Dobias S L, Ma L, Wu H, Bell J R, Maxson R
Department of Biochemistry and Molecular Biology, University of Southern California School of Medicine, Los Angeles 90033, USA.
Mech Dev. 1997 Jan;61(1-2):37-48. doi: 10.1016/s0925-4773(96)00617-x.
Msx- class homeobox genes, characterized by a distinct and highly conserved homeodomain, have been identified in a wide variety of metazoans from vertebrates to coelenterates. Although there is evidence that they participate in inductive tissue interactions that underlie vertebrate organogenesis, including those that pattern the neural crest, there is little information about their function in simple deuterostomes. Both to learn more about the ancient function of Msx genes, and to shed light on the evolution of developmental mechanisms within the lineage that gave rise to vertebrates, we have isolated and characterized Msx genes from ascidians and echinoderms. Here we describe the sequence and expression of a sea urchin (Strongylocentrotus purpouratus) Msx gene whose homeodomain is very similar to that of vertebrate Msx2. This gene, designated SpMsx, is first expressed in blastula stage embryos, apparently in a non-localized manner. Subsequently, during the early phases of gastrulation, SpMsx transcripts are expressed intensely in the invaginating archenteron and secondary mesenchyme, and at reduced levels in the ectoderm. In the latter part of gastrulation, SpMsx transcripts are concentrated in the oral ectoderm and gut, and continue to be expressed at those sites through the remainder of embryonic development. That vertebrate Msx genes are regulated by inductive tissue interactions and growth factors suggested to us that the restriction of SpMsx gene expression to the oral ectoderm and derivatives of the vegetal plate might similarly be regulated by the series of signaling events that pattern these embryonic territories. As a first test of this hypothesis, we examined the influence of exogastrulation and cell-dissociation on SpMsx gene expression. In experimentally-induced exogastrulae, SpMsx transcripts were distributed normally in the oral ectoderm, evaginated gut, and secondary mesenchyme. However, when embryos were dissociated into their component cells, SpMsx transcripts failed to accumulate. These data show that the localization of SpMsx transcripts in gastrulae does not depend on interactions between germ layers, yet the activation and maintenance of SpMsx expression does require cell-cell or cell-matrix interactions.
Msx类同源框基因的特征是具有一个独特且高度保守的同源结构域，已在从脊椎动物到腔肠动物的多种后生动物中被鉴定出来。尽管有证据表明它们参与了脊椎动物器官发生基础的诱导性组织相互作用，包括那些为神经嵴定型的相互作用，但关于它们在简单的后口动物中的功能却知之甚少。为了更多地了解Msx基因的古老功能，并阐明产生脊椎动物的谱系中发育机制的进化，我们从海鞘和棘皮动物中分离并鉴定了Msx基因。在这里，我们描述了一种海胆（紫球海胆）Msx基因的序列和表达，其同源结构域与脊椎动物Msx2的非常相似。这个基因被命名为SpMsx，最初在囊胚期胚胎中表达，显然是以一种非定位的方式。随后，在原肠胚形成的早期阶段，SpMsx转录本在向内凹陷的原肠和次生间充质中强烈表达，在外胚层中表达水平较低。在原肠胚形成的后期，SpMsx转录本集中在口外胚层和肠道中，并在胚胎发育的剩余阶段继续在这些部位表达。脊椎动物Msx基因受诱导性组织相互作用和生长因子调控，这使我们推测SpMsx基因表达局限于口外胚层和植物极板衍生物可能同样受这些胚胎区域定型的一系列信号事件调控。作为对这一假设的首次检验，我们研究了外原肠胚形成和细胞解离对SpMsx基因表达的影响。在实验诱导的外原肠胚中，SpMsx转录本正常分布在口外胚层、外翻的肠道和次生间充质中。然而，当胚胎被解离成其组成细胞时，SpMsx转录本无法积累。这些数据表明，SpMsx转录本在原肠胚中的定位不依赖于胚层之间的相互作用，但SpMsx表达的激活和维持确实需要细胞间或细胞与基质的相互作用。
