The number and shape of dendritic spines are influenced by activity and regulated by molecules that organize the actin cytoskeleton of spines. Cortactin is an F-actin binding protein and activator of the Arp2/3 actin nucleation machinery that also interacts with the postsynaptic density (PSD) protein Shank. Cortactin is concentrated in dendritic spines of cultured hippocampal neurons, and the N-terminal half of the protein containing the Arp2/3 and F-actin binding domains is necessary and sufficient for spine targeting. Knockdown of cortactin protein by short-interfering RNA (siRNA) results in depletion of dendritic spines in hippocampal neurons, whereas overexpression of cortactin causes elongation of spines. In response to synaptic stimulation and NMDA receptor activation, cortactin redistributes rapidly from spines to dendritic shaft, correlating with remodeling of the actin cytoskeleton, implicating cortactin in the activity-dependent regulation of spine morphogenesis.

Synaptic plasticity involves the reorganization of synapses at the protein and the morphological levels. Here, we report activity-dependent remodeling of synapses by serum-inducible kinase (SNK). SNK was induced in hippocampal neurons by synaptic activity and was targeted to dendritic spines. SNK bound to and phosphorylated spine-associated Rap guanosine triphosphatase activating protein (SPAR), a postsynaptic actin regulatory protein, leading to degradation of SPAR. Induction of SNK in hippocampal neurons eliminated SPAR protein, depleted postsynaptic density-95 and Bassoon clusters, and caused loss of mature dendritic spines. These results implicate SNK as a mediator of activity-dependent change in the molecular composition and morphology of synapses.

Calcium ions are crucial messengers in the regulation of synaptic efficacy. In the postsynaptic neuron, this is exemplified by the tight temporal and spatial co-segregation of calcium ions with calcium-dependent signal transduction protein complexes in dendritic spines. Over the last several years optical imaging, physiological, structural, and biological studies have clarified the molecular mechanisms underlying differential calcium signaling within the spine. In this review, we discuss how calcium signaling "microdomains" are organized and regulated. We emphasize the structural and functional features of precisely regulated supramolecular complexes incorporating proteins involved in calcium influx, calcium efflux, and signal transduction. These complexes act in concert to orchestrate the sophisticated postsynaptic calcium signaling that underlies synaptic plasticity.

In Drosophila, the frizzled gene plays a critical role in the establishment of tissue polarity, but the function of the Frizzled family of proteins in mammals is largely unknown. Recent evidence suggested that Frizzleds are receptors for the Wnt family of secreted glycoproteins which are involved in cell fate determination. However, it is unclear how Frizzled receptors transduce Wnt signals to intracellular signaling components. Here we show that the mouse Frizzled-1, -2, -4 and -7 can bind to proteins of the PSD-95 family, which are implicated in the assembly and localization of multiprotein signaling complexes in the brain. Moreover, PSD-95 can form a ternary complex with Frizzled-2 and the adenomatous polyposis coli protein, a negative regulator of Wnt signaling, suggesting that members of the PSD-95 family may serve to recruit intracellular signaling molecules of the Wnt/Frizzled pathway into the vicinity of the receptor.

Interaction with the multi-PDZ protein GRIP is required for the synaptic targeting of AMPA receptors, but the underlying mechanism is unknown. We show that GRIP binds to the liprin-alpha/SYD2 family of proteins that interact with LAR receptor protein tyrosine phosphatases (LAR-RPTPs) and that are implicated in presynaptic development. In neurons, liprin-alpha and LAR-RPTP are enriched at synapses and coimmunoprecipitate with GRIP and AMPA receptors. Dominant-negative constructs that interfere with the GRIP-liprin interaction disrupt the surface expression and dendritic clustering of AMPA receptors in cultured neurons. Thus, by mediating the targeting of liprin/GRIP-associated proteins, liprin-alpha is important for postsynaptic as well as presynaptic maturation.

The clustering of glycine receptors and major subtypes of GABA(A) receptors at inhibitory synapses is mediated by the tubulin-binding protein gephyrin. In an attempt to identify additional components of inhibitory postsynaptic specializations, we performed a yeast two-hybrid screen using gephyrin as bait. Multiple positive clones encoded either the dynein light chain-1 (Dlc-1), also known as dynein LC8 and protein inhibitor of neuronal nitric oxide synthase, or its homolog Dlc-2. Dlc-1 protein bound efficiently to gephyrin in in vitro binding assays and colocalized with gephyrin during coexpression in HEK293 cells. The binding site for Dlc was mapped to a fragment of 63 amino acids within the central linker domain of gephyrin. In hippocampal neurons, endogenous Dlc protein was enriched at synaptic sites identified by synaptophysin and gephyrin immunostaining. Immunoelectron microscopy in spinal cord sections revealed Dlc immunoreactivity at the edges of postsynaptic differentiations, in close contact with cytoskeletal structures and at the periphery of the Golgi apparatus. Because Dlc-1 and Dlc-2 have been described as stoichiometric components of cytoplasmic dynein and myosin-Va complexes, our results suggest that motor proteins are involved in the subcellular localization of gephyrin.