Oral glycine administration increases brain glycine/creatine ratios in men: a proton magnetic resonance spectroscopy study

Glycine and aging: Evidence and mechanisms

The restriction of calories, branched-chain amino acids, and methionine have all been shown to extend lifespan in model organisms. Recently, glycine was shown to significantly boost longevity in genetically heterogenous mice. This simple amino acid similarly extends lifespan in rats and improves health in mammalian models of age-related disease. While compelling data indicate that glycine is a pro-longevity molecule, divergent mechanisms may underlie its effects on aging. Glycine is abundant in collagen, a building block for glutathione, a precursor to creatine, and an acceptor for the enzyme Glycine N-methyltransferase (GNMT). A review of the literature strongly implicates GNMT, which clears methionine from the body by taking a methyl group from S-adenosyl-L-methionine and methylating glycine to form sarcosine. In flies, Gnmt is required for reduced insulin/insulin-like growth factor 1 signaling and caloric restriction to fully extend lifespan. The geroprotector spermidine requires Gnmt to upregulate autophagy genes and boost longevity. Moreover, the overexpression of Gnmt is sufficient to extend lifespan and reduce methionine levels. Sarcosine, or methylglycine, declines with age in multiple species and is capable of inducing autophagy both in vitro and in vivo. Taken all together, existing evidence suggests that glycine prolongs life by mimicking methionine restriction and activating autophagy.

The glycine-containing dipeptide leucine-glycine raises accumbal dopamine levels in a subpopulation of rats presenting a lower endogenous dopamine tone

Interventions that elevate glycine levels and target the glycine receptor (GlyR) in the nucleus Accumbens (nAc) reduce ethanol intake in rats, supposedly by acting on the brain reward system via increased basal and attenuated ethanol-induced nAc dopamine release. Glycine transport across the blood brain barrier (BBB) appears inefficient, but glycine-containing dipeptides elevate whole brain tissue dopamine levels in mice. This study explores whether treatment with the glycine-containing dipeptides leucine-glycine (Leu-Gly) and glycine-leucine (Gly-Leu) by means of a hypothesized, facilitated BBB passage, alter nAc glycine and dopamine levels and locomotor activity in two rodent models. The acute effects of Leu-Gly and Gly-Leu (1–1000 mg/kg, i.p.) alone or Leu-Gly in combination with ethanol on locomotion in male NMRI mice were examined in locomotor activity boxes. Striatal and brainstem slices were obtained for ex vivo HPLC analyses of tissue levels of glycine and dopamine. Furthermore, the effects of Leu-Gly i.p. (1–1000 mg/kg) on glycine and dopamine output in the nAc were examined using in vivo microdialysis coupled to HPLC in freely moving male Wistar rats. Leu-Gly and Gly-Leu did not significantly alter locomotion, ethanol-induced hyperlocomotor activity or tissue levels of glycine or dopamine, apart from Gly-Leu 10 mg/kg that slightly raised nAc dopamine. Microdialysis revealed no significant alterations in nAc glycine or dopamine levels when regarding all rats as a homogenous group. In a subgroup of rats defined as dopamine responders, a significant elevation of nAc dopamine (20%) was seen following Leu-Gly 10–1000 mg/kg i.p, and this group of animals presented lower baseline dopamine levels compared to dopamine non-responders. To conclude, peripheral injection of glycine-containing dipeptides appears inefficient in elevating central glycine levels but raises accumbal dopamine levels in a subgroup of rats with a lower endogenous dopamine tone. The tentative relationship between dopamine baseline and ensuing response to glycinergic treatment and presumptive direct interactions between glycine-containing dipeptides and the GlyR bear insights for refinement of the glycinergic treatment concept for alcohol use disorder (AUD).

GlyCEST: Magnetic Resonance Imaging of Glycine—Distribution in the Normal Murine Brain and Alterations in 5xFAD Mice

The glycine level in the brain is known to be altered in neuropsychiatric disorders, such as schizophrenia and Alzheimer's disease (AD). Several studies have reported the in vivo measurement of glycine concentrations in the brain using proton magnetic resonance spectroscopy (¹H-MRS), but ¹H-MRS is not capable of imaging the distribution of glycine concentration with high spatial resolution. Chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI) is a new technology that can detect specific molecules, including amino acids, in tissues. To validate the measurements of glycine concentrations in living tissues using CEST from glycine to water (GlyCEST), we extracted the brain tissues from mice and performed biochemical tests. In wild-type C57BL/6 mice, GlyCEST effects were found to be higher in the thalamus than in the cerebral cortex (P < 0.0001, paired t-test), and this result was in good agreement with the biochemical results. In 5xFAD mice, an animal model of AD, GlyCEST measurements demonstrated that glycine concentrations in the cerebral cortex (P < 0.05, unpaired t-test) and thalamus (P < 0.0001, unpaired t-test), but not in the hippocampus, were decreased compared to those in wild-type mice. These findings suggest that we have successfully applied the CEST-MRI technique to map the distribution of glycine concentrations in the murine brain. The present method also captured the changes in cerebral glycine concentrations in mice with AD. Imaging the distribution of glycine concentrations in the brain can be useful in investigating and elucidating the pathological mechanisms of neuropsychiatric disorders.

The potential of 1H-MRS in CNS drug development

RATIONALE

Proton magnetic resonance spectroscopy (¹H-MRS) is a cross-species neuroimaging technique that can measure concentrations of several brain metabolites, including glutamate and GABA. This non-invasive method has promise in developing centrally acting drugs, as it can be performed repeatedly within-subjects and be used to translate findings from the preclinical to clinical laboratory using the same imaging biomarker.

OBJECTIVES

This review focuses on the utility of single-voxel ¹H-MRS in developing novel glutamatergic or GABAergic drugs for the treatment of psychiatric disorders and includes research performed in rodent models, healthy volunteers and patient cohorts.

RESULTS

Overall, these studies indicate that ¹H-MRS is able to detect the predicted pharmacological effects of glutamatergic or GABAergic drugs on voxel glutamate or GABA concentrations, although there is a shortage of studies examining dose-related effects. Clinical studies have applied ¹H-MRS to better understand drug therapeutic mechanisms, including the glutamatergic effects of ketamine in depression and of acamprosate in alcohol dependence. There is an emerging interest in identifying patient subgroups with 'high' or 'low' brain regional ¹H-MRS glutamate levels for more targeted drug development, which may require ancillary biomarkers to improve the accuracy of subgroup discrimination.

CONCLUSIONS

Considerations for future research include the sensitivity of single-voxel ¹H-MRS in detecting drug effects, inter-site measurement reliability and the interpretation of drug-induced changes in ¹H-MRS metabolites relative to the known pharmacological molecular mechanisms. On-going technological development, in single-voxel ¹H-MRS and in related complementary techniques, will further support applications within CNS drug discovery.

Evidence for a unique association between fronto-cortical glycine levels and recent heavy drinking in treatment naïve individuals with alcohol use disorder

Although the neurotransmitters/modulators glutamate and, more recently, glycine have been implicated in the development and maintenance of Alcohol Use Disorder (AUD) in preclinical research, human proton magnetic resonance spectroscopy (1H-MRS) studies have focused solely on the measurement of glutamate. The purpose of the present analysis was to examine the relative associations of brain glutamate and glycine levels with recent heavy drinking in 41 treatment naïve individuals with AUD using 1H-MRS. The present study is the first that we are aware of to report in vivo brain glycine levels from an investigation of addiction. Dorsal Anterior Cingulate Cortex (dACC) glutamate and glycine concentration estimates were obtained using Two-Dimensional J-Resolved Point Resolved Spectroscopy at 3 Tesla, and past 2-week summary estimates of alcohol consumption were assessed via the Timeline Followback method. Glutamate (β = -0.44, t = -3.09, p = 0.004) and glycine (β = -0.68, t = -5.72, p <  0.001) were each significantly, inversely associated with number of heavy drinking days when considered alone. However, when both variables were simultaneously entered into a single regression model, the effect of glutamate was no longer significant (β = -0.11, t = -0.81, p = 0.42) whereas the effect of glycine remained significant (β = -0.62, t = -4.38, p < 0.001). The present study extends the literature by demonstrating a unique, inverse association of brain glycine levels with recent heavy drinking in treatment naïve individuals with AUD. If replicated and extended, these data could lead to enhanced knowledge of how glycinergic systems change with alcohol consumption and AUD progression leading to pharmacological interventional/preventative strategies that modulate brain glycine levels.

Dietary glycine decreases both tumor volume and vascularization in a combined colorectal liver metastasis and chemotherapy model

Rationale: Chemotherapy (CTx) with FOLFOX is indicated prior to resection of liver metastases; however, its effect is limited due to chemoresistance and its toxicity prevents from aggressive surgery needed in some cases. Hepatoprotective glycine has been shown to have anti-tumorigenic properties in various cancers. Thus, this study was designed to evaluate the effects of glycine combined with FOLFOX on colorectal liver metastases (CRLM).

Methods: The effect of glycine combined with 5-fluorouracil and oxaliplatin was investigated in vitro on colorectal cancer (CC531). Further, Wag/Rij rats with CRLM were treated with 5% dietary glycine ± FOLFOX. µCT liver scan, anti-Ki67, and anti-CD31 were compared.

Results: Glycine alone and combined with CTx has no effect on both CC531 viability in vitro and tumor proliferation in vivo; however, glycine significantly decreased tumor volume to about 42-35% of controls in vivo (p<0.05) with a 60% decreased tumor microvascular density (MVD) (p=0.004). Further glycine doesn't counteract anti-tumor properties of CTx.

Conclusions: This study nicely demonstrates that glycine inhibits the growth of CRLM and does not decrease CTx effectiveness. Underlying mechanisms most likely include a decreased tumor MVD. Clinical trials are warranted to implement non-toxic hepatoprotective glycine in novel anti-cancer strategies in humans.

In Vivo Brain Glycine and Glutamate Concentrations in Patients With First-Episode Psychosis Measured by Echo Time-Averaged Proton Magnetic Resonance Spectroscopy at 4T

BACKGROUND

Accumulating evidence suggests the involvement of abnormal glutamateric neurotransmission and N-methyl-D-aspartate receptor hypofunction in the pathophysiology of psychotic disorders. The purpose of this study was to quantify in vivo glutamate (Glu) and glycine (Gly) levels in patients with first-episode psychosis as well as age-matched healthy control subjects with magnetic resonance spectroscopy (MRS).

METHODS

The subjects were 46 patients with first-episode psychosis (20 with a schizophrenia spectrum disorder, 26 with bipolar disorder) and 50 age-matched healthy control subjects. Glu and Gly levels were measured in vivo in the anterior cingulate cortex and posterior cingulate cortex of the subjects by using the echo time-averaged proton MRS technique at 4T (i.e., modified point resolved spectroscopy sequence: 24 echo time steps with 20-ms increments). Metabolite levels were quantified using LCModel with simulated basis sets.

RESULTS

Significantly higher Glu and Gly levels were found in both the anterior cingulate cortex and posterior cingulate cortex of patients with first-episode psychosis as compared with healthy control subjects. Glu and Gly levels were positively correlated in patients. Patients with a schizophrenia spectrum disorder and bipolar disorder showed similar abnormalities.

CONCLUSIONS

Our findings demonstrate abnormally elevated brain Glu and Gly levels in patients with first-episode psychosis by means of echo time-averaged proton MRS at 4T. The findings implicate dysfunction of N-methyl-D-aspartate receptor and glutamatergic neurotransmission in the pathophysiology of the acute early phase of psychotic illnesses.

Edited 1 H magnetic resonance spectroscopy in vivo: Methods and metabolites

The Proton magnetic resonance (1 H-MRS) spectrum contains information about the concentration of tissue metabolites within a predefined region of interest (a voxel). The conventional spectrum in some cases obscures information about less abundant metabolites due to limited separation and complex splitting of the metabolite peaks. One method to detect these metabolites is to reduce the complexity of the spectrum using editing. This review provides an overview of the one-dimensional editing methods available to interrogate these obscured metabolite peaks. These methods include sequence optimizations, echo-time averaging, J-difference editing methods (single BASING, dual BASING, and MEGA-PRESS), constant-time PRESS, and multiple quantum filtering. It then provides an overview of the brain metabolites whose detection can benefit from one or more of these editing approaches, including ascorbic acid, γ-aminobutyric acid, lactate, aspartate, N-acetyl aspartyl glutamate, 2-hydroxyglutarate, glutathione, glutamate, glycine, and serine. Magn Reson Med 77:1377-1389, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Neurometabolic abnormalities in schizophrenia and depression observed with magnetic resonance spectroscopy at 7 T

BACKGROUND

Examining neurometabolic abnormalities in critical brain areas in schizophrenia and major depressive disorder (MDD) may help guide future pharmacological interventions including glutamate-modulating treatments.

AIMS

To measure metabolite concentrations within the anterior cingulate cortex (ACC) and thalamus of people with schizophrenia and people with MDD.

METHODS

Spectra were acquired from 16 volunteers with schizophrenia, 17 with MDD and 18 healthy controls using magnetic resonance spectroscopy on a 7 Tesla scanner.

RESULTS

In the thalamus, there were lower glycine concentrations in the schizophrenia group relative to control (P=0.017) and MDD groups (P=0.012), and higher glutamine concentrations relative to healthy controls (P=0.009). In the thalamus and the ACC, the MDD group had lower myo-inositol concentrations than the control (P=0.014, P=0.009, respectively) and schizophrenia (P=0.004, P=0.002, respectively) groups.

CONCLUSION

These results support the glutamatergic theory of schizophrenia and indicate a potential glycine deficiency in the thalamus. In addition, reduced myo-inositol concentrations in MDD suggest its involvement in the disorder.

DECLARATION OF INTEREST

None.

COPYRIGHT AND USAGE

© The Royal College of Psychiatrists 2017. This is an open access article distributed under the terms of the Creative Commons Non-Commercial, No Derivatives (CC BY-NC-ND) license.

Glycine inhibits angiogenesis in colorectal cancer: role of endothelial cells

Neo-angiogenesis is important for tumor growth. Glycine is a non-toxic amino acid with suspected anti-angiogenic effects. This study was designed to evaluate anti-angiogenic effects of glycine in colorectal cancer. Glycine was added to cultures of human and rat colorectal cancer cells (CRC), and endothelial cells (HUVEC). Glycine's direct impact was monitored using MTT assays. Angiogenesis in HUVEC was monitored using 3D sprouting and migration assays. VEGF and CRC-conditioned media were used to stimulate angiogenesis. The glycine receptor (GlyR) was detected using Western blotting and inhibited using strychnine. The WAG-Rij/CC-531 model of metastatic CRC was used to evaluate glycine's impact in vivo. Tumor growth and vessel density were monitored in rats fed with or without 5 % glycine for 14 days. VEGF and conditioned media significantly increased proliferation, migration, and capillary formation to up to 267 %. Glycine completely neutralized this effect and strychnine completely blunted glycine's effect. GlyR was detected in HUVEC. Tumor volume, weight, and vessel density decreased by 35 % (p = 0.02), 34 % (p = 0.03), and 55 % (p = 0.04) in glycine-fed animals. Glycine inhibits angiogenic signaling of endothelial cells and tumor growth. Glycine would be a promising additive to standard and targeted cancer therapies.

In vivo assessment of neurotransmitters and modulators with magnetic resonance spectroscopy: application to schizophrenia

In vivo measurement of neurotransmitters and modulators is now feasible with advanced proton magnetic resonance spectroscopy ((1)H MRS) techniques. This review provides a basic tutorial of MRS, describes the methods available to measure brain glutamate, glutamine, γ-aminobutyric acid, glutathione, N-acetylaspartylglutamate, glycine, and serine at magnetic field strengths of 3T or higher, and summarizes the neurochemical findings in schizophrenia. Overall, (1)H MRS holds great promise for producing biomarkers that can serve as treatment targets, prediction of disease onset, or illness exacerbation in schizophrenia and other brain diseases.

Use of proton magnetic resonance spectroscopy in the treatment of psychiatric disorders: a critical update

Because of the wide availability of hardware as well as of standardized analytic quantification tools, proton magnetic resonance spectroscopy ((1)H-MRS) has become widely used to study psychiatric disorders. (1)H-MRS allows measurement of brain concentrations of more traditional singlet neurometabolites like N-acetylaspartate, choline, and creatine. More recently, quantification of the more complex multiplet spectra for glutamate, glutamine, inositol, and γ-aminobutyric acid have also been implemented. Here we review applications of (1)H-MRS in terms of informing treatment options in schizophrenia, bipolar disorder, and major depressive disorders. We first discuss recent meta-analytic studies reporting the most reliable findings. Then we evaluate the more sparse literature focused on 1H-MRS-detected neurometabolic effects of various treatment approaches in psychiatric populations. Finally we speculate on future developments that may result in translation of these tools to improve the treatment of psychiatric disorders.

The glycine deportation system and its pharmacological consequences

The glycine deportation system is an essential component of glycine catabolism in man whereby 400 to 800mg glycine per day are deported into urine as hippuric acid. The molecular escort for this deportation is benzoic acid, which derives from the diet and from gut microbiota metabolism of dietary precursors. Three components of this system, involving hepatic and renal metabolism, and renal active tubular secretion help regulate systemic and central nervous system levels of glycine. When glycine levels are pathologically high, as in congenital nonketotic hyperglycinemia, the glycine deportation system can be upregulated with pharmacological doses of benzoic acid to assist in normalization of glycine homeostasis. In congenital urea cycle enzymopathies, similar activation of the glycine deportation system with benzoic acid is useful for the excretion of excess nitrogen in the form of glycine. Drugs which can substitute for benzoic acid as substrates for the glycine deportation system have adverse reactions that may involve perturbations of glycine homeostasis. The cancer chemotherapeutic agent ifosfamide has an unacceptably high incidence of encephalopathy. This would appear to arise as a result of the production of toxic aldehyde metabolites which deplete ATP production and sequester NADH in the mitochondrial matrix, thereby inhibiting the glycine deportation system and causing de novo glycine synthesis by the glycine cleavage system. We hypothesize that this would result in hyperglycinemia and encephalopathy. This understanding may lead to novel prophylactic strategies for ifosfamide encephalopathy. Thus, the glycine deportation system plays multiple key roles in physiological and neurotoxicological processes involving glycine.

Measurement of glycine in gray and white matter in the human brain in vivo by 1H MRS at 7.0 T

The concentration of glycine (Gly) was measured in gray matter (GM) and white matter (WM) in the human brain using single-voxel localized (1)H MRS at 7 T. A point-resolved spectroscopy sequence with echo time = 150 ms was used for measuring Gly levels in various regions of the frontal and occipital lobes in 11 healthy volunteers and one subject with a glioblastoma. The point-resolved spectroscopy spectra were analyzed with LCModel using basis functions generated from density matrix simulations that included the effects of volume localized radio-frequency and gradient pulses. The fraction of GM and white matter within the voxels was obtained from T(1)-weighted image segmentation. The metabolite concentrations within the voxels, estimated with respect to the GM + WM water concentrations, were fitted to a linear function of fractional GM content. The Gly concentrations in pure GM and white matter were estimated to be 1.1 and 0.1 mM, with 95% confidence intervals 1.0-1.2 and 0.0-0.2, respectively.

MR spectroscopy in schizophrenia

The purpose of this study is to review the MR spectroscopic literature regarding schizophrenia. However, as there are over 250 primary MRS articles and dozens of MRS review articles on the subject already, this study will take a different approach. First, the clinical features of schizophrenia will be described. The background neuroanatomy and biochemistry relevant to schizophrenia will be reviewed, as many readers of this journal are unlikely to be familiar with these fields. A current model of the abnormal neural circuitry in schizophrenia will be presented, and predictions extrapolated about relevant metabolite changes over time. Finally, the existing MRS literature will be reviewed in the context of our existing anatomical and chemical knowledge, and future MRS research directions will be elaborated.

Measurement of glycine in the human brain in vivo by 1H-MRS at 3 T: application in brain tumors

Glycine is a key metabolic intermediate required for the synthesis of proteins, nucleic acids, and other molecules, and its detection in cancer could, therefore, provide biologically relevant information about the growth of the tumor. Here, we report measurement of glycine in human brain and gliomas by an optimized point-resolved spectroscopy sequence at 3 T. Echo time dependence of the major obstacle, myo-inositol (mI) multiplet, was investigated with numerical simulations, incorporating the 3D volume localization. The simulations indicated that a subecho pair (TE(1) , TE(2) ) = (60, 100) ms permits detection of both glycine and mI with optimum selectivity. In vivo validation of the optimized point-resolved spectroscopy was conducted on the right parietal cortex of five healthy volunteers. Metabolite signals estimated from LC Model were normalized with respect to the brain water signal, and the concentrations were evaluated assuming the total creatine concentration at 8 mM. The glycine concentration was estimated as 0.6 ± 0.1 mM (mean ± SD, n = 5), with a mean Cramér-Rao lower bound of 9 ± 1%. The point-resolved spectroscopy sequence was applied to measure the glycine levels in patients with glioblastoma multiforme. Metabolite concentrations were obtained using the water signal from the tumor mass. The study revealed that a subset of human gliomas contains glycine levels elevated 1.5-8 fold relative to normal.