Human VPS34 is required for internal vesicle formation within multivesicular endosomes

The role of exosomes in cancer biology by shedding light on their lipid contents

Exosomes are extracellular bilayer membrane nanovesicles released by cells after the fusion of multivesicular bodies (MVBs) with the plasma membrane. One of the interesting features of exosomes is their ability to carry and transfer various molecules, including lipids, proteins, nucleic acids, and therapeutic cargoes among cells. As intercellular signaling organelles, exosomes participate in various signaling processes such as tumor growth, metastasis, angiogenesis, epithelial-to-mesenchymal transition (EMT), and cell physiology such as cell-to-cell communication. Moreover, these particles are considered good vehicles to shuttle vaccines and drugs for therapeutic applications regarding cancers and tumor cells. These bioactive vesicles are also rich in various lipid molecules such as cholesterol, sphingomyelin (SM), glycosphingolipids, and phosphatidylserine (PS). These lipids play an important role in the formation, release, and function of the exosomes and interestingly, some lipids are used as biomarkers in cancer diagnosis. This review aimed to focus on exosomes lipid content and their role in cancer biology.

AUTOPHAGY IN THE EYE: FROM PHYSIOLOGY TO PATHOPHYSOLOGY

Autophagy is a catabolic self-degradative pathway that promotes the degradation and recycling of intracellular material through the lysosomal compartment. Although first believed to function in conditions of nutritional stress, autophagy is emerging as a critical cellular pathway, involved in a variety of physiological and pathophysiological processes. Autophagy dysregulation is associated with an increasing number of diseases, including ocular diseases. On one hand, mutations in autophagy-related genes have been linked to cataracts, glaucoma, and corneal dystrophy; on the other hand, alterations in autophagy and lysosomal pathways are a common finding in essentially all diseases of the eye. Moreover, LC3-associated phagocytosis, a form of non-canonical autophagy, is critical in promoting visual cycle function. This review collects the latest understanding of autophagy in the context of the eye. We will review and discuss the respective roles of autophagy in the physiology and/or pathophysiology of each of the ocular tissues, its diurnal/circadian variation, as well as its involvement in diseases of the eye.

The Machinery of Exosomes: Biogenesis, Release, and Uptake

Exosomes are a subtype of membrane-contained vesicles 40-200 nm in diameter that are secreted by cells into their surroundings. By transporting proteins, lipids, mRNA, miRNA, lncRNA, and DNA, exosomes are able to perform such vital functions as maintaining cellular homeostasis, removing cellular debris, and facilitating intercellular and interorgan communication. Exosomes travel in all body fluids and deliver their molecular messages in autocrine, paracrine as well as endocrine manners. In recent years, there has been an increased interest in studying exosomes as diagnostic markers and therapeutic targets, since in many disease conditions this machinery becomes dysregulated or hijacked by pathological processes. Additionally, delivery of exosomes and exosomal miRNA has already been shown to improve systemic metabolism and inhibit progression of cancer development in mice. However, the subcellular machinery of exosomes, including their biogenesis, release and uptake, remains largely unknown. This review will bring molecular details of these processes up to date with the goal of expanding the knowledge basis for designing impactful exosome experiments in the future.

Can Soluble Immune Checkpoint Molecules on Exosomes Mediate Inflammation?

Immune checkpoints (ICPs) are major co-signaling pathways that trigger effector functions in immune cells, with isoforms that are either membrane bound, engaging in direct cell to cell activation locally, or soluble, acting at distant sites by circulating freely or potentially via extracellular vesicles (EVs). Exosomes are small EVs secreted by a variety of cells carrying various proteins and nucleic acids. They are distributed extensively through biological fluids and have major impacts on infectious diseases, cancer, and neuroinflammation. Similarly, ICPs play key roles in a variety of disease conditions and have been extensively utilized as a prognostic tool for various cancers. Herein, we explored if the association between exosomes and ICPs could be a significant contributor of inflammation, particularly in the setting of cancer, neuroinflammation and viral infections, wherein the up regulation in both exosomal proteins and ICPs correlate with immunosuppressive effects. The detailed literature review of existing data highlights the significance and complexity of these two important pathways in mediating cancer and potentiating neuroinflammation via modulating overall immune response. Cells increasingly secret exosomes in response to intracellular signals from invading pathogens or cancerous transformations. These exosomes can carry a variety of cargo including proteins, nucleic acids, cytokines, and receptors/ligands that have functional consequences on recipient cells. Illustration generated using BioRender software.

Activation Mechanisms of the VPS34 Complexes

Phosphatidylinositol-3-phosphate (PtdIns(3)P) is essential for cell survival, and its intracellular synthesis is spatially and temporally regulated. It has major roles in two distinctive cellular pathways, namely, the autophagy and endocytic pathways. PtdIns(3)P is synthesized from phosphatidylinositol (PtdIns) by PIK3C3C/VPS34 in mammals or Vps34 in yeast. Pathway-specific VPS34/Vps34 activity is the consequence of the enzyme being incorporated into two mutually exclusive complexes: complex I for autophagy, composed of VPS34/Vps34-Vps15/Vps15-Beclin 1/Vps30-ATG14L/Atg14 (mammals/yeast), and complex II for endocytic pathways, in which ATG14L/Atg14 is replaced with UVRAG/Vps38 (mammals/yeast). Because of its involvement in autophagy, defects in which are closely associated with human diseases such as cancer and neurodegenerative diseases, developing highly selective drugs that target specific VPS34/Vps34 complexes is an essential goal in the autophagy field. Recent studies on the activation mechanisms of VPS34/Vps34 complexes have revealed that a variety of factors, including conformational changes, lipid physicochemical parameters, upstream regulators, and downstream effectors, greatly influence the activity of these complexes. This review summarizes and highlights each of these influences as well as clarifying key questions remaining in the field and outlining future perspectives.

His domain protein tyrosine phosphatase and Rabaptin-5 couple endo-lysosomal sorting of EGFR with endosomal maturation

His domain protein tyrosine phosphatase (HD-PTP; also known as PTPN23) collaborates with endosomal sorting complexes required for transport (ESCRTs) to sort endosomal cargo into intralumenal vesicles, forming the multivesicular body (MVB). Completion of MVB sorting is accompanied by maturation of the endosome into a late endosome, an event that requires inactivation of the early endosomal GTPase Rab5 (herein referring to generically to all isoforms). Here, we show that HD-PTP links ESCRT function with endosomal maturation. HD-PTP depletion prevents MVB sorting, while also blocking cargo from exiting Rab5-rich endosomes. HD-PTP-depleted cells contain hyperphosphorylated Rabaptin-5 (also known as RABEP1), a cofactor for the Rab5 guanine nucleotide exchange factor Rabex-5 (also known as RABGEF1), although HD-PTP is unlikely to directly dephosphorylate Rabaptin-5. In addition, HD-PTP-depleted cells exhibit Rabaptin-5-dependent hyperactivation of Rab5. HD-PTP binds directly to Rabaptin-5, between its Rabex-5- and Rab5-binding domains. This binding reaction involves the ESCRT-0/ESCRT-III binding site in HD-PTP, which is competed for by an ESCRT-III peptide. Jointly, these findings indicate that HD-PTP may alternatively scaffold ESCRTs and modulate Rabex-5–Rabaptin-5 activity, thereby helping to coordinate the completion of MVB sorting with endosomal maturation.

Summary: Sorting of endocytic cargo to the multivesicular body is accompanied by endosomal maturation. Here, we provide a potential mechanism by which these two processes are linked.

Crosstalk between autophagy inhibitors and endosome-related secretory pathways: a challenge for autophagy-based treatment of solid cancers

Autophagy is best known for its role in organelle and protein turnover, cell quality control, and metabolism. The autophagic machinery has, however, also adapted to enable protein trafficking and unconventional secretory pathways so that organelles (such as autophagosomes and multivesicular bodies) delivering cargo to lysosomes for degradation can change their mission from fusion with lysosomes to fusion with the plasma membrane, followed by secretion of the cargo from the cell. Some factors with key signalling functions do not enter the conventional secretory pathway but can be secreted in an autophagy-mediated manner.Positive clinical results of some autophagy inhibitors are encouraging. Nevertheless, it is becoming clear that autophagy inhibition, even within the same cancer type, can affect cancer progression differently. Even next-generation inhibitors of autophagy can have significant non-specific effects, such as impacts on endosome-related secretory pathways and secretion of extracellular vesicles (EVs). Many studies suggest that cancer cells release higher amounts of EVs compared to non-malignant cells, which makes the effect of autophagy inhibitors on EVs secretion highly important and attractive for anticancer therapy. In this review article, we discuss how different inhibitors of autophagy may influence the secretion of EVs and summarize the non-specific effects of autophagy inhibitors with a focus on endosome-related secretory pathways. Modulation of autophagy significantly impacts not only the quantity of EVs but also their content, which can have a deep impact on the resulting pro-tumourigenic or anticancer effect of autophagy inhibitors used in the antineoplastic treatment of solid cancers.

Centrosomal P4.1-associated protein (CPAP) positively regulates endocytic vesicular transport and lysosome targeting of EGFR

Centrosomal P4.1-associated protein (CPAP) plays a critical role in restricting the centriole length in human cells. Here, we report a novel, positive regulatory influence for CPAP on endocytic vesicular transport (EVT) and lysosome targeting of internalized-cell surface receptor EGFR. We observed that higher CPAP levels cause an increase in the abundance of multi-vesicular body (MVB) and EGFR is detectable in CPAP-overexpression induced puncta. The surface and cellular levels of EGFR are higher under CPAP deficiency and lower under CPAP overexpression. While ligand-engagement induced internalization or routing of EGFR into early endosomes is not influenced by cellular levels of CPAP, we found that targeting of ligand-activated, internalized EGFR to lysosome is impacted by CPAP levels. Transport of ligand-bound EGFR from early endosome to late endosome/MVB and lysosome is diminished in CPAP-depleted cells. Moreover, CPAP depleted cells appear to show a diminished ability to form MVB structures upon EGFR activation. These observations suggest a positive regulatory effect of CPAP on EVT of ligand-bound EGFR-like cell surface receptors to MVB and lysosome. Overall, identification of a non-centriolar function of CPAP in endocytic trafficking provides new insights in understanding the non-canonical cellular functions of CPAP.

INPP4B promotes PI3Kα-dependent late endosome formation and Wnt/β-catenin signaling in breast cancer

INPP4B suppresses PI3K/AKT signaling by converting PI(3,4)P₂ to PI(3)P and INPP4B inactivation is common in triple-negative breast cancer. Paradoxically, INPP4B is also a reported oncogene in other cancers. How these opposing INPP4B roles relate to PI3K regulation is unclear. We report PIK3CA-mutant ER⁺ breast cancers exhibit increased INPP4B mRNA and protein expression and INPP4B increased the proliferation and tumor growth of PIK3CA-mutant ER⁺ breast cancer cells, despite suppression of AKT signaling. We used integrated proteomics, transcriptomics and imaging to demonstrate INPP4B localized to late endosomes via interaction with Rab7, which increased endosomal PI3Kα-dependent PI(3,4)P₂ to PI(3)P conversion, late endosome/lysosome number and cargo trafficking, resulting in enhanced GSK3β lysosomal degradation and activation of Wnt/β-catenin signaling. Mechanistically, Wnt inhibition or depletion of the PI(3)P-effector, Hrs, reduced INPP4B-mediated cell proliferation and tumor growth. Therefore, INPP4B facilitates PI3Kα crosstalk with Wnt signaling in ER⁺ breast cancer via PI(3,4)P₂ to PI(3)P conversion on late endosomes, suggesting these tumors may be targeted with combined PI3K and Wnt/β-catenin therapies.

Phosphatidylinositol 3,5-bisphosphate regulates Ca2+ transport during yeast vacuolar fusion through the Ca2+ ATPase Pmc1

The transport of Ca²⁺ across membranes precedes the fusion and fission of various lipid bilayers. Yeast vacuoles under hyperosmotic stress become fragmented through fission events that requires the release of Ca²⁺ stores through the TRP channel Yvc1. This requires the phosphorylation of phosphatidylinositol-3-phosphate (PI3P) by the PI3P-5-kinase Fab1 to produce transient PI(3,5)P₂ pools. Ca²⁺ is also released during vacuole fusion upon trans-SNARE complex assembly, however, its role remains unclear. The effect of PI(3,5)P₂ on Ca²⁺ flux during fusion was independent of Yvc1. Here, we show that while low levels of PI(3,5)P₂ were required for Ca²⁺ uptake into the vacuole, increased concentrations abolished Ca²⁺ efflux. This was as shown by the addition of exogenous dioctanoyl PI(3,5)P₂ or increased endogenous production of by the hyperactive fab1^T2250A mutant. In contrast, the lack of PI(3,5)P₂ on vacuoles from the kinase dead fab1^EEE mutant showed delayed and decreased Ca²⁺ uptake. The effects of PI(3,5)P₂ were linked to the Ca²⁺ pump Pmc1, as its deletion rendered vacuoles resistant to the effects of excess PI(3,5)P₂ . Experiments with Verapamil inhibited Ca²⁺ uptake when added at the start of the assay, while adding it after Ca²⁺ had been taken up resulted in the rapid expulsion of Ca²⁺ . Vacuoles lacking both Pmc1 and the H⁺ /Ca²⁺ exchanger Vcx1 lacked the ability to take up Ca²⁺ and instead expelled it upon the addition of ATP. Together these data suggest that a balance of efflux and uptake compete during the fusion pathway and that the levels of PI(3,5)P₂ can modulate which path predominates.

Exosomal integrins and their influence on pancreatic cancer progression and metastasis

As one of the most lethal and untreatable types of cancer so far, pancreatic cancer is not benefitting from advancements in research. Despite all the efforts, this malignancy is still very difficult to diagnose in time, resistant to treatments, and prone to relapses. The appearance of metastasis-notoriously difficult to fight and a signal of unfortunate prognosis-is the event most dreaded by every cancer patient, especially by those with pancreatic cancer. Strategies for early detection and treatment of metastases are limited, and new action plans are desperately awaited. Recently, the importance of cell-secreted vesicles, or exosomes, in cell-cell communication and, particularly, their key role in promoting pathological conditions, such as infectious diseases and cancer, have attracted the attention of the scientific community. The discovery of some exosome membrane components, such as adhesion receptors and integrins, and their ability to influence cancer cell functions and metastasis progression, has added some important understanding of the metastatic process and will hopefully open the door to the development of new tools for identifying and targeting metastases. The aim of this review is to discuss the role played by integrins in exosomal-mediated pancreatic cancer progression and metastasis.

MicroRNA regulation of vascular smooth muscle cells and its significance in cardiovascular diseases

Cardiovascular disease (CVD) is among the leading causes of death worldwide. MicroRNAs (miRNAs), regulatory molecules that repress protein expression, have attracted considerable attention in CVD research. The vasculature plays a big role in CVD development and progression and dysregulation of vascular cells underlies the root of many vascular diseases. This review provides a brief introduction of the biogenesis of miRNAs and exosomes, followed by overview of the regulatory mechanisms of miRNAs in vascular smooth muscle cells (VSMCs) intracellular signaling during phenotypic switching, senescence, calcification, and neointimal hyperplasia. Evidence of extracellular signaling of VSMCs and other cells via exosomal and circulating miRNAs is also presented. Lastly, current drawbacks and limitations of miRNA studies in CVD research and potential ways to overcome these disadvantages are discussed in detail. In-depth understanding of VSMC regulation via miRNAs will add substantial knowledge and advance research in diagnosis, disease progression, and (or) miRNA-derived therapeutic approaches in CVD research.

Toll-like receptor-mediated innate immunity against herpesviridae infection: a current perspective on viral infection signaling pathways

Background

In the past decades, researchers have demonstrated the critical role of Toll-like receptors (TLRs) in the innate immune system. They recognize viral components and trigger immune signal cascades to subsequently promote the activation of the immune system.

Main body

Herpesviridae family members trigger TLRs to elicit cytokines in the process of infection to activate antiviral innate immune responses in host cells. This review aims to clarify the role of TLRs in the innate immunity defense against herpesviridae, and systematically describes the processes of TLR actions and herpesviridae recognition as well as the signal transduction pathways involved.

Conclusions

Future studies of the interactions between TLRs and herpesviridae infections, especially the subsequent signaling pathways, will not only contribute to the planning of effective antiviral therapies but also provide new molecular targets for the development of antiviral drugs.

Autophagy Is Required for Maturation of Surfactant-Containing Lamellar Bodies in the Lung and Swim Bladder

Autophagy is an intracellular degradation system, but its physiological functions in vertebrates are not yet fully understood. Here, we show that autophagy is required for inflation of air-filled organs: zebrafish swim bladder and mouse lung. In wild-type zebrafish swim bladder and mouse lung type II pulmonary epithelial cells, autophagosomes are formed and frequently fuse with lamellar bodies. The lamellar body is a lysosome-related organelle that stores a phospholipid-containing surfactant complex that lines the air-liquid interface and reduces surface tension. We find that autophagy is critical for maturation of the lamellar body. Accordingly, atg-deficient zebrafish fail to maintain their position in the water, and type-II-pneumocyte-specific Fip200-deficient mice show neonatal lethality with respiratory failure. Autophagy suppression does not affect synthesis of the surfactant phospholipid, suggesting that autophagy supplies lipids and membranes to lamellar bodies. These results demonstrate an evolutionarily conserved role of autophagy in lamellar body maturation.

Inhibition of class I PI3K enhances chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is the most selective form of lysosomal proteolysis, where individual peptides, recognized by a consensus motif, are translocated directly across the lysosomal membrane. CMA regulates the abundance of many disease-related proteins, with causative roles in neoplasia, neurodegeneration, hepatosteatosis, and other pathologies relevant to human health and aging. At the lysosomal membrane, CMA is inhibited by Akt-dependent phosphorylation of the CMA regulator GFAP. The INS-PI3K-PDPK1 pathway regulates Akt, but its role in CMA is unclear. Here, we report that inhibition of class I PI3K or PDPK1 activates CMA. In contrast, selective inhibition of class III PI3Ks does not activate CMA. Isolated liver lysosomes from mice treated with either of two orally bioavailable class I PI3K inhibitors, pictilisib or buparlisib, display elevated CMA activity, and decreased phosphorylation of lysosomal GFAP, with no change in macroautophagy. The findings of this study represent an important first step in repurposing class I PI3K inhibitors to modulate CMA in vivo.

Mesenchymal stem cell-derived exosome: a promising alternative in the therapy of Alzheimer's disease

Alzheimer's disease (AD) has been a devastating public health with the development of global aging. Approaches for reducing the current AD epidemic are becoming a primary focus of human healthcare due to the lack of achieved lasting and complete remission strategies to treat AD with the characteristics of heterogeneity and complexity. Exosomes, which is the new emerging approach to intercellular communication, provide novel perspective on identified therapeutic strategies of AD. Mesenchymal stem cell-derived exosomes (MSC-exos) are emerging to be an appealing therapeutic tool for AD, with the donor-derived properties and the characteristics of minimal immunogenicity, effortless storage, nature delivery vehicles, and low risks of tumor formation based on the previous researches. In this review, we elaborate the mechanism of MSC-exos in the treatment of AD and discuss limitations in the clinical application.

Beclin 1 Promotes Endosome Recruitment of Hepatocyte Growth Factor Tyrosine Kinase Substrate to Suppress Tumor Proliferation

Beclin 1 has nonautophagic functions that include its ability to regulate endocytic receptor trafficking. However, the contribution of this function to tumor suppression is poorly understood. Here, we provide in vivo evidence that Beclin 1 suppresses tumor proliferation by regulating the endocytic trafficking and degradation of the EGFR and transferrin (TFR1) receptors. Beclin 1 promoted endosomal recruitment of hepatocyte growth factor tyrosine kinase substrate (HRS), which was necessary for sorting surface receptors to intraluminal vesicles for signal silencing and lysosomal degradation. In tumors with low Beclin 1 expression, endosomal HRS recruitment was diminished and receptor function was sustained. Collectively, our results demonstrate a novel role for Beclin 1 in impeding tumor growth by coordinating the regulation of key growth factor and nutrient receptors. These data provide an explanation for how low levels of Beclin 1 facilitate tumor proliferation and contribute to poor cancer outcomes. SIGNIFICANCE: Beclin 1 controls the trafficking fate of growth regulatory receptors to suppress tumor proliferation.

Aurone derivatives as Vps34 inhibitors that modulate autophagy

We report in this study the identification of a natural product-like antagonist (1a) of Vps34 as a potent autophagy modulator via structure-based virtual screening. Aurone derivative 1a strongly inhibited Vps34 activity in cell-free and cell-based assays. Significantly, 1a prevents autophagy in human cells induced either by starvation or by an mTOR inhibitor. In silico modeling and kinetic data revealed that 1a could function as an ATP-competitive inhibitor of Vps34. Moreover, it suppressed autophagy in vivo and without inducing heart or liver damage in mice. 1a could be utilized as a new motif for more selective and efficacious antagonists of Vps34 for the potential treatment of autophagy-related human diseases.

Co-expression of HIF-1α, MDR1 and LAPTM4B in peripheral blood of solid tumors

The hypoxic tumor microenvironment is the major contributor of chemotherapy resistance in solid tumors. One of the key regulators of hypoxic responses within the cell is the hypoxia inducible factor-1α (HIF-1α) that is involved in transcription of genes promoting cell survival and chemotherapy resistance. Multidrug resistance gene-1 (MDR1) and Lysosome-associated protein transmembrane 4B-35 (LAPTM4B-35) are among those notable players which augment their responses to cellular hypoxia. MDR1 is the hypoxia responsive gene involved in multidrug resistance phenotype while LAPTM4B-35 is involved in chemotherapy resistance by stabilizing HIF-1α and overexpressing MDR1. Overexpression of HIF-1α, MDR1 and LAPTM4B has been associated with poor disease outcome in many cancers when studied individually at tissue level. However, accessibility of the tissues following the course of chemotherapy for ascertaining chemotherapy resistance is difficult and sometimes not clinically feasible. Therefore, indication of hypoxic biomarkers in patient’s blood can significantly alter the clinical outcome. Hence there is a need to identify a blood based marker to understand the disease progression. In the current study the expression of hypoxia associated chemotherapy resistance genes were studied in the peripheral blood lymphocytes of solid tumor patients and any potential correlation with disease progression were explored. The expression of HIF-1α, MDR1 and LAPTM4B was studied in blood of 72 breast, 42 ovarian, 32 colon and 21 prostate cancer patients through real time PCR analysis using delta cycle threshold method. The statistical scrutiny was executed through Fisher’s Exact test and the Spearman correlation method. There was 12–13 fold increased in expression of HIF-1α, two fold increased in MDR1 and 13–14 fold increased in LAPTM4B mRNA level in peripheral blood of breast, ovarian, prostate and colon cancer patients. In the current study there was an association of HIF-1α, MDR1 and LAPTM4B expression with advanced tumor stage, metastasis and chemotherapy treated group in breast, ovarian, prostate and colon cancer patients. The Spearman analysis also revealed a positive linear association among HIF-1α, MDR1 and LAPTM4B in all the studied cancer patients. The elevated expression of HIF-1α, MDR1 and LAPTM4B in peripheral blood of solid tumor patients can be a predictor of metastasis, disease progression and treatment response in these cancers. However, larger studies are needed to further strengthen their role as a potential biomarker for cancer prognosis.

Concerted ESCRT and clathrin recruitment waves define the timing and morphology of intraluminal vesicle formation

The endosomal sorting complex required for transport (ESCRT) machinery mediates cargo sorting, membrane deformation and membrane scission on the surface of endosomes, generating intraluminal vesicles (ILVs) to degrade signaling receptors. By live-cell imaging of individual endosomes in human cells, we find that ESCRT proteins are recruited in a repetitive pattern: ESCRT-0 and -I show a gradual and linear recruitment and dissociation, whereas ESCRT-III and its regulatory ATPase VPS4 display fast and transient dynamics. Electron microscopy shows that ILVs are formed consecutively, starting immediately after endocytic uptake of cargo proteins and correlating with the repeated ESCRT recruitment waves, unraveling the timing of ILV formation. Clathrin, recruited by ESCRT-0, is required for timely ESCRT-0 dissociation, efficient ILV formation, correct ILV size and cargo degradation. Thus, cargo sorting and ILV formation occur by concerted, coordinated and repetitive recruitment waves of individual ESCRT subcomplexes and are controlled by clathrin.

Intraluminal vesicles are formed by the endosomal sorting complex required for transport (ESCRT) machinery. Here, the authors unravel the timing of vesicle budding, and that endosomal clathrin regulates concerted recruitment of ESCRT subcomplexes, required for efficient membrane remodeling.

Abrus agglutinin stimulates BMP-2-dependent differentiation through autophagic degradation of β-catenin in colon cancer stem cells

Eradicating cancer stem cells (CSCs) in colorectal cancer (CRC) through differentiation therapy is a promising approach for cancer treatment. Our retrospective tumor-specimen analysis elucidated alteration in the expression of bone morphogenetic protein 2 (BMP-2) and β-catenin during the colon cancer progression, indicating that their possible intervention through "forced differentiation" in colon cancer remission. We reveal that Abrus agglutinin (AGG) induces the colon CSCs differentiation, and enhances sensitivity to the anticancer therapeutics. The low dose AGG (max. dose = 100 ng/mL) decreased the expression of stemness-associated molecules such as CD44 and β-catenin in the HT-29 cell derived colonospheres. Further, AGG augmented colonosphere differentiation, as demonstrated by the enhanced CK20/CK7 expression ratio and induced alkaline phosphatase activity. Interestingly, the AGG-induced expression of BMP-2 and the AGG-induced differentiation were demonstrated to be critically dependent on BMP-2 in the colonospheres. Similarly, autophagy-induction by AGG was associated with colonosphere differentiation and the gene silencing of BMP-2 led to the reduced accumulation of LC3-II, suggesting that AGG-induced autophagy is dependent on BMP-2. Furthermore, hVps34 binds strongly to BMP-2, indicating a possible association of BMP-2 with the process of autophagy. Moreover, the reduction in the self-renewal capacity of the colonospheres was associated with AGG-augmented autophagic degradation of β-catenin through an interaction with the autophagy adaptor protein p62. In the subcutaneous HT-29 xenograft model, AGG profoundly inhibited the growth of tumors through an increase in BMP-2 expression and LC3-II puncta, and a decrease in β-catenin expression, confirming the antitumor potential of AGG through induction of differentiation in colorectal cancer.

Unconventional Trafficking of Mammalian Phospholipase D3 to Lysosomes

Variants in the phospholipase D3 (PLD3) gene have genetically been linked to late-onset Alzheimer's disease. We present a detailed biochemical analysis of PLD3 and reveal its endogenous localization in endosomes and lysosomes. PLD3 reaches lysosomes as a type II transmembrane protein via a (for mammalian cells) uncommon intracellular biosynthetic route that depends on the ESCRT (endosomal sorting complex required for transport) machinery. PLD3 is sorted into intraluminal vesicles of multivesicular endosomes, and ESCRT-dependent sorting correlates with ubiquitination. In multivesicular endosomes, PLD3 is subjected to proteolytic cleavage, yielding a stable glycosylated luminal polypeptide and a rapidly degraded N-terminal membrane-bound fragment. This pathway closely resembles the delivery route of carboxypeptidase S to the yeast vacuole. Our experiments reveal a biosynthetic route of PLD3 involving proteolytic processing and ESCRT-dependent sorting for its delivery to lysosomes in mammalian cells.

Atg5 Disassociates the V1V0-ATPase to Promote Exosome Production and Tumor Metastasis Independent of Canonical Macroautophagy

Autophagy and autophagy-related genes (Atg) have been attributed prominent roles in tumorigenesis, tumor growth, and metastasis. Extracellular vesicles called exosomes are also implicated in cancer metastasis. Here, we demonstrate that exosome production is strongly reduced in cells lacking Atg5 and Atg16L1, but this is independent of Atg7 and canonical autophagy. Atg5 specifically decreases acidification of late endosomes where exosomes are produced, disrupting the acidifying V₁V₀-ATPase by removing a regulatory component, ATP6V1E1, into exosomes. The effect of Atg5 on exosome production promotes the migration and in vivo metastasis of orthotopic breast cancer cells. These findings uncover mechanisms controlling exosome release and identify means by which autophagy-related genes can contribute to metastasis in autophagy-independent pathways.

Dual Inhibition of PIK3C3 and FGFR as a New Therapeutic Approach to Treat Bladder Cancer

Purpose: MPT0L145 has been developed as a FGFR inhibitor exhibiting significant anti-bladder cancer activity in vitro and in vivo via promoting autophagy-dependent cell death. Here, we aim to elucidate the underlying mechanisms.Experimental Design: Autophagy flux, morphology, and intracellular organelles were evaluated by Western blotting, transmission electron microscope, and fluorescence microscope. Molecular docking and surface plasmon resonance assay were performed to identify drug-protein interaction. Lentiviral delivery of cDNA or shRNA and CRISPR/Cas9-mediated genome editing was used to modulate gene expression. Mitochondrial oxygen consumption rate was measured by a Seahorse XFe24 extracellular flux analyzer, and ROS level was measured by flow cytometry.Results: MPT0L145 persistently increased incomplete autophagy and phase-lucent vacuoles at the perinuclear region, which were identified as enlarged and alkalinized late-endosomes. Screening of a panel of lipid kinases revealed that MPT0L145 strongly inhibits PIK3C3 with a K_d value of 0.53 nmol/L. Ectopic expression of PIK3C3 reversed MPT0L145-increased cell death and incomplete autophagy. Four residues (Y670, F684, I760, D761) at the ATP-binding site of PIK3C3 are important for the binding of MPT0L145. In addition, MPT0L145 promotes mitochondrial dysfunction, ROS production, and DNA damage, which may in part, contribute to cell death. ATG5-knockout rescued MPT0L145-induced cell death, suggesting simultaneous induction of autophagy is crucial to its anticancer activity. Finally, our data demonstrated that MPT0L145 is able to overcome cisplatin resistance in bladder cancer cells.Conclusions: MPT0L145 is a first-in-class PIK3C3/FGFR inhibitor, providing an innovative strategy to design new compounds that increase autophagy, but simultaneously perturb its process to promote bladder cancer cell death. Clin Cancer Res; 24(5); 1176-89. ©2017 AACR.

Local detection of PtdIns3P at autophagosome biogenesis membrane platforms

Phosphatidylinositol 3-phosphate (PtdIns3P) is a key player of membrane trafficking regulation, mostly synthesized by the PIK3C3 lipid kinase. The presence of PtdIns3P on endosomes has been demonstrated; however, the role and dynamics of the pool of PtdIns3P dedicated to macroautophagy/autophagy remains elusive. Here we addressed this question by studying the mobilization of PtdIns3P in time and space during autophagosome biogenesis. We compared different dyes known to specifically detect PtdIns3P by fluorescence microscopy analysis, based on PtdIns3P-binding FYVE and PX domains, and show that these transfected dyes induce defects in endosomal dynamics as well as artificial and sustained autophagosome formation. In contrast, indirect use of recombinant FYVE enabled us to track and discriminate endosomal and autophagosomal pools of PtdIns3P. We used this method to analyze localization and dynamics of PtdIns3P subdomains on the endoplasmic reticulum, at sites of pre-autophagosome associated protein recruitment such as the PtdIns3P-binding ZFYVE1/DFCP1 and WIPI2 autophagy regulators. This approach thus revealed the presence of a specific pool of PtdIns3P at the site where autophagosome assembly is initiated.

EGF receptor lysosomal degradation is delayed in the cells stimulated with EGF-Quantum dot bioconjugate but earlier key events of endocytic degradative pathway are similar to that of native EGF

Quantum dots (QDs) complexed to ligands recognizing surface receptors undergoing internalization are an attractive tool for live cell imaging of ligand-receptor complexes behavior and for specific tracking of the cells of interest. However, conjugation of quasi-multivalent large QD-particle to monovalent small growth factors like EGF that bound their tyrosine-kinase receptors may affect key endocytic events tightly bound to signaling. Here, by means of confocal microscopy we have addressed the key endocytic events of lysosomal degradative pathway stimulated by native EGF or EGF-QD bioconjugate. We have demonstrated that the decrease in endosome number, increase in mean endosome integrated density and the pattern of EEA1 co-localization with EGF-EGFR complexes at early stages of endocytosis were similar for the both native and QD-conjugated ligands. In both cases enlarged hollow endosomes appeared after wortmannin treatment. This indicates that early endosomal fusions and their maturation proceed similar for both ligands. EGF-QD and native EGF similarly accumulated in juxtanuclear region, and live cell imaging of endosome motion revealed the behavior described elsewhere for microtubule-facilitated motility. Finally, EGF-QD and the receptor were found in lysosomes. However, degradation of receptor part of QD-EGF-EGFR-complex was delayed compared to native EGF, but not inhibited, while QDs fluorescence was detected in lysosomes even after 24 hours. Importantly, in HeLa and A549 cells the both ligands behaved similarly. We conclude that during endocytosis EGF-QD behaves as a neutral marker for degradative pathway up to lysosomal stage and can also be used as a long-term cell marker.

Schwann cell-specific deletion of the endosomal PI 3-kinase Vps34 leads to delayed radial sorting of axons, arrested myelination, and abnormal ErbB2-ErbB3 tyrosine kinase signaling

The PI 3-kinase Vps34 (Pik3c3) synthesizes phosphatidylinositol 3-phosphate (PI3P), a lipid critical for both endosomal membrane traffic and macroautophagy. Human genetics have implicated PI3P dysregulation, and endosomal trafficking in general, as a recurring cause of demyelinating Charcot-Marie-Tooth (CMT) peripheral neuropathy. Here, we investigated the role of Vps34, and PI3P, in mouse Schwann cells by selectively deleting Vps34 in this cell type. Vps34-Schwann cell knockout (Vps34^SCKO ) mice show severe hypomyelination in peripheral nerves. Vps34^-/- Schwann cells interact abnormally with axons, and there is a delay in radial sorting, a process by which large axons are selected for myelination. Upon reaching the promyelinating stage, Vps34^-/- Schwann cells are significantly impaired in the elaboration of myelin. Nerves from Vps34^SCKO mice contain elevated levels of the LC3 and p62 proteins, indicating impaired autophagy. However, in the light of recent demonstrations that autophagy is dispensable for myelination, it is unlikely that hypomyelination in Vps34^SCKO mice is caused by impaired autophagy. Endosomal trafficking is also disturbed in Vps34^-/- Schwann cells. We investigated the activation of the ErbB2/3 receptor tyrosine kinases in Vps34^SCKO nerves, as these proteins, which play essential roles in Schwann cell myelination, are known to traffic through endosomes. In Vps34^SCKO nerves, ErbB3 was hyperphosphorylated on a tyrosine known to be phosphorylated in response to neuregulin 1 exposure. ErbB2 protein levels were also decreased during myelination. Our findings suggest that the loss of Vps34 alters the trafficking of ErbB2/3 through endosomes. Abnormal ErbB2/3 signaling to downstream targets may contribute to the hypomyelination observed in Vps34^SCKO mice.

The VPS34 PI3K negatively regulates RAB-5 during endosome maturation

The GTPase Rab5 and phosphatidylinositol-3 phosphate [PI(3)P] coordinately regulate endosome trafficking. Rab5 recruits Vps34, the class III phosphoinositide 3-kinase (PI3K), to generate PI(3)P and recruit PI(3)P-binding proteins. Loss of Rab5 and loss of Vps34 have opposite effects on endosome size, suggesting that our understanding of how Rab5 and PI(3)P cooperate is incomplete. Here, we report a novel regulatory loop whereby Caenorhabditis elegans VPS-34 inactivates RAB-5 via recruitment of the TBC-2 Rab GTPase-activating protein. We found that loss of VPS-34 caused a phenotype with large late endosomes, as with loss of TBC-2, and that Rab5 activity (mice have two Rab5 isoforms, Rab5a and Rab5b) is increased in Vps34-knockout mouse embryonic fibroblasts (Vps34 is also known as PIK3C3 in mammals). We found that VPS-34 is required for TBC-2 endosome localization and that the pleckstrin homology (PH) domain of TBC-2 bound PI(3)P. Deletion of the PH domain enhanced TBC-2 localization to endosomes in a VPS-34-dependent manner. Thus, PI(3)P binding of the PH domain might be permissive for another PI(3)P-regulated interaction that recruits TBC-2 to endosomes. Therefore, VPS-34 recruits TBC-2 to endosomes to inactivate RAB-5 to ensure the directionality of endosome maturation.

Vps34 regulates Rab7 and late endocytic trafficking through recruitment of the GTPase-activating protein Armus

The class III phosphoinositide 3-kinase (PI3K) Vps34 (also known as PIK3C3 in mammals) produces phosphatidylinositol 3-phosphate [PI(3)P] on both early and late endosome membranes to control membrane dynamics. We used Vps34-deficient cells to delineate whether Vps34 has additional roles in endocytic trafficking. In Vps34^-/- mouse embryonic fibroblasts (MEFs), transferrin recycling and EEA1 membrane localization were unaffected despite elevated Rab5-GTP levels. Strikingly, a large increase in Rab7-GTP levels, an accumulation of enlarged late endosomes, and decreased EGFR degradation were observed in Vps34-deficient cells. The hyperactivation of Rab7 in Vps34-deficient cells stemmed from the failure to recruit the Rab7 GTPase-activating protein (GAP) Armus (also known as TBC1D2), which binds to PI(3)P, to late endosomes. Protein-lipid overlay and liposome-binding assays reveal that the putative pleckstrin homology (PH) domain in Armus can directly bind to PI(3)P. Elevated Rab7-GTP led to the failure of intraluminal vesicle (ILV) formation and lysosomal maturation. Rab7 silencing and Armus overexpression alleviated the vacuolization seen in Vps34-deficient cells. Taken together, these results demonstrate that Vps34 has a previously unknown role in regulating Rab7 activity and late endosomal trafficking.

Role of Class III phosphoinositide 3-kinase in the brain development: possible involvement in specific learning disorders

Class III phosphoinositide 3-kinase (PIK3C3 or mammalian vacuolar protein sorting 34 homolog, Vps34) regulates vesicular trafficking, autophagy, and nutrient sensing. Recently, we reported that PIK3C3 is expressed in mouse cerebral cortex throughout the developmental process, especially at early embryonic stage. We thus examined the role of PIK3C3 in the development of the mouse cerebral cortex. Acute silencing of PIK3C3 with in utero electroporation method caused positional defects of excitatory neurons during corticogenesis. Time-lapse imaging revealed that the abnormal positioning was at least partially because of the reduced migration velocity. When PIK3C3 was silenced in cortical neurons in one hemisphere, axon extension to the contralateral hemisphere was also delayed. These aberrant phenotypes were rescued by RNAi-resistant PIK3C3. Notably, knockdown of PIK3C3 did not affect the cell cycle of neuronal progenitors and stem cells at the ventricular zone. Taken together, PIK3C3 was thought to play a crucial role in corticogenesis through the regulation of excitatory neuron migration and axon extension. Meanwhile, when we performed comparative genomic hybridization on a patient with specific learning disorders, a 107 Kb-deletion was identified on 18q12.3 (nt. 39554147-39661206) that encompasses exons 5-23 of PIK3C3. Notably, the above aberrant migration and axon growth phenotypes were not rescued by the disease-related truncation mutant (172 amino acids) lacking the C-terminal kinase domain. Thus, functional defects of PIK3C3 might impair corticogenesis and relate to the pathophysiology of specific learning disorders and other neurodevelopmental disorders. Acute knockdown of Class III phosphoinositide 3-kinase (PIK3C3) evokes migration defects of excitatory neurons during corticogenesis. PIK3C3-knockdown also disrupts axon outgrowth, but not progenitor proliferation in vivo. Involvement of PIK3C3 in neurodevelopmental disorders might be an interesting future subject since a deletion mutation in PIK3C3 was detected in a patient with specific learning disorders (SLD).

Negative regulation of phosphatidylinositol 3-phosphate levels in early-to-late endosome conversion

C. elegans SORF-1 and SORF-2 and their mammalian homologs WDR91 and WDR81 maintain appropriate PtdIns3P levels in early-to-late endosome conversion by forming a complex with the Beclin1 subunit of the PI3K complex.

Phosphatidylinositol 3-phosphate (PtdIns3P) plays a central role in endosome fusion, recycling, sorting, and early-to-late endosome conversion, but the mechanisms that determine how the correct endosomal PtdIns3P level is achieved remain largely elusive. Here we identify two new factors, SORF-1 and SORF-2, as essential PtdIns3P regulators in Caenorhabditis elegans. Loss of sorf-1 or sorf-2 leads to greatly elevated endosomal PtdIns3P, which drives excessive fusion of early endosomes. sorf-1 and sorf-2 function coordinately with Rab switching genes to inhibit synthesis of PtdIns3P, allowing its turnover for endosome conversion. SORF-1 and SORF-2 act in a complex with BEC-1/Beclin1, and their loss causes elevated activity of the phosphatidylinositol 3-kinase (PI3K) complex. In mammalian cells, inactivation of WDR91 and WDR81, the homologs of SORF-1 and SORF-2, induces Beclin1-dependent enlargement of PtdIns3P-enriched endosomes and defective degradation of epidermal growth factor receptor. WDR91 and WDR81 interact with Beclin1 and inhibit PI3K complex activity. These findings reveal a conserved mechanism that controls appropriate PtdIns3P levels in early-to-late endosome conversion.

AP-1/σ1A and AP-1/σ1B adaptor-proteins differentially regulate neuronal early endosome maturation via the Rab5/Vps34-pathway

The σ1 subunit of the AP-1 clathrin-coated-vesicle adaptor-protein complex is expressed as three isoforms. Tissues express σ1A and one of the σ1B and σ1C isoforms. Brain is the tissue with the highest σ1A and σ1B expression. σ1B-deficiency leads to severe mental retardation, accumulation of early endosomes in synapses and fewer synaptic vesicles, whose recycling is slowed down. AP-1/σ1A and AP-1/σ1B regulate maturation of these early endosomes into multivesicular body late endosomes, thereby controlling synaptic vesicle protein transport into a degradative pathway. σ1A binds ArfGAP1, and with higher affinity brain-specific ArfGAP1, which bind Rabex-5. AP-1/σ1A-ArfGAP1-Rabex-5 complex formation leads to more endosomal Rabex-5 and enhanced, Rab5^GTP-stimulated Vps34 PI3-kinase activity, which is essential for multivesicular body endosome formation. Formation of AP-1/σ1A-ArfGAP1-Rabex-5 complexes is prevented by σ1B binding of Rabex-5 and the amount of endosomal Rabex-5 is reduced. AP-1 complexes differentially regulate endosome maturation and coordinate protein recycling and degradation, revealing a novel molecular mechanism by which they regulate protein transport besides their established function in clathrin-coated-vesicle formation.

Active vacuolar H+ ATPase and functional cycle of Rab5 are required for the vacuolation defect triggered by PtdIns(3,5)P2 loss under PIKfyve or Vps34 deficiency

The two evolutionarily conserved mammalian lipid kinases Vps34 and PIKfyve are involved in an important physiological relationship, whereby the former produces phosphatidylinositol (PtdIns) 3P that is used as a substrate for PtdIns(3,5)P2 synthesis by the latter. Reduced production of PtdIns(3,5)P2 in proliferating mammalian cells is phenotypically manifested by the formation of multiple translucent cytoplasmic vacuoles, readily rescued upon exogenous delivery of PtdIns(3,5)P2 or overproduction of PIKfyve. Although the aberrant vacuolation phenomenon has been frequently used as a sensitive functional measure of localized PtdIns(3,5)P2 reduction, cellular factors governing the appearance of cytoplasmic vacuoles under PtdIns3P-PtdIns(3,5)P2 loss remain elusive. To gain further mechanistic insight about the vacuolation process following PtdIns(3,5)P2 reduction, in this study we sought for cellular mechanisms required for manifestation of the aberrant endomembrane vacuoles triggered by PIKfyve or Vps34 dysfunction. The latter was achieved by various means such as pharmacological inhibition, gene disruption, or dominant-interference in several proliferating mammalian cell types. We report here that inhibition of V-ATPase with bafilomycin A1 as well as inactivation of the GTP-GDP cycle of Rab5a GTPase phenotypically rescued or completely precluded the cytoplasmic vacuolization despite the continued presence of inactivated PIKfyve or Vps34. Bafilomycin A1 also restored the aberrant EEA1-positive endosomes, enlarged upon short PIKfyve inhibition with YM201636. Together, our work identifies for the first time that factors such as active V-ATPase or functional Rab5a cycle are acting coincidentally with the PtdIns(3,5)P2 reduction in triggering formation of aberrant cytoplasmic vacuoles under PIKfyve or Vps34 dysfunction.

Lysosomal signaling in control of degradation pathways

Autophagy mediates the (non-)selective bulk degradation of cytoplasm, protein aggregates, damaged organelles and certain pathogens. The endosomal membrane system uses multivesicular bodies (MVBs) to selectively deliver ubiquitinated membrane proteins together with extracellular components into lysosomes. Microautophagy (MA) and chaperone-mediated autophagy (CMA) additionally contribute to the selective delivery of cargo into lysosomes. The coordinated function of these lysosomal degradation pathways is essential to maintain cellular homeostasis. Their activity is controlled by mTOR (mammalian target of rapamycin) signaling and thus coupled to metabolic processes during cell growth. Here, we will discuss how TORC1 on lysosomes and TORC2 at the plasma membrane coordinate the different membrane biogenesis pathways with cargo selection, vesicle transport and fusion with lysosomes in response to intracellular and extracellular cues.

Exosome Biogenesis, Regulation, and Function in Viral Infection

Exosomes are extracellular vesicles released upon fusion of multivesicular bodies (MVBs) with the cellular plasma membrane. They originate as intraluminal vesicles (ILVs) during the process of MVB formation. Exosomes were shown to contain selectively sorted functional proteins, lipids, and RNAs, mediating cell-to-cell communications and hence playing a role in the physiology of the healthy and diseased organism. Challenges in the field include the identification of mechanisms sustaining packaging of membrane-bound and soluble material to these vesicles and the understanding of the underlying processes directing MVBs for degradation or fusion with the plasma membrane. The investigation into the formation and roles of exosomes in viral infection is in its early years. Although still controversial, exosomes can, in principle, incorporate any functional factor, provided they have an appropriate sorting signal, and thus are prone to viral exploitation. This review initially focuses on the composition and biogenesis of exosomes. It then explores the regulatory mechanisms underlying their biogenesis. Exosomes are part of the endocytic system, which is tightly regulated and able to respond to several stimuli that lead to alterations in the composition of its sub-compartments. We discuss the current knowledge of how these changes affect exosomal release. We then summarize how different viruses exploit specific proteins of endocytic sub-compartments and speculate that it could interfere with exosome function, although no direct link between viral usage of the endocytic system and exosome release has yet been reported. Many recent reports have ascribed functions to exosomes released from cells infected with a variety of animal viruses, including viral spread, host immunity, and manipulation of the microenvironment, which are discussed. Given the ever-growing roles and importance of exosomes in viral infections, understanding what regulates their composition and levels, and defining their functions will ultimately provide additional insights into the virulence and persistence of infections.

Autophagy in the lens

The lens of the eye is a transparent tissue composed of lens fiber cells that differentiate from lens epithelial cells and degrade all cytoplasmic organelles during terminal differentiation. Autophagy is a major intracellular degradation system in which cytoplasmic proteins and organelles are degraded in the lysosome. Although autophagy is constitutively activated in the lens and has been proposed to be involved in lens organelle degradation, its precise role is not well understood. Recent genetic studies in mice have demonstrated that autophagy is critically important for intracellular quality control in the lens but can be dispensable for lens organelle degradation. Here, we review recent findings on the roles of autophagy and lysosomes in organelle degradation and intracellular quality control in the lens, and discuss their possible involvement in the development of human cataract.

Protein kinase D1/2 is involved in the maturation of multivesicular bodies and secretion of exosomes in T and B lymphocytes

Multivesicular bodies (MVBs) are endocytic compartments that enclose intraluminal vesicles (ILVs) formed by inward budding from the limiting membrane of endosomes. In T lymphocytes, these ILV contain Fas ligand (FasL) and are secreted as 'lethal exosomes' following activation-induced fusion of the MVB with the plasma membrane. Diacylglycerol (DAG) and diacylglycerol kinase α (DGKα) regulate MVB maturation and polarized traffic, as well as subsequent secretion of pro-apoptotic exosomes, but the molecular basis underlying these phenomena remains unclear. Here we identify protein kinase D (PKD) family members as DAG effectors involved in MVB genesis and secretion. We show that the inducible secretion of exosomes is enhanced when a constitutively active PKD1 mutant is expressed in T lymphocytes, whereas exosome secretion is impaired in PKD2-deficient mouse T lymphoblasts and in PKD1/3-null B cells. Analysis of PKD2-deficient T lymphoblasts showed the presence of large, immature MVB-like vesicles and demonstrated defects in cytotoxic activity and in activation-induced cell death. Using pharmacological and genetic tools, we show that DGKα regulates PKD1/2 subcellular localization and activation. Our studies demonstrate that PKD1/2 is a key regulator of MVB maturation and exosome secretion, and constitutes a mediator of the DGKα effect on MVB secretory traffic.

Influence of cellular trafficking pathway on bluetongue virus infection in ovine cells

Bluetongue virus (BTV), a non-enveloped arbovirus, causes hemorrhagic disease in ruminants. However, the influence of natural host cell proteins on BTV replication process is not defined. In addition to cell lysis, BTV also exits non-ovine cultured cells by non-lytic pathways mediated by nonstructural protein NS3 that interacts with virus capsid and cellular proteins belonging to calpactin and ESCRT family. The PPXY late domain motif known to recruit NEDD4 family of HECT ubiquitin E3 ligases is also highly conserved in NS3. In this study using a mixture of molecular, biochemical and microscopic techniques we have analyzed the importance of ovine cellular proteins and vesicles in BTV infection. Electron microscopic analysis of BTV infected ovine cells demonstrated close association of mature particles with intracellular vesicles. Inhibition of Multi Vesicular Body (MVB) resident lipid phosphatidylinositol-3-phosphate resulted in decreased total virus titre suggesting that the vesicles might be MVBs. Proteasome mediated inhibition of ubiquitin or modification of virus lacking the PPXY in NS3 reduced virus growth. Thus, our study demonstrated that cellular components comprising of MVB and exocytic pathways proteins are involved in BTV replication in ovine cells.

Unravelling the pivotal role of Alix in MVB sorting and silencing of the activated EGFR

Endosomal sorting complex required for transport (ESCRT)-III-mediated membrane invagination and scission are a critical step in multivesicular body (MVB) sorting of ubiquitinated membrane receptors, and generally thought to be required for degradation of these receptors in lysosomes. The adaptor protein Alix is critically involved in multiple ESCRT-III-mediated, membrane-remodelling processes in mammalian cells. However, Alix knockdown does not inhibit degradation of the activated epidermal growth factor receptor (EGFR) in mammalian cell lines, leading to a widely held notion that Alix is not critically involved in MVB sorting of ubiquitinated membrane receptors in mammalian cells. In the present study, we demonstrate that, despite its non-essential role in degradation of the activated EGFR, Alix plays a critical role in its MVB sorting and silencing Epidermal growth factor (EGF) stimulation of mammalian cell lines induces Alix's interaction with the ubiquitinated EGFR via the Alix V domain, and increases Alix's association with membrane-bound charged multivesicular body protein 4 (CHMP4) via the Alix Bro1 domain. Under both continuous and pulse-chase EGF stimulation conditions, inhibition of Alix's interaction with membrane-bound CHMP4, inhibition of Alix dimerization through the V domain or Alix knockdown dramatically inhibits MVB sorting of the activated EGFR and promotes sustained activation of extracellular-signal regulated kinase (ERK)1/2. Under the continuous EGF stimulation conditions, these cell treatments also retard degradation of the activated EGFR. These findings indicate that Alix is critically involved in MVB sorting of ubiquitinated membrane receptors in mammalian cells.

Prion strains are differentially released through the exosomal pathway

Cell-to-cell transfer of prions is a crucial step in the spreading of prion infection through infected tissue. At the cellular level, several distinct pathways including direct cell-cell contacts and release of various types of infectious extracellular vesicles have been described that may potentially lead to infection of naïve cells. The relative contribution of these pathways and whether they may vary depending on the prion strain and/or on the infected cell type are not yet known. In this study we used a single cell type (RK13) infected with three different prion strains. We showed that in each case, most of the extracellular prions resulted from active cell secretion through the exosomal pathway. Further, quantitative analysis of secreted infectivity indicated that the proportion of prions eventually secreted was dramatically dependent on the prion strain. Our data also highlight that infectious exosomes secreted from cultured cells might represent a biologically pertinent material for spiking experiments. Also discussed is the appealing possibility that abnormal PrP from different prion strains may differentially interact with the cellular machinery to promote secretion.

Class III PI 3-kinase is the main source of PtdIns3P substrate and membrane recruitment signal for PIKfyve constitutive function in podocyte endomembrane homeostasis

The evolutionarily conserved PIKfyve, which synthesizes PtdIns5P from PtdIns, and PtdIns(3,5)P2 from PtdIns3P, requires PtdIns3P as both an enzyme substrate and a membrane recruitment signal. Whereas the PtdIns3P source is undetermined, class III PI3K (Vps34), the only evolutionarily conserved of the eight mammalian PI3Ks, is presumed as a main candidate. A hallmark of PIKfyve deficiency is formation of multiple translucent cytoplasmic vacuoles seen by light microscopy in cells cultured in complete media. Such an aberrant phenotype is often observed in cells from conditional Vps34 knockout (KO) mice. To clarify the mechanism of Vps34 KO-triggered vacuolation and the PtdIns3P source for PIKfyve functionality, here we have characterized a podocyte cell type derived from Vps34fl/fl mice, which, upon Cre-mediated gene KO, robustly formed cytoplasmic vacuoles resembling those in PikfyveKO MEFs. Vps34wt, expressed in Vps34KO podocytes restored the normal morphology, but only if the endogenous PIKfyve activity was intact. Conversely, expressed PIKfyvewt rescued completely the vacuolation only in PikfyveKO MEFs but not in Vps34KO podocytes. Analyses of phosphoinositide profiles by HPLC and localization patterns by a PtdIns3P biosensor revealed that Vps34 is the main supplier of localized PtdIns3P not only for PIKfyve activity but also for membrane recruitment. Concordantly, Vps34KO podocytes had severely reduced steady-state levels of both PtdIns(3,5)P2 and PtdIns5P, along with PtdIns3P. We further revealed a plausible physiologically-relevant Vps34-independent PtdIns3P supply for PIKfyve, operating through activated class I PI3Ks. Our data provide the first evidence that the vacuolation phenotype in Vps34KO podocytes is due to PIKfyve dysfunction and that Vps34 is a main PtdIns3P source for constitutive PIKfyve functionality.

PI(5)P regulates autophagosome biogenesis

Phosphatidylinositol 3-phosphate (PI(3)P), the product of class III PI3K VPS34, recruits specific autophagic effectors, like WIPI2, during the initial steps of autophagosome biogenesis and thereby regulates canonical autophagy. However, mammalian cells can produce autophagosomes through enigmatic noncanonical VPS34-independent pathways. Here we show that PI(5)P can regulate autophagy via PI(3)P effectors and thereby identify a mechanistic explanation for forms of noncanonical autophagy. PI(5)P synthesis by the phosphatidylinositol 5-kinase PIKfyve was required for autophagosome biogenesis, and it increased levels of PI(5)P, stimulated autophagy, and reduced the levels of autophagic substrates. Inactivation of VPS34 impaired recruitment of WIPI2 and DFCP1 to autophagic precursors, reduced ATG5-ATG12 conjugation, and compromised autophagosome formation. However, these phenotypes were rescued by PI(5)P in VPS34-inactivated cells. These findings provide a mechanistic framework for alternative VPS34-independent autophagy-initiating pathways, like glucose starvation, and unravel a cytoplasmic function for PI(5)P, which previously has been linked predominantly to nuclear roles.

•

PI(5)P positively regulates autophagy

•

PI(5)P is associated with autophagy effectors that bind PI(3)P

•

PI(5)P sustains noncanonical autophagy in PI(3)P-depleted cells

•

PI(5)P is essential for VPS34-independent, glucose-starvation-induced autophagy

Characterization of VPS34-IN1, a selective inhibitor of Vps34, reveals that the phosphatidylinositol 3-phosphate-binding SGK3 protein kinase is a downstream target of class III phosphoinositide 3-kinase

The Vps34 (vacuolar protein sorting 34) class III PI3K (phosphoinositide 3-kinase) phosphorylates PtdIns (phosphatidylinositol) at endosomal membranes to generate PtdIns(3)P that regulates membrane trafficking processes via its ability to recruit a subset of proteins possessing PtdIns(3)P-binding PX (phox homology) and FYVE domains. In the present study, we describe a highly selective and potent inhibitor of Vps34, termed VPS34-IN1, that inhibits Vps34 with 25 nM IC₅₀in vitro, but does not significantly inhibit the activity of 340 protein kinases or 25 lipid kinases tested that include all isoforms of class I as well as class II PI3Ks. Administration of VPS34-IN1 to cells induces a rapid dose-dependent dispersal of a specific PtdIns(3)P-binding probe from endosome membranes, within 1 min, without affecting the ability of class I PI3K to regulate Akt. Moreover, we explored whether SGK3 (serum- and glucocorticoid-regulated kinase-3), the only protein kinase known to interact specifically with PtdIns(3)P via its N-terminal PX domain, might be controlled by Vps34. Mutations disrupting PtdIns(3)P binding ablated SGK3 kinase activity by suppressing phosphorylation of the T-loop [PDK1 (phosphoinositide-dependent kinase 1) site] and hydrophobic motif (mammalian target of rapamycin site) residues. VPS34-IN1 induced a rapid ~50–60% loss of SGK3 phosphorylation within 1 min. VPS34-IN1 did not inhibit activity of the SGK2 isoform that does not possess a PtdIns(3)P-binding PX domain. Furthermore, class I PI3K inhibitors (GDC-0941 and BKM120) that do not inhibit Vps34 suppressed SGK3 activity by ~40%. Combining VPS34-IN1 and GDC-0941 reduced SGK3 activity ~80–90%. These data suggest SGK3 phosphorylation and hence activity is controlled by two pools of PtdIns(3)P. The first is produced through phosphorylation of PtdIns by Vps34 at the endosome. The second is due to the conversion of class I PI3K product, PtdIns(3,4,5)P₃ into PtdIns(3)P, via the sequential actions of the PtdIns 5-phosphatases [SHIP1/2 (Src homology 2-domain-containing inositol phosphatase 1/2)] and PtdIns 4-phosphatase [INPP4B (inositol polyphosphate 4-phosphatase type II)]. VPS34-IN1 will be a useful probe to delineate physiological roles of the Vps34. Monitoring SGK3 phosphorylation and activity could be employed as a biomarker of Vps34 activity, in an analogous manner by which Akt is used to probe cellular class I PI3K activity. Combining class I (GDC-0941) and class III (VPS34-IN1) PI3K inhibitors could be used as a strategy to better analyse the roles and regulation of the elusive class II PI3K.

We characterize VPS34-IN, a potent and selective inhibitor of class III Vps34 PI3K. Using VPS34-IN1, we demonstrate that PtdIns(3)P, produced by Vps34 controls phosphorylation and activity of the SGK3 protein kinase.

Crimean-Congo hemorrhagic fever virus entry into host cells occurs through the multivesicular body and requires ESCRT regulators

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne bunyavirus causing outbreaks of severe disease in humans, with a fatality rate approaching 30%. There are no widely accepted therapeutics available to prevent or treat the disease. CCHFV enters host cells through clathrin-mediated endocytosis and is subsequently transported to an acidified compartment where the fusion of virus envelope with cellular membranes takes place. To better understand the uptake pathway, we sought to identify host factors controlling CCHFV transport through the cell. We demonstrate that after passing through early endosomes in a Rab5-dependent manner, CCHFV is delivered to multivesicular bodies (MVBs). Virus particles localized to MVBs approximately 1 hour after infection and affected the distribution of the organelle within cells. Interestingly, blocking Rab7 activity had no effect on association of the virus with MVBs. Productive virus infection depended on phosphatidylinositol 3-kinase (PI3K) activity, which meditates the formation of functional MVBs. Silencing Tsg101, Vps24, Vps4B, or Alix/Aip1, components of the endosomal sorting complex required for transport (ESCRT) pathway controlling MVB biogenesis, inhibited infection of wild-type virus as well as a novel pseudotyped vesicular stomatitis virus (VSV) bearing CCHFV glycoprotein, supporting a role for the MVB pathway in CCHFV entry. We further demonstrate that blocking transport out of MVBs still allowed virus entry while preventing vesicular acidification, required for membrane fusion, trapped virions in the MVBs. These findings suggest that MVBs are necessary for infection and are the sites of virus-endosome membrane fusion.

Crimean-Congo hemorrhagic fever virus (CCHFV) is the cause of a severe, often fatal disease in humans. While it has been demonstrated that CCHFV cell entry depends on clathrin-mediated endocytosis, low pH, and early endosomes, the identity of the endosomes where virus penetrates into cell cytoplasm to initiate genome replication is unknown. Here, we showed that CCHFV was transported through early endosomes to multivesicular bodies (MVBs). We also showed that MVBs were likely the last organelle virus encountered before escaping into the cytoplasm. Our work has identified new cellular factors essential for CCHFV entry and potential novel targets for therapeutic intervention against this pathogen.

Molecular modulation of the copper and cisplatin transport function of CTR1 and its interaction with IRS-4

The copper influx transporter CTR1 is also a major influx transporter for cisplatin (cDDP) in tumor cells. It influences the cytotoxicity of cDDP both in vivo and in vitro. Whereas Cu triggers internalization of CTR1 from the plasma membrane, cDDP does not. To investigate the mechanisms of these effects, myc-tagged forms of wild type hCTR1 and variants in which Y103 was converted to alanine, C189 was converted to serine, or the K178/K179 dilysine motif was converted to alanines were re-expressed in mouse embryo cells in which both alleles of CTR1 had been knocked out and also in HEK293T cells. The Y103A mutation and to a lesser extent the C189S mutation reduced internalization of CTR1 induced by Cu while the K178A/K179A had little effect. Both Y103 and C189 were required for Cu and cDDP transport whereas the K178/K179 motif was not. While Y103 lies in an YXXM motif that, when phosphorylated, is a potential docking site for phosphatidylinositol 3-kinase and other proteins involved in endocytosis, Western blot analysis of immunoprecipitated myc-CTR1, and proteomic analysis of peptides derived from CTR1, failed to identify any basal or Cu-induced phosphorylation. However, proteomic analysis did identify an interaction of CTR1 with IRS-4 and this was confirmed by co-immunoprecipitation from HEK cells expressing either FLAG-CTR1 or myc-CTR1. The interaction was greater in the Y103A-expressing cells. We conclude that Y103 is required for the internalization of hCTR1 in response to Cu, that this occurs by a mechanism other than phosphorylation and that mutation of Y103 modulates the interaction with IRS-4.

The complex ultrastructure of the endolysosomal system

Live-cell imaging reveals the endolysosomal system as a complex and highly dynamic network of interacting compartments. Distinct types of endosomes are discerned by kinetic, molecular, and morphological criteria. Although none of these criteria, or combinations thereof, can capture the full complexity of the endolysosomal system, they are extremely useful for experimental purposes. Some membrane domain specializations and specific morphological characteristics can only be seen by ultrastructural analysis after preparation for electron microscopy (EM). Immuno-EM allows a further discrimination of seemingly identical compartments by their molecular makeup. In this review we provide an overview of the ultrastructural characteristics and membrane organization of endosomal compartments, along with their organizing machineries.

ARHI (DIRAS3) induces autophagy in ovarian cancer cells by downregulating the epidermal growth factor receptor, inhibiting PI3K and Ras/MAP signaling and activating the FOXo3a-mediated induction of Rab7

The process of autophagy has been described in detail at the molecular level in normal cells, but less is known of its regulation in cancer cells. Aplasia Ras homolog member I (ARHI; DIRAS3) is an imprinted tumor suppressor gene that is downregulated in multiple malignancies including ovarian cancer. Re-expression of ARHI slows proliferation, inhibits motility, induces autophagy and produces tumor dormancy. Our previous studies have implicated autophagy in the survival of dormant ovarian cancer cells and have shown that ARHI is required for autophagy induced by starvation or rapamycin treatment. Re-expression of ARHI in ovarian cancer cells blocks signaling through the PI3K and Ras/MAP pathways, which, in turn, downregulates mTOR and initiates autophagy. Here we show that ARHI is required for autophagy-meditated cancer cell arrest and ARHI inhibits signaling through PI3K/AKT and Ras/MAP by enhancing internalization and degradation of the epidermal growth factor receptor. ARHI-mediated downregulation of PI3K/AKT and Ras/ERK signaling also decreases phosphorylation of FOXo3a, which sequesters this transcription factor in the nucleus. Nuclear retention of FOXo3a induces ATG4 and MAP-LC3-I, required for maturation of autophagosomes, and also increases the expression of Rab7, required for fusion of autophagosomes with lysosomes. Following the knockdown of FOXo3a or Rab7, autophagolysosome formation was observed but was markedly inhibited, resulting in numerous enlarged autophagosomes. ARHI expression correlates with LC3 expression and FOXo3a nuclear localization in surgical specimens of ovarian cancer. Thus, ARHI contributes to the induction of autophagy through multiple mechanisms in ovarian cancer cells.

Monitoring phosphatidylinositol 3-phosphate in multivesicular endosome biogenesis

The endocytic pathway comprises a variety of intracellular compartments that regulate sorting of internalized plasma membrane constituents as well as extracellular material. A major sorting station on this route is the early endosome, where internalized receptors destined for degradation are trafficked from the limiting membrane into the interior of the endosome by formation of intraluminal vesicles (ILVs). This invagination and budding process leads to the biogenesis of multivesicular endosomes (MVEs). The formation of ILVs depends on the sequential action of protein complexes that are partly recruited in a phosphatidylinositol 3-phosphate (PtdIns3P)-dependent manner. The underlying mechanisms of the biogenesis of MVEs are still not completely understood and it is therefore of great interest to study the sorting of PtdIns3P in this process. We are describing several methods to track these sorting events by both light and electron microscopy and combination of both methods.