Tubulins & Microtubules
The formation of the brain is instructed by information encoded within stretches of our DNA called genes. Within our DNA, there are a family of related genes called the tubulins, which contain information to produce tubulin proteins.
There are different classes of these proteins which, in humans, include alpha-, beta- and gamma-tubulin. The most common are alphas and betas. These two tubulin proteins combine to form something called a heterodimer. These heterodimers subsequently co-assemble to form long, scaffold-like structures within our cells called Microtubules.
Microtubules are dynamic structures that characteristically undergo periods of steady growth/elongation (addition of new heterodimers) before sudden and rapid collapse (loss of heterodimers). This cycling behaviour between growth/collapse is harnessed by practically every cell in the body but is extremely important during brain formation. This is because microtubules generate the intracellular forces required for all three stages of brain development: neuronal production, neuronal migration, and neuronal connection.
Reflecting the multiple important roles performed by microtubules during brain development, a number of different alpha- and beta-tubulin genes are active in the brain, some almost exclusively during its development. These include alpha-tubulin TUBA1A, beta-tubulins TUBB (sometimes known as TUBB5), TUBB2A, TUBB2B, TUBB3, TUBB4A, and also gamma-tubulin TUBG1. And whilst the pictures might suggest that Microtubules are composed of uniform alpha- and beta-tubulin subunits, they are actually made up of multiple slightly different tubulin proteins, encoded by these different genes.
These different tubulin proteins are almost identical in structure, and are all compatible with the microtubule framework. Not every tubulin is active in every cell, however. There some which are most active in neuron-producing cells (e.g., TUBB), whilst others are only expressed in more mature neurons (e.g., TUBB3) and it is thought that each can modify microtubule behaviour in slightly different ways to carry out slightly different cellular tasks.
It is for this reason that diseases associated with different tubulins often present with slightly distinctive clinical features. Collectively, these diseases are known as the Tubulinopathies.