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Assembly and positioning of actomyosin rings by contractility and planar cell polarity.

Sehring IM, Recho P, Denker E, Kourakis M, Mathiesen B, Hannezo E, Dong B, Jiang D - Elife (2015)

Bottom Line: Intriguingly, rings always form at the cells' anterior edge before migrating towards the center as contractility increases, reflecting a novel dynamical property of the cortex.We develop a simple model of the physical forces underlying this tug-of-war, which quantitatively reproduces our results.We thus propose a quantitative framework for dissecting the relative contribution of contractility and PCP to the self-assembly and repositioning of cytoskeletal structures, which should be applicable to other morphogenetic events.

View Article: PubMed Central - PubMed

Affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.

ABSTRACT
The actomyosin cytoskeleton is a primary force-generating mechanism in morphogenesis, thus a robust spatial control of cytoskeletal positioning is essential. In this report, we demonstrate that actomyosin contractility and planar cell polarity (PCP) interact in post-mitotic Ciona notochord cells to self-assemble and reposition actomyosin rings, which play an essential role for cell elongation. Intriguingly, rings always form at the cells' anterior edge before migrating towards the center as contractility increases, reflecting a novel dynamical property of the cortex. Our drug and genetic manipulations uncover a tug-of-war between contractility, which localizes cortical flows toward the equator and PCP, which tries to reposition them. We develop a simple model of the physical forces underlying this tug-of-war, which quantitatively reproduces our results. We thus propose a quantitative framework for dissecting the relative contribution of contractility and PCP to the self-assembly and repositioning of cytoskeletal structures, which should be applicable to other morphogenetic events.

No MeSH data available.


Talin localization at the equator is not affected by lower contractility.Notochord cells are labeled simultaneously with Lifeact-mEGFP for actin (A, A') and mCherry-talinA I/LWEQ (B, B'). Both actin and talin are enriched in the equatorial cortex before the blebbistatin treatment (A, B). After 60-min blebbistatin treatment, actin ring is shifted to the anterior pole (white arrowhead in A'), whereas talin remains at the equator (white arrowheads in B'). Anterior to the left in all panels. Scale bars, 10 μm.DOI:http://dx.doi.org/10.7554/eLife.09206.008
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fig2s2: Talin localization at the equator is not affected by lower contractility.Notochord cells are labeled simultaneously with Lifeact-mEGFP for actin (A, A') and mCherry-talinA I/LWEQ (B, B'). Both actin and talin are enriched in the equatorial cortex before the blebbistatin treatment (A, B). After 60-min blebbistatin treatment, actin ring is shifted to the anterior pole (white arrowhead in A'), whereas talin remains at the equator (white arrowheads in B'). Anterior to the left in all panels. Scale bars, 10 μm.DOI:http://dx.doi.org/10.7554/eLife.09206.008

Mentions: To characterize the anterior shift of the equatorial actin ring triggered by blebbistatin treatment, we recorded the actin dynamics during drug treatment in elongated cells possessing an equatorial constriction and a broad actin ring. High-speed time-lapse recordings revealed a migration of the cortical actin ring from the equator to the anterior pole (Figure 2C,C′; Video 2), mirroring exactly the reverse sequence of migration of the ring towards the center in normal development. The front of the ring moved at a velocity of 91.8 ± 10.5 nm/min (n = 10). The speed of the shift was blebbistatin dose dependent; halving the blebbistatin concentration reduced the velocity significantly to 27 ± 0.5 nm/min (n = 10; p < 0.0001). The ring width stayed constant when it shifted anteriorly (Figure 2D). Remarkably, the encounter of the ring with the anterior lateral domain did not bring the movement to a halt. Instead, after the ring contacted the lateral domain (red line in Figure 2D), the posterior edge of the ring continued to move anteriorly at an increasing speed of 191.6 ± 21.4 nm/min (n = 10), so that the width of the ring narrowed (Figure 2D), until the entire ring disappeared at the position of the lateral domain (Video 2). The cell length did not significantly change during this process (Figure 2—figure supplement 1A; n = 10). We also examined the dynamic localization of talin, an actin-binding protein that bridges actin filaments and the adhesion apparatus at the cleavage furrow of dividing cells (Sanger et al., 1994; Critchley, 2009; Kanchanawong et al., 2010), and normally colocalized with the cortical actin ring at the equator in Ciona notochord (Sehring et al., 2014) (Figure 2—figure supplement 2B). Live imaging showed that talin concentrated at the cell equator slightly after the ring had been established centrally (data not shown), suggesting that talin actually responds to cortical repositioning rather than driving it. Upon blebbistatin treatment of already established central rings (Figure 2—figure supplement 2A), whereas the cortical actin was shifted to the anterior pole (Figure 2—figure supplement 2A′), talin lagged behind at the equatorial position (Figure 2—figure supplement 2B′). This result suggests not all components of the ring are shifted by blebbistatin.Video 2.Shifting of equatorial actin filaments upon blebbistatin treatment.


Assembly and positioning of actomyosin rings by contractility and planar cell polarity.

Sehring IM, Recho P, Denker E, Kourakis M, Mathiesen B, Hannezo E, Dong B, Jiang D - Elife (2015)

Talin localization at the equator is not affected by lower contractility.Notochord cells are labeled simultaneously with Lifeact-mEGFP for actin (A, A') and mCherry-talinA I/LWEQ (B, B'). Both actin and talin are enriched in the equatorial cortex before the blebbistatin treatment (A, B). After 60-min blebbistatin treatment, actin ring is shifted to the anterior pole (white arrowhead in A'), whereas talin remains at the equator (white arrowheads in B'). Anterior to the left in all panels. Scale bars, 10 μm.DOI:http://dx.doi.org/10.7554/eLife.09206.008
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4612727&req=5

fig2s2: Talin localization at the equator is not affected by lower contractility.Notochord cells are labeled simultaneously with Lifeact-mEGFP for actin (A, A') and mCherry-talinA I/LWEQ (B, B'). Both actin and talin are enriched in the equatorial cortex before the blebbistatin treatment (A, B). After 60-min blebbistatin treatment, actin ring is shifted to the anterior pole (white arrowhead in A'), whereas talin remains at the equator (white arrowheads in B'). Anterior to the left in all panels. Scale bars, 10 μm.DOI:http://dx.doi.org/10.7554/eLife.09206.008
Mentions: To characterize the anterior shift of the equatorial actin ring triggered by blebbistatin treatment, we recorded the actin dynamics during drug treatment in elongated cells possessing an equatorial constriction and a broad actin ring. High-speed time-lapse recordings revealed a migration of the cortical actin ring from the equator to the anterior pole (Figure 2C,C′; Video 2), mirroring exactly the reverse sequence of migration of the ring towards the center in normal development. The front of the ring moved at a velocity of 91.8 ± 10.5 nm/min (n = 10). The speed of the shift was blebbistatin dose dependent; halving the blebbistatin concentration reduced the velocity significantly to 27 ± 0.5 nm/min (n = 10; p < 0.0001). The ring width stayed constant when it shifted anteriorly (Figure 2D). Remarkably, the encounter of the ring with the anterior lateral domain did not bring the movement to a halt. Instead, after the ring contacted the lateral domain (red line in Figure 2D), the posterior edge of the ring continued to move anteriorly at an increasing speed of 191.6 ± 21.4 nm/min (n = 10), so that the width of the ring narrowed (Figure 2D), until the entire ring disappeared at the position of the lateral domain (Video 2). The cell length did not significantly change during this process (Figure 2—figure supplement 1A; n = 10). We also examined the dynamic localization of talin, an actin-binding protein that bridges actin filaments and the adhesion apparatus at the cleavage furrow of dividing cells (Sanger et al., 1994; Critchley, 2009; Kanchanawong et al., 2010), and normally colocalized with the cortical actin ring at the equator in Ciona notochord (Sehring et al., 2014) (Figure 2—figure supplement 2B). Live imaging showed that talin concentrated at the cell equator slightly after the ring had been established centrally (data not shown), suggesting that talin actually responds to cortical repositioning rather than driving it. Upon blebbistatin treatment of already established central rings (Figure 2—figure supplement 2A), whereas the cortical actin was shifted to the anterior pole (Figure 2—figure supplement 2A′), talin lagged behind at the equatorial position (Figure 2—figure supplement 2B′). This result suggests not all components of the ring are shifted by blebbistatin.Video 2.Shifting of equatorial actin filaments upon blebbistatin treatment.

Bottom Line: Intriguingly, rings always form at the cells' anterior edge before migrating towards the center as contractility increases, reflecting a novel dynamical property of the cortex.We develop a simple model of the physical forces underlying this tug-of-war, which quantitatively reproduces our results.We thus propose a quantitative framework for dissecting the relative contribution of contractility and PCP to the self-assembly and repositioning of cytoskeletal structures, which should be applicable to other morphogenetic events.

View Article: PubMed Central - PubMed

Affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.

ABSTRACT
The actomyosin cytoskeleton is a primary force-generating mechanism in morphogenesis, thus a robust spatial control of cytoskeletal positioning is essential. In this report, we demonstrate that actomyosin contractility and planar cell polarity (PCP) interact in post-mitotic Ciona notochord cells to self-assemble and reposition actomyosin rings, which play an essential role for cell elongation. Intriguingly, rings always form at the cells' anterior edge before migrating towards the center as contractility increases, reflecting a novel dynamical property of the cortex. Our drug and genetic manipulations uncover a tug-of-war between contractility, which localizes cortical flows toward the equator and PCP, which tries to reposition them. We develop a simple model of the physical forces underlying this tug-of-war, which quantitatively reproduces our results. We thus propose a quantitative framework for dissecting the relative contribution of contractility and PCP to the self-assembly and repositioning of cytoskeletal structures, which should be applicable to other morphogenetic events.

No MeSH data available.