Urolithin A Protects Chondrocytes From Mechanical Overloading-Induced Injuries.

Yuchen He,Peter G Alexander,Hang Lin,Lauren Yocum,Michael J Jurczak

doi:10.3389/fphar.2021.703847

Abstract

Physiological mechanical stimulation has been shown to promote chondrogenesis, but excessive mechanical loading results in cartilage degradation. Currently, the underlying mechanotransduction pathways in the context of physiological and injurious loading are not fully understood. In this study, we aim to identify the critical factors that dictate chondrocyte response to mechanical overloading, as well as to develop therapeutics that protect chondrocytes from mechanical injuries. Specifically, human chondrocytes were loaded in hyaluronic hydrogel and then subjected to dynamic compressive loading under 5% (DL-5% group) or 25% strain (DL-25% group). Compared to static culture and DL-5%, DL-25% reduced cartilage matrix formation from chondrocytes, which was accompanied by the increased senescence level, as revealed by higher expression of p21, p53, and senescence-associated beta-galactosidase (SA-β-Gal). Interestingly, mitophagy was suppressed by DL-25%, suggesting a possible role for the restoration mitophagy in reducing cartilage degeneration with mechanical overloading. Next, we treated the mechanically overloaded samples (DL-25%) with Urolithin A (UA), a natural metabolite previously shown to enhance mitophagy in other cell types. qRT-PCR, histology, and immunostaining results confirmed that UA treatment significantly increased the quantity and quality of cartilage matrix deposition. Interestingly, UA also suppressed the senescence level induced by mechanical overloading, demonstrating its senomorphic potential. Mechanistic analysis confirmed that UA functioned partially by enhancing mitophagy. In summary, our results show that mechanical overloading results in cartilage degradation partially through the impairment of mitophagy. This study also identifies UA’s novel use as a compound that can protect chondrocytes from mechanical injuries, supporting high-quality cartilage formation/maintenance.

Highlights

Osteoarthritis (OA) is the most common degenerative joint disease affecting up to 240 million people around the world (Vos et al, 2016)
Safranin O/Fast green staining and COL2 Immunohistochemistry Staining (IHC) staining further indicated that DL-5% resulted in more cartilage matrix production from human chondrocytes than static culture (Figures 2B,C)
We found damaging dynamic loading results in increased cellular senescence, elevated inflammation, reduced mitochondrial quality, and impaired mitophagy in chondrocytes, which together leads to a poor cartilage formation

Summary

Introduction

Osteoarthritis (OA) is the most common degenerative joint disease affecting up to 240 million people around the world (Vos et al, 2016). As a result of the limited knowledge of OA pathogenic mechanisms, there are no disease-modifying osteoarthritis drugs (DMOADs) that can halt or reverse OA progression. Given that the main function of the knee joint is to bear weight, mechanical overloading and overuse may result in damage to cartilage, which initiates cartilage degradation and increases the likelihood of OA (Merkely et al, 2018). How these injurious mechanical cues are converted into biochemical signals and subsequently cause detrimental activities in cartilage and chondrocytes has been limited, but insightful. Other mechanisms involved in overloading-induced OA progression include primary cilia destruction, mitochondrial dysfunction, and impaired autophagy (He, Makarczyk, et al 2020, He et al 2016, Tong et al 2019, Zheng et al 2019)

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