RAB-5- and DYNAMIN-1-Mediated Endocytosis of EFF-1 Fusogen Controls Cell-Cell Fusion

Abstract

Cell-cell fusion plays essential roles during fertilization and organogenesis. Previous studies in C. elegans led to the identification of the eukaryotic fusion protein (EFF-1 fusogen), which has structural homology to class II viral fusogens. Transcriptional repression of EFF-1 ensures correct fusion fates, and overexpression of EFF-1 results in embryonic lethality. EFF-1 must be expressed on the surface of both fusing cells; however, little is known regarding how cells regulate EFF-1 surface exposure. Here, we report that EFF-1 is actively removed from the plasma membrane of epidermal cells by dynamin- and RAB-5-dependent endocytosis and accumulates in early endosomes. EFF-1 was transiently localized to apical domains of fusion-competent cells. Effective cell-cell fusion occurred only between pairs of cell membranes in which EFF-1 localized. Downregulation of dynamin or RAB-5 caused EFF-1 mislocalization to all apical membrane domains and excessive fusion. Thus, internalization of EFF-1 is a safety mechanism preventing excessive cell fusion.

Graphical abstract

Figure 1

EFF-1 Localizes to Intracellular Puncta (A–C) EFF-1 expression pattern in the process of cell fusion was revealed by immunostaining with anti-EFF-1 monoclonal antibody (green) and anti-DLG-1 antibody (apical junctions, magenta). EFF-1 vesicular localization (arrows) at developmental stages prior to (A and B) and after epithelial cell fusion (C) in wild-type C. elegans embryos is shown. Colocalization of EFF-1 with apical junction is shown (arrowhead; B). Fused hyp6 and hyp7 syncytia are outlined with a white dashed line in (C). (D) Immunostaining of eff-1(ok1021) mutant at 1.8-fold stage shows no EFF-1 expression, revealing the antibody specificity, and lack of cell fusion. The scale bar represents 10 μm. (E) Schematics of embryonic epithelial cell fusions during elongation from comma (left) to 1.8-fold stage (right). Cell borders (junctions) are drawn with black lines; EFF-1 expression pattern is visualized as green puncta. Cells forming hyp6 and hyp7 syncytia are colored with dark gray and light gray, respectively. Orientation of embryos in all figures is anterior, left and dorsal, top. See also Figure S1.

Figure 2

RAB-5 and DYN-1 RNAi Knockdown Induces EFF-1 Accumulation on the Apical Plasma Membrane (A, B, and F) Live images of EFF-1::GFP (green) and apical junctions, DLG-1::RFP (magenta) in embryos treated with control RNAi (A), rab-5 RNAi (B), and dyn-1 RNAi (F). Right panel represents the magnified region of the inset area in (A) and (B). (A) EFF-1::GFP is in puncta in control RNAi. See also Movies S1 and S2. (B) EFF-1::GFP in rab-5 RNAi embryo appears diffuse on the cytoplasm in smaller vesicles and associated with the apical plasma membranes and junctions. See also Figure S2G and Movies S3 and S4. (C) Intensity profile plotted along the line (white dashed line in A) of EFF-1::GFP (green line on the plot) and DLG-1::RFP (magenta line on the plot) shows no correlation. (D) Intensity profiles plotted along dashed line from (B) represent correlation of EFF-1::GFP and DLG-1::RFP intensities on the cell junctions, showing the enrichment of EFF-1::GFP on the apical plasma membrane. (E) Representative stages from time-lapse images of cell fusion under rab-5 RNAi showing EFF-1::GFP/DLG-1::RFP colocalization (arrows). Cell junction undergoing fusion is marked by an arrowhead. Note that EFF-1 dissociated from the cell junction at 10 min of fusion (arrowhead). Time in min is indicated. See also Movie S3. (F) EFF-1::GFP apical plasma membrane expression (arrows) caused by dyn-1 RNAi is shown on the embryo surface focus (upper panels) and center focus (lower panels). Embryogenesis defects and hyperfusion are visualized by DLG-1::RFP pattern (Movie S5). (G) EFF-1 localization and embryonic defects caused by rab-5 and dyn-1 downregulation. The first bar graph represents percentage of embryos showing EFF-1 localization with plasma membrane (white), plasma membrane together with vesicular puncta (gray), and vesicular expression (black). The second bar represents the percentage of embryos presenting defective embryogenesis (light green) and hyperfusion (dark green). Most rab-5(RNAi) and dyn-1(RNAi) embryos arrest at early embryogenesis before EFF-1 expression and before the time of hypodermal fusions (elongation/morphogenesis). Most embryos that escape the early arrest show hyperfusion and EFF-1 in apical plasma membranes. Mean of three independent experiments; n ≥ 20 embryos per experiment. The scale bars represent 10 μm in (A), (B), and (F) and 2 μm in (E).

Figure 3

Loss of rab-5 Function Induces eff-1-Mediated Ectopic Fusion (A) Ectopic fusion (asterisk) and EFF-1 mislocalization to the plasma membrane (arrows) caused by rab-5(ok2605). Immunofluorescence with anti-EFF-1 (green) and anti-DLG-1 antibody (magenta) followed by SIM is shown. Magnifications of the inset region represent EFF-1 and DLG-1 fluorescence in separate channels and merged. (B–E) Epistasis analysis between rab-5 and eff-1 reveals rab-5 as a negative regulator of eff-1. Fusion pattern was visualized by live imaging of junction marker DLG-1::RFP expressed in rab-5 and eff-1 single and double mutants. (B) Heterozygous rab-5 mutant exhibits wild-type fusion. Fused cells are outlined with white stroke, junctions that will fuse later in embryogenesis (arrowheads). (C) Hyperfusion induced by rab-5 homozygous mutation is shown (asterisk). (D) No fusion in double rab-5(+/−); eff-1(−/−) mutant is shown. (E) Fusion is blocked in double rab-5(−/−); eff-1(−/−) mutant, unfused junctions (arrowheads). (F) Scheme of RAB-5-negative regulation of EFF-1 derived from the epistasis analysis. The scale bars represent 10 μm.

Figure 4

EFF-1 Localization to Intracellular Compartments (A) The localization of EFF-1 revealed by an anti-EFF-1 monoclonal antibody (green) is compared with the apical junction (anti-DLG-1 antibody, magenta) using superresolution microscopy (SIM). (B–D) EFF-1 colocalization with stably expressed GFP-tagged markers of different membrane-bound organelles, detected with anti-GFP antibodies, magenta: Golgi complex, MANS::GFP (B); early endosomes, RAB-5::GFP (C), and lysosomes, LMP-1::GFP (D). Lower panel represents inset areas enlarged and shown in separate channels and merged. (E) Quantification of EFF-1 colocalization with different markers represents the ratio of EFF-1 puncta that overlay the puncta of indicated marker (number of colocalized EFF-1 puncta/total number of EFF-1 puncta as percentage). Bars represent mean percentage of colocalization calculated in 5–20 embryos (100–1,000 cells) ± SEM. The colocalization above 5% is shown in the graph. The full list of intracellular markers tested, number of puncta, and number of embryos per marker are shown in Table S1. See also Figure S4. The scale bar represents 10 μm.

Figure 5

The Dynamics of EFF-1::GFP during Cell Fusion Live imaging of EFF-1::GFP (green) and apical junction marker DLG-1::RFP (magenta) in the process of dorsal fusions. Fusing cells are highlighted in (A), and higher magnification inset is shown below in separate channels: DLG-1::RFP (left); EFF-1::GFP (middle); and merged image (right panel). (B–E) Early stages of apical junction disassembly (arrows). Dynamic colocalization of EFF-1 and DLG-1 on plasma membranes (arrowheads). Time points (in minutes and seconds) of image acquisition are presented in the upper right corners. Time 0’ indicates the beginning of Movie S1. (A) EFF-1::GFP fluorescence is barely detectable in cells that are going to fuse. (B) EFF-1::GFP appears in the cell cytoplasm within a pair of bright vesicles (arrowheads) and diffuse where the cell junction disassembles (arrow). (C–E) EFF-1::GFP arrives at the cell junction from both fusing cells (C, arrowheads) and move along the junction (D and E, arrowheads). Arrows mark the edge of the cell junction undergoing disassembly. (F) EFF-1 puncta coming from opposite cells join together on the cell junction (arrowhead). (G and H) At the end of the first dorsal cell fusion, EFF-1::GFP puncta are distributed in the cytoplasm of the syncytium. (I) Second junction discontinuity revealing the second cell fusion (arrow). (J) EFF-1::GFP vesicles become larger, brighter, and aligned in an anterior-posterior line (arrowheads) within the intermediate syncytium. The scale bar represents 10 μm. See Movies S1 and S2.

Figure 6

EFF-1 Is Constitutively Transported to All Apical Plasma Membrane Domains (A) EFF-1::GFP (green) puncta colocalization with the apical junctions marked DLG-1::RFP (magenta). On the picture and on the schematics, EFF-1-expressing cells are marked with “+,” epidermal cell that does not express EFF-1 is marked by “−“; d, deirid; s, seam cell. EFF-1::GFP is transiently localized to the junctions between different cell types: −/+; +/+; and +/s. Schematics represent highlighted cells from the picture; EFF-1::GFP (green puncta) transiently colocalizes with apical junctions (see Movie S2). (B) After rab-5(RNAi), EFF-1::GFP is spread to all the junctions of −/+, +/+, and +/s types. In schematics, accumulation of EFF-1::GFP is visualized by the green line on the magenta apical junctions (see Movie S6). (C) Categorical scatterplot of EFF-1::GFP vesicle colocalization to the different junctions per 10 μm of junction length per hour, measured in wild-type embryos. Each point represents number of events (colocalization) per junction over time. n = 8 embryos with a total of 6–10 junctions analyzed over ∼50 min. Middle line reflects the mean; upper and lower lines show SD. Thus, EFF-1-containing vesicles transiently associate (e.g., fuse) to all types of junctions. There is no statistical difference between data sets (Student’s t test). (D) EFF-1::GFP relative intensity measured on the cell junctions in embryos treated with rab-5(RNAi). Five junctions of each type over ten time points were analyzed (n = 50); movies of three independent embryos were used (normalized to EFF-1::GFP gray values × 10³/10 μm). Data are presented as mean (middle line) ± SD (upper and lower lines). There is no difference between intensities measured on −/+ and on +/s junction. ^∗p < 0.05 (Student’s t test).

Figure 7

Model of Cell Fusion in C. elegans Embryonic Hypodermis EFF-1 (green) is expressed in all surrounding plasma membranes (blue line) without relation to the future fusion-fated membranes. Cell that does not express EFF-1 (−) is colored in light khaki (left cell); fusion-fated cells that express EFF-1 (+) are colored in gradient of purple to khaki (central and right cells). The excess EFF-1 is actively removed from the cell surface via endocytosis in a dynamin- (yellow line) and RAB-5- (red dashed line) dependent mechanism. EFF-1 accumulates in the cytoplasm within early endosomes (circles outlined with the red dashed line). Only when EFF-1 is simultaneously present in both neighboring membranes, cell fusion is activated.