Induced pluripotent stem cell technologies for tissue engineering

Tissue Engineering and Regeneration in Dentistry: Current Strategies, First Edition. Edited by Rachel J. Waddington and Alastair J. Sloan. © 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd. Induced pluripotent stem cells (iPSCs) were first esta­ blished by delivering the four factors c‐Myc/Klf4/Oct4/ Sox2 or Lin28/Nanog/Oct4/Sox2 into dermal fibroblasts via a viral vector‐based approach (Takahashi et al., 2007; Takahashi and Yamanaka, 2006; Yu et al., 2007). To avoid permanent integration of these introduced exogenous genes, plus the vector that carries them, significant efforts have been put into removing the transgenes and vectors from cells after they have been reprogrammed into iPSCs (Gonzalez et al., 2009; Kaji et al., 2009; Soldner et al., 2009; Woltjen et al., 2009; Yu et al., 2009a). Because of the reactivation of endogenous pluripotent genes that function to maintain the pluripotent state after repro­ gramming, these exogenous transgenes can be removed without affecting the reprogrammed status. In fact, removing these exogenous transgenes renders iPSCs more similar to human embryonic stem cells (hESCs) (Soldner et al., 2009). Besides using viral vector systems to repro­ gram cells, other methods that can completely circumvent the use of vectors have been utilised, including delivery of recombinant protein‐based or synthetic mRNAs of the four factors to generate iPSCs (reviewed by Rao and Malik, 2012). There are many applications that iPSCs can contribute to; among others, this chapter focuses on (1) cell‐based tissue regeneration and (2) generation of patient‐specific iPSCs to study disease mechanisms. With respect to the source of cells for human iPSC gen­ eration, various cell types are capable of converting into iPSCs, although dermal fibroblasts are most commonly used due to their relative ease of access and availability (Aasen et al., 2008; Giorgetti et al., 2009; Giorgetti et al., 2010; Li et al., 2009; Loh et al., 2009; Miyoshi et al., 2010a; Nakagawa et al., 2008; Park et al., 2008b; Sun et al., 2009; Takahashi et al., 2007; Yan et al., 2010). In general it is easier to reprogram more immature cells than more differ­ entiated cells. From the perspective of clinical applications, cells that are not easily accessible, such as neural stem cells, are not a suitable cell source for iPSC generation. The oral cavity harbours a rich source of mesenchymal stem cells (MSCs), including those from various dental tissues, gingival/mucosal tissues, and alveolar bone (Huang et al., 2009; Morsczeck et al., 2013). Extracted teeth are consid­ ered biomedical waste and gingival/mucosal tissues are easily accessible and available. Oral MSCs are also relatively robust in respect to cell proliferation and population doubling (Huang et al., 2009); therefore, these cells may be one of the best sources for generating iPSCs. While many aspects of iPSCs require investigation concerning their clinical safety, utilising iPSCs for cell therapy is anticipated to take place in the future. Studies focusing on guiding iPSCs to differentiate into various cell types for regeneration purposes have been rigorously undertaken. This chapter will overview current progress in this area, particularly emphasising neurogenesis. Additionally, utilising iPSCs as a tool for studying genetics and disease mechanisms will also be reviewed.