FGF-signalling in the differentiation of mouse ES cells towards ...
FGF-signalling in the differentiation of mouse ES cells towards ...
FGF-signalling in the differentiation of mouse ES cells towards ...
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transplantation is <strong>the</strong> spread <strong>of</strong> animal diseases to humans. Some <strong>of</strong> <strong>the</strong> risk factors may beelim<strong>in</strong>ated by breed<strong>in</strong>g homozygous m<strong>in</strong>i-pigs <strong>in</strong> controlled environments.EpiSCs, h<strong>ES</strong>Cs and iPSCsIt has recently been shown that epiblast stem <strong>cells</strong> (epiSCs) derived from post-implantation<strong>mouse</strong> blastocysts show characteristics <strong>of</strong> h<strong>ES</strong> <strong>cells</strong> <strong>in</strong> <strong>the</strong>ir need for pluripotency-ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>gfactors activ<strong>in</strong> and <strong>FGF</strong>2 (Brons et al. 2007; Tesar et al. 2007; Vallier et al. 2009). Also, <strong>the</strong>irresponse to <strong>differentiation</strong>-<strong>in</strong>duc<strong>in</strong>g factors is more similar to h<strong>ES</strong> <strong>cells</strong> than what is seen form<strong>ES</strong> <strong>cells</strong> (Vallier et al. 2009). This close resemblance to h<strong>ES</strong> <strong>cells</strong> may make epiSCs a bettermodel for study<strong>in</strong>g <strong>differentiation</strong>, as extrapolation <strong>of</strong> knowledge to <strong>the</strong> h<strong>ES</strong> cell field mayprove easier and more valuable. One major advantage is that exist<strong>in</strong>g <strong>ES</strong> cell l<strong>in</strong>es can beconverted <strong>in</strong>to epiSCs without new derivation from <strong>mouse</strong> embryos (Guo et al. 2009), mak<strong>in</strong>galready established transgenic cell l<strong>in</strong>es readily transferable by a low work load. This may holdgreat potential for better <strong>in</strong>ter-species protocol transfer between epiSCs and h<strong>ES</strong> <strong>cells</strong>.In 2006, <strong>the</strong> Yamanaka-group showed that mature somatic <strong>cells</strong>, i.e. sk<strong>in</strong> fibroblasts, can be<strong>in</strong>duced to achieve an <strong>ES</strong> cell-like phenotype, i.e. become pluripotent and are reported tobehave <strong>in</strong> <strong>the</strong> same way as m<strong>ES</strong> or h<strong>ES</strong> <strong>cells</strong> upon <strong>differentiation</strong>. These <strong>in</strong>duced pluripotentstem (iPS) <strong>cells</strong> were generated by <strong>in</strong>troduction <strong>of</strong> four transcription factors Oct4, Sox2, Klf4,and C-myc (Takahashi and Yamanaka 2006). This was done <strong>in</strong> mice, and <strong>the</strong> protocol has s<strong>in</strong>cebeen modified <strong>in</strong> several ways and has been transferred to human <strong>cells</strong> (Takahashi et al. 2007;Yamanaka 2009). iPS <strong>cells</strong> hold <strong>the</strong> potential for development <strong>of</strong> patient-specific pluripotentstem cell l<strong>in</strong>es, which can be differentiated <strong>in</strong>to any cell type <strong>of</strong> choice. They <strong>the</strong>reforerepresent a source <strong>of</strong> transplantable <strong>cells</strong>, which elim<strong>in</strong>ates <strong>the</strong> need for immune-suppress<strong>in</strong>gagents to a large degree. Although this is a very positive future application, <strong>in</strong> reality it mayprove much too expensive for actual treatment. iPS <strong>cells</strong> will likely be important tools formodell<strong>in</strong>g <strong>of</strong> and <strong>in</strong>vestigat<strong>in</strong>g <strong>the</strong> aetiology <strong>of</strong> (<strong>in</strong>herited) human diseases, which are notdiscovered until <strong>the</strong> disease state is complete. For <strong>in</strong>stance, type I diabetes is normally notdiscovered until patients suffer from high blood glucose levels at which time po<strong>in</strong>t <strong>the</strong>ir β cellmass is practically obsolete (Maehr et al. 2009). Whe<strong>the</strong>r iPS <strong>cells</strong> will serve as material for cellreplacement-<strong>the</strong>rapies is still to be seen. Of major concern is to ensure that <strong>the</strong> genomicreprogramm<strong>in</strong>g <strong>of</strong> <strong>the</strong> <strong>cells</strong> is complete, a trait believed necessary for <strong>the</strong> <strong>cells</strong> to adopt <strong>the</strong>correct fate upon exposure to <strong>differentiation</strong>-<strong>in</strong>duc<strong>in</strong>g conditions (Yamanaka 2009). Also,teratoma formation from fully reprogrammed iPS <strong>cells</strong> cannot be avoided so far, mak<strong>in</strong>g <strong>the</strong>munsuited for treatment <strong>in</strong> humans at this po<strong>in</strong>t. In general, avoid<strong>in</strong>g teratoma-formation fromdifferentiated cell populations is a major concern <strong>in</strong> transplantation. It is not acceptable to curefor <strong>in</strong>stance diabetes but at <strong>the</strong> same time <strong>in</strong>duce a cancerous condition, and as long as this riskexists with cell <strong>the</strong>rapy-protocols, <strong>the</strong>y will not be approved for treatment.Generation <strong>of</strong> β <strong>cells</strong> from exist<strong>in</strong>g cell sources <strong>in</strong> <strong>the</strong> pancreasThe presence <strong>of</strong> a pancreatic stem cell, which has clonogenic potential, is multipotent and canbe <strong>in</strong>duced to generate <strong>in</strong>sul<strong>in</strong>-produc<strong>in</strong>g <strong>cells</strong> <strong>in</strong> vitro has been suggested <strong>in</strong> both mice andhumans (Ramiya et al. 2000; Seaberg et al. 2004; Zhao et al. 2007). However, it is speculatedthat <strong>the</strong>se <strong>cells</strong> only show such stem cell-like properties due to <strong>the</strong> <strong>in</strong> vitro culture conditions(Baeyens and Bouwens 2008). A more conv<strong>in</strong>c<strong>in</strong>g <strong>in</strong> vivo study showed <strong>the</strong> presence <strong>of</strong> isletprecursors that could be activated upon serious tissue <strong>in</strong>jury by <strong>the</strong> so-called partial ductligation,<strong>in</strong> which facultative multipotent progenitor <strong>cells</strong> <strong>in</strong> <strong>the</strong> ductal l<strong>in</strong><strong>in</strong>g differentiate andproliferate <strong>in</strong>to functional β <strong>cells</strong> (Xu et al. 2008).An alternative approach is to generate β <strong>cells</strong> by reprogramm<strong>in</strong>g <strong>of</strong> exist<strong>in</strong>g endocr<strong>in</strong>e orexocr<strong>in</strong>e <strong>cells</strong> <strong>in</strong> <strong>the</strong> pancreas. Follow<strong>in</strong>g pancreatectomy to a mild or severe degree (70% and95% respectively), regeneration <strong>of</strong> β cell mass is achieved through ei<strong>the</strong>r replication <strong>of</strong> exist<strong>in</strong>gβ <strong>cells</strong> or through neogenesis <strong>of</strong> precursor <strong>cells</strong> <strong>in</strong> addition to replication (Dor et al. 2004;Bouwens and Rooman 2005). Exocr<strong>in</strong>e ductal <strong>cells</strong> show conv<strong>in</strong>c<strong>in</strong>g potential as <strong>the</strong>y seemable to contribute to glucose-responsive β <strong>cells</strong> through reprogramm<strong>in</strong>g (Baeyens and Bouwens87