Identification of a serotonin receptor coupled to adenylyl cyclase involved in learning-related heterosynaptic facilitation in Aplysia Yong-Seok Leea,1, Sun-Lim Choia,1, Seung-Hee Leea, Hyoung Kima, Hyungju Parka, Nuribalhae Leea, Sue-Hyun Leea, Yeon-Su Chaea, Deok-Jin Janga, Eric R. Kandelb, and Bong-Kiun Kaanga,c,2 aNational Creative Research Initiative Center for Memory and Institute of Molecular Biology and Genetics, Departments of Biological Sciences and cBrain and Cognitive Sciences, College of Natural Sciences, Seoul National University, 599 Gwanangno, Gwanak-gu, Seoul 151–747, Korea; and bCenter for Neurobiology and Behavior, College of Physicians and Surgeons of Columbia University, 1051 Riverside Drive, New York, NY 10032 Contributed by Eric R. Kandel, July 8, 2009 (sent for review November 11, 2008) Serotonin (5-HT) plays a critical role in modulating synaptic plasticity G protein-coupled 5-HT receptors are classified on the basis of the in the marine mollusc Aplysia and in the mammalian nervous system. second messenger systems to which they are coupled (14). The In Aplysia sensory neurons, 5-HT can activate several signal cascades, 5-HT1 and 5-HT5 receptors inhibit adenylyl cyclase, whereas 5-HT4, including PKA and PKC, presumably via distinct types of G protein- 5-HT6, and 5-HT7 activate adenylyl cyclase, and the 5-HT2 receptor coupled receptors. However, the molecular identities of these recep- stimulates phospholipase C (PLC). Molecular evolutionary analy- tors have not yet been identified. We here report the cloning and ses indicate that primordial 5-HT receptors differentiated into functional characterization of a 5-HT receptor that is positively cou- 5-HT1, 5-HT2, and 5-HT6 approximately 800 million years ago pled to adenylyl cyclase in Aplysia neurons. The cloned receptor, (mya) (15). Since vertebrates differentiated from invertebrates 600 5-HTapAC1, stimulates the production of cAMP in HEK293T cells and in mya, one might predict that invertebrates may have 5-HT receptor Xenopus oocytes. Moreover, the knockdown of 5-HTapAC1 expression families that are homologous to at least three subtypes of vertebrate by RNA interference blocked 5-HT-induced cAMP production in Aply- 5-HT receptors: 5-HT1, 5-HT2 and 5-HT6. However, considering sia sensory neurons and blocked synaptic facilitation in nondepressed that a hundred million years have passed since they have diverged, or partially depressed sensory-to-motor neuron synapses. These data it is by no means certain that the invertebrate receptors have similar implicate 5-HTapAC1 as a major modulator of learning related synaptic pharmacological characteristics to the vertebrate receptors within facilitation in the direct sensory to motor neuron pathway of the gill each family (16). withdrawal reflex. Aplysia 5-HT receptors have been characterized mainly by phar- macological means. For example, Abrams et al. characterized the 5-HT receptor ͉ memory ͉ cAMP ͉ protein kinase A 5-HT receptor(s) that are positively coupled to adenylyl cyclase (5-HTapAC) pharmacologically (17, 18). So far, four full-length -Hydroxytryptamine (5-HT), or serotonin, is a key neurotrans- 5-HT receptors have been cloned in Aplysia (19, 20), two of 5mitter that modulates a variety of behaviors in both invertebrate which—5-HTap1 and 5-HTap2—inhibit adenylyl cyclase activity (19, and vertebrate animals and is involved in the regulation of mood 20). However, no receptor positively coupled to adenylyl cyclase and mood disorders in humans (1). Serotonin also modulates and involved in synaptic facilitation has yet been cloned, leaving synaptic plasticity in the marine mollusc Aplysia (2, 3). Synaptic questions about the receptors that initiate synaptic facilitation facilitation of the connections between sensory and motor neurons unanswered. Here, we report the molecular cloning and functional of the gill-withdrawal reflex is mediated by 5-HT, and this form of characterization of a 5-HT receptor that is positively coupled to synaptic plasticity has been found to be a critical cellular mechanism adenylyl cyclase in the Aplysia nervous system. of behavioral sensitization (4–6). A number of pharmacological studies have found that, depending on the behavioral history and Results pattern of sensory stimulation, 5-HT stimulates several downstream The Molecular Cloning of 5-HTapAC1 from the Aplysia Nervous System. signaling pathways, including protein kinase A (PKA), protein To isolate genes encoding 5-HT receptors that are positively kinase C (PKC), and mitogen-activated protein kinase (MAPK), coupled to adenylyl cyclase in Aplysia, we designed two degenera- suggesting that serotonin acts on more than one receptor type (2, tive PCR primers based on the peptide sequences of the fifth and 3, 7, 8). Of these signaling cascades, the adenylyl cyclase-cAMP- seventh transmembrane domains of invertebrate 5-HT7 receptors PKA cascade has been most extensively investigated because of its (Fig. 1A). In addition to the highly conserved amino acids se- important roles in both behavioral sensitization and synaptic facil- quences, NPXXY, in the seventh transmembrane domains of G itation (3, 4, 9, 10). Historically, this was the initially identified protein-coupled receptors, we found that another motif (QIYATL) second-messenger system involved in the regulation of synaptic is strikingly conserved only in invertebrate 5-HT7 receptors. We plasticity, behavior, and memory storage (4). obtained a 519-bp PCR product showing Ϸ80% sequence homol- A single pulse of 5-HT activates PKA, which phosphorylates and ogy with the 5-HT7 receptor of Helisoma trivolvis. Using this inactivates potassium channels (11) and subsequently increases synaptic strength at nondepressed synapses. At depressed synapses, Author contributions: B.-K.K. designed research; Y.-S.L., S.-L.C., Seung-Hee Lee, H.K., H.P., however, PKC becomes the major downstream kinase to be acti- N.L., Sue-Hyun Lee, and Y.-S.C. performed research; Y.-S.L., S.-L.C., Seung-Hee Lee, H.K., vated by a single pulse of 5-HT (8). In addition, repetitive exposures H.P., N.L., Sue-Hyun Lee, D.-J.J., and B.-K.K. analyzed data; and Y.-S.L., S.-L.C., D.-J.J., E.R.K., to 5-HT that induce long-term facilitation result in the activation of and B.-K.K. wrote the paper. additional kinases, including MAPK (12), that translocate to the The authors declare no conflict of interest. nucleus to induce gene expression. However, the molecular mech- The sequence reported in this paper has been deposited in the GenBank database (acces- anism for this dynamic coupling specificity of downstream signaling sion no. FJ477896). pathways is not known. 1Y.-S.L. and S.-L.C. contributed equally to this work. In vertebrates, seven families of 5-HT receptors have been 2To whom correspondence should be addressed. E-mail: [email protected]. characterized; six of these include G protein-coupled receptors, and This article contains supporting information online at www.pnas.org/cgi/content/full/ only the 5-HT3 family is composed of ionotropic receptors (13). The 0907502106/DCSupplemental. 14634–14639 ͉ PNAS ͉ August 25, 2009 ͉ vol. 106 ͉ no. 34 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0907502106 Downloaded by guest on October 1, 2021 The deduced amino acid sequences of 5-HTapAC1 show the hallmarks of known G protein-coupled receptors, including seven transmembrane domains, the tripeptide DRY (Asp-169–Arg-170– Tyr-171) required for G protein coupling (22), and the NPXXY motif (Asn-401–Pro-402–Tyr-405) for receptor desensitization and internalization (23) (Fig. 1B). Four potential sites for PKC phos- phorylation and two sites for PKA are found within the third intracellular loop. Three additional PKC sites are also found in the second intracellular loop and C-terminal intracellular tail. In ad- dition, six consensus asparagine residues for N-glycosylation are found in the extracellular N terminus. 5-HTapAC1 also contains, at its C terminus, a putative PDZ-binding motif (Ser-483–Lys-484– Ile-485), suggesting that this receptor may be clustered by scaffold- ing proteins to form a signaling complex. We next examined the tissue distribution of the cloned receptor by RT-PCR analysis. For this purpose we used total RNA isolated from the abdominal ganglia, the buccal ganglia, the buccal mass, the central ganglia containing the pleural-pedal ganglia, and the gill and ovotestis of Aplysia (Fig. 1C). A specific fragment was detected in the CNS tissue, with the strongest signal coming from the abdom- inal ganglia. The mRNA was also detected in muscle and gill tissue, which is consistent with an earlier study that used an [3H]LSD binding assay (24). To further validate the expression of 5-HTapAC1 in specific neurons involved in synaptic facilitation and behavioral sensitization, we performed single cell RT-PCR analysis (Fig. 1D). Our results clearly showed that 5-HTapAC1 is expressed both in the sensory neurons and in the L7 and LFS motor neurons. The expression pattern of 5-HTapAC1 was further examined by in NEUROSCIENCE situ hybridization in the abdominal and pleural ganglia of A. kurodai. In situ staining revealed that the cloned receptor is widely expressed in the nervous system, including sensory neurons in the sensory clusters of both abdominal and pleural ganglia, as well as L7 motor neurons in abdominal ganglia (Fig. 1E). Expression of 5-HTapAC1 in pleural sensory neurons and in LFS motor neurons Fig. 1. Molecular cloning and expression pattern of 5-HTapAC1.(A) Multiple se- was also confirmed by in situ hybridization in sensory-to-motor quence alignments of TM5 and TM7 of invertebrate 5-HT7 receptors. (B) Deduced coculture (Fig. 1F). amino acid sequence of 5-HTapAC1. Seven transmembrane (TM) domains are indi- cated and numbered. Predicted phosphorylation sites for PKA (F) and PKC ( ) are * Subcellular Localization of 5-HT . marked. Triangles (Œ) indicate N-glycosylation sites.