Sedzinski J et al. (2016), Emergence of an Apical Epithelial Cell Surface ...

XB-ART-52201

Dev Cell 2016 Jan 11;361:24-35. doi: 10.1016/j.devcel.2015.12.013.

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Emergence of an Apical Epithelial Cell Surface In Vivo.

Sedzinski J , Hannezo E , Tu F , Biro M , Wallingford JB .

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Epithelial sheets are crucial components of all metazoan animals, enclosing organs and protecting the animal from its environment. Epithelial homeostasis poses unique challenges, as addition of new cells and loss of old cells must be achieved without disrupting the fluid-tight barrier and apicobasal polarity of the epithelium. Several studies have identified cell biological mechanisms underlying extrusion of cells from epithelia, but far less is known of the converse mechanism by which new cells are added. Here, we combine molecular, pharmacological, and laser-dissection experiments with theoretical modeling to characterize forces driving emergence of an apical surface as single nascent cells are added to a vertebrate epithelium in vivo. We find that this process involves the interplay between cell-autonomous actin-generated pushing forces in the emerging cell and mechanical properties of neighboring cells. Our findings define the forces driving this cell behavior, contributing to a more comprehensive understanding of epithelial homeostasis.

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Species referenced: Xenopus
Genes referenced: fmn1 mcc rhoa utrn
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	FigureÂ 1. Apical Emergence Is Selectively Driven in the Apical Region of Nascent Multiciliated Cells (MCCs)(A) Schematic of the apical emergence process. Upon insertion at the tricellular junction, a MCC precursor (green) expands its apical surface within the surrounding goblet cells (gray).(B) Image sequence of apically emerging MCC expressing actin marker utrophin (UtrCH, green) under MCC-specific Î±-tubulin promoter; goblet cells visualized by expression of UtrCH-RFP (magenta) under goblet-specific nectin promoter. Scale bar, 10Â Î¼m.(C) Orthogonal projections, corresponding to (B), ofÂ apically emerging MCC visualized by Î±-tubulin UtrCH-GFP. Scale bar, 10Â Î¼m.(D) Three phases of apical area expansion process. Data represent mean (green line) and variance (black bars); nÂ = 9 cells from 9 embryos.(E) Schematic of potential autonomous forces (basolateral constriction, magenta arrows; apicobasal squeezing, purple arrows; apical pushing, black arrows) and non-autonomous forces (neighboring pulling, blue arrows) that might contribute to apical expansion process.(F) Representative surface area dynamics of the apical (black), middle (blue) and basal (magenta) region of MCC during apical emergence. Percentage of surface area change is calculated as a ratio of the surface area at the consecutive time point to the surface area at the beginning of the emergence process. Insets: schematic of MCC at the beginning and at the end of the apical emergence process; average percentage changes of surface areas at given region and SD are: apical side 618.5%Â Â± 28.03%, middle side 23.99%Â Â± 28.95%, basal side 3.24%Â Â± 5.15%; nÂ = 5 cells from 5 embryos.(G) Volume (green) and height (purple) changes of MCCs throughout apical emergence process.See also Figures S1 and S2.
	FigureÂ 2. Pulling Forces of Cells Adjacent toÂ MCCs Do Not Drive Apical Emergence Process(A) Image sequence of apically expanding MCC expressing actin marker utrophin (UtrCH, green) under MCC-specific Î±-tubulin promoter; epithelial cells visualized by expression of membrane marker CAAX-RFP mRNA (magenta).(B) Orthogonal projections, corresponding to (A).(C) Image sequence of MCC apical domain upon laser ablation in the center (indicated by yellow bolt and circle) of the neighboring goblet cell. Laser ablation leads to excessive constriction of goblet cell (outlined by blue dotted line) but does not result in MCC apical domain collapse.(D) Orthogonal projections of MCC (outlined by yellow dotted line), corresponding to (C). Constricting goblet cell exerts pulling force on the MCC apical domain, as assessed by change of the angle between apical and lateral side of MCC.(E) Parameters measured upon laser ablation (yellow bolt and circle) of goblet cell adjacent to MCC: apical area of ablated goblet cell, blue; apical area of MCC adjacent to ablated goblet cell, red; angles between apical and lateral side, Ï†1 (ablated side) and Ï†2 (non-ablated side).(F and G) Dynamics of parameters described in (E) inÂ control (F) and upon laser ablation in the center ofÂ the goblet cell (G). Despite the evident pulling by the neighboring goblet cell (increase of Ï†1 in G), the apical area of MCC does not increase compared with (F).Scale bar, 10Â Î¼m. See also FigureÂ S3.
	FigureÂ 3. MCC-Generated Pushing Forces Majorly Drive Apical Expansion Process(A) Schematic of the theoretical model of forces acting on an apical domain of an MCC. Apical domain-expanding forces: effective 2D pressure, Î´P (black arrows) and neighboring junctional pulling forces, Î» (red arrows) acting against MCC cortical tension, Î³ (blue arrows) and elasticity from the surrounding cells, E.(B and C) (B) Simulation of the MCC apical domain shape upon dominant pushing forces or (C) dominant junctional pulling forces.(D) Image sequence of an apically expanding MCC (visualized by Î±-tubulin UtrCH-GFP, green) within goblet cells (visualized by nectin UtrCH-RFP, magenta). Scale bar, 10Â Î¼m.(E) Experimental (black) and theoretical (red) curvature changes, defined by kurtosis, in time of the cell shown in (D).(F) Kurtosis values for the consecutive apical domain sizes, categorized by binned mean radius, in control cells. Boxes extend from the 25th to 75th percentiles, with a line at the median. Mann-Whitney U test (number of embryos, n > 5), âˆ—âˆ—âˆ—pÂ < 0.001; n.s., not significant.(G) Schematic of laser ablation of junctions perpendicular to MCC. Red bolt and circle represents the ablation region.(H) Initial recoil velocities upon laser ablation of junctions described in (G). Black circles, experimental data; solid red line indicates fit.(I) Shape simulation of the MCC apical domain based on the theoretical model.(J) Inferred neighboring junctional tension, Î» andÂ pressure, Î´P (K) from the simulated shape shown in (D).See also FigureÂ S4.
	FigureÂ 4. Formation of Medial Apical Actin of MCCs Highly Correlates with Expansion Dynamics of the Apical Domain(A) Segmentation schematic of the cortical (green) and the medial (red) regions of a MCC within the plane of the exposed apical area.(B) Representative image sequence of automatically segmented cortical and medial region within apically expanding MCC (visualized by Î±-tubulin UtrCH-GFP). Scale bar, 10Â Î¼m.(C) Representative effective actin (ratio of mean medial to mean cortical actin) dynamics (red) during the apical area (black) expansion process. Crosses, data points; line, data smoothed by factor of 5.(D) Effective apical actin concentration for the consecutive apical domain sizes, categorized by binned area, in control cells, nÂ = 9 cells from 9 embryos. Data represent mean and variance.(E) Effective apical actin concentration as a function of apical area in control cells, nÂ = 9 cells from 9 embryos.
	FigureÂ 5. Actin Polymerization and Subsequent Actin Network Formation Drive Apical Expansion of MCCs(A) Phase diagram of possible apical domain expansion dynamics, as a function of target MCC cortical tension Î³ and pressure Î´P. Blue and orange lines indicate the transition zones between different phases. Insets represent simulations of the apical area in time for the three regimes.(B) Image sequence of apically collapsing MCC (4 out of 4 cells) (visualized by Î±-tubulin UtrCH-GFP, green) upon mosaic co-injection of FMN1 morpholino (FMN1 MO with UtrCH-RFP mRNA, magenta). Note that FMN1 MO is present only in MCC, as indicated by UtrCH-RFP mRNA expression.(C) Image sequence of apically expanding MCC (visualized by Î±-tubulin UtrCH-GFP, green) upon mosaic co-injection of FMN1 morpholino (FMN1 MO with UtrCH-RFP mRNA, magenta). Note that FMN1 MO is present only in goblet cells and that MCC expands normally (4 out of 4 cells).(D) Image sequence of MCC (visualized by Î±-tubulin UtrCH-GFP, green) expressing dominant-negative FMN1 (DN-FMN1, visualized by Î±-tubulin DN-FMN1-RFP, magenta).(E) Apical area dynamics of MCC upon mosaic injection of FMN1 MO (black: FMN1 MO present in MCCs only, shown in B; blue: FMN1 MO present in goblet cells only, shown in C).(F) Apical area dynamics of MCC expressing dominant-negative FMN1 (DN-FMN1), representing an â€œovershootingâ€ regime within the phase diagram.(G) Apical expansion rate during linear growth phase in controls (black), MCCs FMN1 KD (blue), and MCCs expressing dominant-negative FMN1 (DN-FMN1); number of embryos, n > 15. Boxes extend from the 25th to 75th percentiles, with a line at the median. Mann-Whitney U test, âˆ—âˆ—âˆ—pÂ < 0.001.(H) Image sequence of apically collapsing MCC (visualized by Î±-tubulin E-Cadherin-GFP, green) within goblet cells (visualized by nectin RFP-UtrCH, magenta) upon treatment with 50Â Î¼M of SMIFH2. Time indicates minutes after drug addition to the medium.(I) Laser ablation schematics of MCC junctions. Blue bolt and circle represents the ablation region.(J) Initial recoil velocities upon laser ablation of MCC junctions described in (I). Black circles, experimental data; solid blue line indicates fit.Scale bar, 10Â Î¼m. See also FigureÂ S5.
	FigureÂ 6. Apical Emergence of MCCs is Influenced by External Rigidity of the Surrounding Cells(A) Schematic of nectin-driven expression (goblet cell specific) of DN-RhoA or CA-RhoA.(B) Image sequence of apically expanding MCC, outlined by expression of UtrCH-GFP under nectin promoter (top, green), and within goblet cells expressing CA-RhoA under nectin promoter (bottom, magenta). Scale bar, 10Â Î¼m.(C) Apical expansion rate during linear growth phase in controls (black), and MCCs surrounded by goblet cells expressing CA-RhoA (magenta) or DN-RhoA (blue). âˆ—âˆ—pÂ < 0.01; n.s., not significant.(D) Final size of the apical domain of MCCs for experimental conditions described in (C). âˆ—âˆ—pÂ < 0.01, âˆ—pÂ < 0.05.(E) Effective apical actin concentration in MCCs as a function of apical area in controls (black), and MCC surrounded by goblet cells expressing CA-RhoA (magenta) or DN-RhoA (blue); data represent mean and variance (number of embryos, n > 5). Boxes extend from the 25th to 75th percentiles, with a line at the median. Mann-Whitney U test (number of embryos, n > 5) not significant (n.s.).

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