INVITED REVIEW
Function and spatial distribution of ion channels and transporters in cell migration

Albrecht Schwab

Physiologisches Institut, D-97070 Würzburg, Germany


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INTRODUCTION
CYTOSKELETAL MECHANISMS OF CELL...
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Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

cytoskeleton; cell volume; sorting


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CELL MIGRATION PLAYS A PIVOTAL role in many physiological and pathophysiological processes during the entire lifespan. Cell migration begins during embryogenesis, long before birth. For example, neural crest cells are migrating through the embryo to form the nervous system (48). Neuroblasts are moving within the central nervous system even after birth to reach their final place of work (47). Leukocytes are chasing invading bacteria or other pathogens, or they are "crawling" into sites of inflammation. Fibroblasts are migrating into a wound, thereby contributing to its closure (41). Migration of epithelial cells constitutes a rapid repair mechanism when gaps in an epithelial layer are to be closed. Such "epithelial wounds" frequently occur in the gastrointestinal (3) or the respiratory tract (110), or after acute renal failure (39, 98). Angiogenesis requires migration of endothelial cells (18). Finally, cell migration contributes to disease states that may be responsible for terminating life. For instance, atherosclerosis involves migration of smooth muscle cells (82), and tumor cell migration is one of the crucial steps in the metastatic cascade (33).

Despite these differences in function, all migrating cells share many similarities. Their polarization within the plane of movement along a front-rear axis is one of their most characteristic properties (67, 83). One can easily distinguish front and rear ends. When cells are crawling over a two-dimensional surface, the front is formed by a flat (~300-nm-thin), organelle-free, fanlike process, the so-called "lamellipodium," or by filopodia, which are pointed protrusions. The rear end is formed by the prominent cell body, which extends into a uropod (1, 38). The ability to establish and to maintain such a polarized state is the basis for migration. Then, repeated cycles of protrusion of the lamellipodium and retraction of the rear end of a migrating cell will result in its directional displacement. The morphological polarization of a migrating cell is highly dynamic, and external cues can lead to a rapid reorientation. For example, granulocytes change their direction of migration by 180° within a few minutes when the concentration of the chemoattractant N-formylmethionyl-leucyl-phenylalanine is reduced (2). They do so by either dissolving their original lamellipodium and forming a new one at the opposite cell pole or by making a "U-turn." Because neutrophils can be forced repetitively to flip their axis of polarization by 180°, it was speculated that they can "remember" their previous direction of locomotion (2).

Migrating cells share the ability to generate functionally distinct domains with epithelial cells and nerve cells whereby "apical" and "basolateral" membrane domains or dendrites and axons, respectively, can be distinguished (5, 40, 79, 107). However, the morphological polarization of epithelial and nerve cells is much more rigid than that of migrating cells, which need a high degree of flexibility when they encounter different extracellular cues while "en route." Thus migrating cells have no tight junction-like structures that separate apical and basolateral membrane from each other in epithelial cells (14). It is also not known whether there is a diffusion barrier separating the plasma membrane of cell body and lamellipodium. Such a diffusion barrier was shown in nerve cells whereby it limits the diffusion of axonal membrane proteins out of the axon (106). Nonetheless, recent studies indicate that some of the mechanisms underlying the polarization of migrating cells and of epithelial cells follow similar principles. This review will focus on one particular aspect of the polarization of migrating cells: the differential function and distribution of ion channels and ion transporters in migrating cells. To provide a framework for the role of ion channels and transporters in cell migration, a brief overview will be given first of those cytoskeletal mechanisms that potentially interact or act in concert with ion transport during locomotion.


    CYTOSKELETAL MECHANISMS OF CELL MIGRATION
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Cytoskeletal mechanisms of migration have been studied in great detail (see Refs. 50 and 63 for reviews). Different cytoskeletal mechanisms take place at the front and at the rear of migrating cells to allow efficient locomotion. Actin filaments play a central role in cell migration (63). They form a dense meshwork in the lamellipodium (96). Actin filaments grow in the direction of locomotion at the leading edge of the lamellipodium (103) and thereby drive the protrusion of this cell pole (64). Actin filaments interact with a great variety of other proteins that are important for their polymerization, dynamic turnover, and structural organization into a cross-linked network (50, 63). The retraction of the rear end of a migrating cell is thought to be related to the contraction of the cortical actomyosin network underneath the cell membrane. This view is supported, among other things, by the finding that myosin II is found predominantly at the rear end of some migrating cells (15).

The functional polarization of cytoskeletal mechanisms is also maintained by a gradient of the intracellular Ca2+ concentration ([Ca2+]i), with [Ca2+]i being higher at the rear end than in the lamellipodium (12, 27, 28, 31, 87). High [Ca2+]i in the rear promotes the retraction of the rear part of a migrating cell. In contrast, low [Ca2+]i in the front will favor the protrusion of the lamellipodium (15). This spatial component of Ca2+ signaling on the intracellular "migration machinery" has superimposed on it a temporal component involving fluctuations in [Ca2+]i in migrating cells (27, 47, 52, 56, 57, 59, 87). Thus the alternating predominance of mechanisms leading to the retraction of the rear part or to the protrusion of the lamellipodium appears to be necessary for optimal cell locomotion.

The cytoskeleton can only translocate a cell when the forces generated by the cellular migration machinery are transmitted to the surrounding extracellular matrix. When there is too little friction and cells can form no contacts with their substratum, locomotion is impaired. Similarly, locomotion is also impaired when the substratum is too "sticky" and cells cannot release their contacts. Thus the interaction between the extracellular matrix and cell adhesion receptors has to be a highly coordinated process (37). It depends on the concentration of extracellular matrix proteins and the expression level of integrins, the cellular adhesion receptors (71). Moreover, the matrix contacts are dynamic and asymmetric. Thus a migrating cell forms new contacts at its front, and contacts are released at its rear part through Ca2+-dependent phosphatases (34). Integrins are brought back to the cell front by oriented endocytic recycling (24, 73). Depending on the adhesiveness of the substrate, integrins can also be left behind (shed), so that one can visualize a cell's path retrospectively (25, 70).


    ION CHANNELS AND TRANSPORTERS MODULATE CELL MIGRATION
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General Considerations

The function of ion channels and transporters is closely related to the actin cytoskeleton. On one hand, ion channels and transporters can be regulated by the state of actin filaments (60, 62). Thus the ability of shrunk or swollen cells to restore their normal volume by means of activating ion channels and transporters is impaired when normal actin filament turnover is disturbed by drugs such as cytochalasin (49). In addition, the organization of the actin filament network by accessory proteins also modulates the ability of a cell to regulate its volume (13). On the other hand, changes in cell volume themselves in turn influence the actin cytoskeleton. Such volume changes can be elicited by activating or inhibiting ion channels and transporters or by treating cells with anisosmotic solutions. Cell swelling is accompanied by a disintegration of actin filaments, and cell shrinkage is followed by an assembly of actin filaments (32, 91). This mutual dependence of actin filaments and cell volume indicates that the "correct" cell volume and thereby the correct activity of ion channels and transporters must play an important role in cell migration, a process critically relying on a rapid turnover of actin filaments (63). By setting the correct cell volume, ion channels and transporters create the intracellular milieu that is required for the optimal operation of the cytoskeletal migration machinery. Thus it is not surprising that migration of a number of cell types is modulated by the activity of ion channels and transporters.

Ion Channels Required for Cell Migration

Human melanoma cells, which lack the actin cross-linking protein ABP-280, are unable to migrate (17). Interestingly, they are also unable to activate K+ channels appropriately during volume regulation. Both defects are "rescued" by transfecting these cells with ABP-280 (13, 17). However, migration of these rescued melanoma cells is impaired when Ca2+-sensitive K+ channels (IK1) are blocked (90). This type of K+ channel is required for migration of other cell types, too (80, 90, 93). Thus IK1 channel activity appears to be a general requirement for cell migration. Accordingly, IK1 channels are predominantly found in cells with the ability to migrate. Voltage-dependent K+ channels (Kv1.3 and Kv3.1) have been related to migration of lymphocytes (54) and of embryonic nerve cells (35). The outgrowth of neurites, a process resembling the protrusion of lamellipodia, is also modulated by the activity of K+ channels (4). Migration of granular cells from the cerebellum depends on the activity of N-type Ca2+ channels and N-methyl-D-aspartate receptors (45, 46). Stretch-activated, Ca2+-permeable nonselective cation channels are involved in migration of fish epithelial keratocytes (52). Locomotion of glioma cells (97) and of transformed renal epithelial (MDCK-F) cells (68) is regulated by the activity of Cl- channels (93). Moreover, spreading of granulocytes, a process related to migration, depends on Cl- efflux (61).

Functional Polarization of Ion Channels in Migrating Cells

As mentioned above, ion channels and transporters have a great impact on cell volume. By mediating salt efflux or influx, followed by osmotic water flux, they elicit cell shrinkage or cell swelling. This implies that changing the activity of ion channels or transporters in migrating cells will have an effect on their volume. These considerations apply in particular to Ca2+-sensitive IK1 channels, which are intermittently activated in migrating cells by fluctuations in [Ca2+]i (26, 92, 93). Studies on volume regulation show that activation of these K+ channels results in cell shrinkage (49). Such volume loss due to activation of IK1 channels also occurs in migrating cells. Oscillations in [Ca2+]i trigger a massive intermittent loss of cellular K+ (19, 88) and a loss of up to 20% of the volume of MDCK-F cells (85). Because blockade of IK1 channels inhibits migration, we postulated that K+ channel-mediated volume fluctuations are part of or modulate the cellular migration machinery. In a similar way, a Cl- channel was suggested to modulate migration of glioma cells by controlling their cell volume (97). Thus it is likely that the observed volume loss after K+ channel stimulation (85) is caused by the simultaneous efflux of K+ and Cl- ions.

However, to contribute to cell migration, such cell volume changes should optimally be polarized within the cell. This should be reflected by a polarized distribution of the respective channel or transporter activity in migrating cells. This hypothesis can be tested by topically applying appropriate blockers to either the front or the rear part of migrating cells. Indeed, inhibition of IK1 channels at the rear and front of migrating MDCK-F cells has differential effects on migration. Cells are only slowed down when the rear part of a crawling cell is exposed to the scorpion venom charybdotoxin, a blocker of IK1 channels (88). When applied to the lamellipodium, charybdotoxin has no effect on migration. Consequently, IK1 channel-mediated volume changes almost exclusively affect the rear part of migrating cells (85). Thus IK1 channels facilitate the retraction of the rear part of migrating cells by inducing a local cell shrinkage at this cell pole. So far it is not known whether Cl- channel activity is also polarized in migrating cells.

Ion Transporters Required for Cell Migration

Ion transporters play an important role in cell migration, too. The Na+/H+ exchanger (NHE1) is the best studied transporter in this context. It is a member of a gene family consisting of at least six isoforms, NHE1-NHE6 (16). The major functions of the ubiquitously expressed "housekeeping" isoform NHE1 are regulation of intracellular pH and of cell volume. However, it is also involved in cell migration. It is activated on chemotactic stimulation of neutrophil granulocytes, thereby increasing their cell volume and facilitating their migratory response (78, 81, 95). NHE activity is also required for migration of keratinocytes (6), human melanoma, and MDCK-F cells, whereby it operates in parallel with the anion exchanger AE2 and the Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter NBC1 (42, 43). In breast carcinoma cells, increased NHE1 activity augments cell motility and invasive ability (77). The role of NHE in migration can at least partially be taken over by a H+-K+-ATPase in neutrophils and MDCK-F cells (43, 78). Finally, the Na+-K+-2Cl- cotransporter has also been linked to migration of epithelial cells (55, 93).

Functional Polarization of Ion Transporters in Migrating Cells

If retraction of the rear part of migrating cells involves local cell shrinkage, one might anticipate that local cell swelling at the front contributes to the protrusion of the lamellipodium. In such a scenario, the activity of transporters mediating salt and osmotically obliged water uptake (e.g., the Na+/H+ or Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchangers) should be limited to the front of migrating cells. Inhibition of these two transporters, which are required for optimal migration (6, 43, 78, 81, 95), reduces the rate of migration only when the respective blockers are applied to the front of MDCK-F cells. The blockers do not slow down migration when directed to the rear part of crawling cells (43). These experiments provide circumstantial evidence that the protrusion of the lamellipodium is accompanied by local solute and volume uptake. The parallel operation of Na+/H+ or Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchangers at the cell front thereby corresponds to a form of isosmotic regulatory volume increase.

Model of Ion Transport in Cell Migration

On the basis of these studies, a model was developed that describes migration as temporally and spatially separated phases of local cell swelling and cell shrinkage. We proposed the following cycle of events by which polarized activity of ion channels and transporters contributes to cell migration (91; see Fig. 1). When [Ca2+]i is at its resting level, IK1 channel activity is minimal. Cell volume is gradually increased, among other things, by the parallel operation of Na+/H+ and Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchangers as well as by Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport. Because these transporters are found at the front of migrating cells (30, 42, 43), they act in concert with gelosmotic swelling at the leading edge of the lamellipodium (69). The growing lamellipodium and the gradual cell swelling increase the tension of the plasma membrane and eventually activate Ca2+-permeable mechanosensitive cation channels (52). [Ca2+]i rises, leading to the activation of IK1 channels. The resulting local shrinkage of the rear part (85) facilitates Ca2+-sensitive cytoskeletal mechanisms underlying the retraction of this pole of a migrating cell, such as contraction of the cortical actomyosin network (50) or the release of integrins from the extracellular matrix. After volume loss and retraction of the rear part, mechanosensitive Ca2+ entry stops, [Ca2+]i returns to basal levels, and the cycle starts over again.


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Fig. 1.   Model summarizing the function of ion channels and transporters in migrating cells. Salt and osmotically obliged water uptake mediated by the parallel operation of Na+/H+ and Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchange as well as Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport at the front of migrating cells contribute to the extension of the lamellipodium (A and B). Increasing volume and membrane tension eventually trigger a rise in intracellular Ca2+ concentration ([Ca2+]i) via activation of Ca2+-permeable stretch-activated cation channels (B). The rise in [Ca2+]i induces the retraction of the rear part of a migrating cell, which is paralleled by massive K+ efflux and shrinkage of the cell pole (C).

Although there is experimental evidence supporting this model, that ion channels and transporters modulate cell migration by additional mechanisms as well cannot be ruled out. For example, the rise of [Ca2+]i after stimulation of neutrophil granulocytes is attenuated by a depolarization of the membrane potential (20). Accordingly, it has been shown in a neutrophil cell line (HL-60) that the IK1 channel blocker clotrimazole decreases Ca2+ influx (44). Hence, it is conceivable that IK1 channel blockade impairs migration not only by preventing fluctuations of the cell volume but also by altering the intracellular Ca2+ homeostasis. This could impair migration, among other things, by inhibiting Ca2+-dependent release of integrins from their substrate.


    STRUCTURAL POLARIZATION OF ION CHANNELS AND TRANSPORTERS IN MIGRATING CELLS
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Functional polarization of ion channels or transporters in migrating cells can be achieved by either inserting them into the cell membrane in a polarized way, differentially regulating them at the front or rear part of the cell, or a combination of both. There is experimental evidence for all three possibilities.

Structural Polarization of Ion Channels

Voltage-dependent Kv1.4 channels are clustered at the leading edge of the lamellipodium when transfected into migrating cells (76). The same holds true for cloned Ca2+-sensitive IK1 channels (94). This is surprising insofar as functional experiments provided no evidence for IK1 channel activity at this cell pole (85, 88). That is, structural and functional polarization of IK1 channels in migrating cells do not match. One explanation for this discrepancy is that the functional polarization of IK1 channels is due to a differential regulation of the channel protein in the front and rear part of migrating cells. IK1 channel activity could be limited to the rear part of locomoting cells by a gradient of [Ca2+]i, the major regulator of IK1 channels, whereby it is higher than at the front (12, 27, 28, 31, 87). Regulation of IK1 channels by protein kinase C (89) could be another cause for the predominance of IK1 channel activity in the rear part of migrating cells. At least in migrating lymphocytes, protein kinase C is found preferentially in their rear part (23). Finally, the mechanical properties of the lamellipodium compared with those of the cell body could be responsible for limiting IK1 channel-mediated effects to the cell body. The lamellipodium is much stiffer than the cell body (75), and it is attached more firmly to the substratum than the cell body (85). It is therefore conceivable that the rigid actin gel and the tight adherence to the substratum prevent the lamellipodium from decreasing its volume to the same extent as the cell body.

Structural Polarization of Ion Transporters

The Na+/H+ exchanger NHE1 is concentrated at the leading edge of the lamellipodium of fibroblasts, where it colocalizes with components of focal adhesion complexes (30, 74). Assuming that NHE1 is distributed in a similar way in other cells, too, the structural polarization of the protein explains the functional polarization of Na+/H+ exchange activity in migrating cells (43). Possibly, polarized Na+/H+ exchange activity is reinforced by an additional regulatory effect. NHE1 activity is involved in signaling events triggered by integrin-mediated cell adhesion, which also include RhoA- regulated cytoskeletal restructuring (100, 101). Two other transporters are also distributed in a polarized way in migrating cells: the Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchanger AE2 (43) and the Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter NBC1 (42) are concentrated at the leading edge of locomoting MDCK-F cells. Thus the functional polarization of the Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchanger in migrating cells has a structural basis, too.

Mechanisms for Polarization of Ion Channels and Transporters in Migrating Cells

All ion channels and transporters, which are clustered at the leading edge of the lamellipodium of migrating cells (Kv1.4 and IK1 channels; NHE1, AE2, NBC1; see Table 1), have one property in common. They are all inserted into the basolateral membrane when expressed in polarized epithelial cells (9, 53, 84, 99, 102). By contrast, an apical K+ channel (ROMK2; 108) or an apical transporter (Na+/H+ exchanger NHE3; 102) is not concentrated at the leading edge but appears to be distributed diffusely on the cell surface (22, 104). These observations suggest that basolateral and apical transport proteins are recognized as such by migrating cells and delivered in specific ways to the cell surface. This implies the existence of basolateral and apical routes from the trans-Golgi network (TGN) to the cell surface of migrating cells. Studies with viral marker proteins of the apical and basolateral membrane lend support to this notion. Apical (influenza virus hemagglutinin) and basolateral markers (vesicular stomatitis virus G protein and Semliki Forest virus spike glycoprotein) are transported via two distinct routes from the TGN to the plasma membrane even in unpolarized cells (66, 109). These transport routes correspond to apical and basolateral routes in polarized epithelia.

                              
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Table 1.   Localization of ion channels and transporters as well as viral markers in migrating cells

A polarized distribution of basolateral viral markers is induced by stimulating fibroblast migration (7) or by coexpression with rab8 (7). rab8 Is a small Ras-like GTPase that regulates polarized membrane transport to the basolateral membrane of epithelial cells and to dendrites in neurons (5). Basolateral membrane markers accumulate at the leading edge of motile fibroblasts or in protrusions of baby hamster kidney cells. Apical markers are apparently distributed randomly on the cell surface (72; see Table 1). These studies are important for understanding the distribution of ion channels and transporters in migrating cells. By analogy, one can hypothesize that basolateral ion channels and transporters are delivered to the leading edge of the lamellipodium by the same rab8-dependent mechanism, too. Interestingly, it was also demonstrated that expression of rab8 in formerly unpolarized cells results in the appearance of cell processes reminiscent of lamellipodia (72). Thus the delivery of membrane proteins to these processes and the development of a polarized morphology go hand in hand.

This is of particular importance for migrating cells, the polarized morphology of which is constantly "renewed" by a process likely involving the exocytotic insertion of membrane vesicles at the leading edge (10, 11). As indicated in Fig. 2, these vesicles are probably derived from two sources. One pool serves the transport of newly synthesized basolateral membrane proteins from the TGN to the plasma membrane at the cell front (7, 72). Moreover, such vesicles can originate from an endocytic cycle that brings membrane proteins, endocytosed over the cell surface, back to the cell front (10, 11, 24, 36, 51, 73). Sorting of basolateral proteins and the endocytic cycle rely, among other things, on overlapping tyrosine- or dileucine-based signaling sequences in the COOH termini of the respective proteins (5, 40). We observed that IK1 channels are no longer concentrated at the leading edge of migrating cells when a dileucine motif in their COOH terminus is mutated (94; Schwab A, and Schulz C, unpublished observations). Future studies will have to show whether this is due to missorting and/or a defect in endocytic recycling of the channel protein. Nonetheless, these observations point to the importance of basolateral sorting and/or endocytic recycling in the maintenance of polarized ion channel distribution in migrating cells.


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Fig. 2.   Tentative model depicting possible routes of intracellular trafficking of "basolateral" ion channels and transporters in migrating cells, which leads to their accumulation at the cell front. Proteins are recognized as basolateral and delivered from the trans-Golgi network (TGN) to the cell front. They are then moving within the plasma membrane toward the rear part of the cell, where they are endocytosed and recycled to the cell front.

There is also evidence that so-called "glycolipid rafts" are involved in the polarization of membrane proteins (e.g., chemokine receptors) in migrating breast adenocarcinoma cells (MCF-7) (58). So far, it is not known whether the association of ion channels and transporters with glycolipid rafts plays a role in their subcellular distribution in migrating cells, too. However, this study shows that multiple sorting pathways exist in locomoting cells. It remains to be elucidated whether they are cell-type specific or operating in parallel.


    PERSPECTIVES
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Studies on the role of ion channels and ion transporters in cell migration are in an early stage, and many questions need to be clarified. For example, the role of ion channels and transporters in migration of epithelial cells during the closure of epithelial lesions has not yet been studied in detail. Initial studies indicate that these membrane proteins, which are responsible for many of the epithelial functions and are abundantly expressed in epithelial cells, are utilized in this process, too (55). What are the exact molecular mechanisms by which ion transport affects the cytoskeletal migration machinery? How and why are some ion channels and transporters directed to the leading edge of the lamellipodium whereas others are not? Are PDZ-domain binding proteins involved in clustering ion channels and transporters at the leading edge and in linking them to the cytoskeleton? In this context, it is noteworthy that the Na+/H+ exchanger regulatory factor (NHE-RF) interacts with one of its two PDZ domains with members of the family of MERM proteins (merlin, ezrin, radixin, and moesin), which function as possible linkers between integral membrane proteins and the cytoskeleton (105). Both NHE-RF and merlin colocalize to membrane ruffles and filopodia (29, 65), i.e., to similar structures as shown for several ion channels and transporters (30, 42, 43, 76, 94). Interestingly, NHE-RF regulates Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport activity (8), which points to a possible physical interaction between NHE-RF and one of the transporters concentrated at the leading edge of migrating cells. However, the direct demonstration of such an interaction still remains to be shown.

The last few years witnessed great progress in our knowledge of cell polarization. It became clear that many of the critical mechanisms have been conserved throughout the evolution from yeast to mammals, as well as in different cell types (21). This knowledge provides a framework for the development of testable hypotheses with respect to the spatial distribution of ion channels and transporters in migrating cells. In particular, the discovery that apical and basolateral sorting pathways also exist in migrating cells shows that the comparison with epithelial cells is very fruitful. Studies on ion transport and cell migration suggest that ion channels and transporters play a prominent role in this important physiological and pathophysiological process. Thereby, they can also be of potential therapeutic use in disease states in which cell migration plays a critical pathophysiological role. Finally, it is conceivable that the localization of ion channels or transporters in migrating cancer cells may be of diagnostic value. Ninety percent of malignant tumors originate from epithelial cells. Tumor formation in epithelia leads to the loss of their normal polarity along the apicobasolateral axis (86). However, cancer cells become polarized along a front-rear axis when they start to migrate to form metastases. On the basis of present knowledge, it is likely that the acquisition of a migratory phenotype leads to a characteristic redistribution of ion channels and transporters in migrating cancer cells, which might be helpful in identifying metastasizing cancer cells. Thus unraveling the mechanisms by which ion channels and transporters modulate migration and are positioned in migrating cells will be a rewarding challenge for the coming years.


    ACKNOWLEDGEMENTS

I thank Stefan Silbernagl and, in particular, Hans Oberleithner. Without their continuous support and friendship, much of our work on ion transport in cell migration would not have been possible. The work from my laboratory represents the combined efforts of Magnus Klein, Agnes Röll, Christoph Schulz, Barbara Schuricht, Ponke Seeger, Dietmar Weinhold, and Andrea Wulf, in collaboration with Jürgen Reinhardt and Stefan W. Schneider.


    FOOTNOTES

This work was supported by Deutsche Forschungsgemeinsschaft Grants SFB 176 A6 and Schw 407/7-1.

Address for reprint requests and other correspondence: A. Schwab, Physiologisches Institut, Röntgenring 9, D-97070 Würzburg, Germany (E-mail: albrecht.schwab{at}mail.uni-wuerzburg.de).


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ABSTRACT
INTRODUCTION
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1.   Abercrombie, M, Heaysman JEM, and Pegrum SM. The locomotion of fibroblasts in culture. IV. Electron microscopy of the leading lamella. Exp Cell Res 67: 359-367, 1971[ISI][Medline].

2.   Albrecht, E, and Petty HR. Cellular memory: neutrophil orientation reverses during temporally decreased chemoattractant concentration. Proc Natl Acad Sci USA 95: 5039-5044, 1998[Abstract/Free Full Text].

3.   Allen, A, Flemström G, Garner A, and Kivilaakso E. Gastroduodenal mucosal protection. Physiol Rev 73: 823-857, 1993[Abstract/Free Full Text].

4.   Arcangeli, A, Becchetti A, Mannini A, Mugnai G, De Filippi P, Tarone G, Del Bene MR, Barletta E, Wanke E, and Olivotto M. Integrin-mediated neurite outgrowth in neuroblastoma cells depends on the activation of potassium channels. J Cell Biol 122: 1131-1143, 1993[Abstract].

5.   Aroeti, B, Okhrimenko H, Reich V, and Orzech E. Polarized trafficking of plasma membrane proteins: emerging roles for coats, SNAREs, GTPases and their link to the cytoskeleton. Biochim Biophys Acta 1376: 57-90, 1998[ISI][Medline].

6.   Bereiter-Hahn, J, and Voth M. Ionic control of locomotion and shape of epithelial cells. II. Role of monovalent cations. Cell Motil Cytoskeleton 10: 528-536, 1988[ISI][Medline].

7.   Bergmann, JE, Kupfer A, and Singer SJ. Membrane insertion at the leading edge of motile fibroblasts. Proc Natl Acad Sci USA 80: 1367-1371, 1983[Abstract].

8.   Bernardo, AA, Kear FT, Santos AVP, Ma J, Steplock D, Robey RB, and Weinman Basolateral EJ. Na+/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransport activity is regulated by the dissociable Na+/H+ regulatory factor. J Clin Invest 104: 195-201, 1999[Abstract/Free Full Text].

9.   Bleich, M, Riedemann N, Warth R, Kerstan D, Leipziger J, Hör M, Van Driesche W, and Greger R. Ca2+ regulated K+ and non-selective cation channels in the basolateral membrane of rat colonic crypt base cells. Pflügers Arch 432: 1011-1022, 1996[ISI][Medline].

10.   Bretscher, MS, and Aguado-Velasco C. Membrane traffic during cell locomotion. Curr Opin Cell Biol 10: 537-541, 1998[ISI][Medline].

11.   Bretscher, MS, and Aguado-Velasco C. EGF induces recycling membrane to form ruffles. Curr Biol 8: 721-724, 1998[ISI][Medline].

12.   Brundage, RA, Fogarty KE, Tuft RA, and Fay FS. Calcium gradients underlying polarization and chemotaxis of eosinophils. Science 254: 703-706, 1991[ISI][Medline].

13.   Cantiello, HF, Prat AG, Bonventre JV, Cunningham CC, Hartwig JH, and Ausiello DA. Actin-binding protein contributes to cell volume regulatory ion channel activation in melanoma cells. J Biol Chem 268: 4596-4599, 1993[Abstract/Free Full Text].

14.   Caplan, MJ. Membrane polarity in epithelial cells: protein sorting and establishment of polarized domains. Am J Physiol Renal Physiol 272: F425-F429, 1997[Abstract/Free Full Text].

15.   Conrad, PA, Giuliano KA, Fisher G, Collins K, Matsudaira PT, and Taylor DL. Relative distribution of actin, myosin I, and myosin II during the wound healing response of fibroblasts. J Cell Biol 120: 1381-1391, 1993[Abstract].

16.   Counillon, L, and Pouyssegur J. The expanding family of eucaryotic Na+/H+ exchangers. J Biol Chem 275: 1-4, 2000[Free Full Text].

17.   Cunningham, CC, Gorlin JB, Kwiatkowski DJ, Hartwig JH, Janmey PA, Byers HR, and Stossel TP. Actin-binding protein requirement for cortical stability and efficient locomotion. Science 255: 325-327, 1992[ISI][Medline].

18.   Daniel, TO, and Abrahamson D. Endothelial signal integration in vascular assembly. Annu Rev Physiol 62: 649-671, 2000[ISI][Medline].

19.   Danker, T, Oberleithner H, Gassner B, and Schwab A. Extracellular detection of K+ loss during migration of transformed Madin-Darby canine kidney cells. Pflügers Arch 433: 71-76, 1996[ISI][Medline].

20.   Di Virgilio, F, Lew PD, Andersson T, and Pozzan T. Plasma membrane potential modulates chemotactic peptide-stimulated cytosolic free Ca2+ changes in human neutrophils. J Biol Chem 262: 4574-4579, 1987[Abstract/Free Full Text].

21.   Drubin, DG, and Nelson WJ. Origins of cell polarity. Cell 84: 335-344, 1996[ISI][Medline].

22.   D'Souza, S, Garcia-Cabado A, Yu F, Teter K, Lukacs G, Skorecki K, Moore HP, Orlowski J, and Grinstein S. The epithelial sodium-hydrogen antiporter Na+/H+ exchanger 3 accumulates and is functional in recycling endosomes. J Biol Chem 273: 2035-2043, 1998[Abstract/Free Full Text].

23.   Entschladen, F, Niggemann B, Zänker KS, and Friedl P. Differential requirement of protein tyrosine kinases and protein kinase C in the regulation of T cell locomotion in three-dimensional matrices. J Immunol 159: 3203-3210, 1997[Abstract].

24.   Fabbri, M, Fumagalli L, Bossi G, Bianchi E, Bender JR, and Pardi R. A tyrosine-based sorting signal in the beta 2 integrin cytoplasmic domain mediates its recycling to the plasma membrane and is required for ligand-supported migration. EMBO J 18: 4915-4925, 1999[Abstract/Free Full Text].

25.   Friedl, P, Maaser K, Klein CE, Niggemann B, and Zänker KS. Migration of highly aggressive MV3 melanoma cells in 3-D collagen lattices results in local matrix reorganization and shedding of beta 1 integrins and CD44. Cancer Res 57: 2061-2070, 1997[Abstract].

26.   Gallin, EK. Evidence for a Ca-activated rectifying K channel in human macrophages. Am J Physiol Cell Physiol 257: C77-C85, 1989[Abstract/Free Full Text].

27.   Gilbert, SH, Perry K, and Fay FS. Mediation of chemoattractant-induced changes in [Ca2+]i and cell shape, polarity, and locomotion by InsP3, DAG, and protein kinase C in newt eosinophils. J Cell Biol 127: 489-503, 1994[Abstract].

28.   Gollnick, F, Meyer R, and Stockem W. Visualization and measurement of calcium transients in Amoeba proteus by fura-2 fluorescence. Eur J Cell Biol 55: 262-271, 1991[ISI][Medline].

29.   Gonzalez-Agosti, C, Xu L, Pinney D, Beauchamp R, Hobbs W, Gusella J, and Ramesh V. The merlin tumor suppressor localizes preferentially in membrane ruffles. Oncogene 13: 1239-1247, 1996[ISI][Medline].

30.   Grinstein, S, Woodside M, Waddell TK, Downey GP, Orlowski J, Pouyssegur J, Wong DCP, and Foskett JK. Focal localization of the NHE-1 isoform of the Na+/H+ antiport: assessment of effects on intracellular pH. EMBO J 12: 5209-5218, 1993[Abstract].

31.   Hahn, K, DeBiasio R, and Taylor DL. Patterns of elevated free calcium and calmodulin activation in living cells. Nature 359: 736-738, 1992[ISI][Medline].

32.   Hallows, KR, Packman CH, and Knauf PA. Acute cell volume changes in anisotonic media affect F-actin content of HL-60 cells. Am J Physiol Cell Physiol 261: C1154-C1161, 1991[Abstract/Free Full Text].

33.   Hart, IR, Goode NT, and Wilson RE. Molecular aspects of the metastatic cascade. Biochim Biophys Acta 989: 65-84, 1989[ISI][Medline].

34.   Hendey, B, Klee CB, and Maxfield FR. Inhibition of neutrophil chemotaxis on vitronectin by inhibitors of calcineurin. Science 258: 296-299, 1992[ISI][Medline].

35.   Hendriks, R, Morest DK, and Kaczmarek LK. Role in neuronal cell migration for high-threshold potassium currents in the chick hindbrain. J Neurosci Res 58: 805-814, 1999[ISI][Medline].

36.   Hopkins, CR, Gibson A, Shipman M, Strickland DK, and Trowbridge IS. In migrating fibroblasts, recycling receptors are concentrated in narrow tubules in the pericentriolar area, and then routed to the plasma membrane of the leading lamella. J Cell Biol 125: 1265-1274, 1994[Abstract].

37.   Huttenlocher, A, Sandborg RR, and Horwitz AF. Adhesion in cell migration. Curr Opin Cell Biol 7: 697-706, 1995[ISI][Medline].

38.   Ingram, VM. A side view of moving fibroblasts. Nature 222: 641-644, 1969[ISI][Medline].

39.   Kartha, S, and Toback FG. Adenine nucleotides stimulate migration in wounded cultures of kidney epithelial cells. J Clin Invest 90: 288-292, 1992[ISI][Medline].

40.   Keller, P, and Simons K. Post-Golgi biosynthetic trafficking. J Cell Sci 110: 3001-3009, 1997[Abstract/Free Full Text].

41.   Kirsner, RS, and Eaglstein WH. The wound healing process. Dermatol Clin 11: 629-640, 1993[ISI][Medline].

42.   Klein, M, Rossmann H, Seidler U, and Schwab A. Cellular knock-out of Na+/H+ exchange reveals a role for the Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter for cell migration (Abstract). Pflügers Arch 439: R320, 2000.

43.   Klein, M, Seeger P, Schuricht B, Alper SL, and Schwab A. Polarization of Na+/H+ and Cl-/HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> exchangers in migrating renal epithelial cells. J Gen Physiol 115: 599-607, 2000[Abstract/Free Full Text].

44.   Koch, BD, Faurot GF, Kopanista MV, and Swinnney DC. Pharmacology of Ca2+-influx pathway activated by emptying the intracellular Ca2+ stores in HL-60 cells: evidence that a cytochrome P-450 is not involved. Biochem J 302: 187-190, 1994[ISI][Medline].

45.   Komuro, H, and Rakic P. Selective role of N-type calcium channels in neuronal migration. Science 257: 806-809, 1992[ISI][Medline].

46.   Komuro, H, and Rakic P. Modulation of neuronal migration by NMDA receptors. Science 260: 95-97, 1993[ISI][Medline].

47.   Komuro, H, and Rakic P. Orchestration of neuronal migration by activity of ion channels, neurotransmitter receptors, and intracellular Ca2+ fluctuations. J Neurobiol 37: 110-130, 1998[ISI][Medline].

48.   Kulesa, PM, and Fraser SE. In ovo time-lapse analysis of chick hindbrain neural crest cell migration shows cell interaction during migration to the branchial arches. Development 127: 1161-1172, 2000[Abstract/Free Full Text].

49.   Lang, F, Busch GL, Ritter M, Völkl H, Waldegger S, Gulbins E, and Häussinger D. Functional significance of cell volume regulatory mechanisms. Physiol Rev 78: 247-306, 1998[Abstract/Free Full Text].

50.   Lauffenburger, DA, and Horwitz AF. Cell migration: a physically integrated molecular process. Cell 84: 359-369, 1996[ISI][Medline].

51.   Lawson, MA, and Maxfield FR. Ca2+- and calcineurin-dependent recycling of an integrin to the front of migrating neutrophils. Nature 376: 75-79, 1995.

52.   Lee, J, Ishihara A, Oxford G, Johnson B, and Jacobson K. Regulation of cell movement is mediated by stretch-activated calcium channels. Nature 400: 382-386, 1999[ISI][Medline].

53.   Le Maout, S, Sewing S, Coudrier E, Elalouf JM, Pongs O, and Merot J. Polarized targeting of a shaker-like (A-type) K+-channel in the polarized epithelial cell line MDCK. Mol Membr Biol 13: 143-147, 1996[ISI][Medline].

54.   Levite, M, Cahalon L, Peretz A, Hershkoviz R, Sobko A, Ariel A, Desai R, Attali B, and Lider O. Extracellular K+ and opening of voltage-gated potassium channels activate T cell migration and functional association between Kv1.3 channels and beta 1 integrins. J Exp Med 191: 1167-1176, 2000[Abstract/Free Full Text].

55.   Lotz, M, Wang H, Pories S, and Matthews J. Gut epithelial wound healing in vitro is modulated by effectors of Cl- secretion (Abstract). Gastroenterology 118: A825, 2000[ISI].

56.   Mandeville, JTH, Gosh RN, and Maxfield FR. Intracellular calcium levels correlate with speed and persistent forward motion of migrating neutrophils. Biophys J 68: 1207-1217, 1995[Abstract].

57.   Mandeville, JTH, and Maxfield FR. Effects of buffering intracellular free calcium on neutrophil migration through three-dimensional matrices. J Cell Physiol 171: 168-178, 1997[ISI][Medline].

58.   Manes, S, Mira E, Gómez-Moutón C, Lacalle RA, Keller P, Labrador JP, and Martínez AC. Membrane raft microdomains mediate front-rear polarity in migrating cells. EMBO J 18: 6211-6220, 1999[Abstract/Free Full Text].

59.   Marks, PW, and Maxfield FR. Transient increases in cytosolic free calcium appear to be required for the migration of adherent human neutrophils. J Cell Biol 110: 43-52, 1990[Abstract].

60.   Matthews, JB, Awtrey CS, and Madara JL. Microfilament-dependent activation of Na+/K+/2Cl- cotransport by cAMP in intestinal epithelial monolayers. J Clin Invest 90: 1608-1613, 1992[ISI][Medline].

61.   Menegazzi, R, Busetto S, Decleva E, Cramer R, Dri P, and Patriarca P. Triggering of chloride ion efflux from human neutrophils as a novel function of leukocyte beta 2 integrins: relationship with spreading and activation of the respiratory burst. J Immunol 162: 423-434, 1999[Abstract/Free Full Text].

62.   Mills, JW, and Mandel LJ. Cytoskeletal regulation of membrane transport events. FASEB J 8: 1161-1165, 1994[Abstract/Free Full Text].

63.   Mitchison, TJ, and Cramer LP. Actin-based cell motility and cell locomotion. Cell 84: 371-379, 1996[ISI][Medline].

64.   Mogilner, A, and Oster G. Cell motility driven by actin polymerization. Biophys J 71: 3030-3045, 1996[Abstract].

65.   Murthy, A, Gonzalez-Agosti C, Cordero E, Pinney D, Candia C, Solomon F, Gusella F, and Ramesh V. NHE-RF, a regulatory cofactor for Na+/H+ exchange, is a common interactor for merlin and ERM (MERM) proteins. J Biol Chem 273: 1273-1276, 1998[Abstract/Free Full Text].

66.   Müsch, A, Xu H, Shields D, and Rodriguez-Boulan E. Transport of vesicular stomatitis virus G protein to the cell surface is signal mediated in polarized and nonpolarized cells. J Cell Biol 133: 543-558, 1996[Abstract].

67.   Nabi, IR. The polarization of a motile cell. J Cell Sci 112: 1803-1811, 1999[Abstract/Free Full Text].

68.   Oberleithner, H, Westphale HJ, and Gassner B. Alkaline stress transforms Madin-Darby canine kidney cells. Pflügers Arch 419: 418-420, 1991[ISI][Medline].

69.   Oster, GF, and Perelson AS. The physics of cell motility. J Cell Sci Suppl 8: 35-54, 1987[Medline].

70.   Palecek, SP, Huttenlocher A, Horwitz AF, and Lauffenburger DA. Physical and biochemical regulation of integrin release during rear detachment of migrating cells. J Cell Sci 111: 929-940, 1998[Abstract/Free Full Text].

71.   Palecek, SP, Loftus JC, Ginsberg MH, Lauffenburger DA, and Horwitz AF. Integrin-ligand binding properties govern cell migration speed through cell-substratum adhesiveness. Nature 385: 537-540, 1997[ISI][Medline].

72.   Peränen, J, Auvinen P, Virta H, Wepf R, and Simons K. Rab8 promotes polarized transport through reorganization of actin and microtubules in fibroblasts. J Cell Biol 135: 153-167, 1996[Abstract].

73.   Pierini, LM, Lawson MA, Eddy RJ, Hendey B, and Maxfiled FR. Oriented endocytic recycling of alpha 5beta 1 in motile neutrophils. Blood 95: 2471-2481, 2000[Abstract/Free Full Text].

74.   Plopper, GE, McNamee HP, Dike IE, Bojanowski K, and Ingber DE. Convergence of integrin and growth factor receptor signalling pathways within the focal adhesion complex. Mol Biol Cell 6: 1349-1365, 1995[Abstract].

75.   Radmacher, M, Fritz M, Kacher CM, Cleveland JP, and Hansma PK. Measuring the viscoelastic properties of human platelets with the atomic force microscope. Biophys J 70: 556-567, 1996[Abstract].

76.   Reinhardt, J, Golenhofen N, Pongs O, Oberleithner H, and Schwab A. Migrating transformed MDCK cells are able to structurally polarize a voltage-activated K+ channel. Proc Natl Acad Sci USA 95: 5378-5382, 1998[Abstract/Free Full Text].

77.   Reshkin, SJ, Bellizzi A, Albarini V, Guerra L, Tommasino M, Paradiso A, and Casavola V. Phosphoinositide 3-kinase is involved in the tumor-specific activation of human breast cancer cell Na+/H+ exchange, motility, and invasion induced by serum deprivation. J Biol Chem 275: 5361-5369, 2000[Abstract/Free Full Text].

78.   Ritter, M, Schratzberger P, Rossmann H, Wöll E, Seiler K, Seidler U, Reinisch U, Kahler CM, Zwierzina H, Lang HJ, Lang F, and Wiedermann CJ. Effect of inhibitors of Na+/H+ exchange and gastric H+/K+ ATPase on cell volume, intracellular pH and migration of human polymorphonuclear leucocytes. Br J Pharmacol 124: 627-638, 1998[Abstract].

79.   Rodriguez-Boulan, E, and Powell SK. Polarity of epithelial and neuronal cells. Annu Rev Cell Biol 8: 395-427, 1992[ISI].

80.   Röll, A, Wulf A, Schuricht B, and Schwab A. K+ channel (hIK1) dependent migration of human neutrophil granulocytes (Abstract). Pflügers Arch 439: R447, 2000.

81.   Rosengren, S, Henson PM, and Worthen GS. Migration-associated volume changes in neutrophils facilitate the migratory process in vitro. Am J Physiol Cell Physiol 267: C1623-C1632, 1994[Abstract/Free Full Text].

82.   Ross, R. The pathogenesis of atherosclerosis: a perpective for the 1990s. Nature 362: 801-809, 1993[ISI][Medline].

83.   Sánchez-Madrid, F, and del Pozo MA. Leukocyte polarization in cell migration and immune interactions. EMBO J 18: 501-511, 1999[Abstract/Free Full Text].

84.   Schmitt, BM, Biemesderfer D, Romero MF, Boulpeap EL, and Boron WF. Immunolocalization of the electrogenic Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter in mammalian and amphibian kidney. Am J Physiol Renal Physiol 276: F27-F36, 1999[Abstract/Free Full Text].

85.   Schneider, SW, Pagel P, Rotsch C, Danker T, Oberleithner H, Radmacher M, and Schwab A. Volume dynamics in migrating epithelial cells imaged with atomic force microscopy. Pflügers Arch 439: 297-303, 2000[ISI][Medline].

86.   Schoenenberger, CA, and Matlin KS. Cell polarity and oncogenesis. Trends Cell Biol 1: 87-92, 1991.

87.   Schwab, A, Finsterwalder F, Kersting U, Danker T, and Oberleithner H. Intracellular Ca2+ distribution in migrating transformed renal epithelial cells. Pflügers Arch 434: 70-76, 1997[ISI][Medline].

88.   Schwab, A, Gabriel K, Finsterwalder F, Folprecht G, Greger R, Kramer A, and Oberleithner H. Polarized ion transport during migration of transformed Madin-Darby canine kidney cells. Pflügers Arch 430: 802-807, 1995[ISI][Medline].

89.   Schwab, A, Geibel J, Wang W, Oberleithner H, and Giebisch G. Mechanism of activation of K+ channels by Minoxidil-sulfate in Madin-Darby canine kidney cells. J Membr Biol 132: 125-136, 1993[ISI][Medline].

90.   Schwab, A, Reinhardt J, Schneider SW, Gassner B, and Schuricht B. K+ channel dependent migration of fibroblasts and human melanoma cells. Cell Physiol Biochem 9: 126-132, 1999[ISI][Medline].

91.   Schwab, A, Reinhardt J, Seeger P, Schuricht B, and Dartsch PC. Migration of transformed renal epithelial cells is regulated by K+ channel modulation of actin cytoskeleton and cell volume. Pflügers Arch 438: 330-337, 1999[ISI][Medline].

92.   Schwab, A, Westphale HJ, Wojnowski L, Wünsch S, and Oberleithner H. Spontaneously oscillating K+ channels in alkali-transformed MDCK cells. J Clin Invest 92: 218-223, 1993[ISI][Medline].

93.   Schwab, A, Wojnowski L, Gabriel K, and Oberleithner H. Oscillating activity of a Ca2+-sensitive K+ channel---a prerequisite for migration of alkali-transformed Madin-Darby canine kidney (MDCK-F) cells. J Clin Invest 93: 1631-1636, 1994[ISI][Medline].

94.   Schwab, A, Wulf A, Reinhardt J, Schulz C, Schuricht B, Hebert SC, and Silbernagl S. Cloning, regulation and subcellular localization of a Ca2+ sensitive K+ channel (cIK1) in migrating cells (Abstract). Pflügers Arch 439: R310, 2000.

95.   Simchowitz, L, and Cragoe EJ, Jr. Regulation of human neutrophil chemotaxis by intracellular pH. J Biol Chem 261: 6492-6500, 1986[Abstract/Free Full Text].

96.   Small, JV, Herzog M, and Anderson K. Actin filament organization in the keratocyte lamellipodium. J Cell Biol 129: 1275-1286, 1995[Abstract].

97.   Soroceanu, L, Manning TJ, and Sontheimer H. Modulation of glioma cell migration and invasion using Cl- and K+ ion channel blockers. J Neurosci 19: 5942-5954, 1999[Abstract/Free Full Text].

98.   Sponsel, HT, Breckon R, and Anderson RJ. Adenine nucleotide and protein kinase C regulation of renal tubular epithelial cell wound healing. Kidney Int 48: 85-92, 1995[ISI][Medline].

99.   Stuart-Tilley, A, Sardet C, Pouysségur JD, Schwartz MA, Brown D, and Alper SL. Immunolocalization of anion exchanger AE2 and cation exchanger NHE1 in distinct, adjacent cells of gastric mucosa. Am J Physiol Cell Physiol 266: C559-C568, 1994[Abstract/Free Full Text].

100.   Tominaga, T, and Barber DL. Na-H exchange acts downstream of RhoA to regulate integrin-induced cell adhesion and spreading. Mol Biol Cell 9: 2287-2303, 1998[Abstract/Free Full Text].

101.   Vexler, ZS, Symons M, and Barber DL. Activation of Na+-H+ exchange is necessary for RhoA-induced stress fiber formation. J Biol Chem 271: 22281-22284, 1996[Abstract/Free Full Text].

102.   Wakabayashi, S, Shigekawa M, and Pouyssegur J. Molecular physiology of vertebrate Na+/H+ exchangers. Physiol Rev 77: 51-77, 1997[Abstract/Free Full Text].

103.   Wang, YL. Exchange of actin subunits at the leading edge of living fibroblasts: possible role of treadmilling. J Cell Biol 101: 597-602, 1985[Abstract].

104.   Weinhold, D, Schuricht B, Wulf A, and Schwab A. Distribution of an apical potassium channel, ROMK2, in migrating renal epithelial cells (Abstract). Pflügers Arch 439: R381, 2000.

105.   Weinman, EJ, Minkoff C, and Shenolikar S. Signal complex regulation of renal transport proteins: NHERF and regulation of NHE3 by PKA. Am J Physiol Renal Physiol 279: F393-F399, 2000[Abstract/Free Full Text].

106.   Winckler, B, Forscher P, and Mellman I. A diffusion barrier maintains distribution of membrane proteins in polarized neurons. Nature 397: 698-701, 1999[ISI][Medline].

107.   Winckler, B, and Mellman I. Neuronal polarity: controlling the sorting and diffusion of membrane components. Neuron 23: 637-640, 1999[ISI][Medline].

108.   Xu, JZ, Hall AE, Peterson LN, Bienkowski MJ, Eessalu TE, and Hebert SC. Localization of the ROMK protein on apical membranes of rat kidney nephron segments. Am J Physiol Renal Physiol 273: F739-F748, 1997[ISI][Medline].

109.   Yoshimori, T, Keller P, Roth MG, and Simons K. Different biosynthetic transport routes to the plasma membrane in BHK and CHO cells. J Cell Biol 133: 247-256, 1996[Abstract].

110.   Zahm, JM, Kaplan H, Hérard AL, Doriot F, Pierrot D, Somelette P, and Puchelle E. Cell migration and proliferation during the in vitro wound repair of the respiratory epithelium. Cell Motil Cyoskeleton 37: 33-43, 1997[ISI][Medline].


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