|Short Term Potentiation
May Result from Tubulin Inhibition of NMDA Receptors
Experimentally, long-term potentiation (LTP) can be induced by several different protocols (Shipton and Paulsen, 2013, Table 1). One of them, referred to as "tetanus", applies 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). In most cases, the time course of f-EPSP consists of two phases: an initial decaying phase and a stable phase (Figure 1). LTP refers to the stable phase while the initial decaying phase is known as short-term potentiation (STP) (Volianskis et al., 2013; Park et al., 2013).
The mechanism of STP is not known. Several lines of evidence suggest that it differs from LTP (Schulz and Fitzgibbons, 1997). This paper will show that STP could originate from microtubule invasion into spines, resulting in the inhibition of NR2B-containing NMDA receptors (NMDARs) by tubulin and CRMP2 (Paper 15).
Microtubule Invasion into Spines
The evidence that microtubules might play a role in synaptic plasticity came in 2008, when three independent groups reported that microtubules could enter spines in an activity-dependent manner (reviewed in Dent, 2017). Upon strong synaptic stimulation, microtubules were shown to polymerize all the way to postsynaptic density (PSD) (Figure 2) and, within 20 seconds to 30 minutes, depolymerize back to the dendritic shaft (Hu et al., 2008). The functional role of the transient entry into spines is not clear.
The major function of microtubules is to transport various cargoes such as proteins and mitochondria. Presumably, during the short visit to spines, microtubules must bring in certain cargoes that are important for synaptic plasticity. However, the particular cargoes remain elusive. CaMKII is an essential protein in spines, but its entry into spines and PSD depends on F-actin, not microtubules (see this figure). PSD-95, another crucial protein in spines, is regulated by brain-derived neurotrophic factor (BDNF). Although the BDNF-mediated increase of PSD-95 in spines requires dynamic microtubule invasion, PSD-95 was not directly transported along microtubules into dendritic spines (Hu et al., 2011).
The Tubulin Inhibition Hypothesis suggests that the major cargoes transported by the dynamic microtubules into spines could be tubulin and CRMP2. Tubulin is the canonical binding partner of CRMP2. It has been demonstrated that the tubulin/CRMP2 complex can be transported by microtubules via the motor protein Kinesin-1 (Kimura et al., 2005).
The Mechanism of STP
STP might originate from microtubule invasion into spines, resulting in the inhibition of NR2B-containing NMDARs by tubulin and CRMP2. The NR2A-containing NMDARs will not be affected, contributing to the sustained phase of f-EPSPs (i.e., LTP). This hypothesis is supported by the following findings.
The Binding Site of Tubulin
Paper 15 proposes that the NR2B-containing NMDARs can be inhibited by tubulin. Experiments have revealed that tubulin binds to the C-terminal domain of NR2B (van Rossum et al., 1999), but the exact binding site was not elucidated. It has been known for over two decades that protein kinase A (PKA) regulates the Ca2+ influx through NMDARs:
More specifically, PKA inhibition decreased the open probability of NR2B-, but not NR2A-containing NMDARs (Aman et al., 2014), suggesting that PKA targets NR2B (also known as GluN2B), not NR2A (GluN2A). Therefore, PKA seems to counteract the inhibition of NR2B-containing NMDARs by tubulin. If this is indeed the case, tubulin should bind to NR2B around the PKA phosphorylation site (Figure 3). Recently, the PKA phosphorylation site on NR2B has been identified as Serine 1166 (Murphy et al., 2014). Tubulin is a highly negatively charged protein while phosphorylation is a process that adds a negatively charged phosphate group PO43- to a protein. Hence, phosphorylation of this site should disrupt tubulin binding, thereby increasing the open probability of NR2B-containing NMDARs.
Memory extinction and retrieval could be fundamentally governed by tubulin inhibition of NR2B-containing NMDARs, which in turn depends on the phosphorylation state of S1166. Details are presented in the book, Born to Forget, Die to Remember.
Author: Frank Lee