A human oocyte and its supportive cells, known as granulosa cells, are interdependent throughout ovarian development and ovulation for successful fertilization [1
]. Oocytes isolated from primordial follicles fail to grow without the aid of granulosa cells [2
]. Cumulus cells are the innermost subgroup of granulosa cells surrounding the oocyte and play a key role in oocyte maturation and fertilization. Cumulus cell function depends on the gap junctions between cumulus cells and the oocyte, which allow rapid transfer of molecules to the oocyte [4
]; so, the quality of cumulus cells is thought to reflect oocyte quality. Therefore, transcriptomic profiling of cumulus cells is useful in identifying biomarkers indicating the developmental ability of oocyte maturation (Figure 1
The classification of mature cumulus–oocyte complexes (COCs), based on morphology, was first described in the 1980s [6
] and has been used for many decades [7
In the human menstrual cycle, only less than 1% of follicles lead to ovulation, and more than 99% of follicles lead to atretic follicles [11
]. We have used these criteria for more than a decade and reported that the oocytes classified as “dysmature” exhibit a remarkably lower fertilization rate, compared to COCs that are in the process of maturation [12
]. Our previous studies revealed significant differences in gene expression of DCCs compared to COCs that are in the process of maturation [13
], and DCCs are reportedly associated with apoptosis [13
] and autophagy-independent cell death [1
To date, several studies using next-generation sequencing (NGS) to examine cumulus cells or granulosa cells have been conducted. Using NGS, it is possible to comprehensively investigate gene expression in samples, because it can determine millions of sequences at once. For example, Hu et al. [14
] reported the cause of insulin resistance in polycystic ovarian syndrome (PCOS) analyzing cumulus cells. Xu et al. [15
] searched the difference in gene expression of mouse cumulus cells with and without blastocyst formation. Fuchs et al. [16
] studied the difference in the gene expression of human cumulus granulosa cells based on the types of drugs used to trigger IVF cycles. Although other studies using NGS for cumulus cells are also present [17
], there have been a few studies on the gene expression related to COC maturity.
As described above, the evaluation of COC maturation is useful to estimate oocyte quality. Furthermore, the procedure used to assess cumulus cells is more straightforward than assessing the oocyte without damaging it. Nevertheless, there have been a few studies that estimate the difference in gene expression based on the maturity of COCs.
In the present study, we compared DCCs, which would become atretic follicles, with COCs that are in the process of maturation, and used NGS to search for candidate genes associated with follicular development, lower fertilization rate, apoptosis, and autophagy.
2. Materials and Methods
This study was reviewed and approved by the ethical committee of the National Center for Child Health and Development (# 581, 3 July 2012). Women presenting unexplained infertility aged 33–42 years who underwent assisted reproductive technology (ART) were included. Briefly, cumulus cells were collected from patients who consented to participate in this study. Based on the morphological criteria, DCCs were selected, and COCs that are in the process of maturation were categorized as control (Supplemental Table S1
). Twenty samples of cumulus cells were collected from patients who provided consent for their samples to be used for research purposes.
The patients underwent standard in vitro fertilization (IVF) treatment, which included gonadotropin administration to induce multiple follicular development, and transvaginal oocyte aspiration. COC retrieval was performed by vaginal puncture under ultrasound echo-guidance 36 h after human chorionic gonadotropin (hCG) administration (10,000 IU). The classification of COCs was based on the following morphological criteria: We defined DCCs as the presence of dark spots in the layers of cumulus cells, with or without complete expansion with a visible halo of the cumulus layer, to distinguish them from maturation cumulus cells (Figure 1
b). The cumulus cells were mechanically removed at 40 h after hCG administration, before conventional IVF or intracytoplasmic sperm injection, washed in Sydney IVF fertilization (COOK Medical, Brisbane, Australia) medium, and frozen individually before total RNA extraction. The sample collection flow is shown in Figure 2
2.2. RNA Extraction and NGS
Total RNA of cumulus cells from oocytes were extracted using the RNeasy mini kit (Qiagen, Valencia, CA, USA) following the manufacturer’s protocol. The extracted RNA quality and quantity were assessed on a 2100 Bioanalyzer, using the RNA 6000 Nano kit (Agilent Technologies, Palo Alto, CA, USA). Libraries were prepared using a TruSeq RNA Sample Preparation Kit (Illumina Inc., San Diego, CA, USA) to generate clusters on the Illumina cBot (Illumina). Sequencing was performed on the Hiseq 2000/2500 (Illumina) as 50-bp reads, while image analysis and base calling were performed with CASAVA ver.1.7.0/1.8.2 (Illumina), following the manufacturer’s instructions. High-quality sequences, which passed the default quality-filtering parameters in the Illumina pipeline GERALD stage were retained and exported into Strand NGS (Agilent Technologies) for subsequent data analysis. The next-generation Web sequence data were deposited in the Japanese Genotype-phenotype Archive (JGA) [Accession #: JGAS00000000212].
Sequences were aligned with the human genome sequence (Feb. 2009/hg19), using the gene annotation file downloaded from the UCSC sequence and annotation database (http://hgdownload.cse.ucsc.edu/downloads.html#human
), and Strand NGS (Agilent Technologies). The downloaded UCSC annotation database included 40,437 genes and transcripts consisting of 19,805 protein-coding genes, 12,221 pseudogenes, and 7365 non-coding RNA (Supplemental Table S2
2.3. Gene Expression Profiling Analysis
Uniquely aligned reads were quantified, and both gene and exon-level reads per kilobase per million mapped read (RPKM) values were calculated. The RPKM values represent counts of reads mapping a feature (gene, or exon), normalized to both overall sequencing coverage and the size of the feature [19
Before gene expression analysis and the use of log-transformed and base-lined counts, we performed principal component analysis (PCA) and hierarchical clustering to overview the expression profiles of all samples. The values were normalized to mean 0 and variance 1 before applying log transformation.
Differentially expressed genes in DCCs were identified based on a 2-fold change compared to the mean value of the control, and statistical significance based on p-value < 0.05, using one-way analysis of variance (ANOVA), followed by post hoc analysis using Student–Newman–Keuls. The list of differentially expressed genes was utilized for gene ontology (GO) analysis for biological/molecular function, using Strand NGS with a false discovery rate-corrected p-value < 0.01. Furthermore, the list of differentially expressed genes was analyzed for pathway enrichment analysis using WikiPathway, and statistical significance was set at a p-value < 0.05.
Cumulus cells prepared as described above were permeabilized with 0.5% Triton X-100 in phosphate-buffered saline (PBS) for 5 min at 25 °C and blocked with 1% BSA and 0.1% Tween 20 in PBS for 1 h at 25 °C. The samples were incubated overnight at 4 °C with monoclonal anti-β-Catenin-Cy3 conjugate (C7738, Sigma, St Louis, MO, USA), prepared at a 1:500 dilution in Dako antibody diluents (Dako, Carpinteria, CA, USA). The complexes were then washed three times with 0.1% Tween 20 in PBS. The nuclei were then counterstained with 4′,6-diamidino-2-phenylindole (DAPI) at a final concentration of 10 μg/mL in 0.1% Tween 20 in PBS for 30 min at 25 °C. Thereafter, samples were washed three times with 0.1% Tween 20 in PBS, immersed in Vectashield (Vector Laboratories Inc., Burlingame, CA, USA), and covered with coverslips. Microphotographs were captured using a confocal laser scanning microscope LSM510 Meta (Carl Zeiss MicroImaging, Inc., Thornwood, NY, USA). Photographs captured from three different areas of each sample were analyzed using Olympus DP Controller and DP Manager (Olympus, Tokyo, Japan). In each experiment, at least 300 cells were measured in randomly selected microscopic fields.
In this study, we found that DCCs showed the most variations in the activation of the Wnt signaling pathway compared to the control. It has been reported that the importance of the Wnt/β-catenin signaling pathway in human cumulus cell function is related to follicle development and folliculogenesis [20
]. DCCs were suggested to have different follicular development from that of control. The Wnt signaling pathway has two activation mechanisms, depending on β-catenin or not. Cytoplasmic β-catenin is kept at low levels through continuous proteasome-mediated degradation, which is controlled by the GSK-3/APC/AXIN complex. When ligands bind to receptors FZD and lipoprotein receptor-related proteins 5 and 6 (LRP5/6), the degradation of β-catenin is inhibited due to dissociation of β-catenin degradation complex, and consequently, β-catenin accumulates in the cytoplasm and nucleus. Nuclear β-catenin interacts with transcription factors, such as lymphoid enhancer-binding factor 1/T cell-specific transcription factor (LEF/TCF) to affect transcription [20
Among the 19 Wnt ligands identified in humans, DCC showed a remarkable decrease in the expression of two ligands, WNT10B
reportedly functions in the Wnt/β-catenin signaling pathway [21
]. While there are no reports directly associating WNT9B
with Wnt/β-catenin signaling, WNT9B
are closely located on the same chromosome of 17q21 or 12q13. WNT1
, and 3a
] ligands have strong transformation abilities, leading to the activation of the Wnt/β-catenin signaling pathway, and conceivably, it may be associated with β-catenin activation. Especially, WNT1
strongly transforms C57MC mammary epithelial cells [25
] and participates in the activation of the Wnt/β-catenin signaling pathway. Accordingly, WNT9B
likely disturb the activation of the Wnt/β-catenin signaling pathway. In immunostaining, localization of β-catenin in DCCs was different from that in the control, and the number of β-catenin-expressing cells in the nucleus was also significantly lower. In addition, in the control, the nuclei are round and regular, while the nuclei of DCC shows irregular form, likely nuclear fragmentation, implying that DCC may be a state of apoptosis during atresia. These results suggested that the Wnt/β-catenin signaling pathway is inactivated in DCCs. Ten FZD members are capable of functioning as Wnt receptors [26
]. There is no published report describing a direct relationship between FZD7
and cumulus cells. Conceivably, FZD7
expression is considered to be reduced, because the Wnt/β-catenin signaling pathway is not active. When the Wnt/β-catenin signaling pathway is inactive, AXIN forms a complex with APC and GSK-3β to act on β-catenin degradation [23
]. DCCs also showed a decrease in AXIN expression, possibly due to inadequate transcription of AXIN
, without the complex formation in the Wnt/β-catenin signaling pathway. Furthermore, due to lower levels of intracellular β-catenin, the target genes, such as CCND1
, could not be transcribed, resulting in a decreased expression.
Besides the genes related to Wnt signaling pathway and pluripotency, we found that six pathways were significantly downregulated (Table 1
). Among them, G protein-coupled receptors (GPCRs) and Netrin-1 are reportedly associated with the Wnt signaling pathway. GPCRs comprise a very large family of proteins that play an essential role in cell proliferation, differentiation, neurotransmission, physiological processes, development, and apoptosis [27
]. FZD family, the receptors for Wnt signaling, include GPCR [28
], and the present results suggested that downregulation of FZD7 led to a decrease in GPCR activity. Netrin-1 is the first axon guidance molecule found in vertebrates that has potent chemotaxis functions for axon guidance, cell migration, morphogenesis, and angiogenesis [29
]. Netrin-1 has been reported to significantly promote β-catenin, which is vital for Wnt signaling after spinal cord injury [30
], and reduced Netrin-1 activity in cumulus cells may negatively affect Wnt signaling activity. Moreover, regarding the other four pathways, the pancreatic cancer pathway is comprehensive, including the cell cycle and apoptosis. It is reportedly involved in the inhibition of pancreatic cancer via apoptosis [31
] and may also be involved in cumulus cell apoptosis. Platelet homeostasis pathway reportedly plays an important role in the regulation of cell coagulation and hemostasis, as well as being involved in the regulation of angiogenesis and apoptosis [32
]. Cumulus cells may be similarly affected by the decline in the platelet homeostasis pathway. Since arachidonic acid metabolism by cyclooxygenase (COX) is a major pathway for platelet activation [33
], a decrease in arachidonic acid metabolic activity may have also reduced platelet homeostasis activity and led to apoptosis. In the integration of the energy metabolism, mitochondria play a central role, and they may be associated with apoptosis regulation [34
]. Although these four pathways induce apoptosis as a result of their decreased activity, they are not associated with follicular development. Therefore, we did not consider them important and concentrated on the Wnt signaling pathway.
In conclusion, the DCC developed via controlled ovarian stimulation during the oocyte retrieval cycle suggests that dysfunction of the Wnt/β-catenin signaling pathway may be a factor, resulting in follicle atresia, lower fertilization rate, or abnormal embryogenesis compared to other cumulus cells. These results suggest that DCCs are indeed present in atretic follicles and can be distinguished from other cumulus cells in the maturation process by morphological evaluation. Gene expression analysis of cumulus cells is a useful and safe method that evaluates the condition of oocytes. Based on the gene expression patterns of cumulus cells obtained in this study, there is evidence that they largely contribute to the development of future ART.