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Phys Biol. 2013 Dec;10(6):065006. doi: 10.1088/1478-3975/10/6/065006. Epub 2013 Dec 4.

Cooperative binding of Annexin A5 to phosphatidylserine on apoptotic cell membranes.

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Department of Otorhinolaryngology, Head and Neck Surgery, Section for Experimental Oncology and Nanomedicine (SEON), University Hospital, Waldstraße 1/Glückstraße 10, D-91054 Erlangen, Germany. Department for Internal Medicine 3, University Hospital Erlangen, Friedrich-Alexander University of Erlangen-Nuremberg, Ulmenweg 18, D-91054 Erlangen, Germany.


Healthy cells exhibit an asymmetric plasma membrane with phosphatidylserine (PS) located on the cytoplasmic leaflet of the plasma membrane bilayer. Annexin A5-FITC, a PS binding protein, is commonly used to evaluate apoptosis in flow cytometry. PS exposed by apoptotic cells serves as a major 'eat-me' signal for phagocytes. Although exposition of PS has been observed after alternative stimuli, no clearance of viable, PS exposing cells has been detected. Thus, besides PS exposure, membranes of viable and apoptotic cells might exhibit specific characteristics. Here, we show that Annexin A5 binds in a cooperative manner to different types of dead cells. Shrunken apoptotic cells thereby showed the highest Hill coefficient values. Contrarily, parafomaldehyde fixation of apoptotic cells completely abrogates the cooperativity effect seen with dead and dying cells. We tend to speculate that the cooperative binding of Annexin A5 to the membranes of apoptotic cells reflects higher fluidity of the exposed membranes facilitating PS clustering.

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