The identification of mutations in the SRY-related SOX9 gene in patients with campomelic dysplasia, a severe skeletal malformation syndrome, and the abundant expression of Sox9 in mouse chondroprogenitor cells and fully differentiated chondrocytes during embryonic development have suggested the hypothesis that SOX9 might play a role in chondrogenesis. Our previous experiments with the gene (Col2a1) for collagen II, an early and abundant marker of chondrocyte differentiation, identified a minimal DNA element in intron 1 which directs chondrocyte-specific expression in transgenic mice. This element is also a strong chondrocyte-specific enhancer in transient transfection experiments. We show here that Col2a1 expression is closely correlated with high levels of SOX9 RNA and protein in chondrocytes. Our experiments indicate that the minimal Col2a1 enhancer is a direct target for Sox9. Indeed, SOX9 binds to a sequence of the minimal Col2a1 enhancer that is essential for activity in chondrocytes, and SOX9 acts as a potent activator of this enhancer in cotransfection experiments in nonchondrocytic cells. Mutations in the enhancer that prevent binding of SOX9 abolish enhancer activity in chondrocytes and suppress enhancer activation by SOX9 in nonchondrocytic cells. Other SOX family members are ineffective. Expression of a truncated SOX9 protein lacking the transactivation domain but retaining DNA-binding activity interferes with enhancer activation by full-length SOX9 in fibroblasts and inhibits enhancer activity in chondrocytes. Our results strongly suggest a model whereby SOX9 is involved in the control of the cell-specific activation of COL2A1 in chondrocytes, an essential component of the differentiation program of these cells. We speculate that in campomelic dysplasia a decrease in SOX9 activity would inhibit production of collagen II, and eventually other cartilage matrix proteins, leading to major skeletal anomalies.Acquisition of the chondrocyte phenotype is one of the major pathways of mesenchymal cell differentiation. During and after condensation of mesenchymal cells, cartilage-specific genes are switched on (2, 12). The products of these genes, which include collagen types II, IX, and XI, the link protein, and aggrecan, form the characteristic extracellular matrix of cartilages. In recent years, differentiation of mesenchymal cells into myocytes and adipocytes has been shown to be controlled by cell-specific transcription factors belonging to different protein families (10,28,39,41,46). By analogy, we speculate that specific transcription factors could also control the differentiation of mesenchymal cells into chondrocytes and activate cartilage-specific genes.In order to identify transcription factors which control chondrocyte differentiation, we have used the gene for type II collagen (Col2a1), an early and abundant marker of chondrocytes (3), and have delineated a minimal sequence in this gene that is sufficient to direct chondrocyte-specific expression both in transgenic mice and in transient transfectio...

Liver mass depends on one or more unidentified humoral signals that drive regeneration when liver functional capacity is diminished. Bile acids are important liver products, and their levels are tightly regulated. Here, we identify a role for nuclear receptor-dependent bile acid signaling in normal liver regeneration. Elevated bile acid levels accelerate regeneration, and decreased levels inhibit liver regrowth, as does the absence of the primary nuclear bile acid receptor FXR. We propose that FXR activation by increased bile acid flux is a signal of decreased functional capacity of the liver. FXR, and possibly other nuclear receptors, may promote homeostasis not only by regulating expression of appropriate metabolic target genes but also by driving homeotrophic liver growth.

In humans, SOX9 heterozygous mutations cause the severe skeletal dysmorphology syndrome campomelic dysplasia. Except for clinical descriptions, little is known about the pathogenesis of this disease. We have generated heterozygous Sox9 mutant mice that phenocopy most of the skeletal abnormalities of this syndrome. The Sox9 ؉/؊ mice died perinatally with cleft palate, as well as hypoplasia and bending of many skeletal structures derived from cartilage precursors. In embryonic day (E)14.5 heterozygous embryos, bending of radius, ulna, and tibia cartilages was already prominent. In E12.5 heterozygotes, all skeletal elements visualized by using Alcian blue were smaller. In addition, the overall levels of Col2a1 RNA at E10.5 and E12.5 were lower than in wild-type embryos. We propose that the skeletal abnormalities observed at later embryonic stages were caused by delayed or defective precartilaginous condensations. Furthermore, in E18.5 embryos and in newborn heterozygotes, premature mineralization occurred in many bones, including vertebrae and some craniofacial bones. Because Sox9 is not expressed in the mineralized portion of the growth plate, this premature mineralization is very likely the consequence of allele insufficiency existing in cells of the growth plate that express Sox9. Because the hypertrophic zone of the heterozygous Sox9 mutants was larger than that of wild-type mice, we propose that Sox9 also has a role in regulating the transition to hypertrophic chondrocytes in the growth plate. Despite the severe hypoplasia of cartilages, the overall organization and cellular composition of the growth plate were otherwise normal. Our results suggest the hypothesis that two critical steps of the chondrocyte differentiation pathway are sensitive to Sox9 dosage. First, an early step presumably at the stage of mesenchymal condensation of cartilage primordia, and second, a later step preceding the transition of chondrocytes into hypertrophic chondrocytes.T he differentiation of chondrocytes from mesenchymal cells occurs along a multistep pathway, during which committed mesenchymal cells first aggregate together to form precartilaginous condensations that prefigure the overall shape of future bones. Then expression of cartilage-specific proteins is initiated, and the cells become surrounded by abundant extracellular matrix (1). In the growth plate of endochondral bones, chondrocytes become flattened and undergo a unidirectional proliferation that is primarily responsible for the longitudinal growth of bones. After these cells stop proliferating, they change their genetic program and become hypertrophic. The extracellular matrix surrounding the hypertrophic chondrocytes that are closest to the metaphyses become mineralized and the cells undergo apoptosis, leaving behind a calcified cartilaginous matrix that is degraded subsequently and replaced by bone matrix (2).Recent studies have identified SOX9 as an essential transcription factor in chondrogenesis (3). In the absence of Sox9, there is a complete block in chondroc...

The farnesoid X receptor (FXR) is a nuclear receptor that plays key roles in hepatoprotection by maintaining the homeostasis of liver metabolism. FXR null mice display strong hepatic inflammation and develop spontaneous liver tumors. In this report, we demonstrate that FXR is a negative modulator of nuclear factor B (