Muscle-specific and age-related changes in protein synthesis and protein degradation in response to hindlimb unloading in rats

Abstract

Disuse is a potent inducer of muscle atrophy, but the molecular mechanisms driving this loss of muscle mass are highly debated. In particular, the extent to which disuse triggers decreases in protein synthesis or increases in protein degradation, and whether these changes are uniform across muscles or influenced by age, is unclear. We aimed to determine the impact of disuse on protein synthesis and protein degradation in lower limb muscles of varied function and fiber type in adult and old rats. Alterations in protein synthesis and degradation were measured in the soleus, medial gastrocnemius, and tibialis anterior (TA) muscles of adult and old rats subjected to hindlimb unloading (HU) for 3, 7, or 14 days. Loss of muscle mass was progressive during the unloading period, but highly variable (-9 to -38%) across muscle types and between ages. Protein synthesis decreased significantly in all muscles, except for the old TA. Atrophy-associated gene expression was only loosely associated with protein degradation as muscle RING finger-1, muscle atrophy F-box (MAFbx), and Forkhead box O1 expression significantly increased in all muscles, but an increase in proteasome activity was only observed in the adult soleus. MAFbx protein levels were significantly higher in the old muscles compared with adult muscles, despite the old having higher expression of microRNA-23a. These results indicate that adult and old muscles respond similarly to HU, and the greatest loss in muscle mass occurs in predominantly slow-twitch extensor muscles due to a concomitant decrease in protein synthesis and increase in protein degradation. In this study, we showed that age did not intensify the atrophy response to unloading in rats, but rather that the degree of atrophy was highly variable across muscles, indicating that changes in protein synthesis and protein degradation occur in a muscle-specific manner. Our data emphasize the importance of studying muscles of varying fiber-type and physiological function at multiple time points to fully understand the molecular mechanisms responsible for disuse atrophy.

Early endplate remodeling and skeletal muscle signaling events following rat hindlimb suspension

“…Prolonged mechanical unloading of skeletal muscle as well as reduced neuromuscular activity leads to muscle wasting and functional decline (Baehr et al, 2017;Baldwin et al, 2013;Bodine, 2013) including the loss of neuromuscular junction integrity (Gonzalez-Freire et al, 2014;Nishimune et al, 2014;Rudolf et al, 2014;Tintignac et al, 2015;Wilson & Deschenes, 2005). A more complete understanding of the early disuse-induced molecular events is needed to identify important signaling pathways that lead to endplate disturbance.…”

“…Much less is known about the early molecular events which precede atrophy and operate within first days and even hours of disuse. These signaling events are of a special interest, highly debated and needs multiple time points of study to discover the molecular mechanisms that trigger disuse‐induced functional alterations (Baehr et al, ; Baldwin et al, ; Vilchinskaya et al, ). Endplates specifically contribute to maintenance of the safety factor for neuromuscular transmission (Ruff, ; Wood & Slater, ) and endplate lipid raft disturbance is among the earliest remodeling events induced by skeletal muscle disuse (Petrov et al, ).…”

Muscle unloading: A comparison between spaceflight and ground‐based models

“…These slow to fast fibre‐type transitions in humans and rodent muscles are probably an adaptation mechanism to restore power in the muscle undergoing atrophy and weakness. While the fibre‐type transformation is evident after 10‐14 days of unloading, corresponding changes at the transcript level start much earlier as only 1 day of HU results in ≈50% reduction in the transcript level of the type I MyHC pre‐mRNA . This reduction is in part because of reduced expression of the transcription factors that bind to the type I MyHC promoter region and also involves epigenetic changes impacting histone modification .…”

UBR5 is a novel E3 ubiquitin ligase involved in skeletal muscle hypertrophy and recovery from atrophy

“…For these experiments as a result of reference gene instability, after first assessing total RNA per mg tissue and then taking the C t expression value, the mRNA was described relative to tissue weight and subsequently reported as the fold change relative to control muscles, as described previously (Heinemeier et al 2009). An analysis that has previously been used extensively by the group (Baehr et al 2016;Baehr et al 2017;Hughes et al 2018).…”