Application of NanoBiT for Monitoring Dimerization of the Null Hong Kong Variant of α-1-Antitrypsin, NHK, in Living Cells

Lumit: A Homogeneous Bioluminescent Immunoassay for Detecting Diverse Analytes and Intracellular Protein Targets

Traditional immunoassays to detect secreted or intracellular proteins can be tedious, require multiple washing steps, and are not easily adaptable to a high-throughput screening (HTS) format. To overcome these limitations, we developed Lumit, a novel immunoassay approach that combines bioluminescent enzyme subunit complementation technology and immunodetection. This bioluminescent immunoassay does not require washes or liquid transfers and takes less than 2 h to complete in a homogeneous "Add and Read" format. In this chapter, we describe step-by-step protocols to create Lumit immunoassays for the detection of (1) secreted cytokines from cells, (2) phosphorylation levels of a specific signaling pathway node protein, and (3) a biochemical protein-protein interaction between a viral surface protein and its human receptor.

Bioluminescence goes portable: recent advances in whole-cell and cell-free bioluminescence biosensors

Recent advancements in synthetic biology, organic chemistry, and computational models have allowed the application of bioluminescence in several fields, ranging from well established methods for detecting microbial contamination to in vivo imaging to track cancer and stem cells, from cell-based assays to optogenetics. Moreover, thanks to recent technological progress in miniaturized and sensitive light detectors, such as photodiodes and imaging sensors, it is possible to implement laboratory-based assays, such as cell-based and enzymatic assays, into portable analytical devices for point-of-care and on-site applications. This review highlights some recent advances in the development of whole-cell and cell-free bioluminescence biosensors with a glance on current challenges and different strategies that have been used to turn bioassays into biosensors with the required analytical performance. Critical issues and unsolved technical problems are also highlighted, to give the reader a taste of this fascinating and challenging field.

Dimerization/oligomerization of the extracellular domain of the GLP-1 receptor and the negative cooperativity in its ligand binding revealed by the improved NanoBiT

The glucagon-like peptide-1 receptor (GLP-1R), a family B G-protein coupled receptor (GPCR), regulates the insulin secretion following stimulation by ligands. The transmembrane domain (TM) mediates GLP-1R homodimerization, which modulates its ligand binding and signaling. We investigated the possible involvement of the N-terminal extracellular domain (NTD) in dimerization/oligomerization and dimer-associated ligand binding by NanoLuc Binary Technology (NanoBiT). With improved NanoBiT detection using a decreasing substrate concentration, the negative cooperativity of ligand binding to the NTD was confirmed by accelerated dissociation and Scatchard analysis. The dimerization/oligomerization of the isolated NTD was observed by NanoBiT and validated by analytical ultracentrifugation, deriving the comparable dimerization affinity (~10⁵  M^-1 ). The NTD was also involved in the dimerization/oligomerization of the full-length GLP-1R with mutated TM4 at the cellular level. In an analysis of the parameters of the NTD binding, the K_d for the probe GLP-1 (7-36, A8G) was similar (6-8 μM) in both the 1:1 binding model and the receptor dimerization model. Compared with GLP-1 and dulaglutide, exenatide showed two-site binding with K_i values of 1.4 pM and 8.7 nM. Our study indicates the involvement of NTD in the GLP-1R dimerization/oligomerization and suggests that further investigations on the role in other family B GPCRs are needed.

A homogeneous bioluminescent immunoassay to probe cellular signaling pathway regulation

Monitoring cellular signaling events can help better understand cell behavior in health and disease. Traditional immunoassays to study proteins involved in signaling can be tedious, require multiple steps, and are not easily adaptable to high throughput screening (HTS). Here, we describe a new immunoassay approach based on bioluminescent enzyme complementation. This immunoassay takes less than two hours to complete in a homogeneous "Add and Read" format and was successfully used to monitor multiple signaling pathways' activation through specific nodes of phosphorylation (e.g pIκBα, pAKT, and pSTAT3). We also tested deactivation of these pathways with small and large molecule inhibitors and obtained the expected pharmacology. This approach does not require cell engineering. Therefore, the phosphorylation of an endogenous substrate is detected in any cell type. Our results demonstrate that this technology can be broadly adapted to streamline the analysis of signaling pathways of interest or the identification of pathway specific inhibitors.

Characterization of IRE1α in Neuro2a cells by pharmacological and CRISPR/Cas9 approaches

IRE1 is the most conserved endoplasmic reticulum (ER)-resident stress sensor. Its activation not only splices XBP1 but also participates in a variety of cell signaling. We elucidated the role of IRE1α in Neuro2a cells by establishing IRE1α-deficient cells and applying four IRE1 inhibitors. IRE1α deficiency prevented almost all spliced XBP1 (sXBP1) protein expression by treatment with thapsigargin (Tg) and tunicamycin (Tm); these phenomena paralleled the values measured by our two Nanoluciferase-based IRE1 assays. However, cell viability and protein expression of other ER stress-responsive factors in the IRE1α-deficient cells were comparable to those in the parental wild-type cells with or without Tm treatment. Next, we elucidated the IRE1 inhibitory actions and cytotoxicity of four compounds: STF083010, KIRA6, 4μ8C, and toyocamycin. KIRA6 attenuated IRE1 activity in a dose-dependent manner, but it showed severe cytotoxicity even in the IRE1α-deficient cells at a low concentration. The IRE1α-deficient cells were slightly resistant to KIRA6 at 0.1 μM in both the presence and absence of ER stress; however, resistance was not observed at 0.02 μM. Treatment with only KIRA6 at 0.1 μM for 12 h remarkably induced LC3 II, an autophagic marker, in both parental and IRE1α-deficient cells. Co-treatment with KIRA6 and Tm induced LC3 II, cleaved caspase-9, and cleaved caspase-3; however, IRE1α-deficiency did not abolish the expression of these two cleaved caspases. On the other hand, KIRA6 prohibited Tm-induced ATF4 induction in an IRE1-independent manner; however, co-treatment with KIRA6 and Tm also induced LC3 II and two cleaved caspases in the ATF4-deficient Neuro2a cells. Thus, we demonstrate that IRE1α deficiency has little impact on cell viability and expression of ER stress-responsive factors in Neuro2a cells, and the pharmacological actions of KIRA6 include IRE1-independent ways.

Elucidation of the Molecular Characteristics of Wild-Type and ALS-Linked Mutant SOD1 Using the NanoLuc Complementation Reporter System

Previously, we evaluated human SOD1 (hSOD1) dimerization in living cells using the NanoLuc complementation reporter system and found that homodimerization of G85R and G93A mutant SOD1 was lower than that of wild-type hSOD1. Since these assays were performed only using N-terminal NanoBiT-tagged hSOD1 constructs in our previous study, we constructed additional hSOD1 genes with NanoBiT-tags at the C-terminus and evaluated the NanoBiT luciferase activities. Among the tested combinations, the luciferase activity in cells expressing NanoBiT-tagged wild-type hSOD1 was higher than that in cells expressing G85R or G93A mutant hSOD1. The NanoBiT luciferase activities were detected both inside and outside of cells; however, the extracellular luciferase activities were minimally dampened by treatment with brefeldin A, which inhibits canonical ER-Golgi transport. In addition to studies on the homodimerization of SOD1, we investigated the interaction between hSOD1 and three chaperone proteins, copper chaperone for SOD1 (CCS), FKBP, and GRP78. The NanoBiT luciferase activities in cells expressing NanoBiT-tagged SOD1 and CCS were relatively high, but weak signals were also observed in cells expressing SOD1 together with FKBP or GRP78. These luciferase activities were different between wild-type and mutant hSOD1. Finally, we investigated the effects of two selenocompounds, ebselen and Se-methylselenocysteine, on SOD1 dimerization and found that ebselen increased the NanoBiT luciferase activity in cells expressing wild-type and mutant hSOD1. In conclusion, we show the differential molecular characteristics of wild-type and mutant hSOD1 in live cells by transfection with NanoBiT-tagged hSOD1 and chaperone genes and demonstrate that this assay might be useful for the development and re-evaluation of chemical compounds modulating the SOD1 conformation.