Progressive encephalopathy after routine 4-month immunizations in a patient with NAXD genetic variant.

Metabolic pathways are known to generate byproducts-some of which have no clear metabolic function and some of which are toxic. Nicotinamide adenine dinucleotide phosphate hydrate (NAD(P)HX) is a toxic metabolite that is produced by stressors such as a fever, infection, or physical stress. Nicotinamide adenine dinucleotide phosphate hydrate dehydratase (NAXD) and nicotinamide adenine dinucleotide phosphate hydrate epimerase (NAXE) are part of the nicotinamide repair system that function to break down this toxic metabolite. Deficiency of NAXD and NAXE interrupts the critical intracellular repair of NAD(P)HX and allows for its accumulation. Clinically, deficiency of NAXE manifests as progressive, early onset encephalopathy with brain edema and/or leukoencephalopathy (PEBEL) 1, while deficiency of NAXD manifests as PEBEL2. In this report, we describe a case of probable PEBEL2 in a patient with a variant of unknown significance (c.362C>T, p.121L) in the NAXD gene who presented after routine immunizations with significant skin findings and in the absence of fevers.

Dopaminergic neuromodulation of prefrontal cortex activity requires the NMDA receptor coagonist d-serine.

Prefrontal control of cognitive functions critically depends upon glutamatergic transmission and N-methyl D-aspartate (NMDA) receptors, the activity of which is regulated by dopamine. Yet whether the NMDA receptor coagonist d-serine is implicated in the dopamine-glutamate dialogue in the prefrontal cortex (PFC) and other brain areas remains unexplored. Here, using electrophysiological recordings, we show that d-serine is required for the fine-tuning of glutamatergic neurotransmission, neuronal excitability, and synaptic plasticity in the PFC through the actions of dopamine at D1 and D3 receptors. Using in vivo microdialysis, we show that D1 and D3 receptors exert a respective facilitatory and inhibitory influence on extracellular levels and activity of d-serine in the PFC, with actions expressed primarily via the cAMP/protein kinase A (PKA) signaling cascade. Further, using functional magnetic resonance imaging (fMRI) and behavioral assessment, we show that d-serine is required for the potentiation of cognition by D3R blockade as revealed in a test of novel object recognition memory. Collectively, these results unveil a key role for d-serine in the dopaminergic neuromodulation of glutamatergic transmission and PFC activity, findings with clear relevance to the pathogenesis and treatment of diverse brain disorders involving alterations in dopamine-glutamate cross-talk.

NAXE deficiency: A neurometabolic disorder of NAD(P)HX repair amenable for metabolic correction.

The NAD(P)HX repair system is a metabolite damage repair mechanism responsible for restoration of NADH and NADPH after their inactivation by hydration. Deficiency in either of its two enzymes, NAD(P)HX dehydratase (NAXD) or NAD(P)HX epimerase (NAXE), causes a fatal neurometabolic disorder characterized by decompensations precipitated by inflammatory stress. Clinical findings include rapidly progressive muscle weakness, ataxia, ophthalmoplegia, and motor and cognitive regression, while neuroimaging abnormalities are subtle or nonspecific, making a clinical diagnosis challenging. During stress, nonenzymatic conversion of NAD(P)H to NAD(P)HX increases, and in the absence of repair, NAD(P)H is depleted, and NAD(P)HX accumulates, leading to decompensation; however, the contribution of each to the metabolic derangement is not established. Herein, we summarize the clinical knowledge of NAXE deficiency from 30 cases and lessons learned about disease pathogenesis from cell cultures and model organisms and describe a metabolomics signature obtained by untargeted metabolomics analysis in one case at the time of crisis and after initiation of treatment. Overall, biochemical findings support a model of acute depletion of NAD+, signs of mitochondrial dysfunction, and altered lipidomics. These findings are further substantiated by untargeted metabolomics six months post-crisis showing that niacin supplementation reverses primary metabolomic abnormalities concurrent with improved clinical status.

Increased excitation-inhibition balance and loss of GABAergic synapses in the serine racemase knockout model of NMDA receptor hypofunction.

There is substantial evidence that both N-methyl-D-aspartate receptor (NMDAR) hypofunction and dysfunction of GABAergic neurotransmission contribute to schizophrenia, though the relationship between these pathophysiological processes remains largely unknown. Although models using cell-type-specific genetic deletion of NMDARs have been informative, they display overly pronounced phenotypes extending beyond those of schizophrenia. Here, we used the serine racemase knockout (SRKO) mice, a model of reduced NMDAR activity rather than complete receptor elimination, to examine the link between NMDAR hypofunction and decreased GABAergic inhibition. The SRKO mice, in which there is a >90% reduction in the NMDAR coagonist d-serine, exhibit many of the neurochemical and behavioral abnormalities observed in schizophrenia. We found a significant reduction in inhibitory synapses onto CA1 pyramidal neurons in the SRKO mice. This reduction increases the excitation/inhibition balance resulting in enhanced synaptically driven neuronal excitability without changes in intrinsic excitability. Consistently, significant reductions in inhibitory synapse density in CA1 were observed by immunohistochemistry. We further show, using a single-neuron genetic deletion approach, that the loss of GABAergic synapses onto pyramidal neurons observed in the SRKO mice is driven in a cell-autonomous manner following the deletion of SR in individual CA1 pyramidal cells. These results support a model whereby NMDAR hypofunction in pyramidal cells disrupts GABAergic synapses leading to disrupted feedback inhibition and impaired neuronal synchrony.NEW & NOTEWORTHY Recently, disruption of excitation/inhibition (E/I) balance has become an area of considerable interest for psychiatric research. Here, we report a reduction in inhibition in the serine racemase knockout mouse model of schizophrenia that increases E/I balance and enhances synaptically driven neuronal excitability. This reduced inhibition was driven cell-autonomously in pyramidal cells lacking serine racemase, suggesting a novel mechanism for how chronic NMDA receptor hypofunction can disrupt information processing in schizophrenia.

Asymptomatic retinal dysfunction in alpha-methylacyl-CoA racemase deficiency.

Purpose: Alpha-methylacyl-CoA racemase (AMACR) deficiency is a peroxisomal disorder due to biallelic mutations in AMACR. At least 13 genetically confirmed patients have been reported to date. Seven had obvious pigmentary retinopathy; however, for the other six, no retinal phenotype was mentioned. The purpose of this report is to document subtle retinal findings in an additional affected family. Methods: Retrospective case series (three affected siblings and their unaffected parents). Results: Three Arab siblings (16, 19, and 22 years old) with prior juvenile cholelithiasis had been diagnosed with AMACR deficiency based on biochemical analysis, whole exome sequencing, and confirmatory segregation analysis (AMACR NM_001167595.1: c.877T>C; p.C293R). For all three, there were no visual complaints, but retinal multimodal imaging and electroretinography suggested subtle retinal dysfunction. Conclusions: Retinal dysfunction is a parameter that should be measured in patients with known or suspected AMACR deficiency even in the absence of visual symptoms. This may be helpful with clinical diagnosis and monitoring response to dietary interventions.

MCEE Mutations in an Adult Patient with Parkinson's Disease, Dementia, Stroke and Elevated Levels of Methylmalonic Acid.

Methylmalonic aciduria (MMA-uria) is seen in several inborn errors of metabolism (IEM) affecting intracellular cobalamin pathways. Methylmalonyl-CoA epimerase (MCE) is an enzyme involved in the mitochondrial cobalamin-dependent pathway generating succinyl-CoA. Homozygous mutations in the corresponding MCEE gene have been shown in children to cause MCE deficiency with isolated MMA-uria and a variable clinical phenotype. We describe a 78-year-old man with Parkinson's disease, dementia and stroke in whom elevated serum levels of methylmalonic acid had been evident for many years. Metabolic work-up revealed intermittent MMA-uria and increased plasma levels of propionyl-carnitine not responsive to treatment with high-dose hydroxycobalamin. Whole genome sequencing was performed, with data analysis targeted towards genes known to cause IEM. Compound heterozygous mutations were identified in the MCEE gene, c.139C>T (p.Arg47X) and c.419delA (p.Lys140fs), of which the latter is novel. To our knowledge, this is the first report of an adult patient with MCEE mutations and MMA-uria, thus adding novel data to the possible phenotypical spectrum of MCE deficiency. Although clinical implications are uncertain, it can be speculated whether intermittent hyperammonemia during episodes of metabolic stress could have precipitated the patient's ongoing neurodegeneration attributed to Parkinson's disease.

Potential deficit from decreased cerebellar granule cell migration in serine racemase-deficient mice is reversed by increased expression of GluN2B and elevated levels of NMDAR agonists.

Inward migration of cerebellar granule cells (CGCs) after birth is critical for lamination in the cerebellar cortex. N-methyl-d-aspartate (NMDA) subtype of glutamate receptor (NMDAR) tethering CGCs into Bergmann glial fibers mediates the inward movement during the glial-dependent migratory phase. Activation of NMDAR depends on simultaneous binding of the GluN2 subunit by glutamate, and of the GluN1 subunit by d-serine or glycine; d-serine is believed to be an endogenous ligand of NMDAR. We hypothesized that lamination of the cerebellar cortex may be compromised in Srr (the gene for serine racemase (SR)) mutated mice (Srrnull) because of significantly low levels of d-serine per se. Indeed, the external germinal cell layer (EGL) in Srrnull was thicker than in sibling wild-type (WT) mice on postnatal day7 (P7), which accords with decreased CGC migration in Srrnull mice. However, the cerebellar laminar structure in Srrnull mice was normal on P12 and later. Feeding d-serine to pregnant mice abrogated the increased EGL thickness in Srrnull mice on P7. To determine the underlying mechanism of abnormal laminar structure during cerebellar development in Srrnull mice, we examined NMDAR subunits and their ligands. We found increased GluN2B on P10 and increased glycine during P7-12 in the cerebellar homogenates from Srrnull mice compared with the corresponding values from sibling WT mice. In summary, the study revealed how the potential defect in early cerebellar development caused by Srr mutation is circumvented by a compensatory mechanism. This knowledge advances understanding of the adaptation of cerebellar development under the condition of Srr mutation.

Serine racemase deficiency attenuates choroidal neovascularization and reduces nitric oxide and VEGF levels by retinal pigment epithelial cells.

Choroidal neovascularization (CNV) is a leading cause of blindness in age-related macular degeneration. Production of vascular endothelial growth factor (VEGF) and macrophage recruitment by retinal pigment epithelial cells (RPE) significantly contributes to the process of CNV in an experimental CNV model. Serine racemase (SR) is expressed in retinal neurons and glial cells, and its product, d-serine, is an endogenous co-agonist of N-methyl-d-aspartate receptor. Activation of the receptor results in production of nitric oxide (. NO), a molecule that promotes retinal and choroidal neovascularization. These observations suggest possible roles of SR in CNV. With laser-injured CNV mice, we found that inactivation of SR-coding gene (Srrnull ) significantly reduced CNV volume, neovascular density, and invading macrophages. We exploited the underlying mechanism in vivo and ex vivo. RPE from wild-type (WT) mice expressed SR. To explore the possible downstream target of SR inactivation, we showed that choroid/RPE homogenates extracted from laser-injured Srrnull mice contained less inducible nitric oxide synthase and decreased phospho-VEGFR2 compared to amounts in WT mice. In vitro, inflammation-primed WT RPEs expressed more inducible NOS, produced more. NO and VEGF than did inflammation-primed Srrnull RPEs. When co-cultured with inflammation-primed Srrnull RPE, significantly fewer RF/6A-a cell line of choroidal endothelial cell, migrated to the opposite side of the insert membrane than did cells co-cultured with pre-treated WT RPE. Altogether, SR deficiency reduces RPE response to laser-induced inflammatory stimuli, resulting in decreased production of a cascade of pro-angiogenic cytokines, including. NO and VEGF, and reduced macrophage recruitment, which contribute synergistically to attenuated angiogenesis.

Alpha-methylacyl-CoA racemase deletion has mutually counteracting effects on T-cell responses, associated with unchanged course of EAE.

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. Altering the metabolism of immune cells is an attractive strategy to modify their activity during autoimmunity in MS. We investigated the effect of modulating fatty acid metabolism in an animal model of MS, EAE. Alpha-methylacyl-CoA racemase (AMACR) converts R-configuration branched fatty acids into the S-configuration, thereby preparing them for ß-oxidation. We observed a significant, disease-dependent elevation of AMACR expression in monocytes and T cells from blood, draining lymph nodes and spleen of EAE mice during the preclinical phase. In vitro analysis revealed that the proliferation of T cells was inhibited in AMACR KO mice, but T-cell polarization was switched toward a pathogenic state involving the production of more IFN-γ and IL-17, but less IL-4. These opposing effects appeared to cancel out each other in vivo, because AMACR KO EAE mice showed a marginal increase in the severity of early clinical symptoms. AMACR was not regulated in the white blood cells of MS patients. Our data show that AMACR is regulated in immune cells during EAE, but it is not a suitable target for the treatment of MS due to its opposing effects.

Methylmalonyl-CoA Epimerase Deficiency Mimicking Propionic Aciduria.

Methylmalonyl-CoA epimerase (MCE) converts d-methylmalonyl-CoA epimer to l-methylmalonyl-CoA epimer in the propionyl-CoA to succinyl-CoA pathway. Only seven cases of MCE deficiency have been described. In two cases, MCE deficiency was combined with sepiapterin reductase deficiency. The reported clinical pictures of isolated MCE are variable, with two asymptomatic patients and two other patients presenting with metabolic acidosis attacks. For combined MCE and sepiapterin reductase deficiency, the clinical picture is dominated by neurologic alterations. We report isolated MCE deficiency in a boy who presented at five years of age with acute metabolic acidosis. Metabolic investigations were consistent with propionic aciduria (PA). Unexpectedly, propionyl-CoA carboxylase activity was within the reference range. Afterward, apparently intermittent and mild excretion of methylmalonic acid (MMA) was discovered. Methylmalonic pathway gene set analysis using the next-generation sequencing approach allowed identification of the common homozygous nonsense pathogenic variant (c.139C > T-p.Arg47*) in the methylmalonyl-CoA epimerase gene (MCEE). Additional cases of MCE deficiency may help provide better insight regarding the clinical impact of this rare condition. MCE deficiency could be considered a cause of mild and intermittent increases in methylmalonic acid.

Phytol is lethal for Amacr-deficient mice.

α-Methylacyl-CoA racemase (Amacr) catalyzes the racemization of the 25-methyl group in C27-intermediates in bile acid synthesis and in methyl-branched fatty acids such as pristanic acid, a metabolite derived from phytol. Consequently, patients with Amacr deficiency accumulate C27-bile acid intermediates, pristanic and phytanic acid and display sensorimotor neuropathy, seizures and relapsing encephalopathy. In contrast to humans, Amacr-deficient mice are clinically symptomless on a standard laboratory diet, but failed to thrive when fed phytol-enriched chow. In this study, the effect and the mechanisms behind the development of the phytol-feeding associated disease state in Amacr-deficient mice were investigated. All Amacr-/- mice died within 36weeks on a phytol diet, while wild-type mice survived. Liver failure was the main cause of death accompanied by kidney and brain abnormalities. Histological analysis of liver showed inflammation, fibrotic and necrotic changes, Kupffer cell proliferation and fatty changes in hepatocytes, and serum analysis confirmed the hepatic disease. Pristanic and phytanic acids accumulated in livers of Amacr-/- mice after a phytol diet. Microarray analysis also revealed changes in the expression levels of numerous genes in wild-type mouse livers after two weeks of the phytol diet compared to a control diet. This indicates that detoxification of phytol metabolites in liver is accompanied by activation of multiple pathways at the molecular level and Amacr-/- mice are not able to respond adequately. Phytol causes primary failure in liver leading to death of Amacr-/- mice thus emphasizing the indispensable role of Amacr in detoxification of α-methyl-branched fatty acids.

Endogenous co-agonists of the NMDA receptor modulate contextual fear in trace conditioning.

We have used mutant mice to probe the roles of the endogenous co-agonists of the NMDA receptor (NMDAR), D-serine and glycine, in fear learning and memory. Serine racemase knockout (SR-/-) mice have less than 15% of wild type forebrain levels of D-serine, whereas glycine transporter 1 heterozygous knockout (GlyT1+/-) mice have elevated synaptic glycine. While cued fear was normal in both delay and trace conditioned mice of both mutant genotypes, contextual fear was affected in trace conditioned subjects: SR-/- mice showed decreased contextual freezing, whereas GlyT1+/- mice showed elevated contextual freezing. These results indicate that endogenous co-agonists of the NMDAR modulate the conditioning of contextual fear responses, particularly in trace conditioning. They further suggest that endogenous glycine can compensate for the D-serine deficiency in cued and contextual fear following delay conditioning.

Clinical and Biochemical Pitfalls in the Diagnosis of Peroxisomal Disorders.

Peroxisomal disorders are a heterogeneous group of genetic metabolic disorders, caused by a defect in peroxisome biogenesis or a deficiency of a single peroxisomal enzyme. The peroxisomal disorders include the Zellweger spectrum disorders, the rhizomelic chondrodysplasia punctata spectrum disorders, X-linked adrenoleukodystrophy, and multiple single enzyme deficiencies. There are several core phenotypes caused by peroxisomal dysfunction that clinicians can recognize. The diagnosis is suggested by biochemical testing in blood and urine and confirmed by functional assays in cultured skin fibroblasts, followed by mutation analysis. This review describes the phenotype of the main peroxisomal disorders and possible pitfalls in (laboratory) diagnosis to aid clinicians in the recognition of this group of diseases.

Multiple risk pathways for schizophrenia converge in serine racemase knockout mice, a mouse model of NMDA receptor hypofunction.

Schizophrenia is characterized by reduced hippocampal volume, decreased dendritic spine density, altered neuroplasticity signaling pathways, and cognitive deficits associated with impaired hippocampal function. We sought to determine whether this diverse pathology could be linked to NMDA receptor (NMDAR) hypofunction, and thus used the serine racemase-null mutant mouse (SR(-/-)), which has less than 10% of normal brain D-serine, an NMDAR coagonist. We found that D-serine was necessary for the maintenance of long-term potentiation in the adult hippocampal dentate gyrus and for full NMDAR activity on granule cells. SR(-/-) mice had reduced dendritic spines and hippocampal volume. These morphological changes were paralleled by diminished BDNF/Akt/mammalian target of rapamycin (mTOR) signaling and impaired performance on a trace-conditioning memory task. Chronic D-serine treatment normalized the electrophysiological, neurochemical, and cognitive deficits in SR(-/-) mice. These results demonstrate that NMDAR hypofunction can reproduce the numerous hippocampal deficits associated with schizophrenia, which can be reversed by chronic peripheral D-serine treatment.

Availability of N-Methyl-d-Aspartate Receptor Coagonists Affects Cocaine-Induced Conditioned Place Preference and Locomotor Sensitization: Implications for Comorbid Schizophrenia and Substance Abuse.

Schizophrenia is associated with high prevalence of substance abuse. Recent research suggests that dysregulation of N-methyl-d-aspartate receptor (NMDAR) function may play a role in the pathophysiology of both schizophrenia and drug addiction, and thus, may account for this high comorbidity. Our laboratory has developed two transgenic mouse lines that exhibit contrasting NMDAR activity based on the availability of the glycine modulatory site (GMS) agonists d-serine and glycine. Glycine transporter 1 knockdowns (GlyT1(+/-)) exhibit NMDAR hyperfunction, whereas serine racemase knockouts (SR(-/-)) exhibit NMDAR hypofunction. We characterized the behavior of these lines in a cocaine-induced (20 mg/kg) conditioned place preference (CPP) and locomotor sensitization paradigm. Compared with wild-type mice, GlyT1(+/-) mice displayed hastened extinction of CPP and robust cocaine-induced reinstatement. SR(-/-) mice appeared to immediately "forget" the learned preference, because they did not exhibit cocaine-induced reinstatement and also displayed attenuated locomotor sensitization. Treatment of GlyT1(+/-) mice with gavestinel (10 mg/kg on day 1; 5 mg/kg on days 2-17), a GMS antagonist, attenuated cocaine-induced CPP and caused them to immediately "forget" the learned preference. Treatment of SR(-/-) mice with d-serine (300 mg/kg on day 1; 150 mg/kg on days 2-17) to normalize brain levels caused them to avoid the cocaine-paired side of the chamber during extinction. These results highlight NMDAR dysfunction as a possible neural mechanism underlying comorbid schizophrenia and substance abuse. Also, these findings suggest drugs that directly or indirectly activate the NMDAR GMS could be an effective treatment of cocaine abuse.

Role of AMACR (α-methylacyl-CoA racemase) and MFE-1 (peroxisomal multifunctional enzyme-1) in bile acid synthesis in mice.

Cholesterol is catabolized to bile acids by peroxisomal ß-oxidation in which the side chain of C27-bile acid intermediates is shortened by three carbon atoms to form mature C24-bile acids. Knockout mouse models deficient in AMACR (α-methylacyl-CoA racemase) or MFE-2 (peroxisomal multifunctional enzyme type 2), in which this ß-oxidation pathway is prevented, display a residual C24-bile acid pool which, although greatly reduced, implies the existence of alternative pathways of bile acid synthesis. One alternative pathway could involve Mfe-1 (peroxisomal multifunctional enzyme type 1) either with or without Amacr. To test this hypothesis, we generated a double knockout mouse model lacking both Amacr and Mfe-1 activities and studied the bile acid profiles in wild-type, Mfe-1 and Amacr single knockout mouse line and Mfe-1 and Amacr double knockout mouse lines. The total bile acid pool was decreased in Mfe-1-/- mice compared with wild-type and the levels of mature C24-bile acids were reduced in the double knockout mice when compared with Amacr-deficient mice. These results indicate that Mfe-1 can contribute to the synthesis of mature bile acids in both Amacr-dependent and Amacr-independent pathways.

Histomorphometric and Immunohistochemical Diagnosis of Renal Cell Carcinoma: Review Article.

Renal cell carcinoma (RCC) is derived from renal tubular epithelial cells and is among the 10 most common cancers worldwide. Incidence of renal cell carcinoma is 400,000 individuals worldwide per year. The age of diagnosis is approximately 60years, and twice as many men are diagnosed as women. African Americans have a slightly higher rate of RCC than do White peoples. The reasons for this are not clear. Inherited syndromes in family, long term dialysis, smoking individuals who had quit smoking >10 years prior had a lower risk when compared to those who had quit <10 years. 22.5 pack-year smokers had a more than 50.0% increased RCC risk compared to nonsmokers, high body mass index i.e. 5kg/m2 increase in body mass index (BMI) was found to be strongly associated with RCC. BMI >35kg/m2 is associated with higher incidence of Cancer raise blood pressure- Higher BMI and hypertension were independently shown to increase the long-term risk of RCC in men. A rise of blood pressure of 10mmHg is associated with 10-22 percent risk of RCC. Clear cell carcinoma is the most common variety of renal cell carcinoma as compared to other varieties of renal cell carcinomas (68.0-75.0%). It has also been found that CAIX is positive for all papillary renal cell carcinoma and negative for CK7, AMACR & TEF. We also found that CK7, EMA, CD117 and CAIX are most commonly positive for all chromophobe renal cell carcinoma. It has been found that clear cell carcinoma is the most common variety of renal cell carcinoma as compared to other varieties of renal cell carcinomas (68.0-75.0%). Again it has also been found that CAIX is positive for all papillary renal cell carcinoma and negative for CK7, AMACR and TEF. Here it has been found that chromophobe carcinoma is most commonly positive for CK7, EMA, CD117 and CAIX. In a patient coming with signs and symptoms of renal cell carcinoma can be confirmed with the help of histoimmunological markers and in that case one can plan for a proper planning of management.

Mouse models for peroxisome biogenesis defects and ß-oxidation enzyme deficiencies.

Peroxisome biogenesis and peroxisomal ß-oxidation defects are rare inherited metabolic disorders in which several organs can be affected. A panel of mouse models has been created in which genes crucial to these processes were inactivated and the ensuing pathologies studied. In mice with enzyme defects of peroxisomal ß-oxidation, the disease state strongly depends on the kind of substrates that are metabolized by the enzyme and the dietary composition. Because mice with generalized biogenesis defects seldom reach adulthood, conditional knockout models were generated to study the consequences of peroxisome deficiency in hepatocytes, different brain cell types and Sertoli cells. Although the precise relationship between the biochemical anomalies and pathologies was often not resolved, the mouse models allowed to document in detail histological abnormalities, metabolic and gene expression deregulations some of which are mediated by PPARα, and to uncover the essential role of peroxisomes in some unsuspected cell types.

Paradoxical roles of serine racemase and D-serine in the G93A mSOD1 mouse model of amyotrophic lateral sclerosis.

D-serine is an endogenous neurotransmitter that binds to the NMDA receptor, thereby increasing the affinity for glutamate, and the potential for excitotoxicity. The primary source of D-serine in vivo is enzymatic racemization by serine racemase (SR). Regulation of D-serine in vivo is poorly understood, but is thought to involve a combination of controlled production, synaptic reuptake by transporters, and intracellular degradation by D-amino acid oxidase (DAO). However, SR itself possesses a well-characterized eliminase activity, which effectively degrades D-serine as well. D-serine is increased two-fold in spinal cords of G93A Cu,Zn-superoxide dismutase (SOD1) mice--the standard model of amyotrophic lateral sclerosis (ALS). ALS mice with SR disruption show earlier symptom onset, but survive longer (progression phase is slowed), in an SR-dependent manner. Paradoxically, administration of D-serine to ALS mice dramatically lowers cord levels of D-serine, leading to changes in the onset and survival very similar to SR deletion. D-serine treatment also increases cord levels of the alanine-serine-cysteine transporter 1 (Asc-1). Although the mechanism by which SOD1 mutations increases D-serine is not known, these results strongly suggest that SR and D-serine are fundamentally involved in both the pre-symptomatic and progression phases of disease, and offer a direct link between mutant SOD1 and a glial-derived toxic mediator.

The NMDA receptor co-agonists, D-serine and glycine, regulate neuronal dendritic architecture in the somatosensory cortex.

There is substantial evidence, both pharmacological and genetic, that hypofunction of the N-methyl-d-aspartate receptor (NMDAR) is a core pathophysiological feature of schizophrenia. There are morphological brain changes associated with schizophrenia, including perturbations in the dendritic morphology of cortical pyramidal neurons and reduction in cortical volume. Our experiments investigated whether these changes in dendritic morphology could be recapitulated in a genetic model of NMDAR hypofunction, the serine racemase knockout (SR-/-) mouse. Pyramidal neurons in primary somatosensory cortex (S1) of SR-/- mice had reductions in the complexity, total length, and spine density of apical and basal dendrites. In accordance with reduced cortical neuropil, SR-/- mice also had reduced cortical volume as compared to wild type mice. Analysis of S1 mRNA by DNA microarray and gene expression analysis revealed gene changes in SR-/- that are associated with psychiatric and neurologic disorders, as well as neurodevelopment. The microarray analysis also identified reduced expression of brain derived neurotrophic factor (BDNF) in SR-/- mice. Follow-up analysis by ELISA confirmed a reduction of BDNF protein levels in the S1 of SR-/- mice. Finally, S1 pyramidal neurons in glycine transporter heterozygote (GlyT1+/-) mutants, which display enhanced NMDAR function, had increased dendritic spine density. These results suggest that proper NMDAR function is important for the arborization and spine density of pyramidal neurons in cortex. Moreover, they suggest that NMDAR hypofunction might, in part, be contributing to the dendritic and synaptic changes observed in schizophrenia and highlight this signaling pathway as a potential target for therapeutic intervention.