A comparative analysis of human and mouse islet G-protein coupled receptor expression

Abstract

G-protein coupled receptors (GPCRs) are essential for islet function, but most studies use rodent islets due to limited human islet availability. We have systematically compared the GPCR mRNA expression in human and mouse islets to determine to what extent mouse islets can be used as surrogates for human islets to study islet GPCR function, and we have identified species-specific expression of several GPCRs. The A₃ receptor (ADORA3) was expressed only in mouse islets and the A₃ agonist MRS 5698 inhibited glucose-induced insulin secretion from mouse islets, with no effect on human islets. Similarly, mRNAs encoding the galanin receptors GAL₁ (GALR1), GAL₂ (GALR2) and GAL₃ GALR3) were abundantly expressed in mouse islets but present only at low levels in human islets, so that it reads (GALR3) and galanin inhibited insulin secretion only from mouse islets. Conversely, the sst1 receptor (SSTR1) was abundant only in human islets and its selective activation by CH 275 inhibited insulin secretion from human islets, with no effect on mouse islets. Our comprehensive human and mouse islet GPCR atlas has demonstrated that species differences do exist in islet GPCR expression and function, which are likely to impact on the translatability of mouse studies to the human context.

Figure 1. Comparison of the relative expression of 341 GPCR mRNAs in islets isolated from the outbred ICR and inbred C57 mouse strains.

Insert panel A has been enlarged to allow visualisation of GPCRs expressed above trace level only in ICR mouse islets. Data are presented as mean GPCR expression relative to the reference genes Actb, Gapdh, Ppia, Tbp and Tfrc from non-pooled biological replicates from four C57 and four ICR mice.

Figure 2

Summary of the relative expression of the ten most abundant GPCR mRNAs in outbred ICR mouse islets (a), inbred C57 mouse islets (b) and human islets (c). Colour coding in mouse islets according to similarities with human islet GPCR expression. White fields represent mouse islet GPCR mRNAs that are not among the ten most abundant GPCR mRNAs in human islets. Data for each GPCR are presented as mean % of the mRNA expression of all GPCRs in each islet type. (d,e) Species/strain distribution of all islet GPCR mRNAs in C57 and ICR mouse islets and human islets with all GPCR mRNAs (d) and only with GPCR mRNAs expressed above trace level (e).

Figure 3

(a,b) Mean relative mRNA expression of human GPCRs in human islets and mouse GPCR orthologues in islets from ICR (a) and C57 (b) mice. Data are presented as mean GPCR mRNA expression ± SEM relative to the reference genes ACTB, GAPDH, PPIA, TBP and TFRC. Data generated using non-pooled islet RNA from four ICR and four C57 mice and four non-pooled human islet preparations. Insert panels A and B represent enlarged areas of the scatter plots, added to allow visualisation of individual GPCR mRNAs. *: excluding bitter taste receptors (TAS2Rs).

Figure 4

(a,b) Species enrichment of the 121 islet GPCR mRNAs expressed above trace level in both human and mouse islets. Mean ICR and C57 GPCR mRNA expression levels were compared to the mRNA expression of human islet GPCRs. mRNAs encoding 68 GPCRs were >3 fold enriched in mouse islets (a), 17 of these GPCRs (Adra2a, CalCrl, Cckar, Chrm3, Fzd3, Galr1, Gipr, Glp1r, Gpr116, Gpr125, Gpr158, Gpr56, Gprc5c, Lgr4, Oxtr, Ptger3, Vipr1) are also among the 25 most abundant mouse islet GPCR mRNAs (highlighted with * in figure a). Four GPCR mRNAs were >3 fold enriched in human islets (b). Data are presented as a ratio of the mean GPCR expression relative to the reference genes ACTB, GAPDH, PPIA, TBP and TFRC in human and mouse islets (i.e. mean of GPCR expression in ICR and C57 islets), and was generated using non-pooled islet RNA from four ICR and four C57 mice and four non-pooled human islet preparations.

Figure 5. Relative expression in human islets of GPCRs that are absent in the mouse genome.

Data are presented as mean ± SEM GPCR expression relative to the reference genes ACTB, GAPDH, PPIA, TBP and TFRC. n = four human islet donors. T denotes trace expression.

Figure 6. Confirmation of mouse islet-specific expression and function of the A₃ (Adora3) receptor.

The A3 receptor agonist MRS 5698 had no effect on insulin secretion from human islets (a), but significantly inhibited glucose-stimulated insulin secretion from mouse islets (b). The G_i-coupled control agonist clonidine had significant inhibitory effects in both mouse and human islets. *p < 0.05; ***p < 0.001 versus secretion at 20 mM glucose.

Figure 7

The effect on insulin secretion by the galanin receptor agonist galanin in human (a) and mouse (b) islets. Human galanin-(1–30) had no effect on insulin secretion from human islets, whereas insulin secretion from mouse islets was significantly inhibited by mouse galanin-(2–29) (b). n = 8 islets in each treatment group. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 8

The effect of the Sst1 receptor agonist CH 275 on the secretion of insulin from human (a) and mouse (b) islets at 2 and 20 mM glucose. A dose-dependent inhibition of insulin secretion by CH 275 was observed in human islets (a), whereas CH275 had no effect on insulin secretion from mouse islets (b). *p < 0.05; ***p < 0.001.