The tailless Ortholog nhr-67 Regulates Patterning of Gene Expression and Morphogenesis in the C. elegans Vulva

Regulation of spatio-temporal gene expression in diverse cell and tissue types is a critical aspect of development. Progression through Caenorhabditis elegans vulval development leads to the generation of seven distinct vulval cell types (vulA, vulB1, vulB2, vulC, vulD, vulE, and vulF), each with its own unique gene expression profile. The mechanisms that establish the precise spatial patterning of these mature cell types are largely unknown. Dissection of the gene regulatory networks involved in vulval patterning and differentiation would help us understand how cells generate a spatially defined pattern of cell fates during organogenesis. We disrupted the activity of 508 transcription factors via RNAi and assayed the expression of ceh-2, a marker for vulB fate during the L4 stage. From this screen, we identified the tailless ortholog nhr-67 as a novel regulator of gene expression in multiple vulval cell types. We find that one way in which nhr-67 maintains cell identity is by restricting inappropriate cell fusion events in specific vulval cells, namely vulE and vulF. nhr-67 exhibits a dynamic expression pattern in the vulval cells and interacts with three other transcriptional regulators cog-1 (Nkx6.1/6.2), lin-11 (LIM), and egl-38 (Pax2/5/8) to generate the composite expression patterns of their downstream targets. We provide evidence that egl-38 regulates gene expression in vulB1, vulC, vulD, vulE, as well as vulF cells. We demonstrate that the pairwise interactions between these regulatory genes are complex and vary among the seven cell types. We also discovered a striking regulatory circuit that affects a subset of the vulval lineages: cog-1 and nhr-67 inhibit both one another and themselves. We postulate that the differential levels and combinatorial patterns of lin-11, cog-1, and nhr-67 expression are a part of a regulatory code for the mature vulval cell types.

时空特异性基因表达在多样细胞与组织类型中的调控，是个体发育的关键核心环节。秀丽隐杆线虫（Caenorhabditis elegans）的阴门发育进程可产生7种截然不同的阴门细胞类型（vulA、vulB1、vulB2、vulC、vulD、vulE及vulF），每一类均拥有独特的基因表达谱。目前，调控这些成熟细胞类型形成精准空间模式的分子机制尚不完全明确。解析参与阴门模式构建与细胞分化的基因调控网络，将有助于我们理解器官发生过程中，细胞如何生成具有空间特异性的细胞命运格局。我们通过RNA干扰（RNA interference, RNAi）技术敲低了508个转录因子的活性，并检测了L4期阴门B细胞命运标志物ceh-2的表达水平。通过该筛选实验，我们鉴定出尾同源框基因的同源物nhr-67，作为调控多种阴门细胞类型基因表达的新型调控因子。我们发现，nhr-67维持细胞身份的途径之一，是限制特定阴门细胞（即vulE与vulF细胞）发生异常的细胞融合事件。nhr-67在阴门细胞中呈现动态表达模式，并与另外三种转录调控因子cog-1（Nkx6.1/6.2）、lin-11（LIM）及egl-38（Pax2/5/8）相互作用，共同调控下游靶基因的复合表达格局。我们提供的实验证据表明，egl-38可调控vulB1、vulC、vulD、vulE及vulF细胞中的基因表达。我们证实，这些调控基因之间的成对相互作用极为复杂，且在7种细胞类型中存在显著差异。我们还发现了一条显著调控部分阴门细胞谱系的调控环路：cog-1与nhr-67可相互抑制，同时亦可自我抑制。我们推测，lin-11、cog-1及nhr-67的表达水平差异与组合模式，共同构成了成熟阴门细胞类型的调控密码。