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  • br Tau NFTs are insoluble aggregates within


    Tau NFTs are insoluble 2-NBDG ic50 within neurons, thought to be primarily made-up of misfolded tau, and comprised of paired helical filaments (PHFs) [58,59]. These aggregates are associated with neuronal dysfunction and death and are at the center of the tau hypothesis of AD pathogenesis. Normal, adult tau is a soluble, naturally unfolded, microtubule-associated protein (MAP) that has 6 common isoforms (352–441 amino acids) [39]. All isoforms of tau can appear in the hyperphosphorylated form in PHFs of AD brains [39]. While normal tau is known to stabilize microtubule assembly, hyperphosphorylated tau does the opposite, by disassembling microtubules and aiding in the proliferation of insoluble PHFs, which interfere with axonal transport [60]. The longest isoform of tau (441 amino acids) contains 85 possible phosphorylation sites. In the normal case, 30 of these sites are phosphorylated [61]. In AD pathology however, additional phosphorylation has been documented to occur at specific sites [61]. What causes increased tau phosphorylation is intensively studied but still relatively unknown. Braak and Braak discovered that, unlike senile plaque localization, NFTs followed a consistent and uniform pattern while spreading throughout the brain [62]. Because tau pathology, alone, can cause neuronal loss, it has become a focus of AD pathogenesis research [38]. The accumulation of NFTs has been shown to occur primarily in the hippocampus (an area of the brain important in memory) [62] and NFTs have been shown to occupy much of the space within neurons which might lead to their eventual death [63]. However, because tau pathology does not resolve in the proliferation of senile plaques, yet AβPP and PSEN1/PSEN2 mutations have been shown to yield both senile plaques and NFTs, tau pathology is placed downstream from amyloid-beta pathology in relation to AD disease onset and progression [38,60]. Recently, some focus has shifted to the smaller soluble tau oligomers being the actual neurotoxic species rather than the insoluble NFTs (which have been theorized to be neuroprotective), thus allowing for modified tau to gain more focus in AD research. Protein phosphorylation is the most common PTM [1]. This type of PTM is reversible and occurs primarily at serine residues but can also occur at threonine and tyrosine residues, albeit to a much lesser extent [64]. Phosphorylation takes place by the regulated transfer of the γ-phosphate group of ATP to the hydroxyl group of a target residue [65]. This occurs in conjunction with the hydrolysis of the newly formed phosphoester bonds and thus, if there is a shift in the equilibrium of these two enzymatic activities, hyper- or hypo-phosphorylation can be observed at the phosphorylation sites, causing physiological imbalances [1,24,66]. In studies of AD pathology, phosphorylation is most often linked to tau [1,24,66]. It is a consensus that the incredibly stable, insoluble NFTs are primarily made up of phosphorylated tau [67]. Furthermore, phosphorylated tau can undergo additional modifications (that either promote or inhibit further aggregation) that account for the complexity and uncertainty surrounding the roles of the protein in the onset and progression of AD [68,69]. Lysine residues are the primary amino acid important in tau aggregation and toxicity [70]. The ubiquitin proteasome system is thought to be the primary means of ubiquitylated tau degradation [66,71,72]. Therefore, impaired function of the ubiquitin proteasome system is thought to be a principal aspect of PHF tau build-up. Ubiquitylation sites have been observed and identified on tau isolated from AD brains and were shown to be localized to the microtuble-binding region which further suggests that a lack of adequate de-ubiquitylation, positively correlates degradation resistant tau aggregation [66,73]. A 2017 article by Thomas and Yang, outlines the ways in which LC-MS/MS can be utilized to clarify the different types of lysine residue modifications and their roles in AD neuropathology. The methodology in this paper will be discussed in more detail shortly.