|The Role of Tubulin/Microtubule in Epilepsy||MT|
Epilepsy is characterized by neuronal hyperexcitability and hypersynchronization. As discussed in Chapter 6 and Chapter 7, microtubules may play a crucial role in both excitability and synchronization through their interaction with the sodium channel Nav1.6 (SCN8A) and Ankyrin-G at the axon initial segment (AIS). Co-localization of Nav1.6 and Ankyrin-G has been implicated in epileptogenesis by regulating neuronal excitability (Chen et al., 2009; Feng et al., 2013). SCN8A mutations have also been discovered in patients with epilepsy (Veeramah et al., 2012; O'Brien and Meisler, 2013; Blanchard et al., 2015; Boerma et al., 2015). The importance of microtubules in epilepsy is further underscored by the microtubule-associated protein Tau.
The Role of Tau Protein in Epilepsy
According to the model proposed in Chapter 7, microtubules may regulate neuronal excitability through association with the membrane mediated by Ankyrin-G at AIS. Tau proteins may modulate excitability by interfering with the binding between microtubules and Ankyrin-G (see Chapter 9 and Chapter 10 in Alzheimer's Disease). Elevation of Tau proteins, especially 4-repeat Tau, increases the average distance between microtubules and the membrane, thereby enhancing excitability. This mechanism is supported by the following observations.
The Role of Tubulin in Preventing Seizures
A microtubule is composed of α and β tubulin heterodimers. The α tubulin has 65 acidic residues (Asp and Glu) and 40 basic residues (Lys and Arg) while in the β tubulin 62 residues are acidic and 37 residues are basic (Minoura and Muto, 2006). Therefore, at the physiological pH value, a tubulin dimer contains about 50 net negative residues. These negative charges may exert a hyperpolarizing field on the ion channels in the membrane. However, unlike microtubules which regulate excitability via association with the AIS membrane, tubulin may influence excitability through association with the synaptic membrane.
Tubulin is present in the postsynaptic density (PSD), a protein complex attached to the postsynaptic membrane of the dendritic spine. By using a Nano-Depth-Tagging method, the positions of various proteins in PSD have been determined (Yun-Hong et al., 2011, Figure 9). Tubulin is located just underneath the postsynaptic membrane which contains a number of ion channels such as NMDA receptor (NMDAR), AMPA receptor (AMPAR) and voltage-gated calcium channels (VGCC) (Higley and Sabatini, 2012). The hyperpolarizing field from tubulin may reduce the open probability of VGCC, but has no effect on the ligand-gated AMPAR.
Although NMDAR is intrinsically ligand-gated (by glutamate), its open probability also depends on Mg2+ block (Mayer et al., 1984). A depolarizing field may repel the Mg2+ ion out of the pore to facilitate channel opening, whereas a hyperpolarizing field can strengthen the Mg2+ block. The Mg2+ block plays a critical role in preventing seizures. By removing Mg2+ ions from the medium, the rat hippocampal slice exhibited spontaneous seizure-like events (Anderson et al., 1986). Hence, the highly negatively charged tubulin may attenuate seizure activity by enhancing Mg2+ block on NMDAR and reducing the open probability of VGCC. In patients with temporal lobe epilepsy, significant reduction of α and β tubulin has been observed (Yang et al., 2006). More specifically, their expression at PSD is down-regulated (Conference Abstract, 2009). Furthermore, mutations in the β-tubulin gene TUBB2A cause infantile-onset epilepsy (Cushion et al., 2014).
CaMKII and Epilepsy
Localization of tubulin to PSD is regulated by Ca2+/calmodulin-dependent protein kinase II (CaMKII), which is the major target of Ca2+ ions that enter the spine through NMDA receptors. In the presence of calmodulin, the transient Ca2+ influx induces persistent activation of CaMKII (Figure 2). The activated CaMKII regulates synaptic strength and spine size by phosphorylating a wide variety of proteins in PSD (Pi et al., 2010), including tubulin (Yoshimura et al., 2000).
The activation (phosphorylation) of CaMKII also promotes its association with PSD. Inactivation will cause CaMKII to slowly exit PSD (Yoshimura and Yamauchi, 1997), together with the associated proteins such as tubulin. Hence, reduced CaMKII activity is linked to epileptogenesis (Churn et al., 2000; Singleton et al., 2005). The anti-epileptic drug, Ganoderma lucidum polysaccharides, has been shown to stimulate the expression of CaMKII (Wang et al., 2014).
Author: Frank Lee