Osteoarthritis osteochondral tissue based in vitro model : potential testing platform for emerging treatments

Abstract

Osteoarthritis is a common degenerative joint disease that affects a large number of people across the globe. However, the actual disease mechanism still remains elusive and an effective intervening treatment is still lacking. In vitro models of osteoarthritis (OA) offers a promising and convenient way of studying OA pathophysiology and evaluation of emerging OA therapies. Two in vitro models were developed in the current study based on human osteoarthritic specimens collected from total knee replacement surgery. In the first approach, a novel 3D culture model – collagen microencapsulation was compared with 2D monolayer culture and a well-established 3D pellet culture method. The ability of collagen microencapsulation in resuming the chondrogenic phenotypes of in vitro dedifferentiated OA chondrocytes were evaluated. Isolated OA chondrocytes were firstly microencapsulated into collagen microspheres after expansion. The change in chondrocyte phenotypes including Sox9 and collagen II expression as well as the OA phenotypes including Runx2, collagen X and MMP13 were evaluated over 2-3 weeks of culture. Overall, microencapsulation was able to resume both chondrogenic and OA phenotypes of monolayer cultured cells. Furthermore, by combining with other biomimetic strategies such as serum-free medium and hypoxia, the phenotypic maintenance of hOA cells under microencapsulation was further improved. Our results therefore showed that collagen microencapsulation possesses the potential of being an in vitro OA cartilage model. In our second approach, we have developed and characterized a human osteoarthritic mimicking osteochondral organ culture model which allows researchers to study OA pathophysiology, evaluation of emerging OA therapies such as tissue engineered cartilage and to screen for a wide range of external factors that could optimize the treatment outcome. Specifically, excised joint tissues from total knee replacement surgeries were carved into miniaturized and standardized osteochondral plugs for OA organ culture. The organ cultures were characterized in detail, before using it for co-culture with the tissue engineered cartilage. The change during co-culture, including the expression of major matrix metalloproteases MMP13 and ADAMTS-5, the histology and the collagen II and GAG deposition of engineered cartilage under OA-mimicking microenvironment was then evaluated. We demonstrate that cells in the OA organ culture were viable while both the typical chondrogenic phenotype and the characteristic OA phenotype were maintained for long period of time. We then demonstrate that upon co-culture with the OA-mimicking organ culture, the engineered cartilage initially exhibited a more fibrocartilage phenotype but progressively reverted back to the chondrogenic phenotype upon long term co-culture up to 8 weeks. The engineered cartilage was also found to be sensitive to all biomimetic environmental factors screened (oxygen tension, serum and compression). Moreover, under the effect of a MMP inhibitor, the chondrogenic phenotype of engineered cartilage was better maintained. Our study demonstrated the feasibility and potential of using organ culture model as an in vitro evaluation tool. Taken together, we anticipate that these two platforms could facilitate the understanding of OA and development of new OA treatments.