Uncoupling of CD71 shedding with mitochondrial clearance in reticulocytes in a subset of myelodysplastic syndromes.

Reticulocytes shed CD71 from the cell membrane and eliminate mitochondria during terminal maturation, but it is unknown whether these two events are coordinated. We demonstrate that timely removal of CD71 is coupled with mitochondrial clearance, which can be disrupted by null mutation of immediate early response gene X-1 (IEX-1), leading to generation of aberrant CD71-positive and mitochondria-negative (CD71+Mito-) reticulocytes. CD71+Mito- reticulocytes were also present in a subset of patients with myelodysplastic syndromes (MDS) in direct proportion to reduced mitochondrial membrane potential (∆ψm). Mitochondrial abnormality caused by either IEX-1 deficiency or agents that dissipate ∆ψm could trigger premature clearance of mitochondria in reticulocytes. Premature clearance of mitochondria or addition of anti-oxidants lowered intracellular reactive oxygen species (ROS) that in turn hindered CD71 shedding and reticulocyte maturation. In contrast, introduction of ROS accelerated CD71 shedding via release of exosomes that contained a high proportion of Fe3+ over Fe2+, suggesting dual functions of CD71 shedding both in removal of toxic Fe3+ from reticulocytes and in limiting importation of Fe3+ into the cells. These observations emphasize the coordination of mitochondrial and CD71 clearance in erythroid terminal maturation and offer new insights into a role for mitochondrial degeneration in the pathogenesis of some MDS-associated anemia.