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Endothelial cell self-fusion during vascular pruning.

Lenard A, Daetwyler S, Betz C, Ellertsdottir E, Belting HG, Huisken J, Affolter M - PLoS Biol. (2015)

Bottom Line: Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail.In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection.Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion.

View Article: PubMed Central - PubMed

Affiliation: Biozentrum der Universität Basel, Basel, Switzerland.

ABSTRACT
During embryonic development, vascular networks remodel to meet the increasing demand of growing tissues for oxygen and nutrients. This is achieved by the pruning of redundant blood vessel segments, which then allows more efficient blood flow patterns. Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail. Here, we present the subintestinal vein (SIV) plexus of the zebrafish embryo as a novel model to study pruning at the cellular level. We show that blood vessel regression is a coordinated process of cell rearrangements involving lumen collapse and cell-cell contact resolution. Interestingly, the cellular rearrangements during pruning resemble endothelial cell behavior during vessel fusion in a reversed order. In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection. Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion. In a second step, the unicellular connection is resolved unilaterally, and the pruning cell rejoins the opposing branch. Thus, we show for the first time that various cellular activities are coordinated to achieve blood vessel pruning and define two different morphogenetic pathways, which are selected by the flow environment.

No MeSH data available.


Related in: MedlinePlus

The mechanism of cell self-fusion.Stills from a time-lapse movie illustrating cell self-fusion during pruning of an arterial connection in the formation of a segmental vein in the fish trunk at ~40 hpf in a transgenic embryo Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5. Cell–cell junctions are green, and cell cytoplasm is red (A). Only the self-fusing cell is labeled with EGFP-ZO-1; the green channel alone is shown in B and modeled in C. The model in D shows all cells in the branch. E shows a simplified 3-D model of self-fusion: the junctions are black, the apical/luminal membrane is green, and the outer/basal membrane is yellow. The labeled cell is part of a multicellular tube (1). The cell reaches around the lumen and establishes a self-contact (2, arrow) when the neighboring cells move away from each other (blue cells in D). The cell expands the self-contact (3, arrow), and its membranes fuse, as no junctional connection is visible along the tube. Two junctional rings connect the cell to its neighbors: to the intersegmental vessel (ISV) on top and to the dorsal aorta at the bottom. The cell forms a funnel-shaped unicellular tube (4). The arrow shows the tube length, and the asterisk marks the nucleus. Scale bars: 10 μm. See also S12 and S13 Movie.
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pbio.1002126.g004: The mechanism of cell self-fusion.Stills from a time-lapse movie illustrating cell self-fusion during pruning of an arterial connection in the formation of a segmental vein in the fish trunk at ~40 hpf in a transgenic embryo Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5. Cell–cell junctions are green, and cell cytoplasm is red (A). Only the self-fusing cell is labeled with EGFP-ZO-1; the green channel alone is shown in B and modeled in C. The model in D shows all cells in the branch. E shows a simplified 3-D model of self-fusion: the junctions are black, the apical/luminal membrane is green, and the outer/basal membrane is yellow. The labeled cell is part of a multicellular tube (1). The cell reaches around the lumen and establishes a self-contact (2, arrow) when the neighboring cells move away from each other (blue cells in D). The cell expands the self-contact (3, arrow), and its membranes fuse, as no junctional connection is visible along the tube. Two junctional rings connect the cell to its neighbors: to the intersegmental vessel (ISV) on top and to the dorsal aorta at the bottom. The cell forms a funnel-shaped unicellular tube (4). The arrow shows the tube length, and the asterisk marks the nucleus. Scale bars: 10 μm. See also S12 and S13 Movie.

Mentions: Our time-lapse analyses revealed that type II pruning involves the formation of transient unicellular tubes formed by a single endothelial cell, which wraps itself around the lumen. Surprisingly, upon contact with its contralateral side the cell starts to self-fuse its cell membrane in a zipper-like fashion, thereby transforming the cell into a seamless, unicellular tube with a transcellular lumen (Fig 4). To demonstrate endothelial cell self-fusion more directly, we performed single cell labeling experiments with the junctional marker EGFP-ZO-1 (Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5), which tends to be expressed in a mosaic fashion [13]. Since this transgene is hardly active in venous vessels, we examined whether endothelial cell self-fusion occurs during regression of segmental arteries (SeAs), which takes place during segmental vein formation (SeV) in the trunk. SeVs sprout from the posterior cardinal vein and form by anastomosis with SeA, thereby transforming the latter into SeVs [21]. This fusion event is accompanied with the regression of the proximal segment of the SeA (S5 Fig and S12 Movie). Using the UAS:EGFP-ZO1 marker, we were able to follow a single endothelial cell as it formed a unicellular tube (Fig 4 and S13 Movie). The junctional transformation of this cell evidenced by ZO-1 expression was entirely consistent with endothelial cell self-fusion. Initially, the SeA was connected to the dorsal aorta with at least two cells, one of them labeled with EGFP-ZO-1 (Fig 4A, green cell). The arterial segment, initially multicellular, undergoes rearrangements similar to the ones described for the SIVs, also in two possible variations (S5 Fig). Fig 4 shows pruning type II, in which lumen was maintained during cell rearrangements and the remaining “last bridging” cell wrapped around the lumen and self-fused to form a unicellular tube. The cell had a funnel-like shape and connected the aorta (bottom, large junctional ring) to the SeA (top, small junctional ring). Eventually, the SeA connection to the aorta was resolved, and the cell incorporated completely into the aorta (S13 Movie).


Endothelial cell self-fusion during vascular pruning.

Lenard A, Daetwyler S, Betz C, Ellertsdottir E, Belting HG, Huisken J, Affolter M - PLoS Biol. (2015)

The mechanism of cell self-fusion.Stills from a time-lapse movie illustrating cell self-fusion during pruning of an arterial connection in the formation of a segmental vein in the fish trunk at ~40 hpf in a transgenic embryo Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5. Cell–cell junctions are green, and cell cytoplasm is red (A). Only the self-fusing cell is labeled with EGFP-ZO-1; the green channel alone is shown in B and modeled in C. The model in D shows all cells in the branch. E shows a simplified 3-D model of self-fusion: the junctions are black, the apical/luminal membrane is green, and the outer/basal membrane is yellow. The labeled cell is part of a multicellular tube (1). The cell reaches around the lumen and establishes a self-contact (2, arrow) when the neighboring cells move away from each other (blue cells in D). The cell expands the self-contact (3, arrow), and its membranes fuse, as no junctional connection is visible along the tube. Two junctional rings connect the cell to its neighbors: to the intersegmental vessel (ISV) on top and to the dorsal aorta at the bottom. The cell forms a funnel-shaped unicellular tube (4). The arrow shows the tube length, and the asterisk marks the nucleus. Scale bars: 10 μm. See also S12 and S13 Movie.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4401649&req=5

pbio.1002126.g004: The mechanism of cell self-fusion.Stills from a time-lapse movie illustrating cell self-fusion during pruning of an arterial connection in the formation of a segmental vein in the fish trunk at ~40 hpf in a transgenic embryo Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5. Cell–cell junctions are green, and cell cytoplasm is red (A). Only the self-fusing cell is labeled with EGFP-ZO-1; the green channel alone is shown in B and modeled in C. The model in D shows all cells in the branch. E shows a simplified 3-D model of self-fusion: the junctions are black, the apical/luminal membrane is green, and the outer/basal membrane is yellow. The labeled cell is part of a multicellular tube (1). The cell reaches around the lumen and establishes a self-contact (2, arrow) when the neighboring cells move away from each other (blue cells in D). The cell expands the self-contact (3, arrow), and its membranes fuse, as no junctional connection is visible along the tube. Two junctional rings connect the cell to its neighbors: to the intersegmental vessel (ISV) on top and to the dorsal aorta at the bottom. The cell forms a funnel-shaped unicellular tube (4). The arrow shows the tube length, and the asterisk marks the nucleus. Scale bars: 10 μm. See also S12 and S13 Movie.
Mentions: Our time-lapse analyses revealed that type II pruning involves the formation of transient unicellular tubes formed by a single endothelial cell, which wraps itself around the lumen. Surprisingly, upon contact with its contralateral side the cell starts to self-fuse its cell membrane in a zipper-like fashion, thereby transforming the cell into a seamless, unicellular tube with a transcellular lumen (Fig 4). To demonstrate endothelial cell self-fusion more directly, we performed single cell labeling experiments with the junctional marker EGFP-ZO-1 (Tg(fliep:GFF)ubs3,(UAS:mRFP),(UAS:EGFP-ZO-1)ubs5), which tends to be expressed in a mosaic fashion [13]. Since this transgene is hardly active in venous vessels, we examined whether endothelial cell self-fusion occurs during regression of segmental arteries (SeAs), which takes place during segmental vein formation (SeV) in the trunk. SeVs sprout from the posterior cardinal vein and form by anastomosis with SeA, thereby transforming the latter into SeVs [21]. This fusion event is accompanied with the regression of the proximal segment of the SeA (S5 Fig and S12 Movie). Using the UAS:EGFP-ZO1 marker, we were able to follow a single endothelial cell as it formed a unicellular tube (Fig 4 and S13 Movie). The junctional transformation of this cell evidenced by ZO-1 expression was entirely consistent with endothelial cell self-fusion. Initially, the SeA was connected to the dorsal aorta with at least two cells, one of them labeled with EGFP-ZO-1 (Fig 4A, green cell). The arterial segment, initially multicellular, undergoes rearrangements similar to the ones described for the SIVs, also in two possible variations (S5 Fig). Fig 4 shows pruning type II, in which lumen was maintained during cell rearrangements and the remaining “last bridging” cell wrapped around the lumen and self-fused to form a unicellular tube. The cell had a funnel-like shape and connected the aorta (bottom, large junctional ring) to the SeA (top, small junctional ring). Eventually, the SeA connection to the aorta was resolved, and the cell incorporated completely into the aorta (S13 Movie).

Bottom Line: Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail.In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection.Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion.

View Article: PubMed Central - PubMed

Affiliation: Biozentrum der Universität Basel, Basel, Switzerland.

ABSTRACT
During embryonic development, vascular networks remodel to meet the increasing demand of growing tissues for oxygen and nutrients. This is achieved by the pruning of redundant blood vessel segments, which then allows more efficient blood flow patterns. Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail. Here, we present the subintestinal vein (SIV) plexus of the zebrafish embryo as a novel model to study pruning at the cellular level. We show that blood vessel regression is a coordinated process of cell rearrangements involving lumen collapse and cell-cell contact resolution. Interestingly, the cellular rearrangements during pruning resemble endothelial cell behavior during vessel fusion in a reversed order. In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection. Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion. In a second step, the unicellular connection is resolved unilaterally, and the pruning cell rejoins the opposing branch. Thus, we show for the first time that various cellular activities are coordinated to achieve blood vessel pruning and define two different morphogenetic pathways, which are selected by the flow environment.

No MeSH data available.


Related in: MedlinePlus