In MRJ/MpJ mice, there is a genetic mutation of exonuclease 1 (Exol), in which the exon 9 is sometimes deleted. In the present study, to check the gen-eration of the spliced exons, exon 8-intron 8-exon 9 (pCX/Ex/EIE/B and pCX/ Ex/EIE/M) plasmids were temporally transfected in vitro into BALB 3T3 cells, and RT-PCR using appropriate primer pair was carried out 1 day after transfection. In these constructions, pCX/Ex/EIE/B was derived from genomic sequence of C57BL/6 mice, and pCX/Ex/EIE/M was from MRL/MpJ. A spliced band was detected in pCX/Ex/EIE/B , but was present little or very weakly in pCX/Ex/EIE/M . Next, the same spliced band was demonstrated in pCX/Ex/EIE/M(T) plasmid, in which the branchpoint sequence (BPS) of pCX/Ex/EIE/M including the exon 9 was changed into that of pCX/Ex/EIE/B. The splicing did not occur in the del1/B mutant, in which 1960 nucleotides of the intron 8 were deleted, whereas it was detected in the del2/B plasmid deleted 1036 nucleotides in its middle region. These results suggest that the nucleotide T to A mutation of the BPS in the intron 8 is at least a sufficient for generation of splice variants (tr-l and tr-2 Exol).

Effects of oxygen (O2) tension in the gas atmosphere during in vitro maturation (IVM), in vitro fertilization (IVF) and in vitro culture (IVC) on the efficiency of in vitro production of mouse embryos were examined. Mouse oocytes recovered from large antral follicles were subjected to IVM in Waymouth medium for 15, 16 and 17 hr under 5 or 20% O2 and then subjected to IVF and IVC under 5 or 20% O2 tension. Lowering the O2 tension in the gas atmosphere for IVM from 20 to 5% improved the cleavage rate after IVF when the oocytes were subjected to IVM for 15 hr; however, no improvement in the cleavage rate was observed when the culture period for IVM was extended to 16 and 17 hr. Lowering the O2 tension to 5% for IVM and IVC improved the development of the cleaved oocytes to the blastocyst stage, regardless of the culture period for IVM. However, the O2 tension for IVF had no remarkable effect on the subsequent embryonic development. These results demonstrate that 5% O2 is superior to 20% O2 for IVM and IVC, and suggest that 20% O2 for IVM may delay oocyte maturation and/or the acquisition of fertilizability and impair the developmental competence of oocytes.

Distribution of ASCT1, a neutral amino acid transporter, in non-neuronal peripheral tissues of adult and developing mice was examined by immunohis- tochemistry and immunoelectron microscopy. Immunoreactivity for ASCT1 in the digestive system was localized in basal cells of stratified squamous epithe-lia from oral parietes to nonglandular region of the stomach, chief cells of the glandular stomach, acinar cells of the salivary gland and exocrine pancreas, and Paneths cells of the small intestine, in all of which the basolateral mem-brane was selectively immuno-labeled. In the liver of adult mice, ASCT1 immunoreactivity was detected on the plasma membrane of hepatocytes sur-rounding central veins, and a temporal expansion of immunoreactive hepato-cytes was observed in the embryonic and CCI sub(4)-treated adult livers. ASCT1 was also localized on the plasma membranes of proximal uriniferous tubule epithelial cells in the kidney of adult mice, and those of supporting cells in the medulla of adrenal gland. These results suggest that ASCT1 is expressed in various non-neuronal peripheral tissues in mice, and it contributes to the amino acid transport throughout non-neuronal tissues.