BXD NIA Longevity Study

Description

We have studied the effects of a high-fat diet on lifespan in a large and genetically diverse family of mice. A diet that is high in fat reduces longevity by an average of 12% in female members of the BXD family, and as in humans, the risk of cardiovascular disease is elevated on the HFD. However, this is not a universal response. Differences among genomes and GXE effects are strong for both lifespan and weight gain. Even after we correct for multiple comparisons, one strain lives significantly longer and another strain gains no weight on the HFD. We confirm that lower weight at an early age is linked to a longer lifespan and that this effect is also true on a HFD. There is at best only a modest association between weight gain after maturity and lifespan. Weight gain measured later in life—between the ages of 12–18 months—accounts for minimal variation in lifespan. Diet modulates lifespan and has a stronger effect than weight gain per se. Key serum metabolites, including glucose and total cholesterol exhibit a modest negative association with lifespan. Similar effects are seen with key serum hormones like insulin and leptin. While these specific biochemical assays can be linked causally to body weight, they cannot be linked causally to lifespan. By far the strongest effect on lifespan and weight gain is genometype.

A high-fat diet decreases lifespan by an average of almost three months across BXD females, roughly scaling to a decade decrease in humans. The high-fat diet is associated with an average two-fold higher age-adjusted risk of death compared to the control chow diet. Lifespan under the two diets correlates moderately well (r = 0.55, GN traits BXD_18435 and 18441). However, the strains of mice display wide variation in responses to diet, and despite the strong effect, diet only accounts for 5% of the total variance in lifespan. In comparison, strain as a factor accounts for 30% of the variance. Combined across the two diets, lifespan varies from a low of 307 ± 37 days in BXD13 (n = 21) to a high of 852 ± 33 days in BXD168 (n = 23). Some strains are fully resistant to the negative effects of HFD on body weight and lifespan while others are strongly affected. While mean lifespan is shortened by an average of 10% on a HFD, genetic factors account for roughly a two-fold range. At least one strain—BXD8—actually lives significantly longer on the HFD (+37%). Lifespans of other strains, including BXD16 and BXD73, are apparently unaffected by the HFD challenge. Of course, the lifespan of most strains is adversely affected (n = 67), and in the case of BXD65 is cut in half. Weight gain is also characterized by a forceful GXE effect—at least four BXD family members are resistant to weight gain, including BXD16, BXD77, BXD87, and BXD91, gaining at most 5% over 100 days of the HFD. There is a mild positive correlation (r = 0.20, n = 63 strains) between weight change after 100 days on the HFD and median lifespan differences (control minus HFD). This again indicates that weight gain accounts for only 4–5% of the change in lifespan.

Animals

Animals were raised and housed in a specific pathogen-free (SPF) facility at UTHSC (Memphis, TN), at 20–24 °C on a 12-hour light cycle. During the course of this study, serum samples from sentinel mice were tested quarterly for the following 12 pathogens: ectromelia virus, epizootic diarrhea of infant mice (EDIM), lymphocytic choriomeningitis (LCM), Mycoplasma pulmonis, mouse hepatitis virus (MHV), murine norovirus (MNV), mouse parvovirus (MPV), minute virus of mice (MVM), pneumonia virus of mice (PVM), respiratory enteric virus III (REO3), Sendai, and Theiler’s murine encephalomyelitis (TMEV GDVII). We tested twice a year for endoparasites in intestinal contents, and ectoparasites by direct pelt microscopy. All sentinel tests were negative.

From October 2011 through to December 2018, both parental strains, C57BL/6J and DBA/2J, their F1 progeny D2B6F1, and 73 BXD strains were followed from their entry into the aging colony from a large breeding colony —typically around 120 ± 66 days of age but with a wide range, from 26 days to 358 days—until their death (details below on entry age effects). Animals were inspected daily, and deaths were recorded for each animal with a precision of one day. Moribund animals (~10%) were euthanized, and those above the age of 200 days were included in lifespan calculations. Criteria for euthanasia were based on an assessment by our veterinary staff following AAALAC guidelines. All animals were initially raised by dams on the standard chow diet in the breeding colony. Upon entry into the aging colony, females were aged in groups of up to 10 individuals in polypropylene cages (935 cm2) provisioned with Envigo Teklad 7087 soft cob bedding. We provided all cages with enrichment and nesting materials: Bed-r’Nest (www.andersonslabbedding.com/irradiated/bed-rnest), Bio-Huts (www.bio-serv.com/product/Bio_Huts), and torn autoclaved paper towels.

While the aging colony at UTHSC is still in operation, for this analysis we only consider individuals with deaths between April 2012 and November 2018. The colony was moved to a new vivarium in the Translational Science Research Building in April 2016 from the Nash Annex Building, both at UTHSC. Approximately 60% of the individuals lived and died in the original vivarium, ~35% were born in the Nash Annex but lived in both vivaria, and ~5% were born and spent their entire lives in the new facility. We evaluated birth and death data over all seasons for both vivaria and have ruled out any site-specific or seasonal effect on lifespan.

Lifespan cohort

We studied a total of 1348 female individuals (n = 663 on CD, n = 685 on HFD) from 76 strains (Supplemental Table; Lifespan cohort). Animals were labeled using ear tags and were randomly assigned to a diet. Baseline weight was measured at the age of entry into the study. Seventy-seven percent of animals (n = 527) started on HFD at ages between 50–185 days, but some started on the diet as early as 26 days or as late as 358 days. Only 12 cases were placed on HFD at an age greater than 365 days, and these have been excluded from the Results. There was no appreciable correlation between the age at which cases were started on the HFD and lifespan (p = 0.22, r = 0.008, n = 685), and this is also true when the analysis was restricted only to those cases that started on the HFD between 26 and 150 days-of-age (p = 0.60, r = 0.0006, n = 487). Fewer than 20% of animals were retired breeders that entered the study at more than 180 days of age and as shown in the Results, this variable also does not covary with lifespan or weight gain. Individuals were weighed to the nearest 0.1 grams every other month until their death (Supplemental Table; Body weight data).