Expression of indian hedgehog, bone morphogenetic protein 6 and gli during skeletal morphogenesis.

A complex signaling pathway involving members of the Hedgehog, Bone morphogenetic protein (Bmp) and Gli families regulates early patterning events in fetal skeletogenesis (Hui and Joyner, 1993. A mouse model of Greig cephalopolysyndactyly syndrome: the extra-toes mutation contains an intragenic deletion of the Gli3 gene. Nat. Genet. 3, 241-246; Bitgood and McMahon, 1995. Hedgehog and Bmp genes are coexpressed at many diverse sites of cell-cell interaction in the mouse embryo. Dev. Biol. 172, 126-138; Lanske et al., 1996. PTH/PTHrP receptor in early development and Indian hedgehog-regulated bone growth. Science 273, 663-666; Vortkamp et al., 1996. Regulation of rate of cartilage differentiation by Indian hedgehog and PTH-related protein. Science 273, 613-622). Hedgehog genes encode secreted proteins that mediate patterning and growth through the induction of secondary signals (reviewed in Hammerschmidt et al., 1997. The world according to hedgehog. Trends Genet. 13, 14-21). Two potential targets of Ihh are bmp6 and gli (Johnson et al., 1995. Patched overexpression alters wing disc size and pattern: transcriptional and post-transcriptional effects on hedgehog targets. Development 121, 4161-4170; Dominguez et al., 1996. Sending and receiving the hedgehog signal: control by the Drosophila Gli protein Cubitus interruptus. Science 272, 1621-1625; Marigo et al., 1996. Sonic hedgehog differentially regulates expression of GLI and GLI3 during limb development. Dev. Biol. 180, 273-283). We investigated the molecular similarities and differences between fetal and postnatal skeletal development by analyzing the coincident and complimentary expression domains of indian hedgehog (ihh), bmp6 and gli in adjacent sections throughout the process of skeletogenesis. In almost all of the skeletal tissues examined, the expression domains of ihh and bmp6 were adjacent to one another and this region was surrounded by gli-expressing cells. These observations are in keeping with the proposed function of gli as a negative regulator of Ihh signaling and the induction of Bmps by Hedgehog proteins (Roberts et al., 1995. Sonic hedgehog is an endodermal signal inducing Bmp-4 and Hox genes during induction and regionalization of the chick hindgut. Development 121, 3163-3174; Kawakami et al., 1996. BMP signaling during bone pattern determination in the developing limb. Development 122, 3557-3566). By puberty, ihh, bmp6 and gli transcripts were no longer detected in the growth plate, despite the fact that physeal chondrocytes continued to hypertrophy and differentiate. Although bmp6 was expressed, ihh transcripts were not found in primordia of intramembranous bones, nor in cells lining the future articular surfaces. Collectively our findings suggest that ihh participates in, but is not required for chondrocyte hypertrophy.