Mechanism and clinical application of stem cell-derived exosomes in promoting fracture healing

doi:10.3969/j.issn.2095-4344.1565

Abstract

Abstract:

BACKGROUND: Recently, increasing researches have reported the specific function of stem cell-derived exosomes based on its biological features in osteogenic differentiation, angiogenesis and bone mineralization, indicating that stem cell-derived exosome is a potential material in promoting fracture healing.
OBJECTIVE: To review the mechanism and clinical progress of stem cell-derived exosomes in promoting fracture healing, which is expected to provide a new insight into its clinical therapy.
METHODS: The first author retrieved literatures on bone graft materials and bone defect repair in CNKI (Medical Edition) database and PubMed database by the keywords of “stem cell derived exosome, fracture healing, bone regeneration, osteogenic differentiation” in Chinese and English, respectively. Finally, 49 articles were eligible for result analysis
RESULTS AND CONCLUSION: Stem cell-derived exosomes can promote fracture healing by selectively carrying genetic factors, proteins, transcription factors and other biologically active substances. The promotion by stem cell-derived exosomes is mainly implemented by improving fracture microenvironment, promoting osteogenic differentiation, angiogenesis and bone mineralization. However, due to the efficient and economical separation and extraction methods and suitable vectors, the clinical application of stem cell-derived exosomes to promote fracture healing still needs further research.

中国组织工程研究杂志出版内容重点：干细胞；骨髓干细胞；造血干细胞；脂肪干细胞；肿瘤干细胞；胚胎干细胞；脐带脐血干细胞；干细胞诱导；干细胞分化；组织工程

Key words: Stem Cells, Exosomes, Fracture Healing, Tissue Engineering

CLC Number:

Zhu Wanbo, Lu Jinsen, Yang Jiazhao, Fang Shiyuan. Mechanism and clinical application of stem cell-derived exosomes in promoting fracture healing[J]. Chinese Journal of Tissue Engineering Research, 2019, 23(5): 815-820.

Figures/Tables 1

2.1 干细胞源性外泌体促骨折愈合的生物学基础骨折愈合是一个复杂的生理过程。在骨折微环境中，骨母细胞和干细胞向成骨细胞增殖分化并移行至骨缺损和骨吸收区域，未成熟的成骨细胞产生含钙盐和磷酸盐的骨基质，并促使其矿化形成矿化骨是骨折愈合的主要机制[10]。调控干细胞和骨祖细胞的成骨分化是促骨折愈合的基础之一。骨折微环境中的新生血管能为骨折愈合带来必须的营养和无机盐，丰富的血管加快局部废物代谢的同时带来大量迁移的多能干细胞，提高成骨分化和骨重建的效率[11-12]。前期大量研究发现，干细胞源性外泌体可通过调控多能干细胞成骨分化，促进新生血管生成，提高无机盐沉积和基质矿化等途径，促进骨折愈合。这一过程是由干细胞源性外泌体选择性携带的生物活性物质所决定[9]。 2.2 外泌体携带的遗传物质调控促骨折愈合过程干细胞源性外泌体能携带遗传物质如微小RNA(miRNA)和mRNA，在骨细胞间和干细胞间传递遗传信息，是调控干细胞源性外泌体促骨折愈合的重要组成部分[13]。miRNA被认为是成骨细胞相关骨生成和骨重建中重要的转录后调控因子，能开启一系列骨折修复相关反应[14]。在脂质膜的包裹下miRNA能稳定存在于外泌体中免于免疫系统的分解，并最终随外泌体以膜融合或直接胞吞的方式进入靶细胞，发挥效应。 2.2.1 干细胞源性外泌体miRNA具有激活靶细胞成骨分化和血管新生能力，促进骨生成的作用 Xu等[15]于2014年首次发现骨髓间充质干细胞源性外泌体(BMSC-Exo)miRNA在促骨髓间充质干细胞成骨分化过程中的分布具有差异性，其中一系列前期实验发现提高干细胞成骨分化能力的miRNA如let-7a等显著上调。还有一些体内外对比实验也相继发现一些特定的干细胞源性外泌体miRNA具有促成骨分化能力。Furuta等[16]利用外泌体沉默表达的CD9-/-小鼠做动物模型，从间充质干细胞培养基分离得到的骨髓间充质干细胞源性外泌体较对照组能加速骨折愈合，体外分析外泌体内容物发现miR-21、miR-4332等成骨分化相关miRNA与其他细胞来源外泌体相比高表达。有趣的是，被认为具有募集干细胞、促进成骨分化和血管生成功能的单核细胞趋化蛋白(MCP-1、MCP-3)和基质细胞衍生因子(SDF-1)在骨髓间充质干细胞源性外泌体内含量较对照组低，研究者推测成骨分化和血管生成相关miRNA在干细胞源性外泌体内的差异性分布，提示这些具有特殊功能的miRNA才是促骨折愈合的主要机制。表1列举了一部分研究中主要的差异性分布的miRNA及其生物学效应[15-19]。 2.2.2 不同分化阶段干细胞外泌体促成骨分化能力具有差异性体外实验证实分化晚期人间充质干细胞源性外泌体(hMSCs-Exo)具有最强的促成骨分化能力，这可能是由于不同分化阶段干细胞选择性携带的miRNA含量不同所决定[15，19]。miR-31，miR-221和miR-144等具有抑制成骨分化作用的miRNA在分化晚期干细胞源性外泌体中含量显著降低，而miR-21，miR-10b等促成骨分化相关miRNA的含量显著上调，使干细胞源性外泌体对靶细胞成骨分化具有促调控作用[19]。 2.2.3 干细胞源性外泌体miRNA调控破骨细胞分化，促进骨吸收骨折愈合是成骨-破骨分化的动态平衡过程，骨折微环境中骨吸收是启动一系列骨折愈合机制的开始[13]。骨髓间充质干细胞源性外泌体中携带的miR-503等miRNA具有促进干细胞破骨分化和骨吸收的作用[20]。这些miRNA通过调控RANK-RANKL信号通路调节成骨-破骨的动态平衡，促进骨折愈合[13]。 2.2.4 一些不同干细胞源性外泌体mRNA也具备激发骨折愈合的能力 Tooi等[21]发现人胚胎间充质干细胞来源外泌体(hPMSC-Exo)与骨形态发生蛋白2共培养时，细胞外基质标记物碱性磷酸酶和成骨细胞特异性转录因子OSX的mRNA表达显著上调，成纤维细胞成骨分化加速，骨矿物沉积增加。体外实验发现人脂肪来源干细胞源性外泌体(ASCs-Exo)能通过上调成骨相关基因RUNX2、ALP和COL1A1的mRNA表达，提高靶细胞成骨能力，促进骨生成[22]。干细胞源性外泌体miRNA和mRNA是遗传信息的载体，其促进骨折愈合作用需要信号通路、蛋白质和细胞因子的共同参与。 2.3 信号通路与转录因子介导促骨折愈合过程干细胞源性外泌体选择性携带的遗传物质和功能性蛋白作用于靶细胞后，通过借助一系列信号通路和转录因子介导促骨折愈合过程。 RUNX2是调控成骨分化的转录因子，其表达可指导多能干细胞向成骨细胞系分化并抑制成骨细胞成熟，维持未成熟成骨细胞持续的成骨能力，在干细胞修复骨损伤中扮演重要角色[23]。Qi等[24]通过体内实验发现人诱导性多能间充质干细胞源性外泌体(hiPS-MSC-Exo)在上调RUNX2等转录因子表达后，激发成骨分化，促进新生血管生成，提高动物骨折愈合率。干细胞源性外泌体miRNA在RUNX2的上调中至关重要[25]。分化晚期间充质干细胞源性外泌体中含量大幅降低的miR-31影响RUNX2下游基因Osterix和SATB2表达下调，是潜在的RUNX2作用位点[26]。体外实验证实降低或抑制miR-31表达可促进成骨分化，提高骨含量和骨密度[27]。"

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