Threonine 175 , a novel pathological phosphorylation site on tau protein linked to multiple tauopathies
In undertaking these studies, we were specifically interested in determining whether the pathogenic phospho-tau species recognized by antibodies against pThr175 tau and pThr231 tau as well as oligomeric tau (T22) were expressed across a broad range of tauopathies. We were also interested in determining whether these pathological tau species were colocalized in ALS and ALSci. It is known that the phosphorylation of tau protein at Thr231 is of both physiological and pathological significance in mediating the dissociation of tau from microtubules [3, 24, 25]. Thr231 is phosphorylated by activated GSK3? physiologically and in pathological states [25–29]. We have previously shown that pThr175 tau induces GSK3? phosphorylation and that this in turn leads to Thr231 tau phosphorylation resulting in tau fibril formation and cell death in vitro [17].
Although the number of cases studied here is limited, the intent was not to undertake a detailed topographic analysis of tau deposition across all tauopathies, but rather to determine whether the proposed pathway of pThr175 tau mediated induction of pThr231 tau formation with its attendant pathological tau fibril formation (as recognized by T22) was evident. It is noteworthy therefore that we observed that in each tauopathy studied, pThr175 tau, pThr231 tau and T22 immunoreactivity co-localized to the same inclusion-containing neuronal populations. In each case, neuronal pThr175 tau colocalized with pThr231 tau. This, paired with the prior identification of pThr175 tau in AD brain tissue but not controls [30] and the lack of identified pThr175 in fetal tau [15, 16] suggests that pThr175 is a key point in pathological tau metabolism, as it is not a physiologically utilized site involved in the regulation of tau function during development or microtubule reorganization. This suggests that the downstream events triggered by pThr175 tau, including toxic monomer formation, are common to each of these diseases.
To further assess the pathogenicity of pThr175 and pThr231, we investigated each epitope in the hippocampus of control cases across three decades of life where tau pathology has been shown to increase with age [22]. We observed no pThr175 tau pathology in the 6th decade with minimal immunoreactive neuronal inclusions in the 7th decade. pThr175 tau immunoreactivity was most prominent in the 8th decade. In each case in which we observed pThr175 tau immunostaining, we also observed T22 immunoreactivity. Similarly, we never observed T22 immunoreactivity in the absence of either pThr175 tau or pThr231 tau immunoreactivity. In contrast, pThr231 tau immunoreactivity was frequently observed in the absence of either pThr175 tau or T22 staining in younger individuals and when present, was within healthy appearing neurons and axonal processes. pThr175 and T22 did not show pathology in hippocampal regions spared from pThr231 pathology, and T22 was only positive in cases showing prominent pThr175 pathology.
Glial pathology was recognized to a greater degree by pThr217 tau and PHF tau than by the pThr175 tau antibody, suggesting that different pathological processes are at play in these cells. This is supported by the lack of identifiable glial pathology by pThr231 tau and T22. This paired with the low frequency of pThr175 tau glial pathology further strengthens the correlation between pThr175 and pThr231 in the induction of neuronal pathology and provides evidence that this pair of phosphorylation sites may be exerting specific neuronal toxicity in the disease process across multiple tauopathies.
Although limbic regions universally presented tau pathology, frontal and ACC tau pathology was present mainly in AD, VD, ALSci, FTLD, mDLBD and MSA. This paired with the deeper layer pathology in this region may indicate that tau pathology did not originate here but instead propagated from other regions. If tau originates in limbic structures, propagating along the Papez circuit, it is possible that it would arrive in ACC through thalamic projections to layer IV and V which could act as a hub for propagation to other brain regions such as frontal cortex through this well connected region. Regardless of the induction cause or place, tau protein toxicity is undeniable once initiated [23, 31], and must be considered when attempting to understand the underlying biology of many neurodegenerative diseases. This hypothesis also implies that disease entities such as primary age-related tauopathies (PART) [32] may be in fact not age-related, but neuronal stress related, as increasing age would indicate longer time periods for stresses on neurons to become pathological through stochastic processes [33]. Therefore, tau protein deposition should not be considered a simple function of normal ageing, but ageing should be considered a risk factor for tauopathy among a plethora of neuronal stresses. Of note as well is the frontal involvement in ALSci, which can be concluded is not likely a result of PART, which spares the neocortex by definition [22, 32]. We cannot conclude, however if the layer distribution of tau pathology resembles PART, as this was not described in the consensus report.