Bhat Ramray, Lerea Kenneth M, Peng Hong, Kaltner Herbert, Gabius Hans-Joachim, Newman Stuart A
Department of Cell Biology and Anatomy, New York Medical College, Valhalla, NY 10595, USA.
BMC Dev Biol. 2011 Feb 1;11:6. doi: 10.1186/1471-213X-11-6.
The skeletal elements of vertebrate embryonic limbs are prefigured by rod- and spot-like condensations of precartilage mesenchymal cells. The formation of these condensations depends on cell-matrix and cell-cell interactions, but how they are initiated and patterned is as yet unresolved.
Here we provide evidence that galectins, β-galactoside-binding lectins with β-sandwich folding, play fundamental roles in these processes. We show that among the five chicken galectin (CG) genes, two, CG-1A, and CG-8, are markedly elevated in expression at prospective sites of condensation in vitro and in vivo, with their protein products appearing earlier in development than any previously described marker. The two molecules enhance one another's gene expression but have opposite effects on condensation formation and cartilage development in vivo and in vitro: CG-1A, a non-covalent homodimer, promotes this process, while the tandem-repeat-type CG-8 antagonizes it. Correspondingly, knockdown of CG-1A inhibits the formation of skeletal elements while knockdown of CG-8 enhances it. The apparent paradox of mutual activation at the gene expression level coupled with antagonistic roles in skeletogenesis is resolved by analysis of the direct effect of the proteins on precartilage cells. Specifically, CG-1A causes their aggregation, whereas CG-8, which has no adhesive function of its own, blocks this effect. The developmental appearance and regulation of the unknown cell surface moieties ("ligands") to which CG-1A and CG-8 bind were indicative of specific cognate- and cross-regulatory interactions.
Our findings indicate that CG-1A and CG-8 constitute a multiscale network that is a major mediator, earlier-acting than any previously described, of the formation and patterning of precartilage mesenchymal condensations in the developing limb. This network functions autonomously of limb bud signaling centers or other limb bud positional cues.
脊椎动物胚胎肢体的骨骼元件由前软骨间充质细胞的杆状和点状凝聚物预先形成。这些凝聚物的形成取决于细胞与基质以及细胞与细胞之间的相互作用，但它们如何启动和形成模式尚未得到解决。
在这里，我们提供证据表明，半乳糖凝集素，即具有β-折叠结构的β-半乳糖苷结合凝集素，在这些过程中发挥着重要作用。我们表明，在五个鸡半乳糖凝集素（CG）基因中，两个，即CG-1A和CG-8，在体外和体内凝聚的预期部位表达明显升高，它们的蛋白质产物在发育中出现的时间比任何先前描述的标记物都要早。这两种分子相互增强彼此的基因表达，但在体内和体外对凝聚物形成和软骨发育具有相反的作用：CG-1A，一种非共价同二聚体，促进这一过程，而串联重复型的CG-8则起拮抗作用。相应地，敲低CG-1A会抑制骨骼元件的形成，而敲低CG-8则会增强它。通过分析蛋白质对前软骨细胞的直接作用，解决了基因表达水平上相互激活与骨骼发生中拮抗作用这一明显的矛盾。具体而言，CG-1A导致它们聚集，而自身没有粘附功能的CG-8则阻断这种作用。CG-1A和CG-8所结合的未知细胞表面部分（“配体”）的发育出现和调节表明存在特定的同源和交叉调节相互作用。
我们的研究结果表明，CG-1A和CG-8构成了一个多尺度网络，它是发育中的肢体中前软骨间充质凝聚物形成和模式化的主要介导者，比任何先前描述的都更早发挥作用。该网络独立于肢芽信号中心或其他肢芽位置线索发挥作用。
