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Biomedical subjects

K Iqbal

Publications and source records attributed to K Iqbal.

At least 55 records · Page 3Linked to original sources
PubMed1999

Hyperphosphorylated tau in SY5Y cells: similarities and dissimilarities to abnormally hyperphosphorylated tau from Alzheimer disease brain.

Unlike normal tau, abnormally hyperphosphorylated tau (AD P-tau) from Alzheimer disease (AD) does not promote but instead inhibits microtubule assembly and disrupts already formed microtubules. Tau in the human neuroblastoma cell line SH-SY5Y is hyperphosphorylated at several of the same sites as AD P-tau, and accumulates in the cell body without any association to the cellular microtubule network. The aim of the present study was to elucidate why the SY5Y tau does not affect the viability of the cells. We found that, like AD P-tau, SY5Y tau because of hyperphosphorylation does not bind to microtubules and inhibits the tau-promoted assembly of microtubules. However, the tau/HMW MAP ratio is about 10 times less in SY5Y cells than in AD brain. These findings suggest that the hyperphosphorylated tau from SY5Y cells has similar biological characteristics as AD P-tau from AD brain, but is not lethal to the SY5Y cells because of its low tau/HMW MAP ratio.

Alzheimer Disease↗
PubMed1999

Improved discrimination of AD patients using beta-amyloid(1-42) and tau levels in CSF.

OBJECTIVE: To evaluate CSF levels of beta-amyloid(1-42) (Abeta42) alone and in combination with CSF tau for distinguishing AD from other conditions. METHODS: At 10 centers in Europe and the United States, 150 CSF samples from AD patients were analyzed and compared with 100 CSF samples from healthy volunteers or patients with disorders not associated with pathologic conditions of the brain (CON), 84 patients with other neurologic disorders (ND), and 79 patients with non-Alzheimer types of dementia (NAD). Sandwich ELISA techniques were used on site for measuring Abeta42 and tau. RESULTS: Median levels of Abeta42 in CSF were significantly lower in AD (487 pg/mL) than in CON (849 pg/mL; p = 0.001), ND (643 pg/mL; p = 0.001), and NAD (603 pg/mL; p = 0.001). Discrimination of AD from CON and ND was significantly improved by the combined assessment of Abeta42 and tau. At 85% sensitivity, specificity of the combined test was 86% (95% CI: 81% to 91%) compared with 55% (95% CI: 47% to 62%) for Abeta42 alone and 65% (95% CI: 58% to 72%) for tau. The combined test at 85% sensitivity was 58% (95% CI: 47% to 69%) specific for NAD. The APOE e4 gene load was negatively correlated with Abeta42 levels not only in AD but also in NAD. CONCLUSIONS: The combined measure of CSF Abeta42 and tau meets the requirements for clinical use in discriminating AD from normal aging and specific neurologic disorders.

Adult↗
PubMed1999

Simulated movement termination for balance recovery: can movement strategies be sought to maintain stability in the presence of slipping or forced sliding?

Slipping during various kinds of movement often leads to potentially dangerous incidents of falling. The purpose of this study was to determine whether there was evidence to support the theory that movement strategies could be used by individuals to regain stability during an episode of slipping and whether forced sliding from a moving platform accurately simulated the effect of slipping on stability and balance. A single-link-plus-foot biomechanical model was used to mathematically simulate base of support (BOS) translation and body segment rotation during movement termination in sagittal plane. An optimization routine was used to determine region of stability [defined at given COM locations as the feasible range of horizontal velocities of the center of mass (COM) of human subject that can be reduced to zero with respect to the BOS while still allowing the COM to traverse within the BOS limits]. We found some 30% overlap in the region of stability for slipping and non-slipping conditions. This finding supports the theory that movement strategies can be sought for restoring stability and balance even if slipping unexpectedly occurs. We also found that forced sliding produces effects on stability that are similar to those of slipping, indicated by over 50% overlap in the regions of stability for the two conditions. In addition, forced sliding has distinctive effects on stability, including a "shift" of the region of stability extended beyond the BOS in the direction of sliding. These findings may provide quantifiable guidance for balance training aimed at reducing fall incidents under uncertain floor surface conditions.

Accidental Falls↗
PubMed1999

Distribution of active glycogen synthase kinase 3beta (GSK-3beta) 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 neuropathological hallmarks of Alzheimer disease (AD). The principal protein subunit of PHFs is the abnormally hyperphosphorylated tau. Glycogen synthase kinase 3beta (GSK-3beta) 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-3beta is active in tangle bearing neurons. In the present study, we investigated the regional and intracellular distributions of active and inactive forms of GSK-3beta in brains staged for neurofibrillary changes. We found that neurons with tangle-like inclusions positive for active, but not inactive, GSK-3beta appear initially in the Pre-alpha 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-3beta 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-3beta in PHF-tau formation.

Adult↗
PubMed1999

Silent myocardial ischemia and cardiac autonomic neuropathy in diabetics.

OBJECTIVE: To evaluate the association of silent myocardial ischemia (SMI) with cardiac autonomic neuropathy in asymptomatic diabetic patients. MATERIAL AND METHODS: Two hundred asymptomatic patients of diabetes mellitus were assessed for evidence of cardiac autonomic neuropathy. Of these, 30 (15 males, 15 females; mean age 44.7 +/- 8.8 years) were found to have cardiac autonomic neuropathy. Thirty (30) age and sex matched diabetic patients (mean age 42.4 +/- 7.6 years) who had no evidence autonomic neuropathy were included in the study as control group. Both the groups of patients were evaluated for SMI by 24 hour ambulatory electrocardiographic (ECG) monitoring. RESULTS: Incidence of SMI was significantly higher in patients with autonomic neuropathy 12/30 (40%) compared to those without 3/30 (10%) p < 0.001. Duration of diabetes was more (13 +/- 1.59 years) in patients with autonomic neuropathy compared to the control group (8.66 +/- 1.55 years) p < 0.001. Serum cholesterol and triglyceride levels were significantly higher in patients with autonomic neuropathy in comparison to control group < 0.05 and < 0.01, respectively. There was no difference in the pattern of SMI in the two groups (p = N.S). CONCLUSION: Cardiac autonomic neuropathy predisposes patients with diabetes mellitus to SMI. Twenty four hour ambulatory ECG monitoring provides useful diagnostic information in early detection and evaluation of SMI in asymptomatic diabetic patients.

Adult↗
PubMed1998

Ser-262 in human recombinant tau protein is a markedly more favorable site for phosphorylation by CaMKII than PKA or PhK.

Several kinases have been shown to phosphorylate tau protein at Ser-262, an important site involved in the regulation of the binding of tau to microtubules. In this study we compared the phosphorylation of tau at Ser-262 by CaMKII, PhK and PKA in vitro as determined by radioimmunoblots developed by the monoclonal antibody 12E8 which recognizes P-Ser-262 and P-Ser-356; and Ab-262, a polyclonal antibody which is specific to unphosphorylated Ser-262 in tau. We found that the phosphorylation at Ser-262 was several times more effective by CaMKII than PKA or PhK. Employing rat brain extract as a source of all brain kinases and KN-62, a specific inhibitor of CaMKII, we found that CaMKII accounts for approximately 45% of phosphorylation at Ser-262. Furthermore, in rat brain slices kept metabolically active in oxygenated artificial CSF, phosphorylation of tau at Ser-262 was (i) increased up to 120% in the presence of bradykinin, a CaMKII activator, and (ii) inhibited by approximately 35% in the presence of KN-62. Thus, CaMKII is a major tau Ser-262 kinase in mammalian brain.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1998

Tau is phosphorylated by GSK-3 at several sites found in Alzheimer disease and its biological activity markedly inhibited only after it is prephosphorylated by A-kinase.

Alzheimer disease is characterized by a specific type of neuronal degeneration in which the microtubule associated protein tau is abnormally hyperphosphorylated causing the disruption of the microtubule network. We have found that the phosphorylation of human tau (tau3L) by A-kinase, GSK-3 or CK-1 inhibits its microtubule assembly-promoting and microtubule-binding activities. However, the inhibition of these activities of tau by GSK-3 is significantly increased if tau is prephosphorylated by A-kinase or CK-1. The most potent inhibition is observed by combination phosphorylation of tau with A-kinase and GSK-3. Under these conditions, only very few microtubules are seen by electron microscopy. Sequencing of 32P-labeled trypsin phosphopeptides from tau prephosphorylated by A-kinase (using unlabeled ATP) and further phosphorylated by GSK-3 in the presence of [gamma-32P]ATP revealed that Ser-195, Ser-198, Ser-199, Ser-202, Thr-205, Thr-231, Ser-235, Ser-262, Ser-356 and Ser-404 are phosphorylated, whereas if tau is not prephosphorylated by A-kinase, GSK-3 phosphorylates it at Thr-181, Ser-184, Ser-262, Ser-356 and Ser-400. These data suggest that (i) prephosphorylation of tau by A-kinase makes additional and different sites accessible for phosphorylation by GSK-3; (ii) phosphorylation of tau at these additional sites further inhibits the biological activity of tau in its ability to bind to microtubules and promote microtubule assembly. Thus a combined role of A-kinase and GSK-3 should be considered in Alzheimer neurofibrillary degeneration.

Alzheimer Disease↗
PubMed1998

Phosphorylation of tau at both Thr 231 and Ser 262 is required for maximal inhibition of its binding to microtubules.

The paired helical filaments (PHFs) found in Alzheimer's disease (AD) brains are composed primarily of the microtubule-associated protein tau. PHF-tau is in a hyperphosphorylated state and is unable to promote microtubule assembly. We investigated whether the inhibition of tau binding to microtubules is increased when tau is phosphorylated by different kinases in combination with GSK-3. We found that when tau was first phosphorylated by A-kinase, C-kinase, cdk5, or CaM kinase II and then by GSK-3, its binding to microtubules was inhibited by 45, 61, 78, and 79%, respectively. Further, the kinase combinations cdk5/GSK-3 and CaM kinase II/GSK-3 rapidly phosphorylated the sites Thr 231 and Ser 235. When these sites were individually replaced by Ala and the phosphorylation experiments repeated, tau binding to microtubules was inhibited by 54 and 71%, respectively. By comparison, when Ser 262 was replaced by Ala, tau binding to microtubules was inhibited by only 8% after phosphorylation by CaM kinase II. From these observations we estimate that the phosphorylation of Thr 231, Ser 235, and Ser 262 contributes approximately 26, approximately 9, and approximately 33%, respectively, of the overall inhibition of tau binding to microtubules. Together, our results indicate that the binding of tau to microtubules is controlled by the phosphorylation of several sites, among which are Thr 231, Ser 235, and Ser 262.

Amino Acid Sequence↗
PubMed1998

Levels of tau phosphorylation at different sites in Alzheimer disease brain.

The microtubule-associated protein tau is abnormally hyperphosphorylated in Alzheimer's disease (AD) brain. To date, 21 phosphorylated sites of tau have been identified. In the present study the levels of phosphorylation at Ser199/Ser202, Thr231/Ser235, Ser262/Ser356 and Ser396/Ser404 of tau in AD brain homogenate and its 100,000 x g supernatant were determined using radioimmuno-dot-blot assay. In homogenate, Ser199/Ser202 and Ser262/Ser356 were phosphorylated to similar level and were more phosphorylated than Thr231 or Ser396/Ser404. In supernatant, there was no significant difference in phosphorylated tau level among the investigated sites except for Thr231/Ser235 which was least phosphorylated. These results suggest that Ser199/Ser202 and Ser262/Ser356 are major sites of phosphorylation of tau in AD brain.

Aged↗
PubMed1998

Accumulation of cyclin-dependent kinase 5 (cdk5) in neurons with early stages of Alzheimer's disease neurofibrillary degeneration.

Cyclin-dependent kinase 5 (cdk5) is one of the candidate kinases involved in the abnormal hyperphosphorylation of tau. To have a direct effect on tau hyperphosphorylation, cdk5 protein levels and enzyme activity should be upregulated in especially those neurons that develop neurofibrillary tangles (NFTs). We studied the distribution of cdk5 immunoreactivity in neurons with or without early- and late-stage NFTs in hippocampal, entorhinal, transentorhinal, temporal and frontal cortices, and cerebellum of Alzheimer's disease (AD) and control brain. The immunocytochemical localisation of cdk5 was compared with that obtained using antibodies to PHF-tau (tau in paired helical filaments of NFTs, mAb AT8) and ubiquitin as markers of early and late stage NFTs, respectively. Immunoreactivities of cdk5 and PHF-tau were found in neuronal perikarya and processes of hippocampal, entorhinal, transentorhinal, temporal and frontal, and cerebellar cortices. An apparent increase of cdk5 immunoreactivity was seen in pretangle neurons and in neurons bearing early stage NFTs. These findings suggest that this kinase might be involved in the formation of NFTs at a relatively early stage in the neocortex.

Aged↗
PubMed1998

The regulation of phosphorylation of tau in SY5Y neuroblastoma cells: the role of protein phosphatases.

In Alzheimer disease brain the microtubule associated protein (MAP) tau is abnormally hyperphosphorylated. The role of protein phosphatases (PP) in the regulation of phosphorylation of tau was studied in undifferentiated SY5Y cells. In cells treated with 10 nM okadaic acid (OA), a PP-2A/PP-1 inhibitor, the PP-1 and -2A activities decreased by 60% and 100% respectively and the activities of MAPKs, cdc2 kinase and cdk5, but not of GSK-3, increased. OA increased the phosphorylation of tau at Thr-231/Ser-235 and Ser-3961404, but not at Ser-262/356 or Ser-199/202. An increase in tyrosinated/detyrosinated tubulin ratio, a decrease in the microtubule binding activities of tau, MAP1b and MAP2, and cell death were observed. Treatment with 1 microm taxol partially inhibited the cell death. These data suggest (1) that OA induced hyperphosphorylation of tau is probably the result of activated MAPK and cdks in addition to decreased PP-2A and PP-1 activities and (2) that in SY5Y cells the OA induced cell death is associated with a decrease in stable microtubules.

Cell Survival↗
PubMed1998

Mechanisms of neurofibrillary degeneration and the formation of neurofibrillary tangles.

Alzheimer disease (AD) has polyetiology. Independent of the etiology the disease is characterized histopathologically by the intraneuronal accumulation of paired helical filaments (PHF), forming neurofibrillary tangles, neuropil threads and dystrophic neurites surrounding the extracellular deposits of beta-amyloid in plaques, the second major lesion. The clincal expression of AD correlates with the presence of neurofibrillary degeneration; beta-amyloid alone does not produce the disease clinically. Thus arresting neurofibrillary degeneration offers a promising key target for therapeutic intervention of AD. The major protein subunit of PHF is the microtubule-associated protein tau. Tau in AD brain, especially PHF, is abnormally hyperphosphorylated and glycosylated. With maturation, the tangles are increasingly ubiquitinated. Levels of tau and conjugated ubiquitin are elevated both in AD brain and CSF. The AD abnormally phosphorylated tau (AD P-tau) does not promote microtubule assembly, but on dephosphorylation its microtubule promoting activity is restored to approximately that of the normal tau. The AD P-tau competes with tubulin in binding to normal tau, MAP1 and MAP2 and inhibits their microtubule assembly promoting activities. Furthermore, the AD P-tau sequesters normal MAPs from microtubules. The association of AD P-tau with normal tau but not with MAP1 or MAP2 results in the formation of tangles of 3.3 +/- 0.5 mm filaments. Deglycosylation of Alzheimer neurofibrillary tangles with endoglycosidase F/N-glycosidase F untwists the PHF resulting in tangles of thin filaments similar to those formed by association between the AD P-tau and normal tau. Dephosphorylation or deglycosylation plus dephosphorylation but not deglycosylation alone restores the microtubule assembly promoting activity of tau. In vitro AD P-tau can be dephosphorylated by protein phosphatases PP-2B, PP-2A and PP-1 but not PP-2C and all the three tau phosphatases are present in brain neurons. Tau phosphatase activity is decreased by approximately 30% in AD brain. Inhibition of PP-2A and PP-1 activities in SY5Y neuroblastoma by 10 nM okadaic acid causes breakdown of microtubules and the degeneration of these cells. It is suggested (I) that a defect(s) in the protein phosphorylation/dephosphorylation system(s) leads to a hyperphosphorylation of tau, (ii) that this altered tau causes disassembly of microtubules and consequently a retrograde neuronal degeneration; (iii) a pharmacological approach to AD is to enhance the tau phosphatase activity; and (iv) that CSF tau and conjugated ubiquitin levels are promising markers of AD brain pathology.

Aged↗
PubMed1998

Subcellular distribution of protein phosphatases and abnormally phosphorylated tau in the temporal cortex from Alzheimer's disease and control brains.

Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer's disease (AD). In vitro studies have shown that protein phosphatases PP-2A and PP-2B can convert Alzheimer like tau to its normal state and that the activities of PP-1, PP-2A, and phosphotyrosyl-protein phosphatase (PTP) are reduced in AD brain. However, to have a direct effect on the regulation of phosphorylation on tau, these enzymes have to exist in neurons. Using specific polyclonal antibodies the levels of protein phosphatases PP-1, PP-2A, and PP-2B were determined by indirect ELISA in superior temporal cortical gray matter of AD and control brains. The protein levels of PP-2A and PP-2B were significantly increased in postsynaptosomal supernatant 2 (S2) of the AD group, and this alteration showed a significant linear correlation with levels of hyperphosphorylated tau. PP-1 and PTP-1B levels were not significantly changed in any of the AD fractions. Because of the large variation from case to case, the activity levels of none of the phosphatases investigated were significantly different between the AD and control groups. However, the PP-2B specific activity (activity/protein) showed a significant linear inverse correlation with hyperphosphorylated tau. These studies suggest that any attempt by the AD brain to compensate for the decreased tau phosphatase activity remains unsuccessful and that the decrease in phosphatase activity might contribute to increased levels of abnormally phosphorylated tau.

Aged↗
PubMed1998

Supraventricular arrhythmia: a complication of 5-fluorouracil therapy.

5-Fluorouracil is an S-phase-specific, synthetic pyrimidine antimetabolite, which is used as a cytostatic agent for a variety of malignant lesions, either singly or in multidrug regimens. Gastrointestinal toxicity and myelosuppression are the most common adverse reactions, but, of late, clinical cardiotoxicity has been reported in both prospective and retrospective studies. We present our experience of clinical cardiotoxicity in five patients.

Adenocarcinoma↗
PubMed1998

Tau phosphatase activity as a therapeutic target for AD.

In the normal brain, tau promotes the assembly of tubulin into microtubules and maintains the structure of microtubules, which are required for axonal transport. Tau in AD brain is abnormally hyperphosphorylated ("AD P-tau") and does not promote the in vitro assembly of microtubules, bind to microtubules or stabilize their structure. Employing phosphorylation-dependent antibodies to tau, site-specific dephosphorylation of AD P-tau has been investigated. The phos-phoseryl/phosphothreonyl protein phosphatases (PSPs) PP-1, PP-2A and PP-2B rapidly dephosphorylated AD P-tau in vitro. The dephosphorylation of neurofibrillary tangles of paired helical filaments by the two major tau phosphatases, PP-2A and PP-2B, has been demonstrated to produce marked biochemical, biological and structural alterations. Thus, by increasing the activity of one more of these tau phosphatases, it might be possible to prevent and inhibit neuronal degeneration and consequently both the sporadic and the familial forms of AD.

Journal Article↗