Osteoarthritis (OA) is the most common joint disease in adults worldwide, characterized by the degenerative destruction of articular cartilage. The pathological changes in OA-affected joints manifest as pain and restricted mobility, leading to a significant decline in the quality of life for those affected (Litwic et al., 2013; Martel-Pelletier et al., 2016). The financial and social burdens induced by OA are increasingly severe, particularly in the older population (Adebayo et al., 2017). Despite treatments such as joint replacement and specific medications, the pathogenesis of OA remains unclear (Baldini et al., 2015; Marshall et al., 2014; McAlindon et al., 2014).

Age is a significant factor influencing the prevalence of knee and hip osteoarthritis in the elderly (Prieto-Alhambra et al., 2014; Long et al., 2022; GBD, 2021 Other Musculoskeletal Disorders Collaborators, 2023). Data from the Johnson County Osteoarthritis Project showed that the prevalence of radiographic knee osteoarthritis increased from 26.2% in the 55–64 age group to nearly 50% in the 75+ age group. Similarly, the prevalence of symptomatic hip osteoarthritis rose from 5.9% in the 45–54 age group to 17% in the 75+ age group (Ambrose et al., 2024). While many studies have focused on clinical evidence of age as a factor in OA prevalence, further research is needed to elucidate the pathogenesis of age-induced osteoarthritis (Hou et al., 2018; Liu et al., 2023). Age-related activation of endoplasmic reticulum (ER) stress is strongly associated with the development of several age-related diseases (Liu et al., 2018; Taylor, 2016; Pinky et al., 2023; Garcia et al., 2023).

Age-related activation of endoplasmic reticulum (ER) stress is strongly associated with the development of several age-related diseases (Liu et al., 2018; Taylor, 2016; Pinky et al., 2023; Garcia et al., 2023). Numerous studies have described a link between ER stress and OA (Li et al., 2016; Takada et al., 2011; Rellmann et al., 2021). In chondrocytes from OA patients of different ages, there is evidence of age-dependent ER stress and the unfolded protein response (UPR). Aged articular cartilage chondrocytes exhibit decreased expression of calnexin and increased immunohistochemical staining for ER stress markers, suggesting that reduced expression of molecular chaperones during aging induces ER stress (Tan et al., 2020). Recent research has indicated that ER stress can contribute to various diseases through the activation of the YAP pathway (Liu et al., 2021; Panda et al., 2022; Anerillas et al., 2023; Wang et al., 2022).

The Hippo/YAP pathway is critically involved in cartilage development and plays a key role in cell fate and tissue regeneration (Yang et al., 2017; Hansen et al., 2015), and finally leads to Osteoarthritis (Sun et al., 2023). Increasing evidence has indicated that the Hippo/YAP pathway is also tightly involved in cartilage development (Karystinou et al., 2015), YAP was found as a negative regulator of chondrocyte differentiation as it promotes early chondrocyte proliferation via binding TEADs for direct upregulation of Sox6 but inhibits subsequent chondrocyte hypertrophy and maturation (Deng et al., 2016). Connective tissue growth factor (CTGF), a significant downstream protein of YAP, is abundantly expressed in chondrocytes of patients with severe osteoarthritis (Shome et al., 2020; Dudek et al., 2021; MacDonald et al., 2021; Tang et al., 2018). It is suggested that the activity of YAP-CTGF is tightly related to the OA progression (Zhang et al., 2020).

This study aims to investigate the relationship between ER stress and YAP signaling in the context of cartilage phenotype loss in OA. By providing an in-depth analysis of the pathogenesis of osteoarthritis, we aim to propose a new direction for future research based on our findings.

In this study, using knee cartilage samples from rats and humans of different ages, we demonstrated that age-related ER stress-induced YAP overexpression leads to the loss of chondrocyte phenotypes. After confirming through in vitro experiments that YAP overexpression is a key factor in chondrocyte phenotype loss, we developed a transgenic mouse model with YAP overexpression specifically in the knee joint. This mouse model spontaneously exhibited signs of osteoarthritis without requiring osteoarthritis induction. Through both in vivo and in vitro experiments, Pamrevlumab was proven effective in preventing osteoarthritis. We propose a novel conclusion that aging-induced ER stress activation triggers YAP-CTGF signaling, causing chondrocyte phenotype loss and leading to osteoarthritis. This conclusion offers a new research direction in the pathogenesis of osteoarthritis for future studies.

ER stress is involved in the development of several aging-related diseases and is an important target for the study of degenerative diseases (Chen et al., 2023; Gao et al., 2018). Researchers have pointed out that age-related increases in ER stress are significant triggers for the development of osteoarthritis (Dreier et al., 2022). Several studies have suggested that activation of ER stress is involved in the pathophysiologic process of osteoarthritis by causing chondrocyte death (Li et al., 2016; Rellmann et al., 2021; Tan et al., 2020; Yang et al., 2020). Furthermore, as an essential physiological process regulating cellular function, ER stress is involved in regulating various cellular phenotypes (Raines et al., 2022; He et al., 2021; Aghadi et al., 2022; Chen and Cubillos-Ruiz, 2021; Gao et al., 2020). Dibyendu K et al. pointed out that PERK-mediated phosphorylation of eIF2α drives the selective translation of ATF4 and CHOP, enhancing YAP expression (Panda et al., 2022). Wu et al. (Wu et al., 2015) demonstrated in a study on liver cancer that the PERK kinase-eIF2α axis can activate Yap, while prolonged ER stress can create a negative feedback loop to inhibit Yap and promote apoptosis. Another study showed that in gastric cancer, YAP may regulate ER stress by activating the ERK signaling pathway (Liu et al., 2019). Additionally, as a key downstream protein of the IRE1a signaling axis, XBP1s is also considered to participate in the regulation of the YAP/TAZ signaling pathway (Fan et al., 2020). In our study, we have identified for the first time the interaction between YAP activation and ER stress in osteoarthritis, which is completely different from previous research. Based on existing studies (Takaguri et al., 2017; Yap et al., 2021; Yap et al., 2020), we speculate that signaling molecules such as PERK and IRE1 must play a crucial role. Moving forward, we will continue to investigate the effects of the three classical downstream pathways of ER stress on the YAP signaling axis to clarify the specific mechanisms involved in osteoarthritis. Our study suggests that ER stress regulates chondrocyte phenotypes through the activation of the YAP pathway. Research on downstream signaling pathways following ER stress in chondrocytes is still ongoing.

The YAP signaling pathway plays an essential role in cell aging (Sladitschek-Martens et al., 2022). It is regarded as an age-related protein, upregulated in various cells, and is associated with a wide variety of degenerative diseases, including cardiogenesis, skeletal muscle aging, vascular senescence, macular degeneration, and hair cell regeneration (Monroe et al., 2019; Setiawan et al., 2021; Pan et al., 2021; Rudolf et al., 2020). Maintaining YAP function can rejuvenate old cells, while YAP overexpression can lead to a series of aging symptoms (Zhang et al., 2018; Cha et al., 2022). These studies confirm the correlation between the YAP pathway and age-related diseases. In this study, we also found a close correlation between YAP and aging in young and old human samples, as well as in rats of different ages, which is consistent with most research findings. Additionally, YAP-related proteins and pathways have been shown to have significant associations with osteoarthritis (OA) in numerous previous studies (Zhang et al., 2011). Recent studies indicate its critical role in the pathogenesis of osteoarthritis. Gong et al. found an evident association between increased total expression of YAP and degenerative cartilage from OA patients (Gong et al., 2019). YAP was markedly upregulated both in mRNA and protein levels in OA mice (Zhang et al., 2019). The activation of YAP is thought to cause the phenotypic loss of chondrocytes, contributing to osteoarthritis development. Conditional knockout of YAP in mice preserves collagen II expression and protects cartilage from degeneration in osteoarthritis models (Zhang et al., 2020). As a degenerative disease, osteoarthritis is closely related to endochondral bone formation, cartilage degeneration, and structural disorganization of subchondral bone (Cox et al., 2013; Hosaka et al., 2013; Saito et al., 2010). In osteoarthritis, MSCs show disordered chondrogenic differentiation. Recent studies found that chondrocytes and osteoblasts are not independent lineages. In the pathological process of osteoarthritis, hypertrophic chondrocytes survive the cartilage-to-bone transition and become osteogenic cells, explaining both cartilage degeneration and endochondral bone formation (Yang et al., 2014; Zhou et al., 2014; Staines et al., 2013).

CTGF is identified as a direct YAP target gene important for cell growth (Fan et al., 2013; Chan et al., 2011; Zhao et al., 2008). The YAP-CTGF signaling axis plays an indispensable role in endochondral ossification in osteoarthritis (Delve et al., 2020). Although research on this signaling axis in the field of osteoarthritis is relatively limited, it has been well studied in the fields of fibrosis and cancer, and may be mediated by TEAD (Qing et al., 2022; Li et al., 2022; Gabdulkhakova et al., 2023). According to previous research, when the Hippo pathway is activated, MST1/2 activates LATS1/2. The LATS1/2 kinase phosphorylates YAP at the Ser127 site. Phosphorylated YAP binds to 14-3-3 proteins, leading to its retention in the cytoplasm and preventing it from functioning as a transcriptional co-activator. Additionally, phosphorylated YAP is more prone to ubiquitination and degradation, which further reduces the level of active YAP. Thus, YAP phosphorylation plays a role in the negative regulation of YAP (Mannaerts et al., 2015; Li et al., 2024). YAP chondrocyte nuclear translocation is regulated by differential phosphorylation in response to anabolic and catabolic stimuli and contribute to reduced anabolic activity and promotion of further cartilage loss (Cui et al., 2023). Some studies have manipulated the YAP signaling axis by inhibiting or increasing YAP phosphorylation (Zhang et al., 2024). In this study, we used Pamrevlumab to directly inhibit CTGF, and therefore did not address the impact of YAP phosphorylation on this pathway.

Previous studies show its contribution to joint homeostasis and osteoarthritis by controlling matrix sequestration. The deletion of CTGF increased the thickness of the articular cartilage and protected mice from osteoarthritis (Tang et al., 2018). Pamrevlumab, a humanized anti-CTGF antibody, has been tested in clinical trials for muscular dystrophy, pancreatic cancer, liver fibrosis, idiopathic pulmonary fibrosis, and type 1 and 2 diabetes (Ramazani et al., 2018). In this study, Pamrevlumab was effective in preventing cartilage phenotype loss and osteoarthritis-like lesions caused by YAP overexpression. Despite the complex mechanism of osteoarthritis, it remains a potential medication for treating and preventing osteoarthritis (Kaplon and Reichert, 2021; Xie et al., 2021).

This study still has some limitations. First, we proposed that aging establishes a connection with the YAP-CTGF signaling axis through endoplasmic reticulum stress. However, it is undeniable that there are likely many interactions involving other signaling pathways. YAP is not the only protein associated with the development of osteoarthritis, but we believe our research introduces a new direction, providing a foundation for future basic research and targeted therapeutic drug development. Second, due to the limitations of experimental conditions, the sample size in the study is relatively small. A smaller sample size and manual analysis of images may lead to bias in the experimental results. However, we believe that these limited samples can still yield valid experimental results. In this study, we did not perform large-scale RNA sequencing analysis, But the results from existing literature related to RNA sequencing data support our view (Ji et al., 2019; Pemmari et al., 2020; Soul et al., 2018). Third, although the research on this drug shows promising prospects, there is still a long way to go before it can be applied clinically. Further exploration is needed regarding its long-term therapeutic effects, potential side effects, and optimal dosage.

This study found that the YAP-CTGF axis, activated by age-related ER stress, causes the loss of the chondrocyte phenotype. We established a transgenic mice model with YAP overexpressed in cartilage and observed osteoarthritis-like pathological changes in the knee tissue of the mice. Pamrevlumab inhibited this loss of chondrocyte phenotype due to YAP overexpression and prevented osteoarthritis development in vivo. This study suggests an important role for ER stress-induced YAP upregulation in degenerative knee diseases and provides new possibilities for the pharmacological prevention of osteoarthritis.