Recombinant insect cell expression and purification of human beta-secretase (BACE-1) for X-ray crystallography

AAV-mediated delivery of an anti-BACE1 VHH alleviates pathology in an Alzheimer's disease model

Single domain antibodies (VHHs) are potentially disruptive therapeutics, with important biological value for treatment of several diseases, including neurological disorders. However, VHHs have not been widely used in the central nervous system (CNS), largely because of their restricted blood-brain barrier (BBB) penetration. Here, we propose a gene transfer strategy based on BBB-crossing Adeno-associated virus (AAV)-based vectors to deliver VHH directly into the CNS. As a proof-of-concept, we explored the potential of AAV-delivered VHH to inhibit BACE1, a well-characterized target in Alzheimer's disease. First, we generated a panel of VHHs targeting BACE1, one of which, VHH-B9, shows high selectivity for BACE1 and efficacy in lowering BACE1 activity in vitro. We further demonstrate that a single systemic dose of AAV-VHH-B9 produces positive long-term (12 months plus) effects on amyloid load, neuroinflammation, synaptic function, and cognitive performance, in the App^NL-G-F Alzheimer's disease mouse model. These results constitute a novel therapeutic approach forneurodegenerative diseases, which is applicable to a range of CNS disease targets.

Advancements in molecular design and bioprocessing of recombinant adeno-associated virus gene delivery vectors using the insect-cell baculovirus expression platform

Despite rapid progress in the field, scalable high-yield production of adeno-associated virus (AAV) is still one of the critical bottlenecks the manufacturing sector is facing. The insect cell-baculovirus expression vector system (IC-BEVS) has emerged as a mainstream platform for the scalable production of recombinant proteins with clinically approved products for human use. In this review, we provide a detailed overview of the advancements in IC-BEVS for rAAV production. Since the first report of baculovirus-induced production of rAAV vector in insect cells in 2002, this platform has undergone significant improvements, including enhanced stability of Bac-vector expression and a reduced number of baculovirus-coinfections. The latter streamlining strategy led to the eventual development of the Two-Bac, One-Bac, and Mono-Bac systems. The one baculovirus system consisting of an inducible packaging insect cell line was further improved to enhance the AAV vector quality and potency. In parallel, the implementation of advanced manufacturing approaches and control of critical processing parameters have demonstrated promising results with process validation in large-scale bioreactor runs. Moreover, optimization of the molecular design of vectors to enable higher cell-specific yields of functional AAV particles combined with bioprocess intensification strategies may also contribute to addressing current and future manufacturing challenges.

Large scale expression and purification of secreted mouse hephaestin

Hephaestin is a large membrane-anchored multicopper ferroxidase involved in mammalian iron metabolism. Newly absorbed dietary iron is exported across the enterocyte basolateral membrane by the ferrous iron transporter ferroportin, but hephaestin increases the efficiency of this process by oxidizing the transported iron to its ferric form and promoting its release from ferroportin. Deletion or mutation of the hephaestin gene leads to systemic anemia with iron accumulation in the intestinal epithelium. The crystal structure of human ceruloplasmin, another multicopper ferroxidase with 50% sequence identity to hephaestin, has provided a framework for comparative analysis and modelling. However, detailed structural information for hephaestin is still absent, leaving questions relating to metal coordination and binding sites unanswered. To obtain structural information for hephaestin, a reliable protocol for large-scale purification is required. Here, we present an expression and purification protocol of soluble mouse hephaestin, yielding milligram amounts of enzymatically active, purified protein using the baculovirus/insect cell system.

Tnao38, high five and Sf9--evaluation of host-virus interactions in three different insect cell lines: baculovirus production and recombinant protein expression

PURPOSE OF WORK

A comparative analysis of new and established insect cell lines, in regard to process relevant parameters, provide data that can be exploited for designing more robust and effective protein production processes. The baculovirus-insect cell expression system has been efficiently used for the production of heterologous proteins. Three different insect cell lines Tnao38, High Five and Sf9 were compared in terms of virus susceptibility, baculovirus production and product yield of an intra-cellularly (YFP) and extra-cellularly (influenza A virus hemagglutinin)-expressed recombinant protein. The Tnao38 and High Five cell lines exhibited higher (tenfold) susceptibility to baculovirus infection than Sf9 cells, whereas Sf9 cells showed a higher (100-fold) capacity for production of infectious virus particles. Analysis of recombinant protein expression revealed considerably higher product yields in Tnao38 and High Five cells as compared to Sf9 cells, for both model proteins. Overall, the two Trichoplusia ni-derived cell lines, High Five and Tnao38, were significantly more efficient in terms of secreting proteins such as the glycoprotein hemagglutinin of influenza A virus.

Expression analysis of human β-secretase in transgenic tomato fruits

An emerging strategy in biomanufacturing involves using transgenic plants to express recombinant pharmaceutical and industrial proteins in large quantities. β-Site APP cleaving enzyme 1 (β-secretase 1, BACE1) is an enzyme involved in the abnormal production of Aβ42, the major component of senile plaques in Alzheimer's disease (AD). Thus, BACE1 represents a key target protein in the development of new potential drugs to treat Alzheimer's disease. We aimed to develop a tomato-derived recombinant BACE1 (rBACE1) protein to serve as a vaccine antigen that would promote an immune response. We utilized a plant expression cassette, pE8BACE, to optimize BACE1 expression in tomato fruits. Polyemerase chain reaction and Southern blot analyses verified integration of the BACE1 gene into the plant genome. Northern and Western blot analyses demonstrated successful mRNA and protein expression of rBACE1, respectively; the Sensizyme assay kit estimated the expression level of rBACE1 protein at 136 ± 7 ng mg⁻¹ total soluble protein. The tomato-derived rBACE1 retains its activity for a long storage period at cool or room temperature, and is highly resistant to degradation in conditions such as low acidity. Tomato-derived rBACE1 was severely degraded by heat or boiling. The proteolytic activity of tomato-derived rBACE1, confirmed by fluorescence resonance transfer assay, was similar to that of a commercial sample of Escherichia coli-derived BACE1.

High-level expression of a human β-site APP cleaving enzyme in transgenic tobacco chloroplasts and its immunogenicity in mice

Plastid transformation has to date been applied to the expression of heterologous genes involved in agronomic traits and to the production of useful recombinant proteins. Here, we report a feasibility study for producing the human β-site APP cleaving enzyme (BACE) via transformation of tobacco chloroplasts. Stable integration of human BACE into the plastome was confirmed by PCR. Genomic Southern blot analysis detected the presence of the tobacco aadA and human BACE genes between trnI and trnA in the plastome. Northern blot analysis revealed that the aadA and BACE genes were both properly transcribed into a dicistronic transcriptional unit. Human BACE protein expression in transplastomic tobacco was determined by western blot analysis. ELISA analysis revealed that, based on a dilution series of E. coli-derived BACE as a standard, transplastomic lines accumulated BACE to levels of 2.0% of total soluble proteins. When mice were gavaged with the transplastomic tobacco extracts, they showed an immune response against the BACE antigen. The successful production of plastid-based BACE protein has the potential for developing a plant-based vaccine against Alzheimer disease.

2,2',4'-trihydroxychalcone from Glycyrrhiza glabra as a new specific BACE1 inhibitor efficiently ameliorates memory impairment in mice

Alzheimer's disease (AD) characterizes a progressive neurodegenerative disorder of the brain, while AD patients are afflicted with irreversible loss of neurons and further the intellectual abilities including memory and reasoning. One of the typical hallmarks of AD is the deposition of senile plaque that is contributed mainly by amyloid-beta (Abeta), whose production is initiated by beta-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1). Inhibition of BACE1 is thereby regarded as an attractive strategy for anti-AD drug discovery. Here, we reported that the natural product 2,2',4'-trihydroxychalcone (TDC) from Glycyrrhiza glabra functioned as a specific non-competitive inhibitor against BACE1 enzyme, and potently repressed beta-cleavage of APP and production of Abeta in human embryo kidney cells-APPswe cells. Moreover, the amelioration ability of this compound against the in vivo memory impairment was further evaluated by APP-PS1 double transgenic mice model. It is discovered that treatment of 9 mg/kg/day of TDC could obviously decrease Abeta production and Abeta plaque formation, while efficiently improve the memory impairment based on Morris water maze test. Our findings thus demonstrated that the natural product TDC as a new BACE1 inhibitor could ameliorate memory impairment in mice, and is expected to be potentially used as a lead compound for further anti-AD reagent development.

2-Amino-3,4-dihydroquinazolines as Inhibitors of BACE-1 (β-Site APP Cleaving Enzyme): Use of Structure Based Design to Convert a Micromolar Hit into a Nanomolar Lead

A new aspartic protease inhibitory chemotype bearing a 2-amino-3,4-dihydroquinazoline ring was identified by high-throughput screening for the inhibition of BACE-1. X-ray crystallography revealed that the exocyclic amino group participated in a hydrogen bonding array with the two catalytic aspartic acids of BACE-1 (Asp(32), Asp(228)). BACE-1 inhibitory potency was increased (0.9 microM to 11 nM K(i)) by substitution into the unoccupied S(1)' pocket.

Purification, analysis, and crystal structure of integrins

Integrins are large modular cell-surface receptors that regulate almost every aspect of cellular function through bidirectional signals transmitted across the lipid bilayer. Regulation of integrin activity is accomplished by complex and still incompletely understood biochemical pathways that modify integrin ligand binding, clustering, trafficking, and signaling functions. The dynamic tertiary and quaternary changes required to channel some of these activities have hampered, until recently, the crystal structure determination of these heterodimeric receptors. In this chapter, we review the methods used to purify and characterize these proteins biophysically and functionally, and to derive their three-dimensional structures.

Expression and characterization of recombinant beta-secretase from Trichoplusia ni BTI Tn5B1-4 cells transformed with cDNAs encoding human beta1,4-galactosyltransferase and Gal beta1,4-GlcNAc alpha 2,6-sialytransferase

Beta-Secretase (betaSEC) was expressed in Trichoplusia ni BTI Tn5B1-4 (Tn5B1-4) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Gal beta1,4-GlcNAc alpha 2,6-sialyltransferase (ST). The apparent molecular weight of recombinant beta-secretase was increased from 57 to 59 k Da. A lectin blot analysis indicated that recombinant beta-secretase from Tn5B1-4 betaSEC/GalT-ST cells (Tn5B1-4 cells co-transformed with cDNAs encoding beta-secretase, glycosyltransferases, GalT, and ST) contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. Two-dimensional electrophoresis revealed that recombinant beta-secretase from Tn5B1-4 beta SEC/GalT-ST cells had a lower isoelectric point than beta-secretase from control Tn5B1-4 betaSEC cells (Tn5B1-4 cells transformed only with beta-secretase cDNA). The enzyme activity of recombinant beta-secretase from Tn5B1-4 betaSEC/GalT-ST cells was enhanced up to 77% compared to control Tn5B1-4 betaSEC cells. The concentrations at half-maximum inhibition (IC(50)) values estimated from inhibition analyses using purified beta-secretases from Tn5B1-4/betaSEC and Tn5B1-4/betaSEC/GalT-ST cells were 32 and 290 nM, respectively.

Enhanced activity of recombinant beta-secretase from Drosophila melanogaster S2 cells transformed with cDNAs encoding human beta1,4-galactosyltransferase and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase

beta-Secretase (betaSEC) was expressed in Drososphila melanogaster Schneider 2 (S2) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase (ST). The apparent molecular weight of recombinant beta-secretase was increased from 56kDa to 61kDa. A lectin blot analysis indicated that recombinant beta-secretase from S2betaSEC/GalT-ST cells (S2 cells co-transformed with cDNAs encoding beta-secretase, glycosyltransferases, GalT, and ST) contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. Two dimensional electrophoresis revealed that recombinant beta-secretase from S2betaSEC/GalT-ST cells had a lower isoelectric point compared to beta-secretase from control S2betaSEC cells (S2 cells transformed only with beta-secretase cDNA). Recombinant beta-secretase from transformed S2 cells was also present as heterogeneous forms. The enzyme activity of recombinant beta-secretase from S2betaSEC/GalT-ST cells was enhanced up to 260% compared to control S2betaSEC cells. We have shown that an exogeneous human glycosyltransferases cDNA can be introduced into S2 cells to extend the N-glycan processing capabilities of the insect cell line, and that the extended glycosylation improves the activity of recombinant beta-secretase.

A general procedure for the purification of human beta-secretase expressed in Escherichia coli

Expression and purification of human beta-secretase (BACE1) in bacteria have been plagued with issues concerning solubility, inhomogeneous N-terminus, and lack of enzymic activity. Several forms of the mature human BACE1 have been expressed in Escherichia coli with different N-terminal extensions and without the C-terminus transmembrane domain. Although each of the proteins expresses in inclusion bodies, a generalized protocol has been developed to solubilize, refold, and purify these BACE1 variants. The resultant proteins are homogeneous and monodispersed in solution. Each possesses a unique N-terminus. Activity assays using the peptide substrate 7-methoxycoumarin-4-yl-SEVNLDAEFK-2,4-dinitrophenyl-RR, corresponding to the beta-secretase cleavage sequence in the amyloid precursor protein with the Swedish mutations of N(670)L(671) substituting for the residues K(670)M(671), reveal a kcat and KM of 9.3 min(-1) and 55 microM, respectively.

Apo and inhibitor complex structures of BACE (beta-secretase)

Human BACE, also known as beta-secretase, shows promise as a potential therapeutic target for Alzheimer's disease. We determined the apo structure of BACE to 1.75 A, and a structure of a hydroxyethylamine inhibitor complex derived by soaking. These show significant active-site movements compared to previously described BACE structures. Additionally, the structures reveal two pockets that could be targeted by structure-based drug design.

Crystallization of glycosylated human BACE protease domain expressed in Trichoplusia ni

Human beta-amyloid precursor protein cleaving enzyme (beta-secretase, or BACE) belongs to the aspartyl protease family, and is responsible for generating the N-terminus of beta-amyloid peptide (Abeta). BACE is a type I transmembrane glycoprotein with pre-, pro- and catalytic domains, a short transmembrane helix and a cytoplasmic region. In this study, a truncated form was engineered to produce the authentic catalytic domain of BACE in Trichoplusia ni (High 5) cells. The glycosylated BACE zymogen (proBACE) was secreted into the conditioned medium for facile purification by metal chelate and gel filtration chromatographies. The mature catalytic domain was obtained by a trans cleavage event under acidic conditions and crystallized in the absence of a bound inhibitor. A complete 3.4 A data set was collected on a single orthorhombic crystal with unit cell parameters a=74 A, b=130 A, c=134A. Successful molecular replacement shows two BACE molecules in the asymmetric unit.

Measuring human beta-secretase (BACE1) activity using homogeneous time-resolved fluorescence

The human beta-secretase enzyme, BACE1, mediates a critical step in the production of A beta(40) and A beta(42) peptides which are responsible for the severe neuronal cell death and insoluble amyloid plaques of Alzheimer's disease (AD). Several lines of evidence suggest that potent BACE1 inhibitors represent an attractive A beta-lowering strategy for AD. We designed a simple homogeneous time-resolved fluorescence (HTRF) assay which utilizes the fluorescence resonance energy transfer (FRET) pair europium and allophycocyanin for measuring BACE1 enzymatic activity in a high-throughput manner. Robust FRET was observed when an 18-amino-acid APP Swedish-synthetic peptide that was N-terminally labeled with europium cryptate and C-terminally biotinylated was incubated with streptavidin-coupled cross-linked allophycocyanin (SA-XL665). Purified BACE1 enzyme caused a time- and concentration-dependent linear change in FRET at low nanomolar enzyme concentrations. This assay was used to compare the autoprocessed "mature" BACE1 enzyme (sautoBACe1) and the soluble proBACE1 for activity and inhibition by selected peptidic BACE inhibitors. sautoBACE1 displayed only a modest increase in activity compared to sproBACE1 and this activity was uninhibited by the BACE1 prodomain peptide. Interestingly, the BACE1 prodomain peptide was able to partially inhibit sproBACE1 activity. IC(50s) for a P10-P4' statine BACE1 inhibitor, OM99-2, and OM-003 determined using the HTRF assay were in good agreement with those reported in the literature. The primary advantages of the HTRF-formatted BACE1 protease assay include appropriate reflection of native BACE1 activity, high sensitivity, low variability, and intrinsic quench correction afforded by ratiometric measurements made between EuK and SA-XL665 fluorophores.