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

2-Guanidine-4-methylquinazoline acts as a novel competitive antagonist of A type γ-aminobutyric acid receptors

The pentameric A type γ-aminobutyric acid receptors (GABAARs) are the major inhibitory neurotransmitter receptors in the nervous system and have long been considered as important pharmaceutical targets for the treatment of multiple neurological or psychological disorders. Here, we show that 2-guanidine-4-methylquinazoline (GMQ), a recently identified acid-sensing ion channel (ASIC) modulator, strongly and preferentially inhibits GABAAR among the major neurotransmitter-gated ion channels in cultured rat hippocampal neurons. GMQ inhibited GABA (1 μM)-induced currents in a competitive manner, with an IC50 (0.39±0.05 μM) comparable to that of bicuculline. Schild analysis revealed a slope of 1.04±0.06 for GMQ on α1β2 GABAARs expressed in HEK293T cells. Single-channel analysis showed that GMQ decreased open probability of GABAARs without affecting conductance. Moreover, GMQ inhibited GABAergic neurotransmission in hippocampal neurons, while having no significant effect on the basal field excitatory postsynaptic potentials (fEPSPs) and the intrinsic excitability of neurons. Using site-directed mutagenesis, we further demonstrated that mutations at Glu155 of β2 subunit and Phe64 of α1 subunit, both located inside the GABA binding pocket, profoundly decreased the sensitivity of the receptor to both GABA and GMQ. Interestingly, these mutations did not significantly affect the inhibition by amiloride, a diuretic structurally similar to GMQ and a known GABAAR inhibitor. We conclude that GMQ represents a novel chemical structure that acts, possibly, by competing with GABA binding to GABAARs. It is anticipated that GMQ and its analogs will facilitate the development of new chemical probes for GABAARs.

Guanidinoacetate methyltransferase deficiency: first steps to newborn screening for a treatable neurometabolic disease

BACKGROUND

GAMT deficiency is an autosomal recessive disorder of creatine biosynthesis resulting in severe neurological complications in untreated patients. Currently available treatment is only successful to stop disease progression, but is not sufficient to reverse neurological complications occurring prior to diagnosis. Normal neurodevelopmental outcome in a patient, treated in the newborn period, highlights the importance of early diagnosis.

METHODS

Targeted mutation analysis (c.59G>C and c.327G>A) in the GAMT gene by the QIAxcel system and GAA measurement by a novel two-tier method were performed in 3000 anonymized newborn blood dot spot cards.

RESULTS

None of the targeted mutations were detected in any newborn. Two novel heterozygous variants (c.283_285dupGTC; p.Val95dup and c.278_283delinsCTCGATGCAC; p.Asp93AlafsX35) were identified by coincidence. Carrier frequency for these insertion/deletion types of GAMT mutations was 1/1475 in this small cohort of newborns. GAA levels were at or above the 99th percentile (3.12 μmol/l) in 4 newborns. Second-tier testing showed normal results for 4 newborns revealing 0.1% false positive rate. No GAMT mutations were identified in 4 of the newborns with elevated GAA levels in the first tier testing.

CONCLUSION

This is the first two-tier study to investigate carrier frequency of GAMT deficiency in the small cohort of newborn population to establish evidence base for the first steps toward newborn screening for this treatable neurometabolic disorder.

L-arginine:glycine amidinotransferase deficiency protects from metabolic syndrome

Phosphorylated creatine (Cr) serves as an energy buffer for ATP replenishment in organs with highly fluctuating energy demand. The central role of Cr in the brain and muscle is emphasized by severe neurometabolic disorders caused by Cr deficiency. Common symptoms of inborn errors of creatine synthesis or distribution include mental retardation and muscular weakness. Human mutations in l-arginine:glycine amidinotransferase (AGAT), the first enzyme of Cr synthesis, lead to severely reduced Cr and guanidinoacetate (GuA) levels. Here, we report the generation and metabolic characterization of AGAT-deficient mice that are devoid of Cr and its precursor GuA. AGAT-deficient mice exhibited decreased fat deposition, attenuated gluconeogenesis, reduced cholesterol levels and enhanced glucose tolerance. Furthermore, Cr deficiency completely protected from the development of metabolic syndrome caused by diet-induced obesity. Biochemical analyses revealed the chronic Cr-dependent activation of AMP-activated protein kinase (AMPK), which stimulates catabolic pathways in metabolically relevant tissues such as the brain, skeletal muscle, adipose tissue and liver, suggesting a mechanism underlying the metabolic phenotype. In summary, our results show marked metabolic effects of Cr deficiency via the chronic activation of AMPK in a first animal model of AGAT deficiency. In addition to insights into metabolic changes in Cr deficiency syndromes, our genetic model reveals a novel mechanism as a potential treatment option for obesity and type 2 diabetes mellitus.

Stuck at the bench: Potential natural neuroprotective compounds for concussion

Background:

While numerous laboratory studies have searched for neuroprotective treatment approaches to traumatic brain injury, no therapies have successfully translated from the bench to the bedside. Concussion is a unique form of brain injury, in that the current mainstay of treatment focuses on both physical and cognitive rest. Treatments for concussion are lacking. The concept of neuro-prophylactic compounds or supplements is also an intriguing one, especially as we are learning more about the relationship of numerous sub-concussive blows and/or repetitive concussive impacts and the development of chronic neurodegenerative disease. The use of dietary supplements and herbal remedies has become more common place.

Methods:

A literature search was conducted with the objective of identifying and reviewing the pre-clinical and clinical studies investigating the neuroprotective properties of a few of the more widely known compounds and supplements.

Results:

There are an abundance of pre-clinical studies demonstrating the neuroprotective properties of a variety of these compounds and we review some of those here. While there are an increasing number of well-designed studies investigating the therapeutic potential of these nutraceutical preparations, the clinical evidence is still fairly thin.

Conclusion:

There are encouraging results from laboratory studies demonstrating the multi-mechanistic neuroprotective properties of many naturally occurring compounds. Similarly, there are some intriguing clinical observational studies that potentially suggest both acute and chronic neuroprotective effects. Thus, there is a need for future trials exploring the potential therapeutic benefits of these compounds in the treatment of traumatic brain injury, particularly concussion.

Transport characteristics of guanidino compounds at the blood-brain barrier and blood-cerebrospinal fluid barrier: relevance to neural disorders

Guanidino compounds (GCs), such as creatine, phosphocreatine, guanidinoacetic acid, creatinine, methylguanidine, guanidinosuccinic acid, γ-guanidinobutyric acid, β-guanidinopropionic acid, guanidinoethane sulfonic acid and α-guanidinoglutaric acid, are present in the mammalian brain. Although creatine and phosphocreatine play important roles in energy homeostasis in the brain, accumulation of GCs may induce epileptic discharges and convulsions. This review focuses on how physiologically important and/or neurotoxic GCs are distributed in the brain under physiological and pathological conditions. Transporters for GCs at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier (BCSFB) have emerged as substantial contributors to GCs distribution in the brain. Creatine transporter (CRT/solute carrier (SLC) 6A8) expressed at the BBB regulates creatine concentration in the brain, and represents a major pathway for supply of creatine from the circulating blood to the brain. CRT may be a key factor facilitating blood-to-brain guanidinoacetate transport in patients deficient in S-adenosylmethionine:guanidinoacetate N-methyltransferase, the creatine biosynthetic enzyme, resulting in cerebral accumulation of guanidinoacetate. CRT, taurine transporter (TauT/SLC6A6) and organic cation transporter (OCT3/SLC22A3) expressed at the BCSFB are involved in guanidinoacetic acid or creatinine efflux transport from CSF. Interestingly, BBB efflux transport of GCs, including guanidinoacetate and creatinine, is negligible, though the BBB has a variety of efflux transport systems for synthetic precursors of GCs, such as amino acids and neurotransmitters. Instead, the BCSFB functions as a major cerebral clearance system for GCs. In conclusion, transport of GCs at the BBB and BCSFB appears to be the key determinant of the cerebral levels of GCs, and changes in the transport characteristics may cause the abnormal distribution of GCs in the brain seen in patients with certain neurological disorders.

Defining the pathogenicity of creatine deficiency syndrome

This work examined nine patients with creatine deficiency syndrome (CDS): six with a creatine transport (CRTR) defect and three with a GAMT defect. Eleven nucleotide variations were detected: six in SLC6A8 and five in GAMT. These changes were analyzed at the mRNA level and specific alleles (most of which bore premature stop codons) were selected as nulls because they provoked nonsense-mediated decay activation. The impact of these CDS mutations on metabolic stress (ROS production, p38MAPK activation, aberrant proliferation and apoptosis) was analyzed in patient fibroblast cultures. Oxidative stress contributed toward the severe form of CDS, with increases seen in the intracellular ROS content and the percentage of apoptotic cells. An altered cell cycle was also seen in a number of CRTR and GAMT fibroblast cell lines (mostly those carrying null alleles). p38MAPK activation only correlated with oxidative stress in the CRTR cells. Based on intracellular creatine levels, the contribution of energy depletion toward metabolic stress was demonstrable only in selected CRTR cells. Together, these findings suggest that the apoptotic response to genotoxic damage in the present CDS cells may have been triggered by different cell signaling pathways. They also suggest that reducing oxidative stress could be helpful in treating CDS. Hum Mutat 32:1-10, 2011. © 2011 Wiley-Liss, Inc.

Treatment of intractable epilepsy in a female with SLC6A8 deficiency

A female heterozygous for a novel, disease causing, missense mutation in the X-linked cerebral creatine transporter (SLC6A8) gene (c.1067G>T, p.Gly356Val) presented with intractable epilepsy, mild intellectual disability and moderately reduced cerebral creatine levels. Treatment with creatine monohydrate, to enhance cerebral creatine transport, combined with L-arginine and L-glycine, to enhance cerebral creatine synthesis, resulted in complete resolution of seizures. Heterozygous SLC6A8 deficiency is a potentially treatable condition and should be considered in females with intractable epilepsy and developmental delay/intellectual disability.

Creatine and creatine deficiency syndromes: biochemical and clinical aspects

Creatine deficiency syndromes, which have only recently been described, represent a group of inborn errors of creatine synthesis (L-arginine-glycine amidinotransferase deficiency and guanidinoacetate methyltransferase deficiency) and transport (creatine transporter deficiency). Patients with creatine deficiency syndromes present with mental retardation expressive speech and language delay, and epilepsy. Patients with guanidinoacetate methyltransferase deficiency or creatine transporter deficiency may exhibit autistic behavior. The common denominator of these disorders is the depletion of the brain creatine pool, as demonstrated by in vivo proton magnetic resonance spectroscopy. For diagnosis, laboratory investigations start with analysis of guanidinoacetate, creatine, and creatinine in plasma and urine. Based on these findings, enzyme assays or DNA mutation analysis may be performed. The creatine deficiency syndromes are underdiagnosed, so the possibility should be considered in all children affected by unexplained mental retardation, seizures, and speech delay. Guanidinoacetate methyltransferase deficiency and arginine-glycine amidinotransferase deficiency are treatable by oral creatine supplementation, but patients with creatine transporter deficiency do not respond to this type of treatment.

Guanidinoacetate methyltransferase deficiency (GAMT)

An increasing number of disorders of metabolism are becoming amenable to the treatment, and GAMT deficiency is one of them. The symptoms and signs are reviewed, emphasising that delayed language development is a particular feature. Other symptoms include learning disorders, autistic behaviour, epileptic seizures, and movement disorders. The condition is inherited in an autosomal recessive manner, and mutations in the GAMT gene severely affect the activity of guanidinoacetate. The MRI scan shows an increased signal in the globus pallidus, and the diagnosis is confirmed by finding increased guanidinoacetate in the urine and a low plasma creatine. Other methods of diagnosis are discussed. Treatment is based on giving creatine supplementation orally and a low-protein diet with restricted arginine and increased ornithine. This results in improvement of many of the symptoms, especially of the epileptic seizures and the abnormal movements. It is justifiable to consider this condition in any patient with unexplained learning disorders.

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.

Activation of cerebellar granule cells GABA(A) receptors by guanidinoacetate

The extracellular concentration of guanidinoacetate (GAA) in the brain increases in guanidino acetate methyl transferase (GAMT) deficiency, an inherited disorder. We tested whether the levels which this substance can reach in the brain in GAMT deficiency are able to activate GABA(A) receptors in key cerebellar neurons such as the cerebellar granules. GAA in fact activates these receptors in rat cerebellar granules in culture although at quite high concentrations, in the millimolar range. However, these millimolar GAA levels are not reached extracellularly in the brain in GAMT deficiency. In addition, GAA does not act as a partial agonist on granules' GABA(A) receptors. This appears to deny an effect by this molecule on cerebellar function in the disease via interference with granule cells' GABA(A) receptors. Study of partial blockage by furosemide of chloride currents activated by GABA and GAA in granule cells allowed us to distinguish two populations of GABA(A) receptors presumably involved in granule cells' tonic inhibition. One is devoid of alpha6 subunit and another one contains it. The latter when activated by GABA has a decay kinetics much slower than the former. GAA does not distinguish between these two populations. In any case, the very high extracellular GAA concentrations able to activate them are not likely to be reached in GAMT deficiency.

Guanidinoacetate administration increases acetylcholinesterase activity in striatum of rats and impairs retention of an inhibitory avoidance task

Guanidinoacetate methyltransferase deficiency (GAMT-deficiency) is an inborn error of metabolism biochemically characterized by accumulation of guanidinoacetate (GAA) and depletion of creatine; the pathogenesis of brain dysfunction in this disorder is not yet established. In the present study we investigated the effect of intrastriatal administration of GAA on acetylcholinesterase (AChE) activity and on memory acquisition, consolidation and retrieval of step-down inhibitory avoidance task in rat. Results showed that GAA significantly increased AChE activity in rat striatum 30 min (50%) and 3 h (25%), but not 6 h after drug administration. GAA impaired test session performance when applied 30 min before training or after training, and before testing sessions, i.e., impaired memory acquisition, consolidation and retrieval. When injected with a 6 hour interval, GAA affected only memory retrieval. Although the mechanisms of action of GAA on AChE activity and on memory are unclear, these findings suggest that the accumulation of GAA found in patients with GAMT-deficiency may be one of the mechanisms involved in neural dysfunction. Further studies are necessary to evaluate these mechanisms.

AGAT, GAMT and SLC6A8 distribution in the central nervous system, in relation to creatine deficiency syndromes: a review

Creatine deficiency syndromes, either due to AGAT, GAMT or SLC6A8 deficiencies, lead to a complete absence, or a very strong decrease, of creatine within the brain, as measured by magnetic resonance spectroscopy. While the mammalian central nervous system (CNS) expresses AGAT, GAMT and SLC6A8, the lack of SLC6A8 in astrocytes around the blood-brain barrier limits the brain capacity to import creatine from the periphery, and suggests that the CNS has to rely mainly on endogenous creatine synthesis through AGAT and GAMT expression. This seems contradictory with SLC6A8 deficiency, which, despite AGAT and GAMT expression, also leads to creatine deficiency in the CNS. We present novel data showing that in cortical grey matter, AGAT and GAMT are expressed in a dissociated way: e.g. only a few cells co-express both genes. This suggests that to allow synthesis of creatine within the CNS, at least for a significant part of it, guanidinoacetate must be transported from AGAT- to GAMT-expressing cells, possibly through SLC6A8. This would explain the creatine deficiency observed in SLC6A8-deficient patients. By bringing together creatine deficiency syndromes, AGAT, GAMT and SLC6A8 distribution in CNS, as well as a synthetic view on creatine and guanidinoacetate levels in the brain, this review presents a comprehensive framework, including new hypotheses, on brain creatine metabolism and transport, both in normal conditions and in case of creatine deficiency.

Protective effects of some creatine derivatives in brain tissue anoxia

Some derivatives more lipophylic than creatine, thus theoretically being capable to better cross the blood-brain barrier, were studied for their protective effect in mouse hippocampal slices. We found that N-amidino-piperidine is harmful to brain tissue, and that phosphocreatine is ineffective. Creatine, creatine-Mg-complex (acetate) and phosphocreatine-Mg-complex (acetate) increased the latency to population spike disappearance during anoxia. Creatine and creatine-Mg-complex (acetate) also increased the latency of anoxic depolarization, while the delay induced by phosphocreatine-Mg-complex (acetate) was of borderline significance (P = 0.056). Phosphocreatine-Mg-complex (acetate) significantly reduced neuronal hyperexcitability during anoxia, an effect that no other compound (including creatine itself) showed. For all parameters except reduced hyperexcitability the effects statistically correlated with tissue levels of creatine or phosphocreatine. Summing up, exogenous phosphocreatine and N-amidino piperidine are not useful for brain protection, while chelates of both creatine and phosphocreatine do replicate some of the known protective effects of creatine. In addition, phosphocreatine-Mg-complex (acetate) also reduced neuronal hyperexcitability during anoxia.

Evidence that the inhibitory effects of guanidinoacetate on the activities of the respiratory chain, Na+,K+-ATPase and creatine kinase can be differentially prevented by taurine and vitamins E and C administration in rat striatum in vivo

Guanidinoacetate methyltransferase (GAMT) deficiency is an inherited neurometabolic disorder biochemically characterized by tissue accumulation of guanidinoacetate (GAA) and depletion of creatine. Affected patients present epilepsy and mental retardation whose etiopathogeny is unclear. In a previous study we showed that instrastriatal administration of GAA caused a reduction of Na(+),K(+)-ATPase and creatine kinase (CK) activities, as well as an increase in TBARS (an index of lipid peroxidation). In the present study we investigated the in vitro and in vivo effects of GAA on glucose uptake from [U-(14)C] acetate (citric acid cycle activity) and on the activities of complexes II, II-III, III and IV of the respiratory chain in striatum of rats. Results showed that 50 and 100 microM GAA (in vitro studies) and GAA administration (in vivo studies) significantly inhibited complexes II and II-III, respectively, but did not alter complexes III and IV, as well as CO(2) production. We also studied the influence of taurine or vitamins E and C on the inhibitory effects caused by intrastriatal administration of GAA on complexes II and II-III, Na(+),K(+)-ATPase and CK activities, and on TBARS in rat striatum. Pre-treatment with taurine and vitamins E and C revealed that taurine prevents the effects of intrastriatal administration of GAA on the inhibition of complex II, complex II-III, and Na(+),K(+)-ATPase activities. Vitamins E and C prevent the effects of intrastriatal administration of GAA on the inhibition of CK and Na(+),K(+)-ATPase activities, and on the increase of TBARS. The data suggest that GAA in vivo and in vitro treatment disturbs important parameters of striatum energy metabolism and that oxidative damage may be mediating these effects. It is presumed that defects in striatum bioenergetics might be involved in the pathophysiology of striatum damage characteristic of patients with GAMT-deficiency.

Cerebral creatine deficiency syndromes: clinical aspects, treatment and pathophysiology

Cerebral creatine deficiency syndromes (CCDSs) are a group of inborn errors of creatine metabolism comprising two autosomal recessive disorders that affect the biosynthesis of creatine--i.e. arginine:glycine amidinotransferase deficiency (AGAT; MIM 602360) and guanidinoacetate methyltransferase deficiency (GAMT; MIM 601240)--and an X-linked defect that affects the creatine transporter, SLC6A8 deficiency (SLC6A8; MIM 300036). The biochemical hallmarks of these disorders include cerebral creatine deficiency as detected in vivo by 1H magnetic resonance spectroscopy (MRS) of the brain, and specific disturbances in metabolites of creatine metabolism in body fluids. In urine and plasma, abnormal guanidinoacetic acid (GAA) levels are found in AGAT deficiency (reduced GAA) and in GAMT deficiency (increased GAA). In urine of males with SLC6A8 deficiency, an increased creatine/creatinine ratio is detected. The common clinical presentation in CCDS includes mental retardation, expressive speech and language delay, autistic like behaviour and epilepsy. Treatment of the creatine biosynthesis defects has yielded clinical improvement, while for creatine transporter deficiency, successful treatment strategies still need to be discovered. CCDSs may be responsible for a considerable fraction of children and adults affected with mental retardation of unknown etiology. Thus, screening for this group of disorders should be included in the differential diagnosis of this population. In this review, also the importance of CCDSs for the unravelling of the (patho)physiology of cerebral creatine metabolism is discussed.

Expression and function of AGAT, GAMT and CT1 in the mammalian brain

In mammals, creatine is taken up from the diet and can be synthesized endogenously by a two-step mechanism involving the enzymes arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT). Creatine (Cr) is taken up by cells through a specific transporter, CT1. While the major part of endogenous synthesis of Cr is thought to occur in kidney, pancreas and liver, the brain widely expresses AGAT, GAMT and CT1, both during development and in adulthood. The adult central nervous system (CNS) has a limited capacity to take up Cr from periphery, and seems to rely more on its endogenous Cr synthesis. In contrast, the embryonic CNS might be more dependent on Cr supply from periphery than on endogenous synthesis. This review will focus on the expression and function of AGAT, GAMT and CT1 in the mammalian CNS, both during development and in adulthood. Emphasis will also be placed on their specific roles in the different cell types of the brain, to analyze which brain cells are responsible for the CNS capacity of (i) endogenous Cr synthesis and (ii) Cr uptake from the periphery, and which brain cells are the main Cr consumers. The potential role of CT1 as guanidinoacetate transporter between "AGAT-only" and "GAMT-only" expressing cells will also be explored.

Pre-symptomatic treatment of creatine biosynthesis defects

Recent observations in two patients, one with AGAT deficiency (AGAT-D) and one with GAMT deficiency (GAMT-D), both diagnosed already at birth, provide first evidence for important therapeutic effects of pre-symptomatic treatment with creatine (Cr) supplementation in AGAT-D and Cr supplementation plus guanidinoacetate lowering strategies in GAMT-D. Although long-term data are lacking, the results suggest that complete prevention of neurological sequelae in early treated patients could be feasible (Battini et al., 2006; Schulze et al., 2006).

Are cerebral creatine deficiency syndromes on the radar screen?

Cerebral creatine deficiency syndromes (CCDS) are responsible for a considerable proportion of the population affected with mental retardation. CCDS are caused by either an inborn error of the proteins involved in creatine biosynthesis or in the creatine transporter. Besides mental retardation, the clinical characteristics of CCDS are speech and language delay, epilepsy and features of autism. CCDS can be diagnosed by proton magnetic resonance spectroscopy of the brain and/or by biochemical and molecular analysis. Treatment of the defects in creatine biosynthesis has yielded favorable outcomes, while treatments for creatine transporter deficiency are still under investigation at this time. The relatively large contribution of the CCDS to the monogenic causes of mental retardation emphasizes the importance of including CCDS in the differential diagnosis of mental retardation of unknown etiology. Pathophysiology is not yet unravelled, although it is known that creatine plays an important role in energy storage and transmission. Moreover, in vitro data indicate that creatine acts as a neuromodulator in the brain.

Intrastriatal administration of guanidinoacetate inhibits Na+, K+-ATPase and creatine kinase activities in rat striatum

Guanidinoacetate methyltransferase deficiency (GAMT deficiency) is an inherited neurometabolic disorder clinically characterized by epilepsy and mental retardation and biochemically by accumulation of guanidinoacetate (GAA) and depletion of creatine. Although this disease is predominantly characterized by severe neurological findings, the underlying mechanisms of brain injury are not yet established. In the present study, we investigated the effect of intrastriatal administration of GAA on Na+, K+-ATPase activity, total (tCK), cytosolic (Cy-CK), and mitochondrial (Mi-CK) creatine kinase (CK) activities in rat striatum. We verified that Na+, K+-ATPase, tCK, and Mi-CK activities were significantly inhibited by GAA, in contrast to Cy-CK which was not affected by this guanidino compound. Since these enzyme activities can be affected by reactive species, we also investigated the effect of intrastriatal administration of GAA on thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation in rats. We found that this metabolite significantly increased this oxidative stress parameter. Considering the importance of Na+, K+-ATPase and CK activities for brain metabolism homeostasis, our results suggest that the inhibition of these enzymes by increased intracerebral levels of GAA may contribute to the neuropathology observed in patients with GAMT-deficiency.

Increased guanidino species in murine and human succinate semialdehyde dehydrogenase (SSADH) deficiency

Mice with targeted deletion of the GABA-degradative enzyme succinate semialdehyde dehydrogenase (SSADH; Aldh5a1; OMIM 271,980) manifest globally elevated GABA and regionally decreased arginine in brain extracts. We examined the hypothesis that arginine-glycine amidinotransferase catalyzed the formation of guanidinobutyrate (GB) from increased GABA by quantifying guanidinoacetate (GA), guanidinopropionate (GP) and GB in brain extracts employing stable isotope dilution gas chromatographic-mass spectrometry. GA and GB were up to 4- and 22-fold elevated, respectively, in total and regional (cerebellum, hippocampus, cortex) brain extracts derived from SSADH(-/-) mice. Corresponding analyses of urine and cerebrospinal fluid derived from SSADH-deficient patients revealed significant (P<0.05) elevations of GA and GB in urine, as well as GB levels in CSF. These data suggest that GB may be an additional marker of SSADH deficiency, implicate additional pathways of pathophysiology, and identify the second instance of elevated GB in a human inborn error of metabolism.

A mutation on exon 6 of guanidinoacetate methyltransferase (GAMT) gene supports a different function for isoform a and b of GAMT enzyme

A new patient affected by Guanidinoacetate methyltransferase (GAMT) deficiency was reported. This 13-year-old girl presented with mental retardation, as main symptom, associated with a typical pattern of biochemical and neurochemical (brain magnetic resonance spectroscopy) alterations. Molecular study detected a L197P transition on exon 6 of the GAMT gene. Since this mutation leaves the isoform B of the GAMT enzyme unaffected, the occurrence of biochemical alterations and disease in this subject testifies against the possibility that isoform b had GAMT activity.

Exocytotic release of creatine in rat brain

The guanidino compound creatine has been shown to occur throughout the brain affecting energy metabolism and mental performance and to act at central GABAA receptors as a partial agonist. Therefore, we examined the possibility that creatine may in fact represent a neuromodulator that is released in the brain in an action-potential dependent manner. To that end, we studied the uptake of [3H]creatine and its electrically evoked release from superfused rat brain slices as well as the evoked release of endogenously synthesized creatine. [3H]creatine was accumulated in neocortex slices in a Na+-dependent manner, consistent with the involvement of the Na+-dependent SLC6A8 creatine transporter. Most importantly, the electrically evoked release of [3H]creatine from neocortex slices (like that from caudate putamen and hippocampus slices) as well as the evoked release of endogenous (unlabeled) creatine was abolished when Ca2+ was omitted from the superfusion medium or in the presence of the Na+-channel blocker tetrodotoxin (TTX). Moreover, blockade of K+-channels by 4-aminopyridine (4-AP) strongly enhanced the electrically evoked release of [3H]creatine as well as that of endogenous creatine. These in vitro data indicate that creatine is not only synthesized and taken up by central neurons, but also released in an action-potential dependent (exocytotic) manner, providing strong evidence for its role as a neuromodulator in the brain.

Lower force and impaired performance during high-intensity electrical stimulation in skeletal muscle of GAMT-deficient knockout mice

Force characteristics of skeletal muscle of knockout mice lacking creatine (Cr) due to a deletion of guanidinoacetate methyltransferase (GAMT) were studied in situ. Medial gastrocnemius muscles of anesthetized GAMT-deficient (GAMT−/−) and control (Con) littermates were stimulated at optimum length via the sciatic nerve at different stimulation frequencies (60–250 Hz). GAMT−/− mice showed reduced maximal tetanic and twitch force, reduced relative force at 60 Hz, and increased relaxation times. High-intensity fatigue protocols consisting of 30 successive isometric or dynamic contractions showed a strong reduction in force at the beginning of the series in GAMT−/− mice, followed by a smaller reduction compared with Con littermates toward the end of the series. Cr supplementation for 2 days in GAMT−/− animals (GAMT[Formula: see text]) resulted in normalization to Con values for relaxation times, relative force at lower stimulation frequencies, and relative force during 30 isometric contractions. Force per muscle mass, however, remained decreased. Furthermore,GAMT[Formula: see text] mice showed differences compared with both Con and unsupplemented animals in maximal rates of force rise and relaxation times during the isometric protocol as well as in force during the dynamic protocol. Our results show that the absence of Cr plays a direct role in relaxation times, maximal rate of force rise, and force production during high-intensity fatigue protocols. The lower force per muscle mass, however, is probably caused by other factors; i.e., high intracellular guanidinoacetate concentrations.

Biochemical and behavioural phenotyping of a mouse model for GAMT deficiency

Deficiency of guanidinoacetate N-methyltransferase (GAMT) is the first described creatine (CT) deficiency syndrome in man, biochemically characterized by accumulation of guanidinoacetic acid (GAA) and depletion of CT. Patients exhibit severe developmental and muscular problems. We created a mouse model for GAMT deficiency, which exerts biochemical changes comparable with those found in human GAMT-deficient subjects. CT and creatinine (CTN) levels are significantly decreased and GAA is increased in knockout (KO) mice. In patients, other guanidino compounds (GCs) appear to be altered as well, which may also contribute to the symptomatology. Extensive evaluation of GCs levels in the GAMT mouse model was therefore considered appropriate. Concentrations of 13 GCs in plasma, 24-h urine, brain and muscle of GAMT mice were measured. We also report on the detailed behavioural characterization of this model for GAMT deficiency. Besides an increase of GAA and a decrease of CT and CTN in plasma, 24-h urine, brain and muscle of KO mice, we observed a significant increase of other GCs in brain and muscle that was sometimes reflected in plasma and/or urine. KO mice displayed mild cognitive impairment. In general, it could be concluded that the GAMT mouse model is very useful for biochemical research of GAMT deficiency, but shows only a mild cognitive deficit.

Creatine metabolism in combined methylmalonic aciduria and homocystinuria

Methylation is an important aspect of many fundamental biological processes including creatine biosynthesis. We studied five patients with an inborn error of cobalamin metabolism to characterize the relation between homocysteine and creatine metabolism. Plasma guanidinoacetate concentrations were increased, 14.9 +/- 4.8 micromol/L (p < 0.0001), whereas plasma creatine concentrations were in the low reference range, 43.8 +/- 20.7 micromol/L (p = not significant). Individuals with combined methylmalonic aciduria and homocystinuria have a functional impairment of the creatine synthetic pathway probably secondary to a relative depletion of labile methyl groups. The neurotoxic effects of guanidinoacetate may be partly responsible for the observed neurological phenotype.

Brain creatine functions to attenuate acute stress responses through GABAnergic system in chicks

The involvement of brain creatine in the adaptation to acute stress responses was investigated in chicks. In experiment 1, brain creatine content of chicks exposed to social separation stress was significantly increased compared with control chicks. The effects of i.c.v. injection of creatine (2 mug) on vocalizations, spontaneous activity and plasma corticosterone concentration in chicks under social separation stress were investigated in experiment 2. All measurements were attenuated by the i.c.v. injection of creatine compared with the controls under separation stress. Creatine also significantly decreased the active posture, but increased the motionless eye-opened posture, compared with the control. To clarify the relationship between creatine function and GABA receptors, the i.c.v. co-injection of creatine with picrotoxin, a GABA-A receptor antagonist, or CGP54626, a GABA-B receptor antagonist, was investigated in experiments 3 and 4. The effects of creatine on vocalizations and spontaneous activity were attenuated by co-injection of picrotoxin. In this case, active postures decreased by creatine were recovered by co-injection with picrotoxin. However, these effects were not obtained with CGP54626. The results suggest that central creatine functions within the CNS to attenuate the acute stress response by acting through GABA-A receptors in chicks.

Evaluation of the mechanism underlying the inhibitory effect of guanidinoacetate on brain Na + , K + ‐ATPase activity

Guanidinoacetate methyltransferase deficiency (GAMT-deficiency) is an inherited neurometabolic disorder clinically characterized by epilepsy and mental retardation and biochemically by accumulation of guanidinoacetate (GAA) and depletion of creatine. Although the neurological symptoms are predominant, the pathogenesis of the brain dysfunction in this disorder is not yet established. In the present study we investigated the in vitro effect of GAA on Na+, K+-ATPase and Mg2+-ATPase activities in synaptic plasma membrane from hippocampus of young rats. Results showed that GAA significantly inhibited Na+, K+-ATPase activity without affecting Mg2+-ATPase activity. We also evaluated the effect of glutathione (GSH), trolox, Nomega-nitro-L-arginine methyl ester (L-NAME) and taurine (Tau) on the inhibition elicited by GAA on Na+, K+-ATPase activity. GSH, trolox, L-NAME and Tau per se did not alter Na+, K+-ATPase activity. However, L-NAME and taurine prevented the inhibitory effect of GAA on this enzyme activity. Our findings suggest that the inhibition of Na+, K+-ATPase activity caused by GAA is possibly mediated by nitric oxide (NO) formation and/or synaptic membrane alteration. The present data may contribute to the understanding of the neurological dysfunction characteristic of GAMT-deficient patients.

Inhibition of Na+, K+-ATPase activity in rat striatum by guanidinoacetate

The aim of this work was to investigate the effect of guanidinoacetate (GAA), the principal metabolite accumulating in guanidinoacetate methyltransferase (GAMT)-deficiency, on Na(+), K(+)-ATPase, Mg(2+)-ATPase and acetylcholinesterase (AChE) activities in striatum of young rats. We also studied the kinetics of the inhibition of Na(+), K(+)-ATPase activity caused by guanidinoacetate. Guanidinoacetate did not alter acetylcholinesterase and Mg(2+)-ATPase activities, but significantly inhibited Na(+), K(+)-ATPase activity. The apparent K(m) and V(max) of Na(+), K(+)-ATPase for ATP as substrate were 0.20mM and 0.82nmol inorganic phosphate (Pi) released per min per mg of protein, respectively. K(i) value was 7.18mM, and the inhibition was of the uncompetitive type. The results also showed a competition between guanidinoacetate and argininic acid (AA), suggesting a common binding site for the guanidino compounds (GC) on the enzyme. It is proposed that Na(+), K(+)-ATPase inhibition by guanidinoacetate may be one of the mechanisms involved in the neuronal dysfunction observed in GAMT-deficiency and in other diseases which accumulate guanidinoacetate.