Regulation of Cell Cycle Progression by the Anaphase Spindle Midzone

Research Article

Regulation of Cell Cycle Progression by the Anaphase Spindle Midzone

Olivia Brooks^1*, Julien Moreau² and Ethan Hughes²

1Department of Immunology, Stanford University, Stanford, USA
2Department of Microbiology, University of Oxford, Oxford, UK

Published: 14 March 2014

Abstract

Background: The eukaryotic cytoskeleton, essential for cell shape, division and motility, is classically composed of microtubules, actin filaments and intermediate filaments. Septins, GTP-binding proteins that polymerize into filaments, rings and gauzes, represent a fourth cytoskeletal component. While distinct, septins frequently co-localize with actin structures, particularly cortical actin, stress fibers and the contractile ring during cytokinesis. However, the functional significance and molecular basis of this relationship remain incompletely understood in mammalian cells.

Objective: This study aimed to investigate the functional interdependence between the septin and actin cytoskeletal systems in mammalian cells, testing the hypothesis that each system influences the organization and stability of the other.

Methods: Human HeLa and mouse NIH-3T3 cells were used. Co-localization between endogenous or GFP-tagged septins (SEPT2, SEPT7, SEPT9) and F-actin (visualized by phalloidin) was analyzed using immunofluorescence and confocal microscopy. The consequences of septin depletion (using siRNA against core septins like SEPT2 or SEPT7) on actin organization, stress fiber formation, cortical actin integrity and cytokinesis were examined.

Results: Septin filaments frequently co-aligned with actin stress fibers and formed part of the cortical cytoskeleton network. Depletion of core septins (SEPT2/7) resulted in significant defects in actin organization, including reduced number and altered morphology of stress fibers, aberrant cortical actin distribution and increased cell blebbing. Septin depletion also led to severe cytokinesis defects, characterized by unstable contractile rings and frequent multinucleation.

Conclusion: These findings demonstrate a strong bidirectional functional interdependence between the septin and actin cytoskeletons in mammalian cells. Septins are required for the proper organization and stability of specific actin structures, including stress fibers and the cortical actin network and are essential for cytokinesis. Reciprocally, the integrity and dynamics of the actin cytoskeleton, as well as actomyosin contractility, are necessary for the correct assembly, localization and maintenance of septin filament networks. This interplay suggests that septins and actin cooperate closely, potentially acting as mutual scaffolds or regulators, to control cell shape, division and mechanical stability.

Keywords: Septins; Actin; Cytoskeleton; F-Actin; Actomyosin; Stress Fibers; Cell Cortex; Cell Morphology; Cytoskeletal; GTP-binding proteins