The development of a multi-organ structure requires the temporally and spatially coordinated regulation of gene expression. Neighboring groups of cells, which initially share a common gene expression programme, will adopt different fates by expressing different sets of genes. This is realised by actively regulated initiation and termination of transcription, which in turn depends on the presence of specific activating and repressing transcription factors, and importantly on their ability to access regulatory gene elements. It has been shown that chromatin modifications, such as methylation or acetylation are central to regulating gene expression (Jenuwein and Allis, 2001; Kouzarides, 2007
). DNA methylation represents a stable and heritable mechanism for epigenetic silencing of transcription (Goll and Bestor, 2005
). In contrast, histone acetylation mediated by Histone acetyltransferases prevents chromatin condensation, thus allowing transcriptional activation. Conversely, removal of acetyl groups leads to chromatin compaction resulting in transcriptional repression. De-acetylation is mediated by Histone deacetylases, which are grouped into four classes based on their homology to yeast: Hdac1, 2, 3 and 8 (class I), Hdac4, 5, 6, 7, 9 and 10 (class II), Sir2-like Hdac (class III) and Hdac 11. The latter shares similarity with class I and II enzymes (de Ruijter et al., 2003
Although the importance of Hdacs in gene regulation is well established, and their specific roles in differentiation of embryonic stem cells, as well as hepatic and pancreatic cancer are emerging (Glozak and Seto, 2007
), their specific roles in embryonic development are still poorly understood. For instance, in mice, depletion of Hdac1 in the entire embryo leads to widespread proliferation defects during gastrulation and early lethality that are at least partly due to up-regulation of the cell-cycle inhibitor p21 (Lagger et al., 2002
). In contrast, because of maternal contribution zebrafish hdac1
mutant embryos pass through gastrulation exhibiting mild patterning defects in a subset of tissues, but without severe early morphological defects (Nambiar and Henion, 2004; Nambiar et al., 2007
). Thus, Hdac1 dependent processes occurring at later stages of embryonic development can be examined, such as neurogenesis, eye or fin development (Cunliffe, 2004; Stadler et al., 2005; Yamaguchi et al., 2005
). Hence, zebrafish is a highly suitable model for elucidating the role(s) of Hdac1 in endodermal organogenesis.
The endodermal organ system consists of the digestive tract and its accessory organs — liver, pancreas and lungs or the inner lining of the swim bladder, in mammals and zebrafish, respectively. The organs arise in close temporal and spatial proximity from the foregut endoderm (Grapin-Botton, 2005
). The foregut and the organs derived from it express different combinations of transcription factors, such as members of the Gata, FoxA and Hnf families, which play different roles in organ specification and differentiation (Duncan, 2000; Kaestner, 2005; Zaret, 2002
). In zebrafish, the endocrine pancreas is the first to develop from the dorsal side, by aggregation of the endocrine islet (Argenton et al., 1999; Biemar et al., 2001
). This is closely followed by specification of the liver on the ventral side, anterior to the endocrine islet (Field et al., 2003b
). Hepatoblasts, the liver precursor cells, express the transcription factors Hhex and Prox1 (Ober et al., 2003; Wallace and Pack, 2003
) and differentiate into mature hepatocytes and biliary cells. Liver specification requires the interaction between the foregut endoderm and the neighboring lateral plate mesoderm (Grapin-Botton, 2005; Zaret, 2002
). The LPM releases hepatoblast-inducing factors that include Fgf, Bmp and Wnt family of signalling molecules (Grapin-Botton, 2005; Ober et al., 2006
). Next, a second, exocrine pancreatic primordium arises from the ventral foregut endoderm close to the forming hepatic bud. The exocrine and endocrine primordium fuse and ultimately become connected by a common extrahepatopancreatic duct (Field et al., 2003a; Wallace and Pack, 2003; Yee et al., 2001
). A number of transcription factors have been implicated in specific endocrine or exocrine development, such as NeuroD1 and Neurogenin3, and Hes1 and Ptf1a, respectively (Cano et al., 2007
). Similar to the interactions required during hepatic development, the mesoderm adjacent to the presumptive pancreatic tissues releases signals such Retinoic acid (RA) and members of the Fgf and Bmp families of secreted molecules (Cano et al., 2007; Grapin-Botton, 2005
) that regulate pancreatic organogenesis. In pancreatic and hepatic development, the respective inductive signalling cascades regulate transcription of genes specific for the induction and differentiation of each organ. Investigating the roles of factors controlling the accessibility of regulatory elements mediating this transcription, will further our understanding of how organ-specific gene expression programmes are realised.
Here, we describe the mutant line s436, a novel allele of hdac1 in zebrafish, which despite its broad expression displays distinct defects in endodermal organogenesis. In hdac1 mutants hepatic and exocrine pancreatic specification and differentiation are severely affected. This is accompanied by defects in extrahepatopancreatic duct formation and an expansion of foregut tissue. Moreover in hdac1 mutants, we observe ectopic endocrine islet formation. Our genetic studies reveal that Hdac1 is required for the establishment of hepatic and exocrine pancreatic cell fates within the foregut, which occurs at the expense of the tissue forming the alimentary canal, suggesting a model in which an epigenetic enzyme mediates a fate switch at the organ level.
Taken together, we present very different yet crucial roles for the chromatin modification factor hdac1 in hepatic, pancreatic and foregut organogenesis in the zebrafish embryo.