The role of oxygen as a regulator of stem cell fate during fracture repair in TSP2-null mice

Bone-derived extracellular matrix hydrogel from thrombospondin-2 knock-out mice for bone repair

Bone extracellular matrix (ECM) has been shown to mimic aspects of the tissue's complex microenvironment, suggesting its potential role in promoting bone repair. However, current ECM-based therapies suffer from limitations such as inefficient scale-up, lack of mechanical integrity, and sub-optimal efficacy. Here, we fabricated hydrogels from decellularized ECM (dECM) from wild type (WT) and thrombospondin-2 knock-out (TSP2KO) mouse bones. TSP2KO bone ECM hydrogel was found to have distinct mechanical properties and collagen fibril assembly from WT. Furthermore, TSP2KO hydrogel promoted mesenchymal stem cell (MSC) attachment, spreading, and invasion in vitro. Similarly, it promoted formation of tube-like structures by human umbilical vein endothelial cells (HUVECs). When applied to a murine calvarial defect model, TSP2KO hydrogel enhanced repair, in part, due to increased angiogenesis. Our study suggests the pro-angiogenic therapeutic potential of TSP2KO bone ECM hydrogel in bone repair. STATEMENT OF SIGNIFICANCE: The study describes the first successful preparation of a novel hydrogel made from decellularized bones from wild-type mice and mice lacking thrombospondin-2 (TSP2). Hydrogels from TSP2 knock-out (TSP2KO) bones have unique characteristics in structure and biomechanics. These gels interacted well with cells in vitro and helped repair damaged bone in a mouse model. Therefore, TSP2KO bone-derived hydrogel has translational potential for accelerating repair of bone defects that are otherwise difficult to heal. This study not only creates a new material with promise for accelerated healing, but also validates tunability of native biomaterials by genetic engineering.

Thrombospondins modulate cell function and tissue structure in the skeleton

Thrombospondins (TSPs) belong to a functional class of ECM proteins called matricellular proteins that are not primarily structural, but instead influence cellular interactions within the local extracellular environment. The 3D arrangement of TSPs allow interactions with other ECM proteins, sequestered growth factors, and cell surface receptors. They are expressed in mesenchymal condensations and limb buds during skeletal development, but they are not required for patterning. Instead, when absent, there are alterations in musculoskeletal connective tissue ECM structure, organization, and function, as well as altered skeletal cell phenotypes. Both functional redundancies and unique contributions to musculoskeletal tissue structure and physiology are revealed in mouse models with compound TSP deletions. Crucial roles of individual TSPs are revealed during musculoskeletal injury and regeneration. The interaction of TSPs with mesenchymal stem cells (MSC), and their influence on cell fate, function, and ultimately, musculoskeletal phenotype, suggest that TSPs play integral, but as yet poorly understood roles in musculoskeletal health. Here, unique and overlapping contributions of trimeric TSP1/2 and pentameric TSP3/4/5 to musculoskeletal cell and matrix physiology are reviewed. Opportunities for new research are also noted.

From hurdle to springboard: The macrophage as target in biomaterial-based bone regeneration strategies

The past decade has seen a growing appreciation for the role of the innate immune response in mediating repair and biomaterial directed tissue regeneration. The long-held view of the host immune/inflammatory response as an obstacle limiting stem cell regenerative activity, has given way to a fresh appreciation of the pivotal role the macrophage plays in orchestrating the resolution of inflammation and launching the process of remodelling and repair. In the context of bone, work over the past decade has established an essential coordinating role for macrophages in supporting bone repair and sustaining biomaterial driven osteogenesis. In this review evidence for the role of the macrophage in bone regeneration and repair is surveyed before discussing recent biomaterial and drug-delivery based approaches that target macrophage modulation with the goal of accelerating and enhancing bone tissue regeneration.

The Role of Long Non-Coding RNAs and Circular RNAs in Bone Regeneration: Modulating MiRNAs Function

Although bone is a self-healing organ and is able to repair and restore most fractures, large bone fractures, about 10%, are not repairable. Bone grafting, as a gold standard, and bone tissue engineering using biomaterials, growth factors, and stem cells have been developed to restore large bone defects. Since bone regeneration is a complex and multiple-step process and the majority of the human genome, about 98%, is composed of the non-protein-coding regions, non-coding RNAs (ncRNAs) play essential roles in bone regeneration. Recent studies demonstrated that long ncRNAs (lncRNAs) and circular RNAs (circRNAs), as members of ncRNAs, are widely involved in bone regeneration by interaction with microRNAs (miRNAs) and constructing a lncRNA or circRNA/miRNA/mRNA regulatory network. The constructed network regulates the differentiation of stem cells into osteoblasts and their commitment to osteogenesis. This review will present the structure and biogenesis of lncRNAs and circRNAs, the mechanism of bone repair, and the bone tissue engineering in bone defects. Finally, we will discuss the role of lncRNAs and circRNAs in osteogenesis and bone fracture healing through constructing various lncRNA or circRNA/miRNA/mRNA networks and the involved pathways. This article is protected by copyright. All rights reserved.

The Cellular Choreography of Osteoblast Angiotropism in Bone Development and Homeostasis

Interaction between endothelial cells and osteoblasts is essential for bone development and homeostasis. This process is mediated in large part by osteoblast angiotropism, the migration of osteoblasts alongside blood vessels, which is crucial for the homing of osteoblasts to sites of bone formation during embryogenesis and in mature bones during remodeling and repair. Specialized bone endothelial cells that form "type H" capillaries have emerged as key interaction partners of osteoblasts, regulating osteoblast differentiation and maturation and ensuring their migration towards newly forming trabecular bone areas. Recent revolutions in high-resolution imaging methodologies for bone as well as single cell and RNA sequencing technologies have enabled the identification of some of the signaling pathways and molecular interactions that underpin this regulatory relationship. Similarly, the intercellular cross talk between endothelial cells and entombed osteocytes that is essential for bone formation, repair, and maintenance are beginning to be uncovered. This is a relatively new area of research that has, until recently, been hampered by a lack of appropriate analysis tools. Now that these tools are available, greater understanding of the molecular relationships between these key cell types is expected to facilitate identification of new drug targets for diseases of bone formation and remodeling.

Recent Advances in Biomaterials for the Treatment of Bone Defects

Bone defects or fractures generally heal in the absence of major interventions due to the high regenerative capacity of bone tissue. However, in situations of severe/large bone defects, these orchestrated regeneration mechanisms are impaired. With advances in modern medicine, natural and synthetic bio-scaffolds from bioceramics and polymers that support bone growth have emerged and gained intense research interest. In particular, scaffolds that recapitulate the molecular cues of extracellular signals, particularly growth factors, offer potential as therapeutic bone biomaterials. The current challenges for these therapies include the ability to engineer materials that mimic the biological and mechanical properties of the real bone tissue matrix, whilst simultaneously supporting bone vascularization. In this review, we discuss the very recent innovative strategies in bone biomaterial technology, including those of endogenous biomaterials and cell/drug delivery systems that promote bone regeneration. We present our understanding of their current value and efficacy, and the future perspectives for bone regenerative medicine.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Regeneration of Osteochondral Defects Using Developmentally Inspired Cartilaginous Templates

IMPACT STATEMENT

Successfully treating osteochondral defects involves regenerating both the damaged articular cartilage and the underlying subchondral bone, in addition to the complex interface that separates these tissues. In this study, we demonstrate that a cartilage template, engineered using bone marrow-derived mesenchymal stem cells, can enhance the regeneration of such defects and promote the development of a more mechanically functional repair tissue. We also use a computational mechanobiological model to understand how joint-specific environmental factors, specifically oxygen levels and tissue strains, regulate the conversion of the engineered template into cartilage and bone in vivo.

Finite-Element Syntheses of Callus and Bone Remodeling: Biomechanical Study of Fracture Healing in Long Bones

Computational simulations of fracture healing are a valuable tool to improve fracture treatment and implants. Several finite-element models have been established to predict callus formation due to mechanobiological rules. This work provides a comprehensive simulation for virtual implantation through the combination of callus simulation and finite-element structural synthesis (FESS) of (re-)modeling during and after healing based on Pauwel's theory of histogenesis and Wolff's law. The simulation is based on a linear elastic material model and includes generation of fracture hematoma and initial mesenchymal stem cell concentration out of an unspecified solid, cell proliferation, migration, and differentiation due to mechanical stimuli and time-dependent axial loading. Three nondisplaced femoral shaft fractures with initial interfragmentary movement of 0.2, 0.6, and 1 mm and one fracture with 4 mm translation are modeled. The predictions of interfragmentary movement during fracture healing, healing success, and healing time agree with observed clinical outcome, animal models, and other numerical models. Initial interfragmentary movement between 0.2 and 1 mm leads to healing success, with the fastest healing occurring at 0.2 mm. The model of the dislocated fractures shows no further bending after remodeling and is loaded with physiological stress of -13 MPa. Ideal load-time graphs may give insight into the bone's ability to withstand loads as healing time progresses, and thus holds potential for applications in rehabilitation planning. Better knowledge of the forces present during fracture healing is needed to deploy simulations for surgical planning and manufacturing of patient individualized implants. Anat Rec, 301:2112-2121, 2018. © 2018 Wiley Periodicals, Inc.

The cytokines and micro-environment of fracture haematoma: Current evidence

Fracture haematoma formation is the first and foremost important stage of fracture healing. It orchestrates the inflammatory and cellular processes leading to the formation of callus and the restoration of the continuity of the bone. Evidence suggests that blocking this initial stage could lead to an impairment of the overall bone healing process. This review aims to analyse the existing evidence of molecular contributions to bone healing within fracture haematoma and to determine the potential to modify the molecular response to fracture in the haematoma with the aim of improving union times. A comprehensive search of literature documenting fracture haematoma cytokine content was performed. Suitable papers according to prespecified criteria were identified and analysed according to Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines. A total of 89 manuscripts formed the basis of this analysis. Low oxygen tension, high acidity, and high calcium characterised initially the fracture haematoma micro-environment. In addition, a number of cytokines have been measured with concentrations significantly higher than those found in peripheral circulation. Growth factors have also been isolated, with an observed increase in bone morphogenetic proteins, platelet-derived growth factor, and transforming growth factor. Although molecular modification of fracture haematoma has been attempted, more research is required to determine a suitable biological response modifier leading to therapeutic effects. The cytokine content of fracture haematoma gives insight into processes occurring in the initial stages of fracture healing. Manipulation of signalling molecules represents a promising pathway to target future therapies aiming to upregulate the osteogenesis.

HIF-1α represses the expression of the angiogenesis inhibitor thrombospondin-2

Thrombospondin-2 (TSP2) is a potent inhibitor of angiogenesis whose expression is dynamically regulated following injury. In the present study, it is shown that HIF-1α represses TSP2 transcription. Specifically, in vitro studies demonstrate that the prolyl hydroxylase inhibitor DMOG or hypoxia decrease TSP2 expression in fibroblasts. This effect is shown to be via a transcriptional mechanism as hypoxia does not alter TSP2 mRNA stability and this effect requires the TSP2 promoter. In addition, the documented repressive effect of nitric oxide (NO) on TSP2 is shown to be non-canonical and involves stabilization of hypoxia inducible factor-1a (HIF-1α). The regulation of TSP2 by hypoxia is supported by the in vivo observation that TSP2 has spatiotemporal expression distinct from regions of hypoxia in gastrocnemius muscle following murine hindlimb ischemia (HLI). A role for TSP2 regulation by HIF-1α is supported by the dysregulation of TSP2 expression in SM22α-cre HIF-1α KO mice following HLI. Indeed, there is a reduction in blood flow recovery in the SM22a-cre HIF-1α KO mice compared to littermate controls following HLI surgery, associated with impaired recovery and increased TSP2 levels. Moreover, SM22α-cre HIF-1α KO smooth muscle cells mice have increased TSP2 mRNA levels that persist in hypoxia. These findings identify a novel, ischemia-induced pro-angiogenic mechanism involving the transcriptional repression of TSP2 by HIF-1α.

Multiscale Modeling of Bone Healing: Toward a Systems Biology Approach

Bone is a living part of the body that can, in most situations, heal itself after fracture. However, in some situations, healing may fail. Compromised conditions, such as large bone defects, aging, immuno-deficiency, or genetic disorders, might lead to delayed or non-unions. Treatment strategies for those conditions remain a clinical challenge, emphasizing the need to better understand the mechanisms behind endogenous bone regeneration. Bone healing is a complex process that involves the coordination of multiple events at different length and time scales. Computer models have been able to provide great insights into the interactions occurring within and across the different scales (organ, tissue, cellular, intracellular) using different modeling approaches [partial differential equations (PDEs), agent-based models, and finite element techniques]. In this review, we summarize the latest advances in computer models of bone healing with a focus on multiscale approaches and how they have contributed to understand the emergence of tissue formation patterns as a result of processes taking place at the lower length scales.

Stimulating Fracture Healing in Ischemic Environments: Does Oxygen Direct Stem Cell Fate during Fracture Healing?

Bone fractures represent an enormous societal and economic burden as one of the most prevalent causes of disability worldwide. Each year, nearly 15 million people are affected by fractures in the United States alone. Data indicate that the blood supply is critical for fracture healing; as data indicate that concomitant bone and vascular injury are major risk factors for non-union. However, the various role(s) that the vasculature plays remains speculative. Fracture stabilization dictates stem cell fate choices during repair. In stabilized fractures stem cells differentiate directly into osteoblasts and heal the injury by intramembranous ossification. In contrast, in non-stable fractures stem cells differentiate into chondrocytes and the bone heals through endochondral ossification, where a cartilage template transforms into bone as the chondrocytes transform into osteoblasts. One suggested role of the vasculature has been to participate in the stem cell fate decisions due to delivery of oxygen. In stable fractures, the blood vessels are thought to remain intact and promote osteogenesis, while in non-stable fractures, continual disruption of the vasculature creates hypoxia that favors formation of cartilage, which is avascular. However, recent data suggests that non-stable fractures are more vascularized than stable fractures, that oxygen does not appear associated with differentiation of stem cells into chondrocytes and osteoblasts, that cartilage is not hypoxic, and that oxygen, not sustained hypoxia, is required for angiogenesis. These unexpected results, which contrast other published studies, are indicative of the need to better understand the complex, spatio-temporal regulation of vascularization and oxygenation in fracture healing. This work has also revealed that oxygen, along with the promotion of angiogenesis, may be novel adjuvants that can stimulate healing in select patient populations.

Recombinant human bone morphogenic protein-2 Induces the Differentiation and Mineralization of Osteoblastic Cells Under Hypoxic Conditions via Activation of Protein Kinase D and p38 Mitogen-Activated Protein Kinase Signaling Pathways

Hypoxia suppresses osteoblastic differentiation and the bone-forming capacity. As the leading osteoinductive growth factor used clinically in bone-related regenerative medicine, recombinant human bone morphogenic protein-2 (rhBMP-2) has yielded promising results in unfavorable hypoxic clinical situations. Although many studies have examined the effects of rhBMP-2 on osteoblastic differentiation, mineralization and the related signaling pathways, those of rhBMP-2 on osteoblastic cells remain unknown, particularly under hypoxic conditions. Therefore, this study was conducted under a 1% oxygen tension to examine the differentiating effects of rhBMP-2 on osteoblastic cells under hypoxia. rhBMP-2 could also induce the differentiation and mineralization of Osteoblastic (MC3T3-E1) cells under 1% hypoxic conditions. rhBMP-2 could also induce the differentiation and mineralization of MC3T3-E1 cells under 1% hypoxic conditions. rhBMP-2 increased the alkaline phosphatase {ALP} activity in a time dependent manner, and expression of ALP, collagen type-1 (Col-1) and osteocalcin (OC) mRNA were up-regulated significantly in a time- and concentration-dependent manner. In addition, the area of the mineralized nodules increased gradually in a concentration-dependent manner. Western blot analysis, which was performed to identify the signaling pathways underlying rhBMP-2-induced osteoblastic differentiation under hypoxic conditions, showed that rhBMP-2 significantly promoted the phosphorylation of the p38 mitogen-activated protein kinase (MAPK) in a time-dependent manner. A pretreatment with SB203580, a p38 MAPK inhibitor, inhibited the rhBMP-2-mediated differentiation and mineralization. Moreover, the phosphorylation of p38 induced by rhBMP-2 was inhibited in response to a pretreatment of the cells with Go6976, a protein kinase D {PKD) inhibitor. These findings suggest that rhBMP-2 induces the differentiation and mineralization of MC3T3-E1 cells under hypoxic conditions via activation of the PKD and p38 MAPK signaling pathways.

Mechanisms of bone response to injury

Bone, despite its relatively inert appearance, is a tissue that is capable of adapting to its environment. Wolff’s law, first described in the 19th century, describes the ability of bone to change structure depending on the mechanical forces applied to it. The mechanostat model extended this principle and suggested that the amount of strain a bone detects depends on bone strength and the amount of muscle force applied to the bone. Experimental studies have found that low-magnitude, high-frequency mechanical loading is considered to be the most effective at increasing bone formation. The osteocyte is considered to be the master regulator of the bone response to mechanical loading. Deformation of bone matrix by mechanical loading is thought to result in interstitial fluid flow within the lacunar–canalicular system, which may activate osteocyte mechanosensors, leading to changes in osteocyte gene expression and ultimately increased bone formation and decreased bone resorption. However, repetitive strain applied to bone can result in microcracks, which may propagate and coalesce, and if not repaired predispose to catastrophic fracture. Osteocytes are a key component in this process, whereby apoptotic osteocytes in an area of microdamage promote targeted remodeling of the damaged bone. If fractures do occur, fracture repair can be divided into 2 types: primary and secondary healing. Secondary fracture repair is the most common and is a multistage process consisting of hematoma formation and acute inflammation, callus formation, and finally remodeling, whereby bone may return to its original form.

In silico bone mechanobiology: modeling a multifaceted biological system

Mechanobiology, the study of the influence of mechanical loads on biological processes through signaling to cells, is fundamental to the inherent ability of bone tissue to adapt its structure in response to mechanical stimulation. The immense contribution of computational modeling to the nascent field of bone mechanobiology is indisputable, having aided in the interpretation of experimental findings and identified new avenues of inquiry. Indeed, advances in computational modeling have spurred the development of this field, shedding new light on problems ranging from the mechanical response to loading by individual cells to tissue differentiation during events such as fracture healing. To date, in silico bone mechanobiology has generally taken a reductive approach in attempting to answer discrete biological research questions, with research in the field broadly separated into two streams: (1) mechanoregulation algorithms for predicting mechanobiological changes to bone tissue and (2) models investigating cell mechanobiology. Future models will likely take advantage of advances in computational power and techniques, allowing multiscale and multiphysics modeling to tie the many separate but related biological responses to loading together as part of a larger systems biology approach to shed further light on bone mechanobiology. Finally, although the ever‐increasing complexity of computational mechanobiology models will inevitably move the field toward patient‐specific models in the clinic, the determination of the context in which they can be used safely for clinical purpose will still require an extensive combination of computational and experimental techniques applied to in vitro and in vivo applications. WIREs Syst Biol Med 2016, 8:485–505. doi: 10.1002/wsbm.1356

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Role of oxygen as a regulator of stem cell fate during the spontaneous repair of osteochondral defects

The complexity of the in vivo environment makes it is difficult to isolate the effects of specific cues on regulating cell fate during regenerative events such as osteochondral defect repair. The objective of this study was to develop a computational model to explore how joint specific environmental factors regulate mesenchymal stem cell (MSC) fate during osteochondral defect repair. To this end, the spontaneous repair process within an osteochondral defect was simulated using a tissue differentiation algorithm which assumed that MSC fate was regulated by local oxygen levels and substrate stiffness. The developed model was able to predict the main stages of tissue formation observed by a number of in vivo studies. Following this, a parametric study was conducted to better understand why interventions that modulate angiogenesis dramatically impact the outcome of osteochondral defect healing. In the simulations where angiogenesis was reduced, by week 12, the subchondral plate was predicted to remain below the native tidemark, although the chondral region was composed entirely of cartilage and fibrous tissue. In the simulations where angiogenesis was increased, more robust cell proliferation and cartilage formation were observed during the first 4 weeks, however, by week 12 the subchondral plate had advanced above the native tidemark although any remaining tissue was either hypertrophic cartilage or fibrous tissue. These results suggest that osteochondral defect repair could be enhanced by interventions where angiogenesis is promoted but confined to within the subchondral region of the defect. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1026-1036, 2016.

Mineral particles modulate osteo-chondrogenic differentiation of embryonic stem cell aggregates

Pluripotent stem cell aggregates offer an attractive approach to emulate embryonic morphogenesis and skeletal development. Calcium phosphate (CaP) based biomaterials have been shown to promote bone healing due to their osteoconductive and potential osteoinductive properties. In this study, we hypothesized that incorporation of CaP-coated hydroxyapatite mineral particles (MPs) within murine embryonic stem cell (ESC) aggregates could promote osteo-chondrogenic differentiation. Our results demonstrated that MP alone dose-dependently promoted the gene expression of chondrogenic and early osteogenic markers. In combination with soluble osteoinductive cues, MPs enhanced the hypertrophic and osteogenic phenotype, and mineralization of ESC aggregates. Additionally, MPs dose-dependently reduced ESC pluripotency and thereby decreased the size of teratomas derived from MP-incorporated ESC aggregates in vivo. Our data suggested a novel yet simple means of using mineral particles to control stem cell fate and create an osteochondral niche for skeletal tissue engineering applications.

STATEMENT OF SIGNIFICANCE

Directing stem cell differentiation and morphogenesis via biomaterials represents a novel strategy to promote cell fates and tissue formation. Our study demonstrates the ability of calcium phosphate-based mineral particles to promote osteochondrogenic differentiation of embryonic stem cell aggregates as well as modulate teratoma formation in vivo. This hybrid biomaterial-ESC aggregate approach serves as an enabling platform to evaluate the ability of biomaterials to regulate stem cell fate and regenerate functional skeletal tissues for clinical applications.

Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

Uncovering the mechanisms of the sensitivity of bone healing to mechanical factors is critical for understanding the basic biology and mechanobiology of the skeleton, as well as for enhancing clinical treatment of bone injuries. This study refined an experimental method of measuring the strain microenvironment at the site of a bone injury during bone healing. This method used a rat model in which a well-controlled bending motion was applied to an osteotomy to induce the formation of pseudarthrosis that is composed of a range of skeletal tissues, including woven bone, cartilage, fibrocartilage, fibrous tissue, and clot tissue. The goal of this study was to identify both the features of the strain microenvironment associated with formation of these different tissues and the expression of proteins frequently implicated in sensing and transducing mechanical cues. By pairing the strain measurements with histological analyses that identified the regions in which each tissue type formed, we found that formation of the different tissue types occurs in distinct strain microenvironments and that the type of tissue formed is correlated most strongly to the local magnitudes of extensional and shear strains. Weaker correlations were found for dilatation. Immunohistochemical analyses of focal adhesion kinase and rho family proteins RhoA and CDC42 revealed differences within the cartilaginous tissues in the calluses from the pseudarthrosis model as compared to fracture calluses undergoing normal endochondral bone repair. These findings suggest the involvement of these proteins in the way by which mechanical stimuli modulate the process of cartilage formation during bone healing.

Extracellular signaling molecules to promote fracture healing and bone regeneration

To date, the delivery of signaling molecules for bone regeneration has focused primarily on factors that directly affect the bone formation pathways (osteoinduction) or that serve to increase the number of bone forming progenitor cells. The first commercialized growth factors approved for bone regeneration, Bone Morphogenetic Protein 2 and 7 (BMP2 and BMP7), are direct inducers of osteoblast differentiation. As well, newer generations of potential therapeutics that target the Wnt signaling pathway are also direct osteoinducers. On the other hand, some signaling molecules may play a role as mitogens and serve to increase the number of bone producing cells or may increase vascularization. This is true for factors such as Platelet Derived Growth Factor (PDGF) or Fibroblast Growth Factor (FGF). Vascular Endothelial Growth Factor (VEGF) likely has a special role. Not only does it induce new blood vessel formation, it also has direct effects on osteoblasts through endothelial cell-based BMP production. In addition to these pathways that classically have targeted bone production, there are also opportunities to target other aspects of the bone healing process such as inflammation, vascularization, and cell ingress to the fracture site. Bone regeneration is highly complex with defined, yet overlapping stages of healing. We will review established and novel extracellular signaling factors associated with various stages of fracture healing that could be targeted to promote enhanced bone regeneration. Importantly, multiple potential cell and tissues could be targeted to enhance healing in addition to focusing solely on osteoinductive therapeutics.

Mechanical regulation of mesenchymal stem cell differentiation

Biophysical cues play a key role in directing the lineage commitment of mesenchymal stem cells or multipotent stromal cells (MSCs), but the mechanotransductive mechanisms at play are still not fully understood. This review article first describes the roles of both substrate mechanics (e.g. stiffness and topography) and extrinsic mechanical cues (e.g. fluid flow, compression, hydrostatic pressure, tension) on the differentiation of MSCs. A specific focus is placed on the role of such factors in regulating the osteogenic, chondrogenic, myogenic and adipogenic differentiation of MSCs. Next, the article focuses on the cellular components, specifically integrins, ion channels, focal adhesions and the cytoskeleton, hypothesized to be involved in MSC mechanotransduction. This review aims to illustrate the strides that have been made in elucidating how MSCs sense and respond to their mechanical environment, and also to identify areas where further research is needed.

The convergence of fracture repair and stem cells: interplay of genes, aging, environmental factors and disease

The complexity of fracture repair makes it an ideal process for studying the interplay between the molecular, cellular, tissue, and organ level events involved in tissue regeneration. Additionally, as fracture repair recapitulates many of the processes that occur during embryonic development, investigations of fracture repair provide insights regarding skeletal embryogenesis. Specifically, inflammation, signaling, gene expression, cellular proliferation and differentiation, osteogenesis, chondrogenesis, angiogenesis, and remodeling represent the complex array of interdependent biological events that occur during fracture repair. Here we review studies of bone regeneration in genetically modified mouse models, during aging, following environmental exposure, and in the setting of disease that provide insights regarding the role of multipotent cells and their regulation during fracture repair. Complementary animal models and ongoing scientific discoveries define an increasing number of molecular and cellular targets to reduce the morbidity and complications associated with fracture repair. Last, some new and exciting areas of stem cell research such as the contribution of mitochondria function, limb regeneration signaling, and microRNA (miRNA) posttranscriptional regulation are all likely to further contribute to our understanding of fracture repair as an active branch of regenerative medicine.

Melatonin promotes osteoblast differentiation and mineralization of MC3T3-E1 cells under hypoxic conditions through activation of PKD/p38 pathways

Osteoblastic differentiation and bone-forming capacity are known to be suppressed under hypoxic conditions. Melatonin has been shown to influence cell differentiation. A number of in vitro and in vivo studies have suggested that melatonin also has an anabolic effect on bone, by promoting osteoblastic differentiation. However, the precise mechanisms and the signaling pathways involved in this process, particularly under hypoxic conditions, are unknown. This study investigated whether melatonin could promote osteoblastic differentiation and mineralization of preosteoblastic MC3T3-E1 cells under hypoxic conditions. Additionally, we examined the molecular signaling pathways by which melatonin mediates this process. We found that melatonin is capable of promoting differentiation and mineralization of MC3T3-E1 cells cultured under hypoxic conditions. Melatonin upregulated ALP activity and mRNA levels of Alp, Osx, Col1, and Ocn in a time- and concentration-dependent manner. Alizarin red S staining showed that the mineralized matrix in hypoxic MC3T3-E1 cells formed in a manner that was dependent on melatonin concentration. Moreover, melatonin stimulated phosphorylation of p38 Mapk and Prkd1 in these MC3T3-E1 cells. We concluded that melatonin promotes osteoblastic differentiation of MC3T3-E1 cells under hypoxic conditions via the p38 Mapk and Prkd1 signaling pathways.

Biological perspectives of delayed fracture healing

Fracture healing is a complex biological process that requires interaction among a series of different cell types. Maintaining the appropriate temporal progression and spatial pattern is essential to achieve robust healing. We can temporally assess the biological phases via gene expression, protein analysis, histologically, or non-invasively using biomarkers as well as imaging techniques. However, determining what leads to normal versus abnormal healing is more challenging. Since the ultimate outcome of fracture healing is to restore the original functions of bone, assessment of fracture healing should include not only monitoring the restoration of structure and mechanical function, but also an evaluation of the restoration of normal bone biology. Currently few non-invasive measures of biological factors of healing exist; however, recent studies that have correlated non-invasive measures with fracture healing outcome in humans have shown that serum TGFbeta1 levels appear to be an indicator of healing versus non-healing. In the future, developing additional measures to assess biological healing will improve the reliability and permit us to assess stages of fracture healing. Additionally, new functional imaging technologies could prove useful for better understanding both normal fracture healing and predicting dysfunctional healing in human patients.

Thrombospondin-2 and extracellular matrix assembly

BACKGROUND

Numerous proteins and small leucine-rich proteoglycans (SLRPs) make up the composition of the extracellular matrix (ECM). Assembly of individual fibrillar components in the ECM, such as collagen, elastin, and fibronectin, is understood at the molecular level. In contrast, the incorporation of non-fibrillar components and their functions in the ECM are not fully understood.

SCOPE OF REVIEW

This review will focus on the role of the matricellular protein thrombospondin (TSP) 2 in ECM assembly. Based on findings in TSP2-null mice and in vitro studies, we describe the participation of TSP2 in ECM assembly, cell-ECM interactions, and modulation of the levels of matrix metalloproteinases (MMPs).

MAJOR CONCLUSIONS

Evidence summarized in this review suggests that TSP2 can influence collagen fibrillogenesis without being an integral component of fibrils. Altered ECM assembly and excessive breakdown of ECM can have both positive and negative consequences including increased angiogenesis during tissue repair and compromised cardiac tissue integrity, respectively.

GENERAL SIGNIFICANCE

Proper ECM assembly is critical for maintaining cell functions and providing structural support. Lack of TSP2 is associated with increased angiogenesis, in part, due to altered endothelial cell-ECM interactions. Therefore, minor changes in ECM composition can have profound effects on cell and tissue function. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.