Strategies for Articular Cartilage Repair; Mesenchymal Stem Cell-Based Targeting of the Joint Niche
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Osteoarthritis (OA) is a degenerative disease characterized by loss of articular cartilage, synovial inflammation and bone remodelling. Due to the low cellularity and avascular nature of articular cartilage it does not spontaneously repair. Mesenchymal stem cells (MSCs) are a potential therapeutic for cartilage regeneration, with delivery of MSCs shown to slow the progression of cartilage degradation. Yet our understanding of the mechanism of action remains unclear. Further to this, loss of transplanted cells from the desired site remains a key biological obstacle. It is now recognised that the synovial joint environment contains tissue-specific progenitor cells, activation of which would signify an additional treatment approach by initiating an endogenous repair programme. Given that biomaterials, such as microspheres have been used as model delivery system to enhance MSC differentiation and also recruit endogenous cells for repair, the first aim was to investigate the use of Hyaluronan (HA) microspheres as growth factor delivery vehicles for in situ differentiation of hMSCs and assess whether functionalization of these microspheres with an antibody to a degraded cartilage epitope would enable specific targeting to OA cartilage. Secondly, the presence of a mesenchyme progenitor cell within the joint was investigated using a Prx1-CreERT2; R26LacZ mouse model. Prx1 is a paired-class homeobox gene that is expressed in undifferentiated limb bud mesenchyme that gives rise to all mesenchymal cell populations, including osteo-chondroprogenitors. Additionally, the fate and biological behaviour of mesenchyme progenitor cells, isolated from foetal limb buds at embryonic stage 11.5 (E11.5) on delivery into OA knee joints was investigated. Specifically, localization and persistence of these cells were assessed and the affects they had on catabolic and anabolic gene expression was analysed. In vitro/ex-vivo results support the use of transforming growth factor beta-3 (TGF-beta3)-loaded HA microspheres for in situ differentiation of hMSCs. Furthermore, antibody functionalization of HA microspheres enabled localization to human OA cartilage in an ex-vivo model system. With regards to the presence of a progenitor population, lineage tracing of Prx1-CreERT2; R26LacZ cells from 2, 4, 6 and 12 week old animals identified Prx1-positive cells in locations consistent with stem cell niches. Furthermore, following intra-articular injection of E11.5 mesenchyme progenitor cells, cells homed to sites previously identified as stem cell niche areas and elicited a paracrine anti-inflammatory effect. These findings indicate that HA microspheres could be utilized as targeted growth factor reservoirs for the differentiation of exogenously delivered MSC/progenitor cells or coupled with other bioactive compounds could potentially recruit endogenous progenitor cells for repair, such as the Prx1 expressing cells identified. Furthermore, we confirmed an anti-inflammatory role of mesenchyme progenitor cells within an OA joint environment.
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