|Microtubules As the Receiving Antennas||MT|
The receiving antenna is an essential element in wireless communication which could also be employed by the brain (see previous chapter). Its purpose is to convert the received electromagnetic (EM) waves into other physical forms that can directly affect the receiving system. For instance, the EM waves cannot interact directly with the electric circuit in a radio or television. They must be converted into electric currents by the metal antenna. In the brain, neuronal excitability is fundamentally governed by the opening and closing of ion channels, which in turn depends on membrane potential (membrane voltage). The EM waves cannot modify directly the channel gating or membrane potential. Some kind of "antenna" is required to enable EM waves to modulate excitability. As discussed in Chapter 1, the microtubule is highly negatively charged, distributed over the entire molecule. This property is well suited for serving as an antenna in a neuron.
How the Metal Antenna Works
The metal rod is a conductor, in which electrons can flow in either direction, giving rise to electric currents. The EM wave consists of electric and magnetic fields, both can exert a combined force (Lorentz force) on the electrons, causing them to flow along the rod. Figure 1 shows only the effects of the electric field.
How the Microtubule Antenna Works
The association of a negatively charged microtubule with the membrane from the intracellular side is equivalent to application of a hyperpolarizing field on the membrane. Dissociation of the microtubule from the membrane has the same effect as depolarization. Hence, microtubules can be used to regulate neuronal excitability, especially if they are localized to the axon initial segment (AIS), which contains high density of voltage gated ion channels. Nerve impulses (action potentials) are initiated at the AIS (Buffington and Rasband, 2011).
The AIS can be divided into three layers as illustrated in Figure 2. During early development, the thickness of the submembrane coat varies in the range 3–11 nm (Jones et al., 2014). From electron micrographs (Figure 4), the coat thickness is about the same as the diameter of a microtubule (25 nm). Therefore, the distance between the center of a microtubule and the middle of the membrane cannot be shorter than 40 nm. At this minimum distance, the electric field produced at the membrane by a tubulin dimer is on the same order of magnitude as the resting membrane potential field (Chapter 1). The diameter of AIS is about 1500 nm while translocation from 40 nm to 130 nm is sufficient to reduce the hyperpolarizing field from a microtubule by an order of magnitude. Thus, translocation of a microtubule within the AIS can have significant impact on channel gating. This property could play a crucial role in the excitability modulated by EM waves and ultrasound.
Ankyrin-G, end-binding proteins and the Tau protein have been demonstrated to regulate the association between microtubules and the membrane. In the absence of external forces, a microtubule fascicle may be anchored to the membrane by Ankyrin-G and the microtubule end-binding protein, EB1 or EB3 (denoted by EB1/3). The membrane-bound microtubule fascicle has inhibitory effects on neuronal firing. Similar to the interaction with metal antenna, the EM wave may exert a force on the negatively charged microtubules. Its alternating electric field may cause the membrane-bound microtubules to vibrate in the transverse direction, resulting in dissociation from the membrane (Figure 3). This should increase excitability.
The Role of Ankyrin-G
Ankyrin-G, encoded by the ANK3 gene, plays a pivotal role in anchoring various proteins to the membrane, including microtubules (Figure 2). The "giant anchor" is located predominately at AIS and nodes of Ranvier (Iqbal et al., 2013). In experiments using TsA201 cells, it was found that Ankyrin-G could suppress the sodium current through Nav1.6 (Shirahata et al., 2006), which is a type of sodium channels that do not inactivate (O'Brien and Meisler, 2013). This observation supports the view that anchoring of microtubules to the membrane by Ankyrin-G has the same effects as hyperpolarization.
The persistent (non-inactivating) sodium channel, Nav1.6, is enriched in AIS, with a crucial role for resonance amplification, namely, enhancing membrane potential oscillation without shifting resonance frequency (Hutcheon and Yarom, 2000; PDF). Many cortical neurons exhibit spontaneous membrane potential oscillation below threshold at the resonance frequency that depends on the intrinsic membrane properties. Then a slight enhancement by the opening of persistent sodium channels can elicit a train of action potentials (spikes) with the original resonance frequency. This feature is important for long range synchronization. Whenever spikes are generated synchronously in one brain region, the emitted EM waves will immediately be received by any neurons in the brain, but only those with the same resonance frequency (e.g., alpha band, theta band, etc.) can join the synchronization.
Microtubule Fascicles at AIS
A microtubule fascicle is a bundle of several individual microtubules that are parallel with each another and cross linked. An AIS may contain 1 - 7 fascicles and the number of microtubules in each fascicle varies between 2 and 25. Its average number depends on neuronal types. In the motor neurons of the spinal cord, the number of microtubules per fascicle ranges from three to five, but in the pyramidal neurons of the cerebral cortex, the number can reach 22. Single or isolated microtubules are rarely observed in AIS (Palay et al., 1968).
The bundling of microtubules into fascicles is a unique feature of the AIS. Although the nodes of Ranvier resemble AIS in many aspects, their microtubules are not organized into fascicles. Since AIS is the initiation site of nerve impulses and enriched with voltage-gated ion channels, one may expect the microtubule fascicles to play a role in neuronal excitability, as already did by Palay and colleagues. In 1968, they postulated that " the regulated contraction of the microtubules could change the shape of the initial segment and thus alter the configuration of the plasmalemma in this region, and consequently its permeability, with a resultant change in excitability".
In the past few years, experimental studies have provided great insights into the interaction between microtubules and Ankyrin-G, which allows for a more detailed description of the underlying mechanism (Figure 3). It turns out that the microtubule end-binding protein and the microtubule-associated protein Tau are also involved. The Tau protein is a central player in Alzheimer's disease and other neurodegenerative disorders. Its involvement in excitability opens the door for the fundamental understanding of neurodegeneration (discussed in the book, Alzheimer's Disease).
The Role of End-Binding Proteins
The microtubule end-binding protein EB1/3 is required for the binding between the microtubule and Ankyrin-G (Leterrier et al., 2011; Fréal et al., 2016). Normally, EB1/3 binds to the microtubule's plus end, regulating its dynamic growth. However, at the AIS, the microtubule fascicles are decorated by EB1/3 over the entire lattice. This is because the microtubule also possesses other binding sites for EB1/3, albeit weaker. At low concentration, EB1/3 binds preferentially at the plus end which has stronger binding affinity. The weaker binding sites may be occupied when the EB1/3 concentration is high (Bu and Su, 2001).
Ankyrin-G can also bind with EB1/3 through its tail domain (Fréal et al., 2016), which can extend into the intracellular AIS shaft at a maximum depth of ~140 nm, with an average of only 26 nm below the submembrane coat (Jones and Svitkina, 2016). The diameter of the AIS is about 1500 nm. Therefore, most region within the AIS shaft is beyond the reach of Ankyrin-G. The fascicle has ample room to move randomly and exert little influence on channel gating. As the fascicle moves toward the membrane and within the reach of Ankyrin-G, it will be anchored to the membrane, thereby reducing excitability. On the other hand, the EM force may disrupt the interaction among Ankyrin-G, EB1/3, and the microtubule fascicle, causing the fascicle to dissociate from the membrane, thus increasing excitability (Figure 3).
The Role of Tau Proteins
The Tau protein can directly interact with and recruit EB1/3 to the microtubule bundle (Sayas et al., 2015). Thus Tau may interfere with the interaction among Ankyrin-G, EB1/3, and the microtubule fascicle, preventing the fascicle association with the membrane, and consequently enhancing excitability. In animal models, Tau reduction or knockout has been demonstrated to attenuate hyperexcitability (Holth et al., 2013; DeVos et al., 2013; Li et al., 2014).
Author: Frank Lee