Towards a correlation of specific posttranslational modifications of tau protein with characteristics of amyloid deposition

Tau is a microtubule-associated protein mainly responsible for stabilizing the neuronal microtubule network in the brain. Under normal conditions, tau is a highly soluble protein adopting an ‘unfolded’ monomeric conformation. However, it undergoes conformational changes resulting in a less soluble form with weakened stabilizing properties. Altered tau forms pathogenic inclusions characteristically seen in Alzheimer’s disease and related tauopathies. Tau aggregates have been observed to be deposited surrounding prion protein amyloid plaques in mouse brains infected with the 87V murine adapted scrapie (87V-VM). Although tau hyperphosphorylation is widely considered as the major trigger of tau malfunction, tau is subject to a variety of other post-translational modifications, the site-specific impact of which on tau physiology and pathology remains unclear. Therefore, we used mass spectrometry to map post-translationally modified sites on tau purified from normal and 87V-VM mouse brains. We identified five types of site-dependent modifications in normal soluble tau, seven types in soluble tau extracted form 87V-VM brains and six types in insoluble aggregated tau. In preliminary, we showed that we can use LC-MS with a multiple reaction monitoring approach to determine relative levels of specific post-translational modifications of tau after only crude extractions. Once optimized, this workflow could be used to correlate the abundance of a wide variety of different modifications with specific properties of tau, such as solubility or morphologies of deposits