Xenopus system model

Thcfrojj embryo teratogenesis assay (FETAX) is a similar ex vivo test based on a Xenopus laevis model. Fertilized eggs in the mid- to late-blastula stage are exposed to test compounds and scored for lethality, growth retardation, and malformations. An interlaboratory validation study with 12 compounds [13], including a metabolic activation system, provided repeatable data, and a recent publication from a pharma industry reported very positive results testing over 400 chemicals with 81 % predictivity and a minimal proportion of false-positive compounds [14]. 

Xenopus oocytes and embryos present a model system for understanding the interface between cell cycle controls and developmental decisions. Maturation of... 

Today, there are a wide variety of laboratory protein expression systems available, ranging from cell-free systems over bacterial and yeast cultures to eukaryotic models including the Xenopus oocytes or insect and mammalian cell cultures, some of which even form polarised epithelial-like cells layers. In Table 24.1, an overview of the most important systems, as well as their particular strength and weaknesses in the expression of transmembrane transport proteins is provided. 

Recently, a putative olfactory receptor from Drosophila, Or43a (Clyne et al., 1999 Vosshall et al., 1999), has been expressed in Xenopus laevis oocytes (Wetzel et al., 2001). The receptor expressed in a heterologous cell system was activated by four odorants, i.e. cyclohexanone, cyclohexanol, benzaldehyde, and benzyl alcohol (Wetzel et al., 2001). These experiments not only provided direct evidence for the function of the Or gene, but also demonstrated that the olfactory receptor can be stimulated without an odorant-binding protein. It was demonstrated earlier that PBP was not necessary to obtain pheromone-dependent responses in cultured olfactory receptor neurons of Manduca sexta (Stengl et al., 1992). The possibility that OBPs have been produced in vitro and were present in cultured ORNs could not be excluded. The same argument can not be raised for the heterologous expression of the Drosophila olfactory receptor. While the evidence that Xenopus oocytes responded to odorants in the absence of OBPs does not support the OBP-odorant complex model, it also demonstrated that OBPs are essential for the kinetics of the olfactory system (see below). 

Currently, it is standard procedure to develop ion channel-specific antibodies for immunocytochemistry, to perform Western and Northern blot analyses, ion channel in situ hybridization, or reverse transcription polymerase chain reaction (RT-PCR). The introduction of the single-cell RT-PCR in combination with the patch-clamp method in the 1990s made it possible to identify gene transcripts and to correlate them with functional data for the same individual cell. Finally, one of the most powerful cell biological techniques in the study of ion channels is based on artificial expression systems such as microinjection of mRNA encoding channel subunits into Xenopus oocytes and selective expression of native ion channels or with different subunit composition (e.g., Ky channel subunits). Because the Xenopus oocytes are large, they are a perfect model to study artificially expressed channels. Another good model for artificial ion channel expression is the Chinese hamster ovary (CHO) cell line. 

Creech Kraft, J. Juchau, M.R. Xenopus laevis A model system for the study of embryonic retinoid metabolism. III. Isomerization and metabolism of all-trares-retinoic acid and 9-cis-retinoic acid and their dysmorphogenic effects in embryos during neurulation. Drug Metab.Dispos., 1995, 23, 1058-1071... 

Sato K, Fukami Y, Stith BJ. 2006a. Signal transduction pathways leading to Ca2+ release in a vertebrate model system lessons from Xenopus eggs. Semin CeU Dev Biol 17(2) 285-292. 

Creech Kraft J, Kimelman D, Juchau MR (1995) Xenopus laevis A Model System for the Study of Retinoid Metabolism 1. Embryonic metabolism of 9-cis- and all-rran5-retinals and retinols to their corresponding acid forms Drug Metab Dispos 23 72-82... 

Screens for small-molecule modulators of biological pathways typically utilize cultured cell lines, purified proteins, or, recently, model organisms (e.g., zebrafish. Drosophila C. elegans). Herein, we describe a method for using Xenopus laevis egg extract, a biologically active and highly tractable cell-free system that recapitulates a legion of complex chemical reactions foimd in intact cells. Specifically, we focus on the use of a luciferase-based fusion system to identify small-molecule modulators that affect protein turnover. 

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