Twin Promotes the Maintenance and Differentiation of Germline Stem Cell Lineage through Modulation of Multiple Pathways.

The central question in stem cell regulation is how the balance between self-renewal and differentiation is controlled at the molecular level. This study uses germline stem cells (GSCs) in the Drosophila ovary to demonstrate that the Drosophila CCR4 homolog Twin is required intrinsically to promote both GSC self-renewal and progeny differentiation. Twin/CCR4 is one of the two catalytic subunits in the highly conserved CCR4-NOT mRNA deadenylase complex. Twin works within the CCR4-NOT complex to intrinsically maintain GSC self-renewal, at least partly by sustaining E-cadherin-mediated GSC-niche interaction and preventing transposable element-induced DNA damage. It promotes GSC progeny differentiation by forming protein complexes with differentiation factors Bam and Bgcn independently of other CCR4-NOT components. Interestingly, Bam can competitively inhibit the association of Twin with Pop2 in the CCR4-NOT complex. Therefore, this study demonstrates that Twin has important intrinsic roles in promoting GSC self-renewal and progeny differentiation by functioning in different protein complexes.

Drosophila lin-52 acts in opposition to repressive components of the Myb-MuvB/dREAM complex.

The Drosophila melanogaster Myb-MuvB/dREAM complex (MMB/dREAM) participates in both the activation and repression of developmentally regulated genes and origins of DNA replication. Mutants in MMB subunits exhibit diverse phenotypes, including lethality, eye defects, reduced fecundity, and sterility. Here, we used P-element excision to generate mutations in lin-52, which encodes the smallest subunit of the MMB/dREAM complex. lin-52 is required for viability, as null mutants die prior to pupariation. The generation of somatic and germ line mutant clones indicates that lin-52 is required for adult eye development and for early embryogenesis via maternal effects. Interestingly, the maternal-effect embryonic lethality, larval lethality, and adult eye defects could be suppressed by mutations in other subunits of the MMB/dREAM complex. These results suggest that a partial MMB/dREAM complex is responsible for the lethality and eye defects of lin-52 mutants. Furthermore, these findings support a model in which the Lin-52 and Myb proteins counteract the repressive activities of the other members of the MMB/dREAM complex at specific genomic loci in a developmentally controlled manner.

New advances in stem cell biology: A perspective from gametogenesis.

Regeneration of adult tissues relies on a small population of adult stem cells located in a specialized microenvironment. The adult stem cells divide continuously to produce new stem cells, as well as differentiated daughter cells to replenish lost cells due to damage or aging. The molecular mechanisms controlling their ability to divide, self-renew and differentiate remain largely undiscovered. The Drosophila reproductive systems have proven to be excellent models to understand the basic mechanisms regulating stem cell function. This report summarizes some of the recent advances in this field that were presented at the 49(th) Drosophila Research Conference held in San Diego in April 2008.

Identification of genes that influence gurken expression.

Axial patterning in Drosophila relies on the deployment of patterning proteins at specific regions within the developing oocyte. This process involves transport of mRNAs from the nurse cells to the oocyte, localization of mRNAs within the oocyte, and translational regulation of these mRNAs to restrict the final distribution of the proteins. Despite extensive analysis, the events of deployment are not fully understood and it seems certain that many contributing factors remain to be identified. We describe the development and application of a sensitized genetic screen to reveal such additional factors. Overexpression of Imp, a factor implicated in regulation of gurken mRNA, causes a weak dorsalization that can be enhanced by reducing the level of other factors acting in the same pathway. A collection of deficiency mutants was screened using this assay, leading to the identification of 5 genes that are candidates to contribute to axial patterning. Three of the genes were characterized in greater detail. The mushroom body expressed gene was implicated in axial patterning, with overexpression leading to a range of patterning abnormalities that can be explained by a more primary defect in organization of the cytoskeleton. Two mitotic cell cycle control factors, cyclin E and E2f1, were also implicated, raising the possibility that a mitotic cell cycle checkpoint may impinge on grk expression, much as meiotic checkpoints can alter expression of this gene.

Staufen- and FMRP-containing neuronal RNPs are structurally and functionally related to somatic P bodies.

Local control of mRNA translation modulates neuronal development, synaptic plasticity, and memory formation. A poorly understood aspect of this control is the role and composition of ribonucleoprotein (RNP) particles that mediate transport and translation of neuronal RNAs. Here, we show that staufen- and FMRP-containing RNPs in Drosophila neurons contain proteins also present in somatic "P bodies," including the RNA-degradative enzymes Dcp1p and Xrn1p/Pacman and crucial components of miRNA (argonaute), NMD (Upf1p), and general translational repression (Dhh1p/Me31B) pathways. Drosophila Me31B is shown to participate (1) with an FMRP-associated, P body protein (Scd6p/trailer hitch) in FMRP-driven, argonaute-dependent translational repression in developing eye imaginal discs; (2) in dendritic elaboration of larval sensory neurons; and (3) in bantam miRNA-mediated translational repression in wing imaginal discs. These results argue for a conserved mechanism of translational control critical to neuronal function and open up new experimental avenues for understanding the regulation of mRNA function within neurons.

Imp associates with squid and Hrp48 and contributes to localized expression of gurken in the oocyte.

Localization and translational control of Drosophila melanogaster gurken and oskar mRNAs rely on the hnRNP proteins Squid and Hrp48, which are complexed with one another in the ovary. Imp, the Drosophila homolog of proteins acting in localization of mRNAs in other species, is also associated with Squid and Hrp48. Notably, Imp is concentrated at sites of gurken and oskar mRNA localization in the oocyte, and alteration of gurken localization also alters Imp distribution. Imp binds gurken mRNA with high affinity in vitro; thus, the colocalization with gurken mRNA in vivo is likely to be the result of direct binding. Imp mutants support apparently normal regulation of gurken and oskar mRNAs. However, loss of Imp activity partially suppresses a gurken misexpression phenotype, indicating that Imp does act in control of gurken expression but has a largely redundant role that is only revealed when normal gurken expression is perturbed. Overexpression of Imp disrupts localization of gurken mRNA as well as localization and translational regulation of oskar mRNA. The opposing effects of reduced and elevated Imp activity on gurken mRNA expression indicate a role in gurken mRNA regulation.