Role of Aplysia cell adhesion molecules during 5-HT-induced long-term functional and structural changes

Structural Components of Synaptic Plasticity and Memory Consolidation

Consolidation of implicit memory in the invertebrate Aplysia and explicit memory in the mammalian hippocampus are associated with remodeling and growth of preexisting synapses and the formation of new synapses. Here, we compare and contrast structural components of the synaptic plasticity that underlies these two distinct forms of memory. In both cases, the structural changes involve time-dependent processes. Thus, some modifications are transient and may contribute to early formative stages of long-term memory, whereas others are more stable, longer lasting, and likely to confer persistence to memory storage. In addition, we explore the possibility that trans-synaptic signaling mechanisms governing de novo synapse formation during development can be reused in the adult for the purposes of structural synaptic plasticity and memory storage. Finally, we discuss how these mechanisms set in motion structural rearrangements that prepare a synapse to strengthen the same memory and, perhaps, to allow it to take part in other memories as a basis for understanding how their anatomical representation results in the enhanced expression and storage of memories in the brain.

Learning-related synaptic growth mediated by internalization of Aplysia cell adhesion molecule is controlled by membrane phosphatidylinositol 4,5-bisphosphate synthetic pathway

Long-term facilitation in Aplysia is accompanied by the growth of new synaptic connections between the sensory and motor neurons of the gill-withdrawal reflex. One of the initial steps leading to the growth of these synapses is the internalization, induced by 5-HT, of the transmembrane isoform of Aplysia cell-adhesion molecule (TM-apCAM) from the plasma membrane of sensory neurons (Bailey et al., 1992). However, the mechanisms that govern the internalization of TM-apCAM and how this internalization is coupled to the molecular events that initiate the structural changes are not fully understood. Here, we report that the synthesis of membrane phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], which is known to be mediated by a signaling cascade through Aplysia Sec7 protein (ApSec7) and phosphatidylinositol-4-phosphate 5-kinase type I α (PIP5KIα) is required for both the internalization of TM-apCAM and the initiation of synaptic growth during 5-HT-induced long-term facilitation. Pharmacological blockade of PI(4,5)P(2) synthesis by the application of the inhibitor phenylarsine oxide blocked the internalization of apCAM. Furthermore, perturbation of the endogenous activation of ApSec7 and its downstream target PIP5KIα also blocked 5-HT-mediated internalization of TM-apCAM and synaptic growth. Finally, long-term facilitation was specifically impaired by blocking the ApSec7 signaling pathway at sensory-to-motor neuron synapses. These data indicate that the ApSec7/PIP5KIα signaling pathway is actively recruited during learning-related 5-HT signaling and acts as a key regulator of apCAM internalization associated with the formation of new synaptic connections during long-term facilitation.

A first partial Aplysia californica proteome

Aplysia proteins have not been studied systematically and it was therefore the aim of the study to carry out protein profiling in ganglia from Aplysia californica (AC). AC ganglia were extirpated, proteins extracted and run on 2DE with subsequent in-gel digestion, followed by identification of proteins by nano-LC-ESI-MS/MS on an ion trap. Proteins were identified based upon a public Aplysia EST database. Out of 408 picked spots, 276 spots were identified corresponding to 172 ESTs and 118 individual proteins. The range of sequence coverage was between 14 and 80% and the average amount of peptides used for the identification of proteins was 9 (from 3 to 24). Mean score for protein identification was 516. Comparison of protein levels between cerebral, pleural, pedal and abdominal ganglia revealed a series of significant differences including: signaling, metabolism, cytoskeleton and structural, redox, chaperone, replication/transcription and electron/proton transport proteins. The generation of a protein map complements transcriptional studies carried out in AC ganglia. The findings provide the basis for investigation into post-translational modifications, splice variants and assist in the generation of antibodies against AC proteins. Moreover, differences in protein expression between ganglia may be valuable for the design of future studies in neurobiology of AC.

The molecular and cellular biology of enhanced cognition

Most molecular and cellular studies of cognitive function have focused on either normal or pathological states, but recent research with transgenic mice has started to address the mechanisms of enhanced cognition. These results point to key synaptic and nuclear signalling events that can be manipulated to facilitate the induction or increase the stability of synaptic plasticity, and therefore enhance the acquisition or retention of information. Here, we review these surprising findings and explore their implications to both mechanisms of learning and memory and to ongoing efforts to develop treatments for cognitive disorders. These findings represent the beginning of a fundamental new approach in the study of enhanced cognition.

Transcriptome analysis and identification of regulators for long-term plasticity in Aplysia kurodai

The marine mollusk Aplysia is a useful model organism for studying the cellular bases of behavior and plasticity. However, molecular studies of Aplysia have been limited by the lack of genomic information. Recently, a large scale characterization of neuronal transcripts was performed in A. californica. Here, we report the analysis of a parallel set of neuronal transcripts from a closely related species A. kurodai found in the northwestern Pacific. We collected 4,859 nonredundant sequences from the nervous system tissue of A. kurodai. By performing microarray and real-time PCR analyses, we found that ApC/EBP, matrilin, antistasin, and eIF3e clones were significantly up-regulated and a BAT1 homologous clone was significantly down-regulated by 5-HT treatment. Among these, we further demonstrated that the Ap-eIF3e plays a key role in 5-HT-induced long-term facilitation (LTF) as a positive regulator.

Transcriptional regulation of long-term memory in the marine snail Aplysia

Whereas the induction of short-term memory involves only covalent modifications of constitutively expressed preexisting proteins, the formation of long-term memory requires gene expression, new RNA, and new protein synthesis. On the cellular level, transcriptional regulation is thought to be the starting point for a series of molecular steps necessary for both the initiation and maintenance of long-term synaptic facilitation (LTF). The core molecular features of transcriptional regulation involved in the long-term process are evolutionally conserved in Aplysia, Drosophila, and mouse, and indicate that gene regulation by the cyclic AMP response element binding protein (CREB) acting in conjunction with different combinations of transcriptional factors is critical for the expression of many forms of long-term memory. In the marine snail Aplysia, the molecular mechanisms that underlie the storage of long-term memory have been extensively studied in the monosynaptic connections between identified sensory neuron and motor neurons of the gill-withdrawal reflex. One tail shock or one pulse of serotonin (5-HT), a modulatory transmitter released by tail shocks, produces a transient facilitation mediated by the cAMP-dependent protein kinase leading to covalent modifications in the sensory neurons that results in an enhancement of transmitter release and a strengthening of synaptic connections lasting minutes. By contrast, repeated pulses of 5-hydroxytryptamine (5-HT) induce a transcription- and translation-dependent long-term facilitation (LTF) lasting more than 24 h and trigger the activation of a family of transcription factors in the presynaptic sensory neurons including ApCREB1, ApCREB2 and ApC/EBP. In addition, we have recently identified novel transcription factors that modulate the expression of ApC/EBP and also are critically involved in LTF. In this review, we examine the roles of these transcription factors during consolidation of LTF induced by different stimulation paradigms.

Nuclear translocation of CAM-associated protein activates transcription for long-term facilitation in Aplysia

Repeated pulses of serotonin (5-HT) induce long-term facilitation (LTF) of the synapses between sensory and motor neurons of the gill-withdrawal reflex in Aplysia. To explore how apCAM downregulation at the plasma membrane and CREB-mediated transcription in the nucleus, both of which are required for the formation of LTF, might relate to each other, we cloned an apCAM-associated protein (CAMAP) by yeast two-hybrid screening. We found that 5-HT signaling at the synapse activates PKA which in turn phosphorylates CAMAP to induce the dissociation of CAMAP from apCAM and the subsequent translocation of CAMAP into the nucleus of sensory neurons. In the nucleus, CAMAP acts as a transcriptional coactivator for CREB1 and is essential for the activation of ApC/EBP required for the initiation of LTF. Combined, our data suggest that CAMAP is a retrograde signaling component that translocates from activated synapses to the nucleus during synapse-specific LTF.

PKA-activated ApAF-ApC/EBP heterodimer is a key downstream effector of ApCREB and is necessary and sufficient for the consolidation of long-term facilitation

Long-term memory requires transcriptional regulation by a combination of positive and negative transcription factors. Aplysia activating factor (ApAF) is known to be a positive transcription factor that forms heterodimers with ApC/EBP and ApCREB2. How these heterodimers are regulated and how they participate in the consolidation of long-term facilitation (LTF) has not, however, been characterized. We found that the functional activation of ApAF required phosphorylation of ApAF by PKA on Ser-266. In addition, ApAF lowered the threshold of LTF by forming a heterodimer with ApCREB2. Moreover, once activated by PKA, the ApAF-ApC/EBP heterodimer transactivates enhancer response element-containing genes and can induce LTF in the absence of CRE- and CREB-mediated gene expression. Collectively, these results suggest that PKA-activated ApAF-ApC/EBP heterodimer is a core downstream effector of ApCREB in the consolidation of LTF.

BDNF mobilizes synaptic vesicles and enhances synapse formation by disrupting cadherin-beta-catenin interactions

Neurons of the vertebrate central nervous system have the capacity to modify synapse number, morphology, and efficacy in response to activity. Some of these functions can be attributed to activity-induced synthesis and secretion of the neurotrophin brain-derived neurotrophic factor (BDNF); however, the molecular mechanisms by which BDNF mediates these events are still not well understood. Using time-lapse confocal analysis, we show that BDNF mobilizes synaptic vesicles at existing synapses, resulting in small clusters of synaptic vesicles “splitting” away from synaptic sites. We demonstrate that BDNF's ability to mobilize synaptic vesicle clusters depends on the dissociation of cadherin–β-catenin adhesion complexes that occurs after tyrosine phosphorylation of β-catenin. Artificially maintaining cadherin–β-catenin complexes in the presence of BDNF abolishes the BDNF-mediated enhancement of synaptic vesicle mobility, as well as the longer-term BDNF-mediated increase in synapse number. Together, this data demonstrates that the disruption of cadherin–β-catenin complexes is an important molecular event through which BDNF increases synapse density in cultured hippocampal neurons.

Regulation of ApC/EBP mRNA by the Aplysia AU-rich element-binding protein, ApELAV, and its effects on 5-hydroxytryptamine-induced long-term facilitation

Aplysia CCAAT enhancer-binding protein (ApC/EBP), a key molecular switch in 5-hydroxytryptamine (5-HT)-induced long-term facilitation of Aplysia, is quickly and transiently expressed in response to a 5-HT stimulus, but the mechanism underlying this dynamic expression profile remains obscure. Here, we report that the dynamic expression of ApC/EBP during long-term facilitation is regulated at the post-transcriptional level by AU-rich element (ARE)-binding proteins. We found that the 3'UTR of ApC/EBP mRNA contains putative sequences for ARE, which is a representative post-transcriptional cis-acting regulatory element that modulates the stability and/or the translatability of a distinct subset of labile mRNAs. We cloned the Aplysia homologue of embryonic lethal abnormal visual system homologue (ELAV/Hu) protein, one of the best-studied RNA-binding proteins that associate with ARE, and elucidated the involvement of Aplysia ELAV/Hu protein in ApC/EBP gene expressional regulation. Cloned Aplysia ELAV/Hu protein, Aplysia embryonic lethal abnormal visual system (ApELAV), bound to an AU-rich region within the 3'UTR of ApC/EBP mRNA. Additionally, ApELAV controlled the expression of ApC/EBP 3'UTR-containing reporter gene by functioning as a stability-enhancing factor. In particular, 5-HT-induced long-term facilitation was impaired when the AU-rich region within the 3'UTR of ApC/EBP was over-expressed, which suggests the significance of this region in 5-HT-induced ApC/EBP expression, and in the resultant formation of long-term facilitation. Our results imply that the Aplysia ARE-binding protein, ApELAV, can regulate ApC/EBP gene expression at the mRNA level, and accordingly, ARE-mediated post-transcriptional mechanism may serve a crucial function in regulating the expression of ApC/EBP in response to a 5-HT stimulus.

Common molecular mechanisms in explicit and implicit memory

Cellular and molecular studies of both implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded and stored within the brain. In this review, we focus on recent advances in our understanding of two types of memory storage: (i) sensitization in Aplysia, a simple form of implicit memory, and (ii) formation of explicit spatial memories in the mouse hippocampus. These two processes share common molecular mechanisms that have been highly conserved through evolution.

Molecular mechanisms of memory storage in Aplysia

Cellular studies of implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded, processed, and stored within the brain. In this review, we focus on recent advances in our understanding of the molecular mechanisms that underlie short-term, intermediate-term, and long-term forms of implicit memory in the marine invertebrate Aplysia californica, and consider how the conservation of common elements in each form may contribute to the different temporal phases of memory storage.

Suppression of long-term facilitation by Rab3-effector protein interaction

Long-term facilitation (LTF) in Aplysia is achieved by the modulation of presynaptic release. However, the underlying mechanism that might be related with the regulation of synaptic vesicle release remains unknown. Since Rab3, a neuronal GTP-binding protein, is known to be a key regulator of synaptic vesicle fusion, we investigated the involvement of Rab3 in LTF. To address this issue, we examined the effect of overexpression of wild type Aplysia Rab3 (apRab3) and its mutant forms on LTF. Overexpression of either apRab3 Q80L, a constitutively active apRab3 mutant, or wild type apRab3 completely inhibited LTF. This inhibitory role of apRab3 appears to be mediated by an interaction with an effector molecule(s), possibly Rim. Expression of apRab3 Q80L, V54E double mutant, which do not bind effector molecules such as Rim or Rabphilin, had no effect on LTF. Furthermore, expression of apRab3 Q80L, F18L, D19E triple mutant, which has reduced binding activity with Rim but normally binds with Rabphilin, enhanced evoked basal synaptic release, and the increase in synaptic strength occluded LTF. In conclusion, our data suggest that apRab3 may act as a negative clamp of LTF through the interaction with effector protein(s), possibly Rim.

Cofilin expression induces cofilin-actin rod formation and disrupts synaptic structure and function in Aplysia synapses

Cofilin-actin rods are inclusion-like structures that are induced by certain chemical or physical stresses in cultured cells, and the rods formed in neurons are thought to be associated with neurodegeneration. Here, we cloned an Aplysia cofilin homolog and overexpressed it in cultured neurons. Overexpressed cofilin formed rod-like structures that included actin. The overall neuronal morphology was unaffected by cofilin overexpression; however, a decrease in number of synaptic varicosities was observed. Consistent with this structural change by cofilin overexpression, the synaptic strength was reduced, and furthermore, the long-term facilitation elicited by repeated pulses of 5-hydroxytryptamine was impaired in sensory-to-motor synapses. However, cofilin overexpression did not induce programmed cell death. These findings suggest that the formation of cofilin-actin rod-like structures can lead to neurodegeneration, and this might be a mechanism of rundown of neuronal and synaptic function without cell death in neurodegenerative diseases.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.

The Persistence of Long-Term Memory

Recent cellular and molecular studies of both implicit and explicit memory storage suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these diverse forms of memory are encoded and stored. For both forms of memory storage, some type of synaptic growth is thought to represent the stable cellular change that maintains the long-term process. In this review, we discuss recent findings on the molecular events that underlie learning-related synaptic growth in Aplysia and discuss the possibility that an active, prion-based mechanism is important for the maintenance of the structural change and for the persistence of long-term memory.