Mechanochemical feedback control of dynamin independent endocytosis modulates membrane tension in adherent cells

Changes in the properties of membrane tethers in response to HP1α depletion in MCF7 cells

Plasma membrane tension is known to regulate many cell functions, such as motility and membrane trafficking. Membrane tether pulling is an effective method for measuring the apparent membrane tension of cells and exploring membrane-cytoskeleton interactions. In this article, the mechanical properties of HP1α-depleted MCF7 breast cancer cells are explored in comparison to controls, by pulling membrane tethers using optical tweezers. These studies were inspired by previous findings that a loss of HP1α correlates with an increase in the invasive potential of malignant cancer cells. Specifically, the membrane tension and force relaxation curves for tethers pulled from MCF7 breast cancer cells with HP1α knockdown and their matched controls were measured, and shown to be significantly different.

Effective cell membrane tension is independent of polyacrylamide substrate stiffness

Most animal cells are surrounded by a cell membrane and an underlying actomyosin cortex. Both structures are linked, and they are under tension. In-plane membrane tension and cortical tension both influence many cellular processes, including cell migration, division, and endocytosis. However, while actomyosin tension is regulated by substrate stiffness, how membrane tension responds to mechanical substrate properties is currently poorly understood. Here, we probed the effective membrane tension of neurons and fibroblasts cultured on glass and polyacrylamide substrates of varying stiffness using optical tweezers. In contrast to actomyosin-based traction forces, both peak forces and steady-state tether forces of cells cultured on hydrogels were independent of substrate stiffness and did not change after blocking myosin II activity using blebbistatin, indicating that tether and traction forces are not directly linked. Peak forces in fibroblasts on hydrogels were about twice as high as those in neurons, indicating stronger membrane-cortex adhesion in fibroblasts. Steady-state tether forces were generally higher in cells cultured on hydrogels than on glass, which we explain by a mechanical model. Our results provide new insights into the complex regulation of effective membrane tension and pave the way for a deeper understanding of the biological processes it instructs.

Endocytosis and exocytosis protect cells against severe membrane tension variations

The ability of cells to regulate their shape and volume is critical for many cell functions. How endocytosis and exocytosis, as important ways of membrane trafficking, affect cellular volume regulation is still unclear. Here, we develop a theoretical framework to study the dynamics of cell volume, endocytosis, and exocytosis in response to osmotic shocks and mechanical loadings. This model can not only explain observed dynamics of endocytosis and exocytosis during osmotic shocks but also predict the dynamics of endocytosis and exocytosis during cell compressions. We find that a hypotonic shock stimulates exocytosis, while a hypertonic shock stimulates endocytosis; and exocytosis in turn allows cells to have a dramatic change in cell volume but a small change in membrane tension during hyposmotic swelling, protecting cells from rupture under high tension. In addition, we find that cell compressions with various loading speeds induce three distinct dynamic modes of endocytosis and exocytosis. Finally, we show that increasing endocytosis and exocytosis rates reduce the changes in cell volume and membrane tension under fast cell compression, whereas they enhance the changes in cell volume and membrane tension under slow cell compression. Together, our findings reveal critical roles of endocytosis and exocytosis in regulating cell volume and membrane tension.

Plant cell polarity as the nexus of tissue mechanics and morphogenesis

How reproducible body patterns emerge from the collective activity of individual cells is a key question in developmental biology. Plant cells are encaged in their walls and unable to migrate. Morphogenesis thus relies on directional cell division, by precise positioning of division planes, and anisotropic cellular growth, mediated by regulated mechanical inhomogeneity of the walls. Both processes require the prior establishment of cell polarity, marked by the formation of polar domains at the plasma membrane, in a number of developmental contexts. The establishment of cell polarity involves biochemical cues, but increasing evidence suggests that mechanical forces also play a prominent instructive role. While evidence for mutual regulation between cell polarity and tissue mechanics is emerging, the nature of this bidirectional feedback remains unclear. Here we review the role of cell polarity at the interface of tissue mechanics and morphogenesis. We also aim to integrate biochemistry-centred insights with concepts derived from physics and physical chemistry. Lastly, we propose a set of questions that will help address the fundamental nature of cell polarization and its mechanistic basis.

Novel suction-based in vivo cutaneous DNA transfection platform

This work reports a suction-based cutaneous delivery method for in vivo DNA transfection. Following intradermal Mantoux injection of plasmid DNA in a rat model, a moderate negative pressure is applied to the injection site, a technique similar to Chinese báguàn and Middle Eastern hijama cupping therapies. Strong GFP expression was demonstrated with pEGFP-N1 plasmids where fluorescence was observed as early as 1 hour after dosing. Modeling indicates a strong correlation between focal strain/stress and expression patterns. The absence of visible and/or histological tissue injury contrasts with current in vivo transfection systems such as electroporation. Specific utility was demonstrated with a synthetic SARS-CoV-2 DNA vaccine, which generated host humoral immune response in rats with notable antibody production. This method enables an easy-to-use, cost-effective, and highly scalable platform for both laboratorial transfection needs and clinical applications for nucleic acid–based therapeutics and vaccines.

Superelasticity of Plasma- and Synthetic Membranes Resulting from Coupling of Membrane Asymmetry, Curvature, and Lipid Sorting

Biological cells are contained by a fluid lipid bilayer (plasma membrane, PM) that allows for large deformations, often exceeding 50% of the apparent initial PM area. Isolated lipids self-organize into membranes, but are prone to rupture at small (<2-4%) area strains, which limits progress for synthetic reconstitution of cellular features. Here, it is shown that by preserving PM structure and composition during isolation from cells, vesicles with cell-like elasticity can be obtained. It is found that these plasma membrane vesicles store significant area in the form of nanotubes in their lumen. These act as lipid reservoirs and are recruited by mechanical tension applied to the outer vesicle membrane. Both in experiment and theory, it is shown that a "superelastic" response emerges from the interplay of lipid domains and membrane curvature. This finding allows for bottom-up engineering of synthetic biomaterials that appear one magnitude softer and with threefold larger deformability than conventional lipid vesicles. These results open a path toward designing superelastic synthetic cells possessing the inherent mechanics of biological cells.

Quantifying force transmission through fibroblasts: changes of traction forces under external shearing

Mammalian cells have evolved complex mechanical connections to their microenvironment, including focal adhesion clusters that physically connect the cytoskeleton and the extracellular matrix. This mechanical link is also part of the cellular machinery to transduce, sense and respond to external forces. Although methods to measure cell attachment and cellular traction forces are well established, these are not capable of quantifying force transmission through the cell body to adhesion sites. We here present a novel approach to quantify intracellular force transmission by combining microneedle shearing at the apical cell surface with traction force microscopy at the basal cell surface. The change of traction forces exerted by fibroblasts to underlying polyacrylamide substrates as a response to a known shear force exerted with a calibrated microneedle reveals that cells redistribute forces dynamically under external shearing and during sequential rupture of their adhesion sites. Our quantitative results demonstrate a transition from dipolar to monopolar traction patterns, an inhomogeneous distribution of the external shear force to the adhesion sites as well as dynamical changes in force loading prior to and after the rupture of single adhesion sites. Our strategy of combining traction force microscopy with external force application opens new perspectives for future studies of force transmission and mechanotransduction in cells.

Cargo-specific recruitment in clathrin- and dynamin-independent endocytosis

Spatially controlled, cargo-specific endocytosis is essential for development, tissue homeostasis and cancer invasion. Unlike cargo-specific clathrin-mediated endocytosis, the clathrin- and dynamin-independent endocytic pathway (CLIC-GEEC, CG pathway) is considered a bulk internalization route for the fluid phase, glycosylated membrane proteins and lipids. While the core molecular players of CG-endocytosis have been recently defined, evidence of cargo-specific adaptors or selective uptake of proteins for the pathway are lacking. Here we identify the actin-binding protein Swiprosin-1 (Swip1, EFHD2) as a cargo-specific adaptor for CG-endocytosis. Swip1 couples active Rab21-associated integrins with key components of the CG-endocytic machinery-Arf1, IRSp53 and actin-and is critical for integrin endocytosis. Through this function, Swip1 supports integrin-dependent cancer-cell migration and invasion, and is a negative prognostic marker in breast cancer. Our results demonstrate a previously unknown cargo selectivity for the CG pathway and a role for specific adaptors in recruitment into this endocytic route.

Shutting the gate: targeting endocytosis in acute leukemia

Endocytosis entails selective packaging of cell surface cargos in cytoplasmic vesicles, thereby controlling key intrinsic cellular processes as well as the response of normal and malignant cells to their microenvironment. The purpose of this review is to outline the latest advances in the development of endocytosis-targeting therapeutic strategies in hematological malignancies.

Endocytosis in the context-dependent regulation of individual and collective cell properties

Endocytosis allows cells to transport particles and molecules across the plasma membrane. In addition, it is involved in the termination of signalling through receptor downmodulation and degradation. This traditional outlook has been substantially modified in recent years by discoveries that endocytosis and subsequent trafficking routes have a profound impact on the positive regulation and propagation of signals, being key for the spatiotemporal regulation of signal transmission in cells. Accordingly, endocytosis and membrane trafficking regulate virtually every aspect of cell physiology and are frequently subverted in pathological conditions. Two key aspects of endocytic control over signalling are coming into focus: context-dependency and long-range effects. First, endocytic-regulated outputs are not stereotyped but heavily dependent on the cell-specific regulation of endocytic networks. Second, endocytic regulation has an impact not only on individual cells but also on the behaviour of cellular collectives. Herein, we will discuss recent advancements in these areas, highlighting how endocytic trafficking impacts complex cell properties, including cell polarity and collective cell migration, and the relevance of these mechanisms to disease, in particular cancer.

Amyloid-β induced membrane damage instigates tunneling nanotube-like conduits by p21-activated kinase dependent actin remodulation

Alzheimer's disease (AD) pathology progresses gradually via anatomically connected brain regions. Direct transfer of amyloid-β_1-42 oligomers (oAβ) between connected neurons has been shown, however, the mechanism is not fully revealed. We observed formation of oAβ induced tunneling nanotubes (TNTs)-like nanoscaled f-actin containing membrane conduits, in differentially differentiated SH-SY5Y neuronal models. Time-lapse images showed that oAβ propagate from one cell to another via TNT-like structures. Preceding the formation of TNT-like conduits, we detected oAβ_-induced plasma membrane (PM) damage and calcium-dependent repair through lysosomal-exocytosis, followed by massive endocytosis to re-establish the PM. Massive endocytosis was monitored by an influx of the membrane-staining dye TMA-DPH and PM damage was quantified by propidium iodide influx in the absence of Ca²⁺. The massive endocytosis eventually caused accumulation of internalized oAβ in Lamp1 positive multivesicular bodies/lysosomes via the actin cytoskeleton remodulating p21-activated kinase1 (PAK1) dependent endocytic pathway. Three-dimensional quantitative confocal imaging, structured illumination superresolution microscopy, and flowcytometry quantifications revealed that oAβ induces activation of phospho-PAK1, which modulates the formation of long stretched f-actin extensions between cells. Moreover, the formation of TNT-like conduits was inhibited by preventing PAK1-dependent internalization of oAβ using the small-molecule inhibitor IPA-3, a highly selective cell-permeable auto-regulatory inhibitor of PAK1. The present study reveals that the TNT-like conduits are probably instigated as a consequence of oAβ induced PM damage and repair process, followed by PAK1 dependent endocytosis and actin remodeling, probably to maintain cell surface expansion and/or membrane tension in equilibrium.

Cholera Toxin as a Probe for Membrane Biology

Cholera toxin B-subunit (CTxB) has emerged as one of the most widely utilized tools in membrane biology and biophysics. CTxB is a homopentameric stable protein that binds tightly to up to five GM1 glycosphingolipids. This provides a robust and tractable model for exploring membrane structure and its dynamics including vesicular trafficking and nanodomain assembly. Here, we review important advances in these fields enabled by use of CTxB and its lipid receptor GM1.

Dynamic interplay between cell membrane tension and clathrin-mediated endocytosis

Deformability of the plasma membrane, the outermost surface of metazoan cells, allows cells to be dynamic, mobile and flexible. Factors that affect this deformability, such as tension on the membrane, can regulate a myriad of cellular functions, including membrane resealing, cell motility, polarisation, shape maintenance, membrane area control and endocytic vesicle trafficking. This review focuses on mechanoregulation of clathrin-mediated endocytosis (CME). We first delineate the origins of cell membrane tension and the factors that yield to its spatial and temporal fluctuations within cells. We then review the recent literature demonstrating that tension on the membrane is a fast-acting and reversible regulator of CME. Finally, we discuss tension-based regulation of endocytic clathrin coat formation during physiological processes.

Regulating the uptake of poly(N-(2-hydroxypropyl) methacrylamide)-based micelles in cells cultured on micropatterned surfaces

Cellular uptake of nanoparticles plays a crucial role in cell-targeted biomedical applications. Despite abundant studies trying to understand the interaction between nanoparticles and cells, the influence of cell geometry traits such as cell spreading area and cell shape on the uptake of nanoparticles remains unclear. In this study, poly(vinyl alcohol) is micropatterned on polystyrene cell culture plates using ultraviolet photolithography to control the spreading area and shape of individual cells. The effects of these factors on the cellular uptake of poly(N-(2-hydroxypropyl)methacrylamide)-based micelles were investigated at a single-cell level. Human carcinoma MCF-7 and A549 cells as well as normal Hs-27 and MRC-5 fibroblasts were cultured on micropatterned surfaces. MCF-7 and A549 cells, both with larger sizes, had a higher total micelle uptake. However, the uptake of Hs-27 and MRC-5 cells decreased with increasing spreading area. In terms of cell shapes, MCF-7 and A549 cells with round shapes showed a higher micelle uptake, while those with a square shape had a lower cellular uptake. On the other hand, Hs-27 and MRC-5 cells showed opposite behaviors. The results indicate that the geometry of cells can influence the nanoparticle uptake and may shed light on the design of functional nanoparticles.

Strategies to target SARS-CoV-2 entry and infection using dual mechanisms of inhibition by acidification inhibitors

Many viruses utilize the host endo-lysosomal network for infection. Tracing the endocytic itinerary of SARS-CoV-2 can provide insights into viral trafficking and aid in designing new therapeutic strategies. Here, we demonstrate that the receptor binding domain (RBD) of SARS-CoV-2 spike protein is internalized via the pH-dependent CLIC/GEEC (CG) endocytic pathway in human gastric-adenocarcinoma (AGS) cells expressing undetectable levels of ACE2. Ectopic expression of ACE2 (AGS-ACE2) results in RBD traffic via both CG and clathrin-mediated endocytosis. Endosomal acidification inhibitors like BafilomycinA1 and NH4Cl, which inhibit the CG pathway, reduce the uptake of RBD and impede Spike-pseudoviral infection in both AGS and AGS-ACE2 cells. The inhibition by BafilomycinA1 was found to be distinct from Chloroquine which neither affects RBD uptake nor alters endosomal pH, yet attenuates Spike-pseudovirus entry. By screening a subset of FDA-approved inhibitors for functionality similar to BafilomycinA1, we identified Niclosamide as a SARS-CoV-2 entry inhibitor. Further validation using a clinical isolate of SARS-CoV-2 in AGS-ACE2 and Vero cells confirmed its antiviral effect. We propose that Niclosamide, and other drugs which neutralize endosomal pH as well as inhibit the endocytic uptake, could provide broader applicability in subverting infection of viruses entering host cells via a pH-dependent endocytic pathway.

Unconventional endocytic mechanisms

Endocytosis mediates the uptake of extracellular proteins, micronutrients and transmembrane cell surface proteins. Importantly, many viruses, toxins and bacteria hijack endocytosis to infect cells. The canonical pathway is clathrin-mediated endocytosis (CME) and is active in all eukaryotic cells to support critical house-keeping functions. Unconventional mechanisms of endocytosis exit in parallel of CME, to internalize specific cargoes and support various cellular functions. These clathrin-independent endocytic (CIE) routes use three distinct mechanisms: acute signaling-induced membrane remodeling drives macropinocytosis, activity-dependent bulk endocytosis (ADBE), massive endocytosis (MEND) and EGFR non-clathrin endocytosis (EGFR-NCE). Cargo capture and local membrane deformation by cytosolic proteins is used by fast endophilin-mediated endocytosis (FEME), IL-2Rβ endocytosis and ultrafast endocytosis at synapses. Finally, the formation of endocytic pits by clustering of extracellular lipids or cargoes according to the Glycolipid-Lectin (GL-Lect) hypothesis mediates the uptake of SV40 virus, Shiga and cholera toxins, and galectin-clustered receptors by the CLIC/GEEC and the endophilin-A3-mediated CIE.

EpCAM promotes endosomal modulation of the cortical RhoA zone for epithelial organization

At the basis of cell shape and behavior, the organization of actomyosin and its ability to generate forces are widely studied. However, the precise regulation of this contractile network in space and time is unclear. Here, we study the role of the epithelial-specific protein EpCAM, a contractility modulator, in cell shape and motility. We show that EpCAM is required for stress fiber generation and front-rear polarity acquisition at the single cell level. In fact, EpCAM participates in the remodeling of a transient zone of active RhoA at the cortex of spreading epithelial cells. EpCAM and RhoA route together through the Rab35/EHD1 fast recycling pathway. This endosomal pathway spatially organizes GTP-RhoA to fine tune the activity of actomyosin resulting in polarized cell shape and development of intracellular stiffness and traction forces. Impairment of GTP-RhoA endosomal trafficking either by silencing EpCAM or by expressing Rab35/EHD1 mutants prevents proper myosin-II activity, stress fiber formation and ultimately cell polarization. Collectively, this work shows that the coupling between co-trafficking of EpCAM and RhoA, and actomyosin rearrangement is pivotal for cell spreading, and advances our understanding of how biochemical and mechanical properties promote cell plasticity.

Key principles and methods for studying the endocytosis of biological and nanoparticle therapeutics

Endocytosis is a critical step in the process by which many therapeutic nanomedicines reach their intracellular targets. Our understanding of cellular uptake mechanisms has developed substantially in the past five years. However, these advances in cell biology have not fully translated to the nanoscience and therapeutics literature. Misconceptions surrounding the role of different endocytic pathways and how to study these pathways are hindering progress in developing improved nanoparticle therapies. Here, we summarize the latest insights into cellular uptake mechanisms and pathways. We highlight limitations of current systems to study endocytosis, particularly problems with non-specific inhibitors. We also summarize alternative genetic approaches to robustly probe these pathways and discuss the need to understand how cells endocytose particles in vivo. We hope that this critical assessment of the current methods used in studying nanoparticle uptake will guide future studies at the interface of cell biology and nanomedicine.

Endomembrane Tension and Trafficking

Eukaryotic cells employ diverse uptake mechanisms depending on their specialized functions. While such mechanisms vary widely in their defining criteria: scale, molecular machinery utilized, cargo selection, and cargo destination, to name a few, they all result in the internalization of extracellular solutes and fluid into membrane-bound endosomes. Upon scission from the plasma membrane, this compartment is immediately subjected to extensive remodeling which involves tubulation and vesiculation/budding of the limiting endomembrane. This is followed by a maturation process involving concomitant retrograde transport by microtubule-based motors and graded fusion with late endosomes and lysosomes, organelles that support the degradation of the internalized content. Here we review an important determinant for sorting and trafficking in early endosomes and in lysosomes; the control of tension on the endomembrane. Remodeling of endomembranes is opposed by high tension (caused by high hydrostatic pressure) and supported by the relief of tension. We describe how the timely and coordinated efflux of major solutes along the endocytic pathway affords the cell control over such tension. The channels and transporters that expel the smallest components of the ingested medium from the early endocytic fluid are described in detail as these systems are thought to enable endomembrane deformation by curvature-sensing/generating coat proteins. We also review similar considerations for the lysosome where resident hydrolases liberate building blocks from luminal macromolecules and transporters flux these organic solutes to orchestrate trafficking events. How the cell directs organellar trafficking based on the luminal contents of organelles of the endocytic pathway is not well-understood, however, we propose that the control over membrane tension by solute transport constitutes one means for this to ensue.

Progress in Nanorobotics for Advancing Biomedicine

Nanorobotics, which has long been a fantasy in the realm of science fiction, is now a reality due to the considerable developments in diverse fields including chemistry, materials, physics, information and nanotechnology in the past decades. Not only different prototypes of nanorobots whose sizes are nanoscale are invented for various biomedical applications, but also robotic nanomanipulators which are able to handle nano-objects obtain substantial achievements for applications in biomedicine. The outstanding achievements in nanorobotics have significantly expanded the field of medical robotics and yielded novel insights into the underlying mechanisms guiding life activities, remarkably showing an emerging and promising way for advancing the diagnosis & treatment level in the coming era of personalized precision medicine. In this review, the recent advances in nanorobotics (nanorobots, nanorobotic manipulations) for biomedical applications are summarized from several facets (including molecular machines, nanomotors, DNA nanorobotics, and robotic nanomanipulators), and the future perspectives are also presented.

Forcing a growth factor response - tissue-stiffness modulation of integrin signaling and crosstalk with growth factor receptors

Research throughout the 90s established that integrin crosstalk with growth factor receptors stimulates robust growth factor signaling. These insights were derived chiefly from comparing adherent versus suspension cell cultures. Considering the new understanding that mechanosensory inputs tune adhesion signaling, it is now timely to revisit this crosstalk in different mechanical environments. Here, we present a brief historical perspective on integrin signaling against the backdrop of the mechanically diverse extracellular microenvironment, then review the evidence supporting the mechanical regulation of integrin crosstalk with growth factor signaling. We discuss early studies revealing distinct signaling consequences for integrin occupancy (binding to matrix) and aggregation (binding to immobile ligand). We consider how the mechanical environments encountered in vivo intersect with this diverse signaling, focusing on receptor endocytosis. We discuss the implications of mechanically tuned integrin signaling for growth factor signaling, using the epidermal growth factor receptor (EGFR) as an illustrative example. We discuss how the use of rigid tissue culture plastic for cancer drug screening may select agents that lack efficacy in the soft in vivo tissue environment. Tuning of integrin signaling via external mechanical forces in vivo and subsequent effects on growth factor signaling thus has implications for normal cellular physiology and anti-cancer therapies.

Membrane Tension Gates ERK-Mediated Regulation of Pluripotent Cell Fate

Cell fate transitions are frequently accompanied by changes in cell shape and mechanics. However, how cellular mechanics affects the instructive signaling pathways controlling cell fate is poorly understood. To probe the interplay between shape, mechanics, and fate, we use mouse embryonic stem cells (ESCs), which change shape as they undergo early differentiation. We find that shape change is regulated by a β-catenin-mediated decrease in RhoA activity and subsequent decrease in the plasma membrane tension. Strikingly, preventing a decrease in membrane tension results in early differentiation defects in ESCs and gastruloids. Decreased membrane tension facilitates the endocytosis of FGF signaling components, which activate ERK signaling and direct the exit from the ESC state. Increasing Rab5a-facilitated endocytosis rescues defective early differentiation. Thus, we show that a mechanically triggered increase in endocytosis regulates early differentiation. Our findings are of fundamental importance for understanding how cell mechanics regulates biochemical signaling and therefore cell fate.

Caveolae: Mechanosensing and mechanotransduction devices linking membrane trafficking to mechanoadaptation

Mechanical forces (extracellular matrix stiffness, vascular shear stress, and muscle stretching) reaching the plasma membrane (PM) determine cell behavior. Caveolae are PM-invaginated nanodomains with specific lipid and protein composition. Being highly abundant in mechanically challenged tissues (muscles, lungs, vessels, and adipose tissues), they protect cells from mechanical stress damage. Caveolae flatten upon increased PM tension, enabling both force sensing and accommodation, critical for cell mechanoprotection and homeostasis. Thus, caveolae are highly plastic, ranging in complexity from flattened membranes to vacuolar invaginations surrounded by caveolae-rosettes-which also contribute to mechanoprotection. Caveolar components crosstalk with mechanotransduction pathways and recent studies show that they translocate from the PM to the nucleus to convey stress information. Furthermore, caveolae components can regulate membrane traffic from/to the PM to adapt to environmental mechanical forces. The interdependence between lipids and caveolae starts to be understood, and the relevance of caveolae-dependent membrane trafficking linked to mechanoadaption to different physiopathological processes is emerging.

Vesicular Transport Machinery in Brain Endothelial Cells: What We Know and What We Do not

The vesicular transport machinery regulates numerous essential functions in cells such as cell polarity, signaling pathways, and the transport of receptors and their cargoes. From a pharmaceutical perspective, vesicular transport offers avenues to facilitate the uptake of therapeutic agents into cells and across cellular barriers. In order to improve receptor-mediated transcytosis of biologics across the blood-brain barrier and into the diseased brain, a detailed understanding of intracellular transport mechanisms is essential. The vesicular transport machinery is a highly complex network and involves an array of protein complexes, cytosolic adaptor proteins, and the subcellular structures of the endo-lysosomal system. The endo-lysosomal system includes several types of vesicular entities such as early, late, and recycling endosomes, exosomes, ectosomes, retromer-coated vesicles, lysosomes, trans-endothelial channels, and tubules. While extensive research has been done on the trafficking system in many cell types, little is known about vesicular trafficking in brain endothelial cells. Consequently, assumptions on the transport system in endothelial cells are based on findings in polarised epithelial cells, although recent studies have highlighted differences in the endothelial system. This review highlights aspects of the vesicular trafficking machinery in brain endothelial cells, including recent findings, limitations, and opportunities for further studies.

Caveolin1 Tyrosine-14 Phosphorylation: Role in Cellular Responsiveness to Mechanical Cues

The plasma membrane is a dynamic lipid bilayer that engages with the extracellular microenvironment and intracellular cytoskeleton. Caveolae are distinct plasma membrane invaginations lined by integral membrane proteins Caveolin1, 2, and 3. Caveolae formation and stability is further supported by additional proteins including Cavin1, EHD2, Pacsin2 and ROR1. The lipid composition of caveolar membranes, rich in cholesterol and phosphatidylserine, actively contributes to caveolae formation and function. Post-translational modifications of Cav1, including its phosphorylation of the tyrosine-14 residue (pY14Cav1) are vital to its function in and out of caveolae. Cells that experience significant mechanical stress are seen to have abundant caveolae. They play a vital role in regulating cellular signaling and endocytosis, which could further affect the abundance and distribution of caveolae at the PM, contributing to sensing and/or buffering mechanical stress. Changes in membrane tension in cells responding to multiple mechanical stimuli affects the organization and function of caveolae. These mechanical cues regulate pY14Cav1 levels and function in caveolae and focal adhesions. This review, along with looking at the mechanosensitive nature of caveolae, focuses on the role of pY14Cav1 in regulating cellular mechanotransduction.

Endomembranes: Unsung Heroes of Mechanobiology?

Mechanical stimuli have profound effects on the cellular architecture and functions. Over the past two decades, considerable progress has been made in unraveling the molecular machineries that confer cells the ability to sense and transduce mechanical input into biochemical signals. This has resulted in the identification of several force-sensing proteins or mechanically activated ion channels distributed throughout most cell types, whereby the plasma membrane, cytoskeleton, and the nucleus have garnered much attention. Although organelles from the endomembrane system make up significant portion of cell volume and play pivotal roles in the spatiotemporal distribution of signaling molecules, they have received surprisingly little attention in mechanobiology. In this mini-review, we summarize results that document participation of the endomembrane system in sensing and responding to mechanical cues.

Pay attention to membrane tension: Mechanobiology of the cell surface

The cell surface is a mechanobiological unit that encompasses the plasma membrane, its interacting proteins, and the complex underlying cytoskeleton. Recently, attention has been directed to the mechanics of the plasma membrane, and in particular membrane tension, which has been linked to diverse cellular processes such as cell migration and membrane trafficking. However, how tension across the plasma membrane is regulated and propagated is still not completely understood. Here, we review recent efforts to study the interplay between membrane tension and the cytoskeletal machinery and how they control cell form and function. We focus on factors that have been proposed to affect the propagation of membrane tension and as such could determine whether it can act as a global or local regulator of cell behavior. Finally, we discuss the limitations of the available tool kit as new approaches that reveal its dynamics in cells are needed to decipher how membrane tension regulates diverse cellular processes.

Mechanobiology of microvesicle release, uptake, and microvesicle-mediated activation

Microvesicles are small, membrane-bound vesicles that are shed from the plasma membrane of cells into the extracellular space. Microvesicles contain a variety of cargo not typically thought to be released from cells, including receptor tyrosine kinases, cytosolic signaling proteins, and microRNAs, which are transferred from donor cells to recipient cells. The transfer of microvesicle cargo can result in the transformation of recipient cells thereby supporting disease progression, including modified fibroblast metabolism, epithelial cell contractility, vascular remodeling, and immune cell inflammatory signaling. Additionally, microvesicles are believed to play prominent roles in cell-cell communication and disease progression as they are detected at elevated concentrations in diseased tissues. As microvesicle uptake by recipient cells can modulate cell function to promote disease progression, understanding the mechanisms and mechanosensitivity of microvesicle release, internalization, and the resulting signaling is crucial to fully comprehend their functions in disease. Here, we review recent advances in the understanding of actomyosin-regulated microvesicle biogenesis, microvesicle uptake via pinocytosis, and the resulting cellular transformation. We discuss the effects of altered cell contractility, mode of cell migration, and extracellular matrix compliance on microvesicle signaling, with direct implications in disease progression and identifying future therapeutic targets.

A molecular sensor to quantify the localization of proteins, DNA and nanoparticles in cells

Intracellular trafficking governs receptor signaling, pathogenesis, immune responses and fate of nanomedicines. These processes are typically tracked by observing colocalization of fluorescent markers using confocal microscopy. However, this method is low throughput, limited by the resolution of microscopy, and can miss fleeting interactions. To address this, we developed a localization sensor composed of a quenched SNAP-tag substrate (SNAP_Switch) that can be conjugated to biomolecules using click chemistry. SNAP_Switch enables quantitative detection of trafficking to locations of interest within live cells using flow cytometry. Using SNAP_Switch, we followed the trafficking of DNA complexes from endosomes into the cytosol and nucleus. We show that antibodies against the transferrin or hyaluronan receptor are initially sorted into different compartments following endocytosis. In addition, we can resolve which side of the cellular membrane material was located. These results demonstrate SNAP_Switch is a high-throughput and broadly applicable tool to quantitatively track localization of materials in cells.

Determining the trafficking of intracellular material is commonly done by colocalisation analysis using microscopy. Here the authors monitor trafficking of select cargo by measuring the conversion of quenched SNAP-tag substrates by subcellularly-localised SNAP-tag and detection by flow cytometry.

Membrane Heterogeneity Controls Cellular Endocytic Trafficking

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

Role of Membrane Tension Sensitive Endocytosis and Rho GTPases in the Uptake of the Alzheimer's Disease Peptide Aβ(1-42)

Intraneuronal accumulation of amyloid-β (Aβ) is an early pathological signum of Alzheimer's disease, and compartments of the endolysosomal system have been implicated in both seeding and cell-cell propagation of Aβ aggregation. We have studied how clathrin-independent mechanisms contribute to Aβ endocytosis, exploring pathways that are sensitive to changes in membrane tension and the regulation of Rho GTPases. Using live cell confocal microscopy and flow cytometry, we show the uptake of monomeric Aβ(1-42) into endocytic vesicles and vacuole-like dilations, following relaxation of osmotic pressure-induced cell membrane tension. This indicates Aβ(1-42) uptake via clathrin independent carriers (CLICs), although overexpression of the bar-domain protein GRAF1, a key regulator of CLICs, had no apparent effect. We furthermore report reduced Aβ(1-42) uptake following overexpression of constitutively active forms of the Rho GTPases Cdc42 and RhoA, whereas modulation of Rac1, which is linked to macropinosome formation, had no effect. Our results confirm that uptake of Aβ(1-42) is clathrin- and dynamin-independent and point to the involvement of a new and distinct clathrin-independent endocytic mechanism which is similar to uptake via CLICs or macropinocytosis but that also appear to involve yet uncharacterized molecular players.

Mechanical Effect on Gene Transfection Based on Dielectric Elastomer Actuator

Gene transfection has been widely applied in genome function and gene therapy. Although many efforts have been focused on designing carrier materials and transfection methods, the influence of mechanical stimulation on gene transfection efficiency has rarely been studied. Herein, dielectric elastomer actuator (DEA)-based stimulation bioreactors are designed to generate tensile and contractile stress on cells simultaneously. With the example of the EGFP transfection, cells with high membrane tension in the stretching stimulation regions had lower transfection efficiency, while the transfection efficiency of cells in the compressing regions tended to increase. Besides, the duty cycle and loading frequency of the applied stress on cells were also important factors that affect gene transfection efficiency. Furthermore, the pathways of cell endocytosis with the effect of mechanical stimulation were explored on the mechanism for the change of EGFP transfection efficiency. This design of the DEA-based bioreactor, as a strategy to study gene transfection efficiency, could be helpful for developing efficient transfection methods.

Mechanical Regulation of Endocytosis: New Insights and Recent Advances

Endocytosis is a mechanosensitive process. It involves remodeling of the plasma membrane from a flat shape to a budded morphology, often at the sub-micrometer scale. This remodeling process is energy-intensive and is influenced by mechanical factors such as membrane tension, membrane rigidity, and physical properties of cargo and extracellular surroundings. The cellular responses to a variety of mechanical factors by distinct endocytic pathways are important for cells to counteract rapid and extreme disruptions in the mechanohomeostasis of cells. Recent advances in microscopy and mechanical manipulation at the cellular scale have led to new discoveries of mechanoregulation of endocytosis by the aforementioned factors. While factors such as membrane tension and membrane rigidity are generally shown to inhibit endocytosis, other mechanical stimuli have complex relationships with endocytic pathways. At this juncture, it is now possible to utilize experimental techniques to interrogate theoretical predictions on mechanoregulation of endocytosis in cells and even living organisms.

Osmotic Concentration-Controlled Particle Uptake and Wrapping-Induced Lysis of Cells and Vesicles

In vivo, high protein and ion concentrations determine the preferred volumes of cells, organelles, and vesicles. Deformations of their lipid-bilayer membranes by nanoparticle wrapping reduce the interior volumes available to solutes and thus induce large osmotic pressure differences. Osmotic concentration can therefore be an important control parameter for wrapping of nanoparticles. We employ a curvature-elasticity model of the membrane and contact interaction with spherical particles to study their wrapping at initially spherical vesicles. Although the continuous particle-binding transition is independent of the presence of solutes, the discontinuous envelopment transition shifts to higher adhesion strengths and the corresponding energy barrier increases with increasing osmotic concentration. High osmotic concentrations stabilize partial-wrapped, membrane-bound states for both, particle attachment to the inside and the outside. In this regime, wrapping of particles controls membrane tension, with power-law dependencies on osmotic concentration and adhesion strength. For high adhesion strengths, particle wrapping can lead to the opening of mechanosensitive channels in cell membranes and to lysis. Membrane tension-induced stabilization of partial-wrapped states as well as wrapping-induced lysis play important roles not only for desired mechano-bacteriocidal effects of engineered nanomaterials but may also determine viral burst sizes of bacteria and control endocytosis for mammalian cells.

Integrins as biomechanical sensors of the microenvironment

Integrins, and integrin-mediated adhesions, have long been recognized to provide the main molecular link attaching cells to the extracellular matrix (ECM) and to serve as bidirectional hubs transmitting signals between cells and their environment. Recent evidence has shown that their combined biochemical and mechanical properties also allow integrins to sense, respond to and interact with ECM of differing properties with exquisite specificity. Here, we review this work first by providing an overview of how integrin function is regulated from both a biochemical and a mechanical perspective, affecting integrin cell-surface availability, binding properties, activation or clustering. Then, we address how this biomechanical regulation allows integrins to respond to different ECM physicochemical properties and signals, such as rigidity, composition and spatial distribution. Finally, we discuss the importance of this sensing for major cell functions by taking cell migration and cancer as examples.

Cell spread area and traction forces determine myosin-II-based cortex thickness regulation

Actomyosin network under the plasma membrane of cells forms a cortical layer that regulates cellular deformations during different processes. What regulates the cortex? Characterized by its thickness, it is believed to be regulated by actin dynamics, filament-length regulators and myosin motor proteins. However, its regulation by cellular morphology (e.g. cell spread area) or mechanical microenvironment (e.g. substrate stiffness) has remained largely unexplored. In this study, super- and high-resolution imaging of actin in CHO cells demonstrates that at high spread areas (>450 μm2), the cortex is thinner, better separated as layers, and sensitive to deactivation of myosin II motors or reduction of substrate stiffness (and traction forces). In less spread cells (<400 μm2) such perturbations do not elicit a response. Myosin IIA's mechanosensing is limited here due to its lowered actin-bound fraction and higher turnover rate. Cofilin, in line with its competitive inhibitory role, is found to be overexpressed in these cells. To establish the causal relation, we initiate a spread area drop by de-adhesion and find enhanced actin dynamics and fragmentation along with oscillations and increase in thickness. This is more correlated to the reduction of traction forces than the endocytosis-based reduction in cell volume. Cortex thickness control by spread area is also found be true during differentiation of THP-1 monocytes to macrophages. Thus, we propose that spread area regulates cortex and its thickness by traction-based mechanosensing of myosin II.

Cell Nanomechanics Based on Dielectric Elastomer Actuator Device

As a frontier of biology, mechanobiology plays an important role in tissue and biomedical engineering. It is a common sense that mechanical cues under extracellular microenvironment affect a lot in regulating the behaviors of cells such as proliferation and gene expression, etc. In such an interdisciplinary field, engineering methods like the pneumatic and motor-driven devices have been employed for years. Nevertheless, such techniques usually rely on complex structures, which cost much but not so easy to control. Dielectric elastomer actuators (DEAs) are well known as a kind of soft actuation technology, and their research prospect in biomechanical field is gradually concerned due to their properties just like large deformation (> 100%) and fast response (< 1 ms). In addition, DEAs are usually optically transparent and can be fabricated into small volume, which make them easy to cooperate with regular microscope to realize real-time dynamic imaging of cells. This paper first reviews the basic components, principle, and evaluation of DEAs and then overview some corresponding applications of DEAs for cellular mechanobiology research. We also provide a comparison between DEA-based bioreactors and current custom-built devices and share some opinions about their potential applications in the future according to widely reported results via other methods.

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.

Artificial Cell Membranes Interfaced with Optical Tweezers: A Versatile Microfluidics Platform for Nanomanipulation and Mechanical Characterization

Cell lipid membranes are the site of vital biological processes, such as motility, trafficking, and sensing, many of which involve mechanical forces. Elucidating the interplay between such bioprocesses and mechanical forces requires the use of tools that apply and measure piconewton-level forces, e.g., optical tweezers. Here, we introduce the combination of optical tweezers with free-standing lipid bilayers, which are fully accessible on both sides of the membrane. In the vicinity of the lipid bilayer, optical trapping would normally be impossible due to optical distortions caused by pockets of the solvent trapped within the membrane. We solve this by drastically reducing the size of these pockets via tuning of the solvent and flow cell material. In the resulting flow cells, lipid nanotubes are straightforwardly pushed or pulled and reach lengths above half a millimeter. Moreover, the controlled pushing of a lipid nanotube with an optically trapped bead provides an accurate and direct measurement of important mechanical properties. In particular, we measure the membrane tension of a free-standing membrane composed of a mixture of dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) to be 4.6 × 10^-6 N/m. We demonstrate the potential of the platform for biophysical studies by inserting the cell-penetrating trans-activator of transcription (TAT) peptide in the lipid membrane. The interactions between the TAT peptide and the membrane are found to decrease the value of the membrane tension to 2.1 × 10^-6 N/m. This method is also fully compatible with electrophysiological measurements and presents new possibilities for the study of membrane mechanics and the creation of artificial lipid tube networks of great importance in intra- and intercellular communication.

An acute decrease in plasma membrane tension induces macropinocytosis via PLD2 activation

Internalization of macromolecules and membrane into cells through endocytosis is critical for cellular growth, signaling and plasma membrane (PM) tension homeostasis. Although endocytosis is responsive to both biochemical and physical stimuli, how physical cues modulate endocytic pathways is less understood. Contrary to the accumulating discoveries on the effects of increased PM tension on endocytosis, less is known about how a decrease of PM tension impacts on membrane trafficking. Here, we reveal that an acute decrease of PM tension results in phosphatidic acid (PA) production, F-actin and phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂]-enriched dorsal membrane ruffling and subsequent macropinocytosis in myoblasts. The PA production induced by decreased PM tension depends on phospholipase D2 (PLD2) activation via PLD2 nanodomain disintegration. Furthermore, the 'decreased PM tension-PLD2-macropinocytosis' pathway is prominent in myotubes, reflecting a potential mechanism of PM tension homeostasis upon intensive muscle stretching and relaxation. Together, we identify a new mechanotransduction pathway that converts an acute decrease in PM tension into PA production and then initiates macropinocytosis via actin and PI(4,5)P₂-mediated processes.

The plasma membrane as a mechanochemical transducer

Cells are constantly submitted to external mechanical stresses, which they must withstand and respond to. By forming a physical boundary between cells and their environment that is also a biochemical platform, the plasma membrane (PM) is a key interface mediating both cellular response to mechanical stimuli, and subsequent biochemical responses. Here, we review the role of the PM as a mechanosensing structure. We first analyse how the PM responds to mechanical stresses, and then discuss how this mechanical response triggers downstream biochemical responses. The molecular players involved in PM mechanochemical transduction include sensors of membrane unfolding, membrane tension, membrane curvature or membrane domain rearrangement. These sensors trigger signalling cascades fundamental both in healthy scenarios and in diseases such as cancer, which cells harness to maintain integrity, keep or restore homeostasis and adapt to their external environment. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

Structural and signaling role of lipids in plasma membrane repair

The plasma membrane forms the physical barrier between the cytoplasm and extracellular space, allowing for biochemical reactions necessary for life to occur. Plasma membrane damage needs to be rapidly repaired to avoid cell death. This relies upon the coordinated action of the machinery that polarizes the repair response to the site of injury, resulting in resealing of the damaged membrane and subsequent remodeling to return the injured plasma membrane to its pre-injury state. As lipids comprise the bulk of the plasma membrane, the acts of injury, resealing, and remodeling all directly impinge upon the plasma membrane lipids. In addition to their structural role in shaping the physical properties of the plasma membrane, lipids also play an important signaling role in maintaining plasma membrane integrity. While much attention has been paid to the involvement of proteins in the membrane repair pathway, the role of lipids in facilitating plasma membrane repair remains poorly studied. Here we will discuss the current knowledge of how lipids facilitate plasma membrane repair by regulating membrane structure and signaling to coordinate the repair response, and will briefly note how lipid involvement extends beyond plasma membrane repair to the tissue repair response.

Forces and constraints controlling podosome assembly and disassembly

Podosomes are a singular category of integrin-mediated adhesions important in the processes of cell migration, matrix degradation and cancer cell invasion. Despite a wealth of biochemical studies, the effects of mechanical forces on podosome integrity and dynamics are poorly understood. Here, we show that podosomes are highly sensitive to two groups of physical factors. First, we describe the process of podosome disassembly induced by activation of myosin-IIA filament assembly. Next, we find that podosome integrity and dynamics depends upon membrane tension and can be experimentally perturbed by osmotic swelling and deoxycholate treatment. We have also found that podosomes can be disrupted in a reversible manner by single or cyclic radial stretching of the substratum. We show that disruption of podosomes induced by osmotic swelling is independent of myosin-II filaments. The inhibition of the membrane sculpting protein, dynamin-II, but not clathrin, resulted in activation of myosin-IIA filament formation and disruption of podosomes. The effect of dynamin-II inhibition on podosomes was, however, independent of myosin-II filaments. Moreover, formation of organized arrays of podosomes in response to microtopographic cues (the ridges with triangular profile) was not accompanied by reorganization of myosin-II filaments. Thus, mechanical elements such as myosin-II filaments and factors affecting membrane tension/sculpting independently modulate podosome formation and dynamics, underlying a versatile response of these adhesion structures to intracellular and extracellular cues. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

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.

Cholesterol Depletion by MβCD Enhances Cell Membrane Tension and Its Variations-Reducing Integrity

Cholesterol depletion by methyl-β-cyclodextrin (MβCD) remodels the plasma membrane’s mechanics in cells and its interactions with the underlying cytoskeleton, whereas in red blood cells, it is also known to cause lysis. Currently it’s unclear if MβCD alters membrane tension or only enhances membrane-cytoskeleton interactions—and how this relates to cell lysis. We map membrane height fluctuations in single cells and observe that MβCD reduces temporal fluctuations robustly but flattens spatial membrane undulations only slightly. Utilizing models explicitly incorporating membrane confinement besides other viscoelastic factors, we estimate membrane mechanical parameters from the fluctuations’ frequency spectrum. This helps us conclude that MβCD enhances membrane tension and does so even on ATP-depleted cell membranes where this occurs despite reduction in confinement. Additionally, on cholesterol depletion, cell membranes display higher intracellular heterogeneity in the amplitude of spatial undulations and membrane tension. MβCD also has a strong impact on the cell membrane’s tenacity to mechanical stress, making cells strongly prone to rupture on hypo-osmotic shock with larger rupture diameters—an effect not hindered by actomyosin perturbations. Our study thus demonstrates that cholesterol depletion increases membrane tension and its variability, making cells prone to rupture independent of the cytoskeletal state of the cell.

Recent advances in clathrin-independent endocytosis

Endocytic pathways are broadly classified into clathrin dependent and independent on the basis of the requirement for the coat protein, clathrin. The molecular pathways and mechanisms underlying the formation of clathrin-independent pathways are still being explored, and this review summarizes recent advances and emerging functional roles of these diverse pathways. In particular, this review will discuss the growing consensus on the role of BAR domain proteins and the actin machinery in different clathrin-independent pathways and its significance to the functions fulfilled by these endocytic pathways.