|1. Introduction to Microtubules||MT|
Microtubules have a well-established function: intracellular transport. This book will present evidence for a novel function: regulation of neuronal excitability, which is crucial for long range synchronization of neural oscillations that give rise to brain waves. This function can be achieved by its simple physical property: highly negatively charged, which allows the microtubule and its building block, tubulin, to influence voltage-gated ion channels, thereby modulating neuronal excitability.
Basic Structure and Function
In most cases, a microtubule consists of 13 protofilaments, which form a hollow tube with a diameter of 25 nanometers (nm). Each protofilament is made up of tubulin dimers: α and β. The α subunit of one dimer is attached to the β subunit of the next dimer. Thus, in a protofilament, one end (called "minus end") has the α subunit exposed while another end (called "plus" end) has the β subunit exposed. Note that the definition of "+" and "-" on both ends does not mean that the microtubule is an electric dipole with the plus end dominated by positive charges. In fact, the microtubule is highly negatively charged along the entire molecule (see Baker et al., 2001 and next section).
Tubulin dimers can be incorporated into an existing microtubule if its concentration exceeds a critical value. The polymerization process usually occurs at the plus end. Both α and β subunits can bind to GTP (a small molecule similar to ATP). The GTP bound to α-tubulin is stable but the GTP bound to β-tubulin may be hydrolyzed to GDP shortly after assembly. A GDP-bound tubulin at the plus end tends to fall off, whereas a GDP-bound tubulin in the middle of a microtubule cannot spontaneously dissociate from the polymer.
When the polymerization speed overtakes GTP hydrolysis, a GTP cap is created at the plus end. If the GTP hydrolysis becomes faster, the tubulin at the plus end will fall off, resulting in depolymerization and shrinkage. This transition is called catastrophe. The feature that microtubules can switch between assembly and disassembly is known as "dynamic instability". The stability of a microtubule may increase through post-translational modifications of tubulin (polyamination, acetylation, etc.) or binding of microtubule associated proteins.
Highly Negatively Charged
The tubulin dimer is enriched with acidic residues (aspartate and glutamate). In a solution at the physiological pH value (~ 7), these amino acids become negatively charged. Another amino acid, histidine, also has significant probability to become negatively charged at pH = 7. From its amino acid sequence, the net charge on a tubulin dimer can be calculated to be 50.9 e– at pH = 6.7 (Minoura and Muto, 2006).
With this electric property, the microtubule will experience a force from electric field. Thus, applied electric fields can direct microtubules moving toward the anode (Kim et al., 2007). In a solution, microtubules are surrounded by counterions and polar water molecules which may reduce the electrostatic interaction between microtubules and external fields. The effective charge on a tubulin dimer was estimated to be 12 - 20 e– (van den Heuvel et al., 2006; Minoura and Muto, 2006).
Transportable Microtubules at AIS
It has been well established that action potentials initiate at the axon initial segment (AIS) which contains voltage gated ion channels on the membrane. Ideally, the microtubules capable of regulating neuronal excitability should be located at AIS. In addition, they should be stable, short and free to move toward or away from the AIS membrane so that their negative electric fields may dynamically change the open probability of voltage-gated ion channels. Such unconventional microtubules have been discovered several decades ago, referred to as "transportable microtubules" (tMTs).
Recently, α-synuclein has been demonstrated to promote the assembly of tMTs (see this article). α-Synuclein is a central player in Parkinson's disease and Dementia with Lewy Bodies. Dementia refers to the loss of cognitive functions—thinking, remembering, and reasoning—which are governed by brain waves. Thus, α-synuclein could influence cognitive functions via tMTs.
The next chapter will describe the AIS structure in more detail.
Author: Frank Lee