Differential sensitivity to 5-bromodeoxyuridine during the S phase of synchronized myogenic cells

Reduced G-protein-coupled-receptor kinase 2 activity results in impairment of osteoblast function

Rapid phosphorylation of many G-protein-coupled receptors (GPCRs) by G-protein-coupled receptor kinases (GRKs) accompanies stimulus-driven desensitization. Recent evidence suggests that GRKs and their associated arresting proteins, beta-arrestins, function as essential elements in the GPCR-mediated mitogen-activated protein (MAP) kinase signaling cascade. We investigated the interaction between GRKs and MAP kinase activation by growth factors in UMR 106-H5 osteoblastic cells stably expressing a dominant negative mutant of GRK2 (K220R). Expression of K220R in osteoblastic cells results in reduced cellular proliferation, both basally and in response to insulin-like growth factor-1 (IGF-1), and blunting of IGF-1- and EGF-induced MAP kinase activation. Reduced MAP kinase activation is not associated with alterations in IGF-1-receptor autophosphorylation. Both a constitutively active Ras mutant and PMA fully activate MAP kinase in K220R cells. We found that disruption of the GRK2 gene results in: (1) reduced osteoblast proliferation in response to growth factors, and (2) impaired receptor tyrosine kinase activation of mitogenic signaling pathways. Thus, GRK2 may regulate growth factor responsiveness in osteoblasts by modulating multiprotein complex formation following receptor tyrosine kinase activation.

During secondary myotube formation, primary myotubes preferentially absorb new nuclei at their ends

Developing muscles contain at least two types of myoblasts. Early myoblasts are the first myoblast to form and are the only myoblasts present during primary myotube formation. By the time secondary myotube formation begins, early myoblasts are rare and late myoblasts are common. The late myoblasts have been postulated to give rise to secondary myotubes. While this is generally accepted, it is unclear whether late myoblasts also contribute to the growth of primary myotubes. One study has produced evidence that myoblasts present during secondary myogenesis selectively fuse with each other or with secondary myotubes, but not with primary myotubes (Harris et al. [1989a] Development 107:771-784). However, the sizes of primary myotubes increase during secondary myotube formation. We have therefore re-examined the question of whether primary myotubes absorb new nuclei during secondary myotube formation. Pregnant rats were given a single intraperitoneal injection of 5 mg of 5-bromodeoxyuridine (BrdU) on one embryonic day (from E13 to E19) and their embryos removed on E20. The brominated-nuclei were labelled with an antibody to BrdU and the myotubes were marked with anti-myosin antibodies. Double labelled sections from the soleus, tibialis anterior, and extensor digitorum longus muscles were examined with a confocal microscope. The numbers and locations of labelled nuclear profiles in primary and secondary myotubes were counted and recorded. The results show: (1) that primary myotubes absorb nuclei at all stages of development, including the period of secondary myotube formation; (2) that in the early stages of secondary myotube formation, more myoblasts fuse with primary than secondary myotubes whereas this situation is reversed by the end of secondary myotube formation; and (3) that the nuclei added to primary and secondary myotubes during the early stages of their formation are located within the middle of E20 muscles. The nuclei added to growing myotubes are preferentially located at the ends of the muscles.

Temporal differences in desmin expression between myoblasts from embryonic and adult chicken skeletal muscle

Desmin expression by myoblasts cultured from embryonic and adult chicken breast muscle was examined employing indirect immunofluorescence. The study was performed in conjunction with [3H]thymidine autoradiography and analysis of skeletal myosin expression in order to determine whether the desmin-expressing cells were terminally differentiated. Following 2 h of labeling with [3H]thymidine, 0.55%, 2.60%, and 15.10% of the cells in mass cultures from 10-day-old embryos, 18-day-old embryos and adults, respectively, incorporated [3H]thymidine and were desmin-positive but did not express skeletal-muscle-specific myosin. Using the same approach we determined that 0.07%, 1.25%, and 7.59% of the mononucleated cells in myogenic clones from 10-day-old embryos, 18-day-old embryos and adults, respectively, were desmin-positive, myosin-negative, [3H]thymidine-positive. We suggest that these desmin-positive, myosin-negative myoblasts are proliferating cells, and we conclude that the progeny of adult myoblasts exhibit more desmin-expressing cells of this type than embryonic myoblasts do.

Histones synthesized at different stages of myogenesis are differentially degraded in myotube cells

We recently reported that cultures of terminally differentiating myotube cells synthesize histones in reduced but significant amounts in comparison with proliferating myoblasts (Wunsch et al., 1987, Dev. Biol., 119: 85-93). In this study, the stability of myotube histone has been determined, comparing the degradation of de novo-synthesized histones in nascent (day 3) and maturing (day 4) myotubes with histones in the same cells that had been previously made during myoblast proliferation (day 1). Histones synthesized in proliferating myoblasts and myotubes were pulse-labeled with 3H-lysine and chased up to seven days, followed by determinations of radioactivity remaining in histone bands using fluorography of one- and two-dimensional polyacrylamide gels. Considered in aggregate, core histones synthesized de novo in nascent (day 3) myotubes were degraded most rapidly, followed by myotube histones that had been previously made during the proliferative phase (day 1) of myogenesis. De novo-synthesized histones in maturing (day 4) myotubes were relatively stable. Individual histone classes were degraded in the following order of increasing half-life, regardless of the differentiative stage at which they were synthesized: H2A.Z, H2A, H2B, H3(.2, day 1; .3, days 3 and 4), H4.

The initial accumulation of carbamoylphosphate synthetase in embryonic rat hepatocytes, and the cell cycle

The hormone-induced expression of the hepatocyte-specific enzyme carbamoylphosphate synthetase can take place in each phase of the cell cycle and is not restricted to the G1 or the G0 phase. To arrive at this conclusion, the cell cycle parameters of embryonic day 14 rat hepatocytes in vitro were determined by autoradiography after labeling with (3H)-TdR or with (3H)- and (14C)-TdR. An S-phase of approximately 14 h, a G2 + M-phase of 8 h, a G1-phase of 8-13 h and a total cell cycle of 30-35 h were measured. Freshly isolated embryonic hepatocytes have exponential growth parameter values, but shift to a steady state growth under culture conditions in the presence of hormones (glucocorticosteroids, thyroid hormones and cyclic AMP). The length of the S-phase and of the total cell cycle remain constant during the culture time. The time course of accumulation of carbamoylphosphate synthetase protein in embryonic hepatocytes is identical in all phases of the cell cycle. It is suggested that hormones, in particular glucocorticosteroids, simultaneously and independently regulate growth mode and gene expression in developing hepatocytes. The nucleotide-analogue 5-bromodeoxyuridine inhibits the hormone-induced expression of carbamoylphosphate synthetase only in cells that are exposed to the drug during early S-phase, indicating replication of the carbamoylphosphate synthetase gene in that part of the cell cycle.

Immunocytochemical localization of transient DNA strand breaks in differentiating myotubes using in situ nick-translation

We have localized DNA strand breaks during in vitro chicken myogenesis by repairing nicks in nuclei of fixed cell monolayers in situ with biotin-11-dUTP, followed by immunocytochemical detection of incorporated biotin with rabbit anti-biotin and FITC-labeled goat anti-rabbit antibodies. No accumulations of biotin sufficient for immunocytochemical detection were observed in 23-hr cultures of dividing cells. In 33- and 43-hr cultures, biotin was first detected in only 3% of the nuclei, all of which appeared to be in fusing myoblasts or small myotubes. In contrast, cultures of young, highly fused myotubes (56 hr) exhibited 18% biotinylated nuclei; virtually all of these nuclei, most of which were grouped as aggregates, were within myotubes. In older cultures (73 and 94 hr) incorporation of biotin into myotube nuclei markedly decreased, while increases were noted in nuclei of mononuclear cells. These results indicate that extensive single-stranded DNA nicking occurs in nuclei of young myotubes, followed by repair as terminal differentiation ensues.

Epigenesis in developing avian scales. III. Stage-specific alterations of the developmental program caused by 5-bromodeoxyuridine

As an approach to the study of a developmental program, 5-bromodeoxyuridine (BrdU) was administered to chick embryos in ovo at various stages of avian scale formation. This brought about stage-specific alterations in morphogenesis in the anterior tarsometatarsus such as feathered scales, from Day 6 through Day 6 1/2; feathers only, from Day 6 3/4 through Day 7 1/4; scalelessness and rudimentary scales, from Day 7 7/8 through Day 8 1/8; and partial ridge scales, from Day 8 1/8 through Day 10. The effects of BrdU were completely nullified by an excess dose of thymidine which instantly suppressed BrdU incorporation into nuclear DNA. Effects of BrdU causing scalelessness were further examined. The percentage of BrdU labeled cells was immunohistochemically detected. It increased linearly in both the epidermis and dermis, reaching nearly 100% 24 hr following its injection on Day 8. However, scale forming potency, as assayed by the area of scale epidermis on Day 11, decreased with the duration of BrdU incorporation into the cells and disproportionately dropped at 15 hr when about 50% of the cells had incorporated BrdU. Scalelessness was also produced when the period of the incorporation of BrdU exceeded 15 hr. Time sequence observations demonstrated epidermal cell shape, polarity, alignment, and packing density to be remarkably disordered so that the placode and interplacode failed to develop on Day 9 1/4. Epidermal-dermal recombinations were carried out by exchanging normal tissues with those treated with BrdU in the anterior tarsometatarsus. The results clearly showed defects in the dermis at the time of reassociation, giving rise to scalelessness.

Synthesis and ubiquitination of histones during myogenesis

One and two-dimensional polyacrylamide gel electrophoresis have revealed that cultures of postmitotic (G0) chicken skeletal myotube cells synthesize significant but reduced quantities of histone proteins as compared to their proliferating myoblast precursors. In addition, modulation of variant synthesis within the histone H2A and H3 classes may accompany myotube formation. That the histone bands contain no nonhistone contaminants was shown by exclusion of [3H]tryptophan. It is unlikely that these results reflect synthesis of histone by contaminating replicating cells, since a single treatment with cytosine arabinoside at the time of fusion effectively removed unfused cells while suppressing synthesis of DNA in the myotube cultures. The relatively sparse incorporation of label by major variants of the H2A class in dividing myoblasts was shown to be caused by heterogeneity due to phosphorylation and extensive ubiquitination, which decline at the time of myotube formation. As determined by quantitative Western-blotting, dividing myoblasts and myotubes contain an average of 1.0 and 0.4 molecules of ubiquitinated H2A (uH2A), respectively, per 10 nucleosomes.

Replication timing of genes and middle repetitive sequences

DNA replication in mammals is temporally bimodal. "Housekeeping" genes, which are active in all cells, replicate during the first half of the S phase of cell growth. Tissue-specific genes replicate early in those cells in which they are potentially expressed, and they usually replicate late in tissues in which they are not expressed. Replication during the first half of the S phase is, therefore, a necessary but not sufficient condition for gene transcription. A change in the replication timing of a tissue-specific gene appears to reflect the commitment of that gene to transcriptional competence or to quiescence during ontogeny. Most families of middle repetitive sequences replicate either early or late. These data are consistent with a model in which two functionally distinct genomes coexist in the nucleus.

Induction of muscle genes in neural cells

The regulation of skeletal muscle genes was examined in heterokaryons formed by fusing differentiated chick skeletal myocytes to four different rat neural cell lines. Highly enriched populations of heterokaryons isolated using irreversible biochemical inhibitors were labeled with [35S]methionine and analyzed on two-dimensional gels. Rat skeletal myosin light chains were induced in three of the four cell combinations. The one exception, the S-20 cholinergic cell line, not only failed to synthesize rat muscle proteins but also suppressed chick myogenic functions. Experiments with heterokaryons between chick myocytes and cells from whole embryonic rat brain cultures demonstrated that rat skeletal myosin light chains are inducible in normal diploid neural cells as well as in established neural cell lines. In contrast, dividing cell hybrids between rat myoblasts and rat glial cells were nonmyogenic. These results demonstrate that although neural cells may contain factors that prevent the decision to differentiate along myogenic lines in cell hybrids, most neural cell lines do not dominantly suppress the expression of muscle structural genes in heterokaryons. Furthermore, the skeletal myosin light chain genes in most neural cell lines are regulated by a mechanism that permits them to respond to putative chick skeletal myocyte-inducing factors. The "open" state of these myogenic genes may explain many of the reports of apparent "transdifferentiation" to muscle in neural cultures and neural tumors.

Analysis of the myogenic lineage in chick embryos. IV. Effects of conditioned medium

Immunocytochemical analysis of small myogenic clones was used to compare the effects of fresh medium (FM) and conditioned medium (CM) on muscle differentiation. In order to compare the same population of cells, clones were initiated in FM and then switched to either new FM or to CM. Clones were fixed at 12-hour intervals up to 76 hours, then assayed for the presence of post-mitotic myoblasts by immunoperoxidase staining for muscle myosin heavy chain (MHC) or M-creatine kinase (MCK). In both media, myogenic cells occurred predominantly in homogeneous positive clones (all cells (MHC +/MCK +) which contained 2" cells. At 76 hours, the percentages of 1-, 2-, and 4-cell positive clones did not differ statistically in the two conditions; however, the percentages of 8- and 16-cell positive clones were significantly reduced in CM, and the percentages of small negative clones were concomitantly increased. We conclude from these data that CM affects myogenesis by slowing progression through a predetermined lineage rather than by changing the number of mitoses an individual cell will undergo before terminally differentiating. These results further support the idea that progress through the myogenic lineage is mediated by cell divisions.

Differential transcription of early and late-replicating DNA in human cells

Several studies suggest that most of the functional genes of mammalian cells are contained in the early-replicating sequences of DNA. There is little direct evidence, however, to support this view. To determine whether the extent of transcription of different DNA sequences is related to the order in which these sequences are replicated, I have selectively labelled early- and late-replicating DNA with [3H]thymidine ([3H[TdR) and determined the extent to which the 3H-labelled non-repeated sequences of each DNA preparation are hybridized by increasing concentrations of RNA from exponentially growing KB cells. The fraction of DNA hybridized at infinite RNA concentration was then estimated by a plotting method which linearizes the data. Selective labelling of DNA was achieved by synchronizing a culture of KB cells by a double block of DNA synthesis and then labelling portions of the culture 0-3 h (early) or 6-9 h (late) after release of the cells from the second block. At all RNA concentrations tested, the fraction of early-replicating DNA hybridized was significantly greater than that of late-replicating DNA. At infinite RNA concentration the value for early-replicating DNA was 3-4 times as great as that of late-replicating DNA. If it is assumed that the fraction of DNA hybridized at infinite RNA concentration is proportional to the fraction of DNA which is transcribed, it can be concluded that 3-4 times as much early-replicating DNA is transcribed as late-replicating DNA in exponentially growing KB cells.

Regulation of rat myosin light-chain synthesis in heterokaryons between 5-bromodeoxyuridine-blocked rat myoblasts and differentiated chick myocytes

Terminal cell differentiation in a variety of model systems is inhibited by the thymidine analogue 5-bromodeoxyuridine (BUdR). We investigated the mode of action of BUdR by forming heterokaryons between undifferentiated BUdR-blocked rat myoblasts and differentiated chick skeletal myocytes. We analyzed newly synthesized proteins on two-dimensional polyacrylamide gels. The induction of rat skeletal myosin light-chain synthesis was reduced fivefold, as compared with controls, when chick myocytes were fused to BUdR-blocked rat myoblasts. This indicates that plasma membrane effects cannot be the proximate cause for the inhibition of myogenesis by BUdR, since BUdR is able to block the effect of chick inducing factors even when a differentiated chick myocyte is in direct cytoplasmic continuity with the BUdR-blocked rat nucleus. The observation that chick cells required an 80% substitution of BUdR for thymidine to block myogenesis, whereas L6 rat myoblasts required only a 20% substitution led to a hypothesis involving a DNA-mediated action of BUdR. This model yielded three testable predictions: (a) putative chick inducing molecules should be present in limiting quantities, (b) exploiting gene-dosage effects to increase the quantity of putative chick inducing factors might overcome the inhibition produced in the rat myoblasts by a 35% BUdR for thymidine substitution, and (c) these gene-dosage effects should be abolished by increasing the level of BUdR substitution in the rat myoblast to 60-80%. All three of these predictions have been verified, providing strong indirect evidence that the inhibition of myogenesis produced by BUdR is a direct result of its incorporation into cellular DNA.

Modulation of in vitro myogenesis by submillimolar concentrations of sodium butyrate

The effects of submillimolar concentrations of sodium butyrate on the differentiation of cultured chick myoblasts have been studied. The continuous presence of 0.5 mM butyrate inhibited myoblast fusion, creatine kinase (CK) isozyme transition, and synthesis of total RNA and protein until the 4th day of myogenesis, after which the fusion index reached control values and total CK activity was elevated. The latter continued to exhibit daily increases over control levels, largely reflecting activity of the MM-CK muscle-specific isozyme which increased to twice the control level by the 8th day. Similar but less striking patterns of early inhibition followed by stimulation were observed for total protein content and synthesis of total protein and RNA. On the other hand, DNA content was slightly but significantly depressed in treated cultures at all times. Butyrate treatment did not reverse 5'-bromodeoxyuridine (BUdR) inhibition of MM-CK differentiation. It was also noted that continuous treatment with 0.5 mM butyrate prevented the unexplained sporadic deterioration of myotubes sometimes observed at the 4th day.

The BrdU content of DNA is decreased during reversal of inhibition of myogenesis by deoxycytidine

The mechanism by which 5-bromodeoxyuridine (BrdU) inhibits cell differentiation is unresolved. The ability of deoxycytidine to reverse the inhibition of myogenesis produced by BrdU has been cited as evidence that the inhibition is not a direct result of the incorporation of BrdU into cellular DNA. In contrast to previous work, the present study demonstrates a direct correlation between the effects of deoxycytidine on myogenic cells and a reduction in the substitution of BrdU for thymidine in the DNA. Further-more, the reversal occurs at the same degree of BrdU substitution (20-30%) as is required to inhibit myogenesis when cells are grown in BrdU alone or with deoxycytidine in a medium that prevents the conversion of deoxycytidine to thymidine. The effects of deoxycytidine thus do not support a mechanism of action of BrdU in myogenic cells independent of its effects on DNA.

Inducer-mediated commitment of murine erythroleukemia cells to differentiation: a multistep process

There are a number of agents which, when added to cultures of murine erythroleukemia cells (MELC), markedly increase the probability of commitment to express the characteristics of terminal erythroid differentiation, including loss of proliferative capacity and increased accumulation of globin mRNA and hemoglobin. Some characteristics of inducer-mediated commitment of MELC to terminal erythroid differentiation were examined by determining the effects of dexamethasone (an inhibitor of inducer-mediated MELC differentiation) and of hemin (an inducer of globin mRNA accumulation). Previously, it was shown that exposure of MELC to hexamethylene-bisacetamide (HMBA) leads to commitment, detectable within 12 hr. MELC cultured with both HMBA and dexamethasone do not express commitment. MELC transferred from culture with HMBA and dexamethasone to cloning medium without these agents express commitment to terminal erythroid differentiation, indicating that MELC retain a "memory" for some early HMBA-mediated changes leading to commitment which occur even in the presence of the inhibitory steroid. The kinetics of commitment in experiments in which exposure to HMBA is interrupted, or dexamethasone is added to the culture in HMBA, suggest that there is a rate-limiting step early in the commitment process. The memory for this step persists for more than one cell cycle. Addition of hemin to cultures with HMBA and dexamethasone initiated accumulation of globin mRNA but does not reverse the steroid-mediated inhibition of terminal cell division (that is, the cells retain their proliferative capacity). Inducer-mediated MELC commitment is associated with accumulation of the chromatin protein IP25; dexamethasone does not inhibit this accumulation. Accumulation of IP25 may be inducer-related, but it is not sufficient to cause expression of terminal erythroid differentiation.

Replication of alpha and beta globin DNA sequences occurs during early S phase in murine erythroleukemia cells

Murine erythroleukemia cells (MELC) can be induced to express the characteristics of erythroid differentiation by a variety of agents. Previous studies indicate that an action of inducer, occurring during early S phase, may be critical to the expression of differentiated characteristics such as initiation of accumulation of newly synthesized alpha and beta globin mRNAs. In this investigation, the time of replication of globin genes in MELC was studied. DNA was isolated from synchronous populations of cells obtained by centrifugal elutriation. Newly replicated DNA sequences were prepared from synchronized cells cultured for 1 1/2 hr with 5-bromodeoxyuridine; bromodeoxyuridine-containing DNA was isolated by CsCl gradient centrifugation. By employing cloned probes for hybridization to newly synthesized DNA, it was found that alpha and beta globin gene sequences are replicated early in S phase, while ribosomal RNA gene sequences are replicated to about the same extent in early, middle, and late S phases.

Perturbation of growth and differentiation of Friend murine erythroleukemia cells by 5-bromodeoxyuridine incorporation in early S phase

Cultured Friend murine erythroleukemia cells (Friend cells) are induced to undergo erythroid differentiation when grown in the presence of dimethylsulfoxide (DMSO) and other compounds. The effects of unifilar substitution of bromouracil (BU) for thymidine in the DNA (BU-DNA) of Friend cells were examined. Cells were grown in the presence of 5-bromodeoxy-uridine (BrdU) for one generation, then centrifuged and resuspended in medium containing DMSO without BrdU. These cells exhibited a delay in the appearance of heme-producing, benzidine-reative (B+) cells and a decreased rate of cell proliferation in comparison to the control not containing BU-DNA. A transient inhibition of entry into S phase was observed when control cells or cells containing BU-DNA were grown in the presence of DMSO) for 10 to 20 hours. This transient inhibition was increased in the BrdU culture. Thus BU-substitution in Friend cells alters other cellular functions in addition to erythroid differentiation. The rate of increase in the percent of cells committed to differentiate (those forming B+ colonies in plasma clots) was similar in the BrdU and control cultures until 40 to 50 hours. After this time, a delay in the appearance of committed cells was observed in the BrdU culture. The effect of BrdU on the appearance of B+ cells was more pronounced and occurred earlier than its effect on the rate of commitment. Therefore, the delay in the appearance of B+ cells in the BrdU culture was due primarily to perturbation of post-commitment events such as the accumulation of hemoglobin. We also examined the effect on growth and differentiation after BrdU was incorporated during different intervals of S phase in cells synchronized by centrifugal elutriation or by double thymidine block and hydroxyurea treatment. The delay in the appearance of B+ cells and inhibition of cell proliferation were only observed when BrdU was incorporated in the first half of S phase. BrdU (10 muM) had no effect on growth or differentiation when present during late S or G1 and G2. These results, using two very different methods to achieve cell synchrony, indicate that the effects of BrdU on growth and differentiation described above are due to its incorporation into DNA sequences replicating during early S.