[Myelodysplastic syndromes in two young brothers]

Chondrocytic ephrin B2 promotes cartilage destruction by osteoclasts in endochondral ossification

The majority of the skeleton arises by endochondral ossification, whereby cartilaginous templates expand and are resorbed by osteoclasts then replaced by osteoblastic bone formation. Ephrin B2 is a receptor tyrosine kinase expressed by osteoblasts and growth plate chondrocytes that promotes osteoblast differentiation and inhibits osteoclast formation. We investigated the role of ephrin B2 in endochondral ossification using Osx1Cre-targeted gene deletion. Neonatal Osx1Cre.Efnb2(Δ/Δ) mice exhibited a transient osteopetrosis demonstrated by increased trabecular bone volume with a high content of growth plate cartilage remnants and increased cortical thickness, but normal osteoclast numbers within the primary spongiosa. Osteoclasts at the growth plate had an abnormal morphology and expressed low levels of tartrate-resistant acid phosphatase; this was not observed in more mature bone. Electron microscopy revealed a lack of sealing zones and poor attachment of Osx1Cre.Efnb2(Δ/Δ) osteoclasts to growth plate cartilage. Osteoblasts at the growth plate were also poorly attached and impaired in their ability to deposit osteoid. By 6 months of age, trabecular bone mass, osteoclast morphology and osteoid deposition by Osx1Cre.Efnb2(Δ/Δ) osteoblasts were normal. Cultured chondrocytes from Osx1Cre.Efnb2(Δ/Δ) neonates showed impaired support of osteoclastogenesis but no significant change in Rankl (Tnfsf11) levels, whereas Adamts4 levels were significantly reduced. A population of ADAMTS4(+) early hypertrophic chondrocytes seen in controls was absent from Osx1Cre.Efnb2(Δ/Δ) neonates. This suggests that Osx1Cre-expressing cells, including hypertrophic chondrocytes, are dependent on ephrin B2 for their production of cartilage-degrading enzymes, including ADAMTS4, and this might be required for attachment of osteoclasts and osteoblasts to the cartilage surface during endochondral ossification.

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

1,25-Dihydroxvitamin D3 [1,25(OH)2D3] is the hormonally active form of vitamin D. The genomic mechanism of 1,25(OH)2D3 action involves the direct binding of the 1,25(OH)2D3 activated vitamin D receptor/retinoic X receptor (VDR/RXR) heterodimeric complex to specific DNA sequences. Numerous VDR co-regulatory proteins have been identified, and genome-wide studies have shown that the actions of 1,25(OH)2D3 involve regulation of gene activity at a range of locations many kilobases from the transcription start site. The structure of the liganded VDR/RXR complex was recently characterized using cryoelectron microscopy, X-ray scattering, and hydrogen deuterium exchange. These recent technological advances will result in a more complete understanding of VDR coactivator interactions, thus facilitating cell and gene specific clinical applications. Although the identification of mechanisms mediating VDR-regulated transcription has been one focus of recent research in the field, other topics of fundamental importance include the identification and functional significance of proteins involved in the metabolism of vitamin D. CYP2R1 has been identified as the most important 25-hydroxylase, and a critical role for CYP24A1 in humans was noted in studies showing that inactivating mutations in CYP24A1 are a probable cause of idiopathic infantile hypercalcemia. In addition, studies using knockout and transgenic mice have provided new insight on the physiological role of vitamin D in classical target tissues as well as evidence of extraskeletal effects of 1,25(OH)2D3 including inhibition of cancer progression, effects on the cardiovascular system, and immunomodulatory effects in certain autoimmune diseases. Some of the mechanistic findings in mouse models have also been observed in humans. The identification of similar pathways in humans could lead to the development of new therapies to prevent and treat disease.

Expansion of murine periosteal progenitor cells with fibroblast growth factor 2 reveals an intrinsic endochondral ossification program mediated by bone morphogenetic protein 2

The preservation of the bone-forming potential of skeletal progenitor cells during their ex vivo expansion remains one of the major challenges for cell-based bone regeneration strategies. We report that expansion of murine periosteal cells in the presence of FGF2, a signal present during the early stages of fracture healing, is necessary and sufficient to maintain their ability to organize in vivo into a cartilage template which gives rise to mature bone. Implantation of FGF2-primed cells in a large bone defect in mice resulted in complete healing, demonstrating the feasibility of using this approach for bone tissue engineering purposes. Mechanistically, the enhanced endochondral ossification potential of FGF2-expanded periosteal cells is predominantly driven by an increased production of BMP2 and is additionally linked to an improved preservation of skeletal progenitor cells in the cultures. This characteristic is unique for periosteal cells, as FGF2-primed bone marrow stromal cells formed significantly less bone and progressed exclusively through the intramembranous pathway, revealing essential differences between both cell pools. Taken together, our findings provide insight in the molecular regulation of fracture repair by identifying a unique interaction between periosteal cells and FGF2. These insights may promote the development of cell-based therapeutic strategies for bone regeneration which are independent of the in vivo use of growth factors, thus limiting undesired side effects.

Extra-intestinal calcium handling contributes to normal serum calcium levels when intestinal calcium absorption is suboptimal

The active form of vitamin D, 1,25(OH)2D, is a crucial regulator of calcium homeostasis, especially through stimulation of intestinal calcium transport. Lack of intestinal vitamin D receptor (VDR) signaling does however not result in hypocalcemia, because the increased 1,25(OH)2D levels stimulate calcium handling in extra-intestinal tissues. Systemic VDR deficiency, on the other hand, results in hypocalcemia because calcium handling is impaired not only in the intestine, but also in kidney and bone. It remains however unclear whether low intestinal VDR activity, as observed during aging, is sufficient for intestinal calcium transport and for mineral and bone homeostasis. To this end, we generated mice that expressed the Vdr exclusively in the gut, but at reduced levels. We found that ~15% of intestinal VDR expression greatly prevented the Vdr null phenotype in young-adult mice, including the severe hypocalcemia. Serum calcium levels were, however, in the low-normal range, which may be due to the suboptimal intestinal calcium absorption, renal calcium loss, insufficient increase in bone resorption and normal calcium incorporation in the bone matrix. In conclusion, our results indicate that low intestinal VDR levels improve intestinal calcium absorption compared to Vdr null mice, but also show that 1,25(OH)2D-mediated fine-tuning of renal calcium reabsorption and bone mineralization and resorption is required to maintain fully normal serum calcium levels.

Enhanced FGF23 production in mice expressing PI3K-insensitive GSK3 is normalized by β-blocker treatment

Glycogen synthase kinase (GSK)-3 is a ubiquitously expressed kinase inhibited by insulin-dependent Akt/PKB/SGK. Mice expressing Akt/PKB/SGK-resistant GSK3α/GSK3β (gsk3(KI)) exhibit enhanced sympathetic nervous activity and phosphaturia with decreased bone density. Hormones participating in phosphate homeostasis include fibroblast growth factor (FGF)-23, a bone-derived hormone that inhibits 1,25-dihydroxyvitamin D3 (1,25(OH)2D3; calcitriol) formation and phosphate reabsorption in the kidney and counteracts vascular calcification and aging. FGF23 secretion is stimulated by the sympathetic nervous system. We studied the role of GSK3-controlled sympathetic activity in FGF23 production and phosphate metabolism. Serum FGF23, 1,25(OH)2D3, and urinary vanillylmandelic acid (VMA) were measured by ELISA, and serum and urinary phosphate and calcium were measured by photometry in gsk3(KI) and gsk3(WT) mice, before and after 1 wk of oral treatment with the β-blocker propranolol. Urinary VMA excretion, serum FGF23, and renal phosphate and calcium excretion were significantly higher, and serum 1,25(OH)2D3 and phosphate concentrations were lower in gsk3(KI) mice than in gsk3(WT) mice. Propranolol treatment decreased serum FGF23 and loss of renal calcium and phosphate and increased serum phosphate concentration in gsk3(KI) mice. We conclude that Akt/PKB/SGK-sensitive GSK3 inhibition participates in the regulation of FGF23 release, 1,25(OH)2D3 formation, and thus mineral metabolism, by controlling the activity of the sympathetic nervous system.

Deletion of macrophage Vitamin D receptor promotes insulin resistance and monocyte cholesterol transport to accelerate atherosclerosis in mice

Intense effort has been devoted to understanding predisposition to chronic systemic inflammation because it contributes to cardiometabolic disease. We demonstrate that deletion of the macrophage vitamin D receptor (VDR) in mice (KODMAC) is sufficient to induce insulin resistance by promoting M2 macrophage accumulation in the liver as well as increasing cytokine secretion and hepatic glucose production. Moreover, VDR deletion increases atherosclerosis by enabling lipid-laden M2 monocytes to adhere, migrate, and carry cholesterol into the atherosclerotic plaque and by increasing macrophage cholesterol uptake and esterification. Increased foam cell formation results from lack of VDR-SERCA2b interaction, causing SERCA dysfunction, activation of ER stress-CaMKII-JNKp-PPARγ signaling, and induction of the scavenger receptors CD36 and SR-A1. Bone marrow transplant of VDR-expressing cells into KODMAC mice improved insulin sensitivity, suppressed atherosclerosis, and decreased foam cell formation. The immunomodulatory effects of vitamin D in macrophages are thus critical in diet-induced insulin resistance and atherosclerosis in mice.

Chondrocytes-Specific Expression of Osteoprotegerin Modulates Osteoclast Formation in Metaphyseal Bone

Bone marrow stromal cells/osteoblasts were originally thought to be the major player in regulating osteoclast differentiation through expressing RANKL/OPG cytokines. Recent studies have established that chondrocytes also express RANKL/OPG and support osteoclast formation. Till now, the in vivo function of chondrocyte-produced OPG in osteoclast formation and postnatal bone growth has not been directly investigated. In this study, chondrocyte-specific Opg transgenic mice were generated by using type II collagen promoter. The Col2-Opg transgenic mice showed delayed formation of secondary ossification center and localized increase of bone mass in proximal metaphysis of tibiae. TRAP staining showed that osteoclast numbers were reduced in both secondary ossification center and proximal metaphysis. This finding was further confirmed by in vitro chondrocyte/spleen cell co-culture assay. In contrast, the mineral apposition rates were not changed in Col2-Opg transgenic mice. TUNEL staining revealed more apoptotic hypertrophic chondrocytes in the growth plate of Col2-Opg mice. Flow cytometry analysis showed fewer RANK-expressing cells in the marrow of Col2a1-Opg mice, suggesting the role of OPG in blocking the differentiation of early mesenchymal progenitors into RANK-expressing pre-osteoclasts. Our results demonstrated that OPG expression in chondrocyte increases bone mass in the proximal metaphysis of tibiae through negative regulation of osteoclast formation.

The androgen receptor has no direct antiresorptive actions in mouse osteoclasts

Androgen deficiency or androgen receptor knockout (ARKO) causes high-turnover osteopenia, but the target cells for this effect remain unclear. To examine whether AR in osteoclasts directly suppresses bone resorption, we crossed AR-floxed with cathepsin K-Cre mice. Osteoclast-specific ARKO (ocl-ARKO) mice showed no changes neither in osteoclast surface nor in bone microarchitecture nor in the response to orchidectomy and androgen replacement, indicating that the AR in osteoclasts is not critical for bone maintenance. In line with the lack of a bone phenotype, the levels of AR were very low in osteoclast-enriched cultures derived from bone marrow (BM) and undetectable in osteoclasts generated from spleen precursors. Since tibiae of ubiquitous ARKO mice displayed increased osteoclast counts, the role of AR was further explored using cell cultures from these animals. Osteoclast generation and activity in vitro were similar between ARKO and wildtype control (WT) mice. In co-culture experiments, BM stromal cells (BMSCs) were essential for the suppressive action of AR on osteoclastogenesis and osteoclast activity. Stimulation with 1,25(OH)2 vitamin D3 increased Rankl and decreased Tnfsf11 (osteoprotegerin, Opg) gene expression in BMSCs more than in osteoblasts. This increase in the Rankl/Opg ratio following 1,25(OH)2D3 stimulation was lower, not higher, in ARKO mice. Runx2 expression in BMSCs was however higher in ARKO vs. WT, suggesting that ARKO mice may more readily commit osteoprogenitor cells to osteoblastogenesis. In conclusion, the AR does not seem to suppress bone resorption through direct actions in osteoclasts. BMSCs may however represent an alternative AR target in the BM milieu.

Vitamin D signaling in calcium and bone homeostasis: a delicate balance

Loss-of-function mutations in genes involved in the vitamin D/vitamin D receptor system have clearly evidenced its critical role for mineral and skeletal homeostasis. Adequate levels of 1,25-dihydroxyvitamin D [1,25(OH)2D], the active form of vitamin D are therefore required and depend on sufficient sunlight exposure or dietary intake. Intestinal calcium absorption is a primary target of 1,25(OH)2D action and this pathway indirectly promotes calcium incorporation in bone. Severe vitamin D deficiency may thus decrease bone quality and leads to osteomalacia, whereas less severe deficiency increases the risk of osteoporosis and bone fractures. On the other hand, high vitamin D levels together with low dietary calcium intake will increase bone resorption and decrease bone mineralization in order to maintain normal serum calcium levels. Appropriate dietary calcium intake and sufficient serum vitamin D levels are thus important for skeletal health. Dosing of calcium and vitamin D supplements is still debated and requires further investigation.

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.

Intestinal epithelial vitamin D receptor deletion leads to defective autophagy in colitis

OBJECTIVE

Vitamin D and the vitamin D receptor (VDR) appear to be important immunological regulators of inflammatory bowel diseases (IBD). Defective autophagy has also been implicated in IBD, where interestingly, polymorphisms of genes such as ATG16L1 have been associated with increased risk. Although vitamin D, the microbiome and autophagy are all involved in pathogenesis of IBD, it remains unclear whether these processes are related or function independently.

DESIGN

We investigated the effects and mechanisms of intestinal epithelial VDR in healthy and inflamed states using cell culture models, a conditional VDR knockout mouse model (VDR(ΔIEC)), colitis models and human samples.

RESULTS

Absence of intestinal epithelial VDR affects microbial assemblage and increases susceptibility to dextran sulfate sodium-induced colitis. Intestinal epithelial VDR downregulates expressions of ATG16L1 and lysozyme, and impairs antimicrobial function of Paneth cells. Gain and loss-of-function assays showed that VDR levels regulate ATG16L1 and lysozyme at the transcriptional and translational levels. Moreover, low levels of intestinal epithelial VDR correlated with reduced ATG16L1 and representation by intestinal Bacteroides in patients with IBD. Administration of the butyrate (a fermentation product of gut microbes) increases intestinal VDR expression and suppresses inflammation in a colitis model.

CONCLUSIONS

Our study demonstrates fundamental relationship between VDR, autophagy and gut microbial assemblage that is essential for maintaining intestinal homeostasis, but also in contributing to the pathophysiology of IBD. These insights can be leveraged to define therapeutic targets for restoring VDR expression and function.

Fibroblast Growth Factor 23

Traditionally, control of phosphorus in the body has been considered secondary to the tighter control of calcium by parathyroid hormone and vitamin D. However, over the past decade, substantial advances have been made in understanding the control of phosphorus by the so-called phosphatonin system, the lynchpin of which is fibroblast growth factor 23 (FGF23). FGF23 binds to the klotho/FGFR1c receptor complex in renal tubular epithelial cells, leading to upregulation of Na/Pi cotransporters and subsequent excretion of phosphorus from the body. In addition, FGF23 inhibits parathyroid hormone and the renal 1α-hydroxylase enzyme, while it stimulates 24-hydroxylase, leading to decreased 1,25-dihydroxyvitamin D3. FGF23 is intimately involved in the pathogenesis of a number of diseases, particularly the hereditary hypophosphatemic rickets group and chronic kidney disease, and is a target for the development of new treatments in human medicine. Little work has been done on FGF23 or the other phosphatonins in veterinary medicine, but increases in FGF23 are seen with chronic kidney disease in cats, and increased FGF23 expression has been found in soft tissue sarcomas in dogs.

Pathophysiology of parathyroid hyperplasia in chronic kidney disease: preclinical and clinical basis for parathyroid intervention

Secondary hyperparathyroidism is characterised by excessive secretion of parathyroid hormone and parathyroid hyperplasia, resulting in both skeletal and extraskeletal consequences. Recent basic and clinical studies have brought considerable advances in our understanding of the pathophysiology of parathyroid hyperplasia and have also provided practical therapeutic approaches, especially with regard to indications for parathyroid intervention. In this context, it is quite important to recognize the development of nodular hyperplasia, because the cells in nodular hyperplasia are usually resistant to calcitriol treatment. Patients with nodular hyperplasia should undergo parathyroid intervention including percutaneous ethanol injection therapy (PEIT). Selective PEIT of the parathyroid gland is an effective approach in which the enlarged parathyroid gland with nodular hyperplasia is 'selectively' destroyed by ethanol injection, and other glands with diffuse hyperplasia are then managed by medical therapy. With a more focused attention to applying parathyroid intervention, we can expect significant improvement in the management of secondary hyperparathyroidism in dialysis patients.

Physiological functions of vitamin D: what we have learned from global and conditional VDR knockout mouse studies

The physiological role of vitamin D depends on calcium supply and calcium balance. When the calcium balance is normal, the major target of vitamin D is intestine. Vitamin D stimulates mainly active intestinal calcium transport mechanism. During a negative calcium balance, bone effects of vitamin D become dominant. Thus, the role of vitamin D in maintaining normocalcemia appears to have priority over skeletal integrity in these situations.

The effects of 1α, 25-dihydroxyvitamin D3 and transforming growth factor-β3 on bone development in an ex vivo organotypic culture system of embryonic chick femora

Transforming growth factor-beta3 (TGF-β3) and 1α,25-dihydroxyvitamin D₃ (1α,25 (OH) ₂D₃) are essential factors in chondrogenesis and osteogenesis respectively. These factors also play a fundamental role in the developmental processes and the maintenance of skeletal integrity, but their respective direct effects on these processes are not fully understood. Using an organotypic bone rudiment culture system the current study has examined the direct roles the osteotropic factors 1α,25 (OH)₂D₃ and TGF-β3 exert on the development and modulation of the three dimensional structure of the embryonic femur. Isolated embryonic chick femurs (E11) were organotypically cultured for 10 days in basal media, or basal media supplemented with either 1α,25 (OH) ₂D₃ (25 nM) or TGF-β3 (5 ng/mL & 15 ng/mL). Analyses of the femurs were undertaken using micro-computed tomography (μCT), histology and immunohistochemistry. 1α,25 (OH)2D3 supplemented cultures enhanced osteogenesis directly in the developing femurs with elevated levels of osteogenic markers such as type 1 collagen. In marked contrast organotypic femur cultures supplemented with TGF-β3 (5 ng/mL & 15 ng/mL) demonstrated enhanced chondrogenesis with a reduction in osteogenesis. These studies demonstrate the efficacy of the ex vivo organotypic embryonic femur culture employed to elucidate the direct roles of these molecules, 1α,25 (OH) ₂D₃ and TGF-β3 on the structural development of embryonic bone within a three dimensional framework. We conclude that 1α,25(OH)₂D and TGF-β3 modify directly the various cell populations in bone rudiment organotypic cultures effecting tissue metabolism resulting in significant changes in embryonic bone growth and modulation. Understanding the roles of osteotropic agents in the process of skeletal development is integral to developing new strategies for the recapitulation of bone tissue in later life.

Role of Vitamin D in Osteoarthritis: Molecular, Cellular, and Clinical Perspectives

Osteoarthritis is a debilitating and degenerative disease which affects millions of people worldwide. The causes and mechanisms of osteoarthritis remain to be fully understood. Vitamin D has been hypothesised to play essential roles in a number of diseases including osteoarthritis. Many cell types within osteoarthritic joints appear to experience negative effects often at increased sensitivity to vitamin D. These findings contrast clinical research which has identified vitamin D deficiency to have a worryingly high prevalence among osteoarthritis patients. Randomised-controlled trial is considered to be the most rigorous way of determining the effects of vitamin D supplementation on the development of osteoarthritis. Studies into the effects of low vitamin D levels on pain and joint function have to date yielded controversial results. Due to the apparent conflicting effects of vitamin D in knee osteoarthritis, further research is required to fully elucidate its role in the development and progression of the disease as well as assess the efficacy and safety of vitamin D supplementation as a therapeutic strategy.

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

Minireview: nuclear receptor regulation of osteoclast and bone remodeling

Osteoclasts are bone-resorbing cells essential for skeletal remodeling and regeneration. However, excessive osteoclasts often contribute to prevalent bone degenerative diseases such as osteoporosis, arthritis, and cancer bone metastasis. Osteoclast dysregulation is also associated with rare disorders such as osteopetrosis, pycnodysostosis, Paget's disease, and Gorham-Stout syndrome. The nuclear receptor (NR) family of transcription factors functions as metabolic sensors that control a variety of physiological processes including skeletal homeostasis and serves as attractive therapeutic targets for many diseases. In this review, we highlight recent findings on the new players and the new mechanisms for how NRs regulate osteoclast differentiation and bone resorption. An enhanced understanding of NR functions in osteoclastogenesis will facilitate the development of not only novel osteoprotective medicine but also prudent strategies to minimize the adverse skeletal effects of certain NR-targeting drugs for a better treatment of cancer and metabolic diseases.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Environmental temperature impact on bone and cartilage growth

Environmental temperature can have a surprising impact on extremity growth in homeotherms, but the underlying mechanisms have remained elusive for over a century. Limbs of animals raised at warm ambient temperature are significantly and permanently longer than those of littermates housed at cooler temperature. These remarkably consistent lab results closely resemble the ecogeographical tenet described by Allen's "extremity size rule," that appendage length correlates with temperature and latitude. This phenotypic growth plasticity could have adaptive significance for thermal physiology. Shortened extremities help retain body heat in cold environments by decreasing surface area for potential heat loss. Homeotherms have evolved complex mechanisms to maintain tightly regulated internal temperatures in challenging environments, including "facultative extremity heterothermy" in which limb temperatures can parallel ambient. Environmental modulation of tissue temperature can have direct and immediate consequences on cell proliferation, metabolism, matrix production, and mineralization in cartilage. Temperature can also indirectly influence cartilage growth by modulating circulating levels and delivery routes of essential hormones and paracrine regulators. Using an integrated approach, this article synthesizes classic studies with new data that shed light on the basis and significance of this enigmatic growth phenomenon and its relevance for treating human bone elongation disorders. Discussion centers on the vasculature as a gateway to understanding the complex interconnection between direct (local) and indirect (systemic) mechanisms of temperature-enhanced bone lengthening. Recent advances in imaging modalities that enable the dynamic study of cartilage growth plates in vivo will be key to elucidating fundamental physiological mechanisms of long bone growth regulation.

Impaired bone formation in Pdia3 deficient mice

1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] is crucial for normal skeletal development and bone homeostasis. Protein disulfide isomerase family A, member 3 (PDIA3) mediates 1α,25(OH)₂D₃ initiated-rapid membrane signaling in several cell types. To understand its role in regulating skeletal development, we generated Pdia3-deficient mice and examined the physiologic consequence of Pdia3-disruption in embryos and Pdia3^+/− heterozygotes at different ages. No mice homozygous for the Pdia3-deletion were found at birth nor were there embryos after E12.5, indicating that targeted disruption of the Pdia3 gene resulted in early embryonic lethality. Pdia3-deficiency also resulted in skeletal manifestations as revealed by µCT analysis of the tibias. In comparison to wild type mice, Pdia3 heterozygous mice displayed expanded growth plates associated with decreased tether formation. Histomorphometry also showed that the hypertrophic zone in Pdia3^+/− mice was more cellular than seen in wild type growth plates. Metaphyseal trabecular bone in Pdia3^+/− mice exhibited an age-dependent phenotype with lower BV/TV and trabecular numbers, which was most pronounced at 15 weeks of age. Bone marrow cells from Pdia3^+/− mice exhibited impaired osteoblastic differentiation, based on reduced expression of osteoblast markers and mineral deposition compared to cells from wild type animals. Collectively, our findings provide in vivo evidence that PDIA3 is essential for normal skeletal development. The fact that the Pdia3^+/− heterozygous mice share a similar growth plate and bone phenotype to nVdr knockout mice, suggests that PDIA3-mediated rapid membrane signaling might be an alternative mechanism responsible for 1α,25(OH)₂D₃’s actions in regulating skeletal development.

The influence of 1α.25-dihydroxyvitamin d3 coating on implant osseointegration in the rabbit tibia

Objectives

This study aims to evaluate bone response to an implant surface modified by 1α,25-dihydroxyvitamin D3 [1.25-(OH)₂D₃] in vivo and the potential link between 1.25-(OH) 2D3 surface concentration and bone response.

Material and Methods

Twenty-eight implants were divided into 4 groups (1 uncoated control, 3 groups coated with 1.25-(OH)₂D₃ in concentrations of 10^-8, 10^-7 and 10^-6 M respectively), placed in the rabbit tibia for 6 weeks. Topographical analyses were carried out on coated and uncoated discs using interferometer and atomic-force-microscope (AFM). Twenty-eight implants were histologically observed (bone-to-implant-contact [BIC] and new-bone-area [NBA]).

Results

The results showed that the 1.25-(OH)₂D₃ coated implants presented a tendency to osseointegrate better than the non-coated surfaces, the differences were not significant (P > 0.05).

Conclusions

The effect of 1.25-(OH)₂D₃ coating to implants suggested possible dose dependent effects, however no statistical differences could be found. It is thought that the base substrate topography (turned) could not sustain sufficient amount of 1.25-(OH)₂D₃ enough to present significant biologic responses. Thus, development a base substrate that can sustain 1.25-(OH)₂D₃ for a long period is necessary in future studies.

Ephrin B2/EphB4 mediates the actions of IGF-I signaling in regulating endochondral bone formation

Ephrin B2/EphB4 mediates interactions among osteoblasts (OBs), osteoclasts (OCLs), and chondrocytes to regulate their differentiation. We investigated the role of ephrin B2/EphB4 signaling in mediating the anabolic effects of insulin-like growth factor-I (IGF-I) and parathyroid hormone (PTH) on those cells and overall endochondral bone formation. Immunohistochemistry demonstrated that the expression of ephrin B2 in OBs, OCLs, and osteocytes, and the expression of EphB4 in OBs and osteocytes was dramatically decreased in global IGF-I knockout mice. Inactivation of EphB4 by EphB4 small, interfering RNA (siRNA) in cultured bone marrow stromal cells significantly decreased the mRNA levels of OB differentiation markers and abolished the stimulatory effects of IGF-I on these markers. Blocking the interaction of EphB4 and ephrin B2 in the OB-OCL cocultures with the EphB4 specific peptide TNYL-RAW or deletion of ephrin B2 in OCL prior to coculture led to fewer and smaller tartrate-resistant acid phosphatase (TRAP)-positive cells, decreased expression of OB differentiation markers, and blunted response to IGF-I for both OCL and OB differentiation. In the growth plate, both ephrin B2 and EphB4 are expressed in late stage proliferating and prehypertrophic chondrocytes, and their expression was decreased in mice lacking the IGF-I receptor specifically in chondrocytes. In vitro, blocking the interaction of EphB4 and ephrin B2 in chondrogenic ATDC5 cells with TNYL-RAW significantly decreased both basal and IGF1-induced expression of type II and type X collagen. In the cocultures of ATDC5 cells and spleen cells (osteoclast precursors), TNYL-RAW decreased the numbers of TRAP-positive cells and the expression of nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and receptor activator of NF-κB (RANK), and blocked their stimulation by IGF-I. Our data indicate that IGF-I/IGF-IR signaling promotes OB, OCL, and chondrocyte differentiation via ephrin B2/EphB4 mediated cell-cell communication.

Vitamin D promotes vascular regeneration

BACKGROUND

Vitamin D deficiency in humans is frequent and has been associated with inflammation. The role of the active hormone 1,25-dihydroxycholecalciferol (1,25-dihydroxy-vitamin D3; 1,25-VitD3) in the cardiovascular system is controversial. High doses induce vascular calcification; vitamin D3 deficiency, however, has been linked to cardiovascular disease because the hormone has anti-inflammatory properties. We therefore hypothesized that 1,25-VitD3 promotes regeneration after vascular injury.

METHODS AND RESULTS

In healthy volunteers, supplementation of vitamin D3 (4000 IU cholecalciferol per day) increased the number of circulating CD45-CD117+Sca1+Flk1+ angiogenic myeloid cells, which are thought to promote vascular regeneration. Similarly, in mice, 1,25-VitD3 (100 ng/kg per day) increased the number of angiogenic myeloid cells and promoted reendothelialization in the carotid artery injury model. In streptozotocin-induced diabetic mice, 1,25-VitD3 also promoted reendothelialization and restored the impaired angiogenesis in the femoral artery ligation model. Angiogenic myeloid cells home through the stromal cell-derived factor 1 (SDF1) receptor CXCR4. Inhibition of CXCR4 blocked 1,25-VitD3-stimulated healing, pointing to a role of SDF1. The combination of injury and 1,25-VitD3 increased SDF1 in vessels. Conditioned medium from injured, 1,25-VitD3-treated arteries elicited a chemotactic effect on angiogenic myeloid cells, which was blocked by SDF1-neutralizing antibodies. Conditional knockout of the vitamin D receptor in myeloid cells but not the endothelium or smooth muscle cells blocked the effects of 1,25-VitD3 on healing and prevented SDF1 formation. Mechanistically, 1,25-VitD3 increased hypoxia-inducible factor 1-α through binding to its promoter. Increased hypoxia-inducible factor signaling subsequently promoted SDF1 expression, as revealed by reporter assays and knockout and inhibitory strategies of hypoxia-inducible factor 1-α.

CONCLUSIONS

By inducing SDF1, vitamin D3 is a novel approach to promote vascular repair.

Influence of diabetic state and vitamin D deficiency on bone repair in female mice

Type 1 diabetes is associated with an increased fracture risk, an impaired fracture healing, and an increased vitamin D insufficiency. However, the role of vitamin D in diabetic bone repair process remains unclear. We therefore examined the effects of vitamin D deficiency on the impaired bone repair in streptozotocin (STZ)-induced diabetes using female mice. Diabetes was induced by STZ injection into female mice after feeding with normal or vitamin D-deficient diet for 6weeks from the age of 4weeks. A femoral bone defect was induced in mice 4 weeks after induction of diabetes. The repair of damaged site on the femur was significantly delayed at days 7 and 10 after bone defect by diabetic state in mice, as assessed by quantitative computed tomography, while vitamin D deficiency did not affect the bone repair both in mice with normal and diabetic state. The decreases in bone mineral density (BMD) at cortical and trabecular bone by diabetic state were significantly augmented by vitamin D deficiency in tibia at the undamaged side in mice. Diabetic state blunted the levels of osteogenic and chondrogenic genes enhanced by vitamin D deficiency. Moreover, vitamin D deficiency significantly aggravated the decreases in osteocalcin and IGF-1 mRNA by diabetic state. In conclusion, our study showed that vitamin D deficiency aggravates the decrease in BMD by diabetic state in female mice, although vitamin D deficiency did not affect bone repair delayed by diabetic state.

Chondrocyte β-catenin signaling regulates postnatal bone remodeling through modulation of osteoclast formation in a murine model

OBJECTIVE

To investigate whether β-catenin signaling in chondrocytes regulates osteoclastogenesis, thereby contributing to postnatal bone growth and bone remodeling.

METHODS

Mice with conditional knockout (cKO) or conditional activation (cAct) of chondrocyte-specific β-catenin were generated. Changes in bone mass, osteoclast numbers, and osteoblast activity were examined. The mechanisms by which β-catenin signaling in chondrocytes regulates osteoclast formation were determined.

RESULTS

The β-catenin cKO mice developed localized bone loss, whereas cAct mice developed a high bone mass phenotype. Histologic findings suggested that these phenotypes were caused primarily by impaired osteoclast formation, rather than impaired bone formation. Further molecular signaling analyses revealed that β-catenin signaling controlled this process by regulating the expression of the RANKL and osteoprotegerin (OPG) genes in chondrocytes. Activation of β-catenin signaling in chondrocytes suppressed Rankl gene transcription through a glucocorticoid receptor-dependent mechanism. The severe bone loss phenotype observed in β-catenin cKO mice was largely restored by treatment with human recombinant OPG or transgenic overexpression of Opg in chondrocytes.

CONCLUSION

β-catenin signaling in chondrocytes plays a key role in postnatal bone growth and bone remodeling through its regulation of osteoclast formation.

Identification of the vitamin D receptor in osteoblasts and chondrocytes but not osteoclasts in mouse bone

Bone is clearly a target of vitamin D and as expected, the vitamin D receptor (VDR) is expressed in osteoblasts. However, the presence of VDR in other cells such as osteocytes, osteoclasts, chondroclasts, and chondrocytes is uncertain. Because of difficulties in obtaining sections of undecalcified adult bone, identification of the site of VDR expression in adult bone tissue has been problematic. In addition, the antibodies to VDR used in previous studies lacked specificity, a property crucial for unambiguous conclusions. In the present study, VDR in the various cells from neonatal and adult mouse bone tissues was identified by a highly specific and sensitive immunohistochemistry method following bone decalcification with EGTA. For accurate evaluation of weak immunosignals, samples from Demay VDR knockout mice were used as negative control. Molecular markers were used to identify cell types. Our results showed that EGTA-decalcification of bone tissue had no detectable effect on the immunoreactivity of VDR. VDR was found in osteoblasts and hypertrophic chondrocytes but not in the multinucleated osteoclasts, chondroclasts, and bone marrow stromal cells. Of interest is the finding that immature osteoblasts contain large amounts of VDR, whereas the levels are low or undetectable in mature osteoblasts including bone lining cells and osteocytes. Proliferating chondrocytes appear devoid of VDR, although low levels were found in the hypertrophic chondrocytes. These data demonstrate that osteoblasts and chondrocytes are major targets of 1α,25-dihydroxyvitamin D, but osteoclasts and chondroclasts are minor targets or not at all. A high level of VDR was found in the immature osteoblasts located in the cancellous bone, indicating that they are major targets of 1α,25-dihydroxyvitamin D. Thus, the immature osteoblasts are perhaps responsible for the vitamin D hormone signaling resulting in calcium mobilization and in osteogenesis.

Vitamin D: direct effects of vitamin D metabolites on bone: lessons from genetically modified mice

The vitamin D endocrine system has clear beneficial effects on bone as demonstrated by prevention of rickets in children and by reducing the risk of osteomalacia or osteoporosis in adults or elderly subjects. Depending on the design of the study of genetically modified animals, however, 1,25(OH)2D and the vitamin D receptor (VDR) may have no effect, beneficial or even deleterious direct effects on bone. We present here a comprehensive model of the direct effects of vitamin D on bone. In case of sufficient calcium supply, vitamin D and its metabolites can improve the calcium balance and facilitate mineral deposition in bone matrix largely without direct effects on bone cells, although some beneficial effects may occur via mature osteoblasts, as demonstrated in mice with osteoblast-specific overexpression of VDR or 1α-hydroxylase. In case of calcium deficiency, however, 1,25(OH)2D enhances bone resorption, whereas simultaneously inhibiting bone mineralization, so as to defend serum calcium homeostasis at the expense of bone mass. This dual role probably provides a survival benefit for land vertebrates living in a calcium-poor environment.

Cardiovascular Risk Factors and Chronic Kidney Disease-FGF23: A Key Molecule in the Cardiovascular Disease

Patients with chronic kidney disease (CKD) are at increased risk of mortality, mainly from cardiovascular disease. Moreover, abnormal mineral and bone metabolism, the so-called CKD-mineral and bone disorder (MBD), occurs from early stages of CKD. This CKD-MBD presents a strong cardiovascular risk for CKD patients. Discovery of fibroblast growth factor 23 (FGF23) has altered our understanding of CKD-MBD and has revealed more complex cross-talk and endocrine feedback loops between the kidney, parathyroid gland, intestines, and bone. During the past decade, reports of clinical studies have described the association between FGF23 and cardiovascular risks, left ventricular hypertrophy, and vascular calcification. Recent translational reports have described the existence of FGF23-Klotho axis in the vasculature and the causative effect of FGF23 on cardiovascular disease. These findings suggest FGF23 as a promising target for novel therapeutic approaches to improve clinical outcomes of CKD patients.

Mice Deficient in NF-κB p50 and p52 or RANK Have Defective Growth Plate Formation and Post-natal Dwarfism

NF-κBp50/p52 double knockout (dKO) and RANK KO mice have no osteoclasts and develop severe osteopetrosis associated with dwarfism. In contrast, Op/Op mice, which form few osteoclasts, and Src KO mice, which have osteoclasts with defective resorptive function, are osteopetrotic, but they are not dwarfed. Here, we compared the morphologic features of long bones from p50/p52 dKO, RANK KO, Op/Op and Src KO mice to attempt to explain the differences in their long bone lengths. We found that growth plates in p50/p52 dKO and RANK KO mice are significantly thicker than those in WT mice due to a 2-3-fold increase in the hypertrophic chondrocyte zone associated with normal a proliferative chondrocyte zone. This growth plate abnormality disappears when animals become older, but their dwarfism persists. Op/Op or Src KO mice have relatively normal growth plate morphology. In-situ hybridization study of long bones from p50/p52 dKO mice showed marked thickening of the growth plate region containing type 10 collagen-expressing chondrocytes. Treatment of micro-mass chondrocyte cultures with RANKL did not affect expression levels of type 2 collagen and Sox9, markers for proliferative chondrocytes, but RANKL reduced the number of type 10 collagen-expressing hypertrophic chondrocytes. Thus, RANK/NF-κB signaling plays a regulatory role in post-natal endochondral ossification that maintains hypertrophic conversion and prevents dwarfism in normal mice.

Relationship between vitamin D receptor gene (VDR) polymorphisms, vitamin D status, osteoarthritis and intervertebral disc degeneration

The vitamin D endocrine system is involved in bony and cartilaginous metabolisms and alterations in the homeostasis of this system could be associated to pathological conditions of cartilaginous tissue. In this context, the presence of polymorphisms in the vitamin D receptor gene (VDR), in association with the susceptibility to common osteochondral diseases, was largely investigated. The aim of this review was to summarize data present in literature, analyzing the association of the VDR polymorphisms, vitamin D status and knee cartilage and intervertebral disc pathologies, trying to suggest links between the different specific pathologies analyzed. Concerning the association between VDR polymorphisms and cartilaginous tissue diseases, we found controversial reports. However, the great majority of papers reported an association with lumbar disc degeneration, whereas about half of the studies found an association with osteoarthritis. A further association between VDR polymorphisms (in linkage disequilibrium) and the presence of specific characteristics of these diseases, in particular the formation of osteophytes, was evidenced. Finally, the influence of vitamin D status on these pathologies was evaluated, trying to evidence the relation between the presence of particular genetic variants in the VDR and vitamin D levels or to show whether a particular vitamin D status could predispose to the development or progression of such diseases, however, no significant associations were found. In the future, given the role of vitamin D system in the cartilaginous tissue metabolism, it could be interesting to perform functional and tissue specific studies to analyze the interplay between the different VDR variants and its ligand.

Maternal hypervitaminosis D reduces fetal bone mass and mineral acquisition and leads to neonatal lethality

Pregnancy challenges maternal calcium handling because sufficient calcium has to be transferred to the fetus to ensure fetal bone mass acquisition. 1,25(OH)2 vitamin D [1,25(OH)2D] is an important regulator of calcium homeostasis during adulthood, yet its role seems redundant for the maternal adaptations to pregnancy as well as during fetal development. However, not only deficiency but also excess of 1,25(OH)2D can be harmful and we therefore questioned whether high maternal 1,25(OH)2D levels may injure fetal development or neonatal outcome, as maternal-fetal transport of 1,25(OH)2D has been largely disputed. To this end, vitamin D receptor (VDR) null (Vdr(-/-)) females, displaying high 1,25(OH)2D levels, were mated with Vdr(+/-) males to obtain pregnancies with fetuses that are responsive (Vdr(+/-)) or resistant (Vdr(-/-)) to 1,25(OH)2D. Surprisingly, most of the Vdr(+/-) neonates died shortly after birth, whereas none of the Vdr(-/-). Mechanistically, we noticed that in Vdr(+/-) embryos, serum calcium levels were normal, but that skeletal calcium storage was reduced as evidenced by decreased mineralized bone mass as well as bone mineral content. More precisely, bone formation was decreased and the level of bone mineralization inhibitors was increased. This decreased fetal skeletal calcium storage may severely compromise calcium balance and survival at birth. In conclusion, these data indicate that high maternal 1,25(OH)2D levels are transferred across the placental barrier and adversely affect the total amount of calcium stored in fetal bones which is accompanied by neonatal death.

Historically significant events in the discovery of RANK/RANKL/OPG

After it was suggested 30 years ago that the osteoblast lineage controlled the formation of osteoclasts, methods were developed that established this to be the case, but the molecular controls were elusive. Over more than a decade much evidence was obtained for signaling mechanisms that regulated the production of a membrane - bound regulator of osteoclastogenesis, in the course of which intercellular communication in bone was revealed in its complexity. The discovery of regulation by tumor necrosis factor ligand and receptor families was made in the last few years of the twentieth century, leading since then to a new physiology of bone, and to exciting drug development.

Inactivation of the integrin-linked kinase (ILK) in osteoblasts increases mineralization

In osteoblasts, Integrin-Linked Kinase (ILK)-dependent phosphorylation of the cJUN transcriptional coactivator, αNAC, induces the nuclear accumulation of the coactivator and potentiates cJUN-dependent transcription. Mutation of the ILK phosphoacceptor site within the αNAC protein leads to cytoplasmic retention of the coactivator and cell-autonomous increases in osteoblastic activity. In order to gain further insight into the ILK-αNAC signaling cascade, we inactivated ILK using RNA knockdown in osteoblastic cells and engineered mice with specific ablation of ILK in osteoblasts. ILK knockdown in MC3T3-E1 osteoblast-like cells reduced phosphorylation of its downstream target glycogen synthase kinase 3β (GSK3β), which led to cytoplasmic retention of αNAC and increased mineralization with augmented expression of the osteoblastic differentiation markers, pro-α1(I) collagen (col1A1), Bone Sialoprotein (Bsp) and Osteocalcin (Ocn). Cultured ILK-deficient primary osteoblasts also showed increased cytoplasmic αNAC levels, and augmented mineralization with higher Runx2, Col1a1 and Bsp expression. Histomorphometric analysis of bones from mutant mice with ILK-deficient osteoblasts (Col1-Cre;Ilk(-/fl)) revealed transient changes, with increased bone volume in newborn animals that was corrected by two weeks of age. Our data suggest that the ILK-αNAC cascade acts to reduce the pace of osteoblast maturation. We propose that in vivo, functional redundancy is able to compensate for the loss of ILK activity, leading to the absence of an obvious phenotype when osteoblast-specific Ilk-deficient mice reach puberty.

Regulation of mineral metabolism by lithium

Lithium, an inhibitor of glycogen synthase kinase 3 (GSK3), is widely used for the treatment of mood disorders. Side effects of lithium include nephrogenic diabetes insipidus, leading to renal water loss. Dehydration has in turn been shown to downregulate Klotho, which is required as co-receptor for the downregulation of 1,25(OH)2D3 formation by fibroblast growth factor 23 (FGF23). FGF23 decreases and 1,25(OH)2D3 stimulates renal tubular phosphate reabsorption. The present study explored whether lithium influences renal Klotho expression, FGF23 serum levels, 1,25(OH)2D3 formation, and renal phosphate excretion. To this end, mice were analyzed after a 14-day period of sham treatment or of treatment with lithium (200 mg/kg/day subcutaneously). Serum antidiuretic hormone (ADH), FGF23, and 1,25(OH)2D3 concentrations were determined by ELISA or EIA, renal Klotho protein abundance and GSK3 phosphorylation were analyzed by Western blotting, and serum phosphate and calcium concentration by photometry. Lithium treatment significantly increased renal GSK3 phosphorylation, enhanced serum ADH and FGF23 concentrations, downregulated renal Klotho expression, stimulated renal calcium and phosphate excretion, and decreased serum 1,25(OH)2D3 and phosphate concentrations. In conclusion, lithium treatment upregulates FGF23 formation, an effect paralleled by substantial decrease of serum 1,25(OH)2D3, and phosphate concentrations and thus possibly affecting tissue calcification.

FGF-23 and secondary hyperparathyroidism in chronic kidney disease

The metabolic changes that occur in patients with chronic kidney disease (CKD) have a profound influence on mineral and bone metabolism. CKD results in altered levels of serum phosphate, vitamin D, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23); the increased levels of serum phosphate, PTH and FGF-23 contribute to the increased cardiovascular mortality in affected patients. FGF-23 is produced by osteocytes and osteoblasts and acts physiologically in the kidney to induce phosphaturia and inhibit the synthesis of 1,25-dihydroxyvitamin D3. PTH acts directly on osteocytes to increase FGF-23 expression. In addition, the high levels of PTH associated with CKD contribute to changes in bone remodelling that result in decreased levels of dentin matrix protein 1 and the release of low-molecular-weight fibroblast growth factors from the bone matrix, which stimulate FGF-23 transcription. A prolonged oral phosphorus load increases FGF-23 expression by a mechanism that includes local changes in the ratio of inorganic phosphate to pyrophosphate in bone. Other factors such as dietary vitamin D compounds, calcium, and metabolic acidosis all increase FGF-23 levels. This Review discusses the mechanisms by which secondary hyperparathyroidism associated with CKD stimulates bone cells to overexpress FGF-23 levels.

The delicate balance between vitamin D, calcium and bone homeostasis: lessons learned from intestinal- and osteocyte-specific VDR null mice

The serum calcium levels and the calcium content of the skeleton are highly interdependent. Indeed, bone requires calcium to preserve its strength, but it is at the same time also the predominant calcium storage from which calcium can be mobilized to supply the serum pool. The active form of vitamin D [1,25(OH)2D] plays a crucial role in regulating the transfer of calcium between blood and bone, evidenced by experimental data obtained from systemic, intestinal-specific and osteocyte-specific vitamin D receptor (Vdr) null mice. In fact, 1,25(OH)2D is required to maintain normocalcemia and bone health by enhancing intestinal calcium absorption when dietary calcium intake is normal/low. When, however, insufficient calcium is absorbed via the intestine, 1,25(OH)2D levels will increase and will act on mature osteoblasts and osteocytes to minimize calcium levels in bone tissue in favor of the blood calcium pool. Mechanistically, the high 1,25(OH)2D levels enhance bone remodeling which leads to osteopenia, and suppress bone matrix mineralization by increasing the levels of mineralization inhibitors, which causes hyperosteoidosis and hypomineralization. Thus, depending on the intestinal calcium acquisition, 1,25(OH)2D will target the intestine and/or the skeleton to maintain calcium levels in serum within a normal range.

Vitamin D signaling in osteocytes: effects on bone and mineral homeostasis

The active form of vitamin D [1,25(OH)2D] is an important regulator of calcium and bone homeostasis, as evidenced by the consequences of 1,25(OH)2D inactivity in man and mice, which include hypocalcemia, hypophosphatemia, secondary hyperparathyroidism and bone abnormalities. The recent generation of tissue-specific (intestine, osteoblast/osteocyte, chondrocyte) vitamin D receptor (Vdr) null mice has provided mechanistic insight in the cell-specific actions of 1,25(OH)2D and their contribution to the integrative physiology of VDR signaling that controls bone and mineral metabolism. These studies have demonstrated that even with normal dietary calcium intake, 1,25(OH)2D is crucial to maintain normal calcium and bone homeostasis and accomplishes this primarily through stimulation of intestinal calcium transport. When, moreover, insufficient calcium is acquired from the diet (severe dietary calcium restriction, lack of intestinal VDR activity), 1,25(OH)2D levels will increase and will directly act on osteoblasts and osteocytes to enhance bone resorption and to suppress bone matrix mineralization. Although this system is essential to maintain normal calcium levels in blood during a negative calcium balance, the consequences for bone are disastrous and generate an increased fracture risk. These findings evidently demonstrate that preservation of serum calcium levels has priority over skeletal integrity. Since vitamin D supplementation is an essential part of anti-osteoporotic therapy, mechanistic insight in vitamin D actions is required to define the optimal therapeutic regimen, taking into account the amount of dietary calcium supply, in order to maximize the targeted outcome and to avoid side-effects. We will review the current understanding concerning the functions of osteoblastic/osteocytic VDR signaling which not only include the regulation of bone metabolism, but also comprise the control of calcium and phosphate homeostasis via fibroblast growth factor (FGF) 23 secretion and the maintenance of the hematopoeitic stem cell (HSC) niche, with special focus on the experimental data obtained from systemic and osteoblast/osteocyte-specific Vdr null mice.

Vitamin d status and spine surgery outcomes

There is a high prevalence of hypovitaminosis D in patients with back pain regardless of whether or not they require surgical intervention. Furthermore, the risk of hypovitaminosis D is not limited to individuals with traditional clinical risk factors. Vitamin D plays an essential role in bone formation, maintenance, and remodeling, as well as muscle function. Published data indicate that hypovitaminosis D could adversely affect bone formation and muscle function in multiple ways. The literature contains numerous reports of myopathy and/or musculoskeletal pain associated with hypovitaminosis D. In terms of spinal fusion outcomes, a patient may have a significant decrease in pain and the presence of de novo bone on an X-ray, yet their functional ability may remain severely limited. Hypovitaminosis D may be a contributing factor to the persistent postoperative pain experienced by these patients. Indeed, hypovitaminosis D is not asymptomatic, and symptoms can manifest themselves independent of the musculoskeletal pathological changes associated with conditions like osteomalacia. It appears that vitamin D status is routinely overlooked, and there is a need to raise awareness about its importance among all healthcare practitioners who treat spine patients.

The ras-GTPase activity of neurofibromin restrains ERK-dependent FGFR signaling during endochondral bone formation

The severe defects in growth plate development caused by chondrocyte extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) gain or loss-of-function suggest that tight spatial and temporal regulation of mitogen-activated protein kinase signaling is necessary to achieve harmonious growth plate elongation and structure. We provide here evidence that neurofibromin, via its Ras guanosine triphosphatase -activating activity, controls ERK1/2-dependent fibroblast growth factor receptor (FGFR) signaling in chondrocytes. We show first that neurofibromin is expressed in FGFR-positive prehypertrophic and hypertrophic chondrocytes during growth plate endochondral ossification. Using mice lacking neurofibromin 1 (Nf1) in type II collagen-expressing cells, (Nf1col2(-/-) mutant mice), we then show that lack of neurofibromin in post-mitotic chondrocytes triggers a number of phenotypes reminiscent of the ones observed in mice characterized by FGFR gain-of-function mutations. Those include dwarfism, constitutive ERK1/2 activation, strongly reduced Ihh expression and decreased chondrocyte proliferation and maturation, increased chondrocytic expression of Rankl, matrix metalloproteinase 9 (Mmp9) and Mmp13 and enhanced growth plate osteoclastogenesis, as well as increased sensitivity to caspase-9 mediated apoptosis. Using wildtype (WT) and Nf1(-/-) chondrocyte cultures in vitro, we show that FGF2 pulse-stimulation triggers rapid ERK1/2 phosphorylation in both genotypes, but that return to the basal level is delayed in Nf1(-/-) chondrocytes. Importantly, in vivo ERK1/2 inhibition by daily injection of a recombinant form of C-type natriuretic peptide to post-natal pups for 18 days was able to correct the short stature of Nf1col2(-/-) mice. Together, these results underscore the requirement of neurofibromin and ERK1/2 for normal endochondral bone formation and support the notion that neurofibromin, by restraining RAS-ERK1/2 signaling, is a negative regulator of FGFR signaling in differentiating chondrocytes.

Vitamin D metabolism and effects on pluripotency genes and cell differentiation in testicular germ cell tumors in vitro and in vivo

Testicular germ cell tumors (TGCTs) are classified as either seminomas or nonseminomas. Both tumors originate from carcinoma in situ (CIS) cells, which are derived from transformed fetal gonocytes. CIS, seminoma, and the undifferentiated embryonal carcinoma (EC) retain an embryonic phenotype and express pluripotency factors (NANOG/OCT4). Vitamin D (VD) is metabolized in the testes, and here, we examined VD metabolism in TGCT differentiation and pluripotency regulation. We established that the VD receptor (VDR) and VD-metabolizing enzymes are expressed in human fetal germ cells, CIS, and invasive TGCTs. VD metabolism diminished markedly during the malignant transformation from CIS to EC but was reestablished in differentiated components of nonseminomas, distinguished by coexpression of mesodermal markers and loss of OCT4. Subsequent in vitro studies confirmed that 1,25(OH)(2)D(3) (active VD) downregulated NANOG and OCT4 through genomic VDR activation in EC-derived NTera2 cells and, to a lesser extent, in seminoma-derived TCam-2 cells, and up-regulated brachyury, SNAI1, osteocalcin, osteopontin, and fibroblast growth factor 23. To test for a possible therapeutic effect in vivo, NTera2 cells were xenografted into nude mice and treated with 1,25(OH)(2)D(3), which induced down-regulation of pluripotency factors but caused no significant reduction of tumor growth. During NTera2 tumor formation, down-regulation of VDR was observed, resulting in limited responsiveness to cholecalciferol and 1,25(OH)(2)D(3) treatment in vivo. These novel findings show that VD metabolism is involved in the mesodermal transition during differentiation of cancer cells with embryonic stem cell characteristics, which points to a function for VD during early embryonic development and possibly in the pathogenesis of TGCTs.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

Advances in osteoclast biology reveal potential new drug targets and new roles for osteoclasts

Osteoclasts are multinucleated myeloid lineage cells formed in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by fusion of bone marrow-derived precursors that circulate in the blood and are attracted to sites of bone resorption in response to factors, such as sphingosine-1 phosphate signaling. Major advances in understanding of the molecular mechanisms regulating osteoclast functions have been made in the past 20 years, mainly from mouse and human genetic studies. These have revealed that osteoclasts express and respond to proinflammatory and anti-inflammatory cytokines. Some of these cytokines activate NF-κB and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling to induce osteoclast formation and activity and also regulate communication with neighboring cells through signaling proteins, including ephrins and semaphorins. Osteoclasts also positively and negatively regulate immune responses and osteoblastic bone formation. These advances have led to development of new inhibitors of bone resorption that are in clinical use or in clinical trials; and more should follow, based on these advances. This article reviews current understanding of how bone resorption is regulated both positively and negatively in normal and pathologic states.

Role and regulation of vascularization processes in endochondral bones

Adequate vascularization is an absolute requirement for bone development, growth, homeostasis, and repair. Endochondral ossification during fetal skeletogenesis is typified by the initial formation of a prefiguring cartilage template of the future bone, which itself is intrinsically avascular. When the chondrocytes reach terminal hypertrophic differentiation they become invaded by blood vessels. This neovascularization process triggers the progressive replacement of the growing cartilage by bone, in a complex multistep process that involves the coordinated activity of chondrocytes, osteoblasts, and osteoclasts, each standing in functional interaction with the vascular system. Studies using genetically modified mice have started to shed light on the molecular regulation of the cartilage neovascularization processes that drive endochondral bone development, growth, and repair, with a prime role being played by vascular endothelial growth factor and its isoforms. The vasculature of bone remains important throughout life as an intrinsic component of the bone and marrow environment. Bone remodeling, the continual renewal of bone by the balanced activities of osteoclasts resorbing packets of bone and osteoblasts building new bone, takes place in close spatial relationship with the vascular system and depends on signals, oxygen, and cellular delivery via the bloodstream. Conversely, the integrity and functionality of the vessel system, including the exchange of blood cells between the hematopoietic marrow and the circulation, rely on a delicate interplay with the cells of bone. Here, the current knowledge on the cellular relationships and molecular crosstalk that coordinate skeletal vascularization in bone development and homeostasis will be reviewed.

Engineering vascularized bone: osteogenic and proangiogenic potential of murine periosteal cells

One of the key challenges in bone tissue engineering is the timely formation of blood vessels that promote the survival of the implanted cells in the construct. Fracture healing largely depends on the presence of an intact periosteum but it is still unknown whether periosteum-derived cells (PDC) are critical for bone repair only by promoting bone formation or also by inducing neovascularization. We first established a protocol to specifically isolate murine PDC (mPDC) from long bones of adult mice. Mesenchymal stem cells were abundantly present in this cell population as more than 50% of the mPDC expressed mesenchymal markers (CD73, CD90, CD105, and stem cell antigen-1) and the cells exhibited trilineage differentiation potential (chondrogenic, osteogenic, and adipogenic). When transplanted on a collagen-calcium phosphate scaffold in vivo, mPDC attracted numerous blood vessels and formed mature bone which comprises a hematopoiesis-supportive stroma. We explored the proangiogenic properties of mPDC using in vitro culture systems and showed that mPDC promote the survival and proliferation of endothelial cells through the production of vascular endothelial growth factor. Coimplantation with endothelial cells demonstrated that mPDC can enhance vasculogenesis by adapting a pericyte-like phenotype, in addition to their ability to stimulate blood vessel ingrowth from the host. In conclusion, these findings demonstrate that periosteal cells contribute to fracture repair, not only through their strong osteogenic potential but also through their proangiogenic features and thus provide an ideal cell source for bone regeneration therapies.