DNA replication and differentiation in rat myoblasts studied with monoclonal antibodies against 5-bromodeoxyuridine, actin, and alpha 2-macroglobulin

Calreticulin couples calcium release and calcium influx in integrin-mediated calcium signaling

The engagement of integrin alpha7 in E63 skeletal muscle cells by laminin or anti-alpha7 antibodies triggered transient elevations in the intracellular free Ca(2+) concentration that resulted from both inositol triphosphate-evoked Ca(2+) release from intracellular stores and extracellular Ca(2+) influx through voltage-gated, L-type Ca(2+) channels. The extracellular domain of integrin alpha7 was found to associate with both ectocalreticulin and dihydropyridine receptor on the cell surface. Calreticulin appears to also associate with cytoplasmic domain of integrin alpha7 in a manner highly dependent on the cytosolic Ca(2+) concentration. It appeared that intracellular Ca(2+) release was a prerequisite for Ca(2+) influx and that calreticulin associated with the integrin cytoplasmic domain mediated the coupling of between the Ca(2+) release and Ca(2+) influx. These findings suggest that calreticulin serves as a cytosolic activator of integrin and a signal transducer between integrins and Ca(2+) channels on the cell surface.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Quantitation of satellite cell proliferation in vivo using image analysis

A nonisotopic, double fluorescence technique was developed to study myogenic satellite cell proliferation in posthatch turkey skeletal muscle. Labeled satellite cell nuclei were identified on enzymatically isolated myofiber segments using a mouse monoclonal antibody (anti-BrdU) followed by fluorescein-5-isothiocyanate (FITC) conjugated goat anti-mouse IgG secondary antibody. Myofiber nuclei (myonuclei+satellite cell nuclei) were counterstained with propidium iodide (PI). The myofiber segment length, myofiber segment diameter, and the number of PI and FITC labeled nuclei contained in each segment was determined using a Nikon fluorescence microscope, a SIT video camera and Image-1 software. Data collected by three different operators of the image analysis system revealed 5.0 +/- 1.4 satellite cell nuclei per 1000 myofiber nuclei and 5284 +/- 462 microns3 of cytoplasm surrounding each myofiber nucleus in the pectoralis thoracicus of 9-week-old tom turkeys. BrdU immunohistochemistry coupled with the new approach of PI staining of whole myofiber mounts is an effective combination to allow the use of an efficient semi-automated image analysis protocol.

Bromodeoxyuridine (BrdU) immunocytochemistry by exonuclease III (Exo III) digestion

A new procedure is described to generate single-stranded DNA by exonuclease III (Exo III) digestion for bromodeoxyuridine (BrdU) immunocytochemistry on tissue sections. We compared this procedure with the most widely used procedure of DNA denaturation with 2 N HCl. In vivo and in vitro pulse and continuous labelling of tissues and cells were used. The specimens were fixed in formalin, ethanol, glutaraldehyde, Carnoy's, Bouin's or Zamboni's fixative and embedded in paraffin or used unfixed as cryostat sections or cytospin preparations. After Exo III digestion, BrdU substituted DNA was detected irrespective of the fixation procedure applied. The optimal protocol for nuclease digestion appeared to be simultaneous incubation, of 10 Units Exo III per ml EcoRI buffer and anti-BrdU monoclonal antibody at 37 degrees C. The advantages of Exo III digestion for BrdU immunocytochemistry compared to acid denaturation were: less non-specific nuclear background reactivity, no DNA renaturation, less DNA loss, optimal nuclear morphology, increase in antibody efficiency and the possibility for simultaneous detection of acid-sensitive tissue constituents. Disadvantages of the Exo III digestion are decreased sensitivity and the need for more rigorous pepsin pretreatment. We conclude that Exo III digestion of DNA is an appropriate alternative for acid denaturation for BrdU immunocytochemistry on sections of pulse-labelled specimens.

Comparison of intradermal injections of bromodeoxyuridine and tritiated thymidine in the in vivo measurement of epidermal cell turnover time in goats

Study of keratinisation defects such as seborrhoea may require investigation of epidermal cell kinetics in clinical cases. For this to be practically useful a safe, reliable and quick method is essential. This study compared epidermal cell kinetics in the goat following the use of intradermal injection of tritiated thymidine ([3H]TdR) and bromodeoxyuridine (BURD) to label S phase nuclei. Turnover time for goat epithelial cells was not significantly different for the two agents; 26.3 +/- 6.0 days for thymidine and 21.8 +/- 5.2 days for BURD. It was concluded that intradermal injection of BURD will be a suitable technique for investigating epidermal cell kinetics in vivo and could have applications in a clinical situation because it is quicker and safer than [3H]TdR.

Measurement of cell proliferation by enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody to bromodeoxyuridine

An ELISA was developed and optimized to measure cell proliferation using a monoclonal antibody to bromodeoxyuridine (BrdUrd). Incorporation of BrdUrd into myoblast monolayers, measured as the optical density at 492 nm, increased in response to fetal calf serum, IGF-I and EGF, the ELISA data correlated closely with data obtained by BrdUrd immunocytochemistry (r = 0.984), cell counting (r = 0.972) and tritiated thymidine uptake by liquid scintillation counting (r = 0.990). The BrdUrd ELISA is a useful alternative to measurement of tritiated thymidine uptake by scintillation counting, and has the added advantages of dispensing with the use of radioactivity and of being less labour intensive.

A combined method for detection of cell surface marker expression and bromodeoxyuridine (BrdU) uptake by epidermal cells in suspension

A rapid and convenient immunocytochemical method is presented for the simultaneous detection of surface antigen expression and S-phase cells using monoclonal antibodies and cells in suspension. The first reagent permits the nature of the cells to be determined while the second immunostaining stage reveals which of these cells are proliferating (BrdU-positive) cells. Staining and fixation conditions have been defined for the detection of cell surface markers by red fluorescence and BrdU labelling by green fluorescence. All the steps were carried out on cells in suspension and the membrane antigen distribution was not affected by the BrdU staining protocol. The method has been applied to two human epidermal cell subpopulations: Langerhans' cells and basal keratinocytes. This technique should have general applicability to the study of the cytokinetic characteristics of phenotypically defined cells in heterogeneous populations.

Replicating myoblasts express a muscle-specific phenotype

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The developmentally regulated, muscle-specific, integral membrane protein H36 and the intermediate-filament protein desmin were scored as markers of the myogenic phenotype. The percentage of BrdUrd+ (i.e., proliferative) cells among H36+ and desmin+ myoblasts was equal to the percentage of BrdUrd+ cells in the entire population, indicating that the expression of H36 and desmin is uniformly characteristic of replicating myoblasts. Inhibition of protein synthesis before and during growth in BrdUrd did not alter the frequency of desmin and H36 immunofluorescence in BrdUrd+ cells. Thus, desmin and H36 were present in the replicating myoblasts prior to the onset of growth in BrdUrd. These results were confirmed using H36+ cells selected by flow cytometry: these purified H36+ myoblasts replicate, express desmin, and differentiate. Similar results were obtained with mouse myoblasts. Desmin expression in these mammalian cells differs from that in chicken embryo myoblasts: only a small proportion of replicating chicken embryo myoblasts express desmin. That replicating mammalian myoblasts have a muscle-specific phenotype serves to define a distinct stage in myogenic development and a specific cell in the myogenic lineage. Further, it implies that there is a regulatory event activated during myogenesis that precedes terminal differentiation and that is required for expression of those genes whose products distinguish the replicating myoblast.

A laminin substrate promotes myogenesis in rat skeletal muscle cultures: analysis of replication and development using antidesmin and anti-BrdUrd monoclonal antibodies

Cells from newborn rat hindlimb show greatly enhanced myogenicity when grown on surfaces coated with poly-L-lysine followed by laminin (PLL/Lam) instead of the collagens routinely used. Coating with poly-L-lysine (PLL) alone or with PLL followed by collagen does not enhance myogenicity. Both myogenic and nonmyogenic cells, as distinguished by a monoclonal antibody specific for desmin, attach equally well to collagen- and laminin-coated surfaces, but there is a two- to five-fold increase in the number of myogenic cells on PLL/Lam by 72 hr, followed by increased myotube formation. To determine whether this increase in myogenic cells was a consequence of a selective increase in proliferation on PLL/Lam, incorporation of 5-bromodeoxyuridine into DNA followed by labeling with anti-BrdUrd antibody was used as an index of cell proliferation. The results indicate that desmin is expressed in replicating rat myoblasts, and that replication of myogenic cells is greatly enhanced on laminin compared to collagen. The rate of replication of nonmyogenic cells is the same on both substrates. Addition of 10 micrograms/ml laminin to the medium of cells seeded on PLL or collagen has no effect on myogenicity. We conclude that a laminin substrate enhances skeletal myogenesis in vitro by promoting selectively the replication of myoblasts. Cultures prepared from fetuses at 17 and 19 days gestation also show enhanced myogenicity when grown on PLL/Lam, while those from 15-day fetuses do not. Growth and development of fetal myoblasts on collagen were very poor, whereas myoblasts from the newborn rat do proliferate and differentiate on this substrate. Thus myogenic cells at different stages of fetal and neonatal development may require and respond to different extracellular environments. Myotube formation in the E63 clone of L8 rat myoblasts is inhibited by PLL/Lam.

In vitro DNA synthesis in the macronuclear replication band of Euplotes eurystomus

Isolated macronuclei from the hypotrichous ciliated protozoan Euplotes eurystomus incorporate biotinylated dUTP specifically into the replication band (RB) as detected with immunofluorescence, using rabbit anti-biotin antibodies followed by fluorescein-conjugated goat anti-rabbit IgG. When gold-conjugated goat anti-rabbit IgG was used in a preembedded reaction, subsequent immunoelectron microscopic analysis demonstrated that the biotinylated nucleotide appeared more concentrated in the rear zone of the RB, with almost no labeling in the forward zone. It was possible to use the immunofluorescent assay to establish that incorporation of biotinylated dUTP is inhibited by simultaneous addition of N-ethyl maleimide or aphidicolin, and by omission of any one of the other unlabeled dNTPs. In addition, prolonged heat shock of the intact cells, before lysis and in vitro assay, yielded markedly reduced incorporation. Comparison with published data on the in vivo incorporation of [3H]thymidine into Euplotes eurystomus RBs indicates the fidelity of the in vitro reaction.

Endocytosis of alpha 2-macroglobulin is developmentally regulated during myogenesis

A monoclonal antibody, H143, reacts with an intracellular antigen present and accumulated in E63 rat myoblasts. H143 is directed against a species-specific determinant on purified equine serum alpha 2-macroglobulin. Immunofluorescence analyses of differentiating myoblasts grown in horse serum demonstrate that the capacity to take up alpha 2-macroglobulin is stage-specific: the rapid uptake of alpha 2-macroglobulin characteristic of myoblasts ceases prior to their fusion to form multinucleate fibers (myotubes). Neither rat fibroblasts nor a developmentally defective mutant of E63 exhibit this change in alpha 2-macroglobulin uptake. The temperature and calcium requirements for the uptake of H143 antigen, and its accumulation as effected by lysosomotropic amines, indicate that alpha 2-macroglobulin is taken up by myoblasts via a developmentally regulated endocytic process. Electron microscopy using equine alpha 2-macroglobulin labeled with colloidal gold supports this finding.

Mapping of the pattern of DNA replication in polytene chromosome from Chironomus thummi using monoclonal anti-bromodeoxyuridine antibodies

We present results from a nonautoradiographic study of DNA replication in polytene chromosomes from dipteran larvae. Monoclonal antibodies with specificity for 5-bromodeoxyuridine (BrdUrd) were used to localize by indirect immunofluorescence the sites of BrdUrd incorporation and to follow the dynamics of DNA synthesis in salivary gland cells of 4th instar Chironomus thummi larvae. This technique presents numerous advantages over autoradiographic procedures and allows mapping of DNA synthesis patterns at the level of resolution of one chromosomal band. Several replication patterns were observed, classified according to characteristic features, and tentatively assigned to specific periods of the S-phase. In early S-phase, DNA synthesis is first detectable in puffs and interbands, later in bands. Most chromosomal bands appear to initiate DNA synthesis synchronously; however, in bands within centromeric and heterochromatic regions the start of synthesis is delayed. At mid S-phase, all the bands show uniform staining. Subsequent staining patterns are increasingly differential with the bands displaying characteristic fluorescence intensities. As replication progresses through the late S-phase period, the chromosomes show a decreasing number of fluorescent bands. The last bands to terminate replication are located in centromeric and heterochromatic DNA-rich regions and a few bands of low DNA content in region IIAa-c.

Fluorescence digital imaging microscopy in cell biology

Developments in microscope, sensor, and image-processing technologies have led to integrated systems for the quantification of low-light-level emission signals from biological samples. Specificity is provided in the form of monoclonal antibodies and other ligands or enzyme substrates conjugated with efficient fluorophores. Fluorescent probes are also available for cellular macromolecular constituents and for free ions of biological interest such as H+ and Ca2+. The entire spectrum of photophysical phenomena can be exploited. Representative data are presented from studies of DNA conformation and architecture in polytene chromosomes and from studies of receptor-mediated endocytosis, calcium distribution, and the organization of the contractile apparatus in muscle cells.