Is Measuring Total Tau Enough to Understand Neurodegeneration?

For just one protein, tau has a big impact. Tau has been implicated in over 26 diseases, which are referred to as tauopathies due to the significant role this one protein has in their pathogenesis. Oftentimes, tau is associated with diseases like Alzheimer’s disease, dementia, and other neurodegenerative diseases like chronic traumatic encephalopathy. Despite an abundance of literature related to tau, the vast range that this one protein has over so many diseases makes it challenging to narrow down what is going on at a molecular level. As such, researchers rely on anti-tau antibodies to help realize mechanisms that cause tau to associate with disease. 

Tau is a major regulator of microtubules, which are the structures that allow for transport and structural stability within cells. In other words, microtubules are essentially the railroads of the cell, and tau is traffic control. What makes tau unique compared to other microtubule-associated proteins is that it is primarily expressed in neurons and other central nervous system cells.

Not all microtubules are regulated by tau. Concentrations of this protein vary from tissue to tissue!

To associate with microtubules, tau contains a microtubule-binding region (MBTR), where electrostatic interactions favorably associate with microtubules. This MBTR is positively charged, containing a slight basic character, which has an affinity for microtubules that are slightly negative due to a high concentration of glutamate and aspartate. 

Tau binds microtubules to help to stabilize microtubules so that cargo can be efficiently transported, and it also physically can act as a roadblock for certain types of cargo. This protein must be highly responsive and adaptive, and tau is able to do this through integration into a vast array of cellular signaling pathways. Studies have reported tau being implicated in PI3K/Akt signaling, mTOR signaling, MAPK signaling, and many more pathways. Since it is such a valuable protein for neuronal function, so many processes exist to precisely modulate its role as a regulator of microtubules. Particularly, post-translational modifications (PTMs) like phosphorylation, acetylation, and glycosylation are highly important for cellular control over tau dynamics by chemically altering how this protein behaves. Phosphorylation is especially important for modulating tau behavior, since this PTM is reversible, thus allowing for rapid adaptations to various cellular stimuli.

How does tau get dysregulated during disease states?

A major school of thought in how tau dysregulation is currently understood actually depends on its PTM status, particularly regarding the phosphorylation of tau. Consider the interaction between tau and microtubules – this is an “opposites attract” relationship where the positively-charged tau and negatively-charged microtubules associate. One of the hallmark characteristics of phosphorylation, however, is that this PTM bears a large negative charge. When serine, threonine, or tyrosine become phosphorylated within tau’s MBTR region, this decreases the strength of tau-microtubule binding, which is one of the two defining characteristics of tauopathies.

Beyond directly disrupting microtubule binding, the chemical structure of phosphorylation on tau causes this intrinsically disordered protein to adopt new conformations. These new conformations can promote aggregation, which is the second main characteristic of tauopathies, and some conformations expose parts of tau that are susceptible to kinase active sites. 

Which phospho-site is important for understanding the mechanisms of tauopathies?

For some proteins, there is a key phospho-site that researchers are able to isolate and track. Tau, on the other hand, has 85 phosphorylation sites reported in the literature. This creates a complex code that must be cracked: which enzyme phosphorylates which residue, what is the functional consequence, what is the trigger for this phosphorylation event? 

One of the phosphorylation sites that has been best characterized is Thr231, which is known to be phosphorylated by GSKβ. Phospho-Thr231 has been found to prime the C-terminus for phosphorylation, and C-terminal phosphorylation is thought to increase tau aggregation activity. This site has also been found to increase the activity of tau seeding, which is the prion-like behavior that causes tau dysregulation to spread from cell to cell.

Not all phospho-tau sites are as well characterized as Thr231. For example, Ser396 and Ser404 have conflicting reports regarding whether their phosphorylation inhibits or, surprisingly, may help microtubule association with tau. Other sites, like Thr181 and Thr217, are not well understood mechanistically, yet these sites are used diagnostically in patient samples. The nature of tau phosphorylation is combinatorial, which has established a major gap in how we are able to reconcile this protein’s molecular characteristics with the development of highly complex tauopathies.

Because phosphorylation is such an ephemeral PTM, it is quite difficult to track this PTM through spectrometric means alone, so using phospho-specific antibodies is extremely valuable for tau researchers. At HUABIO, we offer a wide range of phospho-tau antibodies, raised against 11 different phospho-sites and validated for applications like western blot, immunofluorescence, immunohistochemistry, and immunoprecipitation.