Males and females differ in the regulation and engagement of, but not requirement for, protein degradation in the amygdala during fear memory formation.

Over the last decade, strong evidence has emerged that protein degradation mediated by the ubiquitin-proteasome system is critical for fear memory formation in the amygdala. However, this work has been done primarily in males, leaving unanswered questions about whether females also require protein degradation during fear memory formation. Here, we found that male and female rats differed in their engagement and regulation of, but not need for, protein degradation in the amygdala during fear memory formation. Male, but not female, rats had increased protein degradation in the nuclei of amygdala cells after fear conditioning. Conversely, females had elevated baseline levels of overall ubiquitin-proteasome activity in amygdala nuclei. Gene expression and DNA methylation analyses identified that females had increased baseline expression of the ubiquitin coding gene Uba52, which had increased DNA 5-hydroxymethylation (5hmc) in its promoter region, indicating a euchromatin state necessary for increased levels of ubiquitin in females. Consistent with this, persistent CRISPR-dCas9 mediated silencing of Uba52 and proteasome subunit Psmd14 in the amygdala reduced baseline protein degradation levels and impaired fear memory in male and female rats, while enhancing baseline protein degradation in the amygdala of both sexes promoted fear memory formation. These results suggest that while both males and females require protein degradation in the amygdala for fear memory formation, they differ in their baseline regulation and engagement of this process following learning. These results have important implications for understanding the etiology of sex-related differences in fear memory formation.

Males and Females Differ in the Subcellular and Brain Region Dependent Regulation of Proteasome Activity by CaMKII and Protein Kinase A.

The ubiquitin-proteasome system (UPS) controls the degradation of ~90% of short-lived proteins in cells and is involved in activity- and learning-dependent synaptic plasticity in the brain. Calcium/calmodulin dependent protein kinase II (CaMKII) and Protein Kinase A (PKA) can regulate activity of the proteasome. However, there have been a number of conflicting reports regarding under what conditions CaMKII and PKA regulate proteasome activity in the brain. Furthermore, this work has been done exclusively in males, leaving questions about whether these kinases also regulate the proteasome in females. Here, using subcellular fractionation protocols in combination with in vitro pharmacology and proteasome activity assays, we investigated the conditions under which CaMKII and PKA regulate proteasome activity in the brains of male and female rats. In males, nuclear proteasome chymotrypsin activity was regulated by PKA in the amygdala but CaMKII in the hippocampus. Conversely, in females CaMKII regulated nuclear chymotrypsin activity in the amygdala, but not hippocampus. Additionally, in males CaMKII and PKA regulated proteasome trypsin activity in the cytoplasm of hippocampal, but not amygdala cells, while in females both CaMKII and PKA could regulate this activity in the nucleus of cells in both regions. Proteasome peptidylglutamyl activity was regulated by CaMKII and PKA activity in the nuclei of amygdala and hippocampus cells in males. However, in females PKA regulated nuclear peptidylglutamyl activity in the amygdala, but not hippocampus. Collectively, these results suggest that CaMKII- and PKA-dependent regulation of proteasome activity in the brain varies significantly across subcellular compartments and between males and females.

GluR2 endocytosis-dependent protein degradation in the amygdala mediates memory updating.

Associations learned during Pavlovian fear conditioning are rapidly acquired and long lasting, providing an ideal model for studying long-term memory formation, storage, and retrieval. During retrieval, these memories can "destabilize" and become labile, allowing a transient "reconsolidation" window during which the memory can be updated, suggesting that reconsolidation could be an attractive target for the modification of memories related to past traumatic experiences. This memory destabilization process is regulated by protein degradation and GluR2-endocytosis in the amygdala. However, it is currently unknown if retrieval-dependent GluR2-endocytosis in the amygdala is critical for incorporation of new information into the memory trace. We examined whether the addition of new information during memory retrieval required GluR2-endocytosis to modify the original memory. The presentation of two foot shocks of weaker intensity during retrieval resulted in GluR2 endocytosis-dependent increase in fear responding on a later test, suggesting modification of the original memory. This increase in fear expression was associated with increased protein degradation and zif268 expression in the same population of cells in the amygdala, indicating increased destabilization processes and cellular activity, and both were lost following blockade of GluR2-endocytosis. These data suggest that the endocytosis of GluR2-containing AMPA receptors in the amygdala regulates retrieval-induced strengthening of memories for traumatic events by modulating cellular destabilization and activity.

Distinct subcellular changes in proteasome activity and linkage-specific protein polyubiquitination in the amygdala during the consolidation and reconsolidation of a fear memory.

Numerous studies have supported a critical role for the ubiquitin-proteasome system (UPS) in the memory consolidation and reconsolidation processes. The protein targets and functional role of ubiquitin-proteasome activity can vary widely across cellular compartments, however, it is unknown how UPS activity changes within the nuclear, cytoplasmic, and synaptic regions in response to learning or memory retrieval. Additionally, while previous studies have focused on degradation-specific protein polyubiquitination, it is unknown how learning alters other polyubiquitin tags that are not targeted by the proteasome. Using cellular fractionation protocols in combination with linkage-specific polyubiquitin antibodies, we examined subcellular changes in ubiquitin-proteasome activity in the amygdala during memory consolidation and reconsolidation. Following memory acquisition, overall protein ubiquitination and proteasome activity simultaneously increased in the nucleus and decreased in the synaptic and cytoplasmic regions. The nuclear increases were associated with upregulation of degradation-specific (K48) and degradation-independent (K63, M1) polyubiquitin tags, suggesting multiple functions for ubiquitin signaling within this region. Interestingly, retrieval induced a very different pattern of ubiquitin-proteasome activity in the amygdala, consisting of increases in overall protein ubiquitination and proteasome activity and K48-, K63-, and M1-polyubiquitin tags in the synaptic, but not nuclear or cytoplasmic regions. Collectively, learning and memory retrieval dynamically and differentially alter degradation-dependent and degradation-independent ubiquitin-proteasome activity across different cellular compartments, suggesting that the UPS may serve unique functions during memory consolidation and reconsolidation.