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Novel Substrates for BACE1 Beg the Question: What's the Biological Function of β-Secretase?
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Stefan Lichtenthaler led this live discussion on 17 December 2003. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.
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 View Transcript of Live Discussion — Posted 23 August 2006
Background Text
By Stefan Lichtenthaler, Ludwig-Maximilian University, Munich, Germany.
Introduction
The amyloid hypothesis of Alzheimer's disease ascribes the pathogenesis of the disease to accumulation of the Aβ peptide, a proteolytic fragment of the amyloid precursor protein APP(1). Two protease activities referred to as β- and γ-secretase cleave APP at the N- and C-terminus of the Aβ peptide, respectively. Alternatively, APP can be cleaved by the protease activity of α-secretase, which cleaves within the Aβ domain and thus precludes Aβ-generation. The α-secretase is a member of the ADAM-family of proteases (A Disintegrin And Metalloprotease)(2-4), the β-secretase is the aspartyl protease BACE1(5-9), and γ-secretase is a protein complex consisting of presenilin, nicastrin, Aph-1 and Pen-2 (10). Since BACE1 cleaves APP at the N-terminus of the Aβ-peptide domain, it catalyzes the first step in Aβ-generation. Moreover, BACE1 has been shown to be strictly required for Aβ-generation. Mice with a targeted deletion of BACE1 do not produce any Aβ?(11-13), and are viable and fertile. Thus, inhibition of BACE1-activity is considered to be a highly promising approach to treat Alzheimer's disease(1,14,15). But what does this protease normally do? We should find out while academic and pharmaceutical labs are busy generating inhibitors.
Biological function of BACE1
The biological function of BACE1 is not understood. BACE1 knockout mice have not provided clues to its function, since they have been reported to be healthy and phenotypically normal(11-13). Indeed, at last month's Annual Meeting of the Society for Neuroscience in New Orleans, Alexander Harper, Colin Dingwall, and colleagues at GlaxoSmithKline reported that the BACE1/2 double-knockout is viable and apparently as normal as BACE1 single-knockout strains (see SfN abstract 842.12 at the SfN/ScholarOne website).
For two reasons, we assume that APP is not the major physiological substrate of BACE1. First, APP is poorly cleaved by BACE1 because its amino acid sequence around the BACE1 cleavage site does not fit with the substrate specificity of BACE1 determined in in-vitro experiments(16). Second, in the MDCK cell line—a system to study polarized sorting mechanisms of proteins—the majority of BACE1 is sorted to the apical surface, where very little APP is observed(17). Instead, APP undergoes sorting to the opposite, basolateral side. Thus, BACE1 may cleave additional proteins besides APP.
It is now clear that BACE1 is also involved in the proteolytic processing of two proteins with an important immunological function: the P-selectin glycoprotein ligand-1(18), which mediates leukocyte adhesion, and the sialyl-transferase ST6Gal I(19, 20), an enzyme that is secreted after cleavage and is involved in regulating immune responses. In this regard, it is important to note that a possible immunological phenotype in BACE1 knockout mice might have been overlooked, since these animals have not yet been challenged immunologically.
Given that BACE1 has been implicated in the proteolytic processing of three membrane proteins (APP, PSGL-1, and ST6Gal I), it is tempting to speculate that more BACE1 substrates are yet to be discovered. In this case, BACE1 could have a general role in the secretion of membrane proteins. Indeed, Doo Yeon Kim and colleagues in Dora Kovacs' lab at Massachusetts General Hospital presented unpublished data in New Orleans that the voltage-gated sodium channel 2-subunit (SCN2β) is sequentially cleaved, similarly to APP, by BACE1- and γ-secretase-like activities (see abstract 239.4).
Does BACE1 function in the ectodomain shedding of transmembrane proteins?
APP is one of a large number of membrane proteins that are proteolytically converted to their soluble counterparts. This process is referred to as ectodomain shedding and is an important way of regulating the biological activity of membrane proteins(21, 22). Ectodomain shedding has been described in many multicellular organisms, such as C. elegans, Drosophila melanogaster, mice, and humans, and is important in embryonic development, the inflammatory response, and other biological processes(22-24). The ectodomain shedding cleavage is typically carried out by members of the ADAM family—similar to the α-secretase cleavage of APP(21, 22). Since APP is mainly cleaved by a metalloprotease of the ADAM family and only to a smaller extent by BACE1(1), other BACE1 substrates may be found among the proteins undergoing ectodomain shedding. If, and to what extent, this could complicate therapeutic inhibition of BACE1 remains unclear at this point.
Let's discuss the following questions (and more) during the Live Chat:
- What is the biological function of the BACE1-cleavage of membrane proteins? Is it to generate a biologically active, secreted protein? Or is it merely a first step in the degradation of a membrane protein?
- Might BACE1 play a general role in the ectodomain shedding of membrane proteins, like the ADAM metalloproteases do?
- What proteins may be additional substrates of BACE1?
- Does any of this detract from BACE1's status as a drug target?
- In AD, what factors "divert" BACE1 from its physiological substrates and toward more APP cleavage?
References:
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14. Vassar R. The beta-secretase, BACE: a prime drug target for Alzheimer's disease. J Mol Neurosci. 2001 Oct;17(2):157-70. Abstract
15. Citron, M. (2002) Nat Neurosci 5 Suppl, 1055-7.
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17. Capell A, Meyn L, Fluhrer R, Teplow DB, Walter J, Haass C. Apical sorting of beta-secretase limits amyloid beta-peptide production. J Biol Chem. 2002 Feb 15;277(7):5637-43. Abstract
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20. Kitazume S, Tachida Y, Oka R, Shirotani K, Saido TC, Hashimoto Y. Alzheimer's beta-secretase, beta-site amyloid precursor protein-cleaving enzyme, is responsible for cleavage secretion of a Golgi-resident sialyltransferase. Proc Natl Acad Sci U S A. 2001 Nov 20;98(24):13554-9. Abstract
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