The Insulin Receptor Substrate of 53 kDa (IRSp53) Limits Hippocampal Synaptic Plasticity

Abstract

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...

Fragile X Mental Retardation Protein Regulates the Levels of Scaffold Proteins and Glutamate Receptors in Postsynaptic Densities

“…An increased postsynaptic abundance of IRSp53 in FMRP-deficient brains may play a similar role in FXS pathogenesis as both IRSp53 and SAPAPs are able to connect PSD-95 to Shank scaffold proteins (32). In accordance with this hypothesis, IRSp53-deficient mice exhibit impaired learning (72,73).…”

Different functional modes of BAR domain proteins in formation and plasticity of mammalian postsynapses

“…Although these IRSp53-mediated connections between major factors in postsynapse formation and organization are reflected in decreased spine size and density upon acute IRSp53 loss-of-function (Choi et al, 2005), both the density and the ultrastructure of dendritic spines were unchanged in IRSp53-knockout mice (Kim et al, 2009). However, these mice have defects in spatial learning and novel object recognition suggesting that despite putative compensatory mechanisms that ensure synapse formation, proper brain function might nevertheless require the presence of the integrative component IRSp53 in postsynapses ( Kim et al, 2009;Sawallisch et al, 2009). Whether and to what extent the synaptic functions of IRSp53 indeed involve any membrane shaping or at least binding of the I-BAR domain to membranes has to our knowledge not been addressed so far.…”

High-Throughput LC–MS/MS Proteomic Analysis of a Mouse Model of Mesiotemporal Lobe Epilepsy Predicts Microglial Activation Underlying Disease Development

“…Most of these proteins were also found to be downregulated at 30dpi, suggesting a sustaining phenomenon of neurodegeneration (Table 4). More specifically, Brain-specific angiogenesis inhibitor 1-associated protein 2, an abundant component of the postsynaptic density, shown to interact with Rac and ProSAP/Shank proteins and considered a regulator of hippocampal synaptic plasticity 54,55 as well as Brain-specific serine/threonine-protein kinase 1, a presynaptic regulator of neurotransmitter release 56 , were significantly downregulated (0.37 fold, 3dpi; 0.16 fold, 30dpi) or detected only in NaCl-injected animals at 3dpi, respectively. Shank3, which interacts with metabotropic glutamate receptors, was again detected only in NaCl-injected animals at 3dpi, as in the case of 1dpi.…”