New views on the neural crest epithelial–mesenchymal transition and neuroepithelial interkinetic nuclear migration

Abstract

By developing a technique for imaging the avian neural crest epithelial–mesenchymal transition (EMT), we have discovered cellular behaviors that challenge current thinking on this important developmental event, including the probability that complete disassembly of the adherens junctions may not control whether or not a neural epithelial cell undergoes an EMT. Further, neural crest cells can adopt multiple modes of cell motility in order to emigrate from the neuroepithelium. We also gained insights into interkinetic nuclear migration (INM). For example, the movement of the nucleus from the basal to apical domain may not require microtubule motors nor an intact nuclear envelope, and the nucleus does not always need to reach the apical surface in order for cytokinesis to occur. These studies illustrate the value of live-cell imaging to elucidate cellular processes.

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Acknowledgements

We would like to thank Dan Starr and the Erickson lab members for their helpful comments on the manuscript and discussions as this work progressed. We also acknowledge that our interpretation of INM events were shaped by ideas originating from Philip Spear, who is currently pursuing mechanisms of INM in the Erickson lab. This research was supported by grants from the NIH and the American Heart Association to CAE.

Figures and Tables

Figure 1 Following mitosis, dorsal neural tube cells can reinsert into the neuroepithelium. (A) A neural epithelial cell labeled with γ-tubulin-GFP (green) and mCherry-tubulin (red) and indicated within the white box is shown in the time-lapse series below. (A′) As this cell (white dot) undergoes interkinetic nuclear migration and mitosis at the apical surface, the centrosome (white arrowhead) leaves the apical surface and travels basally (0:14). In the next time frame (0:21), the nucleus moves towards the apical domain and meets the duplicated centrosomes in a non-apical location (white and blue arrowheads), and then the centrosomes and the nucleus travel together to the apical surface, where cytokinesis occurs (completed by 0:56). Note that the plane of cell division separates the basal-most daughter cell (white dot) from the apical surface. Following cell division, the centrosome (blue arrowhead) of the apical-most daughter cell (blue dot) is found at the lumen, whereas the centrosome (white arrowhead) belonging to the basal-most daughter cell (white dot) is found at a non-apical position. As the time-series progresses, the cell tail with the non-apical centrosome (white arrowhead) returns to the apical surface (complete by 4:26). Scale bars are 10 µm. D = dorsal, nt = neural tube, nc = neural crest, ap = apical.

Figure 2 Neural crest emigration accompanied by filopodia and blebs. (A) The mem-EGFP-labeled neural epithelial cell indicated within the white box is shown in the time-lapse series below. (A′) Note that there are two different mem-EGFP-labeled cells close together within the field of view, a dorsal neural tube cell (red dot) and a neural crest cell (no dot). The neural epithelial cell (red dot) detaches from the lumen and migrates out of the neural tube. The formation of filopodia are indicated with red arrowheads and membrane blebs with red arrows. Scale bars are 10 µm.

Figure 3 Examples of neural epithelial cells undergoing INM. (A–C) Neuroepithelial cells were labeled with GFP-α-catenin (green) and mCherry-tubulin (red). Cells within the white boxes are shown in the time-lapse series below. (A′) Prior to nuclear migration in an apical direction, the mCherry-tubulin signal in the cell tail is reduced (white arrowheads) and mCherry-tubulin enters the nucleus (white arrow, t = 0:21). The nucleus is then transported from a basal position to the apical surface, where the mitotic spindle forms (bright mCherry-tubulin signal) and cytokinesis occurs. The nuclei of the daughter cells (white and blue dots) are then transported basally. (B′) Before nuclear migration from the basal to apical surface, the mCherry-tubulin signal in the cell tail is reduced (white arrowheads) and mCherry-tubulin enters the nucleus (white arrow, t = 0:28). Mitotic spindle formation also occurs while the nucleus is in a basal position (bright mCherry-tubulin signal, t = 0:28). The nucleus is then transported to the apical surface where cytokinesis occurs. The daughter cell nuclei (white and blue dots) are subsequently shuttled to a basal position. (C′) Note the presence of two different mCherry-tubulin cell tails in the viewing field. Prior to nuclear migration from the basal to apical surface, the mCherry-tubulin signal in one cell tail is reduced (white arrowheads) and mCherry-tubulin enters the nucleus (white arrow, t = 0:28). Nuclear migration in the apical direction and spindle formation occurs at t = 0:35, but the nucleus does not make it all the way to the apical surface before cell division occurs (t = 0:56). After cytokinesis, the nuclei of the daughter cells (white and blue dots) are transported in a basal direction. Scale bars are 10 µm.

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