Apoptotic mechanisms during competition of ribosomal protein mutant cells: roles of the initiator caspases Dronc and Dream/Strica.

Heterozygosity for mutations in ribosomal protein genes frequently leads to a dominant phenotype of retarded growth and small adult bristles in Drosophila (the Minute phenotype). Cells with Minute genotypes are subject to cell competition, characterized by their selective apoptosis and removal in mosaic tissues that contain wild-type cells. Competitive apoptosis was found to depend on the pro-apoptotic reaper, grim and head involution defective genes but was independent of p53. Rp/+ cells are protected by anti-apoptotic baculovirus p35 expression but lacked the usual hallmarks of 'undead' cells. They lacked Dronc activity, and neither expression of dominant-negative Dronc nor dronc knockdown by dsRNA prevented competitive apoptosis, which also continued in dronc null mutant cells or in the absence of the initiator caspases dredd and dream/strica. Only simultaneous knockdown of dronc and dream/strica by dsRNA was sufficient to protect Rp/+ cells from competition. By contrast, Rp/Rp cells were also protected by baculovirus p35, but Rp/Rp death was dronc-dependent, and undead Rp/Rp cells exhibited typical dronc-dependent expression of Wingless. Independence of p53 and unusual dependence on Dream/Strica distinguish competitive cell death from noncompetitive apoptosis of Rp/Rp cells and from many other examples of cell death.

Cell proliferation, survival, and death in the Drosophila eye.

The Drosophila retina has a precise repeating structure based on the unit eye, or ommatidium. This review summarizes studies of the cell proliferation and survival episodes that affect the number of cells available to make each ommatidium. Late in larval development, as differentiation and patterning begin, the retinal epithelium exhibits striking regulation of the cell cycle including a transient G1 arrest of all cells, followed by a "Second Mitotic Wave" cell cycle that is regulated at the G2/M transition by local intercellular signals. Reiterated episodes of cell death also contribute to precise regulation of retinal cell number. The EGF receptor homolog has multiple roles in retinal proliferation and survival.

Master regulatory genes; telling them what to do.

In 1995, the eyeless (ey) gene was dubbed the "master-regulator" of eye development in Drosophila. Not only is ey required for eye development, but its misexpression can convert many other tissues into eye, including legs, wings and antennae.(1) ey is remarkable for its ability to drive coordinate differentiation of the multiple cell types that have to differentiate in a very precise pattern to construct the fly eye, and for its power to override the previous differentiation programs of many other diverse tissues. Even more remarkable, the ey homolog Pax6 and homologs of other eye determination genes from Drosophila are also required for eye development in vertebrates,(2,3) prompting reassessment of the evolution of vision throughout the animal kingdom.(4,5) Now Kumar and Moses have published a study that throws a new light on ey function in Drosophila.(6) According to their work, ey becomes a master regulator of eye development much later than previously thought, and is regulated by signalling through the Notch and EGFR signaling pathways.

The EGF receptor defines domains of cell cycle progression and survival to regulate cell number in the developing Drosophila eye.

The number of cells in developing organs must be controlled spatially by extracellular signals. Our results show how cell number can be regulated by cell interactions controlling proliferation and survival in local neighborhoods in the case of the Drosophila compound eye. Intercellular signals act during the second mitotic wave, a cell cycle that generates a pool of uncommitted cells used for most ommatidial fates. We find that G1/S progression to start the cell cycle requires EGF receptor inactivity. EGF receptor activation is then required for progression from G2 to M phase of the same cells, and also prevents apoptosis. EGF receptor activation depends on short-range signals from five-cell preclusters of photoreceptor neurons not participating in the second mitotic wave. Through proliferation and survival control, such signals couple the total number of uncommitted cells being generated to the neural patterning of the retina.

Role of the EGFR/Ras/Raf pathway in specification of photoreceptor cells in the Drosophila retina.

The Drosophila EGF receptor is required for differentiation of many cell types during eye development. We have used mosaic analysis with definitive null mutations to analyze the effects of complete absence of EGFR, Ras or Raf proteins during eye development. The Egfr, ras and raf genes are each found to be essential for recruitment of R1-R7 cells. In addition Egfr is autonomously required for MAP kinase activation. EGFR is not essential for R8 cell specification, either alone or redundantly with any other receptor that acts through Ras or Raf, or by activating MAP kinase. As with Egfr, loss of ras or raf perturbs the spacing and arrangement of R8 precursor cells. R8 cell spacing is not affected by loss of argos in posteriorly juxtaposed cells, which rules out a model in which EGFR acts through argos expression to position R8 specification in register between adjacent columns of ommatidia. The R8 spacing role of the EGFR was partially affected by simultaneous deletion of spitz and vein, two ligand genes, but the data suggest that EGFR activation independent of spitz and vein is also involved. The results prove that R8 photoreceptors are specified and positioned by distinct mechanisms from photoreceptors R1-R7.

Proneural enhancement by Notch overcomes Suppressor-of-Hairless repressor function in the developing Drosophila eye.

BACKGROUND: The receptor protein Notch plays a conserved role in restricting neural-fate specification during lateral inhibition. Lateral inhibition requires the Notch intracellular domain to coactivate Su(H)-mediated transcription of the Enhancer-of-split Complex. During Drosophila eye development, Notch plays an additional role in promoting neural fate independently of Su(H) and E(spl)-C, and this finding suggests an alternative mechanism of Notch signal transduction. RESULTS: We used genetic mosaics to analyze the proneural enhancement pathway. As in lateral inhibition, the metalloprotease Kuzbanian, the EGF repeat 12 region of the Notch extracellular domain, Presenilin, and the Notch intracellular domain were required. By contrast, proneural enhancement became constitutive in the absence of Su(H), and this led to premature differentiation and upregulation of the Atonal and Senseless proteins. Ectopic Notch signaling by Delta expression ahead of the morphogenetic furrow also caused premature differentiation. CONCLUSIONS: Proneural enhancement and lateral inhibition use similar ligand binding and receptor processing but differ in the nuclear role of Su(H). Prior to Notch signaling, Su(H) represses neural development directly, not indirectly through E(spl)-C. During proneural enhancement, the Notch intracellular domain overcomes the repression of neural differentiation. Later, lateral inhibition restores the repression of neural development by a different mechanism, requiring E(spl)-C transcription. Thus, Notch restricts neurogenesis temporally to a narrow time interval between two modes of repression.

The scabrous protein can act as an extracellular antagonist of notch signaling in the Drosophila wing.

Notch (N) is a receptor for signals that inhibit neural precursor specification [1-6]. As N and its ligand Delta (DI) are expressed homogeneously, other molecules may be differentially expressed or active to permit neural precursor cells to arise intermingled with nonneural cells [7,8]. During Drosophila wing development, the glycosyltransferase encoded by the gene fringe (fng) promotes N signaling in response to DI, but inhibits N signaling in response to Serrate (Ser), which encodes a ligand that is structurally similar to DI. Dorsal expression of Fng protein localizes N signaling to the dorsoventral (DV) wing margin [9-11]. The secreted protein Scabrous (Sca) is a candidate for modulation of N in neural cells. Mutations at the scabrous (sca) locus alter the locations where precursor cells form in the peripheral nervous system [12,13]. Unlike fringe, sca mutations act cell non-autonomously [12]. Here, we report that targeted misexpression of Sca during wing development inhibited N signaling, blocking expression of all N target genes. Sca reduced N activation in response to DI more than in response to Ser. Ligand-independent signaling by overexpression of N protein, or by expression of activated truncated N molecules, was not inhibited by Sca. Our results indicate that Sca can act on N to reduce its availability for paracrine and autocrine interactions with DI and Ser, and can act as an antagonist of N signaling.

Notch signaling in the nervous system. Pieces still missing from the puzzle.

Notch has been known for many years as a receptor for inhibitory signals that shapes the pattern of the nervous system during its development. Genes in the Notch pathway function to prevent neural determination so that only a subset of the available ectodermal cells become neural precursors. The localization of Notch signaling is crucial for determining where neural precursor cells arise on a cell-by-cell basis. The unresolved problem is that studies of the expression of Notch protein and its ligands are inconsistent with the pattern of neurogenesis. During neural cell fate specification, distributions of Notch protein and of its ligand Delta appear uniform. Under the reigning paradigm, such widespread expression should lead to N signal transduction in all cells and thereby prevent any neural specification. Yet, contrary to this expectation, neural elements still form, in characteristic patterns, hence, Notch signal transduction must have been inactive in the precursor cells. The mechanism preventing Notch signaling in certain cells must be posttranslational but it has not yet been identified. This review will outline the experimental evidence supporting this view of Notch signaling, and briefly evaluate some of the possible mechanisms that have been suggested.

Several levels of EGF receptor signaling during photoreceptor specification in wild-type, Ellipse, and null mutant Drosophila.

Dominant Ellipse mutant alleles of the Drosophila EGF receptor homologue (DER) dramatically suppress ommatidium development in the eye and induce ectopic vein development in the wing. Their phenotype suggests a possible role for DER in specifying the founder R8 photoreceptor cells for each ommatidium. Here we analyze the basis of Ellipse mutations and use them to probe the role of DER in eye development. We show that Elp mutations result from a single amino acid substitution in the kinase domain which activates tyrosine kinase activity and MAP kinase activation in tissue culture cells. Transformant studies confirmed that the mutation is hypermorphic in vivo, but the DER function was elevated less than by ectopic expression of the ligand spitz. Ectopic spi promoted photoreceptor differentiation, even in the absence of R8 cells. Pathways downstream of DER activation were assessed to explore the basis of these distinct outcomes. Elp mutations caused overexpression of the Notch target gene E(spl) mdelta and required function of Notch to suppress ommatidium formation. The Elp phenotype also depended on the secreted protein argos and was reverted in Elp aos double mutants. Complete loss of DER function in clones of null mutant cells led to delay in R8 specification and subsequently to loss of mutant cells. The DER null phenotype was distinct from that of either spitz or vein mutants, suggesting that a combination of these or other ligands was required for aspects of DER function. In normal development DER protein was expressed in most retinal cells, but at distinct levels. We used an antibody specific for diphospho-ERK as well as expression of the DER target gene argos to assess the pattern of DER activity, finding highest activity in the intermediate groups of cells in the morphogenetic furrow. However, studies of mutant genotypes suggested that this activity may not be required for normal ommatidium development. Since we saw distinct phenotypic effects of four different levels of DER activity associated with wild-type, null mutant, Elp mutant, or fully activated DER function, we propose that multiple thresholds separate several aspects of DER function. These include activation of N signaling to repress R8 specification, turning on argos expression, and recruiting photoreceptors R1-R7. It is possible that during normal eye development these thresholds are attained by different cells, contributing to the pattern of retinal differentiation.

Functional analysis of the fibrinogen-related scabrous gene from Drosophila melanogaster identifies potential effector and stimulatory protein domains.

The scabrous (sca) gene encodes a secreted dimeric glycoprotein with putative coiled-coil domains N-terminally and a C-terminal region related to the blood clot protein fibrinogen. Homozygous sca mutants have extra bristle organs and rough eyes. We describe a GAL4-based expression system for testing rescue of the sca mutant phenotype by altered SCA proteins and for misexpression. We find that deletion of the fibrinogen-related domain (FReD) greatly decreases SCA function, confirming the importance of this conserved region. SCA function could not be restored by FReDs from human fibrinogen chain genes. However, proteins lacking any FReD still showed some function in both rescue and misexpression experiments, suggesting that putative effector-binding regions lie outside this domain. Consistent with this, proteins expressing only the FReD had no rescuing activity but were recessive negative; i.e., they enhanced the phenotype of sca mutations but had no phenotype in the presence of a wild-type sca allele. This suggests that the FReD contributes to SCA function by binding to other components of the bristle determination pathway, increasing the activity of the linked N-terminal region.

The R8-photoreceptor equivalence group in Drosophila: fate choice precedes regulated Delta transcription and is independent of Notch gene dose.

It has been suggested that lateral specification of cell fate by Notch signaling depends on feedback on Notch (N) and Delta (Dl) transcription to establish reciprocal distributions of the receptor and its ligand at the protein level. In Drosophila neurogenesis the predicted reciprocal protein distributions have not been observed. Either this model of lateral specification or the description of N and/or Dl protein distributions must be incomplete. We have reexamined R8 photoreceptor specification in the developing eye to resolve this question for this example of lateral specification. N and Dl protein levels were assessed in the cell as a whole and at the cell surface, where these proteins were mostly found at the intercellular cell junctions. Protein levels did not correspond to Notch signaling in wild type. However, Dl transcription and protein levels did correlate with altered N signaling in mutant genotypes. Our findings suggest the difference relates to the speed of lateral specification in vivo. The time required for N signaling to inhibit ato expression was at most 90 min, but changes in the Dl protein distribution in mutant genotypes arose more slowly. N expression was little regulated by N signaling, but protein encoded by the Nts1 allele was temperature-sensitive for appearance at the cell surface. Some aspects of the pattern of Dl protein appeared to be due to endocytosis. We conclude that feedback of N signaling on Dl transcription does occur but is too slow to account for the pattern of R8 specification. Studies of ommatidia mosaic for a Notch duplication, or for the Nts1 allele at semi-restrictive temperatures, found that cells beginning with less N activity were not necessarily predisposed to be selected for R8 differentiation. Our data argue that other signals may be responsible for the pattern of R8 cell fate allocation by N. Potential relevance to other neurogenic regions is discussed.

A subset of notch functions during Drosophila eye development require Su(H) and the E(spl) gene complex.

The Notch signalling pathway is involved in many processes where cell fate is decided. Previous work showed that Notch is required at successive steps during R8 specification in the Drosophila eye. Initially, Notch enhances atonal expression and promotes atonal function. After atonal autoregulation has been established, Notch signalling represses atonal expression during lateral specification. In this paper we investigate which known components of the Notch pathway are involved in each signalling process. Using clonal analysis we show that a ligand of Notch, Delta, is required along with Notch for both proneural enhancement and lateral specification, while the downstream components Suppressor-of-Hairless and Enhancer-of-Split are involved only in lateral specification. Our data point to a distinct signal transduction pathway during proneural enhancement by Notch. Using misexpression experiments we also show that particular Enhancer-of-split bHLH genes can differ greatly in their contribution to lateral specification.

Molecular characterization of a novel A kinase anchor protein from Drosophila melanogaster.

Activation of protein kinase A (PKA) at discrete intracellular sites facilitates oogenesis and development in Drosophila. Thus, PKA-anchor protein complexes may be involved in controlling these crucial biological processes. Evaluation of this proposition requires knowledge of PKA binding/targeting proteins in the fly. We now report the discovery and characterization of cDNAs encoding a novel, Drosophila A kinase anchor protein, DAKAP550. DAKAP550 is a large (>2300 amino acids) acidic protein that is maximally expressed in anterior tissues. It binds regulatory subunits (RII) of both mammalian and Drosophila PKAII isoforms. The tethering region of DAKAP550 includes two proximal, but non-contiguous RII-binding sites (B1 and B2). The B1 domain (residues 1406-1425) binds RII approximately 20-fold more avidly than B2 (amino acids 1350-1369). Affinity-purified anti-DAKAP550 IgGs were exploited to demonstrate that the anchor protein is expressed in many cells in nearly all tissues throughout the lifespan of the fly. However, DAKAP550 is highly enriched and asymmetrically positioned in subpopulations of neurons and in apical portions of cells in gut and trachea. The combination of RII (PKAII) binding activity with differential expression and polarized localization is consistent with a role for DAKAP550 in creating target loci for the reception of signals carried by cAMP. The DAKAP550 gene was mapped to the 4F1.2 region of the X chromosome; flies that carry a deletion for this portion of the X chromosome lack DAKAP550 protein.

Proneural function of neurogenic genes in the developing Drosophila eye.

BACKGROUND: . Intercellular signals are major determinants of cell fate during development. Certain signals and receptors are important for many different cell-fate decisions, suggesting that cellular responses to similar signals change during development. Few transitions between such distinct cellular responses have been studied. The Drosophila genes Notch and hedgehog function during intracellular signaling at various stages of development. In the specific case of development of the Drosophila eye, expression of the proneural gene atonal is induced in response to Hedgehog signaling and then becomes subject to autoregulation. The receptor protein Notch has previously been reported to function in the selection of single founder photoreceptor cells (R8 cells) by inhibiting atonal expression. On this basis, complete elimination of Notch gene function would be expected to cause neural hyperplasia in the eye. RESULTS: . Contrary to expectation, we detect a reduction in neural differentiation both in cells expressing a conditional Notch allele and in those lacking expression of either Notch or its ligand Delta. We show here that Notch signaling acts after the initial Hedgehog-driven expression of atonal to enhance proneural competence of the atonal-expressing cells and also to terminate their response to the Hedgehog signals. This occurs before the Notch-induced lateral inhibition of atonal expression within the same cells. CONCLUSION: . Notch has sequentially opposite effects on the same cells, by first promoting and then inhibiting proneural gene function. This apparently paradoxical sequence of events has two possible consequences. Firstly, coupling of alternative cellular responses to the same receptor may prevent them from occurring simultaneously. Secondly, consecutive regulatory processes become temporally coupled, so that these events follow on from each other, without gaps or overlaps.

Evolution of proneural atonal expression during distinct regulatory phases in the developing Drosophila eye.

BACKGROUND: Receptors of the Notch family affect the determination of many cell types. In the Drosophila eye, Notch antagonises the basic helix-loop-helix (bHLH) protein atonal, which is required for R8 photoreceptor determination. Similar antagonism between Notch and proneural bHLH proteins regulates most neural cell determination, however, it is uncertain whether the mechanisms are similar in all cases. Here, we have analyzed the sensitivity of atonal expression to Notch signalling using a temperature-sensitive Notch allele, by the expression of activated Notch or of the ligand Serrate, and by monitoring expression of the atonal-dependant gene scabrous and of the Notch-dependent Enhancer of split genes. RESULTS: The atonal expression pattern evolves from general "prepattern' expression, through transient "intermediate groups' to R8 precursor-specific expression. Successive phases of atonal expression differ in sensitivity to Notch. Prepattern expression of atonal is not inhibited. Inhibition begins at the intermediate group stage, corresponding to the period when atonal gene function is required for its own expression. At the transition to R8 cell-specific expression, Notch is activated in all intermediate group cells except the R8 cell precursor. R8 cells remain sensitive to inhibition in columns 0 and 1, but become less sensitive thereafter; non-R8 cells do not require Notch activity to keep atonal expression inactive. Thus, Notch signaling is coupled to atonal repression for only part of the atonal expression pattern. Accordingly, the Enhancer-of-split m delta protein is expressed reciprocally to atonal at the intermediate group and early R8 stages, but is expressed in other patterns before and after. CONCLUSIONS: In eye development, inhibition by Notch activity is restricted to specific phases of proneural gene expression, beginning when prepattern decays and is replaced by autoregulation. We suggest that Notch signalling inhibits atonal autoregulation, but not expression by other mechanisms, and that a transition from prepattern to autoregulation is necessary for patterning neural cell determination. Distinct neural tissues might differ in their proneural prepatterns, but use Notch in a similar mechanism.

gp300sca is not a high affinity Notch ligand.

The Notch protooncogene encodes a receptor important for determination and differentiation of many cell types, and is conserved between vertebrates and invertebrates. It has been suggested that the secreted protein scabrous (sca) might be a Notch ligand acting in the peripheral nervous system. The sca protein was purified and a cell line expressing 18,000 Notch molecules per cell surface used to test sca binding by coimmunoprecipitation, cell adhesion assays, and binding with labeled sca. No interaction was detected between gp300sca and Notch or the related protein Delta, suggesting that sca acts through a distinct mechanism.

The scabrous gene encodes a secreted glycoprotein dimer and regulates proneural development in Drosophila eyes.

R8 photoreceptor cells play a primary role in the differentiation of Drosophila eyes. In scabrous (sca) mutants, the pattern of R8 photoreceptor differentiation is altered. The sca gene is predicted to encode a secreted protein related in part to fibrinogen and tenascins. Using expression in Drosophila Schneider cells, we showed that sca encoded a dimeric glycoprotein which was secreted and found in soluble form in the tissue culture medium. The sca protein contained both N- and O-linked carbohydrates and interacted with heparin. This Schneider cell protein was similar to protein detected in embryos. We showed that sca mutations, along with conditional alleles of Notch (N) and Delta (Dl), each affected the pattern of cells expressing atonal (ato), the proneural gene required for R8 differentiation. In normal development, about 1 cell in 20 differentiates into an R8 cell; in the others, ato is repressed. N and Dl were required to repress ato in the vicinity of R8 cells, whereas sca had effects over several cell diameters. Certain antibodies detected uptake of sca protein several cells away from its source. The overall growth factor-like structure of sca protein, its solubility, and its range of effects in vivo are consistent with a diffusible role that complements mechanisms involving direct cell contact. We propose that as the morphogenic furrow advances, cell secreting sca protein control the pattern of the next ommatidial column.

Molecular analysis of scabrous mutant alleles from Drosophila melanogaster indicates a secreted protein with two functional domains.

Mutations at the scabrous locus (sca) affect cell-cell signaling during neural development. Twenty-one mutant alleles of scabrous have been analyzed. Many synthesize no sca protein. In others, a defective protein is arrested intracellularly. Two mutants in which protein is not arrested must affect sca protein function outside the cell. Both affect the fibrinogen related domain (FReD), a 200-amino acid segment conserved in fibrinogen, tenascins, and other proteins. In fibrinogen, this region is involved in protein interactions and is altered in human mutations affecting blood clotting. In sca(UM2), an invariant Asp residue is replaced by Asn. In sca(MSKF) allele has dominant negative properties, indicating that the truncated amino-terminal portion interferes with the function of so me other gene product. These mutations show that the conserved FReD is essential for wild-type sca function, but suggest that the amino-terminal domain also interacts with other proteins, but other neural mutations were without effect. Models for the role of a two-domain protein in neural development are discussed.

Characterization of the isolated cAMP-binding B domain of cAMP-dependent protein kinase.

A 14.4-kDa cAMP-binding fragment was generated during bacterial expression and purification of recombinant bovine cAMP-dependent protein kinase type I alpha regulatory subunit (RI alpha). The full-length RI alpha from which the fragment was derived contained a point mutation allowing its B domain to bind both cAMP and cGMP with high affinity while leaving its A domain highly cAMP selective. The NH2 terminus of the fragment was Ser-252, indicating that it encompassed the entire predicted B domain. Although the [3H]cAMP and [3H]cGMP exchange rates of the isolated B domain were increased relative to the B domain in intact RI alpha, the [3H]cAMP exchange rate was comparable to that of the B domain of full-length RI alpha containing an unoccupied A domain. A plasmid encoding only the isolated B domain was overexpressed in Escherichia coli, and a monomeric form of the B domain was purified that had identical properties to the proteolytically generated fragment, indicating that all of the elements for the high-affinity cAMP-binding B domain are contained within the 128 amino acid carboxyl terminus of the R subunit. Prolonged induction of the B domain in E. coli or storage of the purified protein resulted in the formation of a dimer that could be reverted to the monomer by incubation in 2-mercaptoethanol. Dimerization caused an approximate fivefold increase in the rate of cyclic nucleotide exchange relative to the monomer. The results show that an isolated cAMP-binding domain can function independently of any other domain structures of the R subunit.

Drosophila eye development: Notch and Delta amplify a neurogenic pattern conferred on the morphogenetic furrow by scabrous.

Loss of function mutations of scabrous and conditional alleles of Notch and Delta affect the pattern of morphogenetic furrow development. By studying differentiation of R8 cells, the first photoreceptor neuron subtype to differentiate, we show that all furrow cells pass through an R8-competent stage. Function of Notch and scabrous is necessary if most of these cells are to attain other cell fates. The scabrous gene confers a regular pattern on the morphogenetic furrow, restricting R8 differentiation to alternating groups of cells. Notch and Delta function to restrict the R8 fate to a single cell in each group. Without scabrous gene function, action of Notch and Delta on the entire morphogenetic furrow results in a disorganised pattern of ommatidia arising from a disorganised array of single R8 cells. Aspects of the scabrous mutant phenotype also suggest a secondary role in selecting a single R8 cell from competent clusters. We show that scabrous expression preceeds changes in the apical profiles of morphogenetic furrow cells that identify ommatidial precurf1p4cells, and also preceeds changes in levels of Notch and Delta expression. The pattern of initiation of sca expression depends on sca gene function, indicating that patterning of the morphogenetic furrow depends on the pattern of posterior columns. Our results suggest that in the eye, Notch and Delta amplify and refine a morphogenetic landscape generated by scabrous. Cell determination in other tissues and organisms might also be molded in a two-step process where initial inhomogeneities determined by one protein provide a context for subsequent development.