and 2) it functions as a neurotoxicant, causing oxidative stress, DNA damage and mitochondrial dysfunction. Ethanol inhibition of insulin signaling is mediated at the insulin receptor (IR) level and caused by both impaired receptor binding and increased activation of phosphatases that reverse IR tyrosine kinase activity. As a result, insulin activation of PI3K-Akt, which mediates neuronal survival, motility, energy metabolism, and plasticity, is impaired. The neurotoxicant effects of ethanol promote DNA damage, which could contribute to mitochondrial dysfunction and oxidative stress. Therefore, chronic in utero ethanol exposure produces a dual state of CNS insulin resistance and oxidative stress, which we postulate plays a major role in ethanol neurobehavioral teratogenesis. We propose that many of the prominent adverse effects of chronic prenatal exposure to ethanol on CNS development and function may be prevented or reduced by treatment with peroxisome-proliferated activated receptor (PPAR) agonists which enhance insulin sensitivity by increasing expression and function of insulin-responsive genes, and reducing cellular oxidative stress.

FIG. 1

Schematic of ethanol’s adverse effects on insulin/IGF signaling in the brain. Ethanol inhibits phosphorylation and activation of insulin/IGF-1 receptor tyrosine kinases, as well as downstream mediators of neuronal growth, survival, plasticity, energy metabolism, migration, and neurotransmitter function. In addition, ethanol stimulates GSK-3β activity through inhibition of PI3K and activation of PTEN phosphatase.

FIG. 2A & 2B

Therapeutic benefit of PPAR-delta (δ treatment in an ethanol exposure model. Long Evans rat pups were treated with ethanol (3 mg/g) or vehicle (Veh) by i.p. injection (50 μl) on postnatal days (P) 4, 6, and 8. Rats were also treated with vehicle or L-165,041 (2 μg/Kg), a PPAR-δ agonist by i.p. injection on P5, P7, and P9. Eight male rats were included in each sub-group. (A) On P16, the rats were evaluated by rotarod testing of latency to fall using an incremental (1-5 rpm) fixed speed protocol. Rotarod data from trials 7-10 (speed= 5 rpm) were grouped and analyzed by two-way ANOVA (F=4.98, P=0.028 for interaction of group by treatment; F=4.754, P=0.031 for treatment effect; F=3.04, P=0.084 for group effect). The Bonferroni post-hoc test revealed that the mean latency to fall was significantly shorter for the Ethanol (EtOH) +Veh group compared with all other groups (P<0.05). (B) From P27 to P30, rats were evaluated by Morris Water Maze testing with 3 trials per day. On Day 1, the platform was visible, but on Days 2-4, the platform was submerged. On Days 3 and 4, the water entry quadrant was randomized. The latency for locating and landing on the platform was recorded. Data were analyzed using area-under-the-curve (AUC) calculations corresponding to performance over the 3 trials each day. The graph depicts the mean ± S.E.M. of AUC latencies for each group on each day of testing. Inter-group comparisons were made using repeated measured mixed model ANOVA (F=20.83, P<0.0001 for treatment; and F=95.72, P<0.0001 for Trial Day). Post-hoc Bonferroni tests demonstrated significant differences between Control+Vehicle and Ethanol+Vehicle on Trial Days 1 (P<0.05) and 4 (P<0.01), between Control+ PPAR-δ and Ethanol+Vehicle on Trial Days 1 (P<0.001), 2 (P<0.001), 3 (P<0.05), and 4 (P<0.01), and Control+PPAR-δx and Ethanol+PPAR-δ on Trial Days 1 (P<0.001), 2 (P<0.01), and 4 (P<0.01). (*P<0.05; **P<0.01; ***P<0.001)

FIG. 3

Dual mechanisms hypothesis of alcohol-mediated CNS teratogenesis. Chronic gestational exposure to ethanol causes oxidative stress and insulin/IGF resistance in the brain. The oxidative stress is caused by mitochondrial dysfunction, which promotes DNA damage, lipid peroxidation, pro-inflammatory cytokine activation, and apoptosis. Insulin/IGF resistance in brain impairs gene expression required for neuronal genesis, myelin maintenance, cell migration, neurotransmitter function, and energy metabolism. Oxidative stress and pro-inflammatory cytokine activation exacerbate the adverse effects of insulin/IGF resistance. Together, these abnormalities lead to deficits in neuronal plasticity, learning and memory.