BetaA amyloid peptide (25-35) induced APP expression in cultured astrocytes

The Effect of Aβ₁₋₄₂ Oligomers on APP Processing and Aβ₁₋₄₀ Generation in Cultured U-373 Astrocytes

BACKGROUND

Amyloid-β (Aβ) peptides are a family of proteins that are considered to be a principal aspect of Alzheimer's disease (AD), the most common cause of senile dementia affecting elderly individuals. These peptides result from the proteolytic processing of amyloid precursor protein (APP) by sequential cleavage mediated via β- and x03B3;-secretases. Evidence suggests that an overproduction and/or a lack of degradation may increase brain Aβ levels which, in turn, contribute to neuronal loss and development of AD.

OBJECTIVES

In this study, we seek to determine what effect Aβ has on APP processing in cultured astrocytes.

METHODS

Using the human astrocytoma cell line U-373, we investigated the effects induced by oligomeric Aβ1-42 treatment on the cellular levels/expression of APP and its products, C-terminal fragments αCTF and βCTF, and Aβ1-40. In conjunction with these experiments, we examined the relative levels and activity of β- and x03B3;-secretases in Aβ-treated astrocytes.

RESULTS

We report here that Aβ1-42 treatment of astrocytes increased the expression of APP and its cleaved products including Aβ1-40 in a time-dependent manner.

CONCLUSIONS

These results suggest that activated astrocytes can contribute to the development of AD by enhancing levels and processing of APP leading to an increased production/secretion of Aβ-related peptides.

Complexing Aβ prevents the cellular anomalies induced by the Peptide alone

Retention of intracellular secreted APP (isAPP) can be provoked by the neurotoxic peptide Aβ. The latter decreases in the cerebrospinal fluid of Alzheimer’s disease (AD) patients, as a consequence of its cerebral accumulation and deposition into senile plaques. Of similar relevance, secreted APP (sAPP) levels can be associated with AD. The studies here presented, reinforce the link between sAPP and Aβ and address putative therapeutic strategies. Laminin and gelsolin are potential candidates; both prevent Aβ fibril formation by complexing with Aβ, thus attenuating its neurotoxicity. We show that preincubation of Aβ with laminin and gelsolin has the effect of rendering it less potent to isAPP accumulation in cortical neurons. This appears to be related to a decrease in F-actin polymerization, whereas Aβ alone induces the polymerization. Further, Aβ decreases gelsolin levels, and the latter is involved in Aβ removal. Our data indicates that Aβ-laminin and Aβ-gelsolin complexes are less neurotoxic and also less potent than fibrillar Aβ at inducing isAPP retention. These results validate the potential of these proteins as therapeutic strategies that prevent the Aβ-induced effects. In hence, given that Aβ decreases the levels of proteins involved in its own clearance, this may contribute to the mechanisms underlying AD pathology.

Amyloid precursor protein revisited: neuron-specific expression and highly stable nature of soluble derivatives

APP processing and amyloid-β production play a central role in Alzheimer disease pathogenesis. APP has been considered a ubiquitously expressed protein. In addition to amyloid-β, α- or β-secretase-dependent cleavage of APP also generates soluble secreted APP (APPsα or APPsβ, respectively). Interestingly, APPsβ has been shown to be subject to further cleavage to create an N-APP fragment that binds to the DR6 death receptor and mediates axon pruning and degeneration under trophic factor withdrawal conditions. By performing APP immunocytochemical staining, we found that, unexpectedly, many antibodies yielded nonspecific staining in APP-null samples. Screening of a series of antibodies allowed us to identify a rabbit monoclonal antibody Y188 that is highly specific for APP and prompted us to re-examine the expression, localization, and stability of endogenous APP and APPsβ in wild-type and in APPsβ knock-in mice, respectively. In contrast to earlier studies, we found that APP is specifically expressed in neurons and that its expression cannot be detected in major types of glial cells under basal or neuroinflammatory conditions. Both APPsα and APPsβ are highly stable in the central nervous system (CNS) and do not undergo further cleavage with or without trophic factor support. Our results clarify several key questions with regard to the fundamental properties of APP and offer critical cellular insights into the pathophysiology of APP.

Alphabeta hinders nuclear targeting of AICD and Fe65 in primary neuronal cultures

The intracellular domain of the Alzheimer's amyloid precursor protein (AICD) has been described as an important player in the transactivation of specific genes. It results from proteolytic processing of the Alzheimer's amyloid precursor protein (APP), as does the neurotoxic Abeta peptide. Although normally produced in cells, Abeta is typically considered to be a neurotoxic peptide, causing devastating effects. By exposing primary neuronal cultures to relatively low Abeta concentrations, this peptide was shown to affect APP processing. Our findings indicate that APP C-terminal fragments are increased with concomitant reduction in the expression levels of APP itself. AICD nuclear immunoreactivity detected under control conditions was dramatically reduced in response to Abeta exposure. Additionally, intracellular protein levels of Fe65 and GSK3 were also decreased in response to Abeta. APP nuclear signaling is altered by Abeta, affecting not only AICD production but also its nuclear translocation and complex formation with Fe65. In effect, Abeta can trigger a physiological negative feedback mechanism that modulates its own production.

Beta-amyloid expression, release and extracellular deposition in aged rat brain slices

Alzheimer's disease (AD) is characterized by beta-amyloid plaques, tau pathology, cholinergic cell death and inflammation. The aim of this study was to investigate whether beta-amyloid is generated, released and extracellularly deposited in organotypic brain slices. In developing slices, no amyloid-precursor protein (APP) was detectable; however, there was a strong upregulation in aging slices. In such slices, rat beta-amyloid(1-42) and -(1-40) peptides were found using four sequence-specific antibodies. APP and beta-amyloid were expressed in neurons and to a lesser extent in astrocytes. Beta-amyloid was secreted into the medium. Beta-amyloid was located extracellularly when aging slices were incubated with medium at pH 6.0 including apolipoprotein E4 (ApoE4). It is concluded that aging organotypic brain slices express beta-amyloid and that acidosis induces cell death with efflux of beta-amyloid and extracellular depositions, which is triggered by ApoE4. This novel in vitro model may enable us to investigate further the pathological cascade for AD and may be useful to explore future therapeutics.

Effect of cell density on intracellular levels of the Alzheimer's amyloid precursor protein

The precise function of APP (Alzheimer's amyloid precursor protein) remains to be fully elucidated, but various lines of evidence suggest that it may be involved in cell adhesion processes. Because APP is a transmembrane glycoprotein, variations in its expression level may have direct bearing on its putative role in cell adhesion. Our results revealed that although APP levels did not change markedly with increasing cell density (ICD), there was a small but reproducible increase in APP expression at subconfluent conditions. Higher expression APP levels led to corresponding increases in the amount of APP processed and secreted APP (sAPP) released into the cell media. Given that phorbol esters stimulate the non-amyloidogenic pathway at the expense of reducing production of Abeta (the peptide found deposited as neuritic plaques in the brains of patients with Alzheimer's disease), thus providing an interesting therapeutic focus, we tested the effect of the phorbol 12-myristate 13-acetate (PMA) on APP processing at ICD. PMA not only stimulated sAPP release at all densities tested, but also produced a corresponding decrease in the intracellular levels of APP. Further experimentation revealed that increased APP expression with ICD was dependent on factors present in conditioned medium. Interestingly, exposing cells to the Abeta peptide itself could mimic these results, thus providing evidence for a potential positive feedback mechanism between Abeta production and intracellular APP levels.

Protein phosphorylation and APP metabolism

Numerous lines of evidence place signal transduction cascades at the core of many processes having a direct role in neurodegeneration and associated disorders. Key players include neurotransmitters, growth factors, cytokines, hormones, and even binding and targeting proteins. Indeed, abnormal phosphorylation of key control proteins has been detected in many cases and is thought to underlie the associated cellular dysfunctions. Several signaling cascades have been implicated, affecting processes as varied as protein processing, protein expression, and subcellular protein localization, among others. The Alzheimer's amyloid precursor protein (APP) is a phosphoprotein, with well-defined phosphorylation sites but whose function is not clearly understood. The factors and pathways regulating the processing of APP have been particularly elusive, both in normal ageing and the Alzheimer's disease (AD) condition. Not surprisingly, the physiological function(s) of the protein remain(s) to be elucidated, although many hypotheses have been advanced. Nonetheless, considerable data has accumulated over the last decade, placing APP in key positions to be modulated both directly and indirectly by phosphorylation and phosphorylation-dependent events. The pathological end product of APP processing is the main proteinaceous component of the hallmark senile plaques found in the brains of AD patients, that is, a toxic peptide termed Abeta. In this minireview we address the importance of phosphorylation and signal transduction cascades in relation to APP processing and Abeta production. The possible use of the identified molecular alterations as therapeutic targets is also addressed.

The neuroprotective peptide NAP inhibits the aggregation of the beta-amyloid peptide

Alzheimer's disease (AD) is characterized by brain plaques containing the beta-amyloid peptide (Abeta). One approach for treating AD is by blocking Abeta aggregation. Activity-dependent neuroprotective protein contains a peptide, NAP that protects neurons in culture against Abeta toxicity. Here, NAP was shown to inhibit Abeta aggregation using: (1) fluorimetry; (2) electron microscopy; (3) high-throughput screening of Abeta deposition onto a synthetic template (synthaloid); and (4) Congo Red staining of neurons. Further assays showed biotin-NAP binding to Abeta. These results suggest that part of the neuroprotective mechanism exerted by NAP is through modulation of toxic protein folding in the extracellular milieu.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

High level of amyloid precursor protein expression in neurite-promoting olfactory ensheathing glia (OEG) and OEG-derived cell lines

During all the life of a mammal, olfactory ensheathing glia (OEG) permit the entry and navigation of olfactory neuron axons from peripheral to central nervous system (CNS) territory. This physiological characteristic of OEG has been successfully used for promotion of axonal regeneration after CNS injury in animal models. However, cellular and molecular properties responsible for OEG regenerative ability remain to be unveiled. Two approaches may be followed: to carry out genomic or proteomic analysis to detect secreted and/or membrane bound molecules or to examine the expression of molecules previously described as neuritogenic. This is the case of amyloid precursor protein (APP), a neurite-promoting molecule. We have studied the expression of APP by OEG and OEG-derived clonal lines, immortalised with the large T antigen of SV40 (TEG lines). OEG express high levels of APP in vivo and in culture. TEG lines maintained high expression of APP. Western blot analysis showed the presence of high molecular weight forms of APP in OEG, corresponding probably to glycosylated forms and/or to higher expression of the full length APPs. The main APP isoforms present in OEG cultures were APP770 and 751. L-APP isoforms without the exon 15, which are those corresponding with proteoglycan forms, are predominant in glial cells. Our data showed that OEG had three times as much L-APP as astrocytes, which may correlate with OEG neuritogenic capacity. In conclusion APP, a neurite-promoting molecule, is produced by OEG. Its nexin activity, dependent on the Kunitz family of serine protease inhibitors (KPI) domain and/or in combination with its glycosylation level might contribute with other factors to the ability of these cells to foster axonal elongation.

Fibronectin modulation by A beta amyloid peptide (25-35) in cultured astrocytes of newborn rat cortex

Fibronectin appears to be present in Senile Plaques of Alzheimer's disease brains. These senile or neuritic plaques are surrounded by dystrophic neurites, activated microglia and reactive astrocytes. The purpose of this work was to establish if a direct correlation exists between the production of Fibronectin (FN) by astrocytes and the presence of amyloid, analysing the modification of this protein produced after the treatment of cultured astrocytes with amyloid peptide (25-35). Our data showed that the addition of previously polymerised A beta-peptide to cultured astrocytes induced a marked increase in FN immunoreactivity that is in part dependent on phosphatases 2A or phosphatase 1, since was partially inhibited by okadaic acid. The increased amount of FN did not appear to be associated to any specific single isoform of which are mainly present in the rat brain. Our data suggest that in vivo FN accumulated in senile plaques may be the result, at least in part, of the response of reactive astrocyte to the presence of amyloid peptide. The importance of FN up-regulation in vivo, as part of a 'positive' response of the astrocytes to produce molecules that favours neurite outgrowth, is discussed.

Regulation of cytokine secretion and amyloid precursor protein processing by proinflammatory amyloid beta (A beta)

Neurodegenerative processes in Alzheimer's disease (AD) are thought to be driven, in part, by the deposition of amyloid beta (A beta), a 39-43-aminoacid peptide product resulting from an alternative cleavage of amyloid precursor protein (APP). In addition to its neurotoxic properties, A beta may influence neuropathology by stimulating glial cell cytokine and acute phase protein secretion in affected areas of the brain (e.g., cortex, hippocampus). Using an in vitro human astrocyte model (U-373 MG astrocytoma cells), the effects of A beta treatment on acute phase protein (APP and alpha-1-antichymotrypsin [alpha 1-ACT]) and interleukin-8 (IL-8) were examined. U-373 MG cells secreted increased levels of alpha 1-ACT and neurotrophic/neuroprotective alpha-cleaved APP (alpha APP) after exposure to interleukin-1 beta (IL-1 beta) for 24 hours. A beta treatment resulted in a similar, but modest increase in alpha 1-ACT secretion, a two- to threefold stimulation of IL-8 production, and, conversely, a profound reduction in the levels of secreted alpha APPs. A beta inhibited alpha APP secretion by U-373 MG cells in a concentration- and conformation-dependent manner. Moreover, the reduction in alpha APP secretion was accompanied by an increase in cell-associated APP. Another proinflammatory amyloidogenic peptide, human amylin, similarly affected APP processing in U-373 astrocytoma cells. These data suggest that A beta may contribute to Alzheimer's-associated neuropathology by lowering the production of neuroprotective/neurotrophic alpha APPs. Moreover, the concomitant increase in cell-associated APP may provide increased substrate for the generation of amyloidogenic peptides within astrocytes.

A beta peptides and calcium influence secretion of the amyloid protein precursor from chick sympathetic neurons in culture

The major constituent of amyloid plaques in the Alzheimer disease (AD) brain is the amyloid protein (A beta). A beta has been shown to be neurotoxic to cells, but the exact mechanism of its effects are still not known. Most studies have focussed on A beta neurotoxicity, but little is known about the effect of A beta peptides on cellular protein metabolism and secretion. To examine the effect of A beta peptides on APP secretion, chick sympathetic neurons were metabolically labeled with [(35)S]methionine and the amounts of radiolabeled APP and A beta quantitated. Several A beta peptides (A beta(25-35), [pyroglu(3)]A beta(3-40), and [pyroglu(11)]A beta(11-40)) inhibited secretion of [(35)S]APP and increased cell-associated [(35)S]APP. There was also a 2-2.5-fold increase in secretion of several other proteins when cells were incubated with A beta(25-35). However, the amount of A beta secreted into the medium was decreased. Treatment of cells with the calcium ionophore A23187 caused a 1.5-fold increase in secreted [(35)S]APP and a decrease in cell-associated [(35)S]APP. Although L-type voltage-dependent calcium channels (VDCC) have been implicated in A beta toxicity, the effect of L-type VDCC on APP secretion has not previously been examined. The L-type VDCC antagonists nifedipine and diltiazem both increased [(35)S]APP secretion into the medium but did not influence the effect of A beta on [(35)S]APP secretion. These studies suggest that A beta interferes with the secretory pathway of APP. Insofar as secreted APP has been proposed to have a neuroprotective function, the accumulation of A beta in the AD brain could decrease secreted APP and thereby indirectly increase A beta toxicity.