Xenopus Oocyte Wound Healing as a Model System for Analysis of Microtubule-Actin Interactions. In: Zhou J, editor. Microtubule Protocols. Totowa: Humana Press; 2007. pp. 181-8. [DOI: 10.1007/978-1-59745-442-1_13]

Tension modulation of actomyosin ring assembly and RhoGTPases activity: Perspectives from the Xenopus oocyte wound healing model

Cells are remarkably resilient structures; they are able to recover from injuries to their plasma membrane (PM) and cytoskeleton that would normally constitute existential threats. This capacity is exemplified by Xenopus laevis oocytes which can recover from very large PM defects through exocytotic and endocytic events and can repair damaged cortical cytoskeleton structures through the formation of a contractile actomyosin ring (AMR). Formation of the AMR involves the localized Ca²⁺ -dependent activation of RhoA and Cdc42, and the pre-patterning of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). However, this model fails to account for observations that suggest a link between cytoskeletal dynamics, intracellular tension, and AMR formation. It also does not explain why the formation of an AMR is not involved in the cytoskeletal repair program of adherent cells. We show here evidence for the support of tension as an essential regulatory signal for the formation of AMR. Indeed, oocytes in which global tension has been experimentally reduced were unable to form a functional AMR following injury, showing severely diminished RhoA activity at the wound site. These new insights place the cytoskeleton at the center of events involving changes in cell shape such as cytokinesis which also involves the formation and closure of an AMR.

The direct permeation of nanoparticles through the plasma membrane transiently modifies its properties

The exposure to metal nanoparticles (NPs) has increased with their widespread use in industry, research and medicine. It is well known that NPs may enter cells and that this mechanism is crucial to exert both the therapeutic and toxicity effects. The main cellular entrance route is endocytosis-based, however, recent experimental studies, have reported that NPs can also enter the cell crossing directly the plasma membrane, it is thus important to investigate this alternative internalization mechanism. Size, surface chemistry, solubility and shape play a role in NP ability of entering the cell, but it is still to be elucidated how these properties act on cell membrane. We have demonstrated that a direct permeation of metal oxide NPs through the lipid bilayer of the cell membrane can occur, giving direct access to the cytoplasm. In this paper, using the powerful tool of Xenopus laevis oocytes and two electrode Voltage Clamp, we have investigated several parameters that can influence the direct crossing. The most significant of them is the NP hydrodynamic size as clearly shown by the comparison of the behaviour between Co₃O₄ and NiO NPs. By collecting biophysical membrane parameters in different conditions, we have shown that NPs that are able to cross the membrane share the ability to maintain a hydrodynamic size lower than 200 nm. The presence of this route of entrance must be considered for a better comprehension of the effect at intracellular level considering possible mechanism in order to a safer design of engineered NPs.

Growth-arrest-specific protein 2 inhibits cell division in Xenopus embryos

BACKGROUND

Growth-arrest-specific 2 gene was originally identified in murine fibroblasts under growth arrest conditions. Furthermore, serum stimulation of quiescent, non-dividing cells leads to the down-regulation of gas2 and results in re-entry into the cell cycle. Cytoskeleton rearrangements are critical for cell cycle progression and cell division and the Gas2 protein has been shown to co-localize with actin and microtubules in interphase mammalian cells. Despite these findings, direct evidence supporting a role for Gas2 in the mechanism of cell division has not been reported.

METHODOLOGY AND PRINCIPAL FINDINGS

To determine whether the Gas2 protein plays a role in cell division, we over-expressed the full-length Gas2 protein and Gas2 truncations containing either the actin-binding CH domain or the tubulin-binding Gas2 domain in Xenopus laevis embryos. We found that both the full-length Gas2 protein and the Gas2 domain, but not the CH domain, inhibited cell division and resulted in multinucleated cells. The observation that Gas2 domain alone can arrest cell division suggests that Gas2 function is mediated by microtubule binding. Gas2 co-localized with microtubules at the cell cortex of Gas2-injected Xenopus embryos using cryo-confocal microscopy and co-sedimented with microtubules in cytoskeleton co-sedimentation assays. To investigate the mechanism of Gas2-induced cell division arrest, we showed, using a wound-induced contractile array assay, that Gas2 stabilized microtubules. Finally, electron microscopy studies demonstrated that Gas2 bundled microtubules into higher-order structures.

CONCLUSION AND SIGNIFICANCE

Our experiments show that Gas2 inhibits cell division in Xenopus embryos. We propose that Gas2 function is mediated by binding and bundling microtubules, leading to cell division arrest.