Noggin and Sclerostin Bone Morphogenetic Protein Antagonists Form a Mutually Inhibitory Complex

Abstract

Noggin and sclerostin are bone morphogenetic protein (BMP) antagonists that modulate mitogenic activity through sequestering BMPs. Little is known of the interactions among this class of proteins. We show that recombinant sclerostin and noggin bound to each other with high affinity (K D ‫؍‬ 2.92 nM). This observation has been extended to naturally expressed noggin and sclerostin from the rat osteosarcoma cell line, ROS 17/ 2.8, supporting a role for the complex in natural systems. The noggin-sclerostin complex was competitive with BMP binding and mutually attenuated the activity of each BMP antagonist. Collectively, the data demonstrate a novel and exquisite paradigm for the regulation of BMP activity through direct neutralization of the BMP and activation by co-localized BMP antagonist expression. The pleiotrophic nature of noggin and sclerostin represents a novel mechanism for the fine-tuning of BMP activity in bone homeostasis.Sclerostin, the protein product of the SOST gene, is absent in sclerosteosis, a skeletal disease characterized by bone overgrowth and strong dense bones (1, 2). We have demonstrated that sclerostin, a bone morphogenetic protein (BMP) 1 antagonist highly expressed by osteocytes, is a key regulator of bone formation in vivo and in vitro (3). A number of BMP antagonists have been described that historically were associated with important regulatory activity in developmental systems. Recent studies have expanded the roles of these proteins beyond the described embryonic roles. Noggin has been shown to be expressed in mineralizing tissue, and in in vivo experiments an essential role in joint formation was established (4 -9). The overexpression of either sclerostin or noggin in bone cells leads to osteopenia in rodents, demonstrating the importance of BMP proteins in bone homeostasis and suggesting that sclerostin and noggin function as negative regulators of bone density (3,4,9).BMPs have been shown to induce the expression of BMP antagonists in mesenchymal (preosteoblastic) and osteoblastic cell lines (10,11). In vivo, the highest expression of sclerostin in adult human bone was seen in hypertrophic chondrocytes and osteocytes throughout the skeleton (3). In comparison, in vivo, noggin expression was reported to be associated with chondrocytes (12) and osteoblasts of cranial sutures (7). Given the close localization of these two BMP antagonist proteins and the similarity of their activities in vitro, we asked how these proteins might interact in regulating bone formation via limiting BMP activity. We report that these proteins coordinate their activity but do so through an unexpected pathway. Sclerostin and noggin bind to each other with high affinity and attenuate the BMP antagonist activity of each other. We propose that these BMP antagonists are pleiotrophic. EXPERIMENTAL PROCEDURESThe Binding of Noggin and Sclerostin-Anti-FLAG M2-agarose beads (Sigma) were washed with IP buffer (20 mM Tris, pH 7.6, 150 mM NaCl, 1 mM EDTA, 1% Triton X-100, 1.4 mM ␤ME, 10% glycerol) before incubation in t...

Targeting Bone Remodeling for the Treatment of Osteoporosis: Summary of the Proceedings of an ASBMR Workshop

“…(79) In mice, deletion of the Sost gene leads to increased bone mass, (80) and, conversely, mice overexpressing Sost exhibit low bone mass. (81,82) Sclerostin expression is negatively regulated by hormonal and mechanical stimuli that lead to increased bone formation. Thus, daily injections of PTH as well as chronic excess of this hormone suppress sclerostin expression in mice.…”

Sclerostin: A gem from the genome leads to bone-building antibodies

“…In addition to these Wnt pathway findings, sclerostin had been reported to regulate the bone morphogenetic protein (BMP) pathway by binding to and antagonizing the activity of BMPs, (39,40) as well as Noggin, (41) itself a well-known BMP antagonist.…”

The prognostic significance of BMP-6 signaling in prostate cancer

“…13 In addition, noggin and sclerostin also interact with each other to regulate BMPs activity. 14 Noggin has been shown to modulate the invasiveness and the osteoblastic activity of prostate cancer cells. 7,15 Overexpression of noggin in prostate cancer cells results in decreased ability of the cells to form osteolytic or osteoblastic lesions in tibias of immunosuppressed mouse.…”