Abstract
The AMPA-type glutamate receptor (AMPAR), which is a tetrameric complex composed of four subunits (GluA1-4) with several combinations, mediates the majority of rapid excitatory synaptic transmissions in the nervous system. Cell surface expression levels of AMPAR modulate synaptic plasticity, which is considered one of the molecular bases for learning and memory formation. To date, a unique trisaccharide (HSO3-3GlcAβ1-3Galβ1-4GlcNAc), human natural killer-1 (HNK-1) carbohydrate, was found expressed specifically on N-linked glycans of GluA2 and regulated the cell surface expression of AMPAR and the spine maturation process. However, evidence that the HNK-1 epitope on N-glycans of GluA2 directly affects these phenomena is lacking. Moreover, it is thought that other N-glycans on GluA2 also have potential roles in the regulation of AMPAR functions. In the present study, using a series of mutants lacking potential N-glycosylation sites (N256, N370, N406, and N413) within GluA2, we demonstrated that the mutant lacking the N-glycan at N370 strongly suppressed the intracellular trafficking of GluA2 from the endoplasmic reticulum (ER) in HEK293 cells. Cell surface expression of GluA1, which is a major subunit of AMPAR in neurons, was also suppressed by co-expression of the GluA2 N370S mutant. The N370S mutant and wild-type GluA2 were co-immunoprecipitated with GluA1, suggesting that N370S was properly associated with GluA1. Moreover, we found that N413 was the main potential site of the HNK-1 epitope that promoted the interaction of GluA2 with N-cadherin, resulting in enhanced cell surface expression of GluA2. The HNK-1 epitope on N-glycan at the N413 of GluA2 was also involved in the cell surface expression of GluA1. Thus, our data suggested that site-specific N-glycans on GluA2 regulate the intracellular trafficking and cell surface expression of AMPAR.
Highlights
Glycosylation is one of the major post-translational protein modifications with important roles in the structural and functional diversity of proteins
We previously showed that loss of the human natural killer-1 (HNK-1) epitope greatly increases internalization of AMPA-type glutamate receptor (AMPAR) in cultured hippocampal neurons and in heterologous cells, which indicates the HNK-1 epitope is an important factor in controlling the cell surface expression of the AMPAR [8]
We previously determined that the HNK-1 epitope was attached to the non-reducing terminus of N-glycans on GluA2 [8], but it remains unclear at which N-glycosylation site(s) the HNK-1 epitope is expressed
Summary
Glycosylation is one of the major post-translational protein modifications with important roles in the structural and functional diversity of proteins. The human natural killer-1 (HNK-1) glyco-epitope is highly expressed on several cell adhesion molecules and extracellular matrix molecules in the nervous system [1]. This carbohydrate epitope, which exhibits a unique trisaccharide structure, (HSO3-3GlcAß1-3Galß1-4GlcNAc), is biosynthesized sequentially by galactosyltransferase (ß4GalT2) [2,3], one of two glucuronyltransferases (GlcAT-P and GlcAT-S) [4], and a sulfotransferase (HNK-1ST) [5]. We reported previously that GlcAT-P gene-deficient mice, which showed an almost complete loss of HNK-1 expression in the brain, exhibited an aberration in spatial learning and memory formation and a reduction of long-term potentiation in the hippocampal CA1 region [6]. We identified a candidate HNK-1-carrier protein, which is responsible for the defects in synaptic plasticity observed in GlcAT-P-deficient mice, as GluA2, a subunit of the AMPA-type glutamate receptor (AMPAR) [8]
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