Mitofusin-2 Maintains Mitochondrial Structure and Contributes to Stress-Induced Permeability Transition in Cardiac Myocytes ▿ †

Mitofusin-2 (Mfn-2) is a dynamin-like protein that is involved in the rearrangement of the outer mitochondrial membrane. Research using various experimental systems has shown that Mfn-2 is a mediator of mitochondrial fusion, an evolutionarily conserved process responsible for the surveillance of mitochondrial homeostasis. Here, we find that cardiac myocyte mitochondria lacking Mfn-2 are pleiomorphic and have the propensity to become enlarged. Consistent with an underlying mild mitochondrial dysfunction, Mfn-2-deficient mice display modest cardiac hypertrophy accompanied by slight functional deterioration. The absence of Mfn-2 is associated with a marked delay in mitochondrial permeability transition downstream of Ca2+ stimulation or due to local generation of reactive oxygen species (ROS). Consequently, Mfn-2-deficient adult cardiomyocytes are protected from a number of cell death-inducing stimuli and Mfn-2 knockout hearts display better recovery following reperfusion injury. We conclude that in cardiac myocytes, Mfn-2 controls mitochondrial morphogenesis and serves to predispose cells to mitochondrial permeability transition and to trigger cell death

Ca2+ dynamics in the mitochondria - state of the art

The importance of [Ca(2+)] in the mitochondrial matrix, [Ca(2+)](mito), had been proposed by early work of Carafoli and others [1], [2] and [3]. The key suggestion in the 1970s [4] was that regulatory [Ca(2+)](mito) played a role in controlling the rate of activation of tricarboxylic acid cycle dehydrogenases, important in the regulation of ATP production by the electron transport chain (ETC) during oxidative phosphorylation. This view is now established [5] and [6] and the key questions currently debated are to what extent do the mitochondria acquire and release Ca(2+), and what impact do mitochondria have on the dynamic Ca(2+) signal in the cardiac ventricular myocyte [7]. Although investigations of Ca(2+) dynamics in mitochondria have been problematic, disparate and inconclusive, they have also been both provocative and exciting. A recent special issue of this journal presented contrasting perspectives on the speed, extent and mechanisms of changes in [Ca(2+)](mito), and how these changes may influence cellular spatio-temporal [Ca(2+)](i) dynamics [8]. An audio discussion is also available online [9]. The uncertain nature of the signaling pathways is noted in Table 1 (see below) which shows mitochondrial proteins and processes that are of current focus and which remain contentious. Each of the “items” listed is largely unsettled, or is a “work in progress”. There may be advocates for opposing positions noted or recent discoveries that must still be tested at multiple levels by diverse laboratories. Currently, the first item, the mitochondrial sodium/calcium exchanger (NCLX) [10], appears the most solid with respect to the molecular identification and physiological function, whereas, the recently described candidates of the mitochondrial Ca(2+) uniporter (MCU) [11] and [12] still need to be verified and broadly examined by the scientific community

Calcium movement in cardiac mitochondria

Existing theory suggests that mitochondria act as significant, dynamic buffers of cytosolic calcium ([Ca2+]i) in heart. These buffers can remove up to one-third of the Ca2+ that enters the cytosol during the [Ca2+]i transients that underlie contractions. However, few quantitative experiments have been presented to test this hypothesis. Here, we investigate the influence of Ca2+ movement across the inner mitochondrial membrane during both subcellular and global cellular cytosolic Ca2+ signals (i.e., Ca2+ sparks and [Ca2+]i transients, respectively) in isolated rat cardiomyocytes. By rapidly turning off the mitochondria using depolarization of the inner mitochondrial membrane potential (ΔΨm), the role of the mitochondria in buffering cytosolic Ca2+ signals was investigated. We show here that rapid loss of ΔΨm leads to no significant changes in cytosolic Ca2+ signals. Second, we make direct measurements of mitochondrial [Ca2+] ([Ca2+]m) using a mitochondrially targeted Ca2+ probe (MityCam) and these data suggest that [Ca2+]m is near the [Ca2+]i level (∼100 nM) under quiescent conditions. These two findings indicate that although the mitochondrial matrix is fully buffer-capable under quiescent conditions, it does not function as a significant dynamic buffer during physiological Ca2+ signaling. Finally, quantitative analysis using a computational model of mitochondrial Ca2+ cycling suggests that mitochondrial Ca2+ uptake would need to be at least ∼100-fold greater than the current estimates of Ca2+ influx for mitochondria to influence measurably cytosolic [Ca2+] signals under physiological conditions. Combined, these experiments and computational investigations show that mitochondrial Ca2+ uptake does not significantly alter cytosolic Ca2+ signals under normal conditions and indicates that mitochondria do not act as important dynamic buffers of [Ca2+]i under physiological conditions in heart