Ras1-Mediated Photoreceptor Development in Drosophila

The jing and ras1 pathways are functionally related during CNS midline and tracheal development

The Drosophila jing gene encodes a zinc finger protein required for the differentiation and survival of embryonic CNS midline and tracheal cells. We show that there is a functional relationship between jing and the Egfr pathway in the developing CNS midline and trachea. jing function is required for Egfr pathway gene expression and MAPK activity in both the CNS midline and trachea. jing over-expression effects phenocopy those of the Egfr pathway and require Egfr pathway function. Activation of the Egfr pathway in loss-of-function jing mutants partially rescues midline cell loss. Egfr pathway genes and jing show dominant genetic interactions in the trachea and CNS midline. Together, these results show that jing regulates signal transduction in developing midline and tracheal cells.

A genetic screen for modifiers of a kinase suppressor of Ras-dependent rough eye phenotype in Drosophila

kinase suppressor of Ras (ksr) encodes a putative protein kinase that by genetic criteria appears to function downstream of RAS in multiple receptor tyrosine kinase (RTK) pathways. While biochemical evidence suggests that the role of KSR is closely linked to the signal transduction mechanism of the MAPK cascade, the precise molecular function of KSR remains unresolved. To further elucidate the role of KSR and to identify proteins that may be required for KSR function, we conducted a dominant modifier screen in Drosophila based on a KSR-dependent phenotype. Overexpression of the KSR kinase domain in a subset of cells during Drosophila eye development blocks photoreceptor cell differentiation and results in the external roughening of the adult eye. Therefore, mutations in genes functioning with KSR might modify the KSR-dependent phenotype. We screened approximately 185,000 mutagenized progeny for dominant modifiers of the KSR-dependent rough eye phenotype. A total of 15 complementation groups of Enhancers and four complementation groups of Suppressors were derived. Ten of these complementation groups correspond to mutations in known components of the Ras1 pathway, demonstrating the ability of the screen to specifically identify loci critical for Ras1 signaling and further confirming a role for KSR in Ras1 signaling. In addition, we have identified 4 additional complementation groups. One of them corresponds to the kismet locus, which encodes a putative chromatin remodeling factor. The relevance of these loci with respect to the function of KSR and the Ras1 pathway in general is discussed.

A screen for mutations that prevent lethality caused by expression of activated sevenless and Ras1 in the Drosophila embryo

Ras1 plays a critical role in receptor tyrosine kinase (RTK) signal transduction pathways that function during Drosophila development. We demonstrate that mis-expression of constitutively active forms of Ras1 (Ras1V12) and the Sevenless (Sev) RTK (SevS11) during embryogenesis causes lethality due to inappropriate activation of RTK/Ras1 signaling pathways. Genetic and molecular data indicate that the rate of SevS11/sev-Ras1V12 lethality is sensitive to the expression level of both transgenes. To identify genes that encode components of RTK/Ras1 signaling pathways or modulators of RNA polymerase II transcription, we took advantage of the dose-sensitivity of the system and screened for second site mutations that would dominantly suppress the lethality. The collection of identified suppressors includes the PR55 subunit of Protein Phosphatase 2A indicating that downstream of Sev and Ras1 this subunit acts as a negative regulator of phosphatase activity. The isolation of mutations in the histone deacetylase RPD3 suggests that it functions as positive regulator of sev enhancer-driven transcription. Finally, the isolation of mutations in the Trithorax group gene devenir and the characterized allelism with the Breathless RTK encoding gene provides evidence for Ras1-mediated regulation of homeotic genes.

TAF250 is required for multiple developmental events in Drosophila

The TFIID transcription initiation complex is composed of TBP and multiple TAFs. Studies in unicellular systems indicate that TAF250 is required for progression through G(1)/S of the cell cycle and repression of apoptosis. Here we extend these in vivo studies by determining the developmental requirements for TAF250 in a multicellular organism, Drosophila. TAF250 mutants were isolated in a genetic screen that also yielded TAF60 and TAF110 mutants, indicating that TAFs function coordinately to regulate transcription. Null alleles of TAF250 are recessive larval lethal. However, combinations of weak loss-of-function TAF250 alleles survive to adulthood and reveal requirements for TAF250 during ovary, eye, ocelli, wing, bristle, and terminalia development as well as overall growth of the fly. These phenotypes suggest roles for TAF250 in regulating the cell cycle, cell differentiation, cell proliferation, and cell survival. Finally, molecular analysis of TAF250 mutants reveals that the observed phenotypes are caused by mutations in a central region of TAF250 that is conserved among metazoan organisms. This region is contained within the TAF250 histone acetyltransferase domain, but the mutations do not alter the histone acetyltransferase activity of TAF250 in vitro, indicating that some other aspect of TAF250 function is affected. Because this region is not conserved in the yeast TAF250 homologue, TAF145, it may define an activity for TAF250 that is unique to higher eukaryotes.

PTP-ER, a novel tyrosine phosphatase, functions downstream of Ras1 to downregulate MAP kinase during Drosophila eye development

Activation of ERK/MAPK is a key event downstream of RAS. The duration, extent, and timing of MAPK activity is integral to signal specificity. Consequently, inactivation of MAPK by phosphatases has emerged as a critical element in the precise control of signal output. We have cloned and characterized a novel cytoplasmic protein tyrosine phosphatase, PTP-ER, which is related to mammalian PCPTP1, LC-PTP/HePTP, and STEP tyrosine phosphatases. PTP-ER mutants produce extra R7 cells and enhance activated Ras1 signaling. Ectopic expression of PTP-ER dramatically inhibits RAS1/MAPK signaling. PTP-ER binds to and inactivates Drosophila ERK/MAPK; however, it is unable to dephosphorylate and downregulate Drosophila MAPKSevenmaker. Resistance to PTP-ER activity partially accounts for the Sevenmaker mutant phenotype.

A fly's eye view of biology

Determining how genes function in developmentally complex multicellular organisms can be a formidable task. Obstacles arise from the fact that inactivation of most genes results in subtle or undetectable phenotypic alterations, and when phenotypes are observed they are often difficult to interpret because most genes play multiple roles in development. New techniques that have been applied to studying genes in the developing Drosophila eye promise to circumvent these obstacles. The advent of these techniques combined with the existing wealth of information about cellular pattern formation in the Drosophila eye make the eye a powerful model system for deciphering the function of genes in biological processes.

Ectopic expression of constitutively activated Ral GTPase inhibits cell shape changes during Drosophila eye development

The small GTP-binding protein Ral is activated by RalGDS, one of the effector molecules for Ras. Active Ral binds to a GTPase activating protein for CDC42 and Rac. Although previous studies suggest a role for Ral in the regulation of CDC42 and Rac, which are involved in arranging the cytoskeleton, its in vivo function is largely unknown. To examine the effect of overexpressing Ral on development, transgenic Drosophila were generated that overexpress wild-type or mutated Ral during eye development. While wild-type Ral caused no developmental defects, expression of a constitutively activated protein resulted in a rough eye phenotype. Activated Ral did not affect cell fate determination in the larval eye discs but caused severe disruption of the ommatidial organization later in pupal development. Phalloidin staining showed that activated Ral perturbed the cytoskeletal structure and cell shape changes during pupal development. This phenotype is similar to that caused by RhoA overexpression. In addition, the phenotype was synergistically enhanced by the coexpression of RhoA. These results suggest that Ral functions to control the cytoskeletal structure required for cell shape changes during Drosophila development.

The Drosophila gene hid is a direct molecular target of Ras-dependent survival signaling

Extracellular growth factors are required for the survival of most animal cells. They often signal through the activation of the Ras pathway. However, the molecular mechanisms by which Ras signaling inhibits the intrinsic cell death machinery are not well understood. Here, we present evidence that in Drosophila, activation of the Ras pathway specifically inhibits the proapoptotic activity of the gene head involution defective (hid). By using transgenic animals and cultured cells, we show that MAPK phosphorylation sites in Hid are critical for this response. These findings define a novel mechanism by which growth factor signaling directly inactivates a critical component of the intrinsic cell death machinery. These studies provide further insights into the function of ras as an oncogene.