Mechanical Overloading Induced-Activation of mTOR Signaling in Tendon Stem/Progenitor Cells Contributes to Tendinopathy Development.

James H.-C. Wang,Wang Wang,Daibang Nie,Yiqin Zhou,Wang Wang,Jianying Zhang

doi:10.3389/fcell.2021.687856

Abstract

Despite the importance of mechanical loading in tendon homeostasis and pathophysiology, the molecular responses involved in the mechanotransduction in tendon cells remain unclear. In this study, we found that in vitro mechanical loading activated the mammalian target of rapamycin (mTOR) in rat patellar tendon stem/progenitor cells (TSCs) in a stretching magnitude-dependent manner. Application of rapamycin, a specific inhibitor of mTOR, attenuated the phosphorylation of S6 and 4E-BP1 and as such, largely inhibited the mechanical activation of mTOR. Moreover, rapamycin significantly decreased the proliferation and non-tenocyte differentiation of PTSCs as indicated by the reduced expression levels of LPL, PPARγ, SOX-9, collagen II, Runx-2, and osteocalcin genes. In the animal studies, mice subjected to intensive treadmill running (ITR) developed tendon degeneration, as evidenced by the formation of round-shaped cells, accumulation of proteoglycans, and expression of SOX-9 and collagen II proteins. However, daily injections of rapamycin in ITR mice reduced all these tendon degenerative changes. Collectively, these findings suggest that mechanical loading activates the mTOR signaling in TSCs, and rapamycin may be used to prevent tendinopathy development by blocking non-tenocyte differentiation due to mechanical over-activation of mTOR in TSCs.

Highlights

Tendons are connective tissues that transmit mechanical force to bone to enable joint movement
We found that both 4 and 8% stretching increased the expression of phospho-S6, compared to the control Patellar TSCs (PTSCs) cultured in the same medium but without stretching (Figure 1A)
This indicates that mechanical loading activates mammalian target of rapamycin (mTOR) signaling in a loading magnitude-dependent manner, and such mechanical activation of mTOR can be inhibited by rapamycin treatment

Summary

Introduction

Tendons are connective tissues that transmit mechanical force (i.e., muscle contraction) to bone to enable joint movement. Tendons contain stem/progenitor cells (Bi et al, 2007; Zhang and Wang, 2010a) These tendon stem cells (TSCs) possess common adult stem cell characteristics, and they play a crucial role in tendon development, homeostasis, and repair (Popov et al, 2015; Zhang et al, 2019). We showed that excessive mechanical loading can cause aberrant differentiation of TSCs (Zhang and Wang, 2010b). Repetitive mechanical loading induces high levels of prostaglandin E2 (PGE2) production in tendons, which may induce the differentiation of TSCs into non-tenocytes (Zhang and Wang, 2010c). Years of research have generally concluded that excessive mechanical loading might lead to the development of degenerative tendinopathy commonly seen in clinical settings (Soslowsky et al, 2000; Glazebrook et al, 2008; Abraham et al, 2011; Ng et al, 2011; Zhang and Wang, 2013)

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Conclusion