Kibra Functions as a Tumor Suppressor Protein that Regulates Hippo Signaling in Conjunction with Merlin and Expanded

SummaryThe Hippo signaling pathway regulates organ size and tissue homeostasis from Drosophila to mammals. Central to this pathway is a kinase cascade wherein Hippo (Hpo), in complex with Salvador (Sav), phosphorylates and activates Warts (Wts), which in turn phosphorylates and inactivates the Yorkie (Yki) oncoprotein, known as the YAP coactivator in mammalian cells. The FERM domain proteins Merlin (Mer) and Expanded (Ex) are upstream components that regulate Hpo activity through unknown mechanisms. Here we identify Kibra as another upstream component of the Hippo signaling pathway. We show that Kibra functions together with Mer and Ex in a protein complex localized to the apical domain of epithelial cells, and that this protein complex regulates the Hippo kinase cascade via direct binding to Hpo and Sav. These results shed light on the mechanism of Ex and Mer function and implicate Kibra as a potential tumor suppressor with relevance to neurofibromatosis

FADS3 is a Δ4Z sphingoid base desaturase that contributes to gender differences to the human plasma sphingolipidome

Sphingolipids (SL) are structurally diverse lipids that are defined by the presence of a long chain base (LCB) backbone. Typically, LCBs contain a single Δ4E DB (mostly d18:1), whereas the dienic LCB sphingadienine (d18:2) contains a second DB at Δ14Z position. The enzyme introducing the Δ14Z DB is unknown. We analyzed the LCB plasma profile in a gender-, age-, and BMI-matched subgroup of the CoLaus cohort (n = 658). Sphingadienine levels showed a significant association with gender, being in average ~30% higher in females. A genome-wide association study (GWAS) revealed variants in the fatty-acid desaturase 3 (FADS3) gene to be significantly associated with the plasma d18:2/d18:1 ratio (p = -log 7.9). Metabolic labeling assays, FADS3 overexpression and knockdown approaches, as well as plasma LCB profiling in FADS3-deficient mice confirmed that FADS3 is a bona-fide LCB desaturase and required for the introduction of the Δ14Z double bound. Moreover, we showed that FADS3 is required for the conversion of the atypical cytotoxic 1-deoxysphinganine (1-deoxySA, m18:0) to 1-deoxysphingosine (1-deoxySO, m18:1). HEK293 cells overexpressing FADS3, were more resistant to m18:0 toxicity than WT cells. In summary, using a combination of metabolic profiling and GWAS, we identified FADS3 to be essential for forming Δ14Z DB containing LCBs, such as d18:2 and m18:1. Our results unravel FADS3 as a Δ14Z LCB desaturase, thereby disclosing the last missing enzyme of the SL de novo synthesis pathway

Autopalmitoylation of TEAD Proteins Regulates Transcriptional Output of Hippo Pathway

TEA domain (TEAD) transcription factors bind to the co-activator YAP/TAZ, and regulate the transcriptional output of Hippo pathway, playing critical roles in organ size control and tumorigenesis. Protein S-palmitoylation attaches fatty acid (palmitate) to cysteine residues, and regulates protein trafficking, membrane localization and signaling activities. Using activity-based chemical probes, we discovered that human TEADs possess intrinsic palmitoylating enzyme-like activities, and undergo autopalmitoylation at evolutionarily conserved cysteine residues under physiological conditions. We determined the crystal structures of lipid-bound TEADs, and found that the lipid chain of palmitate inserts into a conserved deep hydrophobic pocket. Strikingly, palmitoylation is required for TEAD’s binding to YAP/TAZ, but dispensable for the binding to Vgll4 tumor suppressor. In addition, palmitoylation does not alter TEAD’s localization. Moreover, TEAD palmitoylation-deficient mutants impaired TAZ-mediated muscle differentiation in vitro, and Yorkie-mediated tissue overgrowth in Drosophila in vivo. Our study directly linked autopalmitoylation to the transcriptional regulation of Hippo pathway

Merlin controls the repair capacity of Schwann cells after injury by regulating Hippo/YAP activity

Loss of the Merlin tumor suppressor and activation of the Hippo signaling pathway play major roles in the control of cell proliferation and tumorigenesis. We have identified completely novel roles for Merlin and the Hippo pathway effector Yes-associated protein (YAP) in the control of Schwann cell (SC) plasticity and peripheral nerve repair after injury. Injury to the peripheral nervous system (PNS) causes a dramatic shift in SC molecular phenotype and the generation of repair-competent SCs, which direct functional repair. We find that loss of Merlin in these cells causes a catastrophic failure of axonal regeneration and remyelination in the PNS. This effect is mediated by activation of YAP expression in Merlin-null SCs, and loss of YAP restores axonal regrowth and functional repair. This work identifies new mechanisms that control the regenerative potential of SCs and gives new insight into understanding the correct control of functional nerve repair in the PNS

TSC1 and TSC2 tumor suppressors antagonize insulin signaling in cell growth

Tuberous sclerosis is a human disease caused by mutations in the TSC1 or the TSC2 tumor suppressor gene. Previous studies of a Drosophila TSC2 homolog suggested a role for the TSC genes in maintaining DNA content, with loss of TSC2 leading to polyploidy and increased cell size. We have isolated mutations in the Drosophila homolog of the TSC1 gene. We show that TSC1 and TSC2 form a complex and function in a common pathway to control cellular growth. Unlike previous studies, our work shows that TSC1(–) or TSC2(–) cells are diploid. We find that, strikingly, the heterozygosity of TSC1 or TSC2 is sufficient to rescue the lethality of loss-of-function insulin receptor mutants. Further genetic analyses suggest that the TSC genes act in a parallel pathway that converges on the insulin pathway downstream from Akt. Taken together, our studies identified the TSC tumor suppressors as novel negative regulators of insulin signaling

Tsc2 is not a critical target of Akt during normal Drosophila development

Signaling by insulin and target of rapamycin are both required for cell growth, but their interrelationships remain poorly defined. It was reported that Akt, an essential component of the insulin pathway, stimulates growth by phosphorylating and inhibiting tuberous sclerosis complex 2 (TSC2). Here we evaluate this model genetically in Drosophila by engineering Tsc2 mutants in which the Akt phosphorylation sites are changed to nonphosphorylatable or phospho-mimicking residues. Strikingly, such mutants completely rescue the lethality and cell growth defects of Tsc2-null mutants. Taken together, our data suggest that Tsc2 is not a critical substrate of Akt in normal Drosophila development

Premetazoan Origin of the Hippo Signaling Pathway

Nonaggregative multicellularity requires strict control of cell number. The Hippo signaling pathway coordinates cell proliferation and apoptosis and is a central regulator of organ size in animals. Recent studies have shown the presence of key members of the Hippo pathway in nonbilaterian animals, but failed to identify this pathway outside Metazoa. Through comparative analyses of recently sequenced holozoan genomes, we show that Hippo pathway components, such as the kinases Hippo and Warts, the coactivator Yorkie, and the transcription factor Scalloped, were already present in the unicellular ancestors of animals. Remarkably, functional analysis of Hippo components of the amoeboid holozoan Capsaspora owczarzaki, performed in Drosophila melanogaster, demonstrate that the growth-regulatory activity of the Hippo pathway is conserved in this unicellular lineage. Our findings show that the Hippo pathway evolved well before the origin of Metazoa and highlight the importance of Hippo signaling as a key developmental mechanism predating the origin of Metazoa

Premetazoan Origin of the Hippo Signaling Pathway

Nonaggregative multicellularity requires strict control of cell number. The Hippo signaling pathway coordinates cell proliferation and apoptosis and is a central regulator of organ size in animals. Recent studies have shown the presence of key members of the Hippo pathway in nonbilaterian animals, but failed to identify this pathway outside Metazoa. Through comparative analyses of recently sequenced holozoan genomes, we show that Hippo pathway components, such as the kinases Hippo and Warts, the coactivator Yorkie, and the transcription factor Scalloped, were already present in the unicellular ancestors of animals. Remarkably, functional analysis of Hippo components of the amoeboid holozoan Capsaspora owczarzaki, performed in Drosophila melanogaster, demonstrate that the growth-regulatory activity of the Hippo pathway is conserved in this unicellular lineage. Our findings show that the Hippo pathway evolved well before the origin of Metazoa and highlight the importance of Hippo signaling as a key developmental mechanism predating the origin of Metazoa. © 2012 The Authors.This work was supported by an Institució Catalana per a la Recerca i Estudis Avançats contract, a European Research Council Starting Grant (ERC-2007-StG-206883), a grant (BFU2008-02839/BMC) from Ministerio de Ciencia e Innovación (MICINN) to I. R.-T., and a National Institutes of Health grant (R01 EY015708) to D.P. A.S-P. was supported by a pregraduate Formacion Profesorado Universitario grant from MICINN. D.P. is an investigator of the Howard Hughes Medical Institute.Peer Reviewe

hippo Encodes a Ste-20 Family Protein Kinase that Restricts Cell Proliferation and Promotes Apoptosis in Conjunction with salvador and warts

AbstractThe coordination between cell proliferation and cell death is essential to maintain homeostasis within multicellular organisms. The mechanisms underlying this regulation are yet to be completely understood. Here, we report the identification of hippo (hpo) as a gene that regulates both cell proliferation and cell death in Drosophila. hpo encodes a Ste-20 family protein kinase that binds to and phosphorylates the tumor suppressor protein Salvador (Sav), which is known to interact with the Warts (Wts) protein kinase. Loss of hpo results in elevated transcription of the cell cycle regulator cyclin E and the cell-death inhibitor diap1, leading to increased proliferation and reduced apoptosis. Further, we show that hpo, sav, and wts define a pathway that regulates diap1 at the transcriptional level. A human homolog of hpo completely rescues the overgrowth phenotype of Drosophila hpo mutants, suggesting that hpo might play a conserved role for growth control in mammals