Notch signaling controls chondrocyte hypertrophy via indirect regulation of Sox9

Abstract

RBPjk-dependent Notch signaling regulates both the onset of chondrocyte hypertrophy and the progression to terminal chondrocyte maturation during endochondral ossification. It has been suggested that Notch signaling can regulate Sox9 transcription, although how this occurs at the molecular level in chondrocytes and whether this transcriptional regulation mediates Notch control of chondrocyte hypertrophy and cartilage development is unknown or controversial. Here we have provided conclusive genetic evidence linking RBPjk-dependent Notch signaling to the regulation of Sox9 expression and chondrocyte hypertrophy by examining tissue-specific Rbpjk mutant (Prx1Cre;Rbpjkf/f), Rbpjk mutant/Sox9 haploinsufficient (Prx1Cre;Rbpjkf/f;Sox9f/+), and control embryos for alterations in SOX9 expression and chondrocyte hypertrophy during cartilage development. These studies demonstrate that Notch signaling regulates the onset of chondrocyte maturation in a SOX9-dependent manner, while Notch-mediated regulation of terminal chondrocyte maturation likely functions independently of SOX9. Furthermore, our in vitro molecular analyses of the Sox9 promoter and Notch-mediated regulation of Sox9 gene expression in chondrogenic cells identified the ability of Notch to induce Sox9 expression directly in the acute setting, but suppresses Sox9 transcription with prolonged Notch signaling that requires protein synthesis of secondary effectors.

HES factors regulate specific aspects of chondrogenesis and chondrocyte hypertrophy during cartilage development

“…S2Dg,Dh). Surprisingly, neither Hes1 LOF or Hes1,5 LOF mutants appeared to exhibit an expanded hypertrophic zone at E18.5, which has been observed previously in several Notch LOF mutant mice and is indicative of a continuous delay in terminal chondrocyte hypertrophy and cartilage matrix catabolism Mead and Yutzey, 2009;Kohn et al, 2012;Kohn et al, 2015). Based on these data, combined with the observation that HEY factor expression increases in maturing and hypertrophic chondrocytes (Fig.…”

“…Genetic removal of various Notch signaling components ( presenilin1, presenilin2, Notch1, Notch2 and RBPjκ) within MPCs using Prx1Cre or within cartilage progenitor cells using a Col2Cre transgene delays the onset and progression of chondrocyte hypertrophy and cartilage matrix catabolism Kohn et al, 2012), whereas activation of NICD in committed chondrocytes both in vivo and in vitro promotes chondrocyte hypertrophy and cartilage matrix catabolism (Mead and Yutzey, 2009;Kohn et al, 2012). Recently, we have also demonstrated that several of the Notch-mediated effects on cartilage development occur in an RBPjκ-dependent manner (Dong et al, 2010) and are likely to be the consequence of an indirect transcriptional regulation of Sox9 (Kohn et al, 2015). Although the importance of Notch signaling in cartilage development has been well documented, the precise molecular mechanism(s) by which Notch regulates these distinct processes remain unclear or unknown.…”

“…Previous studies have demonstrated RBPjκ-dependent Notch regulation of Sox9; however, the exact mechanism remains unknown or controversial (Mead and Yutzey, 2009;Dong et al, 2010;Kohn et al, 2012;Chen et al, 2013;Kohn et al, 2015). Recent data have suggested that the Notch-mediated transcriptional regulation of Sox9 occurs indirectly through secondary effectors (Kohn et al, 2015).…”

“…We have shown previously that RBPjκ-dependent Notch signaling is an important regulator of the onset and progression of chondrocyte hypertrophy Kohn et al, 2012;Kohn et al, 2015). To determine whether HES1 regulates the onset and progression of chondrocyte hypertrophy, we first analyzed Hes1 LOF and WT embryos at E14.5.…”

Oncogenic role of the Notch pathway in primary liver cancer

“…Constitutive Notch2 overexpression causes HPCs to spontaneously develop into dedifferentiated HCC cells (79). In addition, Notch-induced malignant hepatocyte transformation is associated with downregulation of hepatocyte-associated genes and Sox9 expression (80). Fate-mapping studies have demonstrated that clear-cell adenocarcinoma (CCA) cells derived from hepatocytes may be converted to a biliary K-19 positive phenotype (81).…”