Neurodegeneration in Methylmalonic Aciduria Involves Inhibition of Complex II and the Tricarboxylic Acid Cycle, and Synergistically Acting Excitotoxicity

Abstract

Methylmalonic acidurias are biochemically characterized by an accumulation of methylmalonate (MMA) and alternative metabolites. There is growing evidence for basal ganglia degeneration in these patients. The pathomechanisms involved are still unknown, a contribution of toxic organic acids, in particular MMA, has been suggested. Here we report that MMA induces neuronal damage in cultures of embryonic rat striatal cells at a concentration range encountered in affected patients. MMA-induced cell damage was reduced by ionotropic glutamate receptor antagonists, antioxidants, and succinate. These results suggest the involvement of secondary excitotoxic mechanisms in MMA-induced cell damage. MMA has been implicated in inhibition of respiratory chain complex II. However, MMA failed to inhibit complex II activity in submitochondrial particles from bovine heart. To unravel the mechanism underlying neuronal MMA toxicity, we investigated the formation of intracellular metabolites in MMA-loaded striatal neurons. There was a time-dependent intracellular increase in malonate, an inhibitor of complex II, and 2-methylcitrate, a compound with multiple inhibitory effects on the tricarboxylic acid cycle, suggesting their putative implication in MMA neurotoxicity. We propose that neuropathogenesis of methylmalonic aciduria may involve an inhibition of complex II and the tricarboxylic acid cycle by accumulating toxic organic acids, and synergistic secondary excitotoxic mechanisms.

Methylmalonic Acid, a Biochemical Hallmark of Methylmalonic Acidurias but No Inhibitor of Mitochondrial Respiratory Chain

“…However, recently we have demonstrated in striatal neuronal cultures from rat embryos that MMA-induced neuronal damage involves intracellular formation of the competitive complex II inhibitor malonate (MA) and 2-methylcitrate, a compound with multiple inhibitory properties on the tricarboxylic acid cycle (15). Therefore, we suggest that neuronal damage is mainly driven via metabolites that derive from alterna-tive oxidation pathways of propionyl-CoA rather than by MMA itself.…”

“…Protein was determined according to Lowry et al (21) with modifications of Helenius and Simons (22) using bovine serum albumin as the standard. The catalytic activity of respiratory chain complex II was investigated in submitochondrial particles (SMPs) from bovine heart as previously described using decylubiquinone as the electron mediator (15,23). In brief, steady state activities of mitochondrial complex II were recorded using a computer tunable spectrophotometer (SPECTRAmax Plus 384 Microplate Reader, Molecular Devices, Sunnyvale, CA) operating in the dual wavelength mode.…”

“…MMA induced cell damage in different neuronal culture systems (13)(14)(15) and evokes rotational behavior, seizures, and striatal lesions in rats after intrastriatal administration (16,17). MMA-induced changes were prevented by succinate, antagonists of ionotropic glutamate receptors, and antioxidants (15,16,18). Consequently, MMA has been suggested to induce so-called "secondary" or "weak" excitotoxicity (12,19).…”

“…It has been hypothesized that MMA induced inhibition of complex II (synonym, succinate:ubiquinone oxidoreductase), a multiprotein assembly imparted in the tricarboxylic acid cycle and the mitochondrial respiratory chain, has become a focus of interest (12). MMA induced cell damage in different neuronal culture systems (13)(14)(15) and evokes rotational behavior, seizures, and striatal lesions in rats after intrastriatal administration (16,17). MMA-induced changes were prevented by succinate, antagonists of ionotropic glutamate receptors, and antioxidants (15,16,18).…”

Expanding the Spectrum of Methylmalonic Acid-Induced Pallidal Stroke: First Reported Case of Metabolic Globus Pallidus Stroke in Transcobalamin II Deficiency

“…Cobalamin ultimately has an important role in the metabolism of both homocysteine and methylmalonic acid as a cofactor for methionine synthase and methylmalonyl-CoA mutase, respectively (Takahashi-Iñiguez et al 2012;Banerjee and Ragsdale 2003). MethylmalonylCoA mutase isomerizes mitochondrial methylmalonyl-CoA to succinyl-CoA, a crucial substrate in energy provision via the citric acid cycle and complex II of the respiratory chain (Dutra et al 1993;Okun et al 2002). Inherited deficiency of TCII has been described in over 40 patients (Watkins and Rosenblatt 2011).…”