MPCs are decidedly different from

MPCs are decidedly different from the recently described CPCs (Koelling et al., 2009; Koelling and Miosge, 2010) not only due to their tissue origin and stem cell marker pattern, but also due to their ability to produce collagen type I and type II. However, they are similar as they also appear to be modulated via a balance between RUNX2 and SOX9 (Muhammad et al., 2013). Despite the presence of these multipotent moexipril in diseased human meniscus, they are not able to contribute substantially to regeneration efforts. Although the cell clusters found might be interpreted as signs of proliferation of the progenitor cells, it is likely that the numerous mediators of inflammation and tissue degeneration present, especially, in the late stages of OA investigated here, inhibit the regenerative capacity of any stem cell present in the diseased tissue. In articular cartilage, deletion of the type 2 TGF-β receptor results in the upregulation of RUNX2, MMP13, and ADAMTS5, which correlates with the progression of OA disease (Shen et al., 2013), and stimulation of SMAD2/3 signaling in articular chondrocytes has been suggested as a potential therapeutic manipulation for the treatment of OA (van der Kraan et al., 2012). Here, we demonstrate that the treatment of MPCs with TGF-β3 results in an upregulation of SOX9 and a downregulation of RUNX2 similar to findings in articular cartilage. The shift in articular chondrocytes from SMADS2/3 to SMADS1/5 appears to be important for the development of OA, and, recently, a balancing role of endoglin (CD 105) for this shift has been highlighted (Finnson et al., 2010). This change in pathway mediators may also be the case in the diseased human meniscus, because we found lower levels of SMAD2/3 and higher levels of RUNX2 in diseased tissue compared with healthier meniscus tissue. The complex regulatory role of the signaling balance between TGF-β/BMP in OA pathology is widely acknowledged (Finnson et al., 2010; van der Kraan et al., 2010), and our data suggest this interaction also governs meniscal cell behavior. We observe that MPCs from less diseased meniscus respond to TGF-β via the ALK5 receptor, and that this promotes chondrogenesis, whereas MPCs from more diseased meniscus are prone toward osteogenesis. Furthermore, our findings suggest that specific components of the TGF-β pathway may be suitable targets for regenerative therapies directed at diseased human meniscus in late stages of OA. As OA is a very complex disease, and multiple tissues that are integral to joint function are impacted in different ways (Loeser et al., 2012), we find it unlikely that direct TGF-β treatment of the meniscus will have significant therapeutic benefit, and undesirable side effects of TGF-β therapy have been reported, including synovial hyperplasia, inflammation, or even osteophyte formation (Blaney Davidson et al., 2006; van Beuningen et al., 1998). Our current finding supports the notion that a more nuanced understanding of the regulatory events governing MPCs biology will be necessary to identify a cell-based therapy and the modulatory factors that support using these cells for meniscal regeneration. Identification and characterization of MPCs provides direct evidence that meniscal repair cells are present in highly diseased tissue and also allows for detailed study of the signaling pathways that govern their proliferation and differentiation, a first step in identifying drug targets for the regeneration of diseased meniscus.
Experimental Procedures
Author Contributions
Acknowledgments
Introduction Human pluripotent stem cells (hPSCs) offer unprecedented opportunities to study the earliest stages of human development in vitro, to model human disease, to perform drug tests in culture, and to develop unlimited new sources of cells for possible therapeutic applications. To realize this potential, it is essential to be able to control hPSC differentiation to somatic lineages with high efficiency and reproducibility in a scalable and inexpensive manner (Ashton et al., 2011; Burridge et al., 2012; Kinney et al., 2014; Murry and Keller, 2008).