Vitamin D receptor overexpression in osteoblasts and osteocytes prevents bone loss during vitamin D-deficiency

Real-world usage of Chronic Kidney Disease - Mineral Bone Disorder (CKD-MBD) biomarkers in nephrology practices

Background

Chronic kidney disease mineral bone disorder (CKD-MBD) is a condition characterized by alterations of calcium, phosphate, parathyroid hormone (PTH), and fibroblast growth factor 23 (FGF-23) metabolism that in turn promote bone disorders, vascular calcifications, and increase cardiovascular (CV) risk. Nephrologists' awareness of diagnostic, prognostic, and therapeutic tools to manage CKD-MBD plays a primary role in adequately preventing and managing this condition in clinical practice.

Methods

A national survey (composed of 15 closed questions) was launched to inquire about the use of bone biomarkers in the management of CKD-MBD patients by nephrologists and to gain knowledge about the implementation of guideline recommendations in clinical practice.

Results

One hundred and six Italian nephrologists participated in the survey for an overall response rate of about 10%. Nephrologists indicated that the laboratories of their hospitals were able to satisfy request of ionized calcium levels, 105 (99.1%) of both PTH and alkaline phosphatase (ALP), 100 (94.3%) of 25(OH)D, and 61 (57.5%) of 1.25(OH)2D; while most laboratories did not support the requests of biomarkers such as FGF-23 (intact: 88.7% and c-terminal: 93.4%), Klotho (95.3%; soluble form: 97.2%), tartrate-resistant acid phosphatase 5b (TRAP-5b) (92.5%), C-terminal telopeptide (CTX) (71.7%), and pro-collagen type 1 N-terminal pro-peptide (P1NP) (88.7%). As interesting data regarding Italian nephrologists' behavior to start treatment of secondary hyperparathyroidism (sHPT), the majority of clinicians used KDOQI guidelines (n = 55, 51.9%). In contrast, only 40 nephrologists (37.7%) relied on KDIGO guidelines, which recommended referring to values of PTH between two and nine times the upper limit of the normal range.

Conclusion

Results point out a marked heterogeneity in the management of CKD-MBD by clinicians as well as a suboptimal implementation of guidelines in Italian clinical practice.

Effects of rearing systems on the eggshell quality, bone parameters and expression of genes related to bone remodeling in aged laying hens

Public concerns regarding animal welfare are changing the selection of rearing systems in laying hens. This study investigated the effects of rearing systems on eggshell quality, bone parameters and relative expression levels of genes related to bone remodeling in aged laying hens. A total of 2,952 55-day-old Jing Tint Six pullets were randomly assigned to place in the conventional caging system (CCS) or aviary system (AVS) and kept until 95 weeks of age. The AVS group delayed the decrease of eggshell quality and alleviated the symptoms of osteoporosis in the humerus rather than in the femur. Eggshell breaking strength, thickness, weight, weight ratio, stiffness and fracture toughness were decreased linearly with age (from 55 to 95 weeks of age, p < 0.05). The AVS group had higher eggshell breaking strength, stiffness and fracture toughness than the CCS group (p < 0.05). Higher total calcium and phosphorus per egg were presented in the AVS group at 95 weeks of age (p < 0.05). At 95 weeks of age, the AVS group had a humerus with higher weight, volume, length, midpoint perimeter, cortical index, fat-free dry weight, ash content, total calcium per bone, total phosphorus per bone, average bone mineral density, strength, stiffness and work to fracture compared to the CCS group (p < 0.05). Such differences did not appear in the femur. The relative expression levels of alkaline phosphatase (ALP) and osteocalcin (OCN) genes in the femur and hormone receptors (vitamin D receptor (VDR), estrogen receptor alpha (ERα) and fibroblast growth factor 23 (FGF23)) genes in the humerus were significantly upregulated (p < 0.05) in the AVS group. The level of tartrate-resistant acid phosphatase (TRAP) transcripts was also increased (p < 0.05) in the femur of the AVS group. Overall, compared with the CCS, the AVS alleviated the deterioration of eggshell and bone qualities of aged laying hens, which may be related to the changes in the expression of genes associated with bone remodeling.

Renal hyperparathyroidism

The number of the patients with chronic kidney disease is now increasing in the world. The pathophysiology of renal hyperparathyroidism is closely associated with Klotho-FGF-endocrine axes, which must be solved definitively as early as possible. It was revealed that the expression of fgf23 is activated by calciprotein particles, which induces vascular ossification. And it is well known that phosphorus overload directly increases parathyroid hormone and hyperparathyroid bone disease develops in those subjects. On the other hand, low turnover bone disease is often recently. Both the patients with chronic kidney disease suffering from hyperparathyroid bone disease or low turnover bone disease are associated with increased fracture risk. Micropetrosis may be one of the causes of increased fracture risk in the subjects with low turnover bone disease. In this chapter, we now describe the diagnosis, pathophysiology and treatments of renal hyperparathyroidism.

High-Throughput Absolute Quantification Sequencing Revealed Osteoporosis-Related Gut Microbiota Alterations in Han Chinese Elderly

Objective

Accumulative evidence suggests that gut microbiota play an important role in bone remodeling and hence bone health maintenance. This study aimed to explore the association of gut microbiota with the risk of osteoporosis and to identify potential disease-related taxa, which may be promising targets in osteoporosis prevention and treatment in the future.

Methods

Absolute quantification 16S ribosomal RNA gene sequencing was used to detect absolute and relative abundances of gut microbiota in 44 patients with osteoporosis and 64 controls. In combination with one of our previous studies, a total of 175 samples were involved in the relative abundance analysis.

Results

Compared with the controls, the patients with osteoporosis had higher absolute and relative abundances of Bacteroidetes phylum, and Bacteroides and Eisenbergiella genera. The absolute abundances of Clostridium_XlVa, Coprococcus, Lactobacillus, and Eggerthella genera increased, and that of the Veillonella genus decreased in the osteoporosis group. As for relative abundance, that of the Parabacteroides and Flavonifractor genera increased, whereas that of the Raoultella genus decreased in the osteoporosis group. Controlling for potential confounders, the associations of Clostridium_XlVa, Coprococcus, and Veillonella genera with the risk of osteoporosis did not maintain significance. Ridge regression analysis suggested that Bacteroides is associated with reduced bone mineral density (BMD) and T-score at lumbar spines, and Anaerovorax is associated with increased BMD at the femoral neck. Functional predictions revealed that 10 Kyoto Encyclopedia of Genes and Genomes pathways were enriched in the osteoporosis group.

Conclusions

Gut microbiota compositions may contribute to the risk of osteoporosis. Several specific taxa and functional pathways are identified to associate with reduced bone density, thus providing epidemiologic evidence for the potential role of aberrant gut microbiota in osteoporosis pathogenesis.

1,25(OH)2D3 ameliorates palmitate-induced lipotoxicity in human primary osteoblasts leading to improved viability and function

Contributing to bone loss with aging is a progressive reduction in osteoblast number and function leading to decreased bone formation. In aging bone, mesenchymal stem cells decrease in number and their differentiation potential into osteoblasts is reduced. Instead, there is a shift towards adipogenic differentiation and increased lipid accumulation in the marrow of osteoporotic bones. Bone marrow adipocytes produce palmitic acid (PA), a saturated fatty acid, which is toxic to osteoblasts in vitro. Vitamin D (1,25(OH)₂D₃) stimulates osteoblastogenesis and has known anti-apoptotic effects on osteoblasts, as such it may protect human primary osteoblasts from PA-induced lipotoxicity. Here, the effects of PA (250 μM) or 1,25(OH)₂D₃ (10^-8 M), alone or in combination, on osteoblast differentiation and mineralization, viability and autophagy were investigated. In PA-treated osteoblasts, 1,25(OH)₂D₃ ameliorated the decrease in the mRNA transcript abundance of representative palmitoylation (ZDHHC1, ZDHHC2 and ZDHHC12) and osteogenic (alkaline phosphatase and osteocalcin) genes. Collectively these gene regulate signaling pathways pertinent to osteoblastogenesis. In osteoblasts treated with PA and 1,25(OH)₂D₃, the capacity to undergo differentiation and mineralization was recovered and cell viability was increased when compared to osteoblasts treated with PA alone. 1,25(OH)₂D₃, irrespective of PA treatment, increased the expression of key osteogenic signaling proteins; specifically, SMAD1-3,5, Runx2 and β-catenin. 1,25(OH)₂D₃ also attenuated the high level of impaired autophagy induced by PA and potentiated a shift towards activated, functional autophagy and increased flux through autolysosomes. Altogether, these findings provide in vitro evidence regarding the potential of 1,25(OH)₂D₃ to protect osteoblasts from lipotoxicity by modulating autophagy and facilitating cell differentiation, which may enhance bone formation in an osteoporotic microenvironment with a high level of marrow adipose tissue.

The Importance of Biologically Active Vitamin D for Mineralization by Osteocytes After Parathyroidectomy for Renal Hyperparathyroidism

Hypomineralized matrix is a factor determining bone mineral density. Increased perilacunar hypomineralized bone area is caused by reduced mineralization by osteocytes. The importance of vitamin D in the mineralization by osteocytes was investigated in hemodialysis patients who underwent total parathyroidectomy (PTX) with immediate autotransplantation of diffuse hyperplastic parathyroid tissue. No previous reports on this subject exist. The study was conducted in 19 patients with renal hyperparathyroidism treated with PTX. In 15 patients, the serum calcium levels were maintained by subsequent administration of alfacalcidol (2.0 μg/day), i.v. calcium gluconate, and oral calcium carbonate for 4 weeks after PTX (group I). This was followed in a subset of 4 patients in group I by a reduced dose of 0.5 μg/day until 1 year following PTX; this was defined as group II. In the remaining 4 patients, who were not in group I, the serum calcium (Ca) levels were maintained without subsequent administration of alfacalcidol (group III). Transiliac bone biopsy specimens were obtained in all groups before and 3 or 4 weeks after PTX to evaluate the change of the hypomineralized bone area. In addition, patients from group II underwent a third bone biopsy 1 year following PTX. A significant decrease of perilacunar hypomineralized bone area was observed 3 or 4 weeks after PTX in all group I and II patients. The area was increased again in the group II patients 1 year following PTX. In group III patients, an increase of the hypomineralized bone area was observed 4 weeks after PTX. The maintenance of a proper dose of vitamin D is necessary for mineralization by osteocytes, which is important to increase bone mineral density after PTX for renal hyperparathyroidism. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Osteocytic perilacunar/canalicular turnover in hemodialysis patients with high and low serum PTH levels

Osteocytic perilacunar/canalicular turnover in hemodialysis patients has not yet been reported. Osteocyte lacunae in lamellar bone and woven bone were classified as eroded surface-, osteoid surface-, and quiescent surface-predominant osteocyte lacunae (ES-Lc, OS-Lc, QS-Lc, respectively) in 55 hemodialysis patients with either high- (n = 45) or low- (n = 10) parathyroid hormone levels, and 19 control subjects without chronic kidney disease. We calculated the area and number of ES-Lc, OS-Lc, and QS-Lc. The mineralized surface on the osteocyte lacunar walls was measured in each group, and compared among the three groups. The shapes of the osteocyte lacunar walls were validated by backscattered electron microscopy. While the number of ES-Lc per bone area (N.ES-Lc/B.Ar) was higher than the number of OS-Lc per bone area (N.OS-Lc/B.Ar) in all groups, N.ES-Lc/B.Ar and N.OS-Lc/B.Ar were greater in high-parathyroid hormone group than in low-parathyroid hormone and control groups. The total volume of ES-Lc per bone area (ES-Lc.Ar/B.Ar) was greater than the total volume of OS-Lc per bone area (OS-Lc.Ar/B.Ar) in both parathyroid hormone groups. However, both lacunar erosion and lacunar formation increased proportionally, suggesting that global coupling between them was maintained. N.ES-Lc/B.Ar was higher in woven bone than in lamellar bone. The rate of OS-Lc stained by tetracycline hydrochloride, the mineralized lacunar surface and the mean area of OS-Lc with Tc obtained from both parathyroid hormone groups were greater than those in the control group. We conclude that osteocytic perilacunar/canalicular turnover is increased in hemodialysis patients with high parathyroid hormone levels. Osteocytic perilacunar/canalicular turnover depends, at least in part, on serum parathyroid hormone level. However, the ideal PTH level for osteocytic perilacunar/canalicular turnover could not be determined but osteocytic osteolysis was predominant in both the high- and low-PTH groups in this study. Thus, attention should be paid to bone loss from the viewpoint of osteocytic perilacunar/canalicular turnover in hemodialysis patients.

Case report: Electron microscopic evaluation of bone from a patient treated with cinacalcet hydrochloride, maxacalcitol, and alfacalcidol for hyperparathyroid bone disease with secondary hyperparathyroidism

Evaluation of bone is of great importance in chronic kidney disease patients, as these patients are at an increased risk for fractures. We treated a hemodialysis patient suffering from hyperparathyroid bone disease with cinacalcet hydrochloride and concurrent administration of maxacalcitol and alfacalcidol for a year. Hyperparathyroid bone disease is characterized by cortical thinning, increased cortical porosity, reduced trabecular bone volume, and increased hypomineralized matrix volume, and there is little information to date about the effects of treatment with cinacalcet hydrochloride on the bone fragility in patients with hyperparathyroid bone disease. In the present study, histological and backscattered electron microscopic evaluation of this combination treatment revealed an excellent improvement of both bone volume and bone morphology. This treatment improved cortical thinning, cortical porosity, and trabecular thinning. Furthermore, the treatment also reduced hypomineralized matrix volume, indicative of improved mineralization by osteocytes. We speculate that the intermittent maxacalcitol administration may have effectively stimulated the vitamin D receptors expressed on osteocytes and osteoblasts, resulting in increased mineralization. Our approach for evaluating the bone in patients with chronic kidney disease by backscattered electron microscopy is novel.

Vitamin D regulates osteocyte survival and perilacunar remodeling in human and murine bone

Osteocytes are the most abundant bone cells and are highly regulated by external stimuli. Vitamin D and osteocytes cooperatively regulate bone remodeling as well as phosphate and calcium homeostasis. However, it is unclear if vitamin D regulates osteocyte number, connectivity or size in the setting of altered bone formation or impaired mineralization. Sixty iliac crest biopsies of patients with varying vitamin D levels were examined to analyze osteocyte number, osteocyte connectivity and osteocyte viability using high-resolution imaging. Osteocyte parameters were also quantified in mice lacking the vitamin D receptor (Vdr-/-) and in wildtype littermates. The cortical and cancellous bone of patients with vitamin D deficiency exhibited a significant decrease in the number of viable osteocytes, as well as increased osteocyte apoptosis and impaired osteocyte connectivity, based on evaluation of the canalicular network. The number of osteocytes was also decreased in Vdr-deficient mice, in comparison to wildtype controls, and this was accompanied by enlargement of osteocyte lacunae. A high calcium diet normalized the osteocyte lacunar area in Vdr-deficient mice, but failed to normalize osteocyte number. Thus, a diet-independent decrease in osteocyte number in Vdr-deficient mice suggests a mechanism that is directly dependent on the VDR, since vitamin D may promote the transition from osteoblasts to osteocytes. The increase in lacunar area the in Vdr-deficient mice, which is normalized by the high calcium diet suggests this phenotype is due to osteocytic osteolysis. These investigations demonstrate that vitamin D plays a role in the regulation of osteocyte number and perilacunar remodeling.

Vitamin D Activity and Metabolism in Bone

PURPOSE OF REVIEW

In addition to the actions of the endocrine hormone, 1alpha,25-dihydroxyvitamin D (1,25(OH)₂D) in stimulating intestinal calcium absorption, the regulation of bone mineral metabolism by 1,25(OH)₂D is also considered an important contributor to calcium homeostasis. However, recent evidence suggest that 1,25(OH)₂D acting either via endocrine or autocrine pathways plays varied roles in bone, which suggests that vitamin D contributes to the maintenance of bone mineral in addition to its catabolic roles. This review highlights the contrasting evidence for the direct action for vitamin D metabolism and activity in bone.

RECENT FINDINGS

Numerous cells within bone express vitamin D receptor (VDR), synthesise and catabolise 1,25(OH)₂D via 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1), and 25-hydroxyvitamin D 24-hydroxylase (CYP24A1) enzymes, respectively. Recent evidence suggests that all three genes are required to regulate processes of bone resorption, mineralization and fracture repair. The actions of vitamin D in bone appear to negatively or positively regulate bone mineral depending on the physiological and pathological circumstances, suggesting that vitamin D plays pleiotropic roles in bone.

LSD1 dual function in mediating epigenetic corruption of the vitamin D signaling in prostate cancer

BACKGROUND

Lysine-specific demethylase 1A (LSD1) is a key regulator of the androgen (AR) and estrogen receptors (ER), and LSD1 levels correlate with tumor aggressiveness. Here, we demonstrate that LSD1 regulates vitamin D receptor (VDR) activity and is a mediator of 1,25(OH)₂-D₃ (vitamin D) action in prostate cancer (PCa).

METHODS

Athymic nude mice were xenografted with CWR22 cells and monitored weekly after testosterone pellet removal. Expression of LSD1 and VDR (IHC) were correlated with tumor growth using log-rank test. TRAMP tumors and prostates from wild-type (WT) mice were used to evaluate VDR and LSD1 expression via IHC and western blotting. The presence of VDR and LSD1 in the same transcriptional complex was evaluated via immunoprecipitation (IP) using nuclear cell lysate. The effect of LSD1 and 1,25(OH)₂-D₃ on cell viability was evaluated in C4-2 and BC1A cells via trypan blue exclusion. The role of LSD1 in VDR-mediated gene transcription was evaluated for Cdkn1a, E2f1, Cyp24a1, and S100g via qRT-PCR-TaqMan and via chromatin immunoprecipitation assay. Methylation of Cdkn1a TSS was measured via bisulfite sequencing, and methylation of a panel of cancer-related genes was quantified using methyl arrays. The Cancer Genome Atlas data were retrieved to identify genes whose status correlates with LSD1 and DNA methyltransferase 1 (DNMT1). Results were correlated with patients' survival data from two separate cohorts of primary and metastatic PCa.

RESULTS

LSD1 and VDR protein levels are elevated in PCa tumors and correlate with faster tumor growth in xenograft mouse models. Knockdown of LSD1 reduces PCa cell viability, and gene expression data suggest a dual coregulatory role of LSD1 for VDR, acting as a coactivator and corepressor in a locus-specific manner. LSD1 modulates VDR-dependent transcription by mediating the recruitment of VDR and DNMT1 at the TSS of VDR-targeted genes and modulates the epigenetic status of transcribed genes by altering H3K4me2 and H3K9Ac and DNA methylation. Lastly, LSD1 and DNMT1 belong to a genome-wide signature whose expression correlates with shorter progression-free survival and overall survival in primary and metastatic patients' samples, respectively.

CONCLUSIONS

Results demonstrate that LSD1 has a dual coregulatory role as corepressor and coactivator for VDR and defines a genomic signature whose targeting might have clinical relevance for PCa patients.

Effect of Prenatal versus Postnatal Vitamin D Deficiency on Pulmonary Structure and Function in Mice

Epidemiologic studies have linked gestational vitamin D deficiency to respiratory diseases, although mechanisms have not been defined. We hypothesized that antenatal vitamin D deficiency would impair airway development and alveolarization in a mouse model. We studied the effect of antenatal vitamin D deficiency by inducing it in pregnant mice and then compared lung development and function in their offspring to littermate controls. Postnatal vitamin D deficiency and sufficiency models from each group were also studied. We developed a novel tracheal ultrasound imaging technique to measure tracheal diameter in vivo. Histological analysis estimated tracheal cartilage total area and thickness. We found that vitamin D-deficient pups had reduced tracheal diameter with decreased tracheal cartilage minimal width. Vitamin D deficiency increased airway resistance and reduced lung compliance, and led to alveolar simplification. Postnatal vitamin D supplementation improved lung function and radial alveolar count, a parameter of alveolar development, but did not correct tracheal narrowing. We conclude that antenatal vitamin D deficiency impairs airway and alveolar development and limits lung function. Reduced tracheal diameter, cartilage irregularity, and alveolar simplification in vitamin D-deficient mice may contribute to increased airways resistance and diminished lung compliance. Vitamin D supplementation after birth improved lung function and, potentially, alveolar simplification, but did not improve defective tracheal structure. This mouse model offers insight into the mechanisms of vitamin D deficiency-associated lung disease and provides an in vivo model for investigating preclinical preventive and therapeutic strategies.

Nuclear Receptors in Skeletal Homeostasis

Nuclear receptors are a family of transcription factors that can be activated by lipophilic ligands. They are fundamental regulators of development, reproduction, and energy metabolism. In bone, nuclear receptors enable bone cells, including osteoblasts, osteoclasts, and osteocytes, to sense their dynamic microenvironment and maintain normal bone development and remodeling. Our views of the molecular mechanisms in this process have advanced greatly in the past decade. Drugs targeting nuclear receptors are widely used in the clinic for treating patients with bone disorders such as osteoporosis by modulating bone formation and resorption rates. Deficiency in the natural ligands of certain nuclear receptors can cause bone loss; for example, estrogen loss in postmenopausal women leads to osteoporosis and increases bone fracture risk. In contrast, excessive ligands of other nuclear receptors, such as glucocorticoids, can also be detrimental to bone health. Nonetheless, the ligand-induced osteoprotective effects of many other nuclear receptors, e.g., vitamin D receptor, are still in debate and require further characterizations. This review summarizes previous studies on the roles of nuclear receptors in bone homeostasis and incorporates the most recent findings. The advancement of our understanding in this field will help researchers improve the applications of agonists, antagonists, and selective modulators of nuclear receptors for therapeutic purposes; in particular, determining optimal pharmacological drug doses, preventing side effects, and designing new drugs that are more potent and specific.

Sex-related differences in the skeletal phenotype of aged vitamin D receptor global knockout mice

The role of the vitamin D receptor (VDR) in maintaining skeletal health appears to be complex and dependent on the physiological context. Global Vdr deletion in a mouse model (Vdr^-/-) results in hypocalcemia, secondary hyperparathyroidism and bone features typical of vitamin D-dependent rickets type II. When weanling Vdr^-/- mice are fed a diet containing high levels of calcium, phosphorus and lactose, termed the rescue diet, normalisation of serum calcium, phosphate and parathyroid hormone levels results in prevention of rickets at 10 weeks of age. However, 17 week old male Vdr^-/- mice, fed the rescue diet, have been reported as osteopenic due to a decrease in bone formation when compared to wild type mice. We now report confirmation of this finding with further data on the effect of the rescue diet on appendicular and axial skeletal structures in male and female Vdr^-/- mice at 26 weeks of age compared to Vdr^+/- controls. All Vdr^-/- mice were normocalcemic with no evidence of any mineralization defect. However, male Vdr^-/- mice exhibited significantly reduced mineral in femoral and vertebral bones when compared to control littermate Vdr^+/- mice, consistent with the previously reported data. In contrast, 26-week-old female Vdr^-/- mice demonstrated significantly increased femoral trabecular bone volume although there was decreased vertebral trabecular bone volume, similar to males, and femoral cortical bone volume was unchanged. Thus, the Vdr^-/- mouse model displays sex- and site-specific differences in skeletal structures with long-term feeding of a rescue diet. Although the global Vdr^-/- ablation does not permit the determination of skeletal mechanisms producing these differences, these data confirm skeletal changes even when fed the rescue diet.

Skeletal characterization of an osteoblast-specific vitamin D receptor transgenic (ObVDR-B6) mouse model

BACKGROUND

Overexpression of the human vitamin D receptor (hVDR) transgene under control of the human osteocalcin promoter in FVB/N mice (OSVDR) was previously demonstrated to exhibit increased cortical and trabecular bone volume and strength due to decreased bone resorption and increased bone formation. An important question to address is whether the OSVDR bone phenotype persists on an alternative genetic background such as C57Bl6/J.

METHODS

OSVDR mice (OSV3 line) were backcrossed onto the C57Bl6/J genetic background for at least 6 generations to produce OSVDR mice with 98.4% C57Bl6/J congenicity (ObVDR-B6 mice). Hemizygous male and female ObVDR-B6 and littermate wild-type (WT) mice were fed a standard laboratory chow diet and killed at 3, 9 and 20 weeks of age for analyses of biochemical and structural variables and dynamic indices of bone histomorphometry.

RESULTS

At 9 weeks of age, both cortical and trabecular femoral bone volumes were increased in both male and female ObVDR-B6 mice, when compared to WT levels (P<0.05), without systemic changes to calciotropic parameters. The increase in femoral trabecular bone volume was associated with increase in MAR (P<0.01) and reduced osteoclast size (P<0.05). However, in female mice trabecular bone volume was unchanged in femoral metaphysis of 20 weeks mice and in vertebra both at 9 and 20 weeks of age. Increased cortical bone in both male and female ObVDR-B6 mice was due largely to increased periosteal expansion and was associated with increased cortical strength at 20 weeks of age.

CONCLUSION

Overexpression of the human VDR gene in mature osteoblasts of C57Bl6/J mice increases cortical and trabecular bone volumes and confirms the previous reports of increased bone in OSVDR mice on the FVB/N background. However, site-specific and gender-related differences in bone volume suggest that the effects of osteoblast-specific VDR overexpression are more complex than hitherto recognised.

Long-term vitamin D deficiency in older adult C57BL/6 mice does not affect bone structure, remodeling and mineralization

Animal models show that vitamin D deficiency may have severe consequences for skeletal health. However, most studies have been performed in young rodents for a relatively short period, while in older adult rodents the effects of long-term vitamin D deficiency on skeletal health have not been extensively studied. Therefore, the first aim of this study was to determine the effects of long-term vitamin D deficiency on bone structure, remodeling and mineralization in bones from older adult mice. The second aim was to determine the effects of long-term vitamin D deficiency on mRNA levels of genes involved in vitamin D metabolism in bones from older adult mice. Ten months old male C57BL/6 mice were fed a diet containing 0.5% calcium, 0.2% phosphate and 0 (n=8) or 1 (n=9) IU vitamin D₃/gram for 14 months. At an age of 24 months, mice were sacrificed for histomorphometric and micro-computed tomography (micro-CT) analysis of humeri as well as analysis of CYP27B1, CYP24 and VDR mRNA levels in tibiae and kidneys using RT-qPCR. Plasma samples, obtained at 17 and 24 months of age, were used for measurements of 25-hydroxyvitamin D (25(OH)D) (all samples), phosphate and parathyroid hormone (PTH) (terminal samples) concentrations. At the age of 17 and 24 months, mean plasma 25(OH)D concentrations were below the detection limit (<4nmol/L) in mice receiving vitamin D deficient diets. Plasma phosphate and PTH concentrations did not differ between both groups. Micro-CT and histomorphometric analysis of bone mineral density, structure and remodeling did not reveal differences between control and vitamin D deficient mice. Long-term vitamin D deficiency did also not affect CYP27B1 mRNA levels in tibiae, while CYP24 mRNA levels in tibiae were below the detection threshold in both groups. VDR mRNA levels in tibiae from vitamin D deficient mice were 0.7 fold lower than those in control mice. In conclusion, long-term vitamin D deficiency in older adult C57BL/6 mice, accompanied by normal plasma PTH and phosphate concentrations, does not affect bone structure, remodeling and mineralization. In bone, expression levels of CYP27B1 are also not affected by long-term vitamin D deficiency in older adult C57BL/6 mice. Our results suggest that mice at old age have a low or absent response to vitamin D deficiency probably due to factors such as a decreased bone formation rate or a reduced response of bone cells to 25(OH)D and 1,25(OH)₂D. Older adult mice may therefore be less useful for the study of the effects of vitamin D deficiency on bone health in older people.

Animal models for glucocorticoid-induced postmenopausal osteoporosis: An updated review

Glucocorticoid-induced postmenopausal osteoporosis is a severe osteoporosis, with high risk of major osteoporotic fractures. This severe osteoporosis urges more extensive and deeper basic study, in which suitable animal models are indispensable. However, no relevant review is available introducing this model systematically. Based on the recent studies on GI-PMOP, this brief review introduces the GI-PMOP animal model in terms of its establishment, evaluation of bone mass and discuss its molecular mechanism. Rat, rabbit and sheep with their respective merits were chosen. Both direct and indirect evaluation of bone mass help to understand the bone metabolism under different intervention. The crucial signaling pathways, miRNAs, osteogenic- or adipogenic- related factors and estrogen level may be the predominant contributors to the development of glucocorticoid-induced postmenopausal osteoporosis.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Bone muscle interactions and vitamin D

Beyond the established roles of vitamin D in bone and mineral homeostasis, we are becoming increasingly aware of its diverse effects in skeletal muscle. Subjects with severe vitamin D deficiency or mutations of the vitamin D receptor develop generalized atrophy of muscle and bone, suggesting coordinated effects of vitamin D in musculoskeletal physiology. At a mechanistic level, vitamin D exerts wide-ranging effects in muscle and bone calcium handling, differentiation and development. Vitamin D also modulates muscle and bone-derived hormones, facilitating cross-talk between these tissues. In this review, we discuss emerging evidence that vitamin D regulates bone and muscle in a direct, integrated fashion, positioning the vitamin D pathway as a potential therapeutic target for musculoskeletal diseases. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Dietary diallyl disulfide supplementation attenuates ethanol-mediated pulmonary vitamin D speciate depletion in C57Bl/6 mice

Background

Slightly more than 5 % of the United States population heavily consumes ethanol, i.e., more than 14 drinks for men and 7 drinks for women a week. Chronic ethanol consumption can result in increased liver disease, reduced recovery from burn injury, and more frequent and severe respiratory infections. Chronic ethanol over-consumption also leads to vitamin D dysmetabolism and depletion. Vitamin D is a fat-soluble pro-hormone that regulates musculoskeletal health, cellular proliferation/differentiation, and innate and adaptive immune response.

Methods

In this study, C57BL/6 mice were fed 20 % ethanol in their water ad libitum for 7 weeks. Some mice were fed either a standard chow or a modified diet containing 0.15 μg/day of diallyl disulfide (DADS). Whole blood, lung tissue, and bronchial alveolar lavage fluid (BALF) were collected at sacrifice and analyzed for 25(OH) D₃, 1,25 (OH)₂D₃, vitamin D receptor VDR, CYP2E1, and CYP27B1 levels.

Results

Ethanol reduced 25(OH) D₃ and 1,25 (OH)₂D₃ in lung tissue and BALF on average 31 %. The largest ethanol-mediated reduction was in the 1,25 (OH)₂D₃ (42 %) measured in the BALF. Dietary supplementation of DADS restored BALF and lung tissue protein of 25(OH) D₃ and 1,25(OH)₂D₃ to control levels. Chronic ethanol consumption also resulted in tissue increases of vitamin D response (VDR) protein, Cyp2E1, and reductions in vitamin D-activating enzyme CYP27B1. All three of these effects were attenuated by dietary supplementation of DADS.

Conclusions

In conclusion, the pulmonary metabolic disturbances mediated by chronic ethanol consumption as measured by 1,25(OH)₂D₃ protein levels, epithelial lining fluid, and lung tissue can be ameliorated by dietary supplementation of DADS in C57BL/6 mice.

Vitamin D, muscle and bone: Integrating effects in development, aging and injury

Beyond the established effects of muscle loading on bone, a complex network of hormones and growth factors integrates these adjacent tissues. One such hormone, vitamin D, exerts broad-ranging effects in muscle and bone calcium handling, differentiation and development. Vitamin D also modulates muscle and bone-derived hormones, potentially facilitating cross-talk between these tissues. In the clinical setting, vitamin D deficiency or mutations of the vitamin D receptor result in generalized atrophy of muscle and bone, suggesting coordinated effects of vitamin D at these sites. In this review, we discuss emerging evidence that vitamin D exerts specific effects throughout the life of the musculoskeletal system - in development, aging and injury. From this holistic viewpoint, we offer new insights into an old debate: whether vitamin D's effects in the musculoskeletal system are direct via local VDR signals or indirect via its systemic effects in calcium and phosphate homeostasis.

Vitamin D Receptor Gene Ablation in the Conceptus Has Limited Effects on Placental Morphology, Function and Pregnancy Outcome

Vitamin D deficiency has been implicated in the pathogenesis of several pregnancy complications attributed to impaired or abnormal placental function, but there are few clues indicating the mechanistic role of vitamin D in their pathogenesis. To further understand the role of vitamin D receptor (VDR)-mediated activity in placental function, we used heterozygous Vdr ablated C57Bl6 mice to assess fetal growth, morphological parameters and global gene expression in Vdr null placentae. Twelve Vdr^+/- dams were mated at 10–12 weeks of age with Vdr^+/- males. At day 18.5 of the 19.5 day gestation in our colony, females were euthanised and placental and fetal samples were collected, weighed and subsequently genotyped as either Vdr^+/+, Vdr^+/-or Vdr^-/-. Morphological assessment of placentae using immunohistochemistry was performed and RNA was extracted and subject to microarray analysis. This revealed 25 genes that were significantly differentially expressed between Vdr^+/+ and Vdr^-/- placentae. The greatest difference was a 6.47-fold change in expression of Cyp24a1 which was significantly lower in the Vdr^-/- placentae (P<0.01). Other differentially expressed genes in Vdr^-/- placentae included those involved in RNA modification (Snord123), autophagy (Atg4b), cytoskeletal modification (Shroom4), cell signalling (Plscr1, Pex5) and mammalian target of rapamycin (mTOR) signalling (Deptor and Prr5). Interrogation of the upstream sequence of differentially expressed genes identified that many contain putative vitamin D receptor elements (VDREs). Despite the gene expression differences, this did not contribute to any differences in overall placental morphology, nor was function affected as there was no difference in fetal growth as determined by fetal weight near term. Given our dams still expressed a functional VDR gene, our results suggest that cross-talk between the maternal decidua and the placenta, as well as maternal vitamin D status, may be more important in determining pregnancy outcome than conceptus expression of VDR.

Inferences from genetically modified mouse models on the skeletal actions of vitamin D

Vitamin D has pleiotropic extra-skeletal effects which have been noted in mouse models of deletion of either the 25-hydroxy vitamin D 1α-hydroxylase enzyme, cyp27b1 (1OHase(-/-) mice) or of the vitamin D receptor (Vdr(-/-) mice); these may be preventable or reversible by either restoring normal signaling of the 1,25(OH)2D/VDR system, or in some cases by restoring normal mineral homeostasis. However, effects on skeletal and mineral homeostasis are clearly the major phenotype observed in humans with loss-of-function mutations in either CYP27B1 or VDR. In mouse phenocopies of these human disorders, correction of hypocalcemia and hypophosphatemia reduce elevated circulating parathyroid hormone concentrations and normalize impaired bone mineralization, but restoration of normal 1,25(OH)2D/VDR signaling may be required for optimal bone formation. Induction of high endogenous 1,25(OH)2D concentrations in genetically modified mouse models may cause increased bone resorption and decreased mineralization. Transgenic Vdr overexpression and conditional Vdr deletion in cells of the osteoblastic lineage have also provided insights into the stages of osteoblast differentiation which may mediate these actions. These anabolic and catabolic effects of the 1,25(OH)2D system on bone may therefore be a function of both the ambient concentration of circulating 1,25(OH)2D and the stage of differentiation of the osteoblast. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Vitamin D and its receptor during late development

Expression of the vitamin D receptor (VDR) is widespread but may vary depending on the developmental stage of the animal, and therefore may differentially influence phenotypic function. Thus, the major role of the 1,25-dihydroxyvitamin D [1,25(OH)2D]/VDR system is to regulate mineral and skeletal homeostasis, although mainly after birth. Post-natally, under conditions of low dietary calcium, the 1,25(OH)2D/VDR system enhances intestinal transcellular transport of calcium and possibly paracellular calcium entry by regulating genes that are critical for these functions. This process, by providing adequate calcium, is essential for normal development of the skeletal growth plate and mineralization of bone. Furthermore, blood calcium and phosphorus homeostasis is maintained by an interplay between feedback loops of the 1,25(OH)2D/VDR system with parathyroid hormone and with fibroblast-growth factor (FGF) 23 respectively. The 1,25(OH)2D/VDR system can also modulate the expression of genes involved in both bone formation and resorption post-natally. Ligand independent activity of the VDR normally influences mammalian hair cycling after birth by potentiating Wnt and bone morphogenetic protein (BMP) signaling. Nevertheless ligand bound VDR may also modulate epidermal cell proliferation/differentiation by regulating the balance in function of c-MYC and its antagonist the transcriptional repressor MAD1/MXD1 in skin epithelia. The 1,25(OH)2D/VDR system can also modulate innate immune cells and promote a more tolerogenic immunological status and may therefore influence inflammation and the development of autoimmunity; whether this impacts the fetus is uncertain. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Therapies for musculoskeletal disease: can we treat two birds with one stone?

Musculoskeletal diseases are highly prevalent with staggering annual health care costs across the globe. The combined wasting of muscle (sarcopenia) and bone (osteoporosis)-both in normal aging and pathologic states-can lead to vastly compounded risk for fracture in patients. Until now, our therapeutic approach to the prevention of such fractures has focused solely on bone, but our increasing understanding of the interconnected biology of muscle and bone has begun to shift our treatment paradigm for musculoskeletal disease. Targeting pathways that centrally regulate both bone and muscle (eg, GH/IGF-1, sex steroids, etc.) and newly emerging pathways that might facilitate communication between these 2 tissues (eg, activin/myostatin) might allow a greater therapeutic benefit and/or previously unanticipated means by which to treat these frail patients and prevent fracture. In this review, we will discuss a number of therapies currently under development that aim to treat musculoskeletal disease in precisely such a holistic fashion.