Serotonin-induced cleavage of the atypical protein kinase C Apl III in Aplysia

Wings of Change: aPKC/FoxP-dependent plasticity in steering motor neurons underlies operant self-learning in Drosophila

Background

Motor learning is central to human existence, such as learning to speak or walk, sports moves, or rehabilitation after injury. Evidence suggests that all forms of motor learning share an evolutionarily conserved molecular plasticity pathway. Here, we present novel insights into the neural processes underlying operant self-learning, a form of motor learning in the fruit fly Drosophila.

Methods

We operantly trained wild type and transgenic Drosophila fruit flies, tethered at the torque meter, in a motor learning task that required them to initiate and maintain turning maneuvers around their vertical body axis (yaw torque). We combined this behavioral experiment with transgenic peptide expression, CRISPR/Cas9-mediated, spatio-temporally controlled gene knock-out and confocal microscopy.

Results

We find that expression of atypical protein kinase C (aPKC) in direct wing steering motoneurons co-expressing the transcription factor FoxP is necessary for this type of motor learning and that aPKC likely acts via non-canonical pathways. We also found that it takes more than a week for CRISPR/Cas9-mediated knockout of FoxP in adult animals to impair motor learning, suggesting that adult FoxP expression is required for operant self-learning.

Conclusions

Our experiments suggest that, for operant self-learning, a type of motor learning in Drosophila, co-expression of atypical protein kinase C (aPKC) and the transcription factor FoxP is necessary in direct wing steering motoneurons. Some of these neurons control the wing beat amplitude when generating optomotor responses, and we have discovered modulation of optomotor behavior after operant self-learning. We also discovered that aPKC likely acts via non-canonical pathways and that FoxP expression is also required in adult flies.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Characterization of Neuronal RNA Modifications during Non-associative Learning in Aplysia Reveals Key Roles for tRNAs in Behavioral Sensitization

Subtle changes in the landscape of post-transcriptional modifications have emerged as putative regulators of central nervous system plasticity and activity-induced protein synthesis. However, simultaneous characterization of multiple RNA modifications and their covariation during learning and memory paradigms has been impeded by the complexity of animal models and lack of untargeted approaches for identifying pathway-relevant RNA modifications in small-volume samples. Here, we used mass spectrometry to profile spatiotemporal changes in dozens of neuronal RNA modifications in Aplysia californica during behavioral sensitization of a simple defensive reflex. Unique RNA modification patterns were observed in the major ganglia of trained and naı̇ve animals, with two tRNA modifications, namely, 5-methoxycarbonylmethyl-2-thiouridine (mcm⁵s²U) and 1-methyladenosine (m¹A), at significantly higher levels in trained subjects. We report that tRNAs, and their modifications, correlate with increased polyglutamine synthesis and excitability in neurons, characterizing the first link between noncoding RNA modifications and non-associative learning.

Possible novel features of synaptic regulation during long-term facilitation in Aplysia

Most studies of molecular mechanisms of synaptic plasticity have focused on the sequence of changes either at individual synapses or in the cell nucleus. However, studies of long-term facilitation at Aplysia sensory neuron–motor neuron synapses in isolated cell culture suggest two additional features of facilitation. First, that there is also regulation of the number of synaptic contacts between two neurons, which may occur at the level of cell pair-specific branch points in the neuronal arbor. Branch points contain many molecules that are involved in protein synthesis-dependent long-term facilitation including neurotrophins and the RNA binding protein CPEB. Second, the regulation involves homeostatic feedback and tends to keep the total number of contacts between two neurons at a fairly constant level both at rest and following facilitation. That raises the question of how facilitation and homeostasis can coexist. A possible answer is suggested by the findings that they both involve spontaneous transmission and postsynaptic Ca²⁺, which can have bidirectional effects similar to LTP and LTD in hippocampus. In addition, long-term facilitation can involve a change in the set point of homeostasis, which could be encoded by plasticity molecules such as CPEB and/or PKM. A computational model based on these ideas can qualitatively simulate the basic features of both facilitation and homeostasis of the number of contacts.

The central importance of nuclear mechanisms in the storage of memory

The neurobiological nature of the memory trace (engram) remains controversial. The most widely accepted hypothesis at present is that long-term memory is stored as stable, learning-induced changes in synaptic connections. This hypothesis, the synaptic plasticity hypothesis of memory, is supported by extensive experimental data gathered from over 50 years of research. Nonetheless, there are important mnemonic phenomena that the synaptic plasticity hypothesis cannot, or cannot readily, account for. Furthermore, recent work indicates that epigenetic and genomic mechanisms play heretofore underappreciated roles in memory. Here, we critically assess the evidence that supports the synaptic plasticity hypothesis and discuss alternative non-synaptic, nuclear mechanisms of memory storage, including DNA methylation and retrotransposition. We argue that long-term encoding of memory is mediated by nuclear processes; synaptic plasticity, by contrast, represents a means of relatively temporary memory storage. In addition, we propose that memories are evaluated for their mnemonic significance during an initial period of synaptic storage; if assessed as sufficiently important, the memories then undergo nuclear encoding.

Calpain fosters the hyperexcitability of motoneurons after spinal cord injury and leads to spasticity

Up-regulation of the persistent sodium current (INaP) and down-regulation of the potassium/chloride extruder KCC2 lead to spasticity after spinal cord injury (SCI). We here identified calpain as the driver of the up- and down-regulation of INaP and KCC2, respectively, in neonatal rat lumbar motoneurons. Few days after SCI, neonatal rats developed behavioral signs of spasticity with the emergence of both hyperreflexia and abnormal involuntary muscle contractions on hindlimbs. At the same time, in vitro isolated lumbar spinal cords became hyperreflexive and displayed numerous spontaneous motor outputs. Calpain-I expression paralleled with a proteolysis of voltage-gated sodium (Nav) channels and KCC2. Acute inhibition of calpains reduced this proteolysis, restored the motoneuronal expression of Nav and KCC2, normalized INaP and KCC2 function, and curtailed spasticity. In sum, by up- and down-regulating INaP and KCC2, the calpain-mediated proteolysis of Nav and KCC2 drives the hyperexcitability of motoneurons which leads to spasticity after SCI.

aPKC in neuronal differentiation, maturation and function

The atypical Protein Kinase Cs (aPKCs)—PRKCI, PRKCZ and PKMζ—form a subfamily within the Protein Kinase C (PKC) family. These kinases are expressed in the nervous system, including during its development and in adulthood. One of the aPKCs, PKMζ, appears to be restricted to the nervous system. aPKCs are known to play a role in a variety of cellular responses such as proliferation, differentiation, polarity, migration, survival and key metabolic functions such as glucose uptake, that are critical for nervous system development and function. Therefore, these kinases have garnered a lot of interest in terms of their functional role in the nervous system. Here we review the expression and function of aPKCs in neural development and in neuronal maturation and function. Despite seemingly paradoxical findings with genetic deletion versus gene silencing approaches, we posit that aPKCs are likely candidates for regulating many important neurodevelopmental and neuronal functions, and may be associated with a number of human neuropsychiatric diseases.

Isoform Specificity of PKMs during Long-Term Facilitation in Aplysia Is Mediated through Stabilization by KIBRA

Persistent activity of protein kinase M (PKM), the truncated form of protein kinase C (PKC), can maintain long-term changes in synaptic strength in many systems, including the hermaphrodite marine mollusk, Aplysia californica Moreover, different types of long-term facilitation (LTF) in cultured Aplysia sensorimotor synapses rely on the activities of different PKM isoforms in the presynaptic sensory neuron and postsynaptic motor neuron. When the atypical PKM isoform is required, the kidney and brain expressed adaptor protein (KIBRA) is also required. Here, we explore how this isoform specificity is established. We find that PKM overexpression in the motor neuron, but not the sensory neuron, is sufficient to increase synaptic strength and that this activity is not isoform-specific. KIBRA is not the rate-limiting step in facilitation since overexpression of KIBRA is neither sufficient to increase synaptic strength, nor to prolong a form of PKM-dependent intermediate synaptic facilitation. However, the isoform specificity of dominant-negative-PKMs to erase LTF is correlated with isoform-specific competition for stabilization by KIBRA. We identify a new conserved region of KIBRA. Different splice isoforms in this region stabilize different PKMs based on the isoform-specific sequence of an α-helix "handle" in the PKMs. Thus, specific stabilization of distinct PKMs by different isoforms of KIBRA can explain the isoform specificity of PKMs during LTF in Aplysia SIGNIFICANCE STATEMENT Long-lasting changes in synaptic plasticity associated with memory formation are maintained by persistent protein kinases. We have previously shown in the Aplysia sensorimotor model that distinct isoforms of persistently active protein kinase Cs (PKMs) maintain distinct forms of long-lasting synaptic changes, even when both forms are expressed in the same motor neuron. Here, we show that, while the effects of overexpression of PKMs are not isoform-specific, isoform specificity is defined by a "handle" helix in PKMs that confers stabilization by distinct splice forms in a previously undefined domain of the adaptor protein KIBRA. Thus, we define new regions in both KIBRA and PKMs that define the isoform specificity for maintaining synaptic strength in distinct facilitation paradigms.

A role for Numb in Protein kinase M (PKM)-mediated increase in surface AMPA receptors during facilitation in Aplysia

There is considerable evidence from both vertebrates and invertebrates that persistently active protein kinases maintain changes in synaptic strength that underlie memory. In the hermaphrodite marine mollusk, Aplysia californica, truncated forms of protein kinase C (PKC) termed protein kinase Ms have been implicated in both intermediate- and long-term facilitation, an increase in synaptic strength between sensory neurons and motor neurons thought to underlie behavioural sensitization in the animal. However, few substrates have been identified as candidates that could mediate this increase in synaptic strength. PKMs have been proposed to maintain synaptic strength through preventing endocytosis of AMPA receptors. Numb is a conserved regulator of endocytosis that is modulated by phosphorylation. We have identified and cloned Aplysia Numb (ApNumb). ApNumb contains three conserved PKC phosphorylation sites and PKMs generated from classical and atypical Aplysia PKCs can phosphorylate ApNumb in vitro and in cells. Over-expression of ApNumb that lacks the conserved PKC phosphorylation sites blocks increases in surface levels of a pHluorin-tagged Aplysia glutamate receptor measured using live imaging after intermediate- or long-term facilitation. Over-expression of this form of ApNumb did not block increases in synaptic strength seen during intermediate-term facilitation, but did block increases in synaptic strength seen during long-term facilitation. There was no effect of over-expression of this form of ApNumb on other putative Numb targets as measured using increases in calcium downstream of neurotrophins or agonists of metabotropic glutamate receptors. These results suggest that in Aplysia neurons, Numb specifically regulates AMPA receptor trafficking and is an attractive candidate for a target of PKMs in long-term maintenance of synaptic strength. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Experiments with Snails Add to Our Knowledge about the Role of aPKC Subfamily Kinases in Learning

Protein kinase Mζ is considered important for memory formation and maintenance in different species, including invertebrates. PKMζ participates in multiple molecular pathways in neurons, regulating translation initiation rate, AMPA receptors turnover, synaptic scaffolding assembly, and other processes. Here, for the first time, we established the sequence of mRNA encoding PKMζ homolog in land snail Helix lucorum. We annotated important features of this mRNA: domains, putative capping sites, translation starts, and splicing sites. We discovered that this mRNA has at least two isoforms, and one of them lacks sequence encoding C1 domain. C1 deletion may be unique for snail because it has not been previously found in other species. We performed behavioral experiments with snails, measured expression levels of identified isoforms, and confirmed that their expression correlates with one type of learning.

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.

The zinc fingers of the small optic lobes calpain bind polyubiquitin

The small optic lobes (SOL) calpain is a highly conserved member of the calpain family expressed in the nervous system. A dominant negative form of the SOL calpain inhibited consolidation of one form of synaptic plasticity, non-associative facilitation, in sensory-motor neuronal cultures in Aplysia, presumably by inhibiting cleavage of protein kinase Cs (PKCs) into constitutively active protein kinase Ms (PKMs) (Hu et al. 2017a). SOL calpains have a conserved set of 5-6 N-terminal zinc fingers. Bioinformatic analysis suggests that these zinc fingers could bind to ubiquitin. In this study, we show that both the Aplysia and mouse SOL calpain (also known as Calpain 15) zinc fingers bind ubiquitinated proteins, and we confirm that Aplysia SOL binds poly- but not mono- or diubiquitin. No specific zinc finger is required for polyubiquitin binding. Neither polyubiquitin nor calcium was sufficient to induce purified Aplysia SOL calpain to autolyse or to cleave the atypical PKC to PKM in vitro. In Aplysia, over-expression of the atypical PKC in sensory neurons leads to an activity-dependent cleavage event and an increase in nuclear ubiquitin staining. Activity-dependent cleavage is partially blocked by a dominant negative SOL calpain, but not by a dominant negative classical calpain. The cleaved PKM was stabilized by the dominant negative classical calpain and destabilized by a dominant negative form of the PKM stabilizing protein KIdney/BRAin protein. These studies provide new insight into SOL calpain's function and regulation. Open Data: Materials are available on https://cos.io/our-services/open-science-badges/ https://osf.io/93n6m/.

PAR3-PAR6-atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3-PAR6-aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3-PAR6-aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.

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.

The genetics of PKMζ and memory maintenance

Elucidating the molecular mechanisms that maintain long-term memory is a fundamental goal of neuroscience. Accumulating evidence suggests that persistent signaling by the atypical protein kinase C (PKC) isoform protein kinase Mζ (PKMζ) might maintain synaptic long-term potentiation (LTP) and long-term memory. However, the role of PKMζ has been challenged by genetic data from PKMζ-knockout mice showing intact LTP and long-term memory. Moreover, the PKMζ inhibitor peptide ζ inhibitory peptide (ZIP) reverses LTP and erases memory in both wild-type and knockout mice. Data from four papers using additional isoform-specific genetic approaches have helped to reconcile these conflicting findings. First, a PKMζ-antisense approach showed that LTP and long-term memory in PKMζ-knockout mice are mediated through a compensatory mechanism that depends on another ZIP-sensitive atypical isoform, PKCι/λ. Second, short hairpin RNAs decreasing the amounts of individual atypical isoforms without inducing compensation disrupted memory in different temporal phases. PKCι/λ knockdown disrupted short-term memory, whereas PKMζ knockdown specifically erased long-term memory. Third, conditional PKCι/λ knockout induced compensation by rapidly activating PKMζ to preserve short-term memory. Fourth, a dominant-negative approach in the model system Aplysia revealed that multiple PKCs form PKMs to sustain different types of long-term synaptic facilitation, with atypical PKM maintaining synaptic plasticity similar to LTP. Thus, under physiological conditions, PKMζ is the principal PKC isoform that maintains LTP and long-term memory. PKCι/λ can compensate for PKMζ, and because other isoforms could also maintain synaptic facilitation, there may be a hierarchy of compensatory mechanisms maintaining memory if PKMζ malfunctions.

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.

Selective Erasure of Distinct Forms of Long-Term Synaptic Plasticity Underlying Different Forms of Memory in the Same Postsynaptic Neuron

Generalization of fear responses to non-threatening stimuli is a feature of anxiety disorders. It has been challenging to target maladaptive generalized memories without affecting adaptive memories. Synapse-specific long-term plasticity underlying memory involves the targeting of plasticity-related proteins (PRPs) to activated synapses. If distinct tags and PRPs are used for different forms of plasticity, one could selectively remove distinct forms of memory. Using a stimulation paradigm in which associative long-term facilitation (LTF) occurs at one input and non-associative LTF at another input to the same postsynaptic neuron in an Aplysia sensorimotor preparation, we found that each form of LTF is reversed by inhibiting distinct isoforms of protein kinase M (PKM), putative PRPs, in the postsynaptic neuron. A dominant-negative (dn) atypical PKM selectively reversed associative LTF, while a dn classical PKM selectively reversed non-associative LTF. Although both PKMs are formed from calpain-mediated cleavage of protein kinase C (PKC) isoforms, each form of LTF is sensitive to a distinct dn calpain expressed in the postsynaptic neuron. Associative LTF is blocked by dn classical calpain, whereas non-associative LTF is blocked by dn small optic lobe (SOL) calpain. Interfering with a putative synaptic tag, the adaptor protein KIBRA, which protects the atypical PKM from degradation, selectively erases associative LTF. Thus, the activity of distinct PRPs and tags in a postsynaptic neuron contribute to the maintenance of different forms of synaptic plasticity at separate inputs, allowing for selective reversal of synaptic plasticity and providing a cellular basis for developing therapeutic strategies for selectively reversing maladaptive memories.

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 protein synthesis and DNA methylation in the consolidation and maintenance of long-term memory in Aplysia

Previously, we reported that long-term memory (LTM) in Aplysia can be reinstated by truncated (partial) training following its disruption by reconsolidation blockade and inhibition of PKM (Chen et al., 2014). Here, we report that LTM can be induced by partial training after disruption of original consolidation by protein synthesis inhibition (PSI) begun shortly after training. But when PSI occurs during training, partial training cannot subsequently establish LTM. Furthermore, we find that inhibition of DNA methyltransferase (DNMT), whether during training or shortly afterwards, blocks consolidation of LTM and prevents its subsequent induction by truncated training; moreover, later inhibition of DNMT eliminates consolidated LTM. Thus, the consolidation of LTM depends on two functionally distinct phases of protein synthesis: an early phase that appears to prime LTM; and a later phase whose successful completion is necessary for the normal expression of LTM. Both the consolidation and maintenance of LTM depend on DNA methylation.

A PKM generated by calpain cleavage of a classical PKC is required for activity-dependent intermediate-term facilitation in the presynaptic sensory neuron of Aplysia

Atypical PKM, a persistently active form of atypical PKC, is proposed to be a molecular memory trace, but there have been few examinations of the role of PKMs generated from other PKCs. We demonstrate that inhibitors used to inhibit PKMs generated from atypical PKCs are also effective inhibitors of other PKMs. In contrast, we demonstrate that dominant-negative PKMs show isoform-specificity. A dominant-negative PKM from the classical PKC Apl I blocks activity-dependent intermediate-term facilitation (a-ITF) when expressed in the sensory neuron, while a dominant-negative PKM from the atypical PKC Apl III does not. Consistent with a specific role for PKM Apl I in activity-dependent facilitation, live imaging FRET-based cleavage assays reveal that activity leads to cleavage of the classical PKC Apl I, but not the atypical PKC Apl III in the sensory neuron varicosities of Aplysia. In contrast, massed intermediate facilitation (m-ITF) induced by 10 min of 5HT is sufficient for cleavage of the atypical PKC Apl III in the motor neuron. Interestingly, both cleavage of PKC Apl I in the sensory neuron during a-ITF and cleavage of PKC Apl III in the motor neuron during m-ITF are inhibited by a dominant-negative form of a penta-EF hand containing classical calpain cloned from Aplysia. Consistent with a role for PKMs in plasticity, this dominant-negative calpain also blocks both a-ITF when expressed in the sensory neuron and m-ITF when expressed in the motor neuron. This study broadens the role of PKMs in synaptic plasticity in two significant ways: (i) PKMs generated from multiple isoforms of PKC, including classical isoforms, maintain memory traces; (ii) PKMs play roles in the presynaptic neuron.

Differential role of calpain-dependent protein cleavage in intermediate and long-term operant memory in Aplysia

In addition to protein synthesis, protein degradation or protein cleavage may be necessary for intermediate (ITM) and long-term memory (LTM) to remove molecular constraints, facilitate persistent kinase activity and modulate synaptic plasticity. Calpains, a family of conserved calcium dependent cysteine proteases, modulate synaptic function through protein cleavage. We used the marine mollusk Aplysia californica to investigate the in vivo role of calpains during intermediate and long-term operant memory formation using the learning that food is inedible (LFI) paradigm. A single LFI training session, in which the animal associates a specific netted seaweed with the failure to swallow, generates short (30min), intermediate (4-6h) and long-term (24h) memory. Using the calpain inhibitors calpeptin and MDL-28170, we found that ITM requires calpain activity for induction and consolidation similar to the previously reported requirements for persistent protein kinase C activity in intermediate-term LFI memory. The induction of LTM also required calpain activity. In contrast to ITM, calpain activity was not necessary for the molecular consolidation of LTM. Surprisingly, six hours after LFI training we found that calpain activity was necessary for LTM, although this is a time at which neither persistent PKC activity nor protein synthesis is required for the maintenance of long-term LFI memory. These results demonstrate that calpains function in multiple roles in vivo during associative memory formation.

Persistent Associative Plasticity at an Identified Synapse Underlying Classical Conditioning Becomes Labile with Short-Term Homosynaptic Activation

Synapses express different forms of plasticity that contribute to different forms of memory, and both memory and plasticity can become labile after reactivation. We previously reported that a persistent form of nonassociative long-term facilitation (PNA-LTF) of the sensorimotor synapses in Aplysia californica, a cellular analog of long-term sensitization, became labile with short-term heterosynaptic reactivation and reversed when the reactivation was followed by incubation with the protein synthesis inhibitor rapamycin. Here we examined the reciprocal impact of different forms of short-term plasticity (reactivations) on a persistent form of associative long-term facilitation (PA-LTF), a cellular analog of classical conditioning, which was expressed at Aplysia sensorimotor synapses when a tetanic stimulation of the sensory neurons was paired with a brief application of serotonin on 2 consecutive days. The expression of short-term homosynaptic plasticity [post-tetanic potentiation or homosynaptic depression (HSD)], or short-term heterosynaptic plasticity [serotonin-induced facilitation or neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFa)-induced depression], at synapses expressing PA-LTF did not affect the maintenance of PA-LTF. The kinetics of HSD was attenuated at synapses expressing PA-LTF, which required activation of protein kinase C (PKC). Both PA-LTF and the attenuated kinetics of HSD were reversed by either a transient blockade of PKC activity or a homosynaptic, but not heterosynaptic, reactivation when paired with rapamycin. These results indicate that two different forms of persistent synaptic plasticity, PA-LTF and PNA-LTF, expressed at the same synapse become labile when reactivated by different stimuli.

SIGNIFICANCE STATEMENT

Activity-dependent changes in neural circuits mediate long-term memories. Some forms of long-term memories become labile and can be reversed with specific types of reactivations, but the mechanism is complex. At the cellular level, reactivations that induce a reversal of memory must evoke changes in neural circuits underlying the memory. What types of reactivations induce a labile state at neural connections that lead to reversal of different types of memory? We find that a critical neural connection in Aplysia, which is modified with different stimuli that mediate different types of memory, becomes labile with different types of reactivations. These results provide insights for developing strategies in alleviating maladaptive memories accompanying anxiety disorders.

The Role of Functional Prion-Like Proteins in the Persistence of Memory

Prions are a self-templating amyloidogenic state of normal cellular proteins, such as prion protein (PrP). They have been identified as the pathogenic agents, contributing to a number of diseases of the nervous system. However, the discovery that the neuronal RNA-binding protein, cytoplasmic polyadenylation element-binding protein (CPEB), has a prion-like state that is involved in the stabilization of memory raised the possibility that prion-like proteins can serve normal physiological functions in the nervous system. Here, we review recent experimental evidence of prion-like properties of neuronal CPEB in various organisms and propose a model of how the prion-like state may stabilize memory.

Homolog of protein kinase Mζ maintains context aversive memory and underlying long-term facilitation in terrestrial snail Helix

It has been shown that a variety of long-term memories in different regions of the brain and in different species are quickly erased by local inhibition of protein kinase Mζ (PKMζ), a persistently active protein kinase. Using antibodies to mammalian PKMζ, we describe in the present study the localization of immunoreactive molecules in the nervous system of the terrestrial snail Helix lucorum. Presence of a homolog of PKMζ was confirmed with transcriptomics. We have demonstrated in behavioral experiments that contextual fear memory disappeared under a blockade of PKMζ with a selective peptide blocker of PKMζ zeta inhibitory peptide (ZIP), but not with scrambled ZIP. If ZIP was combined with a “reminder” (20 min in noxious context), no impairment of the long-term contextual memory was observed. In electrophysiological experiments we investigated whether PKMζ takes part in the maintenance of long-term facilitation (LTF) in the neural circuit mediating tentacle withdrawal. LTF of excitatory synaptic inputs to premotor interneurons was induced by high-frequency nerve stimulation combined with serotonin bath applications and lasted at least 4 h. We found that bath application of 2 × 10⁻⁶ M ZIP at the 90th min after the tetanization reduced the EPSP amplitude to the non-tetanized EPSP values. Applications of the scrambled ZIP peptide at a similar time and concentration didn’t affect the EPSP amplitudes. In order to test whether effects of ZIP are specific to the synapses, we performed experiments with LTF of somatic membrane responses to local glutamate applications. It was shown earlier that serotonin application in such an “artificial synapse” condition elicits LTF of responses to glutamate. It was found that ZIP had no effect on LTF in these conditions, which may be explained by the very low concentration of PKMζ molecules in somata of these identified neurons, as evidenced by immunochemistry. Obtained results suggest that the Helix homolog of PKMζ might be involved in post-induction maintenance of long-term changes in the nervous system of the terrestrial snail.

Bidirectional regulation of eEF2 phosphorylation controls synaptic plasticity by decoding neuronal activity patterns

At the sensory-motor neuron synapse of Aplysia, either spaced or continuous (massed) exposure to serotonin (5-HT) induces a form of intermediate-term facilitation (ITF) that requires new protein synthesis but not gene transcription. However, spaced and massed ITF use distinct molecular mechanisms to maintain increased synaptic strength. Synapses activated by spaced applications of 5-HT generate an ITF that depends on persistent protein kinase A (PKA) activity, whereas an ITF produced by massed 5-HT depends on persistent protein kinase C (PKC) activity. In this study, we demonstrate that eukaryotic elongation factor 2 (eEF2), which catalyzes the GTP-dependent translocation of the ribosome during protein synthesis, acts as a biochemical sensor that is tuned to the pattern of neuronal stimulation. Specifically, we find that massed training leads to a PKC-dependent increase in phosphorylation of eEF2, whereas spaced training results in a PKA-dependent decrease in phosphorylation of eEF2. Importantly, by using either pharmacological or dominant-negative strategies to inhibit eEF2 kinase (eEF2K), we were able to block massed 5-HT-dependent increases in eEF2 phosphorylation and subsequent PKC-dependent ITF. In contrast, pharmacological inhibition of eEF2K during the longer period of time required for spaced training was sufficient to reduce eEF2 phosphorylation and induce ITF. Finally, we find that the massed 5-HT-dependent increase in synaptic strength requires translation elongation, but not translation initiation, whereas the spaced 5-HT-dependent increase in synaptic strength is partially dependent on translation initiation. Thus, bidirectional regulation of eEF2 is critical for decoding distinct activity patterns at synapses by activating distinct modes of translation regulation.

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.

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.

Primary cilia in stem cells and neural progenitors are regulated by neutral sphingomyelinase 2 and ceramide

Human embryonic stem and induced pluripotent stem cell–derived neuroprogenitors (NPs) develop primary cilia. Ciliogenesis depends on the sphingolipid ceramide and its interaction with atypical PKC, both of which distribute to the primary cilium and the apicolateral cell membrane in NP rosettes.

We show here that human embryonic stem (ES) and induced pluripotent stem cell–derived neuroprogenitors (NPs) develop primary cilia. Ciliogenesis depends on the sphingolipid ceramide and its interaction with atypical PKC (aPKC), both of which distribute to the primary cilium and the apicolateral cell membrane in NP rosettes. Neural differentiation of human ES cells to NPs is concurrent with a threefold elevation of ceramide—in particular, saturated, long-chain C_16:0 ceramide (N-palmitoyl sphingosine) and nonsaturated, very long chain C_24:1 ceramide (N-nervonoyl sphingosine). Decreasing ceramide levels by inhibiting ceramide synthase or neutral sphingomyelinase 2 leads to translocation of membrane-bound aPKC to the cytosol, concurrent with its activation and the phosphorylation of its substrate Aurora kinase A (AurA). Inhibition of aPKC, AurA, or a downstream target of AurA, HDAC6, restores ciliogenesis in ceramide-depleted cells. Of importance, addition of exogenous C_24:1 ceramide reestablishes membrane association of aPKC, restores primary cilia, and accelerates neural process formation. Taken together, these results suggest that ceramide prevents activation of HDAC6 by cytosolic aPKC and AurA, which promotes acetylation of tubulin in primary cilia and, potentially, neural processes. This is the first report on the critical role of ceramide generated by nSMase2 in stem cell ciliogenesis and differentiation.

Cellular, molecular, and epigenetic mechanisms in non-associative conditioning: implications for pain and memory

Sensitization is a form of non-associative conditioning in which amplification of behavioral responses can occur following presentation of an aversive or noxious stimulus. Understanding the cellular and molecular underpinnings of sensitization has been an overarching theme spanning the field of learning and memory as well as that of pain research. In this review we examine how sensitization, both in the context of learning as well as pain processing, shares evolutionarily conserved behavioral, cellular/synaptic, and epigenetic mechanisms across phyla. First, we characterize the behavioral phenomenon of sensitization both in invertebrates and vertebrates. Particular emphasis is placed on long-term sensitization (LTS) of withdrawal reflexes in Aplysia following aversive stimulation or injury, although additional invertebrate models are also covered. In the context of vertebrates, sensitization of mammalian hyperarousal in a model of post-traumatic stress disorder (PTSD), as well as mammalian models of inflammatory and neuropathic pain is characterized. Second, we investigate the cellular and synaptic mechanisms underlying these behaviors. We focus our discussion on serotonin-mediated long-term facilitation (LTF) and axotomy-mediated long-term hyperexcitability (LTH) in reduced Aplysia systems, as well as mammalian spinal plasticity mechanisms of central sensitization. Third, we explore recent evidence implicating epigenetic mechanisms in learning- and pain-related sensitization. This review illustrates the fundamental and functional overlay of the learning and memory field with the pain field which argues for homologous persistent plasticity mechanisms in response to sensitizing stimuli or injury across phyla.

Calcium, calpain, and calcineurin in low-frequency depression of transmitter release

Low-frequency depression (LFD) of transmitter release occurs at phasic synapses with stimulation at 0.2 Hz in both isolated crayfish (Procambarus clarkii) neuromuscular junction (NMJ) preparations and in intact animals. LFD is regulated by presynaptic activity of the Ca(2+)-dependent phosphatase calcineurin (Silverman-Gavrila and Charlton, 2009). Since the fast Ca(2+) chelator BAPTA-AM inhibits LFD but the slow chelator EGTA-AM does not, the Ca(2+) sensor for LFD may be close to a Ca(2+) source at active zones. Calcineurin can be activated by the Ca(2+)-activated protease calpain, and immunostaining showed that both proteins are present at nerve terminals. Three calpain inhibitors, calpain inhibitor I, MDL-28170, and PD150606, but not the control compound PD145305, inhibit LFD both in the intact animal as shown by electromyograms and by intracellular recordings at neuromuscular junctions. Analysis of mini-EPSPs indicated that these inhibitors had minimal postsynaptic effects. Proteolytic activity in CNS extract, detected by a fluorescent calpain substrate, was modulated by Ca(2+) and calpain inhibitors. Western blot analysis of CNS extract showed that proteolysis of calcineurin to a fragment consistent with the constitutively active form required Ca(2+) and was blocked by calpain inhibitors. Inhibition of LFD by calpain inhibition blocks the reduction in phosphoactin and the depolymerization of tubulin that normally occurs in LFD, probably by blocking the dephosphorylation of cytoskeletal proteins by calcineurin. In contrast, high-frequency depression does not involve protein phosphorylation- or calpain-dependent mechanisms. LFD may involve a specific pathway in which local Ca(2+) signaling activates presynaptic calpain and calcineurin at active zones and causes changes of tubulin cytoskeleton.

PKM and the maintenance of memory

How can memories outlast the molecules from which they are made? Answers to this fundamental question have been slow coming but are now emerging. A novel kinase, an isoform of protein kinase C (PKC), PKMzeta, has been shown to be critical to the maintenance of some types of memory. Inhibiting the catalytic properties of this kinase can erase well-established memories without altering the ability of the erased synapse to be retrained. This article provides an overview of the literature linking PKMzeta to memory maintenance and identifies some of the controversial issues that surround the bold implications of the existing data. It concludes with a discussion of the future directions of this domain.

Memory maintenance by PKMζ--an evolutionary perspective

Long-term memory is believed to be maintained by persistent modifications of synaptic transmission within the neural circuits that mediate behavior. Thus, long-term potentiation (LTP) is widely studied as a potential physiological basis for the persistent enhancement of synaptic strength that might sustain memory. Whereas the molecular mechanisms that initially induce LTP have been extensively characterized, the mechanisms that persistently maintain the potentiation have not. Recently, however, a candidate molecular mechanism linking the maintenance of LTP and the storage of long-term memory has been identified. The persistent activity of the autonomously active, atypical protein kinase C (aPKC) isoform, PKMζ, is both necessary and sufficient for maintaining LTP. Furthermore, blocking PKMζ activity by pharmacological or dominant negative inhibitors disrupts previously stored long-term memories in a variety of neural circuits, including spatial and trace memories in the hippocampus, aversive memories in the basolateral amygdala, appetitive memories in the nucleus accumbens, habit memory in the dorsal lateral striatum, and elementary associations, extinction, and skilled sensorimotor memories in the neocortex. During LTP and memory formation, PKMζ is synthesized de novo as a constitutively active kinase. This molecular mechanism for memory storage is evolutionarily conserved. PKMζ formation through new protein synthesis likely originated in early vertebrates ~500 million years ago during the Cambrian period. Other mechanisms for forming persistently active PKM from aPKC are found in invertebrates, and inhibiting this atypical PKM disrupts long-term memory in the invertebrate model systems Drosophila melanogaster and Aplysia californica. Conversely, overexpressing PKMζ enhances memory in flies and rodents. PKMζ persistently enhances synaptic strength by maintaining increased numbers of AMPA receptors at postsynaptic sites, a mechanism that might have evolved from the general function of aPKC in trafficking membrane proteins to the apical compartment of polarized cells. This mechanism of memory may have had adaptive advantages because it is both stable and reversible, as demonstrated by the downregulation of experience-dependent, long-term increases in PKMζ after extinction and reconsolidation blockade that attenuate learned behavior. Thus, PKMζ, the “working end” of LTP, is a component of an evolutionarily conserved molecular mechanism for the persistent, yet flexible storage of long-term memory.