Autophagy guards tendon homeostasis

AMP-activated protein kinase (AMPK) is essential for tendon homeostasis and prevents premature senescence and ectopic calcification

Tendinopathy is a debilitating tendon disorder affecting millions of people, characterized by pain, swelling, and diminished biomechanical properties. While the precise mechanisms underlying tendon homeostasis remain unclear, metabolic regulation plays a critical role. In this study, we combine transcriptomic analysis of human tendinopathic samples with a conditional mouse model in which Prkaa1 (encoding AMPKα1) is selectively deleted in tendon progenitors to elucidate the role of AMPK signaling in tendon homeostasis. RNA sequencing of diseased human tendons revealed downregulation of key metabolic genes, including several involved in the mitochondrial electron transport chain and AMPK signaling pathways, alongside an increase in markers associated with senescence and a secretory inflammatory profile. In parallel, mice with loss of Prkaa1 function exhibited normal postnatal development; however, by one month of age, tendons demonstrated widespread transcriptional alterations related to cell cycle regulation and ECM organization. By three months, AMPKα1-deficient tendons showed significant reductions in mechanical strength and increased expression of senescence markers p21 and p16, progressing to prominent ectopic calcification with age. In vitro studies further confirmed that tendon fibroblasts lacking AMPKα1 have altered ECM substrate adhesion profiles. Importantly, voluntary exercise partially rescued these deficits by enhancing ECM organization and reducing senescence marker expression. Collectively, our findings demonstrate that AMPKα1 is critical for maintaining energy balance, regulating ECM remodeling, and preventing premature cellular senescence in tendon. These insights highlight AMPK signaling as a promising therapeutic target and underscore the beneficial role of exercise in mitigating tendinopathic changes.

Yanghe decoction alleviates osteoarthritis by AMPK-SIRT3 positive feedback loop-mediated mitochondrial autophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Yanghe Decoction(YHD) is a traditional Chinese medicine compound known for its efficacy in treating osteoarthritis (OA).

AIM OF THE STUDY

We aimed to explore the underlying mechanisms of YHD in relation to OA.

MATERIALS AND METHODS

UHPLC-MS technology was used to identify the material basis of YHD. In vivo, OA rat model was induced by the modified Hulth method and then treated with YHD at three different doses (0.625, 1.3 and 2.6 g/kg/d). In vitro,YHD-Contained serum was prepared and administrated into rat chondrocytes, followed by simulation of Lipopolysaccharide(LPS). The protective mechanism was determined by observation of morphology, Flow cytometry and Protein level detection.

RESULTS

In vivo, YHD reduced chondrocyte apoptosis and joint inflammation while promoting mitophagy. It also elevated the protein levels of p-AMPK, SIRT3, PINK1, Parkin, and LC3II/I. In vitro, YHD-Contained Serum reduced chondrocyte apoptosis, decreased mitochondrial ROS, enhanced mitochondrial membrane potential, and upregulated the protein expressions of p-AMPK, SIRT3, PINK1, Parkin, and LC3II/I.

CONCLUSION

Through this study, we demonstrated YHD protect chondrocytes against apoptosis, and its underlying mechanisms may involve the regulation of AMPK-SIRT3 positive feedback loop and activation of PINK1/Parkin mediated mitophagy.

Autophagy modulates tenogenic differentiation of cartilage-derived stem cells in response to mechanical tension via FGF signaling

BACKGROUND

In our previous study, we demonstrated that cartilage-derived stem cells (CDSCs) possess multi-differentiation potential, enabling direct bone-to-tendon structure regeneration after transplantation in a rat model. Therefore, the objective of this study is to investigate whether CDSCs are a suitable candidate for achieving biological regeneration of tendon injuries.

METHODS

Tenogenic differentiation was evaluated through cell morphology observation, PCR, and Western blot (WB) analysis. Autophagic flux, transmission electron microscopy, and WB analysis were employed to elucidate the role of autophagy during CDSC tenogenic differentiation. Cell survival and tenogenesis of transplanted CDSCs were assessed using fluorescence detection of gross and frozen section images. Heterotopic ossification and quality of tendon healing were evaluated by immunofluorescence, hematoxylin-eosin (H&E), and Safrinin O/Fast Green stains.

RESULTS

We found autophagy is activated in CDSCs when treated with cyclic tensile stress, which facilitates the preservation of their chondrogenic potential while impeding tenogenic differentiation. Inhibiting autophagy with chloroquine promoted tenogenic differentiation of CDSCs in response to cyclic tensile stress through activation of the Fgf2/Fgfr2 signaling pathway. This mechanism was further validated by 2 mouse transplantation models, revealed that autophagy inhibition could enhance the tendon regeneration efficacy of transplanted CDSCs at the patellar tendon resection site.

CONCLUSION

Our findings provide insights into CDSC transplantation for achieving biological regeneration of tendon injuries, and demonstrate how modulation of autophagy in CDSCs can promote tenogenic differentiation in response to tensile stress both in vivo and in vitro.

Effect of human myoblasts on tenogenic progression in 2D and 3D culture models

Tendon injuries and disorders associated with mechanical tendon overuse are common musculoskeletal problems. Even though tendons play a central role in human movement, the intrinsic healing process of tendon is very slow. So far, it is known that tendon cell activity is supported by several interstitial cells within the tendon. However, the interplay between the tendon and the adjacent muscle for tendon regeneration and development processes has not been fully investigated. Here, we tested whether factors released from muscle derived myogenic cells (myoblasts) enhance tenogenic progressions of human tendon derived cells (tendon fibroblasts) using two-dimensional (2D) culture model and a three-dimensional (3D)-engineered tendon construct culture model, which mimics tendon regeneration and development. The conditioned media from myoblasts and unconditioned media as control were applied to tendon fibroblasts. In 2D, immunofluorescence analysis revealed increased collagen type I expressing area and increased migration potential when conditioned media from myoblasts were applied. In the 3D-engineered human tendon construct model, wet weight, diameter, and cross-sectional area of the tendon constructs were increased in response to the application of conditioned media from myoblasts, whereas the collagen density was lower and mechanical function was reduced both at the functional level (maximum stiffness) and the material level (maximum stress and modulus). These results indicate that myoblast-derived factors extend collagen expressing area and enhance migration of tendon fibroblasts, while factors involved in the robustness of extra-cellular matrix deposition of tissue-engineered tendon constructs are lacking. Our findings suggest that adjacent muscle affects the signaling interplay in tendons.

Eupalinolide A attenuates trauma-induced heterotopic ossification of tendon in mice by promoting YAP degradation through TOLLIP-mediated selective autophagy

BACKGROUND

Inhibiting the aberrant osteogenic differentiation of tendon-derived stem cells (TDSCs) is an effective strategy for treating traumatic heterotopic ossification (HO) in tendons.

PURPOSE

This study aimed to investigate whether eupalinolide A (EA) could prevent tendon HO progression by suppressing the osteogenic differentiation of TDSCs.

METHODS

The effects of EA on osteogenic differentiation and key signaling pathways in TDSCs were examined in vitro to assess its therapeutic potential and elucidate the underlying molecular mechanisms. Furthermore, the therapeutic efficacy of EA was evaluated in a mouse model of trauma-induced tendon HO via local injection therapy.

RESULTS

EA significantly inhibited the osteogenic differentiation of TDSCs by targeting YAP in vitro. Specifically, EA facilitated the recruitment of E3 ubiquitin ligase HECW1, which mediated K27-linked polyubiquitination of YAP, leading to its degradation via the TOLLIP-mediated selective autophagy pathway. In vivo, EA mitigated trauma-induced tendon HO by inhibiting the YAP pathway.

CONCLUSIONS

EA could be a potential therapeutic agent for treating traumatic tendon HO. The therapeutic target HECW1 involved in YAP degradation via autophagy presents a new therapeutic avenue to attenuate the progression of traumatic tendon HO.

Disruption of day-to-night changes in circadian gene expression with chronic tendinopathy

Overuse injury in tendon tissue (tendinopathy) is a frequent and costly musculoskeletal disorder and represents a major clinical problem with unsolved pathogenesis. Studies in mice have demonstrated that circadian clock-controlled genes are vital for protein homeostasis and important in the development of tendinopathy. We performed RNA sequencing, collagen content and ultrastructural analyses on human tendon biopsies obtained 12 h apart in healthy individuals to establish whether human tendon is a peripheral clock tissue and we performed RNA sequencing on patients with chronic tendinopathy to examine the expression of circadian clock genes in tendinopathic tissues. We found time-dependent expression of 280 RNAs including 11 conserved circadian clock genes in healthy tendons and markedly fewer (23) differential RNAs with chronic tendinopathy. Further, the expression of COL1A1 and COL1A2 was reduced at night but was not circadian rhythmic in synchronised human tenocyte cultures. In conclusion, day-to-night changes in gene expression in healthy human patellar tendons indicate a conserved circadian clock as well as the existence of a night reduction in collagen I expression. KEY POINTS: Tendinopathy is a major clinical problem with unsolved pathogenesis. Previous work in mice has shown that a robust circadian rhythm is required for collagen homeostasis in tendons. The use of circadian medicine in the diagnosis and treatment of tendinopathy has been stifled by the lack of studies on human tissue. Here, we establish that the expression of circadian clock genes in human tendons is time dependent, and now we have data to corroborate that circadian output is reduced in diseased tendon tissues. We consider our findings to be of significance in advancing the use of the tendon circadian clock as a therapeutic target or preclinical biomarker for tendinopathy.

Mitochondrial destabilization in tendinopathy and potential therapeutic strategies

Tendinopathy is a prevalent aging-related disorder characterized by pain, swelling, and impaired function, often resulting from micro-scarring and degeneration caused by overuse or trauma. Current interventions for tendinopathy have limited efficacy, highlighting the need for innovative therapies. Mitochondria play an underappreciated and yet crucial role in tenocytes function, including energy production, redox homeostasis, autophagy, and calcium regulation. Abnormalities in mitochondrial function may lead to cellular senescence. Within this context, this review provides an overview of the physiological functions of mitochondria in tendons and presents current insights into mitochondrial dysfunction in tendinopathy. It also proposes potential therapeutic strategies that focus on targeting mitochondrial health in tenocytes. These strategies include: (1) utilizing reactive oxygen species (ROS) scavengers to mitigate the detrimental effects of aberrant mitochondria, (2) employing mitochondria-protecting agents to reduce the production of dysfunctional mitochondria, and (3) supplementing with exogenous normal mitochondria. In conclusion, mitochondria-targeted therapies hold great promise for restoring mitochondrial function and improving outcomes in patients with tendinopathy. The translational potential of this article: Tendinopathy is challenging to treat effectively due to its poorly understood pathogenesis. This review thoroughly analyzes the role of mitochondria in tenocytes and proposes potential strategies for the mitochondrial treatment of tendinopathy. These findings establish a theoretical basis for future research and the clinical translation of mitochondrial therapy for tendinopathy.

Aging and Autophagy: Roles in Musculoskeletal System Injury

Aging is a multifactorial process that ultimately leads to a decline in physiological function and a consequent reduction in the health span, and quality of life in elderly population. In musculoskeletal diseases, aging is often associated with a gradual loss of skeletal muscle mass and strength, resulting in reduced functional capacity and an increased risk of chronic metabolic diseases, leading to impaired function and increased mortality. Autophagy is a highly conserved physiological process by which cells, under the regulation of autophagy-related genes, degrade their own organelles and large molecules by lysosomal degradation. This process is unique to eukaryotic cells and is a strict regulator of homeostasis, the maintenance of energy and substance balance. Autophagy plays an important role in a wide range of physiological and pathological processes such as cell homeostasis, aging, immunity, tumorigenesis and neurodegenerative diseases. On the one hand, under mild stress conditions, autophagy mediates the restoration of homeostasis and proliferation, reduction of the rate of aging and delay of the aging process. On the other hand, under more intense stress conditions, an inadequate suppression of autophagy can lead to cellular aging. Conversely, autophagy activity decreases during aging. Due to the interrelationship between aging and autophagy, limited literature exists on this topic. Therefore, the objective of this review is to summarize the current concepts on aging and autophagy in the musculoskeletal system. The aim is to better understand the mechanisms of age-related changes in bone, joint and muscle, as well as the interaction relationship between autophagy and aging. Its goal is to provide a comprehensive perspective for the improvement of diseases of the musculoskeletal system.

Pristimerin suppresses AIM2 inflammasome by modulating AIM2-PYCARD/ASC stability via selective autophagy to alleviate tendinopathy

Macroautophagy/autophagy plays an important role in regulating cellular homeostasis and influences the pathogenesis of degenerative diseases. Tendinopathy is characterized by tendon degeneration and inflammation. However, little is known about the role of selective autophagy in tendinopathy. Here, we find that pristimerin (PM), a quinone methide triterpenoid, is more effective in treating tendinopathy than the first-line drug indomethacin. PM inhibits the AIM2 inflammasome and alleviates inflammation during tendinopathy by promoting the autophagic degradation of AIM2 through a PYCARD/ASC-dependent manner. A mechanistic study shows that PM enhances the K63-linked ubiquitin chains of PYCARD/ASC at K158/161, which serves as a recognition signal for SQSTM1/p62-mediated autophagic degradation of the AIM2-PYCARD/ASC complex. We further identify that PM binds the Cys53 site of deubiquitinase USP50 through the Michael-acceptor and blocks the binding of USP50 to PYCARD/ASC, thereby reducing USP50-mediated cleavage of K63-linked ubiquitin chains of PYCARD/ASC. Finally, PM treatment in vivo generates an effect comparable to inflammasome deficiency in alleviating tendinopathy. Taken together, these findings demonstrate that PM alleviates the progression of tendinopathy by modulating AIM2-PYCARD/ASC stability via SQSTM1/p62-mediated selective autophagic degradation, thus providing a promising autophagy-based therapeutic for tendinopathy.Abbreviations: 3-MA: 3-methyladenine; AIM2: absent in melanoma 2; AT: Achilles tenotomy; ATP: adenosine triphosphate; BMDMs: bone marrow-derived macrophages; CHX: cycloheximide; Col3a1: collagen, type III, alpha 1; CQ: chloroquine; Cys: cysteine; DARTS: drug affinity responsive target stability; DTT: dithiothreitol; DUB: deubiquitinase; gDNA: genomic DNA; GSH: glutathione; His: histidine; IL1B/IL-1β: interleukin 1 beta; IND: indomethacin; IP: immunoprecipitation; LPS: lipopolysaccharide; MMP: mitochondrial membrane potential; NLRP3: NLR family, pyrin domain containing 3; PM: pristimerin; PYCARD/ASC: PYD and CARD domain containing; SN: supernatants; SOX9: SRY (sex determining region Y)-box 9; SQSTM1: sequestosome 1; Tgfb: transforming growth factor, beta; TIMP3: tissue inhibitor of metalloproteinase 3; TNMD: tenomodulin; TRAF6: TNF receptor-associated factor 6; Ub: ubiquitin; USP50: ubiquitin specific peptidase 50; WCL: whole cell lysates.

The pathology of oxidative stress-induced autophagy in a chronic rotator cuff enthesis tear

Partial-thickness rotator cuff tears (PTRCTs) are often found in daily orthopedic practice, with most of the tears occurring in middle-aged patients. An anaerobic process and imbalanced oxygenation have been observed in PTRCTs, resulting in oxidative stress. Studies have shown the roles of oxidative stress in autophagy and the potential of unregulated mechanisms causing disturbance in soft tissue healing. This article aims to review literature works and summarize the potential pathology of oxidative stress and unregulated autophagy in the rotator cuff enthesis correlated with chronicity. We collected and reviewed the literature using appropriate keywords, in addition to the manually retrieved literature. Autophagy is a normal mechanism of tissue repair or conversion to energy needed for the repair of rotator cuff tears. However, excessive mechanisms will degenerate the tendon, resulting in an abnormal state. Chronic overloading of the enthesis in PTRCTs and the hypovascular nature of the proximal tendon insertion will lead to hypoxia. The hypoxia state results in oxidative stress. An autophagy mechanism is induced in hypoxia via hypoxia-inducible factors (HIFs) 1/Bcl-2 adenovirus E1B 19-kDa interacting protein (BNIP) 3, releasing beclin-1, which results in autophagy induction. Reactive oxygen species (ROS) accumulation would induce autophagy as the regulator of cell oxidation. Oxidative stress will also remove the mammalian target of rapamycin (mTOR) from the induction complex, causing phosphorylation and initiating autophagy. Hypoxia and endoplasmic reticulum (ER) stress would initiate unfolded protein response (UPR) through protein kinase RNA-like ER kinase (PERK) and activate transcription factor 4, which induces autophagy. Oxidative stress occurring in the hypovascularized chronic rotator cuff tear due to hypoxia and ROS accumulation would result in unregulated autophagy directly or autophagy mediated by HIF-1, mTOR, and UPR. These mechanisms would disrupt enthesis healing.