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

Respiratory insufficiency as a rare presentation in a child with vitamin D-dependent rickets type 1

Rickets in children usually present with skeletal manifestations. However, they can also rarely present with extraskeletal manifestations, one of them being respiratory insufficiency. We present an unusual case of a girl in early childhood with respiratory insufficiency, which turned out to be due to the underlying vitamin D-dependent rickets (VDDR). The girl was born to third-degree consanguineous parentage and had progressive skeletal deformities, acquired loss of motor milestones, recurrent lower respiratory tract infections and florid signs of rickets. She was hospitalised for respiratory insufficiency, which required prolonged oxygen supplementation. X-rays showed multiple fractures and florid rickets. Laboratory parameters revealed low serum calcium and phosphorus, high alkaline phosphatase, parathyroid hormone and 25-OH vitamin D levels with a normal 1,25-OH2D level. Genetic testing confirmed autosomal recessive CYP27B1 mutation proving VDDR1A. She was started on calcitriol and calcium which led to her gradual improvement.

Ligand-Independent Vitamin D Receptor Actions Essential for Keratinocyte Homeostasis in the Skin

Recently, we demonstrated that the alopecia observed in vitamin D receptor gene-deficient (Vdr-KO) rats is not seen in rats with a mutant VDR(R270L/H301Q), which lacks ligand-binding ability, suggesting that the ligand-independent action of VDR plays a crucial role in maintaining the hair cycle. Since Vdr-KO rats also showed abnormalities in the skin, the relationship between alopecia and skin abnormalities was examined. To clarify the mechanism of actions of vitamin D and VDR in the skin, protein composition, and gene expression patterns in the skin were compared among Vdr-KO, Vdr-R270L/H301Q, and wild-type (WT) rats. While Vdr-R270L/H301Q rats exhibited normal skin formation similar to WT rats, Vdr-KO rats showed remarkable hyperkeratosis and trans-epidermal water loss in the skin. RNA sequencing and proteomic analysis revealed that the gene and protein expression patterns in Vdr-KO rats significantly differed from those in WT and Vdr-R270L/H301Q rats, with a marked decrease in the expression of factors involved in Shh, Wnt, and Bmp signaling pathways, a dramatic reduction in the expression of hair keratins, and a substantial increase in the expression of epidermal keratins. This study clearly demonstrated that non-liganded VDR is significantly involved in the differentiation, proliferation, and cell death of keratinocytes in hair follicles and the epidermis.

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.

Gene therapy for alopecia in type II rickets model rats using vitamin D receptor-expressing adenovirus vector

Type II rickets is a hereditary disease caused by a mutation in the vitamin D receptor (VDR) gene. The main symptoms of this disease are bone dysplasia and alopecia. Bone dysplasia can be ameliorated by high calcium intake; however, there is no suitable treatment for alopecia. In this study, we verified whether gene therapy using an adenoviral vector (AdV) had a therapeutic effect on alopecia in Vdr-KO rats. The VDR-expressing AdV was injected into six 7-week-old female Vdr-KO rats (VDR-AdV rats). On the other hand, control-AdV was injected into 7-week-old female rats (control-AdV rats); non-infected Vdr-KO rats (control rats) were also examined. The hair on the backs of the rats was shaved with hair clippers, and VDR-AdV or control-AdV was intradermally injected. Part of the back skin was collected from each rat after AdV administration. Hair follicles were observed using hematoxylin and eosin staining, and VDR expression was examined using immunostaining and western blotting. VDR-AdV rats showed significant VDR expression in the skin, enhanced hair growth, and low cyst formation, whereas control-AdV and non-infected rats did not show any of these effects. The effect of VDR-AdV lasted for nearly 60 days. These results indicate that gene therapy using VDR-AdV may be useful to treat alopecia associated with type II rickets, if multiple injections are possible after a sufficient period of time.

Vitamin D in hypoparathyroidism: insight into pathophysiology and perspectives in clinical practice

Hypoparathyroidism (HypoPT) is a rare endocrine disorder characterized by the absence or insufficient parathyroid hormone production resulting in chronic hypocalcemia. Complications of HypoPT include perturbation of several target organs. The conventional treatment consists of the administration of active vitamin D, namely calcitriol. Regarding vitamin D status, few data are available, mostly in HypoPT subjects supplemented with parent vitamin D. In addition, perturbation of vitamin D metabolism has been poorly investigated, as well as the contribution of altered vitamin D status on the clinical expression of the disease. The most recent consensus on the management of chronic HypoPT suggests the baseline evaluation of serum 25-hydroxy-vitamin D [25(OH)D] and supplementation with parent vitamin D with the aim to achieve and maintain serum 25(OH)D levels in the range of 30-50 ng/mL. The rationale for using supplementation with parent vitamin D (either ergocalciferol or cholecalciferol) in HypoPT would be to provide sufficient 25(OH)D substrate to the residual 1-α-hydroxylase activity, thus ensuring its conversion to active vitamin D in renal and extra-renal tissues. More data from experimental and clinical studies are needed for better assessing how these mechanisms may significantly influence metabolic control in HypoPT and eventually skeletal and extra-skeletal manifestation of the disease. Finally, future data will clarify how the currently available parent vitamin D compounds (ergocalciferol, cholecalciferol, calcifediol) would perform in addressing these specific issues.

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.

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)₂D_3, 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).

The role of vitamin D on redox regulation and cellular senescence

Vitamin D is considered an essential micronutrient for human health that is metabolized into a multifunctional secosteroid hormone. We can synthesize it in the skin through ultraviolet B (UVB) rays or acquire it from the diet. Its deficiency is a major global health problem that affects all ages and ethnic groups. Furthermore, dysregulation of vitamin D homeostasis has been associated with premature aging, driven by various cellular processes, including oxidative stress and cellular senescence. Various studies have shown that vitamin D can attenuate oxidative stress and delay cellular senescence, mainly by inducing the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and Klotho and improving mitochondrial homeostasis, proposing this vitamin as an excellent candidate for delaying aging. However, the mechanisms around these processes are not yet fully explored. Therefore, in this review, the effects of vitamin D on redox regulation and cellular senescence are discussed to propose new lines of research and clinical applications of vitamin D in the context of age-related diseases.

Recent Advances in the Production of Genome-Edited Rats

The rat is an important animal model for understanding gene function and developing human disease models. Knocking out a gene function in rats was difficult until recently, when a series of genome editing (GE) technologies, including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the type II bacterial clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated Cas9 (CRISPR/Cas9) systems were successfully applied for gene modification (as exemplified by gene-specific knockout and knock-in) in the endogenous target genes of various organisms including rats. Owing to its simple application for gene modification and its ease of use, the CRISPR/Cas9 system is now commonly used worldwide. The most important aspect of this process is the selection of the method used to deliver GE components to rat embryos. In earlier stages, the microinjection (MI) of GE components into the cytoplasm and/or nuclei of a zygote was frequently employed. However, this method is associated with the use of an expensive manipulator system, the skills required to operate it, and the egg transfer (ET) of MI-treated embryos to recipient females for further development. In vitro electroporation (EP) of zygotes is next recognized as a simple and rapid method to introduce GE components to produce GE animals. Furthermore, in vitro transduction of rat embryos with adeno-associated viruses is potentially effective for obtaining GE rats. However, these two approaches also require ET. The use of gene-engineered embryonic stem cells or spermatogonial stem cells appears to be of interest to obtain GE rats; however, the procedure itself is difficult and laborious. Genome-editing via oviductal nucleic acids delivery (GONAD) (or improved GONAD (i-GONAD)) is a novel method allowing for the in situ production of GE zygotes existing within the oviductal lumen. This can be performed by the simple intraoviductal injection of GE components and subsequent in vivo EP toward the injected oviducts and does not require ET. In this review, we describe the development of various approaches for producing GE rats together with an assessment of their technical advantages and limitations, and present new GE-related technologies and current achievements using those rats in relation to human diseases.

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.

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 T_max 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.

Rat models of human diseases and related phenotypes: a systematic inventory of the causative genes

The laboratory rat has been used for a long time as the model of choice in several biomedical disciplines. Numerous inbred strains have been isolated, displaying a wide range of phenotypes and providing many models of human traits and diseases. Rat genome mapping and genomics was considerably developed in the last decades. The availability of these resources has stimulated numerous studies aimed at discovering causal disease genes by positional identification. Numerous rat genes have now been identified that underlie monogenic or complex diseases and remarkably, these results have been translated to the human in a significant proportion of cases, leading to the identification of novel human disease susceptibility genes, helping in studying the mechanisms underlying the pathological abnormalities and also suggesting new therapeutic approaches. In addition, reverse genetic tools have been developed. Several genome-editing methods were introduced to generate targeted mutations in genes the function of which could be clarified in this manner [generally these are knockout mutations]. Furthermore, even when the human gene causing a disease had been identified without resorting to a rat model, mutated rat strains (in particular KO strains) were created to analyze the gene function and the disease pathogenesis. Today, over 350 rat genes have been identified as underlying diseases or playing a key role in critical biological processes that are altered in diseases, thereby providing a rich resource of disease models. This article is an update of the progress made in this research and provides the reader with an inventory of these disease genes, a significant number of which have similar effects in rat and humans.