The axon section and the basal body make up primary cilia, which are organelles that project from the cell surface into the ECM. The basal body of the primary cilia anchors to the inner cell membrane, while the axon section of the cilia protrudes from the membrane’s surface. The basal body, a modified version of the centriole, is the source of primary cilia, which are microtubule-based organelles (Tao et al., 2020). The primary cilium’s roles in the articular cartilage are as an antenna to sense the biomechanical environment, control the secretion of ECM components, and store cellular positioning information. Numerous ion channels and signaling receptors are found in primary cilia (McGlashan et al., 2006; Lee et al., 2015).

Using immunofluorescence and confocal imaging, the ECM receptors on chondrocyte plasma membranes and the main cilia have been examined. On the plasma membrane, every receptor that was tested showed a punctate distribution. On the primary cilia, integrins α2, α3, β1, and NG2 were also visible (McGlashan et al., 2006). This study is the first to show that integrins and NG2 are expressed on primary cilia in chondrocyte. Using a FRET-based biosensor fused to ARL13B, Lee et al. report the first observations of Ca²⁺ signaling within primary cilia in osteocyte. They demonstrated that fluid shear stress causes Ca²⁺ increases in primary cilia of osteocytes, which are dependent on both intracellular Ca²⁺ release and external Ca²⁺ entry (Lee et al., 2015).

As one of the transient receptor potential polycystic of ion channels, polycystin-2 (PC2) and polycystin-1 (PC1) have been linked to cilia-mediated mechanotransduction in epithelial cells. Uniaxial cyclic tensile strain (CTS) was used to mechanically stimulate isolated chondrocytes in order to study the impact on PC2 ciliary location and matrix gene expression. The cilium was discovered to have a higher level of PC2 localisation, which co-localized with the ciliary marker acetylated -tubulin (Figure 7A). Response to mechanical stimulation results in an increase in PC2 ciliary localisation (Figure 7B) (Thompson et al., 2021). Furthermore, mechanical stimulation from the ECM can change the degree of deflection (Jensen et al., 2004), length (Fu et al., 2019; Zhao et al., 2020), and orientation (Farnum and Wilsman, 2011) of primary cilia, indicating that primary cilia can sense mechanical signals in bone formation and growth.

The ciliary axoneme was visible interdigitating between collagen fibers and condensed proteoglycans using tomography and TEM. The primary cilium is bent as a result of mechanical stimuli conveyed through matrix macromolecules, suggesting that it may function as a mechanosensor for skeletal patterning and growth (Jensen et al., 2004). Primary cilia in chondrocytes are sensitive to mechanical forces, and when subjected to cyclic tensile strain or hyperosmotic stresses, their length decreases dramatically. The principal cilial length in healthy cartilage is 1.5 mm in the deep layer and 1.1 mm in the superficial layer (Zhao et al., 2020). Treatment with IL-1β at 1 ng/mL produced a statistically significant increase (of 14%) in cilia length. However, this effect was completely inhibited under mechanical loading (Fu et al., 2019). Additionally, there were differences between load-bearing and non-load-bearing zone in the direction of the chondrocyte primary cilia. The axoneme extends from the cellular surface towards the subchondral bone in load-bearing areas of the superficial zone. This uniformity disappears in areas not supporting loads (Farnum and Wilsman, 2011).

During skeletal development, primary cilia proteins, which are involved in the transduction of biological and physiochemical signals, regulate the maturation of cartilage. Researchers tested the effects of the ciliary protein intraflagellar transport protein 88 (IFT88) on postnatal cartilage in mice with the Ift88 gene conditionally knocked out (Ift88-KO). The findings show that IFT88 acts as a chondroprotector in articular cartilage by preventing cartilage from calcifying by maintaining a Hh signaling threshold under physiological loading (Coveney et al., 2022b).

Coveney et al. used a cartilage-specific, inducible Cre (AggrecanCreERT2 Ift88fl/fl) to conditionally target the ciliary gene intraflagellar transport protein 88 (Ift88fl/fl) in the juvenile and adolescent skeleton, in order to investigate the role of primary cilia (Coveney et al., 2022a). IFT88 deletion in cartilage altered chondrocyte differentiation and mineralization by reducing ciliation in the growth plate. These effects were mostly limited to the peripheral tibial regions under the knee’s load-bearing chambers (Figure 7C). AggrecanCreER T2; Ift88 fl/fl mice had fewer and lower density bone bridges than controls. This decrease in bridging was notably noticeable on the medial side of the leg (Figure 7D). The authors argue that ciliary IFT88 protects coordinated ossification of the growth plate from an disruptive heterogeneity of physiological mechanical stimuli during this critical stage in adolescent skeletal maturation.

Primary cilia are essential for the formation of mammalian tissues. Although primary cilia are important for chondrocyte function, their specific roles in postnatal articular cartilage morphogenesis are unknown. Rux et al. used a mouse conditional loss-of-function method (Ift88-flox) targeting joint-lineage progenitors (Gdf5Cre) to investigate the mechanisms. They discovered that tidemark patterning and hedgehog signaling were substantially disturbed, and that specificity was demonstrated based on regional load-bearing functions of articular cartilage (Rux et al., 2022).

Integrins consisting of α and β subunits, translocate across the cell membrane. Numerous experiments have established that chondrocytes express a number of integrins, including integrins α5β1, αVβ3, αVβ5, α6β1, α1β1, α2β1, α10β1, and α3β1 (Loeser et al., 2000; Lahiji et al., 2004; Shattil et al., 2010; Tian et al., 2015). The protein complex is given the name “integrin” to signify its function as an integral complex involved in the transmembrane interaction between the cytoskeleton and ECM (Tamkun et al., 1986). Integrins are well-known as ECM receptors and cell adhesion molecules. They are also thought to affect intracellular signaling pathways physically and chemically as mechanoreceptors. Through integrin-mediated adhesion, cells detect and react to the elastic properties of ECM. Integrins are a class of well-known mechanosensors in cells that alternate between the inactive, bound, and dissociated states based on the various forces acting on them (Xu et al., 2014). When osteoarthritic chondrocytes and normal chondrocytes are mechanically stimulated, there are obvious differences in the cellular responses. These differences could be connected to variations in integrin expression and function [61].

The traction force between the αV integrin and its ligand is increased by mechanical force, suggesting that the αV integrin-RGD link may survive molecular tensions of up to 54 pN in chondrocytes under mechanical stress. When αV integrin bonds with its RGD-containing ligands, such as latent TGF, in chondrocytes under mechanical stress or not, Zhen et al. used a double-stranded deoxyribonucleic acid (dsDNA) tether as a tension gauge to test the tolerance to tension (Figures 8A,B) (Zhen et al., 2021). Mechanical stimulation of healthy chondrocytes resulted in enhanced GAG production, which was prevented by antibodies to α5 and αVβ5 integrins, as well as CD47 (Holledge et al., 2008). These findings show that αVβ5 integrin plays a significant roles in influencing chondrocytes responses to biomechanical stimuli. The subunits of integrins β1 and 3 are both necessary for osteocyte mechanotransduction. In osteocytes, inhibition of these integrin subunits resulted in poorer responses to fluid shear stress (Geoghegan et al., 2019). Integrin α1β1 has been identified within the integrin family as a critical participant in transducing hypo-osmotic stress. It has been demonstrated that deleting the integrin α1 subunit inhibits chondrocytes’ ex vivo and in vitro production of Ca²⁺ transients in response to hypo-osmotic stress (Jablonski et al., 2014).

The constant external stimulation that chondrocytes experience controls remodeling. The maintenance of chondrocyte homeostasis requires an ideal degree of mechanical stimulation, but excessive mechanical stress results in the production of inflammatory cytokines and proteases such as matrix metalloproteinases (MMPs). Using an integrin receptor antagonist (cilengitide), Hirose et al. investigated the relation between integrins (αVβ3 and αVβ5) and the production of inflammatory markers in chondrocytes under mechanical loading. Interleukin-1 (IL-1), tumor necrosis factor (TNF), matrix metalloproteinase-3 (MMP-3), and MMP-13 gene expression that was increased by severe mechanical stress was inhibited by cilengitide (Hirose et al., 2020). Through the activation of the TGF-1/CCN2/integrin-5 pathway, high mechanical stress also causes chondrocyte fibrosis, and halting the expression of TGF-1, CCN2, or integrin-5 can reduce the fibrous development (Huang Y. Z. et al., 2021).

Recent study evidence indicates that excessive mechanical stress (eMS) is an important contributor in the development of OA. In a rat instability of the medial meniscus model, histologic and proteomic analysis of osteoarthritic cartilage revealed increased expression of integrin αVβ3 as well as more severe cartilage degeneration in the medial weight-bearing region (Figure 8C) (Song et al., 2023).

Based on their gating methods, ion channels can be divided into a number of groups, including voltage-gated, ligand-gated, and mechanically-gated. The chondrocyte has different types of channels, but mechanically gated ion channels are particularly intriguing since they can trigger rapid mechanosensory signal transduction. Mechanosensitive ion channels control ECM generation and matrix protein synthesis in articular chondrocytes. This mechanism involves intracellular cation efflux, extracellular cation influx, and mobilization of Ca²⁺ as a result of large Ca²⁺ release from storage (Zhang K. et al., 2021). The initial responses of chondrocyte mechanotransduction involve changes in mitochondrial activity and calcium influx, which take place in seconds to minutes (Delco and Bonassar, 2021). The levels of calcium, an universal messenger, regulate a number of critical cellular functions, such as exocytosis, apoptosis, motility, gene transcription, and differentiation (Uzieliene et al., 2018).

TRPV4 and Piezo 1/2 are arguably the most significant calcium channels. TRPV4 was initially identified in articular chondrocytes as an osmotically sensitive Ca²⁺ ion channel, and it was later demonstrated to be sensitive to physiological dynamic compression (Delco and Bonassar, 2021). Additionally, it was found that the mechanically gated Ca²⁺ channels Piezo 1/2 in articular chondrocytes respond to high (supraphysiologic) strain and are in charge of mechanically inducing chondrocyte death (Zhang M. et al., 2022). The potential roles of TRPV4, Piezo1/2 in translating different magnitudes of repetitive mechanical stimuli in chondrocytes were identified in a study. To investigate this, TRPV4, and Piezo1/2 specific siRNAs were transfected into cultured primary chondrocytes to inhibit the expression of TRPV4, Piezo1, or Piezo2, respectively. These cells were then referred to as TRPV4-KD (knock down), Piezo1-KD, or Piezo2-KD cells. Stretch-evoked Ca²⁺ fluctuations were markedly reduced in TRPV4-KD, Piezo1-KD, or Piezo2-KD cells as compared to control siRNA-treated cells, demonstrating the necessity of these channels for Ca²⁺ signaling in chondrocytes generated by stretch stimulation. Notably, these channels responded differently to the calcium oscillation brought on by different stretch stimulation intensities. More specifically, Piezo2-mediated Ca²⁺ signaling was critical for chondrocyte response to damaging levels of strain (18% of strain), whereas TRPV4-mediated Ca2+ signaling was critical for chondrocyte response to normal levels of strain (3% and 8% of strain) (Du et al., 2020). The idea of therapeutically targeting Piezo2-mediated mechanotransduction for the therapy of cartilage disease is prompted by the results, which serve as a foundation for further research into mechanotransduction in cartilage.

GsMTx4, a PIEZO-blocking peptide, and Piezo1/2-specific siRNA blocked mechanically induced Ca²⁺ transients produced by atomic force microscopy in primary articular chondrocytes (Lee et al., 2014), proposing a potential therapeutic approach to attenuate Piezo-mediated cartilage mechanotransduction of damaging stresses to reduce cartilage injury and posttraumatic osteoarthritis. Interleukin-1 (IL-1) was discovered to upregulate Piezo1 in porcine chondrocytes. The enhanced Piezo1 function caused excess intracellular Ca²⁺ both at rest and in response to mechanical deformation. High resting state Ca²⁺ enhanced mechanically generated deformation microtrauma via rarefying the F-actin cytoskeleton (Lee et al., 2021).

Osteoarthritis and joint arthropathy are linked to the loss of TRPV4 function, which is a Ca²⁺-permeable osmomechano-TRP channel that is extensively expressed in articular chondrocytes. The acute, mechanically mediated regulation of proanabolic and anticatabolic genes has been demonstrated to be prevented by TRPV4 inhibition during dynamic loading. It has also been shown to impede the loading-induced augmentation of matrix accumulation and mechanical characteristics. Additionally, in the absence of mechanical loading, pharmacological stimulation of TRPV4 by the agonist GSK1016790A promoted anabolic and inhibited catabolic gene expression, potently increased matrix production, and improved the mechanical characteristics of the construct (O'Conor et al., 2014). These results lend credence to the idea that mechanical cues that maintain joint health and cartilage extracellular matrix preservation are primarily transmitted by TRPV4-mediated Ca²⁺ signaling.

A study shows that loss of TRPV4-mediated cartilage mechanotransduction in adulthood lessens the severity of aging-associated OA by using tissue-specific, inducible TRPV4 gene-targeted mice. These findings point to a unique disease-modifying strategy for the treatment of OA associated with aging by therapeutically targeting the TRPV4-mediated mechanotransduction pathway (O’Conor et al., 2016). However, blocking TRPV4-mediated calcium ion transmission was insufficient to stop the progression of OA on its own. Ion channels belonging to the Piezo family have recently been found to regulate chondrocyte damage response and cell death, as well as giving chondrocytes mechanosensitivity to high stresses (Lee et al., 2014). Therefore, multimodal therapy strategies may be required.

The growth factor is produced as the inactive latent complex that cannot attach to membrane receptors cause a cellular biological response. Recent research has shown that mechanical stresses may activate dormant growth factors. Growth factors are trapped within the pericellular matrix. Increased bioavailability of these upon mechanical stimulation leads to chondrocyte activation. The PCM and territorial matrix’s composition and structure affect the bioavailability of sequestered growth factors like fibroblast growth factor (FGF) (Vincent et al., 2007; Vincent, 2011), transforming growth factor-β (TGFβ) (Albro et al., 2012; Li et al., 2012) and insulin-like growth factor (IGF) (Martin et al., 2002). ECM-sequestered factors are released during deformation or destruction to interact with cell membrane receptors, thus activating downstream intracellular signaling cascades.

The pericellular matrix of articular chondrocytes contains a highly abundant growth factor called FGF2. The location of FGF-2 storage in articular cartilage, the proteoglycan to which it was linked, and its function in chondrocyte mechanotransduction were all uncovered by a study. In articular cartilage, heparan sulphate proteoglycan traps FGF-2. In the type VI collagen-rich pericellular matrix of pig and human articular cartilage, perlecan and FGF-2 co-localize. Chondrocytes enclosed with alginate had the capacity to build up pericellular perlecan and FGF-2 in culture and to activate ERK in a FGF-dependent manner when loaded (Vincent et al., 2007). Studies exploring the function of FGF-2 have shown inconsistent results. The two main articular cartilage FGF receptors, FGFR1 and FGFR3, may have changed in balance, which could explain variations in responses to FGF-2. The majority of FGF2’s catabolic and anti-anabolic actions are mediated by FGFR1, whereas the positive effects are handled by FGFR3 (Vincent, 2011).

It has been discovered that delayed compressive stress induces endogenous TGF-1 gene transcription, protein expression, and subsequent activation even when exogenous TGF-1 stimulation is stopped (Li et al., 2012). Shearing synovial fluid may have extra metabolic effects on diarthrodial joints. It was also shown that TGF-β could be activated in cell-free scaffolds, proving that mechanical stress alone is, at least in part, responsible for the observed activation (Albro et al., 2012). Further, an investigation revealed the functions of TGF-β/SMAD and integrin signaling, indicating cross-talk between these two signaling pathways in controlling the development of compression-driven hypertrophy (Zhang et al., 2015).

A number of lines of research indicate that the anabolic cytokine IGF-I is important for maintaining articular cartilage and may even be involved in cartilage repair. IGF-I increases chondrocyte synthesis of matrix macromolecules. The high co-localization of the pure IGFBP-3 and fibronectintwo in the cartilage matrix and the direct binding between them provide evidence in favor of the theory that these two proteins work together to control local IGF-I levels (Martin et al., 2002).

A number of cell signaling pathways involved in cellular proliferation, the production of the extracellular matrix (ECM), cell survival, and the mediation of pain are controlled by the mitogen-activated protein kinase (MAPK). The MAPK pathway, which is involved in mechanotransduction and is controlled by mechanosensory stimuli, is essential for chondrocyte differentiation. TRPV4, p38, and primary cilia are all necessary to activate ERK.

According to one study, ex vivo cartilage compression increases the activation of the JNK pathway enzymes SEK1, p38 MAPK, and ERK1/2. This work also indicates unique temporal patterns of MAPK signaling in response to mechanical stress and that mechanical compression alone can activate MAPK signaling in healthy cartilage (Fanning et al., 2003). Various clinical studies have found that the MAPK pathway plays an essential role in the cartilage development (Xiang et al., 2019), cartilage catabolism (Zhang H. et al., 2022), expression of inflammation-related factors (Yanoshita et al., 2018), and enhancement of chondrogenesis (Xie et al., 2021).

Magnitude-dependent effects of mechanical stress on chondrocyte survival, phenotypic, and proliferation have been observed. The expression of autophagy and mechanical stress-regulated ERK/mTOR signaling in chondrocytes depend on the structural integrity of primary cilia (Xiang et al., 2019). In mice articular cartilage and cultured chondrocytes, mechanical overloading speeds up senescence. Mechanical overloading reduces the transcription of the F-box and WD repeat domain containing 7 (FBXW7) mRNA and the MKK7 degradation caused by FBXW7, which in turn stimulates JNK signaling. As evidenced by the overexpression of p16INK4A, p21, and Colx and the downregulation of Col2a1 and ACAN, FBXW7 deletion in chondrocytes has been found to cause chondrocyte senescence and accelerate cartilage catabolism in mice, which led to the worsening of OA (Zhang H. et al., 2022).

The non-receptor tyrosine kinase focal adhesion kinase (FAK) is connected to numerous signaling proteins. The phosphorylation of FAK, p-38, ERK, and JNK was triggered by cyclic tensile strain, which also elevated the expression of the genes encoding COX-2, IL-1, and TNF-α. Through MAPK pathways, FAK seems to control inflammation in chondrocytes exposed to cyclic tensile strain (Yanoshita et al., 2018). Dynamic compression raised the compressive moduli of manufactured cartilage tissues and encouraged the formation of cartilage matrix. ACAN and COL2 levels were increased in constructs cultivated under dynamic loading conditions, confirming the function of dynamic loading with 5% strain as a chondro-supportive agent (Xie et al., 2021).

Under high-strain activation, downstream signaling molecules MAPK/ERK1/2 and MAPK/ERK5 cause late excessive death of chondrocytes through the action of Piezo1 channels. Through the traditional MAPK/ERK1/2 signaling pathway, Piezo1 plays a crucial role in the apoptosis of the human chondrocytes (Li et al., 2016). Following apoptosis, the chondrocyte stimulates the joint’s surroundings and releases a lot of oxygen free radicals and inflammatory mediators (including IL-1, TNF, and PE) that harm the newly formed cartilage tissue and blood vessels (Liu et al., 2022). After pre-incubation at 380 mOsmol, it was discovered that exposure to hyperosmotic conditions (550 mOsmol) initially reduced the rate of 35S-sulphate incorporation. However, after 24 h of culture, rates bounced back and even exceeded their pre-exposure values. This reaction was eliminated by MAP kinase inhibitors, which suggests that they are involved in the adaption mechanism (Hopewell and Urban, 2003). Therefore, it is believed that a number of mechanosensory stimuli have the MAPKERK pathway as a downstream target.

Chondrogenesis, chondrocyte maturation, and hypertrophy are all controlled by the transcriptional cofactors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), which together make up a crucial mechanosignaling complex. The roles of YAP and TAZ as nuclear relays of mechanical stimuli exerted by ECM stiffness and cell shape were identified (Dupont et al., 2011).

The transcriptional cofactors YAP and TAZ form an important mechanosignaling complex and are involved in regulating mechanical stress-mediated apoptosis of chondrocytes (Sun et al., 2021), the degeneration of chondrocytes (Ding et al., 2022), chondrocyte hypertrophy (Lee et al., 2020) and cartilage degradation (Gong et al., 2019).

The expression of the G protein coupled estrogen receptor (GPER) is negatively linked with the pathophysiology of OA cartilage degradation. By encouraging the expression of YAP and ARHGAP29 as well as YAP nuclear localization, GPER suppresses Piezo1 and the mechanical-stress-mediated RhoA/LIMK/cofilin pathway as well as actin polymerization (Sun et al., 2021).

In mechanical-tension-mediated degenerative discs, YAP1 is a crucial regulator. In degenerative human endplate cartilage tissue, YAP1 expression has been shown to be dramatically reduced with higher mechanical stimulation intensity and duration. Additionally, it has been demonstrated in the cartilage endplate tissue in vitro that increasing the expression level of YAP1 can postpone the degeneration of endplate cartilage (Ding et al., 2022).

Although biomaterial design solutions for repairing injured articular cartilage have advanced significantly, preventing stem-cell-derived chondrocyte hypertrophy and the subsequent creation of inferior tissue remains a significant difficulty (Lee et al., 2020).

Furthermore, it has been discovered that OA cartilage tissue expresses higher levels of YAP1, and that YAP1 that is overexpressed interacts with Beclin 1 to advance OA. In a mouse model of mechano-induced OA, researchers attempted to employ siRNA to block YAP1, and they discovered that doing so avoided cartilage breakdown and improved OA development (Gong et al., 2019).

Both in MSCs and chondrocytes, the amount of fluid flow stress controls the expression of YAP. An increase in the stimulation magnitude enhanced the expression of YAP, increasing osteogenesis and initiating dedifferentiation for chondrocytes (Zhong et al., 2013). According to Karystinou et al., YAP is a negative regulator of MSCs’ chondrogenic development. Through the derepression of chondrogenic signaling, YAP must be downregulated for chondrogenesis (Karystinou et al., 2015). A promising future in rheumatology involves therapeutic targeting of YAP to promote cartilage repair and avoid subsequent osteoarthritis. These findings demonstrate the importance of YAP/TAZ as effectors that may transmit mechanical stress into the nucleus and support healthy chondrocyte formation, maturation, and homeostasis.

Wnt signaling has established roles in the expression of hypertrophic markers (Lee et al., 2020), in decreasing the ECM content of cartilage (Praxenthaler et al., 2018), chondrocyte metabolism and oxidative stress (Cheleschi et al., 2020), pro-chondrogenic effects (Ma et al., 2022), and osteoblast osteogenic differentiation (Song et al., 2021). The Wnt signaling pathways that support increased expression of hypertrophic markers in cartilage can affect how transplanted articular or hypertrophic phenotypes behave. Encapsulated hMSCs were pretreated with Wnt inhibitors for 21 days in order to stop them from developing along hypertrophic pathways. Wnt inhibitor supplementation reduced the expression of indicators linked to hypertrophic chondrogenesis in comparison to controls without the addition of inhibitors, which showed an increase in the expression of hypertrophic markers (Lee et al., 2020).

WNT/-catenin and pSmad1/5/9 levels decreased as cartilage’s ECM content rose. In mature constructs, the Wnt agonist CHIR reduced load-induced SOX9-and GAG activation by increasing -catenin levels. IWP-2, a WNT antagonist, on the other hand, had the ability to lessen the GAG-suppression caused by load in developing constructs. In conclusion, a stronger anabolic response of chondrocytes to physiological loading was enabled by either ECM accumulation-associated or chemically induced silencing of WNT-levels (Praxenthaler et al., 2018).

According to one study, hydrostatic pressure (HP) controls chondrocyte metabolism and oxidative stress via the Wnt/-catenin pathway in part by silencing certain miRNAs. Low cyclical HP substantially lowered the amounts of apoptosis, MMP-13, ADAMTS5, miRNA, superoxide anion generation, and mRNA for antioxidant enzymes. On the other hand, Col2a1 and BCL2 genes showed greater expression. The application of continuous static HP produced opposite consequences. Finally, miRNA silencing improved low HP and blocked effects of ongoing HP (Cheleschi et al., 2020). In a different study, mechanical loading by cyclic hydrostatic pressure (CHP) had a pro-chondrogenic impact, whereas mechanical unloading by simulated microgravity (SMG) produced OA-like gene expression in manufactured cartilage. Each sex group displayed a unique gene profile. For instance, the NOTUM gene, which is a part of the Wnt signaling pathway, was considerably elevated in the CHP for the female cohort by 6.7-fold but only by 1.8-fold in the CHP for the male cohort. However, SMG had a negligible impact on NOTUM regulation, and it revealed the opposite direction between male and female cohorts (Ma et al., 2022).

In addition to inhibiting osteoblast osteogenic development, severe mechanical stretching of osteoblasts also caused chondrocyte catabolism and apoptosis. This was accomplished by the Wnt/catenin signaling pathway (Song et al., 2021).

Suppressing canonical Wnt signaling may enhance the chondrogenesis of MSCs and attenuate the progression of OA. In comparison to control hydrogels, encapsulating hMSCs in these self-assembled N-cadherin mimic peptide hydrogels resulted in increased expression of chondrogenic marker genes and deposition of extracellular matrix specific to cartilage that is rich in proteoglycan and Type II collagen. Western blot assessment revealed a substantially reduced level of -catenin and a significantly higher expression of active glycogen synthase kinase-3 (GSK-3), which phosphorylates catenin and promotes ubiquitin-mediated destruction. In N-cadherin mimicking peptide hydrogels, immunofluorescence labeling showed much less nucleus localization of catenin. According to the results, N-cadherin peptide hydrogels increase the chondrogenesis of hMSCs by increasing catenin nuclear translocation and the transcriptional activity of the catenin/LEF-1/TCF complex and suppressing canonical Wnt signaling in hMSCs (Li et al., 2017).

GSK3β is also a downstream protein of TRPV4. In order to regulate GSK3 activation, normal chondrocytes’ intracellular calcium levels, which are controlled by TRPV4 ion channels, fluctuate in response to the viscoelasticity of the ECM. Additionally, osteoarthritic chondrocytes’ TRPV4-GSK3 molecular axis has been damaged, which prevents OA patients’ cells from sensing and reacting to the changed viscoelasticity of the surrounding matrix (Agarwal et al., 2021).

The development of osteoarthritis is heavily influenced by the excessive forces that the articular cartilage is exposed to. Under cyclic strain or hydrostatic pressure loading, Gremlin-1 is found to be a mechanical loading-inducible factor in chondrocytes and is strongly expressed in the middle and deep layers of cartilage. Nuclear factor-B signaling is activated by Gremlin-1, which causes the production of catabolic enzymes (Chang et al., 2019). Osteoarthritis progression is slowed in mice by intra-articular infusion of Gremlin-1 antibody or chondrocyte-specific Gremlin-1 deletion, but this progression is sped up by intra-articular administration of recombinant Gremlin-1 (Chang et al., 2019). These findings point to NF-B’s pivotal function in mechanoinflammation as OA progresses, but perhaps more significantly, they point to several intriguing treatment targets.

In addition to being triggered by cytokine signaling, the physical pressures within chondrocytes can also regulate NF-B. NF-κB activation can activate inflammation (Sun et al., 2020), mimic the negative loading effects (Lückgen et al., 2022), and lead to the induction of catabolic enzymes (Chang et al., 2019). A study found evidence connecting the development of NLRP3 inflammasome with Piezo1-mediated inflammation in nucleus pulposus cells. A unique pathogenic mechanism driving the development of intervertebral disc degeneration is the activation of NLRP3 inflammasome in nucleus pulposus cells via Piezo1 through the Ca²⁺/NF-B pathway (Sun et al., 2020). While catabolic NF-B signaling prevents load-induced deleterious effects on ECM synthesis in MSC-derived neocartilage, NF-B activation mimics negative loading effects and increases PGE2 production (Lückgen et al., 2022). The mechanical characteristic of neocartilage generated from mesenchymal stromal cells is increased by the NF-B suppression.

The new PCL-PTHF urethane electrospun nanofibers with collagen I from calf skin were found to be more effective at inducing chondrogenic differentiation in vitro and cartilage regeneration in vivo than the stiffer P PCL-PTHF urethane nanofibers. This was true even in the absence of additional chondrogenesis inducers. The researchers discovered that the PC worked better than P at initiating chondrogenesis by specifically blocking the NF-B signaling pathway to reduce inflammation (Jiang et al., 2018). Another study found that the AMPK/NF-B signaling pathway might be used to regulate the sensitivity of articular cartilage and chondrocytes to the inflammatory response. By promoting AMP-activated protein kinase (AMPK) activation and inhibiting nuclear factor (NF)-B translocation, cyclic tensile strain (CTS) may reduce the chondrocyte damage brought on by IL-1 (Yang et al., 2019).

Autologous chondrocyte implantation (ACI) on a collagen type I/III scaffold was investigated by Nixon and coworkers, and it appeared to promote cartilage regeneration in a critical-sized lesion in the equine model over the course of 6 months (Nixon et al., 2015). The chondro-inductive effects of 3D collagen and hyaluronic acid hydrogels—self-assembled collagen hydrogel (Col), self-assembled collagen hydrogel cross-linked with genipin (Cgp), and methacrylated hyaluronic acid hydrogel (HA)—on the encapsulated BMSCs were assessed in a different study. In the subsequent stage, there was not enough room in the hydrogels for cell proliferation due to the extreme shrinkage of Col and Cgp. In contrast, the relatively stable mechanical environment of HA supported the maintenance of the ongoing synthesis of the cartilage matrix in the final stage (Yang et al., 2021).

Agarose-based biomaterials are crucial in cartilage tissue healing because of their special qualities, including reversible thermogelling behavior and tissue-like mechanical properties (Salati et al., 2020). Induction, gelation, and quasi-equilibrium are the three phases of the agarose gelation process. The initial phase involves the formation of a number of agarose nuclei, which are then grown into networks (Figure 10A).

Scaffolds were made from elastin, collagen, fibrin, and electrospun polycaprolactone (PCL) with varying ratios to overcome the drawbacks of natural and synthetic polymers by combining them to create a synergistic relationship (Figure 10B). As a result, it was possible to modify the physical and biological characteristics of PCL-based composites, in order to create a beneficial connection for a variety of applications in tissue regeneration (Sawadkar et al., 2020).

Polymeric hydrogels hold promise as potential replacement materials for the cartilage that has been injured. As a matrix for autologous chondrocyte implantation, an injectable polyethylene glycol-crosslinked albumin gel (AG) supplemented with hyaluronic acid holds promise as a beneficial implant for cartilage tissue while also exhibiting non-permissive properties for pathological blood vessel formation (Scholz et al., 2010).

The mechanical properties of the original substrate can be increased by incorporating an interpenetrating polymer network into a single polymer network. By transferring this idea from purely synthetic materials to natural-synthetic hybrid systems, the mechanical properties of bulk biological substrates can also be strengthened (Cooper et al., 2016). This procedure improves the osteoarthritic cartilage’s inferior compressive capabilities to those of healthy cartilage. An effective treatment method for the regeneration of posttraumatic or osteoarthritic lesions of the knee, according to a clinical follow-up, is implanting BioSeed-C, which is based on a bioresorbable two-component gel-polymer scaffold (Ossendorf et al., 2007).

Healthy articular cartilage has a complex structure that helps the joint bear external forces, maintain interstitial fluid to reduce strains on its soft tissue, and reduce friction between the cartilages. When the cartilage is under loading, the surrounding matrix reduces the fluid outflow velocity to provide hydrostatic pressure. Early signs of osteoarthritis include the decrease of glycosaminoglycans and the destruction of the collagen network. An innovative polymeric cartilage supplement that recreates the extracellular matrix’s hydrophilic characteristics by forming a charged interpenetrating polymer network (IPN) has been developed (Mäkelä et al., 2018). The hydrophilic ECM is principally strengthened by the IPN in order to recreate the material characteristics of cartilage. The reestablishment of the fluid phase is also impacted by this strengthening of the solid phase.

Hydrogels made of polyacrylamide are frequently used as potential substitutes for cartilage. Their application, however, is severely limited by their low mechanical durability and puncture resistance. The strength of polyacrylamide (PAM) hydrogels was increased by using titanium oxide (TiO2) and carbon nanotubes (CNTs) independently or in combination in a PAM matrix, which was interconnected by bonding between nanoparticles and polymers using a density functional theory (DFT) approach. The PAM-TiO2-CNT hydrogel’s outstanding compressive strength, elastic modulus (>0.43 and 2.340 MPa, respectively), and puncture resistance (estimated using the needle insertion test) are attributed to the synergistic effect and solid interfacial bonding, making it potentially a remarkable nanocomposite hydrogel for cartilage repair applications (Figure 10C) (Awasthi et al., 2021).

The researchers devised and manufactured a strong bilayer nanocomposite acrylamide-acrylic acid hydrogel reinforced with silica nanoparticles (SNPs). By using only 0.6 wt% SNPs, the mechanical properties revealed a considerable improvement in compressive strength up to 1.4 MPa and a doubled elastic modulus (240 kPa) compared to the non-reinforced hydrogel. With appropriate ratios of monomers and SNPs, samples could be compressed without damage until reaching 85% strain (Mostakhdemin et al., 2021). Bilayer silica-reinforced nanocomposite hydrogels are possible choices for synthetic cartilage due to their mechanical properties.

A biomineralized bottom layer that mimics the calcium phosphate (CaP)-rich bone microenvironment is attached to a cryogel middle layer with an anisotropic pore architecture, and a hydrogel top layer is attached to the trilayer scaffold that is depicted in Figure 10D. The macroporous middle layer and hydrogel top layer were intended to support the production of cartilage tissue, whilst the mineralized bottom layer was intended to support the formation of bones. The transplanted cells continuously differentiated to form cartilage tissue, while endogenous cells were recruited through the mineralized bottom layer to build bone tissue, resulting in the creation of osteochondral tissue (Kang et al., 2018).

The capacity of some materials to produce electric signals in response to mechanical stress is known as the piezoelectric effect (Zhang et al., 2018). When cartilage tissue is rebuilding naturally, the extracellular matrix (ECM) creates an electric signal that is reminiscent to that produced by piezoelectric biomaterials. In a physiological environment, the compressive strain on collagen fibers in the ECM causes the dipole moment to reorganize, generating negative charges. As a result, an electrical signal enters the cell membrane, causing voltage-gated calcium channels to open.

According to a study, flexible, 3D fibrous scaffolds made of piezoelectric materials can promote ECM development and human mesenchymal stem cell differentiation under physiological loading circumstances. Piezoelectric scaffolds with a high voltage output encourage osteogenic differentiation while those with a low voltage output or streaming potential encourage chondrogenic differentiation. More considerable differentiation was discovered to be induced by electromechanical stimuli than by mechanical loading alone (Damaraju et al., 2017). Using poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and barium titanate (BaTiO3), a clever piezoelectric nanohybrid was created. Comparatively to the control (pure PHBV) and unpolarized scaffolds, the polarized scaffolds greatly enhance cell adhesion, proliferation, and COLIIA1 gene expression (Jacob et al., 2019).

For cartilage tissue engineering, the ideal adaptable scaffolds should evolve dynamically and spatiotemporally in response to the physiological microenvironments at each stage of cartilage repair. ECMs’ mechanical strength changes as a result of growing tissue adjusting to the body’s requirements (Peng et al., 2023b). Therefore, adaptive tissue engineering scaffolds should change their mechanical strength to meet the needs of cells as they grow and mature. In addition, osteochondral tissue is a gradient construct with a seamless transition from cartilage to subchondral bone, involving changes in collagen type and orientation, chondrocyte morphologies, ECM components, and cytokines. To meet the anisotropic properties of osteochondral matrices, bioinspired scaffolds have been created by replicating gradient characteristics in heterogeneous tissues, such as the pores, components, and osteochondrogenesis-inducing substances (Peng et al., 2023a). Bioinspired gradient scaffolds can restore osteochondral defects by modifying the microenvironments of cell development to promote osteochondrogenesis.

Rheumatoid arthritis (RA), is an autoimmune illness brought on by both external (hostile environment and virus invasion) and endogenous (cellular, genic, and neurological issues) components. Today, oral or injectable medications are typically used to treat RA. However, there is an urgent need to create a new method of medication delivery because of the adverse effects such as poor patient compliance, gastrointestinal disturbances, and other toxicity hazards connected with current administrations (Conforti et al., 2021). Hence, transdermal drug delivery systems, represented by microneedles, are becoming popular in the treatments of RA (Zhang et al., 2023). The melittin-loaded MeHA microneedles in the Adjuvant-Induced Arthritis (AIA) animal model prevented the rat’s paw from swelling (Ye et al., 2016). TNF and IL-17 levels also decreased as regulatory CD4+T cells increased, which had a suppressive effect on RA and a protective effect on articular cartilage.

In addition, some new drug-delivery scaffolds can promote angiogenesis and subchondral bone formation, indirectly promoting joint repair (Xu et al., 2023a; Xu et al., 2023b). For the best bone regeneration, designing a multifunctional scaffold with osteogenic and angiogenic capabilities offers promise. Exosomes from human bone mesenchymal stem cells (hBMSCs) were immobilized on porous polymer meshes created by PLGA and Cu-based MOF (PLGA/CuBDC@Exo) in one study to create an innovative scaffold. The manufactured exosome-laden scaffold can deliver a dual cooperative controlled release of bioactive copper ions and exosomes that encourage osteogenesis and angiogenesis, resulting in cell-free bone repair (Xu et al., 2023a). A pro-angiogenic small molecule medication (dimethyloxallyl glycine, or DMOG) was loaded onto an iron-based metal-organic framework (MIL-88) and subsequently embedded into PLGA nanofibrous scaffolds to repair cranial lesions in rats in another study (Xu et al., 2023b). Co-delivery system considerably aided angiogenesis by enhancing endothelial cell migration, tube formation, and osteogenesis by enhancing the production of proteins associated with osteoblasts, according to an in vitro study.

If we can combine mechanical stimuli with these state-of-the-art biomaterial scaffolds in cartilage tissue engineering, it is expected to construct grafts that are more consistent with natural cartilage and promote joint repair.