Background: Iron is a critical molecule for normal cellular and physiologic processes including mitochondrial energy production and oxygen transport in the blood. Previous work in our lab has established the RNA-binding protein TTP as an iron-responsive gene whose expression is induced by iron deprivation. TTP functions by binding to AU-rich elements in the 3’UTR of mRNAs and promotes their degradation. Iron deprivation is a form of cellular stress linked to alterations in mitochondrial dynamics. Optic atrophy 1 (OPA1) is a critical component of the mitochondrial fusion machinery and, under conditions of cellular stress, OPA1 is proteolytically cleaved to promote mitochondrial fission and recycling of damaged mitochondria. OPA1 expression and cleavage is altered in cardiomyopathy, and heterozygous mice develop spontaneous cardiomyopathy. We hypothesize that induction of TTP by low iron reduces the level of OPA1 to fragment the mitochondria and allow for mitophagic recycling of non-functional mitochondria.
Results: Iron chelation using desferoxamine (DFO) induced TTP expression at both the mRNA and protein levels in a dose-dependent manner. In silico analysis of the OPA1 3’UTR revealed multiple AU-rich elements, many of which are evolutionarily conserved. Basal levels of OPA1 were increased in TTP KO cells and iron chelation decreased OPA1 mRNA expression in WT but not TTP KO cells. Confocal imaging of mitochondria showed profound mitochondrial fragmentation in WT cells exposed to iron chelation that was abrogated in TTP KO cells. Additionally, mitochondrial fragmentation in WT cells was accompanied by induction of mitophagy, which was blocked in TTP KO cells. Knockdown of OPA1 rescued the defects in mitochondrial fission in TTP KO cells with iron chelation.
Conclusions: Our studies show that TTP expression is upregulated under conditions of iron deficiency and in turn represses the expression of the mitochondrial fusion protein OPA1 to alter mitochondrial dynamics and permit mitophagy. By remodeling the mitochondria under conditions of iron deprivation we propose a critical role for TTP in coordinating mitochondrial quality control with nutrient stress in the heart.