Filopodia-like actin cables position nuclei in association with perinuclear actin in Drosophila nurse cells

Abstract

Controlling the position of the nucleus is vital for a number of cellular processes from yeast to humans. In Drosophila nurse cells, nuclear positioning is crucial during dumping, when nurse cells contract and expel their contents into the oocyte. We provide evidence that in nurse cells, continuous filopodia-like actin cables, growing from the plasma membrane and extending to the nucleus, achieve nuclear positioning. These actin cables move nuclei away from ring canals. When nurse cells contract, actin cables associate laterally with the nuclei, in some cases inducing nuclear turning so that actin cables become partially wound around the nuclei. Our data suggest that a perinuclear actin meshwork connects actin cables to nuclei via actin-crosslinking proteins such as the filamin Cheerio. We provide a revised model for how actin structures position nuclei in nurse cells, employing evolutionary conserved machinery.

Graphical abstract

Figure 1

Actin Cables of Nurse Cells Are Filopodia-like Structures (A) Illustration showing representative areas within the Drosophila egg chambers, from which most pictures were taken for this article; stage 10A is before actin cable formation, stage 10B is after formation of actin cable at the start of dumping. (B–D) Cross-sections through nurse cell illustrating the membrane areas with actin cables. (B) A schematic drawing of four nurse cells with their nuclei and actin cables, surrounded by stretched follicle cells. (C) A cross-section through nurse cells of a stage 10B egg chamber (single z section). (D) A cross-section through nurse cells of a stage late10B/11 chamber (Venus-Cheerio in green, actin in red, nuclei in blue, projection of four z sections covering 6 μm). Note the size of the nuclei compared to the length of actin cables. (E) Live confocal microscopy images from Movie S1 showing the parts of two nurse cells expressing GFP-Act5C (stage 10B). E₁ shows the actin cables before bleaching, E₂ the bleached area depicted in red, and E₃ the postbleach image. Note that the smaller bleached area affected the indentation containing the tip complex. E₄ and E₅ display two time frames of Movie S1 from the bleached cables (each a projection of four z sections covering 3 μm). (F) Kymograph of bleached points along one of the actin cables in Movie S1. Horizontal bar represents 10 μm, vertical bar represents 20 min. (G) Live-imaged actin cables marked with Lifeact-GFP. (H) Actin cables marked with Lifeact-GFP and fixed with 1% formaldehyde (projection of two z sections covering 1 μm). (I) Actin cables marked with Lifeact-GFP and fixed with 8% formaldehyde (projection of two z sections covering 1 μm). (J) Enabled localized to the tip complex of each actin cable, which resided in a small membrane indentation extending into the neighboring cell (J, αEnabled; J′, actin; J″, overlay). (K) Enlargement highlighting the tip complexes in membrane indentations (K, αEnabled; K′, actin; K″, overlay). Scale bar represents 5 μm. Unless otherwise stated, scale bars represent 20 μm in all figures, and each picture is a single confocal section. See also Figure S1 and Movie S1.

Figure 2

E-Cadherin Is Required for Actin Cable Orientation (A) E-cadherin complexes (red) were interspersed between tip complexes of actin cables marked with PicoGFP (green) along the plasma membrane between nurse cells. (B) Enlargement showing the E-cadherin in green and the tip complexes in red (αEnabled). (C) In wild-type nurse cells, actin cables extended to the nucleus perpendicular to the cortex. Tip complexes, green (αEnabled); actin, red. (D) In shg¹ germline clones, actin cables formed, had a tip complex, but do not span the cytoplasm but lie on the membrane marked by ring canal and tip complexes (αEnabled, green; actin, red). See also Movies S2 and S3. (E and F) Egg chambers mutant for shg are rescued by ubiquitous expression of E-cadherin-GFP (genotype: shg^R69/shg¹; ubi::E-cadherin-GFP). Actin cables are oriented toward the nucleus as in wild-type egg chambers (E and F). Msp-300 (E) and Cheerio (F) localize to the perinuclear ends of actin cables. (G) Projection of four z sections spanning 3 μm of nurse cells shown in (D) with actin cables lying on the membrane marked by tip complexes (αEnabled, green; actin, red). See also Movies S2, S3, and Figure S2.

Figure 3

Actin Cables Associate Laterally around Turning Nuclei (A) Six time frames from Movie S4A showing the formation of actin cables and their behavior during dumping. Following the nurse cell at the bottom right (marked with an asterisk, t = 0′), actin cables extended toward the nucleus (t = 10′), pushed against the nucleus (t = 20′), and forced the nucleus toward the side of the nurse cell (t = 30′). Subsequently, the nucleus turned with associated actin cables, which grew over it (t = 40′ and t = 50′). Each frame is a projection of five z sections covering 4 μm; due to the projection, actin cables appear sometimes segmented. Note that the entire length of some actin cables are not in the projected focal planes and so falsely appear unconnected to the plasma membrane. (B) Actin cables pushing nuclei to the outside in a stage 11 egg chamber; note the deformation of the asterisked nucleus (projection of three z sections covering 2 μm). (C) During nurse cell contraction actin cables continue growing, bundle at the nucleus, and start to associate laterally with it (projection of three z sections covering 4 μm). (D) After contraction at stage 14, actin cables localize around the nurse cell nuclei (projection of six z sections covering 5 μm). See also Movies S4A–S4C.

Figure 4

Actin-Binding Proteins at the Perinuclear Ends of Actin Cables (A) The spectraplakin Shot and the adducin-like HtsRC localized in gradients along actin cables with their highest expression at the perinuclear end of actin cables (A, αShot; A′, αHtsRC; A″, overlay with actin staining). (B) The perinuclear end of actin cables is marked by Cheerio and Msp-300 (B, αCheerio; B′, αMsp-300; B″, overlay with actin staining). See also Figure S3.

Figure 5

LINC Complexes Are Not Essential to Link Actin Cables to the Nucleus in Nurse Cells (A and B) Deletion of the KASH domain eliminates localization of Msp-300 from actin cables but does not affect the actin cables or their contact to the nucleus (A, αMsp-300; A′, overlay of αHtsRC with actin; B, overlay of αCheerio with actin). (C) Knockdown of Msp-300 eliminates localization of Msp-300 from actin cables but does not affect actin cables nor nuclear positioning (C, αMsp-300; C′, overlay of αCheerio with actin). (D) Eliminating the SUN protein Koi in koi^HRko80 mutants does not affect the localization of Cheerio to the distal ends of actin cables nor nuclear positioning (overlay of αCheerio in green with actin in red). (E) Box and whisker graph of egg length quantification; boxes are extending from the 25th to the 75th percentile, the line in the middle shows the median, whiskers are extending from the 5th to the 95th percentile, points below or above the whiskers are shown as individual dots (shg¹ n = 39, shg¹/shg^R69;ubi::E-cadherin-GFP n = 105, w¹¹¹⁸ n = 142, msp-300ΔKASH/msp-300Δ3′ n = 193; ^∗∗∗p < 0.001, Student’s t test, n.s. = not significant). See also Figure S4.

Figure 6

Two Mechanisms of Protein Concentration at the Perinuclear Ends of Actin Cables (A) In shg mutants, HtsRC and Shot still bound to actin cables with no contact to the nucleus. Note, the actin-microtubule crosslinker Shot also localized to microtubules (arrowhead in A₄ and A₅). (B–D) In contrast, Msp-300 (B) and Cheerio (C and D) did not bind actin cables lying on the membrane of shg mutants. They only localized to actin cables that appeared to be in contact with the nucleus (arrows in B and D). Images of egg chambers derived from germline clones of the E-cadherin mutants shg¹ (A, C, D) or shg^R69 (B).

Figure 7

The Perinuclear Actin Meshwork of Nurse Cells (A) Image of Cheerio marked with a viable insertion of YFP into filamin repeat 12 (CPTI-001399) at stage 9 of oogenesis. Cheerio marked a perinuclear filamentous structure (arrows) before the formation of actin cables. (B) Image of the YFP insertion CPTI-001403 disrupting the actin binding domain of Cheerio trans-heterozygous over the deficiency Df(3R)Exel6176. The insertion abolished the localization of Cheerio to the perinuclear ends of actin cables (B, Cheerio; B′, overlay of Cheerio with actin and Msp-300). The mutant protein still localized to ring canals (yellow arrowheads in B) but ring canal development is severely disrupted. (C) Lifeact-GFP marked perinuclear actin at stage 9 of oogenesis, before the formation of actin cables. The arrows highlight GFP-positive lines around nuclei. (D) Lifeact-GFP also marked the perinuclear actin during dumping (highlighted by the arrows), in the presence of actin cables (inverted image). (E) Actin5C-GFP also marked perinuclear actin before the formation of actin cables. The arrows highlight GFP-positive lines around nuclei. (F) Overexpression of the actin-binding domain of Shot stabilized the perinuclear actin network (F, Cheerio; F′, GFP; F″, overlay of Cheerio with actin and GFP). (G) Control egg chamber showed perinuclear accumulations of Cheerio-Venus before the formation of actin cables (stage 10a). (H) At the same developmental stage, 500 μM LatB abolished perinuclear Cheerio-Venus. (I) After the formation of actin cables (stage 10b), LatB reduced Cheerio-Venus at the perinuclear ends of actin cables (Cheerio-Venus in green, actin in red). The yellow arrowheads within nuclei point to the border between nuclei and cytoplasm (G-I). See also Figure S5 and Movies S5A–S5D.