Xenopus model

Mammalian (e.g., CHO, HEK-293, COS-7) rather than Xenopus oocytes should be selected for expressing hERG since a major limitation of Xenopus model is that test substances can accumulate in the oocyte yolk, resulting in significant variability and error in potency estimates. 

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

A main consideration in modeling the pyrethroid receptor on insect sodium channels in the open state was the determination that channel modification by cypermethrin and deltamethrin of cloned insect sodium channels expressed in Xenopus oocytes occurred only following repeated depolarizations [33, 55]. This use-dependency of some pyrethroids was the basis for the widely held opinion that these pyrethroids bind preferentially to open sodium channels and that state-dependent modification of sodium channels by pyrethroids was an important consideration for any receptor modeling. 

There have been several studies that underscore the importance of unbound concentration in cell-based studies of receptor function. In a model study of the effect of plasma protein binding on the renal transport of organic anions using the expression of various organic anion transporters (OATPs) in Xenopus oocytes, the transport of ochratoxin A, methotrexate, and estrone sulfate was found to be strongly inhibited by the addition of human serum albumin to the culture medium [16]. Similarly, the addition of oq-acid glycoprotein was found to reverse the blockade of sodium-ion current by cocaine in a preparation of cardiac myocytes [17]. 

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. 

With the widespread availability of cell culture facilities, the reduced costs of media and reagents and above all, the commercialisation of a variety of transfection and expression kits, mammalian cells have now become probably the standard for functional studies of transmembrane transporters. Unsurpassed predictivity of the mammalian models may outweigh the higher costs and the lengthiness of the process, compared with bacterial cultures or Xenopus oocytes. Nevertheless, structural studies may require larger amounts than those easily produced in mammalian cells and the appeal of insect cell cultures for... 

B. Novak and J. J. Tyson, Numerical analysis of a comprehensive model of M-phase control in Xenopus oocyte extracts and intact embryos. J. Cell Sci. 106, 1153-1168 (1993). 

Awayda et al. [278] suggested that CPZ-induced alteration of epithelial sodium channels (ENaC) could be a result of the influence exerted by the phe-niothiazine derivative on the membrane fluidity. To demonstrate it they used epithelial sodium channels expressed in Xenopus laevis oocytes as a model, and compared the effects of CPZ (9) and temperature on the ENaC activity. They have shown that a decrease of membrane order caused either by the increase of temperature or by incorporation of drug molecules into the membrane affected the conductance of epithelial sodium channels in a very... 

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). 

---