The energy demand of the human heart is largely covered by fatty acid oxidation in mitochondria. Critical enzymes of transport and oxidation of fatty acids are located in mitochondrial membranes, where cardiolipin (CL) is an essential phospholipid. An inherited defect in the biosynthesis of CL in Barth Syndrome (BTHS) results in cardiomyopathy, skeletal myopathy, growth retardation and neutropenia. CL deficiency causes structural remodeling of the respiratory chain, defects in the Krebs cycle and in the mitochondrial calcium uniporter (MCU). Calcium signals ensure the energetic coupling of mitochondrial and cytosolic metabolism, which is required for maintaining the redox balance under conditions of increased cardiac workload. Defects in redox homeostasis provides the trigger and substrate for arrhythmias in BTHS. Consistent with the structural defect in the mitochondria, defects in energy conversion and an inability to accelerate workload under exercise conditions signify the metabolism in BTHS patients.

How dysfunctional mitochondria affect cellular metabolism to cause defects in energy conversion and redox homeostasis in BTHS is currently unresolved. Using a mouse model and patient iPSC derived cardiomyocytes, we measured lower rates of fatty acid oxidation in isolated mitochondria (Oroboros) and cells (Seahorse). Western blot analysis, qPCR and microscopic tracing of fluorescently labeled fatty acids revealed a defect not only in oxidation but also in the transport of fatty acids. Consequently, PET-CT imaging revealed substantially reduced cardiac fatty acid oxidation rates, in vivo. The defect in energy conversion triggers retrograde signaling pathways and substantial changes in cellular metabolism. Transcriptome analysis revealed stress induced signaling pathways involving the Atf4 transcription factor, which induces metabolic pathways related to amino acid metabolism. Using carbon tracing experiments we found increased metabolic flux into the biogenesis of glutathione biosynthesis, which serves to compensate defects in redox homeostasis. PET-CT imaging using the novel ¹⁸F-FASu radiotracer revealed elevated activity of the system xCT amino acid transporter in the BTHS mouse heart to support biosynthesis of glutathione precursors. Stress induced signaling pathways also induce an elevated anaplerotic glutamine metabolism, which supplements Krebs cycle and energy converting pathways. In vivo metabolomic flux analysis in the BTHS mouse heart confirmed the extensive metabolic remodeling. In summary, these data highlight a novel role of amino acid metabolism to compensate for metabolic defects in heart disease. Understanding the complex metabolic remodeling driven by retrograde signaling pathways allows the development of new strategies for therapeutic intervention in cardiomyopathies with mitochondrial dysfunctions.