生物谷：據(jù)上周Cell雜志兩篇文章報(bào)道，發(fā)育過(guò)程中的果蠅,，基本結(jié)構(gòu)（body plan）的構(gòu)建機(jī)制非常精確。這項(xiàng)發(fā)現(xiàn)顛覆“關(guān)鍵蛋白的表達(dá)水 平不受?chē)?yán)格控制”早期研究結(jié)果,。
二十多年前,，研究人員發(fā)現(xiàn)轉(zhuǎn)錄因子Bicoid，調(diào)控一系列決定果蠅基本結(jié)構(gòu)前軸和后軸的基因的時(shí)空表達(dá),。這種轉(zhuǎn)錄因子在胚胎中形成一定的濃度梯度——軀體前端高濃度的Bicoid激活前端特異基因,，后端低水平的Bicoid激活后端特異基因。研究人員將果蠅作為研究動(dòng)物身體形態(tài)（body patterning）的模式系統(tǒng),。
Cell文章作者,、普林斯頓大學(xué)Thomas Gregor說(shuō)，之前的研究結(jié)果提示,，Bicoid的表達(dá)水平?jīng)]有被嚴(yán)密控制,，但下游的模式形成（patterning）基因的表達(dá)水平很精確。為何無(wú)章法的Bicoid表達(dá)能夠轉(zhuǎn)換出精確的靶標(biāo)表達(dá),，一直是發(fā)育生物學(xué)領(lǐng)域的一道未解之謎,。由于之前的研究是在固定的組織中進(jìn)行的，
此次，Gregor與其同事決定在活體果蠅中檢測(cè)Bicoid的表達(dá),。他們將融合了綠色熒光色素蛋白基因的Bicoid基因插入黑腹果蠅的基因組中,，利用雙光子激光掃描顯微技術(shù)（two-photon laser-scanning microscopy，TPLSM）對(duì)胚胎發(fā)育頭三個(gè)小時(shí)中的Bicoid梯度進(jìn)行定量,。
第一篇文章中,，作者介紹說(shuō)受精后不到一小時(shí)，胚胎中出現(xiàn)Bicoid梯度,。每次的核分裂過(guò)程中,，子細(xì)胞核中的Bicoid濃度都會(huì)翻番。每次循環(huán)的開(kāi)始,，核Bicoid的濃度回到原先水平,。
貫穿單次核周期的Bicoid核濃度的四倍性變化“真的令人震驚，”Hanes說(shuō),。仍不清楚的是這種變化是怎樣影響基因表達(dá)的,，但是“這種研究Bicoid刺激其靶基因表達(dá)的機(jī)制的新方法將會(huì)對(duì)這個(gè)問(wèn)題進(jìn)行解釋。”
他們還對(duì)Bicoid在細(xì)胞中的擴(kuò)散進(jìn)行了檢測(cè),，以判斷其梯度的形成方式,。兩個(gè)獨(dú)立實(shí)驗(yàn)顯示，Bicoid在細(xì)胞中擴(kuò)散非常慢,，但實(shí)際上,，Bicoid濃度梯度形成的過(guò)程不到一個(gè)小時(shí)。作者推測(cè),，胚胎不同組織部位,、胚胎發(fā)生的不同時(shí)刻，擴(kuò)散速度有所不同,。“系統(tǒng)工作的方式比我們想象的要復(fù)雜的多,”Patel說(shuō),，“簡(jiǎn)單擴(kuò)散不足以解釋。”
原始出處:
Cell, Vol 130, 141-152, 13 July 2007
Article
Stability and Nuclear Dynamics of the Bicoid Morphogen Gradient
Thomas Gregor,1,2,3,4, Eric F. Wieschaus,3,4 Alistair P. McGregor,5 William Bialek,1,2 and David W. Tank1,2,3
1 Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA
2 Joseph Henry Laboratories of Physics, Princeton University, Princeton, New Jersey 08544, USA
3 Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA
4 Howard Hughes Medical Institute, Princeton University, Princeton, New Jersey 08544, USA
5 Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA
Corresponding author
Thomas Gregor
tg2@princeton.edu
Patterning in multicellular organisms results from spatial gradients in morphogen concentration, but the dynamics of these gradients remain largely unexplored. We characterize, through in vivo optical imaging, the development and stability of the Bicoid morphogen gradient in Drosophila embryos that express a Bicoid-eGFP fusion protein. The gradient is established rapidly (∼1 hr after fertilization), with nuclear Bicoid concentration rising and falling during mitosis. Interphase levels result from a rapid equilibrium between Bicoid uptake and removal. Initial interphase concentration in nuclei in successive cycles is constant (±10%), demonstrating a form of gradient stability, but it subsequently decays by approximately . Both direct photobleaching measurements and indirect estimates of Bicoid-eGFP diffusion constants (D ≤ 1 μm2/s) provide a consistent picture of Bicoid transport on short (∼min) time scales but challenge traditional models of long-range gradient formation. A new model is presented emphasizing the possible role of nuclear dynamics in shaping and scaling the gradient.
