Modulation of ionotropic glutamate receptor function by vertebrate galectins

Abstract

Key pointsr Lectins are a family of small evolutionarily conserved sugar-binding proteins that regulate a diverse array of physiological processes ranging from immune system activation to cancer cell metastasis.r Ionotropic glutamate receptor function can be modulated by plant-derived lectins, but the physiological relevance of this activity is unclear as no analogous function has been identified in animal lectins.r We found that a variety of vertebrate lectins, including human brain-expressed galectin-1, modulated glutamate receptor kinetics in a subunit and lectin-dependent manner, which critically depended on complex oligosaccharide processing.r Galectin application slowed neuronal kainate receptor currents from nociceptive dorsal root ganglion neurons.r We propose that brain-expressed galectins are potential endogenous modulators of neuronal glutamate receptors, which may play important roles in diseases of altered cellular excitability, such as epilepsy or chronic pain.Abstract AMPA and kainate receptors are glutamate-gated ion channels whose function is known to be altered by a variety of plant oligosaccharide-binding proteins, or lectins, but the physiological relevance of this activity has been uncertain because no lectins with analogous allosteric modulatory effects have been identified in animals. We report here that members of the prototype galectin family, which are β-galactoside-binding lectins, exhibit subunit-specific allosteric modulation of desensitization of recombinant homomeric and heteromeric AMPA and kainate receptors. Galectin modulation of GluK2 kainate receptors was dependent upon complex oligosaccharide processing of N-glycosylation sites in the amino-terminal domain and downstream linker region. The sensitivity of GluA4 AMPA receptors to human galectin-1 could be enhanced by supplementation of culture media with uridine and N-acetylglucosamine (GlcNAc), precursors for the hexosamine pathway that supplies UDP-GlcNAc for synthesis of complex oligosaccharides. Neuronal kainate receptors in dorsal root ganglia were sensitive to galectin modulation, whereas AMPA receptors in cultured hippocampal neurons were insensitive, which could be a reflection of differential N-glycan processing or receptor subunit selectivity. Because glycan content of integral proteins can be modified dynamically, we postulate that physiological or pathological conditions in the CNS could arise in which galectins alter excitatory neurotransmission or neuronal excitability through their actions on AMPA or kainate receptors.

Altered fucosyltransferase expression in the superior temporal gyrus of elderly patients with schizophrenia

“…In humans, AMPAR subunits GluA1-4 each have several N -glycosylation sites that exert regulatory effects on glutamate-signaling. Specific N -glycans expressed on AMPAR subunits modulate neurotransmission by influencing receptor assembly, trafficking, and synaptic stability, as well as by amending interactions between receptors and activity-modulating compounds (Copits et al, 2014; Everts et al, 1997; Kawamoto et al, 1994; Pasternack et al, 2003; Standley et al, 1998; Standley and Baudry, 2000; Takeuchi et al, 2015). N -glycosylation of N370 in GluA2 is necessary for the ER exit of GluA2 as well as the cell surface expression of both GluA2 and GluA1; furthermore, when a specific glyco-epitope (the HNK1 epitope, a unique N -glycan terminal trisaccharide) is expressed on GluA2 N413, both GluA1 and GluA2 demonstrated increased cell surface expression (Takeuchi et al, 2015).…”

Two N-glycosylation Sites in the GluN1 Subunit Are Essential for Releasing N-methyl-d-aspartate (NMDA) Receptors from the Endoplasmic Reticulum

“…We suggest that the differences in the expression of the native GluN subunits and/or specific protein complexes regulating the assembly, ER retention, and/or intracellular trafficking of NMDARs among the various cell types could account for this phenomenon. Interestingly, recent studies reported that the N-glycosylation pattern of AMPA receptors in the cortex is altered in schizophrenia (42), the ionotropic glutamate receptor function is modulated by vertebrate galectins (43), and O-GlcNAcylation of the GluA2 subunit is associated with long-term depression in hippocampal synapses (44). Therefore, it is likely that future studies will reveal more information regarding the precise roles that specific glycosylation of NMDARs play in regulating neuronal function.…”

Vertebrate galectins: endogenous regulators of ionotropic glutamate receptors?

“…Now, a study from Geoff Swanson's lab in this issue of The Journal of Physiology (Copits et al . ) investigates the ability of vertebrate galectins, including recombinant human galectin‐1, and congerin I and II from the conger eel, to regulate native and recombinant AMPA and KA receptors. Galectins bind specifically to β‐galactoside glycoconjugates and the three galectins studied by Copits et al .…”

“…Galectins bind specifically to β‐galactoside glycoconjugates and the three galectins studied by Copits et al . () can each form a bivalent homodimer. In contrast to tetravalent ConA, which substantially increases currents mediated by most AMPA and KA receptor isoforms, galectin application produced a remarkable diversity of effects on recombinant iGluRs depending on the specific subunits that were expressed and on which galectin was applied.…”

“…Thus, Swanson and colleagues (Copits et al . ) have established the potential for native receptors to be modulated by endogenous lectins, but much work remains to determine where and when such regulation actually occurs.…”