Effect of bone morphogenetic protein 2 signal transduction pathway on osteogenesis, bone development and damage repair

doi:10.3969/j.issn.2095-4344.2013.02.028

Abstract

Abstract:

BACKGROUND: Bone morphogenetic proteins play an important role in bone formation, fracture healing and bone induction after bone destruction, and it can be used as a start regulating factor to promote the new bone formation with other signal paths. The regulation of endogenous bone morphogenetic protein 2 can be used as a new target in the future field of bone repair.
OBJECTIVE: Through analyzing and summarizing the mechanism of the bone morphogenetic protein signal transduction pathway in endochondral ossification reported from 1988 to now, adult bone growth and development, to explore the role of the bone morphogenetic protein signal pathways in the bone growth and damage repair.
METHODS: A computer-based search was performed to search relevant articles about bone morphogenetic protein signaling pathways and its function from VIP and PubMed databases (1998-01/2009-12), respectively by using the key words of “bone morphogenetic protein, endochondral ossification, bone repair” in Chinese and “bone reduction” in English. Literatures irrelevant to research purpose or containing a similar content were eliminated. Finally, 34 articles were chosen for further analysis.
RESULTS AND CONCLUSION: Because of its osteoinductive, bone morphogenetic protein signal pathway has become a hot spot in spinal surgery, fracture healing and bone induction after bone destruction. During the bone formation, the bone morphogenetic protein also serves as a start regulating factor to promote the new bone formation with other signal paths. In the bone micro environment, the bone morphogenetic protein and other signal paths that influence bone dynamic balance offer a new target for anabolic therapy that induces the bone formation, which provides a new thought on bone morphogenetic protein control to the formation of new bone following bone repair.

Key words: tissue construction, academic discussion in tissue construction, bone morphogenetic protein 2, signaling pathway, bone, damage repair, endochondral bone, microenvironment, bone regeneration, bone formation, osteoblasts, Sox9, provincial grants-supported paper

CLC Number:

R318

Hou Bo, Wang Yi, Shen Yu-hui. Effect of bone morphogenetic protein 2 signal transduction pathway on osteogenesis, bone development and damage repair[J]. Chinese Journal of Tissue Engineering Research, 2013, 17(2): 342-348.

Figures/Tables 1

2.1 BMP信号对软骨内成骨的作用小鼠和鸡的动物实验证明了BMP信号在软骨内成骨过程中是非常重要的。鼠体内BMPs复合体被去除表明了骨骼表型的多样性。这个结论和单独的BMPs在特定的骨骼位点上维持BMP活性的正常水平的观点是一致的[3]。这些研究也表明BMPs在直接调节软骨形成方面要优于骨形成。通过用头蛋白和腱蛋白来抵消BMP的活性，Barna和Niswander[4]检测了BMP介导的信号通路。这是BMP信号介导骨形成的早期发现。由于它出现在Sox9的表达之前，BMP的活性对于激活Sox9的表达式必要的，并且可以维持早期凝结期Sox9的表达水平。一旦凝结出现，持续的BMP信号可以调控成软骨细胞的扩增。 Lyons及其合作者的研究表明，从Co12a表达的细胞中去除BMP受体可导致PCNA染色的丧失[5-6]。这也表明了BMPs对于成软骨细胞的重要性。随后在肢体发生中，BMP活性的缺失阻碍了前成软骨细胞向成软骨细胞的分化。BMPs也参与了成软骨细胞的增生[7-8]。除了在软骨形成过程中的作用，BMP信号也有助于形成肢体交叉指型间质。在肢体胚胎发育时期，BMP信号通过调节AER细胞产生的FGF来直接介导交叉指型间叶细胞凋亡[9]。由于中胚层中细胞凋亡的衰减和细胞增殖的增多，在额外的足趾、足趾分歧和交叉指状蹼化中AER细胞BMP2和BMP4表达缺失[10]。BMP信号在关节形态发生中的作用也被发现。在这个过程中，BMP家族成员在区分骨骼和关节形成的界限上发挥了重要作用[11-12]。典型的BMP信号作为BMPs激活靶细胞的主要通路，在BMPs与Ⅰ型和Ⅱ型BMP受体结合时被识别。这两个受体分别是跨膜的丝氨酸/苏氨酸激酶，并形成一个多聚体受体配基复合物。在这个复合物中，基本磷酸化的Ⅱ型BMP受体通过磷酸化作用激活Ⅰ型BMP受体，开始了经典的BMP信号级联。激活的Ⅰ型BMP受体通过磷酸化的BMP特定蛋白1，5，8来传导BMP信号。这些BMP特定蛋白复合体和Smad4一起转移到细胞核内并作为靶基因复制的催化剂和阻遏剂。BMPs还可以通过非经典通路接受软骨形成过程中TAK1必须的数据，从而为经典的BMP信号通路和MAPK非经典信号通路提供潜在联系[13]。当单独的BMPs，Ⅰ型BMP受体和BMP特定蛋白从肢体中被去除，显型观察表明相当数量的多余的功能性因子存在于经典BMP信号通路中。在BMP2，BMP4，BMP5，BMP6和BMP7缺乏的情况下，胚胎骨骼发育可以发生。并且当BMPs被去除的时候，它们仅收到轻微的损害[3]。最近关于检测单独Ⅰ型BMP受体在软骨内成骨中的作用研究表明BMPR1A和BMPR1B在成软骨细胞发育过程中有重叠功能，尽管BMPR1A在这个过程中扮演着更为重要的角色[14]。经典BMP信号RSmads1，5和8下游介质的有条件的失活表明经典BMP信号对软骨形成是必不可少的，并且在Smads1和Smads5之间存在着功能上的分区[15-16]。 2.2 BMP信号在维持成人骨骼生长发育中的作用在初生动物中，BMPs对于快速的骨形成是必不可少的。出生后的小鼠由成骨细胞引起的BMP拮抗剂的过表达所引起的内源性BMP活性的普遍缺失会导致骨质缺乏、骨质脆性增加以及无意识的骨折[17-18]。关节软骨中缺乏BMP信号的鼠会形成一种骨关节炎样的表型。它最初在鼠7周龄时在膝关节中出现，到9月龄的时候骨关节炎就会表现得非常明显。它以关节表面软骨的丧失为特征，同时伴随着严重的运动功能损害[19]。在成人组织和器官中BMPs的功能之一体现在干细胞的维持和发育。而这或许可以解释当BMP信号在出生后的骨骼中减弱时所观察到的骨骼表型。BMP被认为是支持鼠的上皮干细胞和果蝇的卵巢干细胞必不可少的组成部分之一。在此BMP活性随着年龄增长而衰退的现象得到反转，可以储存和提高卵巢干细胞的功能并延长衰老干细胞的寿命[20-21]。在鼠的骨骼中，通过Ⅰ型BMP受体AIK3的BMP信号对于维持骨髓基质间隔内的造血干细胞表达水平是必不可少的[22]。这是关于骨髓基质细胞调节造血功能的一个发现。在骨髓的微环境内，骨祖细胞或间质干细胞产生这些细胞，但是当BMP活性过低时并不能维持其表达水平。事实上，内源性的BMPs在人类间质干细胞中作为生长因子已经被识别[23-24]。上述发现强调了BMP信号在成人骨中维持骨骼干细胞水平的作用。 2.3 BMP2在骨再生中的作用单独的BMPs断肢特定的靶基因使用证明了BMP2在出生后骨骼中的重要作用。缺乏肢体特定的BMP2基因表达的鼠有一个随着年龄增长逐渐恶化的骨骼表型。BMP2骨骼细胞的缺失导致了继发性骨化中心形成骨的延迟。早在出生后2周，缺乏BMP2的骨就有明显的微骨折。这个发现通常与诸如压力或张力等高负载条件下易导致成人骨抗压性的损害相联系[25]。如果这个损害不能被修复，骨折就会在短期内发生。而受BMP2缺失所影响的成骨细胞系内的特定细胞并没有被识别。一个微损伤修复的工作模式表明成骨细胞监测定位损伤并且发送信号来去除受到损伤的骨，以促进新骨形成[26]。从BMP2基因缺乏小鼠中对其骨祖细胞的监测表明它们在扩增、分化成完全有功能的成骨细胞过程中表现出了严重的缺陷[27]。这些发现表明BMP2基因的缺乏阻止了从骨祖细胞分化为骨细胞的进程。在分子水平，BMP2基因缺乏小鼠的骨祖细胞降低了Osterix，Wnt1，Lrp5，Fzd1，Axin1和Axin2的表达水平。由于Osterix是BMP信号和Wnt信号共同的靶基因，所以Osterix表达水平的下降可能是BMP活性降低的直接原因。或者它可能反映BMP2缺乏时Wnt信号通路的减退。BMP2和Wnt信号通路之间的联系可能是成人骨骼中的重要影响因素。在新形成的骨组织中，通过β连环蛋白的Wnt信号对Runx2的激活并不是非常重要的[28]。在出生后的骨骼中，经典的Wnt信号通路控制间叶祖细胞分化为成骨细胞系的过程、骨祖细胞的扩增、成熟基质产生成骨细胞的功能以及BMPs所调节的活性。 BMPs和Wnt在成骨细胞分化的不同时期是如何联系到目前为止仍然未知，并且从体内外模型体系中获得的数据有时是矛盾的。例如，Mbalaviele等[29]报道了在出生后的小鼠中激活的β环蛋白本身并不能诱导成骨细胞分化，但是它可以协同增强BMPs和它们的成骨活性。Chen等[30]观察到在Wnt受体中异位位点上移植的BMP2可以激活Wnt信号。并且在这个位点上，阻遏性β环蛋白在很大程度上降低了骨形成的数量。这些研究表明在成骨细胞系以及随后的成骨细胞分化过程中，BMPs和Wnt之间有一个可以改变的关系存在。出生后骨骼中BMPs和Wnt之间的相互关系是相关细胞所特有的。并且在很大程度上取决于BMP/Wnt相互作用的成骨细胞系通路[31]。在microRNA中一个激动人心的发现是其不同生物学功能的负性调节作用。Li等[32]最近研究表明BMP2处理的C2C12细胞会导致22microRNAs的下调，而这些与负性调节骨形成的包括Wnts、BMPs和FGAs在内的细胞因子所形成的靶基因相关联，甚至互相重叠。在这个模型中，BMP2调节细胞转化为成骨细胞系，并通过对microRNAs的调控来抑制肌细胞生成。在非洲爪蟾蜍胚胎中，BMP和BMP受体复合体结合激发Smad1的3个序列磷酸化作用，用以控制BMP信号的持续[33]。Ⅰ型BMP受体引发的第一步磷酸化导致了BMP信号的活化，这对于后来由MAPK介导的第二步磷酸化，以及GSK3β引起的最终磷酸化是必要的。在Wnts存在时，GSK3β的功能是不可用的，Smad1保持其活性，提高了BMO信号的持续性。如果骨组织中BMP靶细胞中相同的作用存在，它将与几个信号通路共同对骨骼发育产生深远的影响。除了受损的成骨细胞的功能，缺乏特定的BMP2骨基因表达的小鼠不能对骨折做出一个愈合反应。当涉及到正常愈合过程的细胞和分子被检测，发现最初的创伤愈合反应就会发生。但是在BMP2缺乏的情况下，骨细胞并没有被激活。对照组小鼠则表现出了强烈的骨膜活化反应，它可以在原先的位点上以及充满了高度增殖的缺陷位点上直接形成新的骨组织。缺乏BMP2的股骨有一些骨膜反应，从而产生反应性新骨。Chandler等[34]认为BMP2缺失时，损伤愈合的过程也变得缓慢。"

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