In a recent review published in the journal Nature, a group of researchers investigated how preconception high-fat diet (HFD) affects epididymal spermatozoa and the role of mitochondrial transfer Ribonucleic Acid (mt-tRNAs) in influencing offspring metabolic health.
Study: Epigenetic inheritance of diet-induced and sperm-borne mitochondrial RNAs. Image Credit: Prostock-studio / Shutterstock
Background
Apart from Mendelian inheritance, fathers use alternative routes for intergenerational information transfer, including a complex, dynamic, and environment-sensitive pool of small non-coding RNAs (sncRNAs) in mature spermatozoa, which influence embryonic development and adult phenotypes. Spermatozoa production involves spermatogenesis and epididymal maturation, both potential windows of environmental susceptibility. Further research is needed to fully understand the mechanisms behind diet-induced epigenetic changes in sperm and their long-term effects on offspring health.
Epididymal sperm susceptibility to diet
To investigate how epididymal spermatozoa respond to environmental influences and to discern the contributions of epididymal and spermatogenic information to paternal intergenerational effects, the team conducted an experiment on 6-week-old male mice. These mice were fed either an HFD or a low-fat diet (LFD) for 2 weeks. After this dietary challenge, some of the treated males were mated directly to unexposed females to generate the F1 generation (Early High-Fat Diet (eHFD) group). Others were first allowed to mate to clear their epididymis and then returned to a regular diet for 4 weeks before mating again (Standard High-Fat Diet (sHFD) group).
Impact on spermatogenesis and reproductive fitness
The HFD feeding did not affect spermatogenesis or male reproductive fitness. This was confirmed by testis histology, seminiferous tubule diameter, sperm motility, and rates of successful fertilization and pre-implantation development. Both bulk and single-cell transcriptomic analyses of round spermatids and testis indicated normal spermatogenesis and minimal impact from the HFD.
Effects of HFD on body weight and glucose tolerance
A 2-week HFD exposure resulted in a slight but significant increase in body weight and adiposity and a reduction in whole-body glucose tolerance in the exposed mice. These phenotypes were reversed after 4 weeks on a regular diet. Notably, a 2-week HFD challenge did not affect the offspring's body weight or composition but caused about 30% of male offspring to develop glucose intolerance. This glucose intolerance was consistent across different cohorts and remained stable over time. The offspring of sHFD groups showed no alterations in body weight, body composition, or glucose tolerance.
Transcriptional signatures in offspring
The differences in glucose tolerance between HFD-tolerant (HFDt) and HFD-intolerant (HFDi) offspring were associated with unique transcriptional signatures in metabolically relevant tissues. About 30% of the genes differentially expressed in HFDi mice were also linked to childhood obesity in humans. These genes are clustered into pathways related to mitochondrial function and inflammation.
Paternal body mass index (BMI) and offspring health
Parental obesity is a significant risk factor for early-onset obesity in children. Analysis of data from the Lifestyle Intervention for Everyone (LIFE) Child Study revealed that paternal BMI independently correlated with offspring BMI, with paternal BMI contributing an additional 6.5% to offspring BMI variation. Paternal overweight doubled the risk of obesity in offspring, especially when mothers were lean. This effect was compounded by paternal obesity and was associated with insulin resistance. These results emphasize the importance of paternal preconceptional body weight for offspring metabolic health.
Role of mt-tRNAs
Spermatozoa sncRNAs are potential mediators of paternal epigenetic effects. We profiled sncRNAs from round spermatids and cauda spermatozoa of mice subjected to HFD. The analysis revealed that about 25% of the sperm sncRNA pool is sensitive to the HFD challenge, with significant upregulation of mt-tRNAs and their fragments. These findings suggest that Mitochondrial Transfer Small Ribonucleic Acids (mt-tsRNAs) and Mitochondrial Ribosomal Ribonucleic Acids (mt-rRNAs) are predominantly upregulated in response to HFD.
Human studies on mt-tsRNAs
In a study involving young Finnish volunteers, mt-tsRNAs were found to be positively associated with BMI. Although obtained from a small cohort, the findings suggest that mt-sncRNAs play important roles in responding to metabolic challenges in mice and humans.
Active transcription of mt-tRNAs in spermatozoa
Mature spermatozoa can actively transcribe mt-tRNAs, and these transcripts were upregulated following HFD-feeding. Analysis of sncRNA-seq datasets showed that mt-tsRNAs are primarily present in spermatozoa, supporting their role as dynamic molecular signals responsive to environmental changes.
Epigenetic inheritance of mt-tRNAs
Using hybrid embryos and genetic tracking, we demonstrated the paternal transfer of mtRNAs to oocytes at fertilization. Male embryos sired by HFD-fed males showed significant overexpression of mt-tRNAs, suggesting a direct link between paternal diet and early embryo transcriptional changes.
Mitochondrial dysfunction mimics HFD effects
Comparative tissue transcriptomics indicated consistent downregulation of genes involved in mitochondrial metabolism in diet-exposed mice. This was coupled with the upregulation of mtDNA transcriptional machinery in cauda spermatozoa, suggesting a compensatory response to diet-induced mitochondrial dysfunction. These findings were supported by data from the International Mouse Phenotyping Consortium, which showed paternal effects on offspring adiposity and glucose intolerance.
Conclusions
To summarize, the study demonstrates that acute HFD feeding or genetic induction of mitochondrial dysfunction in male mice leads to impaired glucose homeostasis in their male offspring. This is linked to an accumulation of mt-tRNAs in mature spermatozoa, which are transferred to the oocyte at fertilization, resulting in altered gene transcription in early embryos. This reversible mechanism suggests that paternal metabolic health can influence offspring through mitochondrial signals. Although other sncRNAs may also play a role, mt-tRNAs are promising candidates for monitoring preconception lifestyle interventions to prevent metabolic disorders.
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