Plasma membrane expansion: a neuron's Herculean task

GTP hydrolysis of TC10 promotes neurite outgrowth through exocytic fusion of Rab11- and L1-containing vesicles by releasing exocyst component Exo70

The use of exocytosis for membrane expansion at nerve growth cones is critical for neurite outgrowth. TC10 is a Rho family GTPase that is essential for specific types of vesicular trafficking to the plasma membrane. Recent studies have shown that TC10 and its effector Exo70, a component of the exocyst tethering complex, contribute to neurite outgrowth. However, the molecular mechanisms of the neuritogenesis-promoting functions of TC10 remain to be established. Here, we propose that GTP hydrolysis of vesicular TC10 near the plasma membrane promotes neurite outgrowth by accelerating vesicle fusion by releasing Exo70. Using Förster resonance energy transfer (FRET)-based biosensors, we show that TC10 activity at the plasma membrane decreased at extending growth cones in hippocampal neurons and nerve growth factor (NGF)-treated PC12 cells. In neuronal cells, TC10 activity at vesicles was higher than its activity at the plasma membrane, and TC10-positive vesicles were found to fuse to the plasma membrane in NGF-treated PC12 cells. Therefore, activity of TC10 at vesicles is presumed to be inactivated near the plasma membrane during neuronal exocytosis. Our model is supported by functional evidence that constitutively active TC10 could not rescue decrease in NGF-induced neurite outgrowth induced by TC10 depletion. Furthermore, TC10 knockdown experiments and colocalization analyses confirmed the involvement of Exo70 in TC10-mediated trafficking in neuronal cells. TC10 frequently resided on vesicles containing Rab11, which is a key regulator of recycling pathways and implicated in neurite outgrowth. In growth cones, most of the vesicles containing the cell adhesion molecule L1 had TC10. Exocytosis of Rab11- and L1-positive vesicles may play a central role in TC10-mediated neurite outgrowth. The combination of this study and our previous work on the role of TC10 in EGF-induced exocytosis in HeLa cells suggests that the signaling machinery containing TC10 proposed here may be broadly used for exocytosis.

MAPping out distribution routes for kinesin couriers

In the crowded environment of eukaryotic cells, diffusion is an inefficient distribution mechanism for cellular components. Long-distance active transport is required and is performed by molecular motors including kinesins. Furthermore, in highly polarised, compartmentalised and plastic cells such as neurons, regulatory mechanisms are required to ensure appropriate spatio-temporal delivery of neuronal components. The kinesin machinery has diversified into a large number of kinesin motor proteins as well as adaptor proteins that are associated with subsets of cargo. However, many mechanisms contribute to the correct delivery of these cargos to their target domains. One mechanism is through motor recognition of sub-domain-specific microtubule (MT) tracks, sign-posted by different tubulin isoforms, tubulin post-translational modifications, tubulin GTPase activity and MT-associated proteins (MAPs). With neurons as a model system, a critical review of these regulatory mechanisms is presented here, with a particular focus on the emerging contribution of compartmentalised MAPs. Overall, we conclude that - especially for axonal cargo - alterations to the MT track can influence transport, although in vivo, it is likely that multiple track-based effects act synergistically to ensure accurate cargo distribution.

Mutations in BICD2 cause dominant congenital spinal muscular atrophy and hereditary spastic paraplegia

Dominant congenital spinal muscular atrophy (DCSMA) is a disorder of developing anterior horn cells and shows lower-limb predominance and clinical overlap with hereditary spastic paraplegia (HSP), a lower-limb-predominant disorder of corticospinal motor neurons. We have identified four mutations in bicaudal D homolog 2 (Drosophila) (BICD2) in six kindreds affected by DCSMA, DCSMA with upper motor neuron features, or HSP. BICD2 encodes BICD2, a key adaptor protein that interacts with the dynein-dynactin motor complex, which facilitates trafficking of cellular cargos that are critical to motor neuron development and maintenance. We demonstrate that mutations resulting in amino acid substitutions in two binding regions of BICD2 increase its binding affinity for the cytoplasmic dynein-dynactin complex, which might result in the perturbation of BICD2-dynein-dynactin-mediated trafficking, and impair neurite outgrowth. These findings provide insight into the mechanism underlying both the static and the slowly progressive clinical features and the motor neuron pathology that characterize BICD2-associated diseases, and underscore the importance of the dynein-dynactin transport pathway in the development and survival of both lower and upper motor neurons.

Measuring nonequilibrium vesicle dynamics in neurons under tension

Vesicle transport in neurons is a highly complex nonequilibrium process. Their subcellular environment is undergoing constant fluctuations from thermal energy and molecular motors. Vesicle transport is an interplay between random motion (passive) and directed motion (active) driven by molecular motors along cytoskeletal filaments. It has been shown that growth, guidance, and vesicle dynamics of neurons is affected by mechanical tension. Here we present a method to analyze vesicle transport via a temporal Mean Square Displacement (tMSD) analysis while applying mechanical strain to neurons. The tMSD analysis allows characterization of active and passive vesicle motion as well as many other parameters including: power law scaling, velocity, direction, and flux. Our results suggest: (1) The tMSD analysis is able to capture vesicle motion alternating between passive and active states, and indicates that vesicle motion in Aplysia neurons is primarily passive (exhibiting active motion for ~8% of the time). (2) Under mechanical stretch (increased neurite tension), active transport of vesicles increases to ~13%, while vesicle velocity remains unchanged. (3) Upon unstretching (decreased tension), the level of active transport returns to normal but vesicle velocity decreases. These results suggest that vesicle transport in neurons is highly sensitive to mechanical stimulation. Our method allows precise characterization of vesicle dynamics in response to applied mechanical strain.

Controlled lateral packing of insulin monolayers influences neuron polarization in solid-supported cultures

Neurons are highly polarized cells, composed of one axon and several branching dendrites. One important issue in neurobiology is to understand the molecular factors that determine the neuron to develop polarized structures. A particularly early event, in neurons still lacking a discernible axon, is the segregation of IGF-1 (Insulin like Growth Factor-1) receptors in one neurite. This receptor can be activated by insulin in bulk, but, it is not known if changes of insulin organization as a monomolecular film may affect neuron polarization. To this end, in this work we developed solid-supported Langmuir-Blodgett films of insulin with different surface packing density. Hyppocampal pyramidal neurons, in early stage of differentiation, were cultured onto those substrates and polarization was studied after 24 h by confocal microscopy. Also we used surface reflection interference contrast microscopy and confocal microscopy to study attachment patterns and morphology of growth cones. We observed that insulin films packed at 14 mN/m induced polarization in a similar manner to high insulin concentration in bulk, but insulin packed at 44 mN/m did not induce polarization. Our results provide novel evidence that the neuron polarization through IGF-1 receptor activation can be selectively modulated by the lateral packing of insulin organized as a monomolecular surface for cell growth.

Rab33a mediates anterograde vesicular transport for membrane exocytosis and axon outgrowth

Axon outgrowth requires plasma membrane expansion, which results from post-Golgi vesicular transport and fusion. However, the molecular mechanisms regulating post-Golgi vesicular trafficking for membrane expansion and axon outgrowth remain unclear. Here, we show that Rab33a expression became upregulated during axon outgrowth of cultured rat hippocampal neurons. Rab33a was preferentially localized to the Golgi apparatus and to synaptophysin-positive vesicles that are transported along the growing axon. Previous studies showed that synaptophysin is localized to post-Golgi vesicles transported by fast axonal transport in developing neurons. Reduction of Rab33a expression by RNAi (RNA interference) inhibited the anterograde transport of synaptophysin-positive vesicles, leading to their decrease in axonal tips. Furthermore, this treatment reduced membrane fusion of synaptophysin-positive vesicles at the growth cones and inhibited axon outgrowth. Overexpression of Rab33a, on the other hand, induced excessive accumulation of synaptophysin-positive vesicles and concurrent formation of surplus axons. These data suggest that Rab33a participates in axon outgrowth by mediating anterograde axonal transport of synaptophysin-positive vesicles and their concomitant fusion at the growth cones.

Syntaxin 1 is required for DCC/Netrin-1-dependent chemoattraction of migrating neurons from the lower rhombic lip

Directed cell migration and axonal guidance are essential steps in neural development that share many molecular mechanisms. The guidance of developing axons and migrating neurons is likely to depend on the precise control of plasmalemma turnover in selected regions of leading edges and growth cones, respectively. Previous results provided evidence of a signaling mechanism that couples chemotropic deleted in colorectal cancer (DCC)/Netrin-1 axonal guidance and exocytosis through Syntaxin1(Sytx1)/TI-VAMP SNARE proteins. Here we studied whether Netrin-1-dependent neuronal migration relies on a similar SNARE mechanism. We show that migrating neurons in the lower rhombic lip (LRL) express several SNARE proteins, and that DCC co-associates with Sytx1 and TI-VAMP in these cells. We also demonstrate that cleavage of Sytx1 by botulinum toxin C1 (BoNT/C1) abolishes Netrin-1-dependent chemoattraction of migrating neurons, and that interference of Sytx1 functions with shRNAs or Sytx1-dominant negatives disrupts Netrin-1-dependent chemoattraction of LRL neurons. These findings indicate that a Sytx1/DCC interaction is required for Netrin-1 guidance of migrating neurons, thereby highlighting a relationship between guidance signaling and SNARE proteins that regulate membrane turnover.

Neuronal polarity: demarcation, growth and commitment

In a biological sense, polarity refers to the extremity of the main axis of an organelle, cell, or organism. In neurons, morphological polarity begins with the appearance of the first neurite from the cell body. In multipolar neurons, a second phase of polarization occurs when a single neurite initiates a phase of rapid growth to become the neuron's axon, while the others later differentiate as dendrites. Finally, during a third phase, axons and dendrites develop an elaborate architecture, acquiring special morphological and molecular features that commit them to their final identities. Mechanistically, each phase must be preceded by spatial restriction of growth activity. We will review recent work on the mechanisms underlying the polarized growth of neurons.

A molecular network for the transport of the TI-VAMP/VAMP7 vesicles from cell center to periphery

The compartmental organization of eukaryotic cells is maintained dynamically by vesicular trafficking. SNARE proteins play a crucial role in intracellular membrane fusion and need to be targeted to their proper donor or acceptor membrane. The molecular mechanisms that allow for the secretory vesicles carrying the v-SNARE TI-VAMP/VAMP7 to leave the cell center, load onto microtubules, and reach the periphery to mediate exocytosis are largely unknown. Here, we show that the TI-VAMP/VAMP7 partner Varp, a Rab21 guanine nucleotide exchange factor, interacts with GolginA4 and the kinesin 1 Kif5A. Activated Rab21-GTP in turn binds to MACF1, an actin and microtubule regulator, which is itself a partner of GolginA4. These components are required for directed movement of TI-VAMP/VAMP7 vesicles from the cell center to the cell periphery. The molecular mechanisms uncovered here suggest an integrated view of the transport of vesicles carrying a specific v-SNARE toward the cell surface.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

Intracellular signaling and membrane trafficking control bidirectional growth cone guidance

The formation of precise neuronal networks is critically dependent on the motility of axonal growth cones. Extracellular gradients of guidance cues evoke localized Ca(2+) elevations to attract or repel the growth cone. Recent studies strongly suggest that the polarity of growth cone guidance, with respect to the localization of Ca(2+) signals, is determined by Ca(2+) release from the endoplasmic reticulum (ER) in the following manner: Ca(2+) signals containing ER Ca(2+) release cause growth cone attraction, while Ca(2+) signals without ER Ca(2+) release cause growth cone repulsion. Recent studies have also shown that exocytic and endocytic membrane trafficking can drive growth cone attraction and repulsion, respectively, downstream of Ca(2+) signals. Most likely, these two mechanisms underlie cue-induced axon guidance, in which a localized imbalance between exocytosis and endocytosis dictates bidirectional growth cone steering. In this Update Article, I summarize recent advances in growth cone research and propose that polarized membrane trafficking plays an instructive role to spatially localize steering machineries, such as cytoskeletal components and adhesion molecules.

Calcium-activated chloride current expression in axotomized sensory neurons: what for?

Calcium-activated chloride currents (CaCCs) are activated by an increase in intracellular calcium concentration. Peripheral nerve injury induces the expression of CaCCs in a subset of adult sensory neurons in primary culture including mechano- and proprioceptors, though not nociceptors. Functional screenings of potential candidate genes established that Best1 is a molecular determinant for CaCC expression among axotomized sensory neurons, while Tmem16a is acutely activated by inflammatory mediators in nociceptors. In nociceptors, such CaCCs are preferentially activated under receptor-induced calcium mobilization contributing to cell excitability and pain. In axotomized mechano- and proprioceptors, CaCC activation does not promote electrical activity and prevents firing, a finding consistent with electrical silencing for growth competence of adult sensory neurons. In favor of a role in the process of neurite growth, CaCC expression is temporally correlated to neurons displaying a regenerative mode of growth. This perspective focuses on the molecular identity and role of CaCC in axotomized sensory neurons and the future directions to decipher the cellular mechanisms regulating CaCC during neurite (re)growth.

Functional complexity of the axonal growth cone: a proteomic analysis

The growth cone, the tip of the emerging neurite, plays a crucial role in establishing the wiring of the developing nervous system. We performed an extensive proteomic analysis of axonal growth cones isolated from the brains of fetal Sprague-Dawley rats. Approximately 2000 proteins were identified at ≥ 99% confidence level. Using informatics, including functional annotation cluster and KEGG pathway analysis, we found great diversity of proteins involved in axonal pathfinding, cytoskeletal remodeling, vesicular traffic and carbohydrate metabolism, as expected. We also found a large and complex array of proteins involved in translation, protein folding, posttranslational processing, and proteasome/ubiquitination-dependent degradation. Immunofluorescence studies performed on hippocampal neurons in culture confirmed the presence in the axonal growth cone of proteins representative of these processes. These analyses also provide evidence for rough endoplasmic reticulum and reveal a reticular structure equipped with Golgi-like functions in the axonal growth cone. Furthermore, Western blot revealed the growth cone enrichment, relative to fetal brain homogenate, of some of the proteins involved in protein synthesis, folding and catabolism. Our study provides a resource for further research and amplifies the relatively recently developed concept that the axonal growth cone is equipped with proteins capable of performing a highly diverse range of functions.

Single vesicle imaging indicates distinct modes of rapid membrane retrieval during nerve growth

Background

During nerve growth, cytoplasmic vesicles add new membrane preferentially to the growth cone located at the distal tip of extending axons. Growth cone membrane is also retrieved locally, and asymmetric retrieval facilitates membrane remodeling during growth cone repulsion by a chemorepellent gradient. Moreover, growth inhibitory factors can stimulate bulk membrane retrieval and induce growth cone collapse. Despite these functional insights, the processes mediating local membrane remodeling during axon extension remain poorly defined.

Results

To investigate the spatial and temporal dynamics of membrane retrieval in actively extending growth cones, we have used a transient labeling and optical recording method that can resolve single vesicle events. Live-cell confocal imaging revealed rapid membrane retrieval by distinct endocytic modes based on spatial distribution in Xenopus spinal neuron growth cones. These modes include endocytic "hot-spots" triggered at the base of filopodia, at the lateral margins of lamellipodia, and along dorsal ridges of the growth cone. Additionally, waves of endocytosis were induced when individual filopodia detached from the substrate and fused with the growth cone dorsal surface or with other filopodia. Vesicle formation at sites of membrane remodeling by self-contact required F-actin polymerization. Moreover, bulk membrane retrieval by macroendocytosis correlated positively with the substrate-dependent rate of axon extension and required the function of Rho-family GTPases.

Conclusions

This study provides insight into the dynamic membrane remodeling processes essential for nerve growth by identifying several distinct modes of rapid membrane retrieval in the growth cone during axon extension. We found that endocytic membrane retrieval is intensified at specific subdomains and may drive the dynamic membrane ruffling and re-absorption of filopodia and lamellipodia in actively extending growth cones. The findings offer a platform for determining the molecular mechanisms of distinct endocytic processes that may remodel the surface distribution of receptors, ion channels and other membrane-associated proteins locally to drive growth cone extension and chemotactic guidance.

Small GTPase Rab17 regulates dendritic morphogenesis and postsynaptic development of hippocampal neurons

Neurons are compartmentalized into two morphologically, molecularly, and functionally distinct domains: axons and dendrites, and precise targeting and localization of proteins within these domains are critical for proper neuronal functions. It has been reported that several members of the Rab family small GTPases that are key mediators of membrane trafficking, regulate axon-specific trafficking events, but little has been elucidated regarding the molecular mechanisms that underlie dendrite-specific membrane trafficking. Here we show that Rab17 regulates dendritic morphogenesis and postsynaptic development in mouse hippocampal neurons. Rab17 is localized at dendritic growth cones, shafts, filopodia, and mature spines, but it is mostly absent in axons. We also found that Rab17 mediates dendrite growth and branching and that it does not regulate axon growth or branching. Moreover, shRNA-mediated knockdown of Rab17 expression resulted in a dramatically reduced number of dendritic spines, probably because of impaired filopodia formation. These findings have revealed the first molecular link between membrane trafficking and dendritogenesis.

The vesicular SNARE Synaptobrevin is required for Semaphorin 3A axonal repulsion

Semaphorin 3A-mediated signaling and axonal repulsion in the mouse brain require Synaptobrevin-dependent vesicular traffic.

Attractive and repulsive molecules such as Semaphorins (Sema) trigger rapid responses that control the navigation of axonal growth cones. The role of vesicular traffic in axonal guidance is still largely unknown. The exocytic vesicular soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor (SNARE) Synaptobrevin 2 (Syb2) is known for mediating neurotransmitter release in mature neurons, but its potential role in axonal guidance remains elusive. Here we show that Syb2 is required for Sema3A-dependent repulsion but not Sema3C-dependent attraction in cultured neurons and in the mouse brain. Syb2 associated with Neuropilin 1 and Plexin A1, two essential components of the Sema3A receptor, via its juxtatransmembrane domain. Sema3A receptor and Syb2 colocalize in endosomal membranes. Moreover, upon Sema3A treatment, Syb2-deficient neurons failed to collapse and transport Plexin A1 to cell bodies. Reconstitution of Sema3A receptor in nonneuronal cells revealed that Sema3A further inhibited the exocytosis of Syb2. Therefore, Sema3A-mediated signaling and axonal repulsion require Syb2-dependent vesicular traffic.

A signaling mechanism coupling netrin-1/deleted in colorectal cancer chemoattraction to SNARE-mediated exocytosis in axonal growth cones

Directed cell migration and axonal guidance are essential steps in neural development. Both processes are controlled by specific guidance cues that activate the signaling cascades that ultimately control cytoskeletal dynamics. Another essential step in migration and axonal guidance is the regulation of plasmalemma turnover and exocytosis in leading edges and growth cones. However, the cross talk mechanisms linking guidance receptors and membrane exocytosis are not understood. Netrin-1 is a chemoattractive cue required for the formation of commissural pathways. Here, we show that the Netrin-1 receptor deleted in colorectal cancer (DCC) forms a protein complex with the t-SNARE (target SNARE) protein Syntaxin-1 (Sytx1). This interaction is Netrin-1 dependent both in vitro and in vivo, and requires specific Sytx1 and DCC domains. Blockade of Sytx1 function by using botulinum toxins abolished Netrin-1-dependent chemoattraction of axons in mouse neuronal cultures. Similar loss-of-function experiments in the chicken spinal cord in vivo using dominant-negative Sytx1 constructs or RNAi led to defects in commissural axon pathfinding reminiscent to those described in Netrin-1 and DCC loss-of-function models. We also show that Netrin-1 elicits exocytosis at growth cones in a Sytx1-dependent manner. Moreover, we demonstrate that the Sytx1/DCC complex associates with the v-SNARE (vesicle SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) and that knockdown of TI-VAMP in the commissural pathway in the spinal cord results in aberrant axonal guidance phenotypes. Our data provide evidence of a new signaling mechanism that couples chemotropic Netrin-1/DCC axonal guidance and Sytx1/TI-VAMP SNARE proteins regulating membrane turnover and exocytosis.

Astrocyte stellation, a process dependent on Rac1 is sustained by the regulated exocytosis of enlargeosomes

Cultured astrocytes exhibit a flat/epitelioid phenotype much different from the star-like phenotype of tissue astrocytes. Upon exposure to treatments that affect the small GTPase Rho and/or its effector ROCK, however, flat astrocytes undergo stellation, with restructuring of cytoskeleton and outgrowth of processes with lamellipodia, assuming a phenotype closer to that exhibited in situ. The mechanisms of this change are known only in part. Using the ROCK blocker drug Y27632, which induces rapid (tens of min), dose-dependent and reversible stellations, we focused on two specific aspects of the process: its dependence on small GTPases and the large surface expansion of the cells. Contrary to previous reports, we found stellation to be governed by the small G protein Rac1, up to disappearance of the process when Rac1 was downregulated or blocked by a specific drug. In contrast cdc42, the other G-protein often involved in phenotype changes, appeared not involved. The surface expansion concomitant to cytoskeleton restructuring, also dependent on Rac1, was found to be at least partially sustained by the exocytosis of enlargeosomes, small vesicles distinct from classical cell organelles, which are abundant in astrocytes. Exhaustion of stellation induced by repeated administrations of Y27632 correlated with the decrease of the enlargeosome pool. A whole-cell process like stellation of cultured astrocytes might be irrelevant in the brain tissue. However, local restructuring of the cytoskeleton coordinate with surface expansion, occurring at critical cell sites and sustained by mechanisms analogous to those of stellation, might be of importance in both astrocyte physiology and pathology. © 2011 Wiley Periodicals, Inc.

Insight into the role of Ca2+-binding protein 5 in vesicle exocytosis

PURPOSE

CaBP5 is a neuronal calmodulin-like Ca(2+)-binding protein that is expressed in the retina and in the cochlea. Although CaBP5 knockout mice displayed reduced sensitivity of retinal ganglion cell light responses, the function of CaBP5 in vivo is still unknown. To gain further insight into CaBP5 function, the authors screened for CaBP5-interacting partners.

METHODS

Potential retinal interacting partners for CaBP5 were identified using affinity chromatography followed by mass spectrometry and by yeast two-hybrid screening of a bovine retina cDNA library. Interacting partners were further analyzed using coimmunoprecipitation. Immunohistochemistry and subcellular fractionation were performed to determine their colocalization in the retina. The effect of CaBP5 on dopamine release and neurite outgrowth of PC12 cells was analyzed using ELISA and fluorescent labeling.

RESULTS

Using affinity chromatography, the authors identified Munc18-1 and myosin VI as interacting partners for CaBP5. Munc18-1 was also identified using the yeast two-hybrid system. Colocalization and coimmunoprecipitation of CaBP5 with these two proteins in retinal tissue further established their physiological interactions. Furthermore, CaBP5 expression in NGF-stimulated PC12 cells stimulates neurite outgrowth and dopamine exocytosis.

CONCLUSIONS

This study shows that CaBP5 interacts with Munc18-1 and myosin VI, two proteins involved in the synaptic vesicle cycle. Together with the effect of CaBP5 in stimulating neurite outgrowth and vesicle exocytosis in PC12 cells, these results suggest that CaBP5 plays a role in neurotransmitter release.

Lgl1 activation of rab10 promotes axonal membrane trafficking underlying neuronal polarization

Directed membrane trafficking is believed to be crucial for axon development during neuronal morphogenesis. However, the underlying mechanisms are poorly understood. Here, we report a role of Lgl1, the mammalian homolog of Drosophila tumor suppressor Lethal giant larvae, in controlling membrane trafficking underlying axonal growth. We find that Lgl1 is associated with plasmalemmal precursor vesicles and enriched in developing axons. Lgl1 upregulation promoted axonal growth, whereas downregulation attenuated it as well as directional membrane insertion. Interestingly, Lgl1 interacted with and activated Rab10, a small GTPase that mediates membrane protein trafficking, by releasing GDP dissociation inhibitor (GDI) from Rab10. Furthermore, Rab10 lies downstream of Lgl1 in axon development and directional membrane insertion. Finally, both Lgl1 and Rab10 are required for neocortical neuronal polarization in vivo. Thus, the Lgl1 regulation of Rab10 stimulates the trafficking of membrane precursor vesicles, whose fusion with the plasmalemma is crucial for axonal growth.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

Roles of membrane trafficking in nerve repair and regeneration

Successful axonal repair following injury is critical for nerve regeneration and functional recovery. Nerve repair relies on three functionally distinct events involving membrane trafficking. First, axonally transported vesicles accumulate, while others are generated at the cut end to restore a selective barrier to the severed axon. Then, retrograde transport of vesicles along microtubules informs the cell body that damage has occurred in the distal axon. Finally, membrane addition to a newly formed growth cone, or to the axonal membrane is required to promote axonal re-growth and elongation. Yet, how these membrane trafficking events are regulated and what are the identities of the molecules and organelles involved remains largely unknown. Several potential factors have been recently identified. Members of the SNARE machinery appear to regulate fusion of vesicles in a calcium-dependent manner to promote axolemmal resealing. Retrograde transport of endosomes powered by the dynein-dynactin molecular motor complex represents a potential injury-signaling platform. Several classes of secretory and endocytic vesicles may coordinate axonal membrane extension and re-growth. Here we discuss recent advances in understanding the mechanisms of the membrane trafficking involved in nerve repair.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Polarizing membrane dynamics and adhesion for growth cone navigation

Neuronal network formation relies on the motile behavior of growth cones at the tip of navigating axons. Accumulating evidence indicates that growth cone motility requires spatially controlled endocytosis and exocytosis that can redistribute bulk membrane and functional cargos such as cell adhesion molecules. For axon elongation, the growth cone recycles cell adhesion molecules from its rear to its leading front through endosomes, thereby polarizing growth cone adhesiveness along the axis of migration direction. In response to extracellular guidance cues, the growth cone turns by retrieving membrane components from the retractive side or by supplying them to the side facing the new direction. We propose that polarized membrane trafficking creates adhesion gradients along and across the front-to-rear axis of growth cones that are essential for axon elongation and turning, respectively. This review will examine how growth cone adhesiveness can be patterned by spatially coordinated endocytosis and exocytosis of cell adhesion molecules. This article is part of a Special Issue entitled 'Neuronal Function'.

Second messengers and membrane trafficking direct and organize growth cone steering

Graded distributions of extracellular cues guide developing axons toward their targets. A network of second messengers - Ca(2+) and cyclic nucleotides - shapes cue-derived information into either attractive or repulsive signals that steer growth cones bidirectionally. Emerging evidence suggests that such guidance signals create a localized imbalance between exocytosis and endocytosis, which in turn redirects membrane, adhesion and cytoskeletal components asymmetrically across the growth cone to bias the direction of axon extension. These recent advances allow us to propose a unifying model of how the growth cone translates shallow gradients of environmental information into polarized activity of the steering machinery for axon guidance.

Lack of the endosomal SNAREs vti1a and vti1b led to significant impairments in neuronal development

Fusion between membranes is mediated by specific SNARE complexes. Here we report that fibroblasts survive the absence of the trans-Golgi network/early endosomal SNARE vti1a and the late endosomal SNARE vti1b with intact organelle morphology and minor trafficking defects. Because vti1a and vti1b are the only members of their SNARE subclass and the yeast homolog Vti1p is essential for cell survival, these data suggest that more distantly related SNAREs acquired the ability to function in endosomal traffic during evolution. However, absence of vti1a and vti1b resulted in perinatal lethality. Major axon tracts were missing, reduced in size, or misrouted in Vti1a(-/-) Vti1b(-/-) embryos. Progressive neurodegeneration was observed in most Vti1a(-/-) Vti1b(-/-) peripheral ganglia. Neurons were reduced by more than 95% in Vti1a(-/-) Vti1b(-/-) dorsal root and geniculate ganglia at embryonic day 18.5. These data suggest that special demands for endosomal membrane traffic could not be met in Vti1a(-/-) Vti1b(-/-) neurons. Vti1a(-/-) and Vti1b(-/-) single deficient mice were viable without these neuronal defects, indicating that they can substitute for each other in these processes.

Expression and function of neuronal growth-associated proteins (nGAPs) in PC12 cells

The growth cone plays crucial roles in neural wiring, synapse formation, and axonal regeneration. Continuous rearrangement of cytoskeletal elements and targeting of transported vesicles to the plasma membrane are essential to growth cone motility; however, the proteins directly involved in these processes and their specific functions are not well established. We recently identified 17 proteins as functional marker proteins of the mammalian growth cone and as neuronal growth-associated proteins in rat cortical neurons (nGAPs; Nozumi et al., 2009). To determine whether these 17 proteins are growth cone markers in other neuronal cell types, we examined their expression and function in PC12D cells. We found that all 17 nGAPs were highly concentrated in the growth cones of PC12D cells, and that knockdown of all of them by RNAi reduced or inhibited neurite outgrowth, indicating that all of the 17 nGAPs may be general growth cone markers. Among them, eight proteins were shown to regulate the amount of F-actin in PC12D growth cones. Two of these nGAP that are cytoskeletal proteins, Cap1 and Sept2, increased the mean growth cone area and the mean neurite length by regulating the amount of F-actin; Sept2 also induced filopodial growth. Taken together, our data suggested that some of the nGAPs were generalized markers of the growth cone in multiple neuronal cell types and some of them, such as Cap1 and Sept2, regulated growth cone morphology through rearrangement of F-actin and thereby controlled neurite outgrowth.

Reelin and stk25 have opposing roles in neuronal polarization and dendritic Golgi deployment

The Reelin ligand regulates a Dab1-dependent signaling pathway required for brain lamination and normal dendritogenesis, but the specific mechanisms underlying these actions remain unclear. We find that Stk25, a modifier of Reelin-Dab1 signaling, regulates Golgi morphology and neuronal polarization as part of an LKB1-Stk25-Golgi matrix protein 130 (GM130) signaling pathway. Overexpression of Stk25 induces Golgi condensation and multiple axons, both of which are rescued by Reelin treatment. Reelin stimulation of cultured neurons induces the extension of the Golgi into dendrites, which is suppressed by Stk25 overexpression. In vivo, Reelin and Dab1 are required for the normal extension of the Golgi apparatus into the apical dendrites of hippocampal and neocortical pyramidal neurons. This demonstrates that the balance between Reelin-Dab1 signaling and LKB1-Stk25-GM130 regulates Golgi dispersion, axon specification, and dendrite growth and provides insights into the importance of the Golgi apparatus for cell polarization.

Outgrowth of neurites is a dual process

In neurons and neurosecretory (nerve) cells, neurite outgrowth requires the enlargement of the plasma membrane sustained by the exocytosis of specific vesicles. The well known, slow canonical form of outgrowth induced in pheochromocytoma PC12 cells by NGF, as well as the outgrowth taking place in neurons, involve vesicles positive for the vSNARE Ti-VAMP. Working in defective PC12 clones expressing high levels of the transcriptional repressor REST, we have identified now a new, rapid form of outgrowth, triggered by activation of a small GTPase, Rac1. This form is sustained by the exocytosis of another type of vesicles, taking place locally at the tip of neurite growth cones, the enlargeosomes (vSNARE: VAMP4). This new form, which is positively controlled by REST, requires the dynamics of microtubules, but not of microfilaments. Its signaling remains undefined because established second messengers, (Ca(2+), DAG, cAMP) seem not involved. Using a high REST/enlargeosome-rich PC12 clone transfected with TrkA we have found that the NGF-induced outgrowth is not always slow, but can be fast in cells expressing high levels of the receptor involved, TrkA; that PC12 can express together the two distinct forms of outgrowth, canonical and new, activated independently from each other. Their comparative characterization in terms of changes in the cytoskeleton has now been initiated. The two forms are present also in neurons where the new one seems to predominate in the initial phases of development, the canonical one later on. Our results identify a new aspect of the REST impact in nerve cell specificity/function. The existence of two distinct forms of neurite outgrowth may cope better than a single form with the variable needs of nerve cells in the subsequent stages of their development.

Lipoprotein particles cross the blood-brain barrier in Drosophila

The blood-brain barrier (BBB) regulates passage of nutrients and signaling molecules from the circulation into the brain. Whether lipoproteins cross the BBB in vivo has been controversial, and no clear requirement for circulating lipoproteins in brain development has been shown. We address these issues in Drosophila, which has an functionally conserved BBB, and lipoproteins that resemble those of vertebrates. We show that the Drosophila lipoprotein lipophorin exists in two isoforms. Both isoforms cross the BBB, but accumulate on distinct subsets of cells within the brain. In addition to acting as a lipid carrier, lipophorin carries both sterol-linked and GPI-linked proteins into the circulation and transports them across the BBB. Finally, lipophorin promotes neuroblast proliferation by a mechanism that does not depend on delivery of dietary lipids. Transport of lipophorin and its cargo across the BBB represents a novel mechanism by which peripherally synthesized proteins might enter the brain and influence its development. Furthermore, lipid-linkage may be an efficient method to transport therapeutic molecules across the BBB.

Emerging themes of ER organization in the development and maintenance of axons

The endoplasmic reticulum (ER) is a continuous membrane system comprising the nuclear envelope, polyribosome-studded peripheral sheets, and a polygonal network of smooth tubules extending throughout the cell. Though protein biosynthesis, transport, and quality control in the ER have been extensively studied, mechanisms underlying the heterogeneous architecture of the ER have been clarified more recently. These insights have increased interest in ER morphology changes associated with the development of neuronal axons and dendrites as well as their integration with presynaptic and postsynaptic signaling pathways. A number of proteins involved in shaping and distributing the ER network are mutated in neurological disorders, particularly the hereditary spastic paraplegias, emphasizing the importance of proper ER morphology for the establishment and maintenance of highly polarized neurons.

Integrin signaling switches the cytoskeletal and exocytic machinery that drives neuritogenesis

Neurons establish their unique morphology by elaborating multiple neurites that subsequently form axons and dendrites. Neurite initiation entails significant surface area expansion, necessitating addition to the plasma membrane. We report that regulated membrane delivery coordinated with the actin cytoskeleton is crucial for neuritogenesis and identify two independent pathways that use distinct exocytic and cytoskeletal machinery to drive neuritogenesis. One pathway uses Ena/VASP-regulated actin dynamics coordinated with VAMP2-mediated exocytosis and involves a novel role for Ena/VASP in exocytosis. A second mechanism occurs in the presence of laminin through integrin-dependent activation of FAK and src and uses coordinated activity of the Arp2/3 complex and VAMP7-mediated exocytosis. We conclude that neuritogenesis can be driven by two distinct pathways that differentially coordinate cytoskeletal dynamics and exocytosis. These regulated changes and coordination of cytoskeletal and exocytic machinery may be used in other physiological contexts involving cell motility and morphogenesis.

Elevated oxidative membrane damage associated with genetic modifiers of Lyst-mutant phenotypes

LYST is a large cytosolic protein that influences the biogenesis of lysosome-related organelles, and mutation of the encoding gene, LYST, can cause Chediak-Higashi syndrome. Recently, Lyst-mutant mice were recognized to also exhibit an iris disease resembling exfoliation syndrome, a common cause of glaucoma in humans. Here, Lyst-mutant iris phenotypes were used in a search for genes that influence Lyst pathways. In a candidate gene–driven approach, albino Lyst-mutant mice homozygous for a mutation in Tyr, whose product is key to melanin synthesis within melanosomes, exhibited complete rescue of Lyst-mutant iris phenotypes. In a genetic background–driven approach using a DBA/2J strain of congenic mice, an interval containing Tyrp1 enhanced Lyst-dependent iris phenotypes. Thus, both experimental approaches implicated the melanosome, an organelle that is a potential source of oxidative stress, as contributing to the disease phenotype. Confirming an association with oxidative damage, Lyst mutation resulted in genetic context–sensitive changes in iris lipid hydroperoxide levels, being lowest in albino and highest in DBA/2J mice. Surprisingly, the DBA/2J genetic background also exposed a late-onset neurodegenerative phenotype involving cerebellar Purkinje-cell degeneration. These results identify an association between oxidative damage to lipid membranes and the severity of Lyst-mutant phenotypes, revealing a new mechanism that contributes to pathophysiology involving LYST.

LYST is a poorly understood protein involved in hereditary disease. Mutations in the encoding gene cause Chediak-Higashi syndrome, a rare lethal disease affecting multiple tissues of the body. Mutations in Lyst also recapitulate features of exfoliation syndrome, a common disease affecting the anterior chamber of the eye. Unfortunately, the Lyst gene is quite large, rendering it difficult to study by many molecular and cellular approaches. Here, we use a genetic approach in mice to identify additional genetic pathways which might modify, or prevent, the ill consequences associated with Lyst mutation. Our experiments demonstrate that Lyst mutation results in elevated levels of oxidative damage to lipid membranes. These results identify a previously unrecognized consequence of Lyst mutation and a modifiable pathway of potential clinical relevance in humans. Ultimately, knowledge of these events will contribute to the design of new therapeutic strategies allowing a similar alleviation of disease in humans.

Neurite outgrowth: this process, first discovered by Santiago Ramon y Cajal, is sustained by the exocytosis of two distinct types of vesicles

Neurite outgrowth is a fundamental process in the differentiation of neurons. The first, seminal study documenting the generation of "appendages" (now known as filopodia and lamellipodia) on the "cones d'accroissement," the specialized growth cones at the tips of neurites, was reported by Cajal still in the XIXth century, investigating chicken neurons embryos stained by the Golgi's reazione nera. Since then, studies have continued using, in addition to brain tissues, powerful in vitro models, i.e. primary cultures of pyramidal neurons from the hippocampus and neurosecretory cell lines, in particular PC12 cells. These studies have documented that neuronal neurites, upon sprouting from the cell body, give rise to both axons and dendrites. The specificity of these differentiated neurites depends on the diffusion barrier established at the initial segment of the axon and on the specialized domains, spines and presynaptic boutons, assembled around complexes of scaffold proteins. The two main, coordinate mechanisms that support neurite outgrowth are (a) the rearrangement of the cytoskeleton and (b) the expansion of the plasma membrane due to the exo/endocytosis of specific vesicles, distinct from those filled with neurotransmitters (clear and dense-core vesicles). The latter process is the main task of this review. In axons the surface-expanding exocytoses are concentrated at the growth cones; in dendrites they may be more distributed along the shaft. At least two types of exocytic vesicles appear to be involved, the enlargeosomes, positive for VAMP4, during early phases of development, and Ti-VAMP-positive vesicles later on. Outgrowth studies, that are now intensely pursued, have already yielded results of great importance in brain cell biology and function, and are playing an increasing role in pathology and medicine.

Clustering of excess growth resources within leading growth cones underlies the recurrent "deposition" of varicosities along developing neurites

Varicosities (VRs) are ubiquitous neuronal structures that are considered to serve as presynaptic structures. The mechanisms of their assembly are unknown. Using cultured Aplysia neurons, we found that in the absence of postsynaptic targets, VRs form at the leading edge of extending neurites when anterogradely transported organelles accumulate within the palm of the growth cone (GC) at a rate that exceeds their utilization by the GC machinery. The aggregation of excess organelles at the palm of the GC leads to slowdown of the GC's advance. As the size of the organelle clusters increases, the rate of organelle sequestration diminishes and the supply of building blocks to the GC resumes. The GCs' advance is re-initiated, "leaving behind" an organelle-loaded nascent VR. These mechanisms account for the recurrent "deposition" of almost equally spaced VRs by advancing GCs. Consistent with the view that VRs serve as "ready-to-go" presynaptic terminals, we found that a short train of action potentials leads to exocytosis of labeled vesicles within the varicosities. We propose that the formation and spacing of VRs by advancing GCs is the default outcome of the balance between the rate of supply of growth-supporting resources and the usage of these resources by the GC's machinery at the leading edges of specific neurites.

Asymmetric clathrin-mediated endocytosis drives repulsive growth cone guidance

Asymmetric Ca(2+) elevations across the axonal growth cone mediate its turning responses to attractive and repulsive guidance cues. Here we show that clathrin-mediated endocytosis acts downstream of Ca(2+) signals as driving machinery for growth cone turning. In dorsal root ganglion neurons, the formation of clathrin-coated pits is facilitated asymmetrically across the growth cone by a directionally applied chemorepellent, semaphorin 3A, or by Ca(2+) signals that mediate repulsive guidance. In contrast, coated pit formation remains symmetric in the presence of attractive Ca(2+) signals. Inhibition of clathrin-mediated endocytosis abolishes growth cone repulsion, but not attraction, induced by Ca(2+) or extracellular physiological cues. Furthermore, asymmetric perturbation of the balance of endocytosis and exocytosis in the growth cone is sufficient to initiate its turning toward the side with less endocytosis or more exocytosis. With our previous finding that growth cone attraction involves asymmetric exocytosis, we propose that the balance between membrane addition and removal dictates bidirectional axon guidance.

Branching out: mechanisms of dendritic arborization

Type-specific dendrite morphology is a hallmark of the neuron and has important functional implications in determining what signals a neuron receives and how these signals are integrated. During the past two decades, studies on dendritic arborization neurons in Drosophila melanogaster have started to identify mechanisms of dendrite morphogenesis that may have broad applicability to vertebrate species. Transcription factors, receptor-ligand interactions, various signalling pathways, local translational machinery, cytoskeletal elements, Golgi outposts and endosomes have been identified as contributors to the organization of dendrites of individual neurons and the placement of these dendrites in the neuronal circuitry. Further insight into these mechanisms will improve our understanding of how the nervous system functions and might help to identify the underlying causes of some neurological and neurodevelopmental disorders.

Pericentrosomal targeting of Rab6 secretory vesicles by Bicaudal-D-related protein 1 (BICDR-1) regulates neuritogenesis

Membrane and secretory trafficking are essential for proper neuronal development. However, the molecular mechanisms that organize secretory trafficking are poorly understood. Here, we identify Bicaudal-D-related protein 1 (BICDR-1) as an effector of the small GTPase Rab6 and key component of the molecular machinery that controls secretory vesicle transport in developing neurons. BICDR-1 interacts with kinesin motor Kif1C, the dynein/dynactin retrograde motor complex, regulates the pericentrosomal localization of Rab6-positive secretory vesicles and is required for neural development in zebrafish. BICDR-1 expression is high during early neuronal development and strongly declines during neurite outgrowth. In young neurons, BICDR-1 accumulates Rab6 secretory vesicles around the centrosome, restricts anterograde secretory transport and inhibits neuritogenesis. Later during development, BICDR-1 expression is strongly reduced, which permits anterograde secretory transport required for neurite outgrowth. These results indicate an important role for BICDR-1 as temporal regulator of secretory trafficking during the early phase of neuronal differentiation.

The TC10-Exo70 complex is essential for membrane expansion and axonal specification in developing neurons

Axonal elongation is one of the hallmarks of neuronal polarization. This phenomenon requires axonal membrane growth by exocytosis of plasmalemmal precursor vesicles (PPVs) at the nerve growth cone, a process regulated by IGF-1 activation of the PI3K (phosphatidylinositol-3 kinase) pathway. Few details are known, however, about the targeting mechanisms for PPVs. Here, we show, in cultured hippocampal pyramidal neurons and growth cones isolated from fetal rat brain, that IGF-1 activates the GTP-binding protein TC10, which triggers translocation to the plasma membrane of the exocyst component exo70 in the distal axon and growth cone. We also show that TC10 and exo70 function are necessary for addition of new membrane and, thus, axon elongation stimulated by IGF-1. Moreover, expression silencing of either TC10 or exo70 inhibit the establishment of neuronal polarity by hindering the insertion of IGF-1 receptor in one of the undifferentiated neurites. We conclude that, in hippocampal pyramidal neurons in culture, (1) membrane expansion at the axonal growth cone is regulated by IGF-1 via a cascade involving TC10 and the exocyst complex, (2) TC10 and exo70 are essential for the polarized externalization of IGF-1 receptor, and (3) this process is necessary for axon specification.

Emerging new roles of the pre-Golgi intermediate compartment in biosynthetic-secretory trafficking

The intermediate compartment (IC) between the endoplasmic reticulum (ER) and the Golgi apparatus appears to constitute an autonomous organelle composed of spatially and functionally distinct, but interconnected, vacuolar and tubular subdomains. In mammalian cells the IC network is stably anchored at the cell center, communicating directly with the endocytic pathway via a pericentrosomal membrane system (PCMS). This finding suggests that the secretory pathway divides at the level of the IC, which functions as a sorting station both in Golgi-dependent and -independent trafficking. The tubular subdomain of the IC is capable of expansion in accordance with its proposed biosynthetic functions such as cholesterol synthesis.

Gene expression profiling in cells with enhanced gamma-secretase activity

Background

Processing by γ-secretase of many type-I membrane protein substrates triggers signaling cascades by releasing intracellular domains (ICDs) that, following nuclear translocation, modulate the transcription of different genes regulating a diverse array of cellular and biological processes. Because the list of γ-secretase substrates is growing quickly and this enzyme is a cancer and Alzheimer's disease therapeutic target, the mapping of γ-secretase activity susceptible gene transcription is important for sharpening our view of specific affected genes, molecular functions and biological pathways.

Methodology/Principal Findings

To identify genes and molecular functions transcriptionally affected by γ-secretase activity, the cellular transcriptomes of Chinese hamster ovary (CHO) cells with enhanced and inhibited γ-secretase activity were analyzed and compared by cDNA microarray. The functional clustering by FatiGO of the 1,981 identified genes revealed over- and under-represented groups with multiple activities and functions. Single genes with the most pronounced transcriptional susceptibility to γ-secretase activity were evaluated by real-time PCR. Among the 21 validated genes, the strikingly decreased transcription of PTPRG and AMN1 and increased transcription of UPP1 potentially support data on cell cycle disturbances relevant to cancer, stem cell and neurodegenerative diseases' research. The mapping of interactions of proteins encoded by the validated genes exclusively relied on evidence-based data and revealed broad effects on Wnt pathway members, including WNT3A and DVL3. Intriguingly, the transcription of TERA, a gene of unknown function, is affected by γ-secretase activity and was significantly altered in the analyzed human Alzheimer's disease brain cortices.

Conclusions/Significance

Investigating the effects of γ-secretase activity on gene transcription has revealed several affected clusters of molecular functions and, more specifically, 21 genes that hold significant potential for a better understanding of the biology of γ-secretase and its roles in cancer and Alzheimer's disease pathology.