Mechanisms for generating the autonomous cAMP-dependent protein kinase required for long-term facilitation in Aplysia

The ubiquitin-proteasome system and learning-dependent synaptic plasticity - A 10 year update

Over 25 years ago, a seminal paper demonstrated that the ubiquitin-proteasome system (UPS) was involved in activity-dependent synaptic plasticity. Interest in this topic began to expand around 2008 following another seminal paper showing that UPS-mediated protein degradation controlled the "destabilization" of memories following retrieval, though we remained with only a basic understanding of how the UPS regulated activity- and learning-dependent synaptic plasticity. However, over the last 10 years there has been an explosion of papers on this topic that has significantly changed our understanding of how ubiquitin-proteasome signaling regulates synaptic plasticity and memory formation. Importantly, we now know that the UPS controls much more than protein degradation, is involved in plasticity underlying drugs of abuse and that there are significant sex differences in how ubiquitin-proteasome signaling is used for memory storage processes. Here, we aim to provide a critical 10-year update on the role of ubiquitin-proteasome signaling in synaptic plasticity and memory formation, including updated cellular models of how ubiquitin-proteasome activity could be regulating learning-dependent synaptic plasticity in the brain.

The proteasome: A key modulator of nervous system function, brain aging, and neurodegenerative disease

The proteasome is a large multi-subunit protease responsible for the degradation and removal of oxidized, misfolded, and polyubiquitinated proteins. The proteasome plays critical roles in nervous system processes. This includes maintenance of cellular homeostasis in neurons. It also includes roles in long-term potentiation via modulation of CREB signaling. The proteasome also possesses roles in promoting dendritic spine growth driven by proteasome localization to the dendritic spines in an NMDA/CaMKIIα dependent manner. Proteasome inhibition experiments in varied organisms has been shown to impact memory, consolidation, recollection and extinction. The proteasome has been further shown to impact circadian rhythm through modulation of a range of 'clock' genes, and glial function. Proteasome function is impaired as a consequence both of aging and neurodegenerative diseases. Many studies have demonstrated an impairment in 26S proteasome function in the brain and other tissues as a consequence of age, driven by a disassembly of 26S proteasome in favor of 20S proteasome. Some studies also show proteasome augmentation to correct age-related deficits. In amyotrophic lateral sclerosis Alzheimer's, Parkinson's and Huntington's disease proteasome function is impaired through distinct mechanisms with impacts on disease susceptibility and progression. Age and neurodegenerative-related deficits in the function of the constitutive proteasome are often also accompanied by an increase in an alternative form of proteasome called the immunoproteasome. This article discusses the critical role of the proteasome in the nervous system. We then describe how proteasome dysfunction contributes to brain aging and neurodegenerative disease.

Defective synaptic plasticity in a model of Coffin-Lowry syndrome is rescued by simultaneously targeting PKA and MAPK pathways

Empirical and computational methods were combined to examine whether individual or dual-drug treatments can restore the deficit in long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse observed in a cellular model of Coffin-Lowry syndrome (CLS). The model was produced by pharmacological inhibition of p90 ribosomal S6 kinase (RSK) activity. In this model, coapplication of an activator of the mitogen-activated protein kinase (MAPK) isoform ERK and an activator of protein kinase A (PKA) resulted in enhanced phosphorylation of RSK and enhanced LTF to a greater extent than either drug alone and also greater than their additive effects, which is termed synergism. The extent of synergism appeared to depend on another MAPK isoform, p38 MAPK. Inhibition of p38 MAPK facilitated serotonin (5-HT)-induced RSK phosphorylation, indicating that p38 MAPK inhibits activation of RSK. Inhibition of p38 MAPK combined with activation of PKA synergistically activated both ERK and RSK. Our results suggest that cellular models of disorders that affect synaptic plasticity and learning, such as CLS, may constitute a useful strategy to identify candidate drug combinations, and that combining computational models with empirical tests of model predictions can help explain synergism of drug combinations.

Dynamics and Mechanisms of ERK Activation after Different Protocols that Induce Long-Term Synaptic Facilitation in Aplysia

Phosphorylation of the MAPK family member extracellular signal-regulated kinase (ERK) is required to induce long-term synaptic plasticity, but little is known about its persistence. We examined ERK activation by three protocols that induce long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse - the standard protocol (five 5-min pulses of 5-HT with interstimulus intervals (ISIs) of 20 min), the enhanced protocol (five pulses with irregular ISIs, which induces greater and longer-lasting LTF) and the two-pulse protocol (two pulses with ISI 45 min). Immunofluorescence revealed complex ERK activation. The standard and two-pulse protocols immediately increased active, phosphorylated ERK (pERK), which decayed within 5 h. A second wave of increased pERK was detected 18 h post-treatment for all protocols. This late phase was blocked by inhibitors of protein kinase A, TrkB and TGF-β. These results suggest that complex interactions among kinase pathways and growth factors contribute to the late increase of pERK. ERK activity returned to basal 24 h after the standard or two-pulse protocols, but remained elevated 24 h for the enhanced protocol. This 24-h elevation was also dependent on PKA and TGF-β, and partly on TrkB. These results begin to characterize long-lasting ERK activation, plausibly maintained by positive feedback involving growth factors and PKA, that appears essential to maintain LTF and LTM. Because many processes involved in LTF and late LTP are conserved among Aplysia and mammals, these findings highlight the importance of examining the dynamics of kinase cascades involved in vertebrate long-term memory.

Autophagy regulates neuronal excitability by controlling cAMP/protein kinase A signaling at the synapse

Autophagy provides nutrients during starvation and eliminates detrimental cellular components. However, accumulating evidence indicates that autophagy is not merely a housekeeping process. Here, by combining mouse models of neuron-specific ATG5 deficiency in either excitatory or inhibitory neurons with quantitative proteomics, high-content microscopy, and live-imaging approaches, we show that autophagy protein ATG5 functions in neurons to regulate cAMP-dependent protein kinase A (PKA)-mediated phosphorylation of a synapse-confined proteome. This function of ATG5 is independent of bulk turnover of synaptic proteins and requires the targeting of PKA inhibitory R1 subunits to autophagosomes. Neuronal loss of ATG5 causes synaptic accumulation of PKA-R1, which sequesters the PKA catalytic subunit and diminishes cAMP/PKA-dependent phosphorylation of postsynaptic cytoskeletal proteins that mediate AMPAR trafficking. Furthermore, ATG5 deletion in glutamatergic neurons augments AMPAR-dependent excitatory neurotransmission and causes the appearance of spontaneous recurrent seizures in mice. Our findings identify a novel role of autophagy in regulating PKA signaling at glutamatergic synapses and suggest the PKA as a target for restoration of synaptic function in neurodegenerative conditions with autophagy dysfunction.

The APP intracellular domain promotes LRRK2 expression to enable feed-forward neurodegenerative mechanisms in Parkinson's disease

Gain-of-function mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are common in familial forms of Parkinson's disease (PD), which is characterized by progressive neurodegeneration that impairs motor and cognitive function. We previously demonstrated that LRRK2-mediated phosphorylation of β-amyloid precursor protein (APP) triggers the production and nuclear translocation of the APP intracellular domain (AICD). Here, we connected LRRK2 to AICD in a feed-forward cycle that enhanced LRRK2-mediated neurotoxicity. In cooperation with the transcription factor FOXO3a, AICD promoted LRRK2 expression, thus increasing the abundance of LRRK2 that promotes AICD activation. APP deficiency in LRRK2^G2019S mice suppressed LRRK2 expression, LRRK2-mediated mitochondrial dysfunction, α-synuclein accumulation, and tyrosine hydroxylase (TH) loss in the brain, phenotypes associated with toxicity and loss of dopaminergic neurons in PD. Conversely, AICD overexpression increased LRRK2 expression and LRRK2-mediated neurotoxicity in LRRK2^G2019S mice. In LRRK2^G2019S mice or cultured dopaminergic neurons from LRRK2^G2019S patients, treatment with itanapraced reduced LRRK2 expression and was neuroprotective. Itanapraced showed similar effects in a neurotoxin-induced PD mouse model, suggesting that inhibiting the AICD may also have therapeutic benefits in idiopathic PD. Our findings reveal a therapeutically targetable, feed-forward mechanism through which AICD promotes LRRK2-mediated neurotoxicity in PD.

A Putative Role for Ubiquitin-Proteasome Signaling in Estrogenic Memory Regulation

Sex steroid hormones such as 17β-estradiol (E₂) are critical neuromodulators of hippocampal synaptic plasticity and hippocampus-dependent memory in both males and females. However, the mechanisms through which E₂ regulates memory formation in both sexes remain unclear. Research to date suggests that E₂ regulates hippocampus-dependent memory by activating numerous cell-signaling cascades to promote the synthesis of proteins that support structural changes at hippocampal synapses. However, this work has largely overlooked the equally important contributions of protein degradation mediated by the ubiquitin proteasome system (UPS) in remodeling the synapse. Despite being critically implicated in synaptic plasticity and successful formation of long-term memories, it remains unclear whether protein degradation mediated by the UPS is necessary for E₂ to exert its beneficial effects on hippocampal plasticity and memory formation. The present article provides an overview of the receptor and signaling mechanisms so far identified as critical for regulating hippocampal E₂ and UPS function in males and females, with a particular emphasis on the ways in which these mechanisms overlap to support structural integrity and protein composition of hippocampal synapses. We argue that the high degree of correspondence between E₂ and UPS activity warrants additional study to examine the contributions of ubiquitin-mediated protein degradation in regulating the effects of sex steroid hormones on cognition.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

The role of CREB and BDNF in neurobiology and treatment of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia worldwide. β-amyloid peptide (Aβ) is currently assumed to be the main cause of synaptic dysfunction and cognitive impairments in AD, but the molecular signaling pathways underlying its neurotoxic consequences have not yet been completely explored. Additional investigations regarding these pathways will contribute to development of new therapeutic targets. In context, developing evidence suggest that Aβ decreases brain-derived neurotrophic factor (BDNF) mostly by lowering phosphorylated cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) protein. In fact, it has been observed that brain or serum levels of BDNF appear to be beneficial markers for cognitive condition. In addition, the participation of transcription mediated by CREB has been widely analyzed in the memory process and AD development. Designing pharmacologic or genetic therapeutic approaches based on the targeting of CREB-BDNF signaling could be a promising treatment potential for AD. In this review, we summarize data demonstrating the role of CREB-BDNF signaling pathway in cognitive status and mediation of Aβ toxicity in AD. Finally, we also focus on the developing intervention methods for improvement of cognitive decline in AD based on targeting of CREB-BDNF pathway.

Post-translational modification and protein sorting to small extracellular vesicles including exosomes by ubiquitin and UBLs

Exosomes, a type of small extracellular vesicles (sEVs), are secreted membrane vesicles that are derived from various cell types, including cancer cells, mesenchymal stem cells, and immune cells via multivesicular bodies (MVBs). These sEVs contain RNAs (mRNA, miRNA, lncRNA, and rRNA), lipids, DNA, proteins, and metabolites, all of which mediate cell-to-cell communication. This communication is known to be implicated in a diverse set of diseases such as cancers and their metastases and degenerative diseases. The molecular mechanisms, by which proteins are modified and sorted to sEVs, are not fully understood. Various cellular processes, including degradation, transcription, DNA repair, cell cycle, signal transduction, and autophagy, are known to be associated with ubiquitin and ubiquitin-like proteins (UBLs). Recent studies have revealed that ubiquitin and UBLs also regulate MVBs and protein sorting to sEVs. Ubiquitin-like 3 (UBL3)/membrane-anchored Ub-fold protein (MUB) acts as a post-translational modification (PTM) factor to regulate efficient protein sorting to sEVs. In this review, we focus on the mechanism of PTM by ubiquitin and UBLs and the pathway of protein sorting into sEVs and discuss the potential biological significance of these processes.

Balanced actions of protein synthesis and degradation in memory formation

Storage of long-term memory requires not only protein synthesis but also protein degradation. In this article, we overview recent publications related to this issue, stressing that the balanced actions of protein synthesis and degradation are critical for long-term memory formation. We particularly focused on the brain-derived neurotrophic factor signaling that leads to protein synthesis; proteasome- and autophagy-dependent protein degradation that removes molecular constraints; the role of Fragile X mental retardation protein in translational suppression; and epigenetic modifications that control gene expression at the genomic level. Numerous studies suggest that an imbalance between protein synthesis and degradation leads to intellectual impairment and cognitive disorders.

How can memories last for days, years, or a lifetime? Proposed mechanisms for maintaining synaptic potentiation and memory

With memory encoding reliant on persistent changes in the properties of synapses, a key question is how can memories be maintained from days to months or a lifetime given molecular turnover? It is likely that positive feedback loops are necessary to persistently maintain the strength of synapses that participate in encoding. Such feedback may occur within signal-transduction cascades and/or the regulation of translation, and it may occur within specific subcellular compartments or within neuronal networks. Not surprisingly, numerous positive feedback loops have been proposed. Some posited loops operate at the level of biochemical signal-transduction cascades, such as persistent activation of Ca²⁺/calmodulin kinase II (CaMKII) or protein kinase Mζ. Another level consists of feedback loops involving transcriptional, epigenetic and translational pathways, and autocrine actions of growth factors such as BDNF. Finally, at the neuronal network level, recurrent reactivation of cell assemblies encoding memories is likely to be essential for late maintenance of memory. These levels are not isolated, but linked by shared components of feedback loops. Here, we review characteristics of some commonly discussed feedback loops proposed to underlie the maintenance of memory and long-term synaptic plasticity, assess evidence for and against their necessity, and suggest experiments that could further delineate the dynamics of these feedback loops. We also discuss crosstalk between proposed loops, and ways in which such interaction can facilitate the rapidity and robustness of memory formation and storage.

Proteolytic Degradation of Hippocampal STEP61 in LTP and Learning

Striatal-enriched protein tyrosine phosphatase (STEP) modulates key signaling molecules involved in synaptic plasticity and neuronal function. It is postulated that STEP opposes the development of long-term potentiation (LTP) and that it exerts a restraint on long-term memory (LTM). Here, we examined whether STEP₆₁ levels are regulated during hippocampal LTP and after training in hippocampal-dependent tasks. We found that after inducing LTP by high frequency stimulation or theta-burst stimulation STEP₆₁ levels were significantly reduced, with a concomitant increase of STEP₃₃ levels, a product of calpain cleavage. Importantly, inhibition of STEP with TC-2153 improved LTP in hippocampal slices. Moreover, we observed that after training in the passive avoidance and the T-maze spontaneous alternation task, hippocampal STEP₆₁ levels were significantly reduced, but STEP₃₃ levels were unchanged. Yet, hippocampal BDNF content and TrkB levels were increased in trained mice, and it is known that BDNF promotes STEP degradation through the proteasome. Accordingly, hippocampal pTrkB^Tyr816, pPLCγ^Tyr783, and protein ubiquitination levels were increased in T-SAT trained mice. Remarkably, injection of the TrkB antagonist ANA-12 (2 mg/Kg, but not 0.5 mg/Kg) elicited LTM deficits and promoted STEP₆₁ accumulation in the hippocampus. Also, STEP knockout mice outperformed wild-type animals in an age- and test-dependent manner. Summarizing, STEP₆₁ undergoes proteolytic degradation in conditions leading to synaptic strengthening and memory formation, thus highlighting its role as a molecular constrain, which is removed to enable the activation of pathways important for plasticity processes.

Memory Synapses Are Defined by Distinct Molecular Complexes: A Proposal

Synapses are diverse in form and function. While there are strong evidential and theoretical reasons for believing that memories are stored at synapses, the concept of a specialized “memory synapse” is rarely discussed. Here, we review the evidence that memories are stored at the synapse and consider the opposing possibilities. We argue that if memories are stored in an active fashion at synapses, then these memory synapses must have distinct molecular complexes that distinguish them from other synapses. In particular, examples from Aplysia sensory-motor neuron synapses and synapses on defined engram neurons in rodent models are discussed. Specific hypotheses for molecular complexes that define memory synapses are presented, including persistently active kinases, transmitter receptor complexes and trans-synaptic adhesion proteins.

Spinal activation of protein kinase C elicits phrenic motor facilitation

The protein kinase C family regulates many cellular functions, including multiple forms of neuroplasticity. The novel PKCθ and atypical PKCζ isoforms have been implicated in distinct forms of spinal, respiratory motor plasticity, including phrenic motor facilitation (pMF) following acute intermittent hypoxia or inactivity, respectively. Although these PKC isoforms are critical in regulating spinal motor plasticity, other isoforms may be important for phrenic motor plasticity. We tested the impact of conventional/novel PKC activator, phorbol 12-myristate 13-acetate (PMA) on pMF. Rats given cervical intrathecal injections of PMA exhibited pMF, which was abolished by pretreatment of broad-spectrum PKC inhibitors bisindolymalemide 1 (BIS) or NPC-15437 (NPC). Because PMA fails to activate atypical PKC isoforms, and NPC does not block PKCθ, this finding demonstrates that classical/novel PKC isoforms besides PKCθ are sufficient to elicit pMF. These results advance our understanding of mechanisms producing respiratory motor plasticity, and may inspire new treatments for disorders that compromise breathing, such as ALS, spinal injury and obstructive sleep apnea.

Novel calpain families and novel mechanisms for calpain regulation in Aplysia

Calpains are a family of intracellular proteases defined by a conserved protease domain. In the marine mollusk Aplysia californica, calpains are important for the induction of long-term synaptic plasticity and memory, at least in part by cleaving protein kinase Cs (PKCs) into constitutively active kinases, termed protein kinase Ms (PKMs). We identify 14 genes encoding calpains in Aplysia using bioinformatics, including at least one member of each of the four major calpain families into which metazoan calpains are generally classified, as well as additional truncated and atypical calpains. Six classical calpains containing a penta-EF-hand (PEF) domain are present in Aplysia. Phylogenetic analysis determined that these six calpains come from three separate classical calpain families. One of the classical calpains in Aplysia, AplCCal1, has been implicated in plasticity. We identify three splice cassettes and an alternative transcriptional start site in AplCCal1. We characterize several of the possible isoforms of AplCCal1 in vitro, and demonstrate that AplCCal1 can cleave PKCs into PKMs in a calcium-dependent manner in vitro. We also find that AplCCal1 has a novel mechanism of auto-inactivation through N-terminal cleavage that is modulated through its alternative transcriptional start site.

Memory Takes Time

Memory is an adaptation to particular temporal properties of past events, such as the frequency of occurrence of a stimulus or the coincidence of multiple stimuli. In neurons, this adaptation can be understood in terms of a hierarchical system of molecular and cellular time windows, which collectively retain information from the past. We propose that this system makes various timescales of past experience simultaneously available for future adjustment of behavior. More generally, we propose that the ability to detect and respond to temporally structured information underlies the nervous system's capacity to encode and store a memory at molecular, cellular, synaptic, and circuit levels.

Superior long-term synaptic memory induced by combining dual pharmacological activation of PKA and ERK with an enhanced training protocol

Developing treatment strategies to enhance memory is an important goal of neuroscience research. Activation of multiple biochemical signaling cascades, such as the protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) pathways, is necessary to induce long-term synaptic facilitation (LTF), a correlate of long-term memory (LTM). Previously, a computational model was developed which correctly predicted a novel enhanced training protocol that augmented LTF by searching for the protocol with maximal overlap of PKA and ERK activation. The present study focused on pharmacological approaches to enhance LTF. Combining an ERK activator, NSC, and a PKA activator, rolipram, enhanced LTF to a greater extent than did either drug alone. An even greater increase in LTF occurred when rolipram and NSC were combined with the Enhanced protocol. These results indicate superior memory can be achieved by enhanced protocols that take advantage of the structure and dynamics of the biochemical cascades underlying memory formation, used in conjunction with combinatorial pharmacology.

Cell-Specific PKM Isoforms Contribute to the Maintenance of Different Forms of Persistent Long-Term Synaptic Plasticity

Multiple kinase activations contribute to long-term synaptic plasticity, a cellular mechanism mediating long-term memory. The sensorimotor synapse of Aplysia expresses different forms of long-term facilitation (LTF)-nonassociative and associative LTF-that require the timely activation of kinases, including protein kinase C (PKC). It is not known which PKC isoforms in the sensory neuron or motor neuron L7 are required to sustain each form of LTF. We show that different PKMs, the constitutively active isoforms of PKCs generated by calpain cleavage, in the sensory neuron and L7 are required to maintain each form of LTF. Different PKMs or calpain isoforms were blocked by overexpressing specific dominant-negative constructs in either presynaptic or postsynaptic neurons. Blocking either PKM Apl I in L7, or PKM Apl II or PKM Apl III in the sensory neuron 2 d after 5-hydroxytryptamine (5-HT) treatment reversed persistent nonassociative LTF. In contrast, blocking either PKM Apl II or PKM Apl III in L7, or PKM Apl II in the sensory neuron 2 d after paired stimuli reversed persistent associative LTF. Blocking either classical calpain or atypical small optic lobe (SOL) calpain 2 d after 5-HT treatment or paired stimuli did not disrupt the maintenance of persistent LTF. Soon after 5-HT treatment or paired stimuli, however, blocking classical calpain inhibited the expression of persistent associative LTF, while blocking SOL calpain inhibited the expression of persistent nonassociative LTF. Our data suggest that different stimuli activate different calpains that generate specific sets of PKMs in each neuron whose constitutive activities sustain long-term synaptic plasticity.SIGNIFICANCE STATEMENT Persistent synaptic plasticity contributes to the maintenance of long-term memory. Although various kinases such as protein kinase C (PKC) contribute to the expression of long-term plasticity, little is known about how constitutive activation of specific kinase isoforms sustains long-term plasticity. This study provides evidence that the cell-specific activities of different PKM isoforms generated from PKCs by calpain-mediated cleavage maintain two forms of persistent synaptic plasticity, which are the cellular analogs of two forms of long-term memory. Moreover, we found that the activation of specific calpains depends on the features of the stimuli evoking the different forms of synaptic plasticity. Given the recent controversy over the role of PKMζ maintaining memory, these findings are significant in identifying roles of multiple PKMs in the retention of memory.

Role of proteasome-dependent protein degradation in long-term operant memory in Aplysia

We investigated the in vivo role of protein degradation during intermediate (ITM) and long-term memory (LTM) in Aplysia using an operant learning paradigm. The proteasome inhibitor MG-132 inhibited the induction and molecular consolidation of LTM with no effect on ITM. Remarkably, maintenance of steady-state protein levels through inhibition of protein synthesis using either anisomycin or rapamycin in conjunction with proteasome inhibition permitted the formation of robust 24 h LTM. Our studies suggest a primary role for proteasomal activity in facilitation of gene transcription for LTM and raise the possibility that synaptic mechanisms are sufficient to sustain 24 h memory.

The role of the ubiquitin proteasome system in the memory process

Quite intuitive is the notion that memory formation and consolidation is orchestrated by protein synthesis because of the synaptic plasticity necessary for those processes. Nevertheless, recent advances have begun accumulating evidences of a high requirement for protein degradation on the molecular mechanisms of the memory process in the mammalian brain. Because degradation determines protein half-life, degradation has been increasingly recognized as an important intracellular regulatory mechanism. The proteasome is the main player in the degradation of intracellular proteins. Proteasomal substrates are mainly degraded after a post-translational modification by a poly-ubiquitin chain. Latter process, namely poly-ubiquitination, is highly regulated at the step of the ubiquitin molecule transferring to the protein substrate mediated by a set of proteins whose genes represent almost 2% of the human genome. Understanding the role of polyubiquitin-mediated protein degradation has challenging researchers in many fields of investigation as a new source of targets for therapeutic intervention, e.g. E3 ligases that transfer ubiquitin moieties to the substrate. The goal of present work was to uncover mechanisms underlying memory processes regarding the role of the ubiquitin-proteasome system (UPS). For that purpose, preceded of a short review on UPS and memory processes a top-down systems biology approach was applied to establish central proteins involved in memory formation and consolidation highlighting their cross-talking with the UPS. According to that approach, the pattern of expression of several elements of the UPS were found overexpressed in regions of the brain involved in processing cortical inputs.

Proteolysis, synaptic plasticity and memory

Protein degradation has many critical functions in the nervous system such as refinement of synaptic connections during development and synaptic plasticity and memory in the adult organisms. A major cellular machinery of proteolysis is the ubiquitin-proteasome pathway (UPP). The UPP precisely regulates proteolysis by covalently attaching ubiquitin, a small protein, to substrates through sequential enzymatic reactions and the proteins marked with the ubiquitin tag are degraded by complex containing many subunits called the proteasome. Research over the years has shown a role for the UPP in regulating presynaptic and postsynaptic proteins critical for neurotransmission and synaptic plasticity. Studies have also revealed a role for the UPP in various forms of memory. Mechanistic investigations suggest that the function of the UPP in neurons is not homogenous and is subject to local regulation in different neuronal sub-compartments. In both invertebrate and vertebrate model systems, local roles have been found for enzymes that attach ubiquitin to substrate proteins as well as for enzymes that remove ubiquitin from substrates. The proteasome also has disparate functions in different parts of the neuron. In addition to the UPP, proteolysis by the lysosome and autophagy play a role in synaptic plasticity and memory. This review details the functions of proteolysis in synaptic plasticity and summarizes the findings on the connection between proteolysis and memory mainly focusing on the UPP including its local roles.

Morphine-induced synaptic plasticity in the VTA is reversed by HDAC inhibition

Dopamine (DA) dysfunction originating from the ventral tegmental area (VTA) occurs as a result of synaptic abnormalities following consumption of drugs of abuse and underlies behavioral plasticity associated with drug abuse. Drugs of abuse can cause changes in gene expression through epigenetic mechanisms in the brain that underlie some of the lasting neuroplasticity and behavior associated with addiction. Here we investigated the function of histone acetylation and histone deacetylase (HDAC)2 in the VTA in recovery of morphine-induced synaptic modifications following a single in vivo exposure to morphine. Using a combination of immunohistochemistry, Western blot, and whole cell patch-clamp recording in rat midbrain slices, we show that morphine increased HDAC2 activity in VTA DA neurons and reduced histone H3 acetylation at lysine 9 (Ac-H3K9) in the VTA 24 h after the injection. Morphine-induced synaptic changes at glutamatergic synapses involved endocannabinoid signaling to reduce GABAergic synaptic strength onto VTA DA neurons. Both plasticities were recovered by in vitro incubation of midbrain slices with a class I-specific HDAC inhibitor (HDACi), CI-994, through an increase in acetylation of histone H3K9. Interestingly, HDACi incubation also increased levels of Ac-H3K9 and triggered GABAergic and glutamatergic plasticities in DA neurons of saline-treated rats. Our results suggest that acute morphine-induced changes in VTA DA activity and synaptic transmission engage HDAC2 activity locally in the VTA to maintain synaptic modifications through histone hypoacetylation.

Transforming growth factor β recruits persistent MAPK signaling to regulate long-term memory consolidation in Aplysia californica

In this study, we explore the mechanistic relationship between growth factor signaling and kinase activity that supports the protein synthesis-dependent phase of long-term memory (LTM) consolidation for sensitization of Aplysia. Specifically, we examine LTM for tail shock-induced sensitization of the tail-elicited siphon withdrawal (T-SW) reflex, a form of memory that requires both (i) extracellular signal-regulated kinase (ERK1/2; MAPK) activity within identified sensory neurons (SNs) that mediate the T-SW and (ii) the activation of transforming growth factor β (TGFβ) signaling. We now report that repeated tail shocks that induce intermediate-term (ITM) and LTM for sensitization, also induce a sustained post-training phase of MAPK activity in SNs (lasting at least 1 h). We identified two mechanistically distinct phases of post-training MAPK: (i) an immediate phase that does not require ongoing protein synthesis or TGFβ signaling, and (ii) a sustained phase that requires both protein synthesis and extracellular TGFβ signaling. We find that LTM consolidation requires sustained MAPK, and is disrupted by inhibitors of protein synthesis and TGFβ signaling during the consolidation window. These results provide strong evidence that TGFβ signaling sustains MAPK activity as an essential mechanistic step for LTM consolidation.

Stochasticity and bifurcations in a reduced model with interlinked positive and negative feedback loops of CREB1 and CREB2 stimulated by 5-HT

The cyclic AMP (cAMP)-response element-binding protein (CREB) family of transcription factors is crucial in regulating gene expression required for long-term memory (LTM) formation. Upon exposure of sensory neurons to the neurotransmitter serotonin (5-HT), CREB1 is activated via activation of the protein kinase A (PKA) intracellular signaling pathways, and CREB2 as a transcriptional repressor is relieved possibly via phosphorylation of CREB2 by mitogen-activated protein kinase (MAPK). Song et al. [18] proposed a minimal model with only interlinked positive and negative feedback loops of transcriptional regulation by the activator CREB1 and the repressor CREB2. Without considering feedbacks between the CREB proteins, Pettigrew et al. [8] developed a computational model characterizing complex dynamics of biochemical pathways downstream of 5-HT receptors. In this work, to describe more simply the biochemical pathways and gene regulation underlying 5-HT-induced LTM, we add the important extracellular sensitizing stimulus 5-HT as well as the product Ap-uch into the Song's minimal model. We also strive to examine dynamical properties of the gene regulatory network under the changing concentration of the stimulus, [5-HT], cooperating with the varying positive feedback strength in inducing a high state of CREB1 for the establishment of long-term memory. Different dynamics including monostability, bistability and multistability due to coexistence of stable steady states and oscillations is investigated by means of codimension-2 bifurcation analysis. At the different positive feedback strengths, comparative analysis of deterministic and stochastic dynamics reveals that codimension-1 bifurcation with respect to [5-HT] as the parameter can predict diverse stochastic behaviors resulted from the finite number of molecules, and the number of CREB1 molecules more and more preferentially resides near the high steady state with increasing [5-HT], which contributes to long-term memory formation.

Repeated Mu-Opioid Exposure Induces a Novel Form of the Hyperalgesic Priming Model for Transition to Chronic Pain

The primary afferent nociceptor was used as a model system to study mechanisms of pain induced by chronic opioid administration. Repeated intradermal injection of the selective mu-opioid receptor (MOR) agonist DAMGO induced mechanical hyperalgesia and marked prolongation of prostaglandin E2 (PGE2) hyperalgesia, a key feature of hyperalgesic priming. However, in contrast to prior studies of priming induced by receptor-mediated (i.e., TNFα, NGF, or IL-6 receptor) or direct activation of protein kinase Cε (PKCε), the pronociceptive effects of PGE2 in DAMGO-treated rats demonstrated the following: (1) rapid induction (4 h compared with 3 d); (2) protein kinase A (PKA), rather than PKCε, dependence; (3) prolongation of hyperalgesia induced by an activator of PKA, 8-bromo cAMP; (4) failure to be reversed by a protein translation inhibitor; (5) priming in females as well as in males; and (6) lack of dependence on the isolectin B4-positive nociceptor. These studies demonstrate a novel form of hyperalgesic priming induced by repeated administration of an agonist at the Gi-protein-coupled MOR to the peripheral terminal of the nociceptor. Significance statement: The current study demonstrates the molecular mechanisms involved in the sensitization of nociceptors produced by repeated activation of mu-opioid receptors and contributes to our understanding of the painful condition observed in patients submitted to chronic use of opioids.

The relationship between protein synthesis and protein degradation in object recognition memory

For decades there has been a consensus that de novo protein synthesis is necessary for long-term memory. A second round of protein synthesis has been described for both extinction and reconsolidation following an unreinforced test session. Recently, it was shown that consolidation and reconsolidation depend not only on protein synthesis but also on protein degradation by the ubiquitin-proteasome system (UPS), a major mechanism responsible for protein turnover. However, the involvement of UPS on consolidation and reconsolidation of object recognition memory remains unknown. Here we investigate in the CA1 region of the dorsal hippocampus the involvement of UPS-mediated protein degradation in consolidation and reconsolidation of object recognition memory. Animals with infusion cannulae stereotaxically implanted in the CA1 region of the dorsal hippocampus, were exposed to an object recognition task. The UPS inhibitor β-Lactacystin did not affect the consolidation and the reconsolidation of object recognition memory at doses known to affect other forms of memory (inhibitory avoidance, spatial learning in a water maze) while the protein synthesis inhibitor anisomycin impaired the consolidation and the reconsolidation of the object recognition memory. However, β-Lactacystin was able to reverse the impairment caused by anisomycin on the reconsolidation process in the CA1 region of the hippocampus. Therefore, it is possible to postulate a direct link between protein degradation and protein synthesis during the reconsolidation of the object recognition memory.

Rescue of impaired long-term facilitation at sensorimotor synapses of Aplysia following siRNA knockdown of CREB1

Memory impairment is often associated with disrupted regulation of gene induction. For example, deficits in cAMP response element-binding protein (CREB) binding protein (CBP; an essential cofactor for activation of transcription by CREB) impair long-term synaptic plasticity and memory. Previously, we showed that small interfering RNA (siRNA)-induced knockdown of CBP in individual sensory neurons significantly reduced levels of CBP and impaired 5-HT-induced long-term facilitation (LTF) in sensorimotor cocultures from Aplysia. Moreover, computational simulations of the biochemical cascades underlying LTF successfully predicted training protocols that restored LTF following CBP knockdown. We examined whether simulations could also predict a training protocol that restores LTF impaired by siRNA-induced knockdown of the transcription factor CREB1. Simulations based on a previously described model predicted rescue protocols that were specific to CREB1 knockdown. Empirical studies demonstrated that one of these rescue protocols partially restored impaired LTF. In addition, the effectiveness of the rescue protocol was enhanced by pretreatment with rolipram, a selective cAMP phosphodiesterase inhibitor. These results provide further evidence that computational methods can help rescue disruptions in signaling cascades underlying memory formation. Moreover, the study demonstrates that the effectiveness of computationally designed training protocols can be enhanced with complementary pharmacological approaches.

cJun and CREB2 in the postsynaptic neuron contribute to persistent long-term facilitation at a behaviorally relevant synapse

Basic region leucine zipper (bZIP) transcription factors regulate gene expression critical for long-term synaptic plasticity or neuronal excitability contributing to learning and memory. At sensorimotor synapses of Aplysia, changes in activation or expression of CREB1 and CREB2 in sensory neurons are required for long-term synaptic plasticity. However, it is unknown whether concomitant stimulus-induced changes in expression and activation of bZIP transcription factors in the postsynaptic motor neuron also contribute to persistent long-term facilitation (P-LTF). We overexpressed various forms of CREB1, CREB2, or cJun in the postsynaptic motor neuron L7 in cell culture to examine whether these factors contribute to P-LTF. P-LTF is evoked by 2 consecutive days of 5-HT applications (2 5-HT), while a transient form of LTF is produced by 1 day of 5-HT applications (1 5-HT). Significant increases in the expression of both cJun and CREB2 mRNA in L7 accompany P-LTF. Overexpressing each bZIP factor in L7 did not alter basal synapse strength, while coexpressing cJun and CREB2 in L7 evoked persistent increases in basal synapse strength. In contrast, overexpressing cJun and CREB2 in sensory neurons evoked persistent decreases in basal synapse strength. Overexpressing wild-type cJun or CREB2, but not CREB1, in L7 can replace the second day of 5-HT applications in producing P-LTF. Reducing cJun activity in L7 blocked P-LTF evoked by 2 5-HT. These results suggest that expression and activation of different bZIP factors in both presynaptic and postsynaptic neurons contribute to persistent change in synapse strength including stimulus-dependent long-term synaptic plasticity.

Nonassociative learning in invertebrates

The simplicity and tractability of the neural circuits mediating behaviors in invertebrates have facilitated the cellular/molecular dissection of neural mechanisms underlying learning. The review has a particular focus on the general principles that have emerged from analyses of an example of nonassociative learning, sensitization in the marine mollusk Aplysia. Learning and memory rely on multiple mechanisms of plasticity at multiple sites of the neuronal circuits, with the relative contribution to memory of the different sites varying as a function of the extent of training and time after training. The same intracellular signaling cascades that induce short-term modifications in synaptic transmission can also be used to induce long-term changes. Although short-term memory relies on covalent modifications of preexisting proteins, long-term memory also requires regulated gene transcription and translation. Maintenance of long-term cellular memory involves both intracellular and extracellular feedback loops, which sustain the regulation of gene expression and the modification of targeted molecules.

Synaptic therapy in Alzheimer's disease: a CREB-centric approach

Therapeutic attempts to cure Alzheimer's disease (AD) have failed, and new strategies are desperately needed. Motivated by this reality, many laboratories (including our own) have focused on synaptic dysfunction in AD because synaptic changes are highly correlated with the severity of clinical dementia. In particular, memory formation is accompanied by altered synaptic strength, and this phenomenon (and its dysfunction in AD) has been a recent focus for many laboratories. The molecule cyclic adenosine monophosphate response element-binding protein (CREB) is at a central converging point of pathways and mechanisms activated during the processes of synaptic strengthening and memory formation, as CREB phosphorylation leads to transcription of memory-associated genes. Disruption of these mechanisms in AD results in a reduction of CREB activation with accompanying memory impairment. Thus, it is likely that strategies aimed at these mechanisms will lead to future therapies for AD. In this review, we will summarize literature that investigates 5 possible therapeutic pathways for rescuing synaptic dysfunction in AD: 4 enzymatic pathways that lead to CREB phosphorylation (the cyclic adenosine monophosphate cascade, the serine/threonine kinases extracellular regulated kinases 1 and 2, the nitric oxide cascade, and the calpains), as well as histone acetyltransferases and histone deacetylases (2 enzymes that regulate the histone acetylation necessary for gene transcription).

Local ubiquitin-proteasome-mediated proteolysis and long-term synaptic plasticity

The ubiquitin-proteasome pathway (UPP) of protein degradation has many roles in synaptic plasticity that underlies memory. Work on both invertebrate and vertebrate model systems has shown that the UPP regulates numerous substrates critical for synaptic plasticity. Initial research took a global view of ubiquitin-protein degradation in neurons. Subsequently, the idea of local protein degradation was proposed a decade ago. In this review, we focus on the functions of the UPP in long-term synaptic plasticity and discuss the accumulated evidence in support of the idea that the components of the UPP often have disparate local roles in different neuronal compartments rather than a single cell-wide function.

Reinstatement of long-term memory following erasure of its behavioral and synaptic expression in Aplysia

Long-term memory (LTM) is believed to be stored in the brain as changes in synaptic connections. Here, we show that LTM storage and synaptic change can be dissociated. Cocultures of Aplysia sensory and motor neurons were trained with spaced pulses of serotonin, which induces long-term facilitation. Serotonin (5HT) triggered growth of new presynaptic varicosities, a synaptic mechanism of long-term sensitization. Following 5HT training, two antimnemonic treatments-reconsolidation blockade and inhibition of PKM--caused the number of presynaptic varicosities to revert to the original, pretraining value. Surprisingly, the final synaptic structure was not achieved by targeted retraction of the 5HT-induced varicosities but, rather, by an apparently arbitrary retraction of both 5HT-induced and original synapses. In addition, we find evidence that the LTM for sensitization persists covertly after its apparent elimination by the same antimnemonic treatments that erase learning-related synaptic growth. These results challenge the idea that stable synapses store long-term memories.

Protein degradation and protein synthesis in long-term memory formation

Long-term memory (LTM) formation requires transient changes in the activity of intracellular signaling cascades that are thought to regulate new gene transcription and de novo protein synthesis in the brain. Consistent with this, protein synthesis inhibitors impair LTM for a variety of behavioral tasks when infused into the brain around the time of training or following memory retrieval, suggesting that protein synthesis is a critical step in LTM storage in the brain. However, evidence suggests that protein degradation mediated by the ubiquitin-proteasome system (UPS) may also be a critical regulator of LTM formation and stability following retrieval. This requirement for increased protein degradation has been shown in the same brain regions in which protein synthesis is required for LTM storage. Additionally, increases in the phosphorylation of proteins involved in translational control parallel increases in protein polyubiquitination and the increased demand for protein degradation is regulated by intracellular signaling molecules thought to regulate protein synthesis during LTM formation. In some cases inhibiting proteasome activity can rescue memory impairments that result from pharmacological blockade of protein synthesis, suggesting that protein degradation may control the requirement for protein synthesis during the memory storage process. Results such as these suggest that protein degradation and synthesis are both critical for LTM formation and may interact to properly “consolidate” and store memories in the brain. Here, we review the evidence implicating protein synthesis and degradation in LTM storage and highlight the areas of overlap between these two opposing processes. We also discuss evidence suggesting these two processes may interact to properly form and store memories. LTM storage likely requires a coordinated regulation between protein degradation and synthesis at multiple sites in the mammalian brain.

Persistent long-term facilitation at an identified synapse becomes labile with activation of short-term heterosynaptic plasticity

Short-term and long-term synaptic plasticity are cellular correlates of learning and memory of different durations. Little is known, however, how these two forms of plasticity interact at the same synaptic connection. We examined the reciprocal impact of short-term heterosynaptic or homosynaptic plasticity at sensorimotor synapses of Aplysia in cell culture when expressing persistent long-term facilitation (P-LTF) evoked by serotonin [5-hydroxytryptamine (5-HT)]. Short-term heterosynaptic plasticity induced by 5-HT (facilitation) or the neuropeptide FMRFa (depression) and short-term homosynaptic plasticity induced by tetanus [post-tetanic potentiation (PTP)] or low-frequency stimulation [homosynaptic depression (HSD)] of the sensory neuron were expressed in both control synapses and synapses expressing P-LTF in the absence or presence of protein synthesis inhibitors. All forms of short-term plasticity failed to significantly affect ongoing P-LTF in the absence of protein synthesis inhibitors. However, P-LTF reversed to control levels when either 5-HT or FMRFa was applied in the presence of rapamycin. In contrast, P-LTF was unaffected when either PTP or HSD was evoked in the presence of either rapamycin or anisomycin. These results indicate that synapses expressing persistent plasticity acquire a "new" baseline and functionally express short-term changes as naive synapses, but the new baseline becomes labile following selective activations-heterosynaptic stimuli that evoke opposite forms of plasticity-such that when presented in the presence of protein synthesis inhibitors produce a rapid reversal of the persistent plasticity. Activity-selective induction of a labile state at synapses expressing persistent plasticity may facilitate the development of therapies for reversing inappropriate memories.

Proteasome modulates positive and negative translational regulators in long-term synaptic plasticity

Proteolysis by the ubiquitin-proteasome pathway appears to have a complex role in synaptic plasticity, but its various functions remain to be elucidated. Using late phase long-term potentiation (L-LTP) in the hippocampus of the mouse as a model for long-term synaptic plasticity, we previously showed that inhibition of the proteasome enhances induction but blocks maintenance of L-LTP. In this study, we investigated the possible mechanisms by which proteasome inhibition has opposite effects on L-LTP induction and maintenance. Our results show that inhibiting phosphatidyl inositol-3 kinase or blocking the interaction between eukaryotic initiation factors 4E (eIF4E) and 4G (eIF4G) reduces the enhancement of L-LTP induction brought about by proteasome inhibition suggesting interplay between proteolysis and the signaling pathway mediated by mammalian target of rapamycin (mTOR). Also, proteasome inhibition leads to accumulation of translational activators in the mTOR pathway such as eIF4E and eukaryotic elongation factor 1A (eEF1A) early during L-LTP causing increased induction. Furthermore, inhibition of the proteasome causes a buildup of translational repressors, such as polyadenylate-binding protein interacting protein 2 (Paip2) and eukaryotic initiation factor 4E-binding protein 2 (4E-BP2), during late stages of L-LTP contributing to the blockade of L-LTP maintenance. Thus, the proteasome plays a critical role in regulating protein synthesis during L-LTP by tightly controlling translation. Our results provide novel mechanistic insights into the interplay between protein degradation and protein synthesis in long-term synaptic plasticity.

Dysfunctional autophagy in Alzheimer's disease: pathogenic roles and therapeutic implications

Neuronal autophagy is essential for neuronal survival and the maintenance of neuronal homeostasis. Increasing evidence has implicated autophagic dysfunction in the pathogenesis of Alzheimer's disease (AD). The mechanisms underlying autophagic failure in AD involve several steps, from autophagosome formation to degradation. The effect of modulating autophagy is context-dependent. Stimulation of autophagy is not always beneficial. During the implementation of therapies that modulate autophagy, the nature of the autophagic defect, the timing of intervention, and the optimal level and duration of modulation should be fully considered.

The less things change, the more they are different: contributions of long-term synaptic plasticity and homeostasis to memory

An important cellular mechanism contributing to the strength and duration of memories is activity-dependent alterations in the strength of synaptic connections within the neural circuit encoding the memory. Reversal of the memory is typically correlated with a reversal of the cellular changes to levels expressed prior to the stimulation. Thus, for stimulus-induced changes in synapse strength and their reversals to be functionally relevant, cellular mechanisms must regulate and maintain synapse strength both prior to and after the stimuli inducing learning and memory. The strengths of synapses within a neural circuit at any given moment are determined by cellular and molecular processes initiated during development and those subsequently regulated by the history of direct activation of the neural circuit and system-wide stimuli such as stress or motivational state. The cumulative actions of stimuli and other factors on an already modified neural circuit are attenuated by homeostatic mechanisms that prevent changes in overall synaptic inputs and excitability above or below specific set points (synaptic scaling). The mechanisms mediating synaptic scaling prevent potential excitotoxic alterations in the circuit but also may attenuate additional cellular changes required for learning and memory, thereby apparently limiting information storage. What cellular and molecular processes control synaptic strengths before and after experience/activity and its reversals? In this review we will explore the synapse-, whole cell-, and circuit level-specific processes that contribute to an overall zero sum-like set of changes and long-term maintenance of synapse strengths as a consequence of the accommodative interactions between long-term synaptic plasticity and homeostasis.

The slow afterhyperpolarization: a target of β1-adrenergic signaling in hippocampus-dependent memory retrieval

In rodents, adrenergic signaling by norepinephrine (NE) in the hippocampus is required for the retrieval of intermediate-term memory. NE promotes retrieval via the stimulation of β1-adrenergic receptors, the production of cAMP, and the activation of both protein kinase A (PKA) and the exchange protein activated by cAMP. However, a final effector for this signaling pathway has not been identified. Among the many targets of adrenergic signaling in the hippocampus, the slow afterhyperpolarization (sAHP) is an appealing candidate because its reduction by β1 signaling enhances excitatory neurotransmission. Here we report that reducing the sAHP is critical for the facilitation of retrieval by NE. Direct blockers of the sAHP, as well as blockers of the L-type voltage-dependent calcium influx that activates the sAHP, rescue retrieval in mutant mice lacking either NE or the β1 receptor. Complementary to this, a facilitator of L-type calcium influx impairs retrieval in wild-type mice. In addition, we examined the role of NE in the learning-related reduction of the sAHP observed ex vivo in hippocampal slices. We find that this reduction in the sAHP depends on the induction of persistent PKA activity specifically in conditioned slices. Interestingly, this persistent PKA activity is induced by NE/β1 signaling during slice preparation rather than during learning. These observations suggest that the reduction in the sAHP may not be present autonomously in vivo, but is likely induced by neuromodulatory input, which is consistent with the idea that NE is required in vivo for reduction of the sAHP during memory retrieval.

The ubiquitin-proteasome system as a critical regulator of synaptic plasticity and long-term memory formation

Numerous studies have supported the idea that de novo protein synthesis is critical for synaptic plasticity and normal long-term memory formation. This requirement for protein synthesis has been shown for several different types of fear memories, exists in multiple brain regions and circuits, and is necessary for different stages of memory creation and storage. However, evidence has recently begun to accumulate suggesting that protein degradation through the ubiquitin-proteasome system is an equally important regulator of memory formation. Here we review those recent findings on protein degradation and memory formation and stability and propose a model explaining how protein degradation may be contributing to various aspects of memory and synaptic plasticity. We conclude that protein degradation may be the major factor regulating many of the molecular processes that we know are important for fear memory formation and stability in the mammalian brain.

Local synaptic integration of mitogen-activated protein kinase and protein kinase A signaling mediates intermediate-term synaptic facilitation in Aplysia

It is widely appreciated that memory processing engages a wide range of molecular signaling cascades in neurons, but how these cascades are temporally and spatially integrated is not well understood. To explore this important question, we used Aplysia californica as a model system. We simultaneously examined the timing and subcellular location of two signaling molecules, MAPK (ERK1/2) and protein kinase A (PKA), both of which are critical for the formation of enduring memory for sensitization. We also explored their interaction during the formation of enduring synaptic facilitation, a cellular correlate of memory, at tail sensory-to-motor neuron synapses. We find that repeated tail nerve shock (TNS, an analog of sensitizing training) immediately and persistently activates MAPK in both sensory neuron somata and synaptic neuropil. In contrast, we observe immediate PKA activation only in the synaptic neuropil. It is followed by PKA activation in both compartments 1 h after TNS. Interestingly, blocking MAPK activation during, but not after, TNS impairs PKA activation in synaptic neuropil without affecting the delayed PKA activation in sensory neuron somata. Finally, by applying inhibitors restricted to the synaptic compartment, we show that synaptic MAPK activation during TNS is required for the induction of intermediate-term synaptic facilitation, which leads to the persistent synaptic PKA activation required to maintain this facilitation. Collectively, our results elucidate how MAPK and PKA signaling cascades are spatiotemporally integrated in a single neuron to support synaptic plasticity underlying memory formation.

A role of protein degradation in memory consolidation after initial learning and extinction learning in the honeybee (Apis mellifera)

Protein degradation is known to affect memory formation after extinction learning. We demonstrate here that an inhibitor of protein degradation, MG132, interferes with memory formation after extinction learning in a classical appetitive conditioning paradigm. In addition, we find an enhancement of memory formation when the same inhibitor is applied after initial learning. This result supports the idea that MG132 targets an ongoing consolidation process. Furthermore, we demonstrate that the sensitivity of memory formation after initial learning and extinction learning to MG132 depends in the same way on the number of CS-US trials and the intertrial interval applied during initial learning. This supports the idea that the learning parameters during acquisition are critical for memory formation after extinction and that protein degradation in both learning processes might be functionally linked.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

Proteolytic regulation of the mitochondrial cAMP-dependent protein kinase

The mitochondrial cAMP-dependent protein kinase (PKA) is activatable in a cAMP-independent fashion. The regulatory (R) subunits of the PKA holoenzyme (R(2)C(2)), but not the catalytic (C) subunits, suffer proteolysis upon exposure of bovine heart mitochondria to digitonin, Ca(2+), and a myriad of electron transport inhibitors. Selective loss of both the RI- and RII-type subunits was demonstrated via Western blot analysis, and activation of the C subunit was revealed by phosphorylation of a validated PKA peptide substrate. Selective proteolysis transpires in a calpain-dependent fashion as demonstrated by exposure of the R and C subunits of PKA to calpain and by attenuation of R and C subunit proteolysis in the presence of calpain inhibitor I. By contrast, exposure of mitochondria to cAMP fails to promote R subunit degradation, although it does result in enhanced C subunit catalytic activity. Treatment of mitochondria with electron transport chain inhibitors rotenone, antimycin A, sodium azide, and oligomycin, as well as an uncoupler of oxidative phosphorylation, also elicits enhanced C subunit activity. These results are consistent with the notion that signals, originating from cAMP-independent sources, elicit enhanced mitochondrial PKA activity.

Computational design of enhanced learning protocols

Learning and memory are influenced by the temporal pattern of training stimuli. However, the mechanisms that determine the effectiveness of a particular training protocol are not well understood. We tested the hypothesis that the efficacy of a protocol is determined in part by interactions among biochemical cascades that underlie learning and memory. Previous findings suggest that the protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) cascades are necessary to induce long-term synaptic facilitation (LTF) in Aplysia, a neuronal correlate of memory. We developed a computational model of the PKA and ERK cascades and used it to identify a training protocol that maximized PKA and ERK interactions. In vitro studies confirmed that the protocol enhanced LTF. Moreover, the protocol enhanced the levels of phosphorylation of the transcription factor CREB1. Behavioral training confirmed that long-term memory also was enhanced by the protocol. These results illustrate the feasibility of using computational models to design training protocols that improve memory.

Altered neurotransmitter release machinery in mice deficient for the deubiquitinating enzyme Usp14

Homozygous ataxic mice (ax(J)) express reduced levels of the deubiquitinating enzyme Usp14. They develop severe tremors by 2-3 wk of age, followed by hindlimb paralysis, and death by 6-8 wk. While changes in the ubiquitin proteasome system often result in the accumulation of ubiquitin protein aggregates and neuronal loss, these pathological markers are not observed in the ax(J) mice. Instead, defects in neurotransmission were observed in both the central and peripheral nervous systems of ax(J) mice. We have now identified several new alterations in peripheral neurotransmission in the ax(J) mice. Using the two-microelectrode voltage clamp technique on diaphragm muscles of ax(J) mice, we observed that under normal neurotransmitter release conditions ax(J) mice lacked paired-pulse facilitation and exhibited a frequency-dependent increase in rundown of the end plate current at high-frequency stimulation (HFS). Combined electrophysiology and styryl dye staining revealed a significant reduction in quantal content during the initial and plateau portions of the HFS train. In addition, uptake of styryl dyes (FM dye) during HFS demonstrated that the size of the readily releasable vesicle pool was significantly reduced. Destaining rates for styryl dyes suggested that ax(J) neuromuscular junctions are unable to mobilize a sufficient number of vesicles during times of intense activity. These results imply that ax(J) nerve terminals are unable to recruit a sufficient number of vesicles to keep pace with physiological rates of transmitter release. Therefore, ubiquitination of synaptic proteins appears to play an important role in the normal operation of the neurotransmitter release machinery and in regulating the size of pools of synaptic vesicles.

Protein degradation and memory formation

Long-term memories are created when labile short-term memory traces are converted to more enduring forms. This process, called consolidation, is associated with changes in the synthesis of proteins that alter the biophysical properties of neurons and the strength of their synaptic connections. Recently, it has become clear that the consolidation process requires not only protein synthesis but also degradation. Here, we discuss recent findings on the roles of ubiquitination and protein degradation in synaptic plasticity and learning and memory.

Control of PKA stability and signalling by the RING ligase praja2

Activation of G-protein-coupled receptors (GPCRs) mobilizes compartmentalized pulses of cyclic AMP. The main cellular effector of cAMP is protein kinase A (PKA), which is assembled as an inactive holoenzyme consisting of two regulatory (R) and two catalytic (PKAc) subunits. cAMP binding to R subunits dissociates the holoenzyme and releases the catalytic moiety, which phosphorylates a wide array of cellular proteins. Reassociation of PKAc and R components terminates the signal. Here we report that the RING ligase praja2 controls the stability of mammalian R subunits. Praja2 forms a stable complex with, and is phosphorylated by, PKA. Rising cAMP levels promote praja2-mediated ubiquitylation and subsequent proteolysis of compartmentalized R subunits, leading to sustained substrate phosphorylation by the activated kinase. Praja2 is required for efficient nuclear cAMP signalling and for PKA-mediated long-term memory. Thus, praja2 regulates the total concentration of R subunits, tuning the strength and duration of PKA signal output in response to cAMP.

Deconstruction for reconstruction: the role of proteolysis in neural plasticity and disease

The brain changes in response to experience and altered environment. This neural plasticity is largely mediated by morphological and functional modification of synapses, a process that depends on both synthesis and degradation of proteins. It is now clear that regulated proteolysis plays a critical role in the remodeling of synapses, learning and memory, and neurodevelopment. Here, we highlight the mechanisms and functions of proteolysis in synaptic plasticity and discuss its alteration in disease states.