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M G Achen

Publications and source records attributed to M G Achen.

31 records · Page 2Linked to original sources
PubMed1998

Vascular endothelial growth factor D (VEGF-D) is a ligand for the tyrosine kinases VEGF receptor 2 (Flk1) and VEGF receptor 3 (Flt4).

We have identified a member of the VEGF family by computer-based homology searching and have designated it VEGF-D. VEGF-D is most closely related to VEGF-C by virtue of the presence of N- and C-terminal extensions that are not found in other VEGF family members. In adult human tissues, VEGF-D mRNA is most abundant in heart, lung, skeletal muscle, colon, and small intestine. Analyses of VEGF-D receptor specificity revealed that VEGF-D is a ligand for both VEGF receptors (VEGFRs) VEGFR-2 (Flk1) and VEGFR-3 (Flt4) and can activate these receptors. However. VEGF-D does not bind to VEGFR-1. Expression of a truncated derivative of VEGF-D demonstrated that the receptor-binding capacities reside in the portion of the molecule that is most closely related in primary structure to other VEGF family members and that corresponds to the mature form of VEGF-C. In addition, VEGF-D is a mitogen for endothelial cells. The structural and functional similarities between VEGF-D and VEGF-C define a subfamily of the VEGFs.

Adult↗
PubMed1998

The vascular endothelial growth factor family; proteins which guide the development of the vasculature.

The development of the vascular tree during embryogenesis involves vasculogenesis, angiogenesis and tissue-specific differentiation of endothelium which gives rise to many different vessel types. These processes are physiologically complex and are therefore difficult to study in vitro. However, the discovery of endothelial cell-specific receptors and cognate ligands has led to the generation of transgenic and knockout mouse models which have shed light on the molecular mechanisms that regulate the development of blood and lymphatic vessels during embryogenesis. Such mouse models have demonstrated that members of the vascular endothelial growth factor (VEGF) family of proteins and the VEGF receptors are critical regulators of vasculogenesis, angiogenesis and endothelial cell differentiation. The availability of purified VEGF family members and of inhibitors of these growth factors may provide a means to modulate blood vessel growth for the treatment of cancer, retinopathies and diseases of ischemia.

Adult↗
PubMed1997

Placenta growth factor and vascular endothelial growth factor are co-expressed during early embryonic development.

We have used the polymerase chain reaction to identify mouse proteins similar in primary structure to the endothelial cell mitogen Vascular Endothelial Growth Factor (VEGF). One amplified product encoded mouse Placenta Growth Factor (PIGF). The pattern of PIGF gene expression in mouse embryos was studied by in situ hybridization. Transcripts encoding mouse PIGF were abundant in trophoblastic giant cells associated with the parietal yolk sac at early stages of embryogenesis. VEGF transcripts were also detected in trophoblastic giant cells raising the possibility that these cells may secrete heterodimers consisting of one PIGF subunit and one VEGF subunit. The secretion of PIGF and VEGF by trophoblastic giant cells is likely to be the signal which initiates and co-ordinates vascularization in the deciduum and placenta during early embryogenesis.

Amino Acid Sequence↗
PubMed1995

Hypoxia-induced transcriptional activation and increased mRNA stability of vascular endothelial growth factor in C6 glioma cells.

Vascular endothelial growth factor (VEGF) is an endothelial specific angiogenic mitogen secreted from various cell types including tumor cells. Increasing evidence suggests that VEGF is a major regulator of physiological and pathological angiogenesis, and the VEGF/VEGF receptor system has been shown to be necessary for glioma angiogenesis. Hypoxia seems to play a critical role in the induction of VEGF expression during glioma progression. C6 glioma cells provide an in vivo glioma model for the study of tumor angiogenesis, and the expression of VEGF in C6 cells has been shown to be up-regulated by hypoxia in vitro. However, little is known about the molecular mechanism of hypoxic induction of VEGF. Here, we demonstrate that hypoxic induction of VEGF in C6 cells is due to both transcriptional activation and increased stability of mRNA. Nuclear run-on assays revealed a fast and lasting transcriptional activation, whereas the determination of mRNA half-life showed a slower increase of mRNA stability during hypoxia. Reporter gene studies revealed that hypoxia responsive transcription-activating elements were present in the 5'-flanking region of the VEGF gene. These results suggested that several distinct molecular mechanisms were involved in hypoxia-induced gene expression and were activated in a biphasic manner.

Animals↗
PubMed1995

The non-receptor tyrosine kinase Lyn is localised in the developing murine blood-brain barrier.

The blood-brain barrier, formed by brain endothelium, is critical for brain function. The development of the blood-brain barrier involves brain angiogenesis and endothelial cell differentiation, processes which require active signal transduction pathways. The differentiation of brain endothelial cells to the "blood-brain-barrier phenotype" involves cytoskeletal changes which modulate the tightness of the barrier. In order to identify signal transduction proteins involved in blood-brain barrier development, cDNA from bovine and murine brain endothelial cells was used in a polymerase chain reaction for cloning of DNA encoding Src homology 3 domains. Src homology 3 domains are structural domains found in many signal transduction proteins. These domains often mediate interaction of signaling proteins with the cytoskeleton and therefore may play a role in the regulation of the cytoskeletal changes which occur during blood-brain-barrier development. Unexpectedly, all bovine and murine clones analyzed from polymerase chain reactions encoded the Src homology 3 domain of one protein, namely the non-receptor tyrosine kinase, Lyn, which is involved in signal transduction in cells of the hemopoietic system. In situ hybridization analyses confirmed the presence of lyn mRNA in developing blood vessels in embryonic and early post-natal mouse brain, but not in endothelium outside the brain. In bovine brain endothelial cells in primary culture, p53lyn is highly abundant and present in two forms which have different patterns of tyrosine phosphorylation. These data suggest that Lyn may be involved in transduction of growth and differentiation signals required for blood-brain-barrier development.

Animals↗
PubMed1993

Transthyretin gene expression in choroid plexus first evolved in reptiles.

The presence of transthyretin in mammals and birds, but not amphibia, suggested that transthyretin expression first appeared in stem reptiles. Therefore, transthyretin synthesis was studied in a lizard. Transthyretin synthesis in choroid plexus pieces from Tiliqua rugosa was demonstrated by incorporation of radiactive amino acids. Oligonucleotides corresponding to conserved regions of transthyretin were used as primers in polymerase chain reaction with lizard choroid plexus cDNA. Amplified DNA was used to screen a lizard choroid plexus cDNA library. A full-length transthyretin cDNA clone was isolated and sequenced. A three-dimensional model of lizard transthyretin was obtained by homology modeling. The central channel of transthyretin, containing the thyroxine-binding site, was found to be completely conserved between reptiles and mammals. Transthyretin expression was not detected in lizard liver. These data suggest that transthyretin first evolved in the choroid plexus of the brain. Due to a change in tissue distribution of gene expression, occurring much later during evolution, transthyretin also became a plasma protein, synthesized in the liver.

Amino Acid Sequence↗
PubMed1992

Protein synthesis at the blood-brain barrier. The major protein secreted by amphibian choroid plexus is a lipocalin.

Among the proteins secreted by choroid plexus of vertebrates, one protein is much more abundant than all others. In mammals, birds, and reptiles this protein is transthyretin, a tetramer of identical 15-kDa subunits. In this study choroid plexus from frogs, tadpoles, and toads incubated in vitro were found to synthesize and secrete one predominant protein. However, this consisted of one single 20-kDa polypeptide chain. It was expressed throughout amphibian metamorphosis. Part of its amino acid sequence was determined and used for construction of oligonucleotides for polymerase chain reaction. The amplified DNA was used to screen a toad choroid plexus cDNA library. Full-length cDNA clones were isolated and sequenced. The derived amino acid sequence for the encoded protein was 183 amino acids long, including a 20-amino acid presegment. The calculated molecular weight of the mature protein was 18,500. Sequence comparison with other proteins showed that the protein belonged to the lipocalin superfamily. Its expression was highest in choroid plexus, much lower in other brain areas, and absent from liver. Since no transthyretin was detected in proteins secreted from amphibian choroid plexus, abundant synthesis and secretion of transthyretin in choroid plexus must have evolved only after the stage of the amphibians.

Amino Acid Sequence↗
PubMed1991

Isolation, characterization, cDNA cloning and gene expression of an avian transthyretin. Implications for the evolution of structure and function of transthyretin in vertebrates.

A chicken liver cDNA library was constructed in bacteriophage lambda gt10. A full-length transthyretin cDNA clone was identified by screening with rat transthyretin cDNA and was sequenced. A three-dimensional model of chicken transthyretin was obtained by computer-graphics-based prediction from the derived amino acid sequence for chicken transthyretin and from the structure of human transthyretin determined by X-ray diffraction analysis [Blake, C.C.F., Geisow, M.J., Oatley, S.J., Rérat, B. & Rérat, C. (1978) J. Mol. Biol. 121, 339-356]. The similarity of the amino acid sequences of chicken and human transthyretins was 75% overall and 100% for the central channel containing the thyroxine-binding site. Also, the organization of the transthyretin gene into exons and introns and the tissue specificity of expression of the transthyretin gene were similar in chicken and mammals, despite an evolutionary distance of about 3 x 10(8) years from their common ancestor, the Cotylosaurus. By far the highest levels of transthyretin mRNA were found in choroid plexus. The data suggest a fundamental role for the cerebral expression of transthyretin in all vertebrates. It has been proposed that this role is the transport of thyroxine from the bloodstream to the brain [Schreiber, G., Aldred, A.R., Jaworowski, A., Nilsson, C., Achen, M.G. & Segal, M.B. (1990) Am. J. Physiol. 258, R338-R345].

Amino Acid Sequence↗
PubMed1991

The distribution of cerebral expression of the transferrin gene is species specific.

Various plasma proteins, for example, transferrin, are synthesized not only in the liver, but also in the brain. The proportion of transferrin mRNA in total RNA from different regions of brains from various mammalian species was studied by Northern blot analysis. Absolute amounts of transferrin mRNA were determined in brain, choroid plexus, and liver from rats, sheep, and pigs by hybridization in solution followed by ribonuclease protection assay. Corrections for differences in yields of RNA were made using internal RNA standards. Large proportions of transferrin mRNA in total RNA and high absolute levels of transferrin mRNA in choroid plexus were found only in rats. Small proportions of transferrin mRNA were observed in RNA from choroid plexus from mice, dogs, and rabbits, while no transferrin mRNA at all was detected in choroid plexus from humans, sheep, pigs, cows, and guinea pigs. In further analysis of sheep and pigs, various amounts of transferrin mRNA were found in many parts of the brain, in contrast to the absence of transferrin mRNA from choroid plexus. In conclusion, a striking species specificity was observed for the pattern of cerebral expression of the transferrin gene.

Amino Acid Sequence↗
PubMed1990

Thyroxine transport from blood to brain via transthyretin synthesis in choroid plexus.

The transport of thyroxine from the bloodstream to the brain and the synthesis and secretion of transthyretin (formerly called prealbumin) were studied in rats and in sheep choroid plexus perfused in vitro. Rat choroid plexus contained 4.4 micrograms and rat liver 0.39 micrograms transthyretin mRNA per gram wet tissue. The specific radioactivity of transthyretin isolated from cerebrospinal fluid of rats 60 min after intravenous injection of [14C]leucine was greater than 50 times that of transthyretin from serum. After adding [14C]leucine to the perfusion medium of an in vitro perfused sheep choroid plexus, highly radioactive transthyretin was isolated from freshly secreted cerebrospinal fluid collected from the exposed choroid plexus surface. Secretion of newly synthesized transthyretin into the perfusion medium could not be demonstrated. After intravenous injection of [125I]-thyroxine into rats, a maximum in the curve of radioactivity in tissue plotted against time after injection was observed first for choroid plexus, thereafter for cerebrospinal fluid, and still later for cortex and striatum. Based on the obtained data, a hypothesis is derived for the mechanism of the transport of thyroid hormones from the bloodstream to the brain involving transthyretin synthesized in choroid plexus and secreted into the cerebrospinal fluid.

Animals↗
PubMed1986

Construction of plasmid vectors for the detection of streptococcal promoters.

Plasmid vectors have been constructed for detecting DNA fragments that exhibit promoter activity in Streptococcus sanguis. The plasmids are able to replicate in both S. sanguis and Escherichia coli, and contain an erythromycin resistance marker which is expressed in both hosts. Selection for promoter activity is dependent upon the insertion of appropriate DNA fragments upstream from a promoterless chloramphenicol acetyl transferase gene (cat) from Staphylococcus aureus. To facilitate this insertion, a pair of vectors, pMU1327 and pMU1328, were constructed with the polylinker from M13mp 18 in either orientation. The to transcriptional terminator of phage lambda is present downstream from cat. Translation stop codons in all reading frames are located between the polylinker and the initiation codon of cat. These plasmids have been used to isolate DNA fragments from S. sanguis, S. lactis and S. cremoris that exhibit promoter activity in S. sanguis.

Base Sequence↗