Identification of Notch-1 expression in the limbal basal epithelium
To determine whether Notch-1, a ligand-activated transmembrane receptor known to maintain cells in an undifferentiated state, primarily progenitor cells in other systems, could be used as a stem cell marker in human limbal epithelium. Human corneoscleral tissues obtained from the Doheny Eye & Tissue Transplant Bank were prepared for cross section and whole mount analysis. Tissue for whole mount was incubated in dispase; the epithelial sheet was removed and fixed in 4% paraformaldehyde. Sections and whole mount were stained with antibodies against Notch-1, Notch-2, beta-1 integrin, alpha-6, and the G2 subtype member of the ATP binding cassette transporter (ABCG2). Specificity of the Notch-1 antibody was determined by western blot analysis with Cos-7 cells transfected with Notch-1. Explant culture was performed and only primary cultures were used in this experiment. Notch-1 was found to be expressed in the limbal basal region where stem cells reside. Notch-1 antigenicity was more pronounced in cell clusters, mainly in the palisades of Vogt. The central cornea was almost devoid of Notch-1. The intensity of Notch-1 staining in cultured cells from the limbal explants was high in only a few cells. The Notch-1 signal was diminished in dividing cells. Expression in cultured cells was more cytoplasmic; few cells showed additional nuclear staining. The Notch-1-stained whole mount showed only a few cells in the limbal region. A 300 kDa and a 110 kDa band confirmed the specificity of the antibody in Cos-7 cells transfected with Notch-1. Double staining for ABCG2 and Notch-1 showed some ABCG2-positive cells co-expressing Notch-1 in the limbal basal epithelium, indicating that Notch-1-expressing cells might be a unique subpopulation of cells with stem cell properties. Immunofluorescence data shows that Notch-1 could be a possible marker for the stem cells in the limbal basal epithelium. Further studies and characterization of the Notch pathway in corneal development will provide valuable clues for the identification of stem cells.