BEHAVIORAL, MOLECULAR AND EPIGENETIC CONSEQUENCES OF EARLY LIFE STRESS EXPOSURE AND THEIR IMPACT ON ADULT PSYCHOPATHOLOGY

There is now consistent evidence that psychiatric diseases may often represent the consequence of exposure to adverse events early in life, which may disrupt the correct program of brain maturation thus leading to long-lasting changes in brain function. Accordingly, exposure to stress during gestation in rats has a strong impact on brain development and can cause long-term abnormalities in adult behavior (Fumagalli et al., 2007; Seckl, 1998). In this context, the study of environmental manipulations in animal models offers the possibility to investigate the mechanisms that may be responsible for functional deterioration, with the advantage of keeping the influence of various factors such as the timing and intensity of the adverse condition, the growth environment and the genetic background under control. Given all these premises, in this study we first set up and employed a paradigm of prenatal stress in rodents in order to reproduce early life adversities that may encompass pregnancy and early postnatal life. Indeed, gestational stress has long-lasting effects on the hypothalamic-pituitary-adrenal (HPA) axis and on the behavior of the dams, suggesting that alterations in maternal behavior following exposure to prenatal stress could also contribute to the long-term effects (Maccari et al., 2003; Maccari and Morley-Fletcher, 2007) of this environmental stressor. In particular, the paradigm we employed consisted in restraining the dams during the last week of gestation for 45 minutes three times a day under bright light, from gestation day 14 until delivery. We next sacrificed the pups, both males and females, at different postnatal time points, in order to create a time profile of the modifications under investigation. First, we tested the cognitive functionality of adult animals with the object recognition test, since cognitive disabilities are one of the common symptoms that characterize different psychiatric conditions (Disner et al., 2011; Lapiz-Bluhm et al., 2008; Lesh et al., 2011; Lewis et al., 2012). Next, we performed a detailed analysis of two candidate systems whose deterioration could contribute to the development of the diseased phenotype, namely the glucocorticoid receptor (GR) and the neurotrophin brain-derived neurotrophic factor (BDNF), through basal and functional analyses at gene and protein levels. These two systems have emerged as the most vulnerable elements of exposure to stress during development and can be considered markers of the dysfunctions associated with psychiatric disorders. The HPA axis is involved in the response to stressful events (Maccari et al., 2003), whereas neuronal plasticity represents an array of mechanisms involved in the adaptive capacity to environmental changes (Calabrese et al., 2009; Duman and Monteggia, 2006). We performed the analyses at various stages of development, trying to establish how early the molecular alterations become manifest and their persistence in time. Notably this bears the possibility to evaluate the potential of early pharmacological interventions that may prove effective in preventing the molecular and functional alterations set in motion by prenatal stress exposure, leading to long-term beneficial effects on the brain function. Several animal models and human studies suggest that the effect of exposure to stress early in life on lifelong phenotypes is mediated by epigenetic regulation of gene expression involving changes in DNA methylation (McGowan and Szyf, 2010; Weaver, 2007). A further aspect of this experimental work was thus to determine the methylome profile of the hippocampus and the prefrontal cortex of adult rats exposed to prenatal stress. In order to do this, we combined methylated DNA immunoprecipitation (MeDIP) followed by the hybridization on a custom designed high-density oligonucleotide arrays, in order to identify, with an unbiased approach, the genes that are persistently affected by gestational exposure to stress at expression level through changes in the methylation of their promoters. Last, we aimed at identifying novel candidate markers in a translational approach, by comparing the methylome results obtained in the rat model with a non-human primate model based on different rearing condition, and with a human model of maternal adversities. The identification of genes that show a similar response to early adversities in the brain and in peripheral tissues, in three different species and across the lifespan, is critical for the development of novel diagnostic tools and for therapeutic interventions.