Protein-mediated membrane remodeling

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

Investigators

Linked publications & trials

Abstract

Bone-resorbing osteoclasts are responsible for essential, life-long skeletal remodeling, and their dysfunction is a major contributor to bone diseases affecting >200 million individuals worldwide, including osteoporosis, fibrous dysplasia (FD), Pagets disease and osteopetrosis. Multinucleated osteoclasts are formed by the successive fusion of mononucleated precursor cells, where each fusion event raises bone-resorbing activity of osteoclasts. Despite the fundamental role of cell-cell fusion in osteoclast formation, the mechanisms of this fusion process in normal physiology and in disease remain to be fully understood. In our recent study, we discovered that osteoclastogenesis involves lupus La protein (SSB gene product). La, also referred to as LARP3 and La autoantigen, is generally recognized as an abundant and ubiquitous mostly nuclear RNA-binding protein. The best characterized function of nuclear La is to protect precursor tRNAs from exonuclease digestion through specific interactions between Las highly conserved, N-terminal La domain and the 3 ends of tRNA. We found that La protein has an additional function as an osteoclast fusion regulator. Monocyte-to-osteoclast differentiation starts with a drastic decrease in La levels. As fusion begins, La reappears at the osteoclast surface where it promotes fusion. Osteoclast formation is accompanied by and depends on drastic changes in the steady-state level, molecular species, and intracellular localization of La protein. Las novel role in promoting membrane fusion stage of formation of multinucleated osteoclasts is independent of canonical La-RNA interactions and involves direct interactions between La and Annexin A5, which anchors La to transiently exposed phosphatidylserine at the surface of fusing osteoclasts. Our findings indicate that, while La protein plays an ancient, well-described and essential role in the RNA biology of all eukaryotes, La has also been adapted in mammals to serve as an osteoclast fusion manager. In this new, highly specific role on the surface of fusing osteoclasts, La may present a promising target for treatment of bone diseases stemming from perturbed bone turnover. In a follow up study, we focused on another aspect of bone biology: interactions between osteoclasts and bone-forming cells, osteoblasts. Diverse skeletal diseases are underpinned by breakdown in the poorly understood coordination of osteoclast-osteoblast functions. Since the paucity of simple, activatable, biologically relevant models of osteoclast-osteoblast coordination has hindered our understanding of the osteoblast-osteoclast crosstalk, we developed an inducible ex vivo model of osteoclast-osteoblast progenitor coordination. Our experimental system utilizes conditional, tetracycline inducible mouse model of FD. Induction activates the release of osteoclastogenic factors from osteoprogenitors, which elicits the differentiation and fusion of neighboring preosteoclasts. In turn, multinucleated osteoclasts promote osteoprogenitor proliferation by releasing soluble coupling factors and RANK+ extracellular vesicles. Our model recapitulates several aspects of perturbed coordination in diseases linked to excessive osteoclast formation. We expect this model to expedite the investigation of cell-cell fusion, osteoclast-osteoblast progenitor coordination, and extracellular vesicle signaling during bone remodeling and offer a powerful tool for evaluating signaling cascades and novel therapeutic interventions in osteoclast-linked skeletal disease.