The erythroid cells of anaemic Xenopus laevis. I. Studies on cellular morphology and protein and nucleic acid synthesis during differentiation

Haemopoietic System of the Anurans: The Role of Bone Marrow and Liver

The haemopoietic activity of the frog,Pelophylax ridibunduswas investigated during the year. Liver and bone marrow myelograms were examined in the different seasons using the special indexes and coefficients. It was shown the presence of the erythroid and granulocytic differentiation during the year in the both organs. In the bone marrow is changing the total number of the non-diferentiated haemopoietic cells, but ratio between erythroid and granulocytic progenitors is stabile. The haemopoietic activity of the liver has more prominent season variation in comparison with the marrow, but their total significance is comparable. The erythrocytic differentiation is more prominent during the summer and autumn, but granulocytic one took place in the spring and summer.

Identification of erythroid progenitors induced by erythropoietic activity in Xenopus laevis

Oxygen is essential for the survival of animals. Red blood cells in the circulation, i.e. peripheral erythrocytes, are responsible for transporting oxygen to tissues. The regulation of erythropoiesis in vertebrates other than mammals is yet to be elucidated. Recently we identified erythropoietin, a primary regulator of erythropoiesis, in Xenopus laevis, which should enable us to identify target cells, including erythroid progenitors, and to investigate the production and development of erythroid cells in amphibians. Here, we established a semi-solid colony-forming assay in Xenopus laevis to clarify the existence of colony-forming unit-erythroid cells, the functional erythroid progenitors identified in vitro. Using this assay, we showed that recombinant xlEPO induces erythroid colony formation in vitro and detected an increased level of erythropoietin activity in blood serum during acute anemic stress. In addition, our study demonstrated the possible presence of multiple, non-xlEPO, factors in anemic serum supportive of erythroid colony formation. These results indicate that erythropoiesis mediated by erythropoietin is present in amphibian species and, furthermore, that the regulatory mechanisms controlling peripheral erythrocyte number may vary among vertebrates.

4SR, a novel zinc-finger protein with SR-repeats, is expressed during early development of Xenopus

The protein C4SR contains two cysteine(4) (C(4)) zinc-finger motifs at its amino terminus, a stretch of acidic residues in the middle and a series of serine-arginine (SR) repeats at its carboxyl terminus. A cDNA clone encoding the zinc-finger domain was first selected from a Xenopus laevis oocyte expression library on the basis of the ability of the fusion protein to stably bind an RNA probe. The mRNA encoding C4SR is expressed during oogenesis, and the protein is present at a constant level in oocytes and early embryos. The C4SR protein is expressed in transcriptionally active erythroblasts but not in transcriptionally inert mature erythrocytes. An epitope-tagged C4SR protein, expressed in oocytes, associates with nascent transcripts at many loci in lampbrush chromosomes and is absent from storage particles (snurposomes) containing the normally recognized complement of RNA splicing components. It is likely that C4SR is involved in pre-mRNA transcription/packaging rather than in exon splicing. The zinc-finger motif, present as two copies in C4SR, is also present in a range of transcription-associated proteins. We suggest the descriptor (DW)C(4), in which DW refers to the invariant aspartic acid (D)/tryptophan (W) dipeptide that precedes the first cysteine residue, for this distinctive zinc-finger structure.

DNA replication in quiescent cell nuclei: regulation by the nuclear envelope and chromatin structure

Quiescent nuclei from differentiated somatic cells can reacquire pluripotence, the capacity to replicate, and reinitiate a program of differentiation after transplantation into amphibian eggs. The replication of quiescent nuclei is recapitulated in extracts derived from activated Xenopus eggs; therefore, we have exploited this cell-free system to explore the mechanisms that regulate initiation of replication in nuclei from terminally differentiated Xenopus erythrocytes. We find that these nuclei lack many, if not all, pre-replication complex (pre-RC) proteins. Pre-RC proteins from the extract form a stable association with the chromatin of permeable nuclei, which replicate in this system, but not with the chromatin of intact nuclei, which do not replicate, even though these proteins cross an intact nuclear envelope. During extract incubation, the linker histones H1 and H1(0) are removed from erythrocyte chromatin by nucleoplasmin. We show that H1 removal facilitates the replication of permeable nuclei by increasing the frequency of initiation most likely by promoting the assembly of pre-RCs on chromatin. These data indicate that initiation in erythrocyte nuclei requires the acquisition of pre-RC proteins from egg extract and that pre-RC assembly requires the loss of nuclear envelope integrity and is facilitated by the removal of linker histone H1 from chromatin.

Comparative study of immature erythroid cells of the diploid Bufo ictericus and the tetraploid Odontophrynus americanus (Amphibia, Anura): ultrastructural cytochemical detection of nucleic acids and polysaccharides, and mapping of the element phosphorus

In the present study, we used ultrastructural cytochemistry to analyze the distribution of nuclear and cytoplasmic nucleic acids and polysaccharides, and electron spectroscopic imaging to map the element phosphorus in immature erythroid cells taken from two amphibians, the diploid Bufo ictericus and the tetraploid Odontophrynus americanus. In the cytoplasm of cells from the tetraploid species, we detected numerous inclusions containing a material that was similar to the dispersed chromatin seen in the nucleus of these cells. The RNase-gold complex labeled both the dispersed nuclear chromatin and the cytoplasmic inclusions. The Thiéry technique showed that glycoconjugates were present in all the membranous complexes of the erythroid cells of both types of amphibians under study, although they were absent within or around the cytoplasmic RNA inclusions. Electron spectroscopic imaging revealed the presence of phosphorus in these inclusions. These data suggest that an increase in RNA synthesis occurs in tetraploid amphibian cells, probably as a result of an alteration in the mechanisms of gene regulation.

Genomic potential of erythroid and leukocytic cells of Rana pipiens analyzed by nuclear transfer into diplotene and maturing oocytes

In order to determine whether differentiated somatic cells maintain genetic totipotency, nuclear transplantations from several differentiated somatic cell types into eggs and oocytes were performed previously in Rana pipiens and Xenopus laevis. The formation of postneurula embryos and tadpoles under the direction of the test nuclei demonstrated their genetic multipotency. In addition, Rana erythrocyte nuclei transplanted to oocytes directed more extensive tadpole development than those injected into eggs. We have extended our studies of the genomic potential of differentiated somatic nuclei from the peripheral blood of Rana pipiens. First, we show that the developmental potential of erythrocyte nuclei injected into oocytes at first meiotic metaphase was greater than those injected into diplotene oocytes. Second, we demonstrate that erythroblast and leukocyte nuclei transplanted to oocytes at first meiotic metaphase promoted more advanced tadpole development than those previously injected into Xenopus eggs. Third, erythrocyte nuclei were more successful in promoting advanced tadpole development compared with erythroblast and leukocyte nuclei. The results show that differentiated somatic nuclei transferred to the cytoplasm of oocytes at first meiotic metaphase display enhanced genomic and developmental potential over those transplanted to diplotene oocytes and eggs, at least for the three nuclear cell types tested from the peripheral blood.

The multiple beta-tubulin genes of Xenopus: isolation and developmental expression of a germ-cell isotype beta-tubulin gene

In this report, we demonstrate the presence of multiple beta-tubulin genes in Xenopus and begin to explore the regulation of isotypes within the beta-tubulin family by focusing on the characterization of a specific beta-tubulin cDNA derived from a Xenopus oocyte library. This clone (XLOT: Xenopus laevis oocyte beta-tubulin) contains the entire protein coding and 3'-untranslated regions of the gene, and is only missing approximately eleven nucleotides from the start of transcription. The XLOT transcript is ubiquitously expressed, but steady-state amounts are highest in immature oocytes and in testes. Consistent with the present understanding of this type of autoregulation, levels of oocyte beta-tubulin transcript vary in accordance with fluctuating polymeric/monomeric tubulin protein ratios both in the developing oocyte and as the late stage oocyte matures to an unfertilized egg. In addition, steady-state levels of the oocyte beta-tubulin transcript do not increase as the total number of cells per embryo increase during embryogenesis. Although one major and three minor transcriptional start sites are utilized in immature oocytes and adult tissues, usage of each individual site varies during oogenesis and embryogenesis. The preferential expression in germ cells indicate that the oocyte beta-tubulin transcript may provide a useful marker for gonadal differentiation in early amphibian development.

Supercoiled loop organization of genomic DNA: a close relationship between loop domains, expression units, and replicon organization in rDNA from Xenopus laevis

Analysis of the organization of a specific chromosomal gene, the gene for rRNA in Xenopus laevis, has evidenced a close relationship between loop organization, replication organization, and expression units. The nontranscribed spacer appears to be involved in all three levels of organization. Furthermore the replication origin region appears to be involved in nuclear matrix anchorage and is closely related to the 40 S transcription promoter. This organization suggests how expression domains may be regulated and how this functional organization may be transmitted to daughter cells after DNA replication, thus allowing selected expression patterns not to be lost during development.

Photocrosslinking of proteins to maternal mRNA in Xenopus oocytes

Ultraviolet irradiation was used to covalently crosslink poly(A) RNA and associated proteins in Xenopus oocytes and reticulocytes. Each cell type contained similar as well as unique crosslinked proteins. The somatic cells contained a single 78-kDa 3' poly(A) tract binding protein while oocyte poly(A), however, was bound by this protein and at least three additional proteins. Based on the mass of poly(A) RNA, oocytes in their earliest stages of growth contained crosslinked proteins that were generally more prevalent than in fully grown oocytes. An investigation of possible messenger RNA-specific proteins was undertaken by a series of RNA injection experiments. Two radiolabeled SP6-derived mRNAs were injected into oocytes; the first, globin mRNA, assembled into polysomes, while the second, a maternal mRNA termed G10, entered a nontranslating ribonucleoprotein compartment. Following the induction of oocyte maturation, additional globin mRNA was recruited onto polysomes while G10 mRNA remained a nontranslating mRNP. The proteins that can be crosslinked to these injected mRNAs were detected by 32P nucleotide transfer. Each mRNA associated with shared as well as unique proteins, some of which were detected only in mature oocytes. The possible function of these proteins is discussed.

Changes in heat shock protein synthesis and hsp70 gene transcription during erythropoiesis of Xenopus laevis

As erythroid cells of Xenopus laevis differentiate, their capacity for transcription and protein synthesis becomes progressively restricted. Mature erythrocytes are thought to be transcriptionally inert and have very low levels of protein synthesis (Maclean et al., 1973, Cell Differ. 2, 261-269). We have examined actual and potential synthetic activity during erythropoiesis by comparing the patterns of protein synthesis and measuring the transcriptional activity of selected genes under ambient and heat shock conditions. As erythropoiesis proceeds, there is a progressive decrease in the number of heat shock proteins (hsps) whose synthesis is induced at elevated temperatures. Despite this repression, mature erythrocytes are able to respond to heat shock with the induction of synthesis of one protein, hsp70. In addition, an hsp70-like protein is synthesized constitutively in orthochromatic erythroblasts, the final erythroid cell stage before maturation into erythrocytes. An in vitro nuclear run-off transcription assay was used to study transcription of the hsp70, beta-globin, and ribosomal genes during erythropoiesis. beta-Globin and ribosomal gene transcription were detected at all stages of erythropoiesis, including the erythrocyte stage. However, transcription of the beta-globin gene relative to ribosomal gene transcription declines substantially in erythrocytes as compared to earlier stages. Hsp70 gene transcription shows both stage-specific and heat shock-dependent transcription. Orthochromatic erythroblasts and erythrocytes have heat shock-dependent hsp70 gene transcription, whereas the gene is transcribed at the earlier erythroblast stages at ambient temperature as well as during heat shock. The constitutive hsp70 transcripts made during these earlier stages may be stored for utilization in constitutive synthesis of the hsp70-like protein in orthochromatic erythroblasts. These data indicate that although erythrocytes become synthetically repressed during differentiation, they are less inert than previously thought. The data also reveal that the synthesis of hsps and related proteins is subject to both transcriptional and post-transcriptional control during erythropoiesis.

Different conformations of ribosomal DNA in active and inactive chromatin in Xenopus laevis

The chromatin structure of the ribosomal DNA in Xenopus laevis was studied by micrococcal nuclease digestions of blood, liver and embryonic cell nuclei. We have found that BglI-restricted DNA from micrococcal nuclease-digested blood cell nuclei has an increased electrophoretic mobility compared to the undigested control. Micrococcal nuclease digestion of liver cell nuclei causes a very slight shift in mobility, only in the region of the spacer containing the "Bam Islands". In contrast, the mobility of ribosomal DNA in chromatin of embryonic cells, under identical digestion conditions, remains unaffected by the nuclease activity. Denaturing gels or ligase action on the nuclease-treated DNA abolishes the differences in the electrophoretic mobility. Ionic strength and ethidium bromide influence the relative electrophoretic migration of the two DNA fragment populations, suggesting that secondary structure may play an important role in the observed phenomena. In addition, restriction analysis under native electrophoretic conditions of DNA prepared from blood, liver and embryonic cells shows that blood cell DNA restriction fragments always have a faster mobility than the corresponding fragments of liver and embryo cell DNA. We therefore propose that nicking activity by micrococcal nuclease modifies the electrophoretic mobility of an unusual DNA conformation, present in blood cell, and to a lesser extent, in liver cell ribosomal chromatin. A possible function for these structures is discussed. The differences of the ribosomal chromatin structures in adult and embryonic tissues may reflect the potential of the genes to be expressed.

Uncoupled synthesis of H1o-like histone H1s during late erythropoiesis in Xenopus laevis

This study investigated the synthesis of Xenopus histones during erythropoiesis. Although cessation of DNA replication in the mid-stages of erythroid maturation is accompanied by arrested synthesis of histone H1 and core histones, synthesis of H1o (an H1o-like histone) was found to continue into late stages of erythropoiesis, as has been reported for avian erythrocyte histone H5. This was accompanied by a threefold increase in the relative amount of Xenopus H1s, similar to the accumulation reported for H5 during avian erythropoiesis and for H1o in some differentiated mammalian cells. The structural and metabolic homologies of avian H5, mammalian H1o, and Xenopus H1s imply that these lysine-rich histones have closely related functions distinct from those of H1, and thus represent a subclass of lysine-rich histones.

Activation of dormant genes in specialized cells

In several experimental systems the genomic capacity in specialized cells can be assessed by examining the activation of dormant genes. Since some of these specialized cells can be induced to change cell phenotype, all cell specializations do not necessarily involve irreversible genetic changes.

Globin gene expression in Xenopus laevis: anemia induces precocious globin transition and appearance of adult erythroblasts during metamorphosis

The expression of Xenopus laevis larval and adult globin genes after phenylhydrazine-induced anemia has been investigated at the cellular and molecular levels by means of cloned cDNA probes specific for the four main larval and the four main adult globin mRNA species. In the circulating blood of anemic metamorphic larvae there are at least two distinct populations of erythroblasts containing either larval or adult globin mRNA sequences. The cells expressing adult sequences replace those expressing larval ones at the end of metamorphosis. Under the influence of anemia the qualitative pattern of transcribed RNA species is not changed, but the larval to adult transition takes place earlier during development. This would imply that this transition is not strongly correlated with the morphological changes of metamorphosis. The abundance of the different larval globin mRNA species is similar and, as compared to control animals, not affected by the phenylhydrazine treatment. In anemic adults no reactivation of larval gene expression has been observed. The different adult globin mRNA species are present in comparable abundance, but the phenylhydrazine treatment appears to enhance expression of the alpha A II and beta A II genes, as compared to nonanemic individuals.

Gene reactivation in erythrocytes: nuclear transplantation in oocytes and eggs of Rana

Adult erythrocyte nuclei of Rana, transplanted and incubated in the cytoplasm of maturing oocytes, direct matured oocytes to form swimming tadpoles. These results demonstrate that nuclei of noncycling and terminally differentiated erythrocytes contain the genes to specify tadpole development, and conditioning these nuclei in the cytoplasm of oocytes leads to a widespread reactivation of dormant genes.

The activation of RNA synthesis by somatic nuclei injected into amphibian oocytes

Previous work has shown that nuclei of cultured cells or erythrocytes are transcriptionally activated when injected into the germinal vesicle of Xenopus oocytes. We now find that the total amount of stable RNA synthesized by an oocyte with injected nuclei is about twice that of an uninjected oocyte (approximately 15 ng/day). At least half of the RNA transcribed by the injected nuclei is low-molecular-weight RNA of discrete sizes, and is transcribed by RNA polymerase III. Part of this is attributable to a new class of gene (referred to as OAX) which makes a transcript about 180 nucleotides long, and which is activated by at least 50-fold in somatic nuclei injected into oocytes. OAX RNA is not a degradation or processing variant of ribosomal or 5 S RNA, and is not coded by mitochondrial DNA. It enters oocyte cytoplasm, but turns over with a half-life of less than 3 hr, as judged by the culture of enucleate oocyte cytoplasms for several days during which 4 and 5 S RNAs are stable. OAX RNA synthesis is not detected by labelling cultured cells or their nuclei. The results emphasize the selectivity with which components of an oocyte germinal vesicle activate genes in injected somatic nuclei.

The reactivation of developmentally inert 5S genes in somatic nuclei injected into Xenopus oocytes

In the frog Xenopus, the somatic-type 5S ribosomal RNA genes are active in all cells; the oocyte-type 5S genes are active in oocytes but not in somatic cells. A new method of native gel electrophoresis resolves the two types of 5S RNA which are of the same length but different sequence. When somatic nuclei were injected into oocytes, their inactive oocyte-type 5S genes often remained inactive, and had thus conserved their developmentally regulated condition. Other conditions, which include treatment with 0.35 M NaCl, reactivated the genes.

Chromatin organization within nuclear blebs in leukocytes of Xenopus laevis

The nuclei from leukocytes of peripheral blood, liver and spleen of an individual anaemic Xenopus laevis have been found to possess numerous nuclear blebs or projections. There structures were found to be very variable in size and shape as viewed in electron micrographs, but commonly included an enclosed mass of cytoplasm bound on one side by a very thin section of nuclear material. Such sections are membrane bounded on each side and frequently display an interesting ordered array of chromatin.