The organisation of the cell membrane: do proteins rule lipids?

Disease Mechanisms of Multiple System Atrophy: What a Parallel Between the Form of Pasta and the Alpha-Synuclein Assemblies Involved in MSA and PD Tells Us

Intracellular deposits rich in aggregated alpha-synuclein that appear within the central nervous system are intimately associated to Parkinson's disease and multiple system atrophy. While it is understandable that the aggregation of proteins, which share no primary structure identity, such as alpha-synuclein and tau protein, leads to different diseases, that of a given protein yielding distinct pathologies is counterintuitive. This short review relates molecular and mechanistic processes to the observed pathological diversity associated to alpha-synuclein aggregation.

Dynamic interfaces for contact-time control of colloidal interactions

Understanding pairwise interactions between colloidal particles out of equilibrium has a profound impact on dynamical processes such as colloidal self assembly. However, traditional colloidal interactions are effectively quasi-static on colloidal timescales and cannot be modulated out of equilibrium. A mechanism to dynamically tune the interactions during colloidal contacts can provide new avenues for self assembly and material design. In this work, we develop a framework based on polymer-coated colloids and demonstrate that in-plane surface mobility and mechanical relaxation of polymers at colloidal contact interfaces enable an effective, dynamic interaction. Combining analytical theory, simulations, and optical tweezer experiments, we demonstrate precise control of dynamic pair interactions over a range of pico-Newton forces and seconds timescales. Our model helps further the general understanding of out-of-equilibrium colloidal assemblies while providing extensive design freedom via interface modulation and nonequilibrium processing.

Fifty Years of the Fluid–Mosaic Model of Biomembrane Structure and Organization and Its Importance in Biomedicine with Particular Emphasis on Membrane Lipid Replacement

The Fluid–Mosaic Model has been the accepted general or basic model for biomembrane structure and organization for the last 50 years. In order to establish a basic model for biomembranes, some general principles had to be established, such as thermodynamic assumptions, various molecular interactions, component dynamics, macromolecular organization and other features. Previous researchers placed most membrane proteins on the exterior and interior surfaces of lipid bilayers to form trimolecular structures or as lipoprotein units arranged as modular sheets. Such membrane models were structurally and thermodynamically unsound and did not allow independent lipid and protein lateral movements. The Fluid–Mosaic Membrane Model was the only model that accounted for these and other characteristics, such as membrane asymmetry, variable lateral movements of membrane components, cis- and transmembrane linkages and dynamic associations of membrane components into multimolecular complexes. The original version of the Fluid–Mosaic Membrane Model was never proposed as the ultimate molecular description of all biomembranes, but it did provide a basic framework for nanometer-scale biomembrane organization and dynamics. Because this model was based on available 1960s-era data, it could not explain all of the properties of various biomembranes discovered in subsequent years. However, the fundamental organizational and dynamic aspects of this model remain relevant to this day. After the first generation of this model was published, additional data on various structures associated with membranes were included, resulting in the addition of membrane-associated cytoskeletal, extracellular matrix and other structures, specialized lipid–lipid and lipid–protein domains, and other configurations that can affect membrane dynamics. The presence of such specialized membrane domains has significantly reduced the extent of the fluid lipid membrane matrix as first proposed, and biomembranes are now considered to be less fluid and more mosaic with some fluid areas, rather than a fluid matrix with predominantly mobile components. However, the fluid–lipid matrix regions remain very important in biomembranes, especially those involved in the binding and release of membrane lipid vesicles and the uptake of various nutrients. Membrane phospholipids can associate spontaneously to form lipid structures and vesicles that can fuse with various cellular membranes to transport lipids and other nutrients into cells and organelles and expel damaged lipids and toxic hydrophobic molecules from cells and tissues. This process and the clinical use of membrane phospholipid supplements has important implications for chronic illnesses and the support of healthy mitochondria, plasma membranes and other cellular membrane structures.

Amphiphilic Gold Nanoparticles: A Biomimetic Tool to Gain Mechanistic Insights into Peptide-Lipid Interactions

Functional peptides are now widely used in a myriad of biomedical and clinical contexts, from cancer therapy and tumor targeting to the treatment of bacterial and viral infections. Underlying this diverse range of applications are the non-specific interactions that can occur between peptides and cell membranes, which, in many contexts, result in spontaneous internalization of the peptide within cells by avoiding energy-driven endocytosis. For this to occur, the amphipathicity and surface structural flexibility of the peptides play a crucial role and can be regulated by the presence of specific molecular residues that give rise to precise molecular events. Nevertheless, most of the mechanistic details regulating the encounter between peptides and the membranes of bacterial or animal cells are still poorly understood, thus greatly limiting the biomimetic potential of these therapeutic molecules. In this arena, finely engineered nanomaterials—such as small amphiphilic gold nanoparticles (AuNPs) protected by a mixed thiol monolayer—can provide a powerful tool for mimicking and investigating the physicochemical processes underlying peptide-lipid interactions. Within this perspective, we present here a critical review of membrane effects induced by both amphiphilic AuNPs and well-known amphiphilic peptide families, such as cell-penetrating peptides and antimicrobial peptides. Our discussion is focused particularly on the effects provoked on widely studied model cell membranes, such as supported lipid bilayers and lipid vesicles. Remarkable similarities in the peptide or nanoparticle membrane behavior are critically analyzed. Overall, our work provides an overview of the use of amphiphilic AuNPs as a highly promising tailor-made model to decipher the molecular events behind non-specific peptide-lipid interactions and highlights the main affinities observed both theoretically and experimentally. The knowledge resulting from this biomimetic approach could pave the way for the design of synthetic peptides with tailored functionalities for next-generation biomedical applications, such as highly efficient intracellular delivery systems.

Amyloid polymorphs and pathological diversities

The presence of protein aggregates within the central nervous system is intimately associated to debilitating neurodegenerative diseases. While the aggregation of proteins, that share no primary structure identity, is understandable in different diseases, that of a given protein yielding distinct pathologies is counterintuitive. This short review relates molecular and mechanistic processes to the observed pathological diversity.

Surfactin cyclic lipopeptides change the plasma membrane composition and lateral organization in mammalian cells

The specific structure and composition of the cell plasma membrane (PM) is crucial for many cellular processes and can be targeted by various substances with potential medical applications. In this context, biosurfactants (BS) constitute a promising group of natural compounds that possess several biological functions, including anticancer activity. Despite the efficiency of BS, their mode of action had never been elucidated before. Here, we demonstrate the influence of cyclic lipopeptide surfactin (SU) on the PM of CHO-K1 cells. Both FLIM and svFCS experiments show that even a low concentration of SU causes significant changes in the membrane fluidity and dynamic molecular organization. Further, we demonstrate that SU causes a relevant dose-dependent reduction of cellular cholesterol by extracting it from the PM. Finally, we show that CHO-25RA cells characterized by increased cholesterol levels are more sensitive to SU treatment than CHO-K1 cells. We propose that sterols organizing the PM raft nanodomains, constitute a potential target for SU and other biosurfactants. In our opinion, the anticancer activity of biosurfactants is directly related with the higher cholesterol content found in many cancer cells.

A Brief Introduction to Some Aspects of the Fluid-Mosaic Model of Cell Membrane Structure and Its Importance in Membrane Lipid Replacement

Early cell membrane models placed most proteins external to lipid bilayers in trimolecular structures or as modular lipoprotein units. These thermodynamically untenable structures did not allow lipid lateral movements independent of membrane proteins. The Fluid-Mosaic Membrane Model accounted for these and other properties, such as membrane asymmetry, variable lateral mobilities of membrane components and their associations with dynamic complexes. Integral membrane proteins can transform into globular structures that are intercalated to various degrees into a heterogeneous lipid bilayer matrix. This simplified version of cell membrane structure was never proposed as the ultimate biomembrane description, but it provided a basic nanometer scale framework for membrane organization. Subsequently, the structures associated with membranes were considered, including peripheral membrane proteins, and cytoskeletal and extracellular matrix components that restricted lateral mobility. In addition, lipid-lipid and lipid-protein membrane domains, essential for cellular signaling, were proposed and eventually discovered. The presence of specialized membrane domains significantly reduced the extent of the fluid lipid matrix, so membranes have become more mosaic with some fluid areas over time. However, the fluid regions of membranes are very important in lipid transport and exchange. Various lipid globules, droplets, vesicles and other membranes can fuse to incorporate new lipids or expel damaged lipids from membranes, or they can be internalized in endosomes that eventually fuse with other internal vesicles and membranes. They can also be externalized in a reverse process and released as extracellular vesicles and exosomes. In this Special Issue, the use of membrane phospholipids to modify cellular membranes in order to modulate clinically relevant host properties is considered.

Evaluation of Serum Lysophosphatidic Acid and Lysophosphatidylcholine Levels in Major Depressive Disorder Patients

Background

Major depressive disorder (MDD) is a heterogeneous condition featured with a continuous low mood, feeling of sadness, lack of interest to perform daily activities. Genetic, physiological, biological, social, and environmental factors are associated with the pathophysiology of depression. Though several human studies failed to identify the suitable biological markers for depression, some animal studies showed phospholipids play a vital role in the alteration of emotion. Thus, the current study aimed to measure the serum levels of lysophosphatidic acid (LPA) and lysophosphatidylcholine (LPC) in MDD patients and healthy controls (HCs) to explore their roles and relationship with depression.

Methods

This case-control study enrolled 53 MDD patients and 50 HCs matched by age, gender, and body mass index. Based on the diagnostic and statistical manual of mental disorders, 5^th edition, a qualified psychiatrist diagnosed patients and assessed HCs. We applied the Hamilton depression rating scale (Ham-D) to measure the severity of depression. We used enzyme-linked immunosorbent assay kits to measure serum lysophosphatidic acid and lysophosphatidylcholine levels.

Results

We found no alterations of these parameters in serum levels of MDD patients compared to HCs. We also observed a significant positive correlation between LPA and LPC levels in MDD patients. Moreover, the present study showed no significant associations between target markers and either diagnosis of depression or Ham-D scores, or management of depression.

Conclusion

The present study suggests that LPA and LPC levels probably would not serve as potential biomarkers of MDD. Thus, we recommend further studies with large and more homogeneous populations to explore the exact relationship between serum lipids and MDD.

ABCA1 transporter reduces amphotericin B cytotoxicity in mammalian cells

Amphotericin B (AmB) belongs to a group of polyene antibiotics commonly used in the treatment of systemic mycotic infections. A widely accepted mechanism of action of AmB is based on the formation of an oligomeric pore structure within the plasma membrane (PM) by interaction with membrane sterols. Although AmB binds preferentially to ergosterol, it can also bind to cholesterol in the mammalian PM and cause severe cellular toxicity. The lipid content and its lateral organization at the cell PM appear to be significant for AmB binding. Several ATP-binding cassette (ABC) transporters, including ABCA1, play a crucial role in lipid translocation, cholesterol redistribution and efflux. Here, we demonstrate that cells expressing ABCA1 are more resistant to AmB treatment, while cells lacking ABCA1 expression or expressing non-active ABCA1MM mutant display increased sensitivity. Further, a FLIM analysis of AmB-treated cells reveals a fraction of the antibiotic molecules, characterized by relatively high fluorescence lifetimes (> 6 ns), involved in formation of bulk cholesterol-AmB structures at the surface of ABCA1-expressing cells. Finally, lowering the cellular cholesterol content abolishes resistance of ABCA1-expressing cells to AmB. Therefore, we propose that ABCA1-mediated cholesterol efflux from cells induces formation of bulk cholesterol-AmB structures at the cell surface, preventing AmB cytotoxicity.

Galectin-3 modulation of T-cell activation: mechanisms of membrane remodelling

Galectin-3 (Gal3) is a multifaceted protein which belongs to a family of lectins and binds β-galactosides. Gal3 expression is altered in many types of cancer, with increased expression generally associated with poor prognosis. Although the mechanisms remain unknown, Gal3 has been implicated in several biological processes involved in cancer progression, including suppression of T cell-mediated immune responses. Extracellular Gal3 binding to the plasma membrane of T cells alters membrane organization and the formation of an immunological synapse. Its multivalent capacity allows Gal3 to interact specifically with different membrane proteins and lipids, influencing endocytosis, trafficking and T cell receptor signalling. The ability of Gal3 to inhibit T cell responses may provide a mechanism by which Gal3 aids in cancer progression. In this review, we seek to give an overview of the mechanisms by which Gal3 alters the spatial organization of cell membranes and how these processes impact on T cell activation.

Membrane assembly of Shiga toxin glycosphingolipid receptors and toxin refractiveness of MDCK II epithelial cells

Shiga toxins (Stxs) are the major virulence factors of Stx-producing Escherichia coli (STEC), which cause hemorrhagic colitis and severe extraintestinal complications due to injury of renal endothelial cells, resulting in kidney failure. Since kidney epithelial cells are suggested additional targets for Stxs, we analyzed Madin-Darby canine kidney (MDCK) II epithelial cells for presence of Stx-binding glycosphingolipids (GSLs), determined their distribution to detergent-resistant membranes (DRMs), and ascertained the lipid composition of DRM and non-DRM preparations. Globotriaosylceramide and globotetraosylceramide, known as receptors for Stx1a, Stx2a, and Stx2e, and Forssman GSL as a specific receptor for Stx2e, were found to cooccur with SM and cholesterol in DRMs of MDCK II cells, which was shown using TLC overlay assay detection combined with mass spectrometry. The various lipoforms of GSLs were found to mainly harbor ceramide moieties composed of sphingosine (d18:1) and C24:1/C24:0 or C16:0 FA. The cells were highly refractory toward Stx1a, Stx2a, and Stx2e, most likely due to the absence of Stx-binding GSLs in the apical plasma membrane determined by immunofluorescence confocal laser scanning microscopy. The results suggest that the cellular content of Stx receptor GSLs and their biochemical detection in DRM preparations alone are inadequate to predict cellular sensitivity toward Stxs.

Lipid Cell Biology: A Focus on Lipids in Cell Division

Cells depend on hugely diverse lipidomes for many functions. The actions and structural integrity of the plasma membrane and most organelles also critically depend on membranes and their lipid components. Despite the biological importance of lipids, our understanding of lipid engagement, especially the roles of lipid hydrophobic alkyl side chains, in key cellular processes is still developing. Emerging research has begun to dissect the importance of lipids in intricate events such as cell division. This review discusses how these structurally diverse biomolecules are spatially and temporally regulated during cell division, with a focus on cytokinesis. We analyze how lipids facilitate changes in cellular morphology during division and how they participate in key signaling events. We identify which cytokinesis proteins are associated with membranes, suggesting lipid interactions. More broadly, we highlight key unaddressed questions in lipid cell biology and techniques, including mass spectrometry, advanced imaging, and chemical biology, which will help us gain insights into the functional roles of lipids.

Clustering on Membranes: Fluctuations and More

Clustering of extracellular ligands and proteins on the plasma membrane is required to perform specific cellular functions, such as signaling and endocytosis. Attractive forces that originate in perturbations of the membrane's physical properties contribute to this clustering, in addition to direct protein-protein interactions. However, these membrane-mediated forces have not all been equally considered, despite their importance. In this review, we describe how line tension, lipid depletion, and membrane curvature contribute to membrane-mediated clustering. Additional attractive forces that arise from protein-induced perturbation of a membrane's fluctuations are also described. This review aims to provide a survey of the current understanding of membrane-mediated clustering and how this supports precise biological functions.

How the shapes of seeds can influence pathology

It is widely accepted that the loss of function of different cellular proteins following their aggregation into highly stable aggregates or the gain of pathologic function of the resulting macromolecular assemblies or both processes are tightly associated to distinct debilitating neurodegenerative diseases such as Alzheimer's, Parkinson's, Creutzfeldt-Jacob, Amyotrophic Lateral Sclerosis and Huntington's diseases. How the aggregation of one given protein leads to distinct diseases is unclear. Here, a structural-molecular explanation based on the ability of proteins such as α-synuclein or tau to form assemblies that differ by their intrinsic architecture, stability, seeding capacity, and surfaces is proposed to account for distinct synucleinopathies and tauopathies. The shape and surfaces of the seeds is proposed to define at the same time their seeding capacity, interactome and tropism for defined neuronal cells within the central nervous system.

Mechanisms of protein nanoscale clustering

Due to recent technical developments in microscopy, huge advances have been made in our understanding of the architecture of the cell membrane. It is now well appreciated that nanoscale clustering is a common feature of membrane proteins. Many of these clusters have been implicated in signal initiation and integration platforms. However, the mechanisms that mediate the dynamic nanoscale arrangement of membrane proteins are not fully understood and could involve lipid domains, electrostatic interactions between proteins and lipid, protein scaffolding as well as purely mechanical processes. In this review we summarise these mechanisms giving rise to dynamic nanoscale protein reorganisation in the plasma membrane with reference to recent examples of immune receptor clustering to illustrate general principles.

Role of Different Alpha-Synuclein Strains in Synucleinopathies, Similarities with other Neurodegenerative Diseases

Misfolded protein aggregates are the hallmark of several neurodegenerative diseases in humans. The main protein constituent of these aggregates and the regions within the brain that are affected differ from one neurodegenerative disorder to another. A plethora of reports suggest that distinct diseases have in common the ability of protein aggregates to spread and amplify within the central nervous system. This review summarizes briefly what is known about the nature of the protein aggregates that are infectious and the reason they are toxic to cells. The chameleon property of polypeptides which aggregation into distinct high-molecular weight assemblies is associated to different diseases, in particular, that of alpha-synuclein which aggregation is the hallmark of distinct synucleinopathies, is discussed. Finally, strategies targeting the formation and propagation of structurally distinct alpha-synuclein assemblies associated to different synucleinopathies are presented and their therapeutic and diagnostic potential is discussed.

Membrane bending by protein crowding is affected by protein lateral confinement

Crowding of asymmetrically-distributed membrane proteins has been recently recognized as an important factor in remodeling of biological membranes, for example during transport vesicle formation. In this paper, we theoretically analyze the effect of protein crowding on membrane bending and examine its dependence on protein size, shape, transmembrane asymmetry and lateral confinement. We consider three scenarios of protein lateral organization, which are highly relevant for cellular membranes in general: freely diffusing membrane proteins without lateral confinement, the presence of a diffusion barrier and interactions with a vesicular coat. We show that protein crowding affects vesicle formation even if the proteins are distributed symmetrically across the membrane and that this effect depends significantly on lateral confinement. The largest crowding effect is predicted for the proteins that are confined to the forming vesicle by a diffusion barrier. We calculate the bending properties of a crowded membrane and find that its spontaneous curvature depends primarily on the degree of transmembrane asymmetry, and its effective bending modulus on the type of lateral confinement. Using the example of COPII vesicle formation from the endoplasmic reticulum, we analyze the energetic cost of vesicle formation. The results provide a novel insight into the effects of lateral and transmembrane organization of membrane proteins, and can guide data interpretation and future experimental approaches.

Annexin A6 regulates interleukin-2-mediated T-cell proliferation

Annexin A6 (AnxA6) has been implicated in cell signalling by contributing to the organisation of the plasma membrane. Here we examined whether AnxA6 regulates signalling and proliferation in T cells. We used a contact hypersensitivity model to immune challenge wild-type (WT) and AnxA6(-/-) mice and found that the in vivo proliferation of CD4(+) T cells, but not CD8(+) T cells, was impaired in AnxA6(-/-) relative to WT mice. However, T-cell migration and signalling through the T-cell receptor ex vivo was similar between T cells isolated from AnxA6(-/-) and WT mice. In contrast, interleukin-2 (IL-2) signalling was reduced in AnxA6(-/-) compared with WT T cells. Further, AnxA6-deficient T cells had reduced membrane order and cholesterol levels. Taken together, our data suggest that AnxA6 regulates IL-2 homeostasis and sensitivity in T cells by sustaining a lipid raft-like membrane environment.

Nanoscale dynamics of phospholipids reveals an optimal assembly mechanism of pore-forming proteins in bilayer membranes

Using STED-FCS, we show that the fluidity of the membrane controls the induced dynamical heterogeneity in model membranes upon interacting with pore-forming toxins.

The plasma membrane as a capacitor for energy and metabolism

When considering which components of the cell are the most critical to function and physiology, we naturally focus on the nucleus, the mitochondria that regulate energy and apoptotic signaling, or other organelles such as the endoplasmic reticulum, Golgi, ribosomes, etc. Few people will suggest that the membrane is the most critical element of a cell in terms of function and physiology. Those that consider the membrane critical will point to its obvious barrier function regulated by the lipid bilayer and numerous ion channels that regulate homeostatic gradients. What becomes evident upon closer inspection is that not all membranes are created equal and that there are lipid-rich microdomains that serve as platforms of signaling and a means of communication with the intracellular environment. In this review, we explore the evolution of membranes, focus on lipid-rich microdomains, and advance the novel concept that membranes serve as "capacitors for energy and metabolism." Within this framework, the membrane then is the primary and critical regulator of stress and disease adaptation of the cell.

The aPKC/Par3/Par6 Polarity Complex and Membrane Order Are Functionally Interdependent in Epithelia During Vertebrate Organogenesis

The differential distribution of lipids between apical and basolateral membranes is necessary for many epithelial cell functions, but how this characteristic membrane organization is integrated within the polarity network during ductal organ development is poorly understood. Here we quantified membrane order in the gut, kidney and liver ductal epithelia in zebrafish larvae at 3-11 days post fertilization (dpf) with Laurdan 2-photon microscopy. We then applied a combination of Laurdan imaging, antisense knock-down and analysis of polarity markers to understand the relationship between membrane order and apical-basal polarity. We found a reciprocal relationship between membrane order and the cell polarity network. Reducing membrane condensation by exogenously added oxysterol or depletion of cholesterol reduced apical targeting of the polarity protein, aPKC. Conversely, using morpholino knock down in zebrafish, we found that membrane order was dependent upon the Crb3 and Par3 polarity protein expression in ductal epithelia. Hence our data suggest that the biophysical property of membrane lipid packing is a regulatory element in apical basal polarity.

Chromosome-Membrane Interactions in Bacteria

Prokaryotes, by definition, do not segregate their genetic material from the cytoplasm. Thus, there is no barrier preventing direct interactions between chromosomal DNA and the plasma membrane. The possibility of such interactions in bacteria was proposed long ago and supported by early electron microscopy and cell fractionation studies. However, the identification and characterization of chromosome-membrane interactions have been slow in coming. Recently, this subject has seen more progress, driven by advances in imaging techniques and in the exploration of diverse cellular processes. A number of loci have been identified in specific bacteria that depend on interactions with the membrane for their function. In addition, there is growing support for a general mechanism of DNA-membrane contacts based on transertion-concurrent transcription, translation, and insertion of membrane proteins. This review summarizes the history and recent results of chromosome-membrane associations and discusses the known and theorized consequences of these interactions in the bacterial cell.

Lysophosphatidic Acid signaling in the nervous system

The brain is composed of many lipids with varied forms that serve not only as structural components but also as essential signaling molecules. Lysophosphatidic acid (LPA) is an important bioactive lipid species that is part of the lysophospholipid (LP) family. LPA is primarily derived from membrane phospholipids and signals through six cognate G protein-coupled receptors (GPCRs), LPA1-6. These receptors are expressed on most cell types within central and peripheral nervous tissues and have been functionally linked to many neural processes and pathways. This Review covers a current understanding of LPA signaling in the nervous system, with particular focus on the relevance of LPA to both physiological and diseased states.

Shiga toxin induces membrane reorganization and formation of long range lipid order

Lateral variation of the in-plane orientation of lipids in a bilayer is referred to as texture. The influence of the protein Shiga toxin on orientational membrane texture was studied in phosphatidylcholine lipid bilayers using polarization two-photon fluorescence microscopy and atomic force microscopy. A content of 1% of glycosphingolipid globotriaosylceramide (Gb3) receptor lipids in a bilayer was used to bind the Shiga toxin B-subunit to the surface of gel domains. Binding of the Shiga toxin B-subunit to lipids led to the modulation of orientational membrane texture in gel domains and induced membrane reordering. When Shiga toxin was added above the lipid chain melting temperature, the toxin interaction with the membrane induced rearrangement and clustering of Gb3 lipids that resulted in the long range order and alignment of lipids in gel domains. The toxin induced redistribution of Gb3 lipids inside gel domains is governed by the temperature at which Shiga toxin was added to the membrane: above or below the phase transition. The temperature is thus one of the critical factors controlling lipid organization and texture in the presence of Shiga toxin. Lipid chain ordering imposed by Shiga toxin binding can be another factor driving the reconstruction of lipid organization and crystallization of lipids inside gel domains.

MICAL-L1-related and unrelated mechanisms underlying elongated tubular endosomal network (ETEN) in human dendritic cells

The endosomal pathway constitutes a highly dynamic intracellular transport system, which is composed of vesicular and tubular compartments. Endosomal tubules enable geometry-based discrimination between membrane and luminal content. Extended tubular endosomes were suggested to deliver a steady stream of membrane proteins to one location more reliable and effective than vesicular endosomes. Recently, we demonstrated that human dendritic cells (DCs) form a large elongated tubular endosomal network, e.g. ETEN, upon distinct triggers. LPS-stimulation triggered late endosomal tubulation. Additional clustering of class I MHC and ICAM-1 by a cognate interaction between antigen-laden DC and antigen-specific CD8⁺ T-cells induces formation of transferrin-positive tubules emanating from the endosomal recycling compartment (ERC). We here discuss cell-biological mechanisms that are involved in membrane bending and possibly underlie initiation, elongation, and stabilization of ETEN in human DCs. Using a knock-down approach we demonstrate that MICAL-L1 is necessary for ETEN remodeling originating from ERC in human DCs.