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A more recent version of this article appeared on March 1, 2002
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Submitted on April 10, 2001
Revised on November 27, 2001
Accepted on November 28, 2001
1 Department of Biology, Dickinson College, Carlisle, PA 17013, and Mount Desert Island Biological Laboratory, Salisbury Cove, ME 04672
2 Department of Biology, Dickinson College, Carlisle, PA 17013
* Corresponding author. E-mail address: henson{at}dickinson.edu.
The actomyosin purse string is an evolutionarily conserved contractile structure that is involved in cytokinesis, morphogenesis, and wound healing. Recent studies suggest that an actomyosin purse string is crucial for the closure of wounds in single cells. In the present study, morphological and pharmacological methods are used to investigate the role of this structure in the closure of wounds in the peripheral cytoplasm of sea urchin coelomocytes. These discoidal shaped cells undergo a dramatic form of actin-based centripetal/retrograde flow and occasionally open and close spontaneous wounds in their lamellipodia. Fluorescent phalloidin staining indicates that a well defined fringe of actin filaments assembles from the margin of these holes and drug studies with cytochalasin D and latrunculin A indicates that actin polymerization is required for wound closure. Additional evidence that actin polymerization is involved in wound closure is provided by the localization of components of the Arp2/3 complex to the wound margin. Significantly, myosin II immunolocalization demonstrates that it is not associated with wound margins despite being present in the perinuclear region. Pharmacological evidence for the lack of myosin II involvement in wound closure comes from experiments in which a microneedle is used to produce wounds in cells in which actomyosin contraction has been inhibited by treatment with kinase inhibitors. Wounds produced in kinase inhibitor treated cells close in a manner similar to that seen with control cells. Taken together, our results suggest that an actomyosin purse string mechanism is not responsible for the closure of lamellar wounds in coelomocytes. We hypothesize that the wounds heal by means of a combination of the force produced by actin polymerization alone and centripetal flow. Interestingly, these cells do assemble an actomyosin structure around the margin of phagosome-like membrane invaginations indicating that myosin is not simply excluded from the periphery by some general mechanism. The results indicate that the actomyosin purse string is not the only mechanism that can mediate wound closure in single cells.
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