Role of the proteasome in Alzheimer's disease.

The proteasome is a multicatalytic complex involved in the degradation of polyubiquitinated proteins. Here we review the clues of a possible involvement of the proteasome in Alzheimer's disease neuropathology. Thus, we discuss the fact that the proteasome modulates the intracellular concentrations of presenilins 1 and 2. These two proteins, when mutated, appear responsible for most of early onset forms of Alzheimer's disease and this is thought to be due to the exacerbation of the pathogenic pathway of the maturation of the beta-amyloid precursor protein. Controlling presenilins concentrations could have drastic repercussions on cell physiology as suggested by the fact that proteasome inhibitors drastically potentiate the 'normal' or 'pathogenic' presenilins phenotype related with betaAPP processing. The possibility of considering the proteasome as a potential target for therapeutic intervention in Alzheimer's disease is discussed.

Alpha-synuclein and the Parkinson's disease-related mutant Ala53Thr-alpha-synuclein do not undergo proteasomal degradation in HEK293 and neuronal cells.

Synucleins are neuronal proteins detectable in the neuropathological lesions of several cerebral disorders. Thus, alpha-synuclein immunoreactivity is found in Lewy bodies, the histopathological hallmark of sporadic Parkinson disease-affected brains. When mutated, alpha-synuclein seems to be responsible for some familial forms of Parkinson disease. As Lewy bodies are enriched in ubiquitinated structures and also contain proteasome-related immunoreactivity, it could be hypothesized that the proteasome contributes to the cellular degradation of alpha-synucleins, thereby controlling their concentration-dependent aggregation process. Here, we first demonstrate that alpha-synuclein is not ubiquitinated in HEK293 cells. Furthermore, by means of two specific inhibitors, we show that wild type and Ala53Thr alpha-synuclein do not behave as proteasome substrates in HEK293 cells and murine neurons. Our study indicates that the proteasome does not contribute to the control of cellular synucleins concentration and therefore, unlikely participates to cerebral alpha-synucleinopathies.

Wild-type but not Parkinson's disease-related ala-53 --> Thr mutant alpha -synuclein protects neuronal cells from apoptotic stimuli.

Recent works suggest that alpha-synuclein could play a central role in Parkinson's disease (PD). Thus, two mutations were reported to be associated with rare autosomal dominant forms of the disease. We examined whether alpha-synuclein could modulate the caspase-mediated response and vulnerability of murine neurons in response to various apoptotic stimuli. We established TSM1 neuronal cell lines overexpressing wild-type (wt) alpha-synuclein or the PD-related Ala-53 --> Thr mutant alpha-synuclein. Under basal conditions, acetyl-Asp-Glu-Val-Asp-aldehyde-sensitive caspase activity appears significantly lower in wt alpha-synuclein-expressing cells than in neurons expressing the mutant. Interestingly, wt alpha-synuclein drastically reduces the caspase activation of TSM1 neurons upon three distinct apoptotic stimuli including staurosporine, etoposide, and ceramide C(2) when compared with mock-transfected cells. This inhibitory control of the caspase response triggered by apoptotic agents was abolished by the PD-related pathogenic mutation. Comparison of wild-type and mutated alpha-synuclein-expressing cells also indicates that the former exhibits much less vulnerability in response to staurosporine and etoposide as measured by the sodium 3'-[1-(phenylaminocarbonyl)-3, 4-tetrazolium]-bis(4-methoxy-6-nitro)benzenesulfonic acid assay. Altogether, our study indicates that wild-type alpha-synuclein exerts an antiapoptotic effect in neurons that appears to be abolished by the Parkinson's disease-related mutation, thereby suggesting a possible mechanism underlying both sporadic and familial forms of this neurodegenerative disease.

C-terminal maturation fragments of presenilin 1 and 2 control secretion of APP alpha and A beta by human cells and are degraded by proteasome.

BACKGROUND: Most early-onset forms of Alzheimer's disease are due to missense mutations located on two homologous proteins named presenilin 1 and 2 (PS1 and PS2). Several lines of evidence indicate that PS1 and PS2 undergo various post-transcriptional events including endoproteolytic cleavages, giving rise to 28-30 kD N-terminal (NTF) and 18-20 kD C-terminal (CTF) fragments that accumulate in vivo. Whether the biological activity of presenilins is borne by the processed fragments or their holoprotein precursor remains in question. We have examined the putative control of beta APP maturation by CTF-PS1/PS2 and the catabolic process of the latter proteins by the multicatalytic complex, proteasome. MATERIALS AND METHODS: We transiently and stably transfected HEK293 cells with CTF-PS1 or CTF-PS2 cDNA. We examined these transfectants for their production of A beta 40, A beta 42, and APP alpha by immunoprecipitation using specific polyclonals. The effect of a series of proteases inhibitors on the immunoreactivity of CTF-PS1/PS2 was examined by Western blot. Finally, the influence of proteasome inhibitors on the generation of beta APP fragments by CTF-expressing cells was assessed by combined immunoprecipitation and densitometric analyses. RESULTS: We showed that transient and stable transfection of CTF-PS1 and CTF-PS2 cDNAs in human cells leads to increased secretion of APP alpha and A beta, the maturation products of beta APP. Furthermore, we demonstrated that two proteasome inhibitors, lactacystin and Z-IE(Ot-Bu)A-Leucinal, prevent the degradation of both CTFs. Accordingly, we established that proteasome inhibitors drastically potentiate the phenotypic increased production of APP alpha and A beta elicited by CTF-PS1/PS2. CONCLUSION: Our data establish that the C-terminal products of PS1 and PS2 maturation exhibit biological activity and in particular control beta APP maturation upstream to alpha-and beta/gamma-secretase cleavages. This function is directly controlled by the proteasome that modulates the intracellular concentration of CTFs.

Effect of protein kinase A inhibitors on the production of Abeta40 and Abeta42 by human cells expressing normal and Alzheimer's disease-linked mutated betaAPP and presenilin 1.

1. We previously established that the formation of both alpha- and beta/gamma-secretase-derived products generated by human embryonic kidney 293 cells (HEK293) expressing either wild type or mutant betaAPP could be stimulated by agonists of the cyclic AMP/protein kinase A pathways. This cyclic AMP-dependent effect modulates post-translational events since it is not prevented by actinomycin D or cycloheximide. 2. We show here that two protein kinase A inhibitors, H89 and PKI, both trigger dose-dependent inhibition of the basal constitutive production of Abeta40 and Abeta42 by HEK293 cells expressing wild type betaAPP751. 3. H89 also potently inhibits the total Abeta produced by the neocortical neuronal cell line TSM1. 4. These two inhibitors also drastically reduce the recovery of Abeta40 and Abeta42 produced by HEK293 cells expressing the Swedish (Sw) betaAPP and M146V-presenilin 1 (PS1) mutations responsible for cases of the early-onset forms of Familial Alzheimer's disease (FAD). 5. By contrast, H89 and PKI do not significantly affect the recovery of the physiological alpha-secretase-derived fragment APPalpha. 6. Our study indicates that protein kinase A inhibitors selectively lower the formation of Abeta40 and Abeta42 in human cells expressing normal and mutant betaAPP and PS1 without affecting the physiological alpha-secretase pathway in these cells. Selective inhibitors of protein kinase A may be of therapeutic value in both sporadic and Familial Alzheimer's disease, since they may decrease the production of Abeta that is thought to be responsible for the neurodegenerative process.

Unusual phenotypic alteration of beta amyloid precursor protein (betaAPP) maturation by a new Val-715 --> Met betaAPP-770 mutation responsible for probable early-onset Alzheimer's disease.

We have identified a novel beta amyloid precursor protein (betaAPP) mutation (V715M-betaAPP770) that cosegregates with early-onset Alzheimer's disease (AD) in a pedigree. Unlike other familial AD-linked betaAPP mutations reported to date, overexpression of V715M-betaAPP in human HEK293 cells and murine neurons reduces total Abeta production and increases the recovery of the physiologically secreted product, APPalpha. V715M-betaAPP significantly reduces Abeta40 secretion without affecting Abeta42 production in HEK293 cells. However, a marked increase in N-terminally truncated Abeta ending at position 42 (x-42Abeta) is observed, whereas its counterpart x-40Abeta is not affected. These results suggest that, in some cases, familial AD may be associated with a reduction in the overall production of Abeta but may be caused by increased production of truncated forms of Abeta ending at the 42 position.

Alzheimer's disease-linked mutation of presenilin 2 (N141I-PS2) drastically lowers APPalpha secretion: control by the proteasome.

Most of early onset familial forms of Alzheimer's disease (FAD) are due to inherited mutations located on two homologous proteins, presenilins 1 and 2 (PS1 and PS2) encoded by chromosomes 14 and 1, respectively. Here we show that the expression of wild type (wt)-PS2 in human HEK293 cells increases the production of the physiological alpha-secretase-derived product, APPalpha. By contrast, APPalpha secretion is drastically reduced in cells expressing the FAD-linked N141I-PS2. We establish that wt-PS2, N141I-PS2 and their C-terminal maturation fragment are degraded by the enzymatic multicatalytic complex, proteasome. Interestingly, two selective proteasome inhibitors, Z-IE(Ot-Bu)A-Leucinal and lactacystin potentiate the APPalpha secretion observed in wtPS2-expressing cells and further amplify the N141I-PS2-induced decrease in APPalpha production. By contrast, a series of pharmacological agents unable to affect the proteasome do not modify PS2 immunoreactivities and APPalpha recoveries. Altogether, our data indicate that: 1) wtPS2 positively modulates the alpha-secretase physiological pathway of betaAPP maturation in human cells; 2) N141I mutation on PS2 drastically lowers the secretion of APPalpha; 3) Proteasome inhibitors prevent the degradation of wtPS2, N141I-PS2 and their C-terminal maturation product. This protection against proteasomal degradation directly modulates the APPalpha secretion response elicited by wt- and FAD-linked PS2 expression in human HEK293 cells.

Proteasome inhibitors prevent the degradation of familial Alzheimer's disease-linked presenilin 1 and potentiate A beta 42 recovery from human cells.

BACKGROUND: Several lines of evidence suggest that most of the early-onset forms of familial Alzheimer's disease (FAD) are due to inherited mutations borne by a chromosome 14-encoded protein, presenilin 1 (PS1). This is likely related to an increased production of amyloid beta-peptide (A beta) 42, one of the main components of the extracellular deposits called senile plaques that invade human cortical areas during the disease. MATERIALS AND METHODS: We set up stably transfected HEK293 cells overexpressing wild-type (wt) and various FAD-linked mutated PS1. By Western blot analysis, we examined the influence of specific proteasome inhibitors on PS1-like immunoreactivities. Furthermore, by means of metabolic labeling and immunoprecipitation with A beta 40 and A beta 42-directed specific antibodies, we assessed the effect of the inhibitors on the production of A beta s by wt and mutated PS1-expressing cells transiently transfected with beta APP751. RESULTS: We show that two distinct proteasome inhibitors, Z-IE (Ot-Bu)A-Leucinal and lactacystin, increase in a time- and dose-dependent manner the immunoreactivities of both wt and mutated PS1. Furthermore, we demonstrate that PS1 is polyubiquitinated in these cells. Other inhibitors, ineffective on the proteasome, fail to protect wt and mutated PS1-like immunoreactivities. We also establish that the FAD-linked mutations of PS1 trigger a selective increased formation of A beta 42 as reflected by higher A beta 42 over total A beta ratios when compared with wtPS1-expressing cells. Interestingly, this augmentation was further amplified by proteasome inhibitors in cells expressing mutated but not wtPS1. CONCLUSION: Altogether, our data indicate that PS1 undergoes polyubiquitination in HEK293 cells and that the proteasome contributes to the degradation of wt and FAD-linked PS1, thereby directly influencing the A beta production in human cells.

Post-transcriptional contribution of a cAMP-dependent pathway to the formation of alpha- and beta/gamma-secretases-derived products of beta APP maturation in human cells expressing wild-type and Swedish mutated beta APP.

BACKGROUND: The physiopathological maturation of the beta-amyloid precursor protein can be modulated by effectors targeting a protein kinase C-dependent pathway. These agents increase the recovery of APP alpha, the physiological alpha-secretase-derived product of beta APP processing, and concomittantly lower the production of the pathogenic beta/gamma-secretase-derived A beta fragment. METHODS: We set up stably transfected HEK293 cells expressing wild-type or Swedish mutated beta APP. By combined metabolic labeling and/or immunoprecipitation procedures, we assessed the effect of various cAMP effectors on the production of the beta APP maturation products A beta 40, A beta 42, APP alpha, and its C-terminal counterpart. RESULTS: We show here that the cAMP-dependent protein kinase (PKA) effectors, dibutyryl-cAMP (dBut-cAMP) and forskolin, but not the inactive analog dideoxyforskolin, enhance the secretion of APP alpha and the intracellular production of its C-terminal counterpart (p10) in stably transfected HEK293 cells. The above agonists also drastically increase both A beta 40 and A beta 42 secretions and intracellular A beta recovery. The same influence was observed with HEK293 cells overexpressing the Swedish mutated beta APP. We attempted to delineate the relative contribution of transcriptional and post-transcriptional events in the cAMP-mediated response. We show here that the dBut-cAMP and forskolin-induced increase of APP alpha and A beta s secretions is not prevented by the transcription inhibitor actinomycin D. CONCLUSION: Our data suggest a major contribution of post-transcriptional events in the cAMP-dependent effect on beta APP maturation. It appears likely that cAMP triggers the PKA-dependent phosphorylation of a protein involved in beta APP maturation and occurring upstream to alpha- and beta/gamma-secretase cleavages.

Characterization of new polyclonal antibodies specific for 40 and 42 amino acid-long amyloid beta peptides: their use to examine the cell biology of presenilins and the immunohistochemistry of sporadic Alzheimer's disease and cerebral amyloid angiopathy cases.

BACKGROUND: In Alzheimer's disease (AD), the main histological lesion is a proteinaceous deposit, the senile plaque, which is mainly composed of a peptide called A beta. The aggregation process is thought to occur through enhanced concentration of A beta 40 or increased production of the more readily aggregating 42 amino acid-long A beta 42 species. MATERIALS AND METHODS: Specificity of the antibodies was assessed by dot blot, Western blot, ELISA, and immunoprecipitation procedures on synthetic and endogenous A beta produced by secreted HK293 cells. A beta and p3 production by wild-type and mutated presenilin 1-expressing cells transiently transfected with beta APP751 was monitored after metabolic labeling and immunoprecipitation procedures. Immunohistochemical analysis was performed on brains of sporadic and typical cerebrovascular amyloid angiopathy (CAA) cases. RESULTS: Dot and Western blot analyses indicate that IgG-purified fractions of antisera recognize native and denaturated A beta s. FCA3340 and FCA 3542 display full specificity for A beta 40 and A beta 42, respectively. Antibodies immunoprecipitate their respective synthetic A beta species but also A beta s and their related p3 counterparts endogenously secreted by transfected human kidney 293 cells. This allowed us to show that mutations on presenilin 1 triggered similar increased ratios of A beta 42 and its p 342 counterpart over total A beta and p3. ELISA assays allow detection of about 25-50 pg/ml of A beta s and remain linear up to 750 to 1500 pg/ml without any cross-reactivity. FCA18 and FCA3542 label diffuse and mature plaques of a sporadic AD case whereas FCA3340 only reveals the mature lesions and particularly labels their central dense core. In a CAA case, FCA18 and FCA3340 reveal leptomeningeal and cortical arterioles whereas FCA3542 only faintly labels such structures. CONCLUSIONS: Polyclonal antibodies exclusively recognizing A beta 40 (FCA 3340) or A beta 42 (FCA3542) were obtained. These demonstrated that FAD-linked presenilins similarly affect both p342 and A beta 42, suggesting that these mutations misroute the beta APP to a compartment where gamma-secretase, but not alpha-secretase, cleavages are modified. Overall, these antibodies should prove useful for fundamental and diagnostic approaches, as suggested by their usefulness for biochemical, cell biological, and immunohistochemical techniques.

Alpha-secretase-derived product of beta-amyloid precursor protein is decreased by presenilin 1 mutations linked to familial Alzheimer's disease.

Recent reports indicate that missense mutations on presenilin (PS) 1 are likely responsible for the main early-onset familial forms of Alzheimer's disease (FAD). Consensual data obtained through distinct histopathological, cell biology, and molecular biology approaches have led to the conclusion that these PS1 mutations clearly trigger an increased production of the 42-amino-acid-long species of beta-amyloid peptide (A beta). Here we show that overexpression of wild-type PS1 in HK293 cells increases A beta40 secretion. By contrast, FAD-linked mutants of PS1 trigger increased secretion of both A beta40 and A beta42 but clearly favor the production of the latter species. We also demonstrate that overexpression of the wild-type PS1 augments the alpha-secretase-derived C-terminally truncated fragment of beta-amyloid precursor protein (APP alpha) recovery, whereas transfectants expressing mutated PS1 secrete drastically lower amounts of APP alpha when compared with cells expressing wild-type PS1. This decrease was also observed when comparing double transfectants overexpressing wild-type beta-amyloid precursor protein and either PS1 or its mutated congener M146V-PS1. Altogether, our data indicate that PS mutations linked to FAD not only trigger an increased ratio of A beta42 over total A beta secretion but concomitantly down-regulate the production of APP alpha.