Mutational analysis of the mouse 5-HT7 receptor: importance of the third intracellular loop for receptor-G-protein interaction

Structural determinants for activation of the Tau kinase CDK5 by the serotonin receptor 5-HT7R

BACKGROUND

Multiple neurodegenerative diseases are induced by the formation and deposition of protein aggregates. In particular, the microtubule-associated protein Tau leads to the development of so-called tauopathies characterized by the aggregation of hyperphosphorylated Tau within neurons. We recently showed that the constitutive activity of the serotonin receptor 7 (5-HT7R) is required for Tau hyperphosphorylation and aggregation through activation of the cyclin-dependent kinase 5 (CDK5). We also demonstrated physical interaction between 5-HT7R and CDK5 at the plasma membrane suggesting that the 5-HT7R/CDK5 complex is an integral part of the signaling network involved in Tau-mediated pathology.

METHODS

Using biochemical, microscopic, molecular biological, computational and AI-based approaches, we investigated structural requirements for the formation of 5-HT7R/CDK5 complex.

RESULTS

We demonstrated that 5-HT7R domains responsible for coupling to Gs proteins are not involved in receptor interaction with CDK5. We also created a structural model of the 5-HT7R/CDK5 complex and refined the interaction interface. The model predicted two conserved phenylalanine residues, F278 and F281, within the third intracellular loop of 5-HT7R to be potentially important for complex formation. While site-directed mutagenesis of these residues did not influence Gs protein-mediated receptor signaling, replacement of both phenylalanines by alanine residues significantly reduced 5-HT7R/CDK5 interaction and receptor-mediated CDK5 activation, leading to reduced Tau hyperphosphorylation and aggregation. Molecular dynamics simulations of 5-HT7R/CDK5 complex for wild-type and receptor mutants confirmed binding interface stability of the initial model.

CONCLUSIONS

Our results provide a structural basis for the development of novel drugs targeting the 5-HT7R/CDK5 interaction interface for the selective treatment of Tau-related disorders, including frontotemporal dementia and Alzheimer's disease.

Stimulating DDX3 expression by serotonin 5-HT receptor 7 through phosphorylation of p53 via the AC-PKA-ERK signaling pathway

DDX3 is a host viral factor that can inhibit the hepatitis B virus-induced innate immune responses. In this study, the 20 bioactive compounds have screened the effects on DDX3 and we found that 5-HT upregulated DDX3 promoter activity via the 5-HT₇ receptor on liver hepatocellular cells (HepG2 cells) by using a luciferase assay, reverse transcription-polymerase chain reaction analysis, and Western blot analysis. Furthermore, we are trying to elucidate the pathways involved in the stimulating effect of 5-HT on DDX3 expression to induce innate immune responses against hepatitis B virus infection. A knockdown of the 5-HT₇ receptor by transfection si-5-HT₇ receptors or si-control into HepG2 cells treated by 5-HT (or 5-HT plus agonist) confirmed the role of the 5-HT₇ receptor in DDX3 expression. The IFN-β-Luc expression and level of hepatitis B virus surface Antigen (HBsAg) showed that DDX3 mediated by the 5-HT₇ agonist (AS-19) increased IFN-β expression and inhibited HBV replication. Luciferase assays showed the involvement of 5-HT₇ receptors in DDX3 expression via cAMP/AC/PKA pathways by using protein kinase A (PKA) and adenylyl cyclase inhibitor (MDL 12330A). AS-19 mediated DDX3 promoter activated PKA extracellular signal-regulated kinase ERK signaling the p53 phosphorylation (-1080/-1070) resulted in upregulation of DDX3 promoter transactivation via the 5-HT₇ receptors agonist. Overall, 5-HT₇ was found to be a new potential target to inhibit hepatitis B infection by activating AC/PKA/ERK pathways by phosphorylating p53 via the 5-HT₇ agonist response by mediating DDX3 expression.

Exploring the autoinhibitory domain of the electrogenic Na+ /HCO3- transporter NBCe1-B, from residues 28 to 62

KEY POINTS

Slc4a4 (mouse) encodes at least five variants of the electrogenic sodium/bicarbonate transporter NBCe1. The initial 41 cytosolic amino acids of NBCe1-A and -D are unique; NBCe1-A has high activity. The initial 85 amino acids of NBCe1-B, -C and -E are unique; NBCe1-B and -C have low activity. Previous work showed that deleting residues 1-85 or 40-62 of NBCe1-B, or 1-87 of NBCe1-C, eliminates autoinhibition. These regions also include binding determinants for IRBIT (inositol trisphosphate (IP₃ )-receptor binding protein released with IP₃ ), which relieves autoinhibition. Here, systematically replacing/deleting residues 28-62, we find that only the nine amino acid cationic cluster (residues 40-48) of NBCe1-B is essential for autoinhibition. IRBIT stimulates all but one low-activity construct. We suggest that electrostatic interactions - which IRBIT presumably interrupts - between the cationic cluster and the membrane or other domains of NBCe1 play a central role in tempering the activity of NBCe1-B in the pancreas, brain and other organs.

ABSTRACT

Variant B of the electrogenic Na⁺ /HCO₃^- cotransporter (NBCe1-B) contributes to the vectorial transport of HCO₃^- in epithelia (e.g. pancreatic ducts) and to the maintenance of intracellular pH in the central nervous systems (e.g. astrocytes). NBCe1-B has very low basal activity due to an autoinhibitory domain (AID) located, at least in part, in the unique portion (residues 1-85) of the cytosolic NH₂ -terminus. Previous work has shown that removing 23 amino acids (residues 40-62) stimulates NBCe1-B. Here, we test the hypothesis that a cationic cluster of nine consecutive positively charged amino acids (residues 40-48) is a necessary part of the AID. Using two-electrode voltage clamping of Xenopus oocytes, we assess the activity of human NBCe1-B constructs in which we systematically replace or delete residues 28-62, which includes the cationic cluster. We find that replacing or deleting all residues within the cationic cluster markedly increases NBCe1-B activity (i.e. eliminates autoinhibition). On the background of a cationic clusterless construct, systematically restoring Arg residues restores autoinhibition in two distinct quanta, with one to three Arg residues restoring ∼50%, and four or more Arg residues restoring virtually all autoinhibition. Systematically deleting residues before the cluster reduces autoinhibition by, at most, a small amount. Replacing or deleting residues after the cluster has no effect. For constructs with low NBCe1 activity (but good surface expression, as assessed by biotinylation), co-expression with super-IRBIT (lacking PP1-binding site) restores full activity (i.e. relieves autoinhibition). In summary, the cationic cluster is a necessary component of the AID of NBCe1-B.

Activation and conformational dynamics of a class B G-protein-coupled glucagon receptor

The human glucagon receptor (GCGR) is a class B G-protein-coupled receptor (GPCR). The GCGR can be activated by glucagon and regulates the release of glucose. The GCGR has been proposed to be an important drug target for type 2 diabetes. Based on the structural model of a full-length glucagon-bound GCGR (glu-GCGR), we performed accelerated molecular dynamics (aMD) simulations, potential of mean force (PMF) calculations, cross-correlation analysis and community network analysis to study the activation mechanism and the conformational dynamics during the activation process. The PMF map depicts three different conformational states of the GCGR: the inactive, intermediate and active states. The activation of the GCGR is characterized by the outward movement of the intracellular side of helix VI. In the active state of the GCGR, the Arg173(2.46)-Ser350(6.41) and Glu245(3.50)-Thr351(6.42) hydrogen bonds break, and the χ1 rotamer of Phe322(5.54) changes from perpendicular to parallel to helix VI. The binding of the agonist glucagon decreases the correlated motions of the extracellular loops (ELCs) and the helices around the glucagon-binding site. During the activation of the GCGR, the connections between the intracellular sides of helices become weaker, and the connections between glucagon and ECLs and the extracellular sides of helices become stronger. These facilitate G-protein coupling on the intracellular side and glucagon binding on the extracellular side, and stabilize the GCGR in the active state. We expect that this study can provide useful information on the activation mechanism of the GCGR and facilitate the future design of GCGR inhibitors.

The serotonin 5-HT7 receptors: two decades of research

Like most neurotransmitters, serotonin possesses a simple structure. However, the pharmacological consequences are more complex and diverse. Serotonin is involved in numerous functions in the human body including the control of appetite, sleep, memory and learning, temperature regulation, mood, behavior, cardiovascular function, muscle contraction, endocrine regulation, and depression. Low levels of serotonin may be associated with several disorders, namely increase in aggressive and angry behaviors, clinical depression, Parkinson's disease, obsessive-compulsive disorder, eating disorders, migraine, irritable bowel syndrome, tinnitus, and bipolar disease. These effects are mediated via different serotonin (5-HT) receptors. In this review, we will focus on the last discovered member of this serotonin receptor family, the 5-HT7 receptor. This receptor belongs to the G protein-coupled receptor superfamily and was cloned two decades ago. Later, different splice variants were described but no major functional differences have been described so far. All 5-HT7 receptor variants are coupled to Gαs proteins and stimulate cAMP formation. Recently, several interacting proteins have been reported, which can influence receptor signaling and trafficking.

Similarity between class A and class B G-protein-coupled receptors exemplified through calcitonin gene-related peptide receptor modelling and mutagenesis studies

Modelling class B G-protein-coupled receptors (GPCRs) using class A GPCR structural templates is difficult due to lack of homology. The plant GPCR, GCR1, has homology to both class A and class B GPCRs. We have used this to generate a class A–class B alignment, and by incorporating maximum lagged correlation of entropy and hydrophobicity into a consensus score, we have been able to align receptor transmembrane regions. We have applied this analysis to generate active and inactive homology models of the class B calcitonin gene-related peptide (CGRP) receptor, and have supported it with site-directed mutagenesis data using 122 CGRP receptor residues and 144 published mutagenesis results on other class B GPCRs. The variation of sequence variability with structure, the analysis of polarity violations, the alignment of group-conserved residues and the mutagenesis results at 27 key positions were particularly informative in distinguishing between the proposed and plausible alternative alignments. Furthermore, we have been able to associate the key molecular features of the class B GPCR signalling machinery with their class A counterparts for the first time. These include the [K/R]KLH motif in intracellular loop 1, [I/L]xxxL and KxxK at the intracellular end of TM5 and TM6, the NPXXY/VAVLY motif on TM7 and small group-conserved residues in TM1, TM2, TM3 and TM7. The equivalent of the class A DRY motif is proposed to involve Arg^2.39, His^2.43 and Glu^3.46, which makes a polar lock with T^6.37. These alignments and models provide useful tools for understanding class B GPCR function.

Differential determinants for coupling of distinct G proteins with the class B secretin receptor

The secretin receptor is a prototypic class B G protein-coupled receptor that is activated by binding of its natural peptide ligand. The signaling effects of this receptor are mediated by coupling with Gs, which activates cAMP production, and Gq, which activates intracellular calcium mobilization. We have explored the molecular basis for the coupling of each of these G proteins to this receptor using systematic site-directed mutagenesis of key residues within each of the intracellular loop regions, and studying ligand binding and secretin-stimulated cAMP and calcium responses. Mutation of a conserved histidine in the first intracellular loop (H157A and H157R) markedly reduced cell surface expression, resulting in marked reduction in cAMP and elimination of measurable calcium responses. Mutation of an arginine (R153A) in the first intracellular loop reduced calcium, but not cAMP responses. Mutation of a dibasic motif in the second intracellular loop (R231A/K232A) had no significant effects on any measured responses. Mutations in the third intracellular loop involving adjacent lysine and leucine residues (K302A/L303A) or two arginine residues separated by a leucine and an alanine (R318A/R321A) significantly reduced cAMP responses, while the latter also reduced calcium responses. Additive effects were elicited by combining the effective mutations, while combining all the effective mutations resulted in a construct that continued to bind secretin normally, but that elicited no significant cAMP or calcium responses. These data suggest that, while some receptor determinants are clearly shared, there are also distinct determinants for coupling with each of these G proteins.

The G protein coupled receptor Gpr153 shares common evolutionary origin with Gpr162 and is highly expressed in central regions including the thalamus, cerebellum and the arcuate nucleus

The Rhodopsin family of G protein coupled receptors (GPCRs) includes the phylogenetic α-group consisting of about 100 human members. The α-group is the only group of GPCRs that has many receptors for biogenic amines which are major drug targets. Several members of this group are orphan receptors and their functions are elusive. In this study we present a detailed phylogenetic and anatomical characterization of the Gpr153 receptor and also attempt to study its functional role. We identified the homologue of Gpr153 in the elephant shark genome and phylogenetic and synteny analyses revealed that Gpr162 and Gpr153 share a common ancestor that split most likely through a duplication event before the divergence of the tetrapods and the teleost lineage. A quantitative real-time PCR study reveals widespread expression of Gpr153 in the central nervous system and all the peripheral tissues investigated. Detailed in situ hybridization on mouse brain showed specifically high expression in the thalamus, cerebellum and the arcuate nucleus. The antisense oligodeoxynucleotide knockdown of Gpr153 caused a slight reduction in food intake and the elevated plus maze test showed significant reduction in the percentage of time spent in the centre square, which points towards a probable role in decision making. This report provides the first detailed characterization of the evolution, expression and primary functional properties of the Gpr153 gene.

Cloning, genomic organization and functionality of 5-HT(7) receptor splice variants from mouse brain

The serotonin (5-HT) 5-HT(7) receptors are expressed in both the central nervous system and in peripheral tissues. Receptor distribution studies and pharmacological studies have established that 5-HT(7) receptors play an important role in the control of circadian rhythms and thermoregulation. Selective 5-HT(7) receptor ligands have potential therapeutic applications for the treatment of pain and migraine, schizophrenia, anxiety, cognitive disturbances and inflammation. We have cloned two novel C-terminal splice variants of the 5-HT(7) receptor from mouse brain. These two new splice variants have almost identical sequences as the rat 5-HT(7(b)) and 5-HT(7(c)) splice variants and so were given the same name. Ligand binding assays ([(3)H]5-CT), membrane localization and functional studies in transiently transfected cells indicated that all three splice variants are well expressed on the membrane and no major differences in their respective pharmacology and their ability to activate adenylyl cyclase were observed. This is in analogy with previous reports comparing either the rat or the human variants.

SER-7, a Caenorhabditis elegans 5-HT7-like receptor, is essential for the 5-HT stimulation of pharyngeal pumping and egg laying

Serotonin (5-HT) stimulates both pharyngeal pumping and egg laying in Caenorhabditis elegans. Four distinct 5-HT receptors have been partially characterized, but little is known about their function in vivo. SER-7 exhibits most sequence identity to the mammalian 5-HT7 receptors and couples to a stimulation of adenyl cyclase when expressed in COS-7 cells. However, many 5-HT7-specific agonists have low affinity for SER-7. 5-HT fails to stimulate pharyngeal pumping and the firing of the MC motorneurons in animals containing the putative ser-7(tm1325) and ser-7(tm1728) null alleles. In addition, although pumping on bacteria is upregulated in ser-7(tm1325) animals, pumping is more irregular. A similar failure to maintain "fast pumping" on bacteria also was observed in ser-1(ok345) and tph-1(mg280) animals that contain putative null alleles of a 5-HT2-like receptor and tryptophan hydroxylase, respectively, suggesting that serotonergic signaling, although not essential for the upregulation of pumping on bacteria, "fine tunes" the process. 5-HT also fails to stimulate egg laying in ser-7(tm1325), ser-1(ok345), and ser-7(tm1325) ser-1(ok345) animals, but only the ser-7 ser-1 double mutants exhibit an Egl phenotype. All of the SER-7 mutant phenotypes are rescued by the expression of full-length ser-7gfp translational fusions. ser-7gfp is expressed in several pharyngeal neurons, including the MC, M2, M3, M4, and M5, and in vulval muscle. Interestingly, 5-HT inhibits egg laying and pharyngeal pumping in ser-7 null mutants and the 5-HT inhibition of egg laying, but not pumping, is abolished in ser-7(tm1325);ser-4(ok512) double mutants. Taken together, these results suggest that SER-7 is essential for the 5-HT stimulation of both egg laying and pharyngeal pumping, but that other signaling pathways can probably fulfill similar roles in vivo.

Molecular analysis of the interaction between the intracellular loops of the human serotonin receptor type 6 (5-HT6) and the alpha subunit of GS protein

The serotonin type 6 (5-HT(6)) receptor is a G-protein coupled receptor (GPCR) coupled to a stimulatory G-protein (G(S)). To identify the structural basis for the interaction of the 5-HT(6) receptor with the G(S) protein, we have dissected the interaction between GST-fusion proteins containing the second intracellular loop (iL2), the third intracellular loop (iL3), or the C-terminal tail of the 5-HT(6) receptor and the alpha subunit of G(S) (Galpha(S)). The direct interaction of iL3 and Galpha(S) was demonstrated by co-immunoprecipitation. Furthermore, the kinetic parameters of the interaction between iL3 and Galpha(S) were measured by surface plasmon resonance, and the apparent dissociation constant was determined to be 0.9 x 10(-6)M. In contrast, the second intracellular loop and C-terminal tail regions showed negligible affinity to Galpha(S). The critical residues within the iL3 region for the interaction with Galpha(S) were identified as conserved positively charged residues near the C-terminus of iL3 by measuring the cellular levels of cAMP produced in response to 5-HT stimulation of cells transfected with 5-HT(6) receptor mutants.

Mutations in the carboxy-terminus of the third intracellular loop of the human recombinant VPAC1 receptor impair VIP-stimulated [Ca2+]i increase but not adenylate cyclase stimulation

The vasoactive intestinal polypeptide (VIP) VPAC1 receptor is preferentially coupled to Galphas protein that stimulates adenylate cyclase activity and also to Galphaq and Galphai proteins that stimulate the inositol phosphate/calcium pathway. Previous studies indicated the importance of the third intracellular loop of the receptor for G protein coupling. By site-directed mutation of the human recombinant receptor expressed in Chinese hamster ovary cells, we identified two domains in this loop that contain clusters of basic residues conserved in most of the G-protein-coupled seven transmembrane domains receptors. We found that mutations in the proximal domain (K322) reduced the capability of VIP to increase adenylate cyclase activity without any change in the calcium response, whereas mutations in the distal part of the loop (R338, L339, R341) markedly reduced the calcium increase and Galphai coupling but only weakly the adenylate cyclase activity. Thus, the interaction of different G proteins with the VPAC1 receptor involves different receptor sub-domains.

Impairment of signal transduction in response to stimulation of the naturally occurring Pro279Leu variant of the h5-HT7(a) receptor

The aim of this study, performed in stably transfected HEK293 cells, was to investigate whether expression of the naturally occurring Pro279Leu variant of the h5-HT7(a) receptor (located in the third intracellular loop) is associated with changes in the pharmacological properties and/or second messenger formation compared to the wild-type receptor. Radioligand binding of [3H]5-carboxamidotryptamine ([3H]5-CT) to membranes and stimulation of [3H]cAMP formation in whole cells evoked by 5-HT receptor agonists were determined. Maximum binding (B(max)) to, and affinity (K(D)) of [3H]5-CT for, the variant receptor and the wild-type receptor were equal. All agonists and antagonists investigated exhibited no differences in affinity between the variant receptor and the wild-type receptor. However, the intrinsic activity of the 5-HT receptor agonists 5-HT, 5-CT, RU24969 and 8-OH-DPAT in stimulating [3H]cAMP accumulation in the cells expressing the Pro279Leu variant was almost abolished and their potency was 2.9-4.3-fold lower. Despite its affinity for both receptor isoforms, sumatriptan did not stimulate the accumulation of cAMP. In individuals expressing the Pro279Leu variant of the h5-HT7(a) receptor, a considerable attenuation of its function may be predicted. This may have relevance for the action of new classes of drugs which affect circadian rhythm or psychiatric diseases, such as schizophrenia.

Functional characterization of alternatively spliced 5-HT2 receptor isoforms from the pharynx and muscle of the parasitic nematode, Ascaris suum

Serotonin (5-HT) receptors play key regulatory roles in nematodes and alternatively spliced 5-HT2 receptor isoforms have been identified in the parasitic nematode, Ascaris suum. 5-HT2As1 and 5-HT2As2 contain different C-termini, and 5-HT2As1Delta4 lacks 42 amino acids at the C-terminus of the third intracellular loop. 5-HT2As1 and 5-HT2As2 exhibited identical pharmacological profiles when stably expressed in human embryonic kidney (HEK) 293 cells. Both 5-HT2As isoforms had higher affinity for 5-HT than their closely related Caenorhabditis elegans homolog (5-HT2Ce). This increased 5-HT affinity was not related to the substitution in 5-HT2As1 of F120 for Y in the highly conserved DRY motif found in the second intracellular loop of other 5-HT receptors, since a 5-HT2As1F120Y mutant actually exhibited increased 5-HT affinity compared with that of 5-HT2As1. As predicted, cells expressing either 5-HT2As1 or 5-HT2As2 exhibited a 5-HT-dependent increase in phosphatidylinositol (PI) turnover. In contrast, although 5-HT2As1Delta4 displayed a 10-fold higher affinity for 5-HT and 5-HT agonists than either 5-HT2As1 or 5-HT2As2, 5-HT2As1Delta4 did not couple to either PI turnover or adenyl cyclase activity. Based on RT-PCR, 5-HT2As1 and 5-HT2As2 were more highly expressed in pharynx and body wall muscle and 5-HT2As1Delta4 in nerve cord/hypodermis. This is the first report of different alternatively spliced 5-HT2 receptor isoforms from any system.

Basic charge clusters and predictions of membrane protein topology

The topology predictor SPLIT 4.0 (http://pref.etfos.hr) predicts the sequence location of transmembrane helices by performing an automatic selection of optimal amino acid attribute and corresponding preference functions. The best topological model is selected by choosing the highest absolute bias parameter that combines the bias in basic charge motifs and the bias in positive residues (the "positive inside rule") with the charge difference across the first transmembrane segment. Basic charge motifs, such as the BBB, BXXBB, and BBXXB motifs in alpha-helical integral membrane proteins, are significantly more frequent near cytoplasmic membrane surface than expected from the Arg/Lys (B) frequency. The predictor's accuracy is 99% for predicting 178 transmembrane helices in all membrane proteins or subunits of known 3D structure.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Cloning and expression analysis of a 5HT7-like serotonin receptor cDNA from mosquito Aedes aegypti female excretory and respiratory systems

In the mosquito Aedes aegypti, 5-HT changes the endogenous rhythm of contractions in the female hindgut and increases fluid secretion in the larval Malpighian tubule. The role of 5-HT as a diuretic hormone in adults has been questioned. We cloned a cDNA encoding a serotonin receptor from a female A. aegypti Malpighian tubule library that is similar to the 5-HT7 receptor from Drosophila melanogaster. The transcript was localized in the tracheolar cells associated with the female Malpighian tubules but no signal was detectable in the tubule epithelium. Immunohistochemistry with specific antibodies confirmed the receptor expression in tracheolar cells and hindgut, and western blots of these tissues showed the expected 50 kDa band. The results suggest a role for serotonin in respiration and that this receptor may coordinate the tubule-hindgut response to serotonin during diuresis.

Pharmacological characterization of the human 5-HT(4(d)) receptor splice variant stably expressed in Chinese hamster ovary cells

The recently identified C-terminal splice variant of the human 5-HT(4) receptor, the h5-HT(4(d)) receptor, was stably expressed in a CHO cell line at 493+/-25 fmol mg(-1) protein. We analysed its pharmacological properties by measuring binding affinities and 5-HT(4) ligand-induced cyclic AMP production. The pharmacological binding profile determined in competition studies with the specific antagonist [(3)H]-GR113808 revealed a rank order of affinity of 5-HT(4) ligands for the h5-HT(4(d)) receptor that was consistent with those previously reported for other 5-HT(4) receptor isoforms. In adenylyl cyclase functional assays, the h5-HT(4(d)) receptor displayed equipotent coupling for all 5-HT(4) agonists tested (EC(50) in the range of 1 - 6 nM). EC(50) values were lower than those previously obtained with the 5-HT(4(e)) receptor stably expressed in CHO cells indicating that the 5-HT(4(d)) receptor was more efficiently coupled to its effector than the 5-HT(4(e)) receptor isoform. Moreover, in terms of agonist efficacy (E(max)), the benzamide derivative, renzapride displayed full agonist properties at the h5-HT(4(d)) receptor (same E(max) as 5-HT) whereas it was previously shown to be a partial agonist at the h5-HT(4(e)) receptor. A constitutive activity of the h5-HT(4(d)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand. Surprisingly, two 5-HT(4) ligands, SB204070 and RS39604 which are described as highly potent antagonists in various biological models, revealed partial agonist properties at the h5-HT(4(d)) receptor. We conclude that C-terminal tails of 5-HT(4) receptor isoforms may directly influence their functional properties.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Alternative-splicing of serotonin receptor isoforms in the pharynx and muscle of the parasitic nematode, Ascaris suum

Pharyngeal pumping is essential for nematode feeding and survival and is dramatically stimulated by serotonin (5-HT). In the present study, a cDNA pool was prepared from poly A + RNA isolated from pharynxes dissected from the parasitic nematode, Ascaris suum, and was used as a template for RT-PCR with degenerate primers designed from sequences conserved in 5-HT receptors from a variety of sources. A putative 5-HT receptor cDNA (AS1) was identified which exhibited most identity to the 5-HT2 family of receptors. AS1 was 1925 nucleotides, did not appear to be trans-spliced and contained a 3' untranslated region of 127 nucleotides with a polyadenylation signal (ATTAAA) and a short poly A+ tail. The coding region predicted a protein of 532 amino acids with a molecular weight of 60 176. When AS1 was transiently expressed in COS-7 cells, isolated membranes exhibited the high affinity, saturable binding of [125I]LSD. More importantly, [125I]LSD binding was inhibited by 5-HT, but not other biogenic amines, supporting the identification of AS1 as a 5-HT receptor. Additional cDNAs identical, in part, to AS1 were also identified. AS1deltaIV lacked a predicted 42 amino acids at the carboxy terminus of the third intracellular loop, while AS2 and AS3 contained different COOH-termini, regions implicated in G-protein coupling in other heptahelical receptors. A portion of the gene (5htn) encoding AS1 also was cloned and sequenced. This genomic fragment was about 10 kb, contained the entire AS1 open reading frame and included eight exons and seven introns. From this analysis, it appears that these different AS cDNAs were generated by alternative-splicing, AS1deltaIV from the deletion of exon IV, and AS2 and AS3 from the use of alternative sites within exon VII as 5' splice acceptor sites for exon VIII. Using RT-PCR and primers specific for each of the isoforms, AS1 -3 appeared to be expressed in pharynx, while only AS1 and AS2 were present in body wall muscle. More importantly, the deletion of exon IV appeared to be associated exclusively with AS1 in pharynx and AS2 in muscle.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.