Transsynaptic Signaling by Postsynaptic Synapse-Associated Protein 97

106

Proteins of the PSD-95-like membrane-associated guanylate kinase (PSD-MAGUK) family are vital for trafficking AMPA receptors (AMPARs) to synapses, a process necessary for both basal synaptic transmission and forms of synaptic plasticity. Synapse-associated protein 97 (SAP97) exhibits protein interactions, such as direct interaction with the GluA1 AMPAR subunit, and subcellular localization (synaptic, perisynaptic, and dendritic) unique within this protein family. Due in part to the lethality of the germline knockout of SAP97, this protein's role in synaptic transmission and plasticity is poorly understood. We found that overexpression of SAP97 during early development traffics AMPARs and NMDA receptors (NMDARs) to synapses, and that SAP97 rescues the deficits in AMPAR currents normally seen in PSD-93/-95 double-knockout neurons. Mature neurons that have experienced the overexpression of SAP97 throughout development exhibit enhanced AMPAR and NMDAR currents, as well as faster NMDAR current decay kinetics. In loss-of-function experiments using conditional SAP97 gene deletion, we recorded no deficits in glutamatergic transmission or long-term potentiation. These results support the hypothesis that SAP97 is part of the machinery that traffics glutamate receptors and compensates for other PSD-MAGUKs in knockout mouse models. However, due to functional redundancy, other PSD-MAGUKs can presumably compensate when SAP97 is conditionally deleted during development.AMPA | hippocampus | membrane-associated guanylate kinase | NMDA | postsynaptic density | synaptic transmission | synaptic development T he excitatory postsynaptic density (PSD) stabilizes AMPA receptors (AMPARs) and NMDA receptors (NMDARs) opposed to presynaptic terminals, providing a foundation for synaptic transmission and bidirectional receptor trafficking during plasticity. A major PSD component is the PSD-95-like membrane-associated guanylate kinase (PSD-MAGUK) protein family, which includes PSD-95/SAP90, PSD-93/Chapsyn-110, synapse-associated protein (SAP) 102, and SAP97, the mammalian homolog of Drosophila tumor-suppressor discs large (Dlg1) (1, 2), All PSD-MAGUKs contain protein-protein interaction motifs, most notably three PSD95/Dlg/ZO-1 (PDZ) domains (3-5), which provide a scaffold for synaptic protein complexes. PSD-95 and PSD-93 play critical roles in AMPAR trafficking at mature synapses, whereas SAP102 is most important during synaptogenesis (6-13). PSD-MAGUKs also are important for localizing NMDA receptors, especially during synapse development (14-17).Unlike other PSD-MAGUKs, SAP97β, the major isoform of the protein (also known as DLGH1; encoded by Dlgh1), is expressed at presynaptic and postsynaptic sites (2, 18) and perisynaptically in dendritic spines (19). SAP97β directly binds to AMPAR GluA1 subunits (20-23). Because SAP97 has been resistant to RNAi in our hands (but see 10) and germline KO causes lethal feeding deficits in neonatal mice (24), SAP97's synaptic function is uncertain and is based primarily on dissociated neuronal culture stu...

Section: Sap97 Overexpression Enhances Glutamatergic Synaptic Transmisupporting

confidence: 87%

“…Although SAP97 overexpression has no effect on AMPAR-mediated synaptic transmission (8,11,12), SAP97 rescues AMPAR transmission reduced by RNAi-mediated knockdown of PSD-95 (12). SAP97 overexpression during synapse formation increases presynaptic terminal size and activity in neuronal cultures via a transynaptic signal (26).…”

mentioning

confidence: 97%

The role of SAP97 in synaptic glutamate receptor dynamics

Howard

Elias

et al. 2010

106

“…Postsynaptic expression of PSD-MAGUKs in immature dissociated neuronal cultures has been shown to enhance presynaptic function (35,36). We found that changing the levels of PSD-95 (or SAP102) during synaptogenesis or synapse maturation did not affect the PPR, suggesting that neither PSD-95 nor SAP102 retrogradely regulates presynaptic function in vivo.…”

Section: Discussionmentioning

confidence: 73%

Differential trafficking of AMPA and NMDA receptors by SAP102 and PSD-95 underlies synapse development

Elias

Apostolides

et al. 2008

196

190

The development of glutamatergic synapses involves changes in the number and type of receptors present at the postsynaptic density. To elucidate molecular mechanisms underlying these changes, we combine in utero electroporation of constructs that alter the molecular composition of developing synapses with dual whole-cell electrophysiology to examine synaptic transmission during two distinct developmental stages. We find that SAP102 mediates synaptic trafficking of AMPA and NMDA receptors during synaptogenesis. Surprisingly, after synaptogenesis, PSD-95 assumes the functions of SAP102 and is necessary for two aspects of synapse maturation: the developmental increase in AMPA receptor transmission and replacement of NR2B-NMDARs with NR2A-NMDARs. In PSD-95/PSD-93 double-KO mice, the maturational replacement of NR2B-with NR2A-NMDARs fails to occur, and PSD-95 expression fully rescues this deficit. This study demonstrates that SAP102 and PSD-95 regulate the synaptic trafficking of distinct glutamate receptor subtypes at different developmental stages, thereby playing necessary roles in excitatory synapse development.AMPAR and NMDAR trafficking ͉ membrane-associated guanylate kinase ͉ synaptogenesis ͉ postsynaptic density ͉ synaptic transmission A fundamental goal of developmental neurobiology is to identify the sequence of molecular events underlying excitatory synapse development, a process that can be divided into two distinct stages: synaptogenesis and synapse maturation. Synaptogenesis follows the specification of cell-to-cell contacts mediated by cell adhesion molecules (1, 2) and involves the initiation of chemical communication through the recruitment of pre-and postsynaptic proteins necessary for fast synaptic transmission (3, 4), such as AMPA receptors (AMPARs) and NMDA receptors (NMDARs). Synapse maturation is characterized by two functional events: an increase in the strength of AMPAR-mediated transmission (5, 6) and a switch in the subunit composition of synaptic NMDARs (7). NMDARs are composed of two obligatory NR1 subunits and two NR2 subunits, of which there are four members (NR2A-D) (8). NR2B-NMDARs are expressed during synaptogenesis and are replaced by NR2A-NMDARs during synapse maturation (9-11), a replacement that accounts for the developmental decrease in the NMDAR excitatory postsynaptic current (EPSC) decay time (12, 13) and the loss of sensitivity to the NR2B antagonist ifenprodil (14). The precise molecular mechanisms underlying the differential synaptic trafficking of AMPAR and NMDAR during developmental synaptogenesis and maturation remain largely unknown.PSD-95 is a member of a family of proteins collectively known as membrane-associated guanylate kinases (MAGUKs) (15-17). The PSD-95-like subfamily of neuronal MAGUKs (PSD-MAGUKs) includes PSD-93, SAP102, and SAP97 (15-17). Comparative studies emphasize the remarkable similarities among PSD-MAGUKs in terms of protein-protein interactions (18,19) and overlapping functions in synaptic trafficking of AMPARs at mature synapses (20-24). On the ot...

“…Postsynaptic clustering of ion channels and adhesion molecules is controlled by scaffolding proteins (37,38), and overexpression of postsynaptic scaffolding proteins retrogradely enhances maturation of presynaptic terminals (5,39). The MAGI type postsynaptic scaffolding protein S-SCAM (26,29) has been described to potentially interact with catenins via binding with its PDZ5 domain to the PDZ-binding domains of β-catenin and δ-catenin (28,40).…”

Section: Discussionmentioning

confidence: 99%

“…The yet relatively unexplored molecular control mechanisms involved in vesicle clustering have been hypothesized to use retrograde signaling via transsynaptic adhesion molecules (2)(3)(4)(5)(6). Transsynaptic adhesion/signaling systems are based either on a homophilic or on a heterophilic protein-protein interaction across the synaptic cleft (7).…”

mentioning

confidence: 99%

Essential cooperation of N-cadherin and neuroligin-1 in the transsynaptic control of vesicle accumulation

Stan¹,

Pielarski²,

Brigadski

et al. 2010

117

116

Cell adhesion molecules are key players in transsynaptic communication, precisely coordinating presynaptic differentiation with postsynaptic specialization. At glutamatergic synapses, their retrograde signaling has been proposed to control presynaptic vesicle clustering at active zones. However, how the different types of cell adhesion molecules act together during this decisive step of synapse maturation is largely unexplored. Using a knockout approach, we show that two synaptic adhesion systems, N-cadherin and neuroligin-1, cooperate to control vesicle clustering at nascent synapses. Live cell imaging and fluorescence recovery after photobleaching experiments at individual synaptic boutons revealed a strong impairment of vesicle accumulation in the absence of N-cadherin, whereas the formation of active zones was largely unaffected. Strikingly, also the clustering of synaptic vesicles triggered by neuroligin-1 overexpression required the presence of N-cadherin in cultured neurons. Mechanistically, we found that N-cadherin acts by postsynaptically accumulating neuroligin-1 and activating its function via the scaffolding molecule S-SCAM, leading, in turn, to presynaptic vesicle clustering. A similar cooperation of N-cadherin and neuroligin-1 was observed in immature CA3 pyramidal neurons in an organotypic hippocampal network. Moreover, at mature synapses, N-cadherin was required for the increase in release probability and miniature EPSC frequency induced by expressed neuroligin-1. This cooperation of two cell adhesion systems provides a mechanism for coupling bidirectional synapse maturation mediated by neuroligin-1 to cell type recognition processes mediated by classical cadherins.synapse maturation | synaptic adhesion molecules | vesicle clustering