Male mice that are more easily stressed than others appear to produce offspring that are similarly vulnerable to stressful situations due to genetic alterations in their sperm, according to researchers from the Icahn School of Medicine at Mount Sinai.

Their study, published Monday in JNeuroSci, expands on prior research that showed how stressful events can alter gene expression in mice, resulting in changes in behavior that can be passed down through generations. The findings could have implications for understanding the heritability of stress and generational trauma in humans, as well as the source of various mood disorders, along with updated treatment targets.

Prior studies had not considered the variation between how different mice react to stress. 

"Our study is the first to segregate fathers into resilient and susceptible groups," said lead author Ashley Cunningham, a first-year Ph.D. student studying molecular neuroscience at Mount Sinai's Icahn School of Medicine. "I think that's super important because people are complicated, and people who go through the same stressors aren't going to have the same response."

While prior research has described how stress can change various traits in offspring through a father's sperm, the mechanism behind how this occurs is not completely understood, according to Cunningham. A 2020 study on mice, published in Nature Communications, suggested that stress alters the cellular components involved in the development of sperm, which could change how sperm influences stress-related traits in offspring. 

To investigate how differences in fathers' susceptibility to stress manifests in offspring, Cunningham and her colleagues first performed stress exercises on adult male mice. The researchers placed the experiment subjects in contact with aggressive, retired breeder mice for 10 minutes at a time over the course of 10 days. 

Then, to determine which mice were more prone to stress, the researchers allowed the mice to explore a testing area, first without the presence of an aggressor, then with. They measured how much time each mouse spent in the testing space when the alpha male mouse was present to figure out which ones were stress-susceptible. 

After 30 days, the scientists let the test subjects breed naturally with control females, then artificially inseminated others using sperm from the same males. Once the offspring were born, a male and female from each litter were subjected to various stressors. 

In addition to other tests, the researchers assessed stress responsiveness in the offspring by seeing how willingly or how quickly they would enter an open space to eat their food. Mice are naturally afraid of open spaces.

Once they finished their tests, the researchers found that mice born from fathers more vulnerable to stress were more hesitant to perform the assessment tasks, such as eating a piece of corn placed in the middle of a large box. 

Also important, they found that, more often than not, mice offspring that had been born through artificial insemination also experienced the same heightened stress response from more stressed fathers.

"We know that mom matters, we know that maternal experience matters, but we also found that dad matters, too," Cunningham said in an interview with The Academic Times. "Dad is able to experience things in life such as stress, and that stress can lead to long-lasting changes in the sperm itself."

The researchers also analyzed the transcriptome, or all of the RNA transcripts, of mouse sperm before and after applying stress, and found that it altered over 1,400 genes in vulnerable mice, compared with only 62 in stress-resistant mice.

Studies such as these, where researchers are trying to better understand what contributes to mood disorders, could eventually lead to identifying more molecules and proteins that new drugs could target to improve treatment, according to Cunningham, or use existing drugs to prevent undesirable stress reactions.   

"Understanding how these environmental stressors and factors that one generation experiences may influence their offspring I think is important when we're thinking about and trying to develop new therapeutic tools to potentially target and help people with mood disorders," Cunningham said. 

And while her study shows that sperm plays a significant role in mediating stress traits, Cunningham says that it isn't everything, and how other influences can mediate these traits remains unclear. These variables could, for a mouse, include extra bedding, more toys or the company of friends. 

For future research, Cunningham and her colleagues want to better understand how long the effects of stress last through each brood. To do that, they would try to see how many litters the fathers can have and still transmit traits showing a stronger stress response. The team would also like to examine how the changes seen in a father's sperm manifest in the brains of offspring. For example, Cunningham would like to know if these changes are only experienced during stressful situations, or are present during a pre-stress baseline state. 

"With this study, we're able to think about how not only are your experiences important, but your parents' experiences might also be important," Cunningham said. "When we go to the doctor, they ask for our family's history, so we already have a sprinkling of that, but maybe there is more to that that we could be investigating."

The study, "Sperm transcriptional state associated with paternal transmission of stress phenotypes," published June 7 in JNeuroSci, was authored by Ashley M. Cunningham, Aarthi Ramakrishnan, Orna Issler, Caleb Browne, Scott J. Russo, Li Shen and Eric J. Nestler, Icahn School of Medicine at Mount Sinai; Deena M. Walker, Icahn School of Medicine at Mount Sinai and Oregon Health and Science University; Marie A. Doyle, Icahn School of Medicine at Mount Sinai and Vanderbilt University; Rosemary C. Bagot, Icahn School of Medicine at Mount Sinai and McGill University; Hannah M. Cates, Icahn School of Medicine at Mount Sinai and Adelphi University; Catherine J. Peña, Icahn School of Medicine at Mount Sinai and Princeton University; and Casey Lardner,  Icahn School of Medicine at Mount Sinai and Columbia University Medical Center.