Electrostimulation during hindlimb unloading modulates PI3K-AKT downstream targets without preventing soleus atrophy and restores slow phenotype through ERK

Abstract

Our aim was to analyze the role of phosphatidylinositol 3-kinase (PI3K)-AKT and MAPK signaling pathways in the regulation of muscle mass and slow-to-fast phenotype transition during hindlimb unloading (HU). For that purpose, we studied, in rat slow soleus and fast extensor digitorum longus muscles, the time course of anabolic PI3K-AKT-mammalian target of rapamycin, catabolic PI3K-AKT-forkhead box O (FOXO), and MAPK signaling pathway activation after 7, 14, and 28 days of HU. Moreover, we performed chronic low-frequency soleus electrostimulation during HU to maintain exclusively contractile phenotype and so to determine more precisely the role of these signaling pathways in the modulation of muscle mass. HU induced a downregulation of the anabolic AKT, mammalian target of rapamycin, 70-kDa ribosomal protein S6 kinase, 4E-binding protein 1, and glycogen synthase kinase-3β targets, and an upregulation of the catabolic FOXO1 and muscle-specific RING finger protein-1 targets correlated with soleus muscle atrophy. Unexpectedly, soleus electrostimulation maintained 70-kDa ribosomal protein S6 kinase, 4E-binding protein 1, FOXO1, and muscle-specific RING finger protein-1 to control levels, but failed to reduce muscle atrophy. HU decreased ERK phosphorylation, while electrostimulation enabled the maintenance of ERK phosphorylation similar to control level. Moreover, slow-to-fast myosin heavy chain phenotype transition and upregulated glycolytic metabolism were prevented by soleus electrostimulation during HU. Taken together, our data demonstrated that the processes responsible for gradual disuse muscle plasticity in HU conditions involved both PI3-AKT and MAPK pathways. Moreover, electrostimulation during HU restored PI3K-AKT activation without counteracting soleus atrophy, suggesting the involvement of other signaling pathways. Finally, electrostimulation maintained initial contractile and metabolism properties in parallel to ERK activation, reinforcing the idea of a predominant role of ERK in the regulation of muscle slow phenotype.

Key Markers of mTORC1-Dependent and mTORC1-Independent Signaling Pathways Regulating Protein Synthesis in Rat Soleus Muscle During Early Stages of Hindlimb Unloading

“…It is well established, that proteinkinase mTORC1 is involved in the regulation of mRNA translation by phosphorylation of its downstream targets p70s6k and 4E-BP1 [11]. Surprisingly, we observed an increase in p70s6k phosphorylation after 1-and 3-day HS, although previously it had been shown a significant downregulation of p70s6k phosphorylation during longer periods of HS [19][20][21][22]. There is a growing body of evidence suggesting that mTOR-mediated p70s6k phosphorylation is not essential to sustain the normal maintenance of global protein synthesis.…”

“…For example, it has been shown that 14-day HS induces a significant reduction in phosphorylated AKT (Ser473) [19,20], p70s6k (Thr389) [19][20][21] and 4E-BP1 [19] in rat soleus muscle. At the same time, some authors did not find an increase in phosphorylated p70s6k (Thr389) in soleus after 10 days of mechanical unloading [22].…”

Electric Stimulation at 448 kHz Promotes Proliferation of Human Mesenchymal Stem Cells

“…Among these pathways, the mitogen-activated protein kinases (MAPK-ERK1/2) are molecules typically involved in transduction of proliferative signals from extracellular origin. The family of serine/threonine kinases to which ERK belong, can be activated by a large variety of chemical [47] and physical stimuli [54,55,56]. ERK1 and ERK2 are fully activated through phosphorylation by MEKs (MAPK/ERK kinases).…”

From physical inactivity to immobilization: Dissecting the role of oxidative stress in skeletal muscle insulin resistance and atrophy

“…Interestingly, skeletal muscle exhibits alteration in PI3K/Akt/mTORC1 axis during immobilization [75,76,[80][81][82]. This phenomenon, called anabolic resistance, emphasizes the reduced response to anabolic stimuli [80,82].…”