Inge Grundke-Iqbal

Hyperphosphorylation induces self-assembly of τ into tangles of paired helical filaments/straight filaments

The microtubule-associated protein τ is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in the form of paired helical filaments (PHF) in the brains of patients with Alzheimers disease (AD) and patients with several other tauopathies. Here, we show that the abnormally hyperphosphorylated τ from AD brain cytosol (AD P-τ) self-aggregates into PHF-like structures on incubation at pH 6.9 under reducing conditions at 35°C during 90 min. In vitro dephosphorylation, but not deglycosylation, of AD P-τ inhibits its self-association into PHF. Furthermore, hyperphosphorylation induces self-assembly of each of the six τ isoforms into tangles of PHF and straight filaments, and the microtubule binding domains/repeats region in the absence of the rest of the molecule can also self-assemble into PHF. Thus, it appears that τ self-assembles by association of the microtubule binding domains/repeats and that the abnormal hyperphosphorylation promotes the self-assembly of τ into tangles of PHF and straight filaments by neutralizing the inhibitory basic charges of the flanking regions.

Alzheimer's disease hyperphosphorylated tau sequesters normal tau into tangles of filaments and disassembles microtubules

Microtubule–associated protein tau becomes abnormally hyperphosphorylated in Alzheimers disease (AD) and accumulates as tangles of paired helical filaments in neurons undergoing degeneration. We now show that in solution normal tau associates with the AD hyperphosphorylated tau (AD P–tau) in a nonsaturable fashion, forming large tangles of filaments 3.3 ± 0.7 nm in diameter. These tangles, which are not detected in identically treated normal tau or AD P–tau alone, are made up of filaments several microns in length and are labeled with tau antibodies. Dephosphorylation with alkaline phosphatase abolishes the ability of AD P–tau to aggregate with normal tau and prevents tangle formation. AD P–tau disassembles microtubules assembled from normal tau and tubulin. These data provide insight into how the hyperphosphorylation of tau might lead to the formation of the neurofibrillary tangles and the degeneration of the affected neurons in AD.

Contributions of protein phosphatases PP1, PP2A, PP2B and PP5 to the regulation of tau phosphorylation

Abnormal hyperphosphorylation of tau is believed to lead to neurofibrillary degeneration in Alzheimers disease (AD) and other tauopathies. Recent studies have shown that protein phosphatases (PPs) PP1, PP2A, PP2B and PP5 dephosphorylate tau in vitro, but the exact role of each of these phosphatases in the regulation of site‐specific phosphorylation of tau in the human brain was unknown. Hence, we investigated the contributions of these PPs to the regulation of tau phosphorylation quantitatively. We found that these four phosphatases all dephosphorylated tau at Ser199, Ser202, Thr205, Thr212, Ser214, Ser235, Ser262, Ser396, Ser404 and Ser409, but with different efficiencies toward different sites. The Km values of tau dephosphorylation catalysed by PP1, PP2A and PP5 were 8–12 µm, similar to the intraneuronal tau concentration of human brain, whereas the Km of PP2B was fivefold higher. PP2A, PP1, PP5 and PP2B accounted for ∼ 71%, ∼ 11%, ∼ 10% and ∼ 7%, respectively, of the total tau phosphatase activity of human brain. The total phosphatase activity and the activities of PP2A and PP5 toward tau were significantly decreased, whereas that of PP2B was increased in AD brain. PP2A activity negatively correlated to the level of tau phosphorylation at the most phosphorylation sites in human brains. Our findings indicate that PP2A is the major tau phosphatase that regulates its phosphorylation at multiple sites in human brain. The abnormal hyperphosphorylation of tau is partially due to a downregulation of PP2A activity in AD brain.

Mechanisms of tau-induced neurodegeneration

Alzheimer disease (AD) and related tauopathies are histopathologically characterized by a specific type of slow and progressive neurodegeneration, which involves the abnormal hyperphosphorylation of the microtubule associated protein (MAP) tau. This hallmark, called neurofibrillary degeneration, is seen as neurofibrillary tangles, neuropil threads, and dystrophic neurites and is apparently required for the clinical expression of AD, and in related tauopathies it leads to dementia in the absence of amyloid plaques. While normal tau promotes assembly and stabilizes microtubules, the non-fibrillized, abnormally hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, and disrupts microtubules. The abnormal hyperphosphorylation of tau, which can be generated by catalysis of several different combinations of protein kinases, also promotes its misfolding, decrease in turnover, and self-assembly into tangles of paired helical and or straight filaments. Some of the abnormally hyperphosphorylated tau ends up both amino and C-terminally truncated. Disruption of microtubules by the non-fibrillized abnormally hyperphosphorylated tau as well as its aggregation as neurofibrillary tangles probably impair axoplasmic flow and lead to slow progressive retrograde degeneration and loss of connectivity of the affected neurons. Among the phosphatases, which regulate the phosphorylation of tau, protein phosphatase-2A (PP2A), the activity of which is down-regulated in AD brain, is by far the major enzyme. The two inhibitors of PP-2A, I1PP2A and I2PP2A, which are overexpressed in AD, might be responsible for the decreased phosphatase activity. AD is multifactorial and heterogeneous and involves more than one etiopathogenic mechanism.

Phosphoprotein Phosphatase Activities in Alzheimer Disease Brain

Abstract: Microtubule‐associated protein τ is known to be hyperphosphorylated in Alzheimer disease brain and this abnormal hyperphosphorylation is associated with an inability of τ to promote the assembly of microtubule in the affected neurons. Our previous studies demonstrated that abnormally phosphorylated τ could be dephosphorylated after treatment with alkaline phosphatase, thereby suggesting that the abnormal phosphorylation of τ might in part be the result of a deficiency of the phosphoprotein phosphatase system in patients with Alzheimer disease. In the present study we used 32P‐labeled phosphorylase kinase and poly(Glu.Tyr) 4:1 as substrates to measure phosphoprotein phosphatase activities in Alzheimer disease and control brains. The activities of phosphoseryl/ phosphothreonyl‐protein phosphatase types 1, 2A, 2B, and 2C and of phosphotyrosyl‐protein phosphatase in frontal gray and white matters from 13 Alzheimer brains were determined and compared with those from 12 age‐matched control brains. The activities of type 1 phosphatase and phosphotyrosyl phosphatase in gray matter and of type 2A phosphatase in both gray and white matters were significantly lower in Alzheimer disease brains than in controls. These findings suggest that the hyperphosphorylation of τ in Alzheimer disease brain could result from a protein dephosphorylation defect in vivo. The decrease in the phosphatase activities in Alzheimer disease might also be involved in the formation of β‐amyloid by augmenting the amyloidogenic pathway processing of β‐amyloid precursor protein.

Distribution of active glycogen synthase kinase 3β (GSK-3β) in brains staged for Alzheimer disease neurofibrillary changes

Accumulation of paired helical filaments (PHFs) in neurofibrillary tangles, neuropil threads, and dystrophic neurites is one of the major ncuropathological hallmarks of Alzheimer disease (AD). The principal protein subunit of PHFs is the abnormally hyperphosphorylated tau. Glycogen synthase kinase 3B (GSK-3B) is one of the candidate kinases involved in PHF-tau formation. To play a role in PHF-tau formation, it would be expected that GSK-3B is active in tangle bearing neurons. In the present study, we investigated the regional and intracellular distributions of active and inactive forms of GSK-3B in brains staged for neurofibrillary changes. We found that neurons with tangle-like inclusions positive for active, but not inactive, GSK-3β appear initially in the Pre-α layer of the entorhinal cortex and extend to other brain regions, coincident with the sequence of the development of neurofibrillary changes. Active, but not inactive, GSK-3β was found to initially accumulate in the cytoplasm of pretangle neurons. These data provide direct in situ evidence that is consistent with the involvement of GSK-3β in PHF-tau formation.

Phosphatase Activity Toward Abnormally Phosphorylated τ: Decrease in Alzheimer Disease Brain

Abstract: Microtubule‐associated protein τ is abnormally hyperphosphorylated and aggregated in affected neurons of Alzheimer disease brain. This hyperphosphorylated τ can be dephosphorylated at some of the abnormal phosphorylated sites by purified protein phosphatase‐1, 2A, and 2B in vitro. In the present study, we have developed an assay to measure protein phosphatase activity toward τ‐1 sites (Ser199/Ser202) using the hyperphosphorylated τ isolated from Alzheimer disease brain as substrate. Using this assay, we have identified that in normal brain, protein phosphatase‐2A and 2B and, to a lesser extent, 1 are involved in the dephosphorylation of τ. The Km values of dephosphorylation of the hyperphosphorylated τ by protein phosphatase‐2A and 2B are similar. The τ phosphatase activity is decreased by ∼30% in brain of Alzheimer disease patients compared with those of age‐matched controls. These findings suggest that a defect of protein phosphatase could be the cause of the abnormal hyperphosphorylation of τ in Alzheimer disease.

Occurrence of neuropil threads in the senile human brain and in Alzheimer's disease: A third location of paired helical filaments outside of neurofibrillary tangles and neuritic plaques

Paired helical filaments, pathologically changed components of the cytoskeleton of human nerve cells, are demonstrated by a specific silver staining technique, immunostaining and electron microscopy. Accumulations of these filaments are found in the telencephalic cortex of old-aged individuals and patients suffering from Alzheimers disease. Apart from neurofibrillary tangles and neuritic plaques, paired helical filaments are encountered in neuropil threads. At this third location, pathological filaments occur in small and inconspicuous profiles scattered throughout both allocortical and isocortical areas. The pattern of distribution and packing density of neuropil threads varies between different cortical areas and layers.

DEFECTIVE BRAIN MICROTUBULE ASSEMBLY IN ALZHEIMER'S DISEASE

Brains obtained within 2-4 hours post mortem and histopathologically confirmed for Alzheimers disease and non-Alzheimer brains from age-matched controls were examined for in-vitro assembly of microtubules and neurofilaments. Microtubule assembly was observed only in control but not in Alzheimer brains, and neurofilaments were obtained from both types of brain. The microtubule-associated protein tau, which stimulates assembly of microtubules from tubulin, was abnormally phosphorylated in Alzheimer but not in control brain microtubule preparations. Alzheimer brains did not show the presence of any inhibitor of microtubule assembly or any abnormality of tubulin. DEAE-dextran, a polycation which mimics tau in stimulating microtubule assembly, induced the assembly of microtubules in Alzheimer brain. Tubulin from both normal and Alzheimer brains was labelled on western blots by a monoclonal antibody to the tyrosinylated carboxy-terminal epitope of alpha tubulin. These studies suggest that in Alzheimers disease tubulin can be assembled into brain microtubules, but the process is defective, probably because of abnormal phosphorylation of tau. This post-translational alteration of tau might be the cause of the neurofibrillary abnormality in Alzheimers disease.

Kinases and phosphatases and tau sites involved in Alzheimer neurofibrillary degeneration

Microtubule associated protein (MAP) tau is abnormally hyperphosphorylated in Alzheimers disease (AD) and related tauopathies; in this form it is the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles. However, the nature of protein kinases and phosphatases and tau sites involved in this lesion has been elusive. We investigated self‐assembly and microtubule assembly promoting activities of hyperphosphorylated tau isolated from Alzheimer disease brain cytosol, the AD abnormally hyperphosphorylated tau (AD P‐tau) before and after dephosphorylation by phosphoseryl/phosphothreonyl protein phosphatase‐2A (PP‐2A), and then rephosphorylation by cyclic AMP‐dependent protein kinase (PKA), calcium, calmodulin‐dependent protein kinase II (CaMKII), glycogen synthase kinase‐3β (GSK‐3β) and cyclin‐dependent protein kinase 5 (cdk5) in different kinase combinations. We found that (i) dephosphorylation of AD P‐tau by PP‐2A inhibits its polymerization into PHF/straight filaments (SF) and restores its binding and ability to promote assembly of tubulin into microtubules; (ii) rephosphorylation of PP‐2A‐dephosphorylated AD P‐tau by sequential phosphorylation by PKA, CaMKII and GSK‐3β or cdk5, and as well as by cdk5 and GSK‐3β, promotes its self‐assembly into tangles of PHF similar to those seen in Alzheimer brain, and (iii) phosphorylation of tau sites required for this pathology are Thr231 and Ser262, along with several sites flanking the microtubule binding repeat region. Phosphorylation of recombinant human brain tau441 yielded similar results as the PP‐2A dephosphorylated AD P‐tau, except that mostly SF were formed. The conditions for the abnormal hyperphosphorylation of tau that promoted its self‐assembly also induced the microtubule assembly inhibitory activity. These findings suggest that activation of PP‐2A or inhibition of either both GSK‐3β and cdk5 or one of these two kinases plus PKA or CaMKII might be required to inhibit Alzheimer neurofibrillary degeneration.