Dynamin triple knockout cells reveal off target effects of commonly used dynamin inhibitors

Arrestin-Dependent and -Independent Internalization of G Protein-Coupled Receptors: Methods, Mechanisms, and Implications on Cell Signaling

Agonist-induced endocytosis is a key regulatory mechanism for controlling the responsiveness of the cell by changing the density of cell surface receptors. In addition to the role of endocytosis in signal termination, endocytosed G protein-coupled receptors (GPCRs) have been found to signal from intracellular compartments of the cell. Arrestins are generally believed to be the master regulators of GPCR endocytosis by binding to both phosphorylated receptors and adaptor protein 2 (AP-2) or clathrin, thus recruiting receptors to clathrin-coated pits to facilitate the internalization process. However, many other functions have been described for arrestins that do not relate to their role in terminating signaling. Additionally, there are now more than 30 examples of GPCRs that internalize independently of arrestins. Here we review the methods, pharmacological tools, and cellular backgrounds used to determine the role of arrestins in receptor internalization, highlighting their advantages and caveats. We also summarize key examples of arrestin-independent GPCR endocytosis in the literature and their suggested alternative endocytosis pathway (e.g., the caveolae-dependent and fast endophilin-mediated endocytosis pathways). Finally, we consider the possible function of arrestins recruited to GPCRs that are endocytosed independently of arrestins, including the catalytic arrestin activation paradigm. Technological improvements in recent years have advanced the field further, and, combined with the important implications of endocytosis on drug responses, this makes endocytosis an obvious parameter to include in molecular pharmacological characterization of ligand-GPCR interactions. SIGNIFICANCE STATEMENT: G protein-coupled receptor (GPCR) endocytosis is an important means to terminate receptor signaling, and arrestins play a central role in the widely accepted classical paradigm of GPCR endocytosis. In contrast to the canonical arrestin-mediated internalization, an increasing number of GPCRs are found to be endocytosed via alternate pathways, and the process appears more diverse than the previously defined "one pathway fits all."

Wbox2: A clathrin terminal domain-derived peptide inhibitor of clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) occurs via the formation of clathrin-coated vesicles from clathrin-coated pits (CCPs). Clathrin is recruited to CCPs through interactions between the AP2 complex and its N-terminal domain, which in turn recruits endocytic accessory proteins. Inhibitors of CME that interfere with clathrin function have been described, but their specificity and mechanisms of action are unclear. Here we show that overexpression of the N-terminal domain with (TDD) or without (TD) the distal leg inhibits CME and CCP dynamics by perturbing clathrin interactions with AP2 and SNX9. TDD overexpression does not affect clathrin-independent endocytosis or, surprisingly, AP1-dependent lysosomal trafficking from the Golgi. We designed small membrane–permeant peptides that encode key functional residues within the four known binding sites on the TD. One peptide, Wbox2, encoding residues along the W-box motif binding surface, binds to SNX9 and AP2 and potently and acutely inhibits CME.

VPS34 regulates dynamin to determine the endocytosis of mitochondria-targeted zinc oxide nanoparticles in human osteosarcoma cells

In our previous study, zinc oxide nanoparticles (ZnO NPs) presented satisfying therapeutic effects with cancer cell selectivity in osteosarcoma cells and, thus, have been considered as a potential nanomedicine for human osteosarcoma treatment. However, the poorly investigated internalization process, including their endocytic pathway into tumor cells and intracellular fate, limits the clinical application. Here, we further clarified these aspects. First, ZnO NPs were rapidly internalized by osteosarcoma cells and accumulated in mitochondria, before being entrapped into lysosomes. Second, dynasore (a dynamin inhibitor) was demonstrated to be the most effective in blocking ZnO NP uptake and rescuing ZnO NP-induced osteosarcoma cell autophagic death and apoptosis. Third, we confirmed the key role of dynamin 2 in ZnO NP endocytosis and subsequent autophagic cell death in vitro and in vivo. Furthermore, we proved that VPS34 transferred from cell cytoplasm to cell membrane to interact with dynamin under ZnO NP treatment. Altogether, combined with our previous study, the current research further revealed that ZnO NPs entered human osteosarcoma cells through the VPS34/dynamin 2-dependent endocytic pathway, directly targeting and damaging the mitochondria before being entrapped into the lysosomes, thereby initiating mitophagy-Zn²⁺-reactive oxygen species-mitophagy axis mediated cell apoptosis.

Extracellular MIF, but not its homologue D-DT, promotes fibroblast motility independently of its receptor complex CD74/CD44

Macrophage migration inhibitory factor (MIF) and its homologue D-dopachrome tautomerase (D-DT) are widely expressed pro-inflammatory cytokines with chemokine-like functions that coordinate a wide spectrum of biological activities, such as migration. Here, we biotin-tagged intracellular MIF/D-DT in vivo to identify important cytosolic interactors and found a plethora of actin cytoskeleton-associated proteins. Although the receptor complex between CD74 and CD44 (CD74/CD44) is essential for signalling transduction in fibroblasts via extracellular MIF/D-DT, our interactome data suggested direct effects. We, thus, investigated whether MIF/D-DT can modulate cell migration independently of CD74/CD44. To distinguish between receptor- and non-receptor-mediated motility, we used fibroblasts that are either deficient or that express CD74/CD44 proteins, and treated them with recombinant MIF/D-DT. Interestingly, only MIF could stimulate chemokinesis in the presence or absence of CD74/CD44. The pro-migratory effects of MIF depended on lipid raft/caveolae-mediated but not clathrin-mediated endocytosis, on its tautomerase activity and, probably, on its thiol protein oxidoreductase activity. As MIF treatment restrained actin polymerisation in vitro, our findings establish a new intracellular role for MIF/D-DT in driving cell motility through modulation of the actin cytoskeleton.

SARS-CoV-2 infects cells after viral entry via clathrin-mediated endocytosis

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, so understanding its biology and infection mechanisms is critical to facing this major medical challenge. SARS-CoV-2 is known to use its spike glycoprotein to interact with the cell surface as a first step in the infection process. As for other coronaviruses, it is likely that SARS-CoV-2 next undergoes endocytosis, but whether or not this is required for infectivity and the precise endocytic mechanism used are unknown. Using purified spike glycoprotein and lentivirus pseudotyped with spike glycoprotein, a common model of SARS-CoV-2 infectivity, we now demonstrate that after engagement with the plasma membrane, SARS-CoV-2 undergoes rapid, clathrin-mediated endocytosis. This suggests that transfer of viral RNA to the cell cytosol occurs from the lumen of the endosomal system. Importantly, we further demonstrate that knockdown of clathrin heavy chain, which blocks clathrin-mediated endocytosis, reduces viral infectivity. These discoveries reveal that SARS-CoV-2 uses clathrin-mediated endocytosis to gain access into cells and suggests that this process is a key aspect of virus infectivity.

Scavenger receptor MARCO contributes to cellular internalization of exosomes by dynamin-dependent endocytosis and macropinocytosis

Macrophage receptor with collagenous structure (MARCO) is a scavenger receptor class-A protein that is expressed on the cell surface of macrophages. MARCO mediates binding and ingestion of unopsonized environmental particles, including nano-sized materials. Exosomes are cell-derived, nano-sized vesicles (40–150 nm) that can contain lipids, RNA, DNA, and various proteins. Exosomes play an essential role in cell-to-cell communication via body fluids. However, mechanisms for the recognition and internalization of exosomes by recipient cells remain poorly characterized. In this study, cellular association of serum-derived fluorescent exosomes and 20-nm fluorescent nanoparticles (positive control) was compared between MARCO-expressing (CHO-MARCO) and control (CHO-CT) CHO-K1 cells to examine whether MARCO expression by recipient cells mediates the cellular uptake of exosomes and environmental nanoparticles. Fluorescence microscopic studies and quantitative analyses revealed that the cellular associations of both exosomes and 20-nm nanoparticles were greater in CHO-MARCO cells than in CHO-CT cells. Exosomes and nanoparticles colocalized with green fluorescent protein (GFP)-MARCO in cells transfected with GFP-MARCO-encoding constructs . Furthermore, inhibitory studies showed that actin reorganization and dynamin are involved in the MARCO-mediated cellular internalization of exosomes. These results indicated that MARCO plays a role in the uptake of exosomes.

Dynasore protects ocular surface mucosal epithelia subjected to oxidative stress by maintaining UPR and calcium homeostasis

The mucosal epithelia of the ocular surface protect against external threats to the eye. Using a model of human stratified corneal epithelial cells with mucosal differentiation, we previously demonstrated that a small molecule inhibitor of dynamin GTPases, dynasore, prevents damage to cells and their transcellular barriers when subjected to oxidative stress. Investigating mechanisms, we now report the novel finding that dynasore acts by maintaining Ca+2 homeostasis, thereby inhibiting the PERK branch of the unfolded protein response (UPR) that promotes cell death. Dynasore was found to protect mitochondria by preventing mitochondrial permeability transition pore opening (mPTP), but, unlike reports using other systems, this was not mediated by dynamin family member DRP1. Necrostatin-1, an inhibitor of RIPK1 and lytic forms of programmed cell death, also inhibited mPTP opening and further protected the plasma membrane barrier. Significantly, necrostatin-1 did not protect the mucosal barrier. Oxidative stress increased mRNA for sXBP1, a marker of the IRE1 branch of the UPR, and CHOP, a marker of the PERK branch. It also stimulated phosphorylation of eIF2α, the upstream regulator of CHOP, as well as an increase in intracellular Ca2+. Dynasore selectively inhibited the increase in PERK branch markers, and also prevented the increase intracellular Ca2+ in response to oxidative stress. The increase in PERK branch markers were also inhibited when cells were treated with the cell permeable Ca2+ chelator, BAPTA-AM. To our knowledge, this is the first time that dynasore has been shown to have an effect on the UPR and suggests therapeutic applications.

Mammalian Homologue NME3 of DYNAMO1 Regulates Peroxisome Division

Peroxisomes proliferate by sequential processes comprising elongation, constriction, and scission of peroxisomal membrane. It is known that the constriction step is mediated by a GTPase named dynamin-like protein 1 (DLP1) upon efficient loading of GTP. However, mechanism of fuelling GTP to DLP1 remains unknown in mammals. We earlier show that nucleoside diphosphate (NDP) kinase-like protein, termed dynamin-based ring motive-force organizer 1 (DYNAMO1), generates GTP for DLP1 in a red alga, Cyanidioschyzon merolae. In the present study, we identified that nucleoside diphosphate kinase 3 (NME3), a mammalian homologue of DYNAMO1, localizes to peroxisomes. Elongated peroxisomes were observed in cells with suppressed expression of NME3 and fibroblasts from a patient lacking NME3 due to the homozygous mutation at the initiation codon of NME3. Peroxisomes proliferated by elevation of NME3 upon silencing the expression of ATPase family AAA domain containing 1, ATAD1. In the wild-type cells expressing catalytically-inactive NME3, peroxisomes were elongated. These results suggest that NME3 plays an important role in peroxisome division in a manner dependent on its NDP kinase activity. Moreover, the impairment of peroxisome division reduces the level of ether-linked glycerophospholipids, ethanolamine plasmalogens, implying the physiological importance of regulation of peroxisome morphology.

β-Coronaviruses Use Lysosomes for Egress Instead of the Biosynthetic Secretory Pathway

β-Coronaviruses are a family of positive-strand enveloped RNA viruses that includes the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Much is known regarding their cellular entry and replication pathways, but their mode of egress remains uncertain. Using imaging methodologies and virus-specific reporters, we demonstrate that β-coronaviruses utilize lysosomal trafficking for egress rather than the biosynthetic secretory pathway more commonly used by other enveloped viruses. This unconventional egress is regulated by the Arf-like small GTPase Arl8b and can be blocked by the Rab7 GTPase competitive inhibitor CID1067700. Such non-lytic release of β-coronaviruses results in lysosome deacidification, inactivation of lysosomal degradation enzymes, and disruption of antigen presentation pathways. β-Coronavirus-induced exploitation of lysosomal organelles for egress provides insights into the cellular and immunological abnormalities observed in patients and suggests new therapeutic modalities.

Application of advances in endocytosis and membrane trafficking to drug delivery

Multidisciplinary research efforts in the field of drug delivery have led to the development of a variety of drug delivery systems (DDS) designed for site-specific delivery of diagnostic and therapeutic agents. Since efficient uptake of drug carriers into target cells is central to effective drug delivery, a comprehensive understanding of the biological pathways for cellular internalization of DDS can facilitate the development of DDS capable of precise tissue targeting and enhanced therapeutic outcomes. Diverse methods have been applied to study the internalization mechanisms responsible for endocytotic uptake of extracellular materials, which are also the principal pathways exploited by many DDS. Chemical inhibitors remain the most commonly used method to explore endocytotic internalization mechanisms, although genetic methods are increasingly accessible and may constitute more specific approaches. This review highlights the molecular basis of internalization pathways most relevant to internalization of DDS, and the principal methods used to study each route. This review also showcases examples of DDS that are internalized by each route, and reviews the general effects of biophysical properties of DDS on the internalization efficiency. Finally, options for intracellular trafficking and targeting of internalized DDS are briefly reviewed, representing an additional opportunity for multi-level targeting to achieve further specificity and therapeutic efficacy.

Effects of pharmacological modulators of α-synuclein and tau aggregation and internalization

Parkinson's disease (PD) and Alzheimer's disease (AD) are common neurodegenerative disorders of the elderly and, therefore, affect a growing number of patients worldwide. Both diseases share, as a common hallmark, the accumulation of characteristic protein aggregates, known as Lewy bodies (LB) in PD, and neurofibrillary tangles in AD. LBs are primarily composed of misfolded α-synuclein (aSyn), and neurofibrillary tangles are primarily composed of tau protein. Importantly, upon pathological evaluation, most AD and PD/Lewy body dementia cases exhibit mixed pathology, with the co-occurrence of both LB and neurofibrillary tangles, among other protein inclusions. Recent studies suggest that both aSyn and tau pathology can spread and propagate through neuronal connections. Therefore, it is important to investigate the mechanisms underlying aggregation and propagation of these proteins for the development of novel therapeutic strategies. Here, we assessed the effects of different pharmacological interventions on the aggregation and internalization of tau and aSyn. We found that anle138b and fulvic acid decrease aSyn and tau aggregation, that epigallocatechin gallate decreases aSyn aggregation, and that dynasore reduces tau internalization. Establishing the effects of small molecules with different chemical properties on the aggregation and spreading of aSyn and tau will be important for the development of future therapeutic interventions.

Endosomal signaling of delta opioid receptors is an endogenous mechanism and therapeutic target for relief from inflammatory pain

Whether G protein-coupled receptors signal from endosomes to control important pathophysiological processes and are therapeutic targets is uncertain. We report that opioids from the inflamed colon activate δ-opioid receptors (DOPr) in endosomes of nociceptors. Biopsy samples of inflamed colonic mucosa from patients and mice with colitis released opioids that activated DOPr on nociceptors to cause a sustained decrease in excitability. DOPr agonists inhibited mechanically sensitive colonic nociceptors. DOPr endocytosis and endosomal signaling by protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) pathways mediated the sustained inhibitory actions of endogenous opioids and DOPr agonists. DOPr agonists stimulated the recruitment of Gα_i/o and β-arrestin1/2 to endosomes. Analysis of compartmentalized signaling revealed a requirement of DOPr endocytosis for activation of PKC at the plasma membrane and in the cytosol and ERK in the nucleus. We explored a nanoparticle delivery strategy to evaluate whether endosomal DOPr might be a therapeutic target for pain. The DOPr agonist DADLE was coupled to a liposome shell for targeting DOPr-positive nociceptors and incorporated into a mesoporous silica core for release in the acidic and reducing endosomal environment. Nanoparticles activated DOPr at the plasma membrane, were preferentially endocytosed by DOPr-expressing cells, and were delivered to DOPr-positive early endosomes. Nanoparticles caused a long-lasting activation of DOPr in endosomes, which provided sustained inhibition of nociceptor excitability and relief from inflammatory pain. Conversely, nanoparticles containing a DOPr antagonist abolished the sustained inhibitory effects of DADLE. Thus, DOPr in endosomes is an endogenous mechanism and a therapeutic target for relief from chronic inflammatory pain.

Understanding SARS-CoV-2 endocytosis for COVID-19 drug repurposing

The quest for the effective treatment against coronavirus disease 2019 pneumonia caused by the severe acute respiratory syndrome (SARS)‐coronavirus 2(CoV‐2) coronavirus is hampered by the lack of knowledge concerning the basic cell biology of the infection. Given that most viruses use endocytosis to enter the host cell, mechanistic investigation of SARS‐CoV‐2 infection needs to consider the diversity of endocytic pathways available for SARS‐CoV‐2 entry in the human lung epithelium. Taking advantage of the well‐established methodology of membrane trafficking studies, this research direction allows for the rapid characterisation of the key cell biological mechanism(s) responsible for SARS‐CoV‐2 infection. Furthermore, 11 clinically approved generic drugs are identified as potential candidates for repurposing as blockers of several potential routes for SARS‐CoV‐2 endocytosis. More broadly, the paradigm of targeting a fundamental aspect of human cell biology to protect against infection may be advantageous in the context of future pandemic outbreaks.

When trafficking and signaling mix: How subcellular location shapes G protein-coupled receptor activation of heterotrimeric G proteins

G protein-coupled receptors (GPCRs) physically connect extracellular information with intracellular signal propagation. Membrane trafficking plays a supportive role by "bookending" signaling events: movement through the secretory pathway delivers GPCRs to the cell surface where receptors can sample the extracellular environment, while endocytosis and endolysosomal membrane trafficking provide a versatile system to titrate cellular signaling potential and maintain homeostatic control. Recent evidence suggests that, in addition to these important effects, GPCR trafficking actively shapes the cellular signaling response by altering the location and timing of specific receptor-mediated signaling reactions. Here, we review key experimental evidence underlying this expanding view, focused on GPCR signaling mediated through activation of heterotrimeric G proteins located in the cytoplasm. We then discuss lingering and emerging questions regarding the interface between GPCR signaling and trafficking.

Cellular Uptake Evaluation of Amphiphilic Polymer Assemblies: Importance of Interplay between Pharmacological and Genetic Approaches

Understanding the cellular uptake mechanism of materials is of fundamental importance that would be beneficial for materials design with enhanced biological functions. Herein, we report the interplay of pharmacological and genetic approaches to minimize the possible misinterpretation on cellular uptake mechanism. A library of amphiphilic polymers was used as a model system to evaluate the reliability of such methodological interplay. To probe the cellular uptake of amphiphilic polymers, we utilized an orthogonal end-group labeling strategy to conjugate one fluorescent molecule on each polymer chain. The results from the methodological interplay with these labeled polymers revealed the off-target effects of dynasore, a well-known dynamin inhibitor. Instead of dynamin, actin was found to be an essential cellular component during the cellular uptake of these amphiphilic polymers. Our study demonstrates the importance of interplaying pharmacological and genetic approaches when evaluating the endocytic mechanism of functional materials, providing insights on understanding the cellular uptake of future therapeutic materials.

Targeted enhancement of flotillin-dependent endocytosis augments cellular uptake and impact of cytotoxic drugs

Cellular uptake is limiting for the efficacy of many cytotoxic drugs used to treat cancer. Identifying endocytic mechanisms that can be modulated with targeted, clinically-relevant interventions is important to enhance the efficacy of various cancer drugs. We identify that flotillin-dependent endocytosis can be targeted and upregulated by ultrasound and microbubble (USMB) treatments to enhance uptake and efficacy of cancer drugs such as cisplatin. USMB involves targeted ultrasound following administration of encapsulated microbubbles, used clinically for enhanced ultrasound image contrast. USMB treatments robustly enhanced internalization of the molecular scaffold protein flotillin, as well as flotillin-dependent fluid-phase internalization, a phenomenon dependent on the protein palmitoyltransferase DHHC5 and the Src-family kinase Fyn. USMB treatment enhanced DNA damage and cell killing elicited by the cytotoxic agent cisplatin in a flotillin-dependent manner. Thus, flotillin-dependent endocytosis can be modulated by clinically-relevant USMB treatments to enhance drug uptake and efficacy, revealing an important new strategy for targeted drug delivery for cancer treatment.

Synaptic vesicle generation from activity-dependent bulk endosomes requires a dephosphorylation-dependent dynamin-syndapin interaction

Activity‐dependent bulk endocytosis generates synaptic vesicles (SVs) during intense neuronal activity via a two‐step process. First, bulk endosomes are formed direct from the plasma membrane from which SVs are then generated. SV generation from bulk endosomes requires the efflux of previously accumulated calcium and activation of the protein phosphatase calcineurin. However, it is still unknown how calcineurin mediates SV generation. We addressed this question using a series of acute interventions that decoupled the generation of SVs from bulk endosomes in rat primary neuronal culture. This was achieved by either disruption of protein–protein interactions via delivery of competitive peptides, or inhibition of enzyme activity by known inhibitors. SV generation was monitored using either a morphological horseradish peroxidase assay or an optical assay that monitors the replenishment of the reserve SV pool. We found that SV generation was inhibited by, (i) peptides that disrupt calcineurin interactions, (ii) an inhibitor of dynamin I GTPase activity and (iii) peptides that disrupt the phosphorylation‐dependent dynamin I–syndapin I interaction. Peptides that disrupted syndapin I interactions with eps15 homology domain‐containing proteins had no effect. This revealed that (i) calcineurin must be localized at bulk endosomes to mediate its effect, (ii) dynamin I GTPase activity is essential for SV fission and (iii) the calcineurin‐dependent interaction between dynamin I and syndapin I is essential for SV generation. We therefore propose that a calcineurin‐dependent dephosphorylation cascade that requires both dynamin I GTPase and syndapin I lipid‐deforming activity is essential for SV generation from bulk endosomes.

Spoiled for Choice: Diverse Endocytic Pathways Function at the Cell Surface

Endocytosis has long been identified as a key cellular process involved in bringing in nutrients, in clearing cellular debris in tissue, in the regulation of signaling, and in maintaining cell membrane compositional homeostasis. While clathrin-mediated endocytosis has been most extensively studied, a number of clathrin-independent endocytic pathways are continuing to be delineated. Here we provide a current survey of the different types of endocytic pathways available at the cell surface and discuss a new classification and plausible molecular mechanisms for some of the less characterized pathways. Along with an evolutionary perspective of the origins of some of these pathways, we provide an appreciation of the distinct roles that these pathways play in various aspects of cellular physiology, including the control of signaling and membrane tension.

S-acylated Golga7b stabilises DHHC5 at the plasma membrane to regulate cell adhesion

S-acylation (palmitoylation) is the only fully reversible lipid modification of proteins; however, little is known about how protein S-acyltransferases (PATs) that mediate it are regulated. DHHC5 is a PAT that is mainly localised at the plasma membrane with roles in synaptic plasticity, massive endocytosis and cancer cell growth/invasion. Here, we demonstrate that DHHC5 binds to and palmitoylates a novel accessory protein Golga7b. Palmitoylation of Golga7b prevents clathrin-mediated endocytosis of DHHC5 and stabilises it at the plasma membrane. Proteomic analysis of the composition of DHHC5/Golga7b-associated protein complexes reveals a striking enrichment in adhesion proteins, particularly components of desmosomes. We show that desmoglein-2 and plakophilin-3 are substrates of DHHC5 and that DHHC5 and Golga7b are required for localisation of desmoglein-2 to the plasma membrane and for desmosomal patterning. Loss of DHHC5/Golga7b causes functional impairments in cell adhesion, suggesting these proteins have a wider role in cell adhesion beyond desmosome assembly. This work uncovers a novel mechanism of DHHC5 regulation by Golga7b and demonstrates a role for the DHHC5/Golga7b complex in the regulation of cell adhesion.

Functional recruitment of dynamin requires multimeric interactions for efficient endocytosis

During clathrin mediated endocytosis (CME), the concerted action of dynamin and its interacting partners drives membrane scission. Essential interactions occur between the proline/arginine-rich domain of dynamin (dynPRD) and the Src-homology domain 3 (SH3) of various proteins including amphiphysins. Here we show that multiple SH3 domains must bind simultaneously to dynPRD through three adjacent motifs for dynamin’s efficient recruitment and function. First, we show that mutant dynamins modified in a single motif, including the central amphiphysin SH3 (amphSH3) binding motif, partially rescue CME in dynamin triple knock-out cells. However, mutating two motifs largely prevents that ability. Furthermore, we designed divalent dynPRD-derived peptides. These ligands bind multimers of amphSH3 with >100-fold higher affinity than monovalent ones in vitro. Accordingly, dialyzing living cells with these divalent peptides through a patch-clamp pipette blocks CME much more effectively than with monovalent ones. We conclude that dynamin drives vesicle scission via multivalent interactions in cells.

During clathrin mediated endocytosis (CME), membrane scission is achieved by the concerted action of dynamin and its interacting partners such as amphiphysins. Here authors show that efficient recruitment and function of dynamin requires simultaneous binding of multiple amphiphysin SH3 domains.

Dynamin and reverse-mode sodium calcium exchanger blockade confers neuroprotection from diffuse axonal injury

Mild traumatic brain injury (mTBI) is a frequently overlooked public health concern that is difficult to diagnose and treat. Diffuse axonal injury (DAI) is a common mTBI neuropathology in which axonal shearing and stretching induces breakdown of the cytoskeleton, impaired axonal trafficking, axonal degeneration, and cognitive dysfunction. DAI is becoming recognized as a principal neuropathology of mTBI with supporting evidence from animal model, human pathology, and neuroimaging studies. As mitochondrial dysfunction and calcium overload are critical steps in secondary brain and axonal injury, we investigated changes in protein expression of potential targets following mTBI using an in vivo controlled cortical impact model. We show upregulated expression of sodium calcium exchanger1 (NCX1) in the hippocampus and cortex at distinct time points post-mTBI. Expression of dynamin-related protein1 (Drp1), a GTPase responsible for regulation of mitochondrial fission, also changes differently post-injury in the hippocampus and cortex. Using an in vitro model of DAI previously reported by our group, we tested whether pharmacological inhibition of NCX1 by SN-6 and of dynamin1, dynamin2, and Drp1 by dynasore mitigates secondary damage. Dynasore and SN-6 attenuate stretch injury-induced swelling of axonal varicosities and mitochondrial fragmentation. In addition, we show that dynasore, but not SN-6, protects against H2O2-induced damage in an organotypic oxidative stress model. As there is currently no standard treatment to mitigate cell damage induced by mTBI and DAI, this work highlights two potential therapeutic targets for treatment of DAI in multiple models of mTBI and DAI.

Cigarette Smoke Exposure Induces Retrograde Trafficking of CFTR to the Endoplasmic Reticulum

Chronic obstructive pulmonary disease (COPD), which is most commonly caused by cigarette smoke (CS) exposure, is the third leading cause of death worldwide. The cystic fibrosis transmembrane conductance regulator (CFTR) is an apical membrane anion channel that is widely expressed in epithelia throughout the body. In the airways, CFTR plays an important role in fluid homeostasis and helps flush mucus and inhaled pathogens/toxicants out of the lung. Inhibition of CFTR leads to mucus stasis and severe airway disease. CS exposure also inhibits CFTR, leading to the decreased anion secretion/hydration seen in COPD patients. However, the underlying mechanism is poorly understood. Here, we report that CS causes CFTR to be internalized in a clathrin/dynamin-dependent fashion. This internalization is followed by retrograde trafficking of CFTR to the endoplasmic reticulum. Although this internalization pathway has been described for bacterial toxins and cargo machinery, it has never been reported for mammalian ion channels. Furthermore, the rapid internalization of CFTR is dependent on CFTR dephosphorylation by calcineurin, a protein phosphatase that is upregulated by CS. These results provide new insights into the mechanism of CFTR internalization, and may help in the development of new therapies for CFTR correction and lung rehydration in patients with debilitating airway diseases such as COPD.

Membrane capacitance recordings resolve dynamics and complexity of receptor-mediated endocytosis in Wnt signalling

Receptor-mediated endocytosis is an essential process in signalling pathways for activation of intracellular signalling cascades. One example is the Wnt signalling pathway that seems to depend on endocytosis of the ligand-receptor complex for initiation of Wnt signal transduction. To date, the roles of different endocytic pathways in Wnt signalling, molecular players and the kinetics of the process remain unclear. Here, we monitored endocytosis in Wnt3a and Wnt5a-mediated signalling with membrane capacitance recordings of HEK293 cells. Our measurements revealed a swift and substantial increase in the number of endocytic vesicles. Extracellular Wnt ligands specifically triggered endocytotic activity, which started immediately upon ligand binding and ceased within a period of ten minutes. By using specific inhibitors, we were able to separate Wnt-induced endocytosis into two independent pathways. We demonstrate that canonical Wnt3a is taken up mainly by clathrin-independent endocytosis whereas noncanonical Wnt5a is exclusively regulated via clathrin-mediated endocytosis. Our findings show that membrane capacitance recordings allow the resolution of complex cellular processes in plasma membrane signalling pathways in great detail.

Open and cut: allosteric motion and membrane fission by dynamin superfamily proteins

Cells have evolved diverse protein-based machinery to reshape, cut, or fuse their membrane-delimited compartments. Dynamin superfamily proteins are principal components of this machinery and use their ability to hydrolyze GTP and to polymerize into helices and rings to achieve these goals. Nucleotide-binding, hydrolysis, and exchange reactions drive significant conformational changes across the dynamin family, and these changes alter the shape and stability of supramolecular dynamin oligomers, as well as the ability of dynamins to bind receptors and membranes. Mutations that interfere with the conformational repertoire of these enzymes, and hence with membrane fission, exist in several inherited human diseases. Here, we discuss insights from new x-ray crystal structures and cryo-EM reconstructions that have enabled us to infer some of the allosteric dynamics for these proteins. Together, these studies help us to understand how dynamins perform mechanical work, as well as how specific mutants of dynamin family proteins exhibit pathogenic properties.

Herpes Simplex Virus 1 Can Enter Dynamin 1 and 2 Double-Knockout Fibroblasts

Dynamin GTPases, best known for their role in membrane fission of endocytic vesicles, provide a target for viruses to be exploited during endocytic uptake. Recently, we found that entry of herpes simplex virus 1 (HSV-1) into skin cells depends on dynamin, although our results supported that viral internalization occurs via both direct fusion with the plasma membrane and via endocytic pathways. To further explore the role of dynamin for efficient HSV-1 entry, we utilized conditional dynamin 1 and dynamin 2 double-knockout (DKO) fibroblasts as an experimental tool. Strikingly, HSV-1 entered control and DKO fibroblasts with comparable efficiencies. For comparison, we infected DKO cells with Semliki Forest virus, which is known to adopt clathrin-mediated endocytosis as its internalization pathway, and observed efficient virus entry. These results support the notion that the DKO cells provide alternative pathways for viral uptake. Treatment of cells with the dynamin inhibitor dynasore confirmed that HSV-1 entry depended on dynamin in the control fibroblasts. As expected, dynasore did not interfere with viral entry into DKO cells. Electron microscopy of HSV-1-infected cells suggests viral entry after fusion with the plasma membrane and by endocytosis in both dynamin-expressing and dynamin-deficient cells. Infection at low temperatures where endocytosis is blocked still resulted in HSV-1 entry, although at a reduced level, which suggests that nonendocytic pathways contribute to successful entry. Overall, our results strengthen the impact of dynamin for HSV-1 entry, as only cells that adapt to the lack of dynamin allow dynamin-independent entry.IMPORTANCE The human pathogen herpes simplex virus 1 (HSV-1) can adapt to a variety of cellular pathways to enter cells. In general, HSV-1 is internalized by fusion of its envelope with the plasma membrane or by endocytic pathways, which reflects the high adaptation to differences in its target cells. The challenges are to distinguish whether multiple or only one of these internalization pathways leads to successful entry and, furthermore, to identify the mode of viral uptake. In this study, we focused on dynamin, which promotes endocytic vesicle fission, and explored how the presence and absence of dynamin can influence viral entry. Our results support the idea that HSV-1 entry into mouse embryonic fibroblasts depends on dynamin; however, depletion of dynamin still allows efficient viral entry, suggesting that alternative pathways present upon dynamin depletion can accomplish viral internalization.

Dynamin 2 is required for GPVI signaling and platelet hemostatic function in mice

Receptor-mediated endocytosis, which contributes to a wide range of cellular functions, including receptor signaling, cell adhesion, and migration, requires endocytic vesicle release by the large GTPase dynamin 2. Here, the role of dynamin 2 was investigated in platelet hemostatic function using both pharmacological and genetic approaches. Dnm2^fl/fl Pf4-Cre (Dnm2^Plt−/−) mice specifically lacking dynamin 2 within the platelet lineage developed severe thrombocytopenia and bleeding diathesis and Dnm2^Plt−/− platelets adhered poorly to collagen under arterial shear rates. Signaling via the collagen receptor GPVI was impaired in platelets treated with the dynamin GTPase inhibitor dynasore, as evidenced by poor protein tyrosine phosphorylation, including that of the proximal tyrosine kinase Lyn on its activating tyrosine 396 residue. Platelet stimulation via GPVI resulted in a slight decrease in GPVI, which was maintained by dynasore treatment. Dynasore-treated platelets had attenuated function when stimulated via GPVI, as evidenced by reduced GPIbα downregulation, α-granule release, integrin αIIbβ3 activation, and spreading onto immobilized fibrinogen. By contrast, responses to the G-protein coupled receptor agonist thrombin were minimally affected by dynasore treatment. GPVI expression was severely reduced in Dnm2^Plt−/− platelets, which were dysfunctional in response to stimulation via GPVI, and to a lesser extent to thrombin. Dnm2^Plt−/− platelets lacked fibrinogen in their α-granules, but retained von Willebrand factor. Taken together, the data show that dynamin 2 plays a proximal role in signaling via the collagen receptor GPVI and is required for fibrinogen uptake and normal platelet hemostatic function.

Dynamin inhibitors impair platelet-derived growth factor β-receptor dimerization and signaling

The role of plasma membrane composition and dynamics in the activation process of receptor tyrosine kinases (RTKs) is still poorly understood. In this study we have investigated how signaling via the RTK, platelet-derived growth factor β-receptor (PDGFR-β) is affected by Dynasore or Dyngo-4a, which are commonly used dynamin inhibitors. PDGFR-β preferentially internalizes via clathrin-coated pits and in this pathway, Dynamin II has a major role in the formation and release of vesicles from the plasma membrane by performing the membrane scission. We have found that dynamin inhibitors impedes the activation of PDGFR-β by impairing ligand-induced dimerization of the receptor monomers, which leads to a subsequent lack of phosphorylation and activation both of receptors and downstream effectors, such as ERK1/2 and AKT. In contrast, dynamin inhibitors did not affect epidermal growth factor receptor (EGFR) dimerization and phosphorylation. Our findings suggest that there is a link between plasma membrane dynamics and PDGFR-β activation, and that this link is not shared with the epidermal growth factor receptor.

Influenza virus entry via the GM3 ganglioside-mediated platelet-derived growth factor receptor β signalling pathway

The possible resistance of influenza virus against existing antiviral drugs calls for new therapeutic concepts. One appealing strategy is to inhibit virus entry, in particular at the stage of internalization. This requires a better understanding of virus-host interactions during the entry process, including the role of receptor tyrosine kinases (RTKs). To search for cellular targets, we evaluated a panel of 276 protein kinase inhibitors in a multicycle antiviral assay in Madin-Darby canine kidney cells. The RTK inhibitor Ki8751 displayed robust anti-influenza A and B virus activity and was selected for mechanistic investigations. Ki8751 efficiently disrupted the endocytic process of influenza virus in different cell lines carrying platelet-derived growth factor receptor β (PDGFRβ), an RTK that is known to act at GM3 ganglioside-positive lipid rafts. The more efficient virus entry in CHO-K1 cells compared to the wild-type ancestor (CHO-wt) cells indicated a positive effect of GM3, which is abundant in CHO-K1 but not in CHO-wt cells. Entering virus localized to GM3-positive lipid rafts and the PDGFRβ-containing endosomal compartment. PDGFRβ/GM3-dependent virus internalization involved PDGFRβ phosphorylation, which was potently inhibited by Ki8751, and desialylation of activated PDGFRβ by the viral neuraminidase. Virus uptake coincided with strong activation of the Raf/MEK/Erk cascade, but not of PI3K/Akt or phospholipase C-γ. We conclude that influenza virus efficiently hijacks the GM3-enhanced PDGFRβ signalling pathway for cell penetration, providing an opportunity for host cell-targeting antiviral intervention.

Dynamin-related protein 1 has membrane constricting and severing abilities sufficient for mitochondrial and peroxisomal fission

Dynamin-related protein 1 (Drp1) is essential for mitochondrial and peroxisomal fission. Recent studies propose that Drp1 does not sever but rather constricts mitochondrial membranes allowing dynamin 2 (Dnm2) to execute final scission. Here, we report that unlike Drp1, Dnm2 is dispensable for peroxisomal and mitochondrial fission, as these events occurred in Dnm2 knockout cells. Fission events were also observed in mouse embryonic fibroblasts lacking Dnm1, 2 and 3. Using reconstitution experiments on preformed membrane tubes, we show that Drp1 alone both constricts and severs membrane tubes. Scission required the membrane binding, self-assembling and GTPase activities of Drp1 and occurred on tubes up to 250 nm in radius. In contrast, Dnm2 exhibited severely restricted fission capacity with occasional severing of tubes below 50 nm in radius. We conclude that Drp1 has both membrane constricting and severing abilities and is the dominant dynamin performing mitochondrial and peroxisomal fission.

Drp1 and Dnm2 have been implicated in mitochondrial fission events, although their specific activities in constriction and scission have been unclear. Here, the authors demonstrate that Drp1 is sufficient to constrict and sever mitochondrial and peroxisomal membranes in the absence of Dnm proteins.

Dynamin Is Required for Efficient Cytomegalovirus Maturation and Envelopment

Cytomegalovirus secondary envelopment occurs in a virus-induced cytoplasmic assembly compartment (vAC) generated via a drastic reorganization of the membranes of the secretory and endocytic systems. Dynamin is a eukaryotic GTPase that is implicated in membrane remodeling and endocytic membrane fission events; however, the role of dynamin in cellular trafficking of viruses beyond virus entry is only partially understood. Mouse embryonic fibroblasts (MEF) engineered to excise all three isoforms of dynamin were infected with mouse cytomegalovirus (MCMV-K181). Immediate-early (IE1; m123) viral protein was detected in these triple dynamin knockout (TKO) cells, as well as in mock-induced parental MEF, at early times postinfection, although levels were reduced in TKO cells, indicating that virus entry was affected but not eliminated. Levels of IE1 protein and another viral early protein (m04) were normalized by 48 h postinfection; however, late protein (m55; gB) expression was reduced in infected TKO cells compared to parental MEF. Ultrastructural analysis revealed intact stages of nuclear virus maturation in both cases with equivalent numbers of nucleocapsids containing packaged viral DNA (C-capsids), indicating successful viral DNA replication, capsid assembly, and genome packaging. Most importantly, severe defects in virus envelopment were visualized in TKO cells but not in parental cells. Dynamin inhibitor (dynasore)-treated MEF showed a phenotype similar to TKO cells upon mouse cytomegalovirus infection, confirming the role of dynamin in late maturation processes. In summary, dynamin-mediated endocytic pathways are critical for the completion of cytoplasmic stages of cytomegalovirus maturation.IMPORTANCE Viruses are known to exploit specific cellular functions at different stages of their life cycle in order to replicate, avoid immune recognition by the host and to establish a successful infection. Cytomegalovirus (CMV)-infected cells are characterized by a prominent cytoplasmic inclusion (virus assembly compartment [vAC]) that is the site of virus maturation and envelopment. While endocytic membranes are known to be the functional components of vAC, knowledge of specific endocytic pathways implicated in CMV maturation and envelopment is lacking. We show here that dynamin, which is an integral part of host endocytic machinery, is largely dispensable for early stages of CMV infection but is required at a late stage of CMV maturation. Studies on dynamin function in CMV infection will help us understand the host-virus interaction pathways amenable to targeting by conventional small molecules, as well as by newer generation nucleotide-based therapeutics (e.g., small interfering RNA, CRISPR/CAS gRNA, etc.).

Modulation of dynamin function by small molecules

Dynamins are essential as membrane remodelers in various cellular processes, like receptor-mediated endocytosis, synaptic vesicle recycling and spermatogenesis. Moreover, dynamin is involved in the internalization of numerous viruses and in the motility of several cancer cell lines. As tools for dissecting the underlying mechanisms of these important biological processes and as potential future therapeutics, small molecules have been developed in the last two decades that modulate the functions of dynamin. In this review we give an overview of the compound classes that are currently in use and describe how they affect dynamin function.

Mechanochemical feedback control of dynamin independent endocytosis modulates membrane tension in adherent cells

Plasma membrane tension regulates many key cellular processes. It is modulated by, and can modulate, membrane trafficking. However, the cellular pathway(s) involved in this interplay is poorly understood. Here we find that, among a number of endocytic processes operating simultaneously at the cell surface, a dynamin independent pathway, the CLIC/GEEC (CG) pathway, is rapidly and specifically upregulated upon a sudden reduction of tension. Moreover, inhibition (activation) of the CG pathway results in lower (higher) membrane tension. However, alteration in membrane tension does not directly modulate CG endocytosis. This requires vinculin, a mechano-transducer recruited to focal adhesion in adherent cells. Vinculin acts by controlling the levels of a key regulator of the CG pathway, GBF1, at the plasma membrane. Thus, the CG pathway directly regulates membrane tension and is in turn controlled via a mechano-chemical feedback inhibition, potentially leading to homeostatic regulation of membrane tension in adherent cells.

Plasma membrane tension is an important factor that regulates many key cellular processes. Here authors show that a specific dynamin-independent endocytic pathway is modulated by changes in tension via the mechano-transducer vinculin.

Dynasore-induced potent ubiquitylation of the exon 19 deletion mutant of epidermal growth factor receptor suppresses cell growth and migration in non-small cell lung cancer

Lung cancer is a leading cause of death worldwide, with mutations in EGFR frequently detected that render this receptor tyrosine kinase constantly active. Targeted therapy against EGFR has proved effective in lung cancer treatment, but secondary mutations in EGFR frequently cause drug resistance. In the efforts made to investigate alternative ways to inhibit mutant EGFR, we observed that the dynamin inhibitor dynasore effectively suppressed the exon 19-deleted mutant of EGFR. This agent inhibited cell proliferation, colony formation, cell migration, and cell cycle progression of HCC827 and H1650 cells driven by the exon 19-deleted EGFR mutant. From a mechanistic point of view, dynasore suppressed the activation of AKT and MEK in HCC827 and H1650 cells. However, dynasore failed to alter the subcellular distribution of EGFR, and another dynamin inhibitor, dyngo-4a, did not phenocopy the effects of dynasore, suggesting a dynamin activity-independent effect of dynasore. Finally, we show that dynasore induced the potent ubiquitylation of the exon 19-deleted mutant of EGFR. Our observations will shed light on the development of alternative therapeutic strategies that target mutant EGFR in lung cancer.

The scaffolding protein ZO-1 coordinates actomyosin and epithelial apical specializations in vitro and in vivo

Polarized epithelia assemble into sheets that compartmentalize organs and generate tissue barriers by integrating apical surfaces into a single, unified structure. This tissue organization is shared across organs, species, and developmental stages. The processes that regulate development and maintenance of apical epithelial surfaces are, however, undefined. Here, using an intestinal epithelial-specific knockout (KO) mouse and cultured epithelial cells, we show that the tight junction scaffolding protein zonula occludens-1 (ZO-1) is essential for development of unified apical surfaces in vivo and in vitro We found that U5 and GuK domains of ZO-1 are necessary for proper apical surface assembly, including organization of microvilli and cortical F-actin; however, direct interactions with F-actin through the ZO-1 actin-binding region (ABR) are not required. ZO-1 lacking the PDZ1 domain, which binds claudins, rescued apical structure in ZO-1-deficient epithelia, but not in cells lacking both ZO-1 and ZO-2, suggesting that heterodimerization with ZO-2 restores PDZ1-dependent ZO-1 interactions that are vital to apical surface organization. Pharmacologic F-actin disruption, myosin II motor inhibition, or dynamin inactivation restored apical epithelial structure in vitro and in vivo, indicating that ZO-1 directs epithelial organization by regulating actomyosin contraction and membrane traffic. We conclude that multiple ZO-1-mediated interactions contribute to coordination of epithelial actomyosin function and genesis of unified apical surfaces.

Identifying Small-Molecule Inhibitors of the Clathrin Terminal Domain

Clathrin-mediated endocytosis (CME) is a universal and evolutionarily conserved process that enables the internalization of numerous cargo proteins, including receptors for nutrients and signaling molecules, as well as synaptic vesicle reformation. Multiple genetic and chemical approaches have been developed to interfere with this process. However, many of these tools do not selectively block CME, for example by targeting components shared with clathrin-independent endocytosis pathways or by interfering with other cellular processes that indirectly affect CME.Clathrin, via interactions of endocytic proteins with its terminal domain (TD), serves as a central interaction hub for coat assembly in CME. Here, we describe an ELISA-based, high-throughput screening method used to identify small molecules that inhibit these interactions. In addition, we provide protocols for the purification of recombinant protein domains used for screening, e.g., the clathrin TD and the amphiphysin B/C domain. The screen has been applied successfully in the past, and ultimately led to the discovery of the Pitstop® family of inhibitors, but remains in use to evaluate the inhibitory potency of derivatives of these compounds, and to screen for completely novel inhibitor families.

Septins suppress the release of vaccinia virus from infected cells

Septins are conserved components of the cytoskeleton that play important roles in many fundamental cellular processes including division, migration, and membrane trafficking. Septins can also inhibit bacterial infection by forming cage-like structures around pathogens such as Shigella We found that septins are recruited to vaccinia virus immediately after its fusion with the plasma membrane during viral egress. RNA interference-mediated depletion of septins increases virus release and cell-to-cell spread, as well as actin tail formation. Live cell imaging reveals that septins are displaced from the virus when it induces actin polymerization. Septin loss, however, depends on the recruitment of the SH2/SH3 adaptor Nck, but not the activity of the Arp2/3 complex. Moreover, it is the recruitment of dynamin by the third Nck SH3 domain that displaces septins from the virus in a formin-dependent fashion. Our study demonstrates that septins suppress vaccinia release by "entrapping" the virus at the plasma membrane. This antiviral effect is overcome by dynamin together with formin-mediated actin polymerization.

Aberrant endocytosis leads to the loss of normal mitotic spindle orientation during epithelial glandular morphogenesis

Epithelial cells form tissues with many functions, including secretion and environmental separation and protection. Glandular epithelial tissues comprise cysts and tubules that are formed from a polarized, single-epithelial cell layer surrounding a central, fluid-filled lumen. The pathways regulating key processes in epithelial tissue morphogenesis such as mitotic spindle formation are incompletely understood, but are important to investigate, as their dysregulation is a signature of epithelial tumors. Here, we describe a signaling axis that manifests in a defect in mitotic spindle orientation during epithelial growth and cystogenesis. We found that activation of the small GTPase ADP-ribosylation factor 6 (ARF6) results in the sustained internalization of cell-surface components such as the cMet receptor and the cell-adhesion molecule E-cadherin. The spindle orientation defect arising from elevated levels of ARF6-GTP required an increase in cMet endocytosis, but was independent of E-cadherin internalization or elevated extracellular signal-regulated kinase (ERK) activity resulting from internalized receptor signaling on endosomes. Misorientation of the mitotic spindle resulted in the development of epithelial cysts with structural abnormalities, the most conspicuous of which was the presence of multiple intercellular lumens. Abnormal mitotic spindle orientation was necessary but insufficient to disrupt glandular development, as blocking the strong prosurvival signal resulting from ERK hyperactivation yielded structurally normal cysts despite continued manifestation of spindle orientation defects. Our findings highlight a previously unknown link between ARF6 activation, cMet receptor internalization, and mitotic spindle orientation during epithelial glandular morphogenesis.

Protease-activated receptor-2 in endosomes signals persistent pain of irritable bowel syndrome

Once activated at the surface of cells, G protein-coupled receptors (GPCRs) redistribute to endosomes, where they can continue to signal. Whether GPCRs in endosomes generate signals that contribute to human disease is unknown. We evaluated endosomal signaling of protease-activated receptor-2 (PAR2), which has been proposed to mediate pain in patients with irritable bowel syndrome (IBS). Trypsin, elastase, and cathepsin S, which are activated in the colonic mucosa of patients with IBS and in experimental animals with colitis, caused persistent PAR2-dependent hyperexcitability of nociceptors, sensitization of colonic afferent neurons to mechanical stimuli, and somatic mechanical allodynia. Inhibitors of clathrin- and dynamin-dependent endocytosis and of mitogen-activated protein kinase kinase-1 prevented trypsin-induced hyperexcitability, sensitization, and allodynia. However, they did not affect elastase- or cathepsin S-induced hyperexcitability, sensitization, or allodynia. Trypsin stimulated endocytosis of PAR2, which signaled from endosomes to activate extracellular signal-regulated kinase. Elastase and cathepsin S did not stimulate endocytosis of PAR2, which signaled from the plasma membrane to activate adenylyl cyclase. Biopsies of colonic mucosa from IBS patients released proteases that induced persistent PAR2-dependent hyperexcitability of nociceptors, and PAR2 association with β-arrestins, which mediate endocytosis. Conjugation to cholestanol promoted delivery and retention of antagonists in endosomes containing PAR2 A cholestanol-conjugated PAR2 antagonist prevented persistent trypsin- and IBS protease-induced hyperexcitability of nociceptors. The results reveal that PAR2 signaling from endosomes underlies the persistent hyperexcitability of nociceptors that mediates chronic pain of IBS. Endosomally targeted PAR2 antagonists are potential therapies for IBS pain. GPCRs in endosomes transmit signals that contribute to human diseases.

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

Endocytosis is required for CXC chemokine receptor type 4 (CXCR4)-mediated Akt activation and antiapoptotic signaling

Signaling activated by binding of the CXC motif chemokine ligand 12 (CXCL12) to its cognate G protein-coupled receptor (GPCR), chemokine CXC motif receptor 4 (CXCR4), is linked to metastatic disease. However, the mechanisms governing CXCR4 signaling remain poorly understood. Here, we show that endocytosis and early endosome antigen 1 (EEA1), which is part of the endosome fusion machinery, are required for CXCL12-mediated AKT Ser/Thr kinase (Akt) signaling selective for certain Akt substrates. Pharmacological inhibition of endocytosis partially attenuated CXCL12-induced phosphorylation of Akt, but not phosphorylation of ERK-1/2. Similarly, phosphorylation of Akt, but not ERK-1/2, stimulated by CXCL13, the cognate ligand for the chemokine receptor CXCR5, was also attenuated by inhibited endocytosis. Furthermore, siRNA-mediated depletion of the Rab5-effector EEA1, but not of adaptor protein, phosphotyrosine-interacting with PH domain and leucine zipper 1 (APPL1), partially attenuated Akt, but not ERK-1/2, phosphorylation promoted by CXCR4. Attenuation of Akt phosphorylation through inhibition of endocytosis or EEA1 depletion was associated with reduced signaling to Akt substrate forkhead box O1/3a but not the Akt substrates TSC complex subunit 2 or glycogen synthase kinase 3β. This suggested that endocytosis and endosomes govern discrete aspects of CXCR4- or CXCR5-mediated Akt signaling. Consistent with this hypothesis, depletion of EEA1 reduced the ability of CXCL12 to attenuate apoptosis in suspended, but not adherent, HeLa cells. Our results suggest a mechanism whereby compartmentalized chemokine-mediated Akt signaling from endosomes suppresses the cancer-related process known as anoikis. Targeting this signaling pathway may help inhibit metastatic cancer involving receptors such as CXCR4.

Dynasore inhibition on productive infection of HIV-1 in commonly used cell lines is independent of transferrin endocytosis

The route of HIV-1 entry for productive infection in CD₄₊ host cells is a fundamental question for the molecular understanding of HIV-1 infection and transmission. Although direct fusion has long been thought to be the mode of entry, recent studies have suggested that productive entry of HIV-1 may actually occur through dynamin-dependent endocytosis. In several of these studies, dynasore, a noncompetitive inhibitor of the GTPase activity of dynamin, has been used to support this conclusion. Here we show that dynasore does produce inhibitory effects on the productive infection of HIV-1 in several commonly used cell lines. This effect is present regardless of the methods used to facilitate the infection of HIV-1. However, transferrin uptake remains fully functional in these cell lines upon dynasore treatment. Therefore, the inhibition on HIV-1 infection by dynasore in these cell lines is due to an effect that is independent of transferrin endocytosis. The use of dynasore in probing the role of endocytosis in HIV-1 infection should be corroborated by other methods.

Morphological changes of plasma membrane and protein assembly during clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) proceeds through a series of morphological changes of the plasma membrane induced by a number of protein components. Although the spatiotemporal assembly of these proteins has been elucidated by fluorescence-based techniques, the protein-induced morphological changes of the plasma membrane have not been fully clarified in living cells. Here, we visualize membrane morphology together with protein localizations during CME by utilizing high-speed atomic force microscopy (HS-AFM) combined with a confocal laser scanning unit. The plasma membrane starts to invaginate approximately 30 s after clathrin starts to assemble, and the aperture diameter increases as clathrin accumulates. Actin rapidly accumulates around the pit and induces a small membrane swelling, which, within 30 s, rapidly covers the pit irreversibly. Inhibition of actin turnover abolishes the swelling and induces a reversible open–close motion of the pit, indicating that actin dynamics are necessary for efficient and irreversible pit closure at the end of CME.

Cells communicate with their environments via the plasma membrane and various membrane proteins. Clathrin-mediated endocytosis (CME) plays a central role in such communication and proceeds with a series of multiprotein assembly, deformation of the plasma membrane, and production of a membrane vesicle that delivers extracellular signaling molecules into the cytoplasm. In this study, we utilized our home-built correlative imaging system comprising high-speed atomic force microscopy (HS-AFM) and confocal fluorescence microscopy to simultaneously image morphological changes of the plasma membrane and protein localization during CME in a living cell. The results revealed a tight correlation between the size of the pit and the amount of clathrin assembled. Actin dynamics play multiple roles in the assembly, maturation, and closing phases of the process, and affects membrane morphology, suggesting a close relationship between endocytosis and dynamic events at the cell cortex. Knock down of dynamin also affected the closing motion of the pit and showed functional correlation with actin.

Distinct functions of dynamin isoforms in tumorigenesis and their potential as therapeutic targets in cancer

Dynamins and their related proteins participate in the regulation of neurotransmission, antigen presentation, receptor internalization, growth factor signalling, nutrient uptake, and pathogen infection. Recently, emerging findings have shown dynamin proteins can also contribute to the genesis of cancer. This up-to-date review herein focuses on the functionality of dynamin in cancer development. Dynamin 1 and 2 both enhance cancer cell proliferation, tumor invasion and metastasis, whereas dynamin 3 has tumor suppression role. Antisense RNAs encoded on the DNA strand opposite a dynamin gene regulate the function of dynamin, and manipulate oncogenes and tumor suppressor genes. Certain dynamin-related proteins are also upregulated in distinct cancer conditions, resulting in apoptotic resistance, cell migration and poor prognosis. Altogether, dynamins are potential biomarkers as well as representing promising novel therapeutic targets for cancer treatment. This study also summarizes the current available dynamin-targeted therapeutics and suggests the potential strategy based on signalling pathways involved, providing important information to aid the future development of novel cancer therapeutics by targeting these dynamin family members.

Targeting GLP-1 receptor trafficking to improve agonist efficacy

Glucagon-like peptide-1 receptor (GLP-1R) activation promotes insulin secretion from pancreatic beta cells, causes weight loss, and is an important pharmacological target in type 2 diabetes (T2D). Like other G protein-coupled receptors, the GLP-1R undergoes agonist-mediated endocytosis, but the functional and therapeutic consequences of modulating GLP-1R endocytic trafficking have not been clearly defined. Here, we investigate a series of biased GLP-1R agonists with variable propensities for GLP-1R internalization and recycling. Compared to a panel of FDA-approved GLP-1 mimetics, compounds that retain GLP-1R at the plasma membrane produce greater long-term insulin release, which is dependent on a reduction in β-arrestin recruitment and faster agonist dissociation rates. Such molecules elicit glycemic benefits in mice without concomitant increases in signs of nausea, a common side effect of GLP-1 therapies. Our study identifies a set of agents with specific GLP-1R trafficking profiles and the potential for greater efficacy and tolerability as T2D treatments.

Endocytosis sustains release at photoreceptor ribbon synapses by restoring fusion competence

Endocytosis is an essential process at sites of synaptic release. Not only are synaptic vesicles recycled by endocytosis, but the removal of proteins and lipids by endocytosis is needed to restore release site function at active zones after vesicle fusion. Synaptic exocytosis from vertebrate photoreceptors involves synaptic ribbons that serve to cluster vesicles near the presynaptic membrane. In this study, we hypothesize that this clustering increases the likelihood that exocytosis at one ribbon release site may disrupt release at an adjacent site and therefore that endocytosis may be particularly important for restoring release site competence at photoreceptor ribbon synapses. To test this, we combined optical and electrophysiological techniques in salamander rods. Pharmacological inhibition of dynamin-dependent endocytosis rapidly inhibits release from synaptic ribbons and slows recovery of ribbon-mediated release from paired pulse synaptic depression. Inhibiting endocytosis impairs the ability of second-order horizontal cells to follow rod light responses at frequencies as low as 2 Hz. Inhibition of endocytosis also increases lateral membrane mobility of individual Ca2+ channels, showing that it changes release site structure. Visualization of single synaptic vesicles by total internal reflection fluorescence microscopy reveals that inhibition of endocytosis reduces the likelihood of fusion among vesicles docked near ribbons and increases the likelihood that they will retreat from the membrane without fusion. Vesicle advance toward the membrane is also reduced, but the number of membrane-associated vesicles is not. Endocytosis therefore appears to be more important for restoring later steps in vesicle fusion than for restoring docking. Unlike conventional synapses in which endocytic restoration of release sites is evident only at high frequencies, endocytosis is needed to maintain release from rod ribbon synapses even at modest frequencies.

Neurokinin 1 receptor signaling in endosomes mediates sustained nociception and is a viable therapeutic target for prolonged pain relief

Typically considered to be cell surface sensors of extracellular signals, heterotrimeric GTP-binding protein (G protein)-coupled receptors (GPCRs) control many pathophysiological processes and are the target of 30% of therapeutic drugs. Activated receptors redistribute to endosomes, but researchers have yet to explore whether endosomal receptors generate signals that control complex processes in vivo and are viable therapeutic targets. We report that the substance P (SP) neurokinin 1 receptor (NK₁R) signals from endosomes to induce sustained excitation of spinal neurons and pain transmission and that specific antagonism of the NK₁R in endosomes with membrane-anchored drug conjugates provides more effective and sustained pain relief than conventional plasma membrane-targeted antagonists. Pharmacological and genetic disruption of clathrin, dynamin, and β-arrestin blocked SP-induced NK₁R endocytosis and prevented SP-stimulated activation of cytosolic protein kinase C and nuclear extracellular signal-regulated kinase, as well as transcription. Endocytosis inhibitors prevented sustained SP-induced excitation of neurons in spinal cord slices in vitro and attenuated nociception in vivo. When conjugated to cholestanol to promote endosomal targeting, NK₁R antagonists selectively inhibited endosomal signaling and sustained neuronal excitation. Cholestanol conjugation amplified and prolonged the antinociceptive actions of NK₁R antagonists. These results reveal a critical role for endosomal signaling of the NK₁R in the complex pathophysiology of pain and demonstrate the use of endosomally targeted GPCR antagonists.

Sensing Exocytosis and Triggering Endocytosis at Synapses: Synaptic Vesicle Exocytosis-Endocytosis Coupling

The intact synaptic structure is critical for information processing in neural circuits. During synaptic transmission, rapid vesicle exocytosis increases the size of never terminals and endocytosis counteracts the increase. Accumulating evidence suggests that SV exocytosis and endocytosis are tightly connected in time and space during SV recycling, and this process is essential for synaptic function and structural stability. Research in the past has illustrated the molecular details of synaptic vesicle (SV) exocytosis and endocytosis; however, the mechanisms that timely connect these two fundamental events are poorly understood at central synapses. Here we discuss recent progress in SV recycling and summarize several emerging mechanisms by which synapses can “sense” the occurrence of exocytosis and timely initiate compensatory endocytosis. They include Ca²⁺ sensing, SV proteins sensing, and local membrane stress sensing. In addition, the spatial organization of endocytic zones adjacent to active zones provides a structural basis for efficient coupling between SV exocytosis and endocytosis. Through linking different endocytosis pathways with SV fusion, these mechanisms ensure necessary plasticity and robustness of nerve terminals to meet diverse physiological needs.