Discrimination of Cell Nuclei in Early S-phase,Mid-to-late S-phase,and G 2 /M-phase by Sequential Administration of 5-Bromo-2'-Deoxyuridine and 5-Chloro-2'-Deoxyuridine

Abstract

5-Bromo-2'-deoxyuridine (BrdU) and 5-chloro-2'-deoxyuridine (CldU) were sequentially administered intraperitoneally into mice at 1-hr intervals. After one additional hr, the small intestines were resected, fixed, and embedded in paraffin. In histological sections stained with monoclonal antibody Br-3 reactive to both BrdU and CldU, and CldU antibody reactive only to CldU, three types of staining could be identified in the proliferating zone. Cells with nuclei stained only with Br-3 antibody were estimated to have completed DNA replication during the first 1 hr and were fixed in G₂/M-phase. Those nuclei were frequently found in apical areas of the simple columnar epithelium of the intestine, whereas other nuclei were located basally in the cells. This observation suggested intracellular movement of cell nuclei in G₂/M-phase. Identification of cells in early S-phase became possible using these antibodies in combination with DAB and fluorescence stainings. Replication sites in early S-phase nuclei were found to be numerous, whereas in late S-phase they were larger in size and much smaller in number.

Keywords

thymidine analog double labeling cell cycle S-phase

It is known that halogenated deoxyuridines such as 5-bromo-2'-deoxyuridine (BrdU), 5-iodo-2'-deoxyuridine (IdU), and 5-chloro-2'-deoxyuridine (CldU) may be incorporated into DNA instead of thymidine during replication so that their application, together with specific antibodies, allows proliferating cells to be identified in histological sections of various organs (Takahashi et al. 1992; Patel et al. 1993; Vanky et al. 1998; Calvo et al. 2000; Uni et al. 2000). For estimation of the length of S-phase or a cell cycle in living animals, many animals need to be injected with one of these thymidine analogs and killed after different intervals (Nowakowski et al. 1989; Takahashi et al. 1993; Kornack and Rakic 1998), followed by laborious counting of labeled cells. As an alternative, sequential administration of IdU and BrdU with a certain interval and use of anti-BrdU and anti-IdU antibodies for identifying the cells incorporating these thymidine analogs has been employed to determine cell kinetic parameters in cultured cells and tumors (Shibui et al. 1989; Miller et al. 1991; Hoshino et al. 1992; Yanik et al. 1992; Shibuya et al. 1993; Ritter et al. 1994; Pollack et al. 1995; Raza et al. 1997). This approach greatly simplifies the procedures for calculating the duration of S-phase and the length of a cell cycle. Using the doublelabeling technique, DNA replication sites in early, mid-to-late, or very late S-phase or G₂-phase were studied in synchronized cultured cells (Ma et al. 1998). In the present study, we raised and purified a rabbit polyclonal antibody specifically reactive to CldU and studied its usefulness for the double-labeling technique. We also examined the possibility of discriminating cells in early S-phase, mid-to-late S-phase, and G₂/M-phase in histological sections of animals and studied characteristics of labeling pattern in each subphase as well as intracellular movement of the nucleus of each cell in a cell cycle.

Materials and Methods

Antibody

Ten mg of 5-fluorouridine (Sigma-Aldrich; St Louis, MO) was dissolved in 500 μl of distilled water and mixed with 10 mg of sodium periodate dissolved in 500 μl of distilled water. After 30 min, 10 mg of rabbit serum albumin (Calzyme Laboratories; San Luis Obispo, CA) dissolved in 500 μl of 0.2 M phosphate buffer (pH 7.4) was added and the mixture was left for 2 hr. After addition of 5 mg of sodium boro-hydride, the 5-fluorouridine-RSA conjugate was dialyzed against 20 mM PBS (pH 7.4) overnight, diluted to make up 10 ml of PBS solution, and stored frozen until use. One ml of the conjugate was mixed with 1 ml of Freund's adjuvant and injected monthly into the back of a rabbit. The immunoglobulin fraction prepared from antiserum obtained from the rabbit was added with BrdU (approximately 6 mg/10 ml antiserum) and left overnight. The mixture was then applied to an affinity column coupled with 5-fluorouridine on amino cellulofine (Seikagakukogyo; Tokyo, Japan). The column was washed and eluted with glycine-HCl buffer (pH 2.5) to obtain fractions reactive with CldU. The eluate was again applied to a BrdU affinity column to remove BrdU-reactive immunoglobulins. The final pass-through fraction was called the CldU antibody in the present study.

DNA Double Labeling

Male mice aged 4 or 10 weeks (Slc/ICR) were injected intraperitoneally with 50 mg/kg body weight of BrdU (TCI Tokyo Kasei Kogyo Co., Ltd.; Tokyo, Japan) dissolved in physiological saline. After 1 hr, they received an injection of 50 mg/kg of CldU (ICN Biomedicals; Aurora, OH). After an additional hr, the animals were sacrificed under deep anesthesia, and the small intestines were removed, fixed in 70% ethanol, and routinely processed to 5-μm-thick paraffin sections.

To establish the specificity of the antibody, mice injected with either CldU or BrdU were prepared and their intestines were used for examination of binding.

Immunohistochemistry

After deparaffinization and rehydration through graded series of ethanol, sections were pretreated in a microwave oven in sodium citrate buffer (pH 6.0) for 5 min at 100C. After they were cooled down in NaCl-TB (300 mM NaCl in 50 mM Tris buffer, pH 7.4) for 5 min, they were incubated with 1% skimmed milk in NaCl-TB for 5 min to block nonspecific binding.

For double staining with DAB and fluorescent dye, sections were incubated with CldU antibody (10 μg/ml) overnight. After they were exposed to biotinylated goat anti-rabbit IgG (Lab Vision Corporation; Fremont, CA) for 60 min, they were incubated with 1:2000 diluted horseradish peroxidase avidin D (Vector Laboratories; Burlingame, CA) for 60 min, then 0.02% of 3, 3'-diaminobenzidine tetrahydrochloride (DAB, Dojinkagaku, Kumamoto, Japan) in 100 mM imidazole-HCl solution, pH 7.4, with 300 mM NaCl and 0.006% of H₂O₂ for 15 min. After they were washed, they were treated again in a microwave oven in sodium citrate buffer for 5 min at 100C and incubated with Br-3 antibody (1:10000) overnight, followed by Alexa Fluor 594 goat antimouse IgG (1:400; Molecular Probes, Eugene, OR) in NaCl-TB with skimmed milk for 3 hr. Finally they were mounted in mounting medium containing DAPI (Vector Laboratories).

For double immunofluorescent staining, after pretreatment in a microwave oven, sections were incubated with CldU (10μg/ml) and Br-3 (1:10000) antibodies overnight. Then the sections were incubated with a mixture of Alexa Fluor 594 goat anti-rabbit IgG (1:400) and Alexa Fluor 488 goat anti-mouse IgG (1:400) for 3 hr and mounted in the medium containing DAPI. Sections from mice injected with either CldU or BrdU were also treated with the same method to determine reactivities of antibodies used in the present study.

All stained sections were examined under a fluorescent microscope equipped with a digital camera (Nikon DXM 1200; Tokyo, Japan). Photographs of DAB stained sections were treated with a computer program (Photoshop) to reverse the tone of the pictures and merged with corresponding fluorescence images.

DNA Double Labeling of Cultured Cells

Neural progenitor cells were cultured according to the method described by Shinohara et al. (2004). Dissociated cells (4–5 × 10⁵/dish) were seeded onto 35-mm dishes with 12 × 9 mm coverslips precoated with poly-L-lysine (Sigma) and propagated in serum-free medium (SFM) with fibroblast growth factor-2 (FGF-2) for 24 hr. After 24 hr, some cultured cells were incubated in SFM with FGF-2 and BrdU (10 μM) for 60 min. After the incubation, the culture medium was exchanged to SFM with FGF-2 and CldU (10 μM), and the cells were cultured for another 60 min (pulse-chase-pulse-chase experiment). Other cultured cells were incubated in SFM with FGF-2 and BrdU (10 μM) for 60 min. Then CldU (10 μM) was added to the culture medium and the cells were incubated for another 60 min (no-chase experiment). Finally, the cultured cells were fixed in 70% ethanol for 20 min and then stored in PBS with sodium azide until use.

The methods of double immunostaining were the same as those of histological sections.

Estimation of Cell Cycle Length

To calculate the duration of S-phase (Ts) and the length of a cell cycle (Tc), we counted the number of nuclei of the intestinal epithelial cells that were stained only with Br-3 antibody, and with both Br-3 and CldU antibodies, respectively, and the total number of nuclei comprising the proliferating zone in the same section. The duration of S-phase (Ts) was calculated as the number of double stained cells/Br-3 stained cells × t, where t is the time interval between injection of the two labels (in this study 1 hr). The length of a cell cycle (Tc) was expressed as Ts/CldU labeling index (LI) × GF, where GF is the “growth fraction,” assumed to be 100% in this case. The CldU LI is the percentage of CldU-labeled nuclei among the DAPI positive nuclei.

Results

The CldU antibody recovered from a 5-fluorouridine affinity column and passed through a 5-bromouridine affinity column proved reactive with nuclei of CldU injected mice but not those of BrdU-injected mice, indicating high specificity to CldU. On the other hand, Br-3 was reactive to both BrdU and CldU (Figure 1). Specimens from non-injected mice were not immunostained with either CldU or Br-3 antibodies, and positive immunostaining was not seen when the primary antibody was replaced by non-immune mouse or rabbit IgG (Figure 2).

Figure 1.

Specificity of CldU and Br-3 antibodies. (A) the reactivity of CldU antibody to BrdU; (B) the reactivity of Br-3 antibody to BrdU; (C) the reactivity of CldU antibody to CldU; (D) the reactivity of Br-3 antibody to CldU.

When a mouse was injected with BrdU, it was incorporated by cells in S-phase (Figure 3A). After the mouse was injected with CldU, cells in S-phase incorporated both BrdU and CldU (Figure 3B). However, cells that had finished DNA replication between the injections did not incorporate CldU (Figure 3B). Numbers of nuclei stained only with Br-3 antibody and with both Br-3 and CldU antibodies, and those labeled with DAPI (i.e., total number of cells) were counted in 4- and 10-week-old mice (Table 1). On the cultured cells of the pulse-chase-pulse-chase experiment and the no-chase experiment, numbers of nuclei stained only with Br-3 antibody and with both Br-3 and CldU antibodies, and those labeled with DAPI were counted similarly (Table 2). Based on the data, the durations of S-phase and a cell cycle were calculated for each mouse and culture experiment. The duration of S-phase (Ts) and the length of a cell cycle (Tc) of the histological sections of one 4-week-old and two 10-week-old mice were 4.6, 4.6, 5.1 hr and 10.9, 10.3, 13.1 hr, respectively (Table 1). On the other hand, Ts and Tc of the cultured cells of pulse-chase-pulse-chase experiment and no-chase experiment were 13.3 and 12.5 hr and 31.8 and 29.7 hr, respectively (Table 2).

When the two photographs of DAB and fluorescent dye were merged, three types of cells were identified; those stained only with the fluorescent dye, those stained only with DAB and those stained with both DAB and the fluorescent dye (Figure 4C). They were estimated to be the cells which had finished DNA replication in the first 1 hr, cells entering S-phase within the second 1 hr, and those in mid to late S-phase, respectively. On the other hand, two types of cells were recognized in pulse-chase-pulse-chase experiment and no-chase experiment of the cultured cells; those stained only with the green fluorescent dye and those stained with both the red and green fluorescent dyes (Figure 5). In higher magnification of the sections and the cultured cells, stainings of the cells that had finished DNA replication in the first 1 hr were sometimes found as a small number of relatively large dots localized at the periphery of the nuclei (Figure 4 and Figure 5). In histological sections, these nuclei were frequently found to be localized in the apical regions of the cytoplasm while other nuclei were found in basal regions of the cells (Figure 4).

Figure 2.

Negative controls of the non-injected mice (A, B) and non-immune sera (C, D). Immunostainings with either Br-3 (A) or CldU (B) antibodies of specimens from non-injected mice. Immunostainings using non-immune mouse (C) or rabbit (D) IgG as the primary antibody.

Figure 3.

Findings after injection of both BrdU and CldU. When BrdU was injected, it was incorporated by cells in S-phase (A). One hr later, when CldU was injected, cells that already completed DNA replication could not incorporate the new label (B, a), whereas cells in S-phase or newly entering S-phase during the interval incorporated both CldU and BrdU (B, b).

Discussion

Thymidine analogs such as BrdU, IdU and CldU are known to be incorporated into cellular DNA during DNA replication, and have been widely used for identification of proliferating cells (Vanky et al. 1998; Calvo et al. 2000; Uni et al. 2000). They have also been employed to reveal the duration of S-phase and the length of a cell cycle (Nowakowski et al. 1989; Takahashi et al. 1993; Kornack and Rakic 1998). Although BrdU and IdU have been used for double labeling of proliferating cells, IdU is difficult to dissolve in water and a large volume of solution is necessary for administration to mice. Moreover, it was difficult in our hands to obtain antibodies reactive to IdU but not to BrdU from IdU-conjugate injected chicken. In contrast, CldU is readily soluble in water and the volume for injection to mice is small, so that its application is much easier. In addition we could readily purify an antibody fraction reactive to CldU but not to BrdU from our 5-fluorouridine-conjugate injected rabbit (Figure 1). Using the CldU and Br-3 antibodies, it was here possible ble to count the number of nuclei stained by each of the antibody and calculate the duration of S-phase and length of a cell cycle in one animal. The values obtained were almost the same as those reported from previous laborious work (Potten et al. 1979). Because BrdU may remain in the animal body after the second injection (CldU), cells in S-phase incorporate both CldU and BrdU. We then tried a pulse-chase-pulsechase experiment of cultured cells and compared the result with that of a no-chase experiment. The duration of S-phase and the length of a cell cycle in the cultured cells were not different between the two experiments (Table 2). It seems that the present CldU antibody and application of it to a double labeling study is useful and reliable for calculating the duration of S-phase or cell cycle in histological tissue sections as well as in cultured cells.

Table 1

Duration of S-phase and length of a cell cycle of the epithelium of 4- and 10-week-old mice intestine

	No.	Br-3(+) CldU(-)	Br-3(+) CldU(+)	Total cells	Ts (hr)	Tc (hr)	CldU LI(%)
4 weeks	1	415	1893	4541	4.6	10.9	41.7
10 weeks	1	515	2369	5330	4.6	10.3	44.5
10 weeks	2	111	561	1459	5.1	13.1	38.5

Table 2

Duration of S-phase and length of a cell cycle of the cultured cells

		Br-3(+) CldU(-)	Br-3(+) CldU(+)	Total cells	Ts (hr)	Tc (hr)	CldU LI(%)
Pulse-chase-pulse-chase	42	559	1336	13.3	31.8	41.8
No-chase	46	576	1368	12.5	29.7	42.1

Figure 4.

Double staining with CldU and Br-3 antibodies in mice. (A) cell nuclei stained with Br-3 antibody (red fluorescent dye); (B) cell nuclei stained with CldU antibody (the brown color of DAB was changed to green using the computer program Photoshop); (C) merged photographs of A, B; D, nuclei stained with DAPI; arrowheads, nuclei stained only with Br-3 antibody; arrows, nuclei stained only with CldU antibody.

Figure 5.

Double staining with CldU and Br-3 antibodies in cultured cells. (A-D) pulse-chase-pulse-chase experiment; (E-H) no-chase experiment; (A, E) cell nuclei stained with Br-3 antibody (green fluorescent dye); (B, F) cell nuclei stained with CldU antibody (red fluorescent dye); (C, G) merged photographs of (A, B, E, F), respectively; (D, H) nuclei stained with DAPI; arrowheads, nuclei stained only with Br-3 antibody.

Identification of cells in early S-phase has been performed in cultured cells (Nakayasu and Berezney 1989; O'Keefe et al. 1992; Manders et al. 1992, 1996; Ma et al. 1998). In the present study, identification of those cells in tissue sections was attempted. It is known that eukaryotic chromosomal DNA is divided into hundreds to thousands of independent subunits of replication named replicons (Hand 1978). When BrdU and CldU were incorporated into a replication site simultaneously, DAB pigmentation due to CldU staining can quench the colocalized fluorescence due to BrdU, while when they were incorporated sequentially, fluorescence of BrdU will not be quenched by subsequent DAB staining because the replication sites moved during the 1 hr interval between the administrations. In the present study, cells entered the S-phase during the second 60 min were expected to incorporate both BrdU and CldU simultaneously and will be stained only with DAB (Figure 4). In practice, cell nuclei stained only with DAB were found and the staining pattern was diffuse suggesting many replication sites. In previous reports on cultured cells, replication sites in early S-phase consisted of many small domains scattered throughout the nucleoplasm, whereas in late S-phase they were larger in size and smaller in number (Manders et al. 1992, 1996; Ma et al. 1998). An electron microscopic study revealed that cells in early S-phase display a diffuse pattern of labeling with many spots, whereas cells in late S-phase show labeling confined to large domains, often at the periphery of the nucleus or associated with nucleolus (Jaunin et al. 1998). Thus, the present observation was in good agreement with these reports on cultured cells (Figure 4 and Figure 5).

Considering the good correlation between the present double-labeling study and the previous ones, we can conclude that our double labeling technique using CldU and BrdU in combination with DAB and fluorescent staining enabled to identify cells in early S-phase in tissue sections. By shortening the interval between the 2nd injection and the fixation, it will be possible to identify the cells in very early S-phase.

In embryos, cell nuclei of the neuroepithelium, a kind of pseudostratified epithelium, are positioned in a variety of levels and were reported to change the level depending on their cell kinetics (Fujita 1962). In the present study, most of nuclei of the intestinal columnar epithelium are positioned in basal level, while nuclei stained only with Br-3 antibody were frequently found in apical level (Figure 4). Because the cells stained only with Br-3 antibody were thought to have finished DNA synthesis during the first 60 min, the displaced nuclei stained only with Br-3 antibody suggest intracellular movement of nuclei in the G₂- or M-phase in columnar epithelium. Because the displaced nuclei were frequently not oval in shape, they may be in M-phase.

Footnotes

Acknowledgements

We wish to thank Dr. Tomiko Asano (Institute for Developmental Research, Aichi Human Service Center) for her support through the cell culture experiment.

References

Calvo

Boya

Carbonell

García-Mauriño

(2000) Cell proliferation in the developing rat pineal gland. A bromodeoxyuridine immunohistochemical study. Histol Histopathol 15:1005–1010

Fujita

(1962) Kinetics of cellular proliferation. Exp Cell Res 28:52–60

Hand

(1978) Eucaryotic DNA: organization of the genome for replication. Cell 15:315–325

Hoshino

Ito

Asai

Shibuya

Prados

Dodson

Davis

. (1992) Cell kinetic analysis of human brain tumors by in situ double labelling with bromodeoxyuridine and iododeoxyuridine. Int J Cancer 50:1–5

Jaunin

Visser

Cmarko

Aten

Fakan

(1998) A new immunocytochemical technique for ultrastructural analysis of DNA replication in proliferating cells after application of two halogenated deoxyuridines. J Histochem Cytochem 46:1203–1209

Kornack

Rakic

(1998) Changes in cell-cycle kinetics during the development and evolution of primate neocortex. Proc Natl Acad Sci USA 95:1242–1246

Samarabandu

Devdhar

Acharya

Cheng

Meng

Berezney

(1998) Spatial and temporal dynamics of DNA replication sites in mammalian cells. J Cell Biol 143:1415–1425

Manders

EMM

Stap

Brakenhoff

van Driel

Aten

(1992) Dynamics of three-dimensional replication patterns during the S-phase, analysed by double labeling of DNA and confocal microsopy. J Cell Sci 103:857–862

Manders

EMM

Stap

Strackee

van Driel

Aten

(1996) Dynamic behavior of DNA replication domains. Exp Cell Res 226:328–335

10.

Miller

Mazewski

Yousuf

Sheikh

White

Yanik

Hyams

. (1991) Simultaneous immunohistochemical detection of IUdR and BrdU infused intravenously to cancer patients. J Histochem Cytochem 39:407–412

11.

Nakayasu

Berezney

(1989) Mapping replicational sites in the eucaryotic cell nucleus. J Cell Biol 108:1–11

12.

Nowakowski

Lewin

Miller

(1989) Bromodeoxyuridine immunohistochemical determination of the lengths of the cell cycle and the DNA-synthetic phase for an anatomically defined population. J Neurocytol 18:311–318

13.

O'Keefe

Henderson

Spector

(1992) Dynamic organization of DNA replication in mammalian cell nuclei: spatially and temporally defined replication of chromosome-specific α-satellite DNA sequences. J Cell Biol 116:1095–1110

14.

Patel

Rew

Taylor

Potten

Owen

Roberts

(1993) Study of the proliferation in human gastric mucosa after in vivo bromodeoxyuridine labelling. Gut 34:893–896

15.

Pollack

Terry

Wise

White

Meistrich

(1995) Specific staining of iododeoxyuridine and bromodeoxyuridine in tumors double labeled in vivo: a cell kinetic analysis. Cytometry 20:53–61

16.

Potten

Schofield

Lajtha

(1979) A comparison of cell replacement in bone marrow, testis and three regions of surface epithelium. Biochim Biophys Acta 560:281–299

17.

Raza

Alvi

Broady-Robinson

Showel

Cartlidge

Mundle

Shetty

. (1997) Cell cycle kinetic studies in 68 patients with myelodysplastic syndromes following intravenous iodo- and/or bromodeoxyuridine. Exp Hematol 25:530–535

18.

Ritter

Fowler

Kim

Gilchrist

Morrissey

Kinsella

(1994) Tumor cell kinetics using two labels and flow cytometry. Cytometry 16:49–58

19.

Shibui

Hoshino

Vanderlaan

Gray

(1989) Double labeling with Iodo- and Bromodeoxyuridine for cell kinetics studies. J Histochem Cytochem 37:1007–1011

20.

Shibuya

Ito

Davis

Wilson

Hoshino

(1993) A new method for analyzing the cell kinetics of human brain tumors by double labeling with bromodeoxyuridine in situ and with iododeoxyuridine in vitro. Cancer 71:3109–3113

21.

Shinohara

Udagawa

Morishita

Ueda

Otani

Semba

Kato

. (2004) G_i2 signaling enhances proliferation of neural progenitor cells in the developing brain. J Biol Chem 279:41141–41148

22.

Takahashi

Nowakowski

Caviness

Jr (1992) BUdR as an S-phase marker for quantitative studies of cytokinetic behaviour in the murine cerebral ventricular zone. J Neurocytol 21:185–197

23.

Takahashi

Nowakowski

Caviness

Jr (1993) Cell cycle parameters and patterns of nuclear movement in the neocortical proliferative zone of the fetal mouse. J Neurosci 13:820–833

24.

Uni

Geyra

Ben-Hur

Sklan

(2000) Small intestinal development in the young chick: crypt formation and enterocyte proliferation and migration. Br Poult Sci 41:544–551

25.

Vanky

Brockstedt

Hjerpe

Wikström

(1998) Kinetic studies on epiphyseal growth cartilage in the normal mouse. Bone 22:331–339

26.

Yanik

Yousuf

Miller

Swerdlow

Lampkin

Raza

(1992) In vivo determination of cell cycle kinetics of non-Hodgkin's lymphomas using iododeoxyuridine and bromodeoxyuridine. J Histochem Cytochem 40:723–728