Increased mitochondrial nanotunneling activity, induced by calcium imbalance, affects intermitochondrial matrix exchanges

Abstract

Exchanges of matrix contents are essential to the maintenance of mitochondria. Cardiac mitochondrial exchange matrix content in two ways: by direct contact with neighboring mitochondria and over longer distances. The latter mode is supported by thin tubular protrusions, called nanotunnels, that contact other mitochondria at relatively long distances. Here, we report that cardiac myocytes of heterozygous mice carrying a catecholaminergic polymorphic ventricular tachycardia-linked RyR2 mutation (A4860G) show a unique and unusual mitochondrial response: a significantly increased frequency of nanotunnel extensions. The mutation induces Ca 2+ imbalance by depressing RyR2 channel activity during excitation-contraction coupling, resulting in random bursts of Ca 2+ release probably due to Ca 2+ overload in the sarcoplasmic reticulum. We took advantage of the increased nanotunnel frequency in RyR2 A4860G+/− cardiomyocytes to investigate and accurately define the ultrastructure of these mitochondrial extensions and to reconstruct the overall 3D distribution of nanotunnels using electron tomography. Additionally, to define the effects of communication via nanotunnels, we evaluated the intermitochondrial exchanges of matrix-targeted soluble fluorescent proteins, mtDsRed and photoactivable mtPA-GFP, in isolated cardiomyocytes by confocal microscopy. A direct comparison between exchanges occurring at short and long distances directly demonstrates that communication via nanotunnels is slower. mitochondria | nanotunnels | CPVT | RyR2 | mitochondrial dynamics

Mitochondrial fusion dynamics is robust in the heart and depends on calcium oscillations and contractile activity

“…Thus, it might be speculated that the IMM undergoes fast intermittent fusion pore openings as well. Second, the observation of narrow connectors that resemble those previously described in heart mitochondria (16,34), along with the results of Lavorato et al (47) showing that in a model of catecholaminergic polymorphic ventricular tachycardia (RyR2-A4860G), nanotunnel structures coupled with slow matrix mixing events become more frequent, support the idea of nanotunnels mediating slow mitochondrial fusion in AVCMs. Third, ample ultrastructural evidence shows that the IMM is highly folded and the numerous cristae define narrow compartments of the matrix specifically in cardiac mitochondria (12,48,49), which might have more restricted communication compared with the matrix areas of mitochondria in other tissues.…”

“…Importantly, stopping and restarting of ECC activity does not occur under physiological conditions; thus, we can only claim that the maintenance of mitochondrial fusion activity involves a Ca 2+ oscillation/contraction-dependent factor in the heart. The potential pathophysiological significance of this factor is highlighted by the work of Lavorato et al (47), who report that a mutation in RyR2 (A4860G), which alters the normal Ca 2+ oscillation phenotype, causes a striking change in mitochondrial fusion dynamics.…”

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

“…Complete fusions are believed to result from the merge of two mitochondria moving towards each other along the same microtubule, and transient fusion events occur between mitochondria traveling along separate microtubules (Liu et al, 2009). In this latter case, merging mitochondria first appear to quickly engage oblique interactions before undergoing Drp1-induced fission due to the pulling forces exerted by the microtubule motor proteins, in a transient or kiss-and-run manner, and the mechanisms are currently under investigation (Jiang et al, 2017; Lavorato et al, 2017; Picard et al, 2015). …”

Tracking intra‐ and inter‐organelle signaling of mitochondria

“…These intermitochondrial contact sites are thought to transmit Ca 2+ or apoptotic signals across mitochondria or even a coupling of the mitochondrial membrane potential (Δψ m ) of neighboring mitochondria . Thereby, kissing and nanotunneling of mitochondria represent an alternative form of intermitochondrial communication . Notably, exchange of, for instance, matrix proteins through nanotunnels follows a slower kinetic compared to conventional fusion most likely due to their small diameter of < 100 nm .…”