|Introduction to Microtubules||MT|
The microtubule is well known for its functions in cell division and intracellular transport. This book will present evidence that it may also play important roles in neuron-specific processes such as long range synchronization, long-term memory, and synaptic reactivation. It will be shown that many problems in neuroscience can be resolved by its simple physical property: highly negatively charged. As explained in this chapter, the property allows the microtubule and its building block, tubulin, to regulate neuronal excitability. In fact, this underappreciated function has been directly demonstrated more than three decades ago (Matsumoto et al., 1979; Sakai et al., 1985). Recent studies also provided strong evidence that the Tau protein, a microtubule associated protein, can modulate neuronal excitability (Holth et al., 2013; DeVos et al., 2013; Li et al., 2014). The Tau protein is a central player in Alzheimer's disease and other neurodegenerative disorders, which are usually preceded by abnormal hyperexcitability. The mechanism of neurodegeneration is discussed in another book, Alzheimer's Disease, and a series of research papers.
Basic Structure and Function
In most types of cells, 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 over 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". In a living cell, microtubule dynamics is regulated by a host of 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).
Regulation of Excitability
Neuronal excitability is fundamentally governed by the opening and closing of ion channels, which in turn depends on the membrane voltage. By definition, the voltage between two points is given by the integration of electric fields from one point to another. In a nerve membrane, the electric fields may arise from various sources, including ions in the intracellular and extracellular solutions, charges on surface molecules and the microtubules. The effects of surface charges on channel gating and excitability have been reported (Cukierman et al., 1988; Genet and Cohen, 1996), but the contribution from microtubules was largely ignored. As shown below, a microtubule can significantly modulate the membrane potential field (and thus excitability) when it localizes near the membrane (Figure 2).
Let rm be the distance between the center of a tubulin dimer and the middle of the membrane. The electric field produced at the membrane by the tubulin dimer is given by
E = kQ/rm2
where k is the Coulomb's constant and Q represents the effective charge on a tubulin dimer. Assuming Q = 12 e–, at rm = 40 nm, we have E ~ 107 N/C = 107 V/m. On the other hand, the membrane thickness (d) is about 7 nm and the resting membrane voltage (Vm) equals to 70 mV. This gives the resting membrane potential field,
E = Vm/d ~ 107 V/m
Hence, the contribution of a tubulin dimer to the membrane potential field is on the same order of magnitude as the resting membrane potential field when it is close to the membrane. Since a tubulin (or microtubule) is negatively charged, its translocation away from the membrane should have the same effects on channel gating as membrane depolarization. The long microtubule may use this basic principle to modulate excitability at the axon initial segment (Chapter 2) while the small tubulin may regulate excitability throughout the neuron.
Author: Frank Lee