Bone morphogenetic proteins (BMPs) are multifunctional
cytokines that belong to the
transforming growth factor-β family. BMPs were originally identified based on their unique activity, inducing heterotopic bone formation in skeletal muscle. This unique BMP activity is transduced by specific type I and type II transmembrane
kinase receptors. Among the downstream pathways activated by these receptors, the Smad1/5/8
transcription factors appear to play critical roles in BMP activity. Smad1/5/8
transcription factors are phosphorylated at the C-terminal SVS motif by BMP type I receptors and then induce the transcription of early BMP-responsive genes by binding to conserved sequences in their enhancer regions. The linker regions of Smad1/5/8 contain multiple
kinase phosphorylation sites, and phosphorylation and dephosphorylation of these sites regulate the transcriptional activity of
Smad proteins. Gain-of-function mutations in one
BMP type I receptor have been identified in patients with
fibrodysplasia ossificans progressiva, a rare
genetic disorder that is characterized by progressive heterotopic bone formation in the skeletal muscle. The mutant receptors activate the Smad signaling pathway even in the absence of BMPs, therefore novel inhibitors for the
BMP receptor - Smad axis are being developed to prevent heterotopic bone formation in
fibrodysplasia ossificans progressiva. Taken together, the data in the literature show that the
BMP type I receptor - Smad signaling axis is the critical pathway for the unique activity of BMPs and is a potential therapeutic target for pathological conditions caused by inappropriate BMP activity.