PSD-95 and PSD-93 Play Critical But Distinct Roles in Synaptic Scaling Up and Down

Abstract

Synaptic scaling stabilizes neuronal firing through the homeostatic regulation of postsynaptic strength, but the mechanisms by which chronic changes in activity lead to bidirectional adjustments in synaptic AMPAR abundance are incompletely understood. Further, it remains unclear to what extent scaling up and scaling down utilize distinct molecular machinery. PSD-95 is a scaffold protein proposed to serve as a binding “slot” that determines synaptic AMPAR content, and synaptic PSD-95 abundance is regulated by activity, raising the possibility that activity-dependent changes in the synaptic abundance of PSD-95 or other MAGUKs drives the bidirectional changes in AMPAR accumulation during synaptic scaling. We found that synaptic PSD-95 and SAP102 (but not PSD-93) abundance were bidirectionally regulated by activity, but these changes were not sufficient to drive homeostatic changes in synaptic strength. Although not sufficient, the PSD-95-MAGUKs were necessary for synaptic scaling, but scaling up and down were differentially dependent on PSD-95 and PSD-93. Scaling down was completely blocked by reduced or enhanced PSD-95, through a mechanism that depended on the PDZ1/2 domains. In contrast scaling up could be supported by either PSD-95 or PSD-93 in a manner that depended on neuronal age, and was unaffected by a superabundance of PSD-95. Taken together, our data suggest that scaling up and down of quantal amplitude is not driven by changes in synaptic abundance of PSD-95-MAGUKs, but rather that the PSD-95 MAGUKs serve as critical synaptic organizers that utilize distinct protein-protein interactions to mediate homeostatic accumulation and loss of synaptic AMPAR.

PSD-95 promotes the stabilization of young synaptic contacts

“…This is consistent with our previous finding that PSD-95 knockdown reduces metabotropic excitatory postsynaptic current frequency in old but not young neurons [29]. Between DIV 7 and DIV 14 the levels of synaptic PSD-95 increase substantially [28,29], and it has been shown that postsynaptic densities share and compete for PSD-95 [5,7]. As the acquisition of PSD-95 is probably regulated by changes in activity and experience [5, 45,46] when PSD-95 levels are low early in development, competition for PSD-95 could help determine which synapses are maintained and which are lost.…”

“…This is consistent with our previous finding that PSD-95 knockdown reduces metabotropic excitatory postsynaptic current frequency in old but not young neurons [29]. Between DIV 7 and DIV 14 the levels of synaptic PSD-95 increase substantially [28,29], and it has been shown that postsynaptic densities share and compete for PSD-95 [5,7].…”

“…Interestingly, PDS-95 overexpression did not affect the ratio of contact gain to loss, as this ratio remained close to unity across ages and PSD-95 expression levels (figure 5b). These data show that PSD-95 overexpression in young neurons enhances the stability of putative synaptic contacts, while having little impact on older synapses that express more endogenous PSD-95 [29].…”

“…PSD-95 binds indirectly to a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), regulates synaptic strength, and has been proposed to serve as a 'slot' protein that directly determines synaptic AMPAR content [36][37][38][39]. Manipulations of PSD-95 can dramatically alter evoked synaptic transmission [23,25,37,[40][41][42][43]], but we found previously that PSD-95 overexpression or knockdown in this same culture system did not significantly increase AMPA quantal amplitude [29], in keeping with other reports of only modest effects [5,23,37,40,41,44]. The modest impact of PSD-95 on quantal amplitude, coupled with the substantial impact on synapse stability shown here and suggested previously [31], suggest that the effects of PSD-95 overexpression on evoked transmission are primarily due to an enhancement of the number of stable, functional synapses.…”

“…We have shown so far that stable AD contacts have a higher ratio of active to inactive CaMKII and an enhanced accumulation of PSD-95. The synaptic localization of PSD-95 increases developmentally and with time in vitro [28,29], and there is ample evidence that PSD-95 overexpression can enhance the number of synapses in immature neurons [20,21,23,35], suggesting an important role for PSD-95 in some aspect of excitatory synapse formation. However, it is not known whether PSD-95 exerts its effects by enhancing the rate of synapse formation, or by enhancing the stability of newly formed contacts.…”