Trends in Pharmacological Sciences
Reviewβ-Secretase: its biology as a therapeutic target in diseases
Introduction
In 1999, BACE1 was simultaneously discovered by five groups to be an aspartic proteinase 1, 2, 3, 4, 5. The various research teams used very different experimental approaches to identify exactly the same protein. Since then, many clinicians, basic scientists, and industrial pharmacologists have made significant increases in the understanding of BACE1, and this progress has elucidated the role of BACE1 in multiple diseases, such as AD, schizophrenia, and epileptic behavior. However, most studies on BACE1 have focused on disease-related mechanisms or its pathological effects rather than its normal physiological properties. Thus, a deeper understanding of the basic pharmacological, biochemical, and molecular biological properties of BACE1 is necessary not only for elucidation of disease pathogenesis but also for drug discovery. Therefore, in this review, we provide an update on several important recent discoveries, including:
- (i)
the primary structure of BACE1 and its basic post-translational modifications, such as glycosylation, phosphorylation, palmitoylation, ubiquitination, and acetylation;
- (ii)
the enzymatic activity, substrates, and physiological functions of BACE1;
- (iii)
BACE1-binding partners; and
- (iv)
recombinant BACE1 in different expression systems, from Escherichia coli to mammals.
We will also discuss additional biological and physiological features of BACE1, particularly its biosynthesis, subcellular location, and degradation.
Understanding the biology of BACE1 will be crucial for development of safe and effective inhibitors or modulators for neurological disorder therapies.
There are two homologs of BACE1 [β-site amyloid precursor protein (APP) cleaving enzyme or β-secretase]. BACE1 and BACE2 comprise a new subfamily of membrane-anchored aspartyl proteases [6]. BACE1 [3.4.23.46] (Asp-2, membrane-bound aspartic proteinase or Memapsin 2) exists in neurons and cleaves APP at the Asp+1 site. BACE2 [3.4.23.45] (Asp1, Memapsin 1, or DRAP) shares ∼60% homology with BACE1 but exists in peripheral tissues and cleaves APP at the Phe+19 or Phe+20 site. Both homologs of BACE (BACE1 and BACE2) share similar structure. They share 51% similarity in amino acid sequences, and also both proteins have a catalytic domain formed via DTG and DSG active site motifs, a single transmembrane domain, and a short C-terminal tail. BACE1 has been identified as the Alzheimer's β-secretase, whereas BACE2 was mapped to the region of human chromosome 21 that is involved in Down syndrome [6], and is also related to diabetes 7, 8. The expression of BACE2 in the brain is significantly lower compared with BACE1 6, 9. Thus, it can be distinguished from BACE1 by pro-segment autoprocessing, APP processing, and subcellular localization, as well as the distinct expression patterns in the brain [9].
Section snippets
Primary sequence
BACE1, a ∼75-kD protein that has ∼30% sequence homology with its pepsin family members, has two conserved active sites [DTGS (93–96) and DSGT (289–292)] and three pairs of disulfide bonds (216/420, 278/443, 330/380) (Figure 1a). Mutations in these disulfide bonds result in incomplete glycosylation and pro-peptide processing. Among them, the disulfide bond (330/380) in the C-terminal lobe is of significant importance in maintaining catalytic activity and proper structure, and the other two seem
The enzymatic reactions of BACE1
Pro-BACE1, monomeric BACE1, dimeric BACE1, and high molecular complex BACE1 are four species that exist naturally. Because monomeric BACE1 undergoes autoproteolysis in vitro, its activity fluctuates. In the endogenous environment, dimeric BACE1 and high molecular complex BACE1 have much higher kinetic activity, and the pro-domain itself has little effect on kinetic parameters in a pro-BACE1 activity assay [21]. For recombinant BACE1, BACE1-FL (full-length BACE1) is more active than BACE1-NT
Substrates of BACE1
BACE1 accommodates 12 subsite residues (P4′–P8) in its substrate-binding cleft. The substrate residue preference is shown in Table 1. P1 is the most stringent site, and in P subsites, the inner ones are more stringent than the outer residues. Physiologically, BACE1 also cleaves substrates for various functions (Table 2).
Binding proteins
To function properly, BACE1 must bind with other proteins (Table 3). These binding partners are briefly reviewed below.
Recombinant BACE1 in different expression systems
In E. coli, recombinant pro-BACE1 cannot be cleaved by furins. In insect cells, furin-like enzymes typically cannot cleave recombinant BACE1 at putative furin processing sites, and recombinant BACE1 is always a mixture of pro- and mature enzymes [17].
Glycosylation is important for BACE1 to maintain its proteolytic activity and to allow it to be properly identified by furins because glycosylation has direct effects on substrate binding and protein interactions. Thus, in E. coli, lack of
Biosynthesis, subcellular location, and degradation
The BACE1 gene is 30.6 kb in length and shows no AD-related mutations. Alternative splicing in exons 3 and 4 generates four variants with 501, 476, 457, and 432 amino acids, respectively, and the shorter variants have no cleavage activity on APP [82].
In the ER, BACE1 exists as an immature, N-glycosylated pro-protein (∼60 kDa). Subsequently, pro-BACE1 is transported to Golgi for further modification. Then, the pro-peptide is removed and the mature enzyme (>16 h half-life) is trafficked to
Concluding remarks
In conclusion, BACE1 activity is closely related to AD and other CNS disease pathogenesis. However, the use BACE1 as a potential early biomarker for early diagnosis or drug treatment monitoring requires complete understanding of its function and activities. Although a great deal of information about structure and functions of BACE1 is available, many questions remain to be answered, such as: what are the basic physiological functions of BACE1 in the brain versus the peripheral system? Is it
Acknowledgments
This paper is supported by grants from the National Institute on Aging (NIHR01AG032441 and RO1AG025888), the Alzheimer's Association (Zenith Award, IIRG-07-59510 and IIRG-09-61521), and the American Health Assistance Foundation (G2006-118).
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