Cardiolipin content controls mitochondrial coupling and energetic efficiency in muscle

Abstract

Unbalanced energy partitioning participates in the rise of obesity, a major public health concern in many countries. Increasing basal energy expenditure has been proposed as a strategy to fight obesity yet raises efficiency and safety concerns. Here, we show that mice deficient for a muscle-specific enzyme of very-long-chain fatty acid synthesis display increased basal energy expenditure and protection against high-fat diet-induced obesity. Mechanistically, muscle-specific modulation of the very-long-chain fatty acid pathway was associated with a reduced content of the inner mitochondrial membrane phospholipid cardiolipin and a blunted coupling efficiency between the respiratory chain and adenosine 5'-triphosphate (ATP) synthase, which was restored by cardiolipin enrichment. Our study reveals that selective increase of lipid oxidative capacities in skeletal muscle, through the cardiolipin-dependent lowering of mitochondrial ATP production, provides an effective option against obesity at the whole-body level.

Fig. 1. Increased energy expenditure protects Hacd1-KO mice against HFD-induced obesity.

(A) Spontaneous distance traveled per night by wild-type (WT) and Hacd1-KO mice. (B) Body weight evolution over life. (C) Body weight evolution of mice fed during 9 weeks with HFD, compared to age-matched mice fed with normal diet (ND). (D) Morphology of WT and Hacd1-KO mice after 9 weeks of HFD. (E) Glycemia measured after fasting and assessed during 120 min after an intraperitoneal glucose injection at T₀ in mice fed during 9 weeks with HFD. (F) Area under the glycemia curves displayed in (E). a.u., arbitrary units. (G) Total body fat percentage (adiposity index) after 9 weeks of ND or HFD, expressed as a percentage of the eviscerated body mass (lean body mass, LBM). (H) Feed efficiency during the 9-week period of HFD (body weight, BW). (I) Circadian energy expenditure measured by indirect calorimetry under HFD; the active period of night is shaded. Mean hourly energy expenditure during the assessment period is represented as histogram. (J) Estimated basal metabolism during the assessment period of (I) during daylight and night. Error bars ± SE; *P < 0.05, **P < 0.01, and ***P < 0.001 versus respective WT values; ^$$$P < 0.001 versus respective ND values. Photo credit for (D): Alexandre Prola, EnvA.

Fig. 2. Skeletal muscle of Hacd1-KO mice displays increased oxidative activity.

(A) Glycerol-3-phosphate dehydrogenase (GPDH), succinate dehydrogenase (SDH), and reduced form of nicotinamide adenine dinucleotide (NADH) dehydrogenase (NADH tetrazolium reductase reaction, NADH-TR) activity on tibialis anterior muscle sections. Note the decreased GPDH (glycolytic) and increased SDH and NADH-TR (oxidative) activity on Hacd1-KO mice sections. (B) Cytochrome c oxidase (COX; or complex IV) activity in superficial gastrocnemius and soleus muscles. (C) CS activity in superficial gastrocnemius and soleus muscles. IU, International Unit. (D and E) Representative immunoblots (D) and quantification (E) of CS, ATP5A, and VDAC normalized to β-actin in superficial gastrocnemius muscle. (F) Circadian fat oxidation under ND measured by indirect calorimetry; the active period of night is shaded. (G) Fat oxidation during the assessment period of (F) during daylight and night. (H) ¹⁴C-labeled linoleic acid consumption rates in isolated gastrocnemius muscle. Scale bars, 200 μm (A). Error bars ± SE; *P < 0.05, **P < 0.01, and ***P < 0.001 versus respective WT values.

Fig. 3. Remodeled structure and lowered respiratory coupling of mitochondria lead to reduced ATP production in muscle of Hacd1-KO mice.

(A) Transmission electron microscopy of longitudinal sections of myofibers from the superficial gastrocnemius muscle. Images are from datasets taken at a low (×2500, left), intermediate (×10,000, middle), and high (×30,000, right) magnification. The Z line is delimited by arrows. Cristae are regular tubular-shaped invaginations (white arrow heads) of the IMM, unambiguously identified on the same plane than the outer mitochondrial membrane (black arrow heads). Excessive dilation of cristae tips (asterisk) is frequently observed in Hacd1-KO myofibers. (B and C) Morphometric quantification of mitochondria containing cristae with excessively dilated tips (maximal diameter > 15 nm, in percentage) (B) and the mean of the maximal diameter of cristae (C). (D) Oxidation rate of freshly isolated mitochondria from tibialis anterior muscle in the presence of pyruvate plus ADP (phosphorylating), oligomycin (nonphosphorylating), and FCCP (uncoupled). (E) Mitochondrial coupling ratio for pyruvate (respiratory control ratio [RCR; state 3/state 4)] of isolated mitochondria from tibialis anterior muscle. (F) ATP production measured on isolated mitochondria from tibialis anterior muscle. (G) ATP/O ratio calculated from simultaneous recording of O₂ consumption and ATP production on isolated mitochondria from tibialis anterior muscle. (H) ATP content after submaximal exercise on treadmill in tibialis anterior muscle of Hacd1-KO mice, compared to WT mice, set to 1.0. (I to J) Representative immunoblots (I) and quantification (J) of phospho-AMPK (adenosine 5′-monophosphate–activated protein kinase) and AMPK in tibialis anterior muscle after submaximal exercise on treadmill. (K) Blood lactate concentration (lactatemia) before (at rest) and after (exercised) a submaximal exercise on treadmill. Scale bars, 100 nm (A). Error bars ± SE; *P < 0.05 and **P < 0.01 versus respective WT values; ^$$$P < 0.001 versus respective at rest values.

Fig. 4. Cardiolipin deficit in muscle mitochondria of Hacd1-KO mice is responsible for reduced mitochondrial coupling.

(A) Total cardiolipin content of mitochondria isolated from the tibialis anterior muscle, normalized to the mitochondrial protein content. (B) Heatmap of individually identified cardiolipin species in mitochondria isolated from tibialis anterior muscle of Hacd1-KO mice, compared to WT mice. Colors show row z scores. (C) Diagram depicting the experimental steps allowing to enrich mitochondrial membranes with phospholipids. (D) ATP/O ratio calculated from simultaneous recording of O₂ consumption and ATP production of native, cardiolipin-enriched (+CL), and phosphatidylcholine-enriched (+PC) mitochondria isolated from tibialis anterior muscle of WT and Hacd1-KO mice. Error bars ± SEM.; *P < 0.05 versus WT, ^$P ≤ 0.05 and ^$$P ≤ 0.01 versus native, and ^££P ≤ 0.01 versus +CL.