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

Macropinocytic cups function as signal platforms for the mTORC2-AKT pathway to modulate LPS-induced cytokine expression in macrophages

Macropinocytosis is a large-scale endocytosis process primarily observed in phagocytes as part of their cellular function to ingest antigens. Once phagocytes encounter gram-negative bacteria, the receptor proteins identify lipopolysaccharides (LPSs), which trigger radical membrane ruffles that gradually change to cup-like structures. The open area of the cups closes to generate vesicles called macropinosomes. The target bacteria are isolated by the cups and engulfed by the cells as the cups close. In addition to its ingestion function, macropinocytosis also regulates the AKT pathway in macrophages. In the current study, we report that macropinocytic cups are critical for LPS-induced AKT phosphorylation (pAKT) and cytokine expression in macrophages. High-resolution scanning electron microscope observations detailed the macropinocytic cup structures induced by LPS stimulation. Confocal microscopy revealed that AKT and the kinase molecule mTORC2 were localized in the cups. The biochemical analysis showed that macropinocytosis inhibition blocked LPS-induced pAKT. RNA sequencing, quantitative polymerase chain reaction, and enzyme-linked immunosorbent assay analyses revealed that the inhibition of macropinocytosis or the AKT pathway causes a decrease in the expression of proinflammatory cytokines interlukin-6 and interlukin-1α. Moreover, activation of the transcription factor nuclear factor κB, which regulates the cytokine expression downstream of the AKT/IκB pathway, was hindered when macropinocytosis or AKT was inhibited. These results indicate that LPS-induced macropinocytic cups function as signal platforms for the AKT pathway to regulate the cytokine expression by modulating nuclear factor κB activity in LPS-stimulated macrophages. Based on these findings, we propose that macropinocytosis may be a good therapeutic target for controlling cytokine expression.

Mannose receptor (MRC1) mediates uptake of dextran in macrophages via receptor-mediated endocytosis

Macrophages maintain surveillance of their environment using receptor-mediated endocytosis and pinocytosis. Receptor-mediated endocytosis allows macrophages to recognize and internalize specific ligands whereas macropinocytosis non-selectively internalizes extracellular fluids and solutes. Here, CRISPR/Cas9 whole-genome screens were used to identify genes regulating constitutive and growth factor-stimulated dextran uptake in murine bone-marrow derived macrophages (BMDM). The endocytic mannose receptor c-type 1 ( Mrc1 , also known as CD206) was a top hit in the screen. Targeted gene disruptions of Mrc1 reduced dextran uptake but had little effect on uptake of Lucifer yellow, a fluid-phase marker. Other screen hits also differentially affected the uptake of dextran and Lucifer yellow, indicating the solutes are internalized by different mechanisms. We further deduced that BMDMs take up dextran via MRC1-mediated endocytosis by showing that competition with mannan, a ligand of MRC1, as well as treatment with Dyngo-4a, a dynamin inhibitor, reduced dextran uptake. Finally, we observed that IL4-treated BMDM internalize more dextran than untreated BMDM by upregulating MRC1 expression. These results demonstrate that dextran is not an effective marker for the bulk uptake of fluids and solutes by macropinocytosis since it is internalized by both macropinocytosis and receptor-mediated endocytosis in cells expressing MRC1. This report identifies numerous genes that regulate dextran internalization in primary murine macrophages and predicts cellular pathways and processes regulating MRC1. This work lays the groundwork for identifying specific genes and regulatory networks that regulate MRC1 expression and MRC1-mediated endocytosis in macrophages.

Significance Statement

Macrophages constantly survey and clear tissues by specifically and non-specifically internalizing debris and solutes. However, the molecular mechanisms and modes of regulation of these endocytic and macropinocytic processes are not well understood. Here, CRISPR/Cas9 whole genome screens were used to identify genes regulating uptake of dextran, a sugar polymer that is frequently used as a marker macropinocytosis, and compared with Lucifer yellow, a fluorescent dye with no known receptors. The authors identified the mannose receptor as well as other proteins regulating expression of the mannose receptor as top hits in the screen. Targeted disruption of Mrc1 , the gene that encodes mannose receptor, greatly diminished dextran uptake but had no effect on cellular uptake of Lucifer yellow. Furthermore, exposure to the cytokine IL4 upregulated mannose receptor expression on the cell surface and increased uptake of dextran with little effect on Lucifer yellow uptake. Studies seeking to understand regulation of macropinocytosis in macrophages will be confounded by the use of dextran as a fluid-phase marker. MRC1 is a marker of alternatively activated/anti-inflammatory macrophages and is a potential target for delivery of therapeutics to macrophages. This work provides the basis for mechanistic underpinning of how MRC1 contributes to the receptor-mediated uptake of carbohydrates and glycoproteins from the tissue milieu and distinguishes genes regulating receptor-mediated endocytosis from those regulating the bona fide fluid-phase uptake of fluids and solutes by macropinocytosis.

SP-R210 isoforms of Myosin18A modulate endosomal sorting and recognition of influenza A virus infection in macrophages

Influenza A virus (IAV) infection causes acute and often lethal inflammation in the lung. The role of macrophages in this adverse inflammation is partially understood. The surfactant protein A receptor 210 (SP-R210) consists of two isoforms, a long (L) SP-R210L and a short (S) SP-R210S isoform encoded by alternative splicing of the myosin 18A gene. We reported that disruption of SP-R210L enhances cytosolic and endosomal antiviral response pathways. Here, we report that SP-R210L antagonizes type I interferon β (IFNβ), as depletion of SP-R210L potentiates IFNβ secretion. SP-R210 antibodies enhance and attenuate IFNβ secretion in SP-R210L replete and deficient macrophages, respectively, indicating that SP-R210 isoform stoichiometry alters macrophage function intrinsically. This reciprocal response is coupled to unopposed and restricted expression of viral genes in control and SP-R210L-deficient macrophages, respectively. Human monocytic cells with sub-stoichiometric expression of SP-R210L resist IAV infection, whereas alveolar macrophages with increased abundance of SP-R210L permit viral gene expression similar to murine macrophages. Uptake and membrane binding studies show that lack of SP-R210 isoforms does not impair IAV binding and internalization. Lack of SP-R210L, however, results in macropinocytic retention of the virus that depends on both SP-R210S and interferon-inducible transmembrane protein-3 (IFITM3). Mass spectrometry and Western blot analyses indicate that SP-R210 isoforms modulate differential recruitment of the Rho-family GTPase RAC1 and guanine nucleotide exchange factors. Our study suggests that SP-R210 isoforms modulate RAC-dependent macropinosomal sorting of IAV to discrete endosomal and lysosomal compartments that either permit or prevent endolysosomal escape and inflammatory sensing of viral genomes in macrophages.

Identification of circular dorsal ruffles as signal platforms for the AKT pathway in glomerular podocytes

Circular dorsal ruffles (CDRs) are rounded membrane ruffles induced by growth factors to function as precursors of the large-scale endocytosis called macropinocytosis. In addition to their role in cellular uptake, recent research using cell line systems has shown that CDRs/macropinocytosis regulate the canonical AKT-mTORC1 growth factor signaling pathway. However, as CDRs have not been observed in tissues, their physiological relevance has remained unclear. Here, utilizing ultrahigh-resolution scanning electron microscopy, we first report that CDRs are expressed in glomerular podocytes ex vivo and in vivo, and we visually captured the transformation process to macropinocytosis. Moreover, through biochemical and imaging analyses, we show that AKT phosphorylation localized to CDRs upstream of mTORC1 activation in podocyte cell lines and isolated glomeruli. These results demonstrate the physiological role of CDRs as signal platforms for the AKT-mTORC1 pathway in glomerular podocytes at the tissue level. As mTORC1 plays critical roles in podocyte metabolism, and aberrant activation of mTORC1 triggers podocytopathies, our results strongly suggest that targeting CDR formation could represent a potential therapeutic approach for these diseases.

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.

Chaperone-assisted selective autophagy targets filovirus VP40 as a client and restricts egress of virus particles

The filovirus VP40 protein directs virion egress, which is regulated either positively or negatively by select VP40-host interactions. We demonstrate that host BAG3 and HSP70 recognize VP40 as a client and inhibit the egress of VP40 virus-like particles (VLPs) by promoting degradation of VP40 via Chaperone-assisted selective autophagy (CASA). Pharmacological inhibition of either the early stage formation of the VP40/BAG3/HSP70 tripartite complex, or late stage formation of autolysosomes, rescued VP40 VLP egress back to WT levels. The mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of autophagy, and we found that surface expression of EBOV GP on either VLPs or an infectious VSV recombinant virus, activated mTORC1. Notably, pharmacological suppression of mTORC1 signaling by rapamycin activated CASA in a BAG3-dependent manner to restrict the egress of both VLPs and infectious EBOV in Huh7 cells. In sum, our findings highlight the involvement of the mTORC1/CASA axis in regulating filovirus egress.

ATM inhibition drives metabolic adaptation via induction of macropinocytosis

Macropinocytosis is a nonspecific endocytic process that may enhance cancer cell survival under nutrient-poor conditions. Ataxia-Telangiectasia mutated (ATM) is a tumor suppressor that has been previously shown to play a role in cellular metabolic reprogramming. We report that the suppression of ATM increases macropinocytosis to promote cancer cell survival in nutrient-poor conditions. Combined inhibition of ATM and macropinocytosis suppressed proliferation and induced cell death both in vitro and in vivo. Supplementation of ATM-inhibited cells with amino acids, branched-chain amino acids (BCAAs) in particular, abrogated macropinocytosis. Analysis of ATM-inhibited cells in vitro demonstrated increased BCAA uptake, and metabolomics of ascites and interstitial fluid from tumors indicated decreased BCAAs in the microenvironment of ATM-inhibited tumors. These data reveal a novel basis of ATM-mediated tumor suppression whereby loss of ATM stimulates protumorigenic uptake of nutrients in part via macropinocytosis to promote cancer cell survival and reveal a potential metabolic vulnerability of ATM-inhibited cells.

Piezo1 activation using Yoda1 inhibits macropinocytosis in A431 human epidermoid carcinoma cells

Macropinocytosis is a type of endocytosis accompanied by actin rearrangement-driven membrane deformation, such as lamellipodia formation and membrane ruffling, followed by the formation of large vesicles, macropinosomes. Ras-transformed cancer cells efficiently acquire exogenous amino acids for their survival through macropinocytosis. Thus, inhibition of macropinocytosis is a promising strategy for cancer therapy. To date, few specific agents that inhibit macropinocytosis have been developed. Here, focusing on the mechanosensitive ion channel Piezo1, we found that Yoda1, a Piezo1 agonist, potently inhibits macropinocytosis induced by epidermal growth factor (EGF). The inhibition of ruffle formation by Yoda1 was dependent on the extracellular Ca²⁺ influx through Piezo1 and on the activation of the calcium-activated potassium channel KCa3.1. This suggests that Ca²⁺ ions can regulate EGF-stimulated macropinocytosis. We propose the potential for macropinocytosis inhibition through the regulation of a mechanosensitive channel activity using chemical tools.

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.

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.

Macropinocytosis: Biology and mechanisms

Macropinocytosis is a form of endocytosis performed by ruffles and cups of the plasma membrane. These close to entrap droplets of medium into micron-sized vesicles, which are trafficked through the endocytic system, their contents digested and useful products absorbed. Macropinocytosis is constitutive in certain immune cells and stimulated in many other cells by growth factors. It occurs across the animal kingdom and in amoebae, implying a deep evolutionary history. Its scientific history goes back 100 years, but increasingly work is focused on its medical importance in the immune system, cancer cell feeding, and as a backdoor into cells for viruses and drugs. Macropinocytosis is driven by the actin cytoskeleton whose dynamics can be appreciated with lattice light sheet microscopy: this reveals a surprising variety of routes for forming macropinosomes. In Dictyostelium amoebae, macropinocytic cups are organized around domains of PIP3 and active Ras and Rac in the plasma membrane. These attract activators of the Arp2/3 complex to their periphery, creating rings of actin polymerization that shape the cups. The size of PIP3 domains is controlled by RasGAPs, such as NF1, and the lipid phosphatase, PTEN. It is likely that domain dynamics determine the shape, evolution and closing of macropinocytic structures.

Dark-Side of Exosomes

Exosomes are nanoscale extracellular vesicles that can transport cargos of proteins, lipids, DNA, various RNA species and microRNAs (miRNAs). Exosomes can enter cells and deliver their contents to recipient cell. Owing to their cargo exosomes can transfer different molecules to the target cells and change the phenotype of these cells. The fate of the contents of an exosome depends on its target destination. Various mechanisms for exosome uptake by target cells have been proposed, but the mechanisms responsible for exosomes internalization into cells are still debated. Exosomes exposed cells produce labeled protein kinases, which are expressed by other cells. This means that these kinases are internalized by exosomes, and transported into the cytoplasm of recipient cells. Many studies have confirmed that exosomes are not only secreted by living cells, but also internalized or accumulated by the other cells. The "next cell hypothesis" supports the notion that exosomes constitute communication vehicles between neighboring cells. By this mechanism, exosomes participate in the development of diabetes and its associated complications, critically contribute to the spreading of neuronal damage in Alzheimer's disease, and non-proteolysed form of Fas ligand (mFasL)-bearing exosomes trigger the apoptosis of T lymphocytes. Furthermore, exosomes derived from human B lymphocytes induce antigen-specific major histocompatibility complex (MHC) class II-restricted T cell responses. Interestingly, exosomes secreted by cancer cells have been demonstrated to express tumor antigens, as well as immune suppressive molecules. This process is defined as "exosome-immune suppression" concept. The interplay via the exchange of exosomes between cancer cells and between cancer cells and the tumor stroma promote the transfer of oncogenes and onco-miRNAs from one cell to other. Circulating exosomes that are released from hypertrophic adipocytes are effective in obesity-related complications. On the other hand, the "inflammasome-induced" exosomes can activate inflammatory responses in recipient cells. In this chapter protein kinases-related checkpoints are emphasized considering the regulation of exosome biogenesis, secretory traffic, and their impacts on cell death, tumor growth, immune system, and obesity.

LRRK2 and Rab10 coordinate macropinocytosis to mediate immunological responses in phagocytes

Genetic variation in LRRK2 associates with the susceptibility to Parkinson's disease, Crohn's disease, and mycobacteria infection. High expression of LRRK2 and its substrate Rab10 occurs in phagocytic cells in the immune system. In mouse and human primary macrophages, dendritic cells, and microglia-like cells, we find that Rab10 specifically regulates a specialized form of endocytosis known as macropinocytosis, without affecting phagocytosis or clathrin-mediated endocytosis. LRRK2 phosphorylates cytoplasmic PI(3,4,5)P3-positive GTP-Rab10, before EEA1 and Rab5 recruitment to early macropinosomes occurs. Macropinosome cargo in macrophages includes CCR5, CD11b, and MHCII, and LRRK2-phosphorylation of Rab10 potently blocks EHBP1L1-mediated recycling tubules and cargo turnover. EHBP1L1 overexpression competitively inhibits LRRK2-phosphorylation of Rab10, mimicking the effects of LRRK2 kinase inhibition in promoting cargo recycling. Both Rab10 knockdown and LRRK2 kinase inhibition potently suppress the maturation of macropinosome-derived CCR5-loaded signaling endosomes that are critical for CCL5-induced immunological responses that include Akt activation and chemotaxis. These data support a novel signaling axis in the endolysosomal system whereby LRRK2-mediated Rab10 phosphorylation stalls vesicle fast recycling to promote PI3K-Akt immunological responses.

Macropinocytosis: Insights from immunology and cancer

Macropinocytosis is increasingly recognized for its versatile adaptations and functions as a highly conserved, ubiquitous pathway for the bulk uptake of fluid, particulate cargo, and membranes. Innate immune cells and transformed cancer cells share the capacity for both constitutive and induced macropinocytosis, which is used for immune surveillance, ingestion of pathogens, immune response shaping, and enhancement of scavenging for nutrients as fuel for cell survival and proliferation. Immunology and cancer biology are leading a resurgence of interest in defining the molecular and physiological regulation of macropinocytosis, partly in pursuit of ways to control macropinocytic uptake in disease settings. New approaches, including high-resolution live imaging, screening of cell surface molecular inventories, biophysics, and exploration of cell microenvironments, have converged to provide new insights into macropinosome induction, formation, and maturation. Recent studies reveal mechanisms for fluid control in and by macrophage macropinosomes that impinge on membrane trafficking and cell migration. EGFR, PTEN, V-ATPase, syndecan 1, and galectin-3 have roles variably in the metabolic regulation of Ras or PI3K signaling for Rac1-mediated macropinocytosis in cancer. These molecular pathways and mechanisms contribute to the impressive adaptability of macropinocytosis in many cells and tissues and in disease.

STIM1 controls calcineurin/Akt/mTOR/NFATC2-mediated osteoclastogenesis induced by RANKL/M-CSF

Store-operated Ca²⁺ entry (SOCE) is the stable calcium channel influx in most cells. It consists of the cytoplasmic ion channel ORAI and endoplasmic reticulum receptor stromal interaction molecule 1 (STIM1). Abolition of SOCE function due to ORAI1 and STIM1 gene defects may cause non-perspiration, ectoderm dysplasia and skeletal malformations with severe combined immunodeficiency (CID). Calcineurin/mammalian target of rapamycin (mTOR)/nuclear factor of activated T cells 2 (NFATC2) is an important signalling cascade for osteoclast development. Calcineurin is activated by Ca²⁺ via SOCE during osteoclastogenesis, which is induced by receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). However, the underlying mechanism has remained to be fully elucidated, which was therefore the aim of the present study. In the current study, flow cytometry was used to examine the effect of a number of STIM1 mutations on proliferation, differentiation, and expression of osteolysis-associated proteins in Bone marrow-derived mononuclear macrophages (BMDM). The calcineurin/AKT/mTOR/NFATC2 signaling cascade activation were also assessed. BMDMs were obtained from three patients with STIM1 mutations (p.E136X, p.R429C and p.R304W). These mutations, which exhibited abolished (p.E136X, p.R429C) or constitutively activated (p.R304W) SOCE, failed to respond to RANKL/M-CSF-mediated induction of normal osteoclastogenesis. In addition, activation of the calcineurin/Akt/mTOR/NFATC2 signalling cascade induced by RANKL/M-CSF was abnormal in the BMDMs with STIM1 mutants compared with that in BMDMs from healthy subjects. In addition, overexpression of wild-type STIM1 restored SOCE in p.R429C- and p.E136X-mutant BMDMs, but not in p.R304W-mutant BMDMs. Of note, calcineurin, cyclosporin A, mTOR inhibitor rapamycin and NFATC2-specific small interfering RNA restored the function of SOCE in p.R304W-mutant BMDMs. The present study suggests a role for SOCE in calcineurin/Akt/mTOR/NFATC2-mediated osteoclast proliferation, differentiation and function.

Tailored Lipoprotein-Like miRNA Delivery Nanostructure Suppresses Glioma Stemness and Drug Resistance through Receptor-Stimulated Macropinocytosis

Glioma initiating cells (GICs) function as the seed for the propagation and relapse of glioma. Designing a smart and efficient strategy to target the GICs and to suppress the multiple signaling pathways associated with stemness and chemoresistance is essential to achieving a cancer cure. Inspired by the metabolic difference in endocytosis between GICs, differentiated glioma cells, and normal cells, a tailored lipoprotein-like nanostructure is developed to amplify their internalization into GICs through receptor-stimulated macropinocytosis. As CXCR4 is highly expressed on GICs and glioma tumor sites, meanwhile, the activation of CXCR4 induces the receptor-stimulated macropinocytosis pathway in GICs, this CXCR4 receptor-stimulated lipoprotein-like nanoparticle (SLNP) achieves efficient accumulation in GICs in vitro and in vivo. By carrying microRNA-34a in the core, this tailored SLNP reduces sex-determining region Y-box 2 and Notch1 expression, powerfully inhibits GICs stemness and chemoresistance, and significantly prolongs the survival of GICs-bearing mice. Taken together, a tailored lipoprotein-based nanostructure realizes efficient GICs accumulation and therapeutic effect through receptor-stimulated macropinocytosis, providing a powerful nanoplatform for RNA interference drugs to combat glioma.

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'.

Rab8a localisation and activation by Toll-like receptors on macrophage macropinosomes

Macropinocytosis is a prevalent and essential pathway in macrophages where it contributes to anti-microbial responses and innate immune cell functions. Cell surface ruffles give rise to phagosomes and to macropinosomes as multi-functional compartments that contribute to environmental sampling, pathogen entry, plasma membrane turnover and receptor signalling. Rapid, high resolution, lattice light sheet imaging demonstrates the dynamic nature of macrophage ruffling. Pathogen-mediated activation of surface and endosomal Toll-like receptors (TLRs) in macrophages upregulates macropinocytosis. Here, using multiple forms of imaging and microscopy, we track membrane-associated, fluorescently-tagged Rab8a expressed in live macrophages, using a variety of cell markers to demonstrate Rab8a localization and its enrichment on early macropinosomes. Production of a novel biosensor and its use for quantitative FRET analysis in live cells, pinpoints macropinosomes as the site for TLR-induced activation of Rab8a. We have previously shown that TLR signalling, cytokine outputs and macrophage programming are regulated by the GTPase Rab8a with PI3 Kγ as its effector. Finally, we highlight another effector, the phosphatase OCRL, which is located on macropinosomes and interacts with Rab8a, suggesting that Rab8a may operate on multiple levels to modulate phosphoinositides in macropinosomes. These findings extend our understanding of macropinosomes as regulatory compartments for innate immune function in macrophages. 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'.

Using macropinocytosis for intracellular delivery of therapeutic nucleic acids to tumour cells

Nucleic acids are a rapidly emerging therapeutic modality with the potential to become the third major drug modality alongside antibodies and small molecules. Owing to the unfavourable physico-chemical characteristics of nucleic acids, such as large size and negative charge, intracellular delivery remains a fundamental challenge to realizing this potential. Delivery technologies such as lipids, polymers and peptides have been used to facilitate delivery, with many of the most successful technologies using macropinocytosis to gain cellular entry; mostly by default rather than design. Fundamental knowledge of macropinocytosis is rapidly growing, presenting opportunities to better tailor design strategies to target this pathway. Furthermore, certain types of tumour cells have been observed to have high levels of macropinocytic activity and traffic cargo to favourable destinations within the cell for endosomal release, providing unique opportunities to further use this entry route for drug delivery. In this article, we review the delivery systems reported to be taken up by macropinocytosis and what is known about the mechanisms for regulating macropinocytosis in tumour cells. From this analysis, we identify new opportunities for exploiting this pathway for the intracellular delivery of nucleic acids to tumour cells. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

JMJD3 promotes the epithelial-mesenchymal transition and migration of glioma cells via the CXCL12/CXCR4 axis

Histone H3K27 demethylase Jumonji domain-containing protein 3 (JMJD3) is involved in somatic cell differentiation and tumor progression; however, the underlying mechanisms of JMJD3 in cancer progression are yet to be fully explored. To improve understanding regarding the function of JMJD3 in brain tumor cells, the present study investigated the effects of JMJD3 on the epithelial-mesenchymal transition (EMT) and migration in glioma cells, and the underlying mechanisms involving the C-X-C motif chemokine ligand 12 (CXCL12)/C-X-C motif chemokine receptor 4 (CXCR4) axis. Immunohistochemical staining of a tissue microarray of glioma samples confirmed that JMJD3 overexpression could stratify highly metastatic glioma. The overexpression of JMJD3 induced a spindle-shaped morphology, promoted N-cadherin expression, inhibited E-cadherin expression and enhanced the migration ability of U-251MG and U-87MG American Type Culture Collection cells. The expression of E-cadherin and N-cadherin were assessed by western blotting and reverse transcription-quantitative polymerase chain reaction, and cell migration was evaluated using a Transwell migration assay and wound-healing. The overexpression of JMJD3 upregulated CXCL12 expression in a demethylase activity-dependent manner as ChIP assays revealed a decrease in H3K27 trimethylation at the CXCL12 promoter following overexpression of JMJD3 in U-87MG ATCC cells. Accordingly, CXCL12 overexpression was sufficient to rescue the suppressive effects of JMJD3 inhibition on the EMT and migration in glioma cells. In addition, CXCR4 expression was not regulated by JMJD3, but the interruption of CXCR4 caused by the CXCR4 inhibitor AMD3100 abolished the promotional effect of JMJD3 on EMT and migration in glioma cells. Collectively, these results suggested that JMJD3 promoted EMT and migration in glioma cells via the CXCL12/CXCR4 axis. The present study described a novel epigenetic mechanism regulating tumor cell EMT and migration, and provided a novel direction for glioma diagnosis and treatment.

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.

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

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'.

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.

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.

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.

Amplification of PIP3 signalling by macropinocytic cups

In a role distinct from and perhaps more ancient than that in signal transduction, PIP3 and Ras help to spatially organize the actin cytoskeleton into macropinocytic cups. These large endocytic structures are extended by actin polymerization from the cell surface and have at their core an intense patch of active Ras and PIP3, around which actin polymerizes, creating cup-shaped projections. We hypothesize that active Ras and PIP3 self-amplify within macropinocytic cups, in a way that depends on the structural integrity of the cup. Signalling that triggers macropinocytosis may therefore be amplified downstream in a way that depends on macropinocytosis. This argument provides a context for recent findings that signalling to Akt (an effector of PIP3) is sensitive to cytoskeletal and macropinocytic inhibitors.

Rac1-stimulated macropinocytosis enhances Gβγ activation of PI3Kβ

Phosphoinositide 3-kinases (PI 3-kinases) are regulated by a diverse range of upstream activators, including receptor tyrosine kinases (RTKs), G-protein-coupled receptors (GPCRs), and small GTPases from the Ras, Rho and Rab families. For the Class IA PI 3-kinase PI3Kβ, two mechanisms for GPCR-mediated regulation have been described: direct binding of Gβγ subunits to the C2-helical domain linker of p110β, and Dock180/Elmo1-mediated activation of Rac1, which binds to the Ras-Binding Domain of p110β. We now show that the integration of these dual pathways is unexpectedly complex. In breast cancer cells, expression of constitutively activated Rac1 (CA-Rac1) along with either GPCR stimulation or expression of Gβγ led to an additive PI3Kβ-dependent activation of Akt. Whereas CA-Rac1-mediated activation of Akt was blocked in cells expressing a mutated PI3Kβ that cannot bind Gβγ, Gβγ and GPCR-mediated activation of Akt was preserved when Rac1 binding to PI3Kβ was blocked. Surprisingly, PI3Kβ-dependent CA-Rac1 signaling to Akt was still seen in cells expressing a mutant p110β that cannot bind Rac1. Instead of directly binding to PI3Kβ, CA-Rac1 acts by enhancing Gβγ coupling to PI3Kβ, as CA-Rac1-mediated Akt activation was blocked by inhibitors of Gβγ. Cells expressing CA-Rac1 exhibited a robust induction of macropinocytosis, and inhibitors of macropinocytosis blocked the activation of Akt by CA-Rac1 or lysophosphatidic acid. Our data suggest that Rac1 can potentiate the activation of PI3Kβ by GPCRs through an indirect mechanism, by driving the formation of macropinosomes that serve as signaling platforms for Gβγ coupling to PI3Kβ.