Differential roles of Rap1 and Rap2 small GTPases in neurite retraction and synapse elimination in hippocampal spiny neurons

HNK-1 (human natural killer-1) glyco-epitope is essential for normal spine morphogenesis in developing hippocampal neurons

The human natural killer-1 (HNK-1) glyco-epitope possesses a unique structural feature, a sulfated glucuronic acid attached to lactosamine on the non-reducing termini of glycans. The expression of HNK-1 is temporally and spatially regulated by glucuronyltransferase (GlcAT-P) in the brain. Our previous report showed that mice lacking GlcAT-P almost completely lost HNK-1 expression in the brain and exhibited reduced long-term potentiation (LTP) at hippocampal CA1 synapses. GlcAT-P-deficient mice also showed impaired hippocampus-dependent spatial learning. Although HNK-1 plays an essential role in synaptic plasticity and memory formation, it remains unclear how HNK-1 regulates these functions. In this study, we showed that loss of the HNK-1 epitope resulted in an increase of filopodium-like immature spines and a decrease of mushroom-like mature spines in both the early postnatal mouse hippocampus and cultured hippocampal neurons. However, HNK-1 had no influence on spine density or filopodium formation. Immunofluorescence staining revealed that loss of HNK-1 altered the distribution of postsynaptic proteins such as alpha-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA)-type glutamate receptor subunit GluR2 and PSD-95 from spine heads onto dendritic shafts without affecting synapse formation, resulting in an increase of shaft synapses in cultured GlcAT-P-deficient neurons. GluR2, a major HNK-1 carrier glycoprotein in postsynaptic density, has the ability to promote spine morphogenesis. Overexpression of GluR2 promoted spine growth in both wild-type and GlcAT-P-deficient neurons, but the increase in GlcAT-P-deficient neurons was lower than that in wild-type neurons. This is the first evidence that HNK-1 is a key factor for normal dendritic spine maturation and is involved in the distribution of postsynaptic proteins.

Activity patterns govern synapse-specific AMPA receptor trafficking between deliverable and synaptic pools

In single neurons, glutamatergic synapses receiving distinct afferent inputs may contain AMPA receptors (-Rs) with unique subunit compositions. However, the cellular mechanisms by which differential receptor transport achieves this synaptic diversity remain poorly understood. In lateral geniculate neurons, we show that retinogeniculate and corticogeniculate synapses have distinct AMPA-R subunit compositions. Under basal conditions at both synapses, GluR1-containing AMPA-Rs are transported from an anatomically defined reserve pool to a deliverable pool near the postsynaptic density (PSD), but further incorporate into the PSD or functional synaptic pool only at retinogeniculate synapses. Vision-dependent activity, stimulation mimicking retinal input, or activation of CaMKII or Ras signaling regulated forward GluR1 trafficking from the deliverable pool to the synaptic pool at both synapses, whereas Rap2 signals reverse GluR1 transport at retinogeniculate synapses. These findings suggest that synapse-specific AMPA-R delivery involves constitutive and activity-regulated transport steps between morphological pools, a mechanism that may extend to the site-specific delivery of other membrane protein complexes.

Combinatorial morphogenesis of dendritic spines and filopodia by SPAR and alpha-actinin2

Rap small GTPases regulate excitatory synaptic strength and morphological plasticity of dendritic spines. Changes in spine structure are mediated by the F-actin cytoskeleton, but the link between Rap activity and actin dynamics is unclear. Here, we report a novel interaction between SPAR, a postsynaptic inhibitor of Rap, and alpha-actinin, a family of actin-cross-linking proteins. SPAR and alpha-actinin engage in bidirectional structural plasticity of dendritic spines: SPAR promotes spine head enlargement, whereas increased alpha-actinin2 expression favors dendritic spine elongation and thinning. Surprisingly, SPAR and alpha-actinin2 can function in an additive rather than antagonistic fashion at the same dendritic spine, generating combination spine/filopodia hybrids. These data identify a molecular pathway bridging the actin cytoskeleton and Rap at synapses, and suggest that formation of spines and filopodia are not necessarily opposing forms of structural plasticity.

The effects of amyloid precursor protein on postsynaptic composition and activity

The amyloid precursor protein (APP) is cleaved to produce the Alzheimer disease-associated peptide Abeta, but the normal functions of uncleaved APP in the brain are unknown. We found that APP was present in the postsynaptic density of central excitatory synapses and coimmunoprecipitated with N-methyl-d-aspartate receptors (NMDARs). The presence of APP in the postsynaptic density was supported by the observation that NMDARs regulated trafficking and processing of APP; overexpression of the NR1 subunit increased surface levels of APP, whereas activation of NMDARs decreased surface APP and promoted production of Abeta. We transfected APP or APP RNA interference into primary neurons and used electrophysiological techniques to explore the effects of APP on postsynaptic function. Reduction of APP decreased (and overexpression of APP increased) NMDAR whole cell current density and peak amplitude of spontaneous miniature excitatory postsynaptic currents. The increase in NMDAR current by APP was due to specific recruitment of additional NR2B-containing receptors. Consistent with these findings, immunohistochemical experiments demonstrated that APP increased the surface levels and decreased internalization of NR2B subunits. These results demonstrate a novel physiological role of postsynaptic APP in enhancing NMDAR function.

Morphological differences between wild-type and transgenic superoxide dismutase 1 lumbar motoneurons in postnatal mice

Quantitative analysis of the dendritic arborizations of wild-type (WT) and superoxide dismutase 1 (SOD1) postnatal mouse motoneurons was performed following intracellular staining and 3D reconstructions with Neurolucida system. The population of lumbar motoneurons was targeted in the caudal part of the L5 segment, and all labeled motoneurons were located within the same ventrolateral pool. Despite the similar size of the soma and the mean diameter of primary dendrites, the dendritic arborizations of the WT and SOD1 motoneurons showed significant differences in terms of their morphometric parameters. The metric and topological parameters of dendrites show that the total dendritic length and surface area and total number of segments, branching nodes, and tips per motoneuron were significantly higher in SOD1 motoneurons. Our main finding concerns a proliferation of dendritic branches starting at about 100 microm from the soma in the SOD1 motoneurons. However, the longest and mean dendritic paths from soma to terminations were similar, giving a comparable envelope of the dendritic fields. Indeed, the SOD1 motoneurons were larger as a result of abnormal branching. The results suggest that a defect in pruning mechanisms occurs during this developmental period. The abnormal growth of the dendritic arborizations and the reduced excitability of postnatal SOD1 motoneurons could be a neuroprotective response and would represent an early compensatory mechanism against the activity-induced toxicity.

Rit signaling contributes to interferon-gamma-induced dendritic retraction via p38 mitogen-activated protein kinase activation

The proinflammatory cytokine interferon-gamma (IFNgamma) alters neuronal connectivity via selective regressive effects on dendrites but the signaling pathways that mediate this effect are poorly understood. We recently demonstrated that signaling by Rit, a member of the Ras family of GTPases, modulates dendritic growth in primary cultures of sympathetic and hippocampal neurons. In this study, we investigated a role for Rit signaling in IFNgamma-induced dendritic retraction. Expression of a dominant negative Rit mutant inhibited IFNgamma-induced dendritic retraction in cultured embryonic rat sympathetic and hippocampal neurons. In pheochromacytoma cells and hippocampal neurons, IFNgamma caused rapid Rit activation as indicated by increased GTP binding to Rit. Silencing of Rit by RNA interference suppressed IFNgamma-elicited activation of p38 MAPK in pheochromacytoma cells, and pharmacological inhibition of p38 MAPK significantly attenuated the dendrite-inhibiting effects of IFNgamma in cultured sympathetic and hippocampal neurons without altering signal transducer and activator of transcription 1 activation. These observations identify Rit as a downstream target of IFNgamma and suggest that a novel IFNgamma-Rit-p38 signaling pathway contributes to dendritic retraction and may, therefore, represent a potential therapeutic target in diseases with a significant neuroinflammatory component.

Constitutively active Rap2 transgenic mice display fewer dendritic spines, reduced extracellular signal-regulated kinase signaling, enhanced long-term depression, and impaired spatial learning and fear extinction

Within the Ras superfamily of GTPases, Rap1 and Rap2 are the closest homologs to Ras. In non-neural cells, Rap signaling can antagonize Ras signaling. In neurons, Rap also seems to oppose Ras in terms of synaptic function. Whereas Ras is critical for long-term potentiation (LTP), Rap1 has been shown to be required for long-term depression (LTD), and Rap2 has been implicated in depotentiation. Moreover, active Rap1 and Rap2 cause loss of surface AMPA receptors and reduced miniature EPSC amplitude and frequency in cultured neurons. The role of Rap signaling in vivo, however, remains poorly understood. To study the function of Rap2 in the brain and in behavior, we created transgenic mice expressing either constitutively active (Rap2V12) or dominant-negative (Rap2N17) mutants of Rap2 in postnatal forebrain. Multiple lines of Rap2N17 mice showed only weak expression of the transgenic protein, and no phenotype was observed. Rap2V12 mice displayed fewer and shorter dendritic spines in CA1 hippocampal neurons, and enhanced LTD at CA3-CA1 synapses. Behaviorally, Rap2V12 mice showed impaired spatial learning and defective extinction of contextual fear, which correlated with reduced basal phosphorylation of extracellular signal-regulated kinase (ERK) and blunted activation of ERK during fear extinction training. Our data support the idea that Rap2 opposes Ras-ERK signaling in the brain, thereby inhibiting dendritic spine development/maintenance, promoting synaptic depression rather than LTP, and impairing learning. The findings also implicate Rap2 signaling in fear extinction mechanisms, which are thought to be aberrant in anxiety disorders and posttraumatic stress disorder.

Activity-dependent regulation of synaptic AMPA receptor composition and abundance by beta3 integrins

At synapses, cell adhesion molecules (CAMs) provide the molecular framework for coordinating signaling events across the synaptic cleft. Among synaptic CAMs, the integrins, receptors for extracellular matrix proteins and counterreceptors on adjacent cells, are implicated in synapse maturation and plasticity and memory formation. However, little is known about the molecular mechanisms of integrin action at central synapses. Here, we report that postsynaptic beta3 integrins control synaptic strength by regulating AMPA receptors (AMPARs) in a subunit-specific manner. Pharmacological perturbation targeting beta3 integrins promotes endocytosis of GluR2-containing AMPARs via Rap1 signaling, and expression of beta3 integrins produces robust changes in the abundance and composition of synaptic AMPARs without affecting dendritic spine structure. Importantly, homeostatic synaptic scaling induced by activity deprivation elevates surface expression of beta3 integrins, and in turn, beta3 integrins are required for synaptic scaling. Our findings demonstrate a key role for integrins in the feedback regulation of excitatory synaptic strength.

Critical role of CDK5 and Polo-like kinase 2 in homeostatic synaptic plasticity during elevated activity

Homeostatic plasticity keeps neuronal spiking output within an optimal range in the face of chronically altered levels of network activity. Little is known about the underlying molecular mechanisms, particularly in response to elevated activity. We report that, in hippocampal neurons experiencing heightened activity, the activity-inducible protein kinase Polo-like kinase 2 (Plk2, also known as SNK) was required for synaptic scaling-a principal mechanism underlying homeostatic plasticity. Synaptic scaling also required CDK5, which acted as a "priming" kinase for the phospho-dependent binding of Plk2 to its substrate SPAR, a postsynaptic RapGAP and scaffolding molecule that is degraded following phosphorylation by Plk2. RNAi knockdown of SPAR weakened synapses, and overexpression of a SPAR mutant resistant to Plk2-dependent degradation prevented synaptic scaling. Thus, priming phosphorylation of the Plk2 binding site in SPAR by CDK5, followed by Plk2 recruitment and SPAR phosphorylation-degradation, constitutes a molecular pathway for neuronal homeostatic plasticity during chronically elevated activity.

The rap GTPases regulate B cell morphology, immune-synapse formation, and signaling by particulate B cell receptor ligands

B lymphocytes spread and extend membrane processes when searching for antigens and form immune synapses upon contacting cells that display antigens on their surface. Although these dynamic morphological changes facilitate B cell activation, the signaling pathways underlying these processes are not fully understood. We found that activation of the Rap GTPases was essential for these changes in B cell morphology. Rap activation was important for B cell receptor (BCR)- and lymphocyte-function-associated antigen-1 (LFA-1)-induced spreading, for BCR-induced immune-synapse formation, and for particulate BCR ligands to induce localized F-actin assembly and membrane-process extension. Rap activation and F-actin assembly were also required for optimal BCR signaling in response to particulate antigens but not soluble antigens. Thus by controlling B cell morphology and cytoskeletal organization, Rap might play a key role in the activation of B cells by particulate and cell-associated antigens.

Integrin signaling is integral to regeneration

The inability of the adult injured mammalian spinal cord to successfully regenerate is not well understood. Studies suggest that both extrinsic and intrinsic factors contribute to regeneration failure. In this review, we focus on intrinsic factors that impact regeneration, in particular integrin receptors and their downstream signaling pathways. We discuss studies that address the impact of integrins and integrin signaling pathways on growth cone guidance and motility and how lessons learned from these studies apply to spinal cord regeneration in vivo.

The EphA4 receptor regulates neuronal morphology through SPAR-mediated inactivation of Rap GTPases

Eph receptors play critical roles in the establishment and remodeling of neuronal connections, but the signaling pathways involved are not fully understood. We have identified a novel interaction between the C terminus of the EphA4 receptor and the PDZ domain of the GTPase-activating protein spine-associated RapGAP (SPAR). In neuronal cells, this binding mediates EphA4-dependent inactivation of the closely related GTPases Rap1 and Rap2, which have recently been implicated in the regulation of dendritic spine morphology and synaptic plasticity. We show that SPAR-mediated inactivation of Rap1, but not Rap2, is critical for ephrin-A-dependent growth cone collapse in hippocampal neurons and decreased integrin-mediated adhesion in neuronal cells. Distinctive effects of constitutively active Rap1 and Rap2 on the morphology of growth cones and dendritic spines support the idea that these two GTPases have different functions in neurons. Together, our data implicate SPAR as an important signaling intermediate that links the EphA4 receptor with Rap GTPase function in the regulation of neuronal morphology.

Fyn modulation of Dab1 effects on amyloid precursor protein and ApoE receptor 2 processing

Dab1 is an intracellular adaptor protein that interacts with amyloid precursor protein (APP) and apoE receptor 2 (apoEr2), increases their levels on the cell surface, and increases their cleavage by alpha-secretases. To investigate the mechanism underlying these alterations in processing and trafficking of APP and apoEr2, we examined the effect of Fyn, an Src family-tyrosine kinase known to interact with and phosphorylate Dab1. Co-immunoprecipitation, co-immunostaining, and fluorescence lifetime imaging demonstrated an association between Fyn and APP. Fyn induced phosphorylation of APP at Tyr-757 of the (757)YENPTY(762) motif and increased cell surface expression of APP. Overexpression of Fyn alone did not alter levels of sAPPalpha or cytoplasmic C-terminal fragments, although it significantly decreased production of Abeta. However, in the presence of Dab1, Fyn significantly increased sAPPalpha and C-terminal fragments. Fyn-induced APP phosphorylation and cell surface levels of APP were potentiated in the presence of Dab1. Fyn also induced phosphorylation of apoEr2 and increased its cell surface levels and, in the presence of Dab1, affected processing of its C-terminal fragment. In vivo studies showed that sAPPalpha was decreased in the Fyn knock-out, supporting a role for Fyn in APP processing. These data demonstrate that Fyn, due in part to its effects on Dab1, regulates the phosphorylation, trafficking, and processing of APP and apoEr2.

The metalloprotease inhibitor TIMP-3 regulates amyloid precursor protein and apolipoprotein E receptor proteolysis

Cellular cholesterol levels alter the processing of the amyloid precursor protein (APP) to produce Abeta. Activation of liver X receptors (LXRs), one cellular mechanism to regulate cholesterol homeostasis, has been found to alter Abeta levels in vitro and in vivo. To identify genes regulated by LXR, we treated human neuroblastoma cells with an LXR agonist (TO-901317) and examined gene expression by microarray. As expected, TO-901317 upregulated several cholesterol metabolism genes, but it also decreased expression of a metalloprotease inhibitor, TIMP-3. We confirmed this finding using real-time PCR and by measuring TIMP-3 protein in glia, SY5Y cells, and COS7 cells. TIMP-3 is a member of a family of metalloproteinase inhibitors and blocks A disintegrin and metalloproteinase-10 (ADAM-10) and ADAM-17, two APP alpha-secretases. We found that TIMP-3 inhibited alpha-secretase cleavage of APP and an apolipoprotein E (apoE) receptor, ApoER2. TIMP-3 decreased surface levels of ADAM-10, APP, and ApoER2. These changes were accompanied by increased APP beta-C-terminal fragment and Abeta production. These data suggest that TIMP-3 preferentially routes APP and ApoER2 away from the cell surface and alpha-secretase cleavage and encourages endocytosis and beta-secretase cleavage. In vivo, TO-901317 decreased brain TIMP-3 levels. TIMP-3 protein levels were increased in human Alzheimer's disease (AD) brain and in APP transgenic mice, suggesting that increased levels of TIMP-3 in AD may contribute to higher levels of Abeta.

SPAR2, a novel SPAR-related protein with GAP activity for Rap1 and Rap2

Spine-associated RapGAP 2 (SPAR2) is a novel GTPase activating protein (GAP) for the small GTPase Rap that shows significant sequence homology to SPAR, a synaptic RapGAP that was reported to regulate spine morphology in hippocampal neurons. SPAR2, like SPAR, interacts with the recently described synaptic scaffolding protein ProSAP-interacting protein (ProSAPiP), which in turn binds to the PDZ domain of ProSAP/Shank post-synaptic density proteins. In subcellular fractionation experiments, SPAR2 is enriched in synaptosomes and post-synaptic density fractions indicating that it is a synaptic protein. Furthermore, we could show using in vitro GAP assays that SPAR2 has GAP activity for Rap1 and Rap2. Expression in COS-7 cells, however, revealed different actin-binding properties of SPAR2 and SPAR. Additionally, over-expression of SPAR2 in cultured hippocampal neurons did not affect spine morphology as it was reported for SPAR. In situ hybridization studies also revealed a differential tissue distribution of SPAR and SPAR2 with SPAR2 transcripts being mainly expressed in cerebellar and hippocampal granule cells. Moreover, in the cerebellum SPAR2 is developmentally regulated with a peak of expression around the period of synapse formation. Our results imply that SPAR2 is a new RapGAP with specific functions in cerebellar and hippocampal granule cells.

Characterization of Interactions of Adapter Protein RAPL/Nore1B with RAP GTPases and Their Role in T Cell Migration

Using a model of integrin-triggered random migration of T cells, we show that stimulation of LFA-1 integrins leads to the activation of Rap1 and Rap2 small GTPases. We further show that Rap1 and Rap2 have distinct roles in adhesion and random migration of these cells and that an adapter protein from the Ras association domain family (Rassf), RAPL, has a role downstream of Rap2 in addition to its link to Rap1. Further characterization of the RAPL protein and its interactions with small GTPases from the Ras family shows that RAPL forms more stable complexes with Rap2 and classical Ras proteins compared with Rap1. The different interaction pattern of RAPL with Rap1 and Rap2 is not affected by the disruption of the C-terminal SARAH domain that we identified as the alpha-helical region responsible for RAPL dimerization in vitro and in cells. Based on mutagenesis and three-dimensional modeling, we propose that interaction surfaces in RAPL-Rap1 and RAPL-Rap2 complexes are different and that a single residue in the switch I region of Rap proteins (residue 39) contributes considerably to the different kinetics of these protein-protein interactions. Furthermore, the distinct role of Rap2 in migration of T cells is lost when this critical residue is converted to the residue present in Rap1. Together, these observations suggest a wider role for Rassf adapter protein RAPL and Rap GTPases in cell motility and show that subtle differences between highly similar Rap proteins could be reflected in distinct interactions with common effectors and their cellular function.