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

Chehade Karam,Heping Cheng,Yajuan Xiao,Jianjie Ma,Kaitao Li,Jingsong Zhou,Carlo Manno,Kamal Dhakal,Jianxun Yi,Jiejia Xu,Lin Zhang,Xuejun Li

doi:10.1186/s13395-017-0123-0

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.

Highlights

Motor neurons control muscle contraction by initiating action potentials in muscle
Denervation leads to drastic increase of mitoflash signal in skeletal muscle fibers Based on the biochemical study of Muller et al [10], it has been shown that the reactive oxygen species (ROS) production increases 30fold in skeletal muscle after 7 days of denervation
Our study reveals that physiological Ca2+ transient is important for maintaining the normal mitochondrial metabolic function by regulating the mitochondrial permeability transition pore (mPTP)-associated mitochondrial ROS generation

Summary

Introduction

Motor 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. Since denervation isolates muscle from motor neurons and deprives it from any electric stimulation, no action potentials are initiated, and 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. Skeletal muscle is responsible for voluntary movements of the entire body Because it comprises around 40% of whole-body lean mass of a human, skeletal muscle is essential for maintaining the homeostasis of the whole-body metabolism [1]. The initial cause of the mitochondrial ROS production in denervated skeletal muscle remains elusive [11]

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Conclusion