Adaptive thermogenesis and thermal conductance in wild-type and UCP1-KO mice

Abstract

We compared maximal cold-induced heat production (HPmax) and cold limits between warm (WA; 27°C), moderate cold (MCA; 18°C), or cold acclimated (CA; 5°C) wild-type and uncoupling-protein 1 knockout (UCP1-KO) mice. In wild-type mice, HPmax was successively increased after MCA and CA, and the cold limit was lowered to -8.3°C and -18.0°C, respectively. UCP1-KO mice also increased HPmax in response to MCA and CA, although to a lesser extent. Direct comparison revealed a maximal cold-induced recruitment of heat production by +473 mW and +227 mW in wild-type and UCP1-KO mice, respectively. The increase in cold tolerance of UCP1-KO mice from -0.9°C in MCA to -10.1°C in CA could not be directly related to changes in HPmax, indicating that UCP1-KO mice used the dissipated heat more efficiently than wild-type mice. As judged from respiratory quotients, acutely cold-challenged UCP1-KO mice showed a delayed transition toward lipid oxidation, and 5-h cold exposure revealed diminished physical activity and less variability in the control of metabolic rate. We conclude that BAT is required for maximal adaptive thermogenesis but also allows metabolic flexibility and a rapid switch toward sustained lipid-fuelled thermogenesis as an acute response to cold. In both CA groups, expression of contractile proteins (myosin heavy-chain isoforms) showed minor training effects in skeletal muscles, while cardiac muscle of UCP1-KO mice had novel expression of beta cardiac isoform. Neither respiration nor basal proton conductance of skeletal muscle mitochondria were different between genotypes. In subcutaneous white adipose tissue of UCP1-KO mice, cold exposure increased cytochrome-c oxidase activity and expression of the cell death-inducing DFFA-like effector A by 3.6-fold and 15-fold, respectively, indicating the recruitment of mitochondria-rich brown adipocyte-like cells. Absence of functional BAT leads to remodeling of white adipose tissue, which may significantly contribute to adaptive thermogenesis during cold acclimation.

Fig. 1.

Body temperature (A), metabolic rate (B), thermal conductance (C), and respiratory quotient (RQ) (D) of warm (WA; 27°C) and moderate cold (MCA; 18°C) acclimated wild-type (solid symbols) and uncoupling protein 1-knockout (UCP1-KO) mice (open symbols) maintained at 30°C and 5°C (cold endurance test). Each data point represents the means ± SE of 10–15 readings collected during 30 min in the metabolic cage. Mice (n = 5) in each group initially entered the experiment, but numbers in the WA groups progressively decreased (A) because mice became hypothermic and had to be removed from the metabolic cage.

Fig. 2.

Representative original tracings of activity readings (bars), metabolic rate (line and scatterplot) and body temperature (line plot) in individual wild-type and UCP1-KO mice during a cold endurance test (∼4 h at 30°C, up to 5 h at 5°C, temperature switch indicated with an arrow). Previously WA mice of either genotype had to be removed from the experiment because of hypothermia (final body temperature <31°C). In contrast, the mice acclimated to MCA (18°C) remained normothermic. Note the reduced activity and the less variable metabolic rates of the WA wild-type (WT) mouse and the two UCP1-KO mice.

Fig. 3.

Means ± SD resting metabolic rate (RMR), body temperature (T_b), thermal conductance, and RQ of wild-type and UCP1-KO mice (n = 5–7 mice per group) acutely exposed to stepwise decreasing ambient temperatures. The mice had been previously acclimated to either 27°C (warm, WA), 18°C (moderate cold; MCA), or 5°C (cold, CA) for at least 3 wk.

Fig. 4.

Summarizing figure depicting RMRt (resting metabolic rate at thermoneutrality) and maximal norepinephrine-induced heat production (NEmax) of WT and UCP1-KO mice acclimated to different ambient temperatures: WA: 27°C, MCA: 18°C; cold acclimated (CA): 5°C, and in relation to maximal cold-induced heat production (HPmax). The corresponding cold limits for each group of mice are indicated. Data are indicated as means ± SD; n = 4–7 individuals per group. Note that HPmax and NEmax were determined in two different sets of mice for each genotype. RMRt was obtained prior to the injection of norepinephrine (for details, see text).

Fig. 5.

Dependence of proton leak rate (measured as the respiration rate driving proton leak) on membrane potential of isolated skeletal muscle mitochondria of WA (27°C), CA (5°C), WT, and UCP1-KO mice. Duplicate measurements were performed on each mitochondrial preparation and averaged. Values are expressed as means ± SD from 5 independent preparations.

Fig. 6.

Cytochrome c oxidase acitivity (A and B) and Cidea expression (C) in white adipose tissues (eWAT, epididymal white adipose tissue; iWAT, inguinal white adipose tissue) of WT and UCP1-KO mice either acclimated to 27°C (WA) or 5°C (CA). Values are expressed as means ± SD; n = 4, except n = 3 for the WA KO group.