[Primary cilia and hedgehog signaling]

The primary cilium is an immotile organelle protruding from the cell surface in almost all vertebrate cells. Many molecules inside the primary cilia coordinately play a pivotal role, so genetic defects of these components result in diverse congenital malformations of the brain, eye, liver, kidney, and skeleton. Hedgehog signaling is a highly conserved pathway regulating morphogenesis in early development and tumorigenesis postnatally. Recently, advanced molecular biology has revealed that components of hedgehog signaling such as PTCH1, SMO, and GLI specifically translocate within the primary cilium upon the ligand binding of the hedgehog protein, and transduce the biological growth signal from the cell surface to the nucleus. Haploinsufficiency of the components in the primary cilium would inhibit the hedgehog pathway, resulting in developmental anomalies like ventral neural tube defects. Since the hedgehog-dependent pathway is critical for vertebrate development, it is crucial to elucidate the functional roles of hedgehog-related proteins in the primary cilium.