Immobilization after injury alters extracellular matrix and stem cell fate

Diatomite-incorporated hierarchical scaffolds for osteochondral regeneration

Osteochondral regeneration involves the highly challenging and complex reconstruction of cartilage and subchondral bone. Silicon (Si) ions play a crucial role in bone development. Current research on Si ions mainly focuses on bone repair, by using silicate bioceramics with complex ion compositions. However, it is unclear whether the Si ions have important effect on cartilage regeneration. Developing a scaffold that solely releases Si ions to simultaneously promote subchondral bone repair and stimulate cartilage regeneration is critically important. Diatomite (DE) is a natural diatomaceous sediment that can stably release Si ions, known for its abundant availability, low cost, and environmental friendliness. Herein, a hierarchical osteochondral repair scaffold is uniquely designed by incorporating gradient DE into GelMA hydrogel. The adding DE microparticles provides a specific Si source for controlled Si ions release, which not only promotes osteogenic differentiation of rBMSCs (rabbit bone marrow mesenchymal stem cells) but also enhances proliferation and maturation of chondrocytes. Moreover, DE-incorporated hierarchical scaffolds significantly promoted the regeneration of cartilage and subchondral bone. The study suggests the significant role of Si ions in promoting cartilage regeneration and solidifies their foundational role in enhancing bone repair. Furthermore, it offers an economic and eco-friendly strategy for developing high value-added osteochondral regenerative bioscaffolds from low-value ocean natural materials.

Suppression of TNF-α activity by immobilization rescues Mkx expression and attenuates tendon ossification in a mouse Achilles tenotomy model

Traumatic heterotopic ossification is a condition in which extraskeletal bone formation occurs in soft tissues after injury. It most commonly occurs in patients who had major orthopedic surgery and in those with severe extremity injuries. The lesion causes local pain and can impair motor function of the affected limb, but there is currently no established prophylaxis or treatment for this condition. In this study, we show that immobilization at an early stage of the inflammatory response after injury can attenuate ossification formation in a murine Achilles tenotomy model. Gene expression analysis revealed a decrease in the expression of Tnf and an increase in the expression of Mkx, which encodes one of the master regulators of tendon differentiation, Mohawk. Notably, we found that TNF-α suppressed the expression of Mkx transcripts and accelerated the osteogenic differentiation of tendon-derived mesenchymal stem cells (MSCs), suggesting that TNF-α acts as a negative regulator of Mkx transcription. Consistent with these findings, pharmaceutical inhibition of TNF-α increased the expression of Mkx transcripts and suppressed bone formation in this mouse model. These findings reveal the previously unrecognized involvement of TNF-α in regulating tendon MSC fate through suppression of Mkx expression and suggest that TNF-α is a potential target for preventing traumatic heterotopic ossification.

Autofusion in growing rod surgery for early onset scoliosis; what do we know so far?

The evolving landscape of early onset scoliosis management has shifted from the traditional paradigm of early definitive spinal fusion towards modern growth-friendly implants, particularly Growing Rods (GR). Despite the initial classification of GR treatment as a fusionless procedure, the phenomenon of autofusion has emerged as a critical consideration in understanding its outcomes. Studies have demonstrated the presence of autofusion since the early 1980s. The consequences of autofusion are extensive, impacting curve correction, diminishing trunk growth rate, and contributing to the "law of diminishing returns" in growing rod surgery. The literature suggests that autofusion may complicate definitive fusion surgery, leading to prolonged and intricate procedures involving multiple osteotomies. Additionally, it poses challenges in identifying anatomical landmarks during surgery, potentially increasing the risk of complications and revisions. While autofusion poses challenges to achieving optimal outcomes in growing rod treatment, it cannot be considered a standalone replacement for definitive fusion. Recent advances aim to limit autofusion and enhance treatment outcomes. In this review, we will delve into the existing literature on autofusion, examining studies that have documented its presence, probable causes, pathophysiology, potential implications for long-term patient outcomes, and possible new implants and techniques that decrease its incidence.

The roles of Hippo/YAP signaling pathway in physical therapy

Cellular behavior is regulated by mechanical signals within the cellular microenvironment. Additionally, changes of temperature, blood flow, and muscle contraction also affect cellular state and the development of diseases. In clinical practice, physical therapy techniques such as ultrasound, vibration, exercise, cold therapy, and hyperthermia are commonly employed to alleviate pain and treat diseases. However, the molecular mechanism about how these physiotherapy methods stimulate local tissues and control gene expression remains unknow. Fortunately, the discovery of YAP filled this gap, which has been reported has the ability to sense and convert a wide variety of mechanical signals into cell-specific programs for transcription, thereby offering a fresh perspective on the mechanisms by which physiotherapy treat different diseases. This review examines the involvement of Hippo/YAP signaling pathway in various diseases and its role in different physical therapy approaches on diseases. Furthermore, we explore the potential therapeutic implications of the Hippo/YAP signaling pathway and address the limitations and controversies surrounding its application in physiotherapy.

Vegfc-expressing cells form heterotopic bone after musculoskeletal injury

Heterotopic ossification (HO) is a challenging condition that occurs after musculoskeletal injury and is characterized by the formation of bone in non-skeletal tissues. While the effect of HO on blood vessels is well established, little is known about its impact on lymphatic vessels. Here, we use a mouse model of traumatic HO to investigate the relationship between HO and lymphatic vessels. We show that injury triggers lymphangiogenesis at the injury site, which is associated with elevated vascular endothelial growth factor C (VEGF-C) levels. Through single-cell transcriptomic analyses, we identify mesenchymal progenitor cells and tenocytes as sources of Vegfc. We demonstrate by lineage tracing that Vegfc-expressing cells undergo osteochondral differentiation and contribute to the formation of HO. Last, we show that Vegfc haploinsufficiency results in a nearly 50% reduction in lymphangiogenesis and HO formation. These findings shed light on the complex mechanisms underlying HO formation and its impact on lymphatic vessels.

Regional distribution prevalence of heterotopic ossification in the elbow joint: a 3D study of patients after surgery for traumatic elbow injury

BACKGROUND

Heterotopic ossification (HO) is a common complication after elbow fracture surgery and can lead to severe upper extremity disability. The radiographic localization of postoperative HO has been reported previously. However, there is no literature examining the distribution of postoperative HO at the three-dimensional (3D) level. This study aimed to investigate 1) the distribution characteristics of postoperative HO and 2) the possible risk factors affecting the severity of postoperative HO at a 3D level.

METHODS

A retrospective review was conducted of patients who presented to our institution with HO secondary to elbow fracture between 13 January 2020 and 16 February 2023. Computed tomography scans of 56 elbows before elbow release surgery were reconstructed in 3D. HO was identified using density thresholds combined with manual identification and segmentation. The elbow joint and HO were divided into six regions according to three planes: the transepicondylar plane, the lateral ridge of the trochlear plane, and the radiocapitellar joint and coronoid facet plane. The differences in the volume of regional HO associated with different initial injuries were analyzed.

RESULTS

Postoperative HO was predominantly present in the medial aspect of the capsule in 52 patients (93%), in the lateral aspect of the capsule in 45 patients (80%), in the medial supracondylar in 32 patients (57%), and in the lateral supracondylar, radial head, and ulnar region in the same number of 28 patients (50%). The median and interquartile range volume of total postoperative HO was 1683 (777-4894) mm3. The median and interquartile range volume of regional postoperative HO were: 584 (121-1454) mm3 at medial aspect of capsule, 207 (5-568) mm3 at lateral aspect of capsule, 25 (0-449) mm3 at medial supracondylar, 1 (0-288) at lateral supracondylar, 2 (0-478) at proximal radius and 7 (0-203) mm3 at the proximal ulna. In the subgroups with Injury Severity Score > or = 16, Gustilo-Anderson II, normal uric acid levels, elevated alkaline phosphatase, and body mass index > or = 24, the median HO volume exceeds that of the respective control groups.

CONCLUSION

The medial aspect of the capsule was the area with the highest frequency and median volume of postoperative HO among all initial elbow injury types. Patients with higher Gustilo-Anderson grade, Injury Severity Score, alkaline phosphatase or Body Mass Index had higher median volume of postoperative HO.

Shear stress activates the Piezo1 channel to facilitate valvular endothelium-oriented differentiation and maturation of human induced pluripotent stem cells

Valvular endothelial cells (VECs) derived from human induced pluripotent stem cells (hiPSCs) provide an unlimited cell source for tissue engineering heart valves (TEHVs); however, they are limited by their low differentiation efficiency and immature function. In our study, we applied unidirectional shear stress to promote hiPSCs differentiation into valvular endothelial-like cells (VELs). Compared to the static group, shear stress efficiently promoted the differentiation and functional maturation of hiPSC-VELs, as demonstrated by the efficiency of endothelial differentiation reaching 98.3% in the high shear stress group (45 dyn/cm2). Furthermore, we found that Piezo1 served as a crucial mechanosensor for the differentiation and maturation of VELs. Mechanistically, the activation of Piezo1 by shear stress resulted in the influx of calcium ions, which in turn initiated the Akt signaling pathway and promoted the differentiation of hiPSCs into mature VELs. Moreover, VELs cultured on decellularized heart valves (DHVs) exhibited a notable propensity for proliferation, robust adhesion properties, and antithrombotic characteristics, which were dependent on the activation of the Piezo1 channel. Overall, our study demonstrated that proper shear stress activated the Piezo1 channel to facilitate the differentiation and maturation of hiPSC-VELs via the Akt pathway, providing a potential cell source for regenerative medicine, drug screening, pathogenesis, and disease modeling. STATEMENT OF SIGNIFICANCE: This is the first research that systematically analyzes the effect of shear stress on valvular endothelial-like cells (VELs) derived from human induced pluripotent stem cells (hiPSCs). Mechanistically, unidirectional shear stress activates Piezo1, resulting in an elevation of calcium levels, which triggers the Akt signaling pathway and then facilitates the differentiation of functional maturation VELs. After exposure to shear stress, the VELs exhibited enhanced proliferation, robust adhesion capabilities, and antithrombotic characteristics while being cultured on decellularized heart valves. Thus, it is of interest to develop hiPSCs-VELs using shear stress and the Piezo1 channel provides insights into the functional maturation of valvular endothelial cells, thereby serving as a catalyst for potential applications in the development of therapeutic and tissue-engineered heart valves in the future.

The HIF-1α/PLOD2 axis integrates extracellular matrix organization and cell metabolism leading to aberrant musculoskeletal repair

While hypoxic signaling has been shown to play a role in many cellular processes, its role in metabolism-linked extracellular matrix (ECM) organization and downstream processes of cell fate after musculoskeletal injury remains to be determined. Heterotopic ossification (HO) is a debilitating condition where abnormal bone formation occurs within extra-skeletal tissues. Hypoxia and hypoxia-inducible factor 1α (HIF-1α) activation have been shown to promote HO. However, the underlying molecular mechanisms by which the HIF-1α pathway in mesenchymal progenitor cells (MPCs) contributes to pathologic bone formation remain to be elucidated. Here, we used a proven mouse injury-induced HO model to investigate the role of HIF-1α on aberrant cell fate. Using single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics analyses of the HO site, we found that collagen ECM organization is the most highly up-regulated biological process in MPCs. Zeugopod mesenchymal cell-specific deletion of Hif1α (Hoxa11-CreERT2; Hif1afl/fl) significantly mitigated HO in vivo. ScRNA-seq analysis of these Hoxa11-CreERT2; Hif1afl/fl mice identified the PLOD2/LOX pathway for collagen cross-linking as downstream of the HIF-1α regulation of HO. Importantly, our scRNA-seq data and mechanistic studies further uncovered that glucose metabolism in MPCs is most highly impacted by HIF-1α deletion. From a translational aspect, a pan-LOX inhibitor significantly decreased HO. A newly screened compound revealed that the inhibition of PLOD2 activity in MPCs significantly decreased osteogenic differentiation and glycolytic metabolism. This suggests that the HIF-1α/PLOD2/LOX axis linked to metabolism regulates HO-forming MPC fate. These results suggest that the HIF-1α/PLOD2/LOX pathway represents a promising strategy to mitigate HO formation.

Engineered stem cells by emerging biomedical stratagems

Stem cell therapy holds immense potential as a viable treatment for a widespread range of intractable disorders. As the safety of stem cell transplantation having been demonstrated in numerous clinical trials, various kinds of stem cells are currently utilized in medical applications. Despite the achievements, the therapeutic benefits of stem cells for diseases are limited, and the data of clinical researches are unstable. To optimize tthe effectiveness of stem cells, engineering approaches have been developed to enhance their inherent abilities and impart them with new functionalities, paving the way for the next generation of stem cell therapies. This review offers a detailed analysis of engineered stem cells, including their clinical applications and potential for future development. We begin by briefly introducing the recent advances in the production of stem cells (induced pluripotent stem cells (iPSCs), embryonic stem cells (ESCs), mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs)). Furthermore, we present the latest developments of engineered strategies in stem cells, including engineered methods in molecular biology and biomaterial fields, and their application in biomedical research. Finally, we summarize the current obstacles and suggest future prospects for engineered stem cells in clinical translations and biomedical applications.

Enhanced BMP signaling in Cathepsin K-positive tendon progenitors induces heterotopic ossification

Heterotopic ossification (HO) is abnormal bone growth in soft tissues that results from injury, trauma, and rare genetic disorders. Bone morphogenetic proteins (BMPs) are critical osteogenic regulators which are involved in HO. However, it remains unclear how BMP signaling interacts with other extracellular stimuli to form HO. To address this question, using the Cre-loxP recombination system in mice, we conditionally expressed the constitutively activated BMP type I receptor ALK2 with a Q207D mutation (Ca-ALK2) in Cathepsin K-Cre labeled tendon progenitors (hereafter "Ca-Alk2:Ctsk-Cre"). Ca-Alk2:Ctsk-Cre mice were viable but they formed spontaneous HO in the Achilles tendon. Histological and molecular marker analysis revealed that HO is formed via endochondral ossification. Ectopic chondrogenesis coincided with enhanced GLI1 production, suggesting that elevated Hedgehog (Hh) signaling is involved in the pathogenesis of HO. Interestingly, focal adhesion kinase, a critical mediator for the mechanotransduction pathway, was also activated in Ca-Alk2:Ctsk-Cre mice. Our findings suggest that enhanced BMP signaling may elevate Hh and mechanotransduction pathways, thereby causing HO in the regions of the Achilles tendon.

TrkA-mediated sensory innervation of injured mouse tendon supports tendon sheath progenitor cell expansion and tendon repair

Peripheral neurons terminate at the surface of tendons partly to relay nociceptive pain signals; however, the role of peripheral nerves in tendon injury and repair remains unclear. Here, we show that after Achilles tendon injury in mice, there is new nerve growth near tendon cells that express nerve growth factor (NGF). Conditional deletion of the Ngf gene in either myeloid or mesenchymal mouse cells limited both innervation and tendon repair. Similarly, inhibition of the NGF receptor tropomyosin receptor kinase A (TrkA) abrogated tendon healing in mouse tendon injury. Sural nerve transection blocked the postinjury increase in tendon sensory innervation and the expansion of tendon sheath progenitor cells (TSPCs) expressing tubulin polymerization promoting protein family member 3. Single cell and spatial transcriptomics revealed that disruption of sensory innervation resulted in dysregulated inflammatory signaling and transforming growth factor-β (TGFβ) signaling in injured mouse tendon. Culture of mouse TSPCs with conditioned medium from dorsal root ganglia neuron further supported a role for neuronal mediators and TGFβ signaling in TSPC proliferation. Transcriptomic and histologic analyses of injured human tendon biopsy samples supported a role for innervation and TGFβ signaling in human tendon regeneration. Last, treating mice after tendon injury systemically with a small-molecule partial agonist of TrkA increased neurovascular response, TGFβ signaling, TSPC expansion, and tendon tissue repair. Although further studies should investigate the potential effects of denervation on mechanical loading of tendon, our results suggest that peripheral innervation is critical for the regenerative response after acute tendon injury.

The influence of microenvironment stiffness on endothelial cell fate: Implication for occurrence and progression of atherosclerosis

Atherosclerosis, the primary cause of cardiovascular diseases (CVDs), is characterized by phenotypic changes in fibrous proliferation, chronic inflammation and lipid accumulation mediated by vascular endothelial cells (ECs) and vascular smooth muscle cells (SMCs) which are correlated with the stiffening and ectopic remodeling of local extracellular matrix (ECM). The native residents, ECs and SMCs, are not only affected by various chemical factors including inflammatory mediators and chemokines, but also by a range of physical stimuli, such as shear stress and ECM stiffness, presented in the microenvironmental niche. Especially, ECs, as a semi-selective barrier, can sense mechanical forces, respond quickly to changes in mechanical loading and provide context-specific adaptive responses to restore homeostasis. However, blood arteries undergo stiffening and lose their elasticity with age. Reports have shown that the ECM stiffening could influence EC fate by changing the cell adhesion, spreading, proliferation, cell to cell contact, migration and even communication with SMCs. The cell behaviour changes mediated by ECM stiffening are dependent on the activation of a signaling cascade of mechanoperception and mechanotransduction. Although the substantial evidence directly indicates the importance of ECM stiffening on the native ECs, the understanding about this complex interplay is still largely limited. In this review, we systematically summarize the roles of ECM stiffening on the behaviours of endothelial cells and elucidate the underlying details in biological mechanism, aiming to provide the process of how ECs integrate ECM mechanics and the highlights for bioaffinity of tissue-specific engineered scaffolds.

Neutrophil and NETosis Modulation in Traumatic Heterotopic Ossification

OBJECTIVE

To characterize the role of neutrophil extracellular traps (NETs) in heterotopic ossification (HO) formation and progression and to use mechanical and pharmacological methods to decrease NETosis and mitigate HO formation.

BACKGROUND

Traumatic HO is the aberrant osteochondral differentiation of mesenchymal progenitor cells after traumatic injury, burns, or surgery. While the innate immune response has been shown to be necessary for HO formation, the specific immune cell phenotype and function remain unknown. Neutrophils, one of the earliest immune cells to respond after HO-inducing injuries, can extrude DNA, forming highly inflammatory NETs. We hypothesized that neutrophils and NETs would be diagnostic biomarkers and therapeutic targets for the detection and mitigation of HO.

METHODS

C57BL6J mice underwent burn/tenotomy (a well-established mouse model of HO) or a non-HO-forming sham injury. These mice were either (1) ambulated ad libitum, (2) ambulated ad libitum with daily intraperitoneal hydroxychloroquine, ODN-2088 (both known to affect NETosis pathways), or control injections, or (3) had the injured hind limb immobilized. Single-cell analysis was performed to analyze neutrophils, NETosis, and downstream signaling after the HO-forming injury. Immunofluorescence microscopy was used to visualize NETosis at the HO site and neutrophils were identified using flow cytometry. Serum and cell lysates from HO sites were analyzed using enzyme-linked immunosorbent assay for myeloperoxidase-DNA and ELA2-DNA complexes to identify NETosis. Micro-computerized tomography was performed on all groups to analyze the HO volume.

RESULTS

Molecular and transcriptional analyses revealed the presence of NETs within the HO injury site, which peaked in the early phases after injury. These NETs were highly restricted to the HO site, with gene signatures derived from both in vitro NET induction and clinical neutrophil characterizations showing a high degree of NET "priming" at the site of injury, but not in neutrophils in the blood or bone marrow. Cell-cell communication analyses revealed that this localized NET formation coincided with high levels of toll-like receptor signaling specific to neutrophils at the injury site. Reducing the overall neutrophil abundance within the injury site, either pharmacologically through treatment with hydroxychloroquine, the toll-like receptor 9 inhibitor OPN-2088, or mechanical treatment with limb offloading, results in the mitigation of HO formation.

CONCLUSIONS

These data provide a further understanding of the ability of neutrophils to form NETs at the injury site, clarify the role of neutrophils in HO, and identify potential diagnostic and therapeutic targets for HO mitigation.

Itaconate-producing neutrophils regulate local and systemic inflammation following trauma

Modulation of the immune response to initiate and halt the inflammatory process occurs both at the site of injury as well as systemically. Due to the evolving role of cellular metabolism in regulating cell fate and function, tendon injuries that undergo normal and aberrant repair were evaluated by metabolic profiling to determine its impact on healing outcomes. Metabolomics revealed an increasing abundance of the immunomodulatory metabolite itaconate within the injury site. Subsequent single-cell RNA-Seq and molecular and metabolomic validation identified a highly mature neutrophil subtype, not macrophages, as the primary producers of itaconate following trauma. These mature itaconate-producing neutrophils were highly inflammatory, producing cytokines that promote local injury fibrosis before cycling back to the bone marrow. In the bone marrow, itaconate was shown to alter hematopoiesis, skewing progenitor cells down myeloid lineages, thereby regulating systemic inflammation. Therapeutically, exogenous itaconate was found to reduce injury-site inflammation, promoting tenogenic differentiation and impairing aberrant vascularization with disease-ameliorating effects. These results present an intriguing role for cycling neutrophils as a sensor of inflammation induced by injury - potentially regulating immune cell production in the bone marrow through delivery of endogenously produced itaconate - and demonstrate a therapeutic potential for exogenous itaconate following tendon injury.

Ultrasound-Driven Healing: Unleashing the Potential of Chondrocyte-Derived Extracellular Vesicles for Chondrogenesis in Adipose-Derived Stem Cells

Repairing cartilage defects represents a significant clinical challenge. While adipose-derived stem cell (ADSC)-based strategies hold promise for cartilage regeneration, their inherent chondrogenic potential is limited. Extracellular vesicles (EVs) derived from chondrocytes (CC-EVs) have shown potential in enhancing chondrogenesis, but their role in promoting chondrogenic differentiation of ADSCs remains poorly understood. Moreover, the clinical application of EVs faces limitations due to insufficient quantities for in vivo use, necessitating the development of effective methods for extracting significant amounts of CC-EVs. Our previous study demonstrated that low-intensity ultrasound (LIUS) stimulation enhances EV secretion from mesenchymal stem cells. Here, we identified a specific LIUS parameter for chondrocytes that increased EV secretion by 16-fold. CC-EVs were found to enhance cell activity, proliferation, migration, and 21-day chondrogenic differentiation of ADSCs in vitro, while EVs secreted by chondrocytes following LIUS stimulation (US-CC-EVs) exhibited superior efficacy. miRNA-seq revealed that US-CC-EVs were enriched in cartilage-regeneration-related miRNAs, contributing to chondrogenesis in various biological processes. In conclusion, we found that CC-EVs can enhance the chondrogenesis of ADSCs in vitro. In addition, our study introduces ultrasound-driven healing as an innovative method to enhance the quantity and quality of CC-EVs, meeting clinical demand and addressing the limited chondrogenic potential of ADSCs. The ultrasound-driven healing unleashes the potential of CC-EVs for chondrogenesis possibly through the enrichment of cartilage-regeneration-associated miRNAs in EVs, suggesting their potential role in cartilage reconstruction. These findings hold promise for advancing cartilage regeneration strategies and may pave the way for novel therapeutic interventions in regenerative medicine.

Effect of mechanical stimulation on tissue heterotopic ossification: an in vivo experimental study

Background: Heterotopic ossification of tendons and ligaments (HOTL) is a common clinical condition characterized by the absence of discernible features and a lack of effective treatment. In vitro experiments have demonstrated that mechanical stimulation can induce cell differentiation toward osteogenesis, thereby promoting heterotopic ossification. Currently, there are few experimental designs aimed at inducing ligament stretching in mice, and the mechanism of heterotopic ossification may not entirely mirror that observed in clinical cases. Therefore, there is an urgent imperative to develop a novel and feasible animal model. Methods: In this study, all the Enpp1 gene deficiency mice (a mouse model with heterotopic ossification of multiple ligaments) were divided into three groups: the control group, the spinal brake group, and the hyperactive group (treadmill training group). An external spinal fixation device was designed to restrict mice's spinal flexion and extension at 6 weeks of age. The brace was adjusted weekly according to the changes in the size of the mice. Additionally, treadmill training was used to increase activity in the spinal ligaments and Achilles tendons of the mice. Micro-CT scanning and HE staining were performed at 12, 20, and 28 W to evaluate the degree of ossification in the spinal ligament and Achilles tendon. What's more, As one of the mechanical stimulation transduction signals, YAP plays a crucial role in promoting osteogenic differentiation of cells. Immunofluorescence was utilized to assess YAP expression levels for the purpose of determining the extent of mechanical stimulation in tissues. Results: Our findings showed that a few ossification lesions were detected behind the vertebral space of mice at 8 weeks of age. Spinal immobilization effectively restricts the flexion and extension of cervical and thoracic vertebrae in mice, delaying spinal ligament ossification and reducing chronic secondary spinal cord injury. Running exercises not only enhance the ossification area of the posterior longitudinal ligament (PLL) and Achilles tendons but also exacerbate secondary spinal cord injury. Further immunofluorescence results revealed a notable increase in YAP expression levels in tissues with severe ossification, suggesting that these tissues may be subjected to higher mechanical stimulation. Conclusion: Mechanical stimulation plays a pivotal role in the process of heterotopic ossification in tissues. Our study provided valid animal models to further explore the pathological mechanism of mechanical stimulation in HOTL development.

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Adult tissue-specific stem cell interaction: novel technologies and research advances

Adult tissue-specific stem cells play a dominant role in tissue homeostasis and regeneration. Various in vivo markers of adult tissue-specific stem cells have been increasingly reported by lineage tracing in genetic mouse models, indicating that marked cells differentiation is crucial during homeostasis and regeneration. How adult tissue-specific stem cells with indicated markers contact the adjacent lineage with indicated markers is of significance to be studied. Novel methods bring future findings. Recent advances in lineage tracing, synthetic receptor systems, proximity labeling, and transcriptomics have enabled easier and more accurate cell behavior visualization and qualitative and quantitative analysis of cell-cell interactions than ever before. These technological innovations have prompted researchers to re-evaluate previous experimental results, providing increasingly compelling experimental results for understanding the mechanisms of cell-cell interactions. This review aimed to describe the recent methodological advances of dual enzyme lineage tracing system, the synthetic receptor system, proximity labeling, single-cell RNA sequencing and spatial transcriptomics in the study of adult tissue-specific stem cells interactions. An enhanced understanding of the mechanisms of adult tissue-specific stem cells interaction is important for tissue regeneration and maintenance of homeostasis in organisms.

Engineering Cell-ECM-Material Interactions for Musculoskeletal Regeneration

The extracellular microenvironment regulates many of the mechanical and biochemical cues that direct musculoskeletal development and are involved in musculoskeletal disease. The extracellular matrix (ECM) is a main component of this microenvironment. Tissue engineered approaches towards regenerating muscle, cartilage, tendon, and bone target the ECM because it supplies critical signals for regenerating musculoskeletal tissues. Engineered ECM-material scaffolds that mimic key mechanical and biochemical components of the ECM are of particular interest in musculoskeletal tissue engineering. Such materials are biocompatible, can be fabricated to have desirable mechanical and biochemical properties, and can be further chemically or genetically modified to support cell differentiation or halt degenerative disease progression. In this review, we survey how engineered approaches using natural and ECM-derived materials and scaffold systems can harness the unique characteristics of the ECM to support musculoskeletal tissue regeneration, with a focus on skeletal muscle, cartilage, tendon, and bone. We summarize the strengths of current approaches and look towards a future of materials and culture systems with engineered and highly tailored cell-ECM-material interactions to drive musculoskeletal tissue restoration. The works highlighted in this review strongly support the continued exploration of ECM and other engineered materials as tools to control cell fate and make large-scale musculoskeletal regeneration a reality.

Influence of PHA Substrate Surface Characteristics on the Functional State of Endothelial Cells

The needs of modern regenerative medicine for biodegradable polymers are wide and varied. Restoration of the viability of the vascular tree is one of the most important components of the preservation of the usefulness of organs and tissues. The creation of vascular implants compatible with blood is an important task of vascular bioengineering. The function of the endothelial layer of the vessel, being largely responsible for the development of thrombotic complications, is of great importance for hemocompatibility. The development of surfaces with specific characteristics of biomaterials that are used in vascular technologies is one of the solutions for their correct endothelialization. Linear polyhydroxyalkanoates (PHAs) are biodegradable structural polymeric materials suitable for obtaining various types of implants and tissue engineering, having a wide range of structural and physicomechanical properties. The use of PHA of various monomeric compositions in endothelial cultivation makes it possible to evaluate the influence of material properties, especially surface characteristics, on the functional state of cells. It has been established that PHA samples with the inclusion of 3-hydroxyhexanoate have optimal characteristics for the formation of a human umbilical vein endothelial cell, HUVEC, monolayer in terms of cell morphology as well as the levels of expression of vinculin and VE-cadherin. The obtained results provide a rationale for the use of PHA copolymers as materials for direct contact with the endothelium in vascular implants.

Heterotopic mineral deposits in intact rat Achilles tendons are characterized by a unique fiber-like structure

Heterotopic mineralization entails pathological mineral formation inside soft tissues. In human tendons mineralization is often associated with tendinopathies, tendon weakness and pain. In Achilles tendons, mineralization is considered to occur through heterotopic ossification (HO) primarily in response to tendon pathologies. However, refined details regarding HO deposition and microstructure are unknown. In this study, we characterize HO in intact rat Achilles tendons through high-resolution phase contrast enhanced synchrotron X-ray tomography. Furthermore, we test the potential of studying local tissue injury by needling intact Achilles tendons and the relation between tissue microdamage and HO. The results show that HO occurs in all intact Achilles tendons at 16 weeks of age. HO deposits are characterized by an elongated ellipsoidal shape and by a fiber-like internal structure which suggests that some collagen fibers have mineralized. The data indicates that deposition along fibers initiates in the pericellular area, and propagates into the intercellular area. Within HO deposits cells are larger and more rounded compared to tenocytes between unmineralized fibers, which are fewer and elongated. The results also indicate that multiple HO deposits may merge into bigger structures with time by accession along unmineralized fibers. Furthermore, the presence of unmineralized regions within the deposits may indicate that HOs are not only growing, but mineral resorption may also occur. Additionally, phase contrast synchrotron X-ray tomography allowed to distinguish microdamage at the fiber level in response to needling. The needle injury protocol could in the future enable to elucidate the relation between local inflammation, microdamage, and HO deposition.

Discoidin domain receptor 2 regulates aberrant mesenchymal lineage cell fate and matrix organization

Extracellular matrix (ECM) interactions regulate both the cell transcriptome and proteome, thereby determining cell fate. Traumatic heterotopic ossification (HO) is a disorder characterized by aberrant mesenchymal lineage (MLin) cell differentiation, forming bone within soft tissues of the musculoskeletal system following traumatic injury. Recent work has shown that HO is influenced by ECM-MLin cell receptor signaling, but how ECM binding affects cellular outcomes remains unclear. Using time course transcriptomic and proteomic analyses, we identified discoidin domain receptor 2 (DDR2), a cell surface receptor for fibrillar collagen, as a key MLin cell regulator in HO formation. Inhibition of DDR2 signaling, through either constitutive or conditional Ddr2 deletion or pharmaceutical inhibition, reduced HO formation in mice. Mechanistically, DDR2 perturbation alters focal adhesion orientation and subsequent matrix organization, modulating Focal Adhesion Kinase (FAK) and Yes1 Associated Transcriptional Regulator and WW Domain Containing Transcription Regulator 1 (YAP/TAZ)-mediated MLin cell signaling. Hence, ECM-DDR2 interactions are critical in driving HO and could serve as a previously unknown therapeutic target for treating this disease process.

Predicting the formation of different tissue types during Achilles tendon healing using mechanoregulated and oxygen-regulated frameworks

During Achilles tendon healing in rodents, besides the expected tendon tissue, also cartilage-, bone- and fat-like tissue features have been observed during the first twenty weeks of healing. Several studies have hypothesized that mechanical loading may play a key role in the formation of different tissue types during healing. We recently developed a computational mechanobiological framework to predict tendon tissue production, organization and mechanical properties during tendon healing. In the current study, we aimed to explore possible mechanobiological related mechanisms underlying formation of other tissue types than tendon tissue during tendon healing. To achieve this, we further developed our recent framework to predict formation of different tissue types, based on mechanobiological models established in other fields, which have earlier not been applied to study tendon healing. We explored a wide range of biophysical stimuli, i.e., principal strain, hydrostatic stress, pore pressure, octahedral shear strain, fluid flow, angiogenesis and oxygen concentration, that may promote the formation of different tissue types. The numerical framework predicted spatiotemporal formation of tendon-, cartilage-, bone- and to a lesser degree fat-like tissue throughout the first twenty weeks of healing, similar to recent experimental reports. Specific features of experimental data were captured by different biophysical stimuli. Our modeling approach showed that mechanobiology may play a role in governing the formation of different tissue types that have been experimentally observed during tendon healing. This study provides a numerical tool that can contribute to a better understanding of tendon mechanobiology during healing. Developing these tools can ultimately lead to development of better rehabilitation regimens that stimulate tendon healing and prevent unwanted formation of cartilage-, fat- and bone-like tissues.

Exercise Promotes Bone Marrow Microenvironment by Inhibiting Adipsin in Diet-Induced Male Obese Mice

Obesity is a growing global epidemic linked to many diseases, including diabetes, cardiovascular diseases, and musculoskeletal disorders. Exercise can improve bone density and decrease excess bone marrow adipose tissue (BMAT) in obese individuals. However, the mechanism of exercise regulating bone marrow microenvironment remains unclear. This study examines how exercise induces bone marrow remodeling in diet-induced obesity. We employed unbiased RNA-Seq to investigate the effect of exercise on the bone marrow of diet-induced obese male mice. Bone mesenchymal stem cells (BMSCs) were isolated to explore the regulatory effects of exercise in vitro. Our data demonstrated that exercise could slow down the progression of obesity and improve trabecular bone density. RNA-seq data revealed that exercise inhibited secreted phosphoprotein 1 (Spp1), which was shown to mediate bone resorption through mechanosensing mechanisms. Interactome analysis of Spp1 using the HINT database showed that Spp1 interacted with the adipokine adipsin. Moreover, exercise decreased BMAT, which induced osteoclast differentiation and promoted bone loss. Our study reveals that exercise improves the bone marrow microenvironment by at least partially inhibiting the adipsin-Spp1 signaling pathway so as to inhibit the alternative complement system from activating osteoclasts in diet-induced obese mice.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell–cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell–cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C–C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

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Sox9-positive skeletal progenitors contributed to the calvaria bone repair process.

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scRNA-seq analysis revealed a heterogeneous cell population at bone defect sites.

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Skeletal progenitors had a bifurcated lineages of osteogenesis and adipogenesis.

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Ccl9 was identified as an important signaling molecule regulating bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell mapping of regenerative and fibrotic healing responses after musculoskeletal injury

After injury, a cascade of events repairs the damaged tissue, including expansion and differentiation of the progenitor pool and redeposition of matrix. To guide future wound regeneration strategies, we compared single-cell sequencing of regenerative (third phalangeal element [P3]) and fibrotic (second phalangeal element [P2]) digit tip amputation (DTA) models as well as traumatic heterotopic ossification (HO; aberrant). Analyses point to a common initial response to injury, including expansion of progenitors, redeposition of matrix, and activation of transforming growth factor β (TGF-β) and WNT pathways. Surprisingly, fibrotic P2 DTA showed greater transcriptional similarity to HO than to regenerative P3 DTA, suggesting that gene expression more strongly correlates with healing outcome than with injury type or cell origin. Differential analysis and immunostaining revealed altered activation of inflammatory pathways, such as the complement pathway, in the progenitor cells. These data suggests that common pathways are activated in response to damage but are fine tuned within each injury. Modulating these pathways may shift the balance toward regenerative outcomes.

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Regenerative and fibrotic injuries share common early response mechanisms

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Transcriptomes correlate with healing outcome more than injury type or cell source

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Matrix composition after injury-induced tissue repair is highly injury type dependent

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Inflammatory cascades are activated in immune and mesenchymal cells

Single-Cell Analyses of Heterotopic Ossification: Characteristics of Injury-Related Senescent Fibroblasts

Introduction

Injury-related cellular senescence may be involved in heterotopic ossification, and no research has been performed about this before.

Methods and Results

The study utilized integrated single-cell RNA-sequencing (scRNA-seq) data from heterotopic ossification samples. The number of senescent cells increased from day 3 and reached the highest level at day 21. However, the expression level of Cyclin Dependent Kinase Inhibitor 2A (Cdkn2a) has no such tendency as the change of cell amount, indicating that the expression level of Cdkn2a may be different in different types of senescent cells or the same time of senescent cell at different time points. The expression level of SASPs (senescence associated secret phenotypes) was also different in different types of senescent cells or at different time points. The GO (gene ontology) analysis revealed that the senescent cells were significantly correlated with the ossification processes, like ECM organization, cell adhesion, ossification, cartilage development, etc. Trajectory analysis showed that injury-related senescent fibroblasts (day 7 and 21) and age-related senescent fibroblasts (day 0 and 42) were in different branches. GO analysis demonstrated that injury-related senescent fibroblasts were mainly related to ossification and ECM remodeling. The KEGG (Kyoto Encyclopedia of Genes and Genomes) results revealed that the ossification was significantly corrected with protein processing in PI3K-Akt signaling, MAPK signaling, focal adhesion, etc.

Conclusion

Consequently, we demonstrated that, unlike age-related senescence, the injury-related senescence demonstrated significantly different SASP phenotypes. The injury-related senescence of fibroblasts is associated with heterotopic ossification formation and may act through PI3K/Akt-induced SASPs.

The levels of oxidative stress in a combination of stress factors

We studied the effect of the combined action of ionizing radiation and induced immobilization stress on the lipid peroxidation process and antioxidant protection of organs (mesenteric lymph nodes, spleen, adrenal glands, thymus, and liver) and immune cels - the blood lymphocytes. Results were obtained on the role of free-radical oxidation in combination with exposure to ionizing radiation and immobilization stress at an early stage in the experiment. Gamma radiation in the acute period resulted in significant changes in lipoperoxidation and antioxidant systems. The first period of immobilization stress was marked by the imbalance of LPO-AOS systems disturbance with an accumulation of toxic compounds in tissues which had affected their function. The combined sublethal gamma radiation and immobilization stress disturbed the functional activity of adaptive systems of the body in the early stage of adaptation syndrome. Furthermore, the results show the dominant role of ionizing radiation in it.

A new prognostic nomogram for heterotopic ossification formation after elbow trauma : the Shanghai post-Traumatic Elbow Heterotopic Ossification Prediction (STEHOP) model

AIMS

Heterotopic ossification (HO) is a common complication after elbow trauma and can cause severe upper limb disability. Although multiple prognostic factors have been reported to be associated with the development of post-traumatic HO, no model has yet been able to combine these predictors more succinctly to convey prognostic information and medical measures to patients. Therefore, this study aimed to identify prognostic factors leading to the formation of HO after surgery for elbow trauma, and to establish and validate a nomogram to predict the probability of HO formation in such particular injuries.

METHODS

This multicentre case-control study comprised 200 patients with post-traumatic elbow HO and 229 patients who had elbow trauma but without HO formation between July 2019 and December 2020. Features possibly associated with HO formation were obtained. The least absolute shrinkage and selection operator regression model was used to optimize feature selection. Multivariable logistic regression analysis was applied to build the new nomogram: the Shanghai post-Traumatic Elbow Heterotopic Ossification Prediction model (STEHOP). STEHOP was validated by concordance index (C-index) and calibration plot. Internal validation was conducted using bootstrapping validation.

RESULTS

Male sex, obesity, open wound, dislocations, late definitive surgical treatment, and lack of use of non-steroidal anti-inflammatory drugs were identified as adverse predictors and incorporated to construct the STEHOP model. It displayed good discrimination with a C-index of 0.80 (95% confidence interval 0.75 to 0.84). A high C-index value of 0.77 could still be reached in the internal validation. The calibration plot showed good agreement between nomogram prediction and observed outcomes.

CONCLUSION

The newly developed STEHOP model is a valid and convenient instrument to predict HO formation after surgery for elbow trauma. It could assist clinicians in counselling patients regarding treatment expectations and therapeutic choices. Cite this article: Bone Joint J 2022;104-B(8):963-971.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Acetabular Reaming Is a Reliable Model to Produce and Characterize Periarticular Heterotopic Ossification of the Hip

Heterotopic ossification (HO) is a pathologic process characterized by the formation of bone tissue in extraskeletal locations. The hip is a common location of HO, especially as a complication of arthroplasty. Here, we devise a first-of-its-kind mouse model of post-surgical hip HO and validate expected cell sources of HO using several HO progenitor cell reporter lines. To induce HO, an anterolateral surgical approach to the hip was used, followed by disclocation and acetabular reaming. Animals were analyzed with high-resolution roentgenograms and micro-computed tomography, conventional histology, immunohistochemistry, and assessments of fluorescent reporter activity. All the treated animals' developed periarticular HO with an anatomical distribution similar to human patients after arthroplasty. Heterotopic bone was found in periosteal, inter/intramuscular, and intracapsular locations. Further, the use of either PDGFRα or scleraxis (Scx) reporter mice demonstrated that both cell types gave rise to periarticular HO in this model. In summary, acetabular reaming reproducibly induces periarticular HO in the mouse reproducing human disease, and with defined mesenchymal cellular contributors similar to other experimental HO models. This protocol may be used in the future for further detailing of the cellular and molecular mediators of post-surgical HO, as well as the screening of new therapies.

Key Genes Identified in Nonsyndromic Microtia by the Analysis of Transcriptomics and Proteomics

As one of the common birth defects worldwide, nonsyndromic microtia is a complex disease that results from interactions between environmental and genetic factors. However, the underlying causes of nonsyndromic microtia are currently not well understood. The present study determined transcriptomic and proteomic profiles of auricular cartilage tissues in 10 patients with third-degree nonsyndromic microtia and five control subjects by RNA microarray and tandem mass tag-based quantitative proteomics technology. Relative mRNA and protein abundances were compared and evaluated for their function and putative involvement in nonsyndromic microtia. A total of 3971 differentially expressed genes and 256 differentially expressed proteins were identified. Bioinformatics analysis demonstrated that some of these genes and proteins showed potential associations with nonsyndromic microtia. Thirteen proteins with the same trend at the mRNA level obtained by the integrated analysis were validated by parallel reaction monitoring analysis. Several key genes, namely, LAMB2, COMP, APOA2, APOC2, APOC3, and A2M, were found to be dysregulated, which could contribute to nonsyndromic microtia. The present study is the first report on the transcriptomic and proteomic integrated analysis of nonsyndromic microtia using the same auricular cartilage sample. Additional studies are required to clarify the roles of potential key genes in nonsyndromic microtia.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Comparative proteomic analysis identifies differentially expressed proteins and reveals potential mechanisms of traumatic heterotopic ossification progression

Background

Traumatic Heterotopic Ossification (tHO) is one of complications of elbow fractures to the detriment of patients' rehabilitation, and the severity of tHO corresponds to the size of ectopic bone. It has yet to be elucidated which proteins and pathways underlying the progression of tHO, and biomarkers to predict the severity of tHO at early stage of the disease also need further investigation.

Methods

In this study, a new rat model with distinct volume of ectopic bone was established first. Then a data-independent acquisition proteomics approach was used to investigate injured site tissues sequentially obtained from these rats (2, 7, 14, and 28 days post-injury). Differentially expressed analysis, functional annotation and co-expression analysis and protein-protein interaction network were performed to explore the pathways and hub proteins in the tHO progression. Clinical samples from a nest case-control study were used to validate the selected proteins for predicting the severity of tHO.

Results

The Achilles Tenotomy (AT) induced significantly larger sizes of ectopic bone compared to Partial Achilles Tenotomy (PAT) in rat models. A total of 3547 quantifiable proteins were screened for differential expression analysis among the AT, PAT and control groups. The hierarchical clustering and expression pattern analysis revealed more apparent difference in the pathways such as oxidative phosphorylation, mitochondrial function, and sirtuin signaling between AT and PAT group at the early stage (2 dpi) of tHO. The co-expression analysis identified five hub proteins, UBA1, EIF3E, RPL17, RPL27, and RPS28. qPCR assay, immunoblot assay and immunohistochemistry assay verified that these proteins had higher expression level in the tissue samples of clinically relevant HO patients and clinically irrelevant HO patients than HO negative patients.

Conclusion

The new established animal model and proteome profile could serve as a solid foundation for the comprehensive investigation of the progression of traumatic heterotopic ossification. And the identified 5 proteins (UBA1, EIF3E, RPL17, RPL27, and RPS28) may serve as potential biomarkers to predict the severity of tHO.

The translational potential of this article

The proteins identified in this study may be the potential biomarkers and therapeutic targets for predicting and treating the tHO at early stage.

Inflammation-Stimulated MSC-Derived Small Extracellular Vesicle miR-27b-3p Regulates Macrophages by Targeting CSF-1 to Promote Temporomandibular Joint Condylar Regeneration

Small extracellular vesicles (sEVs) secreted by mesenchymal stem cells (MSCs) have been extensively studied in recent years. sEV contents change with the secreting cell state. When MSCs are exposed to an inflammatory environment, they release more functional growth factors, exosomes, and chemokines. Herein, MSCs are stimulated to alter sEV cargos and functions to regulate the inflammatory microenvironment and promote tissue regeneration. Sequencing of sEV miRNAs shows that certain RNAs conducive to cell function are upregulated. In this study, in vitro cell function experiments show that both inflammation-stimulated adipose-derived MSC (ADSC)-derived sEV (IAE) and normal ADSC-derived sEV (AE) promote cell proliferation; IAE also significantly improves cell migration. Regarding macrophage polarization regulation, IAE significantly promotes M2 macrophage differentiation. RNA-sequencing analysis indicates that high miR-27b-3p expression levels in IAE may regulate macrophages by targeting macrophage colony-stimulating factor-1 (CSF-1). In vivo, a rabbit temporomandibular joint (TMJ) condylar osteochondral defect model shows that both AE and IAE promote TMJ regeneration, with IAE having the most significant therapeutic effect. Therefore, the authors confirm that exposing MSCs to an inflammatory environment can feasibly enhance sEV functions and that modified sEVs achieve better therapeutic effects.

Heterotopic Ossification: Clinical Features, Basic Researches, and Mechanical Stimulations

Heterotopic ossification (HO) is defined as the occurrence of extraskeletal bone in soft tissue. Although this pathological osteogenesis process involves the participation of osteoblasts and osteoclasts during the formation of bone structures, it differs from normal physiological osteogenesis in many features. In this article, the primary characteristics of heterotopic ossification are reviewed from both clinical and basic research perspectives, with a special highlight on the influence of mechanics on heterotopic ossification, which serves an important role in the prophylaxis and treatment of HO.

Aberrant Bone Regulation in Albright Hereditary Osteodystrophy dueto Gnas Inactivation: Mechanisms and Translational Implications

PURPOSE OF REVIEW

This review highlights the impact of Gnas inactivation on both bone remodeling and the development of heterotopic subcutaneous ossifications in Albright hereditary osteodystrophy (AHO). Here we discuss recent advancements in understanding the pathophysiologic mechanisms of the aberrant bone development in AHO as well as potential translational implications.

RECENT FINDINGS

Gnas inactivation can regulate the differentiation and function of not only osteoblasts but also osteoclasts and osteocytes. Investigations utilizing a mouse model of AHO generated by targeted disruption of Gnas have revealed that bone formation and resorption are differentially affected based upon the parental origin of the Gnas mutation. Data suggest that Gnas inactivation leads to heterotopic bone formation within subcutaneous tissue by changing the connective tissue microenvironment, thereby promoting osteogenic differentiation of tissue-resident mesenchymal progenitors. Observed variations in bone formation and resorption based upon the parental origin of the Gnas mutation warrant future investigations and may have implications in the management and treatment of AHO and related conditions. Additionally, studies of heterotopic bone formation due to Gnas inactivation have identified an essential role of sonic hedgehog signaling, which could have therapeutic implications not only for AHO and related conditions but also for heterotopic bone formation in a wide variety of settings in which aberrant bone formation is a cause of significant morbidity.

Transplantation of umbilical cord-derived mesenchymal stem cells promotes the recovery of thin endometrium in rats

The endometrium plays a critical role in embryo implantation and pregnancy, and a thin uterus is recognized as a key factor in embryo implantation failure. Umbilical cord mesenchymal stem cells (UC-MSCs) have attracted interest for the repair of intrauterine adhesions. The current study investigated the repair of thin endometrium in rats using the UC-MSCs and the mechanisms involved. Rats were injected with 95% ethanol to establish a model of thin endometrium. The rats were randomly divided into normal, sham, model, and UC-MSCs groups. Endometrial morphological alterations were observed by hematoxylin-eosin staining and Masson staining, and functional restoration was assessed by testing embryo implantation. The interaction between UC-MSCs and rat endometrial stromal cells (ESCs) was evaluated using a transwell 3D model and immunocytochemistry. Microarray mRNA and miRNA platforms were used for miRNA-mRNA expression profiling. Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) analyses were performed to identify the biological processes, molecular functions, cellular components, and pathways of endometrial injury and UC-MSCs transplantation repair and real-time quantitative reverse transcription PCR (qRT-PCR) was performed to further identify the expression changes of key molecules in the pathways. Endometrium thickness, number of glands, and the embryo implantation numbers were improved, and the degree of fibrosis was significantly alleviated by UC-MSCs treatment in the rat model of thin endometrium. In vitro cell experiments showed that UC-MSCs migrated to injured ESCs and enhanced their proliferation. miRNA microarray chip results showed that expression of 45 miRNAs was downregulated in the injured endometrium and upregulated after UC-MSCs transplantation. Likewise, expression of 39 miRNAs was upregulated in the injured endometrium and downregulated after UC-MSCs transplantation. The miRNA-mRNA interactions showed the changes in the miRNA and mRNA network during the processes of endometrial injury and repair. GO and KEGG analyses showed that the process of endometrial injury was mainly attributed to the decomposition of the extracellular matrix (ECM), protein degradation and absorption, and accompanying inflammation. The process of UC-MSCs transplantation and repair were accompanied by the reconstruction of the ECM, regulation of chemokines and inflammation, and cell proliferation and apoptosis. The key molecules involved in ECM-receptor interaction pathways were further verified by qRT-PCR. Itga1 and Thbs expression decreased in the model group and increased by UC-MSCs transplantation, while Laminin and Collagen expression increased in both the model group and MSCs group, with greater expression observed in the latter. This study showed that UC-MSCs transplantation could promote recovery of thin endometrial morphology and function. Furthermore, it revealed the expression changes of miRNA and mRNA after endometrial injury and UC-MSCs transplantation repair processed, and signaling pathways that may be involved in endometrial injury and repair.

A self-amplifying loop of YAP and SHH drives formation and expansion of heterotopic ossification

Heterotopic ossification (HO) occurs as a common complication after injury or in genetic disorders. The mechanisms underlying HO remain incompletely understood, and there are no approved prophylactic or secondary treatments available. Here, we identify a self-amplifying, self-propagating loop of Yes-associated protein (YAP)-Sonic hedgehog (SHH) as a core molecular mechanism underlying diverse forms of HO. In mouse models of progressive osseous heteroplasia (POH), a disease caused by null mutations in GNAS, we found that Gnas^-/- mesenchymal cells secreted SHH, which induced osteoblast differentiation of the surrounding wild-type cells. We further showed that loss of Gnas led to activation of YAP transcription activity, which directly drove Shh expression. Secreted SHH further induced YAP activation, Shh expression, and osteoblast differentiation in surrounding wild-type cells. This self-propagating positive feedback loop was both necessary and sufficient for HO expansion and could act independently of Gnas in fibrodysplasia ossificans progressiva (FOP), another genetic HO, and nonhereditary HO mouse models. Genetic or pharmacological inhibition of YAP or SHH abolished HO in POH and FOP and acquired HO mouse models without affecting normal bone homeostasis, providing a previously unrecognized therapeutic rationale to prevent, reduce, and shrink HO.

High Frequency Spectral Ultrasound Imaging Detects Early Heterotopic Ossification in Rodents

Heterotopic ossification (HO) is a devastating condition in which ectopic bone forms inappropriately in soft tissues following traumatic injuries and orthopedic surgeries as a result of aberrant mesenchymal progenitor cell (MPC) differentiation. HO leads to chronic pain, decreased range of motion, and an overall decrease in quality of life. While several treatments have shown promise in animal models, all must be given during early stages of formation. Methods for early determination of whether and where endochondral ossification/soft tissue mineralization (HO anlagen) develop are lacking. At-risk patients are not identified sufficiently early in the process of MPC differentiation and soft tissue endochondral ossification for potential treatments to be effective. Hence, a critical need exists to develop technologies capable of detecting HO anlagen soon after trauma, when treatments are most effective. In this study, we investigate high frequency spectral ultrasound imaging (SUSI) as a noninvasive strategy to identify HO anlagen at early time points after injury. We show that by determining quantitative parameters based on tissue organization and structure, SUSI identifies HO anlagen as early as 1-week postinjury in a mouse model of burn/tenotomy and 3 days postinjury in a rat model of blast/amputation. We analyze single cell RNA sequencing profiles of the MPCs responsible for HO formation and show that the early tissue changes detected by SUSI match chondrogenic and osteogenic gene expression in this population. SUSI identifies sites of soft tissue endochondral ossification at early stages of HO formation so that effective intervention can be targeted when and where it is needed following trauma-induced injury. Furthermore, we characterize the chondrogenic to osteogenic transition that occurs in the MPCs during HO formation and correlate gene expression to SUSI detection of the HO anlagen.

Sport Performance and Manual Therapies: A Review on the Effects on Mitochondrial, Sarcoplasmatic and Ca2+ Flux Response

The present narrative review aims to highlight the possible effects manual therapies could have on cells and mitochondria, as these effects could improve athletic performance management. To this aim, this review summarizes the relationship between mechanical stimulation, with a special focus on physical activity, and cell response based on the most recent mechanobiology findings. Mechanobiology analyzes how cells respond to mechanical stressors coming from the environment. Indeed, endogenous (e.g., blood pressure, heartbeat and gastrointestinal motility) and exogenous (e.g., physical activity and manual therapies) stimuli can induce biochemical and epigenetic modifications that alter protein synthesis with heavy consequences on cell behavior. Mechanical stress can also influence mitochondrial behavior (i.e., biogenesis, autophagy, fusion, fission and energy production), sarcoplasmic response and calcium ion (Ca²⁺) flux. Since manual therapies have been shown to affect the extracellular matrix, which represents a primary source of mechanical stress that may alter both the cytoskeleton and mitochondrial metabolism, it is conceivable manual therapies could also affect cellular and mitochondrial behavior. Lastly, by suggesting possible directions for future laboratory and clinical studies, the authors expect this review to inspire further research on how manual therapies could affect bioenergetic metabolism and, thus, athletic performance.

Mechanisms of bone development and repair

Bone development occurs through a series of synchronous events that result in the formation of the body scaffold. The repair potential of bone and its surrounding microenvironment - including inflammatory, endothelial and Schwann cells - persists throughout adulthood, enabling restoration of tissue to its homeostatic functional state. The isolation of a single skeletal stem cell population through cell surface markers and the development of single-cell technologies are enabling precise elucidation of cellular activity and fate during bone repair by providing key insights into the mechanisms that maintain and regenerate bone during homeostasis and repair. Increased understanding of bone development, as well as normal and aberrant bone repair, has important therapeutic implications for the treatment of bone disease and ageing-related degeneration.