rst Transcriptional Activity Influences kirre mRNA Concentration in the Drosophila Pupal Retina during the Final Steps of Ommatidial Patterning

Background: Drosophila retinal architecture is laid down between 24-48 hours after puparium formation, when some of the still uncommitted interommatidial cells (IOCs) are recruited to become secondary and tertiary pigment cells while the remaining ones undergo apoptosis. This choice between survival and death requires the product of the roughest (rst) gene, an immunoglobulin superfamily transmembrane glycoprotein involved in a wide range of developmental processes. Both temporal misexpression of Rst and truncation of the protein intracytoplasmic domain, lead to severe defects in which IOCs either remain mostly undifferentiated and die late and erratically or, instead, differentiate into extra pigment cells. Intriguingly, mutants not expressing wild type protein often have normal or very mild rough eyes. Methodology/Principal Findings: By using quantitative real time PCR to examine rst transcriptional dynamics in the pupal retina, both in wild type and mutant alleles we showed that tightly regulated temporal changes in rst transcriptional rate underlie its proper function during the final steps of eye patterning. Furthermore we demonstrated that the unexpected wild type eye phenotype of mutants with low or no rst expression correlates with an upregulation in the mRNA levels of the rst paralogue kin-of-irre (kirre), which seems able to substitute for rst function in this process, similarly to their role in myoblast fusion. This compensatory upregulation of kirre mRNA levels could be directly induced in wild type pupa upon RNAi-mediated silencing of rst, indicating that expression of both genes is also coordinately regulated in physiological conditions. Conclusions/Significance: These findings suggest a general mechanism by which rst and kirre expression could be fine tuned to optimize their redundant roles during development and provide a clearer picture of how the specification of survival and apoptotic fates by differential cell adhesion during the final steps of retinal morphogenesis in insects are controlled at the transcriptional level

"In vivo" toxicity of a truncated version of the Drosophila Rst-IrreC protein is dependent on the presence of a glutamine-rich region in its intracellular domain

The roughest-irregular chiasm C ( rst-irreC) gene of Drosophila melanogaster encodes a transmembrane glycoprotein containing five immunoglobulin-like domains in its extracellular portion and an intracytoplasmic tail rich in serine and threonine as well some conserved motifs suggesting signal transduction activity. In the compound eye, loss-of-function rst-irreC mutants lack the characteristic wave of programmed cell death happening in early pupa and which is essential for the elimination of the surplus interommatidial cells. Here we report an investigation on the role played by the Rst-irreC molecule in triggering programmed cell death. "In vivo" transient expression assays showed that deletion of the last 80 amino acids of the carboxyl terminus produces a form of the protein that is highly toxic to larvae. This toxicity is suppressed if an additional 47 amino acid long, glutamine-rich region ("opa-like domain"), is also removed from the protein. The results suggest the possibility that the opa-like domain and the carboxyl terminus act in concert to modulate rst-irreC function in apoptosis, and we discuss this implication in the context of the general mechanisms causing glutamine-rich neurodegenerative diseases in humans.<br>O gene roughest-irregular chiasm C ( rst-irreC) de Drosophila melanogaster, codifica uma glicoproteína transmembranar contendo cinco domínios semelhantes a imunoglobulina em sua porção extracelular e uma cauda intracitoplasmática rica em serina e treonina, assim como alguns sequências conservadas que sugerem atividade transdutora de sinais. No olho composto, mutantes rst-irreC de perda de função não apresentam uma característica "onda" de morte celular programada que ocorre no início do período pupal e que é essencial para a eliminação de células interomatidiais em excesso. Aqui descrevemos uma investigação sobre o papel desempenhado pela molécula Rst-IrreC no disparo da morte celular programada. Ensaios de expressão transiente "in vivo" demonstraram que a remoção dos 80 aminoácidos finais da porção carboxil terminal produz uma isoforma da proteína que é extremamente tóxica para as larvas. Esta toxicidade é suprimida se 47 aminoácidos adicionais, contendo uma região rica em glutamina (região "opa-like"), forem também removidos da proteína. Os resultados sugerem a possibilidade de que o domínio "opa-like" e o terminal carboxila ajam em conjunto para modular a função de rst-irreC na apoptose, e esta implicação é discutida dentro do contexto dos mecanismos gerais causadores de doenças neurodegenerativas associados a proteínas ricas em glutamina em humanos