Nature, nurture and epigenetics

Abstract

Real life by definition combines heritability (e.g., the legacy of exposures) and experience (e.g. stress during sensitive or 'critical' periods), but how to study or even model this interaction has proven difficult. The hoary concept of evaluating traits according to nature versus nurture continues to persist despite repeated demonstrations that it retards, rather than advances, our understanding of biological processes. Behavioral genetics has proven the obvious, that genes influence behavior and, vice versa, that behavior influences genes. The concept of Genes X Environment (G X E) and its modern variants was viewed as an improvement on nature-nurture but has proven that, except in rare instances, it is not possible to fractionate phenotypes into these constituent elements. The entanglement inherent in terms such as nature-nurture or G X E is a Gordian knot that cannot be dissected or even split. Given that the world today is not what it was less than a century ago, yet the arbitrator (differential survival and reproduction) has stayed constant, de novo principles and practices are needed to better predict what the future holds. Put simply, the transformation that is now occurring within and between individuals as a product of global endocrine disruption is quite independent of what has been regarded as evolution by selection. This new perspective should focus on how epigenetic modifications might revise approaches to understand how the phenotype and, in particular its components, is shaped. In this review we summarize the literature in this developing area, focusing on our research on the fungicide vinclozolin.

Keywords: Adolescence; Epigenetic; Stress; Synchronicity; Transgenerational; Vinclozolin.

Figure 1

The “two-hit 3 generations apart” model for examining the effects of germline-dependent and context-dependent epigenetic modifications. Inherited (germline-dependent epigenetic modifications) and experienced (context-dependent epigenetic modifications) challenges act singly and in combination to create new phenotypes. Illustrated are the experimental design and the important comparisons. These are referred to as First (comparison 1 & 3), Second (comparison 2 & 4), and Third Order (comparison 5) Effects. Main effects (First Order) would be the result of ancestral exposure to vinclozolin or the result of chronic restraint stress (CRS) during adolescence. Interaction effects (Second Order) refer to those observed when comparing one variable in the context of the other as well as the relative contributions of ancestral exposure to vinclozolin and CRS experienced during adolescence (V X S); the latter is commonly the statistical interaction designated by analysis of variance. The Third Order effect is the combined effects of important heritable and experienced phenomena that are not causally connected, but may co-occur, resulting in an altered phenotype that cannot be attributed to either the heritable component or the experienced component. Animals experiencing the combined effects of these two events separated by 3 generations, when compared to the control, stress condition animals, reveal the altered phenotype that cannot be attributed to either of the two-hits alone and is not evident in the V X S interaction.

Figure 2

Exposure to Vinclozolin 3 generations previously changes circulating concentration of corticosterone in adult male and female rats. The effect of chronic restraint stress during adolescence (STRESS) within Control-Lineage animals is illustrated in the left column). The effect of ancestral exposure to Vinclozolin 3 generations previously (LINEAGE) is evident in the middle column.) The effect of ancestral exposure to Vinclozolin and chronic restraint stress during adolescence compared to Control-NonStress animals is in the right column. As expected, control, NonStress females (red) have significantly higher circulating concentrations of CORT relative to males (blue). Comparison of V&S to C-NS animals reveals a striking sex difference is seen. That is, ancestral exposure to vinclozolin significantly increases the effects of CRS during adolescence in the descendant females, but there is no apparent effect in males.

Figure 3

Sex differences in the pattern of expression in targeted genes in a neural network of brain nuclei involved in social, affiliative, and anxiety-related behavioral tests via Taqman low-density PCR arrays (TLDAs). Six hypothalamic and hippocampal nuclei are represented. All differences are equal to or greater than a 2-fold difference relative to control, NonStress (C-NS) male and female rats. Note that each region has unique gene expression changes, with a subset of genes identified in multiple nuclei (Avp in males; Esr1 and Pomc in females). Region abbreviations: CA3 and CA1 – areas of the hippocampus, CeAmy – central amygdaloid nucleus; BLA – basolateral amygdaloid nucleus; BnST – bed nucleus of the stria terminalis; LH – lateral hypothalamic nuclei. Gene abbreviations: Ar – androgen receptor, Avp – arginine vasopressin, Bdnf – brain-derived neurotrophic factor, Drd2 – dopamine receptor D2, Esr1 – estrogen receptor alpha, Esr2 – estrogen receptor beta, Gnrhr – gonadotropin releasing hormone, Lepr – leptin receptor, Mc4r – melanocortin 4 receptor, Negr1 – neuronal growth factor, Oxt – oxytocin prepropeptide, Pomc – proopiomelanocortin, Pgr – progesterone receptor, Ptgds – prostaglandin D2 synthase, Tgfa transforming growth factor alpha, Th – tyrosine hydroxylase. Green shaded numbers indicate up-regulation level of significance (two-tailed). Red shaded numbers indicate down-regulation and level of significance (two-tailed).

Figure 4

Changes in global methylation in a network of six brain nuclei are shown using functional landscapes. Each landscape represents the percent change in average global methylation levels for each brain nucleus. Peaks indicate higher levels in the group indicated while valleys represent higher levels in the control group (see Figure 1). Illustrated are First, Second, and Third Order effects. First Order effects (Top Row) are statistical main effects. In this instance the consequences of CRS during adolescence (top left) or ancestral exposure to vinclozolin (top right) in the descendant animals 3 generations removed. Second Order effects (Middle Row) are the interactions observed when comparing one variable in the context of the other. In this instance the effect of CRS in animals from the vinclozolin-lineage (middle left) or the effect of vinclozolin in animals that had received CRS (middle right). Finally, Third Orders effects (Bottom Row) are of two types: interaction (V X S) from analysis of variance (bottom left) or synchronicity (bottom right). Note, in the first instance represented is the percent in average methylation levels. In other words, the interaction term simply indicates whether the variables contribute significantly to the variance. The latter instance is synchronicity represents the difference between vinclozolin X Stress animals in relation to control, NonStress individuals. The combined effects of important heritable and experienced phenomena are not causally connected, but co-occur, resulting in an altered phenotype that cannot be attributed to either the heritable component or the experienced component. The nodes are equivalent to brain nuclei (shown as inserts) in clockwise fashion: lateral hypothalamus (LH), medial preoptic area (MPOA), medial amygdaloid nucleus (MeA), central amygdaloid nucleus (CeA), bed nucleus of the stria terminalis (BnST), and the ventromedial nucleus of the hypothalamus (VMH).

Figure 5

Determination of a transgenerational epigenetic imprint on mate preference behavior. The left panel shows that 3 generations separate the gestational exposure to vinclozolin. The upper right panel is a picture (under red light when testing occurred) of an experimental animal showing facial investigation of the stimulus animal). The lower right panel is a schematic of the testing apparatus for mate preference. Third generation females from the vinclozolin lineage and the Control (DMSO) lineage were tested with males from both lineages in simultaneous mate preference tests; males from the vinclozolin lineage (indicated by red-filled male symbols) and the control lineage (not shown) were similarly tested with females of both stimulus types. The experimental animal (here a female) was placed in the center of the chamber; a stimulus male from each lineage type was at each end of the apparatus. The female could move freely in their chamber but were separated from the stimulus males by a wire mesh. This enabled the animals to communicate by olfactory, pheromonal, or behavioral cues, but physical interaction was limited to touching across the wire mesh. (Modified from Anway and Skinner, 2005, and Crews et al., 2007).

Figure 6

Third-generation female rats whose progenitors were exposed to vinclozolin are epigenetically altered and prefer males from the unexposed control-lineage. Males do not show this preference. Both females and males from control (DMSO) lineage and vinclozolin lineage were tested with pairs of control- and vinclozolin-lineage stimulus partners. Average differences in the time spent in three behaviors directed to stimulus animal (Plexiglas, Facial Investigation, and Wire Mesh). Top panel: Behaviors exhibited by males from control- and vinclozolin-lineages towards females from control-lineage (positive, right side) and vinclozolin-lineage (negative, left side). Bottom Panel: Behaviors exhibited by females from control- and vinclozolin-lineages towards males from control-lineage (positive, right side) and vinclozolin-lineage (negative, left side). (Modified from Crews et al. (2007).

Figure 7

Studies integrating mate preference behaviors and, in particular facial investigation (upper left panel), brain nuclei and circuits and the patterns of gene expression and transcriptomics therein (lower left panel), lead to better understanding how the environment influences epigenetic modifications that lead to genomic, physiological and neuroendocrine changes that may influence evolutionary trajectories (right panel). (Modified from Skinner et al., 2014)