Characterization of a transgenic short hairpin RNA-induced murine model of Tafazzin deficiency

SS-31 treatment ameliorates cardiac mitochondrial morphology and defective mitophagy in a murine model of Barth syndrome

Barth syndrome (BTHS) is a lethal rare genetic disorder, which results in cardiac dysfunction, severe skeletal muscle weakness, immune issues and growth delay. Mutations in the TAFAZZIN gene, which is responsible for the remodeling of the phospholipid cardiolipin (CL), lead to abnormalities in mitochondrial membrane, including alteration of mature CL acyl composition and the presence of monolysocardiolipin (MLCL). The dramatic increase in the MLCL/CL ratio is the hallmark of patients with BTHS, which is associated with mitochondrial bioenergetics dysfunction and altered membrane ultrastructure. There are currently no specific therapies for BTHS. Here, we showed that cardiac mitochondria isolated from TAFAZZIN knockdown (TazKD) mice presented abnormal ultrastructural membrane morphology, accumulation of vacuoles, pro-fission conditions and defective mitophagy. Interestingly, we found that in vivo treatment of TazKD mice with a CL-targeted small peptide (named SS-31) was able to restore mitochondrial morphology in tafazzin-deficient heart by affecting specific proteins involved in dynamic process and mitophagy. This agrees with our previous data showing an improvement in mitochondrial respiratory efficiency associated with increased supercomplex organization in TazKD mice under the same pharmacological treatment. Taken together our findings confirm the beneficial effect of SS-31 in the amelioration of tafazzin-deficient dysfunctional mitochondria in a BTHS animal model.

Activation of the integrated stress response rewires cardiac metabolism in Barth syndrome

Barth Syndrome (BTHS) is an inherited cardiomyopathy caused by defects in the mitochondrial transacylase TAFAZZIN (Taz), required for the synthesis of the phospholipid cardiolipin. BTHS is characterized by heart failure, increased propensity for arrhythmias and a blunted inotropic reserve. Defects in Ca2+-induced Krebs cycle activation contribute to these functional defects, but despite oxidation of pyridine nucleotides, no oxidative stress developed in the heart. Here, we investigated how retrograde signaling pathways orchestrate metabolic rewiring to compensate for mitochondrial defects. In mice with an inducible knockdown (KD) of TAFAZZIN, and in induced pluripotent stem cell-derived cardiac myocytes, mitochondrial uptake and oxidation of fatty acids was strongly decreased, while glucose uptake was increased. Unbiased transcriptomic analyses revealed that the activation of the eIF2α/ATF4 axis of the integrated stress response upregulates one-carbon metabolism, which diverts glycolytic intermediates towards the biosynthesis of serine and fuels the biosynthesis of glutathione. In addition, strong upregulation of the glutamate/cystine antiporter xCT increases cardiac cystine import required for glutathione synthesis. Increased glutamate uptake facilitates anaplerotic replenishment of the Krebs cycle, sustaining energy production and antioxidative pathways. These data indicate that ATF4-driven rewiring of metabolism compensates for defects in mitochondrial uptake of fatty acids to sustain energy production and antioxidation.

Phenotypic Characterization of Female Carrier Mice Heterozygous for Tafazzin Deletion

Barth syndrome (BTHS) is caused by mutations in tafazzin resulting in deficits in cardiolipin remodeling that alter major metabolic processes. The tafazzin gene is encoded on the X chromosome, and therefore BTHS primarily affects males. Female carriers are typically considered asymptomatic, but age-related changes have been reported in female carriers of other X-linked disorders. Therefore, we examined the phenotype of female mice heterozygous for deletion of the tafazzin gene (Taz-HET) at 3 and 12 months of age. Food intakes, body masses, lean tissue and adipose depot weights, daily activity levels, metabolic measures, and exercise capacity were assessed. Age-related changes in mice resulted in small but significant genotype-specific differences in Taz-HET mice compared with their female Wt littermates. By 12 months, Taz-HET mice weighed less than Wt controls and had smaller gonadal, retroperitoneal, and brown adipose depots and liver and brain masses, despite similar food consumption. Daily movement, respiratory exchange ratio, and total energy expenditure did not vary significantly between the age-matched genotypes. Taz-HET mice displayed improved glucose tolerance and insulin sensitivity at 12 months compared with their Wt littermates but had evidence of slightly reduced exercise capacity. Tafazzin mRNA levels were significantly reduced in the cardiac muscle of 12-month-old Taz-HET mice, which was associated with minor but significant alterations in the heart cardiolipin profile. This work is the first to report the characterization of a model of female carriers of heterozygous tafazzin deficiency and suggests that additional study, particularly with advancing age, is warranted.

Elevated liver glycogenolysis mediates higher blood glucose during acute exercise in Barth syndrome

Barth syndrome (BTHS) is an X-linked recessive genetic disorder due to mutations in the Tafazzin (TAFAZZIN) gene that lead to cardiac and skeletal muscle mitochondrial dysfunction. Previous studies in humans with BTHS demonstrate that the defects in muscle mitochondrial oxidative metabolism result in an enhanced reliance on anaerobic metabolism during exercise to meet energy demands of muscular work. During exercise, the liver normally increases glucose production via glycogenolysis and gluconeogenesis to match the elevated rate of muscle glucose uptake and meet the ATP requirements of working muscle. However, the impact of Tafazzin deficiency on hepatic glucose production and the pathways contributing to hepatic glucose production during exercise is unknown. Therefore, the purpose of this study was to quantify in vivo liver gluconeogenesis and glycogenolysis in Tafazzin knockdown mice at rest and during acute exercise.

METHODS

Male TAFAZZIN shRNA transgenic (TG) and wild-type (WT) mice completed exhaustive treadmill running protocols to test exercise tolerance. Mice underwent 2H- and 13C-stable isotope infusions at rest and during a 30-minute treadmill running bout to quantify hepatic glucose production and associated nutrient fluxes under sedentary conditions and during acute exercise. Circulating and tissue (skeletal muscle and liver) samples were obtained during and following exercise to assess static metabolite levels.

RESULTS

TG mice reached exhaustion sooner during exhaustive treadmill running protocols and exhibited higher plasma lactate concentrations after exhaustive exercise compared to WT mice. Arterial glucose levels were comparable between genotypes at rest, but higher in TG mice compared to WT mice during exercise. Consistent with the higher blood glucose, TG mice showed increased endogenous glucose production owing to elevated glycogenolysis compared to WT mice during exercise. Total gluconeogenesis, gluconeogenesis from glycerol, gluconeogenesis from phosphoenolpyruvate, pyruvate cycling, total cataplerosis, and anaplerotic fluxes were similar between TG and WT mice at rest and during exercise. However, lactate dehydrogenase flux and TCA cycle fluxes trended higher in TG mice during exercise. Liver glycogen content in TG was higher in TG vs. controls.

CONCLUSION

Our data in the Tafazzin knockdown mouse suggest that elevated anaerobic metabolism during rest and exercise previously reported in humans with BTHS are supported by the finding of higher hepatic glycogenolysis.

Genetic modifiers modulate phenotypic expression of tafazzin deficiency in a mouse model of Barth syndrome

Barth syndrome is an X-linked disorder caused by loss-of-function mutations in Tafazzin (TAZ), an acyltransferase that catalyzes remodeling of cardiolipin, a signature phospholipid of the inner mitochondrial membrane. Patients develop cardiac and skeletal muscle weakness, growth delay and neutropenia, although phenotypic expression varies considerably between patients. Taz knockout mice recapitulate many of the hallmark features of the disease. We used mouse genetics to test the hypothesis that genetic modifiers alter the phenotypic manifestations of Taz inactivation. We crossed TazKO/X females in the C57BL6/J inbred strain to males from eight inbred strains and evaluated the phenotypes of first-generation (F1) TazKO/Y progeny, compared to TazWT/Y littermates. We observed that genetic background strongly impacted phenotypic expression. C57BL6/J and CAST/EiJ[F1] TazKO/Y mice developed severe cardiomyopathy, whereas A/J[F1] TazKO/Y mice had normal heart function. C57BL6/J and WSB/EiJ[F1] TazKO/Y mice had severely reduced treadmill endurance, whereas endurance was normal in A/J[F1] and CAST/EiJ[F1] TazKO/Y mice. In all genetic backgrounds, cardiolipin showed similar abnormalities in knockout mice, and transcriptomic and metabolomic investigations identified signatures of mitochondrial uncoupling and activation of the integrated stress response. TazKO/Y cardiac mitochondria were small, clustered and had reduced cristae density in knockouts in severely affected genetic backgrounds but were relatively preserved in the permissive A/J[F1] strain. Gene expression and mitophagy measurements were consistent with reduced mitophagy in knockout mice in genetic backgrounds intolerant of Taz mutation. Our data demonstrate that genetic modifiers powerfully modulate phenotypic expression of Taz loss-of-function and act downstream of cardiolipin, possibly by altering mitochondrial quality control.

Weight loss increases skeletal muscle mitochondrial energy efficiency in obese mice

Weight loss from an overweight state is associated with a disproportionate decrease in whole-body energy expenditure that may contribute to the heightened risk for weight regain. Evidence suggests that this energetic mismatch originates from lean tissue. Although this phenomenon is well documented, the mechanisms have remained elusive. We hypothesized that increased mitochondrial energy efficiency in skeletal muscle is associated with reduced expenditure under weight loss. Wildtype (WT) male C57BL6/N mice were fed with high fat diet for 10 weeks, followed by a subset of mice that were maintained on the obesogenic diet (OB) or switched to standard chow to promote weight loss (WL) for additional 6 weeks. Mitochondrial energy efficiency was evaluated using high-resolution respirometry and fluorometry. Mass spectrometric analyses were employed to describe the mitochondrial proteome and lipidome. Weight loss promoted ~50% increase in the efficiency of oxidative phosphorylation (ATP produced per O2 consumed, or P/O) in skeletal muscle. However, weight loss did not appear to induce significant changes in mitochondrial proteome, nor any changes in respiratory supercomplex formation. Instead, it accelerated the remodeling of mitochondrial cardiolipin (CL) acyl-chains to increase tetralinoleoyl CL (TLCL) content, a species of lipids thought to be functionally critical for the respiratory enzymes. We further show that lowering TLCL by deleting the CL transacylase tafazzin was sufficient to reduce skeletal muscle P/O and protect mice from diet-induced weight gain. These findings implicate skeletal muscle mitochondrial efficiency as a novel mechanism by which weight loss reduces energy expenditure in obesity.

Phenotypic Characterization of Male Tafazzin-Knockout Mice at 3, 6, and 12 Months of Age

Barth syndrome (BTHS) is an X-linked mitochondrial disease caused by mutations in the gene encoding for tafazzin (TAZ), a key enzyme in the remodeling of cardiolipin. Mice with a germline deficiency in Taz have been generated (Taz-KO) but not yet fully characterized. We performed physiological assessments of 3-, 6-, and 12-month-old male Taz-KO mice, including measures of perinatal survival, growth, lifespan, gross anatomy, whole-body energy and substrate metabolism, glucose homeostasis, and exercise capacity. Taz-KO mice displayed reduced viability, with lower-than-expected numbers of mice recorded at 4 weeks of age, and a shortened lifespan due to disease progression. At all ages, Taz-KO mice had lower body weights compared with wild-type (Wt) littermates despite similar absolute food intakes. This finding was attributed to reduced adiposity and diminutive organs and tissues, including heart and skeletal muscles. Although there were no differences in basal levels of locomotion between age-matched genotypes, indirect calorimetry studies showed higher energy expenditure measures and respiratory exchange ratios in Taz-KO mice. At the youngest age, Taz-KO mice had comparable glucose tolerance and insulin action to Wt mice, but while these measures indicated metabolic impairments in Wt mice with advancing age that were likely associated with increasing adiposity, Taz-KO mice were protected. Comparisons across the three age-cohorts revealed a significant and more severe deterioration of exercise capacity in Taz-KO mice than in their Wt littermate controls. The Taz-KO mouse model faithfully recapitulates important aspects of BTHS, and thus provides an important new tool to investigate pathophysiological mechanisms and potential therapies.

Beneficial effects of SS-31 peptide on cardiac mitochondrial dysfunction in tafazzin knockdown mice

Barth Syndrome (BTHS), a genetic disease associated with early-onset cardioskeletal myopathy, is caused by loss-of-function mutations of the TAFAZZIN gene, which is responsible for remodeling the mitochondrial phospholipid cardiolipin (CL). Deregulation of CL biosynthesis and maturation in BTHS mitochondria result in a dramatically increased monolysocardiolipin (MLCL)/CL ratio associated with bioenergetic dysfunction. One of the most promising therapeutic approaches for BTHS includes the mitochondria-targeted tetrapeptide SS-31, which interacts with CL. Here, we used TAFAZZIN knockdown (Taz^KD) mice to investigate for the first time whether in vivo administration of SS-31 could affect phospholipid profiles and mitochondrial dysfunction. The CL fingerprinting of Taz^KD cardiac mitochondria obtained by MALDI-TOF/MS revealed the typical lipid changes associated with BTHS. Taz^KD mitochondria showed lower respiratory rates in state 3 and 4 together with a decreased in maximal respiratory rates. Treatment of Taz^KD mice with SS-31 improved mitochondrial respiratory capacity and promoted supercomplex organization, without affecting the MLCL/CL ratio. We hypothesize that SS-31 exerts its effect by influencing the function of the respiratory chain rather than affecting CL directly. In conclusion, our results indicate that SS-31 have beneficial effects on improving cardiac mitochondrial dysfunction in a BTHS animal model, suggesting the peptide as future pharmacologic agent for therapy.

Cardiolipin Regulates Mitochondrial Ultrastructure and Function in Mammalian Cells

Cardiolipin (CL) is a unique, tetra-acylated diphosphatidylglycerol lipid that mainly localizes in the inner mitochondria membrane (IMM) in mammalian cells and plays a central role in regulating mitochondrial architecture and functioning. A deficiency of CL biosynthesis and remodeling perturbs mitochondrial functioning and ultrastructure. Clinical and experimental studies on human patients and animal models have also provided compelling evidence that an abnormal CL content, acyl chain composition, localization, and level of oxidation may be directly linked to multiple diseases, including cardiomyopathy, neuronal dysfunction, immune cell defects, and metabolic disorders. The central role of CL in regulating the pathogenesis and progression of these diseases has attracted increasing attention in recent years. In this review, we focus on the advances in our understanding of the physiological roles of CL biosynthesis and remodeling from human patients and mouse models, and we provide an overview of the potential mechanism by which CL regulates the mitochondrial architecture and functioning.

A new murine model of Barth syndrome neutropenia links TAFAZZIN deficiency to increased ER stress-induced apoptosis

Barth syndrome is an inherited X-linked disorder that leads to cardiomyopathy, skeletal myopathy, and neutropenia. These symptoms result from the loss of function of the enzyme TAFAZZIN, a transacylase located in the inner mitochondrial membrane that is responsible for the final steps of cardiolipin production. The link between defective cardiolipin maturation and neutropenia remains unclear. To address potential mechanisms of neutropenia, we examined myeloid progenitor development within the fetal liver of TAFAZZIN knockout (KO) animals as well as within the adult bone marrow of wild-type recipients transplanted with TAFAZZIN-KO hematopoietic stem cells. We also used the ER-Hoxb8 system (estrogen receptor fused to Hoxb8) of conditional immortalization to establish a new murine model system for the ex vivo study of TAFAZZIN-deficient neutrophils. The TAFAZZIN-KO cells demonstrated the expected dramatic differences in cardiolipin maturation that result from a lack of TAFAZZIN enzyme activity. Contrary to our hypothesis, we did not identify any significant differences in neutrophil development or neutrophil function across a variety of assays including phagocytosis and the production of cytokines or reactive oxygen species. However, transcriptomic analysis of the TAFAZZIN-deficient neutrophil progenitors demonstrated an upregulation of markers of endoplasmic reticulum stress and confirmatory testing demonstrated that the TAFAZZIN-deficient cells had increased sensitivity to certain ER stress-mediated and non-ER stress-mediated triggers of apoptosis. Although the link between increased sensitivity to apoptosis and the variably penetrant neutropenia phenotype seen in some patients with Barth syndrome remains to be clarified, our studies and new model system set a foundation for further investigation.

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.

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.

Current and future treatment approaches for Barth syndrome

Barth Syndrome is an X-linked disorder of mitochondrial cardiolipin metabolism caused by pathogenic variants in TAFAZZIN with pleiotropic effects including cardiomyopathy, neutropenia, growth delay, and skeletal myopathy. Management requires a multidisciplinary approach to the organ-specific manifestations including specialists from cardiology, hematology, nutrition, physical therapy, genetics, and metabolism. Currently, treatment is centered on management of specific clinical features, and is not targeted toward remediating the underlying biochemical defect. However, two clinical trials have been recently undertaken which target the mitochondrial pathology of this disease: a study to examine the effects of elamipretide, a cardiolipin targeted agent, and a study to examine the effects of bezafibrate, a peroxisome proliferator-activated receptor (PPAR) agonist. Treatments to directly target the defective TAFAZZIN pathway are under development, including enzyme and gene therapies.

Loss of Mitochondrial Ca2+ Uniporter Limits Inotropic Reserve and Provides Trigger and Substrate for Arrhythmias in Barth Syndrome Cardiomyopathy

BACKGROUND

Barth syndrome (BTHS) is caused by mutations of the gene encoding tafazzin, which catalyzes maturation of mitochondrial cardiolipin and often manifests with systolic dysfunction during early infancy. Beyond the first months of life, BTHS cardiomyopathy typically transitions to a phenotype of diastolic dysfunction with preserved ejection fraction, blunted contractile reserve during exercise, and arrhythmic vulnerability. Previous studies traced BTHS cardiomyopathy to mitochondrial formation of reactive oxygen species (ROS). Because mitochondrial function and ROS formation are regulated by excitation-contraction coupling, integrated analysis of mechano-energetic coupling is required to delineate the pathomechanisms of BTHS cardiomyopathy.

METHODS

We analyzed cardiac function and structure in a mouse model with global knockdown of tafazzin (Taz-KD) compared with wild-type littermates. Respiratory chain assembly and function, ROS emission, and Ca²⁺ uptake were determined in isolated mitochondria. Excitation-contraction coupling was integrated with mitochondrial redox state, ROS, and Ca²⁺ uptake in isolated, unloaded or preloaded cardiac myocytes, and cardiac hemodynamics analyzed in vivo.

RESULTS

Taz-KD mice develop heart failure with preserved ejection fraction (>50%) and age-dependent progression of diastolic dysfunction in the absence of fibrosis. Increased myofilament Ca²⁺ affinity and slowed cross-bridge cycling caused diastolic dysfunction, in part, compensated by accelerated diastolic Ca²⁺ decay through preactivated sarcoplasmic reticulum Ca²⁺-ATPase. Taz deficiency provoked heart-specific loss of mitochondrial Ca²⁺ uniporter protein that prevented Ca²⁺-induced activation of the Krebs cycle during β-adrenergic stimulation, oxidizing pyridine nucleotides and triggering arrhythmias in cardiac myocytes. In vivo, Taz-KD mice displayed prolonged QRS duration as a substrate for arrhythmias, and a lack of inotropic response to β-adrenergic stimulation. Cellular arrhythmias and QRS prolongation, but not the defective inotropic reserve, were restored by inhibiting Ca²⁺ export through the mitochondrial Na⁺/Ca²⁺ exchanger. All alterations occurred in the absence of excess mitochondrial ROS in vitro or in vivo.

CONCLUSIONS

Downregulation of mitochondrial Ca²⁺ uniporter, increased myofilament Ca²⁺ affinity, and preactivated sarcoplasmic reticulum Ca²⁺-ATPase provoke mechano-energetic uncoupling that explains diastolic dysfunction and the lack of inotropic reserve in BTHS cardiomyopathy. Furthermore, defective mitochondrial Ca²⁺ uptake provides a trigger and a substrate for ventricular arrhythmias. These insights can guide the ongoing search for a cure of this orphaned disease.

Depletion of cardiac cardiolipin synthase alters systolic and diastolic function

Cardiolipin (CL) is a major cardiac mitochondrial phospholipid maintaining regular mitochondrial morphology and function in cardiomyocytes. Cardiac CL production includes its biosynthesis and a CL remodeling process. Here we studied the impact of CL biosynthesis and the enzyme cardiolipin synthase (CLS) on cardiac function. CLS and cardiac CL species were significantly downregulated in cardiomyocytes following catecholamine-induced cardiac damage in mice, accompanied by increased oxygen consumption rates, signs of oxidative stress, and mitochondrial uncoupling. RNAi-mediated cardiomyocyte-specific knockdown of CLS in Drosophila melanogaster resulted in marked cardiac dilatation, severe impairment of systolic performance, and slower diastolic filling velocity assessed by fluorescence-based heart imaging. Finally, we showed that CL72:8 is significantly decreased in cardiac samples from patients with heart failure with reduced ejection fraction (HFrEF). In summary, we identified CLS as a regulator of cardiac function. Considering the cardiac depletion of CL species in HFrEF, pharmacological targeting of CLS may be a promising therapeutic approach.

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Cardiolipin synthase (CLS) is reduced in isoproterenol (ISO)-induced cardiac damage

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This is accompanied by alterations of cardiolipins and mitochondrial function

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CLS mutant Drosophila melanogaster exhibit mild cardiac changes

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Cardiomyocyte-CLS knockdown in Drosophila results in severe cardiac dysfunction

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.

The Influence of Supplemental Dietary Linoleic Acid on Skeletal Muscle Contractile Function in a Rodent Model of Barth Syndrome

Barth syndrome is a rare and incurable X-linked (male-specific) genetic disease that affects the protein tafazzin (Taz). Taz is an important enzyme responsible for synthesizing biologically relevant cardiolipin (for heart and skeletal muscle, cardiolipin rich in linoleic acid), a critical phospholipid of mitochondrial form and function. Mutations to Taz cause dysfunctional mitochondria, resulting in exercise intolerance due to skeletal muscle weakness. To date, there has been limited research on improving skeletal muscle function, with interventions focused on endurance and resistance exercise. Previous cell culture research has shown therapeutic potential for the addition of exogenous linoleic acid in improving Taz-deficient mitochondrial function but has not been examined in vivo. The purpose of this study was to examine the influence of supplemental dietary linoleic acid on skeletal muscle function in a rodent model of Barth syndrome, the inducible Taz knockdown (TazKD) mouse. One of the main findings was that TazKD soleus demonstrated an impaired contractile phenotype (slower force development and rates of relaxation) in vitro compared to their WT littermates. Interestingly, this impaired contractile phenotype seen in vitro did not translate to altered muscle function in vivo at the whole-body level. Also, supplemental linoleic acid attenuated, to some degree, in vitro impaired contractile phenotype in TazKD soleus, and these findings appear to be partially mediated by improvements in cardiolipin content and resulting mitochondrial supercomplex formation. Future research will further examine alternative mechanisms of dietary supplemental LA on improving skeletal muscle contractile dysfunction in TazKD mice.

Berberine elevates cardiolipin in heart of offspring from mouse dams with high fat diet-induced gestational diabetes mellitus

Berberine (BBR) is an isoquinoline alkaloid from plants known to improve cardiac mitochondrial function in gestational diabetes mellitus (GDM) offspring but the mechanism is poorly understood. We examined the role of the mitochondrial phospholipid cardiolipin (CL) in mediating this cardiac improvement. C57BL/6 female mice were fed either a Lean-inducing low-fat diet or a GDM-inducing high-fat diet for 6 weeks prior to breeding. Lean and GDM-exposed male offspring were randomly assigned a low-fat, high-fat, or high-fat diet containing BBR at weaning for 12 weeks. The content of CL was elevated in the heart of GDM offspring fed a high fat diet containing BBR. The increase in total cardiac CL was due to significant increases in the most abundant and functionally important CL species, tetralinoleoyl-CL and this correlated with an increase in the expression of the CL remodeling enzyme tafazzin. Additionally, BBR treatment increased expression of cardiac enzymes involved in fatty acid uptake and oxidation and electron transport chain subunits in high fat diet fed GDM offspring. Thus, dietary BBR protection from cardiac dysfunction in GDM exposed offspring involves improvement in mitochondrial function mediated through increased synthesis of CL.

Skeletal Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

Neurological & psychological aspects of Barth syndrome: Clinical manifestations and potential pathogenic mechanisms

Barth syndrome is a rare X-linked multisystem mitochondrial disease that is caused by variants in the tafazzin gene leading to deficient and abnormal cardiolipin. Previous research has focused on the cardiomyopathy and neutropenia in individuals with Barth syndrome, yet just as common are the least explored neurological aspects of Barth syndrome. This review focuses on the major neuropsychological and neurophysiological phenotypes that affect the quality of life of individuals with Barth syndrome, including difficulties in sensory perception and feeding, fatigue, and cognitive and psychological challenges. We propose selected pathogenetic mechanisms underlying these phenotypes and draw parallels to other relevant disorders. Finally, avenues for future research are also suggested.

Cardiolipin Remodeling Defects Impair Mitochondrial Architecture and Function in a Murine Model of Barth Syndrome Cardiomyopathy

BACKGROUND

Cardiomyopathy is a major clinical feature in Barth syndrome (BTHS), an X-linked mitochondrial lipid disorder caused by mutations in Tafazzin (TAZ), encoding a mitochondrial acyltransferase required for cardiolipin remodeling. Despite recent description of a mouse model of BTHS cardiomyopathy, an in-depth analysis of specific lipid abnormalities and mitochondrial form and function in an in vivo BTHS cardiomyopathy model is lacking.

METHODS

We performed in-depth assessment of cardiac function, cardiolipin species profiles, and mitochondrial structure and function in our newly generated Taz cardiomyocyte-specific knockout mice and Cre-negative control mice (n≥3 per group).

RESULTS

Taz cardiomyocyte-specific knockout mice recapitulate typical features of BTHS and mitochondrial cardiomyopathy. Fewer than 5% of cardiomyocyte-specific knockout mice exhibited lethality before 2 months of age, with significantly enlarged hearts. More than 80% of cardiomyocyte-specific knockout displayed ventricular dilation at 16 weeks of age and survived until 50 weeks of age. Full parameter analysis of cardiac cardiolipin profiles demonstrated lower total cardiolipin concentration, abnormal cardiolipin fatty acyl composition, and elevated monolysocardiolipin to cardiolipin ratios in Taz cardiomyocyte-specific knockout, relative to controls. Mitochondrial contact site and cristae organizing system and F1F0-ATP synthase complexes, required for cristae morphogenesis, were abnormal, resulting in onion-shaped mitochondria. Organization of high molecular weight respiratory chain supercomplexes was also impaired. In keeping with observed mitochondrial abnormalities, seahorse experiments demonstrated impaired mitochondrial respiration capacity.

CONCLUSIONS

Our mouse model mirrors multiple physiological and biochemical aspects of BTHS cardiomyopathy. Our results give important insights into the underlying cause of BTHS cardiomyopathy and provide a framework for testing therapeutic approaches to BTHS cardiomyopathy, or other mitochondrial-related cardiomyopathies.

Phospholipid ebb and flow makes mitochondria go

Mitochondria, so much more than just being energy factories, also have the capacity to synthesize macromolecules including phospholipids, particularly cardiolipin (CL) and phosphatidylethanolamine (PE). Phospholipids are vital constituents of mitochondrial membranes, impacting the plethora of functions performed by this organelle. Hence, the orchestrated movement of phospholipids to and from the mitochondrion is essential for cellular integrity. In this review, we capture recent advances in the field of mitochondrial phospholipid biosynthesis and trafficking, highlighting the significance of interorganellar communication, intramitochondrial contact sites, and lipid transfer proteins in maintaining membrane homeostasis. We then discuss the physiological functions of CL and PE, specifically how they associate with protein complexes in mitochondrial membranes to support bioenergetics and maintain mitochondrial architecture.

Barth syndrome: cardiolipin, cellular pathophysiology, management, and novel therapeutic targets

Barth syndrome is a rare X-linked genetic disease classically characterized by cardiomyopathy, skeletal myopathy, growth retardation, neutropenia, and 3-methylglutaconic aciduria. It is caused by mutations in the tafazzin gene localized to chromosome Xq28.12. Mutations in tafazzin may result in alterations in the level and molecular composition of the mitochondrial phospholipid cardiolipin and result in large elevations in the lysophospholipid monolysocardiolipin. The increased monolysocardiolipin:cardiolipin ratio in blood is diagnostic for the disease, and it leads to disruption in mitochondrial bioenergetics. In this review, we discuss cardiolipin structure, synthesis, and function and provide an overview of the clinical and cellular pathophysiology of Barth Syndrome. We highlight known pharmacological management for treatment of the major pathological features associated with the disease. In addition, we discuss non-pharmacological management. Finally, we highlight the most recent promising therapeutic options for this rare mitochondrial disease including lipid replacement therapy, peroxisome proliferator-activated receptor agonists, tafazzin gene replacement therapy, induced pluripotent stem cells, mitochondria-targeted antioxidants and peptides, and the polyphenolic compound resveratrol.

Mechanical Stiffness Controls Dendritic Cell Metabolism and Function

Stiffness in the tissue microenvironment changes in most diseases and immunological conditions, but its direct influence on the immune system is poorly understood. Here, we show that static tension impacts immune cell function, maturation, and metabolism. Bone-marrow-derived and/or splenic dendritic cells (DCs) grown in vitro at physiological resting stiffness have reduced proliferation, activation, and cytokine production compared with cells grown under higher stiffness, mimicking fibro-inflammatory disease. Consistently, DCs grown under higher stiffness show increased activation and flux of major glucose metabolic pathways. In DC models of autoimmune diabetes and tumor immunotherapy, tension primes DCs to elicit an adaptive immune response. Mechanistic workup identifies the Hippo-signaling molecule, TAZ, as well as Ca²⁺-related ion channels, including potentially PIEZO1, as important effectors impacting DC metabolism and function under tension. Tension also directs the phenotypes of monocyte-derived DCs in humans. Thus, mechanical stiffness is a critical environmental cue of DCs and innate immunity.

Cardiolipin deficiency in Barth syndrome is not associated with increased superoxide/H2 O2 production in heart and skeletal muscle mitochondria

Barth syndrome (BTHS) is a rare X-linked genetic disorder caused by mutations in the gene encoding the transacylase tafazzin and characterized by loss of cardiolipin and severe cardiomyopathy. Mitochondrial oxidants have been implicated in the cardiomyopathy in BTHS. Eleven mitochondrial sites produce superoxide/hydrogen peroxide (H₂ O₂ ) at significant rates. Which of these sites generate oxidants at excessive rates in BTHS is unknown. Here, we measured the maximum capacity of superoxide/H₂ O₂ production from each site and the ex vivo rate of superoxide/H₂ O₂ production in the heart and skeletal muscle mitochondria of the tafazzin knockdown mice (tazkd) from 3 to 12 months of age. Despite reduced oxidative capacity, superoxide/H₂ O₂ production was indistinguishable between tazkd mice and wild-type littermates. These observations raise questions about the involvement of mitochondrial oxidants in BTHS pathology.

Mitochondrial respiratory supercomplexes in mammalian cells: structural versus functional role

Mitochondria are recognized as the main source of ATP to meet the energy demands of the cell. ATP production occurs by oxidative phosphorylation when electrons are transported through the electron transport chain (ETC) complexes and develop the proton motive force across the inner mitochondrial membrane that is used for ATP synthesis. Studies since the 1960s have been concentrated on the two models of structural organization of ETC complexes known as "solid-state" and "fluid-state" models. However, advanced new techniques such as blue-native gel electrophoresis, mass spectroscopy, and cryogenic electron microscopy for analysis of macromolecular protein complexes provided new data in favor of the solid-state model. According to this model, individual ETC complexes are assembled into macromolecular structures known as respiratory supercomplexes (SCs). A large number of studies over the last 20 years proposed the potential role of SCs to facilitate substrate channeling, maintain the integrity of individual ETC complexes, reduce electron leakage and production of reactive oxygen species, and prevent excessive and random aggregation of proteins in the inner mitochondrial membrane. However, many other studies have challenged the proposed functional role of SCs. Recently, a third model known as the "plasticity" model was proposed that partly reconciles both "solid-state" and "fluid-state" models. According to the "plasticity" model, respiratory SCs can co-exist with the individual ETC complexes. To date, the physiological role of SCs remains unknown, although several studies using tissue samples of patients or animal/cell models of human diseases revealed an associative link between functional changes and the disintegration of SC assembly. This review summarizes and discusses previous studies on the mechanisms and regulation of SC assembly under physiological and pathological conditions.

Metabolic Alterations Caused by Defective Cardiolipin Remodeling in Inherited Cardiomyopathies

The heart is the most energy-consuming organ in the human body. In heart failure, the homeostasis of energy supply and demand is endangered by an increase in cardiomyocyte workload, or by an insufficiency in energy-providing processes. Energy metabolism is directly associated with mitochondrial redox homeostasis. The production of toxic reactive oxygen species (ROS) may overwhelm mitochondrial and cellular ROS defense mechanisms in case of heart failure. Mitochondria are essential cell organelles and provide 95% of the required energy in the heart. Metabolic remodeling, changes in mitochondrial structure or function, and alterations in mitochondrial calcium signaling diminish mitochondrial energy provision in many forms of cardiomyopathy. The mitochondrial respiratory chain creates a proton gradient across the inner mitochondrial membrane, which couples respiration with oxidative phosphorylation and the preservation of energy in the chemical bonds of ATP. Akin to other mitochondrial enzymes, the respiratory chain is integrated into the inner mitochondrial membrane. The tight association with the mitochondrial phospholipid cardiolipin (CL) ensures its structural integrity and coordinates enzymatic activity. This review focuses on how changes in mitochondrial CL may be associated with heart failure. Dysfunctional CL has been found in diabetic cardiomyopathy, ischemia reperfusion injury and the aging heart. Barth syndrome (BTHS) is caused by an inherited defect in the biosynthesis of cardiolipin. Moreover, a dysfunctional CL pool causes other types of rare inherited cardiomyopathies, such as Sengers syndrome and Dilated Cardiomyopathy with Ataxia (DCMA). Here we review the impact of cardiolipin deficiency on mitochondrial functions in cellular and animal models. We describe the molecular mechanisms concerning mitochondrial dysfunction as an incitement of cardiomyopathy and discuss potential therapeutic strategies.

Cardiolipin is required for membrane docking of mitochondrial ribosomes and protein synthesis

The mitochondrial inner membrane contains a unique phospholipid known as cardiolipin (CL), which stabilises the protein complexes embedded in the membrane and supports its overall structure. Recent evidence indicates that the mitochondrial ribosome may associate with the inner membrane to facilitate co-translational insertion of the hydrophobic oxidative phosphorylation (OXPHOS) proteins into the inner membrane. We generated three mutant knockout cell lines for the CL biosynthesis gene Crls1 to investigate the effects of CL loss on mitochondrial protein synthesis. Reduced CL levels caused altered mitochondrial morphology and transcriptome-wide changes that were accompanied by uncoordinated mitochondrial translation rates and impaired respiratory chain supercomplex formation. Aberrant protein synthesis was caused by impaired formation and distribution of mitochondrial ribosomes. Reduction or loss of CL resulted in divergent mitochondrial and endoplasmic reticulum stress responses. We show that CL is required to stabilise the interaction of the mitochondrial ribosome with the membrane via its association with OXA1 (also known as OXA1L) during active translation. This interaction facilitates insertion of newly synthesised mitochondrial proteins into the inner membrane and stabilises the respiratory supercomplexes.

Tafazzin deficiency impairs CoA-dependent oxidative metabolism in cardiac mitochondria

Barth syndrome is a mitochondrial myopathy resulting from mutations in the tafazzin (TAZ) gene encoding a phospholipid transacylase required for cardiolipin remodeling. Cardiolipin is a phospholipid of the inner mitochondrial membrane essential for the function of numerous mitochondrial proteins and processes. However, it is unclear how tafazzin deficiency impacts cardiac mitochondrial metabolism. To address this question while avoiding confounding effects of cardiomyopathy on mitochondrial phenotype, we utilized Taz-shRNA knockdown (Taz^KD ) mice, which exhibit defective cardiolipin remodeling and respiratory supercomplex instability characteristic of human Barth syndrome but normal cardiac function into adulthood. Consistent with previous reports from other models, mitochondrial H₂O₂ emission and oxidative damage were greater in Taz^KD than in wild-type (WT) hearts, but there were no differences in oxidative phosphorylation coupling efficiency or membrane potential. Fatty acid and pyruvate oxidation capacities were 40-60% lower in Taz^KD mitochondria, but an up-regulation of glutamate oxidation supported respiration rates approximating those with pyruvate and palmitoylcarnitine in WT. Deficiencies in mitochondrial CoA and shifts in the cardiac acyl-CoA profile paralleled changes in fatty acid oxidation enzymes and acyl-CoA thioesterases, suggesting limitations of CoA availability or "trapping" in Taz^KD mitochondrial metabolism. Incubation of Taz^KD mitochondria with exogenous CoA partially rescued pyruvate and palmitoylcarnitine oxidation capacities, implicating dysregulation of CoA-dependent intermediary metabolism rather than respiratory chain defects in the bioenergetic impacts of tafazzin deficiency. These findings support links among cardiolipin abnormalities, respiratory supercomplex instability, and mitochondrial oxidant production and shed new light on the distinct metabolic consequences of tafazzin deficiency in the mammalian heart.

Cardiolipin deficiency elevates susceptibility to a lipotoxic hypertrophic cardiomyopathy

Cardiolipin (CL) is a unique tetra-acyl phospholipid localized to the inner mitochondrial membrane and essential for normal respiratory function. It has been previously reported that the failing human heart and several rodent models of cardiac pathology have a selective loss of CL. A rare genetic disease, Barth syndrome (BTHS), is similarly characterized by a cardiomyopathy due to reduced levels of cardiolipin. A mouse model of cardiolipin deficiency was recently developed by knocking-down the cardiolipin biosynthetic enzyme tafazzin (TAZ KD). These mice develop an age-dependent cardiomyopathy due to mitochondrial dysfunction. Since reduced mitochondrial capacity in the heart may promote the accumulation of lipids, we examined whether cardiolipin deficiency in the TAZ KD mice promotes the development of a lipotoxic cardiomyopathy. In addition, we investigated whether treatment with resveratrol, a small cardioprotective nutraceutical, attenuated the aberrant lipid accumulation and associated cardiomyopathy. Mice deficient in tafazzin and the wildtype littermate controls were fed a low-fat diet, or a high-fat diet with or without resveratrol for 16 weeks. In the absence of obesity, TAZ KD mice developed a hypertrophic cardiomyopathy characterized by reduced left-ventricle (LV) volume (~36%) and 30-50% increases in isovolumetric contraction (IVCT) and relaxation times (IVRT). The progression of cardiac hypertrophy with tafazzin-deficiency was associated with several underlying pathological processes including altered mitochondrial complex I mediated respiration, elevated oxidative damage (~50% increase in reactive oxygen species, ROS), the accumulation of triglyceride (~250%) as well as lipids associated with lipotoxicity (diacylglyceride ~70%, free-cholesterol ~44%, ceramide N:16-35%) compared to the low-fat fed controls. Treatment of TAZ KD mice with resveratrol maintained normal LV volumes and preserved systolic function of the heart. The beneficial effect of resveratrol on cardiac function was accompanied by a significant improvement in mitochondrial respiration, ROS production and oxidative damage to the myocardium. Resveratrol treatment also attenuated the development of cardiac steatosis in tafazzin-deficient mice through reduced de novo fatty acid synthesis. These results indicate for the first time that cardiolipin deficiency promotes the development of a hypertrophic lipotoxic cardiomyopathy. Furthermore, we determined that dietary resveratrol attenuates the cardiomyopathy by reducing ROS, cardiac steatosis and maintaining mitochondrial function.

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.

AAV Gene Therapy Prevents and Reverses Heart Failure in a Murine Knockout Model of Barth Syndrome

RATIONALE

Barth syndrome is an X-linked cardiac and skeletal myopathy caused by mutation of the gene Tafazzin (TAZ). Currently, there is no targeted treatment for Barth syndrome. Lack of a proper genetic animal model that recapitulates the features of Barth syndrome has hindered understanding of disease pathogenesis and therapeutic development.

OBJECTIVE

We characterized murine germline TAZ knockout mice (TAZ-KO) and cardiomyocyte-specific TAZ knockout mice models and tested the efficacy of adeno-associated virus (AAV)-mediated gene replacement therapy with human TAZ (hTAZ).

METHODS AND RESULTS

TAZ-KO caused embryonic and neonatal lethality, impaired growth, dilated cardiomyopathy, and skeletal myopathy. TAZ-KO mice that survived the neonatal period developed progressive, severe cardiac dysfunction, and fibrosis. Cardiomyocyte-specific inactivation of floxed Taz in cardiomyocytes using Myh6-Cre caused progressive dilated cardiomyopathy without fetal or perinatal loss. Using both constitutive and conditional knockout models, we tested the efficacy and durability of Taz replacement by AAV gene therapy. Neonatal AAV-hTAZ rescued neonatal death, cardiac dysfunction, and fibrosis in TAZ-KO mice, and both prevented and reversed established cardiac dysfunction in TAZ-KO and cardiomyocyte-specific TAZ knockout mice models. However, both neonatal and adult therapies required high cardiomyocyte transduction (≈70%) for durable efficacy.

CONCLUSIONS

TAZ-KO and cardiomyocyte-specific TAZ knockout mice recapitulate many of the key clinical features of Barth syndrome. AAV-mediated gene replacement is efficacious when a sufficient fraction of cardiomyocytes are transduced.

Defective Mitochondrial Cardiolipin Remodeling Dampens HIF-1α Expression in Hypoxia

Mitochondria fulfill vital metabolic functions and act as crucial cellular signaling hubs, integrating their metabolic status into the cellular context. Here, we show that defective cardiolipin remodeling, upon loss of the cardiolipin acyl transferase tafazzin, decreases HIF-1α signaling in hypoxia. Tafazzin deficiency does not affect posttranslational HIF-1α regulation but rather HIF-1α gene expression, a dysfunction recapitulated in iPSC-derived cardiomyocytes from Barth syndrome patients with tafazzin deficiency. RNA-seq analyses confirmed drastically altered signaling in tafazzin mutant cells. In hypoxia, tafazzin-deficient cells display reduced production of reactive oxygen species (ROS) perturbing NF-κB activation and concomitantly HIF-1α gene expression. Tafazzin-deficient mice hearts display reduced HIF-1α levels and undergo maladaptive hypertrophy with heart failure in response to pressure overload challenge. We conclude that defective mitochondrial cardiolipin remodeling dampens HIF-1α signaling due to a lack of NF-κB activation through reduced mitochondrial ROS production, decreasing HIF-1α transcription.

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Defective remodeling of mitochondrial cardiolipin dampens HIF-1α signaling

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ROS-mediated NF-κB activation is impaired in cardiolipin-deficient cells

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Defective NF-κB-mediated HIF-1α gene induction decreases the cellular response to hypoxia

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Deregulated cardiac response to pressure overload in Barth syndrome mouse

Metabolic Alterations in Inherited Cardiomyopathies

The normal function of the heart relies on a series of complex metabolic processes orchestrating the proper generation and use of energy. In this context, mitochondria serve a crucial role as a platform for energy transduction by supplying ATP to the varying demand of cardiomyocytes, involving an intricate network of pathways regulating the metabolic flux of substrates. The failure of these processes results in structural and functional deficiencies of the cardiac muscle, including inherited cardiomyopathies. These genetic diseases are characterized by cardiac structural and functional anomalies in the absence of abnormal conditions that can explain the observed myocardial abnormality, and are frequently associated with heart failure. Since their original description, major advances have been achieved in the genetic and phenotype knowledge, highlighting the involvement of metabolic abnormalities in their pathogenesis. This review provides a brief overview of the role of mitochondria in the energy metabolism in the heart and focuses on metabolic abnormalities, mitochondrial dysfunction, and storage diseases associated with inherited cardiomyopathies.

Cardiolipin is essential for early embryonic viability and mitochondrial integrity of neurons in mammals

Cardiolipin (CL) is a hallmark phospholipid of mitochondria and plays a significant role in maintaining the mitochondrial structure and functions. Despite the physiological importance of CL, mutant organisms, yeast, Arabidopsis, C elegans, and Drosophila, which lack CL synthase (Crls1) gene and consequently are deprived of CL, are viable. Here we report conditional Crls1-deficient mice using targeted insertion of loxP sequences flanking the functional domain of CRLS1 enzyme. Homozygous null mutant mice exhibited early embryonic lethality at the peri-implantation stage. We generated neuron-specific Crls1 knockout (cKO) mice by crossing with Camk2α-Cre mice. Neuronal loss and gliosis were gradually manifested in the forebrains, where CL levels were significantly decreased. In the surviving neurons, malformed mitochondria with bubble-like or onion-like inner membrane structures were observed. We showed decreased supercomplex assembly and reduced enzymatic activities of electron transport chain complexes in the forebrain of cKO mice, resulting in affected mitochondrial calcium dynamics, a slower rate of Ca²⁺ uptake and a smaller calcium retention capacity. These observations clearly demonstrate indispensable roles of CL as well as of Crls1 gene in mammals.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

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.

Cardiac ageing: extrinsic and intrinsic factors in cellular renewal and senescence

Cardiac ageing manifests as a decline in function leading to heart failure. At the cellular level, ageing entails decreased replicative capacity and dysregulation of cellular processes in myocardial and nonmyocyte cells. Various extrinsic parameters, such as lifestyle and environment, integrate important signalling pathways, such as those involving inflammation and oxidative stress, with intrinsic molecular mechanisms underlying resistance versus progression to cellular senescence. Mitigation of cardiac functional decline in an ageing organism requires the activation of enhanced maintenance and reparative capacity, thereby overcoming inherent endogenous limitations to retaining a youthful phenotype. Deciphering the molecular mechanisms underlying dysregulation of cellular function and renewal reveals potential interventional targets to attenuate degenerative processes at the cellular and systemic levels to improve quality of life for our ageing population. In this Review, we discuss the roles of extrinsic and intrinsic factors in cardiac ageing. Animal models of cardiac ageing are summarized, followed by an overview of the current and possible future treatments to mitigate the deleterious effects of cardiac ageing.

Tetra-linoleoyl cardiolipin depletion plays a major role in the pathogenesis of sarcopenia

Sarcopenia, the progressive loss of muscle mass, strength, and physical performance that occurs during aging, is highly prevalent among the elderly. Sarcopenia increases the risk of falls, disability, and death. The biological basis for sarcopenia is not well understood. There are no specific preventive or therapeutic strategies for sarcopenia except exercise. The elucidation of biological pathways and identification of therapeutic targets for treating or preventing sarcopenia remain a high priority in aging research. Mitochondria play a critical role in skeletal muscle by providing energy in the form of ATP, regulation of signaling, calcium homeostasis, autophagy, and other functions. Cardiolipin, a unique dimeric phospholipid specific to mitochondria and an essential component of mitochondrial membranes, is involved in mitochondrial protein transport, maintaining structural organization of mitochondrial membranes, cellular signaling, regulating enzymes involved in β-oxidation of fatty acids, and facilitating normal electron transport chain (ETC) function and generation of ATP. The fatty acid species composition of cardiolipin is critical to mitochondrial bioenergetics, as cardiolipin affects membrane biophysical properties, binds and stabilizes ETC protein complexes, and shapes the curvature of the mitochondrial cristae. Tetra-linoleoyl cardiolipin (18:2)₄ comprises ∼80% of cardiolipin in mitochondria in normal human skeletal and cardiac muscle and is optimal for effective ETC function and ATP generation. Aging is associated with a decrease in cardiolipin content, decrease in tetra-linoleoyl cardiolipin (18:2)₄ and replacement of linoleic acid (18:2) with other fatty acids in cardiolipin composition, decline of ETC function, and increased generation of reactive oxygen species in muscle. Together, these findings from the literature prompt the hypothesis that depletion of the cardiolipin (18:2)₄ species may be at the root of mitochondrial dysfunction with aging, in turn leading to sarcopenia. Corroboration of the tetra-linoleoyl cardiolipin depletion hypothesis suggests new leads for the prevention and treatment of sarcopenia by enhancing the biosynthesis, accretion, and integrity of tetra-linoleoyl cardiolipin.

The Mitochondrial Transacylase, Tafazzin, Regulates for AML Stemness by Modulating Intracellular Levels of Phospholipids

Tafazzin (TAZ) is a mitochondrial transacylase that remodels the mitochondrial cardiolipin into its mature form. Through a CRISPR screen, we identified TAZ as necessary for the growth and viability of acute myeloid leukemia (AML) cells. Genetic inhibition of TAZ reduced stemness and increased differentiation of AML cells both in vitro and in vivo. In contrast, knockdown of TAZ did not impair normal hematopoiesis under basal conditions. Mechanistically, inhibition of TAZ decreased levels of cardiolipin but also altered global levels of intracellular phospholipids, including phosphatidylserine, which controlled AML stemness and differentiation by modulating toll-like receptor (TLR) signaling.

Mitochondrial dysfunctions in barth syndrome

Barth syndrome (BTHS) is a rare multisystemic genetic disorder caused by mutations in the TAZ gene. TAZ encodes a mitochondrial enzyme that remodels the acyl chain composition of newly synthesized cardiolipin, a phospholipid unique to mitochondrial membranes. The clinical abnormalities observed in BTHS patients are caused by perturbations in various mitochondrial functions that rely on remodeled cardiolipin. However, the contribution of different cardiolipin-dependent mitochondrial functions to the pathology of BTHS is not fully understood. In this review, we will discuss recent findings from different genetic models of BTHS, including the yeast model of cardiolipin deficiency that has uncovered the specific in vivo roles of cardiolipin in mitochondrial respiratory chain biogenesis, bioenergetics, intermediary metabolism, mitochondrial dynamics, and quality control. We will also describe findings from higher eukaryotic models of BTHS that highlight a link between cardiolipin-dependent mitochondrial function and its impact on tissue and organ function. © 2019 IUBMB Life, 9999(9999):1-11, 2019.

AAV9-TAZ Gene Replacement Ameliorates Cardiac TMT Proteomic Profiles in a Mouse Model of Barth Syndrome

Barth syndrome (BTHS) is a rare mitochondrial disease that causes severe cardiomyopathy and has no disease-modifying therapy. It is caused by recessive mutations in the gene tafazzin (TAZ), which encodes tafazzin-an acyltransferase that remodels the inner mitochondrial membrane lipid cardiolipin. To identify novel mechanistic pathways involved in BTHS and evaluate the effects of gene therapy on proteomic profiles, we performed a multiplex tandem mass tagging (TMT) quantitative proteomics analysis to compare protein expression profiles from heart lysates isolated from BTHS, healthy wild-type (WT), and BTHS treated with adeno-associated virus serotype 9 (AAV9)-TAZ gene replacement as neonates or adults. 197 proteins with ≥2 unique peptides were identified. Of these, 91 proteins were significantly differentially expressed in BTHS compared to WT controls. Cause-effect relationships between tafazzin deficiency and altered protein profiles were confirmed through demonstrated significant improvements in expression levels following administration of AAV9-TAZ. The importance of TMEM65 in Cx43 localization to cardiac intercalated discs was revealed as a novel consequence of tafazzin deficiency that was improved following gene therapy. This study identifies novel mechanistic pathways involved in the pathophysiology of BTHS, demonstrates the ability of gene delivery to improve protein expression profiles, and provides support for clinical translation of AAV9-TAZ gene therapy.

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.

The role of mitochondrial cardiolipin in heart function and its implication in cardiac disease

Mitochondria play an essential role in the energy metabolism of the heart. Many of the essential functions are associated with mitochondrial membranes and oxidative phosphorylation driven by the respiratory chain. Mitochondrial membranes are unique in the cell as they contain the phospholipid cardiolipin. The important role of cardiolipin in cardiovascular health is highlighted by several cardiac diseases, in which cardiolipin plays a fundamental role. Barth syndrome, Sengers syndrome, and Dilated cardiomyopathy with ataxia (DCMA) are genetic disorders, which affect cardiolipin biosynthesis. Other cardiovascular diseases including ischemia/reperfusion injury and heart failure are also associated with changes in the cardiolipin pool. Here, we summarize molecular functions of cardiolipin in mitochondrial biogenesis and morphology. We highlight the role of cardiolipin for the respiratory chain, metabolite carriers, and mitochondrial metabolism and describe links to apoptosis and mitochondria specific autophagy (mitophagy) with possible implications in cardiac disease.

Aberrant cardiolipin metabolism is associated with cognitive deficiency and hippocampal alteration in tafazzin knockdown mice

Cardiolipin (CL) is a key mitochondrial phospholipid essential for mitochondrial energy production. CL is remodeled from monolysocardiolipin (MLCL) by the enzyme tafazzin (TAZ). Loss-of-function mutations in the gene which encodes TAZ results in a rare X-linked disorder called Barth Syndrome (BTHS). The mutated TAZ is unable to maintain the physiological CL:MLCL ratio, thus reducing CL levels and affecting mitochondrial function. BTHS is best known as a cardiac disease, but has been acknowledged as a multi-syndrome disorder, including cognitive deficits. Since reduced CL levels has also been reported in numerous neurodegenerative disorders, we examined how TAZ-deficiency impacts cognitive abilities, brain mitochondrial respiration and the function of hippocampal neurons and glia in TAZ knockdown (TAZ kd) mice. We have identified for the first time the profile of changes that occur in brain phospholipid content and composition of TAZ kd mice. The brain of TAZ kd mice exhibited reduced TAZ protein expression, reduced total CL levels and a 19-fold accumulation of MLCL compared to wild-type littermate controls. TAZ kd brain exhibited a markedly distinct profile of CL and MLCL molecular species. In mitochondria, the activity of complex I was significantly elevated in the monomeric and supercomplex forms with TAZ-deficiency. This corresponded with elevated mitochondrial state I respiration and attenuated spare capacity. Furthermore, the production of reactive oxygen species was significantly elevated in TAZ kd brain mitochondria. While motor function remained normal in TAZ kd mice, they showed significant memory deficiency based on novel object recognition test. These results correlated with reduced synaptophysin protein levels and derangement of the neuronal CA1 layer in hippocampus. Finally, TAZ kd mice had elevated activation of brain immune cells, microglia compared to littermate controls. Collectively, our findings demonstrate that TAZ-mediated remodeling of CL contributes significantly to the expansive distribution of CL molecular species in the brain, plays a key role in mitochondria respiratory activity, maintains normal cognitive function, and identifies the hippocampus as a potential therapeutic target for BTHS.

Targeted overexpression of catalase to mitochondria does not prevent cardioskeletal myopathy in Barth syndrome

Barth Syndrome (BTHS) is an X-linked recessive disorder characterized by cardiomyopathy and muscle weakness. The underlying cause of BTHS is a mutation in the tafazzin (TAZ) gene, a key enzyme of cardiolipin biosynthesis. The lack of CL arising from loss of TAZ function results in destabilization of the electron transport system, promoting oxidative stress that is thought to contribute to development of cardioskeletal myopathy. Indeed, in vitro studies demonstrate that mitochondria-targeted antioxidants improve contractile capacity in TAZ-deficient cardiomyocytes. The purpose of the present study was to determine if resolving mitochondrial oxidative stress would be sufficient to prevent cardiomyopathy and skeletal myopathy in vivo using a mouse model of BTHS. To this end we crossed mice that overexpress catalase in the mitochondria (MCAT mice) with TAZ-deficient mice (TAZKD) to produce TAZKD mice that selectively overexpress catalase in the mitochondria (TAZKD+MCAT mice). TAZKD+MCAT mice exhibited decreased mitochondrial H₂O₂ emission and lipid peroxidation compared to TAZKD littermates, indicating decreased oxidative stress. Despite the improvements in oxidative stress, TAZKD+MCAT mice developed cardiomyopathy and mild muscle weakness similar to TAZKD littermates. These findings indicate that resolving oxidative stress is not sufficient to suppress cardioskeletal myopathy associated with BTHS.

Loss of tafazzin results in decreased myoblast differentiation in C2C12 cells: A myoblast model of Barth syndrome and cardiolipin deficiency

Barth syndrome (BTHS) is an X-linked genetic disorder resulting from mutations in the tafazzin gene (TAZ), which encodes the transacylase that remodels the mitochondrial phospholipid cardiolipin (CL). While most BTHS patients exhibit pronounced skeletal myopathy, the mechanisms linking defective CL remodeling and skeletal myopathy have not been determined. In this study, we constructed a CRISPR-generated stable tafazzin knockout (TAZ-KO) C2C12 myoblast cell line. TAZ-KO cells exhibit mitochondrial deficits consistent with other models of BTHS, including accumulation of monolyso-CL (MLCL), decreased mitochondrial respiration, and increased mitochondrial ROS production. Additionally, tafazzin deficiency was associated with impairment of myocyte differentiation. Future studies should determine whether alterations in myogenic determination contribute to the skeletal myopathy observed in BTHS patients. The BTHS myoblast model will enable studies to elucidate mechanisms by which defective CL remodeling interferes with normal myocyte differentiation and skeletal muscle ontogenesis.

The Effects of PPAR Stimulation on Cardiac Metabolic Pathways in Barth Syndrome Mice

Aim: Tafazzin knockdown (TazKD) in mice is widely used to create an experimental model of Barth syndrome (BTHS) that exhibits dilated cardiomyopathy and impaired exercise capacity. Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor proteins that play essential roles as transcription factors in the regulation of carbohydrate, lipid, and protein metabolism. We hypothesized that the activation of PPAR signaling with PPAR agonist bezafibrate (BF) may ameliorate impaired cardiac and skeletal muscle function in TazKD mice. This study examined the effects of BF on cardiac function, exercise capacity, and metabolic status in the heart of TazKD mice. Additionally, we elucidated the impact of PPAR activation on molecular pathways in TazKD hearts.

Methods: BF (0.05% w/w) was given to TazKD mice with rodent chow. Cardiac function in wild type-, TazKD-, and BF-treated TazKD mice was evaluated by echocardiography. Exercise capacity was evaluated by exercising mice on the treadmill until exhaustion. The impact of BF on metabolic pathways was evaluated by analyzing the total transcriptome of the heart by RNA sequencing.

Results: The uptake of BF during a 4-month period at a clinically relevant dose effectively protected the cardiac left ventricular systolic function in TazKD mice. BF alone did not improve the exercise capacity however, in combination with everyday voluntary running on the running wheel BF significantly ameliorated the impaired exercise capacity in TazKD mice. Analysis of cardiac transcriptome revealed that BF upregulated PPAR downstream target genes involved in a wide spectrum of metabolic (energy and protein) pathways as well as chromatin modification and RNA processing. In addition, the Ostn gene, which encodes the metabolic hormone musclin, is highly induced in TazKD myocardium and human failing hearts, likely as a compensatory response to diminished bioenergetic homeostasis in cardiomyocytes.

Conclusion: The PPAR agonist BF at a clinically relevant dose has the therapeutic potential to attenuate cardiac dysfunction, and possibly exercise intolerance in BTHS. The role of musclin in the failing heart should be further investigated.

Evaluation of cardiolipin nanodisks as lipid replacement therapy for Barth syndrome

Barth syndrome (BTHS) is a mitochondrial disorder characterized by cardiomyopathy and skeletal muscle weakness. Disease results from mutations in the tafazzin (TAZ) gene, encoding a phospholipid transacylase. Defective tafazzin activity results in an aberrant cardiolipin (CL) profile. The feasibility of restoring the intracellular CL profile was tested by in vivo administration of exogenous CL in nanodisk (ND) delivery particles. Ninety mg/kg CL (as ND) was administered to doxycycline-inducible taz shRNA knockdown (KD) mice once a week. After 10 weeks of CL-ND treatment, the mice were sacrificed and tissues harvested. Liquid chromatography-mass spectrometry of extracted lipids revealed that CL-ND administration failed to alter the CL profile of taz KD or WT mice. Thus, although CL-ND were previously shown to be an effective means of delivering CL to cultured cells, this effect does not extend to an in vivo setting. We conclude that CL-ND administration is not a suitable therapy option for BTHS.