Biomedical subjects

E M Gerhardt

Publications and source records attributed to E M Gerhardt.

4 recordsLinked to original sources

PubMed1992

Transferrin receptor gene is hyperexpressed and transcriptionally regulated in differentiating erythroid cells.

We have analyzed the developmental pattern of expression of the chicken transferrin receptor (CTR) gene in various chick embryonic tissues. Northern analyses of RNA from embryonic tissues at different stages of development and cultured chick embryonic fibroblasts (CEFs) show that CTR is hyperexpressed in differentiating erythroid cells such that the steady-state level of CTR mRNA in these cells could be 200 or more times higher than in nonerythroid cells. In vitro nuclear transcription assays using nuclei from embryonic erythroid and brain cells, as well as CEFs, demonstrate that the vast differences in CTR mRNA levels in these cells are reflected in their respective CTR gene transcriptional activities. During development, the steady-state level of CTR mRNA declines in all tissues and, in erythroid cells, this pattern is accompanied by a similar decline in beta-globin mRNA levels. These changes are concurrent with the decreases in CTR and beta-globin mRNA transcriptional activities during erythroid maturation. Taken together, our results indicate that the hyperexpression of the CTR gene in differentiating erythroid cells is regulated to a significant degree at the transcriptional level. We also demonstrate that, in erythroid cells, neither CTR gene transcription nor CTR mRNA stability is regulated by intracellular iron levels.

Animals↗

PubMed1991

The cDNA sequence and primary structure of the chicken transferrin receptor.

Recombinant cDNA clones encoding the chicken transferrin receptor (cTR) have been isolated and sequenced. Comparison of the deduced primary structure of cTR with those of the human transferrin receptor (hTR) and mouse transferrin receptor (mTR) shows that their size, hydropathy profile, location of sites for posttranslational modifications, and domain organization are highly similar. The cytoplasmic domain of cTR contains the motif Tyr-Xaa-Arg-Phe (YXRF) that is the recognition signal for high-efficiency endocytosis of hTR. The cTR has several highly conserved regions within its extracellular domain, including those flanking the putative N-glycosylation sites. Overall, however, the extracellular domain of cTR is only 53% identical to the extracellular domains of hTR and mTR. The cTR also lacks three of the six Cys residues found in the extracellular domains of the mammalian TRs. These differences can account for functional and structural properties that distinguish cTR and mammalian TRs.

Amino Acid Sequence↗

PubMed1989

Chicken transferrin receptor gene: conservation 3' noncoding sequences and expression in erythroid cells.

Recombinant clones of the chicken transferrin receptor gene and cDNA have been isolated and sequenced. Two highly conserved regions have been identified in the 3' noncoding sequence of the human and chicken TR gene. The conserved regions include sequences that have been shown to be involved in the iron-dependent regulation of human TR mRNA stability. These sequences can be modeled as two different types of RNA secondary structures, one containing stem-loop structures that are similar to the iron-responsive elements found in ferritin mRNA and the other being a stable, duplex/stem-loop structure. Both forms show considerable similarity between chicken and human mRNA. The expression of TR is developmentally regulated during erythroid maturation, and immature erythroid cells express exceptionally high levels of TR mRNA.

Animals↗

PubMed1989

A cytosolic protein binds to structural elements within the iron regulatory region of the transferrin receptor mRNA.

The level of mRNA encoding the transferrin receptor (TfR) is regulated by iron, and this regulation is mediated by a portion of the 3' untranslated region (UTR) of the TfR transcript. This portion of 3' UTR of the human TfR mRNA contains five RNA elements that have structural similarity to the iron-responsive element (IRE) found as a single copy in the 5' UTR of the mRNA for ferritin, whose translation is regulated by iron. Moreover, five very similar elements are also contained in the 3' UTR of the chicken TfR mRNA. Cytosolic extracts of human cell lines are shown by a gel shift assay involving RNase T1 protection to contain an IRE-binding protein capable of specific interaction with the human TfR 3' UTR. When the protecting protein is removed, the protected RNA can be digested with RNase T1 to yield oligoribonucleotide fragments characteristic of two of the IREs contained in the TfR 3' UTR. As judged by cross-competition experiments, the same IRE-binding protein interacts with the ferritin IRE. The apparent affinity of RNA sequence elements for the IRE-binding protein is shown to depend upon the sequence of the RNA. A comprehensive secondary structure for the regulatory region of the TfR mRNA is proposed based on the experimentally demonstrated presence of at least two IRE-like structural elements.

Binding Sites↗