Review
β-Secretase: its biology as a therapeutic target in diseases

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β-Secretase (BACE1, β-site APP cleaving enzyme 1) is an aspartic proteinase that has multiple functions in various physiological processes, such as cell differentiation, immunoregulation, and cell death. There is increasing evidence that changes in BACE1 activity are involved in many diseases, such as Alzheimer's disease (AD), schizophrenia, epileptic behavior, and others. However, a deeper understanding of the molecular biology of BACE1 is necessary for further exploration of cell development, immunological regulation, and disease pathogenesis. Here, we review the molecular and cellular biology of BACE1, including its enzymatic properties, structure, biosynthesis, and physiological functions to provide a new perspective and rational assessment of drugability. Lastly, we discuss proposed strategies to control BACE1 activity for possible therapeutic application.

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).

References (122)

  • A. Capell

    Maturation and pro-peptide cleavage of beta-secretase

    J. Biol. Chem.

    (2000)
  • S. Jin

    Evidence for dimeric BACE-mediated APP processing

    Biochem. Biophys. Res. Commun.

    (2010)
  • G.G. Westmeyer

    Dimerization of beta-site beta-amyloid precursor protein-cleaving enzyme

    J. Biol. Chem.

    (2004)
  • R. Yan

    The transmembrane domain of the Alzheimer's beta-secretase (BACE1) determines its late Golgi localization and access to beta-amyloid precursor protein (APP) substrate

    J. Biol. Chem.

    (2001)
  • K.K. Freude

    Soluble amyloid precursor protein induces rapid neural differentiation of human embryonic stem cells

    J. Biol. Chem.

    (2011)
  • N.D. Belyaev

    The transcriptionally active amyloid precursor protein (APP) intracellular domain is preferentially produced from the 695 isoform of APP in a β-secretase-dependent pathway

    J. Biol. Chem.

    (2010)
  • X. Luo

    Cleavage of neuregulin-1 by BACE1 or ADAM10 protein produces differential effects on myelination

    J. Biol. Chem.

    (2011)
  • D.Y. Kim

    Reduced sodium channel Na(v)1.1 levels in BACE1-null mice

    J. Biol. Chem.

    (2011)
  • T. Huth

    Non-proteolytic effect of β-site APP-cleaving enzyme 1 (BACE1) on sodium channel function

    Neurobiol. Dis.

    (2009)
  • S.F. Lichtenthaler

    The cell adhesion protein P-selectin glycoprotein ligand-1 is a substrate for the aspartyl protease BACE1

    J. Biol. Chem.

    (2003)
  • M.B. Jones

    Role for hepatic and circulatory ST6Gal-1 sialyltransferase in regulating myelopoiesis

    J. Biol. Chem.

    (2010)
  • A.V. Woodard-Grice

    Proteolytic shedding of ST6Gal-I by BACE1 regulates the glycosylation and function of α4β1 integrins

    J. Biol. Chem.

    (2008)
  • C.A. von Arnim

    The low density lipoprotein receptor-related protein (LRP) is a novel β-secretase (BACE1) substrate

    J. Biol. Chem.

    (2005)
  • T. Kihara

    Aβ-induced BACE-1 cleaves N-terminal sequence of mPGES-2

    Biochem. Biophys. Res. Commun.

    (2010)
  • P.H. Kuhn

    Regulated intramembrane proteolysis of the interleukin-1 receptor II by alpha-, beta-, and gamma-secretase

    J. Biol. Chem.

    (2007)
  • L. Zhou

    The neural cell adhesion molecules L1 and CHL1 are cleaved by BACE1 in vivo

    J. Neurochem.

    (2012)
  • G. Tesco

    Depletion of GGA3 stabilizes BACE and enhances beta-secretase activity

    Neuron

    (2007)
  • S. Kametaka

    Identification of phospholipid scramblase 1 as a novel interacting molecule with beta-secretase (beta-site amyloid precursor protein (APP) cleaving enzyme (BACE))

    J. Biol. Chem.

    (2003)
  • G.M. Finan

    BACE1 retrograde trafficking is uniquely regulated by the cytoplasmic domain of sortilin

    J. Biol. Chem.

    (2011)
  • J. Xie et al.

    PAR-4 is involved in regulation of beta-secretase cleavage of the Alzheimer amyloid precursor protein

    J. Biol. Chem.

    (2005)
  • C. Supnet et al.

    Presenilins function in ER calcium leak and Alzheimer's disease pathogenesis

    Cell Calcium

    (2011)
  • X.P. Huang

    Internalization of exogenously added memapsin 2 (beta-secretase) ectodomain by cells is mediated by amyloid precursor protein

    J. Biol. Chem.

    (2004)
  • H.H. Griffiths

    Prion protein interacts with BACE1 protein and differentially regulates its activity toward wild type and Swedish mutant amyloid precursor protein

    J. Biol. Chem.

    (2011)
  • L.R. Fischer

    Absence of SOD1 leads to oxidative stress in peripheral nerve and causes a progressive distal motor axonopathy

    Exp. Neurol.

    (2012)
  • J. Charlwood

    Characterization of the glycosylation profiles of Alzheimer's beta-secretase protein Asp-2 expressed in a variety of cell lines

    J. Biol. Chem.

    (2001)
  • S.M. Harrison

    BACE1 (beta-secretase) transgenic and knockout mice: identification of neurochemical deficits and behavioral changes

    Mol. Cell. Neurosci.

    (2003)
  • K.R. Mowrer et al.

    Promotion of BACE1 mRNA alternative splicing reduces amyloid beta-peptide production

    J. Biol. Chem.

    (2008)
  • T. O’Connor

    Phosphorylation of the translation initiation factor eIF2α increases BACE1 levels and promotes amyloidogenesis

    Neuron

    (2008)
  • S.S. Hebert

    MicroRNA regulation of Alzheimer's amyloid precursor protein expression

    Neurobiol. Dis.

    (2009)
  • X. Zhang

    Hypoxia-inducible factor 1alpha (HIF-1alpha)-mediated hypoxia increases BACE1 expression and beta-amyloid generation

    J. Biol. Chem.

    (2007)
  • K.R. Sadleir et al.

    Cdk5 protein inhibition and Aβ42 increase BACE1 protein level in primary neurons by a post-transcriptional mechanism: implications of CDK5 as a therapeutic target for Alzheimer disease

    J. Biol. Chem.

    (2012)
  • D.G. Jo

    Evidence that gamma-secretase mediates oxidative stress-induced beta-secretase expression in Alzheimer's disease

    Neurobiol. Aging

    (2010)
  • L. Kalvodova

    Lipids as modulators of proteolytic activity of BACE: involvement of cholesterol, glycosphingolipids, and anionic phospholipids in vitro

    J. Biol. Chem.

    (2005)
  • S.L. Cole

    Statins cause intracellular accumulation of amyloid precursor protein, beta-secretase-cleaved fragments, and amyloid beta-peptide via an isoprenoid-dependent mechanism

    J. Biol. Chem.

    (2005)
  • S. Sinha

    Purification and cloning of amyloid precursor protein beta-secretase from human brain

    Nature

    (1999)
  • R. Vassar

    Beta-secretase cleavage of Alzheimer's amyloid precursor protein by the transmembrane aspartic protease BACE

    Science

    (1999)
  • R. Yan

    Membrane-anchored aspartyl protease with Alzheimer's disease beta-secretase activity

    Nature

    (1999)
  • X. Lin

    Human aspartic protease memapsin 2 cleaves the beta-secretase site of beta-amyloid precursor protein

    Proc. Natl. Acad. Sci. U.S.A.

    (2000)
  • S. Casas

    BACE2 plays a role in the insulin receptor trafficking in pancreatic β-cells

    Am. J. Physiol. Endocrinol. Metab.

    (2010)
  • T. Shiba

    Insights into the phosphoregulation of β-secretase sorting signal by the VHS domain of GGA1

    Traffic

    (2004)
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