Truncated Variants of Gaussia Luciferase with Tyrosine Linker for Site-Specific Bioconjugate Applications

Bioluminescent Systems for Theranostic Applications

Bioluminescence, the light produced by biochemical reactions involving luciferases in living organisms, has been extensively investigated for various applications. It has attracted particular interest as an internal light source for theranostic applications due to its safe and efficient characteristics that overcome the limited penetration of conventional external light sources. Recent advancements in protein engineering technologies and protein delivery platforms have expanded the application of bioluminescence to a wide range of theranostic areas, including bioimaging, biosensing, photodynamic therapy, and optogenetics. This comprehensive review presents the fundamental concepts of bioluminescence and explores its recent applications across diverse fields. Moreover, it discusses future research directions based on the current status of bioluminescent systems for further expansion of their potential.

Targeted Bioluminescent Imaging of Pancreatic Ductal Adenocarcinoma Using Nanocarrier-Complexed EGFR-Binding Affibody-Gaussia Luciferase Fusion Protein

In vivo imaging has enabled impressive advances in biological research, both preclinical and clinical, and researchers have an arsenal of imaging methods available. Bioluminescence imaging is an advantageous method for in vivo studies that allows for the simple acquisition of images with low background signals. Researchers have increasingly been looking for ways to improve bioluminescent imaging for in vivo applications, which we sought to achieve by developing a bioluminescent probe that could specifically target cells of interest. We chose pancreatic ductal adenocarcinoma (PDAC) as the disease model because it is the most common type of pancreatic cancer and has an extremely low survival rate. We targeted the epidermal growth factor receptor (EGFR), which is frequently overexpressed in pancreatic cancer cells, using an EGFR-specific affibody to selectively identify PDAC cells and delivered a Gaussia luciferase (GLuc) bioluminescent protein for imaging by engineering a fusion protein with both the affibody and the bioluminescent protein. This fusion protein was then complexed with a G5-PAMAM dendrimer nanocarrier. The dendrimer was used to improve the protein stability in vivo and increase signal strength. Our targeted bioluminescent complex had an enhanced uptake into PDAC cells in vitro and localized to PDAC tumors in vivo in pancreatic cancer xenograft mice. The bioluminescent complexes could delineate the tumor shape, identify multiple masses, and locate metastases. Through this work, an EGFR-targeted bioluminescent-dendrimer complex enabled the straightforward identification and imaging of pancreatic cancer cells in vivo in preclinical models. This argues for the targeted nanocarrier-mediated delivery of bioluminescent proteins as a way to improve in vivo bioluminescent imaging.

Localization of the Catalytic Domain of Copepod Luciferases: Analysis of Truncated Mutants of the Metridia longa Luciferase

Luciferases from copepods Metridia longa and Gaussia princeps are successfully used as bioluminescent reporters for in vivo and in vitro assays. Here, we report the minimal sequence of copepod luciferases required for bioluminescence activity that was revealed by gradual deletions of sequence encoding the smallest MLuc7 isoform of M. longa luciferase. The single catalytic domain is shown to reside within the G32-A149 MLuc7 sequence and to be formed by both non-identical repeats, including 10 conserved Cys residues. Because this part of MLuc7 displays high homology with those of other copepod luciferases, our suggestion is that the determined boundaries of the catalytic domain are the same for all known copepod luciferases. The involvement of the flexible C-terminus in the retention of the bioluminescent reaction product in the substrate-binding cavity was confirmed by structural modeling and kinetics study. We also demonstrate that the ML7-N10 mutant (15.4 kDa) with deletion of ten amino acid residues at the N-terminus can be successfully used as a miniature bioluminescent reporter in living cells. Application of a shortened reporter may surely reduce the metabolic load on the host cells and decrease steric and functional interference at its use as a part of hybrid proteins.

Miniaturization of Bright Light-Emitting Luciferase ALuc: picALuc

Luciferases are widely used as sensitive reporters in various fields ranging from basic biology to medical diagnosis, public health, and food inspection. Scientists have isolated novel luciferases from bioluminescent organisms and concentrated on improving their brightness and thermostability. Recently, small bright luciferases such as artificial luciferase (ALuc) (21 kDa), NanoLuc (19 kDa), GLuc (18 kDa), and TurboLuc (16 kDa) have been reported. However, smaller, brighter, and more stable luciferases are desired for further applications. Here, we constructed the smallest and bright mutant of ALuc, named "picALuc" (13 kDa). picALuc retained the luminescence activity of the full-length ALuc; moreover, its brightness and thermostability were at the same levels as NanoLuc. Furthermore, we showed the advantage of picALuc for the bioluminescence resonance energy transfer-based assay due to its smallness. Our development has opened the door for wider and more practical applications of luciferases.

Strategic Decoy Peptides Interfere with MSI1/AGO2 Interaction to Elicit Tumor Suppression Effects

Simple Summary

Peptide drugs that can specifically target undesirable protein–protein interactions that lead to oncogenic developments have emerged as the next era of future medicine for cancers. To combat GBM tumor progression, our study offers an alternative therapeutic strategy via targeting the protein–protein interaction between MSI1 and AGO2 with synthetic peptides identified from the C-terminus of MSI1 in peptide arrays. Our present data revealed for the first time that peptidic disruption to the MSI1/AGO2 complex known for promoting cancer stemness and progression could lead to encouraging therapeutic efficacy at both in vitro and in vivo levels. The significantly suppressed tumor growth and prolonged survival rates in PDX tumor models by decoy peptides evidently provided a new rationale for stratifying patients with MSI1/AGO2-targeted therapeutics.

Abstract

Peptide drugs that target protein–protein interactions have attracted mounting research efforts towards clinical developments over the past decades. Increasing reports have indicated that expression of Musashi 1 (MSI1) is tightly correlated to high grade of cancers as well as enrichment of cancer stem cells. Treatment failure in malignant tumors glioblastoma multiform (GBM) had also been correlated to CSC-regulating properties of MSI1. It is thus imperative to develop new therapeutics that could effectively improve current regimens used in clinics. MSI1 and AGO2 are two emerging oncogenic molecules that both contribute to GBM tumorigenesis through mRNA regulation of targets involved in apoptosis and cell cycle. In this study, we designed peptide arrays covering the C-terminus of MSI1 and identified two peptides (Pep#11 and Pep#26) that could specifically interfere with the binding with AGO2. Our Biacore analyses ascertained binding between the identified peptides and AGO2. Recombinant reporter system Gaussian luciferase and fluorescent bioconjugate techniques were employed to determine biological functions and pharmacokinetic characteristics of these two peptides. Our data suggested that Pep#11 and Pep#26 could function as decoy peptides by mimicking the interaction function of MSI1 with its binding partner AGO2 in vitro and in vivo. Further experiments using GMB animal models corroborated the ability of Pep#11 and Pep#26 in disrupting MSI1/AGO2 interaction and consequently anti-tumorigenicity and prolonged survival rates. These striking therapeutic efficacies orchestrated by the synthetic peptides were attributed to the decoy function to C-terminal MSI1, especially in malignant brain tumors and glioblastoma.

Flash properties of Gaussia luciferase are the result of covalent inhibition after a limited number of cycles

Luciferases are widely used as reporters for gene expression and for sensitive detection systems. The luciferase (GLuc) from the marine copepod Gaussia princeps, has gained popularity, primarily because it is secreted and displays a very high light intensity. While firefly luciferase is characterized by kinetic behavior which is consistent with conventional steady-state Michaelis-Menten kinetics, GLuc displays what has been termed "flash" kinetics, which signify a burst in light emission followed by a rapid decay. As the mechanistic background for this behavior was unclear, we decided to decipher this in more detail. We show that decay in light signal is not due to depletion of substrate, but rather is caused by the irreversible inactivation of the enzyme. Inactivation takes place after between 10 and 200 reaction cycles, depending on substrate concentration and can be described by the sum of two exponentials with associated rate constants. The dominant of these increases linearly with substrate concentration while the minor is substrate-concentration independent. In terms of rate of initial luminescence reaction, this increases with the substrate concentration to the power of 1.5 and shows no signs of saturation up to 10 μM coelenterazine. Finally, we find that the inactivated form of the enzyme has a larger apparent size in both size exclusion chromatography and SDS-PAGE analysis and shows a fluorescence peak at 410 nm when excited at 333 nm. These findings indicate that the "flash" kinetics in Gaussia luciferase are caused by an irreversible covalent binding to a substrate derivative during catalysis.

Solution structure of Gaussia Luciferase with five disulfide bonds and identification of a putative coelenterazine binding cavity by heteronuclear NMR

Gaussia luciferase (GLuc) is a small luciferase (18.2 kDa; 168 residues) and is thus attracting much attention as a reporter protein, but the lack of structural information is hampering further application. Here, we report the first solution structure of a fully active, recombinant GLuc determined by heteronuclear multidimensional NMR. We obtained a natively folded GLuc by bacterial expression and efficient refolding using a Solubility Enhancement Petide (SEP) tag. Almost perfect assignments of GLuc’s ¹H, ¹³C and ¹⁵N backbone signals were obtained. GLuc structure was determined using CYANA, which automatically identified over 2500 NOEs of which > 570 were long-range. GLuc is an all-alpha-helix protein made of nine helices. The region spanning residues 10–18, 36–81, 96–145 and containing eight out of the nine helices was determined with a C_α-atom RMSD of 1.39 Å ± 0.39 Å. The structure of GLuc is novel and unique. Two homologous sequential repeats form two anti-parallel bundles made by 4 helices and tied together by three disulfide bonds. The N-terminal helix 1 is grabbed by these 4 helices. Further, we found a hydrophobic cavity where several residues responsible for bioluminescence were identified in previous mutational studies, and we thus hypothesize that this is a catalytic cavity, where the hydrophobic coelenterazine binds and the bioluminescence reaction takes place.

Bioluminescent detection of zearalenone using recombinant peptidomimetic Gaussia luciferase fusion protein

The development of a bioluminescent immunosensor is reported for the determination of zearalenone (ZEA) based on a peptide mimetic identified by phage display. The peptide mimetic GW, with a peptide sequence GWWGPYGEIELL, was used to create recombinant fusion proteins with the bioluminescent Gaussia luciferase (GLuc) that were directly used as tracers for toxin detection in a competitive immunoassay without the need for secondary antibodies or further labeling. The bioluminescent sensor, based on protein G-coupled magnetic beads for antibody immobilization, enabled determination of ZEA with a detection limit of 4.2 ng mL^-1 (corresponding to 420 μg kg^-1 in food samples) and an IC₅₀ value of 11.0 ng mL^-1. The sensor performance was evaluated in spiked maize and wheat samples, with recoveries ranging from 87 to 106% (RSD < 20%, n = 3). Finally, the developed method was applied to the analysis of a naturally contaminated reference matrix material and good agreement with the reported concentrations was obtained.Graphical abstract.

Non-invasive Reporter Gene Imaging of Cell Therapies, including T Cells and Stem Cells

Cell therapies represent a rapidly emerging class of new therapeutics. They are intended and developed for the treatment of some of the most prevalent human diseases, including cancer, diabetes, and for regenerative medicine. Currently, they are largely developed without precise assessment of their in vivo distribution, efficacy, or survival either clinically or preclinically. However, it would be highly beneficial for both preclinical cell therapy development and subsequent clinical use to assess these parameters in situ to enable enhancements in efficacy, applicability, and safety. Molecular imaging can be exploited to track cells non-invasively on the whole-body level and can enable monitoring for prolonged periods in a manner compatible with rapidly expanding cell types. In this review, we explain how in vivo imaging can aid the development and clinical translation of cell-based therapeutics. We describe the underlying principles governing non-invasive in vivo long-term cell tracking in the preclinical and clinical settings, including available imaging technologies, reporter genes, and imaging agents as well as pitfalls related to experimental design. Our emphasis is on adoptively transferred T cell and stem cell therapies.

How Non-invasive in vivo Cell Tracking Supports the Development and Translation of Cancer Immunotherapies

Immunotherapy is a relatively new treatment regimen for cancer, and it is based on the modulation of the immune system to battle cancer. Immunotherapies can be classified as either molecular or cell-based immunotherapies, and both types have demonstrated promising results in a growing number of cancers. Indeed, several immunotherapies representing both classes are already approved for clinical use in oncology. While spectacular treatment successes have been reported, particularly for so-called immune checkpoint inhibitors and certain cell-based immunotherapies, they have also been accompanied by a variety of severe, sometimes life-threatening side effects. Furthermore, not all patients respond to immunotherapy. Hence, there is the need for more research to render these promising therapeutics more efficacious, more widely applicable, and safer to use. Whole-body in vivo imaging technologies that can interrogate cancers and/or immunotherapies are highly beneficial tools for immunotherapy development and translation to the clinic. In this review, we explain how in vivo imaging can aid the development of molecular and cell-based anti-cancer immunotherapies. We describe the principles of imaging host T-cells and adoptively transferred therapeutic T-cells as well as the value of traceable cancer cell models in immunotherapy development. Our emphasis is on in vivo cell tracking methodology, including important aspects and caveats specific to immunotherapies. We discuss a variety of associated experimental design aspects including parameters such as cell type, observation times/intervals, and detection sensitivity. The focus is on non-invasive 3D cell tracking on the whole-body level including aspects relevant for both preclinical experimentation and clinical translatability of the underlying methodologies.

Shining Light on the Secreted Luciferases of Marine Copepods: Current Knowledge and Applications

Copepod luciferases-a family of small secretory proteins of 18.4-24.3 kDa, including a signal peptide-are responsible for bright secreted bioluminescence of some marine copepods. The copepod luciferases use coelenterazine as a substrate to produce blue light in a simple oxidation reaction without any additional cofactors. They do not share sequence or structural similarity with other identified bioluminescent proteins including coelenterazine-dependent Renilla and Oplophorus luciferases. The small size, strong luminescence activity and high stability, including thermostability, make secreted copepod luciferases very attractive candidates as reporter proteins which are particularly useful for nondisruptive reporter assays and for high-throughput format. The most known and extensively investigated representatives of this family are the first cloned GpLuc and MLuc luciferases from copepods Gaussia princeps and Metridia longa, respectively. Immediately after cloning, these homologous luciferases were successfully applied as bioluminescent reporters in vivo and in vitro, and since then, the scope of their applications continues to grow. This review is an attempt to systemize and critically evaluate the data scattered through numerous articles regarding the main structural features of copepod luciferases, their luminescent and physicochemical properties. We also review the main trends of their application as bioluminescent reporters in cell and molecular biology.

Gaussia princeps luciferase: a bioluminescent substrate for oxidative protein folding

Gaussia princeps luciferase (GLuc) generates an intense burst of blue light when exposed to coelenterazine in the absence of ATP. Here we show that this 5-disulfide containing enzyme can be used as a facile and convenient substrate for studies of oxidative protein folding. Reduced GLuc (rGLuc), with 10 free cysteine residues, is completely inactive as a luciferase but >60% bioluminescence activity, compared to controls, can be recovered using a range of oxidizing regimens in the absence of the exogenous shuffling activity of protein disulfide isomerase (PDI). The sulfhydryl oxidase QSOX1 can be assayed using rGLuc in a simple bioluminescence plate reader format. Similarly, low concentrations of rGLuc can be oxidized by millimolar levels of dehydroascorbate, hydrogen peroxide or much lower concentrations of sodium tetrathionate. The oxidative refolding of rGLuc in the presence of a range of glutathione redox buffers is only marginally accelerated by micromolar levels of PDI. This modest rate enhancement probably results from a relatively simple disulfide connectivity in native GLuc; reflecting two homologous domains each carrying two disulfide bonds with a single interdomain disulfide. When GLuc is reoxidized under denaturing conditions the resulting scrambled protein (sGLuc) can be used in a sensitive bioluminescence assay for reduced PDI in the absence of added exogenous thiols. Finally, the general facility by which rGLuc can recover bioluminescent activity in vitro provides a sensitive method for the assessment of inhibitors of oxidative protein folding.

Bioluminescent and structural features of native folded Gaussia luciferase

The secreted luciferases responsible for light emission of marine copepods have gained popularity for being used in noninvasive imaging of intracellular events. The secreted luciferase of copepod Gaussia princeps is a one-subunit protein catalyzing coelenterazine oxidation to emit blue light. It consists of the N-terminal variable part that bears a signal peptide for secretion and the C-terminal catalytic domain containing ten highly conserved Cys residues supposing the existence of up to five SS bonds. Despite wide application of Gaussia luciferase in biomedical research, its biochemical properties are still insufficiently studied due to the general problem of obtaining the proper folded Cys-rich proteins in bacterial cells. Here we report the properties of the proper folded Gaussia luciferase produced in insect cells using baculovirus expression system. This high purity luciferase reveals the highest activity at 15-20 °C but retains only ~20% activity at 37 °C that may hamper its application for in vivo assays. The maximum of bioluminescent activity of GpLuc is found at NaCl concentrations in the range of 1.0-1.5 M and, furthermore, a high NaCl concentration enhances luciferase stability to thermal denaturation, i.e. Gaussia luciferase displays the features characteristic of halophilic enzymes. The studies on bioluminescence kinetics at different coelenterazine concentrations obviously show a positive cooperativity of Gaussia luciferase with coelenterazine (Hill coefficient - 1.8 ± 0.2; K_0.5-2.14 ± 0.17 μM). We suggest this effect to be rather due to the so-called kinetic cooperativity conditioned by conformational changes in response to substrate binding than to the presence of two catalytic sites.

The disulfide-rich Metridia luciferase refolded from E. coli inclusion bodies reveals the properties of a native folded enzyme produced in insect cells

The bioluminescence of a marine copepod Metridia longa is determined by a small secreted coelenterazine-dependent luciferase that uses coelenterazine as a substrate of enzymatic reaction to generate light (λ_max=480nm). To date, four different isoforms of the luciferase differing in size, sequences, and properties have been cloned by functional screening. All of them contain ten conserved Cys residues that suggests up to five SS intramolecular bonds per luciferase molecule. Whereas the use of copepod luciferases as bioluminescent reporters in biomedical research in vivo is growing from year to year, their application for in vitro assays is still limited by the difficulty in obtaining significant amounts of luciferase. The most cost-effective host for producing recombinant proteins is Escherichia coli. However, prokaryotic and eukaryotic cells maintain the reductive environment in cytoplasm that hinders the disulfide bond formation and consequently the proper folding of luciferase. Here we report the expression of the MLuc7 isoform of M. longa luciferase in E. coli cells and the efficient procedure for refolding from inclusion bodies yielding a high-active monomeric protein. Furthermore, in a set of identical experiments we demonstrate that bioluminescent and structural features of MLuc7 produced in bacterial cells are identical to those of MLuc7 isoform produced from culture medium of insect cells. Although the yield of high-purity protein is only 6mg/L, the application of E. coli cells to produce the luciferase is simpler and more cost-effective than the use of insect cells. We expect that the suggested technology of Metridia luciferase production allows obtaining of sufficient amounts of protein both for the development of novel in vitro analytical assays with the use of MLuc7 as a label and for structural studies.

Bioorthogonal Protein Conjugation: Application to the Development of a Highly Sensitive Bioluminescent Immunoassay for the Detection of Interferon-γ

Bioorthogonal conjugation eliminates the shortcomings of classical conjugation methods. The conjugation of antibodies to reporter proteins, such as bioluminescent protein, can be controlled with orthogonal conjugation methods. Here we report a bioluminescent immunoassay for the sensitive detection of interferon-γ (IFN-γ) that utilizes orthogonal conjugation of bioluminescent protein, Gaussia luciferase to anti-IFN-γ antibody. The IFN-γ is produced by the immune system and the detection of the IFN-γ is pivotal for the detection of persistent viral and bacterial infections. A bioorthogonal conjugation approach is used to conjugate an anti-IFN-γ antibody with a GLuc mutant containing the N-terminal tyrosine using formylbenzene diazonium hexafluorophosphate reagent (FBDP) in hydrophilic mild pH environment yielding high conjugation efficiency (60%). This reagent is shown to be specific for tyrosine (Tyr) residues. Therefore, conjugation through Tyr was orthogonal and not detrimental to the bioluminescence activity of GLuc. The immunoassay described in this paper is a sandwich type assay and involves a capture and a detection antibody. The assay was validated for its robustness, precision, accuracy, limit of detection, and recovery.

Expression of a soluble truncated Vargula luciferase in Escherichia coli

Marine luciferases are regularly employed as useful reporter molecules across a range of various applications. However, attempts to transition expression from their native eukaryotic environment into a more economical prokaryotic, i.e. bacterial, expression system often presents several challenges. Specifically, bacterial protein expression inherently lacks chaperone proteins to aid in the folding process, while Escherichia coli presents a reducing cytoplasmic environment in. These conditions contribute to the inhibition of proper folding of cysteine-rich proteins, leading to incorrect tertiary structure and ultimately inactive and potentially insoluble protein. Vargula luciferase (Vluc) is a cysteine-rich marine luciferase that exhibits glow-type bioluminescence through a reaction between its unique native substrate and molecular oxygen. Because most other commonly used bioluminescent proteins exhibit flash-type emission kinetics, this emission characteristic of Vluc is desirable for high-throughput applications where stability of emission is required for the duration of data collection. A truncated form of Vluc that retains considerable bioluminescence activity (55%) compared to the native full-length protein has been reported in the literature. However, expression and purification of this luciferase from bacterial systems has proven difficult. Herein, we demonstrate the expression and purification of a truncated form of Vluc from E. coli. This truncated Vluc (tVluc) was subsequently characterized in terms of both its biophysical and bioluminescence properties.