The transmembrane protein deleted in colorectal cancer (DCC) and its ligand, netrin-1, regulate synaptogenesis during development, but their function in the mature central nervous system is unknown. Given that DCC promotes cell-cell adhesion, is expressed by neurons, and activates proteins that signal at synapses, we hypothesized that DCC expression by neurons regulates synaptic function and plasticity in the adult brain. We report that DCC is enriched in dendritic spines of pyramidal neurons in wild-type mice, and we demonstrate that selective deletion of DCC from neurons in the adult forebrain results in the loss of long-term potentiation (LTP), intact long-term depression, shorter dendritic spines, and impaired spatial and recognition memory. LTP induction requires Src activation of NMDA receptor (NMDAR) function. DCC deletion severely reduced Src activation. We demonstrate that enhancing NMDAR function or activating Src rescues LTP in the absence of DCC. We conclude that DCC activation of Src is required for NMDAR-dependent LTP and certain forms of learning and memory.

The molecular mechanisms underlying the elaboration of branched processes during the later stages of oligodendrocyte maturation are not well understood. Here we describe a novel role for the chemotropic guidance cue netrin 1 and its receptor deleted in colorectal carcinoma (Dcc) in the remodeling of oligodendrocyte processes. Postmigratory, premyelinating oligodendrocytes express Dcc but not netrin 1, whereas mature myelinating oligodendrocytes express both. We demonstrate that netrin 1 promotes process extension by premyelinating oligodendrocytes in vitro and in vivo. Addition of netrin 1 to mature oligodendrocytes in vitro evoked a Dcc-dependent increase in process branching. Furthermore, expression of netrin 1 and Dcc by mature oligodendrocytes was required for the elaboration of myelin-like membrane sheets. Maturation of oligodendrocyte processes requires intracellular signaling mechanisms involving Fyn, focal adhesion kinase (FAK), neuronal Wiscott-Aldrich syndrome protein (N-WASP) and RhoA; however, the extracellular cues upstream of these proteins in oligodendrocytes are poorly defined. We identify a requirement for Src family kinase activity downstream of netrin-1-dependent process extension and branching. Using oligodendrocytes derived from Fyn knockout mice, we demonstrate that Fyn is essential for netrin-1-induced increases in process branching. Netrin 1 binding to Dcc on mature oligodendrocytes recruits Fyn to a complex with the Dcc intracellular domain that includes FAK and N-WASP, resulting in the inhibition of RhoA and inducing process remodeling. These findings support a novel role for netrin 1 in promoting oligodendrocyte process branching and myelin-like membrane sheet formation. These essential steps in oligodendroglial maturation facilitate the detection of target axons, a key step towards myelination.

The netrin‐1 receptor Deleted in Colorectal Cancer (DCC) is required for the formation of major axonal projections by embryonic cortical neurons, including the corpus callosum, hippocampal commissure, and cortico‐thalamic tracts. The presentation of DCC by axonal growth cones is tightly regulated, but the mechanisms regulating DCC trafficking within neurons are not well understood. Here, we investigated the mechanisms regulating DCC recruitment to the plasma membrane of embryonic cortical neurons. In embryonic spinal commissural neurons, protein kinase A (PKA) activation recruits DCC to the plasma membrane and enhances axon chemoattraction to netrin‐1. We demonstrate that PKA activation similarly recruits DCC and increases embryonic cortical neuron axon extension, which, like spinal commissural neurons, respond to netrin‐1 as a chemoattractant. We then determined if depolarization might recruit DCC to the plasma membrane. Neither netrin‐1 induced axon extension, nor levels of plasma membrane DCC, were altered by depolarizing embryonic spinal commissural neurons with elevated levels of KCl. In contrast, depolarizing embryonic cortical neurons increased the amount of plasma membrane DCC, including at the growth cone, and increased axon outgrowth evoked by netrin‐1. Inhibition of PKA, phosphatidylinositol‐3‐kinase, protein kinase C, or exocytosis blocked the depolarization‐induced recruitment of DCC and suppressed axon outgrowth. Inhibiting protein synthesis did not affect DCC recruitment, nor were the distributions of trkB or neural cell adhesion molecule (NCAM) influenced by depolarization, consistent with selective mobilization of DCC. These findings identify a role for membrane depolarization modulating the response of axons to netrin‐1 by regulating DCC recruitment to the plasma membrane.

J. Neurochem. (2012) 122, 147–161. Abstract The mechanisms that regulate synapse formation and maintenance are incompletely understood. In particular, relatively few inhibitors of synapse formation have been identified. Receptor protein tyrosine phosphatase σ (RPTPσ), a transmembrane tyrosine phosphatase, is widely expressed by neurons in developing and mature mammalian brain, and functions as a receptor for chondroitin sulfate proteoglycans that inhibits axon regeneration following injury. In this study, we address RPTPσ function in the mature brain. We demonstrate increased axon collateral branching in the hippocampus of RPTPσ null mice during normal aging or following chemically induced seizure, indicating that RPTPσ maintains neural circuitry by inhibiting axonal branching. Previous studies demonstrated a role for pre‐synaptic RPTPσ promoting synaptic differentiation during development; however, subcellular fractionation revealed enrichment of RPTPσ in post‐synaptic densities. We report that neurons lacking RPTPσ have an increased density of pre‐synaptic varicosities in vitro and increased dendritic spine density and length in vivo. RPTPσ knockouts exhibit an increased frequency of miniature excitatory post‐synaptic currents, and greater paired‐pulse facilitation, consistent with increased synapse density but reduced synaptic efficiency. Furthermore, RPTPσ nulls exhibit reduced long‐term potentiation and enhanced novel object recognition memory. We conclude that RPTPσ limits synapse number and regulates synapse structure and function in the mature CNS.

Neuronal injury and loss are recognized features of neuroinflammatory disorders, including acute and chronic encephalitides and multiple sclerosis; destruction of astrocytes has been demonstrated in cases of Rasmussen encephalitis. Here, we show that innate immune cells (i.e. natural killer [NK] and gammadelta T cells) cause loss of neurons from primary human neuron-enriched cultures by destroying the supporting astrocytes. Interleukin 2-activated NK cells caused loss of astrocytes within 1 hour, whereas neurons were lost at 4 hours. Time-lapse imaging indicated that delayed neuron loss was due to early destruction of supporting astrocytes. Selective blocking of astrocyte death with anti-NKG2D antibodies reduced neuron loss, as did blocking of CD54 on astrocytes. gammadelta T cells also induced astrocyte cytotoxicity, leading to subsequent neuronal displacement. In astrocytes, NK cells caused caspase-dependent fragmentation of the intermediate filament proteins glial fibrillary acidic protein and vimentin, whereas anti-CD3-activated T cells produced fragmentation to a lesser extent and without measurable cytotoxicity. Glial fibrillary acidic protein fragmentation was also demonstrated in lysates from chronic multiple sclerosis plaques but not from normal control white matter. These data suggest that non-major histocompatibility complex-restricted immune effector cells may contribute to neuron loss in neuroinflammatory disorders indirectly through injury of glia.

J. Neurochem. (2010) 10.1111/j.1471‐4159.2010.06582.x Abstract EphA4, a receptor tyrosine kinase, is expressed in various pre‐, post‐ and peri‐synaptic organelles and implicated in the regulation of morphological and physiological properties of synapses. It regulates synaptic plasticity by acting as a binding partner for glial ephrin‐A3 and possibly other pre‐ or post‐synaptic ephrins. Now, its trafficking mechanisms remain unknown. In this study, we examine the association of EphA4 with transport, clathrin‐coated and synaptic vesicles using cell fractionation, vesicle immunoisolation and electron microscopy. EphA4 was found in highly purified fractions of clathrin‐coated or synaptic vesicles. It was also detected in vesicles immuno‐isolated with antibodies anti‐synaptophysin, anti‐vesicular glutamate transporter or anti‐vesicular GABA transporter; demonstrating its presence in synaptic vesicles. However, it was not detected in immuno‐isolated piccolo–bassoon transport vesicles. In vivo and in dissociated cultures, EphA4 was localized by immunoelectron microscopy in vesicular glutamate transporter 1‐positive terminals of hippocampal neurons. Remarkably, the cell surface immunofluorescence of EphA4 increased markedly in cultured hippocampal neurons following KCl depolarization. These observations indicate that EphA4 is present in subsets of synaptic vesicles, can be externalized during depolarization, and internalized within clathrin‐coated vesicles. This trafficking itinerary may serve to regulate the levels of EphA4 in the synaptic plasma membrane and thereby modulate signaling events that contribute to synaptic plasticity.

The majority of hermaphroditic animals mate on a given occasion as either male or female, but terrestrial snails and slugs generally mate reciprocally with each partner participating in both sexual roles. This manner of mating requires that the genitalia be exactly opposed prior to copulation attempts, a task made difficult in snails and slugs by the absence of hearing and very limited vision. In the brown garden snail, Cornu aspersum (Müller, 1774), we found that a small protruding structure associated with the genital atrium plays an important role in positioning the snails prior to copulation. Lesions of the penial lobe reduced mating success rates, delayed mating, increased the number of attempted intromissions, and increased the number of unilateral intromissions. The sensory capacity of the penial lobe is demonstrated by histological and electrophysiological evidence, and behavioral data suggest that the lobe is also a stimulus for the partner snail. A literature review suggests that structures functionally equivalent to the penial lobe may be present in many gastropod molluscs that mate simultaneously and reciprocally, but in none that mate in other ways.

Directional axon extension and cell migration require both cytoskeletal reorganization and membrane insertion to extend the leading edge of a cell. In this issue, Cotrufo et al. (2012) provide evidence that the chemoattractant netrin-1 directs membrane extension during neuronal cell migration by promoting local exocytosis.Netrins are chemotropic guidance cues that direct axon extension, cell migration and neurite branching during neural development (Lai Wing Sun et al., 2011). Signal transduction mechanisms activated by the netrin-1 receptor DCC (Deleted in Colorectal Cancer) regulate cytoskeletal organization, and understanding how chemotropic guidance cues regulate the cytoskeleton has been a major focus of the field. The findings presented by Cotrufo et al. (2012) now fundamentally expand the role of DCC, providing evidence that netrin-1 binding may cause DCC to direct a localized increase in membrane insertion, facilitating the formation of cellular protrusions.Cotrufo et al. (2011) had previously used hippocampal neurons to determine that netrin-1 increases the association between DCC and syntaxin-1, which is a plasma membrane target membrane-Soluble NSF Attachment Protein Receptor (SNARE) with a well-established function in synaptic vesicle exocytosis. In the current study, the authors extend these findings to cell migration using a well-characterized assay of chemotropic neuronal cell migration in the embryonic mammalian brain. During early development, post-mitotic neurons are directed by netrin-1 to migrate from the lower rhombic lip to form the pontine nuclei (Alcantara et al., 2000). The authors provide evidence for a direct interaction between DCC and syntaxin-1 in migrating neurons, and demonstrate that disrupting syntaxin-1 function blocks the chemoattractant migration of these cells to netrin-1. During synaptic vesicle release, syntaxin-1 forms a canonical SNARE complex with SNAP25 (Synaptosomal-associated protein 25), a plasma membrane SNARE, and synaptobrevin, a vesicle-SNARE on synaptic vesicles. Interestingly, disrupting the function of either SNAP25 or synaptobrevin-2 [also called vesicle-associated membrane protein (VAMP)2] had no effect on netrin-mediated axon guidance (Cotrufo et al., 2011) or cell migration (Cotrufo et al., 2012), suggesting that, if DCC functions with syntaxin-1 to promote exocytosis, this may occur via a novel, and perhaps atypical, complex of SNARE proteins. Along these lines, the authors detect a complex of DCC and the vesicle-SNARE tetanus neurotoxin-insensitive VAMP (also called VAMP7) in migrating cells (Cotrufo et al., 2012) and have shown that disrupting tetanus neurotoxin-insensitive VAMP function blocks the chemoattractant axon extension to netrin-1 (Cotrufo et al., 2011). The authors propose a model where netrin-1 binding recruits syntaxin-1 to DCC, which then promotes the fusion of vesicles required for local membrane extension.Of greatest significance to the field, the interaction between DCC and syntaxin-1 provides a possible link between polarized membrane...