Abstract
Convergent extension (CE) is a conserved morphogenetic movement that drives axial lengthening of the primary body axis and depends on the planar cell polarity (PCP) pathway. In Drosophila epithelia, a polarised subcellular accumulation of PCP core components, such as Dishevelled (Dvl) protein, is associated with PCP function. Dvl has long been thought to accumulate in the mediolateral protrusions in Xenopus chordamesoderm cells undergoing CE. Here we present a quantitative analysis of Dvl intracellular localisation in Xenopus chordamesoderm cells. We find that, surprisingly, accumulations previously observed at mediolateral protrusions of chordamesodermal cells are not protrusion-specific but reflect yolk-free cytoplasm and are quantitatively matched by the distribution of the cytoplasm-filling lineage marker dextran. However, separating cell cortex-associated from bulk Dvl signal reveals a statistical enrichment of Dvl in notochord–somite boundary-(NSB)-directed protrusions, which is dependent upon NSB proximity. Dvl puncta were also observed, but only upon elevated overexpression. These puncta showed no statistically significant spatial bias, in contrast to the strongly posteriorly-enriched GFP-Dvl puncta previously reported in zebrafish. We propose that Dvl distribution is more subtle and dynamic than previously appreciated and that in vertebrate mesoderm it reflects processes other than protrusion as such.
Highlights
Convergent extension (CE) is a conserved, early and critical morphogenetic movement that establishes the elongated trunk of the vertebrate body (Keller and Danilchik, 1988)
We use new image analysis algorithms to quantifiy Dvl abundance at the cell cortex, where it is generally thought to be active in planar cell polarity (PCP) (Rothbacher et al, 2000; Wallingford et al, 2000; Yang-Snyder et al, 1996), and show that Dvl is accumulated statistically more than dextran in mediolateral cortex, and most conspicuously in the cortex of protrusions directed towards the notochord–somite boundary (NSB), more so in cells just prior to capture by the NSB
We chose to analyse embryos around mid-gastrulation stage, when protrusive activity becomes stabilised in the mediolateral plane and both notochord and pre-somitic mesoderm are still undergoing massive cell intercalation-driven CE (Keller et al, 1989; Keller and Tibbetts, 1989; Shih and Keller, 1992a, 1992b; Wilson and Keller, 1991)
Summary
Convergent extension (CE) is a conserved, early and critical morphogenetic movement that establishes the elongated trunk of the vertebrate body (Keller and Danilchik, 1988). Models for PCP in vertebrates have proposed similar PCP protein accumulations in specific cellular quadrants, namely bipolar enrichment in mediolateral protrusions of intercalating cells (Kinoshita et al, 2003; Mlodzik, 2006; Shindo et al, 2008; Wallingford et al, 2002).
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