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Oligopotent Mesenchymal Stem Cell-Like Clone Becomes Multinucleated Following Phorbol Ester TPA Stimulation.

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THE ANATOMICAL RECORD 120:1256–1267 (2007)
Oligopotent Mesenchymal Stem
Cell-Like Clone Becomes Multinucleated
Following Phorbol Ester,
TPA Stimulation
Department of Histology and Cell Biology, Yokohama City University School of Medicine,
Kanazawa-ku, Yokohama, Japan
We established a mesenchymal stem cell clone, 5F9A, from rat bone
marrow substrate adherent cells by repeated limiting dilutions. The cells
have a fibroblastic shape and form intimate contacts with adjacent cells
with interdigitations and junctions similar to adherence and tight junctions in a semi-confluent culture. Analysis of the phenotypes of these cells
by RT-PCR and FACS demonstrated that they resembled mesenchymal
stem cells, and the cells could differentiate into adiopocytes and osteoblasts under appropriate conditions in vitro showing their oligopotency.
Furthermore, the cells were induced to become multinuclear cells by TPA
(12-o-tetradecanoylphorbol 13-acetate) stimulation. Anat Rec, 290:1256–
1267, 2007. Ó 2007 Wiley-Liss, Inc.
Key words: mesenchymal stem cell; junction; phorbol ester;
TPA; multinucleation
Mesenchymal stem cells (MSCs) reside in the bone
marrow, like hematopoietic stem cells, and can differentiate into various types of cells in appropriate conditions,
both in vitro and in vivo (Pittenger et al., 1999; Jiang
et al., 2002). MSCs can differentiate into tendon, endothelial, and nerve cells, as well as osteoblasts, adipocytes, and chondrocytes (Pittenger et al., 1999; Dezawa
et al., 2001, 2004; Jiang et al., 2002). MSCs can also
become stromal cells, supporting the differentiation of
hematopoietic stem cells into mature blood cells (Dennis
and Charbord, 2002).
MSCs are fibroblastic in shape, adhere to culture substrates, and do not phagocytose particles (Castro-Malaspina et al., 1980). By fluorescent-activated cell sorting
(FACS) analysis, MSCs were classified into small and
agranular cells (Colter et al., 2000). As for the cell surface markers, MSCs express CD105 (SH-2; Barry et al.,
1999), CD73 (SH-3 and SH-4; Barry et al., 2001), CD90,
STRO-1, and CD106, but not the hematopoietic markers
CD45 and CD34 (Tocci and Forte, 2003). However, it
was also reported that MSCs freshly prepared from bone
marrow aspirates are phenotypically different from
those cultured in vitro for some time (Gronthos et al.,
2003). Jiang et al. (2002) suggested that a single MSC
could differentiate into almost all types of cells in the
Many cells develop junctional structures when they
make contact with each other. These structures include
tight junctions, adherence junctions, gap junctions, desmosomes, and interdigitations, and cells communicate
through these junctions. Although these junctional
structures are not prominent for cells of mesenchymal
origin, human mesenchymal stem cells are reported to
build gap junctions among themselves or with cardiomyocytes in some in vitro culture systems (Valiunas et al.,
Grant sponsor: The Ministry of Education, Culture, Sports,
Science and Technology, Japan Grant-in-Aid for Scientific
Research; Grant numbers: 14370008 and 16659048.
*Correspondence to: Keiichiro Yoshida, Department of Histology and Cell Biology, Yokohama City University School of Medicine, Fukuura 3–9, Kanazawa-ku, Yokohama, Japan 236-0004.
Fax: 81-45-787-2568. E-mail:
Received 29 November 2006; Accepted 12 July 2007
DOI 10.1002/ar.20590
Published online in Wiley InterScience (www.interscience.wiley.
2004; Beeres et al., 2005). These cells interact electrically and can repair experimentally induced conduction
TPA is a potent tumor promoter and known to be a
protein kinase C (PKC) activator. TPA has been reported
to induce formation of multinuclear cells, either by cell–
cell fusion (syncytium; Hassan et al., 1989; David et al.,
1990) or by nuclear division without cell division (plasmodium; Menaya and Clemens, 1991).
Syncytia occur in striated muscle cells and macrophage-related cells, and their formation results in the
generation of multinuclear cells (Anderson, 2000; Taylor,
2003). Plasmodia were reported in some tumors stimulated with TPA (Menaya and Clemens, 1991), and it was
reported that this effect was dependent on PKCa and
PKCd activation (Watanabe et al., 1992; Yamaguchi
et al., 1995).
TPA has also been reported to induce endomitosis
(Murate et al., 1991; Bermejo et al., 2002). An endomitotic cell has multiple sets of its genome, for example,
4n, 8n, 16n, or more in one nucleus (Vitrat et al., 1998).
Endomitosis occurs in leukemic cells (e.g., K562, HEL
MEG-01) stimulated by TPA, and these cells change into
megakaryocytes (Murate et al., 1991; Bermejo et al.,
In this report, we established an oligopotent (Smith,
2006) mesenchymal stem cell clone that could differentiate into adipocytes and osteoblasts, and we investigated
the fine structure of these cells. Upon stimulation with
TPA, these cells converted into multinucleated cells.
Cell Culture
Normal rat bone marrow (BM) was obtained from the
femurs of DA/Slc rats (Japan SLC. Inc., Hamamatsu, Japan). Eagle’s minimum essential medium with alpha
modification (a-MEM; Sigma, St. Louis, MO) including
10% fetal calf serum (Lot.S04301S1820, BioWest, Miami,
FL), 300 mg/ml L-glutamine (WAKO, Osaka, Japan), and
60 mg/ml kanamycin sulfate (WAKO) was used for culturing cells. Culture was performed at 378C in a humidified atmosphere containing 5% CO2 and 95% air. The
medium was changed every third day. A few MSC clones
were obtained through successive limiting dilutions, and
clone 5F9A was used in this study.
Transmission Electron Microscopy
Cells were fixed with 1% glutaraldehyde and 2% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4) overnight at 48C, post-fixed with 1% OsO4 for 1 hr on ice,
and further stained with 0.5% uranyl acetate for 30 min
at room temperature. Fixed and stained cells were dehydrated with ascending concentrations of ethanol, and
embedded in Epon 812. Ultrathin sections were obtained
and stained with 2% uranyl acetate in 70% ethanol and
0.4% lead citrate, and observed using a H-7500 transmission electron microscope (Hitachi, Tokyo, Japan)
operated at 80 kV.
Cells on coverslips were fixed with PLP solution
(0.01 M NaIO4, 0.075 M lysine, 0.0375 M phosphate
buffer, and 2% paraformaldehyde, pH 6.2), blocked with
0.4% bovine serum albumin in phosphate buffered saline
(BSA-PBS), and permeabilized with 0.1% Triton X-100
in BSA-PBS. Thereafter, the cells were labeled with the
primary antibodies and secondary antibodies (Alexa
Fluor1488 goat anti-mouse IgG and anti-rabbit IgG;
Invitrogen, Carlsbad, CA). The stained samples were observed by confocal laser scanning microscopy (LSM510,
Carl Zeiss Ltd. Oberkochen, Germany).
For the primary antibodies, rabbit anti–ZO-1 antibody
(Invitrogen), mouse anti–b-catenin monoclonal antibody
(BD Transduction Lab., Lexington, KY), mouse anti-desmoglein antibody (BD Transduction Lab.), and mouse
anti–E-cadherin monoclonal antibody (BD Transduction
Lab.) were used in this study.
Reverse-Transcription Polymerase
Chain Reaction
Total RNA was extracted from 5F9A cells using TRI
REAGENTTM RNA, DNA, and protein isolation solution
(Sigma) according to the manufacturer’s instructions.
cDNA was synthesized by reverse transcription with
M-MLV reverse transcriptase (Invitrogen). Polymerase
chain reaction (PCR) was performed in a PCR thermal
cycler (Takara, Tokyo, Japan), and the products were
subjected to electrophoresis on 2% agarose gels after 35
cycles of amplification and stained with 100 ng/ml ethidium bromide. The sizes of the PCR products were estimated by comparison with Marker 4 (WAKO). Oligonucleotide primers used in PCR are shown in Table 1.
For FACS analysis, we used biotin–anti-CD29 antibody, fluorescein isothiocyanate (FITC) –anti-CD45 antibody (BD Biosciences Pharmingen, San Diego, CA), antiCD11b antibody, FITC–anti-CD44 antibody (Chemicon,
Tamecula, CA), and FITC–anti-CD90 antibody (eBioscience, San Diego, CA). As a secondary antibody, we
used FITC-Streptavidin (Vector, Burlingame, CA) and
FITC goat anti-mouse IgG (Fab0 ) antibody (PROTOS
IMMUNORESEARCH, San Francisco, CA). The expression of the antigens was analyzed using a MoFlo flow cytometer (Dako Cytomation, Fort Collins, CO).
Phagocytosis of Latex Beads
5F9A cells were cultured in medium containing latex
beads (1.07-mm diameter, Sigma) for 3 hr. These cells
were washed gently with PBS, fixed with methanol, and
stained with Giemsa solution (MERCK, Dermstadt, Germany). The cells were observed using a light microscope.
Induction of Differentiation
For 5F9A cells to differentiate into osteoblasts, they
were cultured in a-MEM containing 50 mM ascorbate-2phosphate (Asc-2-P, Sigma), 10 mM b-glycerophosphate
(Sigma), and 0.1 mM dexamethasone (WAKO; Zuk et al.,
2001). The medium was changed every third day. After
5 weeks, total RNA was extracted from the cells and
applied to reverse transcription (RT) PCR, and the cells
were stained by von Kossa staining for the detection of
calcium. Concerning the differentiation into adipocytes,
the cells were cultured in a-MEM containing 4 mg/ml
TABLE 1. Primers used for reverse-transcription polymerase chain reaction
Oligonucleotide sequence
Rat CD14
Rat CD29
Rat CD90
Rat CD104
Rat CD106
Rat CD126
Rat c-kit
Rat CD45
Rat CD44
Rat CD34
Mouse CD73
Rat CD105
Rat E-Cadherin
Rat N-Cadherin
Rat Runx2
Rat Osteonectin
Rat MyoD
Rat Myf-5
Mouse Myogenin
Rat albumin
Rat a-Fetoprotein
Rat Integrin aV
Rat Integrin b3
Rat Cathepsin K
Rat c-fms
Rat b-actin
Product (base pair)
Annealing (8C)
Fig. 2. Junctional structures between 5F9A cells. A–C: An adherence junction-like structure (A), a tight junction-like structure (B), and
an interdigitation-like structure (C) are indicated by arrows. D: Fluorescence micrograph of anti–ZO-1 antibody staining of 5F9A cells. The
borders of the cells were stained by the antibody (arrow). Scale bars
5 600 nm in A, 100 nm in B, 200 nm in C, 100 mm in D.
insulin (Sigma), 500 mM 3-isobutyl-1-methylxanthine
(IBM, WAKO), 0.4 mM dexamethasone (WAKO), and
60 mM indomethacin (WAKO) for 3 weeks, changing the
medium twice a week, and stained with Sudan III for
the detection of lipids. In both cases, the culture dishes
were coated with Growth Factor Reduced MATRIGEL1Matrix (40 mg/ml; BD Biosciences, Discovery Labware,
Bedford, MA), and the initial cell density was 2 3 104
cells per 2 ml of medium in a 35 mm dish.
Induction of Multinuclear Cells
For the induction of multinuclear cells (MNCs), cells
were cultured in a 35-mm dish in the presence of 20 ng/
ml TPA (Sigma) for 3 days. For light microscopy, the
cells were fixed with absolute methanol for 5 min and
stained with Giemsa solution (MERCK).
Additionally, TPA-stimulated cells were fixed with
10% formalin, permeabilized with 0.5% Triton X-100 in
PBS, and stained with SYBR Green I (SYBR-I; Molecular Probes, Inc., Eugene, OR). Twenty optical sections of
multinuclei along the z-axis (0.9 mm in thickness and
0.45 mm in focus step) were analyzed using confocal
laser scanning microscopy (LSM510, Carl Zeiss Ltd.).
Fig. 1. The 5F9A cells. A: Photograph of 5F9A cells. The 5F9A was
established from adherent cells in DA/Slc rat bone marrow by
repeated limiting dilutions. The cells were fixed with absolute methanol
and stained by Giemsa solution. B: Electron micrographs of 5F9A
cells. The arrow indicates heterochromatin, the arrowhead indicates
Golgi apparatus, the asterisk indicates polysomes, and the white
arrow indicates rough endoplasmic reticulum. C: Actin bundle is indicated beneath the plasma membrane at the basal surface (arrow).
Scale bars 5 25 mm in A, 1 mm in B,C.
Fig. 3. Phenotype of 5F9A cells. A: Reverse transcription-polymerase chain reaction was performed using total RNA extracted from
nontreated and TPA-treated 5F9A cells with specific primers, which
are shown in Table 1. The left column shows the results with total
RNA from 5F9A cells without TPA stimulation. The column in the cen-
ter shows the results from the TPA-stimulated cells. Total RNA prepared from normal rat spleen cells was used as a control (right column). B: Phenotype analyses using fluorescent-activated cell sorting.
5F9A cells stained with fluorescence were analyzed using a MoFlo
flow cytometer.
5F9A cells
The 5F9A cell clone was obtained through repeated
limiting dilutions of bone marrow adherent cells. After
establishment, this clone was maintained continually by
dilution with fresh medium because it needed neither
feeder cells nor special supplements. The cells spread on
plastic surfaces and exhibited a fibroblast-like shape
(Fig. 1A). The population doubling time was approximately 20 hr.
Disperse heterochromatin could be observed in the
nuclei by electron microscopy. Golgi apparati, polysomes,
and rough endoplasmic reticulum developed in the cytoplasm, which suggested the presence of active protein
synthesis in 5F9A cells (Fig. 1B). Thick actin bundles
could be observed on the substratum at the basal surface
beneath the plasma membrane (Fig. 1C).
Junctional Structures Between 5F9A Cells
Conspicuously, the cells formed numerous junctional
structures among themselves. This observation is unique
considering their probable mesenchymal origin. Some
junctions resembled adherence junctions (Fig. 2A), some
resembled tight junctions (Fig. 2B), and some seemed to
be interdigitations formed by protrusions of one cell
ensheathed by the plasma membrane of the adjacent cell
(Fig. 2C). Actin bundles were also seen in the vicinity of
the adherence junction-like structures. The protrusions
were longer than 1 mm and the longest one we observed
was approximately 3 mm long. The width appeared to be
100–200 nm. The interdigitations contained actin filaments extending along their long axis. However, no
thickening or undercoating of plasma membranes reminiscent of junctional specialization could be observed
between these protrusions and their sheaths (Fig. 2C).
Next, we stained 5F9A cells with antibodies that recognize the molecules forming the junctions. This cell
clone did not react with anti–E-cadherin antibody (data
not shown). Neither could we detect any signal of b-catenin in the cells (data not shown). These findings suggest
that typical adherence junctions are not constructed
between 5F9A cells. However, TEM analysis suggested
the presence of adherence junction-like structures (Fig.
2A), which may indicate the presence of an atypical
member of the adherence junction family.
An antibody against the ZO-1 molecule, which is one
of the components of tight junctions, stained borders
between 5F9A cells (Fig. 2D), and, furthermore, a junction-like structure similar to a tight junction was
observed during TEM analysis. Fused outer leaflets of
the plasma membrane could also be detected (Fig. 2B).
In addition, we could not stain 5F9A cells with an
anti-desmogrein antibody that reacts with desmosomes
(data not shown), which suggests that desmosomes did
not exist between the cells. Therefore, the junctional
structures between 5F9A cells likely consist of tight
junction- and adherence junction-like structures, and
protrusions with no communication with the sheath.
Phenotype of 5F9A Cells
To examine the phenotype of the 5F9A clone, we performed RT-PCR using specific primers for various
markers (Table 1; Fig. 3A). The cells showed positive
Fig. 4. The 5F9A cells did not ingest latex beads. The 5F9A cells
were cultured with latex beads (1.07 mm diameter). Rat peritoneal adherent cells were used as a positive control, and they could internalize
beads (arrows). Scale bar 5 25 mm.
reactions for CD90 (Thy-1), CD29 (b1-integrin), CD73
(Ecto-50 -nucleotidase), CD105 (Endoglin), CD44, CD14,
and N-Cadherin, but did not express CD126 (IL-6 receptor), CD104 (b4-integrin), CD106 (VCAM-1), E-Cadherin,
CD45, CD34, or c-kit. The results of FACS were consistent with the results by RT-PCR. The expression of
CD29, CD44, and CD90 but not CD11b and CD45 could
be observed both in the presence and absence of TPA
(Fig. 3B). Additionally, each histogram shows a single
peak of staining except CD44 with TPA, which suggests
that the population of cells was homogenous. However,
TPA seemed to induce some changes of CD44 expression
in the cells. Because the 5F9A clone did not express
CD45, CD34, or c-kit (Fig. 3A,B), this clone did not
likely originate from hematopoietic lineage cells. Although this clone was positive for CD14 (Fig. 3A), which
is a macrophage marker, it did not ingest latex beads,
indicating that it was not a phagocyte (Fig. 4). The negative expression of CD11b (Fig. 3B) confirms these
5F9A Cells Are Oligopotent
5F9A cells were able to differentiate to contain lipid
droplets positive for Sudan III staining in the cytoplasm
when cultured in a-MEM medium containing insulin,
IBM, dexamethasone, and indomethacin for 3 weeks
(Fig. 5A,B). This finding indicates that 5F9A cells can
differentiate into adipocytes, as previously reported for
mesenchymal stem cells. After reaching confluence, the
cell sheets often rolled up to form aggregates of cells. In
these aggregates, adipocytes were very often observed
(Fig. 5C).
When the cells were cultured for 5 weeks in the presence of Asc-2-P, b-glycerophosphate, and dexamethasone
(Zuk et al., 2001), brown deposits were detected on the
cells by von Kossa staining (Fig. 6A,B). These deposits
were regarded to be calcium, and the cells, therefore,
seemed to differentiate into osteoblasts. To confirm
osteoblastic differentiation, analysis by RT-PCR was performed. Cells cultured for 5 weeks in osteoblast differentiation medium were shown to express Runx-2 and bone
sialoprotein, which are markers of osteoblasts (Laino
et al., 2006; Friedman et al., 2006; Fig. 6C), whereas
control cells did not express these markers. These
results indicated that the 5F9A clone maintained the
capacity to differentiate into both adipocytes and osteoblasts under suitable culture conditions, suggesting its
oligopotent nature.
Fig. 5. Differentiation into adipocytes. The 5F9A cells were cultured
in adipogenic differentiation medium, as described in the Materials
and Methods section. A: The cells were fixed and stained by Sudan III
staining for adipocytes. Lipid droplets were stained (arrows). B: Sudan
III staining of cells cultured without supplements. C: Photograph of
transmission electron microscopy of an aggregate of cultured 5F9A
cells. There are many lipid droplets in the cells (arrows). Scale bars 5
25 mm in A,B, 5 mm in C.
Fig. 6. Differentiation into osteoblasts. The 5F9A cells were cultured in osteogenic differentiation medium, as described in Materials
and Methods. A: The cells were fixed and stained by von Kossa staining for osteoblasts. Calcium deposits could be observed (arrows). B:
Shown is the von Kossa staining of the cells cultured without supplements. C: Reverse transcription-polymerase chain reaction analysis of
the expression of the osteoblast specific genes. Total RNA was
extracted from cells cultured for 5 weeks in the osteogenic differentiation medium (1) and the control medium (without supplements) (2),
and used as templates with the primers listed in Table 1. Asterisks
indicate specific bands of Runx-2 and bone sialoprotein. Scale bars 5
25 mm in A,B.
5F9A Cells Can Be Induced to Develop Into
Multinuclear Cells
optimal conditions, 1020% of the cells were multinucleated. Although some 5F9A cells spontaneously
became multinuclear, the frequency was less than 3%,
and the difference between nontreated and TPA-treated
5F9A cells was easily recognizable by microscopic examination. When 5F9A cells were seeded in a 35-mm culture vessel at cell densities of 1 3 104/ml (2 3 104/dish),
2.0 3 1044.0 3 104 cells/dish were multinuclear by day
3 when the cells reached confluence (approximately
When 5F9A cells were cultured in the presence of
TPA, they developed multiple nuclei, some of which
were large and irregular in shape (Fig. 7). This phenomenon was confirmed by confocal laser scanning microscopy, which showed that more than one distinct nucleus
existed in one cell following TPA stimulation (Fig. 8). In
Fig. 7. TPA-induced multinucleation. The 5F9A cells were cultured
in the presence or absence of 20 ng/ml of TPA for 3 days. Cells were
stained with Giemsa solution. Multinucleated cells were induced
(arrows). Scale bar 5 25 mm.
2.0 3 105/dish). Upon TPA stimulation, multinuclear cells
spread (Fig. 7), and highly distinct stress fibers could be
observed. It also appears that cells grew very slowly after
becoming multinucleated (data not shown). TPA stimulation did not change the phenotype of 5F9A cells as
assessed by RT-PCR (Fig. 3A), and this may reflect that
only approximately 1020% cells became multinucleated.
There are some naturally occurring multinuclear cells,
including megakaryocytes (although strictly speaking,
they are not multinucleated), striated muscle cells, hepatocytes, and osteoclasts. In RT-PCR analyses, neither
nonstimulated nor TPA-stimulated cells expressed GPIIb
or GPV (megakaryocyte markers; Lepage et al., 2000),
MyoD, Myf-5, or myogenin (striated muscle cell markers;
Berkes and Tapscott, 2005), or albumin or a-fetoprotein
(hepatocyte markers; Hay et al., 2007; Fig. 9). However,
some osteoclastic markers were expressed in 5F9A cells.
These cells were positive for integrin aV, integrin b3
(Ptaff and Jurdic, 2001), cathepsin K, M-CSF (macrophage-colony stimulating factor) receptor (c-fms), and
TRAP (tartrate-resistant acid phosphatase; Fujisaki
et al., 2007), but negative for RANK (receptor activator
of NF-jB) and CTR (calcitonin receptor; Myers et al.,
1999) both in the presence and absence of TPA stimulation
(Fig. 9). Because 5F9A cells are oligopotent, they would
likely be stem cells that retain some osteoclast markers.
We, therefore, could not acquire any evidence that
5F9A cells differentiated into megakaryocytes, striated
muscle, hepatocytes, or osteoclasts after TPA stimulation.
Although we tried to induce multinuclear cells with
various substances other than TPA during short-term
culturing (3 days), a significant number of multinuclear
cells could not be induced. The substances included dibutyryl cyclic-AMP (250 mg/ml), staurosporine (0.2 mM),
activin (10 ng/ml), vitamin A (50 pg/ml), retinoic acid
(50 nM), insulin (5 mg/ml), concanavalin A (2 mg/ml), lipopolysaccharide (10 mg/ml), and lysophosphatidic acid
(300 ng/ml; data of all materials listed above are not
shown). Additionally, rat normal skin fibroblasts did not
become multinucleated following TPA stimulation (data
not shown), which may suggest that multinucleation is a
specific phenomenon characteristic of 5F9A cells.
5F9A Cells Are Phenotypically Similar to
Mesenchymal Stem Cells
RT-PCR and FACS (Fig. 3) revealed that 5F9A cells
express CD14, CD29, CD44, CD90, CD73, CD105, and
N-Cadherin. They do not express E-Cadherin, CD104,
CD106, CD126, CD34, c-kit, CD45, or CD11b (Fig. 3).
Deans and Moseley (2000) reported that the phenotypes
of MSC were CD90, CD29, and CD106 positive, and
CD14, CD34, and CD45 negative. On the contrary, Reyes
et al. (2001) reported they were CD44 and CD90 positive, and CD34, CD106, c-Kit, and CD45 negative. The
phenotypes of 5F9A cells in the present study were consistent with the results of Reyes et al. and those of Deans
and Moseley except CD14 and CD106. 5F9A cells were
negative for CD45, CD34, and c-Kit, which suggests that
5F9A cells are not hematopoietic. In human mesenchymal stem cells, CD73 is recognized by SH-3 and SH-4
antibodies (Barry, 2001) and CD105 is recognized
by SH-2 antibodies (Barry et al., 1999); Therefore, these
antibodies may be applied to the separation and characterization of rat mesenchymal stem cells by FACS or
other methods.
5F9A cells are CD44 positive. Deans and Moseley
(2000) also reported that CD34-positive hematopoietic
stem cells require CD44-mediated signals expressed by
mesenchymal stem cells for hematopoiesis. 5F9A cells
may, therefore, have the ability to maintain hematopoiesis.
5F9A cells are also positive for CD14, which is one of
the components of the LPS (lipopolysaccharide) receptor
(Miller et al., 2005) and is regarded as a specific marker
for macrophages and granulocytes (Fig. 3A). This finding
may suggest that 5F9A cells are of macrophage/granulocyte lineage. However, 5F9A cells were CD11b negative
and could not internalize latex beads (Figs. 3B, 4). Together with their lack of many hematopoietic markers,
we consider that 5F9A cells are not of macrophage/granulocyte lineage.
In RT-PCR analyses, 5F9A cells expressed many but
not all osteoclast markers. They were positive for integrin aV, integrin b3, cathepsin K, c-fms, and TRAP, but
negative for RANK and CTR (Fig. 9). Although osteoclasts were reported to be positive for CD45 and negative for CD14 (Athanasou and Quinn, 1990), 5F9A cells
showed the opposite pattern of results (Fig. 3). Although
these results suggest that 5F9A cells might be related to
osteoclasts, we could not definitively define 5F9A cells
as osteoclasts because of some phenotypic differences
between 5F9A cells and osteoclasts. When taken into
consideration that 5F9A cells had oligopotency (Figs. 5,
6), it is possible that they may instead be stem cells
expressing some osteoclast markers.
5F9A Cells Are Oligopotent
5F9A cells could differentiate into adipocytes and
osteoblasts in the presence of adipogenic and osteogenic
differentiation cocktails, as determined by Sudan III
staining and electron microscopy for adipocytes, and von
Kossa staining and RT-PCR analysis for osteoblasts
(Figs. 5, 6). Although osteonectin and PTHR (parathy-
Fig. 8. Three-dimensional analysis of multinuclei in the cell using
confocal microscopy. A,B: SYBR-I stained nuclei in one TPA-treated
cell were observed using confocal microscopy (0.9 mm in thickness
and 0.45 mm in focus step along the z-axis from the upper left to the
lower right). Twenty optical sections were analyzed in each cell, and
more than one distinct nucleus could be observed in one cell. Scale
bar 5 10 mm.
Figure 8.
We detected N-cadherin on the surface of 5F9A cells
(Fig. 3), which was previously reported to be associated
with the osteoblastic niche for hematopoietic stem cells
(Zhang et al., 2003). Although at present, we do not
know if these molecules affect the function of 5F9A cells,
a more intensive study of the junctional structures of
5F9A cells would reveal their important roles in the
interaction of the cells during events such as cell–cell
Multinucleation by TPA
Fig. 9. Analyses of the expression of differentiation markers by
TPA-induced multinucleated cells. Total RNA of 5F9A cells with or
without TPA treatment was analyzed by the reverse transcription-polymerase chain reaction. Total RNA of rat fetal limb was used as a positive control for MyoD, Myf-5, and myogenin (striated muscle markers).
Total RNA of the rat liver was used as a positive control for albumin
and a-fetoprotein (hepatocytes markers). Positive controls of GPIIb
and GPV (megakaryocyte markers) and other groups (osteoclast
markers) were also assessed.
roid hormone receptor) were expressed not only in osteogenically differentiated (Runx-2 and bone sialoprotein
positive), but also in undifferentiated cells (Fig. 6C), previous findings showed that osteonectin was expressed in
bone marrow-derived (undifferentiated) MSCs (Silva
et al., 2003), and MSCs in fetal circulating blood spontaneously became PTHR-positive cells (Naruse et al.,
2004). Therefore, our findings do not necessarily deny
the ability of 5F9A cells to undergo osteoblastic differentiation. Rather, our data suggest that 5F9A cells have
oligopotency. In brief, 5F9A cells would give us a valuable tool for studying rat MSCs because of their species
Cell Communication Between Mesenchymal
Stem Cells
Although adherence junctions, tight junctions, or desmosomes, that could be observed mainly in epithelial
cells have not been reported in MSCs, 5F9A cells were
shown to form junction-like structures by electron microscopy (Fig. 2). One type of junction (an interdigitation-like structure) had a bundle of microfilaments in
the interdigitations along their long axis. Actin cables in
the cells were connected to adherence junction-like
However, we could not detect E-cadherin or b-catenin
in 5F9A cells either by immunofluorescence techniques
or RT-PCR analysis (Fig. 3). This finding may suggest
that typical epithelial-type adherence junctions are not
constructed between these cells. We could not detect desmogrein either, indicating that few, if any, desmosomes
were formed (data not shown).
The tumor promoter TPA could cause 5F9A cells to
become cells with more than one nucleus (multinuclear;
TPA-response; Figs. 7, 8) and it induced 1020% of the
total cell population into multinucleated cells, whereas
nontreated control cells contained less than 3% multinucleated cells.
In general, it has been thought that genome amplification in a cell is carried out by one of the mechanisms of
syncytium, plasmodium, and endomitosis. All these processes could be induced by TPA stimulation, possibly
depending on the cell types (Hassan et al., 1989; David
et al., 1990; Murate et al., 1991; Menaya and Clemens,
1991; Bermejo et al., 2002). This is the first report indicating that MSCs could be multinucleated by TPA stimulation. Further studies should be done to clarify the
relationship between stem cells and multinucleation.
We thank Dr. Y. Ikeda, Dr. K. Ohbo, and Dr. H.
Tanaka for helpful discussions and encouragement during the course of this work.
Anderson JM. 2000. Multinucleated giant cells. Curr Opin Hematol
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