Quantum dots-based probes conjugated to Annexin V for photostable apoptosis detection and imaging

Quantum dots (Qdots) are nanoparticles exhibiting fluorescent properties that are widely applied for cell staining. We present here the development of quantum dots for specific targeting of apoptotic cells, for both apoptosis detection and staining of apoptotic"living" cells. These Qdots are functionalized with Annexin V, a 35-kDa protein that specifically interacts with the membrane of apoptotic cells: Annexin V recognizes and binds to phosphatidylserine (PS) moieties which are present on the outer membrane of apoptotic cells and not on this of healthy or necrotic cells. By using Annexin V, our Qdots probes are made specific for apoptotic cells. For that purpose, Qdots Streptavidin Conjugates are coupled to biotinylated Annexin V. Staining of apoptotic cells was checked using fluorescence and confocal microscopy techniques on nonfixed cells. It is shown here that Qdots are insensitive to bleaching after prolonged and frequent exposure as opposed to organic dyes and this makes them excellent candidates for time-lapse imaging purposes. We illustrate the application of our Qdots-based probes to continuously follow fast changes occurring on the membrane of apoptotic cells.