Memory Activates Epigenetic Changes in Mice Brain Cells
In a 2015 article in Nature Neuroscience, Stefan Bonn and André Fischer reported that when mice were prompted to use their long-term memory to recognize a specific environment, epigenetic changes occurred in their neurons and glia. Epigenetic changes refer to chemical alterations in DNA or histones (which give DNA structure) that increase or decrease the expression of certain genes. Sometimes environmental factors lead to a methyl or acetyl group joining a strand of DNA or histones, changing how easily the genes are turned on or off.
When the mice used their long-term memory, the main change that occurred was DNA methylation in their neurons. There were also changes to histones that were linked to memory acquisition but resulted in few changes in gene expression. The DNA methylation changes, on the other hand, changed neural pathways, leading to “rewiring” of the brain.
Link Between Childhood Trauma and Difficult Course of Bipolar Disorder Clarified
A collaboration between Norwegian and French researchers led by Bruno Etain has clarified the pathway by which childhood trauma is linked to worse outcomes among people with bipolar disorder. The researchers, who presented their work in a poster at the 2015 meeting of the Society of Biological Psychiatry, replicated earlier findings by this editor (Robert Post) that patients who experienced trauma as a child had a more adverse course of bipolar disorder. Etain and colleagues found a link between childhood trauma and an earlier age of onset of bipolar disorder, rapid cycling, suicide attempts, and cannabis misuse.
The researchers identified more than 550 patients with bipolar disorder, who answered questionnaires about their history of bipolar disorder and childhood trauma. Their DNA was also analyzed, and the researchers found that the effect of childhood trauma on age of onset was mediated by the presence of common genetic variants in proteins related to stress (the serotonin transporter) and immune function (Toll-like receptors). They also found that the traits of mood lability (or moodiness) and impulsivity mediated the effects of trauma on clinical outcomes.
The lasting epigenetic effects of child maltreatment and adversity noted in the above abstract are consistent with a large literature showing more epigenetic effects in these individuals than in controls. While genetics are important, the impact of the environment is also substantial.
Maternal Infection in Mice Leads to Three Generations of Behavioral Changes
Epigenetics is the process by which environmental factors affect the way a person’s genes are transcribed. These changes, which may include the addition or subtraction of methyl groups from DNA, change the DNA’s structure (how tightly it is wound around the histones that give it shape) but not its sequence. These structural changes, which affect how easily the DNA is transcribed, can then be passed on to future generations. A new study by Ulrike Stadlbauer and colleagues presented at the Society of Biological Psychiatry explored a particular pathway by which an infection in a pregnant mouse can lead to behavioral changes in three following generations of mice.
Pregnant mice were given injections that produced an infection. A first generation of offspring were interbred to create a second generation of offspring, and these were interbred to create a third generation of offspring. The first generation of offspring had epigenetic changes in methylation and hydroxymethylation to promoter regions of two enzymes that regulate synthesis of the neurotransmitter GABA, and these epigenetic changes were associated with reduced mRNA expression of these two genes.
All three generations of offspring had deficits in social interaction, short-term memory, and cued fear conditioning. Interestingly, the second and third offspring generations also exhibited depression-like behavior that had not been present in the original mothers or the first generation of offspring.
Editor’s Note: This is another fascinating demonstration of how environmental occurrences, which can include stressors, exposure to drugs, and now immune challenges, can have effects across generations, likely through epigenetic changes that persist in ova or sperm. Amazingly, it turns out that the environment can change traits in future generations, not by inducing changes to gene sequences, but through epigenetic changes to the structure of DNA or histones that persist across generations.
Early Experiences Have Lasting Effects on DNA
It is well established that certain early experiences can affect a person’s risk of developing a mental illness. Adversity in childhood, including abuse or the loss of a parent, is a risk factor not only for diagnosis of a mood disorder, but also for a more difficult course of illness. This may occur through epigenetic means. Epigenetics refers to a process by which environmental factors can change the way that DNA is transcribed, for example through the addition of methyl groups to strands of DNA. This tends to inhibit DNA from being transcribed and producing protein growth factors and other neurochemicals that are important for development.
A study by Kieran J. O’Donnell and colleagues presented at the 2015 meeting of the Society of Biological Psychiatry investigated whether epigenetics play a role in the success of a parenting intervention called the Nurse Family Partnership. Participants were 27-year-olds born to women who had received the intervention or a control intervention. Genome-wide DNA methylation was measured in the 188 participants’ blood.
Analysis of the blood revealed that the Nurse Family Partnership intervention was associated with DNA methylation at 1015 sites across 593 genes. Some of these sites were enriched for certain neurodevelopmental processes. Maltreatment in childhood was also associated with methylation at 1552 sites across 878 genes.
Editor’s Note: The take-home message of this landmark study is that maltreatment in childhood exerts lasting effects on the genome via epigenetic mechanisms, but early positive intervention also exerts lasting epigenetic effects, which likely have a normalizing impact.
Childhood Adversity, Gene Methylation, and Internalizing Behaviors Linked
Life experiences such as adversity in childhood have been linked to epigenetic changes to DNA. These changes do not affect the sequence of DNA, but can change how tightly DNA is wound, and thus how easily it is transcribed. One epigenetic change that can occur following adversity in childhood is methylation of the gene for the glucocorticoid receptor (NR3C1). A recent study by Kathryn Ridout and colleagues examined links between early adversity, methylation of this gene, and behavioral problems in childhood. Adversity was linked to methylation of the gene at exons 1D and 1F in the promoter of NR3C1. Methylation of the gene was associated with internalizing behaviors (e.g. depression, anxiety) but not externalizing behaviors (e.g. attention deficit hyperactivity disorder (ADHD) or oppositional defiant disorder) in children of preschool age. The NR3C1 methylation was a significant mediator of the internalizing behaviors in children who had experienced adversity.
Editor’s Note: Similar associations of methylation of the glucocorticoid receptor with childhood adversity have been reported in other clinical and animal studies and provide a mechanism for the long-lasting adverse effects of stressors in childhood.
Heart Attacks, Surgery Lead to Memory Impairment in Mice
Events like surgery or heart attacks that cause inflammation can lead to cognitive deficits or depression for months or years afterward, even though the direct effects of inflammation wear off within weeks. In a recent study, Natalie Tronson and colleagues subjected mice to surgical heart attack, sham surgery, or no operation, and observed how well they absorbed new learning eight weeks later.
Both male and female mice had impairments in fear learning following surgical heart attacks. Female mice that received sham surgery also showed deficits in fear learning. When the researchers dissected the mice, analyzing their blood and hippocampi after the eight-week period, inflammatory cytokine measures had normalized as expected, but the researchers found other abnormalities.
Intracellular signaling was dysregulated, and there had been epigenetic changes in cells of the hippocampus. (Epigenetic changes refer to those that change the structure of DNA, such as how tightly it is wound, rather than its sequence. For example, the addition of acetyl groups to DNA or the histones around which it is wound.) The researchers observed increased histone acetylation and phospho-acetylation following the heart attacks.
The researchers concluded that a systemic inflammatory event, such as heart attack or surgery, can cause long-term memory impairment and changes in mood through epigenetic mechanisms. They compared the findings to those of other studies in which normal aging and memory-impairing treatments such as chemotherapy had also been associated with increases in histone acetylation or decreases in histone deacetylase activity.
Maternal Childhood Adversity Associated with Low Infant Birth Weight
In a study of the effect on infant health of a mother’s experience of adversity in childhood by researcher Deborah Kim and colleagues, both adversity in childhood (such as physical abuse or the loss of a parent) and stress during pregnancy were associated with low infant birth weight and lower gestational age at birth.
Among 146 women enrolled in the study, 58.2% percent scored a 0 on the Adverse Childhood Experience Questionnaire (ACE), 24% scored a 1, and 17.8% scored a 2. Those who scored higher on the ACE also scored higher on a scale measuring perceived stress. A score of 2 or higher on the ACE was associated with lower gestational age at birth, indicating infants born prematurely. Greater stress during pregnancy was associated with lower gestational age at birth and lower infant birth weight. When potential confounding demographic factors were removed from the analyses, ACE scores of 2 or higher were still associated with lower infant birth weight, while perceived stress was no longer associated with either low birth weight or gestational age.
Childhood adversity is associated with increases in inflammation and multiple adverse medical consequences in adults. The researchers called childhood adversity a “significant predictor of poor delivery outcomes” for women.
Editor’s Note: This research shows that a mother’s health and earlier life stressors could have an adverse effect on her child.
Childhood adversity leaves behind a residue of neuroendocrine and neuroclinical alterations that can persist into adulthood. Many are mediated by epigenetic changes, consisting of small chemical marks that attach to DNA and the histones around which it is wrapped.
In addition to these neurobiological alterations mediated by epigenetic effects, there is new evidence that some epigenetic marks can be passed on to the next generation via a mother’s egg or a father’s sperm. Thus, either directly or indirectly, parents’ adverse life experiences can influence the health of their offspring.
Transgenerational Transmission of Drug Exposure and Stress in Rodents
New data suggest that there can be transgenerational transmission of the effects of drug exposure and stress from a paternal rat to its offspring. The father mates with a female who was not exposed to drugs or stress and never has any contact with the offspring. Consensus is now building that this transmission occurs via epigenetic alterations in sperm.
Epigenetic alterations are those that are mediated by chemical changes in the structure of DNA and of the histones around which DNA is wrapped. These changes do not alter the inherited gene sequences but only alter how easy it is for genes encoded in the DNA to be activated (transcribed) or suppressed (inhibited).
There are three common types of epigenetic modifications. One involves the attachment of a methyl or acetyl group to the N-terminals of histones. Methylation typically inhibits transcription while acetylation activates transcription. Histones can also be altered by the addition of other compounds. The second major type of epigenetic change is when the DNA itself is methylated. This usually results in inhibition of the transcription of genes in that area. The third epigenetic mechanism is when microRNA (miRNA) binds to active RNA and changes the degree to which proteins are synthesized.
At a recent scientific meeting, researchers described the various ways epigenetic changes can be passed on to future generations.
Researcher Chris Pierce reported that chronic cocaine administration increased brain-derived neurotrophic factor (BDNF) in the medial prefrontal cortex of rats. (BDNF is important for learning and memory.) The cocaine administration led to acetylation of the promoter for BDNF.
This exposure to cocaine in male rats who then fathered offspring led to two changes in the offspring, presumably conveyed by epigenetic changes to the fathers’ sperm. The first change was a decrease in cocaine reinforcement. The offspring took longer to acquire a cocaine self-administration habit. The second change was long-lasting learning deficits in the male offspring, specifically recognition of novel objects. The deficit was associated with a reduction in long-term potentiation in the offspring. Long-term potentiation is the strengthening of synapses that occurs through repeated patterns of activity. Surprisingly, the following generation also showed deficits in learning and memory, but did not show a loss of long-term potentiation.
Editor’s Note: These data indicate that alterations in sensitivity to cocaine (in this case slower acquisition of cocaine self-administration) can be transferred to a later generation, as can learning deficits in males. These data suggest that fathers’ experience of drugs can influence cocaine responsiveness and learning via epigenetic mechanisms likely mediated via epigenetic changes to the father’s sperm.
This research suggests the possibility that, in a human clinical situation, there would be three ways that a father’s drug abuse could affect his child’s DNA. First, there is the traditional genetic inheritance, where, for example, an increased risk for drug abuse is passed on to the child via the father’s genetic code. Next, drug abuse brings about epigenetic changes to the father’s sperm. (His genetic code remains the same, but acetyl groups attach to the BDNF promoter section of his DNA, changing how those proteins get produced.) Lastly, if the father’s drug abuse added stress to the family environment, this stress could have epigenetic effects on the child’s DNA.
Researcher Alison Rodgers described how epigenetic changes involving miRNA in paternal rats influence endocrine responsivity to stress in their offspring. Rodgers put rats under stress and observed a decrease in hormonal corticosterone response to stress. When a father rat was stressed, nine different miRNAs were altered in its sperm. To prove that this stress response could be passed on transgenerationally via miRNAs, the researchers took sperm from an unstressed father, loaded it with one or all nine miRNAs from the stressed animal, and artificially inseminated female rats. Rodgers found that the sperm containing all nine miRNAs, but not the sperm carrying one randomly selected miRNA, resulted in offspring with a blunted corticosterone response to stress.
Researcher Eric Nestler showed that when a rodent goes through 10 days of defeat stress (being defeated repeatedly by a larger animal), they begin to exhibit behaviors resembling those seen in depression. Social avoidance was the most robust change, and continued for the rest of the animal’s life. Animals did not have to be physically attacked by the bigger animal to show the depression-like effects of defeat stress. Just witnessing the repeated defeats of another rat was sufficient to produce the syndrome. Again, father rats that experienced defeat stress or witnessed it passed this susceptibility to defeat stress on to their offspring (with whom they never had any contact), likely by epigenetic changes to sperm. Read more
Childhood adversity, epigenetics, and hippocampal volume
At the 2014 meeting of the International College of Neuropsychopharmacology, researcher Booij reported that in humans, there is an interaction between adversity experienced during childhood, and an epigenetic variation in the short form of the serotonin transporter (5HT-T ss, or SLC6A4), which can influence hippocampal volume during depression.
Epigenetics refers to environmental influences on the way genes are transcribed. The impact of life experiences such as stress is not registered in DNA sequences, but can influence the structure of DNA or tightness of its packaging. Early life experiences, particularly psychosocial stress, can lead to the accumulation of methyl groups on DNA (a process called methylation), which generally constricts DNA’s ability to start transcription (turning on) of genes and the synthesis of the proteins the genes encode. DNA is tightly wound around proteins called histones, which can also be methylated or acetylated based on events in the environment. When histones are acetylated, meaning that acetyl groups are attached to them, DNA is wound around them more loosely, facilitating gene transcription (i.e. the reading out of the DNA code into messenger RNA, which then arranges amino acids in order to construct proteins). Conversely, histone methylation usually tightens the winding of DNA and represses transcription.
Booij followed 33 children who had experienced some form of adversity at a young age until they were 15 or 16, examining methylation of the serotonin transporter in their T cells and monocytes compared to 36 children who had not experienced adversity during childhood. He found that in children who had experienced abuse in childhood, the degree of that abuse was correlated with methylation of the serotonin transporter and was inversely related to the volume of the hippocampus, as measured using magnetic resonance imaging (MRI). Thus, child abuse yields lasting epigenetic effects (methylation of the serotonin transporter) and has anatomical consequences in teenagers, as seen in smaller hippocampi. These data parallel converse findings by Joan Luby et al. published in the journal PNAS in 2012, in which increased maternal warmth directed toward a child aged 4-7 was associated with increased volume of the hippocampus several years later.
Digestion of Wheat and Milk Releases Peptides that Might Cause Inflammation
Some people have found that gluten-free or casein-free diets have improved their intestinal, autoimmune, or neurological symptoms. (Casein is a protein found in mammals’ milk. Cow milk is high in casein while human milk proteins are 20–45% casein.) One explanation for the good effects of these diets is that peptides that are released during digestion of these foods can create epigenetic changes in gene expression, adding methyl groups to DNA strands that increase inflammation.
As infants transition from getting all of their nutrition from the placenta to using their gastrointestinal tract, their diet may lead to epigenetic modifications that affect their health later in life. Epigenetics refers to changes in genes that do not affect the inherited sequence of DNA, but affect how easily the DNA is transcribed to produce proteins. Methyl or acetyl groups can be added to DNA or the histones around which it is wound.
When a person digests casein (from either human or animal milk) or gliaden (a protein derived from wheat), peptides are released that activate opioid receptors, modulating the uptake of the amino acid cysteine in neurons and in the gastrointestinal tract. This decrease in cysteine uptake is associated with drop in the antioxidant glutathione and a methyl donor (a molecule with a reactive methyl group that can easily become part of another molecule) called S-Adenosyl methionine.
In addition to decreasing cysteine uptake, the peptides also increase DNA methylation and create epigenetic changes in genes involved in redox (changes in oxidation) and methylation homeostasis.
These processes are described in a 2014 article by Malav S.Trivedi et al. in the Journal of Nutritional Biology. Trivedi et al. conclude that milk and wheat can change antioxidant activity and gene expression. Differences in the peptides in human and cow milk may explain developmental differences between children who are breastfed and those who receive formula.
The decrease in antioxidants caused by peptides from wheat and milk can predispose people to inflammation and oxidation, explaining why wheat- or casein-free diets might be useful.
