Immunotherapy, through either passive or active immunization, remains a viable and aggressively pursued strategy for ridding the brain of amyloid β, the major component found in the amyloid plaques that are the hallmark of Alzheimer disease (see ARF related news story). But in this week’s online edition of PNAS, researchers present a slight variation on the theme—antibodies not against Aβ, but against the β-secretase (BACE) site in the Aβ precursor protein (AβPP). Because these antibodies could prevent BACE from proteolytically cleaving AβPP, a prerequisite for Aβ production, they have the potential to prevent the Aβ peptide from forming in the first place.
Beka Solomon and colleagues at Tel Aviv University, Israel, used what they call multiple antigenic peptide (MAP)—an antigen comprising eight copies of the BACE cleavage site—to generate BACE site antibodies in mice. First author Michal Arbel and colleagues then made monoclonal antibodies by fusing hybridoma cells with splenocytes from mice that had the highest MAP antibody titers. Then, selecting the BACE site monoclonal antibody with the highest affinity for MAP, they tested its ability to interfere with Aβ production.
Arbel and colleagues found that when the monoclonal antibody, which they called BSS1, was incubated with Chinese hamster ovary cells overexpressing AβPP, secretion of Aβ into the culture medium was reduced by up to 22 percent. In contrast, antibodies to the N-terminal of AβPP had no effect on Aβ levels. The authors also found that BSS1 had an even greater impact on intracellular Aβ, reducing it by about 50 percent, with a concomitant reduction in the amount of membrane-bound AβPP C-terminal fragments (C99), indicating that BACE cleavage had indeed been affected. The authors also found that the antibody was rapidly internalized by the cells, appearing in early endosomes within about 60 minutes of incubation. The antibody had no effect on α-secretase-mediated cleavage, an indication of its specificity for the BACE site on the protein.
When the authors immunized AβPP transgenic mice with MAP, they found that the animals did better than control littermates in the Morris water maze, a standard test of learning and memory function, though the numbers of animals used in these experiments were small. Also, in these animals the amyloid burden in the brain dropped by over half.
BACE itself has come in for intense scrutiny as a possible therapeutic target, but as Solomon and colleagues point out, BACE activity might be required for cleavage of other proteins, or other putative proteases might be capable of carrying out cleavage of AβPP at the BACE site. The use of anti-BACE site antibodies would circumvent both these potential pitfalls. The fact that the antibodies can be internalized is also good news, given the increasing evidence that intracellular cleavage and toxicity of Aβ may play a role in AD (see ARF live discussion and ARF related news story). In fact, because upregulation of endocytosis might lead to enhanced cleavage of AβPP and increased plaque formation in non-familial cases of AD (see, for example, Nixon et al., 2003), the authors suggest that their immunologic approach may be “a therapeutic strategy in AD treatment that is worthy of further investigation.”—Tom Fagan