Alternative mRNA splicing of G protein-coupled receptors

Targeting of G-protein coupled receptors in sepsis

The Third International Consensus Definitions (Sepsis-3) define sepsis as life-threatening multi-organ dysfunction caused by a dysregulated host response to infection. Sepsis can progress to septic shock-an even more lethal condition associated with profound circulatory, cellular and metabolic abnormalities. Septic shock remains a leading cause of death in intensive care units and carries a mortality of almost 25%. Despite significant advances in our understanding of the pathobiology of sepsis, therapeutic interventions have not translated into tangible differences in the overall outcome for patients. Clinical trials of antagonists of various pro-inflammatory mediators in sepsis have been largely unsuccessful in the past. Given the diverse physiologic roles played by G-protein coupled receptors (GPCR), modulation of GPCR signaling for the treatment of sepsis has also been explored. Traditional pharmacologic approaches have mainly focused on ligands targeting the extracellular domains of GPCR. However, novel techniques aimed at modulating GPCR intracellularly through aptamers, pepducins and intrabodies have opened a fresh avenue of therapeutic possibilities. In this review, we summarize the diverse roles played by various subfamilies of GPCR in the pathogenesis of sepsis and identify potential targets for pharmacotherapy through these novel approaches.

Identification of Three Novel Splicing Variants and Expression Analysis of Chicken GPR1 Gene

GPR1 is a G protein-coupled receptor that plays critical roles in eukaryotic cells: typically, response to glucose stimulation, lipid accumulation, and transmitting nutrition signals to cAMP pathway. However, the alternative splicing of the GPR1 gene and its expression pattern in chicken tissues and ovarian follicles were unknown. In our current study, we used RACE-PCR to identify three GPR1 variants, including the full-length variant (GPR1-va1) and two alternatively spliced variants (GPR1-va2, GPR1-vb). Quantitative real-time PCR examined the expression pattern of GPR1 mRNA in chicken tissues and ovarian follicles. The result reveals that the coding sequence of the three variants cDNA is 1053, 1053, and 627 bp in length, encoding 350, 350, and 208 amino acids, respectively. The three variants of GPR1 show similar tissue distributions; GPR1 expression was abundant in the abdominal fat, lung, and heart. With the follicular development, the expression of GPR1 gene gradually increased, and GPR1-va1 and GPR1-va2 spliced variants expression in F2 were significantly higher than in F5, F4, and prehierarchical follicles (P < 0.05). Taken together, we found three novel variants of GPR1, and the results of GPR1 expression profiling in adipose tissues and ovarian follicles suggest that GPR1 may play a significant role in the lipid accumulation and progression of follicular development.

A genomic variant in IRF9 is associated with serum cytokine levels in pig

The interferon regulatory factor 9 (IRF9) gene is a member of the IRF family and has been shown to play functionally diverse roles in the regulation of the immune system. Previous study revealed the IRF9 gene resides within the reported quantitative trait locus (QTLs) for cytokine levels. The aims of this study were to identify genomic variants in IRF9 and to test the association between the variants and cytokine levels in pig. A synonymous single-nucleotide polymorphism (c.459A>G) was identified in exon 4 of the IRF9 gene. Association analysis in 300 piglets (Landrace, n=68; large white, n=158; and Songliao black, n=74) showed that this variant was significantly associated with the level of interferon (IFN)-γ and the ratio of IFN-γ to IL-10 in serum (P<0.05). Relative quantification of messenger RNA (mRNA) revealed that spleen had the highest expression level and individuals with genotype AA had higher expression than those with genotype AG. Transfection-based mRNA stability assay analysis further showed that the mutant allele G could reduce the RNA stability of IRF9. These findings suggest that the SNP (c.459A>G) could be a causative mutation for the association between IRF9 and the serum cytokine levels in swine.

Identifying a Neuromedin U Receptor 2 Splice Variant and Determining Its Roles in the Regulation of Signaling and Tumorigenesis In Vitro

Neuromedin U (NMU) activates two G protein-coupled receptors, NMUR1 and NMUR2; this signaling not only controls many physiological responses but also promotes tumorigenesis in diverse tissues. We recently identified a novel truncated NMUR2 derived by alternative splicing, namely NMUR2S, from human ovarian cancer cDNA. Sequence analysis, cell surface ELISA and immunocytochemical staining using 293T cells indicated that NMUR2S can be expressed well on the cell surface as a six-transmembrane protein. Receptor pull-down and fluorescent resonance energy transfer assays demonstrated that NMUR1, NMUR2 and this newly discovered NMUR2S can not only form homomeric complexes but also heteromeric complexes with each other. Although not activated by NMU itself, functional assay in combination with receptor quantification and radio-ligand binding in 293T cells indicated that NMUR2S does not alter the translocation and stability of NMUR1 or NMUR2, but rather effectively dampens their signaling by blocking their NMU binding capability through receptor heterodimerization. We further demonstrated that NMU signaling is significantly up-regulated in human ovarian cancers, whereas expression of NMUR2S can block endogenous NMU signaling and further lead to suppression of proliferation in SKOV-3 ovarian cancer cells. In contrast, in monocytic THP-1 cells that express comparable levels of NMUR1 and NMUR2S, depletion of NMUR2S restored both the signaling and effect of NMU. Thus, these results not only reveal the presence of previously uncharacterized heteromeric relationships among NMU receptors but also provide NMUR2S as a potential therapeutic target for the future treatment of NMU signaling-mediated cancers.

MC1R, the cAMP pathway, and the response to solar UV: extending the horizon beyond pigmentation

The melanocortin 1 receptor (MC1R) is a G protein-coupled receptor crucial for the regulation of melanocyte proliferation and function. Upon binding melanocortins, MC1R activates several signaling cascades, notably the cAMP pathway leading to synthesis of photoprotective eumelanin. Polymorphisms in the MC1R gene are a major source of normal variation of human hair color and skin pigmentation, response to ultraviolet radiation (UVR), and skin cancer susceptibility. The identification of a surprisingly high number of MC1R natural variants strongly associated with pigmentary phenotypes and increased skin cancer risk has prompted research on the functional properties of the wild-type receptor and frequent mutant alleles. We summarize current knowledge on MC1R structural and functional properties, as well as on its intracellular trafficking and signaling. We also review the current knowledge about the function of MC1R as a skin cancer, particularly melanoma, susceptibility gene and how it modulates the response of melanocytes to UVR.