MAP4K3 Is a Component of the TORC1 Signalling Complex that Modulates Cell Growth and Viability in Drosophila melanogaster

Abstract

BackgroundMAP4K3 is a conserved Ser/Thr kinase that has being found in connection with several signalling pathways, including the Imd, EGFR, TORC1 and JNK modules, in different organisms and experimental assays. We have analyzed the consequences of changing the levels of MAP4K3 expression in the development of the Drosophila wing, a convenient model system to characterize gene function during epithelial development.Methodology and Principal FindingsUsing loss-of-function mutants and over-expression conditions we find that MAP4K3 activity affects cell growth and viability in the Drosophila wing. These requirements are related to the modulation of the TORC1 and JNK signalling pathways, and are best detected when the larvae grow in a medium with low protein concentration (TORC1) or are exposed to irradiation (JNK). We also show that MAP4K3 display strong genetic interactions with different components of the InR/Tor signalling pathway, and can interact directly with the GTPases RagA and RagC and with the multi-domain kinase Tor.Conclusions and SignificanceWe suggest that MAP4K3 has two independent functions during wing development, one related to the activation of the JNK pathway in response to stress and other in the assembling or activation of the TORC1 complex, being critical to modulate cellular responses to changes in nutrient availability.

Identification of Happyhour/MAP4K as Alternative Hpo/Mst-like Kinases in the Hippo Kinase Cascade

“…Since exon 4 encodes essential part of the kinase domain, we consider these mutants as null alleles. As reported before (Resnik-Docampo and de Celis, 2011), hppy mutant flies are viable and fertile without noticeable morphological malformations. Despite that, hppy mutant clones in pupal retina consistently showed a modest increase of the Hippo target gene Ex (Figure 5A), consistent with enhanced Yki activity upon loss of hppy .…”

“…Since previous studies have implicated Hppy/MAP4K as activators of the JNK and TOR pathways (Bryk et al, 2010; Findlay et al, 2007; Lam et al, 2010; Resnik-Docampo and de Celis, 2011), we examined whether these effector pathways are required for Hppy to promote Hippo signaling. Inhibition of JNK or TOR pathway did not affect Hppy-induced Wts or Mats phosphorylation (Figure S3A–D), suggesting that Hppy may directly phosphorylate and activate Wts.…”

“…hppy was initially identified as homozygous mutant adult flies that display increased resistance to ethanol-induced sedation (Corl et al, 2009). It was later reported that Hppy overexpression can reduce wing size by promoting apoptosis (Lam et al, 2010; Resnik-Docampo and de Celis, 2011). Consistent with the previous studies, overexpression of Hppy resulted in smaller wing size when driven by the poster compartment-specific en-Gal4 driver (Figure 4A).…”

“…Unlike its requirement in LatB-induced Yki nuclear exclusion, loss of hppy did not increase the size of hpo mutant clones (Figure S6). The interpretation of this finding is complicated by the fact that Hppy is a pleiotropic kinase involved in multiple pathways, some of which are growth-promoting, such as TOR pathway (Bryk et al, 2010; Resnik-Docampo and de Celis, 2011) and some of which are growth-suppressing, such as the Hippo pathway. Thus, the size of double mutant clones does not only reflect defects in Hippo signaling.…”

Multiple Functions of Mammalian Germinal Center Kinases

“…Amino acid sufficiency regulates MAP4K3 activity and mTOR (mammalian target of rapamycin) signaling by, together with PP2A (protein phosphatase 2), controlling the auto-phosphorylation status of amino acid S170 on the MAP4K3 A-loop [10]. Meanwhile, drosophila MAP4K3 (CG7097) regulates cell growth, body size, metabolism and viability through modulation of TOR (target of rapamycin) and JNK pathway [11][12].…”