Frustrated endocytosis controls contractility-independent mechanotransduction at clathrin-coated structures

The principles of directed cell migration

Cells have the ability to respond to various types of environmental cues, and in many cases these cues induce directed cell migration towards or away from these signals. How cells sense these cues and how they transmit that information to the cytoskeletal machinery governing cell translocation is one of the oldest and most challenging problems in biology. Chemotaxis, or migration towards diffusible chemical cues, has been studied for more than a century, but information is just now beginning to emerge about how cells respond to other cues, such as substrate-associated cues during haptotaxis (chemical cues on the surface), durotaxis (mechanical substrate compliance) and topotaxis (geometric features of substrate). Here we propose four common principles, or pillars, that underlie all forms of directed migration. First, a signal must be generated, a process that in physiological environments is much more nuanced than early studies suggested. Second, the signal must be sensed, sometimes by cell surface receptors, but also in ways that are not entirely clear, such as in the case of mechanical cues. Third, the signal has to be transmitted from the sensing modules to the machinery that executes the actual movement, a step that often requires amplification. Fourth, the signal has to be converted into the application of asymmetric force relative to the substrate, which involves mostly the cytoskeleton, but perhaps other players as well. Use of these four pillars has allowed us to compare some of the similarities between different types of directed migration, but also to highlight the remarkable diversity in the mechanisms that cells use to respond to different cues provided by their environment.

Antagonistic regulation controls clathrin-mediated endocytosis: AP2 adaptor facilitation vs restraint from clathrin light chains

Orchestration of a complex network of protein interactions drives clathrin-mediated endocytosis (CME). A central role for the AP2 adaptor complex beyond cargo recognition and clathrin recruitment has emerged in recent years. It is now apparent that AP2 serves as a pivotal hub for protein interactions to mediate clathrin coated pit maturation, and couples lattice formation to membrane deformation. As a key driver for clathrin assembly, AP2 complements the attenuating role of clathrin light chain subunits, which enable dynamic lattice rearrangement needed for budding. This review summarises recent insights into AP2 function with respect to CME dynamics and biophysics, and its relationship to the role of clathrin light chains in clathrin assembly.

Physical and mechanical cues affecting biomaterial-mediated plasmid DNA delivery: insights into non-viral delivery systems

BACKGROUND

Whilst traditional strategies to increase transfection efficiency of non-viral systems aimed at modifying the vector or the polyplexes/lipoplexes, biomaterial-mediated gene delivery has recently sparked increased interest. This review aims at discussing biomaterial properties and unravelling underlying mechanisms of action, for biomaterial-mediated gene delivery. DNA internalisation and cytoplasmic transport are initially discussed. DNA immobilisation, encapsulation and surface-mediated gene delivery (SMD), the role of extracellular matrix (ECM) and topographical cues, biomaterial stiffness and mechanical stimulation are finally outlined.

MAIN TEXT

Endocytic pathways and mechanisms to escape the lysosomal network are highly variable. They depend on cell and DNA complex types but can be diverted using appropriate biomaterials. 3D scaffolds are generally fabricated via DNA immobilisation or encapsulation. Degradation rate and interaction with the vector affect temporal patterns of DNA release and transgene expression. In SMD, DNA is instead coated on 2D surfaces. SMD allows the incorporation of topographical cues, which, by inducing cytoskeletal re-arrangements, modulate DNA endocytosis. Incorporation of ECM mimetics allows cell type-specific transfection, whereas in spite of discordances in terms of optimal loading regimens, it is recognised that mechanical loading facilitates gene transfection. Finally, stiffer 2D substrates enhance DNA internalisation, whereas in 3D scaffolds, the role of stiffness is still dubious.

CONCLUSION

Although it is recognised that biomaterials allow the creation of tailored non-viral gene delivery systems, there still are many outstanding questions. A better characterisation of endocytic pathways would allow the diversion of cell adhesion processes and cytoskeletal dynamics, in order to increase cellular transfection. Further research on optimal biomaterial mechanical properties, cell ligand density and loading regimens is limited by the fact that such parameters influence a plethora of other different processes (e.g. cellular adhesion, spreading, migration, infiltration, and proliferation, DNA diffusion and release) which may in turn modulate gene delivery. Only a better understanding of these processes may allow the creation of novel robust engineered systems, potentially opening up a whole new area of biomaterial-guided gene delivery for non-viral systems.

FluoSTEPs: Fluorescent biosensors for monitoring compartmentalized signaling within endogenous microdomains

Growing evidence suggests that many essential intracellular signaling events are compartmentalized within kinetically distinct microdomains in cells. Genetically encoded fluorescent biosensors are powerful tools to dissect compartmentalized signaling, but current approaches to probe these microdomains typically rely on biosensor fusion and overexpression of critical regulatory elements. Here, we present a novel class of biosensors named FluoSTEPs (fluorescent sensors targeted to endogenous proteins) that combine self-complementing split green fluorescent protein, CRISPR-mediated knock-in, and fluorescence resonance energy transfer biosensor technology to probe compartmentalized signaling dynamics in situ. We designed FluoSTEPs for simultaneously highlighting endogenous microdomains and reporting domain-specific, real-time signaling events including kinase activities, guanosine triphosphatase activation, and second messenger dynamics in live cells. A FluoSTEP for 3',5'-cyclic adenosine monophosphate (cAMP) revealed distinct cAMP dynamics within clathrin microdomains in response to stimulation of G protein-coupled receptors, showcasing the utility of FluoSTEPs in probing spatiotemporal regulation within endogenous signaling architectures.

The structure and spontaneous curvature of clathrin lattices at the plasma membrane

Clathrin-mediated endocytosis is the primary pathway for receptor and cargo internalization in eukaryotic cells. It is characterized by a polyhedral clathrin lattice that coats budding membranes. The mechanism and control of lattice assembly, curvature, and vesicle formation at the plasma membrane has been a matter of long-standing debate. Here, we use platinum replica and cryoelectron microscopy and tomography to present a structural framework of the pathway. We determine the shape and size parameters common to clathrin-mediated endocytosis. We show that clathrin sites maintain a constant surface area during curvature across multiple cell lines. Flat clathrin is present in all cells and spontaneously curves into coated pits without additional energy sources or recruited factors. Finally, we attribute curvature generation to loosely connected and pentagon-containing flat lattices that can rapidly curve when a flattening force is released. Together, these data present a universal mechanistic model of clathrin-mediated endocytosis.

Surface distribution of heterogenous clathrin assemblies in resorbing osteoclasts

Osteoclasts seeded on either glass coverslips or apatite pellets have at least two morphologically distinct substrate adhesion sites: actin-based adhesion structures including podosome belts and sealing zones, and adjacent clathrin sheets. Clathrin-coated structures are exclusively localized at the podosome belts and sealing zone, in both of which the plasma membrane forms a tight attachment to the substrate surface. When cultured on apatite osteoclasts can degrade the apatite leading to the formation of resorption lacunae. The sealing zone divides the ventral membrane into different domains, outside and inside of the sealing zones. The former facing the smooth-surfaced intact apatite contains relatively solitary or networks of larger flat clathrin structures; and the latter, facing the rough-surfaced degraded apatite in the resorption lacunae contain clathrin in various shapes and sizes. Clathrin assemblies on the membrane domain facing not only a resorption lacuna, or trails but also intact apatite indeed were observed to be heterogeneous in size and intensity, suggesting that they appeared to follow variations in the surface topography of the apatite surface. These results provide a detailed insight into the flat clathrin sheets that have been suggested to be the sites of adhesion and mechanosensing in co-operation with podosomes.

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.

Integrins Control Vesicular Trafficking; New Tricks for Old Dogs

Integrins are transmembrane receptors that transduce biochemical and mechanical signals across the plasma membrane and promote cell adhesion and migration. In addition, integrin adhesion complexes are functionally and structurally linked to components of the intracellular trafficking machinery and accumulating data now reveal that they are key regulators of endocytosis and exocytosis in a variety of cell types. Here, we highlight recent insights into integrin control of intracellular trafficking in processes such as degranulation, mechanotransduction, cell–cell communication, antibody production, virus entry, Toll-like receptor signaling, autophagy, and phagocytosis, as well as the release and uptake of extracellular vesicles. We discuss the underlying molecular mechanisms and the implications for a range of pathophysiological contexts, including hemostasis, immunity, tissue repair, cancer, and viral infection.

Integrin adhesion complexes control polarized targeting of the intracellular trafficking machinery via microtubules.

Integrin adhesions are exocytic hubs for a variety of vesicles, including lytic and dense granules, lysosome-related organelles, and biosynthetic vesicles.

Integrin-dependent adhesion and signaling is required for degranulation of platelets and leukocytes and controls hemostasis and immunity.

Specialized adhesion complexes containing integrin αvβ5 and clathrin are sites of frustrated endocytosis and hubs for mechanotransduction.

Integrin control of endocytosis regulates Toll-like receptor signaling and autophagy in immune cells.

Integrins control intercellular communication and viral transfer through extracellular vesicles.

Clathrin light chain diversity regulates membrane deformation in vitro and synaptic vesicle formation in vivo

This study reveals that diversity of clathrin light chain (CLC) subunits alters clathrin properties and demonstrates that the two neuronal CLC subunits work together for optimal clathrin function in synaptic vesicle formation. Our findings establish a role for CLC diversity in synaptic transmission and illustrate how CLC variability expands the complexity of clathrin to serve tissue-specific functions.

Clathrin light chain (CLC) subunits in vertebrates are encoded by paralogous genes CLTA and CLTB, and both gene products are alternatively spliced in neurons. To understand how this CLC diversity influences neuronal clathrin function, we characterized the biophysical properties of clathrin comprising individual CLC variants for correlation with neuronal phenotypes of mice lacking either CLC-encoding gene. CLC splice variants differentially influenced clathrin knee conformation within assemblies, and clathrin with neuronal CLC mixtures was more effective in membrane deformation than clathrin with single neuronal isoforms nCLCa or nCLCb. Correspondingly, electrophysiological recordings revealed that neurons from mice lacking nCLCa or nCLCb were both defective in synaptic vesicle replenishment. Mice with only nCLCb had a reduced synaptic vesicle pool and impaired neurotransmission compared to WT mice, while nCLCa-only mice had increased synaptic vesicle numbers, restoring normal neurotransmission. These findings highlight differences between the CLC isoforms and show that isoform mixing influences tissue-specific clathrin activity in neurons, which requires their functional balance.

Frustration of endocytosis potentiates compression-induced receptor signaling

Cells experience mechanical stresses in different physiological and pathological settings. Clathrin-coated structures (CCSs) are sensitive to such perturbations in a way that often results in a mechanical impairment of endocytic budding. Compressive stress is a mechanical perturbation that leads to increased membrane tension and promotes proliferative signals. Here, we report that compression leads to frustration of CCSs and that CCSs are required to potentiate receptor-mediated signaling in these conditions. We show that cell compression stalled CCS dynamics and slowed down the dynamic exchange of CCS components. As previously reported, compression-induced paracrine activation of the epidermal growth factor receptor (EGFR) was the primary cause of ERK (ERK1 and ERK2, also known as MAPK3 and MAPK1, respectively) activation in these conditions. We observed that EGFR was efficiently recruited at CCSs upon compression and that CCSs were required for full ERK activation. In addition, we demonstrated that compression-induced frustrated CCSs could also increase ligand-dependent signaling of other receptors. We thus propose that CCS frustration resulting from mechanical perturbations can potentiate signaling through different receptors, with potential important consequences for the adaptation of the cell to its environment.This article has an associated First Person interview with the first author of the paper.

Imaging endocytic vesicle formation at high spatial and temporal resolutions with the pulsed-pH protocol

Endocytosis is a fundamental process occurring in all eukaryotic cells. Live cell imaging of endocytosis has helped to decipher many of its mechanisms and regulations. With the pulsed-pH (ppH) protocol, one can detect the formation of individual endocytic vesicles (EVs) with an unmatched temporal resolution of 2 s. The ppH protocol makes use of cargo protein (e.g., the transferrin receptor) coupled to a pH-sensitive fluorescent protein, such as superecliptic pHluorin (SEP), which is brightly fluorescent at pH 7.4 but not fluorescent at pH <6.0. If the SEP moiety is at the surface, its fluorescence will decrease when cells are exposed to a low pH (5.5) buffer. If the SEP moiety has been internalized, SEP will remain fluorescent even during application of the low pH buffer. Fast perfusion enables the complete exchange of low and high pH extracellular solutions every 2 s, defining the temporal resolution of the technique. Unlike other imaging-based endocytosis assays, the ppH protocol detects EVs without a priori hypotheses on the dynamics of vesicle formation. Here, we explain how the ppH protocol quantifies the endocytic activity of living cells and the recruitment of associated proteins in real time. We provide a step-by-step procedure for expression of the reporter proteins with transient transfection, live cell image acquisition with synchronized pH changes and automated analysis. The whole protocol can be performed in 2 d to provide quantitative information on the endocytic process being studied.

Crosstalk between Cell Adhesion Complexes in Regulation of Mechanotransduction

Physical forces regulate numerous biological processes during development, physiology, and pathology. Forces between the external environment and intracellular actin cytoskeleton are primarily transmitted through integrin-containing focal adhesions and cadherin-containing adherens junctions. Crosstalk between these complexes is well established and modulates the mechanical landscape of the cell. However, integrins and cadherins constitute large families of adhesion receptors and form multiple complexes by interacting with different ligands, adaptor proteins, and cytoskeletal filaments. Recent findings indicate that integrin-containing hemidesmosomes oppose force transduction and traction force generation by focal adhesions. The cytolinker plectin mediates this crosstalk by coupling intermediate filaments to the actin cytoskeleton. Similarly, cadherins in desmosomes might modulate force generation by adherens junctions. Moreover, mechanotransduction can be influenced by podosomes, clathrin lattices, and tetraspanin-enriched microdomains. This review discusses mechanotransduction by multiple integrin- and cadherin-based cell adhesion complexes, which together with the associated cytoskeleton form an integrated network that allows cells to sense, process, and respond to their physical environment.

Clathrin's life beyond 40: Connecting biochemistry with physiology and disease

Understanding of the range and mechanisms of clathrin functions has developed exponentially since clathrin's discovery in 1975. Here, newly established molecular mechanisms that regulate clathrin activity and connect clathrin pathways to differentiation, disease and physiological processes such as glucose metabolism are reviewed. Diversity and commonalities of clathrin pathways across the tree of life reveal species-specific differences enabling functional plasticity in both membrane traffic and cytokinesis. New structural information on clathrin coat formation and cargo interactions emphasises the interplay between clathrin, adaptor proteins, lipids and cargo, and how this interplay regulates quality control of clathrin's function and is compromised in infection and neurological disease. Roles for balancing clathrin-mediated cargo transport are defined in stem cell development and additional disease states.

Chidamide increases the sensitivity of Non-small Cell Lung Cancer to Crizotinib by decreasing c-MET mRNA methylation

Introduction: Crizotinib is a kinase inhibitor targeting c-MET/ALK/ROS1 used as the first-line chemical for the treatment of non-small cell lung cancer (NSCLC) with ALK mutations. Although c-MET is frequently overexpressed in 35-72% of NSCLC, most NSCLCs are primarily resistant to crizotinib treatment.

Method: A set of NSCLC cell lines were used to test the effect of chidamide on the primary crizotinib resistance in vitro and in vivo. Relationships between the synergistic effect of chidamide and c-MET expression and RNA methylation were systemically studied with a battery of molecular biological assays.

Results: We found for the first time that chidamide could sensitize the effect of crizotinib in a set of ALK mutation-free NSCLC cell lines, especially those with high levels of c-MET expression. Notably, chidamide could not increase the sensitivity of NSCLC cells to crizotinib cultured in serum-free medium without hepatocyte growth factor (HGF; a c-MET ligand). In contrast, the addition of HGF into the serum-/HGF-free medium could restore the synergistic effect of chidamide. Moreover, the synergistic effect of chidamide could also be abolished either by treatment with c-MET antibody or siRNA-knockdown of c-MET expression. While cells with low or no c-MET expression were primarily resistant to chidamide-crizotinib cotreatment, enforced c-MET overexpression could increase the sensitivity of these cells to chidamide-crizotinib cotreatment. Furthermore, chidamide could decrease c-MET expression by inhibiting mRNA N6-methyladenosine (m6A) modification through the downregulation of METTL3 and WTAP expression. Chidamide-crizotinib cotreatment significantly suppressed the activity of c-MET downstream molecules.

Conclusion: Chidamide downregulated c-MET expression by decreasing its mRNA m6A methylation, subsequently increasing the crizotinib sensitivity of NSCLC cells in a c-MET-/HGF-dependent manner.

Mapping Cell Membrane Organization and Dynamics Using Soft Nanoimprint Lithography

Membrane shape is a key feature of many cellular processes, including cell differentiation, division, migration, and trafficking. The development of nanostructured surfaces allowing for the in situ manipulation of membranes in living cells is crucial to understand these processes, but this requires complicated and limited-access technologies. Here, we investigate the self-organization of cellular membranes by using a customizable and benchtop method allowing one to engineer 1D SiO₂ nanopillar arrays of defined sizes and shapes on high-performance glass compatible with advanced microscopies. As a result of this original combination, we provide a mapping of the morphology-induced modulation of the cell membrane mechanics, dynamics and steady-state organization of key protein complexes implicated in cellular trafficking and signal transduction.

Frustrated clathrin-mediated endocytosis – causes and possible functions

Clathrin-mediated endocytosis is the main entry route for most cell surface receptors and their ligands. It is regulated by clathrin-coated structures that are endowed with the ability to cluster receptors and to locally bend the plasma membrane, resulting in the formation of receptor-containing vesicles that bud into the cytoplasm. This canonical role of clathrin-coated structures has been shown to play a fundamental part in many different aspects of cell physiology. However, it has recently become clear that the ability of clathrin-coated structures to deform membranes can be perturbed. In addition to chemical or genetic alterations, numerous environmental conditions can physically prevent or slow down membrane bending and/or budding at clathrin-coated structures. The resulting ‘frustrated endocytosis’ is emerging as not merely a passive consequence, but one that actually fulfils some very specific and important cellular functions. In this Review, we provide an historical and defining perspective on frustrated endocytosis in the clathrin pathway of mammalian cells, before discussing its causes and highlighting the possible functional consequences in physiology and diseases.

Cell matrix adhesion in cell migration

The ability of cells to migrate is a fundamental physiological process involved in embryonic development, tissue homeostasis, immune surveillance and wound healing. In order for cells to migrate, they must interact with their environment using adhesion receptors, such as integrins, and form specialized adhesion complexes that mediate responses to different extracellular cues. In this review, we discuss the role of integrin adhesion complexes (IACs) in cell migration, highlighting the layers of regulation that are involved, including intracellular signalling cascades, mechanosensing and reciprocal feedback to the extracellular environment. We also discuss the role of IACs in extracellular matrix remodeling and how they impact upon cell migration.

Membrane dynamics in cell migration

Migration of cells is required in multiple tissue-level processes, such as in inflammation or cancer metastasis. Endocytosis is an extremely regulated cellular process by which cells uptake extracellular molecules or internalise cell surface receptors. While the role of endocytosis of focal adhesions (FA) and plasma membrane (PM) turnover at the leading edge of migratory cells is wide known, the contribution of endocytic proteins per se in migration has been frequently disregarded. In this review, we describe the novel functions of the most well-known endocytic proteins in cancer cell migration, focusing on clathrin, caveolin, flotillins and GRAF1. In addition, we highlight the relevance of the macropinocytic pathway in amoeboid-like cell migration.

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.

The crosstalk between microtubules, actin and membranes shapes cell division

Mitotic progression is orchestrated by morphological and mechanical changes promoted by the coordinated activities of the microtubule (MT) cytoskeleton, the actin cytoskeleton and the plasma membrane (PM). MTs assemble the mitotic spindle, which assists sister chromatid separation, and contact the rigid and tensile actomyosin cortex rounded-up underneath the PM. Here, we highlight the dynamic crosstalk between MTs, actin and cell membranes during mitosis, and discuss the molecular connections between them. We also summarize recent views on how MT traction forces, the actomyosin cortex and membrane trafficking contribute to spindle positioning in isolated cells in culture and in epithelial sheets. Finally, we describe the emerging role of membrane trafficking in synchronizing actomyosin tension and cell shape changes with cell-substrate adhesion, cell-cell contacts and extracellular signalling events regulating proliferation.

The Hippo Pathway, YAP/TAZ, and the Plasma Membrane

The plasma membrane allows the cell to sense and adapt to changes in the extracellular environment by relaying external inputs via intracellular signaling networks. One central cellular signaling pathway is the Hippo pathway, which regulates homeostasis and plays chief roles in carcinogenesis and regenerative processes. Recent studies have found that mechanical stimuli and diffusible chemical components can regulate the Hippo pathway primarily through receptors embedded in the plasma membrane. Morphologically defined structures within the plasma membrane, such as cellular junctions, focal adhesions, primary cilia, caveolae, clathrin-coated pits, and plaques play additional key roles. Here, we discuss recent evidence highlighting the importance of these specialized plasma membrane domains in cellular feedback via the Hippo pathway.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Clathrin-containing adhesion complexes

An understanding of the mechanisms whereby cell adhesion complexes (ACs) relay signals bidirectionally across the plasma membrane is necessary to interpret the role of adhesion in regulating migration, differentiation, and growth. A range of AC types has been defined, but to date all have similar compositions and are dependent on a connection to the actin cytoskeleton. Recently, a new class of AC has been reported that normally lacks association with both the cytoskeleton and integrin-associated adhesome components, but is rich in components of the clathrin-mediated endocytosis machinery. The characterization of this new type of adhesion structure, which is emphasized by mitotic cells and cells in long-term culture, identifies a hitherto underappreciated link between the adhesion machinery and clathrin structures at the plasma membrane. While this discovery has implications for how ACs are assembled and disassembled, it raises many other issues. Consequently, to increase awareness within the field, and stimulate research, we explore a number of the most significant questions below.

Integrin α6β4E variant is associated with actin and CD9 structures and modifies the biophysical properties of cell-cell and cell-extracellular matrix interactions

Integrin α6β4 is an essential, dynamic adhesion receptor for laminin 332 found on epithelial cells, required for formation of strong cell–extracellular matrix (ECM) adhesion and induced migration, and coordinated by regions of the β4C cytoplasmic domain. β4E, a unique splice variant of β4 expressed in normal tissue, contains a cytoplasmic domain of 231 amino acids with a unique sequence of 114 amino acids instead of β4C’s canonical 1089 amino acids. We determined the distribution of α6β4E within normal human glandular epithelium and its regulation and effect on cellular biophysical properties. Canonical α6β4C expressed in all basal cells, as expected, while α6β4E expressed within a subset of luminal cells. α6β4E expression was induced by three-dimensional culture conditions, activated Src, was reversible, and was stabilized by bortezomib, a proteasome inhibitor. α6β4C expressed in all cells during induced migration, whereas α6β4E was restricted to a subset of cells with increased kinetics of cell–cell and cell–ECM resistance properties. Interestingly, α6β4E presented in “ringlike” patterns measuring ∼1.75 × 0.72 microns and containing actin and CD9 at cell–ECM locations. In contrast, α6β4C expressed only within hemidesmosome-like structures containing BP180. Integrin α6β4E is an inducible adhesion isoform in normal epithelial cells that can alter biophysical properties of cell–cell and cell–ECM interactions.