Use of photoactivation and photobleaching to monitor the dynamic regulation of E-cadherin at the plasma membrane

Tissue-specific transcriptional regulation of epithelial/endothelial and mesenchymal markers during renovascular hypertension

Epithelial-to-mesenchymal transition (EMT) and endothelial-to-mesenchymal transition are processes that can occur under different biological conditions, including tissue healing due to hypertension and oxidative stress. The purpose of the present study was to evaluate the differences in gene expression of epithelial/endothelial and mesenchymal markers in different tissues. A two-kidney, one-clip (2K1C) renovascular hypertension rat model was used. Hypertension was induced by the clipping of the left renal artery; the rats were randomized into sham and 2K1C groups and monitored for up to 4 weeks. The gene expressions of E-cadherin (E-cad), N-cadherin (N-cad), α-smooth muscle actin (α-SMA), collagen I (COL1A1), collagen III (COL3A1) and hepatocyte growth factor (HGF) were determined by reverse transcription-PCR. The levels of the cytokines transforming growth factor-β1, tumor necrosis factor-α, interleukin (IL)-4, IL-6 and IL-10 were evaluated using ELISAs. The levels of thiobarbituric acid reactive substances and thiol groups were measured to evaluate oxidative stress. All analyses were performed on the liver, heart and kidneys tissues of sham and model rats. The 2K1C animals exhibited a higher systolic blood pressure, as well as cardiac hypertrophy and atrophy of the left kidney. Fibrotic alterations in the heart and kidneys were observed, as was an increase in the collagen fiber areas, and higher levels of inflammatory cytokines, which are associated with the increased expression of fibroproliferative and anti-fibrotic genes. Renovascular hypertension regulated epithelial/endothelial and mesenchymal markers, including E-cad, N-cad, α-SMA and COL1A1 in the kidneys and heart. EMT in the kidneys was mediated by an increased level of inflammatory and profibrotic cytokines, as well as by oxidative stress. The data in the present study suggested that the expression of epithelial/endothelial and mesenchymal markers are differentially regulated by hypertension in the liver, heart and kidneys.

Altering integrin engagement regulates membrane localization of Kir2.1 channels

How ion channels localize and distribute on the cell membrane remains incompletely understood. We show that interventions that vary cell adhesion proteins and cell size also affect the membrane current density of inward-rectifier K⁺ channels (K_ir2.1; encoded by KCNJ2) and profoundly alter the action potential shape of excitable cells. By using micropatterning to manipulate the localization and size of focal adhesions (FAs) in single HEK293 cells engineered to stably express K_ir2.1 channels or in neonatal rat cardiomyocytes, we establish a robust linear correlation between FA coverage and the amplitude of K_ir2.1 current at both the local and whole-cell levels. Confocal microscopy showed that K_ir2.1 channels accumulate in membrane proximal to FAs. Selective pharmacological inhibition of key mediators of protein trafficking and the spatially dependent alterations in the dynamics of K_ir2.1 fluorescent recovery after photobleaching revealed that the K_ir2.1 channels are transported to the cell membrane uniformly, but are preferentially internalized by endocytosis at sites that are distal from FAs. Based on these results, we propose adhesion-regulated membrane localization of ion channels as a fundamental mechanism of controlling cellular electrophysiology via mechanochemical signals, independent of the direct ion channel mechanogating.

Spatial and temporal organization of cadherin in punctate adherens junctions

Adherens junctions (AJs) play a fundamental role in tissue integrity; however, the organization and dynamics of the key AJ transmembrane protein, E-cadherin, both inside and outside of AJs, remain controversial. Here we have studied the distribution and motility of E-cadherin in punctate AJs (pAJs) of A431 cells. Using single-molecule localization microscopy, we show that pAJs in these cells reach more than 1 μm in length and consist of several cadherin clusters with crystal-like density interspersed within sparser cadherin regions. Notably, extrajunctional cadherin appears to be monomeric, and its density is almost four orders of magnitude less than observed in the pAJ regions. Two alternative strategies of tracking cadherin motion within individual junctions show that pAJs undergo actin-dependent rapid-on the order of seconds-internal reorganizations, during which dense clusters disassemble and their cadherins are immediately reused for new clusters. Our results thus modify the classical view of AJs by depicting them as mosaics of cadherin clusters, the short lifetimes of which enable stable overall morphology combined with rapid internal rearrangements.

Intravital FRAP Imaging using an E-cadherin-GFP Mouse Reveals Disease- and Drug-Dependent Dynamic Regulation of Cell-Cell Junctions in Live Tissue

E-cadherin-mediated cell-cell junctions play a prominent role in maintaining the epithelial architecture. The disruption or deregulation of these adhesions in cancer can lead to the collapse of tumor epithelia that precedes invasion and subsequent metastasis. Here we generated an E-cadherin-GFP mouse that enables intravital photobleaching and quantification of E-cadherin mobility in live tissue without affecting normal biology. We demonstrate the broad applications of this mouse by examining E-cadherin regulation in multiple tissues, including mammary, brain, liver, and kidney tissue, while specifically monitoring E-cadherin mobility during disease progression in the pancreas. We assess E-cadherin stability in native pancreatic tissue upon genetic manipulation involving Kras and p53 or in response to anti-invasive drug treatment and gain insights into the dynamic remodeling of E-cadherin during in situ cancer progression. FRAP in the E-cadherin-GFP mouse, therefore, promises to be a valuable tool to fundamentally expand our understanding of E-cadherin-mediated events in native microenvironments.

Homophilic interaction and deformation of E-cadherin and cadherin 7 probed by single molecule force spectroscopy

Cadherin-mediated adhesion plays a crucial role in multicellular organisms. Dysfunction within this adhesion system has major consequences in many pathologies, including cancer invasion and metastasis. However, mechanisms controlling cadherin recognition and adhesive strengthening are only partially understood. Here, we investigated the homophilic interactions and mechanical stability of the extracellular (EC) domains of E-cadherin and cadherin 7 using atomic force microscopy and magnetic tweezers. Besides exhibiting stronger interactions, E-cadherin also showed more efficient force-induced self-strengthening of interactions than cadherin 7. In addition, the distributions of the unbinding forces for both cadherins partially overlap with those of the unfolding forces, indicating that partial unfolding/deformation of the cadherin EC domains may take place during their homophilic interactions. These conformational changes may be involved in cadherins physiology function and contribute to the significant differences in adhesive strength mediated by type I and type II cadherins.

α-Catenin-mediated cadherin clustering couples cadherin and actin dynamics

The function of the actin-binding domain of α-catenin, αABD, including its possible role in the direct anchorage of the cadherin-catenin complex to the actin cytoskeleton, has remained uncertain. We identified two point mutations on the αABD surface that interfere with αABD binding to actin and used them to probe the role of α-catenin-actin interactions in adherens junctions. We found that the junctions directly bound to actin via αABD were more dynamic than the junctions bound to actin indirectly through vinculin and that recombinant αABD interacted with cortical actin but not with actin bundles. This interaction resulted in the formation of numerous short-lived cortex-bound αABD clusters. Our data suggest that αABD clustering drives the continuous assembly of transient, actin-associated cadherin-catenin clusters whose disassembly is maintained by actin depolymerization. It appears then that such actin-dependent αABD clustering is a unique molecular mechanism mediating both integrity and reassembly of the cell-cell adhesive interface formed through weak cis- and trans-intercadherin interactions.

Cargo sorting in the endocytic pathway: a key regulator of cell polarity and tissue dynamics

The establishment and maintenance of polarized plasma membrane domains is essential for cellular function and proper development of organisms. Epithelial cells polarize along two fundamental axes, the apicobasal and the planar, both depending on finely regulated protein trafficking mechanisms. Newly synthesized proteins destined for either surface domain are processed along the biosynthetic pathway and segregated into distinct subsets of transport carriers emanating from the trans-Golgi network or endosomes. This exocytic trafficking has been identified as essential for proper epithelial polarization. Accumulating evidence now reveals that endocytosis and endocytic recycling play an equally important role in epithelial polarization and the appropriate localization of key polarity proteins. Here, we review recent work in metazoan systems illuminating the connections between endocytosis, postendocytic trafficking, and cell polarity, both apicobasal and planar, in the formation of differentiated epithelial cells, and how these processes regulate tissue dynamics.

Developments in preclinical cancer imaging: innovating the discovery of therapeutics

Integrating biological imaging into early stages of the drug discovery process can provide invaluable readouts of drug activity within complex disease settings, such as cancer. Iterating this approach from initial lead compound identification in vitro to proof-of-principle in vivo analysis represents a key challenge in the drug discovery field. By embracing more complex and informative models in drug discovery, imaging can improve the fidelity and statistical robustness of preclinical cancer studies. In this Review, we highlight how combining advanced imaging with three-dimensional systems and intravital mouse models can provide more informative and disease-relevant platforms for cancer drug discovery.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Advanced intravital subcellular imaging reveals vital three-dimensional signalling events driving cancer cell behaviour and drug responses in live tissue

The integration of signal transduction pathways plays a fundamental role in governing disease initiation, progression and outcome. It is therefore necessary to understand disease at the signalling level to enable effective treatment and to intervene in its progression. The recent extension of in vitro subcellular image-based analysis to live in vivo modelling of disease is providing a more complete picture of real-time, dynamic signalling processes or drug responses in live tissue. Intravital imaging offers alternative strategies for studying disease and embraces the biological complexities that govern disease progression. In the present review, we highlight how three-dimensional or live intravital imaging has uncovered novel insights into biological mechanisms or modes of drug action. Furthermore, we offer a prospective view of how imaging applications may be integrated further with the aim of understanding disease in a more physiological and functional manner within the framework of the drug discovery process.

Myosin-1c regulates the dynamic stability of E-cadherin-based cell-cell contacts in polarized Madin-Darby canine kidney cells

Myo1c knockdown causes defects in E-cadherin localization, E-cadherin binding, and cell–cell contact of Madin–Darby canine kidney cells. Expression of wild-type Myo1c, but not motor-dead mutants or those unable to bind membrane, reverses the phenotype, evidence that Myo1c modulates the assembly/maintenance of adherens junctions.

Cooperation between cadherins and the actin cytoskeleton controls the formation and maintenance of cell–cell adhesions in epithelia. We find that the molecular motor protein myosin-1c (Myo1c) regulates the dynamic stability of E-cadherin–based cell–cell contacts. In Myo1c-depleted Madin–Darby canine kidney cells, E-cadherin localization was dis­organized and lateral membranes appeared less vertical with convoluted edges versus control cells. In polarized monolayers, Myo1c-knockdown (KD) cells were more sensitive to reduced calcium concentration. Myo1c separated in the same plasma membrane fractions as E-cadherin, and Myo1c KD caused a significant reduction in the amount of E-cadherin recovered in one peak fraction. Expression of green fluorescent protein (GFP)–Myo1c mutants revealed that the phosphatidylinositol-4,5-bisphosphate–binding site is necessary for its localization to cell–cell adhesions, and fluorescence recovery after photobleaching assays with GFP-Myo1c mutants revealed that motor function was important for Myo1c dynamics at these sites. At 18°C, which inhibits vesicle recycling, Myo1c-KD cells accumulated more E-cadherin–positive vesicles in their cytoplasm, suggesting that Myo1c affects E-cadherin endocytosis. Studies with photoactivatable GFP–E-cadherin showed that Myo1c KD reduced the stability of E-cadherin at cell–cell adhesions. We conclude that Myo1c stabilizes E-cadherin at adherens junctions in polarized epithelial cells and that the motor function and ability of Myo1c to bind membrane are critical.

E-cadherin-integrin crosstalk in cancer invasion and metastasis

E-cadherin is a single-pass transmembrane protein that mediates homophilic cell-cell interactions. Tumour progression is often associated with the loss of E-cadherin function and the transition to a more motile and invasive phenotype. This requires the coordinated regulation of both E-cadherin-mediated cell-cell adhesions and integrin-mediated adhesions that contact the surrounding extracellular matrix (ECM). Regulation of both types of adhesion is dynamic as cells respond to external cues from the tumour microenvironment that regulate polarity, directional migration and invasion. Here, we review the mechanisms by which tumour cells control the cross-regulation between dynamic E-cadherin-mediated cell-cell adhesions and integrin-mediated cell-matrix contacts, which govern the invasive and metastatic potential of tumours. In particular, we will discuss the role of the adhesion-linked kinases Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK), and the Rho family of GTPases.

p120-catenin binding masks an endocytic signal conserved in classical cadherins

p120 regulates adhesive junction dynamics through binding to a dual-function motif in classical cadherins that alternately serves as a p120-binding interface and an endocytic signal.

p120-catenin (p120) binds to the cytoplasmic tails of classical cadherins and inhibits cadherin endocytosis. Although p120 regulation of cadherin internalization is thought to be important for adhesive junction dynamics, the mechanism by which p120 modulates cadherin endocytosis is unknown. In this paper, we identify a dual-function motif in classical cadherins consisting of three highly conserved acidic residues that alternately serve as a p120-binding interface and an endocytic signal. Mutation of this motif resulted in a cadherin variant that was both p120 uncoupled and resistant to endocytosis. In endothelial cells, in which dynamic changes in adhesion are important components of angiogenesis and inflammation, a vascular endothelial cadherin (VE-cadherin) mutant defective in endocytosis assembled normally into cell–cell junctions but potently suppressed cell migration in response to vascular endothelial growth factor. These results reveal the mechanistic basis by which p120 stabilizes cadherins and demonstrate that VE-cadherin endocytosis is crucial for endothelial cell migration in response to an angiogenic growth factor.

Combining imaging and pathway profiling: an alternative approach to cancer drug discovery

Conventional drug discovery strategies are typically 'target centric' based on the selection of lead compounds with optimised 'on-target' potency and selectivity profiles. However, high-attrition rates are often the result of compensatory or redundant cancer mechanisms and the fact that tumours do not find it difficult to escape inhibition of a single pathway. In this article, we highlight two emerging and complimentary technologies; namely phenotypic imaging and post-translational pathway profiling, which when combined with relevant disease models can provide pharmacodiagnostic and drug combination strategies that predict and counteract inherent and adaptive drug resistance. The implementation of such approaches at early stages of the drug discovery process enables more informed decisions on candidate drug selection and how to maximise and predict efficacy before clinical development.

Advancing cancer drug discovery towards more agile development of targeted combination therapies

Current drug-discovery strategies are typically 'target-centric' and are based upon high-throughput screening of large chemical libraries against nominated targets and a selection of lead compounds with optimized 'on-target' potency and selectivity profiles. However, high attrition of targeted agents in clinical development suggest that combinations of targeted agents will be most effective in treating solid tumors if the biological networks that permit cancer cells to subvert monotherapies are identified and retargeted. Conventional drug-discovery and development strategies are suboptimal for the rational design and development of novel drug combinations. In this article, we highlight a series of emerging technologies supporting a less reductionist, more agile, drug-discovery and development approach for the rational design, validation, prioritization and clinical development of novel drug combinations.

Src/FAK-mediated regulation of E-cadherin as a mechanism for controlling collective cell movement: insights from in vivo imaging

Recent advances in confocal and multi-photon microscopy, together with fluorescent probe development, have enabled cancer biology studies to go beyond the culture dish and interrogate cancer-associated processes in the complex in vivo environment. Regulation of the tumor suppressor protein E-cadherin plays an important role in cancer development and progression and may contribute to the decision between 'single cell' and 'collective invasion' in vivo. Mounting evidence from in vitro and in vivo experiments places the two non-receptor protein tyrosine kinases Src and Focal Adhesion Kinase, at the heart of E-cadherin regulation, and the crosstalk between integrins and cadherins. Here we discuss recent insights, attained using high resolution fluorescent in vivo imaging, into the regulation of E-cadherin and collective invasion. We focus on the regulatory crosstalk between the Src/FAK signaling axis and E-cadherin in vivo.

Live cell in vitro and in vivo imaging applications: accelerating drug discovery

Dynamic regulation of specific molecular processes and cellular phenotypes in live cell systems reveal unique insights into cell fate and drug pharmacology that are not gained from traditional fixed endpoint assays. Recent advances in microscopic imaging platform technology combined with the development of novel optical biosensors and sophisticated image analysis solutions have increased the scope of live cell imaging applications in drug discovery. We highlight recent literature examples where live cell imaging has uncovered novel insight into biological mechanism or drug mode-of-action. We survey distinct types of optical biosensors and associated analytical methods for monitoring molecular dynamics, in vitro and in vivo. We describe the recent expansion of live cell imaging into automated target validation and drug screening activities through the development of dedicated brightfield and fluorescence kinetic imaging platforms. We provide specific examples of how temporal profiling of phenotypic response signatures using such kinetic imaging platforms can increase the value of in vitro high-content screening. Finally, we offer a prospective view of how further application and development of live cell imaging technology and reagents can accelerate preclinical lead optimization cycles and enhance the in vitro to in vivo translation of drug candidates.

Quantitative in vivo imaging of the effects of inhibiting integrin signaling via Src and FAK on cancer cell movement: effects on E-cadherin dynamics

Most cancer-related deaths are due to the development of metastatic disease, and several new molecularly targeted agents in clinical development have the potential to prevent disease progression. However, it remains difficult to assess the efficacy of antimetastatic agents in the clinical setting, and an increased understanding of how such agents work at different stages of the metastatic cascade is important in guiding their clinical use. We used optical window chambers combined with photobleaching, photoactivation, and photoswitching to quantitatively measure (a) tumor cell movement and proliferation by tracking small groups of cells in the context of the whole tumor, and (b) E-cadherin molecular dynamics in vivo following perturbation of integrin signaling by inhibiting focal adhesion kinase (FAK) and Src. We show that inhibition of Src and FAK suppresses E-cadherin-dependent collective cell movement in a complex three-dimensional tumor environment, and modulates cell-cell adhesion strength and endocytosis in vitro. This shows a novel role for integrin signaling in the regulation of E-cadherin internalization, which is linked to regulation of collective cancer cell movement. This work highlights the power of fluorescent, direct, in vivo imaging approaches in the preclinical evaluation of chemotherapeutic agents, and shows that inhibition of the Src/FAK signaling axis may provide a strategy to prevent tumor cell spread by deregulating E-cadherin-mediated cell-cell adhesions.