Osteoclast-specific inactivation of the integrin-linked kinase (ILK) inhibits bone resorption

The Past, Present, and Future of Genetically Engineered Mouse Models for Skeletal Biology

The ability to create genetically engineered mouse models (GEMMs) has exponentially increased our understanding of many areas of biology. Musculoskeletal biology is no exception. In this review, we will first discuss the historical development of GEMMs and how these developments have influenced musculoskeletal disease research. This review will also update our 2008 review that appeared in BONEKey, a journal that is no longer readily available online. We will first review the historical development of GEMMs in general, followed by a particular emphasis on the ability to perform tissue-specific (conditional) knockouts focusing on musculoskeletal tissues. We will then discuss how the development of CRISPR/Cas-based technologies during the last decade has revolutionized the generation of GEMMs.

Osteoclast-derived IGF1 induces RANKL production in osteocytes and contributes to pagetic lesion formation

We previously reported that measles virus nucleocapsid protein (MVNP) expression in osteoclasts (OCLs) of patients with Paget disease (PD) or targeted to the OCL lineage in MVNP-transgenic mice (MVNP mice) increases IGF1 production in osteoclasts (OCL-IGF1) and leads to development of PD OCLs and pagetic bone lesions (PDLs). Conditional deletion of Igf1 in OCLs of MVNP mice fully blocked development of PDLs. In this study, we examined whether osteocytes (OCys), key regulators of normal bone remodeling, contribute to PD. OCys in PDLs of patients and of MVNP mice expressed less sclerostin, and had increased RANKL expression compared with OCys in bones from WT mice or normal patients. To test whether increased OCL-IGF1 is sufficient to induce PDLs and PD phenotypes, we generated TRAP-Igf1 (T-Igf1) transgenic mice to determine whether increased IGF1 expression in the absence of MVNP in OCLs is sufficient to induce PDLs and pagetic OCLs. We found that T-Igf1 mice at 16 months of age developed PD OCLs, PDLs, and OCys, with decreased sclerostin and increased RANKL, similar to MVNP mice. Thus, pagetic phenotypes could be induced by OCLs expressing increased IGF1. OCL-IGF1 in turn increased RANKL production in OCys to induce PD OCLs and PDLs.

CCN2 Facilitates IL-17 Production and Osteoclastogenesis in Human Osteoarthritis Synovial Fibroblasts by Inhibiting miR-655 Expression

Osteoarthritis (OA) is associated with extensive upregulation of osteoclastogenesis and subsequent bone breakdown. The CCN family protein connective tissue growth factor (CCN2, also called CCN2) enhances inflammatory cytokine production in OA disease. The cytokine interleukin (IL)-17 is known to induce osteoclastogenesis and bone erosion in arthritic disease. Our retrieval of data from the Gene Expression Omnibus (GEO) data set and clinical tissues exhibited higher CCN2 and IL-17 expression in OA synovial sample than in normal healthy samples. We observed the same phenomenon in synovial tissue from rats with anterior cruciate ligament transaction (ACLT)-elicited OA compared with synovial tissue from control healthy rats. We also found that CCN2 facilitated increases in IL-17 synthesis in human OA synovial fibroblasts (OASFs) and promoted osteoclast formation. CCN2 affected IL-17 production by reducing miR-655 expression through the ILK and Syk signaling cascades. Our findings improve our understanding about the effect of CCN2 in OA pathogenesis and, in particular, IL-17 production and osteoclastogenesis, which may help with the design of more effective OA treatments. © 2022 American Society for Bone and Mineral Research (ASBMR).

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Osteoclast-derived apoptotic bodies couple bone resorption and formation in bone remodeling

Bone remodeling is precisely coordinated by bone resorption and formation. Apoptotic osteoclasts generate large amounts of apoptotic bodies (ABs) marking the end of the bone resorption phase, whereas the functions of osteoclast-derived ABs remain largely unknown. Here, we identified the molecular profile of ABs derived from osteoclasts at distinct differentiation stages and investigated their corresponding functions. ABs were isolated from apoptotic bone marrow macrophages, preosteoclasts, and mature osteoclasts induced by staurosporine. Proteomic signature analysis with liquid chromatography-tandem mass spectrometry suggested marked protein cargo differences among the different ABs. Further bioinformatic analysis showed that the proteomic signatures of the ABs were highly similar to those of their parental cells. Functionally, pOC-ABs induced endothelial progenitor cell differentiation and increased CD31^hiEmcn^hi endothelial cell formation in a murine bone defect model via their PDGF-BB cargo. mOC-ABs induced osteogenic differentiation of mesenchymal stem cells and facilitated osteogenesis via RANKL reverse signaling. In summary, we mapped the detailed proteomic landscapes of ABs derived from osteoclasts and showed that their potential biological roles are important in coupling bone formation with resorption during bone remodeling.

Glucocorticoids Disrupt Skeletal Angiogenesis Through Transrepression of NF-κB-Mediated Preosteoclast Pdgfb Transcription in Young Mice

In the growing skeleton, angiogenesis is intimately coupled with osteogenesis. Chronic, high doses of glucocorticoids (GCs) are associated with decreased bone vasculature and induce osteoporosis and growth failure. The mechanism of GC-suppression of angiogenesis and relationship to osteoporosis and growth retardation remains largely unknown. Type H vessels, which are regulated by preosteoclast (POC) platelet-derived growth factor-BB (PDGF-BB), are specifically coupled with bone formation and development. We determined the effect of GCs on POC synthesis of PDGF-BB in relation to type H vessel formation, bone mass, and bone growth in the distal femur of 2-week-old young mice receiving prednisolone or vehicle for 2, 4, or 6 weeks. After 2 weeks of prednisolone, the number of POCs were unchanged while POC synthesis of PDGF-BB was reduced. Longer treatment with prednisolone reduced POCs numbers and PDGF-BB. These changes were associated with a reduction in type H vessels, bone formation rate, bone mass, and bone length at each time point. In vitro, excessive concentrations of prednisolone (10^-6 M) resulted in decreased PDGF-BB concentration and POC numbers. Conditioned medium from POC cultures treated with control concentration of prednisolone (10^-7 M) or recombinant PDGF-BB stimulated endothelial tube formation, whereas conditioned medium from control concentration of prednisolone-treated POC cultures neutralized by PDGF-BB antibody or excessive prednisolone inhibited endothelial tube formation. Administration of excessive prednisolone attenuated the P65 subunit of nuclear factor kappa B (NF-κB) binding to the Pdgfb promoter, resulting in lower Pdgfb transcription. Co-treatment with excessive prednisolone and the glucocorticoid receptor (GR) antagonist (RU486), GR siRNA, or TNFα rescued NF-κB binding to the Pdgfb promoter and endothelial tube formation. These results indicate that PDGF-BB synthesis in POCs is suppressed by GCs through transrepression of GR/NF-κB, thus inhibiting type H vessel formation and associated osteoporosis and growth failure. © 2020 American Society for Bone and Mineral Research.

Osteoclast-derived IGF1 is required for pagetic lesion formation in vivo

We report that transgenic mice expressing measles virus nucleocapsid protein (MVNP) in osteoclasts (OCLs) (MVNP mice) are Paget's disease (PD) models and that OCLs from patients with PD and MVNP mice express high levels of OCL-derived IGF1 (OCL-IGF1). To determine OCL-IGF1's role in PD and normal bone remodeling, we generated WT and MVNP mice with targeted deletion of Igf1 in OCLs (Igf1-cKO) and MVNP/Igf1-cKO mice, and we assessed OCL-IGF1's effects on bone mass, bone formation rate, EphB2/EphB4 expression on OCLs and osteoblasts (OBs), and pagetic bone lesions (PDLs). A total of 40% of MVNP mice, but no MVNP/Igf1-cKO mice, had PDLs. Bone volume/tissue volume (BV/TV) was decreased by 60% in lumbar vertebrae and femurs of MVNP/Igf1-cKO versus MVNP mice with PDLs and by 45% versus all MVNP mice tested. Bone formation rates were decreased 50% in Igf1-cKO and MVNP/Igf1-cKO mice versus WT and MVNP mice. MVNP mice had increased EphB2 and EphB4 levels in OCLs/OBs versus WT and MVNP/Igf1-cKO, with none detectable in OCLs/OBs of Igf1-cKO mice. Mechanistically, IL-6 induced the increased OCL-IGF1 in MVNP mice. These results suggest that high OCL-IGF1 levels increase bone formation and PDLs in PD by enhancing EphB2/EphB4 expression in vivo and suggest OCL-IGF1 may contribute to normal bone remodeling.

The haematopoietic stem cell niche: a new player in cardiovascular disease?

Haematopoiesis, the process of blood production, can be altered during the initiation or progression of many diseases. Cardiovascular disease (CVD) has been shown to be heavily influenced by changes to the haematopoietic system, including the types and abundance of immune cells produced. It is now well established that innate immune cells are increased in people with CVD, and the mechanisms contributing to this can be vastly different depending on the risk factors or comorbidities present. Many of these changes begin at the level of the haematopoietic stem and progenitor cells (HSPCs) that reside in the bone marrow (BM). In general, the HSPCs and downstream myeloid progenitors are expanded via increased proliferation in the setting of atherosclerotic CVD. However, HSPCs can also be encouraged to leave the BM and colonise extramedullary sites (i.e. the spleen). Within the BM, HSPCs reside in specialized microenvironments, often referred to as a niche. To date in depth studies assessing the damage or dysregulation that occurs in the BM niche in varying CVDs are scarce. In this review, we provide a general overview of the complex components and interactions within the BM niche and how they influence the function of HSPCs. Additionally, we discuss the main findings regarding changes in the HSPC niche that influence the progression of CVD. We hypothesize that understanding the influence of the BM niche in CVD will aid in delineating new pathways for therapeutic interventions.

Differentially expressed proteins identified by TMT proteomics analysis in bone marrow microenvironment of osteoporotic patients

We applied tandem mass tag (TMT)-based proteomics to investigate protein changes in bone marrow microenvironment of osteoporotic patients undergoing spine fusion. Multiple bioinformatics tools were used to identify and analyze 219 differentially expressed proteins. These proteins may be associated with the pathogenesis of osteoporosis.

INTRODUCTION

Bone marrow microenvironment is indispensable for the maintenance of bone homeostasis. We speculated that alterations of some factors in the microenvironment of osteoporotic subjects might influence the homeostasis. This study aimed to investigate the changes in the expression of protein factors in the bone marrow environment of osteoporosis.

METHODS

We performed a proteomics analysis in the vertebral body-derived bone marrow supernatant fluid from 8 Chinese patients undergoing posterior lumbar interbody fusion (4 osteoporotic vs. 4 non-osteoporotic) and used micro-CT to analyze the microstructural features of spinous processes from these patients. We further performed western blotting to validate the differential expressions of some proteins.

RESULTS

There was deteriorated bone microstructure in osteoporotic patients. Based on proteomics analysis, 172 upregulated and 47 downregulated proteins were identified. These proteins had multiple biological functions associated with osteoblast differentiation, lipid metabolism, and cell migration, and formed a complex protein-protein interaction network. We identified five major regulatory mechanisms, splicing, translation, protein degradation, cytoskeletal organization, and lipid metabolism, involved in the pathogenesis of osteoporosis.

CONCLUSIONS

There are various protein factors, such as DDX5, PSMC2, CSNK1A1, PLIN1, ILK, and TPM4, differentially expressed in the bone marrow microenvironment of osteoporotic patients, providing new ideas for finding therapeutic targets for osteoporosis.

Macrophage-lineage TRAP+ cells recruit periosteum-derived cells for periosteal osteogenesis and regeneration

The periosteum, a thin tissue that covers almost the entire bone surface, accounts for more than 80% of human bone mass and is essential for bone regeneration. Its osteogenic and bone regenerative abilities are well studied, but much is unknown about the periosteum. In this study, we found that macrophage-lineage cells recruit periosteum-derived cells (PDCs) for cortical bone formation. Knockout of colony stimulating factor-1 eliminated macrophage-lineage cells and resulted in loss of PDCs with impaired periosteal bone formation. Moreover, macrophage-lineage TRAP+ cells induced transcriptional expression of periostin and recruitment of PDCs to the periosteal surface through secretion of platelet-derived growth factor-BB (PDGF-BB), where the recruited PDCs underwent osteoblast differentiation coupled with type H vessel formation. We also found that subsets of Nestin+ and LepR+ PDCs possess multipotent and self-renewal abilities and contribute to cortical bone formation. Nestin+ PDCs are found primarily during bone development, whereas LepR+ PDCs are essential for bone homeostasis in adult mice. Importantly, conditional knockout of Pdgfrβ (platelet-derived growth factor receptor beta) in LepR+ cells impaired periosteal bone formation and regeneration. These findings uncover the essential role of periosteal macrophage-lineage cells in regulating periosteum homeostasis and regeneration.

Mouse Cre Models for the Study of Bone Diseases

PURPOSE

Transgenic Cre lines are a valuable tool for conditionally inactivating or activating genes to understand their function. Here, we provide an overview of Cre transgenic models used for studying gene function in bone cells and discuss their advantages and limitations, with particular emphasis on Cre lines used for studying osteocyte and osteoclast function.

RECENT FINDINGS

Recent studies have shown that many bone cell-targeted Cre models are not as specific as originally thought. To ensure accurate data interpretation, it is important for investigators to test for unexpected recombination events due to transient expression of Cre recombinase during development or in precursor cells and to be aware of the potential for germ line recombination of targeted genes as well as the potential for unexpected phenotypes due to the Cre transgene. Although many of the bone-targeted Cre-deleter strains are imperfect and each model has its own limitations, their careful use will continue to provide key advances in our understanding of bone cell function in health and disease.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

Role and mechanism of action of leucine-rich repeat kinase 1 in bone

Leucine-rich repeat kinase 1 (LRRK1) plays a critical role in regulating cytoskeletal organization, osteoclast activity, and bone resorption with little effect on bone formation parameters. Deficiency of Lrrk1 in mice causes a severe osteopetrosis in the metaphysis of the long bones and vertebrae bones, which makes LRRK1 an attractive alternative drug target for the treatment of osteoporosis and other high-turnover bone diseases. This review summarizes recent advances on the functions of the Lrrk1-related family members, Lrrk1 deficiency-induced skeletal phenotypes, LRRK1 structure–function, potential biological substrates and interacting proteins, and the mechanisms of LRRK1 action in osteoclasts.

Leucine-rich repeat kinase-1 regulates osteoclast function by modulating RAC1/Cdc42 Small GTPase phosphorylation and activation

Leucine-rich repeat kinase-1 (Lrrk1) consists of ankyrin repeats (ANK), leucine-rich repeats (LRR), a GTPase-like domain of Roc (ROC), a COR domain, a serine/threonine kinase domain (KD), and WD40 repeats (WD40). Previous studies have revealed that knockout (KO) of Lrrk1 in mice causes severe osteopetrosis, and a human mutation of Lrrk1 leads to osteosclerotic metaphysial dysplasia. The molecular mechanism by which Lrrk1 regulates osteoclast function is unknown. In this study, we generated a series of Lrrk1 mutants and evaluated their ability to rescue defective bone resorption in Lrrk1-deficient osteoclasts by use of pit formation assays. Overexpression of Lrrk1 or LRR-truncated Lrrk1, but not ANK-truncated Lrrk1, WD40-truncated Lrrk1, Lrrk1-KD, or K651A mutant Lrrk1, rescued bone resorption function of Lrrk1 KO osteoclasts. We next examined whether RAC1/Cdc42 small GTPases are direct substrates of Lrrk1 in osteoclasts. Western blot and pull-down assays revealed that Lrrk1 deficiency in osteoclasts resulted in reduced phosphorylation and activation of RAC1/Cdc42. In vitro kinase assays confirmed that recombinant Lrrk1 phosphorylated RAC1-GST protein, and immunoprecipitation showed that the interaction of Lrrk1 with RAC1 occurred within 10 min after RANKL treatment. Overexpression of constitutively active Q61L RAC1 partially rescued the resorptive function of Lrrk1-deficient osteoclasts. Furthermore, lack of Lrrk1 in osteoclasts led to reduced autophosphorylation of p21 protein-activated kinase-1 at Ser¹⁴⁴, catalyzed by RAC1/Cdc42 binding and activation. Our data indicate that Lrrk1 regulates osteoclast function by directly modulating phosphorylation and activation of small GTPase RAC1/Cdc42 and that its function depends on ANK, ROC, WD40, and kinase domains.

β2-Adrenergic signal transduction plays a detrimental role in subchondral bone loss of temporomandibular joint in osteoarthritis

The present study tested whether activation of the sympathetic tone by aberrant joint loading elicits abnormal subchondral bone remodeling in temporomandibular joint (TMJ) osteoarthritis. Abnormal dental occlusion was created in experimental rats, which were then intraperitoneally injected by saline, propranolol or isoproterenol. The norepinephrine contents, distribution of sympathetic nerve fibers, expression of β-adrenergic receptors (β-ARs) and remodeling parameters in the condylar subchondral bone were investigated. Mesenchymal stem cells (MSCs) from condylar subchondral bones were harvested for comparison of their β-ARs, pro-osteoclastic gene expressions and pro-osteoclastic function. Increases in norepinephrine level, sympathetic nerve fiber distribution and β2-AR expression were observed in the condylar subchondral bone of experimental rats, together with subchondral bone loss and increased osteoclast activity. β-antagonist (propranolol) suppressed subchondral bone loss and osteoclast hyperfunction while β-agonist (isoproterenol) exacerbated those responses. MSCs from experimental condylar subchondral bone expressed higher levels of β2-AR and RANKL; norepinephrine stimulation further increased their RANKL expression and pro-osteoclastic function. These effects were blocked by inhibition of β2-AR or the PKA pathway. RANKL expression by MSCs decreased after propranolol administration and increased after isoproterenol administration. It is concluded that β2-AR signal-mediated subchondral bone loss in TMJ osteoarthritisis associated with increased RANKL secretion by MSCs.

PDGF-BB secreted by preosteoclasts induces angiogenesis during coupling with osteogenesis

Osteogenesis during bone modeling and remodeling is coupled with angiogenesis. A recent study shows that the specific vessel subtype, strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi), couples angiogenesis and osteogenesis. We found that preosteoclasts secrete platelet derived growth factor-BB (PDGF-BB), inducing CD31^hiEmcn^hi vessels during bone modeling and remodeling. Mice with depletion of PDGF-BB in tartrate-resistant acid phosphatase positive (TRAP⁺) cell lineage (Pdgfb^–/–) show significantly lower trabecular and cortical bone mass, serum and bone marrow PDGF-BB concentrations, and CD31^hiEmcn^hi vessels compared to wild-type mice. In the ovariectomized (OVX) osteoporotic mouse model, concentrations of serum and bone marrow PDGF-BB and CD31^hiEmcn^hi vessels are significantly decreased. Inhibition of cathepsin K (CTSK) increases preosteoclast numbers, resulting in higher levels of PDGF-BB to stimulate CD31^hiEmcn^hi vessels and bone formation in OVX mice. Thus, pharmacotherapies that increase PDGF-BB secretion from preosteoclasts offer a novel therapeutic target for osteoporosis to promote angiogenesis for bone formation.

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

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

DICAM inhibits osteoclast differentiation through attenuation of the integrin αVβ3 pathway

Dual immunoglobulin (Ig) domain-containing adhesion molecule (DICAM) is involved in cell-cell adhesion through a heterophilic interaction with αVβ3 integrin, which suggests that DICAM may participate in osteoclast differentiation. DICAM was localized in the plasma membrane of RAW264.7 and THP-1 cells, and its expression gradually increased during osteoclastogenesis in mouse bone marrow-derived macrophages (BMMs) treated with receptor activator of nuclear factor κ-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). Forced expression of DICAM in BMMs and RAW264.7 cells blocked the generation of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts. Conversely, knockdown of DICAM by small hairpin RNA (shRNA) increased osteoclast formation in RAW264.7 cells. DICAM-mediated suppression of osteoclast differentiation was in part due to the inhibition of the p38 mitogen-activated protein (MAP) kinase pathway, which was corroborated by a decrease in the expression of c-Fos and nuclear factor of activated T cells (NFAT)c1. Mechanistically, DICAM directly interacted with integrin β3, which inhibited heterodimerization between integrin αV and β3. Exogenous expression of integrin β3 or high-dose M-CSF rescued DICAM-mediated inhibition of osteoclastogenesis, suggesting crosstalk between the integrin β3 and c-Fms pathways. Finally, recombinant DICAM ectodomain suppressed the RANKL- and M-CSF-induced osteoclastogenesis of BMMs. Collectively, these results indicate that DICAM acts as a negative regulator of osteoclast differentiation by suppressing the integrin αVβ3 pathway.

Therapeutic approaches to myeloma bone disease: an evolving story

Bone disease is a major morbidity factor in patients with multiple myeloma and significantly affects their overall survival. A complex interplay between malignant plasma cells and other marrow cells results in the generation of a microenvironment capable of enhancing both tumor growth and bone destruction. Bisphosphonates have consistently reduced the incidence of skeletal-related events in patients with multiple myeloma and other osteotropic tumors as well. However, their use is burdened with side-effects, including the risks of osteonecrosis of the jaw and kidney failure, suggesting that they should be discontinued after prolonged administration. New molecular targets of cell cross-talk in myeloma bone marrow are therefore under intensive investigation and new drugs are being explored in preclinical and clinical studies of myeloma bone disease. Compounds targeting osteoclast activation pathways, such as receptor activator of nuclear factor-κB/receptor activator of nuclear factor-κB ligand/osteoprotegerin, B-cell activating factor, mitogen-activated protein kinase and macrophage inflammatory protein-1α/chemokine receptor for macrophage inflammatory protein-1α axes, or soluble agents that improve osteoblast differentiation by modulating specific inhibitors such as Dickkopf-1 and transforming growth factor-β, as well as novel approaches of cytotherapy represent a new generation of promising drugs for the treatment of myeloma bone disease.

Genetic mouse models for bone studies--strengths and limitations

Mice have become a preferred model system for bone research because of their genetic and pathophysiological similarities to humans: a relatively short reproductive period, leading to relatively low cost of maintenance and the availability of the entire mouse genome sequence information. The success in producing the first transgenic mouse line that expressed rabbit β-globin protein in mouse erythrocytes three decades ago marked the beginning of the use of genetically engineered mice as model system to study human diseases. Soon afterward the development of cultured pluripotent embryonic stem cells provided the possibility of gene replacement or gene deletion in mice. These technologies have been critical to identify new genes involved in bone development, growth, remodeling, repair, and diseases, but like many other approaches, they have limitations. This review will introduce the approaches that allow the generation of transgenic mice and global or conditional (tissue-specific and inducible) mutant mice. A list of the various promoters used to achieve bone-specific gene deletion or overexpression is included. The limitations of these approaches are discussed, and general guidelines related to the analysis of genetic mouse models are provided.

Osseointegration--communication of cells

BACKGROUND

The article provides the scientific documentation for the 3D animated film - "Osseointegration - Communication of cells".

AIM

The aim of this article and of the film is to visualise the molecular and cellular events during the healing of an osseous wound after installation of a dental implant with special emphasis on the process of osseointegration.

MATERIAL AND RESULTS

In this review article for didactic reasons the concept of the four phases of a healing soft tissue wound was transferred to a bone wound after insertion of a dental implant: haemostasis, inflammatory phase, proliferative phase and remodelling phase. Wound healing throughout these phases is the result of a coordinated action of different cell types which communicate with each other by their interaction using signalling molecules like cytokines, extracellular matrix proteins and small molecules. A regular sequence of cell types controlled by adequate concentrations of signalling molecules results in undisturbed healing. Disturbed healing is associated with a continuation of the early inflammatory phase and the development of a toxic wound environment. The latter is characterized by high counts of polymorphnuclear cells, high concentrations of toxic radicals and proteolytic enzymes and low concentrations of growth factors and extracellular matrix molecules. Clinically the development of a toxic wound environment should be avoided, e.g. by antibacterial measures.

DISCUSSION AND CONCLUSION

Experiencing implant osseointegration as a biological process may provide the clinician new targets to improve the therapy with dental implants.