Department of Biology, The Catholic University of America, Washington, DC 20064, USA.
Dev Dyn. 2012 Mar;241(3):481-92. doi: 10.1002/dvdy.23734. Epub 2012 Jan 31.
The Caenorhabditis elegans basic helix-loop-helix (bHLH) factor HLH-8, the single Twist ortholog in the nematode genome, plays important roles in mesoderm development, including M lineage patterning and differentiation of vulval and enteric muscles. HLH-8 cooperates with HLH-2, the bHLH E/Daughterless ortholog, to regulate downstream target genes, but it is not known whether HLH-2 is an obligate partner for all HLH-8 functions.
Using hlh-2 loss-of-function alleles and RNAi, we discovered that HLH-2 is required in the vulval muscles but not in M patterning or enteric muscle development. Additionally, we found that expressing tethered HLH-8/HLH-8 dimers in hlh-8 null animals rescued M patterning and enteric but not vulval muscle development.
These results support a model whereby HLH-8/HLH-8 homodimers function in M lineage patterning and enteric muscles and HLH-8/HLH-2 heterodimers function in the M-derived vulval muscles. Interestingly, the different dimers function in the same M lineage cells and the switch in dimer function coincides with vulval muscle differentiation. The use of distinct Twist dimers is evolutionarily conserved, and C. elegans provides a paradigm for future dissection of differential promoter regulation by these dimers at a single cell resolution.
秀丽隐杆线虫的基本螺旋-环-螺旋（bHLH）因子 HLH-8 是线虫基因组中唯一的 Twist 直系同源物，在中胚层发育中发挥重要作用，包括 M 谱系模式形成和外胚层及肠肌肉的分化。HLH-8 与 bHLH E/Daughterless 直系同源物 HLH-2 合作调节下游靶基因，但尚不清楚 HLH-2 是否是 HLH-8 所有功能的必需伴侣。
使用 hlh-2 功能丧失等位基因和 RNAi，我们发现 HLH-2 在 外胚层 肌肉中是必需的，但在 M 模式形成或肠肌肉发育中不是必需的。此外，我们发现在 hlh-8 缺失动物中表达束缚的 HLH-8/HLH-8 二聚体可挽救 M 模式形成和肠肌肉，但不能挽救外胚层肌肉的发育。
这些结果支持以下模型，即 HLH-8/HLH-8 同源二聚体在 M 谱系模式形成和肠肌肉中发挥作用，而 HLH-8/HLH-2 异源二聚体在 M 衍生的外胚层肌肉中发挥作用。有趣的是，不同的二聚体在相同的 M 谱系细胞中发挥作用，二聚体功能的转换与外胚层肌肉分化相吻合。不同 Twist 二聚体的使用在进化上是保守的，秀丽隐杆线虫为未来在单细胞分辨率下对这些二聚体的不同启动子调控进行剖析提供了范例。
