br Lineage tracing tools for investigating stem cell

Lineage tracing tools for investigating stem cell biology The investigation of cell lineage involves labeling Wnt-C59 of interest, then tracing the destiny of their progeny. The reliance on the presence of cell markers alone to infer the origin or fate of a cell population is fallible, as marker expression may change as cellular context changes; for example, markers on BM cells can be down-regulated after their relocation to solid organs (Rota et al., 2007; Spees et al., 2008). Evolving techniques for lineage tracking have used “vital” dyes, stable isotopes, radioactive compounds, inter-species chimeras and lineage-specific molecular markers in normal or genetically modified organisms (Steinhauser et al., 2012; Stern and Fraser, 2001). Transgenic animals expressing indelible genetic lineage tracers have allowed the fate of specific cell populations to be followed over very long periods during homeostasis, aging and under various disease conditions such as myocardial infarction (MI). A useful type of surrogate tracing is achieved if a genetic tracer protein, such as GFP, is more stable than the endogenous protein it replaces, the latter often down-regulated during differentiation (Kikuchi et al., 2010; Lepilina et al., 2006).
Diverse origins for cardiac progenitors in development Understanding the progenitors that shape the heart in development (Moorman and Christoffels, 2003; Vincent and Buckingham, 2010) is necessary to appreciate the recent findings on the characteristics and origins of CPCs within the adult heart. A progenitor population termed the “first heart field” (FHF) contributes to the formation of a crescent-shaped epithelium of splanchnic mesoderm at the anterior–lateral boundaries of the early embryo (Fig. 2A). The crescent begins to express cardiac sarcomeric genes and undergoes morphogenesis to form the early tubular heart, which serves as a scaffold for continuing growth. Progenitor cells of the “second heart field” (SHF) are deployed at both the inflow and outflow portals of the primary heart tube and contribute almost exclusively to the right ventricle myocardium and endocardium, as well as outflow tract myocardium, endocardium and smooth muscle (Buckingham et al., 2005) (Fig. 2A). They also contribute extensively to the atria, inflow vessel and nodal elements. Lineage tracing suggests that FHF and SHF cells arise from a common progenitor but become geographically separated during gastrulation (Devine et al., submitted for publication; Lescroart et al., in press; Meilhac et al., 2004). Neural crest progenitor cells are known to contribute to the outflow tract, atrioventricular septum, cardiac valves and ganglia of the developing heart (Creazzo et al., 1998; Nakamura et al., 2006) (Fig. 2B). Additional progenitors are contributed from the proepicardium (PE; Fig. 2B), a convoluted epithelial organoid with a mesenchymal core that comes to be situated adjacent to the atrioventricular canal of the forming heart (Asli and Harvey, 2013). It has its origins in the early cardiac fields (Lescroart et al., in press; Zhou et al., 2008b). The PE seeds progenitors that spread over the myocardium to create the epicardium. A subset of these cells, termed epicardial-derived cells (EPDCs), enter the sub-epicardial matrix and heart interstitium and differentiate into the mural elements of the coronary vasculature, as well as cardiac fibroblasts, mesenchymal stem/progenitor cells and possibly myocardium (Asli et al., 2014; Carmona et al., 2010; Winter and Gittenberger-de Groot, 2007). Markers that distinguish cells within the cardiac progenitor fields have been extensively explored, although few if any definitively define these populations. For example, TFs Mesp1 and Mesp2 are expressed in the anterior primitive streak, then in an anterior embryonic territory that contains progenitors of the cardiac lineages, as well as other lineages (Bondue et al., 2008; S.S. Chan et al., 2013; Kitajima et al., 2000; Saga et al., 1999). Likewise, TFs associated with myocardial development, such as ISL1, NKX2-5, GATA4 and MEF2C, are expressed in the first and second heart progenitor fields as well as at different phases of epicardial, endocardial and/or neural crest development (Engleka et al., 2012; Mommersteeg et al., 2010; Nakano et al., 2013; Peng et al., 2013; Watt et al., 2004; Zhou et al., 2008a).