Social stress shortens lifespan in mice

2.2. Subordination stress shortens lifespan in mice

One critical finding of our study is that Cluster 2 and Cluster 3 had opposite distributions of aggression received/exhibited representative of animals belonging to a low and high social rank, respectively, according to the classical ethological definition of social hierarchy (Bartolomucci, 2007; Sapolsky, 2005). Thus, we reanalyzed our dataset based on the more restrictive standard definition of achieved dominant and subordinate social status (see methods; Figure 2a) previously shown to explain individual vulnerability to cardiovascular (Costoli et al., 2004), metabolic (Sanghez et al., 2013), and psychiatric (Bartolomucci et al., 2010) disorders.

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Figure 2

Achieved subordinate status shortens lifespan in mice. (a) Average amount of aggression exhibited [F(1,115) = 309.35, p < .0001)] and received [F(1,115) = 176.1, p < .0001] by individual according to the achieved status (subordinate = 61, dominant = 56). (b, c) Survival probability and distribution of mortality frequencies as affected by the achieved status (log‐rank test, χ² = 8.9 on 1 degrees of freedom, p < .01). (d) Age at which 90%, etc. of the population survived, the percent survival at median and maximum (10%) survival as well as Cox regression model examining the contribution of achieved status on survival (HR, hazard ratio; CI, confidence interval). *p < .05

Subordinate mice had a significantly shorter lifespan compared to dominant animals (Figures 2b and S1c), with the distribution of frequency of age at death that peaked earlier than for dominant animals (Figure 2c). The curve describing the survival probability of subordinate mice corresponded to a 12.4% decrease in the median lifespan and to a significant decline in the maximal lifespan compared to dominant mice (Figure 2b,d). Subordinate status corresponded to a significant hazard ratio of 1.75 compared to being dominant (Figure 2d). The negative impact of the achieved subordinate status on survival did not interact with the diet (Figure S1d).

Overall, the difference in survival between subordinate and dominant groups largely reproduced the difference between the C2 and the C3 groups (Figure 1e). Importantly, the lifespan of the dominant group was comparable to the C1, C3, and C4 groups (Figure 1e,f) and to the expected survival rate for the C57BL/6J strain thus suggesting that dominant could be considered a reference group for aging‐associated conditions in our study. Overall, these results demonstrate that the low survival probability is an intrinsic characteristic of animals achieving a subordinate status.

2.3. Subordination stress impairs healthspan in mice

To gain insights on the life trajectories produced by the LCPS model, healthspan assessments were conducted regularly throughout the duration of the study (Figure 3). During the CPS phase, subordinate mice developed hyperglycemia when fed both a STD or a HFD (Figure 3a), a finding which is consistent with our previous work (Sanghez et al., 2013) and that could have set the stage for a later metabolic dysregulation and impact later survival outcomes (Fink, Kolterman, Griffin & Olefsky, 1983; Houtkooper et al., 2011). Subordinate mice were also characterized by increased body weight when fed a STD (Figure 3b–d) despite eating comparably to dominant mice (Figure 3e) that resulted in a greater food efficiency (Figure 3f). The vulnerability to HFD‐induced obesity was similar among the two groups probably due to the large hyperphagia shown by the dominant mice (Figure 3b,c). This finding in the C57BL/6J strain is at variance with previous data obtained in CD1 mice (Sanghez et al., 2013; Bartolomucci et al., 2009) thus suggesting a strain difference in metabolism (Fengler et al., 2016; Zou et al., 2013).

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Figure 3

Achieved status impacts healthspan in mice. (a–f) Healthspan parameters during the 4 weeks of chronic psychosocial stress (CPS) exposure: (a) 4‐hr fasting glucose [diet: F(1,112) = 20.82, p < .0001; achieved status: F(1,112) = 14.59, p < .001]. (b) body weight gain [diet: F(1,113) = 186.71, p < .0001; achieved status: F(1,113) = 4.21, p < .05], (c) fat mass gain [diet: F(1,113) = 132.56, p < .0001]; (d) fat‐free mass gain; (e) food intake [diet: F(1,113) = 111.16, p < .0001; achieved status: F(1,113) = 12.12, p < .0001]; (f) food efficiency [diet: F(1,113) = 134.26, p < .0001; achieved status: F(1,113) = 7.90, p < .01]; (g–l) Healthspan parameters during the entire duration of the study: (g) 4‐hr fasting glucose after; (h) body weight; (i) fat mass; (j) fat‐free mass [achieved status: F(1,108) = 5.73, p < .05]; (k) food intake [diet: F(1,107) = 4.41, p < .05; achieved status: F(1,107) = 6.62, p < .05]; (STD, standard diet; HFD, high‐fat diet). While data are presented for the entire study duration (g–k), they were statistically analyzed only until 12 months after stress (shaded areas), a point upon which the number of missing samples started exceeding 5% thus affecting the analytical power [78]; missing data from animals not surviving to 12 months after stress (3.76%) were not treated nor included in this analysis. Different letters represent statistical differences between groups. (l) Fecal corticosterone metabolites [F(1,50) = 19.8, p < .0001]. N = 8/group. *p < .05, #p < .06. Data represent group mean ± SEM

During the aging phase, the effect of HFD on metabolism vanished rapidly after its discontinuation (Figure 3g–k). Nevertheless, subordinate mice developed a significant lifelong hyperphagia (Figure 3k) and increased fat‐free mass (Figure 3j) compared to dominant mice. Subordinate mice also showed elevated levels of fecal corticosterone metabolites (measured after 2 months of the aging phase) compared to dominant mice (Figure 3l), suggesting sustained activation of the HPA axis as a consequence of the achieved status. Overall, these data demonstrate a significant and long‐lasting effect of the achieved subordinate status on mice healthspan. As the metabolic effects of HFD vanished after its discontinuation, diet as an experimental factor was not included in subsequent pathological and biomarker analyses conducted on 17‐months old mice (see below).

2.4. Subordination stress induced an earlier onset of aging‐associated diseases and increased cellular markers of senescence

In an attempt to determine the cause of death, we conducted macroscopic evaluation of lesions detectable at necropsy on all mice. Macroscopic examination revealed that subordinate mice presented an earlier onset of dissectible lesions in most of the organs examined, a finding which is in line with their lower survival probability (Figure 4a,b). However, no organ‐specific prevalence could be identified to account for the differential mortality between the two groups (Figures 4a,b and S2). Interestingly, for both dominant and subordinate mice, the higher the number of lesions, the longer the lifespan, indicating an age‐related accumulation of nonlethal organ lesions and tumors (Figures 4a,b and S3).

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Figure 4

Achieved status affects the onset of aging‐associated organ lesions and cellular markers of senescence. (a, b) Heat map of organ‐specific lesions, age‐dependent distribution of macroscopic dissectible lesions in proportion to their prevalence within each organ, and survival probability as a function of the number of macroscopic dissectible lesions as detected at time of necropsy in dominant and subordinate mice. (c, d) Cell senescence markers in spleen and liver of dominant and subordinate mice [p16^Ink4a, HMGB1 (high mobility group box 1), p53] N = 8/group. *p < .05, ^# p < .06. Data represent group mean ± SEM

To gain a mechanistic comprehension of this outcome, we conducted a second LCPS experiment in which subordinate and dominant mice were sacrificed when 17 months old (Figure S1e), an age chosen to anticipate the expected rapid decline in the survival curve typical of this strain (Baker et al., 2016; Mitchell et al., 2014; Turturro et al., 1999), allowing collection of well‐preserved organs for molecular and histopathological analyses. Firstly, we determined the expression of some of the classic cellular markers of senescence including p53, p16^Ink4a, HMGB1, and telomere length (Baker et al., 2016; Campisi, 2013; Childs et al., 2017) in liver and spleen (organs chosen for showing the highest level of lesions, Figure S2). Subordinate mice showed a significant increase in p53, a tissue‐specific significant decrease in HMGB1, and a nearly statistically significant increase in p16^Ink4a when compared to dominant mice (Figure 4c,d), while telomere length was similar to the one from dominant subjects (Figure S4). Overall, these results are consistent with accelerated cellular senescence in subordinate mice.

2.5. Earlier onset but no increased frequency of tumors in subordinate mice

Given the known association between cellular senescence, cancer, and aging (Campisi, 2013), we asked whether the difference in social status during the LCPS led to differences in cancer prevalence that might underlie the survival effect. Although subordinate mice presented an earlier onset of tumors macroscopically detectable at autopsy, their incidence or spectrum was not different between the two groups, thus excluding solid tumors as a factor explaining the detrimental effect of achieved subordinate status on survival (Figure 4a,b; Figures S2 and S3a–c). Similarly, the analysis of sternum specimens, known to harbor most of the pathologies recognized as cause of death in C57BL/6J male mice (Brayton, Treuting & Ward, 2012), did not return any leukemia‐associated sign (Figure S3d–f), which was therefore discarded as a prominent cause of status‐dependent difference in survival.

4.2. Lifelong chronic psychosocial stress model

The LCPS model consisted of a baseline phase of 5 days, during which all mice were singly housed all in one room, followed by a 4‐week CPS phase in which mice were exposed to daily defeats and sensory contact housing (enabled by a wire mesh partition bisecting the cage into two symmetrical compartments, each with food and water available at libitum), and an aging phase lasting until spontaneous death or moribund status (Figure 1a). In the aging phase, mice were housed in sensory contact thus experiencing a continued degree of threat stress related to the previous encounters.

The CPS was conducted essentially as previously described (Sanghez et al., 2013; Bartolomucci et al., 2009). Briefly each C57BL/6J (N = 172) male mouse, representing the experimental subject, was randomized using a simple randomization procedure of flipping a coin (Suresh, 2011) to be transferred as intruder to the home cage of either a CD1 (N = 86) or Sv129Ev (N = 86) resident mouse. The CD1 strain manifests high territorial aggression, while the Sv129Ev manifests a much lower level of territorial aggression and can thus be considered a weaker dominant (Bartolomucci et al., 2010; Dadomo et al., 2011; Zou et al., 2013). Resident and intruder mice were allowed to freely interact for a maximum of 10 min. After the interaction, resident and intruder mice were separated by a perforated partition, which allowed continuous sensory contact but no physical interaction. The partition was removed daily (between 8:30 and 9:30 a.m.), for a maximum of 10 min. During the social interaction, offensive behaviors of the animals were manually recorded. Furthermore, social status was determined as previously established. Specifically, subordinate social status was defined by the display of upright posture, flight behavior, and squeaking vocalization (Bartolomucci et al., 2010; Dadomo et al., 2011). After 1 week, half the dyads were randomized to a STD or a HFD using a simple randomization procedure of flipping a coin. Only dyads (N = 56 dominant and N = 61 subordinate) that reliably showed a stable dominant/subordinate hierarchy and in which the subordinate showed no attack after the fourth day of interaction were included in the analysis of the rank effect on healthspan and lifespan. As expected, most of the C57BL/6J acquiring a dominant status were paired with a Sv129Ev (43/56), while most of the C57BL/6J acquiring a subordinate status were paired with a CD1 (53/61).

After the conclusion of the CPS phase, the mice entered the aging phase of the study when each subordinate/dominant pair was kept in sensory contact in the same housing cage with the partition in place until spontaneous death occurred. The only procedures performed in this phase were the periodic healthspan assessments as detailed below. During the aging phase, all mice were fed standard diet. Finally, from the age of 10 months, all mice were switched to the mature rodent maintenance diet (D10012M) because of its better balance of essential nutrients tailored for aged rodents (Reeves, Nielsen & Fahey, 1993).

This study was designed to analyze the survival of the C57BL/6J mice as a function of a priori experimental variables such as pairing with either CD1 or Sv129Ev male mouse and diet, as well as a function of unbiased clustering of mice based on aggression received/exhibited. Furthermore, we used a design intended to expand upon past research which had examined the association with spontaneously achieved social status (dominant vs. subordinate) on healthspan (e.g., Bartolomucci et al., 2010; Sanghez et al., 2013) to examine the contribution of social status to longevity. The design employed a randomized experimental manipulation (described above) to increase the variability in achieved status thereby increasing the plausible variance in outcomes explained by the achieved status and, consequently, increasing statistical power and reducing the proportion of any relation between achieved status and subsequent outcomes due to potential confounding variables. In this study, we focus primarily on the proximal relation (and by implication plausible effect) of achieved status on healthspan and lifespan.

Mortality (age at death) was the study primary outcome. Animals were checked daily to determine vital status, and date of death was recorded to the nearest day. Mice found dead at each daily inspection were considered as censored deaths and were necropsied for tumor inspection. Criteria for euthanasia were based on an independent assessment made by an institutional veterinarian, and only cases where the condition of the animal was considered incompatible with continued survival are represented as deaths in the curves and censored to day of euthanasia. Conditions considered cause for euthanasia, as approved by the IACUC, University of Minnesota included the following: prolonged respiratory distress, tumor growth that impedes activity such as motion, eating or drinking, severe deformities or self‐mutilation, inability to eat or drink, or ataxia that prevents normal functions of eating and drinking.

4.3. Healthspan analysis

Body weight and food intake were measured weekly, while body composition (Echo MRI 3‐in‐1, Echo Medical System) and 4‐hr fasting glucose collected via tail snip (Accuceck‐Aviva, Roche) were recorded at baseline and at the end of the CPS phase. During the aging phase, the same health span parameters were assessed every 2 months, from 2 months up until 14 months after the end of the stress phase, and from then to completion every 4 months.

4.4. Corticosterone metabolite analysis

To accurately follow the secretion of glucocorticoids in our mice without interfering with the activation of the HPA axis by repeated handling and blood sampling, a noninvasive technique was applied to monitor adrenocortical activity by measuring corticosterone metabolites (CM) in the feces of mice. This technique of glucocorticoid metabolite quantification in fecal samples has been established in an increasing number of species and has been extensively validated for laboratory mice (Touma, Palme & Sachser, 2004). Fecal samples were collected 2 months after the end of the CPS and were analyzed for immunoreactive corticosterone metabolites using a 5α‐pregnane‐3β,11β,21‐triol‐20‐one enzyme immunoassay (EIA) according to published methods (Heinzmann et al., 2014). Before EIA analysis, the fecal samples were dried (2 hr at 80°C), homogenized, and aliquots of 0.05 g of powdered feces were extracted with 1 ml of 80% methanol. The EIA was performed on protein A‐coated microtitre plates. After overnight incubation (at 4°C) of standards (range: 0.8–200 pg/well) and samples with steroid antibody and biotinylated label, the plates were emptied, washed, and blotted dry, before a streptavidin–horseradish peroxidase conjugate was added. After 45‐min incubation time, plates were emptied, washed, and blotted dry. The substrate (tetramethylbenzidine) was added and incubated for another 45 min at 4°C before the enzymatic reaction was stopped with 1 mol/L sulfuric acid. Then, the optical density (at 450 nm) was recorded with an automatic plate reader, and the steroid concentrations were calculated. The intra‐ and interassay coefficients of variation were 8.8% and 13.4%, respectively.

4.5. Senescence biomarkers and pathological analysis study

In a second experiment, 70 C57BL/6J, 35 CD1, and 35 Sv129Ev mice purchased from the respective vendor at 10 weeks of age and exposed to LCPS. Of these 70 C57BL/6J, 21 achieved dominant (all mice paired with a Sv129Ev) and 31 achieved subordinate social status (26/31 when paired with a CD1). Mice were sacrificed at 17 months of age for the collection of tissue specimens, at which time they were in apparent good health as this age precedes the rapid decline of the lifespan curve typical of this strain. These animals are not included in the survival analysis of the lifespan study. Mice were euthanized by CO₂ inhalation in the morning (between 8 and 10 a.m.). The dissection was performed as rapidly as possible following euthanasia. Major organs were removed, cut into appropriate size pieces, and either flash‐frozen in liquid nitrogen, or placed in 4% PFA for preservation. Flash‐frozen tissue samples were stored at −80°C. After several days of PFA fixation at room temperature, tissue fragments were transferred to 70% ethanol and stored at 4°C. Specimens of sternum were decalcified for 3 hr by “Decalcifying Solution‐Lite” (Sigma‐Aldrich, St. Louis, MO, USA). All dissectible tumors were analyzed and eight animals per group were randomly selected to conduct the senescence markers, heart morphology, and immunohistochemistry analyses as detailed below.

4.6. Histological analysis

Fixed lung, liver, and sternum samples were embedded in paraffin, and 4‐μm‐thick sections were dewaxed in xylene and rehydrated through graded alcohol to water and stained with hematoxylin and eosin. Histopathological analysis of tumors was conducted by a certified pathologist blinded to animal's condition of social status.

The heart morphometric analysis and IHC were conducted at the University of Washington Pathology Core facility. Formalin‐fixed hearts were cut horizontally at midventricular level into two halves and paraffin embedded. The upper part was consecutively cut and all sections collected. Next, six 4‐μm heart sections covering 100 μm of aortic sinuses were selected and stained with Movat, Alizarin Red S, and Mac‐2 staining protocols. To identify calcium in tissue sections, a standard protocol for Alizarin Red S staining was used. Calcium forms an Alizarin Red S‐calcium complex in a chelation process, and the orange‐red product is birefringent. The rat anti–Mac‐2 antibody was used (1:3,000, Cedarlane; Hornby, Ontario, Canada) to detect infiltrating monocytes/macrophages. For each animal, the aortic sinuses were photographed under 100× magnification and quantified by computer image analysis (imagepro plus image analysis software) for lesion size and calcium content. A degree of macrophage infiltration was semiquantitatively scored from 0 to 3. Additionally, a 4‐μm midventricular cross sections of myocardium stained with Picrosirius Red stain (PSR) (Direct Red 80, Sigma, Saint Louis, MO, USA) were analyzed for fibrosis. Stained sections were photographed using standard and polarized light for birefringence properties and morphometrically measured.

4.7. Absolute telomere length

Absolute telomere length was determined according to published protocol (O'Callaghan & Fenech, 2011). DNA from spleen and liver was isolated using DNeasy Blood and Tissue kit (Qiagen) with slight modification of adding 1 mm DTT to eluate. Briefly, a standard curve of known quantities of a synthesized 84 mer oligonucleotide containing only TTAGGG repeats was generated based on CT (cycle threshold) using iQ SYBR Green Supermix (Bio‐Rad). A standard curve using the single copy gene, 36B4, was generated to serve as a control for amplification and to determine genome copies per sample. For both standard curves using synthesized oligomers, 20 ng of plasmid DNA (pBR322 Vector, New England Biolabs) was loaded in all wells. For samples, 20 ng of DNA was loaded in triplicate. Synthesized oligomers and primer sequences for qPCR in Table S2 and all samples were processed on a 96CFX real‐time system (Bio‐Rad).

4.8. Quantitative real‐time polymerase chain reaction

Quantitative real‐time polymerase chain reaction was conducted according to standard protocols (Razzoli et al., 2016). Table S2 lists primer information. The best keeper was generated from beta‐actin and TFIIB reference genes (Pfaffl, Tichopad, Prgomet & Neuvians, 2004) and used to normalize target gene values according to the comparative threshold ΔCT. Gene expression data are presented as normalized linear‐transformed values (2‐ΔCT) relative to the dominant group.

4.9. Western Blot

Protein lysates were obtained by homogenizing tissue in RIPA buffer (50 mm Tris [pH 7.5], 1 mm EDTA, 1% Triton X‐100, 0.1% sodium deoxycholate, 0.1% SDS, 50 mm NaCl). 25 μg of protein was resolved on 4%–15% acrylamide gels, transferred to PVDF membranes, and probed overnight with primary antibodies: HMBP1 (3935S), p53 (32532S), and tubulin (2146S) from Cell Signaling and p16 (Abcam, Ab51243). Membranes were incubated with goat anti‐rabbit‐HRP secondary antibody (Santa Cruz Biotechnology, sc‐2004) and then ECL was applied to generate signal and imaged with ChemiDox MP imaging system (Bio‐Rad). Intensities of protein detection were quantified using imagej2.

Supported by NIH/NIA AG043972 (D.A., A.B.), NIH/NIDDK DK102496 (A.B.), Fesler‐Lampert Chair in Aging Studies, Center on Aging, University of Minnesota (A.B), American Cancer Society Research Professor Award (D.A.L.). Authors wish to thank S. Pletcher for critical comments, K. Kaiser, A. Patki, and D.L. Smith Jr for statistical advice and help with the experimental design, L. Anderson, C. Acosta, C. Ayers, R. Heuer, C. Cero, J. Asturias, A. Jain, D. Svedberg, and B. Sahu for help with the study, T. Wietecha and the University of Washington Pathology Core Facility for the heart morphometric analysis and IHC.