Hallmann A, Kirk D L
Department of Biology, Washington University, St Louis, MO 63130, USA.
J Cell Sci. 2000 Dec;113 Pt 24:4605-17. doi: 10.1242/jcs.113.24.4605.
Volvox is one of the simplest multicellular organisms with only two cell types, yet it has a surprisingly complex extracellular matrix (ECM) containing many region-specific morphological components, making Volvox suitable as a model system for ECM investigations. ECM deposition begins shortly after inversion, which is the process by which the embryo turns itself right-side-out at the end of embryogenesis. It was previously shown that the gene encoding an ECM glycoprotein called ISG is transcribed very transiently during inversion. Here we show that the developmentally controlled ISG accumulates at the bases of the flagella right after inversion, before any morphologically recognizable ECM structures have yet developed. Later, ISG is abundant in the 'flagellar hillocks' that encircle the basal ends of all flagella, and in the adjacent 'boundary zone' that delimits the spheroid. Transgenic Volvox were generated which express a truncated form of ISG. These transgenics exhibit a severely disorganized ECM within which the cells are embedded in a highly chaotic manner that precludes motility. A synthetic version of the C-terminal decapeptide of ISG has a similar disorganizing effect, but only when it is applied during or shortly after inversion. We postulate that ISG plays a critical role in morphogenesis and acts as a key organizer of ECM architecture; at the very beginning of ECM formation ISG establishes an essential initial framework that both holds the somatic cells in an adaptive orientation and acts as the scaffold upon which the rest of the ECM can be properly assembled, assuring that somatic cells of post-inversion spheroids are held in orientations and locations that makes adaptive swimming behavior possible.
团藻是最简单的多细胞生物之一，仅具有两种细胞类型，但它拥有一个惊人复杂的细胞外基质（ECM），其中包含许多区域特异性的形态学成分，这使得团藻适合作为研究ECM的模型系统。ECM沉积在反转后不久开始，反转是胚胎在胚胎发育末期将自身翻转至右侧朝外的过程。先前的研究表明，编码一种名为ISG的ECM糖蛋白的基因在反转过程中非常短暂地转录。在这里，我们表明，受发育控制的ISG在反转后立即在鞭毛基部积累，此时尚未形成任何形态上可识别的ECM结构。后来，ISG在环绕所有鞭毛基部末端的“鞭毛小丘”以及界定球体的相邻“边界区域”中大量存在。我们生成了表达截短形式ISG的转基因团藻。这些转基因团藻表现出严重紊乱的ECM，细胞以高度混乱的方式嵌入其中，从而妨碍了运动。ISG C末端十肽的合成版本具有类似的紊乱作用，但仅在反转期间或反转后不久应用时才会出现这种情况。我们推测，ISG在形态发生中起关键作用，并作为ECM结构的关键组织者；在ECM形成的最初阶段，ISG建立了一个基本的初始框架，该框架既能使体细胞保持适应性方向，又能作为其余ECM可以正确组装的支架，确保反转后球体的体细胞保持在能够实现适应性游泳行为的方向和位置。
