Paternal stress prior to conception alters DNA methylation and behaviour of developing rat offspring
Introduction
Although a relatively new field of investigation, researchers are beginning to examine indirect environmental influences capable of altering neurodevelopmental outcomes. As we have extended the macro-environment of the child to include fathers, paternal influences, as they pertain to maternal behaviour/depression and co-parenting expectations (Field et al., 2006, Outscharoff et al., 2006, Paulson and Bazemore, 2010, Seidel et al., 2011, Mashoodh et al., 2012), have come under scrutiny. Despite this expansion however, the underlying theory has not really changed; mothers are still considered the primary influencer of child neurodevelopment, with fathers influencing offspring only through modification of maternal characteristics. Hence, there has been limited examination of experiences whereby fathers directly influence the neurodevelopment of offspring. Despite emerging evidence that paternal age, nutrition, and drug use, may place offspring at risk of psychopathologies (Kaat et al., 2007, Curley et al., 2010), very few studies have been designed to tease out the mechanisms responsible for these associations. When examining the role of the father from a genetic perspective, the majority of this research has focused on possible paternal contributions to foetal alcohol syndrome (Randall et al., 1982, Abel, 1993, Abel, 2004) or genetic imprinting, with very limited analysis of epigenetic reprogramming of the sperm prior to conception (Franklin et al., 2010, Miller et al., 2010, Mychasiuk et al., 2012).
Owing to the continuous nature of spermatogenesis, experiences that change DNA methylation patterns in sperm before fertilization have the potential to alter epigenetic programming of future offspring. DNA methylation is used by spermatozoa and is uniquely regulated to play an important role in the development of future gametes and embryos (Jenkins and Carrell, 2011). Robust paternal epigenetic contribution to embryogenesis requires that the DNA in spermatozoa contain layers of regulatory elements, including methyl groups that drive gene activation or silencing upon contact with the egg. Although critical to normal development, this abundance of regulatory control also leaves the DNA susceptible to damage from outside agents (Jenkins and Carrell, 2011). Damaging outside agents are commonly believed to be chemical toxins or drugs of abuse, however, prior research has demonstrated that stress impairs spermatogenesis in adult rats (Potemina, 2008), and alters methylation patterns in the germline of F2 male mice (Franklin et al., 2010).
The purpose of this study was to examine the effects of paternal stress prior to copulation on behaviour and the epigenome of developing rat offspring. There is overwhelming literature outlining the acute and long-term effects of maternal stress on offspring development (e.g., Champagne and Meaney, 2006, Kapoor et al., 2006, Mychasiuk et al., 2011, Muhammad et al., 2012) but little examination of how paternal stress influences the same outcomes. In effort to begin investigation of the direct link between paternal experience and offspring sequel, we stressed male rats prior to conception and examined behaviour and methylation patterns in the brain of young offspring. Early behavioural testing included the open field paradigm to measure exploratory behaviour and the negative geotaxis paradigm to examine sensorimotor development. The frontal cortex and hippocampus were the two brain areas chosen for DNA methylation analysis. The frontal cortex receives input from all other cortical regions and is intricately involved in processes such as executive functioning, socio-emotional regulation, and psychopathologies, whereas the hippocampus is highly susceptible to the effects of stress and is imperative for learning and memory. As we were interested in detecting effects in the developing brain and wanted to avoid confounding interactions from pubertal hormones, DNA methylation analysis was conducted on brain tissue derived from offspring on postnatal day 21 (P21).
Section snippets
Subjects and paternal stress procedure
All experiments were carried out in accordance with the Canadian Council of Animal Care and were approved by the University of Lethbridge Animal Care Committee. All of the animals in this experiment were maintained on a 12:12-h light:dark cycle in a temperature controlled (21 °C) breeding room and were given access to food and water ad libitum. All rats were bred in the facility for multiple generations. Ten female Long-Evans rats were mated with 10 different male Long-Evans rats (four control
Parental and litter characteristics
The elevated platform stress paradigm has been demonstrated in the past to induce a significant stress response in rats that does not habituate over time (Wong et al., 2007). The male rats in this study (∼P180 at the time of stressing) also displayed overt signs of substantial stress such as significant hair loss and attenuated weight gain. All male rats were sexually naïve. Sexual behaviours were observed for the first 2.5 h to ensure mating did occur, and although stressed males were
Discussion
Although there is an abundance of literature examining the effects of prenatal maternal stress, to our knowledge this is the first study designed to examine the effects of chronic paternal stress immediately prior to conception. As evolutionary success is not based upon longevity, but rather an individual’s ability to reach reproductive age, very different processes and objectives would be expected to drive epigenetic adaptation in the face of adversity for males and females (Meaney et al., 2007
Conclusion
There is a complex interaction between maternal, paternal, and environmental influences that combine to produce the offspring phenotype that is observed. Although we have not established a concrete mechanism whereby chronic paternal stress alters the epigenome of developing offspring, we hypothesize that chronic stress modifies epigenetic expression of genes in maturing spermatozoa which in turn impacts gene expression and development in the embryo. We acknowledge that we are unable to
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