Cell cycle arrest by a gradient of Dpp signaling during Drosophila eye development

Background  

The secreted morphogen Dpp plays important roles in spatial regulation of gene expression and cell cycle progression in the developing Drosophila eye. Dpp signaling is required for timely cell cycle arrest ahead of the morphogenetic furrow as a prelude to differentiation, and is also important for eye disc growth. The dpp gene is expressed at multiple locations in the eye imaginal disc, including the morphogenetic furrow that sweeps across the eye disc as differentiation initiates.     

Fine tuning of TCR signaling by CD5

Current data indicate that CD5 functions as an inhibitor of TCR signal transduction. Consistent with this role, thymocyte selection in TCR transgenic/CD5(-/-) mice is altered in a manner suggestive of enhanced TCR signaling. However, the impact of CD5 deletion on thymocyte selection varies depending on the transgenic TCR analyzed, ranging from a slight to a marked shift from positive toward negative selection. An explanation for the variable effect of CD5 on selection is suggested by the observation that CD5 surface expression is regulated by TCR signal intensity during development and CD5 surface levels on mature thymocytes and T cells parallel the avidity of the positively selecting TCR/MHC/ligand interaction. In this study, we generated mice that overexpress CD5 during thymocyte development (CD5-tg), and then examined the effect of CD5 overexpression or CD5 deletion (CD5(-/-)) on selection of thymocytes that express the same TCR transgenes. The results demonstrate that the effect on thymocyte selection of altering CD5 expression depends on the avidity of the selecting interaction and, consequently, the level of basal (endogenous) CD5 surface expression. Substitution of endogenous CD5 with a transgene encoding a truncated form of the protein failed to rescue the CD5(-/-) phenotype, demonstrating that the cytoplasmic domain of CD5 is required for its inhibitory function. Together, these results indicate that inducible regulation of CD5 surface expression during thymocyte selection functions to fine tune the TCR signaling response.     

Notch signaling controls lineage specification during Drosophila larval hematopoiesis

Drosophila larval hemocytes originate from a hematopoietic organ called lymph glands, which are composed of paired lobes located along the dorsal vessel. Two mature blood cell populations are found in the circulating hemolymph: the macrophage-like plasmatocytes, and the crystal cells that contain enzymes of the immune-related melanization process. A third class of cells, called lamellocytes, are normally absent in larvae but differentiate after infection by parasites too large to be phagocytosed. Here we present evidence that the Notch signaling pathway plays an instructive role in the differentiation of crystal cells. Loss-of-function mutations in Notch result in severely decreased crystal cell numbers, whereas overexpression of Notch provokes the differentiation of high numbers of these cells. We demonstrate that, in this process, Serrate, not Delta, is the Notch ligand. In addition, Notch function is necessary for lamellocyte proliferation upon parasitization, although Notch overexpression does not result in lamellocyte production. Finally, Notch does not appear to play a role in the differentiation of the plasmatocyte lineage. This study underlines the existence of parallels in the genetic control of hematopoiesis in Drosophila and in mammals.     

Locus Delta in the Notch signaling system: organization and pleiotropic function during Drosophila development

Delta locus is the important component of the Delta-Notch signaling system implicating in a general mechanism of local cell signaling. Delta and Notch encode the evolutionary conserved cell surface proteins that interact and function as ligand (DELTA) and receptor (NOTCH) in a wide variety of cell fate specification events during oogenesis, embryogenesis and metamorphosis.     

Notch signaling controls proliferation through cell-autonomous and non-autonomous mechanisms in the Drosophila eye

During Drosophila eye development, localized Notch signaling at the dorsal ventral (DV)-midline promotes growth of the entire eye field. This long-range action of Notch signaling may be mediated through the diffusible ligand of the Jak/STAT pathway, Unpaired (Upd), which was recently identified as a downstream target of Notch. However, Notch activity has not been shown to be cell-autonomously required for Upd expression and therefore yet another diffusible signal may be required for Notch activation of Upd. Our results clarify the Notch requirement, demonstrating that Notch activity at the DV-midline leads to cell-autonomous expression of Upd as monitored in loss and gain-of-function Notch clones. In addition, mutations in the Jak/STAT pathway interact genetically with the Notch pathway by suppressing Notch mediated overgrowth. N(act) clones show non-autonomous effects on the cell cycle anterior to the furrow, indicating function of the Jak/STAT pathway. However, cell-autonomous effects of Notch within and posterior to the furrow are independent of Upd. Here, Notch autonomously maintains cells in a proliferative state and blocks photoreceptor differentiation.     

Lobe mediates Notch signaling to control domain-specific growth in the Drosophila eye disc

Notch (N) activation at the dorsoventral (DV) boundary of the Drosophila eye is required for early eye primordium growth. Despite the apparent DV mirror symmetry, some mutations cause a preferential loss of the ventral domain, suggesting that the growth of individual domains is asymmetrically regulated. We show that the Lobe (L) gene is required non-autonomously for ventral growth but not dorsal growth, and that it mediates the proliferative effect of midline N signaling in a ventral-specific manner. L encodes a novel protein with a conserved domain. Loss of L suppresses the overproliferation phenotype of constitutive N activation in the ventral, but not in the dorsal eye, and gain of L rescues ventral tissue loss in N mutant background. Furthermore, L is necessary and sufficient for the ventral expression of a N ligand, Serrate (Ser), which affects ventral growth. Our data suggest that the control of ventral Ser expression by L represents a molecular mechanism that governs asymmetrical eye growth.     

Regulation of ganglion cell production by Notch signaling during retinal development

Although progenitor cells in developing vertebrate retina are capable of producing all retinal cell types, they are competent to produce only certain cell types at a given time, and this competence changes as development progresses. We asked whether a change in progenitor cell competence is primarily responsible for ending production of a specific cell type, the retinal ganglion cell. Reducing Notch expression using an antisense oligonucleotide in vitro or in vivo increased ganglion cell genesis. The antisense treatment could reinitiate ganglion cell genesis after it had terminated in a region of the retina, but only for a brief period. The failure of the Notch antisense treatment to reinitiate ganglion cell production after this period was not due to the lack of receptor or ligand expression, as both Notch-1 and Delta-1 were still expressed. The failure of the Notch antisense treatment to reinitiate ganglion cell production is consistent with the suggestion that the intrinsic competence of progenitor cells changes as development progresses. Because reducing Notch signaling can reinitiate ganglion cell production for a brief period after ganglion cell production has normally ceased, it appears that ganglion cell production initially ends in a region of the retina because of cell-cell interactions and not because progenitor cells lose the competence to make ganglion cells. Notch signaling appears to temporarily prevent production of ganglion cells in a region, while some other signal must initiate a change in progenitor cell competence, thus permanently ending the possibility of further ganglion cell production.     

Patterning by cell recruitment in the Drosophila eye.

Patterning of the retinal epithelium in insects involves cellular interactions. Recent molecular genetic characterization of these interactions in Drosophila and some emerging principles of how cell fate is determined in this system are the subject of this review.     

Phospholipid membrane composition affects EGF receptor and Notch signaling through effects on endocytosis during Drosophila development

The role of phospholipids in the regulation of membrane trafficking and signaling is largely unknown. Phosphatidylcholine (PC) is a main component of the plasma membrane. Mutants in the Drosophila phosphocholine cytidylyltransferase 1 (CCT1), the rate-limiting enzyme in PC biosynthesis, show an altered phospholipid composition with reduced PC and increased phosphatidylinositol (PI) levels. Phenotypic features of dCCT1 indicate that the enzyme is not required for cell survival, but serves a role in endocytic regulation. CCT1- cells show an increase in endocytosis and enlarged endosomal compartments, whereas lysosomal delivery is unchanged. As a consequence, an increase in endocytic localization of EGF receptor (Egfr) and Notch is observed, and this correlates with a reduction in signaling strength and leads to patterning defects. A further link between PC/PI content, endocytosis, and signaling is supported by genetic interactions of dCCT1 with Egfr, Notch, and genes affecting endosomal traffic.     

Inhibition of ERK-MAP kinase signaling by RSK during Drosophila development

Although p90 ribosomal S6 kinase (RSK) is known as an important downstream effector of the ribosomal protein S6 kinase/extracellular signal-regulated kinase (Ras/ERK) pathway, its endogenous role, and precise molecular function remain unclear. Using gain-of-function and null mutants of RSK, its physiological role was successfully characterized in Drosophila. Surprisingly, RSK-null mutants were viable, but exhibited developmental abnormalities related to an enhanced ERK-dependent cellular differentiation such as ectopic photoreceptor- and vein-cell formation. Conversely, overexpression of RSK dramatically suppressed the ERK-dependent differentiation, which was further augmented by mutations in the Ras/ERK pathway. Consistent with these physiological phenotypes, RSK negatively regulated ERK-mediated developmental processes and gene expressions by blocking the nuclear localization of ERK in a kinase activity-independent manner. In addition, we further demonstrated that the RSK-dependent inhibition of ERK nuclear migration is mediated by the physical association between ERK and RSK. Collectively, our study reveals a novel regulatory mechanism of the Ras/ERK pathway by RSK, which negatively regulates ERK activity by acting as a cytoplasmic anchor in Drosophila.     

Mutations modulating the Argos-regulated signaling pathway in Drosophila eye development

Argos is a secreted protein that contains an EGF-like domain and acts as an inhibitor of Drosophila EGF receptor activation. To identify genes that function in the Argos-regulated signaling pathway, we performed a genetic screen for enhancers and suppressors of the eye phenotype caused by the overexpression of argos. As a result, new alleles of known genes encoding components of the EGF receptor pathway, such as Star, sprouty, bulge, and clown, were isolated. To study the role of clown in development, we examined the eye and wing phenotypes of the clown mutants in detail. In the eye discs of clown mutants, the pattern of neuronal differentiation was impaired, showing a phenotype similar to those caused by a gain-of-function EGF receptor mutation and overexpression of secreted Spitz, an activating ligand for the EGF receptor. There was also an increased number of pigment cells in the clown eyes. Epistatic analysis placed clown between argos and Ras1. In addition, we found that clown negatively regulated the development of wing veins. These results suggest that the clown gene product is important for the Argos-mediated inhibition of EGF receptor activation during the development of various tissues. In addition to the known genes, we identified six mutations of novel genes. Genetic characterization of these mutants suggested that they have distinct roles in cell differentiation and/or survival regulated by the EGF receptor pathway.     

Keeping an eye on retinoic acid signaling during eye development

Retinoic acid is a metabolic derivative of vitamin A that plays an essential function in cell-cell signaling by serving as a ligand for nuclear receptors that directly regulate gene expression. The final step in the conversion of retinol to retinoic acid is carried out by three retinaldehyde dehydrogenases encoded by Raldh1 (Aldh1a1), Raldh2 (Aldh1a2), and Raldh3 (Aldh1a3). Mouse Raldh gene knockout studies have been instrumental in understanding the mechanism of retinoic acid action during eye development. Retinoic acid signaling in the developing eye is particularly complex as all three Raldh genes contribute to retinoic acid synthesis in non-overlapping locations. During optic cup formation Raldh2 is first expressed transiently in perioptic mesenchyme, then later Raldh1 and Raldh3 expression begins in the dorsal and ventral retina, respectively, and these sources of retinoic acid are maintained in the fetus. Retinoic acid is not required for dorsoventral patterning of the retina as originally thought, but it is required for morphogenetic movements that form the optic cup, ventral retina, cornea, and eyelids. These findings will help guide future studies designed to identify retinoic acid target genes during eye organogenesis.