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© The Rockefeller University Press, 0021-9525/1998//627 $5.00
The Journal of Cell Biology, Volume 140, Number 3, , 1998 627-636

Article

Apoptotic Membrane Blebbing Is Regulated by Myosin Light Chain Phosphorylation



Jason C. Mills, Nicole L. Stone, Joseph Erhardt, and Randall N. Pittman

Department of Pharmacology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104

The evolutionarily conserved execution phase of apoptosis is defined by characteristic changes occurring during the final stages of death; specifically cell shrinkage, dynamic membrane blebbing, condensation of chromatin, and DNA fragmentation. Mechanisms underlying these hallmark features of apoptosis have previously been elusive, largely because the execution phase is a rapid event whose onset is asynchronous across a population of cells. In the present study, a model system is described for using the caspase inhibitor, z-VAD-FMK, to block apoptosis and generate a synchronous population of cells actively extruding and retracting membrane blebs. This model system allowed us to determine signaling mechanisms underlying this characteristic feature of apoptosis. A screen of kinase inhibitors performed on synchronized blebbing cells indicated that only myosin light chain kinase (MLCK) inhibitors decreased blebbing. Immunoprecipitation of myosin II demonstrated that myosin regulatory light chain (MLC) phosphorylation was increased in blebbing cells and that MLC phosphorylation was prevented by inhibitors of MLCK. MLC phosphorylation is also mediated by the small G protein, Rho. C3 transferase inhibited apoptotic membrane blebbing, supporting a role for a Rho family member in this process. Finally, blebbing was also inhibited by disruption of the actin cytoskeleton. Based on these results, a working model is proposed for how actin/myosin II interactions cause cell contraction and membrane blebbing. Our results provide the first evidence that MLC phosphorylation is critical for apoptotic membrane blebbing and also implicate Rho signaling in these active morphological changes. The model system described here should facilitate future studies of MLCK, Rho, and other signal transduction pathways activated during the execution phase of apoptosis.

Abbreviations used in this paper: ABP, actin binding protein; BDM, 2,3-butanedione monoxime; MLC, myosin regulatory light chain; MLCK, myosin light chain kinase; ROK, Rho kinase; STS, staurosporine; z-VAD-FMK, z-Val-Ala-Asp-fluoromethylketone.

J.C. Mills and N.L. Stone contributed equally to this work.

Address all correspondence to Nicole L. Stone, University of Pennsylvania School of Medicine, 155 John Morgan Building, 3620 Hamilton Walk, Philadelphia, PA 19104-6084. Tel.: (215) 898-7099. Fax: (215) 573-2236. E-mail: burkhardt{at}pharm.med.upenn.edu

J.C. Mills' present address is Department of Pathology, Washington University School of Medicine, 550 South Euclid Avenue, Box 8118, St. Louis, MO 63110.


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