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Abstract
Articular cartilage progenitor cells (ACPCs) represent a new and potentially powerful alternative cell source to commonly used cell sources for cartilage repair, such as chondrocytes and bone-marrow derived mesenchymal stem cells (MSCs). This is particularly due to the apparent resistance of ACPCs to hypertrophy. The current study opted to investigate whether human ACPCs (hACPCs) are responsive towards mechanical stimulation and/or adenoviral-mediated overexpression of bone morphogenetic protein 2 (BMP-2). hACPCs were cultured in fibrin-polyurethane composite scaffolds. Cells were cultured in a defined chondro-permissive medium, lacking exogenous growth factors. Constructs were cultured, for 7 or 28 days, under free-swelling conditions or with the application of complex mechanical stimulation, using a custom built bioreactor that is able to generate joint-like movements. Outcome parameters were quantification of BMP-2 and transforming growth factor beta 1 (TGF-β1) concentration within the cell culture medium, biochemical and gene expression analyses, histology and immunohistochemistry. The application of mechanical stimulation alone resulted in the initiation of chondrogenesis, demonstrating the cells are mechanoresponsive. This was evidenced by increased GAG production, lack of expression of hypertrophic markers and a promising gene expression profile (significant up-regulation of cartilaginous marker genes, specifically collagen type II, accompanied by no increase in the hypertrophic marker collagen type X or the osteogenic marker alkaline phosphatase). To further investigate the resistance of ACPCs to hypertrophy, overexpression of a factor associated with hypertrophic differentiation, BMP-2, was investigated. A novel, three-dimensional, transduction protocol was used to transduce cells with an adenovirus coding for BMP-2. Over-expression of BMP-2, independent of load, led to an increase in markers associated with hypertropy. Taken together ACPCs represent a potential alterative cell source for cartilage tissue engineering applications.
Highlights
Hyaline articular cartilage possesses a limited intrinsic repair capacity
In addition we aimed to investigate whether human ACPCs (hACPCs) maintained resistance to hypertrophy in the presence of bone morphogenetic protein 2 (BMP-2 overexpression)
The current study opted to investigate the effect of mechanical stimulation and adenoviral-mediated over-expression of BMP-2, alone or in combination, on the chondrogenesis of monolayerexpanded hACPCs. hACPCs were encapsulated in fibrin and seeded into PU scaffolds
Summary
Hyaline articular cartilage possesses a limited intrinsic repair capacity. Cartilage defects that are not, or not sufficiently, treated render the cartilaginous tissue prone to further degeneration. This might result in the development of secondary osteoarthritis [1]. Available treatment options, such as microfracture [2], mosaicplasty [3], soft-tissue grafts [4] or autologous chondrocyte transplantation [5] still fail to demonstrate reproducible success in articular cartilage regeneration. Tissue engineering (TE) represents a promising alternative treatment option for articular cartilage defects [6]. In TE applications, the starting cell source is still a topic of debate [14]
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