Staining of Membrane Receptors with Fluorescently-labeled DNA Aptamers for Super-resolution Imaging

One of the most prominent applications of fluorescent super-resolution microscopy is the study of nanodomain arrangements of receptors and the endocytic pathway. Staining methods are becoming crucial for answering questions on the nanoscale, therefore, the use of small and monovalent affinity probes is of great interest in super-resolution microscopy with biological samples. One kind of affinity probe is the aptamer. Aptamers are single DNA or RNA sequences that bind with high affinity to their targets and due to their small size they are able to (i) place the fluorophore in close proximity to the protein of interest and (ii) bind to most of the protein of interest overcoming the steric hindrance effect, resulting in better staining density. Here we describe a detailed protocol with which to stain live cells using aptamers and to image them with Stimulated Emission Depletion (STED) microscopy. In this protocol, the stainings were performed with commercially available aptamers that target the epidermal growth factor receptor (EGFR), the human epidermal growth factor receptor 2 (HER2 or ErbB2) and the ephrin type-A receptor 2 (Epha2). Since aptamers can be coupled to most of the popular fluorophores, we believe that the procedure presented here can be extended to the large majority of the current super-resolution microscopy techniques.