Actin plays important roles in a number of synaptic processes, including synaptic vesicle organization and exocytosis, mobility of postsynaptic receptors, and synaptic plasticity. However, little is known about the mechanisms that control actin at synapses. Actin dynamics crucially depend on LIM kinase 1 (LIMK1) that controls the activity of the actin depolymerizing proteins of the ADF/cofilin family. While analyses of mouse mutants revealed the importance of LIMK1 for both pre- and postsynaptic mechanisms, the ADF/cofilin family member n-cofilin appears to be relevant merely for postsynaptic plasticity, and not for presynaptic physiology. By means of immunogold electron microscopy and immunocytochemistry, we here demonstrate the presence of ADF (actin depolymerizing factor), a close homolog of n-cofilin, in excitatory synapses, where it is particularly enriched in presynaptic terminals. Surprisingly, genetic ablation of ADF in mice had no adverse effects on synapse structure or density as assessed by electron microscopy and by the morphological analysis of Golgi-stained hippocampal pyramidal cells. Moreover, a series of electrophysiological recordings in acute hippocampal slices revealed that presynaptic recruitment and exocytosis of synaptic vesicles as well as postsynaptic plasticity were unchanged in ADF mutant mice. The lack of synaptic defects may be explained by the elevated n-cofilin levels observed in synaptic structures of ADF mutants. Indeed, synaptic actin regulation was impaired in compound mutants lacking both ADF and n-cofilin, but not in ADF single mutants. From our results we conclude that n-cofilin can compensate for the loss of ADF in excitatory synapses. Further, our data suggest that ADF and n-cofilin cooperate in controlling synaptic actin content.
References
[1]
Matus A, Ackermann M, Pehling G, Byers HR, Fujiwara K (1982) High actin concentrations in brain dendritic spines and postsynaptic densities. Proc Natl Acad Sci U S A 79: 7590–7594.
[2]
Fifkova E, Delay RJ (1982) Cytoplasmic actin in neuronal processes as a possible mediator of synaptic plasticity. J Cell Biol 95: 345–350.
[3]
Landis DM, Hall AK, Weinstein LA, Reese TS (1988) The organization of cytoplasm at the presynaptic active zone of a central nervous system synapse. Neuron 1: 201–209.
[4]
Cingolani LA, Goda Y (2008) Actin in action: the interplay between the actin cytoskeleton and synaptic efficacy. Nat Rev Neurosci 9: 344–356.
[5]
Renner M, Choquet D, Triller A (2009) Control of the postsynaptic membrane viscosity. J Neurosci 29: 2926–2937.
[6]
Rust MB, Gurniak CB, Renner M, Vara H, Morando L, et al. (2010) Learning, AMPA receptor mobility and synaptic plasticity depend on n-cofilin-mediated actin dynamics. EMBO J 29: 1889–1902.
[7]
Gu J, Lee CW, Fan Y, Komlos D, Tang X, et al. (2010) ADF/cofilin-mediated actin dynamics regulate AMPA receptor trafficking during synaptic plasticity. Nat Neurosci 13: 1208–1215.
[8]
Kuriu T, Inoue A, Bito H, Sobue K, Okabe S (2006) Differential control of postsynaptic density scaffolds via actin-dependent and -independent mechanisms. J Neurosci 26: 7693–7706.
[9]
Tada T, Sheng M (2006) Molecular mechanisms of dendritic spine morphogenesis. Curr Opin Neurobiol 16: 95–101.
[10]
Bernstein BW, Bamburg JR (2010) ADF/cofilin: a functional node in cell biology. Trends Cell Biol 20: 187–195.
[11]
Zhou Q, Homma KJ, Poo MM (2004) Shrinkage of dendritic spines associated with long-term depression of hippocampal synapses. Neuron 44: 749–757.
[12]
Fukazawa Y, Saitoh Y, Ozawa F, Ohta Y, Mizuno K, et al. (2003) Hippocampal LTP is accompanied by enhanced F-actin content within the dendritic spine that is essential for late LTP maintenance in vivo. Neuron 38: 447–460.
Frangiskakis JM, Ewart AK, Morris CA, Mervis CB, Bertrand J, et al. (1996) LIM-kinase1 hemizygosity implicated in impaired visuospatial constructive cognition. Cell 86: 59–69.
[15]
Meng Y, Zhang Y, Tregoubov V, Janus C, Cruz L, et al. (2002) Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice. Neuron 35: 121–133.
[16]
Lena JY, Bamburg JR, Rabie A, Faivre-Sarrailh C (1991) Actin-depolymerizing factor (ADF) in the cerebellum of the developing rat: a quantitative and immunocytochemical study. J Neurosci Res 30: 18–27.
[17]
Bellenchi GC, Gurniak CB, Perlas E, Middei S, Ammassari-Teule M, et al. (2007) N-cofilin is associated with neuronal migration disorders and cell cycle control in the cerebral cortex. Genes Dev 21: 2347–2357.
[18]
Guillemin I, Becker M, Ociepka K, Friauf E, Nothwang HG (2005) A subcellular prefractionation protocol for minute amounts of mammalian cell cultures and tissue. Proteomics 5: 35–45.
[19]
Sholl DA (1953) Dendritic organization in the neurons of the visual and motor cortices of the cat. J Anat 87: 387–406.
[20]
Brewer GJ, Torricelli JR, Evege EK, Price PJ (1993) Optimized survival of hippocampal neurons in B27-supplemented Neurobasal, a new serum-free medium combination. J Neurosci Res 35: 567–576.
[21]
Lee B, Tumu P, Paul IA (2002) Effects of LP-BM5 murine leukemia virus infection on errors and response time in a two-choice serial reaction time task in C57BL/6 mice. Brain Res 948: 1–7.
[22]
Ikeda S, Cunningham LA, Boggess D, Hawes N, Hobson CD, et al. (2003) Aberrant actin cytoskeleton leads to accelerated proliferation of corneal epithelial cells in mice deficient for destrin (actin depolymerizing factor). Hum Mol Genet 12: 1029–1037.
[23]
Racz B, Weinberg RJ (2006) Spatial organization of cofilin in dendritic spines. Neuroscience 138: 447–456.
[24]
Herde MK, Friauf E, Rust MB (2010) Developmental expression of the actin depolymerizing factor ADF in the mouse inner ear and spiral ganglia. J Comp Neurol 518: 1724–1741.
