C. Schmidt, C. Klein, and M. Hollmann (2009).
Xenopus laevis oocytes endogenously express all subunits of the ionotropic glutamate receptor family.
Journal of Molecular Biology 390(2): 182-195.
doi: 10.1016/j.jmb.2009.05.008
Xenopus laevis oocytes are commonly used as a heterologous expression system for the electrophysiological characterization of ionotropic glutamate receptors (iGluRs). Recently, however, several glutamate receptor subunits of the N-methyl-D-aspartate receptor subfamily have been found to be expressed endogenously in Xenopus oocytes, thus limiting the use of this expression system for such receptors. We therefore screened oocytes for the Xenopus homologs of all iGluR subunits known to be expressed in mammals to investigate which additional subunits may be present endogenously in oocytes and what, if any, influence such proteins might have on the functional analysis of heterologously expressed receptors. We found Xenopus homologs of every mammalian iGluR subunit to be expressed at the mRNA level. We then cloned, from oocytes, full-length copies of the four Xenopus α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunits XenGluR1 through XenGluR4 and, additionally, XenGluR6 as a representative subunit for the kainate receptor subfamily and electrophysiologically characterized them. Upon analysis, we found only minor functional differences between homologous subunits from Xenopus and rat. Next, we investigated whether endogenous iGluR subunits can be detected electrophysiologically in oocytes. We found no indication for any functional glutamate receptors in native oocytes; however, after heterologous expression of the auxiliary subunit stargazin, endogenous α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors were detected. These data demonstrate that Xenopus oocytes express glutamate receptor subunits endogenously, albeit at very low levels. Such endogenous receptor proteins can, under certain circumstances, become electrophysiologically detectable and then might influence electrophysiological recordings performed on recombinant receptors in oocytes.