Recycling endosomes undergo rapid closure of a fusion pore on exocytosis in neuronal dendrites

ExoJ - a Fiji/ImageJ2 plugin for automated spatiotemporal detection and analysis of exocytosis

Exocytosis is a dynamic physiological process that enables the release of biomolecules to the surrounding environment via the fusion of membrane compartments to the plasma membrane. Understanding its mechanisms is crucial, as defects can compromise essential biological functions. The development of pH-sensitive optical reporters alongside fluorescence microscopy enables the assessment of individual vesicle exocytosis events at the cellular level. Manual annotation represents, however, a time-consuming task that is prone to selection biases and human operational errors. Here, we introduce ExoJ, an automated plugin based on Fiji/ImageJ2 software. ExoJ identifies user-defined genuine populations of exocytosis events, recording quantitative features including intensity, apparent size and duration. We designed ExoJ to be fully user-configurable, making it suitable for studying distinct forms of vesicle exocytosis regardless of the imaging quality. Our plugin demonstrates its capabilities by showcasing distinct exocytic dynamics among tetraspanins and vesicular SNARE protein reporters. Assessment of performance on synthetic data shows that ExoJ is a robust tool that is capable of correctly identifying exocytosis events independently of signal-to-noise ratio conditions. We propose ExoJ as a standard solution for future comparative and quantitative studies of exocytosis.

Emerging role of oncogenic ß-catenin in exosome biogenesis as a driver of immune escape in hepatocellular carcinoma

Immune checkpoint inhibitors have produced encouraging results in cancer patients. However, the majority of ß-catenin-mutated tumors have been described as lacking immune infiltrates and resistant to immunotherapy. The mechanisms by which oncogenic ß-catenin affects immune surveillance remain unclear. Herein, we highlighted the involvement of ß-catenin in the regulation of the exosomal pathway and, by extension, in immune/cancer cell communication in hepatocellular carcinoma (HCC). We showed that mutated ß-catenin represses expression of SDC4 and RAB27A, two main actors in exosome biogenesis, in both liver cancer cell lines and HCC patient samples. Using nanoparticle tracking analysis and live-cell imaging, we further demonstrated that activated ß-catenin represses exosome release. Then, we demonstrated in 3D spheroid models that activation of β-catenin promotes a decrease in immune cell infiltration through a defect in exosome secretion. Taken together, our results provide the first evidence that oncogenic ß-catenin plays a key role in exosome biogenesis. Our study gives new insight into the impact of ß-catenin mutations on tumor microenvironment remodeling, which could lead to the development of new strategies to enhance immunotherapeutic response.

Rational Engineering of an Improved Genetically Encoded pH Sensor Based on Superecliptic pHluorin

Genetically encoded pH sensors based on fluorescent proteins are valuable tools for the imaging of cellular events that are associated with pH changes, such as exocytosis and endocytosis. Superecliptic pHluorin (SEP) is a pH-sensitive green fluorescent protein (GFP) variant widely used for such applications. Here, we report the rational design, development, structure, and applications of Lime, an improved SEP variant with higher fluorescence brightness and greater pH sensitivity. The X-ray crystal structure of Lime supports the mechanistic rationale that guided the introduction of beneficial mutations. Lime provides substantial improvements relative to SEP for imaging of endocytosis and exocytosis. Furthermore, Lime and its variants are advantageous for a broader range of applications including the detection of synaptic release and neuronal voltage changes.

GTP energy dependence of endocytosis and autophagy in the aging brain and Alzheimer's disease

Increased interest in the aging and Alzheimer's disease (AD)-related impairments in autophagy in the brain raise important questions about regulation and treatment. Since many steps in endocytosis and autophagy depend on GTPases, new measures of cellular GTP levels are needed to evaluate energy regulation in aging and AD. The recent development of ratiometric GTP sensors (GEVALS) and findings that GTP levels are not homogenous inside cells raise new issues of regulation of GTPases by the local availability of GTP. In this review, we highlight the metabolism of GTP in relation to the Rab GTPases involved in formation of early endosomes, late endosomes, and lysosomal transport to execute the autophagic degradation of damaged cargo. Specific GTPases control macroautophagy (mitophagy), microautophagy, and chaperone-mediated autophagy (CMA). By inference, local GTP levels would control autophagy, if not in excess. Additional levels of control are imposed by the redox state of the cell, including thioredoxin involvement. Throughout this review, we emphasize the age-related changes that could contribute to deficits in GTP and AD. We conclude with prospects for boosting GTP levels and reversing age-related oxidative redox shift to restore autophagy. Therefore, GTP levels could regulate the numerous GTPases involved in endocytosis, autophagy, and vesicular trafficking. In aging, metabolic adaptation to a sedentary lifestyle could impair mitochondrial function generating less GTP and redox energy for healthy management of amyloid and tau proteostasis, synaptic function, and inflammation.

The BACE1-generated C-terminal fragment of the neural cell adhesion molecule 2 (NCAM2) promotes BACE1 targeting to Rab11-positive endosomes

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), also known as β-secretase, is an aspartic protease. The sorting of this enzyme into Rab11-positive recycling endosomes regulates the BACE1-mediated cleavage of its substrates, however, the mechanisms underlying this targeting remain poorly understood. The neural cell adhesion molecule 2 (NCAM2) is a substrate of BACE1. We show that BACE1 cleaves NCAM2 in cultured hippocampal neurons and NCAM2-transfected CHO cells. The C-terminal fragment of NCAM2 that comprises the intracellular domain and a small portion of NCAM2’s extracellular domain, associates with BACE1. This association is not affected in cells with inhibited endocytosis, indicating that the interaction of NCAM2 and BACE1 precedes the targeting of BACE1 from the cell surface to endosomes. In neurons and CHO cells, this fragment and BACE1 co-localize in Rab11-positive endosomes. Overexpression of full-length NCAM2 or a recombinant NCAM2 fragment containing the transmembrane and intracellular domains but lacking the extracellular domain leads to an increase in BACE1 levels in these organelles. In NCAM2-deficient neurons, the levels of BACE1 are increased at the cell surface and reduced in intracellular organelles. These effects are correlated with increased levels of the soluble extracellular domain of BACE1 in the brains of NCAM2-deficient mice, suggesting increased shedding of BACE1 from the cell surface. Of note, shedding of the extracellular domain of Sez6, a protein cleaved exclusively by BACE1, is reduced in NCAM2-deficient animals. These results indicate that the BACE1-generated fragment of NCAM2 regulates BACE1 activity by promoting the targeting of BACE1 to Rab11-positive endosomes.

Gain-of-Function Dynamin-2 Mutations Linked to Centronuclear Myopathy Impair Ca2+-Induced Exocytosis in Human Myoblasts

Gain-of-function mutations of dynamin-2, a mechano-GTPase that remodels membrane and actin filaments, cause centronuclear myopathy (CNM), a congenital disease that mainly affects skeletal muscle tissue. Among these mutations, the variants p.A618T and p.S619L lead to a gain of function and cause a severe neonatal phenotype. By using total internal reflection fluorescence microscopy (TIRFM) in immortalized human myoblasts expressing the pH-sensitive fluorescent protein (pHluorin) fused to the insulin-responsive aminopeptidase IRAP as a reporter of the GLUT4 vesicle trafficking, we measured single pHluorin signals to investigate how p.A618T and p.S619L mutations influence exocytosis. We show here that both dynamin-2 mutations significantly reduced the number and durations of pHluorin signals induced by 10 μM ionomycin, indicating that in addition to impairing exocytosis, they also affect the fusion pore dynamics. These mutations also disrupt the formation of actin filaments, a process that reportedly favors exocytosis. This altered exocytosis might importantly disturb the plasmalemma expression of functional proteins such as the glucose transporter GLUT4 in skeletal muscle cells, impacting the physiology of the skeletal muscle tissue and contributing to the CNM disease.

Endocytosis in the axon initial segment maintains neuronal polarity

Neurons are highly polarized cells that face the fundamental challenge of compartmentalizing a vast and diverse repertoire of proteins in order to function properly1. The axon initial segment (AIS) is a specialized domain that separates a neuron's morphologically, biochemically and functionally distinct axon and dendrite compartments2,3. How the AIS maintains polarity between these compartments is not fully understood. Here we find that in Caenorhabditis elegans, mouse, rat and human neurons, dendritically and axonally polarized transmembrane proteins are recognized by endocytic machinery in the AIS, robustly endocytosed and targeted to late endosomes for degradation. Forcing receptor interaction with the AIS master organizer, ankyrinG, antagonizes receptor endocytosis in the AIS, causes receptor accumulation in the AIS, and leads to polarity deficits with subsequent morphological and behavioural defects. Therefore, endocytic removal of polarized receptors that diffuse into the AIS serves as a membrane-clearance mechanism that is likely to work in conjunction with the known AIS diffusion-barrier mechanism to maintain neuronal polarity on the plasma membrane. Our results reveal a conserved endocytic clearance mechanism in the AIS to maintain neuronal polarity by reinforcing axonal and dendritic compartment membrane boundaries.

Spatial regulation of endosomes in growing dendrites

Many membrane proteins are highly enriched in either dendrites or axons. This non-uniform distribution is a critical feature of neuronal polarity and underlies neuronal function. The molecular mechanisms responsible for polarized distribution of membrane proteins has been studied for some time and many answers have emerged. A less well studied feature of neurons is that organelles are also frequently non-uniformly distributed. For instance, EEA1-positive early endosomes are somatodendritic whereas synaptic vesicles are axonal. In addition, some organelles are present in both axons and dendrites, but not distributed uniformly along the processes. One well known example are lysosomes which are abundant in the soma and proximal dendrite, but sparse in the distal dendrite and the distal axon. The mechanisms that determine the spatial distribution of organelles along dendrites are only starting to be studied. In this review, we will discuss the cell biological mechanisms of how the distribution of diverse sets of endosomes along the proximal-distal axis of dendrites might be regulated. In particular, we will focus on the regulation of bulk homeostatic mechanisms as opposed to local regulation. We posit that immature dendrites regulate organelle motility differently from mature dendrites in order to spatially organize dendrite growth, branching and sculpting.

The role of pectin phase separation in plant cell wall assembly and growth

A rapidly increasing body of literature suggests that many biological processes are driven by phase separation within polymer mixtures. Liquid-liquid phase separation can lead to the formation of membrane-less organelles, which are thought to play a wide variety of roles in cell metabolism, gene regulation or signaling. One of the characteristics of these systems is that they are poised at phase transition boundaries, which makes them perfectly suited to elicit robust cellular responses to often very small changes in the cell's "environment". Recent observations suggest that, also in the semi-solid environment of plant cell walls, phase separation not only plays a role in wall patterning, hydration and stress relaxation during growth, but also may provide a driving force for cell wall expansion. In this context, pectins, the major polyanionic polysaccharides in the walls of growing cells, appear to play a critical role. Here, we will discuss (i) our current understanding of the structure-function relationship of pectins, (ii) in vivo evidence that pectin modification can drive critical phase transitions in the cell wall, (iii) how such phase transitions may drive cell wall expansion in addition to turgor pressure and (iv) the periodic cellular processes that may control phase transitions underlying cell wall assembly and expansion.

Auxiliary Subunits Regulate the Dendritic Turnover of AMPA Receptors in Mouse Hippocampal Neurons

Different families of auxiliary subunits regulate the function and trafficking of native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the central nervous system. While a facilitatory role of auxiliary subunits in ER export and forward trafficking of newly synthesized AMPA receptors is firmly established, it is unclear whether auxiliary subunits also control endosomal receptor turnover in dendrites. Here, we manipulated the composition of AMPA receptor complexes in cultured hippocampal neurons by overexpression of two auxiliary subunits, transmembrane AMPAR regulatory protein (TARP) γ-8 or cysteine knot AMPAR-modulating protein (CKAMP) 44a, and monitored dendritic receptor cycling in live-cell imaging experiments. Receptor surface delivery was assayed using a modified AMPA receptor subunit carrying the pH-dependent fluorophore superecliptic pHluorin (SEP-GluA1), which regains its fluorescence during receptor exocytosis, when transiting from the acidic lumen of transport organelles to the neutral extracellular medium. Strikingly, we observed a dramatic reduction in the spontaneous fusion rate of AMPA receptor-containing organelles in neurons overexpressing either type of auxiliary subunit. An analysis of intracellular receptor distribution also revealed a decreased receptor pool in dendritic recycling endosomes, suggesting that incorporation of TARPγ-8 or CKAMP44a in receptor complexes generally diminishes cycling through the endosomal compartment. To directly analyze dendritic receptor turnover, we also generated a new reporter by N-terminal fusion of a self-labeling HaloTag to an AMPA receptor subunit (HaloTag-GluA1), which allows for selective, irreversible staining of surface receptors. Pulse chase-experiments with HaloTag-GluA1 indeed demonstrated that overexpression of TARPγ-8 or CKAMP44a reduces the constitutive internalization rate of surface receptors at extrasynaptic but not synaptic sites. Thus, our data point to a yet unrecognized regulatory function of TARPγ-8 and CKAMP44a, by which these structurally unrelated auxiliary subunits delay local recycling and increase surface lifetime of extrasynaptic AMPA receptors.

The vSNAREs VAMP2 and VAMP4 control recycling and intracellular sorting of post-synaptic receptors in neuronal dendrites

The endosomal recycling system dynamically tunes synaptic strength, which underlies synaptic plasticity. Exocytosis is involved in the expression of long-term potentiation (LTP), as postsynaptic cleavage of the SNARE (soluble NSF-attachment protein receptor) protein VAMP2 by tetanus toxin blocks LTP. Moreover, induction of LTP increases the exocytosis of transferrin receptors (TfRs) and markers of recycling endosomes (REs), as well as post-synaptic AMPA type receptors (AMPARs). However, the interplay between AMPAR and TfR exocytosis remains unclear. Here, we identify VAMP4 as the vesicular SNARE that mediates most dendritic RE exocytosis. In contrast, VAMP2 plays a minor role in RE exocytosis. LTP induction increases the exocytosis of both VAMP2- and VAMP4-labeled organelles. Knock down (KD) of VAMP4 decreases TfR recycling but increases AMPAR recycling. Moreover, VAMP4 KD increases AMPAR-mediated synaptic transmission, which consequently occludes LTP expression. The opposing changes in AMPAR and TfR recycling upon VAMP4 KD reveal their sorting into separate endosomal populations.

NMDAR-dependent long-term depression is associated with increased short term plasticity through autophagy mediated loss of PSD-95

Long-term depression (LTD) of synaptic strength can take multiple forms and contribute to circuit remodeling, memory encoding or erasure. The generic term LTD encompasses various induction pathways, including activation of NMDA, mGlu or P2X receptors. However, the associated specific molecular mechanisms and effects on synaptic physiology are still unclear. We here compare how NMDAR- or P2XR-dependent LTD affect synaptic nanoscale organization and function in rodents. While both LTDs are associated with a loss and reorganization of synaptic AMPARs, only NMDAR-dependent LTD induction triggers a profound reorganization of PSD-95. This modification, which requires the autophagy machinery to remove the T19-phosphorylated form of PSD-95 from synapses, leads to an increase in AMPAR surface mobility. We demonstrate that these post-synaptic changes that occur specifically during NMDAR-dependent LTD result in an increased short-term plasticity improving neuronal responsiveness of depressed synapses. Our results establish that P2XR- and NMDAR-mediated LTD are associated to functionally distinct forms of LTD.

Imaging endocytic vesicle formation at high spatial and temporal resolutions with the pulsed-pH protocol

Endocytosis is a fundamental process occurring in all eukaryotic cells. Live cell imaging of endocytosis has helped to decipher many of its mechanisms and regulations. With the pulsed-pH (ppH) protocol, one can detect the formation of individual endocytic vesicles (EVs) with an unmatched temporal resolution of 2 s. The ppH protocol makes use of cargo protein (e.g., the transferrin receptor) coupled to a pH-sensitive fluorescent protein, such as superecliptic pHluorin (SEP), which is brightly fluorescent at pH 7.4 but not fluorescent at pH <6.0. If the SEP moiety is at the surface, its fluorescence will decrease when cells are exposed to a low pH (5.5) buffer. If the SEP moiety has been internalized, SEP will remain fluorescent even during application of the low pH buffer. Fast perfusion enables the complete exchange of low and high pH extracellular solutions every 2 s, defining the temporal resolution of the technique. Unlike other imaging-based endocytosis assays, the ppH protocol detects EVs without a priori hypotheses on the dynamics of vesicle formation. Here, we explain how the ppH protocol quantifies the endocytic activity of living cells and the recruitment of associated proteins in real time. We provide a step-by-step procedure for expression of the reporter proteins with transient transfection, live cell image acquisition with synchronized pH changes and automated analysis. The whole protocol can be performed in 2 d to provide quantitative information on the endocytic process being studied.

News about non-secretory exocytosis: mechanisms, properties, and functions

The fusion by exocytosis of many vesicles to the plasma membrane induces the discharge to the extracellular space of their abundant luminal cargoes. Other exocytic vesicles, however, do not contain cargoes, and thus, their fusion is not followed by secretion. Therefore, two distinct processes of exocytosis exist, one secretory and the other non-secretory. The present review deals with the knowledge of non-secretory exocytosis developed during recent years. Among such developments are the dual generation of the exocytic vesicles, initially released either from the trans-Golgi network or by endocytosis; their traffic with activation of receptors, channels, pumps, and transporters; the identification of their tethering and soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes that govern membrane fusions; the growth of axons and the membrane repair. Examples of potential relevance of these processes for pathology and medicine are also reported. The developments presented here offer interesting chances for future progress in the field.

A Discrete Presynaptic Vesicle Cycle for Neuromodulator Receptors

A major function of GPCRs is to inhibit presynaptic neurotransmitter release, requiring ligand-activated receptors to couple locally to effectors at terminals. The current understanding of how this is achieved is through receptor immobilization on the terminal surface. Here, we show that opioid peptide receptors, GPCRs that mediate highly sensitive presynaptic inhibition, are instead dynamic in axons. Opioid receptors diffuse rapidly throughout the axon surface and internalize after ligand-induced activation specifically at presynaptic terminals. We delineate a parallel regulated endocytic cycle for GPCRs operating at the presynapse, separately from the synaptic vesicle cycle, which clears activated receptors from the surface of terminals and locally reinserts them to maintain the diffusible surface pool. We propose an alternate strategy for achieving local control of presynaptic effectors that, opposite to using receptor immobilization and enforced proximity, is based on lateral mobility of receptors and leverages the inherent allostery of GPCR-effector coupling.

Listeria monocytogenes exploits host exocytosis to promote cell-to-cell spread

The facultative intracellular pathogen Listeria monocytogenes uses an actin-based motility process to spread within human tissues. Filamentous actin from the human cell forms a tail behind bacteria, propelling microbes through the cytoplasm. Motile bacteria remodel the host plasma membrane into protrusions that are internalized by neighboring cells. A critical unresolved question is whether generation of protrusions by Listeria involves stimulation of host processes apart from actin polymerization. Here we demonstrate that efficient protrusion formation in polarized epithelial cells involves bacterial subversion of host exocytosis. Confocal microscopy imaging indicated that exocytosis is up-regulated in protrusions of Listeria in a manner that depends on the host exocyst complex. Depletion of components of the exocyst complex by RNA interference inhibited the formation of Listeria protrusions and subsequent cell-to-cell spread of bacteria. Additional genetic studies indicated important roles for the exocyst regulators Rab8 and Rab11 in bacterial protrusion formation and spread. The secreted Listeria virulence factor InlC associated with the exocyst component Exo70 and mediated the recruitment of Exo70 to bacterial protrusions. Depletion of exocyst proteins reduced the length of Listeria protrusions, suggesting that the exocyst complex promotes protrusion elongation. Collectively, these results demonstrate that Listeria exploits host exocytosis to stimulate intercellular spread of bacteria.

From cells to sensations: A window to the physics of mind

Principles of methods for studying particles and fields that cannot be sensed by third-person observers by routine methods can be used to understand the physics of first-person properties of mind. Accordingly, whenever a system exhibits disparate features at multiple levels, unique combination of constraints offered by them direct us towards a solution that will be the first principle of that system. Using this method, it was possible to arrive at a third-person observable solution-point of brain-mind interface. Examination of this location identified a set of unique features that can allow an associatively learned (cue) stimulus to spark hallucinations that form units of first-person internal (inner) sensations reminiscent of stimuli from the associatively learned second item in timescales of milliseconds. It allows us to peep into a virtual space of mind where different modifications and integrations of units of internal sensations generate their different net conformations ranging from perception to an inner sense of hidden relationships that form a hypothesis. Since sparking of inner sensations of the late arriving (when far away) or non-arriving (when hidden) features of items started providing survival advantage, the focus of evolution might have been to optimize this property. Hence, the circuity that generates it can be considered as the primary circuitry of the system. The solution provides several testable predictions. By taking readers through the process of deriving the solution and by explaining how it interconnects disparate findings, it is hoped that the factors determining the physics of mind will become evident.

Visualization of Exo- and Endocytosis of AMPA Receptors During Hippocampal Synaptic Plasticity Around Postsynaptic-Like Membrane Formed on Glass Surface

Regulation of exo- and endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor (AMPAR) plays a critical role in the expression of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) at excitatory central synapses. Enhanced AMPAR exocytosis or endocytosis has been suggested to contribute to LTP or LTD, respectively. However, several unsettled fundamental questions have remained about AMPAR exo- and endocytosis in the basal condition and during synaptic plasticity: (1) Does the size of each exo- or endocytosis event, and/or do the frequencies of these events change during LTP or LTD? If they change, what are the time courses of the respective changes? (2) Where does the exo- or endocytosis preferentially occur in each condition: inside or in the vicinity of postsynaptic membrane, or in the extrasynaptic membrane? (3) Do different types of AMPAR, such as GluA1 homo-tetramer, GluA1/2 hetero-tetramer and GluA2/3 hetero-tetramer, show distinct exo- and endocytosis changes? To address these questions, we developed new methods to observe individual events of AMPAR exo- or endocytosis with a high signal to noise (SN) ratio in a culture preparation using total internal reflection fluorescence microscopy (TIRFM). In these studies, hippocampal neurons were cultured on a neurexin (NRX)-coated glass coverslip, which induced formation of postsynaptic-like membrane (PSLM) directly on the glass surface. Then, a super-ecliptic pHluorin (SEP)-tagged AMPAR subunit such as GluA1 (GluA1-SEP) was expressed in neurons and its fluorescence changes during LTP induced by high frequency electrical field stimulation were observed with TIRFM, which showed different time courses of exocytosis changes of GluA1-, GluA2-, or GluA3-SEP in and around PSLM. In addition, a new method to detect individual endocytosis events of AMPAR was developed by combining TIFRM observation of GluA-SEP around PSLM with a rapid extracellular pH exchange method using a U-tube. Recent results on exo- and endocytosis changes of GluA-SEP during N-methyl-D-aspartate (NMDA)-induced LTD suggested that suppression of AMPAR exocytosis rather than enhancement of AMPAR endocytosis primarily contributes to LTD expression, although the NMDA application transiently enhances clathrin-dependent endocytosis of GluA1-containing AMPAR.

Genetically encoded fluorescent indicators for live cell pH imaging

BACKGROUND

Intracellular pH underlies most cellular processes. There is emerging evidence of a pH-signaling role in plant cells and microorganisms. Dysregulation of pH is associated with human diseases, such as cancer and Alzheimer's disease.

SCOPE OF REVIEW

In this review, we attempt to provide a summary of the progress that has been made in the field during the past two decades. First, we present an overview of the current state of the design and applications of fluorescent protein (FP)-based pH indicators. Then, we turn our attention to the development and applications of hybrid pH sensors that combine the capabilities of non-GFP fluorophores with the advantages of genetically encoded tags. Finally, we discuss recent advances in multicolor pH imaging and the applications of genetically encoded pH sensors in multiparameter imaging.

MAJOR CONCLUSIONS

Genetically encoded pH sensors have proven to be indispensable noninvasive tools for selective targeting to different cellular locations. Although a variety of genetically encoded pH sensors have been designed and applied at the single cell level, there is still much room for improvements and future developments of novel powerful tools for pH imaging. Among the most pressing challenges in this area is the design of brighter redshifted sensors for tissue research and whole animal experiments.

GENERAL SIGNIFICANCE

The design of precise pH measuring instruments is one of the important goals in cell biochemistry and may give rise to the development of new powerful diagnostic tools for various diseases.

Recent Findings on AMPA Receptor Recycling

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPA-Rs) are tetrameric protein complexes that mediate most of the fast-excitatory transmission in response to the neurotransmitter glutamate in neurons. The abundance of AMPA-Rs at the surface of excitatory synapses establishes the strength of the response to glutamate. It is thus evident that neurons need to tightly regulate this feature, particularly in the context of all synaptic plasticity events, which are considered the biological correlates of higher cognitive functions such as learning and memory. AMPA-R levels at the synapse are regulated by insertion of newly synthesized receptors, lateral diffusion on the plasma membrane and endosomal cycling. The latter is likely the most important especially for synaptic plasticity. This process starts with the endocytosis of the receptor from the cell surface and is followed by either degradation, if the receptor is directed to the lysosomal compartment, or reinsertion at the cell surface through a specialized endosomal compartment called recycling endosomes. Although the basic steps of this process have been discovered, the details and participation of additional regulatory proteins are still being discovered. In this review article, we describe the most recent findings shedding light on this crucial mechanism of synaptic regulation.

L-Type Voltage-Gated Ca2+ Channels Regulate Synaptic-Activity-Triggered Recycling Endosome Fusion in Neuronal Dendrites

The repertoire and abundance of proteins displayed on the surface of neuronal dendrites are tuned by regulated fusion of recycling endosomes (REs) with the dendritic plasma membrane. While this process is critical for neuronal function and plasticity, how synaptic activity drives RE fusion remains unexplored. We demonstrate a multistep fusion mechanism that requires Ca²⁺ from distinct sources. NMDA receptor Ca²⁺ initiates RE fusion with the plasma membrane, while L-type voltage-gated Ca²⁺ channels (L-VGCCs) regulate whether fused REs collapse into the membrane or reform without transferring their cargo to the cell surface. Accordingly, NMDA receptor activation triggered AMPA-type glutamate receptor trafficking to the dendritic surface in an L-VGCC-dependent manner. Conversely, potentiating L-VGCCs enhanced AMPA receptor surface expression only when NMDA receptors were also active. Thus L-VGCCs play a role in tuning activity-triggered surface expression of key synaptic proteins by gating the mode of RE fusion.

Integration of GPCR Signaling and Sorting from Very Early Endosomes via Opposing APPL1 Mechanisms

Endocytic trafficking is a critical mechanism for cells to decode complex signaling pathways, including those activated by G-protein-coupled receptors (GPCRs). Heterogeneity in the endosomal network enables GPCR activity to be spatially restricted between early endosomes (EEs) and the recently discovered endosomal compartment, the very early endosome (VEE). However, the molecular machinery driving GPCR activity from the VEE is unknown. Using luteinizing hormone receptor (LHR) as a prototype GPCR for this compartment, along with additional VEE-localized GPCRs, we identify a role for the adaptor protein APPL1 in rapid recycling and endosomal cAMP signaling without impacting the EE-localized β2-adrenergic receptor. LHR recycling is driven by receptor-mediated Gαs/cAMP signaling from the VEE and PKA-dependent phosphorylation of APPL1 at serine 410. Receptor/Gαs endosomal signaling is localized to microdomains of heterogeneous VEE populations and regulated by APPL1 phosphorylation. Our study uncovers a highly integrated inter-endosomal communication system enabling cells to tightly regulate spatially encoded signaling.

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GPCRs that internalize to very early endosomes (VEEs) require APPL1 to recycle

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Receptor recycling is driven by cAMP/PKA to phosphorylate serine 410 on APPL1

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cAMP signaling from GPCRs, such as LHR, occurs from distinct VEE microdomains

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APPL1 limits VEE cAMP signaling via opposing mechanisms required for GPCR sorting

RCAN1 Knockdown Reverts Defects in the Number of Calcium-Induced Exocytotic Events in a Cellular Model of Down Syndrome

In humans, Down Syndrome (DS) is a condition caused by partial or full trisomy of chromosome 21. Genes present in the DS critical region can result in excess gene dosage, which at least partially can account for DS phenotype. Although regulator of calcineurin 1 (RCAN1) belongs to this region and its ectopic overexpression in neurons impairs transmitter release, synaptic plasticity, learning and memory, the relative contribution of RCAN1 in a context of DS has yet to be clarified. In the present work, we utilized an in vitro model of DS, the CTb neuronal cell line derived from the brain cortex of a trisomy 16 (Ts16) fetal mouse, which reportedly exhibits acetylcholine release impairments compared to CNh cells (a neuronal cell line established from a normal littermate). We analyzed single exocytotic events by using total internal reflection fluorescence microscopy (TIRFM) and the vesicular acetylcholine transporter fused to the pH-sensitive green fluorescent protein (VAChT-pHluorin) as a reporter. Our analyses showed that, compared with control CNh cells, the trisomic CTb cells overexpress RCAN1, and they display a reduced number of Ca²⁺-induced exocytotic events. Remarkably, RCAN1 knockdown increases the extent of exocytosis at levels comparable to those of CNh cells. These results support a critical contribution of RCAN1 to the exocytosis process in the trisomic condition.

Suppression of AMPA Receptor Exocytosis Contributes to Hippocampal LTD

The decrease in number of AMPA-type glutamate receptor (AMPAR) at excitatory synapses causes LTD, a cellular basis of learning and memory. The number of postsynaptic AMPARs is regulated by the balance of exocytosis and endocytosis, and enhanced endocytosis of AMPAR has been suggested to underlie the LTD expression. However, it remains unclear how endocytosis and exocytosis of AMPAR change during LTD. In this study, we addressed this question by analyzing exocytosis and endocytosis of AMPAR by imaging super-ecliptic pHlorin (SEP)-tagged AMPAR around postsynaptic structure formed directly on the glass surface in the hippocampal culture prepared from rat embryos of both sexes. Contrary to a prevailing view on the LTD expression by endocytosis enhancement, the LTD induction by NMDA application only transiently enhanced endocytosis of SEP-tagged GluA1 subunits of AMPAR, which was counteracted by simultaneous augmentation of exocytosis. As a result, soon after the start of the LTD induction (∼1 min), the surface AMPAR did not markedly decrease. Thereafter, the surface GluA1-SEP gradually decreased (2-5 min) and kept at a low level until the end of observation (>30 min). Surprisingly, this gradual and sustained decrease of surface AMPAR was accompanied not by the enhanced endocytic events of GluA1, but by the suppression of exocytosis. Together, our data highlight an unprecedented mechanism for the LTD expression by attenuation of exocytosis of AMPAR, but not by enhanced endocytosis, together with a reduction of postsynaptic AMPAR scaffolding protein PSD95.SIGNIFICANCE STATEMENT It has been generally assumed that LTD is expressed by enhancement of AMPAR endocytosis. Previous studies reported that endocytosis-related protein was involved in LTD and that significant amount of cell-surface AMPAR moved into intracellular compartments during LTD. Here, we report changes of cell-surface amount of AMPAR, and where and when individual exocytosis and endocytosis occurred during LTD. Cell-surface AMPAR gradually decreased in synchrony with suppression of exocytosis but not with enhancement of endocytosis. These results suggest that the decrease of cell-surface AMPAR amount during LTD was caused not by enhancement of endocytosis but rather by suppression of exocytosis, which revises current understanding of the expression mechanism of LTD.

α-Synuclein promotes dilation of the exocytotic fusion pore

The protein α-synuclein has a central role in the pathogenesis of Parkinson’s disease (PD). Similar to other proteins that accumulate in neurodegenerative disease, however, the function of α-synuclein remains unknown. Localization to the nerve terminal suggests a role in neurotransmitter release and over-expression inhibits regulated exocytosis, but previous work has failed to identify a clear physiological defect in mice lacking all three synuclein isoforms. Using adrenal chromaffin cells and neurons, we now find that both over-expressed and endogenous synuclein serve to accelerate the kinetics of individual exocytotic events, promoting cargo discharge and reducing pore closure (‘kiss-and-run’). Thus, synuclein exerts dose-dependent effects on dilation of the exocytotic fusion pore. Remarkably, mutations that cause PD abrogate this property of α-synuclein without impairing its ability to inhibit exocytosis when over-expressed, indicating a selective defect in normal function.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

Synaptic Vesicle Exocytosis at the Dendritic Lobules of an Inhibitory Interneuron in the Mammalian Retina

Ribbon synapses convey sustained and phasic excitatory drive within retinal microcircuits. However, the properties of retinal inhibitory synapses are less well known. AII-amacrine cells are interneurons in the retina that exhibit large glycinergic synapses at their dendritic lobular appendages. Using membrane capacitance measurements, we observe robust exocytosis elicited by the opening of L-type Ca(2+) channels located on the lobular appendages. Two pools of synaptic vesicles were detected: a small, rapidly releasable pool and a larger and more slowly releasable pool. Depending on the stimulus, either paired-pulse depression or facilitation could be elicited. During early postnatal maturation, the coupling of the exocytosis Ca(2+)-sensor to Ca(2+) channel becomes tighter. Light-evoked depolarizations of the AII-amacrine cell elicited exocytosis that was graded to light intensity. Our results suggest that AII-amacrine cell synapses are capable of providing both phasic and sustained inhibitory input to their postsynaptic partners without the benefit of synaptic ribbons.

Studying endosomes in cultured neurons by live-cell imaging

Endosomes play critical roles on regulating surface receptor levels as well as signaling cascades in all cell types, including neurons. Endocytosis and endosomal trafficking is routinely studied after fixation, but live imaging is increasingly being used to capture the dynamic nature of endosomes and is allowing increasingly sophisticated glimpses into trafficking processes in live neurons. In this chapter, we describe the basics of neuronal primary cultures, methods for expressing fluorescent proteins, and live imaging of cargos and endosomal regulators.

Minireview: Spatial Programming of G Protein-Coupled Receptor Activity: Decoding Signaling in Health and Disease

Probing the multiplicity of hormone signaling via G protein-coupled receptors (GPCRs) has demonstrated the complex signal pathways that underlie the multiple functions these receptors play in vivo. This is highly pertinent for the GPCRs key in reproduction and pregnancy that are exposed to cyclical and dynamic changes in their extracellular milieu. How such functional pleiotropy in GPCR signaling is translated to specific downstream cellular responses, however, is largely unknown. Emerging data strongly support mechanisms for a central role of receptor location in signal regulation via membrane trafficking. In this review, we discuss current progress in our understanding of the role membrane trafficking plays in location control of GPCR signaling, from organized plasma membrane signaling microdomains, potentially provided by both distinct endocytic and exocytic pathways, to more recent evidence for spatial control within the endomembrane system. Application of these emerging mechanisms in their relevance to GPCR activity in physiological and pathophysiological conditions will also be discussed, and in improving therapeutic strategies that exploits these mechanisms in order to program highly regulated and distinct signaling profiles.

Survey of Red Fluorescence Proteins as Markers for Secretory Granule Exocytosis

Fluorescent proteins (FPs) have proven to be valuable tools for high-resolution imaging studies of vesicle transport processes, including exo- and endocytosis. Since the pH of the vesicle lumen changes between acidic and neutral during these events, pH-sensitive FPs with near neutral pKa, such as pHluorin, are particularly useful. FPs with pKa>6 are readily available in the green spectrum, while red-emitting pH-sensitive FPs are rare and often not well characterized as reporters of exo- or endocytosis. Here we tested a panel of ten orange/red and two green FPs in fusions with neuropeptide Y (NPY) for use as secreted vesicle marker and reporter of dense core granule exocytosis and release. We report relative brightness, bleaching rate, targeting accuracy, sensitivity to vesicle pH, and their performance in detecting exocytosis in live cells. Tandem dimer (td)-mOrange2 was identified as well-targeted, bright, slowly bleaching and pH-sensitive FP that performed similar to EGFP. Single exocytosis events were readily observed, which allowed measurements of fusion pore lifetime and the dynamics of the exocytosis protein syntaxin at the release site during membrane fusion and cargo release.

Surface Trafficking of APP and BACE in Live Cells

Amyloid‐β (Aβ)‐peptide, the major constituent of the plaques that develop during Alzheimer's disease, is generated via the cleavage of Aβ precursor protein (APP) by β‐site APP‐cleaving enzyme (BACE). Using live‐cell imaging of APP and BACE labeled with pH‐sensitive proteins, we could detect the release events of APP and BACE and their distinct kinetics. We provide kinetic evidence for the cleavage of APP by α‐secretase on the cellular surface after exocytosis. Furthermore, simultaneous dual‐color evanescent field illumination revealed that the two proteins are trafficked to the surface in separate compartments. Perturbing the membrane lipid composition resulted in a reduced frequency of exocytosis and affected BACE more strongly than APP. We propose that surface fusion frequency is a key factor regulating the aggregation of APP and BACE in the same membrane compartment and that this process can be modulated via pharmacological intervention.

The role of AMPA receptors in postsynaptic mechanisms of synaptic plasticity

In the mammalian central nervous system, excitatory glutamatergic synapses harness neurotransmission that is mediated by ion flow through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs, which are enriched in the postsynaptic membrane on dendritic spines, are highly dynamic, and shuttle in and out of synapses in an activity-dependent manner. Changes in their number, subunit composition, phosphorylation state, and accessory proteins can all regulate AMPARs and thus modify synaptic strength and support cellular forms of learning. Furthermore, dysregulation of AMPAR plasticity has been implicated in various pathological states and has important consequences for mental health. Here we focus on the mechanisms that control AMPAR plasticity, drawing particularly from the extensive studies on hippocampal synapses, and highlight recent advances in the field along with considerations for future directions.

pHuji, a pH-sensitive red fluorescent protein for imaging of exo- and endocytosis

Fluorescent proteins with pH-sensitive fluorescence are valuable tools for the imaging of exocytosis and endocytosis. The Aequorea green fluorescent protein mutant superecliptic pHluorin (SEP) is particularly well suited to these applications. Here we describe pHuji, a red fluorescent protein with a pH sensitivity that approaches that of SEP, making it amenable for detection of single exocytosis and endocytosis events. To demonstrate the utility of the pHuji plus SEP pair, we perform simultaneous two-color imaging of clathrin-mediated internalization of both the transferrin receptor and the β2 adrenergic receptor. These experiments reveal that the two receptors are differentially sorted at the time of endocytic vesicle formation.

Imaging of kiss-and-run exocytosis of surface receptors in neuronal cultures

Transmembrane proteins are continuously shuttled from the endosomal compartment to the neuronal plasma membrane by highly regulated and complex trafficking steps. These events are involved in many homeostatic and physiological processes such as neuronal growth, signaling, learning and memory among others. We have previously shown that endosomal exocytosis of the B2 adrenergic receptor (B2AR) and the GluR1-containing AMPA receptor to the neuronal plasma membrane is mediated by two different types of vesicular fusion. A rapid type of exocytosis in which receptors are delivered to the plasma membrane in a single kinetic step, and a persistent mode in which receptors remain clustered at the insertion site for a variable period of time before delivery to the cell surface. Here, by comparing the exocytosis of multiple receptors in dissociated hippocampal and striatal cultures, we show that persistent events are a general mechanism of vesicular delivery. Persistent events were only observed after 10 days in vitro, and their frequency increased with use of the calcium ionophore A23187 and with depolarization induced by KCl. Finally, we determined that vesicles producing persistent events remain at the plasma membrane, closing and reopening their fusion pore for a consecutive release of cargo in a mechanism reminiscent of synaptic kiss-and-run. These results indicate that the delivery of transmembrane receptors to the cell surface can be dynamically regulated by kiss-and-run exocytosis.