Biomedical subjects

Gerard Karsenty

Publications and source records attributed to Gerard Karsenty.

57 records · Page 4Linked to original sources

PubMed2002

PTHrP and Indian hedgehog control differentiation of growth plate chondrocytes at multiple steps.

In developing murine growth plates, chondrocytes near the articular surface (periarticular chondrocytes) proliferate, differentiate into flat column-forming proliferating cells (columnar chondrocytes), stop dividing and finally differentiate into hypertrophic cells. Indian hedgehog (Ihh), which is predominantly expressed in prehypertrophic cells, stimulates expression of parathyroid hormone (PTH)-related peptide (PTHrP) which negatively regulates terminal chondrocyte differentiation through the PTH/PTHrP receptor (PPR). However, the roles of PTHrP and Ihh in regulating earlier steps in chondrocyte differentiation are unclear. We present novel mouse models with PPR abnormalities that help clarify these roles. In mice with chondrocyte-specific PPR ablation and mice with reduced PPR expression, chondrocyte differentiation was accelerated not only at the terminal step but also at an earlier step: periarticular to columnar differentiation. In these models, upregulation of Ihh action in the periarticular region was also observed. In the third model in which the PPR was disrupted in about 30% of columnar chondrocytes, Ihh action in the periarticular chondrocytes was upregulated because of ectopically differentiated hypertrophic chondrocytes that had lost PPR. Acceleration of periarticular to columnar differentiation was also noted in this mouse, while most of periarticular chondrocytes retained PPR signaling. These data suggest that Ihh positively controls differentiation of periarticular chondrocytes independently of PTHrP. Thus, chondrocyte differentiation is controlled at multiple steps by PTHrP and Ihh through the mutual regulation of their activities.

Animals↗

PubMed2002

Cell-type-dependent up-regulation of in vitro mineralization after overexpression of the osteoblast-specific transcription factor Runx2/Cbfal.

Functional expression of the transcriptional activator Runx2/Cbfal is essential for osteoblastic differentiation and bone formation and maintenance. Forced expression of Runx2 in nonosteoblastic cells induces expression of osteoblast-specific genes, but the effects of Runx2 overexpression on in vitro matrix mineralization have not been determined. To examine whether exogenous Runx2 expression is sufficient to direct in vitro mineralization, we investigated sustained expression of Runx2 in nonosteoblastic and osteoblast-like cell lines using retroviral gene delivery. As expected, forced expression of Runx2 induced several osteoblast-specific genes in NIH3T3 and C3H10T1/2 fibroblasts and up-regulated expression in MC3T3-E1 immature osteoblast-like cells. However, Runx2 expression enhanced matrix mineralization in a cell-type-dependent manner. NIH3T3 and IMR-90 fibroblasts overexpressing Runx2 did not produce a mineralized matrix, indicating that forced expression of Runx2 in these nonosteogenic cell lines is not sufficient to direct in vitro mineralization. Consistent with the pluripotent nature of the cell line, a fraction (25%) of Runx2-expressing C3H10T1/2 fibroblast cultures produced mineralized nodules in a viral supernatant-dependent manner. Notably, bone sialoprotein (BSP) gene expression was detected at significantly higher levels in mineralizing Runx2-infected C3H10T1/2 cells compared with Runx2-expressing cultures which did not mineralize. Treatment of Runx2-infected C3H10T1/2 cultures with dexamethasone enhanced osteoblastic phenotype expression, inducing low levels of mineralization independent of viral supernatant. Finally, Runx2 overexpression in immature osteoblast-like MC3T3-E1 cells resulted in acceleration and robust up-regulation of matrix mineralization compared with controls. These results suggest that, although functional Runx2 is essential to multiple osteoblast-specific activities, in vitro matrix mineralization requires additional tissue-specific cofactors, which supplement Runx2 activity.

3T3 Cells↗

PubMed2001

Physical interaction of the activator protein-1 factors c-Fos and c-Jun with Cbfa1 for collagenase-3 promoter activation.

Previously, we determined that the activator protein-1 (AP-1)-binding site and the runt domain (RD)-binding site and their binding proteins, c-Fos.c-Jun and Cbfa, regulate the collagenase-3 promoter in parathyroid hormone-treated and differentiating osteoblasts. Here we show that Cbfa1 and c-Fos.c-Jun appear to cooperatively bind the RD- and AP-1-binding sites and form ternary structures in vitro. Both in vitro and in vivo co-immunoprecipitation and yeast two-hybrid studies further demonstrate interaction between Cbfa1 with c-Fos and c-Jun in the absence of phosphorylation and without binding to DNA. Additionally, only the runt domain of Cbfa1 was required for interaction with c-Jun and c-Fos. In mammalian cells, overexpression of Cbfa1 enhanced c-Jun activation of AP-1-binding site promoter activity, demonstrating functional interaction. Finally, insertion of base pairs that disrupted the helical phasing between the AP-1- and RD-binding sites also inhibited collagenase-3 promoter activation. Thus, we provide direct evidence that Cbfa1 and c-Fos.c-Jun physically interact and cooperatively bind the AP-1- and RD-binding sites in the collagenase-3 promoter. Moreover, the AP-1- and RD-binding sites appear to be organized in a specific required helical arrangement that facilitates transcription factor interaction and enables promoter activation.

Animals↗