The Hippo pathway is a well conserved signaling cascade that modulates cell proliferation and survival in response to external cues such as cell:cell contact, injury, and nutritional status. Models ...of the Hippo pathway have evolved from a series of genetic interactions defined in the fruit fly
into a complex series of biochemical mechanisms in which transmembrane and cytoskeletal proteins modulate cytoplasmic phosphatase and kinase activities that converge on the serine/threonine kinase Warts (Wts) to regulate nuclear entry of the co-activator protein Yorkie (Yki; vertebrate Yap1). This pathway is well conserved in human cells and broadly implicated in cancer. Progress in understanding biochemical events within the Hippo pathway highlights a need for improved understanding of the cell biological contexts in which these molecular interactions occur. A significant body of data linking Hippo signaling to membranes and proteins involved in intracellular membrane trafficking raise the possibility that some molecular regulatory events occur on the cytoplasmic face of vesicles. In
, a Yki-vesicle link was solidified by discoveries that cytoplasmic Yki concentrates at late-endosomes and physically interacts with two endosomal adaptor proteins, Myopic (Mop) and Leash. These two proteins are required for Yki to transit the endolysosomal pathway and be turned over in lysosomes. Molecules involved in recruiting and tethering Yki along this endosomal route are not defined but are predicted to play key roles in regulating Yki levels and thus Hippo-responsiveness of cells. As Wts is recruited to the apical membrane by upstream Hippo components, endosomal internalization could also affect complexes involved in Yki phosphorylation events that alter nucleocytoplasmic shuttling. Recent work has revealed an unexpected, non-transcriptional role of membrane-associated Yki in triggering actinomyosin contractility via the myosin-regulatory light chain Spaghetti squash (Sqh). How Yki interacts with the membrane and controls Sqh is unclear, but this mechanism represents a novel regulatory mechanism based on induced localization of Yki to a specific membrane compartment. These and other data will be discussed as we review data linking Yki to membrane and vesicular traffic in development and homeostasis and speculate on missing elements of these membrane-linked Yki regulatory mechanisms.
The Drosophila polyadenosine RNA binding protein Nab2, which is orthologous to a human protein lost in a form of inherited intellectual disability, controls adult locomotion, axon projection, ...dendritic arborization, and memory through a largely undefined set of target RNAs. Here, we show a specific role for Nab2 in regulating splicing of ~150 exons/introns in the head transcriptome and focus on retention of a male-specific exon in the sex determination factor Sex-lethal (Sxl) that is enriched in female neurons. Previous studies have revealed that this splicing event is regulated in females by N6-methyladenosine (m6A) modification by the Mettl3 complex. At a molecular level, Nab2 associates with Sxl pre-mRNA in neurons and limits Sxl m6A methylation at specific sites. In parallel, reducing expression of the Mettl3, Mettl3 complex components, or the m6A reader Ythdc1 rescues mutant phenotypes in Nab2 flies. Overall, these data identify Nab2 as an inhibitor of m6A methylation and imply significant overlap between Nab2 and Mettl3 regulated RNAs in neuronal tissue.
In mammalian cells, activation of certain checkpoint pathways as a result of exposure to genotoxic agents results in cell cycle arrest. The integrity of these arrest pathways is critical to the ...ability of the cell to repair mutations that otherwise might compromise viability or contribute to deregulation of cellular growth and proliferation. Here we examine the mechanism through which DNA damaging agents result in a G 1 arrest that depends on the tumor suppressor p53 and its transcriptional target p21. By using primary cell lines lacking specific cell cycle regulators, we demonstrate that this pathway functions through the growth suppressive properties of the retinoblastoma protein (pRB) tumor suppressor. Specifically, γ-irradiation inhibits the phosphorylation of pRB at cyclin-dependent kinase 2-specific, but not cyclin-dependent kinase 4-specific, sites in a p21-dependent manner. Most importantly, we show that pRB is a critical component of this DNA damage checkpoint. These data indicate that the p53 → p21 checkpoint pathway uses the normal cell cycle regulatory machinery to induce the accumulation of the growth suppressive form of pRB and suggest that loss of pRB during the course of tumorigenesis disrupts the function of an important DNA damage checkpoint.
The E2F family of proteins is required to establish the correct cell-cycle-dependent transcription of genes that direct the process of cell division. All previously identified E2F proteins can act in ...a similar manner; depending on whether or not they are associated with the cell cycle inhibitors the retinoblastoma protein (pRB), p107, or p130, they can either repress or activate the transcription of E2F-responsive genes. We now report the cloning and characterization of another E2F family member, E2F-6, whose structure is reminiscent of the dominant inhibitors of other transcription factor families. The dimerization and DNA binding properties of E2F-6 are similar to those of the other E2F family members. However, it is not regulated by pRB, p107, or p130, and it is unable to activate transcription. Instead, it can act to repress the transcription of E2F responsive genes by countering the activity of the other E2F complexes via a pRB-, p107-, or p130-independent mechanism.
The Drosophila Taiman (Tai) protein is homologous to the human steroid-receptor coactivators SRC1–3 and activates transcription in complex with the 20-hydroxyecdysone (20E) receptor (EcR). Tai has ...roles in intestinal homeostasis, germline maintenance, cell motility, and proliferation through interactions with EcR and the coactivator Yorkie (Yki). Tai also promotes invasion of tumor cells in adjacent organs, but this pro-invasive mechanism is undefined. Here, we show that Tai expression transforms sessile pupal wing cells into an invasive mass that penetrates the adjacent thorax during a period of high 20E. Candidate analysis confirms a reliance on elements of the 20E and Hippo pathways, such as Yki and the Yki-Tai target dilp8. Screening the Tai-induced wing transcriptome detects enrichment for innate immune factors, including the Spätzle (Spz) family of secreted Toll ligands that induce apoptosis during cell competition. Tai-expressing wing cells induce immune signaling and apoptosis among adjacent thoracic cells, and genetic reduction of spz, Toll, or the rpr/hid/grim pro-apoptotic factors each suppresses invasion, suggesting an intercellular Spz-Toll circuit supports killing-mediated invasion. Modeling these interactions in larval epithelia confirms that Tai kills neighboring cells via a mechanism involving Toll, Spz factors, and the Spz inhibitor Necrotic. Tai-expressing cells evade death signals by repressing the immune deficiency (IMD) pathway, which operates in parallel to Toll to control nuclear factor κB (NF-κB) activity and independently regulates JNK activity. In sum, these findings suggest that Tai promotes competitive cell killing via Spz-Toll and that this killing mechanism supports pathologic intertissue invasion in Drosophila.
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•Tai expression causes pupal wing cells to invade into the adjacent thorax•Spz-Toll-dependent killing of thorax cells is required for Tai-driven wing invasion•Tai-driven killing of neighbors via Spz-Toll also occurs within the larval wing disc•The competitive success of Tai-expressing wing cells is dependent on IMD repression
Byun et al. show that expressing the Drosophila coactivator Tai in wing cells kills neighbor cells through the Spz-Toll pathway and that Tai uses this mechanism to drive invasion of wing cells into the thorax. Tai “winner” cells can be killed by reducing the IMD inhibitor caspar, indicating their competitive success requires IMD repression.
Genetics, and more recently genomics, reveal striking conservation in the fundamental signaling pathways that underlie normal and aberrant cell processes. Consequently, various genetic model ...organisms are now attracting the interest of biomedical scientists who are focused on therapeutic approaches to human disease. There are now several examples of studies in which Drosophila seems likely to facilitate advances in potential therapies, and a recent report has demonstrated the utility of the fly model for understanding and treating human disease. Basic developmental genetic information first obtained in Drosophila was used to design a therapeutic block to oncogenic Notch signaling that was associated with leukemia in mice. The story of Notch signaling in Drosophila demonstrates the potential for standard Drosophila molecular genetics in developing therapeutic strategies that are relevant to human disease.
The
polyadenosine RNA binding protein Nab2, which is orthologous to a human protein lost in a form of inherited intellectual disability, controls adult locomotion, axon projection, dendritic ...arborization, and memory through a largely undefined set of target RNAs. Here, we show a specific role for Nab2 in regulating splicing of ~150 exons/introns in the head transcriptome and focus on retention of a male-specific exon in the sex determination factor
(
) that is enriched in female neurons. Previous studies have revealed that this splicing event is regulated in females by N6-methyladenosine (m
A) modification by the Mettl3 complex. At a molecular level, Nab2 associates with
pre-mRNA in neurons and limits
m
A methylation at specific sites. In parallel, reducing expression of the Mettl3, Mettl3 complex components, or the m
A reader Ythdc1 rescues mutant phenotypes in
flies. Overall, these data identify Nab2 as an inhibitor of m
A methylation and imply significant overlap between Nab2 and Mettl3 regulated RNAs in neuronal tissue.
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