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A dysfunctional TRPV4–GSK3β pathway prevents osteoarthritic chondrocytes from sensing changes in extracellular matrix viscoelasticity

Abstract

Changes in the composition and viscoelasticity of the extracellular matrix in load-bearing cartilage influence the proliferation and phenotypes of chondrocytes, and are associated with osteoarthritis. However, the underlying molecular mechanism is unknown. Here we show that the viscoelasticity of alginate hydrogels regulates cellular volume in healthy human chondrocytes (with faster stress relaxation allowing cell expansion and slower stress relaxation restricting it) but not in osteoarthritic chondrocytes. Cellular volume regulation in healthy chondrocytes was associated with changes in anabolic gene expression, in the secretion of multiple pro-inflammatory cytokines, and in the modulation of intracellular calcium regulated by the ion-channel protein transient receptor potential cation channel subfamily V member 4 (TRPV4), which controls the phosphorylation of glycogen synthase kinase 3β (GSK3β), an enzyme with pleiotropic effects in osteoarthritis. A dysfunctional TRPV4–GSK3β pathway in osteoarthritic chondrocytes rendered the cells unable to respond to environmental changes in viscoelasticity. Our findings suggest strategies for restoring chondrocyte homeostasis in osteoarthritis.

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Fig. 1: Normal chondrocytes exhibit a robust response to differential stress relaxation with changes in volume, gene expression and secreted factors.
Fig. 2: Slow ECM stress relaxation primes normal chondrocytes for an elevated response to inflammatory cues.
Fig. 3: Intracellular-calcium-dependent changes in cellular signalling landscape in normal chondrocytes.
Fig. 4: Activation of TRPV4 leads to increased intracellular calcium levels, GSK3β phosphorylation and inflammatory phenotype.
Fig. 5: Inhibition of TRPV4 leads to decreased intracellular calcium levels, GSK3β phosphorylation and reduced inflammatory phenotype.
Fig. 6: Response to viscoelasticity is absent in OA chondrocytes.
Fig. 7: OA chondrocytes do not regulate intracellular calcium through TRPV4, leading to chronic high GSK3β and inflammation.
Fig. 8: Matrix viscoelasticity is transduced to chondrocytes via the TRPV4–GSK3β axis in normal cartilage but not OA cartilage.

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Data availability

The main data supporting the results in this study are available within the paper and its Supplementary Information. The raw and analysed datasets are available in figshare with the identifier https://doi.org/10.6084/m9.figshare.13184909 (ref. 76).

References

  1. Glyn-Jones, S. et al. Osteoarthritis. Lancet 386, 376–387 (2015).

    Article  CAS  PubMed  Google Scholar 

  2. Martel-Pelletier, J. et al. Osteoarthritis. Nat. Rev. Dis. Prim. 2, 16072 (2016).

    Article  PubMed  Google Scholar 

  3. Vincent, T. L. Targeting mechanotransduction pathways in osteoarthritis: a focus on the pericellular matrix. Curr. Opin. Pharmacol. 13, 449–454 (2013).

    Article  CAS  PubMed  Google Scholar 

  4. Kim, Y. J., Bonassar, L. J. & Grodzinsky, A. J. The role of cartilage streaming potential, fluid flow and pressure in the stimulation of chondrocyte biosynthesis during dynamic compression. J. Biomech. 28, 1055–1066 (1995).

    Article  CAS  PubMed  Google Scholar 

  5. Gu, W. Y., Lai, W. M. & Mow, V. C. Transport of fluid and ions through a porous-permeable charged-hydrated tissue, and streaming potential data on normal bovine articular cartilage. J. Biomech. 26, 709–723 (1993).

    Article  CAS  PubMed  Google Scholar 

  6. Buschmann, M. D., Gluzband, Y. A., Grodzinsky, A. J. & Hunziker, E. B. Mechanical compression modulates matrix biosynthesis in chondrocyte/agarose culture. J. Cell Sci. 108, 1497–1508 (1995).

    Article  CAS  PubMed  Google Scholar 

  7. Guilak, F. Biomechanical factors in osteoarthritis. Best. Pract. Res. Clin. Rheumatol. 25, 815–823 (2011).

    Article  PubMed  PubMed Central  Google Scholar 

  8. Maldonado, M. & Nam, J. The role of changes in extracellular matrix of cartilage in the presence of inflammation on the pathology of osteoarthritis. Biomed. Res. Int. 2013, 284873 (2013).

    Article  PubMed  PubMed Central  Google Scholar 

  9. Goldring, S. R. & Goldring, M. B. Changes in the osteochondral unit during osteoarthritis: structure, function and cartilage–bone crosstalk. Nat. Rev. Rheumatol. 12, 632–644 (2016).

    Article  PubMed  Google Scholar 

  10. Mow, V. C., Kuei, S. C., Lai, W. M. & Armstrong, C. G. Biphasic creep and stress relaxation of articular cartilage in compression? Theory and experiments. J. Biomech. Eng. 102, 73–84 (1980).

    Article  CAS  PubMed  Google Scholar 

  11. Nia, H. T., Han, L., Li, Y., Ortiz, C. & Grodzinsky, A. Poroelasticity of cartilage at the nanoscale. Biophys. J. 101, 2304–2313 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  12. Mouw, J. K., Case, N. D., Guldberg, R. E., Plaas, A. H. K. & Levenston, M. E. Variations in matrix composition and GAG fine structure among scaffolds for cartilage tissue engineering. Osteoarthr. Cartil. 13, 828–836 (2005).

    Article  CAS  Google Scholar 

  13. Smeriglio, P., Lai, J. H., Yang, F. & Bhutani, N. 3D hydrogel scaffolds for articular chondrocyte culture and cartilage generation. J. Vis. Exp. 104, 53085 (2015).

    Google Scholar 

  14. Jutila, A. A., Zignego, D. L., Schell, W. J. & June, R. K. Encapsulation of chondrocytes in high-stiffness agarose microenvironments for in vitro modeling of osteoarthritis mechanotransduction. Ann. Biomed. Eng. 43, 1132–1144 (2015).

    Article  PubMed  Google Scholar 

  15. Guo, J. F., Jourdian, G. W. & MacCallum, D. K. Culture and growth characteristics of chondrocytes encapsulated in alginate beads. Connect. Tissue Res. 19, 277–297 (1989).

    Article  CAS  PubMed  Google Scholar 

  16. Genes, N. G., Rowley, J. A., Mooney, D. J. & Bonassar, L. J. Effect of substrate mechanics on chondrocyte adhesion to modified alginate surfaces. Arch. Biochem. Biophys. 422, 161–167 (2004).

    Article  CAS  PubMed  Google Scholar 

  17. Degala, S., Zipfel, W. R. & Bonassar, L. J. Chondrocyte calcium signaling in response to fluid flow is regulated by matrix adhesion in 3-D alginate scaffolds. Arch. Biochem. Biophys. 505, 112–117 (2011).

    Article  CAS  PubMed  Google Scholar 

  18. Tan, H., Chu, C. R., Payne, K. A. & Marra, K. G. Injectable in situ forming biodegradable chitosan–hyaluronic acid based hydrogels for cartilage tissue engineering. Biomaterials 30, 2499–2506 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  19. Cheng, H.-W., Tsui, Y.-K., Cheung, K. M. C., Chan, D. & Chan, B. P. Decellularization of chondrocyte-encapsulated collagen microspheres: a three-dimensional model to study the effects of acellular matrix on stem cell fate. Tissue Eng. C 15, 697–706 (2009).

    Article  CAS  Google Scholar 

  20. Conrad, B., Han, L.-H. & Yang, F. Gelatin-based microribbon hydrogels accelerate cartilage formation by mesenchymal stem cells in three dimensions. Tissue Eng. A 24, 1631–1640 (2018).

    Article  CAS  Google Scholar 

  21. Lee, H., Gu, L., Mooney, D. J., Levenston, M. E. & Chaudhuri, O. Mechanical confinement regulates cartilage matrix formation by chondrocytes. Nat. Mater. 16, 1243–1251 (2017).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  22. Richardson, B. M., Wilcox, D. G., Randolph, M. A. & Anseth, K. S. Hydrazone covalent adaptable networks modulate extracellular matrix deposition for cartilage tissue engineering. Acta Biomater. 83, 71–82 (2019).

    Article  CAS  PubMed  Google Scholar 

  23. Gong, Z. et al. Matching material and cellular timescales maximizes cell spreading on viscoelastic substrates. Proc. Natl Acad. Sci. USA 115, E2686–E2695 (2018).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  24. Charrier, E. E., Pogoda, K., Wells, R. G. & Janmey, P. A. Control of cell morphology and differentiation by substrates with independently tunable elasticity and viscous dissipation. Nat. Commun. 9, 449 (2018).

    Article  PubMed  PubMed Central  Google Scholar 

  25. McKinnon, D. D., Domaille, D. W., Cha, J. N. & Anseth, K. S. Biophysically defined and cytocompatible covalently adaptable networks as viscoelastic 3D cell culture systems. Adv. Mater. 26, 865–872 (2014).

    Article  CAS  PubMed  Google Scholar 

  26. Cameron, A. R., Frith, J. E. & Cooper-White, J. J. The influence of substrate creep on mesenchymal stem cell behaviour and phenotype. Biomaterials 32, 5979–5993 (2011).

    Article  CAS  PubMed  Google Scholar 

  27. Chaudhuri, O. et al. Substrate stress relaxation regulates cell spreading. Nat. Commun. 6, 6364 (2015).

    Article  PubMed  Google Scholar 

  28. Chaudhuri, O. et al. Hydrogels with tunable stress relaxation regulate stem cell fate and activity. Nat. Mater. 15, 326–334 (2016).

    Article  CAS  PubMed  Google Scholar 

  29. Lee, H., Stowers, R. & Chaudhuri, O. Volume expansion and TRPV4 activation regulate stem cell fate in three-dimensional microenvironments. Nat. Commun. 10, 529 (2019).

    Article  PubMed  PubMed Central  Google Scholar 

  30. Wisdom, K. M. et al. Matrix mechanical plasticity regulates cancer cell migration through confining microenvironments. Nat. Commun. 9, 4144 (2018).

    Article  PubMed  PubMed Central  Google Scholar 

  31. Lee, J. et al. Early induction of a prechondrogenic population allows efficient generation of stable chondrocytes from human induced pluripotent stem cells. FASEB J. 29, 3399–3410 (2015).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  32. Darling, E. M., Wilusz, R. E., Bolognesi, M. P., Zauscher, S. & Guilak, F. Spatial mapping of the biomechanical properties of the pericellular matrix of articular cartilage measured in situ via atomic force microscopy. Biophys. J. 98, 2848–2856 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  33. Alexopoulos, L. G., Williams, G. M., Upton, M. L., Setton, L. A. & Guilak, F. Osteoarthritic changes in the biphasic mechanical properties of the chondrocyte pericellular matrix in articular cartilage. J. Biomech. 38, 509–517 (2005).

    Article  PubMed  Google Scholar 

  34. McLeod, M. A., Wilusz, R. E. & Guilak, F. Depth-dependent anisotropy of the micromechanical properties of the extracellular and pericellular matrices of articular cartilage evaluated via atomic force microscopy. J. Biomech. 46, 586–592 (2013).

    Article  PubMed  Google Scholar 

  35. Chen, D. et al. Osteoarthritis: toward a comprehensive understanding of pathological mechanism. Bone Res. 5, 16044 (2017).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  36. Wenham, C. Y. J. & Conaghan, P. G. The role of synovitis in osteoarthritis. Ther. Adv. Musculoskelet. Dis. 2, 349–359 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  37. Raghu, H. et al. CCL2/CCR2, but not CCL5/CCR5, mediates monocyte recruitment, inflammation and cartilage destruction in osteoarthritis. Ann. Rheum. Dis. 76, 914–922 (2017).

    Article  CAS  PubMed  Google Scholar 

  38. Zhao, X. Y. et al. CCL3 serves as a potential plasma biomarker in knee degeneration (osteoarthritis). Osteoarthr. Cartil. 23, 1405–1411 (2015).

    Article  CAS  Google Scholar 

  39. Yan, D. et al. Fibroblast growth factor receptor 1 is principally responsible for fibroblast growth factor 2-induced catabolic activities in human articular chondrocytes. Arthritis Res. Ther. 13, R130 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  40. Scanzello, C. R. Chemokines and inflammation in osteoarthritis: insights from patients and animal models. J. Orthop. Res. 35, 735–739 (2017).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  41. Leah, E. Experimental arthritis: GM-CSF mediates pain and disease in a mouse model of osteoarthritis. Nat. Rev. Rheumatol. 8, 634 (2012).

    Article  PubMed  Google Scholar 

  42. Cook, A. D. et al. Granulocyte-macrophage colony-stimulating factor is a key mediator in experimental osteoarthritis pain and disease development. Arthritis Res. Ther. 14, R199 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  43. Erickson, G. R., Alexopoulos, L. G. & Guilak, F. Hyper-osmotic stress induces volume change and calcium transients in chondrocytes by transmembrane, phospholipid, and G-protein pathways. J. Biomech. 34, 1527–1535 (2001).

    Article  CAS  PubMed  Google Scholar 

  44. Loeser, R. F., Erickson, E. A. & Long, D. L. Mitogen-activated protein kinases as therapeutic targets in osteoarthritis. Curr. Opin. Rheumatol. 20, 581–586 (2008).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  45. Saklatvala, J. Inflammatory signaling in cartilage: MAPK and NF-kappaB pathways in chondrocytes and the use of inhibitors for research into pathogenesis and therapy of osteoarthritis. Curr. Drug Targets 8, 305–313 (2007).

    Article  CAS  PubMed  Google Scholar 

  46. Thalhamer, T., McGrath, M. A. & Harnett, M. M. MAPKs and their relevance to arthritis and inflammation. Rheumatology 47, 409–414 (2007).

    Article  Google Scholar 

  47. Ge, H., Zou, F., Li, Y., Liu, A. & Tu, M. JNK pathway in osteoarthritis: pathological and therapeutic aspects. J. Recept. Signal Transduct. 37, 431–436 (2017).

    Article  CAS  Google Scholar 

  48. Melas, I. N. et al. Modeling of signaling pathways in chondrocytes based on phosphoproteomic and cytokine release data. Osteoarthr. Cartil. 22, 509–518 (2014).

    Article  CAS  Google Scholar 

  49. Corr, M. Wnt–β-catenin signaling in the pathogenesis of osteoarthritis. Nat. Rev. Rheumatol. 4, 550–556 (2008).

    Article  CAS  Google Scholar 

  50. Miclea, R. L. et al. Inhibition of Gsk3β in cartilage induces osteoarthritic features through activation of the canonical Wnt signaling pathway. Osteoarthr. Cartil. 19, 1363–1372 (2011).

    Article  CAS  Google Scholar 

  51. Guidotti, S. et al. GSK3β inactivation affects chondrocyte mitochondria leading to oxidative DNA damage, GADD45 beta induction, hypertrophy and cellular senescence. Osteoarthr. Cartil. 22, S163–S164 (2014).

    Article  Google Scholar 

  52. O’Conor, C. J., Leddy, H. A., Benefield, H. C., Liedtke, W. B. & Guilak, F. TRPV4-mediated mechanotransduction regulates the metabolic response of chondrocytes to dynamic loading. Proc. Natl Acad. Sci. USA 111, 1316–1321 (2014).

    Article  PubMed  PubMed Central  Google Scholar 

  53. O’Conor, C. J. et al. Cartilage-specific knockout of the mechanosensory ion channel TRPV4 decreases age-related osteoarthritis. Sci. Rep. 6, 29053 (2016).

    Article  PubMed  PubMed Central  Google Scholar 

  54. Jin, M. et al. Determinants of TRPV4 activity following selective activation by small molecule agonist GSK1016790A. PLoS ONE 6, e0016713 (2011).

    Article  Google Scholar 

  55. Ramos, Y. F. M. et al. Genes involved in the osteoarthritis process identified through genome wide expression analysis in articular cartilage; the RAAK study. PLoS ONE 9, e103056 (2014).

    Article  PubMed  PubMed Central  Google Scholar 

  56. Askari, A. et al. Increased serum levels of IL-17A and IL-23 are associated with decreased vitamin D3 and increased pain in osteoarthritis. PLoS ONE 11, e0164757 (2016).

    Article  PubMed  PubMed Central  Google Scholar 

  57. Taylor, S. E. B. et al. Identification of human juvenile chondrocyte-specific factors that stimulate stem cell growth. Tissue Eng. A 22, 645–653 (2016).

    Article  CAS  Google Scholar 

  58. Aigner, T., Zien, A., Gehrsitz, A., Gebhard, P. M. & McKenna, L. Anabolic and catabolic gene expression pattern analysis in normal versus osteoarthritic cartilage using complementary DNA-array technology. Arthritis Rheum. 44, 2777–2789 (2001).

    Article  CAS  PubMed  Google Scholar 

  59. Guo, M. et al. Cell volume change through water efflux impacts cell stiffness and stem cell fate. Proc. Natl Acad. Sci. USA 114, E8618–E8627 (2017).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  60. Stroka, K. M. et al. Water permeation drives tumor cell migration in confined microenvironments. Cell 157, 611–623 (2014).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  61. Zlotek-Zlotkiewicz, E., Monnier, S., Cappello, G., Berre, M. L. & Piel, M. Optical volume and mass measurements show that mammalian cells swell during mitosis. J. Cell Biol. 211, 765–774 (2015).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  62. Bader, D. L., Salter, D. M. & Chowdhury, T. T. Biomechanical influence of cartilage homeostasis in health and disease. Arthritis 2011, 979032 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  63. Wu, Q. et al. Smurf2 induces degradation of GSK-3β and upregulates β-catenin in chondrocytes: a potential mechanism for Smurf2-induced degeneration of articular cartilage. Exp. Cell Res. 315, 2386–2398 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  64. Boyle, W. J. et al. Activation of protein kinase C decreases phosphorylation of c-Jun at sites that negatively regulate its DNA-binding activity. Cell 64, 573–584 (1991).

    Article  CAS  PubMed  Google Scholar 

  65. Lewis, R., Feetham, C. H. & Barrett-Jolley, R. Cell volume regulation in chondrocytes. Cell. Physiol. Biochem. 28, 1111–1122 (2011).

    Article  CAS  PubMed  Google Scholar 

  66. Servin-Vences, M. Rocio, Moroni, M., Lewin, G. & Poole, K. Direct measurement of TRPV4 and PIEZO1 activity reveals multiple mechanotransduction pathways in chondrocytes. eLife 6, e21074 (2017).

    Article  PubMed  Google Scholar 

  67. O’Connor, C. J. Increased susceptibility of Trpv4-deficient mice to obesity and obesity-induced osteoarthritis with very high-fat diet. Ann. Rheum. Dis. 72, 300–4 (2013).

    Article  Google Scholar 

  68. van der Eerden, B. C. TRPV4 deficiency causes sexual dimorphism in bone metabolism and osteoporotic fracture risk. Bone 57, 443–54 (2013).

    Article  PubMed  Google Scholar 

  69. Clark, A. L., Votta, B. J., Kumar, S., Liedtke, W. & Guilak, F. Chondroprotective role of the osmotically sensitive ion channel transient receptor potential vanilloid 4: age- and sex-dependent progression of osteoarthritis in Trpv4-deficient mice. Arthritis Rheum. 62, 2973–2983 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  70. McNulty, A. L., Leddy, H. A., Liedtke, W. & Guilak, F. TRPV4 as a therapeutic target for joint diseases. Naunyn. Schmiedebergs Arch. Pharmacol. 388, 437–450 (2015).

    Article  CAS  PubMed  Google Scholar 

  71. Jones, R. C. et al. Piezo1 expression is increased in response to non-invasive impact of mouse knee joint. Osteoarthr. Cartil. 26, S113–S114 (2018).

    Article  Google Scholar 

  72. Lee, W. et al. Synergy between Piezo1 and Piezo2 channels confers high-strain mechanosensitivity to articular cartilage. Proc. Natl Acad. Sci. USA 111, E5114–E5122 (2014).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  73. Pritchard, S. & Guilak, F. Effects of interleukin-1 on calcium signaling and the increase of filamentous actin in isolated and in situ articular chondrocytes. Arthritis Rheum. 54, 2164–2174 (2006).

    Article  CAS  PubMed  Google Scholar 

  74. Beekman, B., Verzijl, N., de Roos, J. A. D. M. & TeKoppele, J. M. Matrix degradation by chondrocytes cultured in alginate: IL-1β induces proteoglycan degradation and proMMP synthesis but does not result in collagen degradation. Osteoarthr. Cartil. 6, 330–340 (1998).

    Article  CAS  Google Scholar 

  75. Walczysko, P., Wagner, E. & Albrechtová, J. T. P. Use of co-loaded Fluo-3 and Fura red fluorescent indicators for studying the cytosolic Ca2+ concentrations distribution in living plant tissue. Cell Calcium 28, 23–32 (2000).

    Article  CAS  PubMed  Google Scholar 

  76. Agarwal, P. et al. Dataset for ‘A dysfunctional TRPV4–GSK3β pathway prevents osteoarthritic chondrocytes from sensing changes in extracellular matrix viscoelasticity’. figshare https://doi.org/10.6084/m9.figshare.13184909 (2021).

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Acknowledgements

We thank Y. Rosenberg-Hasson at the Stanford Human Immune Profiling Center for help with the Luminex analysis. These studies were supported by funding from the Stanford Bio-X Interdisciplinary Initiatives Seed Grants Program (IIP) (R9-52 to N.B. and O.C.), National Institutes of Health grants (R01 AR070864 and R01 AR070865 to N.B. and R21 AR074070 to O.C.) and a Stanford Bio-X fellowship (to H.-p.L.).

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P.A., H.-p.L., O.C. and N.B. designed the overall study. P.A. and H.-p.L. conducted experiments and analysed the data. P.S. and F.G. contributed to discussions and data interpretation. S.G. provided samples from human patients with OA. P.A., H.-p.L., O.C. and N.B. wrote the manuscript.

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Correspondence to Ovijit Chaudhuri or Nidhi Bhutani.

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Agarwal, P., Lee, Hp., Smeriglio, P. et al. A dysfunctional TRPV4–GSK3β pathway prevents osteoarthritic chondrocytes from sensing changes in extracellular matrix viscoelasticity. Nat Biomed Eng 5, 1472–1484 (2021). https://doi.org/10.1038/s41551-021-00691-3

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