Synthesis of (22R)-, (22S)-22-Fluoro-, and 22,22-Difluoro-25-hydroxyvitamin D3 and Effects of Side-Chain Fluorination on Biological Activity and CYP24A1-Dependent Metabolism.

Three novel analogues of C22-fluoro-25-hydroxyvitamin D3 (5-7) were synthesized and evaluated to investigate the effects of side-chain fluorination on biological activity and metabolism of vitamin D. These novel analogues were constructed by convergent synthesis applying the Wittig-Horner coupling reaction between CD-ring ketones (41,42,44) and A-ring phosphine oxide (11). The introduction of C22-fluoro units was achieved by stereoselective deoxy-fluorination for synthesizing 5 and 6 or two-step cationic fluorination for 7. The absolute configuration of the C22-fluoro-8-oxo-CD-ring (39) was confirmed by X-ray crystallographic structure determination. The basic biological activity of the side-chain fluorinated analogues, including compounds (5-7), was evaluated. Generally, osteocalcin promoter transactivation activity decreased in the order of C24-fluoro, C23-fluoro, and C22-fluoro analogues. In addition, the metabolic stability of C22-fluoro-25-hydroxyvitamin D3 (5-7) against hCYP24A1 metabolism was also evaluated. 22,22-Difluoro-25(OH)D3 (7) was more stable against hCYP24A1 metabolism compared with its non-fluorinated counterpart 25-hydroxyvitamin D3 (1), but fluorination at the C22 position had little effect on the metabolic stability compared with C24- and C23-fluoro analogues. Our research clarified that side-chain fluorination in vitamin D markedly changes CYP24A1 metabolic stability depending on the fluorinating position.

Synthesis of New 26,27-Difluoro- and 26,26,27,27-Tetrafluoro-25-hydroxyvitamin D3: Effects of Terminal Fluorine Atoms on Biological Activity and Half-life.

As an extension of our research on providing a chemical library of side-chain fluorinated vitamin D3 analogues, we newly designed and synthesized 26,27-difluoro-25-hydroxyvitamin D3 (1) and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D3 (2) using a convergent method applying the Wittig-Horner coupling reaction between CD-ring ketones (13, 14) and A-ring phosphine oxide (5). The basic biological activities of analogues, 1, 2, and 26,26,26,27,27,27-hexafluoro-25-hydroxyvitamin D3 [HF-25(OH)D3] were examined. Although the tetrafluorinated new compound 2 exhibited higher binding affinity for vitamin D receptor (VDR) and resistance to CYP24A1-dependent metabolism compared with the difluorinated 1 and its non-fluorinated counterpart 25-hydroxyvitamin D3 [25(OH)D3], HF-25(OH)D3 showed the highest activity among these compounds. Osteocalcin promoter transactivation activity of these fluorinated analogues was tested, and it decreased in the order of HF-25(OH)D3, 2, 1, and 25(OH)D3 in which HF-25(OH)D3 showed 19-times greater activity than the natural 25(OH)D3.

Elucidation of metabolic pathways of 25-hydroxyvitamin D3 mediated by CYP24A1 and CYP3A using Cyp24a1 knockout rats generated by CRISPR/Cas9 system.

CYP24A1-deficient (Cyp24a1 KO) rats were generated using the CRISPER/Cas9 system to investigate CYP24A1-dependent or -independent metabolism of 25(OH)D3, the prohormone of calcitriol. Plasma 25(OH)D3 concentrations in Cyp24a1 KO rats were approximately twofold higher than in wild-type rats. Wild-type rats showed five metabolites of 25(OH)D3 in plasma following oral administration of 25(OH)D3, and these metabolites were not detected in Cyp24a1 KO rats. Among these metabolites, 25(OH)D3-26,23-lactone was identified as the second major metabolite with a significantly higher Tmax value than others. When 23S,25(OH)2D3 was administered to Cyp24a1 KO rats, neither 23,25,26(OH)3D3 nor 25(OH)D3-26,23-lactone was observed. However, when 23S,25R,26(OH)3D3 was administered to Cyp24a1 KO rats, plasma 25(OH)D3-26,23-lactone was detected. These results suggested that CYP24A1 is responsible for the conversion of 25(OH)D3 to 23,25,26(OH)3D3 via 23,25(OH)2D3, but enzyme(s) other than CYP24A1 may be involved in the conversion of 23,25,26(OH)3D3 to 25(OH)D3-26,23-lactone. Enzymatic studies using recombinant human CYP species and the inhibitory effects of ketoconazole suggested that CYP3A plays an essential role in the conversion of 23,25,26(OH)3D3 into 25(OH)D3-26,23-lactone in both rats and humans. Taken together, our data indicate that Cyp24a1 KO rats are valuable for metabolic studies of vitamin D and its analogs. In addition, long-term administration of 25(OH)D3 to Cyp24a1 KO rats at 110 µg/kg body weight/day resulted in significant weight loss and ectopic calcification. Thus, Cyp24a1 KO rats could represent an important model for studying renal diseases originating from CYP24A1 dysfunction.

The First Convergent Synthesis of 23,23-Difluoro-25-hydroxyvitamin D3 and Its 24-Hydroxy Derivatives: Preliminary Assessment of Biological Activities.

In this paper, we report an efficient synthetic route for the 23,23-difluoro-25-hydroxyvitamin D3 (5) and its 24-hydroxylated analogues (7,8), which are candidates for the CYP24A1 main metabolites of 5. The key fragments, 23,23-difluoro-CD-ring precursors (9-11), were synthesized starting from Inhoffen-Lythgoe diol (12), and introduction of the C23 difluoro unit to α-ketoester (19) was achieved using N,N-diethylaminosulfur trifluoride (DAST). Preliminary biological evaluation revealed that 23,23-F2-25(OH)D3 (5) showed approximately eight times higher resistance to CYP24A1 metabolism and 12 times lower VDR-binding affinity than its nonfluorinated counterpart 25(OH)D3 (1).

Stereoselective Synthesis of 24-Fluoro-25-Hydroxyvitamin D3 Analogues and Their Stability to hCYP24A1-Dependent Catabolism.

Two 24-fluoro-25-hydroxyvitamin D3 analogues (3,4) were synthesized in a convergent manner. The introduction of a stereocenter to the vitamin D3 side-chain C24 position was achieved via Sharpless dihydroxylation, and a deoxyfluorination reaction was utilized for the fluorination step. Comparison between (24R)- and (24S)-24-fluoro-25-hydroxyvitamin D3 revealed that the C24-R-configuration isomer 4 was more resistant to CYP24A1-dependent metabolism than its 24S-isomer 3. The new synthetic route of the CYP24A1 main metabolite (24R)-24,25-dihydroxyvitamin D3 (6) and its 24S-isomer (5) was also studied using synthetic intermediates (30,31) in parallel.

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.

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.

Production of an active form of vitamin D2 by genetically engineered CYP105A1.

Our previous studies revealed that CYP105A1 can convert vitamin D3 (VD3) to its active form, 1α,25-dihydroxyvitamin D3 (1,25D3). Site-directed mutagenesis of CYP105A1 based on its crystal structure dramatically enhanced its activity; the activity of double variants R73A/R84A and R73A/R84V was more than 100-fold higher than that of the wild type of CYP105A1. In contrast, these variants had a low ability to convert vitamin D2 (VD2) to 1α,25-dihydroxyvitamin D2 (1,25D2), whereas they catalyzed the sequential hydroxylation at positions C25 and C26 to produce 25,26D2. A comparison of the docking models of 25D2 and 25D3 into the substrate-binding pocket of R73A/R84A suggests that the side chain of the Met239 inhibits the binding of 25D2 for 1α-hydroxylation. Therefore, the Met239 residue of R73A/R84A was substituted for Ala. As expected, the triple variant R73A/R84A/M239A showed a 22-fold higher 1α-hydroxylation activity towards 25D2. To the best of our knowledge, this is the first report on the generation of microbial cytochrome P450 that converts VD2 to 1,25D2 via 25D2.

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

Active forms of vitamin D regulate the expression of multiple genes that play essential roles in calcium and phosphate homeostasis, cell differentiation, and the immune system via the vitamin D receptor (VDR). Many vitamin D analogs have been synthesized for clinical use in the treatment of type I rickets, osteoporosis, renal osteodystrophy, psoriasis, leukemia, and breast cancer. We have constructed two fusion proteins containing split-luciferase and the ligand binding domain (LBD) of the VDR designated as LucN-LBD-LucC and LucC-LBD-LucN. Remarkably, the LucC-LBD-LucN, which has the C-terminal domain of luciferase at the N-terminus of the fusion protein, was a significantly better biosensor than LucN-LBD-LucC. Addition of the VDR agonists to COS-7 cells expressing LucC-LBD-LucN dramatically reduced luciferase activity. In contrast, the VDR antagonist significantly increased the chimeric luciferase activity in a dose- and time-dependent manner. Our results on chimeric luciferases containing the LBDs of mutant VDRs derived from patients with vitamin D-dependent type II rickets indicated that our system could detect a conformational change of the LBD of the VDR likely based on a positional change of the helix 12, which occurs upon ligand binding. This novel system to detect and discriminate between VDR agonists and antagonists could be useful for the screening and identification of chemical compounds that bind to normal or mutant VDRs with high affinity.

Loss of Scribble causes cell competition in mammalian cells.

In Drosophila, normal and transformed cells compete with each other for survival in a process called cell competition. However, it is not known whether comparable phenomena also occur in mammals. Scribble is a tumor suppressor protein in Drosophila and mammals. In this study we examine the interface between normal and Scribble-knockdown epithelial cells using Madin-Darby Canine Kidney (MDCK) cells expressing Scribble short hairpin RNA (shRNA) in a tetracycline-inducible manner. We observe that Scribble-knockdown cells undergo apoptosis and are apically extruded from the epithelium when surrounded by normal cells. Apoptosis does not occur when Scribble-knockdown cells are cultured alone, suggesting that the presence of surrounding normal cells induces the cell death. We also show that death of Scribble-knockdown cells occurs independently of apical extrusion. Finally, we demonstrate that apoptosis of Scribble-knockdown cells depends on activation of p38 mitogen-activated protein kinase (MAPK). This is the first demonstration that an oncogenic transformation within an epithelium induces cell competition in a mammalian cell culture system.

Discovery of novel adenylyl cyclase inhibitor by cell-based screening.

We screened 2400 compounds to find novel inhibitors of the adenylyl cyclase (AC)-protein kinase A (PKA)-cAMP response-element-binding protein (CREB) signaling pathway (AC/PKA/CREB pathway). Using a multistep cell-based screening system employing split luciferase technique, we narrowed down the candidates effectively from 2400 chemical compounds and identified a novel AC inhibitor (compound 1). Since dysregulation of the AC/PKA/CREB pathway is known to cause diseases not only in the nervous system but also in other organs, compound 1 is expected to be developed as a medicine for these diseases.

Possible involvement of Hcn1 ion channel in learning and memory dysfunction in SAMP8 mice.

The senescence-accelerated mouse prone 8 (SAMP8) strain exhibits age-related learning and memory deficits (LMD) at 2 months of age. Combined linkage analysis of 264 F2 intercross SAMP8 × JF1 mice and RNA-seq analysis identified Hcn1 gene out of 29 genes in the LMD region on chromosome 13. Hcn1 in SAMP8 strain showed 15 times less polyglutamine repetition compared to Japanese fancy mouse 1 (JF1). Whole cell patch clamp analysis showed that Hcn1 ion conductivity was significantly lower in SAMP8 compared to that of JF1, which may be associated with learning and memory deficiency.

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.

Characterization of Rickets Type II Model Rats to Reveal Functions of Vitamin D and Vitamin D Receptor.

Vitamin D has been known to exert a wide range of physiological effects, including calcemic, osteogenic, anticancer, and immune responses. We previously generated genetically modified (GM) rats and performed a comparative analysis of their physiological properties to elucidate the roles of vitamin D and vitamin D receptor (VDR). In this study, our primary goal was to investigate the manifestations of type II rickets in rats with the VDR(H301Q) mutation, analogous to the human VDR(H305Q). Additionally, we created a double-mutant rat with the VDR(R270L/H301Q) mutation, resulting in almost no affinity for 1,25-dihydroxy-vitamin D3 (1,25D3) or 25-hydroxy-vitamin D3 (25D3). Notably, the plasma calcium concentration in Vdr(R270L/H301Q) rats was significantly lower than in wild-type (WT) rats. Meanwhile, Vdr(H301Q) rats had calcium concentrations falling between those of Vdr(R270L/H301Q) and WT rats. GM rats exhibited markedly elevated plasma parathyroid hormone and 1,25D3 levels compared to those of WT rats. An analysis of bone mineral density in the cortical bone of the femur in both GM rats revealed significantly lower values than in WT rats. Conversely, the bone mineral density in the trabecular bone was notably higher, indicating abnormal bone formation. This abnormal bone formation was more pronounced in Vdr(R270L/H301Q) rats than in Vdr(H301Q) rats, highlighting the critical role of the VDR-dependent function of 1,25D3 in bone formation. In contrast, neither Vdr(H301Q) nor Vdr(R270L/H301Q) rats exhibited symptoms of alopecia or cyst formation in the skin, which were observed in the Vdr-KO rats. These findings strongly suggest that unliganded VDR is crucial for maintaining the hair cycle and normal skin. Our GM rats hold significant promise for comprehensive analyses of vitamin D and VDR functions in future research.

Functional analysis of vitamin D receptor (VDR) using adenovirus vector.

Recently, we generated type II rickets model rats, including Vdr(R270L), Vdr(H301Q), Vdr(R270L/H301Q), and Vdr-knockout (KO), by genome editing. All generated animals showed symptoms of rickets, including growth retardation and abnormal bone formation. Among these, only Vdr-KO rats exhibited abnormal skin formation and alopecia. To elucidate the relationship between VDR function and rickets symptoms, each VDR was expressed in human HaCaT-VDR-KO cells using an adenovirus vector. We also constructed an adenovirus vector expressing VDR(V342M) corresponding to human VDR(V346M) which causes alopecia. We compared the nuclear translocation of VDRs after adding 1α,25-dihydroxyvitamin D3 (1,25D3) or 25-hydroxyvitamin D3 (25D3) at final concentrations of 10 and 100 nM, respectively. Both 25D3 and 1,25D3 induced the nuclear translocation of wild type VDR and VDR(V342M). Conversely, VDR(R270L) translocation was observed in the presence of 100 nM 25D3, with almost no translocation following treatment with 10 nM 1,25D3. VDR(R270L/H301Q) failed to undergo nuclear translocation. These results were consistent with their affinity for each ligand. Notably, VDR(R270L/H301Q) may exist in an unliganded form under physiological conditions, and factors interacting with VDR(R270L/H301Q) may be involved in the hair growth cycle. Thus, this novel system using an adenovirus vector could be valuable in elucidating vitamin D receptor functions.

Measuring CREB activation using bioluminescent probes that detect KID-KIX interaction in living cells.

The cyclic adenosine monophosphate response element-binding protein (CREB) is a transcription factor that contributes to memory formation. The transcriptional activity of CREB is induced by its phosphorylation at Ser-133 and subsequent interaction with the CREB-binding protein (CBP)/p300. We designed and optimized firefly split luciferase probe proteins that detect the interaction of the kinase-inducible domain (KID) of CREB and the KIX domain of CBP/p300. The increase in the light intensity of the probe proteins results from the phosphorylation of the responsible serine corresponding to Ser-133 of CREB. Because these proteins have a high signal-to-noise ratio and are nontoxic, it has become possible for the first time to carry out long-term measurement of KID-KIX interaction in living cells. Furthermore, we examined the usefulness of the probe proteins for future high-throughput cell-based drug screening and found several herbal extracts that activated CREB.

Robust osteogenic efficacy of 2α-heteroarylalkyl vitamin D analogue AH-1 in VDR (R270L) hereditary vitamin D-dependent rickets model rats.

Active vitamin D form 1α,25-dihydroxtvitamin D3 (1,25(OH)2D3) plays pivotal roles in calcium homeostasis and osteogenesis via its transcription regulation effect via binding to vitamin D receptor (VDR). Mutated VDR often causes hereditary vitamin D-dependent rickets (VDDR) type II, and patients with VDDR-II are hardly responsive to physiological doses of 1,25(OH)D3. Current therapeutic approaches, including high doses of oral calcium and supraphysiologic doses of 1,25(OH)2D3, have limited success and fail to improve the quality of life of affected patients. Thus, various vitamin D analogues have been developed as therapeutic options. In our previous study, we generated genetically modified rats with mutated Vdr(R270L), an ortholog of human VDR(R274L) isolated from the patients with VDDR-II. The significant reduced affinity toward 1,25(OH)2D3 of rat Vdr(R270L) enabled us to evaluate biological activities of exogenous VDR ligand without 1α-hydroxy group such as 25(OH)D3. In this study, 2α-[2-(tetrazol-2-yl)ethyl]-1α,25(OH)2D3 (AH-1) exerted much higher affinity for Vdr(R270L) in in vitro ligand binding assay than both 25(OH)D3 and 1,25(OH)2D3. A robust osteogenic activity of AH-1 was observed in Vdr(R270L) rats. Only a 40-fold lower dose of AH-1 than that of 25(OH)D3 was effective in ameliorating rickets symptoms in Vdr(R270L) rats. Therefore, AH-1 may be promising for the therapy of VDDR-II with VDR(R274L).

Evaluation of Anti-Adhesion Characteristics of Diamond-Like Carbon Film by Combining Friction and Wear Test with Step Loading and Weibull Analysis.

Anti-adhesion characteristics are important requirements for diamond-like carbon (DLC) films. The failure load corresponding to the anti-adhesion capacity varies greatly on three types of DLC film (hydrogen-free amorphous carbon film (a-C), hydrogenated amorphous carbon film (a-C:H), and tetrahedral hydrogen-free amorphous carbon film (ta-C)) in the friction and wear test with step loading using a high-frequency, linear-oscillation tribometer. Therefore, a new method that estimates a representative value of the failure load was developed in this study by performing a statistical analysis based on the Weibull distribution based on the assumption that the mechanism of delamination of a DLC film obeys the weakest link model. The failure load at the cumulative failure probabilities of 10% and 50% increased in the order ta-C < a-C:H < a-C and ta-C < a-C < a-C:H, respectively. The variation of the failure load, represented by the Weibull slope, was minimum on ta-C and maximum on a-C:H. The rank of the anti-adhesion capacity of each DLC film with respect to the load obtained by a constant load test agreed with the rank of the failure load on each DLC film at the cumulative failure probability of 10% obtained by Weibull analysis. It was found to be possible to evaluate the anti-adhesion capacity of a DLC film under more practical conditions by combining the step loading test and Weibull analysis.

Generation of novel genetically modified rats to reveal the molecular mechanisms of vitamin D actions.

Recent studies have suggested that vitamin D activities involve vitamin D receptor (VDR)-dependent and VDR-independent effects of 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 25-hydroxyvitamin D3 (25(OH)D3) and ligand-independent effects of the VDR. Here, we describe a novel in vivo system using genetically modified rats deficient in the Cyp27b1 or Vdr genes. Type II rickets model rats with a mutant Vdr (R270L), which recognizes 1,25(OH)2D3 with an affinity equivalent to that for 25(OH)D3, were also generated. Although Cyp27b1-knockout (KO), Vdr-KO, and Vdr (R270L) rats each showed rickets symptoms, including abnormal bone formation, they were significantly different from each other. Administration of 25(OH)D3 reversed rickets symptoms in Cyp27b1-KO and Vdr (R270L) rats. Interestingly, 1,25(OH)2D3 was synthesized in Cyp27b1-KO rats, probably by Cyp27a1. In contrast, the effects of 25(OH)D3 on Vdr (R270L) rats strongly suggested a direct action of 25(OH)D3 via VDR-genomic pathways. These results convincingly suggest the usefulness of our in vivo system.