Src inhibitors modulate frataxin protein levels

The Regulation of the Disease-Causing Gene FXN

Friedreich's ataxia (FRDA) is a progressive neurodegenerative disease caused in almost all patients by expanded guanine-adenine-adenine (GAA) trinucleotide repeats within intron 1 of the FXN gene. This results in a relative deficiency of frataxin, a small nucleus-encoded mitochondrial protein crucial for iron-sulfur cluster biogenesis. Currently, there is only one medication, omaveloxolone, available for FRDA patients, and it is limited to patients 16 years of age and older. This necessitates the development of new medications. Frataxin restoration is one of the main strategies in potential treatment options as it addresses the root cause of the disease. Comprehending the control of frataxin at the transcriptional, post-transcriptional, and post-translational stages could offer potential therapeutic approaches for addressing the illness. This review aims to provide a general overview of the regulation of frataxin and its implications for a possible therapeutic treatment of FRDA.

An In Silico Analysis of Genetic Variants and Structural Modeling of the Human Frataxin Protein in Friedreich's Ataxia

Friedreich's Ataxia (FRDA) stands out as the most prevalent form of hereditary ataxias, marked by progressive movement ataxia, loss of vibratory sensitivity, and skeletal deformities, severely affecting daily functioning. To date, the only medication available for treating FRDA is Omaveloxolone (Skyclarys®), recently approved by the FDA. Missense mutations within the human frataxin (FXN) gene, responsible for intracellular iron homeostasis regulation, are linked to FRDA development. These mutations induce FXN dysfunction, fostering mitochondrial iron accumulation and heightened oxidative stress, ultimately triggering neuronal cell death pathways. This study amalgamated 226 FXN genetic variants from the literature and database searches, with only 18 previously characterized. Predictive analyses revealed a notable prevalence of detrimental and destabilizing predictions for FXN mutations, predominantly impacting conserved residues crucial for protein function. Additionally, an accurate, comprehensive three-dimensional model of human FXN was constructed, serving as the basis for generating genetic variants I154F and W155R. These variants, selected for their severe clinical implications, underwent molecular dynamics (MD) simulations, unveiling flexibility and essential dynamic alterations in their N-terminal segments, encompassing FXN42, FXN56, and FXN78 domains pivotal for protein maturation. Thus, our findings indicate potential interaction profile disturbances in the FXN42, FXN56, and FXN78 domains induced by I154F and W155R mutations, aligning with the existing literature.

Frataxin inhibits the sensitivity of the myocardium to ferroptosis by regulating iron homeostasis

RATIONALE

Myocardial ischemia/reperfusion (I/R) injury is characterized by cell death via various cellular mechanisms upon reperfusion. As a new type of cell death, ferroptosis provides new opportunities to reduce myocardial cell death. Ferroptosis is known to be more active during reperfusion than ischemia. However, the mechanisms regulating ferroptosis during ischemia and reperfusion remain largely unknown.

METHODS

The contribution of ferroptosis in ischemic and reperfused myocardium were detected by administered of Fer-1, a ferroptosis inhibitor to C57BL/6 mice, followed by left anterior descending (LAD) ligation surgery. Ferroptosis was evaluated by measurement of cell viability, ptgs2 mRNA level, iron production, malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) levels. H9C2 cells were exposed to hypoxia/reoxygenation to mimic in vivo I/R. We used LC-MS/MS to identify potential E3 ligases that interacted with frataxin in heart tissue. Cardiac-specific overexpression of frataxin in whole heart was achieved by intracardiac injection of frataxin, carried by adeno-associated virus serotype 9 (AAV9) containing cardiac troponin T (cTnT) promoter.

RESULTS

We showed that regulators of iron metabolism, especially iron regulatory protein activity, were increased in the ischemic myocardium or hypoxia cardiomyocytes. In addition, we found that frataxin, which is involved in iron metabolism, is differentially expressed in the ischemic and reperfused myocardium and involved in the regulation of cardiomyocytes ferroptosis. Furthermore, we identified an E3 ligase, NHL repeat-containing 1 (NHLRC1), that mediates frataxin ubiquitination degradation. Cardiac-specific overexpression of frataxin ameliorated myocardial I/R injury through ferroptosis inhibition.

CONCLUSIONS

Through a multi-level study from molecule to animal model, these findings uncover the key role of frataxin in inhibiting cardiomyocyte ferroptosis and provide new strategies and perspectives for the treatment of myocardial I/R injury.

Interferon Gamma Enhances Cytoprotective Pathways via Nrf2 and MnSOD Induction in Friedreich's Ataxia Cells

Friedreich's ataxia (FRDA) is a rare monogenic disease characterized by multisystem, slowly progressive degeneration. Because of the genetic defect in a non-coding region of FXN gene, FRDA cells exhibit severe deficit of frataxin protein levels. Hence, FRDA pathophysiology is characterized by a plethora of metabolic disruptions related to iron metabolism, mitochondrial homeostasis and oxidative stress. Importantly, an impairment of the antioxidant defences exacerbates the oxidative damage. This appears closely associated with the disablement of key antioxidant proteins, such as the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and the mitochondrial superoxide dismutase (MnSOD). The cytokine interferon gamma (IFN-γ) has been shown to increase frataxin expression in FRDA cells and to improve functional deficits in FRDA mice. Currently, IFN-γ represents a potential therapy under clinical evaluation in FRDA patients. Here, we show that IFN-γ induces a rapid expression of Nrf2 and MnSOD in different cell types, including FRDA patient-derived fibroblasts. Our data indicate that IFN-γ signals two separate pathways to enhance Nrf2 and MnSOD levels in FRDA fibroblasts. MnSOD expression increased through an early transcriptional regulation, whereas the levels of Nrf2 are induced by a post-transcriptional mechanism. We demonstrate that the treatment of FRDA fibroblasts with IFN-γ stimulates a non-canonical Nrf2 activation pathway through p21 and potentiates antioxidant responses under exposure to hydrogen peroxide. Moreover, IFN-γ significantly reduced the sensitivity to hydrogen peroxide-induced cell death in FRDA fibroblasts. Collectively, these results indicate the presence of multiple pathways triggered by IFN-γ with therapeutic relevance to FRDA.

The CaMKII/MLC1 Axis Confers Ca2+-Dependence to Volume-Regulated Anion Channels (VRAC) in Astrocytes

Astrocytes, the main glial cells of the central nervous system, play a key role in brain volume control due to their intimate contacts with cerebral blood vessels and the expression of a distinctive equipment of proteins involved in solute/water transport. Among these is MLC1, a protein highly expressed in perivascular astrocytes and whose mutations cause megalencephalic leukoencephalopathy with subcortical cysts (MLC), an incurable leukodystrophy characterized by macrocephaly, chronic brain edema, cysts, myelin vacuolation, and astrocyte swelling. Although, in astrocytes, MLC1 mutations are known to affect the swelling-activated chloride currents (ICl,_swell) mediated by the volume-regulated anion channel (VRAC), and the regulatory volume decrease, MLC1′s proper function is still unknown. By combining molecular, biochemical, proteomic, electrophysiological, and imaging techniques, we here show that MLC1 is a Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII) target protein, whose phosphorylation, occurring in response to intracellular Ca²⁺ release, potentiates VRAC-mediated ICl,_swell. Overall, these findings reveal that MLC1 is a Ca²⁺-regulated protein, linking volume regulation to Ca²⁺ signaling in astrocytes. This knowledge provides new insight into the MLC1 protein function and into the mechanisms controlling ion/water exchanges in the brain, which may help identify possible molecular targets for the treatment of MLC and other pathological conditions caused by astrocyte swelling and brain edema.

Drug Repositioning in Friedreich Ataxia

Friedreich ataxia is a rare neurodegenerative disorder caused by insufficient levels of the essential mitochondrial protein frataxin. It is a severely debilitating disease that significantly impacts the quality of life of affected patients and reduces their life expectancy, however, an adequate cure is not yet available for patients. Frataxin function, although not thoroughly elucidated, is associated with assembly of iron-sulfur cluster and iron metabolism, therefore insufficient frataxin levels lead to reduced activity of many mitochondrial enzymes involved in the electron transport chain, impaired mitochondrial metabolism, reduced ATP production and inefficient anti-oxidant response. As a consequence, neurons progressively die and patients progressively lose their ability to coordinate movement and perform daily activities. Therapeutic strategies aim at restoring sufficient frataxin levels or at correcting some of the downstream consequences of frataxin deficiency. However, the classical pathways of drug discovery are challenging, require a significant amount of resources and time to reach the final approval, and present a high failure rate. Drug repositioning represents a viable alternative to boost the identification of a therapy, particularly for rare diseases where resources are often limited. In this review we will describe recent efforts aimed at the identification of a therapy for Friedreich ataxia through drug repositioning, and discuss the limitation of such strategies.

Mitochondrial dysfunction in neurons in Friedreich's ataxia

Friedreich's ataxia is a multisystemic genetic disorder within the family of mitochondrial diseases that is characterized by reduced levels of the essential mitochondrial protein frataxin. Based on clinical evidence, the peripheral nervous system is affected early, neuronal dysfunction progresses towards the central nervous system, and other organs (such as heart and pancreas) are affected later. However, little attention has been given to the specific aspects of mitochondria function altered by frataxin depletion in the nervous system. For years, commonly accepted views on mitochondria dysfunction in Friedreich's ataxia stemmed from studies using non-neuronal systems and may not apply to neurons, which have their own bioenergetic needs and present a unique, extensive neurite network. Moreover, the basis of the selective neuronal vulnerability, which primarily affects large sensory neurons in the dorsal root ganglia, large principal neurons in the dentate nuclei of the cerebellum, and pyramidal neurons in the cerebral cortex, remains elusive. In order to identify potential misbeliefs in the field and highlight controversies, we reviewed current knowledge on frataxin expression in different tissues, discussed the molecular function of frataxin, and the consequences of its deficiency for mitochondria structural and functional properties, with a focus on the nervous system.

SINEUP non-coding RNAs rescue defective frataxin expression and activity in a cellular model of Friedreich's Ataxia

Friedreich's ataxia (FRDA) is an untreatable disorder with neuro- and cardio-degenerative progression. This monogenic disease is caused by the hyper-expansion of naturally occurring GAA repeats in the first intron of the FXN gene, encoding for frataxin, a protein implicated in the biogenesis of iron-sulfur clusters. As the genetic defect interferes with FXN transcription, FRDA patients express a normal frataxin protein but at insufficient levels. Thus, current therapeutic strategies are mostly aimed to restore physiological FXN expression. We have previously described SINEUPs, natural and synthetic antisense long non-coding RNAs, which promote translation of partially overlapping mRNAs through the activity of an embedded SINEB2 domain. Here, by in vitro screening, we have identified a number of SINEUPs targeting human FXN mRNA and capable to up-regulate frataxin protein to physiological amounts acting at the post-transcriptional level. Furthermore, FXN-specific SINEUPs promote the recovery of disease-associated mitochondrial aconitase defects in FRDA-derived cells. In summary, we provide evidence that SINEUPs may be the first gene-specific therapeutic approach to activate FXN translation in FRDA and, more broadly, a novel scalable platform to develop new RNA-based therapies for haploinsufficient diseases.

Molecular Mechanisms and Therapeutics for the GAA·TTC Expansion Disease Friedreich Ataxia

Friedreich ataxia (FRDA), the most common inherited ataxia, is caused by transcriptional silencing of the nuclear FXN gene, encoding the essential mitochondrial protein frataxin. Currently, there is no approved therapy for this fatal disorder. Gene silencing in FRDA is due to hyperexpansion of the triplet repeat sequence GAA·TTC in the first intron of the FXN gene, which results in chromatin histone modifications consistent with heterochromatin formation. Frataxin is involved in mitochondrial iron homeostasis and the assembly and transfer of iron-sulfur clusters to various mitochondrial enzymes and components of the electron transport chain. Frataxin insufficiency leads to progressive spinocerebellar neurodegeneration, causing symptoms of gait and limb ataxia, slurred speech, muscle weakness, sensory loss, and cardiomyopathy in many patients, resulting in death in early adulthood. Numerous approaches are being taken to find a treatment for FRDA, including excision or correction of the repeats by genome engineering methods, gene activation with small molecules or artificial transcription factors, delivery of frataxin to affected cells by protein replacement therapy, gene therapy, or small molecules to increase frataxin protein levels, and therapies aimed at countering the cellular consequences of reduced frataxin. This review will summarize the mechanisms involved in repeat-mediated gene silencing and recent efforts aimed at development of therapeutics.

Frataxin Structure and Function

Mammalian frataxin is a small mitochondrial protein involved in iron sulfur cluster assembly. Frataxin deficiency causes the neurodegenerative disease Friedreich's Ataxia. Valuable knowledge has been gained on the structural dynamics of frataxin, metal-ion-protein interactions, as well as on the effect of mutations on protein conformation, stability and internal motions. Additionally, laborious studies concerning the enzymatic reactions involved have allowed for understanding the capability of frataxin to modulate Fe-S cluster assembly function. Remarkably, frataxin biological function depends on its interaction with some proteins to form a supercomplex, among them NFS1 desulfurase and ISCU, the scaffolding protein. By combining multiple experimental tools including high resolution techniques like NMR and X-ray, but also SAXS, crosslinking and mass-spectrometry, it was possible to build a reliable model of the structure of the desulfurase supercomplex NFS1/ACP-ISD11/ISCU/frataxin. In this chapter, we explore these issues showing how the scientific view concerning frataxin structure-function relationships has evolved over the last years.

Overexpression of Drosophila frataxin triggers cell death in an iron-dependent manner

Friedreich ataxia (FRDA) is the most important autosomal recessive ataxia in the Caucasian population. FRDA patients display severe neurological and cardiac symptoms that reflect a strong cellular and axonal degeneration. FRDA is caused by a loss of function of the mitochondrial protein frataxin which impairs the biosynthesis of iron-sulfur clusters and in turn the catalytic activity of several enzymes in the Krebs cycle and the respiratory chain leading to a diminished energy production. Although FRDA is due to frataxin depletion, overexpression might also be very helpful to better understand cellular functions of frataxin. In this work, we have increased frataxin expression in neurons to elucidate specific roles that frataxin might play in these tissues. Using molecular, biochemical, histological and behavioral methods, we report that frataxin overexpression is sufficient to increase oxidative phosphorylation, modify mitochondrial morphology, alter iron homeostasis and trigger oxidative stress-dependent cell death. Interestingly, genetic manipulation of mitochondrial iron metabolism by silencing mitoferrin successfully improves cell survival under oxidative-attack conditions, although enhancing antioxidant defenses or mitochondrial fusion failed to ameliorate frataxin overexpression phenotypes. This result suggests that cell degeneration is directly related to enhanced incorporation of iron into the mitochondria. Drosophila frataxin overexpression might also provide an alternative approach to identify processes that are important in FRDA such as changes in mitochondrial morphology and oxidative stress induced cell death.

Nrf2-Inducers Counteract Neurodegeneration in Frataxin-Silenced Motor Neurons: Disclosing New Therapeutic Targets for Friedreich's Ataxia

Oxidative stress is actively involved in Friedreich's Ataxia (FA), thus pharmacological targeting of the antioxidant machinery may have therapeutic value. Here, we analyzed the relevance of the antioxidant phase II response mediated by the transcription factor Nrf2 on frataxin-deficient cultured motor neurons and on fibroblasts of patients. The in vitro treatment of the potent Nrf2 activator sulforaphane increased Nrf2 protein levels and led to the upregulation of phase II antioxidant enzymes. The neuroprotective effects were accompanied by an increase in neurites' number and extension. Sulforaphane (SFN) is a natural compound of many diets and is now being used in clinical trials for other pathologies. Our results provide morphological and biochemical evidence to endorse a neuroprotective strategy that may have therapeutic relevance for FA. The findings of this work reinforce the crucial importance of Nrf2 in FA and provide a rationale for using Nrf2-inducers as pharmacological agents.

Translating HDAC inhibitors in Friedreich's ataxia

INTRODUCTION

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by expansion of a GAA·TTC triplet in the first intron of the FXN gene, encoding the essential mitochondrial protein frataxin. Repeat expansion results in transcriptional silencing through an epigenetic mechanism, resulting in significant decreases in frataxin protein in affected individuals. Since the FXN protein coding sequence is unchanged in FRDA, an attractive therapeutic approach for this disease would be to increase transcription of pathogenic alleles with small molecules that target the silencing mechanism.

AREAS COVERED

We review the evidence that histone postsynthetic modifications and heterochromatin formation are responsible for FXN gene silencing in FRDA, along with efforts to reverse silencing with drugs that target histone modifying enzymes. Chemical and pharmacological properties of histone deacetylase (HDAC) inhibitors, which reverse silencing, together with enzyme target profiles and kinetics of inhibition, are discussed. Two HDAC inhibitors have been studied in human clinical trials and the properties of these compounds are compared and contrasted. Efforts to improve on bioavailability, metabolic stability, and target activity are reviewed.

EXPERT OPINION

2-aminobenzamide class I HDAC inhibitors are attractive therapeutic small molecules for FRDA. These molecules increase FXN gene expression in human neuronal cells derived from patient induced pluripotent stem cells, and in two mouse models for the disease, as well as in circulating lymphocytes in patients treated in a phase Ib clinical trial. Medicinal chemistry efforts have identified compounds with improved brain penetration, metabolic stability and efficacy in the human neuronal cell model. A clinical candidate will soon be identified for further human testing.