Mechanisms through which Sos-1 coordinates the activation of Ras and Rac

Signaling from receptor tyrosine kinases (RTKs)* requires the sequential activation of the small GTPases Ras and Rac. Son of sevenless (Sos-1), a bifunctional guanine nucleotide exchange factor (GEF), activates Ras in vivo and displays Rac-GEF activity in vitro, when engaged in a tricomplex with Eps8 and E3b1–Abi-1, a RTK substrate and an adaptor protein, respectively. A mechanistic understanding of how Sos-1 coordinates Ras and Rac activity is, however, still missing. Here, we demonstrate that (a) Sos-1, E3b1, and Eps8 assemble into a tricomplex in vivo under physiological conditions; (b) Grb2 and E3b1 bind through their SH3 domains to the same binding site on Sos-1, thus determining the formation of either a Sos-1–Grb2 (S/G) or a Sos-1–E3b1–Eps8 (S/E/E8) complex, endowed with Ras- and Rac-specific GEF activities, respectively; (c) the Sos-1–Grb2 complex is disrupted upon RTKs activation, whereas the S/E/E8 complex is not; and (d) in keeping with the previous result, the activation of Ras by growth factors is short-lived, whereas the activation of Rac is sustained. Thus, the involvement of Sos-1 at two distinct and differentially regulated steps of the signaling cascade allows for coordinated activation of Ras and Rac and different duration of their signaling within the cell

Cell cycle-dependent resolution of DNA double-strand breaks

DNA double strand breaks (DSBs) elicit prompt activation of DNA damage response (DDR), which arrests cell-cycle either in G1/S or G2/M in order to avoid entering S and M phase with damaged DNAs. Since mammalian tissues contain both proliferating and quiescent cells, there might be fundamental difference in DDR between proliferating and quiescent cells (or G0-arrested). To investigate these differences, we studied recruitment of DSB repair factors and resolution of DNA lesions induced at site-specific DSBs in asynchronously proliferating, G0-, or G1-arrested cells. Strikingly, DSBs occurring in G0 quiescent cells are not repaired and maintain a sustained activation of the p53-pathway. Conversely, re-entry into cell cycle of damaged G0-arrested cells, occurs with a delayed clearance of DNA repair factors initially recruited to DSBs, indicating an inefficient repair when compared to DSBs induced in asynchronously proliferating or G1-synchronized cells. Moreover, we found that initial recognition of DSBs and assembly of DSB factors is largely similar in asynchronously proliferating, G0-, or G1-synchronized cells. Our study thereby demonstrates that repair and resolution of DSBs is strongly dependent on the cell-cycle state

Extracellular vesicle-mediated transfer of CLIC1 protein is a novel mechanism for the regulation of glioblastoma growth

Little progresses have been made in the treatment of glioblastoma (GBM), the most aggressive and lethal among brain tumors. Recently we have demonstrated that Chloride Intracellular Channel-1 (CLIC1) is overexpressed in GBM compared to normal tissues, with highest expression in patients with poor prognosis. Moreover, CLIC1-silencing in cancer stem cells (CSCs) isolated from human GBM patients negatively influences proliferative capacity and self-renewal properties in vitro and impairs the in vivo tumorigenic potential. Here we show that CLIC1 exists also as a circulating protein, secreted via extracellular vesicles (EVs) released by either cell lines or GBM-derived CSCs. Extracellular vesicles (EVs), comprising exosomes and microvesicles based on their composition and biophysical properties, have been shown to sustain tumor growth in a variety of model systems, including GBM. Interestingly, treatment of GBM cells with CLIC1-containing EVs stimulates cell growth both in vitro and in vivo in a CLIC1-dose dependent manner. EVs derived from CLIC1-overexpressing GBM cells are strong inducers of proliferation in vitro and tumor engraftment in vivo. These stimulations are significantly attenuated by treatment of GBM cells with EVs derived from CLIC1-silenced cells. However, CLIC1 modulation appears to have no direct role in EV structure, biogenesis and secretion. These findings reveal that, apart from the function of CLIC1 cellular reservoir, CLIC1 contained in EVs is a novel regulator of GBM growth

In Vitro FRAP Identifies the Minimal Requirements for Mad2 Kinetochore Dynamics

BACKGROUND: Mad1 and Mad2 are constituents of the spindle-assembly checkpoint, a device coupling the loss of sister-chromatid cohesion at anaphase to the completion of microtubule attachment of the sister chromatids at metaphase. Fluorescence recovery after photobleaching (FRAP) revealed that the interaction of cytosolic Mad2 with kinetochores is highly dynamic, suggesting a mechanism of catalytic activation of Mad2 at kinetochores followed by its release in a complex with Cdc20. The recruitment of cytosolic Mad2 to kinetochores has been attributed to a stable receptor composed of a distinct pool of Mad2 tightly bound to Mad1. Whether specifically this interaction accounts for the kinetochore dynamics of Mad2 is currently unknown. RESULTS: To gain a precise molecular understanding of the interaction of Mad2 with kinetochores, we reconstituted the putative Mad2 kinetochore receptor and developed a kinetochore recruitment assay with purified components. When analyzed by FRAP in vitro, this system faithfully reproduced the previously described in vivo dynamics of Mad2, providing an unequivocal molecular account of the interaction of Mad2 with kinetochores. Using the same approach, we dissected the mechanism of action of p31(comet), a spindle-assembly checkpoint inhibitor. CONCLUSIONS: In vitro FRAP is a widely applicable approach to dissecting the molecular bases of the interaction of a macromolecule with an insoluble cellular scaffold. The combination of in vitro fluorescence recovery after photobleaching with additional fluorescence-based assays in vitro can be used to unveil mechanism, stoichiometry, and kinetic parameters of a macromolecular interaction, all of which are important for modeling protein interaction networks

Correlative Multi-Modal Microscopy: A Novel Pipeline for Optimizing Fluorescence Microscopy Resolutions in Biological Applications

The modern fluorescence microscope is the convergence point of technologies with different performances in terms of statistical sampling, number of simultaneously analyzed signals, and spatial resolution. However, the best results are usually obtained by maximizing only one of these parameters and finding a compromise for the others, a limitation that can become particularly significant when applied to cell biology and that can reduce the spreading of novel optical microscopy tools among research laboratories. Super resolution microscopy and, in particular, molecular localization-based approaches provide a spatial resolution and a molecular localization precision able to explore the scale of macromolecular complexes in situ. However, its use is limited to restricted regions, and consequently few cells, and frequently no more than one or two parameters. Correlative microscopy, obtained by the fusion of different optical technologies, can consequently surpass this barrier by merging results from different spatial scales. We discuss here the use of an acquisition and analysis correlative microscopy pipeline to obtain high statistical sampling, high content, and maximum spatial resolution by combining widefield, confocal, and molecular localization microscopy

Light-sheet confined super-resolution using two-photon photoactivation.

Light-sheet microscopy is a useful tool for performing biological investigations of thick samples and it has recently been demonstrated that it can also act as a suitable architecture for super-resolution imaging of thick biological samples by means of individual molecule localization. However, imaging in depth is still limited since it suffers from a reduction in image quality caused by scattering effects. This paper sets out to investigate the advantages of non-linear photoactivation implemented in a selective plane illumination configuration when imaging scattering samples. In particular, two-photon excitation is proven to improve imaging capabilities in terms of imaging depth and is expected to reduce light-sample interactions and sample photo-damage. Here, two-photon photoactivation is coupled to individual molecule localization methods based on light-sheet illumination (IML-SPIM), allowing super-resolution imaging of nuclear pH2AX in NB4 cells

From Double-Strand Break Recognition to Cell-Cycle Checkpoint Activation: High Content and Resolution Image Cytometry Unmasks 53BP1 Multiple Roles in DNA Damage Response and p53 Action

53BP1 protein has been isolated in-vitro as a putative p53 interactor. From the discovery of its engagement in the DNA-Damage Response (DDR), its role in sustaining the activity of the p53-regulated transcriptional program has been frequently under-evaluated, even in the case of a specific response to numerous DNA Double-Strand Breaks (DSBs), i.e., exposure to ionizing radiation. The localization of 53BP1 protein constitutes a key to decipher the network of activities exerted in response to stress. We present here an automated-microscopy for image cytometry protocol to analyze the evolution of the DDR, and to demonstrate how 53BP1 moved from damaged sites, where the well-known interaction with the DSB marker γH2A.X takes place, to nucleoplasm, interacting with p53, and enhancing the transcriptional regulation of the guardian of the genome protein. Molecular interactions have been quantitatively described and spatiotemporally localized at the highest spatial resolution by a simultaneous analysis of the impairment of the cell-cycle progression. Thanks to the high statistical sampling of the presented protocol, we provide a detailed quantitative description of the molecular events following the DSBs formation. Single-Molecule Localization Microscopy (SMLM) Analysis finally confirmed the p53–53BP1 interaction on the tens of nanometers scale during the distinct phases of the response