CALL FOR PAPERS Mitochondrial Function/Dysfunction in Health and Disease Editorial Focus. A needle in a haystack: focus on “Proteomic alterations of distinct mitochondrial subpopulations in the type 1 diabetic heart”

DIABETES MELLITUS IS INCREASINGLY common throughout the world, due to both an increase in incidence and to decreased mortality of those with diabetes. Unfortunately, diabetes more than doubles the risk of cardiovascular disease in adults, and it even causes its own peculiar form of “diabetic” cardiomyopathy. These facts are conspiring, in turn, to increase the prevalence of heart failure (reviewed in Ref. 2). In fact, cardiovascular disease is now the leading cause of morbidity and mortality among individuals with diabetes, which raises the importance of understanding the mechanism of diabetic cardiomyopathy. Proteomics is a very appealing tool for the of study of complicated disease processes, such as diabetes, since it allows one to compare the protein expression profile of tissues, cells, or even organelles harvested from normal and diseased hearts. With luck, this unbiased approach might reveal the proverbial “needle in a haystack”—a specifically altered protein or pathway offering a potential therapeutic angle. However, as the results reported in the accompanying article by Baseler et al. (1) demonstrate, it is important to be certain that one is searching in the right “haystack.” Then, once the “needle” is found, there is also the not-insignificant task of working out what it means. Whereas the more common type II diabetes is due to loss of insulin sensitivity (“insulin resistance”), up to 10% of cases are due to insufficient insulin production and are categorized as type I. In both types, there is a progressive increase in blood glucose concentration which defines the disease. Paradoxically, despite the plentiful glucose, glucose oxidation rates actually decrease in the diabetic heart, and fatty acid oxidation increases (2). This method of ATP production is less efficient, and contributes to the development of heart failure. Diabetes can, therefore, be considered a metabolic disease, which naturally raises the possible involvement of the mitochondria in diabetic cardiomyopathy. Previous proteomic investigations of the ventricular myocardium of type I diabetic mouse models have detected an increase in mitochondrial proteins involved in fatty acid oxidation (2, 8, 19). Interpretation is complicated, though, by a potentially confounding increase in mitochondrial volume in the diabetic hearts (19). A subsequent proteomic analysis of mitochondria purified from type I diabetic rat hearts apparently confirmed an increase in proteins involved in fatty acid -oxidation, with little change in tricarboxylic acid cycle proteins and a modest decrease in electron transport proteins (21). However, in all studies of cardiac mitochondria, a critical factor is the method used for their separation and purification, since there are at least two distinct populations of mitochondria that require different techniques for their isolation. The mitochondria purified most easily, by simple cellular lysis and centrifugation, are the subsarcolemmal mitochondria (SSM), positioned just beneath the sarcolemma (Fig. 1A). These are believed to be specialized for their role in producing ATP for ion pumps, and hence provide energy for the maintenance of the plasma membrane potential and generation of the cardiac action potential (10% of all cellular energy expenditure) and maintenance of internal stores of calcium (15% energy expenditure) (10). In the above study by Turko and Murad (21), the mitochondria are likely to have been mainly of subsarcolemmal origin, raising the possibility that differences might be detected in other mitochondrial subpopulations (other “haystacks”). The ideal arrangement on a farm, presumably, is to locate the haystacks close to the livestock, i.e., the source of energy is close to its site of utilization. Indeed, one finds just such an arrangement in the cardiomyocyte, wherein the majority of mitochondria are positioned immediately adjacent to the myofibrillar contractile units, providing the energy for contractile force production, which, in itself, accounts for 75% of energy expenditure (10). The close physical association with these interfibrillar mitochondria (IFM) is important for efficient cardiac energy utilization and function (22). When an additional, mild proteolysis step is used to release the IFM from the myofibrils (15), they are found to have an innately higher respiratory rate than the SSM (15). Importantly, these differences are not an artefact of the isolation procedure, since the differences persist even if SSM are exposed to protease (15). On the other hand, a recent study suggesting that mitochondrial respiratory defects only become apparent in aged skeletal muscle after mitochondrial isolation and are not present in the original myofibers, may be cause for concern about the isolation procedure (16). The mitochondrial subpopulations also exhibit differences in internal structure, with the cristae of the IFM being relatively more tubular than the lamelliform cristae of the SSM (Fig. 1B), and these differences are reasonably well preserved during isolation (17). Though the IFM were believed to be relatively “fixed” in place due to the physical constraint of the sarcomeres, recent evidence suggests that they are able fuse into elongated, “thread”-like mitochondria under certain conditions (14). Conversely, in pathological situations, such as ischemia, hypertrophy, and diabetes, they may fragment into smaller individual units, though whether this reflects an injury or a Address for reprint requests and other correspondence: The Hatter Cardiovascular Institute, Univ. College London, 67 Chenies Mews, London, WC1E 6HX, UK (e-mail: [email protected]). Am J Physiol Regul Integr Comp Physiol 300: R183–R185, 2011; doi:10.1152/ajpregu.00751.2010. Editorial Focus