- Email: [email protected]
Cell-to-cell contact and extracellular matrix
Cell-to-cell contact and extracellular matrix Editorial overview Kathleen Green and Fiona Watt Current Opinion in Cell Biology 2004, 16:465–469 Available online 18th August 2004 0955-0674/$ – see front matter ß 2004 Published by Elsevier Ltd. DOI 10.1016/j.ceb.2004.08.004
Kathleen Green Departments of Pathology, Dermatology and the Robert H. Lurie Cancer Center, 303 E. Chicago Ave., Chicago, IL 60611
Kathleen Green is Joseph L. Mayberry Professor and Associate Chair of Research and Graduate Education in the Department of Pathology at Northwestern University Feinberg School of Medicine. Her research program is directed toward understanding the assembly and regulation of cadherin-based intercellular junctions in normal differentiation as well as human inherited disease and cancer. Fiona Watt Cancer Research UK London Research Institute, 44 Lincoln’s Inn Fields, London WC2A 3PX, UK
Fiona Watt is a principal scientist and head of the Keratinocyte Laboratory at the CR-UK London Research Institute. She is interested in the control of differentiation and tissue assembly in mammalian epidermis and in the deregulation of these processes that occurs in cancer.
Abbreviations APC adenomatous polyposis coli CAM cell adhesion molecule ECM extracellular matrix IF intermediate filament ILK integrin linked kinase TNT tunneling nanotube
Even before the acquisition of multicellularity, cells developed ways of gaining traction for motility and communicating with their environment through the development of adhesion systems. With the acquisition of multicellularity, more complex mechanisms were required for coordinating the sophisticated rearrangements occurring during tissue morphogenesis, for maintaining tissue integrity in the embryo and the adult, and for monitoring and responding to the environment. Since the requirement for regulated cell adhesion in the normal functioning of multicellular organisms is so pervasive, to adequately represent the significant strides made in this area during the past year is a daunting task. In putting together this issue, we set out to highlight lessons learned from the molecular to the organismal level in several broad areas: intercellular and integrin-based adhesion, cell junctions, extracellular matrix (ECM) function and modulation, and the receptorbased and intracellular machinery that controls adhesion effector pathways. By drawing comparisons between organisms primitive and advanced, and between plants and animals, we sought to gain new insight into shared and unique adhesive mechanisms. Several themes emerged, among them, a familiar refrain — that adhesion molecules are not just glue anymore. From an evolutionary perspective, it doesn’t seem surprising that signaling systems co-evolved with molecules involved in adhesion and recognition between cells. It is also an economy of efficiency that molecules serving one function can be modified, redesigned or retread biologically in response to intracellular or environmental cues to take on other roles within the cell. Molecules change post-translational states like we change our clothing, to provide new functional attributes that prepare them for specific environments and direct them to interact with new neighbors and neighborhoods. Furthermore, just as one molecule can serve different functions, similar functions can be performed by different molecules. Cellular receptors, and the intracellular cytoskeleton with which they are coupled, also co-evolved with pathogenic organisms. A number of contributions in this issue discuss how pathogenic organisms developed ingenious ways of targeting adhesion systems and co-opting them to their advantage, informing us in the process about the function and regulation of the targets. We begin with an evolutionary perspective on adhesion. We see how strategies developed by primitive organisms either evolve and remain
www.sciencedirect.com
Current Opinion in Cell Biology 2004, 16:465–469
466 Cell-to-cell contact and extracellular matrix
recognizable in modern species, are fashioned anew using bits and pieces from the old, or simply disappear as novel strategies are developed. In their ‘prehistory of adhesion’, Harwood and Coates tell us that familiar molecules and motifs were present in ancient organisms, including cadherin-like molecules, IgG-like cell adhesion molecules (CAMs) and C-type lectins, as were molecules with Fas-1 and thrombospondin domains. Collagens are found in porifera (sponges) and their receptors, the integrins, are also ancient proteins. In Dictyostelium, which exhibits both ameobic and multicellular states, we see the beginnings of ultrastructurally visible intercellular adhesive structures resembling those of higher organisms. During a stage known as fruiting body formation, junctions resembling the adherens junctions of higher vertebrates form in a band of cells around the top of the stalk; these junctions harbor the b-catenin homologue known as aardvark, which in Dictyostelium is required for attachment of actin and for signal transduction, foreshadowing similar roles in more advanced multicellular organisms.
not suprabasally as in vertebrates. However, each of these powerful, genetically tractable organisms has its own unique strengths for addressing particular developmental questions. In their review, Hardin and Lockwood discuss how C. elegans epithelial cells, positioned between the cuticle and ECM overlying the muscle, carry out their integrative functions through coordinated activities of intermediate filament (IF)- and actin-based organelles. C. elegans offers an opportunity to study cytoskeletal cross talk which is unique among invertebrate organisms, as other invertebrates, including Drosophila, do not have IF. Wu and Beitel reveal the power of Drosophila genetics in the analysis of the septate junction’s role in regulating tracheal epithelial tube size. Septate junctions, often compared to vertebrate tight junctions, appear to control tube size through mechanisms unrelated to their function in the diffusion barrier, possibly through patterning of the ECM and regulation of polarity genes. These studies promise to provide insight into the underlying basis of human pathologies like polycystic kidney disease.
Nowhere is cell surface recognition more fundamental to our existence than in the process of fertilization and sperm–egg interactions. In the review by Shur, Ensslin and Rodeheffer, we see an example of how motifs appearing first in the Dictyostelium lectin ‘discoidin’ appear later in the form of discoidin/F5/8 complement domains. These domains are characteristic of a group of galactose and monosacharide binding proteins, including SED1, a peripherally associated sperm protein required for sperm– egg adhesion in mouse. Its discoidin/F5/8 C domains are important for binding both the sperm and zona pellucida. SED1 may have other functions as it is also found in tissues outside of the male reproductive tract, and, in addition, other SED1 related proteins exist. One of these is Del 1, which mediates endothelial interactions with ECM via av integrins and, as detailed in another review in this issue by Sheppard, can activate a pro-angiogenic program by inducing the transcription factor Hox-D3.
Up until this past year it was thought that plants and animals diverged quite dramatically in their strategies to provide a means of direct communication and passage of small molecules from cell to cell. Whereas gap junctions serve this function in animals, direct cytoplasmic connections called plasmodesmata do so in higher plants. In a review by Cilia and Jackson, we learn the importance of plasmodesmata in protein trafficking, cell fate decisions and the regulation of the movement of pathogenic organisms from cell to cell. Though plasmodesmata have long been appreciated in plants, it was only recently discovered that animals may have something similar, termed TNTs or tunneling nanotubes, which allow selective transfer of vesicles and organelles [1]. How similar are tunneling nanotubes to plasmodesmata? TNTs are transient in nature, which is a major difference, as plasmodesmata are very stable structures. Cilia and Jackson note that plasmodesmata of characean algae are somewhat more similar in structure to TNTs, suggesting intriguing ancestral relationships between this species, which is transitional between algae and higher plants, and more advanced organisms that form TNTs. Plasmodesmata provide a major route for pathogenic organisms such as TMV (tobacco mosaic virus), raising the question of whether TNTs might serve this function in higher organisms.
With increasing multicellularity we see the emergence of multiple types of junctional complexes. In some cases these appear to be similar to the familiar vertebrate junctions, which include tight junctions, adherens junctions, desmosomes, gap junctions, hemidesmosomes and focal contacts. Other times similar macromolecular complexes are disguised in different sorts of junctional forms in invertebrate organisms. For instance, instead of an apical junctional complex, Caenorhabditis elegans has a single electron-dense junctional structure containing a phylogenetically conserved adherens junction domain, a new family of claudin-like proteins typically found in tight junctions in vertebrates, a discs large domain, and an apical domain. On the other hand, Drosophila has a clearly distinguishable adherens junction and septate junction; the latter serves some functions of tight junctions although it is located basally to the adherens junction, Current Opinion in Cell Biology 2004, 16:465–469
New data are shattering our notion that connexins simply facilitate traditional modes of communication in vertebrates through gap junctions. Stout, Goodenough and Paul review provocative evidence supporting the idea that connexins play roles in cell growth, resistance to injury, and other physiological functions through hemichannels and perhaps also independently from a recognizable plaque in non-contacting cells. In addition to other functions, connexin hemichannels play a key role www.sciencedirect.com
Editorial overview Green and Watt 467
in bacterial pathogenesis by Shigella, which causes bacterial dysentery. Like viruses, Shigella can undergo spreading from cell to cell in epithelia, and it appears that cells lacking connexins are unable to support the spread of Shigella, but this capability is restored by transfecting in certain connexins, possibly due to hemichannel-dependent release of ATP and consequent activation of purinergic receptors. So connexins aren’t just gap junction proteins; conversely, gap junctions aren’t just made from connexins. Invertebrate connexins, or innexins, represent an unusual, perhaps unique, case of channel proteins not being conserved across phyla. Innexins and connexins are unrelated, but they form gap junctions with remarkably similar properties. It turns out that innexins are not unique to invertebrates; related proteins that also assemble functional channels have been discovered in vertebrates and are collectively referred to as pannexins. So it seems that gap junctions evolved twice, and that the innexins/pannexins represent the more ancient channel building blocks. Several contributions in this issue highlight the diversity of functions that intercellular cadherin-containing adhesive junctions play in vertebrates and underscore the concept that the same molecule can perform multiple functions. These reviews also reveal that adhesion molecules are targets for pathogenic organisms that have developed multiple strategies for breaching epithelial barriers and ensuring their spread. We learn in the review from Sakisaka and Takai that such is the case for nectins, which are Ig-like CAMs, and a newer class of nectin-like molecules that cooperate with E-cadherin in the formation of adherens junctions and tight junctions, in part through their ability to activate the small GTPases rac and cdc42. In addition to their role in the earliest stages of adherens junction assembly, these molecules are also receptors for members of the Herpes virus family. Interaction of nectin-1 or-2 with one of the HSV (herpes simplex virus) envelope glycoproteins sets into motion clustering of other viral glycoproteins and facilitates fusion with the viral envelope and viral entry. Interaction of nectin-1 with the actin binding protein afadin increases the efficiency of intercellular spreading, but whether spread occurs through direct fusion of adjacent plasma membranes, possibly through mechanisms similar to tunneling nanotubes, is not well understood. A newly recognized core function of the adherens junction protein called p120 catenin is the focus of the review by Kowalczyk and Reynolds. p120 catenin has been implicated in both positive and negative regulation of cadherin-based adhesion, and, like nectin, also regulates the activity of small GTPases. p120 binds to the juxtamembrane domain of the cadherin tails but does not directly associate with the actin cytoskeleton — this is a job performed by a- and b-catenin. Recent work has revealed that loss of p120 from cells is correlated with www.sciencedirect.com
depletion of E-cadherin, and that re-introducing p120 into cells restores E-cadherin protein expression and intercellular adhesion. It turns out that this may be due to the ability of p120 to act as a plasma membrane retention signal, preventing cadherins from being endocytosed, perhaps by competing with other factors that target cadherins for internalization. Based on this critical function in vertebrates, it is puzzling that p120 is not required for cadherin stability in non-vertebrate organisms. It seems likely that, along with the appearance of multiple family members with partially redundant functions, this ability of p120 has evolved as a mechanism for fine-tuning cadherin-based adhesion in vertebrates during remodeling of tissues in development and in the adult. Degradation is also something that comes to mind when thinking about regulation of b-catenin, the prototypical dual-functioning armadillo protein, which plays roles in both the cadherin/catenin complex and in Wnt/wingless signaling. The regulator of b-catenin degradation is a protein complex in which APC (adenomatous polyposis coli) is a critical component that directs b-catenin to the proteosome in the absence of Wnt signaling. Like p120, APC has multiple functions. Recent evidence highlighted in the review by Bienz and Hamada suggests that APC serves important functions in adhesion through interacting with the actin-rich cell cortex. Its loss in tumor cells may not just affect b-catenin signaling but perhaps also compromise intercellular adhesion. A possible role for APC in adhesion is an idea that historically is not new, but our attention was diverted a number of years ago to its role in regulating b-catenin degradation, and we are only now coming back to its possible roles in adhesion. The mechanism by which APC regulates adhesion is not known, but authors speculate it could be by inhibiting a GEF called Asef, which is activated by truncated but not wild type APC. Like p120, re-expression of fulllength APC in cells expressing a truncation mutant restores E-cadherin-mediated adhesion. So, APC represents another example where adhesion and signaling may have co-evolved. It seems that uni-functional proteins may be the exception rather than the rule, particularly when it comes to the armadillo family. Another class of anchoring junction molecules that have been co-opted as points of entry into an epithelium by human pathogens is the desmosomal cadherin family. Desmosomal cadherins are related to classic cadherins, but are found in junctions called desmosomes that anchor intermediate filaments to the plasma membrane. Payne, Hanakawa, Amagai and Stanley relate a fascinating story in which the inactivation of the desmosomal cadherin, desmoglein 1, by two completely different pathways leads to epidermal blisters of strikingly similar histology. In one case, antibodies circulating in patients with an autoimmune disease called pemphigus foliaceus bind to Current Opinion in Cell Biology 2004, 16:465–469
468 Cell-to-cell contact and extracellular matrix
the extracellular domains of this cadherin, resulting in separation of the desmosomes and acantholysis — that is, separation between the keratinocyte plasma membranes. Intriguingly, the pathogen Staphylococcus aureus invades and spreads within the epidermis by producing a toxin, exfoliative toxin, which proteolyses desmoglein 1 with exquisite specificity, leading to effective spreading of the infection through blisters produced in the upper layers of the skin where this differentiation-specific cadherin is expressed. So pathogens have developed as many ways to breach intercellular adhesions as there are molecules, in some cases using host molecules as receptors for entry and spread (e.g. in the case of nectins) and in others targeting host molecules for demolition to allow entry and spread (e.g. desmoglein 1). Similar themes emerge as we move to cell–matrix interactions dependent on integrins. Integrins are important for numerous physiologic and pathologic processes including leukocyte trafficking, platelet aggregation, wound healing, invasion and metastasis, development and tissue morphogenesis. We are beginning to understand at an ever increasing level of detail how integrin function can be finely tuned by altering its conformational state. Mould and Humphries review data suggesting that instead of three previously proposed distinct conformational states — inactive, primed and ligandoccupied — it is more likely that there is a continuous spectrum of states regulated by protein interactions, cytokine stimulation and divalent cation occupancy. These states are facilitated by the presence of flexible ‘joints’ and ‘connections’ within the integrin heterodimer, and all of these states can affect ligand-binding affinity. The potential plethora of functional conformations is mind-boggling especially when one considers that data are available on only a limited number of integrins out of the total possible number of integrinligand pairs. But this sort of information will be invaluable for future efforts in drug design, enabling us to tailormake agents that bind to specific conformations to modify integrin function and cell adhesion. The need for drug design is underscored by the number of pathologies in which integrin-ligand interactions are involved. As highlighted in the review by Sheppard, the av integrins are of critical importance in pathways related to human disease. In addition to the role of av integrins in regulating vascular growth and permeability, avb6 and avb8 participate in the activation of TGFb, albeit by completely different mechanisms. avb8 presents latent complexes to one or more cell surface MMPs, which results in the release of TGFb into the milieu. Although activation of TGFb by avb6 does not require an MMP, its loss and the consequent loss of TGFb activation leads to MMP12 induction in macrophages, contributing to development of emphysema. The cooperation of MMPs in modulating integrin–matrix interactions is also highCurrent Opinion in Cell Biology 2004, 16:465–469
lighted in the contribution by Mott and Werb. These enzymes are widely functioning in many physiological processes and pathologies including arthritis and cancer, and recent evidence suggests that cleavage of ECM can expose cryptic information contained within a molecule, thus providing another mechanism to produce molecules with multiple functions that influence processes such as angiogenesis and cell migration. As also discussed in the Sheppard review, MMPs facilitate the release of biologically active factors such as TGFb from the ECM. Interestingly, mice lacking fibrillin-1, an LTBP which is a component of the complex which tethers latent TGFb to the ECM, are unable to sequester TGFb in the matrix. These mice have early postnatal lung abnormalities and impairment of distal alveolar septation and, like mice in which TGFb activation is impaired by loss of avb6, develop emphysema as they age. Integrin–ECM interactions can be regulated from inside the cell, for example by proteins such as ILK (integrin linked kinase). ILK acts as a platform for the recruitment of proteins which participate in the linkage of integrins to the actin cytoskeleton in focal adhesions. Thus, ILK acts as a conduit for transmitting mechanical and chemical signals into the cell from integrin–matrix interactions. In the review by Fa¨ ssler and colleagues we learn that the physiological role of ILK’s kinase activity is controversial. Severe defects in worms and flies lacking ILK can be rescued by kinase-dead forms of ILK, suggesting that at least some important functions do not require kinase activity. Nevertheless, other cell behaviors do require kinase activity, and the fact that tumors have increased expression of ILK and ILK activity underscores the importance of this question. In the review by Yurchenco and Wadsworth, we learn about two families of matrix molecules that play roles in the basement membranes of the early developing embryo: the laminins and netrins. These matrix molecules share N-terminal domain structure but are otherwise only distantly related. However, each family has highly conserved functions across species throughout evolution. Laminins, are heterotrimeric proteins that are required for viability in both Drosophila and C. elegans and are essential for basement membrane assembly. Netrins on the other hand serve as guidance cues (both attractive and repellant) that regulate the cell adhesion and migration of pioneer axons, other neurons and mesodermal cells, and are important for development and epithelial morphogenesis as well. Cues which regulate neuronal behavior are also the topic of a review by Klein, who provides us with an update on the roles of eph/ephrin signaling in neural development and plasticity. Receptor (Eph)–ligand pairs comprise the largest family of receptor tyrosine kinases. They control behaviors in a bi-directional fashion to mediate attraction www.sciencedirect.com
Editorial overview Green and Watt 469
and repulsion or adhesion and de-adhesion and in so doing are critical regulators of neuronal plasticity during morphogenesis and in the adult. Rather than altering nuclear events, Eph/ephrins appear to control behavior primarily through modification of the cytoskeleton by regulating signaling pathways such as small GTPases. Novel modes of regulation by internalization of receptor/ligand pairs dependent on actin polymerization and rac signaling have emerged over the past year. In addition, it has been revealed that different Ephs bind different Rho-GEFS, providing one possible mechanism for regulating the specificity of downstream effector pathways. The regulation of cell protrusive activity and motility by the cytoplasmic actin binding protein fascin is the subject of Adams’ contribution. This small actin-bundling and cross-linking protein is well-conserved from platyhelminths and echinoderms. It provides a functional link between matrix molecules such as thrombospondin and fibronectin and the actin cytoskeletal machinery that regulates cell behavior. Fascin is also another example of a protein that has been co-opted by bacterial pathogens. It has been implicated in Listeria monocytogenes actin tail formation. In addition, Rickettsis conorii activates the
www.sciencedirect.com
host Arp2/3 complex and promotes the formation of actin tails containing fascin, which is thought be responsible for the observed bundling of actin filaments. It is clear that as each year passes we gain a better understanding of the various sticky situations in which we find ourselves and a greater appreciation for the varied strategies developed throughout evolution for providing contact and communication between neighboring cells and organisms and their environments. Experiments of nature continue to provide insight into how adhesion molecules work, and inform us about the vulnerabilities of these systems to attack. Along with the increasing resolution and emerging molecular detail available regarding the molecular adhesion machinery, this knowledge will help direct our efforts towards developing designer drugs with which to combat pathogens and pathologies that co-opt and target adhesion molecules for their own purposes.
References 1.
Rustom A, Saffrich R, Markovic I, Walther P, Gerdes HH: Nanotubular highways for intercellular organelle transport. Science 2004, 303:1007-1010.
Current Opinion in Cell Biology 2004, 16:465–469
Kathleen Green Departments of Pathology, Dermatology and the Robert H. Lurie Cancer Center, 303 E. Chicago Ave., Chicago, IL 60611
Kathleen Green is Joseph L. Mayberry Professor and Associate Chair of Research and Graduate Education in the Department of Pathology at Northwestern University Feinberg School of Medicine. Her research program is directed toward understanding the assembly and regulation of cadherin-based intercellular junctions in normal differentiation as well as human inherited disease and cancer. Fiona Watt Cancer Research UK London Research Institute, 44 Lincoln’s Inn Fields, London WC2A 3PX, UK
Fiona Watt is a principal scientist and head of the Keratinocyte Laboratory at the CR-UK London Research Institute. She is interested in the control of differentiation and tissue assembly in mammalian epidermis and in the deregulation of these processes that occurs in cancer.
Abbreviations APC adenomatous polyposis coli CAM cell adhesion molecule ECM extracellular matrix IF intermediate filament ILK integrin linked kinase TNT tunneling nanotube
Even before the acquisition of multicellularity, cells developed ways of gaining traction for motility and communicating with their environment through the development of adhesion systems. With the acquisition of multicellularity, more complex mechanisms were required for coordinating the sophisticated rearrangements occurring during tissue morphogenesis, for maintaining tissue integrity in the embryo and the adult, and for monitoring and responding to the environment. Since the requirement for regulated cell adhesion in the normal functioning of multicellular organisms is so pervasive, to adequately represent the significant strides made in this area during the past year is a daunting task. In putting together this issue, we set out to highlight lessons learned from the molecular to the organismal level in several broad areas: intercellular and integrin-based adhesion, cell junctions, extracellular matrix (ECM) function and modulation, and the receptorbased and intracellular machinery that controls adhesion effector pathways. By drawing comparisons between organisms primitive and advanced, and between plants and animals, we sought to gain new insight into shared and unique adhesive mechanisms. Several themes emerged, among them, a familiar refrain — that adhesion molecules are not just glue anymore. From an evolutionary perspective, it doesn’t seem surprising that signaling systems co-evolved with molecules involved in adhesion and recognition between cells. It is also an economy of efficiency that molecules serving one function can be modified, redesigned or retread biologically in response to intracellular or environmental cues to take on other roles within the cell. Molecules change post-translational states like we change our clothing, to provide new functional attributes that prepare them for specific environments and direct them to interact with new neighbors and neighborhoods. Furthermore, just as one molecule can serve different functions, similar functions can be performed by different molecules. Cellular receptors, and the intracellular cytoskeleton with which they are coupled, also co-evolved with pathogenic organisms. A number of contributions in this issue discuss how pathogenic organisms developed ingenious ways of targeting adhesion systems and co-opting them to their advantage, informing us in the process about the function and regulation of the targets. We begin with an evolutionary perspective on adhesion. We see how strategies developed by primitive organisms either evolve and remain
www.sciencedirect.com
Current Opinion in Cell Biology 2004, 16:465–469
466 Cell-to-cell contact and extracellular matrix
recognizable in modern species, are fashioned anew using bits and pieces from the old, or simply disappear as novel strategies are developed. In their ‘prehistory of adhesion’, Harwood and Coates tell us that familiar molecules and motifs were present in ancient organisms, including cadherin-like molecules, IgG-like cell adhesion molecules (CAMs) and C-type lectins, as were molecules with Fas-1 and thrombospondin domains. Collagens are found in porifera (sponges) and their receptors, the integrins, are also ancient proteins. In Dictyostelium, which exhibits both ameobic and multicellular states, we see the beginnings of ultrastructurally visible intercellular adhesive structures resembling those of higher organisms. During a stage known as fruiting body formation, junctions resembling the adherens junctions of higher vertebrates form in a band of cells around the top of the stalk; these junctions harbor the b-catenin homologue known as aardvark, which in Dictyostelium is required for attachment of actin and for signal transduction, foreshadowing similar roles in more advanced multicellular organisms.
not suprabasally as in vertebrates. However, each of these powerful, genetically tractable organisms has its own unique strengths for addressing particular developmental questions. In their review, Hardin and Lockwood discuss how C. elegans epithelial cells, positioned between the cuticle and ECM overlying the muscle, carry out their integrative functions through coordinated activities of intermediate filament (IF)- and actin-based organelles. C. elegans offers an opportunity to study cytoskeletal cross talk which is unique among invertebrate organisms, as other invertebrates, including Drosophila, do not have IF. Wu and Beitel reveal the power of Drosophila genetics in the analysis of the septate junction’s role in regulating tracheal epithelial tube size. Septate junctions, often compared to vertebrate tight junctions, appear to control tube size through mechanisms unrelated to their function in the diffusion barrier, possibly through patterning of the ECM and regulation of polarity genes. These studies promise to provide insight into the underlying basis of human pathologies like polycystic kidney disease.
Nowhere is cell surface recognition more fundamental to our existence than in the process of fertilization and sperm–egg interactions. In the review by Shur, Ensslin and Rodeheffer, we see an example of how motifs appearing first in the Dictyostelium lectin ‘discoidin’ appear later in the form of discoidin/F5/8 complement domains. These domains are characteristic of a group of galactose and monosacharide binding proteins, including SED1, a peripherally associated sperm protein required for sperm– egg adhesion in mouse. Its discoidin/F5/8 C domains are important for binding both the sperm and zona pellucida. SED1 may have other functions as it is also found in tissues outside of the male reproductive tract, and, in addition, other SED1 related proteins exist. One of these is Del 1, which mediates endothelial interactions with ECM via av integrins and, as detailed in another review in this issue by Sheppard, can activate a pro-angiogenic program by inducing the transcription factor Hox-D3.
Up until this past year it was thought that plants and animals diverged quite dramatically in their strategies to provide a means of direct communication and passage of small molecules from cell to cell. Whereas gap junctions serve this function in animals, direct cytoplasmic connections called plasmodesmata do so in higher plants. In a review by Cilia and Jackson, we learn the importance of plasmodesmata in protein trafficking, cell fate decisions and the regulation of the movement of pathogenic organisms from cell to cell. Though plasmodesmata have long been appreciated in plants, it was only recently discovered that animals may have something similar, termed TNTs or tunneling nanotubes, which allow selective transfer of vesicles and organelles [1]. How similar are tunneling nanotubes to plasmodesmata? TNTs are transient in nature, which is a major difference, as plasmodesmata are very stable structures. Cilia and Jackson note that plasmodesmata of characean algae are somewhat more similar in structure to TNTs, suggesting intriguing ancestral relationships between this species, which is transitional between algae and higher plants, and more advanced organisms that form TNTs. Plasmodesmata provide a major route for pathogenic organisms such as TMV (tobacco mosaic virus), raising the question of whether TNTs might serve this function in higher organisms.
With increasing multicellularity we see the emergence of multiple types of junctional complexes. In some cases these appear to be similar to the familiar vertebrate junctions, which include tight junctions, adherens junctions, desmosomes, gap junctions, hemidesmosomes and focal contacts. Other times similar macromolecular complexes are disguised in different sorts of junctional forms in invertebrate organisms. For instance, instead of an apical junctional complex, Caenorhabditis elegans has a single electron-dense junctional structure containing a phylogenetically conserved adherens junction domain, a new family of claudin-like proteins typically found in tight junctions in vertebrates, a discs large domain, and an apical domain. On the other hand, Drosophila has a clearly distinguishable adherens junction and septate junction; the latter serves some functions of tight junctions although it is located basally to the adherens junction, Current Opinion in Cell Biology 2004, 16:465–469
New data are shattering our notion that connexins simply facilitate traditional modes of communication in vertebrates through gap junctions. Stout, Goodenough and Paul review provocative evidence supporting the idea that connexins play roles in cell growth, resistance to injury, and other physiological functions through hemichannels and perhaps also independently from a recognizable plaque in non-contacting cells. In addition to other functions, connexin hemichannels play a key role www.sciencedirect.com
Editorial overview Green and Watt 467
in bacterial pathogenesis by Shigella, which causes bacterial dysentery. Like viruses, Shigella can undergo spreading from cell to cell in epithelia, and it appears that cells lacking connexins are unable to support the spread of Shigella, but this capability is restored by transfecting in certain connexins, possibly due to hemichannel-dependent release of ATP and consequent activation of purinergic receptors. So connexins aren’t just gap junction proteins; conversely, gap junctions aren’t just made from connexins. Invertebrate connexins, or innexins, represent an unusual, perhaps unique, case of channel proteins not being conserved across phyla. Innexins and connexins are unrelated, but they form gap junctions with remarkably similar properties. It turns out that innexins are not unique to invertebrates; related proteins that also assemble functional channels have been discovered in vertebrates and are collectively referred to as pannexins. So it seems that gap junctions evolved twice, and that the innexins/pannexins represent the more ancient channel building blocks. Several contributions in this issue highlight the diversity of functions that intercellular cadherin-containing adhesive junctions play in vertebrates and underscore the concept that the same molecule can perform multiple functions. These reviews also reveal that adhesion molecules are targets for pathogenic organisms that have developed multiple strategies for breaching epithelial barriers and ensuring their spread. We learn in the review from Sakisaka and Takai that such is the case for nectins, which are Ig-like CAMs, and a newer class of nectin-like molecules that cooperate with E-cadherin in the formation of adherens junctions and tight junctions, in part through their ability to activate the small GTPases rac and cdc42. In addition to their role in the earliest stages of adherens junction assembly, these molecules are also receptors for members of the Herpes virus family. Interaction of nectin-1 or-2 with one of the HSV (herpes simplex virus) envelope glycoproteins sets into motion clustering of other viral glycoproteins and facilitates fusion with the viral envelope and viral entry. Interaction of nectin-1 with the actin binding protein afadin increases the efficiency of intercellular spreading, but whether spread occurs through direct fusion of adjacent plasma membranes, possibly through mechanisms similar to tunneling nanotubes, is not well understood. A newly recognized core function of the adherens junction protein called p120 catenin is the focus of the review by Kowalczyk and Reynolds. p120 catenin has been implicated in both positive and negative regulation of cadherin-based adhesion, and, like nectin, also regulates the activity of small GTPases. p120 binds to the juxtamembrane domain of the cadherin tails but does not directly associate with the actin cytoskeleton — this is a job performed by a- and b-catenin. Recent work has revealed that loss of p120 from cells is correlated with www.sciencedirect.com
depletion of E-cadherin, and that re-introducing p120 into cells restores E-cadherin protein expression and intercellular adhesion. It turns out that this may be due to the ability of p120 to act as a plasma membrane retention signal, preventing cadherins from being endocytosed, perhaps by competing with other factors that target cadherins for internalization. Based on this critical function in vertebrates, it is puzzling that p120 is not required for cadherin stability in non-vertebrate organisms. It seems likely that, along with the appearance of multiple family members with partially redundant functions, this ability of p120 has evolved as a mechanism for fine-tuning cadherin-based adhesion in vertebrates during remodeling of tissues in development and in the adult. Degradation is also something that comes to mind when thinking about regulation of b-catenin, the prototypical dual-functioning armadillo protein, which plays roles in both the cadherin/catenin complex and in Wnt/wingless signaling. The regulator of b-catenin degradation is a protein complex in which APC (adenomatous polyposis coli) is a critical component that directs b-catenin to the proteosome in the absence of Wnt signaling. Like p120, APC has multiple functions. Recent evidence highlighted in the review by Bienz and Hamada suggests that APC serves important functions in adhesion through interacting with the actin-rich cell cortex. Its loss in tumor cells may not just affect b-catenin signaling but perhaps also compromise intercellular adhesion. A possible role for APC in adhesion is an idea that historically is not new, but our attention was diverted a number of years ago to its role in regulating b-catenin degradation, and we are only now coming back to its possible roles in adhesion. The mechanism by which APC regulates adhesion is not known, but authors speculate it could be by inhibiting a GEF called Asef, which is activated by truncated but not wild type APC. Like p120, re-expression of fulllength APC in cells expressing a truncation mutant restores E-cadherin-mediated adhesion. So, APC represents another example where adhesion and signaling may have co-evolved. It seems that uni-functional proteins may be the exception rather than the rule, particularly when it comes to the armadillo family. Another class of anchoring junction molecules that have been co-opted as points of entry into an epithelium by human pathogens is the desmosomal cadherin family. Desmosomal cadherins are related to classic cadherins, but are found in junctions called desmosomes that anchor intermediate filaments to the plasma membrane. Payne, Hanakawa, Amagai and Stanley relate a fascinating story in which the inactivation of the desmosomal cadherin, desmoglein 1, by two completely different pathways leads to epidermal blisters of strikingly similar histology. In one case, antibodies circulating in patients with an autoimmune disease called pemphigus foliaceus bind to Current Opinion in Cell Biology 2004, 16:465–469
468 Cell-to-cell contact and extracellular matrix
the extracellular domains of this cadherin, resulting in separation of the desmosomes and acantholysis — that is, separation between the keratinocyte plasma membranes. Intriguingly, the pathogen Staphylococcus aureus invades and spreads within the epidermis by producing a toxin, exfoliative toxin, which proteolyses desmoglein 1 with exquisite specificity, leading to effective spreading of the infection through blisters produced in the upper layers of the skin where this differentiation-specific cadherin is expressed. So pathogens have developed as many ways to breach intercellular adhesions as there are molecules, in some cases using host molecules as receptors for entry and spread (e.g. in the case of nectins) and in others targeting host molecules for demolition to allow entry and spread (e.g. desmoglein 1). Similar themes emerge as we move to cell–matrix interactions dependent on integrins. Integrins are important for numerous physiologic and pathologic processes including leukocyte trafficking, platelet aggregation, wound healing, invasion and metastasis, development and tissue morphogenesis. We are beginning to understand at an ever increasing level of detail how integrin function can be finely tuned by altering its conformational state. Mould and Humphries review data suggesting that instead of three previously proposed distinct conformational states — inactive, primed and ligandoccupied — it is more likely that there is a continuous spectrum of states regulated by protein interactions, cytokine stimulation and divalent cation occupancy. These states are facilitated by the presence of flexible ‘joints’ and ‘connections’ within the integrin heterodimer, and all of these states can affect ligand-binding affinity. The potential plethora of functional conformations is mind-boggling especially when one considers that data are available on only a limited number of integrins out of the total possible number of integrinligand pairs. But this sort of information will be invaluable for future efforts in drug design, enabling us to tailormake agents that bind to specific conformations to modify integrin function and cell adhesion. The need for drug design is underscored by the number of pathologies in which integrin-ligand interactions are involved. As highlighted in the review by Sheppard, the av integrins are of critical importance in pathways related to human disease. In addition to the role of av integrins in regulating vascular growth and permeability, avb6 and avb8 participate in the activation of TGFb, albeit by completely different mechanisms. avb8 presents latent complexes to one or more cell surface MMPs, which results in the release of TGFb into the milieu. Although activation of TGFb by avb6 does not require an MMP, its loss and the consequent loss of TGFb activation leads to MMP12 induction in macrophages, contributing to development of emphysema. The cooperation of MMPs in modulating integrin–matrix interactions is also highCurrent Opinion in Cell Biology 2004, 16:465–469
lighted in the contribution by Mott and Werb. These enzymes are widely functioning in many physiological processes and pathologies including arthritis and cancer, and recent evidence suggests that cleavage of ECM can expose cryptic information contained within a molecule, thus providing another mechanism to produce molecules with multiple functions that influence processes such as angiogenesis and cell migration. As also discussed in the Sheppard review, MMPs facilitate the release of biologically active factors such as TGFb from the ECM. Interestingly, mice lacking fibrillin-1, an LTBP which is a component of the complex which tethers latent TGFb to the ECM, are unable to sequester TGFb in the matrix. These mice have early postnatal lung abnormalities and impairment of distal alveolar septation and, like mice in which TGFb activation is impaired by loss of avb6, develop emphysema as they age. Integrin–ECM interactions can be regulated from inside the cell, for example by proteins such as ILK (integrin linked kinase). ILK acts as a platform for the recruitment of proteins which participate in the linkage of integrins to the actin cytoskeleton in focal adhesions. Thus, ILK acts as a conduit for transmitting mechanical and chemical signals into the cell from integrin–matrix interactions. In the review by Fa¨ ssler and colleagues we learn that the physiological role of ILK’s kinase activity is controversial. Severe defects in worms and flies lacking ILK can be rescued by kinase-dead forms of ILK, suggesting that at least some important functions do not require kinase activity. Nevertheless, other cell behaviors do require kinase activity, and the fact that tumors have increased expression of ILK and ILK activity underscores the importance of this question. In the review by Yurchenco and Wadsworth, we learn about two families of matrix molecules that play roles in the basement membranes of the early developing embryo: the laminins and netrins. These matrix molecules share N-terminal domain structure but are otherwise only distantly related. However, each family has highly conserved functions across species throughout evolution. Laminins, are heterotrimeric proteins that are required for viability in both Drosophila and C. elegans and are essential for basement membrane assembly. Netrins on the other hand serve as guidance cues (both attractive and repellant) that regulate the cell adhesion and migration of pioneer axons, other neurons and mesodermal cells, and are important for development and epithelial morphogenesis as well. Cues which regulate neuronal behavior are also the topic of a review by Klein, who provides us with an update on the roles of eph/ephrin signaling in neural development and plasticity. Receptor (Eph)–ligand pairs comprise the largest family of receptor tyrosine kinases. They control behaviors in a bi-directional fashion to mediate attraction www.sciencedirect.com
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and repulsion or adhesion and de-adhesion and in so doing are critical regulators of neuronal plasticity during morphogenesis and in the adult. Rather than altering nuclear events, Eph/ephrins appear to control behavior primarily through modification of the cytoskeleton by regulating signaling pathways such as small GTPases. Novel modes of regulation by internalization of receptor/ligand pairs dependent on actin polymerization and rac signaling have emerged over the past year. In addition, it has been revealed that different Ephs bind different Rho-GEFS, providing one possible mechanism for regulating the specificity of downstream effector pathways. The regulation of cell protrusive activity and motility by the cytoplasmic actin binding protein fascin is the subject of Adams’ contribution. This small actin-bundling and cross-linking protein is well-conserved from platyhelminths and echinoderms. It provides a functional link between matrix molecules such as thrombospondin and fibronectin and the actin cytoskeletal machinery that regulates cell behavior. Fascin is also another example of a protein that has been co-opted by bacterial pathogens. It has been implicated in Listeria monocytogenes actin tail formation. In addition, Rickettsis conorii activates the
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host Arp2/3 complex and promotes the formation of actin tails containing fascin, which is thought be responsible for the observed bundling of actin filaments. It is clear that as each year passes we gain a better understanding of the various sticky situations in which we find ourselves and a greater appreciation for the varied strategies developed throughout evolution for providing contact and communication between neighboring cells and organisms and their environments. Experiments of nature continue to provide insight into how adhesion molecules work, and inform us about the vulnerabilities of these systems to attack. Along with the increasing resolution and emerging molecular detail available regarding the molecular adhesion machinery, this knowledge will help direct our efforts towards developing designer drugs with which to combat pathogens and pathologies that co-opt and target adhesion molecules for their own purposes.
References 1.
Rustom A, Saffrich R, Markovic I, Walther P, Gerdes HH: Nanotubular highways for intercellular organelle transport. Science 2004, 303:1007-1010.
Current Opinion in Cell Biology 2004, 16:465–469