Recent research published in Molecular Psychiatry provides evidence that chronic stress can impact the genetic material in sperm and consequently affect the behavior of offspring. The study reveals that high levels of stress hormones disrupt the long non-coding RNAs in sperm. When these altered RNAs are injected into fertilized eggs, they lead to developmental changes and behavioral alterations relevant to affective disorders such as anxiety and depression.

The researchers aimed to explore how paternal stress before conception influences the behavior of future generations. Previous studies had shown that environmental factors experienced by parents could affect their offspring’s behavior, a phenomenon known as epigenetic inheritance. The team previously found that stress-related changes in small non-coding RNAs in sperm could lead to increased anxiety-like behaviors in male offspring. However, the role of long non-coding RNAs in this process remained unclear.

RNA, or ribonucleic acid, is a molecule essential for coding, decoding, regulation, and expression of genes. It acts as a messenger carrying instructions from DNA for controlling the synthesis of proteins.

Small non-coding RNAs are a category of RNA molecules, typically about 20-30 nucleotides long, that do not code for proteins but play crucial roles in regulating gene expression and maintaining genomic stability. On the other hand, long non-coding RNAs are longer RNA molecules, over 200 nucleotides in length, which are involved in a variety of cellular processes, including chromatin remodeling, gene expression regulation, and the modification of other RNA molecules.

“We published a study 8 years ago (Short AK et al., 2016, Translational Psychiatry) showing that increased stress hormones caused changes to small non-coding RNAs in sperm of father mice, and changes in anxiety and social behavior in their offspring. We therefore wanted to study long non-coding RNAs and investigate whether these changes in sperm RNAs could contribute to changes in offspring brain and behaviour,” explained study author Anthony Hannan of the Florey Institute of Neuroscience and Mental Health at the University of Melbourne.

The study was conducted using mice. The researchers used male mice of the C57BL/6 strain from two different facilities in Australia. These male mice were divided into two groups: one group received corticosterone (a stress hormone) in their drinking water for four weeks to mimic chronic stress, while the control group received normal drinking water.

After the treatment, sperm was collected from the males and analyzed using a technique called CaptureSeq to identify changes in the long non-coding RNAs. To test the impact of these changes, the researchers injected long non-coding RNAs from both the corticosterone-treated and control groups into fertilized mouse eggs. These eggs were then implanted into female mice to produce offspring.

The offspring underwent various behavioral tests to assess anxiety, depression, social dominance, and attractiveness. Tests included the elevated-plus maze, light-dark box, social dominance tube test, mate-choice test, and Porsolt swim test.

The study found that exposure to corticosterone led to significant changes in the long non-coding RNAs profile of sperm. Out of 7,552 long non-coding RNAs, 2,382 were differentially expressed, with 772 being upregulated and 1,610 downregulated. These changes included alterations in specific long non-coding RNAs known to be involved in neural differentiation.

“I was surprised that so many (over 2,000) different large non-coding RNAs were altered by the increase in stress hormone levels,” Hannan told PsyPost.

When the altered long non-coding RNAs were injected into fertilized eggs, the resulting offspring exhibited notable behavioral differences. Male offspring from the microinjection group spent more time in the light zone of the light-dark box, suggesting increased anxiety-like behavior. They also displayed more depressive-like behavior in the Porsolt swim test, spending more time immobile compared to control offspring.

The study also found that the microinjected offspring had increased body weight compared to naturally conceived mice, highlighting that the presence of additional long non-coding RNAs could impact physical growth during early adulthood.

“These findings suggest that increased levels of stress hormone (modelling chronic stress) can cause major changes to the long non-coding RNAs in sperm, and that these changes can contribute to altered behaviour in offspring,” Hannan explained. “These changes in offspring are relevant to depression and anxiety disorders. We need to know whether these changes also occur in human sperm, and whether they contribute to brain disorders in the next generation.”

The study provides compelling evidence linking paternal stress to behavioral changes in offspring, but it is important to note that the research was conducted in mice. Human studies are needed to determine if similar mechanisms are at play.

“The main caveat is that our study was in mice and we urgently need to know whether this also occurs in human sperm,” Hannan noted. “But we do know that human sperm has a similar array of large non-coding RNAs. We also know that the RNA content of human sperm can be altered by environmental exposures, including stress.”

“We urgently want to understand the mechanisms involved, both prior to and after conception. Whilst exploring these mechanisms in mouse models, we also want to pursue human studies, including long-term cohort studies with detailed data on mothers, fathers and their children.”

Understanding these mechanisms could have important implications for human health, particularly in understanding the heritability of affective disorders such as anxiety and depression. This line of research also has a broader impact on our understanding of evolution. The researchers propose that such epigenetic inheritance mechanisms could be an adaptive response, allowing offspring to better cope with the environmental conditions experienced by their parents.

“These kinds of epigenetic inheritance studies have important evolutionary implications,” Hannan said. “I propose that Darwinian evolution (which acts very slowly and involves genetic mutations and natural selection over many generations) has generated ‘Lamarckian mechanisms’ which allow information on changing paternal environmental exposures to be transmitted to offspring so as to enhance adaptive fitness in the next generation.”

The study, “Chronically high stress hormone levels dysregulate sperm long noncoding RNAs and their embryonic microinjection alters development and affective behaviours,” was authored by L. B. Hoffmann, B. Li, Q. Zhao, W. Wei, L. J. Leighton, T. W. Bredy, T. Y. Pang, and A. J. Hannan.