The global rise in obesity and steady decline in sperm quality are two alarming trends that have emerged during recent decades. In parallel, evidence from model organisms shows that paternal diet can affect offspring metabolic health in a process involving sperm tRNA-derived small RNA (tsRNA). Here, we report that human sperm are acutely sensitive to nutrient flux, both in terms of sperm motility and changes in sperm tsRNA. Over the course of a 2-week diet intervention, in which we first introduced a healthy diet followed by a diet rich in sugar, sperm motility increased and stabilized at high levels. Small RNA-seq on repeatedly sampled sperm from the same individuals revealed that tsRNAs were up-regulated by eating a high-sugar diet for just 1 week. Unsupervised clustering identified two independent pathways for the biogenesis of these tsRNAs: one involving a novel class of fragments with specific cleavage in the T-loop of mature nuclear tRNAs and the other exclusively involving mitochondrial tsRNAs. Mitochondrial involvement was further supported by a similar up-regulation of mitochondrial rRNA-derived small RNA (rsRNA). Notably, the changes in sugar-sensitive tsRNA were positively associated with simultaneous changes in sperm motility and negatively associated with obesity in an independent clinical cohort. This rapid response to a dietary intervention on tsRNA in human sperm is attuned with the paternal intergenerational metabolic responses found in model organisms. More importantly, our findings suggest shared diet-sensitive mechanisms between sperm motility and the biogenesis of tsRNA, which provide novel insights about the interplay between nutrition and male reproductive health.

Funding: The study was kindly supported by grants from The Swedish Research Council (2015-03141; https://www.vr.se/english.html ; received by AÖ), Knut and Alice Wallenberg Foundation (Wallenberg Academy Fellow, 2015.0165; https://kaw.wallenberg.org/wallenberg-academy-fellows ; received by AÖ), Ragnar Söderberg (Fellow in Medicine 2015; https://ragnarsoderbergsstiftelse.se/ ; received by AÖ), Strategic Research Area Health Care Science at Karolinska Institutet/Umeå University ( https://ki.se/en/research/strategic-research-area-health-care-science-sfo-v ; received by ML). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Data Availability: All relevant data generated during the current study are contained within the paper and its Supporting Information files, with one exception. S1 Fig was generated from fastq sequence files, which contain personal genetic information. Unrestricted distribution of these files is therefore risking the personal integrity of the study participants, which violates our current ethical permits, participants consents, and Swedish laws. Nevertheless, these files may become available on request, given that the researcher adhere to the existing ethical permits and consents, or extend the ethical permits and consents by applying for this at the Swedish Ethical Review Authority (more information on how to apply contact, registrator@etikprovning.se , or visit https://etikprovningsmyndigheten.se/ ). For specific information about the data, please contact the corresponding authors at daniel.natt@liu.se , anita.natt@liu.se . Data used in the current study, but generated by others, have previously been deposited at the Sequence Read Archive (SRA) and can be accessed through these accession numbers: SRP065418, SRP132262 (a link is also provided to Gene Expression Omnibus by these accessions GSE74426, GSE110190).

Copyright: © 2019 Nätt et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Here, we present the acute effects on human sperm following a 2-step diet intervention. This intervention involved, first, 1 week of healthy diet, to establish a baseline, followed by 1 week of additional sugar intake on top of that. By investigating 3 ejaculates from the same individuals, we found that sperm motility dramatically stabilized at high levels in all individuals during the intervention. Changes in sperm motility were paralleled with a simultaneous increase of tsRNA, primarily from mitochondrial origin, but also of a specific type of nuclear tsRNA. These nuclear tsRNAs, which we name nuclear internal T-loop tsRNA (nitRNA), had a specific cut-site within the conserved TψC region in the T-loop of mature tRNA, indicative of a sugar-sensitive enzyme promoting the biogenesis of this tsRNA subtype. Thus, the sncRNA repertoire in human sperm, as well as sperm motility, show a fast and highly specific response to dietary changes.

In paternal intergenerational metabolic responses, males are exposed to dietary interventions that create robust metabolic ripples that propagate through one or two generations before subsiding [ 15 , 16 ]. Such phenomena have been observed in many organisms, including humans, mice, and fruit flies [ 17 – 19 ]. The best candidate mechanism here involves changes in the sperm load of small noncoding RNA (sncRNA). In general, RNAs are known to play essential roles in establishing epigenetic states, including centromeric heterochromatin [ 20 ] and transposon silencing in the germ cells [ 21 ]. A subtype of sncRNA, tRNA-derived small RNAs (tsRNAs), are known to be abundant in mammalian sperm—including human—and are playing a role in paternal intergenerational metabolic responses in mice [ 22 – 28 ]. The functional significance of tRNA fragments is just being unraveled but has so far been implicated in inhibition of translation, stress granule formation, and control of retrotransposons [ 24 , 29 – 31 ]. Whether tsRNA of human sperm is responsive to dietary interventions, and whether it associates with changes in sperm quality, have not been investigated.

Risk factors for low sperm quality in healthy men involve, for example, male reproductive age, environmental exposures to endocrine disruptors (e.g., pesticides and heavy metals), and lifestyle factors (e.g., tobacco/alcohol and exercise) [ 5 ]. Obesity—with associated pathologies such as diabetes—is also a strong risk factor [ 9 – 12 ]. Interestingly, many of the most frequently studied populations with declines in sperm quality have also experienced recent rises in obesity. It is well known that nutritional and metabolic factors may affect male fertility [ 9 , 13 , 14 ], but little is known about the molecular mechanisms. Clues may, however, be found in recent discoveries of so-called paternal intergenerational metabolic responses in animals.

Epidemiological studies have for decades reported worldwide declines in sperm quality among healthy men [ 1 – 3 ]. While interpretations of these studies, which sometimes reach apocalyptic proportions, are rightly criticized for often being underpowered, regionally dependent, and biased by covariates (for example see [ 4 ]), the consistency is reason enough for concern. A recent meta-analysis of 137 reports estimated a 57% decline in sperm concentration during the past 35 years, where best support for such declines was found in North America, Europe, and Asia [ 5 ]. Ongoing large-scale studies, covering tens to hundreds of thousands of individuals, also suggest that this decline shows no sign of recovery [ 6 – 8 ]. Thus, it has become increasingly urgent to better understand factors that affect sperm quality in humans.

Results