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| FACULTY BIOGRAPHY |
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| Michael Sheetz |
| William R Kenan Jr. Professor |
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| Force-dependent Signaling
The morphology of cells, organs and whole organisms is determined by the generation of forces on the immediate environment, which is either extracellular matrix or adjacent cells. Thus, control of forces controls morphology. Further, a hallmark of cancer is cell transformation, which is defined as a cell’s ability to grow on soft agar, i.e., in the absence force. There are many pathways of force transduction in cells and we have identified two in recent studies, one involving force generation at fibronectin bead binding sites (Galbraith et al., 2002) by a protein tyrosine phosphatase-integrin complex (von Wichert et al., 2003). The second involving the direct alteration of cytoskeleton morphology by force (Sawada and Sheetz, 2002). The protein tyrosine phosphatase is at the leading edge of the cell and is involved in early, low force, activation. Immediately downstream are the Src family kinases (oncogenes) that also affect motility. In the case of the second pathway, our working model is that the cytoskeleton is unfolding with force and thereby binding and activating signaling enzymes. We are currently engaged in studies to understand the detailed molecular mechanisms involved and the extent of the involvement of these two pathways in a variety of phenomena from fertilization to brain function. Further, we are developing several new tools and protocols for measuring cell forces at the molecular level (Jiang et al., 2003), which are revealing many new aspects of how cells can both generate and respond to external forces. An overview of some of the proteins involved and their specific roles in force-dependent signaling and motility is provided at Bead Force Transduction.
Cell Spreading and Force Generation
We have an effort underway to define quantitatively the steps involved in cell adhesion to and spreading on a matrix-coated surface. Using a variety of mouse fibroblast cell lines that are missing proteins in various motility pathways, we are determining the quantitative changes in the spreading process. This will enable us to generate a working model of the process of spreading that is consistent with previous studies as well as our findings. The working model can be accessed at Cell Spreading.
Membrane (Lipid)-Cytoskeleton Adhesion and PIP2
Cell plasma membranes conform to the cytoskeleton shape through an extensive interaction between them. After many studies of the strength of membrane-cytoskeleton adhesion, there appears to be a very tight control of the adhesion energy through the regulation of the free concentration of phosphatidylinositol 4,5 diphosphate (PIP2)(reviewed in Sheetz, 2001) (Raucher et al., 2000). Now we are exploring how the free level of PIP2 is regulated and which cytoskeleton proteins interact with PIP2 in vivo. Much of our effort is focusing on the lipid binding proteins that are in the MARCKS class of proteins. Lipid control of membrane-cytoskeleton adhesion has important implications for membrane structure and the control of a wide variety of cell functions.
Organelle Traffic in Neurons
We have a long-standing interest in the molecular basis of organelle transport in neurons. Recent studies are focused on the control of mitochondrial movements since they are critical organelles that move in two directions on microtubules and interact with myosin. Thus, they are ideal for studying control of motor switching and regulation of motility. Using in vivo and in vitro assays of directional movement of mitochondria on microtubules, we have found a critical role for inositol lipids in the cytoplasmic dynein but not kinesin-dependent movements (De Vos et al., 2003). Now we will analyze whether the basis of control is through vesicle binding or motor activation. Additional projects are focussed on membrane binding sites for kinesin such as kinectin and the role of tension in ER and Golgi networks.
Bacterial Pilus Retraction
The retraction of bacterial pili appears to constitute a robust motility mechanism (Merz et al., 2000) that can generate the largest force of any known single molecular motor (Maier et al., 2002). There are many mutations in the pilus retraction pathway and we are now collaborating with groups who have characterized these mutations to understand the mechanism of retraction and the role of force generation in pathogenesis.
MedLine Listing of Dr. Sheetz's Publications |
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Representative Recent Publications
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- De Vos, K., J. Sable, K.E. Miller, and M.P. Sheetz (2003) Expression of phosphatidylinositol (4.5) bisphosphate-specific pleckstrin homology domains alters direction but not level of axonal transport of mitochondria Mol Biol Cell In Press.
- Jiang, G., G. Giannone, D.R. Critchley, E. Fukumoto, and M.P. Sheetz (2003) Two-piconewton slip bond between fibronectin and the cytoskeleton depends on talin Nature 424: 334-7. Article
- Galbraith, C.G., K.M. Yamada, and M.P. Sheetz (2002) The relationship between force and focal complex development J Cell Biol 159: 695-705. Article
- Maier, B., L. Potter, M. So, C.D. Long, H.S. Seifert, and M.P. Sheetz (2002) Single pilus motor forces exceed 100 pN Proc Natl Acad Sci U S A 99: 16012-7. Article
- Sawada, Y., and M.P. Sheetz (2002) Force transduction by Triton cytoskeletons J Cell Biol 156: 609-15. Article
- Sheetz, M.P. (2001) Cell control by membrane-cytoskeleton adhesion Nat. Rev. Molec. Cell Biol. 2: 392-396. Article
- Merz, A. J., M. So, and M.P. Sheetz. (2000) Pilus retraction powers bacterial twitching motility Nature 407: 98-102. Article
- Miller, K. E. and M.P. Sheetz (2000) Characterization of myosin V binding to brain vesicles J. Biol. Chem. 275: 2598-2606.
- Raucher, D., and M. P. Sheetz. (2000) Cell spreading is regulated by membrane tension J. Cell Biol. 148: 127-136. Article
- Raucher, D., T. Stauffer, W. Chen, K. Shen, S. Guo, J.D. York, M.P. Sheetz, and T. Meyer (2000) Phosphatidylinositol 4,5-bisphosphate functions as a second messenger that regulates cytoskeleton-plasma membrane adhesion Cell 100: 221-228. Article
- Felsenfeld, D.P., Schwartzberg, P. L., Venegas, A., Tse, R. and Sheetz, M.P. (1999) Selective regulation of integrin-cytoskeleton interactions by the tyrosine kinase Src Nature Cell Biol. 1: 200-206.
- Galbraith, C. G. and M.P. Sheetz (1999) Keratocytes pull with similar forces on their dorsal and ventral surfaces J. Cell Biol. 147: 1313-1323.
- Raucher, D., and Sheetz, M. P. (1999) Membrane expansion increases endocytosis rate during mitosis J. Cell Biol. 144: 497-506.
- Choquet, D., D.P. Felsenfeld, and M.P. Sheetz. (1997) Extracellular Matrix Rigidity Causes Strengthening of Integrin-Cytoskeletal Linkages Cell 88: 39-48.
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