CONSERVATION OF PDX-1 STRUCTURE, FUNCTION, AND EXPRESSION IN ZEBRAFISH

Development of the mammalian pancreas has been studied extensively in mice. The stages from budding of the pancreatic anlaga through endocri ne and exocrine cell differentiation and islet formation have been des cribed in detail. Recently, the homeodomain transcription factor PDX-1 has been identified as an important factor in the proliferation and d ifferentiation of the pancreatic buds to form a mature pancreas. To ev aluate the possibility of using zebrafish as a model for the genetic a nalysis of pancreas development, we have cloned and characterized PDX- 1 from this organism. The deduced sequence of zebrafish PDX-1 contains 246 amino acids and is 95% identical to mammalian PDX-1 in the homeod omain. We also cloned zebrafish preproinsulin complementary DNA as a m arker for islet tissue. By in situ hybridization we demonstrate that P DX-1 and insulin are coexpressed during embryonic development and in a dults, although PDX-1 expression appears to be biphasic. Insulin expre ssion apparently begins before 44 hpf, the earliest stage examined in this study. Additionally, very high levels of PDX-1 expression were ob served in the pyloric caeca, the accessory digestive organs that also are derived from the proximal region of the intestine in teleosts, Fin ally, our data show that the evolutionary conservation of zebrafish PD X-1 extends to its DNA binding properties. Zebrafish PDX-1 was equally as effective as mouse PDX-1 in stimulating insulin gene transcription , and maximum promoter activation was dependent on the presence of fou r intact A elements. The demonstration of this capability suggests tha t transcriptional regulatory mechanisms that control pancreatic develo pment and insulin gene expression have been conserved among vertebrate s.