The microbiome has a profound impact on host health that extends to the host’s young ones. Studies in mice have shown that maternal gut bacteria play a role in offspring behavior and placental growth during pregnancy.1,2 Yet, the effects of the paternal microbiome on the health of their progeny remained relatively unexplored.
In a new study, scientists found that altering the gut microbiome of male mice negatively affected the health and lifespan of their offspring through epigenetic changes in the sperm.3 The results, published in Nature, offer insights into a gut-germline axis that mediates the effects of the microbiome on health and disease across generations.
The microbiome can impact almost all organ systems, but its effects on the reproductive system were not well understood, said study coauthor Ayele Argaw-Denboba, a reproductive developmental biologist at the Max Planck Institute of Immunobiology and Epigenetics. “When we started the project, we hypothesized the impact on the reproductive system could expand into the next generation,” he said.
To study the paternal microbiome’s influence on offspring health, the researchers treated male mice with antibiotics or laxatives to cause gut microbial imbalance, or dysbiosis. They then mated these mice with female mice that had healthy microbiomes. Studying hundreds of the resulting pups, both male and female, revealed that they had lower birth weights and higher likelihoods of premature death compared to offspring of fathers with normal microbiomes. The pups’ body weights remained significantly lower throughout development and transcriptional analyses of brain and fat cells revealed differences in several genes related to metabolic processes between offspring of control and dysbiotic mice.
The researchers further showed that the paternal microbiome recovered within eight weeks after stopping antibiotic treatment. Offspring conceived after this restoration were healthy, indicating that the effect of dysbiosis is short-lived.
The gut microbiota of offspring was not disrupted, suggesting that the altered paternal microbiome was not transmitted to the pups. Next, the researchers explored whether the effects of disrupted gut bacteria were passed on to the next generation via the fathers’ sperm. In vitro fertilization using sperm isolated from antibiotic-treated mice revealed that the offspring had lower birth weights and impaired development, indicating a gut-germline axis that affects offspring health. Further experiments showed that some small RNAs were less abundant in the sperm of dysbiotic mice, suggesting the involvement of these epigenetic factors—those that do not alter DNA sequence but can affect gene expression—in transmitting traits across generations.
Once they knew that sperm carried this epigenetic information to their offspring, the researchers wondered how dysbiosis affected the paternal reproductive system. They observed that mice with disrupted microbiomes had significantly smaller testes and lower sperm counts compared to healthy mice.
Metabolic profiling of the testis further revealed that microbiome dysbiosis changed the metabolite landscape, notably metabolites that are involved in germ cell function, and levels of leptin, a hormone that is essential to maintain reproductive function.4 Transcriptomic analysis confirmed that an altered microbiome caused dysregulation of leptin gene expression, implicating leptin signaling as a key component in the gut-germline axis.
To pinpoint the initial source of defects in the offspring, the researchers analyzed the transcriptome of embryos halfway through gestation. When they did not find any differently expressed genes between embryos fathered by healthy versus dysbiotic mice, they turned to studying the placenta at the same gestational stage.
This revealed significant differences depending on the paternal microbiome. The placentas of embryos from antibiotic-treated mice had smaller surface areas for nutrient exchange, fewer blood vessels, and reduced blood supply compared to placentas of embryos with healthy fathers, indicating a higher risk of placental insufficiency.
“We weren’t super surprised with the results,” said Argaw-Denboba, partly because previous studies have shown that paternal stress and diet can influence offspring. “But [it] was very exciting, since the study is the first to directly tie prospective fathers’ gut microbiota to their offspring health.” The research, however, did not explore specifically which microbial species are involved in the gut-germline axis, said Argaw-Denboba, but the lab is pursuing this further.
Oliver Rando, who studies paternal epigenetic inheritance at University of Massachusetts Chan Medical School said the study was thorough and convincing, in part because of the large number of animals utilized. He found it impressive that restoring the microbiota in prospective fathers improved offspring health. “That implies that actually fathers ‘tell’ their kids about things that are much more in the moment, which is quite surprising,” he said. “That really forced me to change how I think about my own field.”
However, he added, the study did not reveal the exact molecular changes in sperm that influence offspring health. Although Rando suspects that the results would hold true in humans, he is not sure, particularly because of the distinct nature of human and mouse placentas.
Argaw-Denboba had similar thoughts and cautioned that these results from mice may not necessarily apply to humans. But if they do, Argaw-Denboba said that identifying the microbes involved could provide diagnostic markers to predict or inform new therapeutic strategies to prevent adverse birth outcomes.