Iterative fractionation of recycling receptors from lysosomally destined ligands in an early sorting endosome

LDLR-Mediated Targeting and Productive Uptake of siRNA-Peptide Ligand Conjugates In Vitro and In Vivo

Small RNA molecules such as microRNA and small interfering RNA (siRNA) have become promising therapeutic agents because of their specificity and their potential to modulate gene expression. Any gene of interest can be potentially up- or down-regulated, making RNA-based technology the healthcare breakthrough of our era. However, the functional and specific delivery of siRNAs into tissues of interest and into the cytosol of target cells remains highly challenging, mainly due to the lack of efficient and selective delivery systems. Among the variety of carriers for siRNA delivery, peptides have become essential candidates because of their high selectivity, stability, and conjugation versatility. Here, we describe the development of molecules encompassing siRNAs against SOD1, conjugated to peptides that target the low-density lipoprotein receptor (LDLR), and their biological evaluation both in vitro and in vivo.

A dual-targeting peptide for glioblastoma screened by phage display peptide library biopanning combined with affinity-adaptability analysis

The obstruction of blood-brain barrier (BBB) and the poor specific targeting are still the major obstacles and challenges of targeted nano-pharmaceutical therapy for glioblastoma (GBM) up to now. It is critical to find appropriate targeting ligands that can effectively mediate the nano-pharmaceuticals to penetrate brain capillary endothelial cells (BCECs) and then specifically bind to glioblastoma cells (GCs). Herein, a dual-targeting ligand for GBM was screened by the combination of phage display peptide library biopanning and affinity-adaptability analysis. Based on the acquisition of sub-library of peptide which exhibited the specific affinity to both BCECs and GCs, a comparison parameter of relative affinity was deliberately introduced to evaluate the relative affinity of candidate peptides to U251-MG cells and bEnd.3 cells. The optimized WTW peptide (sequenced as WTWEYTK) was provided with a high relative affinity (RU/B = 2.44), implying that its high affinity to U251-MG cells and moderate affinity to bEnd.3 cells might synergistically promote its receptor-mediated internalization and transport, the dissociation from bEnd.3, and the binding to U251-MG. The results of BBB model trials in vitro showed that the BBB penetration efficiency and GBM accumulation of WTW peptide were significantly higher than those of WSL peptide, GNH peptide, and REF peptide. Results of orthotopic GBM xenograft model assays in vivo also indicated that WTW peptide had successfully penetrated the BBB and improved accumulation in GBM. The screened WTW peptide might be the potential dual-targeting ligand to motivate the advancement of GBM targeted therapy.

Preparation and In Vitro Validation of a Cucurbit[7]uril-Peptide Conjugate Targeting the LDL Receptor

Here we report the coupling of a cyclic peptide (VH4127) targeting the low density lipoprotein (LDL) receptor (LDLR) noncompetitively to cucurbit[7]uril (CB[7]) to develop a new kind of drug delivery system (DDS), namely, CB[7]-VH4127, with maintained binding affinity to the LDLR. To evaluate the uptake potential of this bismacrocyclic compound, another conjugate was prepared comprising a high-affinity group for CB[7] (adamantyl(Ada)-amine) coupled to the fluorescent tracker Alexa680 (A680). The resulting A680-Ada·CB[7]-VH4127 supramolecular complex demonstrated conserved LDLR-binding potential and improved LDLR-mediated endocytosis and intracellular accumulation potential in LDLR-expressing cells. The combination of two technologies, namely, monofunctionalized CB[7] and the VH4127 LDLR-targeting peptide, opens new avenues in terms of targeting and intracellular delivery to LDLR-expressing tissues or tumors. The versatile transport capacity of CB[7], known to bind a large spectrum of bioactive or functional compounds, makes this new DDS suitable for a wide range of therapeutic or imaging applications.

AP2 Regulates Thickveins Trafficking to Attenuate NMJ Growth Signaling in Drosophila

Compromised endocytosis in neurons leads to synapse overgrowth and altered organization of synaptic proteins. However, the molecular players and the signaling pathways which regulate the process remain poorly understood. Here, we show that σ2-adaptin, one of the subunits of the AP2-complex, genetically interacts with Mad, Medea and Dad (components of BMP signaling) to control neuromuscular junction (NMJ) growth in Drosophila Ultrastructural analysis of σ2-adaptin mutants show an accumulation of large vesicles and membranous structures akin to endosomes at the synapse. We found that mutations in σ2-adaptin lead to an accumulation of Tkv receptors at the presynaptic membrane. Interestingly, the level of small GTPase Rab11 was significantly reduced in the σ2-adaptin mutant synapses. However, expression of Rab11 does not restore the synaptic defects of σ2-adaptin mutations. We propose a model in which AP2 regulates Tkv internalization and endosomal recycling to control synaptic growth.

FERARI and cargo adaptors coordinate cargo flow through sorting endosomes

Cellular organization, compartmentalization and cell-to-cell communication are crucially dependent on endosomal pathways. Sorting endosomes provide a transit point for various trafficking pathways and decide the fate of proteins: recycling, secretion or degradation. FERARI (Factors for Endosome Recycling and Rab Interactions) play a key role in shaping these compartments and coordinate Rab GTPase function with membrane fusion and fission of vesicles through a kiss-and-run mechanism. Here, we show that FERARI also mediate kiss-and-run of Rab5-positive vesicles with sorting endosomes. During these encounters, cargo flows from Rab5-positive vesicles into sorting endosomes and from there in Rab11-positive vesicles. Cargo flow from sorting endosomes into Rab11 structures relies on the cargo adaptor SNX6, while cargo retention in the Rab11 compartment is dependent on AP1. The available cargo amount appears to regulate the duration of kisses. We propose that FERARI, together with cargo adaptors, coordinate the vectorial flow of cargo through sorting endosomes.

Nucleic Acid-Loaded Lipid Nanoparticle Interactions with Model Endosomal Membranes

Lipid nanoparticles (LNPs) are important delivery systems for RNA-based therapeutics, yet the mechanism of their interaction with endosomal membranes remains unclear. Here, the interactions of nucleic acid-loaded LNPs that contain an ionizable lipid with models of the early and late endosomal membranes are studied, for the first time, using different reflectometry techniques. Novel insight is provided with respect to the subphase pH, the stage of the endosome, and the nature of the nucleic acid cargo. It is found that the insertion of lipids from the LNPs into the model membrane is greatest at pH 6.5 and 5.5, whereas at higher pH, lipid insertion is suppressed with evidence instead for the binding of intact LNPs, demonstrating the importance of the pH in the fusion of LNPs undergoing the endosomal pathway. Furthermore, and independently of the pH, the effect of the early- versus late-stage endosomal models is minimal, suggesting that the increased fluidity and anionic nature of the late endosome has little effect on the extent of LNP interaction. Last, there is greater nucleic acid delivery from LNPs containing mRNA than Poly(A), indicating that the extent of interaction can be tuned according to the nature of the nucleic acid cargo. Such new information on the relative impact of factors influencing nucleic acid delivery by LNP interactions with endosomal membranes is important in the design and tuning of vehicles with improved nucleic acid delivery capacities.

Connecting the dots: combined control of endocytic recycling and degradation

Endocytosis is an essential process where proteins and lipids are internalised from the plasma membrane in membrane-bound carriers, such as clathrin-coated vesicles. Once internalised into the cell these vesicles fuse with the endocytic network where their contents are sorted towards degradation in the lysosome or recycling to their origin. Initially, it was thought that cargo recycling is a passive process, but in recent years the identification and characterisation of specialised recycling complexes has established a hitherto unthought-of level of complexity that actively opposes degradation. This review will summarise recent developments regarding the composition and regulation of the recycling machineries and their relationship with the degradative pathways of the endosome.

PCSK9 Inhibitors and Cardiovascular Disease: Impact on Cardiovascular Outcomes

Cardiovascular Disease (CVD) remains the leading cause of morbidity and mortality in the western world. Hypolipidemic drugs have long been used for the primary and secondary prevention of heart disease. However, the high frequency of recurrent events in patients despite hypolipidemic therapy has increased the need for new more targeted therapeutic approaches. Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors are monoclonal antibodies to the PCSK9 gene and represent a new class of drugs that have been shown to further decrease LDL-C when administered as a monotherapy or in combination with statins. In addition to LDL reduction, PCSK9 inhibitors are shown to decrease apolipoprotein B and lipoprotein (a) levels without major adverse effects. Whether or not PCSK9 inhibitors can actually reduce the incidence of cardiovascular events and ameliorate CVD prognosis is yet to be clarified. This review summarizes recent literature on the safety and efficacy of PCSK9 inhibitors on CVD outcome and its potential role in the management of patients with high-risk cardiovascular disease.

Strategies to target SARS-CoV-2 entry and infection using dual mechanisms of inhibition by acidification inhibitors

Many viruses utilize the host endo-lysosomal network for infection. Tracing the endocytic itinerary of SARS-CoV-2 can provide insights into viral trafficking and aid in designing new therapeutic strategies. Here, we demonstrate that the receptor binding domain (RBD) of SARS-CoV-2 spike protein is internalized via the pH-dependent CLIC/GEEC (CG) endocytic pathway in human gastric-adenocarcinoma (AGS) cells expressing undetectable levels of ACE2. Ectopic expression of ACE2 (AGS-ACE2) results in RBD traffic via both CG and clathrin-mediated endocytosis. Endosomal acidification inhibitors like BafilomycinA1 and NH4Cl, which inhibit the CG pathway, reduce the uptake of RBD and impede Spike-pseudoviral infection in both AGS and AGS-ACE2 cells. The inhibition by BafilomycinA1 was found to be distinct from Chloroquine which neither affects RBD uptake nor alters endosomal pH, yet attenuates Spike-pseudovirus entry. By screening a subset of FDA-approved inhibitors for functionality similar to BafilomycinA1, we identified Niclosamide as a SARS-CoV-2 entry inhibitor. Further validation using a clinical isolate of SARS-CoV-2 in AGS-ACE2 and Vero cells confirmed its antiviral effect. We propose that Niclosamide, and other drugs which neutralize endosomal pH as well as inhibit the endocytic uptake, could provide broader applicability in subverting infection of viruses entering host cells via a pH-dependent endocytic pathway.

Endomembrane Tension and Trafficking

Eukaryotic cells employ diverse uptake mechanisms depending on their specialized functions. While such mechanisms vary widely in their defining criteria: scale, molecular machinery utilized, cargo selection, and cargo destination, to name a few, they all result in the internalization of extracellular solutes and fluid into membrane-bound endosomes. Upon scission from the plasma membrane, this compartment is immediately subjected to extensive remodeling which involves tubulation and vesiculation/budding of the limiting endomembrane. This is followed by a maturation process involving concomitant retrograde transport by microtubule-based motors and graded fusion with late endosomes and lysosomes, organelles that support the degradation of the internalized content. Here we review an important determinant for sorting and trafficking in early endosomes and in lysosomes; the control of tension on the endomembrane. Remodeling of endomembranes is opposed by high tension (caused by high hydrostatic pressure) and supported by the relief of tension. We describe how the timely and coordinated efflux of major solutes along the endocytic pathway affords the cell control over such tension. The channels and transporters that expel the smallest components of the ingested medium from the early endocytic fluid are described in detail as these systems are thought to enable endomembrane deformation by curvature-sensing/generating coat proteins. We also review similar considerations for the lysosome where resident hydrolases liberate building blocks from luminal macromolecules and transporters flux these organic solutes to orchestrate trafficking events. How the cell directs organellar trafficking based on the luminal contents of organelles of the endocytic pathway is not well-understood, however, we propose that the control over membrane tension by solute transport constitutes one means for this to ensue.

Genome-scale CRISPR screening for modifiers of cellular LDL uptake

Hypercholesterolemia is a causal and modifiable risk factor for atherosclerotic cardiovascular disease. A critical pathway regulating cholesterol homeostasis involves the receptor-mediated endocytosis of low-density lipoproteins into hepatocytes, mediated by the LDL receptor. We applied genome-scale CRISPR screening to query the genetic determinants of cellular LDL uptake in HuH7 cells cultured under either lipoprotein-rich or lipoprotein-starved conditions. Candidate LDL uptake regulators were validated through the synthesis and secondary screening of a customized library of gRNA at greater depth of coverage. This secondary screen yielded significantly improved performance relative to the primary genome-wide screen, with better discrimination of internal positive controls, no identification of negative controls, and improved concordance between screen hits at both the gene and gRNA level. We then applied our customized gRNA library to orthogonal screens that tested for the specificity of each candidate regulator for LDL versus transferrin endocytosis, the presence or absence of genetic epistasis with LDLR deletion, the impact of each perturbation on LDLR expression and trafficking, and the generalizability of LDL uptake modifiers across multiple cell types. These findings identified several previously unrecognized genes with putative roles in LDL uptake and suggest mechanisms for their functional interaction with LDLR.

From cartoons to quantitative models in Golgi transport

BACKGROUND

Cell biology is evolving to become a more formal and quantitative science. In particular, several mathematical models have been proposed to address Golgi self-organisation and protein and lipid transport. However, most scientific articles about the Golgi apparatus are still using static cartoons that miss the dynamism of this organelle.

RESULTS

In this report, we show that schematic drawings of Golgi trafficking can be easily translated into an agent-based model using the Repast platform. The simulations generate an active interplay among cisternae and vesicles rendering quantitative predictions about Golgi stability and transport of soluble and membrane-associated cargoes. The models can incorporate complex networks of molecular interactions and chemical reactions by association with COPASI, a software that handles ordinary differential equations.

CONCLUSIONS

The strategy described provides a simple, flexible and multiscale support to analyse Golgi transport. The simulations can be used to address issues directly linked to the mechanism of transport or as a way to incorporate the complexity of trafficking to other cellular processes that occur in dynamic organelles.

SIGNIFICANCE

We show that the rules implicitly present in most schematic representations of intracellular trafficking can be used to build dynamic models with quantitative outputs that can be compared with experimental results.

Treating brain diseases using systemic parenterally-administered protein therapeutics: Dysfunction of the brain barriers and potential strategies

The parenteral administration of protein therapeutics is increasingly gaining importance for the treatment of human diseases. However, the presence of practically impermeable blood-brain barriers greatly restricts access of such pharmaceutics to the brain. Treating brain disorders with proteins thus remains a great challenge, and the slow clinical translation of these therapeutics may be largely ascribed to the lack of appropriate brain delivery system. Exploring new approaches to deliver proteins to the brain by circumventing physiological barriers is thus of great interest. Moreover, parallel advances in the molecular neurosciences are important for better characterizing blood-brain interfaces, particularly under different pathological conditions (e.g., stroke, multiple sclerosis, Parkinson's disease, and Alzheimer's disease). This review presents the current state of knowledge of the structure and the function of the main physiological barriers of the brain, the mechanisms of transport across these interfaces, as well as alterations to these concomitant with brain disorders. Further, the different strategies to promote protein delivery into the brain are presented, including the use of molecular Trojan horses, the formulation of nanosystems conjugated/loaded with proteins, protein-engineering technologies, the conjugation of proteins to polymers, and the modulation of intercellular junctions. Additionally, therapeutic approaches for brain diseases that do not involve targeting to the brain are presented (i.e., sink and scavenging mechanisms).

ESCRT-dependent protein sorting is required for the viability of yeast clathrin-mediated endocytosis mutants

Endocytosis regulates many processes, including signaling pathways, nutrient uptake, and protein turnover. During clathrin-mediated endocytosis (CME), adaptors bind to cytoplasmic regions of transmembrane cargo proteins, and many endocytic adaptors are also directly involved in the recruitment of clathrin. This clathrin-associated sorting protein family includes the yeast epsins, Ent1/2, and AP180/PICALM homologs, Yap1801/2. Mutant strains lacking these four adaptors, but expressing an epsin N-terminal homology (ENTH) domain necessary for viability (4Δ+ENTH), exhibit endocytic defects, such as cargo accumulation at the plasma membrane (PM). This CME-deficient strain provides a sensitized background ideal for revealing cellular components that interact with clathrin adaptors. We performed a mutagenic screen to identify alleles that are lethal in 4Δ+ENTH cells using a colony-sectoring reporter assay. After isolating candidate synthetic lethal genes by complementation, we confirmed that mutations in VPS4 led to inviability of a 4Δ+ENTH strain. Vps4 mediates the final step of endosomal sorting complex required for transport (ESCRT)-dependent trafficking, and we found that multiple ESCRTs are also essential in 4Δ+ENTH cells, including Snf7, Snf8 and Vps36. Deletion of VPS4 from an end3Δ strain, another CME mutant, similarly resulted in inviability, and upregulation of a clathrin-independent endocytosis pathway rescued 4Δ+ENTH vps4Δ cells. Loss of Vps4 from an otherwise wild-type background caused multiple cargoes to accumulate at the PM because of an increase in Rcy1-dependent recycling of internalized protein to the cell surface. Additionally, vps4Δ rcy1Δ mutants exhibited deleterious growth phenotypes. Together, our findings reveal previously unappreciated effects of disrupted ESCRT-dependent trafficking on endocytic recycling and the PM.

Target engagement and intracellular delivery of mono- and bivalent LDL receptor-binding peptide-cargo conjugates: Implications for the rational design of new targeted drug therapies

Targeted delivery to specific tissues and subcellular compartments is of paramount importance to optimize therapeutic or diagnostic interventions while minimizing side-effects. Using recently identified LDL receptor (LDLR) -targeting small synthetic peptide-vectors conjugated to model cargos of different nature and size, we investigated in LDLR-expressing cells the impact of vector-cargo molecular engineering and coupling valency, as well as the cellular exposure duration on their target engagement and intracellular trafficking and delivery profiles. All vector-cargo conjugates evaluated were found to be delivered to late compartments together with the natural ligand LDL, although to varying extents and with different kinetics. Partial recycling together with the LDLR was also consistently observed. Under continuous cellular exposure, the extent of intracellular vector-cargo delivery primarily relies on their endosomal unloading potential. In this condition, the highest intracellular delivery potential was observed with a monovalent conjugate displaying a rather high LDLR dissociation rate. On the contrary, under transient cellular exposure followed by chase, low dissociation-rate bivalent conjugates revealed a higher intracellular delivery potential than the monovalent conjugate. This was shown to rely on their ability to undergo multiple endocytosis-recycling rounds, with limited release in the ligand-free medium. The absence of reciprocal competition with the natural ligand LDL on their respective intracellular trafficking was also demonstrated, which is essential in terms of potential safety liabilities. These results demonstrate that not only molecular engineering of new therapeutic conjugates of interest, but also the cellular exposure mode used during in vitro evaluations are critical to anticipate and optimize their delivery potential.

To degrade or not to degrade: mechanisms and significance of endocytic recycling

Newly endocytosed integral cell surface proteins are typically either directed for degradation or subjected to recycling back to the plasma membrane. The sorting of integral cell surface proteins, including signalling receptors, nutrient transporters, ion channels, adhesion molecules and polarity markers, within the endolysosomal network for recycling is increasingly recognized as an essential feature in regulating the complexities of physiology at the cell, tissue and organism levels. Historically, endocytic recycling has been regarded as a relatively passive process, where the majority of internalized integral proteins are recycled via a nonspecific sequence-independent 'bulk membrane flow' pathway. Recent work has increasingly challenged this view. The discovery of sequence-specific sorting motifs and the identification of cargo adaptors and associated coat complexes have begun to uncover the highly orchestrated nature of endosomal cargo recycling, thereby providing new insight into the function and (patho)physiology of this process.

New approaches for solving old problems in neuronal protein trafficking

Fundamental cellular properties are determined by the repertoire and abundance of proteins displayed on the cell surface. As such, the trafficking mechanisms for establishing and maintaining the surface proteome must be tightly regulated for cells to respond appropriately to extracellular cues, yet plastic enough to adapt to ever-changing environments. Not only are the identity and abundance of surface proteins critical, but in many cases, their regulated spatial positioning within surface nanodomains can greatly impact their function. In the context of neuronal cell biology, surface levels and positioning of ion channels and neurotransmitter receptors play essential roles in establishing important properties, including cellular excitability and synaptic strength. Here we review our current understanding of the trafficking pathways that control the abundance and localization of proteins important for synaptic function and plasticity, as well as recent technological advances that are allowing the field to investigate protein trafficking with increasing spatiotemporal precision.

New Method for Quantitation of Lipid Droplet Volume From Light Microscopic Images With an Application to Determination of PAT Protein Density on the Droplet Surface

Determination of lipid droplet (LD) volume has depended on direct measurement of the diameter of individual LDs, which is not possible when LDs are small or closely apposed. To overcome this problem, we describe a new method in which a volume-fluorescence relationship is determined from automated analysis of calibration samples containing well-resolved LDs. This relationship is then used to estimate total cellular droplet volume in experimental samples, where the LDs need not be individually resolved, or to determine the volumes of individual LDs. We describe quantitatively the effects of various factors, including image noise, LD crowding, and variation in LD composition on the accuracy of this method. We then demonstrate this method by utilizing it to address a scientifically interesting question, to determine the density of green fluorescent protein (GFP)-tagged Perilipin-Adipocyte-Tail (PAT) proteins on the LD surface. We find that PAT proteins cover only a minority of the LD surface, consistent with models in which they primarily serve as scaffolds for binding of regulatory proteins and enzymes, but inconsistent with models in which their major function is to sterically block access to the droplet surface.

Early Endosome Morphology in Health and Disease

Early endosomes are organelles that receive macromolecules and solutes from the extracellular environment. The major function of early endosomes is to sort these cargos into recycling and degradative compartments of the cell. Degradation of the cargo involves maturation of early endosomes into late endosomes, which, after acquisition of hydrolytic enzymes, form lysosomes. Endosome maturation involves recruitment of specific proteins and lipids to the early endosomal membrane, which drives changes in endosome morphology. Defects in early endosome maturation are generally accompanied by alterations in morphology, such as increase in volume and/or number. Enlarged early endosomes have been observed in Alzheimer's disease and Niemann Pick Disease type C, which also exhibit defects in endocytic sorting. This article discusses the mechanisms that regulate early endosome morphology and highlights the potential importance of endosome maturation in the retinal pigment epithelium.

Reciprocal regulation of signaling and endocytosis: Implications for the evolving cancer cell

Schmid provides a perspective on exciting new research examining the relationship between signaling and endocytosis in cancer.

Cell surface receptor uptake via clathrin-mediated endocytosis (CME) and subsequent intracellular sorting for degradation or recycling regulates the strength and specificity of downstream signaling. Signaling, in turn, modulates early endocytic trafficking. This reciprocal regulation of signaling and endocytosis provides opportunities for the establishment of feedback loops to enhance or suppress surface-derived signals. Recent studies suggest that dynamin-1, a presumed neuron-specific isoform of the large, membrane fission GTPase, can be activated in nonneuronal cells downstream of cancer-relevant signaling pathways and thereby function as a nexus between signaling and early endocytic trafficking. I speculate that sustained up-regulation and/or acute activation of dynamin-1 in cancer cells contributes to a program of “adaptive” CME that alters signaling to enhance cancer cell survival, migration, and proliferation.

Use of LDL receptor-targeting peptide vectors for in vitro and in vivo cargo transport across the blood-brain barrier

The blood-brain barrier (BBB) prevents the entry of many drugs into the brain and, thus, is a major obstacle in the treatment of CNS diseases. There is some evidence that the LDL receptor (LDLR) is expressed at the BBB and may participate in the transport of endogenous ligands from blood to brain, a process referred to as receptor-mediated transcytosis. We previously described a family of peptide vectors that were developed to target the LDLR. In the present study, in vitro BBB models that were derived from wild-type and LDLR-knockout animals (ldlr^-/- ) were used to validate the specific LDLR-dependent transcytosis of LDL via a nondegradative route. We next showed that LDLR-targeting peptide vectors, whether in fusion or chemically conjugated to an Ab Fc fragment, promote binding to apical LDLR and transendothelial transfer of the Fc fragment across BBB monolayers via the same route as LDL. Finally, we demonstrated in vivo that LDLR significantly contributes to the brain uptake of vectorized Fc. We thus provide further evidence that LDLR is a relevant receptor for CNS drug delivery via receptor-mediated transcytosis and that the peptide vectors we developed have the potential to transport drugs, including proteins or Ab based, across the BBB.-Molino, Y., David, M., Varini, K., Jabès, F., Gaudin, N., Fortoul, A., Bakloul, K., Masse, M., Bernard, A., Drobecq, L., Lécorché, P., Temsamani, J., Jacquot, G., Khrestchatisky, M. Use of LDL receptor-targeting peptide vectors for in vitro and in vivo cargo transport across the blood-brain barrier.

Quantitative Co-Localization and Pattern Analysis of Endo-Lysosomal Cargo in Subcellular Image Cytometry and Validation on Synthetic Image Sets

Late endosomes and lysosomes (LE/LYSs) play a central role in trafficking of endocytic cargo, secretion of exosomes, and hydrolysis of ingested proteins and lipids. Failure in such processes can lead to lysosomal storage disorders in which a particular metabolite accumulates within LE/LYSs. Analysis of endocytic trafficking relies heavily on quantitative fluorescence microscopy, but evaluation of the huge image data sets is challenging and demands computer-assisted statistical tools. Here, we describe how to use SpatTrack ( www.sdu.dk/bmb/spattrack ), an imaging toolbox, which we developed for quantification of the distribution and dynamics of endo-lysosomal cargo from fluorescence images of living cells. First, we explain how to analyze experimental images of endocytic processes in Niemann Pick C2 disease fibroblasts using SpatTrack. We demonstrate how to quantify the location of the sterol-binding protein NPC2 in LE/LYSs relative to cholesterol -rich lysosomal storage organelles (LSOs) stained with filipin. Second, we show how to simulate realistic vesicle patterns in the cell geometry using Markov Chain Monte Carlo and suitable inter-vesicle and cell-vesicle interaction potentials. Finally, we use such synthetic vesicle patterns as "ground truth" for validation of two-channel analysis tools in SpatTrack, revealing their high reliability. An improved version of SpatTrack for microscopy-based quantification of cargo transport through the endo-lysosomal system accompanies this protocol.

Role of EhRab7A in phagocytosis of type 1 fimbriated E. coli by Entamoeba histolytica

Entamoeba histolytica, the causative agent of amoebic colitis and liver abscess in human, ingests the intestinal bacteria and variety of host cells. Phagocytosis of bacteria by the amebic trophozoite has been reported to be important for the virulence of the parasite. Here, we set out to characterize different stages of phagocytosis of type 1 E. coli and investigated the role of a set of amoebic Rab GTPases in the process. The localizations of the Rab GTPases during different stages of the phagocytosis were investigated using laser scanning confocal microscopy and their functional relevance were determined using fluorescence activated cell sorter based assay as well as colony forming unit assay. Our results demonstrate that EhRab7A is localized on the phagosomes and involved in both early and late stages of type 1 E. coli phagocytosis. We further showed that the E. coli or RBC containing phagosomes are distinct from the large endocytic vacuoles in the parasite which are exclusively used to transport human holotransferrin and low density lipoprotein. Remarkably, type 1 E. coli uptake was found to be insensitive to cytochalasin D treatment, suggesting that the initial stage of E. coli phagocytosis is independent of the formation of actin filaments.

NECAP2 controls clathrin coat recruitment to early endosomes for fast endocytic recycling

Endocytic recycling returns receptors to the plasma membrane following internalization and is essential to maintain receptor levels on the cell surface, re-sensitize cells to extracellular ligands and for continued nutrient uptake. Yet, the protein machineries and mechanisms that drive endocytic recycling remain ill-defined. Here, we establish that NECAP2 regulates the endocytic recycling of EGFR and transferrin receptor. Our analysis of the recycling dynamics revealed that NECAP2 functions in the fast recycling pathway that directly returns cargo from early endosomes to the cell surface. In contrast, NECAP2 does not regulate the clathrin-mediated endocytosis of these cargos, the degradation of EGFR or the recycling of transferrin along the slow, Rab11-dependent recycling pathway. We show that protein knockdown of NECAP2 leads to enlarged early endosomes and causes the loss of the clathrin adapter AP-1 from the organelle. Through structure-function analysis, we define the protein-binding interfaces in NECAP2 that are crucial for AP-1 recruitment to early endosomes. Together, our data identify NECAP2 as a pathway-specific regulator of clathrin coat formation on early endosomes for fast endocytic recycling.

PCSK9 inhibition in the management of hyperlipidemia: focus on evolocumab

Proprotein convertase subtilisin/kexin type 9 (PCSK9) increases low-density lipoprotein cholesterol (LDL-C) concentrations through interference with normal physiologic hepatic LDL receptor (LDLR) recycling. Inhibiting PCSK9 results in improved LDLR recycling, increased LDLR availability on hepatocyte cell surfaces, and reduced blood LDL-C levels, making PCSK9 inhibition a novel therapeutic strategy for managing hypercholesterolemia. Monoclonal antibodies directed against PCSK9 have been developed for this purpose. A large number of clinical trials have demonstrated that monoclonal antibodies against PCSK9 yield substantial reductions in LDL-C when administered as monotherapy or in combination with statins to patients with nonfamilial and familial forms of hypercholesterolemia. Data from long-term trials demonstrate that the LDL-C-lowering effect of PCSK9 inhibitors is durable. These agents are generally well tolerated, and few patients discontinue treatment due to adverse events. Moreover, PCSK9 inhibitors do not appear to elicit the hepatic and muscle-related side effects associated with statin use. The ultimate value of PCSK9 inhibitors will be measured by their effect on clinical outcomes. Early evidence of a reduction in cardiovascular events after 1 year of treatment was shown in a prospective exploratory analysis of two ongoing long-term open-label extension evolocumab trials. Similarly, cardiovascular events were reduced in another exploratory analysis after >1 year of therapy with alirocumab. For the primary care physician, PCSK9 inhibitors represent a welcome additional option for lowering LDL-C in patients with familial forms of hypercholesterolemia and those with clinical atherosclerotic cardiovascular disease who are on maximally tolerated statin therapy.

Numb regulates vesicular docking for homotypic fusion of early endosomes via membrane recruitment of Mon1b

Numb is an endocytic protein that plays crucial roles in diverse cellular processes such as asymmetric cell division, cell migration and differentiation. However, the molecular mechanism by which Numb regulates endocytic trafficking is poorly understood. Here, we demonstrate that Numb is a docking regulator for homotypic fusion of early endosomes (EEs). Numb depletion causes clustered but unfused EEs, which can be rescued by overexpressing cytosolic Numb 65 and Numb 71 but not plasma membrane-attached Numb 66 or Numb 72. Time-lapse analysis reveals that paired vesicles tend to tether but not fuse with each other in the absence of Numb. We further show that Numb binds to another docking regulator, Mon1b, and is required for the recruitment of cytosolic Mon1b to the EE membrane. Consistent with this, deletion of Mon1b causes similar defects in EE fusion. Our study thus identifies a novel mechanism by which Numb regulates endocytic sorting by mediating EE fusion.

Specific role of cytoplasmic dynein in the mechanism of action of an antitumor molecule, Amblyomin-X

The Kunitz-type recombinant protein, Amblyomin-X, is an antitumor recombinant molecule from a cDNA library prepared from the salivary glands of the tick Amblyomma cajennense. The primary target of this protein appears to be the proteasome. Amblyomin-X increased gene and protein expression of distinct subunits of the molecular motor dynein, which plays a key role in the intracellular transport. Herein, Amblyomin-X was specifically taken up by tumor cells through lipid-raft endocytic pathways, but not by fibroblasts. Moreover, dynein inhibitor, ciliobrevin A, decreased Amblyomin-X uptake by tumor cells. Furthermore, incubation of tumor cells with Amblyomin-X inhibited trypsin-like activity of the proteasome, which was restored upon pretreatment with ciliobrevin A. Only in tumor cells treated with Amblyomin-X, we identified proteins bounds to dynein that are related to aggresome formation, autophagy inhibition, and early and recycling endosome markers. In addition, Amblyomin-X was found to interact with dynein, increased Rab11A protein expression and Rab11A co-localization with the light-intermediate chain 2 (LIC2) of dynein. Thereby, the results provide new insights on the antitumor mechanism of Amblyomin-X and reveal an unsuspected role of cytoplasmic dynein in its uptake, intracellular trafficking and pro-apoptotic action.

Tick salivary gland as potential natural source for the discovery of promising antitumor drug candidates

Nowadays, the relationship between cancer blood coagulation is well established. Regarding biodiversity and bioprospection, the tick biology has become quite attractive natural source for coagulation inhibitors, since its saliva has a very rich variety of bioactive molecules. For instance, a Kunitz-type FXa inhibitor, named Amblyomin-X, was found through transcriptome of the salivary gland of the Amblyomma cajennense. tick. This TFPI-like inhibitor, after obtained as recombinant protein, has presented anticoagulant, antigionenic, and antitumor properties. Although its effects on blood coagulation could be relevant for antitumor effect, Amblyomin-X acts by non-hemostatic mechanisms, such as proteasome inhibition and autophagy inhibition. Notably, cytotoxicity was not observed on non-tumor cells treated with this protein, suggesting some selectivity for tumor cells. Considering the current efforts in order to develop effective anticancer therapies, the findings presented in this review strongly suggest Amblyomin-X as a promising novel antitumor drug candidate.

Measuring receptor recycling in polarized MDCK cells

Recycling of proteins such as channels, pumps, and receptors is critical for epithelial cell function. In this chapter we present a method to measure receptor recycling in polarized Madin-Darby canine kidney cells using an iodinated ligand. We describe a technique to iodinate transferrin (Tf), we discuss how (125)I-Tf can be used to label a cohort of endocytosed Tf receptor, and then we provide methods to measure the rate of recycling of the (125)I-Tf-receptor complex. We also show how this approach, which is easily adaptable to other proteins, can be used to simultaneously measure the normally small amount of (125)I-Tf transcytosis and degradation.

EhVps32 Is a Vacuole-Associated Protein Involved in Pinocytosis and Phagocytosis of Entamoeaba histolytica

Here, we investigated the role of EhVps32 protein (a member of the endosomal-sorting complex required for transport) in endocytosis of Entamoeba histolytica, a professional phagocyte. Confocal microscopy, TEM and cell fractionation revealed EhVps32 in cytoplasmic vesicles and also located adjacent to the plasma membrane. Between 5 to 30 min of phagocytosis, EhVps32 was detected on some erythrocytes-containing phagosomes of acidic nature, and at 60 min it returned to cytoplasmic vesicles and also appeared adjacent to the plasma membrane. TEM images revealed it in membranous structures in the vicinity of ingested erythrocytes. EhVps32, EhADH (an ALIX family member), Gal/GalNac lectin and actin co-localized in the phagocytic cup and in some erythrocytes-containing phagosomes, but EhVps32 was scarcely detected in late phagosomes. During dextran uptake, EhVps32, EhADH and Gal/GalNac lectin, but not actin, co-localized in pinosomes. EhVps32 recombinant protein formed oligomers composed by rings and filaments. Antibodies against EhVps32 monomers stained cytoplasmic vesicles but not erythrocytes-containing phagosomes, suggesting that in vivo oligomers are formed on phagosome membranes. The involvement of EhVps32 in phagocytosis was further study in pNeoEhvps32-HA-transfected trophozoites, which augmented almost twice their rate of erythrophagocytosis as well as the membranous concentric arrays built by filaments, spirals and tunnel-like structures. Some of these structures apparently connected phagosomes with the phagocytic cup. In concordance, the EhVps32-silenced G3 trophozoites ingested 80% less erythrocytes than the G3 strain. Our results suggest that EhVps32 participates in E. histolytica phagocytosis and pinocytosis. It forms oligomers on erythrocytes-containing phagosomes, probably as a part of the scission machinery involved in membrane invagination and intraluminal vesicles formation.

Trophozoites of E. histolytica represent an excellent model to study endosomal-sorting complex required for transport components due to their high endocytic activity and vesicle trafficking. The key role of EhVps32 on phagocytosis is supported by: i) its presence on phagosomes, ii) its interaction with EhADH (an erythrocytes receptor), Gal/GalNac lectin and actin, iii) the higher rate of erythrophagocytosis showed by EhVps32 overexpressing trophozoites, iv) the diminish rate of phagocytosis in EhVps32-silenced G3 trophozoites, and v) its location in erythrocytes-containing acidic phagosomes. Here, we discovered the presence of membranous concentric helicoidally and tunnel-like structures constituted by EhVps32 and EhADH that may have a dynamic role in membrane remodeling and in the generation of intraluminal vesicles in the phagosomes. Elucidating molecular mechanisms of endocytosis-exocytosis pathways will help us to better understand the pathogenic process of E. histolytica and develop new drugs for diagnosis and vaccine methods.

Visualizing and quantitating sequence-dependent GPCR recycling

Recent advances in direct imaging have given us a new appreciation of the spatial and temporal dynamics of membrane trafficking processes, and have allowed us to ask questions that were difficult to address with traditional methods. A relevant example of this is protein sorting in the endosome, which serves as the primary sorting station for proteins internalized from the cell surface. In this chapter, we discuss fluorescence imaging protocols to directly visualize and quantitate the recycling of G protein-coupled receptors (GPCRs)-a highly physiologically relevant family of signaling receptors-in real time in living cells. The protocols allow direct visualization and quantitation of both GPCR exit from the endosome and GPCR delivery to the cell surface. The methods may be extended to study the endolysosomal sorting of many proteins that undergoes endocytic cycling, and may be adapted to other organelles and systems where proteins are sorted.

Postendocytic Sorting of Adrenergic and Opioid Receptors: New Mechanisms and Functions

The endocytic pathway tightly regulates the activity of G protein-coupled receptors (GPCRs). Much of our understanding of this relationship between GPCR endocytic trafficking and signaling comes from studies done on catecholamine and opioid receptors. After ligand-induced endocytosis, a key sorting step in the endosome determines whether receptors are recycled back to the cell surface, leading to recovery of signaling, or are degraded in the lysosome, leading to desensitization. Recycling of GPCRs, unlike that of many other proteins, is an active process driven by specific sequences on the receptor and proteins that interact with this sequence. Recent data suggest that sequence-dependent recycling plays complex roles in regulating both the timing and location of GPCR signaling. This chapter will describe our current understanding of the mechanisms regulating GPCR sorting in the endosome and discuss emerging ideas on their role in GPCR signaling, focusing on adrenergic and opioid receptors as prototypes.

ER contact sites define the position and timing of endosome fission

Endocytic cargo and Rab GTPases are segregated to distinct domains of an endosome. These domains maintain their identity until they undergo fission to traffic cargo. It is not fully understood how segregation of cargo or Rab proteins is maintained along the continuous endosomal membrane or what machinery is required for fission. Endosomes form contact sites with the endoplasmic reticulum (ER) that are maintained during trafficking. Here, we show that stable contacts form between the ER and endosome at constricted sorting domains, and free diffusion of cargo is limited at these positions. We demonstrate that the site of constriction and fission for early and late endosomes is spatially and temporally linked to contact sites with the ER. Lastly, we show that altering ER structure and dynamics reduces the efficiency of endosome fission. Together, these data reveal a surprising role for ER contact in defining the timing and position of endosome fission.

Form follows function: the importance of endoplasmic reticulum shape

The endoplasmic reticulum (ER) has a remarkably complex structure, composed of a single bilayer that forms the nuclear envelope, along with a network of sheets and dynamic tubules. Our understanding of the biological significance of the complex architecture of the ER has improved dramatically in the last few years. The identification of proteins and forces required for maintaining ER shape, as well as more advanced imaging techniques, has allowed the relationship between ER shape and function to come into focus. These studies have also revealed unexpected new functions of the ER and novel ER domains regulating alterations in ER dynamics. The importance of ER structure has become evident as recent research has identified diseases linked to mutations in ER-shaping proteins. In this review, we discuss what is known about the maintenance of ER architecture, the relationship between ER structure and function, and diseases associated with defects in ER structure.

New automated single-cell technique for segmentation and quantitation of lipid droplets

Lipid droplets are the major organelle for intracellular storage of triglycerides and cholesterol esters. Various methods have been attempted for automated quantitation of fluorescently stained lipid droplets using either thresholding or watershed methods. We find that thresholding methods deal poorly with clusters of lipid droplets, whereas watershed methods require a smoothing step that must be optimized to remove image noise. We describe here a novel three-stage hybrid method for automated segmentation and quantitation of lipid droplets. In this method, objects are initially identified by thresholding. They are then tested for circularity to distinguish single lipid droplets from clusters. Clusters are subjected to a secondary watershed segmentation. We provide a characterization of this method in simulated images. Additionally, we apply this method to images of fixed cells containing stained lipid droplets and GFP-tagged proteins to provide a proof-of-principle that this method can be used for colocalization studies. The circularity measure can additionally prove useful for the identification of inappropriate segmentation in an automated way; for example, of non-cellular material. We will make the programs and source code available to the community under the Gnu Public License. We believe this technique will be of interest to cell biologists for light microscopic studies of lipid droplet biology.

Single point mutation in Rabenosyn-5 in a female with intractable seizures and evidence of defective endocytotic trafficking

BACKGROUND

We report a 6.5 year-old female with a homozygous missense mutation in ZFYVE20, encoding Rabenosyn-5 (Rbsn-5), a highly conserved multi-domain protein implicated in receptor-mediated endocytosis. The clinical presentation includes intractable seizures, developmental delay, microcephaly, dysostosis, osteopenia, craniofacial dysmorphism, macrocytosis and megaloblastoid erythropoiesis. Biochemical findings include transient cobalamin deficiency, severe hypertriglyceridemia upon ketogenic diet, microalbuminuria and partial cathepsin D deficiency.

METHODS AND RESULTS

Whole exome sequencing followed by Sanger sequencing confirmed a rare (frequency:0.003987) homozygous missense mutation, g.15,116,371 G > A (c.1273G > A), in ZFYVE20 resulting in an amino acid change from Glycine to Arginine at position 425 of the Rbsn protein (p.Gly425Arg), as the only mutation segregating with disease in the family. Studies in fibroblasts revealed expression and localization of Rbsn-5G425R in wild-type manner, but a 50% decrease in transferrin accumulation, which is corrected by wild-type allele transfection. Furthermore, the patient's fibroblasts displayed an impaired proliferation rate, cytoskeletal and lysosomal abnormalities.

CONCLUSION

These results are consistent with a functional defect in the early endocytic pathway resulting from mutation in Rbsn-5, which secondarily disrupts multiple cellular functions dependent on endocytosis, leading to a severe multi-organ disorder.

TCR Microclusters pre-exist and contain molecules necessary for TCR signal transduction

TCR-dependent signaling events have been observed to occur in TCR microclusters. We found that some TCR microclusters are present in unstimulated murine T cells, indicating that the mechanisms leading to microcluster formation do not require ligand binding. These pre-existing microclusters increase in absolute number following engagement by low-potency ligands. This increase is accompanied by an increase in cell spreading, with the result that the density of TCR microclusters on the surface of the T cell is not a strong function of ligand potency. In characterizing their composition, we observed a constant number of TCRs in a microcluster, constitutive exclusion of the phosphatase CD45, and preassociation with the signaling adapters linker for activation of T cells and growth factor receptor-bound protein 2. The existence of TCR microclusters prior to ligand binding in a state that is conducive for the initiation of downstream signaling could explain, in part, the rapid kinetics with which TCR signal transduction occurs.

The role of endocytosis during morphogenetic signaling

Morphogens are signaling molecules that are secreted by a localized source and spread in a target tissue where they are involved in the regulation of growth and patterning. Both the activity of morphogenetic signaling and the kinetics of ligand spreading in a tissue depend on endocytosis and intracellular trafficking. Here, we review quantitative approaches to study how large-scale morphogen profiles and signals emerge in a tissue from cellular trafficking processes and endocytic pathways. Starting from the kinetics of endosomal networks, we discuss the role of cellular trafficking and receptor dynamics in the formation of morphogen gradients. These morphogen gradients scale during growth, which implies that overall tissue size influences cellular trafficking kinetics. Finally, we discuss how such morphogen profiles can be used to control tissue growth. We emphasize the role of theory in efforts to bridge between scales.

Role of endosomes and lysosomes in human disease

In addition to their roles in normal cell physiology, endocytic processes play a key role in many diseases. In this review, three diseases are discussed as examples of the role of endocytic processes in disease. The uptake of cholesterol via LDL is central to our understanding of atherosclerosis, and the study of this disease led to many of the key breakthroughs in understanding receptor-mediated endocytosis. Alzheimer's disease is a growing burden as the population ages. Endosomes and lysosomes play important but only partially understood roles in both the formation and the degradation of the amyloid fibrils that are associated with Alzheimer's disease. Inherited lysosomal storage diseases are individually rare, but collectively they affect many individuals. Recent advances are leading to improved enzyme replacement therapy and are also leading to small-molecule drugs to treat some of these diseases.

Membrane-associated cargo recycling by tubule-based endosomal sorting

The endosome system is a collection of organelles that sort membrane-associated proteins and lipids for lysosomal degradation or recycling back to their target organelle. Recycling cargo is captured in a network of membrane tubules emanating from endosomes where tubular carriers pinch off. These tubules are formed and stabilized through the scaffolding properties of cytosolic Bin/Amphiphysin/Rvs (BAR) proteins that comprise phosphoinositide-detecting moieties, recruiting these proteins to specific endosomal membrane areas. These include the protein family of sorting nexins that remodel endosome membrane into tubules by an evolutionary conserved mechanism of dimerization, local membrane curvature detection/induction and oligomerization. How the formation of such a tubular membrane carrier is coordinated with cargo capture is largely unknown. The tubular structure of the membrane carriers could sequester membrane-bound cargo through an iterative mechanism of geometric sorting. Furthermore, the recent identification of cargo adaptors for the endosome protein sorting complex retromer has expanded the sorting signals that retrieve specific sets of cargo away from lysosomal degradation through distinct membrane trafficking pathways.

Rab11a-positive compartments in proximal tubule cells sort fluid-phase and membrane cargo

The proximal tubule (PT) reabsorbs the majority of sodium, bicarbonate, and chloride ions, phosphate, glucose, water, and plasma proteins from the glomerular filtrate. Despite the critical importance of endocytosis for PT cell (PTC) function, the organization of the endocytic pathway in these cells remains poorly understood. We have used immunofluorescence and live-cell imaging to dissect the itinerary of apically internalized fluid and membrane cargo in polarized primary cultures of PTCs isolated from mouse kidney cortex. Cells from the S1 segment could be distinguished from those from more distal PT segments by their robust uptake of albumin and comparatively low expression of γ-glutamyltranspeptidase. Rab11a in these cells is localized to variously sized spherical compartments that resemble the apical vacuoles observed by electron microscopy analysis of PTCs in vivo. These Rab11a-positive structures are highly dynamic and receive membrane and fluid-phase cargo. In contrast, fluid-phase cargoes are largely excluded from Rab11a-positive compartments in immortalized kidney cell lines. The unusual morphology and sorting capacity of Rab11a compartments in primary PTCs may reflect a unique specialization of these cells to accommodate the functional demands of handling a high endocytic load.

A nexus for receptor recycling

Sorting nexin proteins (SNXs) and the cargo-selective retromer complex play key roles in receptor recycling from endosomes to the cell surface. A global proteomics analysis reveals a collection of cell surface proteins that rely on SNX27 and the retromer complex for their cell surface localization at steady state.

Quantitative fluorescence loss in photobleaching for analysis of protein transport and aggregation

Background

Fluorescence loss in photobleaching (FLIP) is a widely used imaging technique, which provides information about protein dynamics in various cellular regions. In FLIP, a small cellular region is repeatedly illuminated by an intense laser pulse, while images are taken with reduced laser power with a time lag between the bleaches. Despite its popularity, tools are lacking for quantitative analysis of FLIP experiments. Typically, the user defines regions of interest (ROIs) for further analysis which is subjective and does not allow for comparing different cells and experimental settings.

Results

We present two complementary methods to detect and quantify protein transport and aggregation in living cells from FLIP image series. In the first approach, a stretched exponential (StrExp) function is fitted to fluorescence loss (FL) inside and outside the bleached region. We show by reaction–diffusion simulations, that the StrExp function can describe both, binding/barrier–limited and diffusion-limited FL kinetics. By pixel-wise regression of that function to FL kinetics of enhanced green fluorescent protein (eGFP), we determined in a user-unbiased manner from which cellular regions eGFP can be replenished in the bleached area. Spatial variation in the parameters calculated from the StrExp function allow for detecting diffusion barriers for eGFP in the nucleus and cytoplasm of living cells. Polyglutamine (polyQ) disease proteins like mutant huntingtin (mtHtt) can form large aggregates called inclusion bodies (IB’s). The second method combines single particle tracking with multi-compartment modelling of FL kinetics in moving IB’s to determine exchange rates of eGFP-tagged mtHtt protein (eGFP-mtHtt) between aggregates and the cytoplasm. This method is self-calibrating since it relates the FL inside and outside the bleached regions. It makes it therefore possible to compare release kinetics of eGFP-mtHtt between different cells and experiments.

Conclusions

We present two complementary methods for quantitative analysis of FLIP experiments in living cells. They provide spatial maps of exchange dynamics and absolute binding parameters of fluorescent molecules to moving intracellular entities, respectively. Our methods should be of great value for quantitative studies of intracellular transport.

Alternative splicing results in RET isoforms with distinct trafficking properties

The RET gene encodes a receptor tyrosine kinase that is alternatively spliced to two protein isoforms that differ in their C-terminal peptide sequences (RET9, RET51). These unique C-terminal tails produce distinct subcellular localizations and intracellular trafficking properties, which affect downstream signaling.

RET encodes a receptor tyrosine kinase that is essential for spermatogenesis, development of the sensory, sympathetic, parasympathetic, and enteric nervous systems and the kidneys, as well as for maintenance of adult midbrain dopaminergic neurons. RET is alternatively spliced to encode multiple isoforms that differ in their C-terminal amino acids. The RET9 and RET51 isoforms display unique levels of autophosphorylation and have differential interactions with adaptor proteins. They induce distinct gene expression patterns, promote different levels of cell differentiation and transformation, and play unique roles in development. Here we present a comprehensive study of the subcellular localization and trafficking of RET isoforms. We show that immature RET9 accumulates intracellularly in the Golgi, whereas RET51 is efficiently matured and present in relatively higher amounts on the plasma membrane. RET51 is internalized faster after ligand binding and undergoes recycling back to the plasma membrane. This differential trafficking of RET isoforms produces a more rapid and longer duration of signaling through the extracellular-signal regulated kinase/mitogen-activated protein kinase pathway downstream of RET51 relative to RET9. Together these differences in trafficking properties contribute to some of the functional differences previously observed between RET9 and RET51 and establish the important role of intracellular trafficking in modulating and maintaining RET signaling.

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.

Getting active: protein sorting in endocytic recycling

Endocytic recycling returns proteins to the plasma membrane in many physiological contexts. Studies of these events have helped to elucidate fundamental mechanisms that underlie recycling. Recycling was for some time considered to be the exception to a general mechanism of active cargo sorting in multiple intracellular pathways. In recent years, studies have begun to reconcile this seeming disparity and also suggest explanations for why early recycling studies did not detect active sorting. Further articulation of this emerging trend has far-reaching implications for a deeper understanding of many physiological and pathological events that require recycling.