Leucine toxicity in a neuronal cell model with inhibited branched chain amino acid catabolism

Branched-chain amino acids in metabolic signalling and insulin resistance

Branched-chain amino acids (BCAAs) are important nutrient signals that have direct and indirect effects. Frequently, BCAAs have been reported to mediate antiobesity effects, especially in rodent models. However, circulating levels of BCAAs tend to be increased in individuals with obesity and are associated with worse metabolic health and future insulin resistance or type 2 diabetes mellitus (T2DM). A hypothesized mechanism linking increased levels of BCAAs and T2DM involves leucine-mediated activation of the mammalian target of rapamycin complex 1 (mTORC1), which results in uncoupling of insulin signalling at an early stage. A BCAA dysmetabolism model proposes that the accumulation of mitotoxic metabolites (and not BCAAs per se) promotes β-cell mitochondrial dysfunction, stress signalling and apoptosis associated with T2DM. Alternatively, insulin resistance might promote aminoacidaemia by increasing the protein degradation that insulin normally suppresses, and/or by eliciting an impairment of efficient BCAA oxidative metabolism in some tissues. Whether and how impaired BCAA metabolism might occur in obesity is discussed in this Review. Research on the role of individual and model-dependent differences in BCAA metabolism is needed, as several genes (BCKDHA, PPM1K, IVD and KLF15) have been designated as candidate genes for obesity and/or T2DM in humans, and distinct phenotypes of tissue-specific branched chain ketoacid dehydrogenase complex activity have been detected in animal models of obesity and T2DM.

Single amino acid supplementation in aminoacidopathies: a systematic review

Aminoacidopathies are a group of rare and diverse disorders, caused by the deficiency of an enzyme or transporter involved in amino acid metabolism. For most aminoacidopathies, dietary management is the mainstay of treatment. Such treatment includes severe natural protein restriction, combined with protein substitution with all amino acids except the amino acids prior to the metabolic block and enriched with the amino acid that has become essential by the enzymatic defect. For some aminoacidopathies, supplementation of one or two amino acids, that have not become essential by the enzymatic defect, has been suggested. This so-called single amino acid supplementation can serve different treatment objectives, but evidence is limited. The aim of the present article is to provide a systematic review on the reasons for applications of single amino acid supplementation in aminoacidopathies treated with natural protein restriction and synthetic amino acid mixtures.

Neurocognitive profile in a case of maple syrup urine disease

Maple Syrup Urine Disease (MSUD) is a metabolic disease with associated enzyme deficiency and an inability to break down amino acids. Neurotoxic levels can occur resulting in neurological sequelae. Information regarding cognitive functions has suggested greater verbal than visuospatial abilities. Specific neuropsychological functions have not been studied. The objective of this study is to examine the neuropsychological profile in a case of MSUD. We present a case study of a 7-year-old female who has a history of late diagnosis (7 days of age) and poor metabolic control. Consistent with existing literature, a profile of stronger verbal reasoning and memory skills compared with visual-perceptual and nonverbal memory was revealed. Additional weaknesses were demonstrated with attention, emerging executive functions, and fine motor control. The results suggest that while previously described nonverbal reasoning and visuospatial impairments are present, there is likely a more complex pattern of neuropsychological impairments in children with MSUD, especially those with poor metabolic control.

Impact of longitudinal plasma leucine levels on the intellectual outcome in patients with classic MSUD

Maple syrup urine disease (MSUD) is an inherited deficiency of branched chain alpha-ketoacid dehydrogenase (BCKDH) activity impairing the degradation of the branched chain amino acids valine, leucine, and isoleucine. Classic MSUD may lead to severe neonatal encephalopathy including coma and impaired cognitive outcome in later life. Early start of dietary treatment and careful metabolic control may improve the outcome of patients with classic MSUD. The aim of this study was to investigate the impact of long-term metabolic control assessed by plasma leucine levels on cognitive outcome in patients with classic MSUD. Plasma leucine levels of 24 patients were obtained retrospectively for the first 6 y of life and yearly medians of mean plasma leucine levels were calculated. At the age of 6 y, IQ tests were performed. Yearly medians of mean plasma leucine levels yielded three homogeneous clusters (low, intermediate, high). Patients of the low cluster showed statistically significant higher IQ scores compared with those of those of intermediate and high clusters. Long-term plasma leucine levels are associated with impaired cognitive outcome in patients with classic MSUD. To achieve the best possible intellectual outcome for affected individuals, we recommend that in infants and preschool children the target range for plasma leucine should not exceed 200 micromol/L.

Human microRNA (miR29b) expression controls the amount of branched chain alpha-ketoacid dehydrogenase complex in a cell

Branched chain amino acids (BCAAs) play critical roles in cell and tissue functions in addition to being important components of protein structure. The multifunctional roles speak to the need for maintaining tight control of their concentration within cells. As the BCAA cannot be made de novo in mammals, their cellular concentration is a function of dietary intake, endogenous protein turnover and catabolism of the three amino acids. The branched chain alpha-ketoacid dehydrogenase (BCKD) complex commits the BCAA to degradation and thus is vital in controlling their concentration within a cell. In mammals, BCKD activity state depends on the presence of a covalently bound phosphate on one protein component of the complex. Phosphate is added to the protein through the action of the complex-specific kinase and results in BCKD inactivation. Here, we demonstrate that another reaction plays a role in determining the total amount of BCKD present in a cell. The microRNA (miR29b) molecule tested is targeted to the mRNA for the dihydrolipoamide branched chain acyltransferase component of BCKD and prevents translation when bound. This is the first demonstration of the use of a microRNA to exert control on a metabolic pathway of amino acid catabolism in mammals and offers an explanation for the observed differences in the amount of the BCKD complex present in different tissues and under varying nutritional states.

Markers associated with inborn errors of metabolism of branched-chain amino acids and their relevance to upper levels of intake in healthy people: an implication from clinical and molecular investigations on maple syrup urine disease

Maple syrup urine disease (MSUD) is caused by a deficiency in the branched-chain alpha-ketoacid dehydrogenase complex. Accumulations of branched-chain amino acids (BCAAs) and branched-chain alpha-ketoacids (BCKAs) in patients with MSUD induce ketoacidosis, neurological disorders, and developmental disturbance. BCAAs and BCKAs influence on the nervous system can be estimated by analyzing these patients. According to clinical investigations on MSUD patients, leucine levels over 400 micromol/L apparently can cause any clinical problem derived from impaired function of the central nervous system. Damage to neuronal cells found in MSUD patients are presumably because of higher concentrations of both blood BCAAs or BCKAs, especially alpha-ketoisocapronic acids. These clinical data from MSUD patients provide a valuable basis on understanding leucine toxicity in the normal subject.

Tissue-specific translation of murine branched-chain alpha-ketoacid dehydrogenase kinase mRNA is dependent upon an upstream open reading frame in the 5'-untranslated region

The committed step in the pathway for leucine, isoleucine, and valine catabolism is catalyzed by branched-chain alpha-ketoacid dehydrogenase (BCKD). This multienzyme complex is itself regulated through reversible subunit phosphorylation by a specific kinase (BCKD-kinase). Although BCKD is present in the mitochondria of all mammalian cells, BCKD-kinase has a tissue-specific pattern of expression. Various experimental, nutritional, and hormonal conditions have been used to alter the expression of BCKD-kinase, yet little is known regarding the regulation of basal BCKD-kinase expression under normal conditions including the mechanism of its tissue specificity in any organism. Here we use tissue-derived cultured cells to explore the mechanisms used to control BCKD-kinase expression. Whereas the amount of BCKD-kinase protein is significantly higher in mitochondria from C2C12 myotubes than in BNL Cl.2 liver cells, gene transcription and stability of BCKD-kinase mRNA share similar properties in these two cell types. Our results show that the amount of protein synthesized is regulated at the level of translation of BCKD-kinase mRNA and that an upstream open reading frame in the 5'-untranslated region of this transcript controls its translation. The location and putative 19-residue peptide are conserved in the mouse, rat, chimpanzee, and human genes. Likewise, gene structure of mouse, chimpanzee, and human BCKD-kinase is conserved, whereas the rat gene has lost intron 9.