Phosphoinositide metabolism during membrane ruffling and macropinosome formation in EGF-stimulated A431 cells

Endocytic highways: Navigating macropinocytosis and other endocytic routes for precision drug delivery

Drug molecules can reach intracellular targets by different mechanisms, such as passive diffusion, active transport, and endocytosis. Endocytosis is the process by which cells engulf extracellular material by forming a vesicle and transporting it into the cells. In addition to its biological functions, endocytosis plays a vital role in the internalization of the therapeutic molecules. Clathrin-mediated endocytosis, caveolar endocytosis, and macropinocytosis are the most researched routes in the field of drug delivery. In addition to conventional small therapeutic molecules, the use of nanoformulations and large molecules, such as nucleic acids, peptides, and antibodies, have broadened the field of drug delivery. Although the majority of small therapeutic molecules can enter cells via passive diffusion, large molecules, and advanced targeted delivery systems, such as nanoparticles, are internalized by the endocytic route. Therefore, it is imperative to understand the characteristics of the endocytic routes in greater detail to design therapeutic molecules or formulations for successful delivery to the intracellular targets. This review highlights the prospects and limitations of the major endocytic routes for drug delivery, with a major emphasis on macropinocytosis. Since macropinocytosis is a non-selective uptake of extracellular matrix, the selective induction of macropinocytosis, using compounds that induce macropinocytosis and modulate macropinosome trafficking pathways, could be a potential approach for the intracellular delivery of diverse therapeutic modalities. Furthermore, we have summarized the characteristics associated with the formulations or drug carriers that can affect the endocytic routes for cellular internalization. The techniques that are used to study the intracellular uptake processes of therapeutic molecules are briefly discussed. Finally, the major limitations for intracellular targeting, endo-lysosomal degradation, and different approaches that have been used in overcoming these limitations, are highlighted in this review.

Massive endocytosis mechanisms are involved in uptake of HIV-1 particles by monocyte-derived dendritic cells

Introduction

HIV-1 exploits dendritic cells (DCs) to spread throughout the body via specific recognition of gangliosides present on the viral envelope by the CD169/Siglec-1 membrane receptor. This interaction triggers the internalization of HIV-1 within a structure known as the sac-like compartment. While the mechanism underlying sac-like compartment formation remains elusive, prior research indicates that the process is clathrin-independent and cell membrane cholesterol-dependent and involves transient disruption of cortical actin. Here, we investigate the potential involvement of massive endocytosis (MEND) in this process.

Methods

We used live cell confocal imaging to measure the dimensions and dynamics of the compartment. We assessed the role of actin and cholesterol in fixed and live cells using confocal microscopy and evaluated the effect of PI3K and protein palmytoilation inhibitors during viral uptake.

Results

Our data demonstrate extensive plasma membrane invagination based on sac-like compartment dimensions (2.9 μm in diameter and 20 μm3 in volume). We showed that the cholesterol concentration doubles within the regions of viral uptake, suggesting lipid-phase separation, and that development of the sac-like compartment is accompanied by transient depolarization of cortical actin. Moreover, we observed that protein palmitoylation and PI3K inhibition reduce the sac-like compartment formation rate from 70% to 20% and 40%, respectively.

Conclusions

Our results indicate the involvement of MEND mechanisms during sac-like compartment formation.

CARMIL1-AA selectively inhibits macropinocytosis while sparing autophagy

Macropinocytosis is reported to fuel tumor growth and drug resistance by allowing cancer cells to scavenge extracellular macromolecules. However, accurately defining the role of macropinocytosis in cancer depends on our ability to selectively block this process. 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) is widely used to inhibit macropinocytosis but affects multiple Na+/H+ exchangers (NHE) that regulate cytoplasmic and organellar pH. Consistent with this, we report that EIPA slows proliferation to a greater extent than can be accounted for by macropinocytosis inhibition and triggers conjugation of ATG8 to single membranes (CASM). Knocking down only NHE1 would not avoid macropinocytosis-independent effects on pH. Moreover, contrary to published reports, NHE1 loss did not block macropinocytosis in multiple cell lines. Knocking down CARMIL1 with CRISPR-Cas9 editing limited macropinocytosis, but only by 50%. In contrast, expressing the CARMIL1-AA mutant inhibits macropinocytosis induced by a wide range of macropinocytic stimuli to a similar extent as EIPA. CARMIL1-AA expression did not inhibit proliferation, highlighting the shortcomings of EIPA as a macropinocytosis inhibitor. Importantly, autophagy, another actin dependent, nutrient-producing process, was not affected by CARMIL1-AA expression. In sum, constitutive or inducible CARMIL1-AA expression reduced macropinocytosis without affecting proliferation, RAC activation, or autophagy, other processes that drive tumor initiation and progression.

Live-cell imaging and CLEM reveal the existence of ACTN4-dependent ruffle-edge lamellipodia acting as a novel mode of cell migration

α-Actinin-4 (ACTN4) expression levels are correlated with the invasive and metastatic potential of cancer cells; however, the underlying mechanism remains unclear. Here, we identified ACTN4-localized ruffle-edge lamellipodia using live-cell imaging and correlative light and electron microscopy (CLEM). BSC-1 cells expressing EGFP-ACTN4 showed that ACTN4 was most abundant in the leading edges of lamellipodia, although it was also present in stress fibers and focal adhesions. ACTN4 localization in lamellipodia was markedly diminished by phosphoinositide 3-kinase inhibition, whereas its localization in stress fibers and focal adhesions remained. Furthermore, overexpression of ACTN4, but not ACTN1, promoted lamellipodial formation. Live-cell analysis demonstrated that ACTN4-enriched lamellipodia are highly dynamic and associated with cell migration. CLEM revealed that ACTN4-enriched lamellipodia exhibit a characteristic morphology of multilayered ruffle-edges that differs from canonical flat lamellipodia. Similar ruffle-edge lamellipodia were observed in A549 and MDA-MB-231 invasive cancer cells. ACTN4 knockdown suppressed the formation of ruffle-edge lamellipodia and cell migration during wound healing in A549 monolayer cultures. Additionally, membrane-type 1 matrix metalloproteinase was observed in the membrane ruffles, suggesting that ruffle-edge lamellipodia have the ability to degrade the extracellular matrix and may contribute to active cell migration/invasion in certain cancer cell types.

Recombinant biosensors for multiplex and super-resolution imaging of phosphoinositides

Phosphoinositides are a small family of phospholipids that act as signaling hubs and key regulators of cellular function. Detecting their subcellular distribution is crucial to gain insights into membrane organization and is commonly done by the overexpression of biosensors. However, this leads to cellular perturbations and is challenging in systems that cannot be transfected. Here, we present a toolkit for the reliable, fast, multiplex, and super-resolution detection of phosphoinositides in fixed cells and tissue, based on recombinant biosensors with self-labeling SNAP tags. These are highly specific and reliably visualize the subcellular distributions of phosphoinositides across scales, from 2D or 3D cell culture to Drosophila tissue. Further, these probes enable super-resolution approaches, and using STED microscopy, we reveal the nanoscale organization of PI(3)P on endosomes and PI(4)P on the Golgi. Finally, multiplex staining reveals an unexpected presence of PI(3,5)P2-positive membranes in swollen lysosomes following PIKfyve inhibition. This approach enables the versatile, high-resolution visualization of multiple phosphoinositide species in an unprecedented manner.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

An emerging master inducer and regulator for epithelial-mesenchymal transition and tumor metastasis: extracellular and intracellular ATP and its molecular functions and therapeutic potential

Despite the rapid development of therapeutic strategies in cancer treatment, metastasis remains the major cause of cancer-related death and scientific challenge. Epithelial-Mesenchymal Transition (EMT) plays a crucial role in cancer invasion and progression, a process by which tumor cells lose cell-cell adhesion and acquire increased invasiveness and metastatic activity. Recent work has uncovered some crucial roles of extracellular adenosine 5'- triphosphate (eATP), a major component of the tumor microenvironment (TME), in promoting tumor growth and metastasis. Intratumoral extracellular ATP (eATP), at levels of 100-700 µM, is 10³-10⁴ times higher than in normal tissues. In the current literature, eATP's function in promoting metastasis has been relatively poorly understood as compared with intracellular ATP (iATP). Recent evidence has shown that cancer cells internalize eATP via macropinocytosis in vitro and in vivo, promoting cell growth and survival, drug resistance, and metastasis. Furthermore, ATP acts as a messenger molecule that activates P2 purinergic receptors expressed on both tumor and host cells, stimulating downstream signaling pathways to enhance the invasive and metastatic properties of tumor cells. Here, we review recent progress in understanding eATP's role in each step of the metastatic cascade, including initiating invasion, inducing EMT, overcoming anoikis, facilitating intravasation, circulation, and extravasation, and eventually establishing metastatic colonization. Collectively, these studies reveal eATP's important functions in many steps of metastasis and identify new opportunities for developing more effective therapeutic strategies to target ATP-associated processes in cancer.

Vav independently regulates synaptic growth and plasticity through distinct actin-based processes

Modulation of presynaptic actin dynamics is fundamental to synaptic growth and functional plasticity; yet the underlying molecular and cellular mechanisms remain largely unknown. At Drosophila NMJs, the presynaptic Rac1-SCAR pathway mediates BMP-induced receptor macropinocytosis to inhibit BMP growth signaling. Here, we show that the Rho-type GEF Vav acts upstream of Rac1 to inhibit synaptic growth through macropinocytosis. We also present evidence that Vav-Rac1-SCAR signaling has additional roles in tetanus-induced synaptic plasticity. Presynaptic inactivation of Vav signaling pathway components, but not regulators of macropinocytosis, impairs post-tetanic potentiation (PTP) and enhances synaptic depression depending on external Ca²⁺ concentration. Interfering with the Vav-Rac1-SCAR pathway also impairs mobilization of reserve pool (RP) vesicles required for tetanus-induced synaptic plasticity. Finally, treatment with an F-actin–stabilizing drug completely restores RP mobilization and plasticity defects in Vav mutants. We propose that actin-regulatory Vav-Rac1-SCAR signaling independently regulates structural and functional presynaptic plasticity by driving macropinocytosis and RP mobilization, respectively.

The Role and Therapeutic Potential of Macropinocytosis in Cancer

Macropinocytosis, a unique endocytosis pathway characterized by nonspecific internalization, has a vital role in the uptake of extracellular substances and antigen presentation. It is known to have dual effects on cancer cells, depending on cancer type and certain microenvironmental conditions. It helps cancer cells survive in nutrient-deficient environments, enhances resistance to anticancer drugs, and promotes invasion and metastasis. Conversely, overexpression of the RAS gene alongside drug treatment can lead to methuosis, a novel mode of cell death. The survival and proliferation of cancer cells is closely related to macropinocytosis in the tumor microenvironment (TME), but identifying how these cells interface with the TME is crucial for creating drugs that can limit cancer progression and metastasis. Substantial progress has been made in recent years on designing anticancer therapies that utilize the effects of macropinocytosis. Both the induction and inhibition of macropinocytosis are useful strategies for combating cancer cells. This article systematically reviews the general mechanisms of macropinocytosis, its specific functions in tumor cells, its occurrence in nontumor cells in the TME, and its application in tumor therapies. The aim is to elucidate the role and therapeutic potential of macropinocytosis in cancer treatment.

Targeting Membrane Trafficking as a Strategy for Cancer Treatment

Membrane trafficking is emerging as an attractive therapeutic strategy for cancer. Recent reports have found a connection between Wnt signaling, receptor-mediated endocytosis, V-ATPase, lysosomal activity, and macropinocytosis through the canonical Wnt pathway. In macropinocytic cells, a massive internalization of the plasma membrane can lead to the loss of cell-surface cadherins, integrins, and other antigens that mediate cell–cell adhesion, favoring an invasive phenotype. V-ATPase is a key regulator in maintaining proper membrane trafficking, homeostasis, and the earliest developmental decisions in the Xenopus vertebrate development model system. Here, we review how the interference of membrane trafficking with membrane trafficking inhibitors might be clinically relevant in humans.

Macropinocytosis and Cancer: From Tumor Stress to Signaling Pathways

Macropinocytosis is an evolutionarily conserved endocytic pathway that mediates the nonselective acquisition of extracellular material via large endocytic vesicles known as macropinosomes. In addition to other functions, this uptake pathway supports cancer cell metabolism through the uptake of nutrients. Cells harboring oncogene or tumor suppressor mutations are known to display heightened macropinocytosis, which confers to the cancer cells the ability to survive and proliferate despite the nutrient-scarce conditions of the tumor microenvironment. Thus, macropinocytosis is associated with cancer malignancy. Macropinocytic uptake can be induced in cancer cells by different stress stimuli, acting as an adaptive mechanism for the cells to resist stresses in the tumor milieu. Here, we review the cellular stresses that are known to promote macropinocytosis, as well as the underlying molecular mechanisms that drive this process.

Revealing macropinocytosis using nanoparticles

Endocytosis mechanisms are one of the methods that cells use to interact with their environments. Endocytosis mechanisms vary from the clathrin-mediated endocytosis to the receptor independent macropinocytosis. Macropinocytosis is a niche of endocytosis that is quickly becoming more relevant in various fields of research since its discovery in the 1930s. Macropinocytosis has several distinguishing factors from other receptor-mediated forms of endocytosis, including: types of extracellular material for uptake, signaling cascade, and niche uses between cell types. Nanoparticles (NPs) are an important tool for various applications, including drug delivery and disease treatment. However, surface engineering of NPs could be tailored to target them inside the cells exploiting different endocytosis pathways, such as endocytosis versus macropinocytosis. Such surface engineering of NPs mainly, size, charge, shape and the core material will allow identification of new adapter molecules regulating different endocytosis process and provide further insight into how cells tweak these pathways to meet their physiological need. In this review, we focus on the description of macropinocytosis, a lesser studied endocytosis mechanism than the conventional receptor mediated endocytosis. Additionally, we will discuss nanoparticle endocytosis (including macropinocytosis), and how the physio-chemical properties of the NP (size, charge, and surface coating) affect their intracellular uptake and exploiting them as tools to identify new adapter molecules regulating these processes.

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.

Roles for 3' Phosphoinositides in Macropinocytosis

The distinct movements of macropinosome formation and maturation have corresponding biochemical activities which occur in a defined sequence of stages and transitions between those stages. Each stage in the process is regulated by variously phosphorylated derivatives of phosphatidylinositol (PtdIns) which reside in the cytoplasmic face of the membrane lipid bilayer. PtdIns derivatives phosphorylated at the 3' position of the inositol moiety, called 3' phosphoinositides (3'PIs), regulate different stages of the sequence. 3'PIs are synthesized by numerous phosphoinositide 3'-kinases (PI3K) and other lipid kinases and phosphatases, which are themselves regulated by small GTPases of the Ras superfamily. The combined actions of these enzymes localize four principal species of 3'PI to distinct domains of the plasma membrane or to discrete organelles, with distinct biochemical activities confined to those domains. Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃) and phosphatidylinositol (3,4)-bisphosphate (PtdIns(3,4)P₂) regulate the early stages of macropinosome formation, which include cell surface ruffling and constrictions of circular ruffles which close into macropinosomes. Phosphatidylinositol 3-phosphate (PtdIns3P) regulates macropinosome fusion with other macropinosomes and early endocytic organelles. Phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P₂) mediates macropinosome maturation and shrinkage, through loss of ions and water, and subsequent traffic to lysosomes. The different characteristic rates of macropinocytosis in different cell types indicate levels of regulation which may be governed by the cell's capacity to generate 3'PIs.

Entry of the Varicellovirus Canid herpesvirus 1 into Madin-Darby canine kidney epithelial cells is pH-independent and occurs via a macropinocytosis-like mechanism but without increase in fluid uptake

Canid herpesvirus 1 (CHV-1) is a Varicellovirus that causes self-limiting infections in adult dogs but morbidity and mortality in puppies. Using a multipronged approach, we discovered the CHV-1 entry pathway into Madin-Darby canine kidney (MDCK) epithelial cells. We found that CHV-1 triggered extensive host cell membrane lamellipodial ruffling and rapid internalisation of virions in large, uncoated vacuoles, suggestive of macropinocytosis. Treatment with inhibitors targeting key macropinocytosis factors, including inhibitors of Na⁺ /H⁺ exchangers, F-actin, myosin light-chain kinase, protein kinase C, p21-activated kinase, phosphatidylinositol-3-kinase and focal adhesion kinase, significantly reduced viral replication. Moreover, the effect was restricted to exposure to the inhibitors early in infection, confirming a role for the macropinocytic machinery during entry. The profile of inhibitors also suggested a role for signalling via integrins and receptor tyrosine kinases in viral entry. In contrast, inhibitors of clathrin, caveolin, microtubules and endosomal acidification did not affect CHV-1 entry into MDCK cells. We found that the virus colocalised with the fluid-phase uptake marker dextran; however, surprisingly, CHV-1 infection did not enhance the uptake of dextran. Thus, our results indicate that CHV-1 uses a macropinocytosis-like, pH-independent entry pathway into MDCK cells, which nevertheless is not based on stimulation of fluid uptake. TAKE AWAYS: CHV-1 enters epithelial cells via a macropinocytosis-like mechanism. CHV-1 induces extensive lamellipodial ruffling. CHV-1 entry into MDCK cells is pH-independent.

CYRI-A limits invasive migration through macropinosome formation and integrin uptake regulation

The Scar/WAVE complex drives actin nucleation during cell migration. Interestingly, the same complex is important in forming membrane ruffles during macropinocytosis, a process mediating nutrient uptake and membrane receptor trafficking. Mammalian CYRI-B is a recently described negative regulator of the Scar/WAVE complex by RAC1 sequestration, but its other paralogue, CYRI-A, has not been characterized. Here, we implicate CYRI-A as a key regulator of macropinosome formation and integrin internalization. We find that CYRI-A is transiently recruited to nascent macropinosomes, dependent on PI3K and RAC1 activity. CYRI-A recruitment precedes RAB5A recruitment but follows sharply after RAC1 and actin signaling, consistent with it being a local inhibitor of actin polymerization. Depletion of both CYRI-A and -B results in enhanced surface expression of the α5β1 integrin via reduced internalization. CYRI depletion enhanced migration, invasion, and anchorage-independent growth in 3D. Thus, CYRI-A is a dynamic regulator of macropinocytosis, functioning together with CYRI-B to regulate integrin trafficking.

The structural dynamics of macropinosome formation and PI3-kinase-mediated sealing revealed by lattice light sheet microscopy

Macropinosomes are formed by shaping actin-rich plasma membrane ruffles into large intracellular organelles in a phosphatidylinositol 3-kinase (PI3K)-coordinated manner. Here, we utilize lattice lightsheet microscopy and image visualization methods to map the three-dimensional structure and dynamics of macropinosome formation relative to PI3K activity. We show that multiple ruffling morphologies produce macropinosomes and that the majority form through collisions of adjacent PI3K-rich ruffles. By combining multiple volumetric representations of the plasma membrane structure and PI3K products, we show that PI3K activity begins early throughout the entire ruffle volume and continues to increase until peak activity concentrates at the base of the ruffle after the macropinosome closes. Additionally, areas of the plasma membrane rich in ruffling had increased PI3K activity and produced many macropinosomes of various sizes. Pharmacologic inhibition of PI3K activity had little effect on the rate and morphology of membrane ruffling, demonstrating that early production of 3'-phosphoinositides within ruffles plays a minor role in regulating their morphology. However, 3'-phosphoinositides are critical for the fusogenic activity that seals ruffles into macropinosomes. Taken together, these data indicate that local PI3K activity is amplified in ruffles and serves as a priming mechanism for closure and sealing of ruffles into macropinosomes.

The structural dynamics of macropinosome formation and PI3-kinase-mediated sealing revealed by lattice light sheet microscopy

Macropinosomes are formed by shaping actin-rich plasma membrane ruffles into large intracellular organelles in a phosphatidylinositol 3-kinase (PI3K)-coordinated manner. Here, we utilize lattice lightsheet microscopy and image visualization methods to map the three-dimensional structure and dynamics of macropinosome formation relative to PI3K activity. We show that multiple ruffling morphologies produce macropinosomes and that the majority form through collisions of adjacent PI3K-rich ruffles. By combining multiple volumetric representations of the plasma membrane structure and PI3K products, we show that PI3K activity begins early throughout the entire ruffle volume and continues to increase until peak activity concentrates at the base of the ruffle after the macropinosome closes. Additionally, areas of the plasma membrane rich in ruffling had increased PI3K activity and produced many macropinosomes of various sizes. Pharmacologic inhibition of PI3K activity had little effect on the rate and morphology of membrane ruffling, demonstrating that early production of 3'-phosphoinositides within ruffles plays a minor role in regulating their morphology. However, 3'-phosphoinositides are critical for the fusogenic activity that seals ruffles into macropinosomes. Taken together, these data indicate that local PI3K activity is amplified in ruffles and serves as a priming mechanism for closure and sealing of ruffles into macropinosomes.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

The role of endolysosomal trafficking in anticancer drug resistance

Multidrug resistance (MDR) remains a major obstacle towards curative treatment of cancer. Despite considerable progress in delineating the basis of intrinsic and acquired MDR, the underlying molecular mechanisms remain to be elucidated. Emerging evidences suggest that dysregulation in endolysosomal compartments is involved in mediating MDR through multiple mechanisms, such as alterations in endosomes, lysosomes and autophagosomes, that traffic and biodegrade the molecular cargo through macropinocytosis, autophagy and endocytosis. For example, altered lysosomal pH, in combination with transcription factor EB (TFEB)-mediated lysosomal biogenesis, increases the sequestration of hydrophobic anti-cancer drugs that are weak bases, thereby producing an insufficient and off-target accumulation of anti-cancer drugs in MDR cancer cells. Thus, the use of well-tolerated, alkalinizing compounds that selectively block Vacuolar H⁺-ATPase (V-ATPase) may be an important strategy to overcome MDR in cancer cells and increase chemotherapeutic efficacy. Other mechanisms of endolysosomal-mediated drug resistance include increases in the expression of lysosomal proteases and cathepsins that are involved in mediating carcinogenesis and chemoresistance. Therefore, blocking the trafficking and maturation of lysosomal proteases or direct inhibition of cathepsin activity in the cytosol may represent novel therapeutic modalities to overcome MDR. Furthermore, endolysosomal compartments involved in catabolic pathways, such as macropinocytosis and autophagy, are also shown to be involved in the development of MDR. Here, we review the role of endolysosomal trafficking in MDR development and discuss how targeting endolysosomal pathways could emerge as a new therapeutic strategy to overcome chemoresistance in cancer.

Rab10-Positive Tubular Structures Represent a Novel Endocytic Pathway That Diverges From Canonical Macropinocytosis in RAW264 Macrophages

Using the optogenetic photo-manipulation of photoactivatable (PA)-Rac1, remarkable cell surface ruffling and the formation of a macropinocytic cup (premacropinosome) could be induced in the region of RAW264 macrophages irradiated with blue light due to the activation of PA-Rac1. However, the completion of macropinosome formation did not occur until Rac1 was deactivated by the removal of the light stimulus. Following PA-Rac1 deactivation, some premacropinosomes closed into intracellular macropinosomes, whereas many others transformed into long Rab10-positive tubules without forming typical macropinosomes. These Rab10-positive tubules moved centripetally towards the perinuclear Golgi region along microtubules. Surprisingly, these Rab10-positive tubules did not contain any endosome/lysosome compartment markers, such as Rab5, Rab7, or LAMP1, suggesting that the Rab10-positive tubules were not part of the degradation pathway for lysosomes. These Rab10-positive tubules were distinct from recycling endosomal compartments, which are labeled with Rab4, Rab11, or SNX1. These findings suggested that these Rab10-positive tubules may be a part of non-degradative endocytic pathway that has never been known. The formation of Rab10-positive tubules from premacropinosomes was also observed in control and phorbol myristate acetate (PMA)-stimulated macrophages, although their frequencies were low. Interestingly, the formation of Rab10-positive premacropinosomes and tubules was not inhibited by phosphoinositide 3-kinase (PI3K) inhibitors, while the classical macropinosome formation requires PI3K activity. Thus, this study provides evidence to support the existence of Rab10-positive tubules as a novel endocytic pathway that diverges from canonical macropinocytosis.

Rab5 regulates macropinocytosis by recruiting the inositol 5-phosphatases OCRL and Inpp5b that hydrolyse PtdIns(4,5)P2

Rab5 is required for macropinosome formation, but its site and mode of action remain unknown. We report that Rab5 acts at the plasma membrane, downstream of ruffling, to promote macropinosome sealing and scission. Dominant-negative Rab5, which obliterates macropinocytosis, had no effect on the development of membrane ruffles. However, Rab5-containing vesicles were recruited to circular membrane ruffles, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent endomembrane fusion was necessary for the completion of macropinocytosis. This fusion event coincided with the disappearance of PtdIns(4,5)P2 that accompanies macropinosome closure. Counteracting the depletion of PtdIns(4,5)P2 by expression of phosphatidylinositol-4-phosphate 5-kinase impaired macropinosome formation. Importantly, we found that the removal of PtdIns(4,5)P2 is dependent on Rab5, through the Rab5-mediated recruitment of the inositol 5-phosphatases OCRL and Inpp5b, via APPL1. Knockdown of OCRL and Inpp5b, or APPL1, prevented macropinosome closure without affecting ruffling. We therefore propose that Rab5 is essential for the clearance of PtdIns(4,5)P2 needed to complete the scission of macropinosomes or to prevent their back-fusion with the plasmalemma.

Phosphatidic acid-mediated binding and mammalian cell internalization of the Vibrio cholerae cytotoxin MakA

Vibrio cholerae is a noninvasive intestinal pathogen extensively studied as the causative agent of the human disease cholera. Our recent work identified MakA as a potent virulence factor of V. cholerae in both Caenorhabditis elegans and zebrafish, prompting us to investigate the potential contribution of MakA to pathogenesis also in mammalian hosts. In this study, we demonstrate that the MakA protein could induce autophagy and cytotoxicity of target cells. In addition, we observed that phosphatidic acid (PA)-mediated MakA-binding to the host cell plasma membranes promoted macropinocytosis resulting in the formation of an endomembrane-rich aggregate and vacuolation in intoxicated cells that lead to induction of autophagy and dysfunction of intracellular organelles. Moreover, we functionally characterized the molecular basis of the MakA interaction with PA and identified that the N-terminal domain of MakA is required for its binding to PA and thereby for cell toxicity. Furthermore, we observed that the ΔmakA mutant outcompeted the wild-type V. cholerae strain A1552 in the adult mouse infection model. Based on the findings revealing mechanistic insights into the dynamic process of MakA-induced autophagy and cytotoxicity we discuss the potential role played by the MakA protein during late stages of cholera infection as an anti-colonization factor.

Vibrio cholerae is the cause of cholera, an infectious disease causing watery diarrhea that can lead to fatal dehydration. The bacteria can readily adapt to different environments, such as from its natural aquatic habitats to the human digestive system. Recently, we reported a novel V. cholerae cytotoxin, MakA that functions as a potent virulence factor in C. elegans and zebrafish. Here we identified phosphatidic acid as a lipid target for MakA interaction with mammalian cells. This interaction promoted macropinocytosis resulting in the formation of an endomembrane-rich aggregate in intoxicated cells that ultimately lead to activation of autophagy. Importantly, data from bacterial colonization in a mouse infection model suggested that MakA might act as an anti-colonization factor of V. cholerae, presumably expressed during later stage(s) of infection. MakA might be explored as a new target for diagnostics and therapeutic developments against V. cholerae infections. Our findings will contribute to further understanding of the virulence, colonization and post-infection spread of V. cholerae.

Phosphoinositides signaling modulates microglial actin remodeling and phagocytosis in Alzheimer's disease

Alzheimer's disease is one of the neurodegenerative diseases, characterized by the accumulation of abnormal protein deposits, which disrupts signal transduction in neurons and other glia cells. The pathological protein in neurodegenerative diseases, Tau and amyloid-β contribute to the disrupted microglial signaling pathways, actin cytoskeleton, and cellular receptor expression. The important secondary messenger lipids i.e., phosphatidylinositols are largely affected by protein deposits of amyloid-β in Alzheimer's disease. Phosphatidylinositols are the product of different phosphatidylinositol kinases and the state of phosphorylation at D3, D4, and D5 positions of inositol ring. Phosphatidylinositol 3,4,5-triphosphate (PI 3, 4, 5-P3) involves in phagocytic cup formation, cell polarization, whereas Phosphatidylinositol 4,5-bisphosphate (PI 4, 5-P2)-mediates the process of phagosomes formation and further its fusion with early endosome.. The necessary activation of actin-binding proteins such as Rac, WAVE complex, and ARP2/3 complex for the actin polymerization in the process of phagocytosis, migration is regulated and maintained by PI 3, 4, 5-P3 and PI 4, 5-P2. The ratio and types of fatty acid intake can influence the intracellular secondary lipid messengers along with the cellular content of phaphatidylcholine and phosphatidylethanolamine. The Amyloid-β deposits and extracellular Tau seeds disrupt phosphatidylinositides level and actin cytoskeletal network that hamper microglial-signaling pathways in AD. We hypothesize that being a lipid species intracellular levels of phosphatidylinositol would be regulated by dietary fatty acids. Further we are interested to understand phosphoinositide-based signaling cascades in phagocytosis and actin remodeling. Video Abstract.

The Dual Role of Macropinocytosis in Cancers: Promoting Growth and Inducing Methuosis to Participate in Anticancer Therapies as Targets

Macropinocytosis is an important mechanism of internalizing extracellular materials and dissolved molecules in eukaryotic cells. Macropinocytosis has a dual effect on cancer cells. On the one hand, cells expressing RAS genes (such as K-RAS, H-RAS) under the stress of nutrient deficiency can spontaneously produce constitutive macropinocytosis to promote the growth of cancer cells by internalization of extracellular nutrients (like proteins), receptors, and extracellular vesicles(EVs). On the other hand, abnormal expression of RAS genes and drug treatment (such as MOMIPP) can induce a novel cell death associated with hyperactivated macropinocytosis: methuosis. Based on the dual effect, there is immense potential for designing anticancer therapies that target macropinocytosis in cancer cells. In view of the fact that there has been little review of the dual effect of macropinocytosis in cancer cells, herein, we systematically review the general process of macropinocytosis, its specific manifestation in cancer cells, and its application in cancer treatment, including anticancer drug delivery and destruction of macropinocytosis. This review aims to serve as a reference for studying macropinocytosis in cancers and designing macropinocytosis-targeting anticancer drugs in the future.

Membrane Heterogeneity Controls Cellular Endocytic Trafficking

Endocytic trafficking relies on highly localized events in cell membranes. Endocytosis involves the gathering of protein (cargo/receptor) at distinct plasma membrane locations defined by specific lipid and protein compositions. Simultaneously, the molecular machinery that drives invagination and eventually scission of the endocytic vesicle assembles at the very same place on the inner leaflet of the membrane. It is membrane heterogeneity - the existence of specific lipid and protein domains in localized regions of membranes - that creates the distinct molecular identity required for an endocytic event to occur precisely when and where it is required rather than at some random location within the plasma membrane. Accumulating evidence leads us to believe that the trafficking fate of internalized proteins is sealed following endocytosis, as this distinct membrane identity is preserved through the endocytic pathway, upon fusion of endocytic vesicles with early and sorting endosomes. In fact, just like at the plasma membrane, multiple domains coexist at the surface of these endosomes, regulating local membrane tubulation, fission and sorting to recycling pathways or to the trans-Golgi network via late endosomes. From here, membrane heterogeneity ensures that fusion events between intracellular vesicles and larger compartments are spatially regulated to promote the transport of cargoes to their intracellular destination.

Coordinated Ras and Rac Activity Shapes Macropinocytic Cups and Enables Phagocytosis of Geometrically Diverse Bacteria

Engulfment of extracellular material by phagocytosis or macropinocytosis depends on the ability of cells to generate specialized cup-shaped protrusions. To effectively capture and internalize their targets, these cups are organized into a ring or ruffle of actin-driven protrusion encircling a non-protrusive interior domain. These functional domains depend on the combined activities of multiple Ras and Rho family small GTPases, but how their activities are integrated and differentially regulated over space and time is unknown. Here, we show that the amoeba Dictyostelium discoideum coordinates Ras and Rac activity using the multidomain protein RGBARG (RCC1, RhoGEF, BAR, and RasGAP-containing protein). We find RGBARG uses a tripartite mechanism of Ras, Rac, and phospholipid interactions to localize at the protruding edge and interface with the interior of both macropinocytic and phagocytic cups. There, we propose RGBARG shapes the protrusion by expanding Rac activation at the rim while suppressing expansion of the active Ras interior domain. Consequently, cells lacking RGBARG form enlarged, flat interior domains unable to generate large macropinosomes. During phagocytosis, we find that disruption of RGBARG causes a geometry-specific defect in engulfing rod-shaped bacteria and ellipsoidal beads. This demonstrates the importance of coordinating small GTPase activities during engulfment of more complex shapes and thus the full physiological range of microbes, and how this is achieved in a model professional phagocyte.

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We identify a new regulator that shapes macropinocytic and phagocytic cups

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Shaping protrusions into cups requires differential regulation of Ras and Rac

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Cups are organized by integrating interactions with phospholipids and multiple GTPases

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Defective cup formation causes a target shape-specific defect in phagocytosis

Endocytosis and the internalization of pathogenic organisms: focus on phosphoinositides

Despite their comparatively low abundance in biological membranes, phosphoinositides are key to the regulation of a diverse array of signaling pathways and direct membrane traffic. The role of phosphoinositides in the initiation and progression of endocytic pathways has been studied in considerable depth. Recent advances have revealed that distinct phosphoinositide species feature prominently in clathrin-dependent and -independent endocytosis as well as in phagocytosis and macropinocytosis. Moreover, a variety of intracellular and cell-associated pathogens have developed strategies to commandeer host cell phosphoinositide metabolism to gain entry and/or metabolic advantage, thereby promoting their survival and proliferation. Here, we briefly survey the current knowledge on the involvement of phosphoinositides in endocytosis, phagocytosis, and macropinocytosis and highlight several examples of molecular mimicry employed by pathogens to either “hitch a ride” on endocytic pathways endogenous to the host or create an entry path of their own.

Macropinosomes as units of signal transduction

Macropinosome formation occurs as a localized sequence of biochemical activities and associated morphological changes, which may be considered a form of signal transduction leading to the construction of an organelle. Macropinocytosis may also convey information about the availability of extracellular nutrients to intracellular regulators of metabolism. Consistent with this idea, activation of the metabolic regulator mechanistic target of rapamycin complex-1 (mTORC1) in response to acute stimulation by growth factors and extracellular amino acids requires internalization of amino acids by macropinocytosis. This suggests that macropinocytosis is necessary for mTORC1-dependent growth of metazoan cells, both as a route for delivery of amino acids to sensors associated with lysosomes and as a platform for growth factor-dependent signalling to mTORC1 via phosphatidylinositol 3-kinase (PI3K) and the Akt pathway. Because the biochemical signals required for the construction of macropinosomes are also required for cell growth, and inhibition of macropinocytosis inhibits growth factor signalling to mTORC1, we propose that signalling by growth factor receptors is organized into stochastic, structure-dependent cascades of chemical reactions that both build a macropinosome and stimulate mTORC1. More generally, as discrete units of signal transduction, macropinosomes may be subject to feedback regulation by metabolism and cell dimensions. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Constitutive and stimulated macropinocytosis in macrophages: roles in immunity and in the pathogenesis of atherosclerosis

Macrophages respond to several stimuli by forming florid membrane ruffles that lead to fluid uptake by macropinocytosis. This type of induced macropinocytosis, executed by a variety of non-malignant and malignant cells, is initiated by transmembrane receptors and is involved in nutrient acquisition and mTOR signalling. However, macrophages also perform a unique type of constitutive ruffling and macropinocytosis that is dependent on the presence of extracellular calcium. Calcium-sensing receptors are responsible for this activity. This distinct form of macropinocytosis enables macrophages to continuously sample their microenvironment for antigenic molecules and for pathogen- and danger-associated molecular patterns, as part of their immune surveillance functions. Interestingly, even within the monocyte lineage, there are differences in macropinocytic ability that reflect the polarized functional roles of distinct macrophage subsets. This review discusses the shared and distinct features of both induced and constitutive macropinocytosis displayed by the macrophage lineage and their roles in physiology, immunity and pathophysiology. In particular, we analyse the role of macropinocytosis in the uptake of modified low-density lipoprotein (LDL) and its contribution to foam cell and atherosclerotic plaque formation. We propose a combined role of scavenger receptors and constitutive macropinocytosis in oxidized LDL uptake, a process we have termed 'receptor-assisted macropinocytosis'. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

PI3Kβ is selectively required for growth factor-stimulated macropinocytosis

Macropinocytosis is an actin-dependent but clathrin-independent endocytic process by which cells nonselectively take up large aliquots of extracellular material. Macropinocytosis is used for immune surveillance by dendritic cells, as a route of infection by viruses and protozoa, and as a nutrient uptake pathway in tumor cells. In this study, we explore the role of class I phosphoinositide 3-kinases (PI3Ks) during ligand-stimulated macropinocytosis. We find that macropinocytosis in response to receptor tyrosine kinase activation is strikingly dependent on a single class I PI3K isoform, namely PI3Kβ (containing the p110β catalytic subunit encoded by PIK3CB). Loss of PI3Kβ expression or activity blocks macropinocytosis at early steps, before the formation of circular dorsal ruffles, but also plays a role in later steps, downstream from Rac1 activation. PI3Kβ is also required for the elevated levels of constitutive macropinocytosis found in tumor cells that are defective for the PTEN tumor suppressor. Our data shed new light on PI3K signaling during macropinocytosis, and suggest new therapeutic uses for pharmacological inhibitors of PI3Kβ.

On the existence of endocytosis driven by membrane phase separations

Large endocytic responses can occur rapidly in diverse cell types without dynamins, clathrin, or actin remodeling. Our experiments suggest that membrane phase separations are crucial with more ordered plasma membrane domains being internalized. Not only do these endocytic processes rely on coalescence of membrane domains, they are promoted by participation of membrane proteins in such domains, one important regulatory influence being palmitoylation. Membrane actin cytoskeleton in general resists membrane phase transitions, and its remodeling may play many roles. Besides membrane 'caging' and 'pinching' roles, typically ascribed to clathrin and dynamins, cytoskeleton remodeling may modify local membrane tension and buckling, as well as the presence and location of actin- and tension-free membrane patches. Endocytosis that depends on membrane phase separations becomes activated in metabolic stress and in response to Ca and PI3 kinase signaling. Internalized membrane traffics normally, and the secretory pathway eventually resupplies membrane to the plasmalemma or directs internalized membrane to other locations, including the extracellular space as exosomes. We describe here that endocytosis driven by membrane phase transitions is regulated by the same signaling mechanisms that regulate macropinocytosis, and it may play diverse roles in cells from nutrient assimilation to membrane recycling, cell migration, and the initiation of quiescent or hibernating cell states. Membrane ordering and phase separations have been shown to promote endocytosis in diverse cell types, including fibroblasts, myocytes, glial cells, and immune cells. We propose that clathrin/dynamin-independent endocytosis represents a continuum of related mechanisms with variable but universal dependence on membrane ordering and actin remodeling. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Akt-ing Up Just About Everywhere: Compartment-Specific Akt Activation and Function in Receptor Tyrosine Kinase Signaling

The serine/threonine kinase Akt is a master regulator of many diverse cellular functions, including survival, growth, metabolism, migration, and differentiation. Receptor tyrosine kinases are critical regulators of Akt, as a result of activation of phosphatidylinositol-3-kinase (PI3K) signaling leading to Akt activation upon receptor stimulation. The signaling axis formed by receptor tyrosine kinases, PI3K and Akt, as well as the vast range of downstream substrates is thus central to control of cell physiology in many different contexts and tissues. This axis must be tightly regulated, as disruption of PI3K-Akt signaling underlies the pathology of many diseases such as cancer and diabetes. This sophisticated regulation of PI3K-Akt signaling is due in part to the spatial and temporal compartmentalization of Akt activation and function, including in specific nanoscale domains of the plasma membrane as well as in specific intracellular membrane compartments. Here, we review the evidence for localized activation of PI3K-Akt signaling by receptor tyrosine kinases in various specific cellular compartments, as well as that of compartment-specific functions of Akt leading to control of several fundamental cellular processes. This spatial and temporal control of Akt activation and function occurs by a large number of parallel molecular mechanisms that are central to regulation of cell physiology.

BMP-dependent synaptic development requires Abi-Abl-Rac signaling of BMP receptor macropinocytosis

Retrograde BMP trans-synaptic signaling is essential for synaptic development. Despite the importance of endocytosis-regulated BMP receptor (BMPR) control of this developmental signaling, the mechanism remains unknown. Here, we provide evidence that Abelson interactor (Abi), a substrate for Abl kinase and component of the SCAR/WAVE complex, links Abl and Rac1 GTPase signaling to BMPR macropinocytosis to restrain BMP-mediated synaptic development. We find that Abi acts downstream of Abl and Rac1, and that BMP ligand Glass bottom boat (Gbb) induces macropinocytosis dependent on Rac1/SCAR signaling, Abl-mediated Abi phosphorylation, and BMPR activation. Macropinocytosis acts as the major internalization route for BMPRs at the synapse in a process driven by Gbb activation and resulting in receptor degradation. Key regulators of macropinocytosis (Rabankyrin and CtBP) control BMPR trafficking to limit BMP trans-synaptic signaling. We conclude that BMP-induced macropinocytosis acts as a BMPR homeostatic mechanism to regulate BMP-mediated synaptic development.

BMP ligands act as retrograde signalling molecules to regulate presynaptic development, and regulation of BMP receptors by endocytosis may be an important component of this signalling pathway. Here, the authors show that Abi-mediated macropinocytosis of BMP receptors in Drosophila larva and contributes to neuromuscular development.

The origins and evolution of macropinocytosis

In macropinocytosis, cells take up micrometre-sized droplets of medium into internal vesicles. These vesicles are acidified and fused to lysosomes, their contents digested and useful compounds extracted. Indigestible contents can be exocytosed. Macropinocytosis has been known for approaching 100 years and is described in both metazoa and amoebae, but not in plants or fungi. Its evolutionary origin goes back to at least the common ancestor of the amoebozoa and opisthokonts, with apparent secondary loss from fungi. The primary function of macropinocytosis in amoebae and some cancer cells is feeding, but the conserved processing pathway for macropinosomes, which involves shrinkage and the retrieval of membrane to the cell surface, has been adapted in immune cells for antigen presentation. Macropinocytic cups are large actin-driven processes, closely related to phagocytic cups and pseudopods and appear to be organized around a conserved signalling patch of PIP3, active Ras and active Rac that directs actin polymerization to its periphery. Patches can form spontaneously and must be sustained by excitable kinetics with strong cooperation from the actin cytoskeleton. Growth-factor signalling shares core components with macropinocytosis, based around phosphatidylinositol 3-kinase (PI3-kinase), and we suggest that it evolved to take control of ancient feeding structures through a coupled growth factor receptor. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Function of small GTPases in Dictyostelium macropinocytosis

Macropinocytosis-the large-scale, non-specific uptake of fluid by cells-is used by Dictyostelium discoideum amoebae to obtain nutrients. These cells form circular ruffles around regions of membrane defined by a patch of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and the activated forms of the small G-proteins Ras and Rac. When this ruffle closes, a vesicle of the medium is delivered to the cell interior for further processing. It is accepted that PIP3 is required for efficient macropinocytosis. Here, we assess the roles of Ras and Rac in Dictyostelium macropinocytosis. Gain-of-function experiments show that macropinocytosis is stimulated by persistent Ras activation and genetic analysis suggests that RasG and RasS are the key Ras proteins involved. Among the activating guanine exchange factors (GEFs), GefF is implicated in macropinocytosis by an insertional mutant. The individual roles of Rho family proteins are little understood but activation of at least some may be independent of PIP3. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Akt and SGK protein kinases are required for efficient feeding by macropinocytosis

Macropinocytosis is an actin-driven process of large-scale and non-specific fluid uptake used for feeding by some cancer cells and the macropinocytosis model organism Dictyostelium discoideum In Dictyostelium, macropinocytic cups are organized by 'macropinocytic patches' in the plasma membrane. These contain activated Ras, Rac and phospholipid PIP3, and direct actin polymerization to their periphery. We show that a Dictyostelium Akt (PkbA) and an SGK (PkbR1) protein kinase act downstream of PIP3 and, together, are nearly essential for fluid uptake. This pathway enables the formation of larger macropinocytic patches and macropinosomes, thereby dramatically increasing fluid uptake. Through phosphoproteomics, we identify a RhoGAP, GacG, as a PkbA and PkbR1 target, and show that it is required for efficient macropinocytosis and expansion of macropinocytic patches. The function of Akt and SGK in cell feeding through control of macropinosome size has implications for cancer cell biology.

Glutathione-Responsive Prodrug Nanoparticles for Effective Drug Delivery and Cancer Therapy

Spurred by recent progress in medicinal chemistry, numerous lead compounds have sprung up in the past few years, although the majority are hindered by hydrophobicity, which greatly challenges druggability. In an effort to assess the potential of platinum (Pt) candidates, the nanosizing approach to alter the pharmacology of hydrophobic Pt(IV) prodrugs in discovery and development settings is described. The construction of a self-assembled nanoparticle (NP) platform, composed of amphiphilic lipid-polyethylene glycol (PEG) for effective delivery of Pt(IV) prodrugs capable of resisting thiol-mediated detoxification through a glutathione (GSH)-exhausting effect, offers a promising route to synergistically improving safety and efficacy. After a systematic screening, the optimized NPs (referred to as P6 NPs) exhibited small particle size (99.3 nm), high Pt loading (11.24%), reliable dynamic stability (∼7 days), and rapid redox-triggered release (∼80% in 3 days). Subsequent experiments on cells support the emergence of P6 NPs as a highly effective means of transporting a lethal dose of cargo across cytomembranes through macropinocytosis. Upon reduction by cytoplasmic reductants, particularly GSH, P6 NPs under disintegration released sufficient active Pt(II) metabolites, which covalently bound to target DNA and induced significant apoptosis. The PEGylation endowed P6 NPs with in vivo longevity and tumor specificity, which were essential to successfully inhibiting the growth of cisplatin-sensitive and -resistant xenograft tumors, while effectively alleviating toxic side-effects associated with cisplatin. P6 NPs are, therefore, promising for overcoming the bottleneck in the development of Pt drugs for oncotherapy.

Dorsal ruffles enhance activation of Akt by growth factors

In fibroblasts, platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) stimulate the formation of actin-rich, circular dorsal ruffles (CDRs) and phosphatidylinositol 3-kinase (PI3K)-dependent phosphorylation of Akt. To test the hypothesis that CDRs increase synthesis of phosphorylated Akt1 (pAkt), we analyzed the contributions of CDRs to Akt phosphorylation in response to PDGF and EGF. CDRs appeared within several minutes of growth factor addition, coincident with a peak of pAkt. Microtubule depolymerization with nocodazole blocked CDR formation and inhibited phosphorylation of Akt in response to EGF but not PDGF. Quantitative immunofluorescence showed increased concentrations of Akt, pAkt and phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), the phosphoinositide product of PI3K that activates Akt, concentrated in CDRs and ruffles. EGF stimulated lower maximal levels of pAkt than did PDGF, which suggests that Akt phosphorylation requires amplification in CDRs only when PI3K activities are low. Accordingly, stimulation with low concentrations of PDGF elicited lower levels of Akt phosphorylation, which, like responses to EGF, were inhibited by nocodazole. These results indicate that when receptor signaling generates low levels of PI3K activity, CDRs facilitate local amplification of PI3K and phosphorylation of Akt.This article has an associated First Person interview with the first author of the paper.

High-throughput Measurement of Dictyostelium discoideum Macropinocytosis by Flow Cytometry

Large-scale non-specific fluid uptake by macropinocytosis is important for the proliferation of certain cancer cells, antigen sampling, host cell invasion and the spread of neurodegenerative diseases. The commonly used laboratory strains of the amoeba Dictyostelium discoideum have extremely high fluid uptake rates when grown in nutrient medium, over 90% of which is due to macropinocytosis. In addition, many of the known core components of mammalian macropinocytosis are also present, making it an excellent model system for studying macropinocytosis. Here, the standard technique to measure internalized fluid using fluorescent dextran as a label is adapted to a 96-well plate format, with the samples analyzed by flow cytometry using a high-throughput sampling (HTS) attachment. Cells are fed non-quenchable fluorescent dextran for a pre-determined length of time, washed by immersion in ice-cold buffer and detached using 5 mM sodium azide, which also stops exocytosis. Cells in each well are then analyzed by flow cytometry. The method can also be adapted to measure membrane uptake and phagocytosis of fluorescent beads or bacteria. This method was designed to allow measurement of fluid uptake by Dictyostelium in a high-throughput, labor and resource efficient manner. It allows simultaneous comparison of multiple strains (e.g. knockout mutants of a gene) and conditions (e.g. cells in different media or treated with different concentrations of inhibitor) in parallel and simplifies time-courses.

Merlin/ERM proteins regulate growth factor-induced macropinocytosis and receptor recycling by organizing the plasma membrane:cytoskeleton interface

The architectural and biochemical features of the plasma membrane are governed by its intimate association with the underlying cortical cytoskeleton. The neurofibromatosis type 2 (NF2) tumor suppressor merlin and closely related membrane:cytoskeleton-linking protein ezrin organize the membrane:cytoskeleton interface, a critical cellular compartment that both regulates and is regulated by growth factor receptors. An example of this poorly understood interrelationship is macropinocytosis, an ancient process of nutrient uptake and membrane remodeling that can both be triggered by growth factors and manage receptor availability. We show that merlin deficiency primes the membrane:cytoskeleton interface for epidermal growth factor (EGF)-induced macropinocytosis via a mechanism involving increased cortical ezrin, altered actomyosin, and stabilized cholesterol-rich membranes. These changes profoundly alter EGF receptor (EGFR) trafficking in merlin-deficient cells, favoring increased membrane levels of its heterodimerization partner, ErbB2; clathrin-independent internalization; and recycling. Our work suggests that, unlike Ras transformed cells, merlin-deficient cells do not depend on macropinocytic protein scavenging and instead exploit macropinocytosis for receptor recycling. Finally, we provide evidence that the macropinocytic proficiency of NF2-deficient cells can be used for therapeutic uptake. This work provides new insight into fundamental mechanisms of macropinocytic uptake and processing and suggests new ways to interfere with or exploit macropinocytosis in NF2 mutant and other tumors.

A Novel Method to Image Macropinocytosis in Vivo

Here we described an experimental protocol for in vivo imaging of macropinocytosis and subsequent intracellular events. By microinjection, we delivered fluorescence dextrans together with or without ATPγS into transparent Drosophila melanogaster embryos. Using a confocal microscope for live imaging, we monitored the generation of dextran-positive macropinosomes and subsequent intracellular events. Our protocol provides a continent and reliable way for investigating macropinocytosis and its underlying mechanisms, especially when combined with genetic strategies.

Macropinocytosis, mTORC1 and cellular growth control

The growth and proliferation of metazoan cells are driven by cellular nutrient status and by extracellular growth factors. Growth factor receptors on cell surfaces initiate biochemical signals that increase anabolic metabolism and macropinocytosis, an actin-dependent endocytic process in which relatively large volumes of extracellular solutes and nutrients are internalized and delivered efficiently into lysosomes. Macropinocytosis is prominent in many kinds of cancer cells, and supports the growth of cells transformed by oncogenic K-Ras. Growth factor receptor signaling and the overall metabolic status of the cell are coordinated in the cytoplasm by the mechanistic target-of-rapamycin complex-1 (mTORC1), which positively regulates protein synthesis and negatively regulates molecular salvage pathways such as autophagy. mTORC1 is activated by two distinct Ras-related small GTPases, Rag and Rheb, which associate with lysosomal membranes inside the cell. Rag recruits mTORC1 to the lysosomal surface where Rheb directly binds to and activates mTORC1. Rag is activated by both lysosomal luminal and cytosolic amino acids; Rheb activation requires phosphoinositide 3-kinase, Akt, and the tuberous sclerosis complex-1/2. Signals for activation of Rag and Rheb converge at the lysosomal membrane, and several lines of evidence support the idea that growth factor-dependent endocytosis facilitates amino acid transfer into the lysosome leading to the activation of Rag. This review summarizes evidence that growth factor-stimulated macropinocytosis is essential for amino acid-dependent activation of mTORC1, and that increased solute accumulation by macropinocytosis in transformed cells supports unchecked cell growth.

Inositol, neural tube closure and the prevention of neural tube defects

Susceptibility to neural tube defects (NTDs), such as anencephaly and spina bifida is influenced by genetic and environmental factors including maternal nutrition. Maternal periconceptional supplementation with folic acid significantly reduces the risk of an NTD-affected pregnancy, but does not prevent all NTDs, and "folic acid non-responsive" NTDs continue to occur. Similarly, among mouse models of NTDs, some are responsive to folic acid but others are not. Among nutritional factors, inositol deficiency causes cranial NTDs in mice while supplemental inositol prevents spinal and cranial NTDs in the curly tail (Grhl3 hypomorph) mouse, rodent models of hyperglycemia or induced diabetes, and in a folate-deficiency induced NTD model. NTDs also occur in mice lacking expression of certain inositol kinases. Inositol-containing phospholipids (phosphoinositides) and soluble inositol phosphates mediate a range of functions, including intracellular signaling, interaction with cytoskeletal proteins, and regulation of membrane identity in trafficking and cell division. Myo-inositol has been trialed in humans for a range of conditions and appears safe for use in human pregnancy. In pilot studies in Italy and the United Kingdom, women took inositol together with folic acid preconceptionally, after one or more previous NTD-affected pregnancies. In nonrandomized cohorts and a randomized double-blind study in the United Kingdom, no recurrent NTDs were observed among 52 pregnancies reported to date. Larger-scale fully powered trials are needed to determine whether supplementation with inositol and folic acid would more effectively prevent NTDs than folic acid alone. Birth Defects Research 109:68-80, 2017. © 2016 The Authors Birth Defects Research Published by Wiley Periodicals, Inc.

Macropinocytosis: A Metabolic Adaptation to Nutrient Stress in Cancer

Oncogenic mutations, such as Ras mutations, drive not only enhanced proliferation but also the metabolic adaptations that confer to cancer cells the ability to sustain cell growth in a harsh tumor microenvironment. These adaptations might represent metabolic vulnerabilities that can be exploited to develop novel and more efficient cancer therapies. Macropinocytosis is an evolutionarily conserved endocytic pathway that permits the internalization of extracellular fluid via large endocytic vesicles known as macropinosomes. Recently, macropinocytosis has been determined to function as a nutrient-scavenging pathway in Ras-driven cancer cells. Macropinocytic uptake of extracellular proteins, and their further degradation within endolysosomes, provides the much-needed amino acids that fuel cancer cell metabolism and tumor growth. Here, we review the molecular mechanisms that govern the process of macropinocytosis, as well as discuss recent work that provides evidence of the important role of macropinocytosis as a nutrient supply pathway in cancer cells.

A plasma membrane template for macropinocytic cups

Macropinocytosis is a fundamental mechanism that allows cells to take up extracellular liquid into large vesicles. It critically depends on the formation of a ring of protrusive actin beneath the plasma membrane, which develops into the macropinocytic cup. We show that macropinocytic cups in Dictyostelium are organised around coincident intense patches of PIP3, active Ras and active Rac. These signalling patches are invariably associated with a ring of active SCAR/WAVE at their periphery, as are all examined structures based on PIP3 patches, including phagocytic cups and basal waves. Patch formation does not depend on the enclosing F-actin ring, and patches become enlarged when the RasGAP NF1 is mutated, showing that Ras plays an instructive role. New macropinocytic cups predominantly form by splitting from existing ones. We propose that cup-shaped plasma membrane structures form from self-organizing patches of active Ras/PIP3, which recruit a ring of actin nucleators to their periphery.

Genetic impairment of parasite myosin motors uncovers the contribution of host cell membrane dynamics to Toxoplasma invasion forces

Background

The several-micrometer-sized Toxoplasma gondii protozoan parasite invades virtually any type of nucleated cell from a warm-blooded animal within seconds. Toxoplasma initiates the formation of a tight ring-like junction bridging its apical pole with the host cell membrane. The parasite then actively moves through the junction into a host cell plasma membrane invagination that delineates a nascent vacuole. Recent high resolution imaging and kinematics analysis showed that the host cell cortical actin dynamics occurs at the site of entry while gene silencing approaches allowed motor-deficient parasites to be generated, and suggested that the host cell could contribute energetically to invasion. In this study we further investigate this possibility by analyzing the behavior of parasites genetically impaired in different motor components, and discuss how the uncovered mechanisms illuminate our current understanding of the invasion process by motor-competent parasites.

Results

By simultaneously tracking host cell membrane and cortex dynamics at the site of interaction with myosin A-deficient Toxoplasma, the junction assembly step could be decoupled from the engagement of the Toxoplasma invasive force. Kinematics combined with functional analysis revealed that myosin A-deficient Toxoplasma had a distinct host cell-dependent mode of entry when compared to wild-type or myosin B/C-deficient Toxoplasma. Following the junction assembly step, the host cell formed actin-driven membrane protrusions that surrounded the myosin A-deficient mutant and drove it through the junction into a typical vacuole. However, this parasite-entry mode appeared suboptimal, with about 40 % abortive events for which the host cell membrane expansions failed to cover the parasite body and instead could apply deleterious compressive forces on the apical pole of the zoite.

Conclusions

This study not only clarifies the key contribution of T. gondii tachyzoite myosin A to the invasive force, but it also highlights a new mode of entry for intracellular microbes that shares early features of macropinocytosis. Given the harmful potential of the host cell compressive forces, we propose to consider host cell invasion by zoites as a balanced combination between host cell membrane dynamics and the Toxoplasma motor function. In this light, evolutionary shaping of myosin A with fast motor activity could have contributed to optimize the invasive potential of Toxoplasma tachyzoites and thereby their fitness.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0316-8) contains supplementary material, which is available to authorized users.

CXCL12-induced macropinocytosis modulates two distinct pathways to activate mTORC1 in macrophages

Although growth factors and chemokines elicit different overall effects on cells-growth and chemotaxis, respectively-and activate distinct classes of cell-surface receptors, nonetheless, they trigger similar cellular activities and signaling pathways. The growth factor M-CSF and the chemokine CXCL12 both stimulate the endocytic process of macropinocytosis, and both activate the mechanistic target of rapamycin complex 1 (mTORC1), a protein complex that regulates cell metabolism. Recent studies of signaling by M-CSF in macrophages identified a role for macropinocytosis in the activation of mTORC1, in which delivery of extracellular amino acids into lysosomes via macropinocytosis was required for activation of mTORC1. Here, we analyzed the regulation of macropinosome (MP) formation in response to CXCL12 and identified 2 roles for macropinocytosis in the activation of mTORC1. Within 5 min of adding CXCL12, murine macrophages increased ruffling, macropinocytosis and amino acid-dependent activation of mTORC1. Inhibitors of macropinocytosis blocked activation of mTORC1, and various isoform-specific inhibitors of type 1 PI3K and protein kinase C (PKC) showed similar patterns of inhibition of macropinocytosis and mTORC1 activity. However, unlike the response to M-CSF, Akt phosphorylation (pAkt) in response to CXCL12 required the actin cytoskeleton and the formation of macropinocytic cups. Quantitative fluorescence microscopy showed that phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), a product of PI3K and an upstream activator of Akt, localized to macropinocytic cups and that pAkt occurred primarily in cups. These results indicate that CXCL12 activates mTORC1 via 2 mechanisms: 1) that the macropinocytic cup localizes Akt signaling and 2) that MPs convey extracellular nutrients to lysosomes.