Modifying MSC Phenotype to Facilitate Bone Healing: Biological Approaches

The m6A Reader YTHDF1 Accelerates the Osteogenesis of Bone Marrow Mesenchymal Stem Cells Partly via Activation of the Autophagy Signaling Pathway

N6-methyladenosine (m6A) mRNA methylation has emerged as an important player in many biological processes by regulating gene expression. As a crucial reader, YTHDF1 usually improves the translation efficiency of its target mRNAs. However, its roles in bone marrow mesenchymal stem cells (BMSCs) osteogenesis remain largely unknown. Here, we reported that YTHDF1, an m6A reader, is highly expressed during osteogenic differentiation of BMSCs. Upregulation of YTHDF1 increased osteogenic differentiation and proliferation capacity of BMSCs. Accordingly, downregulation of YTHDF1 inhibited osteogenic differentiation and proliferation capacity. Possible underlying mechanisms were explored, and analysis revealed that YTHDF1 could affect autophagy levels, thus regulating osteogenesis of BMSCs. In an in vivo study, we found that upregulation of YTHDF1 accelerates fracture healing with elevated bone volume fraction and trabecular thickness. Taken together, our study revealed that m6A reader YTHDF1 accelerates osteogenic differentiation of BMSCs partly via the autophagy signaling pathway. These findings reveal a previously unrecognized mechanism involved in the regulation of BMSCs osteogenesis, providing new ideas and target sites for the treatment of fracture.

RNA-binding Protein QKI Inhibits Osteogenic Differentiation Via Suppressing Wnt Pathway

BACKGROUND

Dysregulation of MSCs differentiation is associated with many pathophysiological processes. Genetically modified MSCs transplantation helps restore bone loss efficiently.

METHODS

BMSCs-specific QKI overexpressing and knockdown mice were built to explore QKI's role in bone formation and fat accumulation. Primary BMSCs with QKI overexpression and knockout were subjected to osteogenic and adipogenic differentiation. ALP staining and oil red O staining were performed to evaluate the differences between the groups. RNA immunoprecipitation was performed to identify the QKI-related pathway. QKI deficient BMSCs were transplanted into mice with glucocorticoid-induced osteoporosis to evaluate its therapeutic potential.

RESULTS

Mice harboring BMSC-specific transgenic QKI exhibited reduced bone mass, while BMSC-specific QKI-deficient mice showed an increase in bone mass. Osteogenic differentiation of QKI deficient BMSCs was promoted and adipogenic differentiation was inhibited, while QKI overexpression in BMSCs displayed the opposite effects. To define the underlying mechanisms, RIP sequencing was performed. Wnt pathway-related genes were the putative direct target mRNAs of QKI, Canonical Wnt pathway activation was involved in QKI's effects on osteogenic differentiation. RNA immunoprecipitation quantitative real-time Polymerase Chain Reaction (PCR) and RNA fluorescence in situ hybridization experiments further validated that QKI repressed the expressions of Wnt5b, Fzd7, Dvl3 and β-catenin via direct binding to their putative mRNA specific sites. Glucocorticoid-induced osteoporotic mice transplanted with QKI deficient BMSCs exhibited less bone loss compared with mice transplanted with control BMSCs.

CONCLUSIONS

QKI suppressed BMSCs osteogenic differentiation by downregulating the expressions of Wnt5b, Fzd7, Dvl3 and β-catenin. Loss of QKI in BMSCs transplantation may provide a new strategy for the treatment of orthopedic diseases such as osteoporosis.

The emerging studies on mesenchymal progenitors in the long bone

Mesenchymal progenitors (MPs) are considered to play vital roles in bone development, growth, bone turnover, and repair. In recent years, benefiting from advanced approaches such as single-cell sequence, lineage tracing, flow cytometry, and transplantation, multiple MPs are identified and characterized in several locations of bone, including perichondrium, growth plate, periosteum, endosteum, trabecular bone, and stromal compartment. However, although great discoveries about skeletal stem cells (SSCs) and progenitors are present, it is still largely obscure how the varied landscape of MPs from different residing sites diversely contribute to the further differentiation of osteoblasts, osteocytes, chondrocytes, and other stromal cells in their respective destiny sites during development and regeneration. Here we discuss recent findings on MPs' origin, differentiation, and maintenance during long bone development and homeostasis, providing clues and models of how the MPs contribute to bone development and repair.

The role of immune cells in modulating chronic inflammation and osteonecrosis

Osteonecrosis occurs when, under continuous stimulation by adverse factors such as glucocorticoids or alcohol, the death of local bone and marrow cells leads to abnormal osteoimmune function. This creates a chronic inflammatory microenvironment, which interferes with bone regeneration and repair. In a variety of bone tissue diseases, innate immune cells and adaptive immune cells interact with bone cells, and their effects on bone metabolic homeostasis have attracted more and more attention, thus developing into a new discipline - osteoimmunology. Immune cells are the most important regulator of inflammation, and osteoimmune disorder may be an important cause of osteonecrosis. Elucidating the chronic inflammatory microenvironment regulated by abnormal osteoimmune may help develop potential treatments for osteonecrosis. This review summarizes the inflammatory regulation of bone immunity in osteonecrosis, explains the pathophysiological mechanism of osteonecrosis from the perspective of osteoimmunology, and provides new ideas for the treatment of osteonecrosis.

Modification of mesenchymal stem cells for cartilage-targeted therapy

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by the destruction of the articular cartilage, sclerosis of the subchondral bone, and joint dysfunction. Its pathogenesis is attributed to direct damage and mechanical destruction of joint tissues. Mesenchymal stem cells (MSCs), suggested as a potential strategy for the treatment of OA, have shown therapeutic effects on OA. However, the specific fate of MSCs after intraarticular injection, including cell attachment, proliferation, differentiation, and death, is still unclear, and there is no guarantee that stem cells can be retained in the cartilage tissue to enact repair. Direct homing of MSCs is an important determinant of the efficacy of MSC-based cartilage repair. Recent studies have revealed that the unique homing capacity of MSCs and targeted modification can improve their ability to promote tissue regeneration. Here, we comprehensively review the homing effect of stem cells in joints and highlight progress toward the targeted modification of MSCs. In the future, developments of this targeting system that accelerate tissue regeneration will benefit targeted tissue repair.

Recent advances to enhance the immunomodulatory potential of mesenchymal stem cells

Considering the unique therapeutic potential of mesenchymal stem cells (MSCs), including their immunosuppressive and immunomodulatory properties as well as their ability to improve tissue regeneration, these cells have attracted the attention of scientists and clinicians for the treatment of different inflammatory and immune system mediated disorders. However, various clinical trials using MSCs for the therapeutic purpose are conflicting and differ from the results of promising preclinical studies. This inconsistency is caused by several factors such as poor migration and homing capacities, low survival rate, low level of proliferation and differentiation, and donor-dependent variation of the cells. Enhancement and retention of persistent therapeutic effects of the cells remain a challenge to overcome in MSC-based therapy. In this review, we summarized various approaches to enhance the clinical outcomes of MSC-based therapy as well as revised current and future perspectives for the creation of cellular products with improved potential for diverse clinical applications.

Effects of rmBMP-7 on Osteoblastic Cells Grown on a Nanostructured Titanium Surface

This study evaluates the effects of the availability of exogenous BMP-7 on osteoblastic cells’ differentiation on a nanotextured Ti surface obtained by chemical etching (Nano-Ti). The MC3T3-E1 and UMR-106 osteoblastic cell lines were cultured for 5 and 7 days, respectively, on a Nano-Ti surface and on a control surface (Control-Ti) in an osteogenic medium supplemented with either 40 or 200 ng/mL recombinant mouse (rm) BMP-7. The results showed that MC3T3-E1 cells exhibited distinct responsiveness when exposed to each of the two rmBMP-7 concentrations, irrespective of the surface. Even with 40 ng/mL rmBMP-7, important osteogenic effects were noticed for Control-Ti in terms of cell proliferation potential; Runx2, Osx, Alp, Bsp, Opn, and Smad1 mRNA expression; and in situ ALP activity. For Nano-Ti, the effects were limited to higher Alp, Bsp, and Opn mRNA expression and in situ ALP activity. On both surfaces, the osteogenic potential of UMR-106 cultures remained unaltered with 40 ng/mL rmBMP-7, but it was significantly reduced when the cultures were exposed to the 200 ng/mL concentration. The availability of rmBMP-7 to pre-osteoblastic cells at the concentrations used alters the expression profile of osteoblast markers, indicative of the acquisition of a more advanced stage of osteoblastic differentiation. This occurs less pronouncedly on the nanotextured Ti and without reflecting in higher mineralized matrix production by differentiated osteoblasts on both surfaces.

Hypoxia-Preconditioned Extracellular Vesicles from Mesenchymal Stem Cells Improve Cartilage Repair in Osteoarthritis

In the past decade, mesenchymal stem cells (MSCs) have been widely used for the treatment of osteoarthritis (OA), and extracellular vesicles (EVs) may play a major role in the efficacy of this treatment. Hypoxia can change the cargo and biological functions of MSC-derived EVs (MSC-EVs). The aim of the present study was to determine whether the effects of hypoxia-preconditioned MSC-EVs on OA cartilage repair are superior to normoxia-preconditioned MSC-EVs. By using in vitro and in vivo OA models, we verified that hypoxia-preconditioned MSC-EVs improved chondrocyte proliferation and migration and suppressed chondrocyte apoptosis to a greater extent than normoxia-preconditioned MSC-EVs. Furthermore, we found that hypoxia altered the microRNA expression in MSC-EVs and identified four differentially expressed microRNAs: hsa-miR-181c-5p, hsa-miR-18a-3p, hsa-miR-376a-5p, and hsa-miR-337-5p. Bioinformatics analysis revealed that hypoxic pretreatment may promote cartilage repair by stimulating chondrocyte proliferation and migration and suppressing chondrocyte apoptosis through the miRNA-18-3P/JAK/STAT or miRNA-181c-5p/MAPK signaling pathway. Therefore, hypoxia-preconditioned EVs may be a novel treatment for OA.

Regulation of the mesenchymal stem cell fate by interleukin-17: Implications in osteogenic differentiation

Bone regeneration is a tightly regulated process that ensures proper repair and functionality after injury. The delicate balance between bone formation and resorption is governed by cytokines and signaling molecules released during the inflammatory response. Interleukin (IL)-17A, produced in the early phase of inflammation, influences the fate of osteoprogenitors. Due to their inherent capacity to differentiate into osteoblasts, mesenchymal stem/stromal cells (MSCs) contribute to bone healing and regeneration. This review presents an overview of IL-17A signaling and the leading cellular and molecular mechanisms by which it regulates the osteogenic differentiation of MSCs. The main findings demonstrating IL-17A’s influence on osteoblastogenesis are described. To this end, divergent information exists about the capacity of IL-17A to regulate MSCs’ osteogenic fate, depending on the tissue context and target cell type, along with contradictory findings in the same cell types. Therefore, we summarize the data showing both the pro-osteogenic and anti-osteogenic roles of IL-17, which may help in the understanding of IL-17A function in bone repair and regeneration.

The Crosstalk between Mesenchymal Stem Cells and Macrophages in Bone Regeneration: A Systematic Review

Bone regeneration is a complex and well-coordinated process that involves crosstalk between immune cells and resident cells in the injury site. Transplantation of mesenchymal stem cells (MSCs) is a promising strategy to enhance bone regeneration. Growing evidence suggests that macrophages have a significant impact on osteogenesis during bone regeneration. However, the precise mechanisms by which macrophage subtypes influence bone regeneration and how MSCs communicate with macrophages have not yet been fully elucidated. In this systematic literature review, we gathered evidence regarding the crosstalk between MSCs and macrophages during bone regeneration. According to the PRISMA protocol, we extracted literature from PubMed and Embase databases by using "mesenchymal stem cells" and "macrophages" and "bone regeneration" as keywords. Thirty-three studies were selected for this review. MSCs isolated from both bone marrow and adipose tissue and both primary macrophages and macrophage cell lines were used in the selected studies. In conclusion, anti-inflammatory macrophages (M2) have significantly more potential to strengthen bone regeneration compared with naïve (M0) and classically activated macrophages (M1). Transplantation of MSCs induced M1-to-M2 transition and transformed the skeletal microenvironment to facilitate bone regeneration in bone fracture and bone defect models. This review highlights the complexity between MSCs and macrophages, providing more insight into the polarized macrophage behavior in this evolving field of osteoimmunology. The results may serve as a useful reference for definite success in MSC-based therapy based on the critical interaction with macrophages.

Apoptotic mesenchymal stromal cells support osteoclastogenesis while inhibiting multinucleated giant cells formation in vitro

In bone regeneration induced by the combination of mesenchymal stromal cells (MSCs) and calcium-phosphate (CaP) materials, osteoclasts emerge as a pivotal cell linking inflammation and bone formation. Favorable outcomes are observed despite short-term engraftments of implanted MSCs, highlighting their major paracrine function and the possible implication of cell death in modulating their secretions. In this work, we focused on the communication from MSCs towards osteoclasts-like cells in vitro. MSCs seeded on a CaP biomaterial or undergoing induced apoptosis produced a conditioned media favoring the development of osteoclasts from human CD14+ monocytes. On the contrary, MSCs’ apoptotic secretion inhibited the development of inflammatory multinucleated giant cells formed after IL-4 stimulation. Components of MSCs’ secretome before and after apoptotic stress were compared using mass spectrometry-based quantitative proteomics and a complementary immunoassay for major cytokines. CXCR-1 and CXCR-2 ligands, primarily IL-8/CXCL-8 but also the growth-regulated proteins CXCL-1, -2 or -3, were suggested as the major players of MSCs’ pro-osteoclastic effect. These findings support the hypothesis that osteoclasts are key players in bone regeneration and suggest that apoptosis plays an important role in MSCs’ effectiveness.

Bone fracture healing: perspectives according to molecular basis

Fractures have a great impact on health all around the world and with fracture healing optimization; this problem could be resolved partially. To make a practical contribution to this issue, the knowledge of bone tissue, cellularity, and metabolism is essential, especially cytoskeletal architecture and its transformations according to external pressures. Special physical and chemical characteristics of the extracellular matrix (ECM) allow the transmission of mechanical stimuli from outside the cell to the plasmatic membrane. The osteocyte cytoskeleton is conformed by a complex network of actin and microtubules combined with crosslinker proteins like vinculin and fimbrin, connecting and transmitting outside stimuli through EMC to cytoplasm. Herein, critical signaling pathways like Cx43-depending ones, MAPK/ERK, Wnt, YAP/TAZ, Rho-ROCK, and others are activated due to mechanical stimuli, resulting in osteocyte cytoskeletal changes and ECM remodeling, altering the tissue and, therefore, the bone. In recent years, the osteocyte has gained more interest and value in relation to bone homeostasis as a great coordinator of other cell populations, thanks to its unique functions. By integrating the latest advances in relation to intracellular signaling pathways, mechanotransmission system of the osteocyte and bone tissue engineering, there are promising experimental strategies, while some are ready for clinical trials. This work aims to show clearly and precisely the integration between cytoskeleton and main molecular pathways in relation to mechanotransmission mechanism in osteocytes, and the use of this theoretical knowledge in therapeutic tools for bone fracture healing.

Phosphorylation inhibition of protein-tyrosine phosphatase 1B tyrosine-152 induces bone regeneration coupled with angiogenesis for bone tissue engineering

A close relationship has been reported to exist between cadherin-mediated cell–cell adhesion and integrin-mediated cell mobility, and protein tyrosine phosphatase 1B (PTP1B) may be involved in maintaining this homeostasis. The stable residence of mesenchymal stem cells (MSCs) and endothelial cells (ECs) in their niches is closely related to the regulation of PTP1B. However, the exact role of the departure of MSCs and ECs from their niches during bone regeneration is largely unknown. Here, we show that the phosphorylation state of PTP1B tyrosine-152 (Y152) plays a central role in initiating the departure of these cells from their niches and their subsequent recruitment to bone defects. Based on our previous design of a PTP1B Y152 region-mimicking peptide (152RM) that significantly inhibits the phosphorylation of PTP1B Y152, further investigations revealed that 152RM enhanced cell migration partly via integrin αvβ3 and promoted MSCs osteogenic differentiation partly by inhibiting ATF3. Moreover, 152RM induced type H vessels formation by activating Notch signaling. Demineralized bone matrix (DBM) scaffolds were fabricated with mesoporous silica nanoparticles (MSNs), and 152RM was then loaded onto them by electrostatic adsorption. The DBM-MSN/152RM scaffolds were demonstrated to induce bone formation and type H vessels expansion in vivo. In conclusion, our data reveal that 152RM contributes to bone formation by coupling osteogenesis with angiogenesis, which may offer a potential therapeutic strategy for bone defects.

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PTP1B plays a dual regulatory role in cadherin- and integrin-related pathways.

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Inhibition of PTP1B Y152 phosphorylation enhances the departure of MSCs from the stem cell niche.

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DBM-MSN/152RM scaffolds coordinate the recruitment of MSCs and ECs.

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DBM-MSN/152RM scaffolds promote bone regeneration and angiogenesis in bone defects.

Inflammation, Bone Healing and Osteonecrosis: From Bedside to Bench

Osteonecrosis of the epiphyseal and metaphyseal regions of major weight-bearing bones of the extremities is a condition that is associated with local death of bone cells and marrow in the afflicted compartment. Chronic inflammation is a prominent feature of osteonecrosis. If the persistent inflammation is not resolved, this process will result in progressive collapse and subsequent degenerative arthritis. In the pre-collapse stage of osteonecrosis, attempt at joint preservation rather than joint replacement in this younger population with osteonecrosis is a major clinical objective. In this regard, core decompression, with/without local injection of bone marrow aspirate concentrate (BMAC), is an accepted and evidence-based method to help arrest the progression and improve the outcome of early-stage osteonecrosis. However, some patients do not respond favorably to this treatment. Thus, it is prudent to consider strategies to mitigate chronic inflammation concurrent with addressing the deficiencies in osteogenesis and vasculogenesis in order to save the affected joint. Interestingly, the processes of inflammation, osteonecrosis, and bone healing are highly inter-related. Therefore, modulating the biological processes and crosstalk among cells of the innate immune system, the mesenchymal stem cell-osteoblast lineage and others are important to providing the local microenvironment for resolution of inflammation and subsequent repair. This review summarizes the clinical and biologic principles associated with osteonecrosis and provides potential cutting-end strategies for modulating chronic inflammation and facilitating osteogenesis and vasculogenesis using local interventions. Although these studies are still in the preclinical stages, it is hoped that safe, efficacious, and cost-effective interventions will be developed to save the host’s natural joint.

Ceramic Scaffolds in a Vacuum Suction Handle for Intraoperative Stromal Cell Enrichment

During total joint replacement, high concentrations of mesenchymal stromal cells (MSCs) are released at the implantation site. They can be found in cell–tissue composites (CTC) that are regularly removed by surgical suction. A surgical vacuum suction handle was filled with bone substitute granules, acting as a filter allowing us to harvest CTC. The purpose of this study was to investigate the osteopromotive potential of CTC trapped in the bone substitute filter material during surgical suction. In the course of 10 elective total hip and knee replacement surgeries, β-tricalcium-phosphate (TCP) and cancellous allograft (Allo) were enriched with CTC by vacuum suction. Mononuclear cells (MNC) were isolated from the CTC and investigated towards cell proliferation and colony forming unit (CFU) formation. Furthermore, MSC surface markers, trilineage differentiation potential and the presence of defined cytokines were examined. Comparable amounts of MNC and CFUs were detected in both CTCs and characterized as MSC‰ of MNC with 9.8 ± 10.7‰ for the TCP and 12.8 ± 10.2‰ for the Allo (p = 0.550). CTCs in both filter materials contain cytokines for stimulation of cell proliferation and differentiation (EGF, PDGF-AA, angiogenin, osteopontin). CTC trapped in synthetic (TCP) and natural (Allo) bone substitute filters during surgical suction in the course of a joint replacement procedure include relevant numbers of MSCs and cytokines qualified for bone regeneration.