Characterisation of the adiponectin receptors: the non-conserved N-terminal region of AdipoR2 prevents its expression at the cell-surface.

Adiponectin is a beneficial adipokine with insulin-sensitizing, anti-inflammatory and anti-atherogenic effects. These effects are mediated by two poorly characterised, closely related, atypical seven-transmembrane receptors. In the current report we have used C-terminal, epitope-tagged AdipoR1 and AdipoR2 constructs to monitor cell-surface expression by indirect immunofluorescence microscopy and quantitative plate-based analysis. We demonstrate that only AdipoR1 is constitutively expressed on the cell-surface. Further investigations, involving characterisation of a number of chimeric and truncated constructs, show the non-conserved region of AdipoR2 (residues 1-81) restricts its cell-surface expression. Introduction or deletion of this region, into AdipoR1 or AdipoR2, resulted in inhibition or promotion of cell-surface expression, respectively. We also confirmed that AdipoR1 and AdipoR2 can form heterodimers when co-expressed and that co-expression leads to the cell-surface expression of AdipoR2. Collectively these studies demonstrate that the non-conserved region of AdipoR2 restricts its cell-surface expression and raise the possibility that the majority of cell-surface AdipoR2 may be present in the form of heterodimers.