Age-Related Kynurenine Accumulation Impairs miRNA and Hdac Epigenetic Regulation of the SDF-1 Axis Resulting in Bone Loss

与衰老相关的骨质疏松症的机制仍不明确，然而，我们最近的研究 小组和其他人提出，这是一种干细胞疾病。我们一直在研究细胞因子SDF-1的作用 (CXCL12)和HDAC3在骨髓间充质干细胞/基质细胞(BMSC)功能和骨稳态中的作用。 一个关键的问题是，衰老会导致神经干细胞的本地化、增殖、存活和分化受损 骨髓(BM)中的成骨祖细胞群体，特别是BMSCs。一个严重的障碍 防止这些变化的关键是识别关键的调控途径，并能够改变或纠正它们。我们 已经在microRNA miR-29b-1-5p中发现了BMSC表观遗传学变化，似乎与组蛋白相互作用 脱乙酰酶3(HDAC3)，第二个表观遗传调控系统。此外，我们已经确认了氧化的 色氨酸(Trp)代谢产物犬尿氨酸(KYN)改变BMSCs中这些特定的表观遗传调节分子， 进而通过SDF-1信号通路直接影响细胞存活、成骨、成骨细胞脂 储存和骨骼形成。值得注意的是，miR-29b-1-5p在BMSCs中随着年龄的增长而增加，属于 MiR29家族，已被证明在细胞外基质动态平衡和 成骨作用。一个新的方面是miR29b-1-5p是通常认为的“乘客链” 降解，留下互补的miR-29b-1-3p作为功能性miRNA；然而，随着年龄的增长， 我们的数据表明，这个客体链是BMSC功能障碍的重要影响因素。重要的是，miR-29b- 靶向SDF-1的1-5P被KYN上调，而HDAC3被下调，导致血脂增加 骨髓成骨祖细胞和成骨细胞的储存和功能障碍。了解上游机制 在miR-29b-1-5p和HDAC3中推动这些以前未知的与年龄相关的变化是一个关键目标，因为 我们已经证明，这两个相互作用的系统调节SDF-1的表达并抑制BMSC 成骨和存活途径，同时增加骨髓脂肪储存。我们建议检验这个假设 与年龄相关的KYN水平升高导致miR-29b-1-5p表达增加，并抑制HDAC3 通过SDF-1及其受体CXCR4表达，或直接通过SDF-1及其受体CXCR4对骨稳态产生下游作用 在骨髓间充质干细胞和成骨细胞中靶向额外的成骨基因。我们的目标是检验新的假设 通过操纵我们确定的随年龄变化的miRNAs和HDAC来获得我们的发现 人/鼠骨髓间充质干细胞在体内和分子水平对骨形成和转换的影响 对BMSC成骨功能的影响。我们将测试减少这些分子表达的新方法， 包括抑制KYN的产生，抑制KYN信号通路，或将合成的抗miRNAs传递到 抑制miR29B-1-5P。这个项目的影响将是阐明与年龄相关的表观遗传学变化的作用。 在相互交谈的BMSC miRNA和HDAC系统中，目标是确定新的目标以减少或 逆转，与年龄相关的骨丢失和骨质疏松症。

The mechanisms involved in aging-related osteoporosis remain poorly defined, however, recent studies from our group, and others, suggest that it is a stem cell disease. We have been studying the role of the cytokine SDF-1 (CXCL12) and Hdac3 in bone marrow mesenchymal stem/stromal cell (BMSC) function and bone homeostasis. A critical problem is that aging triggers impaired localization, proliferation, survival, and differentiation of the osteogenic progenitor cell population in the bone marrow (BM), specifically BMSCs. A critical barrier to preventing these changes is identifying key regulatory pathways, and being able to alter or correct them. We have identified BMSC epigenetic changes in the microRNA miR-29b-1-5p, that appears to cross-talk with histone deacetylase 3 (Hdac3), a second epigenetic regulatory system. Further, we have identified that the oxidized tryptophan (TRP) metabolite kynurenine (KYN) alters these specific epigenetic regulatory molecules in BMSCs, which in turn directly, and via SDF-1 signaling pathways, affect cell survival, osteogenesis, osteoblastic lipid storage, and bone formation. Significantly, miR-29b-1-5p, which increases with aging in BMSCs, belongs to the miR29 family of miRNAs that have been shown to be critical in extracellular matrix homeostasis and osteogenesis. One novel aspect is that miR29b-1-5p is the “passenger strand”, which is normally thought to be degraded leaving the complementary miR-29b-1-3p “guide strand” as the functional miRNA; with aging, however, our data suggest that this passenger strand is an important effector of BMSC dysfunction. Importantly, miR-29b- 1-5p, which targets SDF-1, is upregulated by KYN while Hdac3 is down regulated leading to increased lipid storage and dysfunction in BM osteoprogenitor cells and OBs. Understanding the upstream mechanisms that drive these previously unknown age-associated changes in miR-29b-1-5p and Hdac3 is a critical goal because we have demonstrated that these two interacting systems regulate SDF-1 expression and suppress BMSC osteogenesis and survival pathways while increasing BM fat storage. We propose to test the hypothesis that the elevated, age-related levels of KYN drive the increased expression of miR-29b-1-5p and inhibition of Hdac3 expression, with downstream effects on bone homeostasis via SDF-1 and its receptor CXCR4, or directly by targeting additional osteogenic genes both in BMSCS and OBs. Our objectives are to test new hypotheses derived from our findings by manipulating the miRNAs and Hdacs we identified as changing with age in human/murine BMSCs to determine their effects on bone formation and turnover in vivo and at the molecular level on BMSC osteogenic function. We will test novel methods to reduce expression of these molecules, including inhibiting KYN generation, inhibiting KYN signaling pathways, or delivering synthetic anti-miRNAs to inhibit miR29b-1-5p. The impact of this project will be to clarify the roles of age-associated epigenetic changes in cross-talking BMSC miRNA and Hdac systems with the goal of identifying novel targets for reducing, or reversing, age-related bone loss and osteoporosis.