Control of haemoglobin synthesis: distribution of ribosomes on the messenger RNA for alpha and beta chains

Over 60 Years of Experimental Hematology (without a License)

I am deeply honored to receive the International Society for Experimental Hematology (ISEH) 2020 Donald Metcalf Lecture Award. Although I am not a physician and have had no formal training in hematology, I have had the privilege of working with some of the top hematologists in the world, beginning in 1970 when Dr. David Nathan was a sabbatical visitor in my laboratory and introduced me to hematological diseases.  And I take this award to be given not just to me but to an exceptional group of MD and PhD trainees and visitors in my laboratory who have cloned and characterized many proteins and RNAs important for red cell development and function. Many of these projects involved taking exceptionally large risks in developing and employing novel experimental technologies. Unsurprisingly, all of these trainees have gone on to become leaders in hematology and, more broadly, in molecular cell biology and molecular medicine. To illustrate some of the challenges we have faced and the technologies we had to develop, I have chosen several of our multiyear projects to describe in some detail: elucidating the regulation of translation of α- and β-globin mRNAs and the defect in beta thalassemia in the 1970s; cloning the Epo receptor and several red cell membrane proteins in the 1980s and 1990s; and more recently, determining the function of many microRNAs and long noncoding RNAs in red cell development. I summarize how we are currently utilizing single-cell transcriptomics (scRNAseq) to understand how dividing transit-amplifying burst-forming unit erythroid progenitors balance the need for more progenitor cells with the need for terminally differentiated erythroid cells, and to identify drugs potentially useful in treating Epo-resistant anemias such as Diamond Blackfan anemia. I hope that the lessons I learned in managing these diverse fellows and projects, initially without having grants to support them, will be helpful to others who would like to undertake ambitious and important lines of research in hematology.

Translational control of protein synthesis: the early years

For the past fifty-five years, much of my research has focused on the function and biogenesis of red blood cells, including the cloning and study of many membrane proteins such as glucose and anion transporters and the erythropoietin receptor. We have also elucidated the mechanisms of membrane insertion, folding, and maturation of many plasma membrane and secreted proteins. Despite all of this work and more, I remain extremely proud of our very early work on the regulation of mRNA translation: work on bacteriophage f2 RNA in the 1960s and on translation of α- and β-globin mRNAs in the early 1970s. Using techniques hopelessly antiquated by today's standards, we correctly elucidated many important aspects of translational control, and I thought readers would be interested in learning how we did these experiments.

Historical review: an energy reservoir for mitosis, and its productive wake

The realization that DNA is replicated during only a discrete part of interphase in most cells gave birth to the cell-cycle concept exactly half a century ago. But soon thereafter, a controversy arose as to whether a special reservoir of metabolic energy, built up at the end of interphase, was essential for the entry of cells into mitosis. After a period of intense debate, the 'energy reservoir' concept came to a sudden demise. But the notion that mitotic progression involves the cyclic build-up and consumption of key factors endured and, as we all now know so well, eventually triumphed.

Control of cell-free protein synthesis by amino acids: effects on tRNA charging

In order to resolve the observation that addition of glutamine and glutamate appears to be of particular importance in enhancing the activity of a cell-free protein synthesis system derived from rat liver (Manchester and Tyobeka, 1980), we have measured the KM of the aminoacyl-tRNA synthetases towards amino acids and the extent of aminoacylation of tRNA under the conditions of our earlier experiments. During incubation of the cell-free system in the presence of an amino acid mixture the extent of acylation to tRNA of 15 amino acids studied showed no clear change from initial time values. When incubation took place in the absence of added amino acids, however, the levels of glutamate and glutamine bound to their appropriate tRNAs dropped more rapidly and to lower levels than for other amino acids except tryptophan. The pronounced drop for these two amino acids does not seem to result from an abnormally high KM value for the synthetases towards the respective amino acids, nor an abnormally low Vmax, but probably from the fact that the amounts of glutamyl and glutaminyl-tRNA in the cell-free system are comparatively low.

Increased rates of polypeptide chain elongation in placental explants from human diabetics

Average rates of polypeptide chain elongation were determined in placental explants of first trimester and term placentas from both normal and diabetic human pregnancies. Average ribosome half-transit times were determined by measuring the kinetics of transfer of labeled polypeptides from polysomal-bound to released polypeptides. The average half-transit time decreases from 75 sec per ribosome in first trimester explants to 56 sec per ribosome in term placentas. The average polypeptide molecular weights synthesized by explants from first trimester and in term are 49,300 and 49,600, respectively, which are not significantly different. The average elongation rates for first trimester and term placental explants are 172 and 231 amino acids per minute per ribosome, respectively, which are significantly different. Moreover, the average polypeptide molecular weight synthesized by term placentas from diabetic pregnancies is 48,200, while the average ribosome half-transit time is 40 sec. Thus, ribosomes from explants of term placenta from diabetics move along the average message at a much higher speed than do ribosomes in normal term tissue. The assembly rate of amino acid into polypeptide in explant of placenta of diabetic mothers is 314 amino acids per minute, which is significantly faster than 231 amino acids per minute in normal term tissue. These findings indicate that during placental development and in diabetic pregnancy there is a large change in the actual rates at which amino acids are added to the nascent polypeptide chain--i.e., the rates in polypeptide chain elongation. Therefore, translation-level regulation of protein synthesis in placenta plays a significant part in the magnitude of the response to developmental and other physiological stimulations.

Variable rate of polypeptide chain elongation in vitro. Effect of spermidine

Translation of rabbit globin mRNA and tobacco mosaic virus mRNA was studied in a wheat germ system. After a short pulse with labeled methionine, the reaction products were separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and visualized by fluorography. Discrete bands corresponding to incomplete products were detected both with globin and tobacco mosaic virus RNA as messenger at a time when no release of peptides from tRNA could be detected. In the absence of spermidine certain intermediate products accumulated during the chase period. With increasing incubation time, the radioactivity of most bands was chased into higher-molecular-weight products. This change in size distribution was more pronounced when magnesium was partially replaced by spermidine. Evidence is presented that some of the incomplete products had in part been released from tRNA. No such release of polypeptides could be detected in experiments using the artificial messenger poly(U), indicating that non-specific peptidyl-tRNA hydrolases were not present in the system. The transient accumulation of discrete bands during the chase period indicates that in the wheat germ system polypeptide chain elongation occurs at discontinuous rates. The fact that spermidine increases the rate of elongation and the yield of full-length translation products may be due to its ability to facilitate the movement of the ribosomes beyond regions where the elongation is retarded.

The initiation of fetal hemoglobin biosynthesis

Biosynthesis of the alpha and beta chains of rabbit and human adult hemoglobin is initiated with a methionyl residue, which is removed during elongation of the peptide chain. To study the initiation of biosynthesis of the delta chain of human fetal hemoglobin, fresh placental blood was used for labeling experiments with radioactive amino acids. Labeled nascent peptide chains were purified from the polysomal fraction of placental blood reticulocytes. The number of amino acid residues in nascent gamma chain at the time of removal of its N-terminal methionine was estimated to be 40--60 from the relative yields of labeled tryptic peptides.

Patterns of globin chain synthesis in erythroid colonies grown from sheep marrow of different developmental stages

Erythroid colonies were grown from fetal sheep bone marrow at different stages of development and from adult marrow. Colony numbers increased with erythropoietin concentration, but fetal erythroid cells were more sensitive to the hormone than adult cells. Haemoglobin synthesis was characterized in the colonies and compared to that in control marrow incubations. No beta chain synthesis was detectable in marrow incubations or erythroid colonies from fetal marrow at 87-96 d gestation. Incubations of 115-120 d marrow shoed 5% beta chain synthesis while erythroid colonies synthesised up to 20% beta chains. Incubations and erythroid colonies from 125 d marrows showed about 27% beta chain synthesis, despite a similar increase in colony numbers with erythropoietin as the other marrows. Later in gestation, beta chain synthesis was lower in erythroid colonies than in marrow incubations and declined further in erythroid colonies as erythropoietin concentration increased. Adult marrow showed no detectable gamma chain synthesis but erythroid colonies produced up to 12% gamma chains, the identity of which was confirmed by peptide mapping. All changes in globin synthesis in vitro were correlated with colony numbers. In adult marrow, the maximal level of gamma chain production was inversely related to the cloning efficiency of the marrow samples studied. Potential uses of this model for studying globin gene expression are discussed.

Models for mRNA translation: theory versus experiment

Three models for mRNA translation are discussed in the light of available experimental data. It is concluded that the elongation rates vary along a messenger, possibly as a result of a coupling between ribosome movement and mRNA secondary structure. Some promising areas of further experimentation are indicated.

Studies on the structure of rat-liver polysomes by small-angle X-ray scattering

The spatial structure of rat liver polysomes in dilute solution has been studied by the small-angle X-ray scattering technique. The distance between the mass centres of neighbouring ribosomes on the mRNA molecule is 35 nm, and the distance from next-but-one neighbour, about 55 nm. The spatial arrangement of the ribosomes on the polysome is compact; stretched configurations can be ruled out. The best model is a solenoid with six ribosomes per turn and a pitch of about 33 nm. The greatest diameter of a polysome with eight ribosomes is 130 nm, the radius of gyration 34 nm.

Hemoglobin Riyadh--alpha2beta2 (120(GH3)Lys replaced by Asn). A new variant found in association with alpha-thalassemia and iron deficiency

On a field trip toSaudi Arabia (M.A.F.E.H.) in which the relationship between alpha-thalassemia and iron deficiency was studied, a fast moving hemoglobin variant was noted in a 30 year old Saudi Arabian woman. Analysis of the hemoglobin variant showed that the amino acid substitution was beta120 Lys replaced by Asn. This variant had not been described previously and has been named Hb Riyadh. There was also present an alpha-thalassemia and details are given of the imbalance of globin chain synthesis. It was possible to improve considerably the balance in vitro by the addition of hemin.

Control of haemoglobin synthesis. The effects of iron deprivation, cobalt and temperature on the rate and extent of globin synthesis in reticulocytes

A detailed examination of the kinetics of protein synthesis in rabbit reticulocytes in the presence of the iron chelating agent 2,2'-dipyridyl showed that between 30 degrees C and 42 degrees C there were characteristically two distinct phases of protein synthesis. An initial phase (I), in which no inhibition of protein synthesis was apparent, was followed by a gradual decline in the rate of protein synthesis leading to the second phase (II) in which protein synthesis occurred at a linear but inhibited rate for extended periods. In contrast, below 30 degrees C, incubation in the presence of dipyridyl caused no inhibition of protein synthesis. Between 30 degrees C and 42 degrees C the duration and amount of protein synthesis occurring in phase I before the onset of inhibition were inversely related of the inhibition as was the final rate of incorporation in phase II. During phase II, a partial reversal of the inhibition caused by dipyridyl was obtained by lowering the incubation temperature. This resulted in a burst of protein synthesis at the uninhibited rate until the amount of protein synthesis reached the same level as that in reticulocytes maintained continuously with dipyridyl at the lower incubation temperature. This burst of synthesis was observed in reticulocytes which had been held in phase II for as long as 90 min. It was also possible to reverse the inhibition by addition of haemin to cells in phase II. At any particular incubation temperature, a fixed number of rounds of protein synthesis had to occur before the onset of phase II became apparent. By the use of puromycin we showed that this was not a requirement for the synthesis of globin or of any other protein. We believe that this critical amount of protein synthesis reflects the residual ability of reticulocytes to initiate new protein chains in the absence of concurrent haem synthesis. Reticulocytes preincubated in the presence of cobaltous ions showed almost no inhibition of protein synthesis upon subsequent incubation with dipyridyl. The results are compared to those obtained in reticulocyte lysates and are discussed in terms of current theories to account for control of protein chain initiation by haemin.

Homozygous haemoglobin D Punjab

A homozygote for the gene controlling Hb D Punjab is described. The diagnosis is supported by the peptide analysis of the haemoglobin and the examination of both parents. There was no anaemia or reticulocytosis and there was balanced synthesis of both the alphaA- and betaD-globin chains in reticulocytes. However, the oxygen affinity of the haemolysate had a small but significantly higher oxygen affinity than normal. The only other abnormalities were a somewhat decreased osmotic fragility and a slight anisocytosis of the red cells.

Erythraemia due to haemoglobin San Diego

A 63-year-old man with erythraemia was found to have a high affinity haemoglobin which was subsequently identified as Hb San Diego beta 109 (G11) Val leads to Met, recently described in a Filipino family (Nute et al, 1974). The oxygen affinity of the haemolysates was increased approximately two-fold at pH values between 6.04 and 7.90. The n value was nearly normal (2.5-2.9) in the upper range of saturation (70-95% oxygenation) but was somewhat diminished (1.9-2.1) below this range. In vitro biosynthesis of globin by reticulocytes showed that there was balanced synthesis of alpha and beta chains (alpha:beta ratio = 1).

Specific cyanylation and cleavage at cysteine-104 human hemoglobin alpha-chain. A novel approach to the problem of the alpha-chain tryptic core in the study of haemoglobin variants by "fingerprinting" methods

1. A new approach to the analysis, by "fingerprinting", of the tryptic core region of human haemoglobin alpha-chain is described. 2. The alpha-chain is cyanylated at its single cysteine residue (alpha104) and then split, by exposure to mild alkali, at the N-peptide bond of the resulting beta-thiocyanoalanine residue. 3. The two cleavage fragments, alpha1-103 and alpha104-141, are separated by gel filtration, and the fragment alpha104-141, which contains all the residues of the alpha-chain tryptic core, is digested with pepsin. 4. Preparative "fingerprints" of these peptic peptides yield eight major peptides, which provide complete sequence information for the whole region alpha104-141. 5. The utility of the method is demonstrated by repeating the determination of the substitution in haemoglobin Hopkins-2, a known alpha-chain core variant in which histidine-alpha112 (G19) is replaced by an aspartic acid residue.

Delayed removal of N-terminal methionine from nascent globin chains in sickle-cell anemia reticulocytes

Synthetic alphaT1 and betaT1, the N-terminal tryptic peptides of alpha-chain and beta-chain of hemoglobin, and MetalphaT1 and MetbetaT1, peptides in which N-terminal methionyl residues are peptide-bonded to alphaT1 and betaT1, were prepared by the solid-state method of Merrifield. These synthetic peptides were used to establish conditions for chromatographic purification and analysis. When tryptic digests of nascent globin chains from rabbit and sickle-cell anemia reticulocytes incubated with (35)S- and (3)H-labeled amino acids were analyzed, radioactivity not present in tryptic digests of labeled hemoglobin appeared at the elution positions of MetalphaT1 and MetbetaT1. The fraction of nascent chains with N-terminal methionine was higher in sickle-cell anemia reticulocytes than in rabbit reticulocytes. If rate of peptide-chain elongation in polysomes is uniform, nascent human chains must attain a greater length before removal of the initial methionyl residue. Length of nascent chain at time of removal was calculated from two independent sets of data, one obtained from [(35)S]methionine incorporation into MetalphaT1, MetbetaT1, alphaT5, and betaT5, and the other obtained from [(3)H]lysine and [(3)H]valine incorporation into MetbetaT1 and betaT1.

Specific protein synthesis in cellular differentiation. Production of eggshell proteins by silkmoth follicular cells

Silkmoth follicles, arranged in a precise developmental sequence within the ovariole, yield pure and uniform populations of follicular epithelial cells highly differentiated for synthesis of the proteinaceous eggshell (chorion). These cells can be maintained and labeled efficiently in organ culture; their in vitro (and cell free) protein synthetic activity reflects their activity in vivo. During differentiation the cells undergo dramatic changes in protein synthesis. For 2 days the cells are devoted almost exclusively to production of distinctive chorion proteins of low molecular weight and of unusual amino acid composition. Each protein has its own characteristic developmental kinetics of synthesis. Each is synthesized as a separate polypeptide, apparently on monocistronic messenger RNA (mRNA), and thus reflects the expression of a distinct gene. The rapid changes in this tissue do not result from corresponding changes in translational efficiency. Thus, the peptide chain elongation rate is comparable for chorion and for proteins synthesized at earlier developmental stages (1.3-1.9 amino acids/sec); moreover, the spacing of ribosomes on chorion mRNA (30-37 codons per ribosome) is similar to that encountered in other eukaryotic systems.

Protection of T4 bacteriophage against inactivation during freezing and thawing by addition of peptides

Peptides of rabbit globin produced by tryptic digestion, were found to be highly protective against inactivation of freeze-thawed T4 phage. In concentrations of about 10(-3) M the peptides protected the phage against inactivation by both concentrated NaBr in the unfrozen aqueous phase and the eutectic phase change.Fractionation of the peptides by G25 Sephadex showed that peptide concentration rather than peptide size was the more important factor in determining the degree of protection of the phage by electrolyte effects. In contrast, protection against eutectic injury was strongly dependent on peptide size.Possible mechanisms of action of the protective peptides are discussed.

Translation of -globin m-RNA in -thalassemia and the S and C hemoglobinopathies

Genetic and biochemical evidence indicates that in beta-thalassemia there is impaired synthesis of the beta-globin chains of hemoglobin A. In patients heterozygous for the hemoglobinopathies, hemoglobin S and hemoglobin C, the mutant beta-chain is produced in smaller amounts than normal beta(A). Defective m-RNA translation has been suggested as a possible cause of decreased beta-globin polypeptide synthesis in thalassemia and the hemoglobinopathies. In the present study, the ribosomal assembly of beta-globin chains was examined in the peripheral, nucleated red blood cells and reticulocytes of patients with Cooley's anemia, thalassemia intermedia, sickle thalassemia, sickle cell anemia, hemoglobin C disease, and in hemolytic anemias not associated with a hemoglobinopathy. The translation times of beta(A), beta(S), and beta(C) did not differ significantly (average times; beta(A) = 75 sec, range 43-114, beta(S) = 69 sec, beta(C) = 92 sec). In thalassemia, no evidence was found for a delay in translation as the cause of the marked impairment of beta-globin synthesis. In several specimens of peripheral blood from thalassemic patients, the translation time of the beta-chain was even shorter than in nonthalassemic specimens (average time = 45 sec, range 35-59). The results suggest that the defect in beta-globin synthesis in beta-thalassemia is due to impaired initiation of beta-globin chain assembly or a quantitative deficiency in m-RNA.