Current Biology has a paper out yesterday titled “Integrin α5β1 and Fibronectin Regulate Polarized Cell Protrusions Required for Xenopus Convergence and Extension,” using RNAi knockdown and antibody blocking to study defects in frog embryo gastrulation when fibronectin or its primary cell-suface receptor activities are blocked.
Specifically, Davidson et al. examine the relationship between integrin-fibronectin engagement and the regulation of mediolateral cell intercalation, convergence and extension of cell layers in tissue formation, and polarized orientation of cell-cell contacts in migrating cell sheets.
Davidson et al. suggest two explanations for how the fibrillar fibronectin matrix could regulate mediolateral cell intercalation in gastrulation:
First, fibrils may act to repress protrusions directed along the fibrillar matrix and enhance traction generated by cell-cell adhesions (Figure 8C). This regulatory mechanism would prevent cells from crawling under their neighbors, an event that produces tissue thickening rather than tissue extension (Figure 8D). Successful assembly of a fibrillar matrix could serve to maintain the tissue architecture that guides cell rearrangement into axial extension.
Another way fibrils might regulate mediolateral cell intercalation is to repress short-duration random protrusions and encourage persistent, mediolaterally directed protrusions. This regulatory mechanism directs cell intercalation between mediolateral neighbors rather than between anterior or posterior neighboring cells. Intercalation between anterior and posterior neighbors reduces the effectiveness of directed cell intercalation in driving extension of mesoderm along the anterior-posterior axis. Both of these mechanisms operating at the scale of individual cells would regulate the progress of morphogenesis on the tissue level.
This fits in well with the view of fibronectin, and ECM proteins in general, as both providing mechanical traction and stability, and cell-orientating biochemical signals.
- Integrin alpha(5)beta(1) and Fibronectin Regulate Polarized Cell Protrusions Required for Xenopus Convergence and Extension. Davidson LA, Marsden M, Keller R, Desimone DW. Curr. Biol. 2006 May 9; 16(9):833-44. Pubmed
- Mechanisms of convergence and extension by cell intercalation. Keller R, Davidson L, Edlund A, Elul T, Ezin M, Shook D, Skoglund P. Philos Trans R Soc Lond B Biol Sci. 2000 Jul 29; 355(1399):897-922. Pubmed