Kibra is a regulator of the Salvador/Warts/Hippo signaling network

Abstract

The Salvador (Sav)/Warts (Wts)/Hippo (Hpo) (SWH) network controls tissue growth by inhibiting cell proliferation and promoting apoptosis. The core of the pathway consists of a MST and LATS family kinase cascade that ultimately phosphorylates and inactivates the YAP/Yorkie (Yki) transcription coactivator. The FERM domain proteins Merlin (Mer) and Expanded (Ex) represent one mode of upstream regulation controlling pathway activity. Here, we identify Kibra as a member of the SWH network. Kibra, which colocalizes and associates with Mer and Ex, also promotes the Mer/Ex association. Furthermore, the Kibra/Mer association is conserved in human cells. Finally, Kibra complexes with Wts and kibra depletion in tissue culture cells induces a marked reduction in Yki phosphorylation without affecting the Yki/Wts interaction. We suggest that Kibra is part of an apical scaffold that promotes SWH pathway activity.

Figure 1

Kibra Loss of Function Induces a Phenotype Similar to SWH Network Loss of Function (A–C) Control wings (A) and wings where the kibra (B) or wts (C) genes were silenced by RNAi in the posterior compartment (red dotted line). (D–I) Control fly eyes (D and G) and eyes expressing the same RNAi lines against kibra (E and H) or wts (F and I). In adult eye sections (G–I), interommatidial cells (IOCs) are in red and photoreceptors in blue. (J) Schematic of the kibra locus showing the localization of the kibra^Δ32 allele, generated by excision of the EP747 P element (triangle). The coding sequence is in red; 5′- and 3′-UTRs are in blue. In black is the peptide recognized by the Kib18 antibody. (K–M) 40 hr APF retinas containing kibra mutant clones. (K and L) Cell outlines are visualized by anti-Arm staining (scale bar = 10 μm). (M) Quantification of the IOCs. The p value from a Mann-Whitney test is shown. (N–P) Apoptotic index in 28 hr APF retinas containing kibra mutant clones (absence of GFP). A retina stained for activated Caspase-3 (Cas3) merged with GFP is shown in (N). The Cas3 staining for the whole retina is shown in (O). (P) Quantification of the apoptotic index. The p value from a Mann-Whitney test is shown. (Q–S) Wing disc (Q) and eye disc (R) containing kibra clones (lack of GFP). Scale bars = 20 μm. (S) Quantification of the proliferation advantage of WT or kibra mutant cells by calculating the ratio between total clonal area (no GFP) and total twin spot area (two copies of GFP) for discs containing either WT or kibra clones. The p value from a Mann-Whitney test is shown. (T–V) EdU labeling (shown in gray or red) of WT discs (T) and of discs containing kibra clones (U and V). Posterior is to the right, and the SMW is indicated by a red arrowhead. See also Figure S1. Error bars represent standard deviations.

Figure 2

Kibra Regulates SWH Pathway Targets in Ovarian Posterior Follicle Cells Egg chambers containing heat-shock-induced kibra mutant clones (absence of GFP). Yellow arrows point to kibra mutant cells in the PFC region while white arrowheads point to kibra cells in lateral follicle cells. Scale bars = 20 μm. (A–C′) Stage 10B egg chambers stained for β-galactosidase (gray or red), which monitors the activity of the ex-lacZ reporter. (B–B″) Close-up of the PFC region in (A)–(A″). (D–K″) Egg chambers stained for the N target Hindsight (Hnt) (D–G″) or for Cut (H–K″). Nuclei are shown in blue (Hoechst). (E)–(E″), (G)–(G″), (I)–(I″), and (K)–(K″) are close-ups of the PFC region of stage 10B egg chambers. (D–D″) The arrow points to a stage 8 clone; the arrowhead points to a stage 9 clone. (F–F″ and J–J″) Arrows and arrowheads point to clones in stage 9 egg chambers. See also Figure S2.

Figure 3

kibra Is Epistatic to hpo and Is a Transcriptional Target of the SWH Network (A–A″) Third instar eye imaginal disc expressing kibra under the GMR promoter and containing hpo mutant cells (absence of GFP). Apoptosis is visualized by an anti-activated Caspase-3 staining. Posterior is to the bottom. (B and C) Epistatic relationships between Kibra, Mer and Ex. Examples of the 4 phenotypic classes used to score are shown in (B). The percentages of each class for each genotype are summarized in (C). (D–G′) Third instar imaginal discs containing clones (marked by absence of GFP) of cells mutant for mer;ex (D, D′, F, and F′) or hpo (E, E′, G, and G′). (D)–(E′) are XY sections while (F)–(G′) are XZ transverse sections (apical is to the top). (H–I″) Stage 10A egg chamber containing a hpo clone (marked by absence of GFP) in the PFCs and stained for Kibra (red). (I)–(I″) show a higher magnification of the PFC region in (H). Nuclei are stained with Hoechst (blue). Scale bars = 20 μm (A–A″, D–E′, and H), 10 μm (F–G′ and I–I″). (J) Lysates from S2R+ cells treated with lacZ RNAi or hpo RNAi and probed for Kibra and Tubulin. (K) A graph presenting a comparison of kibra and ex mRNA levels between yki overexpressing and WT wing discs, as measured by qRT-PCR, is shown. p values from Mann-Whitney tests are shown. Error bars represent standard deviations. (L–O″) XY and transverse sections of third instar imaginal wing discs containing hpo mutant clones (labeled by absence of βgal) and stained for Kibra (L–L″ and N–N″), Ex (M–M″ and O–O″), and Mer (L–O″). See also Figure S3.

Figure 4

Kibra Associates with Mer and Ex, and kibra Depletion Strongly Reduces Yki Phosphorylation without Interfering with the Yki/Wts Interaction (A and B) Western blots of coimmunoprecipitation assays between Myc-Kibra and HA-tagged members of the SWH network are shown. (C) Western blots of co-IP assays between Mer-HA and Ex-Flag in presence or absence of Kibra are shown. (D) Lysates of S2R+ cells treated with RNAi against different members of the SWH network and probed for P-Ser168 Yki (P-Yki), pan-Yki, Kibra, and Tubulin are shown (the anti-Kibra blot was performed on a parallel run of the same samples). The bottom panel shows efficiency of the ex dsRNAi treatment. (E) Western blots of co-IP assays between Wts-Flag and different versions of Myc-tagged Kibra are shown. (F) Endogenous co-IP assays between Yki and Wts in control S2 cells and in cells treated with ex and/or kibra RNAi are shown. The anti-Wts input blot was performed on a parallel run of the same samples. See also Figure S4.