Mice lacking osteopontin show normal development and bone structure but display altered osteoclast formation in vitro

OPN, BSP, and Bone Quality-Structural, Biochemical, and Biomechanical Assessment in OPN-/-, BSP-/-, and DKO Mice

Osteopontin (OPN) and Bone Sialoprotein (BSP), abundantly expressed by osteoblasts and osteoclasts, appear to have important, partly overlapping functions in bone. In gene-knockout (KO, -/-) models of either protein and their double (D)KO in the same CD1/129sv genetic background, we analyzed the morphology, matrix characteristics, and biomechanical properties of femur bone in 2 and 4 month old, male and female mice. OPN-/- mice display inconsistent, perhaps localized hypermineralization, while the BSP-/- are hypomineralized throughout ages and sexes, and the low mineralization of young DKO mice recovers with age. The higher contribution of primary bone remnants in OPN-/- shafts suggests a slow turnover, while their lower percentage in BSP-/- indicates rapid remodeling, despite FTIR-based evidence in this genotype of a high maturity of the mineralized matrix. In 3-point bending assays, OPN-/- bones consistently display higher Maximal Load, Work to Max. Load and in young mice Ultimate Stress, an intrinsic characteristic of the matrix. Young male and old female BSP-/- also display high Work to Max. Load along with low Ultimate Stress. Principal Component Analysis confirms the major role of morphological traits in mechanical competence, and evidences a grouping of the WT phenotype with the OPN-/- and of BSP-/- with DKO, driven by both structural and matrix parameters, suggesting that the presence or absence of BSP has the most profound effects on skeletal properties. Single or double gene KO of OPN and BSP thus have multiple distinct effects on skeletal phenotypes, confirming their importance in bone biology and their interplay in its regulation.

SOXC are critical regulators of adult bone mass

Pivotal in many ways for human health, the control of adult bone mass is governed by complex, incompletely understood crosstalk namely between mesenchymal stem cells, osteoblasts and osteoclasts. The SOX4, SOX11 and SOX12 (SOXC) transcription factors were previously shown to control many developmental processes, including skeletogenesis, and SOX4 was linked to osteoporosis, but how SOXC control adult bone mass remains unknown. Using SOXC loss- and gain-of-function mouse models, we show here that SOXC redundantly promote prepubertal cortical bone mass strengthening whereas only SOX4 mitigates adult trabecular bone mass accrual in early adulthood and subsequent maintenance. SOX4 favors bone resorption over formation by lowering osteoblastogenesis and increasing osteoclastogenesis. Single-cell transcriptomics reveals its prevalent expression in Lepr+ mesenchymal cells and ability to upregulate genes for prominent anti-osteoblastogenic and pro-osteoclastogenic factors, including interferon signaling-related chemokines, contributing to these adult stem cells' secretome. SOXC, with SOX4 predominantly, are thus key regulators of adult bone mass.

Evaluating osteogenic effects associated with the incorporation of ascorbic acid in mineralized collagen scaffolds

Current treatments for craniomaxillofacial (CMF) defects motivate the design of instructive biomaterials that can promote osteogenic healing of complex bone defects. We report methods to promote in vitro osteogenesis of human mesenchymal stem cells (hMSCs) within a model mineralized collagen scaffold via the incorporation of ascorbic acid (vitamin C), a key factor in collagen biosynthesis and bone mineralization. An addition of 5 w/v% ascorbic acid into the base mineralized collagen scaffold significantly changes key morphology characteristics including porosity, macrostructure, and microstructure. This modification promotes hMSC metabolic activity, ALP activity, and hMSC-mediated deposition of calcium and phosphorous. Additionally, the incorporation of ascorbic acid influences osteogenic gene expression (BMP-2, RUNX2, COL1A2) and delays the expression of genes associated with osteoclast activity and bone resorption (OPN, CTSK), though it reduces the secretion of OPG. Together, these findings highlight ascorbic acid as a relevant component for mineralized collagen scaffold design to promote osteogenic differentiation and new bone formation for improved CMF outcomes.

Periosteal Bone Formation Varies with Age in Periostin Null Mice

Periostin, also known as osteoblast-specific factor 2, is a matricellular protein predominantly expressed at the periosteum of bone. During growth and development, periostin contributes to periosteal expansion by facilitating osteoblast differentiation and mineralization. Later in life, periosteal expansion provides an adaptive strategy to increase tissue strength without requiring substantial increase in bone mass. However, the function of periostin past skeletal maturity and during advanced aging is relatively unknown. The objective of this study was to examine the function of periostin in maintaining bone mass and tissue strength across different ages. In periostin null mice (Postn-/-), periosteal bone formation was significantly reduced in young (3 months) and adult mice (9 months). The lack of bone formation resulted in reduced bone mass and ultimate strength. Conversely, periosteal bone formation increased at advanced ages in 18-month-old Postn-/- mice. The increase in periosteal mineralization at advanced ages coincides with increased expression of vitronectin and osteopontin. Periosteal progenitors from Postn-/- mice displayed an increased capacity to mineralize when cultured on vitronectin, but not type-1 collagen. Altogether, these findings demonstrate the unique role of periostin in regulating periosteal bone formation at different ages and the potential for vitronectin to compensate in the absence of periostin.

Wnt/β-Catenin Signaling Inhibits Osteogenic Differentiation in Human Periodontal Ligament Fibroblasts

The periodontal ligament is a collagenous tissue that is important for maintaining the homeostasis of cementum and alveolar bone. In tendon cells, Wnt/β-catenin signaling has been reported to regulate the expression level of Scleraxis (Scx) and Mohawk Homeobox (Mkx) gene and maintain the tissue homeostasis, while its role in the periodontal ligament is unclear. The aim of this study was to investigate the effects of Wnt/β-catenin signaling induced by Wnt-3a stimulation on the inhibition of osteogenic differentiation of human periodontal ligament fibroblasts (HPLFs). During osteogenic differentiation of HPLFs, they formed bone nodules independently of alkaline phosphatase (ALP) activity. After stimulation of Wnt-3a, the expression of β-catenin increased, and nuclear translocation of β-catenin was observed. These data indicate that Wnt-3a activated Wnt/β-catenin signaling. Furthermore, the stimulation of Wnt-3a inhibited the bone nodule formation and suppressed the expression of osteogenic differentiation-related genes such as Runx2, Osteopontin and Osteocalcin, and upregulated the gene expression of Type-I collagen and Periostin (Postn). Scx may be involved in the suppression of osteogenic differentiation in HPLFs. In conclusion, Wnt/β-catenin signaling may be an important signaling pathway that inhibits the osteogenic differentiation in HPLFs by the upregulation of Scx gene expression and downregulation of osteogenic differentiation-related genes.

Osteopontin/secreted phosphoprotein-1 harnesses glial-, immune-, and neuronal cell ligand-receptor interactions to sense and regulate acute and chronic neuroinflammation

Osteopontin (OPN) also known by its official gene designation secreted phosphoprotein-1 (SPP1) is a fascinating, multifunctional protein expressed in a number of cell types that functions not only in intercellular communication, but also in the extracellular matrix (ECM). OPN/SPP1 possesses cytokine, chemokine, and signal transduction functions by virtue of modular structural motifs that provide interaction surfaces for integrins and CD44-variant receptors. In humans, there are three experimentally verified splice variants of OPN/SPP1 and CD44's ten exons are also alternatively spiced in a cell/tissue-specific manner, although very little is known about how this is regulated in the central nervous system (CNS). Post-translational modifications of phosphorylation, glycosylation, and localized cleavage by specific proteases in the cells and tissues where OPN/SPP1 functions, provides additional layers of specificity. However, the former make elucidating the exact molecular mechanisms of OPN/SPP1 function more complex. Flexibility in OPN/SPP1 structure and its engagement with integrins having the ability to transmit signals in inside-out and outside-in direction, is likely why OPN/SPP1 can serve as an early detector of inflammation and ongoing tissue damage in response to cancer, stroke, traumatic brain injury, pathogenic infection, and neurodegeneration, processes that impair tissue homeostasis. This review will focus on what is currently known about OPN/SPP1 function in the brain.

The Roles of SIBLING Proteins in Dental, Periodontal and Craniofacial Development

The majority of dental, periodontal, and craniofacial tissues are derived from the neural crest cells and ectoderm. Neural crest stem cells are pluripotent, capable of differentiating into a variety of cells. These cells can include osteoblasts, odontoblasts, cementoblasts, chondroblasts, and fibroblasts which are responsible for forming some of the tissues of the oral and craniofacial complex. The hard tissue forming cells deposit a matrix composed of collagen and non-collagenous proteins (NCPs) that later undergoes mineralization. The NCPs play a role in the mineralization of collagen. One such category of NCPs is the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family of proteins. This family is composed of dentin sialophosphosprotein (DSPP), osteopontin (OPN), dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), and matrix extracellular phosphoglycoprotein (MEPE). The SIBLING family is known to have regulatory effects in the mineralization process of collagen fibers and the maturation of hydroxyapatite crystals. It is well established that SIBLING proteins have critical roles in tooth development. Recent literature has described the expression and role of SIBLING proteins in other areas of the oral and craniofacial complex as well. The objective of the present literature review is to summarize and discuss the different roles the SIBLING proteins play in the development of dental, periodontal, and craniofacial tissues.

Plasma EBF1 as a Novel Biomarker for Postmenopausal Osteoporosis

Postmenopausal osteoporosis (OPO) is one of the most common types of primary osteoporosis. There is currently lack of a plasma biomarker for sensitive and early diagnosis of OPO. Here we aimed to explore the potential of early B cell factor 1 (EBF1) as a new plasma biomarker of OPO. Quantitative real-time PCR was used to measure the plasma EBF1 levels. Absorptiometry markers, such as lumbar spine (LS) bone mineral density (BMD) and LS T score were obtained after X-ray scans. Biochemical analyses used to measure osteopontin (OPN), β-isomerized C-terminal telopeptides and total N-terminal procollagen of type-I collagen levels of patients with osteopenia (OPE, n = 81), osteoporosis (OPO, n = 98) as well as healthy subjects (NC, n = 110). Quantitative real-time PCR was used to measure the plasma levels of PAX5 and GSTP1, which are target genes of EBF1. EBF1 was downregulated in OPO patients. Levels of EBF1 were positively correlated to clinicopathological characteristics, including LS BMD and LS T scores, and negatively correlated to OPN and total N-terminal procollagen of type-I collagen levels. Increased PAX5 and GSTP1 levels also demonstrated strong correlations with higher EBF1, LS BMD and LS T score. Anti-osteoporotic treatment resulted in significant upregulation of EBF1, PAX5 and GSTP1 at 6 mo after treatment. Our study suggests that plasma EBF1 is a potential biomarker for diagnosing and assessing treatment outcome of OPO.

Hallmarks of aging-based dual-purpose disease and age-associated targets predicted using PandaOmics AI-powered discovery engine

Aging biology is a promising and burgeoning research area that can yield dual-purpose pathways and protein targets that may impact multiple diseases, while retarding or possibly even reversing age-associated processes. One widely used approach to classify a multiplicity of mechanisms driving the aging process is the hallmarks of aging. In addition to the classic nine hallmarks of aging, processes such as extracellular matrix stiffness, chronic inflammation and activation of retrotransposons are also often considered, given their strong association with aging. In this study, we used a variety of target identification and prioritization techniques offered by the AI-powered PandaOmics platform, to propose a list of promising novel aging-associated targets that may be used for drug discovery. We also propose a list of more classical targets that may be used for drug repurposing within each hallmark of aging. Most of the top targets generated by this comprehensive analysis play a role in inflammation and extracellular matrix stiffness, highlighting the relevance of these processes as therapeutic targets in aging and age-related diseases. Overall, our study reveals both high confidence and novel targets associated with multiple hallmarks of aging and demonstrates application of the PandaOmics platform to target discovery across multiple disease areas.

Macrophages in epididymal adipose tissue secrete osteopontin to regulate bone homeostasis

Epididymal white adipose tissue (eWAT) secretes an array of cytokines to regulate the metabolism of organs and tissues in high-fat diet (HFD)-induced obesity, but its effects on bone metabolism are not well understood. Here, we report that macrophages in eWAT are a main source of osteopontin, which selectively circulates to the bone marrow and promotes the degradation of the bone matrix by activating osteoclasts, as well as modulating bone marrow-derived macrophages (BMDMs) to engulf the lipid droplets released from adipocytes in the bone marrow of mice. However, the lactate accumulation induced by osteopontin regulation blocks both lipolysis and osteoclastogenesis in BMDMs by limiting the energy regeneration by ATP6V0d2 in lysosomes. Both surgical removal of eWAT and local injection of either clodronate liposomes (for depleting macrophages) or osteopontin-neutralizing antibody show comparable amelioration of HFD-induced bone loss in mice. These results provide an avenue for developing therapeutic strategies to mitigate obesity-related bone disorders.

Aging of the Hematopoietic Stem Cell Niche: New Tools to Answer an Old Question

The hematopoietic stem cell (HSC) niche is a specialized microenvironment, where a complex and dynamic network of interactions across multiple cell types regulates HSC function. During the last years, it became progressively clearer that changes in the HSC niche are responsible for specific alterations of HSC behavior. The aging of the bone marrow (BM) microenvironment has been shown to critically contribute to the decline in HSC function over time. Interestingly, while upon aging some niche structures within the BM are degenerated and negatively affect HSC functionality, other niche cells and specific signals are preserved and essential to retaining HSC function and regenerative capacity. These new findings on the role of the aging BM niche critically depend on the implementation of new technical tools, developed thanks to transdisciplinary approaches, which bring together different scientific fields. For example, the development of specific mouse models in addition to coculture systems, new 3D-imaging tools, ossicles, and ex-vivo BM mimicking systems is highlighting the importance of new technologies to unravel the complexity of the BM niche on aging. Of note, an exponential impact in the understanding of this biological system has been recently brought by single-cell sequencing techniques, spatial transcriptomics, and implementation of artificial intelligence and deep learning approaches to data analysis and integration. This review focuses on how the aging of the BM niche affects HSCs and on the new tools to investigate the specific alterations occurring in the BM upon aging. All these new advances in the understanding of the BM niche and its regulatory function on HSCs have the potential to lead to novel therapeutical approaches to preserve HSC function upon aging and disease.

Smoc1 and Smoc2 regulate bone formation as downstream molecules of Runx2

Runx2 is an essential transcription factor for bone formation. Although osteocalcin, osteopontin, and bone sialoprotein are well-known Runx2-regulated bone-specific genes, the skeletal phenotypes of knockout (KO) mice for these genes are marginal compared with those of Runx2 KO mice. These inconsistencies suggest that unknown Runx2-regulated genes play important roles in bone formation. To address this, we attempted to identify the Runx2 targets by performing RNA-sequencing and found Smoc1 and Smoc2 upregulation by Runx2. Smoc1 or Smoc2 knockdown inhibited osteoblastogenesis. Smoc1 KO mice displayed no fibula formation, while Smoc2 KO mice had mild craniofacial phenotypes. Surprisingly, Smoc1 and Smoc2 double KO (DKO) mice manifested no skull, shortened tibiae, and no fibulae. Endochondral bone formation was also impaired at the late stage in the DKO mice. Collectively, these results suggest that Smoc1 and Smoc2 function as novel targets for Runx2, and play important roles in intramembranous and endochondral bone formation.

Takahata et al. investigate the functional role of SMOC1/2 proteins in skeletal development. They reveal a genetic pathway that includes Bmp2 and Runx2 inducing expression of the paralogous Smoc genes, which may offer novel and effective therapeutic strategies associated with various bone and cartilage diseases.

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.

Osteopontin induces osteogenic differentiation by human periodontal ligament cells via calcium binding domain-activin receptor-like kinase (ALK-1) interaction

BACKGROUND

Recently we have generated recombinant human osteopontin (rhOPN) using a plant platform (Nicotiana benthamiana) and demonstrated, when coated on culture plates, its osteogenic induction capacity of human periodontal ligament (PDL) cells. The aim of this study is to elucidate the molecular mechanism underlying the rhOPN-induced osteogenic differentiation of human PDL cells.

METHODS

Full length rhOPN (FL-OPN) and three constructs of OPN containing integrin binding domain (N142), calcium binding domain (C122) and mutated calcium-binding domain (C122δ) were generated from N. benthamiana. Human PDL cells were isolated from extracted third molars and cultured on FL-OPN, N142, C122, or C122δ-coated surfaces. Real-time PCR and Western blot analyses were used to determine mRNA and protein expression. In vitro calcification was determined by Alizarin red staining. A chemical inhibitor and RNAi silencing were used to elucidate signaling pathways. In silico analyses were performed to predict the protein-protein interaction. In vivo analysis was performed using a rat calvaria defect model.

RESULTS

Human PDL cells seeded on FL-OPN and C122-coated surfaces significantly increased both mRNA and protein expression of osterix (OSX) and enhanced in vitro calcification. Soluble FL-OPN as well as a surface coated with N142 did not affect OSX expression. Inhibition of activin receptor-like kinase (ALK-1) abolished the induction of osterix expression. In silico analysis suggested a possible interaction between the calcium binding domain (CaBD) of OPN and ALK-1 receptor. C122, but not C122δ coated surfaces, induced the expression of p-Smad-1 and this induction was inhibited by an ALK-1 inhibitor and RNAi against ALK-1. In vivo data showed that 3D porous scaffold containing C-122 enhanced new bone formation as compared to scaffold alone.

CONCLUSION

The results suggest that next to full length OPN, the CaBD of OPN, if coated to a surface, induces osteogenic differentiation via interaction with ALK-1 receptor.

RANKL-Induced Btn2a2 - A T Cell Immunomodulatory Molecule - During Osteoclast Differentiation Fine-Tunes Bone Resorption

Butyrophilins, which are members of the extended B7 family of immunoregulators structurally related to the B7 family, have diverse functions on immune cells as co-stimulatory and co-inhibitory molecules. Despite recent advances in the understanding on butyrophilins' role on adaptive immune cells during infectious or autoimmune diseases, nothing is known about their role in bone homeostasis. Here, we analyzed the role of one specific butyrophilin, namely Btn2a2, as we have recently shown that Btn2a2 is expressed on the monocyte/macrophage lineage that also gives rise to bone degrading osteoclasts. We found that expression of Btn2a2 on monocytes and pre-osteoclasts is upregulated by the receptor activator of nuclear factor κ-B ligand (RANKL), an essential protein required for osteoclast formation. Interestingly, in Btn2a2-deficient osteoclasts, typical osteoclast marker genes (Nfatc1, cathepsin K, TRAP, and RANK) were downregulated following RANKL stimulation. In vitro osteoclast assays resulted in decreased TRAP positive osteoclast numbers in Btn2a2-deficient cells. However, Btn2a2-deficient osteoclasts revealed abnormal fusion processes shown by their increased size. In vivo steady state µCT and histological analysis of bone architecture in complete Btn2a2-deficient mice showed differences in bone parameters further highlighting the fine-tuning effect of BTN2a2. Moreover, in rheumatoid arthritis patients and experimental arthritis, we detected significantly decreased serum levels of the secreted soluble Btn2a2 protein. Taken together, we identified the involvement of the immunomodulatory molecule Btn2a2 in osteoclast differentiation with potential future implications in basic and translational osteoimmunology.

FAM20C Overview: Classic and Novel Targets, Pathogenic Variants and Raine Syndrome Phenotypes

FAM20C is a gene coding for a protein kinase that targets S-X-E/pS motifs on different phosphoproteins belonging to diverse tissues. Pathogenic variants of FAM20C are responsible for Raine syndrome (RS), initially described as a lethal and congenital osteosclerotic dysplasia characterized by generalized atherosclerosis with periosteal bone formation, characteristic facial dysmorphisms and intracerebral calcifications. The aim of this review is to give an overview of targets and variants of FAM20C as well as RS aspects. We performed a wide phenotypic review focusing on clinical aspects and differences between all lethal (LRS) and non-lethal (NLRS) reported cases, besides the FAM20C pathogenic variant description for each. As new targets of FAM20C kinase have been identified, we reviewed FAM20C targets and their functions in bone and other tissues, with emphasis on novel targets not previously considered. We found the classic lethal and milder non-lethal phenotypes. The milder phenotype is defined by a large spectrum ranging from osteonecrosis to osteosclerosis with additional congenital defects or intellectual disability in some cases. We discuss our current understanding of FAM20C deficiency, its mechanism in RS through classic FAM20C targets in bone tissue and its potential biological relevance through novel targets in non-bone tissues.

Binding Mode Characterization of Osteopontin on Hydroxyapatite by Solution NMR Spectroscopy

Extracellular matrix glycoproteins play a major role in bone mineralization and modulation of osteogenesis. Among these, the intrinsically disordered protein osteopontin (OPN) is associated with the inhibition of formation, growth and proliferation of the bone mineral hydroxyapatite (HAP). Furthermore, post-translational modifications like phosphorylation can alter conformations and interaction properties of intrinsically disordered proteins (IDPs). Therefore, the actual interaction of OPN with a HAP surface on an atomic level and how this interaction is affected by phosphorylation is of great interest. Here, we study the interaction of full-length OPN on the surface of suspended HAP nanoparticles by solution NMR spectroscopy. We report the binding modes of this IDP and provide evidence for the influence of hyperphosphorylation on the binding character and an explanation for the differing roles in biomineralization. Our study moreover presents an easy and suitable option to measure interaction of nanoparticles in a stable suspension with full-length proteins.

Functions of Matricellular Proteins in Dental Tissues and Their Emerging Roles in Orofacial Tissue Development, Maintenance, and Disease

Matricellular proteins (MCPs) are defined as extracellular matrix (ECM) associated proteins that are important regulators and integrators of microenvironmental signals, contributing to the dynamic nature of ECM signalling. There is a growing understanding of the role of matricellular proteins in cellular processes governing tissue development as well as in disease pathogenesis. In this review, the expression and functions of different MP family members (periostin, CCNs, TSPs, SIBLINGs and others) are presented, specifically in relation to craniofacial development and the maintenance of orofacial tissues, including bone, gingiva, oral mucosa, palate and the dental pulp. As will be discussed, each MP family member has been shown to have non-redundant roles in development, tissue homeostasis, wound healing, pathology and tumorigenesis of orofacial and dental tissues.

Vascular Calcification in Rodent Models-Keeping Track with an Extented Method Assortment

Simple Summary

Arterial vessel diseases are the leading cause of death in the elderly and their accelerated pathogenesis is responsible for premature death in patients with chronic renal failure. Since no functioning therapy concepts exist so far, the identification of the main signaling pathways is of current research interest. To develop therapeutic concepts, different experimental rodent models are needed, which should be subject to the 3R principle of Russel and Burch: “Replace, Reduce and Refine”. This review aims to summarize the current available experimental rodent models for studying vascular calcification and their quantification methods.

Abstract

Vascular calcification is a multifaceted disease and a significant contributor to cardiovascular morbidity and mortality. The calcification deposits in the vessel wall can vary in size and localization. Various pathophysiological pathways may be involved in disease progression. With respect to the calcification diversity, a great number of research models and detection methods have been established in basic research, relying mostly on rodent models. The aim of this review is to provide an overview of the currently available rodent models and quantification methods for vascular calcification, emphasizing animal burden and assessing prospects to use available methods in a way to address the 3R principles of Russel and Burch: “Replace, Reduce and Refine”.

Scaffolds from Self-Assembling Tetrapeptides Support 3D Spreading, Osteogenic Differentiation, and Angiogenesis of Mesenchymal Stem Cells

The apparent rise of bone disorders demands advanced treatment protocols involving tissue engineering. Here, we describe self-assembling tetrapeptide scaffolds for the growth and osteogenic differentiation of human mesenchymal stem cells (hMSCs). The rationally designed peptides are synthetic amphiphilic self-assembling peptides composed of four amino acids that are nontoxic. These tetrapeptides can quickly solidify to nanofibrous hydrogels that resemble the extracellular matrix and provide a three-dimensional (3D) environment for cells with suitable mechanical properties. Furthermore, we can easily tune the stiffness of these peptide hydrogels by just increasing the peptide concentration, thus providing a wide range of peptide hydrogels with different stiffnesses for 3D cell culture applications. Since successful bone regeneration requires both osteogenesis and vascularization, our scaffold was found to be able to promote angiogenesis of human umbilical vein endothelial cells (HUVECs) in vitro. The results presented suggest that ultrashort peptide hydrogels are promising candidates for applications in bone tissue engineering.

Bone Matrix Non-Collagenous Proteins in Tissue Engineering: Creating New Bone by Mimicking the Extracellular Matrix

Engineering biomaterials that mimic the extracellular matrix (ECM) of bone is of significant importance since most of the outstanding properties of the bone are due to matrix constitution. Bone ECM is composed of a mineral part comprising hydroxyapatite and of an organic part of primarily collagen with the rest consisting on non-collagenous proteins. Collagen has already been described as critical for bone tissue regeneration; however, little is known about the potential effect of non-collagenous proteins on osteogenic differentiation, even though these proteins were identified some decades ago. Aiming to engineer new bone tissue, peptide-incorporated biomimetic materials have been developed, presenting improved biomaterial performance. These promising results led to ongoing research focused on incorporating non-collagenous proteins from bone matrix to enhance the properties of the scaffolds namely in what concerns cell migration, proliferation, and differentiation, with the ultimate goal of designing novel strategies that mimic the native bone ECM for bone tissue engineering applications. Overall, this review will provide an overview of the several non-collagenous proteins present in bone ECM, their functionality and their recent applications in the bone tissue (including dental) engineering field.

Osteopontin regulates type I collagen fibril formation in bone tissue

Osteopontin (OPN) is a non-collagenous protein involved in biomineralization of bone tissue. Beyond its role in biomineralization, we show that osteopontin is essential to the quality of collagen fibrils in bone. Transmission electron microscopy revealed that, in Opn-/- tissue, the organization of the collagen fibrils was highly heterogeneous, more disorganized than WT bone and comprised of regions of both organized and disorganized matrix with a reduced density. The Opn-/- bone tissue also exhibited regions in which the collagen had lost its characteristic fibrillar structure, and the crystals were disorganized. Using nanobeam electron diffraction, we show that damage to structural integrity of collagen fibrils in Opn-/- bone tissue and their organization causes mineral disorganization, which could ultimately affect its mechanical integrity. STATEMENT OF SIGNIFICANCE: This study presents new evidence about the role of osteopontin (OPN) - a non-collagenous protein - on the structure and organization of the organic and mineral matrix in bone. In previous work, osteopontin has been suggested to regulate the nucleation and growth of bone mineral crystals and to form sacrificial bonds between mineralized collagen fibrils to enhance bone's toughness. Our findings show that OPN plays a crucial role before mineralization, during the formation of the collagen fibrils. OPN-deficient bones present a lower collagen content compared to wild type bone and, at the tissue level, collagen fibrils organization can be significantly altered in the absence of OPN. Our results suggest that OPN is critical for the formation and/or remodeling of bone collagen matrix. Our findings could lead to the development of new therapeutic strategies of bone diseases affecting collagen formation and remodeling.

Protaetia brevitarsis seulensis Derived Protein Isolate with Enhanced Osteomodulatory and Antioxidative Property

The osteogenic differentiation of stem cells is profoundly affected by their microenvironmental conditions. The differentiation behavior of stem cells can be tuned by changing the niche environments. The proteins or peptides that are derived by living organisms facilitate the osteogenic differentiation of stem cells. Here, we have evaluated the osteoinductive and antioxidative potential of the Protaetia brevitarsis seulensis insect-derived protein for human bone marrow-derived mesenchymal stem cells (hBMSCs). The amino acid contents in the isolated protein were determined by an amino acid analyzer. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were used to analyze the extract’s functional groups and surface morphology. The extracted protein exhibited 51.08% β-sheet conformation. No adverse effects were observed in extract-treated cells, indicating their biocompatibility. The protein isolate showed an excellent antioxidative property. Besides this, an enhancement in the hBMSCs’ mineralization has been observed in the presence of treated protein isolates. Notably, osteogenic marker genes and proteins were effectively expressed in the treated cells. These results indicated that the P. brevitarsis-derived protein isolate can be used as a potential antioxidative biomaterial for bone tissue engineering applications.

The role of extracellular matrix phosphorylation on energy dissipation in bone

Protein phosphorylation, critical for cellular regulatory mechanisms, is implicated in various diseases. However, it remains unknown whether heterogeneity in phosphorylation of key structural proteins alters tissue integrity and organ function. Here, osteopontin phosphorylation level declined in hypo- and hyper- phosphatemia mouse models exhibiting skeletal deformities. Phosphorylation increased cohesion between osteopontin polymers, and adhesion of osteopontin to hydroxyapatite, enhancing energy dissipation. Fracture toughness, a measure of bone’s mechanical competence, increased with ex-vivo phosphorylation of wildtype mouse bones and declined with ex-vivo dephosphorylation. In osteopontin-deficient mice, global matrix phosphorylation level was not associated with toughness. Our findings suggest that phosphorylated osteopontin promotes fracture toughness in a dose-dependent manner through increased interfacial bond formation. In the absence of osteopontin, phosphorylation increases electrostatic repulsion, and likely protein alignment and interfilament distance leading to decreased fracture resistance. These mechanisms may be of importance in other connective tissues, and the key to unraveling cell–matrix interactions in diseases.

RNA-seq analysis of palatal transcriptome changes in all-trans retinoic acid-induced cleft palate of mice

Cleft palate is a common congenital maxillofacial malformation in newborns. All-trans retinoic acid (atRA) is an ideal exogenous stimulus to construct a mouse cleft palate model. However, the precise pathogenic mechanism remains to be elucidated. In our study, to explore the toxicity of atRA on palatal shelves during different stages of palate development, a total of 100 mg/kg atRA was administered to C57BL/6 mice at embryonic day 10.5 (E10.5). Mouse embryonic palatal shelves at E13.5, E14.5, E15.5, and E16.5 were collected for RNA extraction and histological treatment. Changes in gene expression were tested through RNA-seq. Selected differentially expressed genes (DEGs) related to metabolic pathways, such as Ptgds, Ttr, Cyp2g1, Ugt2a1 and Mgst3, were validated and analyzed by Quantitative real-time PCR (qRT-PCR). In addition, Gene Oncology analysis showed that transcriptional changes of genes from extracellular matrix (ECM) components, such as Spp1, and crystallin family might play important role in palatal shelves elevation (E13.5-E14.5). Therefore, the protein expression level of Ttr and Spp1 from E13.5 to E16.5 were tested by immunohistochemistry (IHC). Besides, the mRNA level of Spp1, were down-regulated at E16.5 and the protein were down-regulated at E15.5 and E16.5 in all-trans retinoic acid group, suggesting that atRA may involve in palatal bone formation by regulating Spp1. Overall, gene transcriptional profiles were obviously different at each time point of palate development. Thus, this study summarized some pathways and genes that may be related to palatogenesis and cleft palate through RNA-seq, to provide a direction for subsequent studies on the mechanism and targeted therapy of cleft palate.

Active sites of human MEPE-ASARM regulating bone matrix mineralization

The proteolytic fragment ASARM (acidic serine- and aspartate-rich motif) of MEPE (matrix extracellular phosphoglycoprotein) (MEPE-ASARM) may act as an endogenous anti-mineralization factor involved in X-linked hypophosphatemic rickets/osteomalacia (XLH). We synthesized MEPE-ASARM peptides and relevant peptide fragments with or without phosphorylated Ser residues (pSer) to determine the active site(s) of MEPE-ASARM in a rat calvaria cell culture model. None of the synthetic peptides elicited changes in cell death, proliferation or differentiation, but the peptide (pASARM) with three pSer residues inhibited mineralization without causing changes in gene expression of osteoblast markers tested. The anti-mineralization effect was maintained in peptides in which any one of three pSer residues was deleted. Polyclonal antibodies recognizing pASARM but not ASARM abolished the pASARM effect. Deletion of six N-terminal residues but leaving the recognition sites for PHEX (phosphate regulating endopeptidase homolog, X-linked), a membrane endopeptidase responsible for XLH, intact and two C-terminal amino acid residues did not alter the anti-mineralization activity of pASARM. Our results strengthen understanding of the active sites of MEPE-pASARM and allowed us to identify a shorter more stable sequence with fewer pSer residues still exhibiting hypomineralization activity, reducing peptide synthesis cost and increasing reliability for exploring biological and potential therapeutic effects.

Osteopontin/secreted phosphoprotein-1 behaves as a molecular brake regulating the neuroinflammatory response to chronic viral infection

BACKGROUND

Osteopontin (OPN) as a secreted signaling protein is dramatically induced in response to cellular injury and neurodegeneration. Microglial inflammatory responses in the brain are tightly associated with the neuropathologic hallmarks of neurodegenerative disease, but understanding of the molecular mechanisms remains in several contexts poorly understood.

METHODS

Micro-positron emission tomography (PET) neuroimaging using radioligands to detect increased expression of the translocator protein (TSPO) receptor in the brain is a non-invasive tool used to track neuroinflammation in living mammals.

RESULTS

In humanized, chronically HIV-infected female mice in which OPN expression was knocked down with functional aptamers, uptake of TSPO radioligand DPA-713 was markedly upregulated in the cortex, olfactory bulb, basal forebrain, hypothalamus, and central grey matter compared to controls. Microglia immunoreactive for Iba-1 were more abundant in some HIV-infected mice, but overall, the differences were not significant between groups. TSPO+ microglia were readily detected by immunolabeling of post-mortem brain tissue and unexpectedly, two types of neurons also selectively stained positive for TSPO. The reactive cells were the specialized neurons of the cerebellum, Purkinje cells, and a subset of tyrosine hydroxylase-positive neurons of the substantia nigra.

CONCLUSIONS

In female mice with wild-type levels of osteopontin, increased levels of TSPO ligand uptake in the brain was seen in animals with the highest levels of persistent HIV replication. In contrast, in mice with lower levels of osteopontin, the highest levels of TSPO uptake was seen, in mice with relatively low levels of persistent infection. These findings suggest that osteopontin may act as a molecular brake regulating in the brain, the inflammatory response to HIV infection.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Apolipoprotein D

ApoD is a 25 to 30 kDa glycosylated protein, member of the lipocalin superfamily. As a transporter of several small hydrophobic molecules, its known biological functions are mostly associated to lipid metabolism and neuroprotection. ApoD is a multi-ligand, multi-function protein that is involved lipid trafficking, food intake, inflammation, antioxidative response and development and in different types of cancers. An important aspect of ApoD's role in lipid metabolism appears to involve the transport of arachidonic acid, and the modulation of eicosanoid production and delivery in metabolic tissues. ApoD expression in metabolic tissues has been associated positively and negatively with insulin sensitivity and glucose homeostasis in a tissue dependent manner. ApoD levels rise considerably in association with aging and neuropathologies such as Alzheimer's disease, stroke, meningoencephalitis, moto-neuron disease, multiple sclerosis, schizophrenia and Parkinson's disease. ApoD is also modulated in several animal models of nervous system injury/pathology.

Functional Roles of Matrix Metalloproteinases and Their Inhibitors in Melanoma

The extracellular matrix (ECM) plays an important role in the regulation of the tissue microenvironment and in the maintenance of cellular homeostasis. Several proteins with a proteolytic activity toward several ECM components are involved in the regulation and remodeling of the ECM. Among these, Matrix Metalloproteinases (MMPs) are a class of peptidase able to remodel the ECM by favoring the tumor invasive processes. Of these peptidases, MMP-9 is the most involved in the development of cancer, including that of melanoma. Dysregulations of the MAPKs and PI3K/Akt signaling pathways can lead to an aberrant overexpression of MMP-9. Even ncRNAs are implicated in the aberrant production of MMP-9 protein, as well as other proteins responsible for the activation or inhibition of MMP-9, such as Osteopontin and Tissue Inhibitors of Metalloproteinases. Currently, there are different therapeutic approaches for melanoma, including targeted therapies and immunotherapies. However, no biomarkers are available for the prediction of the therapeutic response. In this context, several studies have tried to understand the diagnostic, prognostic and therapeutic potential of MMP-9 in melanoma patients by performing clinical trials with synthetic MMPs inhibitors. Therefore, MMP-9 may be considered a promising molecule for the management of melanoma patients due to its role as a biomarker and therapeutic target.

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.

The Matrix Revolution: Matricellular Proteins and Restructuring of the Cancer Microenvironment

The extracellular matrix (ECM) surrounding cells is indispensable for regulating their behavior. The dynamics of ECM signaling are tightly controlled throughout growth and development. During tissue remodeling, matricellular proteins (MCP) are secreted into the ECM. These factors do not serve classical structural roles, but rather regulate matrix proteins and cell-matrix interactions to influence normal cellular functions. In the tumor microenvironment, it is becoming increasingly clear that aberrantly expressed MCPs can support multiple hallmarks of carcinogenesis by interacting with various cellular components that are coupled to an array of downstream signals. Moreover, MCPs also reorganize the biomechanical properties of the ECM to accommodate metastasis and tumor colonization. This realization is stimulating new research on MCPs as reliable and accessible biomarkers in cancer, as well as effective and selective therapeutic targets.

The Role of miR-21 in Osteoblasts-Osteoclasts Coupling In Vitro

MiR-21 is being gradually more and more recognized as a molecule regulating bone tissue homeostasis. However, its function is not fully understood due to the dual role of miR-21 on bone-forming and bone-resorbing cells. In this study, we investigated the impact of miR-21 inhibition on pre-osteoblastic cells differentiation and paracrine signaling towards pre-osteoclasts using indirect co-culture model of mouse pre-osteoblast (MC3T3) and pre-osteoclast (4B12) cell lines. The inhibition of miR-21 in MC3T3 cells (MC3T3_inh21) modulated expression of genes encoding osteogenic markers including collagen type I (Coll-1), osteocalcin (Ocl), osteopontin (Opn), and runt-related transcription factor 2 (Runx-2). Inhibition of miR-21 in osteogenic cultures of MC3T3 also inflected the synthesis of OPN protein which is essential for proper mineralization of extracellular matrix (ECM) and anchoring osteoclasts to the bones. Furthermore, it was shown that in osteoblasts miR-21 regulates expression of factors that are vital for survival of pre-osteoclast, such as receptor activator of nuclear factor κB ligand (RANKL). The pre-osteoclast cultured with MC3T3_inh21 cells was characterized by lowered expression of several markers associated with osteoclasts' differentiation, foremost tartrate-resistant acid phosphatase (Trap) but also receptor activator of nuclear factor-κB ligand (Rank), cathepsin K (Ctsk), carbonic anhydrase II (CaII), and matrix metalloproteinase (Mmp-9). Collectively, our data indicate that the inhibition of miR-21 in MC3T3 cells impairs the differentiation and ECM mineralization as well as influences paracrine signaling leading to decreased viability of pre-osteoclasts.

Role of p53 deficiency in socket healing after tooth extractions

p53 is known to advance the cell arrest and cell senescence in human tumors. In this study, we displayed that osteogenic ability of p53-knockout (p53^-/-) mice was significantly increased in the tooth extraction socket compared with wild-type (WT) counterparts. Bone marrow mesenchymal stem cells (BM-MSCs) from mandibular were collected and exhibited with elevated proliferation potential and colony-forming units compared with the control, as well as stronger mineral deposits and osteogenic markers. Besides, the bone mass and bone parameter in p53^-/- mice were markedly enhanced compared with the counterpart after extractions by micro-CT. Masson's trichrome staining and immunohistochemistry also revealed that new bone filling and osterix/osteocalcin (Osx/OCN)-immunopositive staining in p53^-/- mice were remarkably increased at each time point. Furthermore, consistent with the enhanced osteogenic markers, the angiogenic marker of blood vessels (alpha smooth muscle actin, α-SMA) was significantly elevated in p53^-/- mice in contrast to WT mice. Importantly, we found that the osteoclast numbers exhibited an increased trend in p53^-/- mice compared with WT mice during socket healing. Collectively, our result suggest that p53 deficiency could promote the osteogenesis and angiogenesis in the tooth extraction socket and might lend possibility for p53-based therapeutic approaches in acceleration of extraction bone healing.

Critical and diverse roles of phosphates in human bone formation

Humans utilise biomineralisation in the formation of bone and teeth. Human biomineralisation processes are defined by the transformation of an amorphous phosphate-based precursor to highly organised nanocrystals. Interestingly, ionic phosphate species not only provide a fundamental building block of biological mineral, but rather exhibit several diverse roles in mediating mineral formation in the physiological milieu. In this review, we focus on elucidating the complex roles of phosphate ions and molecules within human biomineralisation pathways, primarily referring to the nucleation and crystallisation of bone mineral.

Direct electrical stimulation enhances osteogenesis by inducing Bmp2 and Spp1 expressions from macrophages and preosteoblasts

The capability of electrical stimulation (ES) in promoting bone regeneration has already been addressed in clinical studies. However, its mechanism is still being investigated and discussed. This study aims to investigate the responses of macrophages (J774A.1) and preosteoblasts (MC3T3-E1) to ES and the faradic by-products from ES. It is found that pH of the culture media was not significantly changed, whereas the average hydrogen peroxide concentration was increased by 3.6 and 5.4 µM after 1 and 2 hr of ES, respectively. The upregulation of Bmp2 and Spp1 messenger RNAs was observed after 3 days of stimulation, which is consistent among two cell types. It is also found that Spp1 expression of macrophages was partially enhanced by faradic by-products. Osteogenic differentiation of preosteoblasts was not observed during the early stage of ES as the level of Runx2 expression remains unchanged. However, cell proliferation was impaired by the excessive current density from the electrodes, and also faradic by-products in the case of macrophages. This study shows that macrophages could respond to ES and potentially contribute to the bone formation alongside preosteoblasts. The upregulation of Bmp2 and Spp1 expressions induced by ES could be one of the mechanisms behind the electrically stimulated osteogenesis.

Osteopontin is An Important Regulative Component of the Fetal Bone Marrow Hematopoietic Stem Cell Niche

Osteopontin (OPN) is an important component in both bone and blood regulation, functioning as a bridge between the two. Previously, thrombin-cleaved osteopontin (trOPN), the dominant form of OPN in adult bone marrow (BM), was demonstrated to be a critical negative regulator of adult hematopoietic stem cells (HSC) via interactions with α₄β₁ and α₉β₁ integrins. We now demonstrate OPN is also required for fetal hematopoiesis in maintaining the HSC and progenitor pool in fetal BM. Specifically, we showed that trOPN is highly expressed in fetal BM and its receptors, α₄β₁ and α₉β₁ integrins, are both highly expressed and endogenously activated on fetal BM HSC and progenitors. Notably, the endogenous activation of integrins expressed by HSC was attributed to high concentrations of three divalent metal cations, Ca²⁺, Mg²⁺ and Mn²⁺, which were highly prevalent in developing fetal BM. In contrast, minimal levels of OPN were detected in fetal liver, and α₄β₁ and α₉β₁ integrins expressed by fetal liver HSC were not in the activated state, thereby permitting the massive expansion of HSC and progenitors required during early fetal hematopoiesis. Consistent with these results, no differences in the number or composition of hematopoietic cells in the liver of fetal OPN^-/- mice were detected, but significant increases in the hematopoietic progenitor pool in fetal BM as well as an increase in the BM HSC pool following birth and into adulthood were observed. Together, the data demonstrates OPN is a necessary negative regulator of fetal and neonatal BM progenitors and HSC, and it exhibits preserved regulatory roles during early development, adulthood and ageing.

Knockdown of CD44 expression decreases valve interstitial cell calcification in vitro

The lack of pharmaceutical targets available to treat patients with calcific aortic valve disease (CAVD) necessitates further research into the specific mechanisms of the disease. The significant changes that occur to the aortic valves extracellular matrix (ECM) during the progression of CAVD suggests that these proteins may play an important role in calcification. Exploring the relationship between valve interstitial cells (VICs) and the ECM may lead to a better understand of CAVD mechanisms and potential pharmaceutical targets. In this study, we look at the effect of two ECM components, collagen and hyaluronic acid (HA), on the mineralization of VICs within the context of a two-dimensional, polyacrylamide (PAAM) model system. Using a novel, nondestructive imaging technique, we were able to track calcific nodule development in culture systems over a 3-wk time frame. We saw a significant increase in the size of the nodules grown on HA PAAM gels as compared with collagen PAAM gels, suggesting that HA has a direct effect on mineralization. Directly looking at the two known receptors of HA, CD44 and receptor for HA-mediated motility (RHAMM), and using siRNA knockdown revealed that a decrease in CD44 expression resulted in a reduction of calcification. A decrease in CD44, through siRNA knockdown, reduces mineralization on HA PAAM gels, suggesting a potential new target for CAVD treatment. NEW & NOTEWORTHY Our in vitro model of calcific aortic valve disease shows an interaction between the hyaluronic acid binding protein CD44 with the osteogenic factor OPN as a potential mechanism of aortic valve calcification. Using siRNA knockdown of CD44, we show an upregulation of OPN expression with a decrease in overall mineralization.

Effect of CBX7 deficiency on the socket healing after tooth extractions

CBX7 is shown to down-regulate the expression of osteopontin (OPN) that is associated with osteoblast function. Here, we studied the role of CBX7 in the wound healing of tooth extraction socket in which osteoblast activity is critical via comparison between CBX7-knockout (CBX7^-/-) mice and their wild-type (WT) counterparts of 6 weeks old with maxillary first molar extracted. Mice were euthanized at 7, 14, and 21 days after extractions, and alveolar sockets were assessed by semi-quantitative histomorphometry for hard tissue healing, including new bone fill (Masson's trichrome staining), osteoblast activity (OPN/osterix, Osx), osteoclast activity (tartrate-resistant acid phosphatase, TRAP), and for soft tissue healing, including blood vessels (alpha smooth muscle actin, α-SMA). Also, the bone microarchitecture was evaluated by micro-CT. In radiological analysis, CBX7^-/- mice increased bone mass significantly more than WT mice did. Consistently, both the amount of new bone fill and OPN/Osx-immunopositive cells in the extraction sockets were significantly increased in CBX7^-/- mice at each time point with respect to their WT siblings, while osteoclast number exhibited a trend of more increase in CBX7^-/- mice at all time points as well. In agreement with enhanced bone formation during socket healing, significantly elevated α-SMA-immunopositive area was noted in CBX7^-/- mice in contrast to WT mice. Taken together, these data suggest that CBX7 deficiency has a positive effect on tooth extraction socket healing.

Induction of Osteogenic MC3T3-E1 Cell Differentiation by Nacre and Flesh Lipids of Tunisian Pinctada radiata

The flesh of the Pinctada radiata pearl oyster from coastal Tunisia is considered as a high source of n-3 and n-6 and its shell nacre layer is a promising osteogenic biomaterial. Fatty acid (FA) analysis showed that the major components found in total FA (TFA) were 14:0, 16:0, and 18:0 saturated FA (SFA); 16:1, 18:1, and 20:1 monoenoic FA; 20:4n-6 (ARA), 22:5n-3 (DPA). Characteristically high levels of 20:5n-3 (EPA) and 22:6n-3 (DHA) (6.53-89.75 mg/100 g TFA) polyunsaturated FA (PUFA) were found, respectively, in the TFA of nacre and flesh. Evaluated the effects in vitro of lipids extracted from nacre (Ln) and from flesh (Lc) of P. radiata on growth and the differentiation of osteoblasts. Cytotoxicity tests (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide [MTT] and lactic acid dehydrogenase c [LDH]) demonstrated that both extracts are nontoxic. Alizarin Red staining was used in an osteoblast differentiation model using the osteoblast MC3T3-E1 cell line. It showed that the FA of both extracts induced osteoblast differentiation leading to mineralization. Reverse transcription-polymerase chain reaction (RT-PCR) showed a significantly higher expression of osteocalcin (Bglap) and runt-related transcription (Runx2) in MC3T3-E1 cells in the presence of Ln. No difference of osteopontin (Spp1) and Collagen type I (Col1a1) genes compared to the control was observed. In conclusion, these results supported, obtained from our in vitro experimental model used, the interest/potential of lipids extracted from nacre and P. radiata flesh to stimulate bone formation.

The Retina of Osteopontin deficient Mice in Aging

Osteopontin (OPN) is a secreted glycosylated phosphoprotein that influences cell survival, inflammation, migration, and homeostasis after injury. As the role of OPN in the retina remains unclear, this study issue was addressed by aiming to study how the absence of OPN in knock-out mice affects the retina and the influence of age on these effects. The study focused on retinal ganglion cells (RGCs) and glial cells (astrocytes, Müller cells, and resident microglia) in 3- and 20-month-old mice. The number of RGCs in the retina was quantified and the area occupied by astrocytes was measured. In addition, the morphology of Müller cells and microglia was examined in retinal sections. The deficiency in OPN reduces RGC density by 25.09% at 3 months of age and by 60.37% at 20 months of age. The astrocyte area was also reduced by 51.01% in 3-month-old mice and by 57.84% at 20 months of age, although Müller glia and microglia did not seem to be affected by the lack of OPN. This study demonstrates the influence of OPN on astrocytes and RGCs, whereby the absence of OPN in the retina diminishes the area occupied by astrocytes and produces a secondary reduction in the number of RGCs. Accordingly, OPN could be a target to develop therapies to combat neurodegenerative diseases and astrocytes may represent a key mediator of such effects.

Functional evaluation of transposable elements as enhancers in mouse embryonic and trophoblast stem cells

Transposable elements (TEs) are thought to have helped establish gene regulatory networks. Both the embryonic and extraembryonic lineages of the early mouse embryo have seemingly co-opted TEs as enhancers, but there is little evidence that they play significant roles in gene regulation. Here we tested a set of long terminal repeat TE families for roles as enhancers in mouse embryonic and trophoblast stem cells. Epigenomic and transcriptomic data suggested that a large number of TEs helped to establish tissue-specific gene expression programmes. Genetic editing of individual TEs confirmed a subset of these regulatory relationships. However, a wider survey via CRISPR interference of RLTR13D6 elements in embryonic stem cells revealed that only a minority play significant roles in gene regulation. Our results suggest that a subset of TEs are important for gene regulation in early mouse development, and highlight the importance of functional experiments when evaluating gene regulatory roles of TEs.

Correlations between gene expression and mineralization in the avian leg tendon

Certain avian tendons have been studied previously as a model system for normal mineralization of vertebrates in general. In this regard, the gastrocnemius tendon in the legs of turkeys mineralizes in a well defined temporal and spatial manner such that changes in the initial and subsequent events of mineral formation can be associated with time and specific locations in the tissue. In the present investigation, these parameters and mineral deposition have been correlated with the expression of several genes and the synthesis and secretion of their related extracellular matrix proteins by the composite tenocytes of the tendon. Quantitative polymerase chain reaction analysis demonstrates that mRNA expression of the non-collagenous genes of bone sialoprotein, osteopontin, and osteocalcin corresponds well with the temporal and spatial onset and progression of mineralization. Immunolocalization separately confirms the synthesis and secretion of these matrix molecules. The expression of other non-collagenous genes such as decorin does not show strong correlation with turkey leg tendon mineralization, and expression of vimentin, a cytoskeletal component which may be regulated by biomechanical factors in the tendon, may lead to inhibition of osteocalcin expression during the development and mineralization of the tissue. The overall results of this work provide insight into direct temporal and spatial relations between the genes and proteins of interest as well as the formation and deposition of mineral in the avian tendon model.

JNK inactivation suppresses osteogenic differentiation, but robustly induces osteopontin expression in osteoblasts through the induction of inhibitor of DNA binding 4 (Id4)

Osteoblasts are versatile cells involved in multiple whole-body processes, including bone formation and immune response. Secretory amounts and patterns of osteoblast-derived proteins such as osteopontin (OPN) and osteocalcin (OCN) modulate osteoblast function. However, the regulatory mechanism of OPN and OCN expression remains unknown. Here, we demonstrate that p54/p46 c-jun N-terminal kinase (JNK) inhibition suppresses matrix mineralization and OCN expression but increases OPN expression in MC3T3-E1 cells and primary osteoblasts treated with differentiation inducers, including ascorbic acid, bone morphogenic protein-2, or fibroblast growth factor 2. Preinhibition of JNK before the onset of differentiation increased the number of osteoblasts that highly express OPN but not OCN (OPN-OBs), indicating that JNK affects OPN secretory phenotype at the early stage of osteogenic differentiation. Additionally, we identified JNK2 isoform as being critically involved in OPN-OB differentiation. Microarray analysis revealed that OPN-OBs express characteristic transcription factors, cell surface markers, and cytokines, including glycoprotein hormone α2 and endothelial cell-specific molecule 1. Moreover, we found that inhibitor of DNA binding 4 is an important regulator of OPN-OB differentiation and that dual-specificity phosphatase 16, a JNK-specific phosphatase, functions as an endogenous regulator of OPN-OB induction. OPN-OB phenotype was also observed following LPS from Porphyromonas gingivalis stimulation during osteogenic differentiation. Collectively, these results suggest that the JNK-Id4 signaling axis is crucial in the control of OPN and OCN expression during osteoblastic differentiation.-Kusuyama, J., Amir, M. S., Albertson, B. G., Bandow, K., Ohnishi, T., Nakamura, T., Noguchi, K., Shima, K., Semba, I., Matsuguchi, T. JNK inactivation suppresses osteogenic differentiation, but robustly induces osteopontin expression in osteoblasts through the induction of inhibitor of DNA binding 4 (Id4).

Deletion of OPN in BSP knockout mice does not correct bone hypomineralization but results in high bone turnover

The two SIBLING (Small Integrin Binding Ligand N-linked Glycoproteins), bone sialoprotein (BSP) and osteopontin (OPN) are expressed in osteoblasts and osteoclasts. In mature BSP knockout (KO, ^-/-) mice, both bone formation and resorption as well as mineralization are impaired. OPN^-/- mice display impaired resorption, and OPN is described as an inhibitor of mineralization. However, OPN is overexpressed in BSP^-/- mice, complicating the understanding of their phenotype. We have generated and characterized mice with a double KO (DKO) of OPN and BSP, to try and unravel their respective contributions. Despite the absence of OPN, DKO bones are still hypomineralized. The SIBLING, matrix extracellular phosphoglycoprotein with ASARM motif (MEPE) is highly overexpressed in both BSP^-/- and DKO and may impair mineralization through liberation of its ASARM (Acidic Serine-Aspartate Rich MEPE associated) peptides. DKO mice also display evidence of active formation of trabecular, secondary bone as well as primary bone in the marrow-ablation repair model. A higher number of osteoclasts form in DKO marrow cultures, with higher resorption activity, and DKO long bones display a localized and conspicuous cortical macroporosity. High bone formation and resorption parameters, and high cortical porosity in DKO mice suggest an active bone modeling/remodeling, in the absence of two key regulators of bone cell performance. This first double KO of SIBLING proteins thus results in a singular, non-trivial phenotype leading to reconsider the interpretation of each single KO, concerning in particular matrix mineralization and the regulation of bone cell activity.

Normal inflammation and regeneration of muscle following injury require osteopontin from both muscle and non-muscle cells

Background

Osteopontin is secreted by skeletal muscle myoblasts and macrophages, and its expression is upregulated in muscle following injury. Osteopontin is present in many different structural forms, which vary in their expression patterns and effects on cells. Using a whole muscle autograft model of muscle injury in mice, we have previously shown that inflammation and regeneration of muscle following injury are delayed by the absence of osteopontin. The current study was undertaken to determine whether muscle or non-muscle cells provide the source of osteopontin required for its role in muscle regeneration.

Methods

The extensor digitorum longus muscle of wild-type and osteopontin-null mice was removed and returned to its bed in the same animal (autograft) or placed in the corresponding location in an animal of the opposite genotype (allograft). Grafts were harvested at various times after surgery and analysed by histology, flow cytometry and quantitative polymerase chain reaction. Data were analysed using one- or two-way ANOVA or Kruskal-Wallis test.

Results

Immunohistochemistry confirmed that osteopontin was expressed by macrophages in osteopontin-null muscle allografts in wild-type hosts and by myoblasts in wild-type allografts in osteopontin-null hosts. The decrease in muscle fibre number observed in wild-type autografts following grafting and the subsequent appearance of regenerating fibres were delayed in both types of allografts to a similar extent as in osteopontin-null autografts. Infiltration of neutrophils, macrophages and M1 and M2 macrophage subtypes were also delayed by lack of osteopontin in the muscle and/or host. While the proportion of macrophages showing the M1 phenotype was not affected, the proportion showing the M2 phenotype was decreased by osteopontin deficiency. Expression of tumour necrosis factor-α and interleukin-4 was lower in osteopontin-null than in wild-type autografts, and there was no difference between the osteopontin-null graft types.

Conclusions

Osteopontins from muscle and non-muscle sources are equally important in the acute response of muscle to injury, and cannot substitute for each other, suggesting that they play distinct roles in regulation of cell behaviour. Future studies of mechanisms of osteopontin’s roles in acute muscle inflammation and regeneration will need to investigate responses to osteopontins derived from both myoblasts and macrophages.

Utility of osteopontin in cerebrospinal fluid as a diagnostic marker for neuropsychiatric systemic lupus erythematosus

The whole protein of osteopontin (OPN full) and its cleaved form (OPN N-half) are involved in the immune response and the migration of immune cells to an inflammatory lesion. We have reported that serum OPN full and urine OPN N-half are elevated in lupus nephritis (LN). Neuropsychiatric systemic lupus erythematosus (NPSLE) is a refractory complication of SLE. To investigate whether OPN full and OPN N-half could serve as diagnostic markers for NPSLE, and to elucidate their role in NPSLE pathogenesis, the concentrations of OPN full and OPN N-half in cerebrospinal fluid (CSF) were measured in NPSLE and non-NPSLE patients. We found that the concentration of OPN full in the CSF was significantly higher in NPSLE than in non-NPSLE, and it decreased after treatment. When the cutoff value of OPN full in CSF was set to 963.4 ng/ml, the sensitivity and specificity for the diagnosis of NPSLE were 70% and 100%, respectively. The correlation analysis of OPN full, OPN N-half and various cytokines/chemokines suggested that the cytokines/chemokines could be divided into two clusters: cluster A, which contains OPN full and cluster B, which contains interleukin-6. OPN full in CSF could be a novel diagnostic marker for NPSLE.

Phosphorylation of Extracellular Bone Matrix Proteins and Its Contribution to Bone Fragility

Phosphorylation of bone matrix proteins is of fundamental importance to all vertebrates including humans. However, it is currently unknown whether increase or decline of total protein phosphorylation levels, particularly in hypophosphatemia-related osteoporosis, osteomalacia, and rickets, contribute to bone fracture. To address this gap, we combined biochemical measurements with mechanical evaluation of bone to discern fracture characteristics associated with age-related development of skeletal fragility in relation to total phosphorylation levels of bone matrix proteins and one of the key representatives of bone matrix phosphoproteins, osteopontin (OPN). Here for the first time, we report that as people age the total phosphorylation level declines by approximately 20% for bone matrix proteins and approximately 30% for OPN in the ninth decade of human life. Moreover, our results suggest that the decline of total protein phosphorylation of extracellular matrix (ECM) contributes to bone fragility, but less pronouncedly than glycation. We theorize that the separation of two sources of OPN negative charges, acidic backbone amino acids and phosphorylation, would be nature's means of assuring that OPN functions in both energy dissipation and biomineralization. We propose that total phosphorylation decline could be an important contributor to the development of osteoporosis, increased fracture risk and skeletal fragility. Targeting the enzymes kinase FamC20 and bone alkaline phosphatase involved in the regulation of matrix proteins' phosphorylation could be a means for the development of suitable therapeutic treatments. © 2018 American Society for Bone and Mineral Research.

NR4A1 Regulates Motility of Osteoclast Precursors and Serves as Target for the Modulation of Systemic Bone Turnover

NR4A1 (Nur77 or NGFI-B), an orphan member of the nuclear receptor superfamily, has been identified as a key regulator of the differentiation and function of myeloid, lymphoid, and mesenchymal cells. The detailed role of NR4A1 in bone biology is incompletely understood. Here, we report a role for NR4A1 as novel factor controlling the migration and recruitment of osteoclast precursors during bone remodeling. Myeloid-specific but not osteoblast-specific deletion of NR4A1 resulted in osteopenia due to an increase in the number of bone-lining osteoclasts. Although NR4A1-deficient osteoclast precursors displayed a regular differentiation into mature osteoclasts, they showed a hyper-motile phenotype that was largely dependent on increased osteopontin expression, suggesting that expression of NR4A1 negatively controlled osteopontin-mediated recruitment of osteoclast precursors to the trabecular bone. Pharmacological activation of NR4A1, in turn, inhibited osteopontin expression and osteopontin-dependent migration of osteoclast precursors resulted in reduced abundance of bone-resorbing osteoclasts in vivo as well as in an ameliorated bone loss after ovariectomy in mice. This study identifies NR4A1 as a crucial player in the regulation of osteoclast biology and bone remodeling and highlights this nuclear receptor as a promising target for therapeutic intervention during the treatment of osteoporosis. © 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.