- Email: [email protected]
Editorial overview: Cell-to-cell contact and extracellular matrix
525
Cell-to-cell contact and extracellular matrix Editorial overview Keith Burridge* and Shoichiro Tsukita† Addresses *Department of Cell and Developmental Biology, and Lineberger Comprehensive Cancer Center, CB #7295, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599, USA e-mail: [email protected] † Department of Cell Biology, Kyoto University Facility of Medicine, Sakyo-Ku, Kyoto 606-8501, Japan e-mail: [email protected] Current Opinion in Cell Biology 2001, 13:525–528 0955-0674/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations ECM extracellular matrix MMPs matrix metalloproteinases WAK wall-associated kinases
The field of cell adhesion has enjoyed explosive growth over the past decade. Not long ago, cell adhesion molecules were considered largely in a structural context. On the outside of cells, they were known to provide anchorage to the extracellular matrix (ECM) or to other cells, and on the inside to provide sites of attachment for the cytoskeleton. The idea that adhesion molecules might also have a signaling function was almost heretical. This idea probably gained credence first with integrins a decade ago. Now, however, the pendulum has swung to the other extreme, to the point where the expectation is that every adhesion molecule is involved in signaling. Accordingly, much of the energy of the field has moved to exploring details of signaling pathways downstream from specific adhesion molecules. The field continues to be exciting and we are now able to ask refined questions about how cells explore their environment and respond to their neighbors. Adhesion research is also generating new clinical therapies that would not have been anticipated a few years back. For this issue of Current Opinion in Cell Biology, we were faced with the dilemma of what topics to include when so many aspects of this field are progressing well. Indeed, our initial list of topics was too long. In the end, we have picked several areas for review because these seemed to us to be particularly dynamic or to be in a state of flux with new insights being uncovered.
Plants We start with plants, which arguably possess the most developed ECMs. So many of us who work with animal cells or systems tend to dismiss the plant cell wall as an ECM specialized just for mechanical strength. Recent work, however, is establishing that there are some unexpected similarities between plants and animals with respect to how they interact with their ECMs. For example, one of the hottest areas in cell adhesion research for the past decade has been the characterization of the signaling pathways that involve protein kinases
downstream from integrins that are activated upon adhesion to the ECM. Now, a family of transmembrane kinases has been discovered that link to plant cell walls. Kohorn (pp 529–533) discusses this exciting work, identifying plant cell wall-associated kinases (WAKs). These serine–threonine kinases are transmembrane proteins that link very tightly to the cell wall on the extracellular side. Little is currently known about the signaling pathways downstream from the WAKs, and Kohorn speculates that they may be developmentally important and involved in signaling during events such as cell expansion.
Proteinases Remodeling of metazoan ECMs often occurs by proteolysis. McCawley and Matrisian (pp 534–540) discuss the ECM proteases — matrix metalloproteinases (MMPs). However, a theme of their review is that these proteases are doing so much more than just crudely degrading the ECM. They demonstrate that MMPs have both positive and negative effects on the behavior of cells. Many ECM components have cryptic biological activities that are only exposed following selective proteolysis. Additionally, growth factors and other signaling molecules often bind tenaciously to proteoglycans and other constituents of the ECM and require proteolysis for their release and activity. The action of MMPs is not confined just to the matrix, these proteases can also affect cell surface components. Traditionally, MMPs have been considered in the framework of tumor invasion, where undoubtedly they are important, but they also play critical roles in apoptosis, cell migration, angiogenesis and cell–cell communication — topics discussed elsewhere in this issue. This is an exciting area, and McCawley and Matrisian’s review points to multiple directions in which this field is rapidly moving.
Integrins Integrins are major receptors for adhesion to the metazoan ECM. In general, these heterodimers have short cytoplasmic domains; however, the β4 integrin subunit has an exceptionally long cytoplasmic sequence. Paired with α6, this integrin has usually been considered in the situation of epithelial cell adhesion to laminins within basement membranes. In this circumstance, it is the key adhesion molecule spanning the plasma membrane within hemidesmosomes, and it links keratin intermediate filaments on the cytoplasmic side. Mercurio and coworkers (pp 541–545) review new work indicating that this integrin has signaling functions and that it may play a role in cell migration and tumor invasion. The α6β4 integrin has often been viewed as an oddity among the family of integrins because of its association with intermediate filaments rather than actin. Mercurio and colleagues discuss the evidence that in migrating epithelia this
526
Cell-to-cell contact and extracellular matrix
integrin can be found associated with actin filaments in motile structures such as filopodia and lamellipodia, and it can be responsible for the transmission of force to the matrix during migration. The β4 integrin appears to be more versatile and multifunctional than previously considered. Shortly after integrins were discovered and first characterized, there was considerable interest in whether their short cytoplasmic domains become modified, for example by phosphorylation, during signaling events. Although some evidence for phosphorylation was obtained under certain conditions, this was not extensively pursued. The topic has been revived lately, largely from the work of Phillips and colleagues (pp 546–554) who have demonstrated that the β3 cytoplasmic domain becomes tyrosine phosphorylated during platelet aggregation. The αIIb/β3 integrin is the major integrin on platelets and is responsible for platelet aggregation owing to its binding to fibrin. In their review, Phillips and his coworkers discuss how this tyrosine phosphorylation generates binding sites both for structural components, such as myosin II, and for signaling components, such as Shc. Because the β3 integrin subunit is widely expressed, this tyrosine phosphorylation may be important in many situations besides platelet aggregation. Adhesion affects many aspects of cell behavior. Particularly important is the influence of adhesion on cell survival. Strong selective pressures have ensured that cells, particularly epithelial cells, displaced from their normal environments not only fail to grow but actively enter a form of apoptosis, that has been named anoikis. Frisch and Screaton (pp 555–562) review developments in this area, with emphasis on the signaling pathways that are initiated downstream from integrin engagement that normally prevent the onset of anoikis. When cells detach from a surface to which they are adhering, the loss of adhesion is accompanied by major changes in cytoskeletal organization. The authors describe several interesting connections between cytoskeletal disruption and the onset of anoikis. One example where adhesion and growth factor signaling is highly coordinated is in angiogenesis. Eliceiri and Cheresh (pp 563–568) review the interactions between αv integrins (αvβ3 and αvβ5) and angiogenic growth factors, such as bFGF and VEGF. Several naturally occurring antiangiogenic agents are proteolytic fragments of ECM proteins, and they act by binding to αvβ3 or αvβ5 integrins. This is an area of significant clinical interest, and the authors review the therapeutic potential that these and other anti-angiogenic agents may have. Another area of clinical and pharmaceutical interest is the transmigration of leukocytes across the endothelial lining of blood vessels during inflammation. This involves the coordination of many adhesion molecules both on the leukocytes that are transmigrating as well as on the endothelial cells. Worthylake and Burridge (pp 569–577) discuss these events from the perspective of the leukocyte. An interesting aspect
of leukocyte transmigration is the crosstalk between adhesion molecules on leukocytes and the manner in which there is both negative and positive feedback. The adhesion molecules that become engaged first trigger the activation of subsequent adhesion molecules, but the engagement of these feeds back to inhibit the activity of those involved earlier in the sequence. The authors discuss the cytoskeletal rearrangements that take place during transmigration and the regulation of this process by Rho family GTPases. Space prohibited discussion of the role of endothelial cells in leukocyte transmigration, although this is equally interesting and involves cell adhesion molecules both within and outside the adherens junctions of the endothelium.
Focal adhesions Several reviews deal with different aspects of focal adhesions/focal contacts. In their review, Woods and Couchman (pp 578–583) discuss the role of syndecan-4 in focal adhesion formation. Whereas integrins have long been known to be the primary adhesion molecules within focal adhesions responsible for mediating attachment to the ECM, there is increasing evidence supporting a significant role for the transmembrane heparan sulfate proteoglycan, syndecan-4, acting as a co-receptor with integrins within focal adhesions. Ligation of syndecan-4 promotes assembly of focal adhesions, and the authors review the evidence that syndecan-4 engagement may trigger activation of RhoA. Interestingly, the cytoplasmic domain of syndecan-4 binds to and activates PKCα and may affect signaling pathways through several recently identified binding partners. Mice deficient in syndecan-4 have defects in fetal blood vessels and delayed wound repair. Somewhat surprisingly, fibroblasts from these mice can assemble focal adhesions, raising the possibility that other components may compensate for the lack of this transmembrane proteoglycan. The assembly of focal contacts (another name for focal adhesions) is also addressed by Geiger and Bershadsky (pp 584–592). In particular, they consider the steps that occur when a focal complex, a small integrin cluster under the regulation of Rac, matures into a focal contact (focal adhesion). Normally this is driven by RhoA activity. Downstream from RhoA, two key effectors have been identified: Rho-kinase (ROCK) and mDia. The latter appears to regulate actin polymerization and affects the diameter of stress fibers that attach to focal contacts, whereas the former regulates myosin interaction with actin and consequently governs contractility. Recent work from the authors’ own laboratories has demonstrated that the role of ROCK can be substituted by external mechanical force. This leads them to discuss the idea that focal contacts act as mechanosensors, detecting tension applied to cells. Geiger and Bershadsky discuss various models for how force may be sensed and how this may be translated into further assembly. The complexity of focal adhesions is being increasingly revealed. Not only are new components continuing to be discovered but unexpected associations and properties are
Editorial overview Burridge and Tsukita
also being revealed. Turner and colleagues (pp 593–599) discuss the interaction of paxillin, an adaptor protein within focal adhesions, with members of the ARF GAP family. It seems unlikely that anyone would have predicted this interaction prior to its discovery, and the full consequences of these interactions have yet to be fully appreciated. ARF GAPs regulate the activity of ARFs, low molecular weight GTPases that, in turn, regulate both vesicle trafficking and the actin cytoskeleton. ARF GAPs appear to function at the interface between the Rho and ARF families of regulator proteins. In addition to the association of ARF GAPs with paxillin, this interesting protein also binds to an exchange factor for Rac, as well as to one of its effectors.
Adherens junctions Not only are focal adhesions becoming crowded with newly identified components, but so are adherens junctions. In his review, Nagafuchi (pp 600–603) describes the properties of several of the new adherens junction proteins. One has the sense that adherens junctions are only just behind focal adhesions in terms of the number of signaling and structural components concentrated within these sites. Recent work has also revealed that the asssembly of adherens junctions is regulated by Rho family proteins. In their review, Anastasiadis and Reynolds (pp 604–610) focus their attention on a single adherens junction component, p120catenin, that regulates cadherin function and also affects the activity of several Rho family members. Overexpression of p120catenin has a marked effect on the morphology of cells and on their cytoskeletal organization. The authors review the studies that demonstrate p120catenin’s effect on the activities of RhoA, Rac1 and Cdc42. Not only does this junctional protein regulate the Rho family, it also traffics to the nucleus where it interacts with a newly discovered transcription factor, Kaiso.
Neural cell adhesion molecules The largest family of adhesion molecules is the Ig superfamily. The Ig domain is well suited for binding interactions and has been used repeatedly in many different cell adhesion molecules and has ultimately given rise to that most versatile of recognition systems, immunoglobulins themselves. Brümmendorf and Lemmon (pp 611–618) approach the Ig superfamily from an interesting evolutionary viewpoint. They point out that not only is this one of the largest gene families in the human genome, but the complexity of several family members is hugely increased by extensive splicing. Additionally, the Ig domain appears well suited for pairing in cis with other Ig-domain-containing proteins, with some adhesion molecules being able to form both homo- and hetero-associations. Brümmendorf and Lemmon go on to discuss the role of several Ig superfamily members in axon guidance as well as the links to the cytoskeleton that recent work is revealing. They also discuss the association of several Ig-containing CAMs (cell adhesion molecules) with PDZ adaptor proteins, such as afadin, and the role that these may play in adherens junctions. At least two of the PDZ domain proteins found in
527
adherens and tight junctions have been found to interact with transcription factors. One of the exciting themes that has emerged over the past several years is that proteins that were originally thought only to function within cell adhesions are turning out also to bind transcription factors and to act within the nucleus. What started with β-catenin is becoming not the exception but the rule. Another example is p120catenin described in the review by Anastasiadis and Reynolds (see above). Also on the subject of axon guidance, Liu and Strittmatter (pp 619–626) focus on the Semaphorins and their receptors, Neuropilins and Plexins. Recent work has revealed that these receptors affect the growth cone actin cytoskeleton by acting on the Rho family of GTPases. Although most attention is directed towards RhoA, Rac1 and Cdc42, Liu and Strittmatter point out that other members of the Rho family are expressed in neurons, and recent evidence suggests that these too may contribute to the motile properties of growth cones. In discussing the regulation of the Rho family, the authors indicate that several guanine nucleotide exchange factors are specific for neural tissue and couple directly or indirectly to cell adhesion molecules that affect growth cone behavior. Given the complexity of neural regulation, we suspect that this field is about to explode as signaling pathways downstream from many axon guidance receptors converge on the Rho family, thereby regulating cytoskeletal dynamics and growth cone collapse or extension. An interesting example of signaling derived from cell–cell interactions occurs with Notch and Delta, where the receptor and ligand occur on interacting cells. Critical to the ability of Notch to signal is a series of proteolytic cleavages, with the final cleavage resulting in a cytoplasmic domain that can travel to the nucleus to effect transcriptional events. Fortini (pp 627–634) reviews the proteolytic processing of Notch and, in particular, the recent work on presenilin. Increasing evidence supports the idea that presenilin is the catalytic subunit of a multiprotein complex that executes intramembranous cleavage of target membrane proteins. One of these is Notch. However, much of what has been learned about presenilin derives from its role in Alzheimer’s disease and the generation of amyloid precursor protein. In spite of its importance in human disease, relatively little is known about presenilin and its normal mode of action. Fortini raises a number of important questions about this protein. Given that it is evolutionarily old and that Notch signaling functions in worms and flies, many of the details of its action and regulation are likely to be elucidated from studies in these organisms.
Polarity Many cells reveal polarized organization, and this is particularly striking in epithelia, where apical and basal surfaces are distinct. Ohno (pp 641–648) reviews the considerable progress that has been made in this field in the past few years. Studies of cell polarity in C. elegans revealed a set of
528
Cell-to-cell contact and extracellular matrix
‘partition’ genes (PAR-1-6) responsible for asymmetric cell divisions in the early embryo. Two of the proteins, PAR-3 and PAR-6, form a complex with atypical protein kinase C. The proteins in this complex are highly conserved, and an exciting development has been the identification of their homologues in flies and mammals and the discovery that in these organisms they function in determining the apicalbasal polarity of epithelial cells. Both PAR-3 and PAR-6 contain PDZ domains. The mammalian homologue of PAR-3 interacts via one of its PDZ domains with the junctional adhesion molecule, JAM, which is found in tight junctions. PAR-6 has a binding site for Cdc42 and Rac1, two members of Rho family that stimulate junction formation and that regulate cell polarity. Binding of these GTPases to PAR-6 leads to activation of atypical PKC. In addition to apical–basal polarity, many epithelia also demonstrate planar polarity. Adler and Lee (pp 635–640) review recent progress in understanding planar polarity in the epithelium of the insect wing, which is easily visualized by the pattern of hair organization. Genetic analysis has identified the frizzled pathway as critical for the organization of hairs pointing distally on the Drosophila wing. Recent studies have indicated that Fz, as well as several interacting proteins, accumulate along the distal border of epithelial cells, thereby marking the polarity of individual cells and resulting in initiating hair formation at the distal
vertex. Interestingly for the topic of cell–cell adhesion, one of the proteins that accumulates at the distal cell border is the product of the flamingo/starry night gene, a seven-transmembrane-spanning protein that has several extracellular cadherin domains. It has been suggested that adhesive interactions between this and other cell adhesion molecules on adjacent cells may stabilize the polarity pattern. As with so many cell adhesion molecules, downstream from frizzled, the Rho family of GTPases are implicated in this pattern formation. In a period and in a field in which information is increasing exponentially, reviews serve many obvious functions. We hope that this volume of reviews on various aspects of cell adhesion will be valuable both to those who work in the field as well to those outside this discipline. However, we also hope for more. The juxtaposition of a series of cutting edge reviews offers the chance for readers to make connections that have not previously been made and that have not been recognized by the reviewers themselves. We suspect that within this volume there may be cross-connections that have yet to be recognized. We are excited by the possibility that perhaps some of these will provide sparks that will inspire new ideas or stimulate new directions for research. Finally, as editors of this section, the two of us would like to thank all those who have contributed their reviews to make what we think is an exciting issue.
Cell-to-cell contact and extracellular matrix Editorial overview Keith Burridge* and Shoichiro Tsukita† Addresses *Department of Cell and Developmental Biology, and Lineberger Comprehensive Cancer Center, CB #7295, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599, USA e-mail: [email protected] † Department of Cell Biology, Kyoto University Facility of Medicine, Sakyo-Ku, Kyoto 606-8501, Japan e-mail: [email protected] Current Opinion in Cell Biology 2001, 13:525–528 0955-0674/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations ECM extracellular matrix MMPs matrix metalloproteinases WAK wall-associated kinases
The field of cell adhesion has enjoyed explosive growth over the past decade. Not long ago, cell adhesion molecules were considered largely in a structural context. On the outside of cells, they were known to provide anchorage to the extracellular matrix (ECM) or to other cells, and on the inside to provide sites of attachment for the cytoskeleton. The idea that adhesion molecules might also have a signaling function was almost heretical. This idea probably gained credence first with integrins a decade ago. Now, however, the pendulum has swung to the other extreme, to the point where the expectation is that every adhesion molecule is involved in signaling. Accordingly, much of the energy of the field has moved to exploring details of signaling pathways downstream from specific adhesion molecules. The field continues to be exciting and we are now able to ask refined questions about how cells explore their environment and respond to their neighbors. Adhesion research is also generating new clinical therapies that would not have been anticipated a few years back. For this issue of Current Opinion in Cell Biology, we were faced with the dilemma of what topics to include when so many aspects of this field are progressing well. Indeed, our initial list of topics was too long. In the end, we have picked several areas for review because these seemed to us to be particularly dynamic or to be in a state of flux with new insights being uncovered.
Plants We start with plants, which arguably possess the most developed ECMs. So many of us who work with animal cells or systems tend to dismiss the plant cell wall as an ECM specialized just for mechanical strength. Recent work, however, is establishing that there are some unexpected similarities between plants and animals with respect to how they interact with their ECMs. For example, one of the hottest areas in cell adhesion research for the past decade has been the characterization of the signaling pathways that involve protein kinases
downstream from integrins that are activated upon adhesion to the ECM. Now, a family of transmembrane kinases has been discovered that link to plant cell walls. Kohorn (pp 529–533) discusses this exciting work, identifying plant cell wall-associated kinases (WAKs). These serine–threonine kinases are transmembrane proteins that link very tightly to the cell wall on the extracellular side. Little is currently known about the signaling pathways downstream from the WAKs, and Kohorn speculates that they may be developmentally important and involved in signaling during events such as cell expansion.
Proteinases Remodeling of metazoan ECMs often occurs by proteolysis. McCawley and Matrisian (pp 534–540) discuss the ECM proteases — matrix metalloproteinases (MMPs). However, a theme of their review is that these proteases are doing so much more than just crudely degrading the ECM. They demonstrate that MMPs have both positive and negative effects on the behavior of cells. Many ECM components have cryptic biological activities that are only exposed following selective proteolysis. Additionally, growth factors and other signaling molecules often bind tenaciously to proteoglycans and other constituents of the ECM and require proteolysis for their release and activity. The action of MMPs is not confined just to the matrix, these proteases can also affect cell surface components. Traditionally, MMPs have been considered in the framework of tumor invasion, where undoubtedly they are important, but they also play critical roles in apoptosis, cell migration, angiogenesis and cell–cell communication — topics discussed elsewhere in this issue. This is an exciting area, and McCawley and Matrisian’s review points to multiple directions in which this field is rapidly moving.
Integrins Integrins are major receptors for adhesion to the metazoan ECM. In general, these heterodimers have short cytoplasmic domains; however, the β4 integrin subunit has an exceptionally long cytoplasmic sequence. Paired with α6, this integrin has usually been considered in the situation of epithelial cell adhesion to laminins within basement membranes. In this circumstance, it is the key adhesion molecule spanning the plasma membrane within hemidesmosomes, and it links keratin intermediate filaments on the cytoplasmic side. Mercurio and coworkers (pp 541–545) review new work indicating that this integrin has signaling functions and that it may play a role in cell migration and tumor invasion. The α6β4 integrin has often been viewed as an oddity among the family of integrins because of its association with intermediate filaments rather than actin. Mercurio and colleagues discuss the evidence that in migrating epithelia this
526
Cell-to-cell contact and extracellular matrix
integrin can be found associated with actin filaments in motile structures such as filopodia and lamellipodia, and it can be responsible for the transmission of force to the matrix during migration. The β4 integrin appears to be more versatile and multifunctional than previously considered. Shortly after integrins were discovered and first characterized, there was considerable interest in whether their short cytoplasmic domains become modified, for example by phosphorylation, during signaling events. Although some evidence for phosphorylation was obtained under certain conditions, this was not extensively pursued. The topic has been revived lately, largely from the work of Phillips and colleagues (pp 546–554) who have demonstrated that the β3 cytoplasmic domain becomes tyrosine phosphorylated during platelet aggregation. The αIIb/β3 integrin is the major integrin on platelets and is responsible for platelet aggregation owing to its binding to fibrin. In their review, Phillips and his coworkers discuss how this tyrosine phosphorylation generates binding sites both for structural components, such as myosin II, and for signaling components, such as Shc. Because the β3 integrin subunit is widely expressed, this tyrosine phosphorylation may be important in many situations besides platelet aggregation. Adhesion affects many aspects of cell behavior. Particularly important is the influence of adhesion on cell survival. Strong selective pressures have ensured that cells, particularly epithelial cells, displaced from their normal environments not only fail to grow but actively enter a form of apoptosis, that has been named anoikis. Frisch and Screaton (pp 555–562) review developments in this area, with emphasis on the signaling pathways that are initiated downstream from integrin engagement that normally prevent the onset of anoikis. When cells detach from a surface to which they are adhering, the loss of adhesion is accompanied by major changes in cytoskeletal organization. The authors describe several interesting connections between cytoskeletal disruption and the onset of anoikis. One example where adhesion and growth factor signaling is highly coordinated is in angiogenesis. Eliceiri and Cheresh (pp 563–568) review the interactions between αv integrins (αvβ3 and αvβ5) and angiogenic growth factors, such as bFGF and VEGF. Several naturally occurring antiangiogenic agents are proteolytic fragments of ECM proteins, and they act by binding to αvβ3 or αvβ5 integrins. This is an area of significant clinical interest, and the authors review the therapeutic potential that these and other anti-angiogenic agents may have. Another area of clinical and pharmaceutical interest is the transmigration of leukocytes across the endothelial lining of blood vessels during inflammation. This involves the coordination of many adhesion molecules both on the leukocytes that are transmigrating as well as on the endothelial cells. Worthylake and Burridge (pp 569–577) discuss these events from the perspective of the leukocyte. An interesting aspect
of leukocyte transmigration is the crosstalk between adhesion molecules on leukocytes and the manner in which there is both negative and positive feedback. The adhesion molecules that become engaged first trigger the activation of subsequent adhesion molecules, but the engagement of these feeds back to inhibit the activity of those involved earlier in the sequence. The authors discuss the cytoskeletal rearrangements that take place during transmigration and the regulation of this process by Rho family GTPases. Space prohibited discussion of the role of endothelial cells in leukocyte transmigration, although this is equally interesting and involves cell adhesion molecules both within and outside the adherens junctions of the endothelium.
Focal adhesions Several reviews deal with different aspects of focal adhesions/focal contacts. In their review, Woods and Couchman (pp 578–583) discuss the role of syndecan-4 in focal adhesion formation. Whereas integrins have long been known to be the primary adhesion molecules within focal adhesions responsible for mediating attachment to the ECM, there is increasing evidence supporting a significant role for the transmembrane heparan sulfate proteoglycan, syndecan-4, acting as a co-receptor with integrins within focal adhesions. Ligation of syndecan-4 promotes assembly of focal adhesions, and the authors review the evidence that syndecan-4 engagement may trigger activation of RhoA. Interestingly, the cytoplasmic domain of syndecan-4 binds to and activates PKCα and may affect signaling pathways through several recently identified binding partners. Mice deficient in syndecan-4 have defects in fetal blood vessels and delayed wound repair. Somewhat surprisingly, fibroblasts from these mice can assemble focal adhesions, raising the possibility that other components may compensate for the lack of this transmembrane proteoglycan. The assembly of focal contacts (another name for focal adhesions) is also addressed by Geiger and Bershadsky (pp 584–592). In particular, they consider the steps that occur when a focal complex, a small integrin cluster under the regulation of Rac, matures into a focal contact (focal adhesion). Normally this is driven by RhoA activity. Downstream from RhoA, two key effectors have been identified: Rho-kinase (ROCK) and mDia. The latter appears to regulate actin polymerization and affects the diameter of stress fibers that attach to focal contacts, whereas the former regulates myosin interaction with actin and consequently governs contractility. Recent work from the authors’ own laboratories has demonstrated that the role of ROCK can be substituted by external mechanical force. This leads them to discuss the idea that focal contacts act as mechanosensors, detecting tension applied to cells. Geiger and Bershadsky discuss various models for how force may be sensed and how this may be translated into further assembly. The complexity of focal adhesions is being increasingly revealed. Not only are new components continuing to be discovered but unexpected associations and properties are
Editorial overview Burridge and Tsukita
also being revealed. Turner and colleagues (pp 593–599) discuss the interaction of paxillin, an adaptor protein within focal adhesions, with members of the ARF GAP family. It seems unlikely that anyone would have predicted this interaction prior to its discovery, and the full consequences of these interactions have yet to be fully appreciated. ARF GAPs regulate the activity of ARFs, low molecular weight GTPases that, in turn, regulate both vesicle trafficking and the actin cytoskeleton. ARF GAPs appear to function at the interface between the Rho and ARF families of regulator proteins. In addition to the association of ARF GAPs with paxillin, this interesting protein also binds to an exchange factor for Rac, as well as to one of its effectors.
Adherens junctions Not only are focal adhesions becoming crowded with newly identified components, but so are adherens junctions. In his review, Nagafuchi (pp 600–603) describes the properties of several of the new adherens junction proteins. One has the sense that adherens junctions are only just behind focal adhesions in terms of the number of signaling and structural components concentrated within these sites. Recent work has also revealed that the asssembly of adherens junctions is regulated by Rho family proteins. In their review, Anastasiadis and Reynolds (pp 604–610) focus their attention on a single adherens junction component, p120catenin, that regulates cadherin function and also affects the activity of several Rho family members. Overexpression of p120catenin has a marked effect on the morphology of cells and on their cytoskeletal organization. The authors review the studies that demonstrate p120catenin’s effect on the activities of RhoA, Rac1 and Cdc42. Not only does this junctional protein regulate the Rho family, it also traffics to the nucleus where it interacts with a newly discovered transcription factor, Kaiso.
Neural cell adhesion molecules The largest family of adhesion molecules is the Ig superfamily. The Ig domain is well suited for binding interactions and has been used repeatedly in many different cell adhesion molecules and has ultimately given rise to that most versatile of recognition systems, immunoglobulins themselves. Brümmendorf and Lemmon (pp 611–618) approach the Ig superfamily from an interesting evolutionary viewpoint. They point out that not only is this one of the largest gene families in the human genome, but the complexity of several family members is hugely increased by extensive splicing. Additionally, the Ig domain appears well suited for pairing in cis with other Ig-domain-containing proteins, with some adhesion molecules being able to form both homo- and hetero-associations. Brümmendorf and Lemmon go on to discuss the role of several Ig superfamily members in axon guidance as well as the links to the cytoskeleton that recent work is revealing. They also discuss the association of several Ig-containing CAMs (cell adhesion molecules) with PDZ adaptor proteins, such as afadin, and the role that these may play in adherens junctions. At least two of the PDZ domain proteins found in
527
adherens and tight junctions have been found to interact with transcription factors. One of the exciting themes that has emerged over the past several years is that proteins that were originally thought only to function within cell adhesions are turning out also to bind transcription factors and to act within the nucleus. What started with β-catenin is becoming not the exception but the rule. Another example is p120catenin described in the review by Anastasiadis and Reynolds (see above). Also on the subject of axon guidance, Liu and Strittmatter (pp 619–626) focus on the Semaphorins and their receptors, Neuropilins and Plexins. Recent work has revealed that these receptors affect the growth cone actin cytoskeleton by acting on the Rho family of GTPases. Although most attention is directed towards RhoA, Rac1 and Cdc42, Liu and Strittmatter point out that other members of the Rho family are expressed in neurons, and recent evidence suggests that these too may contribute to the motile properties of growth cones. In discussing the regulation of the Rho family, the authors indicate that several guanine nucleotide exchange factors are specific for neural tissue and couple directly or indirectly to cell adhesion molecules that affect growth cone behavior. Given the complexity of neural regulation, we suspect that this field is about to explode as signaling pathways downstream from many axon guidance receptors converge on the Rho family, thereby regulating cytoskeletal dynamics and growth cone collapse or extension. An interesting example of signaling derived from cell–cell interactions occurs with Notch and Delta, where the receptor and ligand occur on interacting cells. Critical to the ability of Notch to signal is a series of proteolytic cleavages, with the final cleavage resulting in a cytoplasmic domain that can travel to the nucleus to effect transcriptional events. Fortini (pp 627–634) reviews the proteolytic processing of Notch and, in particular, the recent work on presenilin. Increasing evidence supports the idea that presenilin is the catalytic subunit of a multiprotein complex that executes intramembranous cleavage of target membrane proteins. One of these is Notch. However, much of what has been learned about presenilin derives from its role in Alzheimer’s disease and the generation of amyloid precursor protein. In spite of its importance in human disease, relatively little is known about presenilin and its normal mode of action. Fortini raises a number of important questions about this protein. Given that it is evolutionarily old and that Notch signaling functions in worms and flies, many of the details of its action and regulation are likely to be elucidated from studies in these organisms.
Polarity Many cells reveal polarized organization, and this is particularly striking in epithelia, where apical and basal surfaces are distinct. Ohno (pp 641–648) reviews the considerable progress that has been made in this field in the past few years. Studies of cell polarity in C. elegans revealed a set of
528
Cell-to-cell contact and extracellular matrix
‘partition’ genes (PAR-1-6) responsible for asymmetric cell divisions in the early embryo. Two of the proteins, PAR-3 and PAR-6, form a complex with atypical protein kinase C. The proteins in this complex are highly conserved, and an exciting development has been the identification of their homologues in flies and mammals and the discovery that in these organisms they function in determining the apicalbasal polarity of epithelial cells. Both PAR-3 and PAR-6 contain PDZ domains. The mammalian homologue of PAR-3 interacts via one of its PDZ domains with the junctional adhesion molecule, JAM, which is found in tight junctions. PAR-6 has a binding site for Cdc42 and Rac1, two members of Rho family that stimulate junction formation and that regulate cell polarity. Binding of these GTPases to PAR-6 leads to activation of atypical PKC. In addition to apical–basal polarity, many epithelia also demonstrate planar polarity. Adler and Lee (pp 635–640) review recent progress in understanding planar polarity in the epithelium of the insect wing, which is easily visualized by the pattern of hair organization. Genetic analysis has identified the frizzled pathway as critical for the organization of hairs pointing distally on the Drosophila wing. Recent studies have indicated that Fz, as well as several interacting proteins, accumulate along the distal border of epithelial cells, thereby marking the polarity of individual cells and resulting in initiating hair formation at the distal
vertex. Interestingly for the topic of cell–cell adhesion, one of the proteins that accumulates at the distal cell border is the product of the flamingo/starry night gene, a seven-transmembrane-spanning protein that has several extracellular cadherin domains. It has been suggested that adhesive interactions between this and other cell adhesion molecules on adjacent cells may stabilize the polarity pattern. As with so many cell adhesion molecules, downstream from frizzled, the Rho family of GTPases are implicated in this pattern formation. In a period and in a field in which information is increasing exponentially, reviews serve many obvious functions. We hope that this volume of reviews on various aspects of cell adhesion will be valuable both to those who work in the field as well to those outside this discipline. However, we also hope for more. The juxtaposition of a series of cutting edge reviews offers the chance for readers to make connections that have not previously been made and that have not been recognized by the reviewers themselves. We suspect that within this volume there may be cross-connections that have yet to be recognized. We are excited by the possibility that perhaps some of these will provide sparks that will inspire new ideas or stimulate new directions for research. Finally, as editors of this section, the two of us would like to thank all those who have contributed their reviews to make what we think is an exciting issue.