Genetic inducible fate mapping in larval zebrafish reveals origins of adult insulin-producing-cells

INTRODUCTION Embryogenesis leads to the development of the complete body plan, but not necessarily all adult cell types. Instead, pools of stem cells and progenitors are maintained to differentiate later in development or even in the adult in response to injury or other dramatic events. To identify progenitor populations and ascertain their lineage potential, long-range fate mapping is an essential tool. Fate mapping using Cre-based technologies has proved to be a powerful technique with which to indelibly label cells and their progeny (Joyner and Zervas, 2006). This method uses two transgenic elements: (1) a Cre-driver transgene that expresses Cre in cells of interest; and (2) a Cre-responder that contains sites of recombination for Cre. Following Cre-dependent recombination, the responder will continually express a marker protein, thereby labeling the cell and subsequent descendents. Temporal control of Cre is obtained by expressing a fusion of Cre to a mutant estrogen receptor with a high affinity for 4-hydroxytamoxifen (4OHT) (Feil et al., 1997). Expression of this 4OHT-dependent Cre recombinase (CreERT2) has been extensively used to label embryonic pancreatic cells in the mouse and identify their ultimate fate in adult pancreas (Gu et al., 2002; Kawaguchi et al., 2002) as well as their contribution to regenerative tissues following injury (Inada et al., 2008; Solar et al., 2009). As a model organism the zebrafish is unique in its capacity for high-throughput chemical (Zon and Peterson, 2010) and genetic screening (Driever et al., 1996). Another asset of zebrafish biology is their phenomenal capability to regenerate tissue following injury (Jopling et al., 2010; Kikuchi et al., 2010). For these reasons, the zebrafish is increasingly being used as a system to study pancreas organogenesis (Kinkel and Prince, 2009; Tiso et al., 2009) and regeneration (Curado et al., 2007; Moss et al., 2009; Pisharath et al., 2007). In common with mammals, the zebrafish pancreas is a glandular organ consisting of two main compartments: (1) the endocrine pancreas containing hormone producing cells arranged into islets; and (2) the exocrine pancreas containing acinar and ductal cells. Acinar cells are arranged into spherical structures called acini and secrete digestive enzymes (zymogens) from their apical surface. Ductal cells form a conduit to the intestine allowing digestive zymogens to gain access to the gut lumen. Analogous to mammalian pancreas formation, the zebrafish endocrine cells differentiate in two distinct phases. In the embryonic zebrafish the first endocrine cells form in a dorsal region of the gut endoderm. These cells coalesce during the first day of development to form what is called the principal islet (Biemar et al., 2001). These early endocrine cells have limited proliferation and ultimately contribute very little to the adult endocrine mass (Hesselson et al., 2009). The ventral bud of the pancreas can first be detected as ventral group of gut endoderm cells expressing the exocrine marker ptf1a (Lin et al., 2004; Zecchin et al., 2004). These cells migrate in a dorsal and posterior direction to envelop the principal islet (see overview in Fig. 7). In the heart and soul (has) mutation, particular morphological movements are blocked, resulting in a phenotype that includes the failure of the ventral bud to fuse with the principal islet; hence, ventral tissue differentiates in isolation. In has mutant fish, endocrine cells differentiate in the isolated ventral pancreas (Field et al., 2003). This result implies the existence of endocrine progenitors within the ventral bud. Differentiation of these endocrine cells is equivalent to the secondary transition in Development 138, 609-617 (2011) doi:10.1242/dev.059097 © 2011. Published by The Company of Biologists Ltd