Research

My laboratory works on the role of second messengers in signaling cell cycle events. For a number of years we studied cAMP-dependent phosphorylation during mitosis in cultured cells and in fertilized sea urchin eggs. More recently, we studied patterns of Ca⁺⁺ release during the cell cycle and in early development in sea urchins, demonstrating a release of calcium prior to nuclear envelope breakdown in fertilized eggs. The photoprotein aequorin, which luminesces upon binding Ca⁺⁺, was microinjected into cells and photon release was detected by an image photon detector attached to a miroscope. The temporal and spatial pattern of photon release was analyzed by computer to determine the localization, timing, and pattern of Ca⁺⁺ release within the cell. In other experiments, microinjected calcium buffers are used to control intracellular free Ca⁺⁺ concentration to determine the dependence of cell cycle events on free Ca⁺⁺.

Most recently, working in collaboration with Dr. Michael Tytell of the Wake Forest University School of Medicine, we have been using the sea urchin egg as a model system for studying the effect of exogenous heat shock protein (Hsp) on cells. Hsps are molecular chaperones involved in protein folding and translocation in response to physical stresses and during immune and inflammatory responses. Recently it has become clear that they also play a role in regulating the cell cycle. Although Hsps are best understood in the context of their intracellular actions, evidence suggests that they can be passed from cell to cell, expressed on plasma membranes, and taken up from the extracellular medium. Thus exogenous application of Hsps might have clinical benefits. Exogenous Hsp is taken up by fertilized eggs of the sea urchin Lytechinus variegatus, leading to a shortening of the first cell cycle after fertilization. The ability of Hsp to decrease cell cycle time is correlated with their ability to protect proteins against heat-induced denaturation.

The coelomic fluid of sea urchins contains motile, phagocytic cells called coelomocytes that undergo a dramatic spreading reaction when physically stressed or placed in hypotonic medium. The ability of coelomocytes to rapidly and extensively remodel their cytoskeleton in response to external stimuli suggested this as another model for testing the functional effects of exogenous Hsp. We have demonstrated that Hsp 70 inhibits the injury response of sea urchin coelomocytes, suggesting that it acts at the cell surface or is internalized in such a way as to inhibit signals regulating the polymerization of the cytoskeleton.

Selected Publications

Browne, C.L., Swan, J.B., Rankin, E.E., Calvert, H. Griffiths, S. and Tytell, M. (2007) Extracellular heat shock protein 70 has novel functional effects on sea urchin eggs and coelomocytes. J. Exp. Biol. 210:1275-1287

Silver, W.L. and Browne, C.L. (2000) Integration of laboratory exercises in development and neurobiology courses using the Xenopus oocyte expression xsystem. J. Industrial Microbiol. Biotech. 24:353-358.

Browne, C.L., Creton R., Karplus, E., Mohler, P.M., Palazzo, R.E., and Miller A.L. 1996. Analysis of the Calcium Transient at NEB During the First Cell Cycle in Dividing Sea Urchin Eggs. Biol. Bull. 191:5-16.

Browne, C.L., Miller, A.L., Palazzo, R.E., and Jaffe, L.F. 1992. On the calcium pulse during nuclear envelope breakdown (NEB) in sea urchin eggs. Biol. Bull. 183:370-372.

Browne, C.L., Bower, W.A., Palazzo, R.E. and Rebhun, L.I. 1990. Inhibition of mitosis in fertilized sea urchin eggs by inhibition of the cAMP-dependent protein kinase. Exp. Cell Res. 188:122-128.