羅進(jìn)勇

(重慶醫(yī)科大學(xué)臨床檢驗(yàn)診斷學(xué)教育部重點(diǎn)實(shí)驗(yàn)室，重慶 400016)

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羅進(jìn)勇

骨形態(tài)發(fā)生蛋白9促成骨的分子機(jī)制

羅進(jìn)勇

(重慶醫(yī)科大學(xué)臨床檢驗(yàn)診斷學(xué)教育部重點(diǎn)實(shí)驗(yàn)室，重慶 400016)

骨形態(tài)發(fā)生蛋白(bone morphogenetic proteins，BMPs)屬于轉(zhuǎn)化生長(zhǎng)因子β(transforming growth factor-β，TGF-β)超家族的成員，因其具有誘導(dǎo)骨形成的能力而得名。在人類(lèi)中，BMPs家族至少包括15個(gè)以上的成員，其中具有促成骨作用的(成骨性BMPs)包括BMP2、BMP4、BMP6和BMP7，而B(niǎo)MP2和BMP7已經(jīng)在臨床用于促進(jìn)脊柱融合。近年發(fā)現(xiàn)，BMPs中的成員BMP9又稱(chēng)生長(zhǎng)分化因子2(growth differentiation factor 2，GDF2)也具有誘導(dǎo)成骨分化和骨形成的能力，而B(niǎo)MP9促成骨的相關(guān)分子機(jī)制也得到了一定的解析。

1BMP9具有強(qiáng)效促成骨作用

BMP9主要存在于肝臟，其具有誘導(dǎo)和維持胚胎神經(jīng)元的類(lèi)膽堿分化、調(diào)節(jié)葡萄糖和脂肪酸代謝、調(diào)節(jié)體內(nèi)鐵的動(dòng)態(tài)平衡、參與腫瘤發(fā)生及血管生成等多種重要功能。但是對(duì)BMP9在骨形成及骨再生中的作用，長(zhǎng)期以來(lái)都缺乏研究和了解。基于此，Kang等[1]利用重組腺病毒介導(dǎo)的方法，系統(tǒng)研究了BMP2～BMP15共14種BMPs在骨形成中的作用，結(jié)果發(fā)現(xiàn)BMP9也具有誘導(dǎo)成骨活性，且其促成骨能力遠(yuǎn)強(qiáng)于已在臨床應(yīng)用的BMP2和BMP7。在體外細(xì)胞實(shí)驗(yàn)中，BMP9可以促進(jìn)多種細(xì)胞如間充質(zhì)干細(xì)胞(mesenchymal stem cells，MSCs)、小鼠胚胎成纖維細(xì)胞(mouse embryonic fibroblasts，MEFs)、成骨前體細(xì)胞、牙周膜干細(xì)胞(periodontal ligament stem cells，PDLSCs)和肌源干細(xì)胞(muscle-derived stem cells，MDSCs)等定向成骨分化[1-5]。而且，相比于其他的成骨性BMPs，BMP9的促成骨作用很難被BMP3和Noggin抑制[1,6]。在動(dòng)物實(shí)驗(yàn)中，BMP9也顯示出較強(qiáng)的促成骨作用，BMP9對(duì)小鼠骨缺損愈合及大鼠腰椎脊柱融合均有明顯的促進(jìn)作用[7-8]。因此，BMP9是一種強(qiáng)效的促成骨因子。

2BMP9促成骨的信號(hào)轉(zhuǎn)導(dǎo)途徑

2.1BMP9促成骨相關(guān)受體BMPs均可與具有絲氨酸/蘇氨酸激酶活性的跨膜TGF-βⅠ型和Ⅱ型受體結(jié)合，啟動(dòng)信號(hào)轉(zhuǎn)導(dǎo)。因此TGF-β受體作為BMPs早期信號(hào)轉(zhuǎn)導(dǎo)的最重要的分子之一，是BMPs信號(hào)轉(zhuǎn)導(dǎo)的關(guān)鍵起始點(diǎn)，與其誘導(dǎo)成骨活性關(guān)系密切。為鑒定與BMP9促成骨相關(guān)的受體，Luo等[2]構(gòu)建了一系列TGF-βⅠ型和Ⅱ型受體的突變體，并利用其進(jìn)行系統(tǒng)，結(jié)果分析發(fā)現(xiàn)BMP9促成骨相關(guān)的Ⅰ型受體是ALK1和ALK2；而與BMP9促成骨相關(guān)的Ⅱ型受體為BMPRⅡ、ActRⅡ[3]。當(dāng)抑制BMP9促成骨相關(guān)受體的表達(dá)后，BMP9下游的信號(hào)轉(zhuǎn)導(dǎo)和BMP9的促成骨作用均受到抑制。從受體的角度看，BMP9與其他成骨性BMPs既有相同的促成骨受體，同時(shí)BMP9也有其獨(dú)特的促成骨受體如ALK1，見(jiàn)表1。

表1　成骨性BMPs促成骨相關(guān)的TGF-β受體

2.2Smad信號(hào)途徑BMP9與相應(yīng)受體結(jié)合后，可激活下游的信號(hào)途徑。與其他BMPs一樣，BMP9與相應(yīng)受體結(jié)合后，可以促進(jìn)經(jīng)典的Smad信號(hào)活化，導(dǎo)致Smad1、Smad5、Smad8磷酸化和核內(nèi)轉(zhuǎn)移，并使Smad1、Smad5、Smad8的轉(zhuǎn)錄調(diào)控活性增強(qiáng)[2,3,9]。當(dāng)抑制Smad信號(hào)后，BMP9誘導(dǎo)骨形成的能力減弱，因此，經(jīng)典的Smad信號(hào)在BMP9的促成骨作用中發(fā)揮重要作用。值得注意的是，雖然BMP3和Noggin可以抑制BMP2等成骨性BMPs的經(jīng)典Smad信號(hào)的活化，其對(duì)于BMP9所活化的Smad信號(hào)的抑制作用很弱[1,6]，提示了BMP9促成骨作用的獨(dú)特性。

2.3非Smad信號(hào)途徑除經(jīng)典的Smad信號(hào)外，BMP9 亦可活化其他非Smad的信號(hào)途徑如MAPKs、PI3K/AKT和PKA/CREB等[9-12]。BMP9可以激活MAPKs途徑的p38、ERK1/2和JNK[9-10]，但是3者對(duì)于BMP9促成骨作用的調(diào)控效應(yīng)并不相同，當(dāng)抑制p38和JNK信號(hào)時(shí)，BMP9促成骨作用減弱[9-10]；而抑制ERK1/2時(shí)，BMP9的促成骨作用反而增強(qiáng)[10]。由此表明，p38和JNK對(duì)BMP9的促成骨活性可能是正向調(diào)控作用，而ERK1/2則是負(fù)向調(diào)控作用。此外，BMP9也可活化PI3K/AKT和PKA/CREB信號(hào)，當(dāng)抑制AKTPI3K/AKT及PKA/CREB信號(hào)后，BMP9的促成骨作用相應(yīng)減弱[11-12]。

BMP9所活化的經(jīng)典Smad信號(hào)途徑和非Smad途徑存在聯(lián)系，當(dāng)非Smad信號(hào)發(fā)生變化時(shí)，經(jīng)典Smad信號(hào)也發(fā)生相應(yīng)的改變[9-12]，這表明BMP9的經(jīng)典Smad信號(hào)途徑和非Smad途徑同時(shí)存在，相互補(bǔ)充，互有聯(lián)系，形成其信號(hào)轉(zhuǎn)導(dǎo)和功能發(fā)揮的一個(gè)全面的信號(hào)轉(zhuǎn)導(dǎo)體系，并進(jìn)而發(fā)揮促成骨的功能

3BMP9促成骨信號(hào)的下游關(guān)鍵靶分子

BMP9通過(guò)上游的信號(hào)轉(zhuǎn)導(dǎo)，作用于下游的多種靶分子，這些靶分子對(duì)于BMP9的促成骨活性也具有重要的調(diào)控作用，Peng等[13]首先利用基因芯片技術(shù)系統(tǒng)分析BMP9促成骨過(guò)程中的基因表達(dá)變化。以此為基礎(chǔ)，后續(xù)對(duì)其中的某些關(guān)鍵靶分子對(duì)BMP9的促成骨作用的調(diào)控進(jìn)行了分析和研究，目前發(fā)現(xiàn)BMP9促成骨的關(guān)鍵靶分子有Id、CTGF、Crlde2、PPARγ、COX2、HIF1α和Hey1等[14-20]，見(jiàn)表2。

4BMP9促成骨的表觀遺傳調(diào)控

表觀遺傳指DNA序列不發(fā)生變化，但基因表達(dá)卻發(fā)生了可遺傳的改變。表觀遺傳包括DNA甲基化、組蛋白乙?；⒎蔷幋aRNA、染色質(zhì)重塑等。表觀遺傳在骨骼發(fā)育中有重要作用。BMP9促成骨的表觀遺傳調(diào)控目前研究較少，主要也僅集中于MicroRNA(miR)和組蛋白乙酰化[21-23]，見(jiàn)表3。因此，BMP9促成骨的表觀遺傳調(diào)控機(jī)制可能成為今后一個(gè)重要的研究領(lǐng)域。

表2　BMP9促成骨的關(guān)鍵靶分子

表3　BMP9促成骨的表觀遺傳調(diào)控

表4　BMP9促成骨過(guò)程中與其他信號(hào)途徑的交互串話

5BMP9促成骨過(guò)程中與其他信號(hào)分子的交互串話(crosstalk)

在BMP9促成骨過(guò)程中，其他一些關(guān)鍵的信號(hào)分子也可與BMP9發(fā)生相互串話，這些信號(hào)分子包括胰島素樣生長(zhǎng)因子(insulin-like growth factor，IGF)WnT3a、生長(zhǎng)激素(growth hormone，GH)、表皮生長(zhǎng)因子(epidermal growth factor，EGF)、全反式維甲酸(all-trans retinoic acid，ATRA)、成纖維細(xì)胞生長(zhǎng)因子2(fibroblast growth factor 2，F(xiàn)GF2)、TGF-β1等[11,24-29]。但是這些信號(hào)分子對(duì)BMP9促成骨作用的影響并不盡相同，有些可以協(xié)同增強(qiáng)BMP9的促成骨作用[11,24-27]，有些可抑制BMP9的促成骨作用[28]，另有些則因劑量不同而產(chǎn)生不同的效應(yīng)[29],見(jiàn)表4。

5結(jié)語(yǔ)

BMP9是目前BMPs家族中誘導(dǎo)成骨能力最強(qiáng)的成員因子，有希望作為一種促進(jìn)成骨的細(xì)胞因子候選者，具有潛在的臨床應(yīng)用價(jià)值。但由于對(duì)BMP9促成骨能力的研究相對(duì)較晚，因此對(duì)于其促成骨的相關(guān)分子機(jī)制的解析目前還不完善，還有待深入研究，以期為其將來(lái)在臨床的實(shí)際應(yīng)用奠定理論和實(shí)驗(yàn)基礎(chǔ)。

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doi:·專(zhuān)家述評(píng)·10.3969/j.issn.1671-8348.2016.09.001

[中圖分類(lèi)號(hào)]R68

[文獻(xiàn)標(biāo)識(shí)碼]A

[文章編號(hào)]1671-8348(2016)09-1153-03

(收稿日期：2015-11-08修回日期：2016-01-15)

羅進(jìn)勇：(1974-)博士，教授，碩士生導(dǎo)師。主要研究方向?yàn)楦杉?xì)胞定向分化與骨再生，對(duì)于間充質(zhì)干細(xì)胞定向分化相關(guān)信號(hào)分子及網(wǎng)絡(luò)有較深入研究。擔(dān)任國(guó)家自然科學(xué)基金委員會(huì)生命科學(xué)部一審專(zhuān)家、重慶市科委自然科學(xué)基金評(píng)審專(zhuān)家、廣西自然科學(xué)基金評(píng)審專(zhuān)家、天津市自然科學(xué)基金評(píng)審專(zhuān)家。為SCI雜志《J Cell Mol Med》、《Curr Pharm Design》、《Archives of Oral Biology》、《Stem Cells International》審稿人，《生物化學(xué)與生物物理進(jìn)展》、《中國(guó)生物化學(xué)與分子生物學(xué)報(bào)》、《遺傳》、《中國(guó)組織工程研究》雜志審稿專(zhuān)家。