The cell biology of quiescent yeast – a diversity of individual scenarios

International audienceMost 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. Quiescent cell properties also evolve with time, adding another layer of complexity. Studying quiescence is, above all, limited by the fact that a quiescent cell can be recognized as such only after having proved that it is capable of re-proliferating. Recent cellular biology studies in yeast have reported the relocalization of hundreds of proteins and the reorganization of several cellular machineries upon proliferation cessation. These works have revealed that quiescent cells can display various properties, shedding light on a plethora of individual behaviors. The deciphering of the molecular mechanisms beyond these reorganizations, together with the understanding of their cellular functions, have begun to provide insights into the physiology of quiescent cells. In this Review, we discuss recent findings and emerging concepts in Saccharomyces cerevisiae quiescent cell biology

Mitochondria reorganization upon proliferation arrest predicts individual yeast cell fate

International audienceMost 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. These cellular states are hardly distinguishable without triggering proliferation resumption, hampering thus the study of quiescent cells properties. Here we show that quiescent and senescent yeast cells are recognizable based on their mitochondrial network morphology. Indeed, while quiescent yeast cells display numerous small vesicular mitochondria, senescent cells exhibit few globular mitochondria. This allowed us to reconsider at the individual-cell level, properties previously attributed to quiescent cells using population-based approaches. We demonstrate that cell's propensity to enter quiescence is not influenced by replicative age, volume or density. Overall, our findings reveal that quiescent cells are not all identical but that their ability to survive is significantly improved when they exhibit the specific reorganization of several cellular machineries

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. We report that upon cell entry into quiescence, proteasome subunits massively relocalize from the nucleus into motile cytoplasmic structures. We further demonstrate that these structures are proteasome cytoplasmic reservoirs that are rapidly mobilized upon exit from quiescence. Therefore, we have named these previously unknown structures proteasome storage granules (PSGs). Finally, we observe conserved formation and mobilization of these PSGs in the evolutionary distant yeast Schizosaccharomyces pombe. This conservation implies a broad significance for these proteasome reserves

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 G(1) 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. This localization depends on Esc1 but not on the Ku proteins. Telomere hypercluster formation requires the Sir complex but is independent of the nuclear microtubule bundle that specifically assembles in quiescent cells. Importantly, mutants deleted for the linker histone H1 Hho1 or defective in condensin activity or affected for histone H4 Lys-16 deacetylation are impaired, at least in part, for telomere hypercluster formation in quiescence, suggesting that this process involves chromosome condensation. Finally, we establish that telomere hypercluster formation is not necessary for quiescence establishment, maintenance, and exit, raising the question of the physiological raison d’être of this nuclear reorganization

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.We found that MtbHLH1 is expressed in nodule primordia cells derived from pericycle divisions, in nodule vascular bundles (VBs) and in uninfected cells of the nitrogen (N) fixation zone. MtbHLH1 is also expressed in root tips, lateral root primordia cells and root VBs, and induced upon auxin treatment. Altering MtbHLH1 function led to an unusual phenotype, with a modified patterning of nodule VB development and a reduced growth of aerial parts of the plant, even though the nodules were able to fix atmospheric N. Several putative MtbHLH1 regulated genes were identified, including an asparagine synthase and a LOB (lateral organ boundary) transcription factor.Our results suggest that the MtbHLH1 gene is involved in the control of nodule vasculature patterning and nutrient exchanges between nodules and roots

Upper Body Skeletal Posture of the Average Pedestrian Male from Upright High-Resolution X-ray Images – Comparison to the THUMS Pedestrian Model

No Abstrac

The Brazilian Tunable Filter Imager for the SOAR telescope

This paper presents a new Tunable Filter Instrument for the SOAR telescope. The Brazilian Tunable Filter Imager (BTFI) is a versatile, new technology, tunable optical imager to be used in seeing-limited mode and at higher spatial fidelity using the SAM Ground-Layer Adaptive Optics facility at the SOAR telescope. The instrument opens important new science capabilities for the SOAR community, from studies of the centers of nearby galaxies and the insterstellar medium to statistical cosmological investigations. The BTFI takes advantage of three new technologies. The imaging Bragg Tunable Filter concept utilizes Volume Phase Holographic Gratings in a double-pass configuration, as a tunable filter, while a new Fabry-Perot (FP) concept involves technologies which allow a single FP etalon to act over a large range of interference orders and spectral resolutions. Both technologies will be in the same instrument. Spectral resolutions spanning the range between 25 and 30,000 can be achieved through the use of iBTF at low resolution and scanning FPs beyond R ~2,000. The third new technologies in BTFI is the use of EMCCDs for rapid and cyclically wavelength scanning thus mitigating the damaging effect of atmospheric variability through data acquisition. An additional important feature of the instrument is that it has two optical channels which allow for the simultaneous recording of the narrow-band, filtered image with the remaining (complementary) broad-band light. This avoids the uncertainties inherent in tunable filter imaging using a single detector. The system was designed to supply tunable filter imaging with a field-of-view of 3 arcmin on a side, sampled at 0.12" for direct Nasmyth seeing-limited area spectroscopy and for SAM's visitor instrument port for GLAO-fed area spectroscopy. The instrument has seen first light, as a SOAR visitor instrument. It is now in comissioning phase.Comment: accepted in PAS

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

The use of new candidate markers for yeast quiescence reveals that quiescence entry and exit primarily rely on cellular metabolic status and can be uncoupled from the cell cycle

Assembly and architecture of precursor nodes during fission yeast cytokinesis

Mapping of fission yeast precursor node interaction modules and assembly reveals important steps in contractile ring assembly

Analyse de la réponse d'assemblages collés sous des sollicitations en dynamique rapide. Essais et modélisations

Our work gives a contribution to the analysis of bonded assemblies' behaviour under high strain rates in a range from 105 s-1 to 107 s-1. We investigate the response of assemblies of aluminium 6061T6 substrates bonded with an adhesive material. The influence of the adhesive materials' nature is studied by using two kind of glue: an epoxy resin and a silicon resin. The behaviours of these materials under shock wave loading are characterized and modelling approaches are proposed. The response of bonded assemblies under shock wave loading is characterized with several experimental techniques (lasers, GEPI, gas-guns). The influence of the pulse duration on the tensile strength of the structure is investigated on a range from 3 ns to 500 ns. Laser tests help us to make microscopic observation of samples after experiments. The link between damage and rear surface velocity is established. Comparisons of measures and numerical computations with plate impact or isentropic compression experiments show that the mode of failure is mainly adhesive. Over more, the numerical computations help us to determine the tensile stress threshold with each kind of experiments and to evaluate the pulse duration influence on this threshold. These results will be used for the optimization of a shock adhesion test.Cette thèse apporte une contribution à la compréhension et à la modélisation de la réponse d'assemblages collés sous des sollicitations en dynamique rapide générant des vitesses de déformation comprises entre 105 s-1 et 107 s-1. L'analyse présentée porte sur des assemblages aluminium/colle/aluminium. L'influence de la nature de la colle est étudiée par le biais de l'utilisation de deux types d'adhésifs : une résine époxyde et une résine silicone. Ces deux matériaux font l'objet d'une caractérisation expérimentale en régime dynamique et des approches de modélisation de leur réponse sous choc sont proposées. La réponse sous choc des assemblages collés est caractérisée à l'aide d'une grande diversité de moyens expérimentaux (laser, GEPI, lanceurs). L'influence de la durée de sollicitation et de la vitesse de déformation sur les seuils de rupture des empilements est ainsi investiguée sur une gamme de 3 ns à plus de 500 ns. Les essais laser permettent une récupération des échantillons pour des observations post-mortem. Le lien entre l'état final de l'empilement et la vitesse de face arrière mesurée est alors établi. Sur les essais d'impact de plaques et de compression isentropique, le couplage entre les mesures et la simulation numérique permet de mettre en évidence le caractère essentiellement adhésif des ruptures des empilements. La simulation numérique permet également de déterminer les seuils de rupture sur chacun des types d'essais et d'évaluer l'influence de la durée de sollicitation sur ces seuils. L'ensemble de ces résultats est destiné à être utilisé dans le cadre de l'optimisation d'un test d'adhérence par choc sur des assemblages millimétriques