Experimental evidence that overexpression of NR2B glutamate receptor subunit is associated with brain vacuolation in adult glutaryl-CoA dehydrogenase deficient mice: A potential role for glutamatergic-induced excitotoxicity in GA I neuropathology

Systemic delivery of AAV-GCDH ameliorates HLD-induced phenotype in a glutaric aciduria type I mouse model

Glutaric aciduria type 1 (GA1) is a rare inherited metabolic disorder caused by a deficiency of glutaryl-coenzyme A dehydrogenase (GCDH), with accumulation of neurotoxic metabolites, resulting in a complex movement disorder, irreversible brain damage, and premature death in untreated individuals. While early diagnosis and a lysine restricted diet can extend survival, they do not prevent neurological damage in approximately one-third of treated patients, and more effective therapies are required. Here we report the efficacy of adeno-associated virus 9 (AAV9)-mediated systemic delivery of human GCDH at preventing a high lysine diet (HLD)-induced phenotype in Gcdh -/- mice. Neonatal treatment with AAV-GCDH restores GCDH expression and enzyme activity in liver and striatum. This treatment protects the mice from HLD-aggressive phenotype with all mice surviving this exposure; in stark contrast, a lack of treatment on an HLD triggers very high accumulation of glutaric acid, 3-hydroxyglutaric acid, and glutarylcarnitine in tissues, with about 60% death due to brain accumulation of toxic lysine metabolites. AAV-GCDH significantly ameliorates the striatal neuropathology, minimizing neuronal dysfunction, gliosis, and alterations in myelination. Magnetic resonance imaging findings show protection against striatal injury. Altogether, these results provide preclinical evidence to support AAV-GCDH gene therapy for GA1.

Modeling Glutaric Aciduria Type I in human neuroblastoma cells recapitulates neuronal damage that can be rescued by gene replacement

Glutaric Aciduria type I (GA1) is a rare neurometabolic disorder caused by mutations in the GDCH gene encoding for glutaryl-CoA dehydrogenase (GCDH) in the catabolic pathway of lysine, hydroxylysine and tryptophan. GCDH deficiency leads to increased concentrations of glutaric acid (GA) and 3-hydroxyglutaric acid (3-OHGA) in body fluids and tissues. These metabolites are the main triggers of brain damage. Mechanistic studies supporting neurotoxicity in mouse models have been conducted. However, the different vulnerability to some stressors between mouse and human brain cells reveals the need to have a reliable human neuronal model to study GA1 pathogenesis. In the present work we generated a GCDH knockout (KO) in the human neuroblastoma cell line SH-SY5Y by CRISPR/Cas9 technology. SH-SY5Y-GCDH KO cells accumulate GA, 3-OHGA, and glutarylcarnitine when exposed to lysine overload. GA or lysine treatment triggered neuronal damage in GCDH deficient cells. SH-SY5Y-GCDH KO cells also displayed features of GA1 pathogenesis such as increased oxidative stress vulnerability. Restoration of the GCDH activity by gene replacement rescued neuronal alterations. Thus, our findings provide a human neuronal cellular model of GA1 to study this disease and show the potential of gene therapy to rescue GCDH deficiency.

Disturbance of Mitochondrial Dynamics, Endoplasmic Reticulum-Mitochondria Crosstalk, Redox Homeostasis, and Inflammatory Response in the Brain of Glutaryl-CoA Dehydrogenase-Deficient Mice: Neuroprotective Effects of Bezafibrate

Patients with glutaric aciduria type 1 (GA1), a neurometabolic disorder caused by deficiency of glutaryl-CoA dehydrogenase (GCDH) activity, commonly manifest acute encephalopathy associated with severe striatum degeneration and progressive cortical and striatal injury whose pathogenesis is still poorly known. We evaluated redox homeostasis, inflammatory response, mitochondrial biogenesis and dynamics, endoplasmic reticulum (ER)-mitochondria crosstalk, and ER stress in the brain of GCDH-deficient (Gcdh^-/-) and wild-type (Gcdh^+/+) mice fed a high Lys chow, which better mimics the human neuropathology mainly characterized by striatal lesions. Increased lipid peroxidation and altered antioxidant defenses, including decreased concentrations of reduced glutathione and increased activities of superoxide dismutase, catalase, and glutathione transferase, were observed in the striatum and cerebral cortex of Gcdh^-/- mice. Augmented Iba-1 staining was also found in the dorsal striatum and neocortex, whereas the nuclear content of NF-κB was increased, and the cytosolic content of IκBα decreased in the striatum of the mutant animals, indicating a pro-inflammatory response. Noteworthy, in vivo treatment with the pan-PPAR agonist bezafibrate normalized these alterations. It was also observed that the ER-mitochondria crosstalk proteins VDAC1 and IP3R were reduced, whereas the ER stress protein DDIT3 was augmented in Gcdh^-/- striatum, signaling disturbances of these processes. Finally, DRP1 content was elevated in the striatum of Gcdh^-/- mice, indicating activated mitochondrial fission. We presume that some of these novel pathomechanisms may be involved in GA1 neuropathology and that bezafibrate should be tested as a potential adjuvant therapy for GA1.

Protective effects of L-carnitine on behavioral alterations and neuroinflammation in striatum of glutaryl-COA dehydrogenase deficient mice

Glutaric acidemia type 1 (GA1) is caused by glutaryl-CoA dehydrogenase deficiency that leads to a blockage in the metabolic route of the amino acids lysine and tryptophan and subsequent accumulation of glutaric acid (GA), 3-hydroxyglutaric acids and glutarylcarnitine (C5DC). Patients predominantly manifest neurological symptoms, associated with acute striatal degeneration, as well as progressive cortical and striatum injury whose pathogenesis is not yet fully established. Current treatment includes protein/lysine restriction and l-carnitine supplementation of (L-car). The aim of this work was to evaluate behavior parameters and pro-inflammatory factors (cytokines IL-1β, TNF-α and cathepsin-D levels), as well as the anti-inflammatory cytokine IL10 in striatum of knockout mice (Gcdh-/-) and wild type (WT) mice submitted to a normal or a high Lys diet. The potential protective effects of L-car treatment on these parameters were also evaluated. Gcdh-/- mice showed behavioral changes, including lower motor activity (decreased number of crossings) and exploratory activity (reduced number of rearings). Also, Gcdh-/- mice had significantly higher concentrations of glutarylcarnitine (C5DC) in blood and cathepsin-D (CATD), interleukin IL-1β and tumor factor necrosis alpha (TNF-α) in striatum than WT mice. Noteworthy, L-car treatment prevented most behavioral alterations, normalized CATD levels and attenuated IL-1β levels in striatum of Gcdh-/- mice. Finally, IL-1β was positively correlated with CATD and C5DC levels and L-car was negatively correlated with CATD. Our results demonstrate behavioral changes and a pro-inflammatory status in striatum of the animal model of GA1 and, most importantly, L-car showed important protective effects on these alterations.

Lipopolysaccharide-Elicited Systemic Inflammation Induces Selective Vulnerability of Cerebral Cortex and Striatum of Developing Glutaryl-CoA Dehydrogenase Deficient (Gcdh-/-) Mice to Oxidative Stress

We investigated redox homeostasis in cerebral and peripheral tissues of wild type (WT) and glutaryl-CoA dehydrogenase knockout mice (Gcdh^-/-) submitted to inflammation induced by lipopolysaccharide (LPS) since patients with glutaric aciduria type I (GA I) manifest acute encephalopathy during catabolic events triggered by inflammation. WT and Gcdh^-/- mice fed a low (0.9%) or high (4.7%) Lys chow were euthanized 4 h after LPS intraperitoneal injection. Cerebral cortex of Lys-restricted Gcdh^-/- animals presented no alterations of redox homeostasis, whereas those fed a high Lys chow showed increased malondialdehyde (MDA) levels and superoxide dismutase (SOD) activity, compared to WT mice. Furthermore, Gcdh^-/- mice receiving low Lys and injected with LPS presented elevated MDA levels and decreased reduced glutathione (GSH) concentrations, glutathione peroxidase (GPx), and glutathione reductase (GR) activities in cerebral cortex. LPS administration also decreased GSH values, as well as GPx and GR activities in cerebral cortex of Gcdh^-/- mice receiving Lys overload. Further experiments performed in WT and Gcdh^-/- mice injected with LPS and receiving either a low or high Lys chow revealed increased MDA levels and decreased GSH concentrations in cerebral cortex and striatum, but not in hippocampus, liver and heart of Gcdh^-/- mice, suggesting a selective vulnerability of these cerebral structures to oxidative stress during an inflammatory process. LPS administration also increased S100B and NF-κF protein levels in brain of Gcdh^-/- mice receiving high Lys. These data support the hypothesis that low Lys diet is beneficial in GA I by preventing redox imbalance, whereas a high Lys diet or systemic inflammation per se or combined induce oxidative stress in striatum and cerebral cortex that are mainly damaged in this disorder.

The lysine degradation pathway: Subcellular compartmentalization and enzyme deficiencies

Lysine degradation via formation of saccharopine is a pathway confined to the mitochondria. The second pathway for lysine degradation, the pipecolic acid pathway, is not yet fully elucidated and known enzymes are localized in the mitochondria, cytosol and peroxisome. The tissue-specific roles of these two pathways are still under investigation. The lysine degradation pathway is clinically relevant due to the occurrence of two severe neurometabolic disorders, pyridoxine-dependent epilepsy (PDE) and glutaric aciduria type 1 (GA1). The existence of three other disorders affecting lysine degradation without apparent clinical consequences opens up the possibility to find alternative therapeutic strategies for PDE and GA1 through pathway modulation. A better understanding of the mechanisms, compartmentalization and interplay between the different enzymes and metabolites involved in lysine degradation is of utmost importance.

Oxidative damage in glutaric aciduria type I patients and the protective effects of l-carnitine treatment

The deficiency of the enzyme glutaryl-CoA dehydrogenase, known as glutaric acidemia type I (GA-I), leads to the accumulation of glutaric acid (GA) and glutarilcarnitine (C5DC) in the tissues and body fluids, unleashing important neurotoxic effects. l-carnitine (l-car) is recommended for the treatment of GA-I, aiming to induce the excretion of toxic metabolites. l-car has also demonstrated an important role as antioxidant and anti-inflammatory in some neurometabolic diseases. This study evaluated GA-I patients at diagnosis moment and treated the oxidative damage to lipids, proteins, and the inflammatory profile, as well as in vivo and in vitro DNA damage, reactive nitrogen species (RNS), and antioxidant capacity, verifying if the actual treatment with l-car (100 mg kg^-1  day^-1 ) is able to protect the organism against these processes. Significant increases of GA and C5DC were observed in GA-I patients. A deficiency of carnitine in patients before the supplementation was found. GA-I patients presented significantly increased levels of isoprostanes, di-tyrosine, urinary oxidized guanine species, and the RNS, as well as a reduced antioxidant capacity. The l-car supplementation induced beneficial effects reducing these biomarkers levels and increasing the antioxidant capacity. GA, in three different concentrations, significantly induced DNA damage in vitro, and the l-car was able to prevent this damage. Significant increases of pro-inflammatory cytokines IL-6, IL-8, GM-CSF, and TNF-α were shown in patients. Thus, the beneficial effects of l-car presented in the treatment of GA-I are due not only by increasing the excretion of accumulated toxic metabolites, but also by preventing oxidative damage.

Impairment of GABAergic system contributes to epileptogenesis in glutaric acidemia type I

OBJECTIVES

Glutaric acidemia type I (GA-I) is an inherited neurometabolic disorder caused by deficiency of glutaryl-CoA dehydrogenase (GCDH) and characterized by increased levels of glutaric, 3-OH-glutaric, and glutaconic acids in the brain parenchyma. The increment of these organic acids inhibits glutamate decarboxylase (GAD) and consequently lowers the γ-aminobutyric acid (GABA) synthesis. Untreated patients exhibit severe neurologic deficits during development, including epilepsy, especially following an acute encephalopathy outbreak. In this work, we evaluated the role of the GABAergic system on epileptogenesis in GA-I using the Gcdh^-/- mice exposed to a high lysine diet (Gcdh^-/- -Lys).

METHODS

Spontaneous recurrent seizures (SRS), seizure susceptibility, and changes in brain oscillations were evaluated by video-electroencephalography (EEG). Cortical GABAergic synaptic transmission was evaluated using electrophysiologic and neurochemical approaches.

RESULTS

SRS were observed in 72% of Gcdh^-/- -Lys mice, whereas no seizures were detected in age-matched controls (Gcdh^+/+ or Gcdh^-/- receiving normal diet). The severity and number of PTZ-induced seizures were higher in Gcdh^-/- -Lys mice. EEG spectral analysis showed a significant decrease in theta and gamma oscillations and predominant delta waves in Gcdh^-/- -Lys mice, associated with increased EEG left index. Analysis of cortical synaptosomes revealed a significantly increased percentage of glutamate release and decreased GABA release in Gcdh^-/- -Lys mice that were associated with a decrease in cortical GAD immunocontent and activity and confirmed by reduced frequency of inhibitory events in cortical pyramidal cells.

SIGNIFICANCE

Using an experimental model with a phenotype similar to that of GA-I in humans-the Gcdh^-/- mice under high lysine diet (Gcdh^-/- -Lys)-we provide evidence that a reduction in cortical inhibition of Gcdh^-/- -Lys mice, probably induced by GAD dysfunction, leads to hyperexcitability and increased slow oscillations associated with neurologic abnormalities in GA-I. Our findings offer a new perspective on the pathophysiology of brain damage in GA-I.

Higher Vulnerability of Menadione-Exposed Cortical Astrocytes of Glutaryl-CoA Dehydrogenase Deficient Mice to Oxidative Stress, Mitochondrial Dysfunction, and Cell Death: Implications for the Neurodegeneration in Glutaric Aciduria Type I

Patients affected by glutaric aciduria type I (GA-I) show progressive cortical leukoencephalopathy whose pathogenesis is poorly known. In the present work, we exposed cortical astrocytes of wild-type (Gcdh ^+/+ ) and glutaryl-CoA dehydrogenase knockout (Gcdh ^-/- ) mice to the oxidative stress inducer menadione and measured mitochondrial bioenergetics, redox homeostasis, and cell viability. Mitochondrial function (MTT and JC1-mitochondrial membrane potential assays), redox homeostasis (DCFH oxidation, nitrate and nitrite production, GSH concentrations and activities of the antioxidant enzymes SOD and GPx), and cell death (propidium iodide incorporation) were evaluated in primary cortical astrocyte cultures of Gcdh ^+/+ and Gcdh ^-/- mice unstimulated and stimulated by menadione. We also measured the pro-inflammatory response (TNFα levels, IL1-β and NF-ƙB) in unstimulated astrocytes obtained from these mice. Gcdh ^-/- mice astrocytes were more vulnerable to menadione-induced oxidative stress (decreased GSH concentrations and altered activities of the antioxidant enzymes), mitochondrial dysfunction (decrease of MTT reduction and JC1 values), and cell death as compared with Gcdh ^+/+ astrocytes. A higher inflammatory response (TNFα, IL1-β and NF-ƙB) was also observed in Gcdh ^-/- mice astrocytes. These data indicate a higher susceptibility of Gcdh ^-/- cortical astrocytes to oxidative stress and mitochondrial dysfunction, probably leading to cell death. It is presumed that these pathomechanisms may contribute to the cortical leukodystrophy observed in GA-I patients.