Fetal stress-mediated hypomethylation increases the brain susceptibility to hypoxic–ischemic injury in neonatal rats
Introduction
Increasing evidence suggests that epigenetic mechanisms are of critical importance in regulating gene expression patterns, profoundly impacting normal brain development and programming of adaptive/maladaptive phenotypes in response to various environmental cues (Choi and Friso, 2010, Dauncey, 2012, Li et al., 2012a, Mehler, 2008). DNA methylation is the most characterized epigenetic mechanism, once considered as an inherently stable mark incapable of rapid change (Godfrey et al., 2007, Moore et al., 2013). Recently, emerging evidence has demonstrated that DNA undergoes rapid methylation and demethylation in the brain by means of distinct mechanisms in an activity-dependent fashion, which is critical for various types of brain function and physiological activity (Borrelli et al., 2008, Guo et al., 2011a, Guo et al., 2011b). However, pathological DNA methylation or demethylation profiles may result in aberrant gene expression and thus contribute to multiple brain pathologies in various neuropsychiatric conditions (Gräff et al., 2011, Hwang et al., 2013, Ikegame et al., 2013, Levenson and Sweatt, 2005).
Neonatal hypoxic–ischemic encephalopathy (HIE) is one of major causes of acute brain damage and mortality as well as chronic neurological disability in newborns (Chen et al., 2009b, Vannucci, 2000). Due to the poor understanding of the basic pathogenesis, few universally accepted therapy is available for neonatal HIE except that some studies implied the possible therapeutic effects of moderate hypothermia intervention (Perlman, 2006, Rees et al., 2011). Various candidate mechanisms have been proposed to elucidate the underlying pathogenesis in HIE, of which several molecules were considered as players and promising therapeutic targets, including the glucocorticoid receptor, angiotensin II type 2 receptor, hypoxia-inducible factor 1α (HIF-1α) and matrix metalloproteinase (MMP), etc. (Chen et al., 2008, Chen et al., 2009a, Gonzalez-Rodriguez et al., 2014a, Gonzalez-Rodriguez et al., 2014b, Li et al., 2012b, Li et al., 2013). Of great interest, recent studies revealed that adverse intrauterine environment may contribute to aberrant brain development and program a sensitive brain phenotype to neonatal HI insult (Gonzalez-Rodriguez et al., 2014b, Li et al., 2012b, Ma and Zhang, 2015).
Gestational hypoxia is a common stress to the fetal development and increases the risk of neonatal morbidity and mortality (Ma et al., 2014, Ma and Zhang, 2015). Our previous study revealed that chronic fetal hypoxia resulted in increased brain susceptibility to HI injury in neonatal rats (Gonzalez-Rodriguez et al., 2014b), whilst the underlying mechanisms are not fully elucidated. Herein, we presented a novel finding that gestational hypoxia induced a significant decrease in global DNA methylation and a sustained increase in HIF-1α in the developing brain. Of importance, we demonstrated that DNA hypomethylation in the brain significantly increased HI-induced brain injury in neonatal rats in a HIF-1α-dependent manner and worsened long-term neurobehavioral deficits, which may underlie fetal stress-induced programming of HI sensitive phenotype in the developing brain.
Section snippets
Experimental animals
Pregnant Sprague Dawley rats were purchased from Charles River Laboratories (Portage, MI). For the hypoxic treatment, pregnant animals were randomly divided into 2 groups: normoxic control and hypoxic treatment (10.5% O2, days 15 to 21 of gestation), as described previously (Patterson et al., 2010). On day 21 of pregnancy, some animals were killed and brains were isolated from fetuses (E21). Other animals were allowed to give birth, and brains collected from 12-day old (P12) pups and P30
Fetal hypoxia and 5-Aza induced global hypomethylation in the developing brain
Fig. 1A showed a developmental regulation of DNA methylation levels in the brain. Compared with the fetal brain, there was a significant increase in global methylation in the brain of P30 animals. Of importance, gestational hypoxia resulted in a significant decrease in methylation levels in the fetal brain, which persisted in the postnatal development, inhibiting the increase of methylation during the brain development (Fig. 1A). We investigated the role of hypomethylation in fetal
Discussion
The present study confers several novel findings. Firstly, we showed that gestational hypoxia altered normal developmental patterns of DNA methylation, resulting in a persistent global hypomethylation status in the developing brain. We then demonstrated that DNA hypomethylation in the brain resulted in a significant increase in neonatal brain vulnerability to HI-induced injury and worsened subsequent neurobehavioral dysfunction. Furthermore, we revealed that DNA methylation was an important
Sources of funding
This work was supported by National Institutes of Health grants HL82779 (L.Z.), HL83966 (L.Z.), and HL118861 (L.Z.).
Disclosures
The authors declare no competing financial interests.
Acknowledgments
A portion of this research used the Loma Linda University School of Medicine Advanced Imaging and Microscopy Core, a facility supported in part by the National Science Foundation through the Major Research Instrumentation program of the Division of Biological Infrastructure Grant No. 0923559 and the Loma Linda University School of Medicine.
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