Absence of physiological Ca2+ transients is an initial trigger for mitochondrial dysfunction in skeletal muscle following denervation

Abstract

BackgroundMotor neurons control muscle contraction by initiating action potentials in muscle. Denervation of muscle from motor neurons leads to muscle atrophy, which is linked to mitochondrial dysfunction. It is known that denervation promotes mitochondrial reactive oxygen species (ROS) production in muscle, whereas the initial cause of mitochondrial ROS production in denervated muscle remains elusive. Since denervation isolates muscle from motor neurons and deprives it from any electric stimulation, no action potentials are initiated, and therefore, no physiological Ca2+ transients are generated inside denervated muscle fibers. We tested whether loss of physiological Ca2+ transients is an initial cause leading to mitochondrial dysfunction in denervated skeletal muscle.MethodsA transgenic mouse model expressing a mitochondrial targeted biosensor (mt-cpYFP) allowed a real-time measurement of the ROS-related mitochondrial metabolic function following denervation, termed “mitoflash.” Using live cell imaging, electrophysiological, pharmacological, and biochemical studies, we examined a potential molecular mechanism that initiates ROS-related mitochondrial dysfunction following denervation.ResultsWe found that muscle fibers showed a fourfold increase in mitoflash activity 24 h after denervation. The denervation-induced mitoflash activity was likely associated with an increased activity of mitochondrial permeability transition pore (mPTP), as the mitoflash activity was attenuated by application of cyclosporine A. Electrical stimulation rapidly reduced mitoflash activity in both sham and denervated muscle fibers. We further demonstrated that the Ca2+ level inside mitochondria follows the time course of the cytosolic Ca2+ transient and that inhibition of mitochondrial Ca2+ uptake by Ru360 blocks the effect of electric stimulation on mitoflash activity.ConclusionsThe loss of cytosolic Ca2+ transients due to denervation results in the downstream absence of mitochondrial Ca2+ uptake. Our studies suggest that this could be an initial trigger for enhanced mPTP-related mitochondrial ROS generation in skeletal muscle.Electronic supplementary materialThe online version of this article (doi:10.1186/s13395-017-0123-0) contains supplementary material, which is available to authorized users.

Mitochondrial Calcium Increase Induced by RyR1 and IP3R Channel Activation After Membrane Depolarization Regulates Skeletal Muscle Metabolism

“…Cepia3 mt (Kd 11 μM) exhibited significant preferential mitochondria localization (evaluated through its co-localization with mtDsRed). It was previously reported that in muscle fibers the maximal mitochondrial Ca 2+ increase reached during a prolonged depolarization pulse exhibits a small delay compared to the cytoplasmic Ca 2+ transient (Karam et al, 2017 ). Nevertheless, the kinetics of Cepia3 mt compared to other mitochondrial Ca 2+ sensors appears to be even slower (Rudolf et al, 2004 ; Karam et al, 2017 ); this might be explained by Cepia3 mt characteristics, other than its Kd (Suzuki et al, 2014 ).…”

Distinct signal transductions in fast- and slow- twitch muscles upon denervation

“…Denervation is reported to cause muscle oxidative stress, evidenced by high level of reactive oxygen species (ROS) (Karam et al. ). ROS in turns upregulates PGC1 α expression via AMPK (Irrcher et al.…”

“…AMPK-PGC1a pathway may selectively serve as a compensatory mechanism in soleus, in contrast to EDL that primarily uses AKT as compensatory mechanism during atrophy. Denervation is reported to cause muscle oxidative stress, evidenced by high level of reactive oxygen species (ROS) (Karam et al 2017). ROS in turns upregulates PGC1a expression via AMPK (Irrcher et al 2009).…”

Mitochondrial ROS cause motor deficits induced by synaptic inactivity: Implications for synapse pruning

“…In support, skeletal muscle activity and inactivity decreases and increases mitochondrial ROS, respectively [21] , [22] , [23] , [24] . Further, surgically abolishing skeletal muscle activity increases mitochondrial ROS ex vivo [25] , [26] , [27] , [28] . We propose that: mitochondrial ROS are endogenous synaptic activity sentinels.…”