Glucuronyl C5-epimerase (Hsepi) catalyzes the conversion of glucuronic acid to iduronic acid in the process of heparan sulfate biosynthesis. Targeted interruption of the gene, Glce, in mice resulted in neonatal lethality with varied defects in organ development. To understand the underlying molecular mechanisms of the phenotypes, we used mouse embryonic fibroblasts (MEF) as a model to examine selected signaling pathways. Our earlier studies found reduced activities of FGF-2, GDNF, but increased activity of sonic hedgehog in the mutant cells. In this study, we focused on the bone morphogenetic protein (BMP) signaling pathway. Western blotting detected substantially elevated endogenous Smad1/5/8 phosphorylation in the Hsepi mutant (KO) MEF cells, which is reverted by re-expression of the enzyme in the KO cells. The mutant cells displayed an enhanced proliferation and elevated alkaline phosphatase activitywhen cultured in osteogenic medium. Analysis of the genes involved in the BMP signaling pathway revealed upregulation of a number of BMP ligands, but reduced expression of several Smads and BMP antagonist (Grem1) in the KO MEF cells. The high level of Smad1/5/8 phosphorylation was also found in primary calvarial cells isolated from the KO mice. The results suggest that Hsepi expression modulates BMP signaling activity, which, at least partially, is associated with defected molecular structure of heparan sulfate expressed in the cells.

葡萄糖醛酸C5-表异构酶（Hsepi）在硫酸乙酰肝素的生物合成过程中催化葡萄糖醛酸转化为艾杜糖醛酸。有针对性的基因Glce的中断，小鼠体内的致死导致新生儿致死，器官发育中存在各种缺陷。为了了解表型的潜在分子机制，我们使用小鼠胚胎成纤维细胞（MEF）作为模型来检查选定的信号通路。我们较早的研究发现，FGF-2，GDNF的活性降低，但突变细胞中的声刺猬活性提高。在这项研究中，我们专注于骨形态发生蛋白（BMP）信号传导途径。Western印迹检测到Hsepi突变（KO）MEF细胞中内源性Smad1 / 5/8磷酸化水平显着升高，该酶通过在KO细胞中重新表达该酶而恢复。当在成骨培养基中培养时，突变细胞显示出增强的增殖和升高的碱性磷酸酶活性。KO MEF单元格中的Grem1）。在从KO小鼠分离的原发颅盖细胞中也发现了高水平的Smad1 / 5/8磷酸化。结果表明，Hsepi表达调节BMP信号传导活性，这至少部分与细胞中表达的硫酸乙酰肝素的分子结构缺陷有关。