The metabotropic glutamate receptor 4 is internalized and desensitized upon protein kinase C activation

1 The metabotropic glutamate receptor 4 (mGluR4) is a Ga-i-coupled receptor that modulates glutamatergic neurotransmission. As mGluR4 expression and activation have been implicated in a number of pathological conditions and because the internalization and desensitization properties of this receptor are poorly understood, studies were designed to investigate these aspects of mGluR4 biology. 2 Neither agonist activation by L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) nor L-glutamate caused mGluR4 internalization when cmyc-tagged mGluR4 was expressed in a human embryonic kidney 293 cell line as assessed by cell surface enzyme-linked immunosorbent and immunostaining assays. Instead, a modest increase in mGluR4 surface expression was observed and found to be receptor specific as the competitive antagonist alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) blocked this effect. 3 In contrast, mGluR4 internalized when the protein kinase C (PKC) pathway was activated either by phorbol-12-myristate-13-acetate (PMA) or by the activation of the G alpha(q)-coupled, neurokinin 3 receptor (NK3R) when co-expressed. This process was PKC-dependent as the specific PKC inhibitor GF 109203X inhibited PMA and NK3R-mediated internalization. 4 PKC activation by PMA caused desensitization of mGluR4 as measured by forskolin-stimulated cAMP inhibition, whereas agonist activation had no effect on desensitization. 5 When mGluR4's coupling was redirected from adenylyl cyclase to phospholipase C by coexpression of a chimeric G alpha(qo5) protein, mGluR4 both internalized and desensitized in response to its agonists. 6 These findings demonstrate that mGluR4 internalization and desensitization are agonist-independent unless pathways leading to the activation of PKC are induced.