Mct8-deficient mice have increased energy expenditure and reduced fat mass that is abrogated by normalization of serum T3 levels

Abstract

Children with monocarboxylate transporter 8 (MCT8) deficiency lose weight, even when adequately nourished. Changes in serum markers of thyroid hormone (TH) action compatible with thyrotoxicosis suggested that this might be due to T3 excess in peripheral tissues. Mct8-deficient mice (Mct8KO) replicate the human thyroid phenotype and are thus suitable for metabolic studies so far unavailable in humans. In the current work, compared with wild-type (Wt) mice, Mct8KO mice were leaner due to reduced fat mass. They tended to use more carbohydrates and fewer lipids during the dark phase. Mct8KO mice had increased total energy expenditure (TEE) and food and water intake, with normal total activity, indicating hypermetabolism. To determine whether this is due to the high serum T3, we studied mice deficient in both Mct8 and deiodinase 1 (Mct8D1KO) with serum T3 similar to Wt mice and Wt mice given L-T3 to raise their serum T3 to the level of Mct8KO mice. Contrary to Mct8KO, Mct8D1KO mice had similar fat mass, TEE, and food intake as their D1KO littermates, whereas T3-treated Wt mice showed increased food intake and TEE, similar to Mct8KO mice. In skeletal muscle, Mct8KO mice had increased T3 content and TH action and increased glucose metabolism, which improved in Mct8D1KO mice. These studies indicate that the high serum T3 in MCT8 deficiency increases the TEE and fails to maintain weight despite adequate calorie intake. This is mediated by tissues that are not predominantly MCT8 dependent for TH transport, including skeletal muscle. Normalizing serum T3 level by deleting deiodinase 1 corrects body composition and the metabolic alterations caused by the MCT8 deficiency.

Figure 1.

DEXA study of Wt, Mct8KO, D1KO, and Mct8D1KO mice. A, Body composition. B, Fat and lean mass expressed as percentage of total body mass. Genotypes are indicated. Asterisks represent P values by ANOVA and Newman-Keuls test. Significant differences compared with Wt animals are above the SEM bars. Comparisons of Mct8KO and Mct8D1KO are indicated by horizontal lines. *, P < .05; **, P < .01; ***, P < .001.

Figure 2.

Metabolic cage study of Wt, Mct8KO, D1KO, and Mct8D1KO mice. A, An example of total TEE flow chart. B, Average 24-hour food intake. C, Average 24-hour water intake. D, Average TEE. Genotypes are indicated. Symbols indicating statistical significances are as indicated in the legend to Figure 1.

Figure 3.

RER and substrate oxidation. A, RER, B, Glucose oxidation and C, Lipid oxidation in Wt, Mct8KO, D1KO, and Mct8D1KO mice. Genotypes are indicated. Symbols indicating statistical significances are as indicated in the legend to Figure 1.

Figure 4.

Measurements in skeletal muscle. A, T₃ content. B, Gene expression in soleus. C, Gene expression in gastrocnemius. Genotypes are indicated. Symbols indicating statistical significances are as in the legend to Figure 1. In addition, # and ##, P < .05 and P < .01, respectively, calculated by a 2-tailed Student's t test compared with Wt.

Figure 5.

Measurements in BAT. A, T₃ content. B, D2 enzymatic activity. C, Gene expression. Genotypes are indicated. Symbols indicating statistical significances are as in the legend to Figure 1.

Figure 6.

Metabolic cage study of Wt T₃-treated mice. A, Average 24-hour food intake, water intake, and TEE in Wt T₃-treated mice compared with Wt. B, Average 24-hour RER and glucose and lipid oxidation in Wt T₃-treated and Mct8KO mice compared with Wt. Genotypes are indicated. Statistical analyses were performed by a 2-tailed Student's t test for unpaired observations. *, P < .05; **, P < .01; ***, P < .001.

Figure 7.

Gene expression in gastrocnemius of Wt T₃-treated mice. A, TH target genes in Wt T₃-treated mice compared with Wt. B, Lipid metabolism genes in Wt T₃-treated and Mct8KO mice compared with Wt. Statistical analyses were performed by a 2-tailed Student's t test for unpaired observations. *, P < .05; **, P < .01.