Finding the Cell Center by a Balance of Dynein and Myosin Pulling and Microtubule Pushing: A Computational Study

By comparing computer modeling predictions with observations, we conclude that strong dynein and weaker myosin-generated forces pull the microtubules inward competing with microtubule plus-ends pushing the microtubule aster outward and that the balance of these forces positions the centrosome at the cell center

Centering and Shifting of Centrosomes in Cells

Centrosomes have a nonrandom localization in the cells: either they occupy the centroid of the zone free of the actomyosin cortex or they are shifted to the edge of the cell, where their presence is justified from a functional point of view, for example, to organize additional microtubules or primary cilia. This review discusses centrosome placement options in cultured and in situ cells. It has been proven that the central arrangement of centrosomes is due mainly to the pulling microtubules forces developed by dynein located on the cell cortex and intracellular vesicles. The pushing forces from dynamic microtubules and actomyosin also contribute, although the molecular mechanisms of their action have not yet been elucidated. Centrosomal displacement is caused by external cues, depending on signaling, and is drawn through the redistribution of dynein, the asymmetrization of microtubules through the capture of their plus ends, and the redistribution of actomyosin, which, in turn, is associated with basal-apical cell polarization

Divergent Contribution of the Golgi Apparatus to Microtubule Organization in Related Cell Lines

Membrane trafficking in interphase animal cells is accomplished mostly along the microtubules. Microtubules are often organized radially by the microtubule-organizing center to coordinate intracellular transport. Along with the centrosome, the Golgi often serves as a microtubule-organizing center, capable of nucleating and retaining microtubules. Recent studies revealed the role of a special subset of Golgi-derived microtubules, which facilitates vesicular traffic from this central transport hub of the cell. However, proteins essential for microtubule organization onto the Golgi might be differentially expressed in different cell lines, while many potential participants remain undiscovered. In the current work, we analyzed the involvement of the Golgi complex in microtubule organization in related cell lines. We studied two cell lines, both originating from green monkey renal epithelium, and found that they relied either on the centrosome or on the Golgi as a main microtubule-organizing center. We demonstrated that the difference in their Golgi microtubule-organizing activity was not associated with the well-studied proteins, such as CAMSAP3, CLASP2, GCC185, and GMAP210, but revealed several potential candidates involved in this process

Spatial distribution of refractive index variations induced in bulk fused silica by single ultrashort and short laser pulses

International audienceWe correlate phase-contrast microscopy of modification tracks induced by tightly focused single ultrashort and short laser pulses inside fused silica with numerical simulations of nonlinear laser excitation footprints. Different pulse durations on the femtosecond and picosecond range are compared in order to validate the experimental and theoretical observations on the subsequent refractive index variations in a regime where linear and nonlinear contributions play a comparable role. The nature of the laser-induced structural changes depends essentially on the characteristics of pulse propagation in different regions of the irradiated zone. Numerical simulations of laser pulse propagation in the excited region show that accumulation of excess energy and swift nonlinear absorption contribute to the formation of either positive or negative phase-shift regions within the same single-pulse-induced damage trace. The decrease in the refractive index can be unambiguously correlated with the regions of maximum energy deposition during prolonged exposure times

A Solitary Stalled 80S Ribosome Prevents mRNA Recruitment to Stress Granules

Abstract: In response to stress stimuli, eukaryotic cells typically suppress protein synthesis. This leads to the release of mRNAs from polysomes, their condensation with RNA-binding proteins, and the formation of non-membrane-bound cytoplasmic compartments called stress granules (SGs). SGs contain 40S but generally lack 60S ribosomal subunits. It is known that cycloheximide, emetine, and anisomycin, the ribosome inhibitors that block the progression of 80S ribosomes along mRNA and stabilize polysomes, prevent SG assembly. Conversely, puromycin, which induces premature termination, releases mRNA from polysomes and stimulates the formation of SGs. The same effect is caused by some translation initiation inhibitors, which lead to polysome disassembly and the accumulation of mRNAs in the form of stalled 48S preinitiation complexes. Based on these and other data, it is believed that the trigger for SG formation is the presence of mRNA with extended ribosome-free segments, which tend to form condensates in the cell. In this study, we evaluated the ability of various small-molecule translation inhibitors to block or stimulate the assembly of SGs under conditions of severe oxidative stress induced by sodium arsenite. Contrary to expectations, we found that ribosome-targeting elongation inhibitors of a specific type, which arrest solitary 80S ribosomes at the beginning of the mRNA coding regions but do not interfere with all subsequent ribosomes in completing translation and leaving the transcripts (such as harringtonine, lactimidomycin, or T-2 toxin), completely prevent the formation of arsenite-induced SGs. These observations suggest that the presence of even a single 80S ribosome on mRNA is sufficient to prevent its recruitment into SGs, and the presence of extended ribosome-free regions of mRNA is not sufficient for SG formation. We propose that mRNA entry into SGs may be mediated by specific contacts between RNA-binding proteins and those regions on 40S subunits that remain inaccessible when ribosomes are associated.</p

A Solitary Stalled 80S Ribosome Prevents mRNA Recruitment to Stress Granules

Abstract: In response to stress stimuli, eukaryotic cells typically suppress protein synthesis. This leads to the release of mRNAs from polysomes, their condensation with RNA-binding proteins, and the formation of non-membrane-bound cytoplasmic compartments called stress granules (SGs). SGs contain 40S but generally lack 60S ribosomal subunits. It is known that cycloheximide, emetine, and anisomycin, the ribosome inhibitors that block the progression of 80S ribosomes along mRNA and stabilize polysomes, prevent SG assembly. Conversely, puromycin, which induces premature termination, releases mRNA from polysomes and stimulates the formation of SGs. The same effect is caused by some translation initiation inhibitors, which lead to polysome disassembly and the accumulation of mRNAs in the form of stalled 48S preinitiation complexes. Based on these and other data, it is believed that the trigger for SG formation is the presence of mRNA with extended ribosome-free segments, which tend to form condensates in the cell. In this study, we evaluated the ability of various small-molecule translation inhibitors to block or stimulate the assembly of SGs under conditions of severe oxidative stress induced by sodium arsenite. Contrary to expectations, we found that ribosome-targeting elongation inhibitors of a specific type, which arrest solitary 80S ribosomes at the beginning of the mRNA coding regions but do not interfere with all subsequent ribosomes in completing translation and leaving the transcripts (such as harringtonine, lactimidomycin, or T-2 toxin), completely prevent the formation of arsenite-induced SGs. These observations suggest that the presence of even a single 80S ribosome on mRNA is sufficient to prevent its recruitment into SGs, and the presence of extended ribosome-free regions of mRNA is not sufficient for SG formation. We propose that mRNA entry into SGs may be mediated by specific contacts between RNA-binding proteins and those regions on 40S subunits that remain inaccessible when ribosomes are associated.</p

Flipping the sign of refractive index changes in ultrafast and temporally shaped laser-irradiated borosilicate crown optical glass at high repetition rates

International audienceUltrafast subpicosecond laser exposure usually induces negative refractive index changes in optical glasses with strong thermal expansion such as borosilicate BK7 due to volume expansion and mechanical rarefaction. We show that temporally shaped laser excitation on picosecond scales and at high repetition rates can invert the regular material response resulting in a significant refractive index increase. Simulations of pulse propagation and evolution of heat and strain waves in BK7 glass exposed to different pulse durations were performed to understand mechanisms of refractive index increase. Narrow spatial distribution of energy for optimized picosecond pulses determines shock-induced plastic deformations accompanied by partial healing of the lateral strain due to preferential heat flow. The matter momentum relaxation produces directional on-axis material compaction

Optimization of structural modifications induced in laser-excited bulk transparent materials

International audienceUltrafast laser technology has emerged as a feasible tool to perform advanced processing of band-gap materials for potential applications in optics and photonics. The strong nonlinear localization of energy induces refractive index changes that may add specific functions (light guiding, coupling, diffraction, or amplification) to compact, integrated optical devices. Nevertheless, the result of the laser action depends essentially on the nature of the material and may result in specific electronic and structural changes associated with either increasing or decreasing the refractive index under light exposure. Employing real-time phase contrast microscopy we study the formation dynamics and the resultant transient and permanent morphology of laser-induced modifications in bulk dielectric materials in different conditions of irradiation. Several interaction regimes with respect to the input energy dose can be established based on these observations, emphasizing the role of nonlinear pulse propagation and of the intrinsic material properties. Furthermore we will show that using programmable spatio-temporal shaping of the laser pulse we regulate the energy delivery rate, delineate the spatial extent of excitation, and achieve an upgraded degree of controllability of the spatio-temporal characteristics of excitation. The approach is able to influence the balance of the laser-induced sequence of electronic and structural transformations and to define the end result of the laser action. The technique permits to design the nature of the refractive index change so that complex guiding structures can be induced in materials that do not allow it under normal excitation conditions

Transient response of dielectric materials exposed to ultrafast laser irradiation

International audienceWe present results describing several characteristics of energy coupling into fused silica dielectric materials irradiated by ultrashort laser pulses in a regime close to the surface optical breakdown threshold. The results intend to illustrate the energy balance in the interaction process by observing the spatio-temporal variations of a laser pulse transversing a dielectric slab as a function of its energy. The measurements are based on real-time observations of the self-action of the laser pulse and associated effects on its temporal envelope, as well as on ex-situ phase-contrast microscopy of induced permanent material reactions. The experimental results are accompanied by numerical simulations of the pulse traces inside the dielectric material at different energetic conditions. The optical observations allow insights into the development and the dynamics of the laser-induced free carrier population, emphasizing the role of the bulk effects related to the nonlinear wave propagation into the transparent material during laser exposure

Designing laser-induced refractive index changes in "thermal" glasses

International audienceUltrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction leads to a positive refractive index change and guiding structures may thus be created. From an application perspective, the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the structuring accuracy. The corrective process is becoming important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change