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

Bioactive Porous Composite Implant Guides Mesenchymal Stem Cell Differentiation and Migration to Accelerate Bone Reconstruction

Background

Delayed healing and non-healing of bone defects pose significant challenges in clinical practice, with metal materials increasingly recognized for their significance in addressing these issues. Among these materials, Strontium (Sr) and Zinc (Zn) have emerged as promising agents for promoting bone repair. Building upon this insight, this research evaluates the impact of a porous Sr@Zn@SiO2 nanocomposite implant on bone regeneration, aiming to advance the field of bone repair.

Methods

The preparation of the Sr@Zn@SiO2 composite implant involves various techniques such as roasting, centrifugation, and washing. The material's composition is examined, and its microstructure and element distribution are analyzed using TEM and elemental scanning technology. In vitro experiments entail the isolation and characterization of BMSCs followed by safety assessments of the implant material, evaluation of cell migration capabilities, and relevant proliferation markers. Mechanistically, this study delves into key targets associated with significant changes in the osteogenic process. In vivo experiments involve establishing a rat femur bone defect model, followed by assessment of the osteogenic potential of Sr@Zn@SiO2 using Micro-CT imaging and tissue section staining.

Results

Through in vivo and in vitro investigations, we validate the osteogenic efficacy of the Sr@Zn@SiO2 composite implant. In vitro analyses demonstrate that porous Sr@Zn@SiO2 nanocomposite materials upregulate BMP-2 expression, leading to the activation of Smad1/5/9 phosphorylation and subsequent activation of downstream osteogenic genes, culminating in BMSCs osteogenic differentiation and bone proliferation. And the migration of BMSCs is closely related to the high expression of CXCL12/CXCR4, which will also provide the conditions for osteogenesis. In vivo, the osteogenic ability of Sr@Zn@SiO2 was also confirmed in rats.

Conclusion

In our research, the porous Sr@Zn@SiO2 composite implant displays prominent osteogenic effect and promotes the migration and differentiation of BMSCs to promote bone defect healing. This bioactive implant has surgical application potential in the future.

A GCaMP reporter mouse with chondrocyte specific expression of a green fluorescent calcium indicator

One of the major processes occurring during the healing of a fractured long bone is chondrogenesis, leading to the formation of the soft callus, which subsequently undergoes endochondral ossification and ultimately bridges the fracture site. Thus, understanding the molecular mechanisms of chondrogenesis can enhance our knowledge of the fracture repair process. One such molecular process is calciun (Ca++) signaling, which is known to play a critical role in the development and regeneration of multiple tissues, including bone, in response to external stimuli. Despite the existence of various mouse models for studying Ca++ signaling, none of them were designed to specifically examine the skeletal system or the various musculoskeletal cell types. As such, we generated a genetically engineered mouse model that is specific to cartilage (crossed with Col2a1 Cre mice) to study chondrocytes. Herein, we report on the characterization of this transgenic mouse line using conditional expression of GCaMP6f, a Ca++-indicator protein. Specifically, this mouse line exhibits increased GCaMP6f fluorescence following Ca++ binding in chondrocytes. Using this model, we show real-time Ca++ signaling in embryos, newborn and adult mice, as well as in fracture calluses. Further, robust expression of GCaMP6f in chondrocytes can be easily detected in embryos, neonates, adults, and fracture callus tissue sections. Finally, we also report on Ca++ signaling pathway gene expression, as well as real-time Ca++ transient measurements in fracture callus chondrocytes. Taken together, these mice provide a new experimental tool to study chondrocyte-specific Ca++ signaling during skeletal development and regeneration, as well as various in vitro perturbations.

Activation of Wnt signaling in human fracture callus and nonunion tissues

The Wnt signaling pathway is a key molecular process during fracture repair. Although much of what we now know about the role of this pathway in bone is derived from in vitro and animal studies, the same cannot be said about humans. As such, we hypothesized that Wnt signaling will also be a key process in humans during physiological fracture healing as well as in the development of a nonunion (hypertrophic and oligotrophic). We further hypothesized that the expression of Wnt-signaling pathway genes/proteins would exhibit a differential expression pattern between physiological fracture callus and the pathological nonunion tissues. We tested these two hypotheses by examining the mRNA levels of key Wnt-signaling related genes: ligands (WNT4, WNT10a), receptors (FZD4, LRP5, LRP6), inhibitors (DKK1, SOST) and modulators (CTNNB1 and PORCN). RNA sequencing from calluses as well as from the two nonunion tissue types, revealed that all of these genes were expressed at about the same level in these three tissue types. Further, spatial expression experiments identified the cells responsible of producing these proteins. Robust expression was detected in osteoblasts for the majority of these genes except SOST which displayed low expression, but in contrast, was mostly detected in osteocytes. Many of these genes were also expressed by callus chondrocytes as well. Taken together, these results confirm that Wnt signaling is indeed active during both human physiological fracture healing as well as in pathological nonunions.

Psychostimulants prescribed to children for ADHD following distal radius fractures significantly reduce bone density as a function of duration

Methylphenidate and mixed amphetamine salts (MAS) are psychostimulant medications widely prescribed for various psychiatric disorders. Although these medications are known to adversely impact bone mineral content and density, as well as biomechanical integrity during skeletal development in rats, their effect on bone density in children remains largely unknown. The primary aim of this work was to investigate the effects of methylphenidate and MAS on bone density following distal radius fractures in pediatric populations, and secondarily assess any impact on healing. The retrospective case-control study was designed to assess fracture healing in patients treated with stimulant drugs and matched controls. For the primary outcome, X-rays ( n  = 188) were evaluated using an optical density image analysis technique to compare bone density throughout the bone healing process. Results showed that methylphenidate and MAS significantly reduced bone healing by approximately 20% following distal radius fractures in these children. The data also suggested that duration of psychostimulant use played a role in bone healing; the longer the treatment (1-5 years), the lower the bone density was observed (by approximately 52%) as compared to controls (no medication). However, subjects taking these drugs for longer than 5 years did not show a significant difference. Our results suggested that children taking psychostimulants for up to 5 years had slower bone healing following distal radius fractures. Orthopedic surgeons planning elective surgeries should be cognizant of this as a potential issue in recovery after any elective bone procedures and preoperatively optimize bone health as well as counsel patients and their families.

Vascular restoration through local delivery of angiogenic factors stimulates bone regeneration in critical size defects

Critical size bone defects represent a significant challenge worldwide, often leading to persistent pain and physical disability that profoundly impact patients' quality of life and mental well-being. To address the intricate and complex repair processes involved in these defects, we performed single-cell RNA sequencing and revealed notable shifts in cellular populations within regenerative tissue. Specifically, we observed a decrease in progenitor lineage cells and endothelial cells, coupled with an increase in fibrotic lineage cells and pro-inflammatory cells within regenerative tissue. Furthermore, our analysis of differentially expressed genes and associated signaling pathway at the single-cell level highlighted impaired angiogenesis as a central pathway in critical size bone defects, notably influenced by reduction of Spp1 and Cxcl12 expression. This deficiency was particularly pronounced in progenitor lineage cells and myeloid lineage cells, underscoring its significance in the regeneration process. In response to these findings, we developed an innovative approach to enhance bone regeneration in critical size bone defects. Our fabrication process involves the integration of electrospun PCL fibers with electrosprayed PLGA microspheres carrying Spp1 and Cxcl12. This design allows for the gradual release of Spp1 and Cxcl12 in vitro and in vivo. To evaluate the efficacy of our approach, we locally applied PCL scaffolds loaded with Spp1 and Cxcl12 in a murine model of critical size bone defects. Our results demonstrated restored angiogenesis, accelerated bone regeneration, alleviated pain responses and improved mobility in treated mice.

Human nonunion tissues display differential gene expression in comparison to physiological fracture callus

The healing of bone fractures can become aberrant and lead to nonunions which in turn have a negative impact on patient health. Understanding why a bone fails to normally heal will enable us to make a positive impact in a patient's life. While we have a wealth of molecular data on rodent models of fracture repair, it is not the same with humans. As such, there is still a lack of information regarding the molecular differences between normal physiological repair and nonunions. This study was designed to address this gap in our molecular knowledge of the human repair process by comparing differentially expressed genes (DEGs) between physiological fracture callus and two different nonunion types, hypertrophic (HNU) and oligotrophic (ONU). RNA sequencing data revealed over ∼18,000 genes in each sample. Using the physiological callus as the control and the nonunion samples as the experimental groups, bioinformatic analyses identified 67 and 81 statistically significant DEGs for HNU and ONU, respectively. Out of the 67 DEGs for the HNU, 34 and 33 were up and down-regulated, respectively. Similarly, out of the 81 DEGs for the ONU, 48 and 33 were up and down-regulated, respectively. Additionally, we also identified common genes between the two nonunion samples; 8 (10.8 %) upregulated and 12 (22.2 %) downregulated. We further identified many biological processes, with several statistically significant ones. Some of these were related to muscle and were common between the two nonunion samples. This study represents the first comprehensive attempt to understand the global molecular events occurring in human nonunion biology. With further research, we can perhaps decipher new molecular pathways involved in aberrant healing of human bone fractures that can be therapeutically targeted.

Zinc finger protein 36 like 2-histone deacetylase 1 axis is involved in the bone responses to mechanical stress

The molecular mechanism underlying the promotion of fracture healing by mechanical stimuli remains unclear. The present study aimed to investigate the role of zinc finger protein 36 like 2 (ZFP36L2)-histone deacetylase 1 (HDAC1) axis on the osteogenic responses to moderate mechanical stimulation. Appropriate stimulation of fluid shear stress (FSS) was performed on MC3T3-E1 cells transduced with ZFP36L2 and HDAC1 recombinant adenoviruses, aiming to validate the influence of mechanical stress on the expression of ZFP36L2-HDAC1 and the osteogenic differentiation and mineralization. The results showed that moderate FSS stimulation significantly upregulated the expression of ZFP36L2 in MC3T3-E1 cells (p < 0.01). The overexpression of ZFP36L1 markedly enhanced the levels of osteogenic differentiation markers, including bone morphogenetic protein 2 (BMP2), runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), Osterix, and collagen type I alpha 1 (COL1A1) (p < 0.01). ZFP36L2 accelerated the degradation of HDAC1 by specifically binding to its 3' UTR region, thereby fulfilling its function at the post-transcriptional regulatory gene level and promoting the osteogenic differentiation and mineralization fate of cells. Mechanical unloading notably diminished/elevated the expression of ZFP36L2/HDAC1, decreased bone mineral density and bone volume fraction, hindered the release of osteogenic-related factors and vascular endothelial growth factor in callus tissue (p < 0.01), and was detrimental to fracture healing. Collectively, proper stress stimulation plays a crucial role in facilitating osteogenesis through the promotion of ZFP36L2 and subsequent degradation of HDAC1. Targeting ZFP36L2-HDAC1 axis may provide promising insights to enhance bone defect healing.

Tenascin-C promotes endochondral ossification and fracture healing through Hedgehog and Hippo signaling

Fractures are frequent and severe musculoskeletal injuries. This study aimed to investigate the function of tenascin-C (TNC) in regulating chondrogenic during fracture healing and elucidate the underlying molecular mechanisms. A well-established femur fracture model in male C57BL/6J mice was used to transect the middle diaphysis of the femur. To identify the essential role of TNC, shTNC lentiviruses or TNC protein were administered in the animal model. Micro-CT analysis, histologic analysis, immunostaining assays, and gene expression analysis were employed to investigate the effect of TNC during fracture healing. An in vitro mesenchymal stem cell culture system was developed to investigate the role and molecular mechanism of TNC in regulating chondrogenesis. TNC expression was induced at the inflammatory phase and peaked at the cartilaginous callus phase during fracture healing. Knockdown of TNC expression in callus results in decreased callus formation and impaired fracture healing. Conversely, administration of exogenous TNC promoted chondrogenic differentiation, cartilage template formation and ultimately improved fracture healing. Both the Hedgehog and Hippo signaling pathways were found to be involved in the pro-chondrogenic function of TNC. Our observations demonstrate that TNC is a crucial factor responsible for endochondral ossification in fracture healing and provide a potential therapeutic strategy for promoting fracture healing.

Dnmt3b ablation affects fracture repair process by regulating apoptosis

PURPOSE

Previous studies have shown that DNA methyltransferase 3b (Dnmt3b) is the only Dnmt responsive to fracture repair and Dnmt3b ablation in Prx1-positive stem cells and chondrocyte cells both delayed fracture repair. Our study aims to explore the influence of Dnmt3b ablation in Gli1-positive stem cells in fracture healing mice and the underlying mechanism.

METHODS

We generated Gli1-CreERT2; Dnmt3bflox/flox (Dnmt3bGli1ER) mice to operated tibia fracture. Fracture callus tissues of Dnmt3bGli1ER mice and control mice were collected and analyzed by X-ray, micro-CT, biomechanical testing, histopathology and TUNEL assay.

RESULTS

The cartilaginous callus significantly decrease in ablation of Dnmt3b in Gli1-positive stem cells during fracture repair. The chondrogenic and osteogenic indicators (Sox9 and Runx2) in the fracture healing tissues in Dnmt3bGli1ER mice much less than control mice. Dnmt3bGli1ER mice led to delayed bone callus remodeling and decreased biomechanical properties of the newly formed bone during fracture repair. Both the expressions of Caspase-3 and Caspase-8 were upregulated in Dnmt3bGli1ER mice as well as the expressions of BCL-2.

CONCLUSIONS

Our study provides an evidence that Dnmt3b ablation Gli1-positive stem cells can affect fracture healing and lead to poor fracture healing by regulating apoptosis to decrease chondrocyte hypertrophic maturation.

DNA methylation-mediated Rbpjk suppression protects against fracture nonunion caused by systemic inflammation

Challenging skeletal repairs are frequently seen in patients experiencing systemic inflammation. To tackle the complexity and heterogeneity of the skeletal repair process, we performed single-cell RNA sequencing and revealed that progenitor cells were one of the major lineages responsive to elevated inflammation and this response adversely affected progenitor differentiation by upregulation of Rbpjk in fracture nonunion. We then validated the interplay between inflammation (via constitutive activation of Ikk2, Ikk2ca) and Rbpjk specifically in progenitors by using genetic animal models. Focusing on epigenetic regulation, we identified Rbpjk as a direct target of Dnmt3b. Mechanistically, inflammation decreased Dnmt3b expression in progenitor cells, consequently leading to Rbpjk upregulation via hypomethylation within its promoter region. We also showed that Dnmt3b loss-of-function mice phenotypically recapitulated the fracture repair defects observed in Ikk2ca-transgenic mice, whereas Dnmt3b-transgenic mice alleviated fracture repair defects induced by Ikk2ca. Moreover, Rbpjk ablation restored fracture repair in both Ikk2ca mice and Dnmt3b loss-of-function mice. Altogether, this work elucidates a common mechanism involving a NF-κB/Dnmt3b/Rbpjk axis within the context of inflamed bone regeneration. Building on this mechanistic insight, we applied local treatment with epigenetically modified progenitor cells in a previously established mouse model of inflammation-mediated fracture nonunion and showed a functional restoration of bone regeneration under inflammatory conditions through an increase in progenitor differentiation potential.

Collagen-Coated Hyperelastic Bone Promotes Osteoblast Adhesion and Proliferation

Successfully reconstructing bone and restoring its dynamic function represents a significant challenge for medicine. Critical size defects (CSDs), resulting from trauma, tumor removal, or degenerative conditions, do not naturally heal and often require complex bone grafting. However, these grafts carry risks, such as tissue rejection, infections, and surgical site damage, necessitating the development of alternative treatments. Three-dimensional and four-dimensional printed synthetic biomaterials represent a viable alternative, as they carry low production costs and are highly reproducible. Hyperelastic bone (HB), a biocompatible synthetic polymer consisting of 90% hydroxyapatite and 10% poly(lactic-co-glycolic acid, PLGA), was examined for its potential to support cell adhesion, migration, and proliferation. Specifically, we seeded collagen-coated HB with MG-63 human osteosarcoma cells. Our analysis revealed robust cell adhesion and proliferation over 7 days in vitro, with cells forming uniform monolayers on the external surface of the scaffold. However, no cells were present on the core of the fibers. The cells expressed bone differentiation markers on days 3 and 5. By day 7, the scaffold began to degrade, developing microscopic fissures and fragmentation. In summary, collagen-coated HB scaffolds support cell adhesion and proliferation but exhibit reduced structural support after 7 days in culture. Nevertheless, the intricate 3D architecture holds promise for cellular migration, vascularization, and early osteogenesis.

Cellular senescence in skeletal disease: mechanisms and treatment

The musculoskeletal system supports the movement of the entire body and provides blood production while acting as an endocrine organ. With aging, the balance of bone homeostasis is disrupted, leading to bone loss and degenerative diseases, such as osteoporosis, osteoarthritis, and intervertebral disc degeneration. Skeletal diseases have a profound impact on the motor and cognitive abilities of the elderly, thus creating a major challenge for both global health and the economy. Cellular senescence is caused by various genotoxic stressors and results in permanent cell cycle arrest, which is considered to be the underlying mechanism of aging. During aging, senescent cells (SnCs) tend to aggregate in the bone and trigger chronic inflammation by releasing senescence-associated secretory phenotypic factors. Multiple signalling pathways are involved in regulating cellular senescence in bone and bone marrow microenvironments. Targeted SnCs alleviate age-related degenerative diseases. However, the association between senescence and age-related diseases remains unclear. This review summarises the fundamental role of senescence in age-related skeletal diseases, highlights the signalling pathways that mediate senescence, and discusses potential therapeutic strategies for targeting SnCs.

The Effect of the Hip Flexion Angle in Osteonecrosis of the Femoral Head Based on China-Japan Friendship Hospital Classification - A Finite Element Study

OBJECTIVE

The alteration in the mechanical environment of the necrotic area is the primary cause of the collapse observed in osteonecrosis of the femoral head (ONFH). This study aims to evaluate the biomechanical implications of the China-Japan Friendship Hospital (CJFH) classification system and hip flexion angles on the necrotic area in ONFH using finite element analysis (FEA). The goal is to provide valuable guidance for hip preservation treatments and serve as a reference for clinical diagnosis and therapeutic interventions.

METHODS

Hip tomography CT scan data from a healthy volunteer was used to create a 3D model of the left hip. The model was preprocessed and imported into Solidworks 2018, based on the CJFH classification. Material parameters and boundary conditions were applied to each fractal model in ANSYS 21.0. Von Mises stresses were calculated, and maximum deformation values were obtained to evaluate the biomechanical effects of the load on the necrotic area and post-necrotic femur, as well as assess each fractal model's collapse risk.

RESULTS

(1) At the same hip flexion angle, maximum deformation followed this order: M Type < C Type < L Type. The L3 type necrotic area experienced the most significant deformation at 0, 60, and 110° angles (1.121, 1.7913, and 1.8239 mm respectively). (2) Under the same CJFH classification, maximum deformation values increased with hip flexion angle (0 < 60 < 110°), suggesting a higher risk of collapse at larger angles. (3) Von Mises stress results showed that the maximum stress was not located in the necrotic area but near the inner and outer edge of the femoral neck, indicating decreased stiffness and strength of the subchondral bone after osteonecrosis.

CONCLUSION

The study found that femoral head collapse risk was higher when the necrotic area was located in the lateral column under the same stress load and flexion angle. Mechanical properties of the necrotic area changed, resulting in decreased bone strength and stiffness. Large-angle hip flexion is more likely to cause excessive deformation of the necrotic area; thus, ONFH patients should reduce or avoid large-angle hip flexion during weight-bearing training in rehabilitation activities.

MiRNAs regulate cell communication in osteogenesis-angiogenesis coupling during bone regeneration

Bone regeneration is a complex process that requires not only the participation of multiple cell types, but also signal communication between cells. The two basic processes of osteogenesis and angiogenesis are closely related to bone regeneration and bone homeostasis. H-type vessels are a subtype of bone vessels characterized by high expression of CD31 and EMCN. These vessels play a key role in the regulation of bone regeneration and are important mediators of coupling between osteogenesis and angiogenesis. Molecular regulation between different cell types is important for coordination of osteogenesis and angiogenesis that promotes bone regeneration. MiRNAs are small non-coding RNAs that predominantly regulate gene expression at the post-transcriptional level and are closely related to cell communication. Specifically, miRNAs transduce external stimuli through various cell signaling pathways and cause a series of physiological and pathological effects. They are also deeply involved in the bone repair process. This review focuses on three signaling pathways related to osteogenesis-angiogenesis coupling, as well as the miRNAs involved in these pathways. Elucidation of the molecular mechanisms governing osteogenesis and angiogenesis is of great significance for bone regeneration.

The Use of Hydrogels for the Treatment of Bone Osteosarcoma via Localized Drug-Delivery and Tissue Regeneration: A Narrative Review

Osteosarcoma is a malignant tumor of bone that leads to poor mortality and morbidity. Management of this cancer through conventional methods involves invasive treatment options that place patients at an increased risk of adverse events. The use of hydrogels to target osteosarcoma has shown promising results both in vitro and in vivo to eradicate tumor cells while promoting bone regeneration. The loading of hydrogels with chemotherapeutic drugs provides a route for site-specific targeted therapy for osteosarcoma. Current studies demonstrate tumor regression in vivo and lysis of tumor cells in vitro when exposed to doped hydrogel scaffolds. Additionally, novel stimuli-responsive hydrogels are able to react with the tissue microenvironment to facilitate the controlled release of anti-tumor drugs and with biomechanical properties that can be modulated. This narrative review of the current literature discusses both in vitro and in vivo studies of different hydrogels, including stimuli-responsive, designed to treat bone osteosarcoma. Future applications to address patient treatment for this bone cancer are also discussed.

Identification of the miRNAome in human fracture callus and nonunion tissues

Background

Nonunions remain a challenging post-traumatic complication that often leads to a financial and health burden that affects the patient's quality of life. Despite a wealth of knowledge about fracture repair, especially gene and more recently miRNA expression, much remains unknown about the molecular differences between normal physiological repair (callus tissue) and a nonunion. To probe this lack of knowledge, we embarked on a study that sought to identify and compare the human miRNAome of normal bone to that present in a normal fracture callus and those from two different classic nonunion types, hypertrophic and oligotrophic.

Methods

Normal bone and callus tissue samples were harvested during revision surgery from patients with physiological fracture repair and nonunions (hypertrophic and oligotrophic) and analyzed using histology. Also, miRNAs were isolated and screened using microarrays followed by bioinformatic analyses, including, differential expression, pathways and biological processes, as well as elucidation of target genes.

Results

Out of 30,424 mature miRNAs (from 203 organisms) screened via microarrays, 635 (∼2.1%) miRNAs were found to be upregulated and 855 (∼2.8%) downregulated in the fracture callus and nonunion tissues as compared to intact bone. As our tissue samples were derived from humans, we focused on the human miRNAs and out of the 4223 human miRNAs, 86 miRNAs (∼2.0%) were upregulated and 51 (∼1.2%) were downregulated. Although there were similarities between the three experimental samples, we also found specific miRNAs that were unique to individual samples. We further identified the predicted target genes from these differentially expressed miRNAs as well as the relevant biological processes, including specific signaling pathways that are activated in all three experimental samples.

Conclusion

Collectively, this is the first comprehensive study reporting on the miRNAome of intact bone as compared to fracture callus and nonunion tissues. Further, we identify specific miRNAs involved in normal physiological fracture repair as well as those of nonunions.

The translational potential of this article

The data generated from this study further increase our molecular understanding of the roles of miRNAs during normal and aberrant fracture repair and this knowledge can be used in the future in the development of miRNA-based therapeutics for skeletal regeneration.

Combined computational analysis and cytology show limited depth osteogenic effect on bone defects in negative pressure wound therapy

Background: The treatment of bone defects remains a clinical challenge. The effect of negative pressure wound therapy (NPWT) on osteogenesis in bone defects has been recognized; however, bone marrow fluid dynamics under negative pressure (NP) remain unknown. In this study, we aimed to examine the marrow fluid mechanics within trabeculae by computational fluid dynamics (CFD), and to verify osteogenic gene expression, osteogenic differentiation to investigate the osteogenic depth under NP. Methods: The human femoral head is scanned using micro-CT to segment the volume of interest (VOI) trabeculae. The VOI trabeculae CFD model simulating the bone marrow cavity is developed by combining the Hypermesh and ANSYS software. The effect of trabecular anisotropy is investigated, and bone regeneration effects are simulated under NP scales of -80, -120, -160, and -200 mmHg. The working distance (WD) is proposed to describe the suction depth of the NP. Finally, gene sequence analysis, cytological experiments including bone mesenchymal stem cells (BMSCs) proliferation and osteogenic differentiation are conducted after the BMSCs are cultured under the same NP scale. Results: The pressure, shear stress on trabeculae, and marrow fluid velocity decrease exponentially with an increase in WD. The hydromechanics of fluid at any WD inside the marrow cavity can be theoretically quantified. The NP scale significantly affects the fluid properties, especially those fluid close to the NP source; however, the effect of the NP scale become marginal as WD deepens. Anisotropy of trabecular structure coupled with the anisotropic hydrodynamic behavior of bone marrow; An NP of -120 mmHg demonstrates the majority of bone formation-related genes, as well as the most effective proliferation and osteogenic differentiation of BMSCs compared to the other NP scales. Conclusion: An NP of -120 mmHg may have the optimal activated ability to promote osteogenesis, but the effective WD may be limited to a certain depth. These findings help improve the understanding of fluid mechanisms behind NPWT in treating bone defects.

3D Printed Osteoblast-Alginate/Collagen Hydrogels Promote Survival, Proliferation and Mineralization at Low Doses of Strontium Calcium Polyphosphate

The generation of biomaterials via 3D printing is an emerging biotechnology with novel methods that seeks to enhance bone regeneration. Alginate and collagen are two commonly used biomaterials for bone tissue engineering and have demonstrated biocompatibility. Strontium (Sr) and Calcium phosphate (CaP) are vital elements of bone and their incorporation in composite materials has shown promising results for skeletal repair. In this study, we investigated strontium calcium polyphosphate (SCPP) doped 3D printed alginate/collagen hydrogels loaded with MC3T3-E1 osteoblasts. These cell-laden scaffolds were crosslinked with different concentrations of 1% SCPP to evaluate the effect of strontium ions on cell behavior and the biomaterial properties of the scaffolds. Through scanning electron microscopy and Raman spectroscopy, we showed that the scaffolds had a granular surface topography with the banding pattern of alginate around 1100 cm^-1 and of collagen around 1430 cm^-1. Our results revealed that 2 mg/mL of SCPP induced the greatest scaffold degradation after 7 days and least amount of swelling after 24 h. Exposure of osteoblasts to SCPP induced severe cytotoxic effects after 1 mg/mL. pH analysis demonstrated acidity in the presence of SCPP at a pH between 2 and 4 at 0.1, 0.3, 0.5, and 1 mg/mL, which can be buffered with cell culture medium. However, when the SCPP was added to the scaffolds, the overall pH increased indicating intrinsic activity of the scaffold to buffer the SCPP. Moreover, cell viability was observed for up to 21 days in scaffolds with early mineralization at 0.3, 0.5, and 1 mg/mL of SCPP. Overall, low doses of SCPP proved to be a potential additive in biomaterial approaches for bone tissue engineering; however, the cytotoxic effects due to its pH must be monitored closely.

Operative treatment of nonunions in the elderly: Clinical and radiographic outcomes in patients at minimum 75 years of age

BACKGROUND

Limited information exists on nonunion treatment in the elderly. This retrospective study evaluates whether results of operative treatment of nonunion of the humerus or femur in patients aged ≥ 75 years are comparable to those in younger patients.

METHODS

We identified patients age ≥ 75 years with a nonunion of humerus or femur treated with open reduction and internal fixation. The Non-Union Scoring System was calculated. Complications, clinical outcome, and radiographic findings were assessed. Primary endpoint was nonunion healing. A literature review compared time to healing of humeral and femoral nonunion in younger populations.

RESULTS

We identified 45 patients treated for a nonunion of humerus or femur with > 12 months follow-up. Median age was 79 years (range 75-96). Median time to presentation was 12 months (range 4-127) after injury, median number of prior surgeries was 1 (range 0-4). Union rate was 100%, with median time to union 6 months (range 2-42). Six patients underwent revision for persistent nonunion and healed without further complications.

CONCLUSIONS

Using a protocol of debridement, alignment, compression, stable fixation, bone grafting and early motion, patients aged 75 years or older can reliably achieve healing when faced with a nonunion of the humerus or femur.

LEVEL OF EVIDENCE

IV.

Effects of bisphosphonates on appendicular fracture repair in rodents

The balance between osteoclastic bone resorption and osteoblastic bone formation is ultimately responsible for maintaining a structural and functional skeleton. Despite their strength, bones do break and the main cause of fractures are trauma and decreased bone mineral density as a result of aging and/or pathology that weakens the bone's microarchitecture and subsequently, its material properties. Osteoporosis is a disease marked by increased osteoclast activity and decreased osteoblastic activity tipping the remodeling balance in favor of bone resorption and can be caused by aging, glucocorticoids, disuse and estrogen-deficiency. Ultimately, this leads to brittle and weaker bones which become more prone to trauma or stress-induced fractures. The current treatment for preventing and treating osteoporotic fractures is the use of antiresorptive drugs such as bisphosphonates (BPs) and denosumab, but unfortunately, their long-term use, especially with alendronate and ibandronate, has been associated with increased risk of atypical femoral fractures (AFFs); femoral diaphyseal fractures distal to the lesser trochanter but proximal to the supracondylar flare. The purpose of this review is to examine the information that exists in the literature examining the effects of BPs on fracture repair of long bones in rodent (rat and mouse) models. The focus on rodents stems from the scientific community's unresolved need to develop small animal models to examine the molecular, cellular, tissue and biomechanical mechanisms responsible for the development of AFFs and how best they can be treated.

Negative pressure wound therapy improves bone regeneration by promoting osteogenic differentiation via the AMPK-ULK1-autophagy axis

Deficient bone regeneration causes bone defects or nonunion in a substantial proportion of trauma patients that urges for novel therapies. To develop a reliable therapy, we investigated the effect of negative pressure wound therapy (NPWT) on bone regeneration in vivo in a rat calvarial defect model. Negative pressure (NP) treatment in vitro was mimicked to test its effect on osteoblast differentiation in rat mesenchymal stem cells (MSCs) and MC3T3-E1 cells. Transcriptomic analyses, pharmaceutical interventions, and shRNA knockdowns were conducted to explore the underlying mechanism and their clinical relevance was investigated in samples from patients with nonunion. The potential application of a combined therapy of MSCs in hydrogels with negative pressure was tested in the rat critical-size calvarial defect model. We found that NPWT promoted bone regeneration in vivo and NP treatment induced osteoblast differentiation in vitro. NP induced osteogenesis via activating macroautophagy/autophagy by AMPK-ULK1 signaling that was impaired in clinical samples from patients with nonunion. More importantly, the combined therapy involving MSCs in hydrogels with negative pressure significantly improved bone regeneration in rat critical-size calvarial defect model. Thus, our study identifies a novel AMPK-ULK1-autophagy axis by which negative pressure promotes osteoblast differentiation of MSCs and bone regeneration. NPWT treatment can potentially be adopted for therapy of bone defects.Abbreviations: ADP, adenosine diphosphate; AICAR/Aic, acadesine; ALP, alkaline phosphatase; ALPL, alkaline phosphatase, biomineralization associated; AMP, adenosine monophosphate; AMPK, AMP-activated protein kinase; ARS, alizarin red S staining; ATG7, autophagy related 7; ATP, adenosine triphosphate; BA1, bafilomycin A₁; BGLAP/OCN, bone gamma-carboxyglutamate protein; BL, BL-918; BS, bone surface; BS/TV, bone surface per tissue volume; BV/TV, bone volume per tissue volume; C.C, compound C; CCN1, cellular communication network factor 1; COL1A1, collagen type I alpha 1 chain; COL4A3, collagen type IV alpha 3 chain; COL4A4, collagen type IV alpha 4 chain; COL18A1, collagen type XVIII alpha 1 chain; CQ, chloroquine; GelMA, gelatin methacryloyl hydrogel; GO, Gene Ontology; GSEA, gene set enrichment analysis; HIF1A, hypoxia inducible factor 1 subunit alpha; HPLC, high-performance liquid chromatography; ITGAM/CD11B, integrin subunit alpha M; ITGAX/CD11C, integrin subunit alpha X; ITGB1/CdD9, integrin subunit beta 1; KEGG, Kyoto Encyclopedia of Genes and Genomes; MAP1LC3B/LC3B, microtubule associated protein 1 light chain 3 beta; micro-CT, microcomputed tomography; MSCs, mesenchymal stem cells; MTOR, mechanistic target of rapamycin kinase; NP, negative pressure; NPWT, negative pressure wound therapy; PRKAA1/AMPKα1, protein kinase AMP-activated catalytic subunit alpha 1; PRKAA2, protein kinase AMP-activated catalytic subunit alpha 2; PTPRC/CD45, protein tyrosine phosphatase receptor type C; ROS, reactive oxygen species; RUNX2, RUNX family transcription factor 2; SBI, SBI-0206965; SPP1/OPN, secreted phosphoprotein 1; THY1/CD90, Thy-1 cell surface antigen; SQSTM1, sequestosome 1; TGFB3, transforming growth factor beta 3; ULK1/Atg1, unc-51 like autophagy activating kinase 1.

Low-intensity pulsed ultrasound/nanomechanical force generators enhance osteogenesis of BMSCs through microfilaments and TRPM7

BACKGROUND

Low-intensity pulsed ultrasound (LIPUS) has been reported to accelerate fracture healing, but the mechanism is unclear and its efficacy needs to be further optimized. Ultrasound in combination with functionalized microbubbles has been shown to induce local shear forces and controllable mechanical stress in cells, amplifying the mechanical effects of LIPUS. Nanoscale lipid bubbles (nanobubbles) have high stability and good biosafety. However, the effect of LIPUS combined with functionalized nanobubbles on osteogenesis has rarely been studied.

RESULTS

In this study, we report cyclic arginine-glycine-aspartic acid-modified nanobubbles (cRGD-NBs), with a particle size of ~ 500 nm, able to actively target bone marrow mesenchymal stem cells (BMSCs) via integrin receptors. cRGD-NBs can act as nanomechanical force generators on the cell membrane, and further enhance the BMSCs osteogenesis and bone formation promoted by LIPUS. The polymerization of actin microfilaments and the mechanosensitive transient receptor potential melastatin 7 (TRPM7) ion channel play important roles in BMSCs osteogenesis promoted by LIPUS/cRGD-NBs. Moreover, the mutual regulation of TRPM7 and actin microfilaments promote the effect of LIPUS/cRGD-NBs. The extracellular Ca^2 + influx, controlled partly by TRPM7, could participated in the effect of LIPUS/cRGD-NBs on BMSCs.

CONCLUSIONS

The nanomechanical force generators cRGD-NBs could promote osteogenesis of BMSCs and bone formation induced by LIPUS, through regulation TRPM7, actin cytoskeleton, and intracellular calcium oscillations. This study provides new directions for optimizing the efficacy of LIPUS for fracture healing, and a theoretical basis for the further application and development of LIPUS in clinical practice.

Fracture hematoma micro-architecture influences transcriptional profile and plays a crucial role in determining bone healing outcomes

The hematoma that forms between broken fragments of bone serves as a natural fibrin scaffold, and its removal from the defect site delays bone healing. The hypothesis of this study is that the microarchitectural and mechanical properties of the initially formed hematoma has a significant effect on the regulation of the biological process, which ultimately determines the outcome of bone healing. To mimic three healing conditions in the rat femur (normal, delayed, and non-healing bone defects), three different defect sizes of 0.5, 1.5, and 5.0 mm, are respectively used. The analysis of 3-day-old hematomas demonstrates clear differences in fibrin clot micro-architecture in terms of fiber diameter, fiber density, and porosity of the formed fibrin network, which result in different mechanical properties (stiffness) of the hematoma in each model. Those differences directly affect the biological processes involved. Specifically, RNA-sequencing reveals almost 700 differentially expressed genes between normally healing and non-healing defects, including significantly up-regulated essential osteogenic genes in normally healing defects, also differences in immune cell populations, activated osteogenic transcriptional regulators as well as potential novel marker genes. Most importantly, this study demonstrates that the healing outcome has already been determined during the hematoma phase of bone healing, three days post-surgery.

Spatial transcriptomics reveals metabolic changes underly age-dependent declines in digit regeneration

De novo limb regeneration after amputation is restricted in mammals to the distal digit tip. Central to this regenerative process is the blastema, a heterogeneous population of lineage-restricted, dedifferentiated cells that ultimately orchestrates regeneration of the amputated bone and surrounding soft tissue. To investigate skeletal regeneration, we made use of spatial transcriptomics to characterize the transcriptional profile specifically within the blastema. Using this technique, we generated a gene signature with high specificity for the blastema in both our spatial data, as well as other previously published single-cell RNA-sequencing transcriptomic studies. To elucidate potential mechanisms distinguishing regenerative from non-regenerative healing, we applied spatial transcriptomics to an aging model. Consistent with other forms of repair, our digit amputation mouse model showed a significant impairment in regeneration in aged mice. Contrasting young and aged mice, spatial analysis revealed a metabolic shift in aged blastema associated with an increased bioenergetic requirement. This enhanced metabolic turnover was associated with increased hypoxia and angiogenic signaling, leading to excessive vascularization and altered regenerated bone architecture in aged mice. Administration of the metabolite oxaloacetate decreased the oxygen consumption rate of the aged blastema and increased WNT signaling, leading to enhanced in vivo bone regeneration. Thus, targeting cell metabolism may be a promising strategy to mitigate aging-induced declines in tissue regeneration.

Different Protocols of Combined Application of Photobiomodulation In Vitro and In Vivo Plus Adipose-Derived Stem Cells Improve the Healing of Bones in Critical Size Defects in Rat Models

Introduction: Long bone segmental deficiencies are challenging complications to treat. Hereby, the effects of the scaffold derived from the human demineralized bone matrix (hDBMS) plus human adipose stem cells (hADSs) plus photobiomodulation (PBM) (in vitro and or in vivo) on the catabolic step of femoral bone repair in rats with critical size femoral defects (CDFDs) were evaluated with stereology and high stress load (HSL) assessment methods. Methods: hADSs were exposed to PBM in vitro; then, the mixed influences of hDBMS+hADS+PBM on CSFDs were evaluated. CSFDs were made on both femurs; then hDBMSs were engrafted into both CSFDs of all rats. There were 6 groups (G)s: G1 was the control; in G2 (hADS), hADSs only were engrafted into hDBMS of CSFD; in G3 (PBM) only PBM therapy for CSFD was provided; in G4 (hADS+PBM in vivo), seeded hADSs on hDBMS of CSFDs were radiated with a laser in vivo; in G5 (hADSs+PBM under in vitro condition), hADSs in a culture system were radiated with a laser, then transferred on hDBMS of CSFDs; and in G6 (hADS+PBM in conditions of in vivo and in vitro), laser-exposed hADSs were transplanted on hDBMS of CSFDs, and then CSFDs were exposed to a laser in vivo. Results: Groups 4, 5, and 6 meaningfully improved HSLs of CSFD in comparison with groups 3, 1, and 2 (all, P=0.001). HSL of G5 was significantly more than G4 and G6 (both, P=0.000). Gs 6 and 4 significantly increased new bone volumes of CSFD compared to Gs 2 (all, P=0.000) and 1 (P=0.001 & P=0.003 respectively). HSL of G 1 was significantly lower than G5 (P=0.026). Conclusion: HSLs of CSFD in rats that received treatments of hDBMS plus hADS plus PBM were significantly higher than treatments with hADS and PBM alone and control groups.

Extracellular vesicles-encapsulated microRNA-29b-3p from bone marrow-derived mesenchymal stem cells promotes fracture healing via modulation of the PTEN/PI3K/AKT axis

Bone marrow-derived mesenchymal stem cells (BM-MSCs) are well-established as vital regulators of fracture healing, whereas angiogenesis is one of the critical processes during the course of bone healing. Accordingly, the current study sought to determine the functions of microRNA (miR)-29b-3p from BM-MSCs-derived extracellular vesicles (EVs) on the angiogenesis of fracture healing via the PTEN/PI3K/AKT axis. Firstly, BM-MSCs-EVs were extracted and identified. The lentiviral protocol was adopted to construct miR-29b-3p^KD-BMSCs or miR-negative control-BMSCs, which were then co-cultured with human umbilical vein endothelial cells (HUVECs) in vitro to determine the roles of EVs-encapsulated miR-29b-3p on the proliferation, migration, and angiogenesis of HUVECs in vitro with the help of a CCK-8 assay, scratch test, and tube formation assay. Subsequent database prediction, luciferase activity assay, RT-qPCR, and Western blot assay findings identified the downstream target gene of miR-29b-3p, PTEN, and a signaling pathway, PI3K/AKT. Furthermore, the application of si-PTEN attenuated the effects induced by miR-29b-3p^KD-EVs. Finally, a mouse model of femoral fracture was established with a locally instilled injection of equal volumes of BM-MSCs-EVs and miR-29b-3p^KD-BM-MSCs-EVs. Notably, the mice treated with BMSC-EVs presented with enhanced neovascularization at the fracture site, in addition to increased bone volume (BV), BV/tissue volume, and mean bone mineral density; whereas miR-29b-3p^KD-BMSCs-EVs-treated mice exhibited decreased vessel density with poor fracture healing capacity. Collectively, our findings elicited that BM-MSCs-EVs carrying miR-29b-3p were endocytosed by HUVECs, which consequently suppressed the PTEN expression and activated the PI3K/AKT pathway, thereby promoting HUVEC proliferation, migration, and angiogenesis, and ultimately facilitating fracture healing.

The Potential Use of Mesenchymal Stem Cells and Their Derived Exosomes for Orthopedic Diseases Treatment

Musculoskeletal disorders (MSDs) are conditions that can affect muscles, bones, and joints. These disorders are very painful and severely limit patients' mobility and are more common in the elderly. MSCs are multipotent stem cells isolated from embryonic (such as the umbilical cord) and mature sources (such as adipose tissue and bone marrow). These cells can differentiate into various cells such as osteoblasts, adipocytes, chondrocytes, NP-like cells, Etc. Due to MSC characteristics such as immunomodulatory properties, ability to migrate to the site of injury, recruitment of cells involved in repair, production of growth factors, and large amount production of extracellular vesicles, these cells have been used in many regenerative-related medicine studies. Also, MSCs produce different types of EVs, such as exosomes, to the extracellular environment. Exosomes reflect MSCs' characteristics and do not have cell therapy-associated problems because they are cell-free. These vesicles carry proteins, nucleic acids, and lipids to the host cell and change their function. This review focuses on MSCs and MSCs exosomes' role in repairing dense connective tissues such as tendons, cartilage, invertebrate disc, bone fracture, and osteoporosis treatment.

The risk of revision following total hip arthroplasty in patients with inflammatory bowel disease, a registry based study

Inflammatory bowel disease (IBD) is characterized by chronic inflammation of the intestinal tract and is associated with decreased bone mineral density. IBD patients are at higher risk of osteopenia, osteoporosis and fracture compared to non-IBD patients. The impact of IBD on the performance of orthopedic implants has not been well studied. We hypothesized that a history of IBD at the time of primary total hip arthroplasty (THA) would increase the risk of subsequent failure as assessed by revision surgery. A retrospective implant survival analysis was completed using the Swedish Hip Arthroplasty Registry and the Sweden National Patient Register. A total of 150,073 patients undergoing THA for osteoarthritis within an 18-year period were included in the study. THA patients with (n = 2,604) and without (n = 147,469) a history of IBD at the time of THA were compared with primary revision as the main endpoint and adjusted using sex, age category and comorbidity (Elixhauser scores) as covariates. We found that patients with a history of IBD had a relatively higher risk of revision surgery for septic causes while the non-IBD patients had a relatively higher risk of revision for aseptic causes (p = 0.004). Our findings suggest there may be an association between gut health and THA performance.

Electromagnetic stimulation increases mitochondrial function in osteogenic cells and promotes bone fracture repair

Bone fracture is a growing public health burden and there is a clinical need for non-invasive therapies to aid in the fracture healing process. Previous studies have demonstrated the utility of electromagnetic (EM) fields in promoting bone repair; however, its underlying mechanism of action is unclear. Interestingly, there is a growing body of literature describing positive effects of an EM field on mitochondria. In our own work, we have previously demonstrated that differentiation of osteoprogenitors into osteoblasts involves activation of mitochondrial oxidative phosphorylation (OxPhos). Therefore, it was reasonable to propose that EM field therapy exerts bone anabolic effects via stimulation of mitochondrial OxPhos. In this study, we show that application of a low intensity constant EM field source on osteogenic cells in vitro resulted in increased mitochondrial membrane potential and respiratory complex I activity and induced osteogenic differentiation. In the presence of mitochondrial inhibitor antimycin A, the osteoinductive effect was reversed, confirming that this effect was mediated via increased OxPhos activity. Using a mouse tibial bone fracture model in vivo, we show that application of a low intensity constant EM field source enhanced fracture repair via improved biomechanical properties and increased callus bone mineralization. Overall, this study provides supporting evidence that EM field therapy promotes bone fracture repair through mitochondrial OxPhos activation.

MicroRNA-21: An Emerging Player in Bone Diseases

MicroRNAs (MiRNAs) are small endogenous non-coding RNAs that bind to the 3′-untranslated region of target genes and promote their degradation or inhibit translation, thereby regulating gene expression. MiRNAs are ubiquitous in biology and are involved in many biological processes, playing an important role in a variety of physiological and pathological processes. MiRNA-21 (miR-21) is one of them. In recent years, miR-21 has received a lot of attention from researchers as an emerging player in orthopedic diseases. MiR-21 is closely associated with the occurrence, development, treatment, and prevention of orthopedic diseases through a variety of mechanisms. This review summarizes its effects on osteoblasts, osteoclasts and their relationship with osteoporosis, fracture, osteoarthritis (OA), osteonecrosis, providing a new way of thinking for the diagnosis, treatment and prevention of these bone diseases.

Targeting angiogenesis for fracture nonunion treatment in inflammatory disease

Atrophic fracture nonunion poses a significant clinical problem with limited therapeutic interventions. In this study, we developed a unique nonunion model with high clinical relevance using serum transfer-induced rheumatoid arthritis (RA). Arthritic mice displayed fracture nonunion with the absence of fracture callus, diminished angiogenesis and fibrotic scar tissue formation leading to the failure of biomechanical properties, representing the major manifestations of atrophic nonunion in the clinic. Mechanistically, we demonstrated that the angiogenesis defect observed in RA mice was due to the downregulation of SPP1 and CXCL12 in chondrocytes, as evidenced by the restoration of angiogenesis upon SPP1 and CXCL12 treatment in vitro. In this regard, we developed a biodegradable scaffold loaded with SPP1 and CXCL12, which displayed a beneficial effect on angiogenesis and fracture repair in mice despite the presence of inflammation. Hence, these findings strongly suggest that the sustained release of SPP1 and CXCL12 represents an effective therapeutic approach to treat impaired angiogenesis and fracture nonunion under inflammatory conditions.

Long Non-coding RNAs in Traumatic Brain Injury Accelerated Fracture Healing

It is commonly observed that patients with bone fracture concomitant with traumatic brain injury (TBI) had significantly increased fracture healing, but the underlying mechanisms were not fully revealed. Long non-coding RNAs (lncRNAs) are known to play complicated roles in bone homeostasis, but their role in TBI accelerated fracture was rarely reported. The present study was designed to determine the role of lncRNAs in TBI accelerated fracture via transcriptome sequencing and further bioinformatics analyses. Blood samples from three fracture-only patients, three fracture concomitant with TBI patients, and three healthy controls were harvested and were subsequently subjected to transcriptome lncRNA sequencing. Differentially expressed genes were identified, and pathway enrichment was performed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. High-dimensional data visualization by self-organizing map (SOM) machine learning was applied to further interpret the data. An xCell method was then used to predict cellular behavior in all samples based on gene expression profiles, and an lncRNA-cell interaction network was generated. A total of 874 differentially expressed genes were identified, of which about 26% were lncRNAs. Those identified lncRNAs were mainly enriched on TBI-related and damage repair-related pathways. SOM analyses revealed that those differentially expressed lncRNAs could be divided into three major module implications and were mainly enriched on transcriptional regulation and immune-related signal pathways, which promote us to further explore cellular behaviors based on differentially expressed lncRNAs. We have predicted that basophils, CD8+ T effector memory cells, B cells, and naïve B cells were significantly downregulated, while microvascular endothelial cells were predicted to be significantly upregulated in the Fr/TBI group, was the lowest and highest, respectively. ENSG00000278905, ENSG00000240980, ENSG00000255670, and ENSG00000196634 were the most differentially expressed lncRNAs related to all changes of cellular behavior. The present study has revealed for the first time that several critical lncRNAs may participate in TBI accelerated fracture potentially via regulating cellular behaviors of basophils, cytotoxic T cells, B cells, and endothelial cells.

MicroRNAs and fracture healing: Pre-clinical studies

During the past several years, pre-clinical experiments have established that microRNAs (miRNAs), small non-coding RNAs, serve as key regulatory molecules of fracture healing. Their easy modulation with agonists and antagonists make them highly desirable targets for future therapeutic strategies, especially for pathophysiologic fractures that either do not heal (nonunions) or are delayed. It is now well documented that these problematic fractures lead to human suffering and impairment of life quality. Additionally, financial difficulties are also encountered as work productivity decreases and income is reduced. Moreover, targeting miRNAs may also be an avenue to enhancing normal physiological fracture healing. Herein we present the most current knowledge of the involvement of miRNAs during fracture healing in pre-clinical studies. Following a brief description on the nature of miRNAs and of the fracture healing process, we present data from studies focusing specifically, on miRNA regulation of osteoblast differentiation and osteogenesis (within the context of known signaling pathways), chondrocytes, angiogenesis, and apoptosis, all critical to successful bone repair. Further, we also discuss miRNAs and exosomes. We hope that this manuscript serves as a comprehensive review that will facilitate basic/translational scientists in the orthopaedic arena to realize and further decipher the biological and future therapeutic impact of these small regulatory RNA molecules, especially as they relate to the molecular events of each of the major phases of fracture healing.

Metabolic regulation of skeletal cell fate and function in physiology and disease

The skeleton is diverse in its functions, which include mechanical support, movement, blood cell production, mineral storage and endocrine regulation. This multifaceted role is achieved through an interplay of osteoblasts, chondrocytes, bone marrow adipocytes and stromal cells, all generated from skeletal stem cells. Emerging evidence shows the importance of cellular metabolism in the molecular control of the skeletal system. The different skeletal cell types not only have distinct metabolic demands relating to their particular functions but also are affected by microenvironmental constraints. Specific metabolites control skeletal stem cell maintenance, direct lineage allocation and mediate cellular communication. Here, we discuss recent findings on the roles of cellular metabolism in determining skeletal stem cell fate, coordinating osteoblast and chondrocyte function, and organizing stromal support of haematopoiesis. We also consider metabolic dysregulation in skeletal ageing and degenerative diseases, and provide an outlook on how the field may evolve in the coming years.

Smoking Alters Inflammation and Skeletal Stem and Progenitor Cell Activity During Fracture Healing in Different Murine Strains

Smokers are at a higher risk of delayed union or nonunion after fracture repair. Few specific interventions are available for prevention because the molecular mechanisms that result in these negative sequelae are poorly understood. Murine models that mimic fracture healing in smokers are crucial in further understanding the local cellular and molecular alterations during fracture healing caused by smoking. We exposed three murine strains, C57BL/6J, 129X1/SvJ, and BALB/cJ, to cigarette smoke for 3 months before the induction of a midshaft transverse femoral osteotomy. We evaluated fracture healing 4 weeks after the osteotomy using radiography, micro-computed tomography (μCT), and biomechanical testing. Radiographic analysis demonstrated a significant decrease in the fracture healing capacity of smoking 129X1/SvJ mice. μCT results showed delayed remodeling of fracture calluses in all three strains after cigarette smoke exposure. Biomechanical testing indicated the most significant impairment in the functional properties of 129X1/SvJ in comparison with C57BL/6J and BALB/cJ mice after cigarette smoke exposure. Thus, the 129X1/SvJ strain is most suitable in simulating smoking-induced impaired fracture healing. Furthermore, in smoking 129X1/SvJ murine models, we investigated the molecular and cellular alterations in fracture healing caused by cigarette smoking using histology, flow cytometry, and multiplex cytokine/chemokine analysis. Histological analysis showed impaired chondrogenesis in cigarette smoking. In addition, the important reparative cell populations, including skeletal stem cells and their downstream progenitors, demonstrated decreased expansion after injury as a result of cigarette smoking. Moreover, significantly increased pro-inflammatory mediators and the recruitment of immune cells in fracture hematomas were demonstrated in smoking mice. Collectively, our findings demonstrate the significant cellular and molecular alterations during fracture healing impaired by smoking, including disrupted chondrogenesis, aberrant skeletal stem and progenitor cell activity, and a pronounced initial inflammatory response. © 2020 American Society for Bone and Mineral Research (ASBMR).

Growth Factors, Carrier Materials, and Bone Repair

This chapter provides an overview of the growth factors active in bone regeneration and healing. Both normal and impaired bone healing are discussed, with a focus on the spatiotemporal activity of the various growth factors known to be involved in the healing response. The review highlights the activities of most important growth factors impacting bone regeneration, with a particular emphasis on those being pursued for clinical translation or which have already been marketed as components of bone regenerative materials. Current approaches the use of bone grafts in clinical settings of bone repair (including bone grafts) are summarized, and carrier systems (scaffolds) for bone tissue engineering via localized growth factor delivery are reviewed. The chapter concludes with a consideration of how bone repair might be improved in the future.

Ablation of Ephrin B2 in Col2 Expressing Cells Delays Fracture Repair

Ephrin B2 is critical for endochondral bone development. In this study, we investigated its role in fracture repair by deleting ephrin B2 in type II collagen (Col.2) expressing cells. We used a nonstable tibia fracture model to evaluate fracture repair at 3 sites: intramembranous bone formation, endochondral bone formation, and intramedullary bone formation. We observed that during fracture repair, deletion of ephrin B2 impaired periosteal stem cell activation, inhibited their proliferation, decreased their survival, and blocked their differentiation into osteoblasts and chondrocytes. In addition, deletion of ephrin B2 decreased vascular endothelial growth factor production as well as vascular invasion into the fracture site. These changes led to reduced cartilage to bone conversion in the callus with decreased new bone formation, resulting in impaired fracture repair. Our data indicate that ephrin B2 in Col2-expressing cells is a critical regulator of fracture repair, pointing to a new and potentially targetable mechanism to enhance fracture repair.

Bone Angiogenesis and Vascular Niche Remodeling in Stress, Aging, and Diseases

The bone marrow (BM) vascular niche microenvironments harbor stem and progenitor cells of various lineages. Bone angiogenesis is distinct and involves tissue-specific signals. The nurturing vascular niches in the BM are complex and heterogenous consisting of distinct vascular and perivascular cell types that provide crucial signals for the maintenance of stem and progenitor cells. Growing evidence suggests that the BM niche is highly sensitive to stress. Aging, inflammation and other stress factors induce changes in BM niche cells and their crosstalk with tissue cells leading to perturbed hematopoiesis, bone angiogenesis and bone formation. Defining vascular niche remodeling under stress conditions will improve our understanding of the BM vascular niche and its role in homeostasis and disease. Therefore, this review provides an overview of the current understanding of the BM vascular niches for hematopoietic stem cells and their malfunction during aging, bone loss diseases, arthritis and metastasis.

JAK/STAT Activation: A General Mechanism for Bone Development, Homeostasis, and Regeneration

The Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathway serves as an important downstream mediator for a variety of cytokines, hormones, and growth factors. Emerging evidence suggests JAK/STAT signaling pathway plays an important role in bone development, metabolism, and healing. In this light, pro-inflammatory cytokines are now clearly implicated in these processes as they can perturb normal bone remodeling through their action on osteoclasts and osteoblasts at both intra- and extra-articular skeletal sites. Here, we summarize the role of JAK/STAT pathway on development, homeostasis, and regeneration based on skeletal phenotype of individual JAK and STAT gene knockout models and selective inhibition of components of the JAK/STAT signaling including influences of JAK inhibition in osteoclasts, osteoblasts, and osteocytes.

Identification of osteogenic progenitor cell-targeted peptides that augment bone formation

Activation and migration of endogenous mesenchymal stromal cells (MSCs) are critical for bone regeneration. Here, we report a combinational peptide screening strategy for rapid discovery of ligands that not only bind strongly to osteogenic progenitor cells (OPCs) but also stimulate osteogenic cell Akt signaling in those OPCs. Two lead compounds are discovered, YLL3 and YLL8, both of which increase osteoprogenitor osteogenic differentiation in vitro. When given to normal or osteopenic mice, the compounds increase mineral apposition rate, bone formation, bone mass, and bone strength, as well as expedite fracture repair through stimulated endogenous osteogenesis. When covalently conjugated to alendronate, YLLs acquire an additional function resulting in a “tri-functional” compound that: (i) binds to OPCs, (ii) targets bone, and (iii) induces “pro-survival” signal. These bone-targeted, osteogenic peptides are well suited for current tissue-specific therapeutic paradigms to augment the endogenous osteogenic cells for bone regeneration and the treatment of bone loss.

Activation of osteogenic cells is essential for bone regeneration. Here, the authors screen a peptide library and identify 2 compounds that promote osteogenic progenitor cell differentiation in vitro, and show that they increase bone formation and fracture repair in mice.

Implementation of Endogenous and Exogenous Mesenchymal Progenitor Cells for Skeletal Tissue Regeneration and Repair

Harnessing adult mesenchymal stem/progenitor cells to stimulate skeletal tissue repair is a strategy that is being actively investigated. While scientists continue to develop creative and thoughtful ways to utilize these cells for tissue repair, the vast majority of these methodologies can ultimately be categorized into two main approaches: (1) Facilitating the recruitment of endogenous host cells to the injury site; and (2) physically administering into the injury site cells themselves, exogenously, either by autologous or allogeneic implantation. The aim of this paper is to comprehensively review recent key literature on the use of these two approaches in stimulating healing and repair of different skeletal tissues. As expected, each of the two strategies have their own advantages and limitations (which we describe), especially when considering the diverse microenvironments of different skeletal tissues like bone, tendon/ligament, and cartilage/fibrocartilage. This paper also discusses stem/progenitor cells commonly used for repairing different skeletal tissues, and it lists ongoing clinical trials that have risen from the implementation of these cells and strategies. Lastly, we discuss our own thoughts on where the field is headed in the near future.

The Life of a Fracture: Biologic Progression, Healing Gone Awry, and Evaluation of Union

New knowledge about the molecular biology of fracture-healing provides opportunities for intervention and reduction of risk for specific phases that are affected by disease and medications. Modifiable and nonmodifiable risk factors can prolong healing, and the informed clinician should optimize each patient to provide the best chance for union. Techniques to monitor progression of fracture-healing have not changed substantially over time; new objective modalities are needed.

Regional variations of jaw bone characteristics in an ovariectomized rat model

Postmenopausal osteoporosis causes severe loss of bone quantity and quality in limb bone but has a lesser effect on jaw bone. Thus, the objective of this study was to examine whether ovariectomy (OVX) and mastication alter the regional variation of jaw bone characteristics. Sprague-Dawley female rats (6 months) were given a bilateral OVX or a sham operation (SHAM) (n = 10 for each group). After 2 months post-OVX, the hemi-mandible from each rat was dissected. A micro-computed tomography based mean, standard deviation (SD), the lower and upper 5th percentile (Low₅ and High₅) values of tissue mineral density (TMD) histograms were assessed for whole bone (WB), alveolar bone (AB), cortical bone (CB), and trabecular bone (TB) regions. Morphology of TB and periodontal ligament (PDL) was also obtained. Layers of AB were segmented up to 400 μm from the PDL. Mechanical properties at the tissue level were measured by nanoindentation at the same site by a single loading-unloading cycle of indentation in hydration. The AB and TB regions had significantly lower TMD Mean, Low₅, and High₅ but higher SD than the CB region for both sham and OVX groups (p < 0.01). TMD parameters of the OVX group rapidly increased up to 60 μm away from the PDL and were significantly higher than those of the sham group starting at 280 μm and farther in the CB region (p < 0.05). All values of morphological and nanoindentation parameters were not significantly different between sham and OVX groups (p > 0.06). Estrogen deficiency induced by OVX did not deteriorate bone characteristics including mineral density, morphology, and nanoindentation parameters in rat mandibles. Masticatory loading had an effect on the TMD parameters at the limited region of AB. These results provide insight into why osteoporosis-associated jaw bone fractures are extremely rare.

A segmental defect adaptation of the mouse closed femur fracture model for the analysis of severely impaired bone healing

OBJECTIVE

To better characterize nonunion endochondral bone healing and evaluate novel therapeutic approaches for critical size defect healing in clinically challenging bone repair, a segmental defect model of bone injury was adapted from the three-point bending closed fracture technique in the murine femur.

METHODS

The mouse femur was surgically stabilized with an intramedullary threaded rod with plastic spacers and the defect adjusted to different sizes. Healing of the different defects was analyzed by radiology and histology to 8 weeks postsurgery. To determine whether this model was effective for evaluating the benefits of molecular therapy, BMP-2 was applied to the defect and healing then examined.

RESULTS

Intramedullary spacers were effective in maintaining the defect. Callus bone formation was initiated but was arrested at defect sizes of 2.5 mm and above, with no more progress in callus bone development evident to 8 weeks healing. Cartilage development in a critical size defect attenuated very early in healing without bone development, in contrast to the closed femur fracture healing, where callus cartilage was replaced by bone. BMP-2 therapy promoted osteogenesis of the resident cells of the defect, but there was no further callus development to indicate that healing to pre-surgery bone structure was successful.

CONCLUSIONS

This segmental defect adaptation of the closed femur fracture model of murine bone repair severely impairs callus development and bone healing, reflecting a challenging bone injury. It is adjustable and can be compared to the closed fracture model to ascertain healing deficiencies and the efficacy of therapeutic approaches.

Fabrication of a Biomimetic Nano-Matrix with Janus Base Nanotubes and Fibronectin for Stem Cell Adhesion

A biomimetic NM was developed to serve as a tissue-engineering biological scaffold, which can enhance stem cell anchorage. The biomimetic NM is formed from JBNTs and FN through self-assembly in an aqueous solution. JBNTs measure 200-300 µm in length with inner hydrophobic hollow channels and outer hydrophilic surfaces. JBNTs are positively charged and FNs are negatively charged. Therefore, when injected into a neutral aqueous solution, they are bonded together via noncovalent bonding to form the NM bundles. The self-assembly process is completed within a few seconds without any chemical initiators, heat source, or UV light. When the pH of the NM solution is lower than the isoelectric point of FNs (pI 5.5-6.0), the NM bundles will self-release due to the presence of positively charged FN. NM is known to mimic the extracellular matrix (ECM) morphologically and hence, can be used as an injectable scaffold, which provides an excellent platform to enhance hMSC adhesion. Cell density analysis and fluorescence imaging experiments indicated that the NMs significantly increased the anchorage of hMSCs compared to the negative control.

Protein kinase G1 regulates bone regeneration and rescues diabetic fracture healing

Bone fractures are a major cause of morbidity and mortality, particularly in patients with diabetes, who have a high incidence of fractures and exhibit poor fracture healing. Coordinated expression of osteoblast-derived vascular endothelial growth factor (VEGF) and bone morphogenic proteins (BMPs) is essential for fracture repair. The NO/cGMP/protein kinase G (PKG) signaling pathway mediates osteoblast responses to estrogens and mechanical stimulation, but the pathway's role in bone regeneration is unknown. Here, we used a mouse cortical-defect model to simulate bone fractures and studied osteoblast-specific PKG1-knockout and diabetic mice. The knockout mice had normal bone microarchitecture but after injury exhibited poor bone regeneration, with decreased osteoblasts, collagen deposition, and microvessels in the bone defect area. Primary osteoblasts and tibiae from the knockout mice expressed low amounts of Vegfa and Bmp2/4 mRNAs, and PKG1 was required for cGMP-stimulated expression of these genes. Diabetic mice also demonstrated low Vegfa and Bmp2/4 expression in bone and impaired bone regeneration after injury; notably, the cGMP-elevating agent cinaciguat restored Vegfa and BMP2/4 expression and full bone healing. We conclude that PKG1 is a key orchestrator of VEGF and BMP signaling during bone regeneration and propose pharmacological PKG activation as a novel therapeutic approach to enhance fracture healing.

Dnmt3b ablation impairs fracture repair through upregulation of Notch pathway

We previously established that DNA methyltransferase 3b (Dnmt3b) is the sole Dnmt responsive to fracture repair and that Dnmt3b expression is induced in progenitor cells during fracture repair. In the current study, we confirmed that Dnmt3b ablation in mesenchymal progenitor cells (MPCs) resulted in impaired endochondral ossification, delayed fracture repair, and reduced mechanical strength of the newly formed bone in Prx1-Cre;Dnmt3bf/f (Dnmt3bPrx1) mice. Mechanistically, deletion of Dnmt3b in MPCs led to reduced chondrogenic and osteogenic differentiation in vitro. We further identified Rbpjκ as a downstream target of Dnmt3b in MPCs. In fact, we located 2 Dnmt3b binding sites in the murine proximal Rbpjκ promoter and gene body and confirmed Dnmt3b interaction with the 2 binding sites by ChIP assays. Luciferase assays showed functional utilization of the Dnmt3b binding sites in murine C3H10T1/2 cells. Importantly, we showed that the MPC differentiation defect observed in Dnmt3b deficiency cells was due to the upregulation of Rbpjκ, evident by restored MPC differentiation upon Rbpjκ inhibition. Consistent with in vitro findings, Rbpjκ blockage via dual antiplatelet therapy reversed the differentiation defect and accelerated fracture repair in Dnmt3bPrx1 mice. Collectively, our data suggest that Dnmt3b suppresses Notch signaling during MPC differentiation and is necessary for normal fracture repair.

Bone Substitutes in Orthopaedic Surgery: Current Status and Future Perspectives

Bone replacement materials have been successfully supplied for a long time. But there are cases, especially in critical sized bone defects, in which the therapy is not sufficient. Nowadays, there are multiple bone substitutes available. Autologous bone grafts remain the "gold standard" in bone regeneration. Yet, donor-site morbidity and the available amount of sufficient material are limitations for autologous bone grafting. This study aimed to provide information about the current status in research regarding bone substitutes. We report on the advantages and drawbacks of several bone substitutes. At the end, we discuss the current developments of combining ceramic substitutes with osteoinductive substances.