Only one splice variant of the human TAZ gene encodes a functional protein with a role in cardiolipin metabolism

Valproate regulates inositol synthesis by reducing expression of myo-inositol-3-phosphate synthase

Inositol depletion is a hypothesized mechanism of action of mood stabilization drugs used in the treatment of bipolar disorder. It was previously reported that the mood stabilizer valproate (VPA) increased phosphorylation of myo-inositol-3-phosphate synthases (MIPS), the rate limiting enzyme of inositol synthesis. Phosphosites were identified and examination of site-directed mutants suggested that phosphorylation leads to decreased enzymatic activity. In this study, we examined the extent of MIPS phosphorylation in response to VPA and used two interaction screens to identify protein kinases that interact with MIPS. Using an epitope tagged MIPS construct, we determined the fraction of phosphorylated MIPS to be very low (less than 2% of total), and we could not detect phosphorylation of untagged MIPS in response to VPA. In vitro analyses of phosphorylation revealed that putative protein kinases, PKC and CKII, have low specificity toward MIPS. These findings suggest that VPA likely depletes inositol via a mechanism other than MIPS phosphorylation. Consistent with this, mRNA levels of the MIPS-encoding gene INO1 and MIPS protein levels were significantly reduced during the mid-log growth phase in response to VPA treatment. These findings suggest that the mechanism whereby VPA causes inositol depletion is by reducing expression of the rate-limiting enzyme MIPS.

Drug Drop Test: How to Quickly Identify Potential Therapeutic Compounds for Mitochondrial Diseases Using Yeast Saccharomyces cerevisiae

Mitochondrial diseases (MDs) refer to a group of clinically and genetically heterogeneous pathologies characterized by defective mitochondrial function and energy production. Unfortunately, there is no effective treatment for most MDs, and current therapeutic management is limited to relieving symptoms. The yeast Saccharomyces cerevisiae has been efficiently used as a model organism to study mitochondria-related disorders thanks to its easy manipulation and well-known mitochondrial biogenesis and metabolism. It has been successfully exploited both to validate alleged pathogenic variants identified in patients and to discover potential beneficial molecules for their treatment. The so-called "drug drop test", a phenotype-based high-throughput screening, especially if coupled with a drug repurposing approach, allows the identification of molecules with high translational potential in a cost-effective and time-saving manner. In addition to drug identification, S. cerevisiae can be used to point out the drug's target or pathway. To date, drug drop tests have been successfully carried out for a variety of disease models, leading to very promising results. The most relevant aspect is that studies on more complex model organisms confirmed the effectiveness of the drugs, strengthening the results obtained in yeast and demonstrating the usefulness of this screening as a novel approach to revealing new therapeutic molecules for MDs.

The critical role of cardiolipin in metazoan differentiation, development, and maturation

Cardiolipins are phospholipids that are central to proper mitochondrial functioning. Because mitochondria play crucial roles in differentiation, development, and maturation, we would also expect cardiolipin to play major roles in these processes. Indeed, cardiolipin has been implicated in the mechanism of three human diseases that affect young infants, implying developmental abnormalities. In this review, we will: (1) Review the biology of cardiolipin; (2) Outline the evidence for essential roles of cardiolipin during organismal development, including embryogenesis and cell maturation in vertebrate organisms; (3) Place the role(s) of cardiolipin during embryogenesis within the larger context of the roles of mitochondria in development; and (4) Suggest avenues for future research.

Cytidine diphosphate diacylglycerol synthase is essential for mitochondrial structure and energy production in Arabidopsis thaliana

Cytidine diphosphate diacylglycerol (CDP-DAG), an important intermediate for glycerolipid biosynthesis, is synthesized under the catalytic activity of CDP-DAG synthase (CDS) to produce anionic phosphoglycerolipids such as phosphatidylglycerol (PG) and cardiolipin (CL). Previous studies showed that Arabidopsis CDSs are encoded by a small gene family, termed CDS1-CDS5, the members of which are integral membrane proteins in endoplasmic reticulum (ER) and in plastids. However, the details on how CDP-DAG is provided for mitochondrial membrane-specific phosphoglycerolipids are missing. Here we present the identification of a mitochondrion-specific CDS, designated CDS6. Enzymatic activity of CDS6 was demonstrated by the complementation of CL synthesis in the yeast CDS-deficient tam41Δ mutant. The Arabidopsis cds6 mutant lacking CDS6 activity showed decreased mitochondrial PG and CL biosynthesis capacity, a severe growth deficiency finally leading to plant death. These defects were rescued partly by complementation with CDS6 or supplementation with PG and CL. The ultrastructure of mitochondria in cds6 was abnormal, missing the structures of cristae. The degradation of triacylglycerol (TAG) in lipid droplets and starch in chloroplasts in the cds6 mutant was impaired. The expression of most differentially expressed genes involved in the mitochondrial electron transport chain was upregulated, suggesting an energy-demanding stage in cds6. Furthermore, the contents of polar glycerolipids in cds6 were dramatically altered. In addition, cds6 seedlings lost the capacity for cell proliferation and showed a higher oxidase activity. Thus, CDS6 is indispensable for the biosynthesis of PG and CL in mitochondria, which is critical for establishing mitochondrial structure, TAG degradation, energy production and seedling development.

Clinical variant interpretation and biologically relevant reference transcripts

Clinical variant interpretation is highly dependent on the choice of reference transcript. Although the longest transcript has traditionally been chosen as the reference, APPRIS principal and MANE Select transcripts, biologically supported reference sequences, are now available. In this study, we show that MANE Select and APPRIS principal transcripts are the best reference transcripts for clinical variation. APPRIS principal and MANE Select transcripts capture almost all ClinVar pathogenic variants, and they are particularly powerful over the 94% of coding genes in which they agree. We find that a vanishingly small number of ClinVar pathogenic variants affect alternative protein products. Alternative isoforms that are likely to be clinically relevant can be predicted using TRIFID scores, the highest scoring alternative transcripts are almost 700 times more likely to house pathogenic variants. We believe that APPRIS, MANE and TRIFID are essential tools for clinical variant interpretation.

Re-Expression of Tafazzin Isoforms in TAZ-Deficient C6 Glioma Cells Restores Cardiolipin Composition but Not Proliferation Rate and Alterations in Gene Expression

Tafazzin—an acyltransferase—is involved in cardiolipin (CL) remodeling. CL is associated with mitochondrial function, structure and more recently with cell proliferation. Various tafazzin isoforms exist in humans. The role of these isoforms in cardiolipin remodeling is unknown. Aim of this study was to investigate if specific isoforms like Δ5 can restore the wild type phenotype with respect to CL composition, cellular proliferation and gene expression profile. In addition, we aimed to determine the molecular mechanism by which tafazzin can modulate gene expression by applying promoter analysis and (Ingenuity Pathway Analyis) IPA to genes regulated by TAZ-deficiency. Expression of Δ5 and rat full length TAZ in C6-TAZ- cells could fully restore CL composition and—as proven for Δ5—this is naturally associated with restoration of mitochondrial respiration. A similar restoration of CL-composition could not be observed after re-expression of an enzymatically dead full-length rat TAZ (H69L; TAZMut). Re-expression of only rat full length TAZ could restore proliferation rate. Surprisingly, the Δ5 variant failed to restore wild-type proliferation. Further, as expected, re-expression of the TAZMut variant completely failed to reverse the gene expression changes, whereas re-expression of the TAZ-FL variant largely did so and the Δ5 variant to somewhat less extent. Very likely TAZ-deficiency provokes substantial long-lasting changes in cellular lipid metabolism which contribute to changes in proliferation and gene expression, and are not or only very slowly reversible.

Barth Syndrome Cardiomyopathy: An Update

Barth syndrome (BTHS) is an X-linked mitochondrial lipid disorder caused by mutations in the TAFAZZIN (TAZ) gene, which encodes a mitochondrial acyltransferase/transacylase required for cardiolipin (CL) biosynthesis. Cardiomyopathy is a major clinical feature of BTHS. During the past four decades, we have witnessed many landmark discoveries that have led to a greater understanding of clinical features of BTHS cardiomyopathy and their molecular basis, as well as the therapeutic targets for this disease. Recently published Taz knockout mouse models provide useful experimental models for studying BTHS cardiomyopathy and testing potential therapeutic approaches. This review aims to summarize key findings of the clinical features, molecular mechanisms, and potential therapeutic approaches for BTHS cardiomyopathy, with particular emphasis on the most recent studies.

Targeted Lipidomics Reveal the Effects of Different Phospholipids on the Phospholipid Profiles of Hepatic Mitochondria and Endoplasmic Reticulum in High-Fat/High-Fructose-Diet-Induced Nonalcoholic Fatty Liver Disease Mice

The lipid alternation in mitochondria and endoplasmic reticulum (ER) might be indicative of their abnormal morphology and function, which contribute to development of nonalcoholic fatty liver disease (NAFLD). However, the influence of dietary phospholipids (PLs) on the PL composition of the organellar membrane is largely unknown. High-fat/high-fructose (HFHF)-diet-induced NAFLD mice were administrated with different PLs (2%, w/w) with specific fatty acids and headgroups, including eicosapentaenoic acid (EPA)-phosphatidylcholine (PC)/phosphatidylethanolamine (PE)/phosphatidylserine (PS), docosahexaenoic acid (DHA)-PC/PE/PS, egg-PC/PE/PS, and soy-PC/PE/PS. After 8 weeks of feeding, PLs dramatically decreased hepatic lipid accumulation, in which EPA/DHA-PS had the best efficiency. Furthermore, lipidomic analysis revealed that the HFHF diet narrowed the difference in PL composition between mitochondria and ER, significantly reduced the PC/PE ratio, and changed the unsaturation of cardiolipin in mitochondria. Dietary PLs reversed these alterations. Heatmap analysis indicated that dietary PL groups containing the same fatty acids clustered together. Moreover, dietary PLs significantly increased the ratio of PC/PE in both hepatic mitochondria and ER, especially EPA-PE. This study showed that fatty acid composition of PLs might represent greater impact on the PL composition of the organellar membrane than headgroups.

Experimental models of Barth syndrome

Mutation of the gene Tafazzin (TAZ) causes Barth syndrome, an X-linked disorder characterized by cardiomyopathy, skeletal muscle weakness, and neutropenia. TAZ is an acyltransferase that catalyzes the remodeling of cardiolipin, the signature phospholipid of the inner mitochondrial membrane. Here, we review the major model systems that have been established to study the role of cardiolipin remodeling in mitochondrial function and the pathogenesis of Barth syndrome. We summarize key features of each model and provide examples of how each has contributed to advance our understanding of TAZ function and Barth syndrome pathophysiology.

A simple mechanistic explanation for Barth syndrome and cardiolipin remodeling

Barth syndrome is a multisystem disorder caused by an abnormal metabolism of the mitochondrial lipid cardiolipin. In this review, we discuss physical properties, biosynthesis, membrane assembly, and function of cardiolipin. We hypothesize that cardiolipin reduces packing stress in the inner mitochondrial membrane, which arises as a result of protein crowding. According to this hypothesis, patients with Barth syndrome are unable to meet peak energy demands because they fail to concentrate the proteins of oxidative phosphorylation to a high surface density in the inner mitochondrial membrane.

Cardiolipin function in the yeast S. cerevisiae and the lessons learned for Barth syndrome

Cardiolipin (CL) is the signature phospholipid (PL) of mitochondria and plays a pivotal role in mitochondrial and cellular function. Disruption of the CL remodeling gene tafazzin (TAZ) causes the severe genetic disorder Barth syndrome (BTHS). Our current understanding of the function of CL and the mechanism underlying the disease has greatly benefited from studies utilizing the powerful yeast model Saccharomyces cerevisiae. In this review, we discuss important findings on the function of CL and its remodeling from yeast studies and the implications of these findings for BTHS, highlighting the potential physiological modifiers that may contribute to the disparities in clinical presentation among BTHS patients.

Studying Lipid-Related Pathophysiology Using the Yeast Model

Saccharomyces cerevisiae, commonly known as baker's yeast, is one of the most comprehensively studied model organisms in science. Yeast has been used to study a wide variety of human diseases, and the yeast model system has proved to be an especially amenable tool for the study of lipids and lipid-related pathophysiologies, a topic that has gained considerable attention in recent years. This review focuses on how yeast has contributed to our understanding of the mitochondrial phospholipid cardiolipin (CL) and its role in Barth syndrome (BTHS), a genetic disorder characterized by partial or complete loss of function of the CL remodeling enzyme tafazzin. Defective tafazzin causes perturbation of CL metabolism, resulting in many downstream cellular consequences and clinical pathologies that are discussed herein. The influence of yeast research in the lipid-related pathophysiologies of Alzheimer's and Parkinson's diseases is also summarized.

R. Tafazzin Mutation Affecting Cardiolipin Leads to Increased Mitochondrial Superoxide Anions and Mitophagy Inhibition in Barth Syndrome

Tafazzin is a phospholipid transacylase that catalyzes the remodeling of cardiolipin, a mitochondrial phospholipid required for oxidative phosphorylation. Mutations of the tafazzin gene cause Barth syndrome, which is characterized by mitochondrial dysfunction and dilated cardiomyopathy, leading to premature death. However, the molecular mechanisms underlying the cause of mitochondrial dysfunction in Barth syndrome remain poorly understood. We again highlight the fact that the tafazzin deficiency is also linked to defective oxidative phosphorylation associated with oxidative stress. All the mitochondrial events are positioned in a context where mitophagy is a key element in mitochondrial quality control. Here, we investigated the role of tafazzin in mitochondrial homeostasis dysregulation and mitophagy alteration. Using a HeLa cell model of tafazzin deficiency, we show that dysregulation of tafazzin in HeLa cells induces alteration of mitophagy. Our findings provide some additional insights into mitochondrial dysfunction associated with Barth syndrome, but also show that mitophagy inhibition is concomitant with apoptosis dysfunction through the inability of abnormal mitochondrial cardiolipin to assume its role in cytoplasmic signal transduction. Our work raises hope that pharmacological manipulation of the mitophagic pathway together with mitochondrially targeted antioxidants may provide new insights leading to promising treatment for these highly lethal conditions.

Role of Tafazzin in Mitochondrial Function, Development and Disease

Tafazzin, an enzyme associated with the rare inherited x-linked disorder Barth Syndrome, is a nuclear encoded mitochondrial transacylase that is highly conserved across multiple species and plays an important role in mitochondrial function. Numerous studies have elucidated the mechanisms by which Tafazzin affects mitochondrial function, but its effects on development and susceptibility to adult disease are incompletely understood. The purpose of this review is to highlight previous functional studies across a variety of model organisms, introduce recent studies that show an important role in development, and also to provide an update on the role of Tafazzin in human disease. The profound effects of Tafazzin on cardiac development and adult cardiac homeostasis will be emphasized. These studies underscore the importance of mitochondrial function in cardiac development and disease, and also introduce the concept of Tafazzin as a potential therapeutic modality.

The mitochondrial phosphate carrier TbMCP11 is essential for mitochondrial function in the procyclic form of Trypanosoma brucei

Conserved amongst all eukaryotes is a family of mitochondrial carrier proteins (SLC25A) responsible for the import of various solutes across the inner mitochondrial membrane. We previously reported that the human parasite Trypanosoma brucei possesses 26 SLC25A proteins (TbMCPs) amongst which two, TbMCP11 and TbMCP8, were predicted to function as phosphate importers. The transport of inorganic phosphate into the mitochondrion is a prerequisite to drive ATP synthesis by substrate level and oxidative phosphorylation and thus crucial for cell viability. In this paper we describe the functional characterization of TbMCP11. In procyclic form T. brucei, the RNAi of TbMCP11 blocked ATP synthesis on mitochondrial substrates, caused a drop of the mitochondrial oxygen consumption and drastically reduced cell viability. The functional complementation in yeast and mitochondrial swelling experiments suggested a role for TbMCP11 as inorganic phosphate carrier. Interestingly, procyclic form T. brucei cells in which TbMCP11 was depleted displayed an inability to either replicate or divide the kinetoplast DNA, which resulted in a severe cytokinesis defect.

The Function of Tafazzin, a Mitochondrial Phospholipid-Lysophospholipid Acyltransferase

Tafazzin is a mitochondrial enzyme that exchanges fatty acids between phospholipids by phospholipid-lysophospholipid transacylation. The reaction alters the molecular species composition and, as a result, the physical properties of lipids. In vivo, the most important substrate of tafazzin is the mitochondria-specific lipid cardiolipin. Tafazzin mutations cause the human disease Barth syndrome, which presents with cardiomyopathy, skeletal muscle weakness, fatigue, and other symptoms, probably all related to mitochondrial dysfunction. The reason why mitochondria require tafazzin is still not known, but recent evidence suggests that tafazzin may lower the energy cost associated with protein crowding in the inner mitochondrial membrane.

New C-Terminal Conserved Regions of Tafazzin, a Catalyst of Cardiolipin Remodeling

Cardiolipin interacts with many proteins of the mitochondrial inner membrane and, together with cytochrome C and creatine kinase, activates them. It can be considered as an integrating factor for components of the mitochondrial respiratory chain, which provides for an efficient transfer of electrons and protons. The major, if not the only, factor of cardiolipin maturation is tafazzin. Variations of isoform proportions of this enzyme can cause severe diseases such as Barth syndrome. Using bioinformatic methods, we have found conserved C-terminal regions in many tafazzin isoforms and identified new mammalian species that acquired exon 5 as well as rare occasions of intron retention between exons 8 and 9. The regions in the C-terminal part arise from frameshifts relative to the full-length TAZ transcript after skipping exon 9 or retention of the intron between exons 10 and 11. These modifications demonstrate specific distribution among the orders of mammals. The dependence of the species maximum lifespan, body weight, and mitochondrial metabolic rate on the modifications has been demonstrated. Arguably, unconventional tafazzin isoforms provide for the optimal balance between the increased biochemical activity of mitochondria (resulting from specific environmental or nutritional conditions) and lifespan maintenance; and the functional role of such isoforms is linked to the modification of the primary and secondary structures at their C-termini.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

A critical appraisal of the tafazzin knockdown mouse model of Barth syndrome: what have we learned about pathogenesis and potential treatments?

Pediatric heart failure remains poorly understood, distinct in many aspects from adult heart failure. Limited data point to roles of altered mitochondrial functioning and, in particular, changes in mitochondrial lipids, especially cardiolipin. Barth syndrome is a mitochondrial disorder caused by tafazzin mutations that lead to abnormal cardiolipin profiles. Patients are afflicted by cardiomyopathy, skeletal myopathy, neutropenia, and growth delay. A mouse model of Barth syndrome was developed a decade ago, which relies on a doxycycline-inducible short hairpin RNA to knock down expression of tafazzin mRNA (TAZKD). Our objective was to review published data from the TAZKD mouse to determine its contributions to our pathogenetic understanding of, and potential treatment strategies for, Barth syndrome. In regard to the clinical syndrome, the reported physiological, biochemical, and ultrastructural abnormalities of the mouse model mirror those in Barth patients. Using this model, the peroxisome proliferator-activated receptor pan-agonist bezafibrate has been suggested as potential therapy because it ameliorated the cardiomyopathy in TAZKD mice, while increasing mitochondrial biogenesis. A clinical trial is now underway to test bezafibrate in Barth syndrome patients. Thus the TAZKD mouse model of Barth syndrome has led to important insights into disease pathogenesis and therapeutic targets, which can potentially translate to pediatric heart failure.

Barth syndrome: mechanisms and management

Objectives: Barth syndrome is an ultra-rare, infantile-onset, X-linked recessive mitochondrial disorder, primarily affecting males, due to variants in TAZ encoding for the cardiolipin transacylase tafazzin. This review aimed to summarize and discuss recent and earlier findings concerning the etiology, pathogenesis, clinical presentation, diagnosis, treatment, and outcome of Barth syndrome.

Method: A literature review was undertaken through a MEDLINE search.

Results: The phenotype of Barth syndrome is highly variable but most frequently patients present with hypertrophic/dilated/non-compaction cardiomyopathy, fibroelastosis, arrhythmias, neutropenia, mitochondrial myopathy, growth retardation, dysmorphism, cognitive impairment, and other, rarer features. Lactic acid and creatine kinase, and blood and urine organic acids, particularly 3-methylglutaconic acid and monolysocardiolipin, are often elevated. Cardiolipin is decreased. Biochemical investigations may show decreased activity of various respiratory chain complexes. The diagnosis is confirmed by documentation of a causative TAZ variant. Treatment is symptomatic and directed toward treating heart failure, arrhythmias, neutropenia, and mitochondrial myopathy.

Conclusions: Although Barth syndrome is still an orphan disease, with fewer than 200 cases described so far, there is extensive ongoing research with regard to its pathomechanism and new therapeutic approaches. Although most of these approaches are still experimental, it can be expected that causative strategies will be developed in the near future.

AAV-Mediated TAZ Gene Replacement Restores Mitochondrial and Cardioskeletal Function in Barth Syndrome

Barth syndrome (BTHS) is a rare mitochondrial disease that affects heart and skeletal muscle and has no curative treatment. It is caused by recessive mutations in the X-linked gene TAZ, which encodes tafazzin. To develop a clinically relevant gene therapy to restore tafazzin function and treat BTHS, three different adeno-associated virus serotype 9 vectors were tested and compared to identify the optimal promoter-cytomegalovirus (CMV), desmin (Des), or a native tafazzin promoter (Taz)-for TAZ expression following intravenous administration of 1 × 10¹³ vector genomes/kilogram to a mouse model of BTHS as either neonates (1-2 days of age) or adults (3 months of age). At 5 months of age, evaluations of biodistribution and TAZ expression levels, mouse activity assessments, fatigue in response to exercise, muscle strength, cardiac function, mitochondrial structure, oxygen consumption, and electron transport chain complex activity assays were performed to measure the extent of improvement in treated mice. Each promoter was scored for significant improvement over untreated control mice and significant improvement compared with the other two promoters for every measurement and within each age of administration. All three of the promoters resulted in significant improvements in a majority of the assessments compared with untreated BTHS controls. When scored for overall effectiveness as a gene therapy, the Des promoter was found to provide improvement in the most assessments, followed by the CMV promoter, and finally Taz regardless of injection age. This study provides substantial support for translation of an adeno-associated virus serotype 9-mediated TAZ gene replacement strategy using a Des promoter for human BTHS patients in the clinic.

Barth syndrome cells display widespread remodeling of mitochondrial complexes without affecting metabolic flux distribution

Barth syndrome (BTHS) is a rare X-linked disorder that is characterized by cardiac and skeletal myopathy, neutropenia and growth abnormalities. The disease is caused by mutations in the tafazzin (TAZ) gene encoding an enzyme involved in the acyl chain remodeling of the mitochondrial phospholipid cardiolipin (CL). Biochemically, this leads to decreased levels of mature CL and accumulation of the intermediate monolysocardiolipin (MLCL). At a cellular level, this causes mitochondrial fragmentation and reduced stability of the respiratory chain supercomplexes. However, the exact mechanism through which tafazzin deficiency leads to disease development remains unclear. We therefore aimed to elucidate the pathways affected in BTHS cells by employing proteomic and metabolic profiling assays. Complexome profiling of patient skin fibroblasts revealed significant effects for about 200 different mitochondrial proteins. Prominently, we found a specific destabilization of higher order oxidative phosphorylation (OXPHOS) supercomplexes, as well as changes in complexes involved in cristae organization and CL trafficking. Moreover, the key metabolic complexes 2-oxoglutarate dehydrogenase (OGDH) and branched-chain ketoacid dehydrogenase (BCKD) were profoundly destabilized in BTHS patient samples. Surprisingly, metabolic flux distribution assays using stable isotope tracer-based metabolomics did not show reduced flux through the TCA cycle. Overall, insights from analyzing the impact of TAZ mutations on the mitochondrial complexome provided a better understanding of the resulting functional and structural consequences and thus the pathological mechanisms leading to Barth syndrome.

Substantial Decrease in Plasmalogen in the Heart Associated with Tafazzin Deficiency

Tafazzin is the mitochondrial enzyme that catalyzes transacylation between a phospholipid and a lysophospholipid in remodeling. Mutations in tafazzin cause Barth syndrome, a potentially life-threatening disease with the major symptom being cardiomyopathy. In the tafazzin-deficient heart, cardiolipin (CL) acyl chains become abnormally heterogeneous unlike those in the normal heart with a single dominant linoleoyl species, tetralinoleoyl CL. In addition, the amount of CL decreases and monolysocardiolipin (MLCL) accumulates. Here we determine using high-resolution 31P nuclear magnetic resonance with cryoprobe technology the fundamental phospholipid composition, including the major but oxidation-labile plasmalogens, in the tafazzin-knockdown (TAZ-KD) mouse heart as a model of Barth syndrome. In addition to confirming a lower level of CL (6.4 ± 0.1 → 2.0 ± 0.4 mol % of the total phospholipid) and accumulation of MLCL (not detected → 3.3 ± 0.5 mol %) in the TAZ-KD, we found a substantial reduction in the level of plasmenylcholine (30.8 ± 2.8 → 18.1 ± 3.1 mol %), the most abundant phospholipid in the control wild type. A quantitative Western blot revealed that while the level of peroxisomes, where early steps of plasmalogen synthesis take place, was normal in the TAZ-KD model, expression of Far1 as a rate-determining enzyme in plasmalogen synthesis was dramatically upregulated by 8.3 (±1.6)-fold to accelerate the synthesis in response to the reduced level of plasmalogen. We confirmed lyso-plasmenylcholine or plasmenylcholine is a substrate of purified tafazzin for transacylation with CL or MLCL, respectively. Our results suggest that plasmenylcholine, abundant in linoleoyl species, is important in remodeling CL in the heart. Tafazzin deficiency thus has a major impact on the cardiac plasmenylcholine level and thereby its functions.

Phospholipid subcellular localization and dynamics

Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.

Decreasing cytosolic translation is beneficial to yeast and human Tafazzin-deficient cells

Cardiolipin (CL) optimizes diverse mitochondrial processes, including oxidative phosphorylation (OXPHOS). To function properly, CL needs to be unsaturated, which requires the acyltransferase Tafazzin (TAZ). Loss-of-function mutations in the TAZ gene are responsible for the Barth syndrome (BTHS), a rare X-linked cardiomyopathy, presumably because of a diminished OXPHOS capacity. Herein we show that a partial inhibition of cytosolic protein synthesis, either chemically with the use of cycloheximide or by specific genetic mutations, fully restores biogenesis and the activity of the oxidative phosphorylation system in a yeast BTHS model (taz1Δ). Interestingly, the defaults in CL were not suppressed, indicating that they are not primarily responsible for the OXPHOS deficiency in taz1Δ yeast. Low concentrations of cycloheximide in the picomolar range were beneficial to TAZ-deficient HeLa cells, as evidenced by the recovery of a good proliferative capacity. These findings reveal that a diminished capacity of CL remodeling deficient cells to preserve protein homeostasis is likely an important factor contributing to the pathogenesis of BTHS. This in turn, identifies cytosolic translation as a potential therapeutic target for the treatment of this disease.

Effects of siRNA-dependent knock-down of cardiolipin synthase and tafazzin on mitochondria and proliferation of glioma cells

The mitochondrial phospholipid cardiolipin (CL) has been implicated with mitochondrial morphology, function, and cell proliferation. Changes in CL are often paralleled by changes in the lipid environment of mitochondria that may contribute to mitochondrial function and proliferation. This study aimed to separate the effects of CL content and CL composition from cellular free fatty acid distribution on bioenergetics and proliferation in C6 glioma cells. To this end, cardiolipin synthase and the CL remodelling enzyme, tafazzin, were knocked-down by siRNA in C6 cells. After 72 h of cultivation, we analysed CL composition by means of LC/MS/MS, distribution of cellular fatty acids by means of gas chromatography, and determined oxygen consumption and proliferation. Knock-down of cardiolipin synthase affected the cellular CL content in the presence of linoleic acid (LA) in the culture medium. Knock-down of tafazzin had no consequence with respect to the pattern of cellular fatty acids but caused a decrease in cell proliferation. It significantly changed the distribution of molecular CL species, increased CL content, decreased oxygen consumption, and decreased cell proliferation when cultured in the presence of linoleic acid (LA). The addition of linoleic acid to the culture medium caused significant changes in the pattern of cellular fatty acids and the composition of molecular CL species. These data suggest that tafazzin is required for efficient bioenergetics and for proliferation of glioma cells. Supplementation of fatty acids can be a powerful tool to direct specific changes in these parameters.

Cardiolipin and mitochondrial cristae organization

A fundamental question in cell biology, under investigation for over six decades, is the structural organization of mitochondrial cristae. Long known to harbor electron transport chain proteins, crista membrane integrity is key to establishment of the proton gradient that drives oxidative phosphorylation. Visualization of cristae morphology by electron microscopy/tomography has provided evidence that cristae are tube-like extensions of the mitochondrial inner membrane (IM) that project into the matrix space. Reconciling ultrastructural data with the lipid composition of the IM provides support for a continuously curved cylindrical bilayer capped by a dome-shaped tip. Strain imposed by the degree of curvature is relieved by an asymmetric distribution of phospholipids in monolayer leaflets that comprise cristae membranes. The signature mitochondrial lipid, cardiolipin (~18% of IM phospholipid mass), and phosphatidylethanolamine (34%) segregate to the negatively curved monolayer leaflet facing the crista lumen while the opposing, positively curved, matrix-facing monolayer leaflet contains predominantly phosphatidylcholine. Associated with cristae are numerous proteins that function in distinctive ways to establish and/or maintain their lipid repertoire and structural integrity. By combining unique lipid components with a set of protein modulators, crista membranes adopt and maintain their characteristic morphological and functional properties. Once established, cristae ultrastructure has a direct impact on oxidative phosphorylation, apoptosis, fusion/fission as well as diseases of compromised energy metabolism.

The Basis for Acyl Specificity in the Tafazzin Reaction

Tafazzin is a mitochondrial enzyme that transfers fatty acids from phospholipids to lysophospholipids. Mutations in tafazzin cause abnormal molecular species of cardiolipin and the clinical phenotype of Barth syndrome. However, the mechanism by which tafazzin creates acyl specificity has been controversial. We have shown that the lipid phase state can produce acyl specificity in the tafazzin reaction, but others have reported that tafazzin itself carries enzymatic specificity. To resolve this issue, we replicated and expanded the controversial experiments, i.e. the transfer of different acyl groups from phosphatidylcholine to monolysocardiolipin by yeast tafazzin. Our data show that this reaction requires the presence of detergent and does not take place in liposomes but in mixed micelles. To separate thermodynamic (lipid-dependent) from kinetic (enzyme-dependent) parameters, we followed the accumulation of cardiolipin during the reaction from the initial state to the equilibrium state. The transacylation rates of different acyl groups varied over 2 orders of magnitude and correlated tightly with the concentration of cardiolipin in the equilibrium state (lipid-dependent parameter). In contrast, the rates by which different transacylations approached the equilibrium state were very similar (enzyme-dependent parameter). Furthermore, we found that tafazzin catalyzes the remodeling of cardiolipin by combinations of forward and reverse transacylations, essentially creating an equilibrium distribution of acyl groups. These data strongly support the idea that the acyl specificity of the tafazzin reaction results from the physical properties of lipids.

Overexpression of mitochondrial oxodicarboxylate carrier (ODC1) preserves oxidative phosphorylation in a yeast model of Barth syndrome

Cardiolipin (CL) is a diglycerol phospholipid mostly found in mitochondria where it optimizes numerous processes, including oxidative phosphorylation (OXPHOS). To function properly, CL needs to be unsaturated, which requires the acyltransferase tafazzin. Loss-of-function mutations in this protein are responsible for Barth syndrome (BTHS), presumably because of a diminished OXPHOS capacity. Here, we show that overexpressing Odc1p, a conserved oxodicarboxylic acid carrier located in the mitochondrial inner membrane, fully restores oxidative phosphorylation in a yeast model (taz1Δ) of BTHS. The rescuing activity involves the recovery of normal expression of key components that sustain oxidative phosphorylation, including cytochrome c and electron transport chain complexes IV and III, which are strongly downregulated in taz1Δ yeast. Interestingly, overexpression of Odc1p was also shown previously to rescue yeast models of mitochondrial diseases caused by defects in the assembly of ATP synthase and by mutations in the MPV17 protein that result in hepatocerebral mitochondrial DNA depletion syndrome. These findings define the transport of oxodicarboxylic acids across the inner membrane as a potential therapeutic target for a large spectrum of mitochondrial diseases, including BTHS.

Barth syndrome cardiomyopathy

Barth syndrome (BTHS) is an inherited form of cardiomyopathy, caused by a mutation within the gene encoding the mitochondrial transacylase tafazzin. Tafazzin is involved in the biosynthesis of the unique phospholipid cardiolipin (CL), which is almost exclusively found in mitochondrial membranes. CL directly interacts with a number of essential protein complexes in the mitochondrial membranes including the respiratory chain, mitochondrial metabolite carriers, and proteins, involved in shaping mitochondrial morphology. Here we describe, how in BTHS CL deficiency causes changes in the morphology of mitochondria, structural changes in the respiratory chain, decreased respiration, and increased generation of reactive oxygen species. A large number of cellular and animal models for BTHS have been established to elucidate how mitochondrial dysfunction induces sarcomere disorganization and reduced contractility, resulting in dilated cardiomyopathy in vivo.

A novel mutation in TAZ causes mitochondrial respiratory chain disorder without cardiomyopathy

Tafazzin, encoded by the TAZ gene, is a mitochondrial membrane-associated protein that remodels cardiolipin (CL), an important mitochondrial phospholipid. TAZ mutations are associated with Barth syndrome (BTHS). BTHS is an X-linked multisystemic disorder affecting usually male patients. Through sequence analysis of TAZ, we found one novel mutation c.39_60del p.(Pro14Alafs*19) by whole-exome sequencing and a reported missense mutation c.280C>T p.(Arg94Cys) by Sanger sequencing in two male patients (Pt1 and Pt2). Patient with c.280C>T mutation had dilated cardiomyopathy, while another patient with c.39_60del mutation had no feature of cardiomyopathy. A reported m.1555A>G homoplasmic variant was also identified in the patient having mutation c.39_60del by whole mitochondrial DNA sequencing method. This variant was not considered to be the main cause of mitochondrial dysfunction based on a cytoplasmic hybrid (cybrid) assay. Tafazzin expression was absent in both patient-derived fibroblast cells. Complementation of TAZ expression in fibroblasts from the patient with the novel mutation c.39_60del restored mitochondrial respiratory complex assembly. High-performance liquid chromatography-tandem mass spectrometry-based metabolic analysis revealed the decline of CL and the accumulation of monolysocardiolipin, indicating the loss of tafazzin activity. Owing to phenotypic variability, it is difficult to diagnose BTHS based on clinical features only. We conclude that genetic analysis should be performed to avoid underdiagnosis of this potentially life-threatening inborn error of metabolism.

Cardiolipin Regulates Mitophagy through the Protein Kinase C Pathway

Cardiolipin (CL), the signature phospholipid of mitochondrial membranes, is important for cardiovascular health, and perturbation of CL metabolism is implicated in cardiovascular disease. Although the role of CL in mitochondrial function, biogenesis, and genome stability has been studied, recent findings indicate that it is essential for functions apart from mitochondrial bioenergetics. In this study, we report that mitophagy is perturbed in CL-deficient yeast cells. Mutants of autophagy/mitophagy genes ATG8, ATG18, and ATG32 synthetically interact with CL synthase mutant crd1Δ. CL-deficient cells exhibited decreased GFP-tagged mitochondrial proteins inside the vacuole and decreased free GFP, consistent with decreased mitophagy. Both PKC and high osmolarity glycerol (HOG) MAPK pathways were shown previously to be required for mitophagy. Activation of both MAPKs was defective in CL-deficient cells. Deletion of HOG pathway genes SHO1, SSK1, STE50, and HOG1 exacerbated crd1Δ growth. 1 m sorbitol and 0.2 m NaCl, which induce the HOG pathway, rescued growth of the mutant. Activation of the MAPK Slt2p was defective in crd1Δ cells, and up-regulation of the PKC pathway by expression of the PKC1^R398P gene, which encodes constitutively activated Pkc1p, rescued crd1Δ growth and mitophagy defects. These findings indicate that loss of CL impairs MAPK pathway activation, and decreased activation of the PKC pathway leads to defective mitophagy.

Towards improved genome-scale metabolic network reconstructions: unification, transcript specificity and beyond

Genome-scale metabolic network reconstructions provide a basis for the investigation of the metabolic properties of an organism. There are reconstructions available for multiple organisms, from prokaryotes to higher organisms and methods for the analysis of a reconstruction. One example is the use of flux balance analysis to improve the yields of a target chemical, which has been applied successfully. However, comparison of results between existing reconstructions and models presents a challenge because of the heterogeneity of the available reconstructions, for example, of standards for presenting gene-protein-reaction associations, nomenclature of metabolites and reactions or selection of protonation states. The lack of comparability for gene identifiers or model-specific reactions without annotated evidence often leads to the creation of a new model from scratch, as data cannot be properly matched otherwise. In this contribution, we propose to improve the predictive power of metabolic models by switching from gene-protein-reaction associations to transcript-isoform-reaction associations, thus taking advantage of the improvement of precision in gene expression measurements. To achieve this precision, we discuss available databases that can be used to retrieve this type of information and point at issues that can arise from their neglect. Further, we stress issues that arise from non-standardized building pipelines, like inconsistencies in protonation states. In addition, problems arising from the use of non-specific cofactors, e.g. artificial futile cycles, are discussed, and finally efforts of the metabolic modelling community to unify model reconstructions are highlighted.

Biosynthesis, remodeling and turnover of mitochondrial cardiolipin

Among mitochondrial lipids, cardiolipin occupies a unique place. It is the only phospholipid that is specific to mitochondria and although it is merely a minor component, accounting for 10-20% of the total phospholipid content, cardiolipin plays an important role in the molecular organization, and thus the function of the cristae. This review covers the formation of cardiolipin, a phospholipid dimer containing two phosphatidyl residues, and its assembly into mitochondrial membranes. While a large body of literature exists on this topic, the review focuses on papers that appeared in the past three years. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Influence of tissue, diet, and enzymatic remodeling on cardiolipin fatty acyl profile

Cardiolipin is a specialized phospholipid found primarily in the inner mitochondrial membrane. Because of its unique dimeric structure, cardiolipin plays an important role in mitochondrial function, stability, and membrane fluidity. As such, cardiolipin is subject to a high degree of remodeling by phospholipases, acyltransferases, and transacylases that create a fatty acyl profile that tends to be highly tissue-specific. Despite this overarching regulation, the molecular species of cardiolipin produced are also influenced by dietary lipid composition. A number of studies have characterized the tissue-specific profile of cardiolipin species and have investigated the specific nature of cardiolipin remodeling, including the role of both enzymes and diet. The aim of this review is to highlight tissue specific differences in cardiolipin composition and, collectively, the enzymatic and dietary factors that contribute to these differences. Consequences of aberrant cardiolipin fatty acyl remodeling are also discussed.

Molecular species composition of plant cardiolipin determined by liquid chromatography mass spectrometry

Cardiolipin (CL), an anionic phospholipid of the inner mitochondrial membrane, provides essential functions for stabilizing respiratory complexes and is involved in mitochondrial morphogenesis and programmed cell death in animals. The role of CL and its metabolism in plants are less well understood. The measurement of CL in plants, including its molecular species composition, is hampered by the fact that CL is of extremely low abundance, and that plants contain large amounts of interfering compounds including galactolipids, neutral lipids, and pigments. We used solid phase extraction by anion exchange chromatography to purify CL from crude plant lipid extracts. LC/MS was used to determine the content and molecular species composition of CL. Thus, up to 23 different molecular species of CL were detected in different plant species, including Arabidopsis, mung bean, spinach, barley, and tobacco. Similar to animals, plant CL is dominated by highly unsaturated species, mostly containing linoleic and linolenic acid. During phosphate deprivation or exposure to an extended dark period, the amount of CL decreased in Arabidopsis, accompanied with an increased degree in unsaturation. The mechanism of CL remodeling during stress, and the function of highly unsaturated CL molecular species, remains to be defined.

Defining functional classes of Barth syndrome mutation in humans

The X-linked disease Barth syndrome (BTHS) is caused by mutations in TAZ; TAZ is the main determinant of the final acyl chain composition of the mitochondrial-specific phospholipid, cardiolipin. To date, a detailed characterization of endogenous TAZ has only been performed in yeast. Further, why a given BTHS-associated missense mutation impairs TAZ function has only been determined in a yeast model of this human disease. Presently, the detailed characterization of yeast tafazzin harboring individual BTHS mutations at evolutionarily conserved residues has identified seven distinct loss-of-function mechanisms caused by patient-associated missense alleles. However, whether the biochemical consequences associated with individual mutations also occur in the context of human TAZ in a validated mammalian model has not been demonstrated. Here, utilizing newly established monoclonal antibodies capable of detecting endogenous TAZ, we demonstrate that mammalian TAZ, like its yeast counterpart, is localized to the mitochondrion where it adopts an extremely protease-resistant fold, associates non-integrally with intermembrane space-facing membranes and assembles in a range of complexes. Even though multiple isoforms are expressed at the mRNA level, only a single polypeptide that co-migrates with the human isoform lacking exon 5 is expressed in human skin fibroblasts, HEK293 cells, and murine heart and liver mitochondria. Finally, using a new genome-edited mammalian BTHS cell culture model, we demonstrate that the loss-of-function mechanisms for two BTHS alleles that represent two of the seven functional classes of BTHS mutation as originally defined in yeast, are the same when modeled in human TAZ.

Yeast as a system for modeling mitochondrial disease mechanisms and discovering therapies

Mitochondrial diseases are severe and largely untreatable. Owing to the many essential processes carried out by mitochondria and the complex cellular systems that support these processes, these diseases are diverse, pleiotropic, and challenging to study. Much of our current understanding of mitochondrial function and dysfunction comes from studies in the baker's yeast Saccharomyces cerevisiae. Because of its good fermenting capacity, S. cerevisiae can survive mutations that inactivate oxidative phosphorylation, has the ability to tolerate the complete loss of mitochondrial DNA (a property referred to as ‘petite-positivity’), and is amenable to mitochondrial and nuclear genome manipulation. These attributes make it an excellent model system for studying and resolving the molecular basis of numerous mitochondrial diseases. Here, we review the invaluable insights this model organism has yielded about diseases caused by mitochondrial dysfunction, which ranges from primary defects in oxidative phosphorylation to metabolic disorders, as well as dysfunctions in maintaining the genome or in the dynamics of mitochondria. Owing to the high level of functional conservation between yeast and human mitochondrial genes, several yeast species have been instrumental in revealing the molecular mechanisms of pathogenic human mitochondrial gene mutations. Importantly, such insights have pointed to potential therapeutic targets, as have genetic and chemical screens using yeast.

Summary: In this Review, we discuss the use of budding yeast to understand mitochondrial diseases and help in the search for their treatments.

Barth Syndrome: From Mitochondrial Dysfunctions Associated with Aberrant Production of Reactive Oxygen Species to Pluripotent Stem Cell Studies

Mutations in the gene encoding the enzyme tafazzin, TAZ, cause Barth syndrome (BTHS). Individuals with this X-linked multisystem disorder present cardiomyopathy (CM) (often dilated), skeletal muscle weakness, neutropenia, growth retardation, and 3-methylglutaconic aciduria. Biopsies of the heart, liver and skeletal muscle of patients have revealed mitochondrial malformations and dysfunctions. It is the purpose of this review to summarize recent results of studies on various animal or cell models of Barth syndrome, which have characterized biochemically the strong cellular defects associated with TAZ mutations. Tafazzin is a mitochondrial phospholipidlysophospholipid transacylase that shuttles acyl groups between phospholipids and regulates the remodeling of cardiolipin (CL), a unique inner mitochondrial membrane phospholipid dimer consisting of two phosphatidyl residues linked by a glycerol bridge. After their biosynthesis, the acyl chains of CLs may be modified in remodeling processes involving up to three different enzymes. Their characteristic acyl chain composition depends on the function of tafazzin, although the enzyme itself surprisingly lacks acyl specificity. CLs are crucial for correct mitochondrial structure and function. In addition to their function in the basic mitochondrial function of ATP production, CLs play essential roles in cardiac function, apoptosis, autophagy, cell cycle regulation and Fe-S cluster biosynthesis. Recent developments in tafazzin research have provided strong insights into the link between mitochondrial dysfunction and the production of reactive oxygen species (ROS). An important tool has been the generation of BTHS-specific induced pluripotent stem cells (iPSCs) from BTHS patients. In a complementary approach, disease-specific mutations have been introduced into wild-type iPSC lines enabling direct comparison with isogenic controls. iPSC-derived cardiomyocytes were then characterized using biochemical and classical bioenergetic approaches. The cells are tested in a "heart-on-chip" assay to model the pathophysiology in vitro, to characterize the underlying mechanism of BTHS deriving from TAZ mutations, mitochondrial deficiencies and ROS production and leading to tissue defects, and to evaluate potential therapies with the use of mitochondrially targeted antioxidants.

Cardiolipin metabolism and its causal role in the etiology of the inherited cardiomyopathy Barth syndrome

Cardiolipin (CL) is a phospholipid with many unique characteristics. CL is synthesized in the mitochondria and resides almost exclusively within the mitochondrial inner membrane. Unlike most phospholipids that have two fatty acyl chains, CL possesses four fatty acyl chains resulting in unique biophysical characteristics that impact several biological processes including membrane fission and fusion. In addition, several proteins directly bind CL including proteins within the electron transport chain, the ADP/ATP carrier, and proteins that mediate mitophagy. Tafazzin is an enzyme that remodels saturated fatty acyl chains within CL to unsaturated fatty acyl chains, loss of function mutations in the TAZ gene encoding tafazzin are causal for the inherited cardiomyopathy Barth syndrome. Cells from Barth syndrome patients as well as several models of Barth have reduced mitochondrial functions including impaired electron transport chain function and increased reactive oxygen species (ROS) production. Mitochondria in cells from Barth syndrome patients, as well as several model organism mimics of Barth syndrome, are large and lack cristae consistent with the recently described role of CL participating in the generation of mitochondrial membrane contact sites. Cells with an inactive TAZ gene have also been shown to have a decreased capacity to undergo mitophagy when faced with stresses such as increased ROS or decreased mitochondrial quality control. This review describes CL metabolism and how defects in CL metabolism cause Barth syndrome, the etiology of Barth syndrome, and known modifiers of Barth syndrome phenotypes some of which could be explored for their amelioration of Barth syndrome in higher organisms.

The Role of Cardiolipin in Cardiovascular Health

Cardiolipin (CL), the signature phospholipid of mitochondrial membranes, is crucial for both mitochondrial function and cellular processes outside of the mitochondria. The importance of CL in cardiovascular health is underscored by the life-threatening genetic disorder Barth syndrome (BTHS), which manifests clinically as cardiomyopathy, skeletal myopathy, neutropenia, and growth retardation. BTHS is caused by mutations in the gene encoding tafazzin, the transacylase that carries out the second CL remodeling step. In addition to BTHS, CL is linked to other cardiovascular diseases (CVDs), including cardiomyopathy, atherosclerosis, myocardial ischemia-reperfusion injury, heart failure, and Tangier disease. The link between CL and CVD may possibly be explained by the physiological roles of CL in pathways that are cardioprotective, including mitochondrial bioenergetics, autophagy/mitophagy, and mitogen activated protein kinase (MAPK) pathways. In this review, we focus on the role of CL in the pathogenesis of CVD as well as the molecular mechanisms that may link CL functions to cardiovascular health.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

Tafazzins from Drosophila and mammalian cells assemble in large protein complexes with a short half-life

Tafazzin is a transacylase that affects cardiolipin fatty acid composition and mitochondrial function. Mutations in human tafazzin cause Barth syndrome yet the enzyme has mostly been characterized in yeast. To study tafazzin in higher organisms, we isolated mitochondria from Drosophila and mammalian cell cultures. Our data indicate that tafazzin binds to multiple protein complexes in these organisms, and that the interactions of tafazzin lack strong specificity. Very large tafazzin complexes could only be detected in the presence of cardiolipin, but smaller complexes remained intact even upon treatment with phospholipase A2. In mammalian cells, tafazzin had a half-life of only 3-6h, which was much shorter than the half-life of other mitochondrial proteins. The data suggest that tafazzin is a transient resident of multiple protein complexes.

Mis-sesnse mutations in Tafazzin (TAZ) that escort to mild clinical symptoms of Barth syndrome is owed to the minimal inhibitory effect of the mutations on the enzyme function: In-silico evidence

Tafazzin (EC 2.3.1.23) is a Phospholipid Transacylase involved in Cardiolipin remodeling on mitochondrial membrane and coded by TAZ gene (Cytogenetic Location: Xq28) in human. Its mutations cause Barth syndrome (MIM ID: #302060)/3-Methyl Glutaconyl Aciduria Type II, an inborn error of metabolism often leading to foetal or infantile fatality. Nevertheless, some mis-sense mutations result in mild clinical symptoms. To evaluate the rationale of mild symptoms and for an insight of Tafazzin active site, sequence based and structure based ramifications of wild and mutant Tafazzins were compared in-silico. Sequence based domain predictions, surface accessibilities on substitution & conserved catalytic sites with statistical drifts, as well as thermal stability changes for the mutations and the interaction analysis of Tafazzin were performed. Crystal structure of Tafazzin is not yet resolved experimentally, therefore 3D coordinates of Tafazzin and its mutants were spawned through homology modeling. Energetically minimized and structurally validated models were used for comparative docking simulations. We analyzed active site geometry of the models in addition to calculating overall substrate binding efficiencies for each of the enzyme-ligand complex deduced from binding energies instead of comparing only the docking scores. Also, individual binding energies of catalytic residues on conserved HX4D motif of Acyltransferase superfamily present in Tafazzins were estimated. This work elucidates the basis of mild symptoms in patients with mis-sense mutations, identifies the most pathogenic mutant among others in the study and also divulges the critical role of HX4D domain towards successful transacylation by Taffazin. The in-silico observations are in complete agreement with clinical findings reported for the patients with mutations.

Tafazzin splice variants and mutations in Barth syndrome

Barth syndrome is caused by mutations in the TAZ (tafazzin) gene on human chromosome Xq28. The human tafazzin gene produces four major mRNA splice variants; two of which have been shown to be functional (TAZ lacking exon 5 and full-length) in complementation studies with yeast and Drosophila. This study characterizes the multiple alternative splice variants of TAZ mRNA and their proportions in blood samples from a cohort of individuals with Barth syndrome (BTHS). Because it has been reported that collection and processing methods can affect the expression of various genes, we tested and chose a stabilizing medium for collecting, shipping and processing of the blood samples of these individuals. In both healthy controls and in BTHS individuals, we found a greater variety of alternatively spliced forms than previously described, with a sizeable proportion of minor splice variants besides the four dominant isoforms. Individuals with certain exonic and intronic splice mutations produce additional mutant mRNAs that could be translated into two or more proteins with different amino acid substitutions in a single individual. A fraction of the minor splice variants is predicted to be non-productive.

Deletion of the cardiolipin-specific phospholipase Cld1 rescues growth and life span defects in the tafazzin mutant: implications for Barth syndrome

Cardiolipin (CL) that is synthesized de novo is deacylated to monolysocardiolipin (MLCL), which is reacylated by tafazzin. Remodeled CL contains mostly unsaturated fatty acids. In eukaryotes, loss of tafazzin leads to growth and respiration defects, and in humans, this results in the life-threatening disorder Barth syndrome. Tafazzin deficiency causes a decrease in the CL/MLCL ratio and decreased unsaturated CL species. Which of these biochemical outcomes contributes to the physiological defects is not known. Yeast cells have a single CL-specific phospholipase, Cld1, that can be exploited to distinguish between these outcomes. The cld1Δ mutant has decreased unsaturated CL, but the CL/MLCL ratio is similar to that of wild type cells. We show that cld1Δ rescues growth, life span, and respiratory defects of the taz1Δ mutant. This suggests that defective growth and respiration in tafazzin-deficient cells are caused by the decreased CL/MLCL ratio and not by a deficiency in unsaturated CL. CLD1 expression is increased during respiratory growth and regulated by the heme activator protein transcriptional activation complex. Overexpression of CLD1 leads to decreased mitochondrial respiration and growth and instability of mitochondrial DNA. However, ATP concentrations are maintained by increasing glycolysis. We conclude that transcriptional regulation of Cld1-mediated deacylation of CL influences energy metabolism by modulating the relative contribution of glycolysis and respiration.

Barth syndrome: cellular compensation of mitochondrial dysfunction and apoptosis inhibition due to changes in cardiolipin remodeling linked to tafazzin (TAZ) gene mutation

Cardiolipin is a mitochondrion-specific phospholipid that stabilizes the assembly of respiratory chain complexes, favoring full-yield operation. It also mediates key steps in apoptosis. In Barth syndrome, an X chromosome-linked cardiomyopathy caused by tafazzin mutations, cardiolipins display acyl chain modifications and are present at abnormally low concentrations, whereas monolysocardiolipin accumulates. Using immortalized lymphoblasts from Barth syndrome patients, we showed that the production of abnormal cardiolipin led to mitochondrial alterations. Indeed, the lack of normal cardiolipin led to changes in electron transport chain stability, resulting in cellular defects. We found a destabilization of the supercomplex (respirasome) I+III2+IVn but also decreased amounts of individual complexes I and IV and supercomplexes I+III and III+IV. No changes were observed in the amounts of individual complex III and complex II. We also found decreased levels of complex V. This complex is not part of the supercomplex suggesting that cardiolipin is required not only for the association/stabilization of the complexes into supercomplexes but also for the modulation of the amount of individual respiratory chain complexes. However, these alterations were compensated by an increase in mitochondrial mass, as demonstrated by electron microscopy and measurements of citrate synthase activity. We suggest that this compensatory increase in mitochondrial content prevents a decrease in mitochondrial respiration and ATP synthesis in the cells. We also show, by extensive flow cytometry analysis, that the type II apoptosis pathway was blocked at the mitochondrial level and that the mitochondria of patients with Barth syndrome cannot bind active caspase-8. Signal transduction is thus blocked before any mitochondrial event can occur. Remarkably, basal levels of superoxide anion production were slightly higher in patients' cells than in control cells as previously evidenced via an increased protein carbonylation in the taz1Δ mutant in the yeast. This may be deleterious to cells in the long term. The consequences of mitochondrial dysfunction and alterations to apoptosis signal transduction are considered in light of the potential for the development of future treatments.

The physical state of lipid substrates provides transacylation specificity for tafazzin

Cardiolipin is a mitochondrial phospholipid with a characteristic acyl chain composition that depends on the function of tafazzin, a phospholipid-lysophospholipid transacylase, although the enzyme itself lacks acyl specificity. We incubated isolated tafazzin with various mixtures of phospholipids and lysophospholipids, characterized the lipid phase by (31)P-NMR and measured newly formed molecular species by MS. Substantial transacylation was observed only in nonbilayer lipid aggregates, and the substrate specificity was highly sensitive to the lipid phase. In particular, tetralinoleoyl-cardiolipin, a prototype molecular species, formed only under conditions that favor the inverted hexagonal phase. In isolated mitochondria, <1% of lipids participated in transacylations, suggesting that the action of tafazzin was limited to privileged lipid domains. We propose that tafazzin reacts with non-bilayer-type lipid domains that occur in curved or hemifused membrane zones and that acyl specificity is driven by the packing properties of these domains.

New clinical and molecular insights on Barth syndrome

Background

Barth syndrome (BS) is an X-linked infantile-onset cardioskeletal disease characterized by cardiomyopathy, hypotonia, growth delay, neutropenia and 3-methylglutaconic aciduria. It is caused by mutations in the TAZ gene encoding tafazzin, a protein involved in the metabolism of cardiolipin, a mitochondrial-specific phospholipid involved in mitochondrial energy production.

Methods

Clinical, biochemical and molecular characterization of a group of six male patients suspected of having BS. Three patients presented early with severe metabolic decompensation including respiratory distress, oxygen desaturation and cardiomyopathy and died within the first year of life. The remaining three patients had cardiomyopathy, hypotonia and growth delay and are still alive. Cardiomyopathy was detected during pregnancy through a routine check-up in one patient. All patients exhibited 3-methylglutaconic aciduria and neutropenia, when tested and five of them also had lactic acidosis.

Results

We confirmed the diagnosis of BS with sequence analysis of the TAZ gene, and found five new mutations, c.641A>G p.His214Arg, c.284dupG (p.Thr96Aspfs*37), c.678_691del14 (p.Tyr227Trpfs*79), g.8009_16445del8437 and g.[9777_9814del38; 9911-?_14402del] and the known nonsense mutation c.367C>T (p.Arg123Term). The two gross rearrangements ablated TAZ exons 6 to 11 and probably originated by non-allelic homologous recombination and by Serial Replication Slippage (SRS), respectively. The identification of the breakpoints boundaries of the gross deletions allowed the direct detection of heterozygosity in carrier females.

Conclusions

Lactic acidosis associated with 3-methylglutaconic aciduria is highly suggestive of BS, whilst the severity of the metabolic decompensation at disease onset should be considered for prognostic purposes. Mutation analysis of the TAZ gene is necessary for confirming the clinical and biochemical diagnosis in probands in order to identify heterozygous carriers and supporting prenatal diagnosis and genetic counseling.