Denervation-Induced Activation of the Ubiquitin-Proteasome System Reduces Skeletal Muscle Quantity Not Quality

Abstract

It is well known that the ubiquitin-proteasome system is activated in response to skeletal muscle wasting and functions to degrade contractile proteins. The loss of these proteins inevitably reduces skeletal muscle size (i.e., quantity). However, it is currently unknown whether activation of this pathway also affects function by impairing the muscle’s intrinsic ability to produce force (i.e., quality). Therefore, the purpose of this study was twofold, (1) document how the ubiquitin-proteasome system responds to denervation and (2) identify the physiological consequences of these changes. To induce soleus muscle atrophy, C57BL6 mice underwent tibial nerve transection of the left hindlimb for 7 or 14 days (n = 6–8 per group). At these time points, content of several proteins within the ubiquitin-proteasome system were determined via Western blot, while ex vivo whole muscle contractility was specifically analyzed at day 14. Denervation temporarily increased several key proteins within the ubiquitin-proteasome system, including the E3 ligase MuRF1 and the proteasome subunits 19S, α7 and β5. These changes were accompanied by reductions in absolute peak force and power, which were offset when expressed relative to physiological cross-sectional area. Contrary to peak force, absolute and relative forces at submaximal stimulation frequencies were significantly greater following 14 days of denervation. Taken together, these data represent two keys findings. First, activation of the ubiquitin-proteasome system is associated with reductions in skeletal muscle quantity rather than quality. Second, shortly after denervation, it appears the muscle remodels to compensate for the loss of neural activity via changes in Ca2+ handling.

Denervation-Induced Activation of the Standard Proteasome and Immunoproteasome

“…In fact, α7, β1, β2, β5 and PA700 were all significantly greater at 7 and/or 14 days post-denervation when compared to control muscle (day 0). These results are consistent with that of others who demonstrated similar findings at the mRNA and/or protein level following 3, 7 and/or 14 days after denervation [5,11–14]. We also observed a significant increase in several components associated with the immunoproteasome.…”

“…Mice from the DN group underwent tibial nerve transection on the left hindlimb for either 7 or 14 days, similar to that previously described [2,5,23]. Mice were anesthetized by inhalation of 2.5% isoflurane and prepped for surgery.…”

“…In conditions of skeletal muscle atrophy (e.g., denervation), muscle mass is reduced, which ultimately impairs the muscle’s force producing capacity and ability to function [1–5]. Loss of muscle mass, also referred to as muscle atrophy, is primarily due to an imbalance between protein turnover with degradation exceeding synthesis.…”

Uncoupling protein 1 expression in adipocytes derived from skeletal muscle fibro/adipogenic progenitors is under genetic and hormonal control

“…It would be also important to assess intramuscular Ucp1 expression in other models of atrophy. This will, however, likely require experiments with additional animal species because fatty infiltration is generally not observed during denervation atrophy [66][67][68][69] or cancer cachexia 65,[70][71][72] in mice.…”

Downhill exercise alters immunoproteasome content in mouse skeletal muscle

“…Among the β subunits, β1, β5 and β2 are catalytically active and cleave after acidic, hydrophobic and basic amino acid residues, respectively (Bedford et al 2010;Coux et al 1996;Kloetzel 2001;Tanaka 2009). Several components of the ubiquitin-proteasome system have been shown to increase during atrophic states (e.g., denervation) and following unaccustomed exercise (Attaix et al 2005;Baumann et al 2016b;Reid 2005;Willoughby et al 2003), demonstrating that the standard proteasome is an essential mediator of skeletal muscle remodeling (Reid 2005).…”