Cell type-specific differences in the coupling of recombinant mGlu1α receptors to endogenous G protein sub-populations

In this study the effects of cell background on the coupling of the type 1α metabotropic glutamate (mGlu1α) receptor to different G protein sub-populations by recombinant expression of this receptor subtype in baby hamster kidney (BHK) and Chinese hamster ovary (CHO) cells have been investigated. Receptor–G protein interactions were assessed using [ 35S]GTPγS binding and subsequent Gα subunit-specific immunoprecipitation. In a CHO cell line (CHO–lac–mGlu1α), where mGlu1α receptor expression is under inducible control, stimulation of membranes with the mGlu receptor agonist quisqualate resulted in an increase in specific [ 35S]GTPγS binding to G q/11α only, whereas in a BHK cell line (BHK–mGlu1α) agonist stimulation increased [ 35S]GTPγS binding to G q/11α and also to pertussis toxin (PTx)-sensitive G i/o proteins (assessed using G i1/2α- and G i3/oα-specific antibodies). These data are consistent with our previous observations of dual, antagonistic G q/11/G i/o regulation of phospholipase C (PLC) in BHK–mGlu1α cells, whereas no evidence was found for a G i/o modulation of PLC activity in the CHO–lac–mGlu1α cell line. PTx pre-treatment of either cell line had no effect on either the magnitude or the concentration-dependency of agonist-stimulated [ 35S]GTPγS–G q/11α binding, excluding the possibility that receptor–G i/o uncoupling can unmask an increase in receptor–G q/11 interaction. mGlu1α receptor expression per se had little effect on Gα protein expression levels in either CHO or BHK cell lines, with the possible exception of a small, but consistent increase in G oα expression in BHK–mGlu1α cells compared to the vector-transfected control cell line (BHK-570). Semi-quantitative assessment of mGlu1α receptor immunoreactivity and [ 3H]quisqualate saturation binding analysis demonstrated a ca 10-fold higher mGlu1α receptor content in BHK cells. Whether the higher receptor expression level in BHK–mGlu1α cells underlies the additional G i/o coupling observed in this cell line, or additional factors contribute to the phenomenon are discussed.