Bone remodelling at a glance

Semaphorins in bone development, homeostasis, and disease

Semaphorins were originally identified as axon guidance cues in the development of the nervous system. In recent years, numerous studies have determined that they are also involved in organogenesis, vascularization/angiogenesis, oncogenesis, and immune responses. In addition, the mechanisms underlying the diverse functions of semaphorins and their receptors have been identified. Recently, significant advances have been made in our understanding of the roles of semaphorins in bone remodeling, particularly the regulation of osteoclast and osteoblast differentiation and migration. Moreover, dysregulated semaphorin expression causes severe bone diseases, including osteoporosis and osteopetrosis. This review focuses on advanced findings on the role of semaphorins/receptors and their intracellular signaling in the regulation of bone homeostasis.

Bisphosphonates: focus on inflammation and bone loss

Bisphosphonates are pharmacological compounds that have been used for the prevention and treatment of several pathological conditions including osteoporosis, primary hyperparathyroidism, osteogenesis imperfecta, and other conditions characterized by bone fragility. Many studies have been performed to date to analyze their effects on inflammation and bone remodelling and related pathologies. The aim of this review is, starting from a background on inflammatory processes and bone remodelling, to give an update on the use of bisphosphonates, outlining the possible side effects and proposing new trends for the future. Starting from a brief introduction on inflammation and bone remodelling, we collect and analyze studies involving the use of bisphosphonates for treatment of inflammatory conditions and pathologies characterized by bone loss. Selected articles, including reviews, published between 1976 and 2011, were chosen from Pubmed/Medline on the basis of their content. Bisphosphonates exert a selective activity on inflammation and bone remodelling and related pathologies, which are characterized by an excess in bone resorption. They improve not only skeletal defects, but also general symptoms. Bisphosphonates have found clinical application preventing and treating osteoporosis, osteitis deformans (Paget's disease of bone), bone metastasis (with or without hypercalcaemia), multiple myeloma, primary hyperparathyroidism, osteogenesis imperfecta, and other conditions that feature bone fragility. Further clinical studies involving larger cohorts are needed to optimize the dosage and length of therapy for each of these agents in each clinical field in order to be able to maximize their properties concerning modulation of inflammation and bone remodelling. In the near future, although "old" bisphosphonates will reach the end of their patent life, "new" bisphosphonates will be designed to specifically target a pathological condition.

Cytokine combination therapy prediction for bone remodeling in tissue engineering based on the intracellular signaling pathway

The long-term performance of tissue-engineered bone grafts is determined by a dynamic balance between bone regeneration and resorption. We proposed using embedded cytokine slow-releasing hydrogels to tune this balance toward a desirable final bone density. In this study we established a systems biology model, and quantitatively explored the combinatorial effects of delivered cytokines from hydrogels on final bone density. We hypothesized that: 1) bone regeneration was driven by transcription factors Runx2 and Osterix, which responded to released cytokines, such as Wnt, BMP2, and TGFβ, drove the development of osteoblast lineage, and contributed to bone mass generation; and 2) the osteoclast lineage, on the other hand, governed the bone resorption, and communications between these two lineages determined the dynamics of bone remodeling. In our model, Intracellular signaling pathways were represented by ordinary differential equations, while the intercellular communications and cellular population dynamics were modeled by stochastic differential equations. Effects of synergistic cytokine combinations were evaluated by Loewe index and Bliss index. Simulation results revealed that the Wnt/BMP2 combinations released from hydrogels showed best control of bone regeneration and synergistic effects, and suggested optimal dose ratios of given cytokine combinations released from hydrogels to most efficiently control the long-term bone remodeling. We revealed the characteristics of cytokine combinations of Wnt/BMP2 which could be used to guide the design of in vivo bone scaffolds and the clinical treatment of some diseases such as osteoporosis.

Transcriptional profile of native CD271+ multipotential stromal cells: evidence for multiple fates, with prominent osteogenic and Wnt pathway signaling activity

OBJECTIVE

Controversy surrounds the identity and functionality of rare bone marrow-derived multipotential stromal cells (BM-MSCs), including their differentiation capabilities, their relationship to pericytes and hematopoiesis-supporting stromal cells, and the relevance of their culture-expanded progeny in studies of skeletal biology and development of cell-based therapies. The aim of this study was to clarify the nature of candidate BM-MSCs by profiling transcripts that reflect different aspects of their putative functions in vivo.

METHODS

Rare, sorted BM-derived CD45(-/low) CD271(bright) (CD271) cells were analyzed using 96-gene expression arrays focused on transcripts relevant to mesenchymal-lineage differentiation (toward bone, cartilage, fat, or muscle), hematopoietic and stromal support, and molecules critical to skeletal homeostasis. These cells were compared to matched CD45+ CD271- hematopoietic-lineage cells, culture-expanded MSCs, and skin fibroblasts. When feasible, transcription was validated using flow cytometry.

RESULTS

CD271 cells had a transcriptional profile consistent with the multiple fates of in vivo MSCs, evident from the observed simultaneous expression of osteogenic, adipogenic, pericytic, and hematopoiesis-supporting genes (e.g., SP7 [osterix], FABP4 [fatty acid binding protein 4], ANGPT1 [angiopoietin 1], and CXCL12 [stromal cell-derived factor 1], respectively). Compared to culture-expanded MSCs and fibroblasts, CD271 cells exhibited greater transcriptional activity, particularly with respect to Wnt-related genes (>1,000-fold increased expression of FRZB [secreted frizzled-related protein 3] and WIF1 [Wnt inhibitory factor 1]). A number of transcripts were identified as novel markers of MSCs.

CONCLUSION

The native, BM-derived in vivo MSC population is endowed with a gene signature that is compatible with multiple functions, reflecting the topographic bone niche of these cells, and their signature is significantly different from that of culture-expanded MSCs. This indicates that studies of the biologic functions of MSCs in musculoskeletal diseases, including osteoporosis and osteoarthritis, should focus on in vivo MSCs, rather than their culture-adapted progeny.

Bone balance within a cortical BMU: local controls of bone resorption and formation

Maintaining bone volume during bone turnover by a BMU is known as bone balance. Balance is required to maintain structural integrity of the bone and is often dysregulated in disease. Consequently, understanding how a BMU controls bone balance is of considerable interest. This paper develops a methodology for identifying potential balance controls within a single cortical BMU. The theoretical framework developed offers the possibility of a directed search for biological processes compatible with the constraints of balance control. We first derive general control constraint equations and then introduce constitutive equations to identify potential control processes that link key variables that describe the state of the BMU. The paper describes specific local bone volume balance controls that may be associated with bone resorption and bone formation. Because bone resorption and formation both involve averaging over time, short-term fluctuations in the environment are removed, leaving the control systems to manage deviations in longer-term trends back towards their desired values. The length of time for averaging is much greater for bone formation than for bone resorption, which enables more filtering of variability in the bone formation environment. Remarkably, the duration for averaging of bone formation may also grow to control deviations in long-term trends of bone formation. Providing there is sufficient bone formation capacity by osteoblasts, this leads to an extraordinarily robust control mechanism that is independent of either osteoblast number or the cellular osteoid formation rate. A complex picture begins to emerge for the control of bone volume. Different control relationships may achieve the same objective, and the ‘integration of information’ occurring within a BMU may be interpreted as different sets of BMU control systems coming to the fore as different information is supplied to the BMU, which in turn leads to different observable BMU behaviors.

Membrane trafficking in osteoblasts and osteoclasts: new avenues for understanding and treating skeletal diseases

The endocytic and exocytic/secretory pathways are two major intracellular membrane trafficking routes that regulate numerous cellular functions in a variety of cell types. Osteoblasts and osteoclasts, two major bone cells responsible for bone remodeling and homeostasis, are no exceptions. During the past few years, emerging evidence has pinpointed a critical role for endocytic and secretory pathways in osteoblast and osteoclast differentiation and function. The endosomal membrane provides a platform to integrate bone tropic signals of hormones and growth factors in osteoblasts. In osteoclasts, endocytosis, followed by transcytosis, of degraded bone matrix promotes bone resorption. Secretory pathways, especially lysosome secretion, not only participate in bone matrix deposition by osteoblasts and degradation of mineralized bone matrix by osteoclasts; they may also be involved in the coupling of bone resorption and bone formation during bone remodeling. More importantly, mutations in genes encoding regulatory factors within the endocytic and secretory pathways have been identified as causes for bone diseases. Identification of the molecular mechanisms of these genes in bone cells may provide new therapeutic targets for skeletal disorders.

Bone morphogenetic protein receptor type Ia localization causes increased BMP2 signaling in mice exhibiting increased peak bone mass phenotype

Bone morphogenetic protein 2 (BMP2) is a growth factor that initiates osteoblast differentiation. Recent studies show that BMP2 signaling regulates bone mineral density (BMD). BMP2 interacts with BMP receptor type Ia (BMPRIa) and type II receptor leading to the activation of the Smad signaling pathway. BMPRIa must shuttle between distinct plasma membrane domains, enriched of Caveolin-1 alpha and Caveolin-1 beta isoforms, and receptor activation occurs in these domains. Yet it remains unknown whether the molecular mechanism that regulates BMP2 signaling is driving mineralization and BMD. Therefore, the B6.C3H-1-12 congenic mouse model with increased BMD and osteoblast mineralization was utilized in this study. Using the family image correlation spectroscopy, we determined if BMP2 led to a significant re-localization of BMPRIa to caveolae of the alpha/beta isoforms in bone marrow stromal cells (BMSCs) isolated from B6.C3H-1-12 mice compared to the C57BL/6J mice, which served as controls. The control, C57BL/6J mice, was selected due to only 4 Mb of chromosome 1 from the C3H/HeJ mouse was backcrossed to a C57BL/6J background. Using reporter gene assays, the B6.C3H-1-12 BMSCs responded to BMP2 with increased Smad activation. Furthermore, disrupting caveolae reduced the BMP2-induced Smad signaling in BMSCs isolated from B6.C3H-1-12 and C57BL/6J. This study suggests for the first time a regulatory mechanism of BMPRIa signaling at the plasma membrane of BMSCs that (i) associated with genetic differences in the distal Chromosome 1 segment carried by the B6.C3H-1-12 congenic and (ii) contributes to increase BMD of the B6.C3H-1-12 compared to the C57BL/6J control mice.

Autophagy: a new player in skeletal maintenance?

Imbalances between bone resorption and formation lie at the root of disorders such as osteoporosis, Paget's disease of bone (PDB), and osteopetrosis. Recently, genetic and functional studies have implicated proteins involved in autophagic protein degradation as important mediators of bone cell function in normal physiology and in pathology. Autophagy is the conserved process whereby aggregated proteins, intracellular pathogens, and damaged organelles are degraded and recycled. This process is important both for normal cellular quality control and in response to environmental or internal stressors, particularly in terminally-differentiated cells. Autophagic structures can also act as hubs for the spatial organization of recycling and synthetic process in secretory cells. Alterations to autophagy (reduction, hyperactivation, or impairment) are associated with a number of disorders, including neurodegenerative diseases and cancers, and are now being implicated in maintenance of skeletal homoeostasis. Here, we introduce the topic of autophagy, describe the new findings that are starting to emerge from the bone field, and consider the therapeutic potential of modifying this pathway for the treatment of age-related bone disorders.

Intercellular cross-talk among bone cells: new factors and pathways

Intercellular communication within the bone microenvironment is critical for the maintenance of normal bone structure. Osteoblast-lineage cells at all stages of differentiation, from pluripotent precursors to matrix-embedded osteocytes, produce regulatory factors that modulate the differentiation and activity of both bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoclasts can also release factors that feed back to regulate osteoblast activity. Intercellular cross-talk within the bone microenvironment is not restricted only to these bone cells. Other cells within the bone marrow microenvironment, including adipocytes, T cells, and macrophages, play key roles that influence the processes of bone formation and resorption. This review discusses recent work that provides new insights into some of these communication networks and the factors involved, including osteocytic production of receptor activator of nuclear factor-κB ligand (RANKL) and sclerostin, osteoblastic production of interleukin-33, osteoclast-derived Semaphorin 4D, ephrin signaling, and signals from T helper cells and resident osteal macrophages (osteomacs).

Possible role of calcium permselectivity in bone adaptation

According to the core activity of calcium in the bone cellular expression, a new hypothesis linking calcium transport with the mechanical loading is proposed to explain the mechano-adaptation of bone tissue. Due to the piezoelectric coupling, the tensile and compressive areas of bone produce different electrical environments for the osteocytic cells that are embedded in the lacuno-canalicular porosity. This electrical asymmetry engenders a calcium enrichment-exclusion effect that strongly changes the calcium concentration in the lacuno-canalicular fluid and thus modifies the remodelling process. A bibliographic body of evidence supporting this idea is given and its experimental validation is suggested.

Altered plasma membrane dynamics of bone morphogenetic protein receptor type Ia in a low bone mass mouse model

Bone morphogenetic proteins (BMPs) are growth factors that initiate differentiation of bone marrow stromal cells to osteoblasts and adipocytes, yet the mechanism that decides which lineage the cell will follow is unknown. BMP2 is linked to the development of osteoporosis and variants of BMP2 gene have been reported to increase the development of osteoporosis. Intracellular signaling is transduced by BMP receptors (BMPRs) of type I and type II that are serine/threonine kinase receptors. The BMP type I a receptor (BMPRIa) is linked to osteogenesis and bone mineral density (BMD). BMPRs are localized to caveolae enriched with Caveolin1 alpha/beta and Caveolin beta isoforms to facilitate signaling. BMP2 binding to caveolae was recently found to be crucial for the initiation of the Smad signaling pathway. Here we determined the role of BMP receptor localization within caveolae isoforms and aggregation of caveolae as well as BMPRIa in bone marrow stromal cells (BMSCs) on bone mineral density using the B6.C3H-6T as a model system. The B6.C3H-6T is a congenic mouse with decreased bone mineral density (BMD) with increased marrow adipocytes and decreased osteoprogenitor proliferation. C57BL/6J mice served as controls since only a segment of Chr6 from the C3H/HeJ mouse was backcrossed to a C57BL/6J background. Family of image correlation spectroscopy was used to analyze receptor cluster density and co-localization of BMPRIa and caveolae. It was previously shown that BMP2 stimulation results in an aggregation of caveolae and BMPRIa. Additionally, BMSCs isolated from the B6.C3H-6T mice showed a dispersion of caveolae domains compared to C57BL/6J. The aggregation of BMPRIa that is necessary for signaling to occur was inhibited in BMSCs isolated from B6.C3H-6T. Additionally, we analyzed the co-localization of BMPRIa with caveolin-1 isoforms. There was increased percentage of BMPRIa co-localization with caveolae compared to C57BL/6J. BMP2 stimulation had no effect on the colocalization of BMPRIa with caveolin-1. Disrupting caveolae initiated Smad signaling in the isolated BMSCs from B6.C3H-6T. These data suggest that in congenic 6T mice BMP receptors aggregation is inhibited causing an inhibition of signaling and reduced bone mass.

The skeleton: a multi-functional complex organ: the role of key signalling pathways in osteoclast differentiation and in bone resorption

Osteoclasts are the specialised cells that resorb bone matrix and are important both for the growth and shaping of bones throughout development as well as during the process of bone remodelling that occurs throughout life to maintain a healthy skeleton. Osteoclast formation, function and survival are tightly regulated by a network of signalling pathways, many of which have been identified through the study of rare monogenic diseases, knockout mouse models and animal strains carrying naturally occurring mutations in key molecules. In this review, we describe the processes of osteoclast formation, activation and function and discuss the major transcription factors and signalling pathways (including those that control the cytoskeletal rearrangements) that are important at each stage.

Bone targeting for the treatment of osteoporosis

Osteoporosis represents a major public health burden especially considering the aging populations worldwide. Drug targeting will be important to better meet these challenges and direct the full therapeutic potential of therapeutics to their intended site of action. This review has been organized in modules, such that scientists working in the field can easily gain specific insight in the field of bone targeting for the drug class they are interested in. We review currently approved and emerging treatment options for osteoporosis and discuss these in light of the benefit these would gain from advanced targeting. In addition, established targeting strategies are reviewed and novel opportunities as well as promising areas are presented along with pharmaceutical strategies how to render novel composites consisting of a drug and a targeting moiety responsive to bone-specific or disease-specific environmental stimuli. Successful implementation of these principles into drug development programs for osteoporosis will substantially contribute to the clinical success of anti-catabolic and anabolic drugs of the future.