NanoLuc luciferase as a suitable fusion partner of recombinant antibody fragments for developing sensitive luminescent immunoassays

Enzyme Cascade Amplification-Based Immunoassay Using Alkaline Phosphatase-Linked Single-Chain Variable Fragment Fusion Tracer and MnO2 Nanosheets for Detection of Deoxynivalenol in Corn Samples

Deoxynivalenol (DON) is a common mycotoxin that contaminates cereals. Therefore, the development of sensitive and efficient detection methods for DON is essential to guarantee food safety and human health. In this study, an enzyme cascade amplification-based immunoassay (ECAIA) using a dual-functional alkaline phosphatase-linked single-chain fragment variable fusion tracer (scFv-ALP) and MnO2 nanosheets was established for DON detection. The scFv-ALP effectively catalyzes the hydrolysis of ascorbyl-2-phosphate (AAP) to produce ascorbic acid (AA). This AA subsequently interacts with MnO2 nanosheets to initiate a redox reaction that results in the loss of oxidizing properties of MnO2. In the absence of ALP, MnO2 nanosheets can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to produce the blue oxidized product of TMB, which exhibits a signal at a wavelength of 650 nm for quantitative analysis. After optimization, the ECAIA had a limit of detection of 0.45 ng/mL and a linear range of 1.2-35.41 ng/mL. The ECAIA exhibited good accuracy in recovery experiments and high selectivity for DON. Moreover, the detection results of the actual corn samples correlated well with those from high-performance liquid chromatography. Overall, the proposed ECAIA based on the scFv-ALP and MnO2 nanosheets was demonstrated as a reliable tool for the detection of DON in corn samples.

Development of alkaline phosphatase-linked single-chain variable fragment fusion proteins for one-step immunodetection of deoxynivalenol in cereals

Deoxynivalenol (DON) is a mycotoxin that widely distributes in various foods and seriously threatens food safety. To minimize the consumers' dietary exposure to DON, there is an urgent demand for developing rapid and sensitive detection methods for DON in food. In this study, a bifunctional single-chain variable fragment (scFv) linked alkaline phosphatase (ALP) fusion protein was developed for rapid and sensitive detection of deoxynivalenol (DON). The scFv gene was chemically synthesized and cloned into the expression vector pET25b containing the ALP gene by homologous recombination. The prokaryotic expression, purification, and activity analysis of fusion proteins (scFv-ALP and ALP-scFv) were well characterized and performed. The interactions between scFv and DON were investigated by computer-assisted simulation, which included hydrogen bonds, hydrophobic interactions, and van der Waals forces. The scFv-ALP which showed better bifunctional activity was selected for developing a direct competitive enzyme-linked immunosorbent assay (dc-ELISA) for DON in cereals. The dc-ELISA takes 90 min for one test and exhibits a half inhibitory concentration (IC50) of 11.72 ng/mL, of which the IC50 was 3.08-fold lower than that of the scFv-based dc-ELISA. The developed method showed high selectivity for DON, and good accuracy was obtained from the spike experiments. Furthermore, the detection results of actual cereal samples analyzed by the method correlated well with that determined by high-performance liquid chromatography (R2=0.97165). These results indicated that the scFv-ALP is a promising bifunctional probe for developing the one-step colorimetric immunoassay, providing a new strategy for rapid and sensitive detection of DON in cereals.

Luciola mingrelica firefly luciferase as a marker in bioluminescent immunoassays

Chemical modification of the enzymes with biospecific macromolecules is used in various fields of biotechnology to impart new functions or improve their properties and is a fast and convenient way to get the final products. The preparation of highly active, stable, and functionally active conjugates of the thermostable luciferase through the NH2-groups or free SH-groups of the enzyme with target molecules of different molecular weight (albumin, avidin from chicken eggs, antibodies, and progesterone) is described. The obtained conjugates were successfully tested as a reporter in bioluminescent immunoassay for the detection of the molecules and pathogens. Thus, the luc-albumin (Luc-Alb) and luc-insulin (Luc-Ins) conjugates were used in competitive ELISA for the detection of an analyte (albumin or insulin) in the samples. Luc-progesterone (Luc-Pg) was used in the rapid homogeneous immunoassay of progesterone by the BRET technique with the detection limit of 0.5 ng/ml. Luciferase conjugates with avidin (Luc-Avi) and secondary and primary antibodies (Luc-RAM and Luc-Sal) were used for enzyme immunoassay detection of Salmonella paratyphi A cells with the cell detection limit of 5 × 104 CFU/ml. To reduce the detection limit of Salmonella cells, we developed a pseudo-homogeneous bioluminescent enzyme immunoassay of cells using a new matrix for the analyte capture-polystyrene microparticles coated with Pluronic F108, covalently labeled with Sal antibodies. This allowed to achieve efficient trapping of cells from solution, significantly reduced nonspecific sorption and decreased the cell detection limit to 2.7 × 103 CFU/ml without prior concentration of the sample. The methodology that was developed in this study can be applied for the development of novel bioanalytical systems based on firefly luciferases.

Development and evaluation of a new luciferase immunosorbent assay to detect GII.6 norovirus-specific IgG in different domestic and wild animals

Noroviruses (NoVs) are the leading viral pathogens globally causing acute gastroenteritis (AGE) in humans, posing a significant global health threat and economic burden. Recent investigations revealed that human NoVs had been detected in different animals, which raises concerns about whether NoVs are potential zoonotic diseases. This study developed a novel luciferase immunosorbent assay (LISA) to detect GII.6 NoV IgG based on P protein of VP1. The LISA showed high specificity (99.20%) and sensitivity (92.00%) with 4-16 times more sensitivity compared with an ELISA. NoV-LISA was reproducible with human serum regarding the inter- and intra-assay coefficient of variance values. Potential cross-reactivity was also evaluated using mice serum immunized by other antigens, which showed that NoV-LISA could differentiate GII.6 NoV from rotavirus and various genotypes of NoV. Specific GII.6 NoV IgG was widely detected in different domestic and wild animals, including dogs, pigs, bats, rats, and home shrews, with various IgG-positive rates ranging from 2.5 to 74.4%. In conclusion, our newly developed NoV-LISA assay is suitable for NoV-specific IgG detection in humans and animals. The wide distribution of IgG antibodies against human NoV indicates potential zoonotic transmission between humans and animals.

Mix-and-Read Nanobody-Based Sandwich Homogeneous Split-Luciferase Assay for the Rapid Detection of Human Soluble Epoxide Hydrolase

The soluble epoxide hydrolase (sEH) is possibly both a marker for and target of numerous diseases. Herein, we describe a homogeneous mix-and-read assay for the detection of human sEH based on using split-luciferase detection coupled with anti-sEH nanobodies. Selective anti-sEH nanobodies were individually fused with NanoLuc Binary Technology (NanoBiT), which consists of a large and small portion of NanoLuc (LgBiT and SmBiT, respectively). Different orientations of the LgBiT and SmBiT-nanobody fusions were expressed and investigated for their ability to reform the active NanoLuc in the presence of the sEH. After optimization, the linear range of the assay could reach 3 orders of magnitude with a limit of detection (LOD) of 1.4 ng/mL. The assay has a high sensitivity to human sEH and reached a similar detection limit to our previously reported conventional nanobody-based ELISA. The procedure of the assay was faster (30 min total) and easy to operate, providing a more flexible and simple way to monitor human sEH levels in biological samples. In general, the immunoassay proposed here offers a more efficient detection and quantification approach that can be easily adapted to numerous macromolecules.

Framework-Directed Amino-Acid Insertions Generated over 55-Fold Affinity-Matured Antibody Fragments That Enabled Sensitive Luminescent Immunoassays of Cortisol

We generated three single-chain Fv fragments (scFvs) specific to cortisol according to our original affinity-maturation strategy and verified their utility in developing immunoassays. These scFv mutants (m-scFvs) had insertion of one, four, or six amino acid(s) in the framework region 1 of the VH-domain and showed >55-fold higher affinity (Ka, 2.0 - 2.2 × 1010 M-1) than the unmodified scFv (wt-scFv). Each m-scFv was fused with NanoLuc luciferase (NLuc) for the use in enzyme-linked immunosorbent assays (ELISAs). In these ELISA, the m-scFv-NLuc fusions were competitively reacted with immobilized cortisol residues and cortisol standards, and then the bound NLuc activity was monitored luminometrically. The luminescent ELISAs generated dose-response curves with extremely low midpoints (approx. 3 pg/assay) and were >150-fold more sensitive than the colorimetric ELISAs using wt-scFv and >8000-fold more sensitive than the ELISA using the parental native antibody. The luminescent ELISAs showed acceptable cross-reactivity patterns with related steroids, and the determination of control sera afforded cortisol levels in the reference range with satisfactory parallelism.

Protein A-Nanoluciferase fusion protein for generalized, sensitive detection of immunoglobulin G

Detection and quantification of antibodies, especially immunoglobulin G (IgG), is a cornerstone of ELISAs, many diagnostics, and the development of antibody-based drugs. Current state-of-the-art immunoassay techniques for antibody detection require species-specific secondary antibodies and carefully-controlled bioconjugations. Poor conjugation efficiency degrades assay performance and increases the risk of clinical false positives due to non-specific binding. We developed a generic, highly-sensitive platform for IgG quantification by fusing the IgG-Fc binding Z domain of Staphylococcal Protein A with the ultrabright bioluminescence reporter Nanoluc-luciferase (Nluc). We demonstrated the application of this fusion protein in a sandwich IgG detection immunoassay using surface-bound antigens to capture target IgG and protein A-Nanoluc fusion as the detector. We optimized the platform's sensitivity by incorporating multiple repeats of the Z domain into the fusion protein constructs. Using rabbit and mouse anti-SARS-CoV-2 Nucleoprotein IgGs as model analytes, we performed ELISAs in two different formats, either with SARS-CoV-2 Nucleoprotein as the capture antigen or with polyclonal chicken IgY as the capture antibody. Using standard laboratory equipment, the platform enabled the quantitation of antibody analytes at concentrations as low as 10 pg/mL (67 fM).

Retrieving Dissociation-Resistant Antibody Mutants: An Efficient Strategy for Developing Immunoassays with Improved Sensitivities

Previously, we generated high-affinity antibody mutants that enabled sensitive immunoassays by exploring diverse libraries of single-chain Fv fragments (scFvs) displayed on bacteriophage. To isolate rarely-occurring desirable clones, "panning" has commonly been performed but is often unsuccessful. Therefore, we previously developed a clonal array profiling (CAP) method, wherein scFv-displaying phage (scFv-Ph) clones in a library were examined individually regarding their ability to target antigens immobilized on microwells. Clones that showed strong reactivity were recovered via dissociation using an acidic treatment. The CAP successfully discovered cortisol-specific scFvs showing 17-31-fold improved K_a from libraries generated via site-directed insertions in a prototype anti-cortisol scFv (wt-scFv; K_a, 3.6 × 10⁸ M^-1), but their K_a did not exceed 1.1 × 10¹⁰ M^-1. In this study, to break this possible affinity ceiling, we devised a new system employing a dissociation-independent recovery. scFv-Phs were individually reacted to target antigen (cortisol) immobilized on microwells via a linker containing a disulfide bond. Following acidic and basic treatments to eliminate scFv-Phs with "ordinary affinities," dissociation-resistant scFv-Phs remaining on the microwells were retrieved via reductive cleavage of the disulfide bonds. This system allowed for a straightforward and efficient discovery of scFv mutants with 33-56-fold increased K_a (1.2-2.0 × 10¹⁰ M^-1), exceeding the previous affinity ceiling. These scFvs enabled an enzyme-linked immunosorbent assay for cortisol with 18-51-fold higher sensitivity than the assay performed using wt-scFv.

Comparative Analysis of NanoLuc Luciferase and Alkaline Phosphatase Luminescence Reporter Systems for Phage-Based Detection of Bacteria

Reporter phage assays are a promising alternative to culture-based assays for rapidly detecting viable bacteria. The reporter systems used in phage-based detection are typically enzymes and their corresponding substrates that provide a signal following infection and expression. While several reporter systems have been developed, comparing reporter systems based on reported bacteria detection limits from literature can be challenging due to factors other than the reporter system that influence detection capabilities. To advance the development of phage-based assays, a systematic comparison and understanding of the components are necessary. The objective of this study was to directly compare two common enzyme-mediated luminescence reporter systems, NanoLuc/Nano-Glo and alkaline phosphatase (ALP*)/DynaLight, for phage-based detection of bacteria. The detection limits of the purified enzymes were determined, as well as the expression levels and bacteria detection capabilities following engineering of the coding genes into T7 phage and infection of E. coli BL21. When comparing the sensitivity of the purified enzymes, NLuc/Nano-Glo enzyme/substrate system demonstrated a lower detection limit than ALP*/DynaLight. In addition, the expression of the NLuc reporter following phage infection of E. coli was greater than ALP*. The lower detection limit combined with the higher expression resulted in a greater than 100-fold increase in sensitivity for the NLuc/Nano-Glo^® reporter system compared to ALP*/DynaLight when used for the detection of E. coli in a model system. These findings provide a comparative analysis of two common reporter systems used for phage-based detection of bacteria and a foundational understanding of these systems for engineering future reporter phage assays.

Engineered Amber-Emitting Nano Luciferase and Its Use for Immunobioluminescence Imaging In Vivo

The NanoLuc luciferase (NLuc) and its furimazine (FRZ) substrate have revolutionized bioluminescence (BL) assays and imaging. However, the use of the NLuc-FRZ luciferase-luciferin pair for mammalian tissue imaging is hindered by the low tissue penetration of the emitting blue photons. Here, we present the development of an NLuc mutant, QLuc, which catalyzes the oxidation of a synthetic QTZ luciferin for bright and red-shifted emission peaking at ∼585 nm. Compared to other small single-domain NLuc mutants, this amber-light-emitting luciferase exhibited improved performance for imaging deep-tissue targets in live mice. Leveraging this novel bioluminescent reporter, we further pursued in vivo immunobioluminescence imaging (immunoBLI), which used a fusion protein of a single-chain variable antibody fragment (scFv) and QLuc for molecular imaging of tumor-associated antigens in a xenograft mouse model. As one of the most red-shifted NLuc variants, we expect QLuc to find broad applications in noninvasive mammalian imaging. Moreover, the immunoBLI method complements immunofluorescence imaging and immuno-positron emission tomography (immunoPET), serving as a convenient and nonradioactive molecular imaging tool for animal models in basic and preclinical research.

Derivatization-assisted immunoassays: application for group-specific detection of potent methamphetamine and amphetamine enantiomers

Reliable and feasible tools for detecting (S)-methamphetamine [(S)-MAP] and (S)-amphetamine [(S)-AP] are required for regulating their illicit circulation. Antibodies that react equally to these stimulants are desirable for this purpose, but have been difficult to generate because of the crucial difference between their characteristic structures: i.e., N-methylamino (MAP) and amino (AP) groups. Furthermore, their small molecular masses (M_r < 150) have hampered the generation of high-affinity antibodies. To overcome these problems, we converted (S)-MAP and -AP into their 2-(trimethylsilyl)ethyl carbamate forms, Teoc-(S)-MAP and -AP, respectively, as surrogate analytes. The Teoc-derivatization not only increases their molecular masses, but also masks their structural differences. We generated a novel monoclonal antibody that showed a satisfactory affinity to Teoc-(S)-MAP residues (K_d = 13 nM as the IgG form) and developed a competitive enzyme-linked immunosorbent assay (ELISA) using microplates containing immobilized Teoc-(S)-MAP residues. Almost overlapping dose-response curves were obtained for Teoc-(S)-MAP and -AP, with the limit of detection of 0.078 and 0.10 ng per assay, respectively. A fixed amount of test powder sample (1 mg) was derivatized with Teoc-O-succinimidyl for 5 min, and subjected to ELISA using Teoc-(S)-MAP as the calibration standard. Under this protocol, (S)-MAP and -AP were converted to their Teoc derivatives with 30% and 34% yield, respectively, determined using ELISA as "Teoc-(S)-MAP equivalent," being distinguished from the derivatization products of (R)-MAP, (R)-AP, ephedrine, (S)-methylenedioxymethamphetamine, tyramine, dopamine, and β-alanine. This ELISA detected as little as 10 μg of (S)-MAP and -AP, and (S)-MAP in urine obtained from (S)-MAP-administered rats. Immunochromatography devices were also developed using gold nanoparticles coated with the monoclonal antibody, with which 0.10 mg of (S)-MAP and -AP was detected by the naked eye. We conclude that the present derivatization-assisted immunoassays may be useful for the detection of (S)-MAP and/or -AP in early stage screening of suspicious substances.

A Series of Furimazine Derivatives for Sustained Live-Cell Bioluminescence Imaging and Application to the Monitoring of Myogenesis at the Single-Cell Level

Bioluminescence (BL) imaging, which utilizes light emitted through the enzymatic reaction of luciferase oxidizing its substrate luciferin, enables sensitive and noninvasive monitoring of life phenomena. Herein, we developed a series of caged furimazine (FMZ) derivatives by introducing a protective group at the C-3 position and a hydroxy group at the C-6 phenyl ring to realize long-term live-cell BL imaging based on the NanoLuc (NLuc)/NanoKAZ (NKAZ)-FMZ system. The membrane permeability and cytotoxicity of the substrates were evaluated and related to their hydrophobicity. Among the series, the derivative with the bulkiest protective group (adamantanecarbonyl group) and a hydroxy substituent (named Ad-FMZ-OH) showed significantly prolonged and constant BL signal in cells expressing NLuc compared to the native FMZ substrate. This derivative enabled continuous BL imaging at the single-cell level for 24 h. Furthermore, we applied Ad-FMZ-OH to BL imaging of myocyte fusion and succeeded in the consecutive and sensitive monitoring at a single-cell level over a day. In summary, NLuc/NKAZ-caged FMZ derivatives have the potential to be applied to live-cell BL imaging of various life phenomena that require long-term observation.

[Dengue virus E protein-based luciferase immunosorbent assay for detecting dengue virus IgG antibody]

OBJECTIVE

To establish a luciferase immunosorbent assay (DENV-LISA) based on dengue virus (DENV) E protein, a specific antigen of DENV, for detection of DENV IgG antibody.

METHODS

The fused expression plasmids of DENV1-E1 and DENV2-E2 with luciferase were constructed. The plasmids were transfected into 293T cells, and the fusion protein containing the specific antigen and luciferase was obtained for establishing DENV-LISA. The specificity and sensitivity of DENV-LISA were assessed and compared with those of commercial DENV IgG antibody detection kit (ELISA).

RESULTS

The established DENV-LISA had a positive detection rate of 32.4% and a specificity of 96.6%, showing a similar positive detection rate with the commercial ELISA kit (35.3%; P>0.05). DENV-LISA was capable of detecting positive samples with a 1: 6400 dilution with a high sensitivity. The test values of DENV-LISA did not differ significantly between plates or within plates in the same batch (P> 0.05), suggesting a good reproducibility of the test.

CONCLUSION

The luciferase immunosorbent assay based on DENV E protein has high specificity and sensitivity for detecting DENV IgG antibody, and can be used for early screening, surveillance and epidemiological investigation of DENV infection.