A novel signaling cascade controlling actin polymerization in response to extracellular signals regulates filopodia formation and likely also neuronal synapse formation.

IRSp53 is an essential intermediate between the activation of Rac and Cdc42GTPases and the formation of cellular protrusions; it affects cell shape by coupling membrane-deforming activity with the actin cytoskeleton. IRSp53 is highly expressed in neurons where it is also an abundant component of the postsynaptic density (PSD). Here we analyze the physiological function of this protein in the mouse brain by generating IRSp53-deficient mice. Neurons in the hippocampus of young and adult knock-out (KO) mice do not exhibit morphological abnormalities in vivo. Conversely, primary cultured neurons derived from IRSp53 KO mice display retarded dendritic development in vitro. On a molecular level, Eps8 cooperates with IRSp53 to enhance actin bundling and interacts with IRSp53 in developing neurons. However, postsynaptic Shank proteins which are expressed at high levels in mature neurons compete with Eps8 to block actin bundling. In electrophysiological experiments the removal of IRSp53 increases synaptic plasticity as measured by augmented long term potentiation and pairedpulse facilitation. A primarily postsynaptic role of IRSp53 is underscored by the decreased size of the PSDs, which display increased levels of N-methyl-D-aspartate receptor subunits in IRSp53 KO animals. Our data suggest that the incorporation of IRSp53 into the PSD enables the protein to limit the number of postsynaptic glutamate receptors and thereby affect synaptic plasticity rather than dendritic morphology. Consistent with altered synaptic plasticity, IRSp53-deficient mice exhibit cognitive deficits in the contextual fear-conditioning paradigm.Rho GTPases such as Cdc42, Rac, and Rho control key events in neuronal cell biology, including the generation of neuronal polarity and morphology, establishment of dendritic spines, the generation of postsynaptic specializations and synaptic plasticity (1, 2). Specificity in these processes is thought to arise through control of different downstream targets which are recognized and activated by the active, GTP-bound forms of Rho family members. The insulin receptor substrate of 53 kDa (IRSp53) 3 is an essential mediator between activated Rac or Cdc42 and the formation of lamellipodia or filopodia, respectively. GTPase binding to IRSp53 enables interactions of its SH3 domain with downstream effectors WAVE2, Mena, Eps8, or N-WASP, all of which are known regulators of actin dynamics (3-6). In addition, the N-terminal IRSp53/missing in metastasis homology domain of IRSp53 assists in generating cellular protrusions by bundling actin filaments (5, 7, 8) and promoting membrane curvature (9, 10). Expression of IRSp53 is particularly high in the brain, and consequently IRSp53 contributes to the formation of dendritic spines in the cultured hippocampal neuron model (11).Via the SH3 domain and a C-terminal PDZ binding motif, IRSp53 also bridges postsynaptic shank and PSD-95 family members (11)(12)(13)(14). A significant enrichment in the postsynaptic density (PSD) of excitatory synapses suggests that Rac/Cdc42 signalin...

The authors have realized that the composition of the gel image in S5D Fig is not clearly explained in the figure legend, nor is it marked adequately in the figure itself. We provide therefore an amended legend and figure, which clarify the compilation of the S5D Fig and the presentation of the data.In addition, and at the editors' request, the original data related to Supporting Information S5 Fig. (D) MAPK-mediated phosphorylation does not affect the binding of Eps8 (535-821) to F-actin. Eps8 C-terminal fragment (GST-535-821) (1 μM) incubated with MAPK in the presence or absence of an excess of ATP were mixed with 5 μM of F-actin and subjected to co-sedimentation assay by high-speed ultracentrifugation. Each lane shows the supernatant (S) and the pellet (P) after ultracentrifugation. The panels show a composite of two gels. All conditions but +ATP/+MAPK were run in one gel, while the sample marked +ATP/+MAPK, including its molecular weight marker (M), were run in a different gel. (PDF) S6 Fig. (related to Fig. 6D), primary data. Eps8 phosphomutants bind Abi1 similar to Eps8 WT. Lysates of cells expressing GFP-Eps8 WT or SATA or SETE (indicated at the top) were incubated with immobilized GST, or GST-PIN1, used as negative control, or GST-Abi1 as described in Fig