Mechanistic analysis of massive endocytosis in relation to functionally defined surface membrane domains

Endocytic pathways of pathogenic protein aggregates in neurodegenerative diseases

Endocytosis is the fundamental uptake process through which cells internalize extracellular materials and species. Neurodegenerative diseases (NDs) are characterized by a progressive accumulation of intrinsically disordered protein species, leading to neuronal death. Misfolding in many proteins leads to various NDs such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS) and other disorders. Despite the significance of disordered protein species in neurodegeneration, their spread between cells and the cellular uptake of extracellular species is not entirely understood. This review discusses the major internalization mechanisms of the different conformer species of these proteins and their endocytic mechanisms. We briefly introduce the broad types of endocytic mechanisms found in cells and then summarize what is known about the endocytosis of monomeric, oligomeric and aggregated conformations of tau, Aβ, α-Syn, Huntingtin, Prions, SOD1, TDP-43 and other proteins associated with neurodegeneration. We also highlight the key players involved in internalizing these disordered proteins and the several techniques and approaches to identify their endocytic mechanisms. Finally, we discuss the obstacles involved in studying the endocytosis of these protein species and the need to develop better techniques to elucidate the uptake mechanisms of a particular disordered protein species.

Dynamin-Independent Mechanisms of Endocytosis and Receptor Trafficking

Endocytosis is a fundamental mechanism by which cells perform housekeeping functions. It occurs via a variety of mechanisms and involves many regulatory proteins. The GTPase dynamin acts as a “molecular scissor” to form endocytic vesicles and is a critical regulator among the proteins involved in endocytosis. Some GTPases (e.g., Cdc42, arf6, RhoA), membrane proteins (e.g., flotillins, tetraspanins), and secondary messengers (e.g., calcium) mediate dynamin-independent endocytosis. These pathways may be convergent, as multiple pathways exist in a single cell. However, what determines the specific path of endocytosis is complex and challenging to comprehend. This review summarizes the mechanisms of dynamin-independent endocytosis, the involvement of microRNAs, and factors that contribute to the cellular decision about the specific route of endocytosis.

Phospholipid Scramblases: Role in Cancer Progression and Anticancer Therapeutics

Phospholipid scramblases (PLSCRs) that catalyze rapid mixing of plasma membrane lipids result in surface exposure of phosphatidyl serine (PS), a lipid normally residing to the inner plasma membrane leaflet. PS exposure provides a chemotactic eat-me signal for phagocytes resulting in non-inflammatory clearance of apoptotic cells by efferocytosis. However, metastatic tumor cells escape efferocytosis through alteration of tumor microenvironment and apoptotic signaling. Tumor cells exhibit altered membrane features, high constitutive PS exposure, low drug permeability and increased multidrug resistance through clonal evolution. PLSCRs are transcriptionally up-regulated in tumor cells leading to plasma membrane remodeling and aberrant PS exposure on cell surface. In addition, PLSCRs interact with multiple cellular components to modulate cancer progression and survival. While PLSCRs and PS exposed on tumor cells are novel drug targets, many exogenous molecules that catalyze lipid scrambling on tumor plasma membrane are potent anticancer therapeutic molecules. In this review, we provide a comprehensive analysis of scramblase mediated signaling events, membrane alteration specific to tumor development and possible therapeutic implications of scramblases and PS exposure.

FXYD protein isoforms differentially modulate human Na/K pump function

Tight regulation of the Na/K pump is essential for cellular function because this heteromeric protein builds and maintains the electrochemical gradients for Na+ and K+ that energize electrical signaling and secondary active transport. We studied the regulation of the ubiquitous human α1β1 pump isoform by five human FXYD proteins normally located in muscle, kidney, and neurons. The function of Na/K pump α1β1 expressed in Xenopus oocytes with or without FXYD isoforms was evaluated using two-electrode voltage clamp and patch clamp. Through evaluation of the partial reactions in the absence of K+ but presence of Na+ in the external milieu, we demonstrate that each FXYD subunit alters the equilibrium between E1P(3Na) and E2P, the phosphorylated conformations with Na+ occluded and free from Na+, respectively, thereby altering the apparent affinity for Na+. This modification of Na+ interaction shapes the small effects of FXYD proteins on the apparent affinity for external K+ at physiological Na+. FXYD6 distinctively accelerated both the Na+-deocclusion and the pump-turnover rates. All FXYD isoforms altered the apparent affinity for intracellular Na+ in patches, an effect that was observed only in the presence of intracellular K+. Therefore, FXYD proteins alter the selectivity of the pump for intracellular ions, an effect that could be due to the altered equilibrium between E1 and E2, the two major pump conformations, and/or to small changes in ion affinities that are exacerbated when both ions are present. Lastly, we observed a drastic reduction of Na/K pump surface expression when it was coexpressed with FXYD1 or FXYD6, with the former being relieved by injection of PKA's catalytic subunit into the oocyte. Our results indicate that a prominent effect of FXYD1 and FXYD6, and plausibly other FXYDs, is the regulation of Na/K pump trafficking.

TMEM16F and dynamins control expansive plasma membrane reservoirs

Cells can expand their plasma membrane laterally by unfolding membrane undulations and by exocytosis. Here, we describe a third mechanism involving invaginations held shut by the membrane adapter, dynamin. Compartments open when Ca activates the lipid scramblase, TMEM16F, anionic phospholipids escape from the cytoplasmic monolayer in exchange for neutral lipids, and dynamins relax. Deletion of TMEM16F or dynamins blocks expansion, with loss of dynamin expression generating a maximally expanded basal plasma membrane state. Re-expression of dynamin2 or its GTPase-inactivated mutant, but not a lipid binding mutant, regenerates reserve compartments and rescues expansion. Dynamin2-GFP fusion proteins form punctae that rapidly dissipate from these compartments during TMEM16F activation. Newly exposed compartments extend deeply into the cytoplasm, lack numerous organellar markers, and remain closure-competent for many seconds. Without Ca, compartments open slowly when dynamins are sequestered by cytoplasmic dynamin antibodies or when scrambling is mimicked by neutralizing anionic phospholipids and supplementing neutral lipids. Activation of Ca-permeable mechanosensitive channels via cell swelling or channel agonists opens the compartments in parallel with phospholipid scrambling. Thus, dynamins and TMEM16F control large plasma membrane reserves that open in response to lateral membrane stress and Ca influx.

Cholesterol-dependent plasma membrane order (Lo) is critical for antigen-specific clonal expansion of CD4+ T cells

Early “T cell activation” events are initiated within the lipid microenvironment of the plasma membrane. Role of lipid membrane order (L_o) in spatiotemporal signaling through the antigen receptor in T cells is posited but remains unclear. We have examined the role of membrane order (L_o)/disorder (L_d) in antigen specific CD4⁺ T cell activation and clonal expansion by first creating membrane disorder, and then reconstituting membrane order by inserting cholesterol into the disordered plasma membrane. Significant revival of antigen specific CD4⁺ T cell proliferative response was observed after reconstituting the disrupted membrane order with cholesterol. These reconstitution experiments illustrate Koch’s postulate by demonstrating that cholesterol-dependent membrane order (L_o) is critical for responses generated by CD4⁺ T cells and point to the importance of membrane order and lipid microenvironment in signaling through T cell membrane antigen receptors.

Dosage-controlled intracellular delivery mediated by acoustofluidics for lab on a chip applications

Biological research and many cell-based therapies rely on the successful delivery of cargo materials into cells. Intracellular delivery in an in vitro setting refers to a variety of physical and biochemical techniques developed for conducting rapid and efficient transport of materials across the plasma membrane. Generally, the techniques that are time-efficient (e.g., electroporation) suffer from heterogeneity and low cellular viability, and those that are precise (e.g., microinjection) suffer from low-throughput and are labor-intensive. Here, we present a novel in vitro microfluidic strategy for intracellular delivery, which is based on the acoustic excitation of adherent cells. Strong mechanical oscillations, mediated by Lamb waves, inside a microfluidic channel facilitate the cellular uptake of different size (e.g., 3-500 kDa, plasmid encoding EGFP) cargo materials through endocytic pathways. We demonstrate successful delivery of 500 kDa dextran to various adherent cell lines with unprecedented efficiency in the range of 65-85% above control. We also show that actuation voltage and treatment duration can be tuned to control the dosage of delivered substances. High viability (≥91%), versatility across different cargo materials and various adherent cell lines, scalability to hundreds of thousands of cells per treatment, portability, and ease-of-operation are among the unique features of this acoustofluidic strategy. Potential applications include targeting through endocytosis-dependant pathways in cellular disorders, such as lysosomal storage diseases, which other physical methods are unable to address. This novel acoustofluidic method achieves rapid, uniform, and scalable delivery of material into cells, and may find utility in lab-on-a-chip applications.

Regulation of NCX1 by palmitoylation

Palmitoylation (S-acylation) is the reversible conjugation of a fatty acid (usually C16 palmitate) to intracellular cysteine residues of proteins via a thioester linkage. Palmitoylation anchors intracellular regions of proteins to membranes because the palmitoylated cysteine is recruited to the lipid bilayer. NCX1 is palmitoylated at a single cysteine in its large regulatory intracellular loop. The presence of an amphipathic α-helix immediately adjacent to the NCX1 palmitoylation site is required for NCX1 palmitoylation. The NCX1 palmitoylation site is conserved through most metazoan phlya. Although palmitoylation does not regulate the normal forward or reverse ion transport modes of NCX1, NCX1 palmitoylation is required for its inactivation: sodium-dependent inactivation and inactivation by PIP2 depletion are significantly impaired for unpalmitoylatable NCX1. Here we review the role of palmitoylation in regulating NCX1 activity, and highlight future questions that must be addressed to fully understand the importance of this regulatory mechanism for sodium and calcium transport in cardiac muscle.

Targeted enhancement of flotillin-dependent endocytosis augments cellular uptake and impact of cytotoxic drugs

Cellular uptake is limiting for the efficacy of many cytotoxic drugs used to treat cancer. Identifying endocytic mechanisms that can be modulated with targeted, clinically-relevant interventions is important to enhance the efficacy of various cancer drugs. We identify that flotillin-dependent endocytosis can be targeted and upregulated by ultrasound and microbubble (USMB) treatments to enhance uptake and efficacy of cancer drugs such as cisplatin. USMB involves targeted ultrasound following administration of encapsulated microbubbles, used clinically for enhanced ultrasound image contrast. USMB treatments robustly enhanced internalization of the molecular scaffold protein flotillin, as well as flotillin-dependent fluid-phase internalization, a phenomenon dependent on the protein palmitoyltransferase DHHC5 and the Src-family kinase Fyn. USMB treatment enhanced DNA damage and cell killing elicited by the cytotoxic agent cisplatin in a flotillin-dependent manner. Thus, flotillin-dependent endocytosis can be modulated by clinically-relevant USMB treatments to enhance drug uptake and efficacy, revealing an important new strategy for targeted drug delivery for cancer treatment.

Spoiled for Choice: Diverse Endocytic Pathways Function at the Cell Surface

Endocytosis has long been identified as a key cellular process involved in bringing in nutrients, in clearing cellular debris in tissue, in the regulation of signaling, and in maintaining cell membrane compositional homeostasis. While clathrin-mediated endocytosis has been most extensively studied, a number of clathrin-independent endocytic pathways are continuing to be delineated. Here we provide a current survey of the different types of endocytic pathways available at the cell surface and discuss a new classification and plausible molecular mechanisms for some of the less characterized pathways. Along with an evolutionary perspective of the origins of some of these pathways, we provide an appreciation of the distinct roles that these pathways play in various aspects of cellular physiology, including the control of signaling and membrane tension.

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.

TMEM16F activation by Ca2+ triggers plasma membrane expansion and directs PD-1 trafficking

TMEM16F is a Ca2+ -gated ion channel that is required for Ca2+ -activated phosphatidylserine exposure on the surface of many eukaryotic cells. TMEM16F is widely expressed and has roles in platelet activation during blood clotting, bone formation and T cell activation. By combining microscopy and patch clamp recording we demonstrate that activation of TMEM16F by Ca2+ ionophores in Jurkat T cells triggers large-scale surface membrane expansion in parallel with phospholipid scrambling. With continued ionophore application,TMEM16F-expressing cells then undergo extensive shedding of ectosomes. The T cell co-receptor PD-1 is selectively incorporated into ectosomes. This selectivity depends on its transmembrane sequence. Surprisingly, cells lacking TMEM16F not only fail to expand surface membrane in response to elevated cytoplasmic Ca2+, but instead undergo rapid massive endocytosis with PD-1 internalisation. These results establish a new role for TMEM16F as a regulator of Ca2+ activated membrane trafficking.

Greasing the wheels or a spanner in the works? Regulation of the cardiac sodium pump by palmitoylation

The ubiquitous sodium/potassium ATPase (Na pump) is the most abundant primary active transporter at the cell surface of multiple cell types, including ventricular myocytes in the heart. The activity of the Na pump establishes transmembrane ion gradients that control numerous events at the cell surface, positioning it as a key regulator of the contractile and metabolic state of the myocardium. Defects in Na pump activity and regulation elevate intracellular Na in cardiac muscle, playing a causal role in the development of cardiac hypertrophy, diastolic dysfunction, arrhythmias and heart failure. Palmitoylation is the reversible conjugation of the fatty acid palmitate to specific protein cysteine residues; all subunits of the cardiac Na pump are palmitoylated. Palmitoylation of the pump's accessory subunit phospholemman (PLM) by the cell surface palmitoyl acyl transferase DHHC5 leads to pump inhibition, possibly by altering the relationship between the pump catalytic α subunit and specifically bound membrane lipids. In this review, we discuss the functional impact of PLM palmitoylation on the cardiac Na pump and the molecular basis of recognition of PLM by its palmitoylating enzyme DHHC5, as well as effects of palmitoylation on Na pump cell surface abundance in the cardiac muscle. We also highlight the numerous unanswered questions regarding the cellular control of this fundamentally important regulatory process.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Soluble klotho binds monosialoganglioside to regulate membrane microdomains and growth factor signaling

Soluble klotho, the shed ectodomain of the antiaging membrane protein α-klotho, is a pleiotropic endocrine/paracrine factor with no known receptors and poorly understood mechanism of action. Soluble klotho down-regulates growth factor-driven PI3K signaling, contributing to extension of lifespan, cardioprotection, and tumor inhibition. Here we show that soluble klotho binds membrane lipid rafts. Klotho binding to rafts alters lipid organization, decreases membrane's propensity to form large ordered domains for endocytosis, and down-regulates raft-dependent PI3K/Akt signaling. We identify α2-3-sialyllactose present in the glycan of monosialogangliosides as targets of soluble klotho. α2-3-Sialyllactose is a common motif of glycans. To explain why klotho preferentially targets lipid rafts we show that clustering of gangliosides in lipid rafts is important. In vivo, raft-dependent PI3K signaling is up-regulated in klotho-deficient mouse hearts vs. wild-type hearts. Our results identify ganglioside-enriched lipid rafts to be receptors that mediate soluble klotho regulation of PI3K signaling. Targeting sialic acids may be a general mechanism for pleiotropic actions of soluble klotho.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Ultrasound Microbubble Treatment Enhances Clathrin-Mediated Endocytosis and Fluid-Phase Uptake through Distinct Mechanisms

Drug delivery to tumors is limited by several factors, including drug permeability of the target cell plasma membrane. Ultrasound in combination with microbubbles (USMB) is a promising strategy to overcome these limitations. USMB treatment elicits enhanced cellular uptake of materials such as drugs, in part as a result of sheer stress and formation of transient membrane pores. Pores formed upon USMB treatment are rapidly resealed, suggesting that other processes such as enhanced endocytosis may contribute to the enhanced material uptake by cells upon USMB treatment. How USMB regulates endocytic processes remains incompletely understood. Cells constitutively utilize several distinct mechanisms of endocytosis, including clathrin-mediated endocytosis (CME) for the internalization of receptor-bound macromolecules such as Transferrin Receptor (TfR), and distinct mechanism(s) that mediate the majority of fluid-phase endocytosis. Tracking the abundance of TfR on the cell surface and the internalization of its ligand transferrin revealed that USMB acutely enhances the rate of CME. Total internal reflection fluorescence microscopy experiments revealed that USMB treatment altered the assembly of clathrin-coated pits, the basic structural units of CME. In addition, the rate of fluid-phase endocytosis was enhanced, but with delayed onset upon USMB treatment relative to the enhancement of CME, suggesting that the two processes are distinctly regulated by USMB. Indeed, vacuolin-1 or desipramine treatment prevented the enhancement of CME but not of fluid phase endocytosis upon USMB, suggesting that lysosome exocytosis and acid sphingomyelinase, respectively, are required for the regulation of CME but not fluid phase endocytosis upon USMB treatment. These results indicate that USMB enhances both CME and fluid phase endocytosis through distinct signaling mechanisms, and suggest that strategies for potentiating the enhancement of endocytosis upon USMB treatment may improve targeted drug delivery.

Defying death: Cellular survival strategies following plasmalemmal injury by bacterial toxins

The perforation of the plasmalemma by pore-forming toxins causes an influx of Ca(2+) and an efflux of cytoplasmic constituents. In order to ensure survival, the cell needs to identify, plug and remove lesions from its membrane. Quarantined by membrane folds and isolated by membrane fusion, the pores are removed from the plasmalemma and expelled into the extracellular space. Outward vesiculation and microparticle shedding seem to be the strategies of choice to eliminate toxin-perforated membrane regions from the plasmalemma of host cells. Depending on the cell type and the nature of injury, the membrane lesion can also be taken up by endocytosis and degraded internally. Host cells make excellent use of an initial, moderate rise in intracellular [Ca(2+)], which triggers containment of the toxin-inflicted damage and resealing of the damaged plasmalemma. Additional Ca(2+)-dependent defensive cellular actions range from the release of effector molecules in order to warn neighbouring cells, to the activation of caspases for the initiation of apoptosis in order to eliminate heavily damaged, dysregulated cells. Injury to the plasmalemma by bacterial toxins can be prevented by the early sequestration of bacterial toxins. Artificial liposomes can act as a decoy system preferentially binding and neutralizing bacterial toxins.

Dealing with damage: plasma membrane repair mechanisms

Eukaryotic cells have developed repair mechanisms, which allow them to reseal their membrane in order to prevent the efflux of cytoplasmic constituents and the uncontrolled influx of calcium. After injury, the Ca(2+)-concentration gradient fulfils a dual function: it provides guidance cues for the repair machinery and directly activates the molecules, which have a repair function. Depending on the nature of injury, the morphology of the cell and the severity of injury, the membrane resealing can be effected by lysosomal exocytosis, microvesicle shedding or a combination of both. Likewise, exocytosis is often followed by the endocytic uptake of lesions. Additionally, since plasmalemmal resealing must be attempted, even after extensive injury in order to prevent cell lysis, the restoration of membrane integrity can be achieved by ceramide-driven invagination of the lipid bilayer, during which the cell is prepared for apoptotic disposal. Plasmalemmal injury can be contained by a surfeit of plasma membrane, which serves as a trap for toxic substances: either passively by an abundance of cellular protrusions, or actively by membrane blebbing.

Damage control: cellular mechanisms of plasma membrane repair

When wounded, eukaryotic cells reseal in a few seconds. Ca(2+) influx induces exocytosis of lysosomes, a process previously thought to promote repair by 'patching' wounds. New evidence suggests that resealing involves direct wound removal. Exocytosis of lysosomal acid sphingomyelinase (ASM) triggers endocytosis of lesions followed by intracellular degradation. Characterization of injury-induced endosomes revealed a role for caveolae, sphingolipid-enriched plasma membrane invaginations that internalize toxin pores and are abundant in mechanically stressed cells. These findings provide a novel mechanistic explanation for the muscle pathology associated with mutations in caveolar proteins. Membrane remodeling by the ESCRT complex was also recently shown to participate in small-wound repair, emphasizing that cell resealing involves previously unrecognized mechanisms for lesion removal that are distinct from the patch model.

Massive palmitoylation-dependent endocytosis during reoxygenation of anoxic cardiac muscle

In fibroblasts, large Ca transients activate massive endocytosis (MEND) that involves membrane protein palmitoylation subsequent to mitochondrial permeability transition pore (PTP) openings. Here, we characterize this pathway in cardiac muscle. Myocytes with increased expression of the acyl transferase, DHHC5, have decreased Na/K pump activity. In DHHC5-deficient myocytes, Na/K pump activity and surface area/volume ratios are increased, the palmitoylated regulatory protein, phospholemman (PLM), and the cardiac Na/Ca exchanger (NCX1) show greater surface membrane localization, and MEND is inhibited in four protocols. Both electrical and optical methods demonstrate that PTP-dependent MEND occurs during reoxygenation of anoxic hearts. Post-anoxia MEND is ablated in DHHC5-deficient hearts, inhibited by cyclosporine A (CsA) and adenosine, promoted by staurosporine (STS), reduced in hearts lacking PLM, and correlates with impaired post-anoxia contractile function. Thus, the MEND pathway appears to be deleterious in severe oxidative stress but may constitutively contribute to cardiac sarcolemma turnover in dependence on metabolic stress.

DOI:http://dx.doi.org/10.7554/eLife.01295.001

Many people who survive a stroke or heart attack experience substantial tissue damage when the blood supply is restored. Much of this damage can be caused by the mitochondria inside the cells releasing a protein called cytochrome c that can cause cells to die in a process called apoptosis. The cytochrome c is released as the outer membrane of the mitochondria becomes permeable and pores called permeability transition pores open up in the inner membrane. Now Lin et al. explore if additional molecules released from the mitochondria might also initiate important cellular responses during the reoxygenation of oxygen-deprived tissue.

Lin and co-workers recently showed that the mitochondria of some cells can release a small enzyme cofactor, coenzyme A, which then promotes a cellular response called massive endocytosis. This process can cause up to 70% of the cell surface membrane to be absorbed into the interior of the cell in the form of membrane vesicles. Most forms of endocytosis involve a much smaller fraction of the cell membrane and employ a set of well-known endocytic proteins that are not involved in massive endocytosis. Now, Lin et al. investigate the role of massive endocytosis in cardiac muscle.

Electrical and optical measurements reveal that massive endocytosis occurs as cardiac cells that have been deprived of oxygen are reoxygenated. Lin et al. also find that an enzyme called DHHC5 must be present to allow endocytosis to take place during reoxygenation. DHHC5 is an enzyme that catalyzes a process called acylation – the transfer of acyl groups to proteins at the cell surface. Moreover, the deletion of DHHC5 has a beneficial impact on the performance of cardiac muscle after oxygen deprivation, which implies that molecules that inhibit protein acylation might protect the heart from damage during reoxygenation. Together, these results establish new pathological and physiological roles for the acylation, which is one of the most common biochemical modifications made to membrane proteins after they are synthesized.

DOI:http://dx.doi.org/10.7554/eLife.01295.002

The mother of all endocytosis

Massive endocytosis is initiated by a series of steps that involve a sudden influx of calcium ions, changes in mitochondria, and modification of surface proteins by lipids. A better understanding of this process could lead to new approaches to reducing the tissue damage that is caused by heart attacks.

Massive endocytosis triggered by surface membrane palmitoylation under mitochondrial control in BHK fibroblasts

Large Ca transients cause massive endocytosis (MEND) in BHK fibroblasts by nonclassical mechanisms. We present evidence that MEND depends on mitochondrial permeability transition pore (PTP) openings, followed by coenzyme A (CoA) release, acyl CoA synthesis, and membrane protein palmitoylation. MEND is blocked by inhibiting mitochondrial Ca uptake or PTP openings, depleting fatty acids, blocking acyl CoA synthesis, metabolizing CoA, or inhibiting palmitoylation. It is triggered by depolarizing mitochondria or promoting PTP openings. After mitochondrial MEND blockade, MEND is restored by cytoplasmic acyl CoA or CoA. MEND is blocked by siRNA knockdown of the plasmalemmal acyl transferase, DHHC5. When acyl CoA is abundant, transient H2O2 oxidative stress or PKC activation initiates MEND, but the immediate presence of H2O2 prevents MEND. The PTP inhibitor, NIM811, significantly increases plasmalemma in normally growing cells. Thus, the MEND pathway may contribute to constitutive as well as pathological plasmalemma turnover in dependence on mitochondrial stress signaling. DOI: http://dx.doi.org/10.7554/eLife.01293.001.

Biophysics of cell membrane lipids in cancer drug resistance: Implications for drug transport and drug delivery with nanoparticles

In this review, we focus on the biophysics of cell membrane lipids, particularly when cancers develop acquired drug resistance, and how biophysical changes in resistant cell membrane influence drug transport and nanoparticle-mediated drug delivery. Recent advances in membrane lipid research show the varied roles of lipids in regulating membrane P-glycoprotein function, membrane trafficking, apoptotic pathways, drug transport, and endocytic functions, particularly endocytosis, the primary mechanism of cellular uptake of nanoparticle-based drug delivery systems. Since acquired drug resistance alters lipid biosynthesis, understanding the role of lipids in cell membrane biophysics and its effect on drug transport is critical for developing effective therapeutic and drug delivery approaches to overcome drug resistance. Here we discuss novel strategies for (a) modulating the biophysical properties of membrane lipids of resistant cells to facilitate drug transport and regain endocytic function and (b) developing effective nanoparticles based on their biophysical interactions with membrane lipids to enhance drug delivery and overcome drug resistance.

Proton-induced endocytosis is dependent on cell membrane fluidity, lipid-phase order and the membrane resting potential

Recently it has been shown that decreasing the extracellular pH of cells stimulates the formation of inward membrane invaginations and vesicles, accompanied by an enhanced uptake of macromolecules. This type of endocytosis was coined as proton-induced uptake (PIU). Though the initial induction of inward membrane curvature was rationalized in terms of proton-based increase of charge asymmetry across the membrane, the dependence of the phenomenon on plasma membrane characteristics is still unknown. The present study shows that depolarization of the membrane resting potential elevates PIU by 25%, while hyperpolarization attenuates it by 25%. Comparison of uptake in suspended and adherent cells implicates that the resting-potential affects PIU through remodeling the actin-cytoskeleton. The pH at the external interface of the cell membrane rather than the pH gradient across it determines the extent of PIU. PIU increases linearly upon temperature increase in the range of 4-36°C, in correlation with the membrane fluidity. The plasma membrane fluidity and the lipid phase order are modulated by enriching the cell's membrane with cholesterol, tergitol, dimethylsulfoxide, 6-ketocholestanol and phloretin and by cholesterol depletion. These treatments are shown to alter the extent of PIU and are better correlated with membrane fluidity than with the lipid phase order. We suggest that the lipid phase order and fluidity influence PIU by regulating the lipid order gradient across the perimeter of the lipid-condensed microdomains (rafts) and alter the characteristic tension line that separates the higher ordered lipid-domains from the lesser ordered ones.

Toward an understanding of the complete NCX1 lifetime in the cardiac sarcolemma

The density of Na/Ca exchangers (NCX1) in the cardiac sarcolemma, like all plasma membrane proteins, will be influenced by (and ultimately determined by) the function of membrane insertion and retrieval processes (i.e., exo- and endocytic mechanisms). Progress in understanding these processes in cardiac muscle faces many biological and methodological complexities and hurdles. As described here, we are attempting to overcome these hurdles to study more adequately the assembly and disassembly of the cardiac sarcolemma, in general, and the control of NCX1 by membrane trafficking processes in particular. First, we have developed improved noninvasive methods to monitor the cellular capacitance of cardiac tissue (NIC) over periods of hours. Thus, we can study long-term changes of total membrane area. Second, we have developed mice that express fusion proteins of NCX1 with the pHluorin green protein. Thus, we can determine the membrane disposition of NCX1, and changes thereof, on-line in intact cardiac muscle.

Ceramide in plasma membrane repair

The perforation of the plasmalemma by pore-forming toxins causes an influx of Ca(2+) and an efflux of cytoplasmic proteins. In order to ensure cellular survival, lesions have to be identified, plugged and removed from the membrane. The Ca(2+)-driven fusion of lysosomes with the plasma membrane leads to hydrolysis of sphingomyelin by acid sphingomyelinase and a formation of ceramide platforms in the outer leaflet of the lipid bilayer. We propose that the negative curvature, promoted by tighter packing of lipids in the outer layer, leads to an inward vesiculation of the damaged area for its endocytotic uptake and internal degradation. In contrast, the activation of neutral sphingomyelinase triggers the production of ceramide within the inner leaflet of the lipid bilayer, thereby promoting an outward curvature, which enables the cell to shed the membrane-containing toxin pore into the extracellular space. In this process, ceramide is supported by members of the annexin protein family which act as Ca(2+) sensors and as membrane fusion agents.

The targeting of plasmalemmal ceramide to mitochondria during apoptosis

Ceramide is a key lipid mediator of cellular processes such as differentiation, proliferation, growth arrest and apoptosis. During apoptosis, ceramide is produced within the plasma membrane. Although recent data suggest that the generation of intracellular ceramide increases mitochondrial permeability, the source of mitochondrial ceramide remains unknown. Here, we determine whether a stress-mediated plasmalemmal pool of ceramide might become available to the mitochondria of apoptotic cells. We have previously established annexin A1—a member of a family of Ca²⁺ and membrane-binding proteins—to be a marker of ceramide platforms. Using fluorescently tagged annexin A1, we show that, upon its generation within the plasma membrane, ceramide self-associates into platforms that subsequently invaginate and fuse with mitochondria. An accumulation of ceramide within the mitochondria of apoptotic cells was also confirmed using a ceramide-specific antibody. Electron microscopic tomography confirmed that upon the formation of ceramide platforms, the invaginated regions of the plasma membrane extend deep into the cytoplasm forming direct physical contacts with mitochondrial outer membranes. Ceramide might thus be directly transferred from the plasma membrane to the mitochondrial outer membrane. It is conceivable that this “kiss-of-death” increases the permeability of the mitochondrial outer membrane thereby triggering apoptosis.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Massive endocytosis driven by lipidic forces originating in the outer plasmalemmal monolayer: a new approach to membrane recycling and lipid domains

The roles that lipids play in endocytosis are the subject of debate. Using electrical and imaging methods, we describe massive endocytosis (MEND) in baby hamster kidney (BHK) and HEK293 cells when the outer plasma membrane monolayer is perturbed by the nonionic detergents, Triton X-100 (TX100) and NP-40. Some alkane detergents, the amphipathic drugs, edelfosine and tamoxifen, and the phospholipase inhibitor, U73122, are also effective. Uptake of the membrane tracer, FM 4–64, into vesicles and loss of reversible FM 4–64 binding confirm that 40–75% of the cell surface is internalized. Ongoing MEND stops in 2–4 s when amphipaths are removed, and amphipaths are without effect from the cytoplasmic side. Thus, expansion of the outer monolayer is critical. As found for Ca-activated MEND, vesicles formed are <100 nm in diameter, membrane ruffles are lost, and β-cyclodextrin treatments are inhibitory. However, amphipath-activated MEND does not require Ca transients, adenosine triphosphate (ATP) hydrolysis, G protein cycling, dynamins, or actin cytoskeleton remodeling. With elevated cytoplasmic ATP (>5 mM), MEND can reverse completely and be repeated multiple times in BHK and HEK293 cells, but not cardiac myocytes. Reversal is blocked by N-ethylmaleimide and a nitric oxide donor, nitroprusside. Constitutively expressed Na/Ca exchangers internalize roughly in proportion to surface membrane, whereas Na/K pump activities decrease over-proportionally. Sodium dodecyl sulfate and dodecylglucoside do not cause MEND during their application, but MEND occurs rapidly when they are removed. As monitored capacitively, the binding of these detergents decreases with MEND, whereas TX100 binding does not decrease. In summary, nonionic detergents can fractionate the plasma membrane in vivo, and vesicles formed connect immediately to physiological membrane-trafficking mechanisms. We suggest that lateral and transbilayer inhomogeneities of the plasma membrane provide potential energies that, when unbridled by triggers, can drive endocytosis by lipidic forces.