The β-Secretase BACE1 in Alzheimer's Disease

Abstract

BACE1 (beta-site amyloid precursor protein cleaving enzyme 1) was initially cloned and characterized in 1999. It is required for the generation of all monomeric forms of amyloid-β (Aβ), including Aβ₄₂, which aggregates into bioactive conformational species and likely initiates toxicity in Alzheimer's disease (AD). BACE1 concentrations and rates of activity are increased in AD brains and body fluids, thereby supporting the hypothesis that BACE1 plays a critical role in AD pathophysiology. Therefore, BACE1 is a prime drug target for slowing down Aβ production in early AD. Besides the amyloidogenic pathway, BACE1 has other substrates that may be important for synaptic plasticity and synaptic homeostasis. Indeed, germline and adult conditional BACE1 knockout mice display complex neurological phenotypes. Despite BACE1 inhibitor clinical trials conducted so far being discontinued for futility or safety reasons, BACE1 remains a well-validated therapeutic target for AD. A safe and efficacious compound with high substrate selectivity as well as a more accurate dose regimen, patient population, and disease stage may yet be found. Further research should focus on the role of Aβ and BACE1 in physiological processes and key pathophysiological mechanisms of AD. The functions of BACE1 and the homologue BACE2, as well as the biology of Aβ in neurons and glia, deserve further investigation. Cellular and molecular studies of BACE1 and BACE2 knockout mice coupled with biomarker-based human research will help elucidate the biological functions of these important enzymes and identify their substrates and downstream effects. Such studies will have critical implications for BACE1 inhibition as a therapeutic approach for AD.

Keywords: Alzheimer’s disease; BACE1 inhibitors; Biomarkers; Clinical trials; Soluble amyloid; Synaptic.

Figure 1.

Schematic representation of amyloid precursor protein (APP) processing pathways. Aβ, amyloid-β; Aη-α, amyloid-η-α; Aη-β, amyloid-η-β; BACE2, beta-site amyloid precursor protein cleaving enzyme 2; CTF, C-terminal fragment; LM, lipid membrane; p3, p3 fragment of the amyloid precursor protein; sAPP, soluble amyloid precursor protein. [Adapted with permission from Barão et al. (143).]

Figure 2.

BACE1 (beta-site amyloid precursor protein cleaving enzyme 1) and the amyloid-β (Aβ) cycle. Despite the fact that several clinical trials investigating anti-Aβ compounds did not reach primary end points, Aβ peptides, oligomers, protofibrils, and plaques still remain attractive targets. Of note, the nature of the toxic Aβ species remains unclear. Evidence suggests that, besides fibrils, dimeric or oligomeric Aβ species, but not monomeric Aβ peptides, cause neuronal hyperactivity and downstream toxicity in the vicinity of Aβ plaques.