Asymmetric clathrin-mediated endocytosis drives repulsive growth cone guidance

A central role for Numb/Nbl in multiple Shh-mediated axon repulsion processes

Sonic hedgehog (Shh) is an axon guidance molecule that can act as either a chemorepellent or a chemoattractant, depending on the neuron type and their developmental stage. In the developing spinal cord, Shh initially attracts commissural axons to the floor plate and later repels them after they cross the midline. In the developing visual system, Shh repels ipsilateral retinal ganglion cell (iRGC) axons at the optic chiasm. Although Shh requires the endocytic adaptor Numb for attraction of spinal commissural axons, the molecular mechanisms underlying Shh dual function in attraction and repulsion are still unclear. In this study, we show that Numb is essential for two Shh-mediated repulsion processes: iRGC axon repulsion at the optic chiasm and antero-posterior commissural axon repulsion in the spinal cord. Therefore, Numb is required for Shh-mediated attraction and repulsion. These results position Numb as a central player in the non-canonical Shh signaling pathway mediating axon repulsion.

Invasion of glioma cells through confined space requires membrane tension regulation and mechano-electrical coupling via Plexin-B2

Glioblastoma (GBM) is a malignant brain tumor with diffuse infiltration. Here, we demonstrate how GBM cells usurp guidance receptor Plexin-B2 for confined migration through restricted space. Using live-cell imaging to track GBM cells negotiating microchannels, we reveal endocytic vesicle accumulation at cell front and filamentous actin assembly at cell rear in a polarized manner. These processes are interconnected and require Plexin-B2 signaling. We further show that Plexin-B2 governs membrane tension and other membrane features such as endocytosis, phospholipid composition, and inner leaflet surface charge, thus providing biophysical mechanisms by which Plexin-B2 promotes GBM invasion. Together, our studies unveil how GBM cells regulate membrane tension and mechano-electrical coupling to adapt to physical constraints and achieve polarized confined migration.

TRIM9 Controls Growth Cone Responses to Netrin Through DCC and UNC5C

The guidance cue netrin-1 promotes both growth cone attraction and growth cone repulsion. How netrin-1 elicits diverse axonal responses, beyond engaging the netrin receptor DCC and UNC5 family members, remains elusive. Here, we demonstrate that murine netrin-1 induces biphasic axonal responses in cortical neurons: Attraction at lower concentrations and repulsion at higher concentrations using both a microfluidic-based netrin-1 gradient and bath application of netrin-1. We find that repulsive turning in a netrin gradient is blocked by knockdown of UNC5C, whereas attractive turning is impaired by knockdown of DCC. TRIM9 is a brain-enriched E3 ubiquitin ligase previously shown to bind and cluster the attractive receptor DCC at the plasma membrane and regulate netrin-dependent attractive responses. However, whether TRIM9 also regulated repulsive responses to netrin-1 remained to be seen. In this study, we show that TRIM9 localizes and interacts with both the attractive netrin receptor DCC and the repulsive netrin receptor, UNC5C. We find that deletion of murine Trim9 alters both attractive and repulsive axon turning and changes in growth cones size in response to murine netrin-1. TRIM9 was required for netrin-1-dependent changes in the surface levels of DCC and UNC5C in the growth cone during morphogenesis. We demonstrate that DCC at the membrane regulates the growth cone area and show that TRIM9 negatively regulates FAK activity in the absence of both repulsive and attractive concentrations of netrin-1. Together, our work demonstrates that TRIM9 interacts with and regulates both DCC and UNC5C during attractive and repulsive axonal responses to netrin-1.

TRIM9 controls growth cone responses to netrin through DCC and UNC5C

The guidance cue netrin-1 promotes both growth cone attraction and growth cone repulsion. How netrin-1 elicits these diverse axonal responses, beyond engaging the attractive receptor DCC and repulsive receptors of the UNC5 family, remains elusive. Here we demonstrate that murine netrin-1 induces biphasic axonal responses in cortical neurons: attraction at lower concentrations and repulsion at higher concentrations using both a microfluidic-based netrin-1 gradient and bath application of netrin-1. TRIM9 is a brain-enriched E3 ubiquitin ligase previously shown to bind and cluster the attractive receptor DCC at the plasma membrane and regulate netrin-dependent attractive responses. However, whether TRIM9 also regulated repulsive responses to netrin-1 remained to be seen. In this study, we show that TRIM9 localizes and interacts with both the attractive netrin receptor DCC and the repulsive netrin receptor, UNC5C, and that deletion of murine Trim9 alters both attractive and repulsive responses to murine netrin-1. TRIM9 was required for netrin-1-dependent changes in surface levels of DCC and total levels of UNC5C in the growth cone during morphogenesis. We demonstrate that DCC at the membrane regulates growth cone area and show that TRIM9 negatively regulates FAK activity in the absence of netrin-1. We investigate membrane dynamics of the UNC5C receptor using pH-mScarlet fused to the extracellular domain of UNC5C. Minutes after netrin addition, levels of UNC5C at the plasma membrane drop in a TRIM9-independent fashion, however TRIM9 regulated the mobility of UNC5C in the plasma membrane in the absence of netrin-1. Together this work demonstrates that TRIM9 interacts with and regulates both DCC and UNC5C during attractive and repulsive axonal responses to netrin-1.

Invasion of glioma cells through confined space requires membrane tension regulation and mechano-electrical coupling via Plexin-B2

Glioblastoma (GBM) is a malignant brain tumor with uncontrolled invasive growth. Here, we demonstrate how GBM cells usurp guidance receptor Plexin-B2 to gain biomechanical plasticity for polarized migration through confined space. Using live-cell imaging to track GBM cells negotiating microchannels, we reveal active endocytosis at cell front and filamentous actin assembly at rear to propel GBM cells through constrictions. These two processes are interconnected and governed by Plexin-B2 that orchestrates cortical actin and membrane tension, shown by biomechanical assays. Molecular dynamics simulations predict that balanced membrane and actin tension are required for optimal migratory velocity and consistency. Furthermore, Plexin-B2 mechanosensitive function requires a bendable extracellular ring structure and affects membrane internalization, permeability, phospholipid composition, as well as inner membrane surface charge. Together, our studies unveil a key element of membrane tension and mechanoelectrical coupling via Plexin-B2 that enables GBM cells to adapt to physical constraints and achieve polarized confined migration.

Syntaxin-1 is necessary for UNC5A-C/Netrin-1-dependent macropinocytosis and chemorepulsion

Introduction

Brain connectivity requires correct axonal guidance to drive axons to their appropriate targets. This process is orchestrated by guidance cues that exert attraction or repulsion to developing axons. However, the intricacies of the cellular machinery responsible for the correct response of growth cones are just being unveiled. Netrin-1 is a bifunctional molecule involved in axon pathfinding and cell migration that induces repulsion during postnatal cerebellar development. This process is mediated by UNC5 homolog receptors located on external granule layer (EGL) tracts.

Methods

Biochemical, imaging and cell biology techniques, as well as syntaxin-1A/B (Stx1A/B) knock-out mice were used in primary cultures and brain explants.

Results and discussion

Here, we demonstrate that this response is characterized by enhanced membrane internalization through macropinocytosis, but not clathrin-mediated endocytosis. We show that UNC5A, UNC5B, and UNC5C receptors form a protein complex with the t-SNARE syntaxin-1. By combining botulinum neurotoxins, an shRNA knock-down strategy and Stx1 knock-out mice, we demonstrate that this SNARE protein is required for Netrin1-induced macropinocytosis and chemorepulsion, suggesting that Stx1 is crucial in regulating Netrin-1-mediated axonal guidance.

Molecular machinery regulating organelle dynamics during axon growth and guidance

Axon growth and guidance in the developing nervous system rely on intracellular membrane dynamics that involve endosome maturation and transport, as well as its regulated tethering to the endoplasmic reticulum (ER). Recent studies have identified several key molecules, such as protrudin, which plays a dynamic role at membrane contact sites between the ER and endosomes/lysosomes, and myosin Va, which acts as a sensor for ER-derived Ca²⁺ that triggers peri-ER membrane export. These molecules form different types of multiprotein complexes at the interface of organelles and, in response to their surrounding microenvironments, such as Ca²⁺ concentrations and lipid contents, regulate the directional movement of endosomal vesicles in extending axons. Here, we review the molecular mechanisms underlying membrane dynamics and inter-organelle interactions during neuronal morphogenesis.

Axon pathfinding and targeting: (R)evolution of insights from in vitro assays

Investigating axonal behaviors while neurons are connecting with each other has been a challenge since the early studies on nervous system development. While molecule-driven axon pathfinding has been theorized by observing neurons at different developmental stages in vivo, direct observation and measurements of axon guidance behaviors required the invention of in vitro systems enabling to test the impact of molecules or cellular extracts on axons growing in vitro. With time, the development of novel in vivo approaches has confirmed the mechanisms highlighted in culture and has led in vitro systems to be adapted for cellular processes that are still inaccessible in intact organisms. We here review the evolution of these in vitro assays, which started with crucial contributions from the Bonhoeffer lab.

Clathrin-coated structures support 3D directed migration through local force transmission

Migrating cells navigate in complex environments through sensing and interpreting biochemical and/or mechanical cues. Here, we report that recently identified tubular clathrin/AP-2 lattices (TCALs), a subset of clathrin-coated structures (CCSs) that pinch collagen fibers, mechanically control directed migration along fibers decorated with ligands of CCS cargoes in three-dimensional (3D) environments. We observed that epidermal growth factor or low-density lipoprotein bound to collagen fibers leads to increased local nucleation and accumulation of TCALs. By using engineered, mixed collagen networks, we demonstrate that this mechanism selectively increases local forces applied on ligand-decorated fibers. We show that these effects depend on the ligand’s receptors but do not rely on their ability to trigger signaling events. We propose that the preferential accumulation of TCALs along ligand-decorated fibers steers migration in 3D environments. We conclude that ligand-regulated, local TCAL accumulation results in asymmetric force distribution that orients cell migration in 3D environments.

Migration cues interpretation by clathrin-coated structures

Cell migration is oriented by cues from the environment. Such cues are read and interpreted by the cell and translated into a reorganization of the migration machinery to steer migration. Receptors at the cell surface are central to detect these cues. These receptors can be internalized and this plays an important role in the decision-making process leading to choosing a migration direction. Independently of endocytosis, recent findings suggest that regulation of these receptors and translation of the information they carry into a phenotype is facilitated by their clustering at discrete locations of the plasma membrane. Clathrin-coated structures are archetypal clustering assemblies and thus provide the cell with a finely tunable mechanism for controlling receptor availability. In addition, clathrin-coated structures can be regulated by many factors playing a role in cell migration and thus take part in feedback loop mechanisms that are instrumental in defining a migration direction.

Brain-specific functions of the endocytic machinery

Endocytosis is an essential cellular process required for multiple physiological functions, including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. In a broad sense, endocytosis is accomplished through either constitutive or ligand-induced invagination of the plasma membrane, which results in the formation of the plasma membrane-retrieved endocytic vesicles, which can either be sent for degradation to the lysosomes or recycled back to the PM. This additional function of endocytosis in membrane retrieval has been adopted by excitable cells, such as neurons, for membrane equilibrium maintenance at synapses. The last two decades were especially productive with respect to the identification of brain-specific functions of the endocytic machinery, which additionally include but not limited to regulation of neuronal differentiation and migration, maintenance of neuron morphology and synaptic plasticity, and prevention of neurotoxic aggregates spreading. In this review, we highlight the current knowledge of brain-specific functions of endocytic machinery with a specific focus on three brain cell types, neuronal progenitor cells, neurons, and glial cells.

SH3BP4 promotes neuropilin-1 and α5-integrin endocytosis and is inhibited by Akt

Cells probe their surrounding matrix for attachment sites via integrins that are internalized by endocytosis. We find that SH3BP4 regulates integrin surface expression in a signaling-dependent manner via clathrin-coated pits (CCPs). Dephosphorylated SH3BP4 at S246 is efficiently recruited to CCPs, while upon Akt phosphorylation, SH3BP4 is sequestered by 14-3-3 adaptors and excluded from CCPs. In the absence of Akt activity, SH3BP4 binds GIPC1 and targets neuropilin-1 and α5/β1-integrin for endocytosis, leading to inhibition of cell spreading. Similarly, chemorepellent semaphorin-3a binds neuropilin-1 to activate PTEN, which antagonizes Akt and thus recruits SH3BP4 to CCPs to internalize both receptors and induce cell contraction. In PTEN mutant non-small cell lung cancer cells with high Akt activity, expression of non-phosphorylatable active SH3BP4-S246A restores semaphorin-3a induced cell contraction. Thus, SH3BP4 links Akt signaling to endocytosis of NRP1 and α5/β1-integrins to modulate cell-matrix interactions in response to intrinsic and extrinsic cues.

High throughput error corrected Nanopore single cell transcriptome sequencing

Droplet-based high throughput single cell sequencing techniques tremendously advanced our insight into cell-to-cell heterogeneity. However, those approaches only allow analysis of one extremity of the transcript after short read sequencing. In consequence, information on splicing and sequence heterogeneity is lost. To overcome this limitation, several approaches that use long-read sequencing were introduced recently. Yet, those techniques are limited by low sequencing depth and/or lacking or inaccurate assignment of unique molecular identifiers (UMIs), which are critical for elimination of PCR bias and artifacts. We introduce ScNaUmi-seq, an approach that combines the high throughput of Oxford Nanopore sequencing with an accurate cell barcode and UMI assignment strategy. UMI guided error correction allows to generate high accuracy full length sequence information with the 10x Genomics single cell isolation system at high sequencing depths. We analyzed transcript isoform diversity in embryonic mouse brain and show that ScNaUmi-seq allows defining splicing and SNVs (RNA editing) at a single cell level.

Droplet-based high throughput single cell sequencing techniques can often lose information on transcript splicing and heterogenity. Here the authors introduce ScNaUmi-seq, which uses Oxford Nanopore sequencing and barcoding to generate high accuracy full length sequences.

Involvement of the synaptotagmin/stonin2 system in vesicular transport regulated by semaphorins in Caenorhabditis elegans epidermal cells

Vesicular transport serves as an important mechanism for cell shape regulation during development. Although the semaphorin signaling molecule, a well-known regulator of axon guidance, induces endocytosis in the growth cone and the axonal transport of vertebrate neurons, the underlying molecular mechanisms remain largely unclear. Here, we show that the Caenorhabditis elegans SNT-1/synaptotagmin-UNC-41/stonin2 system, whose role in synaptic vesicle recycling in neurons has been studied extensively, is involved in semaphorin-regulated vesicular transport in larval epidermal cells. Mutations in the snt-1/unc-41 genes strongly suppressed the cell shape defects of semaphorin mutants. The null mutation in the semaphorin receptor gene, plx-1, altered the expression and localization pattern of endocytic and exocytic markers in the epidermal cells while repressing the transport of SNT-1-containing vesicles toward late endosome/lysosome pathways. Our findings suggest that the nematode semaphorins regulate the vesicular transport in epidermal cells in a manner distinct from that of vertebrate semaphorins in neurons.

A hormone receptor pathway cell-autonomously delays neuron morphological aging by suppressing endocytosis

Neurons have a lifespan that parallels that of the organism and are largely irreplaceable. Their unusually long lifespan predisposes neurons to neurodegenerative disease. We sought to identify physiological mechanisms that delay neuron aging in Caenorhabditis elegans by asking how neuron morphological aging is arrested in the long-lived, alternate organismal state, the dauer diapause. We find that a hormone signaling pathway, the abnormal DAuer Formation (DAF) 12 nuclear hormone receptor (NHR) pathway, functions cell-intrinsically in the dauer diapause to arrest neuron morphological aging, and that same pathway can be cell-autonomously manipulated during normal organismal aging to delay neuron morphological aging. This delayed aging is mediated by suppressing constitutive endocytosis, which alters the subcellular localization of the actin regulator T cell lymphoma Invasion And Metastasis 1 (TIAM-1), thereby decreasing age-dependent neurite growth. Intriguingly, we show that suppressed endocytosis appears to be a general feature of cells in diapause, suggestive that this may be a mechanism to halt the growth and other age-related programs supported by most endosome recycling.

FIGNL1 associates with KIF1Bβ and BICD1 to restrict dynein transport velocity during axon navigation

Atkins et al. identify a new role for Fidgetin-like 1 in motor axon navigation via its regulation of bidirectional axonal transport. They show that Fidgetin-like 1 binds Kif1bβ and the opposed polarity-directed motor dynein/dynactin in a molecular complex and controls circuit wiring by reducing dynein velocity in developing motor axons.

Neuronal connectivity relies on molecular motor-based axonal transport of diverse cargoes. Yet the precise players and regulatory mechanisms orchestrating such trafficking events remain largely unknown. We here report the ATPase Fignl1 as a novel regulator of bidirectional transport during axon navigation. Using a yeast two-hybrid screen and coimmunoprecipitation assays, we showed that Fignl1 binds the kinesin Kif1bβ and the dynein/dynactin adaptor Bicaudal D-1 (Bicd1) in a molecular complex including the dynactin subunit dynactin 1. Fignl1 colocalized with Kif1bβ and showed bidirectional mobility in zebrafish axons. Notably, Kif1bβ and Fignl1 loss of function similarly altered zebrafish motor axon pathfinding and increased dynein-based transport velocity of Rab3 vesicles in these navigating axons, pinpointing Fignl1/Kif1bβ as a dynein speed limiter complex. Accordingly, disrupting dynein/dynactin activity or Bicd1/Fignl1 interaction induced motor axon pathfinding defects characteristic of Fignl1 gain or loss of function, respectively. Finally, pharmacological inhibition of dynein activity partially rescued the axon pathfinding defects of Fignl1-depleted larvae. Together, our results identify Fignl1 as a key dynein regulator required for motor circuit wiring.

Postsynaptic Proteome of Non-Demented Individuals with Alzheimer's Disease Neuropathology

Some individuals, here referred to as Non-Demented with Alzheimer’s Neuropathology (NDAN), retain their cognitive function despite the presence of amyloid plaques and tau tangles typical of symptomatic Alzheimer’s disease (AD). In NDAN, unlike AD, toxic amyloid-β oligomers do not localize to the postsynaptic densities (PSDs). Synaptic resistance to amyloid-β in NDAN may thus enable these individuals to remain cognitively intact despite the AD-like pathology. The mechanism(s) responsible for this resistance remains unresolved and understanding such protective biological processes could reveal novel targets for the development of effective treatments for AD. The present study uses a proteomic approach to compare the hippocampal postsynaptic densities of NDAN, AD, and healthy age-matched persons to identify protein signatures characteristic for these groups. Subcellular fractionation followed by 2D gel electrophoresis and mass spectrometry were used to analyze the PSDs. We describe fifteen proteins which comprise the unique proteomic signature of NDAN PSDs, thus setting them apart from control subjects and AD patients.

PTENα regulates endocytosis and modulates olfactory function

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) α is the first identified isoform of the well-known tumor suppressor PTEN. PTENα has an evolutionarily conserved 173-aa N terminus compared with canonical PTEN. Recently, PTENα has been shown to play roles in multiple biologic processes including learning and memory, cardiac homeostasis, and antiviral immunity. Here, we report that PTENα maintains mitral cells in olfactory bulb (OB), regulates endocytosis in OB neurons, and controls olfactory behaviors in mice. We show that PTENα directly dephosphorylates the endocytic protein amphiphysin and promotes its binding to adaptor-related protein complex 2 subunit β1 (Ap2b1). In addition, we identified mutations in the N terminus of PTENα in patients with Parkinson disease and Lewy-body dementia, which are neurodegenerative disorders with early olfactory loss. Overexpression of PTENα mutant H169N in mice OB reduces odor sensitivity. Our data demonstrate a role of PTENα in olfactory function and provide insight into the mechanism of olfactory dysfunction in neurologic disorders.-Yuan, Y., Zhao, X., Wang, P., Mei, F., Zhou, J., Jin, Y., McNutt, M. A., Yin, Y. PTENα regulates endocytosis and modulates olfactory function.

Opposing functions of F-BAR proteins in neuronal membrane protrusion, tubule formation, and neurite outgrowth

Neurite formation is a fundamental antecedent to axon and dendrite formation, but the mechanisms that underlie this important process are poorly characterized. Here, we demonstrate that two F-BAR proteins, CIP4 and FBP17, have opposing functions in early cortical neuron development.

The F-BAR family of proteins play important roles in many cellular processes by regulating both membrane and actin dynamics. The CIP4 family of F-BAR proteins is widely recognized to function in endocytosis by elongating endocytosing vesicles. However, in primary cortical neurons, CIP4 concentrates at the tips of extending lamellipodia and filopodia and inhibits neurite outgrowth. Here, we report that the highly homologous CIP4 family member, FBP17, induces tubular structures in primary cortical neurons and results in precocious neurite formation. Through domain swapping and deletion experiments, we demonstrate that a novel polybasic region between the F-BAR and HR1 domains is required for membrane bending. Moreover, the presence of a poly-PxxP region in longer splice isoforms of CIP4 and FBP17 largely reverses the localization and function of these proteins. Thus, CIP4 and FBP17 function as an antagonistic pair to fine-tune membrane protrusion, endocytosis, and neurite formation during early neuronal development.

Boc Acts via Numb as a Shh-Dependent Endocytic Platform for Ptch1 Internalization and Shh-Mediated Axon Guidance

During development, Shh attracts commissural axons toward the floor plate through a non-canonical, transcription-independent signaling pathway that requires the receptor Boc. Here, we find that Shh induces Boc internalization into early endosomes and that endocytosis is required for Shh-mediated growth-cone turning. Numb, an endocytic adaptor, binds to Boc and is required for Boc internalization, Shh-mediated growth-cone turning in vitro, and commissural axon guidance in vivo. Similar to Boc, Ptch1 is also internalized by Shh in a Numb-dependent manner; however, the binding of Shh to Ptch1 alone is not sufficient to induce Ptch1 internalization nor growth-cone turning. Therefore, the binding of Shh to Boc is required for Ptch1 internalization and growth-cone turning. Our data support a model where Boc endocytosis via Numb is required for Ptch1 internalization and Shh signaling in axon guidance. Thus, Boc acts as a Shh-dependent endocytic platform gating Ptch1 internalization and Shh signaling.

A crucial role for Arf6 in the response of commissural axons to Slit

A switch in the response of commissural axons to the repellent Slit is crucial for ensuring that they cross the ventral midline only once. However, the underlying mechanisms remain to be elucidated. We have found that both endocytosis and recycling of Robo1 receptor are crucial for modulating Slit sensitivity in vertebrate commissural axons. Robo1 endocytosis and its recycling back to the cell surface maintained the stability of axonal Robo1 during Slit stimulation. We identified Arf6 guanosine triphosphatase and its activators, cytohesins, as previously unknown components in Slit-Robo1 signalling in vertebrate commissural neurons. Slit-Robo1 signalling activated Arf6. The Arf6-deficient mice exhibited marked defects in commissural axon midline crossing. Our data showed that a Robo1 endocytosis-triggered and Arf6-mediated positive-feedback strengthens the Slit response in commissural axons upon their midline crossing. Furthermore, the cytohesin-Arf6 pathways modulated this self-enhancement of the Slit response before and after midline crossing, resulting in a switch that reinforced robust regulation of axon midline crossing. Our study provides insights into endocytic trafficking-mediated mechanisms for spatiotemporally controlled axonal responses and uncovers new players in the midline switch in Slit responsiveness of commissural axons.

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.

A cell surface protein controls endocrine ring gland morphogenesis and steroid production

Identification of signals for systemic adaption of hormonal regulation would help to understand the crosstalk between cells and environmental cues contributing to growth, metabolic homeostasis and development. Physiological states are controlled by precise pulsatile hormonal release, including endocrine steroids in human and ecdysteroids in insects. We show in Drosophila that regulation of genes that control biosynthesis and signaling of the steroid hormone ecdysone, a central regulator of developmental progress, depends on the extracellular matrix protein Obstructor-A (Obst-A). Ecdysone is produced by the prothoracic gland (PG), where sensory neurons projecting axons from the brain integrate stimuli for endocrine control. By defining the extracellular surface, Obst-A promotes morphogenesis and axonal growth in the PG. This process requires Obst-A-matrix reorganization by Clathrin/Wurst-mediated endocytosis. Our data identifies the extracellular matrix as essential for endocrine ring gland function, which coordinates physiology, axon morphogenesis, and developmental programs. As Obst-A and Wurst homologs are found among all arthropods, we propose that this mechanism is evolutionary conserved.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

APache Is an AP2-Interacting Protein Involved in Synaptic Vesicle Trafficking and Neuronal Development

Synaptic transmission is critically dependent on synaptic vesicle (SV) recycling. Although the precise mechanisms of SV retrieval are still debated, it is widely accepted that a fundamental role is played by clathrin-mediated endocytosis, a form of endocytosis that capitalizes on the clathrin/adaptor protein complex 2 (AP2) coat and several accessory factors. Here, we show that the previously uncharacterized protein KIAA1107, predicted by bioinformatics analysis to be involved in the SV cycle, is an AP2-interacting clathrin-endocytosis protein (APache). We found that APache is highly enriched in the CNS and is associated with clathrin-coated vesicles via interaction with AP2. APache-silenced neurons exhibit a severe impairment of maturation at early developmental stages, reduced SV density, enlarged endosome-like structures, and defects in synaptic transmission, consistent with an impaired clathrin/AP2-mediated SV recycling. Our data implicate APache as an actor in the complex regulation of SV trafficking, neuronal development, and synaptic plasticity.

Reciprocal link between cell biomechanics and exocytosis

A cell is able to sense the biomechanical properties of the environment such as the rigidity of the extracellular matrix and adapt its tension via regulation of plasma membrane and underlying actomyosin meshwork properties. The cell's ability to adapt to the changing biomechanical environment is important for cellular homeostasis and also cell dynamics such as cell growth and motility. Membrane trafficking has emerged as an important mechanism to regulate cell biomechanics. In this review, we summarize the current understanding of the role of cell mechanics in exocytosis, and reciprocally, the role of exocytosis in regulating cell mechanics. We also discuss how cell mechanics and membrane trafficking, particularly exocytosis, can work together to regulate cell polarity and motility.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Visualizing and Characterizing Semaphorin Endocytic Events Using Quantum Dot-Conjugated Proteins

Neurons can endocytose soluble semaphorins to either initiate or interrupt signaling at the cell membrane. Depending on the cell type and even on the specific subcellular domain, the endocytic process will differ in intensity, speed, and modality, and will subsequently facilitate diverse actions of semaphorin molecules. Therefore, in order to understand the physiology of guidance cues like semaphorins it is important to visualize endocytic events with good spatial and temporal resolution. Here, we describe methods to visualize endocytosed Semaphorin3A (Sema3A) molecules and to characterize the rate and pathway of internalization in primary rat neuronal cultures using semiconductor quantum dot nanoparticles (Q-dots).

How does calcium interact with the cytoskeleton to regulate growth cone motility during axon pathfinding?

The precision with which neurons form connections is crucial for the normal development and function of the nervous system. The development of neuronal circuitry in the nervous system is accomplished by axon pathfinding: a process where growth cones guide axons through the embryonic environment to connect with their appropriate synaptic partners to form functional circuits. Despite intense efforts over many years to understand how this process is regulated, the complete repertoire of molecular mechanisms that govern the growth cone cytoskeleton and hence motility, remain unresolved. A central tenet in the axon guidance field is that calcium signals regulate growth cone behaviours such as extension, turning and pausing by regulating rearrangements of the growth cone cytoskeleton. Here, we provide evidence that not only the amplitude of a calcium signal is critical for growth cone motility but also the source of calcium mobilisation. We provide an example of this idea by demonstrating that manipulation of calcium signalling via L-type voltage gated calcium channels can perturb sensory neuron motility towards a source of netrin-1. Understanding how calcium signals can be transduced to initiate cytoskeletal changes represents a significant gap in our current knowledge of the mechanisms that govern axon guidance, and consequently the formation of functional neural circuits in the developing nervous system.

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.

Structure analyses reveal a regulated oligomerization mechanism of the PlexinD1/GIPC/myosin VI complex

The GIPC family adaptor proteins mediate endocytosis by tethering cargo proteins to the myosin VI motor. The structural mechanisms for the GIPC/cargo and GIPC/myosin VI interactions remained unclear. PlexinD1, a transmembrane receptor that regulates neuronal and cardiovascular development, is a cargo of GIPCs. GIPC-mediated endocytic trafficking regulates PlexinD1 signaling. Here, we unravel the mechanisms of the interactions among PlexinD1, GIPCs and myosin VI by a series of crystal structures of these proteins in apo or bound states. GIPC1 forms a domain-swapped dimer in an autoinhibited conformation that hinders binding of both PlexinD1 and myosin VI. PlexinD1 binding to GIPC1 releases the autoinhibition, promoting its interaction with myosin VI. GIPCs and myosin VI interact through two distinct interfaces and form an open-ended alternating array. Our data support that this alternating array underlies the oligomerization of the GIPC/Myosin VI complexes in solution and cells.

DOI:http://dx.doi.org/10.7554/eLife.27322.001

A functional coupling between CRMP1 and Nav1.7 for retrograde propagation of Semaphorin3A signaling

Semaphorin3A (Sema3A) is a secreted type of axon guidance molecule that regulates axon wiring through complexes of neuropilin-1 (NRP1) with PlexinA protein receptors. Sema3A regulates the dendritic branching through tetrodotoxin (TTX)-sensitive retrograde axonal transport of PlexA proteins and tropomyosin-related kinase A (TrkA) complex. We here demonstrate that Na_v1.7 (encoded by SCN9A), a TTX-sensitive Na⁺ channel, by coupling with collapsin response mediator protein 1 (CRMP1), mediates the Sema3A-induced retrograde transport. In mouse dorsal root ganglion (DRG) neurons, Sema3A increased co-localization of PlexA4 and TrkA in the growth cones and axons. TTX treatment and RNAi knockdown of Na_v1.7 sustained Sema3A-induced colocalized signals of PlexA4 and TrkA in growth cones and suppressed the subsequent localization of PlexA4 and TrkA in distal axons. A similar localization phenotype was observed in crmp1^-/- DRG neurons. Sema3A induced colocalization of CRMP1 and Na_v1.7 in the growth cones. The half maximal voltage was increased in crmp1^-/- neurons when compared to that in wild type. In HEK293 cells, introduction of CRMP1 lowered the threshold of co-expressed exogenous Na_v1.7. These results suggest that Na_v1.7, by coupling with CRMP1, mediates the axonal retrograde signaling of Sema3A.

Visualization of Clathrin-Mediated Endocytosis During Semaphorin-Guided Axonal Growth

Semaphorin3A (Sema3A) guides axonal growth during neuronal network development. Accumulating evidence indicates that Sema3A-induced growth cone collapse and repulsion involve endocytic membrane trafficking in the growth cone. It is now possible to visualize endocytic processes in living cells using total internal reflection fluorescence microscopy (TIRFM), a powerful tool for imaging dynamic subcellular events at the plasma membrane. In this chapter, we describe a method for TIRFM observation and analysis of clathrin-mediated endocytosis in growth cones of chicken dorsal root ganglion neurons that receive an extracellular concentration gradient of Sema3A in a culture medium.

Multi-phasic bi-directional chemotactic responses of the growth cone

The nerve growth cone is bi-directionally attracted and repelled by the same cue molecules depending on the situations, while other non-neural chemotactic cells usually show uni-directional attraction or repulsion toward their specific cue molecules. However, how the growth cone differs from other non-neural cells remains unclear. Toward this question, we developed a theory for describing chemotactic response based on a mathematical model of intracellular signaling of activator and inhibitor. Our theory was first able to clarify the conditions of attraction and repulsion, which are determined by balance between activator and inhibitor, and the conditions of uni- and bi-directional responses, which are determined by dose-response profiles of activator and inhibitor to the guidance cue. With biologically realistic sigmoidal dose-responses, our model predicted tri-phasic turning response depending on intracellular Ca²⁺ level, which was then experimentally confirmed by growth cone turning assays and Ca²⁺ imaging. Furthermore, we took a reverse-engineering analysis to identify balanced regulation between CaMKII (activator) and PP1 (inhibitor) and then the model performance was validated by reproducing turning assays with inhibitions of CaMKII and PP1. Thus, our study implies that the balance between activator and inhibitor underlies the multi-phasic bi-directional turning response of the growth cone.

Tracking Differential Endocytosis and Trafficking of Semaphorin Receptor Complexes in Responding Nerve Growth Cones

The study of receptor endocytosis is important to our understanding of the signal transduction events initiated by axon guidance cues in growth cones. Fab fragments of antibodies to guidance receptors and endocytic cargoes like transferrin and cholera toxin-B are the tools of choice for studying the dynamics of receptor internalization and intracellular trafficking by different pathways. We describe a method where in vitro cultures of growth cones are incubated with these ligands in the presence or absence of Sema3A, followed by stripping of remaining ligand on cell-surface and analysis by immunofluorescence techniques. These techniques can be employed for studying the endocytosis of any axon guidance receptor in response to attractive or repulsive guidance cues and, in particular, to allow the differential trafficking of specific receptor components to be revealed.

Regulation of dendritic development by semaphorin 3A through novel intracellular remote signaling

Numerous cell adhesion molecules, extracellular matrix proteins and axon guidance molecules participate in neuronal network formation through local effects at axo-dendritic, axo-axonic or dendro-dendritic contact sites. In contrast, neurotrophins and their receptors play crucial roles in neural wiring by sending retrograde signals to remote cell bodies. Semaphorin 3A (Sema3A), a prototype of secreted type 3 semaphorins, is implicated in axon repulsion, dendritic branching and synapse formation via binding protein neuropilin-1 (NRP1) and the signal transducing protein PlexinAs (PlexAs) complex. This review focuses on Sema3A retrograde signaling that regulates dendritic localization of AMPA-type glutamate receptor GluA2 and dendritic patterning. This signaling is elicited by activation of NRP1 in growth cones and is propagated to cell bodies by dynein-dependent retrograde axonal transport of PlexAs. It also requires interaction between PlexAs and a high-affinity receptor for nerve growth factor, toropomyosin receptor kinase A. We propose a control mechanism by which retrograde Sema3A signaling regulates the glutamate receptor localization through trafficking of cis-interacting PlexAs with GluA2 along dendrites; this remote signaling may be an alternative mechanism to local adhesive contacts for neural network formation.

Beyond the cytoskeleton: The emerging role of organelles and membrane remodeling in the regulation of axon collateral branches

The generation of axon collateral branches is a fundamental aspect of the development of the nervous system and the response of axons to injury. Although much has been discovered about the signaling pathways and cytoskeletal dynamics underlying branching, additional aspects of the cell biology of axon branching have received less attention. This review summarizes recent advances in our understanding of key factors involved in axon branching. This article focuses on how cytoskeletal mechanisms, intracellular organelles, such as mitochondria and the endoplasmic reticulum, and membrane remodeling (exocytosis and endocytosis) contribute to branch initiation and formation. Together this growing literature provides valuable insight as well as a platform for continued investigation into how multiple aspects of axonal cell biology are spatially and temporally orchestrated to give rise to axon branches. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1293-1307, 2016.

ESCRT-II controls retinal axon growth by regulating DCC receptor levels and local protein synthesis

Endocytosis and local protein synthesis (LPS) act coordinately to mediate the chemotropic responses of axons, but the link between these two processes is poorly understood. The endosomal sorting complex required for transport (ESCRT) is a key regulator of cargo sorting in the endocytic pathway, and here we have investigated the role of ESCRT-II, a critical ESCRT component, in Xenopus retinal ganglion cell (RGC) axons. We show that ESCRT-II is present in RGC axonal growth cones (GCs) where it co-localizes with endocytic vesicle GTPases and, unexpectedly, with the Netrin-1 receptor, deleted in colorectal cancer (DCC). ESCRT-II knockdown (KD) decreases endocytosis and, strikingly, reduces DCC in GCs and leads to axon growth and guidance defects. ESCRT-II-depleted axons fail to turn in response to a Netrin-1 gradient in vitro and many axons fail to exit the eye in vivo. These defects, similar to Netrin-1/DCC loss-of-function phenotypes, can be rescued in whole (in vitro) or in part (in vivo) by expressing DCC. In addition, ESCRT-II KD impairs LPS in GCs and live imaging reveals that ESCRT-II transports mRNAs in axons. Collectively, our results show that the ESCRT-II-mediated endocytic pathway regulates both DCC and LPS in the axonal compartment and suggest that ESCRT-II aids gradient sensing in GCs by coupling endocytosis to LPS.

Optogenetic activation of axon guidance receptors controls direction of neurite outgrowth

Growth cones of extending axons navigate to correct targets by sensing a guidance cue gradient via membrane protein receptors. Although most signaling mechanisms have been clarified using an in vitro approach, it is still difficult to investigate the growth cone behavior in complicated extracellular environment of living animals due to the lack of tools. We develop a system for the light-dependent activation of a guidance receptor, Deleted in Colorectal Cancer (DCC), using Arabidopsis thaliana Cryptochrome 2, which oligomerizes upon blue-light absorption. Blue-light illumination transiently activates DCC via its oligomerization, which initiates downstream signaling in the illuminated subcellular region. The extending axons are attracted by illumination in cultured chick dorsal root ganglion neurons. Moreover, light-mediated navigation of the growth cones is achieved in living Caenorhabditis elegans. The photo-manipulation system is applicable to investigate the relationship between the growth cone behavior and its surrounding environment in living tissue.

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.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

Draxin from neocortical neurons controls the guidance of thalamocortical projections into the neocortex

The thalamocortical tract carries sensory information to the neocortex. It has long been recognized that the neocortical pioneer axons of subplate neurons are essential for thalamocortical development. Herein we report that an axon guidance cue, draxin, is expressed in early-born neocortical neurons, including subplate neurons, and is necessary for thalamocortical development. In draxin^−/− mice, thalamocortical axons do not enter the neocortex. This phenotype is sufficiently rescued by the transgenic expression of draxin in neocortical neurons. Genetic interaction data suggest that draxin acts through Deleted in colorectal cancer (DCC) and Neogenin (Neo1), to regulate thalamocortical projections in vivo. Draxin promotes the outgrowth of thalamic axons in vitro and this effect is abolished in thalamic neurons from Dcc and Neo1 double mutants. These results suggest that draxin from neocortical neurons controls thalamocortical projections into the neocortex, and that this effect is mediated through the DCC and Neo1 receptors.

During neural development thalamocortical axons follow corticofugal projections into the neocortex. Here, using a combination of knock down and rescue experiments, the authors show that Draxin expression in neocortical cells promotes thalamic axon projections from the internal capsule.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

Mechanochemical regulation of growth cone motility

Neuronal growth cones are exquisite sensory-motor machines capable of transducing features contacted in their local extracellular environment into guided process extension during development. Extensive research has shown that chemical ligands activate cell surface receptors on growth cones leading to intracellular signals that direct cytoskeletal changes. However, the environment also provides mechanical support for growth cone adhesion and traction forces that stabilize leading edge protrusions. Interestingly, recent work suggests that both the mechanical properties of the environment and mechanical forces generated within growth cones influence axon guidance. In this review we discuss novel molecular mechanisms involved in growth cone force production and detection, and speculate how these processes may be necessary for the development of proper neuronal morphogenesis.

Regulation of patterned dynamics of local exocytosis in growth cones by netrin-1

Axonal guidance and synaptic specification depends on specific signaling mechanisms that occur in growth cones. While several signaling pathways implicated in cone navigation have been identified, membrane dynamics in growth cones remains largely unknown. We took advantage of SynaptopHluorin and high-speed optical recordings to monitor the patterns of membrane dynamics in rat hippocampal growth cones. We show that exocytosis occurs both at the peripheral and central domains, including filopodia, and that SynaptopHluorin signals occur as spontaneous patterned peaks. Such transients average approximately two per minute and last ∼30 s. We also demonstrate that the chemoattractant Netrin-1 elicits increases in the frequency and slopes of these transients, with peaks averaging up to six per minute in the peripheral domain. Netrin-1-dependent regulation of exocytotic events requires the activation of the Erk1/2 and SFK pathways. Furthermore, we show that domains with high SynaptopHluorin signals correlate with high local calcium concentrations and that local, spontaneous calcium increases are associated with higher SynaptopHluorin signals. These findings demonstrate highly stereotyped, spontaneous transients of local exocytosis in growth cones and that these transients are positively regulated by chemoattractant molecules such as Netrin-1.

Asymmetric formation of coated pits on dorsal and ventral surfaces at the leading edges of motile cells and on protrusions of immobile cells

Clathrin/AP2-coated vesicles are the principal endocytic carriers originating at the plasma membrane. In the experiments reported here, we used spinning-disk confocal and lattice light-sheet microscopy to study the assembly dynamics of coated pits on the dorsal and ventral membranes of migrating U373 glioblastoma cells stably expressing AP2 tagged with enhanced green fluorescence (AP2-EGFP) and on lateral protrusions from immobile SUM159 breast carcinoma cells, gene-edited to express AP2-EGFP. On U373 cells, coated pits initiated on the dorsal membrane at the front of the lamellipodium and at the approximate boundary between the lamellipodium and lamella and continued to grow as they were swept back toward the cell body; coated pits were absent from the corresponding ventral membrane. We observed a similar dorsal/ventral asymmetry on membrane protrusions from SUM159 cells. Stationary coated pits formed and budded on the remainder of the dorsal and ventral surfaces of both types of cells. These observations support a previously proposed model that invokes net membrane deposition at the leading edge due to an imbalance between the endocytic and exocytic membrane flow at the front of a migrating cell.