Runx2 is a known master transcription factor for osteoblast differentiation, as well as an essential regulator for chondrocyte maturation. Recently, more and more data has shown that Runx2 regulates hypertrophic chondrocyte-specific type X collagen gene (Col10a1) expression in different species. However, how Runx2 regulation of Col10a1 expression impacts chondrocyte maturation, an essential step of endochondral bone formation, remains unknown. We have recently generated transgenic mice in which flag-tagged Runx2 was driven by a cell-specific Col10a1 control element. Significantly increased level of Runx2 and Col10a1 mRNA transcripts were detected in transgenic mouse limbs at both E17.5 (embryonic day 17.5) and P1 (postnatal day1) stages, suggesting an in vivo correlation of Runx2 and Col10a1 expression. Surprisingly, skeletal staining suggested delayed ossification in both the axial and the appendicular skeleton of transgenic mice from E14.5 until P6. Histological analysis showed elongated hypertrophic zones in transgenic mice, with less von Kossa and TUNEL staining in long bone sections at both E17.5 and P1 stages, suggesting defective mineralization due to delayed chondrocyte maturation or apoptosis. Indeed, we detected increased level of anti-apoptotic genes Bcl-2, Opn, and Sox9 in transgenic mice by real-time RT-PCR. Moreover, immunohistochemistry and Western blotting analysis also suggested increased Sox9 expression in hypertrophic chondrocytes of transgenic mice. Together, our data suggest that targeting Runx2 in hypertrophic chondrocytes upregulates expression of Col10a1 and other marker genes (such as Sox9 etc.). This will change the local matrix environment, delay chondrocyte maturation, reduce apoptosis and matrix mineralization, and eventually, lead to impaired endochondral ossification.

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In this work, we investigated the correlation between the molar mass and the rheological properties of cellulose/1-butyl-3-methylimidazolium chloride (BmimCl) solutions, and provided the depolymerization kinetics of cellulose in BmimCl. Gel permeation chromatography was used to track the change in molar mass and kinetics as a function of the dissolution time. The molar mass of cellulose in BmimCl decreased significantly as the dissolution time increased, following a zeroth order rate law. The decrease of inter-chain friction induced by depolymerization resulted in a lower viscosity, shorter relaxation time, and lower activation energy. The activation energies for flow were distinctly different above and below the critical molar mass, which indicates that the relaxation mechanisms were not identical above and below the critical molar mass. The transition behavior of liquid crystalline phase also changed at the critical molar mass, which strongly demonstrated the effect of chain length on the formation of cholesteric phase. The exponents of Mark-Houwink-Sakurada and the radius of gyration showed that cellulose in BmimCl existed as a Gaussian chain in a theta solvent.