At various stages during development, epithelial cells undergo epithelial-mesenchymal transition (EMT), becoming nonadherent and motile, a process that involves coordination of distinct events (such as loss of polarity, disruption of intercellular junctions, and degradation of the basement membrane) and that also occurs in carcinoma progression (see Levayer and Lecuit). Nakaya et al. characterized the expression patterns of various EMT markers in gastrulation-stage chick embryos and found that breakdown of the basement membrane was the earliest identifiable EMT event. Electroporation of RhoA into medial epiblast cells (mesoderm precursors) in the primitive streak interfered with their ability to undergo EMT. It led to the retention of the basement membrane marker laminin in nascent mesoderm cells and in mesoderm precursors without affecting laminin expression in lateral epiblast cells. Excess RhoA also led to the formation of aggregates of nascent mesoderm cells at the streak midline. These defects were rescued by concurrent electroporation of the Rho inhibitor C3 exoenzyme. Immunofluorescence analysis revealed RhoA in the epiblast but not the mesoderm; further, whereas RhoA was identified in both apical and basal regions of lateral epiblast cells, it was only apparent in the apical portions of medial cells. Expression of the RhoA guanine nucleotide exchange factor (GEF) Net1 was basally restricted in lateral epiblast cells and down-regulated or absent in medial epiblast cells. Antisense morpholino oligonucleotides directed against RhoA and Net1 elicited premature breakdown of laminin, as did expression of dominant-negative RhoA; moreover, the Net1-directed morpholinos reduced basal RhoA activity in epiblast cells without affecting the activity of apical RhoA. Nocodazole, which destabilizes microtubules, promoted breakdown of the basement membrane, whereas taxol, which stabilizes them, promoted retention of the basement membrane in the medial epiblast. Misexpression of RhoA caused the retention in medial epiblast cells of a posttranslationally modified form of β-tubulin, typically found near the basal cell membrane of lateral epiblast cells but absent in medial epiblast cells. Thus, the authors propose that the decreased activity of basal RhoA and destabilization of basal microtubules lead to disruption of interactions between epithelial cells and basement membrane, and thereby trigger basement membrane breakdown, at the onset of the EMT during gastrulation.
Y. Nakaya, E. W. Sukowati, Y. Wu, G. Sheng, RhoA and microtubule dynamics control cell-basement membrane interaction in EMT during gastrulation. Nat. Cell Biol. 10, 765-775 (2008). [PubMed]
R Levayer, T Lecuit, Breaking down EMT. Nat. Cell Biol. 10, 757-759 (2008). [PubMed]