Abstract
G-protein-coupled receptors are well known for converting an extracellular signal into an intracellular response. Here we showed that the metabotropic glutamate receptor 5 (mGlu5) plays a dynamic intracellular role in signal transduction. Activation of endogenously expressed mGlu5 on striatal nuclear membranes leads to rapid, sustained calcium (Ca2+) responses within the nucleoplasm that can be blocked by receptor-specific antagonists. Extracellular ligands such as glutamate and quisqualate reach nuclear receptors via both sodium-dependent transporters and cystine glutamate exchangers. Inhibition of either transport system blocks radiolabeled agonist uptake as well as agonist-induced nuclear Ca2+ changes. Impermeable antagonists like LY393053 and LY367366 not only blocked [3H]quisqualate binding but also prevented nontransported agonists such as (RS)-3,5-dihydroxyphenylglycine from inducing intracellular Ca2+ changes in heterologous cells. In contrast, neither LY compound prevented quisqualate or glutamate from activating intracellular receptors leading to Ca2+ responses. Inasmuch as Ca2+ can enter the nucleoplasm via the nuclear pore complex or from the nuclear lumen, the presence of nuclear mGlu5 receptors appeared to amplify the latter process generating a faster nuclear response in heterologous cells. In isolated striatal nuclei, nuclear receptor activation results in the de novo appearance of phosphorylated CREB protein. Thus, activation of nuclear mGlu5 receptors initiates a signaling cascade that is known to alter gene transcription and regulate many paradigms of synaptic plasticity. These studies demonstrated that mGlu5 receptors play a dynamic role in signaling both on and off the plasma membrane.
Highlights
G-protein-coupled receptors are well known for converting an extracellular signal into an intracellular response
Wells stained on the 9th day in vitro (DIV) showed numerous immunofluorescent neurons in which metabotropic glutamate receptor 5 (mGlu5) could be visualized on cell processes, intracellularly, and on the nuclear membrane (Fig. 1B)
Contrary to the idea that intracellular mGlu5 receptors are nonfunctional and merely constitute an internal reserve of receptors waiting to go to the cell surface, the present results demonstrate that mGlu5 receptors play dynamic intracellular roles in signal transduction
Summary
Activation of endogenously expressed mGlu on striatal nuclear membranes leads to rapid, sustained calcium (Ca2؉) responses within the nucleoplasm that can be blocked by receptor-specific antagonists. Extracellular ligands such as glutamate and quisqualate reach nuclear receptors via both sodiumdependent transporters and cystine glutamate exchangers. Immunofluorescent staining of embryonic or neonatal striatal cultures shows widespread mGlu expression; agonist exposure elicits an intracellular rise in Ca2ϩ typified by an initial increase followed by a sustained plateau This response can be blocked by application of the mGlu5-specific antagonist MPEP [15]. Agonist-induced nuclear receptor activation leads to phosphorylation of CREB Taken together, these data challenge existing paradigms by suggesting a new mode of intracellular signal transduction mediated in situ by G-protein-coupled receptors
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have