NHE3 kinase A regulatory protein E3KARP binds the epithelial brush border Na+/H+ exchanger NHE3 and the cytoskeletal protein ezrin

Structure of the ERM protein moesin reveals the FERM domain fold masked by an extended actin binding tail domain

The ezrin-radixin-moesin (ERM) protein family link actin filaments of cell surface structures to the plasma membrane, using a C-terminal F-actin binding segment and an N-terminal FERM domain, a common membrane binding module. ERM proteins are regulated by an intramolecular association of the FERM and C-terminal tail domains that masks their binding sites. The crystal structure of a dormant moesin FERM/tail complex reveals that the FERM domain has three compact lobes including an integrated PTB/PH/ EVH1 fold, with the C-terminal segment bound as an extended peptide masking a large surface of the FERM domain. This extended binding mode suggests a novel mechanism for how different signals could produce varying levels of activation. Sequence conservation suggests a similar regulation of the tumor suppressor merlin.

Molecular mechanisms of sodium transport inhibition in proximal tubule during acute hypertension

Acute hypertension provokes a rapid decrease in proximal tubule salt and water reabsorption that increases the levels of sodium chloride at the macula densa, the error signal to increase arteriolar resistance to autoregulate renal blood flow and glomerular filtration rate, and contributes to pressure natriuresis. The molecular mechanisms responsible for this critical homeostatic adjustment are beginning to be dissected: apical sodium transporters in the proximal tubule are redistributed out of the brush border to intermicrovillar and endosomal stores and sodium pump activity is inhibited. These responses are strikingly similar to the cellular responses to parathyroid hormone, and are mediated by similar signalling pathways.

Actin filament organization is required for proper cAMP-dependent activation of CFTR

Previous studies have indicated a role of the actin cytoskeleton in the regulation of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel. However, the exact molecular nature of this regulation is still largely unknown. In this report human epithelial CFTR was expressed in human melanoma cells genetically devoid of the filamin homologue actin-cross-linking protein ABP-280 [ABP(-)]. cAMP stimulation of ABP(-) cells or cells genetically rescued with ABP-280 cDNA [ABP(+)] was without effect on whole cell Cl(-) currents. In ABP(-) cells expressing CFTR, cAMP was also without effect on Cl(-) conductance. In contrast, cAMP induced a 10-fold increase in the diphenylamine-2-carboxylate (DPC)-sensitive whole cell Cl(-) currents of ABP(+)/CFTR(+) cells. Further, in cells expressing both CFTR and a truncated form of ABP-280 unable to cross-link actin filaments, cAMP was also without effect on CFTR activation. Dialysis of ABP-280 or filamin through the patch pipette, however, resulted in a DPC-inhibitable increase in the whole cell currents of ABP(-)/CFTR(+) cells. At the single-channel level, protein kinase A plus ATP activated single Cl(-) channels only in excised patches from ABP(+)/CFTR(+) cells. Furthermore, filamin alone also induced Cl(-) channel activity in excised patches of ABP(-)/CFTR(+) cells. The present data indicate that an organized actin cytoskeleton is required for cAMP-dependent activation of CFTR.

Interdomain interaction of merlin isoforms and its influence on intermolecular binding to NHE-RF

Merlin, the neurofibromatosis 2 tumor suppressor protein, has two major isoforms with alternate C termini and is related to the ERM (ezrin, radixin, moesin) proteins. Regulation of the ERMs involves intramolecular and/or intermolecular head-to-tail associations between family members. We have determined whether merlin undergoes similar interactions, and our findings indicate that the C terminus of merlin isoform 1 is able to associate with its N-terminal domain in a head-to-tail fashion. However, the C terminus of isoform 2 lacks this property. Similarly, the N terminus of merlin can also associate with C terminus of moesin. We have also explored the effect of merlin self-association on binding to the regulatory cofactor of Na(+)-H(+) exchanger (NHE-RF), an interacting protein for merlin and the ERMs. Merlin isoform 2 captures more NHE-RF than merlin isoform 1 in affinity binding assays, suggesting that in full-length merlin isoform 1, the NHE-RF binding site is masked because of the self-interactions of merlin. Treatment with a phospholipid known to decrease self-association of ERMs enhances the binding of merlin isoform 1 to NHE-RF. Thus, although isoform 1 resembles the ERM proteins, which transition between inactive (closed) and active (open) states, isoform 2 is distinct, existing only in the active (open) state and presumably constitutively more available for interaction with other protein partners.

Yes-associated protein 65 localizes p62(c-Yes) to the apical compartment of airway epithelia by association with EBP50

We recently showed that the COOH terminus of the cystic fibrosis transmembrane conductance regulator associates with the submembranous scaffolding protein EBP50 (ERM-binding phosphoprotein 50 kD; also called Na(+)/H(+) exchanger regulatory factor). Since EBP50 associates with ezrin, this interaction links the cystic fibrosis transmembrane conductance regulator (CFTR) to the cortical actin cytoskeleton. EBP50 has two PDZ domains, and CFTR binds with high affinity to the first PDZ domain. Here, we report that Yes-associated protein 65 (YAP65) binds with high affinity to the second EBP50 PDZ domain. YAP65 is concentrated at the apical membrane in airway epithelia and interacts with EBP50 in cells. The COOH terminus of YAP65 is necessary and sufficient to mediate association with EBP50. The EBP50-YAP65 interaction is involved in the compartmentalization of YAP65 at the apical membrane since mutant YAP65 proteins lacking the EBP50 interaction motif are mislocalized when expressed in airway epithelial cells. In addition, we show that the nonreceptor tyrosine kinase c-Yes is contained within EBP50 protein complexes by association with YAP65. Subapical EBP50 protein complexes, containing the nonreceptor tyrosine kinase c-Yes, may regulate apical signal transduction pathways leading to changes in ion transport, cytoskeletal organization, or gene expression in epithelial cells.

The apical Na(+)/H(+) exchanger isoform NHE3 is regulated by the actin cytoskeleton

The epithelial isoform of the Na(+)/H(+) exchanger, NHE3, associates with at least two related regulatory factors called NHERF1/EBP50 and NHERF2/TKA-1/E3KARP. These factors in addition interact with the cytoskeletal protein ezrin, which in turn binds to actin. The possible linkage of NHE3 with the cytoskeleton prompted us to test the effect of actin-modifying agents on NHE3 activity. Cytochalasins B and D and latrunculin B, which interfere with actin polymerization, induced a profound inhibition of NHE3 activity. The effect was isoform-specific inasmuch as the "housekeeping" exchanger NHE1 was virtually unaffected. Cytoskeletal disorganization was associated with a subcellular redistribution of NHE3, which accumulated at sites where actin aggregated, suggesting a physical interaction of exchangers with the cytoskeleton. An interaction was further suggested by the co-sedimentation of a detergent-insoluble fraction of NHE3 with the actin cytoskeleton. Inhibition of transport was not due to diminution in the number of transporters at the plasmalemma. Functional analyses of NHE1/NHE3 chimeras revealed that the cytoplasmic domain of NHE3 conferred sensitivity to cytochalasin B. Progressive carboxyl-terminal and internal deletions of the cytoplasmic region of NHE3 indicated that the region between residues 650 and 684 is critical for this response. This region overlaps with the domain reported to interact with NHERF and also contains a putative ezrin-binding site; hence, it likely plays a role in interactions with the cytoskeleton.

Assembly of signaling complexes by the sodium-hydrogen exchanger regulatory factor family of PDZ-containing proteins

The sodium-hydrogen exchanger regulatory factor (NHERF) was first identified as an essential cofactor for cyclic AMP-mediated inhibition of the epithelial isoform of rabbit kidney sodium-hydrogen exchanger (NHE3). More recent work shows that NHERF constitutes a family of PSD-95/DIg/ZO-1 (PDZ) domain-containing adapter proteins, only some of which associate with the NHE3 antiporter. Other targets of the NHERF proteins include members of the ezrin-radixin-moesin family of cytoskeletal proteins. In the current model for NHE3 regulation, NHERF links NHE3 to the protein kinase A-anchoring protein, ezrin, and thereby facilitates its phosphorylation and inhibition by protein kinase A. Recent studies have also established the interaction of NHERF and its homologs with the beta2-adrenergic receptor and the platelet-derived growth factor receptor tyrosine kinase that facilitates signal transduction by these receptors. Association with NHERF may also regulate the cystic fibrosis transmembrane conductance regulator and the sodium-bicarbonate transporter. With the rapid increase in the intracellular targets identified for NHERF, the emerging data point to a broad role for these PDZ-containing proteins in the organization of signaling complexes and control of cell physiology.

Expression level, subcellular distribution and rho-GDI binding affinity of merlin in comparison with Ezrin/Radixin/Moesin proteins

Merlin, a neurofibromatosis type-2 tumor suppressor, shows significant sequence similarity to ERM (Ezrin/Radixin/Moesin) proteins, general actin filament/plasma membrane cross-linkers, which are regulated in a Rho-dependent manner. To understand its physiological functions, we compared merlin with ERM proteins in vivo and in vitro. Quantitative immunoblotting revealed that the molar ratio of merlin/ERM in cultured epithelial or non-epithelial cells was approximately 0.14 or approximately 0.05, respectively. After centrifugation of cell homogenate, merlin was mostly recovered in the insoluble fraction, whereas almost half of ERM proteins were found in the soluble fraction. Merlin and ERM proteins were concentrated at microvilli when introduced into fibroblasts. In contrast, in epithelial cells, introduced merlin was co-distributed with E-cadherin in lateral membranes, whereas ERM proteins were concentrated in apical microvilli. Finally, we examined the binding affinity of merlin to Rho GDP dissociation inhibitor (Rho-GDI), to which N-terminal halves of ERM proteins but not the full-length molecules specifically bind. In vitro binding assays revealed that the N-terminal halves of merlin isoform-I and -II as well as full-length merlin isoform-II bound to Rho-GDI with similar binding affinity to ERM proteins. Immunoprecipitation confirmed these findings in vivo. These findings do not favor the notion that merlin functions simply in a redundant or competitive manner to ERM proteins.

G protein-coupled receptor kinase 6A phosphorylates the Na(+)/H(+) exchanger regulatory factor via a PDZ domain-mediated interaction

The Na(+)/H(+) exchanger regulatory factor (NHERF) is constitutively phosphorylated in cells, but the site(s) of this phosphorylation and the kinase(s) responsible for it have not been identified. We show here that the primary site of constitutive NHERF phosphorylation in human embryonic kidney 293 (HEK-293) cells is Ser(289), and that the stoichiometry of phosphorylation is near 1 mol/mol. NHERF contains two PDZ domains that recognize the sequence S/T-X-L at the carboxyl terminus of target proteins, and thus we examined the possibility that kinases containing this motif might associate with and phosphorylate NHERF. Overlay experiments and co-immunoprecipitation studies revealed that NHERF binds with high affinity to a splice variant of the G protein-coupled receptor kinase 6, GRK6A, which terminates in the motif T-R-L. NHERF does not associate with GRK6B or GRK6C, alternatively spliced variants that differ from GRK6A at their extreme carboxyl termini. GRK6A phosphorylates NHERF efficiently on Ser(289) in vitro, whereas GRK6B, GRK6C, and GRK2 do not. Furthermore, the endogenous "NHERF kinase" activity in HEK-293 cell lysates is sensitive to treatments that alter the activity of GRK6A. These data suggest that GRK6A phosphorylates NHERF via a PDZ domain-mediated interaction and that GRK6A is the kinase in HEK-293 cells responsible for the constitutive phosphorylation of NHERF.

Basolateral Na(+)/HCO(3)(-) cotransport activity is regulated by the dissociable Na(+)/H(+) exchanger regulatory factor

In the renal proximal tubule, the activities of the basolateral Na(+)/HCO(3)(-) cotransporter (NBC) and the apical Na(+)/H(+) exchanger (NHE3) uniformly vary in parallel, suggesting that they are coordinately regulated. PKA-mediated inhibition of NHE3 is mediated by a PDZ motif-containing protein, the Na(+)/H(+) exchanger regulatory factor (NHE-RF). Given the common inhibition of these transporters after protein kinase A (PKA) activation, we sought to determine whether NHE-RF also plays a role in PKA-regulated NBC activity. Renal cortex immunoblot analysis using anti-peptide antibodies directed against rabbit NHE-RF demonstrated the presence of this regulatory factor in both brush-border membranes (BBMs) and basolateral membranes (BLMs). Using a reconstitution assay, we found that limited trypsin digestion of detergent solubilized rabbit renal BLM preparations resulted in NBC activity that was unaffected by PKA activation. Co-reconstitution of these trypsinized preparations with a recombinant protein corresponding to wild-type rabbit NHE-RF restored the inhibitory effect of PKA on NBC activity in a concentration-dependent manner. NBC activity was inhibited 60% by 10(-8)M NHE-RF; this effect was not observed in the absence of PKA. Reconstitution with heat-denatured NHE-RF also failed to attenuate NBC activity. To establish further a physiologic role for NHE-RF in NBC regulation, the renal epithelial cell line B-SC-1, which lacks detectable endogenous NHE-RF expression, was engineered to express stably an NHE-RF transgene. NHE-RF-expressing B-SC-1 cells (B-SC-RF) exhibited markedly lower basal levels of NBC activity than did wild-type controls. Inhibition of NBC activity in B-SC-RF cells was enhanced after 10 microM of forskolin treatment, consistent with a postulated role for NHE-RF in mediating the inhibition of NBC activity by PKA. These findings not only suggest NHE-RF involvement in PKA-regulated NBC activity, but also provide a unique molecular mechanism whereby basolateral NBC and apical NHE3 activities may be coordinately regulated in renal proximal tubule cells.

Effects of pH on kinetic parameters of the Na-HCO3 cotransporter in renal proximal tubule

The effects of pH on cotransporter kinetics were studied in renal proximal tubule cells. Cells were grown to confluence on permeable support, mounted in an Ussing-type chamber, and permeabilized apically to small monovalent ions with amphotericin B. The steady-state, dinitrostilbene-disulfonate-sensitive current (DeltaI) was Na+ and HCO3- dependent and therefore was taken as flux through the cotransporter. When the pH of the perfusing solution was changed between 6.0 and 8.0, the conductance attributable to the cotransporter showed a maximum between pH 7.25 and pH 7.50. A similar profile was observed in the presence of a pH gradient when the pH of the apical solutions was varied between 7.0 and 8.0 (basal pH lower by 1), but not when the pH of the basal solution was varied between 7.0 and 8.0 (apical pH lower by 1 unit). To delineate the kinetic basis for these observations, DeltaI-voltage curves were obtained as a function of Na+ and HCO3- concentrations and analyzed on the basis of a kinetic cotransporter model. Increases in pH from 7.0 to 8.0 decreased the binding constants for the intracellular and extracellular substrates by a factor of 2. Furthermore, the electrical parameters that describe the interaction strength between the electric field and substrate binding or charge on the unloaded transporter increased by four- to fivefold. These data can be explained by a channel-like structure of the cotransporter, whose configuration is modified by intracellular pH such that, with increasing pH, binding of substrate to the carrier is sterically hindered but electrically facilitated.

ERM proteins in cell adhesion and membrane dynamics

Ezrin, radixin and moesin, collectively known as the ERM proteins, are a group of closely related membrane-cytoskeleton linkers that regulate cell adhesion and cortical morphogenesis. ERM proteins can self-associate through intra- and inter-molecular interactions, and these interactions mask several binding sites on the proteins. ERM activation involves unfolding of the molecule, and allows the protein to bind to plasma membrane components either directly, or indirectly through linker proteins. The discovery that the tumour-suppressor NF2, also known as merlin/schwannomin, is related to ERM proteins has added a new impetus to investigations of their roles. This review discusses current understanding of the structure and function of members of the ERM family of proteins.

Dual mechanisms of regulation of Na/H exchanger NHE-3 by parathyroid hormone in rat kidney

Parathyroid hormone (PTH) is a potent inhibitor of mammalian renal proximal tubule sodium absorption via suppression of the apical membrane Na/H exchanger (NHE-3). We examined the mechanisms by which PTH inhibits NHE-3 activity by giving an acute intravenous PTH bolus to parathyroidectomized rats. Parathyroidectomy per se increased apical membrane NHE-3 activity and antigen. Acute infusion of PTH caused a time-dependent decrease in NHE-3 activity as early as 30 min. Decrease in NHE-3 activity at 30 and 60 min was accompanied by increased NHE-3 phosphorylation. In contrast to the rapid changes in NHE-3 activity and phosphorylation, decrease in apical membrane NHE-3 antigen was not detectable until 4-12 h after the PTH bolus. The decrease in apical membrane NHE-3 occurred in the absence of changes in total renal cortical NHE-3 antigen. Pretreatment of the animals with the microtubule-disrupting agent colchicine blocked the PTH-induced decrease in apical NHE-3 antigen. We propose that PTH acutely cause a decrease in NHE-3 intrinsic transport activity possibly via a phosphorylation-dependent mechanism followed by a decrease in apical membrane NHE-3 antigen via changes in protein trafficking.

Merlin: the neurofibromatosis 2 tumor suppressor

In recent years, it has become clear that the ERMs occupy a crucial position as protein linkers that both respond to and participate in reorganization of membrane-cytoskeletal interactions. With the identification of new binding partners, the ERMs are also implicated in linked regulation of the activities of particular membrane proteins. Thus, they reside at a junction in a complex web of interactions that must respond to stimuli from both outside and inside the cell. As expected from its structural motifs, merlin behaves in a manner similar to the ERM proteins, but with some notable differences. Chief among these is the absence of intramolecular interaction to mask intermolecular interaction domains in isoform 2. The full range of merlin's intermolecular interactions remains to be delineated, but it can be expected from the comparison to ERMs that merlin also sits within a web of interactions that may involve multiple partners and signaling pathways, some of which it shares with the ERMs. Defining merlin's tumor suppressor function will likely require identifying those differences that are peculiarly important in the target cell types of NF2. However, the fact that inactivation of merlin in the mouse by targeted mutagenesis produces a variety of malignant tumors with a high rate of metastasis [33] suggests that merlin's suppression of tumor formation may involve different partners and pathways in different cell types and genetic backgrounds. Consequently, the disruptions due to merlin inactivation in the progression of malignant mesothelioma may represent a tumor suppressor role operating by a different pathway than that in schwannoma or meningioma.

Regulation of cortical structure by the ezrin-radixin-moesin protein family

Molecules involved in ERM (ezrin-radixin-moesin) based attachment of membrane proteins to the cortical cytoskeleton in cell surface structures have been identified. In lymphocytes, a direct interaction is seen with extracellular matrix receptors and intercellular adhesion molecules. In polarized epithelial cells, an adaptor molecule named EBP50 provides a bridge between the amino-terminal domain of ezrin and the cytoplasmic regions of plasma membrane proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR) and the beta2 adrenergic receptor. ERM proteins are conformationally regulated - binding sites for EBP50 and F actin are masked in the dormant molecules and activation leads to exposure of these sites. The mechanism of activation, however, remains to be fully elucidated. ERM proteins also play a role in the Rho and Rac signaling pathways: activated ERM proteins can dissociate Rho-GDI (GDP dissociation inhibitor) from Rho and thereby activate Rho-dependent pathways.

C-terminal threonine phosphorylation activates ERM proteins to link the cell's cortical lipid bilayer to the cytoskeleton

The plasma membrane consists of a lipid bilayer with integral membrane proteins stabilized by regulated linkages to the cortical actin cytoskeleton. The regulation is necessary for cells to change shape ormigrate. The ERM (ezrin-radixin-moesin) proteins are believed to provide such links, with the N-terminal halves associating with integral membrane proteins, either directly or indirectly through adapter molecules like EBP50 (ERM binding phosphoprotein, 50 kDa), and their C-terminal halves associating with F-actin. However, isolated ERM proteins largely exist in a dormant state by virtue of an intramolecular interaction between amino- and carboxyl-terminal domains, thereby masking membrane and cytoskeletal association sites. C-terminal threonine phosphorylation of a fragment of radixin has been found to destroy its ability to bind the amino-terminal domain without affecting the C-terminal F-actin binding site. Here we show that C-terminal phosphorylation of full-length, dormant ezrin and moesin by protein kinase C-theta simultaneously unmasks both the F-actin and EBP50 binding sites. Increased phosphorylation of moesin in cells correlated with increased association of moesin with the cortical actin cytoskeleton. These results show that activation of ERM proteins can be accomplished by phosphorylation of a single C-terminal threonine residue.

The role of NHERF and E3KARP in the cAMP-mediated inhibition of NHE3

NHE3 is the apically located Na+/H+ exchanger in the gut and in the renal proximal tubule. Acute inhibition of this transporter by cAMP requires the presence of either of two NHE3-associated proteins, NHERF or E3KARP. It has been suggested that these proteins either directly regulate NHE3 activity after being phosphorylated by protein kinase A (PKA) or that they may serve as adapters that localize PKA near NHE3. We studied the role of NHERF and E3KARP in opossum kidney cells, which endogenously express NHE3, NHERF, and ezrin and display cAMP-dependent inhibition of NHE3. In vivo phosphorylation studies showed that NHERF is a phosphoprotein under basal conditions, but does not change its phosphorylation state after 8-bromo-cAMP treatment, and that E3KARP is not phosphorylated at all. Co-immunoprecipitation showed that NHERF and E3KARP bind both NHE3 and ezrin. Using cAMP analogs it was demonstrated that NHE3 activity, measured as sodium-dependent recovery of the intracellular pH after intracellular acidification, is inhibited by PKA type II. Because others have shown that ezrin binds PKA type II and that NHE3 is phosphorylated by PKA we suggest that NHERF and E3KARP are adapters that link NHE3 to ezrin, thereby localizing PKA near NHE3 to allow NHE3 phosphorylation.