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First published online January 10, 2007
doi: 10.1242/10.1242/jcs.03337

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Transmitting on actin: synaptic control of dendritic architecture

¹ Cavalieri Ottolenghi Scientific Institute, Universita degli Studi di Torino, A.O. San Luigi Gonzaga, Regione Gonzole 10, 10043 Orbassano (Torino), Italy
² Department of Human Genetics, Catholic University of Leuven and Department of Developmental and Molecular Genetics, Flanders Interuniversity Institute of Biotechnology (VIB11), Heerestraat 43, 3000 Leuven, Belgium

View larger version (30K): [in this window] [in a new window]   Fig. 1. Putative model of actin-regulatory pathways controlled by ionotropic glutamate receptors. Active RhoA interacts with NMDARs at the excitatory PSD and recruits and activates the ROCK/PII complex, thus stabilizing actin. High levels of Ca2+ induce CaMKII-dependent phosphorylation of spinophilin, detaching it from actin and recruiting it to the membrane, where it interacts with Lcf, which in turn activates RhoA. The actin-severing activity of cofilin is controlled by different kinases and phosphatases. LIMK, for example, is a negative-regulator of cofilin activity. Activation of LIMK can occur in a Rac1-dependent manner through its downstream effector PAK. NMDA stimuli could trigger an increase in local Rac1-activity levels through the Rac1-GEFs PIX and Tiam1, consequently increasing the activity of cofilin, which eventually results in higher actin-turnover rates.

View larger version (33K): [in this window] [in a new window]   Fig. 2. Putative model of actin regulatory pathways mediated by non-glutamate receptors. The actin-polymerizing activity of Arp2/3 and N-WASP depends on cortactin phosphorylation levels, which are controlled in a TrkB- and Src-dependent manner. Activation of EphB recruits and activates kalirin to spines, which through Rac and PAK results in activation of myosin.

View larger version (28K): [in this window] [in a new window]   Fig. 3. Putative network of actin-regulatory proteins present in spines. The actin-severing activity of cofilin depends on a balance of various kinases and phosphatases (e.g. LIMK and CN/PP2B). The binding of cofilin to actin affects filament structure and lowers the actin affinity of drebrin at these sites. The actin stabilizer drebrin prevents actin reorganization by negatively regulating the binding of myosin to actin filaments and by interacting with gelsolin. Myosin stabilizes and contracts F-actin structures. Myosin motor activity can be triggered through PAK but also by other means. Gelsolin severs actin in a Ca2+-dependent manner and caps the barbed ends of filaments, allowing control of actin polymerization at these sites.

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