Expression of bone-specific genes by hypertrophic chondrocytes: Implications of the complex functions of the hypertrophic chondrocyte during endochondral bone development

Relationships between matrix mineralization, oxidative metabolism, and mitochondrial structure during ATDC5 murine chondroprogenitor cell line differentiation

The mechanistic relationships between the progression of growth chondrocyte differentiation, matrix mineralization, oxidative metabolism, and mitochondria content and structure were examined in the ATDC5 murine chondroprogenitor cell line. The progression of chondrocyte differentiation was associated with a statistically significant (p ≤ 0.05) ~2-fold increase in oxidative phosphorylation. However, as matrix mineralization progressed, oxidative metabolism decreased. In the absence of mineralization, cartilage extracellular matrix mRNA expression for Col2a1, Aggrecan, and Col10a1 were statistically (p ≤ 0.05) ~2-3-fold greater than observed in mineralizing cultures. In contrast, BSP and Phex that are associated with promoting matrix mineralization showed statistically (p ≤ 0.05) higher ~2-4 expression, while FGF23 phosphate regulatory factor was significantly lower (~50%) in mineralizing cultures. Cultures induced to differentiate under both nonmineralizing and mineralizing media conditions showed statistically greater basal oxidative metabolism and ATP production. Maximal respiration and spare oxidative capacity were significantly elevated (p ≤ 0.05) in differentiated nonmineralizing cultures compared to those that mineralized. Increased oxidative metabolism was associated with both an increase in mitochondria volume per cell and mitochondria fusion, while mineralization diminished mitochondrial volume and appeared to be associated with fission. Undifferentiated and mineralized cells showed increased mitochondrial co-localization with the actin cytoskeletal. Examination of proteins associated with mitochondria fission and apoptosis and mitophagy, respectively, showed levels of immunological expression consistent with the increasing fission and apoptosis in mineralizing cultures. These results suggest that chondrocyte differentiation is associated with intracellular structural reorganization, promoting increased mitochondria content and fusion that enables increased oxidative metabolism. Mineralization, however, does not need energy derived from oxidative metabolism; rather, during mineralization, mitochondria appear to undergo fission and mitophagy. In summary, these studies show that as chondrocytes underwent hypertrophic differentiation, they increased oxidative metabolism, but as mineralization proceeds, metabolism decreased. Mitochondria structure also underwent a structural reorganization that was further supportive of their oxidative capacity as the chondrocytes progressed through their differentiation. Thus, the mitochondria first underwent fusion to support increased oxidative metabolism, then underwent fission during mineralization, facilitating their programed death.

Phosphate-induced activation of VEGFR2 leads to caspase-9-mediated apoptosis of hypertrophic chondrocytes

Low circulating phosphate (Pi) leads to rickets, characterized by expansion of the hypertrophic chondrocytes (HCs) in the growth plate due to impaired HC apoptosis. Studies in HCs demonstrate that Pi activates the Raf/MEK/ERK1/2 and mitochondrial apoptotic pathways. To determine how Pi activates these pathways, a small-molecule screen was undertaken to identify inhibitors of Pi-induced ERK1/2 phosphorylation in HCs. Vascular endothelial growth factor receptor 2 (VEGFR2) was identified as a target. In vitro studies in HCs demonstrate that VEGFR2 inhibitors block Pi-induced pERK1/2 and caspase-9 cleavage. Like Pi, rhVEGF activates ERK1/2 and caspase-9 in HCs and induces phosphorylation of VEGFR2, confirming that Pi activates this signaling pathway in HCs. Chondrocyte-specific depletion of VEGFR2 leads to an increase in HCs, impaired vascular invasion, and a decrease in HC apoptosis. Thus, these studies define a role for VEGFR2 in transducing Pi signals and mediating its effects on growth plate maturation.

Bone without borders – Monetite-based calcium phosphate guides bone formation beyond the skeletal envelope

Calcium phosphates (CaP) represent an important class of osteoconductive and osteoinductive biomaterials. As proof-of-concept, we show how a multi-component CaP formulation (monetite, beta-tricalcium phosphate, and calcium pyrophosphate) guides osteogenesis beyond the physiological envelope. In a sheep model, hollow dome-shaped constructs were placed directly over the occipital bone. At 12 months, large amounts of bone (∼75%) occupy the hollow space with strong evidence of ongoing remodelling. Features of both compact bone (osteonal/osteon-like arrangements) and spongy bone (trabeculae separated by marrow cavities) reveal insights into function/need-driven microstructural adaptation. Pores within the CaP also contain both woven bone and vascularised lamellar bone. Osteoclasts actively contribute to CaP degradation/removal. Of the constituent phases, only calcium pyrophosphate persists within osseous (cutting cones) and non-osseous (macrophages) sites. From a translational perspective, this multi-component CaP opens up exciting new avenues for osteotomy-free and minimally-invasive repair of large bone defects and augmentation of the dental alveolar ridge.

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Dome-shaped hollow multi-component calcium phosphate (CaP) constructs were fabricated.

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CaP is 85% monetite, 8% beta-tricalcium phosphate, and 7% calcium pyrophosphate.

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CaP degrades in vivo and remodelled bone occupies the available extraskeletal space.

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CaP loses monetite to form carbonated apatite; calcium pyrophosphate found in tissues.

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Extraskeletal bone micro-/nanostructure and composition comparable to the native bone.

Mesenchymal cell-derived Wnt1 signaling regulates subchondral bone remodeling but has no effects on the development of growth plate or articular cartilage in mice

Chondrocyte differentiation is a principal progress in endochondral ossification and in the formation of secondary ossification center (SOC) during the long bone development. We have previously reported that targeted deletion of Wnt1 in mesenchymal progenitors (Wnt1_Prrx^-/-) leads to spontaneous fractures and severe osteopenia in mouse long bones, suggesting that Wnt1 is a key regulator of bone metabolism. However, the effect of Wnt1 on the regulation of cartilage development and chondrocyte differentiation remained unknown. In this study, WNT1 protein expression was observed in lateral superficial cartilage and growth plate pre-hypertrophic chondrocytes in mice. Wnt1 mRNA expression was detected in epiphyseal cartilage from E16.5 to 3 month-old mice. Detailed histological analyses revealed that the average thickness and chondrocyte density of proximal tibial articular cartilage and growth plate were unchanged between Wnt1_Prrx^-/- and control mice. However, μCT analysis of tibial epiphyses showed that the subchondral bone mass was reduced in Wnt1_Prrx^-/- mice compared to control mice, as demonstrated by decreased bone volume, trabecular number, trabecular thickness, and increased trabecular separation in Wnt1_Prrx^-/- mice. Mechanistically, histomorphometric analyses showed that the reduced subchondral bone mass in Wnt1_Prrx^-/- mice was due to impaired bone formation and enhanced bone resorption. In vitro, exogenous Wnt1 inhibited chondrogenesis and chondrocyte hypertrophy in both cell autonomous and juxtacrine manners, while matrix mineralization and the expression of Mmp13, Mmp9 and Opn were induced in a juxtacrine manner. Taken together, mesenchymal cell-derived Wnt1 is an important regulator of subchondral bone remodeling, although it has no effect on the regulation of growth plate or articular cartilage.

Osteoclasts and Macrophages-Their Role in Bone Marrow Cavity Formation During Mouse Embryonic Development

The formation of the bone marrow cavity is a prerequisite for endochondral ossification. In reviews and textbooks, it is occasionally reported that osteoclasts are essential for bone marrow cavity formation removing hypertrophic chondrocytes. Mice lacking osteoclasts or having functionally defective osteoclasts have osteopetrotic bones, yet they still form a bone marrow cavity. Here, we investigated the role of osteoclasts and macrophages in bone marrow cavity formation during embryogenesis. Macrophages can assist osteoclasts in matrix removal by phagocytosing resorption byproducts. Rank-deficient mice, lacking osteoclasts, and Pu.1-deficient mice, lacking monocytes, macrophages, and osteoclasts, displayed a delay in bone marrow cavity formation and a lengthening of the zone of hypertrophic chondrocytes. F4/80-positive monocyte/macrophage numbers increased by about fourfold in the bone marrow cavity of E18.5 Rank-deficient mice. Based on lineage-tracing experiments, the majority of the excess F4/80 cells were derived from definitive hematopoietic precursors of the fetal liver. In long bones of both Rank^-/- and Pu.1^-/- specimens, Mmp9-positive cells were still present. In addition to monocytes, macrophages, and osteoclasts, Ctsb-positive septoclasts were lost in Pu.1^-/- specimens. The mineralization pattern was altered in Rank^-/- and Pu.1^-/- specimens, revealing a significant rise in transverse-oriented mineralized structures. Taken together, our findings imply that early on during bone marrow cavity formation, osteoclasts facilitate the entry of blood vessels and later the turnover of hypertrophic chondrocytes, whereas macrophages appear to play no major role. Furthermore, the absence of septoclasts in Pu.1^-/- specimens suggests that septoclasts are either derived from Pu.1-dependent precursors or require PU.1 activity for their differentiation. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A Novel Ex Vivo Bone Culture Model for Regulation of Collagen/Apatite Preferential Orientation by Mechanical Loading

The anisotropic microstructure of bone, composed of collagen fibers and biological apatite crystallites, is an important determinant of its mechanical properties. Recent studies have revealed that the preferential orientation of collagen/apatite composites is closely related to the direction and magnitude of in vivo principal stress. However, the mechanism of alteration in the collagen/apatite microstructure to adapt to the mechanical environment remains unclear. In this study, we established a novel ex vivo bone culture system using embryonic mouse femurs, which enabled artificial control of the mechanical environment. The mineralized femur length significantly increased following cultivation; uniaxial mechanical loading promoted chondrocyte hypertrophy in the growth plates of embryonic mouse femurs. Compressive mechanical loading using the ex vivo bone culture system induced a higher anisotropic microstructure than that observed in the unloaded femur. Osteocytes in the anisotropic bone microstructure were elongated and aligned along the long axis of the femur, which corresponded to the principal loading direction. The ex vivo uniaxial mechanical loading successfully induced the formation of an oriented collagen/apatite microstructure via osteocyte mechano-sensation in a manner quite similar to the in vivo environment.

Dissociation of nanosilicates induces downstream endochondral differentiation gene expression program

Bioactive materials harness the body's innate regenerative potential by directing endogenous progenitor cells to facilitate tissue repair. Dissolution products of inorganic biomaterials provide unique biomolecular signaling for tissue-specific differentiation. Inorganic ions (minerals) are vital to biological processes and play crucial roles in regulating gene expression patterns and directing cellular fate. However, mechanisms by which ionic dissolution products affect cellular differentiation are not well characterized. We demonstrate the role of the inorganic biomaterial synthetic two-dimensional nanosilicates and its ionic dissolution products on human mesenchymal stem cell differentiation. We use whole-transcriptome sequencing (RNA-sequencing) to characterize the contribution of nanosilicates and its ionic dissolution products on endochondral differentiation. Our study highlights the modulatory role of ions in stem cell transcriptome dynamics by regulating lineage-specific gene expression patterns. This work paves the way for leveraging biochemical characteristics of inorganic biomaterials to direct cellular processes and promote in situ tissue regeneration.

Collagen X Longitudinal Fracture Biomarker Suggests Staged Fixation in Tibial Plateau Fractures Delays Rate of Endochondral Repair

OBJECTIVES

To use a novel, validated bioassay to monitor serum concentrations of a breakdown product of collagen X in a prospective longitudinal study of patients sustaining isolated tibial plateau fractures. Collagen X is the hallmark extracellular matrix protein present during conversion of soft, cartilaginous callus to bone during endochondral repair. Previous preclinical and clinical studies demonstrated a distinct peak in collagen X biomarker (CXM) bioassay levels after long bone fractures.

SETTING

Level 1 academic trauma facility.

PATIENTS/PARTICIPANTS

Thirty-six patients; isolated tibial plateau fractures.

INTERVENTION

(3) Closed treatment, ex-fix (temporizing/definitive), and open reduction internal fixation.

MAIN OUTCOME MEASUREMENTS

Collagen X serum biomarker levels (CXM bioassay).

RESULTS

Twenty-two men and 14 women (average age: 46.3 y; 22.6-73.4, SD 13.3) enrolled (16 unicondylar and 20 bicondylar fractures). Twenty-five patients (72.2%) were treated operatively, including 12 (33.3%) provisionally or definitively treated by ex-fix. No difference was found in peak CXM values between sexes or age. Patients demonstrated peak expression near 1000 pg/mL (average: male-986.5 pg/mL, SD 369; female-953.2 pg/mL, SD 576). There was no difference in peak CXM by treatment protocol, external fixator use, or fracture severity (Schatzker). Patients treated with external fixation (P = 0.05) or staged open reduction internal fixation (P = 0.046) critically demonstrated delayed peaks.

CONCLUSIONS

Pilot analysis demonstrates a strong CXM peak after fractures commensurate with previous preclinical and clinical studies, which was delayed with staged fixation. This may represent the consequence of delayed construct loading. Further validation requires larger cohorts and long-term follow-up. Collagen X may provide an opportunity to support prospective interventional studies testing novel orthobiologics or fixation techniques.

LEVEL OF EVIDENCE

Level II, prospective clinical observational study.

Characteristic expression of CGRP and osteogenic and vasculogenic markers in the proximal and distal regions of the rib during male mouse development

BACKGROUND

Neuropeptide calcitonin gene-related peptide (CGRP) is a neurotransmitter related to vasculogenesis and osteogenesis during bone formation and organ development. From the foetal period to the postnatal period, the thorax, which is necessary for lung respiration, forms. The thorax exhibits the same cartilage ossification as the bones of the extremities, but a specific system within the thorax exists as costal cartilage after birth. The relationship among CGRP, osteogenesis and vasculogenic markers in the two rib locations during thorax formation is not fully understood.

MATERIALS AND METHODS

In our study, male mice were used to provide ribs under different development conditions on various embryonic days (E12. 5, E14.5, and E17.5) and postnatal days (P1 and P5). The mRNA expression levels of CGRP, vascular endothelial growth factor (VEGF-A), type 1 collagen (Col1a-1), type 2 collagen (Col2a1), neuropeptide Y (NPY), osteocalcin (OCN) and osteopontin (OPN) were analysed by qRT-PCR. We also analysed the mRNA expression of CGRP, VEGF-A and OPN by in situ hybridization. Multivariate modelling with principal component analysis (PCA) was performed to estimate the interactions among the quantitative real-time RT-PCR data.

RESULTS

The mRNA expression levels of CGRP, VEGF-A, Col2a, Col1a-1, OCN, and NPY in the male mouse rib gradually increased during development. An antisense probe for CGRP mRNA was strongly detected in the central region of the mouse rib at E12.5 and the hypertrophic and ossification zones at E17.5 by in situ hybridization. VEGF-A was also located in the same region as CGRP at E12.5 and E17.5. OPN was strongly detected at the rib formation stage from E14.5 to E17.5. The expression of CGRP also differed between the proximal and distal regions of the rib at E17.5. As demonstrated by in situ hybridization, CGRP continuously participates in cartilage formation in the distal regions of the rib after birth. The PCA revealed that the mRNA expression of CGRP was related to that of Col1a-1 and VEGF-A during rib formation.

CONCLUSION

This study shows that CGRP is involved in vascular and bone formation during rib development and may also be involved in cartilage formation after birth. The findings suggest that CGRP may temporarily participate in bone formation and continuously participate in cartilage formation in the rib, which may also be related to the formation of the anterior thoracic wall after birth.

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

Inadequate vascularization of engineered tissue constructs is a main challenge in developing a clinically impactful therapy for large, complex, and recalcitrant bone defects. It is well established that bone and blood vessels form concomitantly during development, as well as during repair after injury. Endothelial cells (ECs) and mesenchymal stromal cells (MSCs) are known to be key players in orthopedic tissue regeneration and vascularization, and these cell types have been used widely in tissue engineering strategies to create vascularized bone. Coculture studies have demonstrated that there is crosstalk between ECs and MSCs that can lead to synergistic effects on tissue regeneration. At the same time, the complexity in fabricating, culturing, and characterizing engineered tissue constructs containing multiple cell types presents a challenge in creating multifunctional tissues. In particular, the timing, spatial distribution, and cell phenotypes that are most conducive to promoting concurrent bone and vessel formation are not well understood. This review describes the processes of bone and vascular development, and how these have been harnessed in tissue engineering strategies to create vascularized bone. There is an emphasis on interactions between ECs and MSCs, and the culture systems that can be used to understand and control these interactions within a single engineered construct. Developmental engineering strategies to mimic endochondral ossification are discussed as a means of generating vascularized orthopedic tissues. The field of tissue engineering has made impressive progress in creating tissue replacements. However, the development of larger, more complex, and multifunctional engineered orthopedic tissues will require a better understanding of how osteogenesis and vasculogenesis are coupled in tissue regeneration. Impact statement Vascularization of large engineered tissue volumes remains a challenge in developing new and more biologically functional bone grafts. A better understanding of how blood vessels develop during bone formation and regeneration is needed. This knowledge can then be applied to develop new strategies for promoting both osteogenesis and vasculogenesis during the creation of engineered orthopedic tissues. This article summarizes the processes of bone and blood vessel development, with a focus on how endothelial cells and mesenchymal stromal cells interact to form vascularized bone both during development and growth, as well as tissue healing. It is meant as a resource for tissue engineers who are interested in creating vascularized tissue, and in particular to those developing cell-based therapies for large, complex, and recalcitrant bone defects.

Ectopic models recapitulating morphological and functional features of articular cartilage

BACKGROUND

Articular cartilage is an extremely specialized connective tissue which covers all diarthrodial joints. Implantation of chondrogenic cells without or with additional biomaterial scaffolds in ectopic locationsin vivo generates substitutes of cartilage with structural and functional characteristics that are used in fundamental investigations while also serving as a basis for translational studies.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant ectopic models, among which subcutaneous, intramuscular, and kidney capsule transplantation and elaborates on implanted cells and biomaterial scaffolds and on their use to recapitulate morphological and functional features of articular cartilage. Although the absence of a physiological joint environment and biomechanical stimuli is the major limiting factor, ectopic models are an established component for articular cartilage research aiming to generate a bridge between in vitro data and the clinically more relevant translational orthotopic in vivo models when their limitations are considered.

A quantitative serum biomarker of circulating collagen X effectively correlates with endochondral fracture healing

Currently, there are no standardized methods for quantitatively measuring fracture repair. Physicians rely on subjective physical examinations and qualitative evaluation of radiographs to detect mineralized tissue. Since most fractures heal indirectly through a cartilage intermediate, these tools are limited in their diagnostic utility of early repair. Prior to converting to the bone, cartilage undergoes hypertrophic maturation, characterized by the deposition of a provisional collagen X matrix. The objective of this study was to characterize the kinetics of a novel collagen X biomarker relative to other biological measurements of fracture healing using a murine model of endochondral fracture repair in which a closed, mid-shaft tibia fracture was created using the classic drop-weight technique. Serum was collected 5 to 42 days post-fracture in male and female mice and compared to uninjured controls (n = 8-12). Collagen X in the serum was quantified using a recently validated ELISA-based bioassay ("Cxm")1 and compared to genetic and histological markers of fracture healing and inflammation. We found the Cxm biomarker reliably increased from baseline to a statistically unique peak 14 days post-fracture that then resolved to pre-fracture levels by 3 weeks following injury. The shape and timing of the Cxm curve followed the genetic and histological expression of collagen X but did not show a strong correlation with local inflammatory states. Assessment of fracture healing progress is crucial to making correct and timely clinical decisions for patients. This Cxm bioassay represents a minimally invasive, inexpensive technique that could provide reliable information on the biology of the fracture to significantly improve clinical care.

Murine Limb Bud Organ Cultures for Studying Musculoskeletal Development

The biological signals that coordinate the three-dimensional outgrowth and patterning of the vertebrate limb bud have been well delineated. These include a number of vital embryonic signaling pathways, including the fibroblast growth factor, WNT, transforming growth factor, and hedgehog. Collectively these signals converge on multiple progenitor populations to drive the formation of a variety of tissues that make up the limb musculoskeletal system, such as muscle, tendon, cartilage, stroma, and bone. The basic mechanisms regulating the commitment and differentiation of diverse limb progenitor populations has been successfully modeled in vitro using high density primary limb mesenchymal or micromass cultures. However, this approach is limited in its ability to more faithfully recapitulate the assembly of progenitors into organized tissues that span the entire musculoskeletal system. Other biological systems have benefitted from the development and availability of three-dimensional organoid cultures which have transformed our understanding of tissue development, homeostasis and regeneration. Such a system does not exist that effectively models the complexity of limb development. However, limb bud organ cultures while still necessitating the use of collected embryonic tissue have proved to be a powerful model system to elucidate the molecular underpinning of musculoskeletal development. In this methods article, the derivation and use of limb bud organ cultures from murine limb buds will be described, along with strategies to manipulate signaling pathways, examine gene expression and for longitudinal lineage tracking.

Concerted actions by MMPs, ADAMTS and serine proteases during remodeling of the cartilage callus into bone during osseointegration of hip implants

INTRODUCTION

Although the number of patients undergoing total hip arthroplasty is constantly on the rise, we only have limited knowledge of the molecular mechanisms necessary for successful osseointegration of implants or the reasons why some fail. Understanding the spatiotemporal characteristics of signaling pathways involved in bone healing of implants is therefore of particular importance for our ability to identify factors causing implants to fail. The current study investigated the role of three families of proteases, i.e. MMPs (matrix metalloproteinases), ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) and serine proteases, as well as their endogenous inhibitors during osseointegration of hip implants that have endured two decades of use without clinical or radiological signs of loosening.

MATERIALS AND METHODS

Twenty-four patients that had undergone primary THA due to one-sided osteoarthritis (OA) were monitored during 18 years (Y) with repeated measurements of plasma biomarkers, clinical variables and radiographs. All implants were clinically and radiographically well-fixed throughout the follow-up. Eighty-one healthy donors divided in three gender and age-matched groups and twenty OA patients awaiting THA, served as controls. Plasma was analyzed for MMP-1, -2, -3, -8, -9, -10, -13, -14, tissue inhibitor of metalloproteinase (TIMP)-1, -2, -3, ADAMTS4, ADAMTS5, the serine proteases neutrophil elastase (NE), proteinase 3 (PR3) and their endogenous inhibitors, secretory leucocyte proteinase inhibitor (SLPI), trappin-2/elafin and serpina1 (α-1 antitrypsin). Cartilage turnover was monitored using two markers of cartilage synthesis, type II procollagen and PIICP (procollagen II C-terminal propeptide), and two markers of cartilage degradation, CTX-II (C-terminal telopeptide fragments of type II collagen) and split products of aggrecan (G1-IGD-G2).

RESULTS

MMP-1, MMP-9, ADAMTS4, NE and PR3 were above healthy in presurgery OA patients but returned to the level of healthy within 6 weeks (W) after surgery. MMPs and serine proteases were counter-regulated during this phase by TIMP-1, SLPI and trappin-2/elafin. Type II procollagen, PIICP and CTX-II increased to a peak 6 W after surgery with a gradual return to the level of controls within weeks. Significant increases by MMP-8, MMP-9, ADAMTS4, ADAMTS5, NE, PR3 and the protease inhibitors, TIMP-3 and serpina1, were seen 5 Y after hip arthroplasty paralleled by a sharp increase in the levels of the cartilage degradation markers, CTX-II and G1-IGD-G2. All the above mediators were normalized before 18 Y, except MMP-1 and MMP-9 that remained above healthy at 18 Y. MMP-14 increased immediately after surgery and remained elevated until 5 Y postsurgery before returning to the level of controls at 7 Y.

CONCLUSION

Notwithstanding temporal differences, the molecular processes of bone repair in arthroplasty patients show great spatial similarities with the classical phases of fracture repair as previously shown in animal models. Cartilagenous callus, produced and remodeled early after hip arthroplasty, is replaced with bone 5 Y to7 Y after surgery by the concerted actions of MMP-8, MMP-9, ADAMTS4, ADAMTS5, NE and PR3, thus suggesting that a complex regulatory cross-talk may exist between different families of proteases during this transitional phase of cartilage degradation. Regulation and fine-tuning of cartilage remodeling by MMPs and ADAMTS is controlled by TIMP-3 whereas serine proteases are regulated by serpina1. Increased MMP-1 and MMP-9 beyond 10Y post-THA support a role during coupled bone remodeling.

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

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

Exendin‑4 promotes osteogenic differentiation of adipose‑derived stem cells and facilitates bone repair

Inflammation-related bone defects pose a heavy burden on patients and orthopedic surgeons. Although stem-cell-based bone repair has developed rapidly, it is of great significance to characterize bio-active molecules that facilitate bone regeneration. It is reported that a glucagon-like peptide 1 receptor agonist, exendin-4, promoted bone regeneration mediated by the transplantation of adipose-derived stem cells in a metaphyseal defect mouse model of femur injury. However, the underlying mechanism is unclear. Bone imaging, immunohistochemistry real-time PCR and western blot analysis were used in the present study, and the results revealed that exendin-4 increased the transcription of the osteogenic differentiation-related genes and induced osteogenic differentiation in situ. Furthermore, the present data obtained from sorted adipose-derived stem cells revealed that exendin-4 promoted osteogenic differentiation and inhibited adipogenic differentiation in vitro. These findings indicated that exendin-4 facilitates osteogenic differentiation of transplanted adipose-derived stem cells for bone repair and illuminated clinical prospects of both adipose-derived stem cells and exendin-4 in stem-cell-based bone defect repair.

Correlation between diffusion tensor imaging parameters of the distal femoral physis and adjacent metaphysis, and subsequent adolescent growth

BACKGROUND

Diffusion-tensor imaging (DTI) depicts the movement of water through columns of cartilage and newly formed bone and provides information about velocity of growth and growth potential.

OBJECTIVE

To determine the correlation between DTI tractography parameters of the distal femoral physis and metaphysis and the height change after DTI in pubertal and post-pubertal children.

MATERIALS AND METHODS

We retrospectively analyzed DTI images of the knee in 47 children with a mean age of 14.1 years in a 2-year period. In sagittal echoplanar DTI studies, regions of interest were placed in the femoral physis. Tractography was performed using a fractional anisotropy threshold of 0.15 and a maximum turning angle of 40°. The sample was divided to assess short-term and long-term growth after DTI. Short-term growth (n=25) was the height change between height at MRI and 1 year later. Long-term growth (n=36) was the height gain between height at MRI and at the growth plateau.

RESULTS

For the short-term group, subjects with larger tract volume (R²=0.40) and longer track lengths (R²=0.38) had larger height gains (P<0.01). For the long-term group, subjects with larger tract volume (R²=0.43) and longer track lengths (R²=0.32) had a larger height gain at the growth plateau (P<0.01). Intra- and inter-observer variability were good-excellent.

CONCLUSION

Follow-up data of growth 1 year after DTI evaluation and at skeletal maturity confirms that DTI parameters are associated with the amount of post-imaging growth.

Sox9+ messenger cells orchestrate large-scale skeletal regeneration in the mammalian rib

Most bones in mammals display a limited capacity for natural large-scale repair. The ribs are a notable exception, yet the source of their remarkable regenerative ability remains unknown. Here, we identify a Sox9-expressing periosteal subpopulation that orchestrates large-scale regeneration of murine rib bones. Deletion of the obligate Hedgehog co-receptor, Smoothened, in Sox9-expressing cells prior to injury results in a near-complete loss of callus formation and rib bone regeneration. In contrast to its role in development, Hedgehog signaling is dispensable for the proliferative expansion of callus cells in response to injury. Instead, Sox9-positive lineage cells require Hh signaling to stimulate neighboring cells to differentiate via an unknown signal into a skeletal cell type with dual chondrocyte/osteoblast properties. This type of callus cell may be critical for bridging large bone injuries. Thus despite contributing to only a subset of callus cells, Sox9-positive progenitors play a major role in orchestrating large-scale bone regeneration.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Enzymatically Cross-Linked Silk Fibroin-Based Hierarchical Scaffolds for Osteochondral Regeneration

Osteochondral (OC) regeneration faces several limitations in orthopedic surgery, owing to the complexity of the OC tissue that simultaneously entails the restoration of articular cartilage and subchondral bone diseases. In this study, novel biofunctional hierarchical scaffolds composed of a horseradish peroxidase (HRP)-cross-linked silk fibroin (SF) cartilage-like layer (HRP-SF layer) fully integrated into a HRP-SF/ZnSr-doped β-tricalcium phosphate (β-TCP) subchondral bone-like layer (HRP-SF/dTCP layer) were proposed as a promising strategy for OC tissue regeneration. For comparative purposes, a similar bilayered structure produced with no ion incorporation (HRP-SF/TCP layer) was used. A homogeneous porosity distribution was achieved throughout the scaffolds, as shown by micro-computed tomography analysis. The ion-doped bilayered scaffolds presented a wet compressive modulus (226.56 ± 60.34 kPa) and dynamic mechanical properties (ranging from 403.56 ± 111.62 to 593.56 ± 206.90 kPa) superior to that of the control bilayered scaffolds (189.18 ± 90.80 kPa and ranging from 262.72 ± 59.92 to 347.68 ± 93.37 kPa, respectively). Apatite crystal formation, after immersion in simulated body fluid (SBF), was observed in the subchondral bone-like layers for the scaffolds incorporating TCP powders. Human osteoblasts (hOBs) and human articular chondrocytes (hACs) were co-cultured onto the bilayered structures and monocultured in the respective cartilage and subchondral bone half of the partitioned scaffolds. Both cell types showed good adhesion and proliferation in the scaffold compartments, as well as adequate integration of the interface regions. Osteoblasts produced a mineralized extracellular matrix (ECM) in the subchondral bone-like layers, and chondrocytes showed GAG deposition. The gene expression profile was different in the distinct zones of the bilayered constructs, and the intermediate regions showed pre-hypertrophic chondrocyte gene expression, especially on the BdTCP constructs. Immunofluorescence analysis supported these observations. This study showed that the proposed bilayered scaffolds allowed a specific stimulation of the chondrogenic and osteogenic cells in the co-culture system together with the formation of an osteochondral-like tissue interface. Hence, the structural adaptability, suitable mechanical properties, and biological performance of the hierarchical scaffolds make these constructs a desired strategy for OC defect regeneration.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

The impact of immune response on endochondral bone regeneration

Tissue engineered cartilage substitutes, which induce the process of endochondral ossification, represent a regenerative strategy for bone defect healing. Such constructs typically consist of multipotent mesenchymal stromal cells (MSCs) forming a cartilage template in vitro, which can be implanted to stimulate bone formation in vivo. The use of MSCs of allogeneic origin could potentially improve the clinical utility of the tissue engineered cartilage constructs in three ways. First, ready-to-use construct availability can speed up the treatment process. Second, MSCs derived and expanded from a single donor could be applied to treat several patients and thus the costs of the medical interventions would decrease. Finally, it would allow more control over the quality of the MSC chondrogenic differentiation. However, even though the envisaged clinical use of allogeneic cell sources for bone regeneration is advantageous, their immunogenicity poses a significant obstacle to their clinical application. The aim of this review is to increase the awareness of the role played by immune cells during endochondral ossification, and in particular during regenerative strategies when the immune response is altered by the presence of implanted biomaterials and/or cells. More specifically, we focus on how this balance between immune response and bone regeneration is affected by the implantation of a cartilaginous tissue engineered construct of allogeneic origin.

Non-sutural basicranium-derived cells undergo a unique mineralization pathway via a cartilage intermediate in vitro

The basicranium serves as a key interface in the mammalian skull, interacting with the calvarium, facial skeleton and vertebral column. Despite its critical function, little is known about basicranial bone formation, particularly on a cellular level. The goal of this study was therefore to cultivate a better understanding of basicranial development by isolating and characterizing the osteogenic potential of cells from the neonatal murine cranial base. Osteoblast-like basicranial cells were isolated, seeded in multicellular aggregates (designated micromasses), and cultured in osteogenic medium in the presence or absence of bone morphogenetic protein-6 (BMP6). A minimal osteogenic response was observed in control osteogenic medium, while BMP6 treatment induced a chondrogenic response followed by up-regulation of osteogenic markers and extensive mineralization. This response appears to be distinct from prior analyses of the calvarium and long bones, as basicranial cells did not mineralize under standard osteogenic conditions, but rather required BMP6 to stimulate mineralization, which occurred via an endochondral-like process. These findings suggest that this site may be unique compared to other cranial elements as well as the limb skeleton, and we propose that the distinct characteristics of these cells may be a function of the distinct properties of the basicranium: endochondral ossification, dual embryology, and complex loading environment.

Identification of circulating murine CD34+OCN+ cells

BACKGROUND AIMS

Previous studies identified a circulating human osteoblastic population that expressed osteocalcin (OCN), increased following fracture and pubertal growth, and formed mineralized colonies in vitro and bone in vivo. A subpopulation expressed CD34, a hematopoietic/endothelial marker. These findings led to our hypothesis that hematopoietic-derived CD34⁺OCN⁺ cells exist in the circulation of mice and are modulated after fracture.

METHODS

Flow cytometry was used to identify CD34⁺OCN⁺ cells in male B6.SJL-Ptprc^aPepc^b/BoyJ and Vav-Cre/mTmG (VavR) mice. Non-stabilized tibial fractures were created by three-point bend. Fractures were longitudinally imaged by micro-computed tomography, and immunofluorescent staining was used to evaluate CD34⁺OCN⁺ cells within fracture callus. AMD3100 (10 mg/kg) was injected subcutaneously for 3 days and the CD34⁺OCN⁺ population was evaluated by flow cytometry.

RESULTS

Circulating CD34⁺OCN⁺ cells were identified in mice and confirmed to be of hematopoietic origin (CD45⁺; Vav1⁺) using two mouse models. Both circulating and bone marrow-derived CD34⁺OCN⁺ cells peaked three weeks post-non-stabilized tibial fracture, suggesting association with cartilage callus transition to bone and early mineralization. Co-expression of CD34 and OCN in the fracture callus at two weeks post-fracture was observed. By three weeks, there was 2.1-fold increase in number of CD34⁺OCN⁺ cells, and these were observed throughout the fracture callus. AMD3100 altered CD34⁺OCN⁺ cell levels in peripheral blood and bone marrow.

DISCUSSION

Together, these data demonstrate a murine CD34⁺OCN⁺ circulating population that may be directly involved in fracture repair. Future studies will molecularly characterize CD34⁺OCN⁺ cells, determine mechanisms regulating their contribution, and examine if their number correlates with improved fracture healing outcomes.

The Chondro-Osseous Continuum: Is It Possible to Unlock the Potential Assigned Within?

Endochondral ossification (EO), by which long bones of the axial skeleton form, is a tightly regulated process involving chondrocyte maturation with successive stages of proliferation, maturation, and hypertrophy, accompanied by cartilage matrix synthesis, calcification, and angiogenesis, followed by osteoblast-mediated ossification. This developmental sequence reappears during fracture repair and in osteoarthritic etiopathology. These similarities suggest that EO, and the cells involved, are of great clinical importance for bone regeneration as it could provide novel targeted approaches to increase specific signaling to promote fracture healing, and if regulated appropriately in the treatment of osteoarthritis. The long-held accepted dogma states that hypertrophic chondrocytes are terminally differentiated and will eventually undergo apoptosis. In this mini review, we will explore recent evidence from experiments that revisit the idea that hypertrophic chondrocytes have pluripotent capacity and may instead transdifferentiate into a specific sub-population of osteoblast cells. There are multiple lines of evidence, including our own, showing that local, selective alterations in cartilage extracellular matrix (ECM) remodeling also indelibly alter bone quality. This would be consistent with the hypothesis that osteoblast behavior in long bones is regulated by a combination of their lineage origins and the epigenetic effects of chondrocyte-derived ECM which they encounter during their recruitment. Further exploration of these processes could help to unlock potential novel targets for bone repair and regeneration and in the treatment of osteoarthritis.

Perfusion Enhances Hypertrophic Chondrocyte Matrix Deposition, But Not the Bone Formation

Perfusion bioreactors have been an effective tool in bone tissue engineering. Improved nutrient delivery and the application of shear forces have stimulated osteoblast differentiation and matrix production, allowing for generation of large, clinically sized constructs. Differentiation of hypertrophic chondrocytes has been considered an alternative strategy for bone tissue engineering. We studied the effects of perfusion on hypertrophic chondrocyte differentiation, matrix production, and subsequent bone formation. Hypertrophic constructs were created by differentiation in chondrogenic medium (2 weeks) and maturation in hypertrophic medium (3 weeks). Bioreactors were customized to study a range of flow rates (0-1200 μm/s). During chondrogenic differentiation, increased flow rates correlated with cartilage matrix deposition and the presence of collagen type X. During induced hypertrophic maturation, increased flow rates correlated with bone template deposition and the increased secretion of chondroprotective cytokines. Following an 8-week implantation into the critical-size femoral defect in nude rats, nonperfused constructs displayed larger bone volume, more compact mineralized matrix, and better integration with the adjacent native bone. Therefore, although medium perfusion stimulated the formation of bone template in vitro, it failed to enhance bone regeneration in vivo. However, the promising results of the less developed template in the critical-sized defect warrant further investigation, beyond interstitial flow, into the specific environment needed to optimize hypertrophic chondrocyte-based constructs for bone repair.

Roles of Chondrocytes in Endochondral Bone Formation and Fracture Repair

The formation of the mandibular condylar cartilage (MCC) and its subchondral bone is an important but understudied topic in dental research. The current concept regarding endochondral bone formation postulates that most hypertrophic chondrocytes undergo programmed cell death prior to bone formation. Under this paradigm, the MCC and its underlying bone are thought to result from 2 closely linked but separate processes: chondrogenesis and osteogenesis. However, recent investigations using cell lineage tracing techniques have demonstrated that many, perhaps the majority, of bone cells are derived via direct transformation from chondrocytes. In this review, the authors will briefly discuss the history of this idea and describe recent studies that clearly demonstrate that the direct transformation of chondrocytes into bone cells is common in both long bone and mandibular condyle development and during bone fracture repair. The authors will also provide new evidence of a distinct difference in ossification orientation in the condylar ramus (1 ossification center) versus long bone ossification formation (2 ossification centers). Based on our recent findings and those of other laboratories, we propose a new model that contrasts the mode of bone formation in much of the mandibular ramus (chondrocyte-derived) with intramembranous bone formation of the mandibular body (non-chondrocyte-derived).

Signaling pathways regulating cartilage growth plate formation and activity

The growth plate is a highly specialized and dynamic cartilage structure that serves many essential functions in skeleton patterning, growth and endochondral ossification in developing vertebrates. Major signaling pathways initiated by classical morphogens and by other systemic and tissue-specific factors are intimately involved in key aspects of growth plate development. As a corollary of these essential functions, disturbances in these pathways due to mutations or environmental factors lead to severe skeleton disorders. Here, we review these pathways and the most recent progress made in understanding their roles in chondrocyte differentiation in growth plate development and activity. Furthermore, we discuss newly uncovered pathways involved in growth plate formation, including mTOR, the circadian clock, and the COP9 signalosome.

Interplay between CaSR and PTH1R signaling in skeletal development and osteoanabolism

Parathyroid hormone (PTH)-related peptide (PTHrP) controls the pace of pre- and post-natal growth plate development by activating the PTH1R in chondrocytes, while PTH maintains mineral and skeletal homeostasis by modulating calciotropic activities in kidneys, gut, and bone. The extracellular calcium-sensing receptor (CaSR) is a member of family C, G protein-coupled receptor, which regulates mineral and skeletal homeostasis by controlling PTH secretion in parathyroid glands and Ca(2+) excretion in kidneys. Recent studies showed the expression of CaSR in chondrocytes, osteoblasts, and osteoclasts and confirmed its non-redundant roles in modulating the recruitment, proliferation, survival, and differentiation of the cells. This review emphasizes the actions of CaSR and PTH1R signaling responses in cartilage and bone and discusses how these two signaling cascades interact to control growth plate development and maintain skeletal metabolism in physiological and pathological conditions. Lastly, novel therapeutic regimens that exploit interrelationship between the CaSR and PTH1R are proposed to produce more robust osteoanabolism.

Spatially organized differentiation of mesenchymal stem cells within biphasic microparticle-incorporated high cell density osteochondral tissues

Giving rise to both bone and cartilage during development, bone marrow-derived mesenchymal stem cells (hMSC) have the unique capacity to generate the complex tissues of the osteochondral interface. Utilizing a scaffold-free hMSC system, biphasic osteochondral constructs are incorporated with two types of growth factor-releasing microparticles to enable spatially organized differentiation. Gelatin microspheres (GM) releasing transforming growth factor-β1 (TGF-β1) combined with hMSC form the chondrogenic phase. The osteogenic phase contains hMSC only, mineral-coated hydroxyapatite microparticles (MCM), or MCM loaded with bone morphogenetic protein-2 (BMP-2), cultured in medium with or without BMP-2. After 4 weeks, TGF-β1 release from GM within the cartilage phase promotes formation of a glycosaminoglycan- and type II collagen-rich matrix, and has a local inhibitory effect on osteogenesis. In the osteogenic phase, type X collagen and osteopontin are produced in all conditions. However, calcification occurs on the outer edges of the chondrogenic phase in some constructs cultured in media containing BMP-2, and alkaline phosphatase levels are elevated, indicating that BMP-2 releasing MCM provides better control over region-specific differentiation. The production of complex, stem cell-derived osteochondral tissues via incorporated microparticles could enable earlier implantation, potentially improving outcomes in the treatment of osteochondral defects.

Dullard/Ctdnep1 regulates endochondral ossification via suppression of TGF-β signaling

Transforming growth factor (TGF)-β signaling plays critical roles during skeletal development and its excessive signaling causes genetic diseases of connective tissues including Marfan syndrome and acromelic dysplasia. However, the mechanisms underlying prevention of excessive TGF-β signaling in skeletogenesis remain unclear. We previously reported that Dullard/Ctdnep1 encoding a small phosphatase is required for nephron maintenance after birth through suppression of bone morphogenetic protein (BMP) signaling. Unexpectedly, we found that Dullard is involved in suppression of TGF-β signaling during endochondral ossification. Conditional Dullard-deficient mice in the limb and sternum mesenchyme by Prx1-Cre displayed the impaired growth and ossification of skeletal elements leading to postnatal lethality. Dullard was expressed in early cartilage condensations and later in growth plate chondrocytes. The tibia growth plate of newborn Dullard mutant mice showed reduction of the proliferative and hypertrophic chondrocyte layers. The sternum showed deformity of cartilage primordia and delayed hypertrophy. Micromass culture experiments revealed that Dullard deficiency enhanced early cartilage condensation and differentiation, but suppressed mineralized hypertrophic chondrocyte differentiation, which was reversed by treatment with TGF-β type I receptor kinase blocker LY-364947. Dullard deficiency induced upregulation of protein levels of both phospho-Smad2/3 and total Smad2/3 in micromass cultures without increase of Smad2/3 mRNA levels, suggesting that Dullard may affect Smad2/3 protein stability. The phospho-Smad2/3 level was also upregulated in perichondrium and hypertrophic chondrocytes in Dullard-deficient embryos. Response to TGF-β signaling was enhanced in Dullard-deficient primary chondrocyte cultures at late, but not early, time point. Moreover, perinatal administration of LY-364947 ameliorated the sternum deformity in vivo. Thus, we identified Dullard as a new negative regulator of TGF-β signaling in endochondral ossification.

In vitro 30 nm silver nanoparticles promote chondrogenesis of human mesenchymal stem cells

Silver nanoparticles positively influence chondrogenesis of human mesenchymal stem cells through promoting expression of chondrogenic markers while reducing hypertrophy.

Chondrocytes transdifferentiate into osteoblasts in endochondral bone during development, postnatal growth and fracture healing in mice

One of the crucial steps in endochondral bone formation is the replacement of a cartilage matrix produced by chondrocytes with bone trabeculae made by osteoblasts. However, the precise sources of osteoblasts responsible for trabecular bone formation have not been fully defined. To investigate whether cells derived from hypertrophic chondrocytes contribute to the osteoblast pool in trabecular bones, we genetically labeled either hypertrophic chondrocytes by Col10a1-Cre or chondrocytes by tamoxifen-induced Agc1-CreERT2 using EGFP, LacZ or Tomato expression. Both Cre drivers were specifically active in chondrocytic cells and not in perichondrium, in periosteum or in any of the osteoblast lineage cells. These in vivo experiments allowed us to follow the fate of cells labeled in Col10a1-Cre or Agc1-CreERT2 -expressing chondrocytes. After the labeling of chondrocytes, both during prenatal development and after birth, abundant labeled non-chondrocytic cells were present in the primary spongiosa. These cells were distributed throughout trabeculae surfaces and later were present in the endosteum, and embedded within the bone matrix. Co-expression studies using osteoblast markers indicated that a proportion of the non-chondrocytic cells derived from chondrocytes labeled by Col10a1-Cre or by Agc1-CreERT2 were functional osteoblasts. Hence, our results show that both chondrocytes prior to initial ossification and growth plate chondrocytes before or after birth have the capacity to undergo transdifferentiation to become osteoblasts. The osteoblasts derived from Col10a1-expressing hypertrophic chondrocytes represent about sixty percent of all mature osteoblasts in endochondral bones of one month old mice. A similar process of chondrocyte to osteoblast transdifferentiation was involved during bone fracture healing in adult mice. Thus, in addition to cells in the periosteum chondrocytes represent a major source of osteoblasts contributing to endochondral bone formation in vivo.

Diffusion-tensor imaging of the growing ends of long bones: pilot demonstration of columnar structure in the physes and metaphyses of the knee

PURPOSE

To determine the feasibility of using in vivo diffusion-tensor imaging and tractography of the physis to examine changes related to rate of growth, location, and age.

MATERIALS AND METHODS

This retrospective study was institutional review board approved and HIPAA compliant and the requirement for informed consent was waived. Diffusion-tensor imaging of the knee was performed at 3.0 T in 31 subjects (nine boys and 22 girls) with a median age of 13.6 years. The mean ages of boys and girls were 14.7 years (range, 12.0-18.3 years) and 13.2 years (range, 7.0-18.6 years), respectively. Regions of interest were placed in the physis of the tibia and femur, and in the epiphyseal and articular cartilage of these bones. Tractography was performed by using a fractional anisotropic threshold of 0.15 and an angle threshold of 40°. The tractographic patterns were qualitatively evaluated and changes related to age were described. The tract-based apparent diffusion coefficient, fractional anistropy, tensor eigenvalues, and tract length were measured. Diffusion parameters were compared between the center and periphery of the physis, and between the distal femur and proximal tibia.

RESULTS

Tractography resulted in parallel tracts in the physis and the adjacent metaphysis. Tractographic pattern changed with age, with individuals approaching physeal closure having shorter tracts in a random arrangement. Patterns of tractography varied with age in the femur (P < .001) and tibia (P < .001). Femoral tracts (median length, 6.5 mm) were longer than tibial tracts (median length, 4.3 mm) (P < .001). Tracts in the periphery of the physes were longer than those in the center (femur, P = .005; tibia, P = .004). In the physis of the femur and tibia, a significant age-related decrease was observed in apparent diffusion coefficient (P < .001 for both), axial diffusion (femur, P = .001; tibia, P < .001), and transverse diffusion [P < .001 for both]), and an age-related increase was seen in fractional anistropy (P < .001, for both).

CONCLUSION

Diffusion-tensor imaging shows the columnar microstructure of the physis and adjacent metaphysis, and provides further insight into normal growth.

Stem cell-derived endochondral cartilage stimulates bone healing by tissue transformation

Although bone has great capacity for repair, there are a number of clinical situations (fracture non-unions, spinal fusions, revision arthroplasty, segmental defects) in which auto- or allografts attempt to augment bone regeneration by promoting osteogenesis. Critical failures associated with current grafting therapies include osteonecrosis and limited integration between graft and host tissue. We speculated that the underlying problem with current bone grafting techniques is that they promote bone regeneration through direct osteogenesis. Here we hypothesized that using cartilage to promote endochondral bone regeneration would leverage normal developmental and repair sequences to produce a well-vascularized regenerate that integrates with the host tissue. In this study, we use a translational murine model of a segmental tibia defect to test the clinical utility of bone regeneration from a cartilage graft. We further test the mechanism by which cartilage promotes bone regeneration using in vivo lineage tracing and in vitro culture experiments. Our data show that cartilage grafts support regeneration of a vascularized and integrated bone tissue in vivo, and subsequently propose a translational tissue engineering platform using chondrogenesis of mesenchymal stem cells (MSCs). Interestingly, lineage tracing experiments show the regenerate was graft derived, suggesting transformation of the chondrocytes into bone. In vitro culture data show that cartilage explants mineralize with the addition of bone morphogenetic protein (BMP) or by exposure to human vascular endothelial cell (HUVEC)-conditioned medium, indicating that endothelial cells directly promote ossification. This study provides preclinical data for endochondral bone repair that has potential to significantly improve patient outcomes in a variety of musculoskeletal diseases and injuries. Further, in contrast to the dogmatic view that hypertrophic chondrocytes undergo apoptosis before bone formation, our data suggest cartilage can transform into bone by activating the pluripotent transcription factor Oct4A. Together these data represent a paradigm shift describing the mechanism of endochondral bone repair and open the door for novel regenerative strategies based on improved biology.

PTHrP produced by myeloma plasma cells regulates their survival and pro-osteoclast activity for bone disease progression

To promote their survival and progression in the skeleton, osteotropic malignancies of breast, lung, and prostate produce parathyroid hormone-related protein (PTHrP), which induces hypercalcemia. PTHrP serum elevations have also been described in multiple myeloma (MM), although their role is not well defined. When we investigated MM cells from patients and cell lines, we found that PTHrP and its receptor (PTH-R1) are highly expressed, and that PTHrP is secreted both as a full-length molecule and as small subunits. Among these subunits, the mid-region, including the nuclear localization sequence (NLS), exerted a proliferative effect because it was accumulated in nuclei of MM cells surviving in starvation conditions. This was confirmed by increased transcription of several genes enrolled in proliferation and apoptosis control. PTHrP was also found to stimulate PTH-R1 in MM cells. PTH-R1's selective activation by the full-length PTHrP molecule or the NH2 -terminal fragment resulted in a significant increase of intracellular Ca(2+) influx, cyclic adenosine monophosphate (cAMP) content, and expression of receptor activator of NF-κB ligand (RANKL) and monocyte chemoattractant protein-1 (MCP-1). Our data definitely clarify the role of PTHrP in MM. The PTHrP peptide is functionally secreted by malignant plasma cells and contributes to MM tumor biology and progression, both by intracrine maintenance of cell proliferation in stress conditions and by autocrine or paracrine stimulation of PTH-R1, which in turn reinforces the production of osteoclastogenic factors. © 2014 American Society for Bone and Mineral Research.

Discordant radiologic and histological dimensions of the zone of provisional calcification in fetal piglets

BACKGROUND

Studies have shown that the fracture plane of the classic metaphyseal lesion (CML) of infant abuse occurs in the region of the primary spongiosa, encompassing a radiodense fracture fragment customarily referred to as the "zone of provisional calcification" or ZPC. However, the zone of provisional calcification is defined differently in the pathology and the imaging literature, potentially impeding efforts to understand the fundamental morphological features of the classic metaphyseal lesion.

OBJECTIVE

We systematically correlated micro-CT data with histology in piglets to explore the differing definitions of the zone of provisional calcification and to elucidate the anatomical basis for divergent definitions.

MATERIALS AND METHODS

The distal tibias of five normal fetal piglets were studied postmortem. The specimens were resected and imaged with digital radiography (50 μm resolution) and micro-CT (45 μm(3) isotropic resolution). Image processing techniques were applied to the micro-CT data for visualization and data analysis. The resected tissue specimens were then processed routinely and the light microscopic features were correlated with the imaging findings.

RESULTS

The longitudinal dimension of the radiologic zone of provisional calcification is greater than the histological ZPC, and these dimensions are statistically distinct (P < 0.0002). The radiologic zone of provisional calcification consists of two adjoining mineralized discoid regions that span the chondro-osseous junction-a thick discoid region that encompasses the densest region of the primary spongiosa, and a thin discoid region (corresponding to the histological ZPC) that is situated in the base of the physis adjacent to the metaphysis.

CONCLUSION

The correlation of the normal histology and micro-CT appearance of this dynamic and complex region provides an anatomical foundation upon which a deeper appreciation of the morphology of the classic metaphyseal lesion can be built.

Cellular reactions to biodegradable magnesium alloys on human growth plate chondrocytes and osteoblasts

PURPOSE

In recent decades operative fracture treatment using elastic stable intramedullary nails (ESINs) has mainly taken precedence over conservative alternatives in children. The development of biodegradable materials that could be used for ESINs would be a further step towards treatment improvement. Due to its mechanical and elastic properties, magnesium seems to be an ideal material for biodegradable implant application. The aim of this study was therefore to investigate the cellular reaction to biodegradable magnesium implants in vitro.

METHODS

Primary human growth plate chondrocytes and MG63 osteoblasts were used for this study. Viability and metabolic activity in response to the eluate of a rapidly and a slower degrading magnesium alloy were investigated. Furthermore, changes in gene expression were assessed and live cell imaging was performed.

RESULTS

A superior performance of the slower degrading WZ21 alloy's eluate was detected regarding cell viability and metabolic activity, cell proliferation and morphology. However, the ZX50 alloy's eluate induced a favourable up-regulation of osteogenic markers in MG63 osteoblasts.

CONCLUSIONS

This study showed that magnesium alloys for use in biodegradable implant application are well tolerated in both osteoblasts and growth plate chondrocytes respectively.

Deletion of beta catenin in hypertrophic growth plate chondrocytes impairs trabecular bone formation

In order to elucidate the role of β-catenin in hypertrophic cartilage zone of the growth plate, we deleted the β-catenin gene ctnnb1specifically from hypertrophic chondrocytes by mating ctnnb1(fl/fl) mice with BAC-Col10a1-Cre-deleter mice. Surprisingly, this resulted in a significant reduction of subchondral trabecular bone formation in BACCol10Cre; ctnnb1(Δ/Δ) (referred to as Cat-ko) mice, although Cre expression was restricted to hypertrophic chondrocytes. The size of the Col10a1 positive hypertrophic zone was normal, but qRT-PCR revealed reduced expression of Mmp13, and Vegfa in Cat-ko hypertrophic chondrocytes, indicating impaired terminal differentiation. Immunohistological and in situ hybridization analysis revealed the substantial deficiency of collagen I positive mature osteoblasts, but equal levels of osterix-positive cells in the subchondral bone marrow space of Cat-ko mice, indicating that the supply of osteoblast precursor cells was not reduced. The fact that in Cat-ko mice subchondral trabeculae were lacking including their calcified cartilage core indicated a strongly enhanced osteoclast activity. In fact, TRAP staining as well as in situ hybridization analysis of Mmp9 expression revealed denser occupation of the cartilage erosion zone with enlarged osteoclasts as compared to the control growth plate, suggesting increased RANKL or reduced osteoprotegerin (Opg) activity in this zone. This notion was confirmed by qRT-PCR analysis of mRNA extracted from cultured hypertrophic chondrocytes or from whole epiphyses, showing increased Rankl mRNA levels in Cat-ko as compared to control chondrocytes, whereas changes in OPG levels were not significant. These results indicate that β-catenin levels in hypertrophic chondrocytes play a key role in regulating osteoclast activity and trabecular bone formation at the cartilage-bone interface by controlling RANKL expression in hypertrophic chondrocytes.

Endochondral bone growth, bone calcium accretion, and bone mineral density: how are they related?

Endochondral bone growth in young growing mammals or adult mammals with persistent growth plates progresses from proliferation, maturation and hypertrophy of growth plate chondrocytes to mineralization of cartilaginous matrix to form an osseous tissue. This complex process is tightly regulated by a number of factors with different impacts, such as genetics, endocrine/paracrine factors [e.g., PTHrP, 1,25(OH)(2)D(3), IGF-1, FGFs, and prolactin], and nutritional status (e.g., dietary calcium and vitamin D). Despite a strong link between growth plate function and elongation of the long bone, little is known whether endochondral bone growth indeed determines bone calcium accretion, bone mineral density (BMD), and/or peak bone mass. Since the process ends with cartilaginous matrix calcification, an increase in endochondral bone growth typically leads to more calcium accretion in the primary spongiosa and thus higher BMD. However, in lactating rats with enhanced trabecular bone resorption, bone elongation is inversely correlated with BMD. Although BMD can be increased by factors that enhance endochondral bone growth, the endochondral bone growth itself is unlikely to be an important determinant of peak bone mass since it is strongly determined by genetics. Therefore, endochondral bone growth and bone elongation are associated with calcium accretion only in a particular subregion of the long bone, but do not necessarily predict BMD and peak bone mass.

Behaviour of human physeal chondro-progenitorcells in early growth plate injury response in vitro

PURPOSE

The aim of this study was to investigate the proliferation and differentiation behaviour of a defined cell population gained from the human growth plate, namely, chondro-progenitorcells (CPCs), in the initial inflammatory phase of growth plate injury response in vitro.

METHODS

Growth plate cells were sorted via FACS and differentiated along adipogenic and osteogenic lineage to confirm their progenitor features. To mimic the inflammatory phase of injury response at the growth plate they were treated with IL-1β and exposed to cyclic mechanical loading. A BrdU assay was used to investigate CPC proliferation. CPC differentiation behaviour was analysed by RT-PCR.

RESULTS

CPCs (CD45-, CD34-, CD73+, CD90+, and CD105+) showed a successful differentiation along adipogenic and osteogenic lineage. Under conditions simulating the inflammatory phase of injury response at the growth plate in vitro CPCs differentiated towards hypertrophy while chondrogenesis and ossification were inhibited. Proliferation was not significantly altered.

CONCLUSION

This study showed that CPCs can be isolated from the human growth plate and expanded in vitro. In the first phase of injury response at the growth plate these cells differentiate towards hypertrophy. As longitudinal growth is obtained by chondrocyte proliferation and volume increase during hypertrophy this maturation might be the first step towards post-traumatic growth disorders such as unwanted premature ossification of the growth plate.

Molecular differentiation between osteophytic and articular cartilage--clues for a transient and permanent chondrocyte phenotype

OBJECTIVE

To identify the molecular differences between the transient and permanent chondrocyte phenotype in osteophytic and articular cartilage.

METHODS

Total RNA was isolated from the cartilaginous layer of osteophytes and from intact articular cartilage from knee joints of 15 adult human donors and subjected to cDNA microarray analysis. The differential expression of relevant genes between these two cartilaginous tissues was additionally validated by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and by immunohistochemistry.

RESULTS

Among 47,000 screened transcripts, 600 transcripts were differentially expressed between osteophytic and articular chondrocytes. Osteophytic chondrocytes were characterized by increased expression of genes involved in the endochondral ossification process [bone gamma-carboxyglutamate protein/osteocalcin (BGLAP), bone morphogenetic protein-8B (BMP8B), collagen type I, alpha 2 (COL1A2), sclerostin (SOST), growth arrest and DNA damage-induced gene 45ß (GADD45ß), runt-related transcription factor 2 (RUNX2)], and genes encoding tissue remodeling enzymes [matrix metallopeptidase (MMP)9, 13, hyaluronan synthase 1 (HAS1)]. Articular chondrocytes expressed increased transcript levels of antagonists and inhibitors of the BMP- and Wnt-signaling pathways [Gremlin-1 (GREM1), frizzled-related protein (FRZB), WNT1 inducible signaling pathway protein-3 (WISP3)], as well as factors that inhibit terminal chondrocyte differentiation and endochondral bone formation [parathyroid hormone-like hormone (PTHLH), sex-determining region Y-box 9 (SOX9), stanniocalcin-2 (STC2), S100 calcium binding protein A1 (S100A1), S100 calcium binding protein B (S100B)]. Immunohistochemistry of tissue sections for GREM1 and BGLAP, the two most prominent differentially expressed genes, confirmed selective detection of GREM1 in articular chondrocytes and that of BGLAP in osteophytic chondrocytes and bone.

CONCLUSIONS

Osteophytic and articular chondrocytes significantly differ in their gene expression pattern. In articular cartilage, a prominent expression of antagonists inhibiting the BMP- and Wnt-pathway may serve to lock and stabilize the permanent chondrocyte phenotype and thus prevent their terminal differentiation. In contrast, osteophytic chondrocytes express genes with roles in the endochondral ossification process, which may account for their transient phenotype.

Transglutaminase 2 as a biomarker of osteoarthritis: an update

Osteoarthritis is a progressive joint disease characterized by cartilage degradation and bone remodelling. Under physiologic conditions, articular cartilage displays a stable chondrocyte phenotype, whereas in osteoarthritis a chondrocyte hypertrophy develops near the sites of cartilage surface damage and associates to the pathologic expression of type X collagen. Transglutaminases (TGs) include a family of Ca(2+)-dependent enzymes that catalyze the formation of γ-glutamyl cross-links. Their substrates include a variety of intracellular and extracellular macromolecular components. TGs are ubiquitously and abundantly expressed and implicated in a variety of physiopathological processes. TGs activity is modulated by inflammatory cytokines. TG2 (also known as tissue transglutaminase) mediates the hypertrophic differentiation of joint chondrocytes and interleukin-1-induced calcification. Histomorphometrical and biomolecular investigations document increased TG2 expression in human and experimental osteoarthritis. Consequently, the level of TG2 expression may represent an adjuvant additional marker to monitor tissue remodelling occurring in osteoarthritic joint tissue. Experimental induction of osteoarthritis in TG2 knockout mice is followed from reduced cartilage destruction and increased osteophyte formation compared to wild-type mice, suggesting a different influence on joint bone and cartilage remodelling. The capacity of transamidation by TG2 to regulate activation of latent TGF-β seems to have a potential impact on the regulation of inflammatory response in osteoarthritic tissues. Additional studies are needed to define TG2-regulated pathways that are differently modulated in osteoblasts and chondrocytes during osteoarthritis.

Pannexin 3 is a novel target for Runx2, expressed by osteoblasts and mature growth plate chondrocytes

Pannexins are a class of chordate channel proteins identified by their homology to insect gap junction proteins. The pannexin family consists of three members, Panx1, Panx2, and Panx3, and the role each of these proteins plays in cellular processes is still under investigation. Previous reports of Panx3 expression indicate enrichment in skeletal tissues, so we have further investigated this distribution by surveying the developing mouse embryo with immunofluorescence. High levels of Panx3 were detected in intramembranous craniofacial flat bones, as well as long bones of the appendicular and axial skeleton. This distribution is the result of expression in both osteoblasts and hypertrophic chondrocytes. Furthermore, the Panx3 promoter contains putative binding sites for transcription factors involved in bone formation, and we show that the sequence between bases -275 and -283 is responsive to Runx2 activation. Taken together, our data suggests that Panx3 may serve an important role in bone development, and is a novel target for Runx2-dependent signaling.

Changes in the chondrocyte and extracellular matrix proteome during post-natal mouse cartilage development

Skeletal growth by endochondral ossification involves tightly coordinated chondrocyte differentiation that creates reserve, proliferating, prehypertrophic, and hypertrophic cartilage zones in the growth plate. Many human skeletal disorders result from mutations in cartilage extracellular matrix (ECM) components that compromise both ECM architecture and chondrocyte function. Understanding normal cartilage development, composition, and structure is therefore vital to unravel these disease mechanisms. To study this intricate process in vivo by proteomics, we analyzed mouse femoral head cartilage at developmental stages enriched in either immature chondrocytes or maturing/hypertrophic chondrocytes (post-natal days 3 and 21, respectively). Using LTQ-Orbitrap tandem mass spectrometry, we identified 703 cartilage proteins. Differentially abundant proteins (q < 0.01) included prototypic markers for both early and late chondrocyte differentiation (epiphycan and collagen X, respectively) and novel ECM and cell adhesion proteins with no previously described roles in cartilage development (tenascin X, vitrin, Urb, emilin-1, and the sushi repeat-containing proteins SRPX and SRPX2). Meta-analysis of cartilage development in vivo and an in vitro chondrocyte culture model (Wilson, R., Diseberg, A. F., Gordon, L., Zivkovic, S., Tatarczuch, L., Mackie, E. J., Gorman, J. J., and Bateman, J. F. (2010) Comprehensive profiling of cartilage extracellular matrix formation and maturation using sequential extraction and label-free quantitative proteomics. Mol. Cell. Proteomics 9, 1296-1313) identified components involved in both systems, such as Urb, and components with specific roles in vivo, including vitrin and CILP-2 (cartilage intermediate layer protein-2). Immunolocalization of Urb, vitrin, and CILP-2 indicated specific roles at different maturation stages. In addition to ECM-related changes, we provide the first biochemical evidence of changing endoplasmic reticulum function during cartilage development. Although the multifunctional chaperone BiP was not differentially expressed, enzymes and chaperones required specifically for collagen biosynthesis, such as the prolyl 3-hydroxylase 1, cartilage-associated protein, and peptidyl prolyl cis-trans isomerase B complex, were down-regulated during maturation. Conversely, the lumenal proteins calumenin, reticulocalbin-1, and reticulocalbin-2 were significantly increased, signifying a shift toward calcium binding functions. This first proteomic analysis of cartilage development in vivo reveals the breadth of protein expression changes during chondrocyte maturation and ECM remodeling in the mouse femoral head.

Identification and characterization of chondrogenic progenitor cells in the fascia of postnatal skeletal muscle

Intramuscular injection of bone morphogenetic proteins (BMPs) has been shown to induce ectopic bone formation. A chondrogenic phase is typically observed in this process, which suggests that there may exist a chondrogenic subpopulation of cells residing in skeletal muscle. Two prospective cell populations were isolated from rat skeletal muscle: fascia-derived cells (FDCs), extracted from gluteus maximus muscle fascia (epimysium) and muscle-derived cells (MDCs) isolated from the muscle body. Both populations were investigated for their cell surface marker profiles (flowcytometry analysis), proliferation rates as well as their myogenic and chondrogenic potentials. The majority of FDCs expressed mesenchymal stromal cell markers but not endothelial cell markers. FDCs underwent chondrogenic differentiation after BMP4 treatment in vitro, but not myogenic differentiation. Although MDCs showed chondrogenic potential, they expressed the myogenic cell marker desmin and readily underwent myogenic differentiation in vitro; however, the chondrogenic potential of the MDCs is confounded by the presence of FDC-like cells residing in the muscle perimysium and endomysium. To clarify the role of the muscle-derived myogenic cells in chondrogenesis, mixed pellets with varying ratios of FDCs and L6 myoblasts were formed and studied for chondrogenic potential. Our results indicated that the chondrogenic potential of the mixed pellets decreased with the increased ratio of myogenic cells to FDCs supporting the role of FDCs in chondrogenesis. Taken together, our results suggest that non-myogenic cells residing in the fascia of skeletal muscle have a strong chondrogenic potential and may represent a novel donor cell source for cartilage regeneration and repair.

Enhanced growth and osteogenic differentiation of human osteoblast-like cells on boron-doped nanocrystalline diamond thin films

Intrinsic nanocrystalline diamond (NCD) films have been proven to be promising substrates for the adhesion, growth and osteogenic differentiation of bone-derived cells. To understand the role of various degrees of doping (semiconducting to metallic-like), the NCD films were deposited on silicon substrates by a microwave plasma-enhanced CVD process and their boron doping was achieved by adding trimethylboron to the CH₄:H₂ gas mixture, the B∶C ratio was 133, 1000 and 6700 ppm. The room temperature electrical resistivity of the films decreased from >10 MΩ (undoped films) to 55 kΩ, 0.6 kΩ, and 0.3 kΩ (doped films with 133, 1000 and 6700 ppm of B, respectively). The increase in the number of human osteoblast-like MG 63 cells in 7-day-old cultures on NCD films was most apparent on the NCD films doped with 133 and 1000 ppm of B (153,000±14,000 and 152,000±10,000 cells/cm², respectively, compared to 113,000±10,000 cells/cm² on undoped NCD films). As measured by ELISA per mg of total protein, the cells on NCD with 133 and 1000 ppm of B also contained the highest concentrations of collagen I and alkaline phosphatase, respectively. On the NCD films with 6700 ppm of B, the cells contained the highest concentration of focal adhesion protein vinculin, and the highest amount of collagen I was adsorbed. The concentration of osteocalcin also increased with increasing level of B doping. The cell viability on all tested NCD films was almost 100%. Measurements of the concentration of ICAM-1, i.e. an immunoglobuline adhesion molecule binding inflammatory cells, suggested that the cells on the NCD films did not undergo significant immune activation. Thus, the potential of NCD films for bone tissue regeneration can be further enhanced and tailored by B doping and that B doping up to metallic-like levels is not detrimental for cells.