Novel mutations introduced at the beta-site of amyloid beta protein precursor enhance the production of amyloid beta peptide by BACE1 in vitro and in cells.

Abnormal production and accumulation of amyloid-beta peptide (Abeta) plays a major role in the pathogenesis of Alzheimer's disease (AD). beta-secretase (BACE1) is responsible for the cleavage at thebeta-site in amyloid beta protein precursor (AbetaPP/APP) to generate the N-terminus of Abeta. Here we report the stepwise identification and characterization of a novel APP-beta-site mutant, "NFEV" (APP_NFEV) in vitro and in cells. In vitro, the APP_NFEV exhibits 100-fold enhanced cleavage rate relative to the "wild-type" substrate (APPwt) and 10-fold increase relative to the Swedish-type mutation variant (APPsw). In cells, it was preferably cleaved among 24 APP beta-site mutations tested. More importantly, the APP_NFEV mutant failed to generate any detectable Abeta peptides in BACE1-KO mouse fibroblast cells. The production of Abeta peptides was restored by co-transfecting human BACE1, demonstrating that BACE1 is the only enzyme responsible for the processing of APP_NFEV in these cells. Analysis of APP_NFEV cleavage products secreted in the media revealed that in cells BACE1 cleaves APP_NFEV at the position between NF and EV, identical to that observed in vitro. A BACE inhibitor blocked the processing of the APP_NFEV beta-site in vitro and in cells. Our data indicates that the "NFEV" mutant is not only an enhanced substrate for BACE1 in vitro, but also a specific substrate for BACE1 in cells.

Beta-secretase cleavage at amino acid residue 34 in the amyloid beta peptide is dependent upon gamma-secretase activity.

The amyloid beta peptides (Abeta) are the major components of the senile plaques characteristic of Alzheimer's disease. Abeta peptides are generated from the cleavage of amyloid precursor protein (APP) by beta- and gamma-secretases. Beta-secretase (BACE), a type-I transmembrane aspartyl protease, cleaves APP first to generate a 99-amino acid membrane-associated fragment (CT99) containing the N terminus of Abeta peptides. Gamma-secretase, a multi-protein complex, then cleaves within the transmembrane region of CT99 to generate the C termini of Abeta peptides. The production of Abeta peptides is, therefore, dependent on the activities of both BACE and gamma-secretase. The cleavage of APP by BACE is believed to be a prerequisite for gamma-secretase-mediated processing. In the present study, we provide evidence both in vitro and in cells that BACE-mediated cleavage between amino acid residues 34 and 35 (Abeta-34 site) in the Abeta region is dependent on gamma-secretase activity. In vitro, the Abeta-34 site is processed specifically by BACE1 and BACE2, but not by cathepsin D, a closely related aspartyl protease. Moreover, the cleavage of the Abeta-34 site by BACE1 or BACE2 occurred only when Abeta 1- 40 peptide, a gamma-secretase cleavage product, was used as substrate, not the non-cleaved CT99. In cells, overexpression of BACE1 or BACE2 dramatically increased the production of the Abeta 1-34 species. More importantly, the cellular production of Abeta 1-34 species induced by overexpression of BACE1 or BACE2 was blocked by a number of known gamma-secretase inhibitors in a concentration-dependent manner. These gamma-secretase inhibitors had no effect on enzymatic activity of BACE1 or BACE2 in vitro. Our data thus suggest that gamma-secretase cleavage of CT99 is a prerequisite for BACE-mediated processing at Abeta-34 site. Therefore, BACE and gamma-secretase activity can be mutually dependent.

Purification and molecular characterization of cGMP-dependent protein kinase from Apicomplexan parasites. A novel chemotherapeutic target.

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (Compound 1) inhibits the growth of Eimeria spp. both in vitro and in vivo. The molecular target of Compound 1 was identified as cGMP-dependent protein kinase (PKG) using a tritiated analogue to purify a approximately 120-kDa protein from lysates of Eimeria tenella. This represents the first example of a protozoal PKG. Cloning of PKG from several Apicomplexan parasites has identified a parasite signature sequence of nearly 300 amino acids that is not found in mammalian or Drosophila PKG and which contains an additional, third cGMP-binding site. Nucleotide cofactor regulation of parasite PKG is remarkably different from mammalian enzymes. The activity of both native and recombinant E. tenella PKG is stimulated 1000-fold by cGMP, with significant cooperativity. Two isoforms of the parasite enzyme are expressed from a single copy gene. NH(2)-terminal sequence of the soluble isoform of PKG is consistent with alternative translation initiation within the open reading frame of the enzyme. A larger, membrane-associated isoform corresponds to the deduced full-length protein sequence. Compound 1 is a potent inhibitor of both soluble and membrane-associated isoforms of native PKG, as well as recombinant enzyme, with an IC(50) of <1 nm.