mRNA localization is a conserved mechanism for spatial control of protein synthesis, with key roles in generating cellular and developmental asymmetry. While different transcripts may be targeted to the same subcellular domain, the extent to which their localization is coordinated is unclear. Using quantitative single molecule imaging, we analyzed the assembly of Drosophila germ plasm mRNA granules inherited by nascent germ cells. We find that the germ cell-destined transcripts nanos, cyclin B, and polar granule component travel within the oocyte as ribonucleoprotein particles containing single mRNA molecules but co-assemble into multi-copy heterogeneous granules selectively at the posterior of the oocyte. The stoichiometry and dynamics of assembly indicate a defined stepwise sequence. Our data suggest that co-packaging of these transcripts ensures their effective segregation to germ cells. In contrast, compartmentalization of the germline determinant oskar mRNA into different granules limits its entry into germ cells. This exclusion is required for proper germline development.

Controlled, transient cytokine production by monocytes depends heavily upon rapid mRNA degradation, conferred by 3 untranslated region-localized AU-rich elements (AREs) that associate with RNA-binding proteins. The ARE-binding protein AUF1 forms a complex with cap-dependent translation initiation factors and heat shock proteins to attract the mRNA degradation machinery. We refer to this protein assembly as the AUF1-and signal transduction-regulated complex, ASTRC. Rapid degradation of ARE-bearing mRNAs (AREmRNAs) requires ubiquitination of AUF1 and its destruction by proteasomes. Activation of monocytes by adhesion to capillary endothelium at sites of tissue damage and subsequent proinflammatory cytokine induction are prominent features of inflammation, and ARE-mRNA stabilization plays a critical role in the induction process. Here, we demonstrate activation-induced subunit rearrangements within ASTRC and identify chaperone Hsp27 as a novel subunit that is itself an ARE-binding protein essential for rapid ARE-mRNA degradation. As Hsp27 has well-characterized roles in protein ubiquitination as well as in adhesion-induced cytoskeletal remodeling and cell motility, its association with ASTRC may provide a sensing mechanism to couple proinflammatory cytokine induction with monocyte adhesion and motility.Many mRNAs encoding proteins transiently required for inflammatory responses, cell proliferation, and intracellular signaling are labile due to AU-rich elements (AREs) in their 3Ј untranslated regions (UTRs) (14,21,57). ARE association by ELAV-like (embryonic lethal, abnormal vision) proteins, such as HuR, blocks ARE-mediated mRNA decay (AMD) (31). By contrast, association of proteins such as AUF1, tristetraprolin (TTP), BRF1 (butyrate-responsive factor-1), K-homology splicing regulatory protein (KSRP), ring finger K-homology domain 1 (RKHD1), polymyositisscleroderma 75-kDa antigen (PM-Scl75), or microRNA miR16 or miR289 with an ARE promotes AMD (6,8,12,18,24,34,43). The phosphorylation state of TTP, BRF1, and AUF1 affects AMD efficiency (3,37,51,56), indicating that signal transduction networks regulate this pathway.AUF1 has four protein isoforms-p37, p40, p42, and p45-generated by alternative pre-mRNA splicing (50). Based upon extensive biochemical studies of AUF1, we proposed an integrated, three-step model for induction of AMD by AUF1 via assembly of a trans-acting complex that targets the mRNA for degradation (52). The first step is dynamic AUF1 dimer binding to an ARE and formation of an oligomeric AUF1 complex (7, 52). Stabilizing ARE-binding proteins (AUBPs) may compete with AUF1 for binding to the ARE during this step, thus preventing AUF1 oligomerization and subsequent factor recruitment (25). Binding of AUF1 to an ARE then permits the second step involving recruitment of additional trans-acting factors including eukaryotic translation initiation factor eIF4G, poly(A)-binding protein, dual-functional heat shock/AUBPs Hsp/Hsc70 (27), and additional unknown proteins, forming a multisubunit AUF1-and signal tra...

SUMMARYAsymmetric mRNA localization is an effective mechanism for establishing cellular and developmental polarity. Posterior localization of oskar in the Drosophila oocyte targets the synthesis of Oskar to the posterior, where Oskar initiates the assembly of the germ plasm. In addition to harboring germline determinants, the germ plasm is required for localization and translation of the abdominal determinant nanos. Consequently, failure of oskar localization during oogenesis results in embryos lacking germ cells and abdominal segments. oskar accumulates at the oocyte posterior during mid-oogenesis through a well-studied process involving kinesin-mediated transport. Through live imaging of oskar mRNA, we have uncovered a second, mechanistically distinct phase of oskar localization that occurs during late oogenesis and results in amplification of the germ plasm. Analysis of two newly identified oskar localization factors, Rumpelstiltskin and Lost, that are required specifically for this late phase of oskar localization shows that germ plasm amplification ensures robust abdomen and germ cell formation during embryogenesis. In addition, our results indicate the importance of mechanisms for adapting mRNAs to utilize multiple localization pathways as necessitated by the dramatic changes in ovarian physiology that occur during oogenesis.

IL-10 is an immunomodulatory cytokine that regulates infl ammatory responses of mononuclear phagocytes (monocytes and macrophages). Mononuclear cells exposed to microbes or microbial products secrete a host of proinfl ammatory cytokines followed by delayed onset of anti-infl ammatory IL-10. IL-10 suppresses immune responses by inhibiting cytokine production by mononuclear phagocytes. Using THP-1, a human promonocytic leukemia cell line, we show that endotoxin/lipopolysaccharide (LPS) exposure induces IL10 expression while IFN-γ blocks this LPS-mediated effect. IFN-γ is an important modulator of IL-10 production during infectious diseases. We show that LPS and IFN-γ regulate IL10 expression in THP-1 cells in part through posttranscriptional mechanisms. Our results demonstrate that 3′-untranslated region (3′-UTR) AU-rich elements (AREs) decrease expression of a chimeric luciferase reporter gene in THP-1 cells. The ARE-binding protein AUF1 binds the IL10 3′-UTR. Depletion of AUF1 by RNAi suppresses LPS-mediated induction of IL10 mRNA and protein without affecting LPS-mediated stabilization of IL10 mRNA. Upon complementation with either RNAi-refractory p37 or p40 AUF1 plasmids, only p40 restores LPS-mediated induction of IL10 mRNA and protein to near normal levels. Thus, the p40 AUF1 isoform selectively plays a critical, positive role in IL10 expression upon LPS exposure.

Summary Localized cytoplasmic determinants packaged as ribonucleoprotein (RNP) particles direct embryonic patterning and cell fate specification in a wide range of organisms. Once established, the asymmetric distributions of such RNP particles must be maintained, often over considerable developmental time. A striking example is the Drosophila germ plasm, which contains RNP particles whose localization to the posterior of the egg during oogenesis results in their asymmetric inheritance and segregation of germline from somatic fates in the embryo. Although actin-based anchoring mechanisms have been implicated, high-resolution live imaging revealed persistent trafficking of germ plasm RNP particles at the posterior cortex of the Drosophila oocyte. This motility relies on newly-identified cortical microtubules, is mediated by kinesin and dynein motors, and requires coordination between the microtubule and actin cytoskeletons. Finally, we show that RNP particle motility is required for long-term germ plasm retention. We propose that anchoring is a dynamic state that renders asymmetries robust to developmental time and environmental perturbations.