Since the identification in 1988 of bone morphogenetic protein 2 (BMP2) as a potent inducer of bone and cartilage formation, BMP superfamily signalling has become one of the most heavily investigated topics in vertebrate skeletal biology. Whereas a large part of this research has focused on the roles of BMP2, BMP4 and BMP7 in the formation and repair of endochondral bone, a large number of BMP superfamily molecules have now been implicated in almost all aspects of bone, cartilage and joint biology. As modulating BMP signalling is currently a major therapeutic target, our rapidly expanding knowledge of how BMP superfamily signalling affects most tissue types of the skeletal system creates enormous potential to translate basic research findings into successful clinical therapies that improve bone mass or quality, ameliorate diseases of skeletal overgrowth, and repair damage to bone and joints. This Review examines the genetic evidence implicating BMP superfamily signalling in vertebrate bone and joint development, discusses a selection of human skeletal disorders associated with altered BMP signalling and summarizes the status of modulating the BMP pathway as a therapeutic target for skeletal trauma and disease.

The mammalian homeobox gene pdx-1 is expressed in pluripotent precursor cells in the dorsal and ventral pancreatic bud and duodenal endoderm, which will produce the pancreas and the rostral duodenum. In the adult, pdx-1 is expressed principally within insulin-secreting pancreatic islet ␤ cells and cells of the duodenal epithelium. Our objective in this study was to localize sequences within the mouse pdx-1 gene mediating selective expression within the islet. Studies of transgenic mice in which a genomic fragment of the mouse pdx-1 gene from kb ؊4.5 to ؉8.2 was used to drive a ␤-galactosidase reporter showed that the control sequences sufficient for appropriate developmental and adult specific expression were contained within this region. Three nuclease-hypersensitive sites, located between bp ؊2560 and ؊1880 (site 1), bp ؊1330 and ؊800 (site 2), and bp ؊260 and ؉180 (site 3), were identified within the 5-flanking region of the endogenous pdx-1 gene. Pancreatic ␤-cell-specific expression was shown to be controlled by sequences within site 1 from an analysis of the expression pattern of various pdx-1-herpes simplex virus thymidine kinase promoter expression constructs in transfected ␤-cell and non-␤-cell lines. Furthermore, we also established that this region was important in vivo by demonstrating that expression from a site 1-driven ␤-galactosidase reporter construct was directed to islet ␤-cells in transgenic mice. The activity of the site 1-driven constructs was reduced substantially in ␤-cell lines by mutating a hepatocyte nuclear factor 3 (HNF3)-like site located between nucleotides ؊2007 and ؊1996. Gel shift analysis indicated that HNF3␤ present in islet ␤ cells binds to this element. Immunohistochemical studies revealed that HNF3␤ was present within the nuclei of almost all islet ␤ cells and subsets of pancreatic acinar cells. Together, these results suggest that HNF3␤, a key regulator of endodermal cell lineage development, plays an essential role in the cell-type-specific transcription of the pdx-1 gene in the pancreas.During mammalian pancreatic development, common multipotential endodermally derived precursors in the early embryo undergo a series of specific changes that lead to the differentiated exocrine and endocrine pancreas. The dorsal and ventral pancreatic primordia first appear as evaginations of the gut endoderm on day 9 postcoitum (p.c.) in the mouse. The first cells to express differentiated pancreatic hormone markers in these buds are found in embryos of about 20 somites, corresponding to 9.5 days p.c. (15,18,41,55). The two buds grow independently, forming both exocrine and endocrine tissues, and finally merge on day 10.5 p.c. (41). Recent studies indicate that expression of the PDX-1 homeoprotein in a common precursor cell population is essential for the development of the endocrine and exocrine compartments of the pancreas, with pancreatic development becoming arrested at a very early post-bud stage in homozygous pdx-1 mutant mice (1,25,35). PDX-1 was also recently shown to be essential...

The ecotropic viral integration site-1 (Evi1) locus was initially identified as a common site of retroviral integration in myeloid tumors of the AKXD-23 recombinant inbred mouse strain. The full-length Evi1 transcript encodes a putative transcription factor, containing ten zinc finger motifs found within two domains of the protein. To determine the biological function of the Evi1 proto-oncogene, the full-length, but not an alternately spliced, transcript was disrupted using targeted mutagenesis in embryonic stem cells. Evi1 homozygous mutant embryos die at approximately 10.5 days post coitum. Mutants were distinguished at 10.5 days post coitum by widespread hypocellularity, hemorrhaging, and disruption in the development of paraxial mesenchyme. In addition, defects in the heart, somites, and cranial ganglia were detected and the peripheral nervous system failed to develop. These results correlated with whole-mount in situ hybridization analyses of embryos which showed expression of the Evi1 proto-oncogene in embryonic mesoderm and neural crest-derived cells associated with the peripheral nervous system. These data suggest that Evi1 has important roles in general cell proliferation, vascularization, and cell-specific developmental signaling, at midgestation.

The cis-acting sequences that mediate insulin gene expression exclusively in pancreatic islet beta-cells are localized within the 5'-flanking region between nucleotides -340 and -91. We have identified an evolutionarily conserved, A+T-rich element at -201/-196 basepairs in the rat insulin II gene that is essential for efficient expression in beta-cells. Affinity-purified antibody to the XIHbox 8 protein super-shifted the major beta-cell-activator factor complex binding to the -201/-196 element. XIHbox 8 is a Xenopus endoderm-specific homeodomain protein whose expression is restricted to the nucleus of endodermal cells of the duodenum and developing pancreas. Antibody to XIHbox 8 specifically interacts with a 47-kilodalton protein present in this DNA complex. Immunohistochemical studies revealed XIHbox 8-like proteins within the nucleus of almost all mouse islet beta-cells and a subset of islet alpha- and beta-cells. These results are consistent with the proposal that an XIHbox 8-related homeoprotein of 47 kilodalton is required for expression of the mammalian insulin gene in beta-cells. Experiments conducted with antiserum raised to somatostatin transcription factor-1 (STF-1), a recently isolated mammalian XIHbox 8-related homeoprotein, indicate that the STF-1 protein is the mammalian homolog of Xenopus XIHbox 8.