Increased lateral microtubule contact at the cell cortex is sufficient to drive mammalian spindle elongation

The spindle is a dynamic structure that changes its architecture and size in response to biochemical and physical cues. For example, a simple physical change, cell confinement, can trigger centrosome separation and increase spindle steady-state length at metaphase. How this occurs is not understood, and is the question we pose here. We find that metaphase and anaphase spindles elongate at the same rate when confined, suggesting that similar elongation forces can be generated independent of biochemical and spindle structural differences. Furthermore, this elongation does not require bipolar spindle architecture or dynamic microtubules. Rather, confinement increases numbers of astral microtubules laterally contacting the cortex, shifting contact geometry from “end-on” to “side-on.” Astral microtubules engage cortically anchored motors along their length, as demonstrated by outward sliding and buckling after ablation-mediated release from the centrosome. We show that dynein is required for confinement-induced spindle elongation, and both chemical and physical centrosome removal demonstrate that astral microtubules are required for such spindle elongation and its maintenance. Together the data suggest that promoting lateral cortex–microtubule contacts increases dynein-mediated force generation and is sufficient to drive spindle elongation. More broadly, changes in microtubule-to-cortex contact geometry could offer a mechanism for translating changes in cell shape into dramatic intracellular remodeling

Increased lateral microtubule contact at the cell cortex is sufficient to drive mammalian spindle elongation.

The spindle is a dynamic structure that changes its architecture and size in response to biochemical and physical cues. For example, a simple physical change, cell confinement, can trigger centrosome separation and increase spindle steady-state length at metaphase. How this occurs is not understood, and is the question we pose here. We find that metaphase and anaphase spindles elongate at the same rate when confined, suggesting that similar elongation forces can be generated independent of biochemical and spindle structural differences. Furthermore, this elongation does not require bipolar spindle architecture or dynamic microtubules. Rather, confinement increases numbers of astral microtubules laterally contacting the cortex, shifting contact geometry from "end-on" to "side-on." Astral microtubules engage cortically anchored motors along their length, as demonstrated by outward sliding and buckling after ablation-mediated release from the centrosome. We show that dynein is required for confinement-induced spindle elongation, and both chemical and physical centrosome removal demonstrate that astral microtubules are required for such spindle elongation and its maintenance. Together the data suggest that promoting lateral cortex-microtubule contacts increases dynein-mediated force generation and is sufficient to drive spindle elongation. More broadly, changes in microtubule-to-cortex contact geometry could offer a mechanism for translating changes in cell shape into dramatic intracellular remodeling

The Trapped Proton Environment in Medium Earth Orbit (MEO)

Energetic proton flux maps of the differential flux intensity in the medium-Earth orbit (MEO) regime (altitudes ~ 7000-15,000 km) are developed from measurements taken by detectors aboard the Combined Release and Radiation Effects Satellite (CRRES), HEO-F1, HEO-F3 and ICO satellites. Measurement errors have been estimated by cross-calibrating to a standard sensor aboard the GOES satellite during solar proton events. Spectral inversion techniques were employed to derive differential flux spectra from the HEO and ICO integral channel dosimeters. Two methods for combining the four different satellite data sets on a standard energy and coordinate grid are presented and the ramifications due to limited spatial and temporal coverage are explored. Comparison to the NASA AP-8 models shows the new model median flux maps to be of approximately equivalent or lower magnitude in the slot region while new model 95th percentile maps are always higher. Implications for the proton dose received by MEO satellites are discussed.United States. Air Force (Contract FA8721-10-C-0007

Radiation and Plasma Effects Working Team: Overview and Progress Report

To track space weather models’ progress and performance over time, the Community Coordinated Modeling Center (CCMC), together with NASA’s Living with a Star program, has initiated an extensive model validation and assessment efforts involving a community of space environment experts, model and application developers, data providers, forecasters and end-users of space weather products and services (https://ccmc.gsfc.nasa.gov/assessment/). The Radiation and Plasma Effects Working Team deals with five different subtopics: Surface Charging from 10s eV to 40 keV electrons, Internal Charging due to energetic electrons from hundreds keV to several MeVs. Single Event Effects from solar energetic particles (SEPs) and galactic cosmic rays (GCRs) (several MeV to TeVs), Total Dose due to accumulation of doses from electrons and protons in a broad energy range, and Radiation Effects from SEPs and GCRs at aviation altitudes