NuclER-QC

Cellular homeostasis is maintained by quality control (QC) pathways that counteract the potentially deleterious effects of intra- or extracellular stresses. Although much is known about the signaling pathways that control individual physiological responses to stress, our understanding of how signaling from different organelles/cellular compartments is integrated into a single cellular network that coordinates a multi-faceted response is still in its infancy. This project aims at unraveling the intricate relationships between a novel yeast nuclear mRNA biogenesis/export QC pathway mediated by the chromatin remodeler Isw1, and two endoplasmic reticulum (ER) stress response pathways, the UPR (Unfolded protein response) and the ERSU (ER Surveillance). The UPR is a transcriptional response that allows to regulate the protein-folding capacity of the ER according to cellular demand. The ERSU monitors the ER status to ensure the transmission of a functional organelle to the progeny. My preliminary data indicate that Isw1, which selectively retains immature messenger ribonucleoparticles (mRNPs) in the nucleus, directly interacts with the mRNA of HAC1, the pivotal ER stress-induced transcription factor of the UPR. Conversely, ISW1 was identified as an UPR transcriptional target, suggesting a previously unrecognized level of communication between the UPR and nuclear mRNA QC. In addition, while cells deleted for ISW1 do not present any discernable phenotype in standard growth conditions, they show an increased sensitivity to ER stress inducing agents. Altogether these results indicate a crucial but overlooked role for Isw1 as a novel mediator of unprecedented cross-talks between nuclear and ER- based QC pathways. In the first axis of this project, we wish to investigate the mechanisms and the cellular consequences of the reciprocal interactions between the Isw1-mediated nuclear mRNP QC pathway and the UPR, asking in particular: (i) How Isw1-dependent mRNA retention regulates the UPR response? (ii) How Isw1 influences the transcriptional output of UPR (iii) How UPR activation in turn controls the expression and the activity of Isw1? In addition, my preliminary studies show that the Mitogen-Activated Protein Kinase Slt2 which controls the ERSU, inhibits nuclear export of bulk poly(A) RNA upon ER stress, hence suggesting it is yet another facet of the cellular ER stress response. This observation is reminiscent of the heat shock (HS) response under which Heat Shock Protein (HSP)-encoding mRNAs are efficiently exported, favoring chaperone production, while poly(A) non-HSP mRNAs are specifically retained within the nucleus. Therefore, the second axis of this proposal will aim at deciphering the molecular mechanism underlying the ER stress-induced and Slt2-driven nuclear poly(A) RNA retention and investigating the potential influence of Slt2 on the Isw1/UPR cross-talk, which will be achieved by: (i) Investigating differential mRNA export upon ER stress (ii) Investigating the molecular mechanisms responsible for the differential nuclear retention of non-ER stress induced transcripts upon ER stress and evaluating their dependency to Slt2. Using a combination of biochemical, genetic, genome-wide and microscopy approaches, this project intends to dissect out the reciprocal signaling between Isw1, the HAC1 mRNA and the nuclear poly(A) retention-mediating kinase Slt2. Although this project is ambitious, we believe that we are in a unique position to successfully achieve its goals, due to our strong preliminary data, our deep knowledge of these QC pathways, the immediate availability of the necessary tools and our strong collaborative network. Altogether, it should allow to establish a new paradigm of nuclear mRNA and endoplasmic reticulum QC pathway crosstalk and to unravel its biological basis.