Nitric oxide prevents Aft1 activation and metabolic remodeling in frataxin-deficient yeast

Abstract

Yeast frataxin homolog (Yfh1) is the orthologue of human frataxin, a mitochondrial protein whose deficiency causes Friedreich Ataxia. Yfh1 deficiency activates Aft1, a transcription factor governing iron homeostasis in yeast cells. Although the mechanisms causing this activation are not completely understood, it is assumed that it may be caused by iron-sulfur deficiency. However, several evidences indicate that activation of Aft1 occurs in the absence of iron-sulfur deficiency. Besides, Yfh1 deficiency also leads to metabolic remodeling (mainly consisting in a shift from respiratory to fermentative metabolism) and to induction of Yhb1, a nitric oxide (NO) detoxifying enzyme. In this work, we have used conditional Yfh1 mutant yeast strains to investigate the relationship between NO, Aft1 activation and metabolic remodeling. We have observed that NO prevents Aft1 activation caused by Yfh1 deficiency. This phenomenon is not observed when Aft1 is activated by iron scarcity or impaired iron-sulfur biogenesis. In addition, analyzing key metabolic proteins by a targeted proteomics approach, we have observed that NO prevents the metabolic remodeling caused by Yfh1 deficiency. We conclude that Aft1 activation in Yfh1-deficient yeasts is not caused by iron-sulfur deficiency or iron scarcity. Our hypothesis is that Yfh1 deficiency leads to the presence of anomalous iron species that can compromise iron bioavailability and activate a signaling cascade that results in Aft1 activation and metabolic remodeling.

Impact of Drosophila Models in the Study and Treatment of Friedreich’s Ataxia

“…An appealing idea might be the possibility to test how the manipulation of different metabolic pathways influences loss of frataxin in the fly. In agreement with this hypothesis, recent studies in isolated platelets from FRDA patients [ 168 ] and in a FRDA yeast model [ 250 ] have reported a metabolic remodeling. As reviewed by Filadi and colleagues, the mitochondria-ER axis is a pivotal element in such metabolic homeostasis [ 251 ].…”

Mitochondrial iron and calcium homeostasis in Friedreich ataxia

“…This assumption seems logical, as Aft1 is activated by iron‐sufur loss 41 and frataxin has been shown to accelerate iron–sulfur biogenesis 42 . However, using conditional deficient frataxin mutants, we have shown that iron overload precedes iron–sulfur loss and that nitric oxide can prevent the Aft1 activation caused by frataxin deficiency, but not its activation by iron–sulfur deficiency 8,26 . Therefore, activation of Aft1 in frataxin‐deficient yeast may be caused by a still unknown mechanism.…”

“…These acidic residues have been reported to coordinate Fe(II) with an estimated Kd on the micromolar range 6,7 . However, this low affinity may not be enough to allow frataxin binding iron “in vivo,” as the estimates about the concentration of chelatable iron in mitochondria range from 1 to 100 μM 8 . In this regard, spectroscopic studies in yeast have shown that the concentration and nature of these labile iron forms may change from ~15 μM in respiring mitochondria to ~150 μM in fermenting mitochondria 9 .…”

Iron in Friedreich Ataxia: A Central Role in the Pathophysiology or an Epiphenomenon?

“…As indicated above, many evidences support that frataxin deficiency causes a dysregulation in iron homeostasis, and it has also been shown in several models that the modulation of iron homeostasis ameliorates several phenotypes caused by frataxin deficiency [ 74 , 85 ]. However, the contribution of iron accumulation to the pathophysiology of FRDA has not been clearly determined.…”

“…Therefore iron-sulfur loss was an epiphenomenon mainly caused by Cth2, which is an Aft1 target that binds to mRNAs from iron-containing proteins and promotes its degradation [ 42 ]. Moreover, we have also observed that nitric oxide can prevent Aft1 activation in Yfh1-deficient cells, but not in cells that are deficient in iron-sulfur biogenesis [ 74 ]. This observation also indicates that in Yfh1 deficient yeast Aft1 may be activated by a mechanism different than iron-sulfur cluster deficiency.…”