The chaperone activity of 4PBA ameliorates the skeletal phenotype of Chihuahua, a zebrafish model for dominant osteogenesis imperfecta

Classical osteogenesis imperfecta (OI) is a bone disease caused by type I collagen mutations and characterized by bone fragility, frequent fractures in absence of trauma and growth deficiency. No definitive cure is available for OI and to develop novel drug therapies, taking advantage of a repositioning strategy, the small teleost zebrafish (Danio rerio) is a particularly appealing model. Its small size, high proliferative rate, embryo transparency and small amount of drug required make zebrafish the model of choice for drug screening studies, when a valid disease model is available. We performed a deep characterization of the zebrafish mutant Chihuahua, that carries a G574D (p.G736D) substitution in the α1 chain of type I collagen. We successfully validated it as a model for classical OI. Growth of mutants was delayed compared with WT. X-ray, µCT, alizarin red/alcian blue and calcein staining revealed severe skeletal deformity, presence of fractures and delayed mineralization. Type I collagen extracted from different tissues showed abnormal electrophoretic migration and low melting temperature. The presence of endoplasmic reticulum (ER) enlargement due to mutant collagen retention in osteoblasts and fibroblasts of mutant fish was shown by electron and confocal microscopy. Two chemical chaperones, 4PBA and TUDCA, were used to ameliorate the cellular stress and indeed 4PBA ameliorated bone mineralization in larvae and skeletal deformities in adult, mainly acting on reducing ER cisternae size and favoring collagen secretion. In conclusion, our data demonstrated that ER stress is a novel target to ameliorate OI phenotype; chemical chaperones such as 4PBA may be, alone or in combination, a new class of molecules to be further investigated for OI treatment.

A novel COL1A2 C-propeptide cleavage site mutation causing high bone mass osteogenesis imperfecta with a regional distribution pattern

Osteogenesis imperfecta (OI) is typically characterized by low bone mass and increased bone fragility caused by heterozygous mutations in the type I procollagen genes (COL1A1/COL1A2). We report two cases of a 56-year-old woman and her 80-year-old mother who suffered from multiple vertebral and non-vertebral fractures with onset in early childhood. A full osteologic assessment including dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses pointed to a high bone mineral density (BMD) in the hip (DXA Z-score + 3.7 and + 3.9) but low to normal bone mass in the spine and preserved bone microstructure in the distal tibia. Serum markers of bone formation and bone resorption were elevated. Using whole exome sequencing, we identified a novel mutation in the COL1A2 gene causing a p. (Asp1120Gly) substitution at the protein level and affecting the type I procollagen C-propeptide cleavage site. In line with previously reported cases, our data independently prove the existence of an unusual phenotype of high bone mass OI caused by a mutation in the procollagen C-propeptide cleavage with a clinically persistent phenotype through adulthood.

Clinical Significance of DXA and HR-pQCT in Autosomal Dominant Osteopetrosis (ADO II)

The main hallmark of high bone mass (HBM) disorders is increased bone mineral density, potentially visible in conventional radiographs and quantifiable by other radiographic methods. While one of the most common forms of HBM is CLCN7-related autosomal dominant osteopetrosis type II (ADO II), there is no consensus on diagnostic thresholds. We therefore wanted to assess whether CLCN7-osteopetrosis patients differ from benign HBM cases in terms of (1) bone mineral density, (2) bone structure, and (3) microarchitectural abnormalities. 16 patients meeting the criteria of HBM (DXA T/Z-score ≥ 2.5 at all sites) were included in this retrospective study. Osteologic assessment using dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses was performed. The presence of CLCN7 and/or other HBM gene mutations affecting bone mass were tested using a custom designed bone panel. While a DXA threshold for ADO II could be implemented (DXA Z-score ≥ + 6.0), the differences in bone microarchitecture were of lesser extent compared to the benign HBM group. All adult patients with ADO II suffered from elevated fracture rates independent from Z-score. In HR-pQCT, structural alterations, such as bone islets were found only inconsistently. In cases of HBM, a DXA Z-score ≥ 6 may be indicative for an inheritable HBM disorder, such as ADO II. Microarchitectural bone alterations might represent local microfracture repair or accumulation of cartilage remnants due to impaired osteoclast function, but seem not to be correlated with fracture risk.

High Bone Turnover in Mice Carrying a Pathogenic Notch2 Mutation Causing Hajdu-Cheney Syndrome

Hajdu-Cheney syndrome (HCS) is a rare autosomal-dominant disorder primarily characterized by acro-osteolysis and early-onset osteoporosis. Genetically, HCS is caused by nonsense or deletion mutations within exon 34 of the NOTCH2 gene, resulting in premature translational termination and production of C-terminally truncated NOTCH2 proteins that are predicted to activate NOTCH2-dependent signaling. To understand the role of Notch2 in bone remodeling, we developed a mouse model of HCS by introducing a pathogenic mutation (6272delT) into the murine Notch2 gene. By μCT and undecalcified histology, we observed generalized osteopenia in two independent mouse lines derived by injection of different targeted embryonic stem (ES) cell clones, yet acro-osteolysis did not occur until the age of 52 weeks. Cellular and dynamic histomorphometry revealed a high bone turnover situation in Notch2^+/HCS mice, since osteoblast and osteoclast indices were significantly increased compared with wild-type littermates. Whereas ex vivo cultures failed to uncover cell-autonomous gain-of-functions within the osteoclast or osteoblast lineage, an unbiased RNA sequencing approach identified Tnfsf11 and Il6 as Notch-signaling target genes in bone marrow cells cultured under osteogenic conditions. Because we further observed that the high-turnover pathology of Notch2^+/HCS mice was fully normalized by alendronate treatment, our results demonstrate that mutational activation of Notch2 does not directly control osteoblast activity but favors a pro-osteoclastic gene expression pattern, which in turn triggers high bone turnover. © 2017 American Society for Bone and Mineral Research.

Mast Cells Are Critical Regulators of Bone Fracture-Induced Inflammation and Osteoclast Formation and Activity

Mast cells, important sensor and effector cells of the immune system, may influence bone metabolism as their number is increased in osteoporotic patients. They are also present during bone fracture healing with currently unknown functions. Using a novel c-Kit-independent mouse model of mast cell deficiency, we demonstrated that mast cells did not affect physiological bone turnover. However, they triggered local and systemic inflammation after fracture by inducing release of inflammatory mediators and the recruitment of innate immune cells. In later healing stages, mast cells accumulated and regulated osteoclast activity to remodel the bony fracture callus. Furthermore, they were essential to induce osteoclast formation after ovariectomy. Additional in vitro studies revealed that they promote osteoclastogenesis via granular mediators, mainly histamine. In conclusion, mast cells are redundant in physiologic bone turnover but exert crucial functions after challenging the system, implicating mast cells as a potential target for treating inflammatory bone disorders. © 2017 American Society for Bone and Mineral Research.

Post-weaning epiphysiolysis causes distal femur dysplasia and foreshortened hindlimbs in fetuin-A-deficient mice

Fetuin-A / α2-Heremans-Schmid-glycoprotein (gene name Ahsg) is a systemic inhibitor of ectopic calcification. Due to its high affinity for calcium phosphate, fetuin-A is highly abundant in mineralized bone matrix. Foreshortened femora in fetuin-A-deficient Ahsg-/- mice indicated a role for fetuin-A in bone formation. We studied early postnatal bone development in fetuin-A-deficient mice and discovered that femora from Ahsg-/- mice exhibited severely displaced distal epiphyses and deformed growth plates, similar to the human disease slipped capital femoral epiphysis (SCFE). The growth plate slippage occurred in 70% of Ahsg-/- mice of both sexes around three weeks postnatal. At this time point, mice weaned and rapidly gained weight and mobility. Epiphysis slippage never occurred in wildtype and heterozygous Ahsg+/- mice. Homozygous fetuin-A-deficient Ahsg-/- mice and, to a lesser degree, heterozygous Ahsg+/- mice showed lesions separating the proliferative zone from the hypertrophic zone of the growth plate. The hypertrophic growth plate cartilage in long bones from Ahsg-/- mice was significantly elongated and V-shaped until three weeks of age and thus prior to the slippage. Genome-wide transcriptome analysis of laser-dissected distal femoral growth plates from 13-day-old Ahsg-/- mice revealed a JAK-STAT-mediated inflammatory response including a 550-fold induction of the chemokine Cxcl9. At this stage, vascularization of the elongated growth plates was impaired, which was visualized by immunofluorescence staining. Thus, fetuin-A-deficient mice may serve as a rodent model of growth plate pathologies including SCFE and inflammatory cartilage degradation.

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.

Severe bone loss and multiple fractures in SCN8A-related epileptic encephalopathy

Mutations in the SCN8A gene encoding the neuronal voltage-gated sodium channel Nav1.6 are known to be associated with epileptic encephalopathy type 13. We identified a novel de novo SCN8A mutation (p.Phe360Ala, c.1078_1079delTTinsGC, Exon 9) in a 6-year-old girl with epileptic encephalopathy accompanied by severe juvenile osteoporosis and multiple skeletal fractures, similar to three previous case reports. Skeletal assessment using dual energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT) and serum analyses revealed a combined trabecular and cortical bone loss syndrome with elevated bone resorption. Likewise, when we analyzed the skeletal phenotype of 2week-old Scn8a-deficient mice we observed reduced trabecular and cortical bone mass, as well as increased osteoclast indices by histomorphometric quantification. Based on this cumulative evidence the patient was treated with neridronate (2mg/kg body weight administered every 3months), which fully prevented additional skeletal fractures for the next 25months. Taken together, our data provide evidence for a negative impact of SCN8A mutations on bone mass, which can be positively influenced by anti-resorptive treatment.

Tissue-specific mineralization defects in the periodontium of the Hyp mouse model of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a dento-osseous disorder caused by inactivating mutations in the PHEX gene, leading to renal phosphate wasting and hypophosphatemia, and impaired mineralization of bones and teeth. In the oral cavity, recent reports suggest a higher susceptibility of XLH patients to periodontitis, where patients present with impaired tooth cementum - a bone-like tissue involved in tooth attachment to the jaw bones and post-eruption tooth positioning - and a higher frequency of intrabony defects. In the present study, the pathobiology of alveolar bone and tooth cementum was investigated in the Hyp mouse, the murine analog of XLH. PHEX deficiency in XLH/Hyp dramatically alters the periodontal phenotype, with hypoplasia of tooth root cementum associated with a lack of periodontal ligament attachment and the presence of an immature apatitic mineral phase of all periodontal mineralized tissues. Challenging the Hyp periodontium in two surgical experimental models - ligature-induced periodontal breakdown and repair, and a model of tooth movement adaptation inducing cementum formation - we show that bone and cementum formation, and their healing, are altered. Bone and cementum mineralization appear similarly disturbed, where hypomineralized pericellular matrix surrounds cells, and where the protein osteopontin (OPN, a mineralization inhibitor) accumulates in a tissue-specific manner, most notably in the perilacunar matrix surrounding osteocytes. Although the pathobiology is different between XLH/Hyp bone and cementum, our results show a major XLH phenotype in oral mineralized tissues consistent with variations in patient susceptibility to periodontal disorders.

Calcium and vitamin-D deficiency marginally impairs fracture healing but aggravates posttraumatic bone loss in osteoporotic mice

Calcium and vitamin-D (Ca/VitD) deficiency is a major risk factor for osteoporosis. It may also contribute to the compromised bone healing frequently observed in osteoporotic patients, since calcium is essential for fracture-callus mineralization. Additionally, clinical data suggest systemic bone loss following fracture, which may aggravate osteoporosis and thus increase the risk for fragility fractures in osteoporotic patients further. However, the role of Ca/VitD in fracture healing and posttraumatic bone turnover has to date been poorly investigated. Here, we studied bone regeneration and posttraumatic bone turnover in C57BL/6 J mice with ovariectomy-induced osteoporosis. Mice were fed a standard or a Ca/VitD-deficient diet. Notably, fracture healing was only marginally disturbed in Ca/VitD-deficient mice. However, deficient mice displayed significantly increased serum parathyroid hormone levels and osteoclast activity, as well as reduced bone mass in the intact skeleton post-fracture, suggesting considerably enhanced calcium mobilization from the intact skeleton during bone regeneration. Ca/VitD supplementation initiated post-fracture prevented posttraumatic bone loss by reducing bone resorption and furthermore improved bone repair. These results imply that adequate Ca/VitD supply post-fracture is essential to provide sufficient calcium for callus-mineralization in order to prevent posttraumatic bone loss and to reduce the risk for secondary fractures in osteoporotic patients with Ca/VitD deficiency.

The Lrp4R1170Q Homozygous Knock-In Mouse Recapitulates the Bone Phenotype of Sclerosteosis in Humans

Sclerosteosis is a rare autosomal recessive bone disorder marked by hyperostosis of the skull and tubular bones. Initially, we and others reported that sclerosteosis was caused by loss-of-function mutations in SOST, encoding sclerostin. More recently, we identified disease-causing mutations in LRP4, a binding partner of sclerostin, in three sclerosteosis patients. Upon binding to sclerostin, LRP4 can inhibit the canonical WNT signaling that is known to be an important pathway in the regulation of bone formation. To further investigate the role of LRP4 in the bone formation process, we generated an Lrp4 mutated sclerosteosis mouse model by introducing the p.Arg1170Gln mutation in the mouse genome. Extensive analysis of the bone phenotype of the Lrp4R1170Q/R1170Q knock-in (KI) mouse showed the presence of increased trabecular and cortical bone mass as a consequence of increased bone formation by the osteoblasts. In addition, three-point bending analysis also showed that the increased bone mass results in increased bone strength. In contrast to the human sclerosteosis phenotype, we could not observe syndactyly in the forelimbs or hindlimbs of the Lrp4 KI animals. Finally, we could not detect any significant changes in the bone formation and resorption markers in the serum of the mutant mice. However, the serum sclerostin levels were strongly increased and the level of sclerostin in the tibia was decreased in Lrp4R1170Q/R1170Q mice, confirming the role of LRP4 as an anchor for sclerostin in bone. In conclusion, the Lrp4R1170Q/R1170Q mouse is a good model for the human sclerosteosis phenotype caused by mutations in LRP4 and can be used in the future for further investigation of the mechanism whereby LRP4 regulates bone formation. © 2017 American Society for Bone and Mineral Research.

Differential effects of Calca-derived peptides in male mice with diet-induced obesity

Key metabolic hormones, such as insulin, leptin, and adiponectin, have been studied extensively in obesity, however the pathophysiologic relevance of the calcitonin family of peptides remains unclear. This family includes calcitonin (CT), its precursor procalcitonin (PCT), and alpha calcitonin-gene related peptide (αCGRP), which are all encoded by the gene Calca. Here, we studied the role of Calca-derived peptides in diet-induced obesity (DIO) by challenging Calcr-/- (encoding the calcitonin receptor, CTR), Calca-/-, and αCGRP-/- mice and their respective littermates with high-fat diet (HFD) feeding for 16 weeks. HFD-induced pathologies were assessed by glucose tolerance, plasma cytokine and lipid markers, expression studies and histology. We found that DIO in mice lacking the CTR resulted in impaired glucose tolerance, features of enhanced nonalcoholic steatohepatitis (NASH) and adipose tissue inflammation compared to wildtype littermates. Furthermore, CTR-deficient mice were characterized by dyslipidemia and elevated HDL levels. In contrast, mice lacking Calca were protected from DIO, NASH and adipose tissue inflammation, and displayed improved glucose tolerance. Mice exclusively lacking αCGRP displayed a significantly less improved DIO phenotype compared to Calca-deficient mice. In summary, we demonstrate that the CT/CTR axis is involved in regulating plasma cholesterol levels while Calca, presumably through PCT, seems to have a detrimental effect in the context of metabolic disease. Our study provides the first comparative analyses of the roles of Calca-derived peptides and the CTR in metabolic disease.

Osteoblast-specific overexpression of complement receptor C5aR1 impairs fracture healing

The anaphylatoxin receptor C5aR1 plays an important role not only in innate immune responses, but also in bone metabolism and fracture healing, being highly expressed on immune and bone cells, including osteoblasts and osteoclasts. C5aR1 induces osteoblast migration, cytokine generation and osteoclastogenesis, however, the exact role of C5aR1-mediated signaling in osteoblasts is not entirely known. Therefore, we hypothesized that osteoblasts are essential target cells for C5a and that fracture healing should be disturbed in mice with an osteoblast-specific C5aR1 overexpression (Col1a1-C5aR1). Osteoblast activity in vitro, bone phenotype and fracture healing after isolated osteotomy and after combined osteotomy with additional thoracic trauma were analyzed. The systemic and local inflammatory reactions were analyzed by determining C5a and IL-6 concentrations in blood, bronchoalveolar lavage fluid and fracture callus and the recruitment of immune cells. In vitro, osteoblast proliferation and differentiation were similar to wildtype cells, and phosphorylation of p38 and expression of IL-6 and RANKL were increased in osteoblasts derived from Col1a1-C5aR1 mice. Bone phenotype and the inflammatory reaction were unaffected in Col1a1-C5aR1 mice. Fracture healing was significantly impaired as demonstrated by significantly reduced bone content, bone mineral density and flexural rigidity, possibly due to significantly increased osteoclast numbers. C5aR1 signaling in osteoblasts might possibly affect RANKL/OPG balance, leading to increased bone resorption. Additional trauma significantly impaired fracture healing, particularly in Col1a1-C5aR1 mice. In conclusion, the data indicate that C5aR1 signaling in osteoblasts plays a detrimental role in bone regeneration after fracture.

A Novel ANO5 Mutation Causing Gnathodiaphyseal Dysplasia With High Bone Turnover Osteosclerosis

Gnathodiaphyseal dysplasia (GDD) is a rare skeletal syndrome that involves an osteopetrosis-like sclerosis of the long bones and fibrous dysplasia-like cemento-osseous lesions of the jawbone. Although the genetic analysis of the respective patients has revealed mutations in the ANO5 gene as an underlying cause, there is still no established consensus regarding the bone status of GDD patients. We report a new case of GDD in a 13-year-old boy with recurrent diaphyseal fractures of the femur, in whom we identified a novel de novo missense mutation in the ANO5 gene, causing a p.Ser500Phe substitution at the protein level. After confirming the presence of GDD-characteristic abnormalities within the jaw bones, we focused on a full osteologic assessment using dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses. We thereby identified increased trabecular bone mass accompanied by elevated serum markers of bone formation and bone resorption. The high turnover bone pathology was further confirmed through the analysis of an iliac crest biopsy, where osteoblast and osteoclast indices were remarkably increased. Taken together, our findings provide evidence for a critical and generalized role of anoctamin-5 (the protein encoded by the ANO5 gene) in skeletal biology. As it is reasonable to speculate that modifying the function of anoctamin-5 might be useful for therapeutically activating bone remodeling, it is now required to analyze its function at a molecular level, for instance in mouse models. © 2016 American Society for Bone and Mineral Research.

Hypochlorhydria-induced calcium malabsorption does not affect fracture healing but increases post-traumatic bone loss in the intact skeleton

Efficient calcium absorption is essential for skeletal health. Patients with impaired gastric acidification display low bone mass and increased fracture risk because calcium absorption is dependent on gastric pH. We investigated fracture healing and post-traumatic bone turnover in mice deficient in Cckbr, encoding a gastrin receptor that affects acid secretion by parietal cells. Cckbr-/- mice display hypochlorhydria, calcium malabsorption, and osteopenia. Cckbr-/- and wildtype (WT) mice received a femur osteotomy and were fed either a standard or calcium-enriched diet. Healed and intact bones were assessed by biomechanical testing, histomorphometry, micro-computed tomography, and quantitative backscattering. Parathyroid hormone (PTH) serum levels were determined by enzyme-linked immunosorbent assay. Fracture healing was unaffected in Cckbr-/- mice. However, Cckbr-/- mice displayed increased calcium mobilization from the intact skeleton during bone healing, confirmed by significantly elevated PTH levels and osteoclast numbers compared to WT mice. Calcium supplementation significantly reduced secondary hyperparathyroidism and bone resorption in the intact skeleton in both genotypes, but more efficiently in WT mice. Furthermore, calcium administration improved bone healing in WT mice, indicated by significantly increased mechanical properties and bone mineral density of the fracture callus, whereas it had no significant effect in Cckbr-/- mice. Therefore, under conditions of hypochlorhydria-induced calcium malabsorption, calcium, which is essential for callus mineralization, appears to be increasingly mobilized from the intact skeleton in favor of fracture healing. Calcium supplementation during fracture healing prevented systemic calcium mobilization, thereby maintaining bone mass and improving fracture healing in healthy individuals whereas the effect was limited by gastric hypochlorhydria. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1914-1921, 2016.

Parathyroid hormone induces expression and proteolytic processing of Rankl in primary murine osteoblasts

Rankl, the major pro-osteoclastogenic cytokine, is synthesized as a transmembrane protein that can be cleaved by specific endopeptidases to release a soluble form (sRankl). We have previously reported that interleukin-33 (IL-33) induces expression of Tnfsf11, the Rankl-encoding gene, in primary osteoblasts, but we failed to detect sRankl in the medium. Since we also found that PTH treatment caused sRankl release in a similar experimental setting, we directly compared the influence of the two molecules. Here we show that treatment of primary murine osteoblasts with PTH causes sRankl release into the medium, whereas IL-33 only induces Tnfsf11 expression. This difference was not explainable by alternative splicing or by PTH-specific induction of endopeptidases previously shown to facilitate Rankl processing. Since sRankl release after PTH administration was blocked in the presence a broad-spectrum matrix metalloprotease inhibitor, we applied genome-wide expression analyses to identify transcriptional targets of PTH in osteoblasts. We thereby confirmed some of the effects of PTH established in other systems, but additionally identified few PTH-induced genes encoding metalloproteases. By comparing expression of these genes following administration of IL-33, PTH and various other Tnfsf11-inducing molecules, we observed that PTH was the only molecule simultaneously inducing sRankl release and Adamts1 expression. The functional relevance of the putative influence of PTH on Rankl processing was further confirmed in vivo, as we found that daily injection of PTH into wildtype mice did not only increase bone formation, but also osteoclastogenesis and sRankl concentrations in the serum. Taken together, our findings demonstrate that transcriptional effects on Tnfsf11 expression do not generally trigger sRankl release and that PTH has a unique activity to promote the proteolytic processing of Rankl.

Inhibition of Midkine Augments Osteoporotic Fracture Healing

The heparin-binding growth and differentiation factor midkine (Mdk) is proposed to negatively regulate osteoblast activity and bone formation in the adult skeleton. As Mdk-deficient mice were protected from ovariectomy (OVX)-induced bone loss, this factor may also play a role in the pathogenesis of postmenopausal osteoporosis. We have previously demonstrated that Mdk negatively influences bone regeneration during fracture healing. Here, we investigated whether the inhibition of Mdk using an Mdk-antibody (Mdk-Ab) improves compromised bone healing in osteoporotic OVX-mice. Using a standardized femur osteotomy model, we demonstrated that Mdk serum levels were significantly enhanced after fracture in both non-OVX and OVX-mice, however, the increase was considerably greater in osteoporotic mice. Systemic treatment with the Mdk-Ab significantly improved bone healing in osteoporotic mice by increasing bone formation in the fracture callus. On the molecular level, we demonstrated that the OVX-induced reduction of the osteoanabolic beta-catenin signaling in the bony callus was abolished by Mdk-Ab treatment. Furthermore, the injection of the Mdk-Ab increased trabecular bone mass in the skeleton of the osteoporotic mice. These results implicate that antagonizing Mdk may be useful for the therapy of osteoporosis and osteoporotic fracture-healing complications.

Antagonizing midkine accelerates fracture healing in mice by enhanced bone formation in the fracture callus

BACKGROUND AND PURPOSE

Previous findings suggest that the growth and differentiation factor midkine (Mdk) is a negative regulator of osteoblast activity and bone formation, thereby raising the possibility that a specific Mdk antagonist might improve bone formation during fracture healing.

EXPERIMENTAL APPROACH

In the present study, we investigated the effects of a monoclonal anti-Mdk antibody (Mdk-Ab) on bone healing using a standardized femur osteotomy model in mice. Additional in vitro experiments using chondroprogenitor and preosteoblastic cells were conducted to analyse the effects of recombinant Mdk and Mdk-Ab on differentiation markers and potential binding partners in these cells.

KEY RESULTS

We demonstrated that treatment with Mdk-Ab accelerated bone healing in mice based on increased bone formation in the fracture callus. In vitro experiments using preosteoblastic cells showed that Mdk-Ab treatment abolished the Mdk-induced negative effects on the expression of osteogenic markers and Wnt/β-catenin target proteins, whereas the differentiation of chondroprogenitor cells was unaffected. Phosphorylation analyses revealed an important role for the low-density lipoproteinLDL receptor-related protein 6 in Mdk signalling in osteoblasts.

CONCLUSIONS AND IMPLICATIONS

We conclude that Mdk-Ab treatment may be a potential novel therapeutic strategy to enhance fracture healing in patients with orthopaedic complications such as delayed healing or non-union formation.

Osteoblast-specific Notch2 inactivation causes increased trabecular bone mass at specific sites of the appendicular skeleton

Notch signaling is a key pathway controlling various cell fate decisions during embryogenesis and adult life. It is activated by binding of specific ligands to four different Notch receptors that are subsequently cleaved by presenilins to release an intracellular domain that enters the nucleus and activates specific transcription factors. While the skeletal analysis of various mouse models with activated or inactivated Notch signaling has demonstrated a general impact of this pathway on bone remodeling, the more recent identification of NOTCH2 mutations in individuals with Hajdu-Cheney syndrome (HCS) has highlighted its human relevance. Since HCS is primarily characterized by skeletal defects, these latter findings led us to analyze the specific role of Notch2 in skeletal remodeling. After observing Notch2 expression in osteoblasts and osteoclasts, we utilized Runx2-Cre and Lyz2-Cre mice to inactivate Notch2 in cells of the osteoblast or osteoclast lineage, respectively. Whereas Notch2(fl/fl)/Lyz2-Cre mice did not display significant alterations of skeletal growth, bone mass or remodeling, Notch2(fl/fl)/Runx2-Cre mice progressively developed skeletal abnormalities in long bones. More specifically, these mice displayed a striking increase of trabecular bone mass in the proximal femur and the distal tibia at 6 and 12months of age. Whereas undecalcified sectioning of the respective regions did not reveal impaired osteocyte differentiation as a potential trigger for the observed phenotype, ex vivo experiments with bone marrow cells identified an increased osteogenic capacity of Notch2(fl/fl)/Runx2-Cre cultures. Collectively, our findings demonstrate that Notch2 physiologically regulates bone remodeling by inhibiting trabecular bone formation in the appendicular skeleton. Understanding the underlying mechanisms may help to improve diagnosis and therapy of HCS.

Mannose 6-phosphate-dependent targeting of lysosomal enzymes is required for normal craniofacial and dental development

Mucolipidosis II (MLII) is a severe systemic genetic disorder caused by defects in mannose 6-phosphate-dependent targeting of multiple lysosomal hydrolases and subsequent lysosomal accumulation of non-degraded material. MLII patients exhibit marked facial coarseness and gingival overgrowth soon after birth, accompanied with delayed tooth eruption and dental infections. To examine the pathomechanisms of early craniofacial and dental abnormalities, we analyzed mice with an MLII patient mutation that mimic the clinical and biochemical symptoms of MLII patients. The mouse data were compared with clinical and histological data of gingiva and teeth from MLII patients. Here, we report that progressive thickening and porosity of calvarial and mandibular bones, accompanied by elevated bone loss due to 2-fold higher number of osteoclasts cause the characteristic craniofacial phenotype in MLII. The analysis of postnatal tooth development by microcomputed tomography imaging and histology revealed normal dentin and enamel formation, and increased cementum thickness accompanied with accumulation of storage material in cementoblasts of MLII mice. Massive accumulation of storage material in subepithelial cells as well as disorganization of collagen fibrils led to gingival hypertrophy. Electron and immunofluorescence microscopy, together with (35)S-sulfate incorporation experiments revealed the accumulation of non-degraded material, non-esterified cholesterol and glycosaminoglycans in gingival fibroblasts, which was accompanied by missorting of various lysosomal proteins (α-fucosidase 1, cathepsin L and Z, Npc2, α-l-iduronidase). Our study shows that MLII mice closely mimic the craniofacial and dental phenotype of MLII patients and reveals the critical role of mannose 6-phosphate-dependent targeting of lysosomal proteins for alveolar bone, cementum and gingiva homeostasis.

Immediate effects of retinoic acid on gene expression in primary murine osteoblasts

Consistent with clinical observations demonstrating that hypervitaminosis A is associated with increased skeletal fracture risk, we have previously found that dietary retinol deprivation partially corrects the bone mineralization defects in a mouse model of X-linked hypophosphatemic rickets. That retinol-dependent signaling pathways impact the skeleton is further supported by various findings demonstrating a negative influence of retinoic acid (RA) on bone-forming osteoblasts. We hypothesized that RA would directly regulate the expression of specific target genes in osteoblasts, and we aimed to identify these by genome-wide expression analyses. Here we show that high dietary retinol intake in mice causes low bone mass associated with increased osteoclastogenesis and decreased osteoblastogenesis, but intact bone matrix mineralization. We additionally found that short-term treatment of primary osteoblasts with RA causes a rapid induction of specific genes involved in either retinol-dependent signaling (i.e. Rara, Crabp2) or skeletal remodeling (i.e. Twist2, Tnfsf11). In contrast, neither expression of established osteoblast differentiation markers nor the proliferation rate was immediately affected by RA administration. Collectively, our data suggest that the negative effects of vitamin A on skeletal integrity are explainable by an immediate influence of RA signaling on specific genes in osteoblasts that in turn influence bone remodeling.

Rsk2, the Kinase Mutated in Coffin-Lowry Syndrome, Controls Cementum Formation

The ribosomal S6 kinase RSK2 is essential for osteoblast function, and inactivating mutations of RSK2 cause osteopenia in humans with Coffin-Lowry syndrome (CLS). Alveolar bone loss and premature tooth exfoliation are also consistently reported symptoms in CLS patients; however, the pathophysiologic mechanisms are unclear. Therefore, aiming to identify the functional relevance of Rsk2 for tooth development, we analyzed Rsk2-deficient mice. Here, we show that Rsk2 is a critical regulator of cementoblast function. Immunohistochemistry, histology, micro-computed tomography imaging, quantitative backscattered electron imaging, and in vitro assays revealed that Rsk2 is activated in cementoblasts and is necessary for proper acellular cementum formation. Cementum hypoplasia that is observed in Rsk2-deficient mice causes detachment and disorganization of the periodontal ligament and was associated with significant alveolar bone loss with age. Moreover, Rsk2-deficient mice display hypomineralization of cellular cementum with accumulation of nonmineralized cementoid. In agreement, treatment of the cementoblast cell line OCCM-30 with a Rsk inhibitor reduces formation of mineralization nodules and decreases the expression of cementum markers. Western blot analyses based on antibodies against Rsk1, Rsk2, and an activated form of the 2 kinases confirmed that Rsk2 is expressed and activated in differentiating OCCM-30 cells. To discriminate between periodontal bone loss and systemic bone loss, we additionally crossed Rsk2-deficient mice with transgenic mice overexpressing the osteoanabolic transcription factor Fra1. Fra1 overexpression clearly increases systemic bone volume in Rsk2-deficient mice but does not protect from alveolar bone loss. Our results indicate that cell autonomous cementum defects are causing early tooth loss in CLS patients. Moreover, we identify Rsk2 as a nonredundant regulator of cementum homeostasis, alveolar bone maintenance, and periodontal health, with all these features being independent of Rsk2 function in systemic bone formation.

Can we induce osteoporosis in animals comparable to the human situation?

Osteoporosis is a chronic systemic bone disease of growing relevance due to the on-going demographic change. Since the underlying regulatory mechanisms of this critical illness are still not fully understood and treatment options are not satisfactorily resolved, there is still a great need for osteoporosis research in general and animal models in particular. Ovariectomized rodents are standard animal models for postmenopausal osteoporosis and highly attractive due to the possibility to specifically modify their genetic background. However, some aspects can only be addressed in large animal models; such as metaphyseal fracture healing and advancement of orthopedic implants. Among other large animal models sheep in particular have been proven invaluable for osteoporosis research in this context. In conclusion, today we are able to influence the bone metabolism in animals causing a more or less pronounced systemic bone loss and structural deterioration comparable to the situation found in patients suffering from osteoporosis. However, there is no perfect model for osteoporosis, but a variety of models appropriate for answering specific questions. Though, the appropriateness of an animal model is not only defined in regard to the similarity to human physiology and the disease itself, but also in regard to acquisition, housing requirements, handling, costs, and particularly ethical concerns and animal welfare.