BMP2/6 ligand formation and function in development and iron homeostasis

About This Grant

ABSTRACT Bone morphogenetic proteins (BMPs) play critical roles in development and adult tissue homeostasis. Class I (BMP2 and 4) and class II (BMP5-7) BMPs are the dominant players in specifying cell fate, embryonic patterning and organ morphogenesis. We previously showed that liver endothelial cell (LEC)-derived BMP2 and BMP6 also have a key role to regulate systemic homeostasis of the essential nutrient iron by controlling expression of the liver hormone hepcidin. Indeed, iron homeostasis regulation is the major non-redundant function of BMP6 in vivo, and impaired BMP signaling is the main cause of the iron overload disorder hereditary hemochromatosis. BMPs are dimeric proteins that are made as precursors comprised of a non-signaling prodomain and an active ligand released by proteolytic cleavage. Our work and others' have shown that prodomains play critical roles in ligand folding, dimerization, activation, and receptor interactions. Moreover, many ligand functions are carried out by heterodimers (comprised of 2 different ligands) rather than homodimers. Notably, little is known about BMP2 or BMP6 dimerization, proteolytic processing, and prodomain function. Moreover, BMP6 prodomain mutations are linked to altered hepcidin regulation and iron overload in humans, but the mechanisms are uncertain. Here, we will show that 1) LEC-secreted BMP2 and BMP6 must work together to regulate hepcidin and iron homeostasis in vivo; 2) co-expressed BMP2 and BMP6 form heterodimers that signal more strongly vs homodimers; 2) BMP2 and BMP6 proteolytic maturation and/or subcellular trafficking differ from their closest homologues BMP4 and BMP7; 3) BMP2 prodomain is essential to generate active BMP6 homodimers or BMP2/6 heterodimers in Xenopus embryos, whereas cognate prodomains are essential in LECs; and 4) BMP6 prodomain mutations linked to iron overload impair ligand activity and hepcidin induction in vitro. We hypothesize that BMP2/6 heterodimers are a key ligand for hepcidin and iron homeostasis regulation and that prodomains have critical roles in BMP2/6 maturation and function. In this proposal, we aim to use structural modeling, Xenopus embryos, a human LEC-hepatocyte cell culture system, and novel knock-in mice to elucidate how BMP2 and BMP6 prodomains, and the process of proteolytic maturation, contribute to heterodimer and/or homodimer formation and function, and how BMP6 prodomain mutations impact these processes to impair hepcidin regulation and cause iron overload. Our long-term goals are to understand how BMP signaling is regulated to control hepcidin expression and systemic iron homeostasis, how this process is perturbed in iron disorders, and ultimately to develop new treatments for iron disorders. We will also gain fundamental insights into BMP ligand maturation and prodomain function that will have broader impacts for many other fields where BMP signaling is important, including developmental biology.