In this regard induced pluripotent stem
In this regard, induced pluripotent stem squalene epoxidase (iPSCs) converge to a better-defined ground state of pluripotency (Hackett et al., 2013). They can be differentiated into all cell types of the organism and—while in pluripotent state—cultured virtually indefinitely without signs of replicative senescence. Epigenetic profiles, such as DNA methylation (DNAm) patterns, are reorganized during reprogramming of somatic cells into iPSCs and closely resemble those of embryonic stem cells (ESCs) (Huang et al., 2014). In particular, senescence-associated DNAm, which is acquired during in vitro expansion (Koch et al., 2013), and age-related DNAm, which accumulate during aging of the organism (Horvath, 2013), are reversed to ground state. In comparison to primary cells, iPSCs are therefore better defined and offer a good starting point for large-scale generation of standardized derivatives, such as iPSC-derived MSCs (iPS-MSCs).
Several groups described strategies to derive MSC-like cells from either ESCs (Barberi et al., 2005; Boyd et al., 2009) or iPSCs (Liu et al., 2012; Diederichs and Tuan, 2014; Zhang et al., 2011). These approaches were based on coculture with primary MSCs, growth factor combinations, or spontaneous differentiation in embryoid bodies (EBs). So far, it has not been analyzed whether DNAm patterns of iPS-MSCs resemble those of primary MSCs.
Discussion Induced pluripotent stem cells are a very good basis for generation of standardized cell types. However, differentiation to specific cell types remains a major challenge. Here, we describe a simple protocol for differentiation of iPSCs toward iPS-MSCs, which may be assisted by the fact that our iPSCs were initially derived from MSCs. Furthermore, the medium used for iPS-MSC induction is the same as for initial culture isolation of MSCs. These culture conditions would be compatible with guidelines of good manufacturing practice (GMP) in cellular therapy. Our iPS-MSCs fulfilled the minimal criteria of MSCs (Dominici et al., 2006), but the propensity for adipogenic differentiation was markedly decreased as compared to primary MSCs. This has also been observed by several other groups that used different protocols for generation of iPS-MSCs (Boyd et al., 2009; Chen et al., 2012; Diederichs and Tuan, 2014). Furthermore, adipogenic differentiation is also quite heterogeneous within primary MSCs (Schellenberg et al., 2012). Hence, a more sophisticated molecular definition is required for MSCs and for iPS-MSCs. MSCs and iPS-MSCs revealed close relationship in gene expression profiles. However, genes associated with T cell activation and immune response were higher expressed in MSCs. Consistent with these observations, iPS-MSCs were impaired in suppressing T cell proliferation, indicating that iPS-MSCs have lower immunomodulatory properties than primary MSCs. Other authors indicated that ESC- and iPSC-derived MSCs are somewhat immunoprivileged and might even have therapeutic efficacy in autoimmune disorder models (de Peppo et al., 2010; Fu et al., 2012; Kimbrel et al., 2014). Therefore, the immunomodulatory function of iPS-MSCs—which is critical for clinical application—deserves further analysis in future studies. Gene expression changes are not necessarily reflected on DNAm level and vice versa. In fact, many recent studies demonstrated no general correlation of DNAm and gene expression (Wagner et al., 2014), despite the common perception of hypermethylation in promoter regions should entail downregulation of gene expression. Although our understanding of the functional relevance of specific DNAm changes is so far limited epigenetic profiles are very well suited to classify cell preparations: DNAm can be provided as absolute β values at single base resolution; it is relatively stable; less prone to growth conditions; and less influenced by subpopulations, which may highly overexpress subsets of genes. In this regard, the large number of differentially methylated CpGs indicates that iPS-MSCs are rather “MSC-like” than direct correlates of primary MSCs—this has to be taken into account, but it does not exclude that iPS-MSCs may be valuable tools for cellular therapy, too.