A heterozygote phenotype is present in the jvs +/- mutant mouse livers

A review of fatty acid oxidation disorder mouse models

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

High-fat diet-induced nonalcoholic steatohepatitis is accelerated by low carnitine and impaired glucose tolerance in novel murine models

Carnitine deficiency and impaired glucose tolerance (IGT) exacerbate liver steatosis. Given the current lack of ideal murine nonalcoholic steatohepatitis (NASH) models, we investigated new NASH models using jvs/+ mice with low carnitine and wild-type mice with low-dose alloxan-induced IGT. The jvs/+ and wild-type mice were divided into jvs/+ mice fed a high-fat diet (HFD) from 3 weeks of age (HF hetero group), wild-type mice with low-dose alloxan treatment fed HFD (AL + HF wild group), wild-type mice fed HFD (HF wild group), and two types of mice fed a normal diet-jvs/+ and wild-type (intact group). All mice were sacrificed at 20 or 40 weeks of age. All male HFD-fed mice showed obesity, IGT, high blood insulin levels, homeostatic model assessment of insulin resistance (HOMA-IR), high liver enzyme levels, and high cholesterol levels. The degree of IGT was the worst in the AL + HF wild group, and blood insulin levels and HOMA-IR score were remarkably increased from 20 to 40 weeks of age. Almost all HFD-fed mice showed steatosis, fibrosis, and lobular inflammation in the centrilobular zone. These changes were accompanied by hepatocyte ballooning and were enhanced at 40 weeks of age. Furthermore, the incidence rate of nodular hyperplasia and adenoma in both the HF hetero and AL + HF wild groups was nearly 30%. We successfully established two novel murine models of NASH using male jvs/+ mice with low carnitine and male wild-type mice with IGT that eventually developed obesity, fatty liver, insulin resistance, liver fibrosis, and tumorigenesis. These results suggest that low carnitine levels and early-stage induction of IGT are important factors in the progression of NASH to tumorigenesis, similar to human NASH.

Slc22a5 haploinsufficiency does not aggravate the phenotype of the long-chain acyl-CoA dehydrogenase KO mouse

Secondary carnitine deficiency is commonly observed in inherited metabolic diseases characterised by the accumulation of acylcarnitines such as mitochondrial fatty acid oxidation (FAO) disorders. It is currently unclear if carnitine deficiency and/or acylcarnitine accumulation play a role in the pathophysiology of FAO disorders. The long-chain acyl-CoA dehydrogenase (LCAD) KO mouse is a model for long-chain FAO disorders and is characterised by decreased levels of tissue and plasma free carnitine. Tissue levels of carnitine are controlled by SLC22A5, the plasmalemmal carnitine transporter. Here, we have further decreased carnitine availability in the LCAD KO mouse through a genetic intervention by introducing one defective Slc22a5 allele (jvs). Slc22a5 haploinsufficiency decreased free carnitine levels in liver, kidney, and heart of LCAD KO animals. The resulting decrease in the tissue long-chain acylcarnitines levels had a similar magnitude as the decrease in free carnitine. Levels of cardiac deoxycarnitine, a carnitine biosynthesis intermediate, were elevated due to Slc22a5 haploinsufficiency in LCAD KO mice. A similar increase in heart and muscle deoxycarnitine was observed in an independent experiment using Slc22a5^jvs/jvs mice. Cardiac hypertrophy, fasting-induced hypoglycemia and increased liver weight, the major phenotypes of the LCAD KO mouse, were not affected by Slc22a5 haploinsufficiency. This may suggest that secondary carnitine deficiency does not play a major role in the pathophysiology of these phenotypes. Similarly, our data do not support a major role for toxicity of long-chain acylcarnitines in the phenotype of the LCAD KO mouse.

Maternal Plasma l-Carnitine Reduction During Pregnancy Is Mainly Attributed to OCTN2-Mediated Placental Uptake and Does Not Result in Maternal Hepatic Fatty Acid β-Oxidation Decline

l-Carnitine (l-Car) plays a crucial role in fatty acid β-oxidation. However, the plasma l-Car concentration in women markedly declines during pregnancy, but the underlying mechanism and its consequences on maternal hepatic β-oxidation have not yet been clarified. Our results showed that the plasma l-Car level in mice at gestation day (GD) 18 was significantly lower than that in nonpregnant mice, and the mean fetal-to-maternal plasma l-Car ratio in GD 18 mice was 3.0. Carnitine/organic cation transporter 2 (OCTN2) was highly expressed in mouse and human placenta and upregulated as gestation proceeds in human placenta, whereas expressions of carnitine transporter (CT) 1, CT2, and amino acid transporter B^0,+ were extremely low. Further study revealed that renal peroxisome proliferator-activated receptor α (PPARα) and OCTN2 were downregulated and the renal l-Car level was reduced, whereas the urinary excretion of l-Car was lower in late pregnant mice than in nonpregnant mice. Meanwhile, progesterone (pregnancy-related hormone) downregulated the expression of renal OCTN2 via PPARα-mediated pathway, and inhibited the activity of OCTN2, but estradiol, corticosterone, and cortisol did not. Unexpectedly, the maternal hepatic level of l-Car and β-hydroxybutyrate (an indicator of mitochondrial β-oxidation), and mRNA levels of several enzymes involved in fatty acid β-oxidation in GD 18 mice were higher than that in nonpregnant mice. In conclusion, OCTN2 mediated l-Car transfer across the placenta played a major role in maternal plasma l-Car reduction during pregnancy, which did not subsequently result in maternal hepatic fatty acid β-oxidation decrease.

Carnitine deficiency in OCTN2-/- newborn mice leads to a severe gut and immune phenotype with widespread atrophy, apoptosis and a pro-inflammatory response

We have investigated the gross, microscopic and molecular effects of carnitine deficiency in the neonatal gut using a mouse model with a loss-of-function mutation in the OCTN2 (SLC22A5) carnitine transporter. The tissue carnitine content of neonatal homozygous (OCTN2^−/−) mouse small intestine was markedly reduced; the intestine displayed signs of stunted villous growth, early signs of inflammation, lymphocytic and macrophage infiltration and villous structure breakdown. Mitochondrial β-oxidation was active throughout the GI tract in wild type newborn mice as seen by expression of 6 key enzymes involved in β-oxidation of fatty acids and genes for these 6 enzymes were up-regulated in OCTN2^−/− mice. There was increased apoptosis in gut samples from OCTN2^−/− mice. OCTN2^−/− mice developed a severe immune phenotype, where the thymus, spleen and lymph nodes became atrophied secondary to increased apoptosis. Carnitine deficiency led to increased expression of CD45-B220⁺ lymphocytes with increased production of basal and anti-CD3-stimulated pro-inflammatory cytokines in immune cells. Real-time PCR array analysis in OCTN2^−/− mouse gut epithelium demonstrated down-regulation of TGF-β/BMP pathway genes. We conclude that carnitine plays a major role in neonatal OCTN2^−/− mouse gut development and differentiation, and that severe carnitine deficiency leads to increased apoptosis of enterocytes, villous atrophy, inflammation and gut injury.

Cardiac conduction improvement in two heterozygotes for primary carnitine deficiency on L-carnitine supplementation

Expanded newborn screening (NBS) for free carnitine levels has led to the identification of a larger number of heterozygous infants of undiagnosed mothers affected with systemic primary carnitine deficiency (PCD), which in turn leads to the identification of other undiagnosed heterozygous family members. There is an increasing recognition that individuals heterozygous for mutations of genes involved in fatty acid oxidation (FAO) may become symptomatic under environmental stress (fasting, prolonged exercise and illness). Considering the importance of carnitine in FAO, its role in heart and bowel function and in lipid metabolism, what is still little known is the phenotypic variability, biochemical parameters and clinical course of PCD heterozygotes with consistently low-to-normal levels to low levels of carnitine over a lifetime. We report on three generations of a family--an asymptomatic PCD heterozygous infant identified through NBS that led to the diagnosis of her asymptomatic PCD-affected mother and the heterozygous status of the maternal grandparents who report some cardiac symptoms that overlap with PCD that improved with L-carnitine supplementation.

Novel ENU-induced point mutation in scavenger receptor class B, member 1, results in liver specific loss of SCARB1 protein

Cardiovascular disease (CVD) is the largest cause of premature death in human populations throughout the world. Circulating plasma lipid levels, specifically high levels of LDL or low levels of HDL, are predictive of susceptibility to CVD. The scavenger receptor class B member 1 (SCARB1) is the primary receptor for the selective uptake of HDL cholesterol by liver and steroidogenic tissues. Hepatic SCARB1 influences plasma HDL-cholesterol levels and is vital for reverse cholesterol transport. Here we describe the mapping of a novel N-ethyl-N-nitrosourea (ENU) induced point mutation in the Scarb1 gene identified in a C57BL/6J background. The mutation is located in a highly conserved amino acid in the extracellular loop and leads to the conversion of an isoleucine to an asparagine (I179N). Homozygous mutant mice express normal Scarb1 mRNA levels and are fertile. SCARB1 protein levels are markedly reduced in liver (∼90%), but not in steroidogenic tissues. This leads to ∼70% increased plasma HDL levels due to reduced HDL cholesteryl ester selective uptake. Pdzk1 knockout mice have liver-specific reduction of SCARB1 protein as does this mutant; however, in vitro analysis of the mutation indicates that the regulation of SCARB1 protein in this mutant is independent of PDZK1. This new Scarb1 model may help further our understanding of post-translational and tissue-specific regulation of SCARB1 that may aid the important clinical goal of raising functional HDL.

Cardiomyopathy and carnitine deficiency

Carnitine is essential for the transfer of long-chain fatty acids across the mitochondrial membrane for subsequent beta-oxidation. A defect in the high-affinity carnitine transporter OCTN2 causes autosomal recessive primary carnitine deficiency that can present with hypoketotic hypoglycemia, mainly in infancy or cardiomyopathy. Heterozygotes for primary carnitine deficiency can have mildly reduced plasma carnitine levels and can develop benign cardiac hypertrophy. In animal models, heterozygotes for this disease have a higher incidence of cardiomyopathy with aging. This study tested whether heterozygosity for primary carnitine deficiency was associated with cardiomyopathy. The frequency of mutations in the SLC22A5 gene encoding the OCTN2 carnitine transporter was determined in 324 patients with cardiomyopathy and compared to that described in the normal population. Missense variations identified in normal controls and patients with cardiomyopathy were expressed in Chinese Hamster Ovary cells to confirm a functional effect. Exons 2-10 of the SLC22A5 gene were amplified by PCR in the presence of LCGreen I and analyzed by dye-binding/high-resolution thermal denaturation. Exon 1 of the gene was sequenced in all patients. Heterozygosity for a few variants (L144F, T264M, I312V, E317K, and R488H) was found in 6/324 patients with cardiomyopathy. Expression of these variants in CHO cells indicated that T264M decreased, E317K increased, while L144F, I312V, and R488H did not significantly affect carnitine transport. Expression in CHO cells of all the variants identified in a normal population indicated that only two had a functional effect (L17F and Y449D), while L144F, V481I, V481F, M530V, and P549S did not change significantly carnitine transport. The frequency of variants affecting carnitine transport was 2/324 patients with cardiomyopathy (0.61%) not significantly different from frequency of 3/270 (1.11%) in the general population. These results indicate that heterozygosity for primary carnitine deficiency is not more frequent in patients with unselected types of cardiomyopathy and is unlikely to be an important cause of cardiomyopathy in humans.

Pressure overload-induced cardiomyopathy in heterozygous carrier mice of carnitine transporter gene mutation

Primary systemic carnitine deficiency is an autosomal recessive disorder caused by a decreased renal reabsorption of carnitine because of mutations of the carnitine transporter OCTN2 gene, and hypertrophic cardiomyopathy is a common clinical feature of homozygotes. Although heterozygotes for OCTN2 mutations are generally healthy with normal cardiac performance, heterozygotes may be at risk for cardiomyopathy in the presence of additional risk factors, such as hypertension. To test this hypothesis, we investigated the effects of surgically induced pressure overload on the hearts of heterozygous mutants of a murine model of OCTN2 mutation, juvenile visceral steatosis mouse (jvs/+). Eleven-week-old jvs/+ mice and age-matched wild-type mice were used. At baseline, there were no differences in physical characteristics between wild-type and jvs/+ mice. However, plasma and myocardial total carnitine levels in jvs/+ mice were lower than in wild-type mice. Both wild-type and jvs/+ mice were subjected to ascending aortic constriction with or without 1% l-carnitine supplementation for 4 weeks. At 4 weeks after ascending aortic constriction, jvs/+ mice showed an exaggeration of cardiac hypertrophy and pulmonary congestion, further increased gene expression of atrial natriuretic peptide in the left ventricles, further deterioration of left ventricular fractional shortening, reduced myocardial phosphocreatine:adenosine triphosphate ratio, and increased mortality compared with wild-type mice; l-carnitine supplementation prevented these changes in jvs/+ mice subjected to ascending aortic constriction. In conclusion, cardiomyopathy and heart failure with energy depletion may be induced by pressure overload in heterozygotes for OCTN2 mutations and could be prevented by l-carnitine supplementation.

Prolonged effect of single carnitine administration on fasted carnitine-deficient JVS mice regarding their locomotor activity and energy expenditure

Carnitine is an essential cofactor for the oxidation of fatty acid in the mitochondria and an efficient therapeutics for primary carnitine deficiency. We herein analyzed the prolonged effects of carnitine on the reduced locomotor activity and energy metabolism of fasted carnitine-deficient juvenile visceral steatosis (jvs(-/-)) mice. We found that a single carnitine administration to 24-h fasted jvs(-/-) mice in the morning increased both the locomotor activity and oxygen consumption at night not only on the same day, but also on the next day, when the carnitine levels in the blood and tissues were already as low as at the original carnitine-deficient state. We also found that fat utilization for energy production significantly increased under fasting even in jvs(-/-) mice and was stimulated in the carnitine-administrated fasted jvs(-/-) mice at night, in comparison to that observed in the saline-administered jvs(-/-) mice, at least for 2 days even under the low plasma and tissue carnitine levels. These results suggest that the low tissue carnitine levels are therefore not the sole rate-limiting factor of general fatty acid oxidation in carnitine-deficient jvs(-/-) mice.

Functional genetic diversity in the high-affinity carnitine transporter OCTN2 (SLC22A5)

Systemic carnitine deficiency (SCD) is a rare autosomal recessive disease resulting from defects in the OCTN2 (SLC22A5) gene, which encodes the high-affinity plasma membrane carnitine transporter. Although OCTN2 is fairly well studied in its relationship with SCD, little is known about the carrier frequency of disease-causing alleles of OCTN2, or of more common functional polymorphisms in this gene. To address these issues, we screened for genetic variants in the OCTN2 coding region by direct sequencing of the exons and flanking intronic region of OCTN2 in a large sample (n = 276) of ethnically diverse subjects. In addition, we established lymphoblastoid cell lines from subjects homozygous for either allele of the previously identified promoter region variant, -207G>C. We found eight amino acid sequence variants of OCTN2, of which three (Phe17Leu, Leu144Phe, and Pro549Ser) were polymorphic in at least one ethnic group. When assayed for functional activity by expression in human embryonic kidney 293 cells, using as probes both the endogenous substrate (l-carnitine) and the organic cation tetraethylammonium, three variants showed functional differences from the reference OCTN2 (Phe17Leu, Tyr449Asp, Val481Phe; p < 0.05). Further studies of the Phe17Leu polymorphism showed a reduced V(max) for l-carnitine transport to approximately 50% of the reference OCTN2. Confocal microscopy studies using an OCTN2-GFP fusion protein showed that Phe17Leu had distinct subcellular localization from the reference OCTN2, with diffuse cytoplasmic retention of Phe17Leu, in contrast to reference OCTN2, which localized specifically to the plasma membrane. Lymphoblasts from subjects homozygous for the -207G allele showed increased l-carnitine transport compared with the -207C/C homozygotes (p < 0.05). This study suggests that although loss-of-function mutations in OCTN2 are likely to be rare, common variants of OCTN2 found in healthy populations may contribute to variation in the disposition of carnitine and some clinically used drugs.

Carnitine depletion in rat pups from mothers given mildronate: a model of carnitine deficiency in late fetal and neonatal life

Mildronate (3-(2,2,2,-trimethylhydrazinium)propionate), is a butyrobetaine analogue that is known to inhibit gamma-butyrobetaine hydroxylase, the enzyme catalyzing the last step of carnitine biosynthesis. When administered to adult rats it determines a systemic carnitine deficiency and may therefore serve as an animal model for human carnitine depletion. The aim of this study was to evaluate the effect of mildronate administration to pregnant and lactating rats on tissue carnitine concentrations in 4- and 13-day-old rat pups. At 14 days of gestation female rats began to receive mildronate in the diet (200 mg/kg/d) and this continued for entire lactation period. Mildronate treatment determined a large reduction of carnitine levels in the milk of lactating dams. Because organ carnitine concentrations in neonatal rats are directly related to dietary supply, pups from mildronate group had significantly depleted levels of total carnitine in serum, heart, liver, muscle, brain and pancreas relative to controls, at 4 and 13 days of age. Correspondingly, an increase in triglyceride levels was observed in liver, heart and muscle of mildronate pups. This is in agreement with a reduction of basal rate of oxidation of [U-(14)C]-palmitate to (14)CO(2) and (14)C-acid-soluble products observed in liver homogenates from carnitine-deficient pups. All functional and biochemical modifications were compatible with a carnitine deficiency status. In conclusion our results describe a model of carnitine depletion in pups, suitable for the investigation of carnitine deficiency in fetal-neonatal nutrition, without any concomitant mildronate-mediated metabolic alterations.

Carnitine transport: pathophysiology and metabolism of known molecular defects

Early-onset dilatative and/or hypertrophic cardiomyopathy with episodic hypoglycaemic coma and very low serum and tissue concentrations of carnitine should alert the clinician to the probability of the plasmalemmal high-affinity carnitine transporter defect. The diagnosis can be established by demonstration of impaired carnitine uptake in cultured skin fibroblasts or lymphoblasts and confirmed by mutation analysis of the human OCTN2 gene in the affected child and obligate heterozygote parents. The institution of high-dose oral carnitine supplementation reverses the pathology in this otherwise lethal autosomal recessive disease of childhood, and carnitine therapy from birth in prospectively screened siblings may altogether prevent the development of the clinical phenotype. Heterozygotes may be at risk for cardiomyopathy in later adult life, particularly in the presence of additional risk factors such as hypertension and competitive pharmacological agents. OCTN2 belongs to a family of organic cation/carnitine transporters that function primarily in the elimination of cationic drugs and other xenobiotics in kidney, intestine, liver and placenta. The high- and low-affinity human carnitine transporters, OCTN2 and OCTN1, are multifunctional polyspecific organic cation transporters; therefore, defects in these transporters may have widespread implications for the absorption and/or elimination of a number of key pharmacological agents such as cephalosporins, verapamil, quinidine and valproic acid. A third organic/cation carnitine transporter with high specificity for carnitine, Octn3, has been cloned in mice. The juvenile visceral steatosis (jvs) mouse serves as an excellent clinical, biochemical and molecular model for the high-affinity carnitine transporter OCTN2 defect and is due to a spontaneous point mutation in the murine Octn2 gene on mouse chromosome 11, which is syntenic to the human locus at 5q31 that harbours the human OCTN2 gene.