Detection of Mu Opioid Receptor (MOPR) and Its Glycosylation in Rat and Mouse Brains by Western Blot with Anti-μC, an Affinity-Purified Polyclonal Anti-MOPR Antibody

Abstract

Our experience demonstrates that it is difficult to identify MOPR in rat and mouse brains by western blot, in part due to low abundance of the receptor and a wide relative molecular mass (M_r) range of the receptor associated with its heterogeneous glycosylation states. Here, we describe generation and purification of anti-μC (a rabbit polyclonal anti-MOPR antibody), characterization of its specificity in immunoblotting of HA-tagged MOPR expressed in a cell line, and ultimately, unequivocal detection of the MOPR in brain tissues by western blot with multiple rigorous controls. In particular, using brain tissues from MOPR knockout (K/O) mice as the negative controls allowed unambiguous identification of the MOPR band, since the anti-MOPR antibody, even after affinity purification, recognizes nonspecific protein bands. The MOPR was resolved as a faint, broad, and diffuse band with a wide M_r range of 58–84 kDa depending on brain regions and species. Upon deglycosylation to remove N-linked glycans by PNGase F (but not Endo H), the MOPR became a dense and sharp band with M_r of ~43 kDa, close to the theoretical M_r of its deduced amino acid sequences. Thus, MOPRs in rodent brains are differentially glycosylated by complex type of N-linked glycans in brain region- and species-specific manners. Furthermore, we characterized the MOPR in an A112G/N38D-MOPR knockin mouse model that possesses the equivalent substitution of the A118G/N40D SNP in the human MOPR gene. The substitution removes one of the four and five N-linked consensus glycosylation sites of the mouse and human MOPR, respectively. We demonstrated that the M_r of the MOPR in A112G mouse brains was lower than that in wild-type mouse brains, and that the difference was due to lower degrees of N-linked glycosylation.