Alzheimer's Peptide Aβ1–42 Binds to Two β-Sheets of α1-Antichymotrypsin and Transforms It from Inhibitor to Substrate

Abstract

The serpin ␣ 1 -antichymotrypsin is a major component of brain amyloid plaques in Alzheimer's disease. In vitro ␣ 1 -antichymotrypsin interacts with the Alzheimer's amyloid peptide A␤ 1-42 and stimulates both formation and disruption of neurotoxic A␤ 1-42 fibrils in a concentration-dependent manner. We have constructed a new hybrid model of the complex between A␤ 1-42 and ␣ 1 -antichymotrypsin in which both amino and carboxyl sequences of A␤ 1-42 insert into two different ␤-sheets of ␣ 1 -antichymotrypsin. We have tested this model and shown experimentally that full-length and amino-terminal segments of A␤ 1-42 bind to ␣ 1 -antichymotrypsin as predicted. We also show that A␤ 1-42 forms both intraand intermolecular SDS-stable complexes with ␣ 1 -antichymotrypsin and that the binding of A␤ 1-42 to ␣ 1 -antichymotrypsin abolishes the inhibitory activity of the latter and its ability to form stable complex with chymotrypsin. The existence of both inter-as well as intramolecular complexes of A␤ 1-42 explains the nonlinear concentration-dependent effects of ␣ 1 -antichymotrypsin on A␤ 1-42 fibril formation, which we have reinvestigated here over a broad range of A␤ 1-42 :␣ 1 -antichymotrypsin ratios. These data suggest a molecular basis for the distinction between amorphous and fibrillar A␤ 1-42 in vivo. The reciprocal effects of A␤ 1-42 and ␣ 1 -antichymotrypsin could play a role in the etiology of Alzheimer's disease.1 is a serpin serine proteinase inhibitor with specificity for cathepsin and chymotrypsin-like enzymes (1). Its physical properties (2), mechanism (3), and structure (4, 5) conform to the general model (6, 7) that has been established for serpins. These inhibitors present a flexible reactive site loop, which is cleaved at a susceptible residue by target proteinase (8, 9) to form a highly stable, covalent complex (10, 11). As in other serpins (12), the reactive site loop in the cleaved form of ACT is inserted into a preexisting, flexible ␤-sheet (sA) (4), and this cleaved form is stabilized to denaturation relative to the uncleaved native form (2). There is evidence (13, 14) consistent with proposals (10, 11) that insertion of the reactive site loop into sA is a prerequisite for stable proteinase-serpin complex formation. In further support of this, it has been shown that exogenous peptides of restricted sequence can insert into sA of the uncleaved serpin (15-17). These binary peptide⅐serpin complexes are similarly increased in stability to denaturation but have lost inhibitory activity, becoming substrates instead that do not form a stable complex with target proteinase.A possible link between ACT and Alzheimer's disease was established by the observation that ACT occurs in the senile plaques characteristic of this disease (18). Subsequently, it was shown in vitro that ACT can either stimulate formation of the neurotoxic fibrillar form of Alzheimer's peptide A␤ 1-42 (19, 20) or destabilize preformed A␤ 1-40 fibrils (21, 22), depending on the stoichiometry of ACT to A␤. Two models for the binding of A␤...

Metalloendopeptidase EC 3.4.24.15 Is Necessary for Alzheimer's Amyloid-β Peptide Degradation

“…We have shown that ACT binds A␤ to form an SDS-stable complex rendering ACT a weaker inhibitor of chymotrypsin (48). Recently the binding of A␤ to ACT was described in structural detail (49). Binding of ACT to A␤ could render A␤ a poorer substrate for degradation.…”

α-1-Antichymotrypsin Promotes β-Sheet Amyloid Plaque Deposition in a Transgenic Mouse Model of Alzheimer's Disease

“…The human ACT gene shares the highest sequence homology with the family of murine Spi-2 protease inhibitors (Inglis and Hill, 1991). However, the reactive center loop of these protease inhibitors, which is the likely A␤ peptide interacting region in the human ACT protein (Potter et al, 1992;Janciauskiene et al, 1996Janciauskiene et al, , 1998, is hypervariable and differs substantially from that of the human ACT protein (Hill and Hastie, 1987). Here we report on a new transgenic mouse and on experiments pursued to determine the function of ACT as an amyloid promoter in vivo.…”

“…In vitro studies have demonstrated that ACT and ApoE promote the assembly of the A␤ 1-42 peptide into amyloid filaments (Ma et al, 1994(Ma et al, , 1996Sanan et al, 1994;Wisniewski et al, 1994;Janciauskiene et al, 1996Janciauskiene et al, , 1998. However, using the A␤ 1-40 peptide or an equimolar ratio, ApoE and ACT have also been reported to inhibit A␤ filament formation (Fraser et al, 1993;Eriksson et al, 1995;Evans et al, 1995).…”

Plasma and CSF serpins in Alzheimer disease and dementia with Lewy bodies

“…strated by the finding of raised levels in brain homogenates from affected individuals 6 ; the finding that it is tightly associated with virtually all ␤-amyloid plaques 7 ; and the demonstration that it interacts with, 8,9 and affects the clearance of, A␤ . [10][11][12][13][14] Moreover elevated levels of plasma and CSF ␣ 1 -antichymotrypsin correlate with cognitive decline in elderly nondemented persons and those with AD.…”