Heterotopic ossification (HO), the formation of ectopic endochondral bone in skeletal muscle and soft tissues, is a significant cause of morbidity from joint immobility and pain. The precise mechanisms responsible for HO are not known;however, its association with postsurgical and posttraumatic contexts suggests a process of disordered injury repair. Further insights into the causes of HO may be gained from a congenital HO syndrome, fibrodysplasia ossificans progressiva (FOP). FOP is caused by "constitutively-activating" mutations in the bone morphogenetic protein (BMP) type I receptor ALK2, which result in progressive and widespread joint ossifications triggered by minimal trauma or inflammation. Both FOP and acquired forms of HO lack effective therapies. In fact, there is significant evidence that both FOP and HO are caused by inappropriate activation of the BMP signaling pathway. It is not known how enhanced BMP signaling deviates the injury repair program, or which populations of cells mediate the effects of enhanced BMP signaling. To address these questions we have developed a mouse model of FOP in which a constitutively-active mutant form of ALK2 (caALK2) is inducibly expressed. Similar to affected humans, expression of this gene does not spontaneously induce HO, but vigorous ossification and joint fusion occur with additional stimuli of inflammation and muscle injury. Our subsequent studies with this model suggest that these caALK2 proteins may not be constitutively-active, as previously thought, but may sensitize cells to traditional ligand-mediated BMP signals.
In Aim 1 of this grant, we will employ this model to discern the mechanisms by which caALK2 sensitizes cells to BMP signals, testing whether caALK2 requires ligand-mediated signaling or functions independently. To identify the cellular progenitors which mediate the effects of enhanced BMP signaling, and which contribute to the ectopic bone lesions, in Aim 2 we have targeted the expression of mutant caALK2 to several candidate progenitor lineages. Using a complement of cell-based, genetic targeting, and adoptive transfer techniques, we will systematically determine the impact of expressing caALK2 in compartments with known osteogenic potential, including skeletal muscle satellite cells, vascular pericytes, as well as bone-marrow derived lineages.
In Aim 3, we examine the role of innate immune signaling in the development of ectopic bone in this model. Understanding how caALK2 mutations alter the consequences of BMP signaling could highlight novel molecular or cellular targets for management of HO. This proposal seeks to elucidate how enhanced BMP signaling impacts mesenchymal progenitors and governs adaptive and maladaptive osteogenesis at the interface of injury, inflammation, and regeneration.

Public Health Relevance

This proposal asks how bone morphogenetic protein signals, which normally regulate the activity of progenitor cells involved in the repair of muscle, blood vessels, connective tissues, and bone, may become dysregulated to cause inappropriate bone formation in the human diseases of heterotopic ossification and fibrodysplasia ossificans progressiva. In addition to providing insights into the mechanism of these poorly understood processes, these studies may identify much needed novel approaches for their management. These mechanisms have relevance to a broader set of conditions in which inflammation and injury appear to lead to abnormal ossification in autoimmune, cardiac, and vascular disease.

Agency
National Institute of Health (NIH)
Institute
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Type
Research Project (R01)
Project #
5R01AR057374-03
Application #
8325423
Study Section
Skeletal Biology Structure and Regeneration Study Section (SBSR)
Program Officer
Sharrock, William J
Project Start
2010-08-01
Project End
2015-06-30
Budget Start
2012-07-01
Budget End
2013-06-30
Support Year
3
Fiscal Year
2012
Total Cost
$385,560
Indirect Cost
$169,560
Name
Brigham and Women's Hospital
Department
Type
DUNS #
030811269
City
Boston
State
MA
Country
United States
Zip Code
02115
Mayeur, Claire; Lohmeyer, Lisa K; Leyton, Patricio et al. (2014) The type I BMP receptor Alk3 is required for the induction of hepatic hepcidin gene expression by interleukin-6. Blood 123:2261-8
Mohedas, Agustin H; Wang, You; Sanvitale, Caroline E et al. (2014) Structure-activity relationship of 3,5-diaryl-2-aminopyridine ALK2 inhibitors reveals unaltered binding affinity for fibrodysplasia ossificans progressiva causing mutants. J Med Chem 57:7900-15
Mohedas, Agustin H; Xing, Xuechao; Armstrong, Kelli A et al. (2013) Development of an ALK2-Biased BMP Type I Receptor Kinase Inhibitor. ACS Chem Biol :
Bagarova, Jana; Vonner, Ashley J; Armstrong, Kelli A et al. (2013) Constitutively active ALK2 receptor mutants require type II receptor cooperation. Mol Cell Biol 33:2413-24
Sanvitale, Caroline E; Kerr, Georgina; Chaikuad, Apirat et al. (2013) A new class of small molecule inhibitor of BMP signaling. PLoS One 8:e62721
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Derwall, Matthias; Malhotra, Rajeev; Lai, Carol S et al. (2012) Inhibition of bone morphogenetic protein signaling reduces vascular calcification and atherosclerosis. Arterioscler Thromb Vasc Biol 32:613-22