Nuclear pore complex proteins mark the implantation window in human endometrium

Abstract

Nucleolar channel systems (NCSs) are membranous organelles appearing transiently in the epithelial cell nuclei of postovulatory human endometrium. Their characterization and use as markers for a healthy receptive endometrium have been limited because they are only identifiable by electron microscopy. Here we describe the light microscopic detection of NCSs using immunofluorescence. Specifically, the monoclonal nuclear pore complex antibody 414 shows that NCSs are present in about half of all human endometrial epithelial cells but not in any other cell type, tissue or species. Most nuclei contain only a single NCS of uniform 1 μm diameter indicating a tightly controlled organelle. The composition of NCSs is as unique as their structure; they contain only a subset each of the proteins of nuclear pore complexes, inner nuclear membrane, nuclear lamina and endoplasmic reticulum. Validation of our robust NCS detection method on 95 endometrial biopsies defines a 6-day window, days 19-24 (±1) of an idealized 28 day cycle, wherein NCSs occur. Therefore, NCSs precede and overlap with the implantation window and serve as potential markers of uterine receptivity. The immunodetection assay, combined with the hitherto underappreciated prevalence of NCSs, now enables simple screening and further molecular and functional dissection.

Timing the window of implantation by nucleolar channel system prevalence matches the accuracy of the endometrial receptivity array

“…2, #1-24). In contrast, in our previous studies, peak NCS prevalence was closer to 50% (19,21,23). This discrepancy could be caused by fixation with formaldehyde instead of paraformaldehyde and by laboratory-specific differences in tissue processing.…”

“…2, #32). Although LHþ5 specimens were previously documented as pre-receptive (24), NCSs are clearly observed already during that period (19). Therefore, it is possible that NCS prevalence slightly precedes the receptive phase determined by ERA.…”

“…The transcriptomic signature of endometrial receptivity identified by ERA is capable of partitioning the menstrual cycle into 4 distinct phases: proliferative, prereceptive, receptive, and post-receptive (6). In contrast, NCS prevalence solely identifies the receptive window without specifying pre-or post-receptive phases (19,22). For comparison, therefore, the higher time-resolution of the ERA had to be reduced to 2 phases, receptive vs. nonreceptive (Fig.…”

“…NCSs consist of inner nuclear membrane-derived membrane tubules embedded in a matrix of nuclear lamin and pore complex proteins (without containing intact pore complexes) (19). Together these structures form a barrier from the rest of the nucleoplasm.…”

“…More recently, we developed a light, microscopic, quantitative detection method for NCSs, taking advantage of their enrichment in certain nuclear pore complex proteins (nucleoporins) (19,20). Applying our simplified NCS detection method in several studies, we determined that about half of the EECs harbor an NCS at their peak, midluteal presence, and that they are distributed uniformly throughout the upper uterine cavity.…”