JCJC - Jeunes chercheuses et jeunes chercheurs

Remodeling of cellular machineries during quiescence – Quiescent Tools

Submission summary

Most cells spend the majority of their life in quiescence, a state defined as a temporary and reversible absence of proliferation. Establishing quiescence and being able to re-enter the proliferation cycle must be tightly orchestrated to avoid pathological cell proliferation. Hence, deciphering the mechanisms involved in transitions from proliferation to quiescence is an exciting emergent research field. To understand better this cellular state, we have taken an original approach and chosen to characterize the dynamic remodelling of cellular structures associated with quiescence in S. cerevisiae. In this organism, proliferation primarily relies on the availability of nutrients and consequently, entry into- and exit from quiescence can be easily handled. Recently, we have described a new actin cytoskeleton organization, named Actin Bodies, which is specific of quiescent cells. Actin Bodies are small and compact actin filaments-containing structures. Remarkably, by contrast to all other know actin filaments-containing structures, Actin Bodies do not display any detectable actin filaments turn-over. When cells are re-fed, Actin Bodies quickly disassemble and give rise to actin structures that are specific of proliferative cells. Several lines of evidence have led us to propose that Actin Bodies serve as an actin reserve immediately available for cells re-entering the proliferation cycle. Further, we have uncovered the specific reorganization of another cellular machine: the proteasome. Indeed, upon entry into quiescence, we have shown that the proteasome is specifically re-localized form the nucleus into cytoplamic structures that we have named Proteasome Storage Granules (PSGs). PSGs rapidly and fully disassemble upon exit from quiescence and proteasome subunits subsequently re-localize into the nucleus. Importantly, this nuclear re-localization occurs even in the absence of de novo protein synthesis, implying that PSGs are storage structures. Both PSGs formation and nuclear re-localization were observed in the evolutionary distant yeast S. pombe, demonstrating the strong conservation and the broad significance of this phenomenon. The discovery of the specific remodelling of two cellular machineries in yeast quiescent cells not only establish that quiescent cells are not simply G1 resting cells but also demonstrate they are ultrastructurally committed to the quiescent state. Our project is built around three research axes. First we will focus on deciphering the molecular mechanisms and the signaling pathways involved in formation, maintenance and dissociation of the Actin Bodies. Our aim is to uncover the cellular function of such an actin reserve. This quiescent machine displays the unique property of containing stable actin filaments. As such is a very exciting tool that gives the original possibility for deciphering at the molecular level, the network of ABPs interactions that lead to the formation of a specific actin structure. Further, its unusual stability may allow, for the first time, the purification of a native actin structure. Our second axis of research will be devoted to the characterization of the PSGs. Using state-of-the-art cell biology and biochemistry approaches, we intend to characterize the composition of PSGs. Further, we aim to uncover the activity of the proteasome subunits embedded into PSGs. Finally, we will test the requirement of PSGs and its activity for quiescent cell survival and ability to re-enter the proliferative cycle. The need for the activity of the proteasome in quiescent cells is an important issue. Indeed, it has implications in both the comprehension of ageing, with its potential requirement for the degradation of damaged (oxidized) proteins, and the control of cell proliferation, with its critical role in cell cycle progression. Actin Bodies and PSGs are the first unambiguous markers of yeast quiescence. In the third part of our project, we will use these two quiescent machines as innovative tools to decipher a potential 'quiescence program'. Indeed, a major unsolved problem is to evaluate the respective weight of two intricate processes: (1) changes in metabolic status due to nutrient limitation and (2) alteration of the gene expression program. While it seems likely that both processes contribute to the proper establishment of quiescence, our goal is to evaluate their contribution to the remodeling of cellular structures upon entry into quiescence. Due to the evolutionary conservation of basic molecular mechanisms in eukaryotes, the discoveries made in yeast could potentially be transposed to more complex biological systems. Therefore, understanding quiescence in yeast could be relevant for major medical challenges such as cell survival upon cellular ageing or the control of pathological cell proliferation.

Project coordination

Isabelle SAGOT (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 199,000 euros
Beginning and duration of the scientific project: - 48 Months

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