Origins and consequences of mitochondrial decline in nucleated erythrocytes

Cellular aging in nucleated erythrocytes from lower vertebrates is accompanied by losses in mitochondria but it remains unclear (i) how these losses accrue (ii) if these changes alter energetics and (iii) whether such changes increase the propensity for apoptosis. We addressed these questions using trout erythrocytes that were separated into age classes using inherent differences in buoyant density. The oldest cells showed a profound decline in mtDNA transcripts, due to reductions in both transcription (90% decline in total RNA) and mtDNA copy number (35%). No alterations in the ratio of 16S rRNA to COX I mRNA were detected, nor was there an accumulation of unprocessed mtDNA transcripts. While older cells had reduced basal respiration, there were no changes in mitochondrial enzymes stoichiometries, tissue ATP levels or dinitrophenol-induced (maximal) respiration rates. Apoptosis could not be induced in either whole blood, young or old erythrocytes by pro-oxidants, mitochondrial inhibitors or staurosporine. In contrast, cyclosporin A (CsA) caused caspase 3 activation, DNA laddering and LDH leakage, but only in young cells. Both CsA and a combination of azide, oligomycin and dinitrophenol cause mitochondrial depolarization and caspase 9 activation, but only CsA induced caspase 3 and apoptosis. Caspase inhibitor studies support the conclusion that mitochondrial changes may accompany CsA-induced cell death, but are not essential in its progression. While pifithrin failed to induce cell death, it enhanced the effects of CsA, implicating a role for p53. Collectively, these studies suggest that the mitochondrial changes with aging do not compromise cellular function, although trout erythrocytes can initiate apoptosis by non-mitochondrial pathways.