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The roles played by highly truncated splice variants of G protein-coupled receptors
Journal of Molecular Signaling volume 7, Article number: 13 (2012)
Alternative splicing of G protein-coupled receptor (GPCR) genes greatly increases the total number of receptor isoforms which may be expressed in a cell-dependent and time-dependent manner. This increased diversity of cell signaling options caused by the generation of splice variants is further enhanced by receptor dimerization. When alternative splicing generates highly truncated GPCRs with less than seven transmembrane (TM) domains, the predominant effect in vitro is that of a dominant-negative mutation associated with the retention of the wild-type receptor in the endoplasmic reticulum (ER). For constitutively active (agonist-independent) GPCRs, their attenuated expression on the cell surface, and consequent decreased basal activity due to the dominant-negative effect of truncated splice variants, has pathological consequences. Truncated splice variants may conversely offer protection from disease when expression of co-receptors for binding of infectious agents to cells is attenuated due to ER retention of the wild-type co-receptor. In this review, we will see that GPCRs retained in the ER can still be functionally active but also that highly truncated GPCRs may also be functionally active. Although rare, some truncated splice variants still bind ligand and activate cell signaling responses. More importantly, by forming heterodimers with full-length GPCRs, some truncated splice variants also provide opportunities to generate receptor complexes with unique pharmacological properties. So, instead of assuming that highly truncated GPCRs are associated with faulty transcription processes, it is time to reassess their potential benefit to the host organism.
G protein-coupled receptors (GPCRs) constitute the most abundant gene family in most animal species  and are the target of at least 50% of marketed drugs . The number of GPCRs varies widely among different species, with the elephant having almost three times as many genes as the 1265 found in humans . The question then is, “does this matter?” GPCRs are not only receptors for light and for biologically active chemicals, but also respond to stimuli associated with smell and taste. Therefore, the higher number of GPCR genes in the elephant may just reflect its greater reliance on its sense of smell. However, simply counting the number of genes in a genome greatly underestimates the number of gene products actually produced because of a process known as alternative splicing. By overlooking the number and variety of GPCR splice variants, we may misinterpret the fine-tuning options for controling cell signaling activity in health and disease. Furthermore, it has been proposed that GPCR oligomerization also provides a way to increase the number of receptor entities with a limited number of genes [4, 5], and even apparently non-functional highly-truncated splice variants of GPCRs can generate functionally active GPCRs by heterodimerization [6, 7]. To add further to this complexity of GPCR signaling, even though individual cell types typically express more than one hundred different GPCRs, the most highly expressed GPCRs are not necessarily those which have been targeted therapeutically . So even if splice variants are expressed less well than their wild-type counterparts, this does not automatically mean they are of little consequence. Because dysregulation of GPCR activity contributes to many different pathophysiological processes , we still need to carefully consider the specific role of each GPCR and its splice variants. Alternative splicing generates GPCRs with variations in specific structural domains, and in this review we will focus on the highly truncated splice variants which represent approximately 50% of all GPCR isoforms studied to date. We will show that generalizations assuming highly truncated GPCRs are non-functional may be an oversimplification, and we will discuss how GPCR dimerization between wild-type receptors and their splice variants has added a new level of regulation to the complex process of cell signaling. This review will also focus on some unique splice variants offering insights into the physiological role of these GPCR isoforms.
Alternative splicing of genes
Alternative splicing is a major mechanism for modulating the expression of genes and enables a single gene to increase its coding capacity, allowing the synthesis of several structurally and functionally distinct protein isoforms [9, 10]. In humans, a typical primary transcript, or precursor to mRNA (pre-mRNA), contains seven or eight introns and eight or nine exons, which together average more than 27,000 nucleotides in length . The removal of introns (pre-mRNA splicing) is carried out by spliceosomes, ribonucleoprotein complexes that recognize the exon-intron junctions and catalyse the removal of introns and subsequent joining of exons. There are several different types of alternative splicing [11, 12] which allows a single primary transcript to yield different mature RNAs; an example of intron retention is given in Figure 1. It is not unreasonable to assume that a large fraction of all human mutations affect splicing activity , and that the ratio of isoforms will ultimately affect normal cellular function. Any differences in the activities or amounts of general splicing factors and/or gene-specific splicing regulators during development or in different tissues will cause differential patterns of splicing. This means that transcriptome analysis, rather than genome analysis, is needed to assess the real impact of alternative splicing on human diseases . Splice variants do not just represent the end-products of gene transcription, they can also change the regulation of this process. For example, intron 4 of the human neuropeptide Y receptor (Y1) efficiently promotes the increased production of Y1 but this function is missing in the splice variant .
GPCR trafficking – ER retention of highly truncated splice variants
GPCRs are seven-transmembrane (7TM) domain receptors (Figure 1) which are trafficked through the biosynthetic pathway to the cell surface in a tightly regulated mechanism with multiple steps and a stringent quality control system to ensure correct GPCR folding and targeting. Association of GPCRs with accessory proteins or chaperones are a key step for the forward trafficking through the endoplasmic reticulum (ER) and Golgi . The life of GPCRs begins in the ER where they are synthesized, folded and assembled [16, 17]. During their migration to the cell surface, GPCRs undergo post-translational modifications to attain mature status. While knowledge of the structure-function relationship of GPCRs has been extended by recent high-resolution structural studies of β2-adrenoceptors  and muscarinic M2 receptors , little is known of features influencing overall stability of the highly truncated mutant GPCRs. In some cases, alternative splicing generates mutant transcripts which are simply too faulty to be expressed; splice variants of the human endothelin (ETB) receptor do not appear to be translated, or the products are quickly degraded, presumably because of their instability . Nevertheless, the preferential production of this null function ETB by RNA editing/splicing has been proposed to underlie the etiology of Hirshsprung disease , so even the most unstable GPCRs can impact on overall physiology.
Because the ER forms part of the cellular quality control machinery where functionally inactive mutant GPCRs can be prevented from expression at the cell surface , it is not unusual to find highly truncated GPCRs retained in the ER (see Table 1). Conditions such as X-linked nephrogenic-diabetes insipidus, familial hypocalciuric hypercalcemia, familial glucocorticoid deficiency or hypogonadodotropic hypogonadism are associated with mutations in GPCRs which result in intracellular retention in the ER or Golgi compartments . For this reason, alternative splicing which generates truncated GPCRs which are retained in the ER may likely to be associated with pathological conditions. Significant ER retention is increasingly seen as the norm because as much as 50% of all newly synthesized protein fails to meet ER exit quality control criteria . Much of the work in this field has highlighted the importance of C-terminal tails that can influence the efficiency of ER exit as well as the internalization and endosomal sorting of GPCRs . Examples are human gonadotrophin-releasing hormone (GnRH) receptors, luteinizing hormone/choriogonadotropin (LH/CG) receptors , rhodopsin  and vasopressin V2 receptors .
For the GnRH receptor, a single change in net charge is sufficient to tip the balance in favour of the ER and diminish GnRH receptor available at the plasma membrane . Conn et al. (2006)  have proposed that the apparent inefficiency of the GnRH receptor must have evolved under strong and convergent evolutionary pressure, suggesting there must be a strong advantage to generating an inefficiently produced receptor which is highly susceptible to cause a mutational disease. Whether such unexpected evolutionary pressure also exists for the ER-retained highly truncated GPCRs is unknown.
GPCR dimerization and its influence on GPCR trafficking
It is becoming apparent that many GPCRs form homo and/or hetero-dimers or higher order oligomers and that dimerization could both positively and negatively regulate GPCR cell surface targeting . But it is important to realize that some functional consequences that are proposed to originate from heteromeric receptor interactions may also be observed due to intracellular crosstalk between signaling pathways of non-associated GPCRs . Heterodimerization between different GPCR subtypes can significantly modify functional characteristics of the individual protomers, included subcellular localization, ligand binding co-operativity and proximal signaling .
That homodimerization was a prerequisite for cell surface targeting was first identified with the β2-adrenoceptor . A few examples of other GPCRs with constitutive dimers/oligomers which form during biosynthesis are: α1D-adrenoceptors (in complex with β2-adrenoceptors) ; GABAB receptors ; ghrelin receptors ; gonadotrophin hormone (LH/hCG and FSH) receptors [59, 60]; neurotensin NTS1 and NTS2 receptors ; oxytocin receptors ; and vasopressin V1a and V2 receptors . Collectively, most data suggests that receptor oligomers are preassembled in the ER and ‘walk hand-in-hand’ to the cell surface .
The impact of GPCR heterodimerization on cell surface targeting is perhaps best exemplified by GABAB receptors . The GABAB receptor (GABABR1) did not couple effectively to expected signaling pathways until co-expressed with GABABR2 which allowed GABAB1R to escape from the ER . Expression of the GABABR1 subunit on the cell surface was prevented through a C-terminal retention motif which needed to be masked by heterodimerization with GABABR2 . In contrast, the heterodimerization of α1A- and α1D-adrenoceptors primarily involves the hydrophobic core of these receptors as deletion of the C-terminal domains did not affect cell surface expression . While there is evidence for specific heterodimerization between wild-type GPCRs and highly truncated isoforms in the ER , the structural motifs responsible for heterodimerization with truncated splice variants and subsequent ER retention are unknown. It is currently possible to rescue GPCRs retained in the ER using chemical or pharmacological chaperones , and this presents an interesting approach to overcome the dominant-negative effect of the highly truncated GPCR splice variants.
Evidence of heterodimerization of full length GPCRs in vivo has been harder to obtain, , but examples are available associated with pathophysiological conditions: angiotensin-II (AT1) – bradykinin receptor complexes in preeclampsia ; the μ-opioid receptor and α2A-adrenoceptor complex in depression and opioid addiction ; the adenosine A2A – dopamine D2 receptor complex in Parkinson’s disease ; the κ-δ-opioid receptor complex in analgesia and drug abuse  and the metabotropic glutamate (mGlu2) – 5-hydroxytryptamine (5-HT2A) receptor complex in schizophrenia [72, 73]. As highly truncated splice variants are relatively hard to detect in vivo, definite proof of the existence of their heterodimerization with full length GPCRs is harder to obtain (see next section).
To address the issue of whether or not truncated GPCRs are functionally active, we need to consider if ER-retained GPCRs are capable of signaling activity. There is a growing body of evidence supporting the hypothesis that endocytosed receptors can activate specific signal transduction pathways . But what about signaling from the ER before trafficking of receptors to the cell surface? Studies with β2-adrenoceptor have shown that during its trafficking through the ER/Golgi, this GPCR is already pre-associated with its G proteins and effector enzyme (adenylyl cyclase 2) in a signaling complex . Another example of a GPCR with activity in the ER is the ghrelin receptor. Constitutive activation of extracellular signal-regulated kinases (ERK1/2) can be detected in the ER of HEK293 cells expressing ghrelin receptors . Both observations support the idea that GPCR signaling can be activated in the ER, but whether or not GPCR signaling in the ER is fundamentally different from GPCR signaling at the cell surface remains to be determined.
For non-constitutively active GPCRs, there is the issue of access of agonist to the receptor when it is retained in the ER. Nuclear metabotropic glutamate (mGlu5) receptors are orientated with their ligand binding domain inside the nucleoplasmic reticulum, and an active transport process is used so that glutamate can access their binding sites in this intracellular compartment . As the ER is contiguous with the nuclear membrane , it is possible that similar systems exist to facilitate cell signaling of other GPCRs from this intracellular location. For GPCRs such as the thyroid-stimulating hormone (TSH) receptor, activation of adenylyl cyclase in different subcellular compartments regulates different cellular functions . Ultimately, one should not necessarily assume that ER-retained GPCRs and their splice variants will be non-signaling, or that they will serve the same function as receptors on the cell surface.
Splice variants of GPCRs - Introduction
Previously, it was assumed that splice variants of ligand-gated channels were common, whereas each GPCR was far more likely to be a single product of its gene . Thus, initial estimates suggested that approximately 90% of mammalian GPCRs were intronless in their open reading frame, which compares with approximately 35% for other cell surface receptors . Access to additional databases and bioinformatics studies now indicates that perhaps only 50% of mammalian GPCR genes are intronless and do not require post-transcriptional processing . The relative absence of introns in human GPGRs could indicate that most GPCR genes were derived from a single intronless common progenitor relatively recently in evolutionary history . Because intronless genes do not require post-transcriptional splicing, we might expect them to be transcribed more efficiently , but this in turn would decrease diversity. After all, GPCR genes that do possess introns can undergo alternative splicing, generating GPCR subtype isoforms that may differ in their pharmacological, signaling and regulatory properties. Splice variants of GPCRs were often dismissed as the consequence of ‘leaky transcription’ and hence deemed physiologically irrelevant . Despite this, we see that splice variants of GPCRs are not uncommon, and that formation of heterodimers and/or retention of functionally-active GPCRs in different subcellular compartments greatly increases the complexity of GPCR signaling.
The largest group of splice variants per GPCR relates to the C-terminus [79, 82] but distinct differences can exist between human and rat/mouse orthologs such as the EP3 receptor , making it essential to understand the relationship between human and non-human GPCRs before inferring mechanisms based on non-human receptor studies. When genomic libraries were first prepared, the possibility of alternative gene splicing was generally ignored , with the first clear evidence in 1989 pertaining to the rat dopamine D2 receptor . GPCR splice variants often show differential distribution among many tissues and brain regions, consistent with cell-specific control of transcription and splicing , and alternative versions of a gene transcript might be necessary for different tissue types or at different stages of life . Thus, alternative splicing serves as a molecular tool to introduce more diversity into gene expression and this may have been generated as a more economical alternative to gene duplication during evolution .
Given that GPCRs can exist as dimers or oligomers, we might expect to observe dominant-negative effects in some heterozygous individuals, which could relate to defective routing of the complex formed between the wild-type and mutated receptors to the plasma membrane . Some clinical evidence described in obese patients with mutations in melanocortin (MC4) receptor supports this concept, but, with a few exceptions, expression of the disease in heterozygous individuals is usually mild or absent .
Alternative splicing of the GPCR superfamily in human airway smooth muscle has recently been demonstrated to diversify the complement of receptors . Out of the 434 GPCRs detected in airway smooth muscle, 192 GPCRs had, on average, five different expressed receptor isoforms. There was no apparent relationship between ‘wild-type’ expression levels and the occurrence of a particular splicing event. Of note in this study was the relatively low expression levels of GPCRs such as the M3-muscarinic receptor, β2-adrenoceptor, and receptors for leukotrienes and prostanoids which are already targeted therapeutically. Therefore, a low expression level of GPCRs does not necessarily equate to functional insignificance.
The dominant-negative effect of GPCR splice variants with altered or deleted transmembrane domains, generating highly truncated mutant receptors
The majority of truncated GPCR splice variants act as dominant-negative mutations, but there are always the exceptions, with truncated human somatostatin (sst5) variants being surprisingly functional . These sst5 variants maintain the same N-terminal region as full-length sst5 receptors, but have different, shorter C-terminal tails with 4, 2 or 1 TM domains (see Figure 2 and Table 1) [49, 50]. Similarly, mutant chemokine receptors (CCR5 and CXCR4) comprising merely the N-terminus and TM3 to TM7 domains, i.e., lacking TM1 and TM2 (Figure 2), can still function normally when expressed in HEK293 cells . So, the full 7TM domains are definitely not a prerequisite for effective cell signaling. But, if the GPCR had lost TM3, we would expect this to severely compromise GPCR signaling as the highly conserved E/DRY motif in the cytoplasmic surface of TM3 has been shown to be essential for coupling of the GPCRs such as vasopressin V2 receptors, α1B-adrenoceptors and angiotensin-II (AT1A) receptors to G proteins  by forming an ‘ionic lock’ with a glutamate residue in TM6 . Thus, the functional significance of specific TM domains is highly dependent on the particular GPCR.
For GPCRs with relatively large N-terminal ectodomains, alternative splicing can generate truncated receptors lacking the N-terminal domain. Cleaving the N-terminal domain from the remaining TM domains generates a more specific dominant-negative effect due to the released N-terminal domain (Figure 2). Hence, soluble non-membrane anchored ‘receptors’ are seen for the corticotrophin-releasing factor receptor (CRF2) , metabotropic glutamate (mGlu6) receptor  and the relaxin receptor (RXFP1) . These secreted proteins are proposed to act as biological modifiers of endogenous ligand-stimulated activity by mopping up endogenous ligand and preventing activation of the respective wild-type receptors.
Association of truncated GPCRs with pathophysiological conditions
Although there are some exceptions (see sst5 receptors above), splice variants resulting in truncated GPCRs of just 4 – 5 TM domains tends to generate functionally inactive mutant receptors. In Table 1 we have 22 examples of truncated GPCRs and the majority have no cell surface expression and no cell signaling activity. These mutant GPCRs are retained in an intracellular compartment, typically the ER and frequently function as dominant-negative mutants of the wild-type receptors. Retaining the wild-type receptors in the ER can have significant functional consequences, dependent on the constitutive role of the wild-type GPCR and/or its role in mediating responses to pathogenic organisms. For example, the chemokine receptor 5 (CCR5) functions as a co-receptor for HIV-1 infection, thus mutations allow for a protective role against HIV infection . The 5TM ccr5Δ32 splice variant complexes with and retains CCR5 in the ER, so the ccr5Δ32/ccr5Δ32 genotype has been linked with a phenotype that is “highly” protected from HIV-1 infection, whereas the CCR5/ccr5Δ32 genotype confers only “relative” protection . As seen with CCR5 and its 5TM splice variant, formation of specific heterodimers between truncated GPCRs and their associated wild-type receptors has been identified with the dopamine D3 receptor [32, 33], ghrelin receptor mutant polypeptide (GHS-R1b)  and histamine H4 receptor , but the association with a specific pathology remains speculative. The intracellular localization pattern of the other 5TM mutants listed in Table 1 strongly suggests they too form heterodimers with corresponding wild-type GPCRs resulting in decreased cell surface expression of these receptors. Unfortunately, the absence of antibodies capable of distinguishing wild-type and truncated GPCRs in vivo continues to hamper developments in this area.
While intracellular retention of wild-type GPCRs can arise by heterodimerization with 5TM mutants, there are other means to ultimately decrease expression of wild-type GPCRs. In schizophrenia, there is a decrease in dopamine D3 receptor mRNA but no change in 5TM D3nf mutant . The enhanced D3nf-specific splicing of D3 pre-mRNA in schizophrenia may lead to this decreased expression of D3 mRNA, and thus a decrease in D3 receptor protein . To generate D3nf mRNA by alternative splicing requires a rare splice event capable of recognising just 98 nucleotides as an alternative intron within a larger 2675 nucleotide exon . Despite its rarity, this splicing event clearly occurs.
The differential expression of truncated splice variants is perhaps the norm [40, 47, 49, 95, 96], but there are always exceptions. The somatostatin (sst5) receptor splice variants showed no marked differential expression, but they were differentially regulated by changes in hormone/metabolic environment in a tissue- and ligand-dependent manner . An altered hormone/metabolic environment resulting from a high-fat diet changes the ratio of expression of gastric inhibitory polypeptide (GIP) receptor and its 4TM mutant in mice . The relative reduction of truncated GIP receptor expression may be involved in the hypersensitivity of GIP and hyperinsulinemia in these obese mice. Similarly, the expression of luteinizing hormone (LH) receptor and its 5TM truncated splice variant (LHd) in the corpus luteum varies at different stages of the ovarian cycle . This profile of independent regulation for sst5, LH and GIP and their splice variants in response to different physiological conditions contrasts with the prostaglandin F2α (FP) receptor splice variants. FP receptor splice variants are differentially expressed in endothelial cells and in highly vascularised tissues , but no distinct role for PTGFR-v1 or PTGFR-v2 has been identified in relation to an altered cellular environment associated with pregnancy . Ultimately, the functional role of each pair of wild-type GPCR and its truncated splice variant has to be studied under both normal and pathophysiological conditions, but proving a specific role for these highly truncated receptors will remain problematic when there is insufficient sequence difference between splice variant and wild-type GPCR mRNA to selectively knock down the splice variant using siRNA techniques.
Association of truncated GPCRs with constitutively active counterparts
The ghrelin receptor (GHS-R1a) and histamine H3 and H4 receptors are constitutively active and therefore can function independent of agonist [39, 40, 97], and dominant-negative effects have been observed with their truncated splice variants (see Table 1). The human H4 splice variant is differentially expressed in CD34+ cord blood-derived eosinophils and mast cells, but the functional consequence of heterodimerization with wild-type H4 is presently unknown . For the ghrelin receptor, we see marked effects of its truncated 5TM splice variant (GHS-R1b) on constitutive activation of phospholipase C, with no effect on activation of ERK1/2 [36, 58, 98]. Bioluminescence resonance energy transfer (BRET) studies suggested that heterodimers of GHS-R1a/GHS-R1b are concentrated in the ER, whereas homodimers of GHS-R1a are more uniformly distributed throughout the cell . It has been proposed that conflicting findings concerning the role of ghrelin in different tissues could be linked to the presence of this ghrelin receptor polypeptide GHS-R1b [14, 99]. There is no correlation between the expression of GHS-R1a mRNA and GHS-R1b mRNA in different tissues , and the factors regulating the expression of ghrelin receptor isoforms are unknown. The constitutive activity of the ghrelin receptor attenuates apoptosis via a protein kinase C-dependent process in vitro, and because GHS-R1b has a dominant-negative effect on ghrelin receptor function, any changes which increase GHS-R1b relative to GHS-R1a would be predicted to have a functional effect.
For an individual receptor, it is unclear to what degree such ligand-independent receptor signaling is present in the in vivo situation and consequently whether the constitutive signaling is of physiological relevance . Some clues can be gained from other natural mutations of the ghrelin receptor and the melanocortin (MC4) receptor. Two families have been identified in which short stature is segregated with a GHSR mutation (Ala204Glu) that is characterized by substantially decreased basal activity of the receptor , possibly due to decreased expression in vivo. Of the individuals who were identified as heterozygous for this mutation, not all had short stature. This observation is compatible with codominant transmission of the trait, with incomplete penetrance of the phenotype . The constitutive activity of the MC4 receptor is maintained by its N-terminal domain, and mutations in the N-terminal lead to loss of basal activity and functional defects . Obesity-associated mutations in the N-terminal domain of MC4 decrease its constitutive activity, which suggests that in addition to the agonist-mediated activation of MC4, this constitutive activity is also required for the maintenance of the anorexigenic catabolic state and the prevention of obesity in humans . Together, these observations suggest that the physiological mechanisms controlled by ghrelin receptors and MC4 would be highly sensitive to the level of their basal activity. Since co-expression of ghrelin receptors and GHS-R1b in HEK293 cells dramatically decreases constitutive activation of phospholipase C [36, 98], and that this effect may be physiologically relevant, we have proposed that alternative splicing of genes for other constitutively active GPCRs will also dramatically affect cell signaling and functional activity.
Generation of receptor signaling complexes with altered pharmacology
Despite this dominant property of truncated GPCRs acting as dominant-negative mutants, there are examples where some functional activity is maintained. A 6TM μ-opioid receptor (MOR-1) variant in mice can identify ligands which lack the traditional side effects of classical opiates but maintain significant analgesic properties , and a 5TM somatostatin (sst2) receptor also retains functional activity and generates a receptor displaying an altered pharmacology . A large series of splice variants of μ-OR (MOR) have been isolated from mice, rats, and humans with similar splicing patterns . Most MOR-1 knock-out mice with a disruption of exon 1 were unresponsive to morphine, but mice with a series of MOR-1 variants generated from a second, upstream promoter associated with exon 11 had quite different pharmacology. These exon 11-associated variants lacking exon 1 are 6TM truncated variants, lacking the first TM domain of MOR-1 encoded by exon 1. One proposed partner for this inactive 6TM MOR-1 variant is the nociceptin (NOP) receptor . By partnering of the mutant MOR-1 with NOP, the resulting heterodimers have a unique pharmacology which may provide valuable therapeutic targets.
The ghrelin receptor polypeptide (GHS-R1b) is another example of a functionally inactive truncated GPCR which can generate novel pharmacology by heterodimerization with a related GPCR. In this example, GHS-R1b can heterodimerize with the neurotensin receptor 1 (NTS1) to provide a receptor capable of responding to neuromedin U . Treatment of non-small cell lung cancer cells with siRNA for GHS-R1b or NTS1 suppressed their growth in response to autocrine production of neuromedin U . Indeed, when the ghrelin receptor splice variant GHS-R1b was first discovered , there was a rash of papers profiling GHS-R1a and GHS-R1b mRNA expression in human tumours, attempting to link altered expression with the state of malignancy [105–110]. It might be fruitful instead to look for potential novel partners for truncated splice variants in cells and tissues where conventional molecular tools have failed to compliment pharmacological identification of receptor subtypes.
By analogy with GPCR heterodimerization as a means to expand the repertoire of cell signaling , we can see that the availability of splice variant protomers will similarly influence the cell signaling capacity of GPCRs. It is well established that the majority of GPCRs are desensitized, internalized and down-regulated by constant or repeated exposure to agonist. And, it is now established that trafficking of GPCRs to the cell surface is a highly regulated process, and heavily influenced by the expression of highly truncated splice variants. Is it possible that expression of these splice variants allows fine-tuning of these processes? Is it possible that activation of a GPCR ‘auto regulates’ expression of its spliced isoforms or regulates the expression of other GPCR isoforms? At the present time, it is premature to answer these intriguing questions. The expectation of dominant-negative mutants to inactivate cell signaling by retaining the functional wild-type GPCR in the ER is perhaps an oversimplification because retention in the ER does not necessarily mean a loss of cell signaling activity. Furthermore, splice variants with 5TM or 6TM domains can be functionally active and present unique pharmacology by forming heterodimers with 7TM domain receptors. We could start by looking for potential novel partners for truncated splice variants in cells and tissues where conventional molecular tools have failed to compliment pharmacological identification of receptor subtypes. Currently, the incidence and association of splice variants with disease is an area of intense interest as a means to better target GPCR-dependent therapies. However, the association between GPCR polymorphisms and clinical disease are currently too weak to allow clinically meaningful predictions of GPCR variants and their relationship to disease onset or progression, or in drug responses . Perhaps now we need to concentrate on treating each GPCR as a unique entity and thoroughly assess its activity profile in relation to the co-expression of splice variants in a cell-dependent and a time-dependent manner.
Schöneberg T, Hofreiter M, Schulz A, Römpler H: Learning from the past: evolution of GPCR functions. Trends Pharmacol Sci 2007, 28:117–121.
Pierce KL, Premont RT, Lefkowitz RJ: Seven-transmembrane receptors. Nat Rev Mol Cell Biol 2002, 3:639–650.
Insel PA, Snead A, Murray F, Zhang L, Yokouchi H, Katakia T, et al.: GPCR expression in tissues and cells: Are the optimal receptors being used as drug targets? Br J Pharmacol 2012, 165:1613–1616.
Park PSH, Palczewski K: Diversifying the repertoire of G protein-coupled receptors through oligomerization. Proc Natl Acad Sci USA 2005, 102:8793–8794.
Prinster SC, Holmqvist TG, Hall RA: α 2C -Adrenergic receptors exhibit enhanced surface expression and signaling upon association with β 2 -adrenergic receptors. J Pharmacol Exp Ther 2006, 318:974–981.
Majumdar S, Grinnell S, Le Rouzic V, Burgman M, Polikar L, Ansonoff M, et al.: Truncated G protein-coupled mu opioid receptor MOR-1 splice variants are targets for highly potent opioid analgesics lacking side effects. Proc Natl Acad Sci USA 2011, 108:19778–19783.
Takahashi K, Furukawa C, Takano A, Ishikawa N, Kato T, Hayama S, et al.: The neuromedin U-growth hormone secretagogue receptor 1b/neurotensin receptor 1 oncogenic signaling pathway as a therapeutic target for lung cancer. Cancer Res 2006, 66:9408–9419.
Magalhaes AC, Dunn H, Ferguson SS: Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins. Br J Pharmacol 2012, 165:1717–1736.
Cáceres JF, Kornblihtt AR: Alternative splicing: multiple control mechanisms and involvement in human disease. Trends Genet 2002, 18:186–193.
International Human Genome Sequencing Consortium: Initial sequencing and analysis of the human genome. Nature 2001, 409:860–921.
Garcia-Blanco MA, Baraniak AP, Lasda EL: Alternative splicing in disease and therapy. Nat Biotech 2004, 22:535–546.
Smith DJ, Query CC, Konarska MM: "Nought may endure but mutability": Spliceosome dynamics and the regulation of splicing. Mol Cell 2008, 30:657–666.
Marklaund U, Byström M, Gedda K, Larefalk Å, Junebald K, Nyström S, et al.: Intron-mediated expression of the human neuropeptide Y Y1 receptor. Mol Cell Endocrinol 2002, 188:85–97.
Howard AD, Feighner SD, Cully DF, Arena JP, Liberator PA, Rosenblum CI, et al.: A receptor in pituitary and hypothalamus that functions in growth hormone release. Science 1996, 273:974–977.
Jean-Alphonse F, Hanyaloglu AC: Regulation of GPCR signal networks via membrane trafficking. Mol Cell Endocrinol 2010, 331:205–214.
Duvernay MT, Filipeanu CM, Wu G: The regulatory mechanisms of export trafficking of G protein-coupled receptors. Cell Signal 2005, 17:1457–1465.
Tan CM, Brady AE, Nickols HH, Wang Q, Limbird LE: Membrane trafficking of G protein-coupled receptors. Annu Rev Pharmacol Toxicol 2004, 44:559–609.
Rasmussen SGF, DeVree BT, Zou Y, Kruse AC, Chung KY, Kobilka TS, et al.: Crystal structure of the β2 adrenergic receptor-Gs protein complex. Nature 2011, 477:549–555.
Haga K, Kruse AC, Asada H, Yurugi-Kobayashi T, Shiroishi M, Zhang C, et al.: Structure of the human M2 muscarinic acetylcholine receptor bound to an antagonist. Nature 2012, 482:547–551.
Tanoue A, Koshimizu T, Tsuchiya M, Ishii K, Osawa M, Saeki M, et al.: Two novel transcripts for human endothelin B receptor produced by RNA editing/alternative splicing from a single gene. J Biol Chem 2002, 277:33205–33212.
Salahpour A, Angers S, Mercier JF, Lagacé M, Marullo S, Bouvier M: Homodimerization of the β2-adrenergic receptor as a prerequisite for cell surface targeting. J Biol Chem 2004, 279:33390–33397.
Conn PM, Ulloa-Aguirre A, Ito J, Janovick JA: G protein-coupled receptor trafficking in health and disease: lessons learned to prepare for therapeutic mutant rescue in vivo. Pharmacol Rev 2007, 59:225–250.
Ellgaard L, Molinari M, Helenius A: Setting the standards: quality control in the secretory pathway. Science 1999, 286:1882–1888.
Hanyaloglu AC, Von Zastrow M: Regulation of GPCRs by membrane trafficking and its potential implications. Annu Rev Pharmacol Toxicol 2008, 48:537–568.
Rodriguez MC, Xie YB, Wang H, Collison K, Segaloff DL: Effects of truncations of the cytoplasmic tail of the luteinizing hormone/chorionic gonadotropin receptor on receptor-mediated hormone internalization. Mol Endocrinol 1992, 6:327–336.
Tai AW, Chuang JZ, Bode C, Wolfrum U, Sung CH: Rhodopsin's carboxy-terminal cytoplasmic tail acts as a membrane receptor for cytoplasmic dynein by binding to the dynein light chain Tctex-1. Cell 1999, 97:877–887.
Oksche A, Dehe M, Schülein R, Wiesner B, Rosenthal W: Folding and cell surface expression of the vasopressin V2 receptor: requirement of the intracellular C-terminus. FEBS Lett 1998, 424:57–62.
Cogé F, Guenin SP, Renouard-Try A, Rique H, Ouvry C, Fabry N, et al.: Truncated isoforms inhibit [ 3 H]prazosin binding and cellular trafficking of native human α 1A -adrenoceptors. Biochem J 1999, 343:231–239.
Seck T, Pellegrini M, Florea AM, Grignoux V, Baron R, Mierke DF, et al.: The delta e13 isoform of the calcitonin receptor forms a six-transmembrane domain receptor with dominant-negative effects on receptor surface expression and signaling. Mol Endocrinol 2005, 19:2132–2144.
Benkirane M, Jin DY, Chun RF, Koup RA, Jeang KT: Mechanism of transdominant inhibition of CCR5-mediated HIV-1 infection by cc5Δ32. J Biol Chem 1997, 272:30603–30606.
Liu R, Paxton WA, Choe S, Ceradini D, Martin SR, Horuk R, et al.: Homozygous defect in HIV-1 coreceptor accounts for resistance of some multiply-exposed individuals to HIV-1 infection. Cell 1996, 86:367–377.
Karpa KD, Lin R, Kabbani N, Levenson R: The dopamine D3 receptor interacts with itself and the truncated D3 splice variant D3nf: D3-D3nf interaction causes mislocalization of D3 receptors. Mol Pharmacol 2000, 58:677–683.
Elmhurst JL, Xie Z, O'Dowd BF, George SR: The splice variant D3nf reduces ligand binding to the D3 dopamine receptor: evidence for heterooligomerization. Mol Brain Res 2000, 80:63–74.
Isomoto S, Kaibara M, Sakurai-Yamashita Y, Nagayama Y, Uezono Y, Yano K, et al.: Cloning and tissue distribution of novel splice variants of the rat GABAB receptor. Biochem Biophys Res Commun 1998, 253:10–15.
Harada N, Yamada Y, Tsukiyama K, Yamada C, Nakamura Y, Mukai E, et al.: A novel GIP receptor splice variant influences GIP sensitivity of pancreatic β-cells in obese mice. Am J Physiol Endocrinol Metab 2008, 294:E61-E68.
Leung PK, Chow KBS, Lau PN, Chu KM, Chan CB, Cheng CHK, et al.: The truncated ghrelin receptor polypeptide (GHS-R1b) acts as a dominant-negative mutant of the ghrelin receptor. Cell Signal 2007, 19:1011–1022.
Grosse R, Schöneberg T, Schultz G, Gudermann T: Inhibition of gonandotropin-releasing hormone receptor signaling by expression of a splice variant of the human receptor. Mol Endocrinol 1997, 11:1305–1318.
Motomura T, Hashimoto K, Koga M, Arita N, Hayakawa T, Kishimoto T, et al.: Inhibition of signal transduction by a splice variant of the growth hormone−releasing hormone receptor expressed in human pituitary adenomas. Metabolism 1998, 47:804–808.
Bakker RA, Lozada AF, van Marle A, Shenton FC, Drutel G, Karlstedt K, et al.: Discovery of naturally occurring splice variants of the rat histamine H3 receptor that act as dominant-negative isoforms. Mol Pharmacol 2006, 69:1194–1206.
van Rijn RM, van Marle A, Chazot PL, Langemeijer E, Qin Y, Shenton FC, et al.: Cloning and characterization of dominant negative splice variants of the human histamine H4 receptor. Biochem J 2008, 414:121–131.
Einstein R, Jordan H, Zhou W, Brenner M, Moses EG, Liggett SB: Alternative splicing of the G protein-coupled receptor superfamily in human airway smooth muscle diversifies the complement of receptors. Proc Natl Acad Sci USA 2008, 105:5230–5235.
Osuga Y, Hayashi M, Kudo M, Contri M, Kobilka BK, Hsueh AJW: Co-expression of defective luteinizing hormone receptor fragments partially reconstitutes ligand-induced signal generation. J Biol Chem 1997, 272:25006–25012.
Feighner SD, Tan CP, McKee KK, Palyha OC, Hreniuk DL, Pong S-S, et al.: Receptor for motilin identified in the human gastrointestinal system. Science 1999, 284:2184–2188.
Bellucci F, Meini S, Catalioto RM, Catalani C, Giuliani S, Quartara L, et al.: Pharmacological evaluation of α and β human tachykinin NK2 receptor splice variants expressed in CHO cells. Eur J Pharmacol 2004, 499:229–238.
Perron A, Sarret P, Gendron L, Stroh T, Beaudet A: Identification and functional characterization of a 5-transmembrane domain variant isoform of the NTS2 neurotensin receptor in rat central nervous system. J Biol Chem 2005, 280:10219–10227.
Xie G, Ito E, Maruyama K, Pietruck C, Sharma M, Yu LC, et al.: An alternatively spliced transcript of the rat nociceptin receptor ORL1 gene encodes a truncated receptor. Mol Brain Res 2000, 77:1–9.
Vielhauer GA, Fujino H, Regan JW: Cloning and localization of hFP S : a six-transmembrane mRNA splice variant of the human FP prostanoid receptor. Arch Biochem Biophys 2004, 421:175–185.
Hay A, Wood S, Olson D, Slater DM: Labour is associated with decreased expression of the PGF 2α receptor (PTGFR) and a novel PTGFR splice variant in human myometrium but not decidua. Mol Human Reprod 2010, 16:752–760.
Durán-Prado M, Gahete MD, Martínez-Fuentes AJ, Luque RM, Quintero A, Webb SM, et al.: Identification and characterization of two novel truncated but functional isoforms of the somatostatin receptor subtype 5 differentially present in pituitary tumors. J Clin Endocrinol Metab 2009, 94:2634–2643.
Córdoba-Chacón J, Gahete M, Duran-Prado M, Pozo-Salas A, Malagón M, Gracia-Navarro F, et al.: Identification and characterization of new functional truncated variants of somatostatin receptor subtype 5 in rodents. Cell Mol Life Sci 2010, 67:1147–1163.
Gonzalez A, Borquez M, Trigo CA, Brenet M, Sarmiento JM, Figueroa CD, et al.: The splice variant of the V2 vasopressin receptor adopts alternative topologies. Biochemistry 2011, 50:4981–4986.
Conn PM, Knollman PE, Brothers SP, Janovick JA: Protein folding as posttranslational regulation: evolution of a mechanism for controlled plasma membrane expression of a G protein-coupled receptor. Mol Endocrinol 2006, 20:3035–3041.
Milligan G: The role of dimerisation in the cellular trafficking of G-protein-coupled receptors. Curr Opin Pharmacol 2010, 10:23–29.
Vischer HF, Watts AO, Nijmeijer S, Leurs R: G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand? Br J Pharmacol 2011, 163:246–260.
Milligan G: G protein-coupled receptor dimerization: function and ligand pharmacology. Mol Pharmacol 2004, 66:1–7.
Uberti MA, Hague C, Oller H, Minneman KP, Hall RA: Heterodimerization with β2-adrenergic receptors promotes surface expression and functional activity of α1D-adrenergic receptors. J Pharmacol Exp Ther 2005, 313:16–23.
Balasubramanian S, Teissére JA, Raju DV, Hall RA: Hetero-oligomerization between GABA A and GABA B receptors regulates GABA B receptor trafficking. J Biol Chem 2004, 279:18840–18850.
Chow KBS, Sun J, Chu KM, Cheung WT, Cheng CHK, Wise H: The truncated ghrelin receptor polypeptide (GHS-R1b) is localized in the endoplasmic reticulum where it forms heterodimers with ghrelin receptors (GHS-R1a) to attenuate their cell surface expression. Mol Cell Endocrinol 2012, 348:247–254.
Guan R, Feng X, Wu X, Zhang M, Zhang X, Hébert TE, et al.: Bioluminescence resonance energy transfer studies reveal constitutive dimerization of the human lutropin receptor and a lack of correlation between receptor activation and the propensity for dimerization. J Biol Chem 2009, 284:7483–7494.
Guan R, Wu X, Feng X, Zhang M, Hébert TE, Segaloff DL: Structural determinants underlying constitutive dimerization of unoccupied human follitropin receptors. Cell Signal 2010, 22:247–256.
Perron A, Sharif N, Sarret P, Stroh T, Beaudet A: NTS2 modulates the intracellular distribution and trafficking of NTS1 via heterodimerization. Biochem Biophys Res Commun 2007, 353:582–590.
Terrillon S, Durroux T, Mouillac B, Breit A, Ayoub MA, Taulan M, et al.: Oxytocin and vasopressin V1a and V2 receptors form constitutive homo- and heterodimers during biosynthesis. Mol Endocrinol 2003, 17:677–691.
Marshall FH, Jones KA, Kaupmann K, Bettler B: GABA B receptors - the first 7TM heterodimers. Trends Pharmacol Sci 1999, 20:396–399.
Margeta-Mitrovic M, Jan YN, Jan LY: A trafficking checkpoint controls GABA B receptor heterodimerization. Neuron 2000, 27:97–106.
Hague C, Uberti MA, Chen Z, Hall RA, Minneman KP: Cell surface expression of α 1D -adrenergic receptors is controlled by heterodimerization with α 1B -adrenergic receptors. J Biol Chem 2004, 279:15541–15549.
Bernier V, Lagace M, Bichet DG, Bouvier M: Pharmacological chaperones: potential treatment for conformational diseases. Trends Endocrinol & Metabol 2004, 15:222–228.
Pin JP, Neubig R, Bouvier M, Devi L, Filizola M, Javitch JA, et al.: International Union of Basic and Clinical Pharmacology. LXVII. Recommendations for the recognition and nomenclature of G protein-coupled receptor heteromultimers. Pharmacol Rev 2007, 59:5–13.
AbdAlla S, Lother H, El Massiery A, Quitterer U: Increased AT1 receptor heterodimers in preeclampsia mediate enhanced angiotensin II responsiveness. Nat Med 2001, 7:1003–1009.
Jordan BA, Gomes I, Rios C, Filipovska J, Devi LA: Functional interactions between μ opioid and α 2A -adrenergic receptors. Mol Pharmacol 2003, 64:1317–1324.
Ferre S, von Euler G, Johansson B, Fredholm BB, Fuxe K: Stimulation of high-affinity adenosine A 2 receptors decreases the affinity of dopamine D 2 receptors in rat striatal membranes. Proc Natl Acad Sci USA 1991, 88:7238–7241.
Jordan BA, Devi LA: G-protein-coupled receptor heterodimerization modulates receptor function. Nature 1999, 399:697–700.
Gonzalez-Maeso J, Ang RL, Yuen T, Chan P, Weisstaub NV, Lopez-Gimenez JF, et al.: Identification of a serotonin/glutamate receptor complex implicated in psychosis. Nature 2008, 452:93–97.
Fribourg M, Moreno JL, Holloway T, Provasi D, Baki L, Mahajan R, et al.: Decoding the signaling of a GPCR heteromeric complex reveals a unifying mechanism of action of antipsychotic drugs. Cell 2011, 147:1011–1023.
Sorkin A, Von Zastrow M: Endocytosis and signaling: intertwining molecular networks. Nat Rev Mol Cell Biol 2009, 10:609–622.
Dupré DJ, Baragli A, Rebois RV, Éthier N, Hébert TE: Signaling complexes associated with adenylyl cyclase II are assembled during their biosynthesis. Cell Signal 2007, 19:481–489.
Kumar V, Jong Y-JI, O'Malley KL: Activated nuclear metabotropic glutamate receptor mGlu5 couples to nuclear G q/11 proteins to generate inositol 1,4,5-trisphosphate-mediated nuclear Ca 2+ release. J Biol Chem 2008, 283:14072–14083.
Du Y, Ferro-Novick S, Novick P: Dynamics and inheritance of the endoplasmic reticulum. J Cell Sci 2004, 117:2871–2878.
Calebiro D, Nikolaev VO, Persani L, Lohse MJ: Signaling by internalized G-protein-coupled receptors. Trends Pharmacol Sci 2010, 31:221–228.
Minneman KP: Splice variants of G protein-coupled receptors. Mol Interv 2001, 1:108–116.
Gentles AJ, Karlin S: Why are human G-protein-coupled receptors predominantly intronless? Trends Genet 1999, 15:47–49.
Markovic D, Challiss R: Alternative splicing of G protein-coupled receptors: physiology and pathophysiology. Cell Mol Life Sci 2009, 66:3337–3352.
Kilpatrick GJ, Dautzenberg FM, Martin GR, Eglen RM: 7TM receptors: the splicing on the cake. Trends Pharmacol Sci 1999, 20:294–301.
Kotani M, Tanaka I, Ogawa Y, Usui T, Mori K, Ichikawa A, et al.: Molecular cloning and expression of multiple isoforms of human prostaglandin E receptor EP 3 subtype generated by alternative messenger RNA splicing: multiple second messenger systems and tissue-specific distributions. Mol Pharmacol 1995, 48:869–879.
Monsma FJ, McVittie LD, Gerfen CR, Mahan LC, Sibley DR: Multiple D 2 dopamine receptors produced by alternative RNA splicing. Nature 1989, 342:926–929.
Vassart G, Costagliola S: G protein-coupled receptors: mutations and endocrine diseases. Nat Rev Endocrinol 2011, 7:362–372.
Biebermann H, Krude H, Elsner A, Chubanov V, Gudermann T, Grüters A: Autosomal-dominant mode of inheritance of a melanocortin-4 receptor mutation in a patient with severe early-onset obesity is due to a dominant-negative effect caused by receptor dimerization. Diabetes 2003, 52:2984–2988.
Ling K, Wang P, Zhao J, Wu YL, Cheng ZJ, Wu GX, et al.: Five-transmembrane domains appear sufficient for a G protein-coupled receptor: Functional five-transmembrane domain chemokine receptors. Proc Natl Acad Sci USA 1999, 96:7922–7927.
Wilbanks AM, Laporte SA, Bohn LM, Barak LS, Caron MG: Apparent loss-of-function mutant GPCRs revealed as constitutively desensitized receptors. Biochemistry 2002, 41:11981–11989.
Rosenbaum DM, Rasmussen SGF, Kobilka BK: The structure and function of G-protein-coupled receptors. Nature 2009, 459:356–363.
Chen AM, Perrin MH, DiGruccio MR, Vaughan JM, Brar BK, Arias CM, et al.: A soluble mouse brain splice variant of type 2α corticotropin-releasing factor (CRF) receptor binds ligands and modulates their activity. Proc Natl Acad Sci USA 2005, 102:2620–2625.
Valerio A, Ferraboli S, Paterlini M, Spano PF, Barlati S: Identification of novel alternatively-spliced mRNA isoforms of metabotropic glutamate receptor 6 gene in rat and human retina. Gene 2001, 262:99–106.
Scott DJ, Tregear GW, Bathgate RAD: LGR7-truncate is a splice variant of the relaxin receptor LGR7 and is a relaxin antagonist in vitro. Ann N Y Acad Sci 2005, 1041:22–26.
Schmauss C, Haroutunian V, Davis KL, Davidson M: Selective loss of dopamine D 3 -type receptor mRNA expression in parietal and motor cortices of patients with chronic schizophrenia. Proc Natl Acad Sci USA 1993, 90:8942–8946.
Schmauss C: Enhanced cleavage of an atypical intron of dopamine D 3 -receptor pre-mRNA in chronic schizophrenia. J Neurosci 1996, 16:7902–7909.
Chan CB, Cheng CHK: Identification and functional characterization of two alternatively spliced growth hormone secretagogue receptor transcripts from the pituitary of black seabream Acanthopagrus schlegeli . Mol Cell Endocrinol 2004, 214:81–95.
Dickinson RE, Stewart AJ, Myers M, Millar RP, Duncan WC: Differential expression and functional characterization of luteinizing hormone receptor splice variants in human luteal cells: implications for luteolysis. Endocrinology 2009, 150:2873–2881.
Holst B, Cygankiewicz A, Jensen TH, Ankersen M, Schwartz TW: High constitutive signaling of the ghrelin receptor - identification of a potent inverse agonist. Mol Endocrinol 2003, 17:2201–2210.
Chu KM, Chow KBS, Leung PK, Lau PN, Chan CB, Cheng CHK, et al.: Over-expression of the truncated ghrelin receptor polypeptide attenuates the constitutive activation of phosphatidylinositol-specific phospholipase C by ghrelin receptors but has no effect on ghrelin-stimulated extracellular signal-regulated kinase 1/2 activity. Int J Biochem Cell Biol 2007, 39:752–764.
Inoue H, Sakamoto Y, Kangawa N, Kimura C, Ogata T, Fujieda K, et al.: Analysis of expression and structure of the rat GH-secretagogue/ghrelin receptor ( Ghsr ) gene: Roles of epigenetic modifications in transcriptional regulation. Mol Cell Endocrinol 2011, 345:1–15.
Lau PN, Chow KB, Chan CB, Cheng CH, Wise H: The constitutive activity of the ghrelin receptor attenuates apoptosis via a protein kinase C-dependent pathway. Mol Cell Endocrinol 2009, 299:232–239.
Holst B, Mokrosinski J, Lang M, Brandt E, Nygaard R, Frimurer TM, et al.: Identification of an efficacy switch region in the ghrelin receptor responsible for interchange between agonism and inverse agonism. J Biol Chem 2007, 282:15799–15811.
Pantel J, Legendre M, Cabrol S, Hilal L, Hajaji Y, Morisset S, et al.: Loss of constitutive activity of the growth hormone secretagogue receptor in familial short stature. J Clin Invest 2006, 116:760–768.
Liu G, Fortin JP, Beinborn M, Kopin AS: Four missense mutations in the ghrelin receptor result in distinct pharmacological abnormalities. J Pharmacol Exp Ther 2007, 322:1036–1043.
Srinivasan S, Lubrano-Berthelier C, Govaerts C, Picard F, Santiago P, Conklin BR, et al.: Constitutive activity of the melanocortin-4 receptor is maintained by its N-terminal domain and plays a role in energy homeostasis in humans. J Clin Invest 2004, 114:1158–1164.
Korbonits M, Bustin SA, Kojima M, Jordan S, Adams EF, Lowe DG, et al.: The expression of the growth hormone secretagogue receptor ligand ghrelin in normal and abnormal human pituitary and other neuroendocrine tumors. J Clin Endocrinol Metab 2001, 86:881–887.
Jeffery PL, Herington AC, Chopin LK: Expression and action of the growth hormone releasing peptide ghrelin and its receptor in prostate cancer cell lines. J Endocrinol 2002, 172:R7-R11.
Volante M, Allia E, Gugliotta P, Funaro A, Broglio F, Deghenghi R, et al.: Expression of ghrelin and of the GH secretagogue receptor by pancreatic islet cells and related endocrine tumors. J Clin Endocrinol Metab 2002, 87:1300–1308.
Duxbury MS, Waseem T, Ito H, Robinson MK, Zinner MJ, Ashley SW, et al.: Ghrelin promotes pancreatic and adenocarcinoma cellular proliferation and invasiveness. Biochem Biophys Res Commun 2003, 309:464–468.
Arnaldi G, Mancini T, Kola B, Appolloni G, Freddi S, Concettoni C, et al.: Cyclical Cushing's syndrome in a patient with a bronchial neuroendocrine tumor (typical carcinoid) expressing ghrelin and growth hormone secretagogue receptors. J Clin Endocrinol Metab 2003, 88:5834–5840.
Gaytan F, Barreiro ML, Caminos JE, Chopin LK, Herington A, Morales C, et al.: Expression of ghrelin and its functional receptor, the type 1a growth hormone secretagogue receptor, in normal human testis and testicular tumors. J Clin Endocrinol Metab 2004, 89:400–409.
Rozenfeld R, Devi LA: Receptor heteromerization and drug discovery. Trends Pharmacol Sci 2010, 31:124–130.
Insel PA, Tang CM, Hahntow I, Michel MC: Impact of GPCRs in clinical medicine: Monogenic diseases, genetic variants and drug targets. Biochim Biophys Acta 2007, 1768:994–1005.
The author declares that they have no competing interests.
HW is the sole author.
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Wise, H. The roles played by highly truncated splice variants of G protein-coupled receptors. J Mol Signal 7, 13 (2012). https://doi.org/10.1186/1750-2187-7-13
- Alternative splicing
- Cell signaling
- G protein-coupled receptors
- Truncated splice variants