Screening of Chemical Libraries Using Xenopus Embryos and Tadpoles for Phenotypic Drug Discovery

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.

Label-free determination of hemodynamic parameters in the microcirculaton with third harmonic generation microscopy

Determination of blood flow velocity and related hemodynamic parameters is an important aspect of physiological studies which in many settings requires fluorescent labeling. Here we show that Third Harmonic Generation (THG) microscopy is a suitable tool for label-free intravital investigations of the microcirculation in widely-used physiological model systems. THG microscopy is a non-fluorescent multi-photon scanning technique combining the advantages of label-free imaging with restriction of signal generation to a focal spot. Blood flow was visualized and its velocity was measured in adult mouse cremaster muscle vessels, non-invasively in mouse ear vessels and in Xenopus tadpoles. In arterioles, THG line scanning allowed determination of the flow pulse velocity curve and hence the heart rate. By relocating the scan line we obtained velocity profiles through vessel diameters, allowing shear rate calculations. The cell free layer containing the glycocalyx was also visualized. Comparison of the current microscopic resolution with theoretical, diffraction limited resolution let us conclude that an about sixty-fold THG signal intensity increase may be possible with future improved optics, optimized for 1200-1300 nm excitation. THG microscopy is compatible with simultaneous two-photon excited fluorescence detection. It thus also provides the opportunity to determine important hemodynamic parameters in parallel to common fluorescent observations without additional label.

Engineering Xenopus embryos for phenotypic drug discovery screening

Many rare human inherited diseases remain untreatable despite the fact that the disease causing genes are known and adequate mouse disease models have been developed. In vivo phenotypic drug screening relies on isolating drug candidates by their ability to produce a desired therapeutic phenotype in whole organisms. Embryos of zebrafish and Xenopus frogs are abundant, small and free-living. They can be easily arrayed in multi-well dishes and treated with small organic molecules. With the development of novel genome modification tools, such a zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR/Cas, it is now possible to efficiently engineer non-mammalian models of inherited human diseases. Here, we will review the rapid progress made in adapting these novel genome editing tools to Xenopus. The advantages of Xenopus embryos as in vivo models to study human inherited diseases will be presented and their utility for drug discovery screening will be discussed. Being a tetrapod, Xenopus complements zebrafish as an indispensable non-mammalian animal model for the study of human disease pathologies and the discovery of novel therapeutics for inherited diseases.

Chemical genetics and drug discovery in Xenopus

Chemical genetics uses small molecules to modulate protein function and has the potential to perturb any biochemical event in a complex cellular context. The application of chemical genetics to dissect biological processes has become an attractive alternative to mutagenesis screens due to its technical simplicity, inexpensive reagents, and low-startup costs. Xenopus embryos are particularly amenable to whole organism chemical genetic screens. Here we describe the basic protocols we have developed to screen small compound libraries on Xenopus laevis embryos. We score embryos either by observing phenotypic changes in the whole tadpole or by changes in gene expression pattern using automated wholemount in situ hybridization.

An in vivo chemical library screen in Xenopus tadpoles reveals novel pathways involved in angiogenesis and lymphangiogenesis

Angiogenesis and lymphangiogenesis are essential for organogenesis but also play important roles in tissue regeneration, chronic inflammation, and tumor progression. Here we applied in vivo forward chemical genetics to identify novel compounds and biologic mechanisms involved in (lymph)angiogenesis in Xenopus tadpoles. A novel 2-step screening strategy involving a simple phenotypic read-out (edema formation or larval lethality) followed by semiautomated in situ hybridization was devised and used to screen an annotated chemical library of 1280 bioactive compounds. We identified 32 active compounds interfering with blood vascular and/or lymphatic development in Xenopus. Selected compounds were also tested for activities in a variety of endothelial in vitro assays. Finally, in a proof-of-principle study, the adenosine A1 receptor antagonist 7-chloro-4-hydroxy-2-phenyl-1,8-naphthyridine, an inhibitor of blood vascular and lymphatic development in Xenopus, was shown to act also as a potent antagonist of VEGFA-induced adult neovascularization in mice. Taken together, the present chemical library screening strategy in Xenopus tadpoles represents a rapid and highly efficient approach to identify novel pathways involved in (lymph)angiogenesis. In addition, the recovered compounds represent a rich resource for in-depth analysis, and their drug-like features will facilitate further evaluation in preclinical models of inflammation and cancer metastasis.

Paracrine and autocrine mechanisms of apelin signaling govern embryonic and tumor angiogenesis

Apelin and its G protein-coupled receptor APJ play important roles in blood pressure regulation, body fluid homeostasis, and possibly the modulation of immune responses. Here, we report that apelin-APJ signaling is essential for embryonic angiogenesis and upregulated during tumor angiogenesis. A detailed expression analysis demonstrates that both paracrine and autocrine mechanisms mark areas of embryonic and tumor angiogenesis. Knockdown studies in Xenopus reveal that apelin-APJ signaling is required for intersomitic vessel angiogenesis. Moreover, ectopic expression of apelin but not vascular endothelial growth factor A (VEGFA) is sufficient to trigger premature angiogenesis. In vitro, apelin is non-mitogenic for primary human endothelial cells but promotes chemotaxis. Epistasis studies in Xenopus embryos suggest that apelin-APJ signaling functions downstream of VEGFA. Finally, we show that apelin and APJ expression is highly upregulated in microvascular proliferations of brain tumors such as malignant gliomas. Thus, our results define apelin and APJ as genes of potential diagnostic value and promising targets for the development of a new generation of anti-tumor angiogenic drugs.