Kriti Babu ’18 and Shruti Arora ’19


From work to school, social media to politics, there seem to be a growing number of stressors each year. For many of us stress is an everyday part of life while for others stress can be debilitating. According to the American Psychological Association, the stress level in America is increasing and along with it, depression, anxiety, and other mental health issues. Many people don’t know that stress is an evolutionary adaptation to let animals know when they are in serious danger of death. However, when your term paper is due just a day after your chemistry final and you are experiencing a full-blown stress response, you are not in danger of dying. So what is going on here? Scientists have been looking at new mechanisms for why some people cannot control their stress response, so much so that it can lead to other diseases. To do that, they have been looking at your genes.

Your genes are what make you unique. You may have heard of the genetic code, and how it is acts as a blueprint for your body’s cells. But let’s break it down further: Say your genetic code or DNA is the blueprint for building a house. But just as each builder has his/her own way of interpreting the blueprint and implementing his/her style into the house, your genetic code is not read the same way as everyone else’s. In our case, the house can be thought of as gene expression.

A relatively new field of study termed epigenetics is the exploration of modifications to your DNA that don’t involve changing the actual genetic code. Even though the genetic code remains the same, behaviors can be different. Epigenetic modifications are like post it notes on your blueprint or DNA. For example, if I was to place post it notes on the house’s blueprint some parts will be blocked out, and the house will be missing a sink. The house is still a functioning house, it’s just a little different from other houses with different sets of instructions. The genetic code or blueprint is still underneath the post its, and there could be potential for the post-it notes to be removed, but there is also potential for those blocks or post-its to be inherited, or passed on to children from parents. Post-its or over certain regions of DNA that encode particular genes will then lead to silencing of those genes. Silencing means that those genes are not expressed, or they are expressed to a much lesser extent. This can ultimately predispose you and your future generations to diseases.

One type of epigenetic alteration or post it is DNA methylation and it occurs when a methyl group is added onto the DNA. Methyl groups can come from diet, chemicals in your environment, or can be made in your body! DNA methylation can be activating, like turning a gene on, or repressing, like turning a gene off. But, for this discussion, it will be simpler to just consider repressive DNA methylation.

Scientists have found that epigenetic post-it notes can make your body less able to properly cope with stress. Interestingly, there have been many studies that have shown that environment plays a role in how much epigenetic alteration is taking place, providing a biological link between genes and environment. Your environment can be the parental care you receive, or perhaps the chemicals your mother was exposed to while she was pregnant. So does the environment of grandparents and parents leave a fingerprint on later generations? Here we look at how environmentally based epigenetic changes or post its can be passed onto future generations and how this can impact an individual’s response to stressors. We will discuss how the stress response is affected after experiencing two types of negative environment: exposure to chemicals and maternal abuse.

Scientists use mouse models to understand the relationship between environment and behavior. These models are useful in understanding other mammals, like humans. One environmental condition scientists have looked at is exposure to chemicals during pregnancy and its effect on stress. The researchers looked at mice whose “great grandparents” had been treated with an endocrine disruptor called vinclozolin that interferes with hormone signaling. Endocrine disruptors negatively impact hormone signaling and can have many harmful effects on pregnant women and their unborn offspring. One of the effects of an endocrine disrupter is epigenetic: it causes those post-it notes to be placed on the DNA. The researchers reasoned that treating the pregnant mouse causes changes in the unborn offspring. These changes are then passed down to later generations, because the effects that occur on the unborn mouse also occur in the unborn mouse’s developing sex cells, like sperm or egg cells. That means that the the lineage of the toxin-exposed pregnant female could experience negative effects. The below figure might help. The F0 (the first generation) represents the mice that were treated with an endocrine disrupting chemical, vinclozolin, and the F3 (the fourth generation) are the mice that were studied, particularly for anxiety behaviors and response to stress.

But what changes did vinclozolin cause? The researchers found that the disrupter impacted how the mice experienced stress. The researchers found that F3 mice whose lineage had been treated with vinclozolin showed increased anxiety based on behavioral experiments that test for these symptoms. To see if the vinclozolin treated lineage had an impacted stress response, researchers used a model called chronic restraint stress, which exposes the mice to tight spaces for a number of hours each day; think claustrophobia. Just as humans can get stressed out in small spaces, so can mice.

The researchers used F3 mice whose “great grandparents,” the F0, had been treated with vinclozolin and purposefully stressed them out with chronic restraint stress. They compared their results from these F3 mice with F3 mice who were also restrained, but whose lineage had never been treated with a toxic chemical. They found that the vinclozolin F3 mice showed more stress related behaviors, like huddling, and fewer social behaviors. They also found a 10% reduction in brain activity, especially in regions involving learning, memory and emotional control2. Finally, they found that F3 mice whose lineages had been treated with vinclozolin had altered gene expression for many genes associated with disease, but namely ones associated with Huntington disease, Alzheimer’s disease, and Parkinson’s disease2. It seems as though those post-it notes created from the vinclozolin are long lasting across generations and can have some pretty serious effects. Vinclozolin is used in agriculture, but since more research has come out about its effects, its use has declined. However, vinclozolin use is not banned in the United States and mothers can still be exposed. Women who are pregnant are often told to avoid certain foods, medicines etc. but these studies highlights the importance of that avoidance and could serve to educate the public on the potential health risks.

Additional experiments have been done using the same methods described above, but using different chemicals other than vinclozolin. One such chemical, called diethylstilbestrol (DES), like vinclozolin, is an endocrine inhibitor. From the early 1900s to about 1971, DES was used to prevent miscarriages in high risk pregnancies4. This was put to a stop when the daughters of mothers treated with DES were found to have reproductive organ issues, reduced fertility, abnormal pregnancy, immune system disorders, and cancers of reproductive organs. The sons of mothers treated with DES also had reproductive issues such as low fertility. Researchers used chronic restraint stress to model DES effects in mice to fully understand the symptoms in humans. They found that the offspring of DES treated mothers had similar symptoms of reproductive dysfunction as seen in humans and certain genes expressed by the reproductive system were impacted. Finally, they found that these changes persisted in the grandchildren of the treated mice! Based on these findings, scientists predict that epigenetic methylation of DNA, or adding post-it notes, is what causes these inheritable changes in gene expression, leading to individuals with varying responses to stress4. With all this evidence, transgenerational inheritance of environmental epigenetic effects seems increasingly likely to occur in humans. While we always have to tread carefully when interpreting mouse models, these studies showed that health issues related to chemical exposure can persist across generations, corroborating findings observed in humans. This led to changes in public policy regarding the use and distribution of certain chemicals. DES is no longer manufactured or marketed. These studies not only helped to remove toxins from clinical practice, but could also serve to promote discovery of safer and more effective drug targets to deal with stress and anxiety.

So far, we know that certain environmental exposures can cause development of behavioral and emotional disorders and these disorders not only persist through adulthood but can be transmitted to later generations. However, this is not the only source of negative environmental influence. One source of trauma many humans experience is lack of parental care early in life, particularly lack of maternal care. To see if maternal separation can cause transgenerational transmission of behavioral disorders researchers subjected mice pups to unpredictable maternal separation and unpredictable maternal stress (MSUS). Researchers found that mice separated from their mothers and subjected to stressful situations during separation showed an increase in depression-like symptoms and decreased ability to avoid stressful situations. Investigators found that the offspring of these “abandoned” mice exhibited similar depressive symptoms and irregular stress responses as their parents3. The symptoms seen in the behavioral tests were reversed when the mice of both generations were given antidepressants, which further suggests that these symptoms were in fact signs of depression, and could be treated as such.

In addition to behavioral differences, researchers found that the DNA of the abandoned mice and their offspring was also affected! Researchers found that maternal stress and separation affected DNA methylation of both the abandoned mice and their offspring3. Remember that these methylation “post-its” can silence genes. In this case, researchers observed silencing of a hormone receptor gene that plays a vital role in the body’s stress response. Receptors act as a lock on the door that leads into your cells. Receptors receive signals from outside the cell – these signals act as the key and when they connect with the receptor lock, a door is opened and it causes a response within the cell. In abandoned mice, fewer stress response-related receptors were expressed meaning the body could not respond to stress in a normal way. Other important receptors were affected as well, such as the serotonin receptor. Serotonin is an important brain chemical. It is believed to regulate mood and social behavior and has been linked to depression in previous studies. Researchers found that F1 mice that had experienced abandonment and their F2 offspring had fewer serotonin receptors. Since serotonin regulates mood, these mice displayed increased depressive behavior after undergoing stress5. Together these studies show that the combination of maternal abandonment in early life and stress later in life can predispose individuals to depression and an inability to cope with stress. Not only that, these changes can be carried over to the next generation.

We are a product of our experiences. But apparently, also a product of our parents’ and grandparents’ experiences! However, we can’t blame our parents for all our stress related problems just yet. Currently, the field has not come to a conclusive answer on whether effects of environment can be transmitted across generations. Further research into transgenerational epigenetic inheritance and its underlying molecular mechanisms could elucidate the relationship between environment and genetics and promote better health care, such as better medications to cope with depression and stress. The two models of environmental stress discussed here feature neuroendocrine stress and unpredictable maternal separation. Both models result in epigenetic changes to the genome and have lasting effects across generations. In all the studies, the scientists concluded that methylation “post-its” are likely what has contributed to the difference in behaviors and gene expression. Many times, the two types of trauma even affect the mice models in similar ways. For example, in one study researchers found brain changes in mice from a lineage that had been treated with endocrine toxins. Changes in the same brain areas were seen in the maternal abandonment mouse model. Both these models also showed transgenerational changes. Transgenerational epigenetic inheritance also has many social implications. Often times children in low socioeconomic settings whose mothers are constantly working may experience maternal abandonment. One mother unpredictably abandoning her child may not only impact the stress response of her child but also her grandchildren. It is also important to understand the effects of chemicals on pregnancy and future generations. Understanding more about these systems of gene expression can only lead to a healthier society!

Fig. 1 shows how epigenetic effects can be transmitted to a fetus before it is born and after based on experience of the mother7.

References

  1. Chiba, Shuichi, et al. “Chronic Restraint Stress Causes Anxiety- and Depression-like Behaviors, Downregulates Glucocorticoid Receptor Expression, and Attenuates Glutamate Release Induced by Brain-Derived Neurotrophic Factor in the Prefrontal Cortex.” Progress in Neuro-Psychopharmacology and Biological Psychiatry, vol. 39, no. 1, 2012, pp. 112–119., doi:10.1016/j.pnpbp.2012.05.018.
  2. Crews, D., et al. “Epigenetic Transgenerational Inheritance of Altered Stress Responses.”Proceedings of the National Academy of Sciences, vol. 109, no. 23, 2012, pp. 9143–9148., doi:10.1073/pnas.1118514109.
  3. Franklin, Tamara B., et al. “Epigenetic Transmission of the Impact of Early Stress Across Generations.” Biological Psychiatry, vol. 68, no. 5, 2010, pp. 408–415., doi:10.1016/j.biopsych.2010.05.036.
  4. Newbold, Retha R., et al. “Adverse Effects of the Model Environmental Estrogen Diethylstilbestrol Are Transmitted to Subsequent Generations.” Endocrinology, vol. 147, no. 6, 2006, doi:10.1210/en.2005-1164.
  5. Razoux, F, et al. “Transgenerational Disruption of Functional 5-HT1AR-Induced Connectivity in the Adult Mouse Brain by Traumatic Stress in Early Life.” Molecular Psychiatry, vol. 22, no. 4, 2016, pp. 519–526., doi:10.1038/mp.2016.146.
  6. Skinner, Michael K., et al. “Transgenerational Epigenetic Programming of the Brain Transcriptome and Anxiety Behavior.” PLoS ONE, vol. 3, no. 11, 2008, doi:10.1371/journal.pone.0003745.
  7. Sweatt, J. David, et al. Epigenetic Regulation in the Nervous System: Basic Mechanisms and Clinical Impact. Academic Press, 2013.