Effective expression of human proteins on bacterial magnetic particles in an anchor gene deletion mutant of Magnetospirillum magneticum AMB-1

Rapid and sensitive detection of glucocorticoids using engineered magnetosomes functionalized protein A conjugated broad-spectrum monoclonal antibody

Engineered bacterial magnetic nanoparticles (BMPs) fused with protein A (BMP-PA) can bind antibodies, creating immunomagnetic beads that offer an attractive tool for targets screening. In the study, BMP-PA-IgG was formed by attaching broad-spectrum monoclonal antibodies against glucocorticoids (GCs) to BMP-PA. Immunomagnetic assay was developed for analysis of GCs, using the BMP-PA-IgG and hydrocortisone-horseradish peroxidase. The developed assay exhibited broad specificity for GCs, including hydrocortisone (HCS), betamethasone (BMS), dexamethasone (DMS), prednisolone (PNS), beclomethasone (BCMS), cortisone (CS), 6-α-methylprednisone (6-α-MPNS), and fludrocortisone acetate (HFCS), with half inhibitory concentrations (IC50) ranging from 0.88 to 6.57 ng/mL. The proposed assay showed average recoveries of HCS and DMS ranging from 75.6% to 105.2% in chicken and pork samples, which were correlated well with those obtained by LC-MS/MS. This study indicated that the integration of engineered immunomagnetic beads into immunoassay systems offer possibilities for the sensitive and selective detection of GCs.

Fusion expression of nanobodies specific for the insecticide fipronil on magnetosomes in Magnetospirillum gryphiswaldense MSR-1

BACKGROUND

Magnetic nanoparticles such as magnetosomes modified with antibodies allow a high probability of their interaction with targets of interest. Magnetosomes biomineralized by magnetotactic bacteria are in homogeneous nanoscale size and have crystallographic structure, and high thermal and colloidal stability. Camelidae derived nanobodies (Nbs) are small in size, thermal stable, highly water soluble, easy to produce, and fusible with magnetosomes. We aimed to functionalize Nb-magnetosomes for the analysis of the insecticide fipronil.

RESULTS

Three recombinant magnetotactic bacteria (CF, CF+ , and CFFF) biomineralizing magnetosomes with different abundance of Nbs displayed on the surface were constructed. Compared to magnetosomes from the wild type Magnetospirillum gryphiswaldense MSR-1, all of the Nb-magnetosomes biosynthesized by strains CF, CF+ , and CFFF showed a detectable level of binding capability to fipronil-horseradish peroxidase (H2-HRP), but none of them recognized free fipronil. The Nb-magnetosomes from CFFF were oxidized with H₂O₂ or a glutathione mixture consisting of reduced glutathione and oxidized glutathione in vitro and their binding affinity to H2-HRP was decreased, whereas that to free fipronil was enhanced. The magnetosomes treated with the glutathione mixture were employed to develop an enzyme-linked immunosorbent assay for the detection of fipronil in water samples, with average recoveries in a range of 78-101%.

CONCLUSIONS

The economical and environmental-friendly Nb-magnetosomes biomineralized by the bacterial strain MSR-1 can be potentially applied to nanobody-based immunoassays for the detection of fipronil or nanobody-based assays in general.

Magnetotactic bacteria and magnetosomes - Scope and challenges

Geomagnetism aided navigation has been demonstrated by certain organisms which allows them to identify a particular location using magnetic field. This attractive technique to recognize the course was earlier exhibited in numerous animals, for example, birds, insects, reptiles, fishes and mammals. Magnetotactic bacteria (MTB) are one of the best examples for magnetoreception among microorganisms as the magnetic mineral functions as an internal magnet and aid the microbe to move towards the water columns in an oxic-anoxic interface (OAI). The ability of MTB to biomineralize the magnetic particles (magnetosomes) into uniform nano-sized, highly crystalline structure with uniform magnetic properties has made the bacteria an important topic of research. The superior properties of magnetosomes over chemically synthesized magnetic nanoparticles made it an attractive candidate for potential applications in microbiology, biophysics, biochemistry, nanotechnology and biomedicine. In this review article, the scope of MTB, magnetosomes and its challenges in research and industrial application have been discussed in brief. This article mainly focuses on the application based on the magnetotactic behaviour of MTB and magnetosomes in different areas of modern science.

Functional expression of full-length TrkA in the prokaryotic host Magnetospirillum magneticum AMB-1 by using a magnetosome display system

Tropomyosin receptor kinase A (TrkA), a receptor tyrosine kinase, is known to be associated with various diseases. Thus, TrkA has become a major drug-screening target for these diseases. Despite the fact that the production of recombinant proteins by prokaryotic hosts has advantages, such as fast growth and ease of genetic engineering, the efficient production of functional receptor tyrosine kinase by prokaryotic hosts remains a major experimental challenge. Here, we report the functional expression of full-length TrkA on magnetosomes in Magnetospirillum magneticum AMB-1 by using a magnetosome display system. TrkAwas fused with the magnetosome-localized protein Mms13 and expressed on magnetosome surfaces. Recombinant TrkA showed both nerve growth factor (NGF)-binding and autophosphorylation activities. TrkA expressed on magnetosomes has the potential to be used, not only for further functional analysis of TrkA, but also for ligand screening.

Stoichiometrically Controlled Immobilization of Multiple Enzymes on Magnetic Nanoparticles by the Magnetosome Display System for Efficient Cellulose Hydrolysis

The immobilization of multiple cellulase complexes receiving attention for use in the efficient hydrolysis of celluloses. In this study, the magnetosome display system was employed for the preparation of systems mimicking natural multiple cellulase complexes (cellulosomes) on magnetic nanoparticles (MNPs). Initially, two fluorescent proteins, namely, green fluorescent protein and mCherry, were immobilized on MNPs. Fluorescence analysis revealed the close proximity of two different proteins on the MNPs. Enzyme-linked immunosorbent assay analysis showed that stoichiometrically equivalent amounts of the proteins were immobilized on the MNPs. Next, endoglucanase (EG) and β-glucosidase (BG) were immobilized on MNPs to give EG/BG-MNPs. The resulting MNPs were applied for the hydrolysis of celluloses, with rapid hydrolysis of carboxymethyl cellulose being observed. Furthermore, the fusion of the cellulose-binding domain to EG/BG-MNPs promoted improved hydrolysis activity against the insoluble cellulose. We could therefore conclude that the magnetosome display system can expand the possibilities of mimicking natural cellulosome organization on MNPs.

From invagination to navigation: The story of magnetosome-associated proteins in magnetotactic bacteria

Magnetotactic bacteria (MTB) are a group of Gram-negative microorganisms that are able to sense and change their orientation in accordance with the geomagnetic field. This unique capability is due to the presence of a special suborganelle called the magnetosome, composed of either a magnetite or gregite crystal surrounded by a lipid membrane. MTB were first detected in 1975 and since then numerous efforts have been made to clarify the special mechanism of magnetosome formation at the molecular level. Magnetosome formation can be divided into several steps, beginning with vesicle invagination from the cell membrane, through protein sorting, followed by the combined steps of iron transportation, biomineralization, and the alignment of magnetosomes into a chain. The magnetosome-chain enables the sensing of the magnetic field, and thus, allows the MTB to navigate. It is known that magnetosome formation is tightly controlled by a distinctive set of magnetosome-associated proteins that are encoded mainly in a genomically conserved region within MTB called the magnetosome island (MAI). Most of these proteins were shown to have an impact on the magnetism of MTB. Here, we describe the process in which the magnetosome is formed with an emphasis on the different proteins that participate in each stage of the magnetosome formation scheme.

A tailored galK counterselection system for efficient markerless gene deletion and chromosomal tagging in Magnetospirillum gryphiswaldense

Magnetotactic bacteria have emerged as excellent model systems to study bacterial cell biology, biomineralization, vesicle formation, and protein targeting because of their ability to synthesize single-domain magnetite crystals within unique organelles (magnetosomes). However, only few species are amenable to genetic manipulation, and the limited methods for site-specific mutagenesis are tedious and time-consuming. Here, we report the adaptation and application of a fast and convenient technique for markerless chromosomal manipulation of Magnetospirillum gryphiswaldense using a single antibiotic resistance cassette and galK-based counterselection for marker recycling. We demonstrate the potential of this technique by genomic excision of the phbCAB operon, encoding enzymes for polyhydroxyalkanoate (PHA) synthesis, followed by chromosomal fusion of magnetosome-associated proteins to fluorescent proteins. Because of the absence of interfering PHA particles, these engineered strains are particularly suitable for microscopic analyses of cell biology and magnetosome biosynthesis.

Functional expression of an scFv on bacterial magnetic particles by in vitro docking

A Gram-negative, magnetotactic bacterium, Magnetospirillum magneticum AMB-1 produces nano-sized magnetic particles (BacMPs) in the cytoplasm. Although various applications of genetically engineered BacMPs have been demonstrated, such as immunoassay, ligand-receptor interaction or cell separation, by expressing a target protein on BacMPs, it has been difficult to express disulfide-bonded proteins on BacMPs due to lack of disulfide-bond formation in the cytoplasm. Here, we propose a novel dual expression system, called in vitro docking, of a disulfide-bonded protein on BacMPs by directing an immunoglobulin Fc-fused target protein to the periplasm and its docking protein ZZ on BacMPs. By in vitro docking, an scFv-Fc fusion protein was functionally expressed on BacMPs in the dimeric or trimeric form. Our novel disulfide-bonded protein expression system on BacMPs will be useful for efficient screening of potential ligands or drugs, analyzing ligand-receptor interactions or as a magnetic carrier for affinity purification.

Functional expression of thyroid-stimulating hormone receptor on nano-sized bacterial magnetic particles in Magnetospirillum magneticum AMB-1

The measurement of autoantibodies to thyroid-stimulating hormone receptor (TSHR) is important for the diagnosis of autoimmune thyroid disease such as Graves' disease (GD). Although TSHR from porcine thyroid membrane is commonly used for the measurement of TSHR autoantibodies (TRAb), recombinant human TSHR (hTSHR) remains ideal in terms of stable supply and species identity. Here we set out to express recombinant hTSHR on the lipid-bilayer surface of magnetic nanoparticles from a magnetotactic bacterium, Magnetospirillum magneticum AMB-1. Using a tetracycline-inducible expression system, we successfully overexpressed functional hTSHR on bacterial magnetic particles (BacMPs) in AMB-1 via an anchor protein specific for BacMPs. The overexpressed hTSHR was membrane integrated and possessed both ligand and autoantibody binding activity. Our data suggest that hTSHR-displayed BacMPs have potential as novel tools for ligand-receptor interaction analysis or for TRAb immunoassay in GD patients.

Enhanced heterologous protein display on bacterial magnetic particles using a lon protease gene deletion mutant in Magnetospirillum magneticum AMB-1

Bacterial magnetic particles (BacMPs) produced by the magnetotactic bacterium Magnetospirillum magneticum AMB-1, are used as magnetic supports or carriers for a variety of biomedical and environmental applications. Although protein expression systems on BacMPs have been established in previous studies, the expression efficiency was dependent on the introduced protein sequences. Recombinant human proteins are often poorly expressed on BacMPs because of proteolytic degradation by endogenous proteases. We constructed a lon protease gene deletion mutant strain (Δlon) of M. magneticum AMB-1 by homologous recombination to increase the efficiency of functional protein display on BacMPs using Δlon host cells. Wild-type and Δlon-M. magneticum AMB-1 cells were transformed using expression plasmids for human proteins, thyroid-stimulating hormone receptor (TSHR) and the class II major histocompatibility complex (MHC II) molecules onto BacMPs. Although mRNA expression of both TSHR and MHC II was the same level in the wild-type and Δlon transformants, the protein expression levels in Δlon transformants were significantly increased versus wild-type cells. Furthermore, the amounts of two different human proteins on BacMPs were successfully improved. This phenomenon could be due to the reduction of the degradation of target proteins in the Δlon strain. This is the first report to construct a protease deletion mutant in magnetotactic bacteria. The Δlon strain is a useful host to provide BacMPs displaying target proteins for various experimental, and ultimately, clinical applications.