Date Published: February 2, 2010
Publisher: Public Library of Science
Author(s): Asako Shindo, Yusuke Hara, Takamasa S. Yamamoto, Masamichi Ohkura, Junichi Nakai, Naoto Ueno, Mai Har Sham. http://doi.org/10.1371/journal.pone.0008897
Abstract: The establishment of cell polarity is crucial for embryonic cells to acquire their proper morphologies and functions, because cell alignment and intracellular events are coordinated in tissues during embryogenesis according to the cell polarity. Although much is known about the molecules involved in cell polarization, the direct trigger of the process remains largely obscure. We previously demonstrated that the tissue boundary between the chordamesoderm and lateral mesoderm of Xenopus laevis is important for chordamesodermal cell polarity. Here, we examined the intracellular calcium dynamics during boundary formation between two different tissues. In a combination culture of nodal-induced chordamesodermal explants and a heterogeneous tissue, such as ectoderm or lateral mesoderm, the chordamesodermal cells near the boundary frequently displayed intracellular calcium elevation; this frequency was significantly less when homogeneous explants were used. Inhibition of the intracellular calcium elevation blocked cell polarization in the chordamesodermal explants. We also observed frequent calcium waves near the boundary of the dorsal marginal zone (DMZ) dissected from an early gastrula-stage embryo. Optical sectioning revealed that where heterogeneous explants touched, the chordamesodermal surface formed a wedge with the narrow end tucked under the heterogeneous explant. No such configuration was seen between homogeneous explants. When physical force was exerted against a chordamesodermal explant with a glass needle at an angle similar to that created in the explant, or migrating chordamesodermal cells crawled beneath a silicone block, intracellular calcium elevation was frequent and cell polarization was induced. Finally, we demonstrated that a purinergic receptor, which is implicated in mechano-sensing, is required for such frequent calcium elevation in chordamesoderm and for cell polarization. This study raises the possibility that tissue-tissue interaction generates mechanical forces through cell-cell contact that initiates coordinated cell polarization through a transient increase in intracellular calcium.
Partial Text: Convergent extension (CE), one of the most important cell movements in early vertebrate development, elongates the embryo along the anterior-posterior axis. During this process, chordamesodermal cells of the Xenopus gastrula become highly polarized, become spindle-shaped representing polarized intracellular events, and intercalate between each other mediolaterally, via active protrusions , , . Although a signaling pathway initiated by secreted Wnt ligands, such as Wnt11, is essential for establishing the planar cell polarity (PCP)  that governs CE, little is known about the mechanism that initiates the coordinated polarity. In our previous study, we monitored the orientation of microtubule (MT) growth as an indicator of functional cell polarity, and demonstrated that cell-cell or tissue-tissue interaction plays a critical role in establishing cell polarity . The most intriguing finding was that the polarity, the direction of MT elongation and cell alignment, becomes visible in chordamesodermal explants only when they are co-cultured in contact with a Xenopus embryonic tissue possessing different properties, i.e., the lateral mesoderm or ectoderm. Furthermore, the polarity revealed by MT growth was evident only in the chordamesoderm, suggesting that mesodermal differentiation is prerequisite for the establishment of cell polarity. The importance of cell-cell or tissue-tissue interaction for coordinated cell behaviors such as cell migration and cell sorting has recently become a topic of intense interest , , . The reports suggest that physical contact between two different tissues with distinct physical properties as regards cell adhesion and/or surface tension provides a cue for the initiation of cell polarity in cellular morphology and their alignment. These findings prompted us to investigate the very first event that occurs at the boundary of two distinct tissues, using contact cultures.
Our present results, together with those of our previous report, suggest that the boundary formed between two contiguous tissues, namely the chordamesoderm and lateral mesoderm or chordamesoderm and ectoderm, creates a cue for cell polarization. In our previous report, we showed that contact of two tissues, which received different levels of nodal and differentiate into two distinct cell fates such as ectoderm and mesoderm, is important to generate the microtubule polarity and cell alignment. In this study, we focused on the intracellular calcium dynamics near the tissue boundary, which were necessary for cell polarization of the chordamesoderm and CE. The frequency of calcium elevation in the chordamesoderm was dependent on the nature of the neighboring tissue, consistent with our previous observation that the cell alignment and MT elongation establish their polarity according to the boundary with a neighboring tissue. Because one of the earliest cellular responses to an external signal is a calcium event, this could be the earliest response in the cascade that leads to cell polarization.