Lipid metabolism in mitochondrial membranes.
Mayr Johannes A
Journal of inherited metabolic disease
Mitochondrial membranes have a unique lipid composition necessary for proper shape and function of the organelle. Mitochondrial lipid metabolism involves biosynthesis of the phospholipids phosphatidylethanolamine, cardiolipin and phosphatidylglycerol, the latter is a precursor of the late endosomal lipid bis(monoacylglycero)phosphate. It also includes mitochondrial fatty acid synthesis necessary for the formation of the lipid cofactor lipoic acid. Furthermore the synthesis of coenzyme Q takes place in mitochondria as well as essential parts of the steroid and vitamin D metabolism. Lipid transport and remodelling, which are necessary for tailoring and maintaining specific membrane properties, are just partially unravelled. Mitochondrial lipids are involved in organelle maintenance, fission and fusion, mitophagy and cytochrome c-mediated apoptosis. Mutations in TAZ, SERAC1 and AGK affect mitochondrial phospholipid metabolism and cause Barth syndrome, MEGDEL and Sengers syndrome, respectively. In these disorders an abnormal mitochondrial energy metabolism was found, which seems to be due to disturbed protein-lipid interactions, affecting especially enzymes of the oxidative phosphorylation. Since a growing number of enzymes and transport processes are recognised as parts of the mitochondrial lipid metabolism, a further increase of lipid-related disorders can be expected.
Mitochondrial Uncoupling and the Regulation of Glucose Homeostasis.
Giralt Marta,Villarroya Francesc
Current diabetes reviews
INTRODUCTION:Mitochondrial uncoupling is a physiological process that has direct and indirect consequences on glucose homeostasis. Non-shivering thermogenesis in brown adipose tissue, which is the most well-recognized biological process related to the physiological uncoupling of mitochondria, is caused by uncoupling protein-1 (UCP1), which mediates a regulated permeabilization of the mitochondrial inner membrane to protons. CONCLUSION:The uncoupled brown fat mitochondria are specialized to produce heat by oxidizing large amounts of substrates, making brown fat a sink that can actively drain glucose from circulation. This has been confirmed in human studies in which active brown fat was detected by glucose-derivative-based positron emission tomography scans. Thus, UCP1-mediated activation of brown fat appears to be a likely mechanism through which hyperglycemia could be ameliorated. In other tissues, mitochondria are reported to be mildly uncoupled by the UCP1-like proteins, UCP2 and UCP3. The primary role of these other UCPs does not appear to be the oxidation of a metabolic substrate (e.g., glucose) for heat production; instead, they participate in other processes, such as regulating the production of reactive oxygen species and transporting certain metabolites across the mitochondrial membrane. UCP2 activity influences glucose homeostasis by fine tuning intracellular events related to the cellular energy status, thereby controlling insulin secretion, food intake behavior and adiponectin secretion in pancreatic .- cells, brain and white adipose tissue, respectively. UCP3 appears to be more specifically involved in promoting fatty acid oxidation in muscle, and is thus likely to influence glucose metabolism indirectly. Several genetic association studies have related polymorphisms in the genes encoding UCPs with obesity and/or type 2 diabetes phenotypes. In this review, we will focus on what is known about the specific role of mitochondrial uncoupling in glucose metabolism, and its implications in diabetes.