Molecular characterization of a second mouse pancreatic polypeptide receptor and its inactivated human homologue

Illuminating the neuropeptide Y4 receptor and its ligand pancreatic polypeptide from a structural, functional, and therapeutic perspective

The neuropeptide Y4 receptor (Y4R), a rhodopsin-like G protein-coupled receptor (GPCR) and the hormone pancreatic polypeptide (PP) are members of the neuropeptide Y family consisting of four receptors (Y1R, Y2R, Y4R, Y5R) and three highly homologous peptide ligands (neuropeptide Y, peptide YY, PP). In this family, the Y4R is of particular interest as it is the only subtype with high affinity to PP over NPY. The Y4R, as a mediator of PP signaling, has a pivotal role in appetite regulation and energy homeostasis, offering potential avenues for the treatment of metabolic disorders such as obesity. PP as anorexigenic peptide is released postprandial from the pancreas in response to food intake, induces satiety signals and contributes to hamper excessive food intake. Moreover, this system was also described to be associated with different types of cancer: overexpression of Y4R have been found in human adenocarcinoma cells, while elevated levels of PP are related to the development of pancreatic endocrine tumors. The pharmacological relevance of the Y4R advanced the search for potent and selective ligands for this receptor subtype, which will be significantly progressed through the elucidation of the active state PP-Y4R cryo-EM structure. This review summarizes the development of novel PP-derived ligands, like Obinepitide as dual Y2R/Y4R agonist in clinical trials or UR-AK86c as small hexapeptide agonist with picomolar affinity, as well as the first allosteric modulators that selectively target the Y4R, e.g. VU0506013 as potent Y4R positive allosteric modulator or (S)-VU0637120 as allosteric antagonist. Here, we provide valuable insights into the complex physiological functions of the Y4R and PP and the pharmacological relevance of the system in appetite regulation to open up new avenues for the development of tool compounds for targeted therapies with potential applications in metabolic disorders.

Structure-Activity Relationship Study of the High-Affinity Neuropeptide Y4 Receptor Positive Allosteric Modulator VU0506013

Positive allosteric modulators targeting the Y₄ receptor (Y₄R), a G protein-coupled receptor (GPCR) involved in the regulation of satiety, offer great potential in anti-obesity research. In this study, we selected 603 compounds by using quantitative structure-activity relationship (QSAR) models and tested them in high-throughput screening (HTS). Here, the novel positive allosteric modulator (PAM) VU0506013 was identified, which exhibits nanomolar affinity and pronounced selectivity toward the Y₄R in engineered cell lines and mouse descending colon mucosa natively expressing the Y₄R. Based on this lead structure, we conducted a systematic SAR study in two regions of the scaffold and presented a series of 27 analogues with modifications in the N- and C-terminal heterocycles of the molecule to obtain insight into functionally relevant positions. By mutagenesis and computational docking, we present a potential binding mode of VU0506013 in the transmembrane core of the Y₄R. VU0506013 presents a promising scaffold for developing in vivo tools to move toward anti-obesity drug research focused on the Y₄R.

Prognostic and Functional Analysis of NPY6R in Uveal Melanoma Using Bioinformatics

Neuropeptides can mediate tumor cell proliferation and differentiation through autocrine, paracrine, neurosecretory, and endocrine mechanisms. This study investigated the expression and prognostic significance of neuropeptide Y receptor Y6 (NPY6R) in uveal melanoma (UVM) and preliminarily investigated the biological function of NPY6R in UVM. NPY6R was poorly expressed in most tumors and was associated with better prognosis in UVM. Among the clinicopathological features of UVM, NPY6R expression was lower in male patients. The area under the curve (AUC) value of NPY6R for the diagnosis of UVM was 0.676 (95% CI: 0.556–0.795). A nomogram including four clinical predictors was constructed. NPY6R expression was significantly associated with features of the UVM immune microenvironment. ESTIMATE and CIBERSORT algorithms were used to calculate the fraction of immune cells and the percentage of infiltration in each patient, respectively. NPY6R expression-related gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and gene set enrichment analyses were performed. GO and KEGG enrichment analyses revealed that NPY6R-related genes are mainly enriched in pathways and functions related to visual light perception. Gene set enrichment analysis suggested that NPY6R is associated with tumor progression in UVM. NPY6R is involved in the tumor progression of UVM and has a good predictive value as a prognostic marker of UVM.

Differential microRNA expression profile of Trichinella spiralis larvae after exposure to the host small intestinal milieu

The process by which Trichinella spiralis muscle larvae are activated to infect the intestine after exposure to the host small intestinal milieu is crucial for the successful establishment of T. spiralis infection. However, the molecular mechanism underlying the invasion of intestinal epithelial cells by T. spiralis has not been elucidated. MicroRNAs are a class of small noncoding RNAs that participate in parasite growth and development, pathogenic processes, and host-parasite interactions. In the present study, the differential expression profile of miRNAs in T. spiralis after exposure to the mouse small intestinal milieu was analysed using Solexa high-throughput sequencing technology. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to analyse the functions of miRNA target genes, and dual-luciferase reporter assays were subsequently applied to test the regulatory effects of one significantly decreased miRNA (let-7) on its four predicted target genes. In total, 2,000 known miRNAs (930 upregulated and 1070 downregulated) and 43 novel miRNAs (22 upregulated and 21 downregulated) were found to be differentially expressed in intestinal larvae, compared with muscle larvae. The KEGG pathway analysis showed that the predicted target genes of differentially expressed miRNAs were involved in 299 different pathways, and the top 10 pathways were metabolic pathways, biosynthesis of secondary metabolites, neuroactive ligand-receptor interaction, lysosome, focal adhesion, purine metabolism, starch and sucrose metabolism, tight junction, carbohydrate digestion and absorption, and pathways in cancer. As one of the most widely studied miRNA families, the expression of let-7 was significantly decreased in T. spiralis after exposure to host small intestinal milieu. A dual-luciferase reporter assay revealed that neuropeptide Y receptor type 6 and carboxypeptidase E were direct target genes of let-7, and were downregulated by binding with their 3' UTR. GO function analysis showed that carboxypeptidase E had multiple enzymatic activities, suggesting that it might participate in cell membrane damage and larval invasion. These data suggest that the differentially expressed miRNAs in T. spiralis might have a regulatory role in the invasion of host intestinal epithelial cells. This study provides a new insight into the molecular mechanisms of invasion by T. spiralis and the regulatory functions of miRNAs in host-Trichinella interactions.

"ADPKD-omics": determinants of cyclic AMP levels in renal epithelial cells

The regulation of cyclic adenosine monophosphate (cAMP) levels in kidney epithelial cells is important in at least 2 groups of disorders, namely water balance disorders and autosomal dominant polycystic kidney disease. Focusing on the latter, we review genes that code for proteins that are determinants of cAMP levels in cells. We identify which of these determinants are expressed in the 14 kidney tubule segments using recently published RNA-sequencing and protein mass spectrometry data ("autosomal dominant polycystic kidney disease-omics"). This includes G protein-coupled receptors, adenylyl cyclases, cyclic nucleotide phosphodiesterases, cAMP transporters, cAMP-binding proteins, regulator of G protein-signaling proteins, G protein-coupled receptor kinases, arrestins, calcium transporters, and calcium-binding proteins. In addition, compartmentalized cAMP signaling in the primary cilium is discussed, and a specialized database of the proteome of the primary cilium of cultured "IMCD3" cells is provided as an online resource (https://esbl.nhlbi.nih.gov/Databases/CiliumProteome/). Overall, this article provides a general resource in the form of a curated list of proteins likely to play roles in determination of cAMP levels in kidney epithelial cells and, therefore, likely to be determinants of progression of autosomal dominant polycystic kidney disease.

Neuropeptide Y as a risk factor for cardiorenal disease and cognitive dysfunction in CKD: translational opportunities and challenges

Neuropeptide Y (NPY) is a 36-amino-acid peptide member of a family also including peptide YY and pancreatic polypeptide, which are all ligands to Gi/Go coupled receptors. NPY regulates several fundamental biologic functions including appetite/satiety, sex and reproduction, learning and memory, cardiovascular and renal function and immune functions. The mesenteric circulation is a major source of NPY in the blood in man and this peptide is considered a key regulator of gut–brain cross talk. A progressive increase in circulating NPY accompanies the progression of chronic kidney disease (CKD) toward kidney failure and NPY robustly predicts cardiovascular events in this population. Furthermore, NPY is suspected as a possible player in accelerated cognitive function decline and dementia in patients with CKD and in dialysis patients. In theory, interfering with the NPY system has relevant potential for the treatment of diverse diseases from cardiovascular and renal diseases to diseases of the central nervous system. Pharmaceutical formulations for effective drug delivery and cost, as well as the complexity of diseases potentially addressable by NPY/NPY antagonists, have been a problem until now. This in part explains the slow progress of knowledge about the NPY system in the clinical arena. There is now renewed research interest in the NPY system in psychopharmacology and in pharmacology in general and new studies and a new breed of clinical trials may eventually bring the expected benefits in human health with drugs interfering with this system.

Transcriptional Landscaping Identifies a Beige Adipocyte Depot in the Newborn Mouse

The present study sought to identify gene networks that are hallmarks of the developing inguinal subcutaneous adipose tissue (iWAT) and the interscapular brown adipose tissue (BAT) in the mouse. RNA profiling revealed that the iWAT of postnatal (P) day 6 mice expressed thermogenic and lipid catabolism transcripts, along with the abundance of transcripts associated with the beige adipogenesis program. This was an unexpected finding, as thermogenic BAT was believed to be the only site of nonshivering thermogenesis in the young mouse. However, the transcriptional landscape of BAT in P6 mice suggests that it is still undergoing differentiation and maturation, and that the iWAT temporally adopts thermogenic and lipolytic potential. Moreover, P6 iWAT and adult (P56) BAT were similar in their expression of immune gene networks, but P6 iWAT was unique in the abundant expression of antimicrobial proteins and virus entry factors, including a possible receptor for SARS-CoV-2. In summary, postnatal iWAT development is associated with a metabolic shift from thermogenesis and lipolysis towards fat storage. However, transcripts of beige-inducing signal pathways including β-adrenergic receptors and interleukin-4 signaling were underrepresented in young iWAT, suggesting that the signals for thermogenic fat differentiation may be different in early postnatal life and in adulthood.

Fecal microbiota transplantation for irritable bowel syndrome: An intervention for the 21st century

Irritable bowel syndrome (IBS) affects about 12% of the global population. Although IBS does not develop into a serious disease or increase mortality, it results in a considerable reduction in the quality of life. The etiology of IBS is not known, but the intestinal microbiota appears to play a pivotal role in its pathophysiology. There is no effective treatment for IBS, and so the applied treatments clinically focus on symptom relief. Fecal microbiota transplantation (FMT), an old Chinese treatment, has been applied to IBS patients in seven randomized controlled trials (RCTs). Positive effects on IBS symptoms in various degrees were obtained in four of these RCTs, while there was no effect in the remaining three. Across the seven RCTs there were marked differences in the selection processes for the donor and treated patients, the transplant dose, the route of administration, and the methods used to measure how the patients responded to FMT. The present frontier discusses these differences and proposes: (1) criteria for selecting an effective donor (superdonor); (2) selection criteria for patients that are suitable for FMT; (3) the optimal FMT dose; and (4) the route of transplant administration. FMT appears to be safe, with only mild, self-limiting side effects of abdominal pain, cramping, tenderness, diarrhea, and constipation. Although it is early to speculate about the mechanisms underlying the effects of FMT, the available data suggest that changes in the intestinal bacteria accompanied by changes in fermentation patterns and fermentation products (specifically short-chain fatty acids) play an important role in improving the IBS symptoms seen after FMT. FMT appears to be a promising treatment for IBS, but further studies are needed before it can be applied in everyday clinical practice.

Possible role of peptide YY (PYY) in the pathophysiology of irritable bowel syndrome (IBS)

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder of unknown aetiology for which there is no effective treatment. Although IBS does not increase mortality, it reduces the quality of life and is an economic burden to both the patients themselves and society as a whole. Peptide YY (PYY) is localized in endocrine cells located in the ileum, colon and rectum. The concentration of PYY and the density of PYY cells are decreased in both the colon and rectum but unchanged in the ileum of patients with IBS. The low density of PYY cells in the large intestine may be caused by a decreased number of stem cells and their progeny toward endocrine cells. PYY regulates the intestinal motility, secretion and absorption as well as visceral sensitivity via modulating serotonin release. An abnormality in PYY may therefore contribute to the intestinal dysmotility and visceral hypersensitivity seen in IBS patients. Diet management involving consuming a low-FODMAP diet restores the density of PYY cells in the large intestine and improves abdominal symptoms in patients with IBS. This review shows that diet management appears to be a valuable tool for correcting the PYY abnormalities in the large intestine of IBS patients in the clinic.

Neuropeptides at the crossroad of fear and hunger: a special focus on neuropeptide Y

Survival in a natural environment forces an individual into constantly adapting purposive behavior. Specified interoceptive neurons monitor metabolic and physiological balance and activate dedicated brain circuits to satisfy essential needs, such as hunger, thirst, thermoregulation, fear, or anxiety. Neuropeptides are multifaceted, central components within such life‐sustaining programs. For instance, nutritional depletion results in a drop in glucose levels, release of hormones, and activation of hypothalamic and brainstem neurons. These neurons, in turn, release several neuropeptides that increase food‐seeking behavior and promote food intake. Similarly, internal and external threats activate neuronal pathways of avoidance and defensive behavior. Interestingly, specific nuclei of the hypothalamus and extended amygdala are activated by both hunger and fear. Here, we introduce the relevant neuropeptides and describe their function in feeding and emotional‐affective behaviors. We further highlight specific pathways and microcircuits, where neuropeptides may interact to identify prevailing homeostatic needs and direct respective compensatory behaviors. A specific focus will be on neuropeptide Y, since it is known for its pivotal role in metabolic and emotional pathways. We hypothesize that the orexigenic and anorexigenic properties of specific neuropeptides are related to their ability to inhibit fear and anxiety.

Regulation of Feeding and Metabolism by Neuropeptide F and Short Neuropeptide F in Invertebrates

Numerous neuropeptide systems have been implicated to coordinately control energy homeostasis, both centrally and peripherally. However, the vertebrate neuropeptide Y (NPY) system has emerged as the best described one regarding this biological process. The protostomian ortholog of NPY is neuropeptide F, characterized by an RXRF(Y)amide carboxyterminal motif. A second neuropeptide system is short NPF, characterized by an M/T/L/FRF(W)amide carboxyterminal motif. Although both short and long NPF neuropeptide systems display carboxyterminal sequence similarities, they are evolutionary distant and likely already arose as separate signaling systems in the common ancestor of deuterostomes and protostomes, indicating the functional importance of both. Both NPF and short-NPF systems seem to have roles in the coordination of feeding across bilaterian species, but during chordate evolution, the short NPF system appears to have been lost or evolved into the prolactin releasing peptide signaling system, which regulates feeding and has been suggested to be orthologous to sNPF. Here we review the roles of both NPF and sNPF systems in the regulation of feeding and metabolism in invertebrates.

A Short-Length Peptide YY Analogue with Anorectic Effect in Mice

Peripheral administration of PYY_3-36, a fragment of peptide YY (PYY), has been reported to reduce food intake by activating the neuropeptide Y2 receptor (Y2R). An N-terminally truncated PYY analogue, benzoyl-[Ala²⁶,Ile^28,31]PYY(25-36) (1), showed a relatively potent agonist activity for Y2R but a weak anorectic activity by intraperitoneal administration (2000 nmol/kg) in lean mice because of its markedly poor biological stability in the mouse serum. Notably, two cyclohexylalanine (Cha) substitutions for Tyr residues at positions 27 and 36 (4) improved the stability in the mouse serum concomitant with enhanced anorectic activity. Further optimization at positions 27, 28, 30, and 31 revealed that 21, containing Cha²⁸ and Aib³¹ residues, showed a more potent anorectic activity than PYY_3-36 at a low dose of 300 nmol/kg. The minimum effective dose by intraperitoneal administration of 21 was 30 nmol/kg (ca. 52 μg/kg) in mice, suggesting the biologic potential of short-length PYY_3-36 analogues with a potent anorectic effect.

Molecular characterization of neuropeptide Y (NPY) receptors (Y1, Y4 and Y6) and investigation of the tissue expression of their ligands (NPY, PYY and PP) in chickens

Neuropeptide Y (NPY) receptors and its ligands, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are suggested to regulate many physiological processes including food intake in birds. However, our knowledge regarding this avian NPY system remains rather limited. Here, we examined the tissue expression of NPY, PYY and PP and the gene structure, expression and signaling of three NPY receptors (cY1, cY4 and cY6) in chickens. The results showed that 1) NPY is widely expressed in chicken tissues with abundance noted in the hypothalamus via quantitative real-time PCR, whereas PYY is highly expressed in the pancreas, gastrointestinal tract and various brain regions, and PP is expressed almost exclusively in the pancreas; 2) cY1, cY4 and cY6 contain novel non-coding exon(s) at their 5'-UTR; 3) The wide tissue distribution of cY1 and cY4 and cY6 were detected in chickens by quantitative real-time PCR and their expression is controlled by the promoter near exon 1, which displays strong promoter activity in DF-1 cells as demonstrated by Dual-luciferase reporter assay; 4) Monitored by luciferase reporter assays, activation of cY1 and cY4 expressed in HEK293 cells by chicken NPY_1-36, PYY_1-37, and PP_1-36 treatment inhibits cAMP/PKA and activates MAPK/ERK signaling pathways, while cY6-expressing cells show little response to peptide treatment, indicating that cY1 and cY4, and not cY6, can transmit signals in vitro. Taken together, our study offers novel information about the expression and functionality of cY1, cY4, cY6 and their ligands in birds, and helps to decipher their conserved roles in vertebrates.

High molecular weight PEGylation of human pancreatic polypeptide at position 22 improves stability and reduces food intake in mice

BACKGROUND AND PURPOSE

Human pancreatic polypeptide (hPP) is known to suppress appetite and food intake, thereby representing a potential therapeutic approach against obesity and associated metabolic disorders. The aim of this study was to improve hPP stability by covalent PEGylation with diverse molecular weight polyethylene glycols (PEGs) at two positions using promising lead structures while maintaining target activity.

EXPERIMENTAL APPROACH

Modified peptides were synthesized by combined solid-phase and solution-phase peptide synthesis. Their potency was investigated in constitutively expressing human epithelial cells and isolated human colonic mucosa as well as receptor-transfected artificial cell lines. Human blood plasma and porcine liver homogenates were used to examine the in vitro stability of the analogues. The most promising variants were injected s.c. in C57BL/6JRj mice to monitor fasting-induced food intake and bioavailability.

KEY RESULTS

In human epithelia and colonic mucosal preparations, activity of the modified hPP peptides depended on the core sequence and latency of the peptides was related to PEG size. Peptides modified with a 22 kDa PEG (PEG22) remained intact in blood plasma and on incubation with liver homogenates for more than 96 h. Finally, hPP_2-36 , [K²² (PEG22)]hPP_2-36 and [K²² (PEG22),Q³⁴ ]hPP significantly reduced cumulative food intake in mice over 16 h after s.c. administration.

CONCLUSIONS AND IMPLICATIONS

Modification with PEG22 at position 22 stabilizes hPP significantly while extending its biological activities and could be used in drug development prospectively.

The y6 receptor suppresses bone resorption and stimulates bone formation in mice via a suprachiasmatic nucleus relay

The neuropeptide Y system is known to play an important role in the regulation of bone homeostasis and while the functions of its major receptors, Y1R and Y2R, in this process have become clearer, the contributions of other Y-receptors, like the y6 receptor (y6R), are unknown. Y6R expression is restricted to the suprachiasmatic nucleus (SCN) of the hypothalamus, an area known to regulate circadian rhythms, and the testis. Here we show that lack of y6R signalling, results in significant reduction in bone mass, but no changes in bone length. Male and female y6R knockout (KO) mice display reduced cortical and cancellous bone volume in axial and appendicular bones. Mechanistically, the reduction in cancellous bone is the result of an uncoupling of bone remodelling, leading to an increase in osteoclast surface and number, and a reduction in osteoblast number, osteoid surface, mineralizing surface and bone formation rate. y6R KO mice displayed increased numbers of osteoclast precursors and produced greater numbers of osteoclasts in RANKL-treated cultures. They also produced fewer CFU-ALP osteoblast precursors in the marrow and showed reduced mineralization in primary osteoblastic cultures, as well as reduced expression for the osteoblast lineage marker, alkaline phosphatase, in bone isolates. The almost exclusive location of y6Rs in the hypothalamus suggests a critical role of central neuronal pathways controlling this uncoupling of bone remodelling which is in line with known actions or other Y-receptors in the brain. In conclusion, y6R signalling is required for maintenance of bone mass, with loss of y6R uncoupling bone remodelling and resulting in a negative bone balance. This study expands the scope of hypothalamic regulation of bone, highlighting the importance for neural/endocrine coordination and their marked effect upon skeletal homeostasis.

The role of the neuropeptide Y (NPY) family in the pathophysiology of inflammatory bowel disease (IBD)

Inflammatory bowel disease (IBD) includes three main disorders: ulcerative colitis, Crohn's disease, and microscopic colitis. The etiology of IBD is unknown and the current treatments are not completely satisfactory. Interactions between the gut neurohormones and the immune system are thought to play a pivot role in inflammation, especially in IBD. These neurohormones are believed to include members of the neuropeptide YY (NPY) family, which comprises NPY, peptide YY (PYY), and pancreatic polypeptide (PP). Understanding the role of these peptides may shed light on the pathophysiology of IBD and potentially yield an effective treatment tool. Intestinal NPY, PYY, and PP are abnormal in both patients with IBD and animal models of human IBD. The abnormality in NPY appears to be primarily caused by an interaction between immune cells and the NPY neurons in the enteric nervous system; the abnormalities in PYY and PP appear to be secondary to the changes caused by the abnormalities in other gut neurohormonal peptides/amines that occur during inflammation. NPY is the member of the NPY family that can be targeted in order to decrease the inflammation present in IBD.

Regulation of energy homeostasis by the NPY system

Obesity develops when energy intake exceeds energy expenditure over time. Numerous neurotransmitters, hormones, and factors have been implicated to coordinately control energy homeostasis, centrally and peripherally. However, the neuropeptide Y (NPY) system has emerged as the one with the most critical functions in this process. While NPY centrally promotes feeding and reduces energy expenditure, peptide YY (PYY) and pancreatic polypeptide (PP), the other family members, mediate satiety. Importantly, recent research has uncovered additional functions for these peptides that go beyond the simple feeding/satiety circuits and indicate a more extensive function in controlling energy homeostasis. In this review, we will discuss the actions of the NPY system in the regulation of energy balance, with a particular focus on energy expenditure.

Pancreatic polypeptide controls energy homeostasis via Npy6r signaling in the suprachiasmatic nucleus in mice

Y-receptors control energy homeostasis, but the role of Npy6 receptors (Npy6r) is largely unknown. Young Npy6r-deficient (Npy6r(-/-)) mice have reduced body weight, lean mass, and adiposity, while older and high-fat-fed Npy6r(-/-) mice have low lean mass with increased adiposity. Npy6r(-/-) mice showed reduced hypothalamic growth hormone releasing hormone (Ghrh) expression and serum insulin-like growth factor-1 (IGF-1) levels relative to WT. This is likely due to impaired vasoactive intestinal peptide (VIP) signaling in the suprachiasmatic nucleus (SCN), where we found Npy6r coexpressed in VIP neurons. Peripheral administration of pancreatic polypeptide (PP) increased Fos expression in the SCN, increased energy expenditure, and reduced food intake in WT, but not Npy6r(-/-), mice. Moreover, intraperitoneal (i.p.) PP injection increased hypothalamic Ghrh mRNA expression and serum IGF-1 levels in WT, but not Npy6r(-/-), mice, an effect blocked by intracerebroventricular (i.c.v.) Vasoactive Intestinal Peptide (VPAC) receptors antagonism. Thus, PP-initiated signaling through Npy6r in VIP neurons regulates the growth hormone axis and body composition.

The NPY system and its neural and neuroendocrine regulation of bone

The past decade has seen a significant expansion of our understanding of the interaction between the neural system and bone. While innervation of bone was long appreciated, the discovery of central relays from the hypothalamus to the cells of bone has seen the identification of a number of efferent neural pathways to bone. The neuropeptide Y (NPY) system has proven to represent a major central pathway, regulating the activity of osteoblasts and osteoclasts, through signaling of central and peripheral ligands, through specific receptors within the hypothalamus and the osteoblast. Moreover, this pathway is now recognized as acting to coordinate both skeletal and energy homeostasis. This review examines the mechanism and actions of the NPY pathway to regulate bone mass and bone cell activity.

Tobacco addiction and the dysregulation of brain stress systems

Tobacco is a highly addictive drug and is one of the most widely abused drugs in the world. The first part of this review explores the role of stressors and stress-associated psychiatric disorders in the initiation of smoking, the maintenance of smoking, and relapse after a period of abstinence. The reviewed studies indicate that stressors facilitate the initiation of smoking, decrease the motivation to quit, and increase the risk for relapse. Furthermore, people with depression or an anxiety disorder are more likely to smoke than people without these disorders. The second part of this review describes animal studies that investigated the role of brain stress systems in nicotine addiction. These studies indicate that corticotropin-releasing factor, Neuropeptide Y, the hypocretins, and norepinephrine play a pivotal role in nicotine addiction. In conclusion, the reviewed studies indicate that smoking briefly decreases subjective stress levels but also leads to a further dysregulation of brain stress systems. Drugs that decrease the activity of brain stress systems may diminish nicotine withdrawal and improve smoking cessation rates.

NPY receptors as potential targets for anti-obesity drug development

The neuropeptide Y system has proven to be one of the most important regulators of feeding behaviour and energy homeostasis, thus presenting great potential as a therapeutic target for the treatment of disorders such as obesity and at the other extreme, anorexia. Due to the initial lack of pharmacological tools that are active in vivo, functions of the different Y receptors have been mainly studied in knockout and transgenic mouse models. However, over recent years various Y receptor selective peptidic and non-peptidic agonists and antagonists have been developed and tested. Their therapeutic potential in relation to treating obesity and other disorders of energy homeostasis is discussed in this review.

Structure-activity studies of RFamide peptides reveal subtype-selective activation of neuropeptide FF1 and FF2 receptors

Selectivity is a major issue in closely related multiligand/multireceptor systems. In this study we investigated the RFamide systems of hNPFF₁R and hNPFF₂R that bind the endogenous peptide hormones NPFF, NPAF, NPVF, and NPSF. By use of a systematic approach, we characterized the role of the C-terminal dipeptide with respect to agonistic properties using synthesized [Xaa 7]NPFF and [Xaa 8]NPFF analogues. We were able to identify only slight differences in potency upon changing the position of Arg 7, as all modifications resulted in identical behavior at the NPFF₁R and NPFF₂R. However, the C-terminal Phe 8 was able to be replaced by Trp or His with only a minor loss in potency at the NPFF₂R relative to the NPFF₁R. Analogues with shorter side chains, such as α-amino-4-guanidino butyric acid ([Agb 7]NPFF) or phenylglycine ([Phg 8]NPFF), decreased efficacy for the NPFF₁ R to 25-31 % of the maximal response, suggesting that these agonist-receptor complexes are more susceptible to structural modifications. In contrast, mutations to the conserved Asp 6.59 residue in the third extracellular loop of both receptors revealed a higher sensitivity toward the hNPFF₂R receptor than toward hNPFF₁R. These data provide new insight into the subtype-specific agonistic activation of the NPFF₁ and NPFF(2) receptors that are necessary for the development of selective agonists.

Therapeutic potential of neuropeptide Y (NPY) receptor ligands

Neuropeptide Y (NPY) is widely distributed in the human body and contributes to a vast number of physiological processes. Since its discovery, NPY has been implicated in metabolic regulation and, although interest in its role in central mechanisms related to food intake and obesity has somewhat diminished, the topic remains a strong focus of research concerning NPY signalling. In addition, a number of other uses for modulators of NPY receptors have been implied in a range of diseases, although the development of NPY receptor ligands has been slow, with no clinically approved receptor therapeutics currently available. Nevertheless, several interesting small molecule compounds, notably Y2 receptor antagonists, have been published recently, fueling optimism in the field. Herein we review the role of NPY in the pathophysiology of a number of diseases and highlight instances where NPY receptor signalling systems are attractive therapeutic targets.

Regulatory effects of Y4 receptor agonist (BVD-74D) on food intake

The objective of this study was to clarify the role of a novel agonist with high selectivity and affinity for Y4 receptors in the regulation of food intake. The Y4 receptor agonist BVD-74D was administered to C57BL/6J mice that were fed with a normal or high-fat diet, and to fatty liver Shionogi (FLS)-ob/ob mice; the food intake, water intake, and body weight gain were measured in these mice. In the mice fed with a normal diet, the cumulative food intake significantly decreased at 20 min and 1 h after the administration of 1 mg/kg of BVD-74D and at 1, 2, 4, 8, and 24 h after the administration of 10 mg/kg of BVD-74D. Moreover, the cumulative water intake and body weight gain significantly decreased in these mice. Among the mice that were fed with a high-fat diet, the cumulative food intake and water intake significantly decreased 1, 2, and 4 h after BVD-74D (10 mg/kg) administration. Furthermore, the cumulative food intake significantly decreased 2 and 4 h after BVD-74D (10 mg/kg) administration in the FLS-ob/ob mice. Thus, we propose that the novel Y4 receptor agonist BVD-74D has suppressive effects on food intake, water intake, and weight gain in normal mice fed with normal diets and on food intake in normal mice fed with high-fat diets and in FLS-ob/ob mice. These findings indicate that the Y4 receptor and its agonist would be promising targets for obesity.

Chronic psychosocial stress alters NPY system: different effects in rat and tree shrew

The neuropeptide Y (NPY) system has been largely studied in relation to affective disorders, in particular for its role in the mechanisms regulating the pathophysiology of anxiety and depression and in the stress-related behaviours. Although NPY has been previously investigated in a variety of animal models of mood disorders, the receptor subtype mainly involved in the modulation of the stress response has not been identified. In the present study, the chronic psychosocial stress based on the resident-intruder protocol-an ethologically relevant paradigm known to induce behavioural and endocrine modifications which mimic depression-like symptoms-was used. Two different species were investigated: rat and tree shrew (Tupaia belangeri); the latter is regarded as an intermediate between insectivores and primates and it was chosen in this study for its pronounced territoriality. In these animals, the regulation of NPY and of Y(1), Y(2) and Y(5) receptors mRNA expression was evaluated after chronic stress and chronic antidepressant treatment by in situ hybridization in selected brain regions known to be involved in the pathophysiology of mood disorders. The animals were exposed to psychosocial stress for 35 days and concomitant daily fluoxetine treatment (10 mg/kg for rats and 15 mg/kg for tree shrews) after the first week of stress. The results confirmed a major role for hippocampal and hypothalamic NPY system in the pathophysiology of mood disorders. Although there were no evident differences between rat and tree shrew in the NPY system distribution, an opposite effect of chronic psychosocial stress was observed in the two species. Moreover, chronic antidepressant treatment was able to counteract the effects of stress and restored basal expression levels, suggesting the utility of these paradigms as preclinical models of stress-induced depression. Overall, although evident species differences were found in response to chronic psychosocial stress, the present study suggests a role for NPY receptors in the stress response and in the action of antidepressant drugs, providing further support for an involvement of this neuropeptidergic system in the pathophysiology of depression and anxiety.

NPY regulation of bone remodelling

Neuropeptide Y (NPY), a classic neuronal regulator of energy homeostasis, is now also known to be involved in the control of bone homeostasis. Of the five known Y receptors through which the NPY family of ligands signals, the Y1 and Y2 receptors have so far been implicated in the control of osteoblast activity and thus bone formation. Analysis of brain specific NPY overexpressing and Y receptor knockout models has revealed a powerful anabolic pathway likely involving hypothalamic Y2 receptors and osteoblastic Y1 receptors. Furthering our understanding of the mechanisms underlying the involvement of the NPY system in the control of bone could lead to the development of therapies to improve bone mass in patients with diseases such as osteoporosis.

Discovery of trans-N-[1-(2-fluorophenyl)-3-pyrazolyl]-3-oxospiro[6-azaisobenzofuran-1(3H),1'-cyclohexane]-4'-carboxamide, a potent and orally active neuropeptide Y Y5 receptor antagonist

A series of trans-3-oxospiro[(aza)isobenzofuran-1(3H),1'-cyclohexane]-4'-carboxamide derivatives were synthesized to identify potent NPY Y5 receptor antagonists. Of the compounds, 21j showed high Y5 binding affinity, metabolic stability and brain and cerebrospinal fluid (CSF) penetration, and low susceptibility to P-glycoprotein transporters. Oral administration of 21j significantly inhibited the Y5 agonist-induced food intake in rats with a minimum effective dose of 1mg/kg. This compound was selected for proof-of-concept studies in human clinical trials.

Cloning and characterization of rabbit neuropeptide Y receptor subtypes

Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) are structurally related peptides that have numerous functions in both neural and endocrine signaling. These effects are mediated by the NPY receptor family and five members of this family have been cloned in mammals. To better characterize these receptor subtypes, we cloned and expressed the Y1, Y2, Y4 and Y5 receptor subtypes from the rabbit. Comparison of these sequences with human orthologs revealed that the Y1, Y2 and Y5 receptors have generally strong amino-acid sequence conservation, with 91-96% identity, while Y4 receptor showed relatively weak similarity with 82% identity, as with other species. Particularly in the transmembrane regions, Y1, Y2, and Y5 receptor subtypes showed remarkable conservation, with 98-99% amino acid identity. Competitive binding studies by NPY-family peptides and analogs showed that Y1, Y2 and Y5 receptors had similar pharmacological profiles between the respective rabbit and human receptor subtypes. Interestingly, all the tested peptides had a greater affinity for rabbit Y4 receptor than human Y4 receptor. These results suggest that rabbit and human Y1, Y2 and Y5 receptor subtypes are well conserved, whereas Y4 receptors are less well conserved.

Aryl urea derivatives of spiropiperidines as NPY Y5 receptor antagonists

Continuing medicinal chemistry studies to identify spiropiperidine-derived NPY Y5 receptor antagonists are described. Aryl urea derivatives of a variety of spiropiperidines were tested for their NPY Y5 receptor binding affinities. Of the spiropiperidines so far examined, spiro[3-oxoisobenzofurane-1(3H),4'-piperidine] was a useful scaffold for producing orally active NPY Y5 receptor antagonists. Oral administration of 5c significantly inhibited the Y5 agonist-induced food intake in rats with a minimum effective dose of 3mg/kg. In addition, this compound was efficacious in decreasing body weight in diet-induced obese mice.

Identification of novel and orally active spiroindoline NPY Y5 receptor antagonists

A series of spiroindoline-3,4'-piperidine derivatives were synthesized and evaluated for their binding affinities and antagonistic activities at Y5 receptors. Potent Y5 antagonists were tested for their oral bioavailabilities and brain penetration in rats. Some of the antagonists showed good oral bioavailability and/or good brain penetration. In particular, compound 6e was orally bioavailable and brain penetrant, and oral administration of 6e inhibited bPP-induced food intake in rats with a minimum effective dose of 10mg/kg.

Receptor Subtype-specific Docking of Asp6.59 with C-terminal Arginine Residues in Y Receptor Ligands

Y receptors (YRs) are G protein-coupled receptors whose Y(1)R, Y(2)R, and Y(5)R subtypes preferentially bind neuropeptide Y (NPY) and peptide YY, whereas mammalian Y(4)Rs show a higher affinity for pancreatic polypeptide (PP). Comparison of YR orthologs and paralogs revealed Asp(6.59) to be fully conserved throughout all of the YRs reported so far. By replacing this conserved aspartic acid residue with alanine, asparagine, glutamate, and arginine, we now show that this residue plays a crucial role in binding and signal transduction of NPY/PP at all YRs. Sensitivity to distinct replacements is, however, receptor subtype-specific. Next, we performed a complementary mutagenesis approach to identify the contact site of the ligand. Surprisingly, this conserved residue interacts with two different ligand arginine residues by ionic interactions; although in Y(2)R and Y(5)R, Arg(33) is the binding partner of Asp(6.59), in Y(1)R and Y(4)R, Arg(35) of human PP and NPY interacts with Asp(6.59). Furthermore, Arg(25) of PP and NPY is involved in ligand binding only at Y(2)R and Y(5)R. This suggests significant differences in the docking of YR ligands between Y(1/4)R and Y(2/5)R and provides new insights into the molecular binding mode of peptide agonists at GPCRs. Furthermore, the proposed model of a subtype-specific binding mode is in agreement with the evolution of YRs.

Neuropeptide Y receptors; antisecretory control of intestinal epithelial function

This paper reviews the cellular localisation, mechanisms of release and intestinal absorptive actions of neuropeptide Y and its related peptides, peptide YY, pancreatic polypeptide and major fragments NPY(3-36) and PYY(3-36). While NPY is commonly found in inhibitory enteric neurons that can be interneurons, motor neurons or secretomotor-nonvasodilator in nature, its analogue, peptide YY in contrast, is located in neuroendocrine L-cells that predominate in the colorectal mucosa. Peptide YY is released from these cells when nutrients arrive in the small or large bowel, exerting paracrine as well as hormonal actions. Pancreatic polypeptide is found in relatively few, scattered intestinal endocrine cells, the majority of this peptide being produced by, and released from pancreatic islet F-cells in response to food intake. An introduction to the current pharmacology of this family of peptides is provided and the different types of neuropeptide Y (termed Y) receptors, their agonist preferences, antagonism, and preferred signalling pathways, are described. Our current understanding of specific Y receptor localisation within the intestine as determined by immunohistochemistry, is presented as a prelude to an assessment of functional studies that have monitored ion transport across isolated mucosal preparations. It is becoming clear that three Y receptor types are significant functionally in human colon, as well as particular rodent models (e.g. mouse) and these, namely the Y(1), Y(2) and Y(4) receptors, are discussed in detail. Their presence within the basolateral aspect of the epithelial layer (Y(1) and Y(4) receptors) or on enteric neurons (Y(1) and Y(2) receptors) and their activation by endogenous neuropeptide Y, peptide YY (Y(1) and Y(2) receptors) or pancreatic polypeptide (which prefers Y(4) receptors) results consistently in antisecretory/absorptive responses. The recent use of novel mouse knockouts has helped establish loss of specific intestinal functions including Y(1) and Y(2) receptor-mediated absorptive tone in colon mucosa. Progress in this field has been rapid recently, aided by the availability of selective antagonists and mutant mice lacking either one (e.g. Y(4)-/-, for which no antagonists exist at present) or more Y receptor types. It is therefore timely to review this work and present a rational basis for developing stable synthetic Y receptor agonists as novel anti-diarrhoeals.

Design and synthesis of (ant)-agonists that alter appetite and adiposity

Over the past decade, hypothalamic circuits have been described that impact energy homeostasis in rodents and humans. Our drug development efforts for the treatment of obesity and the metabolic syndrome have largely focused on selected genetic and/or pharmacologically validated pathways. The translation of these pathways into therapeutics for the treatment of obesity will find its first clinical successes over the coming decade. Initial efforts have focused on gaining a better understanding of the relevance of rodent pharmacological and genetic observations for the development of therapeutics for the treatment of human obesity. We pursue pathways defined by the expression of the ghrelin receptor, melanin-concentrating hormone receptors, melanocortin receptors, cannabinoid receptors and neuropeptide Y1 and Y5 receptors. In this review, we will discuss drug development efforts for the treatment of obesity, focused on selective melanocortin 4 receptor agonists and neuropeptide Y1 and Y5 receptor antagonists. These drug development efforts required an in-depth understanding of cell-based observations which drive the development of compound structure-activity relationships. These include understanding of receptor function in selected cell-based backgrounds and early evaluation and validation of ex vivo observations in appropriate in vivo models. In order to develop selective and safe anti-obesity drugs, diverse approaches are needed to increase the likelihood of clinical success, including: (i) developing a detailed understanding of the predictive value of rodent pathways for treatment of human disease; (ii) knowledge of the exact location of targeted receptor subtypes for the clinical indication under study in order to derive a suitable compound profile; (iii) predictive measures of in vivo and/or ex vivo receptor occupancy required to bring about a desired physiological effect; (iv) predictive parameters that outline that the drug-derived effects are safe and mechanism-based; and (v) the refinement of selected compound classes, aimed at their clinical use.

Roles of nuclear NPY and NPY receptors in the regulation of the endocardial endothelium and heart functionThis paper is one of a selection of papers published in this Special issue, entitled Second Messengers and Phosphoproteins—12th International Conference

It is now well accepted that the heart is a multifunctional organ in which endothelial cells, and more particularly endocardial endothelial cells (EECs), seem to play an important role in regulating and maintaining cardiac excitation–contraction coupling. Even if major differences exist between vascular endothelial cells (VECs) and EECs, all endothelial cells including EECs release a variety of auto- and paracrine factors such as nitric oxide, endothelin-1, angiotensin II, and neuropeptide Y. All these factors were reported to affect cardiomyocyte contractile performance and rhythmicity. In this review, findings on the morphology of EECs, differences between EECs and other types of endothelial cells, interactions between EECs and the adjacent cardiomyocytes, and effects of NPY on the heart will be presented. We will also show evidence on the presence and localization of NPY and the Y1receptor in the endocardial endothelium and discuss their role in the regulation of cytosolic and nuclear free calcium.

Neuropeptide Y-family receptors Y6 and Y7 in chicken. Cloning, pharmacological characterization, tissue distribution and conserved synteny with human chromosome region

The peptides of the neuropeptide Y (NPY) family exert their functions, including regulation of appetite and circadian rhythm, by binding to G-protein coupled receptors. Mammals have five subtypes, named Y1, Y2, Y4, Y5 and Y6, and recently Y7 has been discovered in fish and amphibians. In chicken we have previously characterized the first four subtypes and here we describe Y6 and Y7. The genes for Y6 and Y7 are located 1 megabase apart on chromosome 13, which displays conserved synteny with human chromosome 5 that harbours the Y6 gene. The porcine PYY radioligand bound the chicken Y6 receptor with a K(d) of 0.80 +/- 0.36 nm. No functional coupling was demonstrated. The Y6 mRNA is expressed in hypothalamus, gastrointestinal tract and adipose tissue. Porcine PYY bound chicken Y7 with a K(d) of 0.14 +/- 0.01 nm (mean +/- SEM), whereas chicken PYY surprisingly had a much lower affinity, with a Ki of 41 nm, perhaps as a result of its additional amino acid at the N terminus. Truncated peptide fragments had greatly reduced affinity for Y7, in agreement with its closest relative, Y2, in chicken and fish, but in contrast to Y2 in mammals. This suggests that in mammals Y2 has only recently acquired the ability to bind truncated PYY. Chicken Y7 has a much more restricted tissue distribution than other subtypes and was only detected in adrenal gland. Y7 seems to have been lost in mammals. The physiological roles of Y6 and Y7 remain to be identified, but our phylogenetic and chromosomal analyses support the ancient origin of these Y receptor genes by chromosome duplications in an early (pregnathostome) vertebrate ancestor.

Endoderm and Pancreatic Islet Lineage Differentiation from Human Embryonic Stem Cells

Human embryonic stem cells (HESCs) are a potential source of insulin-producing tissue for transplantation. Recent studies have begun to define factors that promote definitive endoderm formation from HESCs, but conditions permitting complete islet specification in vitro have not been described. Here, we study spontaneous differentiation of HESCs to definitive endoderm and pancreatic progenitor cells, and begin to determine which aspects of the protocol are required for this cell fate commitment. HESCs were differentiated in culture for up to 10 weeks, including an embryoid body (EB) formation step. Modifications to the protocol included elimination of the EB phase, varying initial cell cluster size when forming EBs, and addition of mesoderm-derived cells to EBs. Differentiated cells were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. HESCs are capable of spontaneous differentiation to cells expressing the definitive endoderm and pancreatic progenitor markers Foxa2, Sox17, and Pdx1, and ultimately, some cells express islet endocrine hormones. This differentiation occurs to a much greater extent when an EB formation step is included. Increased expression of endoderm markers during and after EB formation also correlated strongly with the size of cell clusters used to start EBs, as well as the addition of mesoderm- derived embryonic cells. This study demonstrates that a subset of differentiated HESC progeny adopt an endoderm fate and exhibit the capacity for further pancreatic lineage specification in vitro. Basal conditions were established for examining factors that can commit HESC-derived endoderm cells to specific pancreatic lineages.

The regulation of veratridine-stimulated electrogenic ion transport in mouse colon by neuropeptide Y (NPY), Y1 and Y2 receptors

1 Neuropeptide Y (NPY) is a prominent enteric neuropeptide with prolonged antisecretory effects in mammalian intestine. Veratridine depolarises neurons consequently causing epithelial anion secretion across mouse colon mucosa. Our aim was to characterise functionally, veratridine-stimulated mucosal responses and to determine the roles for NPY, Y(1), and Y(2) receptors in modulating these neurogenic effects. 2 Colon mucosae (with intact submucous innervation) from wild-type mice (+/+) and knockouts lacking either NPY (NPY-/-), Y(1)-/- or Y(2)-/- were placed in Ussing chambers and voltage clamped at 0 mV. Veratridine-stimulated short-circuit current (I(sc)) responses in +/+, Y(1) or Y(2) antagonist pretreated +/+ colon, Y(1)-/- and NPY-/- colon were insensitive to cholinergic blockade by atropine (At; 1 microM) and hexamethonium (Hex; 10 microM). Tetrodotoxin (TTX, 100 nM) abolished veratridine responses, but had no effect upon carbachol (CCh) or vasoactive intestinal polypeptide (VIP)-induced secretory responses. 3 To establish the functional roles for Y(1) and Y(2) receptors, +/+ tissues were pretreated with either the Y(1) or Y(2) receptor antagonist (BIBO3304 (300 nM) or BIIE0246 (1 microM), respectively) and veratridine responses were compared with those from Y(1)-/- or Y(2)-/- colon. Neither BIBO3304 nor Y(1)-/- altered veratridine-induced secretion, but Y(1) agonist responses were abolished in both preparations. In contrast, the Y(2) antagonist BIIE0246 significantly amplified veratridine responses in +/+ mucosa. Unexpectedly, NPY-/- colon exhibited significantly attenuated veratridine responses (between 1 and 5 min). 4 We demonstrate that electrogenic veratridine responses in mouse colon are noncholinergic and that NPY can act directly upon epithelia, a Y(1) receptor effect. The enhanced veratridine response observed in +/+ tissue following BIIE0246, indicates that Y(2) receptors are located on submucosal neurons and that their activation by NPY will inhibit enteric noncholinergic secretory neurotransmission. 5 We also demonstrate Y(1) and Y(2) receptor-mediated antisecretory tone in +/+ colon and show selective loss of each in Y(1) and Y(2) null colon respectively. In NPY-/- tissue, only Y(1)-mediated tone was present, this presumably being mediated by endogenous endocrine peptide YY. Y(2) tone was absent from NPY-/- (and Y(2)-/-) colon and we conclude that NPY activation of neuronal Y(2) receptors attenuates secretory neurotransmission thereby providing an absorptive electrolyte tone in isolated colon.

Characterization of neuropeptide Y2 receptor protein expression in the mouse brain. I. Distribution in cell bodies and nerve terminals

Neuropeptide Y (NPY), a 36-amino-acid peptide, mediates biological effects by activating Y1, Y2, Y5, and y6 receptors. NPY neurons innervate many brain regions, including the hypothalamus, where NPY is involved in regulation of a broad range of homeostatic functions. We examined, by immunohistochemistry with tyramide signal amplification, the expression of the NPY Y2 receptor (Y2R) in the mouse brain with a newly developed rabbit polyclonal antibody. Y2R immunoreactivity was specific with its absence in Y2R knockout (KO) mice and in adjacent sections following preadsorption with the immunogenic peptide (10(-5) M). Y2R-positive processes were located in many brain regions, including the olfactory bulb, some cortical areas, septum, basal forebrain, nucleus accumbens, amygdala, hippocampus, hypothalamus, substantia nigra compacta, locus coeruleus, and solitary tract nucleus. However, colchicine treatment was needed to detect Y2R-like immunoreactivity in cell bodies in many, but not all, areas. The densest distributions of cell bodies were located in the septum basal forebrain, including the bed nucleus, and amygdala, with lower density in the anterior olfactory nucleus, nucleus accumbens, caudal striatum, CA1, CA2, and CA3 hippocampal fields, preoptic nuclei lateral hypothalamus, and A13 DA cells. The widespread distribution of Y2R-positive cell bodies and fibers suggests that NPY signaling through the Y2R is common in the mouse brain. Localization of the Y2R suggests that it is mostly presynaptic, a view supported by its frequent absence in cell bodies in the normal mouse and its dramatic increase in cell bodies of colchicine-treated mice.

Peptide and non-peptide G-protein coupled receptors (GPCRs) in skeletal muscle

G-protein coupled receptors (GPCRs) represent a large class of cell surface receptors that mediate a multitude of functions. Over the years, a number of GPCRs and ancillary proteins have been shown to be expressed in skeletal muscle. Unlike the case with other muscle tissues like cardiac and vascular smooth muscle cells, there has been little attempt at systematically analyzing GPCRs in skeletal muscle. Here we have compiled all the GPCRs that are expressed in skeletal muscle. In addition, we review the known function of these receptors in both skeletal muscle tissue and in cultured skeletal muscle cells.