Gregg Gundersen, Ph.D.

Our laboratory is interested in the function of microtubules (MTs) in cell differentiation, polarization and motility. We have been trying to understand how increases in MT stability are triggered during cell polarization, what factors act to selectively stabilize MTs and how stabilized MTs contribute to cell polarization. In the last few years we have established a number of the molecular steps in a signal transduction pathway that selectively activates MT stability and causes the localized accumulation of other organelles (e.g., intermediate filaments) on the stabilized MTs. Our results suggest that MTs function as cytoplasmic organizers capable of positioning cellular organelles in respond to extracellular cues.

As one approach to understanding the factors involved in changing the stability of MTs, we conducted a pharmacological screen and found that Ser/Thr protein phosphatase (PPase) inhibitors were capable of selectively destabilizing the stable MTs in fibroblasts and epithelial cells, without interfering with the dynamic MTs. This has now been confirmed in neurons as well.

Because of our data with the PPase inhibitors and the abundant literature showing that the interaction of MT-associated proteins (MAPs) with MTs was regulated by phosphorylation, we initiated a biochemical approach to determine whether PPases might be directly bound to MTs. The possibility that there was a MT-associated PPase was also intriguing since the MAP, tau, had previously been found in a hyperphosphorylated form in the paired helical filaments that comprise one of the two characteristic deposits in Alzheimer's disease brains, namely neurofibrillary tangles.

We have recently found that there is a Type 1 PPase (PP1) that specifically associates with MTs biochemically prepared from brain extracts. Since the catalytic subunit of PP1 (PP1c) was unable to bind to MTs, we pursued the idea that there was another subunit of the "PP1MT" that was responsible for binding it to MTs. We have now shown that PP1MT is a complex of PP1c and tau. We have shown directly that tau is capable of forming a complex with PP1c and binding PP1c to MTs. The PP1c that binds to MTs via tau shows increased activity suggesting that tau is a targeting subunit for PP1c that regulates its activity.

PP1c is the first protein that has been identified which forms a stable complex with tau. We have initiated a number of projects to further investigate this interaction. We are determining the structural basis for this interaction with the hopes of identifying the domain on tau necessary for binding PP1c. We are surveying various MT-interacting proteins to determine whether they are substrates of PP1MT. We also plan to examine whether the PP1c interaction with tau is essential for MT dependent functions such as neurite outgrowth, neurite stability, and neuronal polarity. Ultimately, we plan to examine tissue from Alzheimer's disease patients to determine whether loss of PP1-tau interaction contributes the known histopathology of Alzheimer's disease.

Selected Publications:

• Cook, T.A., Nagasaki, T., and Gundersen, G.G. Rho GTPase mediates the selective stabilization of microtubules in vivo. J. Cell Biol. 141: 175-185 (1998).

• Liao, G., and Gundersen, G.G. Kinesin is a candidate for crossbridging micro-tubules and intermediate filaments: selective binding of kenesin to detyrosinated tubulin and vimentin. J. Biol. Chem. 273: 9797-9803 (1998).

• Ho, C.-L., Martys, J.L., Mikhailov, A., Guncersen, G.G., and Liem, R.K.H. Novel features of intermediate filament dynamics revealed by green fluorescent protein fusion proteins. J. Cell Sci. 111: 1767-1778 (1998).

• Liao, H., Li, Y., Brautigan, D.L., Lee, E.Y.C., and Gundersen, G.G. Protein phosphatase 1 is targeted to microtubules by the microtubule-associated protein tau. J. Biol. Chem. 273: 21901-21908 (1998).

• Gundersen, G.G., and Cook, T.A. Microtubules and signal transduction. Curr. Opin. Cell Biol. 11: 81-94 (1999).

• Kreitzer, G., Liao, G. and Gundersen, G.G. Detyrosination regulates the interaction of intermediate filaments with microtubules in vivo via a kinesin-dependent mechanism. Mol. Biol. Cell. 10: 1105-1118 (1999).