李琳 杜巖

中藥有效成分石杉?jí)A甲治療阿爾茨海默病的研究進(jìn)展

李琳 杜巖

杜巖 教授

阿爾茨海默病(Alzheimer’s disease, AD)是一種與年齡相關(guān)的,以進(jìn)行性記憶損傷、認(rèn)知功能障礙、行為障礙和人格改變?yōu)樘卣鞯穆陨窠?jīng)退行性疾病。據(jù)世界衛(wèi)生組織估計(jì),全球目前有近3560萬AD患者,其中一半以上的患者居住在低收入和中等收入國家[1]。AD是一種多因素介導(dǎo)的疾病,其病因及發(fā)病機(jī)制尚不明確,迄今臨床實(shí)踐中仍未找到十分有效的治療策略[2]。

石杉?jí)A甲(Huperzine A, Hup A)是從我國中草藥蛇足石杉(Huperziaserrata)中提取出的新型石杉類生物堿有效單體,是一種高效、高選擇性且可逆性的中樞性乙酰膽堿酯酶(acetylcholinesterase, AChE)抑制劑,臨床上常用于治療各種疾病,例如瘀傷、感染、風(fēng)濕病、精神分裂癥、重癥肌無力和發(fā)熱等[3]。已經(jīng)有報(bào)道顯示,Hup A可以逆轉(zhuǎn)或減弱多種不同動(dòng)物模型(包括嚙齒類動(dòng)物和非人靈長類動(dòng)物等)的認(rèn)知損害[4-5],并且可以改善人類的學(xué)習(xí)和記憶缺陷[6]。事實(shí)上,Hup A的神經(jīng)保護(hù)作用,不僅僅是其對(duì)AChE抑制作用的結(jié)果,它具有多個(gè)分子作用的靶點(diǎn)。因此,本文將對(duì)Hup A治療AD相關(guān)機(jī)制的研究進(jìn)展進(jìn)行綜述。

1 調(diào)控APP代謝途徑

淀粉樣蛋白沉積形成的老年斑(senile plaque, SP)是AD的特征性病理改變,SP的核心成分是β-淀粉樣蛋白(amyloid β-protein, Aβ),Aβ由淀粉樣前體蛋白(amyloid precursor protein, APP)水解而來。APP在體內(nèi)的代謝途徑主要分為兩種,即由α-分泌酶介導(dǎo)的非Aβ途徑和由β-分泌酶和γ-分泌酶共同介導(dǎo)的Aβ途徑[7]。分泌型淀粉樣蛋白前體蛋白α(secretory amyloid precursor protein α, sAPPa)由非Aβ途徑產(chǎn)生,對(duì)神經(jīng)具有營養(yǎng)和保護(hù)作用,可以促進(jìn)神經(jīng)的生長,刺激突觸的形成,同時(shí)穩(wěn)定神經(jīng)元內(nèi)的鈣離子平衡等[8]。研究顯示,Hup A調(diào)節(jié)APP的代謝途徑與蛋白激酶C(protein kinase C,PKC)的活化有關(guān),通過激活毒蕈堿乙酰膽堿受體(moscarinic acetylcholine, mACh)亞型M1和M3,細(xì)胞內(nèi)PLC-PKC信號(hào)傳導(dǎo)途徑被激活,而經(jīng)PKC磷酸化的酪氨酸活化α-分泌酶,從而使sAPPa的分泌增加,并降低Aβ的水平[9]。Zhang等[10]的研究發(fā)現(xiàn),通過腦室內(nèi)注射Aβ1-40建立的AD大鼠模型,對(duì)模型鼠腹腔注射Hup A可以明顯改善大鼠體內(nèi)由Aβ導(dǎo)致的sAPPα分泌減少;而在經(jīng)Aβ1-40處理過的HEK293sw細(xì)胞中,給予Hup A后,APP的水平顯著增加。Yan等[11]運(yùn)用mACh受體的特異性拮抗劑和激酶抑制劑,發(fā)現(xiàn)Hup A可以激活M1-mAChR/PKC級(jí)聯(lián)和MAPK信號(hào)傳導(dǎo),從而增強(qiáng)APP的非Aβ途徑。這些研究結(jié)果表明Hup A可能通過上調(diào)PKC,來影響APP的代謝,從而改善AD的學(xué)習(xí)記憶能力和認(rèn)知功能。

2 抗氧化應(yīng)激

氧化應(yīng)激是體內(nèi)自由基產(chǎn)生的負(fù)效應(yīng),并被認(rèn)為是促進(jìn)衰老及其相關(guān)疾病發(fā)生發(fā)展的重要因素之一[12]。Aβ主要通過氧化應(yīng)激作用和自由基的產(chǎn)生對(duì)神經(jīng)細(xì)胞產(chǎn)生毒性。自由基90%產(chǎn)生于細(xì)胞線粒體,因此線粒體功能障礙可以促進(jìn)氧化應(yīng)激、衰老和神經(jīng)變性[13-14]。研究發(fā)現(xiàn),Hup A可以升高AD模型豚鼠的額葉、頂葉、顳葉和小腦中三價(jià)鐵還原抗氧化力、谷胱甘肽還原酶活性和氧化應(yīng)激標(biāo)記物(如轉(zhuǎn)氨酶,血尿素氮和葡萄糖等)水平[15]。Hup A可以明顯改善D-半乳糖誘導(dǎo)的老化小鼠的學(xué)習(xí)記憶障礙,降低腦組織中一氧化氮含量,抑制一氧化氮合酶活性,提高谷胱甘肽過氧化物酶和琥珀酸脫氫酶活性,降低胞漿鈣離子水平[16]。研究表明,在分離的大鼠腦線粒體中,Hup A除了改善Aβ誘導(dǎo)的線粒體呼吸功能下降、三磷酸腺苷(ATP)合成減少、線粒體呼吸鏈酶失活和跨膜電位的降低,還能有效防止Aβ誘導(dǎo)的線粒體腫脹、活性氧(ROS)過度生成和細(xì)胞色素c釋放[17]。Gao等[18]研究發(fā)現(xiàn)用Hup A孵育2h,不僅減弱了PC12模型細(xì)胞中由Aβ引起的細(xì)胞氧化應(yīng)激損傷,而且未受傷害的正常細(xì)胞中的ATP濃度升高和ROS生成減少。上述結(jié)果表明Hup A能提高細(xì)胞的抗氧化能力,在治療AD中起一定的作用。

3 對(duì)中樞神經(jīng)遞質(zhì)的調(diào)控作用

3.1 膽堿能神經(jīng)傳遞系統(tǒng) Hup A是一種高選擇性的可逆性膽堿酯酶抑制劑,易通過血腦屏障,其抑制方式是典型的混合型抑制,兼顧競爭性和非競爭性,抑制作用明顯優(yōu)于加蘭他敏、多奈哌齊和他克林[19]。Hup A通過與AChE可逆性結(jié)合,減少乙酰膽堿(ACh)的水解,使神經(jīng)突觸間隙的ACh含量明顯升高,并通過激活突觸后膜上的N受體,興奮膽堿能神經(jīng)元,增強(qiáng)學(xué)習(xí)記憶相關(guān)腦區(qū)神經(jīng)元的興奮性,從而增強(qiáng)學(xué)習(xí)記憶能力,改善認(rèn)知行為功能[20-21]。有研究表明,Hup A對(duì)丁酰膽堿酯酶(Butyrylcholinesterase, BuChE)抑制作用較弱,因此外周ACh水平升高作用不明顯,從而減少了惡心、嘔吐等外周膽堿能不良反應(yīng)[22]。

3.2 單胺類神經(jīng)傳遞系統(tǒng) 中樞神經(jīng)系統(tǒng)是一個(gè)復(fù)雜的網(wǎng)絡(luò),其中某一神經(jīng)傳遞系統(tǒng)通常會(huì)干擾其他神經(jīng)傳遞系統(tǒng)。研究表明ACh系統(tǒng)與去甲腎上腺素(Noradrenalin, NA)系統(tǒng)、多巴胺(Dopamine, DA)系統(tǒng)之間存在相互作用,共同影響學(xué)習(xí)和記憶能力。這些神經(jīng)傳遞系統(tǒng)的失調(diào)可能會(huì)導(dǎo)致各種神經(jīng)系統(tǒng)退行性疾病,包括AD[23]。Liang等[24]應(yīng)用清醒大鼠雙探針腦微透析及高效液相色譜電化學(xué)檢測技術(shù)研究表明,口服Hup A顯著提高大鼠中前額葉皮層及腹側(cè)海馬內(nèi)ACh水平和中前額葉皮層突觸間隙DA及NA水平,但對(duì)5-羥色胺水平?jīng)]有影響,對(duì)DA的作用強(qiáng)于NA,但對(duì)二者的作用峰值及作用持續(xù)時(shí)間均弱于對(duì)ACh的作用?？偠灾?細(xì)胞外ACh含量的升高可能同時(shí)會(huì)引起NA和DA含量的升高,因此Hup A可以通過提高患者腦組織內(nèi)的ACh及NA、DA等神經(jīng)遞質(zhì)的水平,發(fā)揮對(duì)受損神經(jīng)的保護(hù)作用。

3.3 谷氨酸(glutamate, Glu)介導(dǎo)的興奮性神經(jīng)傳遞系統(tǒng) Glu是大腦中最豐富的神經(jīng)遞質(zhì),與長時(shí)程增強(qiáng)記憶的形成密切相關(guān)[25]。N-甲基-D-天門冬氨酸(N-methyl-D-aspartate, NMDA)受體是Glu受體的一個(gè)亞型。NMDA受體介導(dǎo)的興奮毒性作用,主要有2個(gè)途徑:(1)由NMDA受體過度興奮介導(dǎo)的神經(jīng)細(xì)胞遲發(fā)性損傷;(2)Glu超常釋放和蓄積造成促進(jìn)NMDA受體通道的開放。二者都會(huì)導(dǎo)致細(xì)胞內(nèi)Ca2+超負(fù)荷,引起神經(jīng)元壞死[26]。AD患者腦內(nèi)Glu水平過高提示存在NMDA受體的慢性激活[27]。Hup A作用于Glu介導(dǎo)的興奮性神經(jīng)傳遞系統(tǒng)。有研究表明將神經(jīng)元細(xì)胞用Hup A預(yù)處理45 min后暴露于NMDA,可以抑制NMDA誘導(dǎo)的毒性作用[28]。另外,Hup A選擇性抑制NMDA誘導(dǎo)電流,且與NMDA受體的變構(gòu)調(diào)節(jié)位點(diǎn)的調(diào)節(jié)劑具有競爭作用[29]。這些結(jié)果表明Hup A具有拮抗NMDA受體的作用,可以減弱由Glu介導(dǎo)的興奮性細(xì)胞毒性。

4 調(diào)節(jié)神經(jīng)生長因子信號(hào)

神經(jīng)生長因子(nerve growth factor, NGF)是重要的神經(jīng)細(xì)胞生長調(diào)節(jié)因子。年齡相關(guān)的基底前腦膽堿能神經(jīng)元(BFCN)變性發(fā)生于AD早期,并且與AD的認(rèn)知功能下降相關(guān)。大量的實(shí)驗(yàn)已經(jīng)表明,減少BFCN細(xì)胞體上的NGF水平,會(huì)導(dǎo)致神經(jīng)元標(biāo)記物的丟失和減少,酷似AD患者的典型病變[30]。在AD的終末階段,出現(xiàn)基底核區(qū)含ACh的皮質(zhì)投影神經(jīng)元的選擇性變性,可能與NGF及它的低親和力受體P75和高親和力受體TrkA三者之間的功能紊亂有關(guān)[31]。NGF增強(qiáng)了APP的非Aβ途徑,并減少APP/PS1轉(zhuǎn)基因小鼠腦中的Aβ生成[32]。然而,NGF不能穿過血腦屏障。因此,通過藥物刺激內(nèi)源性NGF的產(chǎn)生,或通過研究能夠穿過血腦屏障的化合物來模擬NGF的活性是保護(hù)中樞神經(jīng)系統(tǒng)的新途徑。Tang等[33]研究發(fā)現(xiàn),Hup A可以顯著增加帶有軸突的PC12細(xì)胞的數(shù)量,但對(duì)細(xì)胞活力無明顯影響,還可以明顯上調(diào)NGF mRNA的水平和低親和力NGF受體p75的表達(dá)。同年,Tang等[34]通過H2O2介導(dǎo)的神經(jīng)毒性細(xì)胞模型實(shí)驗(yàn)進(jìn)行進(jìn)一步的探索,實(shí)驗(yàn)結(jié)果表明Hup A通過TrkA介導(dǎo)的MAPK/ERK途徑來實(shí)現(xiàn)對(duì)NGF信號(hào)傳導(dǎo)的調(diào)節(jié)作用,該調(diào)節(jié)作用可以被K252α(與TrkA受體信號(hào)傳導(dǎo)活化相關(guān)的磷酸化抑制劑)和PD98059(MAPK/ERK抑制劑)所阻斷。結(jié)果表明,Hup A對(duì)神經(jīng)細(xì)胞的保護(hù)作用由上調(diào)的NGF和NGF受體介導(dǎo)。

5 抑制神經(jīng)細(xì)胞凋亡

有研究表明,Aβ的異常沉積、炎癥反應(yīng)和氧化應(yīng)激損傷均能夠促進(jìn)神經(jīng)細(xì)胞的凋亡,其中Aβ的直接神經(jīng)毒性作用是最常見的原因[35]。Aβ的神經(jīng)毒性可造成細(xì)胞活性下降、線粒體功能紊亂、引起細(xì)胞凋亡。在中樞神經(jīng)系統(tǒng)中,核轉(zhuǎn)錄因子-κB(nuclear transcription factor-κB, NF-κB)的活化與神經(jīng)細(xì)胞凋亡的易感性增加相關(guān)[36]。Hup A可通過阻止NF-κB核易位,抑制D-半乳糖誘導(dǎo)的神經(jīng)血管損傷和血腦屏障功能障礙,從而抑制神經(jīng)細(xì)胞凋亡[37]。另外,Hup A通過抑制Aβ1-42誘導(dǎo)的小膠質(zhì)細(xì)胞炎癥反應(yīng),使神經(jīng)干細(xì)胞在神經(jīng)干細(xì)胞和小膠質(zhì)細(xì)胞共培養(yǎng)系統(tǒng)中免受Aβ誘導(dǎo)的細(xì)胞凋亡[38]。

6 總結(jié)與展望

眾所周知,AD是一種多因素介導(dǎo)的致死性神經(jīng)變性疾病,目前可用的藥物療法僅供姑息治療且療效有限。Hup A作為一種高效的AChE抑制劑,對(duì)APP代謝、多種神經(jīng)遞質(zhì)系統(tǒng)、NGF的調(diào)控作用和抗氧化、抗凋亡等作用刺激內(nèi)源性保護(hù)過程,同時(shí)促進(jìn)損傷結(jié)構(gòu)的修復(fù)。因此,這種多重作用使Hup A成為治療AD的高效藥物。由于大多數(shù)神經(jīng)變性疾病的主要特征在于與癥狀發(fā)展相關(guān)的進(jìn)行性神經(jīng)元丟失,Hup A的神經(jīng)保護(hù)作用通過減緩神經(jīng)變性過程,是否同樣有利于其他神經(jīng)變性疾病,如帕金森氏病,肌萎縮性側(cè)索硬化或亨廷頓病等,還有待進(jìn)一步探索和研究。

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110004遼寧省沈陽市,沈陽醫(yī)學(xué)院附屬第二醫(yī)院(北院區(qū))神經(jīng)內(nèi)科

杜巖,Email:duyansy@163.com

R 749.16

A

10.3969/j.issn.1003-9198.2017.07.005

2017-04-05)