Heterologous expression systems Xenopus oocytes

Heterologous expression systems comprise prokaryotic organisms (e.g., E. coli) and eukaryotic cells (e.g., yeast, HEK293, Xenopus oocytes), which are used to functionally express foreign genes or cDNAs. 

Heterologous expression systems hERG current (whole cell patch clamp) Mammalian cell expression (CHO, HEK-293, mouse L-ceUs, COS-7) and Xenopus oocytes expression system Witchel et al. 77 Zou et al. 78 Cavero et al. 79 Martin et al. 80 McDonald et al.81... 

Theodoulou FL, Miller AJ. 1995. Xenopus oocytes as a heterologous expression system. Methods Mol Biol 49 317. 

The use of heterologous expression systems, such as the Xenopus oocyte (toad). 

When Urel is expressed in Xenopus oocytes, there is a large increase in urea uptake that is pH dependent. There is no increase at neutral pH but a large increase with acidification down to pH 4.0, the limit of survival of oocytes. The pH-transport curve can be overlaid on the pH-urease activity curve of intact bacteria. Half-maximal transport occurs at a medium pH of 5.9. The increased uptake is energy independent, temperature independent, and nonsaturable, properties indicating that Urel is a urea channel. This allows extremely fast transport of urea across the inner membrane of the bacteria. Complementation of Urel deletion mutants with wild-type Urel in H. pylori also restore urease activation at acidic pH, showing that Urel is a urea transporter in both the heterologous oocyte expression system and in the native organism. 

Oocytes from the South African clawed frog, Xenopus laevis, provide a reliable and powerful system for the transient heterologous expression of proteins. The use of this expression system has become very popular as oocytes have a high translational capacity and they are able to express multi-subunit proteins derived from exogenously introduced RNA or DNA. Furthermore, the expressed receptors frequently appear to be correctly assembled, post-translationally modified and oriented to the appropriate site. The relative scarcity of endogenous ion channels in the oocyte membrane makes it a versatile tool for the study of a range of heterologously expressed ion channel proteins. 

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). 

The inhibitory effect of cannabinoids on S-HTj-receptor-mediated currents has been confirmed through heterologous systems lacking eannabinoid receptors but expressing homomeric 5-HT3 receptors (Barann et al., 2002 Oz et al., 2002). When Xenopus oocytes were injected with neuroblastoma cell line mRNA, the expressing S-HTj receptors were capable to mediate a 5-HT-... 

Rdl orthologs have since been cloned from many other insect species and, when heterologously expressed in the Xenopus oocyte expression system or in cell lines, give membrane receptors that behave like insect GABA receptors. Rdl... 

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