Cell adhesion: old and new questions

Metazoans clearly need cell adhesion to hold themselves together, but adhesion does much more than that. Adhesion receptors make transmembrane connections, linking extracellular matrix and adjacent cells to the intracellular cytoskeleton, and they also serve as signal transducers. In this article, I briefly summarize our present understanding of the molecular basis and biological consequences of cell adhesion and discuss how our current knowledge sheds light on questions of specificity of cell adhesion. I offer some thoughts and speculations about the evolution of cell-adhesion molecules and processes, consider their inter-relationships with other forms of cell–cell communication and discuss unresolved questions ripe for investigation as we enter the postgenomic era.     

Cell adhesion: old and new questions

Metazoans clearly need cell adhesion to hold themselves together, but adhesion does much more than that. Adhesion receptors make transmembrane connections, linking extracellular matrix and adjacent cells to the intracellular cytoskeleton, and they also serve as signal transducers. In this article, I briefly summarize our present understanding of the molecular basis and biological consequences of cell adhesion and discuss how our current knowledge sheds light on questions of specificity of cell adhesion. I offer some thoughts and speculations about the evolution of cell-adhesion molecules and processes, consider their inter-relationships with other forms of cell–cell communication and discuss unresolved questions ripe for investigation as we enter the postgenomic era.     

Focal Adhesion: A Focal Point in Current Cell Biology and Molecular Medicine

Cell-extracellular matrix (ECM) is an important property of virtually all cells in multi-cellular organisms. Cell-ECM adhesion research, therefore, has broad impact on biology and medicine. Studies over the past three decades have resulted in tremendous advance in our understanding of the molecular basis and functions of cell-ECM adhesion. Here, I focus on some of the general lessons that we have learned from recent studies on cell-ECM adhesion. In addition, I highlight several topics in this rapidly advancing research area. These topics, which include assembly, disassembly and regulation of cell-ECM adhesion structures, the molecular mechanisms of bi-directional signaling through cell-ECM adhesions, and the tissue and organ pathobiology of cell-ECM adhesion, are pertinent to our understanding of cell-ECM adhesion and signaling.Key Words: Focal adhesion, integrins, extracellular matrix, cytoskeleton, cell migrationCell-ECM adhesion is a fundamental process through which cells interact and communicate with the environment. Cell-ECM adhesion is essential for organogenesis during embryonic development. In adult, it is vital for maintenance of tissue integrity and organ functions. Alterations of cell-ECM adhesion hence are frequently associated with human diseases. Because of the broad significance of cell-ECM adhesion in biology and pathology, understanding how cell-ECM adhesion is mediated and regulated and determining how cell-ECM adhesion influences cell behavior have been the subjects of numerous studies. In particular, studies over the past three decades have led to major breakthroughs in our understanding of cell-ECM adhesion. Many of the key discoveries, including identification of integrins as major transmembrane receptors for ECM proteins, demonstration of integrins as bi-directional (outside-in and inside-out) transmembrane signaling machines, identification of talin, focal adhesion kinase (FAK), integrin-linked kinase (ILK) and other cytoplasmic and membrane-associated proteins as key regulators and effectors of integrins, and delineation of multiple downstream signaling pathways that relay signals from cell surface integrins to diverse cytoplasmic and nuclear effectors, have been reviewed in refs. ^1–12. In this brief article, I will focus on some of the general features of cell-ECM adhesion and discuss from my personal perspective several key questions that remain to be answered in future studies.     

Atomic force microscopy: a nanoscopic window on the cell surface

Atomic force microscopy (AFM) techniques provide a versatile platform for imaging and manipulating living cells to single-molecule resolution, thereby enabling us to address pertinent questions in key areas of cell biology, including cell adhesion and signalling, embryonic and tissue development, cell division and shape, and microbial pathogenesis. In this review, we describe the principles of AFM, and survey recent breakthroughs made in AFM-based cell nanoscopy, showing how the technology has increased our molecular understanding of the organization, mechanics, interactions and processes of the cell surface. We also discuss the advantages and limitations of AFM techniques, and the challenges remaining to be addressed in future research.     

Oxygen levels and the regulation of cell adhesion in the nervous system

In this article, I discuss the hallmarks of hypoxia in vitro and in vivo and review work showing that many types of stem cell proliferate more robustly in lowered oxygen. I then discuss recent studies showing that alterations in the levels and the types of cell and substrate adhesion molecules are a notable response to reduced O₂ levels in both cultured primary neural stem cells and brain tissues in response to hypoxia in vivo. The ability of O₂ levels to regulate adhesion molecule expression is linked to the Wnt signaling pathway, which can control and be controlled by adhesion events. The ability of O₂ levels to influence cell adhesion also has far-reaching implications for development, ischemic trauma and neural regeneration, as well as for cancer and other diseases. Finally I discuss the possibility that the fluctuations in O₂ levels known to have occurred over evolutionary time could, by influencing adhesion systems, have contributed to early symbiotic events in unicellular organisms and to the emergence of multicellularity. It is not my intention to be exhaustive in these domains, which are far from my own field of study. Rather this article is meant to provoke and stimulate thinking about molecular evolution involving O₂ sensing and signaling during eras of geologic and atmospheric change that might inform modern studies on development and disease.     

Oxygen levels and the regulation of cell adhesion in the nervous system: A control point for morphogenesis in development,disease and evolution?

In this article, I discuss the hallmarks of hypoxia in vitro and in vivo and review work showing that many types of stem cell proliferate more robustly in lowered oxygen. I then discuss recent studies showing that alterations in the levels and the types of cell and substrate adhesion molecules are a notable response to reduced O₂ levels in both cultured primary neural stem cells and brain tissues in response to hypoxia in vivo. The ability of O₂ levels to regulate adhesion molecule expression is linked to the Wnt signaling pathway, which can control and be controlled by adhesion events. The ability of O₂ levels to influence cell adhesion also has far-reaching implications for development, ischemic trauma and neural regeneration, as well as for cancer and other diseases. Finally I discuss the possibility that the fluctuations in O₂ levels known to have occurred over evolutionary time could, by influencing adhesion systems, have contributed to early symbiotic events in unicellular organisms and to the emergence of multicellularity. It is not my intention to be exhaustive in these domains, which are far from my own field of study. Rather this article is meant to provoke and stimulate thinking about molecular evolution involving O₂ sensing and signaling during eras of geologic and atmospheric change that might inform modern studies on development and disease.     

Integrative systems and synthetic biology of cell-matrix adhesion sites

ABSTRACT

The complexity of cell-matrix adhesion convolves its roles in the development and functioning of multicellular organisms and their evolutionary tinkering. Cell-matrix adhesion is mediated by sites along the plasma membrane that anchor the actin cytoskeleton to the matrix via a large number of proteins, collectively called the integrin adhesome. Fundamental challenges for understanding how cell-matrix adhesion sites assemble and function arise from their multi-functionality, rapid dynamics, large number of components and molecular diversity. Systems biology faces these challenges in its strive to understand how the integrin adhesome gives rise to functional adhesion sites. Synthetic biology enables engineering intracellular modules and circuits with properties of interest. In this review I discuss some of the fundamental questions in systems biology of cell-matrix adhesion and how synthetic biology can help addressing them.     

Making a Little Affinity Go a Long Way: A Topological View of LFA-1 Regulation

Lymphocytes utilize adhesion to navigate in the body and to transiently interact with a variety of potential antigen presenting cells. Interactions of adhesion molecules are governed by the law of mass action and the less understood rules of apposed biological membranes. Biochemical parameters such as adhesion molecule affinity only tell part of the story. Factors such as lateral mobility, membrane alignment and cytoskeletal interactions are equally important in determining the final outcome. Therefore it is important to determine mechanisms by which the properties of cell membranes and the cytoskeleton reinforce or hinder adhesion molecule interactions. Work from my lab has shown that one mechanism by which lymphocyte adhesion molecules cooperate is to align adhering membranes with nanometer precision. Here, I discuss a model for LFA-1 regulation that is dependent on three independent processes: LFA-1 lateral mobility, ligand induced generation of a small amount of high affinity LFA-1 and local membrane alignment. I propose that coordination of these processes allows rapid interconversion between stable adhesion and detachment.     

Global change,biodiversity, and ecosystem services: What can we learn from studies of pollination?

The study of global environmental change and its effect on biodiversity and ecosystem function is at an exciting crossroads, at which ideas developed largely through theory and small-scale experiments are now being tested with ecosystem services as they are delivered to people in real-world landscapes. Pollinators and pollination are emerging as a model system for exploring these questions, which inherently required working large spatio-temporal scales. In this Invited View, I discuss current questions that are at the leading edge of this research. I first point out some surprising knowledge gaps in our understanding of pollinators’ response to global change. I then outline several ways in which current understanding of the biodiversity-ecosystem functioning relationship might be transformed by studies conducted at large spatio-temporal scales. Specifically, I propose two hypotheses that relate to the number of species required to saturate ecosystem function, and to the mechanisms through which biodiversity stabilizes ecosystem function over space or time.     

Agency and intervention

Novel ways to intervene on brain function raise questions about agency and responsibility. Here, I discuss whether direct brain interventions, and in particular, deep brain stimulation, pose a threat to agency in individual cases, or to our general conceptualization of what it is to be a responsible agent. While I do not currently see evidence that these interventions constitute a global challenge to our concept of agency, they do have the potential to diminish agency in individuals. I consider whether the lack of evidence for a global challenge ratifies our folk conceptions, or is a necessary consequence of them. In closing, I propose that our theoretical understanding of agency and our therapeutic approaches could be improved with a more nuanced, multidimensional view of agency.     

Primate disease ecology in comparative and theoretical perspective

Infectious disease plays a major role in the lives of wild primates, and the past decade has witnessed significant strides in our understanding of primate disease ecology. In this review, I briefly describe some key findings from phylogenetic comparative approaches, focusing on analyses of parasite richness that use the Global Mammal Parasite Database. While these studies have provided new answers to fundamental questions, new questions have arisen, including questions about the underlying epidemiological mechanisms that produce the broader phylogenetic patterns. I discuss two examples in which theoretical models have given us new traction on these comparative questions. First, drawing on findings of a positive association between range use intensity and the richness of helminth parasites, we developed a spatially explicit agent-based model to investigate the underlying drivers of this pattern. From this model, we are gaining deeper understanding of how range use intensity results in greater exposure to parasites, thus producing higher prevalence in the simulated populations-and, plausibly, higher parasite richness in comparative analyses. Second, I show how a model of disease spread on social networks provides solid theoretical foundations for understanding the effects of sociality and group size on parasitism across primate species. This study further revealed that larger social groups are more subdivided, which should slow the spread of infectious diseases. This effect could offset the increased disease risk expected in larger social groups, which has yet to receive strong empirical support in our comparative analyses. In addition to these examples, I discuss the need for more meta-analyses of individual-level phenomena documented in the field, and for greater linkage between theoretical modeling and field research.     

Possible Roles for Cognin in Chick Embryo Neural Retina

In this review, we discuss current information about a cell-cellrecognition protein present in chick embryo neural retina. Thisprotein, retina cognin, has cell adhesion or aggregation promotingpropertiesin vitro. We discuss five questions. First, what isretina cognin (R-cognin)? Second, what do we know about cogninin chick retina? We discuss its histological distribution inretina and how that distribution changes during embryonic andearly post-hatching development. Third, where is cognin withincells? We review light microscopy evidence for its localizationin plasma membranes of somas and neurites of selected retinalneurons as an intrinsic membrane protein. Fourth, how is cognindistributed in membranes? We summarize evidence that cogninmight not be uniformly distributed over cellsurfaces and thatit might bind to specific proteins on the surfaces of otherretina cells. From the available information, we ask what wecan deduce about cognin's biological role in the neural retina.     

Mechanisms of elongation in embryogenesis

Here, I discuss selected examples of elongation in embryogenesis to identify common and unique mechanisms, useful questions for further work, and new systems that offer opportunities for answering these questions. Fiber-wound, hydraulic mechanisms of elongation highlight the importance of biomechanical linkages of otherwise unrelated cellular behaviors during elongation. Little-studied examples of elongation by cell intercalation offer opportunities to study new aspects of this mode of elongation. Elongation by oriented cell division highlights the problem of mitotic spindle orientation and the maintenance of cell-packing patterns in anisotropic force environments. The balance of internal cell-adhesion and external traction forces emerges as a key issue in the formation of elongate structures from compact ones by directed migration.     

Cell adhesion in tumor invasion and metastasis: loss of the glue is not enough

Tumor cells often show a decrease in cell–cell and/or cell–matrix adhesion. An increasing body of evidence indicates that this reduction in cell adhesion correlates with tumor invasion and metastasis. Two main groups of adhesion molecules, cadherins and CAMs, have been implicated in tumor malignancy. However, the specific role that these proteins play in the context of tumor progression remains to be elucidated. In this review, we discuss recent data pointing to a causal relationship between the loss of cell adhesion molecules and tumor progression. In addition, the direct involvement of these molecules in specific signal transduction pathways will be considered, with particular emphasis on the alterations of such pathways in transformed cells. Finally, we review recent observations on the molecular mechanisms underlying metastatic dissemination. In many cases, spreading of tumor cells from the primary site to distant organs has been characterized as an active process involving the loss of cell–cell adhesion and gain of invasive properties. On the other hand, various examples of metastases exhibiting a relatively benign (i.e. not invasive) phenotype have been reported. Together with our recent results on a mouse tumor model, these findings indicate that ‘passive’ metastatic dissemination can occur, in particular as a consequence of impaired cell–matrix adhesion and of tumor tissue disaggregation.     

Characterization of E-cadherin-dependent and -independent events in a new model of c-Fos-mediated epithelial-mesenchymal transition

Fos proteins have been implicated in control of tumorigenesis-related genetic programs including invasion, angiogenesis, cell proliferation and apoptosis. In this study, we demonstrate that c-Fos is able to induce mesenchymal transition in murine tumorigenic epithelial cell lines. Expression of c-Fos in MT1TC1 cells led to prominent alterations in cell morphology, increased expression of mesenchymal markers, vimentin and S100A4, DNA methylation-dependent down-regulation of E-cadherin and abrogation of cell-cell adhesion. In addition, c-Fos induced a strong beta-catenin-independent proliferative response in MT1TC1 cells and stimulated cell motility, invasion and adhesion to different extracellular matrix proteins. To explore whether loss of E-cadherin plays a role in c-Fos-mediated mesenchymal transition, we expressed wild-type E-cadherin and two different E-cadherin mutants in MT1TC1/c-fos cells. Expression of wild-type E-cadherin restored epithelioid morphology and enhanced cellular levels of catenins. However, exogenous E-cadherin did not influence expression of c-Fos-dependent genes, only partly suppressed growth of MT1TC1/c-fos cells and produced no effect on c-Fos-stimulated cell motility and invasion in matrigel. On the other hand, re-expression of E-cadherin specifically negated c-Fos-induced adhesion to collagen type I, but not to laminin or fibronectin. Of interest, mutant E-cadherin which lacks the ability to form functional adhesive complexes had an opposite, potentiating effect on cell adhesion to collagen I. These data suggest that cell adhesion to collagen I is regulated by the functional state of E-cadherin. Overall, our data demonstrate that, with the exception of adhesion to collagen I, c-Fos is dominant over E-cadherin in relation to the aspects of mesenchymal transition assayed in this study.     

Unconventional roles for membrane traffic proteins in response to muscle membrane stress

In skeletal muscle fibers, ubiquitous membrane trafficking pathways responsible for transporting newly synthesized proteins, recycling cell surface receptors, and organizing membrane compartmentation have adapted to the high needs of an extremely specialized cell under constant mechanical stress. Membrane remodeling proteins involved in ubiquitous mechanisms such as clathrin-mediated endocytosis, caveolae formation, and membrane fusion have evolved to produce new pathways with sometimes completely different functions such as adhesion and mechanoprotection. In this review, I discuss recent advances in understanding the specialized features of skeletal muscle clathrin-coated plaques, caveolae, and dysferlin-mediated membrane repair. A special emphasis is given on recent findings suggesting that membrane trafficking pathways have evolved to participate into the mechanisms responsible for sarcolemma resistance to mechanical stress and discuss how defects in these pathways result in muscle disease.     

Kinetics and mechanics of cell adhesion

Cell adhesion is mediated by specific interaction between receptors and ligands. Such interaction provides not only physical linkage but also communication between the cell and its environment. The kinetics and mechanics of cell adhesion are coupled, because force can influence the formation and dissociation of receptor-ligand bonds. The kinetic rates and their force dependence determine how likely, how rapidly and how strongly cells bind as well as how long they remain bound. Since adhesion molecules are linked to apposing cellular membranes, their interaction is governed by two-dimensional (2D) kinetics. This is in contrast to the three-dimensional (3D) binding of soluble ligands to cell surface receptors. Unlike the 3D case in which many methods are available for measuring kinetic rates, not until recently have the 2D kinetic rates become experimentally measurable. In this review, I will discuss the recent progress in the experimental methods that enable quantification of the relevant kinetic and mechanical parameters, the fundamental concepts that underlie the physics of the biological phenomena, and the mathematical models that relate functions to the intrinsic properties of the adhesion molecules.     

Integrins during evolution: Evolutionary trees and model organisms

The integrins form a large family of cell adhesion receptors. All multicellular animals express integrins, indicating that the family evolved relatively early in the history of metazoans, and homologous sequences of the component domains of integrin α and β subunits are seen in prokaryotes. Some integrins, however, seem to be much younger. For example, the αI domain containing integrins, including collagen receptors and leukocyte integrins, have been found in chordates only. Here, we will discuss what conclusions can be drawn about integrin function by studying the evolutionary conservation of integrins. We will also look at how studying integrins in organisms such as the fruit fly and mouse has helped our understanding of integrin evolution-function relationships. As an illustration of this, we will summarize the current understanding of integrin involvement in skeletal muscle formation.     

Integrin Signaling: Cytoskeletal Complexes,MAP Kinase Activation,and Regulation of Gene Expression

Members of the integrin family of adhesion receptors mediate interactions of cells with the extracellular matrix. Besides their role in tissue morphogenesis by anchorage of cells to basement membranes and migration along extracellular matrix proteins, integrins are thought to play a key role in mediating the control of gene expression by the extracellular matrix. Studies over the past 10 years have shown that integrin-mediated cell adhesion can trigger signal transduction cascades involving translocation of proteins and protein tyrosine phosphorylation events. In this review, we discuss approaches used in our lab to study early events in integrin signalling as well as further downstream changes.     

Non-SH2/PDZ reverse signaling by ephrins

Great strides have been made regarding our understanding of the processes and signaling events influenced by Eph/ephrin signaling that play a role in cell adhesion and cell movement. However, the precise mechanisms by which these signaling events regulate cell and tissue architecture still need further resolution. The Eph/ephrin signaling pathways and the ability to regulate cell-cell adhesion and motility constitutes an impressive system for regulating tissue separation and morphogenesis (Pasquale, 2005, 2008 [1,2]). Moreover, the de-regulation of this signaling system is linked to the promotion of aggressive and metastatic tumors in humans [2]. In the following section, we discuss some of the interesting mechanisms by which ephrins can signal through their own intracellular domains (reverse signaling) either independent of forward signaling or in addition to forward signaling through a cognate receptor. In this review we discuss how ephrins (Eph ligands) "reverse signal" through their intracellular domains to affect cell adhesion and movement, but the focus is on modes of action that are independent of SH2 and PDZ interactions.