Ismael Santa-Maria

Primary age-related tauopathy (PART): a common pathology associated with human aging

We recommend a new term, “primary age-related tauopathy” (PART), to describe a pathology that is commonly observed in the brains of aged individuals. Many autopsy studies have reported brains with neurofibrillary tangles (NFTs) that are indistinguishable from those of Alzheimer’s disease (AD), in the absence of amyloid (Aβ) plaques. For these “NFT+/Aβ−” brains, for which formal criteria for AD neuropathologic changes are not met, the NFTs are mostly restricted to structures in the medial temporal lobe, basal forebrain, brainstem, and olfactory areas (bulb and cortex). Symptoms in persons with PART usually range from normal to amnestic cognitive changes, with only a minority exhibiting profound impairment. Because cognitive impairment is often mild, existing clinicopathologic designations, such as “tangle-only dementia” and “tangle-predominant senile dementia”, are imprecise and not appropriate for most subjects. PART is almost universally detectable at autopsy among elderly individuals, yet this pathological process cannot be specifically identified pre-mortem at the present time. Improved biomarkers and tau imaging may enable diagnosis of PART in clinical settings in the future. Indeed, recent studies have identified a common biomarker profile consisting of temporal lobe atrophy and tauopathy without evidence of Aβ accumulation. For both researchers and clinicians, a revised nomenclature will raise awareness of this extremely common pathologic change while providing a conceptual foundation for future studies. Prior reports that have elucidated features of the pathologic entity we refer to as PART are discussed, and working neuropathological diagnostic criteria are proposed.

Tau--an inhibitor of deacetylase HDAC6 function.

Analysis of brain microtubule protein from patients with Alzheimer’s disease showed decreased alpha tubulin levels along with increased acetylation of the alpha tubulin subunit, mainly in those microtubules from neurons containing neurofibrillary tau pathology. To determine the relationship of tau protein and increased tubulin acetylation, we studied the effect of tau on the acetylation‐deacetylation of tubulin. Our results indicate that tau binds to the tubulin‐deacetylase, histone deacetylase 6 (HDAC6), decreasing its activity with a consequent increase in tubulin acetylation. As expected, increased acetylation was also found in tubulin from wild‐type mice compared with tubulin from mice lacking tau because of the tau‐mediated inhibition of the deacetylase. In addition, we found that an excess of tau protein, as a HDAC6 inhibitor, prevents induction of autophagy by inhibiting proteasome function.

Cleavage and conformational changes of tau protein follow phosphorylation during Alzheimer’s disease

Phosphorylation, cleavage and conformational changes in tau protein all play pivotal roles during Alzheimer’s disease (AD). In an effort to determine the chronological sequence of these changes, in this study, using confocal microscopy, we compared phosphorylation at several sites (Ser199/202/396/404/422‐Thr205 and the second repeat domain), cleavage of tau (D421) and the canonical conformational Alz‐50 epitope. While all of these posttranslational modifications are found in neurofibrillary tangles (NFTs) at all stages of the disease, we found significantly higher numbers of phospho‐tau positive NFTs when compared with cleaved tau (P = 0.006 in Braak III; P = 0.002 in Braak IV; P = 0.012 in Braak V) or compared with the Alz‐50 epitope (P < 0.05). Consistent with these findings, in a double transgenic mice model (Tet/GSK‐3β/VLW) overexpressing the enzyme glycogen synthase kinase‐3β (GSK‐3β) and tau with a triple FTDP‐17 mutation (VLW) with AD‐like neurodegeneration, phosphorylation at sites Ser199/202‐Thr205 was greater than truncated tau. Taken together, these data strongly support the notion that the conformational changes and truncation of tau occur after the phosphorylation of tau. We propose two probable pathways for the pathological processing of tau protein during AD, either phosphorylation and cleavage of tau followed by the Alz‐50 conformational change or phosphorylation followed by the conformational change and cleavage as the last step.

Paired Helical Filaments from Alzheimer Disease Brain Induce Intracellular Accumulation of Tau Protein in Aggresomes

Background: Misfolded aggregated tau protein released by degenerating neurons could spread toxicity to neighboring cells. Results: We found that cultured cells internalized paired helical filaments and developed intracellular tau aggregates with attributes of aggresomes. Conclusion: Paired helical filaments could mediate spreading of pathological tau aggregation. Significance: Paired helical filament-mediated formation of aggresome-like bodies may be an important step in neurodegeneration. Abnormal folding of tau protein leads to the generation of paired helical filaments (PHFs) and neurofibrillary tangles, a key neuropathological feature in Alzheimer disease and tauopathies. A specific anatomical pattern of pathological changes developing in the brain suggests that once tau pathology is initiated it propagates between neighboring neuronal cells, possibly spreading along the axonal network. We studied whether PHFs released from degenerating neurons could be taken up by surrounding cells and promote spreading of tau pathology. Neuronal and non-neuronal cells overexpressing green fluorescent protein-tagged tau (GFP-Tau) were treated with isolated fractions of human Alzheimer disease-derived PHFs for 24 h. We found that cells internalized PHFs through an endocytic mechanism and developed intracellular GFP-Tau aggregates with attributes of aggresomes. This was particularly evident by the perinuclear localization of aggregates and redistribution of the vimentin intermediate filament network and retrograde motor protein dynein. Furthermore, the content of Sarkosyl-insoluble tau, a measure of abnormal tau aggregation, increased 3-fold in PHF-treated cells. An exosome-related mechanism did not appear to be involved in the release of GFP-Tau from untreated cells. The evidence that cells can internalize PHFs, leading to formation of aggresome-like bodies, opens new therapeutic avenues to prevent propagation and spreading of tau pathology.

Grape derived polyphenols attenuate Tau neuropathology in a mouse model of alzheimer's disease

Aggregation of microtubule-associated protein tau into insoluble intracellular neurofibrillary tangles is a characteristic hallmark of Alzheimers disease (AD) and other neurodegenerative diseases, including progressive supranuclear palsy, argyrophilic grain disease, corticobasal degeneration, frontotemporal dementias with Parkinsonism linked to chromosome 17, and Picks disease. Tau is abnormally hyperphosphorylated in AD and aberrant tau phosphorylation contributes to the neuropathology of AD and other tauopathies. Anti-aggregation and anti-phosphorylation are main approaches for tau-based therapy. In this study, we report that a select grape-seed polyphenol extract (GSPE) could potently interfere with the assembly of tau peptides into neurotoxic aggregates. Moreover, oral administration of GSPE significantly attenuated the development of AD type tau neuropathology in the brain of TMHT mouse model of AD through mechanisms associated with attenuation of extracellular signal-receptor kinase 1/2 signaling in the brain.

Tramiprosate, a drug of potential interest for the treatment of Alzheimer's disease, promotes an abnormal aggregation of tau

Alzheimers disease (AD) is characterized by the presence of two histopathological hallmarks; the senile plaques, or extracellular deposits mainly composed of amyloid-β peptide (Aβ), and the neurofibrillary tangles, or intraneuronal inclusions composed of hyperphosphorylated tau protein.Since Aβ aggregates are found in the pathological cases, several strategies are under way to develop drugs that interact with Aβ to reduce its assembly. One of them is 3-amino-1-propane sulfonic acid (Tramiprosate, 3-APS, Alzhemed™), that was developed as a sulfated glycosaminoglycan mimetic, that could interact with Aβ peptide, preventing its aggregation.However, little is known about the action of 3-APS on tau protein aggregation. In this work, we have tested the action of 3-APS on cell viability, microtubule network, actin organization and tau aggregation. Our results indicate that 3-APS favours tau aggregation, in tau transfected non-neuronal cells, and in neuronal cells. We also found that 3-APS does not affect the binding of tau to microtubules but may prevent the formation of tau-actin aggregates. We like to emphasize the importance of testing on both types of pathology (amyloid and tau) the potential drugs to be used for AD treatment.

Characterization and Molecular Profiling of PSEN1 Familial Alzheimer's Disease iPSC-Derived Neural Progenitors

Presenilin 1 (PSEN1) encodes the catalytic subunit of γ-secretase, and PSEN1 mutations are the most common cause of early onset familial Alzheimers disease (FAD). In order to elucidate pathways downstream of PSEN1, we characterized neural progenitor cells (NPCs) derived from FAD mutant PSEN1 subjects. Thus, we generated induced pluripotent stem cells (iPSCs) from affected and unaffected individuals from two families carrying PSEN1 mutations. PSEN1 mutant fibroblasts, and NPCs produced greater ratios of Aβ42 to Aβ40 relative to their control counterparts, with the elevated ratio even more apparent in PSEN1 NPCs than in fibroblasts. Molecular profiling identified 14 genes differentially-regulated in PSEN1 NPCs relative to control NPCs. Five of these targets showed differential expression in late onset AD/Intermediate AD pathology brains. Therefore, in our PSEN1 iPSC model, we have reconstituted an essential feature in the molecular pathogenesis of FAD, increased generation of Aβ42/40, and have characterized novel expression changes.

Taurine, an inducer for tau polymerization and a weak inhibitor for amyloid-β-peptide aggregation

Taurine is an abundant aminoacid present in brain. Its concentration is decreased in the brain of Alzheimers disease (AD) patients. The chemical structure of taurine is similar to 3-amino-1-propanesulfonic acid, a known compound which interferes with beta-amyloid peptide aggregation. Here, we have tested if taurine show similar properties. Taurine slightly decreases beta-amyloid peptide aggregation at a milimolar concentration. At that concentration, taurine favours the assembly of tau protein into fibrillars polymers. Thus, it is proposed that the negative charge present in taurine may be involved in the binding to tau protein, facilitating its assembly. In addition, the possible role of taurine in Alzheimer disease is commented.

Binding of Hsp90 to Tau Promotes a Conformational Change and Aggregation of Tau Protein

Tau pathology, associated with Alzheimers disease, is characterized by the presence of phosphorylated and aggregated tau. Phosphorylation of tau takes place mainly in the vicinity of the tubulin-binding region of the molecule and its self aggregation is also mediated via this tubulin-binding region. Tau phosphorylation and aggregation have been related with conformational changes of the protein. These changes could be regulated by chaperones such as heat shock proteins, since one of these, heat shock protein 90 (Hsp90), has already been described as a putative tau-binding protein. In this work, we have confirmed the interaction of Hsp90 with tau protein and report that binding of Hsp90 to tau facilitates a conformational change that could result in its phosphorylation by glycogen synthase kinase 3 and its aggregation into filamentous structures.

Ultrastructural alterations of Alzheimer's disease paired helical filaments by grape seed-derived polyphenols

Abnormal folding of the microtubule-associated protein tau leads to aggregation of tau into paired helical filaments (PHFs) and neurofibrillary tangles, the major hallmark of Alzheimers disease (AD). We have recently shown that grape seed polyphenol extract (GSPE) reduces tau pathology in the TMHT mouse model of tauopathy (Wang et al., 2010). In the present studies we assessed the impact of GSPE exposure on the ultrastructure of PHFs isolated from Alzheimers disease brain. Transmission electron microscopy revealed that GSPE induced profound dose- and time-dependent alterations in the morphology of PHFs with partial disintegration of filaments. Filaments showed ∼2-fold enlargement in width and displayed numerous protrusions and splayed ends consistent with unfolding of tau and diminished structural stability. In addition, GSPE induced a reduction in immunogold labeling with antibodies against the C-terminal half (12E8, PHF-1) and the middle region of tau (AT8, Tau5, pSer214 tau, and AT180) but not the C-terminal end (Tau46). In comparison, labeling of N-terminus (Alz50) was enhanced. It is unlikely that alterations in immunogold labeling were due to biochemical alterations, e.g., protein phosphatase or proteolytic activities potentially stimulated by GSPE, because western blotting studies have shown the preservation of full length polypeptides of tau and their phospho-epitopes in GSPE-treated samples. The GSPE mechanism may include a noncovalent interaction of polyphenols with proline residues in the proline-rich domain of tau, with Pin1 sites at P213 and P232 most seriously affected as judged by suppression of labeling. Collectively, our results suggest that GSPE has a significant potential for therapeutic development by neutralizing phospho-epitopes and disrupting fibrillary conformation leading to disintegration of PHFs.