Nucleus Xenopus oocyte

Different classes of RNAs, including mRNAs, tRNAs, and U snRNAs, are transcribed and processed within the nucleus and then exported to the cytoplasm by multiple pathways. Competition experiments with microinjected Xenopus oocytes have revealed that tRNA, mRNA, U snRNA and ribosomal subunits all use saturable pathways, but none of them saturated the export of others (Jarmolowski et al, 1994). For instance,... 

Starting in 1993, reports have appeared that in rat striatum and nucleus accum-bens D2-like autoreceptors, when activated, enhanced removal of dopamine from the extracellular space by means of the dopamine transporter (DAT) as studied by voltammetry (Table2). D2 receptor activation also increased [3H]-dopamine uptake in Xenopus oocytes transfected with both the human D2 receptor and the human DAT (Mayfield and Zahniser 2001), as well as the uptake of a fluorescent DAT substrate in human embryonic kidney cells equally transfected with both the human D2 receptor and the human DAT (Bolan et al. 2007). In accord with this autoreceptor-transporter connection, clearence of dopamine from the striatal extracellular space in vivo was reduced in D2 receptor knockout mice (Dickinson et al. 1999). Unfortunately, however, the data in the literature are by no means consistent. No effect of D2 receptor activation on the DAT was observed in another rat striatum study (Prasad and Amara 2001), in synaptosomes from rat and human neocortex (Feuer-stein, unpublished observations), and in PC 12 cells that possess D2 receptors and the DAT (Pothos et al. 1998). In a second study with D2 receptor knockout mice, uptake of dopamine in the striatum was increased rather than decreased (Schmitz et al. 2002). The reason for these discrepancies is not clear. It has been suggested that the clearance measurements by means of voltammetry were not appropriate for assessment of the function of the DAT (Prasad and Amara 2001). 

Transfection efficiency is dependent on mitotic activity, as cells prevented from going into mitosis after transfection express transgenes much less efficiently than proliferating cells. In search for an explanation, the transport of plasmids across the nuclear membranes has been studied. Plasmids injected into the cytoplasm of quiescent human fibroblasts are not expressed, in contrast to plasmid injected into the nucleus. This has been found to be true for the cationic lipid-based systems as plasmid injected into the cytoplasm of Xenopus oocytes is not expressed, unlike that injected into the nucleus, it must be concluded that the plasmid must dissociate from the cationic lipids before entering into the nucleus. 

Finally, profilins have been reported as residents of the nucleus. Immunofluorescence revealed nuclear profilin I in fibroblasts and epithehal cells (Mayboroda et al. 1997 Skare et al. 2003) and in bovine oocyte germinal vesicles and early embryos (Rawe et al. 2006). Nuclear profilin has also been described in several higher plants (see Hussey et al. 2006 for original references). Additionally, the existence of nuclear profilin-actin complexes can be deduced from studies on HeLa cells and Xenopus oocytes (Striven et al. 2003). In a recent study with GFP-profilin transfected cells and polyclonal anti-serum, profilin II has been shown to accumulate in the nucleus of hippocampal neurons. This concentration was de-... 

Fig. 2 Electron micrograph of a typical transcription unit on a lampbrush chromosome loop from a stage VI Xenopus oocyte nucleus. These arc seen less frequently than endogenous ribosomal genes and injected plasmid genes. It is not possible to identify these genes beyond the fact that they are Pol II genes, but t hey lend to be very long and to have a reasonably high density of nascent transcripts. Scale bar, 1 pm.

The partial agonistic activity was determined via a functional electrophysi-ological assay in Xenopus oocytes at 10 pM of the test compound relative to 10 pM of nicotine, or upon co-administration of both. These competition experiments demonstrated that varenicline possesses only 68% of the efficacy of nicotine, while the efficacy of nicotine itself is reduced to 34 %. Extensive in vivo studies showed that oral administration of varenicline raises the release of dopamine in the Nucleus accumbens, but on the other hand, it attenuates the effect of nicotine. 

Bataille, N., Helser, T and Fried, H. M. (1990). Cytoplasmic transport of ribosomal subunits microin-jected into the Xenopus laevis oocyte nucleus A generalized, facilitated process. J. Cell Biol. Ill, 1571-1582. 

In-cell EPR measurement. An exciting recent development is the demonstration of SDSL measurements in intracellular environment or cell extract.In nucleic acid studies, spin-labeled DNAs or RNAs were injected into live Xenopus laevis oocytes, which provide a nucleus-like environment. After incubation, the samples were flash-frozen, and DEER measurements were carried out to measure... 

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