Kuanyu Li

Iron-dependent regulation of frataxin expression: implications for treatment of Friedreich ataxia.

Friedreich ataxia (FA) is a progressive neurodegenerative disease caused by expansion of a trinucleotide repeat within the first intron of the gene that encodes frataxin. In our study, we investigated the regulation of frataxin expression by iron and demonstrated that frataxin mRNA levels decrease significantly in multiple human cell lines treated with the iron chelator, desferal (DFO). In addition, frataxin mRNA and protein levels decrease in fibroblast and lymphoblast cells derived from both normal controls and from patients with FA when treated with DFO. Lymphoblasts and fibroblasts of FA patients have evidence of cytosolic iron depletion, as indicated by increased levels of iron regulatory protein 2 (IRP2) and/or increased IRE-binding activity of IRP1. We postulate that this inferred cytosolic iron depletion occurs as frataxin-deficient cells overload their mitochondria with iron, a downstream regulatory effect that has been observed previously when mitochondrial iron–sulfur cluster assembly is disrupted. The mitochondrial iron overload and presumed cytosolic iron depletion potentially further compromise function in frataxin-deficient cells by decreasing frataxin expression. Thus, our results imply that therapeutic efforts should focus on an approach that combines iron removal from mitochondria with a treatment that increases cytosolic iron levels to maximize residual frataxin expression in FA patients.

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.

Expression of Human Frataxin Is Regulated by Transcription Factors SRF and TFAP2

Background Friedreich ataxia is an autosomal recessive neurodegenerative disease caused by reduced expression levels of the frataxin gene (FXN) due to expansion of triplet nucleotide GAA repeats in the first intron of FXN. Augmentation of frataxin expression levels in affected Friedreich ataxia patient tissues might substantially slow disease progression. Methodology/Principal Findings We utilized bioinformatic tools in conjunction with chromatin immunoprecipitation and electrophoretic mobility shift assays to identify transcription factors that influence transcription of the FXN gene. We found that the transcription factors SRF and TFAP2 bind directly to FXN promoter sequences. SRF and TFAP2 binding sequences in the FXN promoter enhanced transcription from luciferase constructs, while mutagenesis of the predicted SRF or TFAP2 binding sites significantly decreased FXN promoter activity. Further analysis demonstrated that robust SRF- and TFAP2-mediated transcriptional activity was dependent on a regulatory element, located immediately downstream of the first FXN exon. Finally, over-expression of either SRF or TFAP2 significantly increased frataxin mRNA and protein levels in HEK293 cells, and frataxin mRNA levels were also elevated in SH-SY5Y cells and in Friedreich ataxia patient lymphoblasts transfected with SRF or TFAP2. Conclusions/Significance We identified two transcription factors, SRF and TFAP2, as well as an intronic element encompassing EGR3-like sequence, that work together to regulate expression of the FXN gene. By providing new mechanistic insights into the molecular factors influencing frataxin expression, our results should aid in the discovery of new therapeutic targets for the treatment of Friedreich ataxia.

A Mitochondrion-Targeted Antioxidant Ameliorates Isoflurane-Induced Cognitive Deficits in Aging Mice

Isoflurane possesses neurotoxicity and can induce cognitive deficits, particularly in aging mammals. Mitochondrial reactive oxygen species (mtROS) have been linked to the early pathogenesis of this disorder. However, the role of mtROS remains to be evaluated due to a lack of targeted method to treat mtROS. Here, we determined in aging mice the effects of the mitochondrion-targeted antioxidant SS-31, on cognitive deficits induced by isoflurane, a general inhalation anesthetic. We further investigated the possible mechanisms underlying the effects of SS-31 on hippocampal neuro-inflammation and apoptosis. The results showed that isoflurane induced hippocampus-dependent memory deficit, which was associated with mitochondrial dysfunction including reduced ATP contents, increased ROS levels, and mitochondrial swelling. Treatment with SS-31 significantly ameliorated isoflurane-induced cognitive deficits through the improvement of mitochondrial integrity and function. Mechanistically, SS-31 treatment suppressed pro-inflammatory responses by decreasing the levels of NF-κB, NLRP3, caspase 1, IL-1β, and TNF-α; and inhibited the apoptotic pathway by decreasing the Bax/Bcl-2 ratio, reducing the release of cytochrome C, and blocking the cleavage of caspase 3. Our results indicate that isoflurane-induced cognitive deficits may be attenuated by mitochondrion-targeted antioxidants, such as SS-31. Therefore, SS-31 may have therapeutic potentials in preventing injuries from oxidative stresses that contribute to anesthetic-induced neurotoxicity.

BDNF pathway is involved in the protective effects of SS-31 on isoflurane-induced cognitive deficits in aging mice.

Mitochondrial dysfunction has been linked to the earliest pathogenesis of isoflurane-induced cognitive impairments in developing or aging mammalian brain. However, its molecular mechanism is poorly understood and a pharmacologic treatment to rapidly reverse mitochondrial dysfunction is lacking. Fifteen-month-old male C57BL/6 mice were exposed to isoflurane for two hours following intraperitoneal administration of mitochondrion-targeted peptide SS-31 or vehicle with 30min interval. The hippocampus was immediately removed for biochemical assays and mitochondria isolation after inhalation. Behavioral tests were evaluated by the open field test and fear conditioning test 24h after the experiment. We showed that cognitive deficits induced by exposure of the aging mice to isoflurane were accompanied by mitochondrial dysfunction in hippocampus due to loss of the enzymatic activity of complex I. This loss resulted in the increase of reactive oxygen species production, decrease of ATP production and mitochondrial membrane potential, and opening of mitochondrial permeability transition pore. Further, we provided evidence that the BDNF signaling pathway was involved in this process to regulate synaptic plasticity-related proteins, for instance, downregulation of synapsin 1, PSD-95 and p-CREB, and upregulation of NR2A, NR2B, CaMKIIα and CaMKIIβ. Of note, the isoflurane-induced cognitive deficits were rescued by SS-31 through reversal of mitochondrial dysfunction, which facilitated the regulation of BDNF signaling including the expression reversal of aforementioned important synaptic-signaling proteins in aging mice. Our data demonstrate that reversing mitochondrial dysfunction by SS-31 enhances BDNF signaling pathway and synaptic plasticity, and provides protective effects on cognitive function, thereby support the notion that SS-31 may have therapeutic benefits for elderly humans undertaking anesthesia.

Blockage of mitochondrial calcium uniporter prevents iron accumulation in a model of experimental subarachnoid hemorrhage

Previous studies have shown that iron accumulation is involved in the pathogenesis of brain injury following subarachnoid hemorrhage (SAH) and chelation of iron reduced mortality and oxidative DNA damage. We previously reported that blockage of mitochondrial calcium uniporter (MCU) provided benefit in the early brain injury after experimental SAH. This study was undertaken to identify whether blockage of MCU could ameliorate iron accumulation-associated brain injury following SAH. Therefore, we used two reagents ruthenium red (RR) and spermine (Sper) to inhibit MCU. Sprague-Dawley (SD) rats were randomly divided into four groups including sham, SAH, SAH+RR, and SAH+Sper. Biochemical analysis and histological assays were performed. The results confirmed the iron accumulation in temporal lobe after SAH. Interestingly, blockage of MCU dramatically reduced the iron accumulation in this area. The mechanism was revealed that inhibition of MCU reversed the down-regulation of iron regulatory protein (IRP) 1/2 and increase of ferritin. Iron-sulfur cluster dependent-aconitase activity was partially conserved when MCU was blocked. In consistence with this and previous report, ROS levels were notably reduced and ATP supply was rescued; levels of cleaved caspase-3 dropped; and integrity of neurons in temporal lobe was protected. Taken together, our results indicated that blockage of MCU could alleviate iron accumulation and the associated injury following SAH. These findings suggest that the alteration of calcium and iron homeostasis be coupled and MCU be considered to be a therapeutic target for patients suffering from SAH.

Peptide SS-31 upregulates frataxin expression and improves the quality of mitochondria: implications in the treatment of Friedreich ataxia

Friedreich ataxia is a progressive neurodegenerative disease caused by the expansion of GAA trinucleotide repeats within the first intron of the FXN gene, which encodes frataxin. The pathophysiology of the disease is thought to be derived from the decrease of Fe-S cluster biogenesis due to frataxin deficiency. There is currently no effective treatment for the disease. In our study, we demonstrated that treatment with the mitochondrion-targeted peptide SS-31 reduced frataxin deficiency-induced oxidative stress in lymphoblasts and fibroblasts derived from patients. Interestingly, SS-31 treatment translationally upregulated the protein level of frataxin in a dose-dependent manner. Furthermore, SS-31 treatment increased the enzymatic activities of the iron-sulphur enzymes, including aconitase and complex II and III of the respiratory chain. Further evaluation of the quality of mitochondria showed that mitochondrial membrane potential, ATP content, NAD+/NADH, and the morphology of mitochondria all improved. Our results suggest that SS-31 might potentially be a new drug for the early treatment of Friedreich ataxia.

Chronic administration of mitochondrion-targeted peptide SS-31 prevents atherosclerotic development in ApoE knockout mice fed Western diet

Background Oxidative stress and inflammatory factors are deeply involved in progression of atherosclerosis. Mitochondrion-targeted peptide SS-31, selectively targeting to mitochondrial inner membrane reacting with cardiolipin, has been reported to inhibit ROS generation and mitigate inflammation. The present study was designed to investigate whether SS-31 could suppress the development of atherosclerosis in vivo. Methods Male ApoE-/- mice (8 weeks old) fed with Western diet were treated with normal saline or SS-31 (1 mg/kg/d or 3 mg/kg/d) through subcutaneous injection for 12 weeks. Oil Red O staining was performed to evaluate area and sizes of the plaques. DHE staining and immunohistochemical staining of 8-OHDG was performed to assess the oxidative stress. The aorta ATP contents were assessed by the ATP bioluminescence assay kit. Immunohistochemical staining of CD68 and α-SMA and Masson’s trichrome staining were performed to evaluate the composition of atherosclerotic plaque. Biochemical assays were performed to determine the protein level and activity of superoxide dismutase (SOD). The levels of CD36, LOX-1 and ABCA1 were immunohistochemically and biochemically determined to evaluate the cholesterol transport in aorta and peritoneal macrophages. Inflammatory factors, including ICAM-1, MCP-1, IL-6 and CRP in serum, were detected through ELISA. Results SS-31 administration reduced the area and sizes of western diet-induced atherosclerotic plaques and changed the composition of the plaques in ApoE-/- mice. Oxidative stress was suppressed, as evidenced by the reduced DHE stain, down-regulated 8-OHDG expression, and increased SOD activity after chronic SS-31 administration. Moreover, systemic inflammation was ameliorated as seen by decreasing serum ICAM-1, MCP-1, and IL-6 levels. Most importantly, SS-31 administration inhibited cholesterol influx by down-regulating expression of CD36 and LOX-1 to prevent lipid accumulation to further suppress the foam cell formation and atherosclerotic progression. Conclusion Administration of SS-31 prevents against atherosclerotic formation in ApoE-/- mice suggesting that SS-31 might be considered to be a potential drug to prevent atherosclerotic progression.

Elamipretide (SS-31) Ameliorates Isoflurane-Induced Long-Term Impairments of Mitochondrial Morphogenesis and Cognition in Developing Rats

Mitochondria are supposed to be involved in the early pathogenesis of general anesthesia (GA)-induced neurotoxicity and long-term cognitive deficits in developing brains. However, effective pharmacologic agents targeted on mitochondria during GA exposure are lacking. This study explores the protective effects of mitochondrion-targeted antioxidant elamipretide (SS-31) on mitochondrial morphogenesis and cognition in developing rats exposed to isoflurane. Rat pups at postnatal day (PND) 7 were exposed to 1.5% isoflurane for 6 h following intraperitoneal administration of elamipretide or vehicle with 30 min interval. The hippocampus was immediately removed for biochemical assays. Histopathological studies were conducted at PND 21, and behavioral tests were performed at PND 40 or 60. We found that early exposure to isoflurane caused remarkable reactive oxygen species (ROS) accumulation, mitochondrial deformation and neuronal apoptosis in hippocampus. The injury occurrence ultimately gave rise to long-term cognitive deficits in developing rats. Interestingly, pretreatment with elamipretide not only provided protective effect against oxidative stress and mitochondrial damages, but also attenuated isoflurane-induced cognitive deficits. Our data support the notion that mitochondrial damage is an early and long lasting event of GA-induced injury and suggest that elamipretide might have clinically therapeutic benefits for pediatric patients undertaking GA.

Ferritin is secreted via 2 distinct nonclassical vesicular pathways

Ferritin turnover plays a major role in tissue iron homeostasis, and ferritin malfunction is associated with impaired iron homeostasis and neurodegenerative diseases. In most eukaryotes, ferritin is considered an intracellular protein that stores iron in a nontoxic and bioavailable form. In insects, ferritin is a classically secreted protein and plays a major role in systemic iron distribution. Mammalian ferritin lacks the signal peptide for classical endoplasmic reticulum-Golgi secretion but is found in serum and is secreted via a nonclassical lysosomal secretion pathway. This study applied bioinformatics and biochemical tools, alongside a protein trafficking mouse models, to characterize the mechanisms of ferritin secretion. Ferritin trafficking via the classical secretion pathway was ruled out, and a 2:1 distribution of intracellular ferritin between membrane-bound compartments and the cytosol was observed, suggesting a role for ferritin in the vesicular compartments of the cell. Focusing on nonclassical secretion, we analyzed mouse models of impaired endolysosomal trafficking and found that ferritin secretion was decreased by a BLOC-1 mutation but increased by BLOC-2, BLOC-3, and Rab27A mutations of the cellular trafficking machinery, suggesting multiple export routes. A 13-amino-acid motif unique to ferritins that lack the secretion signal peptide was identified on the BC-loop of both subunits and plays a role in the regulation of ferritin secretion. Finally, we provide evidence that secretion of iron-rich ferritin was mediated via the multivesicular body-exosome pathway. These results enhance our understanding of the mechanism of ferritin secretion, which is an important piece in the puzzle of tissue iron homeostasis.