Ectonucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) protein regulates osteoblast differentiation

The ecto-nucleotide pyrophosphatase/phosphodiesterase 2 promotes early osteoblast differentiation and mineralization in stromal stem cells

The phosphodiesterase enzymes mediate calcium-phosphate deposition in various tissues, although which enzymes are active in bone mineralization is unclear. Using gene array analysis, we found that a member of ecto-nucleotide pyrophosphatase/phosphodiesterase family, ENPP2, was strongly down-regulated with age in stromal stem cells that produce osteoblasts and make bone. This is in keeping with reduced bone formation in older animals. Thus, we hypothesized that ENPP2 is, at least in part, an early mediator of bone formation and thus may reflect reduced bone formation with age. Since ENPP2 has not previously been shown to have a role in osteoblast differentiation, we studied its effect on bone differentiation from stromal stem cells, verified by flow cytometry for stem cell antigens. In these remarkably uniform osteoblast precursors, we did transfection with ENPP2 DsiRNA, scrambled DsiRNA, or no transfection to make cells with normal or greatly reduced ENPP2 and analyzed osteoblast differentiation and mineralization. Osteoblast differentiation down-regulation was shown by alizarin red binding, silver staining, and alkaline phosphatase activity. Differences were confirmed by real-time PCR for alkaline phosphatase (ALPL), osteocalcin (BGLAP), and ENPP2 and by Western Blot for Enpp2. These were decreased, ∼50%, in osteoblasts transfected with ENPP2 DsiRNA compared with cells transfected with a scrambled DsiRNA or not transfected (control) cells. This finding is the first evidence for the role of ENPP2 in osteoblast differentiation and mineralization.NEW & NOTEWORTHY We report the discovery that the ecto-nucleotide pyrophosphatase/phosphodiesterase, ENPP2, is an important regulator of early differentiation of bone-forming osteoblasts.

ENPP1 in Blood and Bone: Skeletal and Soft Tissue Diseases Induced by ENPP1 Deficiency

The enzyme ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) codes for a type 2 transmembrane glycoprotein that hydrolyzes extracellular ATP to generate pyrophosphate (PPi) and adenosine monophosphate, thereby contributing to downstream purinergic signaling pathways. The clinical phenotypes induced by ENPP1 deficiency are seemingly contradictory and include early-onset osteoporosis in middle-aged adults and life-threatening vascular calcifications in the large arteries of infants with generalized arterial calcification of infancy. The progressive overmineralization of soft tissue and concurrent undermineralization of skeleton also occur in the general medical population, where it is referred to as paradoxical mineralization to highlight the confusing pathophysiology. This review summarizes the clinical presentation and pathophysiology of paradoxical mineralization unveiled by ENPP1 deficiency and the bench-to-bedside development of a novel ENPP1 biologics designed to treat mineralization disorders in the rare disease and general medical population.

β-TCP from 3D-printed composite scaffolds acts as an effective phosphate source during osteogenic differentiation of human mesenchymal stromal cells

Introduction: Human bone marrow-derived mesenchymal stromal cells (hBM-MSCs) are often combined with calcium phosphate (CaP)-based 3D-printed scaffolds with the goal of creating a bone substitute that can repair segmental bone defects. In vitro, the induction of osteogenic differentiation traditionally requires, among other supplements, the addition of β-glycerophosphate (BGP), which acts as a phosphate source. The aim of this study is to investigate whether phosphate contained within the 3D-printed scaffolds can effectively be used as a phosphate source during hBM-MSC in vitro osteogenesis. Methods: hBM-MSCs are cultured on 3D-printed discs composed of poly (lactic-co-glycolic acid) (PLGA) and β-tricalcium phosphate (β-TCP) for 28 days under osteogenic conditions, with and without the supplementation of BGP. The effects of BGP removal on various cellular parameters, including cell metabolic activity, alkaline phosphatase (ALP) presence and activity, proliferation, osteogenic gene expression, levels of free phosphate in the media and mineralisation, are assessed. Results: The removal of exogenous BGP increases cell metabolic activity, ALP activity, proliferation, and gene expression of matrix-related (COL1A1, IBSP, SPP1), transcriptional (SP7, RUNX2/SOX9, PPARγ) and phosphate-related (ALPL, ENPP1, ANKH, PHOSPHO1) markers in a donor dependent manner. BGP removal leads to decreased free phosphate concentration in the media and maintained of mineral deposition staining. Discussion: Our findings demonstrate the detrimental impact of exogenous BGP on hBM-MSCs cultured on a phosphate-based material and propose β-TCP embedded within 3D-printed scaffold as a sufficient phosphate source for hBM-MSCs during osteogenesis. The presented study provides novel insights into the interaction of hBM-MSCs with 3D-printed CaP based materials, an essential aspect for the advancement of bone tissue engineering strategies aimed at repairing segmental defects.

Cranial Neural Crest Specific Deletion of Alpl (TNAP) via P0-Cre Causes Abnormal Chondrocyte Maturation and Deficient Cranial Base Growth

Bone growth plate abnormalities and skull shape defects are seen in hypophosphatasia, a heritable disorder in humans that occurs due to the deficiency of tissue nonspecific alkaline phosphatase (TNAP, Alpl) enzyme activity. The abnormal development of the cranial base growth plates (synchondroses) and abnormal skull shapes have also been demonstrated in global Alpl-/- mice. To distinguish local vs. systemic effects of TNAP on skull development, we utilized P0-Cre to knockout Alpl only in cranial neural crest-derived tissues using Alpl flox mice. Here, we show that Alpl deficiency using P0-Cre in cranial neural crest leads to skull shape defects and the deficient growth of the intersphenoid synchondrosis (ISS). ISS chondrocyte abnormalities included increased proliferation in resting and proliferative zones with decreased apoptosis in hypertrophic zones. ColX expression was increased, which is indicative of premature differentiation in the absence of Alpl. Sox9 expression was increased in both the resting and prehypertrophic zones of mutant mice. The expression of Parathyroid hormone related protein (PTHrP) and Indian hedgehog homolog (IHH) were also increased. Finally, cranial base organ culture revealed that inorganic phosphate (Pi) and pyrophosphate (PPi) have specific effects on cell signaling and phenotype changes in the ISS. Together, these results demonstrate that the TNAP expression downstream of Alpl in growth plate chondrocytes is essential for normal development, and that the mechanism likely involves Sox9, PTHrP, IHH and PPi.

Age-related decline in bone mineral transport and bone matrix proteins in osteoblasts from stromal stem cells

We studied osteoblast bone mineral transport and matrix proteins as a function of age. In isolated bone marrow cells from long bones of young (3 or 4 mo) and old (18 or 19 mo) mice, age correlated with reduced mRNA of mineral transport proteins: alkaline phosphatase (ALP), ankylosis (ANK), the Cl-/H+ exchanger ClC3, and matrix proteins collagen 1 (Col1) and osteocalcin (BGLAP). Some proteins, including the neutral phosphate transporter2 (NPT2), were not reduced. These are predominately osteoblast proteins, but in mixed cell populations. Remarkably, in osteoblasts differentiated from preparations of stromal stem cells (SSCs) made from bone marrow cells in young and old mice, differentiated in vitro on perforated polyethylene terephthalate membranes, mRNA confirmed decreased expression with age for most transport-related and bone matrix proteins. Additional mRNAs in osteoblasts in vitro included ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), unchanged, and ENPP2, reduced with age. Decrease with age in ALP activity and protein by Western blot was also significant. Transport protein findings correlated with micro-computed tomography of lumbar vertebra, showing that trabecular bone of old mice is osteopenic relative to young mice, consistent with other studies. Pathway analysis of osteoblasts differentiated in vitro showed that cells from old animals had reduced Erk1/2 phosphorylation and decreased suppressor of mothers against decapentaplegic 2 (Smad2) mRNA, consistent with TGFβ pathway, and reduced β-catenin mRNA, consistent with WNT pathway regulation. Our results show that decline in bone density with age reflects selective changes, resulting effectively in a phenotype modification. Reduction of matrix and mineral transport protein expression with age is regulated by multiple signaling pathways.NEW & NOTEWORTHY This work for the first time showed that specific enzymes in bone mineral transport, and matrix synthesis proteins, in the epithelial-like bone-forming cell layer are downregulated with aging. Results were compared using cells extracted from long bones of young and old mice, or in essentially uniform osteoblasts differentiated from stromal stem cells in vitro. The age effect showed memory in the stromal stem cells, a remarkable finding.

Genome-wide analysis for nanofiber induced global gene expression profile: A study in MC3T3-E1 cells by RNA-Seq

Nanofibers are one of the attractive biomaterials that can provide unique environments to direct cell behaviors. However, how nanofiber structure affects the global gene expression of laden cells remains unclear. Herein, high-throughput mRNA sequencing (RNA-seq) is applied to analyze the transcriptome of the MC3T3-E1 cells (a model osteoblast cell line) cultured on electrospun nanofibers. The cell-adhesive poly(L-lactide) nanofibers and membranes are developed by the mussel-inspired coating of gelatin-dopamine conjugate under H₂O₂-mediated oxidation. The MC3T3-E1 cells cultured on nanofibers exhibit elongated morphology and increased proliferation compared with those on membranes. The differences in global gene expression profiles are determined by RNA-seq, in which 905 differentially expressed genes (DEGs) are identified. Significantly, the DEGs related to cytoskeleton, promotion of cell cycle progression, cell adhesion, and cell proliferation, are higher expressed in the cells on nanofibers, while the DEGs involved in cell-cycle arrest and osteoblast mineralization are up-regulated in the cells on membranes. This study elucidates the roles of nanofiber structure in affecting gene expression of laden cells at the whole transcriptome level, and it will lay the foundation for understanding nanofiber-guided cell behaviors.

Human antibodies targeting ENPP1 as candidate therapeutics for cancers

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is a type II transmembrane glycoprotein expressed in many tissues. High expression levels of ENPP1 have been observed in many cancer types such as lung cancer, ovarian cancer, and breast cancer. Such overexpression has been associated with poor prognosis in these diseases. Hence, ENPP1 is a potential target for immunotherapy across multiple cancers. Here, we isolated and characterized two high-affinity and specific anti-ENPP1 Fab antibody candidates, 17 and 3G12, from large phage-displayed human Fab libraries. After conversion to IgG1, the binding of both antibodies increased significantly due to avidity effects. Based on these antibodies, we generated antibody-drug conjugates (ADCs), IgG-based bispecific T-cell engagers (IbTEs), and CAR T-cells which all exhibited potent killing of ENPP1-expressing cells. Thus, these various antibody-derived modalities are promising therapeutic candidates for cancers expressing human ENPP1.

ENPP1 deficiency: A clinical update on the relevance of individual variants using a locus-specific patient database

Loss-of-function variants in the ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) cause ENPP1 Deficiency, a rare disorder characterized by pathological calcification, neointimal proliferation, and impaired bone mineralization. The consequence of ENPP1 Deficiency is a broad range of age dependent symptoms and morbidities including cardiovascular complications and 50% mortality in infants, autosomal recessive hypophosphatemic rickets type 2 (ARHR2) in children, and joint pain, osteomalacia and enthesopathies in adults. Recent research continues to add to the growing clinical presentation profile as well as expanding the role of ENPP1 itself. Here we review the current knowledge on the spectrum of clinical and genetic findings of ENPP1 Deficiency reported in patients diagnosed with GACI or ARHR2 phenotypes using a comprehensive database of known ENPP1 variants with associated clinical data. A total of 108 genotypes were identified from 154 patients. Of the 109 ENPP1 variants reviewed, 72.5% were demonstrably disease-causing, a threefold increase in pathogenic/likely pathogenic variants over other databases. There is substantial heterogeneity in disease severity, even among patients with the same variant. The approach to creating a continuously curated database of ENPP1 variants accessible to clinicians is necessary to increase the diagnostic yield of clinical genetic testing and accelerate diagnosis of ENPP1 Deficiency.

Catalysis-Independent ENPP1 Protein Signaling Regulates Mammalian Bone Mass

Biallelic ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) deficiency induces vascular/soft tissue calcifications in generalized arterial calcification of infancy (GACI), and low bone mass with phosphate-wasting rickets in GACI survivors (autosomal hypophosphatemic rickets type-2). ENPP1 haploinsufficiency induces early-onset osteoporosis and mild phosphate wasting in adults. Both conditions demonstrate the unusual combination of reduced accrual of skeletal mineral, yet excess and progressive heterotopic mineralization. ENPP1 is the only enzyme that generates extracellular pyrophosphate (PPi), a potent inhibitor of both bone and heterotopic mineralization. Life-threatening vascular calcification in ENPP1 deficiency is due to decreased plasma PPi; however, the mechanism by which osteopenia results is not apparent from an understanding of the enzyme's catalytic activity. To probe for catalysis-independent ENPP1 pathways regulating bone, we developed a murine model uncoupling ENPP1 protein signaling from ENPP1 catalysis, Enpp1T238A mice. In contrast to Enpp1asj mice, which lack ENPP1, Enpp1T238A mice have normal trabecular bone microarchitecture and favorable biomechanical properties. However, both models demonstrate low plasma Pi and PPi, increased fibroblast growth factor 23 (FGF23), and by 23 weeks, osteomalacia demonstrating equivalent phosphate wasting in both models. Reflecting findings in whole bone, calvarial cell cultures from Enpp1asj mice demonstrated markedly decreased calcification, elevated transcription of Sfrp1, and decreased nuclear β-catenin signaling compared to wild-type (WT) and Enpp1T238A cultures. Finally, the decreased calcification and nuclear β-catenin signaling observed in Enpp1asj cultures was restored to WT levels by knockout of Sfrp1. Collectively, our findings demonstrate that catalysis-independent ENPP1 signaling pathways regulate bone mass via the expression of soluble Wnt inhibitors such as secreted frizzled-related protein 1 (SFRP1), whereas catalysis dependent pathways regulate phosphate homeostasis through the regulation of plasma FGF23. © 2022 American Society for Bone and Mineral Research (ASBMR).

Titanium Nanosurface with a Biomimetic Physical Microenvironment to Induce Endogenous Regeneration of the Periodontium

The periodontium supports the teeth by dentoalveolar fibrous joints that serve unique oral functions. Endogenous regeneration of the periodontium around artificial teeth (dental implants) provides a cost-effective solution for the extension of healthy life expectancy but remains a challenge in regenerative medicine. Biomimetics can create smart biomaterials that tune endogenous cells at a tissue-material interface. Here, we created a smart titanium nanosurface mimicking the surface nanotopography and micromechanical properties of the tooth root cementum (TRC), which is essential for the induction of dentoalveolar fibrous joints to regenerate the periodontium. After transplantation into the rat renal capsule, only the titanium artificial tooth with the TRC-mimetic nanosurface formed a complex dentoalveolar fibrous joint structure, with bone tissue, periodontal ligament (PDL), and TRC, in the decellularized jawbone matrix. TRC-mimetic titanium implants induce the formation of functional periodontium, even in a jawbone implantation model, which generally causes osseointegration (ankyloses). In human PDL cells, TRC analogousness in the surface mechanical microenvironment regulates matrix mineralization through bone sialoprotein expression and phosphorus metabolism, which are critical for cementogenesis. Therefore, the titanium nanosurfaces with nanotopographical and mechanical microenvironments mimicking the TRC surface induce dentoalveolar fibrous joints for periodontal regeneration by interfacial tuning of endogenous cells.

Identification of distinct non-myogenic skeletal-muscle-resident mesenchymal cell populations

Mesenchymal progenitors of the lateral plate mesoderm give rise to various cell fates within limbs, including a heterogeneous group of muscle-resident mesenchymal cells. Often described as fibro-adipogenic progenitors, these cells are key players in muscle development, disease, and regeneration. To further define this cell population(s), we perform lineage/reporter analysis, flow cytometry, single-cell RNA sequencing, immunofluorescent staining, and differentiation assays on normal and injured murine muscles. Here we identify six distinct Pdgfra⁺ non-myogenic muscle-resident mesenchymal cell populations that fit within a bipartite differentiation trajectory from a common progenitor. One branch of the trajectory gives rise to two populations of immune-responsive mesenchymal cells with strong adipogenic potential and the capability to respond to acute and chronic muscle injury, whereas the alternative branch contains two cell populations with limited adipogenic capacity and inherent mineralizing capabilities; one of the populations displays a unique neuromuscular junction association and an ability to respond to nerve injury.

Leinroth et al. explore the heterogeneity of Pdgfra⁺ muscle-resident mesenchymal cells, demonstrating that Pdgfra⁺ subpopulations have unique gene expression profiles, exhibit two distinct cell trajectories from a common progenitor, differ in their abilities to respond to muscle injuries, and show variable adipogenic and mineralizing capacities.

Deletion of the Pyrophosphate Generating Enzyme ENPP1 Rescues Craniofacial Abnormalities in the TNAP−/− Mouse Model of Hypophosphatasia and Reveals FGF23 as a Marker of Phenotype Severity

Hypophosphatasia is a rare heritable metabolic disorder caused by deficient Tissue Non-specific Alkaline Phosphatase (TNAP) enzyme activity. A principal function of TNAP is to hydrolyze the tissue mineralization inhibitor pyrophosphate. ENPP1 (Ectonucleotide Pyrophosphatase/Phosphodiesterase 1) is a primary enzymatic generator of pyrophosphate and prior results showed that elimination of ENPP1 rescued bone hypomineralization of skull, vertebral and long bones to different extents in TNAP null mice. Current TNAP enzyme replacement therapy alleviates skeletal, motor and cognitive defects but does not eliminate craniosynostosis in pediatric hypophosphatasia patients. To further understand mechanisms underlying craniosynostosis development in hypophosphatasia, here we sought to determine if craniofacial abnormalities including craniosynostosis and skull shape defects would be alleviated in TNAP null mice by genetic ablation of ENPP1. Results show that homozygous deletion of ENPP1 significantly diminishes the incidence of craniosynostosis and that skull shape abnormalities are rescued by hemi- or homozygous deletion of ENPP1 in TNAP null mice. Skull and long bone hypomineralization were also alleviated in TNAP^−/−/ENPP1^−/− compared to TNAP^−/−/ENPP1^+/+ mice, though loss of ENPP1 in combination with TNAP had different effects than loss of only TNAP on long bone trabeculae. Investigation of a relatively large cohort of mice revealed that the skeletal phenotypes of TNAP null mice were markedly variable. Because FGF23 circulating levels are known to be increased in ENPP1 null mice and because FGF23 influences bone, we measured serum intact FGF23 levels in the TNAP null mice and found that a subset of TNAP^−/−/ENPP1^+/+ mice exhibited markedly high serum FGF23. Serum FGF23 levels also correlated to mouse body measurements, the incidence of craniosynostosis, skull shape abnormalities and skull bone density and volume fraction. Together, our results demonstrate that balanced expression of TNAP and ENPP1 enzymes are essential for microstructure and mineralization of both skull and long bones, and for preventing craniosynostosis. The results also show that FGF23 rises in the TNAP^−/− model of murine lethal hypophosphatasia. Future studies are required to determine if the rise in FGF23 is a cause, consequence, or marker of disease phenotype severity.

Statins as a Therapeutic Approach for the Treatment of Pseudoxanthoma Elasticum Patients: Evaluation of the Spectrum Efficacy of Atorvastatin In Vitro

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder caused by mutations in the ATP-binding cassette sub-family C member 6 gene. Our previous studies revealed that PXE might be associated with premature aging. Treatment with statins showed positive effects not only for PXE but also for other diseases associated with premature aging like Hutchinson–Gilford progeria syndrome. Nevertheless, the molecular mechanisms in the case of PXE remain unclear. Thus, this study was performed to evaluate the efficiency of atorvastatin by analyzing key characteristics of the PXE phenotype in primary human dermal fibroblasts of PXE patients. Our data indicate that an atorvastatin treatment has a positive effect, especially on factors associated with cholesterol biosynthesis and prenylation processes, whereas the effect on age- and calcification-related factors was less pronounced.

Expression of Calcification and Extracellular Matrix Genes in the Cardiovascular System of the Healthy Domestic Sheep (Ovis aries)

The maintenance of a healthy cardiovascular system requires expression of genes that contribute to essential biological activities and repression of those that are associated with functions likely to be detrimental to cardiovascular homeostasis. Vascular calcification is a major disruption to cardiovascular homeostasis, where tissues of the cardiovascular system undergo ectopic calcification and consequent dysfunction, but little is known about the expression of calcification genes in the healthy cardiovascular system. Large animal models are of increasing importance in cardiovascular disease research as they demonstrate more similar cardiovascular features (in terms of anatomy, physiology and size) to humans than do rodent species. We used RNA sequencing results from the sheep, which has been utilized extensively to examine calcification of prosthetic cardiac valves, to explore the transcriptome of the heart and cardiac valves in this large animal, in particular looking at expression of calcification and extracellular matrix genes. We then examined genes implicated in the process of vascular calcification in a wide array of cardiovascular tissues and across multiple developmental stages, using RT-qPCR. Our results demonstrate that there is a balance between genes that promote and those that suppress mineralization during development and across cardiovascular tissues. We show extensive expression of genes encoding proteins involved in formation and maintenance of the extracellular matrix in cardiovascular tissues, and high expression of hematopoietic genes in the cardiac valves. Our analysis will support future research into the functions of implicated genes in the development of valve calcification, and increase the utility of the sheep as a large animal model for understanding ectopic calcification in cardiovascular disease. This study provides a foundation to explore the transcriptome of the developing cardiovascular system and is a valuable resource for the fields of mammalian genomics and cardiovascular research.

Condyle modeling stability, craniofacial asymmetry and ACTN3 genotypes: Contribution to TMD prevalence in a cohort of dentofacial deformities

Craniofacial asymmetry, mandibular condylar modeling and temporomandibular joint disorders are common comorbidities of skeletally disproportionate malocclusions, but etiology of occurrence together is poorly understood. We compared asymmetry, condyle modeling stability and temporomandibular health in a cohort of 128 patients having orthodontics and orthognathic surgery to correct dentofacial deformity malocclusions. We also compared ACTN3 and ENPP1 genotypes for association to clinical conditions. Pre-surgical posterior-anterior cephalometric and panometric radiographic analyses; jaw pain and function questionnaire and clinical examination of TMD; and SNP-genotype analysis from saliva samples were compared to assess interrelationships. Almost half had asymmetries in need of surgical correction, which could be subdivided into four distinct morphological patterns. Asymmetric condyle modeling between sides was significantly greater in craniofacial asymmetry, but most commonly had an unanticipated pattern. Often, longer or larger condyles occurred on the shorter mandibular ramus side. Subjects with longer ramus but dimensionally smaller condyles were more likely to have self-reported TMD symptoms (p = 0.023) and significantly greater clinical diagnosis of TMD (p = 0 .000001), with masticatory myalgia most prominent. Genotyping found two significant genotype associations for ACTN3 rs1671064 (Q523R missense) p = 0.02; rs678397 (intronic SNP) p = 0.04 and one significant allele association rs1815739 (R577X nonsense) p = 0.00. Skeletal asymmetry, unusual condyle modeling and TMD are common and interrelated components of many dentofacial deformities. Imbalanced musculoskeletal functional adaptations and genetic or epigenetic influences contribute to the etiology, and require further investigation.

Differences in intracellular localisation of ANKH mutants that relate to mechanisms of calcium pyrophosphate deposition disease and craniometaphyseal dysplasia

ANKH mutations are associated with calcium pyrophosphate deposition disease and craniometaphyseal dysplasia. This study investigated the effects of these ANKH mutants on cellular localisation and associated biochemistry. We generated four ANKH overexpression-plasmids containing either calcium pyrophosphate deposition disease or craniometaphyseal dysplasia linked mutations: P5L, E490del and S375del, G389R. They were transfected into CH-8 articular chondrocytes and HEK293 cells. The ANKH mutants dynamic differential localisations were imaged and we investigated the interactions with the autophagy marker LC3. Extracellular inorganic pyrophosphate, mineralization, ENPP1 activity expression of ENPP1, TNAP and PIT-1 were measured. P5L delayed cell membrane localisation but once recruited into the membrane it increased extracellular inorganic pyrophosphate, mineralization, and ENPP1 activity. E490del remained mostly cytoplasmic, forming punctate co-localisations with LC3, increased mineralization, ENPP1 and ENPP1 activity with an initial but unsustained increase in TNAP and PIT-1. S375del trended to decrease extracellular inorganic pyrophosphate, increase mineralization. G389R delayed cell membrane localisation, trended to decrease extracellular inorganic pyrophosphate, increased mineralization and co-localised with LC3. Our results demonstrate a link between pathological localisation of ANKH mutants with different degrees in mineralization. Furthermore, mutant ANKH functions are related to synthesis of defective proteins, inorganic pyrophosphate transport, ENPP1 activity and expression of ENPP1, TNAP and PIT-1.

Arterial Stiffness: A Focus on Vascular Calcification and Its Link to Bone Mineralization

This review focuses on the association between vascular calcification and arterial stiffness, highlighting the important genetic factors, systemic and local microenvironmental signals, and underlying signaling pathways and molecular regulators of vascular calcification. Elevated oxidative stress appears to be a common procalcification factor that induces osteogenic differentiation and calcification of vascular cells in a variety of disease conditions such as atherosclerosis, diabetes mellitus, and chronic kidney disease. Thus, the role of oxidative stress and oxidative stress-regulated signals in vascular smooth muscle cells and their contributions to vascular calcification are highlighted. In relation to diabetes mellitus, the regulation of both hyperglycemia and increased protein glycosylation, by AGEs (advanced glycation end products) and O-linked β-N-acetylglucosamine modification, and its role in enhancing intracellular pathophysiological signaling that promotes osteogenic differentiation and calcification of vascular smooth muscle cells are discussed. In the context of chronic kidney disease, this review details the role of calcium and phosphate homeostasis, parathyroid hormone, and specific calcification inhibitors in regulating vascular calcification. In addition, the impact of the systemic and microenvironmental factors on respective intrinsic signaling pathways that promote osteogenic differentiation and calcification of vascular smooth muscle cells and osteoblasts are compared and contrasted, aiming to dissect the commonalities and distinctions that underlie the paradoxical vascular-bone mineralization disorders in aging and diseases.

Hypophosphatasia: Biological and Clinical Aspects, Avenues for Therapy

Hypophosphatasia (HPP) is a rare inherited systemic metabolic disease caused by mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. TNSALP is expressed in the liver, kidney and bone, and its substrates include TNSALP inorganic pyrophosphate, pyridoxal-5'-phosphate (PLP)/vitamin B6 and phosphoethanolamine (PEA). Autosomal recessive and dominant forms of the disease result in a range of clinical entities. Major hallmarks are low alkaline phosphatase (ALP) and elevated PLP and PEA levels. Very severe infantile forms of HPP cause premature death as a result of respiratory insufficiency and also present with hypo-mineralisation leading to deformed limbs with, in some cases, the near-absence of bones and skull altogether. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency are indicative of a poor prognosis. Craniosynostosis is frequent. HPP leads to an unusual presentation of rickets with high levels of calcium and phosphorus, resulting in hypercalciuria, nephrocalcinosis and low ALP levels. Hypercalcaemic crisis, failure to thrive and growth retardation are concerns in infants. Fractures are common in both infantile and adult forms of the disease, concomitantly occurring with unexplained chronic pain and fatigue. Dental clinical presentations, which include the premature loss of teeth, are also commonly found in HPP and specifically manifest as odontohypophosphatasia. A novel enzyme therapy for human HPP, asfotase alfa, which is specifically targeted to mineralised tissues, has been developed in the past decades. While this treatment seems very promising, especially for infantile HPP, many questions regarding its long-term effects, the management of treatment, and any potential secondary adverse effects remain unresolved.

Mandibular dysmorphology due to abnormal embryonic osteogenesis in FGFR2-related craniosynostosis mice

One diagnostic feature of craniosynostosis syndromes is mandibular dysgenesis. Using three mouse models of Apert, Crouzon and Pfeiffer craniosynostosis syndromes, we investigated how embryonic development of the mandible is affected by fibroblast growth factor receptor 2 (Fgfr2) mutations. Quantitative analysis of skeletal form at birth revealed differences in mandibular morphology between mice carrying Fgfr2 mutations and their littermates that do not carry the mutations. Murine embryos with the mutations associated with Apert syndrome in humans (Fgfr2^+/S252W and Fgfr2^+/P253R) showed an increase in the size of the osteogenic anlagen and Meckel's cartilage (MC). Changes in the microarchitecture and mineralization of the developing mandible were visualized using histological staining. The mechanism for mandibular dysgenesis in the Apert Fgfr2^+/S252W mouse resulting in the most severe phenotypic effects was further analyzed in detail and found to occur to a lesser degree in the other craniosynostosis mouse models. Laser capture microdissection and RNA-seq analysis revealed transcriptomic changes in mandibular bone at embryonic day 16.5 (E16.5), highlighting increased expression of genes related to osteoclast differentiation and dysregulated genes active in bone mineralization. Increased osteoclastic activity was corroborated by TRAP assay and in situ hybridization of Csf1r and Itgb3. Upregulated expression of Enpp1 and Ank was validated in the mandible of Fgfr2^+/S252W embryos, and found to result in elevated inorganic pyrophosphate concentration. Increased proliferation of osteoblasts in the mandible and chondrocytes forming MC was identified in Fgfr2^+/S252W embryos at E12.5. These findings provide evidence that FGFR2 gain-of-function mutations differentially affect cartilage formation and intramembranous ossification of dermal bone, contributing to mandibular dysmorphogenesis in craniosynostosis syndromes.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: FGFR2 gain-of-function mutations differentially affect cartilage formation and intramembranous ossification of dermal bone, resulting in abnormal embryonic osteogenesis of the mandible.

The role of ENPP1/PC-1 in osteoinduction by calcium phosphate ceramics

In the past decade, calcium phosphate (CaP) ceramics have emerged as alternatives to autologous bone grafts for the treatment of large, critical-sized bone defects. In order to be effective in the regeneration of such defects, ceramics must show osteoinductive behaviour, defined as the ability to induce de novo heterotopic bone formation. While a set of osteoinductive CaP ceramics has been developed, the exact processes underlying osteoinduction, and the role of the physical and chemical properties of the ceramics, remain largely unknown. Previous studies have focused on the role of the transcriptome to shed light on the mechanism of osteoinduction at the mRNA level. To complement these studies, a proteomic analysis was performed to study the behaviour of hMSCs on osteoinductive and non-osteoinductive CaPs. The results of this analysis suggest that plasma cell glycoprotein 1 (PC-1), encoded by the ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) gene, plays a key role in the process of osteoinduction by CaP ceramics. Validation experiments have confirmed that indeed, the mRNA expression of ENPP1 and the production of PC-1 are higher on osteoinductive than on non-osteoinductive CaP ceramics, a trend that was also observed for other osteogenic markers such as bone morphogenetic protein 2 (BMP2) and osteopontin (OPN), but not for alkaline phosphatase (ALP). Our results also showed that the expression of PC-1 is restricted to those cells which are in direct contact with the CaP ceramic surface, plausibly due to the localised depletion of calcium and inorganic phosphate ions from the supersaturated cell culture medium as CaP crystallises on the ceramic surface. Replicating the surface of the osteoinductive ceramic in polystyrene resulted in a significant decrease in ENPP1 expression, suggesting that surface structural properties alone are not sufficient to induce ENPP1 expression. Finally, knocking down ENPP1 expression in hMSCs resulted in increased BMP2 expression, both at the mRNA and protein level, suggesting that ENPP1 is a negative regulator of BMP-2 signalling. Taken together, this study shows, for the first time, that ENPP1/PC-1 plays an important role in CaP-induced osteogenic differentiation of hMSCs and thus possibly osteoinduction by CaP ceramics. Furthermore, we have identified a crucial role for the interfacial (chemical) events occurring on the CaP ceramic surface in the process of osteoinduction. This knowledge can contribute to the development of new bone graft substitutes, with improved osteoinductive potential.

Small molecules modified biomimetic gelatin/hydroxyapatite nanofibers constructing an ideal osteogenic microenvironment with significantly enhanced cranial bone formation

Background

Repair of nonunion critical-sized bone defects is a significant clinical challenge all over the world. Construction of osteogenic microenvironment that provides osteoconductive and osteoinductive signals is a leading strategy.

Materials and methods

In the present study, ascorbic acid (AA) and β-glycerophosphate disodium salt hydrate (β-GP) modified biomimetic gelatin/hydroxyapatite (GH) nanofibrous scaffolds were developed by electrospinning. Then the scaffolds were crosslinked by N-hydroxysulfo-succinimide sodium salt (NHS) and 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC). The morphology of the non-crosslinked and crosslinked scaffolds was evaluated by scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FT-IR) was used to assess the interacting model between the small molecules and GH scaffold. Then MTT, Alamar Blue, and CCK8 assays were used to investigate the biocompatibility of the various crosslinked scaffolds. Subsequently, the osteogenic genes expression of bone marrow stromal cells (BMSCs) cultured on the scaffolds were detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Finally, the crosslinked scaffolds were implanted in a rat calvarial defect model to assess the osteogenic effects in vivo.

Results

SEM results showed that the various scaffolds presented extracellular matrix (ECM)-like fibrous porous structure. (FT-IR) spectrum indicated that AA and β-GP were covalently bonded with GH scaffolds. The MTT, Alamar Blue, and CCK8 assays demonstrated that all the scaffolds can support BMSCs' growth well. The qRT-PCR results showed that the expression level of Alp and Runx2 in BMSCs on GH/A/B scaffold was about 3.5- and 1.5-fold, respectively, compared with that of GH group on day 7. The results also showed that AA- and β-GP-modified GH scaffolds can significantly induce the higher levels of osteogenic gene expression in a temporal specific manner. Importantly, AA and β-GP synergistically promoted osteoblast differentiation in vitro and dramatically induced bone regeneration in vivo. Impressively, AA and β-GP dual modified GH nanofibrous scaffold could serve as a template for guiding bone regeneration and the bone defects were almost repaired completely (94.28%±5.00%) at 6 weeks. Besides, single AA or β-GP-modified GH nanofibrous scaffolds could repair 62.95%±9.39% and 66.56%±18.45% bone defects, respectively, at 12 weeks in vivo. In addition, AA and β-GP exhibit an anti-inflammatory effect in vivo.

Conclusion

Our data highlighted that, AA, β-GP, and GH nanofibers created a fine osteoconductive and osteoinductive microenvironments for bone regeneration. We demonstrated that AA and β-GP dual modified GH nanofiber is a versatile bone tissue engineering scaffold.

Global gene expression analysis identifies Mef2c as a potential player in Wnt16-mediated transcriptional regulation

Wnt16 is a major Wnt ligand involved in the regulation of postnatal bone homeostasis. Previous studies have shown that Wnt16 promotes bone formation and inhibits bone resorption, suggesting that this molecule could be targeted for therapeutic interventions to treat bone thinning disorders such as osteoporosis. However, the molecular mechanisms by which Wnt16 regulates bone metabolism is not yet fully understood. To better understand the molecular mechanisms by which Wnt16 promotes bone formation and to identify the target genes regulated by Wnt16 in osteoblasts, we treated calvarial osteoblasts purified from C57Bl/6 mice with recombinant Wnt16 and profiled the gene expression changes by RNA-seq at 24 h post-treatment. We also compared gene expression profiles of Wnt16-treated osteoblasts to canonical Wnt3a- and non-canonical Wnt5a-treated osteoblasts. This study identified 576 genes differentially expressed in Wnt16-treated osteoblasts compared to sham-treated controls; these included several members of Wnt pathway (Wnt2b, Wnt7b, Wnt11, Axin2, Sfrp2, Sfrp4, Fzd5 etc.) and TGF-β/BMP signaling pathway (Bmp7, Inhba, Inhbb, Tgfb2 etc.). Wnt16 also regulated a large number of genes with known bone phenotypes. We also found that about 37% (215/576) of the Wnt16 targets overlapped with Wnt3a targets and ~15% (86/576) overlapped with Wnt5a targets, suggesting that Wnt16 activates both canonical and non-canonical Wnt signaling targets in osteoblasts. Transcription factor binding motif enrichment analysis in the promoter regions of Wnt16 targets identified noncanonical Wnt/JNK pathway activated transcription factors Fosl2 and Fosl1 as two of the most significantly enriched transcription factors associated with genes activated by Wnt16 while Mef2c was the most significantly enriched transcription factor associated with genes repressed by Wnt16. We also found that a large number of Mef2c targets overlapped with genes down-regulated by Wnt16 and Mef2c itself was transcriptionally repressed by Wnt16 suggesting that Mef2c plays a role in Wnt16-mediated transcriptional regulation.

Prmt1 regulates craniofacial bone formation upstream of Msx1

Protein arginine methylation has been recently identified as an important form of post-translational modification (PTM). It is carried out by the protein arginine methyltransferase (PRMT) family of enzymes, which in mammals consists of nine members. Among them, PRMT1 is the major arginine methyltransferase and participates in transcription, signal transduction, development and cancer. The function of PRMT1 in craniofacial development remains unclear. We generated Wnt1-Cre;Prmt1^fl/fl mice with cranial neural crest (CNC)-specific deletion of Prmt1 and compared CNC-derived craniofacial bones from newborn control and Wnt1-Cre;Prmt1^fl/fl mice. The size, surface area and volume of the premaxilla, maxilla, palatine bone, frontal bone, and mandible were analyzed using three-dimensional (3D) micro-computed tomography (microCT). We found that Prmt1 deficiency led to alterations in craniofacial bones including the premaxilla, maxilla, palatine bone, frontal bone, and mandible, as well as defects in the incisor and alveolar bone, recapitulating changes seen in Msx1-deficient mice. We further determined that Prmt1 depletion resulted in significant downregulation of Msx1 in calvaria-derived preosteoblast and primordium of frontal bone and mandible. Our study reveals critical roles of PRMT1 in the formation of CNC-derived craniofacial bones and suggests that Prmt1 is an upstream regulator of Msx1 in craniofacial bone development.

Cell Phenotype Transitions in Cardiovascular Calcification

Cardiovascular calcification was originally considered a passive, degenerative process, however with the advance of cellular and molecular biology techniques it is now appreciated that ectopic calcification is an active biological process. Vascular calcification is the most common form of ectopic calcification, and aging as well as specific disease states such as atherosclerosis, diabetes, and genetic mutations, exhibit this pathology. In the vessels and valves, endothelial cells, smooth muscle cells, and fibroblast-like cells contribute to the formation of extracellular calcified nodules. Research suggests that these vascular cells undergo a phenotypic switch whereby they acquire osteoblast-like characteristics, however the mechanisms driving the early aspects of these cell transitions are not fully understood. Osteoblasts are true bone-forming cells and differentiate from their pluripotent precursor, the mesenchymal stem cell (MSC); vascular cells that acquire the ability to calcify share aspects of the transcriptional programs exhibited by MSCs differentiating into osteoblasts. What is unknown is whether a fully-differentiated vascular cell directly acquires the ability to calcify by the upregulation of osteogenic genes or, whether these vascular cells first de-differentiate into an MSC-like state before obtaining a “second hit” that induces them to re-differentiate down an osteogenic lineage. Addressing these questions will enable progress in preventative and regenerative medicine strategies to combat vascular calcification pathologies. In this review, we will summarize what is known about the phenotypic switching of vascular endothelial, smooth muscle, and valvular cells.

NPP1 is responsible for potent extracellular ATP hydrolysis as NTPDase1 in primary cultured murine microglia

The movement of microglia is regulated mainly by P1 and P2 purinergic receptors, which are activated by various nucleotides and their metabolites. Recently, such purinergic signalling has been spotlighted because of potential roles in the pathophysiologies of neurodegenerative and neuropsychiatric disorders. To understand the characteristics of microglia in relation of P1 and P2 signalling, we investigated the ectoenzymes expressed in microglia. At first, we profiled the expression of all known ectoenzymes in cultured microglia. We found that, like NTPDase1 (ectonucleoside triphosphate diphosphohydrolase 1, CD39), NPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1, PC-1) is also highly expressed in primary cultured murine microglia. Knockdown of NPP1 significantly reduced ATP hydrolysis and P_i production in cultured microglia. In addition, the knockdown of NPP1 enhanced basal nucleotide-stimulating responses of cultured microglia, such as phagocytosis and cell migration, and these results were very similar to NTPDase1 knockdown results. Moreover, inhibition of the adenosine receptors by caffeine treatment reduced phagocytosis of NPP1 knock downed-cultured microglia. In conclusion, we suggest that these potent ectoenzymes of primary cultured murine microglia, NPP1 together with CD73 (ecto-5'-nucleotidase) maintain the adenosine levels for triggering nucleotide-stimulating responses.

ATP-degrading ENPP1 is required for survival (or persistence) of long-lived plasma cells

Survival of antibody-secreting plasma cells (PCs) is vital for sustained antibody production. However, it remains poorly understood how long-lived PCs (LLPCs) are generated and maintained. Here we report that ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is preferentially upregulated in bone marrow LLPCs compared with their splenic short-lived counterparts (SLPCs). We studied ENPP1-deficient mice (Enpp1^−/−) to determine how the enzyme affects PC biology. Although Enpp1^−/− mice generated normal levels of germinal center B cells and plasmablasts in periphery, they produced significantly reduced numbers of LLPCs following immunization with T-dependent antigens or infection with plasmodium C. chabaudi. Bone marrow chimeric mice showed B cell intrinsic effect of ENPP1 selectively on generation of bone marrow as well as splenic LLPCs. Moreover, Enpp1^−/− PCs took up less glucose and had lower levels of glycolysis than those of wild-type controls. Thus, ENPP1 deficiency confers an energetic disadvantage to PCs for long-term survival and antibody production.

Overlapping functions of bone sialoprotein and pyrophosphate regulators in directing cementogenesis

Although acellular cementum is essential for tooth attachment, factors directing its development and regeneration remain poorly understood. Inorganic pyrophosphate (PP_i), a mineralization inhibitor, is a key regulator of cementum formation: tissue-nonspecific alkaline phosphatase (Alpl/TNAP) null mice (increased PP_i) feature deficient cementum, while progressive ankylosis protein (Ank/ANK) null mice (decreased PP_i) feature increased cementum. Bone sialoprotein (Bsp/BSP) and osteopontin (Spp1/OPN) are multifunctional extracellular matrix components of cementum proposed to have direct and indirect effects on cell activities and mineralization. Studies on dentoalveolar development of Bsp knockout (Bsp^-/-) mice revealed severely reduced acellular cementum, however underlying mechanisms remain unclear. The similarity in defective cementum phenotypes between Bsp^-/- mice and Alpl^-/- mice (the latter featuring elevated PP_i and OPN), prompted us to examine whether BSP is operating by modulating PP_i-associated genes. Genetic ablation of Bsp caused a 2-fold increase in circulating PP_i, altered mRNA expression of Alpl, Spp1, and Ank, and increased OPN protein in the periodontia. Generation of a Bsp knock-out (KO) cementoblast cell line revealed significantly decreased mineralization capacity, 50% increased PP_i in culture media, and increased Spp1 and Ank mRNA expression. While addition of 2μg/ml recombinant BSP altered Spp1, Ank, and Enpp1 expression in cementoblasts, changes resulting from this dose were not dependent on the integrin-binding RGD motif or MAPK/ERK signaling pathway. Decreasing PP_i by genetic ablation of Ank on the Bsp^-/- mouse background reestablished cementum formation, allowing >3-fold increased acellular cementum volume compared to wild-type (WT). However, deleting Ank did not fully compensate for the absence of BSP. Bsp^-/-; Ank^-/- double-deficient mice exhibited mean 20-27% reduced cementum thickness and volume compared to Ank^-/- mice. From these data, we conclude that the perturbations in PP_i metabolism are not solely driving the cementum pathology in Bsp^-/- mice, and that PP_i is more potent than BSP as a cementum regulator, as shown by the ability to override loss of BSP by lowering PP_i. We propose that BSP and PP_i work in concert to direct mineralization in cementum and likely other mineralized tissues.

IL15RA is required for osteoblast function and bone mineralization

Interleukin-15 receptor alpha (IL15RA) is an important component of interleukin-15 (IL15) pro-inflammatory signaling. In addition, IL15 and IL15RA are present in the circulation and are detected in a variety of tissues where they influence physiological functions such as muscle contractility and overall metabolism. In the skeletal system, IL15RA was previously shown to be important for osteoclastogenesis. Little is known, however, about its role in osteoblast function and bone mineralization. In this study, we evaluated bone structural and mechanical properties of an Il15ra whole-body knockout mouse (Il15ra^-/-) and used in vitro and bioinformatic analyses to understand the role IL15/IL15RA signaling on osteoblast function. We show that lack of IL15RA decreased bone mineralization in vivo and in isolated primary osteogenic cultures, suggesting a cell-autonomous effect. Il15ra^-/- osteogenic cultures also had reduced Rankl/Opg mRNA ratio, indicating defective osteoblast/osteoclast coupling. We analyzed the transcriptome of primary pre-osteoblasts from normal and Il15ra^-/- mice and identified 1150 genes that were differentially expressed at a FDR of 5%. Of these, 844 transcripts were upregulated and 306 were downregulated in Il15ra^-/- cells. The largest functional clusters, highlighted using DAVID analysis, were related to metabolism, immune response, bone mineralization and morphogenesis. The transcriptome analysis was validated by qPCR of some of the most significant hits. Using bioinformatic approaches, we identified candidate genes, including Cd200 and Enpp1, that could contribute to the reduced mineralization. Silencing Il15ra using shRNA in the calvarial osteoblast MC3T3-E1 cell line decreased ENPP1 activity. Taken together, these data support that IL15RA plays a cell-autonomous role in osteoblast function and bone mineralization.

Heritable and acquired disorders of phosphate metabolism: Etiologies involving FGF23 and current therapeutics

Phosphate is critical for many cellular processes and structural functions, including as a key molecule for nucleic acid synthesis and energy metabolism, as well as hydroxyapatite formation in bone. Therefore it is critical to maintain tight regulation of systemic phosphate levels. Based upon its broad biological importance, disruption of normal phosphate homeostasis has detrimental effects on skeletal integrity and overall health. Investigating heritable diseases of altered phosphate metabolism has led to key discoveries underlying the regulation and systemic actions of the phosphaturic hormone Fibroblast growth factor-23 (FGF23). Both molecular and clinical studies have revealed novel targets for the development and optimization of therapies for disorders of phosphate handling. This review will focus upon the bridge between genetic discoveries involving disorders of altered FGF23 bioactivity, as well as describe how these findings have translated into pharmacologic application.

Nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) and its inhibitors

Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1, EC 3.1.4.1) is a metalloenzyme that belongs to the NPP family, which comprises seven subtypes (NPP1-7). NPP1 hydrolyzes a wide range of phosphodiester bonds, e.g. in nucleoside triphosphates, (cyclic) dinucleotides, and nucleotide sugars yielding nucleoside 5'-monophosphates as products. Its main substrate is ATP which is cleaved to AMP and diphosphate. The enzyme is involved in various biological processes including bone mineralization, soft-tissue calcification, insulin receptor signalling, cancer cell proliferation and immune modulation. Therefore, NPP1 inhibitors have potential as novel drugs, e.g. for (immuno)oncology. In the last two decades several inhibitors of NPP1 derived from nucleotide- or non-nucleotide scaffolds have been developed. The most potent and selective NPP1-inhibitory substrate analog is adenosine 5'-α,β-methylene-γ-thiotriphosphate (Ki = 20 nM vs. p-Nph-5'-TMP, human membrane-bound NPP1). Non-nucleotide-derived NPP1 inhibitors comprise polysulfonates, polysaccharides, polyoxometalates and small heterocyclic compounds. The polyoxometalate [TiW11CoO40]8- (PSB-POM141) is the most potent and selective NPP1 inhibitor described to date (Ki = 1.46 nM vs. ATP, human soluble NPP1); it displays an allosteric mechanism of inhibition and represents a useful pharmacological tool for evaluating the potential of NPP1 as a novel drug target.

Effects of Different Variants in the ENPP1 Gene on the Functional Properties of Ectonucleotide Pyrophosphatase/Phosphodiesterase Family Member 1

Ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (E-NPP1), encoded by ENPP1, is a plasma membrane protein that generates inorganic pyrophosphate (PP_i ), a physiologic inhibitor of hydroxyapatite formation. In humans, variants in ENPP1 are associated with generalized arterial calcification of infancy, an autosomal-recessive condition causing premature onset of arterial calcification and intimal proliferation resulting in stenoses. ENPP1 variants also cause pseudoxanthoma elasticum characterized by ectopic calcification of soft connective tissues. To determine the functional impact of ENPP1 missense variants, we analyzed 13 putative pathogenic variants in vitro regarding their functional properties, that is, activity, localization, and PP_i generation. Transfection of eight of the 13 variants led to complete loss of NPP activity, whereas four mutants (c.1412A > G, p.Tyr471Cys; c.1510A > C, p.Ser504Arg; c.1976A > G, p.Tyr659Cys; c.2330A > G, p.His777Arg) showed residual activity compared with wild-type E-NPP1. One putative pathologic variant (c.2462 G > A, p.Arg821His) showed normal activity. The five mutants with normal or residual E-NPP1 enzyme activity were still able to generate PP_i and localized in the plasma membrane. In this study, we identified a functional ENPP1 polymorphism, which was expected to be pathogenic till now. Furthermore, we identified four mutants (p.Tyr471Cys, p.Ser504Arg, p.Tyr659Cys, p.His777Arg) with residual E-NPP1 function, which would be potential therapeutical targets for conformational-stabilizing agents.

The effects of tissue-non-specific alkaline phosphatase gene therapy on craniosynostosis and craniofacial morphology in the FGFR2C342Y/+ mouse model of Crouzon craniosynostosis

OBJECTIVES

Craniosynostosis, the premature fusion of cranial bones, has traditionally been described as a disease of increased bone mineralization. However, multiple mouse models of craniosynostosis display craniosynostosis simultaneously with diminished cranial bone volume and/or density. We propose an alternative hypothesis that craniosynostosis results from abnormal tissue mineralization through the downregulation of tissue-non-specific alkaline phosphatase (TNAP) enzyme downstream of activating mutations in FGFRs.

MATERIAL AND METHODS

Neonatal Crouzon (FGFRC342Y/+) and wild-type (FGFR+/+) mice were injected with lentivirus to deliver a recombinant form of TNAP. Mice were sacrificed at 4 weeks postnatal. Serum was collected to test for alkaline phosphatase (AP), phosphorus, and calcium levels. Craniofacial bone fusion and morphology were assessed by micro-computed tomography.

RESULTS

Injection with the TNAP lentivirus significantly increased serum AP levels (increased serum AP levels are indicative of efficient transduction and production of the recombinant protein), but results were variable and dependent upon viral lot and the litter of mice injected. Morphological analysis revealed craniofacial form differences for inferior surface (p=0.023) and cranial height (p=0.014) regions between TNAP lentivirus-injected and vehicle-injected Crouzon mice. With each unit increase in AP level, the odds of lambdoid suture fusion decreased by 84.2% and these results came close to statistical significance (p=0.068).

CONCLUSION

These results suggest that TNAP deficiency may mediate FGFR2-associated craniosynostosis. Future studies should incorporate injection of recombinant TNAP protein, to avoid potential side effects and variable efficacy of lentiviral gene delivery.

Association of Cross-Sectional and Longitudinal Change in Arterial Stiffness With Gene Expression in the Twins UK Cohort

We investigated whether expression of genes previously implicated in arterial stiffening associates with cross-sectional and longitudinal measures of arterial stiffness. Women from the Twins UK cohort (n=470, aged 39-81 years) had gene expression in lymphoblastoid cell lines measured using an Illumina microarray. Arterial stiffness was measured by carotid-femoral pulse wave velocity and carotid distensibility. A subsample (n=121) of women had repeat vascular measures after a mean±SD follow-up of 4.3±1.4 years. Associations of arterial phenotypes with gene expression levels were examined for 52 genes identified from previous association studies. The gene transcript most closely associated with pulse wave velocity in cross-sectional analysis was ectonucleotide pyrophosphatase/phosphodiesterase (P=0.012). Pleiotropic genetic effects accounted for 14% of the phenotypic correlation between ectonucleotide pyrophosphatase/phosphodiesterase expression and pulse wave velocity. Progression of pulse wave velocity during the follow-up period best related to expression of ectonucleotide pyrophosphatase/phosphodiesterase (β=0.19, P=0.008) and collagen type IV α 1 (β=0.32, P<0.0001). Gene transcripts most closely related to change in carotid distensibility during the follow-up period were endothelial nitric oxide synthase (β=-0.20, P=0.005), angiotensin-converting enzyme (β=-0.15, P=0.035), and B-cell CLL/lymphoma11B (β=0.18, P=0.010). Expression levels of angiotensin-converting enzyme also related to progression in carotid diameter (β=0.21, P=0.012). Expression levels of ectonucleotide pyrophosphatase/phosphodiesterase, involved in arterial calcification, and collagen type IV α 1, involved in collagen formation, correlate with aortic stiffening. These genes may be functional mediators of arterial stiffening.

1,25-Dihydroxyvitamin D3 and extracellular calcium promote mineral deposition via NPP1 activity in a mature osteoblast cell line MLO-A5

While vitamin D supplementation is common, the anabolic mechanisms that improve bone status are poorly understood. Under standard mineralising conditions including media ionised calcium of 1.1 mM, 1,25-dihydroxyvitamin D3 (1,25D) enhanced differentiation and mineral deposition by the mature osteoblast/pre-osteocyte cell line, MLO-A5. This effect was markedly increased with a higher ionised calcium level (1.5 mM). Gene expression analyses revealed that 1,25D-induced mineral deposition was associated with induction of Enpp1 mRNA, coding for nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) and NPP1 protein levels. Since MLO-A5 cells express abundant alkaline phosphatase that was not further modified by 1,25D treatment or exposure to increased calcium, this finding suggested that the NPP1 production of pyrophosphate (PPi) may provide alkaline phosphatase with substrate for the generation of inorganic phosphate (Pi). Consistent with this, co-treatment with Enpp1 siRNA or a NPP1 inhibitor, PPADS, abrogated 1,25D-induced mineral deposition. These data demonstrate that 1,25D stimulates osteoblast differentiation and mineral deposition, and interacts with the extracellular calcium concentration. 1,25D regulates Enpp1 expression, which presumably, in the context of adequate tissue non-specific alkaline phosphatase activity, provides Pi to stimulate mineralisation. Our findings suggest a mechanism by which vitamin D with adequate dietary calcium can improve bone mineral status.

Enzyme replacement for craniofacial skeletal defects and craniosynostosis in murine hypophosphatasia

Hypophosphatasia (HPP) is an inborn-error-of-metabolism disorder characterized by deficient bone and tooth mineralization due to loss-of function mutations in the gene (Alpl) encoding tissue-nonspecific alkaline phosphatase (TNAP). Alpl(-/-) mice exhibit many characteristics seen in infantile HPP including long bone and tooth defects, vitamin B6 responsive seizures and craniosynostosis. Previous reports demonstrated that a mineral-targeted form of TNAP rescues long bone, vertebral and tooth mineralization defects in Alpl(-/-) mice. Here we report that enzyme replacement with mineral-targeted TNAP (asfotase-alfa) also prevents craniosynostosis (the premature fusion of cranial bones) and additional craniofacial skeletal abnormalities in Alpl(-/-) mice. Craniosynostosis, cranial bone volume and density, and craniofacial shape abnormalities were assessed by microscopy, histology, digital caliper measurements and micro CT. We found that craniofacial shape defects, cranial bone mineralization and craniosynostosis were corrected in Alpl(-/-) mice injected daily subcutaneously starting at birth with recombinant enzyme. Analysis of Alpl(-/-) calvarial cells indicates that TNAP deficiency leads to aberrant osteoblastic gene expression and diminished proliferation. Some but not all of these cellular abnormalities were rescued by treatment with inorganic phosphate. These results confirm an essential role for TNAP in craniofacial skeletal development and demonstrate the efficacy of early postnatal mineral-targeted enzyme replacement for preventing craniofacial abnormalities including craniosynostosis in murine infantile HPP.

Midkine-deficiency delays chondrogenesis during the early phase of fracture healing in mice

The growth and differentiation factor midkine (Mdk) plays an important role in bone development and remodeling. Mdk-deficient mice display a high bone mass phenotype when aged 12 and 18 months. Furthermore, Mdk has been identified as a negative regulator of mechanically induced bone formation and it induces pro-chondrogenic, pro-angiogenic and pro-inflammatory effects. Together with the finding that Mdk is expressed in chondrocytes during fracture healing, we hypothesized that Mdk could play a complex role in endochondral ossification during the bone healing process. Femoral osteotomies stabilized using an external fixator were created in wildtype and Mdk-deficient mice. Fracture healing was evaluated 4, 10, 21 and 28 days after surgery using 3-point-bending, micro-computed tomography, histology and immunohistology. We demonstrated that Mdk-deficient mice displayed delayed chondrogenesis during the early phase of fracture healing as well as significantly decreased flexural rigidity and moment of inertia of the fracture callus 21 days after fracture. Mdk-deficiency diminished beta-catenin expression in chondrocytes and delayed presence of macrophages during early fracture healing. We also investigated the impact of Mdk knockdown using siRNA on ATDC5 chondroprogenitor cells in vitro. Knockdown of Mdk expression resulted in a decrease of beta-catenin and chondrogenic differentiation-related matrix proteins, suggesting that delayed chondrogenesis during fracture healing in Mdk-deficient mice may be due to a cell-autonomous mechanism involving reduced beta-catenin signaling. Our results demonstrated that Mdk plays a crucial role in the early inflammation phase and during the development of cartilaginous callus in the fracture healing process.

Mineralisation of collagen rich soft tissues and osteocyte lacunae in Enpp1(-/-) mice

Ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyse nucleotide triphosphates to the corresponding nucleotide monophosphates and the mineralisation inhibitor, pyrophosphate (PPi). This study examined the role of NPP1 in osteocytes, osteoclasts and cortical bone, using a mouse model lacking NPP1 (Enpp1(-/-)). We used microcomputed tomography (μCT) to investigate how NPP1 deletion affects cortical bone structure; excised humerus bones from 8, 15 and 22-week old mice were scanned at 0.9 μm. Although no changes were evident in the cortical bone of 8-week old Enpp1(-/-) mice, significant differences were observed in older animals. Cortical bone volume was decreased 28% in 22-week Enpp1(-/-) mice, whilst cortical porosity was reduced 30% and 60% at 15 and 22-weeks, respectively. This was accompanied by up to a 15% decrease in closed pore diameter and a 55% reduction in the number of pores. Cortical thickness was reduced up to 35% in 15 and 22-week Enpp1(-/-) animals and the endosteal diameter was increased up to 23%. Thus, the cortical bone from Enpp1(-/-) mice was thinner and less porous, with a larger marrow space. Scanning electron microscopy (SEM) revealed a decrease in the size and number of blood vessel channels in the cortical bone as well as a 40% reduction in the mean plan area of osteocyte lacunae. We noted that the number of viable osteocytes isolated from the long bones of Enpp1(-/-) mice was decreased ≤50%. In contrast, osteoclast formation and resorptive activity were unaffected by NPP1 deletion. μCT and histological analysis of Enpp1(-/-) mice also revealed calcification of the joints and vertebrae as well as soft tissues including the whisker follicles, ear pinna and trachea. This calcification worsened as the animals aged. Together, these data highlight the key role of NPP1 in regulating calcification of both soft and skeletal tissues.

Tissue-nonspecific alkaline phosphatase deficiency causes abnormal craniofacial bone development in the Alpl(-/-) mouse model of infantile hypophosphatasia

Tissue-nonspecific alkaline phosphatase (TNAP) is an enzyme present on the surface of mineralizing cells and their derived matrix vesicles that promotes hydroxyapatite crystal growth. Hypophosphatasia (HPP) is an inborn-error-of-metabolism that, dependent upon age of onset, features rickets or osteomalacia due to loss-of function mutations in the gene (Alpl) encoding TNAP. Craniosynostosis is prevalent in infants with HPP and other forms of rachitic disease but how craniosynostosis develops in these disorders is unknown.

OBJECTIVES

Because craniosynostosis carries high morbidity, we are investigating craniofacial skeletal abnormalities in Alpl(-/-) mice to establish these mice as a model of HPP-associated craniosynostosis and determine mechanisms by which TNAP influences craniofacial skeletal development.

METHODS

Cranial bone, cranial suture and cranial base abnormalities were analyzed by micro-CT and histology. Craniofacial shape abnormalities were quantified using digital calipers. TNAP expression was suppressed in MC3T3E1(C4) calvarial cells by TNAP-specific shRNA. Cells were analyzed for changes in mineralization, gene expression, proliferation, apoptosis, matrix deposition and cell adhesion.

RESULTS

Alpl(-/-) mice feature craniofacial shape abnormalities suggestive of limited anterior-posterior growth. Craniosynostosis in the form of bony coronal suture fusion is present by three weeks after birth. Alpl(-/-) mice also exhibit marked histologic abnormalities of calvarial bones and the cranial base involving growth plates, cortical and trabecular bone within two weeks of birth. Analysis of calvarial cells in which TNAP expression was suppressed by shRNA indicates that TNAP deficiency promotes aberrant osteoblastic gene expression, diminished matrix deposition, diminished proliferation, increased apoptosis and increased cell adhesion.

CONCLUSIONS

These findings demonstrate that Alpl(-/-) mice exhibit a craniofacial skeletal phenotype similar to that seen in infants with HPP, including true bony craniosynostosis in the context of severely diminished bone mineralization. Future studies will be required to determine if TNAP deficiency and other forms of rickets promote craniosynostosis directly through abnormal calvarial cell behavior, or indirectly due to deficient growth of the cranial base.

Hypophosphatemic rickets: revealing novel control points for phosphate homeostasis

Rapid and somewhat surprising advances have recently been made toward understanding the molecular mechanisms causing heritable disorders of hypophosphatemia. The results of clinical, genetic, and translational studies have interwoven novel concepts underlying the endocrine control of phosphate metabolism, with far-reaching implications for treatment of both rare Mendelian diseases as well as common disorders of blood phosphate excess such as chronic kidney disease (CKD). In particular, diseases caused by changes in the expression and proteolytic control of the phosphaturic hormone fibroblast growth factor-23 (FGF23) have come to the forefront in terms of directing new models explaining mineral metabolism. These hypophosphatemic disorders as well as others resulting from independent defects in phosphate transport or metabolism will be reviewed herein, and implications for emerging therapeutic strategies based upon these new findings will be discussed.

GATA2 regulates differentiation of bone marrow-derived mesenchymal stem cells

The bone marrow microenvironment comprises multiple cell niches derived from bone marrow mesenchymal stem cells. However, the molecular mechanism of bone marrow mesenchymal stem cell differentiation is poorly understood. The transcription factor GATA2 is indispensable for hematopoietic stem cell function as well as other hematopoietic lineages, suggesting that it may maintain bone marrow mesenchymal stem cells in an immature state and also contribute to their differentiation. To explore this possibility, we established bone marrow mesenchymal stem cells from GATA2 conditional knockout mice. Differentiation of GATA2-deficient bone marrow mesenchymal stem cells into adipocytes induced accelerated oil-drop formation. Further, GATA2 loss- and gain-of-function analyses based on human bone marrow mesenchymal stem cells confirmed that decreased and increased GATA2 expression accelerated and suppressed bone marrow mesenchymal stem cell differentiation to adipocytes, respectively. Microarray analysis of GATA2 knockdowned human bone marrow mesenchymal stem cells revealed that 90 and 189 genes were upregulated or downregulated by a factor of 2, respectively. Moreover, gene ontology analysis revealed significant enrichment of genes involved in cell cycle regulation, and the number of G1/G0 cells increased after GATA2 knockdown. Concomitantly, cell proliferation was decreased by GATA2 knockdown. When GATA2 knockdowned bone marrow mesenchymal stem cells as well as adipocytes were cocultured with CD34-positive cells, hematopoietic stem cell frequency and colony formation decreased. We confirmed the existence of pathological signals that decrease and increase hematopoietic cell and adipocyte numbers, respectively, characteristic of aplastic anemia, and that suppress GATA2 expression in hematopoietic stem cells and bone marrow mesenchymal stem cells.

DLX3 regulates bone mass by targeting genes supporting osteoblast differentiation and mineral homeostasis in vivo

Human mutations and in vitro studies indicate that DLX3 has a crucial function in bone development, however, the in vivo role of DLX3 in endochondral ossification has not been established. Here, we identify DLX3 as a central attenuator of adult bone mass in the appendicular skeleton. Dynamic bone formation, histologic and micro-computed tomography analyses demonstrate that in vivo DLX3 conditional loss of function in mesenchymal cells (Prx1-Cre) and osteoblasts (OCN-Cre) results in increased bone mass accrual observed as early as 2 weeks that remains elevated throughout the lifespan owing to increased osteoblast activity and increased expression of bone matrix genes. Dlx3OCN-conditional knockout mice have more trabeculae that extend deeper in the medullary cavity and thicker cortical bone with an increased mineral apposition rate, decreased bone mineral density and increased cortical porosity. Trabecular TRAP staining and site-specific Q-PCR demonstrated that osteoclastic resorption remained normal on trabecular bone, whereas cortical bone exhibited altered osteoclast patterning on the periosteal surface associated with high Opg/Rankl ratios. Using RNA sequencing and chromatin immunoprecipitation-Seq analyses, we demonstrate that DLX3 regulates transcription factors crucial for bone formation such as Dlx5, Dlx6, Runx2 and Sp7 as well as genes important to mineral deposition (Ibsp, Enpp1, Mepe) and bone turnover (Opg). Furthermore, with the removal of DLX3, we observe increased occupancy of DLX5, as well as increased and earlier occupancy of RUNX2 on the bone-specific osteocalcin promoter. Together, these findings provide novel insight into mechanisms by which DLX3 attenuates bone mass accrual to support bone homeostasis by osteogenic gene pathway regulation.

Dramatic differences in activity of purines metabolizing ecto-enzymes between mesenchymal stem cells isolated from human umbilical cord blood and umbilical cord tissue

The high quality human mesenchymal stem cells (MSCs) with remarkable expansion potential in culture are demonstrated to possess multifold clinical applications. However, their isolation and characterization are difficult and sometimes ambiguous. We exploited nucleotide metabolizing ecto-enzymes for more complete characterization of MSCs. Using standard methods of cell culturing and analyses, we detected significant differences in the activity of ecto-nucleotidases on the surface of MSCs isolated from umbilical cord tissue and MSC-like cells derived from umbilical cord blood. Interestingly, the proliferation rate and the immunophenotypic characteristics of mesenchymal stem cells also correspond to the activities of these enzymes. Compared with the CD90-, CD105-, and CD73-deficient and slowly proliferating UCB-MSC-like cells that had relatively higher ecto-NTPDases activity, the CD90-, CD105-, and CD73-positive and rapidly proliferating UC-MSCs rather had ecto-5′-nucleotidase activity and presented neither ecto-nucleotidases nor adenylate kinase activities. In summary, our results demonstrate for the first time that activity of purine nucleotide metabolizing ecto-enzymes differs significantly between mesenchymal stem cells drawn from different neonatal sources, corresponding with a distinct proliferative potential.

Development of a bioactive implant for repair and potential healing of cranial defects

The repair of complex craniofacial bone defects is challenging and a successful result is dependent on the size of the defect, quality of the soft tissue covering the defect, and choice of reconstruction method. The objective of this study was to develop a bioactive cranial implant that could provide a permanent reconstructive solution to the patient by stimulating bone healing of the defect. In this paper the authors report on the feasibility and clinical results of using such a newly developed device for the repair of a large traumatic and therapy-resistant cranial bone defect. The patient had undergone numerous attempts at repair, in which established methods had been tried without success. A mosaic-designed device was manufactured and implanted, comprising interconnected ceramic tiles with a defined calcium phosphate composition. The clinical outcome 30 months after surgery revealed a restored cranial vault without postoperative complications. Computed tomography demonstrated signs of bone ingrowth. Examination with combined (18)F-fluoride PET and CT provided further evidence of bone healing of the cranial defect.

Androgen receptor agonism promotes an osteogenic gene program in preadipocytes

Androgens regulate body composition by interacting with the androgen receptor (AR) to control gene expression in a tissue-specific manner. To identify novel regulatory roles for AR in preadipocytes, we created a 3T3-L1 cell line stably expressing human AR. We found AR expression is required for androgen-mediated inhibition of 3T3-L1 adipogenesis. This inhibition is characterized by decreased lipid accumulation, reduced expression of adipogenic genes, and induction of genes associated with osteoblast differentiation. Collectively, our results suggest androgens promote an osteogenic gene program at the expense of adipocyte differentiation.

Inorganic polyphosphates: biologically active biopolymers for biomedical applications

Inorganic polyphosphate (polyP) is a widely occurring but only rarely investigated biopolymer which exists in both prokaryotic and eukaryotic organisms. Only in the last few years, this polymer has been identified to cause morphogenetic activity on cells involved in human bone formation. The calcium complex of polyP was found to display a dual effect on bone-forming osteoblasts and bone-resorbing osteoclasts. Exposure of these cells to polyP (Ca(2+) complex) elicits the expression of cytokines that promote the mineralization process by osteoblasts and suppress the differentiation of osteoclast precursor cells to the functionally active mature osteoclasts dissolving bone minerals. The effect of polyP on bone formation is associated with an increased release of the bone morphogenetic protein 2 (BMP-2), a key mediator that activates the anabolic processes leading to bone formation. In addition, polyP has been shown to act as a hemostatic regulator that displays various effects on blood coagulation and fibrinolysis and might play an important role in platelet-dependent proinflammatory and procoagulant disorders.

Enhanced mineralization potential of vascular cells from SM22α-Rankl (tg) mice

Vascular calcification, prevalent in diabetes and chronic kidney disease, contributes to morbidity and mortality. To investigate the effect of receptor activator of NF-kB ligand (RANKL) on vascular calcification in vivo, transgenic mice, where RANKL expression was targeted to vascular smooth muscle cells using the SM22α promoter (SM22α-Rankl ( tg )), were created. Sixteen-month-old male SM22α-Rankl ( tg ) mice had higher body weight and higher serum calcium levels but lower lumbar bone mineral density (BMD) compared with age- and gender-matched wild-type (WT) littermates. BMD of long bones, body fat (percent of weight) of the leg, and serum levels of phosphate and RANKL were not significantly different. No significant differences in these parameters were observed in female mice. Histological analysis did not reveal calcium deposits in the aortic roots of SM22α-Rankl ( tg ) mice. To analyze the osteoblastic differentiation and mineralization potentials of vascular cells, aortic smooth muscle cells (SMCs) were isolated and cultured. Results showed that SM22α-Rankl ( tg ) SMCs had higher baseline alkaline phosphatase (ALP) activity but not baseline matrix calcification. When induced by the PKA agonist forskolin, ALP activity was greater in SM22α-Rankl ( tg ) than in WT SMCs. Real-time RT-qPCR revealed higher baseline expression of ALP and ankylosis genes but lower osteoprotegerin gene in SM22α-Rankl ( tg ) SMCs. Matrix mineralization induced by inorganic phosphate or forskolin was greater in SM22α-Rankl ( tg ) than in WT SMCs. Treatment of these cells with the ALP inhibitor levamisole abolished forskolin-induced matrix mineralization but not inorganic phosphate-induced matrix mineralization. These findings suggest that RANKL overexpression in the vasculature may promote mineralization potential.