The mitochondrion is essential for energy metabolism and production of reactive oxygen species (ROS). In intact cells, respiratory mitochondria exhibit spontaneous "superoxide flashes", the quantal ROS-producing events consequential to transient mitochondrial permeability transition (tMPT). Here we perform the first in vivo imaging of mitochondrial superoxide flashes and tMPT activity in living mice expressing the superoxide biosensor mt-cpYFP, and demonstrate their coupling to whole-body glucose metabolism. Robust tMPT/superoxide flash activity occurred in skeletal muscle and sciatic nerve of anesthetized transgenic mice. In skeletal muscle, imaging tMPT/superoxide flashes revealed labyrinthine three-dimensional networks of mitochondria that operate synchronously. The tMPT/ superoxide flash activity surged in response to systemic glucose challenge or insulin stimulation, in an apparently frequency-modulated manner and involving also a shift in the gating mode of tMPT. Thus, in vivo imaging of tMPTdependent mitochondrial ROS signals and the discovery of the metabolism-tMPT-superoxide flash coupling mark important technological and conceptual advances for the study of mitochondrial function and ROS signaling in health and disease.

Mitochondria in many types of cells are dynamically interconnected through constant fusion and fission, allowing for exchange of mitochondrial contents and repair of damaged mitochondria. However, constrained by the myofibril lattice, the ∼6,000 mitochondria in the adult mammalian cardiomyocyte display little motility, and it is unclear how, if at all, they communicate with each other. By means of target-expressing photoactivatable green fluorescent protein (PAGFP) in the mitochondrial matrix or on the outer mitochondrial membrane, we demonstrated that the local PAGFP signal propagated over the entire population of mitochondria in cardiomyocytes on a time scale of ∼10 h. Two elemental steps of intermitochondrial communications were manifested as either a sudden PAGFP transfer between a pair of adjacent mitochondria (i.e., "kissing") or a dynamic nanotubular tunnel (i.e., "nanotunneling") between nonadjacent mitochondria. The average content transfer index (fractional exchange) was around 0.5; the rate of kissing was 1‰ s −1 per mitochondrial pair, and that of nanotunneling was about 14 times smaller. Electron microscopy revealed extensive intimate contacts between adjacent mitochondria and elongated nanotubular protrusions, providing a structural basis for the kissing and nanotunneling, respectively. We propose that, through kissing and nanotunneling, the otherwise static mitochondria in a cardiomyocyte form one dynamically continuous network to share content and transfer signals.mitochondrial dynamics | nanotubule I n most cells, mitochondria are highly dynamic organelles that constantly undergo shape changes through fusion and fission. This results in a form of communication that allows for the exchange and distribution of soluble and membranous components, including metabolic intermediates, signaling messengers, proteins, lipids, and mitochondrial DNAs, and provides a mechanism for repairing damaged mitochondria and maintaining a healthy mitochondrial population (1-3). Disorders of mitochondrial fusion and fission have been associated with developmental defects, neurodegenerative diseases, and cardiovascular diseases (4-7).In adult mammalian cardiomyocytes, mitochondria occupy ∼40% of the cell volume and are rigidly organized between bundles of myofilaments (interfibrillar mitochondria), under the sarcolemma (subsarcolemmal mitochondria), and around the nucleus (perinuclear mitochondria). This arrangement and the apparent lack of motility, a prerequisite for mitochondrial fusion in other types of cells, raise the question of whether mitochondria in adult cardiomyocyte communicate with each other dynamically. Mitochondrial dynamics in cardiac cell lines or neonatal cardiomyocytes (8) do not provide a direct answer to this question because mitochondria in these cells are not constrained strictly. Limited studies with adult cardiomyocytes have been inconclusive and even controversial. Whereas low-amplitude and high-frequency mitochondrial fluctuations have been visualized and quantified in living adult cardiomyo...

It has been theorized for decades that mitochondria act as the biological clock of ageing, but the evidence is incomplete. Here we show a strong coupling between mitochondrial function and ageing by in vivo visualization of the mitochondrial flash (mitoflash), a frequency-coded optical readout reflecting free-radical production and energy metabolism at the single-mitochondrion level. Mitoflash activity in Caenorhabditis elegans pharyngeal muscles peaked on adult day 3 during active reproduction and on day 9 when animals started to die off. A plethora of genetic mutations and environmental factors inversely modified the lifespan and the day-3 mitoflash frequency. Even within an isogenic population, the day-3 mitoflash frequency was negatively correlated with the lifespan of individual animals. Furthermore, enhanced activity of the glyoxylate cycle contributed to the decreased day-3 mitoflash frequency and the longevity of daf-2 mutant animals. These results demonstrate that the day-3 mitoflash frequency is a powerful predictor of C. elegans lifespan across genetic, environmental and stochastic factors. They also support the notion that the rate of ageing, although adjustable in later life, has been set to a considerable degree before reproduction ceases.