A new case of GABA transaminase deficiency facilitated by proton MR spectroscopy

BACKGROUND: Deficiency of 4-aminobutyrate aminotransferase (GABA-T) is a rare disorder of GABA catabolism, with only a single sibship reported. We report on a third case, a Japanese female infant with severe psychomotor retardation and recurrent episodic lethargy with intractable seizures, with the diagnosis facilitated by proton magnetic resonance (MR) spectroscopy ((1)H-MRS). METHODS: Neuroimaging was performed at the first episode of lethargy. For (1)H-MRS, locations were placed in the semioval center and the basal ganglia. Quantification of metabolite concentrations were derived using the LCModel. We confirmed the diagnosis subsequently by enzyme and molecular studies, which involved direct DNA sequence analysis and the development of a novel multiplex ligation-dependent probe amplification test. RESULTS: (1)H-MRS analysis revealed an elevated GABA concentration in the basal ganglia (2.9 mmol/l). Based on the results of quantitative (1)H-MRS and clinical findings, GABA-T deficiency was suspected and confirmed in cultured lymphoblasts. Molecular studies of the GABA-T gene revealed compound heterozygosity for a deletion of one exon and a missense mutation, 275G>A, which was not detected in 210 control chromosomes. CONCLUSIONS: Our results suggest that excessive prenatal GABA exposure in the central nervous system (CNS) was responsible for the clinical manifestations of GABA transaminase deficiency. Our findings suggest the dual nature of GABA as an excitatory molecule early in life, followed by a functional switch to an inhibitory species later in development. Furthermore, quantitative (1)H-MRS appears to be a useful, noninvasive tool for detecting inborn errors of GABA metabolism in the CNS

Mg2+-sensing mechanism of Mg2+ transporter MgtE probed by molecular dynamics study

Proper regulation of the intracellular ion concentration is essential to maintain life and is achieved by ion transporters that transport their substrates across the membrane in a strictly regulated manner. MgtE is a Mg2+ transporter that may function in the homeostasis of the intracellular Mg2+ concentration. A recent crystallographic study revealed that its cytosolic domain undergoes a Mg2+-dependent structural change, which is proposed to gate the ion-conducting pore passing through the transmembrane domain. However, the dynamics of Mg2+ sensing, i.e., how MgtE responds to the change in the intracellular Mg2+ concentration, remained elusive. Here we performed molecular dynamics simulations of the MgtE cytosolic domain. The simulations successfully reproduced the structural changes of the cytosolic domain upon binding or releasing Mg2+, as well as the ion selectivity. These results suggested the roles of the N and CBS domains in the cytosolic domain and their respective Mg2+ binding sites. Combined with the current crystal structures, we propose an atomically detailed model of Mg2+ sensing by MgtE

Mg2+-dependent gating of bacterial MgtE channel underlies Mg2+ homeostasis

The MgtE family of Mg2+ transporters is ubiquitously distributed in all phylogenetic domains. Recent crystal structures of the full-length MgtE and of its cytosolic domain in the presence and absence of Mg2+ suggested a Mg2+-homeostasis mechanism, in which the MgtE cytosolic domain acts as a ‘Mg2+ sensor' to regulate the gating of the ion-conducting pore in response to the intracellular Mg2+ concentration. However, complementary functional analyses to confirm the proposed model have been lacking. Moreover, the limited resolution of the full-length structure precluded an unambiguous characterization of these regulatory divalent-cation-binding sites. Here, we showed that MgtE is a highly Mg2+-selective channel gated by Mg2+ and elucidated the Mg2+-dependent gating mechanism of MgtE, using X-ray crystallographic, genetic, biochemical, and electrophysiological analyses. These structural and functional results have clarified the control of Mg2+ homeostasis through cooperative Mg2+ binding to the MgtE cytosolic domain