The Hippo pathway in bone and cartilage: implications for development and disease

Hippo in bone stem cells

As the origin of osteoblasts, stem cells play an indispensable role in bone formation. The Hippo signaling pathway is involved in the development of bone stem cells. Most Prx1-positive mesenchymal stem cells (MSCs) contain the Hippo signaling pathway (Gou et al., 2024). Piezo1 is a nonselective calcium channel located in osteoclasts that senses mechanical load and promotes the translocation of YAP into the nucleus. The loss of Piezo1 or Piezo1/2 in MSCs or osteoblast progenitor cells can inhibit the formation of the NFAT/YAP1/β-catenin complex, hinder the development and maturation of osteoblasts, and cause bone loss in mice (Zhou et al., 2020). TAZ expression in adipose-derived stem cells (ADSCs) increases osteogenic differentiation as well as bone regeneration in vivo (Zhu et al., 2018). When the osteogenesis-related gene GNAS was knocked out in mesenchymal stem cells (MSCs), the activation of the Hippo signaling pathway was promoted, and the differentiation of osteoblasts was inhibited (An et al., 2019). Moreover, Hippo can also regulate the renewal and differentiation of embryonic stem cells and induce pluripotent stem cells by affecting a variety of other transcription factors, thereby affecting the development of craniofacial bones (Xiang et al., 2018). Hippo can regulate the development and differentiation of MSCs, but how it affects the development and differentiation of bone marrow stem cells needs further study.

Hippo in cartilage cells

The development and differentiation of chondrocytes are mediated by cell molecules, mechanical transduction, hormones and other signals (Long & Ornitz, 2013). TNC deposition in the endochondral environment, through mechanical signaling, leads to the inactivation of YAP and enhances chondrocyte differentiation (Li et al., 2021b). In addition, the dephosphorylation of YAP is accompanied by nuclear localization and osteogenesis-related gene RUNX2, which inhibits the expression of Col10a1, thereby inhibiting the development and maturation of chondrocytes (Deng et al., 2016). The nuclear localization of YAP inhibits Sox9 and arrests chondrocyte growth and development (Goto et al., 2018). The dephosphorylation and nuclear localization of YAP inhibit the occurrence and development of chondrocytes and hinder bone formation through different pathways.

Hippo in osteoblasts

Osteoblasts originate from mesenchymal stem cells in the mesoderm of the embryonic stage. The inhibition of YAP decreases bmp2a expression in MSs, leading to the accumulation of osteoprogenitor cells (Brandão et al., 2019). The migration-related and osteogenesis-related abilities of bone marrow mesenchymal stem cells are associated with increased expression of YAP (Wang et al., 2019) (Fig. 2).

In addition, MST2, a key upstream signaling molecule of the Hippo signaling pathway, is involved in the regulation of osteogenesis. Lee et al. (2015) reported that ectopic expression of MST2 in osteoblast precursors upregulated the expression of osteoblast-related genes such as RUNX2 and ALP, thereby increasing the transformation of osteoblast precursors into mature osteoblasts.

Hippo in osteoclasts

Hippo signaling pathway plays an important role in the occurrence, differentiation and function regulation of osteoclasts (Fig. 3). The core mechanism involves the activity regulation of downstream effector molecules YAP/TAZ and its interaction with other signaling pathways (Yang et al., 2018). RASSF2 interacts with MST1/2 and stabilizes its activity. Song et al. (2012) found that BMMs from RASSF2^−/− mice form more TRAP positive multinucleated cells, and can enhance the bone resorption ability of osteoclasts. The NF-κB signaling pathway plays a key role in osteoclast differentiation and function. In RASSF2^−/− mice, the NF-κB signaling pathway was over-activated in osteoclast precursor cells, as indicated by the significantly increased phosphorylation level of IkB-α. Osteoclasts are derived from hematopoietic stem cells (HSCs). NF2-deficient mice have a significant increase in the number of HSCs and promote the translocation of HSCs to the bone marrow. Although the relationship between NF2 and Hippo signaling remained directly explored, these changes may regulate the behavior of HSCs by affecting some components of the Hippo signaling pathway. It also affects the growth and development of osteoclasts (Larsson et al., 2008). It was found that cartilage endplate cells release CCL3, a key factor in osteoclast recruitment and activation, in response to abnormal stress. The expression of CCL3 is regulated by YAP, and the activation of YAP can inhibit CCL3 expression, thereby inhibiting osteoclast maturation (Li et al., 2024a). In addition, YAP/TAZ has been found to inhibit the activation of NF-κB signaling pathway by binding to TAK1, thereby hindering osteoclast differentiation (Yang et al., 2020). The RANKL-RANK-OPG system is a major mechanism regulating osteoclast differentiation, activation, and survival. YAP/TAZ also affected OC differentiation by regulating OPG expression. Upregulation of YAP/TAZ promotes OPG expression, which acts as a decoy receptor for RANKL and inhibits RANKL-RANK interaction, thereby reducing OC differentiation and bone resorption (Yang et al., 2021a).

Hippo in osteoporosis

Previous studies have demonstrated that the Hippo signaling pathway is closely related to bone metabolism and that the stability of bone metabolism is based on the balance of the osteogenic and osteoclast microenvironments. On the one hand, the Hippo signaling pathway directly mediates bone metabolism by regulating bone-related cells. On the other hand, by interacting with other signaling pathways, such as the NOTCH, Wnt/β-catenin, and NF-κB pathways, it indirectly regulates the growth and development of osteoblast-osteoclast and controls the balance of the osteoblast-osteoclast microenvironment. Disorders of bone metabolism can lead to a series of metabolic bone diseases, such as osteoporosis (Jensen et al., 2015).

Hippo in arthritis

Osteoarthritis (OA) is a chronic joint disease characterized by degenerative changes in the articular cartilage and secondary hyperosteogeny (Xia et al., 2014; Liu et al., 2022b). The disease affects the articular cartilage or the entire joint, including the subchondral bone (Cardoneanu et al., 2022), the joint capsule (Sanchez-Lopez et al., 2022), the synovium (Liu et al., 2022a), and the muscles surrounding the joint. It is of great physiological and pathological significance to understand the progress of Hippo in OA.

Effect of Hippo on articular cartilage

The expression of YAP is elevated in articular cartilage or chondrocytes in both mouse and human OA (Gong et al., 2019). Bottini et al. (2019) demonstrated that YAP increases TGF-β-dependent SMAD3 nuclear localization and that the inhibition of YAP improves OA symptoms. Zhang et al. (2022) reported that RUNX1 may inhibit OA by controlling the amount of the YAP protein and coordinating many signaling pathways, including the Wnt, Hippo, and TGF pathways. T-2 Mycotoxin damages cartilage, leading to a disease phenotype similar to osteoarthritis, and Li et al. (2022) reported that T-2 Mycotoxin reduces COL2 and PCNA levels in chondrocytes after the inhibition of YAP. Previous studies have suggested that the NF-κB signaling pathway plays an indispensable role in the development and progression of arthritis. Because when joint inflammation occurs, the development of chondrocytes is regulated by many different types of cytokines. Cytokines such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-6 play regulatory roles in the occurrence of arthritis (Yasuda, 2011). Each of these cytokines can further interfere with the activation or inhibition of the NF-κB signaling pathway (Kapoor et al., 2011), thereby promoting arthritis. Deng et al. (2018) reported that Hippo signaling controls articular cartilage homeostasis through YAP-mediated NF-κB signaling. YAP can attenuate the progression of OA by inhibiting the inflammatory response triggered by the NF-κB signaling pathway. In addition to participating in disease progression, differentiated chondrocytes also inhibit cartilage hypertrophy by increasing the expression of YAP1 (Ying et al., 2018).

Effect of Hippo on subchondral bone

Subchondral bone, below the calcified cartilage, maintains and distributes the mechanical stress caused by exercise and loading (Lories & Luyten, 2011). Studies on the effect of Hippo on subchondral bone development are scarce. However, “dentine changes” in subchondral bone are critical in the pathological process of OA. Notably, Xiao et al. (2023) reported that increased miR-6215 expression mediates the dexamethasone (PDE)-induced decrease in subchondral bone mass via FRMD6/YAP1 signaling. However, more studies are needed to explore the role of Hippo in this process.

Effect of Hippo on synovium

Synovial lesions are associated with pain and pathological responses caused by OA and are the typical disease characteristics of OA. In mouse models of antigen-induced arthritis (AIA), inhibition of YAP affects synovial vasculogenesis (Chen et al., 2021) and leads to rheumatoid arthritis (RA) progression. Leonurus regulates the Hippo signaling pathway through the miR-21/YOD1/YAP axis to reduce joint inflammation and bone destruction in CIA mice, thereby inhibiting the growth and inflammation of rheumatoid arthritis fibroblast-like synoviocytes (RA-FLSs) (Ma et al., 2024). In OA, Deng et al. (2018) reported increased numbers of inflammatory cells and synovial lining cells in the synovium of Yap^fl/fl mice. However, further evidence on how YAP modulates synovial cell progression in OA is lacking.

Hippo in degenerative diseases of spine

Intervertebral disc degeneration is the main cause of low back pain. Despite the long history of intervertebral disc degeneration, its onset and progression are poorly understood, and a standard model of disease occurrence is lacking (Murray et al., 2012). Degenerative disc deformation and injury leading to changes in mechanical and biological behavior are important causes of pathological symptoms (Vo et al., 2016). The etiology of natural lumbar disc degeneration (IDD) is complex, with mechanical, inflammatory, and structural causes contributing to increased pain and worsening patients’ symptoms (Feng, Egan & Wang, 2016).

The intervertebral disc (IVD) is composed of heterogeneous cell populations, including those of the nucleus pulposus (NP) and annulus fibrosus (AF), which undergo different ECM and mechanical loads. As a classical signaling pathway related to mechanical stress, the correlation between Hippo and mechanical stress deserves further exploration. Natural IDD patients show a time-dependent decrease in YAP activity with F-actin (Zhang et al., 2021). Zhang et al. (2018) reported that the expression of YAP in cartilage endplate cells (CEs) was consistent with that in NP cells and gradually decreased with age in disc-depleted IDD model mice, suggesting that YAP was positively correlated with IDD repair. In a rat IVDD model, the overexpression of SEPHS1 and the inhibition of Hippo–YAP/Taz attenuated the progression of IVDD (Hu et al., 2024). The LAT/YAP/CTGF signaling pathway promotes the synthesis and catabolism of the ECM in nucleus pulposus cells (NPCs), inhibits the catabolism of the ECM, and delays the progression of IDD (Chen et al., 2022). Matrix stiffness-induced ferroptosis in NP cells can be reversed by inhibiting the nuclear translocation of YAP (Ke et al., 2023). However, a specific inhibitor of YAP1, verteporfin, can effectively alleviate IDD development in rat intervertebral discs (Chen et al., 2019). Croft et al. (2021) reported that extranuclear YAP was observed only under weak loading conditions, that is, under static loading and low-stress conditions. The increased activation and subsequent nuclear localization of YAP under stronger loading conditions confirmed that mechanical stress can trigger the translocation of YAP and TAZ to promote gene expression (Li et al., 2024b).

Hippo in rheumatism

Bone metabolism, growth, development, remodeling and repair are inseparable from the interaction between osteoblasts and osteoclasts. Immune-related active components such as T cells, B cells, dendritic cells, Toll-like receptors, chemokines, and interleukins cooperate with osteoclasts to regulate bone formation and resorption, thereby changing the direction of bone formation (Wang et al., 2020). Autoimmune diseases occur when the body loses immune homeostasis. Rheumatoid arthritis (RA) is a complex chronic inflammatory disease in which multiple joints in the body become inflamed. Synovitis continues to worsen over time (Tomás et al., 2016), with joint degeneration and deformity and loss of function (Cross et al., 2014).

Chen et al. (2021) used antigen-induced arthritis mice. Silencing Ezrin prevents synovitis in these animals by downregulating the phosphorylation of YAP and preventing its nuclear accumulation. Previous studies have demonstrated that YAP may exert some control over the PI3K/Akt pathway (McLoughlin, Mueller & Grossmann, 2018) and that activated YAP stimulates the PI3K/Akt pathway, promotes the proliferation and activity of vascular endothelial cells, and regulates the growth of synovial arteries in mouse RA and the inflammatory process in RA joints. Berberine, a traditional Chinese medicine, inhibits the pyroptosis of RA-FLSs by downregulating the NLRP3 inflammasome and alleviating collagen-induced arthritis by activating the Hippo signaling pathway (Zhang et al., 2024).

Degenerate family member 3 (VGLL3) is a homolog of degenerate-like genes in Drosophila. It may be a transcriptional cofactor of TEADs (Mesrouze et al., 2020). Du et al. (2022) reported that knockdown of VGLL3 increased TAZ expression but had no effect on YAP expression in RA-FLSs. On the other hand, TAZ expression was suppressed by VGLL3 overexpression. Furthermore, VGLL3 activates the Hippo pathway and promotes the proliferation of A549 cells and MDA-MB-231 cells (Hori et al., 2020).

Hippo in osteosarcoma

Osteosarcoma is characterized by a poor prognosis, difficulty in treatment, and poor patient survival, whereas the most common primary osteosarcoma, for example, primarily affects children, adolescents, and young adults, with the second highest incidence among the elderly (Corre et al., 2020). The Hippo signaling pathway further controls the occurrence and development of osteosarcoma. In summary, the process can be divided into three main aspects: the dysregulation of upstream regulatory factors, the abnormal activation of YAP/TAZ, and the interaction with the tumor microenvironment (Rothzerg et al., 2021).

In osteosarcoma cells, the downregulation of MST1/2 and LATS1/2 leads to reduced phosphorylation of YAP/TAZ, thereby increasing the nuclear translocation and activity of YAP/TAZ (Chan et al., 2011). Sox2, as an upstream signaling molecule of the Hippo signaling pathway, inhibits the activation factors NF2 (neurofibromin 2) and KIBRA of the Hippo pathway, resulting in decreased YAP phosphorylation and subsequent nuclear accumulation, which promotes the development and progression of osteosarcoma (Basu-Roy et al., 2015). Wu et al. (2020) reported that in SRPX2-knockdown cells, decreased YAP phosphorylation and reduced YAP protein levels were detected in 143B and U2 osteosarcoma cells. In summary, the dysregulation of the upstream regulators leads to the accumulation of YAP/TAZ, which leads to the abnormal proliferation and differentiation of cells and promotes the occurrence and development of osteosarcoma.

In osteosarcoma, YAP/TAZ is frequently overexpressed and is associated with tumor aggressiveness and poor prognosis. The nuclear localization of YAP/TAZ is significantly increased, playing a crucial role in the progression of osteosarcoma through various mechanisms. MST1 and YAP promote the expression of PLOD2 and tumor cell migration in human osteosarcoma cell lines (Trang et al., 2023). YAP/TAZ interacts with the TEAD transcription factors to activate the expression of downstream genes, thereby promoting cell proliferation and inhibiting apoptosis (Chai, Xu & Guo, 2017). Studies (Zanconato et al., 2015) have indicated that mitogen-activated protein kinase (MAPK) is closely related to YAP in the Hippo signaling pathway, and that AP1 is a downstream signaling molecule of the MAPK pathway. This interaction can enhance the formation of the YAP-TEAD complex, thereby promoting the expression of downstream YAP-TEAD complex-related genes, significantly increasing the growth and development of tumor cells and enhancing their tumorigenicity. Verteporfin attenuates the interaction between YAP1 and TEAD1 (Yang et al., 2021b). Additional studies have shown that verteporfin delays the progression of osteosarcoma in Ctsk-Cre; Trp53f/f/Rb1f/f mice by inhibiting YAP/TAZ and preventing bone erosion, providing new possibilities for the treatment of osteosarcoma (Li, Yang & Yang, 2022). Moreover, the activation of YAP/TAZ is associated with chemoresistance in osteosarcoma cells to chemotherapeutic drugs such as methotrexate and doxorubicin. Activation of YAP/TAZ leads to decreased sensitivity to chemotherapeutic agents, thereby affecting treatment efficacy (Wang et al., 2016b).

The interaction between the Hippo signaling pathway and the tumor microenvironment promotes the malignant transformation of MSCs. Dysregulation of the Hippo pathway (e.g., overexpression of TAZ) enhances the transformation of MSCs into cancer stem cells (CSCs), increasing tumorigenicity and chemoresistance (Jabbari, Sarvestani & Moeinzadeh, 2015). Telomere abnormalities lead to DNA damage and activate the p53 pathway (Velletri et al., 2016). If p53 function is lost, the combination of Hippo pathway dysregulation and telomere instability synergistically promotes genomic instability, accelerating tumorigenesis (Zhou et al., 2019).

Hippo pathway in bone regeneration therapies

In recent years, in-depth investigations into the role of the Hippo signaling pathway in bone development and regeneration have revealed its potential therapeutic value for bone regenerative medicine (Zhong, Jiao & Yu, 2024). The Hippo pathway exerts critical regulatory effects on bone formation and repair by modulating cellular processes including proliferation, differentiation, and apoptosis (Li et al., 2024b). Although drugs directly targeting YAP/TAZ for osteoporosis treatment remain scarce, preclinical studies demonstrate that activating YAP/TAZ may confer partial therapeutic benefits. Estrogen deficiency represents a primary etiology of postmenopausal osteoporosis. Estrogen enhances osteoblast proliferation and differentiation while promoting bone formation through YAP/TAZ pathway-mediated regulation of Mechanical load in bone cells (Shi & Morgan, 2024). MSC-derived extracellular vesicles enriched with miR-23a-3p activate YAP/TAZ signaling via PTEN inhibition, thereby stimulating chondrocyte proliferation and bone tissue regeneration (Piñeiro Ramil et al., 2025). Verteporfin, a clinically approved drug, upregulates 14-3-3σ protein expression to enhance cytoplasmic retention of YAP, thereby suppressing YAP/TAZ-TEAD complex formation and reducing transcriptional activity. This mechanism suggests its potential utility in modulating bone regeneration through context-dependent YAP/TAZ inhibition (Wang et al., 2016a). Multikinase inhibitors (e.g., dasatinib and pazopanib), which target upstream YAP/TAZ activators such as SRC and FAK, have been shown to attenuate YAP/TAZ nuclear localization and activity. These agents may be repurposed to control YAP/TAZ-driven processes during bone regeneration (Oku et al., 2015). Statins, commonly prescribed for hypercholesterolemia, interfere with YAP/TAZ signaling by modulating Rho GTPase activity. Notably, simvastatin has been shown to promote osteoblast differentiation while suppressing osteoclast activity, positioning it as a promising candidate for bone regenerative therapy (Kamel et al., 2017). Therapeutic strategies targeting YAP/TAZ hold considerable potential for treating bone disorders and enhancing osseous repair. Further research is warranted to fully elucidate the complex roles of YAP/TAZ in bone biology and to develop clinically translatable targeted therapies.