Endocytosed cation-independent mannose 6-phosphate receptor traffics via the endocytic recycling compartment en route to the trans-Golgi network and a subpopulation of late endosomes

BLTP3A is associated with membranes of the late endocytic pathway and is an effector of CASM

Recent studies have identified a family of rod-shaped proteins thought to mediate lipid transfer at intracellular membrane contacts by a bridge-like mechanism. We show one such protein, bridge-like lipid transfer protein 3A (BLTP3A)/UHRF1BP1 binds VAMP7 vesicles via its C-terminal region and anchors them to lysosomes via its chorein domain containing N-terminal region to Rab7. Upon lysosome damage, BLTP3A-positive vesicles rapidly (within minutes) dissociate from lysosomes. Lysosome damage is known to activate the CASM (Conjugation of ATG8 to Single Membranes) pathway leading to lipidation and recruitment to lysosomes of mammalian ATG8 (mATG8) proteins. We find that this process drives the reassociation of BLTP3A with damaged lysosomes via an interaction of its LIR motif with mATG8 which coincides with a dissociation from the vesicles. Our findings reveal that BLTP3A is an effector of CASM, potentially as part of a mechanism to help repair or minimize lysosome damage.

AP-1γ2 is an adaptor protein 1 variant required for endosome-to-Golgi trafficking of the mannose-6-P receptor (CI-MPR) and ATP7B copper transporter

Selective retrograde transport from endosomes back to the trans-Golgi network (TGN) is important for maintaining protein homeostasis, recycling receptors, and returning molecules that were transported to the wrong compartments. Two important transmembrane proteins directed to this pathway are the Cation-Independent Mannose-6-phosphate receptor (CI-MPR) and the ATP7B copper transporter. Among CI-MPR functions is the delivery of acid hydrolases to lysosomes, while ATP7B facilitates the transport of cytosolic copper ions into organelles or the extracellular space. Precise subcellular localization of CI-MPR and ATP7B is essential for the proper functioning of these proteins. This study shows that both CI-MPR and ATP7B interact with a variant of the clathrin adaptor 1 (AP-1) complex that contains a specific isoform of the γ-adaptin subunit called γ2. Through synchronized anterograde trafficking and cell-surface uptake assays, we demonstrated that AP-1γ2 is dispensable for ATP7B and CI-MPR exit from the TGN while being critically required for ATP7B and CI-MPR retrieval from endosomes to the TGN. Moreover, AP-1γ2 depletion leads to the retention of endocytosed CI-MPR in endosomes enriched in retromer complex subunits. These data underscore the importance of AP-1γ2 as a key component in the sorting and trafficking machinery of CI-MPR and ATP7B, highlighting its essential role in the transport of proteins from endosomes.

Circular RNA vaccines with long-term lymph node-targeting delivery stability after lyophilization induce potent and persistent immune responses

IMPORTANCE

messenger RNA (mRNA) vaccines are a key technology in combating existing and emerging infectious diseases. However, the inherent instability of mRNA and the nonspecificity of lipid nanoparticle-encapsulated (LNP) delivery systems result in the need for cold storage and a relatively short-duration immune response to mRNA vaccines. Herein, we develop a novel vaccine in the form of circRNAs encapsulated in LNPs, and the circular structure of the circRNAs enhances their stability. Lyophilization is considered the most effective method for the long-term preservation of RNA vaccines. However, this process may result in irreversible damage to the nanoparticles, particularly the potential disruption of targeting modifications on LNPs. During the selection of lymph node-targeting ligands, we found that LNPs modified with mannose maintained their physical properties almost unchanged after lyophilization. Additionally, the targeting specificity and immunogenicity remained unaffected. In contrast, even with the addition of cryoprotectants such as sucrose, the physical properties of LNPs were impaired, leading to an obvious decrease in immunogenicity. This may be attributed to the protective role of mannose on the surface of LNPs during lyophilization. Freshly prepared and lyophilized mLNP-circRNA vaccines elicited comparable immune responses in both the rabies virus model and the SARS-CoV-2 model. Our data demonstrated that mLNP-circRNA vaccines elicit robust immune responses while improving stability after lyophilization, with no compromise in tissue targeting specificity. Therefore, mannose-modified LNP-circRNA vaccines represent a promising vaccine design strategy.

Inhibitors of Immune Checkpoints: Small Molecule- and Peptide-Based Approaches

The revolutionary progress in cancer immunotherapy, particularly the advent of immune checkpoint inhibitors, marks a significant milestone in the fight against malignancies. However, the majority of clinically employed immune checkpoint inhibitors are monoclonal antibodies (mAbs) with several limitations, such as poor oral bioavailability and immune-related adverse effects (irAEs). Another major limitation is the restriction of the efficacy of mAbs to a subset of cancer patients, which triggered extensive research efforts to identify alternative approaches in targeting immune checkpoints aiming to overcome the restricted efficacy of mAbs. This comprehensive review aims to explore the cutting-edge developments in targeting immune checkpoints, focusing on both small molecule- and peptide-based approaches. By delving into drug discovery platforms, we provide insights into the diverse strategies employed to identify and optimize small molecules and peptides as inhibitors of immune checkpoints. In addition, we discuss recent advances in nanomaterials as drug carriers, providing a basis for the development of small molecule- and peptide-based platforms for cancer immunotherapy. Ongoing research focused on the discovery of small molecules and peptide-inspired agents targeting immune checkpoints paves the way for developing orally bioavailable agents as the next-generation cancer immunotherapies.

Loss of the batten disease protein CLN3 leads to mis-trafficking of M6PR and defective autophagic-lysosomal reformation

Batten disease, one of the most devastating types of neurodegenerative lysosomal storage disorders, is caused by mutations in CLN3. Here, we show that CLN3 is a vesicular trafficking hub connecting the Golgi and lysosome compartments. Proteomic analysis reveals that CLN3 interacts with several endo-lysosomal trafficking proteins, including the cation-independent mannose 6 phosphate receptor (CI-M6PR), which coordinates the targeting of lysosomal enzymes to lysosomes. CLN3 depletion results in mis-trafficking of CI-M6PR, mis-sorting of lysosomal enzymes, and defective autophagic lysosomal reformation. Conversely, CLN3 overexpression promotes the formation of multiple lysosomal tubules, which are autophagy and CI-M6PR-dependent, generating newly formed proto-lysosomes. Together, our findings reveal that CLN3 functions as a link between the M6P-dependent trafficking of lysosomal enzymes and lysosomal reformation pathway, explaining the global impairment of lysosomal function in Batten disease.

SHIP164 is a chorein motif lipid transfer protein that controls endosome-Golgi membrane traffic

Cellular membranes differ in protein and lipid composition as well as in the protein-lipid ratio. Thus, progression of membranous organelles along traffic routes requires mechanisms to control bilayer lipid chemistry and their abundance relative to proteins. The recent structural and functional characterization of VPS13-family proteins has suggested a mechanism through which lipids can be transferred in bulk from one membrane to another at membrane contact sites, and thus independently of vesicular traffic. Here, we show that SHIP164 (UHRF1BP1L) shares structural and lipid transfer properties with these proteins and is localized on a subpopulation of vesicle clusters in the early endocytic pathway whose membrane cargo includes the cation-independent mannose-6-phosphate receptor (MPR). Loss of SHIP164 disrupts retrograde traffic of these organelles to the Golgi complex. Our findings raise the possibility that bulk transfer of lipids to endocytic membranes may play a role in their traffic.

Monitoring Endosomal Cargo Retrieval to the Trans-Golgi Network by Microscopic and Biochemical Approaches

The endosomal recycling pathway plays a crucial role in diverse physiologically important biological processes such as cell-to-cell signaling, nutrient uptake, immune response, and autophagy. A selective subset of these recycling cargoes, mostly transmembrane proteins, is retrieved from endosomes to the trans-Golgi network (TGN) by a retrograde transport process. Endosome-to-TGN retrograde trafficking is crucial for maintaining cellular homeostasis and signaling by preventing proteins and lipids from degradation in the lysosome. Many of the membrane sorting machinery, such as the retromer complex and sorting nexins (SNXs) are involved in endosomal retrieval and recycling of various transmembrane proteins. Recent technological advances in the resolution of light microscopy and unbiased analytical approaches in quantitative image analysis enable us to explore and understand the regulation of membrane trafficking pathways in greater detail. In this chapter, we describe quantitative imaging-based methods for analyzing the roles of proteins involved in the retrograde trafficking in retromer dependent or independent fashion, using cation-independent mannose-6-phosphate receptor (CIM6PR) as an example.

Light-mediated discovery of surfaceome nanoscale organization and intercellular receptor interaction networks

The molecular nanoscale organization of the surfaceome is a fundamental regulator of cellular signaling in health and disease. Technologies for mapping the spatial relationships of cell surface receptors and their extracellular signaling synapses would unlock theranostic opportunities to target protein communities and the possibility to engineer extracellular signaling. Here, we develop an optoproteomic technology termed LUX-MS that enables the targeted elucidation of acute protein interactions on and in between living cells using light-controlled singlet oxygen generators (SOG). By using SOG-coupled antibodies, small molecule drugs, biologics and intact viral particles, we demonstrate the ability of LUX-MS to decode ligand receptor interactions across organisms and to discover surfaceome receptor nanoscale organization with direct implications for drug action. Furthermore, by coupling SOG to antigens we achieved light-controlled molecular mapping of intercellular signaling within functional immune synapses between antigen-presenting cells and CD8⁺ T cells providing insights into T cell activation with spatiotemporal specificity. LUX-MS based decoding of surfaceome signaling architectures thereby provides a molecular framework for the rational development of theranostic strategies.

The spatial organization of cell surface receptors is critical for cell signaling and drug action. Here, the authors develop an optoproteomic method for mapping surface protein interactions, revealing cellular responses to antibodies, drugs and viral particles as well as immunosynapse signaling events.

The CD22-IGF2R interaction is a therapeutic target for microglial lysosome dysfunction in Niemann-Pick type C

Lysosome dysfunction is a shared feature of rare lysosomal storage diseases and common age-related neurodegenerative diseases. Microglia, the brain-resident macrophages, are particularly vulnerable to lysosome dysfunction because of the phagocytic stress of clearing dying neurons, myelin, and debris. CD22 is a negative regulator of microglial homeostasis in the aging mouse brain, and soluble CD22 (sCD22) is increased in the cerebrospinal fluid of patients with Niemann-Pick type C disease (NPC). However, the role of CD22 in the human brain remains unknown. In contrast to previous findings in mice, here, we show that CD22 is expressed by oligodendrocytes in the human brain and binds to sialic acid–dependent ligands on microglia. Using unbiased genetic and proteomic screens, we identify insulin-like growth factor 2 receptor (IGF2R) as the binding partner of sCD22 on human myeloid cells. Targeted truncation of IGF2R revealed that sCD22 docks near critical mannose 6-phosphate–binding domains, where it disrupts lysosomal protein trafficking. Interfering with the sCD22-IGF2R interaction using CD22 blocking antibodies ameliorated lysosome dysfunction in human NPC1 mutant induced pluripotent stem cell–derived microglia-like cells without harming oligodendrocytes in vitro. These findings reinforce the differences between mouse and human microglia and provide a candidate microglia-directed immunotherapeutic to treat NPC.

Loss of Christianson Syndrome Na+/H+ Exchanger 6 (NHE6) Causes Abnormal Endosome Maturation and Trafficking Underlying Lysosome Dysfunction in Neurons

Loss-of-function mutations in endosomal Na⁺/H⁺ exchanger 6 (NHE6) cause the X-linked neurologic disorder Christianson syndrome. Patients exhibit symptoms associated with both neurodevelopmental and neurodegenerative abnormalities. While loss of NHE6 has been shown to overacidify the endosome lumen, and is associated with endolysosome neuropathology, NHE6-mediated mechanisms in endosome trafficking and lysosome function have been understudied. Here, we show that NHE6-null mouse neurons demonstrate worsening lysosome function with time in culture, likely as a result of defective endosome trafficking. NHE6-null neurons exhibit overall reduced lysosomal proteolysis despite overacidification of the endosome and lysosome lumen. Akin to Nhx1 mutants in Saccharomyces cerevisiae, we observe decreased endosome-lysosome fusion in NHE6-null neurons. Also, we find premature activation of pH-dependent cathepsin D (CatD) in endosomes. While active CatD is increased in endosomes, CatD activation and CatD protein levels are reduced in the lysosome. Protein levels of another mannose 6-phosphate receptor (M6PR)-dependent enzyme, β-N-acetylglucosaminidase, were also decreased in lysosomes of NHE6-null neurons. M6PRs accumulate in late endosomes, suggesting defective M6PR recycling and retromer function in NHE6-null neurons. Finally, coincident with decreased endosome-lysosome fusion, using total internal reflection fluorescence, we also find a prominent increase in fusion between endosomal multivesicular bodies and the plasma membrane, indicating enhanced exosome secretion from NHE6-null neurons. In summary, in addition to overacidification of endosomes and lysosomes, loss of NHE6 leads to defects in endosome maturation and trafficking, including enhanced exosome release, contributing to lysosome deficiency and potentially leading to neurodegenerative disease.

SIGNIFICANCE STATEMENT Loss-of-function mutations in the endosomal Na⁺/H⁺ exchanger 6 (NHE6) cause Christianson syndrome, an X-linked neurologic disorder. Loss of NHE6 has been shown to overacidify endosomes; however, endosome trafficking mechanisms have been understudied, and the mechanisms leading to neurodegeneration are largely unknown. In NHE6-null mouse neurons in vitro, we find worsening lysosome function with days in culture. Notably, pH-dependent lysosome enzymes, such as cathepsin D, have reduced activity in lysosomes yet increased, precocious activity in endosomes in NHE6-null neurons. Further, endosomes show reduced fusion to lysosomes, and increased fusion to the plasma membrane with increased exosome release. This study identifies new mechanisms involving defective endosome maturation and trafficking that impair lysosome function in Christianson syndrome, likely contributing to neurodegeneration.

Golgi-58K can re-localize to late endosomes upon cellular uptake of PS-ASOs and facilitates endosomal release of ASOs

Phosphorothioate (PS) modified antisense oligonucleotide (ASO) drugs can trigger RNase H1 cleavage of cellular target RNAs to modulate gene expression. Internalized PS-ASOs must be released from membraned endosomal organelles, a rate limiting step that is not well understood. Recently we found that M6PR transport between Golgi and late endosomes facilitates productive release of PS-ASOs, raising the possibility that Golgi-mediated transport may play important roles in PS-ASO activity. Here we further evaluated the involvement of Golgi in PS-ASO activity by examining additional Golgi proteins. Reduction of certain Golgi proteins, including Golgi-58K, GCC1 and TGN46, decreased PS-ASO activity, without substantial effects on Golgi integrity. Upon PS-ASO cellular uptake, Golgi-58K was recruited to late endosomes where it colocalized with PS-ASOs. Reduction of Golgi-58K caused slower PS-ASO release from late endosomes, decreased GCC2 late endosome relocalization, and led to slower retrograde transport of M6PR from late endosomes to trans-Golgi. Late endosome relocalization of Golgi-58K requires Hsc70, and is most likely mediated by PS-ASO-protein interactions. Together, these results suggest a novel function of Golgi-58K in mediating Golgi-endosome transport and indicate that the Golgi apparatus plays an important role in endosomal release of PS-ASO, ensuring antisense activity.

Activated Protein Kinase C (PKC) Is Persistently Trafficked with Epidermal Growth Factor (EGF) Receptor

Protein kinase Cs (PKCs) are activated by lipids in the plasma membrane and bind to a scaffold assembled on the epidermal growth factor (EGF) receptor (EGFR). Understanding how this complex is routed is important, because this determines whether EGFR is degraded, terminating signaling. Here, cells were preincubated in EGF-tagged gold nanoparticles, then allowed to internalize them in the presence or absence of a phorbol ester PKC activator. PKC colocalized with EGF-tagged nanoparticles within 5 min and migrated with EGFR-bearing vesicles into the cell. Two conformations of PKC-epsilon were distinguished by different primary antibodies. One, thought to be enzymatically active, was on endosomes and displayed a binding site for antibody RR (R&D). The other, recognized by Genetex green (GG), was soluble, on actin-rich structures, and loosely bound to vesicles. During a 15-min chase, EGF-tagged nanoparticles entered large, perinuclear structures. In phorbol ester-treated cells, vesicles bearing EGF-tagged nanoparticles tended to enter this endocytic recycling compartment (ERC) without the GG form. The correlation coefficient between the GG (inactive) and RR conformations on vesicles was also lower. Thus, active PKC has a Charon-like function, ferrying vesicles to the ERC, and inactivation counteracts this function. The advantage conferred on cells by aggregating vesicles in the ERC is unclear.

Molecular regulation of the plasma membrane-proximal cellular steps involved in NK cell cytolytic function

Natural killer (NK) cells, cytolytic lymphocytes of the innate immune system, play a crucial role in the immune response against infection and cancer. NK cells kill target cells through exocytosis of lytic granules that contain cytotoxic proteins, such as perforin and granzymes. Formation of a functional immune synapse, i.e. the interface between the NK cell and its target cell enhances lysis through accumulation of polymerized F-actin at the NK cell synapse, leading to convergence of lytic granules to the microtubule organizing center (MTOC) and its subsequent polarization along microtubules to deliver the lytic granules to the synapse. In this review, we focus on the molecular mechanisms regulating the cellular processes that occur after the lytic granules are delivered to the cytotoxic synapse. We outline how - once near the synapse - the granules traverse the clearings created by F-actin remodeling to dock, tether and fuse with the plasma membrane in order to secrete their lytic content into the synaptic cleft through exocytosis. Further emphasis is given to the role of Ca²⁺ mobilization during degranulation and, whenever applicable, we compare these mechanisms in NK cells and cytotoxic T lymphocytes (CTLs) as adaptive immune system effectors.

Targeted co-delivery of Trp-2 polypeptide and monophosphoryl lipid A by pH-sensitive poly (β-amino ester) nano-vaccines for melanoma

Dendritic cell (DC)-targeted vaccines based on nanotechnology are a promising strategy to efficiently induce potent immune responses. We synthesized and manufactured a mannose-modified poly (β-amino ester) (PBAE) nano-vaccines with easily tuneable and pH-sensitive characteristics to co-deliver the tumor-associated antigen polypeptide Trp-2 and the TLR4 agonist monophosphoryl lipid A (MPLA). To reduce immunosuppression in the tumor microenvironment, an immune checkpoint inhibitor, PD-L1 antagonist, was administrated along with PBAE nano-vaccines to delay melanoma development. We found that mannosylated Trp-2 and MPLA-loaded PBAE nano-vaccines can target and mature DCs, consequently boosting antigen-specific cytotoxic T lymphocyte activity against melanoma. The prophylactic study indicates that combination therapy with PD-L1 antagonist further enhanced anti-tumor efficacy by 3.7-fold and prolonged median survival time by 1.6-fold more than free Trp-2/MPLA inoculation. DC-targeting PBAE polymers have a great potential as a nanotechnology platform to design vaccines and achieve synergistic anti-tumor effects with immune checkpoint therapy.

The mannose 6-phosphate/insulin-like growth factor 2 receptor mediates plasminogen-induced efferocytosis

The plasminogen system is harnessed in a wide variety of physiological processes, such as fibrinolysis, cell migration, or efferocytosis; and accordingly, it is essential upon inflammation, tissue remodeling, wound healing, and for homeostatic maintenance in general. Previously, we identified a plasminogen receptor in the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222). Here, we demonstrate by means of genetic knockdown, knockout, and rescue approaches combined with functional studies that M6P/IGF2R is up-regulated on the surface of macrophages, recognizes plasminogen exposed on the surface of apoptotic cells, and mediates plasminogen-induced efferocytosis. The level of uptake of plasminogen-coated apoptotic cells inversely correlates with the TNF-α production by phagocytes indicating tissue clearance without inflammation by this mechanism. Our results reveal an up-to-now undetermined function of M6P/IGF2R in clearance of apoptotic cells, which is crucial for tissue homeostasis.

Infection-Induced Changes Within the Endocytic Recycling Compartment Suggest a Roadmap of Human Cytomegalovirus Egress

Human cytomegalovirus (HCMV) is an important pathogen in developing fetuses, neonates, and individuals with compromised immune systems. Gaps in our understanding of the mechanisms required for virion assembly stand in the way of development of antivirals targeting late stages of viral replication. During infection, HCMV causes a dramatic reorganization of the host endosecretory system, leading to the formation of the cytoplasmic virion assembly complex (cVAC), the site of virion assembly. As part of cVAC biogenesis, the composition and behavior of endosecretory organelles change. To gain more comprehensive understanding of the impact HCMV infection has on components of the cellular endocytic recycling compartment (ERC), we used previously published transcriptional and proteomic datasets to predict changes in the directionality of ERC trafficking. We identified infection-associated changes in gene expression that suggest shifts in the balance between endocytic and exocytic recycling pathways, leading to formation of a secretory trap within the cVAC. Conversely, there was a corresponding shift favoring outbound secretory vesicle trafficking, indicating a potential role in virion egress. These observations are consistent with previous studies describing sequestration of signaling molecules, such as IL-6, and the synaptic vesicle-like properties of mature HCMV virions. Our analysis enabled development of a refined model incorporating old and new information related to the behavior of the ERC during HCMV replication. While limited by the paucity of integrated systems-level data, the model provides an informed basis for development of experimentally testable hypotheses related to mechanisms involved in HCMV virion maturation and egress. Information from such experiments will provide a robust roadmap for rational development of novel antivirals for HCMV and related viruses.

Retrograde Transport by Clathrin-Coated Vesicles is Involved in Intracellular Transport of PrPSc in Persistently Prion-Infected Cells

Intracellular dynamics of an abnormal isoform of prion protein (PrP^Sc) are tightly associated with prion propagation. However, the machineries involved in the intracellular trafficking of PrP^Sc are not fully understood. Our previous study suggested that PrP^Sc in persistently prion-infected cells dynamically circulates between endocytic-recycling compartments (ERCs) and peripheral regions of the cells. To investigate these machineries, we focused on retrograde transport from endosomes to the trans-Golgi network, which is one of the pathways involved in recycling of molecules. PrP^Sc was co-localized with components of clathrin-coated vesicles (CCVs) as well as those of the retromer complex, which are known as machineries for retrograde transport. Fractionation of intracellular compartments by density gradient centrifugation showed the presence of PrP^Sc and the components of CCVs in the same fractions. Furthermore, PrP^Sc was detected in CCVs isolated from intracellular compartments of prion-infected cells. Knockdown of clathrin interactor 1, which is one of the clathrin adaptor proteins involved in retrograde transport, did not change the amount of PrP^Sc, but it altered the distribution of PrP^Sc from ERCs to peripheral regions, including late endosomes/lysosomes. These data demonstrated that some PrP^Sc is transported from endosomes to ERCs by CCVs, which might be involved in the recycling of PrP^Sc.

Budding Yeast Has a Minimal Endomembrane System

The endomembrane system consists of the secretory and endocytic pathways, which communicate by transport to and from the trans-Golgi network (TGN). In mammalian cells, the endocytic pathway includes early, late, and recycling endosomes. In budding yeast, different types of endosomes have been described, but the organization of the endocytic pathway has remained unclear. We performed a spatial and temporal analysis of yeast endosomal markers and endocytic cargoes. Our results indicate that the yeast TGN also serves as an early and recycling endosome. In addition, as previously described, yeast contains a late or prevacuolar endosome (PVE). Endocytic cargoes localize to the TGN shortly after internalization, and manipulations that perturb export from the TGN can slow the passage of endocytic cargoes to the PVE. Yeast apparently lacks a distinct early endosome. Thus, yeast has a simple endocytic pathway that may reflect the ancestral organization of the endomembrane system.

Cargo-selective SNX-BAR proteins mediate retromer trimer independent retrograde transport

Kvainickas et al. show that the retromer cargo CI-MPR does not recycle from endosomes to the trans-Golgi network through interactions with the core retromer trimer. Instead, CI-MPR depends on cargo-selective SNX-BAR proteins, which function independently of the core retromer trimer.

The retromer complex, which recycles the cation-independent mannose 6-phosphate receptor (CI-MPR) from endosomes to the trans-Golgi network (TGN), is thought to consist of a cargo-selective VPS26–VPS29–VPS35 trimer and a membrane-deforming subunit of sorting nexin (SNX)–Bin, Amphyphysin, and Rvs (BAR; SNX-BAR) proteins. In this study, we demonstrate that heterodimers of the SNX-BAR proteins, SNX1, SNX2, SNX5, and SNX6, are the cargo-selective elements that mediate the retrograde transport of CI-MPR from endosomes to the TGN independently of the core retromer trimer. Using quantitative proteomics, we also identify the IGF1R, among more potential cargo, as another SNX5 and SNX6 binding receptor that recycles through SNX-BAR heterodimers, but not via the retromer trimer, in a ligand- and activation-dependent manner. Overall, our data redefine the mechanics of retromer-based sorting and call into question whether retromer indeed functions as a complex of SNX-BAR proteins and the VPS26–VPS29–VPS35 trimer.

Unravelling novel functions of the endosomal transporter mannose 6-phosphate/insulin-like growth factor receptor (CD222) in health and disease: An emerging regulator of the immune system

Properly balanced cellular responses require both the mutual interactions of soluble factors with cell surface receptors and the crosstalk of intracellular molecules. In particular, immune cells exposed unceasingly to an array of positive and negative stimuli must distinguish between what has to be tolerated and attacked. Protein trafficking is one of crucial pathways involved in this labour. The approximately >270-kDa protein transporter called mannose 6- phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) is a type I transmembrane glycoprotein present largely intracellularly in the Golgi apparatus and endosomal compartments, but also at the cell surface. It is expressed ubiquitously in a vast majority of higher eukaryotic cell types. Through binding and trafficking multiple unrelated extracellular and intracellular ligands, CD222 is involved in the regulation of a plethora of functions, and thus implicated in many physiological but also pathophysiological conditions. This review describes, first, general features of CD222, such as its evolution, genomic structure and regulation, protein structure and ligands; and second, its specific functions with a special focus on the immune system.

The endocytic pathway in microglia during health, aging and Alzheimer's disease

Microglia, the main phagocytes of the central nervous system (CNS), are involved in the surveillance and maintenance of nervous tissue. During normal tissue homeostasis, microglia migrates within the CNS, phagocytose dead cells and tissue debris, and modulate synapse pruning and spine formation via controlled phagocytosis. In the event of an invasion by a foreign body, microglia are able to phagocytose the invading pathogen and process it proteolytically for antigen presentation. Internalized substrates are incorporated and sorted within the endocytic pathway and thereafter transported via complex vesicular routes. When targeted for degradation, substrates are delivered to acidic late endosomes and lysosomes. In these, the enzymatic degradation relies on pH and enzyme content. Endocytosis, sorting, transport, compartment acidification and degradation are regulated by complex signaling mechanisms, and these may be altered during aging and pathology. In this review, we discuss the endocytic pathway in microglia, with insight into the mechanisms controlling lysosomal biogenesis and pH regulation. We also discuss microglial lysosome function associated with Alzheimer's disease (AD) and the mechanisms of amyloid-beta (Aβ) internalization and degradation. Finally, we explore some therapies currently being investigated to treat AD and their effects on microglial response to Aβ, with insight in those involving enhancement of lysosomal function.

Spatiotemporal Resolution of Rab9 and CI-MPR Dynamics in the Endocytic Pathway

Rab9 is a small GTPase that localizes to the trans-Golgi Network (TGN) and late endosomes. Its main function has long been connected to the recycling of mannose-6-phosphate receptors (MPRs). However, recent studies link Rab9 also to autophagy and lysosome biogenesis. In this paper, using confocal imaging, we characterize for the first time the live dynamics of the Rab9 constitutively active mutant, Rab9Q66L. We find that it localizes predominantly to late endosomes and that its expression in HeLa cells disperses TGN46 and cation-independent (CI-MPR) away from the Golgi yet, has no effect on the retrograde transport of CI-MPR. We also show that CI-MPR and Rab9 enter the endosomal pathway together at the transition stage between early, Rab5-positive, and late, Rab7a-positive, endosomes. CI-MPR localizes transiently to separate domains on these endosomes, where vesicles carrying CI-MPR attach and detach within seconds. Taken together, our results demonstrate that Rab9 mediates the delivery of CI-MPR to the endosomal pathway, entering the maturing endosome at the early-to-late transition.

Transport of the cholera toxin B-subunit from recycling endosomes to the Golgi requires clathrin and AP-1

The retrograde pathway is defined by the transport of proteins and lipids from the plasma membrane through endosomes to the Golgi complex, and is essential for a variety of cellular activities. Recycling endosomes are important sorting stations for some retrograde cargo. SMAP2, a GTPase-activating protein (GAP) for Arf1 with a putative clathrin-binding domain, has previously been shown to participate in the retrograde transport of the cholera toxin B-subunit (CTxB) from recycling endosomes. Here, we found that clathrin, a vesicle coat protein, and clathrin adaptor protein complex 1 (AP-1) were present at recycling endosomes and were needed for the retrograde transport of CTxB from recycling endosomes to the Golgi, but not from the plasma membrane to recycling endosomes. SMAP2 immunoprecipitated clathrin and AP-1 through a putative clathrin-binding domain and a CALM-binding domain, and SMAP2 mutants that did not interact with clathrin or AP-1 could not localize to recycling endosomes. Moreover, knockdown of Arf1 suppressed the retrograde transport of CTxB from recycling endosomes to the Golgi. These findings suggest that retrograde transport is mediated by clathrin-coated vesicles from recycling endosomes and that the role of the coat proteins is in the recruitment of Arf GAP to transport vesicles.

Intracellular Transport of Vaccinia Virus in HeLa Cells Requires WASH-VPEF/FAM21-Retromer Complexes and Recycling Molecules Rab11 and Rab22

Vaccinia virus, the prototype of the Orthopoxvirus genus in the family Poxviridae, infects a wide range of cell lines and animals. Vaccinia mature virus particles of the WR strain reportedly enter HeLa cells through fluid-phase endocytosis. However, the intracellular trafficking process of the vaccinia mature virus between cellular uptake and membrane fusion remains unknown. We used live imaging of single virus particles with a combination of various cellular vesicle markers, to track fluorescent vaccinia mature virus particle movement in cells. Furthermore, we performed functional interference assays to perturb distinct vesicle trafficking processes in order to delineate the specific route undertaken by vaccinia mature virus prior to membrane fusion and virus core uncoating in cells. Our results showed that vaccinia virus traffics to early endosomes, where recycling endosome markers Rab11 and Rab22 are recruited to participate in subsequent virus trafficking prior to virus core uncoating in the cytoplasm. Furthermore, we identified WASH-VPEF/FAM21-retromer complexes that mediate endosome fission and sorting of virus-containing vesicles prior to virus core uncoating in the cytoplasm.

IMPORTANCE

Vaccinia mature virions of the WR strain enter HeLa cells through fluid phase endocytosis. We previously demonstrated that virus-containing vesicles are internalized into phosphatidylinositol 3-phosphate positive macropinosomes, which are then fused with Rab5-positive early endosomes. However, the subsequent process of sorting the virion-containing vesicles prior to membrane fusion remains unclear. We dissected the intracellular trafficking pathway of vaccinia mature virions in cells up to virus core uncoating in cytoplasm. We show that vaccinia mature virions first travel to early endosomes. Subsequent trafficking events require the important endosome-tethered protein VPEF/FAM21, which recruits WASH and retromer protein complexes to the endosome. There, the complex executes endosomal membrane fission and cargo sorting to the Rab11-positive and Rab22-positive recycling pathway, resulting in membrane fusion and virus core uncoating in the cytoplasm.

A TOCA/CDC-42/PAR/WAVE functional module required for retrograde endocytic recycling

Endosome-to-Golgi transport is required for the function of many key membrane proteins and lipids, including signaling receptors, small-molecule transporters, and adhesion proteins. The retromer complex is well-known for its role in cargo sorting and vesicle budding from early endosomes, in most cases leading to cargo fusion with the trans-Golgi network (TGN). Transport from recycling endosomes to the TGN has also been reported, but much less is understood about the molecules that mediate this transport step. Here we provide evidence that the F-BAR domain proteins TOCA-1 and TOCA-2 (Transducer of Cdc42 dependent actin assembly), the small GTPase CDC-42 (Cell division control protein 42), associated polarity proteins PAR-6 (Partitioning defective 6) and PKC-3/atypical protein kinase C, and the WAVE actin nucleation complex mediate the transport of MIG-14/Wls and TGN-38/TGN38 cargo proteins from the recycling endosome to the TGN in Caenorhabditis elegans. Our results indicate that CDC-42, the TOCA proteins, and the WAVE component WVE-1 are enriched on RME-1-positive recycling endosomes in the intestine, unlike retromer components that act on early endosomes. Furthermore, we find that retrograde cargo TGN-38 is trapped in early endosomes after depletion of SNX-3 (a retromer component) but is mainly trapped in recycling endosomes after depletion of CDC-42, indicating that the CDC-42-associated complex functions after retromer in a distinct organelle. Thus, we identify a group of interacting proteins that mediate retrograde recycling, and link these proteins to a poorly understood trafficking step, recycling endosome-to-Golgi transport. We also provide evidence for the physiological importance of this pathway in WNT signaling.

Mannosylated poly(beta-amino esters) for targeted antigen presenting cell immune modulation

Given the rise of antibiotic resistance and other difficult-to-treat diseases, genetic vaccination is a promising preventative approach that can be tailored and scaled according to the vector chosen for gene delivery. However, most vectors currently utilized rely on ubiquitous delivery mechanisms that ineffectively target important immune effectors such as antigen presenting cells (APCs). As such, APC targeting allows the option for tuning the direction (humoral vs cell-mediated) and strength of the resulting immune responses. In this work, we present the development and assessment of a library of mannosylated poly(beta-amino esters) (PBAEs) that represent a new class of easily synthesized APC-targeting cationic polymers. Polymeric characterization and assessment methodologies were designed to provide a more realistic physiochemical profile prior to in vivo evaluation. Gene delivery assessment in vitro showed significant improvement upon PBAE mannosylation and suggested that mannose-mediated uptake and processing influence the magnitude of gene delivery. Furthermore, mannosylated PBAEs demonstrated a strong, efficient, and safe in vivo humoral immune response without use of adjuvants when compared to genetic and protein control antigens. In summary, the gene delivery effectiveness provided by mannosylated PBAE vectors offers specificity and potency in directing APC activation and subsequent immune responses.

A high-throughput siRNA screen identifies genes that regulate mannose 6-phosphate receptor trafficking

The delivery of newly synthesized soluble lysosomal hydrolases to the endosomal system is essential for lysosome function and cell homeostasis. This process relies on the proper trafficking of the mannose 6-phosphate receptors (MPRs) between the trans-Golgi network (TGN), endosomes and the plasma membrane. Many transmembrane proteins regulating diverse biological processes ranging from virus production to the development of multicellular organisms also use these pathways. To explore how cell signaling modulates MPR trafficking, we used high-throughput RNA interference (RNAi) to target the human kinome and phosphatome. Using high-content image analysis, we identified 127 kinases and phosphatases belonging to different signaling networks that regulate MPR trafficking and/or the dynamic states of the subcellular compartments encountered by the MPRs. Our analysis maps the MPR trafficking pathways based on enzymes regulating phosphatidylinositol phosphate metabolism. Furthermore, it reveals how cell signaling controls the biogenesis of post-Golgi tubular carriers destined to enter the endosomal system through a SRC-dependent pathway regulating ARF1 and RAC1 signaling and myosin II activity.

Radiation-induced autophagy potentiates immunotherapy of cancer via up-regulation of mannose 6-phosphate receptor on tumor cells in mice

There is a significant body of evidence demonstrating that radiation therapy (XRT) enhances the effect of immune therapy. However, the precise mechanisms by which XRT potentiates the immunotherapy of cancer remain elusive. Here, we report that XRT potentiates the effect of immune therapy via induction of autophagy and resultant trafficking of mannose-6-phopsphate receptor (MPR) to the cell surface. Irradiation of different tumor cells caused substantial up-regulation of MPR on the cell surface in vitro and in vivo. Down-regulation of MPR in tumor cells with shRNA completely abrogated the combined effect of XRT and immunotherapy (CTLA4 antibody) in B16F10-bearing mice without changes in the tumor-specific responses of T cells. Radiation-induced MPR up-regulation was the result of redistribution of the receptor to the cell surface. This effect was caused by autophagy with redirection of MPR to autophagosomes in a clathrin-dependent manner. In autophagosomes, MPR lost its natural ligands, which resulted in subsequent trafficking of empty receptor(s) back to the surface. Together, our data demonstrated a novel mechanism by which XRT can enhance the effect of immunotherapy and the molecular mechanism of this process.

From endosomes to the trans-Golgi network

The retrograde trafficking from endosomes to the trans-Golgi network (TGN) is one of the major endocytic pathways to divert proteins and lipids away from lysosomal degradation. Retrograde transported cargos enter the TGN via two itineraries from either the early endosome/recycling endosome or the late endosome and involve various machinery components such as retromer, sorting nexins, clathrin, small GTPases, tethering factors and SNAREs. Recently, the pathway has been recognized for its role in signal transduction, physiology and pathogenesis of human diseases.

Mammalian Mon2/Ysl2 regulates endosome-to-Golgi trafficking but possesses no guanine nucleotide exchange activity toward Arl1 GTPase

Arl1 is a member of Arf family small GTPases that is essential for the organization and function of Golgi complex. Mon2/Ysl2, which shares significant homology with Sec7 family Arf guanine nucleotide exchange factors, was poorly characterized in mammalian cells. Here, we report the first in depth characterization of mammalian Mon2. We found that Mon2 localized to trans-Golgi network which was dependent on both its N and C termini. The depletion of Mon2 did not affect the Golgi localized or cellular active form of Arl1. Furthermore, our in vitro assay demonstrated that recombinant Mon2 did not promote guanine nucleotide exchange of Arl1. Therefore, our results suggest that Mon2 could be neither necessary nor sufficient for the guanine nucleotide exchange of Arl1. We demonstrated that Mon2 was involved in endosome-to-Golgi trafficking as its depletion accelerated the delivery of furin and CI-M6PR to Golgi after endocytosis.

SMAP2 regulates retrograde transport from recycling endosomes to the Golgi

Retrograde transport is where proteins and lipids are transported back from the plasma membrane (PM) and endosomes to the Golgi, and crucial for a diverse range of cellular functions. Recycling endosomes (REs) serve as a sorting station for the retrograde transport and we recently identified evection-2, an RE protein with a pleckstrin homology (PH) domain, as an essential factor of this pathway. How evection-2 regulates retrograde transport from REs to the Golgi is not well understood. Here, we report that evection-2 binds to SMAP2, an Arf GTPase-activating protein. Endogenous SMAP2 localized mostly in REs and to a lesser extent, the trans-Golgi network (TGN). SMAP2 binds evection-2, and the RE localization of SMAP2 was abolished in cells depleted of evection-2. Knockdown of SMAP2, like that of evection-2, impaired the retrograde transport of cholera toxin B subunit (CTxB) from REs. These findings suggest that evection-2 recruits SMAP2 to REs, thereby regulating the retrograde transport of CTxB from REs to the Golgi.

ARF1 and ARF4 regulate recycling endosomal morphology and retrograde transport from endosomes to the Golgi apparatus

The ARF1+ARF4 and ARF1+ARF3 pairs are both required for integrity of recycling endosomes but are involved in distinct transport pathways: the former pair regulates retrograde transport from endosomes to the TGN, whereas the latter is required for the transferrin recycling pathway from endosomes to the plasma membrane.

Small GTPases of the ADP-ribosylation factor (ARF) family, except for ARF6, mainly localize to the Golgi apparatus, where they trigger formation of coated carrier vesicles. We recently showed that class I ARFs (ARF1 and ARF3) localize to recycling endosomes, as well as to the Golgi, and are redundantly required for recycling of endocytosed transferrin. On the other hand, the roles of class II ARFs (ARF4 and ARF5) are not yet fully understood, and the complementary or overlapping functions of class I and class II ARFs have been poorly characterized. In this study, we find that simultaneous depletion of ARF1 and ARF4 induces extensive tubulation of recycling endosomes. Moreover, the depletion of ARF1 and ARF4 inhibits retrograde transport of TGN38 and mannose-6-phosphate receptor from early/recycling endosomes to the trans-Golgi network (TGN) but does not affect the endocytic/recycling pathway of transferrin receptor or inhibit retrograde transport of CD4-furin from late endosomes to the TGN. These observations indicate that the ARF1+ARF4 and ARF1+ARF3 pairs are both required for integrity of recycling endosomes but are involved in distinct transport pathways: the former pair regulates retrograde transport from endosomes to the TGN, whereas the latter is required for the transferrin recycling pathway from endosomes to the plasma membrane.

Emerging roles of recycling endosomes

Cells internalize extracellular solutes, ligands and proteins and lipids in the plasma membrane (PM) by endocytosis. The removal of membrane from the PM is counteracted by endosomal recycling pathways that return the endocytosed proteins and lipids back to the PM. Recycling to the PM can occur from early endosomes. However, many cells have a distinct subpopulation of endosomes that have a mildly acidic pH of 6.5 and are involved in the endosomal recycling. These endosomes are dubbed recycling endosomes (REs). In recent years, studies have begun to reveal that function of REs is not limited to the endosomal recycling. In this review, I summarize the nature of membrane trafficking pathways that pass through REs and the cell biological roles of these pathways.

Regulation of hepatitis B virus infection by Rab5, Rab7, and the endolysosomal compartment

Despite important progress toward deciphering human hepatitis B virus (HBV) entry into host cells, many aspects of the early steps of the life cycle remained completely obscure. Following endocytosis, HBV must travel through the complex network of the endocytic pathway to reach the cell nucleus and initiate replication. In addition to guiding the viral particles to the replication site, the endosomal vesicles may play a crucial role in infection, providing the appropriate environment for virus uncoating and nucleocapsid release. In this work, we investigated the trafficking of HBV particles internalized in permissive cells. Expression of key Rab proteins, involved in specific pathways leading to different intracellular locations, was modulated in HepaRG cells, using a stable and inducible short hairpin RNA (shRNA) expression system. The trafficking properties of the newly developed cells were demonstrated by confocal microscopy and flow cytometry using specific markers. The results showed that HBV infection strongly depends on Rab5 and Rab7 expression, indicating that HBV transport from early to mature endosomes is required for a step in the viral life cycle. This may involve reduction of disulfide bond-linked envelope proteins, as alteration of the redox potential of the endocytic pathway resulted in inhibition of infection. Subcellular fractionation of HBV-infected cells showed that viral particles are further transported to lysosomes. Intriguingly, infection was not dependent on the lysosomal activity, suggesting that trafficking to this compartment is a "dead-end" route. Together, these data add to our understanding of the HBV-host cell interactions controlling the early stages of infection.

SNAP-tagging the retrograde route

We have developed a chemical biology strategy to identify proteins that follow the retrograde transport route from the plasma membrane to the Golgi apparatus, via endosomes. The general principle is the following: plasma membrane proteins are covalently tagged with a first probe. Only the ones that are then transported to trans-Golgi/TGN membranes are covalently bound to a capture reagent that has been engineered into this compartment. Specifically, the first probe is benzylguanine (BG) that is conjugated onto primary amino groups of plasma-membrane proteins. The capture reagent includes an O(6)-alkylguanine-DNA alkyltransferase-derived fragment, the SNAP-tag, which forms a covalent linkage with BG. The SNAP-tag is fused to the GFP-tagged Golgi membrane anchor from galactosyl transferase for proper targeting to trans-Golgi/TGN membranes. Cell-surface BG-tagged proteins that are transported to trans-Golgi/TGN membranes (i.e., that are retrograde cargoes) are thereby covalently captured by the SNAP-tag fusion protein. For identification, the latter is immunopurified using GFP-Trap, and associated retrograde cargo proteins are identified by mass spectrometry. We here provide a step-by-step protocol of this method.

The Scribble polarity protein stabilizes E-cadherin/p120-catenin binding and blocks retrieval of E-cadherin to the Golgi

Several polarity proteins, including Scribble (Scrb) have been implicated in control of vesicle traffic, and in particular the endocytosis of E-cadherin, but through unknown mechanisms. We now show that depletion of Scrb enhances endocytosis of E-cadherin by weakening the E-cadherin-p120catenin interaction. Unexpectedly, however, the internalized E-cadherin is not degraded but accumulates in the Golgi apparatus. Silencing p120-catenin causes degradation of E-cadherin in lysosomes, but degradation is blocked by the co-depletion of Scrb, which diverts the internalized E-cadherin to the Golgi. Loss of Scrb also enhances E-cadherin binding to retromer components, and retromer is required for Golgi accumulation of Scrb, and E-cadherin stability. These data identify a novel and unanticipated function for Scrb in blocking retromer-mediated diversion of E-cadherin to the Golgi. They provide evidence that polarity proteins can modify the intracellular itinerary for endocytosed membrane proteins.

Endosomal maturation, Rab7 GTPase and phosphoinositides in African swine fever virus entry

Here we analyzed the dependence of African swine fever virus (ASFV) infection on the integrity of the endosomal pathway. Using confocal immunofluorescence with antibodies against viral capsid proteins, we found colocalization of incoming viral particles with early endosomes (EE) during the first minutes of infection. Conversely, viral capsid protein was not detected in acidic late endosomal compartments, multivesicular bodies (MVBs), late endosomes (LEs) or lysosomes (LY). Using an antibody against a viral inner core protein, we found colocalization of viral cores with late compartments from 30 to 60 minutes postinfection. The absence of capsid protein staining in LEs and LYs suggested that virus desencapsidation would take place at the acid pH of these organelles. In fact, inhibitors of intraluminal acidification of endosomes caused retention of viral capsid staining virions in Rab7 expressing endosomes and more importantly, severely impaired subsequent viral protein production. Endosomal acidification in the first hour after virus entry was essential for successful infection but not thereafter. In addition, altering the balance of phosphoinositides (PIs) which are responsible of the maintenance of the endocytic pathway impaired ASFV infection. Early infection steps were dependent on the production of phosphatidylinositol 3-phosphate (PtdIns3P) which is involved in EE maturation and multivesicular body (MVB) biogenesis and on the interconversion of PtdIns3P to phosphatidylinositol 3, 5-biphosphate (PtdIns(3,5)P₂). Likewise, GTPase Rab7 activity should remain intact, as well as processes related to LE compartment physiology, which are crucial during early infection. Our data demonstrate that the EE and LE compartments and the integrity of the endosomal maturation pathway orchestrated by Rab proteins and PIs play a central role during early stages of ASFV infection.

Efficiency of immunotoxin cytotoxicity is modulated by the intracellular itinerary

Pseudomonas exotoxin-based immunotoxins, including LMB-2 (antiTac(F(v))-PE38), are proposed to traffic to the trans-Golgi network (TGN) and move by a retrograde pathway to the endoplasmic reticulum, where they undergo translocation to the cytoplasm, a step that is essential for cytotoxicity. The retrograde transport pathways used by LMB-2 are not completely understood, so it is unclear if transit through specific organelles is critical for maximal cytotoxic activity. In this study, we used Chinese hamster ovary (CHO) cell lines that express chimeric constructs of CD25, the Tac antigen, attached to the cytoplasmic domain of the TGN-targeted transmembrane proteins, TGN38 and furin. These chimeras are both targeted to the TGN, but the itineraries they follow are quite different. LMB-2 was incubated with the two cell lines, and the efficiency of cell killing was determined using cell viability and cytotoxicity assays. LMB-2 that is targeted through the endocytic recycling compartment to the TGN via Tac-TGN38 kills the cells more efficiently than immunotoxins delivered through the late endosomes by Tac-furin. Although the processing to the 37 kDa active fragment was more efficient in Tac-furin cells than in Tac-TGN38 cells, this was not associated with enhanced cytotoxicity - presumably because the toxin was also degraded more rapidly in these cells. These data indicate that trafficking through specific organelles is an important factor modulating toxicity by LMB-2.

Microparticle surface modifications targeting dendritic cells for non-activating applications

Microparticulate systems for delivery of therapeutics to DCs for immunotherapy have gained attention recently. However, reports addressing the optimization of DC-targeting microparticle delivery systems are limited, particularly for cases where the goal is to deliver payload to DCs in a non-activating fashion. Here, we investigate targeting DCs using poly (d lactide-co-glycolide) microparticles (MPs) in a non-stimulatory manner and assess efficacy in vitro and in vivo. We modified MPs by surface immobilizing DC receptor targeting molecules - antibodies (anti-CD11c, anti-DEC-205) or peptides (P-D2, RGD), where anti-CD11c antibody, P-D2 and RGD peptides target integrins and anti-DEC-205 antibody targets the c-type lectin receptor DEC-205. Our results demonstrate the modified MPs are neither toxic nor activating, and DC uptake of MPs in vitro is improved by the anti-DEC-205 antibody, the anti-CD11c antibody and the P-D2 peptide modifications. The P-D2 peptide MP modification significantly improved DC antigen presentation in vitro both at immediate and delayed time points. Notably, MP functionalization with P-D2 peptide and anti-CD11c antibody increased the rate and extent of MP translocation in vivo by DCs and MΦs, with the P-D2 peptide modified MPs demonstrating the highest translocation. This work informs the design of non-activating polymeric microparticulate applications such as vaccines for autoimmune diseases.

Retromer guides STxB and CD8-M6PR from early to recycling endosomes, EHD1 guides STxB from recycling endosome to Golgi

Retrograde trafficking transports proteins, lipids and toxins from the plasma membrane to the Golgi and endoplasmic reticulum (ER). To reach the Golgi, these cargos must transit the endosomal system, consisting of early endosomes (EE), recycling endosomes, late endosomes and lysosomes. All cargos pass through EE, but may take different routes to the Golgi. Retromer-dependent cargos bypass the late endosomes to reach the Golgi. We compared how two very different retromer-dependent cargos negotiate the endosomal sorting system. Shiga toxin B, bound to the external layer of the plasma membrane, and chimeric CD8-mannose-6-phosphate receptor (CI-M6PR), which is anchored via a transmembrane domain. Both appear to pass through the recycling endosome. Ablation of the recycling endosome diverted both of these cargos to an aberrant compartment and prevented them from reaching the Golgi. Once in the recycling endosome, Shiga toxin required EHD1 to traffic to the TGN, while the CI-M6PR was not significantly dependent on EHD1. Knockdown of retromer components left cargo in the EE, suggesting that it is required for retrograde exit from this compartment. This work establishes the recycling endosome as a required step in retrograde traffic of at least these two retromer-dependent cargos. Along this pathway, retromer is associated with EE to recycling endosome traffic, while EHD1 is associated with recycling endosome to TGN traffic of STxB.

Dissecting mannose 6-phosphate-insulin-like growth factor 2 receptor complexes that control activation and uptake of plasminogen in cells

The plasminogen (Plg) activation cascade on the cell surface plays a central role in cell migration and is involved in a plethora of physiological and pathological processes. Its regulation is coordinated by many receptors, in particular the urokinase-type plasminogen activator receptor (uPAR, CD87), receptors that physically interact and functionally cooperate with uPAR, and Plg binding molecules. Here we studied the impact of one of the Plg binding molecules, the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P-IGF2R, CD222), on cellular Plg activation. By developing both in vitro and in vivo Plg activation assays on size-fractionated lysates of M6P-IGF2R-silenced cells, we identified Plg-associated complexes with M6P-IGF2R as the regulatory factor. Using lipid raft preserving versus dissolving detergents, we found lipid dependence of the Plg regulatory function of these complexes. Furthermore, M6P-IGF2R-silencing in uPAR-positive human cell lines reduced internalization of Plg, resulting in elevated Plg activation. In contrast, the expression of human M6P-IGF2R in mouse embryonic fibroblasts derived from M6P-IGF2R knock-out mice enhanced Plg internalization. Finally, peptide 18-36 derived from the Plg-binding site within M6P-IGF2R enhanced Plg uptake. Thus, by targeting Plg to endocytic pathways, M6P-IGF2R appears to control Plg activation within cells that might be important to restrict plasmin activity to specific sites and substrates.

SNAP-tag based proteomics approach for the study of the retrograde route

Proteomics is a powerful technique for protein identification at large scales. A number of proteomics approaches have been developed to study the steady state composition of intracellular compartments. Here, we report a novel vectorial proteomics strategy to identify plasma membrane proteins that undergo retrograde transport to the trans-Golgi network (TGN). This strategy is based on the covalent modification of the plasma membrane proteome with a membrane impermeable benzylguanine derivative. Benzylguanine-tagged plasma membrane proteins that are subsequently targeted to the retrograde route are covalently captured by a TGN-localized SNAP-tagged fusion protein, which allows for their identification. The approach was validated step-by-step using a well explored retrograde cargo protein, the B-subunit of Shiga toxin. It was then extended to the proteomics format. Among other hits we found one of the historically first identified cargo proteins that undergo retrograde transport, which further validated our approach. Most of the other hits were kinases, receptors or transporters. In conclusion, we have pioneered a vectorial proteomics approach that complements traditional methods for the study of retrograde protein trafficking. This approach is of generic nature and could in principle be extended to other endocytic pathways.

Clathrin-mediated endocytosis is inhibited during mitosis

A long-standing paradigm in cell biology is the shutdown of endocytosis during mitosis. There is consensus that transferrin uptake is inhibited after entry into prophase and that it resumes in telophase. A recent study proposed that endocytosis is continuous throughout the cell cycle and that the observed inhibition of transferrin uptake is due to a decrease in available transferrin receptor at the cell surface, and not to a shutdown of endocytosis. This challenge to the established view is gradually becoming accepted. Because of this controversy, we revisited the question of endocytic activity during mitosis. Using an antibody uptake assay and controlling for potential changes in surface receptor density, we demonstrate the strong inhibition of endocytosis in mitosis of CD8 chimeras containing any of the three major internalization motifs for clathrin-mediated endocytosis (YXXΦ, [DE]XXXL[LI], or FXNPXY) or a CD8 protein with the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor. The shutdown is not gradual: We describe a binary switch from endocytosis being "on" in interphase to "off" in mitosis as cells traverse the G(2)/M checkpoint. In addition, we show that the inhibition of transferrin uptake in mitosis occurs despite abundant transferrin receptor at the surface of HeLa cells. Our study finds no support for the recent idea that endocytosis continues during mitosis, and we conclude that endocytosis is temporarily shutdown during early mitosis.

Metabolic pathways and intracellular trafficking of gangliosides

Gangliosides constitute a large and heterogeneous family of acidic glycosphingolipids that contain one or more sialic acid residues and are expressed in nearly all vertebrate cells. Their de novo synthesis starts at the endoplasmic reticulum and is continued by a combination of glycosyltransferase activities at the Golgi complex, followed by vesicular delivery to the plasma membrane. At the cell surface, gangliosides participate in a variety of physiological as well as pathological processes. The cloning of genes for most of the glycosyltransferases responsible for ganglioside biosynthesis has produced a better understanding of the cellular and molecular basis of the ganglioside metabolism. In addition, the ability to delete groups of glycosphingolipid structures in mice has been enormously important in determining their physiological roles. Recently, a number of enzymes for ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane, which might contribute to modulate local glycolipid composition, and consequently, the cell function.

A SNX3-dependent retromer pathway mediates retrograde transport of the Wnt sorting receptor Wntless and is required for Wnt secretion

Wnt proteins are lipid-modified glycoproteins that play a central role in development, adult tissue homeostasis and disease. Secretion of Wnt proteins is mediated by the Wnt-binding protein Wntless (Wls), which transports Wnt from the Golgi network to the cell surface for release. It has recently been shown that recycling of Wls through a retromer-dependent endosome-to-Golgi trafficking pathway is required for efficient Wnt secretion, but the mechanism of this retrograde transport pathway is poorly understood. Here, we report that Wls recycling is mediated through a retromer pathway that is independent of the retromer sorting nexins SNX1-SNX2 and SNX5-SNX6. We have found that the unrelated sorting nexin, SNX3, has an evolutionarily conserved function in Wls recycling and Wnt secretion and show that SNX3 interacts directly with the cargo-selective subcomplex of the retromer to sort Wls into a morphologically distinct retrieval pathway. These results demonstrate that SNX3 is part of an alternative retromer pathway that functionally separates the retrograde transport of Wls from other retromer cargo.

SNX27 mediates retromer tubule entry and endosome-to-plasma membrane trafficking of signalling receptors

Endocytic sorting of signalling receptors between recycling and degradative pathways is a key cellular process controlling the surface complement of receptors and, accordingly, the cell's ability to respond to specific extracellular stimuli. The β2 adrenergic receptor (β2AR) is a prototypical seven-transmembrane signalling receptor that recycles rapidly and efficiently to the plasma membrane after ligand-induced endocytosis. β2AR recycling is dependent on the receptor's carboxy-terminal PDZ ligand and Rab4. This active sorting process is required for functional resensitization of β2AR-mediated signalling. Here we show that sequence-directed sorting occurs at the level of entry into retromer tubules and that retromer tubules are associated with Rab4. Furthermore, we show that sorting nexin 27 (SNX27) serves as an essential adaptor protein linking β2ARs to the retromer tubule. SNX27 does not seem to directly interact with the retromer core complex, but does interact with the retromer-associated Wiskott-Aldrich syndrome protein and SCAR homologue (WASH) complex. The present results identify a role for retromer in endocytic trafficking of signalling receptors, in regulating a receptor-linked signalling pathway, and in mediating direct endosome-to-plasma membrane traffic.

The porcine reproductive and respiratory syndrome virus requires trafficking through CD163-positive early endosomes, but not late endosomes, for productive infection

The porcine reproductive and respiratory syndrome virus (PRRSV) enters its target cell via clathrin-mediated endocytosis. Using dominant-negative Rab5 and Rab7 mutants, we show that upon internalization, PRRSV enters early endosomes but does not continue through the endocytic pathway to late endosomes. This was confirmed via colocalization experiments visualizing PRRSV and markers for different compartments of the endocytic pathway. Furthermore, it was shown that PRRSV colocalizes with its internalization receptor, sialoadhesin, on the cell surface and beneath the plasma membrane, while CD163 and PRRSV only meet in early endosomes.