Geon Tubulin Inhibition:
Evidence from GluN2B Phosphorylation by PKA
Memory

 

It has been known for over two decades that protein kinase A (PKA) regulates the Ca2+ influx through NMDA receptors (NMDARs):

  1. The NMDAR-mediated excitatory postsynaptic currents (EPSCs) were enhanced by PKA phosphorylation (Raman et al., 1996).
  2. PKA blockers markedly inhibited NMDAR-mediated Ca2+ rises (Skeberdis et al., 2006).

More specifically, PKA inhibition decreased the open probability of GluN1/GluN2B but not GluN1/GluN2A receptors (Aman et al., 2014), suggesting that PKA targets GluN2B (also known as NR2B), not GluN2A (NR2A). This result agrees with the finding that PKA associates with GluN2B via the anchoring protein AKAP79/150 (Chapter 8). The crucial PKA phosphorylation site has been identified as Serine 1166 (S1166), which is distinct from the CaMKII binding region, around 1290-1309 (Strack et al., 2000). Loss of this single phosphorylation site abolishes PKA-dependent potentiation of NMDAR Ca2+ permeation, synaptic currents, and Ca2+ rises in dendritic spines (Murphy et al., 2014).

Chapter 7 proposes that the GluN2B-containing NMDARs can be inhibited by tubulin. Experiments have revealed that tubulin binds to the C-terminal domain of GluN2B (van Rossum et al., 1999), but the exact binding region is not known. Tubulin is a highly negatively charged protein while phosphorylation is a process that adds a negatively charged phosphate group PO43- to a protein. Therefore, the Tubulin Inhibition Model for memory extinction predicts that tubulin should bind to GluN2B around the PKA phosphorylation site, S1166, as phosphorylation of this site would disrupt tubulin binding, thereby increasing the open probability of GluN2B-containing NMDARs.

Image

Figure 15-1. The crucial sites in the cytoplasmic domain of GluN2B.
(1) S1166 is the target of PKA phosphorylation, which prevents NMDAR inhibition by tubulin. Calcineurin may dephosphorylate S1166, thus promoting tubulin inhibition.
(2) CaMKII binds to the region 1290-1309.
(3) BDNF promotes phosphorylation at Y1472, which prevents GluN2B-containing NMDARs from being internalized, consequently increasing their localization to the membrane (Chapter 12).

Calcineurin (protein phosphatase 2B) catalyzes the dephosphorylation process. It has been shown that calcineurin competes with PKA for the same site in NMDARs (Raman et al., 1996). While PKA prevents tubulin inhibition of the GluN2B-containing NMDARs, dephosphorylation of S1166 by calcineurin should have the opposite effects. Indeed, calcineurin activation has been demonstrated to reduce NMDAR currents (Raman et al., 1996), and play an important role in promoting memory extinction (Lin et al., 2003). The next several chapters will show that memory extinction and retrieval are fundamentally governed by calcineurin and PKA acting on the same site in GluN2B: S1166.

 

Author: Frank Lee
First Published: November, 2017