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 Acvr1 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 Acvr1 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 Acvr1, expression of the man-made, ligand-independent ACVR1 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 Acvr1 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.

f 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. In human disease, somatic or germ line mutations may confer "constitutive" activity to mutant signaling or receptor proteins, permitting the recruitment of downstream effectors independently of upstream signaling events. So-called "constitutively activating" mutations and their gene products contribute to a broad range of clinical conditions ranging from cancer to syndromic developmental defects, including inborn disorders of metabolism, endocrine abnormalities, and sensory defects (1-3). Constitutively activating mutations, commonly referred to as "gain-offunction" mutations, can augment function in multiple ways. Constitutively active mutant receptor proteins may have true constitutive activity that is invariant with stimuli, harbor activity that is hypersensitive or amplified in response to native ligands, or have activity that can be induced with decreased specificity (4).Fibrodysplasia ossificans progressiva (FOP) is a congenital heterotopic ossification (HO) disorder in which afflicted individuals are nearly normal at birth except for subtle skeletal malformations yet are prone to forming endochondral bone lesions in soft tissues such as skeletal muscle, ligaments, and fascia, especially following injury or inflammation (5). The classic FOP phenotype results from highly conserved activating mutations in ACVR1, encoding the bone morphogenetic protein (BMP) type I receptor kinase ALK2, due to an R206H substitution in the glycine-serine-rich (GS) regulatory domain (6, 7). While other...

The sperm mitochondria-associated cysteine-rich protein (Smcp) mRNA is transcribed in step 3 spermatids, and is stored in free mRNPs until translation begins w6 days later in step 11. To identify sequences that control the timing of Smcp mRNA translation, mutations in both UTRs were analyzed in transgenic mice using green fluorescent protein (GFP), squashes of seminiferous tubules, and quantification of polysomal loading in adult and 21 dpp testes in sucrose and Nycodenz gradients. GFP fluorescence is first detected in step 9 spermatids in lines harboring a transgene containing the