Genetic manipulation of the germline stem cell niche in Drosophila ovaries reveals that support cells ensure the maintenance of stem cells by modulating the spread of Hedgehog within the niche.

PIWI proteins and Piwi-interacting RNAs (piRNAs) have established and conserved roles in repressing transposable elements (TEs) in the germline of animals. However, in several biological contexts, a large proportion of piRNAs are not related to TE sequences and, accordingly, functions for piRNAs and PIWI proteins that are independent of TE regulation have been identified. This aspect of piRNA biology is expanding rapidly. Indeed, recent reports have revealed the role of piRNAs in the regulation of endogenous gene expression programs in germ cells, as well as in somatic tissues, challenging dogma in the piRNA field. In this Review, we focus on recent data addressing the biological and developmental functions of piRNAs, highlighting their roles in embryonic patterning, germ cell specification, stem cell biology, neuronal activity and metabolism.

SummaryTranslational regulation plays an essential role in Drosophila ovarian germline stem cell (GSC) biology. GSC self-renewal requires two translational repressors, Nanos (Nos) and Pumilio (Pum), which repress the expression of differentiation factors in the stem cells. The molecular mechanisms underlying this translational repression remain unknown. Here, we show that the CCR4 deadenylase is required for GSC self-renewal and that Nos and Pum act through its recruitment onto specific mRNAs. We identify mei-P26 mRNA as a direct and major target of Nos/Pum/CCR4 translational repression in the GSCs. mei-P26 encodes a protein of the Trim-NHL tumor suppressor family that has conserved functions in stem cell lineages. We show that fine-tuning Mei-P26 expression by CCR4 plays a key role in GSC self-renewal. These results identify the molecular mechanism of Nos/Pum function in GSC self-renewal and reveal the role of CCR4-NOT-mediated deadenylation in regulating the balance between GSC self-renewal and differentiation.

PIWI proteins play essential roles in germ cells and stem cell lineages. In Drosophila, Piwi is required in somatic niche cells and germline stem cells (GSCs) to support GSC self‐renewal and differentiation. Whether and how other PIWI proteins are involved in GSC biology remains unknown. Here, we show that Aubergine (Aub), another PIWI protein, is intrinsically required in GSCs for their self‐renewal and differentiation. Aub needs to be loaded with piRNAs to control GSC self‐renewal and acts through direct mRNA regulation. We identify the Cbl proto‐oncogene, a regulator of mammalian hematopoietic stem cells, as a novel GSC differentiation factor. Aub stimulates GSC self‐renewal by repressing Cbl mRNA translation and does so in part through recruitment of the CCR4‐NOT complex. This study reveals the role of piRNAs and PIWI proteins in controlling stem cell homeostasis via translational repression and highlights piRNAs as major post‐transcriptional regulators in key developmental decisions.

Stem cell activity is tightly regulated during development and in adult tissues through the combined action of local and systemic effectors. While stem cells and their microenvironments are capable of sustaining homeostasis in normal physiological circumstances, they also provide host tissues with a remarkable plasticity to respond to perturbations. Here, we review recent discoveries that shed light on the adaptive response of niches to systemic signals and aging, and on the ability of niches to modulate signaling upon local perturbations. These characteristics of stem cells and their niches give organs an essential advantage to deal with aging, injury or pathological conditions. STEM CELLS 2014;32:852-859

Stem cells possess the unique properties of self-renewal and the ability to give rise to multiple types of differentiated tissue. The fruit fly Drosophila melanogaster retains several populations of stem cells during adulthood as well as transient populations of stem cells during development. Studies of these different populations of stem cells using the genetic tools available to Drosophila researchers have played an important role in understanding many conserved stem cell characteristics. This review aims highlight some of the recent contributions from this important model system to our understanding of the myriad of processes that interact to control stem cell biology.KEY WORDS: niche signaling, niche morphogenesis, somatic stem cell, germline stem cell, siRNA The essence of a stem cellThe self-renewal of stem cell populations is critical for animal development, growth, tissue homeostasis, damage repair and reproduction. Understanding what gives stem cells these unique characteristic is one of the most important aims in biology today. Researchers using a wide range of model systems including vertebrates, invertebrates and plants have made considerable progress in our knowledge of stem cell biology. While stem cell populations in different species and tissues are as divergent as the roles they perform, several common characteristics have also emerged (see reviews Spradling, 2006, Wong et al., 2005).Niche regulation has been one of the key concepts to emerge in our understanding of stem cell self-renewal (See reviews e.g. Fuchs et al., 2004, Lin, 2002, Nystul and Spradling, 2006, Ohlstein et al., 2004, Spradling et al., 2001. A stem cell niche has been defined as "a specific location where stem cells can reside for an indefinite period of time and produce progeny cells while selfrenewing" (Ohlstein et al., 2004). In the strictest sense this has meant a region which is stably maintained even in the absence of stem cells. While different classifications of stem cell niche organization have been proposed (Ohlstein et al., 2004), they all involve the formation of a limited "permissive zone" for self-renewal. Those stem cells forced to leave this zone (e.g. by spatial constraints) lose the factors required for self-renewal and, typically, enter differentiation. Thus, stem cell niche regulation can be viewed as a homeostatic mechanism for controlling the proliferative potential of stem cells, yet affording the plasticity required to respond to changing conditions. Abbreviations used in this paper: BMP, bone morphogenetic protein; CPC, cyst progenitor cell; ESC, escort stem cells; FSC, follicle stem cell; GMC, ganglion mother cell; GSC, germline stem cell; fGSC, female germline stem cell; mGSC, male germline stem cell; RNSC, renal and nephric stem cell; HP, hematopoetic precursor; ISC, intestinal stem cell; JAK/STAT, janus kinase/ signal transducer and activator of transcription; miRNA, microRNA; NB, neuroblast; PGC, primordial germ cell; piRNA, Piwi-interacting RNA; PSC, posterior signaling center; SGP, so...

Drosophila Orb, the homolog of vertebrate CPEB, is a key translational regulator involved in oocyte polarity and maturation through poly(A) tail elongation of specific mRNAs. orb also has an essential function during early oogenesis that has not been addressed at the molecular level. Here, we show that orb prevents cell death during early oogenesis, thus allowing oogenesis to progress. It does so through the repression of autophagy by directly repressing, together with the CCR4 deadenylase, the translation of Autophagy-specific gene 12 (Atg12) mRNA. Autophagy and cell death observed in orb mutant ovaries are reduced by decreasing Atg12 or other Atg mRNA levels. These results reveal a role of Orb in translational repression and identify autophagy as an essential pathway regulated by Orb during early oogenesis. Importantly, they also establish translational regulation as a major mode of control of autophagy, a key process in cell homeostasis in response to environmental cues.

The subcommissural organ (SCO) is an ependymal differentiation located in the dorsal midline of the caudal diencephalon under the posterior commissure. SCO cells synthesize and release glycoproteins into the cerebrospinal fluid (CSF) forming a threadlike structure known as Reissner's fiber (RF), which runs caudally along the ventricular cavities and the central canal of the spinal cord. Numerous monoclonal antibodies have been raised against bovine RF and the secretory material of the SCO. For this study, we selected the 4F7 monoclonal antibody based on its cross-reactivity with chick embryo SCO glycoproteins in vivo. E4 chick embryos were injected with 4F7 hybridoma cells or with the purified monoclonal antibody into the ventricular cavity of the optic tectum. The hybridoma cells survived, synthesized and released antibody into the CSF for at least 13 days after the injection. E5 embryos injected with 4F7 antibody displayed precipitates in the CSF comprising both the monoclonal antibody and anti-RF-positive material. Such aggregates were never observed in control embryos injected with other monoclonal antibodies used as controls. Western blot analysis of CSF from E4-E6 embryos revealed several immunoreactive bands to anti-RF (AFRU) antibody. We also found AFRU-positive material bound to the apical surface of the choroid plexus primordia in E5 embryos. These and other ultrastructural evidence suggest the existence of soluble SCO-related molecules in the CSF of early chick embryos.