The cost of longevity: transgenerational consequences of parental lifespan extension for offspring fitness

The world population is experiencing unprecedented ageing, and, according to the UN Population Division report 1950-2050, in the 21st century this global phenomenon will overburden existing healthcare systems at an ever increasing rate. Dietary restriction (DR), defined as reduced food consumption without malnutrition, is the most robust way to improve health and increase lifespan in a wide range of model organisms. Moreover, DR by temporary fasting improves human health, and the DR-mimicking drug metformin reduces mortality from all causes. Therefore, the DR approach via temporary fasting and DR-mimicking pharmaceuticals is promising to ameliorate the problems of population ageing by improving health into old age.

Improvements in health and longevity arising from DR may be offset by trade-offs with other key life-history traits, with increased lifespan often resulting in reduced fecundity. Remarkably, several DR-based treatments that substantially increase lifespan in model organisms are reported to circumvent such negative effects. Because these treatments are being translated from the laboratory studies on model organisms to humans, it is of paramount importance to understand the underlying mechanisms and to ensure that there are no unintended transgenerational effects on offspring.

We recently proposed that to fully understand the trade-off between longevity and reproduction we need to change our research focus to incorporate the effects of parental lifespan extension on offspring fitness.

There are two key reasons why:

First, increased allocation of resources into the maintenance of the parental somatic cells can lead to reduced investment in the maintenance of parental germ cells leading to an increase in the germline mutation rate and reduced provisioning in the developing embryos. Our pilot studies, using the Caenorhabditis nematode worm model system, support this idea and show that DR by temporary fasting improves lifespan, healthspan and late-life reproduction of parents, at the cost of reduced offspring fitness.

Second, parental lifespan extension can also result in negative consequences for the offspring, stemming from anticipatory parental effects. Parents can program offspring for survival in DR environment, which can result in harm because of the mismatch between the anticipated (DR) and the actual (normal feeding) environments in which offspring will develop and live.

Despite a strong theoretical basis for the hypothesis that boosting parental longevity can deleteriously affect offspring ageing and fitness, we currently lack the empirical data to evaluate this claim. In this project, we will address this knowledge gap using a classic model organism for ageing - namely Caenorhabditis elegans nematodes. First, we will test whether temporary fasting, DR-mimicking pharmaceuticals and gene expression silencing of dietary restriction response genes in C. elegans detrimentally affect the genetic and phenotypic quality of the gametes - thereby reducing offspring and grand-offspring health and fitness. Second, we will test whether DR animals program their offspring for survival in DR environment at the cost to fitness in a normal environment. Third, we will experimentally manipulate DR animals to re-invest their resources to germline maintenance and test whether such investment will improve fitness of their offspring.

This work is important because 1) it will provide a major advance in our understanding of why ageing evolves, by focusing on an overlooked trade-off between parental ageing and offspring fitness; and 2) because it will directly enable future research of DR effects on germline mutation and offspring health in humans. Because DR effects on nutrient-sensing molecular signalling pathways are evolutionarily conserved from worms to humans, this work is directly relevant to UK research priorities of promoting healthy ageing.

We will use three experimental paradigms for lifespan extension whose effects converge on reduced nutrient-sensing molecular signalling: i) reduced nutrient intake (DR); ii) drugs that mimic the DR effect by impeding nutrient-sensing signalling; and iii) RNA interference (RNAi) silencing of genes in nutrient-sensing signalling pathways. This will allow for generality of conclusions, as results common across all three paradigms can be evaluated, thus avoiding any idiosyncrasies of each. We will quantify the effect of lifespan extension resulting from (i) temporary fasting, (ii) three DR-mimicking drugs (rapamycin, spermidine, metformin) and (iii) RNAi of dietary-restriction-response gene drr-2 in C. elegans, on offspring and grand-offspring age-specific mortality rates, lifetime reproductive success and individual fitness (r), as well as age-specific changes in locomotion, feeding and lipofuscin accumulation.

We will run two resequencing experiments to estimate the effect of lifespan extension on germline mutation rate. First, we will use five single isogenic N2 parental hermaphrodites to produce 12 offspring each that will be split into two groups of six individuals per parent per treatment (ad libitum [AL] feeding vs DR). Subsequently, we will source one offspring from each hermaphrodite for 15 generations of mutation accumulation under the same feeding protocol in each generation. In total, 65 individual worms (five initial hermaphrodite parents and 60 descendants) will be re-sequenced. Second, we will use the same approach to quantify mutations following exposure to rapamycin and metformin.

We will also quantify the effect of DR-mediated lifespan extension on parental and offspring healthspan and fitness across DR and normal feeding environments in a full-factorial design. Finally, we will expose DR animals to food odours and estimate health and fitness measures of offspring from the parents kept on AL, AL with food odour, DR and DR with food odour.

The School of Biological Sciences at UEA and Earlham Institute are integral parts of thriving Norwich Research Park and are committed to deliver solutions to the global challenges of healthy ageing, food and nutrition. We aim to fully realise the significance of our work among the potentially wide group of beneficiaries of this research in the public and private sectors. We will measure the success of our impact strategy by attendance of our workshops and Science Festival booths, by national and international press coverage of our research, by successful CASE studentships and by social media indicators (e.g. Altmetrics).

Who will benefit from this research:
1. Academic and medical communities
2. General public and schools
3. Policy makers, politicians and NGOs
4. Industrial partners
5. Early-career researchers

How will they benefit:
Beneficiaries 1-3: We will inform academic and medical communities about our results at conferences, Open Access publications in leading multi-disciplinary journals with broad readership among scientists and general public, and social media. We will organise a symposium on transgenerational effects of lifespan extending treatments at a cross-disciplinary Evolution, Medicine and Public Health society meeting, as well as showcase our findings at the main big conferences such as European Society for Evolutionary Biology and British Society for Research on Ageing. We continue to actively use social media using our public Twitter profiles to disseminate and popularise our key findings among the broadest audience of general public. We will continue to engage both professional scientists and general public through non-peer reviewed educational and policy-related publications and interviews in journals Current Biology, Nature, Nature Ecology and Evolution. We will continue to produce high-end YouTube videos and blog posts highlighting our research. We will also create a special project blog that will be moderated by our team.

Beneficiaries 2 and 3: We will work closely with Science, Art and Writing (SAW) Trust charity that is based in the Norwich Research Park. The Trust allows scientists to present their work in schools alongside professional artists and writers in order to increase engagement from the widest possible audience of children. We will do two SAW workshops per year discussing 1) the relationship between diet, body function and health in animals and humans and 2) the science of longevity and ageing across the animal kingdom and in historical and contemporary human populations. We will develop teaching resources for use in the Teacher Scientist Network, which is actively promoted and supported across the Norwich Research Park. In collaboration with our Engagement Director in the School, we will also develop kit-based resources, grounded in the National Science Curriculum. Finally, we will apply to have a stand on diet and healthy ageing at the Royal Society Summer Science Exhibition 2018 and the Big Bang Science Fair 2019.

Beneficiary 4: In the private sector we will use the UEA Research and Enterprise office staff to further develop CASEing for studentships and other knowledge transfer opportunities to investigate the potential for DR and DR-mimicking pharmaceuticals to affect human health, focusing on within- and transgenerational reproductive ability. Such effects can be transferred to the next generation and should be of interest to companies involved in manufacturing, retail and marketing these treatments for human use.

Beneficiary 5: Our team members will gain new knowledge and skills in modern biogerontology, experimental design, RNA interference techniques, bioinformatics and state-of-the-art statistical analyses, as well as media training. They will gain experience in knowledge transfer from the laboratory to general public by helping to organise school and Open Day events, making promotional videos and running the blog.