Cell biology of stress: Cytoplasmic rearrangements and signaling events

Abstract

Recent reports show that formation of COPII coated vesicles and ER exit are regulated by kinases and that more generally, many components of the secretory pathway have been found phosphorylated. However the conditions under which these events occur are so far poorly understood.

As the main aim of this thesis, we asked how the function and organization of the early secretory pathway respond to nutrient stress. In particular, we examine the behavior the components of the early secretory pathway under nutrient restriction, their contribution to other processes that are also regulated by the same stress (such as protein translation arrest and stress granule formation), and the nature of the signaling events regulating these responses.

We follow up on the results of a kinase screen designed to identify regulators of the early secretory pathway and find that the extracellularly regulated kinase 7 (Erk7, also known as MAPK15) mediates the response of the ERES to serum starvation. These findings establish that the early secretory pathway is sensitive to nutrient signals, that pathways sensing nutrient abundance regulate its functional organization and that the key ERES component, Sec16 is the platform that integrates these signals.

These results fuelled the further experiments to examine the behavior of the ERES components upon amino-acid starvation. We find that this response is even more dramatic than the one observed with serum deprivation. Indeed the ERES are remodeled into the Sec bodies, a novel, non-membrane bound, reversible structure that does not support protein transport and acts as a reservoir for ERES components. We find that the formation of Sec bodies is critical to cell survival and re-adaptation to normal growth conditions after the stress is relieved. Our quest to understand the nature of this novel structure reveals that it behaves like a liquid droplet, linking it to the cytoplasmic reorganization that occurs during stress, a well-documented manifestation of which is the assembly of stress granules.

The similarities we observed between the Sec bodies and the stress granules prompted us to investigate the hypothesis that the formation of two structures is somehow linked. Indeed, we find that even though Sec bodies and stress granules are distinct both morphologically and functionally the specific ERES components that are required for Sec body assembly are also necessary for stress granule formation. These observations reveal a so far unexplored link between ER exit and mRNA sorting and turnover.

Furthermore we study the stress granule assembly under a different type of stress, heat exposure, which does not affect the early secretory pathway. We find that TORC2 signaling is required for their formation. Having characterized the heat sensitivity phenotype of Rictor mutant flies we set out to understand the molecular mechanisms of this phenotype. Using S2 cells initially and then confirming our observations in Drosophila tissues we show that during heat stress TORC2 mediates the assembly of stress granules, possibly via its effector, Akt.