Iron, Friedreich’s Ataxia and Intermediary Metabolism
In a paper recently published in Blood (doi: 10.1182/blood.2020006987), Petit et al identify a mechanism promoting cellular iron overload in Friedreich’s ataxia (FRDA), a neurodegenerative disease. The mechanism is linked to a defect in intermediary metabolism that prevents appropriate palmitoylation of transferrin receptor 1 (TfR1). FRDA is caused by expansions of a GAA trinucleotide repeat within the first intron of the FXN gene. This impairs expression of the gene product frataxin, a mitochondrial protein involved in Fe-S cluster biogenesis. Frataxin deficiency is associated with mitochondrial iron overload and oxidative stress, which primarily affect the central nervous system and the heart.
The impact of frataxin deficiency on overall cellular iron metabolism is not well understood. Moreover, assessment of cytosolic iron status in cells and tissues from FRDA patients and mouse models has yielded conflicting results. It is generally believed that mitochondrial iron accumulation is the result of the increased metabolic need for the metal due to failure to be incorporated into Fe-S clusters. This in turn drives iron flux from the cytosol into mitochondria and causes cytosolic iron depletion that is sensed by the IRE/IRP system.
Nevertheless, Petit et al found that skin fibroblasts from FRDA patients have a high iron content in both cytosolic and mitochondrial compartments. Moreover, these cells exhibit a dramatic susceptibility to iron overload upon treatment with exogenous iron, which cannot be explained by IRP responses. Petit et al noted increased TfR1 expression in iron-loaded FRDA fibroblasts, in spite of low TfR1 mRNA levels. This prompted them to quantify the amount of TfR1 on the plasma membrane and compare it to control cells. In addition, they used live imaging via spinning disc microscopy to monitor endocytosis of transferrin-TfR1 complexes. The experiments identified a higher fraction of TfR1 on the surface of FRDA fibroblasts, increased endocytosis and a significant delay in the recycling of transferrin. This biochemical phenotype has been linked to defective TfR1 palmitoylation (Alvarez et al, J Biol Chem, 1990).
Palmitoylation is the covalent binding of palmitic acid via S-acyl radicals to cysteine residues of membrane proteins. Using an acyl-biotin exchange assay, Petit et al provided strong evidence for poor TfR1 palmitoylation in FRDA fibroblasts. In addition, they managed to decrease iron levels in these cells by using artesunate, an anti-malaria drug that stimulates TfR1 palmitoylation. In fact, the pharmacological rescue of FRDA fibroblasts from iron overload was accompanied by proper TfR1 palmitoylation.
How does frataxin deficiency cause defective TfR1 palmitoylation? The disruption of Fe-S cluster biogenesis in the absence of frataxin has a profound effect on intermediary metabolism by inhibiting generation of acetyl-CoA, the building block of fatty acids including palmitate. Dihydrolipoamide acetyltransferase (PDH-E2), a subunit of pyruvate dehydrogenase (PDH) that gives rise to acetyl-CoA, requires lipoic acid as cofactor. This is produced by lipoic acid synthase (LIAS), an Fe-S cluster enzyme that is inactive in FRDA fibroblasts. Importantly, supplementation of the cells with acetyl-CoA or pharmacological stimulation of PDH activity by dichloroacetate restored PDH lipoylation and TfR1 palmitoylation. Moreover, targeted mitochondrial expression of exogenous frataxin decreased membrane TfR1 levels and corrected iron overload in skin fibroblasts, while artesunate also corrected iron overload in peripheral blood mononuclear cells from FRDA patients.
These data uncover a new mechanism for cellular iron overload in frataxin-deficient cells. Thus, the loss of frataxin inactivates Fe-S proteins including LIAS and thereby abrogates the synthesis of lipoic acid, an essential cofactor for acetyl-CoA production by PDH. Acetyl-CoA insufficiency limits fatty acid biosynthesis and TfR1 palmitoylation, resulting in uncontrolled TfR1 accumulation on the plasma membrane, increased iron transport activity and eventually iron overload.
This work raises the possibility for pharmacological application of agents that restore acetyl-CoA production and TfR1 palmitoylation for the management of FRDA patients. It will be important to corroborate key findings in cells that are relevant to FRDA pathologies, such as neurons and cardiomyocytes, and also in animal models of FRDA.
posted: March 24, 2021