Restoration of biological activity of Alzheimer abnormally phosphorylated tau by dephosphorylation with protein phosphatase-2A, -2B and -1

Microtubule associated protein tau promotes the assembly of microtubules by binding to microtubules and stabilizing their structure. In Alzheimer disease brain, tau is abnormally hyperphosphorylated and the altered tau is unable to promote the in vitro assembly of microtubules. In the present study, we found that dephosphorylation of abnormally phosphorylated tau by protein phosphatase-2A1, -2B or -1 restored its biological activity both in the nucleation and in the assembly of microtubules. Both the amount of phosphate released and the rate of restoration of microtubule assembly promoting activity of the abnormal tau were greater on dephosphorylation by protein phosphatase-2A1 than -2B or -1. During 90 min incubation at 37 degrees C protein phosphatase-2A1, -2B and -1 released respectively approximately 57%, approximately 36% and approximately 30% of tau phosphate. Association of the restoration of the biological activity of the abnormal tau dephosphorylated by different phosphatases and the immunochemical identification of the dephosphorylated sites revealed that Ser-235 is not critical in tau function, and that the Thr-231 is probably involved in the regulation of the nucleation and not the assembly of microtubules. These studies indicate that the phosphorylation of tau in situ might be regulated by protein phosphatase-2A, -2B and -1 and activation of these enzyme activities might arrest the Alzheimer neurofibrillary degeneration.

Differential phosphorylation of human tau isoforms containing three repeats by several protein kinases

The paired helical filaments (PHF) found in the brain of patients with Alzheimer disease (AD) are composed primarily of the microtubule-associated protein tau. Six isoforms of tau have been recognized and all are present in a hyperphosphorylated state in PHF. It is not known whether all tau isoforms serve equally well as substrates for various kinases. In this study we have compared the phosphorylation of human tau isoforms containing three microtubule-binding repeats and zero (tau 3), one (tau 3S), or two (tau 3L) N-terminal inserts. Four kinases (A-kinase, CK-1, CaM kinase II, GSK-3) were used for this purpose. With A-kinase, CK-1, and CaM kinase II the extent of phosphorylation was tau 3L > tau 3S > tau 3. With GSK-3 it was tau 3L approximately = tau 3S > tau 3. Tau 3 was a poor substrate for either CaM kinase II or CK-1, 32P incorporation being only 5 and 11%, respectively, of that observed by these kinases when tau 3L was the substrate. After prephosphorylation of the three tau isoforms by A-kinase, a subsequent phosphorylation by GSK-3 was stimulated several fold over tau that was not prephosphorylated. Under these conditions the extent of 32P incorporation was tau 3L > tau 3S > tau 3. Both CK-1 and GSK-3 phosphorylated ser 396 more rapidly in tau 3L compared to tau 3 or tau 3S. Our results suggest that (1) the presence of N-terminal inserts in tau isoforms are important structural determinants that modulate the specificity of several kinases; (2) the different tau isoforms may be present at different states of phosphorylation in PHF.

Non-proline-dependent protein kinases phosphorylate several sites found in tau from Alzheimer disease brain

Of 21 phosphorylation sites identified in PHF-tau 11 are on ser/thr-X motifs and are probably phosphorylated by non-proline-dependent protein kinases (non-PDPKs). The identities of the non-PDPKs and how they interact to hyperphosphorylate PHF-tau are still unclear. In a previous study we have shown that the rate of phosphorylation of human tau 39 by a PDPK (GSK-3) was increased several fold if tau were first prephosphorylated by non-PDPKs (Singh et al., FEBS Lett 358: 267-272, 1995). In this study we have examined how the specificity of a non-PDPK for different sites on human tau 39 is modulated when tau is prephosphorylated by other non-PDPKs (A-kinase, C-kinase, CK-1, CaM kinase II) as well as a PDPK (GSK-3). We found that the rate of phosphorylation of tau 39 by a non-PDPK can be stimulated if tau were first prephosphorylated by other non-PDPKs. Of the four non-PDPKs only CK-1 can phosphorylate sites (thr 231, ser 396, ser 404) known to be present in PHF-tau. Further, these sites were phosphorylated more rapidly and to a greater extent by CK-1 if tau 39 were first prephosphorylated by A-kinase, CaM kinase II or GSK-3. These results suggest that the site specificities of the non-PDPKs that participate in PHF-tau hyperphosphorylation can be modulated at the substrate level by the phosphorylation state of tau.

Molecular mechanism of Alzheimer's neurofibrillary degeneration and therapeutic intervention

Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer's disease (AD) and the abnormal tau is the major protein subunit of paired helical filaments (PHF). The abnormal phosphorylation of tau probably precedes its polymerization into PHF. The abnormal tau does not bind to tubulin, but competes with tubulin in binding to normal tau and thereby inhibits the assembly of microtubules in the affected neurons. The abnormal tau can be dephosphorylated enzymatically and by this way its microtubule assembly promoting activity can be restored. The activities of protein phosphatases might be decreased in the affected neurons in AD brain, allowing the abnormal hyperphosphorylation of tau. Neurofibrillary degeneration can probably be inhibited by increasing the activities of protein phosphatases in the brain of patients with Alzheimer's disease.

Phosphatase activity toward abnormally phosphorylated tau: decrease in Alzheimer disease brain

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated in affected neurons of Alzheimer disease brain. This hyperphosphorylated tau 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 tau-1 sites (Ser199/Ser202) using the hyperphosphorylated tau 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 tau. The Km values of dephosphorylation of the hyperphosphorylated tau by protein phosphatase-2A and 2B are similar. The tau phosphatase activity is decreased by approximately 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 tau in Alzheimer disease.

Bovine and human tau, highly homologous but less crossreactive: implications for Alzheimer disease

Tau is a highly conserved protein during evolution. However, polymerization of tau into paired helical filaments, a characteristic of Alzheimer disease is unique to humans and has not been seen in animals. The cause of this phenomenon is not known. The cDNA-derived amino acid sequences of bovine and human tau are highly homologous. In this study we show that despite the high homology there are marked differences between the immunoreactivities of human and bovine tau. The four antibodies employed in this study were the monoclonal Tau-1 to the bovine protein, two polyclonal anti-bovine tau 92e and 111e and a polyclonal anti-human tau 113e. The monoclonal antibody Tau-1, the epitope of which lies in the amino acid residues 196-215 of bovine tau and residues 189-207 of human tau differing by a single amino acid, a glycine at position 196 in bovine protein with a proline at position 189 in human protein, reacts 2.3 +/- 0.5 times better with bovine tau than with human tau. Both polyclonal antibodies against bovine tau, 92e and 111e react 25 +/- 7 and 45 +/- 17 folds, respectively, better with bovine tau than human tau, whereas the polyclonal antibody 113e to a human tau peptide does not cross-react with bovine tau. The differences in cross-reactivity that are probably due to differences in conformation of tau from the two species were found using both cytosolic extracts and tau isolated from human and bovine brains.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharyngocutaneous fistula following laryngectomy

Sixty-eight cases of stage III and IV laryngeal cancer were analysed to determine the frequency and management of post-operative pharyngocutaneous fistula (PCF). The overall incidence in our series is 8.9% (6 cases). In five patients PCF healed spontaneously without any surgical intervention while one patient expired due to concomitant other medical illness. The longest time taken by the fistula to heal by conservative treatment was 7 weeks. Retaining the NG tube for a longer period (14-16 days) and adequate and appropriate antibiotic cover reduces the incidence of fistula formation.

Alzheimer paired helical filaments, untreated and pronase digested, studied by vertical platinum-carbon replication and high resolution transmission electron microscopy

Untreated paired helical filaments (PHF) and pronase treated PHF filaments have been stereoscopically imaged with a freeze-drying vertical platinum-carbon replication preparation method for TEM. The untreated PHF have an average wide region, W = 22.8 +/- 2.4 nm, a narrow region width, T = 10.6 +/- 1.7 nm, and a helical turn period, L = 78.6 +/- 13.4. The widths of the pronase treated PHF were significantly reduced and had average measurements of W = 14.8 +/- 1.2 nm, T = 5.7 +/- 1.0 nm, with the helical period unchanged, L = 75.4 +/- 17 nm. The surfaces of the untreated PHF contained approximately 1.0 and approximately 0.4 nm strands, the size of normal and denatured tau monomer. The pronase treated PHF contained approximately 1.0 and approximately 0.4 nm strands as well as approximately 2.0 nm strands. The stereoscopic images of the untreated and the pronase digested PHF do not support a double helical morphology for the PHF. The PHF appear to be long helical ribbons. The approximately 1.0 and approximately 2 nm substructure has been organized both parallel and orthogonal to the PHF-core axis for distances less than 80 nm. The most frequent structural appearance is of a disorganized PHF core. The surfaces of the untreated PHF also have a similar disorganized appearance.

Dephosphorylation of Alzheimer paired helical filaments by protein phosphatase-2A and -2B

Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer disease and in this form is the major protein subunit of the paired helical filaments (PHF), the most prominent lesion of the disease. In this study the dephosphorylation of sparingly soluble PHF, PHF II-tau by brain protein phosphatase (PP)-2A1 and PP-2B, and the resulting biochemical, biological, and structural alterations were investigated. Both of the phosphatases dephosphorylated PHF II-tau at the sites of Ser-199/Ser-202 and partially dephosphorylated it at Ser-396/Ser-404; in addition, PHF II-tau was dephosphorylated at Ser-46 by PP-2A1 and Ser-235 by PP-2B. The relative electrophoretic mobility of PHF II-tau increased after dephosphorylation by either enzyme. Divalent cations, manganese, and magnesium increased the activities of PP-2A1 and PP-2B toward PHF II-tau. Dephosphorylation both by PP-2B and PP-2A1 decreased the resistance of PHF II-tau to proteolysis by the brain calcium-activated neutral proteases (CANP). The ability of PHF II-tau to promote the in vitro microtubule assembly was restored after dephosphorylation by PP-2A1 and PP-2B. Microtubules assembled by the dephosphorylated PHF II-tau were structurally identical to those assembled by bovine tau used as a control. The dephosphorylation both by PP-2A1 and PP-2B caused dissociation of the tangles and the PHF; some of the PHF dissociated into straight protofilaments/subfilaments. Approximately 25% of the total tau was released from PHF on dephosphorylation by PP-2A1. These observations demonstrate that PHF II-tau is accessible to dephosphorylation by PP-2A1 and PP-2B, and dephosphorylation makes PHF dissociate, accessible to proteolysis by CANP, and biologically active in promoting the assembly of tubulin into microtubules.

Cranial fasciitis: presentation as a post-auricular mass

Cranial fasciitis is a rare variant of nodular fasciitis. It is a benign condition with features resembling sarcoma, seen principally in young males. It involves the skull bones and grows at a rapid pace. Accurate diagnosis and surgical excision is the key to management. Prognosis is good with recurrence rare. By 1992, 17 cases had been reported in the literature. The present case is the first reported in Pakistan. It presented as a post-auricular mass.

Phosphorylation of tau protein by casein kinase-1 converts it to an abnormal Alzheimer-like state

The microtubule-associated protein tau is abnormally hyperphosphorylated in Alzheimer's disease. Both proline-dependent protein kinases (PDPKs) and non-PDPKs are involved in this hyperphosphorylation of tau. Several PDPKs can phosphorylate tau in vitro and induce Alzheimer-like epitopes to many phosphorylation-dependent antibodies. A similar induction has not been reported with non-PDPKs. In this study we have evaluated six non-PDPKs [cyclic AMP-dependent (A-kinase), calcium/phospholipid-dependent (C-kinase), casein kinase-1 (CK-1), casein kinase-2 (CK-2), calcium/calmodulin-dependent protein kinase II, and calcium/calmodulin-dependent protein kinase from rat cerebellum] for their abilities to induce Alzheimer-like epitopes on tau. Such epitopes were induced by A-kinase, C-kinase, CK-1, and CK-2, but the degree of induction achieved by CK-1 was much greater than with the other kinases. These results suggest that CK-1 may play an important role in the conversion of tau from the normal to the abnormal phosphorylation state in Alzheimer's disease.

Phosphorylation and accumulation of tau without any concomitant increase in tubulin levels in Chinese hamster ovary cells stably transfected with human tau441

Eucaryotic expression vectors bearing a 1.4 kb cDNA encoding the 4 repeat isoform of human tau, tau441, in either the sense or anti-sense orientation with respect to a cytomegalovirus (CMV) promoter were constructed. The resulting constructs were used to transiently express tau in Chinese Hamster Ovary cells and to generate non-neuronal stable cell lines. Immunocytochemical studies of these cells show that tau is expressed in the sense but not the anti-sense or vector containing lines. Some of the cells expressing tau showed fine elongated processes which were stained by tau antibodies. The general tau immunostaining pattern appeared diffuse and punctuate. The expressed tau was seen both unbound and bound to microtubules. In some cells labeling with antibodies that specifically recognize hyperphosphorylation of tau was observed. The size of this population increased with increasing numbers of cell passages. However, no increase in steady-state tubulin level was observed following tau441 expression. These studies show that tau can accumulate in the cells without a concomitant increase in tubulin.

Abnormally phosphorylated tau in SY5Y human neuroblastoma cells

In Alzheimer disease (AD) the microtubule associated protein (MAP) tau is hyperphosphorylated at several sites. In the present study, like AD tau, tau in the human neuroblastoma SH-SY5Y was found to be hyperphosphorylated, at Ser-199/202, Thr-231, Ser-396 and Ser-404. However, in contrast to AD, the tau in SY5Y cells was not hyperphosphorylated at Ser-235 and there was only one tau isoform. Quantitative analysis revealed that approximately 80% of the SY5Y-tau was phosphorylated at Ser-199/202. The phosphorylated tau was deposited in perikarya and processes of the cells whereas most of the unphosphorylated (at Ser-199/202) tau was localized in the nucleus. Tau from the cell lysates did not bind to taxol-stabilized microtubules. In contrast, MAP1b and MAP2 from cell lysates bound to stabilized microtubules in vitro and were associated to the microtubule network in situ. Phosphorylation of tau at high levels, its inactivity with microtubules and its accumulation in SY5Y cells provide for the first time a cell model of cytoskeletal changes seen in AD.

Guanosine triphosphate binding to beta-subunit of tubulin in Alzheimer's disease brain: role of microtubule-associated protein tau

In Alzheimer's disease, paired helical filaments composed mainly of abnormally phosphorylated tau accumulate in certain selected neurons of the brain, and microtubules are rarely seen in the affected cells. In the present study, the binding of 32P-labeled 8-azidoguanosine triphosphate ([gamma-32P]8N3GTP), the photoaffinity analogue of GTP to the beta-subunit of tubulin in brain homogenates was found to be markedly lower in patients with Alzheimer's disease than in aged control human cases. No significant differences were observed in the levels of the alpha- and beta-subunits of tubulin between Alzheimer's disease and control brains obtained 2-7 h postmortem. In nine of 19 Alzheimer's disease and 11 of 12 control autopsied brains (2-7 h postmortem and stored at -75 degrees C) tubulin was isolated successfully from brain cytosol by in vitro polymerization induced with DEAE-dextran. The GTP binding was observed in the two cycled assembled microtubule preparations from all the normal control, and in eight of nine Alzheimer's disease cases. Alzheimer's disease microtubule preparations contained varying amounts of abnormally phosphorylated tau, whereas no abnormal tau was detected in the control brain preparations. Addition of bovine tau to bovine, normal human, and Alzheimer's disease brain tubulin preparations markedly increased GTP binding to the beta-subunit. An alkaline phosphatase-treated paired helical filament-enriched preparation increased by approximately twofold the GTP binding to bovine brain tubulin. GTP binding to tubulin prepared by phosphocellulose chromatography of two cycled microtubules from three Alzheimer's disease and three normal control brains, revealed insignificant differences between the two groups. These findings have suggested that (1) tau protein promotes the GTP binding to the beta-subunit of tubulin, and (2) the breakdown of the microtubule system in brains of patients with Alzheimer's disease might in part be due to the abnormal phosphorylation of tau which depresses the GTP binding.

Rapid Alzheimer-like phosphorylation of tau by the synergistic actions of non-proline-dependent protein kinases and GSK-3

Tau protein from Alzheimer disease (AD) brain is phosphorylated at eleven Ser/Thr-Pro and nine Ser/Thr-X sites. The former sites are phosphorylated by proline-dependent protein kinases (PDPKs), the latter by non-PDPKs. The identities of both the PDPKs and non-PDPKs involved in AD tau hyperphosphorylation are still to be established. In this study we have analyzed the interactions between a PDPK (GSK-3) and several non-PDPKs (A-kinase, C-kinase, CK-1, CaM kinase II) in the phosphorylation of one isoform (tau 39) of human tau. We found that the rate of phosphorylation of tau 39 by GSK-3 was increased several-fold if tau were first prephosphorylated by the non-PDPKs. Further, several Alzheimer-like epitopes in tau can be induced only slowly after phosphorylation of tau by GSK-3 alone. After a prephosphorylation of tau by the non-PDPKs, however, the rate of induction of these epitopes by GSK-3 is increased several-fold. These results suggest that one role of non-PDPK-catalyzed phosphorylation is the modulation of PDPK-catalyzed phosphorylation of tau in AD brain.

Modulation of GSK-3-catalyzed phosphorylation of microtubule-associated protein tau by non-proline-dependent protein kinases

The phosphorylation of bovine tau, either by GSK-3 alone or by a combination of GSK-3 and several non-proline-dependent protein kinases (non-PDPKs), was studied. GSK-3 alone catalyzed the incorporation of approximately 3 mol 32P/mol tau at a relatively slow rate. Prephosphorylation of tau by A-kinase, C-kinase, or CK-2 (but not by CK-1, CaM kinase II or Gr kinase) increased both the rate and extent of a subsequent phosphorylation catalyzed by GSK-3 by several-fold. These results suggest that the phosphorylation of tau by PDPKs such as GSK-3 (and possibly MAP kinase, cdk5) may be positively modulated at the substrate level by non-PDPK-catalyzed phosphorylations.

Pathological profile in chronic suppurative otitis media--the regional experience

Peroperative findings in 145 consecutive cases of chronic suppurative otitis media, operated at Civil Hospital, Karachi were recorded. The mean age was 24 years. More than half of the patients (51%) had subtotal perforations and majority had damage of more than one ossicle. Involvement of all the three ossicles was seen in 30 (40%) cases. The frequency of damaged malleus along with incus was higher than other series. Granulations and subtotal perforations were commoner and could account for it. Extensive cholesteatoma was present in 30 cases.