Hyperactive BMP signaling induced by ALK2(R206H) requires type II receptor function in a Drosophila model for classic fibrodysplasia ossificans progressiva

Characterization of Fibrodysplasia Ossificans Progessiva relevant Acvr1/Acvr2 Activin receptors in medaka (Oryzias latipes)

Activin and Bone Morphogenetic Protein (BMP) signaling plays crucial roles in vertebrate organ formation, including osteo- and angiogenesis, and tissue homeostasis, such as neuronal maintenance. Activin and BMP signaling needs to be precisely controlled by restricted expression of shared receptors, stoichiometric composition of receptor-complexes and presence of regulatory proteins. A R206H mutation in the human (hs) BMP type I receptor hsACVR1, on the other hand, leads to excessive phosphorylation of Sons of mothers against decapentaplegic (SMAD) 1/5/8. This in turn causes increased inflammation and heterotopic ossification in soft tissues of patients suffering from Fibrodysplasia Ossificans Progressiva (FOP). Several animal models have been established to understand the spontaneous and progressive nature of FOP, but often have inherent limitations. The Japanese medaka (Oryzias latipes, ola) has recently emerged as popular model for bone research. To assess whether medaka is suitable as a potential FOP animal model, we determined the expression of Activin receptor type I (ACVR1) orthologs olaAcvr1 and olaAcvr1l with that of Activin type II receptors olaAcvr2ab, olaAcvr2ba and olaAcvr2bb in embryonic and adult medaka tissues by in situ hybridization. Further, we showed that Activin A binding properties are conserved in olaAcvr2, as are the mechanistic features in the GS-Box of both olaAcvr1 and olaAcvr1l. This consequently leads to FOP-typical elevated SMAD signaling when the medaka type I receptors carry the R206H equivalent FOP mutation. Together, this study therefore provides experimental groundwork needed to establish a unique medaka model to investigate mechanisms underlying FOP.

Reduced GS Domain Serine/Threonine Requirements of Fibrodysplasia Ossificans Progressiva Mutant Type I BMP Receptor ACVR1 in the Zebrafish

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic condition characterized by altered skeletal development and extraskeletal bone formation. All cases of FOP are caused by mutations in the type I bone morphogenetic protein (BMP) receptor gene ACVR1 that result in overactivation of the BMP signaling pathway. Activation of the wild-type ACVR1 kinase requires assembly of a tetrameric type I and II BMP receptor complex followed by phosphorylation of the ACVR1 GS domain by type II BMP receptors. Previous studies showed that the FOP-mutant ACVR1-R206H required type II BMP receptors and presumptive glycine/serine-rich (GS) domain phosphorylation for overactive signaling. Structural modeling of the ACVR1-R206H mutant kinase domain supports the idea that FOP mutations alter the conformation of the GS domain, but it is unclear how this leads to overactive signaling. Here we show, using a developing zebrafish embryo BMP signaling assay, that the FOP-mutant receptors ACVR1-R206H and -G328R have reduced requirements for GS domain phosphorylatable sites to signal compared to wild-type ACVR1. Further, ligand-independent and ligand-dependent signaling through the FOP-mutant ACVR1 receptors have distinct GS domain phosphorylatable site requirements. ACVR1-G328R showed increased GS domain serine/threonine requirements for ligand-independent signaling compared to ACVR1-R206H, whereas it exhibited reduced serine/threonine requirements for ligand-dependent signaling. Remarkably, while ACVR1-R206H does not require the type I BMP receptor partner, Bmpr1, to signal, a ligand-dependent GS domain mutant of ACVR1-R206H could signal independently of Bmpr1 only when Bmp7 ligand was overexpressed. Of note, unlike human ACVR1-R206H, the zebrafish paralog Acvr1l-R203H does not show increased signaling activity. However, in domain-swapping studies, the human kinase domain, but not the human GS domain, was sufficient to confer overactive signaling to the Acvr1l-R203H receptor. Together these results reflect the importance of GS domain activation and kinase domain functions in regulating ACVR1 signaling and identify mechanisms of reduced regulatory constraints conferred by FOP mutations. © 2023 American Society for Bone and Mineral Research (ASBMR).

Polypeptide Substrate Accessibility Hypothesis: Gain-of-Function R206H Mutation Allosterically Affects Activin Receptor-like Protein Kinase Activity

Although structurally similar to type II counterparts, type I or activin receptor-like kinases (ALKs) are set apart by a metastable helix-loop-helix (HLH) element preceding the protein kinase domain that, according to a longstanding paradigm, serves passive albeit critical roles as an inhibitor-to-substrate-binding switch. A single recurrent mutation in the codon of the penultimate residue, directly adjacent the position of a constitutively activating substitution, causes milder activation of ACVR1/ALK2 leading to sporadic heterotopic bone deposition in patients presenting with fibrodysplasia ossificans progressiva, or FOP. To determine the protein structural-functional basis for the gain of function, R206H mutant, Q207D (aspartate-substituted caALK2) and HLH subdomain-truncated (208 Ntrunc) forms were compared to one another and the wild-type enzyme through in vitro kinase and protein-protein interaction analyses that were complemented by signaling read-out (p-Smad) in primary mouse embryonic fibroblasts and Drosophila S2 cells. Contrary to the paradigm, the HLH subdomain actively suppressed the phosphotransferase activity of the enzyme, even in the absence of FKBP12. Unexpectedly, perturbation of the HLH subdomain elevated kinase activity at a distance, i.e., allosterically, at the ATP-binding and polypeptide-interacting active site cleft. Accessibility to polypeptide substrate (BMP Smad C-terminal tails) due to allosterically altered conformations of type I active sites within heterohexameric cytoplasmic signaling complexes-assembled noncanonically by activin-type II receptors extracellularly-is hypothesized to produce a gain of function of the R206H mutant protein responsible for episodic heterotopic ossification in FOP.

Functional comparison of human ACVR1 and zebrafish Acvr1l FOP-associated variants in embryonic zebrafish

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by progressive heterotopic ossification of muscle and connective tissues, is caused by autosomal dominant activating mutations in the type I receptor, ACVR1/ALK2. The classic human FOP variant, ACVR1R206H , shows increased bone morphogenetic protein (BMP) signaling and activation by activins.

RESULTS

Here, we performed in vivo functional characterization of human ACVR1R206H and orthologous zebrafish Acvr1lR203H using early embryonic zebrafish dorsoventral patterning as a phenotypic readout for receptor activity. Our results showed that human ACVR1R206H and zebrafish Acvr1lR203H exhibit functional differences in early embryonic zebrafish, and that human ACVR1R206H retained its signaling activity in the absence of a ligand-binding domain (LBD). We also showed, for the first time, that zebrafish Acvr2ba/Acvr2bb receptors are required for human ACVR1R206H signaling in early embryonic zebrafish.

CONCLUSIONS

Together, these data provide new insight into ACVR1R206H signaling pathways that may facilitate the design of new and effective therapies for FOP patients.

Overexpression of Wild-Type ACVR1 in Fibrodysplasia Ossificans Progressiva Mice Rescues Perinatal Lethality and Inhibits Heterotopic Ossification

Fibrodysplasia ossificans progressiva (FOP) is a devastating disease of progressive heterotopic bone formation for which effective treatments are currently unavailable. FOP is caused by dominant gain-of-function mutations in the receptor ACVR1 (also known as ALK2), which render the receptor inappropriately responsive to activin ligands. In previous studies, we developed a genetic mouse model of FOP that recapitulates most clinical aspects of the disease. In this model, genetic loss of the wild-type Acvr1 allele profoundly exacerbated heterotopic ossification, suggesting the hypothesis that the stoichiometry of wild-type and mutant receptors dictates disease severity. Here, we tested this model by producing FOP mice that conditionally overexpress human wild-type ACVR1. Injury-induced heterotopic ossification (HO) was completely blocked in FOP mice when expression of both the mutant and wild-type receptor were targeted to Tie2-positive cells, which includes fibro/adipogenic progenitors (FAPs). Perinatal lethality of Acvr1^R206H/+ mice was rescued by constitutive ACVR1 overexpression, and these mice survived to adulthood at predicted Mendelian frequencies. Constitutive overexpression of ACVR1 also provided protection from spontaneous abnormal skeletogenesis, and the incidence and severity of injury-induced HO in these mice was dramatically reduced. Analysis of pSMAD1/5/8 signaling both in cultured cells and in vivo indicates that ACVR1 overexpression functions cell-autonomously by reducing osteogenic signaling in response to activin A. We propose that ACVR1 overexpression inhibits HO by decreasing the abundance of ACVR1(R206H)-containing signaling complexes at the cell surface while increasing the representation of activin-A-bound non-signaling complexes comprised of wild-type ACVR1. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

ACVR1 antibodies exacerbate heterotopic ossification in fibrodysplasia ossificans progressiva (FOP) by activating FOP-mutant ACVR1

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder whose most debilitating pathology is progressive and cumulative heterotopic ossification (HO) of skeletal muscles, ligaments, tendons, and fascia. FOP is caused by mutations in the type I BMP receptor gene ACVR1, which enable ACVR1 to utilize its natural antagonist, activin A, as an agonistic ligand. The physiological relevance of this property is underscored by the fact that HO in FOP is exquisitely dependent on activation of FOP-mutant ACVR1 by activin A, an effect countered by inhibition of anti–activin A via monoclonal antibody treatment. Hence, we surmised that anti-ACVR1 antibodies that block activation of ACVR1 by ligands should also inhibit HO in FOP and provide an additional therapeutic option for this condition. Therefore, we generated anti-ACVR1 monoclonal antibodies that block ACVR1’s activation by its ligands. Surprisingly, in vivo, these anti-ACVR1 antibodies stimulated HO and activated signaling of FOP-mutant ACVR1. This property was restricted to FOP-mutant ACVR1 and resulted from anti-ACVR1 antibody–mediated dimerization of ACVR1. Conversely, wild-type ACVR1 was inhibited by anti-ACVR1 antibodies. These results uncover an additional property of FOP-mutant ACVR1 and indicate that anti-ACVR1 antibodies should not be considered as therapeutics for FOP.

Pathogenic ACVR1R206H activation by Activin A-induced receptor clustering and autophosphorylation

Fibrodysplasia ossificans progressiva (FOP) and diffuse intrinsic pontine glioma (DIPG) are debilitating diseases that share causal mutations in ACVR1, a TGF-β family type I receptor. ACVR1R206H is a frequent mutation in both diseases. Pathogenic signaling via the SMAD1/5 pathway is mediated by Activin A, but how the mutation triggers aberrant signaling is not known. We show that ACVR1 is essential for Activin A-mediated SMAD1/5 phosphorylation and is activated by two distinct mechanisms. Wild-type ACVR1 is activated by the Activin type I receptors, ACVR1B/C. In contrast, ACVR1R206H activation does not require upstream kinases, but is predominantly activated via Activin A-dependent receptor clustering, which induces its auto-activation. We use optogenetics and live-imaging approaches to demonstrate Activin A-induced receptor clustering and show it requires the type II receptors ACVR2A/B. Our data provide molecular mechanistic insight into the pathogenesis of FOP and DIPG by linking the causal activating genetic mutation to disrupted signaling.

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.

Fibrodysplasia ossificans progressiva mutant ACVR1 signals by multiple modalities in the developing zebrafish

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic disorder characterized by altered skeletal development and extraskeletal ossification. All cases of FOP are caused by activating mutations in the type I BMP/TGFβ cell surface receptor ACVR1, which over-activates signaling through phospho-Smad1/5 (pSmad1/5). To investigate the mechanism by which FOP-ACVR1 enhances pSmad1/5 activation, we used zebrafish embryonic dorsoventral (DV) patterning as an assay for BMP signaling. We determined that the FOP mutants ACVR1-R206H and -G328R do not require their ligand binding domain to over-activate BMP signaling in DV patterning. However, intact ACVR1-R206H has the ability to respond to both Bmp7 and Activin A ligands. Additionally, BMPR1, a type I BMP receptor normally required for BMP-mediated patterning of the embryo, is dispensable for both ligand-independent signaling pathway activation and ligand-responsive signaling hyperactivation by ACVR1-R206H. These results demonstrate that FOP-ACVR1 is not constrained by the same receptor/ligand partner requirements as WT-ACVR1.

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.

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.

Bone Morphogenetic Protein Glass Bottom Boat (BMP5/6/7/8) and its receptor Wishful Thinking (BMPRII) are required for injury-induced allodynia in Drosophila

Background Chronic pain affects millions of people worldwide; however, its cellular and molecular mechanisms have not been completely elucidated. It is thought that chronic pain is triggered by nociceptive sensitization, which produces elevated nocifensive responses. A model has been developed in Drosophila melanogaster to investigate the underlying mechanisms of chronic pain using ultraviolet-induced tissue injury to trigger thermal allodynia, a nociceptive hypersensitivity to a normally innocuous stimulus. Larvae were assayed for their behavioral latencies to produce a distinct avoidance response under different thermal conditions. Previously, Decapentaplegic, a member of the Bone Morphogenetic Protein (BMP) family and orthologous to mammalian BMP2/4, was shown to be necessary for the induction of allodynia. Here, we further investigate the BMP pathway to identify other essential molecules necessary to activate the nociceptive sensitization pathway. Results Using the GAL4-UAS-RNAi system to induce a cell-specific knockdown of gene expression, we further explored BMP pathway components to identify other key players in the induction of nociceptive sensitization by comparing the responses of manipulated animals to those of controls. Here, we show that a second BMP, Glass Bottom Boat, and its receptor Wishful Thinking are both necessary for injury-induced thermal allodynia since the formation of sensitization was found to be severely attenuated when either of these components was suppressed. The effects on pain perception appear to be specific to the sensitization system, as the ability to respond to a normally noxious stimulus in the absence of injury was left intact, and no nociceptor morphological defects were observed. Conclusion These results provide further support of the hypothesis that the BMP pathway plays a crucial role in the development of nociceptive sensitization. Because of its strong conservation between invertebrates and mammals, the BMP pathway may be worthy of future investigation for the development of targeted treatments to alleviate chronic pain.

An Adult Zebrafish Model of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare human skeletal disease caused by constitutively activating mutations in the gene ACVR1, which encodes a type I BMP/TGFβ family member receptor. FOP is characterized by progressive heterotopic ossification (HO) of fibrous tissues, including skeletal muscle, tendons, and ligaments, as well as malformation of the big toes, vertebral fusions, and osteochondromas. Surgical interventions in patients often result in enhanced HO, which can exacerbate rather than improve diagnostic outcomes. As a result of these difficulties, a variety of animal models are needed to study human FOP. Here we describe the methods for creating and characterizing zebrafish conditionally expressing Acvr1l^Q204D, the first adult zebrafish model for FOP.

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.

Variant BMP receptor mutations causing fibrodysplasia ossificans progressiva (FOP) in humans show BMP ligand-independent receptor activation in zebrafish

The large majority of cases of the autosomal dominant human disease fibrodysplasia ossificans progressiva (FOP) are caused by gain-of-function Arg206His mutations in the BMP type I receptor ACVR1 (ALK2). The Arg206His mutation is located in the GS domain of the type I receptor. This region is normally phosphorylated by the BMP type II receptor, which activates the type I receptor to phosphorylate its substrate, the signal transducer Smad1/5/8. A small subset of patients with FOP carry variant mutations in ACVR1 altering Gly328 to Trp, Glu or Arg. Since these mutations lie outside the GS domain, the mechanism through which ACVR1 Gly328 mutations cause disease remains unclear. We used a zebrafish embryonic development assay to test the signaling of human ACVR1 Gly328 mutant receptors comparing them to the Arg206His mutant. In this assay increased or decreased BMP pathway activation alters dorsal-ventral axial patterning, providing a sensitive assay for altered BMP signaling levels. We expressed the human ACVR1 Gly328 mutant receptors in zebrafish embryos to investigate their signaling activities. We found that all ACVR1 Gly328 human mutations ventralized wild-type embryos and could partially rescue Bmp7-deficient embryos, indicating that these mutant receptors can activate BMP signaling in a BMP ligand-independent manner. The degree of ventralization or rescue was similar among all three Gly328 mutants. Smad1/5 phosphorylation, a readout of BMP receptor signaling, was mildly increased by ACVR1 Gly328 mutations. Gene expression analyses demonstrate expanded ventral and reciprocal loss of dorsal cell fate markers. This study demonstrates that Gly328 mutants increase receptor activation and BMP ligand-independent signaling through Smad phosphorylation.

Variable signaling activity by FOP ACVR1 mutations

Most patients with fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder of heterotopic ossification, have the same causative mutation in ACVR1, R206H. However, additional mutations within the ACVR1 BMP type I receptor have been identified in a small number of FOP cases, often in patients with disease of lesser or greater severity than occurs with R206H mutations. Genotype-phenotype correlations have been suggested in patients, resulting in classification of FOP mutations based on location within different receptor domains and structural modeling. However while each of the mutations induces increased signaling through the BMP-pSmad1/5/8 pathway, the molecular mechanisms underlying functional differences of these FOP variant receptors remained undetermined. We now demonstrate that FOP mutations within the ACVR1 receptor kinase domain are more sensitive to low levels of BMP than mutations in the ACVR1 GS domain. Our data additionally confirm responsiveness of cells with FOP ACVR1 mutations to both BMP and Activin A ligands. We also have determined that constructs with FOP ACVR1 mutations that are engineered without the ligand-binding domain retain increased BMP-pSmad1/5/8 pathway activation relative to wild-type ACVR1, supporting that the mutant receptors can function through ligand-independent mechanisms either directly through mutant ACVR1 or through indirect mechanisms.

Drosophila models of FOP provide mechanistic insight

Fibrodysplasia ossificans progressiva (FOP) is a rare bone disease characterized by episodic events of heterotopic ossification (HO). All cases of FOP have been attributed to mutations in the ACVR1 gene that render the encoded BMP type I ALK2 receptor hypersensitive, resulting in the activation of BMP signaling, at inappropriate times in inappropriate locations. The episodic or sporadic nature of HO associated with FOP rests with the occurrence of specific 'triggers' that push the hypersensitive ALK2-FOP receptor into full signaling mode. Identification of these triggers and their mechanism of action is critical for preventing HO and its devastating consequences in FOP patients. Models of FOP, generated in Drosophila, are shown to activate the highly conserved BMP signaling pathway in both Drosophila cell culture and in developing tissues in vivo. The most common FOP mutation, R206H, in ALK2 and its synonymous mutation, K262H, in the orthologous Drosophila receptor Sax, abolish the ability of wild type receptors to inhibit BMP ligand-induced signaling and lead to ubiquitous pathway activation in both cases but with important differences. When expressed in Drosophila, human ALK2R206H exhibits constitutive signaling. SaxK262H on the other hand can elicit excessive signaling similar to that observed for ALK2R206H in mammalian systems in vivo. For example, hyperactive signaling mediated by SaxK262H is triggered by an increase in ligand or in type II receptors. Interestingly, while the constitutive nature of ALK2R2026H in Drosophila requires activation by the type II receptor, it does not require its ligand binding domain. The differences exhibited by the two Drosophila FOP models enable a valuable comparative analysis poised to reveal critical regulatory mechanisms governing signaling output from these mutated receptors. Modifier screens using these Drosophila FOP models will be extremely valuable in identifying genes or compounds that reduce or prevent the hyperactive BMP signaling that initiates HO associated with FOP.

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.

Alternative cleavage of the bone morphogenetic protein (BMP), Gbb, produces ligands with distinct developmental functions and receptor preferences

The family of TGF-β and bone morphogenetic protein (BMP) signaling proteins has numerous developmental and physiological roles. They are made as proprotein dimers and then cleaved by proprotein convertases to release the C-terminal domain as an active ligand dimer. Multiple proteolytic processing sites in Glass bottom boat (Gbb), the Drosophila BMP7 ortholog, can produce distinct ligand forms. Cleavage at the S1 or atypical S0 site in Gbb produces Gbb15, the conventional small BMP ligand, whereas NS site cleavage produces a larger Gbb38 ligand. We hypothesized that the Gbb prodomain is involved not only in regulating the production of specific ligands but also their signaling output. We found that blocking NS cleavage increased association of the full-length prodomain with Gbb15, resulting in a concomitant decrease in signaling activity. Moreover, NS cleavage was required in vivo for Gbb-Decapentaplegic (Dpp) heterodimer-mediated wing vein patterning but not for Gbb15-Dpp heterodimer activity in cell culture. Gbb NS cleavage was also required for viability through its regulation of pupal ecdysis in a type II receptor Wishful thinking (Wit)-dependent manner. In fact, Gbb38-mediated signaling exhibits a preference for Wit over the other type II receptor Punt. Finally, we discovered that Gbb38 is produced when processing at the S1/S0 site is blocked by O-linked glycosylation in third instar larvae. Our findings demonstrate that BMP prodomain cleavage ensures that the mature ligand is not inhibited by the prodomain. Furthermore, alternative processing of BMP proproteins produces ligands that signal through different receptors and exhibit specific developmental functions.

Modeling congenital disease and inborn errors of development in Drosophila melanogaster

Fly models that faithfully recapitulate various aspects of human disease and human health-related biology are being used for research into disease diagnosis and prevention. Established and new genetic strategies in Drosophila have yielded numerous substantial successes in modeling congenital disorders or inborn errors of human development, as well as neurodegenerative disease and cancer. Moreover, although our ability to generate sequence datasets continues to outpace our ability to analyze these datasets, the development of high-throughput analysis platforms in Drosophila has provided access through the bottleneck in the identification of disease gene candidates. In this Review, we describe both the traditional and newer methods that are facilitating the incorporation of Drosophila into the human disease discovery process, with a focus on the models that have enhanced our understanding of human developmental disorders and congenital disease. Enviable features of the Drosophila experimental system, which make it particularly useful in facilitating the much anticipated move from genotype to phenotype (understanding and predicting phenotypes directly from the primary DNA sequence), include its genetic tractability, the low cost for high-throughput discovery, and a genome and underlying biology that are highly evolutionarily conserved. In embracing the fly in the human disease-gene discovery process, we can expect to speed up and reduce the cost of this process, allowing experimental scales that are not feasible and/or would be too costly in higher eukaryotes.

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

Fibrodysplasia ossificans progressiva (FOP), a congenital heterotopic ossification (HO) syndrome caused by gain-of-function mutations of bone morphogenetic protein (BMP) type I receptor ACVR1, manifests with progressive ossification of skeletal muscles, tendons, ligaments, and joints. In this disease, HO can occur in discrete flares, often triggered by injury or inflammation, or may progress incrementally without identified triggers. Mice harboring an Acvr1R206H knock-in allele recapitulate the phenotypic spectrum of FOP, including injury-responsive intramuscular HO and spontaneous articular, tendon, and ligament ossification. The cells that drive HO in these diverse tissues can be compartmentalized into two lineages: an Scx+ tendon-derived progenitor that mediates endochondral HO of ligaments and joints without exogenous injury, and a muscle-resident interstitial Mx1+ population that mediates intramuscular, injury-dependent endochondral HO. Expression of Acvr1R206H in either lineage confers aberrant gain of BMP signaling and chondrogenic differentiation in response to activin A and gives rise to mutation-expressing hypertrophic chondrocytes in HO lesions. Compared to Acvr1R206H, expression of the man-made, ligand-independent ACVR1Q207D mutation accelerates and increases the penetrance of all observed phenotypes, but does not abrogate the need for antecedent injury in muscle HO, demonstrating the need for an injury factor in addition to enhanced BMP signaling. Both injury-dependent intramuscular and spontaneous ligament HO in Acvr1R206H knock-in mice were effectively controlled by the selective ACVR1 inhibitor LDN-212854. Thus, diverse phenotypes of HO found in FOP are rooted in cell-autonomous effects of dysregulated ACVR1 signaling in nonoverlapping tissue-resident progenitor pools that may be addressed by systemic therapy or by modulating injury-mediated factors involved in their local recruitment.

The Fibrodysplasia Ossificans Progressiva (FOP) mutation p.R206H in ACVR1 confers an altered ligand response

Patients with Fibrodysplasia Ossificans Progressiva (FOP) suffer from ectopic bone formation, which progresses during life and results in dramatic movement restrictions. Cause of the disease are point mutations in the Activin A receptor type 1 (ACVR1), with p.R206H being most common. In this study we compared the signalling responses of ACVR1^WT and ACVR1^R206H to different ligands. ACVR1^WT, but not ACVR1^R206H inhibited BMP signalling of BMP2 or BMP4 in a ligand binding domain independent manner. Likewise, the basal BMP signalling activity of the receptor BMPR1A or BMPR1B was inhibited by ACVR1^WT, but enhanced by ACVR1^R206H. In comparison, BMP6 or BMP7 activated ACVR1^WT and caused a hyper-activation of ACVR1^R206H. These effects were dependent on an intact ligand binding domain. Finally, the neofunction of Activin A in FOP was tested and found to depend on the ligand binding domain for activating ACVR1^R206H. We conclude that the FOP mutation ACVR1^R206H is more sensitive to a number of natural ligands. The mutant receptor apparently lost some essential inhibitory interactions with its ligands and co-receptors, thereby conferring an enhanced ligand-dependent signalling and stimulating ectopic bone formation as observed in the patients.

Cellular Hypoxia Promotes Heterotopic Ossification by Amplifying BMP Signaling

Hypoxia and inflammation are implicated in the episodic induction of heterotopic endochondral ossification (HEO); however, the molecular mechanisms are unknown. HIF-1α integrates the cellular response to both hypoxia and inflammation and is a prime candidate for regulating HEO. We investigated the role of hypoxia and HIF-1α in fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of HEO in humans. We found that HIF-1α increases the intensity and duration of canonical bone morphogenetic protein (BMP) signaling through Rabaptin 5 (RABEP1)-mediated retention of Activin A receptor, type I (ACVR1), a BMP receptor, in the endosomal compartment of hypoxic connective tissue progenitor cells from patients with FOP. We further show that early inflammatory FOP lesions in humans and in a mouse model are markedly hypoxic, and inhibition of HIF-1α by genetic or pharmacologic means restores canonical BMP signaling to normoxic levels in human FOP cells and profoundly reduces HEO in a constitutively active Acvr1(Q207D/+) mouse model of FOP. Thus, an inflammation and cellular oxygen-sensing mechanism that modulates intracellular retention of a mutant BMP receptor determines, in part, its pathologic activity in FOP. Our study provides critical insight into a previously unrecognized role of HIF-1α in the hypoxic amplification of BMP signaling and in the episodic induction of HEO in FOP and further identifies HIF-1α as a therapeutic target for FOP and perhaps nongenetic forms of HEO. © 2016 American Society for Bone and Mineral Research.

Granting immunity to FOP and catching heterotopic ossification in the Act

The progressive transformation of one organ system into another is a fundamental signature of fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of extraskeletal bone formation in humans. In all affected individuals, FOP is caused by heterozygous missense gain-of-function mutations in Activin receptor A type I (ACVR1), a bone morphogenetic protein (BMP) type I receptor. Loss of autoinhibition of the mutant receptor (mACVR1) results in dysregulated BMP pathway signaling, and is necessary for the myriad developmental features of FOP, but does not appear sufficient to induce the episodic flare-ups that lead to disabling post-natal heterotopic endochondral ossification (HEO) and that are a hallmark of the disease. Post-natal FOP flare-ups strongly implicate an underlying immunological trigger involving inflammation and the innate immune system. Recent studies implicate canonical and non-canonical TGFβ/BMP family ligands in the amplification of mACVR1 signaling leading to the formation of FOP lesions and resultant HEO. BMP and Activin ligands that stimulate mACVR1 signaling also have critical regulatory functions in the immune system. Cross-talk between the morphogenetic and immunological pathways that regulate tissue maintenance and wound healing identifies potential robust therapeutic targets for FOP. Here we review current evidence for an immunological trigger for flare-ups and HEO in FOP, propose a working schema for the pathophysiology of observed phenomena, and highlight outstanding questions under investigation.

Neofunction of ACVR1 in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by extraskeletal bone formation through endochondral ossification. FOP patients harbor point mutations in ACVR1 (also known as ALK2), a type I receptor for bone morphogenetic protein (BMP). Two mechanisms of mutated ACVR1 (FOP-ACVR1) have been proposed: ligand-independent constitutive activity and ligand-dependent hyperactivity in BMP signaling. Here, by using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs), we report a third mechanism, where FOP-ACVR1 abnormally transduces BMP signaling in response to Activin-A, a molecule that normally transduces TGF-β signaling but not BMP signaling. Activin-A enhanced the chondrogenesis of induced mesenchymal stromal cells derived from FOP-iPSCs (FOP-iMSCs) via aberrant activation of BMP signaling in addition to the normal activation of TGF-β signaling in vitro, and induced endochondral ossification of FOP-iMSCs in vivo. These results uncover a novel mechanism of extraskeletal bone formation in FOP and provide a potential new therapeutic strategy for FOP.

Mutant activin-like kinase 2 in fibrodysplasia ossificans progressiva are activated via T203 by BMP type II receptors

Fibrodysplasia ossificans progressiva (FOP) is a genetic disorder characterized by progressive heterotopic ossification in soft tissues, such as the skeletal muscles. FOP has been shown to be caused by gain-of-function mutations in activin receptor-like kinase (ALK)-2, which is a type I receptor for bone morphogenetic proteins (BMPs). In the present study, we examined the molecular mechanisms that underlie the activation of intracellular signaling by mutant ALK2. Mutant ALK2 from FOP patients enhanced the activation of intracellular signaling by type II BMP receptors, such as BMPR-II and activin receptor, type II B, whereas that from heart disease patients did not. This enhancement was dependent on the kinase activity of the type II receptors. Substitution mutations at all nine serine and threonine residues in the ALK2 glycine- and serine-rich domain simultaneously inhibited this enhancement by the type II receptors. Of the nine serine and threonine residues in ALK2, T203 was found to be critical for the enhancement by type II receptors. The T203 residue was conserved in all of the BMP type I receptors, and these residues were essential for intracellular signal transduction in response to ligand stimulation. The phosphorylation levels of the mutant ALK2 related to FOP were higher than those of wild-type ALK2 and were further increased by the presence of type II receptors. The phosphorylation levels of ALK2 were greatly reduced in mutants carrying a mutation at T203, even in the presence of type II receptors. These findings suggest that the mutant ALK2 related to FOP is enhanced by BMP type II receptors via the T203-regulated phosphorylation of ALK2.

Bone Morphogenetic Protein (BMP) signaling in development and human diseases

Bone Morphogenetic Proteins (BMPs) are a group of signaling molecules that belongs to the Transforming Growth Factor-β (TGF-β) superfamily of proteins. Initially discovered for their ability to induce bone formation, BMPs are now known to play crucial roles in all organ systems. BMPs are important in embryogenesis and development, and also in maintenance of adult tissue homeostasis. Mouse knockout models of various components of the BMP signaling pathway result in embryonic lethality or marked defects, highlighting the essential functions of BMPs. In this review, we first outline the basic aspects of BMP signaling and then focus on genetically manipulated mouse knockout models that have helped elucidate the role of BMPs in development. A significant portion of this review is devoted to the prominent human pathologies associated with dysregulated BMP signaling.

Wisdom from the fly

Arguably, almost all research in Drosophila can be considered basic research, yet many of the most essential and fundamental concepts of human genetics were first decoded in the fly. Although the fly genome, which is organized into only four chromosomes, is approximately one-twentieth the size of the human genome, it contains roughly the same number of genes, and up to 75% of human disease-related genes have Drosophila homologues [1]. The fly was prized for its simplicity and utility even before such compelling homology with humans was apparent. Since Thomas Hunt Morgan began his seminal experiments over a century ago (Table 1), the Drosophila system has revealed countless key mechanisms by which cells function, including the factors that maintain chromatin and the signaling pathways that control cell fate determination and organism development. More recently, the fly has emerged as a critical neurobiological tool and disease model for a range of genetic disorders. In this review, we present a brief retrospective of Drosophila as an indispensable genetic system and discuss some of the many contributions, past and present, of this facile system to human genetics.

Strategies for exploring TGF-β signaling in Drosophila

The TGF-β pathway is an evolutionarily conserved signal transduction module that mediates diverse biological processes in animals. In Drosophila, both the BMP and Activin branches are required for viability. Studies rooted in classical and molecular genetic approaches continue to uncover new developmental roles for TGF-β signaling. We present an overview of the secreted ligands, transmembrane receptors and cellular Smad transducer proteins that compose the core pathway in Drosophila. An assortment of tools have been developed to conduct tissue-specific loss- and gain-of-function experiments for these pathway components. We discuss the deployment of these reagents, with an emphasis on appropriate usage and limitations of the available tools. Throughout, we note reagents that are in need of further improvement or development, and signaling features requiring further study. A general theme is that comparison of phenotypes for ligands, receptors, and Smads can be used to map tissue interactions, and to separate canonical and non-canonical signaling activities. Core TGF-β signaling components are subject to multiple layers of regulation, and are coupled to context-specific inputs and outputs. In addition to fleshing out how TGF-β signaling serves the fruit fly, we anticipate that future studies will uncover new regulatory nodes and modes and will continue to advance paradigms for how TGF-β signaling regulates general developmental processes.

ACVR1 p.Q207E causes classic fibrodysplasia ossificans progressiva and is functionally distinct from the engineered constitutively active ACVR1 p.Q207D variant

Fibrodysplasia ossificans progressiva (FOP) is a disabling genetic disorder of progressive heterotopic ossification (HO). Here, we report a patient with an ultra-rare point mutation [c.619C>G, p.Q207E] located in a codon adjacent to the most common FOP mutation [c.617G>A, p.R206H] of Activin A Receptor, type 1 (ACVR1) and that affects the same intracellular amino acid position in the GS activation domain as the engineered constitutively active (c.a.) variant p.Q207D. It was predicted that both mutations at residue 207 have similar functional effects by introducing a negative charge. Transgenic p.Q207D-c.a. mice have served as a model for FOP HO in several in vivo studies. However, we found that the engineered ACVR1(Q207D-c.a.) is significantly more active than the classic FOP mutation ACVR1(R206H) when overexpressed in chicken limbs and in differentiation assays of chondrogenesis, osteogenesis and myogenesis. Importantly, our studies reveal that the ACVR1(Q207E) resembles the classic FOP receptor in these assays, not the engineered ACVR1(Q207D-c.a.). Notably, reporter gene assays revealed that both naturally occurring FOP receptors (ACVR1(R206H) and ACVR1(Q207E)) were activated by BMP7 and were sensitive to deletion of the ligand binding domain, whereas the engineered ACVR1(Q207D-c.a.) exhibited ligand independent activity. We performed an in silico analysis and propose a structural model for p.Q207D-c.a. that irreversibly relocates the GS domain into an activating position, where it becomes ligand independent. We conclude that the engineered p.Q207D-c.a. mutation has severe limitations as a model for FOP, whereas the naturally occurring mutations p.R206H and p.Q207E facilitate receptor activation, albeit in a reversible manner.

Fibrodysplasia ossificans progressiva: mechanisms and models of skeletal metamorphosis

Fibrodysplasia ossificans progressiva (FOP; MIM #135100) is a debilitating genetic disorder of connective tissue metamorphosis. It is characterized by malformation of the great (big) toes during embryonic skeletal development and by progressive heterotopic endochondral ossification (HEO) postnatally, which leads to the formation of a second skeleton of heterotopic bone. Individuals with these classic clinical features of FOP have the identical heterozygous activating mutation (c.617G>A; R206H) in the gene encoding ACVR1 (also known as ALK2), a bone morphogenetic protein (BMP) type I receptor. Disease activity caused by this ACVR1 mutation also depends on altered cell and tissue physiology that can be best understood in the context of a high-fidelity animal model. Recently, we developed such a knock-in mouse model for FOP (Acvr1^R206H/+) that recapitulates the human disease, and provides a valuable new tool for testing and developing effective therapies. The FOP knock-in mouse and other models in Drosophila, zebrafish, chickens and mice provide an arsenal of tools for understanding BMP signaling and addressing outstanding questions of disease mechanisms that are relevant not only to FOP but also to a wide variety of disorders associated with regenerative medicine and tissue metamorphosis.

Constitutively active ALK2 receptor mutants require type II receptor cooperation

Constitutively activating mutations in receptor kinases recruit downstream effector pathways independently of upstream signaling, with consequences ranging from developmental syndromes to cancer. Classic fibrodysplasia ossificans progressiva (FOP) is a congenital syndrome resulting from highly conserved activating mutations of the glycine-serine-rich (GS) regulatory domain of ACVR1, encoding bone morphogenetic protein (BMP) type I receptor ALK2, which lead to inappropriate signaling and heterotopic ossification of soft tissues. It is unclear if constitutively active mutant ALK2 receptors (caALK2) can function independently of signaling complexes with type II receptors and ligands. We found that ablation of BmpRII and ActRIIa abrogated BMP ligand-mediated and caALK2-mediated signaling and transcription in cells and disrupted caALK2-induced heterotopic ossification in mice. Signaling via GS domain ALK2 mutants could be restored by the expression of either BMP type II receptor. The contribution of BMP type II receptors was independent of their ligand-binding or kinase function but was dependent upon an intact cytoplasmic domain. These data demonstrate that GS domain ALK2 mutants act independently of upstream signaling but may require a nonenzymatic scaffolding function provided by type II receptors to form functional, apparently ligand-independent signaling complexes. These findings define the minimal requirements for signaling of GS domain ALK2 mutants, with implications for the therapeutic targeting of their activity in disease.

Structure of the bone morphogenetic protein receptor ALK2 and implications for fibrodysplasia ossificans progressiva

Bone morphogenetic protein (BMP) receptor kinases are tightly regulated to control development and tissue homeostasis. Mutant receptor kinase domains escape regulation leading to severely degenerative diseases and represent an important therapeutic target. Fibrodysplasia ossificans progressiva (FOP) is a rare but devastating disorder of extraskeletal bone formation. FOP-associated mutations in the BMP receptor ALK2 reduce binding of the inhibitor FKBP12 and promote leaky signaling in the absence of ligand. To establish structural mechanisms of receptor regulation and to address the effects of FOP mutation, we determined the crystal structure of the cytoplasmic domain of ALK2 in complex with the inhibitors FKBP12 and dorsomorphin. FOP mutations break critical interactions that stabilize the inactive state of the kinase, thereby facilitating structural rearrangements that diminish FKBP12 binding and promote the correct positioning of the glycine-serine-rich loop and αC helix for kinase activation. The balance of these effects accounts for the comparable activity of R206H and L196P. Kinase activation in the clinically benign mutant L196P is far weaker than R206H but yields equivalent signals due to the stronger interaction of FKBP12 with R206H. The presented ALK2 structure offers a valuable template for the further design of specific inhibitors of BMP signaling.