g Recent studies showed that cyclophilin A (CypA) promotes NF-B/p65 nuclear translocation, resulting in enhanced NF-B activity and altered expression of its target genes, such as the Sox9 transcriptional factor, which plays a critical role in chondrogenic differentiation and endochondral ossification. In this report, we unveil the role of CypA in signal-induced chondrogenic differentiation and endochondral ossification. Expression levels of the chondrogenic differentiation markers and transcriptional regulators Sox9 and Runx2 were all significantly lower in CypA knockdown chondrogenic cells than in wild-type cells, indicating that CypA plays a functional role in chondrogenic differentiation. In vitro differentiation studies using micromass cultures of mouse limb bud cells further supported the conclusion that CypA is needed for chondrogenic differentiation. Newborn CypAdeficient pups double stained with alcian blue and alizarin red exhibited generalized, pronounced skeletal defects, while highresolution micro-computed tomography (
Chondrogenesis is an essential process in vertebrates. It leads to the formation of cartilage growth plates, thereby driving bodily growth while providing structural templates and induction signals for the formation of long bones through endochondral ossification (1). On the other hand, defects in chondrogenesis cause various chondrodysostoses and chondrodysplasias, with such skeletal malformations accounting for a significant proportion of human birth defects that often result in embryonic and perinatal lethality (2). To identify the molecular mechanisms that drive chondrocyte differentiation and impact underlying cartilage diseases, the transcriptional mechanisms governing their cartilage-specific expression have been intensely studied. As a general overview, the chondrocyte differentiation pathway corresponds to a succession of major genetic program switches that are likely controlled by a specific set of transcriptional activators, repressors, and associated factors. While some of these factors play essential roles in determining cell fate and differentiation, other factors are found to be mutated in severe diseases of cartilage and bone malformation (3). The framework of the cartilage matrix is a collagen fiber network comprised primarily of type II collagen (Col2) (encoded by the Col2␣1 gene) and secondarily of type IX collagen (encoded by Col9␣1, Col9␣2, and Col9␣3). Collagen type X (Col10) (encoded by Col10␣1) is also produced in abundance, but exclusively by prehypertrophic and hypertrophic chondrocytes (2).Importantly, chondrocyte differentiation requires Sox family transcription factors in the early stages and Runx2 in the late stages (4, 5). In particular, Sox9 has an essential, nonredundant role in specifying the commitment and differentiation of mesenchymal cells toward the chondrogenic lineage in all developing skeletal elements. While Sox9 is turned on in chondrogenic and osteogenic mesenchymal cells prior to condensation and remains highly expressed in prechondrocytes...