Investigation of Anaplerosis from Propionyl-CoA Precursors and Fatty Acid Oxidation in the Brain of VLCAD and Control Mice

Anaplerotic therapy with triheptanoin is currently investigated for the treatment of long-chain fatty acid oxidation disorders. The recently developed mouse model deficient in very long-chain acyl-CoA dehydrogenase (VLCAD mouse) provides a good tool to study anaplerotic therapy in vivo. The goal of this research was to characterize brain fatty acid oxidation and to test the effect of anaplerotic therapy on brain metabolism in VLCAD mice and their controls. First, concentrations of major acyl-CoA esters (C₂-C₂₀) were profiled in the brain of VLCAD and control mice. The data indicated significant reductions of acetyl-CoA, methylmalonyl-CoA, propionyl-CoA, butyryl-CoA, hexanoyl-CoA and octanoyl-CoA in VLCAD brains vs. controls. The data also revealed clear evidence of long-chain fatty acid oxidation defect suggested by the accumulations of stearoyl-CoA (C_18:0-CoA) and linoleoyl-CoA (C_18:2-CoA) in VLCAD brains vs. controls.

Second, concentrations of major citric acid cycle intermediates and related neurotransmitters (glutamate, glutamine, GABA) in the whole brain were found lower in VLCAD mice vs. control mice, with three significant decreases identified (α-ketoglutarate, glutamate, and GABA). These reductions implicated a compromised pool size of the citric acid cycle intermediates which possibly lead to abnormalities in the brain of VLCAD mice.

Last, VLCAD and control mice were infused intravenously with increasing amounts of odd-chain fatty acids or C₅ ketone body (heptanoate, pentanoate, β-ketopentanoate or propionate). Other mice of the two genotypes were infused with non-anaplerotic octanoate. The concentration and labeling pattern of medium-chain acyl-CoAs demonstrated that the C₈, C₇, and C₅ fatty acids were taken up by the brain and activated to form CoA esters. The odd-chain fatty acids infused were metabolized via the β-oxidation cascade and contributed to a substantial fraction of acetyl-CoA in mice brains of both genotypes. Ketone bodies derived from partial oxidation of infused fatty acids in the liver also contributed to the brain acetyl-CoA. All odd-chain fatty acids or C₅ ketone body infused (heptanoate, pentanoate, and propionate) were strongly anaplerotic at low blood concentrations, and more so in VLCAD brains than in control brains. A fraction of anaplerosis from the odd-chain fatty acids involved their conversions in the liver to C₅-ketone bodies which were also anaplerotic in the brain.

In conclusion, data of the present research revealed that medium-chain fatty acids enter the brain as such, where they are metabolized by the fatty acid oxidation cascade. Odd-chain fatty acids and C₅-ketone bodies of propionyl-CoA precursors contribute to brain anaplerosis rapidly in normal mouse brains, and even more rapidly in VLCAD brains. In VLCAD brains the long-chain fatty acid oxidation occurs by a vicariant enzyme system to be identified.