Mito-protective autophagy is impaired in erythroid cells of aged mtDNA-mutator mice

Abstract

• Mitochondrial dysfunction in aged mtDNA-mutator mice is associated with activation of mechanistic target of rapamycin and suppression of autophagy in erythroid cells.• Autophagy maintains mitochondrial function in erythroid progenitors of mtDNA-mutator mice, and disrupting it accelerates onset of anemia.Somatic mitochondrial DNA (mtDNA) mutations contribute to the pathogenesis of age-related disorders, including myelodysplastic syndromes (MDS). The accumulation of mitochondria harboring mtDNA mutations in patients with these disorders suggests a failure of normal mitochondrial quality-control systems. The mtDNA-mutator mice acquire somatic mtDNA mutations via a targeted defect in the proofreading function of the mtDNA polymerase, PolgA, and develop macrocytic anemia similar to that of patients with MDS. We observed an unexpected defect in clearance of dysfunctional mitochondria at specific stages during erythroid maturation in hematopoietic cells from aged mtDNA-mutator mice. Mechanistically, aberrant activation of mechanistic target of rapamycin signaling and phosphorylation of uncoordinated 51-like kinase (ULK) 1 in mtDNA-mutator mice resulted in proteasomemediated degradation of ULK1 and inhibition of autophagy in erythroid cells. To directly evaluate the consequence of inhibiting autophagy on mitochondrial function in erythroid cells harboring mtDNA mutations in vivo, we deleted Atg7 from erythroid progenitors of wildtype and mtDNA-mutator mice. Genetic disruption of autophagy did not cause anemia in wild-type mice but accelerated the decline in mitochondrial respiration and development of macrocytic anemia in mtDNA-mutator mice. These findings highlight a pathological feedback loop that explains how dysfunctional mitochondria can escape autophagy-mediated degradation and propagate in cells predisposed to somatic mtDNA mutations, leading to disease. (Blood. 2015;125(1):162-174)

MtDNA mutagenesis impairs elimination of mitochondria during erythroid maturation leading to enhanced erythrocyte destruction

“…We did not find direct evidence for NAC-induced autophagy or lysosomal biogenesis, to explain NAC-enhanced mitochondrial uncoupling. This suggests that mitochondrial depolarization was not a direct consequence of mitophagy, and that mtDNA mutagenesis associated with decreased capacity to mitophagy, agreeing with a previous report 37 . In non-erythroid cells, mitophagy has been induced by chemical uncoupling of mitochondria, by mechanisms involving Pink1-Parkin-mediated pathway and Ulk1 (refs 38-40).…”

The presence of anaemia negatively influences survival in patients with POLG disease

“…Our findings suggest, therefore, that decreased RBC production and not liver dysfunction is the cause of the anaemia. Animal studies also support this hypothesis showing that POLG mutation was associated with anaemia, haematopoietic progenitor dysfunction and erythroid dysplasia (Trifunovic et al 2004;Chen et al 2009;Ahlqvist et al 2012;Ahlqvist et al 2015;Li-Harms et al 2015).…”

Pharmacological inhibition of LSD1 and mTOR reduces mitochondrial retention and associated ROS levels in the red blood cells of sickle cell disease

“…(9, 16) RBCs were allowed to settle by gravity and the overlying plasma, and buffy coat was removed by aspiration and then further washed twice with phosphate-buffered saline (PBS) to remove plasma and white blood cells. For the quantification of mitochondria in RBCs, hematocrit adjusted cells were stained with 250nM of TMRM (Invitrogen) in DMEM for 30 minutes in a 37°C humid ified incubator.…”