Cell type-specific differences in the coupling of recombinant mGlu1 alpha receptors to endogenous G protein sub-populations

In this study the effects of cell background on the coupling of the type 1 alpha metabotropic glutamate (mGlu1 alpha) 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 invest igated. Receptor-G protein interactions were assessed using [S-35]GTP gamma S binding and subsequent G alpha subunit-specific immunoprecipitation. In a CHO cell line (CHO-lac-mGlu1 alpha), where mGlu1 alpha receptor expression is under inducible control, stimulation of membranes with the mGlu recepto r agonist quisqualate resulted in an increase in specific [S-35]GTP gammaS binding to G(q11)alpha only, whereas in a BHK cell line (BHK-mGlu1 alpha) a gonist stimulation increased [S-35]GTP gammaS binding to G(q/11)alpha and a lso to pertussis toxin (PTx)-sensitive G(i/o) proteins (assessed using G(i1 /2)alpha- and G(i3/o)alpha -specific antibodies). These data are consistent with our previous observations of dual, antagonistic G(q/11)G(i/o) regulat ion of phospholipase C (PLC) in BHK-mGlu1 alpha cells, whereas no evidence was found for a G(i/o) modulation of PLC activity in the CHO-lac-mGlu1 alph a cell line. PTx pre-treatment of either cell line had no effect on either the magnitude or the concentration-dependency of agonist-stimulated [S-35]G TP gammaS-G(q/11)alpha binding, excluding the possibility that receptor-G(1 /o) uncoupling can unmask an increase in receptor-G(q/11) interaction. mGlu 1 alpha receptor expression per se had little effect on G alpha protein exp ression levels in either CHO or BHK cell lines, with the possible exception of a small, but consistent increase in G(o)alpha expression in BHK-mGlu1 a lpha cells compared to the vector-transfected control cell line (BHK-570). Semi-quantitative assessment of mGlu1 alpha receptor immunoreactivity and [ H-3]quisqualate saturation binding analysis demonstrated a ca 10-fold highe r mGlu1 alpha receptor content in BHK cells. Whether the higher receptor ex pression level in BHK-mGlu1 alpha cells underlies the additional G(1/o) cou pling observed in this cell line, or additional factors contribute to the p henomenon are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved .