Actin filaments (F-actin) constitute a dynamic component of the cytoskeleton of eukaryotic cells. Processes of cellular motility and cell division are dependent upon a pool of actin subunits capable of rapid assembly and disassembly in response to extracellular signals. The interaction of actin with the cell membrane and the spatial and temporal changes in actin organization which underlie cell movement and neuronal pathfinding are largely regulated by a number of actin binding proteins, among the most important of which is actin depolymerizing factor (ADF) and its related family member, cofilin.

ADF and cofilin are 18.5kDa proteins first isolated from brain. All eukaryotic organisms including plants and protists have an ADF/cofilin (AC)-like protein. Genetic studies have shown that AC proteins are essential for survival. AC is needed for the reorganization of actin in the cleavage furrow during cytokinesis. Immuno-fluorescence localization of AC showed it to be particularly enriched in the leading edge of cultured fibroblasts and in neuronal growth cones. ADF is spatially and temporally localized with actin and serves as a carrier for axonal transport of actin. Both in vitro and in vivo, ADF serves to rapidly increase the dynamics of actin filaments. Blocking ADF activity completely inhibits cellular processes dependent upon actin reorganization.

ADF activity is pH dependent, but there are several other regulatory mechanisms important to cell function. First, the expression of ADF, but not cofilin is regulated by the level of assembly competent actin. This suggests that there is an integrated mechanism for regulating the expression of the ADF mRNA that is responsive to the cytoskeletal utilization of actin by the cell. Second, under some growth states, ADF and cofilin, which contains nuclear localization sequences, is found in the nucleus of cells where their function is unknown. Under conditions of stress, AC carries actin into the nucleus where the proteins form rod or sheets, which are found with increasing frequency in neurons of the brain during normal aging or in response to strong electrical stimulation. Although the functional significance of these nuclear inclusions is unknown, they may play a role in cell survival and neuronal plasticity. Third, AC is regulated by phosphorylation of a single serine residue. Since transmembrane signaling culminates in changes in the activity of protein kinases or phosphatases, AC is a key protein through which extracellular ligands bring about an alteration in cytoskeletal organization. In neuronal growth cones, this actin reorganization could be the key to pathfinding. Lim kinase, which phosphorylates AC, is the target of many signal transduction pathways and ADF is its only known substrate. Lim kinase is defective in Williams syndrome, a visuo-spatial cognitive defect.

ADF is rapidly dephosphorylated in response to energy depletion or oxidative stress. There are several phosphatases that can dephosphorylate AC but those in the slingshot family seem to be particularly important. AC overactivation results in the formation of cytoplasmic rods containing actin and ADF. Both of these proteins are also components of cytoplasmic inclusion bodies found in aging brain and in many neurodegenerative diseases. Since local energy depletion (via stroke) may be a major factor in the development of Alzheimer's disease, formation of these rods may be an early step leading to the malfunction of the neuronal circuitry.

Current research in my laboratory is addressing the role of ADF in early neuronal development, neuronal sprouting and regeneration, and neurodegeneration. We make and use viral vectors for expressing proteins in neuronal cultures and use computer imaging to analyze cell behavior.

Increase in neurite outgrowth mediated by overexpression of actin depolymerizing factor. P.J. Meberg, and J.R. Bamburg. J. Neurosci. 20, 2459-2469 (2000)

Cdc42 stimulates neurite outgrowth and formation of growth cone filopodia and lammelipodia. M.D. Brown, B.J. Cornejo, T.B. Kuhn and J.R. Bamburg. J. Neurobiol. 43, 352-364 (2000)

Regulating actin filament dynamics in vivo. H. Chen, B.W. Bernstein and J. R. Bamburg, Trends Biochem. Sci. 25, 19-23 (2000)

Regulating actin dynamics in neuronal growth cones by ADF/cofilin and Rho family GTPases. T.B. Kuhn, P.J. Meberg, M.D. Brown, B.W. Bernstein, L.S. Minamide, J. R. Jensen, K. Okada, E. Soda and J.R. Bamburg. J. Neurobiol. 44, 126-144 (2000)

ADF/cofilin and actin dynamics in disease. J.R. Bamburg and O.P. Wiggan. Trends Cell Biol. 12, 598-605 (2002)

Xenopus actin interacting protein 1 (XAip1) enhances cofilin fragmentation of filaments by capping filament ends. K. Okada, L. Blanchoin, H. Chen, H. Abe, T.D. Pollard, and J.R. Bamburg J. Biol. Chem. 277, 43011-43016 (2002)

ADF/cofilin mediates actin cytoskeletal alterations in LLC-PK cells during ATP depletion. S.L. Ashworth, E. Southgate, R. M. Sandoval, P.J. Meberg, J. R. Bamburg and B.A. Molitoris. Am. J. Physiol. 284, F852-F862 (2003)

ADF/cofilin controls cell polarity during fibroblast migration. H.R. Dawe, L.S. Minamide, J.R. Bamburg and L.P. Cramer. Curr. Biol. 13, 252-257 (2003)

Production and Use of Replication Deficient Adenovirus for Transgene Expression in Neurons. L. S. Minamide, A. E. Shaw, P. D. Sarmiere, O. Wiggan, M. T. Maloney, B.W. Bernstein, J. M. Sneider, J.A. Gonzalez and J.R. Bamburg. Methods Cell Biol 71, 387-416 (2003)

Actin-ATP hydrolysis is a major energy drain for neurons. B.W. Bernstein and J.R. Bamburg. J. Neuroscience 23, 1-6 (2003)

Neurons: Methods and Applications for the Cell Biologist. J. R. Bamburg and P. J. Hollenbeck, Eds. Methods in Cell Biology, Vol. 71, 464pp, Academic Press, San Diego, CA (2003)

Regulation of the Neuronal Actin Cytoskeleton by ADF/Cofilin. P. D. Sarmiere and J. R. Bamburg, J. Neurobiol. 58, 103-117 (2004)

In vitro actiivity differences between proteins of the ADF /cofilin family define two distinct subgroups. Chen, B.W. Bernstein, J. Sneider, J.A. Boyle, L.S. Minamide and J.R. Bamburg. Biochemistry 43, 7127-7142 (2004)

BDNF regulation of growth cone filopodial dynamics is mediated through ADF/cofilin. Gehler, A.E. , Shaw, P.D. Sarmiere, J.R. Bamburg and P.C.Letourneau. J. Neurosci. 24, 10741-10749 (2004)

Interplay between components of a novel LIM kinase 1-slingshot phosphatase complex regulate ADF/cofilin acitivity. J. Soosairajah, S. Maiti, O. wiggan, P. Sarmiere, N. Moussi, B. Sarcevic, R. Sampath, J. R. Bamburg and O. Bernard. EMBO J. 24,473-486 (2005)

Listeria actin-based motility reveals major roles for calcium and gelsolin in actin filament recycling." L. Larson, S. Arnaudeau, R. Krause, W. Li, B. Hao, J.R. Bamburg, B. Gibson, D.P. Lew, N.Demaurex and F.southwick. Proc.Nat'l.Acad.SciUSA 102, 1921-1926 (2005)

Micropuncture gene delivery and intravital two-photon visualization of protein expression in rat kidney. G. A. Tanner, R. M. Sandoval, B. A. Molitoris, J. R. Bamburg, and S. L. Ashworth. Am. J. Physiol. 289, F638-643 (2005)

&beta-Secretase-cleaved APP accumulates at actin inclusions induced in neurons by stress or amyloid beta: a feed-forward mechanism for Alzheimers Disease. M.T. Maloney, L.S. Minamide, A.W. Kinley, J.A. Boyle and J.R. Bamburg. J. Neurosci. 25, 11313-11321 (2005)

Cdc42 participates in the regulation of ADF/cofilin and retinal growth cone filopodial dynamics by Brain Derived Neurotrophic Factor. T-j. Chen, S. Gehler, A.E. Shaw, J.R. Bamburg, P.C. Letourneau. J. Neurobiol. 66, 103-114 (2006)

Regulation of LIM-kinase 1 and cofilin in activated platelets. D. Pandey, P. Goyal, J. R Bamburg and W. Siess. Blood 107, 575-583 (2006)

Expression of &alpha&nuβ3 promotes the activation of ADF/cofilin. D. Dang, J. R. Bamburg, and D.M. Ramos. Exp.Cell Res 312, 468-477 (2006)

Cofilin mediates ATP depletion-induced endothelial cell actin alterations. M.V. Suurna, S. Ashworth, M. Hosford, R. M Sandoval, S.E. Wean, B. Shah, J.R. Bamburg and B.A. Molitoris. Am J. Physiol Renal Physiol 290, F1398-1407 (2006)

Essential requirement for Rho family GTPase signaling in Pax3 induced mesenchymal-epithelial transition. O. Wiggan, A. E. Shaw and J. R.Bamburg. Cellular Signalling 18, 1501-1514 (2006)

Formation of actin-ADF/cofilin rods transiently retards decline of mitochondrial potential and ATP in stressed neurons. B.W. Bernstein, H. Chen, J.A. Boyle and J. R. Bamburg. Am J Physiol Cell Physiol. 291, C828-39 (2006)

BMP molecules guide growth cones by a balancing act of LIM kinase and slingshot phosphatase on ADF/cofilin. Z. Wen, L. Han, J. R. Bamburg, S. Shim, G-l. Ming, and J. Q. Zheng. J. Cell Biol. 178, 107-119 (2007)

Cofilin-mediated neurodegeneration in Alzheimer's disease and other amyloidopathies. M.T. Maloney and J.R. Bamburg. Molecular Neurobiology 35, 21-44 (2007)

Mechanisms underlying actin aggregate formation inside the nucleus. A. Domazetovska, B. Ilkovski, S. T. Cooper, M. Ghoddusi, E. C. Hardeman, L. S. Minamide, P. Gunning, J. R. Bamburg and K. N. North. Brain 130, 3275-3284 (2007)

ADF/cofilin controls formation of oriented actin filament bundles in the cells body to trigger fibroblast polarization. T. Mseka, J.R. Bamburg, L. P. Cramer. J Cell Sci. 120, 4332-4344 (2007)

Cdc42 regulates cofilin during the establishment of neuronal polarity. B. K. Garvalov, K. C. Flynn, D. Neukirchen, L. Meyn, N. Teusch, X. Wu, C. Brakebusch, J. R. Bamburg, and F. Bradke. J. Neurosci. 27, 13117-13129 (2007)

Stochastic simulation of actin dynamics reveals the role of annealing and fragmentation. J. Fass, C. W. Pak, J. R. Bamburg, A. Mogilner. J Theor. Biol. (in press)

Actin Binding Proteins Take the Reigns in Growth Cones. C. Pak , K.C. Flynn and J.R. Bamburg. Nature Reviews Neuroscience (in press)