Shaping cells and organs in Drosophila by opposing roles of fat body-secreted Collagen IV and perlecan

Abstract

Basement membranes (BMs) are resilient polymer structures that surround organs in all animals. Tissues, however, undergo extensive morphological changes during development. It is not known whether the assembly of BM components plays an active morphogenetic role. To study in vivo the biogenesis and assembly of Collagen IV, the main constituent of BMs, we used a GFP-based RNAi method (iGFPi) designed to knock down any GFP-trapped protein in Drosophila. We found with this method that Collagen IV is synthesized by the fat body, secreted to the hemolymph (insect blood), and continuously incorporated into the BMs of the larva. We also show that incorporation of Collagen IV determines organ shape, first by mechanically constricting cells and second through recruitment of Perlecan, which counters constriction by Collagen IV. Our results uncover incorporation of Collagen IV and Perlecan into BMs as a major determinant of organ shape and animal form.

Figure 1. iGFPi Is an Effective Method to Knock Down the Expression of GFP-Trapped Proteins

(A) Strategy of in vivo GFP interference (iGFPi). Knockdown of GFP-trapped proteins is achieved through RNAi targeting the GFP-encoding sequence. (B) Expression of nub-Gal4 in the blade region of a third instar (L3) wing disc, revealed by Gal4-dependent expression of RFP (UAS-myrRFP, red). Cell nuclei stained with DAPI (blue) on left. (C) Wild-type adult wing. (D and E) iGFPi driven by nub-Gal4 (nub>iGFPi) reduces expression of GFP-trapped Strawberry notch (Sno) (E, compared to control in D; GFP in green) and causes delta-shaped termination of veins at the margin (E’). (F and G) nub>iGFPi reduces expression of GFP-trapped Scalloped (Sd) and causes loss of distal wing tissue (G’). (H and I) nub>iGFPi reduces expression of GFP-trapped Neuroglian (Nrg) and causes reduction in wing size (I’). (J and K) nub>iGFPi reduces expression of GFP-trapped Discs large (Dlg) and prevents correct differentiation (K’). (L and M) iGFPi driven by He-Gal4 in the salivary gland reduces expression of GFP-trapped Bunched (Bun) and causes reduction in cell and organ size. (N and O) iGFPi driven by ey-Gal4 in the eye of flies expressing GFP-trapped Discs large (Dlg) prevents correct differentiation. See also Figure S1.

Figure 2. iGFPi Reveals a Postembryonic Requirement of Vkg

(A–D) Vkg-GFP expression (green) in a vkg^G454 heterozygous (vkg^G454/+) L3 larva (A), a wild-type larva (B), an actin-Gal4 vkg^G454/+ larva (C) and an actin-Gal4>iGFPi vkg^G454/+ larva (D). Lower right and left subpanels show, respectively, bright-light and red fluorescence pictures of the same larvae (UAS-myrRFP, driven by actin-Gal4). (E and F) Vkg-GFP expression (green) in 24-hr-old vkg^G454/vkg^G454 embryos in the presence of actin-Gal4 (E) and actin-Gal4>iGFPi (F). Red fluorescence pictures in the bottom right corner subpanels (UAS-myrRFP, red, driven by actin-Gal4). (G) vkg^G454/vkg^G454 pupae of the genotypes actin>iGFPi (upper specimen), UAS-GFP.dsRNA (middle specimen) andactin-Gal4 (lower), imaged 24 hr after puparium formation. actin>iGFPi animals arrest development in prepupa stage and die (notice air bubble in abdomen indicated by asterisk). Lower subpanels show green fluorescence (Vkg-GFP) and red fluorescence (UAS-myrRFP, driven by actin-Gal4) in the same pupae.

Figure 3. Vkg Is Synthesized by the Fat Body and Continuously Incorporated into BMs during Larval Development

(A) Confocal image of a wing imaginal disc of a vkg^G454/+ L3 larva. Vkg-GFP in green (left) and white (right). Cell nuclei stained with DAPI (blue on left). (B and C) Vkg-GFP expression in vkg^G454/+ larvae (B) and control wild-type larvae (C). Images show L1 larvae (B and C), L3 larvae (anterior half, B’ and C’) and wing discs (B’’ and C’’; posterior ventral hinge, corresponding to orange and blue rectangles in A). (D–G) Expression of Cg-Gal4 (D), ppl-Gal4 (E), c754-Gal4 (F), and twi-Gal4 (G) revealed with UAS-myrRFP in L1 (D, E, F, and G) and L3 larvae (D’, E’, F’, and G’). Upper subpanels show bright-light images and lower subpanels red fluorescence. (H–K) Vkg-GFP expression in L1 (H–K), L3 (H’–K’), and L3 wing discs (H’’–K’’) from vkg^G454/+ larvae where iGFPi is driven by Cg-Gal4 (H), ppl-Gal4 (I), c754-Gal4 (J), and twi-Gal4 (K). See also Figure S2.

Figure 4. Multiple Requirements for Correct Collagen IV Incorporation into BMs

(A) Anterior half of a vkg^G454/+ L3 larva. Green fluorescence in lower subpanel. (B) Confocal image of fat body cells of a vkg^G454/+ L3 larva. Nuclei in blue (DAPI), Vkg-GFP in green and membranes in red (UAS-myrRFP). B’ shows a surface section. (C) Confocal image of the wing disc (posterior ventral hinge) of a vkg^G454/+ L3 larva. Nuclei in blue (DAPI), Vkg-GFP in green. (D–H) Images of vkg^G454/+ L3 larvae where expression in the fat body of the cargo adaptor Tango1 has been knocked down, showing the anterior half of the larva (D, compare to A), fat body cells (E–G, compare to B) and wing disc (H, compare to C). (I and J) Confocal images of the fat body of vkg^G454/+ L2 larvae where expression of the CopII components Sar1 (I) and Sec23 (J) has been knocked down. (K–M) Images of vkg^G454/+ larvae where SPARC expression in the fat body has been knocked down, showing the anterior half of the larva (K), fat body cells (L, surface in L’), and wing disc (M). (N–P) Images of vkg^G454/+ L3 larvae where PH4-αEFB expression in the fat body has been knocked down, showing the anterior half of the larva (N), bled hemolymph (O, bright-light image on left and green fluorescence on right) and wing disc (P). (Q and R) Anti-GFP Western blots of hemolymph from control larvae and larvae where PH4αEFB expression in the fat body has been knocked down, both vkg^G454/+ (as in O). SDS-PAGE gels were run under reducing conditions (Q) which separate Collagen IV monomers (α) or nonreducing conditions (R), which preserve the Collagen IV trimers (α₃). Predicted molecular weight of a Vkg-GFP monomer is 221 kDa (Vkg 194 kDa + GFP 27 kDa). (S) Clones of mys^M2 (integrin βPS) mutant cells in vkg^G454/+ wing discs. Clones, outlined in yellow, are labeled by myrRFP expression (red on left and white in middle). Arrows point to scars in the Vkg-GFP layer (green on left and white on right). See also Figure S3.

Figure 5. Collagen IV Incorporation into BMs Maintains the Shape of Larval Organs

(A and B) Confocal images of vkg^G454/+ wing discs from control larvae (A) and larvae where expression of vkg in the fat body has been knocked down during larval stages (Cg-Gal4+Gal80^ts>vkgⁱ +96 hr at 29°C). A’ and B’ show transversal sections of the same discs, as indicated by the blue lines. Vkg-GFP in green in all subpanels. Nuclei in blue (DAPI) in left (A, B) and upper (A’, B’) subpanels. (C and D) Confocal images of the central nervous system in vkg^G454/+ larvae of control (C) and Cg-Gal4+Gal80^ts>vkgi genotype (D, +96 hr). Nuclei in blue (DAPI) in left subpanels. Vkg-GFP in green in all subpanels. (E and F) Confocal images of the salivary glands in vkg^G454/+ larvae of control (C) and Cg-Gal4+Gal80^ts>vkgi genotype (D, +96 hr). Nuclei in blue (DAPI) and actin in white (phalloidin) in left subpanels. Vkg-GFP in green in all subpanels. Arrows indicate the width of the lumen at the duct (orange) and imaginal ring (purple). (G) Head-to-head genomic arrangement of the two Drosophila Collagen IV genes. (H-M) Images of vkg^G454/+ control larvae (H-J) and vkg^G454/+ larvae where expression of Cg25C has been knocked down (K-M; CgGal4+Gal80^ts>Cg25C +96 hr). Images show the anterior half of the larva (H and K), a confocal section of the wing disc (I and L) and a drop of hemolymph (J and M). Vkg-GFP in green. (N-Q) Transversal confocal sections across wing discs from a wild-type larva (N) and larvae where expression in the fat body of vkg (O), Cg25C (P), or both (Q) has been knocked down (Cg-Gal4+Gal80^ts +96 hr). Nuclei (DAPI) in blue and F-actin (phalloidin) in white. (R-T) Transversal confocal sections across a control wing disc (R) and wing discs treated with collagenase for 1 min (S and T). Discs were from wild-type (R and S) or Cg-Gal4+Gal80^ts>vkgⁱ +96 hr (T) larvae. F-actin (phalloidin) in magenta and Vkg-GFP in green. (U) Schematic drawing of a section through the wing disc, showing the columnar epithelial organization of the wing blade. (V) Cell shape changes caused in the wing blade by Collagen IV knockdown and collagenase treatment. Graphs represent height of the epithelium (measured in transversal sections), apical area (measured in planar sections), and apical-to-basal length ratios (length in section of a region measured apically divided by the basal length of the same region, both in transversal sections). Measurements from at least five specimens per genotype and treatment were averaged. Differences in cell height and apical area between wild-type and the other conditions are all significant (p < 0.05 in t tests). Error bars represent standard deviations. (W) Schematic summary of cell shape changes caused by absence of Collagen IV and collagenase treatment. See also Figure S4.

Figure 6. Perlecan Incorporation into BMs Requires Collagen IV

Confocal images ofwing discs from wild-type L3larvae (A–C) and larvae where expression of vkg and Cg25C was knocked down in the fat body (D–F; Cg-Gal4+Gal80^ts>vkgⁱ+Cg25Cⁱ+96 hr). Discs were stained with antibodies against Laminin B1 (A and D, green in upper subpanels and white in lower ones), Nidogen (B and E, green in upper subpanels, white in lower ones) and Perlecan (C and F, red in upper subpanels, white in lower ones). Cell nuclei stained with DAPI (blue) in upper subpanels. See also Figure S5.

Figure 7. Perlecan Counters Collagen IV BM Constriction

(A–C) Confocal imagesof wing discs stained with anti-Perlecan (anti-Trol; redinupper subpanels, white inlower ones), dissected from a wild-type larva (A),alarva where expression of Perlecan was knocked down (B; act>trolⁱ) and a larva overexpressing Perlecan (C; act>GSV-trol). (D–I) Confocal images of wing discs (D–F) and ventral nerve cords (G–I) dissected from wild-type (D and G), act>trolⁱ (E and H) and act>GSV-trol larvae (F and I). F-actin in magenta (phalloidin) and Vkg-GFP in green. Transversal sections of wing discs in (D’)–(F’) as indicated by blue lines. Arrows in (H) point to herniations of the VNC (inset shows cell nuclei stained with DAPI). (J–L) Wing discs dissected from wild-type (J) and act>trolⁱ larvae (K and L) imaged in distilled water after 1 min of immersion. Yellow arrows point to breaks in the BM through which the underlying tissue expands. Cells express RFP (red in upper subpanels, driven by act-Gal4). Vkg-GFP in green. (M) Confocal image of an act>trolⁱ wing disc after 1 min immersion in water. Vkg-GFP in green and cell nuclei (DAPI) in blue. (N and O) Electron micrographs showing sections through the BM in wild-type (P) and act>trolⁱ (Q) wing discs. Black arrows indicate BM thickness. Blue arrows point to space between the BM and the cell membrane, reduced in the absence of Perlecan. (P) BM thickness measured from electron micrographs of wild-type and act>trolⁱ wing discs. Thickness was measured infive discs from different larvae (two wild-type, three act>trolⁱ). Measurements from five micrographs were averaged per disc. Error bars represent standard deviations. (Q) Transversal confocal section of an act>trolⁱ wing disc after 1 min collagenase treatment. Vkg-GFP in green. F-actin in magenta (phalloidin, upper subpanel). See also Figure S6.