FGF signals induce Caprin2 expression in the vertebrate lens

The lens of the eye is derived from the non-neural ectoderm situated next to the optic vesicle. Fibroblast growth factor (FGF) signals play a major role at various stages of vertebrate lens development ranging from induction and proliferation to differentiation. Less is however known about the identity of genes that are induced by FGF activity within the lens. We have isolated and characterized mouse cytoplasmic activation/proliferation-associated protein-2 (Caprin2), with domains belonging to both the Caprin family and the C1q and tumour necrosis factor (TNF) super-family. Here we show that Caprin2 is expressed in the developing vertebrate lens in mouse and chick, and that Caprin2 expression is up-regulated in primary lens fiber cells, after the induction of crystallins the earliest known markers for differentiated lens fiber cells. Caprin2 is subsequently down-regulated in the centre of the lens at the time and at the position of the first fiber cell denucleation and terminal differentiation. In vitro analyses of lens fiber cell differentiation provide evidence that FGF activity emanating from neighboring prospective retinal cells is required and that FGF8 activity is sufficient to induce Caprin2 in lens fiber cells. These results not only provide evidence that FGF signals induce the newly characterized protein Caprin2 in the lens, but also support the general idea that FGF signals are required for lens fiber cell differentiation.     

Wingless signaling regulates the maintenance of ovarian somatic stem cells in Drosophila

Identifying the signals involved in maintaining stem cells is critical to understanding stem cell biology and to using stem cells in future regenerative medicine. In the Drosophila ovary, Hedgehog is the only known signal for maintaining somatic stem cells (SSCs). Here we report that Wingless (Wg) signaling is also essential for SSC maintenance in the Drosophila ovary. Wg is expressed in terminal filament and cap cells, a few cells away from SSCs. Downregulation of Wg signaling in SSCs through removal of positive regulators of Wg signaling, dishevelled and armadillo, results in rapid SSC loss. Constitutive Wg signaling in SSCs through the removal of its negative regulators, Axin and shaggy, also causes SSC loss. Also, constitutive wg signaling causes over-proliferation and abnormal differentiation of somatic follicle cells. This work demonstrates that wg signaling regulates SSC maintenance and that its constitutive signaling influences follicle cell proliferation and differentiation. In mammals, constitutive beta-catenin causes over-proliferation and abnormal differentiation of skin cells, resulting in skin cancer formation. Possibly, mechanisms regulating proliferation and differentiation of epithelial cells, including epithelial stem cells, is conserved from Drosophila to man.     

A role for extra macrochaetae downstream of Notch in follicle cell differentiation

The Drosophila ovary provides a model system for studying the mechanisms that regulate the differentiation of somatic stem cells into specific cell types. Ovarian somatic stem cells produce follicle cells, which undergo a binary choice during early differentiation. They can become either epithelial cells that surround the germline to form an egg chamber ('main body cells') or a specialized cell lineage found at the poles of egg chambers. This lineage goes on to make two cell types: polar cells and stalk cells. To better understand how this choice is made, we carried out a screen for genes that affect follicle cell fate specification or differentiation. We identified extra macrochaetae (emc), which encodes a helix-loop-helix protein, as a downstream effector of Notch signaling in the ovary. EMC is expressed in proliferating cells in the germarium, as well as in the main body follicle cells. EMC expression in the main body cells is Notch dependent, and emc mutant cells located on the main body failed to differentiate. EMC expression is reduced in the precursors of the polar and stalk cells, and overexpression of EMC caused dramatic egg chamber fusions, indicating that EMC is a negative regulator of polar and/or stalk cells. EMC and Notch were both required in the main body cells for expression of Eyes Absent (EYA), a negative regulator of polar and stalk cell fate. We propose that EMC functions downstream of Notch and upstream of EYA to regulate main body cell fate specification and differentiation.     

Germ line stem cell differentiation in Drosophila requires gap junctions and proceeds via an intermediate state

Gap junctions coordinate processes ranging from muscle contraction to ovarian follicle development. Here we show that the gap junction protein Zero population growth (Zpg) is required for germ cell differentiation in the Drosophila ovary. In the absence of Zpg the stem cell daughter destined to differentiate dies. The zpg phenotype is novel, and we used this phenotype to genetically dissect the process of stem cell maintenance and differentiation. Our findings suggest that germ line stem cells differentiate upon losing contact with their niche, that gap junction mediated cell-cell interactions are required for germ cell differentiation, and that in Drosophila germ line stem cell differentiation to a cystoblast is gradual.     

Marker genes identify three somatic cell types in the fetal mouse ovary

The two main functions of the ovary are the production of oocytes, which allows the continuation of the species, and secretion of female sex hormones, which control many aspects of female development and physiology. Normal development of the ovaries during embryogenesis is critical for their function and the health of the individual in later life. Although the adult ovary has been investigated in great detail, we are only starting to understand the cellular and molecular biology of early ovarian development. Here we show that the adult stem cell marker Lgr5 is expressed in the cortical region of the fetal ovary and this expression is mutually exclusive to FOXL2. Strikingly, a third somatic cell population can be identified, marked by the expression of NR2F2, which is expressed in LGR5- and FOXL2 double-negative ovarian somatic cells. Together, these three marker genes label distinct ovarian somatic cell types. Using lineage tracing in mice, we show that Lgr5-positive cells give rise to adult cortical granulosa cells, which form the follicles of the definitive reserve. Moreover, LGR5 is required for correct timing of germ cell differentiation as evidenced by a delay of entry into meiosis in Lgr5 loss-of-function mutants, demonstrating a key role for LGR5 in the differentiation of pre-granulosa cells, which ensure the differentiation of oogonia, the formation of the definitive follicle reserve, and long-term female fertility.     

Daughterless coordinates somatic cell proliferation,differentiation and germline cyst survival during follicle formation in Drosophila

During Drosophila oogenesis two distinct stem cell populations produce either germline cysts or the somatic cells that surround each cyst and separate each formed follicle. From analyzing daughterless (da) loss-of-function, overexpression and genetic interaction phenotypes, we have identified several specific requirements for da(+) in somatic cells during follicle formation. First, da is a critical regulator of somatic cell proliferation. Also, da is required for the complete differentiation of polar and stalk cells, and elevated da levels can even drive the convergence and extension that is characteristic of interfollicular stalks. Finally, da is a genetic regulator of an early checkpoint for germline cyst progression: Loss of da function inhibits normally occurring apoptosis of germline cysts at the region 2a/2b boundary of the germarium, while da overexpression leads to postmitotic cyst degradation. Collectively, these da functions govern the abundance and diversity of somatic cells as they coordinate with germline cysts to form functional follicles.     

The Drosophila fragile X gene negatively regulates neuronal elaboration and synaptic differentiation

Fragile X Syndrome (FraX) is the most common form of inherited mental retardation. The disease is caused by the silencing of the fragile X mental retardation 1 (fmr1) gene, which encodes the RNA binding translational regulator FMRP . In FraX patients and fmr1 knockout mice, loss of FMRP causes denser and morphologically altered postsynaptic dendritic spines . Previously, we established a Drosophila FraX model and showed that dFMRP acts as a negative translational regulator of Futsch/MAP1B and negatively regulates synaptic branching and structural elaboration in the peripheral neuromuscular junction (NMJ) . Here, we investigate the role of dFMRP in the central brain, focusing on the mushroom body (MB), the learning and memory center . In MB neurons, dFMRP bidirectionally regulates multiple levels of structural architecture, including process formation from the soma, dendritic elaboration, axonal branching, and synaptogenesis. Drosophila fmr1 (dfmr) null mutant neurons display more complex architecture, including overgrowth, overbranching, and abnormal synapse formation. In contrast, dFMRP overexpression simplifies neuronal structure, causing undergrowth, underbranching, and loss of synapse differentiation. Studies of ultrastructural dfmr mutant neurons reveal enlarged and irregular synaptic boutons with dense accumulation of synaptic vesicles. Taken together, these data show that dFMRP is a potent negative regulator of neuronal architecture and synaptic differentiation in both peripheral and central nervous systems.     

Diet controls Drosophila follicle stem cell proliferation via Hedgehog sequestration and release

A healthy diet improves adult stem cell function and delays diseases such as cancer, heart disease, and neurodegeneration. Defining molecular mechanisms by which nutrients dictate stem cell behavior is a key step toward understanding the role of diet in tissue homeostasis. In this paper, we elucidate the mechanism by which dietary cholesterol controls epithelial follicle stem cell (FSC) proliferation in the fly ovary. In nutrient-restricted flies, the transmembrane protein Boi sequesters Hedgehog (Hh) ligand at the surface of Hh-producing cells within the ovary, limiting FSC proliferation. Upon feeding, dietary cholesterol stimulates S6 kinase–mediated phosphorylation of the Boi cytoplasmic domain, triggering Hh release and FSC proliferation. This mechanism enables a rapid, tissue-specific response to nutritional changes, tailoring stem cell divisions and egg production to environmental conditions sufficient for progeny survival. If conserved in other systems, this mechanism will likely have important implications for studies on molecular control of stem cell function, in which the benefits of low calorie and low cholesterol diets are beginning to emerge.     

Ecdysteroids affect Drosophila ovarian stem cell niche formation and early germline differentiation

Previously, it has been shown that in Drosophila steroid hormones are required for progression of oogenesis during late stages of egg maturation. Here, we show that ecdysteroids regulate progression through the early steps of germ cell lineage. Upon ecdysone signalling deficit germline stem cell progeny delay to switch on a differentiation programme. This differentiation impediment is associated with reduced TGF-β signalling in the germline and increased levels of cell adhesion complexes and cytoskeletal proteins in somatic escort cells. A co-activator of the ecdysone receptor, Taiman is the spatially restricted regulator of the ecdysone signalling pathway in soma. Additionally, when ecdysone signalling is perturbed during the process of somatic stem cell niche establishment enlarged functional niches able to host additional stem cells are formed.     

Micro-RNA mediated regulation of proliferation,self-renewal and differentiation of mammalian stem cells

Metazoan growth and development is maintained by populations of undifferentiated cells, commonly known as stem cells. Stem cells possess several characteristic properties, including dividing through self-renewing divisions and generating progeny that differentiate to have specialized cell fates. Multiple signaling pathways have been identified which coordinate stem cell proliferation with maintenance and differentiation. Relatively recently, the small, non-protein coding microRNAs (miRNAs) have been identified to function as important regulators in stem cell development. Individual miRNAs are capable of directing the translational repression of many mRNAs targets, generating widespread changes in gene expression. In addition, dysfunction of miRNA expression is commonly associated with cancer development. Cancer stem cells, which are likely responsible for initiating and maintaining tumorigenesis, share many similarities with stem cells and some mechanisms of miRNA function may be in common between these two cell types.