Mitochondrial fatty acid biosynthesis and muscle fiber plasticity in very long-chain acyl-CoA dehydrogenase-deficient mice

A review of fatty acid oxidation disorder mouse models

Fatty acid oxidation disorders (FAODs) are a family of rare, genetic disorders that affect any part of the fatty acid oxidation pathway. Patients present with severe phenotypes, such as hypoketotic hypoglycemia, cardiomyopathy, and rhabdomyolysis, and currently manage these symptoms by the avoidance of fasting and maintaining a low-fat, high-carbohydrate diet. Because knowledge about FAODs is limited due to the small number of patients, rodent models have been crucial in learning more about these disorders, particularly in studying the molecular mechanisms involved in different phenotypes and in evaluating treatments for patients. The purpose of this review is to present the different FAOD mouse models and highlight the benefits and limitations of using these models. Specifically, we discuss the phenotypes of the available FAOD mouse models, the potential molecular causes of prominent FAOD phenotypes that have been studied using FAOD mouse models, and how FAOD mouse models have been used to evaluate treatments for patients.

Effect of skeletal muscle mitochondrial phenotype on H2O2 emission

Reactive oxygen species (ROS) are a key output of the skeletal muscle mitochondrial information processing system both at rest and during exercise. In skeletal muscle, mitochondrial ROS release depends on multiple factors; however, fiber-type specific differences remain ambiguous in part owing to the use of mitochondria from mammalian muscle that consist of mixed fibers. To elucidate fiber-type specific differences, we used mitochondria isolated from rainbow trout (Oncorhynchus mykiss) red and white skeletal muscles that consist of spatially distinct essentially pure red and white fibers. We first characterized the assay conditions for measuring ROS production (as H2O2) in isolated fish red and white skeletal muscle mitochondria (RMM and WMM) and thereafter compared the rates of emission during oxidation of different substrates and the responses to mitochondrial electron transport system (ETS) pharmacological modulators. Our results showed that H2O2 emission rates by RMM and WMM can be quantified using the same protein concentration and composition of the Amplex UltraRed-horseradish peroxidase (AUR-HRP) detection system. For both RMM and WMM, protein normalized H2O2 emission rates were highest at the lowest protein concentration tested and decreased exponentially thereafter. However, the absolute values of H2O2 emission rates depended on the calibration curves used to convert fluorescent signals to H2O2 while the trends depended on the normalization strategy. We found substantial qualitative and quantitative differences between RMM and WMM in the H2O2 emission rates depending on the substrates being oxidized and their concentrations. Similarly, pharmacological modulators of the ETS altered the magnitudes and trends of the H2O2 emission differently in RMM and WMM. While comparable concentrations of substrates elicited maximal albeit quantitively different emission rates in RMM and WMM, different concentrations of pharmacological ETS modulators may be required for maximal H2O2 emission rates depending on muscle fiber-type. Taken together, our study suggests that biochemical differences exist in RMM compared with WMM that alter substrate oxidation and responses to ETS modulators resulting in fiber-type specific mitochondrial H2O2 emission rates.

Exhaustive exercise alters native and site-specific H2O2 emission in red and white skeletal muscle mitochondria

Mitochondrial reactive oxygen species (ROS) homeostasis is intricately linked to energy conversion reactions and entails regulation of the mechanisms of ROS production and removal. However, there is limited understanding of how energy demand modulates ROS balance. Skeletal muscle experiences a wide range of energy requirements depending on the intensity and duration of exercise and therefore is an excellent model to probe the effect of altered energy demand on mitochondrial ROS production. Because in most fish skeletal muscle exists essentially as pure spatially distinct slow-twitch red oxidative and fast-twitch white glycolytic fibers, it provides a natural system for investigating how functional specialization affects ROS homeostasis. We tested the hypothesis that acute increase in energy demand imposed by exhaustive exercise will increase mitochondrial H2O2 emission to a greater extent in red muscle mitochondria (RMM) compared with white muscle mitochondria (WMM). We found that native H2O2 emission rates varied by up to 6-fold depending on the substrate being oxidized and muscle fiber type, with RMM emitting at higher rates with glutamate-malate and palmitoylcarnitine while WMM emitted at higher rates with succinate and glyceral-3-phosphate. Exhaustive exercise increased the native and site-specific H2O2 emission rates; however, the maximal emission rates depended on the substrate, fiber type and redox site. The H2O2 consumption capacity and activities of individual antioxidant enzymes including the glutathione- and thioredoxin-dependent peroxidases as well as catalase were higher in RMM compared with WMM indicating that the activity of antioxidant defense system does not explain the differences in H2O2 emission rates in RMM and WMM. Overall, our study suggests that substrate selection and oxidation may be the key factors determining the rates of ROS production in RMM and WMM following exhaustive exercise.

Use of dexamethasone in acute rhabdomyolysis in LPIN1 deficiency

Introduction

LPIN1 deficiency is an autosomal recessive form of early childhood recurrent severe rhabdomyolysis. Although not completely lucid yet, LPIN1 has been shown to modulate endosomal-related pro-inflammatory responses via peroxisome proliferator-activated receptor α (PPARα) and PPARγ coactivator 1α (PGC-1α). Treatment with anti-inflammatory agents such as dexamethasone has been proposed to improve the outcome.

Case

We report a male toddler with recurrent episodes of complicated rhabdomyolysis, requiring prolonged intensive care unit admissions. Whole exome sequencing revealed a common homozygous 1.7 kb intragenic deletion in LPIN1. Despite optimal metabolic cares, the patient presented with an extremely high CK level where he benefited from intravenous dexamethasone (0.6 mg/Kg/day) for a period of 6 days.

Results

Dexamethasone administration shortened the course of active rhabdomyolysis, intensive care admission and rehabilitation. It also prevented rhabdomyolysis-related complications such as kidney injury and compartment syndrome.

Conclusion

Our patient showed a favorable response to parenteral dexamethasone, in addition to hyperhydration with IV fluids, sufficient calorie intake, and restricted dietary fat. The improvement with corticosteroids suggests an uncontrolled inflammatory response as the pathophysiology of LPIN1 deficiency.

Calpain-mediated cleavage generates a ZBTB18 N-terminal product that regulates HIF1A signaling and glioblastoma metabolism

Proteolytic cleavage is an important post-translational mechanism to increase protein variability and functionality. In cancer, this process can be deregulated to shut off tumor-suppressive functions. Here, we report that in glioblastoma (GBM), the tumor suppressor ZBTB18 is targeted for protein cleavage by the intracellular protease calpain. The N-terminal (Nte) ZBTB18 cleaved fragment localizes to the cytoplasm and thus, is unable to exert the gene expression repressive function of the uncleaved protein. Mass spectrometry (MS) analysis indicates that the Nte ZBTB18 short form (SF) interacts with C-terminal (Cte) binding proteins 1 and 2 (CTBP1/2), which appear to be involved in HIF1A signaling activation. In fact, we show that the new ZBTB18 product activates HIF1A-regulated genes, which in turn lead to increased lipid uptake, lipid droplets (LD) accumulation, and enhanced metabolic activity. We propose that calpain-mediated ZBTB18 cleavage represents a new mechanism to counteract ZBTB18 tumor suppression and increase tumor-promoting functions in GBM cells.

Altered Metabolic Flexibility in Inherited Metabolic Diseases of Mitochondrial Fatty Acid Metabolism

In general, metabolic flexibility refers to an organism's capacity to adapt to metabolic changes due to differing energy demands. The aim of this work is to summarize and discuss recent findings regarding variables that modulate energy regulation in two different pathways of mitochondrial fatty metabolism: β-oxidation and fatty acid biosynthesis. We focus specifically on two diseases: very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and malonyl-CoA synthetase deficiency (acyl-CoA synthetase family member 3 (ACSF3)) deficiency, which are both characterized by alterations in metabolic flexibility. On the one hand, in a mouse model of VLCAD-deficient (VLCAD^-/-) mice, the white skeletal muscle undergoes metabolic and morphologic transdifferentiation towards glycolytic muscle fiber types via the up-regulation of mitochondrial fatty acid biosynthesis (mtFAS). On the other hand, in ACSF3-deficient patients, fibroblasts show impaired mitochondrial respiration, reduced lipoylation, and reduced glycolytic flux, which are compensated for by an increased β-oxidation rate and the use of anaplerotic amino acids to address the energy needs. Here, we discuss a possible co-regulation by mtFAS and β-oxidation in the maintenance of energy homeostasis.

Exploring the lncRNAs Related to Skeletal Muscle Fiber Types and Meat Quality Traits in Pigs

The alteration in skeletal muscle fiber is a critical factor affecting livestock meat quality traits and human metabolic diseases. Long non-coding RNAs (lncRNAs) are a diverse class of non-coding RNAs with a length of more than 200 nucleotides. However, the mechanisms underlying the regulation of lncRNAs in skeletal muscle fibers remain elusive. To understand the genetic basis of lncRNA-regulated skeletal muscle fiber development, we performed a transcriptome analysis to identify the key lncRNAs affecting skeletal muscle fiber and meat quality traits on a pig model. We generated the lncRNA expression profiles of fast-twitch Biceps femoris (Bf) and slow-twitch Soleus (Sol) muscles and identified the differentially expressed (DE) lncRNAs using RNA-seq and performed bioinformatics analyses. This allowed us to identify 4581 lncRNA genes among six RNA libraries and 92 DE lncRNAs between Bf and Sol which are the key candidates for the conversion of skeletal muscle fiber types. Moreover, we detected the expression patterns of lncRNA MSTRG.42019 in different tissues and skeletal muscles of various development stages. In addition, we performed a correlation analyses between the expression of DE lncRNA MSTRG.42019 and meat quality traits. Notably, we found that DE lncRNA MSTRG.42019 was highly expressed in skeletal muscle and its expression was significantly higher in Sol than in Bf, with a positive correlation with the expression of Myosin heavy chain 7 (MYH7) (r = 0.6597, p = 0.0016) and a negative correlation with meat quality traits glycolytic potential (r = −0.5447, p = 0.0130), as well as drip loss (r = −0.5085, p = 0.0221). Moreover, we constructed the lncRNA MSTRG.42019–mRNAs regulatory network for a better understanding of a possible mechanism regulating skeletal muscle fiber formation. Our data provide the groundwork for studying the lncRNA regulatory mechanisms of skeletal muscle fiber conversion, and given the importance of skeletal muscle fiber types in muscle-related diseases, our data may provide insight into the treatment of muscular diseases in humans.

Nutritional ketosis improves exercise metabolism in patients with very long-chain acyl-CoA dehydrogenase deficiency

A maladaptive shift from fat to carbohydrate (CHO) oxidation during exercise is thought to underlie myopathy and exercise-induced rhabdomyolysis in patients with fatty acid oxidation (FAO) disorders. We hypothesised that ingestion of a ketone ester (KE) drink prior to exercise could serve as an alternative oxidative substrate supply to boost muscular ATP homeostasis. To establish a rational basis for therapeutic use of KE supplementation in FAO, we tested this hypothesis in patients deficient in Very Long-Chain acyl-CoA Dehydrogenase (VLCAD). Five patients (range 17-45 y; 4 M/1F) patients were included in an investigator-initiated, randomised, blinded, placebo-controlled, 2-way cross-over study. Patients drank either a KE + CHO mix or an isocaloric CHO equivalent and performed 35 minutes upright cycling followed by 10 minutes supine cycling inside a Magnetic Resonance scanner at individual maximal FAO work rate (fatmax; approximately 40% VO₂ max). The protocol was repeated after a 1-week interval with the alternate drink. Primary outcome measures were quadriceps phosphocreatine (PCr), Pi and pH dynamics during exercise and recovery assayed by in vivo ³¹ P-MR spectroscopy. Secondary outcomes included plasma and muscle metabolites and respiratory gas exchange recordings. Ingestion of KE rapidly induced mild ketosis and increased muscle BHB content. During exercise at FATMAX, VLCADD-specific plasma acylcarnitine levels, quadriceps glycolytic intermediate levels and in vivo Pi/PCr ratio were all lower in KE + CHO than CHO. These results provide a rational basis for future clinical trials of synthetic ketone ester supplementation therapy in patients with FAO disorders. Trial registration: ClinicalTrials.gov. Protocol ID: NCT03531554; METC2014.492; ABR51222.042.14.

Therapeutic potential of triheptanoin in metabolic and neurodegenerative diseases

In the past 15 years the potential of triheptanoin for the treatment of several human diseases in the area of clinical nutrition has grown considerably. Use of this triglyceride of the odd-chain fatty acid heptanoate has been proposed and applied for the treatment of several conditions in which the energy supply from citric acid cycle intermediates or fatty acid degradation are impaired. Neurological diseases due to disturbed glucose metabolism or metabolic diseases associated with impaired β-oxidation of long chain fatty acid may especially take advantage of alternative substrate sources offered by the secondary metabolites of triheptanoin. Epilepsy due to deficiency of the GLUT1 transporter, as well as diseases associated with dysregulation of neuronal signalling, have been treated with triheptanoin supplementation, and very recently the advantages of this oil in long-chain fatty acid oxidation disorders have been reported. The present review summarises the published literature on the metabolism of triheptanoin including clinical reports related to the use of triheptanoin.

Disturbance of mitochondrial functions associated with permeability transition pore opening induced by cis-5-tetradecenoic and myristic acids in liver of adolescent rats

Patients affected by very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency commonly present liver dysfunction whose pathogenesis is poorly known. We demonstrate here that major metabolites accumulating in this disorder, namely cis-5-tetradecenoic acid (Cis-5) and myristic acid (Myr), markedly impair mitochondrial respiration, decreasing ATP production in liver mitochondrial preparations from adolescent rats. Other parameters of mitochondrial homeostasis such as membrane potential (ΔΨm) and Ca²⁺retention capacity were strongly compromised by these fatty acids, involving induction of mitochondrial permeability transition. The present data indicate that disruption of mitochondrial bioenergetics and Ca²⁺homeostasis may contribute to the liver dysfunction of VLCAD deficient patients.

Proteomic Phosphosite Analysis Identified Crucial NPM-ALK-Mediated NIPA Serine and Threonine Residues

Anaplastic large-cell lymphoma (ALCL) is an aggressive non-Hodgkin lymphoma that shows in 60% of cases a translocation t(2;5)(p23;q35), which leads to the expression of the oncogenic kinase NPM-ALK. The nuclear interaction partner of ALK (NIPA) defines an E3-SCF ligase that contributes to the timing of mitotic entry. It has been shown that co-expression of NIPA and NPM-ALK results in constitutive NIPA phosphorylation. By mass spectrometry-based proteomics we identified nine serine/threonine residues to be significantly upregulated in NIPA upon NPM-ALK expression. Generation of phospho-deficient mutants of the respective phospho-residues specified five serine/threonine residues (Ser-338, Ser-344, Ser-370, Ser-381 and Thr-387) as key phosphorylation sites involved in NPM-ALK-directed phosphorylation of NIPA. Analysis of the biological impact of NIPA phosphorylation by NPM-ALK demonstrated that the ALK-induced phosphorylation does not change the SCF^NIPA-complex formation but may influence the localization of NIPA and NPM-ALK. Biochemical analyses with phospho-deficient mutants elucidated the importance of NIPA phosphorylation by NPM-ALK for the interaction of the two proteins and proliferation potential of respective cells: Silencing of the five crucial NIPA serine/threonine residues led to a highly enhanced NIPA-NPM-ALK binding capacity as well as a slightly reduced proliferation in Ba/F3 cells.

The fate of medium-chain fatty acids in very long-chain acyl‑CoA dehydrogenase deficiency (VLCADD): A matter of sex?

Medium-chain-triglycerides (MCT) are widely applied in the treatment of long-chain fatty acid oxidation disorders (lcFAOD). Long-term treatment with MCT led to a sexually dimorphic response in the mouse model of very-long-chain-acyl-CoA-dehydrogenase-deficiency (VLCAD^-/-) with the subsequent development of a metabolic syndrome in female mice. In order to evaluate the molecular mechanisms responsible for this sex specific response we performed a comprehensive metabolic phenotyping, SILAC-based quantitative proteomics and characterized the involved signaling pathways by western blot analysis and gene expression. WT and VLCAD^-/- mice showed strong sex-dependent differences in basal metabolism and expression of proteins involved in the distinct metabolic pathways, even more prominent after treatment with octanoate. The investigation of molecular mechanisms responsible for the sexual dimorphisms delineated the selective activation of the ERK/mTORc1 signaling pathway leading to an increased biosynthesis and elongation of fatty acids in VLCAD^-/- females. In contrast, octanoate induced the activation of ERK/PPARγ pathway and the subsequent upregulation of peroxisomal β‑oxidation in males. We here provide first evidence that sex has to be considered as important variable in disease phenotype. These findings may have implications on treatment strategies in the different sexes.

Defective fatty acid oxidation in mice with muscle-specific acyl-CoA synthetase 1 deficiency increases amino acid use and impairs muscle function

Loss of long-chain acyl-CoA synthetase isoform-1 (ACSL1) in mouse skeletal muscle (Acsl1^{M -/-}) severely reduces acyl-CoA synthetase activity and fatty acid oxidation. However, the effects of decreased fatty acid oxidation on skeletal muscle function, histology, use of alternative fuels, and mitochondrial function and morphology are unclear. We observed that Acsl1^{M -/-} mice have impaired voluntary running capacity and muscle grip strength and that their gastrocnemius muscle contains myocytes with central nuclei, indicating muscle regeneration. We also found that plasma creatine kinase and aspartate aminotransferase levels in Acsl1^{M -/-} mice are 3.4- and 1.5-fold greater, respectively, than in control mice (Acsl1^flox/flox ), indicating muscle damage, even without exercise, in the Acsl1^{M -/-} mice. Moreover, caspase-3 protein expression exclusively in Acsl1^{M -/-} skeletal muscle and the presence of cleaved caspase-3 suggested myocyte apoptosis. Mitochondria in Acsl1^{M -/-} skeletal muscle were swollen with abnormal cristae, and mitochondrial biogenesis was increased. Glucose uptake did not increase in Acsl1^{M -/-} skeletal muscle, and pyruvate oxidation was similar in gastrocnemius homogenates from Acsl1^{M -/-} and control mice. The rate of protein synthesis in Acsl1^{M -/-} glycolytic muscle was 2.1-fold greater 30 min after exercise than in the controls, suggesting resynthesis of proteins catabolized for fuel during the exercise. At this time, mTOR complex 1 was activated, and autophagy was blocked. These results suggest that fatty acid oxidation is critical for normal skeletal muscle homeostasis during both rest and exercise. We conclude that ACSL1 deficiency produces an overall defect in muscle fuel metabolism that increases protein catabolism, resulting in exercise intolerance, muscle weakness, and myocyte apoptosis.