Overexpression of human and fly frataxins in Drosophila provokes deleterious effects at biochemical, physiological and developmental levels

C9-Aryl-substituted berberine derivatives with tunable AIE properties for cell imaging application

Berberine (BBR), a widely used isoquinoline alkaloid derived from natural sources, exhibits aggregation-induced emission (AIE) characteristics and has biological applications such as in selective lipid droplet imaging and photodynamic therapy. However, natural BBR suffers from low fluorescence quantum yield (ΦF) and monotonous emission wavelength. In this paper, a series of C9-position-aryl-substituted berberine derivatives with a D-A structure were designed and synthesized. The electronic effect of the substitution groups can tune the intramolecular charge transfer (ICT) effect of the berberine derivatives, resulting in bluish green to NIR (508-682 nm) luminescence with AIE characteristics and enhanced ΦF up to 36% in the solid state. Interestingly, berberine derivatives containing an amino or a pyridyl group can exhibit fluorescence response to TFA. Cell imaging of the berberine derivatives was conducted using Caco-2 cancer cells, demonstrating their multi-color and efficient wash-free imaging capabilities. This work presents a new strategy for developing novel berberine derivatives with tunable AIE properties for application in biological imaging.

Posttranslational regulation of mitochondrial frataxin and identification of compounds that increase frataxin levels in Friedreich's ataxia

Friedreich's ataxia (FRDA) is a degenerative disease caused by a decrease in the mitochondrial protein frataxin (Fxn), which is involved in iron-sulfur cluster (ISC) synthesis. Diminutions in Fxn result in decreased ISC synthesis, increased mitochondrial iron accumulation, and impaired mitochondrial function. Here, we show that conditions that result in increased mitochondrial reactive oxygen species in yeast or mammalian cell culture give rise to increased turnover of Fxn but not of other ISC synthesis proteins. We demonstrate that the mitochondrial Lon protease is involved in Fxn degradation and that iron export through the mitochondrial metal transporter Mmt1 protects yeast Fxn from degradation. We also determined that when FRDA fibroblasts were grown in media containing elevated iron, mitochondrial reactive oxygen species increased and Fxn decreased compared to WT fibroblasts. Furthermore, we screened a library of FDA-approved compounds and identified 38 compounds that increased yeast Fxn levels, including the azole bifonazole, antiparasitic fipronil, antitumor compound dibenzoylmethane, antihypertensive 4-hydroxychalcone, and a nonspecific anion channel inhibitor 4,4-diisothiocyanostilbene-2,2-sulfonic acid. We show that top hits 4-hydroxychalcone and dibenzoylmethane increased mRNA levels of transcription factor nuclear factor erythroid 2-related factor 2 in FRDA patient-derived fibroblasts, as well as downstream antioxidant targets thioredoxin, glutathione reductase, and superoxide dismutase 2. Taken together, these findings reveal that FRDA progression may be in part due to oxidant-mediated decreases in Fxn and that some approved compounds may be effective in increasing mitochondrial Fxn in FRDA, delaying disease progression.

Advantages and Limitations of Gene Therapy and Gene Editing for Friedreich's Ataxia

Friedreich's ataxia (FRDA) is an inherited, multisystemic disorder predominantly caused by GAA hyper expansion in intron 1 of frataxin (FXN) gene. This expansion mutation transcriptionally represses FXN, a mitochondrial protein that is required for iron metabolism and mitochondrial homeostasis, leading to neurodegerative and cardiac dysfunction. Current therapeutic options for FRDA are focused on improving mitochondrial function and increasing frataxin expression through pharmacological interventions but are not effective in delaying or preventing the neurodegeneration in clinical trials. Recent research on in vivo and ex vivo gene therapy methods in FRDA animal and cell models showcase its promise as a one-time therapy for FRDA. In this review, we provide an overview on the current and emerging prospects of gene therapy for FRDA, with specific focus on advantages of CRISPR/Cas9-mediated gene editing of FXN as a viable option to restore endogenous frataxin expression. We also assess the potential of ex vivo gene editing in hematopoietic stem and progenitor cells as a potential autologous transplantation therapeutic option and discuss its advantages in tackling FRDA-specific safety aspects for clinical translation.

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

Friedreich’s ataxia (FRDA) is the most prevalent autosomic recessive ataxia and is associated with a severe cardiac hypertrophy and less frequently diabetes. It is caused by mutations in the gene encoding frataxin (FXN), a small mitochondrial protein. The primary consequence is a defective expression of FXN, with basal protein levels decreased by 70–98%, which foremost affects the cerebellum, dorsal root ganglia, heart and liver. FXN is a mitochondrial protein involved in iron metabolism but its exact function has remained elusive and highly debated since its discovery. At the cellular level, FRDA is characterized by a general deficit in the biosynthesis of iron-sulfur (Fe-S) clusters and heme, iron accumulation and deposition in mitochondria, and sensitivity to oxidative stress. Based on these phenotypes and the proposed ability of FXN to bind iron, a role as an iron storage protein providing iron for Fe-S cluster and heme biosynthesis was initially proposed. However, this model was challenged by several other studies and it is now widely accepted that FXN functions primarily in Fe-S cluster biosynthesis, with iron accumulation, heme deficiency and oxidative stress sensitivity appearing later on as secondary defects. Nonetheless, the biochemical function of FXN in Fe-S cluster biosynthesis is still debated. Several roles have been proposed for FXN: iron chaperone, gate-keeper of detrimental Fe-S cluster biosynthesis, sulfide production stimulator and sulfur transfer accelerator. A picture is now emerging which points toward a unique function of FXN as an accelerator of a key step of sulfur transfer between two components of the Fe-S cluster biosynthetic complex. These findings should foster the development of new strategies for the treatment of FRDA. We will review here the latest discoveries on the biochemical function of frataxin and the implication for a potential therapeutic treatment of FRDA.

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

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by recessive mutations in the frataxin gene that lead to a deficiency of the mitochondrial frataxin (FXN) protein. Alternative forms of frataxin have been described, with different cellular localization and tissue distribution, including a cerebellum-specific cytosolic isoform called FXN II. Here, we explored the functional roles of FXN II in comparison to the mitochondrial FXN I isoform, highlighting the existence of potential cross-talk between cellular compartments. To achieve this, we transduced two human cell lines of patient and healthy subjects with lentiviral vectors overexpressing the mitochondrial or the cytosolic FXN isoforms and studied their effect on the mitochondrial network and metabolism. We confirmed the cytosolic localization of FXN isoform II in our in vitro models. Interestingly, both cytosolic and mitochondrial isoforms have an effect on mitochondrial dynamics, affecting different parameters. Accordingly, increases of mitochondrial respiration were detected after transduction with FXN I or FXN II in both cellular models. Together, these results point to the existence of a potential cross-talk mechanism between the cytosol and mitochondria, mediated by FXN isoforms. A more thorough knowledge of the mechanisms of action behind the extra-mitochondrial FXN II isoform could prove useful in unraveling FRDA physiopathology.

High Levels of Frataxin Overexpression Lead to Mitochondrial and Cardiac Toxicity in Mouse Models

Friedreich ataxia (FA) is currently an incurable inherited mitochondrial disease caused by reduced levels of frataxin (FXN). Cardiac dysfunction is the main cause of premature death in FA. Adeno-associated virus (AAV)-mediated gene therapy constitutes a promising approach for FA, as demonstrated in cardiac and neurological mouse models. While the minimal therapeutic level of FXN protein to be restored and biodistribution have recently been defined for the heart, it is unclear if FXN overexpression could be harmful. Indeed, depending on the vector delivery route and dose administered, the resulting FXN protein level could reach very high levels in the heart, cerebellum, or off-target organs such as the liver. The present study demonstrates safety of FXN cardiac overexpression up to 9-fold the normal endogenous level but significant toxicity to the mitochondria and heart above 20-fold. We show gradual severity with increasing FXN overexpression, ranging from subclinical cardiotoxicity to left ventricle dysfunction. This appears to be driven by impairment of the mitochondria respiratory chain and ultrastructure, which leads to cardiomyocyte subcellular disorganization, cell death, and fibrosis. Overall, this study underlines the need, during the development of gene therapy approaches, to consider appropriate vector expression level, long-term safety, and biomarkers to monitor such events.

Defining Transcription Regulatory Elements in the Human Frataxin Gene: Implications for Gene Therapy

Friedreich's ataxia (FRDA) is the most common inherited form of ataxia in humans. It is caused by severe downregulation of frataxin (FXN) expression instigated by hyperexpansion of the GAA repeats located in intron 1 of the FXN gene. Despite numerous studies focused on identifying compounds capable of stimulating FXN expression, current knowledge regarding cis-regulatory elements involved in FXN gene expression is lacking. Using a combination of episomal and genome-integrated constructs, we defined a minimal endogenous promoter sequence required to efficiently drive FXN expression in human cells. We generated 19 constructs varying in length of the DNA sequences upstream and downstream of the ATG start codon. Using transient transfection, we evaluated the capability of these constructs to drive FXN expression. These analyses allowed us to identify a region of the gene indispensable for FXN expression. Subsequently, selected constructs containing the FXN expression control regions of varying lengths were site specifically integrated into the genome of HEK293T and human-induced pluripotent stem cells (iPSCs). FXN expression was detected in iPSCs and persisted after differentiation to neuronal and cardiac cells, indicating lineage independent function of defined regulatory DNA sequences. Finally, based on these results, we generated AAV encoding miniFXN genes and demonstrated in vivo FXN expression in mice. Results of these studies identified FXN sequences necessary to express FXN in human and mouse cells and provided rationale for potential use of endogenous FXN sequence in gene therapy strategies for FRDA.

Analysis of Putative Epigenetic Regulatory Elements in the FXN Genomic Locus

Friedreich´s ataxia (FRDA) is an autosomal recessive disease caused by an abnormally expanded Guanine-Adenine-Adenine (GAA) repeat sequence within the first intron of the frataxin gene (FXN). The molecular mechanisms associated with FRDA are still poorly understood and most studies on FXN gene regulation have been focused on the region around the minimal promoter and the region in which triplet expansion occurs. Nevertheless, since there could be more epigenetic changes involved in the reduced levels of FXN transcripts, the aim of this study was to obtain a more detailed view of the possible regulatory elements by analyzing data from ENCODE and Roadmap consortia databases. This bioinformatic analysis indicated new putative regulatory regions within the FXN genomic locus, including exons, introns, and upstream and downstream regions. Moreover, the region next to the end of intron 4 is of special interest, since the enhancer signals in FRDA-affected tissues are weak or absent in this region, whilst they are strong in the rest of the analyzed tissues. Therefore, these results suggest that there could be a direct relationship between the absence of enhancer sequences in this specific region and their predisposition to be affected in this pathology.

CRISPR-Cas9 Gene Editing of Hematopoietic Stem Cells from Patients with Friedreich's Ataxia

Friedreich’s ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by expansion of GAA repeats in intron 1 of the frataxin (FXN) gene, leading to significant decreased expression of frataxin, a mitochondrial iron-binding protein. We previously reported that syngeneic hematopoietic stem and progenitor cell (HSPC) transplantation prevented neurodegeneration in the FRDA mouse model YG8R. We showed that the mechanism of rescue was mediated by the transfer of the functional frataxin from HSPC-derived microglia/macrophage cells to neurons/myocytes. In this study, we report the first step toward an autologous HSPC transplantation using the CRISPR-Cas9 system for FRDA. We first identified a pair of CRISPR RNAs (crRNAs) that efficiently removes the GAA expansions in human FRDA lymphoblasts, restoring the non-pathologic level of frataxin expression and normalizing mitochondrial activity. We also optimized the gene-editing approach in HSPCs isolated from healthy and FRDA patients’ peripheral blood and demonstrated normal hematopoiesis of gene-edited cells in vitro and in vivo. The procedure did not induce cellular toxic effect or major off-target events, but a p53-mediated cell proliferation delay was observed in the gene-edited cells. This study provides the foundation for the clinical translation of autologous transplantation of gene-corrected HSPCs for FRDA.

A new tool to determine the cellular metabolic landscape: nanotechnology to the study of Friedreich's ataxia

Understanding the cell response to oxidative stress in disease is an important but difficult task. Here, we demonstrate the feasibility of using a nanomotion sensor to study the cellular metabolic landscape. This nanosensor permits the non-invasive real-time detection at the single-cell level and offers high sensitivity and time resolution. We optimised the technique to study the effects of frataxin overexpression in a cellular model of Friedreich's ataxia, a neurodegenerative disease caused by partial silencing of the FXN gene. Previous studies had demonstrated that FXN overexpression are as toxic as silencing, thus indicating the importance of a tight regulation of the frataxin levels. We probed the effects of frataxin overexpression in the presence of oxidative stress insults and measured the metabolic response by the nanosensor. We show that the nanosensor provides new detailed information on the metabolic state of the cell as a function of time, that agrees with and complements data obtained by more traditional techniques. We propose that the nanosensor can be used in the future as a new and powerful tool to study directly how drugs modulate the effects of oxidative stress on Friedreich's ataxia patients and, more in general, on other neurodegenerative processes.

Oxymatrine exhibits anti-tumor activity in gastric cancer through inhibition of IL-21R-mediated JAK2/STAT3 pathway

Oxymatrine (OMT) as a type of alkaloids collected from Sophora flavescens Ait exerts some biological functions including anticancer properties. Here, we investigated the therapeutic effects of OMT in gastric cancer cells (HGC 27 and AGS). As a result, the exposure of gastric cancer (GC) cells to OMT contributed to the suppression of cell proliferation and invasion. Interleukin 21 receptor (IL-21R) was identified to be differentially expressed between OMT treatment group (4 mg/mL) and control group (0 mg/mL), and knockdown of IL-21R repressed cell proliferation and invasion via inactivation of the JAK2/STAT3 pathway. The rescue experiment showed that IL-21R overexpression attenuated the anti-tumor effects of OMT through activation of the JAK2/STAT3 pathway. Moreover, the expression of IL-21R was significantly upregulated in GC samples compared with the adjacent normal tissues and associated with overall survival (OS) and tumor recurrence of GC patients. Taken together, in this study, we evaluated the anti-tumor effects of OMT on GC by investigating proliferation and invasion ability changes, and our findings show that OMT exhibits effects via regulation of JAK/STAT signaling pathway. Through the mechanism study, we may enlighten the potential therapeutic target for treatment of GC.

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

Drosophila melanogaster has been for over a century the model of choice of several neurobiologists to decipher the formation and development of the nervous system as well as to mirror the pathophysiological conditions of many human neurodegenerative diseases. The rare disease Friedreich’s ataxia (FRDA) is not an exception. Since the isolation of the responsible gene more than two decades ago, the analysis of the fly orthologue has proven to be an excellent avenue to understand the development and progression of the disease, to unravel pivotal mechanisms underpinning the pathology and to identify genes and molecules that might well be either disease biomarkers or promising targets for therapeutic interventions. In this review, we aim to summarize the collection of findings provided by the Drosophila models but also to go one step beyond and propose the implications of these discoveries for the study and cure of this disorder. We will present the physiological, cellular and molecular phenotypes described in the fly, highlighting those that have given insight into the pathology and we will show how the ability of Drosophila to perform genetic and pharmacological screens has provided valuable information that is not easily within reach of other cellular or mammalian models.

Adding a temporal dimension to the study of Friedreich's ataxia: the effect of frataxin overexpression in a human cell model

The neurodegenerative disease Friedreich's ataxia is caused by lower than normal levels of frataxin, an important protein involved in iron-sulfur (Fe-S) cluster biogenesis. An important step in designing strategies to treat this disease is to understand whether increasing the frataxin levels by gene therapy would simply be beneficial or detrimental, because previous studies, mostly based on animal models, have reported conflicting results. Here, we have exploited an inducible model, which we developed using the CRISPR/Cas9 methodology, to study the effects of frataxin overexpression in human cells and monitor how the system recovers after overexpression. Using new tools, which range from high-throughput microscopy to in cell infrared, we prove that overexpression of the frataxin gene affects the cellular metabolism. It also leads to a significant increase of oxidative stress and labile iron pool levels. These cellular alterations are similar to those observed when the gene is partly silenced, as occurs in Friedreich's ataxia patients. Our data suggest that the levels of frataxin must be tightly regulated and fine-tuned, with any imbalance leading to oxidative stress and toxicity.

Expression of the Shrimp wap gene in Drosophila elicits defense responses and protease inhibitory activity

The wap gene encodes a single whey acidic protein (WAP) domain-containing peptide from Chinese white shrimp (Fenneropenaeus chinensis), which shows broad-spectrum antimicrobial activities and proteinase inhibitory activities in vitro. To explore the medical applications of the WAP peptide, a wap gene transgenic Drosophila melanogaster was constructed. In wap-expressing flies, high expression levels of wap gene (>100 times) were achieved, in contrast to those of control flies, by qRT-PCR analysis. The wap gene expression was associated with increased resistance to microbial infection and decreased bacterial numbers in the flies. In addition, the WAP protein extract from wap-expressing flies, compared with control protein extract from control flies, showed improved antimicrobial activities against broad Gram-positive and Gram-negative bacteria, including the clinical drug resistant bacterium of methicillin-resistant S. aureus (MRSA), improved protease inhibitor activities against crude proteinases and commercial proteinases, including elastase, subtilis proteinase A, and proteinase K in vitro, and improved growth rate and microbial resistance, as well as wound-healing in loach and mouse models. These results suggest that wap-expressing flies could be used as a food additive in aquaculture to prevent infections and a potential antibacterial for fighting drug-resistant bacteria.

Drosophila melanogaster Models of Friedreich's Ataxia

Friedreich's ataxia (FRDA) is a rare inherited recessive disorder affecting the central and peripheral nervous systems and other extraneural organs such as the heart and pancreas. This incapacitating condition usually manifests in childhood or adolescence, exhibits an irreversible progression that confines the patient to a wheelchair, and leads to early death. FRDA is caused by a reduced level of the nuclear-encoded mitochondrial protein frataxin due to an abnormal GAA triplet repeat expansion in the first intron of the human FXN gene. FXN is evolutionarily conserved, with orthologs in essentially all eukaryotes and some prokaryotes, leading to the development of experimental models of this disease in different organisms. These FRDA models have contributed substantially to our current knowledge of frataxin function and the pathogenesis of the disease, as well as to explorations of suitable treatments. Drosophila melanogaster, an organism that is easy to manipulate genetically, has also become important in FRDA research. This review describes the substantial contribution of Drosophila to FRDA research since the characterization of the fly frataxin ortholog more than 15 years ago. Fly models have provided a comprehensive characterization of the defects associated with frataxin deficiency and have revealed genetic modifiers of disease phenotypes. In addition, these models are now being used in the search for potential therapeutic compounds for the treatment of this severe and still incurable disease.

Frataxin overexpression in Müller cells protects retinal ganglion cells in a mouse model of ischemia/reperfusion injury in vivo

Müller cells are critical for retinal function and neuronal survival but can become detrimental in response to retinal ischemia and increased oxidative stress. Elevated oxidative stress increases expression of the mitochondrial enzyme frataxin in the retina, and its overexpression is neuroprotective after ischemia. Whether frataxin expression in Müller cells might improve their function and protect neurons after ischemia is unknown. The aim of this study was to evaluate the effect of frataxin overexpression in Müller cells on neuronal survival after retinal ischemia/reperfusion in the mouse in vivo. Retinal ischemia/reperfusion was induced in mice overexpressing frataxin in Müller cells by transient elevation of intraocular pressure. Retinal ganglion cells survival was determined 14 days after lesion. Expression of frataxin, antioxidant enzymes, growth factors and inflammation markers was determined with qRT-PCR, Western blotting and immunohistochemistry 24 hours after lesion. Following lesion, there was a 65% increase in the number of surviving RGCs in frataxin overexpressing mice. Improved survival was associated with increased expression of the antioxidant enzymes Gpx1 and Sod1 as well as the growth factors Cntf and Lif. Additionally, microglial activation was decreased in these mice. Therefore, support of Müller cell function constitutes a feasible approach to reduce neuronal degeneration after ischemia.

Mitofusin-Dependent ER Stress Triggers Glial Dysfunction and Nervous System Degeneration in a Drosophila Model of Friedreich's Ataxia

Friedreich's ataxia (FRDA) is the most important recessive ataxia in the Caucasian population. It is caused by a deficit of the mitochondrial protein frataxin. Despite its pivotal effect on biosynthesis of iron-sulfur clusters and mitochondrial energy production, little is known about the influence of frataxin depletion on homeostasis of the cellular mitochondrial network. We have carried out a forward genetic screen to analyze genetic interactions between genes controlling mitochondrial homeostasis and Drosophila frataxin. Our screen has identified silencing of Drosophila mitofusin (Marf) as a suppressor of FRDA phenotypes in glia. Drosophila Marf is known to play crucial roles in mitochondrial fusion, mitochondrial degradation and in the interface between mitochondria and endoplasmic reticulum (ER). Thus, we have analyzed the effects of frataxin knockdown on mitochondrial morphology, mitophagy and ER function in our fly FRDA model using different histological and molecular markers such as tetramethylrhodamine, ethyl ester (TMRE), mitochondria-targeted GFP (mitoGFP), p62, ATG8a, LAMP1, Xbp1 and BiP/GRP78. Furthermore, we have generated the first Drosophila transgenic line containing the mtRosella construct under the UAS control to study the progression of the mitophagy process in vivo. Our results indicated that frataxin-deficiency had a small impact on mitochondrial morphology but enhanced mitochondrial clearance and altered the ER stress response in Drosophila. Remarkably, we demonstrate that downregulation of Marf suppresses ER stress in frataxin-deficient cells and this is sufficient to improve locomotor dysfunction, brain degeneration and lipid dyshomeostasis in our FRDA model. In agreement, chemical reduction of ER stress by means of two different compounds was sufficient to ameliorate the effects of frataxin deficiency in three different fly FRDA models. Altogether, our results strongly suggest that the protection mediated by Marf knockdown in glia is mainly linked to its role in the mitochondrial-ER tethering and not to mitochondrial dynamics or mitochondrial degradation and that ER stress is a novel and pivotal player in the progression and etiology of FRDA. This work might define a new pathological mechanism in FRDA, linking mitochondrial dysfunction due to frataxin deficiency and mitofusin-mediated ER stress, which might be responsible for characteristic cellular features of the disease and also suggests ER stress as a therapeutic target.

Iron Sulfur and Molybdenum Cofactor Enzymes Regulate the Drosophila Life Cycle by Controlling Cell Metabolism

Iron sulfur (Fe-S) clusters and the molybdenum cofactor (Moco) are present at enzyme sites, where the active metal facilitates electron transfer. Such enzyme systems are soluble in the mitochondrial matrix, cytosol and nucleus, or embedded in the inner mitochondrial membrane, but virtually absent from the cell secretory pathway. They are of ancient evolutionary origin supporting respiration, DNA replication, transcription, translation, the biosynthesis of steroids, heme, catabolism of purines, hydroxylation of xenobiotics, and cellular sulfur metabolism. Here, Fe-S cluster and Moco biosynthesis in Drosophila melanogaster is reviewed and the multiple biochemical and physiological functions of known Fe-S and Moco enzymes are described. We show that RNA interference of Mocs3 disrupts Moco biosynthesis and the circadian clock. Fe-S-dependent mitochondrial respiration is discussed in the context of germ line and somatic development, stem cell differentiation and aging. The subcellular compartmentalization of the Fe-S and Moco assembly machinery components and their connections to iron sensing mechanisms and intermediary metabolism are emphasized. A biochemically active Fe-S core complex of heterologously expressed fly Nfs1, Isd11, IscU, and human frataxin is presented. Based on the recent demonstration that copper displaces the Fe-S cluster of yeast and human ferredoxin, an explanation for why high dietary copper leads to cytoplasmic iron deficiency in flies is proposed. Another proposal that exosomes contribute to the transport of xanthine dehydrogenase from peripheral tissues to the eye pigment cells is put forward, where the Vps16a subunit of the HOPS complex may have a specialized role in concentrating this enzyme within pigment granules. Finally, we formulate a hypothesis that (i) mitochondrial superoxide mobilizes iron from the Fe-S clusters in aconitase and succinate dehydrogenase; (ii) increased iron transiently displaces manganese on superoxide dismutase, which may function as a mitochondrial iron sensor since it is inactivated by iron; (iii) with the Krebs cycle thus disrupted, citrate is exported to the cytosol for fatty acid synthesis, while succinyl-CoA and the iron are used for heme biosynthesis; (iv) as iron is used for heme biosynthesis its concentration in the matrix drops allowing for manganese to reactivate superoxide dismutase and Fe-S cluster biosynthesis to reestablish the Krebs cycle.

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.

Sustained FXN expression in dorsal root ganglia from a nonreplicative genomic HSV-1 vector

BACKGROUND

Friedreich's ataxia (FA) is an autosomal recessive neurodegenerative disease caused by mutations in the frataxin gene (FXN), which lead to reduced levels of the essential mitochondrial protein frataxin. Currently, there is no effective cure.

METHODS

With the aim of developing a gene therapy for FA neuropathology, we describe the construction and preliminary characterization of a high-capacity nonreplicative genomic herpes simplex virus type 1 vector (H24B-FXNlac vector) carrying a reduced version of the human FXN genomic locus, comprising the 5-kb promoter and the FXN cDNA with the inclusion of intron 1.

RESULTS

We show that the transgene cassette contains the elements necessary to preserve physiological neuronal regulation of human FXN expression. Transduction of cultured fetal rat dorsal root ganglia neurons with the H24B-FXNlac vector results in sustained expression of human FXN transcripts and frataxin protein. Rat footpad inoculation with the H24B-FXNlac vector results in human FXN transgene delivery to the dorsal root ganglia, with expression persisting for at least 1 month.

CONCLUSIONS

The results of the present study support the feasibility of using this vector for sustained neuronal expression of human frataxin for FA gene therapy.

Lentivirus-meditated frataxin gene delivery reverses genome instability in Friedreich ataxia patient and mouse model fibroblasts

Friedreich ataxia (FRDA) is a progressive neurodegenerative disease caused by deficiency of frataxin protein, with the primary sites of pathology being the large sensory neurons of the dorsal root ganglia and the cerebellum. FRDA is also often accompanied by severe cardiomyopathy and diabetes mellitus. Frataxin is important in mitochondrial iron-sulfur cluster (ISC) biogenesis and low-frataxin expression is due to a GAA repeat expansion in intron 1 of the FXN gene. FRDA cells are genomically unstable, with increased levels of reactive oxygen species and sensitivity to oxidative stress. Here we report the identification of elevated levels of DNA double strand breaks (DSBs) in FRDA patient and YG8sR FRDA mouse model fibroblasts compared to normal fibroblasts. Using lentivirus FXN gene delivery to FRDA patient and YG8sR cells, we obtained long-term overexpression of FXN mRNA and frataxin protein levels with reduced DSB levels towards normal. Furthermore, γ-irradiation of FRDA patient and YG8sR cells revealed impaired DSB repair that was recovered on FXN gene transfer. This suggests that frataxin may be involved in DSB repair, either directly by an unknown mechanism, or indirectly via ISC biogenesis for DNA repair enzymes, which may be essential for the prevention of neurodegeneration.

Two different pathogenic mechanisms, dying-back axonal neuropathy and pancreatic senescence, are present in the YG8R mouse model of Friedreich's ataxia

Frataxin (FXN) deficiency causes Friedreich’s ataxia (FRDA), a multisystem disorder with neurological and non-neurological symptoms. FRDA pathophysiology combines developmental and degenerative processes of dorsal root ganglia (DRG), sensory nerves, dorsal columns and other central nervous structures. A dying-back mechanism has been proposed to explain the peripheral neuropathy and neuropathology. In addition, affected individuals have non-neuronal symptoms such as diabetes mellitus or glucose intolerance. To go further in the understanding of the pathogenic mechanisms of neuropathy and diabetes associated with the disease, we have investigated the humanized mouse YG8R model of FRDA. By biochemical and histopathological studies, we observed abnormal changes involving muscle spindles, dorsal root axons and DRG neurons, but normal findings in the posterior columns and brain, which agree with the existence of a dying-back process similar to that described in individuals with FRDA. In YG8R mice, we observed a large number of degenerated axons surrounded by a sheath exhibiting enlarged adaxonal compartments or by a thin disrupted myelin sheath. Thus, both axonal damage and defects in Schwann cells might underlie the nerve pathology. In the pancreas, we found a high proportion of senescent islets of Langerhans in YG8R mice, which decreases the β-cell number and islet mass to pathological levels, being unable to maintain normoglycemia. As a whole, these results confirm that the lack of FXN induces different pathogenic mechanisms in the nervous system and pancreas in the mouse model of FRDA: dying back of the sensory nerves, and pancreatic senescence.

Summary: Frataxin deficiency induces different pathogenic mechanisms in the nervous system and pancreas in a YG8R mouse model of Friedreich's ataxia (FRDA). Thus, the degenerative process in FRDA is determined by the cell type.

Delivery of the 135 kb human frataxin genomic DNA locus gives rise to different frataxin isoforms

Friedreich's ataxia (FRDA) is the most common form of hereditary ataxia caused by recessive mutations in the FXN gene. Recent results have indicated the presence of different frataxin isoforms due to alternative gene expression mechanisms. Our previous studies demonstrated the advantages of using high-capacity herpes simplex virus type 1 (HSV-1) amplicon vectors containing the entire FXN genomic locus (iBAC-FXN) as a gene-delivery vehicle capable of ensuring physiologically-regulated and long-term persistence. Here we describe how expression from the 135 kb human FXN genomic locus produces the three frataxin isoforms both in cultured neuronal cells and also in vivo. Moreover, we also observed the correct expression of these frataxin isoforms in patient-derived cells after delivery of the iBAC-FXN. These results lend further support to the potential use of HSV-1 vectors containing entire genomic loci whose expression is mediated by complex transcriptional and posttranscriptional mechanisms for gene therapy applications.

TORC1 Inhibition by Rapamycin Promotes Antioxidant Defences in a Drosophila Model of Friedreich's Ataxia

Friedreich's ataxia (FRDA), the most common inherited ataxia in the Caucasian population, is a multisystemic disease caused by a significant decrease in the frataxin level. To identify genes capable of modifying the severity of the symptoms of frataxin depletion, we performed a candidate genetic screen in a Drosophila RNAi-based model of FRDA. We found that genetic reduction in TOR Complex 1 (TORC1) signalling improves the impaired motor performance phenotype of FRDA model flies. Pharmacologic inhibition of TORC1 signalling by rapamycin also restored this phenotype and increased the lifespan and ATP levels. Furthermore, rapamycin reduced the altered levels of malondialdehyde + 4-hydroxyalkenals and total glutathione of the model flies. The rapamycin-mediated protection against oxidative stress is due in part to an increase in the transcription of antioxidant genes mediated by cap-n-collar (Drosophila ortholog of Nrf2). Our results suggest that autophagy is indeed necessary for the protective effect of rapamycin in hyperoxia. Rapamycin increased the survival and aconitase activity of model flies subjected to high oxidative insult, and this improvement was abolished by the autophagy inhibitor 3-methyladenine. These results point to the TORC1 pathway as a new potential therapeutic target for FRDA and as a guide to finding new promising molecules for disease treatment.

The role of frataxin in fission yeast iron metabolism: implications for Friedreich's ataxia

BACKGROUND

The neurodegenerative disease Friedreich's ataxia is the result of frataxin deficiency. Frataxin is a mitochondrial protein involved in iron-sulfur cluster (Fe-S) cofactor biogenesis, but its functional role in this pathway is debated. This is due to the interconnectivity of iron metabolic and oxidative stress response pathways that make distinguishing primary effects of frataxin deficiency challenging. Since Fe-S cluster assembly is conserved, frataxin overexpression phenotypes in a simple eukaryotic organism will provide additional insight into frataxin function.

METHODS

The Schizosaccharomyces pombe frataxin homologue (fxn1) was overexpressed from a plasmid under a thiamine repressible promoter. The S. pombe transformants were characterized at several expression strengths for cellular growth, mitochondrial organization, iron levels, oxidative stress, and activities of Fe-S cluster containing enzymes.

RESULTS

Observed phenotypes were dependent on the amount of Fxn1 overexpression. High Fxn1 overexpression severely inhibited S. pombe growth, impaired mitochondrial membrane integrity and cellular respiration, and led to Fxn1 aggregation. Cellular iron accumulation was observed at moderate Fxn1 overexpression but was most pronounced at high levels of Fxn1. All levels of Fxn1 overexpression up-regulated oxidative stress defense and mitochondrial Fe-S cluster containing enzyme activities.

CONCLUSIONS

Despite the presence of oxidative stress and accumulated iron, activation of Fe-S cluster enzymes was common to all levels of Fxn1 overexpression; therefore, Fxn1 may regulate the efficiency of Fe-S cluster biogenesis in S. pombe.

GENERAL SIGNIFICANCE

We provide evidence that suggests that dysregulated Fe-S cluster biogenesis is a primary effect of both frataxin overexpression and deficiency as in Friedreich's ataxia.

Gene regulation and epigenetics in Friedreich's ataxia

This is an exciting time in the study of Friedreich's ataxia. Over the last 10 years much progress has been made in uncovering the mechanisms, whereby the Frataxin gene is silenced by (GAA)n repeat expansions and several of the findings are now ripe for testing in the clinic. The discovery that the Frataxin gene is heterochromatinised and that this can be antagonised in vivo has led to the tantalizing possibility that the disease might be amenable to a more radical therapeutic approach involving epigenetic modifiers. Here, we set out to review progress in the understanding of the fundamental mechanisms whereby genes are regulated at this level and how these findings have been applied to achieve a deeper understanding of the dysregulation that occurs as the primary genetic lesion in Friedreich's ataxia.

Iron absorption in Drosophila melanogaster

The way in which Drosophila melanogaster acquires iron from the diet remains poorly understood despite iron absorption being of vital significance for larval growth. To describe the process of organismal iron absorption, consideration needs to be given to cellular iron import, storage, export and how intestinal epithelial cells sense and respond to iron availability. Here we review studies on the Divalent Metal Transporter-1 homolog Malvolio (iron import), the recent discovery that Multicopper Oxidase-1 has ferroxidase activity (iron export) and the role of ferritin in the process of iron acquisition (iron storage). We also describe what is known about iron regulation in insect cells. We then draw upon knowledge from mammalian iron homeostasis to identify candidate genes in flies. Questions arise from the lack of conservation in Drosophila for key mammalian players, such as ferroportin, hepcidin and all the components of the hemochromatosis-related pathway. Drosophila and other insects also lack erythropoiesis. Thus, systemic iron regulation is likely to be conveyed by different signaling pathways and tissue requirements. The significance of regulating intestinal iron uptake is inferred from reports linking Drosophila developmental, immune, heat-shock and behavioral responses to iron sequestration.

Insertion mutants in Drosophila melanogaster Hsc20 halt larval growth and lead to reduced iron-sulfur cluster enzyme activities and impaired iron homeostasis

Despite the prominence of iron-sulfur cluster (ISC) proteins in bioenergetics, intermediary metabolism, and redox regulation of cellular, mitochondrial, and nuclear processes, these proteins have been given scarce attention in Drosophila. Moreover, biosynthesis and delivery of ISCs to target proteins requires a highly regulated molecular network that spans different cellular compartments. The only Drosophila ISC biosynthetic protein studied to date is frataxin, in attempts to model Friedreich's ataxia, a disease arising from reduced expression of the human frataxin homologue. One of several proteins involved in ISC biogenesis is heat shock protein cognate 20 (Hsc20). Here we characterize two piggyBac insertion mutants in Drosophila Hsc20 that display larval growth arrest and deficiencies in aconitase and succinate dehydrogenase activities, but not in isocitrate dehydrogenase activity; phenotypes also observed with ubiquitous frataxin RNA interference. Furthermore, a disruption of iron homeostasis in the mutant flies was evidenced by an apparent reduction in induction of intestinal ferritin with ferric iron accumulating in a subcellular pattern reminiscent of mitochondria. These phenotypes were specific to intestinal cell types that regulate ferritin expression, but were notably absent in the iron cells where ferritin is constitutively expressed and apparently translated independently of iron regulatory protein 1A. Hsc20 mutant flies represent an independent tool to disrupt ISC biogenesis in vivo without using the RNA interference machinery.

Novel frataxin isoforms may contribute to the pathological mechanism of Friedreich ataxia

Friedreich ataxia (FRDA) is an inherited neurodegenerative disease caused by frataxin (FXN) deficiency. The nervous system and heart are the most severely affected tissues. However, highly mitochondria-dependent tissues, such as kidney and liver, are not obviously affected, although the abundance of FXN is normally high in these tissues. In this study we have revealed two novel FXN isoforms (II and III), which are specifically expressed in affected cerebellum and heart tissues, respectively, and are functional in vitro and in vivo. Increasing the abundance of the heart-specific isoform III significantly increased the mitochondrial aconitase activity, while over-expression of the cerebellum-specific isoform II protected against oxidative damage of Fe-S cluster-containing aconitase. Further, we observed that the protein level of isoform III decreased in FRDA patient heart, while the mRNA level of isoform II decreased more in FRDA patient cerebellum compared to total FXN mRNA. Our novel findings are highly relevant to understanding the mechanism of tissue-specific pathology in FRDA.