Secretory cleavage of beta-amyloid precursor protein in the cerebral white matter produces amyloidogenic carboxyl-terminal fragments

Pathophysiological roles of amyloidogenic carboxy-terminal fragments of the beta-amyloid precursor protein in Alzheimer's disease

Several lines of evidence suggest that some of the neurotoxicity in Alzheimer's disease (AD) is attributed to proteolytic fragments of amyloid precursor protein (APP) and beta-amyloid (Abeta) may not be the sole active component involved in the pathogenesis of AD. The potential effects of other cleavage products of APP need to be explored. The CTFs, carboxy-terminal fragments of APP, have been found in AD patients' brain and reported to exhibit much higher neurotoxicity in a variety of preparations than Abeta. Furthermore CTFs are known to impair calcium homeostasis and learning and memory through blocking LTP, triggering a strong inflammatory reaction through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction. Recently, it was reported that CTF translocated into the nucleus, binding with Fe65 and CP2, and in turn, affected transcription of genes including glycogen synthase kinase-3beta, which results in the induction of tau-rich neurofibrillary tangles and subsequently cell death. Spatial memory of transgenic (Tg) mice overexpressing CT100 was significantly impaired and CTFs were detected in the neurons as well as in plaques of the Tg mice and double Tg mice carrying CT100 and mutant tau. In this review, we summarize observations indicating that both CTF and Abeta may participate in the neuronal degeneration in the progress of AD by differential mechanisms.

Carboxyl-terminal peptide of beta-amyloid precursor protein blocks inositol 1,4,5-trisphosphate-sensitive Ca2+ release in Xenopus laevis oocytes

The effects of Alzheimer's disease-related amyloidogenic peptides on inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) mobilization were examined in Xenopus laevis oocytes. Intracellular Ca(2+) was monitored by electrophysiological measurement of the endogenous Ca(2+)-activated Cl(-) current. Application of a hyperpolarizing pulse released intracellular Ca(2+) in oocytes primed by pre-injection of a non-metabolizable inositol 1,4,5-trisphosphate analogue. The carboxyl terminus of the amyloid precursor protein inhibited inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca(2+) release in a dose-dependent manner. Equimolar beta-amyloid peptides Abeta(1-40) or Abeta(1-42) had no effect, and whereas a truncated carboxyl terminus lacking the Abeta domain was equipotent to the full-length one, a carboxyl terminus fragment lacking the NPTY sequence was less effective than the full-length fragment. The inhibition induced by the carboxyl terminus was not associated with the block of the Ca(2+)-dependent Cl(-) channel itself or compromised Ca(2+) influx. We conclude that the carboxyl terminus of the amyloid precursor protein inhibits inositol 1,4,5-trisphosphate-sensitive Ca(2+) release and could thus disrupt Ca(2+) homeostasis and that the carboxyl terminus is much more effective than the beta-amyloid fragments used. By perturbing the coupling of inositol 1,4,5-trisphosphate and Ca(2+) release, the carboxyl terminus of the amyloid precursor protein can potentially be involved in inducing the neural toxicity characteristic of Alzheimer's disease.

Effects of the beta-amyloid and carboxyl-terminal fragment of Alzheimer's amyloid precursor protein on the production of the tumor necrosis factor-alpha and matrix metalloproteinase-9 by human monocytic THP-1

To explore the direct role of beta-amyloid (Abeta) and carboxyl-terminal fragments of amyloid precursor protein in the inflammatory processes possibly linked to neurodegeneration associated with Alzheimer's disease, the effects of the 105-amino acid carboxyl-terminal fragment (CT(105)) of amyloid precursor protein on the production of tumor necrosis factor-alpha (TNF-alpha) and matrix metalloproteinase-9 (MMP-9) were examined in a human monocytic THP-1 cell line and compared with that of Abeta. CT(105) elicited a marked increase in TNF-alpha and MMP-9 production in the presence of interferon-gamma in a dose- and time-dependent manner. Similar patterns were obtained with Abeta despite its low magnitude of induction. Autocrine TNF-alpha is likely to be a main mediator of the induction of MMP-9 because the neutralizing antibody to TNF-alpha inhibits MMP-9 production. Genistein, a specific inhibitor of tyrosine kinase, dramatically diminished both TNF-alpha secretion and subsequent MMP-9 release in response to CT(105) or Abeta. Furthermore, PD98059 and SB202190, specific inhibitors of ERK or p38 MAPK respectively, efficiently suppressed CT(105)-induced effects whereas only PD98059 was effective at reducing Abeta-induced effects. Our results suggest that CT(105) in combination with interferon-gamma might serve as a more potent activator than Abeta in triggering inflammatory processes and that both tyrosine kinase and MAPK signaling pathways may represent potential therapeutic targets for the control of Alzheimer's disease progression.

Roles of A beta and carboxyl terminal peptide fragments of amyloid precursor protein in Alzheimer disease

Several lines of evidence indicate that A beta may play an important role in the pathogenesis of AD. However, there are several discrepancies between the production of A beta and the development of the disease. Thus, A beta may not be the sole active fragment of beta-amyloid precursor protein (betaAPP) in the neurotoxicity assiciated with AD. We focused on the amyloidegenic carboxyl terminal fragments of betaAPP containing the full length of A beta (CT105). We synthesized a recombinant carboxyl-terminal 105 amino acid fragment of betaAPP and examined the effects of CT105 and A beta on cultured neurons, Ca++ uptake into rat brain microsomes, Na+-Ca++ exchange activity, ion channel forming activity in lipid bilayers and passive avoidance performance of mice. Our results suggest that the cytotoxic and channel inducing effects of CT105 are much more potent than that of A beta and toxic mechanisms of CT105 are different from those of A beta. Taken together, these lines of evidence postulate that CT is an alternative toxic element important in the generation of the symptoms common to AD.

Effect of a carboxy-terminal fragment of the Alzheimer's amyloid precursor protein on expression of proinflammatory cytokines in rat glial cells

To explore factors involved in the induction of cytokines that may contribute to the pathogenesis of Alzheimer's disease (AD), the effect of a carboxy terminal 105 amino acid fragment (CT105) of the amyloid precursor protein (APP) on the gene expression of proinflammatory cytokines, IL-1beta and IL-6 was determined in cultured rat cortical glial cells in comparision to amyloid beta protein (A beta). Cells were incubated with 1 microM of insoluble CT105 aggregates or aged A beta1-40 peptide deposits which were mainly composed of stable monomeric and dimeric forms as assessed on Western blots. The levels of mRNAs were analyzed by reverse transcription polymerase chain reaction (RT-PCR). Highest levels of both IL-1beta and IL-6 transcripts were detected in the culture exposed to CT105 aggregates for 4 days. CT105 aggregates markedly increased IL-1beta mRNA level by 3.5 fold of the control level and this effect was more potent than that produced by aged A beta1-40 peptides. Furthermore, CT105 strongly induced accumulation of IL-6 mRNA level by 2 fold of the value potentiated by A beta1-40. Such induction was not observed with A beta 12-28 treatment. On the other hand, CT105 did not significantly alter either APP or glial fibrillary acidic protein (GFAP) mRNA levels. These results together imply that CT peptide besides its cytotoxic potency may act as a potent immunological component, strongly inducing both IL-1beta and IL-6 mRNA levels in the cultured glial cells. This CT peptide associated exacerbation of cytokine expression may be in part responsible for chronic inflammation linked to slowly progressive neurodegeneration characteristic to AD.

Amyloidogenic processing of Alzheimer's amyloid precursor protein in vitro and its modulation by metal ions and tacrine

Recent studies implicate that excessive amyloidogenic pathway of amyloid precursor protein (APP) processing may be the final common pathway involved in the pathogensis of AD. In attempts to identify the proteases or factors leading to excessive amyloid deposition, we evaluated the potential role of acethylcholinesterase (AChE) and its associated protease for amyloidogenic processing of APP in vitro. Prolonged incubation of a recombinant APP770 with AChE produced several amyloidogenic fragments accumulating a relatively stable a 18 kDa A beta (amyloid beta-protein) bearing carboxy terminal peptide, which was further degraded by an increased concentration of AChE. Protease inhibitory profiles confirmed the trypsin-like serine protease activity present in AChE preparation. This observed APP processing was significantly enhanced by Ca2+, Mg2+, or Mn2+ at 1 mM concentration and modulated in concentration dependent manners by metal ions such as Ca2+, Zn2+, Fe2+/Fe3+, Al3+, or a tacrine, a centrally active cholinesterase inhibitor. Our data imply that AChE and its associated protease may be involved in the generation a 18 kDa amyloidogenic peptide under certain physiological condition in vivo and that the gradual changes in their proteolytic activities or locations and the locally disturbed metal homeostasis could be factors associated with abnormal accumulation of APP, eventually leading to amyloid deposition in AD brain. In addition, zinc or tacrine treatment of AD patients with high dosage or in the long term may have effects on the process of amyloidogensis.