Guanidino compounds that are increased in hyperargininemia inhibit GABA and glycine responses on mouse neurons in cell culture

Succinic semialdehyde dehydrogenase deficiency: a metabolic and genomic approach to diagnosis

Genomic sequencing offers an untargeted, data-driven approach to genetic diagnosis; however, variants of uncertain significance often hinder the diagnostic process. The discovery of rare genomic variants without previously known functional evidence of pathogenicity often results in variants being overlooked as potentially causative, particularly in individuals with undifferentiated phenotypes. Consequently, many neurometabolic conditions, including those in the GABA (gamma-aminobutyric acid) catabolism pathway, are underdiagnosed. Succinic semialdehyde dehydrogenase deficiency (SSADHD, OMIM #271980) is a neurometabolic disorder in the GABA catabolism pathway. The disorder is due to bi-allelic pathogenic variants in ALDH5A1 and is usually characterized by moderate-to-severe developmental delays, hypotonia, intellectual disability, ataxia, seizures, hyperkinetic behavior, aggression, psychiatric disorders, and sleep disturbances. In this study, we utilized an integrated approach to diagnosis of SSADHD by examining molecular, clinical, and metabolomic data from a single large commercial laboratory. Our analysis led to the identification of 16 patients with likely SSADHD along with three novel variants. We also showed that patients with this disorder have a clear metabolomic signature that, along with molecular and clinical findings, may allow for more rapid and efficient diagnosis. We further surveyed all available pathogenic/likely pathogenic variants and used this information to estimate the global prevalence of this disease. Taken together, our comprehensive analysis allows for a global approach to the diagnosis of SSADHD and provides a pathway to improved diagnosis and potential incorporation into newborn screening programs. Furthermore, early diagnosis facilitates referral to genetic counseling, family support, and access to targeted treatments-taken together, these provide the best outcomes for individuals living with either GABA-TD or SSADHD, as well as other rare conditions.

Efficacy and safety of pegzilarginase in arginase 1 deficiency (PEACE): a phase 3, randomized, double-blind, placebo-controlled, multi-centre trial

Background

Arginase 1 Deficiency (ARG1-D) is a rare debilitating, progressive, inherited, metabolic disease characterized by marked increases in plasma arginine (pArg) and its metabolites, with increased morbidity, substantial reductions in quality of life, and premature mortality. Effective treatments that can lower arginine and improve clinical outcomes is currently lacking. Pegzilarginase is a novel human arginase 1 enzyme therapy. The present trial aimed to demonstrate efficacy of pegzilarginase on pArg and key mobility outcomes.

Methods

This Phase 3 randomized, double-blind, placebo-controlled, parallel-group clinical trial (clinicaltrials.govNCT03921541, EudraCT 2018-004837-34), randomized patients with ARG1-D 2:1 to intravenously/subcutaneously once-weekly pegzilarginase or placebo in conjunction with their individualized disease management. It was conducted in 7 countries; United States, United Kingdom, Canada, Austria, France, Germany, Italy. Primary endpoint was change from baseline in pArg after 24 weeks; key secondary endpoints were change from baseline at Week 24 in Gross Motor Function Measure part E (GMFM-E) and 2-min walk test (2MWT). Full Analysis Set was used for the analyses.

Findings

From 01 May 2019 to 29 March 2021, 32 patients were enrolled and randomized (pegzilarginase, n = 21; placebo, n = 11). Pegzilarginase lowered geometric mean pArg from 354.0 μmol/L to 86.4 μmol/L at Week 24 vs 464.7 to 426.6 μmol/L for placebo (95% CI: -67.1%, -83.5%; p < 0.0001) and normalized levels in 90.5% of patients (vs 0% with placebo). In addition, clinically relevant functional mobility improvements were demonstrated with pegzilarginase treatment. These effects were sustained long-term through additional 24 weeks of subsequent exposure. Pegzilarginase was well-tolerated, with adverse events being mostly transient and mild/moderate in severity.

Interpretation

These results support pegzilarginase as the first potential treatment to normalize pArg in ARG1-D and achieve clinically meaningful improvements in functional mobility.

Funding

Aeglea BioTherapeutics.

The role and control of arginine levels in arginase 1 deficiency

Arginase 1 Deficiency (ARG1-D) is a rare urea cycle disorder that results in persistent hyperargininemia and a distinct, progressive neurologic phenotype involving developmental delay, intellectual disability, and spasticity, predominantly affecting the lower limbs and leading to mobility impairment. Unlike the typical presentation of other urea cycle disorders, individuals with ARG1-D usually appear healthy at birth and hyperammonemia is comparatively less severe and less common. Clinical manifestations typically begin to develop in early childhood in association with high plasma arginine levels, with hyperargininemia (and not hyperammonemia) considered to be the primary driver of disease sequelae. Nearly five decades of clinical experience with ARG1-D and empirical studies in genetically manipulated models have generated a large body of evidence that, when considered in aggregate, implicates arginine directly in disease pathophysiology. Severe dietary protein restriction to minimize arginine intake and diversion of ammonia from the urea cycle are the mainstay of care. Although this approach does reduce plasma arginine and improve patients' cognitive and motor/mobility manifestations, it is inadequate to achieve and maintain sufficiently low arginine levels and prevent progression in the long term. This review presents a comprehensive discussion of the clinical and scientific literature, the effects and limitations of the current standard of care, and the authors' perspectives regarding the past, current, and future management of ARG1-D.

Epidemiology, methods of diagnosis, and clinical management of patients with arginase 1 deficiency (ARG1-D): A systematic review

BACKGROUND

Arginase 1 Deficiency (ARG1-D) is a rare, progressive, metabolic disorder that is characterized by devastating manifestations driven by elevated plasma arginine levels. It typically presents in early childhood with spasticity (predominately affecting the lower limbs), mobility impairment, seizures, developmental delay, and intellectual disability. This systematic review aims to identify and describe the published evidence outlining the epidemiology, diagnosis methods, measures of disease progression, clinical management, and outcomes for ARG1-D patients.

METHODS

A comprehensive literature search across multiple databases such as MEDLINE, Embase, and a review of clinical studies in ClinicalTrials.gov (with results reported) was carried out per PRISMA guidelines on 20 April 2020 with no date restriction. Pre-defined eligibility criteria were used to identify studies with data specific to patients with ARG1-D. Two independent reviewers screened records and extracted data from included studies. Quality was assessed using the modified Newcastle-Ottawa Scale for non-comparative studies.

RESULTS

Overall, 55 records reporting 40 completed studies and 3 ongoing studies were included. Ten studies reported the prevalence of ARG1-D in the general population, with a median of 1 in 1,000,000. Frequently reported diagnostic methods included genetic testing, plasma arginine levels, and red blood cell arginase activity. However, routine newborn screening is not universally available, and lack of disease awareness may prevent early diagnosis or lead to misdiagnosis, as the disease has overlapping symptomology with other diseases, such as cerebral palsy. Common manifestations reported at time of diagnosis and assessed for disease progression included spasticity (predominately affecting the lower limbs), mobility impairment, developmental delay, intellectual disability, and seizures. Severe dietary protein restriction, essential amino acid supplementation, and nitrogen scavenger administration were the most commonly reported treatments among patients with ARG1-D. Only a few studies reported meaningful clinical outcomes of these interventions on intellectual disability, motor function and adaptive behavior assessment, hospitalization, or death. The overall quality of included studies was assessed as good according to the Newcastle-Ottawa Scale.

CONCLUSIONS

Although ARG1-D is a rare disease, published evidence demonstrates a high burden of disease for patients. The current standard of care is ineffective at preventing disease progression. There remains a clear need for new treatment options as well as improved access to diagnostics and disease awareness to detect and initiate treatment before the onset of clinical manifestations to potentially enable more normal development, improve symptomatology, or prevent disease progression.

Pathogenesis of ventilator-induced lung injury: metabolomics analysis of the lung and plasma

INTRODUCTION

Nowadays,the mechanical ventilation (MV) aims to rest the respiratory muscles while providing adequate gas exchange, and it has been a part of basic life support during general anesthesia as well as in critically ill patients with and without respiratory failure. However, MV itself has the potential to cause or worsen lung injury, which is also known as ventilator-induced lung injury (VILI). Thus, the early diagnosis of VILI is of great importance for the prevention and treatment of VILI.

OBJECTIVE

This study aimed to investigate the metabolomes in the lung and plasma of mice receiving mechanical ventilation (MV).

METHODS

Healthy mice were randomly assigned into control group; (2) high volume tidal (HV) group (30 ml/kg); (3) low volume tidal (LV) group (6 ml/kg). After ventilation for 4 h, mice were sacrificed and the lung tissue and plasma were collected. The lung and plasma were processed for the metabolomics analysis. We also performed histopathological examination on the lung tissue.

RESULTS

We detected moderate inflammatory damage with alveolar septal thickening in the HV group compared with the normal and LV groups.The metabolomics analysis results showed MV altered the metabolism which was characterized by the dysregulation of γ-amino butyric acid (GABA) system and urea cycle (desregulations in plasma and lung guanidinosuccinic acid, argininosuccinic acid, succinic acid semialdehyde and lung GABA ), Disturbance of citric acid cycle (CAC) (increased plasma glutamine and lung phosphoenol pyruvate) and redox imbalance (desregulations in plasma and/or lung ascorbic acid, chenodeoxycholic acid, uric acid, oleic acid, stearidonic acid, palmitoleic acid and docosahexaenoic acid). Moreover, the lung and plasma metabolomes were also significantly different between LV and HV groups.

CONCLUSIONS

Some lung and plasma metabolites related to the GABA system and urea cycle, citric acid cycle and redox balance were significantly altered, and they may be employed for the evaluation of VILI and serve as targets in the treatment of VILI.

Human cytosolic transaminases: side activities and patterns of discrimination towards physiologically available alternative substrates

Transaminases play key roles in central metabolism, transferring the amino group from a donor substrate to an acceptor. These enzymes can often act, with low efficiency, on compounds different from the preferred substrates. To understand what might have shaped the substrate specificity of this class of enzymes, we examined the reactivity of six human cytosolic transaminases towards amino acids whose main degradative pathways do not include any transamination. We also tested whether sugars and sugar phosphates could serve as alternative amino group acceptors for these cytosolic enzymes. Each of the six aminotransferases reacted appreciably with at least three of the alternative amino acid substrates in vitro, albeit at usually feeble rates. Reactions with L-Thr, L-Arg, L-Lys and L-Asn were consistently very slow-a bias explained in part by the structural differences between these amino acids and the preferred substrates of the transaminases. On the other hand, L-His and L-Trp reacted more efficiently, particularly with GTK (glutamine transaminase K; also known as KYAT1). This points towards a role of GTK in the salvage of L-Trp (in cooperation with ω-amidase and possibly with the cytosolic malate dehydrogenase, MDH1, which efficiently reduced the product of L-Trp transamination). Finally, the transaminases were extremely ineffective at utilizing sugars and sugar derivatives, with the exception of the glycolytic intermediate dihydroxyacetone phosphate, which was slowly but appreciably transaminated by some of the enzymes to yield serinol phosphate. Evidence for the formation of this compound in a human cell line was also obtained. We discuss the biological and evolutionary implications of our results.

Current status of surviving patients with arginase 1 deficiency in Japan

Arginase 1 (ARG1) deficiency is a rare urea cycle disorder (UCD), with an estimated frequency of 1 per 2,200,000 births in Japan. Patients with ARG1 deficiency develop symptoms in late infancy or pre-school age with progressive neurological manifestations and sometimes present with severe hepatic disease. We previously investigated the status of UCDs in Japan; however, only one patient was identified as having ARG1 deficiency. Therefore, we aimed to investigate the current status of patients with ARG1 deficiency in 2018–2021 because almost 10 years have passed since the previous study. We present the disease history, clinical outcome, and treatment of five surviving patients with ARG1 deficiency and discuss the features of ARG1 deficiency in Japan. We found that clinicians often face difficulty in diagnosing ARG1 deficiency at the early stage of onset because of interpatient variability in onset time and clinical manifestations. Blood L-arginine and guanidino compounds were considered to be the major factors causing adverse neurodevelopmental outcomes. Therefore, early detection and intervention of ARG1 deficiency is essential for improved neurodevelopmental outcomes. Liver transplantation has been considered an effective treatment option that can dramatically improve the quality of life of patients, prior to the neurological manifestation of symptoms caused by ARG1 deficiency.

The Role of Preclinical Models in Creatine Transporter Deficiency: Neurobiological Mechanisms, Biomarkers and Therapeutic Development

Creatine (Cr) Transporter Deficiency (CTD) is an X-linked metabolic disorder, mostly caused by missense mutations in the SLC6A8 gene and presenting with intellectual disability, autistic behavior, and epilepsy. There is no effective treatment for CTD and patients need lifelong assistance. Thus, the research of novel intervention strategies is a major scientific challenge. Animal models are an excellent tool to dissect the disease pathogenetic mechanisms and drive the preclinical development of therapeutics. This review illustrates the current knowledge about Cr metabolism and CTD clinical aspects, with a focus on mainstay diagnostic and therapeutic options. Then, we discuss the rodent models of CTD characterized in the last decade, comparing the phenotypes expressed within clinically relevant domains and the timeline of symptom development. This analysis highlights that animals with the ubiquitous deletion/mutation of SLC6A8 genes well recapitulate the early onset and the complex pathological phenotype of the human condition. Thus, they should represent the preferred model for preclinical efficacy studies. On the other hand, brain- and cell-specific conditional mutants are ideal for understanding the basis of CTD at a cellular and molecular level. Finally, we explain how CTD models might provide novel insight about the pathogenesis of other disorders, including cancer.

Concentrations of the Selected Biomarkers of Endothelial Dysfunction in Response to Antiepileptic Drugs: A Literature Review

Epilepsy is a disease arising from morphological and metabolic changes in the brain. Approximately 60% of patients with seizures can be controlled with 1 antiepileptic drug (AED), while in others, polytherapy is required. The AED treatment affects a number of biochemical processes in the body, including increasing the risk of cardiovascular diseases (CVDs). It is indicated that the duration of AED therapy with some AEDs significantly accelerates the process of atherosclerosis. Most of AEDs increase levels of homocysteine (HCys) as well as may affect concentrations of new, nonclassical risk factors for atherosclerosis, that is, asymmetric dimethylarginine (ADMA) and homoarginine (hArg). Because of the role of these parameters in the pathogenesis of CVD, knowledge of HCys, ADMA, and hArg concentrations in patients with epilepsia treated with AED, both pediatric and adult, appears to be of significant importance.

Organic and Peptidyl Constituents of Snake Venoms: The Picture Is Vastly More Complex Than We Imagined

Small metabolites and peptides in 17 snake venoms (Elapidae, Viperinae, and Crotalinae), were quantified using liquid chromatography-mass spectrometry. Each venom contains >900 metabolites and peptides. Many small organic compounds are present at levels that are probably significant in prey envenomation, given that their known pharmacologies are consistent with snake envenomation strategies. Metabolites included purine nucleosides and their bases, neurotransmitters, neuromodulators, guanidino compounds, carboxylic acids, amines, mono- and disaccharides, and amino acids. Peptides of 2⁻15 amino acids are also present in significant quantities, particularly in crotaline and viperine venoms. Some constituents are specific to individual taxa, while others are broadly distributed. Some of the latter appear to support high anabolic activity in the gland, rather than having toxic functions. Overall, the most abundant organic metabolite was citric acid, owing to its predominance in viperine and crotaline venoms, where it chelates divalent cations to prevent venom degradation by venom metalloproteases and damage to glandular tissue by phospholipases. However, in terms of their concentrations in individual venoms, adenosine, adenine, were most abundant, owing to their high titers in Dendroaspis polylepis venom, although hypoxanthine, guanosine, inosine, and guanine all numbered among the 50 most abundant organic constituents. A purine not previously reported in venoms, ethyl adenosine carboxylate, was discovered in D. polylepis venom, where it probably contributes to the profound hypotension caused by this venom. Acetylcholine was present in significant quantities only in this highly excitotoxic venom, while 4-guanidinobutyric acid and 5-guanidino-2-oxopentanoic acid were present in all venoms.

Creatine synthesis and exchanges between brain cells: What can be learned from human creatine deficiencies and various experimental models?

While it has long been thought that most of cerebral creatine is of peripheral origin, the last 20 years has provided evidence that the creatine synthetic pathway (AGAT and GAMT enzymes) is expressed in the brain together with the creatine transporter (SLC6A8). It has also been shown that SLC6A8 is expressed by microcapillary endothelial cells at the blood-brain barrier, but is absent from surrounding astrocytes, raising the concept that the blood-brain barrier has a limited permeability for peripheral creatine. The first creatine deficiency syndrome in humans was also discovered 20 years ago (GAMT deficiency), followed later by AGAT and SLC6A8 deficiencies, all three diseases being characterized by creatine deficiency in the CNS and essentially affecting the brain. By reviewing the numerous and latest experimental studies addressing creatine transport and synthesis in the CNS, as well as the clinical and biochemical characteristics of creatine-deficient patients, our aim was to delineate a clearer view of the roles of the blood-brain and blood-cerebrospinal fluid barriers in the transport of creatine and guanidinoacetate between periphery and CNS, and on the intracerebral synthesis and transport of creatine. This review also addresses the question of guanidinoacetate toxicity for brain cells, as probably found under GAMT deficiency.

Expression quantitative trait loci and receptor pharmacology implicate Arg1 and the GABA-A receptor as therapeutic targets in neuroblastoma

The development of targeted therapeutics for neuroblastoma, the third most common tumor in children, has been limited by a poor understanding of growth signaling mechanisms unique to the peripheral nerve precursors from which tumors arise. In this study, we combined genetics with gene-expression analysis in the peripheral sympathetic nervous system to implicate arginase 1 and GABA signaling in tumor formation in vivo. In human neuroblastoma cells, either blockade of ARG1 or benzodiazepine-mediated activation of GABA-A receptors induced apoptosis and inhibited mitogenic signaling through AKT and MAPK. These results suggest that ARG1 and GABA influence both neural development and neuroblastoma and that benzodiazepines in clinical use may have potential applications for neuroblastoma therapy.

Treatment of arginase deficiency revisited: guanidinoacetate as a therapeutic target and biomarker for therapeutic monitoring

Hyperargininaemia is a disorder of the last step of the urea cycle. It is an autosomal recessive disease caused by deficiency of liver arginase-1 and usually presents later in childhood with progressive neurological symptoms including marked spasticity. In contrast with other urea cycle disorders, hyperammonaemia is not usually present but can be a feature. A number of guanidine compounds may accumulate in the blood and cerebrospinal fluid of these patients, which could play an important pathophysiological role. Guanidinoacetate is of particular interest as a well-known potent epileptogenic compound in guanidinoacetate methyltransferase deficiency. We found markedly elevated guanidinoacetate levels in a patient with arginase deficiency, which dropped significantly in response to dietary and medical treatment. Measurement of guanidinoacetate and other guanidino compounds may, therefore, be important for therapeutic monitoring in arginase deficiency.

AAV-based gene therapy prevents neuropathology and results in normal cognitive development in the hyperargininemic mouse

Complete arginase I deficiency is the least severe urea cycle disorder, characterized by hyperargininemia and infrequent episodes of hyperammonemia. Patients suffer from neurological impairment with cortical and pyramidal tract deterioration, spasticity, loss of ambulation, and seizures, and is associated with intellectual disability. In mice, onset is heralded by weight loss beginning around day 15; gait instability follows progressing to inability to stand and development of tail tremor with seizure-like activity and death. Here we report that hyperargininemic mice treated neonatally with an adeno-associated virus expressing arginase and followed long-term lack any presentation consistent with brain dysfunction. Behavioral and histopathological evaluation demonstrated that treated mice are indistinguishable from littermates and that putative compounds associated with neurotoxicity are diminished. In addition, treatment results in near complete resolution of metabolic abnormalities early in life; however there is the development of some derangement later with decline in transgene expression. Ammonium challenging revealed that treated mice are affected by exogenous loading much greater than littermates. These results demonstrate that AAV-based therapy for hyperargininemia is effective and prevents development of neurological abnormalities and cognitive dysfunction in a mouse model of hyperargininemia; however nitrogen challenging reveals that these mice remain impaired in the handling of waste nitrogen.

Guanidino compound levels in blood, cerebrospinal fluid, and post-mortem brain material of patients with argininemia

The paucity of hyperammonemic crises together with spasticity, only seen in human arginase I deficient patients and not in patients with other urea cycle disorders, forces a search for candidates other than ammonia to associate with the pathophysiology and symptomatology. Therefore, we determined arginine together with some catabolites of arginine in blood and cerebrospinal fluid of these patients as well as in extremely rare post-mortem brain material of two patients with argininemia. The levels of alpha-keto-delta-guanidinovaleric acid, argininic acid and alpha-N-acetylarginine correlate with the arginine levels in blood and cerebrospinal fluid of patients with imposed or spontaneous protein restriction. The levels in blood are higher than the upper limit of normal in all studied patients. In addition to the highly increased levels of these same compounds in blood of a child with argininemia, the increase of guanidinoacetic acid, 24h before death, is remarkable. However, the manifest increases of these studied catabolites of arginine are not seen in post-mortem brain material of the same pediatric patient. Otherwise a clear increase of guanidinoacetic acid in post-mortem brain material of an adult patient was shown. A similar, comparable increase of homoarginine in both studied post-mortem brain materials is observed. Therefore the study of the pathobiochemistry of arginine in argininemia must be completed in the future by the determination of the end catabolites of the nitric oxide and agmatine biosynthesis pathways in the knockouts as well as in the patients to evaluate their role, together with the here studied catabolites, as candidates for association with pathophysiology and symptomatology.

Ammonia toxicity to the brain: effects on creatine metabolism and transport and protective roles of creatine

Hyperammonemia can provoke irreversible damage to the developing brain, with the formation of cortical atrophy, ventricular enlargement, demyelination or gray and white matter hypodensities. Among the various pathogenic mechanisms involved, alterations in cerebral energy have been demonstrated. In particular, we could show that ammonia exposure generates a secondary deficiency in creatine in brain cells, by altering the brain expression and activity of the genes allowing creatine synthesis (AGAT and GAMT) and transport (SLC6A8). On the other hand, it is known that creatine administration can exert protective effects in various neurodegenerative processes. We could also show that creatine co-treatment under ammonia exposure can protect developing brain cells from some of the deleterious effects of ammonia, in particular axonal growth impairment. This article focuses on the effects of ammonia exposure on creatine metabolism and transport in developing brain cells, and on the potential neuroprotective properties of creatine in the brain exposed to ammonium.

Increased plasma and tissue guanidino compounds in a mouse model of hyperargininemia

In humans, arginase I (AI)-deficiency results in hyperargininemia, a metabolic disorder with symptoms of progressive neurological and intellectual impairment, spasticity, persistent growth retardation, and episodic hyperammonemia. A deficiency of arginase II (AII) has never been detected and the clinical disorder, if any, associated with its deficiency has not been defined. Since the spasticity and paucity of hyperammonemic crises seen in human AI-deficient patients are not features of the other urea cycle disorders, the likelihood of ammonia as the main neuropathogenic agent becomes extremely low, and the modest elevations of arginine seen in the brains of our mouse model of hyperargininemia make it an unlikely candidate as well. Specific guanidino compounds, direct or indirect metabolites of arginine, are elevated in the blood of patients with uremia. Other guanidino compounds are also increased in plasma and cerebrospinal fluid of hyperargininemic patients making them plausible as neurotoxins in these disorders. We analyzed several guanidino compounds in our arginase single and double knockout animals and found that alpha-keto-delta-guanidinovaleric acid, alpha-N-acetylarginine, and argininic acid were increased in the brain tissue from the AI knockout and double knockout animals. Several compounds were also increased in the plasma, liver, and kidneys. This is the first time that several of the guanidino compounds have been shown to be elevated in the brain tissue of an arginase-deficient mammal, and it further supports their possible role as the neuropathogenic agents responsible for the complications seen in arginase deficiency.

Clinical, biochemical, and molecular spectrum of hyperargininemia due to arginase I deficiency

The urea cycle consists of six consecutive enzymatic reactions that convert waste nitrogen into urea. Urea cycle disorders are a group of inborn errors of hepatic metabolism that often result in life threatening hyperammonemia and hyperglutaminemia. Deficiencies of all of the enzymes of the cycle have been described and although each specific disorder results in the accumulation of different precursors, hyperammonemia and hyperglutaminemia are common biochemical hallmarks of these disorders. Arginase is the enzyme involved in the last step of the urea cycle. It catalyzes the conversion of arginine to urea and ornithine. The latter reenters the mitochondrion to continue the cycle. Hyperargininemia is an autosomal recessive disorder caused by a defect in the arginase I enzyme. Unlike other urea cycle disorders, this condition is not generally associated with a hyperammonemic encephalopathy in the neonatal period. It typically presents later in childhood between 2 and 4 years of age with predominantly neurological features. If untreated, it progresses with gradual developmental regression. A favorable outcome can be achieved if dietary treatment and alternative pathway therapy are instituted early in the disease course. With this approach, further neurological deterioration is prevented and partial recovery of skills ensues. Early diagnosis of this disorder through newborn screening programs may lead to a better outcome. This review article summarizes the clinical characterization of this disorder; as well as its biochemical, enzymatic, and molecular features. Treatment, prenatal diagnosis and diagnosis through newborn screening are also discussed.

L-NAME administration prevents the inhibition of nucleotide hydrolysis by rat blood serum subjected to hyperargininemia

The main objective of the present study was to evaluate the in vivo and in vitro effect of Arg on serum nucleotide hydrolysis. The action of Nomega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, on the effects produced by Arg was also examined. Sixty-day-old rats were treated with a single or a triple (with an interval of 1 h between each injection) intraperitoneal injection of saline (group I), Arg (0.8 g/kg) (group II), L-NAME (2.0 mg/kg or 20 mg/kg) (group III) or Arg (0.8 g/kg) plus L-NAME (2.0 mg/kg or 20 mg/kg) (group IV) and were killed 1 h later. The present results show that a triple Arg administration decreased ATP, ADP and AMP hydrolysis. Simultaneous injection of L-NAME (20 mg/kg) prevented such effects. Arg in vitro did not alter nucleotide hydrolysis. It is suggested that in vivo Arg administration reduces nucleotide hydrolysis in rat serum, probably through nitric oxide or/and peroxynitrite formation.

Clinical consequences of urea cycle enzyme deficiencies and potential links to arginine and nitric oxide metabolism

Urea cycle disorders (UCD) are human conditions caused by the dysregulation of nitrogen transfer from ammonia nitrogen into urea. The biochemistry and the genetics of these disorders were well elucidated. Earlier diagnosis and improved treatments led to an emerging, longer-lived cohort of patients. The natural history of some of these disorders began to point to pathophysiological processes that may be unrelated to the primary cause of acute morbidity and mortality, i.e., hyperammonemia. Carbamyl phosphate synthetase I single nucleotide polymorphisms may be associated with altered vascular resistance that becomes clinically relevant when specific environmental stressors are present. Patients with argininosuccinic aciduria due to a deficiency of argininosuccinic acid lyase are uniquely prone to chronic hepatitis, potentially leading to cirrhosis. Moreover, our recent observations suggest that there may be an increased prevalence of essential hypertension. In contrast, hyperargininemia found in patients with arginase 1 deficiency is associated with pyramidal tract findings and spasticity, without significant hyperammonemia. An intriguing potential pathophysiological link is the dysregulation of intracellular arginine availability and its potential effect on nitric oxide (NO) metabolism. By combining detailed natural history studies with the development of tissue-specific null mouse models for urea cycle enzymes and measurement of nitrogen flux through the cycle to urea and NO in UCD patients, we may begin to dissect the contribution of different sources of arginine to NO production and the consequences on both rare genetic and common multifactorial diseases.

Reduction of energy metabolism in rat hippocampus by arginine administration

Hyperargininemia is an inherited metabolic disease biochemically characterized by tissue accumulation of arginine. Mental retardation and other neurological features are common symptoms in hyperargininemic patients. Considering that the underlying mechanisms of brain damage in this disease are poorly established, in this work we investigated the effect of arginine administration to adult Wistar rats on some parameters of energy metabolism (CO(2) production, glucose uptake, lactate release and the activities of succinate dehydrogenase, complexes II and IV of the respiratory chain) in rat hippocampus. The action of L-NAME, an inhibitor of oxide nitric oxide synthase, on the effects produced by arginine was also tested. Sixty-day-old rats were treated with a single intraperitoneal injection of saline (group I, control), arginine (0.8 g/kg) (group II) or arginine (0.8 g/kg) plus L-NAME (2 mg/kg) (group III) and were killed 1 h later. Results showed that arginine administration significantly increased lactate release and diminished CO(2) production, glucose uptake, succinate dehydrogenase and complex II activities. In contrast, complex IV (cytochrome c oxidase) activity was not changed by this amino acid. Furthermore, simultaneous injection of L-NAME prevented some of these effects, except CO(2) production and lactate release. The present data indicate that in vivo arginine administration impairs some parameters of energy metabolism in hippocampus of rats probably through NO formation.

Activation of GABA(A) receptors by guanidinoacetate: a novel pathophysiological mechanism

Guanidinoacetate methyltransferase (GAMT) deficiency is an autosomal recessively inherited disorder of creatine biosynthesis. The disease occurs in early life with developmental delay or arrest and several neurological symptoms, e.g., seizures and dyskinesia. Both the deficiency of high-energy phosphates in neurons and the neurotoxic action of the accumulated metabolite guanidinoacetate (GAA) are candidate mechanisms for the pathophysiology of this disease. To examine a potential role of GAA accumulation, we analyzed the electrophysiological responses of neurons induced by GAA application in primary culture and acute murine brain slices. GAA evoked picrotoxin- and bicuculline-sensitive GABA(A) receptor-mediated chloride currents with an EC(50) of 167 microM in cortical neurons. Pathophysiologically relevant GAA concentrations hyperpolarized globus pallidus neurons and reduced their spontaneous spike frequency with an EC(50) of 15.1 microM. Furthermore, GAA acted as a partial agonist at heterologously expressed GABA(A) but not GABA(B) receptors. The interaction of GAA with neuronal GABA(A) receptors represents a candidate mechanism explaining neurological dysfunction in GAMT deficiency.

In vitro stimulation of oxidative stress in cerebral cortex of rats by the guanidino compounds accumulating in hyperargininemia

Hyperargininemia is a metabolic disorder biochemically characterized by tissue accumulation of arginine and other guanidino compounds. Convulsions, lethargy and psychomotor delay or cognitive deterioration are predominant clinical features of this disease. Although neurologic symptoms predominate in this disorder, their pathophysiology is still unknown. In the present study we investigated the in vitro effects of arginine, N-acetylarginine, argininic acid and homoarginine on some oxidative stress parameters in rat brain in the hope to identify a possible mechanism for the brain damage in hyperargininemia. Chemiluminescence, total radical-trapping antioxidant potential (TRAP), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities were measured in the cerebral cortex of rats in the presence of various concentrations of these compounds. The results showed that all guanidino compounds tested significantly increased chemiluminescence and decreased TRAP at concentrations similar to those observed in the tissue of hyperargininemic patients. Furthermore, these compounds inhibited CAT and GSH-Px activities to varying extents, with GSH-Px activity being more susceptible to their action. In turn, argininic acid inhibited all enzyme activities, and its main action was also directed towards GSH-Px. The results suggest that oxidative stress caused by guanidino compounds may be involved in the brain dysfunction amongst other potential pathophysiological mechanisms observed in hyperargininemia.

Improving treatment of guanidinoacetate methyltransferase deficiency: reduction of guanidinoacetic acid in body fluids by arginine restriction and ornithine supplementation

Guanidinoacetate methyltransferase (GAMT) deficiency (McKusick 601240), an inborn error of creatine biosynthesis, is characterized by creatine depletion and accumulation of guanidinoacetate (GAA) in the brain. Treatment by oral creatine supplementation had no effect on the intractable seizures. Based on the possible role of GAA as an epileptogenic agent, we evaluated a dietary treatment with arginine restriction and ornithine supplementation in order to achieve reduction of GAA. In an 8-year-old Kurdish girl with GAMT deficiency arginine intake was restricted to 15 mg/kg/day (0.4 g natural protein/kg/day) and ornithine was supplemented with 100 mg/kg/day over a period of 14 months. The diet was enriched with 0.4 g/kg/day of arginine-free essential amino acid mixture and creatine treatment remained unchanged (1.1 g/kg/day). Guanidino compounds in blood, urine, and CSF were measured by means of cation-exchange chromatography. The combination of arginine restriction and ornithine supplementation led to a substantial and permanent decrease of arginine without disturbance of nitrogen detoxification. Formation of GAA was effectively reduced after 4 weeks of treatment and sustained thereafter. Biochemical effects were accompanied by a marked clinical improvement. Distinctly reduced epileptogenic activities in electroencephalography accompanied by almost complete disappearance of seizures demonstrates the positive effect of GAA reduction. This indicates for the first time that GAA may exert an important epileptogenic potential in man. Arginine restriction in combination with ornithine supplementation represents a new and rationale therapeutic approach in GAMT deficiency.

Expression of arginase isozymes in mouse brain

The two forms of arginase (AI and AII) in man, identical in enzymatic function, are encoded in separate genes and are expressed differentially in various tissues. AI is expressed predominantly in the liver cytosol and is thought to function primarily to detoxify ammonia as part of the urea cycle. AII, in contrast, is predominantly mitochondrial, is more widely expressed, and is thought to function primarily to produce ornithine. Ornithine is a precursor in the synthesis of proline, glutamate, and polyamines. This study was undertaken to explore the cellular and regional distribution of AI and AII expression in brain using in situ hybridization and immunohistochemistry. AI and AII were detected only in neurons and not in glial cells. AI presented stronger expression than AII, but AII was generally coexpressed with AI in most cells studied. Expression was particularly high in the cerebral cortex, cerebellum, pons, medulla, and spinal cord neurons. Glutamic acid decarboxylase 65 and glutamic acid decarboxylase 67, postulated to be related to the risk of glutamate excitotoxic and/or gamma-aminobutyric acid inhibitoxic injury, were similarly ubiquitous in their expression and generally paralleled arginase expression patterns, especially in cerebral cortex, hippocampus, cerebellum, pons, medulla, and spinal cord. This study showed that AI is expressed in the mouse brain, and more strongly than AII, and sheds light on the anatomic basis for the arginine-->ornithine-->glutamate-->GABA pathway.

In vitro inhibition of Na+,K(+)-ATPase activity from rat cerebral cortex by guanidino compounds accumulating in hyperargininemia

Hyperargininemia is a metabolic disorder biochemically characterized by tissue accumulation of arginine (Arg) and other guanidino compounds (GC). Convulsions, lethargy and psychomotor delay are predominant clinical features of this disease. Considering that some GC are epileptogenic and cause a decrease in membrane fluidity and that Na+,K(+)-ATPase, a membrane-bound enzyme, is essential for cellular excitability and is decreased in experimental and human epilepsy, in the present study we determined the in vitro effects of Arg, N-acetylarginine (NAA), argininic acid (AA) and homoarginine (HA) on the activity of Na+,K(+)-ATPase in the synaptic plasma membrane from cerebral cortex of young rats in the hope to identify a possible mechanism for the brain damage in hyperargininemia. The results showed that all GC tested, except Arg, significantly inhibited Na+,K(+)-ATPase activity at concentrations similar to those observed in plasma and CSF of patients with hyperargininemia. In addition, competition between NAA, AA and HA for the binding to the enzyme was observed, suggesting a common binding site for the GC. It is therefore possible that the inhibitory effect of GC on Na+,K(+)-ATPase may be related to the brain dysfunction observed in hyperargininemia.

Uraemic guanidino compounds inhibit gamma-aminobutyric acid-evoked whole cell currents in mouse spinal cord neurones

Guanidine, creatinine (CTN), methylguanidine (MG) and guanidinosuccinic acid (GSA) are four endogenous guanidino compounds with proven neuroexcitatory actions, and putative pathophysiological significance as uraemic toxins. The effects of these uraemic guanidino compounds, were studied on whole-cell current evoked by gamma-amino butyric acid (GABA) on mouse spinal cord neurones in vitro. CTN, MG and GSA concentration dependently blocked GABA-evoked current with calculated IC50 values (+/-SE) of 9.6 +/- 0.9, 9.7 +/- 1.5 and 5.1 +/- 0.4 mM, respectively. CTN, MG and GSA were shown to block inward and outward currents to the same extent, demonstrating voltage independent block of GABA-evoked current by these compounds. Guanidine, however, evoked inward whole-cell currents, which were almost completely blocked by strychnine, indicating that the guanidine-evoked current might have been due to glycine receptor activation.

N omega-amino-L-arginine, an inhibitor of nitric oxide synthase, raises vascular resistance but increases mortality rates in awake canines challenged with endotoxin

Inhibitors of nitric oxide synthase (NOS) have been reported to increase mean arterial pressure in animal models of sepsis and recently have been given to patients in septic shock. However, controlled studies to determine the effects of these agents on cardiovascular function and survival in awake animal models of sepsis have not been reported. To examine the therapeutic potential of NOS inhibition in septic shock, we challenged canines with endotoxin (2 or 4 mg/kg i.v.) and treated them with either normal saline or N omega-amino-L-arginine (10 or 1 mg/kg/h), the most specific inhibitor available for the isoform of NOS implicated in septic shock. Endotoxemic animals treated with N omega-amino-L-arginine (n = 11) had higher systemic and pulmonary vascular resistance indices (SVRI and PVRI, p less than or equal to 0.033) and decreased heart rates (p = 0.009), cardiac indices (CI, p = 0.01), oxygen delivery indices (p = 0.027), and oxygen consumption indices (p = 0.046) compared with controls (n = 6). Moreover, N omega-amino-L-arginine increased mortality rates after endotoxin challenge (10 of 11 vs. 1 of 6 controls, p = 0.005). Administration of L-arginine did not improve survival or alter the cardiopulmonary effects of N omega-amino-L-arginine, which suggests that inhibition of NOS may not have been competitive. In normal animals, N omega-amino-L-arginine alone (n = 3) increased SVRI (p = 0.0008) and mean arterial pressure (p = 0.016), and decreased CI (p = 0.01) compared with saline-treated controls (n = 3), but, at the high dose, also produced neuromuscular rigidity and seizure-like activity that was not apparent in the endotoxemic model. Thus, the mortality rate from endotoxemia increased either because of NOS inhibition per se or because of properties unique to N omega-amino-L-arginine, or both.

Chemical models of epilepsy with some reference to their applicability in the development of anticonvulsants

This paper reviews chemical models of epilepsy and their relevance in the identification and characterization of anticonvulsants. For each convulsant we discuss possible modes of administration, clinical type(s) of seizures induced, proposed mechanism(s) of epileptogenesis and, where available, responsiveness of the induced seizures to anticonvulsants. The following compounds are reviewed: pentylenetetrazol, bicuculline, penicillin, picrotoxin, beta-carbolines, 3-mercaptopropionic acid, hydrazides, allylglycine; the glycine antagonist strychnine; gamma-hydroxybutyrate; excitatory amino acids (glutamate, aspartate, N-methyl-D-aspartate, quisqualate, kainate, quinolinic acid); monosubstituted guanidino compounds, metals (alumina, cobalt, zinc, iron); neuropeptides (opioid peptides, corticotropin releasing factor, somatostatin, vasopressin); cholinergic agents (acetylcholine, acetylcholinesterase inhibitors, pilocarpine); tetanus toxin; flurothyl; folates; homocysteine and colchicine. Although there are a multitude of chemical models of epilepsy, only a limited number are applied in the routine screening of potential anticonvulsants. Some chemical models have a predictive value with regard to the clinical profile of efficacy of the tested anticonvulsants. Some chemical models may contribute to a better understanding of possible mechanisms of epileptogenesis.