Microenvironmental Regulation of Chondrocyte Plasticity in Endochondral Repair-A New Frontier for Developmental Engineering

Endochondral Ossification for Spinal Fusion: A Novel Perspective from Biological Mechanisms to Clinical Applications

Degenerative scoliosis (DS), encompassing conditions like spondylolisthesis and spinal stenosis, is a common type of spinal deformity. Lumbar interbody fusion (LIF) stands as a conventional surgical intervention for this ailment, aiming at decompression, restoration of intervertebral height, and stabilization of motion segments. Despite its widespread use, the precise mechanism underlying spinal fusion remains elusive. In this review, our focus lies on endochondral ossification for spinal fusion, a process involving vertebral development and bone healing. Endochondral ossification is the key step for the successful vertebral fusion. Endochondral ossification can persist in hypoxic conditions and promote the parallel development of angiogenesis and osteogenesis, which corresponds to the fusion process of new bone formation in the hypoxic region between the vertebrae. The ideal material for interbody fusion cages should have the following characteristics: (1) Good biocompatibility; (2) Stable chemical properties; (3) Biomechanical properties similar to bone tissue; (4) Promotion of bone fusion; (5) Favorable for imaging observation; (6) Biodegradability. Utilizing cartilage-derived bone-like constructs holds promise in promoting bony fusion post-operation, thus warranting exploration in the context of spinal fusion procedures.

The involvement of signaling pathways in the pathogenesis of osteoarthritis: An update

Osteoarthritis (OA) is one of the most common disabling pathologies, characterized by joint pain and reduced function, significantly worsening the quality of life. Even if important progresses have been made in OA research, little is yet known about the precise cellular and molecular mechanisms underlying OA. Understanding dysregulated signaling networks and their crosstalk in OA may offer a strong opportunity for the development of combined targeted therapies. Hence, this review highlights the recent findings on the main pathways involved in OA development, including Wnt, Notch, Hedgehog, MAPK, AMPK, and JAK/STAT, providing insights on current targeted therapies in OA patients' management.

The translational potential of this article

The identification of key signaling pathways involved in OA development and the investigation of their signaling crosstalk could pave the way for more effective treatments and improved management of OA patients in the future.

Cells transiently expressing periostin are required for intramedullary intramembranous bone regeneration

Intramembranous bone regeneration plays an important role in fixation of intramedullary implants used in joint replacement and dental implants used in tooth replacement. Despite widespread recognition of the importance of intramembranous bone regeneration in these clinical procedures, the underlying mechanisms have not been well explored. A previous study that examined transcriptomic profiles of regenerating bone from the marrow space showed that increased periostin gene expression preceded increases in several osteogenic genes. We therefore sought to determine the role of cells transiently expressing periostin in intramedullary intramembranous bone regeneration. We used a genetic mouse model that allows tamoxifen-inducible fluorescent labeling of periostin expressing cells. These mice underwent ablation of the bone marrow cavity through surgical disruption, a well-established intramembranous bone regeneration model. We found that in intact bones, fluorescently labeled cells were largely restricted to the periosteal surface of cortical bone and were absent in bone marrow. However, following surgical disruption of the bone marrow cavity, cells transiently expressing periostin were found within the regenerating tissue of the bone marrow compartment even though the cortical bone remained intact. The source of these cells is likely heterogenous, including cells occupying the periosteal surface as well as pericytes and endothelial cells within the marrow cavity. We also found that diphtheria toxin-mediated depletion of cells transiently expressing periostin at the time of surgery impaired intramembranous bone regeneration in mice. These data suggest a critical role of periostin expressing cells in intramedullary intramembranous bone regeneration and may lead to novel therapeutic interventions to accelerate or enhance implant fixation.

Harnessing mechanical cues in the cellular microenvironment for bone regeneration

At the macroscale, bones experience a variety of compressive and tensile loads, and these loads cause deformations of the cortical and trabecular microstructure. These deformations produce a variety of stimuli in the cellular microenvironment that can influence the differentiation of marrow stromal cells (MSCs) and the activity of cells of the MSC lineage, including osteoblasts, osteocytes, and chondrocytes. Mechanotransduction, or conversion of mechanical stimuli to biochemical and biological signals, is thus part of a multiscale mechanobiological process that drives bone modeling, remodeling, fracture healing, and implant osseointegration. Despite strong evidence of the influence of a variety of mechanical cues, and multiple paradigms proposed to explain the influence of these cues on tissue growth and differentiation, even a working understanding of how skeletal cells respond to the complex combinations of stimuli in their microenvironments remains elusive. This review covers the current understanding of what types of microenvironmental mechanical cues MSCs respond to and what is known about how they respond in the presence of multiple such cues. We argue that in order to realize the vast potential for harnessing the cellular microenvironment for the enhancement of bone regeneration, additional investigations of how combinations of mechanical cues influence bone regeneration are needed.

Encapsulation of β-NGF in injectable microrods for localized delivery accelerates endochondral fracture repair

Introduction: Currently, there are no non-surgical FDA-approved biological approaches to accelerate fracture repair. Injectable therapies designed to stimulate bone healing represent an exciting alternative to surgically implanted biologics, however, the translation of effective osteoinductive therapies remains challenging due to the need for safe and effective drug delivery. Hydrogel-based microparticle platforms may be a clinically relevant solution to create controlled and localized drug delivery to treat bone fractures. Here, we describe poly (ethylene glycol) dimethacrylate (PEGDMA)-based microparticles, in the shape of microrods, loaded with beta nerve growth factor (β-NGF) for the purpose of promoting fracture repair. Methods: Herein, PEGDMA microrods were fabricated through photolithography. PEGDMA microrods were loaded with β-NGF and in vitro release was examined. Subsequently, bioactivity assays were evaluated in vitro using the TF-1 tyrosine receptor kinase A (Trk-A) expressing cell line. Finally, in vivo studies using our well-established murine tibia fracture model were performed and a single injection of the β-NGF loaded PEGDMA microrods, non-loaded PEGDMA microrods, or soluble β-NGF was administered to assess the extent of fracture healing using Micro-computed tomography (µCT) and histomorphometry. Results: In vitro release studies showed there is significant retention of protein within the polymer matrix over 168 hours through physiochemical interactions. Bioactivity of protein post-loading was confirmed with the TF-1 cell line. In vivo studies using our murine tibia fracture model show that PEGDMA microrods injected at the site of fracture remained adjacent to the callus for over 7 days. Importantly, a single injection of β-NGF loaded PEGDMA microrods resulted in improved fracture healing as indicated by a significant increase in the percent bone in the fracture callus, trabecular connective density, and bone mineral density relative to soluble β-NGF control indicating improved drug retention within the tissue. The concomitant decrease in cartilage fraction supports our prior work showing that β-NGF promotes endochondral conversion of cartilage to bone to accelerate healing. Discussion: We demonstrate a novel and translational method wherein β-NGF can be encapsulated within PEGDMA microrods for local delivery and that β-NGF bioactivity is maintained resulting in improved bone fracture repair.

Standard in vitro evaluations of engineered bone substitutes are not sufficient to predict in vivo preclinical model outcomes

Understanding the optimal conditions required for bone healing can have a substantial impact to target the problem of non-unions and large bone defects. The combination of bioactive factors, regenerative progenitor cells and biomaterials to form a tissue engineered (TE) complex is a promising solution but translation to the clinic has been slow. We hypothesized the typical material testing algorithm used is insufficient and leads to materials being mischaracterized as promising. In the first part of this study, human bone marrow - derived mesenchymal stromal cells (hBM-MSCs) were embedded in three commonly used biomaterials (hyaluronic acid methacrylate, gelatin methacrylate and fibrin) and combined with relevant bioactive osteogenesis factors (dexamethasone microparticles and polyphosphate nanoparticles) to form a TE construct that underwent in vitro osteogenic differentiation for 28 days. Gene expression of relevant transcription factors and osteogenic markers, and von Kossa staining were performed. In the second and third part of this study, the same combination of TE constructs were implanted subcutaneously (cell containing) in T cell-deficient athymic Crl:NIH-Foxn1^rnu rats for 8 weeks or cell free in an immunocompetent New Zealand white rabbit calvarial model for 6 weeks, respectively. Osteogenic performance was investigated via MicroCT imaging and histology staining. The in vitro study showed enhanced upregulation of relevant genes and significant mineral deposition within the three biomaterials, generally considered as a positive result. Subcutaneous implantation indicates none to minor ectopic bone formation. No enhanced calvarial bone healing was detected in implanted biomaterials compared to the empty defect. The reasons for the poor correlation of in vitro and in vivo outcomes are unclear and needs further investigation. This study highlights the discrepancy between in vitro and in vivo outcomes, demonstrating that in vitro data should be interpreted with extreme caution. In vitro models with higher complexity are necessary to increase value for translational studies. STATEMENT OF SIGNIFICANCE: Preclinical testing of newly developed biomaterials is a crucial element of the development cycle. Despite this, there is still significant discrepancy between in vitro and in vivo test results. Within this study we investigate multiple combinations of materials and osteogenic stimulants and demonstrate a poor correlation between the in vitro and in vivo data. We propose rationale for why this may be the case and suggest a modified testing algorithm.

Close-to-native bone repair via tissue-engineered endochondral ossification approaches

In order to solve the clinical challenges related to bone grafting, several tissue engineering (TE) strategies have been proposed to repair critical-sized defects. Generally, the classical TE approaches are designed to promote bone repair via intramembranous ossification. Although promising, strategies that direct the osteogenic differentiation of mesenchymal stem/stromal cells are usually characterized by a lack of functional vascular supply, often resulting in necrotic cores. A less explored alternative is engineering bone constructs through a cartilage-mediated approach, resembling the embryological process of endochondral ossification. The remodeling of an intermediary hypertrophic cartilaginous template triggers vascular invasion and bone tissue deposition. Thus, employing this knowledge can be a promising direction for the next generation of bone TE constructs. This review highlights the most recent biomimetic strategies for applying endochondral ossification in bone TE while discussing the plethora of cell types, culture conditions, and biomaterials essential to promote a successful bone regeneration process.

Activation of canonical Wnt signaling accelerates intramembranous bone regeneration in male mice

Canonical Wnt signaling plays an important role in skeletal development, homeostasis, and both endochondral and intramembranous repair. While studies have demonstrated that the inhibition of Wnt signaling impairs intramembranous bone regeneration, how its activation affects intramembranous bone regeneration has been underexplored. Therefore, we sought to determine the effects of activation of canonical Wnt signaling on intramembranous bone regeneration by using the well-established marrow ablation model. We hypothesized that mice with a mutation in the Wnt ligand coreceptor gene Lrp5 would have accelerated intramembranous bone regeneration. Male and female wild-type and Lrp5-mutant mice underwent unilateral femoral bone marrow ablation surgery in the right femur at 4 weeks of age. Both the left intact and right operated femurs were assessed at Days 3, 5, 7, 10, and 14. The intact femur of Lrp5 mutant mice of both sexes had higher bone mass than wild-type littermates, although to a greater degree in males than females. Overall, the regenerated bone volume in Lrp5 mutant male mice was 1.8-fold higher than that of littermate controls, whereas no changes were observed between female Lrp5 mutant and littermate control mice. In addition, the rate of intramembranous bone regeneration (from Day 3 to Day 7) was higher in Lrp5 mutant male mice compared to their same-sex littermate controls with no difference in the females. Thus, activation of canonical Wnt signaling increases bone mass in intact bones of both sexes, but accelerates intramembranous bone regeneration following an injury challenge only in male mice.

A novel in vitro assay to study chondrocyte-to-osteoblast transdifferentiation

PURPOSE

Endochondral ossification, which involves transdifferentiation of chondrocytes into osteoblasts, is an important process involved in the development and postnatal growth of most vertebrate bones as well as in bone fracture healing. To study the basic molecular mechanisms of this process, a robust and easy-to-use in vitro model is desirable. Therefore, we aimed to develop a standardized in vitro assay for the transdifferentiation of chondrogenic cells towards the osteogenic lineage.

METHODS

Murine chondrogenic ATDC5 cells were differentiated into the chondrogenic lineage for seven days and subsequently differentiated towards the osteogenic direction. Gene expression analysis of pluripotency, as well as chondrogenic and osteogenic markers, cell-matrix staining, and immunofluorescent staining, were performed to assess the differentiation. In addition, the effects of Wnt3a and lipopolysaccharides (LPS) on the transdifferentiation were tested by their addition to the osteogenic differentiation medium.

RESULTS

Following osteogenic differentiation, chondrogenically pe-differentiated cells displayed the expression of pluripotency and osteogenic marker genes as well as alkaline phosphatase activity and a mineralized matrix. Co-expression of Col2a1 and Col1a1 after one day of osteogenic differentiation indicated that osteogenic cells had differentiated from chondrogenic cells. Wnt3a increased and LPS decreased transdifferentiation towards the osteogenic lineage.

CONCLUSION

We successfully established a rapid, standardized in vitro assay for the transdifferentiation of chondrogenic cells into osteogenic cells, which is suitable for testing the effects of different compounds on this cellular process.

A timeseries analysis of the fracture callus extracellular matrix proteome during bone fracture healing

While most bones fully self-heal, certain diseases require bone allograft to assist with fracture healing. Bone allografts offer promise as treatments for such fractures due to their osteogenic properties. However, current bone allografts made of decellularized bone extracellular matrix (ECM) have high failure rates, and thus grafts which improve fracture healing outcomes are needed. Understanding specific changes to the ECM proteome during normal fracture healing would enable the identification of key proteins that could be used enhance osteogenicity of bone allograft. Here, we performed a timeseries analysis of the fracture callus in mice to investigate proteomic and mineralization changes to the ECM at key stages of fracture healing. We found that changes to the ECM proteome largely coincide with the distinct phases of fracture healing. Basement membrane proteins (AGRN, COL4, LAMA), cartilage proteins (COL2A1, ACAN), and collagen crosslinking enzymes (LOXL, PLOD, ITIH) were initially upregulated, followed by bone specific proteoglycans and collagens (IBSP, COL1A1). Various tissue proteases (MMP2, 9, 13, 14; CTSK, CTSG, ELANE) were expressed at different levels throughout fracture healing. These changes coordinated with mineralization of the fracture callus, which increased steeply during the initial stages of healing. Interestingly the later timepoint was characterized by a response to wound healing and high expression of clotting factors (F2, 7, 9, 10). We identified ELANE and ITIH2 as tissue remodeling enzymes having no prior known involvement with fracture healing. This data can be further mined to identify regenerative proteins for enhanced bone graft design.

Systemic Administration of Recombinant Irisin Accelerates Fracture Healing in Mice

To date, pharmacological strategies designed to accelerate bone fracture healing are lacking. We subjected 8-week-old C57BL/6 male mice to closed, transverse, mid-diaphyseal tibial fractures and treated them with intraperitoneal injection of a vehicle or r-irisin (100 µg/kg/weekly) immediately following fracture for 10 days or 28 days. Histological analysis of the cartilaginous callus at 10 days showed a threefold increase in Collagen Type X (p = 0.0012) and a reduced content of proteoglycans (40%; p = 0.0018). Osteoclast count within the callus showed a 2.4-fold increase compared with untreated mice (p = 0.026), indicating a more advanced stage of endochondral ossification of the callus during the early stage of fracture repair. Further evidence that irisin induced the transition of cartilage callus into bony callus was provided by a twofold reduction in the expression of SOX9 (p = 0.0058) and a 2.2-fold increase in RUNX2 (p = 0.0137). Twenty-eight days post-fracture, microCT analyses showed that total callus volume and bone volume were increased by 68% (p = 0.0003) and 67% (p = 0.0093), respectively, and bone mineral content was 74% higher (p = 0.0012) in irisin-treated mice than in controls. Our findings suggest that irisin promotes bone formation in the bony callus and accelerates the fracture repair process, suggesting a possible use as a novel pharmacologic modulator of fracture healing.

Chondrocyte-to-osteoblast transformation in mandibular fracture repair

The majority of fracture research has been conducted using long bone fracture models, with significantly less research into the mechanisms driving craniofacial repair. However, craniofacial bones differ from long bones in both their developmental mechanism and embryonic origin. Thus, it is possible that their healing mechanisms could differ. In this study we utilize stabilized and unstabilized mandible fracture models to investigate the pathways regulating repair. Whereas fully stable trephine defects in the ramus form bone directly, mechanical motion within a transverse fracture across the same anatomical location promoted robust cartilage formation before boney remodeling. Literature investigating long bone fractures show chondrocytes are a direct precursor of osteoblasts during endochondral repair. Lineage tracing with Aggrecan-Cre^ERT2 ::Ai9 tdTomato mice demonstrated that mandibular callus chondrocytes also directly contribute to the formation of new bone. Furthermore, immunohistochemistry revealed that chondrocytes located at the chondro-osseous junction expressed Sox2, suggesting that plasticity of these chondrocytes may facilitate this chondrocyte-to-osteoblast transformation. Based on the direct role chondrocytes play in bone repair, we tested the efficacy of cartilage grafts in healing critical-sized mandibular defects. Whereas empty defects remained unbridged and filled with fibrous tissue, cartilage engraftment produced bony-bridging and robust marrow cavity formation, indicating healthy vascularization of the newly formed bone. Engrafted cartilage directly contributed to new bone formation since a significant portion of the newly formed bone was graft/donor-derived. Taken together these data demonstrate the important role of chondrocyte-to-osteoblast transformation during mandibular endochondral repair and the therapeutic promise of using cartilage as a tissue graft to heal craniofacial defects.

FOXO1 expression in chondrocytes modulates cartilage production and removal in fracture healing

Fracture healing is a multistage process characterized by inflammation, cartilage formation, bone deposition, and remodeling. Chondrocytes are important in producing cartilage that forms the initial anlagen for the hard callus needed to stabilize the fracture site. We examined the role of FOXO1 by selective ablation of FOXO1 in chondrocytes mediated by Col2α1 driven Cre recombinase. Experimental mice with lineage-specific FOXO1 deletion (Col2α1Cre+FOXO1L/L) and negative control littermates (Col2α1Cre-FOXO1L/L) were used for in vivo, closed fracture studies. Unexpectedly, we found that in the early phases of fracture healing, FOXO1 deletion significantly increased the amount of cartilage formed, whereas, in later periods, FOXO1 deletion led to a greater loss of cartilage. FOXO1 was functionally important as its deletion in chondrocytes led to diminished bone formation on day 22. Mechanistically, the early effects of FOXO1 deletion were linked to increased proliferation of chondrocytes through enhanced expression of cell cycle genes that promote proliferation and reduced expression of those that inhibit it and increased expression of cartilage matrix genes. At later time points experimental mice with FOXO1 deletion had greater loss of cartilage, enhanced formation of osteoclasts, increased IL-6 and reduced numbers of M2 macrophages. These results identify FOXO1 as a transcription factor that regulates chondrocyte behavior by limiting the early expansion of cartilage and preventing rapid cartilage loss at later phases.

An in vitro chondro-osteo-vascular triphasic model of the osteochondral complex

The generation of engineered models of the osteochondral complex to study its pathologies and develop possible treatments is hindered by the distinctly different properties of articular cartilage and subchondral bone, with the latter characterized by vascularization. In vitro models of the osteochondral complex have been mainly engineered as biphasic constructs containing just cartilage and bone cells, a condition very dissimilar from the in vivo environment. The different cellular components of the osteochondral complex are governed by interacting biochemical signaling; hence, to study the crosstalk among chondrocytes, osteoblasts, and endothelial cells, we have developed a novel triphasic model of the osteochondral tissue interface. Wet-spun poly(ε-caprolactone) (PCL) and PCL/hydroxyapatite (HA) scaffolds in combination with a methacrylated gelatin (gelMA) hydrogel were used as the polymeric backbone of the constructs. The scaffold components were engineered with human bone marrow derived mesenchymal stem cells (hMSCs) and human umbilical vein endothelial cells (HUVECs), and differentiated using a dual chamber microphysiological system (MPS) bioreactor that allows the simultaneous, separate flow of media of different compositions for induced differentiation of each compartment towards a cartilaginous or osseous lineage. Within the engineered Microphysiological Vascularized Osteochondral System, hMSCs showed spatially distinct chondrogenic and osteogenic markers in terms of histology and gene expression. HUVECs formed a stable capillary-like network in the engineered bone compartment and enhanced both chondrogenic and osteogenic differentiation of hMSCs, resulting in the generation of an in vitro system that mimics a vascularized osteochondral interface tissue.

SOX9 keeps growth plates and articular cartilage healthy by inhibiting chondrocyte dedifferentiation/osteoblastic redifferentiation

Cartilage is essential throughout vertebrate life. It starts developing in embryos when osteochondroprogenitor cells commit to chondrogenesis, activate a pancartilaginous program to form cartilaginous skeletal primordia, and also embrace a growth-plate program to drive skeletal growth or an articular program to build permanent joint cartilage. Various forms of cartilage malformation and degeneration diseases afflict humans, but underlying mechanisms are still incompletely understood and treatment options suboptimal. The transcription factor SOX9 is required for embryonic chondrogenesis, but its postnatal roles remain unclear, despite evidence that it is down-regulated in osteoarthritis and heterozygously inactivated in campomelic dysplasia, a severe skeletal dysplasia characterized postnatally by small stature and kyphoscoliosis. Using conditional knockout mice and high-throughput sequencing assays, we show here that SOX9 is required postnatally to prevent growth-plate closure and preosteoarthritic deterioration of articular cartilage. Its deficiency prompts growth-plate chondrocytes at all stages to swiftly reach a terminal/dedifferentiated stage marked by expression of chondrocyte-specific (Mgp) and progenitor-specific (Nt5e and Sox4) genes. Up-regulation of osteogenic genes (Runx2, Sp7, and Postn) and overt osteoblastogenesis quickly ensue. SOX9 deficiency does not perturb the articular program, except in load-bearing regions, where it also provokes chondrocyte-to-osteoblast conversion via a progenitor stage. Pathway analyses support roles for SOX9 in controlling TGFβ and BMP signaling activities during this cell lineage transition. Altogether, these findings deepen our current understanding of the cellular and molecular mechanisms that specifically ensure lifelong growth-plate and articular cartilage vigor by identifying osteogenic plasticity of growth-plate and articular chondrocytes and a SOX9-countered chondrocyte dedifferentiation/osteoblast redifferentiation process.

Bone formation at Ti-6Al-7Nb scaffolds consisting of 3D honeycomb frame and diamond-like carbon coating implanted into the femur of beagles

We fabricated Ti-6Al-7Nb bone scaffolds with 5 mm diameter and 20 mm length comprise of a three-dimensional (3D) honeycomb frame structure of truncated octahedra created by selective laser sintering 3D printing. The honeycomb frame was then coated with 0.1 μm thick diamond-like carbon (DLC) to increase biocompatibility. A round rod of Ti-6Al-7Nb alloy (ASTM F1295) was as a control material. They were implanted into the femur bones of beagles to evaluate bone morphometrics and to investigate changes in the transcriptome of the new bone tissue using DNA microarray analysis and real-time polymerase chain reaction (PCR). In the present report, the 3D honeycomb material with and without DLC film consisting of a-C:H is referred to as 3D_a-C:H and 3D_non, respectively. At 3 weeks after implantation, the 3D_non had more contact between the new and artificial bones compared with the control, and the 3D_a-C:H had more contact between the new and artificial bones compared with the control and 3D_non. Furthermore, 3D_a-C:H showed even more new bone compared with the control and 3D_non. At 8 weeks after implantation, more appeared lamellar bone with the 3D_a-C:H implant than those with the control and 3D_non. The real-time PCR results at 1 week of implantation revealed higher expression levels of VEGF, RANKL, and NOTCH2 expression with 3D_a-C:H than with 3D_non and control. As a result of real-time PCR at 2 weeks of implantation, OPN and CTSK expressions were found to be higher with 3D_a-C:H and 3D_non than that with the control.

Matrix metalloproteinase (MMP)-degradable tissue engineered periosteum coordinates allograft healing via early stage recruitment and support of host neurovasculature

Despite serving as the clinical "gold standard" treatment for critical size bone defects, decellularized allografts suffer from long-term failure rates of ~60% due to the absence of the periosteum. Stem and osteoprogenitor cells within the periosteum orchestrate autograft healing through host cell recruitment, which initiates the regenerative process. To emulate periosteum-mediated healing, tissue engineering approaches have been utilized with mixed outcomes. While vascularization has been widely established as critical for bone regeneration, innervation was recently identified to be spatiotemporally regulated together with vascularization and similarly indispensable to bone healing. Notwithstanding, there are no known approaches that have focused on periosteal matrix cues to coordinate host vessel and/or axon recruitment. Here, we investigated the influence of hydrogel degradation mechanism, i.e. hydrolytic or enzymatic (cell-dictated), on tissue engineered periosteum (TEP)-modified allograft healing, especially host vessel/nerve recruitment and integration. Matrix metalloproteinase (MMP)-degradable hydrogels supported endothelial cell migration from encapsulated spheroids whereas no migration was observed in hydrolytically degradable hydrogels in vitro, which correlated with increased neurovascularization in vivo. Specifically, ~2.45 and 1.84-fold, and ~3.48 and 2.58-fold greater vessel and nerve densities with high levels of vessel and nerve co-localization was observed using MMP degradable TEP (MMP-TEP) -modified allografts versus unmodified and hydrolytically degradable TEP (Hydro-TEP)-modified allografts, respectively, at 3 weeks post-surgery. MMP-TEP-modified allografts exhibited greater longitudinal graft-localized vascularization and endochondral ossification, along with 4-fold and 2-fold greater maximum torques versus unmodified and Hydro-TEP-modified allografts after 9 weeks, respectively, which was comparable to that of autografts. In summary, our results demonstrated that the MMP-TEP coordinated allograft healing via early stage recruitment and support of host neurovasculature.

Local injections of β-NGF accelerates endochondral fracture repair by promoting cartilage to bone conversion

There are currently no pharmacological approaches in fracture healing designed to therapeutically stimulate endochondral ossification. In this study, we test nerve growth factor (NGF) as an understudied therapeutic for fracture repair. We first characterized endogenous expression of Ngf and its receptor tropomyosin receptor kinase A (TrkA) during tibial fracture repair, finding that they peak during the cartilaginous phase. We then tested two injection regimens and found that local β-NGF injections during the endochondral/cartilaginous phase promoted osteogenic marker expression. Gene expression data from β-NGF stimulated cartilage callus explants show a promotion in markers associated with endochondral ossification such as Ihh, Alpl, and Sdf-1. Gene ontology enrichment analysis revealed the promotion of genes associated with Wnt activation, PDGF- and integrin-binding. Subsequent histological analysis confirmed Wnt activation following local β-NGF injections. Finally, we demonstrate functional improvements to bone healing following local β-NGF injections which resulted in a decrease in cartilage and increase of bone volume. Moreover, the newly formed bone contained higher trabecular number, connective density, and bone mineral density. Collectively, we demonstrate β-NGF’s ability to promote endochondral repair in a murine model and uncover mechanisms that will serve to further understand the molecular switches that occur during cartilage to bone transformation.

Single-cell RNA landscape of intratumoral heterogeneity and immunosuppressive microenvironment in advanced osteosarcoma

Osteosarcoma is the most frequent primary bone tumor with poor prognosis. Through RNA-sequencing of 100,987 individual cells from 7 primary, 2 recurrent, and 2 lung metastatic osteosarcoma lesions, 11 major cell clusters are identified based on unbiased clustering of gene expression profiles and canonical markers. The transcriptomic properties, regulators and dynamics of osteosarcoma malignant cells together with their tumor microenvironment particularly stromal and immune cells are characterized. The transdifferentiation of malignant osteoblastic cells from malignant chondroblastic cells is revealed by analyses of inferred copy-number variation and trajectory. A proinflammatory FABP4+ macrophages infiltration is noticed in lung metastatic osteosarcoma lesions. Lower osteoclasts infiltration is observed in chondroblastic, recurrent and lung metastatic osteosarcoma lesions compared to primary osteoblastic osteosarcoma lesions. Importantly, TIGIT blockade enhances the cytotoxicity effects of the primary CD3+ T cells with high proportion of TIGIT+ cells against osteosarcoma. These results present a single-cell atlas, explore intratumor heterogeneity, and provide potential therapeutic targets for osteosarcoma.

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.

Cartilage and bone tissue engineering using adipose stromal/stem cells spheroids as building blocks

Scaffold-free techniques in the developmental tissue engineering area are designed to mimic in vivo embryonic processes with the aim of biofabricating, in vitro, tissues with more authentic properties. Cell clusters called spheroids are the basis for scaffold-free tissue engineering. In this review, we explore the use of spheroids from adult mesenchymal stem/stromal cells as a model in the developmental engineering area in order to mimic the developmental stages of cartilage and bone tissues. Spheroids from adult mesenchymal stromal/stem cells lineages recapitulate crucial events in bone and cartilage formation during embryogenesis, and are capable of spontaneously fusing to other spheroids, making them ideal building blocks for bone and cartilage tissue engineering. Here, we discuss data from ours and other labs on the use of adipose stromal/stem cell spheroids in chondrogenesis and osteogenesis in vitro. Overall, recent studies support the notion that spheroids are ideal "building blocks" for tissue engineering by “bottom-up” approaches, which are based on tissue assembly by advanced techniques such as three-dimensional bioprinting. Further studies on the cellular and molecular mechanisms that orchestrate spheroid fusion are now crucial to support continued development of bottom-up tissue engineering approaches such as three-dimensional bioprinting.

The Therapeutic Potential of MicroRNAs as Orthobiologics for Skeletal Fractures

The repair of a fractured bone is critical to the well-being of humans. Failure of the repair process to proceed normally can lead to complicated fractures, exemplified by either a delay in union or a complete nonunion. Both of these conditions lead to pain, the possibility of additional surgery, and impairment of life quality. Additionally, work productivity decreases, income is reduced, and treatment costs increase, resulting in financial hardship. Thus, developing effective treatments for these difficult fractures or even accelerating the normal physiological repair process is warranted. Accumulating evidence shows that microRNAs (miRNAs), small noncoding RNAs, can serve as key regulatory molecules of fracture repair. In this review, a brief description of the fracture repair process and miRNA biogenesis is presented, as well as a summary of our current knowledge of the involvement of miRNAs in physiological fracture repair, osteoporotic fractures, and bone defect healing. Further, miRNA polymorphisms associated with fractures, miRNA presence in exosomes, and miRNAs as potential therapeutic orthobiologics are also discussed. This is a timely review as several miRNA-based therapeutics have recently entered clinical trials for nonskeletal applications and thus it is incumbent upon bone researchers to explore whether miRNAs can become the next class of orthobiologics for the treatment of skeletal fractures.

Uncoupling of in-vitro identity of embryonic limb derived skeletal progenitors and their in-vivo bone forming potential

The healing of large bone defects remains a major unmet medical need. Our developmental engineering approach consists of the in vitro manufacturing of a living cartilage tissue construct that upon implantation forms bone by recapitulating an endochondral ossification process. Key to this strategy is the identification of the cells to produce such cartilage intermediates efficiently. We applied a cell selection strategy based on published skeletal stem cell markers using mouse embryonic limb cartilage as cell source and analysed their potential to form bone in an in vivo ectopic assay. FGF2 supplementation to the culture media for expansion blocked dedifferentiation of the embryonic cartilage cells in culture and enriched for stem cells and progenitors as quantified using the recently published CD marker set. However, when the stem cells and progenitors were fractionated from expanded embryonic cartilage cells and assessed in the ectopic assay, a major loss of bone forming potential was observed. We conclude that cell expansion appears to affect the association between cell identity based on CD markers and in vivo bone forming capacity.

Smart bone plates can monitor fracture healing

There are currently no standardized methods for assessing fracture healing, with physicians relying on X-rays which are only useful at later stages of repair. Using in vivo mouse fracture models, we present the first evidence that microscale instrumented implants provide a route for post-operative fracture monitoring, utilizing electrical impedance spectroscopy (EIS) to track the healing tissue with high sensitivity. In this study, we fixed mouse long bone fractures with external fixators and bone plates. EIS measurements taken across two microelectrodes within the fracture gap were able to track longitudinal differences between individual mice with good versus poor healing. We additionally present an equivalent circuit model that combines the EIS data to classify fracture repair states. Lastly, we show that EIS measurements strongly correlated with standard quantitative µCT values and that these correlations validate clinically-relevant operating frequencies for implementation of this technique. These results demonstrate that EIS can be integrated into current fracture management strategies such as bone plating, providing physicians with quantitative information about the state of fracture repair to guide clinical decision-making for patients.

The expression of chondrogenesis-related and arthritis-related genes in human ONFH cartilage with different Ficat stages

Background

It has been well known that the degeneration of hip articular cartilage with osteonecrosis of the femoral head (ONFH) increases the instability of hip and accelerates the development process of ONFH. A better understanding of the expression of chondrogenesis-related and arthritis-related genes of cartilage along with the progression of ONFH seems to be essential for further insight into the molecular mechanisms of ONFH pathogenesis.

Methods

We analyzed the differentially expressed gene profile (GSE74089) of human hip articular cartilage with ONFH. The functions and pathway enrichments of differentially expressed genes (DEGs) were analyzed via GO and KEGG analysis. The expression of six selected critical chondrogenesis-related and four arthritis-related genes in eight human hip articular cartilage with femoral neck fracture (FNF) and 26 human hip articular cartilage with different stages ONFH (6 cases of Ficat stage II, 10 cases of Ficat stage III and 10 cases of Ficat stage IV) were detected.

Results

A total of 2,174 DEGs, including 1,482 up-regulated and 692 down-regulated ones, were obtained in the ONFH cartilage specimens compared to the control group. The GO and KEGG enrichment analysis indicated that the function of these DEGs mainly enriched in extracellular matrix, angiogenesis, antigen processing and presentation. The results showed a significant stepwise up-expression of chondrogenesis-related genes, including MMP13, ASPN, COL1A1, OGN, COL2A1 and BMP2, along with the progression of ONFH. The arthritis-related genes IL1β, IL6 and TNFα were only found up-expressed in Ficat IV stage which indicated that the arthritis-related molecular changes were not significant in the progression of ONFH before Ficat III stage. However, the arthritis-related gene PTGS2 was significant stepwise up-expression along with the progression of ONFH which makes it to be a sensitive arthritis-related biomarker of ONFH.

Conclusion

Expression changes of six chondrogenesis-related and four arthritis-related genes were found in hip articular cartilage specimens with different ONFH Ficat stages. These findings are expected to a get a further insight into the molecular mechanisms of ONFH progression.