Alterations in mitochondrial structure and function are early events of dexamethasone-induced thymocyte apoptosis

In this paper we used a multiparametric approach to analyze extensively the events occurring during apoptotic cell death of thymocytes, and furthermore, we asked whether alterations in mitochondrial structure and function are occurring in early stages of apoptosis. A multiparametric quantitative analysis was performed on normal or apoptotic thymocytes emerging from a few-hour culture performed in culture medium or in the presence of dexamethasone. Simultaneous detection of light scattering properties, integrity of plasma membrane (trypan blue exclusion), chromatin condensation (AO/EB staining of entire cells or PI staining of nuclei), and DNA fragmentation (in situ nick-translation in apoptotic cells) allowed a precise analysis of the preapoptotic and apoptotic stages. Moreover a thorough study of mitochondrial transmembrane potential (delta psi m) assessed following in a time course study the uptake by apoptotic cells of the cationic lipophilic dye DiOC6(3) or the J-aggregate-forming cation JC-1, indicates that a drop in delta psi m occurs very early in thymocyte apoptosis, before DNA fragmentation. This is associated with alteration in mitochondrial structure assessed by cytofluorimetric study of NAO uptake in apoptotic cells. Finally these dramatic alterations in mitochondrial structure and function occurring in early stages of apoptosis were confirmed by confocal and electron microscopy analysis.