Regulation of glycine metabolism by the glycine cleavage system and conjugation pathway in mouse models of non-ketotic hyperglycinemia.

Kit‐Yi Leung,Nicholas D E Greene,Sandra C P De Castro,Stuart Bennett,Simon Heales,Chloe Santos,Helen Prunty,Diana Gold‐Diaz,Dawn Savery,Andrew J Copp

doi:10.1002/jimd.12295

Abstract

Glycine abundance is modulated in a tissue‐specific manner by use in biosynthetic reactions, catabolism by the glycine cleavage system (GCS), and excretion via glycine conjugation. Dysregulation of glycine metabolism is associated with multiple disorders including epilepsy, developmental delay, and birth defects. Mutation of the GCS component glycine decarboxylase (GLDC) in non‐ketotic hyperglycinemia (NKH) causes accumulation of glycine in body fluids, but there is a gap in our knowledge regarding the effects on glycine metabolism in tissues. Here, we analysed mice carrying mutations in Gldc that result in severe or mild elevations of plasma glycine and model NKH. Liver of Gldc‐deficient mice accumulated glycine and numerous glycine derivatives, including multiple acylglycines, indicating increased flux through reactions mediated by enzymes including glycine‐N‐acyltransferase and arginine: glycine amidinotransferase. Levels of dysregulated metabolites increased with age and were normalised by liver‐specific rescue of Gldc expression. Brain tissue exhibited increased abundance of glycine, as well as derivatives including guanidinoacetate, which may itself be epileptogenic. Elevation of brain tissue glycine occurred even in the presence of only mildly elevated plasma glycine in mice carrying a missense allele of Gldc. Treatment with benzoate enhanced hepatic glycine conjugation thereby lowering plasma and tissue glycine. Moreover, administration of a glycine conjugation pathway intermediate, cinnamate, similarly achieved normalisation of liver glycine derivatives and circulating glycine. Although exogenous benzoate and cinnamate impact glycine levels via activity of glycine‐N‐acyltransferase, that is not expressed in brain, they are sufficient to lower levels of glycine and derivatives in brain tissue of treated Gldc‐deficient mice.

Highlights

Glycine, the smallest amino acid, functions as a neurotransmitter, a one-carbon donor in folate one-carbon metabolism and a precursor in biosynthesis of proteins and other molecules
Mutation of glycine decarboxylase (GLDC) or amino methyltransferase (AMT) causes non-ketotic hyperglycinemia (NKH), a severe life-limiting autosomal recessive neurometabolic disorder characterised by accumulation of excess glycine in body fluids and tissue, epilepsy and profound developmental delay.[2,3]
In addition to analysis of GldcGT1/GT1 mice, in which liver glycine cleavage system (GCS) activity is below the limit of detection,[15] we investigated the effect of partial loss of Gldc function in order to determine whether glycine derivatives still accumulate in tissues

Summary

| INTRODUCTION

The smallest amino acid, functions as a neurotransmitter, a one-carbon donor in folate one-carbon metabolism and a precursor in biosynthesis of proteins and other molecules. The cell and tissue-specific abundance of glycine depends on the balance between its use in biosynthetic reactions, catabolism by the glycine cleavage system (GCS) and excretion via the glycine conjugation pathway. Use in biosynthetic reactions and catabolism by the GCS, glycine abundance is modulated by use in conjugation reactions, primarily mediated by glycine-N-acyltransferase (GLYAT) in the liver and kidneys.[4,5] The glycine conjugation system has been proposed to function primarily as a mechanism for removal of benzoate, phenylpropionate, and other aromatic amino acids, produced from dietary polyphenols by gut microbes.[5] These aromatic acids are detoxified by conversion to acyl-CoA-thioesters, which can be conjugated with glycine to form N-acylglycines (Figure 1A) or undergo β-oxidation to benzoyl-CoA.

| RESULTS

| DISCUSSION

Findings

| MATERIALS AND METHODS