![]() ![]() Here we assess the cell types targeted by Osx-Cre in two-month-old mice by monitoring GFP expression from the R26-mT/mG reporter allele. Although in recent years the Osx-Cre mouse line has been increasingly used to study the osteoblast lineage in postnatal mice, ,, ,, the targeting specificity of Osx-Cre in postnatal bones is yet to be formally evaluated. The initial characterization of this mouse line revealed that Cre activity is largely restricted to the osteogenic perichondrium, periosteum and osteoblast-lineage cells within the marrow cavity, but that analysis was limited to the embryo. Osx-Cre ( Osx1- GFP::Cre), a BAC transgenic mouse line expressing a GFP::Cre fusion protein from the regulatory sequence of Osx, was generated to direct gene deletion in the osteoblast lineage. In this system, specificity is achieved by expression of the Cre recombinase under the control of cell type-specific regulatory sequences. The Cre/loxP technology enables gene deletion in specific cell types and has significantly advanced our understanding of gene functions in both physiological and pathological conditions. In addition to its role in embryonic osteoblast differentiation, Osx also plays a critical role in the formation and function of postnatal osteoblast and osteocyte. Thus, Osx functions genetically downstream of Runx2 to control osteoblast differentiation. On the other hand, the expression of Osx is abolished in Runx2-null embryos. In these embryos, Runx2 expression is relatively normal, but other osteoblast markers including Col1a1, Bsp, and osteocalcin are either absent or severely suppressed. In Osx-null embryos, cartilage elements are largely normal but osteoblast differentiation fails to complete, resulting in a complete lack of bone tissue. Genetic studies have revealed the essential role of Osx in osteoblast differentiation. In addition, Osx is also detected in early hypertrophic chondrocytes at a relatively weak level. ![]() Later during development, the perichondrial Osx-expressing osteoprogenitors co-migrate with the blood vessels that invade the hypertrophic cartilage, and to generate osteoblasts responsible for depositing the trabecular bone. During embryonic skeletal development, Osx is initially expressed in the perichondrium flanking the hypertrophic cartilage, where osteoblasts first arise to produce the bone collar (cortical bone). Osterix (Osx or Sp7) is a zinc finger family transcriptional factor critical for osteoblast differentiation. Thus, potential contributions from the non-osteoblast-lineage cells should be considered when Osx-Cre is used to study gene functions in postnatal mice. Beyond the skeleton, Osx-Cre also targets the olfactory glomerular cells, and a subset of the gastric and intestinal epithelium. The targeting of adipocytes and perivascular cells appears to be specific to those residing within the bone marrow, as the same cell types elsewhere are not targeted. By crossing the Osx-Cre mouse with the R26-mT/mG reporter line and analyzing the progenies at two months of age, we find that Osx-Cre targets not only osteoblasts, osteocytes and hypertrophic chondrocytes as expected, but also stromal cells, adipocytes and perivascular cells in the bone marrow. Because the strain has been increasingly used in postnatal studies, it is important to evaluate its targeting specificity in mice after birth. ![]() The mouse strain was initially characterized during embryogenesis, and found to target mainly osteoblast-lineage cells. The Osx-Cre mouse line (also known as Osx1-GFP::Cre) expresses GFP::Cre fusion protein from a BAC transgene containing the Osx regulatory sequence. Osterix (Osx or Sp7) is a zinc-finger-family transcriptional factor essential for osteoblast differentiation in mammals. ![]()
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