The Hedgehog (Hh) family of proteins consists of Indian hedgehog (Ihh), sonic hedgehog (Shh), and desert hedgehog (Dhh). These proteins serve as essential regulators in a variety of developmental events. Ihh is mainly produced and secreted by prehypertrophic chondrocytes and regulates chondrocyte hypertrophy and endochondral bone formation during growth plate development. Tissue-specific deletion of the Ihh gene (targeted by Col2a1-Cre) causes early lethality in mice. Transgenic mice with induced Ihh expression exhibit increased chondrocyte hypertrophy and cartilage damage resembling human osteoarthritis (OA). During OA development, chondrocytes recapitulate the differentiation process that happens during the fetal status and which does not occur to an appreciable degree in adult articular cartilage. Ihh expression is up-regulated in human OA cartilage, and this upregulation correlates with OA progression and changes in chondrocyte morphology. A genetic study in mice further showed that conditional deletion of Ihh in chondrocytes attenuates OA progression, suggesting the possibility that blocking Ihh signaling can be used as a therapeutic approach to prevent or delay cartilage degeneration. However, Ihh gene deletion is currently not a therapeutic option as it is lethal in animals. RNA interference (RNAi) provides a means to knockdown Ihh without the severe side effects caused by chemical inhibitors. The currently available delivery methods for RNAi are nanoparticles and liposomes. Both have problems that need to be addressed. In the future, it will be necessary to develop a safe and effective RNAi delivery system to target Ihh signaling for preventing and treating OA.
Keywords: Cartilage; Indian Hedgehog; RNAi; degeneration; osteoarthritis.