Mitochondrial fatty acid synthesis is an emergent central regulator of mammalian oxidative metabolism

Abstract

Contrary to their well-known functions in nutrient breakdown, mitochondria are also important biosynthetic hubs and express an evolutionarily conserved mitochondrial fatty acid synthesis (mtFAS) pathway. mtFAS builds lipoic acid and longer saturated fatty acids, but its exact products, their ultimate destination in cells, and the cellular significance of the pathway are all active research questions. Moreover, why mitochondria need mtFAS despite their well-defined ability to import fatty acids is still unclear. The identification of patients with inborn errors of metabolism in mtFAS genes has sparked fresh research interest in the pathway. New mammalian models have provided insights into how mtFAS coordinates many aspects of oxidative mitochondrial metabolism and raise questions about its role in diseases such as obesity, diabetes, and heart failure. In this review, we discuss the products of mtFAS, their function, and the consequences of mtFAS impairment across models and in metabolic disease.

Keywords: fatty acids; inborn errors of metabolism; lipid metabolism; lipids; mitochondria; mitochondrial fatty acid synthesis; mouse models; mtFAS.

Figure 1.. The mitochondrial fatty acid synthesis (mtFAS) pathway.

The synthesis of fatty acids within mitochondria relies upon the sequential addition of 2-carbon units to a growing acyl chain that is covalently attached to the acyl carrier protein (ACP). Mitochondrial malonyl-CoA, the committed substrate for mtFAS, enters the pathway via the enzyme MCAT, which transfers a malonyl group from coenzyme A (CoA) to ACP. This malonyl-ACP is condensed with an even-numbered acyl chain already in the cycle by OXSM, releasing CO₂, and extending the growing acyl chain by 2 carbons. Where this even-numbered chain comes from in the first cycle (acetyl-ACP, dashed line) is not well defined. The subsequent enzymes in the cycle (CBR4/HSD17B8, HTD2, MECR) catalyze a series of reduction and dehydration reactions that return the nascent acyl chain to a fully saturated state. Once the growing acyl chains reach 8, 14, or 16 carbons, they leave the pathway to execute their functions in cellular physiology. Blue text indicates enzymes and red text indicates mtFAS products.

Figure 2.. mtFAS mirrors cytosolic FAS.

The enzymatic steps in mtFAS and cytosolic FAS (catalyzed by FASN) are identical, although mtFAS utilizes individual enzymes for each step (blue text) whereas cytoplasmic FAS uses different domains encoded on a single polypeptide (FASN, orange text). The two carbons added during each round of elongation are always derived from malonate and are combined with acetyl-ACP in the first round, producing a 4-carbon acyl chain that becomes the substrate for future rounds. Multiple rounds of this cycle produce a single product in cytoplasmic FAS (palmitoyl-ACP), whereas data support the idea that mtFAS produces octanoyl-ACP and longer chain products (myristoyl-ACP, and palmitoyl-ACP).

Figure 3.. Functional roles of mtFAS.

mtFAS products contribute to the maintenance of oxidative metabolism through a variety of mechanisms. Acyl-ACPs of 8 carbons in length are converted to lipoic acid that is used in the lipoylation of numerous enzymes involved in maintenance of TCA function and the production of reduced cofactors that deliver electrons to the ETC. Longer chain acyl-ACPs interact with members of the LYRM family of proteins that support the assembly of ETC complexes and the production of FeS clusters. Evidence suggests mtFAS-derived acyl-ACPs of unknown length also regulate the production of ETC proteins via modulation of mitochondrial translation, in yeast via RNase P, and possibly in mammalian cells via MALSU1. Blue text indicates enzymes and red text indicates mtFAS products. GCSH = glycine cleavage system protein H, PDH = pyruvate dehydrogenase, OADH = 2-oxoadipate dehydrogenase, BCKDH = branched chain keto acid dehydrogenase, OGDH = 2-oxoglutarate dehydrogenase.

Figure 4.. Reported Tissue-Specific mtFAS Phenotypes in Mouse.

Emerging studies using novel genetic models have begun to provide insights into the role of mtFAS pathway genes in various physiological settings. Clockwise from top right: Cardiac Muscle: Cardiac-specific (Mlc2v-Cre) knockout of ACP results in fatal cardiac abnormalities, and overexpression is cardio-protective against ischemia-reperfusion injury. In contrast, overexpression of mtFAS enzyme MECR in cardiac muscle cells resulted in cardiac dysfunction. Purkinje Cells: Knockout of MECR in purkinje cells (Pcp2-Cre) results in cell death and associated symptoms of motor abnormalities, recapitulating patient phenotypes. Whole-body weight loss is also observed. Skeletal Muscle: Skeletal muscle-specific knockout (Mlc1f-Cre) of ACP is fatal at postnatal day 5, and results in dysregulation of whole-body glucose homeostasis. In contrast, overexpression of ACP (pan-tissue) protects against high-fat diet (HFD) induced metabolic abnormalities. Adipose Tissue: Adipose specific knockouts of HTD2 and MECR result in insulin resistance in white adipose tissue (Adipoq-Cre) and cold intolerance in brown adipose tissue (Ucp1-Cre). Liver: Liver-specific knockout of MECR (albumin-Cre) in mice results in altered glucose homeostasis. Bottom: Whole Body Knockouts: Knockouts of mtFAS machinery have severe symptoms that are incompatible with life^,.

Figure 5.. Looking Forward: The Role of mtFAS in Mammalian Cells.

While recent studies have provided fundamental insights into mtFAS function in cells, there are still many unknowns. Areas of particular interest and importance for the field include: 1) The nutrient sources (glucose, fatty acids, etc.) that contribute to mtFAS acyl chains. While Malonyl-CoA is required for chain elongation, both malonate and acetyl-CoA have been proposed as sources of malonyl-CoA. How mtFAS competes with other carbon uses (ie TCA) in the mitochondria is also unknown. 2) The activation and regulation of mtFAS machinery. The enzyme responsible for activation of mtACP (via 4’-phosphopantetheinylation) is unannotated, and other post-translational modifications of mtFAS machinery have not yet been investigated. Little is known about how potential modifications, protein turnover, and translation of mtFAS enzymes influence pathway activity. 3) Outputs of mtFAS. Known actions of mtFAS acyl chains include octanoate for LA synthesis and acylated ACP for ACP-LYRM interactions. The decision of whether octanoate exits the cycle for LA synthesis or undergoes continued elongation is a crucial control point, but how this decision is controlled is undescribed, and the relative proportions of each of these product pools is unknown. Moreover, for long-chain fatty acid products of mtFAS, their molecular identity (ie chain length), how they are removed from ACP (a thioesterase?), and ultimate destinations (i.e. incorporation into specific mitochondrial lipids? Degradation?) are undefined. 4) Communication of mtFAS activity to the cell. Many models are beginning to show large scale effects of mtFAS dysfunction on cellular phenotypes that include transcriptional rewiring. We do not understand the mechanism whereby mtFAS activity influences whole cell metabolism and gene expression. Whole cell metabolism and gene expression in turn influence cell function, stemness, and could have implications for whole body metabolism. How mtFAS functions change in specific cell types has limited characterization as well.