Branched chain acyl-CoA metabolism and disease

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Defects in the catabolism of branched chain amino acids cause 13 metabolic disorders Yet essentially nothing is known about the physical parameters defining the BCAA metabolon, or the relationship of its disruption to human disease. My lab has defined many of the genes encoding these proteins, and characterized their enzymatic function and the disorders associated with their deficiency. Recently, variants in the ACAD10 gene have been shown to be associated with a risk for development of type 2 diabetes mellitus (T2DM) in Pima Indians, but the mechanism of this risk has not been elucidated. To examine the role of ACAD10 in the development of T2DM, we generated a knock out mouse model. Strikingly, deficient animals gained excessive weight and became insulin resistant. The long-range goal of this project is to characterize the metabolism of branched chain acyl-CoAs and to identify the consequences of its failure in humans. This renewal application has three specific aims. Specific Aim 1 is to examine the mitochondrial architecture of branched chain amino acid metabolism. I hypothesize that the proteins responsible for branched chain amino acid metabolism form a functional complex within mitochondria. Specific Aim 1a is to use immuno- and cryo-electronmicroscopy to visualize the BCAA metabolic complex. Specific Aim 1b will employ co-immunoprecipitation with and without chemical crosslinking to identify binding partners in the BCAA metabolic complex. I predict that the branched chain specific ACADs will interact tightly with the branched chain keto-acid dehydrogenase to form the core of the BCAA metabolic complex. Specific Aim 1c is to examine disruption of the BCAA complex in genetic disorders of of the ACADs. I hypothesize that differential effects caused by the mutations in the various genes are responsible for the broad array of clinical symptoms seen in these disease. Specific Aim 2 is to demonstrate metabolic channeling of branched chain amino acid substrates in normal cells and those deficient in the branched chain specific ACADs. Specific Aim 2a is to use of metabolic flux studies to further characterize BCAA metabolism. I predict that propionyl-CoA generated from a labeled odd chain fatty acid will readily enter the TCA cycle while that derived from labeled BCAA will not. Specific Aim 2b is to characterize the flux through BCAA in cells deficient in one of the branched chain ACADs. I predict that flux of ILE and VAL metabolites through intact pathways in SBCAD and IBDH deficient cell lines will exceed that of leucine metabolites in IVDH cell lines. Specific Aim 3 is to characterize the mechanisms for the development of T2DM in ACAD10 deficient mice. Specific Aim 3a is to characterize the enzymatic function of ACAD10. I propose to perform expanded directed metabolomics studies on various tissues from ACAD10 mice to identify candidate substrates for the enzyme. Specific Aim 3b is to characterize the glucose insulin axis in ACAD10 deficient mice. These experiments will allow a better understanding of the mechanisms that define clinical risk in defects of these enzymes.