DANON D: POLYRIBOSOMES AND PROTEIN SYNTHESIS DURING RETICULOCYTE MATURATION IN VITRO

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.

FINE STRUCTURE OF DIFFERENTIATING MOUSE PANCREATIC EXOCRINE CELLS IN TRANSFILTER CULTURE

Fine structural observations have been made in the 11-day embryonic mouse of exocrine cells in pancreatic epithelium developing in tissue culture transfilter from salivary gland mesenchyme of the 13-day embryonic mouse. After 2 days in culture, the exocrine cells show increased cytoplasmic density, abundant ribosomes in aggregate or "rosette" form, and expanded profiles of rough-surfaced endoplasmic reticulum. After 3 and 4 days in culture, the cells exhibit continued expansion of the profiles of endoplasmic reticulum, increased amounts of Golgi membranes, and large areas of light density (prozymogen granules). After 5 days in culture, dense zymogen granules are present in the most highly differentiated cells. In addition, at the filter-epithelial surface, at 2 days, small fibers can be discerned which, after 4 days in culture, show obvious periodicity and are thought to be collagen. The significance of these changes, in relation to the mesenchymal effect, to the onset of specific synthesis and to the stabilization of differentiation is discussed.

EFFECTS OF PUROMYCIN ON THE NUCLEOPROTEINS OF THE HELA CELL

The effects of several concentrations of puromycin on the nucleoproteins of HeLa cells grown in monolayers were studied by cytochemical and biochemical techniques. The earliest change at all concentrations of puromycin was a decrease in a granular form of ribonucleoprotein (RNP) that is demonstrable in the normal HeLa cell by the toluidine blue-molybdate (TBM) stain. The other types of RNP revealed by the TBM method were unaltered although the cell volume decreased markedly. Treatment with high concentrations of the antimetabolite resulted in pre-prophase inhibition of mitotic division and led to production of inclusions containing RNP in the cytoplasm; lower concentrations resulted in metaphase arrest. Biochemical analyses confirmed the cytochemical observations and indicated that synthesis of RNA and protein was inhibited to the same extent.

A STRUCTURAL STUDY OF RAT LIVER RIBOSOMES

Polyribosomes, ribosomes, and ribosomal subunits were prepared from rat liver using sodium deoxycholate and a variety of ionic media. They were examined in the electron microscope, mainly as negatively or positively stained preparations, and in the analytical ultracentrifuge. The polyribosomes consist of up to twelve or more ribosomes linked by a fine strand, 10 to 15 A in diameter, probably of RNA. The ribosomes are approximately spherical with diameters of 250 to 300 A, and are estimated to be about 50 per cent porous. Possibly because of their high protein content, whole ribosomes show no cleavage furrows. Ribosomes were dissociated in phosphate buffer and the subunits separated on sucrose density gradients containing 10 per cent formalin. Three classes of subunit were obtained with sedimentation coefficients of 71S, 50S, and 31S respectively. The smallest, 31S subunit is about 250 A long by 100 A wide. The largest subunits appear to be clusters of smaller particles. It is estimated from their linear dimensions in electron micrographs that the whole 83S ribosome could contain up to six 31S subunits, or their equivalent.

AN ANALYSIS OF COLLAGEN SECRETION BY ESTABLISHED MOUSE FIBROBLAST LINES

In vitro synthesis of collagen by established mouse fibroblast lines has been examined by electron microscopy. During rapid growth (log phase), when collagen could not be detected in the cultures, the cells lacked a well developed granular ergastoplasm and Golgi system. Upon cessation of growth (stationary phase), collagen accumulated in the cultures and the cells demonstrated highly developed granular and smooth ergastoplasm. Collagen appeared to be synthesized in the rough-surfaced endoplasmic reticulum and to be transported as a soluble protein to the cell surface by vesicular elements of the agranular ergastoplasm. Fusion of the limiting membranes of these vesicles with the cell membrane permitted the discharge of the soluble collagen into the extracellular space, where fibrils of two diameter distributions formed. The secretion of collagen is concluded to be of the merocrine type. Alternative theories of collagen secretion are discussed and the data for established lines compared with the results of other in vitro and in vivo studies of collagen fibrillogenesis.

ALTERATIONS IN POLYRIBOSOMES DURING ERYTHROID CELL MATURATION

This communication presents a morphological study of the changes in ribosome content and organization which occur during the maturation of erythroid cells of the phenylhydrazine-treated rabbit. Electron micrographs of thin sectioned nucleated and non-nucleated erythroid cells have been subjected to a quantitative analysis of the distribution of ribosomes as polyribosomes of various sizes and as single ribosomes. The ribosomes of nucleated erythroid cells of marrow are virtually all arranged in the polyribosome configuration consisting of clusters of 2 to 6 individual ribosomes. These cells are the most active in the erythroid series in protein biosynthesis. During maturation to the non-nucleated reticulocyte stage, found in the circulating blood, there is a decrease in protein synthesizing capacity, a fall in total ribosome content, and, more significantly, a decrease in the number and size of polyribosomes. Maturation to the ribosome-free erythrocyte, either under in vitro or in vivo conditions, entails a further decrease in protein synthesis which correlates with a progressive disaggregation of the biosynthetically active polyribosomes into smaller clusters and inactive single ribosomes. Possible models which may account for the stability of the polyribosome and for the mechanism of polyribosome dissociation are discussed.

THE ISOLATION AND PROPERTIES OF CARDIAC RIBOSOMES AND POLYSOMES

1. A method is described by which good yields of ribosomes and polysomes free of contamination by submitochondrial fragments can be prepared from rat cardiac muscle. These preparations are capable of incorporation of amino acids into protein in vitro. 2. The ribosome preparation consists of 32% of monomeric ribosomes and 68% of ribosomal aggregates or polysomes. The polysome preparation has a decreased monomeric content. Dimers, trimers, tetramers, pentamers and larger components can be differentiated. 3. The polysome aggregate structure is degraded to monomeric ribosomes on incubation with small amounts of ribonuclease or by preparation in the absence of Mg(2+) ions. The degradation in the absence of Mg(2+) ions was not reversible and drastically decreased the incorporation of amino acids in vitro. 4. The cardiac ribosomes contained two major RNA species sedimenting at 19s and 28s in a 1:2.4 ratio. 5. The RNA/protein ratio of cardiac ribosomes and polysomes was consistently lower than that of similar preparations from liver. The concentrations of Na(+) and K(+) ions present during preparation had a great effect on the RNA/protein ratio. 6. Optimum conditions for the incorporation of amino acids into protein in vitro are reported. Cardiac ribosomes have a lower rate of incorporation of amino acids in vitro than liver ribosomes. 7. Heart cell sap is less active than liver cell sap: evidence is presented that a factor, present in liver cell sap and concerned with stimulating the synthesis of the peptide chain, is lacking in heart cell sap. 8. Pulse-labelling of perfused hearts followed by examination of the subcellular structures showed that the ribosomal fraction was the most active in the incorporation of amino acids in vitro.

WOUND HEALING AND COLLAGEN FORMATION. IV. DISTORTION OF RIBOSOMAL PATTERNS OF FIBROBLASTS IN SCURVY

The changes in scorbutic wounds following the administration of ascorbic acid have been investigated using the techniques of electron microscopy, histochemistry, and autoradioggraphy. Particular attention has been paid to the changes seen in the endoplasmic reticulum of the fibroblasts and to the identity of the extracellular filamentous material characteristic of scorbutic wounds. Seven-day-old wounds in scorbutic guinea pigs were examined prior to and from one to 72 hours following the administration of vitamin C. Fibroblasts from wounds of normal animals demonstrate a characteristic configuration of the ribosomes of the endoplasmic reticulum which is suggested to be analogous to polyribosomes described in cells synthesizing protein such as the reticulocyte. Tangential views of the membranes of the ergastoplasm show the ribosomes to be grouped in paired rows which take both straight and curved paths. This configuration is lost in scurvy and can be seen to begin to reappear as early as 4 hours after giving ascorbic acid. With increasing time, the morphology of the ribosomal aggregates approximates that seen in normal cells, so that by 24 hours their reorientation is complete. It is suggested that one of the disturbances in scurvy may relate to an alteration either in messenger RNA, in the ability of the ribosomes to relate to the messenger, or in the membranes of the ergastoplasm. In addition, the lack of formation of hydroxyamino acids necessary for completing collagen synthesis may be related to the architecture of the ribosomal aggregates. Extracellular collagen fibrils appear concomitant with the restoration of ribosomal and ergastoplasmic morphology as early as 12 hours after administration of ascorbic acid, with complete disappearance of the scorbutic extracellular material within 24 hours. Observations of this scorbutic material do not support the concept that it is a collagen precursor.

Neuronal perikarya with dispersed, single ribosomes in the visual cortex of Macaca mulatta

Numerous small and medium-sized neuronal perikarya in layers III and IV of the visual cortex display an unusual pattern of ribosomal distribution. Instead of being aggregated in clusters, spirals, rows, and other regular polysomal configurations, the ribosomes, whether free or attached to the endoplasmic reticulum, are randomly dispersed, with no discernible pattern. The endoplasmic reticulum in such cells is reduced to a few (perhaps only one) meandering, broad cisternae, which delimit broad fields of cytoplasmic matrix occupied almost solely by scattered, single ribosomes. The Golgi apparatus is elaborate. Mitochondria are either small and numerous or large and infrequent. The other organelles, including the nucleus and nucleolus, are not remarkable. Axonal terminals synapse in the normal fashion on the surfaces of these cells and their dendrites. Associated with these cells are more numerous intermediate cells in which a few to many polysomal clusters can be found. It is proposed that the neurons with dispersed, single ribosomes are inactive in protein synthesis and that the suspension of such an important metabolic activity is probably temporary. Thus, these cells are considered to be part of a population undergoing cyclic fluctuations in the intensity of protein synthesis that should be correlated with their specific neural behavior.

[Inhibition and induction of protein syntheses by actinomycin in the growing oocytes ofMusca domestica]

The protein synthesis of growing oocytes ofMusca domestica L. is investigated by radioautographic and electrophoretic methods on an intermediate developmental stage (stage 3), and after the end of growth (stage 6) in the normal development and after treatment with Actinomycin. The very high RNA synthesis of the nurse cells and of the follicle epithelium of growing oocytes is completely blocked by injection of 16 Μg/g body weight Actinomycin. The protein synthesis in the euplasm of a growing oocyte is relatively high, the stage 6 shows a high protein synthesis in spite of the absence of RNA synthesis. The proteins of the ovary are separated electrophoretically on Cellogel-strips. The ratio of the newly-synthetized proteins is determined after incorporation (30 min) of¹⁴C-labelled amino acids. The protein synthesis is stimulated after 4-6 hours treatment with Actinomycin. After a longer influence of the antibiotic, synthesis of the egg-nurse cell unit decreases to below the control value. The ripe oocyte does not react autoradiographically to Actinomycin. Only some fractions of the newly-synthetized euplasmatic proteins are sensitive to treatment with Actinomycin. After intermediate incubation times with Actinomycin new proteins appear in the ovary and their synthesis is demonstrable in part even after treatment for 48 hours. There is no newly-synthetized RNA in the ripe oocyte, which, nevertheless, reacts with Actinomycin to inhibit some and to stimulate some new protein synthesis. In this case the action of the antibiotic is unknown.

Characteristics of RNA degradation in the erythroid cell

The characteristics of ribonucleic acid (RNA) degradation in intact reticulocytes have been investigated. The rate of degradation during in vitro maturation at 37 degrees C is approximately 4% per hr. RNA degradation does not proceed at 0 degrees C, and the rate of degradation is temperature dependent with an optimum at 50 degrees C. The process is not dependent upon glycolysis. Although all types of RNA progressively decrease during in vivo maturation, ribosomes are degraded at a rate greater than that of soluble RNA.RNA within the cell fluxes freely back and forth between the free and membrane-bound forms. The rate of loss of RNA in the membrane-bound compartment, both in the intact cell and in isolated membranes, is approximately threefold that found in the cell as a whole. During maturation the proportion of total cell RNA which is bound to the membrane decreases. The bulk of RNA destroyed is derived from the free compartment, but bound is also destroyed. The cell membrane participates in RNA metabolism and may be a site of RNA degradation in the erythroid cell.

Polysomes of bovine anterior pituitary gland and their role in hormone and protein biosynthesis

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Disproportional synthesis of the adult duck's two hemoglobins during acute anemia

Concurrent synthesis of two or more hemoglobins occurs in normal man, the human hemoglobinopathies, and certain animal species. Duck erythrocytes produced in response to acutely induced anemic hypoxia (hemolysis or blood loss) contained reciprocally altered proportions of Hb I (alpha(2) (I) beta(2) (I)) and Hb II (alpha(2) (II) beta(2) (II)); the relative proportion of Hb II was 50-100% increased. Relative rates of synthesis of the two hemoglobins remained proportional to their new concentrations throughout erythroid maturation. This information favors the proposal that relatively increased activity, not delayed decay, of biosynthetic processes responsible for net synthesis of Hb II had occurred. These studies support the concept that the individual biosyntheses of multiple hemoglobins, presumably under genetic control, are potentially manipulable, and they provide evidence for one mechanism leading in a reproducible fashion to alterations in net synthesis in vivo.

Molecular aspects of lens cell differentiation

I have presented a series of observations on macromolecular interactions which occur during the terminal stages of lens cell differentiation. These are summarized in Fig. 2. Other cell types that undergo similar changes are the erythrocyte and skin cells (epidermis) during the process of keratinization. These other cells are also involved in the synthesis of highly specific proteins, and there are indications that molecular alterations similar to those described for the lens may also occur in these cells (26). Thus, elucidation of a specific series of macromolecular initeractions such as those described may provide a basis for the biochemical definition of the terminal stages of cellular differentiation. Differentiation of the reticulocyte, for example, involves inactivation of the nucleus, stabilization of mRNA, and possibly a ribosomal breakdown such as I have described here (26). Furthermore, elucidation of the mechanisms of reactions involving the initiation of tissue-specific protein synthesis and suLbsequent nuclear inactivation, stabilization of mRNA, and breakdown of the ribosomes may provide a basis for defining the mechanisms of terminal cellular differentiation. The lens cell has reached its highest form of cellular differentiation when it has formed the fiber cell. With respect to the mechanism of lens fiber cell formation, we would like to know whether specific biochemical changes such as gamma-crystallin synthesis are intiniately linked to fiber cell formation-that is, whether gamma-crystallins are required to bring about the formation of a fiber cell. The potential for synthesizing gamma-crystallins is inherent in the genome of the cell. This part of the genome is nonfunctional in the epithelial cell. Can these genes be activated without bringing about a simultaneous cellular elongation, nuclear inactivation and loss of cellular replication, stabilization of mRNA, and breakdown of the ribosomes? The degree of coupling or uncoupling of tissue-specific-protein synthesis to morphogenesis is an important part of the mechanism of cellular differentiation. We feel that we have now reached the stage where we can begin to answer these questions.