Mild cold induced thermogenesis: are BAT and skeletal muscle synergistic partners?

Abstract

There are two well-described thermogenic sites; brown adipose tissue (BAT) and skeletal muscle, which utilize distinct mechanisms of heat production. In BAT, mitochondrial metabolism is the molecular basis of heat generation, while it serves only a secondary role in supplying energy for thermogenesis in muscle. Here, we wanted to document changes in mitochondrial ultrastructure in these two tissue types based upon adaptation to mild (16°C) and severe (4°C) cold in mice. When reared at thermoneutrality (29°C), mitochondria in both tissues were loosely packed with irregular cristae. Interestingly, adaptation to even mild cold initiated ultrastructural remodeling of mitochondria including acquisition of more elaborate cristae structure in both thermogenic sites. The shape of mitochondria in the BAT remained mostly circular, whereas the intermyofibrilar mitochondria in the skeletal muscle became more elongated and tubular. The most dramatic remodeling of mitochondrial architecture was observed upon adaptation to severe cold. In addition, we report cold-induced alteration in levels of humoral factors: fibroblast growth factor 21 (FGF21), IL1α, peptide YY (PYY), tumor necrosis factor α (TNFα), and interleukin 6 (IL6) were all induced whereas both insulin and leptin were down-regulated. In summary, adaptation to cold leads to enhanced cristae formation in mitochondria in skeletal muscle as well as the BAT. Further, the present study indicates that circulating cytokines might play an important role in the synergistic recruitment of the thermogenic program including cross-talk between muscle and BAT.

Keywords: Core body temperature; brown adipose tissue; cold adaptation; cytokines; skeletal muscle; thermogenesis.

Figure 1. Effect of mild and severe cold adaptation on physiological parameters and skeletal muscle structure

(A) Tc of mice was ∼1°C lower than the average Tc at thermoneutrality in the first 6–10 h. After an initial drop in Tc, the mice were able to up-regulate and maintain Tc quite close to optimal. (B,C) VO₂ of mice during cold challenge shifts from that maintained at ambient temperature as indicated by arrow below the graph. (D) Severe, but not mild cold exposure impairs degree of ambulatory movement of the mice. (E) Adaptation to severe cold caused a greater decrease in body weight. (F) A significant increase in food intake was observed in both mild and severe cold conditions (G,H) Representative cross-sections of skeletal muscle fiber stained with H&E. Fiber cross-sectional area decreases after cold adaptation.

Figure 2. Changes in BAT upon adaptation to cold

(A) Representative images of histological sections of BAT stained with H&E. Cold adaptation caused depletion of lipid droplets in BAT adipocytes. Two representative images of TEM of BAT sections were taken at both lower (B) and higher magnifications (C); housing temperature is mentioned on the top of the H&E images. (D) Staining with anti-nectin4 antibody shows increased expression after adaptation to severe cold (4°C).

Figure 3. Ultrastructural changes in the skeletal muscle due to cold adaptation

(A–D) When mice are adapted to thermoneutrality, the number of intermyofibrilar mitochondria is low with fewer cristae, indicating ultrastructurally ill developed mitochondria. However, adaptation to cold increases abundance of mitochondria in the intermyofibrilar region of the quadriceps muscle. Higher magnification images show increased cristae density after adaptation to both mild and severe cold adaptation. Occasionally, lipid droplets were found in the intermyofibrilar area pointed by yellow arrows.

Figure 4. Plasma level of cytokines in mice adapted to different ambient temperatures

FGF21, IL6, IL1α, PYY, TNFα, leptin, insulin, amylin, and VEGF were measured. Data were analyzed using two-way multiple ANOVA. Comparison between FGF21 levels at thermoneutrality and mild cold was performed by Student’s ttest showed a trend for up-regulation although not statistically significant.