Zinc modulation of glycine receptors in acutely isolated rat CA3 neurons

The synaptic vesicle protein SV31 assembles into a dimer and transports Zn2

The integral synaptic vesicle protein SV31 has been shown to bind divalent cations. Here, we demonstrate that SV31 protein synthesized within a cell-free system binds Zn²⁺ and to a lower extent Ni²⁺ and Cu²⁺ ions. Expression with Zn²⁺ stabilized the protein and increased solubility. SV31 was preferentially monomeric in detergent and revealed specific binding of Zn²⁺ . When co-translationally inserted into defined nanodisc bilayers, SV31 assembled into dimeric complexes, resulting in increased binding of Zn²⁺ . Putative Zn²⁺ -binding motifs within SV31 comprise aspartic acid and histidine residues. Site-directed mutagenesis of two conserved aspartic acid residues leads to a potent decrease in Zn²⁺ binding but did not affect dimerization. Chemical modification of histidine residues abolished some of the Zn²⁺ -binding capacity. We demonstrate proton-dependent transport of Zn²⁺ as by accumulation of fluorescent FluoZin-1 inside of SV31-containing proteoliposomes. Transport activity has a K_m value of 44.3 μM and required external Zn²⁺ and internal acidic pH. Our results demonstrate that the synaptic vesicle-integral protein SV31 functions as a proton-dependent Zn²⁺ transporter. SV31 may attribute specific and yet undiscovered functions to subsets of synapses.

SV31 is a Zn2+-binding synaptic vesicle protein

We recently identified in a proteomic screen a novel synaptic vesicle membrane protein of 31 kDa (SV31) of unknown function. According to its membrane topology and its phylogenetic relation SV31 may function as a vesicular transporter. Based on its amino acid sequence similarity to a prokaryotic heavy metal ion transporter we analyzed its metal ion-binding properties and show that recombinant SV31 binds the divalent cations Zn(2+) and Ni(2+) and to a minor extent Cu(2+), but not Fe(2+), Co(2+), Mn(2+), or Ca(2+). Zn(2+)-binding of SV31 in viable cells was verified following heterologous transfection of pheochromocytoma cells 12 (PC12) with recombinant red fluorescent SV31 (SV31-RFP) and the fluorescent zinc indicator FluoZin-3. Sucrose density gradient fractionation of SV31-RFP-transfected PC12 cells revealed a partial overlap of SV31-RFP with synaptic-like vesicle markers and the early endosome marker rab5. Immunocytochemical analysis demonstrated a punctuate distribution in the cell soma and in neuritic processes and in addition in a compartment in vicinity to the plasma membrane that was immunopositive also for synaptosomal-associated protein 25 (SNAP-25) and syntaxin1A. Our data suggest that SV31 represents a novel Zn(2+) -binding protein that in PC12 cells is targeted to endosomes and subpopulations of synaptic-like microvesicles.

Glycinergic inhibition in the hippocampus

Glycine and GABA are the two main inhibitory neurotransmitters in the central nervous system (CNS). While GABA receptors in the hippocampus have been studied in great detail, the role of glycine receptors (GlyRs) in the hippocampus is less understood. Here we examine recent evidence suggesting that GlyRs are present and active throughout the hippocampus. Extracellular glycine levels are controlled through a combination of release and transport mechanisms, both of which, along with the GlyRs themselves, can be modulated by a number of factors. We discuss the role of GlyRs in suppressing excitation by decreasing postsynaptic membrane resistance in the hippocampus, as well as the contribution of GlyRs to both short- and long-term plasticity.