Abstract:
Syntaxins are a family of membrane-integrated proteins that are instrumental in
exocytosis of vesicles. Syntaxin-1 is an essential component of the presynaptic
exocytotic fusion machinery in the brain and interacts with several other proteins.
Syntaxin-1 forms a four-helical bundle complex with proteins SNAP-25 and VAMP2 that
drives fusion of vesicles with the plasma membrane in the active zone (AZ). Little is
known, however, about the ultrastructural localization of syntaxin-1 at the synapse. We
have analyzed the intrasynaptic expression of syntaxin-1 in glutamatergic hippocampal
synapses in detail by using quantitative postembedding immunogold labeling. Syntaxin-1
was present in highest concentrations at the presynaptic AZ, supporting its role
in transmitter release. Presynaptic plasma membrane lateral to the AZ, as well as
presynaptic cytoplasmic (PreCy) vesicles were also labeled. However, syntaxin-1 was
also significantly expressed in postsynaptic spines, where it was localized at the
postsynaptic density (PSD), at postsynaptic lateral membranes and in postsynaptic
cytoplasm. Postsynaptically, syntaxin-1 colocalized in the nanometer range with the
N-methyl-D-aspartate (NMDA) receptor subunit NR2B, but only weakly with the AMPA
receptor subunits GluA2/3. This observation points to the possibility that syntaxin-1 may
be involved with NR2B vesicular trafficking from cytoplasmic stores to the postsynaptic
plasma membrane, thus facilitating synaptic plasticity. Confocal immunofluorescence
double labeling with PSD-95 and ultrastructural fractionation of synaptosomes also
confirm localization of syntaxin-1 at the PSD.
