Synaptic Protein Degradation Underlies Destabilization of Retrieved Fear Memory

Abstract

Reactivated memory undergoes a rebuilding process that depends on de novo protein synthesis. This suggests that retrieval is dynamic and serves to incorporate new information into preexisting memories. However, little is known about whether or not protein degradation is involved in the reorganization of retrieved memory. We found that postsynaptic proteins were degraded in the hippocampus by polyubiquitination after retrieval of contextual fear memory. Moreover, the infusion of proteasome inhibitor into the CA1 region immediately after retrieval prevented anisomycin-induced memory impairment, as well as the extinction of fear memory. This suggests that ubiquitin- and proteasome-dependent protein degradation underlies destabilization processes after fear memory retrieval. It also provides strong evidence for the existence of reorganization processes whereby preexisting memory is disrupted by protein degradation, and updated memory is reconsolidated by protein synthesis.

Response of the Ubiquitin-Proteasome System to Memory Retrieval After Extended-Access Cocaine or Saline Self-Administration

“…This is analogous to a more established role for the UPS in destabilization of fear memories after their retrieval in a fear conditioning paradigm (Lee et al, 2008;Jarome et al, 2011).…”

“…Based on previous work (Lee et al, 2008;Jarome et al, 2011), we hypothesized that the cocaine group would exhibit increased UPS activity in amygdala (and perhaps other regions) but found no differences in levels of polyubiquitinated proteins between saline/retrieval and cocaine/retrieval groups in any brain region at either time-point (Figure 3c and d; all t-tests nonsignificant). For the NAc, this replicates the lack of difference Figure 3 Polyubiquitinated proteins and proteasome activity following a cue-induced seeking/retrieval test during prolonged withdrawal.…”

“…After this withdrawal period, some rats were given a seeking/retrieval test and then killed 30 or 60 min after memory retrieval (retrieval groups; Figure 2a and b). Timing of the retrieval experiments was based on results from other memory retrieval paradigms (Lee et al, 2008;Jarome et al, 2011;Ren et al, 2013). Other rats were taken from home cages for tissue collection without a seeking test (no retrieval groups) to determine if cocaine withdrawal is associated with altered basal UPS activity and to provide a baseline from which to assess seeking/retrieval test-induced changes in polyubiquitinated protein levels.…”

“…Other rats were taken from home cages for tissue collection without a seeking test (no retrieval groups) to determine if cocaine withdrawal is associated with altered basal UPS activity and to provide a baseline from which to assess seeking/retrieval test-induced changes in polyubiquitinated protein levels. After NAc tissue was harvested, the P2 synaptosomal membrane fraction, which is similar to fractions used in previous work (Lee et al, 2008;Jarome et al, 2011;Ren et al, 2013), was immunoblotted using an antibody recognizing polyubiquitinated proteins targeted for proteasomal degradation (see Materials and Methods). Samples from the retrieval and no retrieval groups were processed for SDS-PAGE and immunoblotting in parallel.…”

“…The UPS-mediated degradation of NSF, a protein whose functions include regulation of AMPA receptor trafficking (Hanley, 2014), has been linked to expression of cocaine CPP (Ren et al, 2013). The synaptic scaffolding protein Shank is tagged and degraded by the UPS after retrieval of fear memories (Lee et al, 2008;Jarome et al, 2011). At the 30-min time point, analysis of NSF levels by two-way ANOVA indicated that NSF was decreased in the retrieval groups compared with no retrieval groups (Figure 2e; main effect: retrieval vs no retrieval: F(1,26) = 5.55, Po0.05).…”