Aberrant Protein Phosphorylation Research Articles

P38 kinase is activated in the Alzheimer's disease brain.

The p38 mitogen-activated protein kinase is a stress-activated enzyme responsible for transducing inflammatory signals and initiating apoptosis. In the Alzheimer's disease (AD) brain, increased levels of phosphorylated (active) p38 were detected relative to age-matched normal brain. Intense phospho-p38 immunoreactivity was associated with neuritic plaques, neuropil threads, and neurofibrillary tangle-bearing neurons. The antibody against phosphorylated p38 recognized many of the same structures as an antibody against aberrantly phosphorylated, paired helical filament (PHF) tau, although PHF-positive tau did not cross-react with the phospho-p38 antibody. These findings suggest a neuroinflammatory mechanism in the AD brain, in which aberrant protein phosphorylation affects signal transduction elements, including the p38 kinase cascade, as well as cytoskeletal components.

Centrosome hypertrophy in human breast tumors: implications for genomic stability and cell polarity.

The centrosome plays an important role in maintenance of cell polarity and in progression through the cell cycle by determining the number, polarity, and organization of interphase and mitotic microtubules. By examining a set of 35 high grade human breast tumors, we show that centrosomes of adenocarcinoma cells generally display abnormal structure, aberrant protein phosphorylation, and increased microtubule nucleating capacity in comparison to centrosomes of normal breast epithelial and stromal tissues. These structural and functional centrosome defects have important implications for understanding the mechanisms by which genomic instability and loss of cell polarity develop in solid tumors.

Changes in protein kinases in brain aging and Alzheimer's disease. Implications for drug therapy.

There is ample evidence for the involvement of aberrant protein phosphorylation reactions in aging and age-associated neurological disorders. Alzheimer's disease (AD) in particular. The exact nature of this involvement, however, is not yet elucidated. In the brain tissue of AD patients, there are numerous examples of altered protein phosphorylation pathways. Individual protein kinases and phosphorylation by these kinases in AD brain tissues have been found to be altered. Protein kinases studied include protein kinase C (PKC), protein tyrosine kinase (PTK), casein kinase II (CKII), Ca++/calmodulin-dependent kinase II and mitogen-activated protein (MAP) kinases, all of which are thought to be necessary for cell survival. Interestingly, different protein kinases are involved in different aspects of AD pathology. It is postulated that the perturbation of amyloid beta/A4-protein precursor (APP) metabolism triggers abnormal protein phosphorylation reactions responsible for dysfunction and eventual death of neurons in the brain. The association of APP mutation with certain familial types of AD strongly suggests that there might be a link between aberrant APP metabolism, protein phosphorylation cascades and the eventual expression of AD pathology (plaques and tangles) and neurodegeneration. In summary, recent studies emphasise the prime importance of protein phosphorylation in aging and AD. This raises the possibility that future pharmacological interventions might be devised to interfere with this kinase cascade for the prevention or treatment of age-associated neurological disorders.

チロシン脱リン酸化作用を有する低分子量酸性ボスファターゼのアルツハイマー病における変動

Recent studies in Alzheimer brains have shown aberrant protein phosphorylation, suggesting an alteration in protein kinases and/or phosphoprotein phosphatases. The separation profile on Sephadex G-100 gel filtration chromatography revealed that two major forms of high-molecular-weight (HMW) and low-molecular-weight (LMW) acid phosphatase were present in the crude extracts of human brains. In Alzheimer brains, the LMW acid phosphatase activity was significantly decreased compared to that in control brains. A specific antibody was prepared to analyze the protein level of this enzyme. Western blot analysis indicated that the level of LMW acid phosphatase protein was significantly reduced, whereas the activity of LMW acid phosphatase per enzyme molecule was not changed in Alzheimer brains. These results suggest that the reduction of LMW acid phosphatase activity in Alzheimer brains is due to its decreased protein level in Alzheimer's disease. The endogenous substrate of LMW acid phosphatase in the brain was also investigated. LMW acid phosphatase was purified from bovine brain with a molecular mass of 17 kDa. The LMW acid phosphatase from bovine brain dephosphorylated a Mr. 170 kDa phosphotyrosine protein in rat brain extracts. This Mr. 170 kDa protein was considered to be the epidermal growth factor (EGF) receptor using a specific antibody. These results suggest that LMW acid phosphatase in the brain may regulate EGF receptor-dependent transmembrane signalling by dephosphorylating the phosphorylated receptor.

Aberrant protein phosphorylation at tyrosine is responsible for the growth-inhibitory action of pp60v-src expressed in the yeast Saccharomyces cerevisiae.

Expression of pp60v-src, the transforming protein of Rous sarcoma virus, arrests the growth of the yeast Saccharomyces cerevisiae. To determine the basis of this growth arrest, yeast strains were constructed that expressed either wild-type v-src or various mutant v-src genes under the control of the galactose-inducible, glucose repressible GAL1 promoter. When shifted to galactose medium, cells expressing wild-type v-src ceased growth immediately and lost viability, whereas cells expressing a catalytically inactive mutant (K295M) continued to grow normally, indicating that the kinase activity of pp60v-src is required for its growth inhibitory effect. Mutants of v-src altered in the SH2/SH3 domain (XD4, XD6, SPX1, and SHX13) and a mutant lacking a functional N-terminal myristoylation signal (MM4) caused only a partial inhibition of growth, indicating that complete growth inhibition requires either targeting of the active kinase or binding of the kinase to phosphorylated substrates, or both. Cells arrested by v-src expression displayed aberrant microtubule structures, alterations in DNA content and elevated p34CDC28 kinase activity. Immunoblotting with antiphosphotyrosine antibody showed that many yeast proteins, including the p34CDC28 kinase, became phosphorylated at tyrosine in cells expressing v-src. Both the growth inhibition and the tyrosine-specific protein phosphorylation observed following v-src expression were reversed by co-expression of a mammalian phosphotyrosine-specific phosphoprotein phosphatase (PTP1B). However a v-src mutant with a small insertion in the catalytic domain (SRX5) had the same lethal effect as wild-type v-src, yet induced only very low levels of protein-tyrosine phosphorylation. These results indicate that inappropriate phosphorylation at tyrosine is the primary cause of the lethal effect of pp60v-src expression but suggest that only a limited subset of the phosphorylated proteins are involved in this effect.

Reduction of low‐molecular‐ weight acid phosphatase activity in Alzheimer brains

Recent studies in Alzheimer brains have shown aberrant protein phosphorylation, suggesting an alteration in protein kinases and/or phosphoprotein phosphatases. In the present study, the activity of acid phosphatase was investigated in samples prepared from postmortem normal human and Alzheimer brains. p-Nitrophenyl phosphate, a nonprotein phosphoester, was used as a substrate for acid phosphatase. The separation profile on Sephadex G-100 gel filtration chromatography revealed that two major forms of high-molecular-weight and low-molecular-weight acid phosphatase were present in the crude extracts of both rat and human brains. Another class of zinc ion (Zn2+)-dependent acid p-nitrophenyl phosphatase was also detected in rat and human brains. In Alzheimer brains, the low-molecular-weight acid phosphatase activity was significantly decreased compared to that in control brains; however, the high-molecular-weight and Zn(2+)-dependent acid phosphatase activity in control and Alzheimer brains was not different. These results suggest that reduced activity of the low-molecular-weight acid phosphatase, which possesses phosphotyrosine protein phosphatase activity, might be linked to aberrant protein tyrosine phosphorylation found in Alzheimer brains.