A context-dependent bifurcation in the Pointed transcriptional effector network contributes specificity and robustness to retinal cell fate acquisition

• Published in: PLoS genetics Vol. 16; no. 11; p. e1009216
• Main Authors: Wu, Chudong, Boisclair Lachance, Jean-François, Ludwig, Michael Z, Rebay, Ilaria
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 30.11.2020; Public Library of Science (PLoS)
• Subjects: 5' Untranslated Regions; Alanine; Alleles; Animals; Biology and Life Sciences; Cell development (Biology); Cell Differentiation - genetics; Cell division; Cell fate; Cloning; Deoxyribonucleic acid; DNA; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; Drosophila melanogaster; Drosophila Proteins - genetics; Drosophila Proteins - metabolism; Enhancers; Epidermal growth factor receptors; Ets-1 protein; Ets-2 protein; Exons; Gene expression; Gene Expression Regulation, Developmental; Gene Regulatory Networks; Genetic aspects; Insects; Kinases; MAP kinase; MAP Kinase Signaling System - genetics; Medicine and Health Sciences; Models, Animal; Nerve Tissue Proteins - genetics; Nerve Tissue Proteins - metabolism; Phosphorylation; Photoreceptor Cells, Invertebrate - physiology; Photoreceptors; Physiological aspects; Protein Isoforms - genetics; Protein Isoforms - metabolism; Protein-tyrosine kinase receptors; Proteins; Proto-Oncogene Proteins - genetics; Proto-Oncogene Proteins - metabolism; Ras protein; Research and Analysis Methods; Retina; Signal transduction; Social Sciences; Spatio-Temporal Analysis; Threonine; Transcription; Transcription factors; Transcription Factors - genetics; Transcription Factors - metabolism; United States
• ISSN: 1553-7404; 1553-7390; 1553-7404
• DOI: 10.1371/journal.pgen.1009216

Spatiotemporally precise and robust cell fate transitions, which depend on specific signaling cues, are fundamental to the development of appropriately patterned tissues. The fidelity and precision with which photoreceptor fates are recruited in the Drosophila eye exemplifies these principles. The fly eye consists of a highly ordered array of ~750 ommatidia, each of which contains eight distinct photoreceptors, R1-R8, specified sequentially in a precise spatial pattern. Recruitment of R1-R7 fates requires reiterative receptor tyrosine kinase / mitogen activated protein kinase (MAPK) signaling mediated by the transcriptional effector Pointed (Pnt). However the overall signaling levels experienced by R2-R5 cells are distinct from those experienced by R1, R6 and R7. A relay mechanism between two Pnt isoforms initiated by MAPK activation directs the universal transcriptional response. Here we ask how the generic Pnt response is tailored to these two rounds of photoreceptor fate transitions. We find that during R2-R5 specification PntP2 is coexpressed with a closely related but previously uncharacterized isoform, PntP3. Using CRISPR/Cas9-generated isoform specific null alleles we show that under otherwise wild type conditions, R2-R5 fate specification is robust to loss of either PntP2 or PntP3, and that the two activate pntP1 redundantly; however under conditions of reduced MAPK activity, both are required. Mechanistically, our data suggest that intrinsic activity differences between PntP2 and PntP3, combined with positive and unexpected negative transcriptional auto- and cross-regulation, buffer first-round fates against conditions of compromised RTK signaling. In contrast, in a mechanism that may be adaptive to the stronger signaling environment used to specify R1, R6 and R7 fates, the Pnt network resets to a simpler topology in which PntP2 uniquely activates pntP1 and auto-activates its own transcription. We propose that differences in expression patterns, transcriptional activities and regulatory interactions between Pnt isoforms together facilitate context-appropriate cell fate specification in different signaling environments.