Human d-amino acid oxidase: The inactive G183R variant

Gut Symptoms, Gut Dysbiosis and Gut-Derived Toxins in ALS

Many pathogenetic mechanisms have been proposed for amyotrophic lateral sclerosis (ALS). Recently, there have been emerging suggestions of a possible role for the gut microbiota. Gut microbiota have a range of functions and could influence ALS by several mechanisms. Here, we review the possible role of gut-derived neurotoxins/excitotoxins. We review the evidence of gut symptoms and gut dysbiosis in ALS. We then examine a possible role for gut-derived toxins by reviewing the evidence that these molecules are toxic to the central nervous system, evidence of their association with ALS, the existence of biochemical pathways by which these molecules could be produced by the gut microbiota and existence of mechanisms of transport from the gut to the blood and brain. We then present evidence that there are increased levels of these toxins in the blood of some ALS patients. We review the effects of therapies that attempt to alter the gut microbiota or ameliorate the biochemical effects of gut toxins. It is possible that gut dysbiosis contributes to elevated levels of toxins and that these could potentially contribute to ALS pathogenesis, but more work is required.

Aggregation of E121K mutant D-amino acid oxidase and ubiquitination-mediated autophagy mechanisms leading to amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a terminal adult-onset neuromuscular disorder. Our group has been studying this illness and previously reported novel mutations and rare mutations in a study using next-generation sequencing of DNA samples from Indian ALS patients. In this paper, we focus on the E121K mutation in the DAO gene to understand how it leads to ALS. Our experiments in SH-SY5Y cells indicate that the E121K mutation results in the accumulation of mutant protein aggregates, a change in cell morphology, and the death of neuronal cells. These protein aggregates get ubiquitinated and cause an imbalance in autophagy regulation. We observed an increase in the cellular concentrations of p62, OPTN, and LC3II. Through confocal microscopy studies, we show that the binding of p62 with ubiquitinated aggregates and its recruitment to LC3II mediates autophagosome generation. These relative changes in the key partners in autophagy increase cell death in cells harboring the E121K mutation and is a probable mechanism leading to ALS.

Characterization of E121K mutation of D-amino acid oxidase - Insights into mechanisms leading to amyotrophic lateral sclerosis

D-amino acid oxidase (DAO) maintains the intracellular d-serine level which modulates the activity of the N-methyl-d-aspartate receptor and its dysfunction has been linked to several neurodegenerative disorders. In targeted next-generation sequencing study by our group, E121K mutation in DAO was associated with amyotrophic lateral sclerosis (ALS) in patients from India. However, variations in molecular mechanisms caused by this mutation which leads to ALS have not been studied. Hence, we carried out comparative biophysical characterization and assay studies of the wildtype- and mutant E121K-DAO. We observed that the purified E121K-DAO was inactive and exhibited a lower affinity for the FAD cofactor and benzoate inhibitor. Structural studies revealed that the E121K mutant has higher beta-sheet content, melting temperature, and oligomeric states compared to the wildtype. Kinetic study of aggregation of the variants using thioflavin-T confirmed that the E121K-DAO was more prone to aggregation. Microscopic visualization showed that the aggregation proceeds through an intermediate step involving the formation of fibrillar structures in the E121K mutant. Our results give insights into the underlying mechanisms leading to ALS pathogenesis.

Mouse Models to Study Peroxisomal Functions and Disorders: Overview, Caveats, and Recommendations

During the last three decades many mouse lines were created or identified that are deficient in one or more peroxisomal functions. Different methodologies were applied to obtain global, hypomorph, cell type selective, inducible, and knockin mice. Whereas some models closely mimic pathologies in patients, others strongly deviate or no human counterpart has been reported. Often, mice, apparently endowed with a stronger transcriptional adaptation, have to be challenged with dietary additions or restrictions in order to trigger phenotypic changes. Depending on the inactivated peroxisomal protein, several approaches can be taken to validate the loss-of-function. Here, an overview is given of the available mouse models and their most important characteristics.

Mechanistic insights into the loss-of-function mechanisms of rare human D-amino acid oxidase variants implicated in amyotrophic lateral sclerosis

Impaired enzymatic activity in D-amino acid oxidase (DAAO) caused by missense mutations has been shown to trigger amyotrophic lateral sclerosis (ALS) through an abnormal accumulation of D-serine in the spinal cord. While loss of enzymatic functions of certain ALS-causing DAAO variants have been studied before, a detailed understanding of structure-dynamics-function relationship of the rare DAAO variants has not been investigated hitherto. To address this, we carried out a comprehensive study of all the reported rare DAAO variants. By employing a spectrum of bioinformatics analyses along with extensive structural dynamics simulations, we show that certain rare variants disrupted key interactions with the active site and decreased the conformational flexibility of active site loop comprising residues 216-228, which is essential for substrate binding and product release. Moreover, these variants lost crucial interactions with the cofactor flavin-adenine-dinucleotide, resulting in weaker binding affinity. A detailed inspection revealed that these variants exhibited such characteristics due to the abrogation of specific salt bridges. Taken together, our study provides a gateway into the structural-dynamic features of the rare DAAO variants and highlights the importance of informatics-based integrated analyses in the screening and prioritization of variants a priori to the clinical-functional characterization.

Human D-Amino Acid Oxidase: Structure, Function, and Regulation

D-Amino acid oxidase (DAAO) is an FAD-containing flavoenzyme that catalyzes with absolute stereoselectivity the oxidative deamination of all natural D-amino acids, the only exception being the acidic ones. This flavoenzyme plays different roles during evolution and in different tissues in humans. Its three-dimensional structure is well conserved during evolution: minute changes are responsible for the functional differences between enzymes from microorganism sources and those from humans. In recent years several investigations focused on human DAAO, mainly because of its role in degrading the neuromodulator D-serine in the central nervous system. D-Serine is the main coagonist of N-methyl D-aspartate receptors, i.e., excitatory amino acid receptors critically involved in main brain functions and pathologic conditions. Human DAAO possesses a weak interaction with the FAD cofactor; thus, in vivo it should be largely present in the inactive, apoprotein form. Binding of active-site ligands and the substrate stabilizes flavin binding, thus pushing the acquisition of catalytic competence. Interestingly, the kinetic efficiency of the enzyme on D-serine is very low. Human DAAO interacts with various proteins, in this way modulating its activity, targeting, and cell stability. The known properties of human DAAO suggest that its activity must be finely tuned to fulfill a main physiological function such as the control of D-serine levels in the brain. At present, studies are focusing on the epigenetic modulation of human DAAO expression and the role of post-translational modifications on its main biochemical properties at the cellular level.

Biochemical Properties of Human D-amino Acid Oxidase Variants and Their Potential Significance in Pathologies

The stereoselective flavoenzyme D-amino acid oxidase (DAAO) catalyzes the oxidative deamination of neutral and polar D-amino acids producing the corresponding α-keto acids, ammonia, and hydrogen peroxide. Despite its peculiar and atypical substrates, DAAO is widespread expressed in most eukaryotic organisms. In mammals (and humans in particular), DAAO is involved in relevant physiological processes ranging from D-amino acid detoxification in kidney to neurotransmission in the central nervous system, where DAAO is responsible of the catabolism of D-serine, a key endogenous co-agonist of N-methyl-D-aspartate receptors. Recently, structural and functional studies have brought to the fore the distinctive biochemical properties of human DAAO (hDAAO). It appears to have evolved to allow a strict regulation of its activity, so that the enzyme can finely control the concentration of substrates (such as D-serine in the brain) without yielding to an excessive production of hydrogen peroxide, a potentially toxic reactive oxygen species (ROS). Indeed, dysregulation in D-serine metabolism, likely resulting from altered levels of hDAAO expression and activity, has been implicated in several pathologies, ranging from renal disease to neurological, neurodegenerative, and psychiatric disorders. Only one mutation in DAO gene was unequivocally associated to a human disease. However, several single nucleotide polymorphisms (SNPs) are reported in the database and the biochemical characterization of the corresponding recombinant hDAAO variants is of great interest for investigating the effect of mutations. Here we reviewed recently published data focusing on the modifications of the structural and functional properties induced by amino acid substitutions encoded by confirmed SNPs and on their effect on D-serine cellular levels. The potential significance of the different hDAAO variants in human pathologies will be also discussed.

Focus on the Role of D-serine and D-amino Acid Oxidase in Amyotrophic Lateral Sclerosis/Motor Neuron Disease (ALS)

We have investigated a pathogenic mutation in D-amino acid oxidase (DAO), DAO^R199W, associated with familial Amyotrophic Lateral Sclerosis (ALS) that impairs D-serine metabolism and causes protein aggregation, autophagy and cell death in motor neuron cell lines. These features are consistent with the pathogenic processes occurring in ALS but most importantly, we have demonstrated that activation of the formation of ubiquitinated protein inclusions, increased autophagosome production and apoptotic cell death caused by the mutation in cell lines are attenuated by 5,7-dichlorokynurenic acid (DCKA), a selective inhibitor of the glycine/D-serine binding site of the NMDA receptor. D-serine is an essential co-agonist at this glutamate receptor. This data provides insight into potential upstream mechanisms that involve the action of D-serine at the NMDA receptor and might contribute to neurodegeneration. This is highly relevant to sporadic ALS (SALS), familial ALS, as well as ALS models, where elevated levels of D-serine have been reported and hence has broader clinical therapeutic implications. In order to investigate this further, we have generated a transgenic line expressing the pathogenic mutation, in order to determine whether mice expressing DAO^R199W develop a motor phenotype and whether crossing the SOD1^G93A model of ALS with mice expressing DAO^R199W affects disease progression. We found that heterozygous expression of DAO^R199W led to a significant loss of spinal cord motor neurons at 14 months, which is similar to that found in homozygous mice expressing DAO^G181R. We hypothesize that DAO has potential for development as a therapeutic agent in ALS.