In vivo fate mapping identifies mesenchymal progenitor cells

Tenomodulin knockout mice exhibit worse late healing outcomes with augmented trauma-induced heterotopic ossification of Achilles tendon

Heterotopic ossification (HO) represents a common problem after tendon injury with no effective treatment yet being developed. Tenomodulin (Tnmd), the best-known mature marker for tendon lineage cells, has important effects in tendon tissue aging and function. We have reported that loss of Tnmd leads to inferior early tendon repair characterized by fibrovascular scaring and therefore hypothesized that its lack will persistently cause deficient repair during later stages. Tnmd knockout (Tnmd-/-) and wild-type (WT) animals were subjected to complete Achilles tendon surgical transection followed by end-to-end suture. Lineage tracing revealed a reduction in tendon-lineage cells marked by ScleraxisGFP, but an increase in alpha smooth muscle actin myofibroblasts in Tnmd-/- tendon scars. At the proliferative stage, more pro-inflammatory M1 macrophages and larger collagen II cartilaginous template were detected in this group. At the remodeling stage, histological scoring revealed lower repair quality in the injured Tnmd-/- tendons, which was coupled with higher HO quantified by micro-CT. Tendon biomechanical properties were compromised in both groups upon injury, however we identified an abnormal stiffening of non-injured Tnmd-/- tendons, which possessed higher static and dynamic E-moduli. Pathologically thicker and abnormally shaped collagen fibrils were observed by TEM in Tnmd-/- tendons and this, together with augmented HO, resulted in diminished running capacity of Tnmd-/- mice. These novel findings demonstrate that Tnmd plays a protecting role against trauma-induced endochondral HO and can inspire the generation of novel therapeutics to accelerate repair.

The periosteum: a simple tissue with many faces, with special reference to the antler-lineage periostea

Periosteum is a thin membrane covering bone surfaces and consists of two layers: outer fibrous layer and inner cambium layer. Simple appearance of periosteum has belied its own complexity as a composite structure for physical bone protection, mechano-sensor for sensing mechanical loading, reservoir of biochemical molecules for initiating cascade signaling, niche of osteogenic cells for bone formation and repair, and "umbilical cord" for nourishing bone tissue. Periosteum-derived cells (PDCs) have stem cell attributes: self-renewal (no signs of senescence until 80 population doublings) and multipotency (differentiate into fibroblasts, osteoblasts, chondrocytes, adipocytes and skeletal myocytes). In this review, we summarized the currently available knowledge about periosteum and with special references to antler-lineage periostea, and demonstrated that although periosteum is a type of simple tissue in appearance, with multiple faces in functions; antler-lineage periostea add another dimension to the properties of somatic periostea: capable of initiation of ectopic organ formation upon transplantation and full mammalian organ regeneration when interacted with the covering skin. Very recently, we have translated this finding into other mammals, i.e. successfully induced partial regeneration of the amputated rat legs. We believe further refinement along this line would greatly benefit human health.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Cortical bone development, maintenance and porosity: genetic alterations in humans and mice influencing chondrocytes, osteoclasts, osteoblasts and osteocytes

Cortical bone structure is a crucial determinant of bone strength, yet for many years studies of novel genes and cell signalling pathways regulating bone strength have focused on the control of trabecular bone mass. Here we focus on mechanisms responsible for cortical bone development, growth, and degeneration, and describe some recently described genetic-driven modifications in humans and mice that reveal how these processes may be controlled. We start with embryonic osteogenesis of preliminary bone structures preceding the cortex and describe how this structure consolidates then matures to a dense, vascularised cortex containing an increasing proportion of lamellar bone. These processes include modelling-induced, and load-dependent, asymmetric cortical expansion, which enables the cortex's transition from a highly porous woven structure to a consolidated and thickened highly mineralised lamellar bone structure, infiltrated by vascular channels. Sex-specific differences emerge during this process. With aging, the process of consolidation reverses: cortical pores enlarge, leading to greater cortical porosity, trabecularisation and loss of bone strength. Each process requires co-ordination between bone formation, bone mineralisation, vascularisation, and bone resorption, with a need for locational-, spatial- and cell-specific signalling pathways to mediate this co-ordination. We will discuss these processes, and a number of cell-signalling pathways identified in both murine and human genetic studies to regulate cortical bone mass, including signalling through gp130, STAT3, PTHR1, WNT16, NOTCH, NOTUM and sFRP4.

Cell lineage tracing and functional assessment of supraspinatus tendon healing in an acute repair murine model

Rotator cuff supraspinatus tendon injuries are common with high rates of anatomic failure after surgical repair. The purpose of the study was to define clinically relevant features of a mouse model of supraspinatus tendon injury to determine painful, functional, and structural outcomes; we further investigated two cell populations mediating healing using genetic lineage tracing after full detachment and repair of the supraspinatus tendon in mice. The pain was assessed using the mouse grimace scale and function by gait analysis and tensile testing. Histological and microCT analyses were used to determine enthesis/tendon and bone structure, respectively. Lineage tracing was carried out using inducible Cre lines for ScxCreERT2 (tendon cells) and αSMACreERT2 (myofibroblasts and mesenchymal progenitors). Mice only expressed pain transiently after surgery despite long-term impairment of functional and structural properties. Gait, tensile mechanical properties, and bone properties were significantly reduced after injury and repair. Lineage tracing showed relatively few Scx ^lin tendon cells while αSMA ^lin cells contributed strongly to scar formation. Despite surgical reattachment of healthy tendon, lineage tracing revealed poor preservation of supraspinatus tendon after acute injury and loss of tendon structure, suggesting that tendon degeneration is also a key impediment of successful rotator cuff repair. Scar formation after surgery is mediated largely by αSMA ^lin cells and results in permanently reduced functional and structural properties.

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Tmem100- and Acta2-Lineage Cells Contribute to Implant Osseointegration in a Mouse Model

Metal implants are commonly used in orthopedic surgery. The mechanical stability and longevity of implants depend on adequate bone deposition along the implant surface. The cellular and molecular mechanisms underlying peri-implant bone formation (ie, osseointegration) are incompletely understood. Herein, our goal was to determine the specific bone marrow stromal cell populations that contribute to bone formation around metal implants. To do this, we utilized a mouse tibial implant model that is clinically representative of human joint replacement procedures. Using a lineage-tracing approach, we found that both Acta2.creERT2 and Tmem100.creERT2 lineage cells are involved in peri-implant bone formation, and Pdgfra- and Ly6a/Sca1-expressing stromal cells (PαS cells) are highly enriched in both lineages. Single-cell RNA-seq analysis indicated that PαS cells are quiescent in uninjured bone tissue; however, they express markers of proliferation and osteogenic differentiation shortly after implantation surgery. Our findings indicate that PαS cells are mobilized to repair bone tissue and participate in implant osseointegration after surgery. Biologic therapies targeting PαS cells might improve osseointegration in patients undergoing orthopedic procedures. © 2021 American Society for Bone and Mineral Research (ASBMR).

Fibrocartilage Stem Cells in the Temporomandibular Joint: Insights From Animal and Human Studies

Temporomandibular disorders (TMD) are diseases involving the temporomandibular joint (TMJ), masticatory muscles, and osseous components. TMD has a high prevalence, with an estimated 4.8% of the U.S. population experiencing signs and symptoms, and represents a financial burden to both individuals and society. During TMD progression, the most frequently affected site is the condylar cartilage. Comprising both fibrous and cartilaginous tissues, condylar cartilage has restricted cell numbers but lacks a vascular supply and has limited regenerative properties. In 2016, a novel stem cell niche containing a reservoir of fibrocartilage stem cells (FCSCs) was discovered in the condylar cartilage of rats. Subsequently, FCSCs were identified in mouse, rabbit, and human condylar cartilage. Unlike mesenchymal stem cells or other tissue-specific stem/progenitor cells, FCSCs play a unique role in the development and regeneration of fibrocartilage. More importantly, engraftment treatment of FCSCs has been successfully applied in animal models of TMD. In this context, FCSCs play a major role in the regeneration of newly formed cartilage. Furthermore, FCSCs participate in the regeneration of intramembranous bone by interacting with endothelial cells in bone defects. This evidence highlights the potential of FCSCs as an ideal stem cell source for the regeneration of oral maxillofacial tissue. This review is intended to detail the current knowledge of the characteristics and function of FCSCs in the TMJ, as well as the potential therapeutic applications of FCSCs. A deep understanding of the properties of FCSCs can thus inform the development of promising, biologically based strategies for TMD in the future.

Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis

Angiogenesis and osteogenesis are coupled. However, the cellular and molecular regulation of these processes remains to be further investigated. Both tissues have recently been recognized as endocrine organs, which has stimulated research interest in the screening and functional identification of novel paracrine factors from both tissues. This review aims to elaborate on the novelty and significance of endocrine regulatory loops between bone and the vasculature. In addition, research progress related to the bone vasculature, vessel-related skeletal diseases, pathological conditions, and angiogenesis-targeted therapeutic strategies are also summarized. With respect to future perspectives, new techniques such as single-cell sequencing, which can be used to show the cellular diversity and plasticity of both tissues, are facilitating progress in this field. Moreover, extracellular vesicle-mediated nuclear acid communication deserves further investigation. In conclusion, a deeper understanding of the cellular and molecular regulation of angiogenesis and osteogenesis coupling may offer an opportunity to identify new therapeutic targets.

Heterogeneity of murine periosteum progenitors involved in fracture healing

The periosteum is the major source of cells involved in fracture healing. We sought to characterize progenitor cells and their contribution to bone fracture healing. The periosteum is highly enriched with progenitor cells, including Sca1+ cells, fibroblast colony-forming units, and label-retaining cells compared to the endosteum and bone marrow. Using lineage tracing, we demonstrate that alpha smooth muscle actin (αSMA) identifies long-term, slow-cycling, self-renewing osteochondroprogenitors in the adult periosteum that are functionally important for bone formation during fracture healing. In addition, Col2.3CreER-labeled osteoblast cells contribute around 10% of osteoblasts but no chondrocytes in fracture calluses. Most periosteal osteochondroprogenitors following fracture can be targeted by αSMACreER. Previously identified skeletal stem cell populations were common in periosteum but contained high proportions of mature osteoblasts. We have demonstrated that the periosteum is highly enriched with skeletal progenitor cells, and there is heterogeneity in the populations of cells that contribute to mature lineages during periosteal fracture healing.

The vital role of Gli1+ mesenchymal stem cells in tissue development and homeostasis

The hedgehog (Hh) signaling pathway plays an essential role in both tissue development and homeostasis. Glioma-associated oncogene homolog 1 (Gli1) is one of the vital transcriptional factors as well as the direct target gene in the Hh signaling pathway. The cells expressing the Gli1 gene (Gli1⁺ cells) have been identified as mesenchymal stem cells (MSCs) that are responsible for various tissue developments, homeostasis, and injury repair. This review outlines some recent discoveries on the crucial roles of Gli1⁺ MSCs in the development and homeostasis of varieties of hard and soft tissues.

Diabetes impairs periosteal progenitor regenerative potential

Diabetics are at increased risk for fracture, and experience severely impaired skeletal healing characterized by delayed union or nonunion of the bone. The periosteum harbors osteochondral progenitors that can differentiate into chondrocytes and osteoblasts, and this connective tissue layer is required for efficient fracture healing. While bone marrow-derived stromal cells have been studied extensively in the context of diabetic skeletal repair and osteogenesis, the effect of diabetes on the periosteum and its ability to contribute to bone regeneration has not yet been explicitly evaluated. Within this study, we utilized an established murine model of type I diabetes to evaluate periosteal cell differentiation capacity, proliferation, and availability under the effect of a diabetic environment. Periosteal cells from diabetic mice were deficient in osteogenic differentiation ability in vitro, and diabetic mice had reduced periosteal populations of mesenchymal progenitors with a corresponding reduction in proliferation capacity following injury. Additionally, fracture callus mineralization and mature osteoblast activity during periosteum-mediated healing was impaired in diabetic mice compared to controls. We propose that the effect of diabetes on periosteal progenitors and their ability to aid in skeletal repair directly impairs fracture healing.

MyD88 in myofibroblasts enhances colitis-associated tumorigenesis via promoting macrophage M2 polarization

The signal adaptor MyD88, an essential component of TLR signaling, plays an important role in gut-microbiome interactions. However, its contribution to colitis-associated cancer (CAC) is still controversial. Far less is known about the specific effects of MyD88 signaling in myofibroblasts in CAC development. Here, we used a CAC mouse model in which MyD88 was selectively depleted in myofibroblasts. Myofibroblast MyD88-deficient mice are resistant to azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced tumorigenesis, as evidenced by the decrease in the number and sizes of tumors. MyD88 deficiency in myofibroblasts attenuates intestinal epithelial cell (IEC) proliferation after acute DSS-induced colitis. Furthermore, MyD88 signaling in myofibroblasts increases the secretion of osteopontin (OPN), which promotes macrophage M2 polarization through binding to α_vβ₃ and CD44, leading to activation of the STAT3/PPARγ pathway. Thus, MyD88 signaling in myofibroblasts crucially contributes to colorectal cancer development and provides a promising therapeutic target for the prevention of colitis-associated carcinogenesis.

Characterization of the structure, vascularity, and stem/progenitor cell populations in porcine Achilles tendon (PAT)

This study aimed to characterize porcine Achilles tendon (PAT) in terms of its structural components, vascularity, and resident tendon cells. We found that PAT is composed of a paratenon sheath, a core of fascicles, and an endotenon/interfascicular matrix (IFM) that encases the fascicle bundles. We analyzed each of these three tendon components structurally using tissue sections and by isolating cells from each component and analyzing in vitro. Many blood vessel-like tissues were present in the paratenon and IFM but not in fascicles, and the vessels in the paratenon and IFM appeared to be inter-connected. Cells isolated from the paratenon and IFM displayed characteristics of vascular stem/progenitor cells expressing the markers CD105, CD31, with α-smooth muscle actin (α-SMA) localized surrounding blood vessels. The isolated cells from paratenon and IFM also harbored abundant stem/progenitor cells as evidenced by their ability to form colonies and express stem cell markers including CD73 and CD146. Furthermore, we demonstrate that both paratenon and IFM-isolated cells were capable of undergoing multi-differentiation. In addition, both paratenon and IFM cells expressed elastin, osteocalcin, tubulin polymerization promoting protein (TPPP), and collagen IV, whereas fascicle cells expressed none of these markers, except collagen I. The neurotransmitter substance P (SP) was also found in the paratenon and IFM-localized surrounding blood vessels. The findings of this study will help us to better understand the vascular and cellular mechanisms of tendon homeostasis, injury, healing, and regeneration.

M2-like macrophage infiltration and transforming growth factor-β secretion during socket healing process in mice

OBJECTIVE

Macrophages are involved in tissue inflammation and repair through cytokine secretion. However, the contribution of macrophages to healing and osteogenesis after tooth extraction remains unclear. Therefore, we investigated the distribution of osteoblastic cells and macrophages in the early healing process after tooth extraction.

METHODS

The maxillary first molars of 6-week-old male mice were extracted. The maxilla was collected 1, 3, and 7 days after extraction. The states of socket healing, localization of osteoblastic markers, and macrophage infiltration were sequentially observed by micro-CT imaging and immunohistochemistry.

RESULTS

On day 3 after tooth extraction, α-smooth muscle actin (SMA)-positive cells, osteoprogenitor cells at fracture healing, were observed in the socket. Several α-SMA-positive cells also expressed Runx2, the early osteoblast differentiation marker. The infiltration of F4/80-positive, mature macrophages and CD206-positive, M2-like macrophages was noted in the socket. However, CD169-positive macrophages (Osteomac), which are involved in fracture healing, were not detected in the socket. F4/80-positive and CD206-positive macrophages also showed the localization of transforming growth factor-β (TGF-β), which promotes osteoprogenitor cell proliferation and early differentiation. Phosphorylated Smad3, a downstream mediator of the signal activity of TGF-β, was detected in α-SMA-positive cells. On day 7, the extracted socket contained a large amount of new bone. Tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts were detected on bone surfaces.

CONCLUSION

Our data indicate that M2-like macrophages regulate the proliferation and differentiation of α-SMA-positive cells by secreting TGF-β at the early stage of socket healing, and also suggest the importance of macrophages in healing and bone formation after tooth extraction.

Thrombospondin-2 spatiotemporal expression in skeletal fractures

Fracture healing is a complex process that relies heavily on the carefully orchestrated expansion and differentiation of periosteal mesenchymal progenitor cells (MSC). Identification of new markers for periosteal MSCs is essential for the development of fracture therapeutics. Expression of the matricellular protein thrombospondin-2 (TSP2) increases during early fracture healing; however, it is currently unknown what cell population expresses TSP2. Using a TSP2 GFP reporter mouse and a stabilized murine fracture model, we characterized the expression of TSP2 during the inflammatory, soft callus formation, and hard callus formation phases of fracture healing. In addition, using TSP2 GFP positive cells harvested from reporter mouse cells, we characterized the cell population using flow cytometry and colony formation assays. In uninjured diaphyseal bone, we observed TSP2 expression in the cells located along the inner periosteum. We also observed a population of TSP2 expressing cells in undifferentiated regions of early fracture callus and along the periphery of the callus. Later in callus development, TSP2 cells were broadly distributed in the undifferentiated callus, but GFP was not expressed by chondrocytes. Flow cytometry confirmed that the majority of TSP2 expressing cells were positive for traditional murine MSC markers. Our in vitro assays further supported these findings by demonstrating all adherent and colony-forming cells expressed TSP2. Taken together, our results suggest that TSP2 is expressed by undifferentiated MSCs, but downregulated in chondrocytes. Clinical significance: expression of the matricellular protein TSP2 is a promising new marker to identify MSCs in early fracture healing.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Bone defect reconstruction via endochondral ossification: A developmental engineering strategy

Traditional bone tissue engineering (BTE) strategies induce direct bone-like matrix formation by mimicking the embryological process of intramembranous ossification. However, the clinical translation of these clinical strategies for bone repair is hampered by limited vascularization and poor bone regeneration after implantation in vivo. An alternative strategy for overcoming these drawbacks is engineering cartilaginous constructs by recapitulating the embryonic processes of endochondral ossification (ECO); these constructs have shown a unique ability to survive under hypoxic conditions as well as induce neovascularization and ossification. Such developmentally engineered constructs can act as transient biomimetic templates to facilitate bone regeneration in critical-sized defects. This review introduces the concept and mechanism of developmental BTE, explores the routes of endochondral bone graft engineering, highlights the current state of the art in large bone defect reconstruction via ECO-based strategies, and offers perspectives on the challenges and future directions of translating current knowledge from the bench to the bedside.

Advantages and Limitations of Cre Mouse Lines Used in Skeletal Research

The Cre-LoxP technology permits gene ablation in specific cell lineages, at chosen differentiation stages of this lineage and in an inducible manner. It has allowed tremendous advances in our understanding of skeleton biology and related pathophysiological mechanisms, through the generation of loss/gain of function or cell tracing experiments based on the creation of an expanding toolbox of transgenic mice expressing the Cre recombinase in skeletal stem cells, chondrocytes, osteoblasts, or osteoclasts. In this chapter, we provide an overview of the different Cre-LoxP systems and Cre mouse lines used in the bone field, we discuss their advantages, limitations, and we outline best practices to interpret results obtained from the use of Cre mice.

Human perivascular stem cells prevent bone graft resorption in osteoporotic contexts by inhibiting osteoclast formation

The vascular wall stores mesenchymal progenitor cells which are able to induce bone regeneration, via direct and paracrine mechanisms. Although much is known regarding perivascular cell regulation of osteoblasts, their regulation of osteoclasts, and by extension utility in states of high bone resorption, is not known. Here, human perivascular stem cells (PSCs) were used as a means to prevent autograft resorption in a gonadectomy-induced osteoporotic spine fusion model. Furthermore, the paracrine regulation by PSCs of osteoclast formation was evaluated, using coculture, conditioned medium, and purified extracellular vesicles. Results showed that PSCs when mixed with autograft bone induce an increase in osteoblast:osteoclast ratio, promote bone matrix formation, and prevent bone graft resorption. The confluence of these factors resulted in high rates of fusion in an ovariectomized rat lumbar spine fusion model. Application of PSCs was superior across metrics to either the use of unpurified, culture-defined adipose-derived stromal cells or autograft bone alone. Under coculture conditions, PSCs negatively regulated osteoclast formation and did so via secreted, nonvesicular paracrine factors. Total RNA sequencing identified secreted factors overexpressed by PSCs which may explain their negative regulation of graft resorption. In summary, PSCs reduce osteoclast formation and prevent bone graft resorption in high turnover states such as gonadectomy-induced osteoporosis.

Markers for Identification of Postnatal Skeletal Stem Cells In Vivo

PURPOSE OF REVIEW

The adult skeleton contains stem cells involved in growth, homeostasis, and healing. Mesenchymal or skeletal stem cells are proposed to provide precursors to osteoblasts, chondrocytes, marrow adipocytes, and stromal cells. We review the evidence for existence and functionality of different skeletal stem cell pools, and the tools available for identifying or targeting these populations in mouse and human tissues.

RECENT FINDINGS

Lineage tracing and single cell-based techniques in mouse models indicate that multiple pools of stem cells exist in postnatal bone. These include growth plate stem cells, stem and progenitor cells in the diaphysis, reticular cells that only form bone in response to injury, and injury-responsive periosteal stem cells. New staining protocols have also been described for prospective isolation of human skeletal stem cells. Several populations of postnatal skeletal stem and progenitor cells have been identified in mice, and we have an increasing array of tools to target these cells. Most Cre models lack a high degree of specificity to define single populations. Human studies are less advanced and require further efforts to refine methods for identifying stem and progenitor cells in adult bone.

Bone regeneration via skeletal cell lineage plasticity: All hands mobilized for emergencies: Quiescent mature skeletal cells can be activated in response to injury and robustly participate in bone regeneration through cellular plasticity

An emerging concept is that quiescent mature skeletal cells provide an important cellular source for bone regeneration. It has long been considered that a small number of resident skeletal stem cells are solely responsible for the remarkable regenerative capacity of adult bones. However, recent in vivo lineage-tracing studies suggest that all stages of skeletal lineage cells, including dormant pre-adipocyte-like stromal cells in the marrow, osteoblast precursor cells on the bone surface and other stem and progenitor cells, are concomitantly recruited to the injury site and collectively participate in regeneration of the damaged skeletal structure. Lineage plasticity appears to play an important role in this process, by which mature skeletal cells can transform their identities into skeletal stem cell-like cells in response to injury. These highly malleable, long-living mature skeletal cells, readily available throughout postnatal life, might represent an ideal cellular resource that can be exploited for regenerative medicine.

Modulation of Notch1 signaling regulates bone fracture healing

Fracture healing involves interactions of different cell types, driven by various growth factors, and signaling cascades. Periosteal mesenchymal progenitor cells give rise to the majority of osteoblasts and chondrocytes in a fracture callus. Notch signaling has emerged as an important regulator of skeletal cell proliferation and differentiation. We investigated the effects of Notch signaling during the fracture healing process. Increased Notch signaling in osteochondroprogenitor cells driven by overexpression of Notch1 intracellular domain (NICD1) (αSMACreERT2 mice crossed with Rosa-NICD1) during fracture resulted in less cartilage, more mineralized callus tissue, and stronger and stiffer bones after 3 weeks. Periosteal cells overexpressing NICD1 showed increased proliferation and migration in vitro. In vivo data confirmed that increased Notch1 signaling caused expansion of alpha-smooth muscle actin (αSMA)-positive cells and their progeny including αSMA-derived osteoblasts in the callus without affecting osteoclast numbers. In contrast, anti-NRR1 antibody treatment to inhibit Notch1 signaling resulted in increased callus cartilage area, reduced callus bone mass, and reduced biomechanical strength. Our study shows a positive effect of induced Notch1 signaling on the fracture healing process, suggesting that stimulating the Notch pathway could be beneficial for fracture repair.

New insights on the reparative cells in bone regeneration and repair

Bone possesses a remarkable repair capacity to regenerate completely without scar tissue formation. This unique characteristic, expressed during bone development, maintenance and injury (fracture) healing, is performed by the reparative cells including skeletal stem cells (SSCs) and their descendants. However, the identity and functional roles of SSCs remain controversial due to technological difficulties and the heterogeneity and plasticity of SSCs. Moreover, for many years, there has been a biased view that bone marrow is the main cell source for bone repair. Together, these limitations have greatly hampered our understanding of these important cell populations and their potential applications in the treatment of fractures and skeletal diseases. Here, we reanalyse and summarize current understanding of the reparative cells in bone regeneration and repair and outline recent progress in this area, with a particular emphasis on the temporal and spatial process of fracture healing, the sources of reparative cells, an updated definition of SSCs, and markers of skeletal stem/progenitor cells contributing to the repair of craniofacial and long bones, as well as the debate between SSCs and pericytes. Finally, we also discuss the existing problems, emerging novel technologies and future research directions in this field.

Diverse epithelial cell populations contribute to the regeneration of secretory units in injured salivary glands

Salivary glands exert exocrine secretory function to provide saliva for lubrication and protection of the oral cavity. Its epithelium consists of several differentiated cell types, including acinar, ductal and myoepithelial cells, that are maintained in a lineage-restricted manner during homeostasis or after mild injuries. Glandular regeneration following a near complete loss of secretory cells, however, may involve cellular plasticity, although the mechanism and extent of such plasticity remain unclear. Here, by combining lineage-tracing experiments with a model of severe glandular injury in the mouse submandibular gland, we show that de novo formation of acini involves induction of cellular plasticity in multiple non-acinar cell populations. Fate-mapping analysis revealed that, although ductal stem cells marked by cytokeratin K14 and Axin2 undergo a multipotency switch, they do not make a significant contribution to acinar regeneration. Intriguingly, more than 80% of regenerated acini derive from differentiated cells, including myoepithelial and ductal cells, that appear to dedifferentiate to a progenitor-like state before re-differentiation into acinar cells. The potential of diverse cell populations serving as a reserve source for acini widens the therapeutic options for hyposalivation.

Role of Prx1-expressing skeletal cells and Prx1-expression in fracture repair

The healing capacity of bones after fracture implies the existence of adult regenerative cells. However, information on identification and functional role of fracture-induced progenitors is still lacking. Paired-related homeobox 1 (Prx1) is expressed during skeletogenesis. We hypothesize that fracture recapitulates Prx1's expression, and Prx1 expressing cells are critical to induce repair. To address our hypothesis, we used a combination of in vivo and in vitro approaches, short and long-term cell tracking analyses of progenies and actively expressing cells, cell ablation studies, and rodent animal models for normal and defective fracture healing. We found that fracture elicits a periosteal and endosteal response of perivascular Prx1+ cells that participate in fracture healing and showed that Prx1-expressing cells have a functional role in the repair process. While Prx1-derived cells contribute to the callus, Prx1's expression decreases concurrently with differentiation into cartilaginous and bone cells, similarly to when Prx1+ cells are cultured in differentiating conditions. We determined that bone morphogenic protein 2 (BMP2), through C-X-C motif-ligand-12 (CXCL12) signaling, modulates the downregulation of Prx1. We demonstrated that fracture elicits an early increase in BMP2 expression, followed by a decrease in CXCL12 that in turn down-regulates Prx1, allowing cells to commit to osteochondrogenesis. In vivo and in vitro treatment with CXCR4 antagonist AMD3100 restored Prx1 expression by modulating the BMP2-CXCL12 axis. Our studies represent a shift in the current research that has primarily focused on the identification of markers for postnatal skeletal progenitors, and instead we characterized the function of a specific population (Prx1+ cells) and their expression marker (Prx1) as a crossroad in fracture repair. The identification of fracture-induced perivascular Prx1+ cells and regulation of Prx1's expression by BMP2 and in turn by CXCL12 in the orchestration of fracture repair, highlights a pathway in which to investigate defective mechanisms and therapeutic targets for fracture non-union.

Real-Time Imaging of CCL5-Induced Migration of Periosteal Skeletal Stem Cells in Mice

Periosteal skeletal stem cells (P-SSCs) are essential for lifelong bone maintenance and repair, making them an ideal focus for the development of therapies to enhance fracture healing.  Periosteal cells rapidly migrate to an injury to supply new chondrocytes and osteoblasts for fracture healing. Traditionally, the efficacy of a cytokine to induce cell migration has only been conducted in vitro by performing a transwell or scratch assay. With advancements in intravital microscopy using multiphoton excitation, it was recently discovered that 1) P-SSCs express the migratory gene CCR5 and 2) treatment with the CCR5 ligand known as CCL5 improves fracture healing and the migration of P-SSCs in response to CCL5. These results have been captured in real-time. Described here is a protocol to visualize P-SSC migration from the calvarial suture skeletal stem cell (SSC) niche towards an injury after treatment with CCL5. The protocol details the construction of a mouse restraint and imaging mount, surgical preparation of the mouse calvaria, induction of a calvaria defect, and acquisition of time-lapse imaging.

Identification of Functionally Distinct Mx1+αSMA+ Periosteal Skeletal Stem Cells

The periosteum is critical for bone maintenance and healing. However, the in vivo identity and specific regulatory mechanisms of adult periosteum-resident skeletal stem cells are unknown. Here, we report animal models that selectively and durably label postnatal Mx1+αSMA+ periosteal stem cells (P-SSCs) and establish that P-SSCs are a long-term repopulating, functionally distinct SSC subset responsible for lifelong generation of periosteal osteoblasts. P-SSCs rapidly migrate toward an injury site, supply osteoblasts and chondrocytes, and recover new periosteum. Notably, P-SSCs specifically express CCL5 receptors, CCR3 and CCR5. Real-time intravital imaging revealed that the treatment with CCL5 induces P-SSC migration in vivo and bone healing, while CCL5/CCR5 deletion, CCR5 inhibition, or local P-SSC ablation reduces osteoblast number and delays bone healing. Human periosteal cells express CCR5 and undergo CCL5-mediated migration. Thus, the adult periosteum maintains genetically distinct SSC subsets with a CCL5-dependent migratory mechanism required for bone maintenance and injury repair.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

What do we know about bone morphogenetic proteins and osteochondroprogenitors in inflammatory conditions?

Osteochondroprogenitors are crucial for embryonic bone development and postnatal processes such as bone repair in response to fracture injury, and their dysfunction may contribute to insufficient repair of structural damage in inflammatory arthritides. In the fracture healing, the early inflammatory phase is crucial for normal callus development and new bone formation. This process involves a complex interplay of many molecules and cell types, responsible for recruitment, expansion and differentiation of osteochondroprogenitor populations. In inflammatory arthritides, inflammation induces bone resorption and causes insufficient bone formation, which leads to local and systemic bone loss. While bone loss is a predominant feature in rheumatoid arthritis, inflammation also induces pathologic bone formation at enthesial sites in seronegative spondyloarthropathies. Bone morphogenetic proteins (BMP) are involved in cell proliferation, differentiation and apoptosis, and have fundamental roles in maintenance of postnatal bone homeostasis. They are crucial regulators of the osteochondroprogenitor pool and drive their proliferation, differentiation, and lifespan during bone regeneration. In this review, we summarize the effects of inflammation on osteochondroprogenitor populations during fracture repair and in inflammatory arthritides, with special focus on inflammation-mediated modulation of BMP signaling. We also present data in which we describe a population of murine synovial osteochondroprogenitor cells, which are reduced in arthritis, and characterize their expression of genes involved in regulation of bone homeostasis, emphasizing the up-regulation of BMP pathways in early progenitor subset. Based on the presented data, it may be concluded that during an inflammatory response, innate immune cells induce osteochondroprogenitors by providing signals for their recruitment, by producing BMPs and other osteogenic factors for paracrine effects, and by secreting inflammatory cytokines that may positively regulate osteogenic pathways. On the other hand, inflammatory cells may secrete cytokines that interfere with osteogenic pathways, proapoptotic factors that reduce the pool of osteochondroprogenitor cells, as well as BMP and Wnt antagonists. The net effect is strongly context-dependent and influenced by the local milieu of cells, cytokines, and growth factors. Further elucidation of the interplay between inflammatory signals and BMP-mediated bone formation may provide valuable tools for therapeutic targeting.

Niches for Skeletal Stem Cells of Mesenchymal Origin

With very few exceptions, all adult tissues in mammals are maintained and can be renewed by stem cells that self-renew and generate the committed progeny required. These functions are regulated by a specific and in many ways unique microenvironment in stem cell niches. In most cases disruption of an adult stem cell niche leads to depletion of stem cells, followed by impairment of the ability of the tissue in question to maintain its functions. The presence of stem cells, often referred to as mesenchymal stem cells (MSCs) or multipotent bone marrow stromal cells (BMSCs), in the adult skeleton has long been realized. In recent years there has been exceptional progress in identifying and characterizing BMSCs in terms of their capacity to generate specific types of skeletal cells in vivo. Such BMSCs are often referred to as skeletal stem cells (SSCs) or skeletal stem and progenitor cells (SSPCs), with the latter term being used throughout this review. SSPCs have been detected in the bone marrow, periosteum, and growth plate and characterized in vivo on the basis of various genetic markers (i.e., Nestin, Leptin receptor, Gremlin1, Cathepsin-K, etc.). However, the niches in which these cells reside have received less attention. Here, we summarize the current scientific literature on stem cell niches for the SSPCs identified so far and discuss potential factors and environmental cues of importance in these niches in vivo. In this context we focus on (i) articular cartilage, (ii) growth plate cartilage, (iii) periosteum, (iv) the adult endosteal compartment, and (v) the developing endosteal compartment, in that order.

LGRs in Skeletal Tissues: An Emerging Role for Wnt-Associated Adult Stem Cell Markers in Bone

Leucine-rich repeat-containing G protein-coupled receptors (LGRs) are adult stem cell markers that have been described across various stem cell niches, and expression of LGRs and their corresponding ligands (R-spondins) has now been reported in multiple bone-specific cell types. The skeleton harbors elusive somatic stem cell populations that are exceedingly compartment-specific and under tight regulation from various signaling pathways. Skeletal progenitors give rise to multiple tissues during development and during regenerative processes of bone, requiring postnatal endochondral and intramembranous ossification. The relevance of LGRs and the LGR/R-spondin ligand interaction in bone and tooth biology is becoming increasingly appreciated. LGRs may define specific stem cell and progenitor populations and their behavior during both development and regeneration, and their role as Wnt-associated receptors with specific ligands poses these proteins as unique therapeutic targets via potential R-spondin agonism. This review seeks to outline the current literature on LGRs in the context of bone and its associated tissues, and points to key future directions for studying the functional role of LGRs and ligands in skeletal biology. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Tgfβ signaling is required for tenocyte recruitment and functional neonatal tendon regeneration

Tendon injuries are common with poor healing potential. The paucity of therapies for tendon injuries is due to our limited understanding of the cells and molecular pathways that drive tendon regeneration. Using a mouse model of neonatal tendon regeneration, we identified TGFβ signaling as a major molecular pathway that drives neonatal tendon regeneration. Through targeted gene deletion, small molecule inhibition, and lineage tracing, we elucidated TGFβ-dependent and TGFβ-independent mechanisms underlying tendon regeneration. Importantly, functional recovery depended on canonical TGFβ signaling and loss of function is due to impaired tenogenic cell recruitment from both Scleraxis-lineage and non-Scleraxis-lineage sources. We show that TGFβ signaling is directly required in neonatal tenocytes for recruitment and that TGFβ ligand is positively regulated in tendons. Collectively, these results show a functional role for canonical TGFβ signaling in tendon regeneration and offer new insights toward the divergent cellular activities that distinguish regenerative vs fibrotic healing.

Skeletal Stem Cells for Bone Development and Repair: Diversity Matters

PURPOSE OF REVIEW

Skeletal stem cells (SSCs) are considered to play important roles in bone development and repair. These cells have been historically defined by their in vitro potential for self-renewal and differentiation into "trilineage" cells; however, little is known about their in vivo identity. Here, we discuss recent progress on SSCs and how they potentially contribute to bone development and repair.

RECENT FINDINGS

Bone is composed of diverse tissues, which include cartilage and its perichondrium, cortical bone and its periosteum, and bone marrow and its trabecular bone and stromal compartment. We are now at the initial stage of understanding the precise identity of SSCs in each bone tissue. The emerging concept is that functionally dedicated SSCs are encased by their own unique cellular and extracellular matrix microenvironment, and locally support its own compartment. Diverse groups of SSCs are likely to work in concert to achieve development and repair of the highly functional skeletal organ.

In vivo dynamic analysis of BMP-2-induced ectopic bone formation

Bone morphogenetic protein (BMP)-2 plays a central role in bone-tissue engineering because of its potent bone-induction ability. However, the process of BMP-induced bone formation in vivo remains poorly elucidated. Here, we aimed to establish a method for intravital imaging of the entire process of BMP-2-induced ectopic bone formation. Using multicolor intravital imaging in transgenic mice, we visualized the spatiotemporal process of bone induction, including appearance and motility of osteoblasts and osteoclasts, angiogenesis, collagen-fiber formation, and bone-mineral deposition. Furthermore, we investigated how PTH1-34 affects BMP-2-induced bone formation, which revealed that PTH1-34 administration accelerated differentiation and increased the motility of osteoblasts, whereas it decreased morphological changes in osteoclasts. This is the first report on visualization of the entire process of BMP-2-induced bone formation using intravital imaging techniques, which, we believe, will contribute to our understanding of ectopic bone formation and provide new parameters for evaluating bone-forming activity.

Expression and function of cartilage-derived pluripotent cells in joint development and repair

Cartilage-derived pluripotent cells reside in hyaline cartilage and fibrocartilage. These cells have the potential for multidirectional differentiation; can undergo adipogenesis, osteogenesis, and chondrogenesis; and have been classified as mesenchymal stem cells (MSCs) conforming to the minimal criteria of the International Society for Cellular Therapy. Cartilage tissue is prone to injury and is difficult to repair. As cartilage-derived pluripotent cells are the closest cell source to cartilage tissue, they are expected to have the strongest ability to differentiate into cartilage compared to other MSCs. This review focuses on the organizational distribution, expression, and function of cartilage-derived pluripotent cells in joint development and repair to help explore the therapeutic potential of in situ cartilage-derived pluripotent cells for joint cartilage repair.

Perivascular osteoprogenitors are associated with transcortical channels of long bones

Bone remodeling and regeneration are dependent on resident stem/progenitor cells with the ability to replenish mature osteoblasts and repair the skeleton. Using lineage tracing approaches, we identified a population of Dmp1+ cells that reside within cortical bone and are distinct from osteocytes. Our aims were to characterize this stromal population of transcortical perivascular cells (TPCs) in their resident niche and evaluate their osteogenic potential. To distinguish this population from osteoblasts/osteocytes, we crossed mice containing inducible DMP1CreERT2/Ai9 Tomato reporter (iDMP/T) with Col2.3GFP reporter (ColGFP), a marker of osteoblasts and osteocytes. We observed iDMP/T+;ColGFP- TPCs within cortical bone following tamoxifen injection. These cells were perivascular and located within transcortical channels. Ex vivo bone outgrowth cultures showed TPCs migrated out of the channels onto the plate and expressed stem cell markers such as Sca1, platelet derived growth factor receptor beta (PDGFRβ), and leptin receptor. In a cortical bone transplantation model, TPCs migrate from their vascular niche within cortical bone and contribute to new osteoblast formation and bone tube closure. Treatment with intermittent parathyroid hormone increased TPC number and differentiation. TPCs were unable to differentiate into adipocytes in the presence of rosiglitazone in vitro or in vivo. Altogether, we have identified and characterized a novel stromal lineage-restricted osteoprogenitor that is associated with transcortical vessels of long bones. Functionally, we have demonstrated that this population can migrate out of cortical bone channels, expand, and differentiate into osteoblasts, therefore serving as a source of progenitors contributing to new bone formation.

αSMA Osteoprogenitor Cells Contribute to the Increase in Osteoblast Numbers in Response to Mechanical Loading

Bone is a dynamic tissue that site-specifically adapts to the load that it experiences. In response to increasing load, the cortical bone area is increased, mainly through enhanced periosteal bone formation. This increase in area is associated with an increase in the number of bone-forming osteoblasts; however, the origin of the cells involved remains unclear. Alpha-smooth muscle actin (αSMA) is a marker of early osteoprogenitor cells in the periosteum, and we hypothesized that the new osteoblasts that are activated by loading could originate from αSMA-expressing cells. Therefore, we used an in vivo fate-mapping approach in an established axial loading model to investigate the role of αSMA-expressing cells in the load-induced increase in osteoblasts. Histomorphometric analysis was applied to measure the number of cells of different origin on the periosteal surface in the most load-responsive region of the mouse tibia. A single loading session failed to increase the number of periosteal αSMA-expressing cells and osteoblasts. However, in response to multiple episodes of loading, the caudal, but not the cranial, periosteal surface was lined with an increased number of osteoblasts originating from αSMA-expressing cells 5 days after the initial loading session. The proportion of osteoblasts derived from αSMA-labeled progenitors increased by 70% (p < 0.05), and the proportion of αSMA-labeled cells that had differentiated into osteoblasts was doubled. We conclude that αSMA-expressing osteoprogenitors can differentiate and contribute to the increase in periosteal osteoblasts induced by mechanical loading in a site-specific manner.

Promoters to Study Vascular Smooth Muscle

Smooth muscle cells (SMCs) are a critical component of blood vessel walls that provide structural support, regulate vascular tone, and allow for vascular remodeling. These cells also exhibit a remarkable plasticity that contributes to vascular growth and repair but also to cardiovascular pathologies, including atherosclerosis, intimal hyperplasia and restenosis, aneurysm, and transplant vasculopathy. Mouse models have been an important tool for the study of SMC functions. The development of smooth muscle-expressing Cre-driver lines has allowed for exciting discoveries, including recent advances revealing the diversity of phenotypes derived from mature SMC transdifferentiation in vivo using inducible CreER ^T2 lines. We review SMC-targeting Cre lines driven by the Myh11, Tagln, and Acta2 promoters, including important technical considerations associated with these models. Limitations that can complicate study of the vasculature include expression in visceral SMCs leading to confounding phenotypes, and expression in multiple nonsmooth muscle cell types, such as Acta2-Cre expression in myofibroblasts. Notably, the frequently employed Tagln/ SM22α- Cre driver expresses in the embryonic heart but can also confer expression in nonmuscular cells including perivascular adipocytes and their precursors, myeloid cells, and platelets, with important implications for interpretation of cardiovascular phenotypes. With new Cre-driver lines under development and the increasing use of fate mapping methods, we are entering an exciting new era in SMC research.

An Improved Methodology to Evaluate Cell and Molecular Signals in the Reparative Callus During Fracture Healing

Approximately 5% to 10% of all bone fractures do not heal completely, contributing to significant patient suffering and medical costs. Even in healthy individuals, fracture healing is associated with significant downtime and loss of productivity. However, no pharmacological treatments are currently available to promote efficient bone healing. A better understanding of the underlying molecular mechanisms is crucial for developing novel therapies to hasten healing. The early reparative callus that forms around the site of bone injury is a fragile tissue consisting of shifting cell populations held together by loose connective tissue. The delicate callus is challenging to section and is vulnerable to disintegration during the harsh steps of immunostaining, namely, decalcification, deparaffinization, and antigen retrieval. Here, we describe an improved methodology for processing early-stage fracture calluses and immunofluorescence labeling of the sections to visualize the temporal (timing) and spatial (location) patterns of cellular and molecular events that regulate bone healing. This method has a short turnaround time from sample collection to microscopy as it does not require lengthy decalcification. It preserves the structural integrity of the fragile callus as the method does not entail deparaffinization or harsh methods of antigen retrieval. Our method can be adapted for high-throughput screening of drugs that promote efficacious bone healing.

Amplifying Bone Marrow Progenitors Expressing α-Smooth Muscle Actin Produce Zonal Insertion Sites During Tendon-to-Bone Repair

Traditional tendon-to-bone repair where the tendon is reattached to bone via suture anchors often results in disorganized scar production rather than the formation of a zonal insertion. In contrast, ligament reconstructions where tendon grafts are passed through bone tunnels can yield zonal tendon-to-bone attachments between the graft and adjacent bone. Therefore, ligament reconstructions can be used to study mechanisms that regulate zonal tendon-to-bone repair in the adult. Anterior cruciate ligament (ACL) reconstructions are one of the most common reconstruction procedures and while we know that cells from outside the graft produce the attachments, we have not yet established specific cell populations that give rise to this tissue. To address this knowledge gap, we performed ACL reconstructions in lineage tracing mice where α-smooth muscle actin (αSMACreERT2) was used to label αSMA-expressing progenitors within the bone marrow that produced zonal attachments. Expression of αSMA was increased during early stages of the repair process such that the contribution of SMA-labeled cells to the tunnel integration was highest when tamoxifen was delivered in the first week post-surgery. The zonal attachments shared features with normal entheses, including tidemarks oriented perpendicularly to collagen fibers, Col1a1-expressing cells, alkaline phosphatase activity, and proteoglycan-rich staining. Finally, the integration strength increased with time, requiring 112% greater force to remove the graft from the tunnel at 28 days compared with 14 days post-surgery. Future studies will target these progenitor cells to define the pathways that regulate zonal tendon-to-bone repair in the adult. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:105-116, 2020.

Engraftment of skeletal progenitor cells by bone-directed transplantation improves osteogenesis imperfecta murine bone phenotype

Osteogenesis imperfecta (OI) is a genetic disorder most commonly caused by mutations associated with type I collagen, resulting in a defective collagen bone matrix. Current treatments for OI focus on pharmaceutical strategies to increase the amount of defective bone matrix, but do not address the underlying collagen defect. Introducing healthy donor stem cells that differentiate into osteoblasts producing normal collagen in OI patients has the potential to increase bone mass and correct the mutant collagen matrix. In this study, donor bone marrow stromal cells (BMSCs, also known as bone marrow mesenchymal stem cells) expressing both αSMACreERT2/Ai9 progenitor reporter and osteoblast reporter Col2.3GFP were locally transplanted into the femur of OI murine (OIM) mice. One month post-transplantation, 18% of the endosteal surface was lined by donor Col2.3GFP expressing osteoblasts indicating robust engraftment. Long-term engraftment in the marrow was observed 3 and 6 months post-transplantation. The presence of Col1a2-expressing donor cell-derived cortical bone matrix was detected in transplanted OIM femurs. Local transplantation of BMSCs increased cortical thickness (+12%), the polar moment of inertia (+14%), bone strength (+30%), and stiffness (+30%) 3 months post-transplantation. Engrafted cells expressed progenitor markers CD51 and Sca-1 up to 3 months post-transplantation. Most importantly, 3 months post-transplantation donor cells maintained the ability to differentiate into Col2.3GFP+ osteoblasts in vitro, and in vivo following secondary transplantation into OIM animals. Locally transplanted BMSCs can improve cortical structure and strength, and persist as continued source of osteoblast progenitors in the OIM mouse for at least 6 months.

Bone marrow osteoprogenitors are depleted whereas osteoblasts are expanded independent of the osteogenic vasculature in response to zoledronic acid

Zoledronic acid (ZOL) is an antiresorptive drug used to prevent bone loss in a variety of conditions, acting mainly through suppression of osteoclast activity. There is growing evidence that ZOL can also affect cells of the mesenchymal lineage in bone. We present novel data revealing significant changes in the abundance of perivascular mesenchymal stromal cells (MSCs)/osteoprogenitors and osteoblasts following the injection of ZOL, in vivo. In young mice with high bone turnover and an abundance of perivascular osteoprogenitors, ZOL significantly (P < 0.0001) increased new bone formation. This was accompanied by a decline in osterix-positive osteoprogenitors and a corresponding increase in osteoblasts. However, these effects were not observed in mature mice with low bone turnover. Interestingly, the ZOL-induced changes in cells of the mesenchymal lineage occurred independently of effects on the osteogenic vasculature. Thus, we demonstrate that a single, clinically relevant dose of ZOL can induce new bone formation in microenvironments enriched for perivascular MSC/osteoprogenitors and high osteogenic potential. This arises from the differentiation of perivascular osterix-positive MSC/osteoprogenitors into osteoblasts at sites that are innately osteogenic. Collectively, our data demonstrate that ZOL affects multiple cell types in bone and has differential effects depending on the level of bone turnover.-Hughes, R., Chen, X., Hunter, K. D., Hobbs, J. K., Holen, I., Brown, N. J. Bone marrow osteoprogenitors are depleted whereas osteoblasts are expanded independent of the osteogenic vasculature in response to zoledronic acid.

Proliferation and Activation of Osterix-Lineage Cells Contribute to Loading-Induced Periosteal Bone Formation in Mice

Mechanical loading stimulates bone formation. Bone-lining-cell activation and cell proliferation have been implicated in this process. However, the origin of osteoblasts that form bone following mechanical stimulation remains unknown. Our objective was to identity the contributions of activation, differentiation, and proliferation of osteoblast lineage cells to loading-induced periosteal bone formation. Tamoxifen-inducible Osx-Cre-ERT2;Ai9/TdTomato reporter mice (male and female) were aged to young adult (5 months) and middle age (12 months), and were administered tamoxifen for 5 consecutive days to label osterix-lineage cells. Following a 3-week clearance period, mice were subjected to five consecutive bouts of unilateral axial tibial compression. We first confirmed this protocol stimulated an increase in periosteal bone formation that was primarily lamellar apposition. Next, mice received 5-bromo-2'-deoxyuridine (BrdU) in their drinking water daily to label proliferating cells; calcein was given to label active mineralizing surfaces. Tibias were harvested after the fifth loading day and processed for frozen undecalcified histology. The middiaphyseal periosteal surface in the region of peak bone formation was analyzed. Histology revealed both nonloaded and loaded tibias were covered in osterix positive (Osx+) cells on the periosteal surface of both 5- and 12-month-old animals. There was a significant increase in the mineralizing surface (calcein+) covered with Osx+ cells in loaded versus control limbs. Furthermore, nearly all of the mineralizing surfaces (>95%) were lined with Osx+ cells. We also observed approximately 30% of Osx+ cells were also BrdU+, indicating they arose via proliferation. These results show that following mechanical loading, pre-existing cells of the Osx lineage cover the vast majority of surfaces where there is active loading-induced bone formation, and a portion of these cells proliferated in the 5-day loading period. We conclude the initial anabolic response after mechanical loading is based on the activation and proliferation of Osx lineage cells, not the differentiation of progenitor cells. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

A PDGFRβ-PI3K signaling axis mediates periosteal cell activation during fracture healing

Insufficient and delayed fracture healing remain significant public health problems with limited therapeutic options. Phosphoinositide 3-kinase (PI3K) signaling, a major pathway involved in regulation of fracture healing, promotes proliferation, migration, and differentiation of osteoprogenitors. We have recently reported that knock-in mice with a global increase in PI3K signaling (gCbl^YF) show enhanced femoral fracture healing characterized by an extraordinary periosteal response to injury. Interestingly, of all growth factor receptors involved in fracture healing, PI3K directly binds only to PDGFR. Given these findings, we hypothesized a PDGFR-PI3K interaction is necessary for mediating robust periosteal cell activation following fracture. In this study, we isolated primary periosteal cells from gCbl^YF mice to analyze cross-talk between the PDGFRβ and PI3K signaling pathways. We found PDGFRβ signaling contributes to robust Akt phosphorylation in periosteal cells in comparison with other growth factor signaling pathways. Additionally, we performed femoral fractures on gCbl^YF mice with a conditional removal of PDGFRβ in mesenchymal progenitors using inducible alpha smooth muscle actin (αSMA) CreER^T2 mice. Our studies showed that depletion of PDGFRβ signaling within these progenitors in the early phase of fracture healing significantly abrogates PI3K-mediated periosteal activation and proliferation three days after fracture. Combined, these results suggest that PDGFRβ signaling through PI3K is necessary for robust periosteal activation in the earliest phases of fracture healing.

Human perivascular stem cell-derived extracellular vesicles mediate bone repair

The vascular wall is a source of progenitor cells that are able to induce skeletal repair, primarily by paracrine mechanisms. Here, the paracrine role of extracellular vesicles (EVs) in bone healing was investigated. First, purified human perivascular stem cells (PSCs) were observed to induce mitogenic, pro-migratory, and pro-osteogenic effects on osteoprogenitor cells while in non-contact co-culture via elaboration of EVs. PSC-derived EVs shared mitogenic, pro-migratory, and pro-osteogenic properties of their parent cell. PSC-EV effects were dependent on surface-associated tetraspanins, as demonstrated by EV trypsinization, or neutralizing antibodies for CD9 or CD81. Moreover, shRNA knockdown in recipient cells demonstrated requirement for the CD9/CD81 binding partners IGSF8 and PTGFRN for EV bioactivity. Finally, PSC-EVs stimulated bone repair, and did so via stimulation of skeletal cell proliferation, migration, and osteodifferentiation. In sum, PSC-EVs mediate the same tissue repair effects of perivascular stem cells, and represent an 'off-the-shelf' alternative for bone tissue regeneration.

Phenotypic Characterization of Bone Marrow Mononuclear Cells and Derived Stromal Cell Populations from Human Iliac Crest, Vertebral Body and Femoral Head

(1) In vitro, bone marrow-derived stromal cells (BMSCs) demonstrate inter-donor phenotypic variability, which presents challenges for the development of regenerative therapies. Here, we investigated whether the frequency of putative BMSC sub-populations within the freshly isolated mononuclear cell fraction of bone marrow is phenotypically predictive for the in vitro derived stromal cell culture. (2) Vertebral body, iliac crest, and femoral head bone marrow were acquired from 33 patients (10 female and 23 male, age range 14–91). BMSC sub-populations were identified within freshly isolated mononuclear cell fractions based on cell-surface marker profiles. Stromal cells were expanded in monolayer on tissue culture plastic. Phenotypic assessment of in vitro derived cell cultures was performed by examining growth kinetics, chondrogenic, osteogenic, and adipogenic differentiation. (3) Gender, donor age, and anatomical site were neither predictive for the total yield nor the population doubling time of in vitro derived BMSC cultures. The abundance of freshly isolated progenitor sub-populations (CD45−CD34−CD73+, CD45−CD34−CD146+, NG2+CD146+) was not phenotypically predictive of derived stromal cell cultures in terms of growth kinetics nor plasticity. BMSCs derived from iliac crest and vertebral body bone marrow were more responsive to chondrogenic induction, forming superior cartilaginous tissue in vitro, compared to those isolated from femoral head. (4) The identification of discrete progenitor populations in bone marrow by current cell-surface marker profiling is not predictive for subsequently derived in vitro BMSC cultures. Overall, the iliac crest and the vertebral body offer a more reliable tissue source of stromal progenitor cells for cartilage repair strategies compared to femoral head.

The Unmixing Problem: A Guide to Applying Single-Cell RNA Sequencing to Bone

Bone is composed of a complex mixture of many dynamic cell types. Flow cytometry and in vivo lineage tracing have offered early progress toward deconvoluting this heterogeneous mixture of cells into functionally well-defined populations suitable for further studies. Single-cell sequencing is poised as a key complementary technique to better understand the cellular basis of bone metabolism and development. However, single-cell sequencing approaches still have important limitations, including transcriptional effects of cell isolation and sparse sampling of the transcriptome, that must be considered during experimental design and analysis to harness the power of this approach. Accounting for these limitations requires a deep knowledge of the tissue under study. Therefore, with the emergence of accessible tools for conducting and analyzing single-cell RNA sequencing (scRNA-seq) experiments, bone biologists will be ideal leaders in the application of scRNA-seq to the skeleton. Here we provide an overview of the steps involved with a single-cell sequencing analysis of bone, focusing on practical considerations needed for a successful study. © 2019 American Society for Bone and Mineral Research.

Perivascular Mesenchymal Progenitors for Bone Regeneration

Mesenchymal progenitor cells reside in all assayed vascularized tissues, and are broadly conceptualized to participate in homeostasis/renewal and repair. The application of mesenchymal progenitor cells has been studied for diverse orthopaedic conditions related to skeletal degeneration, regeneration, and tissue fabrication. One common niche for mesenchymal progenitors is the perivascular space, and in both mouse and human tissues, perivascular progenitor cells have been isolated and characterized. Of these "perivascular stem cells" or PSC, pericytes are the most commonly studied cells. Multiple studies have demonstrated the regenerative properties of PSC when applied to bone, including direct osteochondral differentiation, paracrine-induced osteogenesis and vasculogenesis, and immunomodulatory functions. The confluence of these effects have resulted in efficacious bone regeneration across several preclinical models. Yet, key topics of research in perivascular progenitors highlight our lack of knowledge regarding these cell populations. These ongoing areas of study include cellular diversity within the perivascular niche, tissue-specific properties of PSC, and factors that influence PSC-mediated regenerative potential. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1221-1228, 2019.