|MT > 2. Microtubules at the Axon Initial Segment|
This book centers on a novel function of microtubules: regulation of neuronal excitability, which could play key roles in long range synchronization of neural oscillations that give rise to brain waves. It has been well documented that action potentials initiate at the axon initial segment (AIS) (Coombs et al., 1957; Huang and Rasband, 2018). Therefore, AIS is an ideal region for microtubules to modulate excitability.
Structure of AIS
AIS can be divided into three layers: plasma membrane, submembrane coat, and inner AIS shaft, each having AIS-specific features (Figure 1). The plasma membrane contains voltage-gated sodium, potassium and calcium channels. The subtypes of sodium channels at AIS are mainly Nav1.2 and Nav1.6. During development, AIS expressed Nav1.2 first; Nav1.6 appeared later (Boiko et al., 2003). The submembrane coat consists of Ankyrin-G, βIV-spectrin, and actin filaments. Its thickness varies in the range 3–11 nm (Jones et al., 2014). Microtubules are sparsely distributed in the inner AIS shaft.
Ample space at AIS
At AIS, microtubules usually exist in the form of fascicles. A microtubule fascicle is a bundle of several individual microtubules that are parallel with each other 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 mediated by the tripartite motif containing protein, TRIM46 (van Beuningen et al., 2015; Harterink et al., 2019).
The diameter of AIS is about 1500 nm, far greater than the diameter of a microtubule (25 nm). Therefore, as shown in Figures 2 and 3, microtubules are sparsely distributed in the inner AIS shaft. There is ample space for microtubules to move toward and away from the AIS membrane, thereby modulating the open probability of voltage-gated ion channels.
Microtubules may switch between assembly and disassembly, depending on the concentration of free tubulin. This property is known as "dynamic instability". Microtubule stability also depends on post-translational modifications of tubulin (polyamination, acetylation, tyrosination, etc.) and binding of microtubule-associated proteins. The microtubules capable of modulating excitability should be stable, mobile, and shorter than the length of AIS, which varies from 10 to 60 μm (Gutzmann et al., 2014; Höfflin et al., 2017). Remarkably, such unconventional microtubules have been discovered several decades ago. They are referred to as "transportable microtubules" (tMTs).
Why are action potentials initiated at AIS? It was widely believed that the AIS must contain high density of sodium channels to facilitate the generation of action potentials (Kole et al., 2008; Hu et al., 2009; Leterrier, 2018; Huang and Rasband, 2018), despite several studies indicating that the density of sodium channels at the AIS of pyramidal neurons is comparable to the soma (Colbert and Johnston, 1996; Colbert and Pan, 2002; Fleidervish et al., 2010). The next chapter will show that the mobile tMTs at AIS can AMPLIFY the effects of depolarizing fields on channel gating to lower the threshold for action potential generation. In pyramidal neurons, it is the mobile tMTs, rather than high density of sodium channels, that cause action potentials to initiate at AIS.
Author: Frank Lee