Molecular Control of MSC differentiation and Bone Formation by KDM4B

描述（由申请人提供）：本申请的长期目标是了解分子和表观遗传机制如何控制间充质干细胞/基质细胞（MSCs）的成骨分化、成骨细胞功能和骨形成。间充质干细胞是具有自我更新能力和多系分化潜能的多能祖细胞，包括成骨、软骨和脂肪生成。尽管在理解骨髓间充质干细胞分化的转录控制方面取得了重大进展，但对骨形成如何受到表观遗传调控知之甚少。组蛋白甲基化是激活和抑制基因表达的重要过程，在细胞分化的表观遗传调控中起关键作用。尽管越来越多的证据表明组蛋白去甲基化酶在表观遗传学上调控胚胎干细胞的特性和功能，但去甲基化酶对间充质干细胞分化和骨形成的影响在很大程度上是未知的。为了探索去甲基化酶在MSC分化中的作用，我们系统地分析了组蛋白去甲基化酶在bmp刺激的骨髓间充质干细胞中的表达，因为bmp是成骨分化的有效诱导剂。我们发现bmp能快速诱导赖氨酸(K)特异性去甲基化酶（KDM4B，也称为JMJD2B）的表达，该酶能在赖氨酸9 （H3K9me3）上去甲基化三甲基化组蛋白H3。H3K9me3是参与生长发育的基因沉默的标志。一般来说，当干细胞分化程序被触发时，一组特定的基因被激活。我们的初步研究表明，KDM4B促进MSCs的成骨分化，同时抑制成脂分化。此外，我们发现与年轻小鼠相比，衰老小鼠分离的MSCs中Kdm4b的表达明显下调。同时，H3K9me3标记在衰老小鼠和去卵巢小鼠的成骨细胞中显著升高。基于这些新发现，在本应用中，我们假设KDM4B擦除H3K9me3标记在MSCs的体外成骨分化和体内骨形成中起着不可或缺的作用。

DESCRIPTION (provided by applicant): The long-term objectives of this application are to understand how molecular and epigenetic mechanisms control osteogenic differentiation of mesenchymal stem/stromal cells (MSCs), osteoblast function, and bone formation. MSCs are multipotent progenitor cells with self-renewal capabilities and multilineage differentiation potentials including osteogenesis, chondrogenesis and adipogenesis. Although significant progress has been made in understanding transcriptional control of MSC differentiation, little is known about how bone formation is epigenetically regulated. Histone methylation is an important process linked to the activation and repression of gene expression, thus it plays a critical role in epigenetic regulation of cell differentiation. While growing evidence indicates tht histone demethylases epigenetically regulate embryonic stem cell properties and functions, it is largely unknown what affect demethylases have on MSC differentiation and bone formation. To explore the role of demethylases in MSC differentiation, we systemically profiled the expression of histone demethylases in BMP-stimulated MSCs from bone marrow, as BMPs are potent inducers of osteogenic differentiation. We found that BMPs rapidly induced the expression of the lysine (K)-specific demethylase (KDM4B; also known as JMJD2B) that demethylates trimethylated histone H3 at lysine 9 (H3K9me3). H3K9me3 is a hallmark for gene silencing involved in growth and development. In general, a group of specific genes are activated when the stem cell differentiation program is triggered. Our preliminary studies demonstrated that KDM4B promoted osteogenic differentiation of MSCs while inhibiting adipogenic differentiation. Moreover, we found that the expression of Kdm4b was significantly down regulated in MSCs isolated from aging mice compared to young mice. Co-incidentally, H3K9me3 marks were significantly increased in osteoblasts of aging mice or ovariectomized mice. Based on these novel discoveries, in this application, we hypothesize that erasing H3K9me3 marks by KDM4B plays integral roles in osteogenic differentiation of MSCs in vitro and bone formation and in vivo. Three specific aims are proposed to test our hypothesis. Aim 1 is to determine whether KDM4B epigenetically regulate MSC lineage commitment through induction of DLX5. Aim 2 is to explore how erasing H3K9me3 by KDM4B coordinately regulates osteogenic differentiation of MSCs. Aim 3 is to determine whether KDM4B is required for bone formation in vivo and whether dysregulation of KDM4B impairs osteoblast function and bone formation in osteoporosis. Since histone demethylases are chemically modifiable, KDM4B may present as a novel therapeutic target for specifically controlling the differentiation of MSCs in regenerative medicine, and also lead to clues for new treatment in metabolic bone diseases such as, osteoporosis.