Increased release of an amyloidogenic C‐terminal Alzheimer amyloid precursor protein fragment from stressed PC‐12 cells

Abstract

Amyloid plaques, found in characteristically large numbers in specific brain areas of Alzheimer's disease (AD) and Down's Syndrome (DS) patients, are composed of a 41-43 amino acid peptide, A4, derived from a transmembrane glycoprotein, amyloid precursor protein (APP). In transformed cells APP has been shown to be cleaved within the extracellular portion of the A4 region causing the release of 100-120 kDa soluble N-terminal APP products. If this cleavage occurs in human tissue, neither the soluble product nor the remaining 10-12 kDa transmembrane fragment could be further degraded to yield A4. It has been hypothesized that an alternate APP cleavage product containing the intact A4 region is released in increased amounts in AD and DS brain where subsequent extracellular degradation produces the amyloidogenic A4 peptide. In support of this hypothesis, we have found that PC-12 cells maintained in serum-free media with or without additional injurious agents release a 60 kDa protein which has been detected by immunoprecipitation and immunoblot analyses with 9 antisera elicited by 4 distinct peptides within the carboxyl-terminal half of APP. Six of these antisera, elicited by peptides corresponding to the carboxyl-terminal 20 amino acids of APP, or the A4 peptide itself, do not bind the normally released 120 kDa APP product which is detected by 11 other antisera elicited by peptides with the N-terminal portion of APP. Controls in which two 60 kDa-detecting antisera were preabsorbed with the peptides used to elicit them, produced markedly reduced 60 kd bands on immunoblots.(ABSTRACT TRUNCATED AT 250 WORDS)

C-terminal fragments of the amyloid precursor protein in cerebrospinal fluid as potential biomarkers for Alzheimer disease

“…Anyhow, the presence in CSF of soluble forms of full-length membrane proteins containing the transmembrane and intracellular domains is not an unusual finding 16, 17 . Early studies failed to detect short C-terminal fragments of APP in the medium of cultured cells 18 ; but others suggested the release of largest APP-CTF fragments from cultured neural cells 19 . Moreover, a series of APP-CTFs was found to be secreted within exosomes in cultured cells 20 , as well AICD 21 .…”

Cholinergic agonists stimulate secretion of soluble full-length amyloid precursor protein in neuroendocrine cells.

“…However, in the transmembrane topology of APP, the peptide bond cleaved by ,y-secretase is located within the lipid bilayer and may not be easily accessible to proteases. It is therefore possible that A3 is derived from soluble nontransmembrane precursors where the peptide bond cleaved by y-secretase is not protected by the lipid bilayer (5,6 (6,(10)(11)(12). It has been suggested that the latter species are derived from solAPPcyt and may be further degraded to produce AP3 (6).…”

Morphological differentiation and proteoglycan synthesis regulate Alzheimer amyloid precursor protein processing in PC‐12 and human astrocyte cultures

“…Inhibition of the mid-A4 APP cleavage protease, secretase (Esch et al, 1990), may promote an alternative (Baskin et al, 1991;Golde et al, 1992) APP cleavage, yielding intact A4 peptides in AD and DS. Characterization of factors regulating secretase and of the cell types which express it will be important in understanding the pathological mechanisms causing AD and in choosing therapeutic agents which do not worsen this amyloid induced pathology.…”

“…The 15 kDa C-terminal APP seen with C-terminal antisera C7 or Athena 6 is not precipitated with an antisera elicited by A4 residues 1-17 (Palmed et al, 1989), is produced in stoichiometric amounts with the release of protease nexin I1 (Anderson et al, 1991;Baskin et al, 1991;Oltersdorfet al, 1989), and is not seen if 5 pg/ml leupeptin (serine-protease inhibitor) is included in the chase buffer.…”