Endosomal "sort" of signaling control: The role of ESCRT machinery in regulation of receptor-mediated signaling pathways

NFκB and JNK pathways mediate metabolic adaptation upon ESCRT-I deficiency

Endosomal Sorting Complexes Required for Transport (ESCRTs) are crucial for delivering membrane receptors or intracellular organelles for lysosomal degradation which provides the cell with lysosome-derived nutrients. Yet, how ESCRT dysfunction affects cell metabolism remained elusive. To address this, we analyzed transcriptomes of cells lacking TSG101 or VPS28 proteins, components of ESCRT-I subcomplex. ESCRT-I deficiency reduced the expression of genes encoding enzymes involved in oxidation of fatty acids and amino acids, such as branched-chain amino acids, and increased the expression of genes encoding glycolytic enzymes. The changes in metabolic gene expression were associated with Warburg effect-like metabolic reprogramming that included intracellular accumulation of lipids, increased glucose/glutamine consumption and lactate production. Moreover, depletion of ESCRT-I components led to expansion of the ER and accumulation of small mitochondria, most of which retained proper potential and performed ATP-linked respiration. Mechanistically, the observed transcriptional reprogramming towards glycolysis in the absence of ESCRT-I occurred due to activation of the canonical NFκB and JNK signaling pathways and at least in part by perturbed lysosomal degradation. We propose that by activating the stress signaling pathways ESCRT-I deficiency leads to preferential usage of extracellular nutrients, like glucose and glutamine, for energy production instead of lysosome-derived nutrients, such as fatty acids and branched-chain amino acids.

Analysis of the neuromuscular deficits caused by STAM1 deficiency

The endosomal sorting complexes required for transport (ESCRT) pathway is composed of a series of protein complexes that are essential for sorting cargo through the endosome. In neurons, the ESCRT pathway is a key mediator of many cellular pathways that regulate neuronal morphogenesis as well as synaptic growth and function. The ESCRT-0 complex, consisting of HGS (hepatocyte growth factor-regulated tyrosine kinase substrate) and STAM (signal-transducing adaptor molecule), acts as a gate keeper to this pathway, ultimately determining the fate of the endosomal cargo. We previously showed that a single nucleotide substitution in Hgs results in structural and functional changes in the nervous system of teetering mice. To determine if these changes occurred as a function of HGS's role in the ESCRT pathway and its association with STAM1, we investigated if STAM1 deficiency also leads to a similar impairment of the nervous system. In contrast to teetering mice that die within 5 weeks of age and exhibit reduced body mass, 1-month-old Stam1 knockout mice were not visibly different from controls. However, by 3 months of age, STAM1 deficiency caused reduced muscle mass, strength, and motor performance. These changes in motor function did not correlate with either a loss in motor neuron number or abnormal myelination of peripheral nerves. Instead, the motor endplate structure was altered in the Stam1 knockout mice by 1 month of age and continued to degenerate over time, correlating with a significant reduction in muscle fiber size and increased expression of the embryonic γ acetylcholine receptor (AChR) subunit at 3 months of age. There was also a significant reduction in the levels of two presynaptic SNARE proteins, VTI1A and VAMP2, in the motor neurons of the Stam1 knockout mice. As loss of STAM1 expression replicates many of the structural changes at the motor endplates that we have previously reported with loss of HGS, these results suggest that the HGS/STAM1 complex plays a critical role in maintaining synaptic structure and function in the mammalian nervous system.

The Endosomal Sorting Complex, ESCRT, has diverse roles in blood progenitor maintenance, lineage choice and immune response

Most hematological malignancies are associated with reduced expression of one or more components of the Endosomal Sorting Complex Required for Transport (ESCRT). However, the roles of ESCRT in stem cell and progenitor maintenance are not resolved. Parsing signaling pathways in relation to the canonical role of ESCRT poses a challenge. The Drosophila hematopoietic organ, the larval lymph gland, provides a path to dissect the roles of cellular trafficking pathways such as ESCRT in blood development and maintenance. Drosophila has 13 core ESCRT components. Knockdown of individual ESCRTs showed that only Vps28 and Vp36 were required in all lymph gland progenitors. Using the well-conserved ESCRT-II complex as an example of the range of phenotypes seen upon ESCRT depletion, we show that ESCRTs have cell-autonomous as well as non-autonomous roles in progenitor maintenance and differentiation. ESCRT depletion also sensitized posterior lobe progenitors to respond to immunogenic wasp infestation. We also identify key heterotypic roles for ESCRT in position-dependent control of Notch activation to suppress crystal cell differentiation. Our study shows that the cargo sorting machinery determines the identity of progenitors and their adaptability to the dynamic microenvironment. These mechanisms for control of cell fate may tailor developmental diversity in multiple contexts.

Intrinsic Disorder in the Host Proteins Entrapped in Rabies Virus Particles

A proteomics analysis of purified rabies virus (RABV) revealed 47 entrapped host proteins within the viral particles. Out of these, 11 proteins were highly disordered. Our study was particularly focused on five of the RABV-entrapped mouse proteins with the highest levels of disorder: Neuromodulin, Chmp4b, DnaJB6, Vps37B, and Wasl. We extensively utilized bioinformatics tools, such as FuzDrop, D2P2, UniProt, RIDAO, STRING, AlphaFold, and ELM, for a comprehensive analysis of the intrinsic disorder propensity of these proteins. Our analysis suggested that these disordered host proteins might play a significant role in facilitating the rabies virus pathogenicity, immune system evasion, and the development of antiviral drug resistance. Our study highlighted the complex interaction of the virus with its host, with a focus on how the intrinsic disorder can play a crucial role in virus pathogenic processes, and suggested that these intrinsically disordered proteins (IDPs) and disorder-related host interactions can also be a potential target for therapeutic strategies.

Identification of the metaphyseal skeletal stem cell building trabecular bone

Skeletal stem cells (SSCs) that are capable of self-renewal and multipotent differentiation contribute to bone development and homeostasis. Several populations of SSCs at different skeletal sites have been reported. Here, we identify a metaphyseal SSC (mpSSC) population whose transcriptional landscape is distinct from other bone mesenchymal stromal cells (BMSCs). These mpSSCs are marked by Sstr2 or Pdgfrb+Kitl-, located just underneath the growth plate, and exclusively derived from hypertrophic chondrocytes (HCs). These HC-derived mpSSCs have properties of self-renewal and multipotency in vitro and in vivo, producing most HC offspring postnatally. HC-specific deletion of Hgs, a component of the endosomal sorting complex required for transport, impairs the HC-to-mpSSC conversion and compromises trabecular bone formation. Thus, mpSSC is the major source of BMSCs and osteoblasts in bone marrow, supporting the postnatal trabecular bone formation.

The role of SPP/SPPL intramembrane proteases in membrane protein homeostasis

Signal peptide peptidase (SPP) and the four SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 constitute a family of aspartyl intramembrane proteases with homology to presenilins. The different members reside in distinct cellular localisations within the secretory pathway and the endo-lysosomal system. Despite individual cleavage characteristics, they all cleave single-span transmembrane proteins with a type II orientation exhibiting a cytosolic N-terminus. Though the identification of substrates is not complete, SPP/SPPL-mediated proteolysis appears to be rather selective. Therefore, according to our current understanding cleavage by SPP/SPPL proteases rather seems to serve a regulatory function than being a bulk proteolytic pathway. In the present review, we will summarise our state of knowledge on SPP/SPPL proteases and in particular highlight recently identified substrates and the functional and/or (patho)-physiological implications of these cleavage events. Based on this, we aim to provide an overview of the current open questions in the field. These are connected to the regulation of these proteases at the cellular level but also in context of disease and patho-physiological processes. Furthermore, the interplay with other proteostatic systems capable of degrading membrane proteins is beginning to emerge.

The maize ZmVPS23-like protein relocates the nucleotide-binding leucine-rich repeat protein Rp1-D21 to endosomes and suppresses the defense response

Plants often utilize nucleotide-binding leucine-rich repeat (NLR) proteins to perceive pathogen infections and trigger a hypersensitive response (HR). The endosomal sorting complex required for transport (ESCRT) machinery is a conserved multisubunit complex that is essential for the biogenesis of multivesicular bodies and cargo protein sorting. VPS23 is a key component of ESCRT-I and plays important roles in plant development and abiotic stresses. ZmVPS23L, a homolog of VPS23-like in maize (Zea mays), was previously identified as a candidate gene in modulating HR mediated by the autoactive NLR protein Rp1-D21 in different maize populations. Here, we demonstrate that ZmVPS23L suppresses Rp1-D21-mediated HR in maize and Nicotiana benthamiana. Variation in the suppressive effect of HR by different ZmVPS23L alleles was correlated with variation in their expression levels. ZmVPS23 also suppressed Rp1-D21-mediated HR. ZmVPS23L and ZmVPS23 predominantly localized to endosomes, and they physically interacted with the coiled-coil domain of Rp1-D21 and mediated the relocation of Rp1-D21 from the nucleo-cytoplasm to endosomes. In summary, we demonstrate that ZmVPS23L and ZmVPS23 are negative regulators of Rp1-D21-mediated HR, likely by sequestrating Rp1-D21 in endosomes via physical interaction. Our findings reveal the role of ESCRT components in controlling plant NLR-mediated defense responses.

Endosomal trafficking in metabolic homeostasis and diseases

The global prevalences of obesity and type 2 diabetes mellitus have reached epidemic status, presenting a heavy burden on society. It is therefore essential to find novel mechanisms and targets that could be utilized in potential treatment strategies and, as such, intracellular membrane trafficking has re-emerged as a regulatory tool for controlling metabolic homeostasis. Membrane trafficking is an essential physiological process that is responsible for the sorting and distribution of signalling receptors, membrane transporters and hormones or other ligands between different intracellular compartments and the plasma membrane. Dysregulation of intracellular transport is associated with many human diseases, including cancer, neurodegeneration, immune deficiencies and metabolic diseases, such as type 2 diabetes mellitus and its associated complications. This Review focuses on the latest advances on the role of endosomal membrane trafficking in metabolic physiology and pathology in vivo, highlighting the importance of this research field in targeting metabolic diseases.

Vps37a regulates hepatic glucose production by controlling glucagon receptor localization to endosomes

During mammalian energy homeostasis, the glucagon receptor (Gcgr) plays a key role in regulating both glucose and lipid metabolisms. However, the mechanisms by which these distinct signaling arms are differentially regulated remain poorly understood. Using a Cy5-glucagon agonist, we show that the endosomal protein Vps37a uncouples glucose production from lipid usage downstream of Gcgr signaling by altering intracellular receptor localization. Hepatocyte-specific knockdown of Vps37a causes an accumulation of Gcgr in endosomes, resulting in overactivation of the cAMP/PKA/p-Creb signaling pathway to gluconeogenesis without affecting β-oxidation. Shifting the receptor back to the plasma membrane rescues the differential signaling and highlights the importance of the spatiotemporal localization of Gcgr for its metabolic effects. Importantly, since Vps37a knockdown in animals fed with a high-fat diet leads to hyperglycemia, although its overexpression reduces blood glucose levels, these data reveal a contribution of endosomal signaling to metabolic diseases that could be exploited for treatments of type 2 diabetes.

Endocytic trafficking of GAS6-AXL complexes is associated with sustained AKT activation

AXL, a TAM receptor tyrosine kinase (RTK), and its ligand growth arrest-specific 6 (GAS6) are implicated in cancer metastasis and drug resistance, and cellular entry of viruses. Given this, AXL is an attractive therapeutic target, and its inhibitors are being tested in cancer and COVID-19 clinical trials. Still, astonishingly little is known about intracellular mechanisms that control its function. Here, we characterized endocytosis of AXL, a process known to regulate intracellular functions of RTKs. Consistent with the notion that AXL is a primary receptor for GAS6, its depletion was sufficient to block GAS6 internalization. We discovered that upon receptor ligation, GAS6–AXL complexes were rapidly internalized via several endocytic pathways including both clathrin-mediated and clathrin-independent routes, among the latter the CLIC/GEEC pathway and macropinocytosis. The internalization of AXL was strictly dependent on its kinase activity. In comparison to other RTKs, AXL was endocytosed faster and the majority of the internalized receptor was not degraded but rather recycled via SNX1-positive endosomes. This trafficking pattern coincided with sustained AKT activation upon GAS6 stimulation. Specifically, reduced internalization of GAS6–AXL upon the CLIC/GEEC downregulation intensified, whereas impaired recycling due to depletion of SNX1 and SNX2 attenuated AKT signaling. Altogether, our data uncover the coupling between AXL endocytic trafficking and AKT signaling upon GAS6 stimulation. Moreover, our study provides a rationale for pharmacological inhibition of AXL in antiviral therapy as viruses utilize GAS6–AXL-triggered endocytosis to enter cells.

ESCRT-I fuels lysosomal degradation to restrict TFEB/TFE3 signaling via the Rag-mTORC1 pathway

ESCRT-I deficiency impairs lysosome membrane turnover and induces homeostatic responses to lysosomal nutrient starvation including activation of MiT-TFE signaling caused by inhibition of the substrate-specific mTORC1 pathway.

Within the endolysosomal pathway in mammalian cells, ESCRT complexes facilitate degradation of proteins residing in endosomal membranes. Here, we show that mammalian ESCRT-I restricts the size of lysosomes and promotes degradation of proteins from lysosomal membranes, including MCOLN1, a Ca²⁺ channel protein. The altered lysosome morphology upon ESCRT-I depletion coincided with elevated expression of genes annotated to biogenesis of lysosomes due to prolonged activation of TFEB/TFE3 transcription factors. Lack of ESCRT-I also induced transcription of cholesterol biosynthesis genes, in response to inefficient delivery of cholesterol from endolysosomal compartments. Among factors that could possibly activate TFEB/TFE3 signaling upon ESCRT-I deficiency, we excluded lysosomal cholesterol accumulation and Ca²⁺-mediated dephosphorylation of TFEB/TFE3. However, we discovered that this activation occurs due to the inhibition of Rag GTPase–dependent mTORC1 pathway that specifically reduced phosphorylation of TFEB at S122. Constitutive activation of the Rag GTPase complex in cells lacking ESCRT-I restored S122 phosphorylation and prevented TFEB/TFE3 activation. Our results indicate that ESCRT-I deficiency evokes a homeostatic response to counteract lysosomal nutrient starvation, that is, improper supply of nutrients derived from lysosomal degradation.

EhVps23, an ESCRT-I Member, Is a Key Factor in Secretion, Motility, Phagocytosis and Tissue Invasion by Entamoeba histolytica

The EhVps23 protein, an orthologue of the yeast Vps23 and the mammalian TSG101 proteins, is the single member of the ESCRT-I complex of Entamoeba histolytica identified and characterized until now. EhVps23 actively participates in vesicular trafficking and phagocytosis, which influence several cellular events. In this paper, we investigated the role of EhVps23 in virulence-related functions, including the invasive capacity of trophozoites, using transfected trophozoites. Trophozoites overexpressing the EhVps23 protein (Neo-EhVps23) presented helical arrangements in the cytoplasm, similar to the ones formed by EhVps32 for scission of vesicles. By confocal and transmission electron microscopy, EhVps23 was detected in multivesicular bodies, vesicles, and the extracellular space. It was secreted in vesicles together with other proteins, including the EhADH adhesin. Probably, these vesicles carry molecules that participate in the prey capture or in cell-cell communication. Mass spectrometry of precipitates obtained using α-EhVps23 antibodies, evidenced the presence of proteins involved in motility, phagocytosis, vesicular trafficking and secretion. The study of cellular functions, revealed that Neo-EhVps23 trophozoites exhibit characteristics similar to those described for mammalian transformed cells: they grew 50% faster than the control; presented a significant higher rate of phagocytosis, and migrated five-fold faster than the control, in concordance with the low rate of migration exhibited by Ehvps23-knocked down trophozoites. In addition, Neo-EhVps23 trophozoites produced dramatic liver abscesses in experimental animals. In conclusion, our results showed that EhVps23 overexpression gave to the trophozoites characteristics that resemble cancer cells, such as increased cell proliferation, migration, and invasion. The mutant that overexpresses EhVps23 can be a good study model to explore different events related to the transformation of malignant cells.

Denis N, Gingras AC, Lukacs GL, Pause A. Endofin is required for HD-PTP and ESCRT-0 interdependent endosomal sorting of ubiquitinated transmembrane cargoes

Internalized and ubiquitinated signaling receptors are silenced by their intraluminal budding into multivesicular bodies aided by the endosomal sorting complexes required for transport (ESCRT) machinery. HD-PTP, an ESCRT protein, forms complexes with ESCRT-0, -I and -III proteins, and binds to Endofin, a FYVE-domain protein confined to endosomes with poorly understood roles. Using proximity biotinylation, we showed that Endofin forms a complex with ESCRT constituents and Endofin depletion increased integrin α5-and EGF-receptor plasma membrane density and stability by hampering their lysosomal delivery. This coincided with sustained receptor signaling and increased cell migration. Complementation of Endofin- or HD-PTP-depleted cells with wild-type Endofin or HD-PTP, but not with mutants harboring impaired Endofin/HD-PTP association or cytosolic Endofin, restored EGFR lysosomal delivery. Endofin also promoted Hrs indirect interaction with HD-PTP. Jointly, our results indicate that Endofin is required for HD-PTP and ESCRT-0 interdependent sorting of ubiquitinated transmembrane cargoes to ensure efficient receptor desensitization and lysosomal delivery.

The ESCRT-0 subcomplex component Hrs/Hgs is a master regulator of myogenesis via modulation of signaling and degradation pathways

Background

Myogenesis is a highly regulated process ending with the formation of myotubes, the precursors of skeletal muscle fibers. Differentiation of myoblasts into myotubes is controlled by myogenic regulatory factors (MRFs) that act as terminal effectors of signaling cascades involved in the temporal and spatial regulation of muscle development. Such signaling cascades converge and are controlled at the level of intracellular trafficking, but the mechanisms by which myogenesis is regulated by the endosomal machinery and trafficking is largely unexplored. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery composed of four complexes ESCRT-0 to ESCRT-III regulates the biogenesis and trafficking of endosomes as well as the associated signaling and degradation pathways. Here, we investigate its role in regulating myogenesis.

Results

We uncovered a new function of the ESCRT-0 hepatocyte growth factor-regulated tyrosine kinase substrate Hrs/Hgs component in the regulation of myogenesis. Hrs depletion strongly impairs the differentiation of murine and human myoblasts. In the C2C12 murine myogenic cell line, inhibition of differentiation was attributed to impaired MRF in the early steps of differentiation. This alteration is associated with an upregulation of the MEK/ERK signaling pathway and a downregulation of the Akt2 signaling both leading to the inhibition of differentiation. The myogenic repressors FOXO1 as well as GSK3β were also found to be both activated when Hrs was absent. Inhibition of the MEK/ERK pathway or of GSK3β by the U0126 or azakenpaullone compounds respectively significantly restores the impaired differentiation observed in Hrs-depleted cells. In addition, functional autophagy that is required for myogenesis was also found to be strongly inhibited.

Conclusions

We show for the first time that Hrs/Hgs is a master regulator that modulates myogenesis at different levels through the control of trafficking, signaling, and degradation pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01091-4.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Lysosomal Function and Axon Guidance: Is There a Meaningful Liaison?

Axonal trajectories and neural circuit activities strongly rely on a complex system of molecular cues that finely orchestrate the patterning of neural commissures. Several of these axon guidance molecules undergo continuous recycling during brain development, according to incompletely understood intracellular mechanisms, that in part rely on endocytic and autophagic cascades. Based on their pivotal role in both pathways, lysosomes are emerging as a key hub in the sophisticated regulation of axonal guidance cue delivery, localization, and function. In this review, we will attempt to collect some of the most relevant research on the tight connection between lysosomal function and axon guidance regulation, providing some proof of concepts that may be helpful to understanding the relation between lysosomal storage disorders and neurodegenerative diseases.

Concurrent depletion of Vps37 proteins evokes ESCRT-I destabilization and profound cellular stress responses

Molecular details of how endocytosis contributes to oncogenesis remain elusive. Our in silico analysis of colorectal cancer (CRC) patients revealed stage-dependent alterations in the expression of 112 endocytosis-related genes. Among them, transcription of the endosomal sorting complex required for transport (ESCRT)-I component VPS37B was decreased in the advanced stages of CRC. Expression of other ESCRT-I core subunits remained unchanged in the investigated dataset. We analyzed an independent cohort of CRC patients, which also showed reduced VPS37A mRNA and protein abundance. Transcriptomic profiling of CRC cells revealed non-redundant functions of Vps37 proteins. Knockdown of VPS37A and VPS37B triggered p21 (CDKN1A)-mediated inhibition of cell proliferation and sterile inflammatory response driven by the nuclear factor (NF)-κB transcription factor and associated with mitogen-activated protein kinase signaling. Co-silencing of VPS37C further potentiated activation of these independently induced processes. The type and magnitude of transcriptional alterations correlated with the differential ESCRT-I stability upon individual and concurrent Vps37 depletion. Our study provides novel insights into cancer cell biology by describing cellular stress responses that are associated with ESCRT-I destabilization.

Protein crowding mediates membrane remodeling in upstream ESCRT-induced formation of intraluminal vesicles

As part of the lysosomal degradation pathway, the endosomal sorting complexes required for transport (ESCRT-0 to -III/VPS4) sequester receptors at the endosome and simultaneously deform the membrane to generate intraluminal vesicles (ILVs). Whereas ESCRT-III/VPS4 have an established function in ILV formation, the role of upstream ESCRTs (0 to II) in membrane shape remodeling is not understood. Combining experimental measurements and electron microscopy analysis of ESCRT-III-depleted cells with a mathematical model, we show that upstream ESCRT-induced alteration of the Gaussian bending rigidity and their crowding in concert with the transmembrane cargo on the membrane induce membrane deformation and facilitate ILV formation: Upstream ESCRT-driven budding does not require ATP consumption as only a small energy barrier needs to be overcome. Our model predicts that ESCRTs do not become part of the ILV, but localize with a high density at the membrane neck, where the steep decline in the Gaussian curvature likely triggers ESCRT-III/VPS4 assembly to enable neck constriction and scission.

Host Cell Targets of Released Lipid and Secreted Protein Effectors of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is a very successful pathogen, strictly adapted to humans and the cause of tuberculosis. Its success is associated with its ability to inhibit host cell intrinsic immune responses by using an arsenal of virulence factors of different nature. It has evolved to synthesize a series of complex lipids which form an outer membrane and may also be released to enter host cell membranes. In addition, secreted protein effectors of Mtb are entering the host cell cytosol to interact with host cell proteins. We briefly discuss the current model, involving the ESX-1 type seven secretion system and the Mtb lipid phthiocerol dimycoserosate (PDIM), of how Mtb creates pores in the phagosomal membrane to allow Mtb proteins to access to the host cell cytosol. We provide an exhaustive list of Mtb secreted proteins that have effector functions. They modify (mostly inhibit but sometimes activate) host cell pathways such as: phagosome maturation, cell death, cytokine response, xenophagy, reactive oxygen species (ROS) response via NADPH oxidase 2 (NOX2), nitric oxide (NO) response via NO Synthase 2 (NOS2) and antigen presentation via MHC class I and class II molecules. We discuss the host cell targets for each lipid and protein effector and the importance of the Mtb effector for virulence of the bacterium.

Epstein-Barr Virus LMP1 Promotes Syntenin-1- and Hrs-Induced Extracellular Vesicle Formation for Its Own Secretion To Increase Cell Proliferation and Migration

Extracellular vesicles (EVs) are important mediators of cell-to-cell communication that are involved in both normal processes and pathological conditions. Latent membrane protein 1 (LMP1) is a major viral oncogene that is expressed in most Epstein-Barr virus (EBV)-associated cancers and secreted in EVs. LMP1-modified EVs have the ability to influence recipient cell growth, migration, and differentiation and regulate immune cell function. Despite the significance of LMP1-modified EVs in EBV malignancies, very little is understood about how this protein hijacks the host EV pathway for secretion. Using the biotin identification (BioID) method, we identified LMP1-proximal interacting proteins that are known to play roles in EV formation and protein trafficking. Analysis of the identified LMP1-interacting proteins revealed an enrichment in the ESCRT pathway and associated proteins, including CD63, Syntenin-1, Alix, TSG101, Hrs, and charged multivesicular body proteins (CHMPs). LMP1 transcriptionally upregulated and increased the protein expression of EV biogenesis and secretion genes. Nanoparticle tracking and immunoblot analysis revealed reduced levels of LMP1 EV packaging and of vesicle production following the knockdown of Syntenin-1, Alix, Hrs, and TSG101, with altered endolysosomal trafficking observed when Syntenin-1 and Hrs expression was reduced. Knockdown of specific ESCRT-III subunits (CHMP4B, -5, and -6) impaired LMP1 packaging and secretion into EVs. Finally, we demonstrate that the efficient secretion of LMP1-modified EVs promotes cell attachment, proliferation, and migration and tumor growth. Together, these results begin to shed light on how LMP1 exploits host ESCRT machinery to direct the incorporation of the viral oncoprotein into the EV pathway for secretion to alter the tumor microenvironment.IMPORTANCE LMP1 is a notable viral protein that contributes to the modification of EV content and tumor microenvironment remodeling. LMP1-modified EVs enhance tumor proliferation, migration, and invasion potential and promote radioresistance. Currently, the mechanisms surrounding LMP1 incorporation into the host EV pathways are not well understood. This study revealed that LMP1 utilizes Hrs, Syntenin-1, and specific components of the ESCRT-III complex for release from the cell, enhancement of EV production, and metastatic properties of cancer cells. These findings begin to unravel the mechanism of LMP1 EV trafficking and may provide new targets to control EBV-associated cancers.

The ESCRT System Plays an Important Role in the Germination in Candida albicans by Regulating the Expression of Hyphal-Specific Genes and the Localization of Polarity-Related Proteins

Candida albicans is an important opportunistic fungal pathogen, and its pathogenicity is closely related to its ability to form hyphae. ESCRT system was initially discovered as a membrane-budding machinery involved in the formation of multivesicular bodies. More recently, the role of ESCRT is vastly expanded. Early reports showed that the ESCRT system is involved in inducing hyphae under neutral-alkaline environment via the Rim101 pathway. We previously found that in the environment that contains serum, one ESCRT protein, Vps4, is essential for polarity maintenance during hyphal formation, as its deletion causes the formation of multiple hyphae. In this study, we found that Vps4 is also essential for the proper localization of Cdc42 and Cdc3, which may be related to its role in polarity maintenance. We also discovered that deletions of the ESCRT proteins significantly delay germination and cause downregulation of hyphal-specific genes, most prominent of which is HGC1. Since Hgc1 is essential for many aspects of hyphal growth, its downregulation could explain our observed phenotypes. Our further studies show that ESCRT proteins are involved in the dynamics of Ras1. Deletions of VPS4 or SNF7 significantly decrease the recovery rate of GFP-Ras1 in the fluorescence recovery after photobleaching experiment. The decreased Ras1 dynamics may disrupt the signaling pathway and lead to downregulation of hyphal-specific genes. Therefore, in this study we discovered a novel and Rim101 independent mechanism used by the ESCRT system to regulate hyphal induction and polarity maintenance, which could provide insights on the pathogenicity mechanism of Candia albicans.

Synthetic lethality between VPS4A and VPS4B triggers an inflammatory response in colorectal cancer

Somatic copy number alterations play a critical role in oncogenesis. Loss of chromosomal regions containing tumor suppressors can lead to collateral deletion of passenger genes. This can be exploited therapeutically if synthetic lethal partners of such passenger genes are known and represent druggable targets. Here, we report that VPS4B gene, encoding an ATPase involved in ESCRT‐dependent membrane remodeling, is such a passenger gene frequently deleted in many cancer types, notably in colorectal cancer (CRC). We observed downregulation of VPS4B mRNA and protein levels from CRC patient samples. We identified VPS4A paralog as a synthetic lethal interactor for VPS4B in vitro and in mouse xenografts. Depleting both proteins profoundly altered the cellular transcriptome and induced cell death accompanied by the release of immunomodulatory molecules that mediate inflammatory and anti‐tumor responses. Our results identify a pair of novel druggable targets for personalized oncology and provide a rationale to develop VPS4 inhibitors for precision therapy of VPS4B‐deficient cancers.

Exosomal PD-L1: New Insights Into Tumor Immune Escape Mechanisms and Therapeutic Strategies

As a classical immune checkpoint molecule, PD-L1 on the surface of tumor cells plays a pivotal role in tumor immunosuppression, primarily by inhibiting the antitumor activities of T cells by binding to its receptor PD-1. PD-1/PD-L1 inhibitors have demonstrated unprecedented promise in treating various human cancers with impressive efficacy. However, a significant portion of cancer patients remains less responsive. Therefore, a better understanding of PD-L1-mediated immune escape is imperative. PD-L1 can be expressed on the surface of tumor cells, but it is also found to exist in extracellular forms, such as on exosomes. Recent studies have revealed the importance of exosomal PD-L1 (ExoPD-L1). As an alternative to membrane-bound PD-L1, ExoPD-L1 produced by tumor cells also plays an important regulatory role in the antitumor immune response. We review the recent remarkable findings on the biological functions of ExoPD-L1, including the inhibition of lymphocyte activities, migration to PD-L1-negative tumor cells and immune cells, induction of both local and systemic immunosuppression, and promotion of tumor growth. We also discuss the potential implications of ExoPD-L1 as a predictor for disease progression and treatment response, sensitive methods for detection of circulating ExoPD-L1, and the novel therapeutic strategies combining the inhibition of exosome biogenesis with PD-L1 blockade in the clinic.

Vacuolar protein sorting 4 is required for silkworm metamorphosis

Vacuolar protein sorting 4 (Vps4) not only functions with its positive regulator vacuolar protein sorting 20-associated 1 (Vta1) in the multivesicular body (MVB) pathway but also participates alone in MVB-unrelated cellular processes. However, its physiological roles at the organism level remain rarely explored. We previously identified their respective homologues Bombyx mori Vps4 (BmVps4) and BmVta1 from the silkworm, a model organism for insect research. In this study, we performed fluorescence quantitative real-time PCR and Western blot to globally characterize the transcription and protein expression profiles of BmVps4 and BmVta1 during silkworm development and in different silkworm tissues and organs. The results showed that they were significantly up-regulated in metamorphosis, adulthood and embryogenesis relative to larval stages, and displayed a roughly similar tissue-and-organ specificity for transcriptions in silkworm larvae. Importantly, BmVps4 was down-regulated during the early period of the fifth instar, reaching the lowest level of transcription on Day 6, then up-regulated from Day 7 to the wandering, spinning and pupal stages, and down-regulated again in adulthood. Moreover, knocking down BmVps4 by RNA interference significantly inhibited silk gland growth, shortened spinning time, prolonged pupation, reduced pupal size and weight, and increased moth wing defects. Together, our data demonstrate the critical and broad requirements for BmVps4 in silkworm metamorphosis.

The endosomal sorting adaptor HD-PTP is required for ephrin-B:EphB signalling in cellular collapse and spinal motor axon guidance

The signalling output of many transmembrane receptors that mediate cell-cell communication is restricted by the endosomal sorting complex required for transport (ESCRT), but the impact of this machinery on Eph tyrosine kinase receptor function is unknown. We identified the ESCRT-associated adaptor protein HD-PTP as part of an EphB2 proximity-dependent biotin identification (BioID) interactome, and confirmed this association using co-immunoprecipitation. HD-PTP loss attenuates the ephrin-B2:EphB2 signalling-induced collapse of cultured cells and axonal growth cones, and results in aberrant guidance of chick spinal motor neuron axons in vivo. HD-PTP depletion abrogates ephrin-B2-induced EphB2 clustering, and EphB2 and Src family kinase activation. HD-PTP loss also accelerates ligand-induced EphB2 degradation, contrasting the effects of HD-PTP loss on the relay of signals from other cell surface receptors. Our results link Eph function to the ESCRT machinery and demonstrate a role for HD-PTP in the earliest steps of ephrin-B:EphB signalling, as well as in obstructing premature receptor depletion.

Metabolic regulation through the endosomal system

The endosomal system plays an essential role in cell homeostasis by controlling cellular signaling, nutrient sensing, cell polarity and cell migration. However, its place in the regulation of tissue, organ and whole body physiology is less well understood. Recent studies have revealed an important role for the endosomal system in regulating glucose and lipid homeostasis, with implications for metabolic disorders such as type 2 diabetes, hypercholesterolemia and non-alcoholic fatty liver disease. By taking insights from in vitro studies of endocytosis and exploring their effects on metabolism, we can begin to connect the fields of endosomal transport and metabolic homeostasis. In this review, we explore current understanding of how the endosomal system influences the systemic regulation of glucose and lipid metabolism in mice and humans. We highlight exciting new insights that help translate findings from single cells to a wider physiological level and open up new directions for endosomal research.

Apoptotic signalling targets the post-endocytic sorting machinery of the death receptor Fas/CD95

Fas plays a major role in regulating ligand-induced apoptosis in many cell types. It is well known that several cancers demonstrate reduced cell surface levels of Fas and thus escape a potential control system via ligand-induced apoptosis, although underlying mechanisms are unclear. Here we report that the endosome associated trafficking regulator 1 (ENTR1), controls cell surface levels of Fas and Fas-mediated apoptotic signalling. ENTR1 regulates, via binding to the coiled coil domain protein Dysbindin, the delivery of Fas from endosomes to lysosomes thereby controlling termination of Fas signal transduction. We demonstrate that ENTR1 is cleaved during Fas-induced apoptosis in a caspase-dependent manner revealing an unexpected interplay of apoptotic signalling and regulation of endolysosomal trafficking resulting in a positive feedback signalling-loop. Our data provide insights into the molecular mechanism of Fas post-endocytic trafficking and signalling, opening possible explanations on how cancer cells regulate cell surface levels of death receptors.

Fas is a death receptor that regulates apoptosis in many cell types and is downregulated on the cell surface in many cancers. Here, Sharma et al. show that endosome associated trafficking regulator ENTR1 regulates delivery of Fas to lysosomes, thereby controlling its degradation and signalling.

The Lysosome Signaling Platform: Adapting With the Times

Lysosomes are the terminal degradative compartment of autophagy, endocytosis and phagocytosis. What once was viewed as a simple acidic organelle in charge of macromolecular digestion has emerged as a dynamic organelle capable of integrating cellular signals and producing signal outputs. In this review, we focus on the concept that the lysosome surface serves as a platform to assemble major signaling hubs like mTORC1, AMPK, GSK3 and the inflammasome. These molecular assemblies integrate and facilitate cross-talk between signals such as amino acid and energy levels, membrane damage and infection, and ultimately enable responses such as autophagy, cell growth, membrane repair and microbe clearance. In particular, we review how molecular machinery like the vacuolar-ATPase proton pump, sestrins, the GATOR complexes, and the Ragulator, modulate mTORC1, AMPK, GSK3 and inflammation. We then elaborate how these signals control autophagy initiation and resolution, TFEB-mediated lysosome adaptation, lysosome remodeling, antigen presentation, inflammation, membrane damage repair and clearance. Overall, by being at the cross-roads for several membrane pathways, lysosomes have emerged as the ideal surveillance compartment to sense, integrate and elicit cellular behavior and adaptation in response to changing environmental and cellular conditions.

CGRP Receptor Signalling Pathways

Calcitonin gene-related peptide (CGRP) is a promiscuous peptide, similar to many other members of the calcitonin family of peptides. The potential of CGRP to act on many different receptors with differing affinities and efficacies makes deciphering the signalling from the CGRP receptor a challenging task for researchers.Although it is not a typical G protein-coupled receptor (GPCR), in that it is composed not just of a GPCR, the CGRP receptor activates many of the same signalling pathways common for other GPCRs. This includes the family of G proteins and a variety of protein kinases and transcription factors. It is now also clear that in addition to the initiation of cell-surface signalling, GPCRs, including the CGRP receptor, also activate distinct signalling pathways as the receptor is trafficking along the endocytic conduit.Given CGRP's characteristic of activating multiple GPCRs, we will first consider the complex of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) as the CGRP receptor. We will discuss the discovery of the CGRP receptor components, the molecular mechanisms controlling its internalization and post-endocytic trafficking (recycling and degradation) and the diverse signalling cascades that are elicited by this receptor in model cell lines. We will then discuss CGRP-mediated signalling pathways in primary cells pertinent to migraine including neurons, glial cells and vascular smooth muscle cells.Investigation of all the CGRP- and CGRP receptor-mediated signalling cascades is vital if we are to fully understand CGRP's role in migraine and will no doubt unearth new targets for the treatment of migraine and other CGRP-driven diseases.

The Dynamics of TGF-β Signaling Are Dictated by Receptor Trafficking via the ESCRT Machinery

Signal transduction pathways stimulated by secreted growth factors are tightly regulated at multiple levels between the cell surface and the nucleus. The trafficking of cell surface receptors is emerging as a key step for regulating appropriate cellular responses, with perturbations in this process contributing to human diseases, including cancer. For receptors recognizing ligands of the transforming growth factor β (TGF-β) family, little is known about how trafficking is regulated or how this shapes signaling dynamics. Here, using whole genome small interfering RNA (siRNA) screens, we have identified the ESCRT (endosomal sorting complex required for transport) machinery as a crucial determinant of signal duration. Downregulation of ESCRT components increases the outputs of TGF-β signaling and sensitizes cells to low doses of ligand in their microenvironment. This sensitization drives an epithelial-to-mesenchymal transition (EMT) in response to low doses of ligand, and we demonstrate a link between downregulation of the ESCRT machinery and cancer survival.

The enigmatic endosome - sorting the ins and outs of endocytic trafficking

The early endosome (EE), also known as the sorting endosome (SE) is a crucial station for the sorting of cargoes, such as receptors and lipids, through the endocytic pathways. The term endosome relates to the receptacle-like nature of this organelle, to which endocytosed cargoes are funneled upon internalization from the plasma membrane. Having been delivered by the fusion of internalized vesicles with the EE or SE, cargo molecules are then sorted to a variety of endocytic pathways, including the endo-lysosomal pathway for degradation, direct or rapid recycling to the plasma membrane, and to a slower recycling pathway that involves a specialized form of endosome known as a recycling endosome (RE), often localized to the perinuclear endocytic recycling compartment (ERC). It is striking that 'the endosome', which plays such essential cellular roles, has managed to avoid a precise description, and its characteristics remain ambiguous and heterogeneous. Moreover, despite the rapid advances in scientific methodologies, including breakthroughs in light microscopy, overall, the endosome remains poorly defined. This Review will attempt to collate key characteristics of the different types of endosomes and provide a platform for discussion of this unique and fascinating collection of organelles. Moreover, under-developed, poorly understood and important open questions will be discussed.

Molecular Genetics of Frontotemporal Dementia Elucidated by Drosophila Models-Defects in Endosomal⁻Lysosomal Pathway

Frontotemporal dementia (FTD) is the second most common senile neurodegenerative disease. FTD is a heterogeneous disease that can be classified into several subtypes. A mutation in CHMP2B locus (CHMP2B^intron5), which encodes a component of endosomal sorting complex required for transport-III (ESCRT-III), is associated with a rare hereditary subtype of FTD linked to chromosome 3 (FTD-3). ESCRT is involved in critical cellular processes such as multivesicular body (MVB) formation during endosomal–lysosomal pathway and autophagy. ESCRT mutants causes diverse physiological defects primarily due to accumulation of endosomes and defective MVBs resulting in misregulation of signaling pathways. Charged multivesicular body protein 2B (CHMP2B) is important for neuronal physiology which especially rely on precise regulation of protein homeostasis due to their post-mitotic status. Drosophila has proven to be an excellent model for charaterization of mechanistic underpinning of neurodegenerative disorders including FTD. In this review, current understanding of various FTD-related mutations is discussed with a focus on Drosophila models of CHMP2B^intron5-associated FTD.

The topology of lymphotoxin β receptor accumulated upon endolysosomal dysfunction dictates the NF-κB signaling outcome

Cytokine receptors, such as tumor necrosis factor receptor I (TNFRI) and lymphotoxin β receptor (LTβR), activate inflammatory NF-κB signaling upon stimulation. We previously demonstrated that depletion of ESCRT components leads to endosomal accumulation of TNFRI and LTβR, and their ligand-independent signaling to NF-κB. Here, we studied if other perturbations of the endolysosomal system could trigger intracellular accumulation and signaling of ligand-free LTβR. While depletion of CORVET had no effect, knockdown of HOPS or Rab7, or pharmacological inhibition of lysosomal degradation, caused endosomal accumulation of LTβR and its increased interactions with TRAF2/TRAF3 signaling adaptors. However, the NF-κB pathway was not activated under these conditions. We found that knockdown of HOPS or Rab7 led to LTβR sequestration in intraluminal vesicles of endosomes, thus precluding NF-κB signaling. This was in contrast to LTβR localization on the outer endosomal membrane after ESCRT depletion that was permissive for signaling. We propose that the inflammatory response induced by intracellular accumulation of endocytosed cytokine receptors critically depends on the precise receptor topology within endosomal compartments.