Stem cell self-renewal is controlled by concerted actions of extrinsic niche signals and intrinsic factors in a variety of systems. Drosophila ovarian germline stem cells (GSCs) have been one of the most productive systems for identifying the factors controlling self-renewal. The differentiation factor BAM is necessary and sufficient for GSC differentiation, but it still remains expressed in GSCs at low levels. However, it is unclear how its function is repressed in GSCs to maintain self-renewal. Here, we report the identification of the translation initiation factor eIF4A for its essential role in self-renewal by directly inactivating BAM function. eIF4A can physically interact with BAM in Drosophila S2 cells and yeast cells. eIF4A exhibits dosage-specific interactions with bam in the regulation of GSC differentiation. It is required intrinsically for controlling GSC self-renewal and proliferation but not survival. In addition, it is required for maintaining E-cadherin expression but not BMP signaling activity. Furthermore, BAM and BGCN together repress translation of Ecadherin through its 3 UTR in S2 cells. Therefore, we propose that BAM functions as a translation repressor by interfering with translation initiation and eIF4A maintains self-renewal by inhibiting BAM function and promoting E-cadherin expression.BGCN ͉ E-cadherin ͉ niche ͉ translation

The balance between stem cell self-renewal and differentiation is controlled by intrinsic factors and niche signals. In the Drosophila melanogaster ovary, some intrinsic factors promote germline stem cell (GSC) self-renewal, whereas others stimulate differentiation. However, it remains poorly understood how the balance between self-renewal and differentiation is controlled. Here we use D. melanogaster ovarian GSCs to demonstrate that the differentiation factor Bam controls the functional switch of the COP9 complex from self-renewal to differentiation via protein competition. The COP9 complex is composed of eight Csn subunits, Csn1-8, and removes Nedd8 modifications from target proteins. Genetic results indicated that the COP9 complex is required intrinsically for GSC self-renewal, whereas other Csn proteins, with the exception of Csn4, were also required for GSC progeny differentiation. Bam-mediated Csn4 sequestration from the COP9 complex via protein competition inactivated the self-renewing function of COP9 and allowed other Csn proteins to promote GSC differentiation. Therefore, this study reveals a protein-competition-based mechanism for controlling the balance between stem cell self-renewal and differentiation. Because numerous self-renewal factors are ubiquitously expressed throughout the stem cell lineage in various systems, protein competition may function as an important mechanism for controlling the self-renewal-to-differentiation switch.

Homeostasis of Smad phosphorylation at its C-terminal SXS motif is essential for transforming growth factor ␤ (TGF␤) signaling. Whereas it is known that TGF␤ signaling can be terminated by phosphatases, which dephosphorylate R-Smads in the nucleus, it is unclear whether there are any cytoplasmic phosphatase(s) that can attenuate R-Smad phosphorylation and nuclear translocation. Here we demonstrate that myotubularinrelated protein 4 (MTMR4), a FYVE domain-containing dualspecificity protein phosphatase (DSP), attenuates TGF␤ signaling by reducing the phosphorylation level of R-Smads in early endosomes. Co-immunoprecipitation experiments showed that endogenous MTMR4 interacts with phosphorylated R-Smads, and that this interaction is correlated with dephosphorylation of R-Smads. Further analysis showed that overexpression of MTMR4 resulted in the sequestration of activated Smad3 in the early endosomes, thus reducing its nuclear translocation. However, both point mutations at the conserved catalytic site of the phosphatase (MTMR4-C407S) and small interference RNA of endogenous Mtmr4 expression led to sustained Smad3 activation. This work therefore suggests that MTMR4 plays an important role in preventing the overactivation of TGF␤ signaling by dephosphorylating activated R-Smads that have been trafficked to early endosomes. The transforming growth factor ␤ (TGF␤)3 signaling pathway involves two transmembrane serine/threonine kinases, namely type II (T␤RII) and type I TGF␤ receptors (T␤RI) (1, 2). T␤RII, a constitutively active kinase, binds TGF␤ to initiate its heterodimerization with T␤RI. T␤RI then becomes phosphorylated and activates a signaling cascade through a family of intracellular signaling mediators known as Smads.In doing so, T␤RI recruits receptor-regulated Smads (RSmads, including BMP signaling transducers Smad1, -5, and -8 and TGF␤ signaling transducers Smad2, -3) and phosphorylates the two conserved C-terminal serines (SXS) of R-Smads. Activated R-Smads form a trimeric complex with a common mediator Smad (Co-Smad, Smad4), and these complexes translocate into the nucleus to regulate the transcription of an array of target genes (3-7). Recent studies have also indicated that Smad Anchor for Receptor Activation (SARA), an early endosomal protein containing a characteristic FYVE domain (Fab1p, YO1B, Vac1p, and EEA1), serves as an anchor protein by directly recruiting Smad2 to the early endosomes (8, 9) and presenting Smad2 to the internalized T␤RI/II complex (10, 11). Phosphorylated Smad2 then dissociates from SARA and leaves the early endosome. Subsequently, it forms a Smad2-Smad4 complex for nuclear translocation.Despite the fact that TGF␤ signaling activation is surprisingly simple, there are many layers of negative regulation that fine-tune or terminate the signal. The inhibitory Smads (I-Smads, including Smad6 and -7) competitively inhibit the interaction of R-Smads with Smad4 or receptors to provide a negative checkpoint for TGF␤-signaling activation (12). Ubiquitin-dependent degradation of activated...

Immune homeostasis is a prerequisite to protective immunity against gastrointestinal infections. In Drosophila, immune deficiency (IMD) signalling (tumour necrosis factor receptor/interleukin-1 receptor, TNFR/IL-1R in mammals) is indispensable for intestinal immunity against invading bacteria. However, how this local antimicrobial immune response contributes to inflammatory regulation remains poorly defined. Here, we show that flies lacking intestinal Bap180 (a subunit of the chromatin-remodelling switch/sucrose non-fermentable (SWI/SNF) complex) are susceptible to infection as a result of hyper-inflammation rather than bacterial overload. Detailed analysis shows that Bap180 is induced by the IMD-Relish response to both enteropathogenic and commensal bacteria. Upregulated Bap180 can feed back to restrain overreactive IMD signalling, as well as to repress the expression of the pro-inflammatory gene eiger (TNF), a critical step to prevent excessive tissue damage and elongate the lifespan of flies, under pathological and physiological conditions, respectively. Furthermore, intestinal targeting of Baf180 renders mice susceptible to a more aggressive infectious colitis caused by Citrobacter rodentium. Together, Bap180 and Baf180 serve as a conserved transcriptional repressor that is critical for the maintenance of innate immune homeostasis in the intestines.

Background:The intensity and duration of phosphorylation levels of R-Smads are required for precise control of BMP signaling. Results: MTMR4 associated with and dephosphorylated the activated R-Smads in cytoplasm. Conclusion: MTMR4 attenuates BMP signaling via its DUSP activity. Significance: This study describes a novel role of MTMR4 as a negative modulator essentially involved in homeostatic BMP signaling.

Highlights d Intestinal IMD activation induces systemic immunity via a hemocyte-fat body relay d Intestinal IMD activation causes the increase of sugar alcohols in the hemolymph d Aldose reductase in hemocytes mediates interorgan communication for systemic immunity d Polyols in the hemolymph activate fat body IMD via Mmp2mediated cleavage of PRGP-LC

Lipid droplets (LDs), the highly dynamic intracellular organelles, are critical for lipid metabolism. Dynamic alterations in the configurations and functions of LDs during innate immune responses to bacterial infections and the underlying mechanisms, however, remain largely unknown. In this study, we trace the time-course morphology of LDs in fat bodies of Drosophila after transient bacterial infection. Detailed analysis shows that perilipin1 (plin1), a core gene involved in the regulation of LDs, is suppressed by the immune deficiency signaling, one major innate immune pathway in Drosophila. During immune activation, downregulated plin1 promotes the enlargement of LDs, which in turn alleviates immune reaction–associated reactive oxygen species stress. Thus, the growth of LDs is likely an active adaptation to maintain redox homeostasis in response to immune deficiency activation. Therefore, our study provides evidence that plin1 serves as a modulator on LDs’ reconfiguration in regulating infection-induced pathogenesis, and plin1 might be a potential therapeutic target for coordinating inflammation resolution and lipid metabolism.

In this work, we identify for the first time that the nuclear protein Bub1 (budding uninhibited by benzimidazoles 1), a highly conserved subunit of the kinetochore complex regulating chromosome congression, has a novel and important function on the cell membrane to facilitate the virus to enter host cells. Bub1 deficiency empowers the host to have the ability to resist viral infection in Drosophila and a human cell line. Bub1 is involved in the virus entry step through regulating endocytosis. The DCV capsid protein can recruit Bub1, and DCV infection can strengthen the interaction between Bub1 and a clathrin-dependent endocytosis component. The restricted entry of vesicular stomatitis virus (VSV) and Listeria monocytogenes in bub1-deficient flies and cell lines was also observed. Therefore, our data implicate a previously unknown function of Bub1 that can be hijacked by pathogens to facilitate their entry, and Bub1 may serve as a potential antiviral therapy target for limiting viral entry.