Mechanisms of cell-cell adhesion identified by immunofluorescent labelling with quantum dots: a scanning near-field optical microscopy approach.

Scanning near-field optical microscopy (SNOM) has been employed to simultaneously acquire high-resolution fluorescence images along with shear-force atomic force microscopy from cell membranes. Implementing such a technique overcomes the limits of optical diffraction found in standard fluorescence microscopy and also yields vital topographic information. The application of the technique to investigate cell-cell adhesion has revealed the interactions of filopodia and their functional relationship in establishing adherens junctions. This has been achieved via the selective tagging of the cell adhesion protein, E-cadherin, by immunofluorescence labelling. Two labelling routes were explored; Alexa Fluor 488 and semiconductor quantum dots. The quantum dots demonstrated significantly enhanced photostability and high quantum yield making them a versatile alternative to the conventional organic fluorophores often used in such a study. Analysis of individual cells revealed that E-cadherin is predominantly located along the cell periphery but is also found to extend throughout their filopodia. We have demonstrated that with a fully optimised sample preparation methodology, quantum dot labelling in conjunction with SNOM imaging can be successfully applied to interrogate biomolecular localisation within delicate cellular membranes.