P-312 Transcriptional profiling of SSEA-1+ endometrial epithelial cells highlights their role in endometrial regeneration, remodelling and homeostasis
Authors: Al-Lamee H , Soul J , Green D , Drury J , Hill C , Valentijn A , Maclean A , Roachdown J , Drakeley A , Tempest N , Hapangama D 
Published: July 2024

Abstract

Study question
Will the transcriptional profile of SSEA-1+ endometrial epithelial cells (EECs) explain their functional role, which can be explored in an in vitro endometrial organoid model?

Summary answer
SSEA-1+ EECs demonstrate differentially expressed genes (DEGs) and associated functional pathways expected from a basal stem/progenitor cell (SPC) population, agreeing with the in vitro experiments.

What is known already
Endometrial SPCs are postulated to reside in the basalis and to be responsible for the full phenotypic and functional restoration of the endometrial functionalis layer. SSEA-1+ EECs assume the postulated basalis SPC niche. Previous studies demonstrated isolated SSEA-1+ EECs to have a higher capacity to generate organoids in 3D matrix and display growth like that observed after endometrial denudation. They have lower steroid hormone expression and higher telomerase activity with longer telomere lengths, all suggesting a SPC phenotype. SSEA-1+ EECs co-express nuclear SRY-box transcription factor 9 (nSOX9) and nuclear b-catenin, suggesting an activated Wnt pathway.

Study design, size, duration
Endometrial samples were collected from eight pre-menopausal women attending the Liverpool Women’s Hospital for benign gynaecological procedures, aged 30-47 years. These patients were having regular menstrual cycles and had not been on any hormonal therapy for at least three months prior to enrolment. Three further endometrial samples were obtained for 3D EEC organoid culture studies.

Participants/materials, setting, methods
Magnetic-activated cell sorting was used to isolate SSEA-1 enriched/depleted EECs from freshly harvested endometrial samples. Microarray analysis was performed using Agilent human arrays. DEGs and pathway enrichment analysis were explored using R-studio and Ingenuity Pathway Analysis (IPA). Internal and external validation used RT-qPCR, dual-immunofluorescence, and comparison against publicly available endometrial single cell spatial transcriptomic data. Human EEC organoids models were employed to assess hormonal regulation of SSEA-1 expression using immunohistochemistry.

Main results and the role of chance
SSEA-1+ EECs have a distinct transcriptional profile, with 1,059 DEGs compared with SSEA-1- EECs. Pathway analysis highlighted their role in endometrial regeneration, adhesion, remodelling, and neovascularisation, enriching for pathways such as ‘extracellular matrix structural constituent’, ‘epithelial to mesenchymal transition’, ‘angiogenesis’, ‘TNFα signalling via NFKB’ and ‘Notch signalling’. IPA’s biological functions demonstrated their involvement in tissue homeostasis, tumour suppression and their more quiescent SPC phenotype, with activation of ‘organismal death’ and inhibition of ‘cell movement/migration’ and ‘cell movement/migration of tumour cell lines’. The activation of canonical pathways such as ‘PTEN signalling’ and ‘endocannabinoid cancer inhibition’ demonstrated their fine equilibrium to drive proliferation during regeneration, whilst providing protection from hyperplastic/carcinogenic transformation. Ten selected DEGs were internally validated using RT-qPCR, demonstrating concordance. Dual-immunofluorescence of gene products of DEGs MMP7, MMP26, C11orf52 and CD47 confirmed co-localisation alongside SSEA-1 in an external biological cohort of endometrial samples. External in silico validation using the Garcia-Alonso et al endometrial single-cell transcriptomics dataset, mapped significantly upregulated DEGs MMP7 and MMP26 to mainly SOX9+ and SOX9+/LGR5+ EECs. EEC organoids exposed to hormones (including oestradiol) demonstrated an induction of proliferation and higher proportion of organoids expressing SSEA-1 (46.8% vs 67%) simulating endometrial glandular regeneration process.

Limitations, reasons for caution
Freshly isolated/sorted EECs were obtained from women at different menstrual cycle phases. Our organoid model system only contained EECs without the stromal component, thus, was not suitable to assess progestogenic response of EECs that occur in vivo.

Wider implications of the findings
This study provides novel information on human SSEA-1+ EECs, suggesting their pivotal role in endometrial regeneration. Since endometrial epithelial SPCs make a vital contribution to endometrial repair and pregnancy establishment, characterising of these cells in health will allow the development of targeted novel therapies on aberrant SPCs within gynaecological disease.

Trial registration number
Not applicable