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Sperm telomere length as a mediator of paternal effects

Periodic Reporting for period 1 - TELOSPERM (Sperm telomere length as a mediator of paternal effects)

Reporting period: 2017-07-01 to 2019-06-30

In 2009, Elizabeth Blackburn, Carol Greider and Jack Szostak were awarded the Physiology or Medicine Nobel Prize for discovering the fundamental processes whereby chromosome ends are protected by telomeres and the enzyme telomerase. Telomeres are regions of non-coding, but highly structured DNA at the end of linear eukaryotic chromosomes, consisting of tandem repeated highly conserved DNA sequence. Telomeres shorten at each cell division or when cells are exposed to oxidative stress (OS), resulting in telomere shortening with aging in most animals. A permanent arrest in the cell cycle occurs through cellular senescence when telomeres are shortened to a critical length, thereby increasing risk factors in a large number of diseases. Consequently, telomere length (TL) appears to predict remaining lifespan in humans and wild animals, with individuals with the shortest telomeres displaying the poorest survival prospects. Telomere erosion is therefore now considered an essential component of the ageing phenotype.
While gradual telomere attrition is a normal component of the aging process, accelerated telomere erosion and premature senescence occur in individuals that are exposed to stressors. Several cross-sectional studies in humans and experimental studies in wild and captive animals have reported associations between telomere erosion and high levels of psychosocial stress or stress biomarkers. For example, an experimental exposure to the stress hormone cortisol has been shown to dampen telomerase activity, whereas behavioral interventions that reduce stress have been shown to increase telomerase activity and slow down telomere erosion. This suggests that stress-related changes in telomere integrity is a mechanism linking stress and disease susceptibility.
Exposure to stressors is also well known to have transgenerational effects, with stress levels in the parental line having long-term negative consequences on offspring health. Recent studies show that these transgenerational effects might be mediated, at least in part, by telomere attrition. Babies from mothers who experienced stress during pregnancy had shorter telomere at birth and birds experimentally stressed during egg production produced chicks with shorter telomeres than controls. These negative effects of maternal stress levels on offspring TL should not be mediated by changes in gamete TL because the pool of ova is established in utero and TL in ova are thought to be very stable. These negative maternal effects may instead rely on the complex maternal–fetal/neonatal interaction and be mediated by an increased transfer of glucocorticoid hormones from the mother to the developing embryo. This would in turn activate the production of reactive oxygen species (ROS), as well as decrease telomerase activity in the offspring, potentially leading to telomere erosion.
Despite having long been overlooked, paternal effects (the influence of fathers on offspring traits via mechanisms other than the transmission of alleles) are now increasingly recognized as affecting a broad range of phenotypic traits in the next generation in a variety of organisms. Paternal effects can negatively impact their offspring and there is increasing evidence for transgenerational effects of paternal stress levels at the time of reproduction with long-term consequences on offspring health. In most cases, the proximate mechanism mediating paternal influence on offspring remains unknown. This question is intriguing because in most species fathers do not interact with their offspring beyond transferring the ejaculate and mechanisms of paternal effects in these species might be widespread.
Sperm cells are highly vulnerable to OS due to the high proportion of polyunsaturated fatty acids in their membrane, their condensed DNA and reduced transcription machinery, and it is already known that stress exposure entails costs in terms of reduced sperm quality (i.e. sperm number and motility) through systemic OS lev
First, an experiment on the impact of paternal stress on sperm telomere length has been performed by exposing male zebra finches to an immune challenge and to dietary mercury. Sperm and blood samples for telomere analyses have been collected and are currently in the process of being analyzed. Results should be available soon and I am planning to submit the paper on this experiment early next year. Second, an experiment on the importance of sperm telomere length compared to blood parental telomere length for offspring aging trajectory (and blood telomere length) have been performed during the last few weeks of the fellowship (before the early termination of the grant). Samples are currently analyzed and I hope to submit the manuscript on this section of my project in 2019. Due to the early termination of this grant, I have not developed the third aim of my project.
Through the analyses of the samples collected during the two experiments performed through this fellowship, I'll examine the effect of stress exposure on sperm telomere length and determine the links between sperm telomere length and offspring telomere length at birth. Given the early termination of the grant these results are not available yet.
Zebra finches