Mitochondrial control of fuel switching via carnitine biosynthesis.

Environmental adaptation often involves a shift in energy utilization toward mitochondrial fatty acid oxidation, which requires carnitine. Besides dietary sources of animal origin, carnitine biosynthesis from trimethyllysine (TML) is essential, particularly for those who consume plant-based diets; however, its molecular regulation and physiological role remain elusive. Here, we identify SLC25A45 as a mitochondrial TML carrier that controls carnitine biosynthesis and fuel switching. SLC25A45 deficiency decreased the carnitine pool and impaired mitochondrial fatty acid oxidation, shifting reliance to carbohydrate metabolism. Slc25a45-deficient mice were cold-intolerant and resistant to lipid mobilization by glucagon-like peptide-1 receptor agonist (GLP-1RA), rendering them resistant to adipose tissue loss. Our study suggests that mitochondria serve as a regulatory checkpoint in fuel switching, with implications for metabolic adaptation and the efficacy of GLP-1RA–based anti-obesity therapy. Editor's summary: Our bodies have developed adaptations for using carbohydrates, lipids, or proteins as fuels, depending on availability. This flexibility also helps promote survival during fasting by drawing on adipose reserves, and the same mechanism is involved in thermogenesis to maintain body temperature. These adaptations rely on fatty acid oxidation, with carnitine facilitating the transport of long-chain fatty acids into mitochondria. Dietary intake of carnitine is not sufficient, particularly in those who maintain plant-based diets, and thus de novo carnitine synthesis is required for survival. Auger et al. elucidated the role of mitochondrial protein SLC25A45 in de novo carnitine synthesis and demonstrated its effects on both cold tolerance and weight loss on treatment with glucagon-like peptide-1 receptor agonists in mouse models (see the Perspective by Ramos-Lobo and Maechler). —Yevgeniya Nusinovich INTRODUCTION: Organisms seek optimal fuel and adjust their needs in response to environmental changes. Under conditions of limited food availability, peripheral metabolic organs gradually shift their metabolism from using glucose and carbohydrates to burning fat (fatty acid oxidation, FAO), while preserving glucose for the brain, a process known as fuel switching. Similarly, during chronic cold adaptation, brown adipose tissue (BAT) switches its fuel from glucose to fatty acids to maintain body temperature. This metabolic process is crucial for resilience to environmental stress and maintaining long-term metabolic health. In turn, under pathological conditions, such as heart failure, the heart relies more on glucose metabolism, highlighting the versatility of fuel choice in both health and disease. The mechanisms of fuel switching in response to internal and external cues are an important yet poorly understood phenomenon. l-carnitine is an essential metabolite that facilitates the import of long-chain fatty acids into mitochondria for FAO. Although carnitine can be obtained from dietary sources of animal origin, such as meat, the liver generates carnitine from its precursor, trimethyllysine (TML). This pathway—de novo carnitine biosynthesis—is critical, particularly for strict vegetarians, vegans, and herbivores, because most fruits and vegetables contain negligible amounts of carnitine. We sought to understand the molecular regulation of the carnitine biosynthesis pathway and its biological role in fuel switching during adaptation. RATIONALE: Carnitine biosynthesis begins with the import of TML into the mitochondrial matrix. Because the mitochondrial inner membrane is impermeable to metabolites, this step requires a designated carrier protein. Large-scale human genetic studies, combined with transcriptomics analyses, suggest a link between TML and SLC25A45, leading to the hypothesis that SLC25A45 regulates mitochondrial TML transport and carnitine synthesis. RESULTS: Using mouse genetics, cell-based assays, and biochemical reconstitution, we identified SLC25A45 as a mitochondrial carrier required for TML import and carnitine biosynthesis. Mice lacking SLC25A45 exhibited reduced carnitine levels, resulting in impaired FAO and reliance on carbohydrate metabolism. This altered fuel choice was particularly evident during cold adaptation, as mice lacking SLC25A45 were unable to maintain body temperature because of impaired FAO in the BAT. Supplementing carnitine restored cold tolerance, demonstrating that SLC25A45-dependent carnitine synthesis is required for BAT thermogenesis and cold adaptation. We further found that carnitine biosynthesis is essential for optimal efficacy of glucagon-like peptide-1 receptor agonists (GLP-1RAs), widely used anti-obesity and diabetic drugs. GLP-1RA treatment causes body-weight loss that is accompanied by reduced food intake and enhanced FAO; however, mice lacking SLC25A45 showed impaired fat mobilization and oxidation and reliance on carbohydrate metabolism, and were resistant to adipose tissue loss, even though GLP-1RA still lowered blood glucose levels. Restoring carnitine levels facilitated fat accumulation and weight loss, indicating that carnitine availability limits therapeutic efficacy in this context. CONCLUSION: This work highlights the role of mitochondrial metabolite transport as a regulatory checkpoint for fuel choice in fatty acid metabolism during adaptation to cold and fasting conditions. By defining the role of SLC25A45 in carnitine biosynthesis, this work reveals an unexpected role for carnitine in the optimal efficacy of GLP-1RA–based anti-obesity therapy. Mitochondrial carnitine biosynthesis via SLC25A45 is required for fuel switching during adaptation.: Mitochondrial TML import by SLC25A45 is a crucial step in the synthesis of l-carnitine, a necessary cofactor for long-chain fatty acid oxidation. SLC25A45 is required for fatty acid oxidation and thermogenesis during cold adaptation, as well as for optimal body-weight loss with semaglutide, a GLP-1 receptor agonist. CoA, coenzyme A; CPT1/2, carnitine palmitoyltransferase 1/2; HTML, 3-hydroxy-6-N-trimethyllysine; TMLHE, trimethyllysine hydroxylase, epsilon. [Figure produced by BioRender.com] [ABSTRACT FROM AUTHOR]