Differences in the range of thermoneutral zone between mouse strains: potential effects on translational research.

Laboratory mice are commonly used for studies emulating human metabolism. To render human energetics, their ratio of daily (DEE) to basal (BMR) energy expenditure of 1.7-1.8 should be maintained. However, the DEE/BMR ratio strongly depends on whether a given study using a mouse model is carried out above, or below the lower critical temperature (LCT) of the thermoneutral zone, which is rarely considered in translational research. Here, we used mice artificially selected for high or low rates of BMR along with literature data to analyze the effect of ambient temperature on possible systematic bias in DEE/BMR. We demonstrated that the estimated LCTs of mice from the high and low BMR lines differ by more than 7°C. Furthermore, the range of variation of LCTs of mouse strains used in translational research spans from 23 to 33°C. Differences between LCTs in our selected mice and other mouse strains are mirrored by differences in their DEE-to-BMR ratio, on average increasing it at the rate of 0.172°C-1 at temperatures below LCT. Given the wide range of LCTs in different mouse strains, we conclude that the energetic cost of thermoregulation may differ greatly for different mouse strains with a potentially large impact on translational outcomes. Thus, the LCT of a given mouse strain is an important factor that must be considered in designing translational studies.

Larger guts and faster growth in mice selected for high basal metabolic rate.

Postnatal growth in birds and mammals is the time of highest vulnerability and relatively high energy demands and therefore shapes the organisms' future outcomes. Several different factors might impose limitations on growth in juveniles, one of them being the efficiency of the digestive process and size of the gastrointestinal tract. We tested the gut size-growth rate relationship using a unique experimental model-mice from a selection experiment designed to produce two lines with divergent levels of basal metabolic rate (BMR): the high BMR (H-BMR) and low BMR (L-BMR) line types. These lines differ with respect to not only BMR, but also correlated traits-internal organ size and food intake. Applying a cross-fostering design and a thermoregulatory burden imposed by shaving the mothers, we demonstrated that the mass of intestine strongly affected the growth rate, with the H-BMR pups having larger intestines and growing faster, and with reduced growth rate of pups of both lines nursed by shaved L-BMR mothers. Our study also provides a functional link between high growth rate of neonates and high BMR of adults, partly reflecting metabolic costs of maintenance of their guts.

Not that hot after all: no limits to heat dissipation in lactating mice selected for high or low BMR.

Heat dissipation has been suggested as a limit to sustained metabolic effort, e.g. during lactation, when overheating is a possible risk. We tested this hypothesis using mice artificially selected for either high or low BMR that also differed with respect to parental effort. We used fixed size cross-fostered families and recorded litter mass daily until the 14th day of lactation. Midway through the experiment (day 8) half of the mothers from each group had fur from the dorsal body surface removed to increase their thermal conductance and facilitate heat dissipation. Our results showed that neither high nor low BMR mouse lines benefited from increasing their thermal conductance at peak lactation. On the contrary, growth of the litters reared by the low BMR females was compromised. Thus, our results do not support the heat dissipation limitation hypothesis.

Milk output and composition in mice divergently selected for basal metabolic rate.

From an evolutionary perspective, the high basal metabolic rate (BMR) of homeotherms is hypothesised to be a by-product of natural selection for effective parental care. We estimated daily milk output during two consecutive lactation bouts in mice divergently selected for high/low BMR and applied a cross-fostered design to control for potential differences in the between-line suckling abilities of nursed juveniles. Additionally, to remedy the potential limitation imposed by the ability of mother mice to dissipate excess heat, we exposed them to an ambient temperature of 17°C during the most energetically demanding second week of lactation. We found that the mice selected for high BMR produced significantly more milk in a 24 h period in both reproductive bouts. The milk samples obtained from the high BMR females had lower protein concentration and did not differ with respect to fat. However, the concentration of the primary milk carbohydrate ­ lactose ­ was higher. Although all the above between-line differences were statistically significant, their magnitude was too small to unambiguously ascribe them as stemming from a positive genetic correlation between the physiological traits underlying BMR and lactation performance. Nevertheless, our study lends such support at least at the level of phenotypic variation.

Microbial urea-nitrogen recycling in arctic ground squirrels: the effect of ambient temperature of hibernation.

Energy conservation associated with hibernation is maximized at the intersection of low body temperature (Tb), long torpor bouts, and few interbout arousals. In the arctic ground squirrel (Urocitellus parryii), energy conservation during hibernation is best achieved at ambient temperatures (Ta) around 0 °C; however, they spend the majority of hibernation at considerably lower Ta. Because arctic ground squirrels switch to mixed fuel metabolism, including protein catabolism, at extreme low Ta of hibernation, we sought to investigate how microbial urea-nitrogen recycling is used under different thermal conditions. Injecting squirrels with isotopically labeled urea (13C/15N) during hibernation at Ta's of - 16 °C and 2 °C and while active and euthermic allowed us to assess the ureolytic activity of gut microbes and the amount of liberated nitrogen incorporated into tissues. We found greater incorporation of microbially-liberated nitrogen into tissues of hibernating squirrels. Although ureolytic activity appears higher in euthermic squirrels, liberated nitrogen likely makes up a smaller percentage of the available nitrogen pool in active, fed animals. Because non-lipid fuel is a limiting factor for torpor at lower Ta in this species, we hypothesized there would be greater incorporation of liberated nitrogen in animals hibernating at - 16 °C. However, we found higher microbial-ureolytic activity and incorporation of microbially-liberated nitrogen, particularly in the liver, in squirrels hibernating at 2 °C. Likely this is because squirrels hibernating at 2 °C had higher Tb and longer interbout arousals, a combination of factors creating more favorable conditions for gut microbes to thrive and maintain greater activity while giving the host more time to absorb microbial metabolites.

Basal metabolic rate is positively correlated with parental investment in laboratory mice.

The assimilation capacity (AC) hypothesis for the evolution of endothermy predicts that the maternal basal metabolic rate (BMR) should be positively correlated with the capacity for parental investment. In this study, we provide a unique test of the AC model based on mice from a long-term selection experiment designed to produce divergent levels of BMR. By constructing experimental families with cross-fostered litters, we were able to control for the effect of the mother as well as the type of pup based on the selected lines. We found that mothers with genetically determined high levels of BMR were characterized by higher parental investment capacity, measured as the offspring growth rate. We also found higher food consumption and heavier visceral organs in the females with high BMR. These findings suggested that the high-BMR females have higher energy acquisition abilities. When the effect of the line type of a foster mother was controlled, the pup line type significantly affected the growth rate only in the first week of life, with young from the high-BMR line type growing more rapidly. Our results support the predictions of the AC model.

Significance of variation in basal metabolic rate in laboratory mice for translational experiments.

The basal metabolic rate (BMR) accounts for 60-70% of the daily energy expenditure (DEE) in sedentary humans and at least 50% of the DEE in laboratory mice in the thermoneutral zone. Surprisingly, however, the significance of the variation in the BMR is largely overlooked in translational research using such indices as physical activity level (PAL), i.e., the ratio of DEE/BMR. In particular, it is unclear whether emulation of human PAL in mouse models should be carried out within or below the thermoneutral zone. It is also unclear whether physical activity within the thermoneutral zone is limited by the capacity to dissipate heat generated by exercise and obligatory metabolic processes contributing to BMR. We measured PAL and spontaneous physical activity (SPA) in laboratory mice from two lines, divergently selected towards either high or low level of BMR, and acclimated to 30 °C (i.e., the thermoneutral zone), 23 or 4 °C. The mean PAL did not differ between both lines in the mice acclimated to 30 °C but became significantly higher in the low BMR mouse line at the lower ambient temperatures. Acclimation to 30 °C reduced the mean locomotor activity but did not affect the significant difference observed between the selected lines. We conclude that carrying out experiments within the thermoneutral zone can increase the consistency of translational studies aimed at the emulation of human energetics, without affecting the variation in physical activity correlated with BMR.

Long-Term Trait Consistency in Mice Selected for Swim-Induced High Aerobic Capacity.

The majority of studies show that metabolic rates are usually repeatable at the individual level, although their repeatabilities tend to decline with time and to be strongly affected by physiological changes. Changes in individual repeatabilities may therefore affect putative differences between experimental groups or populations. This problem is particularly relevant to artificial selection experiments that apply the selection protocol at early life stages, running the risk of a poor correlation of the trait with itself throughout the life cycle of individuals. Moreover, significant physiological changes (e.g., induced by reproduction) may affect traits under selection and therefore their postreproductive differentiation between selected lines. Here, using a unique animal model-mice from four lines selected for [Formula: see text] during swimming in 25°C water and four random-bred control (reference) lines-we analyzed the long-term consistency of aerobic capacity as well as postswim hypothermia in primiparous and nonreproducing females at 12, 25, and 29 wk of age. Our results show that significant between-line type divergence in [Formula: see text] and hypothermia persists over time and is only weakly affected by past reproduction. Furthermore, both traits are also repeatable within lines at the individual level. More generally, our results suggest that past reproduction events are unlikely to significantly affect between-population and between-individual differences in [Formula: see text] and related traits.

Selection for high aerobic capacity has no protective effect against obesity in laboratory mice.

Aerobic capacity (VO2max measured during intensive physical exercise) both trained and intrinsic (i.e. genetically determined) has recently been deemed a good predictor of cardiometabolic risks. However, the underlying mechanisms linking VO2max and health risk factors are not entirely clear, as it seems that not VO2max per se, but rather some correlated traits, like spontaneous physical activity (SPA) are responsible for sustaining the lean phenotype. Here we investigated the link between genetically determined aerobic capacity, SPA and resistance to diet-induced health risks using replicated lines of mice selected for high aerobic capacity during swimming in mid-cold water (25°C) and Randomly Bred control mice. After four months of consumption of the western type HFat and HCarb diets and no forced nor voluntary training, we found no evidence of protective effects of intrinsic high VO2max. The Selected mice displayed similar levels of blood glucose, cholesterol, triglycerides and body fat as the Random Bred control animals. Most notably we found no correlation between VO2max and SPA levels. Our results therefore call into question the ubiquity of VO2max as a predictor of metabolic health and leanness, at least in animal models.

Metabolic risk factors in mice divergently selected for BMR fed high fat and high carb diets.

Factors affecting contribution of spontaneous physical activity (SPA; activity associated with everyday tasks) to energy balance of humans are not well understood, as it is not clear whether low activity is related to dietary habits, precedes obesity or is a result of thereof. In particular, human studies on SPA and basal metabolic rates (BMR, accounting for >50% of human energy budget) and their associations with diet composition, metabolic thrift and obesity are equivocal. To clarify these ambiguities we used a unique animal model-mice selected for divergent BMR rates (the H-BMR and L-BMR line type) presenting a 50% between-line type difference in the primary selected trait. Males of each line type were divided into three groups and fed either a high fat, high carb or a control diet. They then spent 4 months in individual cages under conditions emulating human "sedentary lifestyle", with SPA followed every month and measurements of metabolic risk indicators (body fat mass %, blood lipid profile, fasting blood glucose levels and oxidative damage in the livers, kidneys and hearts) taken at the end of study. Mice with genetically determined high BMR assimilated more energy and had higher SPA irrespective of type of diet. H-BMR individuals were characterized by lower dry body fat mass %, better lipid profile and lower fasting blood glucose levels, but higher oxidative damage in the livers and hearts. Genetically determined high BMR may be a protective factor against diet-induced obesity and most of the metabolic syndrome indicators. Elevated spontaneous activity is correlated with high BMR, and constitutes an important factor affecting individual capability to sustain energy balance even under energy dense diets.

Effect of reproduction on the consistency of the between-line type divergence in laboratory mice selected on Basal metabolic rate.

Artificial selection experiments are an effective tool for testing evolutionary hypotheses, because they allow one to separate genetic and environmental variances of the phenotype. However, it is unclear whether trait divergence typically selected early in life persists over an animal's life and altered physiological states, such as reproduction. Here we analyzed the long-term consistency of the between-line type divergence in basal metabolic rate (BMR) selected at 12 wk of age in laboratory mice. We measured BMR in nonreproducing and reproducing females at the age of 22 wk and then at 27 wk of age. Our results show that within both the reproducing group and the control group, the between-line type separation in BMR is consistently retained over time and reproductive status. Metabolically active internal organs (heart, liver, kidneys, and small intestine) also consistently differed in size between the two line types with no significant long-term effect of reproduction. The observed consistency of the between-line type divergence in BMR suggests the existence of the persistent effect of the selection on metabolic traits applied early in life. Moreover, BMR variation achieved by means of artificial selection is considerably higher than that found in natural/unmanipulated populations. The latter may therefore be characterized by insufficient variance to statistically resolve correlations involving BMR.