Biology subjects

Gergely, Z. R.

Publications and source records attributed to Gergely, Z. R..

3 recordsLinked to original sources

bioRxiv2023

Quantifying yeast microtubules and spindles using the Toolkit for Automated Microtubule Tracking (TAMiT)

Fluorescently labeled proteins absorb and emit light, appearing as Gaussian spots in fluorescence imaging. When fluorescent tags are added to cytoskeletal polymers such as microtubules, a line of fluorescence and even non-linear structures results. While much progress has been made in techniques for imaging and microscopy, image analysis is less well developed. Current analysis of fluorescent microtubules uses either manual tools, such as kymographs, or automated software. As a result, our ability to quantify microtubule dynamics and organization from light microscopy remains limited. Despite development of automated microtubule analysis tools for in vitro studies, analysis of images from cells often depends heavily on manual analysis. One of the main reasons for this disparity is the low signal-to-noise ratio in cells, where background fluorescence is typically higher than in reconstituted systems. Here, we present the Toolkit for Automated Microtubule Tracking (TAMiT), which automatically detects, optimizes and tracks fluorescent microtubules in living yeast cells with sub-pixel accuracy. Using basic information about microtubule organization, TAMiT detects linear and curved polymers using a geometrical scanning technique. Images are fit via an optimization problem for the microtubule image parameters that is solved using non-linear least squares in Matlab. We benchmark our software using simulated images and show that it reliably detects microtubules, even at low signal-to-noise ratios. Then, we use TAMiT to measure monopolar spindle microtubule bundle number, length, and lifetime in a large dataset that includes several S. pombe mutants that affect microtubule dynamics and bundling. The results from the automated analysis are consistent with previous work, and suggest a direct role for CLASP/Cls1 in bundling spindle microtubules. We also illustrate automated tracking of single curved astral microtubules in S. cerevisiae, with measurement of dynamic instability parameters. The results obtained with our fully-automated software are similar to results using hand-tracked measurements. Therefore, TAMiT can facilitate automated analysis of spindle and microtubule dynamics in yeast cells.

cell biology↗

bioRxiv2022

Kinesin-5/Cut7 moves bidirectionally on fission-yeast spindles with activity that increases in anaphase

Kinesin-5 motors are essential to separate mitotic spindle poles and assemble a bipolar spindle in many organisms. These tetrameric motors crosslink and slide apart antiparallel microtubules via microtubule plus-end-directed motility. However, kinesin-5s typically accumulate more at spindle poles than in the center of the spindle where antiparallel microtubule overlaps are most numerous. While the relevance of this localization has remained unclear, increasing evidence suggests that it occurs due to bidirectional motility or trafficking of kinesin-5 motors. The kinesin-5 motor Cut7p from fission yeast has been shown to move bidirectionally in reconstituted systems. However, bidirectional movement has not been shown in cells and the funtion of the minus-end-directed movement remains unclear. Here, we characterized the motility of kinesin-5/Cut7 on bipolar and monopolar spindles in fission yeast and observed movement both toward plus and minus ends of microtubules. Notably, we found that the activity of the motor increases at the onset on anaphase B. Perturbations to microtubule dynamics did not significantly change the observed Cut7p movement, while Cut7p mutation reduced or abolished observable movement. These results suggest that the directed movement of Cut7p was due to the motility of the motor itself. Mutations of Cut7p that decreased plus-end-directed motility enhanced its spindle-pole localization. In contrast, abolishing Cut7 motility or replacing it with plus-end-directed human Eg5 eliminates the pole localization. Our results suggest a new hypothesis for the function of minus-end-directed motility and spindle-pole localization of kinesin-5s in spindle assembly.

cell biology↗

bioRxiv2019

Mechanisms of chromosome biorientation and bipolar spindle assembly analyzed by computational modeling

The essential functions required for mitotic spindle assembly and chromosome biorientation and segregation are not fully understood, despite extensive study. To illuminate the combinations of ingredients most important to align and segregate chromosomes and simultaneously assemble a bipolar spindle, we developed a computational model of fission-yeast mitosis. Robust chromosome biorientation requires progressive restriction of attachment geometry, destabilization of misaligned attachments, and attachment force dependence. Large spindle length fluctuations can occur when the kinetochore-microtubule attachment lifetime is long. The primary spindle force generators are kinesin-5 motors and crosslinkers in early mitosis, while interkinetochore stretch becomes important after biorientation. The same mechanisms that contribute to persistent biorientation lead to segregation of chromosomes to the poles after anaphase onset. This model therefore provides a framework to interrogate key requirements for robust chromosome biorientation, spindle length regulation, and force generation in the spindle.

cell biology↗