Consequences of splice variation on Secretin family G protein-coupled receptor function

Abstract

The Secretin family of GPCRs are endocrine peptide hormone receptors that share a common genomic organization and are the subject of a wide variety of alternative splicing. All GPCRs contain a central seven transmembrane domain responsible for transducing signals from the outside of the cell as well as extracellular amino and intracellular carboxyl termini. Members of the Secretin receptor family have a relatively large N-terminus and a variety of lines of evidence support a common mode of ligand binding and a common ligand binding fold. These receptors are best characterized as coupling to intracellular signalling pathways via G(αs) and G(αq) but are also reported to couple to a multitude of other signalling pathways. The intracellular loops are implicated in regulating the interaction between the receptor and heterotrimeric G protein complexes. Alternative splicing of exons encoding both the extracellular N-terminal domain as well as the extracellular loops of some family members has been reported and as expected these splice variants display altered ligand affinity as well as differential activation by endogenous ligands. Various forms of alternative splicing have also been reported to alter intracellular loops 1 and 3 as well as the C-terminus and as one might expect these display differences in signalling bias towards downstream effectors. These diverse pharmacologies require that the physiological role of these splice variants be addressed but should provide unique opportunities for drug design and development.

Figure 1

Schematic of the genomic organization of Secretin family GPCRs. Boxes indicate coding exons and are scaled according to amino acid length coded by each exon. The relative location of exons with respect to the overall receptor fold is depicted. The location of transmembrane helices is indicated in red (exon spanning helices shown with pink insert) and ICLs in blue.

Figure 2

Alignment of the extracellular N-termini of Secretin family GPCRs. Amino acids encoded by alternative exons are shown in alternating black and blue text with amino acids encoded by two exons shown in red text. Predicted signal peptides are highlighted as italicized, underlined text. The six conserved cysteine residues are highlighted by red background and have the cysteine number indicated below the alignment. The disulphide bonding pattern for all receptors is 1–4, 2–5 and 3–6. The other conserved residues are highlighted in purple. Above the alignment alpha helices are indicated with H, beta strands with B and beta turns with T.

Figure 3

Cartoon depicting overall topology of Secretin family GPCRs. The N-terminal domain is shown in red with the seven transmembrane bundle in green. The relative locations of AS reported for the family are shown. N-terminal domain exchange is a feature of CRF₂α, CRF₂β and CRF₂γ. CRF₁, PAC₁ and Secretin receptors have 3rd exon deletion variants. PAC₁ has both N-terminal and ICL3 insert variants. CT receptor has variants with ICL1 insert, ECL1 insert and exon 13 deletion and CRF₁ has an ICL1 insert variant.

Figure 4

Schematic of the genomic organization of PAC₁ receptor indicating the human splice variants discussed in this review. The top most exon pattern labelled all shows all coding exons for human PAC₁; however, we are not aware of a splice variant that incorporates all these exons. Boxes indicate coding exons and are scaled according to amino acid length coded by each exon. The relative location of exons with respect to the overall receptor fold is depicted. The location of transmembrane helices is indicated in red (exon spanning helices shown with pink insert) and ICLs in blue.