Correlation of postural balance and knee muscle strength in the sit-to-stand test among women with and without postmenopausal osteoporosis

The task of standing up from a chair forms a part of daily life for all independent individuals. However, this task becomes more difficult with advancing age. Women with postmenopausal osteoporosis presented diminished knee extensor and flexor muscle strength. There was a weak correlation between knee muscle strength (greater with extensor strength) and postural balance during the act of standing up.

INTRODUCTION

This study aims to evaluate postural balance during the transition from sitting to standing and its relationship with knee extensor and flexor strength among women with and without postmenopausal osteoporosis.

METHODS

Assessments were made on 126 women (aged 55-65 years), divided into osteoporosis and control groups according to lumbar bone density. Their balance during the task of standing up from a chair was evaluated using the Balance Master® device. Knee muscle strength was evaluated using an isokinetic dynamometer (Biodex®), in concentric/concentric mode, at a velocity of 60°/s. Spearman's correlation between the variables of muscle strength and postural balance was evaluated. Subsequently, to evaluate the association of each balance variable with the group and with muscle strength, multiple linear regression models were fitted. The significance level was set at 0.05.

RESULTS

There was a difference in knee muscle strength between the groups (P < 0.05). However, independent of the muscle strength values, there were no differences in relation to weight transfer times (P = 0.556) or center of gravity sway velocity (P = 0.952). Transfer time diminished with increasing extensor strength (P = 0.025). The center of gravity sway velocity tended to increase with increasing extensor strength (P = 0.013) and was the same in the two groups (P = 0.264).

CONCLUSION

Women with postmenopausal osteoporosis presented diminished knee extensor and flexor muscle strength. There was a weak correlation between knee muscle strength (greater with extensor strength) and postural balance during the act of standing up.

Na+,K+-ATPase was found to be the membrane component responsible for the hydrophobic behavior of the brain membrane tubulin

We have previously described that the tubulin isolated from brain membranes as a hydrophobic compound by partitioning into Triton X-114 is a peripheral membrane protein [corrected]. The hydrophobic behavior of this tubulin is due to its interaction with membrane protein(s) and the interaction occurs principally with the acetylated tubulin isotype. In the present work we identified the membrane protein that interacts with tubulin as the Na+,K+-ATPase alpha subunit by amino acid sequencing. Using purified brain Na+,K+-ATPase we were able to isolate part of the total hydrophilic tubulin as a hydrophobic compound which contains a high proportion of the acetylated tubulin isotype.

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.

High-density lipoprotein aggregated by oxidation induces degeneration of neuronal cells

We have previously reported that high-density lipoprotein (HDL) exhibits antineuritogenic effects on chicken cerebral cells in culture. In the present study, we show the effects of HDLs, oxidized by UV irradiation or heating, on chicken cerebral neurons in culture. Both treatments produced several physical and chemical changes in the HDLs, i.e., formation of lipid peroxides, enlargement of HDL diameters, an increased exposure of the tryptophan groups of the apolipoprotein A-I to a more hydrophilic environment, formation of bityrosines, and cross-linking of apolipoprotein A-I. When these treatments were performed in the absence of EDTA, most of the modifications described above were more intense and HDLs formed a macroaggregate that displays a rosette-like structure. The aggregated HDLs produced neurodegeneration and death when added to both undifferentiated and differentiated cerebral neurons in culture. This process was accompanied by the disorganization of the cellular microtubular cytoskeleton and hyperphosphorylation of the microtubule-associated protein tau. Native HDL or HDLs treated in the presence of EDTA inhibited the neuritogenesis of undifferentiated neurons but did not show any significant effect on the differentiated neurons in culture. The effects on the cellular cytoskeleton and morphology of aggregated HDLs recall those of the fibrillar beta-amyloid peptide. The present results suggest that aggregated HDLs could participate in neurodegeneration associated with oxidative stress in the CNS.

Conversion of hydrophilic tubulin into a hydrophobic compound. Evidence for the involvement of membrane proteins

Brain membranes contain tubulin that can be isolated as a hydrophobic compound by partitioning into Triton X-114. We have previously postulated: (a) that this kind of tubulin is a peripheral membrane protein that arises from microtubules that in vivo interact with membranes and (b) that the hydrophobic behaviour is due to the interaction of tubulin with a membrane component. Here we report the in vitro conversion of hydrophilic into hydrophobic tubulin by incubating microtubule associated proteins (MAPs) free taxol-stabilized microtubules with Triton X-100 solubilized membranes. After incubation, the microtubules were sedimented, depolymerized and subjected to partition into Triton X-114. Part of the tubulin was isolated in the detergent phase and contained, as observed in native membranes, a high proportion of the acetylated isotype. Because of the high proportion of acetylated tubulin the 'in vitro' conversion resembles the 'in vivo' interaction. Electrophoretic analysis of the detergent phase shows, besides tubulin, two major protein bands of 29 and 100 kDa molecular mass. The ability of the solubilized membranes to convert hydrophilic into hydrophobic tubulin is greatly diminished if the solubilized membrane preparation is preincubated in the presence of trypsin or heated at 90 degrees C for 5 min, indicating that the membrane component that confers the hydrophobic behaviour to tubulin is of proteinaceous nature.

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

Microtubule-associated protein tau becomes abnormally hyperphosphorylated in Alzheimer's 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.

Mechanism of neurofibrillary degeneration in Alzheimer's disease

Neurofibrillary degeneration associated with the formation of intraneuronal neurofibrillary tangles of paired helical filaments (PHF) and 2.1 nm tau filaments is one of the most characteristic brain lesions of Alzheimer's disease. The major polypeptides of PHF are the microtubule associated protein tau. tau in PHF is present in abnormally phosphorylated forms. In addition to the PHF, the abnormal tau is present in soluble non-PHF form in the Alzheimer's disease brain. The level of tau in Alzheimer's disease neocortex is severalfold higher than in aged control brain, and this increase is in the form of the abnormally phosphorylated protein. The abnormally phosphorylated tau does not promote the assembly of tubulin into microtubules in vitro, and it inhibits the normal tau-stimulated microtubule assembly. After in vitro dephosphorylation both PHF and non-PHF abnormal tau stimulate the assembly of tubulin into microtubules. The activities of phosphoseryl/phosphothreonyl protein phosphatase 2A and nonreceptor phosphotyrosyl phosphatase(s) are decreased in AD brain. It is suggested that 1. A defect(s) in the protein phosphorylation/dephosphorylation system is one of the early events in the neurofibrillary pathology in AD; 2. A decrease in protein phosphatase activities, at least in part, allows the hyperphosphorylation of tau; and 3. Abnormal phosphorylation and polymerization of tau into PHF most probably lead to a breakdown of the microtubule system and consequently to neuronal degeneration.

Role of abnormally phosphorylated tau in the breakdown of microtubules in Alzheimer disease

The microtubule assembly-promoting activity of different pools of tau protein isolated from Alzheimer disease (AD) and control brains and the effect of dephosphorylation on this activity were studied. Tau isolated from a 2.5% perchloric extract of AD brain had almost the same activity as that obtained from control brain, and this activity did not change significantly on dephosphorylation. Abnormally phosphorylated tau (AD P-tau) isolated from brain homogenate of AD patients had little activity, and upon dephosphorylation with alkaline phosphatase, its activity increased to approximately the same level as the acid-soluble tau. Addition of AD P-tau to a mixture of normal tau and tubulin inhibited microtubule assembly. AD P-tau bound to normal tau but not to tubulin. These studies suggest that the abnormal phosphorylation of tau might be responsible for the breakdown of microtubules in affected neurons in AD not only because the altered protein has little microtubule-promoting activity but also because it interacts with normal tau, making the latter unavailable for promoting the assembly of tubulin into microtubules.

Microtubule-associated protein tau. Abnormal phosphorylation of a non-paired helical filament pool in Alzheimer disease

The major protein subunit of the paired helical filaments (PHF) of Alzheimer disease (AD) is the microtubule-associated protein tau. Tau is a family of phosphopolypeptides that are abnormally phosphorylated in PHF. In this study, a non-PHF pool of tau abnormally phosphorylated at Ser-199/202, and tau not phosphorylated at this site (AD P-tau and AD tau, respectively) were isolated from the 27,000 x g to 200,000 x g fraction of AD brain homogenate by extraction in 8 M urea, followed by dialysis against Tris buffer. AD P-tau and AD tau were further purified and separated from each other by acid precipitation, glial fibrillary acidic protein affinity chromatography, and phosphocellulose chromatography. The resulting AD P-tau and AD tau preparations were free of cytoskeletal proteins, ubiquitin, and beta-amyloid peptide. Immunochemical and morphological analysis of AD P-tau preparations revealed that most of the protein was of non-PHF origin. The AD P-tau was about 3-4-fold (approximately 8 mol P04/mol protein, M(r) 41,318) more phosphorylated than cytosolic tau from AD and control brains. Unlike PHF, the AD P-tau lacked ubiquitin. In AD brain the levels of cytosolic tau were about half of those in control aged cases. These findings suggest that the abnormal phosphorylation of tau in AD occurs in the cytosol.

Tyrosinatable and non-tyrosinatable tubulin subpopulations in rat muscle in comparison with those in brain

Using immunobinding and enzymatic assays we determined in rat muscle extracts the proportion of tyrosinatable tubulin, that is, tubulin that participates in the tyrosination/detyrosination cycle. We found that in muscle, in contrast with nervous tissue, practically all tubulin molecules are tyrosinatable. In the case of rat brain the non-tyrosinatable tubulin pool accounts for about 50% of the tubulin. In addition, isolectrofocusing of 14C-tyrosinated tubulin from brain and muscle extracts revealed a different composition in tyrosinatable tubulin isotypes. One of the isotypes, which in muscle accounts for 86% of the 14C-tyrosinated tubulin species, was detyrosinated by the action of tubulin carboxypeptidase faster than the rest of the 14C-tyrosinated tubulin isotypes taken in whole. In the case of brain extract, that isotype accounts for only 16% of the labeled tubulin.

Tyrosinated, detyrosinated and acetylated tubulin isotypes in rat brain membranes. Their proportions in comparison with those in cytosol

The heterogeneity of alpha-tubulin and the relative proportions of the tubulin isotypes were investigated in brain membranes of rats of 1, 25 and 180 days of age by using four anti-alpha-tubulin antibodies: a) the monoclonal DM1A antibody, specific for alpha-tubulin; b) the monoclonal 6-11B-1 antibody, specific for acetylated tubulin; c) a polyclonal antibody (Glu antibody), specific for detyrosinated tubulin; and d) a polyclonal antibody (Tyr antibody), specific for tyrosinated tubulin. We found that rat brain membranes contain the three tubulin isotypes mentioned above. The proportions of tyrosinated and detyrosinated tubulin relative to total alpha-tubulin were somewhat lower in membrane than in cytosol in animals of 25 and 180 days of age. At day one of development, the proportions in membrane were similar to those found in cytosol. With respect to the acetylated form, it was about 20 times higher in membrane than in cytosol at the three ages studied. The proportion of acetylated tubulin was determined in different subcellular fractions: myelin, synaptic vesicles, mitochondria, microsomes, and plasma membrane. While the amount of total tubulin differed between the different subcellular fractions, the proportion of acetylated tubulin relative to total alpha-tubulin was constant and similar to that found in total membranes. The proportion of acetylated tubulin was also investigated in non-neural tissues (kidney, liver and lung). Although values for cytosol were about 10-fold higher than that found in brain cytosol, no detectable values for membranes could be obtained in these organs.

Comparison of two methods in the estimation of the pharmacokinetic parameters of phenytoin

This report compares two different methods of estimating the pharmaco-kinetic parameters of phenytoin (PHT) from steady-state serum concentration data obtained in clinical practice: the standard two-stage method (STS) and the extended least squares method (ELS). The study was carried out with the data from 27 compliant adult epileptic patients treated with PHT on a monotherapy regimen. Application of Akaike information criterion (AIC) points to a better estimation with the STS method (AIC = 268.05 for ELS; AIC = 114.13 for STS). This was confirmed by calculating the mean prediction error (mpe) and the standard deviation of EMP, which were 0.59 +/- 6.11 mg/l and -0.12 +/- 2.93 mg/l for ELS, and STS methods, respectively. Despite the well-known limitations of the STS method, a simulation study revealed that it can be successfully used when the patients to be included are selected with great care and when reliable serum concentration data are available.

Comparison of methods of carbamazepine dosage, individualization in epileptic patients

The aim of the present work was to analyse the predictive capacity of different optimization methods for carbamazepine dosage regimens according to population pharmacokinetic parameters and/or serum levels data. Calculations were performed using a multiple non-linear regression program (MULTI 2 BAYES) with two different options: (a) using the ordinary least squares methods and (b) the least squares method based on the Bayesian algorithm. The predictive capacity of the three methods was analysed comparing the serum levels values in patients predicted by each method and those observed later. The performance obtained was poor and even unacceptable when dosage prediction was based on the mean parameters of the population studied (Method A) and when only one data point relating to the serum carbamazepine levels was available (Method B). When two serum levels data were used the standard deviation of the mean prediction error was only clinically acceptable when Bayesian non-linear regression was applied (Method C). When the number of serum levels data points was 3 or 4 the errors were acceptable from a clinical point of view and no significant differences could be found between the use of the simple and Bayesian version at the same level of information.

Relationship between the tyrosination state of tubulin and the activities of tubulin:tyrosine ligase and tubulin carboxypeptidase in rat muscle during development

Tubulin can be post-translationally modified by the incorporation or the release of a tyrosine residue at the COOH-terminus of the alpha subunit. The present study demonstrates that rat muscle soluble preparations contain tubulin carboxypeptidase besides tubulin:tyrosine ligase. The state of tyrosination of tubulin and the activities of both the ligase and the carboxypeptidase were examined in rat muscle during development. The proportion of tyrosinated tubulin with respect to tyrosinable tubulin (tyrosinated plus detyrosinated tubulin) decreased from 83% (new-born rats) to 28% (adult rats) with the corresponding increase in detyrosinated tubulin. The activities of the enzymes decreased continuously and in a near parallel fashion during development. These results indicate that the changes in the tyrosination state of tubulin can not be explained merely by changes in the enzyme activities. We also compared the ability of rat muscle and brain [14C]tyrosinated tubulin to act as substrate of the carboxypeptidase. Muscle tubulin was found to be a less efficient substrate than brain tubulin.