Abstract
In addition to being involved in protein biosynthesis and metabolism, the amino acid glycine is the most important inhibitory neurotransmitter in caudal regions of the brain. These functions require a tight regulation of glycine concentration not only in the synaptic cleft, but also in various intracellular and extracellular compartments. This is achieved not only by confining the synthesis and degradation of glycine predominantly to the mitochondria, but also by the action of high-affinity large-capacity glycine transporters that mediate the transport of glycine across the membranes of presynaptic terminals or glial cells surrounding the synapses. Although most cells at glycine-dependent synapses express more than one transporter with high affinity for glycine, their synergistic functional interaction is only poorly understood. In this review, we summarize our current knowledge of the two high-affinity transporters for glycine, the sodium-dependent glycine transporters 1 (GlyT1; SLC6A9) and 2 (GlyT2; SLC6A5) and the alanine–serine–cysteine-1 transporter (Asc-1; SLC7A10).
Highlights
Glycine (Gly), the simplest amino acid, was first postulated to acts as a neurotransmitter in the central nervous system (CNS) in 1965 [1]
Glycine can be synthesized by the serine hydroxymethyltransferase (SHMT) that catalyzes the transfer of the hydroxymethyl sidechain from serine to H4 -folate (THF) to form glycine and
Asc1 was found among the astrocyte enriched genes [35], and there is a regional heterogeneity of astroglial alanine–serine–cysteine-1 transporter (Asc-1) expression, showing significantly higher Asc-1 levels in the brain regions rich in glycinergic neurotransmission such as the brainstem as compared to other regions such as hippocampus or cortex [36], further supporting the hypothesis that Asc-1 plays an important role in the regulation of glycinergic neurotransmission
Summary
Glycine (Gly), the simplest amino acid, was first postulated to acts as a neurotransmitter in the central nervous system (CNS) in 1965 [1]. At the glycinergic synapses of the mature CNS, glycine is transported into synaptic vesicles by the vesicular inhibitory amino acid transporter (VIAAT or VGAT, SLC32A1) [7]. That catalyzes the import of glycine into synaptic vesicles [7], there numerous plasma membrane transporters that accept glycine as a substrate are known Of these transporters, at least three display a high affinity for glycine and are expressed in the vicinity of glycine-dependent synapses. In vitro data using membrane preparations from caudal brain regions suggested that GlyT1 might be expressed in glycinergic neurons of the brain stem and spinal cord [22]. These data, could not be verified by in vivo approaches [23]. Asc was found among the astrocyte enriched genes [35], and there is a regional heterogeneity of astroglial Asc-1 expression, showing significantly higher Asc-1 levels in the brain regions rich in glycinergic neurotransmission such as the brainstem as compared to other regions such as hippocampus or cortex [36], further supporting the hypothesis that Asc-1 plays an important role in the regulation of glycinergic neurotransmission
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