Synchronization of the cell cycle of Tetrahymena by stravation and refeeding

Absolute quantification of chromosome copy numbers in the polyploid macronucleus of Tetrahymena thermophila at the single-cell level

Amitosis is widespread among eukaryotes, but the underlying mechanisms are poorly understood. The polyploid macronucleus (MAC) of unicellular ciliates divides by amitosis, making ciliates a potentially valuable model system to study this process. However, a method to accurately quantify the copy number of MAC chromosomes has not yet been established. Here we used droplet digital PCR (ddPCR) to quantify the absolute copy number of the MAC chromosomes in Tetrahymena thermophila. We first confirmed that ddPCR is a sensitive and reproducible method to determine accurate chromosome copy numbers at the single-cell level. We then used ddPCR to determine the copy number of different MAC chromosomes by analyzing individual T. thermophila cells in the G1 and the amitotic (AM) phases. The average copy number of MAC chromosomes was 90.9 at G1 phase, approximately half the number at AM phase (189.8). The copy number of each MAC chromosome varied among individual cells in G1 phase and correlated with cell size, suggesting that amitosis accompanied by unequal cytokinesis causes copy number variability. Furthermore, the fact that MAC chromosome copy number is less variable among AM-phase cells suggests that the copy number is standardized by regulating DNA replication. We also demonstrated that copy numbers differ among different MAC chromosomes and that interchromosomal variations in copy number are consistent across individual cells. Our findings demonstrate that ddPCR can be used to model amitosis in T. thermophila and possibly in other ciliates.

An Optimized and Versatile Counter-Flow Centrifugal Elutriation Workflow to Obtain Synchronized Eukaryotic Cells

Cell synchronization is a powerful tool to understand cell cycle events and its regulatory mechanisms. Counter-flow centrifugal elutriation (CCE) is a more generally desirable method to synchronize cells because it does not significantly alter cell behavior and/or cell cycle progression, however, adjusting specific parameters in a cell type/equipment-dependent manner can be challenging. In this paper, we used the unicellular eukaryotic model organism, Tetrahymena thermophila as a testing system for optimizing CCE workflow. Firstly, flow cytometry conditions were identified that reduced nuclei adhesion and improved the assessment of cell cycle stage. We then systematically examined how to achieve the optimal conditions for three critical factors affecting the outcome of CCE, including loading flow rate, collection flow rate and collection volume. Using our optimized workflow, we obtained a large population of highly synchronous G1-phase Tetrahymena as measured by 5-ethynyl-2'-deoxyuridine (EdU) incorporation into nascent DNA strands, bulk DNA content changes by flow cytometry, and cell cycle progression by light microscopy. This detailed protocol can be easily adapted to synchronize other eukaryotic cells.

Bacterial Surface Traits Influence Digestion by Tetrahymena pyriformis and Alter Opportunity to Escape from Food Vacuoles

Endosymbiotic interactions are frequently found in nature, especially in the group of protists. Even though many endosymbioses have been studied in detail, little is known about the mechanistic origins and physiological prerequisites of endosymbiont establishment. A logical step towards the development of endocytobiotic associations is evading digestion and escaping from the host's food vacuoles. Surface properties of bacteria are probably involved in these processes. Therefore, we chemically modified the surface of a transformant strain of Escherichia coli prior to feeding to Tetrahymena pyriformis. N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide allows any substance carrying amino- or carboxyl groups to be bound covalently to the bacterial surface by forming a peptide bond, thus, altering its properties biochemically and biophysically in a predictable manner. The effect of different traits on digestion of T. pyriformis was examined by fluorescence and transmission electron microscopy. The efficiency of digestion differs considerably depending on the coupled substances. Alkaline substances inhibit digestion partially, resulting in incomplete digestion and slightly enhanced escape rates. Increasing hydrophobicity leads to much higher escape frequencies. Both results point to possible mechanisms employed by pathogenic bacteria or potential endosymbionts in evading digestion and transmission to the host's cytoplasm.

Ingestion and digestion studies in Tetrahymena pyriformis based on chemically modified microparticles

Recognition of food and, in consequence, ingestion of digestible particles is a prerequisite for energy metabolism in Tetrahymena pyriformis. Understanding why some particles are ingested and digested, whereas others are not, is important for many fields of research, e.g. survival of pathogens in single-celled organisms or establishment of endosymbiotic relationships. We offered T. pyriformis synthetical bovine-serum-albumin (BSA)-methacrylate microparticles of approximately 5.5 μm diameter and studied the ciliates' ingestion and digestion behaviour. Different staining techniques as well as co-feeding with a transformant strain of Escherichia coli revealed that T. pyriformis considers these particles as natural food source and shows no feeding preference. Further, they are ingested at normal rates and may serve as sole food source. A pivotal advantage of these particles is the convenient modification of their surface by binding different ligands resulting in defined surface properties. Ingestion rate of modified microparticles either increased (additional BSA, enzymes) or decreased (amino acids). Furthermore, we investigated glycosylation patterns by lectin binding. By binding different substances to the surface in combination with various staining techniques, we provide a versatile experimental tool for elucidating details on food recognition and digestion that may allow to study evading digestion by pathogens or potential endosymbionts, too.

Developmental progression of Tetrahymena through the cell cycle and conjugation

The ciliate Tetrahymena thermophila can be said to undergo a variety of developmental programs. During vegetative growth, cells coordinate a variety of cell-cycle operations including macronuclear DNA synthesis and a-mitotic fission, micronuclear DNA synthesis and mitosis, cytokinesis and an elaborate program of cortical morphogenesis that replicates the cortical organelles. When starved, cells undergo oral replacement, transformation into fast-swimming dispersal forms or, when encountering cells of a complementary mating type, conjugation. Conjugation involves a 12 hour program of meiosis, mitosis, nuclear exchange and karyogamy, and two postzygotic divisions of the fertilization nucleus. This chapter reviews experimental data exploring the developmental dependencies associated with both vegetative and conjugal development.

Divider size and the cell cycle after prolonged starvation ofTetrahymena corlissi

Cell growth and division of the ciliateTetrahymena corlissi were examined upon refeeding after prolonged starvation of up to 12 days. Division did not automatically occur when a certain critical cell size was reached. Rather, it varied both with the nutritional history of the cell and the nutrient conditions in which the cell was growing. Upon refeeding, cells starved for 12 days divided at a smaller size and later than cells starved for 6 days. Cells refed at high density took longer to begin division than cells refed at low density. The results are discussed with respect to the "relative starvation" and "critical constituent" models of the cell cycle and in terms of the polymorphic life cycle ofTetrahymena species.

β-Glucan biosynthesis in synchronous cells of Prototheca zopfii

Synchronous cultures of the colorless chlorophyte Prototheca zopfii Kruger were obtained by a shift-down of the basal medium plus addition of 1.5 mM ethylenediaminetetracetic acid for a 10-14-h period, followed by periodic dilutions with the basal medium. The cell cycle has a duration of 10-12 h at 25°C and an average of eight autospores are produced at the end of each cycle. The incorporation of [(14)C]glucose into β-glucans was determined in vivo. Very high incorporation occurs between hours 3 and 4 of culture, coincident with maximal cell expansion. The incorporation of glucose from uridine 5'diphospho-[(14)C]glucose into lipid-linked sugars and glycoproteins, as well as the activity of guanosine 5'-diphosphoglucose:β-glucan synthase found in cell-free preparations are in good agreement with the in-vivo incorporation of glucose into β-glucans.

Regulation of ribosome synthesis in Tetrahymena pyriformis. 2. Coordination of synthesis of ribosomal proteins and ribosomal RNA during nutritional shift-up

The addition of nutrients to long-time-starved cells of Tetrahymena pyriformis leads to a 50-60-fold increase in the rate of synthesis of ribosomal proteins (r-proteins). This is achieved by a 6-fold increase in the relative rate of r-protein synthesis and a 8-10-fold increase in the rate of total protein synthesis. Synthesis of r-proteins constitutes one third of total cellular protein synthesis 2-4 h after refeeding and the absolute rate of r-protein synthesis is approximately three-times greater than in exponentially growing cells. The synthesis of the individual r-proteins is coordinately regulated during a nutritional shift-up, and de novo synthesized r-proteins are stable. Addition of actinomycin D prevents the increase in the rate of r-protein synthesis. The rates of synthesis of rRNA and r-protein increase in concert, implying coordinate regulation. Furthermore, a comparison of the observed accumulation of r-proteins with the predicted accumulation based on the accumulation of rRNA suggests that rRNA and r-protein are synthesized in a stoichiometrically balanced way during the entire refeeding period.

Respiration rates in heterotrophic, free-living protozoa

Published estimates of protozoan respiratory rates are reviewed with the object of clarifying their value in ecological studies. The data show a surprisingly large variance when similarly sized cells or individual species are compared. This is attributed to the range of physiological states in the cells concerned. The concept of basal metabolism has little meaning in protozoa. During balanced growth, energy metabolism is nearly linearly proportional to the growth rate constant; at the initiation of starvation, metabolic rate rapidly declines. Motility requires an insignificant fraction of the energy budget of protozoans. For growing cells, metabolic rate is approximately proportional to weight(0.75) and the data fall nearly exactly on a curve extrapolated from that describing the respiration rates of poikilotherm metazoans as a function of body weight. It is conceivable that protozoan species exist with lower maximum potential growth and metabolic rates than those predicted from cell volume and the equations derived from the available data. However, the lack of information concerning the state of the cells studied prevents verification of this idea. Laboratory measurements of protozoan respiratory rates have no predictive value for protozoa in nature other than delimiting a potential range. For small protozoans, this range may, on an individual basis, represent a factor of 50.

Dolichyl phosphate phosphatase from Tetrahymena pyriformis

A soluble dolichyl phosphate phosphatase from Tetrahymena pyriformis was purified about 68-fold. The enzyme appeared to be specific for dolichyl phosphate and existed in two interrelated forms, one of mol.wt. about 500000 and the other of mol.wt. about 63000. The enzyme was strongly inhibited by 5 mM-Mn2+ and was strongly stimulated by Mg2+. Tetrahymena in the exponential growth phase contained more of this enzymic activity than cells in stationary or lag phase. The dolichyl phosphate phosphatase may be loosely bound to mitochondrial membranes. Two roles proposed for this enzyme are (1) that of releasing dolichol from its phosphorylated biosynthetic form for its use in the cell as unesterified dolichol or dolichyl ester and/or (2) that of regulation of synthesis of glycoproteins or some other glycosylated compound.

Selective release of HMG nonhistone proteins during DNase digestion of Tetrahymena chromatin at different stages of the cell cycle

The possible role of LG-1, a Tetrahymena specific HMG protein found in the macronuclear chromatin (Hamana, K. and Iwai, K. (1979) J. Biochem. 86, 789-794), was examined in relation to the chromatin structure. The chromatin isolated from cells synchronized at different stages of the cell cycle contained about one molecule of LG-1 per nucleosome. Limited digestion of the chromatin with DNase I or micrococcal nuclease selectively released LG-1 with the nucleosomal core histones and H1 remained insoluble, bound to the resistant DNA. Depending on the cell stages several types of chromatin structure were distinguished by their nuclease sensitivity. However, the chromatin at different stages exhibited the similar behavior of the LG-1 release with the nucleases as a function of the degree of chromatin solubilization. The results suggest that LG-1 proteins play a role in the chromatin organization which is rather independent of the cell stages.

Intracellular pH changes during the cell cycle in Tetrahymena

The equilibrium distribution of 5,5-dimethyloxazoladine 2,4-dione (DMO) between intra- and extracellular volume was used to estimate intracellular pH (pHi) in Tetrahymena pyiformis. In control experiments, DMO was found to equilibrate rapidly in response to a pH gradient. Under normal growth conditions, pHi was constant over a finite range of external pH, being maintained near pH 7.1 over the external pH range 5.2 to 7.3. This same range of external pH was also optimal for growth. pHi was monitored during the cell cycle of a synchronous population of T. pyriformis GL. The cells were synchronized either by starvation/refeeding or heat shock. Under both conditions, there were two alkaline shifts of approximately 0.4 pH units per cell cycle. These shifts in pH retained a constant remporal relationship to S phase and were not affected by changes in the time, duration, or magnitude of cytokinesis.

Induction of division synchrony in Tetrahymena pyriformis by a single hypoxic shock. Its use in elucidating control of the cell cycle by adenosine 3':5'-monophosphate

A single hypoxic shock was used to induce division synchrony in Tetrahymena pyriformis. Hypoxia results in accumulation of the cells in the G2 phase of the cell cycle. Cyclic AMP and adenyl cyclase activity were measured in this system. Cell-cycle blockade was associated with extraordinarily high levels of intracellular cyclic AMP. After release from hypoxia, the cells retain a characteristic pattern of modulation of cyclic AMP associated with division that is found in selection-synchronized cells. The results are discussed with reference to other methods of induction synchrony and related studies on the natural cell cycle in this organism.

Reconstruction of oral cavity of Tetrahymena pyriformis utilizing high voltage electron microscopy

Reconstruction of the oral cavity from 17 0.5 mum thick serial sections observed with a high voltage electron microscope (JEM-1000, operated at 1000 kV) has enabled us to describe the in situ shape of the cavity, the orientation of the membranelles, oral ribs, cytostomal lip and forming food vacuole of Tetrahymena pyriformis. The study also showed that many different sets of microtubules encircle the oral cavity forming an interwoven, basket-like structure around the cavity thus providing it with considerable structural rigidity. By correlating results obtained from the reconstruction with results obtained from scanning electron microscopy and freeze-fracturing we have been able to elucidate the probable mechanism of how food particles are propelled into and through the oral cavity.

Effect of cell population density on G2 arrest in Tetrahymena

The ciliated protozoan, Tetrahymena pyriformis strain GL-C, has been used to study the effect of cell population density during starvation on the synchrony obtained after refeeding and on the number of cells arrested in G2 phase of the cell cycle. At high cell densities two peaks of division indices were observed after refeeding while only one was observed at low cell densities. Cell division began earlier in cultures starved at high cell densities. Most importantly, the proportion of cells in G2 was considerably higher in populations starved at high cell densities. When tritiated thymidine was present during the refeeding period, radioautographs of cell samples at different times showed that the first cells to exhibit division furrows contained unlabeled nuclei. The first peak in the division index after refeeding was observed only at higher cell densities and is attributed to the cells arrested in G2. These results suggest that Tetrahymena is an excellent organism to study the concept of resting stages in the cell cycle and their control.

Ciliary membrane differentiations in Tetrahymena pyriformis. Tetrahymena has four types of cilia

We have examined thin sections and replicas of freeze-fractured cilia of Tetrahymena pyriformis. The ciliary necklace located at the base of all freeze-fractured oral and somatic cilia has been studied in thin sections. Since electron-dense linkers have been found to connect both microtubule doublets and triplets to the ciliary membrane at the level of the necklace, the linkers and the associated necklace seem to be related to the transition region between the doublets and triplets of a cilium. Plaque structures, consisting of small rectangular patches of particles located distal to the ciliary necklace, are found in strain GL, but are absent in other strains examined in this study. In freeze-cleaved material, additional structural differentiations are observed in the distal region of the ciliary membranes of somatic and oral cilia. Somatic cilia contain many randomly distributed particles within their membrane. Oral cilia can be divided into three categories on the basis of the morphology of their freeze-fractured membranes: (a) undifferentiated cilia with very few randomly distributed particles: (b) cilia with particles arranged in parallel longitudinal rows spaced at intervals of 810-1080 A that are located on one side of the cilium; and (c) cilia with patches of particles arranged in short rows oriented obliquely to the main axis of the cilium. The latter particles, found on one side of the cilium, seem to serve as attachment sites for bristles 375-750 A long and 100 A wide which extend into the surrounding medium. The particles with bristles are located at the tips of cilia in the outermost membranelle and may be used to detect food particles and/or to modify currents in the oral region so that food particles are propelled more efficiently into the buccal cavity. Examination of thin-sectioned material indicates that the particles in oral cilia which form the longitudinal rows could be linked to microtubule doublets. Linkage between microtubule doublets and adjacent membrane areas on one side of the cilium could modify the form of ciliary beat by restricting the sliding of the microtubules. It is suggested that membrane-microtubule interactions may form the basis for the various forms of ciliary beat observed in different organisms.

Induction of nucleolar and mitochondrial DNA replication in Tetrahymena pyriformis

Selective induction of replication of nucleolar and mitochondrial DNA has been demonstrated in starved-refed cultures of Tetrahymena pyriformis by different techniques. Labeling of starved cells with [(3)H]thymidine during a nutritional shift-up and analysis of the DNA in isopycnic CsCl gradients shows that the two initially labeled species of DNA are two species banding on the heavy and light side of the bulk macronuclear DNA. In isolated macronuclei the radioactivity is found only in the high density fraction, which has been shown to be of nucleolar origin. In sucrose gradients the newly replicated mitochondrial and nucleolar DNAs sediment considerably slower than the bulk DNA, as one discrete band corresponding to a molecular weight of about 3 to 4 x 10(7). Electron microscope autoradiography of cells labeled with [(3)H]thymidine as above shows that the peripheral nucleoli of the macronucleus as well as the mitochondria are labeled before any radioactivity is found in the chromatin granules of the macronucleus. The results clearly indicate that nucleolar and mitochondrial DNA replication are under a control independent of that for the replication of bulk DNA.

Growth and cloning of Tetrahymena pyriformis on solid meduium

A method of obtaining clones of Tetrahymena pyriformis on solid medium has been developed. The medium consists of a basal layer of 1.5% agar topped with 2 ml of 0.3% agar in sterile, plastic petri plates (100 by 15 mm). Both agar layers contain either 2% proteose peptone and 0.1% liver extract (complex medium) or defined medium supplemented with proteose peptone. After drying, 0.5 ml of liquid culture is spread evenly over the top agar, and the plates are then sprinkled lightly and evenly with autoclaved dry Sephadex G-25 (fine). Cell colonies can be observed after 5 days of incubation either by viewing with a microscope or without the aid of a microscope after staining. Plating efficiency is high on either complex or defined medium with a number of strains of Tetrahymena, both micronucleate and amicronucleate. Colonies can be picked and transferred to liquid culture for further growth. The existence of clones was demonstrated by plating a mixture of two different drug-resistant mutants. The method should prove useful in selective procedures for the isolation of mutants and for determining survival after treatments such as ultraviolet irradiation.