Rac and Arp2/3-Nucleated Actin Networks Antagonize Rho During Mitotic and Meiotic Cleavages.

Debadrita Pal,Andrea Ellis,John H Henson,Richard De La Rosa,Torey Salgado,Isabella Terrazas,Silvia P Sepúlveda-Ramírez,Gabriela Reyes,Charles B Shuster

doi:10.3389/fcell.2020.591141

Debadrita Pal, Andrea Ellis + Show 7 more

Open Access

https://doi.org/10.3389/fcell.2020.591141

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Abstract

In motile cells, the activities of the different Rho family GTPases are spatially segregated within the cell, and during cytokinesis there is evidence that this may also be the case. But while Rho’s role as the central organizer for contractile ring assembly is well established, the role of Rac and the branched actin networks it promotes is less well understood. To characterize the contributions of these proteins during cytokinesis, we manipulated Rac and Arp2/3 activity during mitosis and meiosis in sea urchin embryos and sea star oocytes. While neither Rac nor Arp2/3 were essential for early embryonic divisions, loss of either Rac or Arp2/3 activity resulted in polar body defects. Expression of activated Rac resulted in cytokinesis failure as early as the first division, and in oocytes, activated Rac suppressed both the Rho wave that traverses the oocyte prior to polar body extrusion as well as polar body formation itself. However, the inhibitory effect of Rac on cytokinesis, polar body formation and the Rho wave could be suppressed by effector-binding mutations or direct inhibition of Arp2/3. Together, these results suggest that Rac- and Arp2/3 mediated actin networks may directly antagonize Rho signaling, thus providing a potential mechanism to explain why Arp2/3-nucleated branched actin networks must be suppressed at the cell equator for successful cytokinesis.

Highlights

Cytokinesis is the final phase of cell division where in animal and fungal cells, a transient assemblage of actin and myosin II assembles at the former metaphase plate to effect the partitioning of the two daughter cells
Our results suggest that Rac-stimulated branched actin networks may act as a direct antagonist to Rho activity, which may provide an additional layer of spatial regulation to promote efficient daughter cell partitioning during cytokinesis
To better understand the respective roles of Rho and Rac in regulating cell shape change and cytokinesis, we first employed live cell imaging of actin during the first division of the sea urchin embryo to establish a baseline by which further perturbations could be compared (Figure 1)

Summary

Introduction

Cytokinesis is the final phase of cell division where in animal and fungal cells, a transient assemblage of actin and myosin II assembles at the former metaphase plate to effect the partitioning of the two daughter cells. There is a symmetry-breaking event that drives the localized activation of the small GTPases Rac and/or Cdc, which in turn promote the elaboration of viscoelastic, branched actin networks at the leading edge to effect forward protrusion (Blanchoin et al, 2014; Schaks et al, 2019). Direct inhibition of the Arp2/3 complex rescued cytokinesis in Q61L Rac-expressing embryos (Figure 3G). While CK-666 did not affect the dynamics of cytoplasmic actin in Q61L Rac embryos (Figure 3H), Arp2/3 inhibition did rescue Rho-dependent thickening of the cortical layer (Figure 3I). While Rac is capable of activating a variety of downstream effector pathways (Jordan and Canman, 2012), Rac’s promotion of Arp2/3-mediated branched actin networks appeared to be what was responsible for Rac’s deleterious effects on cytokinesis.

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