Rab4 and cellubrevin define different early endosome populations on the pathway of transferrin receptor recycling

SNARE proteins: Core engines of membrane fusion in cancer

Vesicles are loaded with a variety of cargoes, including membrane proteins, secreted proteins, signaling molecules, and various enzymes, etc. Not surprisingly, vesicle transport is essential for proper cellular life activities including growth, division, movement and cellular communication. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion of vesicles with their target compartments that is fundamental for cargo delivery. Recent studies have shown that multiple SNARE family members are aberrantly expressed in human cancers and actively contribute to malignant proliferation, invasion, metastasis, immune evasion and treatment resistance. Here, the localization and function of SNARE proteins in eukaryotic cells are firstly mapped. Then we summarize the expression and regulation of SNAREs in cancer, and describe their contribution to cancer progression and mechanisms, and finally we propose engineering botulinum toxin as a strategy to target SNAREs for cancer treatment.

Unconventional role of Rab4 in the secretory pathway in Leishmania

Leishmania donovani is an auxotroph for heme. Parasite acquires heme by clathrin-mediated endocytosis of hemoglobin by specific receptor. However, the regulation of receptor recycling pathway is not known in Leishmania. Here, we have cloned, expressed and characterized the Rab4 homologue from L. donovani. We have found that LdRab4 localizes in both early endosomes and Golgi in L. donovani. To understand the role of LdRab4 in L. donovani, we have generated transgenic parasites overexpressing GFP-LdRab4:WT, GFP-LdRab4:Q67L, and GFP-LdRab4:S22N. Our results have shown that overexpression of GFP-LdRab4:Q67L or GFP-LdRab4:S22N does not alter the cell surface localization of hemoglobin receptor in L. donovani. Surprisingly, we have found that overexpression of GFP-LdRab4:S22N significantly blocks the transport of Ldgp63 to the cell surface whereas the trafficking of Ldgp63 is induced to the cell surface in GFP-LdRab4:WT and GFP-LdRab4:Q67L overexpressing parasites. Consequently, we have found significant inhibition of gp63 secretion by GFP-LdRab4:S22N overexpressing parasites whereas secretion of Ldgp63 is enhanced in GFP-LdRab4:WT and GFP-LdRab4:Q67L overexpressing parasites in comparison to untransfected control parasites. Moreover, we have found that survival of transgenic parasites overexpressing GFP-LdRab4:S22N is severely compromised in macrophages in comparison to GFP-LdRab4:WT and GFP-LdRab4:Q67L expressing parasites. These results demonstrated that LdRab4 unconventionally regulates the secretory pathway in L. donovani.

IST1 regulates select recycling pathways

ESCRTs (Endosomal Sorting Complex Required for Transports) are a modular set of protein complexes with membrane remodeling activities that include the formation and release of intraluminal vesicles (ILVs) to generate multivesicular endosomes. While most of the 12 ESCRT-III proteins are known to play roles in ILV formation, IST1 has been associated with a wider range of endosomal remodeling events. Here, we extend previous studies of IST1 function in endosomal trafficking and confirm that IST1, along with its binding partner CHMP1B, contributes to scission of early endosomal carriers. Functionally, depleting IST1 impaired delivery of transferrin receptor from early/sorting endosomes to the endocytic recycling compartment and instead increased its rapid recycling to the plasma membrane via peripheral endosomes enriched in the clathrin adaptor AP-1. IST1 is also important for export of mannose 6-phosphate receptor from early/sorting endosomes. Examination of IST1 binding partners on endosomes revealed that IST1 interacts with the MIT domain-containing sorting nexin SNX15, a protein previously reported to regulate endosomal recycling. Our kinetic and spatial analyses establish that SNX15 and IST1 occupy a clathrin-containing subdomain on the endosomal perimeter distinct from those previously implicated in cargo retrieval or degradation. Using live-cell microscopy, we see that SNX15 and CHMP1B alternately recruit IST1 to this subdomain or the base of endosomal tubules. These findings indicate that IST1 contributes to a subset of recycling pathways from the early/sorting endosome.

miR-124 and VAMP3 Act Antagonistically in Human Neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor that affects developing nerve cells in the fetus, infants, and children. miR-124 is a microRNA (miRNA) enriched in neuronal tissues, and VAMP3 (vesicle-associated membrane protein 3) has been reported to be an miR-124 target, although the relationship between NB and miR-124 or VAMP3 is unknown. Our current work identified that miR-124 levels are high in NB cases and that elevated miR-124 correlates with worse NB outcomes. Conversely, depressed VAMP3 correlates with worse NB outcomes. To investigate the mechanisms by which miR-124 and VAMP3 regulate NB, we altered miR-124 or VAMP3 expression in human NB cells and observed that increased miR-124 and reduced VAMP3 stimulated cell proliferation and suppressed apoptosis, while increased VAMP3 had the opposite effects. Genome-wide mRNA expression analyses identified gene and pathway changes which might explain the NB cell phenotypes. Together, our studies suggest that miR-124 and VAMP3 could be potential new markers of NB and targets of NB treatments.

Receptor-mediated internalization promotes increased endosome size and number in a RAB4- and RAB5-dependent manner

Despite their significance in receptor-mediated internalization and continued signal transduction in cells, early/sorting endosomes (EE/SE) remain incompletely characterized, with many outstanding questions that surround the dynamics of their size and number. While several studies have reported increases in EE/SE size and number resulting from endocytic events, few studies have addressed such dynamics in a methodological and quantitative manner. Herein we apply quantitative fluorescence microscopy to measure the size and number of EE/SE upon internalization of two different ligands: transferrin and epidermal growth factor. Additionally, we used siRNA knock-down to determine the involvement of 5 different endosomal RAB proteins (RAB4, RAB5, RAB8A, RAB10 and RAB11A) in EE/SE dynamics. Our study provides new information on the dynamics of endosomes during endocytosis, an important reference for researchers studying receptor-mediated internalization and endocytic events.

IST1 regulates select endosomal recycling pathways

ESCRTs (Endosomal Sorting Complex Required for Transport) are a modular set of protein complexes with membrane remodeling activities that include the formation and release of intralumenal vesicles (ILVs) to generate multivesicular endosomes. While most of the 12 ESCRT-III proteins are known to play roles in ILV formation, IST1 has been associated with a wider range of endosomal remodeling events. Here, we extend previous studies of IST1 function in endosomal trafficking and confirm that IST1, along with its binding partner CHMP1B, contributes to scission of early endosomal carriers. Depleting IST1 impaired delivery of transferrin receptor from early/sorting endosomes to the endocytic recycling compartment and instead increased its rapid recycling to the plasma membrane via peripheral endosomes enriched in the clathrin adaptor AP-1. IST1 is also important for export of mannose 6-phosphate receptor from early/sorting endosomes. Examination of IST1 binding partners on endosomes revealed that IST1 interacts with the MIT domain-containing sorting nexin SNX15, a protein previously reported to regulate endosomal recycling. Our kinetic and spatial analyses establish that SNX15 and IST1 occupy a clathrin-containing subdomain on the endosomal perimeter distinct from those previously implicated in cargo retrieval or degradation. Using live-cell microscopy we see that SNX15 and CHMP1B alternately recruit IST1 to this subdomain or the base of endosomal tubules. These findings indicate that IST1 contributes to a subset of recycling pathways from the early/sorting endosome.

Vesicle-associated membrane protein 2 is a cargo-selective v-SNARE for a subset of GPCRs

Vesicle fusion at the plasma membrane is critical for releasing hormones and neurotransmitters and for delivering the cognate G protein-coupled receptors (GPCRs) to the cell surface. The SNARE fusion machinery that releases neurotransmitters has been well characterized. In contrast, the fusion machinery that delivers GPCRs is still unknown. Here, using high-speed multichannel imaging to simultaneously visualize receptors and v-SNAREs in real time in individual fusion events, we identify VAMP2 as a selective v-SNARE for GPCR delivery. VAMP2 was preferentially enriched in vesicles that mediate the surface delivery of μ opioid receptor (MOR), but not other cargos, and was required selectively for MOR recycling. Interestingly, VAMP2 did not show preferential localization on MOR-containing endosomes, suggesting that v-SNAREs are copackaged with specific cargo into separate vesicles from the same endosomes. Together, our results identify VAMP2 as a cargo-selective v-SNARE and suggest that surface delivery of specific GPCRs is mediated by distinct fusion events driven by distinct SNARE complexes.

KDEL Receptor Trafficking to the Plasma Membrane Is Regulated by ACBD3 and Rab4A-GTP

KDEL receptor-1 maintains homeostasis in the early secretory pathway by capturing and retrieving ER chaperones to the ER during heavy secretory activity. Unexpectedly, a fraction of the receptor is also known to reside in the plasma membrane (PM), although it is largely unknown exactly how the KDEL receptor gets exported from the Golgi and travels to the PM. We have previously shown that a Golgi scaffolding protein (ACBD3) facilitates KDEL receptor localization at the Golgi via the regulating cargo wave-induced cAMP/PKA-dependent signaling pathway. Upon endocytosis, surface-expressed KDEL receptor undergoes highly complex itineraries through the Golgi and the endo-lysosomal compartments, where the endocytosed receptor utilizes Rab14A- and Rab11A-positive recycling endosomes and clathrin-decorated tubulovesicular carriers. In this study, we sought to investigate the mechanism through which the KDEL receptor gets exported from the Golgi en route to the PM. We report here that ACBD3 depletion results in greatly increased trafficking of KDEL receptor to the PM via Rab4A-positive tubular carriers emanating from the Golgi. Expression of constitutively activated Rab4A mutant (Q72L) increases the surface expression of KDEL receptor up to 2~3-fold, whereas Rab4A knockdown or the expression of GDP-locked Rab4A mutant (S27N) inhibits KDEL receptor targeting of the PM. Importantly, KDELR trafficking from the Golgi to the PM is independent of PKA- and Src kinase-mediated mechanisms. Taken together, these results reveal that ACBD3 and Rab4A play a key role in regulating KDEL receptor trafficking to the cell surface.

Autophagy and neurodegeneration: Unraveling the role of C9ORF72 in the regulation of autophagy and its relationship to ALS-FTD pathology

The proper functioning of the cell clearance machinery is critical for neuronal health within the central nervous system (CNS). In normal physiological conditions, the cell clearance machinery is actively involved in the elimination of misfolded and toxic proteins throughout the lifetime of an organism. The highly conserved and regulated pathway of autophagy is one of the important processes involved in preventing and neutralizing pathogenic buildup of toxic proteins that could eventually lead to the development of neurodegenerative diseases (NDs) such as Alzheimer’s disease or Amyotrophic lateral sclerosis (ALS). The most common genetic cause of ALS and frontotemporal dementia (FTD) is a hexanucleotide expansion consisting of GGGGCC (G4C2) repeats in the chromosome 9 open reading frame 72 gene (C9ORF72). These abnormally expanded repeats have been implicated in leading to three main modes of disease pathology: loss of function of the C9ORF72 protein, the generation of RNA foci, and the production of dipeptide repeat proteins (DPRs). In this review, we discuss the normal physiological role of C9ORF72 in the autophagy-lysosome pathway (ALP), and present recent research deciphering how dysfunction of the ALP synergizes with C9ORF72 haploinsufficiency, which together with the gain of toxic mechanisms involving hexanucleotide repeat expansions and DPRs, drive the disease process. This review delves further into the interactions of C9ORF72 with RAB proteins involved in endosomal/lysosomal trafficking, and their role in regulating various steps in autophagy and lysosomal pathways. Lastly, the review aims to provide a framework for further investigations of neuronal autophagy in C9ORF72-linked ALS-FTD as well as other neurodegenerative diseases.

An optimized exosome production strategy for enhanced yield while without sacrificing cargo loading efficiency

Background

Exosome mediated mRNA delivery is a promising strategy for the treatment of multiple diseases. However, the low yield of exosomes is a bottleneck for clinical translation. In this study, we boosted exosome production via simultaneously reducing the expression of genes inhibiting exosome biogenesis and supplementing the culture medium with red cell membrane components.

Results

Among the candidate genes, knocking down of Rab4 was identified to have the highest efficacy in promoting exosome biogenesis while without any obvious cytotoxicity. Additionally, supplementing red cell membrane particles (RCMPs) in the culture medium further promoted exosome production. Combination of Rab4 knockdown and RCMP supplement increased exosome yield up to 14-fold. As a proof-of-concept study, low-density lipoprotein receptor (Ldlr) mRNA was forced expressed in the exosome donor cells and passively encapsulated into the exosomes during biogenesis with this strategy. Though exosome production per cell increased, the booster strategy didn’t alter the loading efficiency of therapeutic Ldlr mRNA per exosome. Consistently, the therapeutic exosomes derived by the strategy alleviated liver steatosis and atherosclerosis in Ldlr^−/− mice, similar as the exosomes produced by routine methods.

Conclusions

Together, the proposed exosome booster strategy conquers the low yield bottleneck to some extent and would certainly facilitate the clinical translation of exosomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01668-3.

Protein phosphatase 2A regulation of GABAB receptors normalizes ischemia-induced aberrant receptor trafficking and provides neuroprotection

One major factor regulating the strength of GABA_B receptor signaling and thereby neuronal excitability is the dynamic control of their cell surface expression. GABA_B receptors are constitutively internalized and recycled back to the plasma membrane to maintain a stable number of receptors at cell surface for appropriate signaling. Protein phosphatase 2A (PP2A) dependent dephosphorylation of serine 783 (S783) in the GABA_B2 subunit is a key event for downregulating GABA_B receptor cell surface expression particularly under conditions associated with excitotoxicity. Here, we investigated the role of PP2A in regulating GABA_B receptor cell surface expression under physiological and excitotoxic conditions. For this purpose, we developed an interfering peptide (PP2A-Pep) that inhibits the interaction of GABA_B receptors with PP2A. Using cultured cortical neurons, we found that PP2A downregulates GABA_B receptor cell surface expression by inhibiting recycling of the receptors and thereby promoting degradation of the receptors. Inhibition of the GABA_B receptor/PP2A interaction by PP2A-Pep in cultured cortical neurons restored GABA_B receptor cell surface expression after excitotoxic stress and inhibited progressing neuronal death even when added 48 h after the insult. To explore the therapeutic potential of PP2A-Pep, we further analyzed effect of PP2A-Pep in the middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia. Incubation of brain slices prepared from MCAO-treated mice with PP2A-Pep restored normal GABA_B receptor expression and GABA_B receptor-mediated inhibition, reduced ischemic-induced overexcitability of neurons, and prevented neuronal death in the ischemic penumbra. This data illustrates the crucial role of regulating GABA_B receptor phosphorylation by PP2A for controlling neuronal inhibition and excitability. The results further suggest that interfering with the GABA_B receptor/PP2A interaction is a promising strategy for the development of specific therapeutic interventions to treat neurological diseases associated with a disturbed excitation/inhibition balance and downregulation of GABA_B receptors.

Membrane Targeting and GTPase Activity of Rab7 Are Required for Its Ubiquitination by RNF167

Rab7 is a GTPase that controls late endosome and lysosome trafficking. Recent studies have demonstrated that Rab7 is ubiquitinated, a post-translational modification mediated by an enzymatic cascade. To date, only one ubiquitin E3 ligase and one deubiquitinase have been identified in regulating Rab7 ubiquitination. Here, we report that RNF167, a transmembrane endolysosomal ubiquitin ligase, can ubiquitinate Rab7. Using immunoprecipitation and in vitro ubiquitination assays, we demonstrate that Rab7 is a direct substrate of RNF167. Subcellular fractionation indicates that RNF167 activity maintains Rab7′s membrane localization. Epifluorescence microscopy in HeLa cells shows that Rab7-positive vesicles are larger under conditions enabling Rab7 ubiquitination by RNF167. Characterization of its ubiquitination reveals that Rab7 must be in its GTP-bound active form for membrane anchoring and, thus, accessible for RNF167-mediated ubiquitin attachment. Cellular distribution analyses of lysosome marker Lamp1 show that vesicle positioning is independent of Rab7 and RNF167 expression and that Rab7 endosomal localization is not affected by RNF167 knockdown. However, both Rab7 and RNF167 depletion affect each other’s lysosomal localization. Finally, this study demonstrates that the RNF167-mediated ubiquitination of Rab7 GTPase is impaired by variants of Charcot–Marie–Tooth Type 2B disease. This study identified RNF167 as a new ubiquitin ligase for Rab7 while expanding our knowledge of the mechanisms underlying the ubiquitination of Rab7.

Endosomal LC3C-pathway selectively targets plasma membrane cargo for autophagic degradation

Autophagy selectively targets cargo for degradation, yet mechanistic understanding remains incomplete. The ATG8-family plays key roles in autophagic cargo recruitment. Here by mapping the proximal interactome of ATG8-paralogs, LC3B and LC3C, we uncover a LC3C-Endocytic-Associated-Pathway (LEAP) that selectively recruits plasma-membrane (PM) cargo to autophagosomes. We show that LC3C localizes to peripheral endosomes and engages proteins that traffic between PM, endosomes and autophagosomes, including the SNARE-VAMP3 and ATG9, a transmembrane protein essential for autophagy. We establish that endocytic LC3C binds cargo internalized from the PM, including the Met receptor tyrosine kinase and transferrin receptor, and is necessary for their recruitment into ATG9 vesicles targeted to sites of autophagosome initiation. Structure-function analysis identified that LC3C-endocytic localization and engagement with PM-cargo requires the extended carboxy-tail unique to LC3C, the TBK1 kinase, and TBK1-phosphosites on LC3C. These findings identify LEAP as an unexpected LC3C-dependent pathway, providing new understanding of selective coupling of PM signalling with autophagic degradation.

On demand expression control of endogenous genes with DExCon, DExogron and LUXon reveals differential dynamics of Rab11 family members

CRISPR technology has made generation of gene knock-outs widely achievable in cells. However, once inactivated, their re-activation remains difficult, especially in diploid cells. Here, we present DExCon (Doxycycline-mediated endogenous gene Expression Control), DExogron (DExCon combined with auxin-mediated targeted protein degradation), and LUXon (light responsive DExCon) approaches which combine one-step CRISPR-Cas9-mediated targeted knockin of fluorescent proteins with an advanced Tet-inducible TRE3GS promoter. These approaches combine blockade of active gene expression with the ability to re-activate expression on demand, including activation of silenced genes. Systematic control can be exerted using doxycycline or spatiotemporally by light, and we demonstrate functional knock-out/rescue in the closely related Rab11 family of vesicle trafficking regulators. Fluorescent protein knock-in results in bright signals compatible with low-light live microscopy from monoallelic modification, the potential to simultaneously image different alleles of the same gene, and bypasses the need to work with clones. Protein levels are easily tunable to correspond with endogenous expression through cell sorting (DExCon), timing of light illumination (LUXon), or by exposing cells to different levels of auxin (DExogron). Furthermore, our approach allowed us to quantify previously unforeseen differences in vesicle dynamics, transferrin receptor recycling, expression kinetics, and protein stability among highly similar endogenous Rab11 family members and their colocalization in triple knock-in ovarian cancer cell lines.

Therapeutic Validation of GEF-H1 Using a De Novo Designed Inhibitor in Models of Retinal Disease

Inflammation and fibrosis are important components of diseases that contribute to the malfunction of epithelia and endothelia. The Rho guanine nucleotide exchange factor (GEF) GEF-H1/ARHGEF-2 is induced in disease and stimulates inflammatory and fibrotic processes, cell migration, and metastasis. Here, we have generated peptide inhibitors to block the function of GEF-H1. Inhibitors were designed using a structural in silico approach or by isolating an inhibitory sequence from the autoregulatory C-terminal domain. Candidate inhibitors were tested for their ability to block RhoA/GEF-H1 binding in vitro, and their potency and specificity in cell-based assays. Successful inhibitors were then evaluated in models of TGFβ-induced fibrosis, LPS-stimulated endothelial cell-cell junction disruption, and cell migration. Finally, the most potent inhibitor was successfully tested in an experimental retinal disease mouse model, in which it inhibited blood vessel leakage and ameliorated retinal inflammation when treatment was initiated after disease diagnosis. Thus, an antagonist that blocks GEF-H1 signaling effectively inhibits disease features in in vitro and in vivo disease models, demonstrating that GEF-H1 is an effective therapeutic target and establishing a new therapeutic approach.

Rab21 Protein Is Degraded by Both the Ubiquitin-Proteasome Pathway and the Autophagy-Lysosome Pathway

Rab21 is a GTPase protein that is functional in intracellular trafficking and involved in the pathologies of many diseases, such as Alzheimer’s disease (AD), glioma, cancer, etc. Our previous work has reported its interaction with the catalytic subunit of gamma-secretase, PS1, and it regulates the activity of PS1 via transferring it from the early endosome to the late endosome/lysosome. However, it is still unknown how Rab21 protein itself is regulated. This work revealed that Rab21 protein, either endogenously or exogenously, can be degraded by the ubiquitin-proteasome pathway and the autophagy-lysosome pathway. It is further observed that the ubiquitinated Rab21 is increased, but the total protein is unchanged in AD model mice. We further observed that overexpression of Rab21 leads to increased expression of a series of genes involved in the autophagy-lysosome pathway. We speculated that even though the ubiquitinated Rab21 is increased due to the impaired proteasome function in the AD model, the autophagy-lysosome pathway functions in parallel to degrade Rab21 to keep its protein level in homeostasis. In conclusion, understanding the characters of Rab21 protein itself help explore its potential as a target for therapeutic strategy in diseases.

Downregulation of beclin 1 restores arsenite-induced impaired autophagic flux by improving the lysosomal function in the brain

Arsenite is a toxic metalloid that causes various adverse effects in the brain. However, the underlying mechanisms of arsenite-induced neurotoxicity remain poorly understood. In this study, both adult beclin 1^+/+ and beclin 1^+/- mice were employed to establish a model of chronic arsenite exposure by treating with arsenite via drinking water for 6 months. The results clearly demonstrated that exposure to arsenite profoundly caused damage to the cerebral cortex, induced autophagy and impaired autophagic flux in the cerebral cortex. Heterozygous disruption of beclin 1 in animals remarkably alleviated the neurotoxic effects of arsenite. To verify the results obtained in the animals, a permanent U251 cell line was used. After treating of cells with arsenite, similar phenomenon was also observed, showing the significant elevation in the expression levels of autophagy-related genes. Importantly, lysosomal dysfunction caused by arsenite was observed in vitro and in vivo. Either knockdown of beclin 1 in cells or heterozygous disruption of beclin 1 in animals remarkably alleviated the lysosomal dysfunction induced by arsenite. These findings indicate that downregulation of beclin 1 could restore arsenite-induced impaired autophagic flux possibly through improving lysosomal function, and correct that regulation of autophagy via beclin 1 would be an alternative approach for the treatment of arsenite neurotoxicity.

The HIV-1 matrix protein p17 does cross the Blood-Brain Barrier

Human immunodeficiency virus type-1(HIV-1)-associated neurocognitive disorder (HAND) remains an important neurological manifestation in HIV-1-infected (HIV⁺) patients. Furthermore, the HIV-1 matrix protein p17 (p17) detection in the central nervous system (CNS) and its ability to form toxic assemblies in the brain has been recently confirmed. Here we show for the first time using both an in vitro blood-brain barrier (BBB) model and in vivo biodistribution studies in healthy mice that p17 can cross the BBB. There is fast brain uptake with 0.35 ± 0.19% of injected activity per gram of tissue (I.A./g) two minutes after administration, followed by brain accumulation with 0.28 ± 0.09% I.A./g after 1 h. The interaction of p17 with the chemokine receptor 2 (CXCR2) at the surface of brain endothelial cells triggers transcytosis. The present study supports the hypothesis of a direct role of free p17 in neuronal dysfunction in HAND by demonstrating its intrinsic ability to reach the CNS. IMPORTANCE The number of patients affected by HIV-1-associated neurocognitive disorder (HAND) ranges from 30 to 50% of HIV-infected (HIV⁺) patients. The mechanisms leading to HAND development need to be elucidated, but the role of secreted viral proteins, chemokines, and proinflammatory molecules appears to be clear. In particular, the blood-brain barrier (BBB) represents a route for entry into the central nervous system (CNS) thus playing an important role in HAND. Several findings suggest a key role for the HIV-1 matrix protein p17 (p17) as a microenvironmental factor capable of inducing neurocognitive disorders. Here we show, the ability of the p17 to cross the BBB and to reach the CNS thus playing a crucial role in neuronal dysfunction in HAND.

Shigella flexneri subverts host polarized exocytosis to enhance cell-to-cell spread

Shigella flexneri is a gram-negative bacterial pathogen that causes dysentery. Critical for disease is the ability of Shigella to use an actin-based motility (ABM) process to spread between cells of the colonic epithelium. ABM transports bacteria to the periphery of host cells, allowing the formation of plasma membrane protrusions that mediate spread to adjacent cells. Here we demonstrate that efficient protrusion formation and cell-to-cell spread of Shigella involves bacterial stimulation of host polarized exocytosis. Using an exocytic probe, we found that exocytosis is locally upregulated in bacterial protrusions in a manner that depends on the Shigella type III secretion system. Experiments involving RNA interference (RNAi) indicate that efficient bacterial protrusion formation and spread require the exocyst, a mammalian multi-protein complex known to mediate polarized exocytosis. In addition, the exocyst component Exo70 and the exocyst regulator RalA were recruited to Shigella protrusions, suggesting that bacteria manipulate exocyst function. Importantly, RNAi-mediated depletion of exocyst proteins or RalA reduced the frequency of protrusion formation and also the lengths of protrusions, demonstrating that the exocyst controls both the initiation and elongation of protrusions. Collectively, our results reveal that Shigella co-opts the exocyst complex to disseminate efficiently in host cell monolayers.

Sustained Baclofen-Induced Activation of GABA B Receptors After Cerebral Ischemia Restores Receptor Expression and Function and Limits Progressing Loss of Neurons

One important function of GABA_B receptors is the control of neuronal activity to prevent overexcitation and thereby excitotoxic death, which is a hallmark of cerebral ischemia. Consequently, sustained activation of GABA_B receptors with the selective agonist baclofen provides neuroprotection in in vitro and in vivo models of cerebral ischemia. However, excitotoxic conditions severely downregulate the receptors, which would compromise the neuroprotective effectiveness of baclofen. On the other hand, recent work suggests that sustained activation of GABA_B receptors stabilizes receptor expression. Therefore, we addressed the question whether sustained activation of GABA_B receptors reduces downregulation of the receptor under excitotoxic conditions and thereby preserves GABA_B receptor-mediated inhibition. In cultured neurons subjected to oxygen and glucose deprivation (OGD), to mimic cerebral ischemia, GABA_B receptors were severely downregulated. Treatment of the cultures with baclofen after OGD restored GABA_B receptor expression and reduced loss of neurons. Restoration of GABA_B receptors was due to enhanced fast recycling of the receptors, which reduced OGD-induced sorting of the receptors to lysosomal degradation. Utilizing the middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia, we verified the severe downregulation of GABA_B receptors in the affected cortex and a partial restoration of the receptors after systemic injection of baclofen. Restored receptor expression recovered GABA_B receptor-mediated currents, normalized the enhanced neuronal excitability observed after MCAO and limited progressive loss of neurons. These results suggest that baclofen-induced restoration of GABA_B receptors provides the basis for the neuroprotective activity of baclofen after an ischemic insult. Since GABA_B receptors regulate multiple beneficial pathways, they are promising targets for a neuroprotective strategy in acute cerebral ischemia.

Conserved and Distinct Elements of Phagocytosis in Human and C. elegans

Endocytosis provides the cellular nutrition and homeostasis of organisms, but pathogens often take advantage of this entry point to infect host cells. This is counteracted by phagocytosis that plays a key role in the protection against invading microbes both during the initial engulfment of pathogens and in the clearance of infected cells. Phagocytic cells balance two vital functions: preventing the accumulation of cell corpses to avoid pathological inflammation and autoimmunity, whilst maintaining host defence. In this review, we compare elements of phagocytosis in mammals and the nematode Caenorhabditis elegans. Initial recognition of infection requires different mechanisms. In mammals, pattern recognition receptors bind pathogens directly, whereas activation of the innate immune response in the nematode rather relies on the detection of cellular damage. In contrast, molecules involved in efferocytosis—the engulfment and elimination of dying cells and cell debris—are highly conserved between the two species. Therefore, C. elegans is a powerful model to research mechanisms of the phagocytic machinery. Finally, we show that both mammalian and worm studies help to understand how the two phagocytic functions are interconnected: emerging data suggest the activation of innate immunity as a consequence of defective apoptotic cell clearance.

Rab7-Mediated Endocytosis Establishes Patterning of Wnt Activity through Inactivation of Dkk Antagonism

Endocytosis has been proposed to modulate cell signaling activities. However, the role of endocytosis in embryogenesis, which requires coordination of multiple signaling inputs, has remained less understood. We previously showed that mouse embryos lacking a small guanosine triphosphate (GTP)-binding protein Rab7 implicated in endocytic flow are defective in gastrulation. Here, we investigate how subcellular defects associated with Rab7 deficiency are related to the observed developmental defects. Rab7-deficient embryos fail to organize mesodermal tissues due to defects in Wnt-β-catenin signaling. Visceral endoderm (VE)-specific ablation of Rab7 results in patterning defects similar to systemic Rab7 deletion. Rab7 mutants accumulate the Wnt antagonist Dkk1 in the extracellular space and in intracellular compartments throughout the VE epithelium. These data indicate that Rab7-dependent endocytosis regulates the concentration and availability of extracellular Dkk1, thereby relieving the epiblast of antagonism. This intercellular mechanism therefore organizes distinct spatiotemporal patterns of canonical Wnt activity during the peri-gastrulation stages of embryonic development.

Novel function for AP-1B during cell migration

The epithelial cell-specific clathrin adaptor protein (AP)-1B has a well-established role in polarized sorting of cargos to the basolateral membrane. Here we show that β1 integrin was dependent on AP-1B and its coadaptor, autosomal recessive hypercholesterolemia protein (ARH), for sorting to the basolateral membrane. We further demonstrate an unprecedented role for AP-1B at the basal plasma membrane during collective cell migration of epithelial sheets. During wound healing, expression of AP-1B (and ARH in AP–1B-positive cells) slowed epithelial-cell migration. We show that AP-1B colocalized with β1 integrin in focal adhesions during cell migration using confocal microscopy and total internal reflection fluorescence microscopy on fixed specimens. Further, AP-1B labeling in cell protrusions was distinct from labeling for the endocytic adaptor complex AP-2. Using stochastic optical reconstruction microscopy we identified numerous AP–1B-coated structures at or close to the basal plasma membrane in cell protrusions. In addition, immunoelectron microscopy showed AP-1B in coated pits and vesicles at the plasma membrane during cell migration. Lastly, quantitative real-time reverse transcription PCR analysis of human epithelial-derived cell lines revealed a loss of AP-1B expression in highly migratory metastatic cancer cells suggesting that AP-1B’s novel role at the basal plasma membrane during cell migration might be an anticancer mechanism.

Molecular Mechanisms of Intercellular Dissemination of Bacterial Pathogens

Several intracellular bacterial pathogens, including Listeria monocytogenes, Shigella flexerni, and Rickettsia spp. use an actin-based motility process to spread in mammalian cell monolayers. Cell-to-cell spread is mediated by protrusive structures that contain bacteria encased in the host cell plasma membrane. These protrusions, which form in infected host cells, are internalized by neighboring cells. In this review, we summarize key findings on cell-to-cell spread, focusing on recent work on mechanisms of protrusion formation and internalization. We also discuss the dynamic behavior of bacterial populations during spread, and highlight recent findings showing that intercellular spread by an extracellular bacterial pathogen.

Assorted dysfunctions of endosomal alkali cation/proton exchanger SLC9A6 variants linked to Christianson syndrome

Genetic screening has identified numerous variants of the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na+,K+)/H+ exchanger 6 (NHE6) gene that cause Christianson syndrome, a debilitating X-linked developmental disorder associated with a range of neurological, somatic, and behavioral symptoms. Many of these variants cause complete loss of NHE6 expression, but how subtler missense substitutions or nonsense mutations that partially truncate its C-terminal cytoplasmic regulatory domain impair NHE6 activity and endosomal function are poorly understood. Here, we describe the molecular and cellular consequences of six unique mutations located in the N-terminal cytoplasmic segment (A9S), the membrane ion translocation domain (L188P and G383D), and the C-terminal regulatory domain (E547*, R568Q, and W570*) of human NHE6 that purportedly cause disease. Using a heterologous NHE6-deficient cell expression system, we show that the biochemical, catalytic, and cellular properties of the A9S and R568Q variants were largely indistinguishable from those of the WT transporter, which obscured their disease significance. By contrast, the L188P, G383D, E547*, and W570* mutants exhibited variable deficiencies in biosynthetic post-translational maturation, membrane sorting, pH homeostasis in recycling endosomes, and cargo trafficking, and they also triggered apoptosis. These findings broaden our understanding of the molecular dysfunctions of distinct NHE6 variants associated with Christianson syndrome.

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.

The Host GTPase Arf1 and Its Effectors AP1 and PICK1 Stimulate Actin Polymerization and Exocytosis To Promote Entry of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterium that causes gastroenteritis, meningitis, or abortion. Listeria induces its internalization (entry) into some human cells through interaction of the bacterial surface protein InlB with its host receptor, the Met tyrosine kinase. InlB and Met promote entry through stimulation of localized actin polymerization and exocytosis. How actin cytoskeletal changes and exocytosis are controlled during entry is not well understood. Here, we demonstrate important roles for the host GTPase Arf1 and its effectors AP1 and PICK1 in actin polymerization and exocytosis during InlB-dependent uptake. Depletion of Arf1 by RNA interference (RNAi) or inhibition of Arf1 activity using a dominant-negative allele impaired InlB-dependent internalization, indicating an important role for Arf1 in this process. InlB stimulated an increase in the GTP-bound form of Arf1, demonstrating that this bacterial protein activates Arf1. RNAi and immunolocalization studies indicated that Arf1 controls exocytosis and actin polymerization during entry by recruiting the effectors AP1 and PICK1 to the plasma membrane. In turn, AP1 and PICK1 promoted plasma membrane translocation of both Filamin A (FlnA) and Exo70, two host proteins previously found to mediate exocytosis during InlB-dependent internalization (M. Bhalla, H. Van Ngo, G. C. Gyanwali, and K. Ireton, Infect Immun 87:e00689-18, 2018, https://doi.org/10.1128/IAI.00689-18). PICK1 mediated recruitment of Exo70 but not FlnA. Collectively, these results indicate that Arf1, AP1, and PICK1 stimulate exocytosis by redistributing FlnA and Exo70 to the plasma membrane. We propose that Arf1, AP1, and PICK1 are key coordinators of actin polymerization and exocytosis during infection of host cells by Listeria.

AMPK phosphorylation of the β1Pix exchange factor regulates the assembly and function of an ENaC inhibitory complex in kidney epithelial cells

The metabolic sensor AMP-activated protein kinase (AMPK) inhibits the epithelial Na⁺ channel (ENaC), a key regulator of salt reabsorption by the kidney and thus total body volume and blood pressure. Recent studies have suggested that AMPK promotes the association of p21-activated kinase-interacting exchange factor-β₁ β₁Pix, 14-3-3 proteins, and the ubiquitin ligase neural precursor cell expressed developmentally downregulated protein (Nedd)4-2 into a complex that inhibits ENaC by enhancing Nedd4-2 binding to ENaC and ENaC degradation. Functional β₁Pix is required for ENaC inhibition by AMPK and promotes Nedd4-2 phosphorylation and stability in mouse kidney cortical collecting duct cells. Here, we report that AMPK directly phosphorylates β₁Pix in vitro. Among several AMPK phosphorylation sites on β₁Pix detected by mass spectrometry, Ser⁷¹ was validated as functionally significant. Compared with wild-type β₁Pix, overexpression of a phosphorylation-deficient β₁Pix-S71A mutant attenuated ENaC inhibition and the AMPK-activated interaction of both β₁Pix and Nedd4-2 to 14-3-3 proteins in cortical collecting duct cells. Similarly, overexpression of a β₁Pix-Δ602-611 deletion tract mutant unable to bind 14-3-3 proteins decreased the interaction between Nedd4-2 and 14-3-3 proteins, suggesting that 14-3-3 binding to β₁Pix is critical for the formation of a β₁Pix/Nedd4-2/14-3-3 complex. With expression of a general peptide inhibitor of 14-3-3-target protein interactions (R18), binding of both β₁Pix and Nedd4-2 to 14-3-3 proteins was reduced, and AMPK-dependent ENaC inhibition was also attenuated. Altogether, our results demonstrate the importance of AMPK-mediated phosphorylation of β₁Pix at Ser⁷¹, which promotes 14-3-3 interactions with β₁Pix and Nedd4-2 to form a tripartite ENaC inhibitory complex, in the mechanism of ENaC regulation by AMPK.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

WIP/ITSN1 complex is involved in cellular vesicle trafficking and formation of filopodia-like protrusions

WIP (WASP interacting protein) together with N-WASP (neural Wiskott-Aldrich syndrome protein) regulates actin polymerization that is crucial for invadopodia and filopodia formation. Recently, we reported the WIP interaction with ITSN1 which is highly implicated in endo-/exocytosis, apoptosis, mitogenic signaling and cytoskeleton rearrangements. Here we demonstrate that the WIP/ITSN1 complex is involved in the transferrin receptor recycling and partially co-localizes with a marker of the fast recycling endosomes, RAB4. Moreover, ITSN1 recruits WIP to RAB4-positive vesicles upon overexpression. Our data indicate that WIP enhances the interaction of N-WASP with ITSN1 and promotes ITSN1/β-actin association. Moreover, the WIP/ITSN1-L complex facilitates formation of filopodia-like protrusions in MCF-7 cells. Thus, WIP/ITSN1 complex is involved in the cellular vesicle trafficking and actin-dependent membrane processes.

Regulation of the Cell Biology of Antigen Cross-Presentation

Antigen cross-presentation is an adaptation of the cellular process of loading MHC-I molecules with endogenous peptides during their biosynthesis within the endoplasmic reticulum. Cross-presented peptides derive from internalized proteins, microbial pathogens, and transformed or dying cells. The physical separation of internalized cargo from the endoplasmic reticulum, where the machinery for assembling peptide-MHC-I complexes resides, poses a challenge. To solve this problem, deliberate rewiring of organelle communication within cells is necessary to prepare for cross-presentation, and different endocytic receptors and vesicular traffic patterns customize the emergent cross-presentation compartment to the nature of the peptide source. Three distinct pathways of vesicular traffic converge to form the ideal cross-presentation compartment, each regulated differently to supply a unique component that enables cross-presentation of a diverse repertoire of peptides. Delivery of centerpiece MHC-I molecules is the critical step regulated by microbe-sensitive Toll-like receptors. Defining the subcellular sources of MHC-I and identifying sites of peptide loading during cross-presentation remain key challenges.

The parasite Toxoplasma sequesters diverse Rab host vesicles within an intravacuolar network

Many intracellular pathogens subvert host membrane trafficking pathways to promote their replication. Toxoplasma multiplies in a membrane-bound parasitophorous vacuole (PV) that interacts with mammalian host organelles and intercepts Golgi Rab vesicles to acquire sphingolipids. The mechanisms of host vesicle internalization and processing within the PV remain undefined. We demonstrate that Toxoplasma sequesters a broad range of Rab vesicles into the PV. Correlative light and electron microscopy analysis of infected cells illustrates that intravacuolar Rab1A vesicles are surrounded by the PV membrane, suggesting a phagocytic-like process for vesicle engulfment. Rab11A vesicles concentrate to an intravacuolar network (IVN), but this is reduced in Δgra2 and Δgra2Δgra6 parasites, suggesting that tubules stabilized by the TgGRA2 and TgGRA6 proteins secreted by the parasite within the PV contribute to host vesicle sequestration. Overexpression of a phospholipase TgLCAT, which is localized to the IVN, results in a decrease in the number of intravacuolar GFP-Rab11A vesicles, suggesting that TgLCAT controls lipolytic degradation of Rab vesicles for cargo release.

Cellubrevin/vesicle-associated membrane protein-3-mediated endocytosis and trafficking regulate platelet functions

Endocytosis is key to fibrinogen (Fg) uptake, trafficking of integrins (αIIbβ₃, α_vβ₃), and purinergic receptors (P2Y₁, P2Y₁₂), and thus normal platelet function. However, the molecular machinery required and possible trafficking routes are still ill-defined. To further identify elements of the platelet endocytic machinery, we examined the role of a vesicle-residing, soluble N-ethylmaleimide factor attachment protein receptor (v-SNARE) called cellubrevin/vesicle-associated membrane protein-3 (VAMP-3) in platelet function. Although not required for normal platelet exocytosis or hemostasis, VAMP-3^-/- mice had less platelet-associated Fg, indicating a defect in Fg uptake/storage. Other granule markers were unaffected. Direct experiments, both in vitro and in vivo, showed that loss of VAMP-3 led to a robust defect in uptake/storage of Fg in platelets and cultured megakaryocytes. Uptake of the fluid-phase marker, dextran, was only modestly affected. Time-dependent uptake and endocytic trafficking of Fg and dextran were followed using 3-dimensional-structured illumination microscopy. Dextran uptake was rapid compared with Fg, but both cargoes progressed through Rab4⁺, Rab11⁺, and von Willebrand factor (VWF)⁺ compartments in wild-type platelets in a time-dependent manner. In VAMP-3^-/- platelets, the 2 cargoes showed limited colocalization with Rab4, Rab11, or VWF. Loss of VAMP-3 also affected some acute platelet functions, causing enhanced spreading on Fg and fibronectin and faster clot retraction compared with wild-type. In addition, the rate of Janus kinase 2 phosphorylation, initiated through the thrombopoietin receptor (TPOR/Mpl) activation, was affected in VAMP-3^-/- platelets. Collectively, our studies show that platelets are capable of a range of endocytosis steps, with VAMP-3 being pivotal in these processes.

The comings and goings of MHC class I molecules herald a new dawn in cross-presentation

MHC class I (MHC-I) molecules are the centerpieces of cross-presentation. They are loaded with peptides derived from exogenous sources and displayed on the plasma membrane to communicate with CD8 T cells, relaying a message of tolerance or attack. The study of cross-presentation has been focused on the relative contributions of the vacuolar versus cytosolic pathways of antigen processing and the location where MHC-I molecules are loaded. While vacuolar processing generates peptides loaded onto vacuolar MHC-I molecules, how and where exogenous peptides generated by the proteasome and transported by TAP meet MHC-I molecules for loading has been a matter of debate. The source and trafficking of MHC-I molecules in dendritic cells have largely been ignored under the expectation that these molecules came from the Endoplasmic reticulum (ER) or the plasma membrane. New studies reveal a concentrated pool of MHC-I molecules in the endocytic recycling compartment (ERC). These pools are rapidly mobilized to phagosomes carrying microbial antigens, and in a signal-dependent manner under the control of Toll-like receptors. The phagosome becomes a dynamic hub receiving traffic from multiple sources, the ER-Golgi intermediate compartment for delivering the peptide-loading machinery and the ERC for deploying MHC-I molecules that alert CD8 T cells of infection.

Select Rab GTPases Regulate the Pulmonary Endothelium via Endosomal Trafficking of Vascular Endothelial-Cadherin

Pulmonary edema occurs in settings of acute lung injury, in diseases, such as pneumonia, and in acute respiratory distress syndrome. The lung interendothelial junctions are maintained in part by vascular endothelial (VE)-cadherin, an adherens junction protein, and its surface expression is regulated by endocytic trafficking. The Rab family of small GTPases are regulators of endocytic trafficking. The key trafficking pathways are regulated by Rab4, -7, and -9. Rab4 regulates the recycling of endosomes to the cell surface through a rapid-shuttle process, whereas Rab7 and -9 regulate trafficking to the late endosome/lysosome for degradation or from the trans-Golgi network to the late endosome, respectively. We recently demonstrated a role for the endosomal adaptor protein, p18, in regulation of the pulmonary endothelium through enhanced recycling of VE-cadherin to adherens junction. Thus, we hypothesized that Rab4, -7, and -9 regulate pulmonary endothelial barrier function through modulating trafficking of VE-cadherin-positive endosomes. We used Rab mutants with varying activities and associations to the endosome to study endothelial barrier function in vitro and in vivo. Our study demonstrates a key role for Rab4 activation and Rab9 inhibition in regulation of vascular permeability through enhanced VE-cadherin expression at the interendothelial junction. We further showed that endothelial barrier function mediated through Rab4 is dependent on extracellular signal-regulated kinase phosphorylation and activity. Thus, we demonstrate that Rab4 and -9 regulate VE-cadherin levels at the cell surface to modulate the pulmonary endothelium through extracellular signal-regulated kinase-dependent and -independent pathways, respectively. We propose that regulating select Rab GTPases represents novel therapeutic strategies for patients suffering with acute respiratory distress syndrome.

Evidence for the subsynaptic zone as a preferential site for CHRN recycling at neuromuscular junctions

Vertebrate skeletal muscle contraction is mediated by nicotinic acetylcholine receptors (CHRN). Endocytosis and recycling of CHRN regulate their proper abundance at nerve-muscle synapses, i.e. neuromuscular junctions. Recent work showed that RAB5 is essential for CHRN endocytosis. Here, using in vivo-imaging of endocytosed CHRN and RAB-GFP fusion proteins, we deliver evidence for differential effects of RAB5-GFP, RAB4-GFP, and RAB11-GFP on CHRN endocytosis. Furthermore, while newly endocytosed CHRN colocalized with RAB5-GFP over large stretches of muscle fibers, RAB4-GFP and RAB11-GFP colocalized with endocytosed CHRN almost exclusively at neuromuscular junctions. In agreement with previous findings, this data suggests the existence of a specialized subsynaptic zone that is particularly relevant for CHRN recycling.

Visualization of self-delivering hydrophobically modified siRNA cellular internalization

siRNAs are a new class of therapeutic modalities with promising clinical efficacy that requires modification or formulation for delivery to the tissue and cell of interest. Conjugation of siRNAs to lipophilic groups supports efficient cellular uptake by a mechanism that is not well characterized. Here we study the mechanism of internalization of asymmetric, chemically stabilized, cholesterol-modified siRNAs (sd-rxRNAs®) that efficiently enter cells and tissues without the need for formulation. We demonstrate that uptake is rapid with significant membrane association within minutes of exposure followed by the formation of vesicular structures and internalization. Furthermore, sd-rxRNAs are internalized by a specific class of early endosomes and show preferential association with epidermal growth factor (EGF) but not transferrin (Tf) trafficking pathways as shown by live cell TIRF and structured illumination microscopy (SIM). In fixed cells, we observe ∼25% of sd-rxRNA co-localizing with EGF and <5% with Tf, which is indicative of selective endosomal sorting. Likewise, preferential sd-rxRNA co-localization was demonstrated with EEA1 but not RBSN-containing endosomes, consistent with preferential EGF-like trafficking through EEA1-containing endosomes. sd-rxRNA cellular uptake is a two-step process, with rapid membrane association followed by internalization through a selective, saturable subset of the endocytic process. However, the mechanistic role of EEA1 is not yet known. This method of visualization can be used to better understand the kinetics and mechanisms of hydrophobic siRNA cellular uptake and will assist in further optimization of these types of compounds for therapeutic intervention.

Vesicle-associated Membrane Protein 3 (VAMP3) Mediates Constitutive Trafficking of the Renal Co-transporter NKCC2 in Thick Ascending Limbs: ROLE IN RENAL FUNCTION AND BLOOD PRESSURE

Renal cells of the thick ascending limb (TAL) reabsorb NaCl via the apical Na⁺/K⁺/2Cl^- co-transporter NKCC2. Trafficking of NKCC2 to the apical surface regulates NKCC2-mediated NaCl absorption and blood pressure. The molecular mechanisms by which NKCC2 reaches the apical surface and their role in renal function and maintenance of blood pressure are poorly characterized. Here we report that NKCC2 interacts with the vesicle fusion protein VAMP3, and they co-localize at the TAL apical surface. We observed that silencing VAMP3 in vivo blocks constitutive NKCC2 exocytic delivery, decreasing the amount of NKCC2 at the TAL apical surface. VAMP3 is not required for cAMP-stimulated NKCC2 exocytic delivery. Additionally, genetic deletion of VAMP3 in mice decreased total expression of NKCC2 in the TAL and lowered blood pressure. Consistent with these results, urinary excretion of water and electrolytes was higher in VAMP3 knock-out mice, which produced more diluted urine. We conclude that VAMP3 interacts with NKCC2 and mediates its constitutive exocytic delivery to the apical surface. Additionally, VAMP3 is required for normal NKCC2 expression, renal function, and blood pressure.

ICAM-1 Binding Rhinoviruses A89 and B14 Uncoat in Different Endosomal Compartments

Human rhinovirus A89 (HRV-A89) and HRV-B14 bind to and are internalized by intercellular adhesion molecule 1 (ICAM-1); as demonstrated earlier, the RNA genome of HRV-B14 penetrates into the cytoplasm from endosomal compartments of the lysosomal pathway. Here, we show by immunofluorescence microscopy that HRV-A89 but not HRV-B14 colocalizes with transferrin in the endocytic recycling compartment (ERC). Applying drugs differentially interfering with endosomal recycling and with the pathway to lysosomes, we demonstrate that these two major-group HRVs productively uncoat in distinct endosomal compartments. Overexpression of constitutively active (Rab11-GTP) and dominant negative (Rab11-GDP) mutants revealed that uncoating of HRV-A89 depends on functional Rab11. Thus, two ICAM-1 binding HRVs are routed into distinct endosomal compartments for productive uncoating.

IMPORTANCE

Based on similarity of their RNA genomic sequences, the more than 150 currently known common cold virus serotypes were classified as species A, B, and C. The majority of HRV-A viruses and all HRV-B viruses use ICAM-1 for cell attachment and entry. Our results highlight important differences of two ICAM-1 binding HRVs with respect to their intracellular trafficking and productive uncoating; they demonstrate that serotypes belonging to species A and B, but entering the cell via the same receptors, direct the endocytosis machinery to ferry them along distinct pathways toward different endocytic compartments for uncoating.

A Remodeled Hsp90 Molecular Chaperone Ensemble with the Novel Cochaperone Aarsd1 Is Required for Muscle Differentiation

Hsp90 is the ATP-consuming core component of a very abundant molecular chaperone machine that handles a substantial portion of the cytosolic proteome. Rather than one machine, it is in fact an ensemble of molecular machines, since most mammalian cells express two cytosolic isoforms of Hsp90 and a subset of up to 40 to 50 cochaperones and regulate their interactions and functions by a variety of posttranslational modifications. We demonstrate that the Hsp90 ensemble is fundamentally remodeled during muscle differentiation and that this remodeling is not just a consequence of muscle differentiation but possibly one of the drivers to accompany and to match the vast proteomic changes associated with this process. As myoblasts differentiate into myotubes, Hsp90α disappears and only Hsp90β remains, which is the only isoform capable of interacting with the novel muscle-specific Hsp90 cochaperone Aarsd1L. Artificially maintaining Hsp90α or knocking down Aarsd1L expression interferes with the differentiation of C2C12 myotubes. During muscle differentiation, Aarsd1L replaces the more ubiquitous cochaperone p23 and in doing so dampens the activity of the glucocorticoid receptor, one of the Hsp90 clients relevant to muscle functions. This cochaperone switch protects muscle cells against the inhibitory effects of glucocorticoids and may contribute to preventing muscle wasting induced by excess glucocorticoids.

Intracellular periodontal pathogen exploits recycling pathway to exit from infected cells

Although human gingival epithelium prevents intrusions by periodontal bacteria, Porphyromonas gingivalis, the most well-known periodontal pathogen, is able to invade gingival epithelial cells and pass through the epithelial barrier into deeper tissues. We previously reported that intracellular P. gingivalis exits from gingival epithelial cells via a recycling pathway. However, the underlying molecular process remains unknown. In the present study, we found that the pathogen localized in early endosomes recruits VAMP2 and Rab4A. VAMP2 was found to be specifically localized in early endosomes, although its localization remained unclear in mammalian cells. A single transmembrane domain of VAMP2 was found to be necessary and sufficient for localizing in early endosomes containing P. gingivalis in gingival epithelial cells. VAMP2 forms a complex with EXOC2/Sec5 and EXOC3/Sec6, whereas Rab4A mediates dissociation of the EXOC complex followed by recruitment of RUFY1/Rabip4, Rab4A effector, and Rab14. Depletion of VAMP2 or Rab4A resulted in accumulation of bacteria in early endosomes and disturbed bacterial exit from infected cells. It is suggested that these novel dynamics allow P. gingivalis to exploit fast recycling pathways promoting further bacterial penetration of gingival tissues.

The endocytic recycling compartment maintains cargo segregation acquired upon exit from the sorting endosome

The endocytic recycling compartment (ERC) is a series of perinuclear tubular and vesicular membranes that regulates recycling to the plasma membrane. Despite evidence that cargo is sorted at the early/sorting endosome (SE), whether cargo mixes downstream at the ERC or remains segregated is an unanswered question. Here we use three-dimensional (3D) structured illumination microscopy and dual-channel and 3D direct stochastic optical reconstruction microscopy (dSTORM) to obtain new information about ERC morphology and cargo segregation. We show that cargo internalized either via clathrin-mediated endocytosis (CME) or independently of clathrin (CIE) remains segregated in the ERC, likely on distinct carriers. This suggests that no further sorting occurs upon cargo exit from SE. Moreover, 3D dSTORM data support a model in which some but not all ERC vesicles are tethered by contiguous "membrane bridges." Furthermore, tubular recycling endosomes preferentially traffic CIE cargo and may originate from SE membranes. These findings support a significantly altered model for endocytic recycling in mammalian cells in which sorting occurs in peripheral endosomes and segregation is maintained at the ERC.

Neovascularization in the pulmonary endothelium is regulated by the endosome: Rab4-mediated trafficking and p18-dependent signaling

Neovascularization, the formation of new blood vessels, requires multiple processes including vascular leak, migration, and adhesion. Endosomal proteins, such as Rabs, regulate trafficking of key signaling proteins involved in neovascularization. The novel endosome protein, p18, enhances vascular endothelial (VE)-cadherin recycling from early endosome to cell junction to improve pulmonary endothelial barrier function. Since endothelial barrier integrity is vital in neovascularization, we sought to elucidate the role for endosome proteins p18 and Rab4, Rab7, and Rab9 in the process of vessel formation within the pulmonary vasculature. Overexpression of wild-type p18 (p18(wt)), but not the nonendosomal-binding mutant (p18(N39)), significantly increased lung microvascular endothelial cell migration, adhesion, and both in vitro and in vivo tube formation. Chemical inhibition of mTOR or p38 attenuated the proneovascularization role of p18(wt). Similar to the effect of p18(wt), overexpression of prorecycling wild-type (Rab4(WT)) and endosome-anchored (Rab4(Q67L)) Rab4 enhanced neovascularization processes, whereas molecular inhibition of Rab4, by using the nonendosomal-binding mutant (Rab4(S22N)) attenuated VEGF-induced neovascularization. Unlike p18, Rab4-induced neovascularization was independent of mTOR or p38 inhibition but was dependent on p18 expression. This study shows for the first time that neovascularization within the pulmonary vasculature is dependent on the prorecycling endocytic proteins Rab4 and p18.

Insights into endosomal maturation of human holo-transferrin in the enteric parasite Entamoeba histolytica: essential roles of Rab7A and Rab5 in biogenesis of giant early endocytic vacuoles

The pathogenic amoeba Entamoeba histolytica is one of the causative agents of health hazards in tropical countries. It causes amoebic dysentery, colitis and liver abscesses in human. Iron is one of the essential nutritional resources for survival and chronic infection caused by the amoeba. The parasite has developed multiple ways to import, sequester and utilize iron from various iron-binding proteins from its host. In spite of its central role in pathogenesis, the mechanism of iron uptake by the parasite is largely unknown. Here, we carried out a systematic study to understand the role of some of the amoebic homologues of mammalian endocytic Rab GTPases (Rab5 and Rab21, Rab7A and Rab7B) in intracellular transport of human holo-transferrin by the parasite. Flow cytometry and quantitative microscopic image analysis revealed that Rab5 and Rab7A are required for the biogenesis of amoebic giant endocytic vacuoles (GEVs) and regulate the early phase of intracellular trafficking of transferrin. Rab7B is involved in the late phase, leading to the degradation of transferrin in the amoebic lysosome-like compartments. Using time-lapse fluorescence imaging in fixed trophozoites, we determined the kinetics of the vesicular transport of transferrin through Rab5-, Rab7A- and Rab7B-positive compartments. The involvement of Rab7A in the early phase of endocytosis by the parasite marks a significant divergence from its host in terms of spatiotemporal regulation by the Rab GTPases.

Angiotensin II stimulates internalization and degradation of arterial myocyte plasma membrane BK channels to induce vasoconstriction

Arterial smooth muscle cells (myocytes) express large-conductance Ca(2+)-activated K(+) (BK) channel α and auxiliary β1 subunits that modulate arterial contractility. In arterial myocytes, β1 subunits are stored within highly mobile rab11A-positive recycling endosomes. In contrast, BKα subunits are primarily plasma membrane-localized. Trafficking pathways for BKα and whether physiological stimuli that regulate arterial contractility alter BKα localization in arterial myocytes are unclear. Here, using biotinylation, immunofluorescence resonance energy transfer (immunoFRET) microscopy, and RNAi-mediated knockdown, we demonstrate that rab4A-positive early endosomes traffic BKα to the plasma membrane in myocytes of resistance-size cerebral arteries. Angiotensin II (ANG II), a vasoconstrictor, reduced both surface and total BKα, an effect blocked by bisindolylmaleimide-II, concanavalin A, and dynasore, protein kinase C (PKC), internalization, and endocytosis inhibitors, respectively. In contrast, ANG II did not reduce BKα mRNA, and sodium nitroprusside, a nitric oxide donor, did not alter surface BKα protein over the same time course. MG132 and bafilomycin A, proteasomal and lysosomal inhibitors, respectively, also inhibited the ANG II-induced reduction in surface and total BKα, resulting in intracellular BKα accumulation. ANG II-mediated BK channel degradation reduced BK currents in isolated myocytes and functional responses to iberiotoxin, a BK channel blocker, and NS1619, a BK activator, in pressurized (60 mmHg) cerebral arteries. These data indicate that rab4A-positive early endosomes traffic BKα to the plasma membrane in arterial myocytes. We also show that ANG II stimulates PKC-dependent BKα internalization and degradation. These data describe a unique mechanism by which ANG II inhibits arterial myocyte BK currents, by reducing surface channel number, to induce vasoconstriction.

Rab11-dependent Recycling of the Human Ether-a-go-go-related Gene (hERG) Channel

The human ether-a-go-go-related gene (hERG) encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium channel (IKr). A reduction in the hERG current causes long QT syndrome, which predisposes affected individuals to ventricular arrhythmias and sudden death. We reported previously that hERG channels in the plasma membrane undergo vigorous internalization under low K(+) conditions. In the present study, we addressed whether hERG internalization occurs under normal K(+) conditions and whether/how internalized channels are recycled back to the plasma membrane. Using patch clamp, Western blot, and confocal imaging analyses, we demonstrated that internalized hERG channels can effectively recycle back to the plasma membrane. Low K(+)-enhanced hERG internalization is accompanied by an increased rate of hERG recovery in the plasma membrane upon reculture following proteinase K-mediated clearance of cell-surface proteins. The increased recovery rate is not due to enhanced protein synthesis, as hERG mRNA expression was not altered by low K(+) exposure, and the increased recovery was observed in the presence of the protein biosynthesis inhibitor cycloheximide. GTPase Rab11, but not Rab4, is involved in the recycling of hERG channels. Interfering with Rab11 function not only delayed hERG recovery in cells after exposure to low K(+) medium but also decreased hERG expression and function in cells under normal culture conditions. We concluded that the recycling pathway plays an important role in the homeostasis of plasma membrane-bound hERG channels.