Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

T-Ag inhibits implantation by EC cell derived embryoid bodies

When introduced into EC cells of a blastocyst, polyomavirus (Py) T-Ag results in mice mosaic for T-Ag but otherwise essentially normal. It had been reported that SV40 T-Ag does not inhibit differentiation of F9 EC cells, but did inhibit endogenous retrovirus (ERV) production. We therefore sought to determine if Py T-Ag had any affect on EC derived embryoid body implantation onto mouse placenta. F9 EC cells were selected for T-Ag maintenance. Like the SV40 transformed cells, we show that these Py T-Ag selected EC cells no longer express IAP transcripts following differentiation into embryoid bodies. Normal and Py T-Ag selected F9 cells were differentiated into embryoid bodies then implanted into pseudopregnant mice. We observe, that normal F9 derived embryoid bodies underwent the initial stages of implantation whereas the Py T-Ag selected embryoid bodied did not implant. The implications of this observation with respect to trophectoderm and ERV function are discussed. We examine the idea that ERVs may be a required element for normal embryo implantation.

Platelet-derived growth factor (PDGF) BB homodimer regulates PDGF A- and PDGF B-chain gene transcription in human mesangial cells

Mesangial cells express platelet-derived growth factor (PDGF) A- and B-chain mRNA and release PDGF. Several polypeptide growth factors, including PDGF itself, induce PDGF A- and B- chain mRNA abundance. To understand the molecular mechanisms associated with the changes in mRNA abundance, we measured the effects of PDGF BB homodimer on PDGF A- and B-chain gene transcription in cultured mesangial cells. The data demonstrate 2- and 4-fold increases in PDGF A-chain gene transcription in response to PDGF BB homodimer at 5 and 24 h time points respectively. PDGF B-chain gene transcription was also induced approximately 3-fold at 2, 5 and 24 h time points in response to treatment with PDGF BB homodimer. The effect of PDGF BB on the half-life of PDGF A- as well as PDGF B-chain mRNA was measured directly by the pulse-chase method. There was no effect on PDGF A-chain mRNA half-life whereas PDGF B-chain mRNA half-life was increased 1.5-fold. These studies indicate that, in human mesangial cells, the increase in the levels of PDGF A- and B-chain mRNA in response to PDGF- receptor(s) activation is mediated at the level of gene transcription. In addition, the regulation of PDGF B- but not PDGF A-chain gene involves increased mRNA stability. Mesangial cells are a useful model for studying molecular mechanisms of PDGF- gene regulation in non-transformed human cells.

Differential stability of c-myc mRNAS in a cell-free system

We have developed a simple cell-free system for studying the stability of different mRNAs in vitro. We demonstrate that the threefold greater stability in vivo of truncated c-myc mRNA (lacking exon 1) compared with that of full-length c-myc mRNA is maintained in our in vitro system. Chimeric mRNAs in which the first exon of c-myc was fused to immunoglobulin C alpha heavy chain or glyceraldehyde-3-phosphate dehydrogenase mRNAs were not rapidly degraded, demonstrating that c-myc exon 1 alone is not sufficient to tag mRNAs for rapid degradation. Competition experiments show that full-length c-myc mRNA is specifically recognized by a factor(s) responsible for its rapid degradation. This system will allow further characterization and purification of these factors.

Differential stability of c-myc mRNAS in a cell-free system

We have developed a simple cell-free system for studying the stability of different mRNAs in vitro. We demonstrate that the threefold greater stability in vivo of truncated c-myc mRNA (lacking exon 1) compared with that of full-length c-myc mRNA is maintained in our in vitro system. Chimeric mRNAs in which the first exon of c-myc was fused to immunoglobulin C alpha heavy chain or glyceraldehyde-3-phosphate dehydrogenase mRNAs were not rapidly degraded, demonstrating that c-myc exon 1 alone is not sufficient to tag mRNAs for rapid degradation. Competition experiments show that full-length c-myc mRNA is specifically recognized by a factor(s) responsible for its rapid degradation. This system will allow further characterization and purification of these factors.

Expression of the intracisternal A-particle is elevated during differentiation of embryonal carcinoma cells

Three cDNA clones coding for the 3' region of the intracisternal A-particle (IAP), a mouse endogenous retrovirus, were isolated during screening of a library for genes whose expression was modulated during the retinoic acid-induced differentiation of the embryonal carcinoma cell line F9 into parietal endoderm-like (PE-like) cells. In contrast to previously reported results, no IAP transcripts were detected in either F9 cells or two pluripotent cell lines tested. Instead, IAP transcripts as well as IAPs were abundant in the PE-like cells PYS-2 and F9AcCl 9 and in retinoic acid-induced F9 cells but not in the other differentiated cell types of teratocarcinoma origin which were examined. A comparison of the nucleotide sequences of the three IAP cDNA clones with a genomically integrated proviral sequence (MIA14) demonstrated heterogeneity in both length and sequence among the clones. The position of the poly(A) addition site was determined to be 15 nucleotides from the proposed poly(A) addition signal and to occur after the sequence CAGA, not CA, as previously proposed. Length heterogeneity was greatest in a region of TC repeats 80 base pairs 5' to the poly(A) addition site. Additionally, the putative TATAA box found in MIA14 was deleted in the cDNA clones and in the long terminal repeat regions from two other genomic clones examined. The heterogeneity evident among the cDNA clones further demonstrated that at least two distinct IAP genes are activated during differentiation. An analysis of the rate of transcription in isolated nuclei indicated that the activation of expression of IAP genes in PE-like cells is the result of transcriptional regulation. Together, these observations suggest that the modulation of IAP transcription is regulated autonomously rather than by the fortuitous integration of an IAP sequence adjacent to a developmentally regulated cellular gene.

Complete sequence of three alpha-tubulin cDNAs in Chinese hamster ovary cells: each encodes a distinct alpha-tubulin isoprotein

The genome of Chinese hamster ovary (CHO) cells contains a complex family of approximately 16 alpha-tubulin genes, many of which may be pseudogenes. We present here the complete cDNA sequences of three expressed alpha-tubulin genes; one of these genes has been identified only in CHO cells. The noncoding regions of these three CHO alpha-tubulin genes differed significantly, but their coding regions were highly conserved. Nevertheless, we observed differences in the predicted amino acid sequences for the three genes. A comparison of the CHO alpha-tubulin sequences with all of the sequences available for mammals allowed assignment of the alpha-tubulin genes to three classes. The proteins encoded by the members of two of these classes showed no class-specific amino acids among the mammalian species examined. The gene belonging to the third class encoded an isoprotein which was clearly distinct, and members of this class may play a unique role in vivo. Sequencing of the three alpha-tubulin genes was also undertaken in CMR795, a colcemid-resistant clonal CHO cell line which has previously been shown to have structural and functional alterations in its tubulin proteins. We found differences in the tubulin nucleotide sequence compared with the parental line; however, no differences in the alpha-tubulin proteins encoded in the two cell lines were observed.

Analysis of the mouse dhfr promoter region: existence of a divergently transcribed gene

The use of murine dihydrofolate reductase (dhfr) gene amplification mutants enabled us to identify important structural and functional features of the dhfr promoter region. We found another transcription unit, at least 14 kilobases in size, which initiates within 130 base pairs of the major dhfr transcript and is transcribed divergently. The 5' ends of both transcripts were analyzed and found to have multiple initiation sites. The major dhfr transcript and the divergent transcript appear to share the same promoter region; the longer transcripts of the dhfr gene overlap with the divergent transcripts and use a different promoter region. The divergent transcript appears to code for a protein; an homologous sequence to its first exon is found in the corresponding location near the human dhfr gene.

Diverse patterns of expression of the cytochrome c oxidase subunit I gene and unassigned reading frames 4 and 5 during the life cycle of Trypanosoma brucei

Transcription of a maxicircle segment from Trypanosoma brucei 164 that contains nucleotide (nt) sequences corresponding to cytochrome c oxidase subunit I (COI) and unassigned reading frames (URFs) 4 and 5 of other mitochondrial systems was investigated. Two major transcripts that differ in size by ca. 200 nt map to each of the COI and URF4 genes, while a single major transcript maps to URF5. In total RNA, the larger COI transcript is more abundant in procyclic forms (PFs) than in bloodstream forms (BFs), the smaller COI and both URF4 transcripts have similar abundances in both forms, and the single URF5 transcript is more abundant in BF than PF. These patterns of expression differ in poly(A)+ RNA as a result of a higher proportion of poly(A)+ mitochondrial transcripts in PFs than in BFs. In addition, small (300- to 500-nt) RNAs that are transcribed from C-rich sequences located between putative protein-coding genes also exhibit diverse patterns of expression between life cycle stages and differences in polyadenylation in PFs compared with BFs. These observations suggest that multiple processes regulate the differential expression of mitochondrial genes in T. brucei.

Complete sequence of three alpha-tubulin cDNAs in Chinese hamster ovary cells: each encodes a distinct alpha-tubulin isoprotein

The genome of Chinese hamster ovary (CHO) cells contains a complex family of approximately 16 alpha-tubulin genes, many of which may be pseudogenes. We present here the complete cDNA sequences of three expressed alpha-tubulin genes; one of these genes has been identified only in CHO cells. The noncoding regions of these three CHO alpha-tubulin genes differed significantly, but their coding regions were highly conserved. Nevertheless, we observed differences in the predicted amino acid sequences for the three genes. A comparison of the CHO alpha-tubulin sequences with all of the sequences available for mammals allowed assignment of the alpha-tubulin genes to three classes. The proteins encoded by the members of two of these classes showed no class-specific amino acids among the mammalian species examined. The gene belonging to the third class encoded an isoprotein which was clearly distinct, and members of this class may play a unique role in vivo. Sequencing of the three alpha-tubulin genes was also undertaken in CMR795, a colcemid-resistant clonal CHO cell line which has previously been shown to have structural and functional alterations in its tubulin proteins. We found differences in the tubulin nucleotide sequence compared with the parental line; however, no differences in the alpha-tubulin proteins encoded in the two cell lines were observed.

Expression of the intracisternal A-particle is elevated during differentiation of embryonal carcinoma cells

Three cDNA clones coding for the 3' region of the intracisternal A-particle (IAP), a mouse endogenous retrovirus, were isolated during screening of a library for genes whose expression was modulated during the retinoic acid-induced differentiation of the embryonal carcinoma cell line F9 into parietal endoderm-like (PE-like) cells. In contrast to previously reported results, no IAP transcripts were detected in either F9 cells or two pluripotent cell lines tested. Instead, IAP transcripts as well as IAPs were abundant in the PE-like cells PYS-2 and F9AcCl 9 and in retinoic acid-induced F9 cells but not in the other differentiated cell types of teratocarcinoma origin which were examined. A comparison of the nucleotide sequences of the three IAP cDNA clones with a genomically integrated proviral sequence (MIA14) demonstrated heterogeneity in both length and sequence among the clones. The position of the poly(A) addition site was determined to be 15 nucleotides from the proposed poly(A) addition signal and to occur after the sequence CAGA, not CA, as previously proposed. Length heterogeneity was greatest in a region of TC repeats 80 base pairs 5' to the poly(A) addition site. Additionally, the putative TATAA box found in MIA14 was deleted in the cDNA clones and in the long terminal repeat regions from two other genomic clones examined. The heterogeneity evident among the cDNA clones further demonstrated that at least two distinct IAP genes are activated during differentiation. An analysis of the rate of transcription in isolated nuclei indicated that the activation of expression of IAP genes in PE-like cells is the result of transcriptional regulation. Together, these observations suggest that the modulation of IAP transcription is regulated autonomously rather than by the fortuitous integration of an IAP sequence adjacent to a developmentally regulated cellular gene.

Diverse patterns of expression of the cytochrome c oxidase subunit I gene and unassigned reading frames 4 and 5 during the life cycle of Trypanosoma brucei

Transcription of a maxicircle segment from Trypanosoma brucei 164 that contains nucleotide (nt) sequences corresponding to cytochrome c oxidase subunit I (COI) and unassigned reading frames (URFs) 4 and 5 of other mitochondrial systems was investigated. Two major transcripts that differ in size by ca. 200 nt map to each of the COI and URF4 genes, while a single major transcript maps to URF5. In total RNA, the larger COI transcript is more abundant in procyclic forms (PFs) than in bloodstream forms (BFs), the smaller COI and both URF4 transcripts have similar abundances in both forms, and the single URF5 transcript is more abundant in BF than PF. These patterns of expression differ in poly(A)+ RNA as a result of a higher proportion of poly(A)+ mitochondrial transcripts in PFs than in BFs. In addition, small (300- to 500-nt) RNAs that are transcribed from C-rich sequences located between putative protein-coding genes also exhibit diverse patterns of expression between life cycle stages and differences in polyadenylation in PFs compared with BFs. These observations suggest that multiple processes regulate the differential expression of mitochondrial genes in T. brucei.

Retention of autoregulatory control of tubulin synthesis in cytoplasts: demonstration of a cytoplasmic mechanism that regulates the level of tubulin expression

Virtually all animal cells rapidly and specifically depress synthesis of new alpha- and beta-tubulin polypeptides in response to microtubule inhibitors that increase the pool of depolymerized subunits, or in response to direct elevation of the cellular tubulin subunit content through microinjection of exogenous tubulin subunits. Collectively, these previous findings have documented the presence of an apparent eucaryotic, autoregulatory control mechanism that specifies the level of expression of tubulin in cultured animal cells. Mechanistically, this regulation of tubulin synthesis is achieved through modulation of tubulin mRNA levels. To dissect further the molecular pathway that underlies this autoregulatory phenomenon, we have now investigated whether enucleated cells still retain the requisite regulatory machinery with which to alter tubulin synthetic levels in response to fluctuations in the pool size of unpolymerized tubulin subunits. Using two-dimensional gel electrophoresis to analyze the patterns of new polypeptide synthesis, we have determined that such cytoplasts can indeed respond to drug-induced microtubule depolymerization by specific repression of new beta-tubulin synthesis. Moreover, the response of cytoplasts is, if anything, greater in magnitude than that of whole cells. We conclude that autoregulatory control of beta-tubulin gene expression must derive principally, if not exclusively, from a cytoplasmic control mechanism that modulates beta-tubulin mRNA stability. For alpha-tubulin, although the response of cytoplasts after drug-induced microtubule depolymerization is quantitatively less dramatic than that of whole cells, at least part of the regulatory machinery must also be activated through a cytoplasmic regulatory event.

Analysis of the mouse dhfr promoter region: existence of a divergently transcribed gene

The use of murine dihydrofolate reductase (dhfr) gene amplification mutants enabled us to identify important structural and functional features of the dhfr promoter region. We found another transcription unit, at least 14 kilobases in size, which initiates within 130 base pairs of the major dhfr transcript and is transcribed divergently. The 5' ends of both transcripts were analyzed and found to have multiple initiation sites. The major dhfr transcript and the divergent transcript appear to share the same promoter region; the longer transcripts of the dhfr gene overlap with the divergent transcripts and use a different promoter region. The divergent transcript appears to code for a protein; an homologous sequence to its first exon is found in the corresponding location near the human dhfr gene.