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Nascone-Yoder, Nanette, PhD
Assistant Professor
Chemical Genetics
The current paucity of information regarding the mechanisms of gut morphogenesis may be due to the fact that such late stage embryonic events are often confounded by pleiotropic effects, or masked by early lethality, in model organisms amenable to classic mutagenesis or gene knock-out experiments. As a means of overcoming these obstacles, we are developing a forward chemical genetic strategy to identify the signaling pathways involved in vertebrate gut morphogenesis, using the uniquely accessible, transparent embryos of the aquatic vertebrates, Xenopus laevis (the African claw-toed frog) and Danio rerio (the zebrafish). In this strategy, embryos are treated with individual chemicals from libraries of drug-like compounds in order to screen for those capable of inducing abnormal gut development; the cellular target protein whose activity is modulated by the chemical is then implicated in normal gut morphogenesis. Employing a chemical genetic approach has several advantages over traditional knockout or mutagenesis strategies, including control over precisely when a particular protein’s activity is modulated, the potential to wash away the chemical and restore normal function at a later time point in development, and the cost-effective convenience of screening for a phenotype in a single generation. The possibilities for semi-automated screening, the increasing availability of compound libraries of bioactive small molecules, and the use of GFP reporters to illuminate developing organs in transgenic animals, make this approach a potent alternative to standard mutagenesis.
Recently, we found that late-stage treatment of Xenopus embryos with specific chemical agents that modulate retinoic acid (RA) signaling elicits very specific types of gut looping anomalies and intestinal malrotations, without affecting other aspects of development. In addition, particular liver and pancreas deformities associated with intestinal malrotation in humans, including biliary atresia and annular pancreas, were concomitantly induced by these pharmacological agents in Xenopus (Lipscomb et al, 2005, in review).
In addition to chemicals that modulate RA signaling, pilot studies with chemical compounds that inhibit Rac, Rho and Cdc42 (Toxin A), Rho kinase (Y27632), and Myosin II (blebbistatin) in both Xenopus and zebrafish embryos indicate that Rho GTPase-mediated cell signaling also plays an important role in large-scale gut looping and elongation events.