Subcellular heterogeneity of mitochondrial function and dysfunction: Evidence obtained by confocal imaging

Beyond their fundamental role in energy metabolism, mitochondria perform a great variety of other important functions ( e. g. in Ca2+ homeostasis, apoptosis, thermogenesis, etc.), thus suggesting their region-specific specializations and intracellular heterogeneity. Although mitochondrial functional heterogeneity has been demonstrated for several cell types, its origin and role under physiological and, in particular, pathophysiological conditions, where the extent of heterogeneity may significantly increase, remain to be elucidated. The present work thus investigated the static and dynamic heterogeneity of mitochondria and mitochondrial function in various cell types in which mitochondria may cope with specific functions including cardiomyocytes, hepatocytes and some cultured carcinoma cells. Modern confocal and two-photon fluorescent microscopy was used for the investigation and direct imaging of region-specific mitochondrial function and heterogeneity. Analysis of the autofluorescence of mitochondrial flavoproteins in hepatocytes and carcinoma cells permitted significant intracellular heterogeneity of mitochondrial redox state to be demonstrated. Comparative homogeneity and clear colocalization of mitochondrial flavoproteins, membrane potential and calcium-sensitive probes were observed in both isolated cardiomyocytes and permeabilized myocardial fibers. After ischemia reperfusion, however, or under conditions of substrate deprivation, significant heterogeneity of all these parameters was detected. Some methodological issues, mechanistic aspects, possible metabolic consequences of mitochondrial functional heterogeneity and its impact under pathological conditions are discussed.