Extra-mitochondrial mouse frataxin and its implications for mouse models of Friedreich’s ataxia

Abstract

Mature frataxin is essential for the assembly of iron–sulfur cluster proteins including a number of mitochondrial enzymes. Reduced levels of mature frataxin (81-20) in human subjects caused by the genetic disease Friedreich’s ataxia results in decreased mitochondrial function, neurodegeneration, and cardiomyopathy. Numerous studies of mitochondrial dysfunction have been conducted using mouse models of frataxin deficiency. However, mouse frataxin that is reduced in these models, is assumed to be mature frataxin (78-207) by analogy with human mature frataxin (81-210). Using immunoaffinity purification coupled with liquid chromatography-high resolution tandem mass spectrometry, we have discovered that mature frataxin in mouse heart (77%), brain (86%), and liver (47%) is predominantly a 129-amino acid truncated mature frataxin (79-207) in which the N-terminal lysine residue has been lost. Mature mouse frataxin (78-207) only contributes 7–15% to the total frataxin protein present in mouse tissues. We have also found that truncated mature frataxin (79-207) is present primarily in the cytosol of mouse liver; whereas, frataxin (78-207) is primarily present in the mitochondria. These findings, which provide support for the role of extra-mitochondrial frataxin in the etiology of Friedreich’s ataxia, also have important implications for studies of mitochondrial dysfunction conducted in mouse models of frataxin deficiency.

Recent Advances in the Elucidation of Frataxin Biochemical Function Open Novel Perspectives for the Treatment of Friedreich’s Ataxia

“…During the first step, the precursor is cleaved into an intermediate form (FXN 42–210 , 18 kDa) that is subsequently shortened to the mature form (FXN 81–210 , 14 kDa) in the second step. A minor fraction of frataxin also exists as isoforms with extra-mitochondrial localization in the cytoplasm and the nucleus that originate from alternative codon initiation and splicing events ( Pianese et al, 2002 ; Condo et al, 2006 ; Xia et al, 2012 ; Abruzzo et al, 2013 ; Pérez-Luz et al, 2015 ; Guo et al, 2018 ; Agrò and Díaz-Nido, 2020 ; Weng et al, 2020 ). Two isoforms have been characterized in human that both contain an elongated N-terminus, FXN II (FXN 76–210 , 15 kDa) and FXN III (FXN 1–5,53–210 , 18 kDa), with an N-acetyl modification in the case of FXN II ( Xia et al, 2012 ; Guo et al, 2018 ).…”

“…Extra-mitochondrial frataxin isoforms apparently contribute to cellular fitness as was shown in cell survival ( Condo et al, 2006 ) and respiration assays ( Agrò and Díaz-Nido, 2020 ), but their exact biochemical functions are still unclear. In mouse, a truncated cytosolic isoform (FXN 79–207 ) is the predominant form, suggesting that it might contribute to the FRDA phenotypes in this model ( Weng et al, 2020 ). However, this isoform is not present in human subjects.…”

“…However, this isoform is not present in human subjects. In human, the proportions of FXN II and III vs. the canonical mature mitochondrial form (FXN 81–210 ) have not been determined, except in the heart of FRDA patients, where the canonical FXN is the predominant form ( Weng et al, 2020 ), which questions the role of the isoforms.…”

Effect of Mitochondrial and Cytosolic FXN Isoform Expression on Mitochondrial Dynamics and Metabolism

“…The presence of cytosolic frataxin isoforms has been recently described in the mouse, being the cytosolic form more abundant than the mitochondrial one in heart, brain and liver. Furthermore, different proteoforms were identified in different tissues, supporting the idea of the existence of different tissue-specific transcripts [37]. We focused our studies on the biological significance of the alternative isoform II described by Xia, mainly due to its enrichment in the cerebellum [27], one of the most affected tissues in FRDA [38,39].…”

Mitochondrial and metabolic dysfunction in Friedreich ataxia: update on pathophysiological relevance and clinical interventions

“…However, cytosolic frataxin may also be involved. Erythrocytes make a specific form of frataxin without a mitochondrial targeting sequence, derived from a specific splice variant [ 75 ]. In mice, cytosolic forms of frataxin appear in multiple tissues, though the mechanism of their origin is not clear [ 76 ].…”