BACE2, a β-secretase homolog, cleaves at the β site and within the amyloid-β region of the amyloid-β precursor protein

Abstract

Production of amyloid-␤ protein (A␤) is initiated by a ␤-secretase that cleaves the A␤ precursor protein (APP) at the N terminus of A␤ (the ␤ site). A recently identified aspartyl protease, BACE, cleaves the ␤ site and at residue 11 within the A␤ region of APP. Here we show that BACE2, a BACE homolog, cleaves at the ␤ site and more efficiently at a different site within A␤. The Flemish missense mutation of APP, implicated in a form of familial Alzheimer's disease, is adjacent to this latter site and markedly increases A␤ production by BACE2 but not by BACE. BACE and BACE2 respond identically to conservative ␤-site mutations, and alteration of a common active site Arg inhibits ␤-site cleavage but not cleavage within A␤ by both enzymes. These data suggest that BACE2 contributes to A␤ production in individuals bearing the Flemish mutation, and that selective inhibition of these highly similar proteases may be feasible and therapeutically advantageous.T he amyloid-␤ protein (A␤) is the principal component of the senile plaques characteristic of Alzheimer's disease (AD), and multiple lines of evidence have implicated cerebral accumulation of A␤ in AD pathogenesis (1, 2). A␤ is produced from the A␤ precursor protein (APP) by two proteolytic events. A ␤-secretase activity cleaves APP at the N terminus of A␤ (␤ site) between amino acids Met-671 and Asp-672 (using the numbering of the 770-aa isoform of APP). Cleavage at the ␤ site yields a membrane-associated APP fragment of 99 aa (C99). A second site within the transmembrane domain of C99 (␥ site) can then be cleaved by a ␥-secretase to release A␤, a peptide of 39-42 aa. APP can alternatively be cleaved within its A␤ region, predominately at the ␣-secretase cleavage site of APP, to produce a C-terminal APP fragment of 83 aa (C83), which can also be further cleaved by ␥-secretase to produce a small secreted peptide, p3.A number of missense mutations in APP have been implicated in forms of early-onset familial AD. All of these are at or near one of the canonical cleavage sites of APP. Thus, the Swedish double mutation (K670N͞M671L) is immediately adjacent to the ␤-cleavage site and increases the efficiency of ␤-secretase activity, resulting in more total A␤ (3). Any of three mutations at APP residue 717, near the ␥ site, increases the proportion of a more amyloidogenic 42-aa form of A␤ [A␤(1-42)] relative to the more common 40-residue form [A(␤1-40)] (4-7).Two additional mutations of APP have been described which are close but not adjacent to the ␣ site. A mutation (A692G, A␤ residue 21) in a Flemish family and a mutation (E693Q, A␤ residue 22) in a Dutch family each have been implicated in distinct forms of familial AD (8-10). The Flemish mutation, in particular, presents as a syndrome of repetitive intracerebral hemorrhages or as an AD-type dementia. The neuropathological findings include senile plaques in the cortex and hippocampus, and usually multiple amyloid deposits in the walls of cerebral microvessels (8,11,12).Recently, a membrane-associated aspartyl protease, B...

β Subunits of Voltage-gated Sodium Channels Are Novel Substrates of β-Site Amyloid Precursor Protein-cleaving Enzyme (BACE1) and γ-Secretase

“…Indeed, as an additional group of BACE1 substrates, the processing site of each VGSC␤ subunit is also comparable with that of rat ␤-galactoside ␣2,6-sialyltransferase (ST6GalI) and P-selectin glycoprotein ligand-1, the two known BACE1 substrates that are also cleaved by BACE1 after leucine residue (8,9,25,26). Furthermore, it is also worth noting that other protease(s), such as BACE2 (27,28), may also process VGSC␤ at the same (or very close) site, because the lack of BACE1 in MEFs and mouse brain only led to the reduction of CTF␤ production (Figs. 2C and 6, A and B).…”

BACE1 Suppression by RNA Interference in Primary Cortical Neurons

“…Using siRNA designed according to the conserved nucleotide sequence between human, rat, and mouse BACE1, we successfully reduced the expression of recombinant human BACE1 and endogenous human and mouse BACE1 without changing the protein levels of recombinant or endogenous BACE2. Both BACE1 and BACE2 make a second cleavage within the A␤ sequence; however, the BACE1 cleavage product remains amyloidogenic, whereas the BACE2 product does not (32,33). Therefore, inhibiting BACE2 is not desirable, and RNA interference provides the specificity to selectively suppress only one of the BACE isoforms.…”

Trafficking and Proteolytic Processing of APP

“…Besides BACE1, a homologous protease called BACE2 was identified (Vassar 2004). However, BACE2 is not involved in amyloidogenesis and may rather exert an antiamyloidogenic activity in non-neuronal cells somewhat similar to a-secretase (Bennett et al 2000a;Farzan et al 2000;Fluhrer et al 2002;Basi et al 2003). BACE1 is the sole b-secretase, because its knockout completely blocks Ab generation (Cai et al 2001;Roberds et al 2001;Luo et al 2003).…”