Blastocyst implantation is a finely tuned process under hormonal control which requires significant remodelling of the luminal uterine epithelial cells (UECs), which includes a reorganisation of the cytoskeleton and collectively is referred to as the plasma membrane transformation. Epithelial keratins are part of the intermediate filament component of the cytoskeleton and consist of ~20 individual keratins that form cell-specific pairs which are unique in simple and stratified epithelia. Keratins are involved in mechanical support of polarised epithelial cells through interaction with the actin cytoskeleton and junctional complexes, as well as apoptosis modulation and cellular signalling. This study identified the presence of both simple and stratified epithelial keratin intermediate filaments in the uterus and their stage specific localisation in simple UECs during normal pregnancy and ovarian hyperstimulation (OH).
PCR was used to identify individual epithelial keratins present in whole rat uterus to select target keratins for further investigation. Immunofluorescence and Western blotting revealed the unique localisation of selected keratin filaments from fertilisation, implantation and post-implantation stages. Keratin-18 was shown to be reduced during the time of implantation in normal pregnancy where it forms an apical filamentous network. However, Keratin-18 is abundant at the equivalent time in OH and absent at the time of fertilisation. Additionally, Keratin-15 is typically expressed only in stratified squamous epithelia but was identified apically in UECs at implantation in OH animals.
This study identified reorganisation of the epithelial keratin cytoskeleton as a novel component of the plasma membrane transformation. Additionally, the expression of a stratified epithelial specific keratin during OH reveals an ability for UECs to exhibit atypical keratins. The observation that Keratin-18 is reduced at the time of implantation in normal pregnancy but is abundant at the same time of OH pregnancy indicates that keratins may play an important role in receptivity of the uterus.