IQGAP1: a key regulator of adhesion and migration

Ubiquitination of the scaffold protein IQGAP1 diminishes its interaction with and activation of the Rho GTPase CDC42

IQ motif-containing GTPase-activating protein 1 (IQGAP1) is a scaffold protein that interacts with numerous binding partners and thereby regulates fundamental biological processes. The functions of IQGAP1 are modulated by several mechanisms, including protein binding, self-association, subcellular localization, and phosphorylation. Proteome-wide screens have indicated that IQGAP1 is ubiquitinated, but the possible effects of this post-translational modification on its function are unknown. Here we characterized and evaluated the function of IQGAP1 ubiquitination. Using MS-based analysis in HEK293 cells, we identified six lysine residues (Lys-556, -1155, -1230, -1465, -1475, and -1528) as ubiquitination sites in IQGAP1. To elucidate the biological consequences of IQGAP1 ubiquitination, we converted each of these lysines to arginine and found that replacing two of these residues, Lys-1155 and Lys-1230, in the GAP-related domain of IQGAP1 (termed IQGAP1 GRD-2K) reduces its ubiquitination. Moreover, IQGAP1 GRD-2K bound a significantly greater proportion of the two Rho GTPases cell division cycle 42 (CDC42) and Rac family small GTPase 1 (RAC1) than did WT IQGAP1. Consistent with this observation, reconstitution of IQGAP1-null cells with IQGAP1 GRD-2K significantly increased the amount of active CDC42 and enhanced cell migration significantly more than WT IQGAP1. Our results reveal that ubiquitination of the CDC42 regulator IQGAP1 alters its ability to bind to and activate this GTPase, leading to physiological effects. Collectively, these findings expand our view of the role of ubiquitination in cell signaling and provide additional insight into CDC42 regulation.

Hyaluronan-binding protein 1 (HABP1) overexpression triggers induction of senescence in fibroblasts cells

Hyaluronan-binding protein 1 (HABP1), a multi-compartmental, multi-functional protein has a wide range of functions, which can be attributed to its ability to associate with a variety of cellular ligands. Earlier we have reported that HABP1 overexpression in rat normal fibroblasts (F-HABP07) shows chronic generation of reactive oxygen species (ROS), induction of autophagy, and apoptosis. However, a significant proportion of cells remained viable after the majority went through apoptosis from 60 to 72 h. In this study, an attempt has been made to delineate the cellular events in the declined population of surviving cells. It has been elucidated here that, these cells at later time points of growth, that is, 72 and 84 h, not only appeared to shrink but also are devoid of autophagic vacuoles and displayed polyploidy. F-HABP07 cells exhibited an altered cytoskeletal structure from their parental cell line F111, assumed to be caused upon inhibition of actin polymerization and decrease in IQ motif-containing GTPase activating protein 1 (IQGAP1), a key protein associated with maintenance of cytoskeletal integrity. Enhanced expression and nuclear localization of AKT observed in F-HABP07 cells appears to be contributing toward the maintenance of high ROS levels in these cells and also potentially modulating the IQGAP1 activity. These observations, in fact have been considered to result in sustained DNA damage, which then leads to increased expression of p53 and activation of p21 and carry out the cellular events responsible for senescence. Subsequent assessment of the presence of positive β-gal staining and enhanced expression of p16^INK4a in F-HABP07, confirmed that HABP1 overexpressing fibroblasts undergo senescence.

Ca2+-Dependent Switch of Calmodulin Interaction Mode with Tandem IQ Motifs in the Scaffolding Protein IQGAP1

IQ domain GTPase-activating scaffolding protein 1 (IQGAP1) mediates cytoskeleton, cell migration, proliferation, and apoptosis events. Calmodulin (CaM) modulates IQGAP1 functions by binding to its four tandem IQ motifs. Exactly how CaM binds the IQ motifs and which functions of IQGAP1 CaM regulates and how are fundamental mechanistic questions. We combine experimental pull-down assays, mutational data, and molecular dynamics simulations to understand the IQ-CaM complexes with and without Ca2+ at the atomic level. Apo-CaM favors the IQ3 and IQ4 motifs but not the IQ1 and IQ2 motifs that lack two hydrophobic residues for interactions with apo-CaM's hydrophobic pocket. Ca2+-CaM binds all four IQ motifs, with both N- and C-lobes tightly wrapped around each motif. Ca2+ promotes IQ-CaM interactions and increases the amount of IQGAP1-loaded CaM for IQGAP1-mediated signaling. Collectively, we describe IQ-CaM binding in atomistic detail and feature the emergence of Ca2+ as a key modulator of the CaM-IQGAP1 interactions.

IQGAP1 restrains T-cell cosignaling mediated by OX40

A costimulatory signal from the tumor necrosis factor receptor (TNFR) family molecule OX40 (CD134), which is induced on activated T cells, is important for T-cell immunity. Aberrant OX40 cosignaling has been implicated in autoimmune and inflammatory disorders. However, the molecular mechanism by which the OX40 cosignaling regulates the T-cell response remains obscure. We found that OX40 associated with a scaffold protein, IQ motif-containing GTPase-activating protein 1 (IQGAP1) after ligation by its ligand OX40L. Naïve CD4⁺ T cells from Iqgap1^-/- mice displayed enhanced proliferation and cytokine secretion upon receiving OX40 cosignaling. A C-terminal IQGAP1 region was responsible for its association with OX40, and TNFR-associated factor 2 (TRAF2) bridged these two proteins. The enhanced cytokine response in Iqgap1^-/- T cells was restored by the expression of the C-terminal IQGAP1. Thus, the IQGAP1 binding limits the OX40 cosignaling. Disease severity of experimental autoimmune encephalomyelitis (EAE) was significantly exacerbated in Iqgap1^-/- mice as compared to wild-type mice. Additionally, recipient mice with Iqgap1^-/- donor CD4⁺ T cells exhibited significantly higher EAE scores than those with their wild-type counterparts, and OX40 blockade led to a significant reduction in the EAE severity. Thus, our study defines an important component of the OX40 cosignaling that restricts inflammation driven by antigen-activated T cells.

IQGAP1 activates PLC-δ1 by direct binding and moving along microtubule with DLC-1 to cell surface

Phospholipase C (PLC)-δ1, activated by p122RhoGTPase-activating protein (GAP)/deleted in liver cancer-1 (p122RhoGAP/DLC-1), contributes to the coronary spastic angina (CSA) pathogenesis. The present study aims to further investigate the p122RhoGAP/DLC-1 protein. We examined molecules assisting this protein and identified a scaffold protein-IQ motif-containing GTPase-activating protein 1 (IQGAP1). IQGAP1-C binds to the steroidogenic acute regulatory-related lipid transfer (START) domain of p122RhoGAP/DLC-1, and PLC-δ1 binds to IQGAP1-N, forming a complex. In fluorescence microscopy, small dots of PLC-δ1 created fine linear arrays like microtubules, and IQGAP1 and p122RhoGAP/DLC-1 were colocated in the cytoplasm with PLC-δ1. Ionomycin induced the raft recruitment of the PLC-δ1, IQGAP1, and p122RhoGAP/DLC-1 complex by translocation to the plasma membrane (PM), indicating the movement of this complex is along microtubules with the motor protein kinesin. Moreover, the IQGAP1 protein was elevated in skin fibroblasts obtained from patients with CSA, and it enhanced the PLC activity and peak intracellular calcium concentration in response to acetylcholine. IQGAP1, a novel stimulating protein, forms a complex with p122RhoGAP/DLC-1 and PLC-δ1 that moves along microtubules and enhances the PLC activity.

The Specificity of EGF-Stimulated IQGAP1 Scaffold Towards the PI3K-Akt Pathway is Defined by the IQ3 motif

Epidermal growth factor receptor (EGFR) and its downstream phosphoinositide 3-kinase (PI3K) pathway are commonly deregulated in cancer. Recently, we have shown that the IQ motif-containing GTPase-activating protein 1 (IQGAP1) provides a molecular platform to scaffold all the components of the PI3K-Akt pathway and results in the sequential generation of phosphatidylinositol-3,4,5-trisphosphate (PI3,4,5P₃). In addition to the PI3K-Akt pathway, IQGAP1 also scaffolds the Ras-ERK pathway. To define the specificity of IQGAP1 for the control of PI3K signaling, we have focused on the IQ3 motif in IQGAP1 as PIPKIα and PI3K enzymes bind this region. An IQ3 deletion mutant loses interactions with the PI3K-Akt components but retains binding to ERK and EGFR. Consistently, blocking the IQ3 motif of IQGAP1 using an IQ3 motif-derived peptide mirrors the effect of IQ3 deletion mutant by reducing Akt activation but has no impact on ERK activation. Also, the peptide disrupts the binding of IQGAP1 with PI3K-Akt pathway components, while IQGAP1 interactions with ERK and EGFR are not affected. Functionally, deleting or blocking the IQ3 motif inhibits cell proliferation, invasion, and migration in a non-additive manner to a PIPKIα inhibitor, establishing the functional specificity of IQ3 motif towards the PI3K-Akt pathway. Taken together, the IQ3 motif is a specific target for suppressing activation of the PI3K-Akt but not the Ras-ERK pathway. Although EGFR stimulates the IQGAP1-PI3K and -ERK pathways, here we show that IQGAP1-PI3K controls migration, invasion, and proliferation independent of ERK. These data illustrate that the IQ3 region of IQGAP1 is a promising therapeutic target for PI3K-driven cancer.

CD13 tethers the IQGAP1-ARF6-EFA6 complex to the plasma membrane to promote ARF6 activation, β1 integrin recycling, and cell migration

Cell attachment to the extracellular matrix (ECM) requires a balance between integrin internalization and recycling to the surface that is mediated by numerous proteins, emphasizing the complexity of these processes. Upon ligand binding in various cells, the β1 integrin is internalized, traffics to early endosomes, and is returned to the plasma membrane through recycling endosomes. This trafficking process depends on the cyclical activation and inactivation of small guanosine triphosphatases (GTPases) by their specific guanine exchange factors (GEFs) and their GTPase-activating proteins (GAPs). In this study, we found that the cell surface antigen CD13, a multifunctional transmembrane molecule that regulates cell-cell adhesion and receptor-mediated endocytosis, also promoted cell migration and colocalized with β1 integrin at sites of cell adhesion and at the leading edge. A lack of CD13 resulted in aberrant trafficking of internalized β1 integrin to late endosomes and its ultimate degradation. Our data indicate that CD13 promoted ARF6 GTPase activity by positioning the ARF6-GEF EFA6 at the cell membrane. In migrating cells, a complex containing phosphorylated CD13, IQGAP1, GTP-bound (active) ARF6, and EFA6 at the leading edge promoted the ARF6 GTPase cycling and cell migration. Together, our findings uncover a role for CD13 in the fundamental cellular processes of receptor recycling, regulation of small GTPase activities, cell-ECM interactions, and cell migration.

Methamphetamine produces cardiac damage and apoptosis by decreasing melusin

Methamphetamine (METH) is an amphetamine-type drug that is highly addictive and widely abused. Many studies have shown that METH exposure causes severe damage not only to the nervous system but also to the cardiovascular system. Melusin protein is a mechanotransducer that plays an important role in maintaining normal heart function. However, the role of melusin in METH-induced cardiotoxicity has not yet been reported. We hypothesized that methamphetamine can produce cardiac damage and apoptosis by decreasing the quantity of melusin. To test this hypothesis, we determined the protein expression of melusin and apoptosis markers in METH-treated rats and primary rat cardiomyocytes. We also established a melusin-overexpressing cell model to assess the importance of melusin in maintaining antiapoptotic pathways. To confirm our findings from the in vitro and animal models, we also evaluated the apoptotic index of cardiomyocytes and the protein expression of apoptotic markers in postmortem heart tissues from deceased METH abusers and age-matched control subjects. The results showed that the apoptosis of cardiomyocytes was increased significantly and that the protein expression of melusin was decreased after exposure to METH in primary rat cardiomyocytes, in rats and in humans. METH treatment also decreased the expression of the downstream proteins FAK, IQGAP1, p-AKT, p-GSK3β, and p-ERK in primary rat cardiomyocytes and in vivo. After overexpression of melusin, the above effects were partially reversed in primary rat cardiomyocytes. We conclude that METH can produce cardiac damage and apoptosis by decreasing melusin, while melusin-activated signaling by phosphorylated AKT, phosphorylated GSK3β, and ERK may be resistant to methamphetamine-induced myocardial apoptosis.

Focus on Cdc42 in Breast Cancer: New Insights, Target Therapy Development and Non-Coding RNAs

Breast cancer is the most common malignant tumors in females. Although the conventional treatment has demonstrated a certain effect, some limitations still exist. The Rho guanosine triphosphatase (GTPase) Cdc42 (Cell division control protein 42 homolog) is often upregulated by some cell surface receptors and oncogenes in breast cancer. Cdc42 switches from inactive guanosine diphosphate (GDP)-bound to active GTP-bound though guanine-nucleotide-exchange factors (GEFs), results in activation of signaling cascades that regulate various cellular processes such as cytoskeletal changes, proliferation and polarity establishment. Targeting Cdc42 also provides a strategy for precise breast cancer therapy. In addition, Cdc42 is a potential target for several types of non-coding RNAs including microRNAs and lncRNAs. These non-coding RNAs is extensively involved in Cdc42-induced tumor processes, while many of them are aberrantly expressed. Here, we focus on the role of Cdc42 in cell morphogenesis, proliferation, motility, angiogenesis and survival, introduce the Cdc42-targeted non-coding RNAs, as well as present current development of effective Cdc42-targeted inhibitors in breast cancer.

Y-27632 preconditioning enhances transplantation of human-induced pluripotent stem cell-derived cardiomyocytes in myocardial infarction mice

AIMS

The effectiveness of cell-based treatments for regenerative myocardial therapy is limited by low rates of cell engraftment. Y-27632 inhibits Rho-associated protein kinase (ROCK), which regulates the cytoskeletal changes associated with cell adhesion, and has been used to protect cultured cells during their passaging. Here, we investigated whether preconditioning of cardiomyocytes, derived from human-induced pluripotent stem cells (hiPSC-CM), with Y-27632 improves their survival and engraftment in a murine model of acute myocardial infarction (MI).

METHODS AND RESULTS

After MI induction, mice were subjected to intramyocardial injections of phosphate-buffered saline, hiPSC-CM cultured under standard conditions (hiPSC-CM-RI), or Y-27632-preconditioned hiPSC-CM (hiPSC-CM+RI). The resulting engraftment rate calculated 4 weeks after implantation was significantly higher and the abundance of apoptotic transplanted cells was significantly lower in hiPSC-CM+RI recipients than in hiPSC-CM-RI animals. In cultured hiPSC-CM, Y-27632-preconditioning reversibly reduced contractile activity and the expression of troponin genes, while increasing their attachment to an underlying mouse cardiomyocyte (HL1) monolayer. Y-27632 preconditioning also increased the expression of N-cadherin and integrin ß1, the two cell junction proteins. hiPSC-CM+RI were also larger in cell area with greater cytoskeletal alignment and a more rod-like shape than hiPSC-CM-RI, both after transplantation (in vivo) and in culture. The effects of Y-27632 preconditioning on contractile activity and morphology of hiPSC-CMs in culture, as well as on their engraftment rate and apoptotic death in MI mouse grafts, could be recapitulated by hiPSC-CM treatment with the L-type calcium-channel blocker verapamil.

CONCLUSION

Preconditioning with the ROCK inhibitor Y-27632 increased the engraftment of transplanted hiPSC-CM in a murine MI model, while reversibly impairing hiPSC-CM contractility and promoting adhesion.

The phenotype of vascular smooth muscle cells co-cultured with endothelial cells is modulated by PDGFR-β/IQGAP1 signaling in LPS-induced intravascular injury

Background Sepsis, a leading cause of death in intensive care units, is generally associated with vascular dysfunction. However, its pathophysiological process has not been fully clarified, lacking in-depth knowledge of its pathophysiological process may hinder the improvement of diagnosis and therapy for sepsis. Hence, as the key parts of the vascular wall, the interaction between endothelial cells (ECs) and smooth muscle cells (SMCs) under septic situation need to be further studied. Methods ECs and SMCs were co-cultured using Transwell plates. Lipopolysaccharide (LPS) was used to induce sepsis. A scratch-wound assay was used to assess cell migration, and western blotting was used to assess the level of redifferentiation of SMCs as well as the expression of PDGFR-β and IQGAP1. Results Co-culture with ECs reduced the redifferentiation of SMCs induced by LPS (10 μg/ml), which was characterized by increased migration ability and decreased expression of contractile proteins (e.g., SM22 and α-SMA). The production of TNF-α could decrease the level of PDGFR-β in SMCs. Treatment of SMCs with the PDGFR-β inhibitor imatinib (5 μM) was able to counteract LPS-induced SMC redifferentiation and reduce IQGAP1 protein expression, especially when SMCs were co-cultured with ECs. Conclusion The phenotype of vascular SMCs co-cultured with ECs was modulated by IQGAP1 through the PDGFR-β pathway, which may lead to vascular remodeling and homeostasis in LPS-induced intravascular injury. This pathway could be a novel target for the treatment of vascular damage.

Essential Role of mTOR Signaling in Human Retinal Pigment Epithelial Cell Regeneration After Laser Photocoagulation

This study assessed the role of mechanistic target of rapamycin (mTOR) pathway in the human adult retinal pigment epithelial (ARPE) cell response after laser photocoagulation (LP). The effect of mTOR inhibition on ARPE-19 cell was investigated by rapamycin treatment after LP. Cell viability and proliferation were explored using MTT and EdU assays, respectively. The expression of mTOR-related proteins and epithelial-mesenchymal transition (EMT) markers was verified by Western blot. Rapamycin retarded the LP area recovery in a dose-dependent manner by the 120 h, while LP+DMSO vehicle-treated cells completely restored the lesion zone (P ≤ 0.01). ARPE-19 cell viability is significantly lower in LP + rapamycin 80 and 160 ng/ml treated cultures compared to LP control at 120 h (P ≤ 0.001). LP control group demonstrated significantly more proliferative cells compared to untreated cells at the 72 and 120 h, whereas EdU-positive cell numbers in cultures treated with rapamycin at concentrations of 80 and 160 ng/ml were similar to baseline values (P ≤ 0.01). mTOR pathway activation is essential for regulation of the RPE cell migration and proliferation after LP. mTOR inhibition with rapamycin effectively blocks the migration and proliferation of the RPE cells. Our results demonstrate that mTOR has an important role in ARPE-19 cell as a regulator of cell behavior under stress conditions, suggesting that mTOR could be a promising therapeutic target for numerous retinal diseases.

Viscoelasticity of single cells-from subcellular to cellular level

This review deals with insights into complex cellular structures and processes obtained by measuring viscoelastic impedances of the cell envelope and the cytoplasm by colloidal bead microrheometry. I first introduce a mechanical cell model that allows us to understand their unique ability of mechanical self-stabilization by actin microtubule crosstalk. In the second part, I show how cell movements can be driven by pulsatile or propagating solitary actin gelatin waves (SAGW) that are generated on nascent adhesion domains by logistically controlled membrane recruitment of functional proteins by electrostatic-hydrophobic forces. The global polarization of cell migration is guided by actin-microtubule crosstalk that is mediated by the Ca⁺⁺ and strain-sensitive supramolecular scaffolding protein IQGAP. In the third part, I introduce the traction force microscopy as a tool to measure the forces between somatic cells and the tissue ´Here I show, how absolute values of viscoelastic impedances of the composite cell envelope can be obtained by deformation field mapping techniques. In the fourth part, it is shown how the dynamic mechanical properties of the active viscoplastic cytoplasmic space can be evaluated using colloidal beads as phantom endosomes. Separate measurements of velocity distributions of directed and random motions of phantom endosomes, yield local values of transport forces, viscosities and life times of directed motion along microtubules. The last part deals with biomimetic experiments allowing us to quantitatively evaluate the mechanical properties of passive and active actin networks on the basis of the percolation theory of gelation.

Copper transporter ATP7A interacts with IQGAP1, a Rac1 binding scaffolding protein: role in PDGF-induced VSMC migration and vascular remodeling

Vascular smooth muscle cell (VSMC) migration contributes to neointimal formation after vascular injury. We previously demonstrated that copper (Cu) transporter ATP7A is involved in platelet-derived growth factor (PDGF)-induced VSMC migration in a Cu- and Rac1-dependent manner. The underlying mechanism is still unknown. Here we show that ATP7A interacts with IQGAP1, a Rac1 and receptor tyrosine kinase binding scaffolding proteins, which mediates PDGF-induced VSMC migration and vascular remodeling. In cultured rat aortic SMCs, PDGF stimulation rapidly promoted ATP7A association with IQGAP1 and Rac1 and their translocation to the lipid rafts and leading edge. Cotransfection assay revealed that ATP7A directly bound to NH₂-terminal domain of IQGAP1. Functionally, either ATP7A or IQGAP1 depletion using siRNA significantly inhibited PDGF-induced VSMC migration without additive effects, suggesting that IQGAP1 and ATP7A are in the same axis to promote migration. Furthermore, IQGAP1 siRNA blocked PDGF-induced ATP7A association with Rac1 as well as its translocation to leading edge, while PDGF-induced IQGAP1 translocation was not affected by ATP7A siRNA or Cu chelator. Overexpression of mutant IQGAP1 lacking a Rac1 binding site prevented PDGF-induced translocation of Rac1, but not ATP7A, to the leading edge, thereby inhibiting lamellipodia formation and VSMC migration. In vivo, ATP7A colocalized with IQGAP1 at neointimal VSMCs in a mice wire injury model, while neointimal formation and extracellular matrix deposition induced by vascular injury were inhibited in ATP7A mutant mice with reduced Cu transporter function. In summary, IQGAP1 functions as ATP7A and Rac1 binding scaffolding protein to organize PDGF-dependent ATP7A translocation to the lamellipodial leading edge, thereby promoting VSMC migration and vascular remodeling.

Telocinobufagin inhibits the epithelial-mesenchymal transition of breast cancer cells through the phosphoinositide 3-kinase/protein kinase B/extracellular signal-regulated kinase/Snail signaling pathway

Telocinobufagin (TBG), an active ingredient of Venenumbufonis, exhibits an immunomodulatory activity. However, its antimetastatic activity in breast cancer remains unknown. The present study investigated whether TBG prevents breast cancer metastasis and evaluated its regulatory mechanism. TBG inhibited the migration and invasion of 4T1 breast cancer cells. Furthermore, TBG triggered the collapse of F-actin filaments in breast cancer. The epithelial-mesenchymal transition (EMT) markers, vimentin and fibronectin, were downregulated following TBG treatment. However, E-cadherin was upregulated following TBG treatment. Snail, a crucial transcriptional factor of EMT, was downregulated following TBG treatment. Signaling pathway markers, including phosphorylated protein kinase B (P-Akt), p-mechanistic target of rapamycin (mTOR) and p-extracellular signal-regulated kinase (ERK), were decreased following TBG treatment. The same results were obtained from in vivo experiments. In conclusion, in vitro and in vivo experiments reveal that TBG inhibited migration, invasion and EMT via the phosphoinositide 3-kinase (PI3K)/Akt/ERK/Snail signaling pathway in breast cancer.

MISP regulates the IQGAP1/Cdc42 complex to collectively orchestrate spindle orientation and mitotic progression

Precise mitotic spindle orientation is essential for both cell fate and tissue organization while defects in this process are associated with tumorigenesis and other diseases. In most animal cell types, the dynein motor complex is anchored at the cell cortex and exerts pulling forces on astral microtubules to position the spindle. The actin-binding protein MISP controls spindle orientation and mitotic progression in human cells. However, the exact underlying mechanism remains to be elucidated. Here we report that MISP interacts with the multidomain scaffolding protein IQGAP1. We further show that MISP binds to the active form of Cdc42 through IQGAP1. Depletion of MISP promotes increased accumulation of IQGAP1 at the cell cortex and a decrease in its Cdc42-binding capacity leading to reduced active Cdc42 levels. Interestingly, overexpression of IQGAP1 can rescue mitotic defects caused by MISP downregulation including spindle misorientation, loss of astral microtubules and prolonged mitosis and also restores active Cdc42 levels. Importantly, we find that IQGAP1 acts downsteam of MISP in regulating astral microtubule dynamics and the localization of the dynactin subunit p150^glued that is crucial for proper spindle positioning. We propose that MISP regulates IQGAP1 and Cdc42 to ensure proper mitotic progression and correct spindle orientation.

Myoepithelial Cells of Submucosal Glands Can Function as Reserve Stem Cells to Regenerate Airways after Injury

Cells demonstrate plasticity following injury, but the extent of this phenomenon and the cellular mechanisms involved remain underexplored. Using single-cell RNA sequencing (scRNA-seq) and lineage tracing, we uncover that myoepithelial cells (MECs) of the submucosal glands (SMGs) proliferate and migrate to repopulate the airway surface epithelium (SE) in multiple injury models. Specifically, SMG-derived cells display multipotency and contribute to basal and luminal cell types of the SMGs and SE. Ex vivo expanded MECs have the potential to repopulate and differentiate into SE cells when grafted onto denuded airway scaffolds. Significantly, we find that SMG-like cells appear on the SE of both extra- and intra-lobular airways of large animal lungs following severe injury. We find that the transcription factor SOX9 is necessary for MEC plasticity in airway regeneration. Because SMGs are abundant and present deep within airways, they may serve as a reserve cell source for enhancing human airway regeneration.

Regulation of Cell Polarity by Exocyst-Mediated Trafficking

One requirement for establishing polarity within a cell is the asymmetric trafficking of intracellular vesicles to the plasma membrane. This tightly regulated process creates spatial and temporal differences in both plasma membrane composition and the membrane-associated proteome. Asymmetric membrane trafficking is also a critical mechanism to regulate cell differentiation, signaling, and physiology. Many eukaryotic cell types use the eight-protein exocyst complex to orchestrate polarized vesicle trafficking to certain membrane locales. Members of the exocyst were originally discovered in yeast while screening for proteins required for the delivery of secretory vesicles to the budding daughter cell. The same eight exocyst genes are conserved in mammals, in which the specifics of exocyst-mediated trafficking are highly cell-type-dependent. Some exocyst members bind to certain Rab GTPases on intracellular vesicles, whereas others localize to the plasma membrane at the site of exocytosis. Assembly of the exocyst holocomplex is responsible for tethering these vesicles to the plasma membrane before their soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated exocytosis. In this review, we will focus on the role and regulation of the exocyst complex in targeted vesicular trafficking as related to the establishment and maintenance of cellular polarity. We will contrast exocyst function in apicobasal epithelial polarity versus front-back mesenchymal polarity, and the dynamic regulation of exocyst-mediated trafficking during cell phenotype transitions.

Reactivation of epigenetically silenced miR-124 reverses the epithelial-to-mesenchymal transition and inhibits invasion in endometrial cancer cells via the direct repression of IQGAP1 expression

Overexpression of IQGAP1 and microRNA (miRNA) dysregulation are frequent in human tumors, but little is known about the role of IQGAP1 and its relationship to miRNA in endometrial carcinogenesis. We demonstrate that IQGAP1 activates the epithelial–mesenchymal transition (EMT) program and that miR-124 directly represses IQGAP1 expression in endometrial cancer (EC) cells. The overexpression of IQGAP1 stimulates EMT features and enhances migration, invasion and proliferation of EC cells, whereas knocking down IQGAP1 expression reverses EMT and inhibits these malignant properties. Using miRNA microarray profiling, we identified 29 miRNAs (let-7b, let-7f, miR-10b, miR-15b, miR-23a, miR-24, miR-25, miR-27a, miR-29b, miR-30a-5p, miR-34a, miR-124, miR-127, miR-130b, miR-148a, miR-155, miR-191*, miR-194, miR-224, miR-362, miR-409-3p, miR-422b, miR-424, miR-453, miR-497, miR-518d, miR-518f*, miR-526a and miR-656) that are significantly down-regulated in an in vitro-selected highly invasive derivative cell line (HEC-50-HI) relative to the parental HEC-50 cells. We further identified miR-124 as a direct regulator of IQGAP1 in EC cells. Enforced expression of miR-124 suppresses EC cell invasion and proliferation. The expression of IQGAP1 mRNA was significantly elevated in EC tissues, while the expression of miR-124 was decreased. The downregulation of miR-124 correlates with a poor survival outcome for patients with EC. Treating EC cells with the demethylating agent 5-aza-2′-deoxycytidine increased miR-124 expression and down-regulated IQGAP1 levels. Our data suggest that IQGAP1 promotes EMT, migration and invasion of EC cells. MiR-124, a novel tumor suppressor miRNA that is epigenetically silenced in EC, can reverse EMT and the invasive properties, by attenuating the expression of the IQGAP1 oncogene.

Ninein is essential for apico-basal microtubule formation and CLIP-170 facilitates its redeployment to non-centrosomal microtubule organizing centres

Differentiation of columnar epithelial cells involves a dramatic reorganization of the microtubules (MTs) and centrosomal components into an apico-basal array no longer anchored at the centrosome. Instead, the minus-ends of the MTs become anchored at apical non-centrosomal microtubule organizing centres (n-MTOCs). Formation of n-MTOCs is critical as they determine the spatial organization of MTs, which in turn influences cell shape and function. However, how they are formed is poorly understood. We have previously shown that the centrosomal anchoring protein ninein is released from the centrosome, moves in a microtubule-dependent manner and accumulates at n-MTOCs during epithelial differentiation. Here, we report using depletion and knockout (KO) approaches that ninein expression is essential for apico-basal array formation and epithelial elongation and that CLIP-170 is required for its redeployment to n-MTOCs. Functional inhibition also revealed that IQGAP1 and active Rac1 coordinate with CLIP-170 to facilitate microtubule plus-end cortical targeting and ninein redeployment. Intestinal tissue and in vitro organoids from the Clip1/Clip2 double KO mouse with deletions in the genes encoding CLIP-170 and CLIP-115, respectively, confirmed requirement of CLIP-170 for ninein recruitment to n-MTOCs, with possible compensation by other anchoring factors such as p150^Glued and CAMSAP2 ensuring apico-basal microtubule formation despite loss of ninein at n-MTOCs.

Identification of key genes in glioma CpG island methylator phenotype via network analysis of gene expression data

Gene expression data were analysed using bioinformatic tools to demonstrate molecular mechanisms underlying the glioma CpG island methylator phenotype (CIMP). A gene expression data set (accession no. GSE30336) was downloaded from Gene Expression Omnibus, including 36 CIMP+ and 16 CIMP- glioma samples. Differential analysis was performed for CIMP+ vs. CIMP‑ samples using the limma package in R. Functional enrichment analysis was subsequently conducted for differentially expressed genes (DEGs) using Database for Annotation, Visualization and Integration Discovery. Protein‑protein interaction (PPI) networks were constructed for upregulated and downregulated genes with information from STRING. MicroRNAs (miRNAs) targeting DEGs were also predicted using WebGestalt. A total of 439 DEGs were identified, including 214 upregulated and 198 downregulated genes. The upregulated genes were involved in extracellular matrix organisation, defence and immune response, collagen fibril organisation and regulation of cell motion and the downregulated genes in cell adhesion, sensory organ development, regulation of system process, neuron differentiation and membrane organisation. A PPI network containing 134 nodes and 314 edges was constructed from the upregulated genes, whereas a PPI network consisting of 85 nodes and 80 edges was obtained from the downregulated genes. miRNAs regulating upregulated and downregulated genes were predicted, including miRNA‑124a and miRNA‑34a. Numerous key genes associated with glioma CIMP were identified in the present study. These findings may advance the understanding of glioma and facilitate the development of appropriate therapies.

Valosin-containing protein (VCP) is a novel IQ motif-containing GTPase activating protein 1 (IQGAP1)-interacting protein

Scaffold proteins play a pivotal role in making protein complexes, and organize binding partners into a functional unit to enhance specific signaling pathways. IQ motif-containing GTPase activating protein 1 (IQGAP1) is an essential protein for spine formation due to its role in scaffolding multiple signal complexes. However, it remains unclear how IQGAP1 interacts within the brain. In the present study, we screened novel IQGAP1-interacting proteins by a proteomic approach. As a novel IQGAP1-interacting protein, we identified valosin-containing protein (VCP) which is a causative gene in patients with inclusion body myopathy with Paget's disease of bone and frontotemporal dementia (IBMPFD). The physiological interaction of IQGAP1 with VCP was confirmed by an immunoprecipitation assay. Both the N-terminal (N-half) and C-terminal (C-half) fragments of IQGAP1 interacted with the N-terminal region of VCP. Co-localization of IQGAP1 and VCP was observed in the growth corn, axonal shaft, cell body, and dendrites in cultured hippocampal neurons at 4 days in vitro (DIV4). In cultured neurons at DIV14, IQGAP1 co-localized with VCP in dendrites. When HEK293T cells were co-transfected with IQGAP1 and VCP, an immunoprecipitation assay revealed that binding of IQGAP1 with disease-related mutant (R155H or A232E) VCP was markedly reduced compared to wild-type (WT) VCP. These results suggest that reduction of IQGAP1 and VCP interaction may be associated with the pathophysiology of IBMPFD.

Microtubules-associated Rac regulation of endothelial barrier: a role of Asef in acute lung injury

The endothelial barrier function regulated by the cytoskeletal reorganizations has been implicated in the pathogenesis of multiple lung diseases including asthma, sepsis, edema, and acute respiratory distress syndrome. The extensive studies have established that activation of small GTPase Rac is a key mechanism in endothelial barrier protection but the role of microtubules-associated Rac in the endothelial functions remains poorly understood. With the emerging evidences that microtubules disassembly also plays a critical role in actin cytoskeleton remodeling leading to endothelial permeability, the knowledge on microtubules-mediated regulation of endothelial barrier is imperative to better understand the etiology of lung injuries as well as to develop novel therapeutics against these disorders. In this regard, our recent studies have revealed some novel aspects of microtubules-mediated regulation of endothelial barrier functions and unraveled a putative role of Rac-specific guanine nucleotide exchange factor Asef in mediating the barrier protective effects of hepatocyte growth factor. In this review, we will discuss the role of this novel Rac activator Asef in endothelial barrier protection and its regulation by microtubules.

LGR5 receptor promotes cell-cell adhesion in stem cells and colon cancer cells via the IQGAP1-Rac1 pathway

Leucine-rich repeat-containing G protein–coupled receptor 5 (LGR5) is a bona fide marker of adult stem cells in several epithelial tissues, most notably in the intestinal crypts, and is highly up-regulated in many colorectal, hepatocellular, and ovarian cancers. LGR5 activation by R-spondin (RSPO) ligands potentiates Wnt/β-catenin signaling in vitro; however, deletion of LGR5 in stem cells has little or no effect on Wnt/β-catenin signaling or cell proliferation in vivo. Remarkably, modulation of LGR5 expression has a major impact on the actin cytoskeletal structure and cell adhesion in the absence of RSPO stimulation, but the molecular mechanism is unclear. Here, we show that LGR5 interacts with IQ motif-containing GTPase-activating protein 1 (IQGAP1), an effector of Rac1/CDC42 GTPases, in the regulation of actin cytoskeleton dynamics and cell–cell adhesion. Specifically, LGR5 decreased levels of IQGAP1 phosphorylation at Ser-1441/1443, leading to increased binding of Rac1 to IQGAP1 and thus higher levels of cortical F-actin and enhanced cell–cell adhesion. LGR5 ablation in colon cancer cells and crypt stem cells resulted in loss of cortical F-actin, reduced cell–cell adhesion, and disrupted localization of adhesion-associated proteins. No evidence of LGR5 coupling to any of the four major subtypes of heterotrimeric G proteins was found. These findings suggest that LGR5 primarily functions via the IQGAP1–Rac1 pathway to strengthen cell–cell adhesion in normal adult crypt stem cells and colon cancer cells.

Alterations in IQGAP1 expression and localization in colorectal carcinoma and liver metastases following oxaliplatin-based chemotherapy

IQGAP1 is a scaffolding protein that serves a key role in cell dynamics by integrating internal and external stimuli to distinct signal outputs. Previous studies have identified several genes that are significantly up- or downregulated in the peripheral white cells (PWCs) of patients with colorectal adenocarcinoma (CRC), who underwent oxaliplatin-based chemotherapy (CT). In addition, screening studies have reported that IQ-motif containing GTPase activating protein 1 (IQGAP1) transcriptional expression levels varied from ‘off’ to ‘on’ following oxaliplatin CT. In order to determine if variations previously described in PWCs are able to be observed at the protein level in tumors and in metastases following CT, the present study performed an immunohistochemical analysis of IQGAP1 in CRC and primary metastases. IQGAP1 expression was observed in the nuclear envelope and in lateral cell membranes and cytoplasm in normal colon tissue. However, in tumor tissue, cells exhibited a diffuse pattern, with variable expression levels of staining in the nuclear membrane and cytoplasm, with the highest expression intensity observed at the invasive front. In healthy and metastasized liver tissue and in the metastases themselves, expression levels varied from cell to cell from no expression to a high level. In the majority of cells, IQGAP1 co-localized with microtubules at the cytoplasmic face of the nuclear envelope. Strong positive expression was observed in areas of the lesion where cells were detaching from the lesion into the lumen. Despite the homogeneous IQGAP1 staining pattern observed in healthy colon tissue sections, CRC demonstrated heterogeneity in staining, which was more marked in metastasized liver tissue resected following CT. However, the most notable findings were the observed effects on the cellular and subcellular distribution and its implications for cancer biology. These results suggest that IQGAP1 may be a putative biomarker, a candidate for clinical diagnostics and a potential novel target for anti-cancer therapeutics.

Sustained α-catenin Activation at E-cadherin Junctions in the Absence of Mechanical Force

Mechanotransduction at E-cadherin junctions has been postulated to be mediated in part by a force-dependent conformational activation of α-catenin. Activation of α-catenin allows it to interact with vinculin in addition to F-actin, resulting in a strengthening of junctions. Here, using E-cadherin adhesions reconstituted on synthetic, nanopatterned membranes, we show that activation of α-catenin is dependent on E-cadherin clustering, and is sustained in the absence of mechanical force or association with F-actin or vinculin. Adhesions were formed by filopodia-mediated nucleation and micron-scale assembly of E-cadherin clusters, which could be distinguished as either peripheral or central assemblies depending on their relative location at the cell-bilayer adhesion. Whereas F-actin, vinculin, and phosphorylated myosin light chain associated only with the peripheral assemblies, activated α-catenin was present in both peripheral and central assemblies, and persisted in the central assemblies in the absence of actomyosin tension. Impeding filopodia-mediated nucleation and micron-scale assembly of E-cadherin adhesion complexes by confining the movement of bilayer-bound E-cadherin on nanopatterned substrates reduced the levels of activated α-catenin. Taken together, these results indicate that although the initial activation of α-catenin requires micron-scale clustering that may allow the development of mechanical forces, sustained force is not required for maintaining α-catenin in the active state.

New model for the interaction of IQGAP1 with CDC42 and RAC1

The specific and rapid formation of protein complexes, involving IQGAP family proteins, is essential for diverse cellular processes, such as adhesion, polarization, and directional migration. Although CDC42 and RAC1, prominent members of the RHO GTPase family, have been implicated in binding to and activating IQGAP1, the exact nature of this protein-protein recognition process has remained obscure. Here, we propose a mechanistic framework model that is based on a multiple-step binding process, which is a prerequisite for the dynamic functions of IQGAP1 as a scaffolding protein and a critical mechanism in temporal regulation and integration of cellular pathways.

RNA Interference of IQ Motif Containing GTPase-Activating Protein 3 (IQGAP3) Inhibits Cell Proliferation and Invasion in Breast Carcinoma Cells

Breast cancer is a highly prevalent disease affecting women. The association of IQ motif containing GTPase-activating protein 3 (IQGAP3) and breast cancer is poorly defined. Here we reported that IQGAP3 is a key regulator of cell proliferation and metastasis during breast cancer progression. The expression of IQGAP3 was significantly increased in breast tissues compared to nontumor tissues at both protein and mRNA levels. Furthermore, IQGAP3 had a high expression level in ZR-75-30 and BT474 compared to other breast cancer cell lines. Depletion of IQGAP3 through RNA interference in ZR-75-30 and BT474 significantly inhibited cell proliferation. More importantly, IQGAP3 silencing in breast cancer cells notably repressed cell migration and invasion. Further analysis suggested that inhibition of cell proliferation and metastasis was associated with some proteins, including p53, MMP9, Snail, CDC42, p-ERK1/2, KIF2C, KIF4A, PCNA, and Twist. Since expression of IQGAP3 seems to be associated with the pathogenesis of breast cancer and suppression of it can inhibit cancer cell growth and metastasis, IQGAP3 may be a potential therapeutic target in human breast cancer.

Hepatitis B Virus X Protein Stimulates Proliferation, Wound Closure and Inhibits Apoptosis of HuH-7 Cells via CDC42

Chronic hepatitis B virus (HBV) infection has been considered as the major cause of hepatocellular carcinoma (HCC). Hepatitis B virus X protein (HBx) has been reported to be oncogenic. The underlying mechanisms of HBV-related HCC are not fully understood, and the role played by the HBx protein in HBV induced carcinogenesis remains controversial. CDC42, a member of the Rho GTPase family, has been reported to be overexpressed in several different cancers, including HBV-related HCC. However, the specific role of CDC42 in HCC development remains unclear. Here, we investigated the cellular mechanisms by which CDC42 was responsible for the higher proliferation of HuH-7 cells mediated by HBx. We found that the expression level of CDC42 and its activity were significantly increased in HuH-7-HBx cells. The deficiency of CDC42 using the CRISPR/Cas9 system and inhibition by specific inhibitor CASIN led to the reduction of HBx-mediated proliferation. Furthermore, we observed that IQ Motif Containing GTPase Activating Protein 1 (IQGAP1), the downstream mediator of the CDC42 pathway, might be involved in the carcinogenesis induced by HBx. Therefore, the HBx/CDC42/IQGAP1 signaling pathway may potentially play an important role in HBx-mediated carcinogenesis.

HECTD1 controls the protein level of IQGAP1 to regulate the dynamics of adhesive structures

Background

Cell migration including collective cell movement and individual cell migration are crucial factors in embryogenesis. During the spreading/migration of cells, several types of adhesive structures physically interacting with the extracellular matrix (ECM) or with another cell have been described and the formation and maturation of adhesion structures are coordinated, however the molecular pathways involved are still not fully understood.

Results

We generated a mouse embryonic fibroblast line (MEF) from homozygous mutant (Hectd1^R/R, Hectd1^Gt(RRC200)) mouse of the E3 ubiquitin ligase for inhibin B receptor (Hectd1). Detailed examination of cell motion on MEF cells demonstrated that loss of Hectd1 resulted in accelerated cell spreading and migration but impaired directionality of migration. In Hectd1^R/R cells paxillin and zyxin were largely mis-localized, whereas their expression levels were unchanged. In addition the formation of focal adhesions (FAs) was impaired and the focal complexes (FXs) were increased. We further identified HECTD1 as a key regulator of IQGAP1. IQGAP1 co-localized together with HECTD1 in the leading edge of cells. HECTD1 interacted with IQGAP1 and regulated its degradation through ubiquitination. Over-expression of IQGAP1 in control MEF phenocopied the spreading and migration defects of Hectd1^R/R cells. In contrast, siRNA-mediated knockdown of IQGAP1 rescued the defects in cellular movement of Hectd1^R/R cells.

Conclusions

The E3 ligase activity of Hectd1 regulates the protein level of IQGAP1 through ubiquitination and therefore mediates the dynamics of FXs including the recruitment of paxillin and actinin. IQGAP1 is one of the effectors of HECTD1.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0156-8) contains supplementary material, which is available to authorized users.

Elevated IQGAP1 and CDC42 levels correlate with tumor malignancy of human glioma

IQGAP1 is a multifunctional scaffold protein involved in cell adhesion and cell migration. The abnormal expression of IQGAP1 widely exists in many cancers, but the combined biological roles of IQGAP1 and CDC42 in human glioma remain to be clarified. In this study, we investigated the associated expression level of IQGAP1, CDC42 and clinical significances in human glioma, as well as its biological functions in glioma progression. Our results revealed that IQGAP1 and CDC42 are frequently elevated in glioma tissues compared with their noncancerous counterparts, and a high expression of IQGAP1 and CDC42 correlates with tumor grades and poor overall survival of glioma patients. Moreover, the overexpression of IQGAP1 improves cell proliferation and migration ability of human glioma cells, whereas the knockdown of IQGAP1 by siRNA reduces cell growth and cell migration in vitro. These results suggest that IQGAP1, CDC42 and their interactions play important roles in human glioma carcinogenesis and progression.

AECHL-1 targets breast cancer progression via inhibition of metastasis, prevention of EMT and suppression of Cancer Stem Cell characteristics

Triple negative breast cancer (TNBC) features among the most aggressive manifestations of cancer due to its enhanced metastatic potential and immunity to therapeutics which target hormone receptors. Under such scenarios, anti-cancer compounds with an ability to influence multiple targets, or an entire process, will have an advantage over specific signal transduction inhibitors. To counter the metastatic threat it is essential to target cellular components central to the processes of cancer cell migration and adaptation. Our previous work on a novel triterpenoid, AECHL-1, explored its anti-cancer potential, and linked it to elevated ER stress in cancer cells, while its anti-angiogenic potential was credited for its ability to manipulate the cytoskeleton. Here, we broaden its range of action by showing that it curbs the metastatic ability of TNBC cells, both in vitro in MDA-MB-231 cell line and in vivo, in mouse models of metastasis. AECHL-1 does so by disrupting the cytoskeletal network, and also suppressing NF-κB and β-Catenin mediated key molecular pathways. These activities also contributed to AECHL-1 mediated suppression of TGF-β/TNF-α induced Epithelial to Mesenchymal Transition (EMT) and cancer stem cell characteristic. Thus, we present AECHL-1 as a promising therapeutic inhibitor of metastatic disease.

Essential role of Cdc42 in cardiomyocyte proliferation and cell-cell adhesion during heart development

Cdc42 is a member of the Rho GTPase family and functions as a molecular switch in regulating cell migration, proliferation, differentiation and survival. However, the role of Cdc42 in heart development remains largely unknown. To determine the function of Cdc42 in heart formation, we have generated a Cdc42 cardiomyocyte knockout (CCKO) mouse line by crossing Cdc42 flox mice with myosin light chain (MLC) 2a-Cre mice. The inactivation of Cdc42 in embryonic cardiomyocytes induced lethality after embryonic day 12.5. Histological analysis of CCKO embryos showed cardiac developmental defects that included thin ventricular walls and ventricular septum defects. Microarray and real-time PCR data also revealed that the expression level of p21 was significantly increased and cyclin B1 was dramatically decreased, suggesting that Cdc42 is required for cardiomyocyte proliferation. Phosphorylated Histone H3 staining confirmed that the inactivation of Cdc42 inhibited cardiomyocytes proliferation. In addition, transmission electron microscope studies showed disorganized sarcomere structure and disruption of cell-cell contact among cardiomyocytes in CCKO hearts. Accordingly, we found that the distribution of N-cadherin/β-Catenin in CCKO cardiomyocytes was impaired. Taken together, our data indicate that Cdc42 is essential for cardiomyocyte proliferation, sarcomere organization and cell-cell adhesion during heart development.

Deletion of IQGAP1 promotes Helicobacter pylori-induced gastric dysplasia in mice and acquisition of cancer stem cell properties in vitro

Helicobacter pylori infection is responsible for gastric carcinogenesis but host factors are also implicated. IQGAP1, a scaffolding protein of the adherens junctions interacting with E-cadherin, regulates cellular plasticity and proliferation. In mice, IQGAP1 deficiency leads to gastric hyperplasia. The aim of this study was to elucidate the consequences of IQGAP1 deletion on H. pylori-induced gastric carcinogenesis.Transgenic mice deleted for iqgap1 and WT littermates were infected with Helicobacter sp., and histopathological analyses of the gastric mucosa were performed. IQGAP1 and E-cadherin expression was evaluated in gastric tissues and in gastric epithelial cell lines in response to H. pylori infection. The consequences of IQGAP1 deletion on gastric epithelial cell behaviour and on the acquisition of cancer stem cell (CSC)-like properties were evaluated. After one year of infection, iqgap1+/- mice developed more preneoplastic lesions and up to 8 times more gastro-intestinal neoplasia (GIN) than WT littermates. H. pylori infection induced IQGAP1 and E-cadherin delocalization from cell-cell junctions. In vitro, knock-down of IQGAP1 favoured the acquisition of a mesenchymal phenotype and CSC-like properties induced by H. pylori infection.Our results indicate that alterations in IQGAP1 signalling promote the emergence of CSCs and gastric adenocarcinoma development in the context of an H. pylori infection.

The IQGAP1 N-Terminus Forms Dimers, and the Dimer Interface Is Required for Binding F-Actin and Calcium-Bound Calmodulin

IQGAP1 is a multidomain scaffold protein involved in many cellular processes. We have determined the crystal structure of an N-terminal fragment spanning residues 1-191 (CHDF hereafter) that contains the entire calponin homology domain. The structure of the CHDF is very similar to those of other type 3 calponin homology domains like those from calponin, Vav, and the yeast IQGAP1 ortholog Rng2. However, in the crystal, two CHDF molecules form a "head-to-head" or parallel dimer through mostly hydrophobic interactions. Binding experiments indicate that the CHDF binds to both F-actin and Ca²⁺/calmodulin, but binding is mutually exclusive. On the basis of the structure, two dimer interface substitutions were introduced. While CHDFL157D disrupts the dimer in gel filtration experiments, oxidized CHDFK161C stabilizes the dimer. These results imply that the CHDF forms the same dimer in solution that is seen in the crystal structure. The disulfide-stabilized dimer displays a reduced level of F-actin binding in sedimentation assays and shows no binding to Ca²⁺/calmodulin in isothermal titration calorimetry (ITC) experiments, indicating that interface residues are utilized for both binding events. The Calmodulin Target Database predicts that residues ⁹³KK⁹⁴ are important for CaM binding, and indeed, the ⁹³EE⁹⁴ double mutation displays a reduced level of binding to Ca²⁺/calmodulin in ITC experiments. Our results indicate that the CHDF dimer interface is used for both F-actin and Ca²⁺/calmodulin binding, and the ⁹³KK⁹⁴ pair, near the interface, is also used for Ca²⁺/calmodulin binding. These results are also consistent with full-length IQGAP1 forming a parallel homodimer.

IQGAP1 Interaction with RHO Family Proteins Revisited: KINETIC AND EQUILIBRIUM EVIDENCE FOR MULTIPLE DISTINCT BINDING SITES

IQ motif-containing GTPase activating protein 1 (IQGAP1) plays a central role in the physical assembly of relevant signaling networks that are responsible for various cellular processes, including cell adhesion, polarity, and transmigration. The RHO family proteins CDC42 and RAC1 have been shown to mainly interact with the GAP-related domain (GRD) of IQGAP1. However, the role of its RASGAP C-terminal (RGCT) and C-terminal domains in the interactions with RHO proteins has remained obscure. Here, we demonstrate that IQGAP1 interactions with RHO proteins underlie a multiple-step binding mechanism: (i) a high affinity, GTP-dependent binding of RGCT to the switch regions of CDC42 or RAC1 and (ii) a very low affinity binding of GRD and a C terminus adjacent to the switch regions. These data were confirmed by phosphomimetic mutation of serine 1443 to glutamate within RGCT, which led to a significant reduction of IQGAP1 affinity for CDC42 and RAC1, clearly disclosing the critical role of RGCT for these interactions. Unlike CDC42, an extremely low affinity was determined for the RAC1-GRD interaction, suggesting that the molecular nature of IQGAP1 interaction with CDC42 partially differs from that of RAC1. Our study provides new insights into the interaction characteristics of IQGAP1 with RHO family proteins and highlights the complementary importance of kinetic and equilibrium analyses. We propose that the ability of IQGAP1 to interact with RHO proteins is based on a multiple-step binding process, which is a prerequisite for the dynamic functions of IQGAP1 as a scaffolding protein and a critical mechanism in temporal regulation and integration of IQGAP1-mediated cellular responses.

Exploring the developmental mechanisms underlying Wolf-Hirschhorn Syndrome: Evidence for defects in neural crest cell migration

Wolf-Hirschhorn Syndrome (WHS) is a neurodevelopmental disorder characterized by mental retardation, craniofacial malformation, and defects in skeletal and heart development. The syndrome is associated with irregularities on the short arm of chromosome 4, including deletions of varying sizes and microduplications. Many of these genotypic aberrations in humans have been correlated with the classic WHS phenotype, and animal models have provided a context for mapping these genetic irregularities to specific phenotypes; however, there remains a significant knowledge gap concerning the cell biological mechanisms underlying these phenotypes. This review summarizes literature that has made recent contributions to this topic, drawing from the vast body of knowledge detailing the genetic particularities of the disorder and the more limited pool of information on its cell biology. Finally, we propose a novel characterization for WHS as a pathophysiology owing in part to defects in neural crest cell motility and migration during development.

Cell-Intrinsic Adaptation Arising from Chronic Ablation of a Key Rho GTPase Regulator

Genome-editing technologies allow systematic inactivation of human genes. Whether knockout phenotypes always reflect gene functions as determined by acute RNAi is an important question. Here we show how the acute knockdown of the Adams-Oliver syndrome (AOS) gene DOCK6, coding for a RAC1/CDC42 guanine nucleotide exchange factor, results in strikingly different phenotypes to those generated by genomic DOCK6 disruption. Cell-intrinsic adaptation compensates for loss of DOCK6 function. Prolonged DOCK6 loss impacts upon the MRTF-A/SRF transcription factor, reducing levels of the ubiquitin-like modifier ISG15. Reduced ISGylation of the IQGAP1 protein increases levels of active CDC42 and RAC1 to compensate for DOCK6 disruption. Similar downregulation of ISG15 in cells from DOCK6 AOS patients indicates that such adaptation can compensate for genetic defects during development. Thus, phenotypes of gene inactivation are critically dependent on the timescale, as acute knockdown reflects a transient state of adjustment to a new equilibrium that is attained following compensation.

A new approach for the direct visualization of the membrane cytoskeleton in cryo-electron microscopy: a comparative study with freeze-etching electron microscopy

An improved unroofing method consisting of tearing off the cell membrane using an adhesive electron microscopy (EM) grid instead of vitreous ice sectioning (cryo-sectioning) has enabled us to panoramically view the membrane cytoskeleton in its native state with extremely high contrast. Grids pre-treated with Alcian blue were placed on cells, and a portion of the dorsal plasma membrane was transferred onto the grid, which was then floated in buffer solution. These membrane fragments contained sufficient cytoskeleton and were of suitable thickness for observation by cryo-EM. Many actin filaments and microtubules were clearly observed on the cytoplasmic surface of the plasma membrane with extremely high contrast because the soluble components of the cytoplasm flowed out and broke away from the cells. Actin filaments extended in all directions in a smooth contour with little branching. Microtubules spread out as far as 3 µm or more while winding gently in their native state. Upon fixation with 1% glutaraldehyde, however, the microtubules became straight and fragmented. Cryo-EM revealed for the first time a smooth endoplasmic reticulum network beneath the cell membrane in native cells. Clathrin coats and caveolae were also observed on the cytoplasmic surface of the plasma membrane, similar to those seen using freeze-etching replica EM (freeze-etching EM). Unroofing was also useful for immuno-labelling in cryo-EM. Antibody-labelled IQGAP1, one of the effector proteins facilitating the formation of actin filament networks, was localized alongside actin filaments. Freeze-etching EM confirmed the morphological findings of cryo-EM.

Role of IQGAP1 in endothelial barrier enhancement caused by OxPAPC

Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphatidylcholine (OxPAPC) attenuates agonist-induced endothelial cell (EC) permeability and increases pulmonary endothelial barrier function via enhancement of both the peripheral actin cytoskeleton and cell junctions mediated by Rac1 and Cdc42 GTPases. This study evaluated the role for the multifunctional Rac1/Cdc42 effector and regulator, IQGAP1, as a molecular transducer of the OxPAPC-mediated EC barrier enhancing signal. IQGAP1 knockdown in endothelial cells by gene-specific siRNA abolished OxPAPC-induced enlargement of VE-cadherin-positive adherens junctions, suppressed peripheral accumulation of actin polymerization regulators, namely cortactin, N-WASP and Arp3, and attenuated remodeling of the peripheral actin cytoskeleton. Inhibition of OxPAPC-induced barrier enhancement by IQGAP1 knockdown was due to suppressed Rac1 and Cdc42 activation. Expression of an IQGAP1 truncated mutant showed that the GTPase regulatory domain (GRD) of IQGAP1 was essential for the OxPAPC-induced membrane localization of cortactin, adherens junction proteins VE-cadherin and p120-catenin as well as for EC permeability response. IQGAP1knockdown attenuated the protective effect of OxPAPC against thrombin-induced cell contraction, cell junction disruption and EC permeability. These results demonstrate for the first time the role of IQGAP1 as a critical transducer of OxPAPC-induced Rac1/Cdc42 signaling to the actin cytoskeleton and adherens junctions which promotes cortical cytoskeletal remodeling and EC barrier protective effects of oxidized phospholipids.

Protein Profiles for Muscle Development and Intramuscular Fat Accumulation at Different Post-Hatching Ages in Chickens

Muscle development and growth influences the efficiency of poultry meat production, and is closely related to deposition of intramuscular fat (IMF), which is crucial in meat quality. To clarify the molecular mechanisms underlying muscle development and IMF deposition in chickens, protein expression profiles were examined in the breast muscle of Beijing-You chickens at ages 1, 56, 98 and 140 days, using isobaric tags for relative and absolute quantification (iTRAQ). Two hundred and four of 494 proteins were expressed differentially. The expression profile at day 1 differed greatly from those at day 56, 98 and 140. KEGG pathway analysis of differential protein expression from pair-wise comparisons (day 1 vs. 56; 56 vs. 98; 98 vs. 140), showed that the fatty acid degradation pathway was more active during the stage from day 1 to 56 than at other periods. This was consistent with the change in IMF content, which was highest at day 1 and declined dramatically thereafter. When muscle growth was most rapid (days 56-98), pathways involved in muscle development were dominant, including hypertrophic cardiomyopathy, dilated cardiomyopathy, cardiac muscle contraction, tight junctions and focal adhesion. In contrast with hatchlings, the fatty acid degradation pathway was downregulated from day 98 to 140, which was consistent with the period for IMF deposition following rapid muscle growth. Changes in some key specific proteins, including fast skeletal muscle troponin T isoform, aldehyde dehydrogenase 1A1 and apolipoprotein A1, were verified by Western blotting, and could be potential biomarkers for IMF deposition in chickens. Protein-protein interaction networks showed that ribosome-related functional modules were clustered in all three stages. However, the functional module involved in the metabolic pathway was only clustered in the first stage (day 1 vs. 56). This study improves our understanding of the molecular mechanisms underlying muscle development and IMF deposition in chickens.

IQGAP1 gene silencing induces apoptosis and decreases the invasive capacity of human hepatocellular carcinoma cells

IQ motif-containing GTPase-activating proteins (IQGAPs) belong to a conserved family, and they are involved in various intracellular processes. IQGAP1 is expressed in all cells, while IQGAP2 and IQGAP3 are mainly expressed in hepatic cells. IQGAP1 has been suggested to be an oncogene, while IQGAP2 is considered a tumor-suppressor gene. However, the relationship between RAS family genes and IQGAP genes remains unclear. We recently demonstrated this interaction in a chemically induced mouse liver cancer. In this study, IQGAP1 expression was partially silenced in human hepatocellular carcinoma (HepG2) cells. We investigated the impact of IQGAP1 silencing on the interactions of IQGAP and RAS with several apoptotic proteins, including caspase-3 (CASP3), BCL2-associated X protein (BAX), and B-cell leukemia/lymphoma 2 (BCL2). Additionally, we investigated the effects of the interactions of these genes on cell viability, proliferation, apoptosis, and invasive capacity. IQGAP1 siRNA-treated HepG2 cells showed lower invasive capacity than the control cells, and this reduction was time- and vector concentration-dependent. In addition, IQGAP1 silencing resulted in significantly lower IQGAP1 level and subsequently higher IQGAP2 and IQGAP3 expression in HepG2 cells than in the control. Flow cytometry analyses indicated that the silencing of IQGAP1 can induce early and late apoptosis in HepG2 cells. Additionally, IQGAP2, IQGAP3, CASP3, and BAX were upregulated whereas IQGAP1 and BCL2 were downregulated in the siRNA-treated cells. Furthermore, we observed that the mRNA levels of HRAS, KRAS, NRAS, and MRAS decreased upon IQGAP1 silencing. These findings indicate that IQGAP1 potentially regulates the expression of IQGAP and RAS gene families and demonstrate its regulatory role in the apoptotic network. Taken together, our findings suggest that IQGAP1 silencing plays crucial roles in the apoptosis of HepG2 cells and lowers their proliferative and invasive capacities.

Proteomic analysis of Rac1 signaling regulation by guanine nucleotide exchange factors

The small GTPase Rac1 is implicated in various cellular processes that are essential for normal cell function. Deregulation of Rac1 signaling has also been linked to a number of diseases, including cancer. The diversity of Rac1 functioning in cells is mainly attributed to its ability to bind to a multitude of downstream effectors following activation by Guanine nucleotide Exchange Factors (GEFs). Despite the identification of a large number of Rac1 binding partners, factors influencing downstream specificity are poorly defined, thus hindering the detailed understanding of both Rac1's normal and pathological functions. In a recent study, we demonstrated a role for 2 Rac-specific GEFs, Tiam1 and P-Rex1, in mediating Rac1 anti- versus pro-migratory effects, respectively. Importantly, via conducting a quantitative proteomic screen, we identified distinct changes in the Rac1 interactome following activation by either GEF, indicating that these opposing effects are mediated through GEF modulation of the Rac1 interactome. Here, we present the full list of identified Rac1 interactors together with functional annotation of the differentially regulated Rac1 binding partners. In light of this data, we also provide additional insights into known and novel signaling cascades that might account for the GEF-mediated Rac1-driven cellular effects.

Scaffolding protein IQGAP1: an insulin-dependent link between caveolae and the cytoskeleton in primary human adipocytes?

The ubiquitously expressed IQ motif-containing GTPase activating protein-1 (IQGAP1) is a scaffolding protein implicated in an array of cellular functions, in particular by binding to cytoskeletal elements and signaling proteins. A role of IQGAP1 in adipocytes has not been reported. We therefore investigated the cellular IQGAP1 interactome in primary human adipocytes. Immunoprecipitation and quantitative mass spectrometry identified caveolae and caveolae-associated proteins as the major IQGAP1 interactors alongside cytoskeletal proteins. We confirmed co-localization of IQGAP1 with the defining caveolar marker protein caveolin-1 by confocal microscopy and proximity ligation assay. Most interestingly, insulin enhanced the number of IQGAP1 interactions with caveolin-1 by five-fold. Moreover, we found a significantly reduced abundance of IQGAP1 in adipocytes from patients with type 2 diabetes compared with cells from nondiabetic control subjects. Both the abundance of IQGAP1 protein and mRNA were reduced, indicating a transcriptional defect in diabetes. Our findings suggest a novel role of IQGAP1 in insulin-regulated interaction between caveolae and cytoskeletal elements of the adipocyte, and that this is quelled in the diabetic state.

ISL1 cardiovascular progenitor cells for cardiac repair after myocardial infarction

Cardiovascular progenitor cells (CPCs) expressing the ISL1-LIM-homeodomain transcription factor contribute developmentally to cardiomyocytes in all 4 chambers of the heart. Here, we show that ISL1-CPCs can be applied to myocardial regeneration following injury. We used a rapid 3D methylcellulose approach to form murine and human ISL1-CPC spheroids that engrafted after myocardial infarction in murine hearts, where they differentiated into cardiomyocytes and endothelial cells, integrating into the myocardium and forming new blood vessels. ISL1-CPC spheroid-treated mice exhibited reduced infarct area and increased blood vessel formation compared with control animals. Moreover, left ventricular (LV) contractile function was significantly better in mice transplanted with ISL1-CPCs 4 weeks after injury than that in control animals. These results provide proof-of-concept of a cardiac repair strategy employing ISL1-CPCs that, based on our previous lineage-tracing studies, are committed to forming heart tissue, in combination with a robust methylcellulose spheroid-based delivery approach.

Microtubule Transport in the Axon: Re-thinking a Potential Role for the Actin Cytoskeleton

Microtubules are transported down the axon as short pieces by molecular motor proteins. One popular idea is that these microtubules are transported by forces generated against the actin cytoskeleton. The motor for such transport is thought to be cytoplasmic dynein. Here, the authors review this model and discuss recent studies that sought to test it. These studies suggest that the model is valid but incomplete. Microtubule transport is bidirectional and can utilize either actin filaments or longer microtubules as a substrate in the anterograde direction but only longer microtubules in the retrograde direction. Cytoplasmic dynein is one participating motor but not the only one. The authors speculate that the category of anterograde microtubule transport that involves actin filaments may have specialized functions. The relevant forces that transport short microtubules may also be crucial for the manner by which the longer immobile microtubules interact with actin filaments during events such as axonal retraction and growth cone turning.

Microtubules Inhibit E-Cadherin Adhesive Activity by Maintaining Phosphorylated p120-Catenin in a Colon Carcinoma Cell Model

Tight regulation of cadherin-mediated intercellular adhesions is critical to both tissue morphogenesis during development and tissue homeostasis in adults. Cell surface expression of the cadherin-catenin complex is often directly correlated with the level of adhesion, however, examples exist where cadherin appears to be inactive and cells are completely non-adhesive. The state of p120-catenin phosphorylation has been implicated in regulating the adhesive activity of E-cadherin but the mechanism is currently unclear. We have found that destabilization of the microtubule cytoskeleton, independent of microtubule plus-end dynamics, dephosphorylates p120-catenin and activates E-cadherin adhesion in Colo 205 cells. Through chemical screening, we have also identified several kinases as potential regulators of E-cadherin adhesive activity. Analysis of several p120-catenin phosphomutants suggests that gross dephosphorylation of p120-catenin rather than that of specific amino acids may trigger E-cadherin adhesion. Uncoupling p120-catenin binding to E-cadherin at the membrane causes constitutive adhesion in Colo 205 cells, further supporting an inhibitory role of phosphorylated p120-catenin on E-cadherin activity.