Chemical heterogeneity in cell death: combined synchrotron IR and fluorescence microscopy studies of single apoptotic and necrotic cells.

The combination of synchrotron IR microspectroscopy and fluorescence microscopy has led to the identification of specific IR signatures of apoptosis and necrosis at a single cell level. Apoptosis was induced by treatment of Fas+ tumor cell lines with anti-Fas monoclonal antibodies. Detection of the early and late stages of apoptosis was performed using conjugated annexin V-fluorescein isothiocyanate (AV-FITC) and propidium iodide. Very early cellular changes were detected by IR before externalization of phosphatidylserine and AV-FITC labeling, and they were probably linked to DNA unwinding. The IR signals at 1044, 1177, and 1222 cm(-1), as well as an intensity variation in the CHx stretching region, are the main signature changes of early and late apoptosis, in line with the hypothesis of DNA fragmentation. The increased intensity of the CHx stretching bands of the lipids was observed only at an early stage of apoptosis. Changes in the relative intensity of CH3 and CH2 stretching accompany this increased intensity, suggesting changes in the relative amount and/or type of lipids concomitant with an increased lipid content. Finally, necrotic cells were characterized by marked changes in their chemical composition because several new vibrational features were observed.