G9a-mediated histone methylation regulates ethanol-induced neurodegeneration in the neonatal mouse brain

Abstract

Rodent exposure to binge-like ethanol during postnatal day 7 (P7), which is comparable to the third trimester of human pregnancy, induces neuronal cell loss. However, the molecular mechanisms underlying these neuronal losses are still poorly understood. Here, we tested the possibility of histone methylation mediated by G9a (lysine dimethyltransferase) in regulating neuronal apoptosis in P7 mice exposed to ethanol. G9a protein expression, which is higher during embryogenesis and synaptogenic period compared to adult brain, is entirely confined to the cell nuclei in the developing brain. We found that ethanol treatment at P7, which induces apoptotic neurodegeneration in neonatal mice, enhanced G9a activity followed by increased histone H3 lysine 9 (H3K9me2) and 27 (H3K27me2) dimethylation. In addition, it appears that increased dimethylation of H3K9 makes it susceptible to proteolytic degradation by caspase-3 in conditions in which ethanol induces neurodegeneration. Further, pharmacological inhibition of G9a activity prior to ethanol treatment at P7 normalized H3K9me2, H3K27me2 and total H3 proteins to basal levels and prevented neurodegeneration in neonatal mice. Together, these data demonstrate that G9a mediated histone H3K9 and K27 dimethylation critically regulates ethanol-induced neurodegeneration in the developing brain. Furthermore, these findings reveal a novel link between G9a and neurodegeneration in the developing brain exposed to postnatal ethanol and may have a role in fetal alcohol spectrum disorders.

Pre-administration of G9a/GLP inhibitor during synaptogenesis prevents postnatal ethanol-induced LTP deficits and neurobehavioral abnormalities in adult mice

“…The developing brain is so sensitive to ethanol exposure that even a single exposure can produce massive losses of neurons in several brain regions (Ikonomidou, et al, 2000) during the first few postnatal days in neonatal mice (postnatal days 4–10 [P4–10]), a developmental period which corresponds with the third trimester pregnancy in humans (Bayer, et al., 1993). Excessive acute ethanol intoxication in P7 mice prompts neurodegeneration in vital brain regions including the hippocampus and cortex (Ikonomidou, et al, 2000, Sadrian, et al, 2012, Subbanna, et al, 2014, Subbanna, et al., 2013a, Subbanna, et al, 2013b, Wilson, et al, 2011), as well as impairments in LTP (Izumi, et al, 2005, Sadrian, et al, 2012, Subbanna, et al, 2013a, Wilson, et al, 2011) and spatial memory task performance in adult mice (Subbanna, et al, 2013a). Similarly, the local and interregional brain circuitry of the olfacto-hippocampal pathway in adult mice is compromised when P7 mice are exposed to acute ethanol (Sadrian, et al, 2012, Wilson, et al, 2011).…”

Epigenetic Modifications, Alcoholic Brain and Potential Drug Targets

“…Recent evidence also shows that acute ethanol exposure alters dimethylation levels on lysines 9 and 27 of histone 3, which partially mediate ethanol's teratogenic effects in the brain [51] . The same research group also showed that ethanol exposure increases the levels of this mark in exon 1 of G9a, an HDM involved in alcoholinduced apoptosis [52] . Furthermore, epigenetic marks are emerging as major regulators of gene-by-environment interactions and have been implicated in the etiology of ethanol-induced neurodegeneration.…”

“…A key role of histone acetylation was observed in a recent study, in which the hyposensitivity to gamma-aminobutyric acid in the ventral tegmental area during alcohol withdrawal in mice was found to be mediated by HDAC [67] . Alcoholism is also suggested to be associated with neurodegeneration; indeed, in a recent study it was found that ethanol-induced acetylation of histone at G9a exon1 leads to neurodegeneration in neonatal mice [52] . Furthermore, in a study, histone acetylation was found to play a crucial role in regulating the transcriptional activity and contribute to the drug-induced alterations in gene expression and behavior.…”

The role of chromatin repressive marks in cognition and disease: A focus on the repressive complex GLP/G9a

“…Ethanol exposure reduced the global expression levels of the HMTs G9a, Suv39 h1 and Setdb1, but not the activity of HATs or HDACs. Moreover, G9a-mediated H3K9me2 was shown to critically regulate ethanol-induced neurodegeneration and associated loss of cognition in rodents Subbanna et al, 2013). Consequently, inhibiting G9a before ethanol exposure at P7 prevented ethanol-induced deficits in LTP, memory and social cognition .…”