Female XX sex chromosomes increase survival and extend lifespan in aging mice

Abstract Female longevity is observed in humans and much of the animal kingdom, but its causes remain elusive. Using a genetic manipulation that generates XX and XY mice, each with either ovaries or testes, we show that the female XX sex chromosome complement increases survival during aging in male and female mice. In combination with ovaries, it also extends lifespan. Understanding causes of sex‐based differences in aging could lead to new pathways to counter age‐induced decline in both sexes.


| INTRODUCTION
Women live longer than men around the world, regardless of culture or socioeconomic status (UnitedNations, 2015;Zarulli et al., 2018).
Female longevity is also observed in the animal kingdom (Barrett & Richardson, 2011;Bronikowski et al., 2011;Clutton-Brock & Isvaran, 2007) due to causes that may be extrinsic, intrinsic, or both. Extrinsic causes of sex difference in invertebrates can signal antagonistic survival strategies: female pheromones reduce male lifespan in Drosophila (Gendron et al., 2014), and male secretions shorten hermaphrodite lifespan in C. elegans (Maures et al., 2014). Intrinsic effects-operating within the organism-underlie longer life in organisms following removal of reproductive cells or organs in C. elegans hermaphrodites (Berman & Kenyon, 2006), male and female dogs (Hoffman, Creevy, & Promislow, 2013), and possibly men as suggested by a study of eunuchs (Min, Lee, & Park, 2012). Nonetheless, causes of intrinsic sex difference in lifespan remain largely unknown. The pervasive nature of female longevity in humans, even in early death during severe epidemics and famine (Zarulli et al., 2018), suggests a role for innate biology in the survival gap between the sexes. Here, we sought to identify intrinsic causes of female longevity in mammalian lifespan.
Sex chromosomes or gonads cause intrinsic sex differences in mammals, but whether they directly contribute to increased female lifespan is unknown in mammalian aging. To dissect these etiologies, we used four core genotypes (FCG) mice (Arnold, 2004). In mice and humans, the Sry gene normally resides on the Y chromosome and codes for a protein (testicular determining Y factor) that induces development of testes and perinatal masculinization. In FCG mice, thus male, testicular phenotype-with or without the Y chromosome.
The genetic manipulation of SRY generates XX and XY mice,  wood et al., 2006;McCullough et al., 2016). In FCG model mice, a sex difference with a main effect that statistically differs by genotype (XX vs. XY) is sex chromosome-mediated; one that differs by phenotype (ovaries vs. testes) is gonadal sex-mediated ( Figure 1b).
Examples of age-relevant FCG mouse studies show that XX improves blood pressure regulation (Pessoa et al., 2015) and attenuates experimental brain injuries (Du et al., 2014;McCullough et al., 2016).
To explore sex-based differences in lifespan, we generated and aged over 200 mice from the FCG model on a congenic C57BL/6J background and investigated aging-dependent mortality from midlife to old age (12-30 months) (Figure 1c). We first examined whether  Table S1). We next measured main effects of sex chromosomes and gonads on survival in aging. XX mice with ovaries or testes lived longer than  Table S2). Collectively, these data indicate that the XX genotype increases survival in aging-and suggest a protective effect of ovaries.  Information Tables S1 and  S2) To further understand benefits of femaleness on survival in aging, we directly compared the four groups of mice. In mice with ovaries, XX increased lifespan compared to XY (Figure 2a; Supporting Information Table S3). In mice with testes, mortality tended to be higher overall and did not differ between XX and XY genotypes ( Figure 2b; Supporting Information Table S3). Ovaries increased lifespan in XX, but not XY mice (Figure 2c,d; Supporting Information Table S4). This suggests that female gonadal hormones, through organizational (long-term) or activational (short-term) effects, increase lifespan in the presence of a second X chromosome.
Since the XX genotype showed a main effect on overall survival, we next tested whether it increases resilience against death anytime during aging. We used the grid search method (Lerman, 1980) to determine the point in time when XX and XY lifespan curves change in relation to each other in mice with matching gonads. We then measured statistical differences between the two curves before and  Table S5). In mice with testes, XX also increased survival, but the benefit was earlier, prior to 23 months, and did not alter maximal lifespan (Figure 2f; Supporting Information Table S6). Thus, independent of maximal lifespan, the XX genotype increased survival during aging in both male and female mice, albeit at different times.