Normal long bone growth and development in type X collagen-null mice.

To investigate the role of type X collagen in skeletal development, we have generated type X collagen-null mice. Surprisingly, mice without type X collagen were viable and fertile and had no gross abnormalities in long bone growth or development. No differences were detected between the type X collagen-null mice and controls when growth plates of both newborn and 3-week old mice were examined by histology and by immunostaining for extracellular matrix components of bone including osteopontin, osteocalcin and type II collagen. Our results suggest that type X collagen is not required for long bone development. However, mice and humans with dominant acting type X collagen mutations have bone abnormalities, suggesting that only the presence of abnormal type X collagen can modify bone growth and development.

Homeotic transformation of cervical vertebrae in Hoxa-4 mutant mice.

Hoxa-4 (previously known as Hox-1.4) is a mouse homeobox-containing gene that is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues during embryogenesis. To understand the role of Hoxa-4 during development, we generated Hoxa-4 mutant mice. Homozygous mutants were viable and fertile. Analysis of neonatal skeletons revealed the development of ribs on the seventh cervical vertebra at variable penetrance and expressivity. A low frequency of alterations in sternal morphogenesis was also observed. In addition, we analyzed the skeletons of transgenic mice that overexpress Hoxa-4 and found that the formation of the small rib anlagen that often develop on the seventh cervical vertebra was suppressed. Analysis of adult homozygous mutant skeletons revealed that the dorsal process normally associated with the second cervical vertebra was also found on the third cervical vertebra. These results demonstrate that Hoxa-4 plays a role in conferring positional information along the anteroposterior axis to specify the identity of the third and the seventh cervical vertebrae.

Mutations in paralogous Hox genes result in overlapping homeotic transformations of the axial skeleton: evidence for unique and redundant function.

Hoxd-4 (previously known as Hox-4.2 and -5.1) is a mouse homeobox-containing gene homologous to the Drosophila homeotic gene Deformed. During embryogenesis, Hoxd-4 is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues. In the adult, Hoxd-4 transcripts are expressed predominantly in the testis and kidney, and to a lesser extent in intestine and heart. To understand the role of Hoxd-4 during mouse embryogenesis, we generated Hoxd-4 mutant mice. Mice heterozygous or homozygous for the Hoxd-4 mutation exhibit homeotic transformations of the second cervical vertebrae (C2) to the first cervical vertebrae (C1) and malformations of the neural arches of C1 to C3 and of the basioccipital bone. The phenotype was incompletely penetrant and showed variable expressivity on both an F2 hybrid and 129 inbred genetic background. The mutant phenotype was detected in the cartilaginous skeleton of 14.5-day (E14.5) mutant embryos but no apparent differences were detected in the somites of E9.5 mutant embryos, suggesting that the abnormalities develop after E9.5 perhaps during or after resegmentation of the somites to form the prevertebrae. These results suggest that Hoxd-4 plays a role in conferring position information along the anteroposterior axis in the skeleton. The phenotypic similarities and differences between Hoxd-4 and previously reported Hoxa-4 and Hoxb-4 mutant mice suggest that Hox gene paralogs have both redundant and unique functions.

Compound mutants for the paralogous hoxa-4, hoxb-4, and hoxd-4 genes show more complete homeotic transformations and a dose-dependent increase in the number of vertebrae transformed.

The Hox gene products are transcription factors involved in specifying regional identity along the anteroposterior body axis. In the mouse, several single mutants for Hox genes show variably penetrant, partial homeotic transformations of vertebrae at their anterior limits of expression, suggesting that compound Hox mutants might show more complete transformations with greater penetrance than the single Hox mutants. Compound mutants for the paralogous group 3 genes, hoxa-3 and hoxd-3, show deletion of a cervical vertebrae, which is not readily interpretable in terms of an alteration in regional identity. Here, we report the skeletal phenotypes of compound mutants in the group 4 Hox genes, hoxa-4, hoxb-4, and hoxd-4. Mice mutant for each of these genes were intercrossed to generate the three possible double mutant combinations and the triple mutant. In contrast to the hoxa-3, hoxd-3 double mutants, group 4 Hox compound mutants displayed clear alterations in regional identity, including a nearly complete transformation of the second cervical vertebrae toward the morphology of the first cervical vertebra in one double mutant combination. In comparing the types of homeotic transformations observed, different double mutant combinations showed different degrees of synergism. These results suggest a certain degree of functional redundancy among paralogous genes in specifying regional identity. Furthermore, there was a remarkable dose-dependent increase in the number of vertebrae transformed to a first cervical vertebra identity, including the second through the fifth cervical vertebrae in the triple mutant. Thus, these genes are required in a larger anteroposterior domain than is revealed by the single mutant phenotypes alone, such that multiple mutations in these genes result in transformations of vertebrae that are not at their anterior limit of expression.

Functional antagonists of sonic hedgehog reveal the importance of the N terminus for activity.

During development, sonic hedgehog functions as a morphogen in both a short-range contact-dependent and in a long-range diffusable mode. Here, we show using a panel of sonic hedgehog variants that regions near the N terminus of the protein play a critical role in modulating these functions. In the hedgehog responsive cell line C3H10T1/2, we discovered that not only were some N-terminally truncated variants inactive at eliciting a hedgehog-dependent response, but they competed with the wild-type protein for function and therefore served as functional antagonists. These variants were indistinguishable from wild-type sonic hedgehog in their ability to bind the receptor patched-1, but failed to induce the hedgehog-responsive markers, Gli-1 and Ptc-1, and failed to promote hedgehog-dependent differentiation of the cell line. They also failed to support the adhesion of C3H10T1/2 cells to hedgehog-coated plates under conditions where wild-type sonic hedgehog supported binding. Structure-activity data indicated that the N-terminal cysteine plays a key regulatory role in modulating hedgehog activity. The ability to dissect patched-1 binding from signaling events in C3H10T1/2 cells suggests the presence of unidentified factors that contribute to hedgehog responses.