Development of Novel Bioluminescent Sensor to Detect and Discriminate between Vitamin D Receptor Agonists and Antagonists in Living Cells

Analysis of vitamin D metabolites in biological samples using a nanoluc-based vitamin D receptor ligand sensing system: NLucVDR

Many assays are currently being developed to measure the levels of vitamin D metabolites in various samples (such as blood, urine, and saliva). This study focused on the measurement of vitamin D metabolites in serum and urine using the NLucVDR assay system, which consists of a split-type nanoluciferase and ligand-binding domain (LBD) of the human vitamin D receptor. Blood and urine samples were collected from 23 participants to validate the NLucVDR assay. The 25(OH)D3 levels in the serum and urine determined by the NLucVDR assay showed good correlations with those determined by standard analytical methods (ECLIA for serum and LC-MS/MS for urine), with correlation coefficients of 0.923 and 0.844 for serum and urine samples, respectively. In the case of serum samples, 25(OH)D3 levels determined by the NLucVDR assay were in good agreement with those determined by ECLIA. Therefore, the NLucVDR assay is a useful tool for measuring serum 25(OH)D3 levels. The contribution of each vitamin D metabolite to the luminescence intensity obtained during the NLucVDR assay depends on its concentration and affinity for NLucVDR. Thus, the contribution of 25(OH)D3 in serum appears to be much higher than that of the other metabolites. In contrast, the 25(OH)D3 levels in the urine determined by the NLucVDR assay were more than 20-fold higher than those determined by a standard analytical method (LC-MS/MS), suggesting that some vitamin D metabolite(s) in the urine remarkably increased the luminescence intensity of the NLucVDR assay. Notably, the 25(OH)D3 concentration in the urine determined by the NLucVDR assay and the serum 25(OH)D3 concentration determined by standard analytical methods showed a significant positive correlation (r = 0.568). These results suggest that the analysis of a small amount of urine using the NLucVDR assay may be useful for predicting the serum 25(OH)D3 levels.

Development of nanoluciferase-based sensing system that can specifically detect 1α,25-dihydroxyvitamin D in living cells

Previously, we reported a FLucN-LXXLL+LBD-FLucC system that detects VDR ligands using split firefly luciferase techniques, ligand binding domain (LBD) of VDR, and LXXLL sequences that interact with LBD after VDR ligand binding. In vivo, 25-hydroxyvitamin D₃ (25(OH)D₃) and 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃) act as VDR ligands that bind to VDR, and regulate bone-related gene expression. Therefore, the amount of 25(OH)D₃ and 1α,25(OH)₂D₃ are indicators of bone-related diseases such as rickets and osteoporosis. In this study, we have developed a novel LgBiT-LXXLL+LBD-SmBiT system using NanoLuc Binary Technology (NanoBiT), which has an emission intensity several times higher than that of the split-type firefly luciferase. Furthermore, by using genetic engineering techniques, we attempted to construct a novel system that can specifically detect 1α,25(OH)₂D₃. Because histidine residues at positions 305 and 397 play important roles in forming a hydrogen bond with a hydroxyl group at position C25 of 25(OH)D₃ and 1α,25(OH)₂D₃, His305 and His397 were each substituted by other amino acids. Consequently, the three mutant VDRs, H305D, H397N, and H397E were equally useful to detect 1α,25(OH)₂D₃ specifically. In addition, among the 58 variants of the LXXLL sequences, LPYEGSLLLKLLRAPVEE showed the greatest increase in luminescence upon the addition of 25(OH)D₃ or 1α,25(OH)₂D₃. Thus, our novel system using NanoBiT appear to be useful for detecting native vitamin D or its derivatives.

Development of In Vitro and In Vivo Evaluation Systems for Vitamin D Derivatives and Their Application to Drug Discovery

We have developed an in vitro system to easily examine the affinity for vitamin D receptor (VDR) and CYP24A1-mediated metabolism as two methods of assessing vitamin D derivatives. Vitamin D derivatives with high VDR affinity and resistance to CYP24A1-mediated metabolism could be good therapeutic agents. This system can effectively select vitamin D derivatives with these useful properties. We have also developed an in vivo system including a Cyp27b1-gene-deficient rat (a type I rickets model), a Vdr-gene-deficient rat (a type II rickets model), and a rat with a mutant Vdr (R270L) (another type II rickets model) using a genome editing method. For Cyp27b1-gene-deficient and Vdr mutant (R270L) rats, amelioration of rickets symptoms can be used as an index of the efficacy of vitamin D derivatives. Vdr-gene-deficient rats can be used to assess the activities of vitamin D derivatives specialized for actions not mediated by VDR. One of our original vitamin D derivatives, which displays high affinity VDR binding and resistance to CYP24A1-dependent metabolism, has shown good therapeutic effects in Vdr (R270L) rats, although further analysis is needed.

Eldecalcitol effects on osteoblastic differentiation and function in the presence or absence of osteoclastic bone resorption

Eldecalcitol (ELD) is an active vitamin D3 analog, possesses anti-resorption properties and is an approved therapeutic drug for the treatment of osteoporosis in Japan. However, the effect of ELD on osteoblasts in a distinct cell microenvironment, including in the presence or absence of osteoclastic bone resorption, is undetermined. In the current study, the effect of bone resorption supernatant on the ELD-mediated regulation of viability, differentiation and receptor activator of ΝF-κB ligand/osteoprotegerin (RANKL/OPG) expression was assessed in MC3T3-E1 pre-osteoblast cells. The murine macrophage-like cell line RAW 264.7 was induced to differentiate into functional osteoblasts. Bone resorption supernatant was prepared by culturing osteoclast with a bovine cortical bone specimen. Mouse MC3T3-E1 cells were subsequently treated with ELD combined with differentiated osteoclast cell culture (OCS) or osteoclast bone resorption model supernatants. Cell counting kit-8, alkaline phosphatase (ALP) activity, reverse transcription-quantitative (RT-q) PCR and western blot analysis were used to assess cell viability, osteogenic activity and RANKL and OPG expression in MC3T3-E1 cells. The OCS and OCS + ELD treatment exhibited significantly increased MC3T3-E1 cell viability when compared with the control group. However, ELD, bone resorption culture supernatant (BRS) and ELD + BRS treatments significantly decreased MC3T3-E1 cell viability. The results of ALP activity analysis, RT-qPCR and western blot analysis demonstrated that ELD treatment and OCS decreased the osteogenic markers (ALP and RUNX2), however, BRS increased them. All treatments enhanced the expression of RANKL and RANKL/OPG ratio. The results of the current study revealed that ELD inhibits osteoblastic differentiation in vitro. However, in the presence of BRS, which mimics the local bone microenvironment in vivo, the net effect on osteogenesis was positive. Furthermore, osteoclasts and bone matrix-derived factors increased the RANKL/OPG ratio, thereby potentiating osteoclastic activity.

Developing a circularly permuted variant of Renilla luciferase as a bioluminescent sensor for measuring Caspase-9 activity in the cell-free and cell-based systems

Biosensors and whole cell biosensors consisting of biological molecules and living cells can sense a special stimulus on a living system and convert it to a measurable signal. A major group of them are the bioluminescent sensors derived from luciferases. This type of biosensors has a broad application in molecular biology and imaging systems. In this project, a luciferase-based biosensor for detecting and measuring caspase-9 activity is designed and constructed using the circular permutation strategy. The spectroscopic method results reveal changes in the biosensor structure. Additionally, its activity is examined in a cell-free coupled assay system. Afterward, the biosensor is utilized for measuring the cellular caspase-9 activity upon apoptosis induction in a cancer cell line. In following the gene of biosensor is sub-cloned into a eukaryotic vector and transfected to HEK293T cell line and then its activity is measured upon apoptosis induction in the presence and absence of a caspase-9 inhibitor. The obtained results show that the designed biosensor detects the caspase-9 activity in the cell-free and cell-based systems.

Novel biosensor using split-luciferase for detecting vitamin D receptor ligands based on the interaction between vitamin D receptor and coactivator

Vitamin D receptor (VDR) ligands, such as 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] and its analogs, have been investigated for their potential clinical use in the treatment of various diseases such as type I rickets, osteoporosis, psoriasis, leukemia, and cancer. Previously, we reported a split-luciferase-based biosensor that can detect VDR ligands and assess their affinity for the ligand binding domain (LBD) of the VDR in a short time. However, a further increase in its sensitivity was required to detect plasma levels of 1α,25(OH)₂D₃ and its analogs. In this study, a novel type of biosensor called LXXLL + LBD was successfully developed. Here, the split luciferase forms a functional complex based on the intermolecular interaction between the LXXLL motif and the ligand-bound form of the LBD. This biosensor has an approximately 10-fold increase in the light intensity compared to the previous versions. Additionally, the binding affinity of the vitamin D analogs for the wild-type and the rickets-associated mutant R274L of VDR was evaluated.

Novel split luciferase-based biosensors for evaluation of vitamin D receptor ligands and their application to estimate CYP27B1 activity in living cells

Recently, we successfully generated a novel detection system for vitamin D receptor (VDR) ligands in vivo and in vitro, using a split-luciferase technique called the LucN-LBD-LucC biosensor that is a chimeric fusion protein of firefly luciferase with the ligand binding domain (LBD) of VDR. In this system, the luciferase light intensity of the LucN-LBD-LucC biosensor was decreased by binding of VDR ligands. Although this system is quite useful for evaluation of VDR ligands in a short time, the sensitivity of the LucN-LBD-LucC biosensor is not high enough. In this study, LXXLL motif peptides involved in the interaction between LBD and coactivators, such as the steroid receptor coactivator-1 (SRC-1), transcriptional intermediary factor 2 (TIF2), and the vitamin D receptor interacting protein 205 (DRIP205) were each inserted between LucN and LBD of the LucN-LBD-LucC biosensor. Surprisingly, the resulting LucN-LXXLL-LBD-LucC biosensor increased the light intensity in response to natural VDR ligands. This high-sensitivity biosensor system may be a powerful tool for discovery of high-affinity ligands for the mutant VDR. In addition, we have successfully estimated the activity of the wild-type and mutant CYP27B1 using the LucN-LXXLL-LBD-LucC biosensor in living cells within 90 min.

Development of a highly sensitive in vitro system to detect and discriminate between vitamin D receptor agonists and antagonists based on split-luciferase technique

Split-luciferase techniques are widely used to detect protein-protein interaction and bioactive small molecules including some hormones and vitamins. Previously, we successfully expressed chimeric proteins of luciferase and the ligand binding domain (LBD) of the vitamin D receptor (VDR), LucC-LBD-LucN in COS-7 cells. The LucC-LBD-LucN biosensor was named split-luciferase vitamin D biosensor (SLDB). This biosensor can detect and discriminate between VDR agonists and antagonists in mammalian cells. In this study, we established an in vitro screening system for VDR ligands using the SLDB proteins expressed in Escherichia coli (E. coli) cells. Our in vitro screening system using cell lysate of recombinant E. coli cells could be completed within 30min, and its activity was unchanged after 10 freeze-thaw cycles. This highly sensitive and convenient system would be quite useful to screen VDR ligands with therapeutic potential for various bone-related diseases, age-related cognitive disorders, cancer, and immune disorders. In addition, our system might be applicable to diagnostic measurement of serum concentrations of 25-hydroxyvitamin D₃ and 1α,25-dihydroxyvitamin D₃.

Novel screening system for high-affinity ligand of heredity vitamin D-resistant rickets-associated vitamin D receptor mutant R274L using bioluminescent sensor

Hereditary vitamin D-resistant rickets (HVDRR) is caused by mutations in the vitamin D receptor (VDR) gene. Arg274 located in the ligand binding domain (LBD) of VDR is responsible for anchoring 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂D₃) by forming a hydrogen bond with the 1α-hydroxyl group of 1α,25(OH)₂D_3. The Arg274Leu (R274L) mutation identified in patients with HVDRR causes a 1000-fold decrease in the affinity for 1α,25(OH)₂D₃, and dramatically reduces vitamin D- related gene expression. Recently, we successfully constructed fusion proteins consisting of split-luciferase and LBD of the VDR. The chimeric protein LucC-LBD-LucN, which displays the C-terminal domain of luciferase (LucC) at its N-terminus, can detect and discriminate between VDR agonists and antagonists. The LucC-LBD (R274L)-LucN was constructed to screen high-affinity ligands for the mutant VDR (R274L). Of the 33 vitamin D analogs, 5 showed much higher affinities for the mutant VDR (R274L) than 1α,25(OH)₂D₃, and 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-(OH)₂D₃ showed the highest affinity. These compounds might be potential therapeutics for HVDRR caused by the mutant VDR (R274L).

Split luciferase-based biosensors for characterizing EED binders

The EED (embryonic ectoderm development) subunit of the Polycomb repressive complex 2 (PRC2) plays an important role in the feed forward regulation of the PRC2 enzymatic activity. We recently identified a new class of allosteric PRC2 inhibitors that bind to the H3K27me3 pocket of EED. Multiple assays were developed and used to identify and characterize this type of PRC2 inhibitors. One of them is a genetically encoded EED biosensor based on the EED[G255D] mutant and the split firefly luciferase. This EED biosensor can detect the compound binding in the transfected cells and in the in vitro biochemical assays. Compared to other commonly used cellular assays, the EED biosensor assay has the advantage of shorter compound incubation with cells. The in vitro EED biosensor is much more sensitive than other label-free biophysical assays (e.g. DSF, ITC). Based on the crystal structure, the DSF data as well as the biosensor assay data, it's most likely that compound-induced increase in the luciferase activity of the EED[G255D] biosensor results from the decreased non-productive interactions between the EED subdomain and other subdomains within the biosensor construct. This new insight of the mechanism might help to broaden the use of the split luciferase based biosensors.