Levin M, Mercola M
Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
Development. 1999 Nov;126(21):4703-14. doi: 10.1242/dev.126.21.4703.
Invariant patterning of left-right asymmetry during embryogenesis depends upon a cascade of inductive and repressive interactions between asymmetrically expressed genes. Different cascades of asymmetric genes distinguish the left and right sides of the embryo and are maintained by a midline barrier. As such, the left and right sides of an embryo can be viewed as distinct and autonomous fields. Here we describe a series of experiments that indicate that the initiation of these programs requires communication between the two sides of the blastoderm. When deprived of either the left or the right lateral halves of the blastoderm, embryos are incapable of patterning normal left-right gene expression at Hensen's node. Not only are both flanks required, suggesting that there is no single signaling source for LR pattern, but the blastoderm must be intact. These results are consistent with our previously proposed model in which the orientation of LR asymmetry in the frog, Xenopus laevis, depends on large-scale partitioning of LR determinants through intercellular gap junction channels (M. Levin and M. Mercola (1998) Developmental Biology 203, 90-105). Here we evaluate whether gap junctional communication is required for the LR asymmetry in the chick, where it is possible to order early events relative to the well-characterized left and right hierarchies of gene expression. Treatment of cultured chick embryos with lindane, which diminishes gap junctional communication, frequently unbiased normal LR asymmetry of Shh and Nodal gene expression, causing the normally left-sided program to be recapitulated symmetrically on the right side of the embryo. A survey of early expression of connexin mRNAs revealed that Cx43 is present throughout the blastoderm at Hamburger-Hamilton stage 2-3, prior to known asymmetric gene expression. Application of antisense oligodeoxynucleotides or blocking antibody to cultured embryos also resulted in bilateral expression of Shh and Nodal transcripts. Importantly, the node and primitive streak at these stages lack Cx43 mRNA. This result, together with the requirement for an intact blastoderm, suggests that the path of communication through gap junction channels circumvents the node and streak. We propose that left-right information is transferred unidirectionally throughout the epiblast by gap junction channels in order to pattern left-sided Shh expression at Hensen's node.
胚胎发育过程中左右不对称的不变模式依赖于不对称表达基因之间一系列诱导和抑制相互作用。不同的不对称基因级联区分胚胎的左右两侧，并由中线屏障维持。因此，胚胎的左右两侧可被视为不同且自主的区域。在此，我们描述了一系列实验，这些实验表明这些程序的启动需要胚盘两侧之间的通讯。当去除胚盘的左侧或右侧外侧半部时，胚胎无法在亨氏结处形成正常的左右基因表达模式。两侧都不可或缺，这表明不存在左右模式的单一信号源，而且胚盘必须完整。这些结果与我们之前提出的模型一致，在该模型中，非洲爪蟾胚胎中左右不对称的方向取决于通过细胞间缝隙连接通道对左右决定因素的大规模分配（M. 莱文和M. 梅尔科拉（1998年）《发育生物学》203, 90 - 105）。在此，我们评估缝隙连接通讯对于鸡的左右不对称是否必要，在鸡中可以相对于已充分表征的左右基因表达层次来确定早期事件的顺序。用林丹处理培养的鸡胚胎，林丹会减少缝隙连接通讯，这经常会使Shh和Nodal基因表达的正常左右不对称偏向，导致通常在左侧的程序在胚胎右侧对称地重现。对连接蛋白mRNA早期表达的调查显示，在已知不对称基因表达之前，Cx43在汉密尔顿 - 汉堡第2 - 3阶段的整个胚盘中都存在。将反义寡脱氧核苷酸或阻断抗体应用于培养的胚胎也导致Shh和Nodal转录本的双侧表达。重要的是，在这些阶段的节点和原条缺乏Cx43 mRNA。这一结果，连同对完整胚盘的需求，表明通过缝隙连接通道的通讯路径绕过了节点和条带。我们提出，左右信息通过缝隙连接通道在整个上胚层中单向传递，以便在亨氏结处形成左侧的Shh表达模式。
