The pond snail Lymnaea stagnalis is an excellent model system in which to study the neuronal and molecular substrates of associative learning and its consolidation into long-term memory. Until now, the presence of cyclic AMP (cAMP)-responsive element binding protein (CREB), which is believed to be a necessary component in the process of a learned behavior that is consolidated into long-term memory, has only been assumed in Lymnaea neurons. We therefore cloned and analyzed the cDNA sequences of homologues of CREB1 and CREB2 and determined the presence of these mRNAs in identifiable neurons of the central nervous system (CNS) of L. stagnalis. The deduced amino acid sequence of Lymnaea CREB1 is homologous to transcriptional activators, mammalian CREB1 and Aplysia CREB1a, in the C-terminal DNA binding (bZIP) and phosphorylation domains, whereas the deduced amino acid sequence of Lymnaea CREB2 is homologous to transcriptional repressors, human CREB2, mouse activating transcription factor-4, and Aplysia CREB2 in the bZIP domain. In situ hybridization revealed that only a relatively few neurons showed strongly positive signals for Lymnaea CREB1 mRNA, whereas all the neurons in the CNS contained Lymnaea CREB2 mRNA. Using one of the neurons (the cerebral giant cell) containing Lymnaea CREB1 mRNA, we showed that the injection of a CRE oligonucleotide inhibited a cAMP-induced, long-lasting synaptic plasticity. We therefore conclude that CREBs are present in Lymnaea neurons and may function as necessary players in behavioral plasticity.

The pond snail Lymnaea stagnalis is capable of learning taste aversion and consolidating this learning into long-term memory (LTM) that is called conditioned taste aversion (CTA). Previous studies showed that some molluscan insulin-related peptides (MIPs) were upregulated in snails exhibiting CTA. We thus hypothesized that MIPs play an important role in neurons underlying the CTA-LTM consolidation process. To examine this hypothesis, we first observed the distribution of MIP II, a major peptide of MIPs, and MIP receptor and determined the amounts of their mRNAs in the CNS. MIP II was only observed in the light green cells in the cerebral ganglia, but the MIP receptor was distributed throughout the entire CNS, including the buccal ganglia. Next, when we applied exogenous mammalian insulin, secretions from MIP-containing cells or partially purified MIPs, to the isolated CNS, we observed a long-term change in synaptic efficacy (i.e., enhancement) of the synaptic connection between the cerebral giant cell (a key interneuron for CTA) and the B1 motor neuron (a buccal motor neuron). This synaptic enhancement was blocked by application of an insulin receptor antibody to the isolated CNS. Finally, injection of the insulin receptor antibody into the snail before CTA training, while not blocking the acquisition of taste aversion learning, blocked the memory consolidation process; thus, LTM was not observed. These data suggest that MIPs trigger changes in synaptic connectivity that may be correlated with the consolidation of taste aversion learning into CTA-LTM in the Lymnaea CNS. IntroductionFormation of long-term memory (LTM) after associative learning is dependent on both protein synthesis and altered gene activity in neurons that play a critical role in memory formation (Inda et al., 2005;Lee et al., 2008;Rosenegger et al., 2010). The pond snail Lymnaea stagnalis is a good model in which to elucidate the causal mechanisms that underlie LTM formation (Ito et al., 1999(Ito et al., , 2012a Sakakibara, 2006;Nikitin et al., 2008;Kemenes and Benjamin, 2009). In conditioned taste aversion (CTA), a form of associative learning, an appetitive stimulus (sucrose) is used as the conditioned stimulus (CS), and an aversive stimulus (KCl) is used as the unconditioned stimulus (US). The CS increases the feeding response in snails, whereas the US inhibits feeding. In CTA training, the CS is paired with the US. After repeated paired presentations, the CS no longer elicits the feeding response, and this aversive conditioning persists as LTM (Kojima et al., 1996).We identified candidate genes necessary for the establishment of CTA-LTM in Lymnaea and found that some genes were upregulated while others were downregulated . Some of the upregulated genes after LTM consolidation were the molluscan insulin-related peptide (MIP I, II, and others) genes. However, it is unclear whether MIPs are necessary for memory consolidation, and if they are, what is their role in the consolidation process.Peptide purification of MIP I-III and V and the additi...

is the pallial line, which is an impression of the pallial muscles lined in an arc in the middle of a mantle. Several types of epithelial cells are recognized in the outer epithelium of a pearl oyster mantle (Tsujii 1960;Wada 1966). The most abundant type is named as outer epithelial cells. They are regarded to play a major role in shell formation and regeneration, and their functions are a main theme of this article.A shell of pearl oyster Pinctada fucata, characterized by its beautiful luster over the inner surface, consists of two layers: periostracum and ostracum. The periostracum is a thin, pliable proteinaceous sheet covering the outer surface of the shell. Components of the periostracum are synthesized in a periostracum gland located at the bottom of a periostracal groove between an outer and a middle fold of a mantle. The periostracum is thought to serve as a barrier that iso- Fundamental Studies on in vivo and in vitro AbstractOuter epithelial cells, which constitute a monolayer epithelium covering the outer surface of pearl oyster mantle, play principal roles in shell and pearl formation. In pearl culture, a fragment of the mantle prepared from a donor is implanted into the recipient's gonad together with a small inorganic bead. Histological studies using pearl oyster Pinctada fucata have revealed that the outer epithelial cells emigrate from the allograft, proliferate, and form a pearl sac surrounding the bead. Following the pearl-sac formation, the pearl-sac epithelia start to form calcium carbonate crystals, such as nacre, on the bead showing morphological characteristics closely related with the crystal structures.To investigate cellular mechanisms of the pearl formation, organ and cell culture methods for the outer epithelial cells of pearl oyster mantle were developed. In the organ culture, crystal formation, deposition of shell matrix-like structure, and DNA synthesis of the outer epithelial cells were observed. The outer epithelial cells separated from the mantle started DNA synthesis in co-culture with hemocytes that revealed a part of cellto-cell interactions during the pearl-sac formation processes. Substitution of the cultured outer epithelial cells for a mantle allograft in pearl culture was tested by injection of the cultured cells; the results of which implied future possibilities for the application of the cultured outer epithelial cells for pearl production.

Insulin-related peptide cDNA was characterized in the Pacific oyster Crassostrea gigas. It was determined that three transcripts with differing lengths of 3 -untranslated region (3 -UTR) were expressed in the visceral ganglia. The insulin-related peptide cDNA contained a number of AUUUA motifs that were typical of adenylate/uridylaterich elements in the 3 -UTR. The deduced preprohormone was a polypeptide of 161 residues and showed a conformation typical of preprohormones of the insulin superfamily, which included conserved amino acids necessary to adopt the globular insulin structure. The expression of the three different transcripts was variable throughout the year, with the highest expression observed in March and lower expression in November and July.

In vitro experiments were conducted to clarify the involvement of the epithelium-amebocyte interaction in epithelial regeneration of bivalves. The outer epithelia of the pallial mantle of the pearl oyster. Pinctada fucata martensii, were separated in cell sheets from the inner connective tissue layers by digestion with Dispase. Clumps of the separated mantle epithelia were inoculated onto the amebocyte layers prepared on the bottom of culture dishes and maintained at 20 degrees C in 5% CO2:95% air for 1 wk. Balanced salt solution with 0.03% (wt/vol) glucose was used as a culture medium. The epithelial cells adhered to the amebocyte layers within 24 h, changed their shape from cuboidal to squamous, and migrated and formed monolayer sheets within 3 d. Electron microscopy confirmed maintenance of epithelial polarity and cell to cell junction in the sheets; 6 d after the inoculation, 5-bromo-2'-deoxyuridine was added to the culture at 30 microns. After labeling for 24 h, the cultures were fixed and stained with anti 5-bromo-2'-deoxyuridine antibody. Cells with immunoreactive nuclei were clearly observed in the epithelial cell sheets, indicating active DNA synthesis in the epithelial sheets. Thus, cocultured with amebocytes, the outer epithelial cells from pallial mantle tissue formed a monolayer sheet and started DNA synthesis. The morphological features of the mantle outer epithelial cells are analogous to those described for the in vivo cutaneous wound healing process, suggesting that the epithelium-amebocyte interaction is important in the regeneration of epithelium in bivalves.