Cytoplasmic domain of the beta-amyloid protein precursor of Alzheimer's disease: function, regulation of proteolysis, and implications for drug development

The beta-amyloid protein precursor (APP) has been extensively studied for its role in amyloid production and the pathogenesis of Alzheimer's disease (AD). However, little is known about the normal function of APP and its biological interactions. In this Mini-Review, the role of the cytoplasmic domain of APP in APP trafficking and proteolysis is described. These studies suggest that proteins that bind to the cytoplasmic domain may be important targets for drug development in AD.     

Tau is not normally degraded by the proteasome

Tau-positive inclusions in neurons are consistent neuropathologic features of the most common causes of dementias such Alzheimer's disease and frontotemporal dementia. Ubiquitinated tau-positive inclusions have been reported in brains of Alzheimer's disease patients, but involvement of the ubiquitin-dependent proteasomal system in tau degradation remains controversial. Before considering the tau degradation in pathologic conditions, it is important to determine whether or not endogenous tau is normally degraded by the proteasome pathway. We therefore investigated this question using two complementary approaches in vitro and in vivo. Firstly, SH-SY5Y human neuroblastoma cells were treated with different proteasome inhibitors, MG132, lactacystin, and epoxomicin. Under these conditions, neither total nor phosphorylated endogenous tau protein levels were increased. Instead, an unexpected decrease of tau protein was observed. Secondly, we took advantage of a temperature-sensitive mutant allele of the 20S proteasome in Drosophila. Genetic inactivation of the proteasome also resulted in a decrease of tau levels in Drosophila. These results obtained in vitro and in vivo demonstrate that endogenous tau is not normally degraded by the proteasome.     

Signal transduction pathways involved in proteolysis-inducing factor induced proteasome expression in murine myotubes

The proteolysis-inducing factor (PIF) is produced by cachexia-inducing tumours and initiates protein catabolism in skeletal muscle. The potential signalling pathways linking the release of arachidonic acid (AA) from membrane phospholipids with increased expression of the ubiquitin-proteasome proteolytic pathway by PIF has been studied using C(2)C(12) murine myotubes as a surrogate model of skeletal muscle. The induction of proteasome activity and protein degradation by PIF was blocked by quinacrine, a nonspecific phospholipase A(2) (PLA(2)) inhibitor and trifluroacetyl AA, an inhibitor of cytosolic PLA(2). PIF was shown to increase the expression of calcium-independent cytosolic PLA(2), determined by Western blotting, at the same concentrations as those inducing maximal expression of 20S proteasome alpha-subunits and protein degradation. In addition, both U-73122, which inhibits agonist-induced phospholipase C (PLC) activation and D609, a specific inhibitor of phosphatidylcholine-specific PLC also inhibited PIF-induced proteasome activity. This suggests that both PLA(2) and PLC are involved in the release of AA in response to PIF, and that this is important in the induction of proteasome expression. The two tyrosine kinase inhibitors genistein and tryphostin A23 also attenuated PIF-induced proteasome expression, implicating tyrosine kinase in this process. PIF induced phosphorylation of p44/42 mitogen-activated protein kinase (MAPK) at the same concentrations as that inducing proteasome expression, and the effect was blocked by PD98059, an inhibitor of MAPK kinase, as was also the induction of proteasome expression, suggesting a role for MAPK activation in PIF-induced proteasome expression.     

Concomitant Differentiation and Partial Proteasome Inhibition Trigger Apoptosis in Neuroblastoma Cells

Proteasome activity is essential during cAMP-induced terminal differentiation of a murine neuroblastoma cell line (NBP2). However, the mechanisms through which proteasome affects NBP2 differentiation have not been characterized. We hypothesized that proteasome is required to implement the differentiation-mediated effects on cell cycle, and its partial inhibition during differentiation may have adverse consequences. Here we show that partial inhibition of proteasome during cAMP-induced differentiation of NBP2 cells causes apoptosis. Whereas differentiation induced growth arrest at G₁ phase, partial proteasome inhibition during differentiation resulted in the accumulation of cells at G₂M phase. Cell cycle data correlated with the level of cyclin-dependent kinase inhibitors p21WAF and p27Kip1, and cyclin A. While the level of p21 and p27 increased, the level of cyclin A decreased upon differentiation. In contrast, cells treated with proteasome inhibitor in the presence of cAMP-inducing agents showed increased levels of p21 and cyclin A early in the course of differentiation. However, the level of p21 and p27, but not cyclin A, decreased later during concomitant differentiation and partial proteasome inhibition when cells were undergoing apoptosis. Our data suggest that differentiation-mediated growth arrest is dependent on the temporal activity of cell cycle proteins. Partial inhibition of proteasome interferes with differentiation events partly by stabilizing cell cycle proteins and this triggers apoptosis. Thus, differentiating drugs combined with partial proteasome inhibition may impart higher therapeutic efficacy than differentiating agents alone for the treatment of neuroblastoma tumors.     

Impairment of protein homeostasis and decline of proteasome activity in microglial cells from adult Wistar rats

Common symptoms of different neurodegenerative diseases start to develop in the second half of the human life. Several of these diseases, including Alzheimer's and Parkinson's disease, are accompanied by severe disturbances of protein metabolism and homeostasis in the brain. Because microglial cells are, to some extent, responsible for the maintenance of this homeostasis, age-related functional changes of the microglia are important. We established, therefore, the preparation of cultures of primary microglial cells isolated from adult animals in comparison to the widely used standard model, primary microglial cells isolated from newborn animals. In addition, we investigated changes in the activation and in the protein homeostasis within these cells. The protein turnover seems to be significantly impaired in microglial cells isolated from adult animals and this seems to be accompanied by a decline in proteasomal function, but not in the protease content. We were also able to demonstrate higher cell surface molecule expression and a higher basal NO release of microglia isolated from adult animals in comparison to the microglia isolated from newborn rats; however, the PMA stimulated oxidative burst was abolished completely in cells from adult animals. Microglia from adult animals were also not able to upregulate their protein metabolism after activation. From these investigations it was concluded that microglial cells from adult animals have significantly different metabolic properties in comparison to the widely used microglial cells from newborn animals.     

Role of proteolysis in polyglutamine disorders

To date, nine polyglutamine disorders have been characterised, including Huntington's disease (HD), spinobulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and spinocerebellar ataxias 1, 2, 3, 6, 7 and 17 (SCAs). Although knockout and transgenic mouse experiments suggest that a toxic gain of function is central to neuronal death in these diseases (with the probable exception of SCA6), the exact mechanisms of neurotoxicity remain contentious. A further conundrum is the characteristic distribution of neuronal damage in each disease, despite ubiquitous expression of the abnormal proteins. One mechanism that could possibly underlie the specific distribution of neuronal toxicity is proteolytic cleavage of the full-length expanded polyglutamine tract-containing proteins. There is evidence found in vitro or in vivo (or both) of proteolytic cleavage in HD, SBMA, DRPLA, and SCAs 2, 3, and 7. In HD, cleavage has been demonstrated to be regionally specific, occurring as a result of caspase activation. These diseases are also characterised by development of intraneuronal aggregates of the abnormal protein that co-localise with components of the ubiquitin-proteasome pathway. It remains unclear whether these aggregates are pathogenic or merely disease markers; however, at least in the case of ataxin-3, cleavage promotes aggregation. Inhibition of specific proteases constitutes a potential therapeutic approach in these diseases.     

An inhibitor of mitochondrial complex I, rotenone, inactivates proteasome by oxidative modification and induces aggregation of oxidized proteins in SH-SY5Y cells

In Parkinson's disease, characteristic pathological features are the cell death of nigrostriatal dopamine neurons and the formation of Lewy bodies composed of oxidized proteins. Mitochondrial dysfunction and aggregation of abnormal proteins have been proposed to cause the pathological changes. However, the relation between these two factors remains to be clarified. In this study, the effects of mitochondrial dysfunction on the oxidative modification and accumulation of proteins were analyzed using an inhibitor of mitochondrial complex I, rotenone, and antibodies against acrolein- and dityrosine-modified proteins. Under conditions inducing mainly apoptosis in neuroblastoma SH-SY5Y cells, rotenone markedly increased oxidized proteins, especially those modified with acrolein, even though the increase in intracellular reactive oxygen and nitrogen species was only transient and was not so marked. In addition, the activity of the proteasome system degrading oxidized proteins was reduced profoundly after treatment with rotenone. The 20S beta subunit of proteasome was modified with acrolein, to which other acrolein-modified proteins were found to bind, as shown by coprecipitation with the antibody against 20S beta subunit. These results suggest that mitochondrial dysfunction, especially decreased activity of complex I, may reduce proteasome activity through oxidative modification of proteasome itself and aggregation with other oxidized proteins. This mechanism might account for the accumulation of modified protein and, at least partially, for cell death of the dopamine neurons in Parkinson's disease.     

蛋白酶体β5亚单位过表达对氧化环境下人晶状体上皮细胞的影响

【目的】探讨蛋白酶体筇亚单位（PSMB5）基因过表达对氧化条件下晶状体上皮细胞的影响。【方法】构建重组质粒pcDNA3．1-PSMB5并将其转染人人晶状体上皮细胞株SRAO1／04中，形成稳定表达，同时设pcDNA3．1空载体为对照组。RT—PCR法及Western blot法分别检测PSMB5基因及蛋白的表达情况。H2O2分别作用PSMB5转染细胞及空载体转染细胞，MTT法检测细胞增殖能力的变化。【结果】转染后用G418筛选3周后，获得PSMB5转染及空载体转染的G418抗性的细胞克隆。转染PSMB5基因的细胞PSMB5 mRNA表达强度明显增高．而空载体转染细胞与未转染细胞无显著差异：转染PSMB5基因的细胞的PSMB5蛋白表达也显著高于空载体转染细胞。低浓度H2O2作用后，转染PSMB5基因的细胞增殖能力高于空载体转染细胞。【结论】低浓度氧化环境下蛋白酶体历亚单位PSMB5过表达能对人晶状体上皮细胞具有保护作用。