If a mom is stressed out during pregnancy, her bundle of joy may be at higher risk for developing conditions like autism, attention-deficit/hyperactivity disorder, and schizophrenia.
But what if dad is stressed?
Over the last five years, neuroscientists have studied how dad’s stress might affect his offspring. They’ve found that the spawn of a stressed-out dad actually stress less, which could be helpful to children growing up in the same adverse conditions as their fathers. But research suggests this calm-and-collected attitude is passed down through multiple generations, and sometimes, less stress isn’t always best. Blunted stress responses have been associated with psychiatric conditions like anxiety, depression and schizophrenia. Scientists are investigating whether dad’s stress could potentially hurt his grandkids and great-grandkids down the line.
The mechanism that enables dad’s cells to record and pass along this information was a mystery. That is, until neuroscientist Tracy Bale announced she may have cracked the case by finding that dad’s sperm can convey his own stressful history to his offspring.
Bale, the director of the Center for Epigenetic Research in Child Health and Brain Development at the University of Maryland, presented her unpublished research at the American Association for the Advancement of Science Annual Meeting in February 2018 in Austin, Texas. Her research indicates that sperm are vulnerable to environmental stressors at a very specific point in their maturation. During this window of time, sperm absorb packets of microRNA—chemicals that alter which genes are expressed, and therefore, which proteins are made. These microRNA signals ultimately reprogram how a child’s brain develops in utero, as well as after birth, and how it reacts to stress.
Understanding dad’s contribution to neurodevelopmental disease could help his children down the line. Bale envisions a time when preventative therapies are delivered to fathers to “normalize” the stress reactivity of their future children. She has successfully tested this approach in mice, laying the groundwork for its eventual application in humans. Bale discussed her research at AAAS with SciCom’s Nicoletta Lanese.
You’ve been doing this work with paternal stress for a number of years. What’s new now?
We’ve now changed the dogma in the field. The reproductive biologists have now accepted the fact that sperm can be vulnerable to the environment. We know that microRNAs can be changed in the sperm. If we’re ever going to translate this into humans, we have to understand the “how.”
So, it’s accepted that microRNA can be impacted by stress and change dad’s DNA over his lifetime. As you’re looking into how this happens, mechanistically, what questions do you need to answer?
How did those microRNA get there? That’s the upstream question. We need to mechanistically understand where along the reproductive tract these changes occur and how. Then there’s the downstream question: now fertilization happens, what then?
We’ve been focusing on the upstream question. Sperm come out of the testes at the end of spermatogenesis and they’re not mature—they can’t fertilize and they can’t swim. They’re pushed into the head of the epididymis, and the tubules in there are lined with cells called epididymal epithelial cells. Those cells are secreting factors, packaged in vesicles, that are essential for the sperm to mature. This period of time is essential, because then the sperm are stored in another part of the epididymis to mature until ejaculation.
We’ve identified these epididymal epithelial cells as the somatic tissue that’s responding to the environment and changing the content of vesicles entering the sperm—that’s what’s new.
You mentioned earlier that these vesicles are produced by every tissue in the body, including mom’s eggs. How can you be sure the vesicles secreted in the epididymis are the key in this process?
We can culture epididymal epithelial cells from mice and collect vesicles from that pure medium. Basically, we’ve created “stress in a dish.” We can expose those cells to glucocorticoids [stress hormones] at a similar physiological stress level as we see in the mouse, and then take the vesicles we’re seeing and sequence them.
The change is not immediate in the mouse—it actually takes a number of days, and we see the same thing in the culture dish. During the glucocorticoid treatment, we don’t see these changes. A week after the treatment, we see the same changes in the dish that we see in the mouse. There’s something about the stress effects and their removal that programs these epididymal epithelial cells. The changes that the stress imparts allow different microRNA to continue to be made after the stress is long over.
"The reproductive biologists have now accepted the fact that sperm can be vulnerable to the environment."
Sex differences in stress reactivity and neurodevelopmental disease have been central to your research, historically. Why is sex such an important aspect of your research?
I have focused on sex differences across my entire career. My lab is particularly interested in neurodevelopmental disorders, where males are exceptionally vulnerable to what happens in utero. If you think about the presentation of known neurodevelopmental disorders, like autism, attention-deficit/hyperactivity disorder, and early-onset schizophrenia, most of them are biased towards males. When you talk with nurses, they’ll tell you there’s more males in the NICU [the neonatal intensive care unit] than females. Girls go home before boys. They’ve known for a long time that, for whatever reason, boys are more vulnerable to prenatal insults. With postnatal insults, girls tend to be more vulnerable. That tells us a lot about what mechanisms to look at.
Could you describe a specific study of yours where that’s been informative?
We have model in my lab looking at maternal stress. So, mom is pregnant, we stress her, and we see males with a very stress-reactive phenotype, but nothing in the female offspring. That’s one of the things about rodents—they have litters, so we can look at sex-specific effects. The male and female gestate in the same uterus, then the males present with the phenotype and the females do not.
That told us a lot about where the mechanism might lie. Mom is perceiving the stress, it’s being equally transmitted to both males and females—why are you seeing the effect in one sex and not the other? What is different about the stress signals in utero? That led us to realize this effect is occurring while the placenta is developing. The placenta doesn’t have a gonad [sex organ], but it does have X and Y chromosomes, which led us to look at which genes are different between the male and female placenta. The differences we see in the epigenetic genes are dramatic, and these differences underlie why the female placenta doesn’t respond to changes in the environment like the male placenta does.
Are you seeing any interesting sex differences between the male and female offspring of stressed dads in your most recent study?
Interestingly, there is no difference. I think that’s because, when dad passes on his sperm, it’s identical except for the X or the Y chromosome. It makes sense that, whatever dad’s effect is, it’s equally transmitted to both sexes.
Now that you’ve worked out these mechanisms in mice, what needs to be done to translate this research into humans?
First, we have to understand what “normal” looks like to know what might be abnormal.
There are hundreds of microRNAs in the human male, and we don’t know which respond to environmental challenges. Furthermore, we don’t understand the time course of their response. If a male comes in and his stress levels that month are particularly high, we don’t know whether to we expect to see a change in his sperm that month or the next. We don’t even know which populations of microRNA are normally expressed in the majority of males’ sperm, or how these populations fluctuate over time.
We’re now finishing up a study I started at the University of Pennsylvania, where we recruited Penn undergraduates, asked about their perceived stress, and paid them nicely to donate semen samples each month over the course of six months. We included a sample during final exams—and the final exams at Penn are very stressful. We are now analyzing the six samples from each male to build a framework to ask, “Do the differences we see in mice in response to stress translate to humans, and what do those changes look like?”
What I hope is that we identify a subset of about five microRNA that are clearly responding to environmental challenge. Then, knowing what our targets might be and how they might change through time, we can go back to the mouse and manipulate those targets to ask what they do at fertilization. There’s no framework out there to even ask this question, much less to say, “Well, how do these microRNA change my child’s chance of developing schizophrenia? What about autism?”
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© 2018 Nicoletta Lanese. Nicoletta reproduces her articles at nicolettalanese.com.
