Functional genetics reveals the contribution of delta opioid receptor to type 2 diabetes and beta-cell function

Abstract

Functional genetics has identified drug targets for metabolic disorders. Opioid use impacts metabolic homeostasis, although mechanisms remain elusive. Here, we explore the OPRD1 gene (encoding delta opioid receptor, DOP) to understand its impact on type 2 diabetes. Large-scale sequencing of OPRD1 and in vitro analysis reveal that loss-of-function variants are associated with higher adiposity and lower hyperglycemia risk, whereas gain-of-function variants are associated with lower adiposity and higher type 2 diabetes risk. These findings align with studies of opium addicts. OPRD1 is expressed in human islets and beta cells, with decreased expression under type 2 diabetes conditions. DOP inhibition by an antagonist enhances insulin secretion from human beta cells and islets. RNA-sequencing identifies pathways regulated by DOP antagonism, including nerve growth factor, circadian clock, and nuclear receptor pathways. Our study highlights DOP as a key player between opioids and metabolic homeostasis, suggesting its potential as a therapeutic target for type 2 diabetes.

Fig. 1. Detection of 34 OPRD1 coding variants in RaDiO.

Black variants are neutral variants according to our in vitro assays; red variants are loss-of-function variants that are genetically null and/or deleterious according to our in vitro assays; green variants are gain-of-function variants according to our in vitro assays. Transmembrane domains are highlighted in yellow.

Fig. 2. Expression of OPRD1 in human pancreatic beta cells and islets, under conditions related to type 2 diabetes.

A Expression of OPRD1, OPRK1, and OPRM1 in human pancreatic islets (n = 11)^,, and expression of Oprd1, Oprk1, and Oprm1 in mouse pancreatic islets (n = 14)^,, assessed via RNA-seq. Data are mean ± SEM of TPM. B Expression of OPRD1, OPRK1, and OPRM1 was assessed in human purified pancreatic beta cells (n = 11) and beta-cell depleted islet preparations (i.e., non-beta cells; n = 5). Data are mean ± SEM of RPKM. C OPRD1 expression in human islets under different conditions and in type 2 diabetes. Brefeldin A (0.1 μg/mL; orange) vs control (green), 24 hours treatment (n = 4); a combination of palmitate (0.5 mM) and high glucose (22.2 mM) (orange) vs control (green), 48 hours treatment (n = 3); high glucose (22.2 mM; orange) vs control (green), 48 hours treatment (n = 4); palmitate (0.5 mM; orange) vs control (green), 48 hours treatment (n = 5); donors with type 2 diabetes (n = 47; orange) vs non-diabetic controls (n = 228; green). Data are box plots (showing the minimum, the median, the maximum, as well as all data). Adjusted P-values are shown relative to control and were calculated according to DESeq2 (v1.28.1; two-sided). ns not significant, RPKM reads per kilobase million, TPM transcripts per million.

Fig. 3. DOP inhibition activates insulin secretion from human beta cells and islets.

(A) Insulin secretion was assessed in EndoCβH5 cells treated with no glucose (green; LG) either with 10 μM NTI (n = 10 independent cell preparations), 5 μM DII (n = 12), a combination of both (n = 12) or no treatment (n = 12), for 40 min. (B) Insulin secretion was assessed in EndoCβH5 cells treated with 20 mM glucose (violet; HG) supplemented with 10 μM NTI (n = 10), 5 μM DII (n = 12), a combination of both (n = 12) or no treatment (n = 11), for 40 min. (C) Insulin secretion was assessed in human islets treated with 2.8 mM glucose (green; LG) supplemented with 10 μM NTI (n = 24 single isolated islets), 5 μM DII (n = 24), a combination of both (n = 29), or no treatment (n = 28), for 120 min. (D) Insulin secretion was assessed in human islets treated with 16.7 mM glucose (violet; HG) supplemented with 10 μM NTI (n = 23), 5 μM DII (n = 24), a combination of both (n = 29) or no treatment (n = 28), for 120 min. Data are box plots (showing the minimum, the median, the maximum, as well as all data). P-values are shown relative to control by unpaired two-tailed Mann-Whitney test. DII, [D-Ala2]-Deltorphin II; HG, high glucose; LG, low glucose; ns, not significant; NTI, naltrindole.