ALTERATIONS IN POLYRIBOSOMES DURING ERYTHROID CELL MATURATION

Human erythrocytes' perplexing behaviour: erythrocytic microRNAs

Erythrocytes have the potential role in erythropoiesis and disease diagnosis. Thought to have lacked nucleic acid content, mammalian erythrocytes are nevertheless able to function for 120-140 days, metabolize heme, maintain oxidative stress, and so on. Mysteriously, erythrocytes proved as largest repositories of microRNAs (miRNAs) some of which are selectively retained and function in mature erythrocytes. They have unique expression patterns and have been found to be linked to specific conditions such as sickle cell anaemia, high-altitude hypoxia, chronic mountain sickness, cardiovascular and metabolic conditions as well as host-parasite interactions. They also have been implicated in cell storage-related damage and the regulation of its survival. However, the mechanism by which miRNAs function in the cell remains unclear. Investigations into the molecular mechanism of miRNAs in erythrocytes via extracellular vesicles have provided important clues in research studies on Plasmodium infection. Erythrocytes are also the primary source of circulating miRNAs but, how they affect the plasma/serum miRNAs profiles are still poorly understood. Erythrocyte-derived exosomal miRNAs, can interact with various body cell types, and have easy access to all regions, making them potentially crucial in various pathophysiological conditions. Which can also improve our understanding to identify potential treatment options and discovery related to non-invasive diagnostic markers. This article emphasizes the importance of erythrocytic miRNAs while focusing on the enigmatic behaviour of erythrocytes. It also sheds light on how this knowledge may be applied in the future to enhance the state of erythrocyte translational research from the standpoint of erythrocytic miRNAs.

Red Blood Cells as Potential Repositories of MicroRNAs in the Circulatory System

The amount of erythrocyte-derived microRNAs (miRNAs) represents the majority of miRNAs expressed in whole blood. miR-451, miR-144, and miR-486, which are abundant in red blood cells (RBCs), are involved in the process of erythropoiesis and disease occurrence. Moreover, erythrocyte-derived miRNAs have been reported to be potential biomarkers of specific diseases. However, the function and underlying mechanisms of miRNAs derived from erythrocytes remain unclear. Based on a review of previously published literature, we discuss several possible pathways by which RBC miRNAs may function and propose that RBCs may serve as repositories of miRNAs in the circulatory system and participate in the regulation of gene expression mainly via the transfer of miRNAs from erythrocyte extracellular vesicles (EVs). In the whole blood, there are still other important cell types such as leukocytes and platelets harboring functional miRNAs, and hemolysis also exists, which limit the abundance of miRNAs as disease biomarkers, and thus, miRNA studies on RBCs may be impacted. In the future, the role of RBCs in the regulation of normal physiological functions of the body and the entire circulatory system under pathological states, if any, remains to be determined.

Cellular dynamics of mammalian red blood cell production in the erythroblastic island niche

Red blood cells, or erythrocytes, make up approximately a quarter of all cells in the human body with over 2 billion new erythrocytes made each day in a healthy adult human. This massive cellular production system is coupled with a set of cell biological processes unique to mammals, in particular, the elimination of all organelles, and the expulsion and destruction of the condensed erythroid nucleus. Erythrocytes from birds, reptiles, amphibians and fish possess nuclei, mitochondria and other organelles: erythrocytes from mammals lack all of these intracellular components. This review will focus on the dynamic changes that take place in developing erythroid cells that are interacting with specialized macrophages in multicellular clusters termed erythroblastic islands. Proerythroblasts enter the erythroblastic niche as large cells with active nuclei, mitochondria producing heme and energy, and attach to the central macrophage via a range of adhesion molecules. Proerythroblasts then mature into erythroblasts and, following enucleation, in reticulocytes. When reticulocytes exit the erythroblastic island, they are smaller cells, without nuclei and with few mitochondria, possess some polyribosomes and have a profoundly different surface molecule phenotype. Here, we will review, step-by-step, the biophysical mechanisms that regulate the remarkable process of erythropoiesis with a particular focus on the events taking place in the erythroblastic island niche. This is presented from the biological perspective to offer insight into the elements of red blood cell development in the erythroblastic island niche which could be further explored with biophysical modelling systems.

Slowed decay of mRNAs enhances platelet specific translation

Platelets are anucleate cytoplasmic fragments that lack genomic DNA, but continue to synthesize protein using a pool of messenger RNAs (mRNAs), ribosomes, and regulatory small RNAs inherited from the precursor megakaryocyte (MK). The regulatory processes that shape the platelet transcriptome and the full scope of platelet translation have remained elusive. Using RNA sequencing (RNA-Seq) and ribosome profiling of primary human platelets, we show the platelet transcriptome encompasses a subset of transcripts detected by RNA-Seq analysis of in vitro-derived MK cells and that these platelet-enriched transcripts are broadly occupied by ribosomes. We use RNA-Seq of synchronized populations of in vitro-derived platelet-like particles to show that mRNA decay strongly shapes the nascent platelet transcriptome. Our data suggest that the decay of platelet mRNAs is slowed by the natural loss of the mRNA surveillance and ribosome rescue factor Pelota.

Translational control mediated by hnRNP K links NMHC IIA to erythroid enucleation

Post-transcriptional regulation is crucial for structural and functional alterations in erythropoiesis. Enucleation of erythroid progenitors precedes reticulocyte release into circulation. In enucleated cells, reticulocyte 15-lipoxygenase (r15-LOX, also known as ALOX15) initiates mitochondria degradation. Regulation of r15-LOX mRNA translation by hnRNP K determines timely r15-LOX synthesis in terminal maturation. K562 cells induced for erythroid maturation recapitulate enucleation and mitochondria degradation. HnRNP K depletion from maturing K562 cells results in enhanced enucleation, which even occurs independently of maturation. We performed RIP-Chip analysis to identify hnRNP K-interacting RNAs comprehensively. Non-muscle myosin heavy chain (NMHC) IIA (also known as MYH9) mRNA co-purified with hnRNP K from non-induced K562 cells, but not from mature cells. NMHC IIA protein increase in erythroid maturation at constant NMHC IIA mRNA levels indicates post-transcriptional regulation. We demonstrate that binding of hnRNP K KH domain 3 to a specific sequence element in the NMHC IIA mRNA 3'UTR mediates translation regulation in vitro Importantly, elevated NMHC IIA expression results in erythroid-maturation-independent enucleation as shown for hnRNP K depletion. Our data provide evidence that hnRNP-K-mediated regulation of NMHC IIA mRNA translation contributes to the control of enucleation in erythropoiesis.

THE RELATIONSHIP BETWEEN RNA SYNTHESIS AND HEMOGLOBIN SYNTHESIS IN AMPHIBIAN ERYTHROPOIESIS : Cytochemical Evidence

A cytochemical study of the relationship between RNA synthesis and hemoglobin synthesis has been performed on splenectomized newts, Triturus viridescens. Employing radioautography, labeled cytidine was incorporated into the RNA of the early developmental stages but was not incorporated in the later stages. Labeled leucine was incorporated into the cellular protein of all stages except mature erythrocytes but was incorporated at a higher level in the later stages. Microphotometric measurements of azure B binding to cytoplasmic RNA revealed a sharp initial increase between the stem cell and proerythroblast followed by a rapid decrease between the basophilic and polychromatophilic stages. The loss of cytoplasmic RNA became more gradual in the late stages and, in the mature erythrocyte, little or no cytoplasmic RNA could be detected. Measurements of cytoplasmic total protein, using fast green staining at pH 2.0, and of heme showed that both curves increased similarly with development, indicating net hemoglobin synthesis. The results are compatible with the hypothesis that, as the stem cell differentiates along erythrocytic lines, a stable "messenger" RNA specifying the production of a given type or types of hemoglobin is formed. This complex probably becomes associated with ribosomal RNA and is retained throughout the process of RBC differentiation.

LOCALIZATION OF ANTIBODIES BY ELECTRON MICROSCOPY IN DEVELOPING ANTIBODY-PRODUCING CELLS

The development of antibody-producing cells in the early stages of the secondary or hyperimmune response has been studied with the electron microscope in lymph nodes of adult chinchilla rabbits immunized with ferritin or apoferritin. The intracellular distribution of antiferritin antibodies was determined in the lymph node cells at 1 to 5 days after a booster injection, employing the labelling technique previously used by the authors (12) to demonstrate the localization of antibodies in mature plasma cells. Antibodies were first detected at 48 hours in blasts; i.e., cells which have a poorly developed endoplasmic reticulum and a cytoplasm filled with many ribosomes grouped in clusters. The label was subsequently found in forms intermediate between blasts and plasma cells (plasmoblasts, immature plasma cells), in which the endoplasmic reticulum appeared progressively more developed. Antiferritin antibodies were also found in cells in mitosis. In all the above cell types, antigen-antibody precipitates were consistently found in the perinuclear space and in the cisternae of the granular endoplasmic reticulum, from an early stage in the development of the latter. Evidence was also obtained for the presence of antibody in the Golgi area. The results are discussed in relation to the possible cellular sites of antibody synthesis.

MicroRNA in erythrocytes

Mammalian erythrocytes are generally thought to lack RNA and therefore to be unable to translate new proteins in response to internal or external signals. Support for this long-standing view has accumulated from diverse studies, most of which have focused on the total content of RNA or the overall level of translation. However, more recent work on specific types of RNA has shown the presence in human erythrocytes of both Y RNA and microRNA. The latter seem particularly incongruous given that their normal role is to attenuate the translation of mRNA. Y RNA binds the Ro autoantigen which may have a role in cellular RNA quality control. Therefore the presence of both of these non-coding RNAs indicates the possible existence of other cryptic RNAs in erythrocytes. It also suggests either the existence of low levels of translation or new uncharacterized processes involving microRNA in these cells.

The role of human ribosomal proteins in the maturation of rRNA and ribosome production

Production of ribosomes is a fundamental process that occurs in all dividing cells. It is a complex process consisting of the coordinated synthesis and assembly of four ribosomal RNAs (rRNA) with about 80 ribosomal proteins (r-proteins) involving more than 150 nonribosomal proteins and other factors. Diamond Blackfan anemia (DBA) is an inherited red cell aplasia caused by mutations in one of several r-proteins. How defects in r-proteins, essential for proliferation in all cells, lead to a human disease with a specific defect in red cell development is unknown. Here, we investigated the role of r-proteins in ribosome biogenesis in order to find out whether those mutated in DBA have any similarities. We depleted HeLa cells using siRNA for several individual r-proteins of the small (RPS6, RPS7, RPS15, RPS16, RPS17, RPS19, RPS24, RPS25, RPS28) or large subunit (RPL5, RPL7, RPL11, RPL14, RPL26, RPL35a) and studied the effect on rRNA processing and ribosome production. Depleting r-proteins in one of the subunits caused, with a few exceptions, a decrease in all r-proteins of the same subunit and a decrease in the corresponding subunit, fully assembled ribosomes, and polysomes. R-protein depletion, with a few exceptions, led to the accumulation of specific rRNA precursors, highlighting their individual roles in rRNA processing. Depletion of r-proteins mutated in DBA always compromised ribosome biogenesis while affecting either subunit and disturbing rRNA processing at different levels, indicating that the rate of ribosome production rather than a specific step in ribosome biogenesis is critical in patients with DBA.

In vitro maturation of nascent reticulocytes to erythrocytes

Most studies of mammalian reticulocyte maturation have used blood reticulocytes.Nascent reticulocytes, as found in bone marrow, have not been available in developmentally synchronized populations. Nascent murine reticulocytes formed in vitro by enucleation of Friend virus–infected erythroblasts were purified and recultured for 110 hours. At 0 hours, all recultured cells were lobulated and contained dense, centralized reticulin. By 110 hours, about 20% to 25% of the cells became biconcave erythrocytes. Most ribosomes and cellular RNAs were degraded within 20 hours, and during that period, heme synthesis declined from a rate equal to that of late erythroblasts to less than 10% of that rate. Many mitochondria appeared normal until they showed outer membrane swelling, degradation, and apparent fusion with intracellular vacuoles at 40 hours of culture. During the period of mitochondrial loss, Bcl-XL, an antiapoptotic protein that accumulates during erythroblast differentiation and maintains mitochondrial membrane integrity, demonstrated progressive decreases and changes consistent with deamidation. Nevertheless, the reticulocytes did not undergo apoptosis, because their apoptotic machinery was degraded. This experimental system that provides a developmentally synchronized population of nascent murine reticulocytes that mature into biconcave erythrocytes in vitro should be useful in further investigations of the cellular events involved in reticulocyte maturation.

Inhibition of 15-lipoxygenase leads to delayed organelle degradation in the reticulocyte

Mammalian cells are characterized by an endomembrane system. Nevertheless, some cells lose these membranes during their terminal differentiation, e.g. red blood cells and lens fiber cells of the eye. 15-Lipoxygenase is believed to be critical for this membrane degradation. Here we use cultivated rabbit reticulocytes in the presence or absence of a lipoxygenase inhibitor to provide further evidence for the importance of 15-lipoxygenase for the in vivo degradation of mitochondria. We find that inhibitor treatment retarded mitochondrial degradation, as shown by persistence of marker proteins and by direct visualization of mitochondria by electron microscopy.

CORRECTION OF POLYRIBOSOME DISTRIBUTIONS AS OBSERVED IN CELL SECTIONS BY ELECTRON MICROSCOPY

Clusters of ribosomes observed by electron microscopy in thin sections of rabbit reticulocytes are of the same order of size as the section thickness of 600 A. Many of the observed clusters must therefore have been transected by the section surfaces and observed as clusters containing fewer ribosomes. A probability method of correcting for this effect is given. Comparison of the results with grid observations of ribosome distributions indicates sufficiently good agreement for application to cell section observations.

Interaction of acidic liposomes with red blood cells. Induction of endocytosis and shedding of particles

Calcium ions induced tight binding of massive amounts of liposomes containing cardiolipin, phosphatidyl-ethanolamine and phosphatidylcholine to erythrocytes. Initially, liposome-liposome fusion occurred and only subsequently the resulting large structures adhered to cells. Large clumps composed of liposomes and cells were formed. Upon prolonged incubation, the clumps were dissipated spontaneously and excess liposomes were released. A constant amount of phospholipid (15-25 nmol/10(8) cells) were incorporated into the cell membranes. Upon disaggregation, the cells shed erythrocyte particles. The latter were isolated and shown to contain lipids from both cellular and liposomal origin. The particles lacked spectrins and contained variable amounts of band 3 content. Liposomes induced endocytosis in reticulocytes but not in mature erythrocytes. In most cases, the liposomes themselves were not engulfed by the cells and remained attached to their surface. The relevance of this phenomenon to delivery to liposome contents into cells is discussed.

Protein-synthesizing machinery in the growing oocyte of the cyclic mouse. A quantitative electron microscopic study

A quantitative ultrastructural evaluation of the oocyte ribosomal population was carried out during the oocyte growth, bearing in mind that this period of the mouse oogenesis displays the greatest activity of ribosomal RNA synthesis. At the onset of growth almost 3/4 of the oocyte ribosomes exist as singles, these become polysomal ribosomes as growth progresses. At the same time the number of ribosomes increases. Once the major growth period has elapsed, the number of ribosomes starts to decrease just when lattice-like structures exhibiting a periodic organization begin to accumulate in the oocyte cytoplasmic matrix. Evidence, like the particulate organization of these lattices, the size of their particles, its digestion by RNase, and the time of the lattice appearance, together with data reported by several authors, allows one to suggest that near the end of the oocyte growth a great part of the ribosomes are stored in the lattices to be used during early development.

Concanavalin A-induced endocytosis in rabbit reticulocytes, and its decrease with reticulocyte maturation

Concanavalin A (Con A) was taken up to a limited extent by endocytosis in rabbit reticulocytes but not in rabbit erythrocytes. This process was observed by the use of ferritin-labeled Con A and transmission electron microscopy of thin sections of plastic-embedded cells. Furthermore, the extent of endocytosis among the reticulocytes decreased with the extent of their maturation, reticulocyte age being measured by ribosome configurations. These results are consistent with the proposal that there are domains in the membranes of reticulocytes in which the Con A receptors are laterally mobile, and can be clustered and endocytosed. These mobile domains exist, or are formed, within a larger framework of immobile membrane. During reticulocyte maturation, these domains are gradually eliminated, eventually disappearing upon formation of the mature erythrocyte. Possible molecular mechanisms for this proposed elimination process are discussed.

The synthesis and degradation of presumptive messenger RNA in cultured mouse leukemia cells during the inhibition of protein synthesis

RNA synthesis in mouse leukemia L5178Y cells was inhibited depending upon the time of treatment by blasticidin S or by ricin, which inhibits specifically protein synthesis. When blasticidin S or ricin blocked protein synthesis by more than 90% of the control, marked accumulation of monosome was accompanied by the decrease of pulse-labeled RNA (20% of that in the control) in the polysomes and monosome fraction. The size distribution of pulse-labeled RNA among polysomal fractions including monosome obtained from the cells treated with either blasticidin S, ricin of L-asparaginase showed that the size of presumptive mRNA was shifted from 18 S to 9--10 S. TReatment of a blasticidin S-resistant (Bla-R) subline derived from L5178Y cells (Kuwano, M., Matsui, K., Takenaka, K., Akiyama, S. and Endo, H. (1977) Int. J. Cancer 20, 296--302) with L-asparaginase or ricin induced smaller size (9--10 S) RNA, but treatment of Bla-R cells with blasticidin S did not. Such shorter RNA fragments could not be observed even when cellular protein synthesis was inhibited by treatment for short time with blasticidin S (40--80% of the control activity). Smaller RNA fragments accumulated after drastic inhibition of protein synthesis were composed of 74% of polyadenylate sequence lacking poly(A)(-)RNA with peak of approx. 10 S and 26% of polyadenylate sequence containing poly(A)(+)RNA with a peak of 18 S, whereas cytoplasmic polysomal RNA of the control contained 46% poly(A)(+) with a peak of 18 S and 54% poly(A)(-)RNA with a 10--18 S peak. Cytoplasmic poly(A)(+)RNA degraded biphasically with half-lives of approx. 2 h and 8--10 h in exponentially growing mouse cells. However, in degradation of poly(A)(+)RNA molecules being formed in the cells pretreated with blasticidin S for 3 h, the rapid phase of decay with a half-life of approx. 2 h was interrupted by successively appearing poly(A)(+)RNA with a longer half-life of 8--10 h in cytoplasm. However, when the cells were pretreated with blasticidin S for 6 h, there appeared no poly(A)(+)RNA population with the rapid-decay in cytoplasm.

Intracellular distribution of ribonuclease activity during erythroid cell development

Five ribonuclease activities, separable by polyacrylamide gel electrophoresis, have been detected in erythroid bone marrow cells from anaemic rabbits. Their intracellular distribution has been investigated and compared with that of the ribonucleases in reticulocytes. Both the acid and alkaline ribonuclease activities of reticulocytes are much lower (30--50 fold) than those of bone marrow erythroid cells. The most marked decrease in enzyme activity occurs in the fractions containing ribosomes and mitochondria plus lysosomes. In these subcellular organelles there was also a qualitative change in the ribonuclease electrophoretic pattern, whereas the cytosol enzymes of marrow erythroid cells and reticulocytes remained largely unchanged. Several ribonucleases released from reticulocyte membranes with urea were similar to those present in the lysosomal plus mitochondrial fraction, as shown by detection of enzyme activity after polyacrylamide gel electrophoresis. The decline in ribonuclease activity was found to begin in the orthochromatic cells, which have a highly condensed nucleus and are no longer active in DNA and RNA synthesis, and to coincide with a decrease in acid phosphatase activity and loss of lysosomes.

The morphology of erythroid cells separated by density gradient centrifugation through Ficoll

The regenerating blood of geese injected with phenylhydrazine was subjected to large scale, zonal centrifugation through density gradients of Ficoll. In this way, erythroid cells were fractionated according to their respective stages of development. Highly enriched fractions were obtained, containing cells that were well preserved as assessed by both light and electron microscopy. The separated cells exhibited ribosome density and nucleic acid and protein staining patterns typically associated with erythrocyte differentiation. Morphometric analysis of nuclei indicated that despite an apparent net increase in the amount of compact chromatin during development, comparatively little difference existed between the volumes of condensed chromatin present in immature and mature cells. Instead, there was a three fold decrease in nuclear volume between young erythroblasts and reticulocytes, coupled with a concomitant decrease in the volume occupied by dispersed chromatin, RNP and nucleoli. These observations are discussed in relation to molecular changes associated with nuclear differentiation in erythroid cells.

Biosynthesis and stability of globin mRNA in cultured erythroleukemic Friend cells

Biosynthesis and stability of the mRNA population in DMSO-induced Friend erythroleukemic cells were studied after labeling the RNA with 3H-uridine and then chasing it with nonlabeled uridine. Globin RNA metabolism was studied by hybridization to excess complementary DNA convalently coupled to oligo(dT)-cellulose. After a labeling period of 120 min, 2-4% of the poly(A)-containing labeled RNA was in globin RNA; it decayed with a half-life of 16-17 hr. The rest of the poly(A)-containing RNA was composed to two kinetic populations: 85-90% decayed with a half-life of about 3 hr, while 10% decayed with a half-life of about 37 hr. The portion of globin RNA in labeled poly(A)-containing RNA behaved in an unexpected fashion during the chase period. During the initial chase period, the percentage of globin RNA increased rapidly, reaching a maximum of about 15% at 20 hr, but it subsequently declined gradually. Based on these findings, a model was built that describes the changes in the proportion of globin mRNA in poly(A)-containing RNA during continuous synthesis and after chase of the labeled RNA. It appears that if the parameters described remain constant during the maturation of erythroblasts, then this model would not account for the almost exclusive presence of globin RNA in the reticulocyte. By far the most effective way to achieve this high level of globin RNA is the destabilization of the mRNA population which is more stable than globin RNA, and not the stabilization of globin RNA itself.

Ultrastructural studies of the bone marrow in sickle cell anaemia. II. The morphology of erythropoietic cells and their response to deoxygenation in vitro

Electron microscopic studies of bone marrow aspirates obtained from patients with homozygous sickle cell anaemia (HbSS) were fixed immediately without attempts to deoxygenate the samples. Erythroblasts and normoblasts in these preparations were devoid of haemoglobin polymers or other indications of sickling. Furthermore, the nucleated erythroid cells from sickle-cell patients presented an ultrastructural morphology indistinguishable from that of identically-processed erythroid cells in marrow samples from normal human volunteers. This report presents a description of the ultrastructural features of pronormoblasts and normoblasts in normal and sickle-cell marrows and stresses the essentially normal appearance of nucleated erythroid elements in sickle cell anaemia. Exposure of sickle-cell marrow aspirates to nitrogen at 37 degrees C for 30 min resulted in haemoglobin polymerization in most erythrocytes and reticulocytes but only in 10-20% of the nucleated erythroid cells. Haemoglobin polymers in the form of intertwining fibre meshworks were observed in reticulocytes, orthochromatic and polychromatophilic normoblasts, but were absent in basophilic normoblasts and pronormoblasts. The results suggest that the concentration of haemoglobin in intramedullary normoblasts may be the limiting factor determining the predisposition of these cells to undergo sickling as well as the pattern of haemoglobin aggregation. Under the physiological conditions prevailing in the marrow, haemoglobin concentration in normoblasts may be insufficient to result in aggregation and polymerization.

Electron microscopic study of erythrocytes in developing rainbow trouts, Salmo gairdnerii irideus, with particular reference to changes in the cell line

The fine structure of the first erythroid cells in the embryonic circulation of the rainbow trout and new erythroid cells appearing after hatching was examined by scanning and transmission electron microscopy. The first erythroid cells in the embryos, where circulation has just begun, are very immature in ultrastructural characteristics; they are spherical, relatively large, and contain a large nucleus with diffuse chromatin and numerous free ribosomes mostly occurring as polyribosomes. The cells divide in the circulatory system. As development proceeds, the entire population undergoes a synchronous maturation, i.e., transformation into round and flat discs, progressive heterochromatization, reduction in number of cytoplasmic organelles, especially of ribosomes, and a concomitant increase of hemoglobin content. The fully mature first erythroid cells persist until after hatching and are gradually replaced by a new population of erythroid cells which begin to appear in the circulatory system within a few days after hatching. The new cells are at first small and spherical, and show immature ultrastructural features, but no dividing cells were encountered. The cytological changes accompanying maturation are almost parallel to those seen in the first class of erythroid cells. In the circulatory system of fry, where yolk absorption has just been completed, almost all the erythroid cells are mature. These cells are uniformly flat and elliptical discs. The differences in ribosome numbers in electron micrographs of erythroid cells from different stages of development are also consistent with the existence of two series of erythroid cell maturation in the peripheral blood of the developing rainbow trout.

Hemoglobin synthesis in murine virus-induced leukemic cells in vitro: stimulation of erythroid differentiation by dimethyl sulfoxide

Cells of a cloned line of murine virus-induced erythroleukemia were stimulated to differentiate along the erythroid pathway by dimethyl sulfoxide at concentrations that did not inhibit growth. A rise in the number of benzidine-positive normoblasts was accompanied by increased synthesis of heme and hemoglobin and a decrease in the malignancy of the cells. This action of dimethyl sulfoxide, which was reversible, may represent the derepression of leukemic cells to permit their maturation.

Erythroid cell RNase: activation by urea and localization to the cell membrane

Assays of ribonuclease activity in components of mature and immature mammalian erythroid cells indicate that RNase activity is present both in the membrane-free hemolysate and the washed membranes. Erythroid cell RNase exists in an active and latent form. The majority of total cell RNase activity is in the latent state, and is localized to the erythroid cell membrane. Both total and latent RNase activity decline as the cell matures. The latent RNase is released from its relatively firm attachment to the cell membrane and activated by centrifugation or, optimally, by exposure to 4 M urea. The active sites of membrane-associated RNase are apparently oriented toward the inner side of the cell membrane. The properties of the latent membrane-bound RNase which is activated by urea, including K(m), pH optimum, inhibition of enzyme activity by cations, and response to metabolic inhibitors, do not differ significantly from those of the soluble RNase in the membrane-free hemolysate, suggesting that there is only one type of RNase in the erythroid cell. Binding of Rnase to the erythroid cell membrane stabilized the enzyme against inactivation during incubation at 37 degrees C, and the findings suggest that membrane-bound RNase may play a particular part in degrading ribosomes. The findings indicate that the cell membrane has a major role in RNA metabolism in the maturing mammalian erythroid cell.