Microtubule reorganization and quiescence: an intertwined relationship.

Quiescence is operationally defined as a reversible proliferation arrest. This cellular state is central for both organism development and homeostasis, its dysregulation causing many pathologies. The quiescent state encompasses very diverse cellular situations depending on the cell type and its environment.

A stable microtubule bundle formed through an orchestrated multistep process controls quiescence exit.

Cells fine-tune microtubule assembly in both space and time to give rise to distinct edifices with specific cellular functions. In proliferating cells, microtubules are highly dynamics, and proliferation cessation often leads to their stabilization. One of the most stable microtubule structures identified to date is the nuclear bundle assembled in quiescent yeast.

Quiescence Through the Prism of Evolution.

Being able to reproduce and survive is fundamental to all forms of life. In primitive unicellular organisms, the emergence of quiescence as a reversible proliferation arrest has most likely improved cell survival under unfavorable environmental conditions. During evolution, with the repeated appearances of multicellularity, several aspects of unicellular quiescence were conserved while new quiescent cell intrinsic abilities arose.

Monitoring single-cell dynamics of entry into quiescence during an unperturbed life cycle.

The life cycle of microorganisms is associated with dynamic metabolic transitions and complex cellular responses. In yeast, how metabolic signals control the progressive choreography of structural reorganizations observed in quiescent cells during a natural life cycle remains unclear. We have developed an integrated microfluidic device to address this question, enabling continuous single-cell tracking in a batch culture experiencing unperturbed nutrient exhaustion to unravel the coordination between metabolic and structural transitions within cells.

Quiescence, an individual journey.

Quiescence is operationally characterized as a temporary and reversible proliferation arrest. There are many preconceived ideas about quiescence, quiescent cells being generally viewed as insignificant sleeping G1 cells. In fact, quiescence is central for organism physiology and its dysregulation involved in many pathologies.

The cell biology of quiescent yeast - a diversity of individual scenarios.

Most cells, from unicellular to complex organisms, spend part of their life in quiescence, a temporary non-proliferating state. Although central for a variety of essential processes including tissue homeostasis, development and aging, quiescence is poorly understood. In fact, quiescence encompasses various cellular situations depending on the cell type and the environmental niche.

Metabolomics and proteomics identify the toxic form and the associated cellular binding targets of the anti-proliferative drug AICAR.

5-Aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR, or acadesine) is a precursor of the monophosphate derivative 5-amino-4-imidazole carboxamide ribonucleoside 5'-phosphate (ZMP), an intermediate in purine biosynthesis. AICAR proved to have promising anti-proliferative properties, although the molecular basis of its toxicity is poorly understood. To exert cytotoxicity, AICAR needs to be metabolized, but the AICAR-derived toxic metabolite was not identified.

Mitochondria reorganization upon proliferation arrest predicts individual yeast cell fate.

Most cells spend the majority of their life in a non-proliferating state. When proliferation cessation is irreversible, cells are senescent. By contrast, if the arrest is only temporary, cells are defined as quiescent.

Yeast quiescence exit swiftness is influenced by cell volume and chronological age.

Quiescence exit swiftness is crucial not only for micro-organisms in competition for an environmental niche, such as yeast, but also for the maintenance of tissue homeostasis in multicellular species. Here we explore the effect of replicative and chronological age on quiescence exit efficiency. Our study reveals that this step strongly relies on the cell volume in quiescence but is not influenced by cell replicative age, at least for cells that have undergone less than 10 divisions.

Quiescent Saccharomyces cerevisiae forms telomere hyperclusters at the nuclear membrane vicinity through a multifaceted mechanism involving Esc1, the Sir complex, and chromatin condensation.

Like other eukaryotes, Saccharomyces cerevisiae spatially organizes its chromosomes within the nucleus. In G1 phase, the yeast's 32 telomeres are clustered into 6-10 foci that dynamically interact with the nuclear membrane. Here we show that, when cells leave the division cycle and enter quiescence, telomeres gather into two to three hyperclusters at the nuclear membrane vicinity.

A stable microtubule array drives fission yeast polarity reestablishment upon quiescence exit.

Cells perpetually face the decision to proliferate or to stay quiescent. Here we show that upon quiescence establishment, Schizosaccharomyces pombe cells drastically rearrange both their actin and microtubule (MT) cytoskeletons and lose their polarity. Indeed, while polarity markers are lost from cell extremities, actin patches and cables are reorganized into actin bodies, which are stable actin filament-containing structures.

Regulation of Rho-GEF Rgf3 by the arrestin Art1 in fission yeast cytokinesis.

Rho GTPases, activated by guanine nucleotide exchange factors (GEFs), are essential regulators of polarized cell growth, cytokinesis, and many other cellular processes. However, the regulation of Rho-GEFs themselves is not well understood. Rgf3 is an essential GEF for Rho1 GTPase in fission yeast.

Actin cable distribution and dynamics arising from cross-linking, motor pulling, and filament turnover.

The growth of fission yeast relies on the polymerization of actin filaments nucleated by formin For3p, which localizes at tip cortical sites. These actin filaments bundle to form actin cables that span the cell and guide the movement of vesicles toward the cell tips. A big challenge is to develop a quantitative understanding of these cellular actin structures.

Microtubules move the nucleus to quiescence.

The nucleus is a cellular compartment that hosts several macro-molecular machines displaying a highly complex spatial organization. This tight architectural orchestration determines not only DNA replication and repair but also regulates gene expression. In budding yeast microtubules play a key role in structuring the nucleus since they condition the Rabl arrangement in G1 and chromosome partitioning during mitosis through their attachment to centromeres via the kinetochore proteins.

Mitochondrial ATP synthases cluster as discrete domains that reorganize with the cellular demand for oxidative phosphorylation.

Mitochondria are double membrane-bounded organelles that form a dynamic tubular network. Mitochondria energetic functions depend on a complex internal architecture. Cristae, inner membrane invaginations that fold into the matrix space, are proposed to be the site of oxidative phosphorylation, reactions by which ATP synthase produces ATP.

An array of nuclear microtubules reorganizes the budding yeast nucleus during quiescence.

The microtubule cytoskeleton is a highly dynamic network. In dividing cells, its complex architecture not only influences cell shape and movement but is also crucial for chromosome segregation. Curiously, nothing is known about the behavior of this cellular machinery in quiescent cells.

α-Actinin and fimbrin cooperate with myosin II to organize actomyosin bundles during contractile-ring assembly.

The actomyosin contractile ring assembles through the condensation of a broad band of nodes that forms at the cell equator in fission yeast cytokinesis. The condensation process depends on actin filaments that interconnect nodes. By mutating or titrating actin cross-linkers α-actinin Ain1 and fimbrin Fim1 in live cells, we reveal that both proteins are involved in node condensation.

MtbHLH1, a bHLH transcription factor involved in Medicago truncatula nodule vascular patterning and nodule to plant metabolic exchanges.

This study aimed at defining the role of a basic helix-loop-helix (bHLH) transcription factor gene from Medicago truncatula, MtbHLH1, whose expression is upregulated during the development of root nodules produced upon infection by rhizobia bacteria. We used MtbHLH1 promoter::GUS fusions and quantitative reverse-transcription polymerase chain reaction analyses to finely characterize the MtbHLH1 expression pattern. We altered MtbHLH1 function by expressing a dominantly repressed construct (CRES-T approach) and looked for possible MtbHLH1 target genes by transcriptomics.

Assembly and architecture of precursor nodes during fission yeast cytokinesis.

The contractile ring is essential for cytokinesis in most fungal and animal cells. In fission yeast, cytokinesis nodes are precursors of the contractile ring and mark the future cleavage site. However, their assembly and architecture have not been well described.

Metabolic status rather than cell cycle signals control quiescence entry and exit.

Quiescence is defined as a temporary arrest of proliferation, yet it likely encompasses various cellular situations. Our knowledge about this widespread cellular state remains limited. In particular, little is known about the molecular determinants that orchestrate quiescence establishment and exit.

Mechanisms of contractile-ring assembly in fission yeast and beyond.

Most eukaryotes including fungi, amoebas, and animal cells assemble an actin/myosin-based contractile ring during cytokinesis. The majority of proteins implied in ring formation, maturation, and constriction are evolutionarily conserved, suggesting that common mechanisms exist among these divergent eukaryotes. Here, we review the recent advances in positioning and assembly of the actomyosin ring in the fission yeast Schizosaccharomyces pombe, the budding yeast Saccharomyces cerevisiae, and animal cells.

Reversible cytoplasmic localization of the proteasome in quiescent yeast cells.

The 26S proteasome is responsible for the controlled proteolysis of a vast number of proteins, including crucial cell cycle regulators. Accordingly, in Saccharomyces cerevisiae, 26S proteasome function is mandatory for cell cycle progression. In budding yeast, the 26S proteasome is assembled in the nucleus, where it is localized throughout the cell cycle.

Skp1-Cullin-F-box-dependent degradation of Aah1p requires its interaction with the F-box protein Saf1p.

When yeast cells enter into quiescence in response to nutrient limitation, the adenine deaminase Aah1p is specifically degraded via a process requiring the F-box protein Saf1p and components of the Skp1-Cullin-F-box complex. In this paper, we show that Saf1p interacts with both Aah1p and Skp1p. Interaction with Skp1p, but not with Aah1p, requires the F-box domain of Saf1p.