Friederike Hörster

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

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.

Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia

Methylmalonic and propionic acidemia (MMA/PA) are inborn errors of metabolism characterized by accumulation of propionic acid and/or methylmalonic acid due to deficiency of methylmalonyl-CoA mutase (MUT) or propionyl-CoA carboxylase (PCC). MMA has an estimated incidence of ~ 1: 50,000 and PA of ~ 1:100-000 -150,000. Patients present either shortly after birth with acute deterioration, metabolic acidosis and hyperammonemia or later at any age with a more heterogeneous clinical picture, leading to early death or to severe neurological handicap in many survivors. Mental outcome tends to be worse in PA and late complications include chronic kidney disease almost exclusively in MMA and cardiomyopathy mainly in PA. Except for vitamin B12 responsive forms of MMA the outcome remains poor despite the existence of apparently effective therapy with a low protein diet and carnitine. This may be related to under recognition and delayed diagnosis due to nonspecific clinical presentation and insufficient awareness of health care professionals because of disease rarity.These guidelines aim to provide a trans-European consensus to guide practitioners, set standards of care and to help to raise awareness. To achieve these goals, the guidelines were developed using the SIGN methodology by having professionals on MMA/PA across twelve European countries and the U.S. gather all the existing evidence, score it according to the SIGN evidence level system and make a series of conclusive statements supported by an associated level of evidence. Although the degree of evidence rarely exceeds level C (evidence from non-analytical studies like case reports and series), the guideline should provide a firm and critical basis to guide practice on both acute and chronic presentations, and to address diagnosis, management, monitoring, outcomes, and psychosocial and ethical issues. Furthermore, these guidelines highlight gaps in knowledge that must be filled by future research. We consider that these guidelines will help to harmonize practice, set common standards and spread good practices, with a positive impact on the outcomes of MMA/PA patients.

Intracerebral accumulation of glutaric and 3‐hydroxyglutaric acids secondary to limited flux across the blood–brain barrier constitute a biochemical risk factor for neurodegeneration in glutaryl‐CoA dehydrogenase deficiency

Glutaric acid (GA) and 3‐hydroxyglutaric acids (3‐OH‐GA) are key metabolites in glutaryl co‐enzyme A dehydrogenase (GCDH) deficiency and are both considered to be potential neurotoxins. As cerebral concentrations of GA and 3‐OH‐GA have not yet been studied systematically, we investigated the tissue‐specific distribution of these organic acids and glutarylcarnitine in brain, liver, skeletal and heart muscle of Gcdh‐deficient mice as well as in hepatic Gcdh–/– mice and in C57Bl/6 mice following intraperitoneal loading. Furthermore, we determined the flux of GA and 3‐OH‐GA across the blood–brain barrier (BBB) using porcine brain microvessel endothelial cells. Concentrations of GA, 3‐OH‐GA and glutarylcarnitine were significantly elevated in all tissues of Gcdh–/– mice. Strikingly, cerebral concentrations of GA and 3‐OH‐GA were unexpectedly high, reaching similar concentrations as those found in liver. In contrast, cerebral concentrations of these organic acids remained low in hepatic Gcdh–/– mice and after intraperitoneal injection of GA and 3‐OH‐GA. These results suggest limited flux of GA and 3‐OH‐GA across the BBB, which was supported in cultured porcine brain capillary endothelial cells. In conclusion, we propose that an intracerebral de novo synthesis and subsequent trapping of GA and 3‐OH‐GA should be considered as a biochemical risk factor for neurodegeneration in GCDH deficiency.

Long-term outcome in methylmalonic acidurias is influenced by the underlying defect (mut0, mut-, cblA, cblB).

Isolated methylmalonic acidurias comprise a heterogeneous group of inborn errors of metabolism caused by defects of methylmalonyl-CoA mutase (MCM) (mut0, mut–) or deficient synthesis of its cofactor 5′-deoxyadenosylcobalamin (AdoCbl) (cblA, cblB). The aim of this study was to compare the long-term outcome in patients from these four enzymatic subgroups. Eighty-three patients with isolated methylmalonic acidurias (age 7–33 y) born between 1971 and 1997 were enzymatically characterized and prospectively followed to evaluate the long-term outcome (median follow-up period, 18 y). Patients with mut0 (n = 42), mut− (n = 10), cblA (n = 20), and cblB (n = 11) defects were included into the study. Thirty patients (37%) died, and 26 patients survived with a severe or moderate neurologic handicap (31%), whereas 27 patients (32%) remained neurologically uncompromised. Chronic renal failure (CRF) was found most frequently in mut0 (61%) and cblB patients (66%), and was predicted by the urinary excretion of methylmalonic acid (MMA) before CRF. Overall, patients with mut0 and cblB defects had an earlier onset of symptoms, a higher frequency of complications and deaths, and a more pronounced urinary excretion of MMA than those with mut− and cblA defects. In addition, long-term outcome was dependent on the age cohort and cobalamin responsiveness.

NMDA receptor activation and respiratory chain complex V inhibition contribute to neurodegeneration in d‐2‐hydroxyglutaric aciduria

The inherited neurometabolic disease d‐2‐hydroxyglutaric aciduria is complicated by progressive neurodegeneration of vulnerable brain regions during infancy and early childhood, frequently presenting with hypotonia, epilepsy and psychomotor retardation. Here, we report that the pathogenetic role of the endogenously accumulating metabolite d‐2‐hydroxyglutarate (D‐2), which is structurally similar to the excitatory amino acid glutamate, is mediated by at least three mechanisms. (i) D‐2‐induced excitotoxic cell damage in primary neuronal cultures from chick and rat involved N‐methyl‐d‐aspartate (NMDA) receptor activation. Indeed, D‐2 activated recombinant NMDA receptors (NR1/NR2A, NR1/NR2B) but not recombinant alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole (AMPA) receptors in HEK293 cells. (ii) Fluorescence microscopy using fura‐2 as a calcium indicator and the oxidant‐sensitive dye dihydrorhodamine‐123 revealed that D‐2 disturbed intracellular calcium homeostasis and elicited the generation of reactive oxygen species. (iii) D‐2 reduced complex V (ATP synthase) activity of the mitochondrial respiratory chain, reflecting an impaired energy metabolism due to inhibition of ATP synthesis but without affecting the electron‐transferring complexes I–IV. Thus, D‐2 stimulates neurodegeneration by mechanisms well‐known for glutamate, NMDA or mitochondrial toxins. In conclusion, excitotoxicity contributes to the neuropathology of d‐2‐hydroxyglutaric aciduria, highlighting new neuroprotective strategies.

Neurodegeneration and chronic renal failure in methylmalonic aciduria—A pathophysiological approach

SummaryIn the last decades the survival of patients with methylmalonic aciduria has been improved. However, the overall outcome of affected patients remains disappointing. The disease course is often complicated by acute life-threatening metabolic crises, which can result in multiple organ failure or even death, resembling primary defects of mitochondrial energy metabolism. Biochemical abnormalities during metabolic derangement, such as metabolic acidosis, ketonaemia/ketonuria, lactic acidosis, hypoglycaemia and hyperammonaemia, suggest mitochondrial dysfunction. In addition, long-term complications such as chronic renal failure and neurological disease are frequently found. Neuropathophysiological studies have focused on various effects caused by accumulation of putatively toxic organic acids, the so-called ‘toxic metabolite’ hypothesis. In previous studies, methylmalonate (MMA) has been considered as the major neurotoxin in methylmalonic aciduria, whereas more recent studies have highlighted a synergistic inhibition of mitochondrial energy metabolism (pyruvate dehydrogenase complex, tricarboxylic acid cycle, respiratory chain, mitochondrial salvage pathway of deoxyribonucleoside triphosphate (dNTP)) induced by propionyl-CoA, 2-methylcitrate and MMA as the key pathomechanism of inherited disorders of propionate metabolism. Intracerebral accumulation of toxic metabolites (‘trapping’ hypothesis’) is considered a biochemical risk factor for neurodegeneration. Secondary effects of mitochondrial dysfunction, such as oxidative stress and impaired mtDNA homeostasis, contribute to pathogenesis of these disorders. The underlying pathomechanisms of chronic renal insufficiency in methylmalonic acidurias are not yet understood. We hypothesize that renal and cerebral pathomechanisms share some similarities, such as an involvement of dicarboxylic acid transport. This review aims to give a comprehensive overview on recent pathomechanistic concepts for methylmalonic acidurias.

Methylmalonic acid, a biochemical hallmark of methylmalonic acidurias but no inhibitor of mitochondrial respiratory chain.

Methylmalonic acidurias are biochemically characterized by an accumulation of methylmalonic acid and alternative metabolites. An impairment of energy metabolism plays a key role in the pathophysiology of this disease, resulting in neurodegeneration of the basal ganglia and renal failure. It has become the subject of intense debates whether methylmalonic acid is the major toxin, inhibiting respiratory chain complex II. To elucidate whether methylmalonic acid is a respiratory chain inhibitor, we used spectrophotometric analysis of complex II activity in submitochondrial particles from bovine heart, radiometric analysis of 14C-labeled substrates (pyruvate, malate, succinate), and analysis of ATP production in muscle from mice. Methylmalonic acid revealed no direct effects on the respiratory chain function, i.e. on single electron transferring complexes I-IV, ATPase, and mitochondrial transporters. However, we identified a variety of variables that must be carefully controlled to avoid an artificial inhibition of complex II activity. In summary, the study verifies our hypothesis that methylmalonic acid is not the major toxic metabolite in methylmalonic acidurias. Inhibition of respiratory chain and tricarboxylic acid cycle is most likely induced by synergistically acting alternative metabolites, in particular 2-methylcitric acid, malonic acid, and propionyl-CoA.

Aromatic l -amino acid decarboxylase deficiency: clinical features, drug therapy and follow-up

SummaryBackgroundAromatic l-amino acid decarboxylase (AADC) deficiency is a disorder of biogenic amine metabolism resulting in generalized combined deficiency of serotonin, dopamine and catecholamines. Main clinical features are developmental delay, muscular hypotonia, dystonia, oculogyric crises and additional extraneurological symptoms. Response to therapy has been variable and unsatisfactory; the overall prognosis is guarded.MethodsTo gain more insight into this rare disorder we collected clinical and laboratory data of nine German patients. All patients were clinically examined by one investigator, and their responses to different drug regimes were evaluated by the patients’ charts.ResultsSymptoms were obvious from early infancy. Later, main neurological features were truncal muscular hypotonia, hypokinesia, oculogyric crises and rigor. Three patients had single seizures. All patients presented distinct extraneurological symptoms, such as hypersalivation, hyperhidrosis, nasal congestion, sleep disturbances and hypoglycaemia. In CSF all patients revealed the pattern typical of AADC with decreased concentrations of homovanillic and 5-hydroxyindoleacetic acid and elevated concentration of 3-ortho-methyldopa. Diagnosis was confirmed by measurement of AADC activity in plasma in all patients. Drug regimes consisted of vitamin B6, dopamine agonists, MAO inhibitors and anticholinergics in different combinations. No patient achieved a complete recovery from neurological symptoms, but partial improvement of mobility and mood could be achieved in some.ConclusionAADC deficiency is a severe neurometabolic disorder, characterized by muscular hypotonia, dystonia, oculogyric crises and additional extraneurological symptoms. Medical treatment is challenging, but a systematic trial of the different drugs is worthwhile.

Chronic treatment with glutaric acid induces partial tolerance to excitotoxicity in neuronal cultures from chick embryo telencephalons.

Glutaryl‐CoA dehydrogenase deficiency (GDD) is characterized biochemically by an accumulation of glutaric (GA) and 3‐hydroxyglutaric (3‐OH‐GA) acids and clinically by the development of acute striatal degeneration. 3‐OH‐GA was recently shown to induce neuronal damage via N‐methyl‐D‐aspartate (NMDA) receptors. The pathogenetic role of GA, however, remains unclear. We demonstrate that GA exerts a dual action in cultured chick embryo neurons. Short‐term incubation with millimolar concentrations of GA induces a weak neuronal damage, adding to 3‐OH‐GA neurotoxicity. In contrast, chronic treatment with subtoxic, micromolar concentrations of GA results in partial tolerance to 3‐OH‐GA‐ and NMDA‐induced cell damage. A downregulation of NMDA receptors, in particular of the NR2B subunit, is critically involved in this GA‐induced effect, resulting in a reduced Ca2+ increase and generation of reactive oxygen species after acute exposure to NMDA or 3‐OH‐GA. Furthermore, GA decreases Na+/K+‐ATPase activity, which is prevented by glutathione, suggesting a modulation of NMDA receptor function via resting membrane potential and Na+‐dependent glutamate transport. In contrast, GA does not inhibit mitochondrial respiratory chain and β‐oxidation of fatty acids, virtually excluding an activation of NMDA receptors secondary to ATP depletion. These results strongly suggest that GA modulates the NMDA receptor‐mediated neurotoxicity of 3‐OH‐GA, providing an explanatory basis for the non‐linear relationship between organic acid concentrations and disease progression in GDD patients. Furthermore, GA‐induced downregulation of NMDA receptors might be involved in the delayed cerebral maturation of GDD patients, resulting in frontotemporal atrophy and a reduced opercularization, which are common neuroradiological findings in GDD patients.

Ca 2+ and Na + Dependence of 3-Hydroxyglutarate-Induced Excitotoxicity in Primary Neuronal Cultures from Chick Embryo Telencephalons

Glutaryl-CoA dehydrogenase deficiency (also known as glutaric aciduria type I) is an autosomal, recessively inherited neurometabolic disorder with a distinct neuropathology characterized by acute encephalopathy during a vulnerable period of brain development. Neuronal damage in this disease was demonstrated to involve N-methyl-d-aspartate (NMDA) receptor-mediated neurotoxicity of the endogenously accumulating metabolite 3-hydroxyglutarate (3-OH-GA). However, it remained unclear whether NMDA receptors are directly or indirectly activated and whether 3-OH-GA disturbs the intracellular Ca2+ homeostasis. Here we report that 3-OH-GA activated recombinant NMDA receptors (e.g. NR1/NR2A) but not recombinant α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (e.g. GluR-A/GluR-B) in HEK293 cells. Fluorescence microscopy using fura-2 as Ca2+ indicator revealed that 3-OH-GA increased intracellular Ca2+ concentrations in the presence of extracellular Ca2+ in cultured chick neurons. Similar to glutamate-induced cell damage, 3-OH-GA neurotoxicity was modulated by extracellular Na+. The large cation N-methyl-d-glucamine, which does not permeate NMDA receptor channels, enhanced 3-OH-GA-induced Ca2+ increase and cell damage. In contrast, 3-OH-GA-induced neurotoxicity was reduced after replacement of Na+ by Li+, which permeates NMDA channels but does not affect the Na+/Ca2+ exchanger in the plasma membrane. Spectrophotometric analysis of respiratory chain complexes I–V in submitochondrial particles from bovine heart revealed only a weak inhibition of 3-OH-GA on complex V at the highest concentration tested (10 mM). In conclusion, the present study revealed that NMDA receptor activation and subsequent disturbance of Ca2+ homeostasis contribute to 3-OH-GA-induced cell damage.