DNMT1, DNMT3A and DNMT3B proteins are differently expressed in mouse oocytes and early embryos

Abstract

DNA methylation is one of the epigenetic mechanisms and plays important roles during oogenesis and early embryo development in mammals. DNA methylation is basically known as adding a methyl group to the fifth carbon atom of cytosine residues within cytosine–phosphate–guanine (CpG) and non-CpG dinucleotide sites. This mechanism is composed of two main processes: de novo methylation and maintenance methylation, both of which are catalyzed by specific DNA methyltransferase (DNMT) enzymes. To date, six different DNMTs have been characterized in mammals defined as DNMT1, DNMT2, DNMT3A, DNMT3B, DNMT3C, and DNMT3L. While DNMT1 primarily functions in maintenance methylation, both DNMT3A and DNMT3B are essentially responsible for de novo methylation. As is known, either maintenance or de novo methylation processes appears during oocyte and early embryo development terms. The aim of the present study is to investigate spatial and temporal expression levels and subcellular localizations of the DNMT1, DNMT3A, and DNMT3B proteins in the mouse germinal vesicle (GV) and metaphase II (MII) oocytes, and early embryos from 1-cell to blastocyst stages. We found that there are remarkable differences in the expressional levels and subcellular localizations of the DNMT1, DNMT3A and DNMT3B proteins in the GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell, morula, and blastocyst stage embryos. The fluctuations in the expression of DNMT proteins in the analyzed oocytes and early embryos are largely compatible with DNA methylation changes and genomic imprintestablishment appearing during oogenesis and early embryo development. To understand precisemolecular biological meaning of differently expressing DNMTs in the early developmental periods, further studies are required.

Full text

Vol.:(0123456789)
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J Mol Hist (2017) 48:417–426
DOI 10.1007/s10735-017-9739-y
ORIGINAL PAPER
DNMT1, DNMT3A andDNMT3B proteins are differently
expressed inmouse oocytes andearly embryos
FatmaUysal1· SaffetOzturk1· GokhanAkkoyunlu1
Received: 26 July 2017 / Accepted: 9 October 2017 / Published online: 13 October 2017
© Springer Science+Business Media B.V. 2017
DNA methylation changes and genomic imprintestablish-
ment appearing during oogenesis and early embryo develop-
ment. To understand precisemolecular biological meaning
of differently expressing DNMTs in the early developmental
periods, further studies are required.
Keywords DNA methyltransferase· DNA methylation·
Oocyte· Early embryo
Introduction
DNA methylation is an epigenetic mechanism and plays crit-
ical roles in transcriptional repression or activation, X-chro-
mosome inactivation, cell differentiation, and tumorogenesis
(Uysal etal. 2015). DNA methylation is specifically cata-
lyzed by various types of DNA methyltransferase (DNMT)
enzymes, and there are two different DNA methylation pro-
cesses: maintenance and de novo. In the maintenance meth-
ylation, previously established DNA methylation patterns
are continued at the end of each round of DNA replication.
By contrast, new DNA methylation marks in the genomic
DNA are created in the de novo methylation process.
TheDNMT enzymes basicallyadd a methyl group to the
fifth carbon atom of cytosine residues commonly in the
cytosine–phosphate–guanine (CpG) and rarely in the non-
CpG islands such as cytosine–phosphate–thymine (CpT),
cytosine–phosphate–adenine (CpA), and cytosine–phos-
phate–cytosine (CpC)by using S-adenosyl methionine
(AdoMet) as a methyl donor (Turek-Plewa and Jagodz-
inski 2005). To date, structurally and functionally six
different DNMTs have been characterized in mammals:
DNMT1, DNMT2, DNMT3A, DNMT3B, DNMT3C,
and DNMT3L. The DNMT1 protein primarily functions
in maintenance methylation by adding methyl groups to
Abstract DNA methylation is one of the epigenetic
mechanisms and plays important roles during oogenesis
and early embryo development in mammals. DNA meth-
ylation is basically known as adding a methyl group to the
fifth carbon atom of cytosine residues within cytosine–
phosphate–guanine (CpG) and non-CpG dinucleotide sites.
This mechanism is composed of two main processes: de
novo methylation and maintenance methylation, both of
which are catalyzed by specific DNA methyltransferase
(DNMT) enzymes. To date, six different DNMTs have been
characterized in mammals defined as DNMT1, DNMT2,
DNMT3A, DNMT3B, DNMT3C, and DNMT3L. While
DNMT1 primarily functions in maintenance methylation,
both DNMT3A and DNMT3B are essentially responsible
for de novo methylation. As is known, either maintenance
or de novo methylation processes appears during oocyte and
early embryo development terms. The aim of the present
study is to investigate spatial and temporal expression lev-
els and subcellular localizations of the DNMT1, DNMT3A,
and DNMT3B proteins in the mouse germinal vesicle (GV)
and metaphase II (MII) oocytes, and early embryos from
1-cell to blastocyst stages. We found that there are remark-
able differences in the expressional levels and subcellular
localizations of the DNMT1, DNMT3A and DNMT3B pro-
teins in the GV and MII oocytes, and 1-cell, 2-cell, 4-cell,
8-cell, morula, and blastocyst stage embryos. The fluctua-
tions in the expression of DNMT proteins in the analyzed
oocytes and early embryos are largely compatible with
* Gokhan Akkoyunlu
gokhan_akkoyunlu@hotmail.com
1 Department ofHistology andEmbryology, Akdeniz
University School ofMedicine, Campus, 07070Antalya,
Turkey

418 J Mol Hist (2017) 48:417–426
1 3
the hemi-methylated DNA strands during DNA replica-
tion (Bestor 2000), and also it partially contributes to de
novo methylation process (Fatemi etal. 2002). And, the
DNMT2 protein can methylate cytosine 38 in the antico-
don loop of aspartic acid transfer RNA instead of methyl-
ating genomic DNA (Goll etal. 2006). On the other hand,
both DNMT3A and DNMT3B essentially implicate in the
de novo methylation process, commonly being appeared in
the unmethylated DNA strands at the CpG islands (Turek-
Plewa and Jagodzinski 2005). DNMT3C is recently identi-
fied de novo methyltransferase which protects male germ
cells from retrotransposon activity and is essential for
mouse fertility (Barau etal. 2016). DNMT3L does not
have any catalytic domain; however, it is capable of induc-
ing DNMT3A and DNMT3B activity, and thereby contrib-
utes to de novo methylation indirectly (Deplus etal. 2002;
Margot etal. 2003).
In mammals, female germ cells are originated from pri-
mordial germ cells (PGCs) arising in the endodermof yolk
sac. When female PGCs reach to the gonadal ridges at the
end of long migration during which they undergo many
mitotic divisions, the PGCs ultimately differentiate into
oogonia. The oogoniaare defined as oocytes after entering
meiotic division, and the oocytes are arrested at prophase I
(PI) stage of first meiotic division in the developing ovary
before birth. The oocytes at the PI stage in the ovarian folli-
cles from primordial to antral follicles are immature oocytes
also described as germinal vesicle (GV) stage. In the ovula-
tory follicles, the GV oocytes fulfill the first meiotic division,
enter into second meiotic division, and become arrested at
the metaphase II (MII) stage known as mature oocytes (Ver-
lhac and Terret 2016). Once the MII oocytes are fertilized
by mature spermatozoa, 1-cell embryos (zygotes) includ-
ing female and male pronuclei are formed. Then, the 1-cell
embryos undergo consecutive mitotic divisions (cleavage)
and develop into 2-cell, 4-cell, 8-cell, morula and blastocyst
stage embryos, respectively. The early embryo development
from 1-cell to blastocysts includes exclusive events such as
cleavage, embryonic genome activation (EGA), compaction,
and cavitation. EGA is shortly defined as synthesis of the
required RNAs from newly formed embryonic genome and
occurs at the 2-cell stage in mice (Bolton etal. 1984). In the
eight-cell stage embryo, the blastomeres enable to create
tight junctions with each other so that compaction is par-
tially commenced. The compaction is thoroughly completed
in the morula stage embryos in which the cellular borders
become fuzzy and cannot be distinguished easily. Following
morula stage, cavitation known as formation of the blasto-
coel cavity filled with fluid take place in the blastocyst stage
embryos. The blastocoel cavity facilitates division of the
embryonic cells into two different layers in the blastocysts:
trophoblast cells (TE) and inner cell mass (ICM) (Fujimori
2010).
During oogenesis and preimplantation embryo devel-
opment, DNA methylation plays critical roles in strictly
regulating stimulation or repression of the development-
related genes and in timely establishing maternal and pater-
nal imprints (Bartolomei and Ferguson-Smith 2011). The
globalDNA methylation in the growing GV oocytes begins
to progressively increase after birth, and reaches to the
highest levels in the MII oocytes at puberty (Saitou etal.
2012; Smallwood and Kelsey 2012) (Fig.4). During early
embryo development, the global DNA methylation gradually
reduces from 1-cell embryos to 16-cell embryos, and it starts
to increase in the blastocyst stage embryos onward (Small-
wood and Kelsey 2012) (Fig.4). Although DNA methylation
profiles during oogenesis and early embryo development are
well-known, the spatial and temporal expression levels and
subcellular localizations of the DNMTs in the oocytes and
early embryos are not characterized in detail. We hypoth-
esized that the DNMT1, DNMT3A, and DNMT3B pro-
teins display different expression patterns and subcellular
localizations during oocyte and early embryo develop-
ment.The aim of thisstudy was to determine the subcellular
localizationsand relative expression levelsof the DNMT1,
DNMT3A, and DNMT3B proteinsin the mouse oocytes and
early embryos.
Materials andmethods
Animals
The experimental protocol was approved by the Animal Care
and Usage Committee of Akdeniz University (Protocol no:
B.30.2.AKD.0.05.07.00/30). The female Balb/C mice at 4–6
weeks and male mice at 8–10 weeks of age were purchased
from Research Animal Laboratory Unit of Akdeniz Univer-
sity Medical Faculty. All mice were hosted with free access
to food and water, and kept in a 12h light/dark cycle.
Collection ofoocytes andearly embryos
The GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell,
morula and blastocyst stage embryos were obtained from 4
to 6week-old female Balb/C mice superovulated with 5IU
pregnant mare’s serum gonadotropin (PMSG; Intervet, Mil-
ton Keynes, UK) and 5IU human chorionic gonadotropin
(hCG; Sigma-Aldrich, St. Louis, MO, USA). It is impor-
tant to note that each oocyte and early embryo type was
recruited at least three different mice. To obtain GV oocytes,
the female mice were injected intraperitoneally (i.p.) with
0.1mL 5IU of PMSG. Twenty hours after PMSG injection,
the female mice were killed and then ovaries were punctured
with a 23-gauge needle in human tubal fluid (HTF) medium
(Vitrolife, Gotheburg, Sweden), and we collected GV

419J Mol Hist (2017) 48:417–426
1 3
oocytes having a visible germinal vesicle by using a mouth-
controlled pipette under a dissecting microscope (Zeiss,
Oberkochen, Germany) as described previously (Akko-
yunlu etal. 2007). For collection of MII oocytes, the female
mice were injected i.p. with 5IU PMSG, and subsequently
5IU hCG was administered 48h after PMSG injection. We
obtained the cumulus–oocyte complexes (COCs) from ovi-
ducts’ ampulla region of the mice 14h after hCG injection.
The COCs were treated with HTF medium including 1mg/
mL hyaluronidase (Sigma-Aldrich) to remove cumulus cells,
and the MII oocytes in normal morphology and having vis-
ible polar bodies are pooled.
To collect early embryos, female mice at 4–6 weeks old
were injected i.p. with 5IU PMSG. Following 48h PMSG
treatment, 5IU hCG were injected to the PMSG-primed
female mice immediately mated overnight with mature male
mice at a rate of 2 female:1 male. The next morning, pres-
ence of vaginal plug has been controlled, and the female
mice including vaginal plug were used to obtain 1-cell,
2-cell, 4-cell, 8-cell and morula stage embryos from oviducts
and blastocysts from uterus. The early embryos were col-
lected at the following time points after hCG injection: 1-cell
embryos at 20h, 2-cell embryos at 42h, 4-cell embryos at
60h, 8-cell embryo at 68h, morula at 72h, and blastocyst
at 96h. Notably, the cumulus cells surrounding the 1-cell
embryos were removed by using hyaluronidase at a concen-
tration of 1mg/mL as described above.
Immunofluorescence staining
Immunofluorescence staining has been applied to charac-
terize the subcellular localizations and relative expression
profiles of the DNMT1, DNMT3A, and DNMT3B pro-
teins in the GV and MII oocytes, and 1-cell, 2-cell, 4-cell,
8-cell, morula, and blastocyst stage embryos obtained from
superovulated female mice (n ≥ 10 oocytes or early embryos
stained per group). All oocytes and early embryos were
fixed in 3% paraformaldehyde (Sigma-Aldrich), and then
permeabilized with 1% tween-20 (Sigma-Aldrich) prepared
in 1 × PBS at room temperature. The fixed oocytes and early
embryos were blocked with blocking solution including 20%
normal goat serum (Vector Laboratory, Burlingame, CA,
USA). Then, we incubated the oocytes and embryos over-
night at 4°C with primary antibodies specific for DNMT1
[Abcam, Cambridge, USA; ab87654, reactive with either
oocyte-specific (DNMT1o) or somatic (DNMT1s) iso-
forms], DNMT3A (Abcam, ab23565) or DNMT3B (Abcam,
ab2851). After washing three times for 10min with 1 × PBS
including 2% bovine serum albumin (BSA) (PBS–BSA;
Sigma-Aldrich), the oocytes and early embryos were incu-
bated with anti-rabbit Alexa 488 secondary antibody (Inv-
itrogen) for 1h at room temperature in the dark. After incu-
bation with secondary antibody, we washed the oocytes and
early embryos three times for 10min with PBS–BSA. After
counterstained with 4′,6-diamidino-2-phenylindole (DAPI;
Sigma-Aldrich, D8417) for 2min at room temperature in
the dark, the oocytes and early embryos were washed three
times for 10min with PBS–BSA. All staining steps were
performed using mini well trays (VWR-Thermo Scientific,
Radnor, PA, USA) in a humidified chamber. We mountedthe
oocytes and early embryos onto slides using mounting solu-
tion (Vector Laboratory). All immunostaining experiments
for the oocytes and early embryos were performed at the
same time periods under the same conditions. Fluores-
cence signals originating from oocytes or early embryos
were detected with a motorized fluorescence microscope at
400x magnification (Olympus BX61, Tokyo, Japan). Then,
we have evaluated the micrographs with Image J software
(National Institutes of Health, Bethesda, Maryland, USA),
and the relative expression levels and subcellular locali-
zations of the DNMT proteins have been characterized in
detail.All negative controls involving no primary antibody
were also analyzed, but no signal was detected. It is impor-
tant to note that we carried out all experiments at least three
times.
Statistical analysis
All experiment results were analyzed by one-way analysis
of variance (one-way ANOVA) followed by Dunn’s post hoc
test. We conducted statistical calculations by using SigmaS-
tat for Windows, version 3.5 (Jandel Scientific Corp). For all
tests, P < 0.05 was considered to be statistically significant.
Results
DNMT1 expression intheoocytes andearly embryos
In the current study, subcellular localizations and relative
expression levels of the DNMT1 protein have been ana-
lyzed in the GV and MII oocytes, and 1-cell, 2-cell, 4-cell,
8-cell, morula, and blastocyst stage embryos. DNMT1
protein was abundantly localized in the cytoplasm of the
oocytes and early embryos, but it was at weak levels in
the nuclear regions (Fig.1a). When we have evaluated the
relative DNMT1 expression from GV oocytes to blasto-
cysts by using ImageJ software, it reachedto the highest
levels in both MII oocytes and 1-cell embryos, whereas it
was at the lowest levels in the 8-cell embryos (Fig.1b).
Additionally, MII oocytes, 1-cell embryos and blastocysts
exhibited significantly higher DNMT1 protein expression
than the remaining oocytes and early embryos (P < 0.05;
Fig. 1b). Importantly, DNMT1 expression gradually
decreased from 2-cell embryos to 8-cell embryos, and it

420 J Mol Hist (2017) 48:417–426
1 3
progressively increased from GV oocytes to MII oocytes
and similarly from morula to blastocysts (Fig.1b). As a
result, we revealed here that DNMT1 protein expression
was differently expressed in the oocytes and early embryos
and displayed remarkable fluctuations from GV oocytes
to blastocysts.
DNMT3A expression intheoocytes andearly embryos
When subcellular localizations of the DNMT3A protein in
the GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell,
morula and blastocyst stage embryos were analyzed, we
found that DNMT3A was strongly localized in the nucleus
of the oocytes and early embryos at all stages (Fig.2a).
Fig. 1 Immunoexpression of the DNMT1 protein in the mouse
oocytes and early embryos. a The immunostaining results of the
DNMT1 protein in the germinal vesicle oocytes (GV), MII oocytes
(MII), 1-cell embryos (1C), 2-cell embryos (2C), 4-cell embryos
(4C), 8-cell embryos (8C), morulae (M) and blastocysts (B) have been
shown. The negative control (NC) included no primary antibody was
used to identify whether there was any non-specific staining. The
DNMT1 protein expression exhibited spatial and temporal differ-
ences from GV oocytes to blastocyst stage embryos. In the micro-
graphs, FITC (green) staining for DNMT1; DAPI (4′,6-diamidino-
2-phenylindole; blue) staining for the nuclei; Merge, combination of
the DAPI-stained nucleus with the green fluorescence signal. b The
relative expression levels of the DNMT1 protein in the GV and MII
oocytes, and 1-cell, 2-cell, 4-cell, 8-cell, morula and blastocyst stage
embryos have been analyzed by using ImageJ software. The oocytes
and early embryos expressed the DNMT1 protein at different levels
showing gradual decreases and progressive increases. The statistical
significance among groups has been determined by using one-way
ANOVA test. The letters on the columns denote statistical signifi-
cance, and P < 0.05 is considered as statistically significant. (Color
figure online)

421J Mol Hist (2017) 48:417–426
1 3
However, there was a weak DNMT3A expression in the
cytoplasm of the oocytes and early embryos (Fig.2a).
In addition to subcellular localization analysis, we have
further evaluated the relative DNM3A protein expression
from GV oocytes to blastocyst stage embryos. The blasto-
cyst stage embryos had the highest, but the 8-cell embryos
possessed the lowest DNMT3A expression levels (P < 0.05;
Fig.2b). Importantly, both 1-cell and blastocyst stage
embryos expressed the DNMT3A significantly higher than
the remaining oocytes and early embryos (P < 0.05; Fig.2b).
Remarkably, DNMT3A protein expression progressively
increased from GV oocytes to 1-cell embryos, and it gradu-
ally decreased from 1-cell embryos to 8-cell stage embryos
(Fig.2b). Similarly, a progressive increase from 8-cell to
Fig. 2 Immunoexpression of the DNMT3A protein in the mouse
oocytes and early embryos. a The immunostaining of the DNMT3A
protein in the germinal vesicle oocytes (GV), MII oocytes (MII),
1-cell embryos (1C), 2-cell embryos (2C), 4-cell embryos (4C),
8-cell embryos (8C), morulae (M) and blastocysts (B) have been pre-
sented. The DNMT3A protein was expressed at the higher levels in
the nuclear region than that of the cytoplasm in the oocytes and early
embryos. The negative control (NC) included no primary antibody
has been used to identify whether there is any non-specific stain-
ing. In the micrographs, FITC (green) staining for DNMT3A; DAPI
(4′,6-diamidino-2-phenylindole; blue) staining for the nuclei; Merge,
combination of the DAPI-stained nucleus with green fluorescence
signal. b The relative expression levels of the DNMT3A protein in
the GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell, morulae and
blastocyst stage embryos have been analyzed by using ImageJ soft-
ware. And, we found that the oocytes and early embryos differently
expressed the DNMT3A protein. The statistical significance among
groups was tested by using one-way ANOVA. The letters on the col-
umns depict statistical significance, and P < 0.05 is considered as sta-
tistically significant. (Color figure online)

422 J Mol Hist (2017) 48:417–426
1 3
blastocyst stage embryos has been observed.Taken together,
DNMT3A expression distributes either in the cytoplasm or
nucleus of the oocytes and early embryos at different levels.
Also, it exhibits remarkable fluctuations from GV oocytes
to blastocyst stage embryos.
DNMT3B expression intheoocytes andearly embryos
The subcellular localizations and relative expression levels
of the DNMT3B protein in the GV and MII oocytes, and
1-cell, 2-cell, 4-cell, 8-cell, morula and blastocyst stage
embryos have been examined. The DNMT3B protein was
highly expressed in the nuclear regions of the GV oocytes,
1-cell, 2-cell and morula stage embryos. On the other hand,
it was localized in either nuclear or cytoplasm regions of the
MII oocytes, 4-cell and 8-cell embryos. In the blastocysts,
DNMT3B was more highly expressed in the cytoplasm than
that of in the nucleus (Fig.3a).
When we evaluated the relative expression levels of
DNMT3B from GV oocytes to blastocysts, it reached the
highest levels in the blastocysts, but it was at the lowest
profile in the 8-cell embryos (Fig.3b). Both MII oocytes
and blastocysts had significantly higher DNMT3B protein
expression than that of the remaining oocytes and early
embryos except for the 1-cell embryos (P < 0.05; Fig.3b).
Following predominant increase of the DNMT3B protein
expression from GV oocytes to MII oocytes, it began to
be gradually reduced from MII oocytes to 8-cell embryos.
Then, the DNMT3B expression progressively increased
from 8-cell embryos to blastocyst stage embryos (Fig.3b).
Overall, DNMT3B intensively located to the nuclear
region of the oocytes and early embryos. Besides, the rela-
tive expression of the DNMT3B protein exhibited gradual
decrease from MII oocytes to 8-cell embryos, and progres-
sively increase from 8-cell to blastocyst stage embryos
except for the GV oocytes remained at low level. It is impor-
tant to note that the expression profiles of DNMT3B protein
in the oocytes and early embryos coincide to a large extent
with that of the DNMT3A.
Discussion
In the present study, we characterized for the first time that
DNMT1, DNMT3A, and DNMT3B proteins exhibited
spatial and temporal expressional differences. And, they
were differently localized in the mouse oocytes and early
embryos. The DNMT1 protein was predominantly resided
in the cytoplasm of the oocytes and early embryos, and
showed gradual decrease and progressive increase from GV
oocytes to blastocyst stage embryos. Unlike DNMT1 subcel-
lular localization, DNMT3A was abundantly localized in the
nucleus of the oocytes and early embryos, and it exhibited
expressional fluctuations from GV oocytes to blastocysts.
Similarly, DNMT3B was more intensively expressed in the
nucleus when compared to its cytoplasmic localization in the
oocytes and early embryos. The DNMT3B expression levels
gradually decreased and progressively increased from GV
oocytes to blastocyst stage embryos, largely coinciding with
the expression distributions of the DNMT3A protein at the
same developmental terms.
DNA methylation is one of the epigenetic mechanisms
and plays critical roles in the expressional regulation of
the development-related genes, which are required during
oogenesis and early embryos. During oogenesis, global DNA
methylation begins to progressively increase in the growing
GV oocytes at the time period of from birth to puberty, and
it reaches the highest levels in the MII oocytes in mouse
(Saitou etal. 2012). Following fertilization, the global DNA
methylation levels gradually decreases from 1-cell to 16-cell
or morula stage embryos, and the paternally and maternally
imprinted genes are also demethylated late in embryogenesis
(Yang etal. 2007). In blastocysts, global DNA methylation
reaches the highest levels because new genomic imprints
are established by activating de novo methylation process
as well as maintenance methylation establishments (Saitou
etal. 2012).
In the present study, we demonstrated that the expression
levels of either DNMT1 or DNMT3B proteins were remark-
ably increased from GV oocytes to MII oocytes except for
DNMT3A which remained at the same levels in both oocyte-
types (Fig.4). This predominant increase of either DNMT1
or DNMT3B expression is largely compatible with the
increasing global DNA methylation levels from GV oocytes
to MII oocytes. Both proteins most likely implicate in estab-
lishing maintenance and de novo methylation processes in
both GV and MII oocytes. On the other hand, DNMT1,
DNMT3A, and DNMT3B protein expression were gradu-
ally decreased from 1-cell embryo to morula, and they were
progressively increased from morula to blastocysts (Fig.4).
The expression patterns of the DNMT proteins parallel with
the gradually decreasing DNA methylation levels from 1-cell
to morula stage embryos because there is no need for highly
expressed DNMTs at this time points of early development.
Since new genomic imprints, do novo and maintenance
methylation begin to be reestablished in the blastocyst stage
embryos, consistent with this we found that the expression
levels of the DNMT1, DNMT3A and DNMT3B proteinsare
at the highest levels in the blastocysts.
It has been reported in the previous studies (Mertineit
etal. 1998; Howell etal. 2001; Hirasawa etal. 2008) that
DNMT1 is intensively localized in the cytoplasm of oocytes
and early embryos as is detected in our current study. Rat-
nam etal. (2002) found that Dnmt1mRNA is present in the
postnatal oocytes obtained from days 1, 5, 10, 15, 21 and 35
in mouse (Ratnam etal. 2002). In bovines, DNMT1 is also

423J Mol Hist (2017) 48:417–426
1 3
identified at all GV oocytestages (from GV0 to GV3) and in
the MII oocytes (Lodde etal. 2009). Similar to the findings
from mouse and bovines, DNMT1 is expressed in human GV
and MII oocytes (Huntriss etal. 2004). Although DNMT1
is predominantly localized in the nucleus of GV oocytes,
it is intensively resided in the cytoplasm of MII oocytes in
humans (Petrussa etal. 2014). These results are moderately
comparable with our findings, and the differences former
and present studies are most likely originates from use of dif-
ferent species samples, analyzing transcript level other than
Fig. 3 Immunoexpression of the DNMT3B protein in the mouse
oocytes and early embryos. a The immunostaining of DNMT3B pro-
tein in the germinal vesicle oocytes (GV), MII oocytes (MII), 1-cell
embryos (1C), 2-cell embryos (2C), 4-cell embryos (4C), 8-cell
embryos (8C), morulae (M) and blastocysts (B) have been presented.
The DNMT3B protein expression differs in the oocytes and early
embryos. The negative control (NC) included no primary antibody
has been used to identify whether there is any non-specific stain-
ing. In the micrographs, FITC (green) staining for DNMT3B; DAPI
(4′,6-diamidino-2-phenylindole; blue) staining for the nuclei; Merge,
combination of the DAPI-stained nuclei with green fluorescence sig-
nal. b The relative expression levels of the DNMT3B protein in the
GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell, morula and
blastocyst stage embryoshave been analyzed by using ImageJ soft-
ware.We observed that DNMT3B exhibited gradual decreases and
progressive increases from GV oocytes to blastocysts. The statisti-
cal significance between groups has been determined by using one-
way ANOVA test. The letters on the columns depict statistical sig-
nificance, and P < 0.05 is considered as statistically significant. (Color
figure online)

424 J Mol Hist (2017) 48:417–426
1 3
protein level in the previous investigations and employing
distinct techniques to determine DNMT1 expression.
The Dnmt3a and Dnmt3bmRNAsare also foundat detect-
able levels in mouse, cat and bovine oocytes (Bourc’his etal.
2001; Kaneda etal. 2004; O’Doherty etal. 2012) as well as
in human oocytes (Huntriss etal. 2004). Ko etal. (2005)
revealed that DNMT3A is highly localized in the cytoplasm
of the 8-cell, morula and blastocyst stage embryos, but it is
intensively resided in the nuclear regions of the 1-cell, 2-cell
and 4-cell embryos in mice (Ko etal. 2005). On the other
hand, Hirasawa etal. (2008) demonstrated that DNMT3A
is abundantly resided in the nuclear region of the GV
oocytes, 1-cell, 2-cell, 4-cell, 8-cell, morula stage embryos
and that there is an intensive cytoplasmic expression in the
MII oocytes in mouse (Hirasawa etal. 2008). Intriguingly,
the blastocyst stage embryos do not exhibit any DNMT3A
expression either in the nuclear or cytoplasm regions
(Hirasawa etal. 2008). The DNMT3B expression has not
been observed in the GV oocytes and 1-cell embryos, and
it is localized in the nuclear region of the 2-cell embryos
and onward in mouse (Hirasawa etal. 2008). In contrast,
Ko etal. (2005) reported that DNMT3B is mainly local-
ized in the cytoplasm of the early embryos from 1-cell to
blastocyst stages (Ko etal. 2005). We have revealed in the
present study that both DNMT3A and DNMT3B are inten-
sively detected in the nucleus, and weakly localized in the
cytoplasm of the oocytes and early embryos. Among the
studies, there are prominent differences about intracellular
intensities of the DNMT3A and DNMT3B proteins in the
mouse oocytes and early embryos, this may derive from that
use of distinct technical applications and/or mouse strains
from which oocytes and early embryos are obtained.
In bovines, Dobbs etal. (2013) observed that global
DNA methylation is gradually decreased from 2-cell to
Fig. 4 Global DNA methylation profile and relative expression levels
of DNMT1, DNMT3A and DNMT3B proteins in the mouse oocytes
and early embryos. Red line, the global DNA methylation levels in
the oocytes and early embryos. We have created this line based on
the studies by Saitou etal. (2012) and Smallwood and Kelsey (2012).
Expression level of DNMT1 gene (pink line), DNMT3A (navy blue
line) and DNMT3B (blue line) in the oocytes and early embryos. The
expressional difference has been shown by distinct colour. GV germi-
nal vesicle oocyte, MII metaphase II oocyte, 1C 1-cell embryo, 2C
2-cell embryo, 4C 4-cell embryo, 8C 8-cell embryo, M morula, B
blastocyst. (Color figure online)

425J Mol Hist (2017) 48:417–426
1 3
6–8 cell stage embryos meaning that the lowest DNA
methylation levels has been detected at the 6–8 cell
embryos, and the methylation reaches the highest lev-
els in the blastocysts (Dobbs etal. 2013). In accordance
with reducing global DNA methylation levels after 2-cell
embryos, DNMT3B mRNA expression decreases from
2-cell to 6–8 cell embryo stages, and remarkably increases
in the blastocyst stage embryos (Dobbs etal. 2013). The
expression patterns of theDNMT3B mRNA from this study
and DNMT3B protein expression from our study argues
that DNMT3B likely functions in the genomic imprint
and maintenance methylation establishments during early
development in mouse and bovines.
As is known, the first lineage differentiation occurs
in the 8-cell embryos, and increaseof intercellular adhe-
sion, polarization and compaction take place based on
the increased cell–cell associations among blastomeres
(Albert and Peters 2009). And, DNA methylation is at low
levels in the 8-cell embryos (Dobbs etal. 2013). Consist-
ent with this DNMT1, DNMT3A and DNMT3B protein
expression is remarkably decreased at this stage. Since
many proteins including E-cadherin, catenin, mitogen-
activated protein kinase (MAPK), phosphatidylinositol
3-kinase (PtdIns3K), Wingless (Wnt), and Notch (Zhang
etal. 2007) are highly expressed in the 8-cell embryos,
the decreased global DNA methylation levels at this stage
may be required for facilitating the expression of these
genes in time.
In conclusion, this is the first detailed study analyz-
ing subcellular localizations and relative expression lev-
els of the DNMT1, DNMT3A and DNMT3B proteins
in the mouse oocytes and early embryos. We found that
DNMT1, DNMT3A and DNMT3B exhibit expressional
fluctuations from GV oocytes to blastocyst stage embryos.
These fluctuations are to large extent compatible with
timely establishing global DNA methylation and genomic
imprints in the oocytes and early embryos.We suppose that
more detailed molecular biological studies are required to
clearly understand the potential interactions of the DNMTs
in establishing DNA methylation and in the regulation of
early development-related genes during oogenesis and
early embryo development.
Funding This study was supported by Akdeniz University Sci-
entific Research Projects Coordination Unit (Project Number:
2014.02.0122.013).
Compliance with ethical standards
Conflict of interest The authors declare that there is no conflict of
interest.
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