Murine osteogenic protein (OP-1): high levels of mRNA in kidney.

The murine OP-1 gene (EMBL accession No. X56906) encoding the homolog of human osteogenic protein-1 was isolated from cDNA and genomic libraries using human OP-1 cDNA as probe. The deduced murine OP-1 amino acid sequence revealed 11 amino acids changes, three of them in the mature protein. Murine OP-1 probes were used for analysis of OP-1 mRNA in mouse embryo and organ tissues. Northern blot hybridization revealed multiple mRNA species: the major species of 2.2 kb, minor species of 1.8 and 2.4 kb and a large 4 kb species, which may represent alternative splices. Tissue specific expression was studied in brain, lung, heart, liver, spleen, kidney, adrenal and bladder tissue. Maximal levels of OP-1 mRNA were found in kidney which may explain the phenomenon of epithelial osteogenesis, first described by Huggins in 1931 using epithelium from the urinary tract. Moreover, our data suggest that kidneys may be the main site of OP-1 synthesis, even though it is distant from its physiological site of action, skeletal bone.

Functional redundancy of the muscle-specific transcription factors Myf5 and myogenin.

The myogenic basic helix-loop-helix transcription factors, Myf5, MyoD, myogenin and MRF4, play key roles in skeletal muscle development. All of them induce myogenic differentiation in cultured non-muscle cells, suggesting that they might be functionally redundant. But the genes are expressed at different times during embryogenesis and mice carrying a mutation in any of the genes have different phenotypes. A rib cage defect was observed in Myf5-deficient mice, which die perinatally. We investigated whether the rib cage defect was due to the failure of the early activation of the gene or to the unique interactions of Myf5 with specific downstream targets. For this we inserted a myogenin complementary DNA into the Myf5 locus by homologous recombination which simultaneously disrupted Myf5 function. We report here that mice homozygous for this myogenin gene knock-in (ki) developed a normal rib cage and were viable, therefore demonstrating functional redundancy of Myf5 and myogenin for rib formation.

MyoD or Myf-5 is required for the formation of skeletal muscle.

Mice carrying null mutations in the myogenic regulatory factors Myf-5 or MyoD have apparently normal skeletal muscle. To address whether these two factors functionally substitute for one another in myogenesis, mice carrying mutant Myf-5 and MyoD genes were interbred. While mice lacking both MyoD and Myf-5 were born alive, they were immobile and died soon after birth. Northern blot and S1 nuclease analyses indicated that Myf-5(-1-);MyoD(-1-) mice expressed no detectable skeletal muscle-specific mRNAs. Histological examination of these mice revealed a complete absence of skeletal muscle. Immunohistochemical analysis indicated an absence of desmin-expressing myoblast-like cells. These observations suggest that either Myf-5 or MyoD is required for the determination of skeletal myoblasts, their propagation, or both during embryonic development and indicate that these factors play, at least in part, functionally redundant roles in myogenesis.

Osteogenic protein-2. A new member of the transforming growth factor-beta superfamily expressed early in embryogenesis.

Osteogenic protein-2, OP-2, a new member of the transforming growth factor-beta (TGF-beta) superfamily, closely related to the osteogenic/bone morphogenetic proteins, was discovered in mouse embryo and human hippocampus cDNA libraries. The TGF-beta domain of OP-2 shows 74% identity to OP-1, 75% to Vgr-1, and 76% to BMP-5, hence OP-2 may also have bone inductive activity. The genomic locus of OP-2 has seven exons, like OP-1, and spans more than 27 kilobases (kb). In the C-terminal TGF-beta domain, OP-2 has a unique additional cysteine. Mouse embryos express relatively high levels of OP-2 mRNA at 8 days, two species of 3 and 5 kb. A careful study of mRNA expression of the osteogenic proteins in specific organs revealed discrete mRNA species for BMP-3, BMP-4, BMP-5, and BMP-6/Vgr-1 in lung or liver of young and adult mice. OP-1 is expressed in kidney; however, OP-2 and BMP-2 mRNAs were not detected in any organs studied, suggesting an early developmental role.

Mice lacking alpha 1 (IX) collagen develop noninflammatory degenerative joint disease.

Type IX collagen is a nonfibrillar collagen composed of three gene products, alpha 1(IX), alpha 2(IX), and alpha 3(IX). Type IX molecules are localized on the surface of type II-containing fibrils and consist of two arms, a long arm that is crosslinked to type II collagen and a short arm that projects into the perifibrillar space. In hyaline cartilage, the alpha 1(IX) collagen transcript encodes a polypeptide with a large N-terminal globular domain (NC4), whereas in many other tissues an alternative transcript encodes an alpha 1(IX) chain with a truncated NC4 domain. It has been proposed that type IX molecules are involved in the interaction of fibrils with each other or with other components of the extracellular matrix. To test this hypothesis, we have generated a mouse strain lacking both isoforms of the alpha 1(IX) chain. Homozygous mutant mice are viable and show no detectable abnormalities at birth but develop a severe degenerative joint disease resembling human osteoarthritis.