Carnitine transport: pathophysiology and metabolism of known molecular defects

Personalized Medicine in Mitochondrial Health and Disease: Molecular Basis of Therapeutic Approaches Based on Nutritional Supplements and Their Analogs

Mitochondrial diseases (MDs) may result from mutations affecting nuclear or mitochondrial genes, encoding mitochondrial proteins, or non-protein-coding mitochondrial RNA. Despite the great variability of affected genes, in the most severe cases, a neuromuscular and neurodegenerative phenotype is observed, and no specific therapy exists for a complete recovery from the disease. The most used treatments are symptomatic and based on the administration of antioxidant cocktails combined with antiepileptic/antipsychotic drugs and supportive therapy for multiorgan involvement. Nevertheless, the real utility of antioxidant cocktail treatments for patients affected by MDs still needs to be scientifically demonstrated. Unfortunately, clinical trials for antioxidant therapies using α-tocopherol, ascorbate, glutathione, riboflavin, niacin, acetyl-carnitine and coenzyme Q have met a limited success. Indeed, it would be expected that the employed antioxidants can only be effective if they are able to target the specific mechanism, i.e., involving the central and peripheral nervous system, responsible for the clinical manifestations of the disease. Noteworthily, very often the phenotypes characterizing MD patients are associated with mutations in proteins whose function does not depend on specific cofactors. Conversely, the administration of the antioxidant cocktails might determine the suppression of endogenous oxidants resulting in deleterious effects on cell viability and/or toxicity for patients. In order to avoid toxicity effects and before administering the antioxidant therapy, it might be useful to ascertain the blood serum levels of antioxidants and cofactors to be administered in MD patients. It would be also worthwhile to check the localization of mutations affecting proteins whose function should depend (less or more directly) on the cofactors to be administered, for estimating the real need and predicting the success of the proposed cofactor/antioxidant-based therapy.

Impacts of Heat Stress on Rabbit Immune Function, Endocrine, Blood Biochemical Changes, Antioxidant Capacity and Production Performance, and the Potential Mitigation Strategies of Nutritional Intervention

Heat stress has become a widespread concern in the world, which is one of the major environmental stressors and causes substantial economic loss in the rabbit industry. Heat stress leads to multiple damages to the health of rabbits, such as organ damage, oxidative stress, disordered endocrine regulation, suppressed immune function and reproductive disorders, ultimately, induces the decreased production performance and increased mortality. Nutritional approaches, including feeding strategies, adjusting feed formula, and supplementing vitamins, minerals, electrolytes, Chinese herbal medicines, and functional active substances to the feed, were reported to mitigate the detrimental effects of heat stress in rabbits. Therefore, elucidating the damage of heat stress to rabbits; proper management and nutritional approaches should be considered to solve the heat stress issue in rabbits. This review highlights the scientific evidence regarding the effects of heat stress on rabbit's immune function, endocrine, blood biochemical changes, antioxidant capacity and production performance, and the potential mitigation strategies of nutritional intervention to alleviate heat stress in rabbits; which could contribute to develop nutritional strategies in relieving heat stress of rabbits.

Interaction of supplementary L-carnitine and dietary energy levels on feed utilization and blood constituents in New Zealand White rabbits reared under summer conditions

In the summer season, the harmful effects of high ambient temperatures on rabbit productivity have attained global interest. Therefore, the use of new nutritional strategies to improve heat tolerance in rabbits has become highly needed. In the current experiment, the effect of the addition of L-carnitine (LC) to normal (NE) or high-energy (HE) diets of growing rabbits reared under summer conditions on their productive performance, physiological indicators, and carcass characteristics was assessed. For this purpose, a 2 × 2 factorial arrangement of treatments was used with two dietary levels of energy of 10.00 and 11.22 MJ kg^-1 diet of NE and HE, respectively, which was fortified with 0 or 50 mg of LC kg^-1 of diet (LC0 and LC50, respectively). The feeding trial was conducted using 80 male rabbits (initial body weight, 630.7 ± 1.3 g; 35-day-old), and it lasted for 56 days. Throughout the experiment, the average temperature humidity index was 35.49 reflecting very severe heat stress conditions. Rectal temperature, heart rate, and respiration rate decreased significantly in rabbits fed NE+LC50, HE+LC0, and HE+LC50 diets compared with those fed NE diets without supplementation. Simultaneously, growth indices and feed conversion ratio were enhanced significantly. Rabbits fed NE+LC50, HE+LC0, and HE+LC50 diets showed significantly higher blood hemoglobin, white blood cell counts, total protein, glucose, and red blood cell counts, compared with those fed a NE+LC0 diet. Rabbits fed NE+LC50 and HE+LC50 showed higher economic efficiency. In conclusion, growing rabbits reared under summer conditions responded positively to LC supplementation and HE diets, in terms of the highest growth performance, feed conversion, and physiological and biochemical indicators, making them potentially safe to use.

Significance of Levocarnitine Treatment in Dialysis Patients

Carnitine is a naturally occurring amino acid derivative that is involved in the transport of long-chain fatty acids to the mitochondrial matrix. There, these substrates undergo β-oxidation, producing energy. The major sources of carnitine are dietary intake, although carnitine is also endogenously synthesized in the liver and kidney. However, in patients on dialysis, serum carnitine levels progressively fall due to restricted dietary intake and deprivation of endogenous synthesis in the kidney. Furthermore, serum-free carnitine is removed by hemodialysis treatment because the molecular weight of carnitine is small (161 Da) and its protein binding rates are very low. Therefore, the dialysis procedure is a major cause of carnitine deficiency in patients undergoing hemodialysis. This deficiency may contribute to several clinical disorders in such patients. Symptoms of dialysis-related carnitine deficiency include erythropoiesis-stimulating agent-resistant anemia, myopathy, muscle weakness, and intradialytic muscle cramps and hypotension. However, levocarnitine administration might replenish the free carnitine and help to increase carnitine levels in muscle. This article reviews the previous research into levocarnitine therapy in patients on maintenance dialysis for the treatment of renal anemia, cardiac dysfunction, dyslipidemia, and muscle and dialytic symptoms, and it examines the efficacy of the therapeutic approach and related issues.

Cardiometabolism as an Interlocking Puzzle between the Healthy and Diseased Heart: New Frontiers in Therapeutic Applications

Cardiac metabolism represents a crucial and essential connecting bridge between the healthy and diseased heart. The cardiac muscle, which may be considered an omnivore organ with regard to the energy substrate utilization, under physiological conditions mainly draws energy by fatty acids oxidation. Within cardiomyocytes and their mitochondria, through well-concerted enzymatic reactions, substrates converge on the production of ATP, the basic chemical energy that cardiac muscle converts into mechanical energy, i.e., contraction. When a perturbation of homeostasis occurs, such as an ischemic event, the heart is forced to switch its fatty acid-based metabolism to the carbohydrate utilization as a protective mechanism that allows the maintenance of its key role within the whole organism. Consequently, the flexibility of the cardiac metabolic networks deeply influences the ability of the heart to respond, by adapting to pathophysiological changes. The aim of the present review is to summarize the main metabolic changes detectable in the heart under acute and chronic cardiac pathologies, analyzing possible therapeutic targets to be used. On this basis, cardiometabolism can be described as a crucial mechanism in keeping the physiological structure and function of the heart; furthermore, it can be considered a promising goal for future pharmacological agents able to appropriately modulate the rate-limiting steps of heart metabolic pathways.

Protein Nutrition in Autism

Autism is a developmental disorder that affects communication and behavior. Although autism can be diagnosed at any age, it is said to be a "developmental disorder" because symptoms generally appear in the first 2 years of life. The primary cause of autism is still not clear and therapy is currently restricted to controlling behavioral abnormalities. However, emerging studies have shown a link between mitochondrial dysfunction and autism. Dietary supplements that promote mitochondrial biogenesis and inhibit the production of oxidative stress have been used to treat autism patients. Dietary adjustments in treating autism is a novel approach to suppress autistic symptoms. Supplementation with antioxidants has been found to not only inhibit cognitive decline but also improve behavioral symptoms in autism. Dietary supplements fortified with vitamins should only be given under the supervision of a physician. A wide range of nutraceuticals are under clinical trials to understand whether they physiologically target mitochondrial pathways and improve the quality of life in autism.

Current Opinion on Usage of L-Carnitine in End-Stage Renal Disease Patients on Peritoneal Dialysis

The advantages of peritoneal dialysis (PD) over hemodialysis (HD) are well-documented. Notwithstanding, only a small proportion of patients with end-stage renal disease (ESRD) are managed with PD. This may be related to the high glucose load that PD solutions in current use have on the patient. The effects of such excess glucose include the relatively early limitation of the ultrafiltration capacity of the peritoneal membrane, and the metabolic effects associated with hyperglycemia, e.g., decreased insulin sensitivity. This article describes the advantages that may be realized by the glucose-sparing effects of substituting part of the glucose load with other osmotically active metabolites, particularly L-carnitine. The latter is anticipated to have metabolic advantages of its own, especially as in PD patients, high plasma concentrations can be achieved in the absence of renal clearance. Besides its better biocompatibility, L-carnitine demonstrates anti-anemia action due to its effects on erythropoiesis, and positive effects on the longevity and deformability of erythrocytes. Observations from our trials on the use of carnitine-enriched PD solutions have demonstrated the effectiveness of L-carnitine as an efficient osmolyte in PD, and its favorable effect on the insulin sensitivity of the patients. The significance of these findings for future developments in the use of PD in the management of patients with ESRD is discussed.

Omics-based responses induced by bosentan in human hepatoma HepaRG cell cultures

Bosentan is well known to induce cholestatic liver toxicity in humans. The present study was set up to characterize the hepatotoxic effects of this drug at the transcriptomic, proteomic, and metabolomic levels. For this purpose, human hepatoma-derived HepaRG cells were exposed to a number of concentrations of bosentan during different periods of time. Bosentan was found to functionally and transcriptionally suppress the bile salt export pump as well as to alter bile acid levels. Pathway analysis of both transcriptomics and proteomics data identified cholestasis as a major toxicological event. Transcriptomics results further showed several gene changes related to the activation of the nuclear farnesoid X receptor. Induction of oxidative stress and inflammation were also observed. Metabolomics analysis indicated changes in the abundance of specific endogenous metabolites related to mitochondrial impairment. The outcome of this study may assist in the further optimization of adverse outcome pathway constructs that mechanistically describe the processes involved in cholestatic liver injury.

Inborn Errors of Metabolism with Myopathy: Defects of Fatty Acid Oxidation and the Carnitine Shuttle System

Fatty acid oxidation disorders (FAODs) and carnitine shuttling defects are inborn errors of energy metabolism with associated mortality and morbidity due to cardiomyopathy, exercise intolerance, rhabdomyolysis, and liver disease with physiologic stress. Hypoglycemia is characteristically hypoketotic. Lactic acidemia and hyperammonemia may occur during decompensation. Recurrent rhabdomyolysis is debilitating. Expanded newborn screening can detect most of these disorders, allowing early, presymptomatic treatment. Treatment includes avoiding fasting and sustained extraneous exercise and providing high-calorie hydration during illness to prevent lipolysis, and medium-chain triglyceride oil supplementation in long-chain FAODs. Carnitine supplementation may be helpful. However, conventional treatment does not prevent all symptoms.

Primary Carnitine Deficiency: Is Foetal Development Affected and Can Newborn Screening Be Improved?

Primary carnitine deficiency (PCD) causes low levels of carnitine in patients potentially leading to metabolic and cardiac symptoms. Newborn screening for PCD is now routine in many countries by measuring carnitine levels in infants. In this study we report Apgar scores, length and weight in newborns with PCD and newborns born to mothers with PCD compared to controls. Furthermore we report how effective different screening algorithms have been to detect newborns with PCD in the Faroe Islands.

RESULTS

Newborns with PCD and newborns born to mothers with PCD did not differ with regard to Apgar scores, length and weight compared to controls. Newborns with PCD and newborns born to mothers with PCD had significantly lower levels of free carnitine (fC0) than controls. Screening algorithms focusing only on fC0 had a high rate of detection of newborns with PCD. Sample collection 4-9 days after birth seems to result in a higher detection rate than the current 2-3 days.

CONCLUSION

The clinical status at birth in infants with PCD and infants born to mothers with PCD does not differ compared to control infants. Screening algorithms for PCD should focus on fC0, and blood samples should be taken when the maternal influence on fC0 has diminished.

Increased maternal consumption of methionine as its hydroxyl analog promoted neonatal intestinal growth without compromising maternal energy homeostasis

BACKGROUND

To determine responses of neonatal intestine to maternal increased consumption of DL-methionine (DLM) or DL-2-hydroxy-4-methylthiobutanoic acid (HMTBA), eighteen primiparous sows (Landrace × Yorkshire) were allocated based on body weight and backfat thickness to the control, DLM and HMTBA groups (n = 6), with the nutritional treatments introduced from postpartum d0 to d14.

RESULTS

The DLM-fed sows showed negative energy balance manifested by lost bodyweight, lower plasma glucose, subdued tricarboxylic acid cycle, and increased plasma lipid metabolites levels. Both villus height and ratio of villus height to crypt depth averaged across the small intestine of piglets were higher in the DLM and HMTBA groups than in the control group. Piglet jejunal oxidized glutathione concentration and ratio of oxidized to reduced glutathione were lower in the HMTBA group than in the DLM and control groups. However, piglet jejunal aminopeptidase A, carnitine transporter 2 and IGF-II precursor mRNA abundances were higher in the DLM group than in the HMTBA and control groups.

CONCLUSION

Increasing maternal consumption of methionine as DLM and HMTBA promoted neonatal intestinal growth by increasing morphological development or up-regulating expression of genes responsible for nutrient metabolism. And increasing maternal consumption of HMTBA promoted neonatal intestinal antioxidant capacity without compromising maternal energy homeostasis during early lactation.

Carnitine levels in skeletal muscle, blood, and urine in patients with primary carnitine deficiency during intermission of L-carnitine supplementation

BACKGROUND

Primary carnitine deficiency (PCD) is a disorder of fatty acid oxidation with a high prevalence in the Faroe Islands. Only patients homozygous for the c.95A>G (p.N32S) mutation have displayed severe symptoms in the Faroese patient cohort. In this study, we investigated carnitine levels in skeletal muscle, plasma, and urine as well as renal elimination kinetics before and after intermission with L-carnitine in patients homozygous for c.95A>G.

METHODS

Five male patients homozygous for c.95A>G were included. Regular L-carnitine supplementation was stopped and the patients were observed during five days. Blood and urine were collected throughout the study. Skeletal muscle biopsies were obtained at 0, 48, and 96 h.

RESULTS

Mean skeletal muscle free carnitine before discontinuation of L-carnitine was low, 158 nmol/g (SD 47.4) or 5.4% of normal. Mean free carnitine in plasma (fC0) dropped from 38.7 (SD 20.4) to 6.3 (SD 1.7) μmol/L within 96 h (p < 0.05). Mean T 1/2 following oral supplementation was approximately 9 h. Renal reabsorption of filtered carnitine following oral supplementation was 23%. The level of mean free carnitine excreted in urine correlated (R (2) = 0.78, p < 0.01) with fC0 in plasma.

CONCLUSION

Patients homozygous for the c.95A>G mutation demonstrated limited skeletal muscle carnitine stores despite long-term high-dosage L-carnitine supplementation. Exacerbated renal excretion resulted in a short T 1/2 in plasma carnitine following the last oral dose of L-carnitine. Thus a treatment strategy of minimum three daily separate doses of L-carnitine is recommended, while intermission with L-carnitine treatment might prove detrimental.

The adverse effects of long-term l-carnitine supplementation on liver and kidney function in rats

Levo-Carnitine (l-carnitine) is widely used in health and food. This study was to focus on the adverse effects of 8-week oral supplementation of l-carnitine (0.3 and 0.6 g/kg) in female and male Sprague Dawley rats. l-carnitine reduced body and fat weights, as well as serum, liver, and kidney lipid levels in rats. Simultaneously, hepatic fatty acid β-oxidation and lipid synthesis were disturbed in l-carnitine-fed rats. Moreover, l-carnitine accelerated reactive oxygen species production in serum and liver, thereby triggering hepatic NOD-like receptor 3 (NLRP3) inflammasome activation to elevate serum interleukin (IL)-1β and IL-18 levels in rats. Alteration of serum alkaline phosphatase levels further confirmed liver dysfunction in l-carnitine-fed rats. Additionally, l-carnitine may potentially disturb kidney function by altering renal protein levels of rat organic ion transporters. These observations may provide the caution information for the safety of long-term l-carnitine supplementation.

Acetyl-L-carnitine increases mitochondrial protein acetylation in the aged rat heart

Previously we showed that in vivo treatment of elderly Fisher 344 rats with acetylcarnitine abolished the age-associated defect in respiratory chain complex III in interfibrillar mitochondria and improved the functional recovery of the ischemic/reperfused heart. Herein, we explored mitochondrial protein acetylation as a possible mechanism for acetylcarnitine's effect. In vivo treatment of elderly rats with acetylcarnitine restored cardiac acetylcarnitine content and increased mitochondrial protein lysine acetylation and increased the number of lysine-acetylated proteins in cardiac subsarcolemmal and interfibrillar mitochondria. Enzymes of the tricarboxylic acid cycle, mitochondrial β-oxidation, and ATP synthase of the respiratory chain showed the greatest acetylation. Acetylation of isocitrate dehydrogenase, long-chain acyl-CoA dehydrogenase, complex V, and aspartate aminotransferase was accompanied by decreased catalytic activity. Several proteins were found to be acetylated only after treatment with acetylcarnitine, suggesting that exogenous acetylcarnitine served as the acetyl-donor. Two-dimensional fluorescence difference gel electrophoresis analysis revealed that acetylcarnitine treatment also induced changes in mitochondrial protein amount; a two-fold or greater increase/decrease in abundance was observed for thirty one proteins. Collectively, our data provide evidence for the first time that in the aged rat heart in vivo administration of acetylcarnitine provides acetyl groups for protein acetylation and affects the amount of mitochondrial proteins.

Primary Carnitine deficiency in the Faroe Islands: health and cardiac status in 76 adult patients diagnosed by screening

BACKGROUND

Carnitine deficiency can cause cardiomyopathy and cardiac arrhythmia. The prevalence in the Faroe Islands is the highest reported in the world (1:300). A nationwide screening program identified 76 Faroese adult patients (15-80 years) with Primary Carnitine Deficiency (PCD). We describe prior and current health status and symptoms in these patients, especially focusing on cardiac characteristics.

METHODS

Upon identification, patients were immediately admitted for physical examination, ECG, blood tests and initiation of L-carnitine supplementation. Medical records were reviewed and patients were interviewed. Echocardiography and blood tests were performed in 35 patients before and after L-carnitine supplementation.

RESULTS

All patients were either asymptomatic or had minor symptoms when diagnosed. Echocardiography including LVEF, global longitudinal strain and dimensions were normal apart from left ventricular hypertrophy with normal systolic function in one young male. Symptoms, e.g. fatigue, were reported in 43 % with a reduction to 12 % (p < 0.01) following initiation of L-carnitine supplementation. Eighty two % reported participation in sports of which 52 % were on a competitive level. ECGs showed limited changes and blood tests were normal. Mean plasma free carnitine increased from 6.1 μmol/L to 15.1 μmol/L (p < 0.01) within 50 days of L-carnitine supplementation.

CONCLUSION

PCD in adults can cause serious symptoms, but adult Faroese patients identified through a screening program were predominantly asymptomatic with a normal cardiac structure and function.

Obesity and lipid stress inhibit carnitine acetyltransferase activity

Carnitine acetyltransferase (CrAT) is a mitochondrial matrix enzyme that catalyzes the interconversion of acetyl-CoA and acetylcarnitine. Emerging evidence suggests that this enzyme functions as a positive regulator of total body glucose tolerance and muscle activity of pyruvate dehydrogenase (PDH), a mitochondrial enzyme complex that promotes glucose oxidation and is feedback inhibited by acetyl-CoA. Here, we used tandem mass spectrometry-based metabolic profiling to identify a negative relationship between CrAT activity and muscle content of lipid intermediates. CrAT specific activity was diminished in muscles from obese and diabetic rodents despite increased protein abundance. This reduction in enzyme activity was accompanied by muscle accumulation of long-chain acylcarnitines (LCACs) and acyl-CoAs and a decline in the acetylcarnitine/acetyl-CoA ratio. In vitro assays demonstrated that palmitoyl-CoA acts as a direct mixed-model inhibitor of CrAT. Similarly, in primary human myocytes grown in culture, nutritional and genetic manipulations that promoted mitochondrial influx of fatty acids resulted in accumulation of LCACs but a pronounced decrease of CrAT-derived short-chain acylcarnitines. These results suggest that lipid-induced antagonism of CrAT might contribute to decreased PDH activity and glucose disposal in the context of obesity and diabetes.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Pathophysiology of fatty acid oxidation disorders and resultant phenotypic variability

Fatty acids are a major fuel for the body and fatty acid oxidation is particularly important during fasting, sustained aerobic exercise and stress. The myocardium and resting skeletal muscle utilise long-chain fatty acids as a major source of energy. Inherited disorders affecting fatty acid oxidation seriously compromise the function of muscle and other highly energy-dependent tissues such as brain, nerve, heart, kidney and liver. Such defects encompass a wide spectrum of clinical disease, presenting in the neonatal period or infancy with recurrent hypoketotic hypoglycaemic encephalopathy, liver dysfunction, hyperammonaemia and often cardiac dysfunction. In older children, adolescence or adults there is often exercise intolerance with episodic myalgia or rhabdomyolysis in association with prolonged aerobic exercise or other exacerbating factors. Some disorders are particularly associated with toxic metabolites that may contribute to encephalopathy, polyneuropathy, axonopathy and pigmentary retinopathy. The phenotypic diversity encountered in defects of fat oxidation is partly explained by genotype/phenotype correlation and certain identifiable environmental factors but there remain many unresolved questions regarding the complex interaction of genetic, epigenetic and environmental influences that dictate phenotypic expression. It is becoming increasingly clear that the view that most inherited disorders are purely monogenic diseases is a naive concept. In the future our approach to understanding the phenotypic diversity and management of patients will be more realistically achieved from a polygenic perspective.

Studies on the chemopreventive effect of carnitine on tumorigenesis in vivo, using two experimental murine models of colon cancer

Carnitine is known for its essential role in intermediary metabolism. In vitro studies suggest that its antioxidant and anti-inflammatory properties are potentially beneficial toward cancer prevention. This study tested effects of carnitine on the development of colon cancer in vivo using 2 murine models: azoxymethane (AOM) treatment as a model of carcinogen-induced colon cancer and a genetically induced model using Apc (Min/+) mice. AOM and Apc (Min/+) mice divided into dietary groups varying in lipid content, with or without carnitine supplementation (0.08%). AOM-exposed mice on a high butterfat diet had significantly increased aberrant crypts (ACF) (9.3 ± 0.88 vs. 6.3 ± 0.65), and macroscopic tumors (3.8 ± 0.95 vs. 2.0 ± 0.25) compared to mice on a control diet. In AOM mice fed the high butterfat diet, carnitine supplementation inhibited ACF (4.9 ± 0.7 vs. 9.3 ± 0.88, P < 0.001), crypt multiciplicity (1.6 ± 0.08 vs. 1.92 ± 0.1, P < 0.01) and tumors (1.5 ± 0.38 vs. 3.8 ± 0.95, P < 0.001). Carnitine supplementation resulted in significantly increased tissue carnitine and acylcarnitine levels. Carnitine inhibited the development of precancerous lesions and macroscopic colonic tumors in AOM-treated mice. However, carnitine did not exert protective effects on intestinal tumors in Apc (Min/+) mice.

Genetics and metabolic cardiomyopathies

Metabolic disorders encompass a heterogeneous group of conditions that commonly affect the heart and contribute adversely to cardiovascular outcomes. As the heart is a metabolically active organ, inborn errors in metabolism (IEMs) often present with cardiac manifestations such as cardiomyopathy, arrhythmia, and valvular dysfunction. More than 40 IEMs are reported to cause cardiomyopathy, including fatty acid oxidation defects, glycogen, lysosomal and perioxisome storage diseases, mitochondrial cardiomyopathies, organic acidaemias, aminoacidopathies and congenital disorders of glycosylation. Studies suggest that IEM account for only 5% of cardiomyopathies; however, their diagnosis is imperative to enable the effective institution of disease-specific management strategies. This review describes the more common genetic defects that affect metabolic pathways and give rise to heart muscle disease.

Relationship between serum fibronectin levels and carnitine administration: an experimental study in rats

We aimed to investigate the relationship between dorsal flap viability and serum fibronectin levels in carnitine-administered rats. A total of 24 rats were equally divided into three groups and operated on. Group 1 (sham group n = 8): following surgery, no agent was given. Group 2 (control group, n = 8): following surgery, sterile saline solution at 0·9% with a dose of 100 mg/kg per day for 7 days was administered intraperitoneally. Group 3 (study group, n = 8): following surgery, carnitine with a dose of 100 mg/kg per day for 7 days was administered intraperitoneally. The flap model used was a 10 × 3 cm dorsal flap extending from the tip of the scapula to the hip joint. This was elevated, and then sutured back to its original site. At the end of postoperative day 8, the animals were anaesthetised and blood samples were collected from intracardiac space. Then, the animals were euthanised. Flap viability was then evaluated measuring the surviving area, using a transparent graph paper. Finally, excised tissue was examined histopathologically. The percentages of viable areas in groups 1, 2 and 3 were 64·68 ± 3·37%, 67·35 ± 5·82% and 75·15 ± 3·56%, respectively. The mean value of fibronectin levels in groups 1, 2 and 3 were 22·3 ± 3·5, 23·1 ± 3·5 and 31 ± 6·8 mg/dl, respectively. The results of this study demonstrated that 100 mg/kg carnitine administration led to an increase in flap viability, and increased serum fibronectin levels might have a role in this process.

Primary carnitine deficiency and pivalic acid exposure causing encephalopathy and fatal cardiac events

BACKGROUND

Several episodes of sudden death among young Faroese individuals have been associated with primary carnitine deficiency (PCD). Patients suffering from PCD have low carnitine levels and can present with metabolic and/or cardiac complications. Pivalic acid exposure decreases carnitine levels. The purpose of this study was to investigate and describe the association and pathophysiology of exposure to antibiotics containing pivalic acid and severe neurological and cardiac complications in six identified subjects suffering from PCD.

METHODS AND MATERIALS

Six cases of PCD were identified and studied through medical records and family interview. Stored biomaterial was analyzed for mutations causing PCD.

RESULTS

Five patients (two children, three adults) died suddenly while one adult patient survived sudden cardiac arrest. Lethal cardiac arrhythmia was documented in five patients, while one patient was not monitored at time of death, but had signs of cardiac arrhythmia a few days earlier. All patients suffered encephalopathy before cardiac arrhythmia. Autopsy showed severe hepatic steatosis and signs of cerebral edema in four out of five. One subject had a dilated heart. All patients were homozygous for the c.95A>G (p.N32S) mutation in SLC22A5 causing PCD. All patients had been treated with antibiotics containing pivalic acid prior to the episode.

CONCLUSION

Exposure to antibiotics containing pivalic acid was associated with encephalopathy and progression to lethal cardiac arrhythmia in patients suffering from PCD.

Regulation of Genes Involved in Carnitine Homeostasis by PPARα across Different Species (Rat, Mouse, Pig, Cattle, Chicken, and Human)

Recent studies in rodents convincingly demonstrated that PPARα is a key regulator of genes involved in carnitine homeostasis, which serves as a reasonable explanation for the phenomenon that energy deprivation and fibrate treatment, both of which cause activation of hepatic PPARα, causes a strong increase of hepatic carnitine concentration in rats. The present paper aimed to comprehensively analyse available data from genetic and animal studies with mice, rats, pigs, cows, and laying hens and from human studies in order to compare the regulation of genes involved in carnitine homeostasis by PPARα across different species. Overall, our comparative analysis indicates that the role of PPARα as a regulator of carnitine homeostasis is well conserved across different species. However, despite demonstrating a well-conserved role of PPARα as a key regulator of carnitine homeostasis in general, our comprehensive analysis shows that this assumption particularly applies to the regulation by PPARα of carnitine uptake which is obviously highly conserved across species, whereas regulation by PPARα of carnitine biosynthesis appears less well conserved across species.

Pharmacological and pathophysiological roles of carnitine/organic cation transporters (OCTNs: SLC22A4, SLC22A5 and Slc22a21)

The carnitine/organic cation transporter (OCTN) family consists of three transporter isoforms, i.e. OCTN1 (SLC22A4) and OCTN2 (SLC22A5) in humans and animals and Octn3 (Slc22a21) in mice. These transporters are physiologically essential to maintain appropriate systemic and tissue concentrations of carnitine by regulating its membrane transport during intestinal absorption, tissue distribution and renal reabsorption. Among them, OCTN2 is a sodium-dependent, high-affinity transporter of carnitine, and a functional defect of OCTN2 due to genetic mutation causes primary systemic carnitine deficiency (SCD). Since carnitine is essential for beta-oxidation of long-chain fatty acids to produce ATP, OCTN2 gene mutation causes a range of symptoms, including cardiomyopathy, skeletal muscle weakness, fatty liver and male infertility. These functional consequences of Octn2 gene mutation can be seen clearly in an animal model, jvs mouse, which exhibits the SCD phenotype. In addition, although the mechanism is not clear, single nucleotide polymorphisms of OCTN1 and OCTN2 genes are associated with increased incidences of rheumatoid arthritis, Crohn's disease and asthma. OCTN1 and OCTN2 accept cationic drugs as substrates and contribute to intestinal and pulmonary absorption, tissue distribution (including to tumour cells), and renal excretion of these drugs. Modulation of the transport activity of OCTN2 by externally administered drugs may cause drug-induced secondary carnitine deficiency. Rodent Octn3 transports carnitine specifically, particularly in male reproductive tissues. Thus, the OCTNs are physiologically, pathologically and pharmacologically important. Detailed characterization of these transporters will greatly improve our understanding of the pathology associated with common diseases caused by functional deficiency of OCTNs.

Miscellaneous central nervous system intoxicants

The topic of central nervous system intoxicants encompasses a multitude of agents. This article focuses on three classes of therapeutic drugs, with specific examples in which overdoses require admission to the intensive care unit. Included are some of the newer antidepressants, the atypical neuroleptic agents, and selected anticonvulsant drugs. The importance of understanding pertinent physiology and applicable supportive care is emphasized.

Expression of genes involved in hepatic carnitine synthesis and uptake in dairy cows in the transition period and at different stages of lactation

Background

In rodents and pigs, it has shown that carnitine synthesis and uptake of carnitine into cells are regulated by peroxisome proliferator-activated receptor α (PPARA), a transcription factor which is physiologically activated during fasting or energy deprivation. Dairy cows are typically in a negative energy balance during early lactation. We investigated the hypothesis that genes of carnitine synthesis and uptake in dairy cows are enhanced during early lactation.

Results

mRNA abundances of PPARA and some of its classical target genes and genes involved in carnitine biosynthesis [trimethyllysine dioxygenase (TMLHE), 4-N-trimethylaminobutyraldehyde dehydrogenase (ALDH9A1), γ-butyrobetaine dioxygenase (BBOX1)] and uptake of carnitine [novel organic cation transporter 2 (SLC22A5)] as well as carnitine concentrations in liver biopsy samples of 20 dairy cows in late pregnancy (3 wk prepartum) and early lactation (1 wk, 5 wk, 14 wk postpartum) were determined. From 3 wk prepartum to 1 wk postpartum, mRNA abundances of PPARΑ and several PPARΑ target genes involved in fatty acid uptake, fatty acid oxidation and ketogenesis in the liver were strongly increased. Simultaneously, mRNA abundances of enzymes of carnitine synthesis (TMLHE: 10-fold; ALDH9A1: 6-fold; BBOX1: 1.8-fold) and carnitine uptake (SLC22A5: 13-fold) and the concentration of carnitine in the liver were increased from 3 wk prepartum to 1 wk postpartum (P < 0.05). From 1 wk to 5 and 14 wk postpartum, mRNA abundances of these genes and hepatic carnitine concentrations were declining (P < 0.05). There were moreover positive correlations between plasma concentrations of non-esterified fatty acids (NEFA) and hepatic carnitine concentrations at 1 wk, 5 wk and 14 wk postpartum (P < 0.05).

Conclusions

The results of this study show for the first time that the expression of hepatic genes of carnitine synthesis and cellular uptake of carnitine is enhanced in dairy cows during early lactation. These changes might provide an explanation for increased hepatic carnitine concentrations observed in 1 wk postpartum and might be regarded as a physiologic means to provide liver cells with sufficient carnitine required for transport of excessive amounts of NEFA during a negative energy balance.

Effect of L-carnitine on the hepatic transcript profile in piglets as animal model

Background

Carnitine has attracted scientific interest due to several health-related effects, like protection against neurodegeneration, mitochondrial decay, and oxidative stress as well as improvement of glucose tolerance and insulin sensitivity. The mechanisms underlying most of the health-related effects of carnitine are largely unknown.

Methods

To gain insight into mechanisms through which carnitine exerts its beneficial metabolic effects, we fed piglets either a control or a carnitine supplemented diet, and analysed the transcriptome in the liver.

Results

Transcript profiling revealed 563 genes to be differentially expressed in liver by carnitine supplementation. Clustering analysis of the identified genes revealed that most of the top-ranked annotation term clusters were dealing with metabolic processes. Representative genes of these clusters which were significantly up-regulated by carnitine were involved in cellular fatty acid uptake, fatty acid activation, fatty acid β-oxidation, glucose uptake, and glycolysis. In contrast, genes involved in gluconeogenesis were down-regulated by carnitine. Moreover, clustering analysis identified genes involved in the insulin signaling cascade to be significantly associated with carnitine supplementation. Furthermore, clustering analysis revealed that biological processes dealing with posttranscriptional RNA processing were significantly associated with carnitine supplementation.

Conclusion

The data suggest that carnitine supplementation has beneficial effects on lipid and glucose homeostasis by inducing genes involved in fatty acid catabolism and glycolysis and repressing genes involved in gluconeogenesis.

A modern approach to the treatment of mitochondrial disease

The treatment of mitochondrial disease varies considerably. Most experts use a combination of vitamins, optimize patients' nutrition and general health, and prevent worsening of symptoms during times of illness and physiologic stress. We agree with this approach, and we agree that therapies using vitamins and cofactors have value, though there is debate about the choice of these agents and the doses prescribed. Despite the paucity of high-quality scientific evidence, these therapies are relatively harmless, may alleviate select clinical symptoms, and theoretically may offer a means of staving off disease progression. Like many other mitochondrial medicine physicians, we have observed significant (and at times life-altering) clinical responses to such pharmacologic interventions. However, it is not yet proven that these therapies truly alter the course of the disease, and some experts may choose not to use these medications at all. At present, the evidence of their effectiveness does not rise to the level required for universal use. Based on our clinical experience and judgment, however, we agree that a therapeutic trial of coenzyme Q10, along with other antioxidants, should be attempted. Although individual specialists differ as to the exact drug cocktail, a common approach involves combinations of antioxidants that may have a synergistic effect. Because almost all relevant therapies are classified as medical foods or over-the-counter supplements, most physicians also attempt to balance the apparent clinical benefit of mitochondrial cocktails with the cost burden that these supplements pose for the family.

Carnitine is necessary to maintain the phenotype and function of brown adipose tissue

The juvenile visceral steatosis (JVS) mouse is a mutant strain with an inherited systemic carnitine deficiency. Mice of this strain show clinical signs attributable to impaired heat production and disturbed energy production. Brown adipose tissue (BAT) is the primary site of non-shivering thermogenesis in the presence of uncoupling protein-1 (UCP-1) in rodents and humans, especially in infants. To investigate the possible cause of impaired heat production in BAT, we studied the morphological features, carnitine concentration, and UCP-1 production of BAT in JVS mice. The effect of carnitine administration on these parameters was also examined. JVS mice aged 5 or 10 days (60 each) and age-matched control mice were used in this study, along with 10-day-old JVS mice treated subcutaneously with L-carnitine once a day between postpartum days 5 and 10. JVS mice showed lower body temperatures and lower concentrations of carnitine in BAT. Morphologically, BAT cells in JVS mice contained large lipid vacuoles and small mitochondria, similar to those present in white adipose tissue cells. In addition, UCP-1 mRNA and protein expression levels were significantly reduced in JVS as compared with control mice. Carnitine treatment resulted in significant increases in body temperature and carnitine concentrations in BAT, together with the recovery of normal morphological features. UCP-1 mRNA and protein expression levels were also significantly increased. These findings strongly suggest that carnitine is essential for maintaining the function and morphology of BAT.

Quantitative structure activity relationship for inhibition of human organic cation/carnitine transporter

Organic cation/carnitine transporter (OCTN2; SLC22A5) is an important transporter for L-carnitine homeostasis, but can be inhibited by drugs, which may cause L-carnitine deficiency and possibly other OCTN2-mediated drug-drug interactions. One objective was to develop a quantitative structure-activity relationship (QSAR) of OCTN2 inhibitors, in order to predict and identify other potential OCTN2 inhibitors and infer potential clinical interactions. A second objective was to assess two high renal clearance drugs that interact with OCTN2 in vitro (cetirizine and cephaloridine) for possible OCTN2-mediated drug-drug interactions. Using previously generated in vitro data of 22 drugs, a 3D quantitative pharmacophore model and a Bayesian machine learning model were developed. The four pharmacophore features include two hydrophobic groups, one hydrogen-bond acceptor, and one positive ionizable center. The Bayesian machine learning model was developed using simple interpretable descriptors and function class fingerprints of maximum diameter 6 (FCFP_6). An external test set of 27 molecules, including 15 newly identified OCTN2 inhibitors, and a literature test set of 22 molecules were used to validate both models. The computational models afforded good capability to identify structurally diverse OCTN2 inhibitors, providing a valuable tool to predict new inhibitors efficiently. Inhibition results confirmed our previously observed association between rhabdomyolysis and C(max)/K(i) ratio. The two high renal clearance drugs cetirizine and cephaloridine were found not to be OCTN2 substrates, and their diminished elimination by other drugs is concluded not to be mediated by OCTN2.

Colon OCTN2 gene expression is up-regulated by peroxisome proliferator-activated receptor gamma in humans and mice and contributes to local and systemic carnitine homeostasis

In the large intestine organic cation transporter type-2 (OCTN2) is recognized as a transporter of compounds such as carnitine and colony sporulation factor, promoting health of the colon intestinal epithelium. Recent reports suggest that OCTN2 expression in small intestine is under control of peroxisome proliferator-activated receptor-alpha (PPARalpha). However, PPARalpha contribution to colonic OCTN2 expression remains controversial. Here we examined the transcriptional regulation of colon OCTN2 gene by PPARgamma. To exclude any additional modulation of other PPAR to OCTN2 expression, we used both in vivo and in vitro PPAR-null models and specific PPAR inhibitors. The PPARgamma agonists thiazolidinediones increased both OCTN2 mRNA and protein expression in colonic epithelial cell lines independently by PPARalpha expression. The induction was blocked only by PPARgamma antagonists or by gammaORF4, a PPARgamma isoform with dominant negative activity, suggesting a PPARgamma-dependent mechanism. A conserved noncanonical PPAR-responsive element was found by computational analysis in the first intron of human OCTN2 gene and validated by EMSA assay. Promoter-reporter assays further confirmed transcriptional functionality of the putative PPAR response element, whereas selective mutation caused complete loss of responsiveness to PPARgamma activation. Finally, adenovirus-mediated overexpression of constitutively active PPARgamma mutant increased colon OCTN2 expression in PPARalpha(-/-) mice. Interestingly, animals overexpressing colon PPARgamma showed a significant increase in plasma carnitine, thus demonstrating the functional contribution of large intestine to systemic carnitine homeostasis. This study reveals a PPARgamma-dependent absorption machinery in colon that is likely involved in the health of colon epithelium, in the microbiota-host interactions and in the absorption of nutraceuticals and drugs.

Inborn errors of energy metabolism associated with myopathies

Inherited neuromuscular disorders affect approximately one in 3,500 children. Structural muscular defects are most common; however functional impairment of skeletal and cardiac muscle in both children and adults may be caused by inborn errors of energy metabolism as well. Patients suffering from metabolic myopathies due to compromised energy metabolism may present with exercise intolerance, muscle pain, reversible or progressive muscle weakness, and myoglobinuria. In this review, the physiology of energy metabolism in muscle is described, followed by the presentation of distinct disorders affecting skeletal and cardiac muscle: glycogen storage diseases types III, V, VII, fatty acid oxidation defects, and respiratory chain defects (i.e., mitochondriopathies). The diagnostic work-up and therapeutic options in these disorders are discussed.

Neutral lipid storage disease with subclinical myopathy due to a retrotransposal insertion in the PNPLA2 gene

An 18-year-old girl referred to a rheumatologist with malar flush and Gottran papules was found to have a markedly elevated serum CK. She was a good student and an avid ballet dancer. A muscle biopsy showed massive triglyceride storage, which was also found in peripheral blood granulocytes (Jordan anomaly) and cultured skin fibroblasts. Assessment using computerized dynamometry and cycle ergometry showed normal strength and muscle energetics, but proton spectroscopy revealed severe triglyceride accumulation in both skeletal and cardiac muscle. Sequencing of PNPLA2, the gene responsible for neutral lipid storage disease with myopathy (NLSDM), revealed a retrotransposal insertion of about 1.8kb in exon 3 that abrogates transcription of PNPLA2. The sequences of CGI-58, the gene responsible for Chanarin-Dorfman syndrome (CDS), another multisystem triglyceride storage disease, and of two genes encoding lipid droplets-associated proteins, perilipin A and adipophilin, were normal. This case shows that NLSDM can be a transposon-associated disease and that massive lipid storage in muscle can present as asymptomatic hyperCKemia.

Enzymology of the carnitine biosynthesis pathway

The water-soluble zwitterion carnitine is an essential metabolite in eukaryotes required for fatty acid oxidation as it functions as a carrier during transfer of activated acyl and acetyl groups across intracellular membranes. Most eukaryotes are able to synthesize carnitine endogenously, besides their capacity to take up carnitine from the diet or extracellular medium through plasma membrane transporters. This review discusses the current knowledge on carnitine homeostasis with special emphasis on the enzymology of the four steps of the carnitine biosynthesis pathway.

Plasma carnitine ester profile in homozygous and heterozygous OCTN2 deficiency

The carnitine ester spectrum was studied using ESI tandem mass spectrometry in a 2.5-year-old male Roma child with homozygous deletion of 844C of the SLC22A5 gene, presenting with hepatopathy and cardiomyopathy. Besides the dramatic decrease of plasma free carnitine (1.38 vs 32.7 mumol/L in controls) all plasma carnitine esters were severely decreased in the proband: the total esters were 31.4% of the controls. In three heterozygous siblings the free carnitine level was 62.3% of the normal controls, while the levels of the individual carnitine esters ranged between 15.5% and 163% (average 70.9%). The heterozygous parents exhibited the same pattern. The proband was supplemented with 50 mg/kg per day of L-carnitine oral solution. After 2 months of treatment, his hepatomegaly, elevated transaminases and the pathological cardiac ultrasound parameters normalized. The plasma free carnitine rose to 12.8 mumol/L (39% of the controls). All of the carnitine esters also increased; however, the individual esters were still 8.5-169.7% of the controls (average 55.5%). After 13 months of treatment there was a further increase in free carnitine (15.9 mumol/L) as well as in the level of the individual esters, ranging between 16.1% and 140.3% of the controls (average 66.9%). The data presented here show that, besides the dramatic decrease of free carnitine, the carnitine ester metabolism is also affected in OCTN2 deficiency; the replenishment of the pools under treatment is slow. Despite an impressive clinical improvement, the carnitine metabolism can be still seriously affected.

Carnitine insufficiency caused by aging and overnutrition compromises mitochondrial performance and metabolic control

In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine diminution was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete beta-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.

Influence of l-carnitine on metabolism and performance of sows

In recent years, l-carnitine has been used increasingly as a supplement in livestock animals. The present review gives an overview of the effects of dietary l-carnitine supplementation on the reproductive performance of sows. Results concerning the effect of l-carnitine supplementation during pregnancy on litter sizes are controversial. There are some studies reporting an increased number of piglets born alive per litter, while others could not find such an effect. In contrast, most studies performed show consistently that l-carnitine supplementation to a sow diet low in native carnitine during gestation increases piglet and litter weights at birth and enhances growth of litters during the suckling period. Biochemical mechanisms underlying the favourable effect of carnitine on intra-uterine growth have not been fully elucidated. There is, however, some evidence that carnitine influences the insulin-like growth factor-axis in sows and leads to greater placentae, which in turn improves intra-uterine nutrition, and stimulates oxidation of glucose in the fetuses. These effects may, at least in part, be responsible for higher birth weights of piglets. The stimulating effect of carnitine on growth of the litters might be due to an improved suckling behaviour of piglets born to l-carnitine-supplemented sows, causing the sows' milk production to rise. In conclusion, recent studies have clearly shown that dietary l-carnitine supplementation increases the reproductive performance of sows. These findings suggest that endogenous de novo synthesis of carnitine is insufficient to meet the metabolic requirement of sows during gestation.

Increased insulin sensitivity in mice lacking collectrin, a downstream target of HNF-1alpha

Collectrin is a downstream target of the transcription factor hepatocyte nuclear factor-1alpha (HNF-1alpha), which is mutated in maturity-onset diabetes of the young subtype 3 (MODY3). Evidence from transgenic mouse models with collectrin overexpression in pancreatic islets suggests divergent roles for collectrin in influencing beta-cell mass and insulin exocytosis. To clarify the function of collectrin in the pancreas, we used a mouse line with targeted deletion of the gene. We examined pancreas morphology, glucose homeostasis by ip glucose tolerance testing (IPGTT) and insulin tolerance testing (IPITT), and pancreas function by in vivo acute-phase insulin response determination and glucose-stimulated insulin secretion from isolated islets. We find no difference in either pancreas morphology or function between wild-type and collectrin-deficient animals (Tmem27(-/y)). However, we note that by 6 months of age, Tmem27(-/y) mice exhibit increased insulin sensitivity by IPITT and decreased adiposity by dual-energy x-ray absorptiometry scanning compared with wild-type. We have previously reported that Tmem27(-/y) mice exhibit profound aminoaciduria due to failed renal recovery. We now demonstrate that Tmem27(-/y) animals also display inappropriate excretion of some short-chain acylcarnitines derived from amino acid and fatty acid oxidation. We provide further evidence for compensatory up-regulation of oxidative metabolism in Tmem27(-/y) mice, along with enhanced protein turnover associated with preserved lean mass even out to 1.5 yr of age. Our studies suggest that collectrin-deficient mice activate a number of adaptive mechanisms to defend energy homeostasis in the setting of ongoing nutrient losses.