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G Valdimarsson

Publications and source records attributed to G Valdimarsson.

10 recordsLinked to original sources
PubMed1996

Structure and expression of the messenger RNA encoding the murine multidrug resistance protein, an ATP-binding cassette transporter.

In vitro, overexpression of the human multidrug-resistance protein (MRP) causes a form of multidrug resistance similar to that conferred by P-glycoprotein, although the two proteins are only very distantly related. Studies with MRP-enriched membrane vesicles have demonstrated that the protein can bind and transport cysteinyl leukotrienes, as well as some other glutathione conjugates, with high affinity. In contrast, there is no direct evidence of the ability of MRP to bind or transport unmodified forms of the drugs to which it confers resistance. To facilitate studies of the physiological function(s) of MRP and its ability to cause multidrug resistance in vivo, we cloned and characterized the mRNA specifying its murine homolog. The murine MRP mRNA encodes a protein of 1528 amino acids that is 88% identical to human MRP. Although detectable by Northern blotting at variable levels in a wide range of tissues, in situ hybridization experiments revealed that MRP mRNA expression in some tissues is cell-type specific. High levels of the mRNA were detected in epithelia lining bronchi and bronchioles, as well as stage-specific expression in the seminiferous epithelium of the testes. Comparison of the predicted hydropathy profiles of human and murine MRP suggests a highly conserved membrane topology, the most distinctive feature of which is an extremely hydrophobic NH2-terminal region containing five or six potential transmembrane sequences. This structural feature is shared with the sulfonylurea receptor and the yeast cadmium factor 1 but is not present in members of the superfamily, such as the cystic fibrosis transmembrane conductance regulator and P-glycoproteins. Finally, we used overlapping cDNAs to construct an episomally replicating murine MRP expression vector that was stably transfected into HeLa cells. MRP-Transfected cell populations expressed markedly elevated levels of a 180-190-kDa protein that cross-reacted with a polyclonal antiserum raised against a peptide that is completely conserved in murine and human MRPs. The MRP transfectants also displayed increased resistance to vincristine (5-6-fold) and doxorubicin (< 2-fold).

ATP-Binding Cassette Transporters↗
PubMed1995

Temporal control of gap junction assembly in preimplantation mouse embryos.

The de novo assembly of gap junctions during compaction in the 8-cell stage of mouse development is a temporally regulated event. We have performed experiments designed to explore the relationship between this event and DNA replication in the second, third, and fourth cell cycles after fertilization. Inhibition of DNA synthesis by continuous treatment with the DNA synthesis inhibitor, aphidicolin, during the third and fourth cell cycles had no effect on the establishment of gap junctional coupling during compaction. However, a delay of 10 hours in DNA synthesis during the second cell cycle caused by a transient aphidicolin treatment resulted in the failure of gap junctional coupling at the time of compaction. Thus the timing of establishment of gap junctional coupling, like the timing of compaction itself, is linked to DNA replication in the 2-cell stage. Immunofluorescence analysis showed that the failure of gap junctional coupling after aphidicolin treatment in the 2-cell stage is correlated with the failure of nascent connexin43 to be inserted into plasma membranes. We propose that the developmental 'clock' that controls gap junction assembly is set in motion by events surrounding the second cycle of DNA replication, and that this 'clock' ultimately controls the post-translational processing of connexin43.

Animals↗
PubMed1994

Overexpression of multidrug resistance-associated protein (MRP) increases resistance to natural product drugs.

Amplification of the gene encoding multidrug resistance-associated protein (MRP) and overexpression of its cognate mRNA have been detected in multidrug-resistant cell lines derived from several different tumor types. To establish whether or not the increase in MRP is responsible for drug resistance in these cell lines, we have transfected HeLa cells with MRP expression vectors. The transfectants display an increase in resistance to doxorubicin that is proportional to the levels of a M(r) 190,000, integral membrane protein recognized by anti-MRP antibodies. The transfectants are also resistant to vincristine and VP-16 but not to cisplatin. The results demonstrate that MRP overexpression confers a multidrug resistance phenotype similar to that formerly associated exclusively with elevated levels of P-glycoprotein.

Carcinoma, Small Cell↗
PubMed1993

Coexpression of gap junction proteins in the cumulus-oocyte complex.

The connexins constitute a family of proteins that make up the intercellular membrane channels of gap junctions. We had previously reported the presence of two members of this protein family, connexins 32 and 43, in mouse one-cell zygotes (Barron et al., Dev Genet 10:318-323, 1989; Valdimarsson et al., Mol Reprod Dev 30:18-26, 1991), implying that both must be present in the mature oocyte and could be involved in mediating the intercellular coupling that occurs between the oocyte and cumulus granulosa during oogenesis. In the present report we provide evidence for this, based on an analysis of the cumulus-oocyte complex (COC) using reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry with a confocal microscope. Transcripts of both connexin32 (Cx32) and connexin43 (Cx43) were detected by RT-PCR in both components of the COC. Cx32 mRNA in the oocyte declined precipitously following human chorionic gonadotropin (hCG) stimulation of pregnant mare serum gonadotropin (PMSG)-primed ovaries, whereas there was no obvious change in Cx43 mRNA. Peptide-specific antibodies against both connexins provided diffuse cytoplasmic staining of oocytes as well as some punctate staining near the oocyte surface, which could not be unequivocally resolved as cumulus-oocyte gap junctions. However, the two antibodies did provide clear evidence of Cx32 and Cx43 in gap junction-like structures between cumulus cells. We could find no evidence of the incorporation of the oocyte's store of Cx32 into gap junctions during postfertilization development.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Connexin trafficking and the control of gap junction assembly in mouse preimplantation embryos.

Gap junction assembly in the preimplantation mouse embryo is a temporally regulated event, beginning a few hours after the third cleavage during the morphogenetic event known as compaction. Recently, we demonstrated that both mRNA and protein corresponding to connexin43, a gap junction protein, accumulate through preimplantation development beginning at least as early as the 4-cell stage. Using an antibody raised against a synthetic C-terminal peptide of connexin43, this protein was shown to assemble into gap junction-like plaques beginning at compaction (G. Valdimarsson, P. A. De Sousa, E. C. Beyer, D. L. Paul and G. M. Kidder (1991). Molec. Reprod. Dev. 30, 18-26). The purpose of the present study was to follow the fate of nascent connexin43 during preimplantation development, from synthesis to plaque insertion, and to learn more about the control of gap junction assembly during compaction. Cell fractionation and reverse transcription-polymerase chain reaction were employed to show that connexin43 mRNA is in polyribosomes at the 4-cell stage, suggesting that synthesis of connexin43 begins at least one cell cycle in advance of when gap junctions first form. The fate of nascent connexin43 was then followed throughout preimplantation development by means of laser confocal microscopy, using two other peptide (C-terminal)-specific antibodies. As was reported previously, connexin43 could first be detected in gap junction-like plaques beginning in the 8-cell stage, at which time considerable intracellular immunoreactivity could be seen as well. Later, connexin43 becomes differentially distributed in the apposed plasma membranes of morulae and blastocysts: a zonular distribution predominates between outside blastomeres and trophectoderm cells whereas plaque-like localizations predominate between inside blastomeres and cells of the inner cell mass. The cytoplasmic immunoreactivity in morulae was deemed to be nascent connexin en route to the plasma membrane since it could be abolished by treatment with cycloheximide, and redistributed by treatment with monensin or brefeldin-A, known inhibitors of protein trafficking. Treatment of uncompacted 8-cell embryos with either monensin or brefeldin-A inhibited the appearance of gap junction-like structures and the onset of gap junctional coupling in a reversible manner. These data demonstrate that the regulated step in the onset of gap junction assembly during compaction is downstream of transcription and translation and involves mobilization of connexin43 through trafficking organelles to plasma membranes.

Animals↗
PubMed1991

Zygotic expression of the connexin43 gene supplies subunits for gap junction assembly during mouse preimplantation development.

De novo assembly of gap junctions begins during compaction in the eight-cell stage of mouse development, and intercellular coupling mediated by gap junctions appears to be required for maintenance of the compacted state. We have begun to explore the expression of the family of genes encoding the connexins, the proteins that form the gap junction channels. We recently reported that a protein with antigenic and size similarity with connexin32, the rat liver gap junction protein, is inherited as an oogenetic product by the mouse zygote, but its gene appears not to be transcribed prior to implantation (Barron et al., Dev Genet 10:318-323, 1989). Here we report that another member of this gene family, connexin43, is transcribed by the embryonic genome from shortly after the time of genomic activation. As revealed by Northern blotting, connexin43 mRNA is absent from ovulated oocytes, becomes detectable in the 4-cell stage, and accumulates steadily thereafter to reach a maximum in blastocysts. In contrast, no transcripts of connexin26 could be detected in any preimplantation stage. A protein with antigenic and size similarity with connexin43 from rat heart was found by Western blotting to accumulate from the four-cell stage onward. Immunofluorescence analysis with embryo whole mounts was used to demonstrate that this protein is incorporated into punctate interblastomeric foci during compaction, consistent with its assembly into gap junction plaques. We conclude that connexin43 is one member of the connexin gene family whose zygotic expression is critical for preimplantation morphogenesis.

Animals↗
PubMed1989

Connexin32, a gap junction protein, is a persistent oogenetic product through preimplantation development of the mouse.

Gap junctions appear de novo during compaction in the eight-cell stage of mouse development. This is a critical event in the life of the embryo, because gap junctional intercellular communication is an essential requirement for maintaining compaction and, hence, for development of the blastocyst. Recently, a family of genes encoding gap junction proteins (connexins) has been identified and cloned, and we have taken advantage of the availability of antibodies and cDNA probes to investigate the expression of these genes in early development. We found that a protein with antigenic and size similarity to the "liver" gap junction protein, connexin32, is present throughout preimplantation development from the zygote through the late morula. Connexin32 mRNA, however, could not be detected in any preimplantation stage. This, and the presence of connexin32 in zygotes before activation of embryonic transcription, leads us to conclude that this protein is inherited as an oogenetic product that persists well beyond the transition from the oogenetic to embryonic program of gene expression. Furthermore, we found that mRNA for another gap junction protein, connexin43, is fairly abundant in preimplantation embryos. We conclude that it is more likely connexin43, and not connexin32, that is used to assemble new connexons as the level of intercellular coupling increases after compaction.

Animals↗
PubMed1989

The development of microtubular arrays in the germ tissue of an insect telotrophic ovary.

The microtubular arrays characteristic of the trophic core and cords of the adult Rhodnius prolixus ovary develop prior and during the larval--adult transformation. Development of the microtubules was revealed by immunocytochemistry, electron microscopy and polyacrylamide electrophoresis and Western blot analysis. Microtubular arrays were first detected in the trophic cords and presumptive trophic core 6 days before the adult molt. Cord microtubules increase in length and numbers as the trophic cords grow. Three microtubule packed cords have formed by 1 day post molt. The microtubule distribution in the presumptive core is non-uniform. Microtubule packed areas are interspersed with areas devoid of microtubules. The adult core begins forming between 1 day before molt and molting. This early adult core arises from the fusion of the anterior portions of microtubule packed cords. A fully mature adult core is not present by 2 days post molt. The microtubule packing density in the core increases from 2 days before to 2 days post molt. Tubulin increases from 6 days to 1 day before the adult molt.

Animals↗
PubMed1984

The microbiology of stockfish during the drying process.

Stockfish is made by drying species of fish in the open air to a final water content of 17-18% (w/w). By this treatment the fish acquires a very characteristic flavour. A study was made of the microbiology of blue ling (Molva dypterygia dypterygia) during this process. Aerobic plate counts at 22 degrees C in the flesh of the fish reached a maximum of 4.1 x 10(7)/g dry weight, after drying for 30 days. Analyses of the aerobic/facultatively anaerobic bacterial flora in the flesh showed initially a dominance of Moraxella and Acinetobacter-like spp. As the drying progressed a Gram positive, catalase negative flora appeared, increasing its proportion to 77% of the bacterial content of the fish. Representative strains of these bacteria were identified as Lactobacillus plantarum.

Animals↗