An mTOR Signaling Modulator Suppressed Heterotopic Ossification of Fibrodysplasia Ossificans Progressiva

Cellular and Molecular Mechanisms of Heterotopic Ossification in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a debilitating genetic disorder characterized by recurrent episodes of heterotopic ossification (HO) formation in muscles, tendons, and ligaments. FOP is caused by a missense mutation in the ACVR1 gene (activin A receptor type I), an important signaling receptor involved in endochondral ossification. The ACVR1R206H mutation induces increased downstream canonical SMAD-signaling and drives tissue-resident progenitor cells with osteogenic potential to participate in endochondral HO formation. In this article, we review aberrant ACVR1R206H signaling and the cells that give rise to HO in FOP. FOP mouse models and lineage tracing analyses have been used to provide strong evidence for tissue-resident mesenchymal cells as cellular contributors to HO. We assess how the underlying mutation in FOP disrupts muscle-specific dynamics during homeostasis and repair, with a focus on muscle-resident mesenchymal cells known as fibro-adipogenic progenitors (FAPs). Accumulating research points to FAPs as a prominent HO progenitor population, with ACVR1R206H FAPs not only aberrantly differentiating into chondro-osteogenic lineages but creating a permissive environment for bone formation at the expense of muscle regeneration. We will further discuss the emerging role of ACVR1R206H FAPs in muscle regeneration and therapeutic targeting of these cells to reduce HO formation in FOP.

Suppression of Overactive Insulin-Like Growth Factor 1 Attenuates Trauma-Induced Heterotopic Ossification in Mice

Heterotopic ossification (HO) is the ectopic bone formation in soft tissues. Aside from hereditary HO, traumatic HO is common after orthopedic surgery, combat-related injuries, severe burns, or neurologic injuries. Recently, mammalian target of rapamycin (mTOR) was demonstrated to be involved in the chondrogenic and osteogenic processes of HO formation. However, its upstream signaling mechanism remains unknown. The current study used an Achilles tendon puncture-induced HO model to show that overactive insulin-like growth factor 1 (IGF-1) was involved in the progression of HO in mice. Micro-computed tomography imaging showed that IGF-1 not only accelerated the rate of osteogenesis and increased ectopic bone volume but also induced spontaneous ectopic bone formation in undamaged Achilles tendons. Blocking IGF-1 activity with IGF-1 antibody or IGF-1 receptor inhibitor picropodophyllin significantly inhibited HO formation. Mechanistically, IGF-1/IGF-1 receptor activates phosphatidylinositol 3-kinase (PI3K)/Akt signaling to promote the phosphorylation of mTOR, resulting in the chondrogenic and osteogenic differentiation of tendon-derived stem cells into chondrocytes and osteoblasts in vitro and in vivo. Inhibitors of PI3K (LY294002) and mTOR (rapamycin) both suppressed the IGF-1-stimulated mTOR signal and mitigated the formation of ectopic bones significantly. In conclusion, these results indicate that IGF-1 mediated the progression of traumatic HO through PI3K/Akt/mTOR signaling, and suppressing IGF-1 signaling cascades attenuated HO formation, providing a promising therapeutic strategy targeting HO.

The HIF-1α and mTOR Pathways Amplify Heterotopic Ossification

Fibrodysplasia ossificans progressiva (FOP; MIM# 135100) is an ultra-rare congenital disorder caused by gain-of-function point mutations in the Activin receptor A type I (ACVR1, also known as ALK2) gene. FOP is characterized by episodic heterotopic ossification (HO) in skeletal muscles, tendons, ligaments, or other soft tissues that progressively causes irreversible loss of mobility. FOP mutations cause mild ligand-independent constitutive activation as well as ligand-dependent bone morphogenetic protein (BMP) pathway hypersensitivity of mutant ACVR1. BMP signaling is also a key pathway for mediating acquired HO. However, HO is a highly complex biological process involving multiple interacting signaling pathways. Among them, the hypoxia-inducible factor (HIF) and mechanistic target of rapamycin (mTOR) pathways are intimately involved in both genetic and acquired HO formation. HIF-1α inhibition or mTOR inhibition reduces HO formation in mouse models of FOP or acquired HO in part by de-amplifying the BMP pathway signaling. Here, we review the recent progress on the mechanisms of the HIF-1α and mTOR pathways in the amplification of HO lesions and discuss the future directions and strategies to translate the targeting of HIF-1α and the mTOR pathways into clinical interventions for FOP and other forms of HO.

Context-dependent TGFβ family signalling in cell fate regulation

The transforming growth factor-β (TGFβ) family are a large group of evolutionarily conserved cytokines whose signalling modulates cell fate decision-making across varying cellular contexts at different stages of life. Here we discuss new findings in early embryos that reveal how, in contrast to our original understanding of morphogen interpretation, robust cell fate specification can originate from a noisy combination of signalling inputs and a broad range of signalling levels. We compare this evidence with novel findings on the roles of TGFβ family signalling in tissue maintenance and homeostasis during juvenile and adult life, spanning the skeletal, haemopoietic and immune systems. From these comparisons, it emerges that in contrast to robust developing systems, relatively small perturbations in TGFβ family signalling have detrimental effects at later stages in life, leading to aberrant cell fate specification and disease, for example in cancer or congenital disorders. Finally, we highlight novel strategies to target and amend dysfunction in signalling and discuss how gleaning knowledge from different fields of biology can help in the development of therapeutics for aberrant TGFβ family signalling in disease.

Human iPSCs as Model Systems for BMP-Related Rare Diseases

Disturbances in bone morphogenetic protein (BMP) signalling contribute to onset and development of a number of rare genetic diseases, including Fibrodysplasia ossificans progressiva (FOP), Pulmonary arterial hypertension (PAH), and Hereditary haemorrhagic telangiectasia (HHT). After decades of animal research to build a solid foundation in understanding the underlying molecular mechanisms, the progressive implementation of iPSC-based patient-derived models will improve drug development by addressing drug efficacy, specificity, and toxicity in a complex humanized environment. We will review the current state of literature on iPSC-derived model systems in this field, with special emphasis on the access to patient source material and the complications that may come with it. Given the essential role of BMPs during embryonic development and stem cell differentiation, gain- or loss-of-function mutations in the BMP signalling pathway may compromise iPSC generation, maintenance, and differentiation procedures. This review highlights the need for careful optimization of the protocols used. Finally, we will discuss recent developments towards complex in vitro culture models aiming to resemble specific tissue microenvironments with multi-faceted cellular inputs, such as cell mechanics and ECM together with organoids, organ-on-chip, and microfluidic technologies.

TNFα-dependent mTOR activity is required for tenotomy-induced ectopic ossification in mice

INTRODUCTION

Ectopic ossifications often occur in skeletal muscles or tendons following local trauma or internal hemorrhage, and occasionally cause severe pain that limits activities of daily living. However, mechanisms underlying their development remain unknown.

MATERIALS AND METHODS

The right Achilles tendon in 8-week-old female or male mice was dissected. Some mice were injected intraperitoneally either with phosphate-buffered saline, dimethyl sulfoxide, cimetidine, rapamycin, celecoxib or loxoprofen for 10 weeks. One week after surgery, immunohistochemical analysis was performed for mTOR, TNFα or F4/80. Ten weeks after surgery, ectopic ossification at the tenotomy site was detected by 3D micro-CT.

RESULTS

Ectopic ossification was seen at dissection sites in all wild-type mice by dissection of the Achilles tendon. mTOR activation was detected at dissection sites, and development of ectopic ossification was significantly inhibited by administration of rapamycin, an mTOR inhibitor, to wild-type mice. Moreover, administration of the histamine 2 blocker cimetidine, which reportedly inhibits ectopic ossification in tendons, was not effective in inhibiting ectopic ossification in our models. TNFα-expressing F4/80-positive macrophages accumulate at dissection sites and that ectopic ossification of the Achilles tendon dissection was significantly inhibited in TNFα-deficient mice in vivo. Ectopic ossification is significantly inhibited by administration of either celecoxib or loxoprofen, both anti-inflammatory agents, in wild-type mice. mTOR activation by Achilles tendon tenotomy is inhibited in TNFα-deficient mice.

CONCLUSION

The TNFα-mTOR axis could be targeted therapeutically to prevent trauma-induced ectopic ossification in tendons.

Induction of functional xeno-free MSCs from human iPSCs via a neural crest cell lineage

Mesenchymal stem/stromal cells (MSCs) are adult multipotent stem cells. Here, we induced MSCs from human induced pluripotent stem cells (iPSCs) via a neural crest cell (NCC) lineage under xeno-free conditions and evaluated their in vivo functions. We modified a previous MSC induction method to work under xeno-free conditions. Bovine serum albumin-containing NCC induction medium and fetal bovine serum-containing MSC induction medium were replaced with xeno-free medium. Through our optimized method, iPSCs differentiated into MSCs with high efficiency. To evaluate their in vivo activities, we transplanted the xeno-free-induced MSCs (XF-iMSCs) into mouse models for bone and skeletal muscle regeneration and confirmed their regenerative potency. These XF-iMSCs mainly promoted the regeneration of surrounding host cells, suggesting that they secrete soluble factors into affected regions. We also found that the peroxidasin and IGF2 secreted by the XF-iMSCs partially contributed to myotube differentiation. These results suggest that XF-iMSCs are important for future applications in regenerative medicine.

Gene Therapy for Fibrodysplasia Ossificans Progressiva: Feasibility and Obstacles

Fibrodysplasia ossificans progressiva (FOP) is a rare and devastating genetic disease, in which soft connective tissue is converted into heterotopic bone through an endochondral ossification process. Patients succumb early as they gradually become trapped in a second skeleton of heterotopic bone. Although the underlying genetic defect is long known, the inherent complexity of the disease has hindered the discovery of effective preventions and treatments. New developments in the gene therapy field have motivated its consideration as an attractive therapeutic option for FOP. However, the immune system's role in FOP activation and the as-yet unknown primary causative cell, are crucial issues which must be taken into account in the therapy design. While gene therapy offers a potential therapeutic solution, more knowledge about FOP is needed to enable its optimal and safe application.

Contemporary perspectives on heterotopic ossification

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

Developmental principles informing human pluripotent stem cell differentiation to cartilage and bone

Human pluripotent stem cells can differentiate into any cell type given appropriate signals and hence have been used to research early human development of many tissues and diseases. Here, we review the major biological factors that regulate cartilage and bone development through the three main routes of neural crest, lateral plate mesoderm and paraxial mesoderm. We examine how these routes have been used in differentiation protocols that replicate skeletal development using human pluripotent stem cells and how these methods have been refined and improved over time. Finally, we discuss how pluripotent stem cells can be employed to understand human skeletal genetic diseases with a developmental origin and phenotype, and how developmental protocols have been applied to gain a better understanding of these conditions.

Pathophysiology and Emerging Molecular Therapeutic Targets in Heterotopic Ossification

The term heterotopic ossification (HO) describes bone formation in tissues where bone is normally not present. Musculoskeletal trauma induces signalling events that in turn trigger cells, probably of mesenchymal origin, to differentiate into bone. The aetiology of HO includes extremely rare but severe, generalised and fatal monogenic forms of the disease; and as a common complex disorder in response to musculoskeletal, neurological or burn trauma. The resulting bone forms through a combination of endochondral and intramembranous ossification, depending on the aetiology, initiating stimulus and affected tissue. Given the heterogeneity of the disease, many cell types and biological pathways have been studied in efforts to find effective therapeutic strategies for the disorder. Cells of mesenchymal, haematopoietic and neuroectodermal lineages have all been implicated in the pathogenesis of HO, and the emerging dominant signalling pathways are thought to occur through the bone morphogenetic proteins (BMP), mammalian target of rapamycin (mTOR), and retinoic acid receptor pathways. Increased understanding of these disease mechanisms has resulted in the emergence of several novel investigational therapeutic avenues, including palovarotene and other retinoic acid receptor agonists and activin A inhibitors that target both canonical and non-canonical signalling downstream of the BMP type 1 receptor. In this article we aim to illustrate the key cellular and molecular mechanisms involved in the pathogenesis of HO and outline recent advances in emerging molecular therapies to treat and prevent HO that have had early success in the monogenic disease and are currently being explored in the common complex forms of HO.

Monitoring and Management of Fibrodysplasia Ossificans Progressiva: Current Perspectives

Fibrodysplasia ossificans progressiva (FOP), sometimes known as myositis ossificans progressiva, is an ultra-rare disease in which bone is formed in muscular tissue, tendons and ligaments. This is known as heterotopic ossification (HO). FOP is caused by a heterozygous mutation in the highly conserved ACVR1/ALK2 gene which affects about 1 in 1.5–2 million individuals. At birth, patients with the predominant R206H mutation only exhibit a bilateral hallux valgus. During childhood, heterotopic bone formation develops in a typical pattern, affecting the axial muscles first before appendicular body parts are involved. HO can start spontaneously but is often elicited by soft tissue trauma or medical procedures. After soft tissue injury, an inflammatory process called a flare-up can start, followed by the formation of HO. HO leads to a limited range of motion, culminating in complete ankylosis of nearly all joints. As a result of HO surrounding the thorax, patients often suffer from thoracic insufficiency syndrome (TIS). TIS is the most common cause of a limited life expectancy for FOP patients, with a median life expectancy of 56 years. Management is focused on preventing soft-tissue injury that can provoke flare-ups. This includes prevention of iatrogenic damage by biopsies, intramuscular injections and surgery. Anti-inflammatory medication is often started when a flare-up occurs but has a poor basis of evidence. Several forms of potential treatment for FOP are being researched in clinical trials. Progression of the disease is monitored using CT and 18F-NaF PET/CT combined with functional assessments. Patients are regularly evaluated for frequently occurring complications such as restrictive lung disease. Here, we review the current management, monitoring and treatment of FOP.

Differentiation of Induced Pluripotent Stem Cells Into Chondrocytes: Methods and Applications for Disease Modeling and Drug Discovery

Induced pluripotent stem cell (iPSC) technology allows pathomechanistic and therapeutic investigation of human heritable disorders affecting tissue types whose collection from patients is difficult or even impossible. Among them are cartilage diseases. Over the past decade, iPSC-chondrocyte disease models have been shown to exhibit several key aspects of known disease mechanisms. Concurrently, an increasing number of protocols to differentiate iPSCs into chondrocytes have been published, each with its respective (dis)advantages. In this review we provide a comprehensive overview of the different differentiation approaches, the hitherto described iPSC-chondrocyte disease models and mechanistic and/or therapeutic insights that have been derived from their investigation, and the current model limitations. Key lessons are that the most appropriate differentiation approach is dependent upon the cartilage disease under investigation and that further optimization is still required to recapitulate the in vivo cartilage. © 2022 American Society for Bone and Mineral Research (ASBMR).

Allele-Selective Locked Nucleic Acid Gapmers for the Treatment of Fibrodysplasia Ossificans Progressiva Knock Down the Pathogenic ACVR1R206H Transcript and Inhibit Osteogenic Differentiation

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder characterized by episodic heterotopic ossification. The median life span of people with this disorder is ∼40 years, and currently, there is no effective treatment available. More than 95% of cases are caused by a recurrent mutation (c.617G>A; R206H) of Activin A receptor, type I (ACVR1)/Activin receptor-like kinase-2 (ALK2), a bone morphogenetic protein type I receptor. The mutation renders ACVR1 responsive to activin A, which does not activate wild-type ACVR1. Ectopic activation of ACVR1^R206H by activin A induces heterotopic ossification. Since ACVR1^R206H is a hyperactive receptor, a promising therapeutic strategy is to decrease the activity of mutated ACVR1. To accomplish this goal, we developed locked nucleic acid (LNA) gapmers. These are short DNA oligonucleotides with LNA modification at both ends. They induce targeted mRNA degradation and specific knockdown of gene expression. We demonstrated that some of these gapmers efficiently knocked down ACVR1^R206H expression at RNA levels, while ACVR1^WT was mostly unaffected in human FOP fibroblasts. Also, the gapmers suppressed osteogenic differentiation induced by ACVR1^R206H and activin A. These gapmers may be promising drug candidates for FOP. This novel strategy will also pave the way for antisense-mediated therapy of other autosomal dominant disorders.

Development of a Simple Spheroid Production Method Using Fluoropolymers with Reduced Chemical and Physical Damage

Establishing an in vitro–based cell culture system that can realistically simulate in vivo cell dynamics is desirable. It is thus necessary to develop a method for producing a large amount of cell aggregates (i.e., spheroids) that are uniform in size and quality. Various methods have been proposed for the preparation of spheroids; however, none of them satisfy all requirements, such as cost, size uniformity, and throughput. Herein, we successfully developed a new cell culture method by combining fluoropolymers and dot patterned extracellular matrix substrates to achieve size-controlled spheroids. First, the spheroids were spontaneously formed by culturing them two-dimensionally, after which the cells were detached with a weak liquid flow and cultured in suspension without enzyme treatment. Stable quality spheroids were easily produced, and it is expected that the introduction and running costs of the technique will be low; therefore, this method shows potential for application in the field of regenerative medicine.

Macrophages in heterotopic ossification: from mechanisms to therapy

Heterotopic ossification (HO) is the formation of extraskeletal bone in non-osseous tissues. It is caused by an injury that stimulates abnormal tissue healing and regeneration, and inflammation is involved in this process. It is worth noting that macrophages are crucial mediators of inflammation. In this regard, abundant macrophages are recruited to the HO site and contribute to HO progression. Macrophages can acquire different functional phenotypes and promote mesenchymal stem cell (MSC) osteogenic differentiation, chondrogenic differentiation, and angiogenesis by expressing cytokines and other factors such as the transforming growth factor-β1 (TGF-β1), bone morphogenetic protein (BMP), activin A (Act A), oncostatin M (OSM), substance P (SP), neurotrophin-3 (NT-3), and vascular endothelial growth factor (VEGF). In addition, macrophages significantly contribute to the hypoxic microenvironment, which primarily drives HO progression. Thus, these have led to an interest in the role of macrophages in HO by exploring whether HO is a "butterfly effect" event. Heterogeneous macrophages are regarded as the "butterflies" that drive a sequence of events and ultimately promote HO. In this review, we discuss how the recruitment of macrophages contributes to HO progression. In particular, we review the molecular mechanisms through which macrophages participate in MSC osteogenic differentiation, angiogenesis, and the hypoxic microenvironment. Understanding the diverse role of macrophages may unveil potential targets for the prevention and treatment of HO.

Leveraging the Genetic Diversity of Human Stem Cells in Therapeutic Approaches

Since their discovery 15 years ago, human pluripotent stem cell (hPSC) technologies have begun to revolutionize science and medicine, rapidly expanding beyond investigative research to drug discovery and development. Efforts to leverage hPSCs over the last decade have focused on increasing both the complexity and in vivo fidelity of human cellular models through enhanced differentiation methods. While these evolutions have fostered novel insights into disease mechanisms and influenced clinical drug discovery and development, there are still several considerations that limit the utility of hPSC models. In this review, we highlight important, yet underexplored avenues to broaden their reach. We focus on (i) the importance of diversifying existing hPSC collections, and their utilization to investigate therapeutic strategies in individuals from different genetic backgrounds, ancestry and sex; (ii) considerations for the selection of therapeutically relevant hPSC-based models; (iii) strategies to adequately increase the scale of cell-based studies; and (iv) the advances and constraints of clinical trials in a dish. Moreover, we advocate for harnessing the translational capabilities of hPSC models along with the use of innovative, scalable approaches for understanding genetic biases and the impact of sex and ancestry on disease mechanisms and drug efficacy and response. The next decade of hPSC innovation is poised to provide vast insights into the genetic basis of human disease and enable rapid advances to develop, repurpose, and ensure the safety of the next generation of disease therapies across diverse human populations.

Do Interactions of Vitamin D3 and BMP Signaling Hold Implications in the Pathogenesis of Fibrodysplasia Ossificans Progressiva?

PURPOSE OF REVIEW

Fibrodysplasia ossificans progressiva (FOP) is a debilitating rare disease known for episodic endochondral heterotopic ossification (HO) caused by gain-of-function mutations in ACVR1/ALK2. However, disease severity varies among patients with identical mutations suggesting disease-modifying factors, including diet, may have therapeutic implications. The roles of vitamin D₃ in calcium metabolism and chondrogenesis are known, but its effects on BMP signaling and chondrogenesis are less studied. This review attempts to assess the possibility of vitamin D's effects in FOP by exploring relevant intersections of VD₃ with mechanisms of FOP flares.

RECENT FINDINGS

In vitro and in vivo studies suggest vitamin D suppresses inflammation, while clinical studies suggest that vitamin D₃ protects against arteriosclerosis and inversely correlates with non-genetic intramuscular HO. However, the enhancement of chondrogenesis, BMP signaling, and possibly Activin A expression by vitamin D may be more relevant in FOP. There appears to be little potential for vitamin D to reduce HO in FOP, but testing the potential for excess vitamin D to promote HO may be warranted.

High Frequency Spectral Ultrasound Imaging Detects Early Heterotopic Ossification in Rodents

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

Saracatinib is an efficacious clinical candidate for fibrodysplasia ossificans progressiva

Currently, no effective therapies exist for fibrodysplasia ossificans progressiva (FOP), a rare congenital syndrome in which heterotopic bone is formed in soft tissues owing to dysregulated activity of the bone morphogenetic protein (BMP) receptor kinase ALK2 (also known as ACVR1). From a screen of known biologically active compounds, we identified saracatinib as a potent ALK2 kinase inhibitor. In enzymatic and cell-based assays, saracatinib preferentially inhibited ALK2, compared with other receptors of the BMP/TGF-β signaling pathway, and induced dorsalization in zebrafish embryos consistent with BMP antagonism. We further tested the efficacy of saracatinib using an inducible ACVR1^Q207D-transgenic mouse line, which provides a model of heterotopic ossification (HO), as well as an inducible ACVR1^R206H-knockin mouse, which serves as a genetically and physiologically faithful FOP model. In both models, saracatinib was well tolerated and potently inhibited the development of HO, even when administered transiently following soft tissue injury. Together, these data suggest that saracatinib is an efficacious clinical candidate for repositioning in FOP treatment, offering an accelerated path to clinical proof-of-efficacy studies and potentially significant benefits to individuals with this devastating condition.

Establishment of a Robust Platform for Induced Pluripotent Stem Cell Research Using Maholo LabDroid

Induced pluripotent stem cells (iPSCs) are attractive for use in early drug discovery because they can differentiate into any cell type. Maintenance cultures and differentiation processes for iPSCs, however, require a high level of technical expertise. To overcome this problem, technological developments such as enhanced automation are necessary to replace manual operation. In addition, a robot system with the flexibility and expandability to carry out maintenance culture and each of the required differentiation processes would also be important. In this study, we established a platform to enable the multiple processes required for iPSC experiments using the Maholo LabDroid, which is a humanoid robotic system with excellent reproducibility and flexibility. The accuracy and robustness of Maholo LabDroid enabled us to cultivate undifferentiated iPSCs for 63 days while maintaining their ability to differentiate into the three embryonic germ layers. Maholo LabDroid maintained and harvested iPSCs in six-well plates, then seeded them into 96-well plates, induced differentiation, and implemented immunocytochemistry. As a result, Maholo LabDroid was confirmed to be able to perform the processes required for myogenic differentiation of iPSCs isolated from a patient with muscular disease and achieved a high differentiation rate with a coefficient of variation (CV) <10% in the first trial. Furthermore, the expandability and flexibility of Maholo LabDroid allowed us to experiment with multiple cell lines simultaneously.

Challenges and Opportunities for Drug Repositioning in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare congenital disease that progresses through intermittent episodes of bone formation at ectopic sites. FOP patients carry heterozygous gene point mutations in activin A receptor type I ACVR1, encoding the bone morphogenetic protein (BMP) type I serine/threonine kinase receptor ALK2, termed activin receptor-like kinase (ALK)2. The mutant ALK2 displays neofunctional responses to activin, a closely related BMP cytokine that normally inhibits regular bone formation. Moreover, the mutant ALK2 becomes hypersensitive to BMPs. Both these activities contribute to enhanced ALK2 signalling and endochondral bone formation in connective tissue. Being a receptor with an extracellular ligand-binding domain and intrinsic intracellular kinase activity, the mutant ALK2 is a druggable target. Although there is no approved cure for FOP yet, a number of clinical trials have been recently initiated, aiming to identify a safe and effective treatment for FOP. Among other targeted approaches, several repurposed drugs have shown promising results. In this review, we describe the molecular mechanisms underlying ALK2 mutation-induced aberrant signalling and ectopic bone formation. In addition, we recapitulate existing in vitro models to screen for novel compounds with a potential application in FOP. We summarize existing therapeutic alternatives and focus on repositioned drugs in FOP, at preclinical and clinical stages.

ActivinA Induced SMAD1/5 Signaling in an iPSC Derived EC Model of Fibrodysplasia Ossificans Progressiva (FOP) Can Be Rescued by the Drug Candidate Saracatinib

Balanced signal transduction is crucial in tissue patterning, particularly in the vasculature. Heterotopic ossification (HO) is tightly linked to vascularization with increased vessel number in hereditary forms of HO, such as Fibrodysplasia ossificans progressiva (FOP). FOP is caused by mutations in the BMP type I receptor ACVR1 leading to aberrant SMAD1/5 signaling in response to ActivinA. Whether observed vascular phenotype in human FOP lesions is connected to aberrant ActivinA signaling is unknown. Blocking of ActivinA prevents HO in FOP mice indicating a central role of the ligand in FOP. Here, we established a new FOP endothelial cell model generated from induced pluripotent stem cells (iECs) to study ActivinA signaling. FOP iECs recapitulate pathogenic ActivinA/SMAD1/5 signaling. Whole transcriptome analysis identified ActivinA mediated activation of the BMP/NOTCH pathway exclusively in FOP iECs, which was rescued to WT transcriptional levels by the drug candidate Saracatinib. We propose that ActivinA causes transcriptional pre-patterning of the FOP endothelium, which might contribute to differential vascularity in FOP lesions compared to non-hereditary HO.

Fibrodysplasia Ossificans Progressiva: What Have We Achieved and Where Are We Now? Follow-up to the 2015 Lorentz Workshop

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare progressive genetic disease effecting one in a million individuals. During their life, patients with FOP progressively develop bone in the soft tissues resulting in increasing immobility and early death. A mutation in the ACVR1 gene was identified as the causative mutation of FOP in 2006. After this, the pathophysiology of FOP has been further elucidated through the efforts of research groups worldwide. In 2015, a workshop was held to gather these groups and discuss the new challenges in FOP research. Here we present an overview and update on these topics.

Nesfatin-1 Promotes the Osteogenic Differentiation of Tendon-Derived Stem Cells and the Pathogenesis of Heterotopic Ossification in Rat Tendons via the mTOR Pathway

Heterotopic ossification (HO) is a pathological condition involved in tendinopathy. Adipokines are known to play a key role in HO of tendinopathy. Nesfatin-1, an 82-amino acid adipokine is closely reportedly associated with diabetes mellitus (DM), which, in turn, is closely related to tendinopathy. In the present study, we aimed to investigate the effects of nesfatin-1 on the osteogenic differentiation of tendon-derived stem cells (TDSCs) and the pathogenesis of tendinopathy in rats. In vitro, TDSCs were incubated in osteogenic induction medium for 14 days with different nesfatin-1 concentration. In vivo, Sprague Dawley rats underwent Achilles tenotomy to evaluate the effect of nesfatin-1 on tendinopathy. Our results showed that the expression of nesfatin-1 expression in tendinopathy patients was significantly higher than that in healthy subjects. Nesfatin-1 affected the cytoskeleton and reduced the migration ability of TDSCs in vitro. Furthermore, nesfatin-1 inhibited the expression of Scx, Mkx, and Tnmd and promoted the expression of osteogenic genes, such as COL1a1, ALP, and RUNX2; these results suggested that nesfatin-1 inhibits cell migration, adversely impacts tendon phenotype, promotes osteogenic differentiation of TDSCs and the pathogenesis of HO in rat tendons. Moreover, we observed that nesfatin-1 suppressed autophagy and activated the mammalian target of rapamycin (mTOR) pathway both in vitro and in vivo. The suppression of the mTOR pathway alleviated nesfatin-1-induced HO development in rat tendons. Thus, nesfatin-1 promotes the osteogenic differentiation of TDSC and the pathogenesis of HO in rat tendons via the mTOR pathway; these findings highlight a new potential therapeutic target for tendinopathy.

Metformin attenuates trauma-induced heterotopic ossification via inhibition of Bone Morphogenetic Protein signalling

AMP‐activated protein kinase (AMPK) is an intracellular sensor of energy homoeostasis that is activated under energy stress and suppressed in energy surplus. AMPK activation leads to inhibition of anabolic processes that consume ATP. Osteogenic differentiation is a process that highly demands ATP during which AMPK is inhibited. The bone morphogenetic proteins (BMPs) signalling pathway plays an essential role in osteogenic differentiation. The present study examines the inhibitory effect of metformin on BMP signalling, osteogenic differentiation and trauma‐induced heterotopic ossification. Our results showed that metformin inhibited Smad1/5 phosphorylation induced by BMP6 in osteoblast MC3T3‐E1 cells, concurrent with up‐regulation of Smad6, and this effect was attenuated by knockdown of Smad6. Furthermore, we found that metformin suppressed ALP activity and mineralization of the cells, an event that was attenuated by the dominant negative mutant of AMPK and mimicked by its constitutively active mutant. Finally, administration of metformin prevented the trauma‐induced heterotopic ossification in mice. In conjuncture, AMPK activity and Smad6 and Smurf1 expression were enhanced by metformin treatment in the muscle of injured area, concurrently with the reduction of ALK2. Collectively, our study suggests that metformin prevents heterotopic ossification via activation of AMPK and subsequent up‐regulation of Smad6. Therefore, metformin could be a potential therapeutic drug for heterotopic ossification induced by traumatic injury.

Higher BMP Expression in Tendon Stem/Progenitor Cells Contributes to the Increased Heterotopic Ossification in Achilles Tendon With Aging

Although the mineralization in tendon tissue has been reported in a series of aging and disease models, the underlying mechanisms remain unknown. This study aimed to describe the appearance of heterotopic ossification in rat Achilles tendon and further verify whether this tissue metaplasia is related to the enhanced osteogenic differentiation of tendon stem/progenitor cells (TSPCs) owing to the higher expression of bone morphogenetic proteins (BMP-2/4/7) with aging. The male SD rats, aged 4, 8, and 20 months (M), were used. The analyses of ossification and BMP expression in tendon were tested by radiological view (X-ray and CT), histological staining [hematoxylin and eosin (HE), Alcian blue, and Alizarin red], immunohistochemistry, and Western blot. The osteogenic differentiation potential and BMP expression of TSPCs were examined by Alizarin red S staining and real-time PCR. TSPCs were treated with BMP-2 or noggin, and the osteogenic differentiation potential was also examined. X-ray and CT showed the appearance of heterotopic ossification in tendon, and the volume and density of ossification was increased with aging. Histological staining showed the appearance of calcified region surrounded by chondrocyte-like cells and the increased osteogenesis-related gene and BMP expression in ossified tendon with aging. Moreover, the osteogenic differentiation potential and BMP expression in TSPCs isolated from ossified tendon were increased with aging. Additionally, BMP-2 increased the calcium nodule formation and osteogenesis-related gene expression in TSPCs. The addition of noggin inhibited BMP-induced enhancement of osteogenic differentiation. Thus, these findings suggested that the enhanced osteogenic differentiation of TSPCs contributes to the increased heterotopic ossification in aged tendon, which might be induced by the higher expression of BMPs with aging.

Characterization of primary cilia features reveal cell-type specific variability in in vitro models of osteogenic and chondrogenic differentiation

Primary cilia are non-motile sensory antennae present on most vertebrate cell surfaces. They serve to transduce and integrate diverse external stimuli into functional cellular responses vital for development, differentiation and homeostasis. Ciliary characteristics, such as length, structure and frequency are often tailored to distinct differentiated cell states. Primary cilia are present on a variety of skeletal cell-types and facilitate the assimilation of sensory cues to direct skeletal development and repair. However, there is limited knowledge of ciliary variation in response to the activation of distinct differentiation cascades in different skeletal cell-types. C3H10T1/2, MC3T3-E1 and ATDC5 cells are mesenchymal stem cells, preosteoblast and prechondrocyte cell-lines, respectively. They are commonly employed in numerous in vitro studies, investigating the molecular mechanisms underlying osteoblast and chondrocyte differentiation, skeletal disease and repair. Here we sought to evaluate the primary cilia length and frequencies during osteogenic differentiation in C3H10T1/2 and MC3T3-E1 and chondrogenic differentiation in ATDC5 cells, over a period of 21 days. Our data inform on the presence of stable cilia to orchestrate signaling and dynamic alterations in their features during extended periods of differentiation. Taken together with existing literature these findings reflect the occurrence of not only lineage but cell-type specific variation in ciliary attributes during differentiation. These results extend our current knowledge, shining light on the variabilities in primary cilia features correlated with distinct differentiated cell phenotypes. It may have broader implications in studies using these cell-lines to explore cilia dependent cellular processes and treatment modalities for skeletal disorders centered on cilia modulation.

Clinical Aspects and Current Therapeutic Approaches for FOP

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare heritable disorder of connective tissues characterized by progressive heterotopic ossification in various skeletal sites. It is caused by gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1)/activin-like kinase 2 (ALK2), a bone morphogenetic protein (BMP) type I receptor. Heterotopic ossification is usually progressive leading to severe deformities in the trunk and extremities. Early clinical diagnosis is important to prevent unnecessary iatrogenic harm or trauma. Clinicians should become aware of early detectable skeletal malformations, including great toe deformities, shortened thumb, neck stiffness associated with hypertrophy of the posterior elements of the cervical spine, multiple ossification centers in the calcaneus, and osteochondroma-like lesions of the long bones. Although there is presently no definitive medical treatment to prevent, stop or reverse heterotopic ossification in FOP, exciting advances of novel pharmacological drugs focusing on target inhibition of the activated ACVR1 receptor, including palovarotene, REGN 2477, rapamycin, and saracatinib, have developed and are currently in clinical trials.

Bone morphogenetic protein receptors: Structure, function and targeting by selective small molecule kinase inhibitors

Bone morphogenetic proteins (BMPs) are secreted cytokines that control the fate and function of many different cell types. They exert their cellular responses via heteromeric complexes of specific BMP type I and type II serine/threonine kinase receptors, e.g. BMPRIA and BMPRII. Three type II and four type I receptors, also termed activin receptor-like kinases (ALKs), have been identified. The constitutively active type II kinase phosphorylates the type I receptor, which upon activation initiates intracellular signaling by phosphorylating SMAD effectors. Auxiliary cell surface receptors without intrinsic enzymatic motifs, such as Endoglin and Repulsive guidance molecules (RGM), can fine-tune signaling by regulating the interaction of the BMP ligands with the BMPRs. The functional annotation of the BMPR encoding genes has helped to understand underlying mechanisms of diseases in which these genes are mutated. Loss of function mutations in BMPRII, Endoglin or RGMc are causally linked to pulmonary arterial hypertension, hereditary hemorrhagic telangiectasia and juvenile hemochromatosis, respectively. In contrast, gain of function mutations in ACVR1, encoding ALK2, are linked to Fibrodysplasia ossificans progressiva and diffuse intrinsic pontine glioma. Here, we discuss BMPR identification, structure and function in health and disease. Moreover, we highlight the therapeutic promise of small chemical compounds that act as selective BMPR kinase inhibitors to normalize overactive BMPR signaling.

The Survey of Cells Responsible for Heterotopic Ossification Development in Skeletal Muscles-Human and Mouse Models

Heterotopic ossification (HO) manifests as bone development in the skeletal muscles and surrounding soft tissues. It can be caused by injury, surgery, or may have a genetic background. In each case, its development might differ, and depending on the age, sex, and patient's conditions, it could lead to a more or a less severe outcome. In the case of the injury or surgery provoked ossification development, it could be, to some extent, prevented by treatments. As far as genetic disorders are concerned, such prevention approaches are highly limited. Many lines of evidence point to the inflammatory process and abnormalities in the bone morphogenetic factor signaling pathway as the molecular and cellular backgrounds for HO development. However, the clear targets allowing the design of treatments preventing or lowering HO have not been identified yet. In this review, we summarize current knowledge on HO types, its symptoms, and possible ways of prevention and treatment. We also describe the molecules and cells in which abnormal function could lead to HO development. We emphasize the studies involving animal models of HO as being of great importance for understanding and future designing of the tools to counteract this pathology.

Prophylactic treatment of rapamycin ameliorates naturally developing and episode -induced heterotopic ossification in mice expressing human mutant ACVR1

Background

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal-dominant disease characterized by heterotopic ossification (HO) in soft tissues and caused by a mutation of the ACVR1A/ALK2 gene. Activin-A is a key molecule for initiating the process of HO via the activation of mTOR, while rapamycin, an mTOR inhibitor, effectively inhibits the Activin-A-induced HO. However, few reports have verified the effect of rapamycin on FOP in clinical perspectives.

Methods

We investigated the effect of rapamycin for different clinical situations by using mice conditionally expressing human mutant ACVR1A/ALK2 gene. We also compared the effect of rapamycin between early and episode-initiated treatments for each situation.

Results

Continuous, episode-independent administration of rapamycin reduced the incidence and severity of HO in the natural course of FOP mice. Pinch-injury induced HO not only at the injured sites, but also in the contralateral limbs and provoked a prolonged production of Activin-A in inflammatory cells. Although both early and injury-initiated treatment of rapamycin suppressed HO in the injured sites, the former was more effective at preventing HO in the contralateral limbs. Rapamycin was also effective at reducing the volume of recurrent HO after the surgical resection of injury-induced HO, for which the early treatment was more effective.

Conclusion

Our study suggested that prophylactic treatment will be a choice of method for the clinical application of rapamycin for FOP.

Targeted pathological collagen delivery of sustained-release rapamycin to prevent heterotopic ossification

Heterotopic ossification (HO) in connective tissues like tendons and ligaments severely damages tissue structure. The pathogenesis of HO remains unclear but may involve mTOR. The results presented here indicate that tendon stem/progenitor cells do not undergo osteochondrogenic differentiation when mTOR signaling is inactivated by gene knockout or rapamycin (RAPA) treatment. Meanwhile, it is necessary to deliver RAPA to the injured sites and avoid disturbing the normal tendon. A RAPA delivery system, developed using collagen hybrid peptide (CHP) to modify the surface of poly(lactic-co-glycolic acid) (PLGA) nanoparticles, targeted RAPA specifically to pathological tendon collagen. The CHP-PLGA-RAPA nanoparticles showed excellent pathological collagen affinity, sustained-release ability, and bioactivity. In a mouse model of tendon HO, CHP-PLGA-RAPA nanoparticles specifically bound to pathological tendon and strongly suppressed HO progression. The mTOR signaling pathway appears to be a viable therapeutic target for tendon HO, and CHP-PLGA nanoparticles may be valuable for the treatment of tendon-related diseases.

ALK2: A Therapeutic Target for Fibrodysplasia Ossificans Progressiva and Diffuse Intrinsic Pontine Glioma

Fibrodysplasia ossificans progressiva (FOP) and diffuse intrinsic pontine glioma (DIPG) are diseases that typically manifest in childhood and are associated with severely reduced life expectancy. However, there are currently no effective therapies for these diseases, which remain incurable. Activin receptor-like kinase-2 (ALK2), encoded by the ACVR1 gene, is a bone morphogenetic protein (BMP) type-I receptor subtype that plays an important physiological role in the development of bones, muscles, brain, and other organs. Constitutively active mutants of ALK2 have been identified as causative of FOP and involved in the tumorigenesis of DIPG owing to abnormal activation of BMP signaling, and therefore have emerged as promising treatment targets. Here, we describe these two diseases, along with the link to ALK2 signal transduction, and highlight potential ALK2 inhibitors that are under development to offer new hope for patients with FOP and DIPG.

When Limb Surgery Has Become the Only Life-Saving Therapy in FOP: A Case Report and Systematic Review of the Literature

Fibrodysplasia ossificans progressiva (FOP) is a rare disease in which heterotopic ossification (HO) is formed in muscles, tendons and ligaments. Traumatic events, including surgery, are discouraged as this is known to trigger a flare-up with risk of subsequent HO. Anesthetic management for patients with FOP is challenging. Cervical spine fusion, ankylosis of the temporomandibular joints, thoracic insufficiency syndrome, restrictive chest wall disease, and sensitivity to oral trauma complicate airway management and anesthesia and pose life-threatening risks. We report a patient with FOP suffering from life-threatening antibiotic resistant bacterial infected ulcers of the right lower leg and foot. The anesthetic, surgical and postoperative challenges and considerations are discussed. In addition, the literature on limb surgeries of FOP patients is systemically reviewed. The 44 year-old female patient was scheduled for a through-knee amputation. Airway and pulmonary evaluation elicited severe abnormalities, rendering standard general anesthesia a rather complication-prone approach in this patient. Thus, regional anesthesia, supplemented with intravenous analgosedation and N₂O-inhalation were performed in this case. The surgery itself was securely planned to avoid any unnecessary tissue damage. Postoperatively the patient was closely monitored for FOP activity by ultrasound and [¹⁸F]PET/CT-scan. One year after surgery, a non-significant amount of HO had formed at the operated site. The systematic review revealed seventeen articles in which thirty-two limb surgeries in FOP patients were described. HO reoccurrence was described in 90% of the cases. Clinical improvement due to improved mobility of the operated joint was noted in 16% of the cases. It should be noted, though, that follow-up time was limited and no or inadequate imaging modalities were used to follow-up in the majority of these cases. To conclude, if medically urgent, limb surgery in FOP is possible even when general anesthesia is not preferred. The procedure should be well-planned, alternative techniques or procedures should be tested prior to surgery and special attention should be paid to the correct positioning of the patient. According to the literature recurrent HO should be expected after surgery of a limb, even though it was limited in the case described.

Direct cell-fate conversion of somatic cells: Toward regenerative medicine and industries

Cells of multicellular organisms have diverse characteristics despite having the same genetic identity. The distinctive phenotype of each cell is determined by molecular mechanisms such as epigenetic changes that occur throughout the lifetime of an individual. Recently, technologies that enable modification of the fate of somatic cells have been developed, and the number of studies using these technologies has increased drastically in the last decade. Various cell types, including neuronal cells, cardiomyocytes, and hepatocytes, have been generated using these technologies. Although most direct reprogramming methods employ forced transduction of a defined sets of transcription factors to reprogram cells in a manner similar to induced pluripotent cell technology, many other strategies, such as methods utilizing chemical compounds and microRNAs to change the fate of somatic cells, have also been developed. In this review, we summarize transcription factor-based reprogramming and various other reprogramming methods. Additionally, we describe the various industrial applications of direct reprogramming technologies.

Fibrodysplasia ossificans progressiva: lessons learned from a rare disease

PURPOSE OF REVIEW

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare and severely disabling autosomal dominant disease that is yet to be clearly understood. The purpose of this review is to present recent literature on pathophysiology, clinical features, diagnosis and treatment of FOP.

RECENT FINDINGS

FOP is characterized by congenital great toe deformity and progressive heterotopic ossifications in connective tissue. Heterotopic ossifications occur after painful flare-ups that can arise spontaneously or can be triggered by minor trauma. Each flare-up ultimately causes restriction of related-joint, and along with the others eventually leads to immobility. Death is usually caused by pulmonary complications because of chest wall involvement. The causative gene of FOP is activin A receptor type 1 (ACVR1), a bone morphogenetic protein-signalling component, which normally acts to inhibit osteoblastogenesis. The treatment of FOP is still preventive and supportive.

SUMMARY

Although there are still gaps in the underlying mechanism of FOP, effective treatment options, such as potential pharmacologic targets and cell-based therapies are promising for the future. Some of these were tested without a clinical trial setting, and are currently in the process of evidence-based research.

Inflammation in Fibrodysplasia Ossificans Progressiva and Other Forms of Heterotopic Ossification

PURPOSE OF REVIEW

Heterotopic ossification (HO) is associated with inflammation. The goal of this review is to examine recent findings on the roles of inflammation and the immune system in HO. We examine how inflammation changes in fibrodysplasia ossificans progressiva, in traumatic HO, and in other clinical conditions of HO. We also discuss how inflammation may be a target for treating HO.

RECENT FINDINGS

Both genetic and acquired forms of HO show similarities in their inflammatory cell types and signaling pathways. These include macrophages, mast cells, and adaptive immune cells, along with hypoxia signaling pathways, mesenchymal stem cell differentiation signaling pathways, vascular signaling pathways, and inflammatory cytokines. Because there are common inflammatory mediators across various types of HO, these mediators may serve as common targets for blocking HO. Future research may focus on identifying new inflammatory targets and testing combinatorial therapies based on these results.

ACVR1 Function in Health and Disease

Activin A receptor type I (ACVR1) encodes for a bone morphogenetic protein type I receptor of the TGFβ receptor superfamily. It is involved in a wide variety of biological processes, including bone, heart, cartilage, nervous, and reproductive system development and regulation. Moreover, ACVR1 has been extensively studied for its causal role in fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder characterised by progressive heterotopic ossification. ACVR1 is linked to different pathologies, including cardiac malformations and alterations in the reproductive system. More recently, ACVR1 has been experimentally validated as a cancer driver gene in diffuse intrinsic pontine glioma (DIPG), a malignant childhood brainstem glioma, and its function is being studied in other cancer types. Here, we review ACVR1 receptor function and signalling in physiological and pathological processes and its regulation according to cell type and mutational status. Learning from different functions and alterations linked to ACVR1 is a key step in the development of interdisciplinary research towards the identification of novel treatments for these pathologies.

Development of Macrocycle Kinase Inhibitors for ALK2 Using Fibrodysplasia Ossificans Progressiva-Derived Endothelial Cells

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare congenital form of heterotopic ossification (HO), caused by heterozygous mutations in the activin A type I receptor (ACVR1), that encodes the bone morphogenetic protein (BMP) type I receptor ALK2. These mutations enable ALK2 to induce downstream signaling in response to activins, thereby turning them into bone-inducing agents. To date, there is no cure for FOP. The further development of FOP patient-derived models may contribute to the discovery of novel biomarkers and therapeutic approaches. Nevertheless, this has traditionally been a challenge, as biopsy sampling often triggers HO. We have characterized peripheral blood-derived endothelial colony-forming cells (ECFCs) from three independent FOP donors as a new model for FOP. FOP ECFCs are prone to undergo endothelial-to-mesenchymal transition and exhibit increased ALK2 downstream signaling and subsequent osteogenic differentiation upon stimulation with activin A. Moreover, we have identified a new class of small molecule macrocycles with potential activity against ALK2 kinase. Finally, using FOP ECFCs, we have selected OD36 and OD52 as potent inhibitors with excellent kinase selectivity profiles that potently antagonize mutant ALK2 signaling and osteogenic differentiation. We expect that these results will contribute to the development of novel ALK2 clinical candidates for the treatment of FOP. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

The role of Activin A in fibrodysplasia ossificans progressiva: a prominent mediator

Heterotopic ossification (HO) is the aberrant formation of mature, lamellar bone in nonosseous tissue. Fibrodysplasia ossificans progressiva (FOP) is a rare and devastating genetic disorder that causes progressive HO in the ligaments, tendons, and muscles throughout the body. FOP is attributed to an autosomal mutation in activin receptor-like kinase 2 (ALK2), a bone morphogenetic protein (BMP) type I receptor. Initial studies show that mutant ALK2 drives HO by constitutively activating the BMP signaling pathway. Recently, mutant ALK2 has been shown to transduce Smad1/5 signaling and enhance chondrogenesis, calcification in response to Activin A, which normally signals through Smad2/3 and inhibits BMP signaling pathway. Furthermore, Activin A induces heterotopic bone formation via mutant ALK2, while inhibition of Activin A blocks spontaneous and trauma-induced HO. In this manuscript, we describe the molecular mechanism of the causative gene ALK2 in FOP, mainly focusing on the prominent role of Activin A in HO. It reveals a potential strategy for prevention and treatment of FOP by inhibition of Activin A. Further studies are needed to explore the cellular and molecular mechanisms of Activin A in FOP in more detail.

Insights into the biology of fibrodysplasia ossificans progressiva using patient-derived induced pluripotent stem cells

The demand for development of new drugs remains on the upward trend because of the large number of patients suffering from intractable diseases for which effective treatment has not been established yet. Recently, several researchers have attempted to apply induced pluripotent stem cell (iPSC) technology as a powerful tool for studying the mechanisms underlying the onset of various diseases and for new drug screening. This technology has made an enormous breakthrough, since it permits us to recapitulate the disease phenotype in vitro, outside of the patient's body. Here, we discuss the latest findings that uncovered a mechanism underlying the pathology of a rare genetic musculoskeletal disease, fibrodysplasia ossificans progressiva (FOP), by modeling the phenotypes with FOP patient-derived iPSCs, and that discovered promising candidate drugs for FOP treatment. We also discussed future directions of FOP research.