Trans-Golgi network delivery of synaptic proteins in synaptogenesis

Modulation of neuronal morphology by antipsychotic drug: Involvement of serotonin receptor 7

Antipsychotic drugs (APDs) are the primary pharmacological treatment for schizophrenia, a complex disorder characterized by altered neuronal connectivity. Atypical or second-generation antipsychotics, such as Risperidone (RSP) and Clozapine (CZP) predominantly block dopaminergic D2 and serotonin receptor 2A (5-HT2A) neurotransmission. Both compounds also exhibit affinity for the 5-HT7R, with RSP acting as an antagonist and CZP as an inverse agonist. Our study aimed to determine whether RSP and CZP can influence neuronal morphology through a 5-HT7R-mediated mechanism. Here, we demonstrated that CZP promotes neurite outgrowth of early postnatal cortical neurons, and the 5-HT7R mediates its effect. Conversely, RSP leads to a reduction of neurite length of early postnatal cortical neurons, in a 5-HT7R-independent way. Furthermore, we found that the effects of CZP, mediated by 5-HT7R activation, require the participation of ERK and Cdk5 kinase pathways. At the same time, the modulation of neurite length by RSP does not involve these pathways. In conclusion, our findings provide valuable insights into the morphological changes induced by these two APDs in neurons and elucidate some of the associated molecular pathways. Investigating the 5-HT7R-dependent signaling pathways underlying the neuronal morphogenic effects of APDs may contribute to the identification of novel targets for the treatment of schizophrenia.

Synaptic logistics: The presynaptic scaffold protein Piccolo a nodal point tuning synaptic vesicle recycling, maintenance and integrity

Properly working synapses are one important guarantor for a functional and healthy brain. They are small, densely packed structures, where information is transmitted through the release of neurotransmitters from synaptic vesicles (SVs). The latter cycle within the presynaptic terminal as they first fuse with the plasma membrane to deliver their neurotransmitter, and afterwards become recycled and prepared for a new release event. The synapse is an autonomous structure functioning mostly independent of the neuronal soma. Dysfunction in synaptic processes associated with local insults or genetic abnormalities can directly compromise synapse function and integrity and subsequently lead to the onset of neurodegenerative diseases. Therefore, measures need to be in place counteracting these threats for instance through the continuous replacement of old and damaged SV proteins. Interestingly recent studies show that the presynaptic scaffolding protein Piccolo contributes to health, function and integrity of synapses, as it mediates the delivery of synaptic proteins from the trans-Golgi network (TGN) towards synapses, as well as the local recycling and turnover of SV proteins within synaptic terminals. It can fulfill these various tasks through its multi-domain structure and ability to interact with numerous binding partners. In addition, Piccolo has recently been linked with the early onset neurodegenerative disease Pontocerebellar Hypoplasia Type 3 (PCH3) further underlying its importance for neuronal health. In this review, we will focus on Piccolo's contributions to synapse function, health and integrity and make a connection how those may contribute to the disease pattern of PCH3.

The BDNF Val66Met Polymorphism is a Relevant, But not Determinant, Risk Factor in the Etiology of Neuropsychiatric Disorders - Current Advances in Human Studies: A Systematic Review

Brain-derived neurotrophic factor (BDNF) is the brain's most-produced neurotrophin during the lifespan, essentially involved in multiple mechanisms of nervous system development and function. The production/release of BDNF requires multi-stage processing that appears to be regulated at various stages in which the presence of a polymorphism "Val66Met" can exert a critical influence.

Aim

To synthesize the knowledge on the BDNF Val66Met polymorphism on intracellular processing and function of BDNF.

Methods

We performed a systematic review and collected all available studies on the post-translation processes of BDNF, regarding the Val66Met polymorphism. Searches were performed up to 21st March 2021.

Results

Out of 129 eligible papers, 18 studies addressed or had findings relating to BDNF post-translation processes and were included in this review.

Discussion

Compilation of experimental findings reveals that the Val66Met polymorphism affects BDNF function by slightly altering the processing, distribution, and regulated release of BDNF. Regarding the critical role of pro-BDNF as a pro-apoptotic factor, such alteration might represent a risk for the development of neuropsychiatric disorders.

Spatiotemporal Patterns of Menin Localization in Developing Murine Brain: Co-Expression with the Elements of Cholinergic Synaptic Machinery

Menin, a product of MEN1 (multiple endocrine neoplasia type 1) gene is an important regulator of tissue development and maintenance; its perturbation results in multiple tumors—primarily of the endocrine tissue. Despite its abundance in the developing central nervous system (CNS), our understanding of menin’s role remains limited. Recently, we discovered menin to play an important role in cholinergic synaptogenesis in the CNS, whereas others have shown its involvement in learning, memory, depression and apoptosis. For menin to play these important roles in the CNS, its expression patterns must be corroborated with other components of the synaptic machinery imbedded in the learning and memory centers; this, however, remains to be established. Here, we report on the spatio-temporal expression patterns of menin, which we found to exhibit dynamic distribution in the murine brain from early development, postnatal period to a fully-grown adult mouse brain. We demonstrate here that menin expression is initially widespread in the brain during early embryonic stages, albeit with lower intensity, as determined by immunohistochemistry and gene expression. With the progression of development, however, menin expression became highly localized to learning, memory and cognition centers in the CNS. In addition to menin expression patterns throughout development, we provide the first direct evidence for its co-expression with nicotinic acetylcholine, glutamate and GABA (gamma aminobutyric acid) receptors—concomitant with the expression of both postsynaptic (postsynaptic density protein PSD-95) and presynaptic (synaptotagamin) proteins. This study is thus the first to provide detailed analysis of spatio-temporal patterns of menin expression from initial CNS development to adulthood. When taken together with previously published studies, our data underscore menin’s importance in the cholinergic neuronal network assembly underlying learning, memory and cognition.

3-O-sulfated heparan sulfate interactors target synaptic adhesion molecules from neonatal mouse brain and inhibit neural activity and synaptogenesis in vitro

Heparan sulfate (HS) chains, covalently linked to heparan sulfate proteoglycans (HSPG), promote synaptic development and functions by connecting various synaptic adhesion proteins (AP). HS binding to AP could vary according to modifications of HS chains by different sulfotransferases. 3-O-sulfotransferases (Hs3sts) produce rare 3-O-sulfated HSs (3S-HSs), of poorly known functions in the nervous system. Here, we showed that a peptide known to block herpes simplex virus by interfering with 3S-HSs in vitro and in vivo (i.e. G2 peptide), specifically inhibited neural activity, reduced evoked glutamate release, and impaired synaptic assembly in hippocampal cell cultures. A role for 3S-HSs in promoting synaptic assembly and neural activity is consistent with the synaptic interactome of G2 peptide, and with the detection of Hs3sts and their products in synapses of cultured neurons and in synaptosomes prepared from developing brains. Our study suggests that 3S-HSs acting as receptors for herpesviruses might be important regulators of neuronal and synaptic development in vertebrates.

Identification of Potential Interacting Proteins With the Extracellular Loops of the Neuronal Glycoprotein M6a by TMT/MS

Nowadays, great efforts are made to gain insight into the molecular mechanisms that underlie structural neuronal plasticity. Moreover, the identification of signaling pathways involved in the development of psychiatric disorders aids the screening of possible therapeutic targets. Genetic variations or alterations in GPM6A expression are linked to neurological disorders such as schizophrenia, depression, and Alzheimer's disease. GPM6A encodes the neuronal surface glycoprotein M6a that promotes filopodia/spine, dendrite, and synapse formation by unknown mechanisms. A substantial body of evidence suggests that the extracellular loops of M6a command its function. However, the proteins that associate with them and that modulate neuronal plasticity have not been determined yet. To address this question, we generated a chimera protein that only contains the extracellular loops of M6a and performed a co-immunoprecipitation with rat hippocampus samples followed by TMT/MS. Here, we report 72 proteins, which are good candidates to interact with M6a's extracellular loops and modify its function. Gene ontology (GO) analysis showed that 63% of the potential M6a's interactor proteins belong to the category "synapse," at both sides of the synaptic cleft, "neuron projections" (51%) and "presynapse" (49%). In this sense, we showed that endogenous M6a interacts with piccolo, synaptic vesicle protein 2B, and synapsin 1 in mature cultured hippocampal neurons. Interestingly, about 28% of the proteins left were related to the "myelin sheath" annotation, suggesting that M6a could interact with proteins at the surface of oligodendrocytes. Indeed, we demonstrated the (cis and trans) interaction between M6a and proteolipid protein (PLP) in neuroblastoma N2a cells. Finally, the 72 proteins were subjected to disease-associated genes and variants screening by DisGeNET. Apart from the diseases that have already been associated with M6a, most of the proteins are also involved in "autistic disorder," "epilepsy," and "seizures" increasing the spectrum of disorders in which M6a could play a role. Data are available via ProteomeXchange with identifier PXD017347.

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

The physical continuity of axons over long cellular distances poses challenges for their maintenance. One organelle that faces this challenge is endoplasmic reticulum (ER); unlike other intracellular organelles, this forms a physically continuous network throughout the cell, with a single membrane and a single lumen. In axons, ER is mainly smooth, forming a tubular network with occasional sheets or cisternae and low amounts of rough ER. It has many potential roles: lipid biosynthesis, glucose homeostasis, a Ca2+ store, protein export, and contacting and regulating other organelles. This tubular network structure is determined by ER-shaping proteins, mutations in some of which are causative for neurodegenerative disorders such as hereditary spastic paraplegia (HSP). While axonal ER shares many features with the tubular ER network in other contexts, these features must be adapted to the long and narrow dimensions of axons. ER appears to be physically continuous throughout axons, over distances that are enormous on a subcellular scale. It is therefore a potential channel for long-distance or regional communication within neurons, independent of action potentials or physical transport of cargos, but involving its physiological roles such as Ca2+ or organelle homeostasis. Despite its apparent stability, axonal ER is highly dynamic, showing features like anterograde and retrograde transport, potentially reflecting continuous fusion and breakage of the network. Here we discuss the transport processes that must contribute to this dynamic behavior of ER. We also discuss the model that these processes underpin a homeostatic process that ensures both enough ER to maintain continuity of the network and repair breaks in it, but not too much ER that might disrupt local cellular physiology. Finally, we discuss how failure of ER organization in axons could lead to axon degenerative diseases, and how a requirement for ER continuity could make distal axons most susceptible to degeneration in conditions that disrupt ER continuity.

The Golgi apparatus in neurorestoration

The central role of the Golgi apparatus in critical cellular processes such as the transport, processing, and sorting of proteins and lipids has placed it at the forefront of cell science. Golgi apparatus dysfunction caused by primary defects within the Golgi or pharmacological and oxidative stress has been implicated in a wide range of neurodegenerative diseases. In addition to participating in disease progression, the Golgi apparatus plays pivotal roles in angiogenesis, neurogenesis, and synaptogenesis, thereby promoting neurological recovery. In this review, we focus on the functions of the Golgi apparatus and its mediated events during neurorestoration.

Tomosyn associates with secretory vesicles in neurons through its N- and C-terminal domains

The secretory pathway in neurons requires efficient targeting of cargos and regulatory proteins to their release sites. Tomosyn contributes to synapse function by regulating synaptic vesicle (SV) and dense-core vesicle (DCV) secretion. While there is large support for the presynaptic accumulation of tomosyn in fixed preparations, alternative subcellular locations have been suggested. Here we studied the dynamic distribution of tomosyn-1 (Stxbp5) and tomosyn-2 (Stxbp5l) in mouse hippocampal neurons and observed a mixed diffuse and punctate localization pattern of both isoforms. Tomosyn-1 accumulations were present in axons and dendrites. As expected, tomosyn-1 was expressed in about 75% of the presynaptic terminals. Interestingly, also bidirectional moving tomosyn-1 and -2 puncta were observed. Despite the lack of a membrane anchor these puncta co-migrated with synapsin and neuropeptide Y, markers for respectively SVs and DCVs. Genetic blockade of two known tomosyn interactions with synaptotagmin-1 and its cognate SNAREs did not abolish its vesicular co-migration, suggesting an interplay of protein interactions mediated by the WD40 and SNARE domains. We hypothesize that the vesicle-binding properties of tomosyns may control the delivery, pan-synaptic sharing and secretion of neuronal signaling molecules, exceeding its canonical role at the plasma membrane.

Intracellular transport and cell surface delivery of the neural cell adhesion molecule (NCAM)

The neural cell adhesion molecule (NCAM) regulates differentiation and functioning of neurons by accumulating at the cell surface where it mediates the interactions of neurons with the extracellular environment. NCAM also induces a number of intracellular signaling cascades, which coordinate interactions at the cell surface with intracellular processes including changes in gene expression, transport and cytoskeleton remodeling. Since NCAM functions at the cell surface, its transport and delivery to the cell surface play a critical role. Here, we review recent advances in our understanding of the molecular mechanisms of the intracellular transport and cell surface delivery of NCAM. We also discuss the data suggesting a possibility of cross talk between activation of NCAM at the cell surface and the intracellular transport and cell surface delivery of NCAM.

The GDNF-GFRα1 complex promotes the development of hippocampal dendritic arbors and spines via NCAM

The formation of synaptic connections during nervous system development requires the precise control of dendrite growth and synapse formation. Although glial cell line-derived neurotrophic factor (GDNF) and its receptor GFRα1 are expressed in the forebrain, the role of this system in the hippocampus remains unclear. Here, we investigated the consequences of GFRα1 deficiency for the development of hippocampal connections. Analysis of conditional Gfra1 knockout mice shows a reduction in dendritic length and complexity, as well as a decrease in postsynaptic density specializations and in the synaptic localization of postsynaptic proteins in hippocampal neurons. Gain- and loss-of-function assays demonstrate that the GDNF-GFRα1 complex promotes dendritic growth and postsynaptic differentiation in cultured hippocampal neurons. Finally, in vitro assays revealed that GDNF-GFRα1-induced dendrite growth and spine formation are mediated by NCAM signaling. Taken together, our results indicate that the GDNF-GFRα1 complex is essential for proper hippocampal circuit development.

Talking to the neighbours: The molecular and physiological mechanisms of clustered synaptic plasticity

Synaptic connectivity forms the basis for neuronal communication and the storage of information. Experiences and learning of new abilities can drive remodelling of this connectivity and promotes the formation of spine clusters; dendritic segments with a higher spine density. Spines located within these segments are frequently co-activated, undergo different dynamics than synapses located outside of this dendritic compartment and have, in general, a longer lifetime. Several lines of evidence have shown that chemical synapses located close to each other share or compete for intracellular signalling molecules and structural resources. This sharing and competition directly influences spine dynamics. Spines can grow, shrink, increase or decrease the surface expression of receptors, channels and adhesion molecules or remain stable and unchanged over extended periods of time. Here we summarize recent discoveries and provide a closer look at spine clustering, dendritic segment-specific signalling and potential molecular mechanisms underlying associative and heterosynaptic plasticity.

The Roles of Microtubule-Based Transport at Presynaptic Nerve Terminals

Targeted intracellular movement of presynaptic proteins plays important roles during synapse formation and, later, in the homeostatic maintenance of mature synapses. Movement of these proteins, often as vesicular packages, is mediated by motor complexes travelling along intracellular cytoskeletal networks. Presynaptic protein transport by kinesin motors in particular plays important roles during synaptogenesis to bring newly synthesized proteins to establish nascent synaptic sites. Conversely, movement of proteins away from presynaptic sites by Dynein motors enables synapse-nuclear signaling and allows for synaptic renewal through degradation of unwanted or damaged proteins. Remarkably, recent data has indicated that synaptic and protein trafficking machineries can modulate each other's functions. Here, we survey the mechanisms involved in moving presynaptic components to and away from synapses and how this process supports presynaptic function.

α-COP binding to the survival motor neuron protein SMN is required for neuronal process outgrowth

Spinal muscular atrophy (SMA), a heritable neurodegenerative disease, results from insufficient levels of the survival motor neuron (SMN) protein. α-COP binds to SMN, linking the COPI vesicular transport pathway to SMA. Reduced levels of α-COP restricted development of neuronal processes in NSC-34 cells and primary cortical neurons. Remarkably, heterologous expression of human α-COP restored normal neurite length and morphology in SMN-depleted NSC-34 cells in vitro and zebrafish motor neurons in vivo. We identified single amino acid mutants of α-COP that selectively abrogate SMN binding, retain COPI-mediated Golgi-ER trafficking functionality, but were unable to support neurite outgrowth in cellular and zebrafish models of SMA. Taken together, these demonstrate the functional role of COPI association with the SMN protein in neuronal development.

Kinesin-1 promotes post-Golgi trafficking of NCAM140 and NCAM180 to the cell surface

The neural cell adhesion molecule (NCAM, also known as NCAM1) is important during neural development, because it contributes to neurite outgrowth in response to its ligands at the cell surface. In the adult brain, NCAM is involved in regulating synaptic plasticity. The molecular mechanisms underlying delivery of NCAM to the neuronal cell surface remain poorly understood. We used a protein macroarray and identified the kinesin light chain 1 (KLC1), a component of the kinesin-1 motor protein, as a binding partner of the intracellular domains of the two transmembrane isoforms of NCAM, NCAM140 and NCAM180. KLC1 binds to amino acids CGKAGPGA within the intracellular domain of NCAM and colocalizes with kinesin-1 in the Golgi compartment. Delivery of NCAM180 to the cell surface is increased in CHO cells and neurons co-transfected with kinesin-1. We further demonstrate that the p21-activated kinase 1 (PAK1) competes with KLC1 for binding to the intracellular domain of NCAM and contributes to the regulation of the membrane insertion of NCAM. Our results indicate that NCAM is delivered to the cell surface through a kinesin-1-mediated transport mechanism in a PAK1-dependent manner.

The neural cell adhesion molecule promotes maturation of the presynaptic endocytotic machinery by switching synaptic vesicle recycling from adaptor protein 3 (AP-3)- to AP-2-dependent mechanisms

Newly formed synapses undergo maturation during ontogenetic development via mechanisms that remain poorly understood. We show that maturation of the presynaptic endocytotic machinery in CNS neurons requires substitution of the adaptor protein 3 (AP-3) with AP-2 at the presynaptic plasma membrane. In mature synapses, AP-2 associates with the intracellular domain of the neural cell adhesion molecule (NCAM). NCAM promotes binding of AP-2 over binding of AP-3 to presynaptic membranes, thus favoring the substitution of AP-3 for AP-2 during formation of mature synapses. The presynaptic endocytotic machinery remains immature in adult NCAM-deficient (NCAM-/-) mice accumulating AP-3 instead of AP-2 and its partner protein AP180 in synaptic membranes and vesicles. NCAM deficiency or disruption of the NCAM/AP-2 complex in wild-type (NCAM+/+) neurons by overexpression of AP-2 binding-defective mutant NCAM interferes with efficient retrieval of the synaptic vesicle v-SNARE synaptobrevin 2. Abnormalities in synaptic vesicle endocytosis and recycling may thus contribute to neurological disorders associated with mutations in NCAM.

High yield derivation of enriched glutamatergic neurons from suspension-cultured mouse ESCs for neurotoxicology research

Background

Recently, there has been a strong emphasis on identifying an in vitro model for neurotoxicity research that combines the biological relevance of primary neurons with the scalability, reproducibility and genetic tractability of continuous cell lines. Derived neurons should be homotypic, exhibit neuron-specific gene expression and morphology, form functioning synapses and consistently respond to neurotoxins in a fashion indistinguishable from primary neurons. However, efficient methods to produce neuronal populations that are suitable alternatives to primary neurons have not been available.

Methods

With the objective of developing a more facile, robust and efficient method to generate enriched glutamatergic neuronal cultures, we evaluated the neurogenic capacity of three mouse embryonic stem cell (ESC) lines (R1, C57BL/6 and D3) adapted to feeder-independent suspension culture. Neurogenesis and neuronal maturation were characterized as a function of time in culture using immunological, genomic, morphological and functional metrics. The functional responses of ESNs to neurotropic toxins with distinctly different targets and mechanisms of toxicity, such as glutamate, α-latrotoxin (LTX), and botulinum neurotoxin (BoNT), were also evaluated.

Results

Suspension-adapted ESCs expressed markers of pluripotency through at least 30 passages, and differentiation produced 97×10⁶ neural progenitor cells (NPCs) per 10-cm dish. Greater than 99% of embryonic stem cell-derived neurons (ESNs) expressed neuron-specific markers by 96 h after plating and rapidly developed complex axodendritic arbors and appropriate compartmentalization of neurotypic proteins. Expression profiling demonstrated the presence of transcripts necessary for neuronal function and confirmed that ESN populations were predominantly glutamatergic. Furthermore, ESNs were functionally receptive to all toxins with sensitivities and responses consistent with primary neurons.

Conclusions

These findings demonstrate a cost-effective, scalable and flexible method to produce a highly enriched glutamatergic neuron population. The functional characterization of pathophysiological responses to neurotropic toxins and the compatibility with multi-well plating formats were used to demonstrate the suitability of ESNs as a discovery platform for molecular mechanisms of action, moderate-throughput analytical approaches and diagnostic screening. Furthermore, for the first time we demonstrate a cell-based model that is sensitive to all seven BoNT serotypes with EC₅₀ values comparable to those reported in primary neuron populations. These data providing compelling evidence that ESNs offer a neuromimetic platform suitable for the evaluation of molecular mechanisms of neurotoxicity.

A herpes simplex virus 1 (McKrae) mutant lacking the glycoprotein K gene is unable to infect via neuronal axons and egress from neuronal cell bodies

We have shown that the herpes simplex virus 1 (HSV-1) gK gene is essential for efficient replication and spread in the corneal epithelium and trigeminal ganglion neuroinvasion in mice (A. T. David, A. Baghian, T. P. Foster, V. N. Chouljenko, and K. G. Kousoulas, Curr. Eye Res. 33:455-467, 2008). To further investigate the role of gK in neuronal infection, we utilized a microfluidic chamber system separating neuronal cell bodies and axonal termini. HSV-1 (McKrae) engineered virus constitutively expressing enhanced green fluorescence protein (GFP) was efficiently transmitted in both a retrograde and an anterograde manner. These results were corroborated by expression of virion structural proteins in either chamber, as well as detection of viral genomes and infectious viruses. In contrast, efficient infection of either chamber with a gK-null virus did not result in infection of the apposed chamber. These results show that gK is an important determinant in virion axonal infection. Moreover, the inability of the gK-null virus to be transmitted in an anterograde manner suggests that virions acquire cytoplasmic envelopes prior to entering axons.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) enters mucosal epithelial cells and neurons via fusion of the viral envelope with cellular membranes, mediated by viral glycoprotein B (gB) in cooperation with other viral glycoproteins. Retrograde transport of virions to neuronal cell bodies (somata) establishes lifelong latent infection in ganglionic neurons. We have previously reported that gK binds gB and is required for gB-mediated membrane fusion (Jambunatathan et al., J. Virol. 85:12910-12918, 2011; V. N. Chouljenko, A. V. Iyer, S. Chowdhury, J. Kim, and K. G. Kousoulas, J. Virol. 84:8596-8606, 2010). In the current study, we constructed a recombinant virus with the gK gene deleted in the highly virulent ocular HSV-1 strain McKrae. This recombinant virus failed to infect rat ganglionic neuronal axons alone or cocultured with Vero cells in microfluidic chambers. In addition, lack of gK expression prevented anterograde transmission of virions. These results suggest that gK is a critical determinant for neuronal infection and transmission.

Synaptic functions of invertebrate varicosities: what molecular mechanisms lie beneath

In mammalian brain, the cellular and molecular events occurring in both synapse formation and plasticity are difficult to study due to the large number of factors involved in these processes and because the contribution of each component is not well defined. Invertebrates, such as Drosophila, Aplysia, Helix, Lymnaea, and Helisoma, have proven to be useful models for studying synaptic assembly and elementary forms of learning. Simple nervous system, cellular accessibility, and genetic simplicity are some examples of the invertebrate advantages that allowed to improve our knowledge about evolutionary neuronal conserved mechanisms. In this paper, we present an overview of progresses that elucidates cellular and molecular mechanisms underlying synaptogenesis and synapse plasticity in invertebrate varicosities and their validation in vertebrates. In particular, the role of invertebrate synapsin in the formation of presynaptic terminals and the cell-to-cell interactions that induce specific structural and functional changes in their respective targets will be analyzed.

Collapsin response mediator proteins regulate neuronal development and plasticity by switching their phosphorylation status

Collapsin response mediator protein (CRMP) was originally identified as a molecule involved in semaphorin3A signaling. CRMPs are now known to consist of five homologous cytosolic proteins, CRMP1-5. All of them are phosphorylated and highly expressed in the developing and adult nervous system. In vitro experiments have clearly demonstrated that CRMPs play important roles in neuronal development and maturation through the regulation of their phosphorylation. Several recent knockout mice studies have revealed in vivo roles of CRMPs in neuronal migration, neuronal network formation, synapse formation, synaptic plasticity, and neuronal diseases. Dynamic spatiotemporal regulation of phosphorylation status of CRMPs is involved in many aspects of neuronal development.

Centrosomal aggregates and Golgi fragmentation disrupt vesicular trafficking of DAT

Lewy bodies containing the centrosomal protein γ-tubulin and fragmentation of Golgi apparatus (GA) have been described in nigral neurons of Parkinson's disease (PD) patients. However, the relevance of these features in PD pathophysiology remains unknown. We analyzed the impact of proteasome inhibition in the formation of γ-tubulin-containing aggregates as well as on GA structure. SH-SY5Y cells were treated with the proteasome inhibitor Z-Leu-Leu-Leu-al (MG132) to induce centrosomal-protein aggregates. Then, microtubules (MTs) and Golgi dynamics, as well as the vesicular transport of dopamine transporter (DAT) were evaluated both in vitro and in living cells. MG132 treatment induced γ-tubulin aggregates which altered microtubule nucleation. MG132-treated cells containing γ-tubulin aggregates showed fragmentation of GA and perturbation of the trans-Golgi network. Under these conditions, the DAT accumulated at the centrosomal-Golgi region indicating that the vesicular transport of DAT was disrupted. Thus, centrosomal aggregates and fragmentation of GA are 2 closely related processes that could result in the disruption of the vesicular transport of DAT toward the plasma membrane in a model of dopaminergic neuronal degeneration.

The neural cell adhesion molecule promotes FGFR-dependent phosphorylation and membrane targeting of the exocyst complex to induce exocytosis in growth cones

The exocyst complex is an essential regulator of polarized exocytosis involved in morphogenesis of neurons. We show that this complex binds to the intracellular domain of the neural cell adhesion molecule (NCAM). NCAM promotes FGF receptor-mediated phosphorylation of two tyrosine residues in the sec8 subunit of the exocyst complex and is required for efficient recruitment of the exocyst complex to growth cones. NCAM at the surface of growth cones induces Ca(2+)-dependent vesicle exocytosis, which is blocked by an inhibitor of L-type voltage-dependent Ca(2+) channels and tetanus toxin. Preferential exocytosis in growth cones underlying neurite outgrowth is inhibited in NCAM-deficient neurons as well as in neurons transfected with phosphorylation-deficient sec8 and dominant-negative peptides derived from the intracellular domain of NCAM. Thus, we reveal a novel role for a cell adhesion molecule in that it regulates addition of the new membrane to the cell surface of growth cones in developing neurons.

NCAM/spectrin complex disassembly results in PSD perforation and postsynaptic endocytic zone formation

Mechanisms inducing perforation of the postsynaptic density (PSD) are poorly understood. We show that neural cell adhesion molecule- deficient (NCAM-/-) hippocampal neurons have an abnormally high percentage of synapses with perforated PSDs. The percentage of synapses with perforated PSDs is also increased in wild-type (NCAM+/+) neurons after the disruption of the NCAM/spectrin complex indicating that the NCAM-assembled spectrin cytoskeleton maintains the structural integrity of PSDs. We demonstrate that PSD perforations contain endocytic zones involved in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalization. Induction of long-term potentiation in NCAM+/+ neurons accompanied by insertion of AMPAR into the neuronal cell surface is subsequently followed by formation of perforated synapses and AMPAR endocytosis suggesting that perforation of PSDs is important for membrane homeostasis in activated synapses. In NCAM-/- or NCAM+/+ neurons with dissociated spectrin meshwork, AMPAR endocytosis is enhanced under conditions of basal activity. An abnormally high rate of postsynaptic membrane endocytosis may thus contribute to brain pathologies associated with mutations in NCAM or spectrin.

Transport of receptors, receptor signaling complexes and ion channels via neuropeptide-secretory vesicles

Stimulus-induced exocytosis of large dense-core vesicles (LDCVs) leads to discharge of neuropeptides and fusion of LDCV membranes with the plasma membrane. However, the contribution of LDCVs to the properties of the neuronal membrane remains largely unclear. The present study found that LDCVs were associated with multiple receptors, channels and signaling molecules, suggesting that neuronal sensitivity is modulated by an LDCV-mediated mechanism. Liquid chromatography-mass spectrometry combined with immunoblotting of subcellular fractions identified 298 proteins in LDCV membranes purified from the dorsal spinal cord, including G-protein-coupled receptors, G-proteins and other signaling molecules, ion channels and trafficking-related proteins. Morphological assays showed that δ-opioid receptor 1 (DOR1), β2 adrenergic receptor (AR), G(αi2), voltage-gated calcium channel α2δ1 subunit and P2X purinoceptor 2 were localized in substance P (SP)-positive LDCVs in small-diameter dorsal root ganglion neurons, whereas β1 AR, Wnt receptor frizzled 8 and dishevelled 1 were present in SP-negative LDCVs. Furthermore, DOR1/G(αi2)/G(β1γ5)/phospholipase C β2 complexes were associated with LDCVs. Blockade of the DOR1/G(αi2) interaction largely abolished the LDCV localization of G(αi2) and impaired stimulation-induced surface expression of G(αi2). Thus, LDCVs serve as carriers of receptors, ion channels and preassembled receptor signaling complexes, enabling a rapid, activity-dependent modulation of neuronal sensitivity.

Increase in polysialyltransferase gene expression following LTP in adult rat dentate gyrus

Neural cell adhesion molecule (NCAM) is frequently associated with polysialic acid (PSA), and its function is highly dependent on this polysialylation. PSA-NCAM plays an important role in synaptic plasticity in the hippocampus. STX and PST are the enzymes responsible for NCAM polysialylation. We investigated whether unilateral long-term potentiation (LTP) induction in vivo, in adult rat dentate gyrus (DG), triggered NCAM polysialylation by STX and PST produced in the hippocampus. We found that levels of STX and PST mRNA increased strongly since the early stage of hippocampal LTP and remained high during the maintenance of DG-LTP for 4 h. This rapid increase in polysialyltransferase gene expression occurred in both the hippocampi, probably resulting from bilateral LTP induction by strong unilateral HFS. Thus, LTP triggers interhemispheric molecular changes in the hippocampal network. This study is the first to describe the effects of LTP induction and maintenance on polysialyl-transferases in vivo. Our findings suggest that hippocampal synaptic remodeling requires NCAM polysialylation.

Investigation of LDHA and COPB1 as candidate genes for muscle development in the MYOD1 region of pig chromosome 2

Porcine MYOD1 gene has been mapped to swine chromosome (SSC) 2p14-p17, which is involved in the regulation of the proliferation and differentiation of skeletal muscle cells. The LDHA (lactate dehydrogenase A) and COPB1 (coatomer protein complex, subunit beta 1) genes, which map close to MYOD1, are involved in energy metabolism and protein transport processes. Both genes might play important roles in muscle development. However, little is known about the porcine LDHA and COPB1 genes. In the present study, the full-length cDNA of these two genes were cloned. The mapping results demonstrated that porcine LDHA and COPB1 were all mapped to SSC 2p14-p17. In this region, there are several QTL for growth and carcass traits, including average backfat thickness, lean and fat percentage. The RT-PCR results revealed that both LDHA and COPB1 were highly expressed in porcine skeletal muscle tissues, implying their potential regulatory function of muscle development. LDHA and COPB1 were then mapped to the region and multipoint analyses generated a best sex-averaged map order of each gene between linked markers: MYOD1_75.2 cM _LDHA_79 cM _CSRP3_83.8 cM _TEF-1_86.5 cM _COPB1_90 cM. Association analyses revealed that the substitution of c.423A>G had a significant effect on average daily gain on test, average backfat thickness (BFT), loin muscle area, lumbar BFT, marbling score, tenth rib BFT, average drip loss and fiber type II ratio. The substitution of c.3096C>T had a significant effect on average BFT, lumbar BFT, tenth rib BFT, carcass weight and last rib BFT. Interestingly, both SNPs were all associated with average BFT, lumbar BFT and tenth rib BFT.

Organization of NMDA receptors at extrasynaptic locations

NMDA receptors are found in neurons both at synapses and in extrasynaptic locations. Extrasynaptic locations are poorly characterized. Here we used preembedding immunoperoxidase and postembedding immunogold electron microscopy and fluorescence light microscopy to characterize extrasynaptic NMDA receptor locations in dissociated hippocampal neurons in vitro and in the adult and postnatal hippocampus in vivo. We found that extrasynaptic NMDA receptors on neurons in vivo and in vitro were usually concentrated at points of contact with adjacent processes, which were mainly axons, axon terminals, or glia. Many of these contacts were shown to contain adhesion factors such as cadherin and catenin. We also found associations of extrasynaptic NMDA receptors with the membrane associated guanylate kinase (MAGUKs), postsynaptic density (PSD)-95 and SAP102. Developmental differences were also observed. At postnatal day 2 in vivo, extrasynaptic NMDA receptors could often be found at sites with distinct densities whereas dense material was seen only rarely at sites of extrasynaptic NMDA receptors in the adult hippocampus in vivo. This difference probably indicates that many sites of extrasynaptic NMDA receptors in early postnatal ages represent synapse formation or possibly sites for synapse elimination. At all ages, as suggested in both in vivo and in vitro studies, extrasynaptic NMDA receptors on dendrites or the sides of spines may form complexes with other proteins, in many cases, at stable associations with adjacent cell processes. These associations may facilitate unique functions for extrasynaptic NMDA receptors.

Neural cell adhesion molecule is required for stability of reinnervated neuromuscular junctions

Studies examining the etiology of motoneuron diseases usually focus on motoneuron death as the defining pathophysiology of the disease. However, impaired neuromuscular transmission and synapse withdrawal often precede cell death, raising the possibility that abnormalities in synaptic function contribute to disease onset. Although little is known about the mechanisms maintaining the synaptic integrity of neuromuscular junctions (NMJs), Drosophila studies suggest that Fasciclin II plays an important role. Inspired by these studies we used a reinnervation model of synaptogenesis to analyze neuromuscular function in mice lacking neural cell adhesion molecule (NCAM), the Fasciclin II vertebrate homolog. Our results showed that the recovery of contractile force was the same in wild-type and NCAM-/- mice at 1 month after nerve injury, indicating that endplates were appropriately reformed. This normality was only transient because the contractile force and myofiber number decreased at 3 months after injury in NCAM-/- mice. Both declined further 3 months later. Myofibers degenerated, not because motoneurons died but because synapses were withdrawn. Although neurotransmission was initially normal at reinnervated NCAM-/- NMJs, it was significantly compromised 3 months later. Interestingly, the selective ablation of NCAM from motoneurons, or muscle fibers, did not mimic the deficits observed in reinnervated NCAM-/- mice. Taken together, these results indicate that NCAM is required to maintain normal synaptic function at reinnervated NMJs, although its loss pre-synaptically or post-synaptically is not sufficient to induce synaptic destabilization. Consideration is given to the role of NCAM in terminal Schwann cells for maintaining synaptic integrity and how NCAM dysfunction may contribute to motoneuron disorders.

Silencing the SPCA1 (secretory pathway Ca2+-ATPase isoform 1) impairs Ca2+ homeostasis in the Golgi and disturbs neural polarity

Neural cell differentiation involves a complex regulatory signal transduction network in which Ca(2+) ions and the secretory pathway play pivotal roles. The secretory pathway Ca(2+)-ATPase isoform 1 (SPCA1) is found in the Golgi apparatus where it is actively involved in the transport of Ca(2+) or Mn(2+) from the cytosol to the Golgi lumen. Its expression during brain development in different types of neurons has been documented recently, which raises the possibility that SPCA1 contributes to neuronal differentiation. In the present study, we investigated the potential impact of SPCA1 on neuronal polarization both in a cell line and in primary neuronal culture. In N2a neuroblastoma cells, SPCA1 was immunocytochemically localized in the juxtanuclear Golgi. Knockdown of SPCA1 by RNA interference markedly delayed the differentiation in these cells. The cells retarded in differentiation showed increased numbers of neurites of reduced length compared with control cells. Ca(2+) imaging assays showed that the lack of SPCA1 impaired Golgi Ca(2+) homeostasis and resulted in disturbed trafficking of different classes of proteins including normally Golgi-localized cameleon GT-YC3.3, bearing a Golgi-specific galactosyltransferase N terminus, and a normally plasma membrane-targeted, glycosyl phosphatidyl inositol-anchored cyan fluorescent protein construct. Also in hippocampal primary neurons, which showed a differential distribution of SPCA1 expression in Golgi stacks depending on differentiation stage, partial silencing of SPCA1 resulted in delayed differentiation, whereas total suppression drastically affected the cell survival. The disturbed overall cellular Ca(2+) homeostasis and/or the altered targeting of organellar proteins under conditions of SPCA1 knockdown highlight the importance of SPCA1 function for normal neural differentiation.

Herpes simplex virus utilizes the large secretory vesicle pathway for anterograde transport of tegument and envelope proteins and for viral exocytosis from growth cones of human fetal axons

Axonal transport of herpes simplex virus (HSV-1) is essential for viral infection and spread in the peripheral nervous system of the host. Therefore, the virus probably utilizes existing active transport and targeting mechanisms in neurons for virus assembly and spread from neurons to skin. In the present study, we used transmission immunoelectron microscopy to investigate the nature and origin of vesicles involved in the anterograde axonal transport of HSV-1 tegument and envelope proteins and of vesicles surrounding partially and fully enveloped capsids in growth cones. This study aimed to elucidate the mechanism of virus assembly and exit from axons of human fetal dorsal root ganglia neurons. We demonstrated that viral tegument and envelope proteins can travel in axons independently of viral capsids and were transported to the axon terminus in two types of transport vesicles, tubulovesicular membrane structures and large dense-cored vesicles. These vesicles and membrane carriers were derived from the trans-Golgi network (TGN) and contained key proteins, such as Rab3A, SNAP-25, GAP-43, and kinesin-1, involved in the secretory and exocytic pathways in axons. These proteins were also observed on fully and partially enveloped capsids in growth cones and on extracellular virions. Our findings provide further evidence to the subassembly model of separate transport in axons of unenveloped capsids from envelope and tegument proteins with final virus assembly occurring at the axon terminus. We postulate that HSV-1 capsids invaginate tegument- and envelope-bearing TGN-derived vesicles and utilize the large secretory vesicle pathway of exocytosis for exit from axons.

Cytoskeletal requirements in axonal transport of slow component-b

Slow component-b (SCb) translocates approximately 200 diverse proteins from the cell body to the axon and axon tip at average rates of approximately 2-8 mm/d. Several studies suggest that SCb proteins are cotransported as one or more macromolecular complexes, but the basis for this cotransport is unknown. The identification of actin and myosin in SCb led to the proposal that actin filaments function as a scaffold for the binding of other SCb proteins and that transport of these complexes is powered by myosin: the "microfilament-complex" model. Later, several SCb proteins were also found to bind F-actin, supporting the idea, but despite this, the model has never been directly tested. Here, we test this model by disrupting the cytoskeleton in a live-cell model system wherein we directly visualize transport of SCb cargoes. We focused on three SCb proteins that we previously showed were cotransported in our system: alpha-synuclein, synapsin-I, and glyceraldehyde-3-phosphate dehydrogenase. Disruption of actin filaments with latrunculin had no effect on the velocity or frequency of transport of these three proteins. Furthermore, cotransport of these three SCb proteins continued in actin-depleted axons. We conclude that actin filaments do not function as a scaffold to organize and transport these and possibly other SCb proteins. In contrast, depletion of microtubules led to a dramatic inhibition of vectorial transport of SCb cargoes. These findings do not support the microfilament-complex model, but instead indicate that the transport of protein complexes in SCb is powered by microtubule motors.

Activity-dependent expression of brain-derived neurotrophic factor in dendrites: facts and open questions

Long-lasting synaptic changes in transmission and morphology at the basis of memory storage, require delivery of newly synthesized proteins to affected synapses. Although many of these proteins are generated in the cell body, several key molecules for plasticity can be delivered in the form of silent mRNAs at synapses in extra somatic compartments where they are locally translated. One of such mRNAs encodes brain-derived neurotrophic factor (BDNF), a key molecule in neuronal development, learning and memory. A single BDNF protein is produced from several splice variants having a different 5' untranslated region. These mRNA variants have a different subcellular localization (soma, proximal or distal dendritic compartment) and may represent a spatial code for a local control of BDNF availability. This review will highlight current knowledge on the mechanisms of spatial and temporal regulation of activity-dependent BDNF mRNA localization in dendrites in relation with synaptic plasticity.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

WITHDRAWN: Role of NCAM in Spine Dynamics and Synaptogenesis

Increasing evidence indicates that adhesion molecules are critically involved in the regulation of mechanisms of synaptic plasticity including synapse formation, but also synaptic remodeling associated to changes in synaptic strength. Among these, the Neural Cell Adhesion Molecule (NCAM) and its polysialylated form PSA-NCAM are important candidates. Here we review recent results that point to a possible role of these two molecules in regulating the structural properties of excitatory synapses and namely the composition and stability of the postsynaptic density, thereby accounting for their contribution to mechanisms of synaptogenesis and activity-dependent synaptic plasticity.

Activity and localization of the secretory pathway Ca2+-ATPase isoform 1 (SPCA1) in different areas of the mouse brain during postnatal development

Ca2+ and Mn2+ play an important role in many events in the nervous system, ranging from neural morphogenesis to neurodegeneration. As part of the homeostatic control of these ions, the Secretory Pathway Ca2+-ATPase isoform 1 (SPCA1) mediates the accumulation of Ca2+ or Mn2+ with high affinity into Golgi reservoirs. This SPCA1 represents a relatively recently characterized P-type pump that is highly expressed in nervous tissue, but information on its involvement in neural maturation is currently lacking. In this study, we have analyzed the expression and distribution of the SPCA1 pump in mouse brain during postnatal development. RT-PCR and Western blot assays showed that SPCA1 is particularly highly expressed at nearly constant levels during this entire period of development in cortex, hippocampus, and cerebellum. In spite of the apparently unchanged expression levels, functional assays showed that SPCA-associated Ca2+-ATPase activity increased with the stage of development in these areas. Immunohistochemical studies pointed to SPCA1 localization in Golgi stacks of the soma and the initial part of primary dendritic trunk in main cortical, hippocampal and cerebellar neurons from the earliest postnatal stages. This suggests a potential role in intracellular signaling and in Golgi secretory processes involved in dendritic growth and in functional maturation of the mouse nervous system.

A role for the spine apparatus in LTP and spatial learning

Long-term potentiation (LTP) of synaptic strength is a long-lasting form of synaptic plasticity that has been linked to information storage. Although the molecular and cellular events underlying LTP are not yet fully understood, it is generally accepted that changes in dendritic spine calcium levels as well as local protein synthesis play a central role. These two processes may be influenced by the presence of a spine apparatus, a distinct neuronal organelle found in a subpopulation of telencephalic spines. Mice lacking spine apparatuses (synaptopodin-deficient mice) show deficits in LTP and impaired spatial learning supporting the involvement of the spine apparatus in synaptic plasticity. In our review, we consider the possible roles of the spine apparatus in LTP1 (protein synthesis-independent), LTP2 (translation-dependent and transcription-independent) and LTP3 (translation- and transcription-dependent) and discuss the effects of the spine apparatus on learning and memory.

Selective targeting of different neural cell adhesion molecule isoforms during motoneuron myotube synapse formation in culture and the switch from an immature to mature form of synaptic vesicle cycling

Characterization of neuromuscular junction formation and function in mice lacking all neural cell adhesion molecule (NCAM) isoforms or only the 180 isoform demonstrated that the 180 isoform was required at adult synapses to maintain effective transmission with repetitive stimulation whereas the 140 and/or 120 isoform(s) were sufficient to mediate the downregulation of synaptic vesicle cycling along the axon after synapse formation. However, the expression and targeting of each isoform and its relationship to distinct forms of synaptic vesicle cycling before and after synapse formation was previously unknown. By transfecting chick motoneurons with fluorescently tagged mouse 180, 140 and 120 isoforms, we show that before myotube contact the 180 and 140 isoforms are expressed in distinct puncta along the axon which are sites of an immature form (Brefeldin A sensitive, L-type Ca2+ channel mediated) of vesicle cycling. After myotube contact the 140 and 180 isoforms are downregulated from the axon and selectively targeted to the presynaptic terminal. This coincided with the downregulation of vesicle cycling along the axon and the expression of the mature form (BFA insensitive, P/Q type Ca2+ channel mediated) of vesicle cycling at the terminal. The synaptic targeting of exogenously expressed 180 and 140 isoforms also occurred when chick motoneurons contacted +/+ mouse myotubes; however only the 180 but not the 140 isoform was targeted on contact with NCAM-/- myotubes. These observations indicate that postsynaptic NCAM is required for the synaptic targeting of presynaptic 140 NCAM but that the localization of presynaptic 180 NCAM occurs via a different mechanism.

Polysialic acid–neural cell adhesion molecule in brain plasticity: From synapses to integration of new neurons

Isoforms of the neuronal cell adhesion molecule (NCAM) carrying the linear homopolymer of alpha 2,8-linked sialic acid (polysialic acid, PSA) have emerged as particularly attractive candidates for promoting plasticity in the nervous system. The large negatively charged PSA chain of NCAM is postulated to be a spacer that reduces adhesion forces between cells allowing dynamic changes in membrane contacts. Accumulating evidence also suggests that PSA-NCAM-mediated interactions lead to activation of intracellular signaling cascades that are fundamental to the biological functions of the molecule. An important role of PSA-NCAM appears to be during development, when its expression level is high and where it contributes to the regulation of cell shape, growth or migration. However, PSA-NCAM does persist in adult brain structures such as the hippocampus that display a high degree of plasticity where it is involved in activity-induced synaptic plasticity. Recent advances in the field of PSA-NCAM research have not only consolidated the importance of this molecule in plasticity processes but also suggest a role for PSA-NCAM in the regulation of higher cognitive functions and psychiatric disorders. In this review, we discuss the role and mode of actions of PSA-NCAM in structural plasticity as well as its potential link to cognitive processes.

Copb1-facilitated axonal transport and translation of kappa opioid-receptor mRNA

mRNA of kappa opioid receptor (KOR) can be transported to nerve fibers, including axons of dorsal root ganglia (DRG), and can be locally translated. Yeast three-hybrid screening identifies Copb1 as a kor mRNA-associated protein that form complexes with endogenous kor mRNA, which are colocalized in the soma and axons of DRG neurons. Axonal transport of kor mRNA is demonstrated, directly, by observing mobilization of biotin-labeled kor mRNA in Campenot chambers. Efficient transport of kor mRNA into the side chamber requires Copb1 and can be blocked by a drug that disrupts microtubules. The requirement for Copb1 in mobilizing kor mRNA is confirmed by using the MS2-GFP mRNA-tagging system. Furthermore, Copb1 also facilitates the translation of kor mRNA in the soma and axons. This study provides evidence for a microtubule-dependent, active axonal kor mRNA-transport process that involves Copb1 and can stimulate localized translation and suggests coupling of transport and translation of mRNAs destined to the remote areas such as axons.

Cholesterol supports the retinoic acid-induced synaptic vesicle formation in differentiating human SH-SY5Y neuroblastoma cells

Synaptic vesicle formation, vesicle activation and exo/endocytosis in the pre-synaptic area are central steps in neuronal communication. The formation and localization of synaptic vesicles in human SH-SY5Y neuroblastoma cells, differentiated with 12-o-tetradecanoyl-phorbol-13-acetate, dibutyryl cyclic AMP, all-trans-retinoic acid (RA) and cholesterol, was studied by fluorescence microscopy and immunocytochemical methods. RA alone or together with cholesterol, produced significant neurite extension and formation of cell-to-cell contacts. Synaptic vesicle formation was followed by anti-synaptophysin (SypI) and AM1-43 staining. SypI was only weakly detected, mainly in cell somata, before 7 days in vitro, after which it was found in neurites. Depolarization of the differentiated cells with high potassium solution increased the number of fluorescent puncta, as well as SypI and AM1-43 co-localization. In addition to increase in the number of synaptic vesicles, RA and cholesterol also increased the number and distribution of lysosome-associated membrane protein 2 labeled lysosomes. RA-induced Golgi apparatus fragmentation was partly avoided by co-treatment with cholesterol. The SH-SY5Y neuroblastoma cell line, differentiated by RA and cholesterol and with good viability in culture, is a valuable tool for basic studies of neuronal metabolism, specifically as a model for dopaminergic neurons.

Wnt-7a induces presynaptic colocalization of alpha 7-nicotinic acetylcholine receptors and adenomatous polyposis coli in hippocampal neurons

Nicotinic acetylcholine receptors (nAChRs) contribute significantly to hippocampal function. Alpha7-nAChRs are present in presynaptic sites in hippocampal neurons and may influence transmitter release, but the factors that determine their presynaptic localization are unknown. We report here that Wnt-7a, a ligand active in the canonical Wnt signaling pathway, induces dissociation of the adenomatous polyposis coli (APC) protein from the beta-catenin cytoplasmic complex and the interaction of APC with alpha7-nAChRs in hippocampal neurons. Interestingly, Wnt-7a induces the relocalization of APC to membranes, clustering of APC in neurites, and coclustering of APC with different, presynaptic protein markers. Wnt-7a also increases the number and size of coclusters of alpha7-nAChRs and APC in presynaptic terminals. These short-term changes in alpha7-nAChRs occur in the few minutes after ligand exposure and involve translocation to the plasma membrane without affecting total receptor levels. Longer-term exposure to Wnt-7a increases nAChR alpha7 subunit levels in an APC-independent manner and increases clusters of alpha7-nAChRs in neurites via an APC-dependent process. Together, these results demonstrate that stimulation through the canonical Wnt pathway regulates the presynaptic localization of APC and alpha7-nAChRs with APC serving as an intermediary in the alpha7-nAChR relocalization process. Modulation by Wnt signaling may be essential for alpha7-nAChR expression and function in synapses.

Rapid and intermittent cotransport of slow component-b proteins

After synthesis in neuronal perikarya, proteins destined for synapses and other distant axonal sites are transported in three major groups that differ in average velocity and protein composition: fast component (FC), slow component-a (SCa), and slow component-b (SCb). The FC transports mainly vesicular cargoes at average rates of approximately 200-400 mm/d. SCa transports microtubules and neurofilaments at average rates of approximately 0.2-1 mm/d, whereas SCb translocates approximately 200 diverse proteins critical for axonal growth, regeneration, and synaptic function at average rates of approximately 2-8 mm/d. Several neurodegenerative diseases are characterized by abnormalities in one or more SCb proteins, but little is known about mechanisms underlying SCb compared with FC and SCa. Here, we use live-cell imaging to visualize and quantify the axonal transport of three SCb proteins, alpha-synuclein, synapsin-I, and glyceraldehyde-3-phosphate dehydrogenase in cultured hippocampal neurons, and directly compare their transport to synaptophysin, a prototypical FC protein. All three SCb proteins move rapidly but infrequently with pauses during transit, unlike synaptophysin, which moves much more frequently and persistently. By simultaneously visualizing the transport of proteins at high temporal and spatial resolution, we show that the dynamics of alpha-synuclein transport are distinct from those of synaptophysin but similar to other SCb proteins. Our observations of the cotransport of multiple SCb proteins in single axons suggest that they move as multiprotein complexes. These studies offer novel mechanistic insights into SCb and provide tools for further investigating its role in disease processes.

BAD-LAMP defines a subset of early endocytic organelles in subpopulations of cortical projection neurons

The brain-associated LAMP-like molecule (BAD-LAMP) is a new member of the family of lysosome associated membrane proteins (LAMPs). In contrast to other LAMPs, which show a widespread expression, BAD-LAMP expression in mice is confined to the postnatal brain and therein to neuronal subpopulations in layers II/III and V of the neocortex. Onset of expression strictly parallels cortical synaptogenesis. In cortical neurons, the protein is found in defined clustered vesicles, which accumulate along neurites where it localizes with phosphorylated epitopes of neurofilament H. In primary neurons, BAD-LAMP is endocytosed, but is not found in classical lysosomal/endosomal compartments. Modification of BAD-LAMP by addition of GFP revealed a cryptic lysosomal retention motif, suggesting that the cytoplasmic tail of BAD-LAMP is actively interacting with, or modified by, molecules that promote its sorting away from lysosomes. Analysis of BAD-LAMP endocytosis in transfected HeLa cells provided evidence that the protein recycles to the plasma membrane through a dynamin/AP2-dependent mechanism. Thus, BAD-LAMP is an unconventional LAMP-like molecule and defines a new endocytic compartment in specific subtypes of cortical projection neurons. The striking correlation between the appearance of BAD-LAMP and cortical synatogenesis points towards a physiological role of this vesicular determinant for neuronal function.

Dynamic aspects of CNS synapse formation

The mammalian central nervous system (CNS) requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between thousands of differentiating neurons. Proper synapse formation during childhood provides the substrate for cognition, whereas improper formation or function of these synapses leads to neurodevelopmental disorders, including mental retardation and autism. Recent work has begun to identify some of the early cellular events in synapse formation as well as the molecular signals that initiate this process. However, despite the wealth of information published on this topic in the past few years, some of the most fundamental questions about how, whether, and where glutamatergic synapses form in the mammalian CNS remain unanswered. This review focuses on the dynamic aspects of the early cellular and molecular events in the initial assembly of glutamatergic synapses in the mammalian CNS.