Two tissue-resident progenitor lineages drive distinct phenotypes of heterotopic ossification

Controlled-release curcumin attenuates progression of tendon ectopic calcification by regulating the differentiation of tendon stem/progenitor cells

Tendon calcification is a common but intractable problem leading to pain and activity limitation when injury or tendinopathy progresses into the late stage. This is because tendon stem/progenitor cells (TSPCs) can undergo aberrant osteogenic differentiation under inflammatory conditions. This study aims to investigate the effect of curcumin, a natural anti-inflammatory agent, on regulating the differentiation of TSPCs in tendon calcification. With inflammatory stimulation, TSPCs showed higher alkaline phosphatase activity and more frequent formation of mineralized nodules which were verified in the culture system; however, curcumin significantly alleviated these pathological changes. In in vivo function analysis, chitosan microsphere-encapsulated curcumin was delivered to injured sites of rat tendon ectopic calcification model. The inflammation in the tendon tissues of the curcumin group was significantly relieved. Controlled-release curcumin partially rescued tendon calcification and enhanced tendon regeneration in animal model. This study demonstrates that controlled-release curcumin can manipulate the fate decision of TSPCs, and that it promotes the tenogenesis and inhibits the osteogenesis of TSPCs in a pathological microenvironment, which provides a possible new therapeutic strategy for tendon disease.

Lineage Tracing Reveals a Subset of Reserve Muscle Stem Cells Capable of Clonal Expansion under Stress

Stem cell heterogeneity is recognized as functionally relevant for tissue homeostasis and repair. The identity, context dependence, and regulation of skeletal muscle satellite cell (SC) subsets remains poorly understood. We identify a minor subset of Pax7⁺ SCs that is indelibly marked by an inducible Mx1-Cre transgene in vivo, is enriched for Pax3 expression, and has reduced ROS (reactive oxygen species) levels. Mx1⁺ SCs possess potent stem cell activity upon transplantation but minimally contribute to endogenous muscle repair, due to their relative low abundance. In contrast, a dramatic clonal expansion of Mx1⁺ SCs allows extensive contribution to muscle repair and niche repopulation upon selective pressure of radiation stress, consistent with reserve stem cell (RSC) properties. Loss of Pax3 in RSCs increased ROS content and diminished survival and stress tolerance. These observations demonstrate that the Pax7⁺ SC pool contains a discrete population of radiotolerant RSCs that undergo clonal expansion under severe stress.

BMP and mTOR signaling in heterotopic ossification: Does their crosstalk provide therapeutic opportunities?

Heterotopic ossification (HO) refers to the pathological formation of ectopic bone in soft tissues, it occurs following severe trauma or in patients with a rare genetic disorder known as fibrodysplasia ossificans progressiva. The pathological process of HO formation is a two-step mechanism: inflammation and destruction of connective tissues, followed by bone formation. The latter is further subdivided into three stages: fibroproliferation/angiogenesis, chondrogenesis, and osteogenesis. Currently, therapeutic options for HO are limited. New potential therapeutics will most likely arise from a more detailed understanding of the signaling pathways implicated in each stage of ectopic bone formation and molecular targets that may be effective at both the early and late stages of HO. Bone morphogenetic protein (BMP) signaling is believed to play a key role in the overall HO process. Recently, the mammalian target of rapamycin (mTOR) signaling pathway has received attention as a critical pathway for chondrogenesis, osteogenesis, and HO. Inhibition of mTOR signaling has been shown to block trauma-induced and genetic HO. Intriguingly, recent studies have revealed crosstalk between mTOR and BMP signaling. Moreover, mTOR has emerged as a factor involved in the early hypoxic and inflammatory stages of HO. We will summarize the current knowledge of the roles of mTOR and BMP signaling in HO, with a particular focus on the crosstalk between mTOR and BMP signaling. We also discuss the activation of AMP activated protein kinase (AMPK) by the most widely used drug for type 2 diabetes, metformin, which exerts a dual negative regulatory effect on mTOR and BMP signaling, suggesting that metformin is a promising drug treatment for HO. The discovery of an mTOR-BMP signaling network may be a potential molecular mechanism of HO and may represent a novel therapeutic target for the pharmacological control of HO.

Morpholino-Mediated Exon Skipping Targeting Human ACVR1/ALK2 for Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal-dominant disorder characterized by progressive heterotopic ossification. More than 95% of cases are caused by a recurrent mutation (617G>A; R206H) of ACVR1/ALK2, a bone morphogenetic protein (BMP) type I receptor. Recent studies revealed that ACVR1^R206H induces heterotopic ossification by aberrant activation in response to activin A. Because ACVR1^R206H is a hyperactive receptor, a promising therapeutic strategy is to decrease the activity of ACVR1 in patients. Here, we describe a method to reduce ACVR1 expression in FOP patient cells by exon skipping in ACVR1 mRNAs using phosphorodiamidate morpholino oligomers (PMOs). This strategy can be applied to the screen to select antisense oligomers to knockdown not only ACVR1 but also genes which cause other autosomal-dominant genetic diseases.

Limb specific Acvr1-knockout during embryogenesis in mice exhibits great toe malformation as seen in Fibrodysplasia Ossificans Progressiva (FOP)

Purpose

This study analyzes Prx1‐specific conditional knockout of Acvr1 aiming to elucidate the endogenous role of Acvr1 during limb formation in early embryonic development. ACVR1 can exhibit activating and inhibiting function in BMP signaling. ACVR1 gain‐of‐function mutations can cause the rare disease fibrodysplasia ossificans progressiva (FOP), where patients develop ectopic bone replacing soft tissue, tendons and ligaments.

Methods

Whole‐mount in situ hybridization and skeletal preparations revealed that following limb‐specific conditional knockout of Acvr1, metacarpals and proximal phalanges were shortened and additional cartilage and bone elements were formed.

Results

The analysis of a set of marker genes including ligands and receptors of BMP signaling as well as genes involved in patterning and tendon and cartilage formation, revealed temporal disturbances with distinct spatial patterns. The most striking result was that in the absence of Acvr1 in mesoderm precursor cells, first digits were drastically malformed.

Conclusion

In FOP, malformation of big toes can serve as a first soft marker in diagnostics. The surprising similarities in phenotype between the described conditional knockout of Acvr1 and the FOP mouse model, indicates a natural inhibitory function of ACVR1. This represents a further step towards better understanding the role of Acvr1 and developing treatment options for FOP.

Limb specific conditional KO of Acvr1 leads to shortened extremities and to heterotopic cartilage and bone formation.

Acvr1 is particularly involved in the development of the first digit.

Phenotypic similarities between the limb specific cKO of Acvr1 and the FOP mouse model, carrying the gain of function mutation p.R206H in Acvr1, indicates a natural inhibitory function of Acvr1.

The effect of type 2 diabetes mellitus and obesity on muscle progenitor cell function

In addition to its primary function to provide movement and maintain posture, the skeletal muscle plays important roles in energy and glucose metabolism. In healthy humans, skeletal muscle is the major site for postprandial glucose uptake and impairment of this process contributes to the pathogenesis of type 2 diabetes mellitus (T2DM). A key component to the maintenance of skeletal muscle integrity and plasticity is the presence of muscle progenitor cells, including satellite cells, fibroadipogenic progenitors, and some interstitial progenitor cells associated with vessels (myo-endothelial cells, pericytes, and mesoangioblasts). In this review, we aim to discuss the emerging concepts related to these progenitor cells, focusing on the identification and characterization of distinct progenitor cell populations, and the impact of obesity and T2DM on these cells. The recent advances in stem cell therapies by targeting diabetic and obese muscle are also discussed.

Coordinating Tissue Regeneration Through Transforming Growth Factor-β Activated Kinase 1 Inactivation and Reactivation

Aberrant wound healing presents as inappropriate or insufficient tissue formation. Using a model of musculoskeletal injury, we demonstrate that loss of transforming growth factor-β activated kinase 1 (TAK1) signaling reduces inappropriate tissue formation (heterotopic ossification) through reduced cellular differentiation. Upon identifying increased proliferation with loss of TAK1 signaling, we considered a regenerative approach to address insufficient tissue production through coordinated inactivation of TAK1 to promote cellular proliferation, followed by reactivation to elicit differentiation and extracellular matrix production. Although the current regenerative medicine paradigm is centered on the effects of drug treatment ("drug on"), the impact of drug withdrawal ("drug off") implicit in these regimens is unknown. Because current TAK1 inhibitors are unable to phenocopy genetic Tak1 loss, we introduce the dual-inducible COmbinational Sequential Inversion ENgineering (COSIEN) mouse model. The COSIEN mouse model, which allows us to study the response to targeted drug treatment ("drug on") and subsequent withdrawal ("drug off") through genetic modification, was used here to inactivate and reactivate Tak1 with the purpose of augmenting tissue regeneration in a calvarial defect model. Our study reveals the importance of both the "drug on" (Cre-mediated inactivation) and "drug off" (Flp-mediated reactivation) states during regenerative therapy using a mouse model with broad utility to study targeted therapies for disease. Stem Cells 2019;37:766-778.

Identification of reference genes for quantitative PCR during C3H10T1/2 chondrogenic differentiation

C3H10T1/2, a mouse mesenchymal stem cell line, is a well-known in vitro model of chondrogenesis that can be easily employed to recapitulate some of the mechanisms intervening in this process. Moreover, these cells can be used to validate the effect of candidate molecules identified by high throughput screening approaches applied to the development of targeted therapy for human disorders in which chondrogenic differentiation may be involved, as in conditions characterized by heterotopic endochondral bone formation. Chondrogenic differentiation of C3H10T1/2 cells can be monitored by applying quantitative polymerase chain reaction (qPCR), one of the most sensitive methods that allows detection of small dynamic changes in gene expression between samples obtained under different experimental conditions. In this work, we have used qPCR to monitor the expression of specific markers during chondrogenic differentiation of C3H10T1/2 cells in micromass cultures. Then we have applied the geNorm approach to identify the most stable reference genes suitable to get a robust normalization of the obtained expression data. Among 12 candidate reference genes (Ap3d1, Csnk2a2, Cdc40, Fbxw2, Fbxo38, Htatsf1, Mon2, Pak1ip1, Zfp91, 18S, ActB, GAPDH) we identified Mon2 and Ap3d1 as the most stable ones during chondrogenesis. ActB, GAPDH and 18S, the most commonly used in the literature, resulted to have an expression level too high compared to the differentiation markers (Sox9, Collagen type 2a1, Collagen type 10a1 and Collagen type 1a1), therefore are actually less recommended for these experimental conditions. In conclusion, we identified nine reference genes that can be equally used to obtain a robust normalization of the gene expression variation during the C3H10T1/2 chondrogenic differentiation.

ACVR1 R206H cooperates with H3.1K27M in promoting diffuse intrinsic pontine glioma pathogenesis

Diffuse intrinsic pontine glioma (DIPG) is an incurable pediatric brain tumor, with approximately 25% of DIPGs harboring activating ACVR1 mutations that commonly co-associate with H3.1K27M mutations. Here we show that in vitro expression of ACVR1 R206H with and without H3.1K27M upregulates mesenchymal markers and activates Stat3 signaling. In vivo expression of ACVR1 R206H or G328V with H3.1K27M and p53 deletion induces glioma-like lesions but is not sufficient for full gliomagenesis. However, in combination with PDGFA signaling, ACVR1 R206H and H3.1K27M significantly decrease survival and increase tumor incidence. Treatment of ACVR1 R206H mutant DIPGs with exogenous Noggin or the ACVR1 inhibitor LDN212854 significantly prolongs survival, with human ACVR1 mutant DIPG cell lines also being sensitive to LDN212854 treatment. Together, our results demonstrate that ACVR1 R206H and H3.1K27M promote tumor initiation, accelerate gliomagenesis, promote a mesenchymal profile partly due to Stat3 activation, and identify LDN212854 as a promising compound to treat DIPG.

Heterotopic Ossification: A Comprehensive Review

Heterotopic ossification (HO) is a diverse pathologic process, defined as the formation of extraskeletal bone in muscle and soft tissues. HO can be conceptualized as a tissue repair process gone awry and is a common complication of trauma and surgery. This comprehensive review seeks to synthesize the clinical, pathoetiologic, and basic biologic features of HO, including nongenetic and genetic forms. First, the clinical features, radiographic appearance, histopathologic diagnosis, and current methods of treatment are discussed. Next, current concepts regarding the mechanistic bases for HO are discussed, including the putative cell types responsible for HO formation, the inflammatory milieu and other prerequisite “niche” factors for HO initiation and propagation, and currently available animal models for the study of HO of this common and potentially devastating condition. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

BMP-dependent, injury-induced stem cell niche as a mechanism of heterotopic ossification

Background

Heterotopic ossification (HO), either acquired (aHO) or hereditary, such as fibrodysplasia ossificans progressiva (FOP), is a serious condition without effective treatment. Understanding of the core process of injury-induced HO is still severely limited.

Methods

Double-pulse thymidine analog labeling was used to explore the distinctive domains evolved in injury-induced lesions in an animal model of HO (Nse-BMP4). Histological studies were performed to see whether a similar zonal pattern is also consistently found in biopsies from patients with aHO and FOP. In vivo clonal analysis with Rainbow mice, genetic loss-of-function studies with diphtheria toxin A (DTA)-mediated depletion and lineage tracing with Zsgreen reporter mice were used to obtain further evidence that Tie2-cre-, Gli1-creERT-, and Glast-creERT-labeled cells contribute to HO as niche-dwelling progenitor/stem cells. Immunohistochemistry was used to test whether vasculature, neurites, macrophages, and mast cells are closely associated with the proposed niche and thus are possible candidate niche supportive cells. Similar methods also were employed to further understand the signaling pathways that regulate the niche and the resultant HO.

Results

We found that distinctive domains evolved in injury-induced lesions, including, from outside-in, a mesenchymal stem cell (MSC) niche, a transient domain and an inner differentiated core in an animal model of HO (Nse-BMP4). A similar zonal structure was found in patients with aHO and FOP. In vivo clonal analysis with Rainbow mice and genetic loss-of-function studies with DTA provided evidence that Tie2-cre-, Gli1-creERT-, and Glast-creERT-labeled cells contribute to HO as niche-dwelling progenitor/stem cells; consistently, vasculature, neurites, macrophages, and mast cells are closely associated with the proposed niche and thus are possible candidate niche supportive cells. Further mechanistic study found that BMP and hedgehog (Hh) signaling co-regulate the niche and the resultant HO.

Conclusions

Available data provide evidence of a potential core mechanism in which multiple disease-specific cellular and extracellular molecular elements form a unique local microenvironment, i.e., an injury-induced stem cell niche, which regulates the proliferation and osteogenic differentiation of mesenchymal stem cells (MSCs). The implication for HO is that therapeutic approaches must consider several different disease specific factors as parts of a functional unit, instead of treating one factor at a time.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1107-7) contains supplementary material, which is available to authorized users.

Therapeutic advances for blocking heterotopic ossification in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease in which heterotopic bone forms in muscle and soft tissue, leading to joint dysfunction and significant disability. FOP is progressive and many patients are wheelchair-bound by the 3rd decade of life. FOP is caused by an activating mutation in the ACVR1 gene, which encodes the activin A Type 1 receptor. Aberrant signalling through this receptor leads to abnormal activation of the pSMAD 1/5/8 pathway and triggers the formation of bone outside of the skeleton. There is no curative therapy for FOP; however, exciting advances in novel therapies have developed recently. Here, we review the clinical and translational pharmacology of three drugs that are currently in clinical trials (palovarotene, REGN 2477 and rapamycin) as well as other emerging treatment strategies for FOP.

Pharmacologic Strategies for Assaying BMP Signaling Function

The bone morphogenetic protein (BMP) signaling pathway, a subset of the transforming growth factor β (TGF-β) signaling family, consists of structurally diverse receptors and ligands whose combinatorial specificity encodes autocrine, paracrine, and endocrine signals essential for regulating tissue growth, differentiation, and survival during embryonic patterning and postnatal tissue remodeling. Aberrant signaling of these receptors and ligands is implicated in a variety of inborn and acquired diseases. The roles of various receptors and their ligands can be explored using small molecule inhibitors of the BMP receptor kinases. Several BMP type I receptor kinase inhibitor tool compounds have been described that exhibit sufficient selectivity to discriminate BMP receptor signaling in vitro or in vivo, with various trade-offs in selectivity, potency, cell permeability, and pharmacokinetics. Several methods for assaying BMP function via pharmacologic inhibition are presented. Two in vitro methods, an In-Cell Western assay of BMP-mediated SMAD1/5/8 phosphorylation and an alkaline phosphatase osteogenic differentiation assay, represent efficient high-throughput methodologies for assaying pharmacologic inhibitors. Two in vivo methods are described for assaying the effects of BMP signaling inhibition in embryonic zebrafish and mouse development. Small molecule inhibitors of BMP receptor kinases represent an important complementary strategy to genetic gain- and loss-of-function and ligand-trap approaches for targeting this signaling system in biology and disease.

Bone morphogenetic protein receptor signal transduction in human disease

Bone morphogenetic proteins (BMPs) are secreted cytokines that were initially discovered on the basis of their ability to induce bone. Several decades of research have now established that these proteins function in a large variety of physiopathological processes. There are about 15 BMP family members, which signal via three transmembrane type II receptors and four transmembrane type I receptors. Mechanistically, BMP binding leads to phosphorylation of the type I receptor by the type II receptor. This activated heteromeric complex triggers intracellular signaling that is initiated by phosphorylation of receptor‐regulated SMAD1, 5, and 8 (also termed R‐SMADs). Activated R‐SMADs form heteromeric complexes with SMAD4, which engage in specific transcriptional responses. There is convergence along the signaling pathway and, besides the canonical SMAD pathway, BMP‐receptor activation can also induce non‐SMAD signaling. Each step in the pathway is fine‐tuned by positive and negative regulation and crosstalk with other signaling pathways. For example, ligand bioavailability for the receptor can be regulated by ligand‐binding proteins that sequester the ligand from interacting with receptors. Accessory co‐receptors, also known as BMP type III receptors, lack intrinsic enzymatic activity but enhance BMP signaling by presenting ligands to receptors. In this review, we discuss the role of BMP receptor signaling and how corruption of this pathway contributes to cardiovascular and musculoskeletal diseases and cancer. We describe pharmacological tools to interrogate the function of BMP receptor signaling in specific biological processes and focus on how these agents can be used as drugs to inhibit or activate the function of the receptor, thereby normalizing dysregulated BMP signaling. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

NF-κB/MAPK activation underlies ACVR1-mediated inflammation in human heterotopic ossification

BACKGROUND

Inflammation helps regulate normal growth and tissue repair. Although bone morphogenetic proteins (BMPs) and inflammation are known contributors to abnormal bone formation, how these pathways interact in ossification remains unclear.

METHODS

We examined this potential link in patients with fibrodysplasia ossificans progressiva (FOP), a genetic condition of progressive heterotopic ossification caused by activating mutations in the Activin A type I receptor (ACVR1/ALK2). FOP patients show exquisite sensitivity to trauma, suggesting that BMP pathway activation may alter immune responses. We studied primary blood, monocyte, and macrophage samples from control and FOP subjects using multiplex cytokine, gene expression, and protein analyses; examined CD14+ primary monocyte and macrophage responses to TLR ligands; and assayed BMP, TGF-β activated kinase 1 (TAK1), and NF-κB pathways.

RESULTS

FOP subjects at baseline without clinically evident heterotopic ossification showed increased serum IL-3, IL-7, IL-8, and IL-10. CD14+ primary monocytes treated with the TLR4 activator LPS showed increased CCL5, CCR7, and CXCL10; abnormal cytokine/chemokine secretion; and prolonged activation of the NF-κB pathway. FOP macrophages derived from primary monocytes also showed abnormal cytokine/chemokine secretion, increased TGF-β production, and p38MAPK activation. Surprisingly, SMAD phosphorylation was not significantly changed in the FOP monocytes/macrophages.

CONCLUSIONS

Abnormal ACVR1 activity causes a proinflammatory state via increased NF-κB and p38MAPK activity. Similar changes may contribute to other types of heterotopic ossification, such as in scleroderma and dermatomyositis; after trauma; or with recombinant BMP-induced bone fusion. Our findings suggest that chronic antiinflammatory treatment may be useful for heterotopic ossification.

Discovery of Heterotopic Bone-Inducing Activity in Hard Tissues and the TGF-β Superfamily

Bone is a unique organ because it can be experimentally induced in soft tissues by implanting a single growth factor, bone morphogenetic protein (BMP). Heterotopic bone-inducing activity was found in demineralized bone matrix in 1965. The characterization of this activity in bone enabled the purification and molecular cloning of BMPs and showed that they are members of the transforming growth factor-β (TGF-β) superfamily. Assay systems developed for this bone-inducing activity revealed the molecular mechanisms of the intracellular signaling of members of the superfamily, including BMPs. Moreover, they are being applied to elucidate molecular mechanisms and to develop novel therapeutics for a disease caused by an abnormality in BMP signaling.

An mTOR Signaling Modulator Suppressed Heterotopic Ossification of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disorder characterized by extraskeletal bone formation through endochondral ossification. FOP patients harbor gain-of-function mutations in ACVR1 (FOP-ACVR1), a type I receptor for bone morphogenetic proteins. Despite numerous studies, no drugs have been approved for FOP. Here, we developed a high-throughput screening (HTS) system focused on the constitutive activation of FOP-ACVR1 by utilizing a chondrogenic ATDC5 cell line that stably expresses FOP-ACVR1. After HTS of 5,000 small-molecule compounds, we identified two hit compounds that are effective at suppressing the enhanced chondrogenesis of FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and suppressed the heterotopic ossification (HO) of multiple model mice, including FOP-ACVR1 transgenic mice and HO model mice utilizing FOP-iPSCs. Furthermore, we revealed that one of the hit compounds is an mTOR signaling modulator that indirectly inhibits mTOR signaling. Our results demonstrate that these hit compounds could contribute to future drug repositioning and the mechanistic analysis of mTOR signaling.

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Established a screening system for fibrodysplasia ossificans progressiva (FOP)

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Identified two hit compounds that are effective in multiple FOP model mice

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An mTOR signaling modulator opens the door to a therapeutic strategy

Palovarotene reduces heterotopic ossification in juvenile FOP mice but exhibits pronounced skeletal toxicity

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by debilitating heterotopic ossification (HO). The retinoic acid receptor gamma agonist, palovarotene, and antibody-mediated activin A blockade have entered human clinical trials, but how these therapeutic modalities affect the behavior of pathogenic fibro/adipogenic progenitors (FAPs) is unclear. Using live-animal luminescence imaging, we show that transplanted pathogenic FAPs undergo rapid initial expansion, with peak number strongly correlating with HO severity. Palovarotene significantly reduced expansion of pathogenic FAPs, but was less effective than activin A inhibition, which restored wild-type population growth dynamics to FAPs. Palovarotene pretreatment did not reduce FAPs’ skeletogenic potential, indicating that efficacy requires chronic administration. Although palovarotene inhibited chondrogenic differentiation in vitro and reduced HO in juvenile FOP mice, daily dosing resulted in aggressive synovial joint overgrowth and long bone growth plate ablation. These results highlight the challenge of inhibiting pathological bone formation prior to skeletal maturation.

Modulating Cell Fate as a Therapeutic Strategy

In injured tissues, regeneration is often associated with cell fate plasticity, in that cells deviate from their normal lineage paths. It is becoming increasingly clear that this plasticity often creates alternative strategies to restore damaged or lost cells. Alternatively, cell fate plasticity is also part and parcel of pathologic tissue transformations that accompany disease. In this Perspective, we summarize a few illustrative examples of physiologic and aberrant cellular plasticity. Then, we speculate on how one could enhance endogenous plasticity to promote regeneration and reverse pathologic plasticity, perhaps inspiring interest in a new class of therapies targeting cell fate modulation.

Injury of Adult Zebrafish Expressing Acvr1lQ204D Does Not Result in Heterotopic Ossification

Fibrodysplasia Ossificans Progressiva (FOP) is a rare, autosomal dominant genetic disorder in humans characterized by the gradual ossification of fibrous tissues, including skeletal muscle, tendons, and ligaments. In humans, mutations in the Type I BMP/TGFβ family member receptor gene, ACVR1, are associated with FOP. Zebrafish acvr1l, previously known as alk8, is the functional ortholog of human ACVR1. We previously created and characterized the first adult zebrafish model for FOP by generating animals harboring heat shock-inducible mCherry-tagged constitutively active Acvr1l (Q204D). Since injury is a known trigger for heterotopic ossification (HO) development in human FOP patients, in this study, we investigated several injury models in Acvr1l^Q204D-expressing zebrafish and the subsequent formation of HO. We performed studies of Activin A injection, cardiotoxin (CTX) injection, and caudal fin clip injury. We found that none of these methods resulted in HO formation at the site of injury. However, some of the cardiotoxin-injected and caudal fin-clipped animals did exhibit HO at distant sites, including the body cavity and along the spine. We describe these results in the context of new and exciting reports on FOP, and discuss future studies to better understand the etiology and progression of this disease.

Recent Topics in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease that is characterized by the formation of heterotopic bone tissues in soft tissues, such as skeletal muscle, ligament, and tendon. It is difficult to remove such heterotopic bones via internal medicine or invasive procedures. The identification of activin A receptor, type I (ACVR1)/ALK2 gene mutations associated with FOP has allowed the genetic diagnosis of FOP. The ACVR1/ALK2 gene encodes the ALK2 protein, which is a transmembrane kinase receptor in the transforming growth factor-β family. The relevant mutations activate intracellular signaling in vitro and induce heterotopic bone formation in vivo. Activin A is a potential ligand that activates mutant ALK2 but not wild-type ALK2. Various types of small chemical and biological inhibitors of ALK2 signaling have been developed to establish treatments for FOP. Some of these are in clinical trials in patients with FOP.

Modeling human somite development and fibrodysplasia ossificans progressiva with induced pluripotent stem cells

Somites (SMs) comprise a transient stem cell population that gives rise to multiple cell types, including dermatome (D), myotome (MYO), sclerotome (SCL) and syndetome (SYN) cells. Although several groups have reported induction protocols for MYO and SCL from pluripotent stem cells, no studies have demonstrated the induction of SYN and D from SMs. Here, we report systematic induction of these cells from human induced pluripotent stem cells (iPSCs) under chemically defined conditions. We also successfully induced cells with differentiation capacities similar to those of multipotent mesenchymal stromal cells (MSC-like cells) from SMs. To evaluate the usefulness of these protocols, we conducted disease modeling of fibrodysplasia ossificans progressiva (FOP), an inherited disease that is characterized by heterotopic endochondral ossification in soft tissues after birth. Importantly, FOP-iPSC-derived MSC-like cells showed enhanced chondrogenesis, whereas FOP-iPSC-derived SCL did not, possibly recapitulating normal embryonic skeletogenesis in FOP and cell-type specificity of FOP phenotypes. These results demonstrate the usefulness of multipotent SMs for disease modeling and future cell-based therapies.

Summary: Protocols for the differentiation of human iPSCs to somite derivatives (myotome, sclerotome, syndetome and dermatome) are developed and applied to the modeling of the bone disease fibrodysplasia ossificans progressiva.

Characterizing the Circulating Cell Populations in Traumatic Heterotopic Ossification

Heterotopic ossification (HO) occurs secondary to trauma, causing pain and functional limitations. Identification of the cells that contribute to HO is critical to the development of therapies. Given that innate immune cells and mesenchymal stem cells are known contributors to HO, we sought to define the contribution of these populations to HO and to identify what, if any, contribution circulating populations have to HO. A shared circulation was obtained using a parabiosis model, established between an enhanced green fluorescent protein-positive/luciferase⁺ donor and a same-strain nonreporter recipient mouse. The nonreporter mouse received Achilles tendon transection and dorsal burn injury to induce HO formation. Bioluminescence imaging and immunostaining were performed to define the circulatory contribution of immune and mesenchymal cell populations. Histologic analysis showed circulating cells present throughout each stage of the developing HO anlagen. Circulating cells were present at the injury site during the inflammatory phase and proliferative period, with diminished contribution in mature HO. Immunostaining demonstrated that most early circulatory cells were from the innate immune system; only a small population of mesenchymal cells were present in the HO. We demonstrate the time course of the participation of circulatory cells in trauma-induced HO and identify populations of circulating cells present in different stages of HO. These findings further elucidate the relative contribution of local and systemic cell populations to HO.

Hypoxia-selective allosteric destabilization of activin receptor-like kinases: A potential therapeutic avenue for prophylaxis of heterotopic ossification

Heterotopic ossification (HO), the pathological extraskeletal formation of bone, can arise from blast injuries, severe burns, orthopedic procedures and gain-of-function mutations in a component of the bone morphogenetic protein (BMP) signaling pathway, the ACVR1/ALK2 receptor serine-threonine (protein) kinase, causative of Fibrodysplasia Ossificans Progressiva (FOP). All three ALKs (-2, -3, -6) that play roles in bone morphogenesis contribute to trauma-induced HO, hence are well-validated pharmacological targets. That said, development of inhibitors, typically competitors of ATP binding, is inherently difficult due to the conserved nature of the active site of the 500+ human protein kinases. Since these enzymes are regulated via inherent plasticity, pharmacological chaperone-like drugs binding to another (allosteric) site could hypothetically modulate kinase conformation and activity. To test for such a mechanism, a surface pocket of ALK2 kinase formed largely by a key allosteric substructure was targeted by supercomputer docking of drug-like compounds from a virtual library. Subsequently, the effects of docked hits were further screened in vitro with purified recombinant kinase protein. A family of compounds with terminal hydrogen-bonding acceptor groups was identified that significantly destabilized the protein, inhibiting activity. Destabilization was pH-dependent, putatively mediated by ionization of a histidine within the allosteric substructure with decreasing pH. In vivo, nonnative proteins are degraded by proteolysis in the proteasome complex, or cellular trashcan, allowing for the emergence of therapeutics that inhibit through degradation of over-active proteins implicated in the pathology of diseases and disorders. Because HO is triggered by soft-tissue trauma and ensuing hypoxia, dependency of ALK destabilization on hypoxic pH imparts selective efficacy on the allosteric inhibitors, providing potential for safe prophylactic use.

Bone Morphogenetic Protein-Based Therapeutic Approaches

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor (TGF)-β family of ligands and exert most of their effects through the canonical effectors Smad1, 5, and 8. Appropriate regulation of BMP signaling is critical for the development and homeostasis of numerous human organ systems. Aberrations in BMP pathways or their regulation are increasingly associated with diverse human pathologies, and there is an urgent and growing need to develop effective approaches to modulate BMP signaling in the clinic. In this review, we provide a wide perspective on diseases and/or conditions associated with dysregulated BMP signal transduction, outline the current strategies available to modulate BMP pathways, highlight emerging second-generation technologies, and postulate prospective avenues for future investigation.

Post-Traumatic Heterotopic Ossification: An Old Problem in Need of New Solutions

Heterotopic ossification (HO) is the formation of pathological bone in ectopic sites and it can have serious consequences for functional outcomes. For many years, its main clinical relevance was as a rare complication of elective joint arthroplasty or CNS injury and a number of prophylaxes were developed to mitigate against it in these settings. As a consequence of changes in patterns of wounding and survival in conflicts since the turn of the century, post-traumatic HO has become much more common and case severity has increased. It represents one of the main barriers to rehabilitation in a large cohort of combat-injured patients. However, extant prophylaxes have not been shown to be effective or appropriate in this patient cohort. In addition, the lack of reliable early detection or means of predicting which patients will develop HO is another barrier to effective prevention. This review examines the current state of understanding of post-traumatic HO including the historical context, epidemiology, pathophysiology, clinical issues, currently prophylaxis and detection, management, and potential future approaches. Our aims are to highlight the current lack of effective means of early detection and prevention of HO after major trauma and to stimulate research into novel solutions to this challenging problem. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1061-1068, 2018.

Shared ACVR1 mutations in FOP and DIPG: Opportunities and challenges in extending biological and clinical implications across rare diseases

Gain-of-function mutations in the Type I Bone Morphogenic Protein (BMP) receptor ACVR1 have been identified in two diseases: Fibrodysplasia Ossificans Progressiva (FOP), a rare autosomal dominant disorder characterized by genetically driven heterotopic ossification, and in 20-25% of Diffuse Intrinsic Pontine Gliomas (DIPGs), a pediatric brain tumor with no effective therapies and dismal median survival. While the ACVR1 mutation is causal for FOP, its role in DIPG tumor biology remains under active investigation. Here, we discuss cross-fertilization between the FOP and DIPG fields, focusing on the biological mechanisms and principles gleaned from FOP that can be applied to DIPG biology. We highlight our current knowledge of ACVR1 in both diseases, and then describe the growing opportunities and barriers to effectively investigate ACVR1 in DIPG. Importantly, learning from other seemingly unrelated diseases harboring similar mutations may uncover novel mechanisms or processes for future investigation.

The obligatory role of Activin A in the formation of heterotopic bone in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder that presents at birth with only minor patterning defects, but manifests its debilitating pathology early in life with episodic, yet progressive and cumulative, heterotopic ossification (HO) of ligaments, tendons, and a subset of major skeletal muscles. The resulting HO lesions are endochondral in nature, and appear to be linked to inflammatory stimuli arising in association with known injuries, or from inflammation linked to normal tissue repair. FOP is caused by gain-of-function mutations in ACVR1, which encodes a type I BMP receptor. Initial studies on the pathogenic mechanism of FOP-causing mutations in ACVR1 focused on the enhanced function of this receptor in response to certain BMP ligands, or independently of ligands, but did not directly address the fact that HO in FOP is episodic and inflammation-driven. Recently, we and others demonstrated that Activin A is an obligate factor for the initiation of HO in FOP, signaling aberrantly via mutant ACVR1 to transduce osteogenic signals and trigger heterotopic bone formation (Hatsell et al., 2015; Hino et al., 2015). Subsequently, we identified distinct tissue-resident mesenchymal progenitor cells residing in muscles and tendons that recognize Activin A as a pro-osteogenic signal (solely in the context of FOP-causing mutant ACVR1), and give rise to the cartilaginous anlagen that form heterotopic bone (Dey et al., 2016). During the course of these studies, we also found that the activity of FOP-causing ACVR1 mutations does not by itself explain the triggered or inflammatory nature of HO in FOP, suggesting the importance of other, inflammation-introduced, factors or processes. This review presents a synthesis of these findings with a focus on the role of Activin A and inflammation in HO, and lays out perspectives for future research.

Application of human induced pluripotent stem cells to model fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a genetic condition characterized by massive heterotopic ossification. FOP patients have mutations in the Activin A type I receptor (ACVR1), a bone morphogenetic protein (BMP) receptor. FOP is a progressive and debilitating disease characterized by bone formation flares that often occur after trauma. Since it is often difficult or impossible to obtain large amounts of tissue from human donors due to the risks of inciting more heterotopic bone formation, human induced pluripotent stem cells (hiPSCs) provide an attractive source for establishing in vitro disease models and for applications in drug screening. hiPSCs have the ability to self-renew, allowing researchers to obtain large amounts of starting material. hiPSCs also have the potential to differentiate into any cell type in the body. In this review, we discuss how the application of hiPSC technology to studying FOP has changed our perspectives on FOP disease pathogenesis. We also consider ongoing challenges and emerging opportunities for the use of human iPSCs in drug discovery and regenerative medicine.

Gli1-labeled adult mesenchymal stem/progenitor cells and hedgehog signaling contribute to endochondral heterotopic ossification

Heterotopic ossification (HO), acquired or hereditary, endochondral or intramembranous, is the formation of true bone outside the normal skeleton. Since perivascular Gli1+ progenitors contribute to injury induced organ fibrosis, and CD133 is expressed by a variety of populations of adult stem cells, this study utilized Cre-lox based genetic lineage tracing to test the contribution to endochondral HO of adult stem/progenitor cells that expressed either Gli1 or CD133. We found that both lineages contributed broadly to different normal tissues with distinct patterns, but that only Gli1-creERT labeled stem/progenitor cells contributed to all stages of endochondral HO in a BMP dependent, injury induced, transgenic mouse model. Hedgehog (Hh) signaling was abnormal at endochondral HO lesion sites with increased signaling surrounding the lesion but diminished signaling within it. Thus, local dysregulation of Hh signaling participates in the pathophysiology of endochondral HO. However, unlike a previous report of intramembranous HO, systemic inhibition of Hh signaling was insufficient to prevent the initiation of the endochondral HO process or to treat the existing endochondral HO, suggesting that Hh participates in, but is not essential for endochondral HO in this model. This could potentially reflect the underlying difference between intramembranous and endochondral HO. Nevertheless, identification of this novel stem/precursor cell population as a HO-contributing cell population provides a potential drugable target.

Heterotopic ossification and the elucidation of pathologic differentiation

Tissue regeneration following acute or persistent inflammation can manifest a spectrum of phenotypes ranging from the adaptive to the pathologic. Heterotopic Ossification (HO), the endochondral formation of bone within soft-tissue structures following severe injury serves as a prominent example of pathologic differentiation; and remains a persistent clinical issue incurring significant patient morbidity and expense to adequately diagnose and treat. The pathogenesis of HO provides an intriguing opportunity to better characterize the cellular and cell-signaling contributors to aberrant differentiation. Indeed, recent work has continued to resolve the unique cellular lineages, and causative pathways responsible for ectopic bone development yielding promising avenues for the development of novel therapeutic strategies shown to be successful in analogous animal models of HO development. This review details advances in the understanding of HO in the context of inciting inflammation, and explains how these advances inform the current standards of diagnosis and treatment.

Heterotopic ossification: Mechanistic insights and clinical challenges

Bone formation is exquisitely controlled both spatially and temporally. Heterotopic ossification (HO) is pathological bone formation in soft tissues that often leads to deleterious outcomes. Inherited genetic forms of HO can be life-threatening and can happen as early as in infancy. However, there is currently no effective treatment for HO as the underlying cellular and molecular mechanisms have not been completely elucidated. Trauma-induced non-genetic forms of HO often occur as a common complication after surgeries or accidents, and the location of HO occurrence largely determines the symptom and outcome. While it has been difficult to determine the complicated factors causing HO, recent advancement in identifying cellular and molecular mechanism causing the genetic forms of HO may provide important insights in all HO. Here in this review, we summarize recent studies on HO to provide a current status of both clinical options of HO treatments and mechanical understanding of HO.

Heterotopic bone induction via BMP signaling: Potential therapeutic targets for fibrodysplasia ossificans progressiva

More than 50years ago, Marshal M. Urist detected "heterotopic bone-inducing activity" in demineralized bone matrix. This unique activity was referred to as "bone morphogenetic protein (BMP)" because it was sensitive to trypsin digestion. Purification of the bone-inducing activity from demineralized bone matrix using a bone-inducing assay in vivo indicated that the original "BMP" consisted of a mixture of new members of the transforming growth factor-β (TGF-β) family. The establishment of new in vitro assay systems that reflect the bone-inducing activity of BMPs in vivo have revealed the functional receptors and downstream effectors of BMPs. Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic bone formation in soft tissues similar to the event induced by the transplantation of BMPs in skeletal muscle. In patients with FOP, genetic mutations have been identified in the ACVR1 gene, which encodes the BMP receptor ALK2. The mutations in ALK2 associated with FOP are hypersensitive to type II receptor kinases. Recently, activin A, a non-osteogenic member of the TGF-β family, was identified as the ligand of the mutant ALK2 in FOP, and various types of signaling inhibitors for mutant ALK2 are currently under development to establish effective treatments for FOP.

Stem cells and heterotopic ossification: Lessons from animal models

Put most simply, heterotopic ossification (HO) is the abnormal formation of bone at extraskeletal sites. HO can be classified into two main subtypes, genetic and acquired. Acquired HO is a common complication of major connective tissue injury, traumatic central nervous system injury, and surgical interventions, where it can cause significant pain and postoperative disability. A particularly devastating form of HO is manifested in the rare genetic disorder, fibrodysplasia ossificans progressiva (FOP), in which progressive heterotopic bone formation occurs throughout life, resulting in painful and disabling cumulative immobility. While the central role of stem/progenitor cell populations in HO is firmly established, the identity of the offending cell type(s) remains to be conclusively determined, and little is known of the mechanisms that direct these progenitor cells to initiate cartilage and bone formation. In this review, we summarize current knowledge of the cells responsible for acquired HO and FOP, highlighting the strengths and weaknesses of animal models used to interrogate the cellular origins of HO.

Structural basis for the potent and selective binding of LDN-212854 to the BMP receptor kinase ALK2

Individuals with the rare developmental disorder fibrodysplasia ossificans progressiva (FOP) experience disabling heterotopic ossification caused by a gain of function mutation in the intracellular region of the BMP type I receptor kinase ALK2, encoded by the gene ACVR1. Small molecule BMP type I receptor inhibitors that block this ossification in FOP mouse models have been derived from the pyrazolo[1,5-a]pyrimidine scaffold of dorsomorphin. While the first derivative LDN-193189 exhibited pan inhibition of BMP receptors, the more recent compound LDN-212854 has shown increased selectivity for ALK2. Here we solved the crystal structure of ALK2 in complex with LDN-212854 to define how its binding interactions compare to previously reported BMP and TGFβ receptor inhibitors. LDN-212854 bound to the kinase hinge region as a typical type I ATP-competitive inhibitor with a single hydrogen bond to ALK2 His286. Specificity arising from the 5-quinoline moiety was associated with a distinct pattern of water-mediated hydrogen bonds involving Lys235 and Glu248 in the inactive conformation favoured by ALK2. The structure of this complex provides a template for the design of future ALK2 inhibitors under development for the treatment of FOP and other related conditions of heterotopic ossification.

The Horizon of a Therapy for Rare Genetic Diseases: A "Druggable" Future for Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic condition characterized by progressive extra-skeletal ossification leading to cumulative and severe disability. FOP has an extremely variable and episodic course and can be induced by trauma, infections, iatrogenic harms, immunization or can occur in an unpredictable way, without any recognizable trigger. The causative gene is ACVR1, encoding the Alk-2 type I receptor for bone morphogenetic proteins (BMPs). The signaling is initiated by BMP binding to a receptor complex consisting of type I and II molecules and can proceed into the cell through two main pathways, a canonical, SMAD-dependent signaling and a p38-mediated cascade. Most FOP patients carry the recurrent R206H substitution in the receptor Glycine-Serine rich (GS) domain, whereas a few other mutations are responsible for a limited number of cases. Mutations cause a dysregulation of the downstream BMP-dependent pathway and make mutated ACVR1 responsive to a non-canonical ligand, Activin A. There is no etiologic treatment for FOP. However, many efforts are currently ongoing to find specific therapies targeting the receptor activity and the downstream aberrant pathway at different levels or targeting cellular components and/or processes that are important in modifying the local environment leading to bone neo-formation.

Inhibition of overactive TGF-β attenuates progression of heterotopic ossification in mice

Acquired heterotopic ossification (HO) is a painful and debilitating disease characterized by extraskeletal bone formation after injury. The exact pathogenesis of HO remains unknown. Here we show that TGF-β initiates and promotes HO in mice. We find that calcified cartilage and newly formed bone resorb osteoclasts after onset of HO, which leads to high levels of active TGF-β that recruit mesenchymal stromal/progenitor cells (MSPCs) in the HO microenvironment. Transgenic expression of active TGF-β in tendon induces spontaneous HO, whereas systemic injection of a TGF-β neutralizing antibody attenuates ectopic bone formation in traumatic and BMP-induced mouse HO models, and in a fibrodysplasia ossificans progressive mouse model. Moreover, inducible knockout of the TGF-β type II receptor in MSPCs inhibits HO progression in HO mouse models. Our study points toward elevated levels of active TGF-β as inducers and promoters of ectopic bone formation, and suggest that TGF-β might be a therapeutic target in HO.

Activin-dependent signaling in fibro/adipogenic progenitors causes fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal-dominant disorder characterized by progressive and profoundly disabling heterotopic ossification (HO). Here we show that fibro/adipogenic progenitors (FAPs) are a major cell-of-origin of HO in an accurate genetic mouse model of FOP (Acvr1^tnR206H). Targeted expression of the disease-causing type I bone morphogenetic protein (BMP) receptor, ACVR1(R206H), to FAPs recapitulates the full spectrum of HO observed in FOP patients. ACVR1(R206H)-expressing FAPs, but not wild-type FAPs, activate osteogenic signaling in response to activin ligands. Conditional loss of the wild-type Acvr1 allele dramatically exacerbates FAP-directed HO, suggesting that mutant and wild-type ACVR1 receptor complexes compete for activin ligands or type II BMP receptor binding partners. Finally, systemic inhibition of activin A completely blocks HO and restores wild-type-like behavior to transplanted Acvr1^R206H/+ FAPs. Understanding the cells that drive HO may facilitate the development of cell-specific therapeutic approaches to inhibit catastrophic bone formation in FOP.

Fibrodysplasia ossificans progressiva is a severe disorder characterized by heterotopic ossification, and is caused by mutations in ACVR1. Here, the authors show that expression of mutant ACVR1 in fibro/adipogenic progenitors recapitulates disease progression, and that this can be halted by systemic inhibition of activin A in mice.

The Expansion of Heterotopic Bone in Fibrodysplasia Ossificans Progressiva Is Activin A-Dependent

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder that is characterized by episodic yet cumulative heterotopic ossification (HO) in skeletal muscles, tendons, and ligaments over a patient's lifetime. FOP is caused by missense mutations in the type I bone morphogenetic protein (BMP) receptor ACVR1. We have determined that the formation of heterotopic bone in FOP requires activation of mutant ACVR1 by Activin A, in part by showing that prophylactic inhibition of Activin A blocks HO in a mouse model of FOP. Here we piece together a natural history of developing HO lesions in mouse FOP, and determine where in the continuum of HO Activin A is required, using imaging (T2-MRI, μCT, ¹⁸ F-NaF PET/CT, histology) coupled with pharmacologic inhibition of Activin A at different times during the progression of HO. First, we show that expansion of HO lesions comes about through growth and fusion of independent HO events. These events tend to arise within a neighborhood of existing lesions, indicating that already formed HO likely triggers the formation of new events. The process of heterotopic bone expansion appears to be dependent on Activin A because inhibition of this ligand suppresses the growth of nascent HO lesions and stops the emergence of new HO events. Therefore, our results reveal that Activin A is required at least up to the point when nascent HO lesions mineralize and further demonstrate the therapeutic utility of Activin A inhibition in FOP. These results provide evidence for a model where HO is triggered by inflammation but becomes "self-propagating" by a process that requires Activin A. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

Animal models of fibrodysplasia ossificans progressiva

Fibrodysplasia Ossificans Progressiva is a rare human disease of heterotopic ossification. FOP patients experience progressive development of ectopic bone within fibrous tissues that contributes to a gradual loss of mobility and can lead to early mortality. Due to lack of understanding of the etiology and progression of human FOP, and the fact that surgical interventions often exacerbate FOP disease progression, alternative therapeutic methods are needed, including modeling in animals, to study and improve understanding of human FOP. In this review we provide an overview of the existing animal models of FOP and the key mechanistic findings from each. In addition, we highlight the specific advantages of a new adult zebrafish model, generated by our lab, to study human FOP. Developmental Dynamics 247:279-288, 2018. © 2017 Wiley Periodicals, Inc.

Peripheral Blood Mononuclear Cell Immunophenotyping in Fibrodysplasia Ossificans Progressiva Patients: Evidence for Monocyte DNAM1 Up-regulation

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder caused by sporadic heterozygous mutations in ACVR1 gene which progressively leads to severe heterotopic ossification. FOP is characterized by episodic flare-ups triggered by different factors such as viral infections, tissue injuries, vaccinations, or occurring without a recognizable cause. The sporadic course of the disease, the documented presence of an important inflammatory reaction in early lesions and the partial response to corticosteroids support the idea that the immune system, and in particular the innate component, may play a role in FOP pathogenesis. However, an extensive expression profile of the peripheral blood mononuclear cells (PBMC) of FOP patients has never been done.

METHODS

In this study, we carried out a wide PBMC immunophenotyping on a cohort of FOP patients and matching controls by multiparametric analysis of the expression of a panel of 37 markers associated with migration, adhesion, inhibition, activation, and cell death of circulating immune cells.

RESULTS

We observed a statistically significant increase of the expression of DNAM1 receptor in patients' monocytes as compared to controls, and little but significant differences in the expression profile of CXCR1 (CD181), CD62L, CXCR4 (CD184), and HLA-DR molecules.

CONCLUSIONS

DNAM1 had been previously shown to play a pivotal role in monocyte migration through the endothelial barrier and the increased expression detected in patients' monocytes might suggest a role of this surface receptor during the early phases of FOP flare-ups in which the activation of the immune response is believed to represent a crucial event. © 2017 International Clinical Cytometry Society.

Fibrodysplasia ossificans progressiva: Basic understanding and experimental models

Fibrodysplasia ossificans progressive (FOP) is an extremely rare autosomal dominant disorder characterized by congenital malformations of the great toes and progressive heterotopic ossification that can induce a disabling second skeleton. Spontaneously occurring flare-ups can cause inflammatory soft tissue to swell, followed by progressive and disabling heterotopic endochondral ossification. FOP is very rare, with an estimated incidence of one case per two million individuals. There is no definitive treatment for FOP, but the longevity of patients with FOP can be extended by early diagnosis and appropriate prevention of flares-up. Some promising treatment strategies and targets have recently been reported. The current review describes the classical phenotype and genotype of FOP, useful methods of diagnosing the condition, therapeutic approaches and commonly used drugs, and experimental models used to study this disease.

The traumatic bone: trauma-induced heterotopic ossification

Heterotopic ossification (HO) is a common occurrence after multiple forms of extensive trauma. These include arthroplasties, traumatic brain and spinal cord injuries, extensive burns in the civilian setting, and combat-related extremity injuries in the battlefield. Irrespective of the form of trauma, heterotopic bone is typically endochondral in structure and is laid down via a cartilaginous matrix. Once formed, the heterotopic bone typically needs to be excised surgically, which may result in wound healing complications, in addition to a risk of recurrence. Refinements of existing diagnostic modalities, like micro- and nano-CT are being adapted toward early intervention. Trauma-induced HO is a consequence of aberrant wound healing, systemic and local immune system activation, infections, extensive vascularization, and innervation. This intricate molecular crosstalk culminates in activation of stem cells that initiate heterotopic endochondral ossification. Development of animal models recapitulating the unique traumatic injuries has greatly facilitated the mechanistic understanding of trauma-induced HO. These same models also serve as powerful tools to test the efficacy of small molecules which specifically target the molecular pathways underlying ectopic ossification. This review summarizes the recent advances in the molecular understanding, diagnostic and treatment modalities in the field of trauma-induced HO.

Activin-A enhances mTOR signaling to promote aberrant chondrogenesis in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disease characterized by extraskeletal bone formation through endochondral ossification. Patients with FOP harbor point mutations in ACVR1, a type I receptor for BMPs. Although mutated ACVR1 (FOP-ACVR1) has been shown to render hyperactivity in BMP signaling, we and others have uncovered a mechanism by which FOP-ACVR1 mistransduces BMP signaling in response to Activin-A, a molecule that normally transduces TGF-β signaling. Although Activin-A evokes enhanced chondrogenesis in vitro and heterotopic ossification (HO) in vivo, the underlying mechanisms have yet to be revealed. To this end, we developed a high-throughput screening (HTS) system using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) to identify pivotal pathways in enhanced chondrogenesis that are initiated by Activin-A. In a screen of 6,809 small-molecule compounds, we identified mTOR signaling as a critical pathway for the aberrant chondrogenesis of mesenchymal stromal cells derived from FOP-iPSCs (FOP-iMSCs). Two different HO mouse models, an FOP model mouse expressing FOP-ACVR1 and an FOP-iPSC-based HO model mouse, revealed critical roles for mTOR signaling in vivo. Moreover, we identified ENPP2, an enzyme that generates lysophosphatidic acid, as a linker of FOP-ACVR1 and mTOR signaling in chondrogenesis. These results uncovered the crucial role of the Activin-A/FOP-ACVR1/ENPP2/mTOR axis in FOP pathogenesis.

Bone morphogenetic protein signaling mediated by ALK-2 and DLX2 regulates apoptosis in glioma-initiating cells

Bone morphogenetic protein (BMP) signaling exerts antitumor activities in glioblastoma; however, its precise mechanisms remain to be elucidated. Here, we demonstrated that the BMP type I receptor ALK-2 (encoded by the ACVR1 gene) has crucial roles in apoptosis induction of patient-derived glioma-initiating cells (GICs), TGS-01 and TGS-04. We also characterized a BMP target gene, Distal-less homeobox 2 (DLX2), and found that DLX2 promoted apoptosis and neural differentiation of GICs. The tumor-suppressive effects of ALK-2 and DLX2 were further confirmed in a mouse orthotopic transplantation model. Interestingly, valproic acid (VPA), an anti-epileptic compound, induced BMP2, BMP4, ACVR1 and DLX2 mRNA expression with a concomitant increase in phosphorylation of Smad1/5. Consistently, we showed that treatment with VPA induced apoptosis of GICs, whereas silencing of ALK-2 or DLX2 expression partially suppressed it. Our study thus reveals BMP-mediated inhibitory mechanisms for glioblastoma, which explains, at least in part, the therapeutic effects of VPA.

Novel Model of Tendon Regeneration Reveals Distinct Cell Mechanisms Underlying Regenerative and Fibrotic Tendon Healing

To date, the cell and molecular mechanisms regulating tendon healing are poorly understood. Here, we establish a novel model of tendon regeneration using neonatal mice and show that neonates heal via formation of a ‘neo-tendon’ that differentiates along the tendon specific lineage with functional restoration of gait and mechanical properties. In contrast, adults heal via fibrovascular scar, aberrant differentiation toward cartilage and bone, with persistently impaired function. Lineage tracing identified intrinsic recruitment of Scx-lineage cells as a key cellular mechanism of neonatal healing that is absent in adults. Instead, adult Scx-lineage tenocytes are not recruited into the defect but transdifferentiate into ectopic cartilage; in the absence of tenogenic cells, extrinsic αSMA-expressing cells persist to form a permanent scar. Collectively, these results establish an exciting model of tendon regeneration and uncover a novel cellular mechanism underlying regenerative vs non-regenerative tendon healing.