The promiscuous enzyme medium-chain 3-keto-acyl-CoA thiolase triggers a vicious cycle in fatty-acid beta-oxidation

The paradox of fatty-acid β-oxidation in muscle insulin resistance: Metabolic control and muscle heterogeneity

The skeletal muscle is a metabolically heterogeneous tissue that plays a key role in maintaining whole-body glucose homeostasis. It is well known that muscle insulin resistance (IR) precedes the development of type 2 diabetes. There is a consensus that the accumulation of specific lipid species in the tissue can drive IR. However, the role of the mitochondrial fatty-acid β-oxidation in IR and, consequently, in the control of glucose uptake remains paradoxical: interventions that either inhibit or activate fatty-acid β-oxidation have been shown to prevent IR. We here discuss the current theories and evidence for the interplay between β-oxidation and glucose uptake in IR. To address the underlying intricacies, we (1) dive into the control of glucose uptake fluxes into muscle tissues using the framework of Metabolic Control Analysis, and (2) disentangle concepts of flux and catalytic capacities taking into account skeletal muscle heterogeneity. Finally, we speculate about hitherto unexplored mechanisms that could bring contrasting evidence together. Elucidating how β-oxidation is connected to muscle IR and the underlying role of muscle heterogeneity enhances disease understanding and paves the way for new treatments for type 2 diabetes.

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

Fifty years of research on mitochondrial fatty acid oxidation disorders: The remaining challenges

Since the identification of the first disorder of mitochondrial fatty acid oxidation defects (FAOD) in 1973, more than 20 defects have been identified. Although there are some differences, most FAOD have similar clinical signs, which are mainly due to energy depletion and toxicity of accumulated metabolites. However, some of them have an unusual clinical phenotype or specific clinical signs. This manuscript focuses on what we have learnt so far on the pathophysiology of these disorders, which present with clinical signs that are not typical of categorical FAOD. It also highlights that some disorders have not yet been identified and tries to make assumptions to explain why. It also deals with new treatments under consideration in FAOD, including triheptanoin and similar anaplerotic substrates, ketone body treatments, RNA and gene therapy approaches. Finally, it suggests challenges for the diagnosis of FAOD in the coming years, both for symptomatic patients and for those diagnosed through newborn screening. The ultimate goal would be to identify all the patients born with FAOD and ensure for them the best possible quality of life.

Acyl-CoA thioesterase-2 facilitates β-oxidation in glycolytic skeletal muscle in a lipid supply dependent manner

Acyl-Coenzyme A (acyl-CoA) thioesters are compartmentalized intermediates that participate in in multiple metabolic reactions within the mitochondrial matrix. The limited availability of free CoA (CoASH) in the matrix raises the question of how the local acyl-CoA concentration is regulated to prevent trapping of CoASH from overload of any specific substrate. Acyl-CoA thioesterase-2 (ACOT2) hydrolyzes long-chain acyl-CoAs to their constituent fatty acids and CoASH, and is the only mitochondrial matrix ACOT refractory to inhibition by CoASH. Thus, we reasoned that ACOT2 may constitutively regulate matrix acyl-CoA levels. Acot2 deletion in murine skeletal muscle (SM) resulted in acyl-CoA build-up when lipid supply and energy demands were modest. When energy demand and pyruvate availability were elevated, lack of ACOT2 activity promoted glucose oxidation. This preference for glucose over fatty acid oxidation was recapitulated in C2C12 myotubes with acute depletion of Acot2 , and overt inhibition of β-oxidation was demonstrated in isolated mitochondria from Acot2 -depleted glycolytic SM. In mice fed a high fat diet, ACOT2 enabled the accretion of acyl-CoAs and ceramide derivatives in glycolytic SM, and this was associated with worse glucose homeostasis compared to when ACOT2 was absent. These observations suggest that ACOT2 supports CoASH availability to facilitate β-oxidation in glycolytic SM when lipid supply is modest. However, when lipid supply is high, ACOT2 enables acyl-CoA and lipid accumulation, CoASH sequestration, and poor glucose homeostasis. Thus, ACOT2 regulates matrix acyl-CoA concentration in glycolytic muscle, and its impact depends on lipid supply.

Pyruvate-supported flux through medium-chain ketothiolase promotes mitochondrial lipid tolerance in cardiac and skeletal muscles

Even-chain acylcarnitine (AC) metabolites, most of which are generated as byproducts of incomplete fatty acid oxidation (FAO), are viewed as biomarkers of mitochondrial lipid stress attributable to one or more metabolic bottlenecks in the β-oxidation pathway. The origins and functional implications of FAO bottlenecks remain poorly understood. Here, we combined a sophisticated mitochondrial phenotyping platform with state-of-the-art molecular profiling tools and multiple two-state mouse models of respiratory function to uncover a mechanism that connects AC accumulation to lipid intolerance, metabolic inflexibility, and respiratory inefficiency in skeletal muscle mitochondria. These studies also identified a short-chain carbon circuit at the C4 node of FAO wherein reverse flux of glucose-derived acetyl CoA through medium-chain ketothiolase enhances lipid tolerance and redox stability in heart mitochondria by regenerating free CoA and NAD⁺. The findings help to explain why diminished FAO capacity, AC accumulation, and metabolic inflexibility are tightly linked to poor health outcomes.

Nicotinamide riboside kinase-2 regulates metabolic adaptation in the ischemic heart

Ischemia-induced metabolic remodeling plays a critical role in the pathogenesis of adverse cardiac remodeling and heart failure however, the underlying molecular mechanism is largely unknown. Here, we assess the potential roles of nicotinamide riboside kinase-2 (NRK-2), a muscle-specific protein, in ischemia-induced metabolic switch and heart failure through employing transcriptomic and metabolomic approaches in ischemic NRK-2 knockout mice. The investigations revealed NRK-2 as a novel regulator of several metabolic processes in the ischemic heart. Cardiac metabolism and mitochondrial function and fibrosis were identified as top dysregulated cellular processes in the KO hearts post-MI. Several genes linked to mitochondrial function, metabolism, and cardiomyocyte structural proteins were severely downregulated in the ischemic NRK-2 KO hearts. Analysis revealed significantly upregulated ECM-related pathways which was accompanied by the upregulation of several key cell signaling pathways including SMAD, MAPK, cGMP, integrin, and Akt in the KO heart post-MI. Metabolomic studies identified profound upregulation of metabolites mevalonic acid, 3,4-dihydroxyphenylglycol, 2-penylbutyric acid, and uridine. However, other metabolites stearic acid, 8,11,14-eicosatrienoic acid, and 2-pyrrolidinone were significantly downregulated in the ischemic KO hearts. Taken together, these findings suggest that NRK-2 promotes metabolic adaptation in the ischemic heart. The aberrant metabolism in the ischemic NRK-2 KO heart is largely driven by dysregulated cGMP and Akt and mitochondrial pathways. KEY MESSAGES: Post-myocardial infarction metabolic switch critically regulates the pathogenesis of adverse cardiac remodeling and heart failure. Here, we report NRK-2 as a novel regulator of several cellular processes including metabolism and mitochondrial function post-MI. NRK-2 deficiency leads to downregulation of genes important for mitochondrial pathway, metabolism, and cardiomyocyte structural proteins in the ischemic heart. It was accompanied by upregulation of several key cell signaling pathways including SMAD, MAPK, cGMP, integrin, and Akt and dysregulation of numerous metabolites essential for cardiac bioenergetics. Taken together, these findings suggest that NRK-2 is critical for metabolic adaptation of the ischemic heart.

Odd- and even-numbered medium-chained fatty acids protect against glutathione depletion in very long-chain acyl-CoA dehydrogenase deficiency

Recent trials have reported the ability of triheptanoin to improve clinical outcomes for the severe symptoms associated with long-chain fatty acid oxidation disorders, including very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. However, the milder myopathic symptoms are still challenging to treat satisfactorily. Myopathic pathogenesis is multifactorial, but oxidative stress is an important component. We have previously shown that metabolic stress increases the oxidative burden in VLCAD-deficient cell lines and can deplete the antioxidant glutathione (GSH). We investigated whether medium-chain fatty acids provide protection against GSH depletion during metabolic stress in VLCAD-deficient fibroblasts. To investigate the effect of differences in anaplerotic capacity, we included both even-(octanoate) and odd-numbered (heptanoate) medium-chain fatty acids. Overall, we show that modulation of the concentration of medium-chain fatty acids in culture media affects levels of GSH retained during metabolic stress in VLCAD-deficient cell lines but not in controls. Lowered glutamine concentration in the culture media during metabolic stress led to GSH depletion and decreased viability in VLCAD deficient cells, which could be rescued by both heptanoate and octanoate in a dose-dependent manner. Unlike GSH levels, the levels of total thiols increased after metabolic stress exposure, the size of this increase was not affected by differences in cell culture medium concentrations of glutamine, heptanoate or octanoate. Addition of a PPAR agonist further exacerbated stress-related GSH-depletion and viability loss, requiring higher concentrations of fatty acids to restore GSH levels and cell viability. Both odd- and even-numbered medium-chain fatty acids efficiently protect VLCADdeficient cells against metabolic stress-induced antioxidant depletion.

Liver immune and lipid metabolism disorders in mice induced by triphenyl phosphate with or without high fructose and high fat diet

Organophosphorus flame retardants (OPFRs) are frequently detected in food and human samples, and epidemiological studies have found that human exposure to aryl-OPFRs (triphenyl phosphate, TPP) is associated with lipid metabolism. Although toxicity studies suggest a potential obesity risk from TPP exposure, the molecular mechanism remains unclear. This study investigated the subchronic dietary effects on mouse liver significantly changed proteins (SCPs) and elucidated the underlying molecular mechanisms of TPP with or without a high-fructose and high-fat (HFF) diet. Male C57BL/6J mice were exposed to low-dose TPP (corresponding to the oral reference dose, 10 μg/kg body weight (bw)/day) and high-dose TPP (1000 μg/kg bw/day) for 12 weeks. The results showed that exposure to TPP generated changes of liver function and organelle damage as well as increases in total cholesterol and triglyceride levels. TPP exposure at a low dose damaged the liver immune system via major histocompatibility complex-related proteins involved in antigen processing and presentation. TPP exposure at a high dose caused disorders of the biosynthesis of unsaturated fatty acids and steroid hormones, thereby inducing lipid accumulation in the liver. Although 10 μg/kg TPP did not cause serious lipid metabolism disorders in the liver, significant overexpression of fatty acid-binding protein 5, malic enzyme 1, and other related SCPs was observed, which led to disorders of cholesterol metabolism and lipogenesis to activate the proliferator-activated receptor signaling pathway and thus induced potential obesity risks. In addition, lipid metabolism disorders related to TPP were aggravated under the HFF diet, impairing liver mitochondrial and endoplasmic reticulum function in mice by altering the activity of cytochrome P450 enzyme subfamilies. These findings provide an in-depth understanding of the molecular toxicity mechanisms and health risks associated with subchronic exposure to TPP under different dietary regimes.

ACAA2 is a ligand-dependent coactivator for thyroid hormone receptor β1

Thyroid hormones (THs) play a critical role in the metabolic phenotype of the heart; and most of the effects involve transcriptional regulation via thyroid hormone receptors (TRs). TRs ability to form combinatorial complexes with an array of partners accounts for TRs physiological flexibility in modulating gene expression. To identify proteins that associate with TRβ1 in the heart we performed a pull-down assay on cardiac tissue using GST-TRβ1 as bait and identified the bound proteins by LC MS/MS. ACAA2, a mitochondrial thiolase enzyme, was identified as a novel interacting protein. We confirmed ACAA2 localized to the nucleus and using a luciferase reporter assay showed ACAA2 acted as a TH-dependent coactivator for TRβ1. ACAA2 showed an ability to bind to TR recognition sequences but did not alter TRβ1 DNA binding ability. Thus, ACAA2 as a novel TRβ1 associating protein opens a new paradigm to understanding how TH/TRs may be manipulated by energetic pathway molecules.

Age-related susceptibility to insulin resistance arises from a combination of CPT1B decline and lipid overload

BACKGROUND

The skeletal muscle plays a central role in glucose homeostasis through the uptake of glucose from the extracellular medium in response to insulin. A number of factors are known to disrupt the normal response to insulin leading to the emergence of insulin resistance (IR). Advanced age and a high-fat diet are factors that increase the susceptibility to IR, with lipid accumulation in the skeletal muscle being a key driver of this phenomenon. It is debated, however, whether lipid accumulation arises due to dietary lipid overload or from a decline of mitochondrial function. To gain insights into the interplay of diet and age in the flexibility of muscle lipid and glucose handling, we combined lipidomics, proteomics, mitochondrial function analysis and computational modelling to investigate young and aged mice on a low- or high-fat diet (HFD).

RESULTS

As expected, aged mice were more susceptible to IR when given a HFD than young mice. The HFD induced intramuscular lipid accumulation specifically in aged mice, including C18:0-containing ceramides and diacylglycerols. This was reflected by the mitochondrial β-oxidation capacity, which was upregulated by the HFD in young, but not in old mice. Conspicuously, most β-oxidation proteins were upregulated by the HFD in both groups, but carnitine palmitoyltransferase 1B (CPT1B) declined in aged animals. Computational modelling traced the flux control mostly to CPT1B, suggesting a CPT1B-driven loss of flexibility to the HFD with age. Finally, in old animals, glycolytic protein levels were reduced and less flexible to the diet.

CONCLUSION

We conclude that intramuscular lipid accumulation and decreased insulin sensitivity are not due to age-related mitochondrial dysfunction or nutritional overload alone, but rather to their combined effects. Moreover, we identify CPT1B as a potential target to counteract age-dependent intramuscular lipid accumulation and thereby IR.

Bezafibrate activation of PPAR drives disturbances in mitochondrial redox bioenergetics and decreases the viability of cells from patients with VLCAD deficiency

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is the most common inborn long-chain fatty acid oxidation (FAO) disorder. VLCAD deficiency is characterized by distinct phenotypes. The severe phenotypes are potentially life-threatening and affect the heart or liver, with a comparatively milder phenotype characterized by myopathic symptoms. There is an unmet clinical need for effective treatment options for the myopathic phenotype. The molecular mechanisms driving the gradual decrease in mitochondrial function and associated alterations of muscle fibers are unclear. The peroxisome proliferator-activated receptor (PPAR) pan-agonist bezafibrate is a potent modulator of FAO and multiple other mitochondrial functions and has been proposed as a potential medication for myopathic cases of long-chain FAO disorders. In vitro experiments have demonstrated the ability of bezafibrate to increase VLCAD expression and activity. However, the outcome of small-scale clinical trials has been controversial. We found VLCAD deficient patient fibroblasts to have an increased oxidative stress burden and deranged mitochondrial bioenergetic capacity, compared to controls. Applying heat stress under fasting conditions to bezafibrate pretreated patient cells, caused a marked further increase of mitochondrial superoxide levels. Patient cells failed to maintain levels of the essential thiol peptide antioxidant glutathione and experienced a decrease in cellular viability. Our findings indicate that chronic PPAR activation is a plausible initiator of long-term pathogenesis in VLCAD deficiency. Our findings further implicate disruption of redox homeostasis as a key pathogenic mechanism in VLCAD deficiency and support the notion that a deranged thiol metabolism might be an important pathogenic factor in VLCAD deficiency.

Mitochondrial Homeostasis Mediates Lipotoxicity in the Failing Myocardium

Heart failure remains the most common cause of death in the industrialized world. In spite of new therapeutic interventions that are constantly being developed, it is still not possible to completely protect against heart failure development and progression. This shows how much more research is necessary to understand the underlying mechanisms of this process. In this review, we give a detailed overview of the contribution of impaired mitochondrial dynamics and energy homeostasis during heart failure progression. In particular, we focus on the regulation of fatty acid metabolism and the effects of fatty acid accumulation on mitochondrial structural and functional homeostasis.

Valproic acid affects fatty acid and triglyceride metabolism in HepaRG cells exposed to fatty acids by different mechanisms

Treatment with valproate is associated with hepatic steatosis, but the mechanisms are not fully elucidated in human cell systems. We therefore investigated the effects of valproate on fatty acid and triglyceride metabolism in HepaRG cells, a human hepatoma cell line. In previously fatty acid loaded HepaRG cells, valproate impaired lipid droplet disposal starting at 1 mM after incubation for 3 or 7 days. Valproate increased the expression of genes associated with fatty acid import and triglyceride synthesis, but did not relevantly affect expression of genes engaged in fatty acid activation. Valproate impaired mitochondrial fatty acid metabolism by inhibiting β-ketothiolase and the function of the electron transport chain, which was associated with increased mitochondrial reactive oxygen species production. Valproate increased the mitochondrial DNA copy number per HepaRG cell, possibly as a consequence of impaired mitochondrial function. Valproate decreased the hepatocellular mRNA and protein expression of the fatty acid binding protein 1 (FABP1) and of the microsomal triglyceride transfer protein (MTTP) at 1 mM and increased the hepatocellular concentration of free fatty acids. Furthermore, valproate decreased protein expression and excretion of ApoB100 in HepaRG cells at 1 mM, reflecting impaired formation and excretion of very low-density lipoprotein (VLDL). In conclusion, valproate increased the hepatocellular triglyceride content by multiple mechanisms, whereby impaired expression of FABP1 and MTTP as well as impaired VLDL formation and excretion appeared to be dominant. Valproate caused cell death mainly by apoptosis, which may be a consequence of mitochondrial oxidative stress and increased hepatocellular concentration of free fatty acids.

Calcium overload decreases net free radical emission in cardiac mitochondria

Elevated calcium and reactive oxygen species (ROS) are responsible for the bulk of cell death occurring in a variety of clinical settings that include acute coronary events, cerebrovascular accidents, and acute kidney injury. It is commonly believed that calcium and ROS participate in a viscous cycle during these events. However, the precise feedback mechanisms are unknown. We quantitatively demonstrate in this study that, on the contrary, calcium does not stimulate free radical production but suppresses it. Isolated mitochondria from guinea pig hearts were energized with a variety of substrates and exposed to calcium concentrations designed to induce moderate calcium overload conditions associated with ischemia/reperfusion injury but do not elicit the well-known mitochondrial permeability transition phenomenon. Metabolic function and free radical emission were simultaneously quantified using high-resolution respirometry and fluorimetry. Membrane potential, high amplitude swelling, and calcium dynamics were also quantified in parallel. Our results reveal that calcium overload does not lead to excessive ROS emission but does decrease ADP stimulated respiration rates for NADH-dependent pathways. Moreover, we developed an empirical model of mitochondrial free radical homeostasis to identify the processes that are different for each substrate and calcium condition. In summary, we show that in healthy guinea pig mitochondria, calcium uptake and free radical generation do not contribute to a viscous cycle and that the relationship between net free radical production and oxygen concentration is hyperbolic. Altogether, these results lay out an important foundation necessary to quantitatively determine the role of calcium in IR injury and ROS production.

Disruption of Acetyl-Lysine Turnover in Muscle Mitochondria Promotes Insulin Resistance and Redox Stress without Overt Respiratory Dysfunction

This study sought to examine the functional significance of mitochondrial protein acetylation using a double knockout (DKO) mouse model harboring muscle-specific deficits in acetyl-CoA buffering and lysine deacetylation, due to genetic ablation of carnitine acetyltransferase and Sirtuin 3, respectively. DKO mice are highly susceptible to extreme hyperacetylation of the mitochondrial proteome and develop a more severe form of diet-induced insulin resistance than either single KO mouse line. However, the functional phenotype of hyperacetylated DKO mitochondria is largely normal. Of the >120 measures of respiratory function assayed, the most consistently observed traits of a markedly heightened acetyl-lysine landscape are enhanced oxygen flux in the context of fatty acid fuel and elevated rates of electron leak. In sum, the findings challenge the notion that lysine acetylation causes broad-ranging damage to mitochondrial quality and performance and raise the possibility that acetyl-lysine turnover, rather than acetyl-lysine stoichiometry, modulates redox balance and carbon flux.

Multiple mitochondrial thioesterases have distinct tissue and substrate specificity and CoA regulation, suggesting unique functional roles

Acyl-CoA thioesterases (Acots) hydrolyze fatty acyl-CoA esters. Acots in the mitochondrial matrix are poised to mitigate β-oxidation overload and maintain CoA availability. Several Acots associate with mitochondria, but whether they all localize to the matrix, are redundant, or have different roles is unresolved. Here, we compared the suborganellar localization, activity, expression, and regulation among mitochondrial Acots (Acot2, -7, -9, and -13) in mitochondria from multiple mouse tissues and from a model of Acot2 depletion. Acot7, -9, and -13 localized to the matrix, joining Acot2 that was previously shown to localize there. Mitochondria from heart, skeletal muscle, brown adipose tissue, and kidney robustly expressed Acot2, -9, and -13; Acot9 levels were substantially higher in brown adipose tissue and kidney mitochondria, as was activity for C4:0-CoA, a unique Acot9 substrate. In all tissues, Acot2 accounted for about half of the thioesterase activity for C14:0-CoA and C16:0-CoA. In contrast, liver mitochondria from fed and fasted mice expressed little Acot activity, which was confined to long-chain CoAs and due mainly to Acot7 and Acot13 activities. Matrix Acots occupied different functional niches, based on substrate specificity (Acot9 versus Acot2 and -13) and strong CoA inhibition (Acot7, -9, and -13, but not Acot2). Interpreted in the context of β-oxidation, CoA inhibition would prevent Acot-mediated suppression of β-oxidation, while providing a release valve when CoA is limiting. In contrast, CoA-insensitive Acot2 could provide a constitutive siphon for long-chain fatty acyl-CoAs. These results reveal how the family of matrix Acots can mitigate β-oxidation overload and prevent CoA limitation.

Transcriptome analysis suggests a compensatory role of the cofactors coenzyme A and NAD+ in medium-chain acyl-CoA dehydrogenase knockout mice

During fasting, mitochondrial fatty-acid β-oxidation (mFAO) is essential for the generation of glucose by the liver. Children with a loss-of-function deficiency in the mFAO enzyme medium-chain acyl-Coenzyme A dehydrogenase (MCAD) are at serious risk of life-threatening low blood glucose levels during fasting in combination with intercurrent disease. However, a subset of these children remains asymptomatic throughout life. In MCAD-deficient (MCAD-KO) mice, glucose levels are similar to those of wild-type (WT) mice, even during fasting. We investigated if metabolic adaptations in the liver may underlie the robustness of this KO mouse. WT and KO mice were given a high- or low-fat diet and subsequently fasted. We analyzed histology, mitochondrial function, targeted mitochondrial proteomics, and transcriptome in liver tissue. Loss of MCAD led to a decreased capacity to oxidize octanoyl-CoA. This was not compensated for by altered protein levels of the short- and long-chain isoenzymes SCAD and LCAD. In the transcriptome, we identified subtle adaptations in the expression of genes encoding enzymes catalyzing CoA- and NAD(P)(H)-involving reactions and of genes involved in detoxification mechanisms. We discuss how these processes may contribute to robustness in MCAD-KO mice and potentially also in asymptomatic human subjects with a complete loss of MCAD activity.

Flux analysis of inborn errors of metabolism

Patients with an inborn error of metabolism (IEM) are deficient of an enzyme involved in metabolism, and as a consequence metabolism reprograms itself to reach a new steady state. This new steady state underlies the clinical phenotype associated with the deficiency. Hence, we need to know the flux of metabolites through the different metabolic pathways in this new steady state of the reprogrammed metabolism. Stable isotope technology is best suited to study this. In this review the progress made in characterizing the altered metabolism will be presented. Studies done in patients to estimate the residual flux through the metabolic pathway affected by enzyme deficiencies will be discussed. After this, studies done in model systems will be reviewed. The focus will be on glycogen storage disease type I, medium-chain acyl-CoA dehydrogenase deficiency, propionic and methylmalonic aciduria, urea cycle defects, phenylketonuria, and combined D,L-2-hydroxyglutaric aciduria. Finally, new developments are discussed, which allow the tracing of metabolic reprogramming in IEM on a genome-wide scale. In conclusion, the outlook for flux analysis of metabolic derangement in IEMs looks promising.