Analysis of Integrin Signaling by Fluorescence Resonance Energy Transfer

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Molecularly engineered surfaces for cell biology: from static to dynamic surfaces

Surfaces with a well-defined presentation of ligands for receptors on the cell membrane can serve as models of the extracellular matrix for studying cell adhesion or as model cell surfaces for exploring cell-cell contacts. Because such surfaces can provide exquisite control over, for example, the density of these ligands or when the ligands are presented to the cell, they provide a very precise strategy for understanding the mechanisms by which cells respond to external adhesive cues. In the present feature article, we present an overview of the basic biology of cell adhesion before discussing surfaces that have a static presentation of immobile ligands. We outline the biological information that such surfaces have given us, before progressing to recently developed switchable surfaces and surfaces that mimic the lipid bilayer, having adhesive ligands that can move around the membrane and be remodeled by the cell. Finally, the feature article closes with some of the biological information that these new types of surfaces could provide.

A homogeneous chemiluminescent immunoassay method

A new homogeneous chemiluminescent immunoassay method featuring the use of specific binding members separately labeled with an acridan-based chemiluminescent compound and a peroxidase is reported. Formation of an immunocomplex brings the chemiluminescent compound and the peroxidase into close proximity. Without any separation steps, a chemiluminescent signal is generated upon addition of a trigger solution, and the intensity is directly correlated to the quantity of the analyte.

Overview: imaging in the study of integrins

Integrins play critical adhesion and signaling roles during development, wound healing, immunity, and cancer. Central to their function is a unique ability to dynamically modulate their adhesiveness and signaling properties through changes in conformation, both homo- and heterotypic protein-protein interactions and cellular distribution. Genetic, biochemical and structural studies have been instrumental in uncovering overall functions, describing ligand and regulatory protein interactions and elucidating the molecular architecture of integrins. However, such approaches alone are inadequate to describe how dynamic integrin behaviors are orchestrated in intact cells. To fill this void, a wide array of distinct light microscopy (largely fluorescence-based) imaging approaches have been developed and employed. Various microscopy technologies, including wide-field, optical sectioning (laser-scanning confocal, spinning-disk confocal, and multiphoton), TIRF and range of novel "Super-Resolution" techniques have been used in combination with diverse imaging modalities (such as IRM, FRET, FRAP, CALI, and fluorescence speckle imaging) to address distinct aspects of integrin function and regulation. This chapter provides an overview of these imaging approaches and how they have advanced our understanding of integrins.

Short hairpin RNA-mediated fibronectin knockdown delays tumor growth in a mouse glioma model

Glioblastoma multiforme is the most common and lethal primary brain tumor. Glioma progression depends on the rapid proliferation of tumor cells accompanied by an acute immunosuppressive environment, facilitated mainly by tumor infiltration of regulatory T cells (Tregs). In this study, we characterize the role of fibronectin, a high-molecular weight extracellular matrix glycoprotein secreted by tumor cells, in controlling glioma progression and in mediating immunosuppression. Fibronectin binds to membrane-spanning integrin receptors and plays an important role in cell signaling, in defining cellular shape, in mobility, and in regulating the cell cycle. We found that inhibition of fibronectin expression in glioma cells, using short hairpin RNA-mediated silencing of gene expression, delayed cell proliferation in vitro. This delayed growth is explained, in part, by the observed reduced expression of integrin β(1) fibronectin receptor, which was restored by the inhibition of proteosomal activity. In our analysis of the downstream signaling targets of integrin β(1), we demonstrated reduced phosphorylation of Src kinase and STAT-3. We also observed reduced survivin expression that induced a three-fold increased accumulation of fibronectin-knockdown cells in the G(2)/M phase. In an experimental animal model, the fibronectin knockdown tumors had a mean survival advantage of 23 days over wild-type tumors. Moreover, brain samples of animals bearing fibronectin-knockdown tumors showed delayed Treg recruitment. Collectively, we propose that fibronectin is a key mediator of glioma progression because its inhibition delays both tumor progression and immunosuppression.

Systems microscopy approaches to understand cancer cell migration and metastasis

Cell migration is essential in a number of processes, including wound healing, angiogenesis and cancer metastasis. Especially, invasion of cancer cells in the surrounding tissue is a crucial step that requires increased cell motility. Cell migration is a well-orchestrated process that involves the continuous formation and disassembly of matrix adhesions. Those structural anchor points interact with the extra-cellular matrix and also participate in adhesion-dependent signalling. Although these processes are essential for cancer metastasis, little is known about the molecular mechanisms that regulate adhesion dynamics during tumour cell migration. In this review, we provide an overview of recent advanced imaging strategies together with quantitative image analysis that can be implemented to understand the dynamics of matrix adhesions and its molecular components in relation to tumour cell migration. This dynamic cell imaging together with multiparametric image analysis will help in understanding the molecular mechanisms that define cancer cell migration.

Optical methods in the study of protein-protein interactions

Förster (or Fluorescence) resonance energy transfer (FRET) is a physical process in which energy is transferred nonradiatively from an excited fluorophore, serving as a donor, to another chromophore (acceptor). Among the techniques related to fluorescence microscopy, FRET is unique in providing signals sensitive to intra- and intermolecular distances in the 1-10 nm range. Because of its potency, FRET is increasingly used to visualize and quantify the dynamics of protein-protein interaction in living cells, with high spatio-temporal resolution. Here we describe the physical bases of FRET, detailing the principal methods applied: (1) measurement of signal intensity and (2) analysis of fluorescence lifetime (FLIM). Although several technical complications must be carefully considered, both methods can be applied fruitfully to specific fields. For example, FRET based on intensity detection is more suitable to follow biological phenomena at a finely tuned spatial and temporal scale. Furthermore, a specific fluorescence signal occurring close to the plasma membrane (< or = 100 nm) can be obtained using a total internal reflection fluorescence (TIRF) microscopy system. When performing FRET experiments, care must be also taken to the method chosen for labeling interacting proteins. Two principal tools can be applied: (1) fluorophore tagged antibodies; (2) recombinant fluorescent fusion proteins. The latter method essentially takes advantage of the discovery and use of spontaneously fluorescent proteins, like the green fluorescent protein (GFP). Until now, FRET has been widely used to analyze the structural characteristics of several proteins, including integrins and ion channels. More recently, this method has been applied to clarify the interaction dynamics of these classes of membrane proteins with cytosolic signaling proteins. We report two examples in which the interaction dynamics between integrins and ion channels have been studied with FRET methods. Using fluorescent antibodies and applying FRET-FLIM, the direct interaction of beta1 integrin with the receptor for Epidermal Growth Factor (EGF-R) has been proved in living endothelial cells. A different approach, based on TIRFM measurement of the FRET intensity of fluorescently labeled recombinant proteins, suggests that a direct interaction also occurs between integrins and the ether-a-go-go-related-gene 1 (hERG1) K+ channel.

Signal co-operation between integrins and other receptor systems

The multicellular nature of metazoans means that all cellular processes need to be tuned by adhesive interactions between cells and their local microenvironment. The spatial organization of cells within tissues requires sophisticated networks of extracellular signals to control their survival and proliferation, movements and positioning, and differentiated function. These cellular characteristics are mediated by multiple inputs from adhesion systems in combination with soluble and developmental signals. In the present review we explore how one class of adhesion receptor, the integrins, co-operate with other types of receptor to control diverse aspects of cell fate. In particular we discuss: (i) how beta3 and beta1 integrins work together with growth factors to control angiogenesis; (ii) how alpha6beta4 integrin co-operates with receptor tyrosine kinases in normal epithelial function and cancer; (iii) the interplay between beta1 integrins and EGF (epidermal growth factor) receptor; (iv) signal integration connecting integrins and cytokine receptors for interleukins, prolactin and interferons; and (v) how integrins and syndecans co-operate in cell migration.

Profiling distinct mechanisms of tumour invasion for drug discovery: imaging adhesion, signalling and matrix turnover

Recent advances in microscopic imaging technology, fluorescent reporter reagents, 3-dimensional (3D) cell models and multiparametric image analysis have enhanced our ability to model and understand complex cell physiology. Extension of these approaches to live cell, kinetic studies allows further spatial and temporal understanding of a multitude of dynamic functional events, including tumour cell invasion. Recent in vivo and 3D in vitro studies reveal how tumour cells utilize a diverse variety of mechanisms to permit invasion through 3D tissue environments. Such high degrees of diversity and plasticity between invasion mechanisms present a significant challenge to the successful treatment of malignant cancer. This review examines how advances in time-resolved imaging has contributed to the characterization of distinct modes of invasion and their associated molecular mechanisms. Specifically, we highlight the development of fluorescent reporter molecules and their incorporation into more predictive 3D in vitro and in vivo models, to enhance mechanistic analysis of tumour invasion. We also highlight the latest advances in kinetic imaging instrumentation applicable to in vitro and in vivo models of tumour invasion. We discuss how multiparametric image analysis can be used to interpret image data generated by these approaches. We further discuss how these approaches can be integrated into drug discovery pipelines to facilitate evaluation and selection of candidate drugs and novel pharmaceutical compositions, targeting multiple invasive mechanisms.