Complementation analysis of carnitine palmitoyltransferase I and II defects

Cyclosporine A: impact on mitochondrial function in endothelial cells

INTRODUCTION

Although cyclosporine A (CSA) is considered to be an efficient immunosuppressive compound in transplantation, vascular side effects like arterial hypertension, neurologic complications and other adverse reactions occur. Interference of CSA with mitochondrial function may be responsible for these side effects.

METHODS

We evaluated the effect of CSA on mitochondrial and glycolytic function by measuring fatty acid oxidation (FAO), activities of respiratory chain complexes (RC) and citratesynthase (CS), lactate/pyruvate-ratios, energy-rich phosphates as well as activities of some glycolytic enzymes in human umbilical vein endothelial cells.

RESULTS

After 48 h of CSA incubation, global FAO, RC-complexes 1 + 3; 4 and 5 as well as CS were compromised while energy charges were not reduced. Lactate/pyruvate-ratios increased; cellular lactate dehydrogenase (LDH)-, hexokinase- and phosphofructokinase-activities were not impaired by CSA. Moderate cellular toxicity, assessed by LDH leakage, appeared only at the highest CSA concentration.

CONCLUSION

Part of CSA toxicity may arise from alterations in mitochondrial function as judged by impaired FAO and respiratory chain enzymes. To some extent, energy balance seems to be maintained by cytosolic energy production. Although only demonstrated for endothelial cells, it is conceivable that such effects will alter energy metabolism of different organs with high oxidative energy demands.

Characterization of carnitine and fatty acid metabolism in the long-chain acyl-CoA dehydrogenase-deficient mouse

In the present paper, we describe a novel method which enables the analysis of tissue acylcarnitines and carnitine biosynthesis intermediates in the same sample. This method was used to investigate the carnitine and fatty acid metabolism in wild-type and LCAD-/- (long-chain acyl-CoA dehydrogenase-deficient) mice. In agreement with previous results in plasma and bile, we found accumulation of the characteristic C14:1-acylcarnitine in all investigated tissues from LCAD-/- mice. Surprisingly, quantitatively relevant levels of 3-hydroxyacylcarnitines were found to be present in heart, muscle and brain in wild-type mice, suggesting that, in these tissues, long-chain 3-hydroxyacyl-CoA dehydrogenase is rate-limiting for mitochondrial beta-oxidation. The 3-hydroxyacylcarnitines were absent in LCAD-/- tissues, indicating that, in this situation, the beta-oxidation flux is limited by the LCAD deficiency. A profound deficiency of acetylcarnitine was observed in LCAD-/- hearts, which most likely corresponds with low cardiac levels of acetyl-CoA. Since there was no carnitine deficiency and only a marginal elevation of potentially cardiotoxic acylcarnitines, we conclude from these data that the cardiomyopathy in the LCAD-/- mouse is caused primarily by a severe energy deficiency in the heart, stressing the important role of LCAD in cardiac fatty acid metabolism in the mouse.

Long-chain fatty acid oxidation during early human development

Patients with very long-chain acyl-CoA dehydrogenase (VLCAD) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD)/mitochondrial trifunctional protein (MTP) deficiency, disorders of the mitochondrial long-chain fatty acid oxidation, can present with hypoketotic hypoglycemia, rhabdomyolysis, and cardiomyopathy. In addition, patients with LCHAD/MTP deficiency may suffer from retinopathy and peripheral neuropathy. Until recently, there was no indication of intrauterine morbidity in these disorders. This observation was in line with the widely accepted view that fatty acid oxidation (FAO) does not play a significant role during fetal life. However, the high incidence of the gestational complications acute fatty liver of pregnancy and hemolysis, elevated liver enzymes, and low platelets syndrome observed in mothers carrying a LCHAD/MTP-deficient child and the recent reports of fetal hydrops due to cardiomyopathy in MTP deficiency, as well as the high incidence of intrauterine growth retardation in children with LCHAD/MTP deficiency, suggest that FAO may play an important role during fetal development. In this study, using in situ hybridization of the VLCAD and the LCHAD mRNA, we report on the expression of genes involved in the mitochondrial oxidation of long-chain fatty acids during early human development. Furthermore, we measured the enzymatic activity of the VLCAD, LCHAD, and carnitine palmitoyl-CoA transferase 2 (CPT2) enzymes in different human fetal tissues. Human embryos (at d 35 and 49 of development) and separate tissues (5-20 wk of development) were used. The results show a strong expression of VLCAD and LCHAD mRNA and a high enzymatic activity of VLCAD, LCHAD, and CPT2 in a number of tissues, such as liver and heart. In addition, high expression of LCHAD mRNA was observed in the neural retina and CNS. The observed pattern of expression during early human development is well in line with the spectrum of clinical signs and symptoms reported in patients with VLCAD or LCHAD/MTP deficiency.

A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARalpha-dependent and -independent pathways

Branched-chain fatty acids (such as phytanic and pristanic acid) are ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in vitro. To investigate the effects of these physiological compounds in vivo, wild-type and PPARalpha-deficient (PPARalpha-/-) mice were fed a phytol-enriched diet. This resulted in increased plasma and liver levels of the phytol metabolites phytanic and pristanic acid. In wild-type mice, plasma fatty acid levels decreased after phytol feeding, whereas in PPARalpha-/- mice, the already elevated fatty acid levels increased. In addition, PPARalpha-/- mice were found to be carnitine deficient in both plasma and liver. Dietary phytol increased liver free carnitine in wild-type animals but not in PPARalpha-/- mice. Investigation of carnitine biosynthesis revealed that PPARalpha is likely involved in the regulation of carnitine homeostasis. Furthermore, phytol feeding resulted in a PPARalpha-dependent induction of various peroxisomal and mitochondrial beta-oxidation enzymes. In addition, a PPARalpha-independent induction of catalase, phytanoyl-CoA hydroxylase, carnitine octanoyltransferase, peroxisomal 3-ketoacyl-CoA thiolase, and straight-chain acyl-CoA oxidase was observed. In conclusion, branched-chain fatty acids are physiologically relevant ligands of PPARalpha in mice. These findings are especially relevant for disorders in which branched-chain fatty acids accumulate, such as Refsum disease and peroxisome biogenesis disorders.

High activity of fatty acid oxidation enzymes in human placenta: implications for fetal-maternal disease

As the human fetus and placenta are considered to be primarily dependent on glucose oxidation for energy metabolism, the cause of the remarkable association between severe maternal pregnancy complications and the carriage of a fetus with an inborn error of mitochondrial long-chain fatty acid oxidation (FAO) has remained obscure. We analysed human term placenta and chorionic villus samples for the activities of a variety of enzymes involved in FAO, and compared the results with those obtained in human liver. All enzymes were found to be expressed, with a very high activity of two enzymes involved in the metabolism of long-chain fatty acids (CPT2 and VLCAD), whereas the activity of medium-chain acyl-CoA dehydrogenase (MCAD) was found to be low, when compared to liver. These results suggest that fatty acid oxidation may play an important role in energy generation in human placenta, and that a deficiency in the placental oxidation of long-chain FAO may result in placental dysfunction, thus causing gestational complications.

Carnitine palmitoyltransferase deficiencies

Carnitine palmitoyltransferase (CPT) deficiencies are common disorders of mitochondrial fatty acid oxidation. The CPT system is made up of two separate proteins located in the outer- (CPT1) and inner- (CPT2) mitochondrial membranes. While CPT2 is a ubiquitous protein, two tissue-specific CPT1 isoforms-the so-called "liver" (L) and "muscle" (M) CPT1s-have been shown to exist. Amino acid and cDNA nucleotide sequences have been identified for all of these proteins. L-CPT1 deficiency (13 families reported) presents as recurrent attacks of fasting hypoketotic hypoglycemia. Two L-CPT1 mutations have been reported to date. M-CPT1 deficiency has not been hitherto identified. CPT2 deficiency has several clinical presentations. The "benign" adult form (more than 150 families reported) is characterized by episodes of rhabdomyolysis triggered by prolonged exercise. The prevalent S113L mutation is found in about 50% of mutant alleles. The infantile-type CPT2 deficiency (10 families reported) presents as severe attacks of hypoketotic hypoglycemia, occasionally associated with cardiac damage commonly responsible for sudden death before 1 year of age. In addition to these symptoms, features of brain and kidney dysorganogenesis are frequently seen in the neonatal-onset CPT2 deficiency (13 families reported), almost always lethal during the first month of life. More than 25 CPT2 mutations (private missense or truncating mutations) have hitherto been detected. Treatment is based upon avoidance of fasting and/or exercise, a low-fat diet enriched with medium chain triglycerides and carnitine ("severe" CPT2 deficiency). Prenatal diagnosis may be offered for pregnancies at a 1/4 risk of infantile/severe-type CPT2 deficiency.

Defects in activation and transport of fatty acids

The oxidation of long-chain fatty acids in mitochondria plays an important role in energy production, especially in skeletal muscle, heart and liver. Long-chain fatty acids, activated to their CoA esters in the cytosol, are shuttled across the barrier of the inner mitochondrial membrane by the carnitine cycle. This pathway includes four steps, mediated by a plasma membrane carnitine transporter, two carnitine palmitoyltransferases (CPT I and CPT II) and a carnitine-acylcarnitine translocase. Defects in activation and uptake of fatty acids affect these four steps: CPT II deficiency leads to either exercise-induced rhabdomyolysis in adults or hepatocardiomuscular symptoms in neonates and children. The three other disorders of the carnitine cycle have an early onset. Hepatic CPT I deficiency is characterized by recurrent episodes of Reye-like syndrome, whereas severe muscular and cardiac signs are associated with episodes of fasting hypoglycaemia in defects of carnitine transport and translocase. Convenient metabolic investigations for reaching the diagnosis of carnitine cycle disorders are determination of plasma free and total carnitine concentrations, determination of plasma acylcarnitine profile by tandem mass spectrometry and in vitro fatty acid oxidation studies, particularly in fresh lymphocytes. Application of the tools of molecular biology has greatly aided the understanding of the carnitine palmitoyltransferase enzyme system and confirmed the existence of different related genetic diseases. Mutation analysis of CPT II defects has given some clues for correlation of genotype and phenotype. The first molecular analyses of hepatic CPT I and translocase deficiencies were recently reported.