Cross talk between tetanus neurotoxin-insensitive vesicle-associated membrane protein-mediated transport and L1-mediated adhesion

VAMP7j: A Splice Variant of Human VAMP7 That Modulates Neurite Outgrowth by Regulating L1CAM Transport to the Plasma Membrane

The vesicle-associated membrane protein 7 (VAMP7) is a SNARE protein of the longin family involved in a wide range of subcellular trafficking events, including neurite sprouting and elongation. The expression of the human gene SYBL1, encoding VAMP7, is finely regulated by alternative splicing. Among the minor isoforms identified so far, VAMP7j is the one most expressed and modulated in the human brain. Therefore, we focused on gaining functional evidence on VAMP7j, which lacks a functional SNARE motif but retains both the longin and transmembrane domains. In human SH-SY5Y cells, we found VAMP7j to modulate neuritogenesis by mediating transport of L1CAM toward the plasma membrane, in a fashion regulated by phosphorylation of the longin domain. VAMP7-mediated regulation of L1CAM trafficking seems at least to differentiate humans from rats, with VAMP7j CNS expression being restricted to primates, including humans. Since L1CAM is a central player in neuritogenesis and axon guidance, these findings suggest the species-specific splicing of SYBL1 is among the fine tuners of human neurodevelopmental complexity.

The Regulatory Mechanism of Rab21 in Human Diseases

Rab proteins are important components of small GTPases and play crucial roles in regulating intracellular transportation and cargo delivery. Maintaining the proper functions of Rab proteins is essential for normal cellular activities such as cell signaling, division, and survival. Due to their vital and irreplaceable role in regulating intracellular vesicle transportation, accumulated researches have shown that the abnormalities of Rab proteins and their effectors are closely related to human diseases. Here, this review focused on Rab21, a member of the Rab family, and introduced the structures and functions of Rab21, as well as the regulatory mechanisms of Rab21 in human diseases, including neurodegenerative diseases, cancer, and inflammation. In summary, we described in detail the role of Rab21 in human diseases and provide insights into the potential of Rab21 as a therapeutic target for diseases.

Secretion of VGF relies on the interplay between LRRK2 and post-Golgi v-SNAREs

The neuropeptide VGF was recently proposed as a neurodegeneration biomarker. The Parkinson's disease-related protein leucine-rich repeat kinase 2 (LRRK2) regulates endolysosomal dynamics, a process that involves SNARE-mediated membrane fusion and could regulate secretion. Here we investigate potential biochemical and functional links between LRRK2 and v-SNAREs. We find that LRRK2 directly interacts with the v-SNAREs VAMP4 and VAMP7. Secretomics reveals VGF secretory defects in VAMP4 and VAMP7 knockout (KO) neuronal cells. In contrast, VAMP2 KO "regulated secretion-null" and ATG5 KO "autophagy-null" cells release more VGF. VGF is partially associated with extracellular vesicles and LAMP1+ endolysosomes. LRRK2 expression increases VGF perinuclear localization and impairs its secretion. Retention using selective hooks (RUSH) assays show that a pool of VGF traffics through VAMP4+ and VAMP7+ compartments, and LRRK2 expression delays its transport to the cell periphery. Overexpression of LRRK2 or VAMP7-longin domain impairs VGF peripheral localization in primary cultured neurons. Altogether, our results suggest that LRRK2 might regulate VGF secretion via interaction with VAMP4 and VAMP7.

Role of SNAREs in Unconventional Secretion—Focus on the VAMP7-Dependent Secretion

Intracellular membrane protein trafficking is crucial for both normal cellular physiology and cell-cell communication. The conventional secretory route follows transport from the Endoplasmic reticulum (ER) to the plasma membrane via the Golgi apparatus. Alternative modes of secretion which can bypass the need for passage through the Golgi apparatus have been collectively termed as Unconventional protein secretion (UPS). UPS can comprise of cargo without a signal peptide or proteins which escape the Golgi in spite of entering the ER. UPS has been classified further depending on the mode of transport. Type I and Type II unconventional secretion are non-vesicular and non-SNARE protein dependent whereas Type III and Type IV dependent on vesicles and on SNARE proteins. In this review, we focus on the Type III UPS which involves the import of cytoplasmic proteins in membrane carriers of autophagosomal/endosomal origin and release in the extracellular space following SNARE-dependent intracellular membrane fusion. We discuss the role of vesicular SNAREs with a strong focus on VAMP7, a vesicular SNARE involved in exosome, lysosome and autophagy mediated secretion. We further extend our discussion to the role of unconventional secretion in health and disease with emphasis on cancer and neurodegeneration.

Nuclear and Cytoplasmatic Players in Mitochondria-Related CNS Disorders: Chromatin Modifications and Subcellular Trafficking

Aberrant mitochondrial phenotypes are common to many central nervous system (CNS) disorders, including neurodegenerative and neurodevelopmental diseases. Mitochondrial function and homeostasis depend on proper control of several biological processes such as chromatin remodeling and transcriptional control, post-transcriptional events, vesicle and organelle subcellular trafficking, fusion, and morphogenesis. Mutation or impaired regulation of major players that orchestrate such processes can disrupt cellular and mitochondrial dynamics, contributing to neurological disorders. The first part of this review provides an overview of a functional relationship between chromatin players and mitochondria. Specifically, we relied on specific monogenic CNS disorders which share features with mitochondrial diseases. On the other hand, subcellular trafficking is coordinated directly or indirectly through evolutionarily conserved domains and proteins that regulate the dynamics of membrane compartments and organelles, including mitochondria. Among these “building blocks”, longin domains and small GTPases are involved in autophagy and mitophagy, cell reshaping, and organelle fusion. Impairments in those processes significantly impact CNS as well and are discussed in the second part of the review. Hopefully, in filling the functional gap between the nucleus and cytoplasmic organelles new routes for therapy could be disclosed.

Graphene-Based Scaffolds for Regenerative Medicine

Leading-edge regenerative medicine can take advantage of improved knowledge of key roles played, both in stem cell fate determination and in cell growth/differentiation, by mechano-transduction and other physicochemical stimuli from the tissue environment. This prompted advanced nanomaterials research to provide tissue engineers with next-generation scaffolds consisting of smart nanocomposites and/or hydrogels with nanofillers, where balanced combinations of specific matrices and nanomaterials can mediate and finely tune such stimuli and cues. In this review, we focus on graphene-based nanomaterials as, in addition to modulating nanotopography, elastic modulus and viscoelastic features of the scaffold, they can also regulate its conductivity. This feature is crucial to the determination and differentiation of some cell lineages and is of special interest to neural regenerative medicine. Hereafter we depict relevant properties of such nanofillers, illustrate how problems related to their eventual cytotoxicity are solved via enhanced synthesis, purification and derivatization protocols, and finally provide examples of successful applications in regenerative medicine on a number of tissues.

Role of the Sec22b-E-Syt complex in neurite growth and ramification

Axons and dendrites are long and often ramified neurites that need particularly intense plasma membrane (PM) expansion during the development of the nervous system. Neurite growth depends on non-fusogenic Sec22b-Stx1 SNARE complexes at endoplasmic reticulum (ER)-PM contacts. Here, we show that Sec22b interacts with members of the extended synaptotagmin (E-Syt) family of ER lipid transfer proteins (LTPs), and this interaction depends on the longin domain of Sec22b. Overexpression of E-Syts stabilizes Sec22b-Stx1 association, whereas silencing of E-Syts has the opposite effect. Overexpression of wild-type E-Syt2, but not mutants unable to transfer lipids or attach to the ER, increase the formation of axonal filopodia and ramification of neurites in developing neurons. This effect is inhibited by a clostridial neurotoxin cleaving Stx1, and expression of the Sec22b longin domain and a Sec22b mutant with an extended linker between the SNARE and transmembrane domains. We conclude that Sec22b-Stx1 ER-PM contact sites contribute to PM expansion by interacting with LTPs, such as E-Syts.This article has an associated First Person interview with the first author of the paper.

Activity-Dependent Secretion of Synaptic Organizer Cbln1 from Lysosomes in Granule Cell Axons

Synapse formation is achieved by various synaptic organizers. Although this process is highly regulated by neuronal activity, the underlying molecular mechanisms remain largely unclear. Here we show that Cbln1, a synaptic organizer of the C1q family, is released from lysosomes in axons but not dendrites of cerebellar granule cells in an activity- and Ca²⁺-dependent manner. Exocytosed Cbln1 was retained on axonal surfaces by binding to its presynaptic receptor neurexin. Cbln1 further diffused laterally along the axonal surface and accumulated at boutons by binding postsynaptic δ2 glutamate receptors. Cbln1 exocytosis was insensitive to tetanus neurotoxin, accompanied by cathepsin B release, and decreased by disrupting lysosomes. Furthermore, overexpression of lysosomal sialidase Neu1 not only inhibited Cbln1 and cathepsin B exocytosis in vitro but also reduced axonal bouton formation in vivo. Our findings imply that co-release of Cbln1 and cathepsin B from lysosomes serves as a new mechanism of activity-dependent coordinated synapse modification.

Syntaxin-1/TI-VAMP SNAREs interact with Trk receptors and are required for neurotrophin-dependent outgrowth

SNARE proteins are essential components of the machinery that regulates vesicle trafficking and exocytosis. Their role is critical for the membrane-fusion processes that occur during neurotransmitter release. However, research in the last decade has also unraveled the relevance of these proteins in membrane expansion and cytoskeletal rearrangements during developmental processes such as neuronal migration and growth cone extension and attraction. Neurotrophins are neurotrophic factors that are required for many cellular functions throughout the brain, including neurite outgrowth and guidance, synaptic formation, and plasticity. Here we show that neurotrophin Trk receptors form a specific protein complex with the t-SNARE protein Syntaxin 1, both in vivo and in vitro. We also demonstrate that blockade of Syntaxin 1 abolishes neurotrophin-dependent growth of axons in neuronal cultures and decreases exocytotic events at the tip of axonal growth cones. 25-kDa soluble N-ethylmaleimide-sensitive factor attachment protein and Vesicle-associated membrane protein 2 do not participate in the formation of this SNARE complex, while tetanus neurotoxin-insensitive vesicle-associated membrane protein interacts with Trk receptors; knockdown of this (v) SNARE impairs Trk-dependent outgrowth. Taken together, our results support the notion that an atypical SNARE complex comprising Syntaxin 1 and tetanus neurotoxin-insensitive vesicle-associated membrane protein is required for axonal neurotrophin function.

Neuronal membrane dynamics as fine regulator of sphingolipid composition

Sphingolipid metabolism is an intricate network of several interdependent and co-regulated pathways. In addition to the mainstream biosynthetic and catabolic pathways, several processes, even if less important in contributing to the final tissue sphingolipid composition from the quantitative point of view, might become relevant when sphingolipid metabolism is for any reason dysregulated and concur to the onset of neuronal pathologies. The main subcellular sites involved in the mainstream metabolic pathway are represented by the Golgi apparatus (for the biosynthesis) and by the lysosomes (for catabolism). On the other hand, the minor collateral pathways are associated with the plasma membrane and membranes of other organelles, and likely play important roles in the local regulation of membrane dynamics and contribute to maintain a perfect membrane organization functional to the physiology of the cell. In this review, we will consider few aspects of the sphingolipid metabolic pathway depending by the dynamic of the membranes that seems to become relevant in neurodegenerative diseases.

Cyclic Nucleotide Control of Microtubule Dynamics for Axon Guidance

Graded distribution of intracellular second messengers, such as Ca(2+) and cyclic nucleotides, mediates directional cell migration, including axon navigational responses to extracellular guidance cues, in the developing nervous system. Elevated concentrations of cAMP or cGMP on one side of the neuronal growth cone induce its attractive or repulsive turning, respectively. Although effector processes downstream of Ca(2+) have been extensively studied, very little is known about the mechanisms that enable cyclic nucleotides to steer migrating cells. Here, we show that asymmetric cyclic nucleotide signaling across the growth cone mediates axon guidance via modulating microtubule dynamics and membrane organelle transport. In embryonic chick dorsal root ganglion neurons in culture, contact of an extending microtubule with the growth cone leading edge induces localized membrane protrusion at the site of microtubule contact. Such a contact-induced protrusion requires exocytosis of vesicle-associated membrane protein 7 (VAMP7)-positive vesicles that have been transported centrifugally along the microtubule. We found that the two cyclic nucleotides counteractively regulate the frequency of microtubule contacts and targeted delivery of VAMP7 vesicles: cAMP stimulates and cGMP inhibits these events, thereby steering the growth cone in the opposite directions. By contrast, Ca(2+) signals elicit no detectable change in either microtubule contacts or VAMP7 vesicle delivery during Ca(2+)-induced growth cone turning. Our findings clearly demonstrate growth cone steering machinery downstream of cyclic nucleotide signaling and highlight a crucial role of dynamic microtubules in leading-edge protrusion for cell chemotaxis.

SIGNIFICANCE STATEMENT

Developing neurons can extend long axons toward their postsynaptic targets. The tip of each axon, called the growth cone, recognizes extracellular guidance cues and navigates the axon along the correct path. Here we show that asymmetric cyclic nucleotide signaling across the growth cone mediates axon guidance through localized regulation of microtubule dynamics and resulting recruitment of specific populations of membrane vesicles to the growth cone's leading edge. Remarkably, cAMP stimulates microtubule growth and membrane protrusion, whereas cGMP promotes microtubule retraction and membrane senescence, explaining the opposite directional polarities of growth cone turning induced by these cyclic nucleotides. This study reveals a novel microtubule-based mechanism through which cyclic nucleotides polarize the growth cone steering machinery for bidirectional axon guidance.

Spastin regulates VAMP7-containing vesicles trafficking in cortical neurons

Alteration of axonal transport has emerged as a common precipitating factor in several neurodegenerative disorders including Human Spastic Paraplegia (HSP). Mutations of the SPAST (SPG4) gene coding for the spastin protein account for 40% of all autosomal dominant uncomplicated HSP. By cleaving microtubules, spastin regulates several cellular processes depending on microtubule dynamics including intracellular membrane trafficking. Axonal transport is fundamental for the viability of motor neurons which often have very long axons and thus require efficient communication between the cell body and its periphery. Here we found that the anterograde velocity of VAMP7 vesicles, but not that of VAMP2, two vesicular-SNARE proteins implicated in neuronal development, is enhanced in SPG4-KO neurons. We showed that this effect is associated with a slight increase of the level of acetylated tubulin in SPG4-KO neurons and correlates with an enhanced activity of kinesin-1 motors. Interestingly, we demonstrated that an artificial increase of acetylated tubulin by drugs reproduces the effect of Spastin KO on VAMP7 axonal dynamics but also increased its retrograde velocity. Finally, we investigated the effect of microtubule targeting agents which rescue axonal swellings, on VAMP7 and microtubule dynamics. Our results suggest that microtubule stabilizing agents, such as taxol, may prevent the morphological defects observed in SPG4-KO neurons not simply by restoring the altered anterograde transport to basal levels but rather by increasing the retrograde velocity of axonal cargoes.

Neuronal polarization: From spatiotemporal signaling to cytoskeletal dynamics

Neuronal polarization establishes distinct molecular structures to generate a single axon and multiple dendrites. Studies over the past years indicate that this efficient separation is brought about by a network of feedback loops. Axonal growth seems to play a major role in fueling those feedback loops and thereby stabilizing neuronal polarity. Indeed, various effectors involved in feedback loops are pivotal for axonal growth by ultimately acting on the actin and microtubule cytoskeleton. These effectors have key roles in interconnecting actin and microtubule dynamics - a mechanism crucial to commanding the growth of axons. We propose a model connecting signaling with cytoskeletal dynamics and neurite growth to better describe the underlying processes involved in neuronal polarization. We will discuss the current views on feedback loops and highlight the current limits of our understanding.

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 SNARE Protein Syntaxin 3 Confers Specificity for Polarized Axonal Trafficking in Neurons

Cell polarity and precise subcellular protein localization are pivotal to neuronal function. The SNARE machinery underlies intracellular membrane fusion events, but its role in neuronal polarity and selective protein targeting remain unclear. Here we report that syntaxin 3 is involved in orchestrating polarized trafficking in cultured rat hippocampal neurons. We show that syntaxin 3 localizes to the axonal plasma membrane, particularly to axonal tips, whereas syntaxin 4 localizes to the somatodendritic plasma membrane. Disruption of a conserved N-terminal targeting motif, which causes mislocalization of syntaxin 3, results in coincident mistargeting of the axonal cargos neuron-glia cell adhesion molecule (NgCAM) and neurexin, but not transferrin receptor, a somatodendritic cargo. Similarly, RNAi-mediated knockdown of endogenous syntaxin 3 leads to partial mistargeting of NgCAM, demonstrating that syntaxin 3 plays an important role in its targeting. Additionally, overexpression of syntaxin 3 results in increased axonal growth. Our findings suggest an important role for syntaxin 3 in maintaining neuronal polarity and in the critical task of selective trafficking of membrane protein to axons.

Membrane traffic during axon development

Brain formation requires the establishment of complex neural circuits between a diverse array of neuronal subtypes in an intricate and ever changing microenvironment and yet with a large degree of specificity and reproducibility. In the last three decades, mounting evidence has established that neuronal development relies on the coordinated regulation of gene expression, cytoskeletal dynamics, and membrane trafficking. Membrane trafficking has been considered important in that it brings new membrane and proteins to the plasma membrane of developing neurons and because it also generates and maintains the polarized distribution of proteins into neuronal subdomains. More recently, accumulating evidence suggests that membrane trafficking may have an even more active role during development by regulating the distribution and degree of activation of a wide variety of proteins located in plasma membrane subdomains and endosomes. In this article the evidence supporting the different roles of membrane trafficking during axonal development, particularly focusing on the role of SNAREs and Rabs was reviewed. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1185-1200, 2016.

The role of non-canonical SNAREs in synaptic vesicle recycling

An increasing number of studies suggest that distinct pools of synaptic vesicles drive specific forms of neurotransmission. Interspersed with these functional studies are analyses of the synaptic vesicle proteome which have consistently detected the presence of so-called “non-canonical” SNAREs that typically function in fusion and trafficking of other subcellular structures within the neuron. The recent identification of certain non-canonical vesicular SNAREs driving spontaneous (e.g., VAMP7 and vti1a) or evoked asynchronous (e.g., VAMP4) release integrates and corroborates existing data from functional and proteomic studies and implies that at least some complement of non-canonical SNAREs resident on synaptic vesicles function in neurotransmission. Here, we discuss the specific roles in neurotransmission of proteins homologous to each member of the classical neuronal SNARE complex consisting of synaptobrevin2, syntaxin-1 and SNAP-25.

Fusion of Golgi-derived vesicles mediated by SNAP-25 is essential for sympathetic neuron outgrowth but relatively insensitive to botulinum neurotoxins in vitro

Sympathetic neurons ramify to innervate multiple cells in target tissues. In compartmentalized cultures of rat superior cervical ganglion neurons, cleavage of synaptosomal-associated protein of Mr  = 25 000 (SNAP-25) in neurites exposed to botulinum neurotoxin type A (BoNT/A) arrested their growth and collapsed interstitial branches, but this required large, nonclinical doses. A protease-inactive mutant proved ineffective, confirming involvement of SNAP-25 in neurite extension and arborization. BoNT/C1 acted like BoNT/A, but BoNT/E caused only mild inhibition, likely due to transient SNAP-25 proteolysis. Near-total lack of susceptibility to BoNT/B or BoNT/D revealed that vesicle-associated membrane protein (VAMPs) isoforms 1-3 are not essential. Neurite length was not reduced when either BoNT/A or BoNT/C1 was applied to the somata, with no detrimental effect on neuron viability being observed. Treatments that protect cells from deprivation of nerve growth factor failed to prevent the toxin-induced loss of neurites. Inactivation of SNAP-25 caused the accumulation at neurite branch sites of Golgi-derived organelles labelled with N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-sphingosine conjugated to bovine serum albumin, prior to the collapse of arbors. Notably, neurite growth was ~ 1000-fold less susceptible to BoNT/A than cholinergic transmission in these neurons. Accordingly, a BoNT/A acceptor synaptic vesicle protein 2 (SV2) was found to be colocalized with VAMP 1-3, but not with VAMP 7, which is implicated in the growth of neurites. In conclusion, neurites depend on SNAP-25 for extension but this is quite resistant to BoNT/A, possibly, because of a low density of SV2 at growth sites that are distant from the highly susceptible regions of neurotransmitter release.

The SNARE Sec22b has a non-fusogenic function in plasma membrane expansion

Development of the nervous system requires extensive axonal and dendritic growth during which neurons massively increase their surface area. Here we report that the endoplasmic reticulum (ER)-resident SNARE Sec22b has a conserved non-fusogenic function in plasma membrane expansion. Sec22b is closely apposed to the plasma membrane SNARE syntaxin1. Sec22b forms a trans-SNARE complex with syntaxin1 that does not include SNAP23/25/29, and does not mediate fusion. Insertion of a long rigid linker between the SNARE and transmembrane domains of Sec22b extends the distance between the ER and plasma membrane, and impairs neurite growth but not the secretion of VSV-G. In yeast, Sec22 interacts with lipid transfer proteins, and inhibition of Sec22 leads to defects in lipid metabolism at contact sites between the ER and plasma membrane. These results suggest that close apposition of the ER and plasma membrane mediated by Sec22 and plasma membrane syntaxins generates a non-fusogenic SNARE bridge contributing to plasma membrane expansion, probably through non-vesicular lipid transfer.

Disassembly of all SNARE complexes by N-ethylmaleimide-sensitive factor (NSF) is initiated by a conserved 1:1 interaction between α-soluble NSF attachment protein (SNAP) and SNARE complex

Vesicle trafficking in eukaryotic cells is facilitated by SNARE-mediated membrane fusion. The ATPase NSF (N-ethylmaleimide-sensitive factor) and the adaptor protein α-SNAP (soluble NSF attachment protein) disassemble all SNARE complexes formed throughout different pathways, but the effect of SNARE sequence and domain variation on the poorly understood disassembly mechanism is unknown. By measuring SNARE-stimulated ATP hydrolysis rates, Michaelis-Menten constants for disassembly, and SNAP-SNARE binding constants for four different ternary SNARE complexes and one binary complex, we found a conserved mechanism, not influenced by N-terminal SNARE domains. α-SNAP and the ternary SNARE complex form a 1:1 complex as revealed by multiangle light scattering. We propose a model of NSF-mediated disassembly in which the reaction is initiated by a 1:1 interaction between α-SNAP and the ternary SNARE complex, followed by NSF binding. Subsequent additional α-SNAP binding events may occur as part of a processive disassembly mechanism.

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.

Harnessing the power of the endosome to regulate neural development

Endocytosis and endosomal trafficking play a multitude of roles in cellular function beyond regulating entry of essential nutrients. In this review, we discuss the cell biological principles of endosomal trafficking, the neuronal adaptations to endosomal organization, and the role of endosomal trafficking in neural development. In particular, we consider how cell fate decisions, polarity, migration, and axon outgrowth and guidance are influenced by five endosomal tricks: dynamic modulation of receptor levels by endocytosis and recycling, cargo-specific responses via cargo-specific endocytic regulators, cell-type-specific endocytic regulation, ligand-specific endocytic regulation, and endosomal regulation of ligand processing and trafficking.

Absence of TI-VAMP/Vamp7 leads to increased anxiety in mice

Vesicular (v)- and target (t)-SNARE proteins assemble in SNARE complex to mediate membrane fusion. Tetanus neurotoxin-insensitive vesicular-associated membrane protein (TI-VAMP/VAMP7), a vesicular SNARE expressed in several cell types including neurons, was previously shown to play a major role in exocytosis involved in neurite growth in cultured neurons. Here we generated a complete constitutive knock-out by deleting the exon 3 of Vamp7. Loss of TI-VAMP expression did not lead to any striking developmental or neurological defect. Knock-out mice displayed decreased brain weight and increased third ventricle volume. Axon growth appeared normal in cultured knock-out neurons. Behavioral characterization unraveled that TI-VAMP knock-out was associated with increased anxiety. Our results thus suggest compensatory mechanisms allowing the TI-VAMP knock-out mice to fulfill major developmental processes. The phenotypic traits unraveled here further indicate an unexpected role of TI-VAMP-mediated vesicular traffic in anxiety and suggest a role for TI-VAMP in higher brain functions.

Expression of the Longin domain of TI-VAMP impairs lysosomal secretion and epithelial cell migration

BACKGROUND INFORMATION

TI-VAMP (tetanus neurotoxin-insensitive vesicle-associated membrane protein; also called VAMP7) belongs to the Longin subfamily of v-SNAREs (vesicular soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors). The regulatory N-terminal extension, called the Longin domain, of TI-VAMP has been shown previously to have a dual biochemical function: it inhibits the capacity of TI-VAMP to form SNARE complexes and it binds to the delta subunit of the AP-3 (adaptor protein 3) complex in early endosomes, thereby targeting TI-VAMP to late endosomes.

RESULTS

We have generated MDCK (Madin-Darby canine kidney) cell lines expressing the Longin domain of TI-VAMP coupled to GFP (green fluorescent protein) in a doxycycline-dependent manner. As expected, AP-3delta (AP-3 delta subunit) is not properly localized in Longin-expressing cells. We have shown that the expression of the Longin domain impairs lysosomal secretion, as determined by the release of a pre-internalized fluorescent fluid-phase marker and by electron microscopy of the membrane-associated released particles. Membrane repair following mechanical wounding, a process requiring lysosomal secretion, is also impaired in cells expressing the Longin domain. Furthermore, cell migration, assessed by wound healing of MDCK monolayers, is also inhibited.

CONCLUSIONS

The results of the present study suggest that the expression of the Longin domain of TI-VAMP regulates lysosomal secretion of epithelial cells and provide molecular evidence for a role of the late endocytic system in cell migration.

v-SNARE composition distinguishes synaptic vesicle pools

Synaptic vesicles belong to two distinct pools, a recycling pool responsible for the evoked release of neurotransmitter and a resting pool unresponsive to stimulation. The uniform appearance of synaptic vesicles has suggested that differences in location or cytoskeletal association account for these differences in function. We now find that the v-SNARE tetanus toxin-insensitive vesicle-associated membrane protein (VAMP7) differs from other synaptic vesicle proteins in its distribution to the two pools, providing evidence that they differ in molecular composition. We also find that both resting and recycling pools undergo spontaneous release, and when activated by deletion of the longin domain, VAMP7 influences the properties of release. Further, the endocytosis that follows evoked and spontaneous release differs in mechanism, and specific sequences confer targeting to the different vesicle pools. The results suggest that different endocytic mechanisms generate synaptic vesicles with different proteins that can endow the vesicles with distinct properties.

The role of VAMP7/TI-VAMP in cell polarity and lysosomal exocytosis in vivo

VAMP7 or tetanus neurotoxin-insensitive vesicle- associated membrane protein (TI-VAMP) has been proposed to regulate apical transport in polarized epithelial cells, axonal transport in neurons and lysosomal exocytosis. To investigate the function of VAMP7 in vivo, we generated VAMP7 knockout mice. Here, we show that VAMP7 knockout mice are indistinguishable from control mice and display a similar localization of apical proteins in the kidney and small intestine and a similar localization of axonal proteins in the nervous system. Neurite outgrowth of cultured mutant hippocampal neurons was reduced in mutant neurons. However, lysosomal exocytosis was not affected in mutant fibroblasts. Our results show that VAMP7 is required in neurons to extend axons to the full extent. However, VAMP7 does not seem to be required for epithelial cell polarity and lysosomal exocytosis.

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'.

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.

L1 syndrome mutations impair neuronal L1 function at different levels by divergent mechanisms

Mutations in the human L1CAM gene cause neurodevelopmental disorders collectively referred to as L1 syndrome. Here, we investigated cellular pathomechanisms underlying two L1 syndrome mutations, R184Q and W1036L. We demonstrate that these mutations cause partial endoplasmic reticulum (ER) retention of L1, reduce L1 cell surface expression, but do not induce ER stress in neuronal NSC-34 cells. We provide evidence that surface trafficking of mutated L1 is affected by defective sorting to ER exit sites and attenuated ER export. However, in differentiated neuronal cultures and long-term cultured hippocampal slices, the L1-R184Q protein is restricted to cell bodies, whereas L1-W1036L also aberrantly localizes to dendrites. These trafficking defects preclude axonal targeting of L1, thereby affecting L1-mediated axon growth and arborization. Our results indicate that L1 syndrome mutations impair neuronal L1 function at different levels, firstly by attenuating ER export and secondly by interfering with polarized neuronal trafficking.

Trafficking guidance receptors

Wiring of the brain relies initially on the correct outgrowth of axons to reach the appropriate target area for innervation. A large number of guidance receptors present in the plasma membrane of axonal growth cones and elsewhere on the neuron read and execute directional cues present in the extracellular environment of the navigating growth cone. The exact timing, levels, and localization of expression of the guidance receptors in the plasma membrane therefore determine the outcome of guidance decisions. Many guidance receptors are localized in exquisitely precise spatial and temporal patterns. The cellular mechanisms ensuring these localization patterns include spatially accurate sorting after synthesis in the secretory pathway, retrieval of inappropriately expressed receptors by endocytosis followed by degradation or recycling, and restriction of diffusion. This article will discuss the machinery and regulation underlying the restricted distribution of membrane receptors, focusing on the currently best-studied example, the L1 cell adhesion molecule. In addition to the long-range mechanisms ensuring appropriate localization, the same mechanisms can act locally to adjust levels and localization of receptors. These local mechanisms are regulated by ligand binding and subsequent activation of local signaling cascades. It is likely that the localization of all guidance receptors is regulated by a combination of sorting, retrieval, recycling and retention, similar to the ones we discuss here for L1.

Role of TI-VAMP and CD82 in EGFR cell-surface dynamics and signaling

The v-SNARE TI-VAMP (VAMP7) mediates exocytosis during neuritogenesis, phagocytosis and lysosomal secretion. It localizes to endosomes and lysosomes but also to the trans-Golgi network. Here we show that depletion of TI-VAMP enhances the endocytosis of activated EGF receptor (EGFR) without affecting constitutive endocytosis of EGFR, or transferrin uptake. This increased EGFR internalization is mainly clathrin dependent. Searching for defects in EGFR regulators, we found that TI-VAMP depletion reduces the cell surface amount of CD82, a tetraspanin known to control EGFR localization in microdomains. We further show that TI-VAMP is required for secretion from the Golgi apparatus to the cell surface, and that TI-VAMP-positive vesicles transport CD82. Quantum dots video-microscopy indicates that depletion of TI-VAMP, or its cargo CD82, restrains EGFR diffusion and the area explored by EGFR at the cell surface. Both depletions also impair MAPK signaling and enhance endocytosis of activated EGFR by increased recruitment of AP-2. These results highlight the role of TI-VAMP in the secretory pathway of a tetraspanin, and support a model in which CD82 allows EGFR entry in microdomains that control its clathrin-dependent endocytosis and signaling.

Multiple roles of the vesicular-SNARE TI-VAMP in post-Golgi and endosomal trafficking

SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are the core machinery of membrane fusion. Vesicular SNAREs (v-SNAREs) interact with their target SNAREs (t-SNAREs) to form SNARE complexes which mediate membrane fusion. Here we review the basic properties and functions of the v-SNARE TI-VAMP/VAMP7 (Tetanus neurotoxin insensitive-vesicle associated membrane protein). TI-VAMP interacts with its t-SNARE partners, particularly plasmalemmal syntaxins, to mediate membrane fusion and with several regulatory proteins especially via its amino-terminal regulatory Longin domain. Partners include AP-3, Hrb/(Human immunodeficiency virus Rev binding) protein, and Varp (Vps9 domain and ankyrin repeats containing protein) and regulate TI-VAMP's function and targeting. TI-VAMP is involved both in secretory and endocytic pathways which mediate neurite outgrowth and synaptic transmission, plasma membrane remodeling and lysosomal secretion.

TI-VAMP/VAMP7 and VAMP3/cellubrevin: two v-SNARE proteins involved in specific steps of the autophagy/multivesicular body pathways

During reticulocyte maturation, some membrane proteins and organelles that are not required in the mature red cell are lost. Several of these proteins are released into the extracellular medium associated with the internal vesicles present in multivesicular bodies (MVBs). Likewise, organelles such as mitochondria and endoplasmic reticulum are wrapped into double membrane vacuoles (i.e., autophagosomes) and degraded via autophagy. Morphological, molecular, and biochemical studies have shown that autophagosomes fuse with MVBs forming the so-called amphisomes, a prelysosomal hybrid organelle. SNAREs are key molecules of the vesicle fusion machinery. TI-VAMP/VAMP7 and VAMP3/cellubrevin are two v-SNARE proteins involved in the endocytic and exocytic pathways. We have previously shown that in the human leukemic K562 cells, Rab11 decorates MVBs and it is necessary for fusion between autophagosomes with MVBs. In the present report, we present evidence indicating that VAMP3 is required for the fusion between MVBs with autophagosomes to generate the amphisome, allowing the maturation of the autophagosome, but it does not seem to be involved in the next step, i. e., fusion with the lysosome. On the other hand, we demonstrate that VAMP7 is necessary for this latter event, allowing the completion of the autophagic pathway. Furthermore, VAMP7 and ATPase NSF, a protein required for SNAREs disassembly, participate in the fusion between MVBs with the plasma membrane to release the internal vesicles (i.e., exosomes) into the extracellular medium.

Role of Varp, a Rab21 exchange factor and TI-VAMP/VAMP7 partner, in neurite growth

The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP/VAMP7) was previously shown to mediate an exocytic pathway involved in neurite growth, but its regulation is still largely unknown. Here we show that TI-VAMP interacts with the Vps9 domain and ankyrin-repeat-containing protein (Varp), a guanine nucleotide exchange factor (GEF) of the small GTPase Rab21, through a specific domain herein called the interacting domain (ID). Varp, TI-VAMP and Rab21 co-localize in the perinuclear region of differentiating hippocampal neurons and transiently in transport vesicles in the shaft of neurites. Silencing the expression of Varp by RNA interference or expressing ID or a form of Varp deprived of its Vps9 domain impairs neurite growth. Furthermore, the mutant form of Rab21, defective in GTP hydrolysis, enhances neurite growth. We conclude that Varp is a positive regulator of neurite growth through both its GEF activity and its interaction with TI-VAMP.

SNAREs in neurons – beyond synaptic vesicle exocytosis (Review)

The paradigm for soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) function in mammalian cells has been built on advancements in our understanding of structural and biochemical aspects of synaptic vesicle exocytosis, involving specifically synaptobrevin, syntaxin 1 and SNAP25. Interestingly, a good number of SNAREs which are not directly involved in neurotransmitter exocytosis, are either brain-enriched or have distinct neuron-specific functions. Syntaxins 12/13 regulates glutamate receptor recycling via its interaction with neuron-enriched endosomal protein of 21 kDa (NEEP21). TI-VAMP/VAMP7 is essential for neuronal morphogenesis and mediates the vesicular transport processes underlying neurite outgrowth. Ykt6 is highly enriched in the cerebral cortex and hippocampus and is targeted to a novel compartment in neurons. Syntaxin 16 has a moderate expression level in many tissues, but is rather enriched in the brain. Here, we review and discuss the neuron-specific physiology and possible pathology of these and other (such as SNAP-29 and Vti1a-beta) members of the SNARE family.

Subcellular localization of the carbohydrate Lewis(x) adhesion structure in hippocampus cell cultures

The Lewis(x) (Le(x)) epitope (Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc-R) has been associated with the development of the central nervous system of diverse species including human and rodents. In this work, Le(x) has been found in the tetanus neurotoxin insensitive vesicle-associated membrane protein (TI-VAMP) compartment of rat hippocampus neurons in culture, at 7 days in vitro (DIV), when neurite extension is abundant. The TI-VAMP compartment is known to be associated with neurite outgrowth. Le(x) was found predominantly in neurites but also in somata and in growth cones. Abundant Le(x)-carrier glycoproteins specific to neurons have been identified at this stage of differentiation. At a later stage of differentiation, at 14 DIV, Le(x) appeared in extrasynaptic sites of GABAergic neurons, and in synaptic sites of glutamatergic neurons.

Quantifying neurite growth mediated by interactions among secretory vesicles, microtubules, and actin networks

Neurite growth is a fundamental process of neuronal development, which requires both membrane expansions by exocytosis and cytoskeletal dynamics. However, the specific contribution of these processes has not been yet assessed quantitatively. To study and quantify the growth process, we construct a biophysical model in which we relate the overall neurite outgrowth rate to the vesicle dynamics. By considering the complex motion of vesicles in the cell soma, we demonstrate from biophysical consideration that the main step of finding the neurite initiation site relies mainly on a two-dimensional diffusion/sequestration/fusion at the cell surface and we obtain a novel formula for the flux of vesicles at the neurite base. In the absence of microtubules, we show that a nascent neurite initiated by vesicular delivery can only reach a small length. By adding the microtubule dynamics to the secretory pathway and using stochastic analysis and simulations, we study the complex dynamics of neurite growth. Within this model, depending on the coupling parameter between the microtubules and the neurite, we find different regimes of growth, which describe dendritic and axonal growth. To validate one aspect of our model, we demonstrate that the experimental flux of TI-VAMP but not Synaptobrevin 2 vesicles contributes to the neurite growth. We conclude that although vesicles can be generated randomly in the cell body, the search for the neurite position using the microtubule network and diffusion is quite fast. Furthermore, when the TI-VAMP vesicular flow is large enough, the interactions between the microtubule bundle and the neurite control the growth process. In addition, all of these processes intimately cooperate to mediate the various modes of neurite growth: the model predicts three different growing modes including, in addition to the stable axonal growth and the stochastic dendritic growth, a fast oscillatory regime. Finally our study demonstrates that cytoskeletal dynamics is necessary to generate long protrusion, while vesicular delivery alone can only generate small neurite.

Role of HRB in clathrin-dependent endocytosis

Human immunodeficiency virus Rev-binding protein (HRB), also called human Rev-interacting protein (hRIP) or Rev/Rex activation domain binding (RAB) is a partner of the tyrosine kinase substrate EPS15, and it has been recovered in the AP-2 interactome. EPS15 and AP-2 are involved in endocytosis, but the function of HRB in this process is still unknown. Here we identified HRB as a partner of the vesicular SNARE tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP, also called VAMP7) in yeast two-hybrid screens and using biochemical assays. In HeLa cells, HRB localized both in the nucleus and in the cytoplasm. In the cytoplasm, HRB colocalized with clathrin-, AP-2-, EPS15-, and transferrin receptor-containing vesicles. We did not see significant colocalization between HRB and TI-VAMP in HeLa cells, and we saw partial colocalization with green fluorescent protein-TI-VAMP in stably expressing Madin-Darby canine kidney cells. Nevertheless using a pHLuorin-tagged TI-VAMP construct, we found that HRB and TI-VAMP colocalize close to the plasma membrane after 5 min of anti-green fluorescent protein antibody uptake. These results suggest that TI-VAMP and HRB may interact only during the early stages of endocytosis. Furthermore uptake experiments followed by fluorescence-activated cell sorting showed that the endocytosis of fluorescent transferrin and pHLuorin-TI-VAMP is strongly reduced in HRB knockdown cells. Altogether these results suggest that HRB is involved in clathrin-dependent endocytosis and recruits TI-VAMP in this process.

The somatodendritic endosomal regulator NEEP21 facilitates axonal targeting of L1/NgCAM

Correct targeting of proteins to axons and dendrites is crucial for neuronal function. We showed previously that axonal accumulation of the cell adhesion molecule L1/neuron-glia cell adhesion molecule (NgCAM) depends on endocytosis (Wisco, D., E.D. Anderson, M.C. Chang, C. Norden, T. Boiko, H. Folsch, and B. Winckler. 2003. J. Cell Biol. 162:1317–1328). Two endocytosis-dependent pathways to the axon have been proposed: transcytosis and selective retrieval/retention. We show here that axonal accumulation of L1/NgCAM occurs via nondegradative somatodendritic endosomes and subsequent anterograde axonal transport, which is consistent with transcytosis. Additionally, we identify the neuronal-specific endosomal protein NEEP21 (neuron-enriched endosomal protein of 21 kD) as a regulator of L1/NgCAM sorting in somatodendritic endosomes. Down-regulation of NEEP21 leads to missorting of L1/NgCAM to the somatodendritic surface as well as to lysosomes. Importantly, the axonal accumulation of endogenous L1 in young neurons is also sensitive to NEEP21 depletion. We propose that small endosomal carriers derived from somatodendritic recycling endosomes can serve to redistribute a distinct set of membrane proteins from dendrites to axons.

SNARE protein mimicry by an intracellular bacterium

Many intracellular pathogens rely on host cell membrane compartments for their survival. The strategies they have developed to subvert intracellular trafficking are often unknown, and SNARE proteins, which are essential for membrane fusion, are possible targets. The obligate intracellular bacteria Chlamydia replicate within an intracellular vacuole, termed an inclusion. A large family of bacterial proteins is inserted in the inclusion membrane, and the role of these inclusion proteins is mostly unknown. Here we identify SNARE-like motifs in the inclusion protein IncA, which are conserved among most Chlamydia species. We show that IncA can bind directly to several host SNARE proteins. A subset of SNAREs is specifically recruited to the immediate vicinity of the inclusion membrane, and their accumulation is reduced around inclusions that lack IncA, demonstrating that IncA plays a predominant role in SNARE recruitment. However, interaction with the SNARE machinery is probably not restricted to IncA as at least another inclusion protein shows similarities with SNARE motifs and can interact with SNAREs. We modelled IncA's association with host SNAREs. The analysis of intermolecular contacts showed that the IncA SNARE-like motif can make specific interactions with host SNARE motifs similar to those found in a bona fide SNARE complex. Moreover, point mutations in the central layer of IncA SNARE-like motifs resulted in the loss of binding to host SNAREs. Altogether, our data demonstrate for the first time mimicry of the SNARE motif by a bacterium.

Emerging aspects of membrane traffic in neuronal dendrite growth

Polarized growth of the neuron would logically require some form of membrane traffic to the tip of the growth cone, regulated in conjunction with other trafficking processes that are common to both neuronal and non-neuronal cells. Unlike axons, dendrites are endowed with membranous organelles of the exocytic pathway extending from the cell soma, including both rough and smooth endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Dendrites also have satellite Golgi-like cisternal stacks known as Golgi outposts that have no membranous connections with the somatic Golgi. Golgi outposts presumably serve both general and specific local trafficking needs, and could mediate membrane traffic required for polarized dendritic growth during neuronal differentiation. Recent findings suggest that dendritic growth, but apparently not axonal growth, relies very much on classical exocytic traffic, and is affected by defects in components of both the early and late secretory pathways. Within dendrites, localized processes of recycling endosome-based exocytosis regulate the growth of dendritic spines and postsynaptic compartments. Emerging membrane traffic processes and components that contribute specifically to dendritic growth are discussed.

Distinct v-SNAREs regulate direct and indirect apical delivery in polarized epithelial cells

SNARE [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] proteins control the membrane-fusion events of eukaryotic membrane-trafficking pathways. Specific vesicular and target SNAREs operate in specific trafficking routes, but the degree of specificity of SNARE functions is still elusive. Apical fusion requires the polarized distribution at the apical surface of the t-SNARE syntaxin 3, and several v-SNAREs including TI-VAMP and VAMP8 operate at the apical plasma membrane in polarized epithelial cells. It is not known, however, whether specific v-SNAREs are involved in direct and indirect routes to the apical surface. Here, we used RNAi to assess the role of two tetanus-neurotoxin-insensitive v-SNAREs, TI-VAMP/VAMP7 and VAMP8, in the sorting of raft- and non-raft-associated apical markers that follow either a direct or a transcytotic delivery, respectively, in FRT or Caco2 cells. We show that TI-VAMP mediates the direct apical delivery of both raft- and non-raft-associated proteins. By contrast, sorting by means of the transcytotic pathway is not affected by TI-VAMP knockdown but does appear to be regulated by VAMP8. Together with the specific role of VAMP3 in basolateral transport, our results demonstrate a high degree of specificity in v-SNARE function in polarized cells.

Recycling endosomes in neuronal membrane traffic

Neurons are highly polarized cells with axonal and somatodendritic membrane surfaces that spatially separate signal-sending from signal-receiving membrane domains. As found in many other cell types, different populations of endosomes are involved in the sorting of synaptic and other membrane cargo in neurons. The exact source of the membrane for neurite extension and process remodelling during neuronal differentiation has remained uncertain, and we do not know exactly how polarized sorting of neuronal membrane proteins is achieved. In the present article, we will provide a brief overview of endosomes and their putative or proven functions in fibroblasts, epithelial cells and neurons. On the basis of insights from non-neuronal cell types and recent studies on the function of recycling endosomes during synaptic plasticity-induced membrane remodelling, we postulate a speculative model regarding the role of recycling endosomes in neuronal differentiation.

Fast turnover of L1 adhesions in neuronal growth cones involving both surface diffusion and exo/endocytosis of L1 molecules

We investigated the interplay between surface trafficking and binding dynamics of the immunoglobulin cell adhesion molecule L1 at neuronal growth cones. Primary neurons were transfected with L1 constructs bearing thrombin-cleavable green fluorescent protein (GFP), allowing visualization of newly exocytosed L1 or labeling of membrane L1 molecules by Quantum dots. Intracellular L1-GFP vesicles showed preferential centrifugal motion, whereas surface L1-GFP diffused randomly, revealing two pathways to address L1 to adhesive sites. We triggered L1 adhesions using microspheres coated with L1-Fc protein or anti-L1 antibodies, manipulated by optical tweezers. Microspheres coupled to the actin retrograde flow at the growth cone periphery while recruiting L1-GFP molecules, of which 50% relied on exocytosis. Fluorescence recovery after photobleaching experiments revealed a rapid recycling of L1-GFP molecules at L1-Fc (but not anti-L1) bead contacts, attributed to a high lability of L1-L1 bonds at equilibrium. L1-GFP molecules truncated in the intracellular tail as well as neuronal cell adhesion molecules (NrCAMs) missing the clathrin adaptor binding sequence showed both little internalization and reduced turnover rates, indicating a role of endocytosis in the recycling of mature L1 contacts at the base of the growth cone. Thus, unlike for other molecules such as NrCAM or N-cadherin, diffusion/trapping and exo/endocytosis events cooperate to allow the fast renewal of L1 adhesions.

Neuronal and non-neuronal functions of the AP-3 sorting machinery

Vesicles selectively exchange lipids, membrane proteins and luminal contents between organelles along the exocytic and endocytic routes. The repertoire of membrane proteins present in these vesicles is crucial for their targeting and function. Vesicle composition is determined at the time of their biogenesis by cytosolic coats. The heterotetrameric protein adaptor protein complex 3 (AP-3), a coat component, participates in the generation of a diverse group of secretory organelles and lysosome-related organelles. Recent work has shed light on the mechanisms that regulate AP-3 and the trafficking pathways controlled by this adaptor. Phenotypic analysis of organisms carrying genetic deficiencies in the AP-3 pathway highlight its role regulating the targeting of lysosomal, melanosomal and synaptic vesicle-specific membrane proteins. Synaptic vesicles from AP-3-deficient mice possess altered levels of neurotransmitter and ion transporters, molecules that ultimately define the type and amount of neurotransmitter stored in these vesicles. These findings reveal a complex picture of how AP-3 functions in multiple tissues, including neuronal tissue, and expose potential links between endocytic sorting mechanisms and the pathogenesis of psychiatric disorders such as schizophrenia.