506A Poster - 06. Regulation of gene expression
Thursday April 07, 2:00 PM - 4:00 PM

Authors:
Suzy SJ Hur ¹; Sebastian Bernasek ²; Nicolás Peláez ³; Richard W. Carthew ⁴; Ilaria Rebay ¹

Affiliations:
1) Ben May Department for Cancer Research, University of Chicago, Chicago, IL; 2) Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL; 3) California Institute of Technology, Pasadena, CA 91125, USA. Hanna H. Gray Fellow of the Howard Hughes Medical Institute (HHMI); 4) Department of Molecular Biosciences, Northwestern University, Evanston, IL

Keywords:
o. canalization/robustness; i. receptor tyrosine kinase/phosphatase

During animal development, a highly organized pattern of tissue-specialized cell fates emerges from a population of initially equivalent progenitor cells. This is mediated by transcription factors (TFs) acting downstream of spatially localized extracellular signals to produce spatially refined expression of fate determining genes. Each gene is transcribed in discontinuous bursts with gene-specific temporal dynamics. How TFs control the mRNA expression dynamics of their targets, and how these dynamics in turn contribute to the specificity and robustness of cell fate transitions remains an important question in developmental biology.
The Drosophila eye development is an ideal system to address this question. In the developing eye disc, a precise complement of eight photoreceptor neuron fates (R1-R8) are specified in each ommatidium from a population of multipotent progenitor cells through the reiterative use of RTK/MAPK signaling. Transition from a progenitor to a specified state is mediated by an antagonistic pair of TFs, Pointed and Yan, which compete for the same binding site and exert opposing functions downstream of RTK/MAPK signaling. Yan represses gene expression to maintain cells in a progenitor state while Pointed activates the expression of distinct photoreceptor fate genes to trigger the transition to a specified state.
We found that increased Pointed-to-Yan ratio in the nucleus, rather than the absolute concentration of either TF, promotes the transition to a specified state. Focusing on the R3/R4 outer and on R7 inner photoreceptor fates, we show that genetic perturbations that elevate the ratio prematurely in the progenitors induce ectopic fate transitions while perturbations that prevent the ratio increase bias cells to remain in the progenitor state. By combining immunostaining, single-molecule RNA fluorescent in-situ hybridization (smFISH), and quantitative imaging, we will simultaneously measure the protein levels of Pnt and Yan and nascent mRNA levels and bursting frequency of the R3/R4 gene salm and the R7 gene pros in individual cells that remain in the progenitor state vs. those entering the specified R3/R4 and R7 states. By comparing tissues with wild type and altered Pnt and Yan levels, we will present our latest mechanistic models for how distinct mRNA transcription dynamics downstream of Pnt and Yan direct precise and reliable tissue patterning.