Abstract
Phenotypic drug discovery assesses the effect of small molecules on the phenotype of cells, tissues, or whole organisms without a priori knowledge of the target or pathway. Using vertebrate embryos instead of cell-based assays has the advantage that the screening of small molecules occurs in the context of the complex biology and physiology of the whole organism. Fish and amphibians are the only classes of vertebrates with free-living larvae amenable to high-throughput drug screening in multiwell dishes. For both animal classes, particularly zebrafish and Xenopus, husbandry requirements are straightforward, embryos can be obtained in large numbers, and they develop ex utero so their development can be monitored easily with a dissecting microscope. At 350 million years, the evolutionary distance between amphibians and humans is significantly shorter than that between fish and humans, which is estimated at 450 million years. This increases the likelihood that drugs discovered by screening in amphibian embryos will be active in humans. Here, we describe the basic protocol for the medium- to high-throughput screening of chemical libraries using embryos of the African clawed frog Xenopus laevis Bioactive compounds are identified by observing phenotypic changes in whole embryos and tadpoles. In addition to the discovery of compounds with novel bioactivities, the phenotypic screening protocol also allows for the identification of compounds with in vivo toxicity, eliminating early hits that are poor drug candidates. We also highlight important considerations for designing chemical screens, choosing chemical libraries, and performing secondary screens using whole mount in situ hybridization or immunostaining.