The adiponectin axis is a major regulator of metabolic, cardiovascular and inflammatory tone, and area of therapeutic opportunity. The receptors for adiponectin, AdipoR1 (R1) and AdipoR2 (R2), are distant relatives of the largest single class of drug targets, the G-protein coupled receptor (GPCR) family. Unlike GPCRs they have intracellular N-termini and extracellular C-termini and signal via atypical pathways. Current understanding of R1 and R2 is rudimentary, constraining our ability to target these receptors. We hypothesised that several of the molecular mechanisms that govern cell-surface expression of the GPCRs would be conserved with the AdipoRs and have employed cellular and bioinformatics approaches to address this.
Using a series of C-terminal, epitope-tagged R1 and R2 WT, chimeric and truncated constructs we demonstrated that only R1 was readily-detected on the cell surface under steady-state conditions. A non-conserved, intracellular, N-terminal region of R2 (R2(1-81)) restricted cell-surface expression. Co-expression of R1 with R2 lead to the formation of hetero-dimers and cell-surface expression of R2. Complementary investigations revealed that a conserved GxxxG dimerisation motif present in both R1 and R2 was required for homo- and hetero-dimerisation and essential for efficient cell-surface expression. A dimerisation incompetent R1 mutant failed to promote cell-surface expression of R2, but was without effect on WT R1, further highlighting the dependence of R2 on functional R1. Finally, we demonstrated that palmitoylation of a conserved Cysteine, situated in the juxtamembrane region of the N-termini of R1 and R2, was required for efficient cell-surface expression of both R1 and R2.
Collectively these results highlight fundamental differences between R1 and R2 and demonstrate that cell-surface expression of both receptors is dependent on obligate-dimerisation and palmitoylation. Further studies are required to elaborate the mechanisms governing AdipoR trafficking and determine whether alterations in these processes contribute to the aetiology of adiponectin resistance and/or can be targeted therapeutically.