The scaffolding protein EBP50 regulates microvillar assembly in a phosphorylation-dependent manner

The molecular sociology of NHERF1 PDZ proteins controlling renal hormone-regulated phosphate transport

Parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) control extracellular phosphate levels by regulating renal NPT2A-mediated phosphate transport by a process requiring the PDZ scaffold protein NHERF1. NHERF1 possesses two PDZ domains, PDZ1 and PDZ2, with identical core-binding GYGF motifs explicitly recognizing distinct binding partners that play different and specific roles in hormone-regulated phosphate transport. The interaction of PDZ1 and the carboxy-terminal PDZ-binding motif of NPT2A (C-TRL) is required for basal phosphate transport. PDZ2 is a regulatory domain that scaffolds multiple biological targets, including kinases and phosphatases involved in FGF23 and PTH signaling. FGF23 and PTH trigger disassembly of the NHERF1-NPT2A complex through reversible hormone-stimulated phosphorylation with ensuing NPT2A sequestration, down-regulation, and cessation of phosphate absorption. In the absence of NHERF1-NPT2A interaction, inhibition of FGF23 or PTH signaling results in disordered phosphate homeostasis and phosphate wasting. Additional studies are crucial to elucidate how NHERF1 spatiotemporally coordinates cellular partners to regulate extracellular phosphate levels.

EBP50 is a key molecule for the Schwann cell-axon interaction in peripheral nerves

Peripheral nerve injury disrupts the Schwann cell-axon interaction and the cellular communication between them. The peripheral nervous system has immense potential for regeneration extensively due to the innate plastic potential of Schwann cells (SCs) that allows SCs to interact with the injured axons and exert specific repair functions essential for peripheral nerve regeneration. In this study, we show that EBP50 is essential for the repair function of SCs and regeneration following nerve injury. The increased expression of EBP50 in the injured sciatic nerve of control mice suggested a significant role in regeneration. The ablation of EBP50 in mice resulted in delayed nerve repair, recovery of behavioral function, and remyelination following nerve injury. EBP50 deficiency led to deficits in SC functions, including proliferation, migration, cytoskeleton dynamics, and axon interactions. The adeno-associated virus (AAV)-mediated local expression of EBP50 improved SCs migration, functional recovery, and remyelination. ErbB2-related proteins were not differentially expressed in EBP50-deficient sciatic nerves following injury. EBP50 binds and stabilizes ErbB2 and activates the repair functions to promote regeneration. Thus, we identified EBP50 as a potent SC protein that can enhance the regeneration and functional recovery driven by NRG1-ErbB2 signaling, as well as a novel regeneration modulator capable of potential therapeutic effects.

MCC is a centrosomal protein that relocalizes to non-centrosomal apical sites during intestinal cell differentiation

The gene mutated in colorectal cancer (MCC) encodes a coiled-coil protein implicated, as its name suggests, in the pathogenesis of hereditary human colon cancer. To date, however, the contributions of MCC to intestinal homeostasis and disease remain unclear. Here, we examine the subcellular localization of MCC, both at the mRNA and protein levels, in the adult intestinal epithelium. Our findings reveal that Mcc transcripts are restricted to proliferating crypt cells, including Lgr5+ stem cells, where the Mcc protein is distinctly associated with the centrosome. Upon intestinal cellular differentiation, Mcc is redeployed to the apical domain of polarized villus cells where non-centrosomal microtubule organizing centers (ncMTOCs) are positioned. Using intestinal organoids, we show that the shuttling of the Mcc protein depends on phosphorylation by casein kinases 1δ and ε, which are critical modulators of WNT signaling. Together, our findings support a role for MCC in establishing and maintaining the cellular architecture of the intestinal epithelium as a component of both the centrosome and ncMTOC.

Intestinal brush border formation requires a TMIGD1-based intermicrovillar adhesion complex

Intestinal epithelial cells absorb nutrients through the brush border, composed of dense arrays of highly ordered microvilli at their apical membranes. A protocadherin-based intermicrovillar adhesion complex localized at microvilli tips mediates microvilli packing and organization. Here, we identified a second adhesion complex localized at the proximal base region of microvilli. This complex contained the immunoglobulin superfamily member TMIGD1, which directly interacted with the microvillar scaffolding proteins EBP50 and E3KARP. Complex formation with EBP50 required the activation of EBP50 by the actin-binding protein ezrin and was enhanced by the dephosphorylation of Ser¹⁶² in the PDZ2 domain of EBP50 by the phosphatase PP1α. Binding of the EBP50-ezrin complex to TMIGD1 enhanced the dynamic turnover of EBP50 at microvilli. Enterocyte-specific inactivation of Tmigd1 in mice resulted in microvillar blebbing, loss of intermicrovillar adhesion, and perturbed brush border formation. Thus, we identified a second adhesion complex in microvilli and propose a mechanism that promotes microvillar formation and dynamics.

ERM-1 Phosphorylation and NRFL-1 Redundantly Control Lumen Formation in the C. elegans Intestine

Reorganization of the plasma membrane and underlying actin cytoskeleton into specialized domains is essential for the functioning of most polarized cells in animals. Proteins of the ezrin-radixin-moesin (ERM) and Na⁺/H⁺ exchanger 3 regulating factor (NHERF) family are conserved regulators of cortical specialization. ERM proteins function as membrane-actin linkers and as molecular scaffolds that organize the distribution of proteins at the membrane. NHERF proteins are PDZ-domain containing adapters that can bind to ERM proteins and extend their scaffolding capability. Here, we investigate how ERM and NHERF proteins function in regulating intestinal lumen formation in the nematode Caenorhabditis elegans. C. elegans has single ERM and NHERF family proteins, termed ERM-1 and NRFL-1, and ERM-1 was previously shown to be critical for intestinal lumen formation. Using CRISPR/Cas9-generated nrfl-1 alleles we demonstrate that NRFL-1 localizes at the intestinal microvilli, and that this localization is depended on an interaction with ERM-1. However, nrfl-1 loss of function mutants are viable and do not show defects in intestinal development. Interestingly, combining nrfl-1 loss with erm-1 mutants that either block or mimic phosphorylation of a regulatory C-terminal threonine causes severe defects in intestinal lumen formation. These defects are not observed in the phosphorylation mutants alone, and resemble the effects of strong erm-1 loss of function. The loss of NRFL-1 did not affect the localization or activity of ERM-1. Together, these data indicate that ERM-1 and NRFL-1 function together in intestinal lumen formation in C. elegans. We postulate that the functioning of ERM-1 in this tissue involves actin-binding activities that are regulated by the C-terminal threonine residue and the organization of apical domain composition through NRFL-1.

Cotranslational interaction of human EBP50 and ezrin overcomes masked binding site during complex assembly

Multiprotein assemblages are the intracellular workhorses of many physiological processes. Assembly of constituents into complexes can be driven by stochastic, domain-dependent, posttranslational events in which mature, folded proteins specifically interact. However, inaccessibility of interacting surfaces in mature proteins (e.g., due to "buried" domains) can obstruct complex formation. Mechanisms by which multiprotein complex constituents overcome topological impediments remain enigmatic. For example, the heterodimeric complex formed by EBP50 and ezrin must address this issue as the EBP50-interacting domain in ezrin is obstructed by a self-interaction that occupies the EBP50 binding site. Here, we show that the EBP50-ezrin complex is formed by a cotranslational mechanism in which the C terminus of mature, fully formed EBP50 binds the emerging, ribosome-bound N-terminal FERM domain of ezrin during EZR mRNA translation. Consistent with this observation, a C-terminal EBP50 peptide mimetic reduces the cotranslational interaction and abrogates EBP50-ezrin complex formation. Phosphorylation of EBP50 at Ser³³⁹ and Ser³⁴⁰ abrogates the cotranslational interaction and inhibits complex formation. In summary, we show that the function of eukaryotic mRNA translation extends beyond "simple" generation of a linear peptide chain that folds into a tertiary structure, potentially for subsequent complex assembly; importantly, translation can facilitate interactions with sterically inaccessible domains to form functional multiprotein complexes.

The RabGAPs EPI64A and EPI64B regulate the apical structure of epithelial cells†

Here we report on the related TBC/RabGAPs EPI64A and EPI64B and show that they function to organize the apical aspect of epithelial cells. EPI64A binds the scaffolding protein EBP50/NHERF1, which itself binds active ezrin in epithelial cell microvilli. Epithelial cells additionally express EPI64B that also localizes to microvilli. However, EPI64B does not bind EBP50 and both proteins are shown to have a microvillar localization domain that spans the RabGAP domains. CRISPR/Cas9 was used to inactivate expression of each protein individually or both in Jeg-3 and Caco2 cells. In Jeg-3 cells, loss of EPI64B resulted in a reduction of apical microvilli, and a further reduction was seen in the double knockout, mostly likely due to misregulation of Rab8 and Rab35. In addition, apical junctions were partially disrupted in cells lacking EPI64A and accentuated in the double knockout. In Caco2 loss of EPI64B resulted in wavy junctions, whereas loss of both EPI64A and EPI64B had a severe phenotype often resulting in cells with a stellate apical morphology. In the knockout cells, the basal region of the cell remained unchanged, so EPI64A and EPI64B specifically localize to and regulate the morphology of the apical domain of polarized epithelial cells.

Effector-mediated ERM activation locally inhibits RhoA activity to shape the apical cell domain

Activated ezrin-radixin-moesin (ERM) proteins link the plasma membrane to the actin cytoskeleton to generate apical structures, including microvilli. Among many kinases implicated in ERM activation are the homologues LOK and SLK. CRISPR/Cas9 was used to knock out all ERM proteins or LOK/SLK in human cells. LOK/SLK knockout eliminates all ERM-activating phosphorylation. The apical domains of cells lacking LOK/SLK or ERMs are strikingly similar and selectively altered, with loss of microvilli and with junctional actin replaced by ectopic myosin-II–containing apical contractile structures. Constitutively active ezrin can reverse the phenotypes of either ERM or LOK/SLK knockouts, indicating that a central function of LOK/SLK is to activate ERMs. Both knockout lines have elevated active RhoA with concomitant enhanced myosin light chain phosphorylation, revealing that active ERMs are negative regulators of RhoA. As RhoA-GTP activates LOK/SLK to activate ERM proteins, the ability of active ERMs to negatively regulate RhoA-GTP represents a novel local feedback loop necessary for the proper apical morphology of epithelial cells.

Loss-of-function of EBP50 is a new cause of hereditary peripheral neuropathy: EBP50 functions in peripheral nerve system

Finding causative genetic mutations is important in the diagnosis and treatment of hereditary peripheral neuropathies. This study was conducted to find new genes involved in the pathophysiology of hereditary peripheral neuropathy. We identified a new mutation in the EBP50 gene, which is co-segregated with neuropathic phenotypes, including motor and sensory deficit in a family with Charcot-Marie-Tooth disease. EBP50 is known to be important for the formation of microvilli in epithelial cells, and the discovery of this gene mutation allowed us to study the function of EBP50 in the nervous system. EBP50 was strongly expressed in the nodal and paranodal regions of sciatic nerve fibers, where Schwann cell microvilli contact the axolemma, and at the growth tips of primary Schwann cells. In addition, EBP50 expression was decreased in mouse models of peripheral neuropathy. Knockout mice were used to study EBP50 function in the peripheral nervous system. Interestingly motor function deficit and abnormal histology of nerve fibers were observed in EBP50^+/- heterozygous mice at 12 months of age, but not 3 months. in vitro studies using Schwann cells showed that NRG1-induced AKT activation and migration were significantly reduced in cells overexpressing the I325V mutant of EBP50 or cells with knocked-down EBP50 expression. In conclusion, we show for the first time that loss of function due to EBP50 gene deficiency or mutation can cause peripheral neuropathy.

New roles of the Na+/H+ exchange regulatory factor 1 scaffolding protein: a review

Na⁺/H⁺ exchange regulatory factor 1 (NHERF1), a member of a PDZ scaffolding protein family, was first identified as an organizer of membrane-bound protein complexes composed of hormone receptors, signal transduction pathways, and electrolyte and mineral transporters and channels. NHERF1 is involved in the regulation of Na⁺/H⁺ exchanger 3, Na⁺-dependent phosphate transporter 2a, and Na⁺-K⁺-ATPase through its ability to scaffold these transporters to the plasma membrane, allowing regulation of these protein complexes with their associated hormone receptors. Recently, NHERF1 has received increased interest in its involvement in a variety of functions, including cell structure and trafficking, tumorigenesis and tumor behavior, inflammatory responses, and tissue injury. In this review, we highlight the evidence for the expansive role of NHERF1 in cell biology and speculate on the implications for renal physiology and pathophysiology.

NHERF1 Is Required for Localization of PMCA2 and Suppression of Early Involution in the Female Lactating Mammary Gland

Prior studies have demonstrated that the calcium pump, plasma membrane calcium ATPase 2 (PMCA2), mediates calcium transport into milk and prevents mammary epithelial cell death during lactation. PMCA2 also regulates cell proliferation and cell death in breast cancer cells, in part by maintaining the receptor tyrosine kinase ErbB2/HER2 within specialized plasma membrane domains. Furthermore, the regulation of PMCA2 membrane localization and activity in breast cancer cells requires its interaction with the PDZ domain-containing scaffolding molecule sodium-hydrogen exchanger regulatory factor (NHERF) 1. In this study, we asked whether NHERF1 also interacts with PMCA2 in normal mammary epithelial cells during lactation. Our results demonstrate that NHERF1 expression is upregulated during lactation and that it interacts with PMCA2 at the apical membrane of secretory luminal epithelial cells. Similar to PMCA2, NHERF1 expression is rapidly reduced by milk stasis after weaning. Examining lactating NHERF1 knockout (KO) mice showed that NHERF1 contributes to the proper apical location of PMCA2, for proper apical-basal polarity in luminal epithelial cells, and that it participates in the suppression of Stat3 activation and the prevention of premature mammary gland involution. Additionally, we found that PMCA2 also interacts with the closely related scaffolding molecule, NHERF2, at the apical membrane, which likely maintains PMCA2 at the plasma membrane of mammary epithelial cells in lactating NHERF1KO mice. Based on these data, we conclude that, during lactation, NHERF1 is required for the proper expression and apical localization of PMCA2, which, in turn, contributes to preventing the premature activation of Stat3 and the lysosome-mediated cell death pathway that usually occur only early in mammary involution.

Dynamic structure of the full-length scaffolding protein NHERF1 influences signaling complex assembly

The Na+/H+ exchange regulatory cofactor 1 (NHERF1) protein modulates the assembly and intracellular trafficking of several transmembrane G protein-coupled receptors (GPCRs) and ion transport proteins with the membrane-cytoskeleton adapter protein ezrin. Here, we applied solution NMR and small-angle neutron scattering (SANS) to structurally characterize full-length NHERF1 and disease-associated variants that are implicated in impaired phosphate homeostasis. Using NMR, we mapped the modular architecture of NHERF1, which is composed of two structurally-independent PDZ domains that are connected by a flexible, disordered linker. We observed that the ultra-long and disordered C-terminal tail of NHERF1 has a type 1 PDZ-binding motif that interacts weakly with the proximal, second PDZ domain to form a dynamically autoinhibited structure. Using ensemble-optimized analysis of SANS data, we extracted the molecular size distribution of structures from the extensive conformational space sampled by the flexible chain. Our results revealed that NHERF1 is a diffuse ensemble of variable PDZ domain configurations and a disordered C-terminal tail. The joint NMR/SANS data analyses of three disease variants (L110V, R153Q, and E225K) revealed significant differences in the local PDZ domain structures and in the global conformations compared with the WT protein. Furthermore, we show that the substitutions affect the affinity and kinetics of NHERF1 binding to ezrin and to a C-terminal peptide from G protein-coupled receptor kinase 6A (GRK6A). These findings provide important insight into the modulation of the intrinsic flexibility of NHERF1 by disease-associated point mutations that alter the dynamic assembly of signaling complexes.

Identification of PKCα-dependent phosphoproteins in mouse retina

Adjusting to a wide range of light intensities is an essential feature of retinal rod bipolar cell (RBC) function. While persuasive evidence suggests this modulation involves phosphorylation by protein kinase C-alpha (PKCα), the targets of PKCα phosphorylation in the retina have not been identified. PKCα activity and phosphorylation in RBCs was examined by immunofluorescence confocal microscopy using a conformation-specific PKCα antibody and antibodies to phosphorylated PKC motifs. PKCα activity was dependent on light and expression of TRPM1, and RBC dendrites were the primary sites of light-dependent phosphorylation. PKCα-dependent retinal phosphoproteins were identified using a phosphoproteomics approach to compare total protein and phosphopeptide abundance between phorbol ester-treated wild type and PKCα knockout (PKCα-KO) mouse retinas. Phosphopeptide mass spectrometry identified over 1100 phosphopeptides in mouse retina, with 12 displaying significantly greater phosphorylation in WT compared to PKCα-KO samples. The differentially phosphorylated proteins fall into the following functional groups: cytoskeleton/trafficking (4 proteins), ECM/adhesion (2 proteins), signaling (2 proteins), transcriptional regulation (3 proteins), and homeostasis/metabolism (1 protein). Two strongly differentially expressed phosphoproteins, BORG4 and TPBG, were localized to the synaptic layers of the retina, and may play a role in PKCα-dependent modulation of RBC physiology. Data are available via ProteomeXchange with identifier PXD012906. SIGNIFICANCE: Retinal rod bipolar cells (RBCs), the second-order neurons of the mammalian rod visual pathway, are able to modulate their sensitivity to remain functional across a wide range of light intensities, from starlight to daylight. Evidence suggests that this modulation requires the serine/threonine kinase, PKCα, though the specific mechanism by which PKCα modulates RBC physiology is unknown. This study examined PKCα phosophorylation patterns in mouse rod bipolar cells and then used a phosphoproteomics approach to identify PKCα-dependent phosphoproteins in the mouse retina. A small number of retinal proteins showed significant PKCα-dependent phosphorylation, including BORG4 and TPBG, suggesting a potential contribution to PKCα-dependent modulation of RBC physiology.

Parathyroid hormone initiates dynamic NHERF1 phosphorylation cycling and conformational changes that regulate NPT2A-dependent phosphate transport

Na⁺-H⁺ exchanger regulatory factor-1 (NHERF1) is a PDZ protein that scaffolds membrane proteins, including sodium-phosphate co-transport protein 2A (NPT2A) at the plasma membrane. NHERF1 is a phosphoprotein with 40 Ser and Thr residues. Here, using tandem MS analysis, we characterized the sites of parathyroid hormone (PTH)-induced NHERF1 phosphorylation and identified 10 high-confidence phosphorylation sites. Ala replacement at Ser⁴⁶, Ser¹⁶², Ser¹⁸¹, Ser²⁶⁹, Ser²⁸⁰, Ser²⁹¹, Thr²⁹³, Ser²⁹⁹, and Ser³⁰² did not affect phosphate uptake, but S290A substitution abolished PTH-dependent phosphate transport. Unexpectedly, Ser²⁹⁰ was rapidly dephosphorylated and rephosphorylated after PTH stimulation, and we found that protein phosphatase 1α (PP1α), which binds NHERF1 through a conserved VxF/W PP1 motif, dephosphorylates Ser²⁹⁰ Mutating ²⁵⁷VPF²⁵⁹ eliminated PP1 binding and blunted dephosphorylation. Tautomycetin blocked PP1 activity and abrogated PTH-sensitive phosphate transport. Using fluorescence lifetime imaging (FLIM), we observed that PTH paradoxically and transiently elevates intracellular phosphate. Added phosphate blocked PP1α-mediated Ser²⁹⁰ dephosphorylation of recombinant NHERF1. Hydrogen-deuterium exchange MS revealed that β-sheets in NHERF1's PDZ2 domain display lower deuterium uptake than those in the structurally similar PDZ1, implying that PDZ1 is more cloistered. Dephosphorylated NHERF1 exhibited faster exchange at C-terminal residues suggesting that NHERF1 dephosphorylation precedes Ser²⁹⁰ rephosphorylation. Our results show that PP1α and NHERF1 form a holoenzyme and that a multiprotein kinase cascade involving G protein-coupled receptor kinase 6A controls the Ser²⁹⁰ phosphorylation status of NHERF1 and regulates PTH-sensitive, NPT2A-mediated phosphate uptake. These findings reveal how reversible phosphorylation modifies protein conformation and function and the biochemical mechanisms underlying PTH control of phosphate transport.

Regulation of actin-based apical structures on epithelial cells

Cells of transporting epithelia are characterized by the presence of abundant F-actin-based microvilli on their apical surfaces. Likewise, auditory hair cells have highly reproducible rows of apical stereocilia (giant microvilli) that convert mechanical sound into an electrical signal. Analysis of mutations in deaf patients has highlighted the critical components of tip links between stereocilia, and related structures that contribute to the organization of microvilli on epithelial cells have been found. Ezrin/radixin/moesin (ERM) proteins, which are activated by phosphorylation, provide a critical link between the plasma membrane and underlying actin cytoskeleton in surface structures. Here, we outline recent insights into how microvilli and stereocilia are built, and the roles of tip links. Furthermore, we highlight how ezrin is locally regulated by phosphorylation, and that this is necessary to maintain polarity. Localized phosphorylation is achieved through an intricate coincidence detection mechanism that requires the membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and the apically localized ezrin kinase, lymphocyte-oriented kinase (LOK, also known as STK10) or Ste20-like kinase (SLK). We also discuss how ezrin-binding scaffolding proteins regulate microvilli and how, despite these significant advances, it remains to be discovered how the cell polarity program ultimately interfaces with these processes.

NHERF1 Between Promises and Hopes: Overview on Cancer and Prospective Openings

Na+/H+ exchanger regulatory factor 1 (NHERF1) is a scaffold protein, with two tandem PDZ domains and a carboxyl-terminal ezrin-binding (EB) region. This particular sticky structure is responsible for its interaction with different molecules to form multi-complexes that have a pivotal role in a lot of diseases. In particular, its involvement during carcinogenesis and cancer progression has been deeply analyzed in different tumors. The role of NHERF1 is not unique in cancer; its activity is connected to its subcellular localization. The literature data suggest that NHERF1 could be a new prognostic/predictive biomarker from breast cancer to hematological cancers. Furthermore, the high potential of this molecule as therapeutical target in different carcinomas is a new challenge for precision medicine. These evidences are part of a future view to improving patient clinical management, which should allow different tumor phenotypes to be treated with tailored therapies. This article reviews the biology of NHERF1, its engagement in different signal pathways and its involvement in different cancers, with a specific focus on breast cancer. It also considers NHERF1 potential role during inflammation related to most human cancers, designating new perspectives in the study of this "Janus-like" protein.

Dynamic relocalization of NHERF1 mediates chemotactic migration of ovarian cancer cells toward lysophosphatidic acid stimulation

NHERF1/EBP50 (Na⁺/H⁺ exchanger regulating factor 1; Ezrin-binding phosphoprotein of 50 kDa) organizes stable protein complexes beneath the apical membrane of polar epithelial cells. By contrast, in cancer cells without any fixed polarity, NHERF1 often localizes in the cytoplasm. The regulation of cytoplasmic NHERF1 and its role in cancer progression remain unclear. In this study, we found that, upon lysophosphatidic acid (LPA) stimulation, cytoplasmic NHERF1 rapidly translocated to the plasma membrane, and subsequently to cortical protrusion structures, of ovarian cancer cells. This movement depended on direct binding of NHERF1 to C-terminally phosphorylated ERM proteins (cpERMs). Moreover, NHERF1 depletion downregulated cpERMs and further impaired cpERM-dependent remodeling of the cell cortex, suggesting reciprocal regulation between these proteins. The LPA-induced protein complex was highly enriched in migratory pseudopodia, whose formation was impaired by overexpression of NHERF1 truncation mutants. Consistent with this, NHERF1 depletion in various types of cancer cells abolished chemotactic cell migration toward a LPA gradient. Taken together, our findings suggest that the high dynamics of cytosolic NHERF1 provide cancer cells with a means of controlling chemotactic migration. This capacity is likely to be essential for ovarian cancer progression in tumor microenvironments containing LPA.

Controllable Activation of Nanoscale Dynamics in a Disordered Protein Alters Binding Kinetics

The phosphorylation of specific residues in a flexible disordered activation loop yields precise control of signal transduction. One paradigm is the phosphorylation of S339/S340 in the intrinsically disordered tail of the multi-domain scaffolding protein NHERF1, which affects the intracellular localization and trafficking of NHERF1 assembled signaling complexes. Using neutron spin echo spectroscopy (NSE), we show salt-concentration-dependent excitation of nanoscale motion at the tip of the C-terminal tail in the phosphomimic S339D/S340D mutant. The "tip of the whip" that is unleashed is near the S339/S340 phosphorylation site and flanks the hydrophobic Ezrin-binding motif. The kinetic association rate constant of the binding of the S339D/S340D mutant to the FERM domain of Ezrin is sensitive to buffer salt concentration, correlating with the excited nanoscale dynamics. The results suggest that electrostatics modulates the activation of nanoscale dynamics of an intrinsically disordered protein, controlling the binding kinetics of signaling partners. NSE can pinpoint the nanoscale dynamics changes in a highly specific manner.

The scaffolding protein NHERF1 regulates the stability and activity of the tyrosine kinase HER2

We examined whether the scaffolding protein sodium-hydrogen exchanger regulatory factor 1 (NHERF1) interacts with the calcium pump PMCA2 and the tyrosine kinase receptor ErbB2/HER2 in normal mammary epithelial cells and breast cancer cells. NHERF1 interacts with the PDZ-binding motif in PMCA2 in both normal and malignant breast cells. NHERF1 expression is increased in HER2-positive breast cancers and correlates with HER2-positive status in human ductal carcinoma in situ (DCIS) lesions and invasive breast cancers as well as with increased mortality in patients. NHERF1 is part of a multiprotein complex that includes PMCA2, HSP90, and HER2 within specific actin-rich and lipid raft-rich membrane signaling domains. Knocking down NHERF1 reduces PMCA2 and HER2 expression, inhibits HER2 signaling, dissociates HER2 from HSP90, and causes the internalization, ubiquitination, and degradation of HER2. These results demonstrate that NHERF1 acts with PMCA2 to regulate HER2 signaling and membrane retention in breast cancers.

Role of the PDZ-scaffold protein NHERF1/EBP50 in cancer biology: from signaling regulation to clinical relevance

The transmission of cellular information requires fine and subtle regulation of proteins that need to interact in a coordinated and specific way to form efficient signaling networks. The spatial and temporal coordination relies on scaffold proteins. Thanks to protein interaction domains such as PDZ domains, scaffold proteins organize multiprotein complexes enabling the proper transmission of cellular information through intracellular networks. NHERF1/EBP50 is a PDZ-scaffold protein that was initially identified as an organizer and regulator of transporters and channels at the apical side of epithelia through actin-binding ezrin-moesin-radixin proteins. Since, NHERF1/EBP50 has emerged as a major regulator of cancer signaling network by assembling cancer-related proteins. The PDZ-scaffold EBP50 carries either anti-tumor or pro-tumor functions, two antinomic functions dictated by EBP50 expression or subcellular localization. The dual function of NHERF1/EBP50 encompasses the regulation of several major signaling pathways engaged in cancer, including the receptor tyrosine kinases PDGFR and EGFR, PI3K/PTEN/AKT and Wnt-β-catenin pathways.

Ras-activated RSK1 phosphorylates EBP50 to regulate its nuclear localization and promote cell proliferation

Differential subcellular localization of EBP50 leads to its controversial role in cancer biology either as a tumor suppressor when it resides at the membrane periphery, or a tumor facilitator at the nucleus. However, the mechanism behind nuclear localization of EBP50 remains unclear. A RNA interference screening identified the downstream effector of the Ras-ERK cascade, RSK1, as the molecule unique for nuclear transport of EBP50. RSK1 binds to EBP50 and phosphorylates it at a conserved threonine residue at position 156 (T156) under the regulation of growth factor. Mutagenesis experiments confirmed the significance of T156 residue in nuclear localization of EBP50, cellular proliferation, and oncogenic transformation. Our study sheds light on a possible therapeutic strategy targeting at this aberrant nuclear expression of EBP50 without affecting the normal physiological function of EBP50 at other subcellular localization.

A sequence upstream of canonical PDZ-binding motif within CFTR COOH-terminus enhances NHERF1 interaction

The development of cystic fibrosis transmembrane conductance regulator (CFTR) targeted therapy for cystic fibrosis has generated interest in maximizing membrane residence of mutant forms of CFTR by manipulating interactions with scaffold proteins, such as sodium/hydrogen exchange regulatory factor-1 (NHERF1). In this study, we explored whether COOH-terminal sequences in CFTR beyond the PDZ-binding motif influence its interaction with NHERF1. NHERF1 displayed minimal self-association in blot overlays (NHERF1, Kd = 1,382 ± 61.1 nM) at concentrations well above physiological levels, estimated at 240 nM from RNA-sequencing and 260 nM by liquid chromatography tandem mass spectrometry in sweat gland, a key site of CFTR function in vivo. However, NHERF1 oligomerized at considerably lower concentrations (10 nM) in the presence of the last 111 amino acids of CFTR (20 nM) in blot overlays and cross-linking assays and in coimmunoprecipitations using differently tagged versions of NHERF1. Deletion and alanine mutagenesis revealed that a six-amino acid sequence 1417EENKVR1422 and the terminal 1478TRL1480 (PDZ-binding motif) in the COOH-terminus were essential for the enhanced oligomerization of NHERF1. Full-length CFTR stably expressed in Madin-Darby canine kidney epithelial cells fostered NHERF1 oligomerization that was substantially reduced (∼5-fold) on alanine substitution of EEN, KVR, or EENKVR residues or deletion of the TRL motif. Confocal fluorescent microscopy revealed that the EENKVR and TRL sequences contribute to preferential localization of CFTR to the apical membrane. Together, these results indicate that COOH-terminal sequences mediate enhanced NHERF1 interaction and facilitate the localization of CFTR, a property that could be manipulated to stabilize mutant forms of CFTR at the apical surface to maximize the effect of CFTR-targeted therapeutics.

The PDZ Protein Na+/H+ Exchanger Regulatory Factor-1 (NHERF1) Regulates Planar Cell Polarity and Motile Cilia Organization

Directional flow of the cerebrospinal fluid requires coordinated movement of the motile cilia of the ependymal epithelium that lines the cerebral ventricles. Here we report that mice lacking the Na⁺/H⁺ Exchanger Regulatory Factor 1 (NHERF1/Slc9a3r1, also known as EBP50) develop profound communicating hydrocephalus associated with fewer and disorganized ependymal cilia. Knockdown of NHERF1/slc9a3r1 in zebrafish embryos also causes severe hydrocephalus of the hindbrain and impaired ciliogenesis in the otic vesicle. Ultrastructural analysis did not reveal defects in the shape or organization of individual cilia. Similar phenotypes have been described in animals with deficiencies in Wnt signaling and the Planar Cell Polarity (PCP) pathway. We show that NHERF1 binds the PCP core genes Frizzled (Fzd) and Vangl. We further show that NHERF1 assembles a ternary complex with Fzd4 and Vangl2 and promotes translocation of Vangl2 to the plasma membrane, in particular to the apical surface of ependymal cells. Taken together, these results strongly support an important role for NHERF1 in the regulation of PCP signaling and the development of functional motile cilia.

Plasticity of the brush border - the yin and yang of intestinal homeostasis

The brush border on the apical surface of enterocytes is a highly specialized structure well-adapted for efficient digestion and nutrient transport, whilst at the same time providing a protective barrier for the intestinal mucosa. The brush border is constituted of a densely ordered array of microvilli, protrusions of the plasma membrane, which are supported by actin-based microfilaments and interacting proteins and anchored in an apical network of actomyosin and intermediate filaments, the so-called terminal web. The highly dynamic, specialized apical domain is both an essential partner for the gut microbiota and an efficient signalling platform that enables adaptation to physiological stimuli from the external and internal milieu. Nevertheless, genetic alterations or various pathological stresses, such as infection, inflammation, and mechanical or nutritional alterations, can jeopardize this equilibrium and compromise intestinal functions. Long-time neglected, the intestinal brush-border shall be enlightening again as the central actor of the complex but essential intestinal homeostasis. Here, we review the processes and components involved in brush border organization and discuss pathological mechanisms that can induce brush border defects and their physiological consequences.

The surprising dynamics of scaffolding proteins

The function of scaffolding proteins is to bring together two or more proteins in a relatively stable configuration, hence their name. Numerous scaffolding proteins are found in nature, many having multiple protein–protein interaction modules. Over the past decade, examples of scaffolding complexes long thought to be stable have instead been found to be surprisingly dynamic. These studies are scattered among different biological systems, and so the concept that scaffolding complexes might not always represent stable entities and that their dynamics can be regulated has not garnered general attention. We became aware of this issue in our studies of a scaffolding protein in microvilli, which forced us to reevaluate its contribution to their structure. The purpose of this Perspective is to draw attention to this phenomenon and discuss why complexes might show regulated dynamics. We also wish to encourage more studies on the dynamics of “stable” complexes and to provide a word of caution about how functionally important dynamic associations may be missed in biochemical and proteomic studies.

NHERF1 regulates actin cytoskeleton organization through modulation of α-actinin-4 stability

The actin cytoskeleton is composed of a highly dynamic network of filamentous proteins, yet the molecular mechanism that regulates its organization and remodeling remains elusive. In this study, Na(+)/H(+) exchanger regulatory factor (NHERF)-1 loss-of-function and gain-of-function experiments reveal that polymerized actin cytoskeleton (F-actin) in HeLa cells is disorganized by NHERF1, whereas actin protein expression levels exhibit no detectable change. To elucidate the molecular mechanism underlying actin cytoskeleton disorganization by NHERF1, a combined 2-dimensional electrophoresis-matrix-assisted laser desorption/ionization-time of flight mass spectrometry approach was used to screen for proteins regulated by NHERF1 in HeLa cells. α-Actinin-4, an actin cross-linking protein, was identified. Glutathione S-transferase pull-down and coimmunoprecipitation studies showed the α-actinin-4 carboxyl-terminal region specifically interacted with the NHERF1 postsynaptic density 95/disc-large/zona occludens-1 domain. The NHERF1/α-actinin-4 interaction increased α-actinin-4 ubiquitination and decreased its expression levels, resulting in actin cytoskeleton disassembly. Our study identified α-actinin-4 as a novel NHERF1 interaction partner and provided new insights into the regulatory mechanism of the actin cytoskeleton by NHERF1.

Role of Na+/H+ exchanger regulatory factor 1 in forward trafficking of the type IIa Na+-Pi cotransporter

Na+/H+ exchanger regulatory factor (NHERF1) plays a critical role in the renal transport of phosphate by binding to Na+-Pi cotransporter (NpT2a) in the proximal tubule. While the association between NpT2a and NHERF1 in the apical membrane is known, the role of NHERF1 to regulate the trafficking of NpT2a has not been studied. To address this question, we performed cell fractionation by sucrose gradient centrifugation in opossum kidney (OK) cells placed in low-Pi medium to stimulate forward trafficking of NpT2a. Immunoblot analysis demonstrated expression of NpT2a and NHERF1 in the endoplasmic reticulum (ER)/Golgi. Coimmunoprecipitation demonstrated a NpT2a-NHERF1 interaction in the ER/Golgi. Low-Pi medium for 4 and 8 h triggered a decrease in NHERF1 in the plasma membrane with a corresponding increase in the ER/Golgi. Time-lapse total internal reflection fluorescence imaging of OK cells placed in low-Pi medium, paired with particle tracking and mean square displacement analysis, indicated active directed movement of NHERF1 at early and late time points, whereas NpT2a showed active movement only at later times. Silence of NHERF1 in OK cells expressing green fluorescent protein (GFP)-NpT2a resulted in an intracellular accumulation of GFP-NpT2a. Transfection with GFP-labeled COOH-terminal (TRL) PDZ-binding motif deleted or wild-type NpT2a in OK cells followed by cell fractionation and immunoprecipitation confirmed that the interaction between NpT2a and NHERF1 was dependent on the TRL motif of NpT2a. We conclude that appropriate trafficking of NpT2a to the plasma membrane is dependent on the initial association between NpT2a and NHERF1 through the COOH-terminal TRL motif of NpT2a in the ER/Golgi and requires redistribution of NHERF1 to the ER/Golgi.

An inducible mouse model for microvillus inclusion disease reveals a role for myosin Vb in apical and basolateral trafficking

Microvillus inclusion disease (MVID) is a rare intestinal enteropathy with an onset within a few days to months after birth, resulting in persistent watery diarrhea. Mutations in the myosin Vb gene (MYO5B) have been identified in the majority of MVID patients. However, the exact pathophysiology of MVID still remains unclear. To address the specific role of MYO5B in the intestine, we generated an intestine-specific conditional Myo5b-deficient (Myo5bfl/fl;Vil-CreERT2) mouse model. We analyzed intestinal tissues and cultured organoids of Myo5bfl/fl;Vil-CreERT2 mice by electron microscopy, immunofluorescence, and immunohistochemistry. Our data showed that Myo5bfl/fl;Vil-CreERT2 mice developed severe diarrhea within 4 d after tamoxifen induction. Periodic Acid Schiff and alkaline phosphatase staining revealed subapical accumulation of intracellular vesicles in villus enterocytes. Analysis by electron microscopy confirmed an almost complete absence of apical microvilli, the appearance of microvillus inclusions, and enlarged intercellular spaces in induced Myo5bfl/fl;Vil-CreERT2 intestines. In addition, we determined that MYO5B is involved not only in apical but also basolateral trafficking of proteins. The analysis of the intestine during the early onset of the disease revealed that subapical accumulation of secretory granules precedes occurrence of microvillus inclusions, indicating involvement of MYO5B in early differentiation of epithelial cells. By comparing our data with a novel MVID patient, we conclude that our mouse model completely recapitulates the intestinal phenotype of human MVID. This includes severe diarrhea, loss of microvilli, occurrence of microvillus inclusions, and subapical secretory granules. Thus, loss of MYO5B disturbs both apical and basolateral trafficking of proteins and causes MVID in mice.

The function and dynamics of the apical scaffolding protein E3KARP are regulated by cell-cycle phosphorylation

We examine the dynamics and function of the apical scaffolding protein E3KARP/NHERF2, which consists of two PDZ domains and a tail containing an ezrin-binding domain. The exchange rate of E3KARP is greatly enhanced during mitosis due to phosphorylation at Ser-303 in its tail region. Whereas E3KARP can substitute for the function of the closely related scaffolding protein EBP50/NHERF1 in the formation of interphase microvilli, E3KARP S303D cannot. Moreover, the S303D mutation enhances the in vivo dynamics of the E3KARP tail alone, whereas in vitro the interaction of E3KARP with active ezrin is unaffected by S303D, implicating another factor regulating dynamics in vivo. A-Raf is found to be required for S303 phosphorylation in mitotic cells. Regulation of the dynamics of EBP50 is known to be dependent on its tail region but modulated by PDZ domain occupancy, which is not the case for E3KARP. Of interest, in both cases, the mechanisms regulating dynamics involve the tails, which are the most diverged region of the paralogues and probably evolved independently after a gene duplication event that occurred early in vertebrate evolution.

Isotropic actomyosin dynamics promote organization of the apical cell cortex in epithelial cells

Apical membrane organization of nonconfluent epithelial cells is driven by a dynamic network of actin and myosin II filaments.

Although cortical actin plays an important role in cellular mechanics and morphogenesis, there is surprisingly little information on cortex organization at the apical surface of cells. In this paper, we characterize organization and dynamics of microvilli (MV) and a previously unappreciated actomyosin network at the apical surface of Madin–Darby canine kidney cells. In contrast to short and static MV in confluent cells, the apical surfaces of nonconfluent epithelial cells (ECs) form highly dynamic protrusions, which are often oriented along the plane of the membrane. These dynamic MV exhibit complex and spatially correlated reorganization, which is dependent on myosin II activity. Surprisingly, myosin II is organized into an extensive network of filaments spanning the entire apical membrane in nonconfluent ECs. Dynamic MV, myosin filaments, and their associated actin filaments form an interconnected, prestressed network. Interestingly, this network regulates lateral mobility of apical membrane probes such as integrins or epidermal growth factor receptors, suggesting that coordinated actomyosin dynamics contributes to apical cell membrane organization.

A molecular switch for the orientation of epithelial cell polarization

The formation of epithelial tissues containing lumens requires not only the apical-basolateral polarization of cells, but also the coordinated orientation of this polarity such that the apical surfaces of neighboring cells all point toward the central lumen. Defects in extracellular matrix (ECM) signaling lead to inverted polarity so that the apical surfaces face the surrounding ECM. We report a molecular switch mechanism controlling polarity orientation. ECM signals through a β1-integrin/FAK/p190RhoGAP complex to downregulate a RhoA/ROCK/Ezrin pathway at the ECM interface. PKCβII phosphorylates the apical identity-promoting Podocalyxin/NHERF1/Ezrin complex, removing Podocalyxin from the ECM-abutting cell surface and initiating its transcytosis to an apical membrane initiation site for lumen formation. Inhibition of this switch mechanism results in the retention of Podocalyxin at the ECM interface and the development instead of collective front-rear polarization and motility. Thus, ECM-derived signals control the morphogenesis of epithelial tissues by controlling the collective orientation of epithelial polarization.

Dynamics of ezrin and EBP50 in regulating microvilli on the apical aspect of epithelial cells

Microvilli are found on the apical surface of epithelial cells. Recent studies on the microvillar proteins ezrin and EBP50 (ezrin/radixin/moesin-binding phosphoprotein of 50 kDa) have revealed both the dynamics and the regulation of microvillar components, and how a dynamic ezrin phosphocycle is necessary to confine microvilli to the apical membrane. In the present review, we first summarize the background to allow us to place these advances in context.

Cordon Bleu serves as a platform at the basal region of microvilli, where it regulates microvillar length through its WH2 domains

The actin nucleator Cordon Bleu (Cobl) is localized to the basal region of microvilli of epithelial cells, where it regulates microvilli length through its WH2 domains. The COBL domain recruits several BAR-containing proteins, including PACSIN 2 and ASAP1, suggesting a role in coordinating microvillar structure with membrane traffic.

Cordon Bleu (Cobl) is a WH2-containing protein believed to act as an actin nucleator. We show that it has a very specific localization in epithelial cells at the basal region of microvilli, a localization unlikely to be involved in actin nucleation. The protein is localized by a central region between the N-terminal COBL domain and the three C-terminal WH2 domains. Ectopic expression of Cobl shortens apical microvilli, and this requires functional WH2 domains. Proteomic studies reveal that the COBL domain binds several BAR-containing proteins, including SNX9, PACSIN 2/syndapin 2, and ASAP1. ASAP1 is recruited to the base of microvilli by binding the COBL domain through its SH3. We propose that Cobl is localized to the basal region of microvilli both to participate in length regulation and to recruit BAR proteins that associate with the curved membrane found at the microvillar base.

ERM proteins at a glance

The cell cortex is a dynamic and heterogeneous structure that governs cell identity and behavior. The ERM proteins (ezrin, radixin and moesin) are major architects of the cell cortex, and they link plasma membrane phospholipids and proteins to the underlying cortical actin cytoskeleton. Recent studies in several model systems have uncovered surprisingly dynamic and complex molecular activities of the ERM proteins and have provided new mechanistic insight into how they build and maintain cortical domains. Among many well-established and essential functions of ERM proteins, this Cell Science at a Glance article and accompanying poster will focus on the role of ERMs in organizing the cell cortex during cell division and apical morphogenesis. These examples highlight an emerging appreciation that the ERM proteins both locally alter the mechanical properties of the cell cortex, and control the spatial distribution and activity of key membrane complexes, establishing the ERM proteins as a nexus for the physical and functional organization of the cell cortex and making it clear that they are much more than scaffolds. This article is part of a Minifocus on Establishing polarity.

NHERF2/NHERF3 protein heterodimerization and macrocomplex formation are required for the inhibition of NHE3 activity by carbachol

NHERF1, NHERF2, and NHERF3 belong to the NHERF (Na(+)/H(+) exchanger regulatory factor) family of PSD-95/Discs-large/ZO-1 (PDZ) scaffolding proteins. Individually, each NHERF protein has been shown to be involved in the regulation of multiple receptors or transporters including Na(+)/H(+) exchanger 3 (NHE3). Although NHERF dimerizations have been reported, results have been inconsistent, and the physiological function of NHERF dimerizations is still unknown. The current study semiquantitatively compared the interaction strength among all possible homodimerizations and heterodimerizations of these three NHERF proteins by pulldown and co-immunoprecipitation assays. Both methods showed that NHERF2 and NHERF3 heterodimerize as the strongest interaction among all NHERF dimerizations. In vivo NHERF2/NHERF3 heterodimerization was confirmed by FRET and FRAP (fluorescence recovery after photobleach). NHERF2/NHERF3 heterodimerization is mediated by PDZ domains of NHERF2 and the C-terminal PDZ domain recognition motif of NHERF3. The NHERF3-4A mutant is defective in heterodimerization with NHERF2 and does not support the inhibition of NHE3 by carbachol. This suggests a role for NHERF2/NHERF3 heterodimerization in the regulation of NHE3 activity. In addition, both PDZ domains of NHERF2 could be simultaneously occupied by NHERF3 and another ligand such as NHE3, α-actinin-4, and PKCα, promoting formation of NHE3 macrocomplexes. This study suggests that NHERF2/NHERF3 heterodimerization mediates the formation of NHE3 macrocomplexes, which are required for the inhibition of NHE3 activity by carbachol.

Interactome analysis reveals ezrin can adopt multiple conformational states

Ezrin, a member of the ezrin-radixin-moesin family (ERM), is an essential regulator of the structure of microvilli on the apical aspect of epithelial cells. Ezrin provides a linkage between membrane-associated proteins and F-actin, oscillating between active/open and inactive/closed states, and is regulated in part by phosphorylation of a C-terminal threonine. In the open state, ezrin can bind a number of ligands, but in the closed state the ligand-binding sites are inaccessible. In vitro analysis has proposed that there may be a third hyperactivated form of ezrin. To gain a better understanding of ezrin, we conducted an unbiased proteomic analysis of ezrin-binding proteins in an epithelial cell line, Jeg-3. We refined our list of interactors by comparing the interactomes using quantitative mass spectrometry between wild-type ezrin, closed ezrin, open ezrin, and hyperactivated ezrin. The analysis reveals several novel interactors confirmed by their localization to microvilli, as well as a significant class of proteins that bind closed ezrin. Taken together, the data indicate that ezrin can exist in three different conformational states, and different ligands "perceive" ezrin conformational states differently.

ß1 integrin binding phosphorylates ezrin at T567 to activate a lipid raft signalsome driving invadopodia activity and invasion

Extracellular matrix (ECM) degradation is a critical process in tumor cell invasion and requires matrix degrading protrusions called invadopodia. The Na⁺/H⁺ exchanger (NHE1) has recently been shown to be fundamental in the regulation of invadopodia actin cytoskeleton dynamics and activity. However, the structural link between the invadopodia cytoskeleton and NHE1 is still unknown. A candidate could be ezrin, a linker between the NHE1 and the actin cytoskeleton known to play a pivotal role in invasion and metastasis. However, the mechanistic basis for its role remains unknown. Here, we demonstrate that ezrin phosphorylated at T567 is highly overexpressed in the membrane of human breast tumors and positively associated with invasive growth and HER2 overexpression. Further, in the metastatic cell line, MDA-MB-231, p-ezrin was almost exclusively expressed in invadopodia lipid rafts where it co-localized in a functional complex with NHE1, EGFR, ß1-integrin and phosphorylated-NHERF1. Manipulation by mutation of ezrins T567 phosphorylation state and/or PIP2 binding capacity or of NHE1s binding to ezrin or PIP2 demonstrated that p-ezrin expression and binding to PIP2 are required for invadopodia-mediated ECM degradation and invasion and identified NHE1 as the membrane protein that p-ezrin regulates to induce invadopodia formation and activity.

The tails of apical scaffolding proteins EBP50 and E3KARP regulate their localization and dynamics

ERM-binding protein of 50 kDa (EBP50) and NHE3 kinase A regulatory protein (E3KARP) are closely related but show dramatically different dynamics in microvilli. The high dynamics of EBP50 is determined by a region in its tail and is inhibited by its PDZ domains, but is activated upon PDZ ligand binding. Proteomic analysis of the effects of EBP50 dynamics identifies a novel PDZ binding partner, IRSp53.

The closely related apical scaffolding proteins ERM-binding phosphoprotein of 50 kDa (EBP50) and NHE3 kinase A regulatory protein (E3KARP) both consist of two postsynaptic density 95/disks large/zona occludens-1 (PDZ) domains and a tail ending in an ezrin-binding domain. Scaffolding proteins are thought to provide stable linkages between components of multiprotein complexes, yet in several types of epithelial cells, EBP50, but not E3KARP, shows rapid exchange from microvilli compared with its binding partners. The difference in dynamics is determined by the proteins’ tail regions. Exchange rates of EBP50 and E3KARP correlated strongly with their abilities to precipitate ezrin in vivo. The EBP50 tail alone is highly dynamic, but in the context of the full-length protein, the dynamics is lost when the PDZ domains are unable to bind ligand. Proteomic analysis of the effects of EBP50 dynamics on binding-partner preferences identified a novel PDZ1 binding partner, the I-BAR protein insulin receptor substrate p53 (IRSp53). Additionally, the tails promote different microvillar localizations for EBP50 and E3KARP, which localized along the full length and to the base of microvilli, respectively. Thus the tails define the localization and dynamics of these scaffolding proteins, and the high dynamics of EBP50 is regulated by the occupancy of its PDZ domains.

Ligand-induced dynamic changes in extended PDZ domains from NHERF1

The multi-domain scaffolding protein NHERF1 modulates the assembly and intracellular trafficking of various transmembrane receptors and ion-transport proteins. The two PDZ (postsynaptic density 95/disk large/zonula occluden 1) domains of NHERF1 possess very different ligand-binding capabilities: PDZ1 recognizes a variety of membrane proteins with high affinity, while PDZ2 only binds limited number of target proteins. Here using NMR, we have determined the structural and dynamic mechanisms that differentiate the binding affinities of the two PDZ domains, for the type 1 PDZ-binding motif (QDTRL) in the carboxyl terminus of cystic fibrosis transmembrane regulator. Similar to PDZ2, we have identified a helix-loop-helix subdomain coupled to the canonical PDZ1 domain. The extended PDZ1 domain is highly flexible with correlated backbone motions on fast and slow timescales, while the extended PDZ2 domain is relatively rigid. The malleability of the extended PDZ1 structure facilitates the transmission of conformational changes at the ligand-binding site to the remote helix-loop-helix extension. By contrast, ligand binding has only modest effects on the conformation and dynamics of the extended PDZ2 domain. The study shows that ligand-induced structural and dynamic changes coupled with sequence variation at the putative PDZ binding site dictate ligand selectivity and binding affinity of the two PDZ domains of NHERF1.

Molecular architecture of the chick vestibular hair bundle

Hair bundles of the inner ear have a specialized structure and protein composition that underlies their sensitivity to mechanical stimulation. Using mass spectrometry, we identified and quantified >1,100 proteins, present from a few to 400,000 copies per stereocilium, from purified chick bundles; 336 of these were significantly enriched in bundles. Bundle proteins that we detected have been shown to regulate cytoskeleton structure and dynamics, energy metabolism, phospholipid synthesis and cell signaling. Three-dimensional imaging using electron tomography allowed us to count the number of actin-actin cross-linkers and actin-membrane connectors; these values compared well to those obtained from mass spectrometry. Network analysis revealed several hub proteins, including RDX (radixin) and SLC9A3R2 (NHERF2), which interact with many bundle proteins and may perform functions essential for bundle structure and function. The quantitative mass spectrometry of bundle proteins reported here establishes a framework for future characterization of dynamic processes that shape bundle structure and function.

Temporal and spatial regulation of ezrin-radixin-moesin-binding phosphoprotein-50-kDa (EBP50) during embryo implantation in mouse uterus

Embryo implantation is a crucial process for successful pregnancy. To date, the mechanism of embryo implantation remains unclear. Ezrin-radixin-moesin-binding protein-50-kDa (EBP50) is a scaffold protein, which has been shown to play an important role in cancer development. Embryo implantation and cancer follow a similar progression. Thus, in this article, we utilized immunohistochemical staining and western blot analyses to examine the spatiotemporal expression and regulation of EBP50 both in the mouse uterus during embryo implantation as well as in other related models. We found that EBP50 was detected in epithelial cells in all of the groups used in our study. During the peri-implantation period, EBP50 mainly localized in apical membranes. At the implantation site (IS) on day 5 (D5) of pregnancy, EBP50 was mainly expressed in the nuclei of stroma cells, whereas from day 6 to day 8 (D6–D8) of pregnancy, the expression of EBP50 was noted in the cytoplasm of decidual cells. The expression of EBP50 was not significantly different in the pseudopregnant uterus and decreased in the uteri subjected to activation of delayed implantation. Artificial decidualization also decreased EBP50 expression. Thus, the expression levels and location were affected by active blastocysts and decidualization during the window of implantation.

Local phosphocycling mediated by LOK/SLK restricts ezrin function to the apical aspect of epithelial cells

Local cycling of LOK/SLK-dependent phosphorylation of ezrin is required for its apical localization and for microvillus formation.

In this paper, we describe how a dynamic regulatory process is necessary to restrict microvilli to the apical aspect of polarized epithelial cells. We found that local phosphocycling regulation of ezrin, a critical plasma membrane–cytoskeletal linker of microvilli, was required to restrict its function to the apical membrane. Proteomic approaches and ribonucleic acid interference knockdown identified lymphocyte-oriented kinase (LOK) and SLK as the relevant kinases. Using drug-resistant LOK and SLK variants showed that these kinases were sufficient to restrict ezrin function to the apical domain. Both kinases were enriched in microvilli and locally activated there. Unregulated kinase activity caused ezrin mislocalization toward the basolateral domain, whereas expression of the kinase regulatory regions of LOK or SLK resulted in local inhibition of ezrin phosphorylation by the endogenous kinases. Thus, the domain-specific presence of microvilli is a dynamic process requiring a localized kinase driving the phosphocycling of ezrin to continually bias its function to the apical membrane.

PDZ interactions regulate rapid turnover of the scaffolding protein EBP50 in microvilli

Scaffolding proteins containing PDZ (postsynaptic density 95/discs large/zonula occludens-1) domains are believed to provide relatively stable linkages between components of macromolecular complexes and in some cases to bridge to the actin cytoskeleton. The microvillar scaffolding protein EBP50 (ERM-binding phosphoprotein of 50 kD), consisting of two PDZ domains and an ezrin-binding site, retains specific proteins in microvilli and is necessary for microvillar biogenesis. Our analysis of the dynamics of microvillar proteins in vivo indicated that ezrin and microvillar membrane proteins had dynamics consistent with actin treadmilling and microvillar lifetimes. However, EBP50 was highly dynamic, turning over within seconds. EBP50 turnover was reduced by mutations that inactivate its PDZ domains and was enhanced by protein kinase C phosphorylation. Using a novel in vitro photoactivation fluorescence assay, the EBP50-ezrin interaction was shown to have a slow off-rate that was dramatically enhanced in a PDZ-regulated manner by addition of cell extract to near in vivo levels. Thus, the linking of relatively stable microvillar components can be mediated by surprisingly dynamic EBP50, a finding that may have important ramifications for other scaffolding proteins.

Ezrin-anchored protein kinase A coordinates phosphorylation-dependent disassembly of a NHERF1 ternary complex to regulate hormone-sensitive phosphate transport

Congenital defects in the Na/H exchanger regulatory factor-1 (NHERF1) are linked to disordered phosphate homeostasis and skeletal abnormalities in humans. In the kidney, these mutations interrupt parathyroid hormone (PTH)-responsive sequestration of the renal phosphate transporter, Npt2a, with ensuing urinary phosphate wasting. We now report that NHERF1, a modular PDZ domain scaffolding protein, coordinates the assembly of an obligate ternary complex with Npt2a and the PKA-anchoring protein ezrin to facilitate PTH-responsive cAMP signaling events. Activation of ezrin-anchored PKA initiates NHERF1 phosphorylation to disassemble the ternary complex, release Npt2a, and thereby inhibit phosphate transport. Loss-of-function mutations stabilize an inactive NHERF1 conformation that we show is refractory to PKA phosphorylation and impairs assembly of the ternary complex. Compensatory mutations introduced in mutant NHERF1 re-establish the integrity of the ternary complex to permit phosphorylation of NHERF1 and rescue PTH action. These findings offer new insights into a novel macromolecular mechanism for the physiological action of a critical ternary complex, where anchored PKA coordinates the assembly and turnover of the Npt2a-NHERF1-ezrin complex.

Cell cycle dependent association of EBP50 with protein phosphatase 2A in endothelial cells

Ezrin-radixin-moesin (ERM)-binding phosphoprotein 50 (EBP50) is a phosphorylatable PDZ domain-containing adaptor protein that is abundantly expressed in epithelium but was not yet studied in the endothelium. We report unusual nuclear localization of EBP50 in bovine pulmonary artery endothelial cells (BPAEC). Immunofluorescent staining and cellular fractionation demonstrated that EBP50 is present in the nuclear and perinuclear region in interphase cells. In the prophase of mitosis EBP50 redistributes to the cytoplasmic region in a phosphorylation dependent manner and during mitosis EBP50 co-localizes with protein phosphatase 2A (PP2A). Furthermore, in vitro wound healing of BPAEC expressing phospho-mimic mutant of EBP50 was accelerated indicating that EBP50 is involved in the regulation of the cell division. Cell cycle dependent specific interactions were detected between EBP50 and the subunits of PP2A (A, C, and Bα) with immunoprecipitation and pull-down experiments. The interaction of EBP50 with the Bα containing form of PP2A suggests that this holoenzyme of PP2A can be responsible for the dephosphorylation of EBP50 in cytokinesis. Moreover, the results underline the significance of EBP50 in cell division via reversible phosphorylation of the protein with cyclin dependent kinase and PP2A in normal cells.

NHERF1 acts as a molecular switch to program metastatic behavior and organotropism via its PDZ domains

Tumor metastasis is the primary cause of death in cancer patients, but the molecular mechanisms driving the evolution of the phenotype toward a specific organ is one of its less understood aspects. The scaffolding protein NHERF1 reprograms the metastatic phenotype and organotropism via the differential function of its PDZ domains.

Metastatic cells are highly plastic for differential expression of tumor phenotype hallmarks and metastatic organotropism. The signaling proteins orchestrating the shift of one cell phenotype and organ pattern to another are little known. Na⁺/H⁺ exchanger regulatory factor (NHERF1) is a molecular pathway organizer, PDZ-domain protein that recruits membrane, cytoplasmic, and cytoskeletal signaling proteins into functional complexes. To gain insight into the role of NHERF1 in metastatic progression, we stably transfected a metastatic breast cell line, MDA-MB-231, with an empty vector, with wild-type NHERF1, or with NHERF1 mutated in either the PDZ1- or PDZ2-binding domains to block their binding activities. We observed that NHERF1 differentially regulates the expression of two phenotypic programs through its PDZ domains, and these programs form the mechanistic basis for metastatic organotropism. The PDZ2 domain promotes visceral metastases via increased invadopodia-dependent invasion and anchorage-independent growth, as well as by inhibition of apoptosis, whereas the PDZ1 domain promotes bone metastases by stimulating podosome nucleation, motility, neoangiogenesis, vasculogenic mimicry, and osteoclastogenesis in the absence of increased growth or invasion. Collectively, these findings identify NHERF1 as an important signaling nexus for coordinating cell structure with metastatic behavior and identifies the “mesenchymal-to-vasculogenic” phenotypic transition as an essential step in metastatic progression.

Phosphorylation of EBP50 negatively regulates β-PIX-dependent Rac1 activity in anoikis

We demonstrated a protein kinase C (PKC)-dependent phosphorylation of canine ezrin/radixin/moesin (ERM)-binding phosphoprotein 50 (EBP50) at serine 347/348 by site-directed mutagenesis and a phospho-specific antibody. Cell fractionation and confocal imaging revealed the relocation of EBP50 from the plasma membrane to cytosol that accompanied this phosphorylation event. Increased phosphorylation at these serine residues led to the dissociation of EBP50 from ezrin and β-PIX, which are two upstream regulators of Rac1 activation. Cells overexpressing an EBP50 mutant, mimicking serine 347/348 phosphorylation, became refractory to hepatocyte growth factor-induced cell spreading and scattering, which is normally mediated by Rac1 activation. Detachment of cells from the substratum also elicited an increase in EBP50 phosphorylation, apparently due to counteracting activities of PKC and protein phosphastase 2A, which resulted in decreased Rac1 activation and induction of anoikis. Cells overexpressing an EBP50 mutant defective in serine 347/348 phosphorylation did not undergo apoptosis in suspension culture. These studies reveal a signaling cascade in which different phosphorylation states and subcellular localization of EBP50 regulate Rac1 function.