Regenerative effects of transplanted mesenchymal stem cells in fracture healing

Recent Advances in Tracking the Transplanted Stem Cells Using Near-Infrared Fluorescent Nanoprobes: Turning from the First to the Second Near-Infrared Window

Stem cell-based regenerative medicine has attracted tremendous attention for its great potential to treat numerous incurable diseases. Tracking and understanding the fate and regenerative capabilities of transplanted stem cells is vital for improving the safety and therapeutic efficacy of stem cell-based therapy, therefore accelerating the clinical application of stem cells. Fluorescent nanoparticles (NPs) have been widely used for in vivo tracking of the transplanted stem cells. Among these fluorescent NPs, near-infrared (NIR) NPs have greatly improved the sensitivity, tissue penetration depth, spatial and temporal resolutions of the fluorescence imaging-based stem cell tracking technologies due to the reduced absorption, scattering, and autofluorescence of NIR fluorescence in tissues. Here, this review summarizes the recent studies regarding the tracking of transplanted stem cells using NIR NPs and emphasizes the recent advances of fluorescence imaging in the second NIR window (NIR-II, 1000-1700 nm). Furthermore, the challenges and future prospects of the NIR NP-based technologies are also discussed.

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.

Osteogenesis and aging: lessons from mesenchymal stem cells

Aging is a high risk factor for the development of osteoporosis, a multifactorial age-related progressive disease characterized by reduced bone mass and increased risk of fractures. At the cellular level, the mesenchymal stem cell pool in the bone marrow niche shows a biased differentiation into adipogenesis at the cost of osteogenesis. This differentiation shift leads to decreased bone formation, contributing to the etiology of osteoporosis. This review will focus on the most recent/relevant molecular findings driving this functional impairment of mesenchymal stem cells in the aging process.

Evidence for Lgr6 as a Novel Marker of Osteoblastic Progenitors in Mice

Bone marrow-derived mesenchymal stem cells are an important source of osteoblasts critical for both bone homeostasis and repair. The ability to isolate, or specifically target, mesenchymal stem cells committed to the osteogenic lineage is necessary for orthopedic translational therapy efforts; however the precise molecular signature of these cells remains elusive. Previously, we identified a population of osteoprogenitor cells expressing the Wnt signaling agonist Lgr6, which contributes to the development and regeneration of the mouse digit tip bone. In our present study we build upon this data and investigate the expression of Lgr6 more broadly in the skeleton. We find that Lgr6, and closely related Lgr4, are expressed in mouse primary calvarial cells, bone marrow cells, and bone marrow-derived mesenchymal stem cells. In addition, our data demonstrates that Lgr4 expression is modestly increased throughout the differentiation and mineralization of mesenchymal stem cells. In contrast, we find Lgr6 expression to be strikingly increased upon osteogenic induction and subsequently decreased upon differentiation and mineralization. These findings provide evidence for Lgr6 as a novel marker of osteoprogenitor cells in bone marrow, which could prove useful for isolation of this population toward future research and clinical applications.

CTLA-4 Mediates Inhibitory Function of Mesenchymal Stem/Stromal Cells

Mesenchymal stem/stromal cells (MSCs) are stem cells of the connective tissue, possess a plastic phenotype, and are able to differentiate into various tissues. Besides their role in tissue regeneration, MSCs perform additional functions as a modulator or inhibitor of immune responses. Due to their pleiotropic function, MSCs have also gained therapeutic importance for the treatment of autoimmune diseases and for improving fracture healing and cartilage regeneration. However, the therapeutic/immunomodulatory mode of action of MSCs is largely unknown. Here, we describe that MSCs express the inhibitory receptor CTLA-4 (cytotoxic T lymphocyte antigen 4). We show that depending on the environmental conditions, MSCs express different isoforms of CTLA-4 with the secreted isoform (sCTLA-4) being the most abundant under hypoxic conditions. Furthermore, we demonstrate that the immunosuppressive function of MSCs is mediated mainly by the secretion of CTLA-4. These findings open new ways for treatment when tissue regeneration/fracture healing is difficult.

MicroRNA-218 Promotes Osteogenic Differentiation of Mesenchymal Stem Cells and Accelerates Bone Fracture Healing

As a regulator of osteogenesis, microRNA-218 (miR-218) is reported to promote osteogenesis of mesenchymal stem cells (MSCs). However, the in vivo osteogenic effect of miR-218 remains elusive. In this study, miR-218 was confirmed to promote osteogenic differentiation of MSCs by stimulating the alkaline phosphatase activity, calcium nodule formation, and osteogenic marker gene expression. For in vivo study, the miR-218-overexpressing BMSCs were locally administrated into the fracture sites in a femur fracture mouse model. Based on the X-rays, micro-computed tomography, mechanical testing, histology, and immunohistochemistry examinations, miR-218 overexpression improved new bone formation and accelerated fracture healing. These findings suggest that miR-218 may be a promising therapeutic target for bone repair in future clinical applications.

Advances in Regenerative Medicine and Tissue Engineering: Innovation and Transformation of Medicine

Humans and animals lose tissues and organs due to congenital defects, trauma, and diseases. The human body has a low regenerative potential as opposed to the urodele amphibians commonly referred to as salamanders. Globally, millions of people would benefit immensely if tissues and organs can be replaced on demand. Traditionally, transplantation of intact tissues and organs has been the bedrock to replace damaged and diseased parts of the body. The sole reliance on transplantation has created a waiting list of people requiring donated tissues and organs, and generally, supply cannot meet the demand. The total cost to society in terms of caring for patients with failing organs and debilitating diseases is enormous. Scientists and clinicians, motivated by the need to develop safe and reliable sources of tissues and organs, have been improving therapies and technologies that can regenerate tissues and in some cases create new tissues altogether. Tissue engineering and/or regenerative medicine are fields of life science employing both engineering and biological principles to create new tissues and organs and to promote the regeneration of damaged or diseased tissues and organs. Major advances and innovations are being made in the fields of tissue engineering and regenerative medicine and have a huge impact on three-dimensional bioprinting (3D bioprinting) of tissues and organs. 3D bioprinting holds great promise for artificial tissue and organ bioprinting, thereby revolutionizing the field of regenerative medicine. This review discusses how recent advances in the field of regenerative medicine and tissue engineering can improve 3D bioprinting and vice versa. Several challenges must be overcome in the application of 3D bioprinting before this disruptive technology is widely used to create organotypic constructs for regenerative medicine.

Bone marrow cell response after injury and during early stage of regeneration is independent of the tissue-of-injury in 2 injury models

Selectively recruiting bone marrow (BM)-derived stem and progenitor cells to injury sites is a promising therapeutic approach. The coordinated action of soluble factors is thought to trigger the mobilization of stem cells from the BM and recruit them to lesions to contribute to tissue regeneration. Nevertheless, the temporal response profile of the major cellular players and soluble factors involved in priming the BM and recruiting BM-derived cells to promote regeneration is unknown. We show that injury alters the BM cellular composition, introducing population-specific fluctuations during tissue regeneration. We demonstrate that injury causes an immediate, transient response of mesenchymal stromal cells and endothelial cells followed by a nonoverlapping increase in hematopoietic stem and progenitor cells. Moreover, BM reaction is identical whether the injury is inflicted on skin and muscle or also involves a bone defect, but these 2 injury paradigms trigger distinct systemic cytokine responses. Together, our results indicate that the BM response to injury in the early stages of regeneration is independent of the tissue-of-injury based on the 2 models used, but the injured tissue dictates the systemic cytokine response.-Leitão, L., Alves, C. J., Alencastre, I. S., Sousa, D. M., Neto, E., Conceição, F., Leitão, C., Aguiar, P., Almeida-Porada, G., Lamghari, M. Bone marrow cell response after injury and during early stage of regeneration is independent of the tissue-of-injury in 2 injury models.

Bone Targeted Delivery of SDF-1 via Alendronate Functionalized Nanoparticles in Guiding Stem Cell Migration

Stem cells are well-known for their great capacity for tissue regeneration. This provides a promising source for cell-based therapies in treating various bone degenerative disorders. However, the major hurdles for their application in transplantation are the poor bone marrow homing and engraftment efficiencies. Stromal cell-derived factor 1 (SDF-1) has been identified as a major stem cell homing factor. With the aims of bone targeted SDF-1 delivery and regulating MSCs migration, alendronate modified liposomal nanoparticles (Aln-Lipo) carrying SDF-1 gene were developed in this study. Alendronate modification significantly increased the mineral binding affinity of liposomes, and facilitated the gene delivery to osteoblastic cells. Up-regulated SDF-1 expression in osteoblasts triggered MSCs migration. Systemic infusion of Aln-Lipo-SDF-1 with fluorescence labeling in mice showed the accumulation in osseous tissue by biophotonic imaging. Corresponding to the delivered SDF-1, the transplanted GFP⁺ MSCs were attracted to bone marrow and contributed to bone regeneration. This study may provide a useful technique in regulating stem cell migration.

Novel Lipid Signaling Mediators for Mesenchymal Stem Cell Mobilization during Bone Repair

Introduction

Mesenchymal stem and progenitor cells (MSCs), which normally reside in the bone marrow, are critical to bone health and can be recruited to sites of traumatic bone injury, contributing to new bone formation. The ability to control the trafficking of MSCs provides therapeutic potential for improving traumatic bone healing and therapy for genetic bone diseases such as hypophosphatasia.

Methods

In this study, we explored the sphingosine-1-phosphate (S1P) signaling axis as a means to control the mobilization of MSCs into blood and possibly to recruit MSCs enhancing bone growth.

Results

Loss of S1P receptor 3 (S1PR3) leads to an increase in circulating CD45-/CD29+/CD90+/Sca1 putative mesenchymal progenitor cells, suggesting that blocking S1PR3 may stimulate MSCs to leave the bone marrow. Antagonism of S1PR3 with the small molecule VPC01091 stimulated acute migration of CD45-/CD29+/CD90+/Sca1+ MSCs into the blood as early as 1.5 hours after treatment. VPC01091 administration also increased ectopic bone formation induced by BMP-2 and significantly increased new bone formation in critically sized rat cranial defects, suggesting that mobilized MSCs may home to injuries to contribute to healing. We also explored the possibility of combining S1P manipulation of endogenous host cell occupancy with exogenous MSC transplantation for potential use in combination therapies. Importantly, reducing niche occupancy of host MSCs with VPC01091 does not impede engraftment of exogenous MSCs.

Conclusions

Our studies suggest that MSC mobilization through S1PR3 antagonism is a promising strategy for endogenous tissue engineering and improving MSC delivery to treat bone diseases.

Bidirectional juxtacrine ephrinB2/Ephs signaling promotes angiogenesis of ECs and maintains self-renewal of MSCs

Co-transplantation of endothelial cells (ECs) and mesenchymal stem cells (MSCs) is an important strategy for repairing complex and large bone defects. However, the ways in which ECs and MSCs interact remain to be fully clarified. We found that forward ephrinB2/Ephs signaling from hBMSCs to hUVECs promoted the tube formation of hUVECs by activating the PI3K/AKT/mTOR pathway. Reverse ephrinB2/Ephs signaling from hUVECs to hBMSCs promoted the proliferation and maintenance of hBMSCs self-renewal via upregulation of OCT4, SOX2, and YAP1. Subcutaneous co-transplantation of ECs and MSCs in nude mice confirmed that forward ephrinB2/Ephs signaling could increase the cross-sectional area of blood vessels in the transplanted area, and reverse ephrinB2/Ephs signaling could maintain the self-renewal of transplanted hBMSCs in vivo. Based on these results, ephrinB2/Ephs bidirectional juxtacrine regulation between ECs and MSCs plays a pivotal role in improving the healing of bone defects by promoting angiogenesis and achieving a sufficient number of MSCs.

Donor Site Location Is Critical for Proliferation, Stem Cell Capacity, and Osteogenic Differentiation of Adipose Mesenchymal Stem/Stromal Cells: Implications for Bone Tissue Engineering

Human adipose mesenchymal stem/stromal cells (Ad-MSCs) have been proposed as a suitable option for bone tissue engineering. However, donor age, weight, and gender might affect the outcome. There is still a lack of knowledge of the effects the donor tissue site might have on Ad-MSCs function. Thus, this study investigated proliferation, stem cell, and osteogenic differentiation capacity of human Ad-MSCs obtained from subcutaneous fat tissue acquired from different locations (abdomen, hip, thigh, knee, and limb). Ad-MSCs from limb and knee showed strong proliferation despite the presence of osteogenic stimuli, resulting in limited osteogenic characteristics. The less proliferative Ad-MSCs from hip and thigh showed the highest alkaline phosphatase (AP) activity and matrix mineralization. Ad-MSCs from the abdomen showed good proliferation and osteogenic characteristics. Interestingly, the observed differences were not dependent on donor age, weight, or gender, but correlated with the expression of Sox2, Lin28A, Oct4α, and Nanog. Especially, low basal Sox2 levels seemed to be pivotal for osteogenic differentiation. Our data clearly show that the donor tissue site affects the proliferation and osteogenic differentiation of Ad-MSCs significantly. Thus, for bone tissue engineering, the donor site of the adipose tissue from which the Ad-MSCs are derived should be adapted depending on the requirements, e.g., cell number and differentiation state.

The influence of age and osteoporosis on bone marrow stem cells from rats

Objectives

This study aimed to assess the effect of age and osteoporosis on the proliferative and differentiating capacity of bone-marrow-derived mesenchymal stem cells (MSCs) in female rats. We also discuss the role of these factors on expression and migration of cells along the C-X-C chemokine receptor type 4 (CXCR-4) / stromal derived factor 1 (SDF-1) axis.

Methods

Mesenchymal stem cells were harvested from the femora of young, adult, and osteopenic Wistar rats. Cluster of differentiation (CD) marker and CXCR-4 expression was measured using flow cytometry. Cellular proliferation was measured using Alamar Blue, osteogenic differentiation was measured using alkaline phosphatase expression and alizarin red production, and adipogenic differentiation was measured using Oil red O. Cells were incubated in Boyden chambers to quantify their migration towards SDF-1. Data was analyzed using a Student’s t-test, where p-values < 0.05 were considered significant.

Results

CD marker expression and proliferation of the MSCs from the three groups was not significantly different. The young MSCs demonstrated significantly increased differentiation into bone and fat and superior migration towards SDF-1. The migration of SDF-1 doubled with young rats compared with the adult rats (p = 0.023) and it was four times higher when compared with cells isolated from ovariectomized (OVX) osteopenic rats (p = 0.013).

Conclusion

Young rat MSCs are significantly more responsive to osteogenic differentiation, and, contrary to other studies, also demonstrated increased adipogenic differentiation compared with cells from adult and ostopenic rats. Young-rat-derived cells also showed superior migration towards SDF-1 compared with MSCs from OVX and adult control rats.

Cite this article: A. Sanghani-Kerai, L. Osagie-Clouard, G. Blunn, M. Coathup. The influence of age and osteoporosis on bone marrow stem cells from rats. Bone Joint Res 2018;7:289–297. DOI: 10.1302/2046-3758.74.BJR-2017-0302.R1.

Alcohol exposure decreases osteopontin expression during fracture healing and osteopontin-mediated mesenchymal stem cell migration in vitro

BACKGROUND

Alcohol consumption is a risk factor for impaired fracture healing, though the mechanism(s) by which this occurs are not well understood. Our laboratory has previously shown that episodic alcohol exposure of rodents negatively affects fracture callus development, callus biomechanics, and cellular signaling which regulates stem cell differentiation. Here, we examine whether alcohol alters chemokine expression and/or signaling activity in the mouse fracture callus during early fracture healing.

METHODS

A mouse model for alcohol-impaired tibia fracture healing was utilized. Early fracture callus was examined for alcohol-effects on tissue composition, expression of chemokines involved in MSC migration to the fracture site, and biomechanics. The effects of alcohol on MSC migration and cell adhesion receptors were examined in an in vitro system.

RESULTS

Mice exposed to alcohol showed decreased evidence of external callus formation, decreased callus-related osteopontin (OPN) expression levels, and decreased biomechanical stiffness. Alcohol exposure decreased rOPN-mediated MSC migration and integrin β1 receptor expression in vitro.

CONCLUSIONS

The effects of alcohol exposure demonstrated here on fracture callus-associated OPN expression, rOPN-mediated MSC migration in vitro, and MSC integrin β1 receptor expression in vitro have not been previously reported. Understanding the effects of alcohol exposure on the early stages of fracture repair may allow timely initiation of treatment to mitigate the long-term complications of delayed healing and/or fracture non-union.

STAT-3 regulation of CXCR4 is necessary for the prenylflavonoid Icaritin to enhance mesenchymal stem cell proliferation, migration and osteogenic differentiation

Mesenchymal stem cell (MSC) dysfunction has been implicated in the pathogenesis of osteoporosis. MSCs derived from osteoporotic subjects demonstrate significant impairment in proliferation, adhesion and chemotaxis, and osteogenic differentiation, leading to reduced functional bone-forming osteoblasts and ultimately nett bone loss and osteoporosis. Epimedium herbs and its active compound Icaritin (ICT) have been used in Chinese ethnopharmacology for the treatment of metabolic bone diseases. Using an in-vitro cell culture model, we investigated the benefits of ICT treatment in enhancing MSC proliferation, migration and osteogenic differentiation, and provide novel data to describe its mechanism of action. ICT enhances MSC proliferation, chemotaxis to stromal cell-derived factor-1 (SDF-1) and osteogenic differentiation through the activation of signal transduction activator transcription factor 3 (STAT-3), with a consequential up-regulation in the expression and activity of cysteine (C)-X-C motif chemokine receptor 4 (CXCR4). These findings provide a strong basis for future clinical studies to confirm the therapeutic potential of ICT for the prevention and treatment of osteoporosis and fragility fractures.

Co-Culture with Human Osteoblasts and Exposure to Extremely Low Frequency Pulsed Electromagnetic Fields Improve Osteogenic Differentiation of Human Adipose-Derived Mesenchymal Stem Cells

Human adipose-derived mesenchymal stem cells (Ad-MSCs) have been proposed as suitable option for cell-based therapies to support bone regeneration. In the bone environment, Ad-MSCs will receive stimuli from resident cells that may favor their osteogenic differentiation. There is recent evidence that this process can be further improved by extremely low frequency pulsed electromagnetic fields (ELF-PEMFs). Thus, the project aimed at (i) investigating whether co-culture conditions of human osteoblasts (OBs) and Ad-MSCs have an impact on their proliferation and osteogenic differentiation; (ii) whether this effect can be further improved by repetitive exposure to two specific ELF-PEMFs (16 and 26 Hz); (iii) and the effect of these ELF-PEMFs on human osteoclasts (OCs). Osteogenic differentiation was improved by co-culturing OBs and Ad-MSCs when compared to the individual mono-cultures. An OB to Ad-MSC ratio of 3:1 had best effects on total protein content, alkaline phosphatase (AP) activity, and matrix mineralization. Osteogenic differentiation was further improved by both ELF-PEMFs investigated. Interestingly, only repetitive exposure to 26 Hz ELF-PEMF increased Trap5B activity in OCs. Considering this result, a treatment with gradually increasing frequency might be of interest, as the lower frequency (16 Hz) could enhance bone formation, while the higher frequency (26 Hz) could enhance bone remodeling.

Antibody-Mediated Osseous Regeneration for Bone Tissue Engineering in Canine Segmental Defects

Among many applications of therapeutic monoclonal antibodies (mAbs), a unique approach for regenerative medicine has entailed antibody-mediated osseous regeneration (AMOR). In an effort to identify a clinically relevant model of craniofacial defect, the present study investigated the efficacy of mAb specific for bone morphogenetic protein- (BMP-) 2 to repair canine segmental mandibular continuity defect model. Accordingly, a 15 mm unilateral segmental defect was created in mandible and fixated with a titanium plate. Anorganic bovine bone mineral with 10% collagen (ABBM-C) was functionalized with 25 μg/mL of either chimeric anti-BMP-2 mAb or isotype-matched mAb (negative control). Recombinant human (rh) BMP-2 served as positive control. Morphometric analyses were performed on computed tomography (CT) and histologic images. Bone densities within healed defect sites at 12 weeks after surgery were 1360.81 ± 10.52 Hounsfield Unit (HU), 1044.27 ± 141.16 HU, and 839.45 ± 179.41 HU, in sites with implanted anti-BMP-2 mAb, rhBMP-2, and isotype mAb groups, respectively. Osteoid bone formation in anti-BMP-2 mAb (42.99% ± 8.67) and rhBMP-2 (48.97% ± 2.96) groups was not significantly different but was higher (p < 0.05) than in sites with isotype control mAb (26.8% ± 5.35). In view of the long-term objective of translational application of AMOR in humans, the results of the present study demonstrated the feasibility of AMOR in a large clinically relevant animal model.

Periosteum contains skeletal stem cells with high bone regenerative potential controlled by Periostin

Bone regeneration relies on the activation of skeletal stem cells (SSCs) that still remain poorly characterized. Here, we show that periosteum contains SSCs with high bone regenerative potential compared to bone marrow stromal cells/skeletal stem cells (BMSCs) in mice. Although periosteal cells (PCs) and BMSCs are derived from a common embryonic mesenchymal lineage, postnatally PCs exhibit greater clonogenicity, growth and differentiation capacity than BMSCs. During bone repair, PCs can efficiently contribute to cartilage and bone, and integrate long-term after transplantation. Molecular profiling uncovers genes encoding Periostin and other extracellular matrix molecules associated with the enhanced response to injury of PCs. Periostin gene deletion impairs PC functions and fracture consolidation. Periostin-deficient periosteum cannot reconstitute a pool of PCs after injury demonstrating the presence of SSCs within periosteum and the requirement of Periostin in maintaining this pool. Overall our results highlight the importance of analyzing periosteum and PCs to understand bone phenotypes.

The periosteum, a tissue lining the bone surface, and the bone marrow are known to contain bone-forming cells. Here the authors show that skeletal stem cells reside in the mouse periosteum, and that periosteal cells have common embryonic origins with bone marrow stromal/stem cells (BMSCs), but are better at bone repair and long-term integration than BMSCs.

Development of controlled drug delivery systems for bone fracture-targeted therapeutic delivery: A review

Impaired fracture healing is a major clinical problem that can lead to patient disability, prolonged hospitalization, and significant financial burden. Although the majority of fractures heal using standard clinical practices, approximately 10% suffer from delayed unions or non-unions. A wide range of factors contribute to the risk for nonunions including internal factors, such as patient age, gender, and comorbidities, and external factors, such as the location and extent of injury. Current clinical approaches to treat nonunions include bone grafts and low-intensity pulsed ultrasound (LIPUS), which realizes clinical success only to select patients due to limitations including donor morbidities (grafts) and necessity of fracture reduction (LIPUS), respectively. To date, therapeutic approaches for bone regeneration rely heavily on protein-based growth factors such as INFUSE, an FDA-approved scaffold for delivery of bone morphogenetic protein 2 (BMP-2). Small molecule modulators and RNAi therapeutics are under development to circumvent challenges associated with traditional growth factors. While preclinical studies has shown promise, drug delivery has become a major hurdle stalling clinical translation. Therefore, this review overviews current therapies employed to stimulate fracture healing pre-clinically and clinically, including a focus on drug delivery systems for growth factors, parathyroid hormone (PTH), small molecules, and RNAi therapeutics, as well as recent advances and future promise of fracture-targeted drug delivery.

Fetal Mesenchymal Stromal Cells: an Opportunity for Prenatal Cellular Therapy

Purpose of Review

The aim of the study is to provide an overview on the possibility of treating congenital disorders prenatally with mesenchymal stromal cells (MSCs).

Recent Findings

MSCs have multilineage potential and a low immunogenic profile and are immunomodulatory and more easy to expand in culture. Their ability to migrate, engraft and differentiate, or act via a paracrine effect on target tissues makes MSCs candidates for clinical therapies. Fetal and extra-fetal MSCs offer higher therapeutic potential compared to MSCs derived from adult sources.

Summary

MSCs may be safely transplanted prenatally via ultrasound-guided injection into the umbilical cord. Due to these characteristics, fetal MSCs are of great interest in the field of in utero stem cell transplantation for treatment of congenital disease.

Loss of Dnmt3b in Chondrocytes Leads to Delayed Endochondral Ossification and Fracture Repair

Despite advanced understanding of signaling mediated by local and systemic factors, the role of epigenetic factors in the regulation of bone regeneration remains vague. The DNA methyltransferases (Dnmts) Dnmt3a and Dnmt3b have tissue specific expression patterns and create unique methylation signatures to regulate gene expression. Using a stabilized murine tibia fracture model we find that Dnmt3b is induced early in fracture healing, peaks at 10 days post fracture (dpf), and declines to nearly undetectable levels by 28 dpf. Dnmt3b expression was cell-specific and stage-specific. High levels were observed in chondrogenic lineage cells within the fracture callus. To determine the role of Dnmt3b in fracture healing, Agc1CreERT2 ;Dnmt3bf/f (Dnmt3bAgc1ER ) mice were generated to delete Dnmt3b in chondrogenic cells. Dnmt3bAgc1ER fracture displayed chondrogenesis and chondrocyte maturation defect, and a delay in the later events of angiogenesis, ossification, and bone remodeling. Biomechanical studies demonstrated markedly reduced strength in Dnmt3bAgc1ER fractures and confirmed the delay in repair. The angiogenic response was reduced in both vessel number and volume at 10 and 14 dpf in Dnmt3bAgc1ER mice. Immunohistochemistry showed decreased CD31 expression, consistent with the reduced angiogenesis. Finally, in vitro angiogenesis assays with human umbilical vein endothelial cells (HUVECs) revealed that loss of Dnmt3b in chondrocytes significantly reduced tube formation and endothelial migration. To identify specific angiogenic factors involved in the decreased callus vascularization, a protein array was performed using conditioned media isolated from control and Dnmt3b loss-of-function chondrocytes. Several angiogenic factors, including CXCL12 and osteopontin (OPN) were reduced in chondrocytes following loss of Dnmt3b. DNA methylation analysis further identified hypomethylation in Cxcl12 promoter region. Importantly, the defects in tube formation and cell migration could be rescued by administration of CXCL12 and/or OPN. Altogether, our findings establish that Dnmt3b positively regulates chondrocyte maturation process, and its genetic ablation leads to delayed angiogenesis and fracture repair. © 2017 American Society for Bone and Mineral Research.

Micro-Nanostructures of Cellulose-Collagen for Critical Sized Bone Defect Healing

Bone tissue engineering strategies utilize biodegradable polymeric matrices alone or in combination with cells and factors to provide mechanical support to bone, while promoting cell proliferation, differentiation, and tissue ingrowth. The performance of mechanically competent, micro-nanostructured polymeric matrices, in combination with bone marrow stromal cells (BMSCs), is evaluated in a critical sized bone defect. Cellulose acetate (CA) is used to fabricate a porous microstructured matrix. Type I collagen is then allowed to self-assemble on these microstructures to create a natural polymer-based, micro-nanostructured matrix (CAc). Poly (lactic-co-glycolic acid) matrices with identical microstructures serve as controls. Significantly higher number of implanted host cells are distributed in the natural polymer based micro-nanostructures with greater bone density and more uniform cell distribution. Additionally, a twofold increase in collagen content is observed with natural polymer based scaffolds. This study establishes the benefits of natural polymer derived micro-nanostructures in combination with donor derived BMSCs to repair and regenerate critical sized bone defects. Natural polymer based materials with mechanically competent micro-nanostructures may serve as an alternative material platform for bone regeneration.

A cell surface clicked navigation system to direct specific bone targeting

Cell therapies are promising up-and-coming therapeutic strategies for many diseases. For maximal therapeutic benefits, injected cells have to navigate their way to a designated area, including organ and tissue; unfortunately, the majority of therapeutic cells are currently administered without a guide or homing device. To improve this serious shortcoming, a functionalization method was developed to equip cells with a homing signal. Its application was demonstrated by applying an Azadibenzocyclooctyne-bisphosphonate (ADIBO-BP) and azide paired bioorthogonal chemistry on cells for bone specific homing. Jurkat T cells and bone marrow derived stromal cells (BMSCs) were cultured with tetraacetylated N-azidoacetyl-d-mannosamine (Ac₄ManNAz) to place unnatural azido groups onto the cell's surface; these azido groups were then reacted with ADIBO-BP. The tethered bisphosphonates were able to bring Jurkat cells to hydroxyapatite, the major component of bone, and mineralized SAOS-2 cells. The incorporated BP groups also enhanced the specific affinity of BMSCs to mouse femur bone fragments in vitro. The introduced navigation strategy is expected to have a broad application in cell therapy, because through the biocompatible ADIBO and azide reactive pair, various homing signals could be efficiently anchored onto therapeutic cells.

Matrix metalloproteinase-1 facilitates MSC migration via cleavage of IGF-2/IGFBP2 complex

The specific mechanism underlying the tumor tropism of human mesenchymal stem cells (MSCs) for cancer is not well defined. We previously showed that the migration potential of MSCs correlated with the expression and protease activity of matrix metalloproteinase (MMP)‐1. Furthermore, highly tumor‐tropic MSCs expressed higher levels of MMP‐1 and insulin‐like growth factor (IGF)‐2 than poorly migrating MSCs. In this study, we examined the functional roles of IGF‐2 and MMP‐1 in mediating the tumor tropism of MSCs. Exogenous addition of either recombinant IGF‐2 or MMP‐1 could stimulate MSC migration. The correlation between IGF‐2, MMP‐1 expression, and MSC migration suggests that MMP‐1 may play a role in regulating MSC migration via the IGF‐2 signaling cascade. High concentrations of IGF binding proteins (IGFBPs) can inhibit IGF‐stimulated functions by blocking its binding to its receptors and proteolysis of IGFBP is an important mechanism for the regulation of IGF signaling. We thus hypothesized that MMP‐1 acts as an IGFBP2 proteinase, resulting in the cleavage of IGF‐2/IGFBP2 complex and extracellular release of free IGF‐2. Indeed, our results showed that conditioned media from highly migrating MSCs, which expressed high levels of MMP‐1, cleaved the IGF‐2/IGFBP2 complex. Taken together, these results showed that the MMP‐1 secreted by highly tumor‐tropic MSCs cleaved IGF‐2/IGFBP2 complex. Free IGF‐2 released from the complex may facilitate MSC migration toward tumor.

Nanomedicine for safe healing of bone trauma: Opportunities and challenges

Historically, high-energy extremity injuries resulting in significant soft-tissue trauma and bone loss were often deemed unsalvageable and treated with primary amputation. With improved soft-tissue coverage and nerve repair techniques, these injuries now present new challenges in limb-salvage surgery. High-energy extremity trauma is pre-disposed to delayed or unpredictable bony healing and high rates of infection, depending on the integrity of the soft-tissue envelope. Furthermore, orthopedic trauma surgeons are often faced with the challenge of stabilizing and repairing large bony defects while promoting an optimal environment to prevent infection and aid bony healing. During the last decade, nanomedicine has demonstrated substantial potential in addressing the two major issues intrinsic to orthopedic traumas (i.e., high infection risk and low bony reconstruction) through combatting bacterial infection and accelerating/increasing the effectiveness of the bone-healing process. This review presents an overview and discusses recent challenges and opportunities to address major orthopedic trauma through nanomedical approaches.

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.

TRIB3 inhibits proliferation and promotes osteogenesis in hBMSCs by regulating the ERK1/2 signaling pathway

Osteogenic differentiation in human bone marrow-derived mesenchymal stem cells (hBMSCs) is regulated by various factors, including bone morphogenetic proteins (BMPs), Notch, growth hormones and mitogen-activated protein kinases (MAPKs). Tribbles homolog 3 (TRIB3), a pseudokinase, plays an important role in cancer cells and adipocytes. However, TRIB3 function in osteogenic differentiation is unknown, although it is involved in regulating signaling pathways associated with osteogenic differentiation. Here, we found that TRIB3 was highly expressed during osteogenic differentiation in hBMSCs. Inhibition of focal adhesion kinase (FAK) or phosphatidylinositol 3-kinase (PI3K) resulted in a significant decrease in TRIB3 expression, and expression of TRIB3 was restored by increasing insulin-like growth factor-1 (IGF-1) via activating phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signaling. TRIB3 knock-down enhanced proliferation and decreased osteogenic differentiation at the middle stage of differentiation, and these effects were reversed by inhibiting the activation of extracellular signal-regulated kinase (ERK)-1/2. In conclusion, TRIB3 plays an important role in proliferation and osteogenic differentiation by regulating ERK1/2 activity at the middle stage of differentiation, and expression of TRIB3 is regulated by FAK in a PI3K/AKT-dependent manner.

Cell-based approaches for augmentation of tendon repair

Cell-based approaches are among the principal interventions in orthobiologics to improve tendon and ligament healing and to combat degenerative processes. The number of options available for investigation are expanding rapidly and investigators have an increasing number of cell types to choose from for research purposes. However, in part due to the current regulatory environment, the list of available cells at clinicians' disposal for therapeutic purposes is still rather limited. In this review, we present an overview of the main cellular categories in current use. Notable recent developments in cell-based approaches include the introduction of diverse sources of mesenchymal stem cells, pluripotent cells of extra-embryonic origin, and the emerging popularity of fully differentiated cells such as tenocytes and endothelial cells. Delivery strategies are discussed and a succinct discussion of the current regulatory environment in the United States is presented.

Osteoimmunology in Bone Fracture Healing

PURPOSE OF REVIEW

In the process of bone fracture healing, inflammation is thought to be an essential process that precedes bone formation and remodeling. We review recent studies on bone fracture healing from an osteoimmunological point of view.

RECENT FINDINGS

Based on previous observations that many types of immune cells infiltrate into the bone injury site and release a variety of molecules, recent studies have addressed the roles of specific immune cell subsets. Macrophages and interleukin (IL)-17-producing γδ T cells enhance bone healing, whereas CD8⁺ T cells impair bone repair. Additionally, IL-10-producing B cells may contribute to bone healing by suppressing excessive and/or prolonged inflammation. Although the involvement of other cells and molecules has been suggested, the precise underlying mechanisms remain elusive. Accumulating evidence has begun to reveal the deeper picture of bone fracture healing. Further studies are required for the development of novel therapeutic strategies for bone fracture.

Interleukin-3 enhances the migration of human mesenchymal stem cells by regulating expression of CXCR4

BACKGROUND

Mesenchymal stem cells (MSCs) represent an important source for cell therapy in regenerative medicine. MSCs have shown promising results for repair of damaged tissues in various degenerative diseases in animal models and also in human clinical trials. However, little is known about the factors that could enhance the migration and tissue-specific engraftment of exogenously infused MSCs for successful regenerative cell therapy. Previously, we have reported that interleukin-3 (IL-3) prevents bone and cartilage damage in animal models of rheumatoid arthritis and osteoarthritis. Also, IL-3 promotes the differentiation of human MSCs into functional osteoblasts and increases their in-vivo bone regenerative potential in immunocompromised mice. However, the role of IL-3 in migration of MSCs is not yet known. In the present study, we investigated the role of IL-3 in migration of human MSCs under both in-vitro and in-vivo conditions.

METHODS

MSCs isolated from human bone marrow, adipose and gingival tissues were used for in-vitro cell migration, motility and wound healing assays in the presence or absence of IL-3. The effect of IL-3 preconditioning on expression of chemokine receptors and integrins was examined by flow cytometry and real-time PCR. The in-vivo migration of IL-3-preconditioned MSCs was investigated using a subcutaneous matrigel-releasing stromal cell-derived factor-1 alpha (SDF-1α) model in immunocompromised mice.

RESULTS

We observed that human MSCs isolated from all three sources express IL-3 receptor-α (IL-3Rα) both at gene and protein levels. IL-3 significantly enhances in-vitro migration, motility and wound healing abilities of MSCs. Moreover, IL-3 preconditioning upregulates expression of chemokine (C-X-C motif) receptor 4 (CXCR4) on MSCs, which leads to increased migration of cells towards SDF-1α. Furthermore, CXCR4 antagonist AMD3100 decreases the migration of IL-3-treated MSCs towards SDF-1α. Importantly, IL-3 also induces in-vivo migration of MSCs towards subcutaneously implanted matrigel-releasing-SDF-1α in immunocompromised mice.

CONCLUSIONS

The present study demonstrates for the first time that IL-3 has an important role in enhancing the migration of human MSCs through regulation of the CXCR4/SDF-1α axis. These findings suggest a potential role of IL-3 in improving the efficacy of MSCs in regenerative cell therapy.

BNIP3 induction by hypoxia stimulates FASN-dependent free fatty acid production enhancing therapeutic potential of umbilical cord blood-derived human mesenchymal stem cells

Mitophagy under hypoxia is an important factor for maintaining and regulating stem cell functions. We previously demonstrated that fatty acid synthase (FASN) induced by hypoxia is a critical lipid metabolic factor determining the therapeutic efficacy of umbilical cord blood-derived human mesenchymal stem cells (UCB-hMSCs). Therefore, we investigated the mechanism of a major mitophagy regulator controlling lipid metabolism and therapeutic potential of UCB-hMSCs. This study revealed that Bcl2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3)-dependent mitophagy is important for reducing mitochondrial reactive oxygen species accumulation, anti-apoptosis, and migration under hypoxia. And, BNIP3 expression was regulated by CREB binding protein-mediated transcriptional actions of HIF-1α and FOXO3. Silencing of BNIP3 suppressed free fatty acid (FFA) synthesis regulated by SREBP1/FASN pathway, which is involved in UCB-hMSC apoptosis via caspases cleavage and migration via cofilin-1-mediated F-actin reorganization in hypoxia. Moreover, reduced mouse skin wound-healing capacity of UCB-hMSC with hypoxia pretreatment by BNIP3 silencing was recovered by palmitic acid. Collectively, our findings suggest that BNIP3-mediated mitophagy under hypoxia leads to FASN-induced FFA synthesis, which is critical for therapeutic potential of UCB-hMSCs with hypoxia pretreatment.

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BNIP3 induction by hypoxia mainly controls mitophagy and mitochondrial ROS production in UCB-hMSCs.

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BNIP3 silencing impairs UCB-hMSC functions such as survival, migration and free fatty acid production under hypoxia.

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BNIP3 silencing suppresses SREBP1/FASN-mediated free fatty acid production via ROS regulation under hypoxia.

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BNIP3 silencing decreased skin wound healing potential of hypoxia-pretreated UCB-hMSCs.

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Palmitic acid addition recovers decreased therapeutic potential of UCB-hMSCs by BNIP3 silencing.

Effect of SDF-1/Cxcr4 Signaling Antagonist AMD3100 on Bone Mineralization in Distraction Osteogenesis

Distraction osteogenesis (DO) is a widely applied technique in orthopedics surgery, which involves rapid stem cell migration, homing, and differentiation. Interactions between the chemokine receptor Cxcr4 and its ligand, stromal derived factor-1 (SDF-1), regulate hematopoietic stem cell trafficking to the ischemic area and induce their subsequent differentiation. Here, we examined SDF-1 expression and further investigated the role of SDF-1/Cxcr4 signaling antagonist AMD3100 during bone regeneration in rat DO model. The results showed that expression levels of SDF-1 and osteogenic genes were higher in DO zones than in the fracture zones, and SDF-1 expression level was the highest at the termination of the distraction phase. Radiological, mechanical, and histological analyses demonstrated that the local administration of AMD3100 (400 μM) to DO rats significantly inhibited new bone formation. In the rat bone marrow mesenchymal stem cells culture, comparing to the group treated with osteogenic induction medium, AMD3100 supplement led to a considerable decrease in the expression of alkaline phosphatase and early osteogenic marker genes. However, the amount of calcium deposits in rat MSCs did not differ between the groups. Therefore, our study demonstrated that the DO process induced higher expression of SDF-1, which collated to rapid induction of callus formation. Local application of SDF-1/Cxcr4 signaling antagonist AMD3100 significantly inhibited bone mineralization and osteogenesis in DO, which may represent a potential therapeutic approach to the enhancement of bone consolidation in patients undergoing DO.

Stem cell therapy for enhancement of bone consolidation in distraction osteogenesis: A contemporary review of experimental studies

Objectives

Distraction osteogenesis (DO) mobilises bone regenerative potential and avoids the complications of other treatments such as bone graft. The major disadvantage of DO is the length of time required for bone consolidation. Mesenchymal stem cells (MSCs) have been used to promote bone formation with some good results.

Methods

We hereby review the published literature on the use of MSCs in promoting bone consolidation during DO.

Results

Studies differed in animal type (mice, rabbit, dog, sheep), bone type (femur, tibia, skull), DO protocols and cell transplantation methods.

Conclusion

The majority of studies reported that the transplantation of MSCs enhanced bone consolidation or formation in DO. Many questions relating to animal model, DO protocol and cell transplantation regime remain to be further investigated. Clinical trials are needed to test and confirm these findings from animal studies.

Cite this article: Y. Yang, S. Lin, B. Wang, W. Gu, G. Li. Stem cell therapy for enhancement of bone consolidation in distraction osteogenesis: A contemporary review of experimental studies. Bone Joint Res 2017;6:385–390. DOI: 10.1302/2046-3758.66.BJR-2017-0023.

MSC/ECM Cellular Complexes Induce Periodontal Tissue Regeneration

Transplantation of mesenchymal stem cells (MSCs), which possess self-renewing properties and multipotency, into a periodontal defect is thought to be a useful option for periodontal tissue regeneration. However, developing more reliable and predictable implantation techniques is still needed. Recently, we generated clumps of an MSC/extracellular matrix (ECM) complex (C-MSC), which consisted of cells and self-produced ECM. C-MSCs can regulate their cellular functions in vitro and can be grafted into a defect site, without any artificial scaffold, to induce bone regeneration. Accordingly, this study aimed to evaluate the effect of C-MSC transplantation on periodontal tissue regeneration in beagle dogs. Seven beagle dogs were employed to generate a premolar class III furcation defect model. MSCs isolated from dog ilium were seeded at a density of 7.0 × 10⁴ cells/well into 24-well plates and cultured in growth medium supplemented with 50 µg/mL ascorbic acid for 4 d. To obtain C-MSCs, confluent cells were scratched using a micropipette tip and were then torn off as a cellular sheet. The sheet was rolled up to make round clumps of cells. C-MSCs were maintained in growth medium or osteoinductive medium (OIM) for 5 or 10 d. The biological properties of C-MSCs were evaluated in vitro, and their periodontal tissue regenerative activity was tested by using a dog class III furcation defect model. Immunofluorescence analysis revealed that type I collagen fabricated the form of C-MSCs. OIM markedly elevated calcium deposition in C-MSCs at day 10, suggesting its osteogenic differentiation capacity. Both C-MSCs and C-MSCs cultured with OIM transplantation without an artificial scaffold into the dog furcation defect induced periodontal tissue regeneration successfully compared with no graft, whereas osteogenic-differentiated C-MSCs led to rapid alveolar bone regeneration. These findings suggested that the use of C-MSCs refined by self-produced ECM may represent a novel predictable periodontal tissue regenerative therapy.

Human Bone Marrow Mesenchymal Stem/Stromal Cells Preserve Their Immunomodulatory and Chemotactic Properties When Expanded in a Human Plasma Derived Xeno-Free Medium

Due to their immunomodulatory and chemotactic properties, hMSC are being explored to treat immune-related diseases. For their use in human therapies, it is necessary to culture hMSC in xeno-free conditions. In this study, the impact that a xeno-free medium based on a human plasma derivate has on these properties was analysed. Bone marrow-derived hMSC preserved their immunosuppressive and immunostimulatory properties, as observed with in vitro assays with hMSC cocultured with mixed leukocyte reactions or with mitogen-stimulated leukocytes. Moreover, hMSC expanded in xeno-free medium were recruited by macrophages in both migration and invasion assays, which indicates that the cells maintained their chemotactic properties. These data suggest that xeno-free expanded hMSC preserved their immunomodulatory and chemotactic properties, indicating that the described xeno-free medium composition is a potential candidate to culture and expand hMSC for human cell therapies.

Xenotransplantation of interferon-gamma-pretreated clumps of a human mesenchymal stem cell/extracellular matrix complex induces mouse calvarial bone regeneration

Background

Three-dimensional cultured clumps of a mesenchymal stem cell (MSC)/extracellular matrix (ECM) complex (C-MSC) consists of cells and self-produced ECM. C-MSC can regulate the cellular function in vitro and induce successful bone regeneration using ECM as a cell scaffold. Potentiating the immunomodulatory capacity of C-MSCs, which can ameliorate the allo-specific immune response, may be helpful in developing beneficial “off-the-shelf” cell therapy for tissue regeneration. It is well reported that interferon (IFN)-γ stimulates the immunosuppressive properties of MSC via upregulation of the immunomodulatory enzyme IDO. Therefore, the aim of this study was to investigate the effect of IFN-γ on the immunomodulatory capacity of C-MSC in vitro and to test the bone regenerative activity of C-MSC or IFN-γ-pretreated C-MSC (C-MSCγ) xenografts in a mice calvarial defect model.

Methods

Human bone marrow-derived MSCs were seeded at a density of 2.0 × 10⁵ cells/well into 24-well plates and cultured with growth medium supplemented with 50 μg/mL L-ascorbic acid for 4 days. To obtain C-MSC, confluent cells that had formed on the cellular sheet were scratched using a micropipette tip and were then torn off. The cellular sheet was rolled to make a round clump of cells. C-MSC was stimulated with IFN-γ and IDO expression, immunosuppressive capacity, and immunophenotype were evaluated in vitro. Moreover, C-MSC or C-MSCγ was xenotransplanted into immunocompetent or immunodeficient mice calvarial defect models without artificial scaffold, respectively.

Results

IFN-γ stimulated IDO expression in C-MSC. C-MSCγ, but not C-MSC, attenuated CD3/CD28-induced T cell proliferation and its suppressive effect was reversed by an IDO inhibitor. C-MSCγ showed upregulation of HLA-DR expression, but its co-stimulatory molecule, CD86, was not detected. Xenotransplantation of C-MSCγ into immunocompetent mice calvarial defect induced bone regeneration, whereas C-MSC xenograft failed and induced T cell infiltration in the grafted area. On the other hand, both C-MSC and C-MSCγ xenotransplantation into immunodeficient mice caused bone regeneration.

Conclusions

Xenotransplantation of C-MSCγ, which exerts immunomodulatory properties via the upregulation of IDO activity in vitro, may attenuate xenoreactive host immune response, and thereby induce bone regeneration in mice. Accordingly, C-MSCγ may constitute a promising novel allograft cell therapy for bone regeneration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0550-1) contains supplementary material, which is available to authorized users.

Bright Polymer Dots Tracking Stem Cell Engraftment and Migration to Injured Mouse Liver

Stem cell therapy holds promise for treatment of intractable diseases and injured organs. For clinical translation, it is pivotal to understand the homing, engraftment, and differentiation processes of stem cells in a living body. Here we report near-infrared (NIR) fluorescent semiconductor polymer dots (Pdots) for bright labeling and tracking of human mesenchymal stem cells (MSCs). The Pdots exhibit narrow-band emission at 775 nm with a quantum yield of 22%, among the highest value for various NIR probes. The Pdots together with a cell penetrating peptide are able to track stem cells over two weeks without disturbing their multipotent properties, as confirmed by the analyses on cell proliferation, differentiation, stem-cell markers, and immunophenotyping. The in vivo cell tracking was demonstrated in a liver-resection mouse model, which indicated that the Pdot-labeled MSCs after tail-vein transplantation were initially trapped in lung, gradually migrated to the injured liver, and then proliferated into cell clusters. Liver-function analysis and histological examination revealed that the inflammation induced by liver resection was apparently decreased after stem cell transplantation. With the bright labeling, superior biocompatibility, and long-term tracking performance, the Pdot probes are promising for stem cell research and regenerative medicine.

Ultrasound as a stimulus for musculoskeletal disorders

Ultrasound is an inaudible form of acoustic sound wave at 20 kHz or above that is widely used in the medical field with applications including medical imaging and therapeutic stimulation. In therapeutic ultrasound, low-intensity pulsed ultrasound (LIPUS) is the most widely used and studied form that generally uses acoustic waves at an intensity of 30 mW/cm², with 200 ms pulses and 1.5 MHz. In orthopaedic applications, it is used as a biophysical stimulus for musculoskeletal tissue repair to enhance tissue regeneration. LIPUS has been shown to enhance fracture healing by shortening the time to heal and reestablishment of mechanical properties through enhancing different phases of the healing process, including the inflammatory phase, callus formation, and callus remodelling phase. Reports from in vitro studies reveal insights in the mechanism through which acoustic stimulations activate cell surface integrins that, in turn, activate various mechanical transduction pathways including FAK (focal adhesion kinase), ERK (extracellular signal-regulated kinase), PI3K, and Akt. It is then followed by the production of cyclooxygenase 2 and prostaglandin E2 to stimulate further downstream angiogenic, osteogenic, and chondrogenic cytokines, explaining the different enhancements observed in animal and clinical studies. Furthermore, LIPUS has also been shown to have remarkable effects on mesenchymal stem cells (MSCs) in musculoskeletal injuries and tissue regeneration. The recruitment of MSCs to injury sites by LIPUS requires the SDF-1 (stromal cell derived factor-1)/CXCR-4 signalling axis. MSCs would then differentiate differently, and this is regulated by the presence of different cytokines, which determines their fates. Other musculoskeletal applications including bone–tendon junction healing, and distraction osteogenesis are also explored, and the results are promising. However, the use of LIPUS is controversial in treating osteoporosis, with negative findings in clinical settings, which may be attributable to the absence of an injury entry point for the acoustic signal to propagate, strong attenuation effect of cortical bone and the insufficient intensity for penetration, whereas in some animal studies it has proven effective.

Mesenchymal stem cells homing to improve bone healing

Cell therapy continues to attract growing interest as a promising approach to treat a variety of diseases. Mesenchymal stem cells (MSCs) have been one of the most intensely studied candidates for cell therapy. Since the homing capacity of MSCs is an important determinant of effective MSC-based therapy, the enhancement of homing efficiency is essential for optimizing the therapeutic outcome. Furthermore, trafficking of endogenous MSCs to damaged tissues, also referred to as endogenic stem cell homing, and the subsequent participation of MSCs in tissue regeneration are considered to be a natural self-healing response. Therefore, strategies to stimulate and reinforce the mobilisation and homing of MSCs have become a key point in regenerative medicine. The current review focuses on advances in the mechanisms and factors governing trafficking of MSCs, and the relationship between MSC mobilisation and skeletal diseases, providing insights into strategies for their potential translational implications.

Prospective Review of Mesenchymal Stem Cells Differentiation into Osteoblasts

Stem cell research has been a popular topic in the past few decades. This review aims to discuss factors that help regulate, induce, and enhance mesenchymal stem cell (MSC) differentiation into osteoblasts for bone regeneration. The factors analyzed include bone morphogenic protein (BMP), transforming growth factor β (TGF-β), stromal cell-derived factor 1 (SDF-1), insulin-like growth factor type 1 (IGF-1), histone demethylase JMJD3, cyclin dependent kinase 1 (CDK1), fucoidan, Runx2 transcription factor, and TAZ transcriptional coactivator. Methods promoting bone healing are also evaluated in this review that have shown promise in previous studies. Methods tested using animal models include low intensity pulsed ultrasound (LIPUS) with MSC, micro motion, AMD3100 injections, BMP delivery, MSC transplantation, tissue engineering utilizing scaffolds, anti-IL-20 monoclonal antibody, low dose photodynamic therapy, and bone marrow stromal cell transplants. Human clinical trial methods analyzed include osteoblast injections, bone marrow grafts, bone marrow and platelet rich plasma transplantation, tissue engineering using scaffolds, and recombinant human BMP-2. These methods have been shown to promote and accelerate new bone formation. These various methods for enhanced bone regeneration have the potential to be used, following further research, in clinical practice.

Trauma and Stem Cells: Biology and Potential Therapeutic Implications

Trauma may cause irreversible tissue damage and loss of function despite current best practice. Healing is dependent both on the nature of the injury and the intrinsic biological capacity of those tissues for healing. Preclinical research has highlighted stem cell therapy as a potential avenue for improving outcomes for injuries with poor healing capacity. Additionally, trauma activates the immune system and alters stem cell behaviour. This paper reviews the current literature on stem cells and its relevance to trauma care. Emphasis is placed on understanding how stem cells respond to trauma and pertinent mechanisms that can be utilised to promote tissue healing. Research involving notable difficulties in trauma care such as fracture non-union, cartilage damage and trauma induced inflammation is discussed further.

Gremlin-1 suppression increases BMP-2-induced osteogenesis of human mesenchymal stem cells

Previous research focusing on rodent cells and animal models has demonstrated that gremlin-1 antagonizes bone morphogenetic proteins (BMPs) in order to suppress osteogenesis. However, the impact of gremlin-1 on osteogenesis in human bone marrow-derived mesenchymal stem cells (MSCs) remains unknown. The aim of the present study was to test the effects of gremlin-1 on viability and in vitro BMP-2-induced osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (MSCs). Gremlin-1-specific small interfering RNA (siRNA) inhibited gremlin-1 mRNA and protein expression in human MSCs. The mRNA expression levels of osteoblastic genes were analyzed using reverse transcription-quantitative polymerase chain reaction, and calcification and enzymatic alkaline phosphatase (ALP) activity assessed the BMP-2-induced osteogenic differentiation of human MSCs. The results indicated that gremlin-1 suppression significantly increased human MSC metabolism and DNA content. The expression levels of osteoblastic genes were also significantly increased by gremlin-1 inhibition. In the gremlin-1-inhibited group, enzymatic ALP activity was significantly increased. In addition, due to BMP-2-inducing osteoblasts, gremlin-1 inhibition increased calcium deposits. The present study indicated that gremlin-1 inhibited the cell viability and osteogenic differentiation of human MSCs and that the suppression of gremlin-1 expression suppressed can increase the cell viability and osteogenic differentiation of human MSCs induced by BMP-2.

Calcium-phosphate ceramics and polysaccharide-based hydrogel scaffolds combined with mesenchymal stem cell differently support bone repair in rats

Research in bone tissue engineering is focused on the development of alternatives to autologous bone grafts for bone reconstruction. Although multiple stem cell-based products and biomaterials are currently being investigated, comparative studies are rarely achieved to evaluate the most appropriate approach in this context. Here, we aimed to compare different clinically relevant bone tissue engineering methods and evaluated the kinetic repair and the bone healing efficiency supported by mesenchymal stem cells and two different biomaterials, a new hydrogel scaffold and a commercial hydroxyapatite/tricalcium phosphate ceramic, alone or in combination.Syngeneic mesenchymal stem cells (5 × 10⁵) and macroporous biphasic calcium phosphate ceramic granules (Calciresorb C35^®, Ceraver) or porous pullulan/dextran-based hydrogel scaffold were implanted alone or combined in a drilled-hole bone defect in rats. Using quantitative microtomography measurements and qualitative histological examinations, their osteogenic properties were evaluated 7, 30, and 90 days after implantation. Three months after surgery, only minimal repair was evidenced in control rats while newly mineralized bone was massively observed in animals treated with either hydrogels (bone volume/tissue volume = 20%) or ceramics (bone volume/tissue volume = 26%). Repair mechanism and resorption kinetics were strikingly different: rapidly-resorbed hydrogels induced a dense bone mineralization from the edges of the defect while ceramics triggered newly woven bone formation in close contact with the ceramic surface that remained unresorbed. Delivery of mesenchymal stem cells in combination with these biomaterials enhanced both bone healing (>20%) and neovascularization after 1 month, mainly in hydrogel.Osteogenic and angiogenic properties combined with rapid resorption make hydrogels a promising alternative to ceramics for bone repair by cell therapy.

Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation

Intermittently administered parathyroid hormone (PTH 1-34) has been shown to promote bone formation in both human and animal studies. The hormone and its analogues stimulate both bone formation and resorption, and as such at low doses are now in clinical use for the treatment of severe osteoporosis. By varying the duration of exposure, parathyroid hormone can modulate genes leading to increased bone formation within a so-called 'anabolic window'. The osteogenic mechanisms involved are multiple, affecting the stimulation of osteoprogenitor cells, osteoblasts, osteocytes and the stem cell niche, and ultimately leading to increased osteoblast activation, reduced osteoblast apoptosis, upregulation of Wnt/β-catenin signalling, increased stem cell mobilisation, and mediation of the RANKL/OPG pathway. Ongoing investigation into their effect on bone formation through 'coupled' and 'uncoupled' mechanisms further underlines the impact of intermittent PTH on both cortical and cancellous bone. Given the principally catabolic actions of continuous PTH, this article reviews the skeletal actions of intermittent PTH 1-34 and the mechanisms underlying its effect.

CITE THIS ARTICLE

L. Osagie-Clouard, A. Sanghani, M. Coathup, T. Briggs, M. Bostrom, G. Blunn. Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation. Bone Joint Res 2017;6:14-21. DOI: 10.1302/2046-3758.61.BJR-2016-0085.R1.

Cellularizing hydrogel-based scaffolds to repair bone tissue: How to create a physiologically relevant micro-environment?

Tissue engineering is a promising alternative to autografts or allografts for the regeneration of large bone defects. Cell-free biomaterials with different degrees of sophistication can be used for several therapeutic indications, to stimulate bone repair by the host tissue. However, when osteoprogenitors are not available in the damaged tissue, exogenous cells with an osteoblast differentiation potential must be provided. These cells should have the capacity to colonize the defect and to participate in the building of new bone tissue. To achieve this goal, cells must survive, remain in the defect site, eventually proliferate, and differentiate into mature osteoblasts. A critical issue for these engrafted cells is to be fed by oxygen and nutrients: the transient absence of a vascular network upon implantation is a major challenge for cells to survive in the site of implantation, and different strategies can be followed to promote cell survival under poor oxygen and nutrient supply and to promote rapid vascularization of the defect area. These strategies involve the use of scaffolds designed to create the appropriate micro-environment for cells to survive, proliferate, and differentiate in vitro and in vivo. Hydrogels are an eclectic class of materials that can be easily cellularized and provide effective, minimally invasive approaches to fill bone defects and favor bone tissue regeneration. Furthermore, by playing on their composition and processing, it is possible to obtain biocompatible systems with adequate chemical, biological, and mechanical properties. However, only a good combination of scaffold and cells, possibly with the aid of incorporated growth factors, can lead to successful results in bone regeneration. This review presents the strategies used to design cellularized hydrogel-based systems for bone regeneration, identifying the key parameters of the many different micro-environments created within hydrogels.

Mechanical Loading Attenuates Radiation-Induced Bone Loss in Bone Marrow Transplanted Mice

Exposure of bone to ionizing radiation, as occurs during radiotherapy for some localized malignancies and blood or bone marrow cancers, as well as during space travel, incites dose-dependent bone morbidity and increased fracture risk. Rapid trabecular and endosteal bone loss reflects acutely increased osteoclastic resorption as well as decreased bone formation due to depletion of osteoprogenitors. Because of this dysregulation of bone turnover, bone’s capacity to respond to a mechanical loading stimulus in the aftermath of irradiation is unknown. We employed a mouse model of total body irradiation and bone marrow transplantation simulating treatment of hematologic cancers, hypothesizing that compression loading would attenuate bone loss. Furthermore, we hypothesized that loading would upregulate donor cell presence in loaded tibias due to increased engraftment and proliferation. We lethally irradiated 16 female C57Bl/6J mice at age 16 wks with 10.75 Gy, then IV-injected 20 million GFP(+) total bone marrow cells. That same day, we initiated 3 wks compression loading (1200 cycles 5x/wk, 10 N) in the right tibia of 10 of these mice while 6 mice were irradiated, non-mechanically-loaded controls. As anticipated, before-and-after microCT scans demonstrated loss of trabecular bone (-48.2% Tb.BV/TV) and cortical thickness (-8.3%) at 3 wks following irradiation. However, loaded bones lost 31% less Tb.BV/TV and 8% less cortical thickness (both p<0.001). Loaded bones also had significant increases in trabecular thickness and tissue mineral densities from baseline. Mechanical loading did not affect donor cell engraftment. Importantly, these results demonstrate that both cortical and trabecular bone exposed to high-dose therapeutic radiation remain capable of an anabolic response to mechanical loading. These findings inform our management of bone health in cases of radiation exposure.

Neuropeptide Y accelerates post-fracture bone healing by promoting osteogenesis of mesenchymal stem cells

Fracture repair is a complex yet well orchestrated regenerative process involving numerous signaling and cell types including osteoblasts. Here we showed that NPY, a neurotransmitter with regulatory functions in bone homeostasis, may contribute to the post-fracture bone healing in patients with traumatic brain injury-fracture combined injuries. Our results suggested NPY levels were increased in patients with the combined injuries, accomplished by arising of bone healing markers, such as ALP, OC, PICP and ICTP, than in those with simple fractures, and NPY have direct actions on MSCs to promote their osteogenic differentiation. Our results provided clinical evidences for NPY participating in the bone healing process in a nonhypothalamic manner, most probably by directly promoting osteogenesis of mesenchymal stem cells.