Glycosylation of Dentin Matrix Protein 1 is critical for osteogenesis

Effect of blockage of Trem1 on the M1 polarization of macrophages in the regulation dental pulp inflammation

Dental pulp inflammation is a common and significant factor related to poor dental prognosis. Current treatment strategies primarily concentrate on managing the inflammatory response, with specific targets for intervention still under investigation. Triggering receptors expressed on myeloid cells (TREMs) are a group of receptor molecules extensively present on myeloid cell surfaces, crucial in the regulation of inflammatory process. Our analysis of transcriptomic sequencing data from clinical pulp samples of dataset GSE77459 and animal models revealed up-regulation of Trem1 during pulpitis. Administration of the Trem1-blocking peptide LP17 led to lower (more than 1-fold) levels of several pro-inflammatory factors and inhibition of M1 macrophage polarization both in vivo and in vitro. This study of the expression patterns and functions of Trem1 in the development of dental pulp inflammation provides novel insights into the therapeutic strategies for clinical pulpitis.

Regulation of Glycosylation in Bone Metabolism

Glycosylation plays a crucial role in the maintenance of homeostasis in the body and at the onset of diseases such as inflammation, neurodegeneration, infection, diabetes, and cancer. It is also involved in bone metabolism. N- and O-glycans have been shown to regulate osteoblast and osteoclast differentiation. We recently demonstrated that ganglio-series and globo-series glycosphingolipids were essential for regulating the proliferation and differentiation of osteoblasts and osteoclasts in glycosyltransferase-knockout mice. Herein, we reviewed the importance of the regulation of bone metabolism by glycoconjugates, such as glycolipids and glycoproteins, including our recent results.

Hematopoietic and stromal DMP1-Cre labeled cells form a unique niche in the bone marrow

Skeletogenesis and hematopoiesis are interdependent. Niches form between cells of both lineages where microenvironmental cues support specific lineage commitment. Because of the complex topography of bone marrow (BM), the identity and function of cells within specialized niches has not been fully elucidated. Dentin Matrix Protein 1 (DMP1)-Cre mice have been utilized in bone studies as mature osteoblasts and osteocytes express DMP1. DMP1 has been identified in CXCL12+ cells and an undefined CD45+ population. We crossed DMP1-Cre with Ai9 reporter mice and analyzed the tdTomato+ (tdT+) population in BM and secondary hematopoietic organs. CD45+tdT+ express myeloid markers including CD11b and are established early in ontogeny. CD45+tdT+ cells phagocytose, respond to LPS and are radioresistant. Depletion of macrophages caused a significant decrease in tdT+CD11b+ myeloid populations. A subset of CD45+tdT+ cells may be erythroid island macrophages (EIM) which are depleted after G-CSF treatment. tdT+CXCL12+ cells are in direct contact with F4/80 macrophages, express RANKL and form a niche with B220+ B cells. A population of resident cells within the thymus are tdT+ and express myeloid markers and RANKL. In conclusion, in addition to targeting osteoblast/osteocytes, DMP1-Cre labels unique cell populations of macrophage and stromal cells within BM and thymus niches and expresses key microenvironmental factors.

Coding and noncoding RNA profile of human heterotopic ossifications - Risk factors and biomarkers

Heterotopic ossification (HO) means the formation of bone in muscles and soft tissues, such as ligaments or tendons. HO could have a genetic history or develop after a traumatic event, as a result of muscle injury, fractures, burns, surgery, or neurological disorders. Many lines of evidence suggest that the formation of HO is related to the pathological differentiation of stem or progenitor cells present within soft tissues or mobilized from the bone marrow. The cells responsible for the initiation and progression of HO are generally called HO precursor cells. The exact mechanisms behind the development of HO are not fully understood. However, several factors have been identified as potential contributors. For example, local tissue injury and inflammation disturb soft tissue homeostasis. Inflammatory cells release growth factors and cytokines that promote osteogenic or chondrogenic differentiation of HO precursor cells. The bone morphogenetic protein (BMP) is one of the main factors involved in the development of HO. In this study, next-generation sequencing (NGS) and RT-qPCR were performed to analyze the differences in mRNA, miRNA, and lncRNA expression profiles between muscles, control bone samples, and HO samples coming from patients who underwent total hip replacement (THR). As a result, crucial changes in the level of gene expression between HO and healthy tissues were identified. The bioinformatic analysis allowed to describe the processes most severely impacted, as well as genes which level differed the most significantly between HO and control samples. Our analysis showed that the level of transcripts involved in leukocyte migration, differentiation, and activation, as well as markers of chronic inflammatory diseases, that is, miR-148, increased in HO, as compared to muscle. Furthermore, the levels of miR-195 and miR-143, which are involved in angiogenesis, were up-regulated in HO, as compared to bone. Thus, we suggested that inflammation and angiogenesis play an important role in HO formation. Importantly, we noticed that HO is characterized by a higher level of TLR3 expression, compared to muscle and bone. Thus, we suggest that infection may also be a risk factor in HO development. Furthermore, an increased level of transcripts coding proteins involved in osteogenesis and signaling pathways, such as ALPL, SP7, BGLAP, BMP8A, BMP8B, SMPD3 was noticed in HO, as compared to muscles. Interestingly, miR-99b, miR-146, miR-204, and LINC00320 were up-regulated in HO, comparing to muscles and bone. Therefore, we suggested that these molecules could be important biomarkers of HO formation and a potential target for therapies.

Quantitative mapping of the in vivo O-GalNAc glycoproteome in mouse tissues identifies GalNAc-T2 O-glycosites in metabolic disorder

The family of GalNAc-Ts (GalNAcpolypeptide:N-Acetylgalactosaminyl transferases) catalyzes the first committed step in the synthesis of O-glycans, which is an abundant and biologically important protein modification. Abnormalities in the activity of individual GalNAc-Ts can result in congenital disorders of O-glycosylation (CDG) and influence a broad array of biological functions. How site-specific O-glycans regulate biology is unclear. Compiling in vivo O-glycosites would be an invaluable step in determining the function of site-specific O-glycans. We integrated chemical and enzymatic conditions that cleave O-glycosites, a higher-energy dissociation product ions-triggered electron-transfer/higher-energy collision dissociation mass spectrometry (MS) workflow and software to study nine mouse tissues and whole blood. We identified 2,154 O-glycosites from 595 glycoproteins. The O-glycosites and glycoproteins displayed consensus motifs and shared functions as classified by Gene Ontology terms. Limited overlap of O-glycosites was observed with protein O-GlcNAcylation and phosphorylation sites. Quantitative glycoproteomics and proteomics revealed a tissue-specific regulation of O-glycosites that the differential expression of Galnt isoenzymes in tissues partly contributes to. We examined the Galnt2-null mouse model, which phenocopies congenital disorder of glycosylation involving GALNT2 and revealed a network of glycoproteins that lack GalNAc-T2-specific O-glycans. The known direct and indirect functions of these glycoproteins appear consistent with the complex metabolic phenotypes observed in the Galnt2-null animals. Through this study and interrogation of databases and the literature, we have compiled an atlas of experimentally identified mouse O-glycosites consisting of 2,925 O-glycosites from 758 glycoproteins.

Constitutive bone marrow adipocytes suppress local bone formation

BM adipocytes (BMAd) are a unique cell population derived from BM mesenchymal progenitors and marrow adipogenic lineage precursors. Although they have long been considered to be a space filler within bone cavities, recent studies have revealed important physiological roles in hematopoiesis and bone metabolism. To date, the approaches used to study BMAd function have been confounded by contributions by nonmarrow adipocytes or by BM stromal cells. To address this gap in the field, we have developed a BMAd-specific Cre mouse model to deplete BMAds by expression of diphtheria toxin A (DTA) or by deletion of peroxisome proliferator-activated receptor gamma (Pparg). We found that DTA-induced loss of BMAds results in decreased hematopoietic stem and progenitor cell numbers and increased bone mass in BMAd-enriched locations, including the distal tibiae and caudal vertebrae. Elevated bone mass appears to be secondary to enhanced endosteal bone formation, suggesting a local effect caused by depletion of BMAd. Augmented bone formation with BMAd depletion protects mice from bone loss induced by caloric restriction or ovariectomy, and it facilitates the bone-healing process after fracture. Finally, ablation of Pparg also reduces BMAd numbers and largely recapitulates high-bone mass phenotypes observed with DTA-induced BMAd depletion.

The role of proteoglycan form of DMP1 in cranial repair

Background

The cranial region is a complex set of blood vessels, cartilage, nerves and soft tissues. The reconstruction of cranial defects caused by trauma, congenital defects and surgical procedures presents clinical challenges. Our previous data showed that deficiency of the proteoglycan (PG) form of dentin matrix protein 1 (DMP1-PG) could lead to abnormal cranial development. In addition, DMP1-PG was highly expressed in the cranial defect areas. The present study aimed to investigate the potential role of DMP1-PG in intramembranous ossification in cranial defect repair.

Methods

Mouse cranial defect models were established by using wild- type (WT) and DMP1-PG point mutation mice. Microcomputed tomography (micro-CT) and histological staining were performed to assess the extent of repair. Immunofluorescence assays and real-time quantitative polymerase chain reaction (RT‒qPCR) were applied to detect the differentially expressed osteogenic markers. RNA sequencing was performed to probe the molecular mechanism of DMP1-PG in regulating defect healing.

Results

A delayed healing process and an abnormal osteogenic capacity of primary osteoblasts were observed in DMP1-PG point mutation mice. Furthermore, impaired inflammatory signaling pathways were detected by using RNA transcription analysis of this model.

Conclusions

Our data indicate that DMP1-PG is an indispensable positive regulator during cranial defect healing.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-022-00443-4.

Identification of Biomarkers That Modulate Osteogenic Differentiation in Mesenchymal Stem Cells Related to Inflammation and Immunity: A Bioinformatics-Based Comprehensive Study

Background: Inducing mesenchymal stem cells (MSCs) osteogenesis may be beneficial in a number of clinical applications. The aim of this study is to identify key novel biomarkers of this process and to analyze the possible regulatory effects on inflammation and immunity. Results: Seven datasets (GSE159137, GSE159138, GSE114117, GSE88865, GSE153829, GSE63754, GSE73087) were obtained from the Gene Expression Omnibus database and were assigned to either the training or the validation dataset. The least absolute shrinkage and selection operator (LASSO) logistic regression model was applied to the training data to select biomarkers of osteogenesis, which were then confirmed using the validation dataset. FK506 binding protein 5 (FKBP5), insulin-like growth factor binding protein (IGFBP2), prostaglandin E receptor 2 (PTGER2), SAM domain and HD domain-containing protein 1 (SAMHD1), and transmembrane tetratricopeptide 1 (TMTC1) were highlighted as potential biomarkers. In addition, the differential expressions of immunity and inflammation-related genes were examined and their correlations with the five identified biomarkers were analyzed. The results from performing RT-qPCR and Western blots confirmed that the levels of each of these biomarkers were all significantly increased following osteogenic differentiation of MSCs. Conclusions: Our results identify five biomarkers related to MSCs osteogenesis and allow us to identify their potential roles in immunoregulation and inflammation. Each biomarker was verified by in vitro experiments.

Osteogenic transdifferentiation of primary human fibroblasts to osteoblast-like cells with human platelet lysate

Inherited bone disorders account for about 10% of documented Mendelian disorders and are associated with high financial burden. Their study requires osteoblasts which play a critical role in regulating the development and maintenance of bone tissue. However, bone tissue is not always available from patients. We developed a highly efficient platelet lysate-based approach to directly transdifferentiate skin-derived human fibroblasts to osteoblast-like cells. We extensively characterized our in vitro model by examining the expression of osteoblast-specific markers during the transdifferentiation process both at the mRNA and protein level. The transdifferentiated osteoblast-like cells showed significantly increased expression of a panel of osteogenic markers. Mineral deposition and ALP activity were also shown, confirming their osteogenic properties. RNA-seq analysis allowed the global study of changes in the transcriptome of the transdifferentiated cells. The transdifferentiated cells clustered separately from the primary fibroblasts with regard to the significantly upregulated genes indicating a distinct transcriptome profile; transdifferentiated osteoblasts also showed significant enrichment in gene expression related to skeletal development and bone mineralization. Our presented in vitro model may potentially contribute to the prospect of studying osteoblast-dependent disorders in patient-derived cells.

Osteocyte-specific dentin matrix protein 1 : the role of mineralization regulation in low-magnitude high-frequency vibration enhanced osteoporotic fracture healing

Aims

There is an increasing concern of osteoporotic fractures in the ageing population. Low-magnitude high-frequency vibration (LMHFV) was shown to significantly enhance osteoporotic fracture healing through alteration of osteocyte lacuno-canalicular network (LCN). Dentin matrix protein 1 (DMP1) in osteocytes is known to be responsible for maintaining the LCN and mineralization. This study aimed to investigate the role of osteocyte-specific DMP1 during osteoporotic fracture healing augmented by LMHFV.

Methods

A metaphyseal fracture was created in the distal femur of ovariectomy-induced osteoporotic Sprague Dawley rats. Rats were randomized to five different groups: 1) DMP1 knockdown (KD), 2) DMP1 KD + vibration (VT), 3) Scramble + VT, 4) VT, and 5) control (CT), where KD was performed by injection of short hairpin RNA (shRNA) into marrow cavity; vibration treatment was conducted at 35 Hz, 0.3 g; 20 minutes/day, five days/week). Assessments included radiography, micro-CT, dynamic histomorphometry and immunohistochemistry on DMP1, sclerostin, E11, and fibroblast growth factor 23 (FGF23). In vitro, murine long bone osteocyte-Y4 (MLO-Y4) osteocyte-like cells were randomized as in vivo groupings. DMP1 KD was performed by transfecting cells with shRNA plasmid. Assessments included immunocytochemistry on osteocyte-specific markers as above, and mineralized nodule staining.

Results

Healing capacities in DMP1 KD groups were impaired. Results showed that DMP1 KD significantly abolished vibration-enhanced fracture healing at week 6. DMP1 KD significantly altered the expression of osteocyte-specific markers. The lower mineralization rate in DMP1 KD groups indicated that DMP1 knockdown was associated with poor fracture healing process.

Conclusion

The blockage of DMP1 would impair healing outcomes and negate LMHFV-induced enhancement on fracture healing. These findings reveal the importance of DMP1 in response to the mechanical signal during osteoporotic fracture healing.

Cite this article: Bone Joint Res 2022;11(7):465–476.

The Role of DMP1 in CKD-MBD

PURPOSE OF REVIEW

Chronic kidney disease-mineral and bone disorder (CKD-MBD) has become a global health crisis with very limited therapeutic options. Dentin matrix protein 1 (DMP1) is a matrix extracellular protein secreted by osteocytes that has generated recent interest for its possible involvement in CKD-MBD pathogenesis. This is a review of DMP1 established regulation and function, and early studies implicating DMP1 in CKD-MBD.

RECENT FINDINGS

Patients and mice with CKD show perturbations of DMP1 expression in bone, associated with impaired osteocyte maturation, mineralization, and increased fibroblast growth factor 23 (FGF23) production. In humans with CKD, low circulating DMP1 levels are independently associated with increased cardiovascular events. We recently showed that DMP1 supplementation lowers circulating FGF23 levels and improves bone mineralization and cardiac outcomes in mice with CKD. Mortality rates are extremely high among patients with CKD and have only marginally improved over decades. Bone disease and FGF23 excess contribute to mortality in CKD by increasing the risk of bone fractures and cardiovascular disease, respectively. Previous studies focused on DMP1 loss-of-function mutations have established its role in the regulation of FGF23 and bone mineralization. Recent studies show that DMP1 supplementation may fill a crucial therapeutic gap by improving bone and cardiac health in CKD.

Lineage tracing of cells expressing the ciliary gene IFT140 during bone development

BACKGROUND

Primary cilia influence cell function and tissue development. Ciliary signaling is mediated by two intraflagellar transport (IFT) protein complexes, IFT-A and IFT-B. The IFT-A complex is responsible for retrograde transport, and IFT140 is a core protein in the A complex. Mutations in IFT140 cause a variety of skeletal disorders. However, the expression and role of IFT140 during bone development remain unclear. In this study, to further explore the potential role of IFT140 in osteogenesis, we used cell lineage tracing and conditional knockout to analyze the distribution and function of IFT140-positive cells during bone formation.

RESULTS

In newborn Ift140-creER; R26RtdTomato mice, IFT140-positive cells were mainly located in the medullary cavity and then migrated to and differentiated on the surface of trabecular and cortical bone. In contrast, the number of IFT140-positive cells significantly decreased in the adult stage, and these cells were only located in the bone marrow cavity for a short time. In Osx-cre; Ift140flox/flox mice, the loss of IFT140 in preosteoblasts caused bone loss in the trabecular bone area at 10 weeks.

CONCLUSION

The results revealed that IFT140-positive cells mainly contribute to the early stage of bone formation.

Trem2 mediated Syk-dependent ROS amplification is essential for osteoclastogenesis in periodontitis microenvironment

Periodontitis is the sixth most prevalent diseases around the globe, which is closely related to many systemic diseases and affects general health. As the leading cause of tooth loss, periodontitis is characterized by irreversible alveolar bone loss and activated osteoclastogenic process, which might be closely related to the activated intracellular reactive oxygen species (ROS) in osteoclasts. Here, we demonstrated triggering receptor expressed on myeloid cells 2 (Trem2) as a key regulator of osteoclastogenesis with the regulation of intracellular ROS signals in periodontitis. In the present study, the expression of Trem2 was significantly upregulated in human alveolar bones diagnosed with chronic periodontitis, as assessed by RNA-seq. In the mice model of periodontitis, the alveolar bone resorption was impeded in the presence of the conditional knockout of Trem2 in osteoclasts. Furthermore, we identified Trem2/DAP12/Syk-dependent cascade as a vital intracellular signaling for the amplification of reactive oxygen species (ROS) signals in osteoclastogenesis, while the accumulation of soluble Aβ₄₂ oligomers (Aβo) in periodontitis microenvironment further strengthened the signals and enhanced osteoclastogenesis through direct interactions with Trem2. Collectively, Trem2 mediated ROS signal amplification cascade was crucial in the process of osteoclastogenesis in periodontitis, suggesting the potential of Trem2 as a target for the prevention and treatment of bone destruction in periodontitis.

Nanoparticle-directed and ionically forced polyphosphate coacervation: a versatile and reversible core-shell system for drug delivery

A drug encapsulation/delivery system using a novel principle is described that is based on an intra-particle migration of calcium ions between a central Ca²⁺-enriched nanoparticle core and the surrounding shell compartment. The supply of Ca²⁺ is needed for the formation of a coacervate shell around the nanoparticles, acting as the core of drug-loadable core–shell particles, using the physiological inorganic polymer polyphosphate (polyP). This polyanion has the unique property to form, at an alkaline pH and in the presence of a stoichiometric surplus of calcium ions, water-insoluble and stabile amorphous nanoparticles. At neutral pH a coacervate, the biologically active form of the polymer, is obtained that is composed of polyP and Ca²⁺. The drug-loaded core–shell particles, built from the Ca–polyP core and the surrounding Ca–polyP shell, were fabricated in two successive steps. First, the formation of the nanoparticle core at pH 10 and a superstoichiometric 2:1 molar ratio between CaCl₂ and Na–polyP into which dexamethasone, as a phosphate derivative, was incorporated. Second, the preparation of the coacervate shell, loaded with ascorbic acid, by exposure of the Ca–polyP core to soluble Na–polyP and L-ascorbate (calcium salt). EDX analysis revealed that during this step the Ca²⁺ ions required for coacervate formation migrate from the Ca–polyP core (with a high Ca:P ratio) to the shell. Electron microscopy of the particles show an electron-dense 150–200 nm sized core surrounded by a less sharply delimited electron-sparse shell. The core–shell particles exhibited strong osteogenic activity in vitro, based on the combined action of polyP and of dexamethasone and ascorbic acid, which reversibly bind to the anionic polyP via ionic Ca²⁺ bonds. Drug release from the particles occurs after contact with a peptide/protein-containing serum, a process which is almost complete after 10 days and accompanied by the conversion of the nanoparticles into a coacervate. Human osteosarcoma SaOS-2 cells cultivated onto or within an alginate hydrogel matrix showed increased growth/viability and mineralization when the hybrid particles containing dexamethasone and ascorbic acid were embedded in the matrix. The polyP-based core–shell particles have the potential to become a suitable, pH-responsive drug encapsulation/release system, especially for bone, cartilage and wound healing.

A clinical and multi‑omics study of Van der Woude syndrome in three generations of a Chinese family

Previous studies have suggested that pathogenic variants in interferon regulatoryse factor 6 (IRF6) can account for almost 70% of familial Van der Woude Syndrome (VWS) cases. However, gene modifiers that account for the phenotypic variability of IRF6 in the context of VWS remain poorly characterized. The aim of this study was to report a family with VWS with variable expressivity and to identify the genetic cause. A 4-month-old boy initially presented with cleft palate and bilateral lower lip pits. Examination of his family history identified similar, albeit milder, clinical features in another four family members, including bilateral lower lip pits and/or hypodontia. Peripheral blood samples of eight members in this three-generation family were subsequently collected, and whole-exome sequencing was performed to detect pathogenic variants. A heterozygous missense IRF6 variant with a c.1198C>T change in exon 9 (resulting in an R400W change at the amino acid level) was detected in five affected subjects, but not in the other three unaffected subjects. Moreover, subsequent structural analysis was indicative of damaged stability to the structure in the mutant IRF protein. Whole-transcriptome sequencing, expression analysis and Gene Ontology enrichment analysis were conducted on two groups of patients with phenotypic diversity from the same family. These analyses identified significant differentially expressed genes and enriched pathways in these two groups. Altogether, these findings provide insight into the mechanism underlying the variable expressivity of VWS.

Glycosylation of DMP1 maintains cranial sutures in mice

Craniosynostosis, a severe craniofacial developmental disease, can only be treated with surgery currently. Recent studies have shown that proteoglycans are involved in the suture development. For the bone matrix protein, dentin matrix protein 1 (DMP1), glycosylation on the N-terminal of it could generate a functional proteoglycan form of DMP1 during osteogenesis. We identified that the proteoglycan form of DMP1 (DMP1-PG) is highly expressed in mineralisation front of suture. But, the potential role of DMP1-PG in suture fusion remain unclear. To investigate the role of DMP1-PG in cranial suture fusion and craniofacial bone development. By using a DMP1 glycosylation site mutation mouse model, DMP1-S89G mice, we compared the suture development in it with control mice. We compared the suture phenotypes, bone formation rate, expression levels of bone formation markers in vivo between DMP1-S89G mice and wild-type mice. Meanwhile, cell culture and organ culture were performed to detect the differences in cell differentiation and suture fusion in vitro. Finally, chondroitin sulphate (CHS), as functional component of DMP1-PG, was employed to test whether it could delay the premature suture fusion and the abnormal differentiation of bone mesenchymal stem cells (BMSCs) of DMP1-PG mice. DMP1-S89G mice had premature closure of suture and shorter skull size. Lack of DMP1-PG accelerated bone formation in cranial suture. DMP1-PG maintained the essential stemness of BMSCs in suture through blocking the premature differentiation of BMSCs to osteoblasts. Finally, chondroitin sulphate, a major component of DMP1-PG, successfully delayed the premature suture fusion by organ culture of skull in vitro. DMP1-PG could inhibit premature fusion of cranial suture and maintain the suture through regulating the osteogenic differentiation of BMSCs.

Glycosylation of DMP1 promotes bone reconstruction in long bone defects

The regeneration of bone defects is necessary for the successful healing. During the process of healing, callus plays crucial roles in providing the stable bone-reconstruction environment. The callus is consisted of various large molecules including collagen proteins, non-collagen proteins and proteoglycans (PGs), which are involved in maintaining mechanical strength and interacting with cytokines and grow factors in the injury sites. Recently, our data have found that the PG form of Dentin Matrix Protein 1 (DMP1-PG), which is a newly identified PG, was richly expressed in the bone defect sites. Previous researches have demonstrated the special role of DMP1-PG in chondrogenesis and endochondral ossification, however, the knowledge about the role of DMP1-PG in bone defect repair is still limited. To further detect the potential function of DMP1-PG in the defect healing, we employed a bone defect intramembranous ossification model using the glycosylation site mutant DMP1-PG (S⁸⁹-G⁸⁹, S89G-DMP1) mouse. The morphologic changes of calluses and abnormal expression levels of osteogenesis genes were displayed in the injury sites in S89G-DMP1 mice. In addition, impaired BMP-Smad signaling pathway was observed due to the deficiency of DMP1-PG. Collectively, our findings indicated that the DMP1-PG is one of key proteoglycans in the process of defect healing via regulating the osteogenesis.

Bone and heart health in chronic kidney disease: role of dentin matrix protein 1

Purpose of review

Chronic kidney disease (CKD) is a condition associated with bone disease and fibroblast growth factor 23 (FGF23) excess that contributes to cardiovascular mortality. Dentin matrix protein 1 (DMP1) is an established regulator of bone mineralization and FGF23 production in osteocytes. To date, DMP1 function has mainly been studied in the context of hereditary hypophosphatemic rickets diseases. This review describes the role of DMP1 as a potential strong candidate to prevent bone disorders, FGF23 elevation and associated cardiac outcomes in CKD.

Recent findings

Patients and mice with CKD show impaired osteocyte maturation and impaired regulation of DMP1 and FGF23 in bone. New data suggest that impaired DMP1 production contributes to CKD-associated bone and mineral metabolism disorders and we show that DMP1 repletion improves osteocyte alterations, bone mineralization and partially prevents FGF23 elevation. As a result, mice with CKD show attenuated left ventricular hypertrophy and improved survival.

Summary

There is an urgent need for new therapeutic strategies to improve bone quality and to lower FGF23 levels in CKD. By preventing osteocyte apoptosis and inhibiting Fgf23 transcription, DMP1 supplementation may represent an ideal approach to improve CKD-associated bone and cardiac outcomes.

Short Pretreatment with Calcitriol Is Far Superior to Continuous Treatment in Stimulating Proliferation and Osteogenic Differentiation of Human Adipose Stem Cells

Objective

This study investigated whether short stimulation (30 minutes) of human adipose stem cells (hASCs) with 1,25-dihydroxyvitamin D₃ (calcitriol or 1,25-(OH)₂VitD₃), fitting within the surgical procedure time frame, suffices to induce osteogenic differentiation, and compared this with continuous treatment with 1,25-(OH)₂VitD₃.

Materials and Methods

In this experimental study, hASCs were pretreated with/without 10 nM calcitriol for 30 minutes, seeded on biphasic calcium phosphate (BCP), and cultured for 3 weeks with/without 1,25-(OH)₂VitD₃. Cell attachment was determined 30 minutes after cell seeding. AlamarBlue assay, alkaline phosphatase (ALP) assay, ALP staining, real-time polymerase chain reaction (PCR), and protein assay were used to evaluate the effect of short calcitriol pretreatment on proliferation and osteogenic differentiation of hASCs up to 3 weeks.

Results

Pretreatment with 1,25-(OH)₂VitD₃ enhanced the attachment of hASCs to BCP by 1.5-fold compared to nontreated cells and increased the proliferation by 3.5-fold at day 14, and 2.6-fold at day 21. In contrast, continuous treatment increased the proliferation by 1.7-fold only at day 14. After 2 weeks, ALP activity was increased by 18.5-fold when hASCs were pretreated with 1,25-(OH)₂VitD₃ for 30 minutes but increased only 2.6-fold when compared with its continuous counterpart. Moreover, after 14 days, pretreatment resulted in significant upregulation of the osteogenic markers RUNX2 and SPARC by 3.6-fold and 2.2-fold, respectively, while this was not observed upon continuous treatment. Finally, 30 minutes pretreatment of hASCs with 1,25-(OH)₂VitD₃ increased VEGF₁₈₉ expression, which may contribute to the process of angiogenesis.

Conclusion

This study is the first research showing that 30 minutes pretreatment of hASCs with 1,25-(OH)₂VitD₃, not only enhanced cell attachment to the scaffold at seeding time, but also promoted the proliferation and osteogenic differentiation of hASCs more strongly than continuous treatment, suggesting that short pre-treatment with 1,25-(OH)₂VitD₃ is a promising approach for the regeneration of bones in a one-step surgical procedure.

Recombinant Human Dentin Matrix Protein 1 (hDMP1) Expressed in Nicotiana benthamiana Potentially Induces Osteogenic Differentiation

Inductive molecules are critical components for successful bone tissue engineering. Dentin matrix protein-1 (DMP1), a non-collagenous protein in the bone matrix, has been shown to play roles in osteogenic differentiation and phosphate homeostasis. This study aimed to produce recombinant human dentin matrix protein-1 (hDMP1) in Nicotiana benthamiana and investigated the ability of this plant-produced DMP1 to induce osteogenesis in human periodontal ligament stem cells (hPDLSCs). The hDMP1 gene was cloned into the geminiviral vector for transient expression in N. benthamiana. We found that hDMP1 was transiently expressed in N. benthamiana leaves and could be purified by ammonium sulphate precipitation followed by nickel affinity chromatography. The effects of hDMP1 on the induction of cell proliferation and osteogenic differentiation were investigated. The results indicated that plant-produced hDMP1 could induce the cell proliferation of hPDLSCs and increase the expression levels of osteogenic genes, including osterix (OSX), type I collagen (COL1), bone morphogenetic protein-2 (BMP2), and Wnt3a. Moreover, the plant-produced hDMP1 promoted calcium deposition in hPDLSCs as determined by alizarin red S staining. In conclusion, our results indicated that plant-produced hDMP1 could induce osteogenic differentiation in hPDLSCs and could potentially be used as a bone inducer in bone tissue engineering.

Essential Role of IFT140 in Promoting Dentinogenesis

Primary cilia, with highly regulated cellular sensory functions, play key roles in tissue development and function maintenance. Intraflagellar transport 140 (IFT140) is a subunit of IFT complex A, which is specialized for retrograde transportation in cilia. Mutations of Ift140 are usually associated with syndromic ciliopathy and may cause isolated diseases such as retinal dystrophy, short ribs, and polycystic kidney. However, the role of IFT140 in tooth development has not been well investigated. In this study, a close relationship between IFT140 and dentin formation is disclosed. During tooth development, IFT140 was highly expressed in odontoblasts. To further understand the role of IFT140 in dentinogenesis, Ift140^flox/flox/Osx-Cre mouse was generated. The dentin thickness of Ift140^flox/flox/Osx-Cre mouse is thinner and the dentin formation is slower than that in control. In vitro, deletion of IFT140 in odontoblasts led to poor odontogenic differentiation, abnormal primary cilia, and decreased Sonic hedgehog signaling molecules. More important, due to loss of primary cilia in odontoblasts by IFT140 deletion, reparative dentin formation was impaired in a tooth-drilling model. These results suggest that cilia gene IFT140 is essential in promoting dentin formation and reparation.

Glycosylation of dentin matrix protein 1 is a novel key element for astrocyte maturation and BBB integrity

The blood-brain barrier (BBB) is a tight boundary formed between endothelial cells and astrocytes, which separates and protects brain from most pathogens as well as neural toxins in circulation. However, detailed molecular players involved in formation of BBB are not completely known. Dentin matrix protein 1 (DMP1)-proteoglycan (PG), which is known to be involved in mineralization of bones and dentin, is also expressed in soft tissues including brain with unknown functions. In the present study, we reported that DMP1-PG was expressed in brain astrocytes and enriched in BBB units. The only glycosylation site of DMP1 is serine89 (S89) in the N-terminal domain of the protein in mouse. Mutant mice with DMP1 point mutations changing S89 to glycine (S89G), which completely eradicated glycosylation of the protein, demonstrated severe BBB disruption. Another breed of DMP1 mutant mice, which lacked the C-terminal domain of DMP1, manifested normal BBB function. The polarity of S89G-DMP1 astrocytes was disrupted and cell-cell adhesion was loosened. Through a battery of analyses, we found that DMP1 glycosylation was critically required for astrocyte maturation both in vitro and in vivo. S89G-DMP1 mutant astrocytes failed to express aquaporin 4 and had reduced laminin and ZO1 expression, which resulted in disruption of BBB. Interestingly, overexpression of wild-type DMP1-PG in mouse brain driven by the nestin promoter elevated laminin and ZO1 expression beyond wild type levels and could effectively resisted intravenous mannitol-induced BBB reversible opening. Taken together, our study not only revealed a novel element, i.e., DMP1-PG, that regulated BBB formation, but also assigned a new function to DMP1-PG.

Extracellular Phosphate Induces the Expression of Dentin Matrix Protein 1 Through the FGF Receptor in Osteoblasts

Dentin matrix protein 1 (Dmp1) is an extracellular matrix protein involved in phosphate metabolism and biomineralization, and its expression markedly increases during the maturation of osteoblasts into osteocytes. We previously reported that an increased level of inorganic phosphate (Pi) in media up-regulated the expression of Dmp1 in primary osteocytes isolated from mouse bones. In the present study, we found that elevated extracellular Pi strongly induced the expression of Dmp1 in osteoblasts and explored its underlying mechanism of action. In an osteoblastic cell line MC3T3-E1, increases in extracellular Pi induced the phosphorylation of ERK1/2 and up-regulated the expression of Dmp1, fibroblast growth factor 2 (Fgf2), and Fgf receptor 1 (Fgfr1). A co-treatment with the MEK inhibitor U0126 abolished the increase in the expression of Dmp1 and Fgfr1 by elevated Pi, suggesting the involvement of the MEK/ERK pathway in this up-regulation. Elevated extracellular Pi also resulted in the phosphorylation of FGF receptor substrate 2α (FRS2α), which was diminished by knockdown of Slc20a1 encoding Pit1 sodium-phosphate co-transporter. The co-treatment with an inhibitor against FGFR (SU5402) abolished the up-regulation of Dmp1 induced by elevated extracellular Pi. In primary osteoblasts, a treatment with 4 mM Pi transiently increased the expression of early growth response 1 (Egr1) before the up-regulation of Dmp1. These results indicate that FGFR mediates the direct effects of extracellular Pi on the expression of Dmp1 in osteoblasts and enhance the close relationship between the signaling evoked by elevated extracellular Pi and FGF/FGFR signaling. J. Cell. Biochem. 118: 1151-1163, 2017. © 2016 Wiley Periodicals, Inc.