|Memory > Regulation of both LTP and LTD by Ca2+|
It has been well established that the two opposing synaptic modulations, long-term potentiation (LTP) and long-term depression (LTD), are controlled primarily by the Ca2+ influx through NMDA receptors (NMDARs). How can Ca2+ ions induce opposing modulations?
An NMDAR consists of two GluN1 (formerly NR1) subunits and two additional subunits which are predominately either GluN2A (NR2A) or GluN2B (NR2B). Other subunits, GluN3, GluN2C and GluN2D, are relatively rare. Decades of intensive research has revealed that either GluN2A- or GluN2B-containing NMDARs are capable of supporting LTP induction, provided they can mediate sufficient Ca2+ influx (Shipton and Paulsen, 2013). However, GluN2B is critical for the NMDAR-dependent LTD as the GluN2B-selective antagonist, ifenprodil or Ro 25-6981, abolishes LTD, but not LTP (Liu et al., 2004; France et al., 2017). In support of the CABT Hypothesis for memory extinction, both CRMP2 and tubulin also interact with GluN2B, not GluN2A (van Rossum et al., 1999; Brustovetsky et al., 2014).
This chapter will explain how the NMDAR-mediated Ca2+ influx can induce opposing synaptic modulations. An important factor is the kinetics of NMDAR currents: upon activation, the GluN2B-containing NMDAR currents rise and fall much slower than GluN2A-containing NMDAR currents (Erreger et al., 2005). Another crucial factor that leads to bi-directional plasticity is the location of A-kinase anchoring protein which associates with GluN2B (Colledge et al., 2000).
The Roles of AKAP79/150
AKAP79/150 refers to either A-kinase anchoring protein 79 (AKAP79) in humans or AKAP150 in rodents. They have essentially the same functions. It is a scaffold protein that organizes protein kinase A (PKA), protein kinase C (PKC) and calcineurin (CaN, also known as PP2B) at a specific subcellular location to restrict their substrate targeting. For instance, the PKA anchored to GluN2B-containing NMDARs should be more effective in phosphorylating GluN2B than the PKA diffusing randomly in the cytoplasm. Importantly, Ca2+ ions can enhance the activities of all three anchored enzymes: PKA, PKC and CaN.
AKAP79/150, together with anchored enzymes, can be recruited to the postsynaptic membrane by interacting with F-actin and phosphatidylinositol-4,5-bisphosphate (PIP2) which are regulated by PKC (Gomez et al., 2002). It also interacts with PSD-95 and other membrane-associated guanylate kinase (MAGUK) such as SAP97 (Sanderson and Dell'Acqua, 2011). Recently, Woolfrey et al. (2018) revealed that removal of AKAP79/150 from spines requires Ca2+/calmodulin-dependent protein kinase II (CaMKII). As we shall see later, this could be the underlying mechanism for persistent memory extinction.
AKAP79/150 has been demonstrated to associate with the GluA1 (formerly GluR1) subunit of AMPA receptors (AMPARs) and the GluN2B subunit of NMDARs (Colledge et al., 2000). LTP or LTD is fundamentally determined by the increase or decrease in synaptic AMPARs, respectively. This in turn depends on the phosphorylation state of S845 and S831 in the GluA1 subunit of AMPARs. S845 is the target of both PKA and CaN while S831 can be phosphorylated by PKC and CaMKII. Phosphorylation on S845 or S831 stimulates synaptic incorporation of AMPARs, thus promoting LTP. CaN can dephosphorylate S845, resulting in LTD (Henley and Wilkinson , 2013; Woolfrey and Dell'Acqua, 2015, Figure 2).
How the Brief Tetanic Stimulation Induces LTP
Experimentally, LTP can be induced by several different protocols (Shipton and Paulsen, 2013, Table 1). One of them, referred to as "tetanus", applies strong high frequency (~100 Hz) stimulation on the presynaptic neuron for about 1 second. This leads to postsynaptic potentiation as monitored by field excitatory postsynaptic potentials (f-EPSPs) (Chapter 17).
Recalling that the GluN2B-containing NMDAR currents have slow kinetics. Therefore, the Ca2+ influx triggered by the brief tetanic stimulation should pass through mainly the GluN2A-containing NMDARs which are NOT associated with AKAP79/150. Hence, the major enzyme that induces LTD, CaN, is not significantly affected. The other two enzymes anchored by AKAP79/150, PKA and PKC, will also remain mostly inactive. In this case, LTP should arise mainly from phosphorylation on AMPARs and stargazin by CaMKII as described in Chapter 5.
Without contributions from PKA and PKC, a single tetanic stimulation typically generates lower levels of potentiation (Huganir and Nicoll, 2013). Multiple tetanic stimulations separated by a few minutes may induce PKA-dependent LTP via phosphorylation of GluA1 at S845, leading to the synaptic incorporation of calcium-permeable AMPARs (Park et al., 2016).
How the Prolonged Low Frequency Stimulation Induces LTD
The most commonly used protocol to induce LTD is a weaker low frequency (~ 1 Hz) stimulation on the presynaptic neuron for about 15 minutes. The prolonged weaker low frequency stimulation (LFS) will be able to trigger substantial Ca2+ influx through GluN2B-containing NMDARs (Erreger et al., 2005; Shipton and Paulsen, 2013). These Ca2+ ions should have significant impact on the activities of anchored enzymes. However, there are two competing processes on synaptic plasticity. CaN catalyzes the dephosphorylation, while PKA stimulates the phosphorylation, of S845. Which will win? The fact that prolonged LFS induces LTD indicates that CaN dominates. How?
As mentioned above, AKAP79/150 is associated with PSD-95 in the postsynaptic density (PSD, a structure beneath the postsynaptic membrane). The Ca2+ influx through NMDARs may activate CaN to dissociate AKAP79/150 from PSD-95, accompanied by F-actin reorganization (Gomez et al., 2002). This structural change favors CaN to dephosphorylate GluA1 at S845, promoting AMPAR endocytosis. Additional activation of CaMKII may cause removal of AKAP79/150 (together with PKA) from spines, resulting in spine shrinkage (Woolfrey et al., 2018). This prevents re-phosphorylation and recycling of AMPARs during LTD (Sanderson and Dell'Acqua, 2011). Importantly, phospholipase C (PLC) is required for LTD (Horne and Dell'Acqua, 2007). PLC activation is known to trigger the release of Ca2+ from the internal store (see this article), which may contribute to the activation of CaMKII. Furthermore, the hydrolysis of PIP2 catalyzed by PLC may release cofilin to depolymerize F-actin (Kanellos and Frame, 2016).
The above mechanism is consistent with LTD at different developmental stages. It has been known for many years that LFS produces robust LTD in hippocampal slices from very young rodents (mice or rats), but not from adult animals (Kemp et al., 2000; Milner et al., 2004). The hippocampal neurons express predominately GluN2B at birth, which then decreases into adulthood while GluN2A increases with age (Dong et al., 2006). Therefore, the young rodents, but not aged, may contain sufficient GluN2B-associated CaN to produce LTD.
Author: Frank Lee