title: N-terminal SAP97 isoforms differentially regulate synaptic structure and postsynaptic surface pools of AMPA receptors.
abstract: The location and density of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors is controlled by scaffolding proteins within the postsynaptic density (PSD). SAP97 is a PSD protein with two N-terminal isoforms, α and β, that have opposing effects on synaptic strength thought to result from differential targeting of AMPA receptors into distinct synaptic versus extrasynaptic locations, respectively. In this study, we have applied dSTORM super resolution imaging in order to localize the synaptic and extrasynaptic pools of AMPA receptors in neurons expressing α or βSAP97. Unexpectedly, we observed that both α and βSAP97 enhanced the localization of AMPA receptors at synapses. However, this occurred via different mechanisms: αSAP97 increased PSD size and consequently the number of receptor binding sites, whilst βSAP97 increased synaptic receptor cluster size and surface AMPA receptor density at the PSD edge and surrounding perisynaptic sites without changing PSD size. αSAP97 also strongly enlarged presynaptic active zone protein clusters, consistent with both presynaptic and postsynaptic enhancement underlying the previously observed αSAP97-induced increase in AMPA receptor-mediated currents. In contrast, βSAP97-expressing neurons increased the proportion of immature filopodia that express higher levels of AMPA receptors, decreased the number of functional presynaptic terminals, and also reduced the size of the dendritic tree and delayed the maturation of mushroom spines. Our data reveal that SAP97 isoforms can specifically regulate surface AMPA receptor nanodomain clusters, with βSAP97 increasing extrasynaptic receptor domains at peri-synaptic and filopodial sites. Moreover, βSAP97 negatively regulates synaptic maturation both structurally and functionally. These data support diverging presynaptic and postsynaptic roles of SAP97 N-terminal isoforms in synapse maturation and plasticity. As numerous splice isoforms exist in other major PSD proteins (e.g., Shank, PSD95, and SAP102), this alternative splicing may result in individual PSD proteins having divergent functional and structural roles in both physiological and pathophysiological synaptic states.
authors.affiliation: Department of Physiology and Centre for Brain Research, University of Auckland, Auckland, New Zealand.
authors.forename: Lucy
authors.initials: L
authors.lastname: Goodman
authors.affiliation: Department of Physiology and Centre for Brain Research, University of Auckland, Auckland, New Zealand.|Department of Cell Biology, Yale School of Medicine, New Haven, Connecticut.
authors.forename: David
authors.initials: D
authors.lastname: Baddeley
authors.affiliation: Department of Physiology and Centre for Brain Research, University of Auckland, Auckland, New Zealand.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Ambroziak
authors.affiliation: Department of Pathology and Cell Biology, Columbia University, New York, New York.
authors.forename: Clarissa L
authors.initials: CL
authors.lastname: Waites
authors.affiliation: German Center for Neurodegenerative Diseases, Charité University, Berlin, Germany.
authors.forename: Craig C
authors.initials: CC
authors.lastname: Garner
authors.affiliation: Department of Physiology and Centre for Brain Research, University of Auckland, Auckland, New Zealand.|Department of Physical and Cell Biology, Physical and Cell Biology, University of Exeter, Exeter, United Kingdom.
authors.forename: Christian
authors.initials: C
authors.lastname: Soeller
authors.affiliation: Department of Physiology and Centre for Brain Research, University of Auckland, Auckland, New Zealand.
authors.forename: Johanna M
authors.identifier: 0000-0002-8714-7456
authors.initials: JM
authors.lastname: Montgomery
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D002478:Cells, Cultured; D006624:Hippocampus / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D057907:Post-Synaptic Density / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D018091:Receptors, AMPA / Q000378:metabolism*; D013569:Synapses / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C501119:Dlg1 protein, rat; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D020033:Protein Isoforms; D018091:Receptors, AMPA; C033652:postsynaptic density proteins
keywords: MAGUK; dSTORM; glutamate receptors; postsynaptic density; synapse