Rab11 activity and PtdIns(3)P turnover removes recycling cargo from endosomes

A competition network connects Rab5 and Rab11 GTPases at the surface of endocytic structures

Specificity in membrane trafficking relies on the interaction between Rab small GTPase proteins and their molecular effectors. However, the evidence that different Rab proteins can bind to common effectors challenges this view. Here, we show that molecular competition between distinct Rab GTPases for a shared protein can link diverse membrane trafficking pathways. Theoretical analysis and experimental data support a role for Zfyve26 as a part of a competitive network that modulates changes in Rab5-Rab11 abundance, activation status, and correlation at the surface of single endocytic structures. By leveraging on the Loop index, a novel metric that couples the GTP-bound fraction and the total amount of Rab GTPase, we infer the saturation of Zfyve26 molecules at the endocytic surface from time-lapse imaging data. Our findings establish that transduction in the endocytic system is governed by stoichiometric constraints determining the trade-off between different trafficking pathways at the surface of a membrane-bound organelle.

Discovery of a Small Molecule with an Inhibitory Role for RAB11

RAB11, a pivotal RabGTPase, regulates essential cellular processes such as endocytic recycling, exocytosis, and autophagy. The protein was implicated in various human diseases, including cancer, neurodegenerative disorders, viral infections, and podocytopathies. However, a small-molecular inhibitor is lacking. The complexity and workload associated with potential assays make conducting large-scale screening for RAB11 challenging. We employed a tiered approach for drug discovery, utilizing deep learning-based computational screening to preselect compounds targeting a specific pocket of RAB11 protein with experimental validation by an in vitro platform reflecting RAB11 activity through the exocytosis of GFP. Further validation included the exposure of Drosophila by drug feeding. In silico pre-screening identified 94 candidates, of which 9 were confirmed using our in vitro platform for Rab11 activity. Focusing on compounds with high potency, we assessed autophagy, which independently requires RAB11, and validated three of these compounds. We further analyzed the dose-response relationship, observing a biphasic, potentially hormetic effect. Two candidate compounds specifically caused a shift in Rab11 vesicles to the cell periphery, without significant impact on Rab5 or Rab7. Drosophila larvae exposed to another candidate compound with predicted oral bioavailability exhibited minimal toxicity, subcellular dispersal of endogenous Rab11, and a decrease in RAB11-dependent nephrocyte function, further supporting an inhibitory role. Taken together, the combination of computational screening and experimental validation allowed the identification of small molecules that modify the function of Rab11. This discovery may further open avenues for treating RAB11-associated disorders.

Formation of a giant unilocular vacuole via macropinocytosis-like process confers anoikis resistance

Cell survival in metazoans depends on cell attachment to the extracellular matrix (ECM) or to neighboring cells. Loss of such attachment triggers a type of programmed cell death known as anoikis, the acquisition of resistance to which is a key step in cancer development. The mechanisms underlying anoikis resistance remain unclear, however. The intracellular F-actin cytoskeleton plays a key role in sensing the loss of cell-ECM attachment, but how its disruption affects cell fate during such stress is not well understood. Here, we reveal a cell survival strategy characterized by the formation of a giant unilocular vacuole (GUVac) in the cytoplasm of the cells whose actin cytoskeleton is disrupted during loss of matrix attachment. Time-lapse imaging and electron microscopy showed that large vacuoles with a diameter of >500 nm accumulated early after inhibition of actin polymerization in cells in suspension culture, and that these vacuoles subsequently coalesced to form a GUVac. GUVac formation was found to result from a variation of a macropinocytosis-like process, characterized by the presence of inwardly curved membrane invaginations. This phenomenon relies on both F-actin depolymerization and the recruitment of septin proteins for micron-sized plasma membrane invagination. The vacuole fusion step during GUVac formation requires PI(3)P produced by VPS34 and PI3K-C2α on the surface of vacuoles. Furthermore, its induction after loss of matrix attachment conferred anoikis resistance. Our results thus show that the formation of a previously unrecognized organelle promotes cell survival in the face of altered actin and matrix environments.

Congenital myopathies: pathophysiological mechanisms and promising therapies

Congenital myopathies (CMs) are a kind of non-progressive or slow-progressive muscle diseases caused by genetic mutations, which are currently defined and categorized mainly according to their clinicopathological features. CMs exhibit pleiotropy and genetic heterogeneity. Currently, supportive treatment and pharmacological remission are the mainstay of treatment, with no cure available. Some adeno-associated viruses show promising prospects in the treatment of MTM1 and BIN1-associated myopathies; however, such gene-level therapeutic interventions target only specific mutation types and are not generalizable. Thus, it is particularly crucial to identify the specific causative genes. Here, we outline the pathogenic mechanisms based on the classification of causative genes: excitation-contraction coupling and triadic assembly (RYR1, MTM1, DNM2, BIN1), actin-myosin interaction and production of myofibril forces (NEB, ACTA1, TNNT1, TPM2, TPM3), as well as other biological processes. Furthermore, we provide a comprehensive overview of recent therapeutic advancements and potential treatment modalities of CMs. Despite ongoing research endeavors, targeted strategies and collaboration are imperative to address diagnostic uncertainties and explore potential treatments.

CMTM6 mediates the Warburg effect and promotes the liver metastasis of colorectal cancer

Liver metastasis of colorectal cancer (CRC) is a leading cause of death among cancer patients. The overexpression of glucose transporter 1 (Glut1) and enhanced glucose uptake that are associated with the Warburg effect are frequently observed in CRC liver metastases, but the underlying mechanisms remain poorly understood. CKLF-like MARVEL transmembrane domain-containing protein 6 (CMTM6) regulates the intracellular trafficking of programmed death-ligand-1 (PD-L1); therefore, we investigated whether CMTM6 regulates Glut1 trafficking and the Warburg effect in CRC cells. We found that knocking down of CMTM6 by shRNA induced the lysosomal degradation of Glut1, decreased glucose uptake and glycolysis in CRC cells, and suppressed subcutaneous CRC growth in nude mice and liver metastasis in C57BL/6 mice. Mechanistically, CMTM6 forms a complex with Glut1 and Rab11 in the endosomes of CRC cells, and this complex is required for the Rab11-dependent transport of Glut1 to the plasma membrane and for the protection of Glut1 from lysosomal degradation. Multiomics revealed global transcriptomic changes in CMTM6-knockdown CRC cells that affected the transcriptomes of adjacent cancer-associated fibroblasts from CRC liver metastases. As a result of these transcriptomic changes, CMTM6-knockdown CRC cells exhibited a defect in the G2-to-M phase transition, reduced secretion of 60 cytokines/chemokines, and inability to recruit cancer-associated fibroblasts to support an immunosuppressive CRC liver metastasis microenvironment. Analysis of TCGA data confirmed that CMTM6 expression was increased in CRC patients and that elevated CMTM6 expression was associated with worse patient survival. Together, our data suggest that CMTM6 plays multiple roles in regulating the Warburg effect, transcriptome, and liver metastasis of CRC.

A novel noncanonical function for IRF6 in the recycling of E-cadherin

Interferon Regulatory Factor 6 (IRF6) is a transcription factor essential for keratinocyte cell-cell adhesions. Previously, we found that recycling of E-cadherin was defective in the absence of IRF6, yet total E-cadherin levels were not altered, suggesting a previously unknown, nontranscriptional function for IRF6. IRF6 protein contains a DNA binding domain (DBD) and a protein binding domain (PBD). The transcriptional function of IRF6 depends on its DBD and PBD, however, whether the PBD is necessary for the interaction with cytoplasmic proteins has yet to be demonstrated. Here, we show that an intact PBD is required for recruitment of cell-cell adhesion proteins at the plasma membrane, including the recycling of E-cadherin. Colocalizations and coimmunoprecipitations reveal that IRF6 forms a complex in recycling endosomes with Rab11, Myosin Vb, and E-cadherin, and that the PBD is required for this interaction. These data indicate that IRF6 is a novel effector of the endosomal recycling of E-cadherin and demonstrate a non-transcriptional function for IRF6 in regulating cell-cell adhesions.

Efficient and Targeted siRNA Delivery to M2 Macrophages by Smart Polymer Blends for M1 Macrophage Repolarization as a Promising Strategy for Future Cancer Treatment

Cancer remains an issue on a global scale. It is estimated that nearly 10 million people succumbed to cancer worldwide in 2020. New treatment options are urgently needed. A promising approach is a conversion of tumor-promoting M2 tumor-associated macrophages (TAMs) as part of the tumor microenvironment to tumor-suppressive M1 TAMs by small interfering RNA (siRNA). In this work, we present a well-characterized polymeric nanocarrier system capable of targeting M2 TAMs by a ligand-receptor interaction. Therefore, we developed a blended PEI-based polymeric nanoparticle system conjugated with mannose, which is internalized after interaction with macrophage mannose receptors (MMRs), showing low cytotoxicity and negligible IL-6 activation. The PEI-PCL-PEI (5 kDa-5 kDa-5 kDa) and Man-PEG-PCL (2 kDa-2 kDa) blended siRNA delivery system was optimized for maximum targeting capability and efficient endosomal escape by evaluation of different polymer and N/P ratios. The nanoparticles were formulated by surface acoustic wave-assisted microfluidics, achieving a size of ∼80 nm and a zeta potential of approximately +10 mV. Special attention was given to the endosomal escape as the so-called bottleneck of RNA drug delivery. To estimate the endosomal escape capability of the nanocarrier system, we developed a prediction method by evaluating the particle stability via the inflection temperature. Our predictions were then verified in an in vitro setting by applying confocal microscopy. For cellular experiments, however, human THP-1 cells were polarized to M2 macrophages by cytokine treatment and validated through MMR expression. To show the efficiency of the nanoparticle system, GAPDH and IκBα knockdown was performed in the presence or absence of an MMR blocking excess of mannan. Cellular uptake, GAPDH knockdown, and NF-κB western blot confirmed efficient mannose targeting. Herein, we presented a well-characterized nanoparticle delivery system and a promising approach for targeting M2 macrophages by a mannose-MMR interaction.

PI3K-C2β limits mTORC1 signaling and angiogenic growth

Phosphoinositide 3-kinases (PI3Ks) phosphorylate intracellular inositol lipids to regulate signaling and intracellular vesicular trafficking. Mammals have eight PI3K isoforms, of which class I PI3Kα and class II PI3K-C2α are essential for vascular development. The class II PI3K-C2β is also abundant in endothelial cells. Using in vivo and in vitro approaches, we found that PI3K-C2β was a critical regulator of blood vessel growth by restricting endothelial mTORC1 signaling. Mice expressing a kinase-inactive form of PI3K-C2β displayed enlarged blood vessels without corresponding changes in endothelial cell proliferation or migration. Instead, inactivation of PI3K-C2β resulted in an increase in the size of endothelial cells, particularly in the sprouting zone of angiogenesis. Mechanistically, we showed that the aberrantly large size of PI3K-C2β mutant endothelial cells was caused by mTORC1 activation, which sustained growth in these cells. Consistently, pharmacological inhibition of mTORC1 with rapamycin normalized vascular morphogenesis in PI3K-C2β mutant mice. Together, these results identify PI3K-C2β as a crucial determinant of endothelial signaling and illustrate the importance of mTORC1 regulation during angiogenic growth.

An Update on the Interplay between LRRK2, Rab GTPases and Parkinson's Disease

Over the last decades, research on the pathobiology of neurodegenerative diseases has greatly evolved, revealing potential targets and mechanisms linked to their pathogenesis. Parkinson's disease (PD) is no exception, and recent studies point to the involvement of endolysosomal defects in PD. The endolysosomal system, which tightly controls a flow of endocytosed vesicles targeted either for degradation or recycling, is regulated by a number of Rab GTPases. Their associations with leucine-rich repeat kinase 2 (LRRK2), a major causative and risk protein of PD, has also been one of the hot topics in the field. Understanding their interactions and functions is critical for unraveling their contribution to PD pathogenesis. In this review, we summarize recent studies on LRRK2 and Rab GTPases and attempt to provide more insight into the interaction of LRRK2 with each Rab and its relationship to PD.

CaSR links endocytic and secretory pathways via MADD, a Rab11A effector that activates Rab27B

Calcium sensing receptor (CaSR), a class C GPCR, regulates essential secretory pathways, involving communication between endocytic and secretory Rab GTPases, via still to be fully defined molecular mechanisms. To address how communication between endocytic and secretory vesicles occurs, we hypothesized that CaSR activates endocytic Rab11A-dependent effector pathways acting upstream of Rab27B-regulated secretion. We found that Rab11A is critical to promote Rab27B-dependent secretion of chemotactic and inflammatory factors, including IL-8, CCL2/MCP-1, and IL1-β, in response to CaSR stimulation. It also attenuates secretion of IL-6. The process is mediated by endosomal PI3-kinases, Vps34 and PI3KC2α, which promote Rab27B activation. Rab11A interacts with and activates MADD, a guanine exchange factor for Rab3, and Rab27A/B. Mechanistically, CaSR drives Rab11A-dependent coupling of recycling endosomes to secretory-vesicles via endosomal PI3K-mediated activation of a MADD/Rab27B pathway.

Multiple functions and dual characteristics of RAB11A in cancers

Vesicle trafficking is an unceasing and elaborate cellular process that functions in material transport and information delivery. Recent studies have identified the small GTPase, Ras-related protein in brain 11A (RAB11A), as a key regulator in this process. Aberrant RAB11A expression has been reported in several types of cancers, suggesting the important functions and characteristics of RAB11A in cancer. These discoveries are of great significance because therapeutic strategies based on the physiological and pathological status of RAB11A might make cancer treatment more effective, as the molecular mechanisms of cancer development have not been completely revealed. However, these studies on RAB11A have not been reviewed and discussed specifically. Therefore, we summarize and discuss the recent findings of RAB11A involvement in different biological processes, including endocytic recycling regulation, receptors and adhesion molecules recycling, exosome secretion, phagophore formation and cytokinesis, as well as regulatory mechanisms in several tumor types. Moreover, contradictory effects of RAB11A have also been observed in different types of cancers, implying the dual characteristics of RAB11A in cancer, which are either oncogenic or tumor-suppressive. This review on the functions and characteristics of RAB11A highlights the value of RAB11A in inducing multiple important phenotypes based on vesicle trafficking and therefore will offer insights for future studies to reveal the molecular mechanisms, clinical significance, and therapeutic targeting of RAB11A in different cancers.

A phosphoinositide switch mediates exocyst recruitment to multivesicular endosomes for exosome secretion

Exosomes are secreted to the extracellular milieu when multivesicular endosomes (MVEs) dock and fuse with the plasma membrane. However, MVEs are also known to fuse with lysosomes for degradation. How MVEs are directed to the plasma membrane for exosome secretion rather than to lysosomes is unclear. Here we report that a conversion of phosphatidylinositol-3-phosphate (PI(3)P) to phosphatidylinositol-4-phosphate (PI(4)P) catalyzed sequentially by Myotubularin 1 (MTM1) and phosphatidylinositol 4-kinase type IIα (PI4KIIα) on the surface of MVEs mediates the recruitment of the exocyst complex. The exocyst then targets the MVEs to the plasma membrane for exosome secretion. We further demonstrate that disrupting PI(4)P generation or exocyst function blocked exosomal secretion of Programmed death-ligand 1 (PD-L1), a key immune checkpoint protein in tumor cells, and led to its accumulation in lysosomes. Together, our study suggests that the PI(3)P to PI(4)P conversion on MVEs and the recruitment of the exocyst direct the exocytic trafficking of MVEs for exosome secretion.

Structural Basis for Highly Selective Class II Alpha Phosphoinositide-3-Kinase Inhibition

Class II phosphoinositide-3-kinases (PI3Ks) play central roles in cell signaling, division, migration, and survival. Despite evidence that all PI3K class II isoforms serve unique cellular functions, the lack of isoform-selective inhibitors severely hampers the systematic investigation of their potential relevance as pharmacological targets. Here, we report the structural evaluation and molecular determinants for selective PI3K-C2α inhibition by a structure-activity relationship study based on a pteridinone scaffold, leading to the discovery of selective PI3K-C2α inhibitors called PITCOINs. Cocrystal structures and docking experiments supported the rationalization of the structural determinants essential for inhibitor activity and high selectivity. Profiling of PITCOINs in a panel of more than 118 diverse kinases showed no off-target kinase inhibition. Notably, by addressing a selectivity pocket, PITCOIN4 showed nanomolar inhibition of PI3K-C2α and >100-fold selectivity in a general kinase panel. Our study paves the way for the development of novel therapies for diseases related to PI3K-C2α function.

CRB1 is required for recycling by RAB11A+ vesicles in human retinal organoids

CRB1 gene mutations can cause early- or late-onset retinitis pigmentosa, Leber congenital amaurosis, or maculopathy. Recapitulating human CRB1 phenotypes in animal models has proven challenging, necessitating the development of alternatives. We generated human induced pluripotent stem cell (iPSC)-derived retinal organoids of patients with retinitis pigmentosa caused by biallelic CRB1 mutations and evaluated them against autologous gene-corrected hiPSCs and hiPSCs from healthy individuals. Patient organoids show decreased levels of CRB1 and NOTCH1 expression at the retinal outer limiting membrane. Proximity ligation assays show that human CRB1 and NOTCH1 can interact via their extracellular domains. CRB1 patient organoids feature increased levels of WDFY1+ vesicles, fewer RAB11A+ recycling endosomes, decreased VPS35 retromer complex components, and more degradative endolysosomal compartments relative to isogenic control organoids. Taken together, our data demonstrate that patient-derived retinal organoids enable modeling of retinal degeneration and highlight the importance of CRB1 in early endosome maturation receptor recycling in the retina.

Sorting of multiple molecular species on cell membranes

Eukaryotic cells maintain their inner order by a hectic process of sorting and distillation of molecular factors taking place on their lipid membranes. A similar sorting process is implied in the assembly and budding of enveloped viruses. To understand the properties of this molecular sorting process, we have recently proposed a physical model [Zamparo et al., Phys. Rev. Lett. 126, 088101 (2021)]10.1103/PhysRevLett.126.088101, based on (1) the phase separation of a single, initially dispersed molecular species into spatially localized sorting domains on the lipid membrane and (2) domain-induced membrane bending leading to the nucleation of submicrometric lipid vesicles, naturally enriched in the molecules of the engulfed sorting domain. The analysis of the model showed the existence of an optimal region of parameter space where sorting is most efficient. Here the model is extended to account for the simultaneous distillation of a pool of distinct molecular species. We find that the mean time spent by sorted molecules on the membrane increases with the heterogeneity of the pool (i.e., the number of distinct molecular species sorted) according to a simple scaling law, and that a large number of distinct molecular species can in principle be sorted in parallel on cell membranes without significantly interfering with each other. Moreover, sorting is found to be most efficient when the distinct molecular species have comparable homotypic affinities. We also consider how valence (i.e., the average number of interacting neighbors of a molecule in a sorting domain) affects the sorting process, finding that higher-valence molecules can be sorted with greater efficiency than lower-valence molecules.

Phosphoinositide 3-kinase (PI3K) classes: From cell signaling to endocytic recycling and autophagy

Lipid signaling is defined as any biological signaling action in which a lipid messenger binds to a protein target, converting its effects to specific cellular responses. In this complex biological pathway, the family of phosphoinositide 3-kinase (PI3K) represents a pivotal role and affects many aspects of cellular biology from cell survival, proliferation, and migration to endocytosis, intracellular trafficking, metabolism, and autophagy. While yeasts have a single isoform of phosphoinositide 3-kinase (PI3K), mammals possess eight PI3K types divided into three classes. The class I PI3Ks have set the stage to widen research interest in the field of cancer biology. The aberrant activation of class I PI3Ks has been identified in 30-50% of human tumors, and activating mutations in PIK3CA is one of the most frequent oncogenes in human cancer. In addition to indirect participation in cell signaling, class II and III PI3Ks primarily regulate vesicle trafficking. Class III PI3Ks are also responsible for autophagosome formation and autophagy flux. The current review aims to discuss the original data obtained from international research laboratories on the latest discoveries regarding PI3Ks-mediated cell biological processes. Also, we unravel the mechanisms by which pools of the same phosphoinositides (PIs) derived from different PI3K types act differently.

Small molecules targeting endocytic uptake and recycling pathways

Over the past years a growing number of studies highlighted the pivotal role of intracellular trafficking in cell physiology. Among the distinct transport itineraries connecting the endocytic system, both internalization (endocytosis) and recycling (endocytic recycling) pathways were found fundamental to ensure cellular sensing, cell-to-cell communication, cellular division, and collective cell migration in tissue specific-contexts. Consistently, the dysregulation of endocytic trafficking pathways is correlated with several human diseases including both cancers and neurodegeneration. Aimed at suppress specific intracellular trafficking routes involved in disease onset and progression, huge efforts have been made to identify small molecule inhibitors with suitable pharmacological properties for in vivo administration. Here, we review most used drugs and recently discovered small molecules able to block endocytosis and endocytic recycling pathways. We characterize such pharmacological inhibitors by emphasizing their target specificity, molecular affinity, biological activity and efficacy in both in vitro and in vivo experimental models.

COMMD3-Mediated Endosomal Trafficking of HER2 Inhibits the Progression of Ovarian Carcinoma

The dysregulated endocytic traffic of oncogenic receptors, such as the EGFR family especially HER2, contributes to the uncontrolled activation of the downstream oncogenic signaling and progression of various carcinomas, including 90% of ovarian carcinoma. However, the key regulators in the intracellular trafficking of HER2 and their impacts for cancer progression remain largely unknown. In this study, through a genome-wide CRISPR/Cas9 screening for key genes affecting the peritoneal disseminated metastasis of ovarian carcinoma, we identified a member of COMMD family, that is, COMMD3, as a key regulator in the endosomal trafficking of HER2. In the patients with high-grade serous ovarian carcinoma (HGSOC), the expression of COMMD3 is dramatically decreased in the peritoneal disseminated ovarian carcinoma cells comparing with that in the primary ovarian carcinoma cells. COMMD3 greatly inhibits the proliferation, migration, and epithelial-mesenchymal transition (EMT) of HGSOC cells, and dramatically suppresses the tumor growth, the formation of malignant ascites, and the peritoneal dissemination of cancer cells in the orthotopic murine model of HGSOC. Further transcriptome analysis reveals that silencing COMMD3 boosts the activation of HER2 downstream signaling. As a component in the Retriever-associated COMMD/CCDC22/CCDC93 complex responsible for the recognition and recycling of membrane receptors, COMMD3 physically interacts with HER2 for directing it to the slow recycling pathway, leading to the attenuated downstream tumor-promoting signaling.

IMPLICATIONS

Collectively, this study reveals a novel HER2 inactivation mechanism with a high value for the clinic diagnosis of new ovarian carcinoma types and the design of new therapeutic strategy.

Endosomal sorting sorted - motors, adaptors and lessons from in vitro and cellular studies

Motor proteins are key players in exerting spatiotemporal control over the intracellular location of membrane-bound compartments, including endosomes containing cargo. In this Review, we focus on how motors and their cargo adaptors regulate positioning of cargoes from the earliest stages of endocytosis and through the two main intracellular itineraries: (1) degradation at the lysosome or (2) recycling back to the plasma membrane. In vitro and cellular (in vivo) studies on cargo transport thus far have typically focussed independently on either the motor proteins and adaptors, or membrane trafficking. Here, we will discuss recent studies to highlight what is known about the regulation of endosomal vesicle positioning and transport by motors and cargo adaptors. We also emphasise that in vitro and cellular studies are often performed at different scales, from single molecules to whole organelles, with the aim to provide a perspective on the unified principles of motor-driven cargo trafficking in living cells that can be learned from these differing scales.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Development of selective inhibitors of phosphatidylinositol 3-kinase C2α

Phosphatidylinositol 3-kinase type 2α (PI3KC2α) and related class II PI3K isoforms are of increasing biomedical interest because of their crucial roles in endocytic membrane dynamics, cell division and signaling, angiogenesis, and platelet morphology and function. Herein we report the development and characterization of PhosphatidylInositol Three-kinase Class twO INhibitors (PITCOINs), potent and highly selective small-molecule inhibitors of PI3KC2α catalytic activity. PITCOIN compounds exhibit strong selectivity toward PI3KC2α due to their unique mode of interaction with the ATP-binding site of the enzyme. We demonstrate that acute inhibition of PI3KC2α-mediated synthesis of phosphatidylinositol 3-phosphates by PITCOINs impairs endocytic membrane dynamics and membrane remodeling during platelet-dependent thrombus formation. PITCOINs are potent and selective cell-permeable inhibitors of PI3KC2α function with potential biomedical applications ranging from thrombosis to diabetes and cancer.

Engineering poly- and micelleplexes for nucleic acid delivery - A reflection on their endosomal escape

For the longest time, the field of nucleic acid delivery has remained skeptical whether or not polycationic drug carrier systems would ever make it into clinical practice. Yet, with the disclosure of patents on polyethyleneimine-based RNA carriers through leading companies in the field of nucleic acid therapeutics such as BioNTech SE and the progress in clinical studies beyond phase I trials, this aloofness seems to regress. As one of the most striking characteristics of polymer-based vectors, the extraordinary tunability can be both a blessing and a curse. Yet, knowing about the adjustment screws and how they impact the performance of the drug carrier provides the formulation scientist committed to its development with a head start. Here, we equip the reader with a toolbox - a toolbox that should advise and support the developer to conceptualize a cutting-edge poly- or micelleplex system for the delivery of therapeutic nucleic acids; to be specific, to engineer the vector towards maximum endosomal escape performance at minimum toxicity. Therefore, after briefly sketching the boundary conditions of polymeric vector design, we will dive into the topic of endosomal trafficking. We will not only discuss the most recent knowledge of the endo-lysosomal compartment but further depict different hypotheses and mechanisms that facilitate the endosomal escape of polyplex systems. Finally, we will combine the different facets introduced in the previous chapters with the fundamental building blocks of polymer vector design and evaluate the advantages and drawbacks. Throughout the article, a particular focus will be placed on cellular peculiarities, not only as an additional barrier, but also to give inspiration to how such cell-specific traits might be capitalized on.

Endosomal lipid signaling reshapes the endoplasmic reticulum to control mitochondrial function

Cells respond to fluctuating nutrient supply by adaptive changes in organelle dynamics and in metabolism. How such changes are orchestrated on a cell-wide scale is unknown. We show that endosomal signaling lipid turnover by MTM1, a phosphatidylinositol 3-phosphate [PI(3)P] 3-phosphatase mutated in X-linked centronuclear myopathy in humans, controls mitochondrial morphology and function by reshaping the endoplasmic reticulum (ER). Starvation-induced endosomal recruitment of MTM1 impairs PI(3)P-dependent contact formation between tubular ER membranes and early endosomes, resulting in the conversion of ER tubules into sheets, the inhibition of mitochondrial fission, and sustained oxidative metabolism. Our results unravel an important role for early endosomal lipid signaling in controlling ER shape and, thereby, mitochondrial form and function to enable cells to adapt to fluctuating nutrient environments.

Myotubularin functions through actomyosin to interact with the Hippo pathway

The Hippo pathway is an evolutionarily conserved developmental pathway that controls organ size by integrating diverse regulatory inputs, including actomyosin-mediated cytoskeletal tension. Despite established connections between the actomyosin cytoskeleton and the Hippo pathway, the upstream regulation of actomyosin in the Hippo pathway is less defined. Here, we identify the phosphoinositide-3-phosphatase Myotubularin (Mtm) as a novel upstream regulator of actomyosin that functions synergistically with the Hippo pathway during growth control. Mechanistically, Mtm regulates membrane phospholipid PI(3)P dynamics, which, in turn, modulates actomyosin activity through Rab11-mediated vesicular trafficking. We reveal PI(3)P dynamics as a novel mode of upstream regulation of actomyosin and establish Rab11-mediated vesicular trafficking as a functional link between membrane lipid dynamics and actomyosin activation in the context of growth control. Our study also shows that MTMR2, the human counterpart of Drosophila Mtm, has conserved functions in regulating actomyosin activity and tissue growth, providing new insights into the molecular basis of MTMR2-related peripheral nerve myelination and human disorders.

LC3-associated endocytosis and the functions of Rubicon and ATG16L1

LC3-associated endocytosis (LANDO) is a noncanonical function of the autophagy machinery, in which LC3 (microtubule-associated protein light chain) is conjugated to rab5-positive endosomes, using a portion of the canonical autophagy pathway. LANDO was initially discovered in a murine model of Alzheimer's disease as a critical regulator of amyloid-β receptor recycling in microglial cells, playing a protective role against neuronal loss and memory impairment. Recent evidence suggests an emerging role of LANDO in cytokine receptor signaling and innate immunity. Here, we discuss the regulation of two crucial effectors of LANDO, Rubicon and ATG16L1, and their impact on endocytosis, autophagy, and phagocytosis.

Dynamic regulation of KIF15 phosphorylation and acetylation promotes focal adhesions disassembly in pancreatic cancer

Pancreatic cancer (PC) is prone to distant metastasis in the early stage, which is attributed to the strong migration ability of tumor cells. Focal adhesion turnover is essential for cancer cell metastasis, and the integrin recycling process is a key activation pathway for focal adhesion depolymerization. To identify the key motor protein involving in the integrin β1 recycling, we screened kinesin proteins involved in integrin β1 recycling using a kinesin family siRNA library and identified kinesin family 15 (KIF15) as a key regulator. KIF15 was upregulated in metastasis PC tissues and promoted PC cell migration and invasion. We identified KIF15 as a key component mediating integrin β1/FAK signaling that accelerated FA disassembly in a FAK-Y397-dependent manner. KIF15 recruited PI3K-C2α to promote integrin β1/FAK signaling and FA disassembly in a RAB11A-dependent manner. The C-terminal tail of KIF15 is required for the PI3K-C2α interaction and RAB11A activation. In addition, we also found that SIRT1-mediated acetylation of KIF15 is essential for KIF15 phosphorylation, which is the key activation event in motor protein function. Together, these findings indicate that KIF15 interacts with PI3K-C2α to promote FA turnover in PC cells by controlling the endosome recycling of integrin β1 in a SIRT1 acetylation modification-dependent manner, eventually promoting focal adhesions turnover and distant metastasis in PC.

Spatial snapshots of amyloid precursor protein intramembrane processing via early endosome proteomics

Degradation and recycling of plasma membrane proteins occurs via the endolysosomal system, wherein endosomes bud into the cytosol from the plasma membrane and subsequently mature into degradative lysosomal compartments. While methods have been developed for rapid selective capture of lysosomes (Lyso-IP), analogous methods for isolation of early endosome intermediates are lacking. Here, we develop an approach for rapid isolation of early/sorting endosomes through affinity capture of the early endosome-associated protein EEA1 (Endo-IP) and provide proteomic and lipidomic snapshots of EEA1-positive endosomes in action. We identify recycling, regulatory and membrane fusion complexes, as well as candidate cargo, providing a proteomic landscape of early/sorting endosomes. To demonstrate the utility of the method, we combined Endo- and Lyso-IP with multiplexed targeted proteomics to provide a spatial digital snapshot of amyloid precursor protein (APP) processing by β and γ-Secretases, which produce amyloidogenic Aβ species, and quantify small molecule modulation of Secretase action on endosomes. We anticipate that the Endo-IP approach will facilitate systematic interrogation of processes that are coordinated on EEA1-positive endosomes.

Host Cell Signatures of the Envelopment Site within Beta-Herpes Virions

Beta-herpesvirus infection completely reorganizes the membrane system of the cell. This system is maintained by the spatiotemporal arrangement of more than 3000 cellular proteins that continuously adapt the configuration of membrane organelles according to cellular needs. Beta-herpesvirus infection establishes a new configuration known as the assembly compartment (AC). The AC membranes are loaded with virus-encoded proteins during the long replication cycle and used for the final envelopment of the newly formed capsids to form infectious virions. The identity of the envelopment membranes is still largely unknown. Electron microscopy and immunofluorescence studies suggest that the envelopment occurs as a membrane wrapping around the capsids, similar to the growth of phagophores, in the area of the AC with the membrane identities of early/recycling endosomes and the trans-Golgi network. During wrapping, host cell proteins that define the identity and shape of these membranes are captured along with the capsids and incorporated into the virions as host cell signatures. In this report, we reviewed the existing information on host cell signatures in human cytomegalovirus (HCMV) virions. We analyzed the published proteomes of the HCMV virion preparations that identified a large number of host cell proteins. Virion purification methods are not yet advanced enough to separate all of the components of the rich extracellular material, including the large amounts of non-vesicular extracellular particles (NVEPs). Therefore, we used the proteomic data from large and small extracellular vesicles (lEVs and sEVs) and NVEPs to filter out the host cell proteins identified in the viral proteomes. Using these filters, we were able to narrow down the analysis of the host cell signatures within the virions and determine that envelopment likely occurs at the membranes derived from the tubular recycling endosomes. Many of these signatures were also found at the autophagosomes, suggesting that the CMV-infected cell forms membrane organelles with phagophore growth properties using early endosomal host cell machinery that coordinates endosomal recycling.

Cell Trafficking and Function of G Protein-coupled Receptors

The G protein-coupled receptors (GPCRs) are plasma membrane proteins that function as sensors of changes in the internal and external milieux and play essential roles in health and disease. They are targets of hormones, neurotransmitters, local hormones (autacoids), and a large proportion of the drugs currently used as therapeutics and for "recreational" purposes. Understanding how these receptors signal and are regulated is fundamental for progress in areas such as physiology and pharmacology. This review will focus on what is currently known about their structure, the molecular events that trigger their signaling, and their trafficking to endosomal compartments. GPCR phosphorylation and its role in desensitization (signaling switching) are also discussed. It should be mentioned that the volume of information available is enormous given the large number and variety of GPCRs. However, knowledge is fragmentary even for the most studied receptors, such as the adrenergic receptors. Therefore, we attempt to present a panoramic view of the field, conscious of the risks and limitations (such as oversimplifications and incorrect generalizations). We hope this will provoke further research in the area. It is currently accepted that GPCR internalization plays a role signaling events. Therefore, the processes that allow them to internalize and recycle back to the plasma membrane are briefly reviewed. The functions of cytoskeletal elements (mainly actin filaments and microtubules), the molecular motors implicated in receptor trafficking (myosin, kinesin, and dynein), and the GTPases involved in GPCR internalization (dynamin) and endosomal sorting (Rab proteins), are discussed. The critical role phosphoinositide metabolism plays in regulating these events is also depicted.

A mechanism for exocyst-mediated tethering via Arf6 and PIP5K1C-driven phosphoinositide conversion

Polarized trafficking is necessary for the development of eukaryotes and is regulated by a conserved molecular machinery. Late steps of cargo delivery are mediated by the exocyst complex, which integrates lipid and protein components to tether vesicles for plasma membrane fusion. However, the molecular mechanisms of this process are poorly defined. Here, we reconstitute functional octameric human exocyst, demonstrating the basis for holocomplex coalescence and biochemically stable subcomplexes. We determine that each subcomplex independently binds to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), which is minimally sufficient for membrane tethering. Through reconstitution and epithelial cell biology experiments, we show that Arf6-mediated recruitment of the lipid kinase PIP5K1C rapidly converts phosphatidylinositol 4-phosphate (PI(4)P) to PI(4,5)P₂, driving exocyst recruitment and membrane tethering. These results provide a molecular mechanism of exocyst-mediated tethering and a unique functional requirement for phosphoinositide signaling on late-stage vesicles in the vicinity of the plasma membrane.

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Complete reconstitution and subunit connectivity of the human exocyst complex

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Binding to PI(4,5)P₂ in trans by each subcomplex enables membrane tethering

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PI(4)P to PI(4,5)P2 conversion is sufficient for exocyst recruitment and tethering

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Arf6 controls phosphoinositide conversion by PIP5K1C in cells and in vitro

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

Early Endosomal Vps34-Derived Phosphatidylinositol-3-Phosphate Is Indispensable for the Biogenesis of the Endosomal Recycling Compartment

Phosphatidylinositol-3-phosphate (PI3P), a major identity tag of early endosomes (EEs), provides a platform for the recruitment of numerous cellular proteins containing an FYVE or PX domain that is required for PI3P-dependent maturation of EEs. Most of the PI3P in EEs is generated by the activity of Vps34, a catalytic component of class III phosphatidylinositol-3-phosphate kinase (PI3Ks) complex. In this study, we analyzed the role of Vps34-derived PI3P in the EE recycling circuit of unperturbed cells using VPS34-IN1 (IN1), a highly specific inhibitor of Vps34. IN1-mediated PI3P depletion resulted in the rapid dissociation of recombinant FYVE- and PX-containing PI3P-binding modules and endogenous PI3P-binding proteins, including EEA1 and EE sorting nexins. IN1 treatment triggered the rapid restructuring of EEs into a PI3P-independent functional configuration, and after IN1 washout, EEs were rapidly restored to a PI3P-dependent functional configuration. Analysis of the PI3P-independent configuration showed that the Vps34-derived PI3P is not essential for the pre-EE-associated functions and the fast recycling loop of the EE recycling circuit but contributes to EE maturation toward the degradation circuit, as previously shown in Vps34 knockout and knockdown studies. However, our study shows that Vps34-derived PI3P is also essential for the establishment of the Rab11a-dependent pathway, including recycling cargo sorting in this pathway and membrane flux from EEs to the pericentriolar endosomal recycling compartment (ERC). Rab11a endosomes of PI3P-depleted cells expanded and vacuolized outside the pericentriolar area without the acquisition of internalized transferrin (Tf). These endosomes had high levels of FIP5 and low levels of FIP3, suggesting that their maturation was arrested before the acquisition of FIP3. Consequently, Tf-loaded-, Rab11a/FIP5-, and Rab8a-positive endosomes disappeared from the pericentriolar area, implying that PI3P-associated functions are essential for ERC biogenesis. ERC loss was rapidly reversed after IN1 washout, which coincided with the restoration of FIP3 recruitment to Rab11a-positive endosomes and their dynein-dependent migration to the cell center. Thus, our study shows that Vps34-derived PI3P is indispensable in the recycling circuit to maintain the slow recycling pathway and biogenesis of the ERC.

Phosphoinositide Conversion Inactivates R-RAS and Drives Metastases in Breast Cancer

Breast cancer is the most prevalent cancer and a major cause of death in women worldwide. Although early diagnosis and therapeutic intervention significantly improve patient survival rate, metastasis still accounts for most deaths. Here it is reported that, in a cohort of more than 2000 patients with breast cancer, overexpression of PI3KC2α occurs in 52% of cases and correlates with high tumor grade as well as increased probability of distant metastatic events, irrespective of the subtype. Mechanistically, it is demonstrated that PI3KC2α synthetizes a pool of PI(3,4)P2 at focal adhesions that lowers their stability and directs breast cancer cell migration, invasion, and metastasis. PI(3,4)P2 locally produced by PI3KC2α at focal adhesions recruits the Ras GTPase activating protein 3 (RASA3), which inactivates R-RAS, leading to increased focal adhesion turnover, migration, and invasion both in vitro and in vivo. Proof-of-concept is eventually provided that inhibiting PI3KC2α or lowering RASA3 activity at focal adhesions significantly reduces the metastatic burden in PI3KC2α-overexpressing breast cancer, thereby suggesting a novel strategy for anti-breast cancer therapy.

Structural basis of phosphatidylinositol 3-kinase C2α function

Phosphatidylinositol 3-kinase type 2α (PI3KC2α) is an essential member of the structurally unresolved class II PI3K family with crucial functions in lipid signaling, endocytosis, angiogenesis, viral replication, platelet formation and a role in mitosis. The molecular basis of these activities of PI3KC2α is poorly understood. Here, we report high-resolution crystal structures as well as a 4.4-Å cryogenic-electron microscopic (cryo-EM) structure of PI3KC2α in active and inactive conformations. We unravel a coincident mechanism of lipid-induced activation of PI3KC2α at membranes that involves large-scale repositioning of its Ras-binding and lipid-binding distal Phox-homology and C-C2 domains, and can serve as a model for the entire class II PI3K family. Moreover, we describe a PI3KC2α-specific helical bundle domain that underlies its scaffolding function at the mitotic spindle. Our results advance our understanding of PI3K biology and pave the way for the development of specific inhibitors of class II PI3K function with wide applications in biomedicine.

Serotonin: an overlooked regulator of endocytosis and endosomal sorting?

Serotonin is a neurotransmitter and a hormone that is typically associated with regulating our mood. However, the serotonin transporter and receptors are expressed throughout the body, highlighting the much broader, systemic role of serotonin in regulating human physiology. A substantial body of data strongly implicates serotonin as a fundamental regulator of endocytosis and endocytic sorting. Serotonin has the potential to enhance endocytosis through three distinct mechanisms - serotonin signalling, serotonylation and insertion into the plasma membrane - although the interplay and relationship between these mechanisms has not yet been explored. Endocytosis is central to the cellular response to the extracellular environment, controlling receptor distribution on the plasma membrane to modulate signalling, neurotransmitter release and uptake, circulating protein and lipid cargo uptake, and amino acid internalisation for cell proliferation. Uncovering the range of cellular and physiological circumstances in which serotonin regulates endocytosis is of great interest for our understanding of how serotonin regulates mood, and also the fundamental understanding of endocytosis and its regulation throughout the body. This article has an associated Future Leader to Watch interview with the first author of the paper.

OUP accepted manuscript

Epilepsy is one of the most frequent neurological diseases, with focal epilepsy accounting for the largest number of cases. The genetic alterations involved in focal epilepsy are far from being fully elucidated.

Here, we show that defective lipid signalling caused by heterozygous ultra-rare variants in PIK3C2B, encoding for the class II phosphatidylinositol 3-kinase PI3K-C2β, underlie focal epilepsy in humans. We demonstrate that patients’ variants act as loss-of-function alleles, leading to impaired synthesis of the rare signalling lipid phosphatidylinositol 3,4-bisphosphate, resulting in mTORC1 hyperactivation. In vivo, mutant Pik3c2b alleles caused dose-dependent neuronal hyperexcitability and increased seizure susceptibility, indicating haploinsufficiency as a key driver of disease. Moreover, acute mTORC1 inhibition in mutant mice prevented experimentally induced seizures, providing a potential therapeutic option for a selective group of patients with focal epilepsy.

Our findings reveal an unexpected role for class II PI3K-mediated lipid signalling in regulating mTORC1-dependent neuronal excitability in mice and humans.

Rab GTPases: The principal players in crafting the regulatory landscape of endosomal trafficking

After endocytosis, diverse cargos are sorted into endosomes and directed to various destinations, including extracellular macromolecules, membrane lipids, and membrane proteins. Some cargos are returned to the plasma membrane via endocytic recycling. In contrast, others are delivered to the Golgi apparatus through the retrograde pathway, while the rest are transported to late endosomes and eventually to lysosomes for degradation. Rab GTPases are major regulators that ensure cargos are delivered to their proper destinations. Rabs are localized to distinct endosomes and play predominant roles in membrane budding, vesicle formation and motility, vesicle tethering, and vesicle fusion by recruiting effectors. The cascades between Rabs via shared effectors or the recruitment of Rab activators provide an additional layer of spatiotemporal regulation of endocytic trafficking. Notably, several recent studies have indicated that disorders of Rab-mediated endocytic transports are closely associated with diseases such as immunodeficiency, cancer, and neurological disorders.

Phosphoinositides as membrane organizers

Phosphoinositides are signalling lipids derived from phosphatidylinositol, a ubiquitous phospholipid in the cytoplasmic leaflet of eukaryotic membranes. Initially discovered for their roles in cell signalling, phosphoinositides are now widely recognized as key integrators of membrane dynamics that broadly impact on all aspects of cell physiology and on disease. The past decade has witnessed a vast expansion of our knowledge of phosphoinositide biology. On the endocytic and exocytic routes, phosphoinositides direct the inward and outward flow of membrane as vesicular traffic is coupled to the conversion of phosphoinositides. Moreover, recent findings on the roles of phosphoinositides in autophagy and the endolysosomal system challenge our view of lysosome biology. The non-vesicular exchange of lipids, ions and metabolites at membrane contact sites in between organelles has also been found to depend on phosphoinositides. Here we review our current understanding of how phosphoinositides shape and direct membrane dynamics to impact on cell physiology, and provide an overview of emerging concepts in phosphoinositide regulation.

Deliver on Time or Pay the Fine: Scheduling in Membrane Trafficking

Membrane trafficking is all about time. Automation in such a biological process is crucial to ensure management and delivery of cellular cargoes with spatiotemporal precision. Shared molecular regulators and differential engagement of trafficking components improve robustness of molecular sorting. Sequential recruitment of low affinity protein complexes ensures directionality of the process and, concomitantly, serves as a kinetic proofreading mechanism to discriminate cargoes from the whole endocytosed material. This strategy helps cells to minimize losses and operating errors in membrane trafficking, thereby matching the appealed deadline. Here, we summarize the molecular pathways of molecular sorting, focusing on their timing and efficacy. We also highlight experimental procedures and genetic approaches to robustly probe these pathways, in order to guide mechanistic studies at the interface between biochemistry and quantitative biology.

The Role of Phosphatidylinositol 3-Kinase Catalytic Subunit Type 3 in the Pathogenesis of Human Cancer

Phosphatidylinositol 3-kinase catalytic subunit type 3 (PIK3C3), the mammalian ortholog of yeast vesicular protein sorting 34 (Vps34), belongs to the phosphoinositide 3-kinase (PI3K) family. PIK3C3 can phosphorylate phosphatidylinositol (PtdIns) to generate phosphatidylinositol 3-phosphate (PI3P), a phospholipid central to autophagy. Inhibition of PIK3C3 successfully inhibits autophagy. Autophagy maintains cell survival when modifications occur in the cellular environment and helps tumor cells resist metabolic stress and cancer treatment. In addition, PIK3C3 could induce oncogenic transformation and enhance tumor cell proliferation, growth, and invasion through mechanisms independent of autophagy. This review addresses the structural and functional features, tissue distribution, and expression pattern of PIK3C3 in a variety of human tumors and highlights the underlying mechanisms involved in carcinogenesis. The implications in cancer biology, patient prognosis prediction, and cancer therapy are discussed. Altogether, the discovery of pharmacological inhibitors of PIK3C3 could reveal novel strategies for improving treatment outcomes for PIK3C3-mediated human diseases.

Phosphatidic acid-PKA signaling regulates p38 and ERK1/2 functions in ligand-independent EGFR endocytosis

Ligand-independent epidermal growth factor receptor (EGFR) endocytosis is inducible by a variety of stress conditions converging upon p38 kinase. A less known pathway involves phosphatidic acid (PA) signaling toward the activation of type 4 phosphodiesterases (PDE4) that decrease cAMP levels and protein kinase A (PKA) activity. This PA/PDE4/PKA pathway is triggered with propranolol used to inhibit PA hydrolysis and induces clathrin-dependent and clathrin-independent endocytosis, followed by reversible accumulation of EGFR in recycling endosomes. Here we give further evidence of this signaling pathway using biosensors of PA, cAMP, and PKA in live cells and then show that it activates p38 and ERK1/2 downstream the PKA inhibition. Clathrin-silencing and IN/SUR experiments involved the activity of p38 in the clathrin-dependent route, while ERK1/2 mediates clathrin-independent EGFR endocytosis. The PA/PDE4/PKA pathway selectively increases the EGFR endocytic rate without affecting LDLR and TfR constitute endocytosis. This selectiveness is probably because of EGFR phosphorylation, as detected in Th1046/1047 and Ser669 residues. The EGFR accumulates at perinuclear recycling endosomes colocalizing with TfR, fluorescent transferrin, and Rab11, while a small proportion distributes to Alix-endosomes. A non-selective recycling arrest includes LDLR and TfR in a reversible manner. The PA/PDE4/PKA pathway involving both p38 and ERK1/2 expands the possibilities of EGFR transmodulation and interference in cancer.

Endosomal Phosphatidylinositol-3-Phosphate-Associated Functions Are Dispensable for Establishment of the Cytomegalovirus Pre-Assembly Compartment but Essential for the Virus Growth

Murine cytomegalovirus (MCMV) initiates the stepwise establishment of the pre-assembly compartment (pre-AC) in the early phase of infection by the expansion of the early endosome (EE)/endosomal recycling compartment (ERC) interface and relocation of the Golgi complex. We depleted Vps34-derived phosphatidylinositol-3-phosphate (PI(3)P) at EEs by VPS34-IN1 and inhibited PI(3)P-associated functions by overexpression of 2xFYVE- and p40PX PI(3)P-binding modules to assess the role of PI(3)P-dependent EE domains in the pre-AC biogenesis. We monitored the accumulation of Rab10 and Evectin-2 in the inner pre-AC and the relocation of GM130-positive cis-Golgi organelles to the outer pre-AC by confocal microscopy. Although PI(3)P- and Vps34-positive endosomes build a substantial part of pre-AC, the PI(3)P depletion and the inhibition of PI(3)P-associated functions did not prevent the establishment of infection and progression through the early phase. The PI(3)P depletion in uninfected and MCMV-infected cells rapidly dispersed PI(3)P-bond proteins and reorganized EEs, including ablation of EE-to-ERC transport and relocation of Rab11 endosomes. The PI(3)P depletion one hour before pre-AC initiation and overexpression of 2xFYVE and p40PX domains neither prevented Rab10- and Evectin-2 accumulation, nor Golgi unlinking and relocation. These data demonstrate that PI(3)P-dependent functions, including the Rab11-dependent EE-to-ERC route, are dispensable for pre-AC initiation. Nevertheless, the virus growth was drastically reduced in PI(3)P-depleted cells, indicating that PI(3)P-associated functions are essential for the late phase of infection.

Endosomal recycling tubule scission and integrin recycling involve the membrane curvature-supporting protein LITAF

Recycling to the cell surface requires the scission of tubular membrane intermediates emanating from endosomes. Here, we identify the monotopic membrane protein LPS-induced TNF-activating factor (LITAF) and the related protein cell death involved p53 target 1 (CDIP1) as novel membrane curvature proteins that contribute to recycling tubule scission. Recombinant LITAF supports high membrane curvature, shown by its ability to reduce proteoliposome size. The membrane domains of LITAF and CDIP1 partition strongly into ∼50 nm diameter tubules labelled with the recycling markers Pacsin2, ARF6 and SNX1, and the recycling cargoes MHC class I and CD59. Partitioning of LITAF into tubules is impaired by mutations linked to Charcot Marie Tooth disease type 1C. Meanwhile, co-depletion of LITAF and CDIP1 results in the expansion of tubular recycling compartments and stabilised Rab11 tubules, pointing to a function for LITAF and CDIP1 in membrane scission. Consistent with this, co-depletion of LITAF and CDIP1 impairs integrin recycling and cell migration.

D-Propranolol Impairs EGFR Trafficking and Destabilizes Mutant p53 Counteracting AKT Signaling and Tumor Malignancy

Simple Summary

Cancer progression is frequently driven by altered functions of EGFR belonging to the tyrosine-kinase family of growth factor receptors and by the transcription factor p53, which is called the “genome guardian”. We report that D-Propranolol, previously used for other purposes in human patients, has antitumor effects involving a redistribution of cell surface EGFR to intracellular compartments and degradation of gain-of-function mutants of p53 (GOF-mutp53). These effects can be seen in cancer cell lines expressing EGFR and GOF-mutp53 and are reproduced in vivo, reducing tumor growth and prolonging survival of xenografted mice. D-Propranolol is proposed as a prototype drug for a new strategy against highly aggressive EGFR- and mutp53-expressing tumors.

Abstract

Cancer therapy may be improved by the simultaneous interference of two or more oncogenic pathways contributing to tumor progression and aggressiveness, such as EGFR and p53. Tumor cells expressing gain-of-function (GOF) mutants of p53 (mutp53) are usually resistant to EGFR inhibitors and display invasive migration and AKT-mediated survival associated with enhanced EGFR recycling. D-Propranolol (D-Prop), the non-beta blocker enantiomer of propranolol, was previously shown to induce EGFR internalization through a PKA inhibitory pathway that blocks the recycling of the receptor. Here, we first show that D-Prop decreases the levels of EGFR at the surface of GOF mutp53 cells, relocating the receptor towards recycling endosomes, both in the absence of ligand and during stimulation with high concentrations of EGF or TGF-α. D-Prop also inactivates AKT signaling and reduces the invasive migration and viability of these mutp53 cells. Unexpectedly, mutp53 protein, which is stabilized by interaction with the chaperone HSP90 and mediates cell oncogenic addiction, becomes destabilized after D-Prop treatment. HSP90 phosphorylation by PKA and its interaction with mutp53 are decreased by D-Prop, releasing mutp53 towards proteasomal degradation. Furthermore, a single daily dose of D-Prop reproduces most of these effects in xenografts of aggressive gallbladder cancerous G-415 cells expressing GOF R282W mutp53, resulting in reduced tumor growth and extended mice survival. D-Prop then emerges as an old drug endowed with a novel therapeutic potential against EGFR- and mutp53-driven tumor traits that are common to a large variety of cancers.

Targeting PI3K/AKT/mTOR Signaling Pathway in Breast Cancer

Breast cancer is the most frequently diagnosed cancer and the primary cause of cancer death in women worldwide. Although early diagnosis and cancer growth inhibition has significantly improved breast cancer survival rate over the years, there is a current need to develop more effective systemic treatments to prevent metastasis. One of the most commonly altered pathways driving breast cancer cell growth, survival, and motility is the PI3K/AKT/mTOR signaling cascade. In the past 30 years, a great surge of inhibitors targeting these key players has been developed at a rapid pace, leading to effective preclinical studies for cancer therapeutics. However, the central role of PI3K/AKT/mTOR signaling varies among diverse biological processes, suggesting the need for more specific and sophisticated strategies for their use in cancer therapy. In this review, we provide a perspective on the role of the PI3K signaling pathway and the most recently developed PI3K-targeting breast cancer therapies.

Physics of compartmentalization: How phase separation and signaling shape membrane and organelle identity

Compartmentalization of cellular functions is at the core of the physiology of eukaryotic cells. Recent evidences indicate that a universal organizing process - phase separation - supports the partitioning of biomolecules in distinct phases from a single homogeneous mixture, a landmark event in both the biogenesis and the maintenance of membrane and non-membrane-bound organelles. In the cell, 'passive' (non energy-consuming) mechanisms are flanked by 'active' mechanisms of separation into phases of distinct density and stoichiometry, that allow for increased partitioning flexibility and programmability. A convergence of physical and biological approaches is leading to new insights into the inner functioning of this driver of intracellular order, holding promises for future advances in both biological research and biotechnological applications.

PI3KC2β inactivation stabilizes VE-cadherin junctions and preserves vascular integrity

Endothelium protection is critical, because of the impact of vascular leakage and edema on pathological conditions such as brain ischemia. Whereas deficiency of class II phosphoinositide 3-kinase alpha (PI3KC2α) results in an increase in vascular permeability, we uncover a crucial role of the beta isoform (PI3KC2β) in the loss of endothelial barrier integrity following injury. Here, we studied the role of PI3KC2β in endothelial permeability and endosomal trafficking in vitro and in vivo in ischemic stroke. Mice with inactive PI3KC2β showed protection against vascular permeability, edema, cerebral infarction, and deleterious inflammatory response. Loss of PI3KC2β in human cerebral microvascular endothelial cells stabilized homotypic cell-cell junctions by increasing Rab11-dependent VE-cadherin recycling. These results identify PI3KC2β as a potential new therapeutic target to prevent aggravating lesions following ischemic stroke.

RBD11, a bioengineered Rab11-binding module for visualizing and analyzing endogenous Rab11

The small GTPase Rab11 (herein referring to the Rab11A and Rab11B isoforms) plays pivotal roles in diverse physiological phenomena, including the recycling of membrane proteins, cytokinesis, neurite outgrowth and epithelial morphogenesis. One effective method of analyzing the function of endogenous Rab11 is to overexpress a Rab11-binding domain from one of its effectors, for example, the C-terminal domain of Rab11-FIP2 (Rab11-FIP2-C), as a dominant-negative construct. However, the drawback of this method is the broader Rab-binding specificity of the effector domain, because Rab11-FIP2-C binds to Rabs other than Rab11, for example, to Rab14 and Rab25. In this study, we bioengineered an artificial Rab11-specific binding domain, named RBD11. Expression of RBD11 allowed visualization of endogenous Rab11 without affecting its localization or function, whereas expression of a tandem RBD11, named 2×RBD11, inhibited epithelial morphogenesis and induced a multi-lumen phenotype characteristic of Rab11-deficient cysts. We also developed two tools for temporally and reversibly analyzing Rab11-dependent membrane trafficking - tetracycline-inducible 2×RBD11 and an artificially oligomerized domain (FM)-tagged RBD11.

A conserved myotubularin-related phosphatase regulates autophagy by maintaining autophagic flux

Macroautophagy (autophagy) targets cytoplasmic cargoes to the lysosome for degradation. Like all vesicle trafficking, autophagy relies on phosphoinositide identity, concentration, and localization to execute multiple steps in this catabolic process. Here, we screen for phosphoinositide phosphatases that influence autophagy in Drosophila and identify CG3530. CG3530 is homologous to the human MTMR6 subfamily of myotubularin-related 3-phosphatases, and therefore, we named it dMtmr6. dMtmr6, which is required for development and viability in Drosophila, functions as a regulator of autophagic flux in multiple Drosophila cell types. The MTMR6 family member MTMR8 has a similar function in autophagy of higher animal cells. Decreased dMtmr6 and MTMR8 function results in autophagic vesicle accumulation and influences endolysosomal homeostasis.

Class II phosphatidylinositol 3-kinase isoforms in vesicular trafficking

Phosphatidylinositol 3-kinases (PI3Ks) are critical regulators of many cellular processes including cell survival, proliferation, migration, cytoskeletal reorganization, and intracellular vesicular trafficking. They are a family of lipid kinases that phosphorylate membrane phosphoinositide lipids at the 3′ position of their inositol rings, and in mammals they are divided into three classes. The role of the class III PI3K Vps34 is well-established, but recent evidence suggests the physiological significance of class II PI3K isoforms in vesicular trafficking. This review focuses on the recently discovered functions of the distinct PI3K-C2α and PI3K-C2β class II PI3K isoforms in clathrin-mediated endocytosis and consequent endosomal signaling, and discusses recently reported data on class II PI3K isoforms in different physiological contexts in comparison with class I and III isoforms.