Lawrence Peter A, Casal José, Struhl Gary
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.
Development. 2004 Oct;131(19):4651-64. doi: 10.1242/dev.01351. Epub 2004 Aug 25.
The integument of the Drosophila adult abdomen bears oriented hairs and bristles that indicate the planar polarity of the epidermal cells. We study four polarity genes, frizzled (fz), prickle (pk), Van gogh/strabismus (Vang/stbm) and starry night/flamingo (stan/fmi), and note what happens when these genes are either removed or overexpressed in clones of cells. The edges of the clones are interfaces between cells that carry different amounts of gene products, interfaces that can cause reversals of planar polarity in the clone and wild-type cells outside them. To explain, we present a model that builds on our earlier picture of a gradient of X, the vector of which specifies planar polarity and depends on two cadherin proteins, Dachsous and Fat. We conjecture that the X gradient is read out, cell by cell, as a scalar value of Fz activity, and that Pk acts in this process, possibly to determine the sign of the Fz activity gradient. We discuss evidence that cells can compare their scalar readout of the level of X with that of their neighbours and can set their own readout towards an average of those. This averaging, when it occurs near the edges of clones, changes the scalar response of cells inside and outside the clones, leading to new vectors that change polarity. The results argue that Stan must be present in both cells being compared and acts as a conduit between them for the transfer of information. And also that Vang assists in the receipt of this information. The comparison between neighbours is crucial, because it gives the vector that orients hairs--these point towards the neighbour cell that has the lowest level of Fz activity. Recently, it has been shown that, for a limited period shortly before hair outgrowth in the wing, the four proteins we study, as well as others, become asymmetrically localised in the cell membrane, and this process is thought to be instrumental in the acquisition of cell polarity. However, some results do not fit with this view--we suggest that these localisations may be more a consequence than a cause of planar polarity.
果蝇成虫腹部的体壁上有定向的毛和刚毛，这些毛和刚毛指示着表皮细胞的平面极性。我们研究了四个极性基因，卷曲（fz）、棘（pk）、梵高/斜视（Vang/stbm）和星夜/火烈鸟（stan/fmi），并观察了这些基因在细胞克隆中被去除或过表达时会发生什么。克隆的边缘是携带不同数量基因产物的细胞之间的界面，这些界面会导致克隆及其外部野生型细胞中平面极性的反转。为了解释这一现象，我们提出了一个模型，该模型建立在我们早期关于X梯度的图景之上，X梯度的向量指定平面极性，并依赖于两种钙黏着蛋白，达索蛋白和脂肪蛋白。我们推测，X梯度逐个细胞地被读取为Fz活性的标量值，并且Pk在这个过程中起作用，可能是为了确定Fz活性梯度的正负号。我们讨论了一些证据，即细胞可以将其对X水平的标量读取与邻居的进行比较，并可以将自己的读取设置为两者的平均值。当这种平均化发生在克隆边缘附近时，会改变克隆内部和外部细胞的标量反应，从而产生改变极性的新向量。结果表明，Stan必须存在于被比较的两个细胞中，并作为它们之间传递信息的管道。而且Vang有助于接收这些信息。邻居之间的比较至关重要，因为它给出了使毛定向的向量——这些毛指向Fz活性水平最低的邻居细胞。最近，研究表明，在翅膀上的毛长出前不久的一段有限时间内，我们研究的这四种蛋白质以及其他蛋白质会在细胞膜上不对称定位，并且这个过程被认为对细胞极性的获得至关重要。然而，一些结果并不符合这一观点——我们认为这些定位可能更多是平面极性的结果而非原因。
