Circulating osteogenic cells: implications for injury, repair, and regeneration

Genetic regulation of injury-induced heterotopic ossification in adult zebrafish

Heterotopic ossification is the inappropriate formation of bone in soft tissues of the body. It can manifest spontaneously in rare genetic conditions or as a response to injury, known as acquired heterotopic ossification. There are several experimental models for studying acquired heterotopic ossification from different sources of damage. However, their tenuous mechanistic relevance to the human condition, invasive and laborious nature and/or lack of amenability to chemical and genetic screens, limit their utility. To address these limitations, we developed a simple zebrafish injury model that manifests heterotopic ossification with high penetrance in response to clinically emulating injuries, as observed in human myositis ossificans traumatica. Using this model, we defined the transcriptional response to trauma, identifying differentially regulated genes. Mutant analyses revealed that an increase in the activity of the potassium channel Kcnk5b potentiates injury response, whereas loss of function of the interleukin 11 receptor paralogue (Il11ra) resulted in a drastically reduced ossification response. Based on these findings, we postulate that enhanced ionic signalling, specifically through Kcnk5b, regulates the intensity of the skeletogenic injury response, which, in part, requires immune response regulated by Il11ra.

Evaluation of the Effectiveness of Bone Grafting in Alveolar Ridge Augmentation Using the Two-Stage Splitting Technique

Stimulation of neoosteogenesis is the main mechanism of osseointegration during installation of dental implants, bone tissue recession, and alveolar process augmentation in adentia. In experiments on miniature pigs, we used the technology of two-stage splitting of the ridge of the alveolar process of the mandible in combination with a xenograft that was placed between the fragments of the split bone plate. The morphology of the reparative process and the distribution of osteogenic differentiation markers in the compact and trabecular bone of the alveolar crest were studied. Signs of reparative osteogenesis were observed in the bone regenerate that had a lamellar structure, formed osteons, and foci of woven tissue. It was found that the xenograft was replaced by newly formed trabecular bone tissue. These sites were characterized by increased expression of osteocalcin and CD44. Augmentation technology through two-stage splitting provides trophic relationship of osteoprogenitor cells and is an effective method of osteogenesis stimulation in the alveolar process.

Pathogenesis of acquired heterotopic ossification: Risk factors, cellular mechanisms, and therapeutic implications

Heterotopic ossification (HO), including hereditary and acquired HO, is the formation of extraskeletal bone in skeletal muscle and surrounding soft tissues. Acquired HO is often caused by range of motion, explosion injury, nerve injury or burns. Severe HO can lead to pain and limited joint activity, affecting functional rehabilitation and quality of life. Increasing evidence shows that inflammatory processes and mesenchymal stem cells (MSCs) can drive HO. However, explicit knowledge about the specific mechanisms that result in HO and related cell precursors is still limited. Moreover, there are no effective methods to prevent or reduce HO formation. In this review, we provide an update of known risk factors and relevant cellular origins for HO. In particular, we focus on the underlying mechanisms of MSCs in acquired HO, which follow the osteogenic program. We also discuss the latest therapeutic value and implications for acquired HO. Our review highlights the current gaps in knowledge regarding the pathogenesis of acquired HO and identifies potential targets for the prevention and treatment of HO.

Bone construction surgery: A case report using recombinant human platelet-derived growth factor-BB

BACKGROUND

The position and condition of bone largely sets the stage for functional and esthetic implant success. In bone construction surgery, creating a sustainable functional matrix is complex, but necessary, to enable long-term mechanotransduction and maintenance of soft tissue support.

METHODS

A patient with a severe peri-implantitis ridge defect in the anterior maxilla underwent bone construction surgery simultaneous with implant removal using a composite bone graft (mineralized freeze dried bone allograft + xenograft) enhanced with recombinant human platelet-derived growth factor (rhPDGF-BB). Space maintenance for bone construct immobility and unimpeded wound healing was ensured via a nonresorbable titanium reinforced polytetrafluoroethylene membrane and an absorbable porcine collagen membrane.

RESULTS

Primary closure was maintained throughout the 6 month healing process at which time implant diagnostics commenced for prosthetically directed implant placement using dynamic navigation and involving soft tissue augmentation. Uncovery was performed 3 months thereafter leading to provisionalization and prosthetic phase completion.

CONCLUSION

This case report highlights a severe maxillary anterior ridge defect secondary to advanced peri-implantitis in a systemically healthy Caucasian male patient. The surgical outcome success, both clinically and radiographically, underscores the complexities of complete regional anatomy rehabilitation after suffering catastrophic and debilitating bone loss from inflammatory peri-implantitis. Further, it demonstrates the importance of incorporating optimized angiogenetic therapeutics to help establish a vascularized functional bone matrix for implant success.

Frailty biomarkers under the perspective of geroscience: A narrative review

Cellular and molecular aging biomarkers might contribute to identify at-risk individuals for frailty before overt clinical manifestations appear. Although studies on the associations of aging biomarkers and frailty exist, no investigation has gathered this information using a structured framework for identifying aging biomarkers; as a result, the evidence on frailty and aging biomarkers is diffuse and incomplete. Therefore, this narrative review aimed to gather information on the associations of the hallmarks of aging and frailty under the perspective of geroscience. The literature on human studies on this topic is sparse and mainly composed of cross-sectional investigations performed in small study samples. The main putative aging biomarkers associated to frailty were: mitochondrial DNA copy number (genomic instability and mitochondrial dysfunction), telomere length (telomere attrition), global DNA methylation (epigenetic alterations), Hsp70 and Hsp72 (loss of proteostasis), IGF-1 and SIRT1 (deregulated nutrient-sensing), GDF-15 (mitochondrial dysfunction, cellular senescence and altered intercellular communication), CD4 + and CD8 + cell percentages (cellular senescence), circulating osteogenic progenitor (COP) cells (stem cell exhaustion), and IL-6, CRP and TNF-alpha (altered intercellular communication). IGF-1, SIRT1, GDF-15, IL-6, CRP and TNF-alpha presented more evidence among these biomarkers, highlighting the importance of inflammation and nutrient sensing on frailty. Further longitudinal studies investigating biomarkers across the hallmarks of aging would provide valuable information on this topic.

Osteosarcopenia: A Narrative Review on Clinical Studies

Osteosarcopenia (OS) is defined by the concurrent presence of osteopenia/osteoporosis and sarcopenia. The pathogenesis and etiology of OS involve genetic, biochemical, mechanical, and lifestyle factors. Moreover, an inadequate nutritional status, such as low intake of protein, vitamin D, and calcium, and a reduction in physical activity are key risk factors for OS. This review aims to increase knowledge about diagnosis, incidence, etiology, and treatment of OS through clinical studies that treat OS as a single disease. Clinical studies show the relationship between OS and the risk of frailty, falls, and fractures and some association with Non-communicable diseases (NCDs) pathologies such as diabetes, obesity, and cardiovascular disease. In some cases, the importance of deepening the related mechanisms is emphasized. Physical exercise with adequate nutrition and nutritional supplementations such as proteins, Vitamin D, or calcium, represent a significant strategy for breaking OS. In addition, pharmacological interventions may confer benefits on muscle and bone health. Both non-pharmacological and pharmacological interventions require additional randomized controlled trials (RCT) in humans to deepen the synergistic effect of exercise, nutritional interventions, and drug compounds in osteosarcopenia.

Association between Circulating Osteoprogenitor Cells and Sarcopenia

BACKGROUND

Circulating osteoprogenitor (COP) cells are a surrogate of the bone marrow mesenchymal stem cells with high levels observed in osteoporosis and the initial stages of fracture healing. Conversely, a low percentage of COP cells (%COP) is strongly associated with frailty and disability. However, it is unknown whether %COP is associated with sarcopenia, a musculoskeletal disease closely related to frailty.

OBJECTIVES

This study sought to determine the associations between %COP and sarcopenia defined using the Sarcopenia Definitions and Outcomes Consortium (SDOC) criteria.

METHODS

Data from a random sample of 73 community-dwelling older persons enrolled in the Nepean Osteoporosis and Frailty study (median age 74 years; 60% female) were analyzed. %COP was quantified by flow cytometry using selective gating of CD45/osteocalcin (OCN) + cells. Sarcopenia was defined using handgrip strength and gait speed with cut points as per the SDOC criteria. Linear regression was used for analysis.

RESULTS

Sarcopenia was identified in 19% of participants, all of whom were frail. After adjusting for age, sex, and interleukin 6, sarcopenic participants had 36% lower %COP (95% confidence interval [CI] -56%, -6%, p = 0.024). Both grip strength and gait speed showed associations with %COP (p = 0.065 and 0.002, respectively); however, after adjusting for age and frailty, only gait speed remained associated with %COP (0.1 m/s increase in gait velocity was associated with a 5% increase in %COP cells (95% CI 0%, 10%, p = 0.052).

CONCLUSIONS

High levels of %COP are associated with better muscle function. Future longitudinal studies are required to elucidate the clinical utility of %COP as a potential biomarker or disease stratifier for sarcopenia.

Uncovering the Bone-Muscle Interaction and Its Implications for the Health and Function of Older Adults (the Wellderly Project): Protocol for a Randomized Controlled Crossover Trial

BACKGROUND

Bone and muscle are closely linked anatomically, biochemically, and metabolically. Acute exercise affects both bone and muscle, implying a crosstalk between the two systems. However, how these two systems communicate is still largely unknown. We will explore the role of undercarboxylated osteocalcin (ucOC) in this crosstalk. ucOC is involved in glucose metabolism and has a potential role in muscle maintenance and metabolism.

OBJECTIVE

The proposed trial will determine if circulating ucOC levels in older adults at baseline and following acute exercise are associated with parameters of muscle function and if the ucOC response to exercise varies between older adults with low muscle quality and those with normal or high muscle quality.

METHODS

A total of 54 men and women aged 60 years or older with no history of diabetes and warfarin and vitamin K use will be recruited. Screening tests will be performed, including those for functional, anthropometric, and clinical presentation. On the basis of muscle quality, a combined equation of lean mass (leg appendicular skeletal muscle mass in kg) and strength (leg press; one-repetition maximum), participants will be stratified into a high or low muscle function group and randomized into the controlled crossover acute intervention. Three visits will be performed approximately 7 days apart, and acute aerobic exercise, acute resistance exercise, and a control session (rest) will be completed in any order. Our primary outcome for this study is the effect of acute exercise on ucOC in older adults with low muscle function and those with high muscle function.

RESULTS

The trial is active and ongoing. Recruitment began in February 2018, and 38 participants have completed the study as of May 26, 2019.

CONCLUSIONS

This study will provide novel insights into bone and muscle crosstalk in older adults, potentially identifying new clinical biomarkers and mechanistic targets for drug treatments for sarcopenia and other related musculoskeletal conditions.

TRIAL REGISTRATION

Australia New Zealand Clinical Trials Registry ACTRN12618001756213; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=375925.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/18777.

Delayed Union of a Diaphyseal Forearm Fracture Associated With Impaired Osteogenic Differentiation of Prospectively Isolated Human Skeletal Stem Cells

Delayed union or nonunion are relatively rare complications after fracture surgery, but when they do occur, they can result in substantial morbidity for the patient. In many cases, the etiology of impaired fracture healing is uncertain and attempts to determine the molecular basis for delayed union and nonunion formation have been limited. Prospectively isolating skeletal stem cells (SSCs) from fracture tissue samples at the time of surgical intervention represent a feasible methodology to determine a patient's biologic risk for compromised fracture healing. This report details a case in which functional in vitro readouts of SSCs derived from human fracture tissue at time of injury predicted a poor fracture healing outcome. This case suggests that it may be feasible to stratify a patient's fracture healing capacity and predict compromised fracture healing by prospectively isolating and analyzing SSCs during the index fracture surgery. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Fibrodysplasia ossificans progressiva: current concepts from bench to bedside

Heterotopic ossification (HO) is a disorder characterised by the formation of ectopic bone in soft tissue. Acquired HO typically occurs in response to trauma and is relatively common, yet its aetiology remains poorly understood. Genetic forms, by contrast, are very rare, but provide insights into the mechanisms of HO pathobiology. Fibrodysplasia ossificans progressiva (FOP) is the most debilitating form of HO. All patients reported to date carry heterozygous gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1). These mutations cause dysregulated bone morphogenetic protein (BMP) signalling, leading to HO at extraskeletal sites including, but not limited to, muscles, ligaments, tendons and fascia. Ever since the identification of the causative gene, developing a cure for FOP has been a focus of investigation, and studies have decoded the pathophysiology at the molecular and cellular levels, and explored novel management strategies. Based on the established role of BMP signalling throughout HO in FOP, therapeutic modalities that target multiple levels of the signalling cascade have been designed, and some drugs have entered clinical trials, holding out hope of a cure. A potential role of other signalling pathways that could influence the dysregulated BMP signalling and present alternative therapeutic targets remains a matter of debate. Here, we review the recent FOP literature, including pathophysiology, clinical aspects, animal models and current management strategies. We also consider how this research can inform our understanding of other types of HO and highlight some of the remaining knowledge gaps.

Summary: Fibrodysplasia ossificans progressiva is a rare disease characterised by progressive heterotopic bone formation. Here, we present a comprehensive summary of the recent literature on this debilitating condition and discuss approaches to solving this clinical puzzle.

The Blood Circulating Rare Cell Population. What is it and What is it Good For?

Blood contains a diverse cell population of low concentration hematopoietic as well as non-hematopoietic cells. The majority of such rare cells may be bone marrow-derived progenitor and stem cells. This paucity of circulating rare cells, in particular in the peripheral circulation, has led many to believe that bone marrow as well as other organ-related cell egress into the circulation is a response to pathological conditions. Little is known about this, though an increasing body of literature can be found suggesting commonness of certain rare cell types in the peripheral blood under physiological conditions. Thus, the isolation and detection of circulating rare cells appears to be merely a technological problem. Knowledge about rare cell types that may circulate the blood stream will help to advance the field of cell-based liquid biopsy by supporting inter-platform comparability, making use of biological correct cutoffs and “mining” new biomarkers and combinations thereof in clinical diagnosis and therapy. Therefore, this review intends to lay ground for a comprehensive analysis of the peripheral blood rare cell population given the necessity to target a broader range of cell types for improved biomarker performance in cell-based liquid biopsy.

Increase in bone metabolic markers and circulating osteoblast-lineage cells after orthognathic surgery

Increased mineralisation rate and bone formation after surgery or fracture is the regional acceleratory phenomenon (RAP), and its systemic impact is the systemic acceleratory phenomenon (SAP). The proportion of circulating osteoblast lineage cells, including osteocalcin-positive (OCN⁺) cells, in the peripheral blood is markedly higher during pubertal growth and in patients with bone fractures. This study aimed to elucidate the dynamic changes in bone metabolic activity after orthognathic surgery by longitudinal prospective observation. Peripheral venous blood samples were collected from patients who had undergone orthognathic surgery, and serum bone metabolic markers and the proportion of OCN⁺ cells were measured. Orthognathic surgery induces systemic dynamic changes in bone metabolic activity by targeting steps in the bone healing process and related proteins, such as surgical stress/inflammation (C-reactive protein), bone resorption (type I collagen C-telopeptide), and bone formation (alkaline phosphatase and bone-specific alkaline phosphatase). During the early post-operative period, the population of OCN⁺ cells significantly increased. Confocal microscopy revealed that OCN proteins were localised in the cytoplasm in Triton X-100-treated OCN⁺ cells. Furthermore, OCN, ALP, and COL1A1 gene expression was detected in OCN⁺ cells, suggesting the contribution of the local maturation of bone marrow-derived OCN⁺ cells at the site of bone healing.

Characterisation of multipotent stem cells from human peripheral blood using an improved protocol

BACKGROUND

A promising approach of bone repair is to use stem cells, such as mesenchymal stem cells (MSCs). Seeking available source of MSCs still remains a great challenge in tissue engineering and cell therapy. Peripheral blood (PB) emerges as an alternative source of MSCs which can be easily acquired with minimal invasiveness. This study was undertaken to evaluate the multipotency of PB-MSCs and effects of human PB-MSCs transplantation on ectopic bone regeneration in nude mice.

METHODS

Human venous blood collected was mixed with heparin and then red blood cells were removed using red blood cell lysis buffer. Cell suspension was cultured in normoxia-culture and hypoxia-culture conditions, respectively. The non-adherent cells were removed by half changing culture media every three days. Cells were selected due to plastic adherence. The adherent cells were then passaged and subjected to multi-differentiation induction assays in vitro and in vivo ectopic bone formation assay.

RESULTS

Characterization assays indicated that cells cultured under hypoxia possessed potent multi-lineage differentiation capacity and expressed Nanog and Lgr5, as well as a series of MSC surface antigens (including CD29, CD90, CD105, and CD73). Additionally, regenerated bone tissues by transplantation of human PB-MSCs in vivo were confirmed by histological examinations of ectopic osteogenesis assay. A purified population of MSCs can be obtained within a short period of time using this protocol with a successful rate of 60%.

CONCLUSION

We reported an effective and reliable method to harvest highly purified MSCs with potent multi-differentiation potential from human peripheral blood. Lgr5 may be a potential biomarker for identification of a subpopulation of PB-MSCs.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

PB-MSCs is an alternative cell source for cell therapy, which may be harvested, culture expanded and PB-MSCs loaded with β-tricalcium phosphate (β-TCP) may be used to promote bone repair.

Lgr5-overexpressing mesenchymal stem cells augment fracture healing through regulation of Wnt/ERK signaling pathways and mitochondrial dynamics

Fracture remains one of the most common traumatic conditions in orthopedic surgery. The use of mesenchymal stem cells (MSCs) to augment fracture repair is promising. Leucine-rich repeat-containing GPCR 5 (Lgr5), a transmembrane protein, has been identified as a novel adult stem cell marker in various organs and tissues. However, the roles of Lgr5 in MSCs are not fully understood. In this study, we investigated cellular functions of Lgr5 in MSCs and its potential implications in treating fracture. Lgr5-overexpressing MSCs (MSCLgr5) were established in murine SV40 promoter-driven luciferase reporter MSC line through virus transfection. Results of real-time quantitative PCR and Western blot analysis confirmed the increased expression of Lgr5 in MSCLgr5. MSCLgr5 exhibited increased osteogenic capacity, which may result from elevated expression of β-catenin and phosphorylated ERK1/2 within the nuclear region of cells. In contrast, inhibition of Lgr5 expression decreased the osteogenic differentiation ability of MSCs, accompanied with increased mitochondrial fragmentation and reduced expression of β-catenin. Local transplantation of MSCLgr5 at fracture sites accelerated fracture healing via enhanced osteogenesis and angiogenesis. MSCLgr5 stimulated the tube formation capacity of HUVECs in a Matrigel coculture system in vitro significantly. Taken together, results suggest that Lgr5 is implicated in the cellular processes of osteogenic differentiation of MSCs through regulation of Wnt and ERK signaling pathways and mitochondrial dynamics in fusion and fission. Inhibition of Lgr5 expression induced increased mitochondrial fragmentation and suppression of osteogenesis. MSCLgr5 exhibited enhanced therapeutic efficacy for fracture healing, which may serve as a superior cell source for bone tissue repair.-Lin, W., Xu, L., Pan, Q., Lin, S., Feng, L., Wang, B., Chen, S., Li, Y., Wang, H., Li, Y., Wang, Y., Lee, W. Y. W., Sun, D., Li, G. Lgr5-overexpressing mesenchymal stem cells augment fracture healing through regulation of Wnt/ERK signaling pathways and mitochondrial dynamics.

Adaptive immune cells in calcific aortic valve disease

Calcific aortic valve disease (CAVD) is highly prevalent and has no pharmaceutical treatment. Surgical replacement of the aortic valve has proved effective in advanced disease but is costly, time limited, and in many cases not optimal for elderly patients. This has driven an increasing interest in noninvasive therapies for patients with CAVD. Adaptive immune cell signaling in the aortic valve has shown potential as a target for such a therapy. Up to 15% of cells in the healthy aortic valve are hematopoietic in origin, and these cells, which include macrophages, T lymphocytes, and B lymphocytes, are increased further in calcified specimens. Additionally, cytokine signaling has been shown to play a causative role in aortic valve calcification both in vitro and in vivo. This review summarizes the physiological presence of hematopoietic cells in the valve, innate and adaptive immune cell infiltration in disease states, and the cytokine signaling pathways that play a significant role in CAVD pathophysiology and may prove to be pharmaceutical targets for this disease in the near future.

TAFA2 Induces Skeletal (Stromal) Stem Cell Migration Through Activation of Rac1-p38 Signaling

Understanding the mechanisms regulating recruitment of human skeletal (stromal or mesenchymal) stem cells (hMSC) to sites of tissue injury is a prerequisite for their successful use in cell replacement therapy. Chemokine‐like protein TAFA2 is a recently discovered neurokine involved in neuronal cell migration and neurite outgrowth. Here, we demonstrate a possible role for TAFA2 in regulating recruitment of hMSC to bone fracture sites. TAFA2 increased the in vitro trans‐well migration and motility of hMSC in a dose‐dependent fashion and induced significant morphological changes including formation of lamellipodia as revealed by high‐content‐image analysis at single‐cell level. Mechanistic studies revealed that TAFA2 enhanced hMSC migration through activation of the Rac1‐p38 pathway. In addition, TAFA2 enhanced hMSC proliferation, whereas differentiation of hMSC toward osteoblast and adipocyte lineages was not altered. in vivo studies demonstrated transient upregulation of TAFA2 gene expression during the inflammatory phase of fracture healing in a closed femoral fracture model in mice, and a similar pattern was observed in serum levels of TAFA2 in patients after hip fracture. Finally, interleukin‐1β was found as an upstream regulator of TAFA2 expression. Our findings demonstrate that TAFA2 enhances hMSC migration and recruitment and thus is relevant for regenerative medicine applications. Stem Cells2019;37:407–416

Circulating osteogenic precursor cells: Building bone from blood

Circulating osteogenic precursor (COP) cells constitute a recently discovered population of circulating progenitor cells with the capacity to form not only bone but other mesenchymal tissues. There is a small, but growing body of literature exploring these cells, but with a great deal of disagreement and contradiction within it. This review explores the origins and biological characterization of these cells, including the identification strategies used to isolate these cells from the peripheral blood. It also examines the available knowledge on the in vitro and in vivo behaviour of these cells, in the areas of plastic adherence, differentiation capacity, proliferation, and cellular homing. We also review the implications for future use of COP cells in clinical practice, particularly in the area of regenerative medicine and the treatment and assessment of musculoskeletal disease.

•

Circulating Osteogenic Precursors are circulating cells with characteristics of bone marrow mesenchymal stem cells.

•

They are able to differentiate into bone, fat, cartilage and muscle, but many other characteristics remain unknown.

•

They are heterogenous, with at least two specific populations present, with displaying different markers and behaviors.

Characterizing the Circulating Cell Populations in Traumatic Heterotopic Ossification

Heterotopic ossification (HO) occurs secondary to trauma, causing pain and functional limitations. Identification of the cells that contribute to HO is critical to the development of therapies. Given that innate immune cells and mesenchymal stem cells are known contributors to HO, we sought to define the contribution of these populations to HO and to identify what, if any, contribution circulating populations have to HO. A shared circulation was obtained using a parabiosis model, established between an enhanced green fluorescent protein-positive/luciferase+ donor and a same-strain nonreporter recipient mouse. The nonreporter mouse received Achilles tendon transection and dorsal burn injury to induce HO formation. Bioluminescence imaging and immunostaining were performed to define the circulatory contribution of immune and mesenchymal cell populations. Histologic analysis showed circulating cells present throughout each stage of the developing HO anlagen. Circulating cells were present at the injury site during the inflammatory phase and proliferative period, with diminished contribution in mature HO. Immunostaining demonstrated that most early circulatory cells were from the innate immune system; only a small population of mesenchymal cells were present in the HO. We demonstrate the time course of the participation of circulatory cells in trauma-induced HO and identify populations of circulating cells present in different stages of HO. These findings further elucidate the relative contribution of local and systemic cell populations to HO.

Granulocyte-colony stimulating factor enhances bone fracture healing

BACKGROUND

Circulating mesenchymal stem cells contribute to bone repair. Their incorporation in fracture callus is correlated to their bioavailability. In addition, Granulocyte-colony stimulating factor induces the release of vascular and mesenchymal progenitors. We hypothesized that this glycoprotein stimulates fracture healing, and analyzed the effects of its administration at low doses on bone healing.

METHODS

27 adult male Sprague-Dawley rats underwent mid-femur osteotomy stabilized by centromedullar pinning. In a post (pre) operative group, rats were subcutaneously injected with 5 μg/kg per day of Granulocyte-colony stimulating factor for 5 days after (before) surgery. In a control group, rats were injected with saline solution for 5 days immediately after surgery. A radiographic consolidation score was calculated. At day 35, femurs were studied histologically and underwent biomechanical tests.

FINDINGS

5 weeks after surgery, mean radiographic scores were significantly higher in the Preop group 7.75 (SD 0.42) and in the Postop group 7.67 (SD 0.52) than in the control group 6.75 (SD 0.69). Biomechanical tests showed femur stiffness to be more than three times higher in both the Preop 109.24 N/mm (SD 51.86) and Postop groups 100.05 N/mm (SD 60.24) than in control 32.01 N/mm (SD 15.78). Mean maximal failure force was twice as high in the Preop group 68.66 N (SD 27.78) as in the control group 34.21 N (SD 11.79). Histological results indicated a later consolidation process in control than in treated groups.

INTERPRETATION

Granulocyte-colony stimulating factor injections strongly stimulated early femur fracture healing, indicating its potential utility in human clinical situations such as programmed osteotomy and fracture.

Circulating osteogentic precursor cells in non-hereditary heterotopic ossification

Non-hereditary heterotopic ossification (NHHO) may occur after musculoskeletal trauma, central nervous system (CNS) injury, or surgery. We previously described circulating osteogenic precursor (COP) cells as a bone marrow-derived type 1 collagen⁺CD45⁺subpopulation of mononuclear adherent cells that are able of producing extraskeletal ossification in a murine in vivo implantation assay. In the current study, we performed a tissue analysis of COP cells in NHHO secondary to defined conditions, including traumatic brain injury, spinal cord injury, cerebrovascular accident, trauma without neurologic injury, and joint arthroplasty. All bone specimens revealed the presence of COP cells at 2-14 cells per high power field. COP cells were localized to early fibroproliferative and neovascular lesions of NHHO with evidence for their circulatory status supported by their presence near blood vessels in examined lesions. This study provides the first systematic evaluation of COP cells as a contributory histopathological finding associated with multiple forms of NHHO. These data support that circulating, hematopoietic-derived cells with osteogenic potential can seed inflammatory sites, such as those subject to soft tissue injury, and due to their migratory nature, may likely be involved in seeding sites distant to CNS injury.

Heterotopic bone induction via BMP signaling: Potential therapeutic targets for fibrodysplasia ossificans progressiva

More than 50years ago, Marshal M. Urist detected "heterotopic bone-inducing activity" in demineralized bone matrix. This unique activity was referred to as "bone morphogenetic protein (BMP)" because it was sensitive to trypsin digestion. Purification of the bone-inducing activity from demineralized bone matrix using a bone-inducing assay in vivo indicated that the original "BMP" consisted of a mixture of new members of the transforming growth factor-β (TGF-β) family. The establishment of new in vitro assay systems that reflect the bone-inducing activity of BMPs in vivo have revealed the functional receptors and downstream effectors of BMPs. Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic bone formation in soft tissues similar to the event induced by the transplantation of BMPs in skeletal muscle. In patients with FOP, genetic mutations have been identified in the ACVR1 gene, which encodes the BMP receptor ALK2. The mutations in ALK2 associated with FOP are hypersensitive to type II receptor kinases. Recently, activin A, a non-osteogenic member of the TGF-β family, was identified as the ligand of the mutant ALK2 in FOP, and various types of signaling inhibitors for mutant ALK2 are currently under development to establish effective treatments for FOP.

Lamin A expression in circulating osteoprogenitors as a potential biomarker for frailty: The Nepean Osteoporosis and Frailty (NOF) Study

Lamin A is a protein of the nuclear lamina. Low levels of lamin A expression are associated with osteosarcopenia in mice. In this study, we hypothesized that low lamin A expression is also associated with frailty in humans. We aimed to develop a non-invasive method to quantify lamin A expression in epithelial and circulating osteoprogenitor (COP) cells, and to determine the relationship between lamin A expression and frailty in older individuals. COP cells and buccal swabs were obtained from 66 subjects (median age 74; 60% female; 26 non-frail, 23 pre-frail, and 17 frail) participating at the Nepean Osteoporosis and Frailty (NOF) Study. We quantified physical performance and disability, and stratified frailty in this population. Lamin A expression in epithelial and COP cells was quantified by flow cytometry. Linear regression models estimated the relationship between lamin A expression in buccal and COP cells, and prevalent disability and frailty. Lamin A expression in buccal cells showed no association with either disability or frailty. Low lamin A expression values in COP cells were associated with frailty. Frail individuals showed 60% lower levels of lamin A compared to non-frail (95% CI -36 to -74%, p<0.001) and 62% lower levels compared to pre-frail (95%CI -40 to -76%, p<0.001). In summary, we have identified lamin A expression in COP cells as a strong indicator of frailty. Further work is needed to understand lamin A expression as a risk stratifier, biomarker, or therapeutic target in frail older persons.

Mesenchymal stem cells in peripheral blood of severely injured patients

PURPOSE

Mesenchymal stem cells (MSCs) are primarily stromal cells present in bone marrow and other tissues that are crucial for tissue regeneration and can be mobilized into peripheral blood after different types of organ damage. However, little is known about MSC appearance in blood in the setting of polytrauma.

METHODS

We conducted a monocentered and longitudinal observational clinical study in 11 polytraumatized patients with an injury severity score (ISS) ≥ 24 to determine the numbers of MSCs in peripheral blood. Blood was collected from healthy volunteers and patients after polytrauma in the emergency room and 4, 12, 24, 48 h, 5 and 10 day later, and cells carrying MSC-surface markers (negative for CD45, positive for CD29, CD73, CD90, CD105, and CD166 in different combinations also employing the more stringent markers STRO1 and MSCA1) were detected and characterized using flow cytometry. Relative numbers of MSC-like cells were correlated with clinical parameters to evaluate if specific injury patterns had an influence on their presence in the blood cell pool.

RESULTS

We were able to detect MSC marker-positive cells in both cohorts; however, the percentage of those cells present in the blood of patients during the first 10 day after injury was mostly similar to healthy volunteers, and significantly lowers starting at 4 h post trauma for one marker combination when compared to controls. Furthermore, the presence of a pelvis fracture was partly correlated with reduced relative numbers of MSC-like cells detectable in blood.

CONCLUSIONS

Polytrauma in humans was associated with partly reduced relative numbers of MSC-like cells detected in peripheral blood in the time course after injury. Further studies need to define if this reduction was due to lower mobilization from the bone marrow or to active migration to the sites of injury.

High density lipoprotein modulates osteocalcin expression in circulating monocytes: a potential protective mechanism for cardiovascular disease in type 1 diabetes

Background

Cardiovascular disease (CVD) is a major cause of mortality in type 1 diabetes (T1D). A pro-calcific drift of circulating monocytes has been linked to vascular calcification and is marked by the surface expression of osteocalcin (OCN). We studied OCN+ monocytes in a unique population with ≥50 years of T1D, the 50-Year Joslin Medalists (J50M).

Methods

CD45 bright/CD14+/OCN+ cells in the circulating mononuclear blood cell fraction were quantified by flow cytometry and reported as percentage of CD45 bright cells. Mechanisms were studied by inducing OCN expression in human monocytes in vitro.

Results

Subjects without history of CVD (n = 16) showed lower levels of OCN+ monocytes than subjects with CVD (n = 14) (13.1 ± 8.4% vs 19.9 ± 6.4%, p = 0.02). OCN+ monocytes level was inversely related to total high density lipoprotein (HDL) cholesterol levels (r = −0.424, p = 0.02), large (r = −0.413, p = 0.02) and intermediate (r = −0.445, p = 0.01) HDL sub-fractions, but not to small HDL. In vitro, incubation with OxLDL significantly increased the number of OCN+ monocytes (p < 0.01). This action of OxLDL was significantly reduced by the addition of HDL in a concentration dependent manner (p < 0.001). Inhibition of the scavenger receptor B1 reduced the effects of both OxLDL and HDL (p < 0.05).

Conclusions

Low OCN+ monocytes levels are associated with lack of CVD in people with long duration T1D. A possible mechanism for the increased OCN+ monocytes could be the elevated levels of oxidized lipids due to diabetes which may be inhibited by HDL. These findings suggest that circulating OCN+ monocytes could be a marker for vascular disease in diabetic patients and possibly modified by HDL elevation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-017-0599-2) contains supplementary material, which is available to authorized users.

Harnessing Endogenous Cellular Mechanisms for Bone Repair

Although autologous tissue transplantation represents a valid approach for bone repair, it has encountered crucial barriers in therapeutic translation, not least the invasive process necessary for stem cell isolation. In recent years, the scientific community has made significant strides for identifying new treatment options, and great emphasis has been placed on the tight interaction between skeletal and immune system in modulating the outcome of bone repair. Within the context of specific injury environmental cues, the cross talk among inflammatory cells and tissue resident and/or circulating progenitor cells is crucial to finely coordinate repair and remodeling processes. The appropriate modulation of the inflammatory response can now be considered a new trend in the field of regenerative medicine, as it raises the attracting possibility to enhance endogenous progenitor cell functions, finally leading to tissue repair. Therefore, new treatment options have been developed considering the wide spectrum of bone–inflammation interplay, considering in particular the cell intrinsic cues responsible for the modulation of the injured environment. In this review, we will provide a panoramic overview focusing on novel findings developed to uphold endogenous bone repair.

Host cell recruitment patterns by bone morphogenetic protein-2 releasing hyaluronic acid hydrogels in a mouse subcutaneous environment

Aim: This study aimed to identify host cell recruitment patterns in a mouse model in response to rhBMP-2 releasing hyaluronic acid hydrogels and influence of added nano-hydroxyapatite particles on rhBMP-2 release and pattern of bone formation. Materials & methods: Implanted gels were retrieved after implantation and cells were enzymatically dissociated for flow cytometric analysis. Percentages of macrophages, progenitor endothelial cells and putative mesenchymal stem cells were measured. Implants were evaluated for BMP-2 release by ELISA and by histology to monitor tissue formation. Results & conclusion: Hyaluronic acid+BMP-2 gels influenced the inflammatory response in the bone healing microenvironment. Host-derived putative mesenchymal stem cells were major contributors. Addition of hydroxyapatite nanoparticles modified the release pattern of rhBMP-2, resulting in enhanced bone formation.

Age, gender, and percentage of circulating osteoprogenitor (COP) cells: The COP Study

Circulating osteoprogenitor (COP) cells are blood-borne cells which express a variety of osteoblastic markers and are able to form bone nodules in vivo. Whereas a high percentage of COP cells (%COP) is associated with vascular calcification, low %COP has been associated with disability and frailty. However, the reference range of %COP in age- and gender-matching populations, and the age-related changes in %COP remain unknown. A cross-sectional study was undertaken in 144 healthy volunteers in Western Sydney (20-90year-old, 10 male and 10 female subjects per decade). %COP was quantified by flow cytometry. A high inter-and intra-rater reliability was found. In average, in this healthy population average of %COP was 0.42. There was no significant difference in %COP among the age groups. Similarly, no significant difference was found in %COP with gender, weight, height or BMI. In addition, we identified a normal reference range of %COP of 0.1-3.8%. In conclusion, in addition to the identification of steady levels of COP cells with age, we also identified a normal reference range of %COP, which could be used in future studies looking at musculoskeletal diseases in older populations.

IGF-1 Receptor Expression on Circulating Osteoblast Progenitor Cells Predicts Tissue-Based Bone Formation Rate and Response to Teriparatide in Premenopausal Women With Idiopathic Osteoporosis

We have previously reported that premenopausal women with idiopathic osteoporosis (IOP) have profound microarchitectural deficiencies and heterogeneous bone remodeling. Those with the lowest bone formation rate have higher baseline serum insulin-like growth factor-1 (IGF-1) levels and less robust response to teriparatide. Because IGF-1 stimulates bone formation and is critical for teriparatide action on osteoblasts, these findings suggest a state of IGF-1 resistance in some IOP women. To further investigate the hypothesis that osteoblast and IGF-1-related mechanisms mediate differential responsiveness to teriparatide in IOP, we studied circulating osteoblast progenitor (COP) cells and their IGF-1 receptor (IGF-1R) expression. In premenopausal women with IOP, peripheral blood mononuclear cells (PBMCs) were obtained at baseline (n = 25) and over 24 months of teriparatide treatment (n = 11). Flow cytometry was used to identify and quantify COPs (non-hematopoetic lineage cells expressing osteocalcin and RUNX2) and to quantify IGF-1R expression levels. At baseline, both the percent of PBMCs that were COPs (%COP) and COP cell-surface IGF-1R expression correlated directly with several histomorphometric indices of bone formation in tetracycline-labeled transiliac biopsies. In treated subjects, both %COP and IGF-1R expression increased promptly after teriparatide, returning toward baseline by 18 months. Although neither baseline %COP nor increase in %COP after 3 months predicted the bone mineral density (BMD) response to teriparatide, the percent increase in IGF-1R expression on COPs at 3 months correlated directly with the BMD response to teriparatide. Additionally, lower IGF-1R expression after teriparatide was associated with higher body fat, suggesting links between teriparatide resistance, body composition, and the GH/IGF-1 axis. In conclusion, these assays may be useful to characterize bone remodeling noninvasively and may serve to predict early response to teriparatide and possibly other bone formation-stimulating medications. These new tools may also have utility in the mechanistic investigation of teriparatide resistance in premenopausal IOP and perhaps in other populations. © 2017 American Society for Bone and Mineral Research.

Flowtaxis of osteoblast migration under fluid shear and the effect of RhoA kinase silencing

Despite the important role of mechanical signals in bone remodeling, relatively little is known about how fluid shear affects osteoblastic cell migration behavior. Here we demonstrated that MC3T3-E1 osteoblast migration could be activated by physiologically-relevant levels of fluid shear in a shear stress-dependent manner. Interestingly, shear-sensitive osteoblast migration behavior was prominent only during the initial period after the onset of the steady flow (for about 30 min), exhibiting shear stress-dependent migration speed, displacement, arrest coefficient, and motility coefficient. For example, cell speed at 1 min was 0.28, 0.47, 0.51, and 0.84 μm min-1 for static, 2, 15, and 25 dyne cm-2 shear stress, respectively. Arrest coefficient (measuring how often cells are paused during migration) assessed for the first 30 min was 0.40, 0.26, 0.24, and 0.12 respectively for static, 2, 15, and 25 dyne cm-2. After this initial period, osteoblasts under steady flow showed decreased migration capacity and diminished shear stress dependency. Molecular interference of RhoA kinase (ROCK), a regulator of cytoskeletal tension signaling, was found to increase the shear-sensitive window beyond the initial period. Cells with ROCK-shRNA had increased migration in the flow direction and continued shear sensitivity, resulting in greater root mean square displacement at the end of 120 min of measurement. It is notable that the transient osteoblast migration behavior was in sharp contrast to mesenchymal stem cells that exhibited sustained shear sensitivity (as we recently reported, J. R. Soc. Interface. 2015; 12:20141351). The study of fluid shear as a driving force for cell migration, i.e., "flowtaxis", and investigation of molecular mechanosensors governing such behavior (e.g., ROCK as tested in this study) may provide new and improved insights into the fundamental understanding of cell migration-based homeostasis.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Abdominal aortic calcification: A reappraisal of epidemiological and pathophysiological data

In men and women, there is a significant association between the risk of cardiovascular event (myocardial infarction, stroke) and risk of major fragility fracture (hip, vertebra). Abdominal aortic calcification (AAC) can be assessed using semiquantitative scores on spine radiographs and spine scans obtained by DXA. Severe AAC is associated with higher risk of major cardiovascular event. Not only does severe AAC reflect poor cardiovascular health status, but also directly disturbs blood flow in the vascular system. Severe (but not mild or moderate) AAC is associated with lower bone mineral density (BMD), faster bone loss and higher risk of major fragility fracture. The fracture risk remains increased after adjustment for BMD and other potential risk factors. The association between severe AAC and fracture risk was found in both sexes, mainly in the follow-ups of less than 10years. Many factors contribute to initiation and progression of AAC: lifestyle, co-morbidities, inorganic ions, dyslipidemia, hormones, cytokines (e.g. inflammatory cytokines, RANKL), matrix vesicles, microRNAs, structural proteins (e.g. elastin), vitamin K-dependent proteins, and medications (e.g. vitamin K antagonists). Osteogenic transdifferentiation of vascular smooth muscle cells (VSMC) and circulating osteoprogenitors penetrating into vascular wall plays a major role in the AAC initiation and progression. Vitamin K-dependent proteins protect vascular tunica media against formation of calcified deposits (matrix GLA protein, GLA-rich protein) and against VSMC apoptosis (Gas6). Further studies are needed to investigate clinical utility of AAC for the assessment of fracture and cardiovascular risk at the individual level and develop new medications permitting to prevent AAC progression.

Ability of circulating human hematopoietic lineage negative cells to support hematopoiesis

Hematopoietic stem cell (HSC) self-renewal is regulated by osteoblast and/or endothelial cells within the hematopoietic niche. However, the true identity of the supporting cells and the nature of the secreted factors remain uncertain. We developed a novel mouse model and analyzed whether circulating human peripheral hematopoietic lineage negative/AP+ (lin-/AP+) cells support hematopoiesis in vivo. Thus, immunocompromised (Rag) mice expressing thymidine kinase (Tk) under the control of the 3.6Col1α1 promoter (Tk-Rag) were treated with ganciclovir, resulting in osteoblast progenitor cell ablation and subsequent loss of hematopoiesis (evaluated by measuring mouse Ter119+ erythroid cells). Following hematopoietic cell depletion, human bone marrow-derived marrow stromal cells (MSCs) or lin-/AP+ cells were infused into Tk-Rag mice and compared with saline infusions. Ganciclovir significantly reduced (7.4-fold) Ter119+ cells in the bone marrow of Tk-Rag mice compared to saline injections. Infusion of either MSCs or lin-/AP+ cells into ganciclovir-treated mice resulted in a 3.3-fold and 2.7-fold increase (P < 0.01), respectively, in Ter119+ cells compared to mice receiving saline. Relative to lin-/AP- cells, lin-/AP+ cells expressed high levels of mesenchymal, endothelial, and hematopoiesis supporting genes. Thus, human peripheral blood lin-/AP+ cells represent a novel cell type capable of supporting hematopoiesis in a manner comparable to MSCs.

Stromal cell-derived factor-1 mediates changes of bone marrow stem cells during the bone repair process

Osteoblasts, osteoclasts, chondrocytes, and macrophages that participate in the bone repair process are derived from hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). However, the roles of these stem cells during the repair of injured bone tissue are still unclear. In the present study, we examined the effects of bone defect on HSCs and MSCs in bone marrow and spleen in 75 mice and its mechanism. We analyzed the HSC and MSC populations in these tissues of a mouse with femoral bone damage by using flow cytometry. The number of HSCs in the bone marrow of mice with damaged femurs was significantly lower than the number of these cells in the bone marrow of the contralateral intact femurs on day 2 after injury. Meanwhile, the number of MSCs in the bone marrow of mice with damaged femurs was significantly higher than that of the contralateral femurs. Both intraperitoneal administration of AMD3100, a C-X-C chemokine receptor 4 (CXCR4) antagonist, and local treatment with an anti-stromal cell-derived factor-1 (SDF-1) antibody blunted the observed decrease in HSC and increase in MSC populations within the bone marrow of injured femurs. In conclusion, the present study revealed that there is a concurrent decrease and increase in the numbers of HSCs and MSCs, respectively, in the bone marrow during repair of mouse femoral bone damage. Furthermore, the SDF-1/CXCR4 system was implicated as contributing to the changes in these stem cell populations upon bone injury.

Skeletal stem cells and their contribution to skeletal fragility: senescence and rejuvenation

Age-related osteoporotic fractures are major health care problem worldwide and are the result of impaired bone formation, decreased bone mass and bone fragility. Bone formation is accomplished by skeletal stem cells (SSC) that are recruited to bone surfaces from bone marrow microenvironment. This review discusses targeting SSC to enhance bone formation and to abolish age-related bone fragility in the context of using stem cells for treatment of age-related disorders. Recent studies are presented that have demonstrated that SSC exhibit impaired functions during aging due to intrinsic senescence-related changes as well as the presence of senescent microenvironment. Also, a number of approaches aiming at increasing bone formation through targeting SSC and that include systemic SSC transplantation, systemic SSC targeting using aptamers or antibodies, use of therapeutic screteome and tissue engineering approaches will be presented and discussed.

Clopidogrel Enhances Mesenchymal Stem Cell Proliferation Following Periodontitis

Bone formation is dependent on the differentiation of osteoblasts from mesenchymal stem cells (MSCs). In addition to serving as progenitors, MSCs reduce inflammation and produce factors that stimulate tissue formation. Upon injury, MSCs migrate to the periodontium, where they contribute to regeneration. We examined the effect of clopidogrel and aspirin on MSCs following induction of periodontitis in rats by placement of ligatures. We showed that after the removal of ligatures, which induces resolution of periodontal inflammation, clopidogrel had a significant effect on reducing the inflammatory infiltrate. It also increased the number of osteoblasts and MSCs. Mechanistically, the latter was linked to increased proliferation of MSCs in vivo and in vitro. When given prior to inducing periodontitis, clopidogrel had little effect on MSC or osteoblasts numbers. Applying aspirin before or after induction of periodontitis did not have a significant effect on the parameters measured. These results suggest that clopidogrel may have a positive effect on MSCs in conditions where a reparative process has been initiated.

Bioactive lipid coating of bone allografts directs engraftment and fate determination of bone marrow-derived cells in rat GFP chimeras

Bone grafting procedures are performed to treat wounds incurred during wartime trauma, accidents, and tumor resections. Endogenous mechanisms of repair are often insufficient to ensure integration between host and donor bone and subsequent restoration of function. We investigated the role that bone marrow-derived cells play in bone regeneration and sought to increase their contributions by functionalizing bone allografts with bioactive lipid coatings. Polymer-coated allografts were used to locally deliver the immunomodulatory small molecule FTY720 in tibial defects created in rat bone marrow chimeras containing genetically-labeled bone marrow for monitoring cell origin and fate. Donor bone marrow contributed significantly to both myeloid and osteogenic cells in remodeling tissue surrounding allografts. FTY720 coatings altered the phenotype of immune cells two weeks post-injury, which was associated with increased vascularization and bone formation surrounding allografts. Consequently, degradable polymer coating strategies that deliver small molecule growth factors such as FTY720 represent a novel therapeutic strategy for harnessing endogenous bone marrow-derived progenitors and enhancing healing in load-bearing bone defects.

Cartilage repair in vivo: the role of migratory progenitor cells

The most common diseases of the joints and its tissues are osteoarthritis and rheumatoid arthritis, with osteoarthritis being anticipated to be the fourth leading cause of disability by the year 2020. To date, no truly causal therapies are available, and this has promoted tissue engineering attempts mainly involving mesenchymal stem cells. The goal of all tissue repairs would be to restore a fully functional tissue, here a hyaline articular cartilage. The hyaline cartilage is the most affected in osteoarthritis, where altered cell–matrix interactions gradually destroy tissue integrity. In rheumatoid arthritis, the inflammatory aspect is more important, and the cartilage tissue is destroyed by the invasion of tumor-like pannus tissue arising from the inflamed synovia. Furthermore, the fibrocartilage of the meniscus is clearly involved in the initiation of osteoarthritis, especially after trauma. Recent investigations have highlighted the role of migratory progenitor cells found in diseased tissues in situ. In osteoarthritis and rheumatoid arthritis, these chondrogenic progenitor cells are involved in regeneration efforts that are largely unsuccessful in diseased cartilage tissue. However, these progenitor cells are interesting targets for a cell-based regenerative therapy for joint diseases.

Arterial calcification: Finger-pointing at resident and circulating stem cells

The term ‘‘Stammzelle’’ (stem cells) originally appeared in 1868 in the works of Ernst Haeckel who used it to describe the ancestor unicellular organism from which he presumed all multicellular organisms evolved. Since then stem cells have been studied in a wide spectrum of normal and pathological conditions; it is remarkable to note that ectopic arterial calcification was considered a passive deposit of calcium since its original discovering in 1877; in the last decades, resident and circulating stem cells were imaged to drive arterial calcification through chondro-osteogenic differentiation thus opening the idea that an active mechanism could be at the basis of the process that clinically shows a Janus effect: calcifications either lead to the stabilization or rupture of the atherosclerotic plaques. A review of the literature underlines that 130 years after stem cell discovery, antigenic markers of stem cells are still debated and the identification of the osteoprogenitor phenotype is even more elusive due to tissue degradation occurring at processing and manipulation. It is necessary to find a consensus to perform comparable studies that implies phenotypic recognition of stem cells antigens. A hypothesis is based on the singular morphology and amitotic mechanism of division of osteoclasts: it constitutes the opening to a new approach on osteoprogenitors markers and recognition. Our aim was to highlight all the present evidences of the active calcification process, summarize the different cellular types involved, and discuss a novel approach to discover osteoprogenitor phenotypes in arterial wall.

In-vivo imaging of the fracture healing in medaka revealed two types of osteoclasts before and after the callus formation by osteoblasts

The fracture healing research, which has been performed in mammalian models not only for clinical application but also for bone metabolism, revealed that generally osteoblasts are induced to enter the fracture site before the induction of osteoclasts for bone remodeling. However, it remains unknown how and where osteoclasts and osteoblasts are induced, because it is difficult to observe osteoclasts and osteoblasts in a living animal. To answer these questions, we developed a new fracture healing model by using medaka. We fractured one side of lepidotrichia in a caudal fin ray without injuring the other soft tissues including blood vessels. Using the transgenic medaka in which osteoclasts and osteoblasts were visualized by GFP and DsRed, respectively, we found that two different types of functional osteoclasts were induced before and after osteoblast callus formation. The early-induced osteoclasts resorbed the bone fragments and the late-induced osteoclasts remodeled the callus. Both types of osteoclasts were induced near the surface on the blood vessels, while osteoblasts migrated from adjacent fin ray. Transmission electron microscopy revealed that no significant ruffled border and clear zone were observed in early-induced osteoclasts, whereas the late-induced osteoclasts had clear zones but did not have the typical ruffled border. In the remodeling of the callus, the expression of cox2 mRNA was up-regulated at the fracture site around vessels, and the inhibition of Cox2 impaired the induction of the late-induced osteoclasts, resulting in abnormal fracture healing. Finally, our developed medaka fracture healing model brings a new insight into the molecular mechanism for controlling cellular behaviors during the fracture healing.

In vivo hypobaric hypoxia performed during the remodeling process accelerates bone healing in mice

We investigated the effects of respiratory hypobaric hypoxia on femoral bone-defect repair in mice because hypoxia is believed to influence both mesenchymal stromal cell (MSC) and hematopoietic stem cell mobilization, a process involved in the bone-healing mechanism. To mimic conditions of non-weight-bearing limb immobilization in patients suffering from bone trauma, our hypoxic mouse model was further subjected to hind-limb unloading. A hole was drilled in the right femur of adult male C57/BL6J mice. Four days after surgery, mice were subjected to hind-limb unloading for 1 week. Seven days after surgery, mice were either housed for 4 days in a hypobaric room (FiO2 at 10%) or kept under normoxic conditions. Unsuspended control mice were housed in either hypobaric or normoxic conditions. Animals were sacrificed on postsurgery day 11 to allow for collection of both contralateral and lesioned femurs, blood, and spleen. As assessed by microtomography, delayed hypoxia enhanced bone-healing efficiency by increasing the closing of the cortical defect and the newly synthesized bone volume in the cavity by +55% and +35%, respectively. Proteome analysis and histomorphometric data suggested that bone-repair improvement likely results from the acceleration of the natural bone-healing process rather than from extended mobilization of MSC-derived osteoprogenitors. Hind-limb unloading had hardly any effect beyond delayed hypoxia-enhanced bone-healing efficiency.

Phospho-NHE3 forms membrane patches and interacts with beta-actin to sense and maintain constant direction during cell migration

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (Vmem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only Vmem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and Vmem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.

Local transplantation is an effective method for cell delivery in the osteogenesis imperfecta murine model

PURPOSE

Osteogenesis imperfecta is a serious genetic disorder that results from improper type I collagen production. We aimed to evaluate whether bone marrow stromal cells (BMSC) delivered locally into femurs were able to engraft, differentiate into osteoblasts, and contribute to formation of normal bone matrix in the osteogenesis imperfect murine (oim) model.

METHODS

Donor BMSCs from bone-specific reporter mice (Col2.3GFP) were expanded in vitro and transplanted into the femoral intramedullary cavity of oim mice. Engraftment was evaluated after four weeks.

RESULTS

We detected differentiation of donor BMSCs into Col2.3GFP+ osteoblasts and osteocytes in cortical and trabecular bone of transplanted oim femurs. New bone formation was detected by deposition of dynamic label in the proximity to the Col2.3GFP+ osteoblasts, and new bone showed more organized collagen structure and expression of type I α2 collagen. Col2.3GFP cells were not found in the contralateral femur indicating that transplanted osteogenic cells did not disseminate by circulation. No osteogenic engraftment was observed following intravenous transplantation of BMSCs. BMSC cultures derived from transplanted femurs showed numerous Col2.3GFP+ colonies, indicating the presence of donor progenitor cells. Secondary transplantation of cells recovered from recipient femurs and expanded in vitro also showed Col2.3GFP+ osteoblasts and osteocytes confirming the persistence of donor stem/progenitor cells.

CONCLUSION

We show that BMSCs delivered locally in oim femurs are able to engraft, differentiate into osteoblasts and osteocytes and maintain their progenitor potential in vivo. This suggests that local delivery is a promising approach for introduction of autologous MSC in which mutations have been corrected.

The alliance of mesenchymal stem cells, bone, and diabetes

Bone fragility has emerged as a new complication of diabetes. Several mechanisms in diabetes may influence bone homeostasis by impairing the action between osteoblasts, osteoclasts, and osteocytes and/or changing the structural properties of the bone tissue. Some of these mechanisms can potentially alter the fate of mesenchymal stem cells, the initial precursor of the osteoblast. In this review, we describe the main factors that impair bone health in diabetic patients and their clinical impact.

Advances in regenerative orthopedics

Orthopedic injuries are common and a source of much misery and economic stress. Several relevant tissues, such as cartilage, meniscus, and intra-articular ligaments, do not heal. And even bone, which normally regenerates spontaneously, can fail to mend. The regeneration of orthopedic tissues requires 4 key components: cells, morphogenetic signals, scaffolds, and an appropriate mechanical environment. Although differentiated cells from the tissue in question can be used, most cellular research focuses on the use of mesenchymal stem cells. These can be retrieved from many different tissues, and one unresolved question is the degree to which the origin of the cells matters. Embryonic and induced pluripotent stem cells are also under investigation. Morphogenetic signals are most frequently supplied by individual recombinant growth factors or native mixtures provided by, for example, platelet-rich plasma; mesenchymal stem cells are also a rich source of trophic factors. Obstacles to the sustained delivery of individual growth factors can be addressed by gene transfer or smart scaffolds, but we still lack detailed, necessary information on which delivery profiles are needed. Scaffolds may be based on natural products, synthetic materials, or devitalized extracellular matrix. Strategies to combine these components to regenerate tissue can follow traditional tissue engineering practices, but these are costly, cumbersome, and not well suited to treating large numbers of individuals. More expeditious approaches make full use of intrinsic biological processes in vivo to avoid the need for ex vivo expansion of autologous cells and multiple procedures. Clinical translation remains a bottleneck.