SMAD3 directly regulates cell cycle genes to maintain arrest in granulosa cells of mouse primordial follicles
SMAD3 directly regulates cell cycle genes to maintain arrest in granulosa cells of mouse primordial follicles
Mark A. Fenwick
Sofia Granados-Aparici 0 1
Kate Hardy 2
Stephen Franks 2
Isam B. sharum 0 3
Sarah L. Waite 0
0 Academic Unit of Reproductive and Developmental Medicine, Department of Oncology and Metabolism, University of Sheffield , Sheffield, S10 2SF , United Kingdom
1 Present address: Department of Obstetrics and Gynecology, McGill University Health centre , Montreal, canada
2 institute of Reproductive and Developmental Biology, imperial College London, Hammersmith Hospital , Du Cane Road, London, W12 0NN , United Kingdom
3 Present address: Department of Surgery and theriogenology, college of Veterinary Medicine, University of Mosul , Mosul, i raq
OPEN Primordial follicles, consisting of granulosa cell (GC)-enveloped oocytes are maintained in a state of developmental arrest until activated to grow. The mechanism that operates to maintain this arrested state in GCs is currently unknown. Here, we show the TGF?-activated transcription factor SMAD3 is expressed in primordial GC nuclei alongside the cell cycle proteins, cyclin D2 (CCND2) and P27. Using neonatal C57/Bl6 mouse ovaries densely populated with primordial follicles, CCND2 protein colocalised and was detected in complex with P27 by immunofluorescence and co-immunoprecipitation, respectively. In the same tissue, SMAD3 co-precipitated with DNA sequences upstream of Ccnd2 and Myc transcription start sites implicating both as direct SMAD3 targets. In older ovaries follicle growth was associated with nuclear exclusion of SMAD3 and reduced P27 and CCND2 in GCs, alongside elevated Myc expression. Brief (2 H) exposure of neonatal ovaries to TGF?1 (10 ng/ml) in vitro led to immediate dissociation of SMAD3 from the Ccnd2 and Myc promoters. This coincided with elevated Myc and phospho-S6, an indicator of mTOR signalling, followed by a small increase in mean primordial GC number after 48 H. These findings highlight a concentration-dependent role for TGF? signalling in the maintenance and activation of primordial follicles, through SMAD-dependent and independent signalling pathways, respectively.
Primordial follicles, each consisting of a central oocyte surrounded by a single layer of supporting granulosa cells
(GCs) are held in a relative state of developmental arrest until activated to grow1?3. Maintenance and regulation of
the arrested state is fundamental for ensuring that a steady supply of oocytes is available for ovulation throughout
the reproductive life time in most mammals4,5; however, the molecular mechanisms that operate to maintain this
quiescent phenotype are currently unclear.
We recently showed that in the mouse ovary, the TGF?-mediated transcription factors SMAD2/3 are
detectable in nuclei of primordial GCs, suggesting that this pathway is active in these cells6. It is well established that
TGF? signalling is important for maintaining growth arrest in a range of cell types by directly inhibiting Myc7?9,
as well as regulating the expression of other cell cycle proteins10,11. Cell proliferation initially involves cell cycle
progression from G0-G112. Typically, this occurs through growth factor stimulated expression of cyclin D, which
then binds to cyclin-dependent kinase (CDK) proteins, precipitating release of E2F transcription factors to drive
cell cycle progression12?14. In many cell types, cyclin D-CDK complexes can also be inhibited by P27, creating a
stable trimeric complex that maintains cell cycle arrest15. In the ovary, in contrast to other D-type cyclins, cyclin
D2 (CCND2) mRNA is detectable in GCs of follicles across a range of stages and knockout of both cyclin D2
(Ccnd2?/?) and p27?/? exhibit dis-regulated early follicle development with major effects on fertility16?18.
We therefore carried out detailed analysis of CCND2 and P27 in the context of arrested primordial and early
developing follicles, focussing on the relationship of these factors with TGF? signalling in GCs. Using the C57/Bl6
represent measurements from d4 and d8 ovaries, respectively. (G) Nuclear/cytoplasmic ratio of SMAD3 in GCs
by follicle stage. Each point represents the % positive pixels in all GC nuclei relative to % positive pixels in the GC
compartment for an individual follicle obtained from d8 ovaries. Dashed lines (E,F) demarcate lower and upper
quartiles to represent proportions of samples that were considered low, medium and high intensity staining,
respectively. (H) Subcellular expression of SMAD3 by western blotting in protein lysates from d4 mouse ovaries.
Samples were separated into cytosolic, membrane-bound organelle, and nuclear fractions. The cytoplasmic
protein GAPDH is included as a control. Gel images have been cropped from originals provided in Fig.?S1. PF,
primordial; T, transitional; P, primary; P + , primary plus, S, secondary follicle. Data in C, D are means ? SEM
(n = 5 ovaries/age) and differences are relative to d4. Data in E, F, G are medians ? interquartile ranges and
differences are relative to the follicle stage in parentheses. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.
mouse as a model, we show that SMAD3 specifically localises to GCs of small follicles where it directly promotes
the expression of Ccnd2 and represses Myc. The CCND2 protein is detectable in complex with the inhibitory factor
P27 and together highlights a mechanism that potentially maintains primordial GC arrest, whilst ensuring they are
poised ready to proliferate. Interestingly, the loss of SMAD3 in GCs of growing follicles is associated with elevated
mTOR signalling, potentially explaining a dual mechanism of TGF? signalling in follicle growth and arrest.
Results and Discussion
SMAD3 is predominantly expressed in GCs of primordial follicles. Both SMAD2 and SMAD3 medi
ate TGF? signalling and regulate gene expression10,11,19; however, whether both or either SMAD is important in
this context is not known. In this study, we have used ovaries from C57/Bl6 mice at postnatal day 4, 8 and 16 as
a natural developmental paradigm to analyse gene expression in primordial, early activated (transitional), and
growing follicles (primary, primary+ , secondary), respectively20,21 (Fig.?1A). Immunofluorescent localisation of
SMAD2 was very weak in ovaries at each age, while SMAD3 was specifically expressed in GCs of small follicles,
notably in the earliest primordial stages as exemplified in sections from day 4 ovaries (Fig.?1B). Consistent with
this, the level of Smad2 mRNA, although detectable, did not vary between the different ovary ages (Fig.?1C),
whereas Smad3 mRNA expression was reduced in day 8 and 16 ovaries in which the relative proportion of
primordial follicles is reduced (Fig.?1D). Based on these findings we focused on SMAD3 for further analyses.
When SMAD3 immunofluorescence intensity values were plotted by follicle stage, a bi-phasic pattern was
evident, with total GC-expression being highest in transitional follicles that are poised to initiate growth, and
lowest in more advanced secondary-staged growing follicles (Fig.?1E). Since SMAD3 is a cytoplasmic signalling
mediator and nuclear transcription factor19,22, we analysed GC expression in both cellular compartments. In GC
nuclei SMAD3 was highly variable; however, the median expression was similar between primordial, transitional
and primary staged follicles, and was lower from the primary plus stage (Fig.?1F). Nuclear SMAD3 in each
follicle was then normalised against cytoplasmic expression to determine if the ratio was equal throughout follicle
development. The results, however, show that SMAD3 was similarly located in the nuclei and cytoplasm of GCs in
primordial and transitional follicles, with a relative reduction in nuclear expression from the primary stage,
indicating that nuclear exclusion occurs prior to the general loss of SMAD3 expression in GCs (Fig.?1G). Sub-cellular
fractionation of protein lysates from day 4 ovaries further confirmed that SMAD3 was detectable in both nuclear
and non-nuclear fractions (Fig.?1H). Since SMAD3 exhibits a DNA binding domain22, our findings support a role
for SMAD3 as a potential transcriptional regulator in GCs of primordial follicles.
CCND2 and P27 are maintained in GCs of primordial follicles but decrease as follicles initiate
growth. The expression of D-type cyclins occurs early in the cell cycle12,15; therefore, the localisation and
regulation of CCND2, as well as the inhibitory partner P27 in quiescent primordial follicles were investigated.
CCND2 was detectable in GC and oocyte nuclei, with relatively strong staining evident in GCs of small, single
layered follicles (Fig.?2A). Strong expression (intensity values >75%) of CCND2 was detectable in GCs of over
50% of primordial follicles analysed, with a relative reduction in expression from the primary stage onwards
(Fig.?2B). Consistent with this, Ccnd2 mRNA levels in whole ovaries were significantly reduced in older ovaries
containing proportionately more growing follicles, relative to day 4 ovaries (Fig.?2C). As with CCND2, P27
protein localised to GC nuclei of small follicles (Fig.?2D). Staining was highly variable between primordial follicles;
however, the nuclear expression of P27 significantly declined in the majority of growing follicles from the primary
stage onwards (Fig.?2E). This loss of expression in developing follicles reflected p27 mRNA expression in whole
ovaries, which was significantly reduced in day 8 and 16 ovaries, relative to day 4 (Fig.?2F).
Both proteins co-localised in GC nuclei of small follicles regardless of age (Fig.?2G). We then focussed on
d4 and d8 ovaries due to the preponderance of primordial and small activated follicles. Although the degree of
co-localisation in each cell varied widely within follicles and between follicle stages, the ratio of P27 to CCND2
declined from the transitional stage onwards. Thus, as follicles activate, P27 is initially reduced while CCND2
persists (Fig.?2H). In support of this, both CCND2 and P27 co-immunoprecipitated in protein lysates from day
4 ovaries. However, in day 16 ovaries, although CCND2 was detectable by western blot, the same protein was
beyond the limits of detection by western blotting following immunoprecipitation with P27 (Fig.?2I). Together
these data indicate that CCND2 and P27 exist in molecular complexes in GCs of primordial follicles. Consistent
with a role for P27 as a cell-cycle inhibitory factor15, it is likely that P27 is inhibiting the activity of CCND2 in
primordial GCs, thereby participating in maintaining the quiescent phenotype of primordial follicles.
SMAD3 directly regulates Ccnd2 and Myc expression in primordial follicles. Since the transcrip
tion factor SMAD3 is expressed in GC nuclei of primordial/transitional follicles alongside CCND2 and P27, we
then investigated whether SMAD3 has the ability to directly bind and regulate the expression of Ccnd2 and p27
in this context. The immediate early gene Myc was also used as a positive control, since it is a well-known target
of SMAD3 and is involved in cell-cycle progression7?9. In day 4 and day 16 ovaries, SMAD3 was detected on the
promoter of Ccnd2 using ChIP-PCR, which was significant when quantified in relation to non-immune control
IgG (Fig.?3A). This is consistent with the observed reduction in Ccnd2 gene expression in ovaries of Smad3?/?
mice23,24. SMAD3 was also detectable on the promoter of the Myc gene in day 4 ovary samples; however, this
binding was not significantly different from control IgG in day 16 ovary samples (Fig.?3B). By comparison, SMAD3
binding to the p27 promoter was undetectable by ChIP-PCR in day 4 ovaries, and barely detectable in day 16
ovaries (Fig.?3C). Therefore, it is unlikely that p27 is a direct target of SMAD3 in small follicles.
Since Myc has a well-ascribed role in promoting cell proliferation25,26, we then quantified expression in
immature ovaries. The increase in Myc mRNA expression in day 8 and day 16 ovaries relative to day 4 ovaries (Fig.?3D)
suggests that SMAD3 acts to repress Myc transcription in primordial follicles. Conversely, the decrease in Ccnd2
mRNA expression in day 16 ovaries relative to day 4 ovaries observed previously (Fig.?2A?C) shows that SMAD3
likely promotes cyclin D2 expression in primordial follicles. These findings imply that TGF?-SMAD3 signalling
acts directly on GCs of primordial follicles to differentially regulate genes involved in cell cycle regulation.
Short term exposure to TGF? 1 promotes mTOR signalling in neonatal mouse ovaries.
Considering the above observations, we then investigated the effect of an altered TGF? environment on small
follicle development in vitro. Previous studies have shown that long-term culture of neonatal rodent ovaries
( > 7 days) with a TGF? ligand can influence primordial follicle viability27?29. In cell culture models, changes in
SMAD3-regulated gene expression have been observed as little as 30 minutes after stimulation with TGF?
ligands30,31. Furthermore, a 60?90 min exposure of kit ligand or the pathway inhibitor imatinib to post-natal day 7
mouse ovaries in culture was sufficient to cause a significant shift in FOXO3a translocation in oocytes32. We
therefore evaluated effects on gene expression and signalling following a brief 2-hour exposure to TGF?1 ligand and
A83-01, a Type I receptor inhibitor, in day 4 ovaries in vitro. Consistent with in vivo findings, SMAD3 bound to
the promoter of Ccnd2 and Myc in unstimulated (control) ovaries. By comparison, binding to Ccnd2 and Myc was
no longer evident in ovaries exposed to either TGF?1, A83-01, or a combination of TGF?1 and A83-01 (Fig.?4A),
indicating that modulation of TGF? signalling causes dissociation of SMAD3 from the promoters of these genes.
Moreover, dissociation of SMAD3 to target gene promoters was not able to be over-ridden by exposure to TGF?1.
Interestingly, this change in binding was associated with elevated expression of the pro-proliferation factor Myc
in ovaries exposed to TGF?1. Myc is an early response gene and in other studies Myc mRNA reaches
maximum levels after 2?3 hours after growth factor stimulation, whereas induction of Ccnd2 mRNA peaks around
4 hours33,34. The latter point may explain why the loss of SMAD3 occupancy on the Ccnd2 promoter did not
effect Ccnd2 mRNA expression in the same way as Myc (Fig.?4B). Elevated transcript levels of Smad7 ? a negative
regulator of TGF? signalling35,36 were also observed, suggesting one effect of increased exposure to TGF?1 is to
inhibit the action of SMAD3 (Fig.?4B). In ovaries exposed to A83-01 or a combination of TGF?1 and A83-01, Myc
expression was unchanged. As Myc is normally repressed in cells with low turnover, it is possible that other
transcriptional repressors independent of TGF? signalling, such as WT137,38, are active in primordial GCs, although
further studies would be required to confirm this.
Since exposure to A83-01 and TGF?1 led to a similar loss of SMAD3 promoter binding, but with differential
effects on Myc and Smad7 expression, we sought to determine if this was associated with elevated SMAD3
signalling, or activation of SMAD-independent pathways. Levels of p-SMAD3, p-AKT and p-ERK1/2 proteins did
not vary between TGF?1, A83-01 or control groups. By comparison, the mTOR mediator p-S6 was significantly
increased in ovaries exposed to TGF?1 in comparison to ovaries exposed to A83-01 or control (Fig.?4C),
indicating that elevated TGF? signalling in neonatal ovaries leads to early effects on the mTOR pathway.
Acute modulation of TGF? signalling causes stage-specific effects on follicle development.
Finally, in order to evaluate the consequence of an acutely altered TGF? environment, ovaries were returned
to control media for an additional two days. Processed ovaries were then stained with SMAD3 and DDX4 as
markers of the GC compartment and oocytes, respectively, for morphological assessment and quantification.
The overall distribution of follicles was similar between groups, although GCs of growing primary follicles in
ovaries exposed to A83-01 had failed to adequately cuboidalise (Fig.?5A). When follicles were classified according
to oocyte size, TGF?1 or A83-01 had no effect on stage of follicle development (Fig.?5B). Likewise, exposure to
TGF?1 or A83-01 had no effect on oocyte size (Fig.?5C). Interestingly, brief exposure to TGF?1 ligand promoted
a small, but measurable increase in the mean GC number of follicles classified as primordial, suggesting that
2 hours of elevated TGF?1 was sufficient to promote GC proliferation but not sufficient enough to promote
significant oocyte growth. By comparison, exposure to A83-01 had no effect on GCs of primordial follicles but caused
a significant reduction in mean GC number in follicles classified as growing, indicating that inhibition of TGF?
receptors inhibits normal GC proliferation in activated follicles (Fig.?5D). It should also be noted that very few
apoptotic cells were detectable in any of the groups as determined by TUNEL labelling (Fig.?S6). The reduction in
GC number along with inadequate cuboidalisation indicates that inhibition of TGF? receptors likely impairs
normal GC proliferation in activated follicles. Thus, although SMAD3 is reduced in GCs of growing follicles (Fig.?1),
TGF? signalling is still essential, which is consistent with the well-ascribed role of other TGF? ligands that signal
through the same Type I receptors, such as GDF939?42 and activin43?46 in supporting early follicle development.
The ovarian reserve consists of a limited pool of primordial follicles held in a state of developmental arrest1,3?5.
The molecular programme that maintains this arrest, and conversely regulates activation and growth is largely
unknown. We recently presented a detailed analysis highlighting an inverse relationship between nuclear
SMAD2/3 expression and cell proliferation in GCs of mouse ovaries, suggesting a role for TGF? signalling in
primordial follicle maintenance and activation6. Here, we show that SMAD3 recapitulates the pattern of SMAD2/3
and is associated with expression of the cell cycle regulators cyclin D2 and P27. The specific localisation of these
proteins was similar in ovaries derived from d4 and d8 mice, each with different populations of developing
follicles, further highlighting a stage-specific, rather than age-dependent role in in GCs of non-growing and
early growing follicles (Fig.?S7). In non-growing follicles, it is unlikely that SMAD3 directly regulates P27 in this
context; instead, SMAD3 binds directly to Ccnd2 and Myc to differentially regulate these genes, which may be
important for ensuring GCs are ?poised? in a state ready to progress through the cell cycle. The presence and
accumulation of CCND2 in complex with P27 suggests a mechanism for primordial follicle maintenance, where P27
may prevent GC cycle progression by inhibiting cyclin-CDK activity15. This hypothesis is consistent with other
studies reporting an inhibitory effect of P27 on early follicle activation16,47. Although the mechanism leading to a
reduction in P27 activity in this context is currently unknown, it is clear that follicle growth is also associated with
a reduction in accumulated CCND2 and SMAD3 ? the latter also allowing de-repression of Myc. CCND2 and
SMAD3 are important regulators of the cell cycle and although their steady-state expression is reduced in growing
follicles, both proteins are still expressed at a?moderate level in GCs (Figs?1 and 2). Furthermore, briefly exposing
ovaries to an elevated environment of TGF? led to a slight increase in GC proliferation. This occurred following
an immediate uncoupling of SMAD3 from Ccnd2 and Myc gene promoters alongside elevated mTOR signalling.
Activation of mTOR signalling in primordial GCs is a key early event in follicle initiation48, has been implicated
in TGF?-driven models of epithelilal-mesenchymal transition49, and plays an important role more generally in
cellular growth and proliferation50.
Based on these findings, we propose a multi-step mechanism of follicle activation that initially involves a basal
level of SMAD3 expression and nuclear activity to prepare GCs for proliferation. It remains to be determined
whether this activity in primordial GCs is dependent on receptor-mediated signalling, and if so, which is the
principle ligand involved; however, increased TGF?1 ligand exposure promotes a shift to a SMAD-independent
pathway, which drives cell proliferation and eventual follicle activation (Fig.?6). The role of other TGF?
superfamily members and the cross-talk involved in regulating SMAD signalling in this context will also be important to
establish. Although this study has used the mouse as a model organism, revelation of the molecular framework
that underscores the phenotype of pre-granulosa cells in other species is essential for understanding how the
ovarian reserve is maintained throughout the reproductive lifetime of the female.
Materials and Methods
Animals and tissue collection. All ovary tissues used in this study were obtained from wild-type C57Bl6
mice housed in the Biological Services Unit at the University of Sheffield in compliance with the Animals
(Scientific Procedures) Act, 1986. Mice were killed by a registered practitioner (Schedule 1 procedures) with
approval from the University of Sheffield Training and Competency Officer. For total RNA, protein extraction
and ChIP experiments, ovaries from immature mice (day 4, 8 and 16) were dissected and either placed in culture
(day 4 only) or snap frozen in liquid nitrogen and stored at ?80 ?C. For immunostaining, ovaries were immersed
in 10% neutral buffered formalin solution (Sigma-Aldrich) for 24 hours and processed in paraffin blocks and
sectioned at 5 ?m.
Neonatal mouse ovary culture. Ovaries were placed in 24 mm Transwell plates containing membrane
inserts (Corning; Sigma). Culture medium consisted of Waymouth medium 752/1 (Life Technologies)
supplemented with 10% fetal bovine serum (FBS; Sigma), 0.23 mM pyruvic acid (Sigma), 10 ? g/ml streptomycin
sulfate (Sigma), 75 ? g/ml penicillin G (PENK; Sigma) and 0.3 mg/mL BSA. Ovaries were maintained in a 37 ?C
incubator (5% CO2) for 24 h prior to any treatment. For the assessment of the molecular changes under
different TGF? conditions (SMAD3 binding and gene regulation) ovaries were exposed for 2 h in serum free culture
media, supplemented with either TGF?1 ligand (10 ng/ml; R&D systems, 240-B-002), A83-01 (1 ?M; MedChem
Press, 2939), a selective inhibitor of Type 1 TGF? receptors ALK4, ALK5 and ALK751, a combination of TGF?1
(10 ng/ml) and A83-01 (1 ? M), or DMSO (control; same volume as for A83-01; Sigma). A single time point
(2 hours) and concentration for TGF?1 ligand (10 ng/ml) and A 83-01 inhibitor (1 ? M) were selected based on
previous studies27?29,51,52. Immediately following treatment, ovaries were collected and either fixed and processed
for ChIP-qPCR or flash frozen in liquid nitrogen for gene expression analysis. For the assessment of the
morphological changes, ovaries were exposed for 2 h in serum free culture media followed by supplementation as above.
In the TGF?1 treatment group, the addition of A83-01 inhibitor for an extra 1 hour was used to block further
TGF?1 signalling as previously described53. Treatments were then washed out and ovaries were left in culture for
two days before processing.
Immunofluorescence staining. Sections were dewaxed before microwaving in 0.1 M citrate buffer
(pH6.0). Sections were blocked with CAS universal blocking reagent (ThermoFisher) for 20minutes at room
temperature to reduce non-specific binding. Specific antibodies against SMAD2 (0.59? g/ml, #5339, Cell Signaling
Technology, MA, USA), SMAD3 (0.75 ?g/ml, #9523, Cell Signaling), SMAD2/3 (0.25 ?g/ml, 133098, Santa Cruz
Biotechnology, TX, USA), CCND2 (0.5 ? g/ml, sc-593, Santa Cruz Biotechnology, TX, USA), DDX4 (2.2 ? g/ml,
ab13840, Abcam, Cambridge, UK) and P27 (1 ? g/ml, sc-528, Santa Cruz or 0.5 ? g/ml, sc-1641 for co-localisation)
were diluted in blocking solution and incubated overnight at 4 ?C. A species-specific isotype control IgG antibody
at the same concentration (normal rabbit IgG I-1000 and normal mouse IgG I-200, Vector Laboratories) were
included in each experiment. Sections were washed in PBS and incubated with secondary antibodies (either
donkey anti-rabbit AlexaFluor? 488 or AlexaFluor? 555; donkey anti-mouse AlexaFluor? 488 and AlexaFluor?
555, each 1:400; Invitrogen). Sections were mounted with a drop of Prolong? Gold anti-fade reagent with DAPI
(ThermoFisher) and were imaged using an inverted Leica SP5 confocal laser-scanning microscope (Leica
Microsystems, Wetzlar, Germany).
Image analysis and quantification of imunofluorescence. Quantification of GC factors in situ
was performed from a total of 7 ovaries (four day 4 and three day 8 ovaries) using the largest cross section per
ovary. Confocal images were stitched together using DoubleTake (http://echoone.com/doubletake/) to create a
high-resolution composite of each section. Images were obtained as an 8-bit RGB stack file and the threshold
for each channel in every composite section was set up with the same criteria across the different sections of the
same staining. Pixels above the threshold were considered as ?positive pixels? while the remaining pixels were
registered as negative. The oocyte cytoplasm was used as a baseline setting (due to lack of specific staining) and
threshold settings were normalised against this to account for variation between slides. Follicles with a visible
oocyte nucleus were selected with the region of interest (ROI) manager tool of ImageJ software (https://imagej.
nih.gov/ij/). In order to quantify each of the candidate proteins in different ROIs, the area of positive pixels ?Area
fraction? was calculated and referred as ?Intensity? for simplicity. ROIs from each follicle were selected manually
using the DAPI channel and the ?measure and label? tool, which allowed the intensity of each ROI in either the
green or red channel to be quantified. Follicles were classified by the number of GCs and oocyte area (Fig.?S5).
For each follicle, measurements were recorded for the intensities of individual GC nuclei. For SMAD3 analysis,
the intensity of the entire GC compartment was also measured and the ratio between the geometric mean of
the intensities for the GC nuclei, and the geometric mean for the entire GC compartment were calculated and
plotted as a measure of nuclear:cytoplasm expression. CCND2 and P27 co-localisation was analysed with the
?Colocalization threshold? plugin from ImageJ. A minimum of 20 individual GC nuclei from primordial and
transitional follicles were selected from a single day 4 and day 8 ovary section. The ratio of intensities between
CCND2 and p27 was represented as (P27/CCND2). For cultured ovaries, sections were stained with antibodies
against DDX4 and SMAD3 and image analysis was performed as described above. When more than one section
from the same ovary was used, a minimum separation of 2 sections (10 ? m) from the largest cross-section was
considered to avoid double counting.
Gene expression analysis. Total RNA was isolated from ovaries using RNeasy? Micro Kits (QIAGEN,
Crawley, UK). Concentration and purity of all RNA samples was assessed using the Agilent 2100 Bioanalyzer.
Samples with an RNA integrity number (RIN) between 8 and 10 were used for gene expression analysis. Fifty ng
of RNA from each sample was converted to cDNA using the SuperScript III Reverse Transcription kit (Invitrogen;
ThermoFisher). Quantivative PCR (qPCR) using 500nM of primers for Smad2, Smad3, Smad7, Myc, Ccnd2 and
p27 (Table?S1) were used with the SensiFAST? SYBR? Hi-ROX kit (Bioline). Efficiencies were 90?130% for all
primers. Samples (1 ?l per reaction) and the same volume of distilled water (negative control) were run in
duplicate in 384 well-plates. qPCR conditions were as follows: activation at 95 ?C for 2 minutes and then 40 cycles of
amplification (3 seconds at 95 ?C for DNA denaturation, 20 seconds at 60 ?C for primer annealing and 10 seconds
at 72 ?C for primer extension) using a 7900HT Fast Real-Time PCR System (Applied Biosystems). Ct values were
normalised against Atpb5 (PrimerDesign, Southampton, UK) as previously described6,21 and fold changes
(relative to control sample) were calculated for each gene according to the 2???Ct method54.
Protein extraction from mouse ovaries. Protein extraction from whole ovaries was performed after
initial disruption with 25 G needles with Pierce IP lysis buffer (Thermo Fisher Scientific) containing protease
inhibitors (cOmpleteTM Mini ULTRA tablets; Roche) and phosphatase inhibitors (PhosSTOPTM, Roche). Pooled ovaries
) from day 4 and 16 mice were used for co-immunoprecipitation or (
) for processing cultured ovaries. Samples
were vortexed, centrifuged and concentration of supernatant was measured using the Pierce bicinchoninic acid
(BCA) protein assay kit (ThermoFisher). For subcellular fractionation, proteins were extracted from six day 4
mouse ovaries using a sequential lysis protocol as described previously55. This protocol utilises a gradient of
detergent lysis buffers to sequentially yield a cytosolic fraction, a membrane-bound organelle fraction and a nuclear
fraction, respectively. Equal amounts (3?5 ?g) of the cytosolic, organelle and nuclear protein fractions were used
for analysis by western blotting.
Co-immunoprecipitation and Western blotting. Twenty-five ? g of total protein was diluted with
Pierce IP lysis buffer (ThermoFisher Scientific) and 2? g of the immunoprecipitating antibody P27 (sc-528,
Santa Cruz) were incubated for 1.5 hours at 4 ?C with rotation. The same amount of protein was used to incubate
with a non-specific IgG (Normal rabbit IgG I-1000, Vector Laboratories). 20 ? l of Protein G magnetic beads
(Merck-Millipore, MA, USA) were added to each aliquot and incubated with rotation for an additional hour.
Bead/protein complexes were washed with IP buffer and gentle centrifugation. After the last wash, 35? l of 5x
loading buffer (EC-887, National Diagnostics, GA, USA) was used to elute the beads. Half of the volume of the
IP samples was loaded on the western blot, and input samples (control for the IP) contained 20% of the initial
amount of protein used for the IPs. Proteins were separated in 12% separating gel and 4% stacking PAGE gels.
Proteins were transferred to a nitrocellulose membrane using a semi-dry transfer. Membranes were washed with
TBS-Tween and blocked in either 5% (w/v) BSA (Sigma) for phospho-protein detection or 5% (w/v) non-fat
dried milk at room temperature for 2 hours. Blots were washed in TBS-Tween before applying primary antibodies
for CCND2 (0.4 ? g/ml; sc-593, Santa Cruz), P27 (0.6 ? g/ml; sc-528, Santa Cruz), p-SMAD3 (1:1000; ab52903,
Abcam), p-AKT (1:1000; 4060, Cell Signaling), p-ERK1/2 (1:1000; 4370, Cell Signaling), p-S6 (1:1000; 2211, Cell
Signaling) overnight at 4 ?C. Antibodies were detected using HRP-conjugated secondary antibodies (1:20,000;
P0448, DAKO) and ECL reagents (Westar Supernova enhanced chemiluminescent HRP substrate; Cyanagen,
Chromatin immunoprecipitation (ChIP) - qPCR. Whole ovaries were fixed with 1% formaldehyde
(Sigma) in Leibovitz?s L-15 medium (Life Technologies) for 30 minutes. An EZ- Magna ChIPTM G kit (Millipore)
was used according to manufacturer?s instructions for the preparation of nuclear extracts and subsequent ChIP.
2% of the initial volume was taken as control for the IPs (Input). Protein G magnetic beads (Merck-Millipore)
were added to each aliquot along with 1 ? g of immunoprecipitating antibody against SMAD2/3 (sc-133098, Santa
Cruz) and non-immune mouse IgG (I-200, Vector Laboratories). As a positive control, an antibody against the
protein FOXL2 was used (0.2 ? g/ml; sc-55655, Santa Cruz), since Ccnd2 and p27 are known targets of this
protein56,57. An equal volume (4 ? l) of IP sample, control or RNAse/DNAse free water (additional negative control)
were analysed by qPCR in duplicate using primers designed to flank predicted FOXL2, SMAD3 and MYC binding
sites (TRANSFAC; Table?S1) in 384 well-plates as described above. Input, IPs and IgG control samples were
analysed using the ?% input? method where the mean Ct value of each IP and IgG sample was normalised against the
input sample. Since the starting input sample was 2% of the initial chromatin, a dilution factor of 50 or 5.64 cycles
(log 2 of 50) was subtracted and calculations were performed as follows: i) Mean Ct INPUT ? 5.64, ii) Mean Ct IP
(or IgG) - Mean Ct INPUT = ?Ct, iii)% INPUT = (2 (??Ct)) ? 100.
Statistical analysis. For immunofluorescent quantification of SMAD3, CCND2 and P27, follicle
stage-specific differences in expression were analysed using a non-parametric Kruskal-Wallis test with a post-hoc
Dunn?s multiple comparisons test. For all gene expression analyses by qPCR, fold changes between groups were
evaluated using one-way ANOVAs with post-hoc Bonferroni multiple comparisons tests. For the ChIP-qPCR
data, non-parametric Mann-Whitney U tests were used to compare % input values in SMAD3 vs IgG control
groups. Western blot data was analysed using one-way ANOVAs with post-hoc Tukey?s multiple comparisons
tests. Effect of different culture conditions on oocyte size was evaluated using a Kruskal-Wallis test with post-hoc
Dunn?s multiple comparisons. All analyses were carried out using Prism (v7.03; Graphpad). Differences were
considered significant when P < 0.05.
All data generated or analysed during this study are included in this published article (and its Supplementary
The authors would like to thank Dr Paul Heath, Dr Neil Chapman and Ms Orla Gallagher (University of Sheffield)
for helpful technical advice. This work was supported by funds received from The Royal Society UK awarded
to M.F. (RG130193), the Infertility Research Trust awarded to M.F. and a University of Sheffield postgraduate
scholarship awarded to S.G.-A.
The study was conceived by S.G.-A., K.H., S.F., and M.A.F. Experimental work was carried out by S.G.-A., I.B.S.,
S.L.W., M.A.F.; Formal analysis and validation S.G.-A., M.A.F.; Writing original draft S.G.-A., M.A.F.; All authors
reviewed the manuscript.
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-42878-4.
Competing Interests: The authors declare no competing interests. Publisher?s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made. The images or other third party material in this
article are included in the article?s Creative Commons license, unless indicated otherwise in a credit line to the
material. If material is not included in the article?s Creative Commons license and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the
copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
1. Adhikari , D. & Liu , K. Molecular Mechanisms Underlying the Activation of Mammalian Primordial Follicles . Endocr Rev 30 , 438 - 464 , https://doi.org/10.1210/er.2008- 0048 ( 2009 ).
2. Da Silva-Buttkus , P. et al. Effect of cell shape and packing density on granulosa cell proliferation and formation of multiple layers during early follicle development in the ovary . J Cell Sci 121 , 3890 - 3900 , https://doi.org/10.1242/jcs.036400 ( 2008 ).
3. McLaughlin , E. A. & McIver , S. C. Awakening the oocyte: controlling primordial follicle development . Reproduction 137 , 1 - 11 , https://doi.org/10.1530/Rep-08- 0118 ( 2009 ).
4. Monniaux , D. et al. The ovarian reserve of primordial follicles and the dynamic reserve of antral growing follicles: what is the link? Biol Reprod 90 , 85 , https://doi.org/10.1095/biolreprod.113.117077 ( 2014 ).
5. Skinner , M. K. Regulation of primordial follicle assembly and development . Hum Reprod Update 11 , 461 - 471 , https://doi. org/10.1093/humupd/dmi020 ( 2005 ).
6. Hardy , K. et al. Nuclear exclusion of SMAD2/3 in granulosa cells is associated with primordial follicle activation in the mouse ovary . J Cell Sci 131 , https://doi.org/10.1242/jcs.218123 ( 2018 ).
7. Pietenpol , J. A. et al. TGF-beta 1 inhibition of c-myc transcription and growth in keratinocytes is abrogated by viral transforming proteins with pRB binding domains . Cell 61 , 777 - 785 ( 1990 ).
8. Seoane , J. et al. TGFbeta influences Myc, Miz-1 and Smad to control the CDK inhibitor p15INK4b . Nat Cell Biol 3 , 400 - 408 , https:// doi.org/10.1038/35070086 ( 2001 ).
9. Yagi , K. et al. c -myc is a downstream target of the Smad pathway . J Biol Chem 277 , 854 - 861 , https://doi.org/10.1074/jbc. M104170200 ( 2002 ).
10. Massague , J. TGFbeta signalling in context . Nat Rev Mol Cell Biol 13 , 616 - 630 , https://doi.org/10.1038/nrm3434 ( 2012 ).
11. Morikawa , M. , Derynck , R. & Miyazono , K. TGF-beta and the TGF-beta Family: Context-Dependent Roles in Cell and Tissue Physiology . Cold Spring Harb Perspect Biol 8 , https://doi.org/10.1101/cshperspect.a021873 ( 2016 ).
12. Sherr , C. J. & Roberts , J. M. CDK inhibitors: positive and negative regulators of G1-phase progression . Genes Dev 13 , 1501 - 1512 ( 1999 ).
13. Haberichter , T. et al. A systems biology dynamical model of mammalian G1 cell cycle progression . Mol Syst Biol 3 , 84 , https://doi. org/10.1038/msb4100126 ( 2007 ).
14. Knudsen , E. S. & Wang , J. Y. Dual mechanisms for the inhibition of E2F binding to RB by cyclin-dependent kinase-mediated RB phosphorylation . Mol Cell Biol 17 , 5771 - 5783 ( 1997 ).
15. Susaki , E. , Nakayama , K. & Nakayama , K. I. Cyclin D2 translocates p27 out of the nucleus and promotes its degradation at the G0-G1 transition . Mol Cell Biol 27 , 4626 - 4640 , https://doi.org/10.1128/ MCB . 00862 - 06 ( 2007 ).
16. Rajareddy , S. et al. p27kip1 (cyclin-dependent kinase inhibitor 1B) controls ovarian development by suppressing follicle endowment and activation and promoting follicle atresia in mice . Mol Endocrinol 21 , 2189 - 2202 , https://doi.org/10.1210/me.2007- 0172 ( 2007 ).
17. Sicinski , P. et al. Cyclin D2 is an FSH-responsive gene involved in gonadal cell proliferation and oncogenesis . Nature 384 , 470 - 474 , https://doi.org/10.1038/384470a0 ( 1996 ).
18. Robker , R. L. & Richards , J. S. Hormone-induced proliferation and differentiation of granulosa cells: a coordinated balance of the cell cycle regulators cyclin D2 and p27Kip1 . Mol Endocrinol 12 , 924 - 940 , https://doi.org/10.1210/mend.12.7. 0138 ( 1998 ).
19. Moustakas , A. & Heldin , C. H. The regulation of TGFbeta signal transduction . Development 136 , 3699 - 3714 , https://doi. org/10.1242/dev.030338 ( 2009 ).
20. Fenwick , M. A. , Mansour , Y. T. , Franks , S. & Hardy , K. Identification and regulation of bone morphogenetic protein antagonists associated with preantral follicle development in the ovary . Endocrinology 152 , 3515 - 3526 , https://doi.org/10.1210/en.2011- 0229 ( 2011 ).
21. Sharum , I. B. , Granados-Aparici , S. , Warrander , F. C. , Tournant , F. P. & Fenwick , M. A. Serine threonine kinase receptor associated protein regulates early follicle development in the mouse ovary . Reproduction 153 , 221 - 231 , https://doi.org/10.1530/REP-16- 0612 ( 2017 ).
22. Hill , C. S. Transcriptional Control by the SMADs . Cold Spring Harb Perspect Biol 8 , https://doi.org/10.1101/cshperspect.a022079 ( 2016 ).
23. Looyenga , B. D. & Hammer , G. D. Genetic removal of Smad3 from inhibin-null mice attenuates tumor progression by uncoupling extracellular mitogenic signals from the cell cycle machinery . Mol Endocrinol 21 , 2440 - 2457 , https://doi.org/10.1210/me.2006- 0402 ( 2007 ).
24. Tomic , D. et al. Ovarian follicle development requires Smad3 . Mol Endocrinol 18 , 2224 - 2240 , https://doi.org/10.1210/me.2003- 0414 ( 2004 ).
25. Adams , M. R. , Sears , R. , Nuckolls , F. , Leone , G. & Nevins , J. R. Complex transcriptional regulatory mechanisms control expression of the E2F3 locus . Mol Cell Biol 20 , 3633 - 3639 ( 2000 ).
26. Leone , G. , DeGregori, J., Sears , R. , Jakoi , L. & Nevins , J. R. Myc and Ras collaborate in inducing accumulation of active cyclin E/ Cdk2 and E2F . Nature 387 , 422 - 426 , https://doi.org/10.1038/387422a0 ( 1997 ).
27. Rosairo , D. , Kuyznierewicz , I. , Findlay , J. & Drummond , A. Transforming growth factor-beta: its role in ovarian follicle development . Reproduction 136 , 799 - 809 , https://doi.org/10.1530/Rep-08- 0310 ( 2008 ).
28. Schindler , R. , Nilsson , E. & Skinner , M. K. Induction of ovarian primordial follicle assembly by connective tissue growth factor CTGF . PLoS One 5 , e12979, https://doi.org/10.1371/journal.pone. 0012979 ( 2010 ).
29. Wang , Z. P. et al. Transforming growth factor-beta signaling participates in the maintenance of the primordial follicle pool in the mouse ovary . J Biol Chem 289 , 8299 - 8311 , https://doi.org/10.1074/jbc.M113. 532952 ( 2014 ).
30. Du , D. et al. Smad3-mediated recruitment of the methyltransferase SETDB1/ESET controls Snail1 expression and epithelialmesenchymal transition . EMBO Rep 19 , 135 - 155 , https://doi.org/10.15252/embr.201744250 ( 2018 ).
31. Zhang , Y. et al. High throughput determination of TGFbeta1/SMAD3 targets in A549 lung epithelial cells . PLoS One 6 , e20319, https://doi.org/10.1371/journal.pone. 0020319 ( 2011 ).
32. Ezzati , M. M. et al. Regulation of FOXO3 subcellular localization by Kit ligand in the neonatal mouse ovary . J Assist Reprod Genet 32 , 1741 - 1747 , https://doi.org/10.1007/s10815-015-0589- 9 ( 2015 ).
33. Lau , L. F. & Nathans , D. Expression of a set of growth-related immediate early genes in BALB/c 3T3 cells: coordinate regulation with c-fos or c-myc . Proc Natl Acad Sci USA 84 , 1182 - 1186 ( 1987 ).
34. Park , Y. et al. Induction of cyclin D2 in rat granulosa cells requires FSH-dependent relief from FOXO1 repression coupled with positive signals from Smad . J Biol Chem 280 , 9135 - 9148 , https://doi.org/10.1074/jbc. M409486200 ( 2005 ).
35. Nakao , A. et al. Identification of Smad7, a TGFbeta-inducible antagonist of TGF-beta signalling . Nature 389 , 631 - 635 , https://doi. org/10.1038/39369 ( 1997 ).
36. Yan , X. et al. Smad7 Protein Interacts with Receptor-regulated Smads (R-Smads) to Inhibit Transforming Growth Factor-beta (TGFbeta)/Smad Signaling . J Biol Chem 291 , 382 - 392 , https://doi.org/10.1074/jbc.M115. 694281 ( 2016 ).
37. Gao , F. et al. Wt1 functions in ovarian follicle development by regulating granulosa cell differentiation . Hum Mol Genet 23 , 333 - 341 , https://doi.org/10.1093/hmg/ddt423 ( 2014 ).
38. Hartkamp , J. , Carpenter , B. & Roberts , S. G. The Wilms' tumor suppressor protein WT1 is processed by the serine protease HtrA2/ Omi . Mol Cell 37 , 159 - 171 , https://doi.org/10.1016/j.molcel. 2009 . 12 .023 ( 2010 ).
39. Elvin , J. A. , Yan , C. , Wang , P. , Nishimori , K. & Matzuk , M. M. Molecular characterization of the follicle defects in the growth differentiation factor 9-deficient ovary . Mol Endocrinol 13 , 1018 - 1034 , https://doi.org/10.1210/mend.13.6. 0309 ( 1999 ).
40. Hayashi , M. et al. Recombinant growth differentiation factor-9 (GDF-9) enhances growth and differentiation of cultured early ovarian follicles . Endocrinology 140 , 1236 - 1244 , https://doi.org/10.1210/endo.140.3. 6548 ( 1999 ).
41. Vitt , U. A. , Hayashi , M. , Klein , C. & Hsueh , A. J. Growth differentiation factor-9 stimulates proliferation but suppresses the folliclestimulating hormone-induced differentiation of cultured granulosa cells from small antral and preovulatory rat follicles . Biol Reprod 62 , 370 - 377 , https://doi.org/10.1095/biolreprod62.2. 370 ( 2000 ).
42. Wang , C. & Roy , S. K. Expression of growth differentiation factor 9 in the oocytes is essential for the development of primordial follicles in the hamster ovary . Endocrinology 147 , 1725 - 1734 , https://doi.org/10.1210/en.2005- 1208 ( 2006 ).
43. Ding , C. C. , Thong , K. J. , Krishna , A. & Telfer , E. E. Activin A inhibits activation of human primordial follicles in vitro . J Assist Reprod Genet 27 , 141 - 147 , https://doi.org/10.1007/s10815-010-9395- 6 ( 2010 ).
44. Liu , X. et al. A comparative study on transforming growth factor-beta and activin A for preantral follicles from adult, immature, and diethylstilbestrol-primed immature mice . Endocrinology 140 , 2480 - 2485 , https://doi.org/10.1210/endo.140.6. 6827 ( 1999 ).
45. Mizunuma , H. et al. Activin from secondary follicles causes small preantral follicles to remain dormant at the resting stage . Endocrinology 140 , 37 - 42 , https://doi.org/10.1210/endo.140.1. 6409 ( 1999 ).
46. Smitz , J. , Cortvrindt , R. , Hu , Y. & Vanderstichele , H. Effects of recombinant activin A on in vitro culture of mouse preantral follicles . Mol Reprod Dev 50 , 294 - 304 , https://doi.org/10.1002/(SICI) 1098 - 2795 ( 199807 )50: 3 < 294 : :AID-MRD5>3.0 .CO;2- E ( 1998 ).
47. Hirashima , Y. , Moniruzzaman , M. & Miyano , T. p27 (Kip1) negatively regulates the activation of murine primordial oocytes . J Reprod Dev 57 , 217 - 222 ( 2011 ).
48. Zhang , H. et al. Somatic cells initiate primordial follicle activation and govern the development of dormant oocytes in mice . Curr Biol 24 , 2501 - 2508 , https://doi.org/10.1016/j.cub. 2014 . 09 .023 ( 2014 ).
49. Lamouille , S. , Connolly , E. , Smyth , J. W. , Akhurst , R. J. & Derynck , R. TGF-beta-induced activation of mTOR complex 2 drives epithelial-mesenchymal transition and cell invasion . J Cell Sci 125 , 1259 - 1273 , https://doi.org/10.1242/jcs.095299 ( 2012 ).
50. Laplante , M. & Sabatini , D. M. mTOR signaling in growth control and disease . Cell 149 , 274 - 293 , https://doi.org/10.1016/j. cell. 2012 . 03 .017 ( 2012 ).
51. Tojo , M. et al. The ALK-5 inhibitor A-83-01 inhibits Smad signaling and epithelial-to-mesenchymal transition by transforming growth factor-beta . Cancer Sci 96 , 791 - 800 , https://doi.org/10.1111/j.1349- 7006 . 2005 . 00103 . x ( 2005 ).
52. Yamamura , S. et al. The activated transforming growth factor-beta signaling pathway in peritoneal metastases is a potential therapeutic target in ovarian cancer . Int J Cancer 130 , 20 - 28 , https://doi.org/10.1002/ijc.25961 ( 2012 ).
53. Clarke , D. C. , Brown , M. L. , Erickson , R. A. , Shi , Y. & Liu , X. Transforming growth factor beta depletion is the primary determinant of Smad signaling kinetics . Mol Cell Biol 29 , 2443 - 2455 , https://doi.org/10.1128/ MCB . 01443 - 08 ( 2009 ).
54. Livak , K. J. & Schmittgen , T. D. Analysis of relative gene expression data using real-time quantitative PCR and the 2(- Delta Delta C(T)) Method . Methods 25 , 402 - 408 , https://doi.org/10.1006/meth. 2001 . 1262 ( 2001 ).
55. Baghirova , S. , Hughes , B. G. , Hendzel , M. J. & Schulz , R. Sequential fractionation and isolation of subcellular proteins from tissue or cultured cells . MethodsX 2 , 440 - 445 , https://doi.org/10.1016/j.mex. 2015 . 11 .001 ( 2015 ).
56. Bentsi-Barnes , I. K. , Kuo , F. T. , Barlow , G. M. & Pisarska , M. D. Human forkhead L2 represses key genes in granulosa cell differentiation including aromatase, P450scc, and cyclin D2 . Fertil Steril 94 , 353 - 356 , https://doi.org/10.1016/j.fertnstert. 2009 . 09 .050 ( 2010 ).
57. Garcia-Ortiz , J. E. et al. Foxl2 functions in sex determination and histogenesis throughout mouse ovary development . BMC Dev Biol 9 , 36 , https://doi.org/10.1186/ 1471 -213X- 9 - 36 ( 2009 ).