The endometrium undergoes dramatic remodelling throughout the menstrual cycle in preparation of implantation. Implantation requires is initiated when the blastocyst adheres to an adequately prepared ‘receptive’ endometrium. Abnormal adhesion, and/or inability to establish receptivity, results in infertility and is the major cause of implantation failure in IVF. However, despite a multitude of studies blastocyst–endometrial molecular dialogue, the molecular mechanisms of receptivity in humans are still unclear.
The Notch family regulates a number of cell processes including cell invasion, survival, apoptosis and differentiation. We have investigated spatial and temporal localisation and staining intensity of the entire Notch signalling family across the menstrual cycle as well as key downstream genes in fertile (n=25) and primary unexplained infertile women (n=10). During the receptive phase of the menstrual cycle Notch-1 and the Ligands DLL-1 (in the glandular epithelium) and Jagged-1 (in the luminal epithelium) were key proteins displaying increased staining intensity (p<0.05 respectively) and dysregrulated in infertile endometrium (p<0.05, p<0.05 and p<0.01 respectively). Conversely, Numb a negative regulator of Notch signalling was decreased in the glandular epithelium of mid secretory phase of the menstrual cycle in fertile women and increased with infertility.
We further investigated the role of DLL1 in adhesion demonstrating that silencing DLL1 (100nM siRNA) inhibited endometrial epithelial (RL95) cell adhesion to fibronectin (p<0.05, n=3, xCELLigence) whereas treatment with recombinant DLL1 protein (0.5µg/ml, n=3, p<0.05) enhanced endometrial epithelial (RL95) -trophoblast (HTR8) cell adhesion. DLL1 can be cleaved at the plasma membrane producing the soluble form; soluble DLL1 is reported to block Notch signalling. In vitro sDLL1 (0.5µg/ml, n=3, p<0.05) impaired endometrial epithelial-trophoblast adhesion. Our study suggests DLL1 is abnormally cleaved in the endometrial epithelium of women with unexplained infertility, leading to reduced endometrial epithelial adhesive properties and direct competitive inhibition of epithelial DLL1 binding to Notch receptors on the blastocyst.