NMDA and β₁-Adrenergic Receptors Differentially Signal Phosphorylation of Glutamate Receptor Type 1 in Area CA1 of Hippocampus

Abstract

Glutamatergic synaptic transmission is mediated primarily through the AMPA-type glutamate receptor (AMPAR); the regulation of this receptor underlies many forms of synaptic plasticity. In particular, phosphorylation of GluR1, an AMPAR subunit, by PKA at serine 845 (S845) increases peak open channel probability and is permissive for both the synaptic expression of the receptor and NMDA-receptor (NMDAR)-dependent long-term potentiation (LTP). Robust NMDAR activation activates PKA as well as other signaling enzymes; however, we find that maximal NMDAR activation dephosphorylates GluR1 at the PKA site S845. Coincident inhibition of phosphatases blocks NMDAR-induced dephosphorylation of S845, but surprisingly does not promote PKA phosphorylation at this site. However, we find that phosphorylation of S845 is increased by the activation of a Gs-coupled receptor, the beta1-adrenergic receptor. Interestingly, this divergent signaling occurs despite a more robust coupling of the NMDAR to cAMP generation. In addition, NMDAR activation plays a dominant role in S845 regulation, because activation of beta1AR after NMDAR activation has no detectable effect on S845 phosphorylation. These data (1) demonstrate highly specific coupling between these receptors and this substrate, (2) provide an example of a substrate critical in NMDAR-dependent LTP that is incompletely regulated by the NMDAR, and (3) highlight the importance of identifying the physiological signals that regulate these critical synaptic substrates.

Novel Blockade of Protein Kinase A-Mediated Phosphorylation of AMPA Receptors

“…However, rather than observing a simple additive effect between phosphorylation and dephosphorylation on the S845 site of GluR1 in response to coactivation of the NMDAR and ␤AR, we have observed a more unconventional form of regulation such that previous NMDAR activation results in no detection of phosphorylation mediated by the ␤AR (Vanhoose and Winder, 2003). These data imply that, during NMDAR activation, a novel form of regulation at the S845 site of GluR1 occurs, such that a signal is generated that blocks GluR1 phosphorylation by PKA.…”

“…Preapplication of NMDA for 3 min followed by the addition of isoproterenol, a ␤AR agonist, for 3 min does not result in an additive effect between dephosphorylation and phosphorylation of GluR1, as might have been predicted. Rather, a dominant NMDAR-mediated dephosphorylation with no detectable phosphorylation of GluR1 at S845 is observed (Vanhoose and Winder, 2003), suggesting that NMDAR activation recruits a signal that mediates blockade of ␤AR-mediated regulation of GluR1.…”

“…NMDAR activation regulates the phosphorylation state of GluR1 at S845 via at least two distinct signaling mechanisms We and others have shown previously that NMDAR activation regulates GluR1 phosphorylation state at S845 through the activation of serine/threonine phosphatases, protein phosphatase 1 (PP1) and/or PP2A and PP2B, which recruit robust dephosphorylation Lee et al, 1998;Vanhoose and Winder, 2003). In addition, we have reported previously a potential second means of NMDAR-mediated regulation of GluR1 phosphorylation state that is manifest when a G s -coupled receptor that recruits GluR1 phosphorylation at S845 is coactivated with the NMDAR.…”

“…Several forms of AMPAR regulation have been identified, but of particular interest are two phosphorylation sites in the C-terminal tail of the GluR1 subunit that play a critical role in channel localization, biophysical properties, and plasticity (Roche et al, 1996;Derkach et al, 1999;Banke et al, 2000;Chao et al, 2002;Derkach, 2003;Esteban et al, 2003;Lee et al, 2003). The serine 831 site (S831) is phosphorylated by calcium/ calmodulin-dependent kinase II (CaMKII) and protein kinase C (PKC) (Roche et al, 1996;Mammen et al, 1997;Derkach et al, 1999) and is readily phosphorylated in response to NMDAR activation (Barria et al, 1997;Lee et al, 2000;Vanhoose and Winder, 2003). A second site of GluR1 regulation occurs at serine 845 (S845), which is phosphorylated by protein kinase A (PKA) (Roche et al, 1996).…”

“…A second site of GluR1 regulation occurs at serine 845 (S845), which is phosphorylated by protein kinase A (PKA) (Roche et al, 1996). Several G s -coupled receptors such, as the ␤ 1 -adrenergic receptor (␤ 1 AR) and the D 1 dopamine receptor, robustly couple to phosphorylation of GluR1 at S845 (Price et al, 1999;Snyder et al, 2000;Chao et al, 2002;Vanhoose and Winder, 2003). Conversely, NMDAR activation readily recruits dephosphorylation of this site Lee et al, 1998;Snyder et al, 2003;Vanhoose and Winder, 2003).…”