|Inhibition of Neuronal Firing by Tubulin and CRMP2||Memory|
The synaptic strength is supposed to increase with increasing number of synaptic AMPA receptors (AMPARs), because they can generate larger amplitude of excitatory postsynaptic potential (EPSP) to facilitate excitation of the postsynaptic neuron. However, in some cases, such as the phosphorylation of CaMKII at T305/T306, synapse is depressed while AMPAR level remains elevated (see Chapter 6). Its underlying mechanism is discussed in this chapter.
Inhibition of NMDAR by Tubulin
Tubulin is the building block of microtubules. It has two isoforms, α and β, that usually form heterodimers. In a tubulin heterodimer, the number of negatively charged amino acids exceeds that of positively charged amino acids by about 50 (Minoura and Muto, 2006). Tubulin has been demonstrated to interacts with NMDA receptors (NMDARs), specifically at the C-terminal domain of NR1 (also known as GluN1) and NR2B (aka. GluN2B) subunits (van Rossum et al., 1999). The opening of NMDAR requires both glutamate binding and the relief of Mg2+ block. The latter depends on electric fields. During associative learning, the Mg2+ block is relieved by the membrane potential field when the postsynaptic neuron fires. If the NMDAR is bound by tubulin, the membrane potential field would not be able to repel Mg2+ out of the pore, because the highly negatively charged tubulin can produce an inward electric field to counteract the outward depolarizing field (Figure 7-1).
The interaction between NMDAR and tubulin alone is likely to be weak, as the amount of tubulin in the postsynaptic density (PSD) increases if the isolated brain slice has been kept on ice for longer period, but decreases if the isolated PSD sample is washed with detergent-containing solutions (Yun-Hong et al., 2011). Interestingly, tubulin is the canonical binding partner of collapsin response mediator protein 2 (CRMP2) (Fukata et al., 2002), which also interacts with NR2B (Brustovetsky et al., 2014). Therefore, CRMP2/tubulin may associate tightly with NR2B to inhibit the opening of NMDARs. Since NMDARs play a crucial role in learning and memory, CRMP2 should be involved in memory processing. Indeed, experiments have demonstrated that the antibody against CRMP2 causes amnesia (Mileusnic and Rose, 2011).
The Role of NMDA Spikes
During associative learning, both presynaptic and postsynaptic neurons should fire simultaneously to activate NMDARs. After learning, the synaptic strength is increased such that the glutamates released from presynaptic neurons suffice to excite a postsynaptic neuron. In response to presynaptic glutamate release, the elevated number of synaptic AMPARs can generate larger amplitude of EPSPs to facilitate postsynaptic firing. However, the contribution of NMDARs to neuronal firing is more complex. The opening of NMDARs requires not only glutamate binding, but also the relief of Mg2+ block. The EPSP generated in a single spine is too small to relieve Mg2+ block. Therefore, even in the absence of the hypothetical tubulin inhibition, NMDARs are already blocked by Mg2+. Then, how can tubulin influence neuronal firing?
After an action potential (AP) is initiated at the axon initial segment, it may propagate not only along the axon, but also backward to dendrites. The backward AP may relieve Mg2+ block, and if coincide with glutamate binding, can open NMDARs. Tubulin may inhibit the opening of NMDARs due to the backward AP. More importantly, tubulin can also control the initiation of AP by inhibiting NMDA spikes (Figure 7-2).
The NMDA spike is a membrane potential change resulting from synchronous activation of 10-50 neighboring synapses that leads to the opening of NMDARs (Schiller et al., 2000; Antic et al., 2010; Chalifoux and Carter, 2011). It plays a crucial role in burst firing (Figure 7-3), a pattern commonly used to convey encoded information. In cortical pyramidal neurons, the AMPAR-only EPSP is too small to trigger postsynaptic firing (Polsky et al., 2009). The larger membrane potential change generated by NMDA spikes is also required. Hence, inhibition of NMDARs by tubulin can have dramatic impact on the initiation of action potentials.
The Mechanism of Memory Extinction and Retrieval
Memory extinction refers to a brain state in which the memory still exists, but requires certain stimuli to recall. In contrast, memory erasure means that the physical memory traces have disappeared forever. The rest of this book will present evidence for the following postulates:
Author: Frank Lee