Effects of brefeldin A-inhibited guanine nucleotide-exchange (BIG) 1 and KANK1 proteins on cell polarity and directed migration during wound healing

The lipid flippase ATP8A1 regulates the recruitment of ARF effectors to the trans-Golgi Network

Formation of transport vesicles requires the coordinate activity of the coating machinery that selects cargo into the nascent vesicle and the membrane bending machinery that imparts curvature to the forming bud. Vesicle coating at the trans-Golgi Network (TGN) involves AP1, GGA2 and clathrin, which are recruited to membranes by activated ARF GTPases. The ARF activation at the TGN is mediated by the BIG1 and BIG2 guanine nucleotide exchange factors (GEFs). Membrane deformation at the TGN has been shown to be mediated by lipid flippases, including ATP8A1, that moves phospholipids from the inner to the outer leaflet of the TGN membrane. We probed a possible coupling between the coating and deformation machineries by testing for an interaction between BIG1, BIG2 and ATP8A1, and by assessing whether such an interaction may influence coating efficiency. Herein, we document that BIG1 and BIG2 co-localize with ATP8A1 in both, static and highly mobile TGN elements, and that BIG1 and BIG2 bind ATP8A1. We show that the interaction involves the catalytic Sec7 domain of the GEFs and the cytosolic C-terminal tail of ATP8A1. Moreover, we report that the expression of ATP8A1, but not ATP8A1 lacking the GEF-binding cytosolic tail, increases the generation of activated ARFs at the TGN and increases the selective recruitment of AP1, GGA2 and clathrin to TGN membranes. This occurs without increasing BIG1 or BIG2 levels at the TGN, suggesting that the binding of the ATP8A1 flippase tail to the Sec7 domain of BIG1/BIG2 increases their catalytic activity. Our results support a model in which a flippase component of the deformation machinery impacts the activity of the GEF component of the coating machinery.

Potential involvement of KANK1 haploinsufficiency in centrosome aberrations

KANK1 was found as a tumor suppressor gene based on frequent deletions in renal cell carcinoma and the inhibitory activity of tumor cell proliferation. Previously, we reported that knockdown of KANK1 induced centrosomal amplification, leading to abnormal cell division, through the hyperactivation of RhoA small GTPase. Here, we investigated the loss of KANK1 function by performing CRISPR/Cas9-based genome editing to knockout the gene. After several rounds of genome editing, however, there were no cell lines with complete loss of KANK1, and the less the wild-type KANK1 dosage, the greater the number of cells with abnormal numbers of centrosomes and rates of cell-doubling and apoptosis, suggesting the involvement of KANK1 haploinsufficiency in centrosome aberrations. The rescue of KANK1-knockdown cells with a KANK1-expressing plasmid restored the rates of cells exhibiting centrosomal amplification to the control level. RNA-sequencing analysis of the cells with reduced dosages of functional KANK1 revealed potential involvement of other cell proliferation-related genes, such as EGR1, MDGA2, and BMP3, which have been reported to show haploinsufficiency when they function. When EGR1 protein expression was reduced by siRNA technology, the number of cells exhibiting centrosomal amplification increased, along with the reduction of KANK1 protein expression, suggesting their functional relationship. Thus, KANK1 haploinsufficiency may contribute to centrosome aberrations through the network of haploinsufficiency-related genes.

The Protective Role of KANK1 in Podocyte Injury

Approximately 30% of steroid-resistant nephrotic syndromes are attributed to monogenic disorders that involve 27 genes. Mutations in KANK family members have also been linked to nephrotic syndrome; however, the precise mechanism remains elusive. To investigate this, podocyte-specific Kank1 knockout mice were generated to examine phenotypic changes. In the initial assessment under normal conditions, Kank1 knockout mice showed no significant differences in the urinary albumin-creatinine ratio, blood urea nitrogen, serum creatinine levels, or histological features compared to controls. However, following kidney injury with adriamycin, podocyte-specific Kank1 knockout mice exhibited a significantly higher albumin-creatinine ratio and a significantly greater sclerotic index than control mice. Electron microscopy revealed more extensive foot process effacement in the knockout mice than in control mice. In addition, KANK1-deficient human podocytes showed increased detachment and apoptosis following adriamycin exposure. These findings suggest that KANK1 may play a protective role in mitigating podocyte damage under pathological conditions.

Circ_0059354 aggravates the progression of papillary thyroid carcinoma by elevating ARFGEF1 through sponging miR-766-3p

BACKGROUND

Circular RNAs (circRNAs) have been identified as vital players in tumors, including papillary thyroid carcinoma (PTC). The purpose of this study is to explore the functions of circ_0059354 on PTC development.

METHODS

Quantitative real-time polymerase chain reaction (qRT-PCR) was conducted to examine the levels of circ_0059354, microRNA-766-3p (miR-766-3p) and ADP ribosylation factor guanine nucleotide exchange factor 1 (ARFGEF1). Cell Counting Kit-8 (CCK-8) assay and colony formation assay were proceeded for cell proliferation ability. Transwell assay was conducted for cell migration and invasion. Tube formation assay was employed to examine the angiogenesis ability. Flow cytometry analysis was adopted for cell apoptosis. Western blot assay was conducted for protein levels. Dual-luciferase reporter assay and RNA immunoprecipitation (RIP) assay were utilized to verify the relationships among circ_0059354, miR-766-3p and ARFGEF1. The murine xenograft model was constructed to analyze the function of circ_0059354 in vivo.

RESULTS

Circ_0059354 level was abnormally increased in PTC tissues and cells. Functionally, circ_0059354 silencing suppressed cell proliferation, migration, invasion and angiogenesis and facilitated apoptosis in PTC cells. Circ_0059354 was identified to sponge miR-766-3p, which directly targeted ARFGEF1. Moreover, circ_0059354 directly targeted miR-766-3p to positively regulated ARFGEF1 expression. MiR-766-3p inhibition reversed circ_0059354 knockdown-mediated effect of PTC cell malignant behaviors. Overexpression of miR-766-3p restrained the malignant behaviors of PTC cells, whereas ARFGEF1 elevation reversed the effects. Additionally, circ_0059354 deficiency blocked tumor growth in vivo.

CONCLUSION

Circ_0059354 served as an oncogene in PTC progression through regulating miR-766-3p/ARFGEF1 axis.

KIF21A regulates breast cancer aggressiveness and is prognostic of patient survival and tumor recurrence

PURPOSE

Invasion of carcinoma cells into surrounding tissue affects breast cancer staging, influences choice of treatment, and impacts on patient outcome. KIF21A is a member of the kinesin superfamily that has been well-studied in congenital extraocular muscle fibrosis. However, its biological relevance in breast cancer is unknown. This study investigated the functional roles of KIF21A in this malignancy and examined its expression pattern in breast cancer tissue.

METHODS

The function of KIF21A in breast carcinoma was studied in vitro by silencing its expression in breast cancer cells and examining the changes in cellular activities. Immunohistochemical staining of breast cancer tissue microarrays was performed to determine the expression patterns of KIF21A.

RESULTS

Knocking down the expression of KIF21A using siRNA in MDA-MB-231 and MCF7 human breast cancer cells resulted in significant decreases in tumor cell migration and invasiveness. This was associated with reduced Patched 1 expression and F-actin microfilaments. Additionally, the number of focal adhesion kinase- and paxillin-associated focal adhesions was increased. Immunohistochemical staining of breast cancer tissue microarrays showed that KIF21A was expressed in both the cytoplasmic and nuclear compartments of carcinoma cells. Predominance of cytoplasmic KIF21A was significantly associated with larger tumors and high grade cancer, and prognostic of cause-specific overall patient survival and breast cancer recurrence.

CONCLUSION

The data demonstrates that KIF21A is an important regulator of breast cancer aggressiveness and may be useful in refining prognostication of this malignant disease.

Digital Image Analysis Applied to Tumor Cell Proliferation, Aggressiveness, and Migration-Related Protein Synthesis in Neuroblastoma 3D Models

Patient-derived cancer 3D models are a promising tool that will revolutionize personalized cancer therapy but that require previous knowledge of optimal cell growth conditions and the most advantageous parameters to evaluate biomimetic relevance and monitor therapy efficacy. This study aims to establish general guidelines on 3D model characterization phenomena, focusing on neuroblastoma. We generated gelatin-based scaffolds with different stiffness and performed SK-N-BE(2) and SH-SY5Y aggressive neuroblastoma cell cultures, also performing co-cultures with mouse stromal Schwann cell line (SW10). Model characterization by digital image analysis at different time points revealed that cell proliferation, vitronectin production, and migration-related gene expression depend on growing conditions and are specific to the tumor cell line. Morphometric data show that 3D in vitro models can help generate optimal patient-derived cancer models, by creating, identifying, and choosing patterns of clinically relevant artificial microenvironments to predict patient tumor cell behavior and therapeutic responses.

BIG1 controls macrophage pro-inflammatory responses through ARF3-mediated PI(4,5)P2 synthesis

Sepsis is caused by a dysregulated host inflammatory response to serious infections resulting in life-threatening organ dysfunction. The high morbidity and mortality make sepsis still a major clinical problem. Here, we investigated the roles of Brefeldin A-inhibited guanine nucleotide-exchange factor 1 (BIG1) in the pathogenesis process of sepsis and the underlying mechanisms. We found myeloid cell-specific BIG1 knockout (BIG1 cKO) significantly reduced the mortality and organ damage in LPS-induced and CLP-induced polymicrobial sepsis mouse model. The serum concentration and mRNA expression of pro-inflammatory cytokines including TNF-α, IL-6, IL-1β, and IL-12 were obviously decreased in BIG1 cKO mice. In bone marrow-derived macrophages or THP-1 cells, BIG1 deficiency caused an inhibited ARF3 activation, which reduced PI(4,5)P2 synthesis and the recruitment of TIRAP to the plasma membrane through inhibiting the activation of PIP5K induced by LPS, and eventually resulted in the inhibitory activity of TLR4-MyD88 signaling pathway. These results reveal a crucial new role of BIG1 in regulating macrophage inflammation responses, and provide evidence for BIG1 as a potential promising therapeutic target in sepsis.

Regulating the regulators: role of phosphorylation in modulating the function of the GBF1/BIG family of Sec7 ARF-GEFs

Membrane traffic between secretory and endosomal compartments is vesicle-mediated and must be tightly balanced to maintain a physiological compartment size. Vesicle formation is initiated by guanine nucleotide exchange factors (GEFs) that activate the ARF family of small GTPases. Regulatory mechanisms, including reversible phosphorylation, allow ARF-GEFs to support vesicle formation only at the right time and place in response to cellular needs. Here, we review current knowledge of how the Golgi-specific brefeldin A-resistance factor 1 (GBF1)/brefeldin A-inhibited guanine nucleotide exchange protein (BIG) family of ARF-GEFs is influenced by phosphorylation and use predictive paradigms to propose new regulatory paradigms. We describe a conserved cluster of phosphorylation sites within the N-terminal domains of the GBF1/BIG ARF-GEFs and suggest that these sites may respond to homeostatic signals related to cell growth and division. In the C-terminal region, GBF1 shows phosphorylation sites clustered differently as compared with the similar configuration found in both BIG1 and BIG2. Despite this similarity, BIG1 and BIG2 phosphorylation patterns are divergent in other domains. The different clustering of phosphorylation sites suggests that the nonconserved sites may represent distinct regulatory nodes and specify the function of GBF1, BIG1, and BIG2.

KANK2 Links αVβ5 Focal Adhesions to Microtubules and Regulates Sensitivity to Microtubule Poisons and Cell Migration

Integrins are heterodimeric glycoproteins that bind cells to extracellular matrix. Upon integrin clustering, multimolecular integrin adhesion complexes (IACs) are formed, creating links to the cell cytoskeleton. We have previously observed decreased cell migration and increased sensitivity to microtubule (MT) poisons, paclitaxel and vincristine, in the melanoma cell line MDA-MB-435S upon transfection with integrin αV-specific siRNA, suggesting a link between adhesion and drug sensitivity. To elucidate the underlying mechanism, we determined αV-dependent changes in IAC composition. Using mass spectrometry (MS)-based proteomics, we analyzed the components of isolated IACs of MDA-MB-435S cells and two MDA-MB-435S-derived integrin αV-specific shRNA-expressing cell clones with decreased expression of integrin αV. MS analysis showed that cells preferentially use integrin αVβ5 for the formation of IACs. The differential analysis between MDA-MB-435S cells and clones with decreased expression of integrin αV identified key components of integrin αVβ5 adhesion complexes as talins 1 and 2, α-actinins 1 and 4, filamins A and B, plectin and vinculin. The data also revealed decreased levels of several components of the cortical microtubule stabilization complex, which recruits MTs to adhesion sites (notably liprins α and β, ELKS, LL5β, MACF1, KANK1, and KANK2), following αV knockdown. KANK2 knockdown in MDA-MB-435S cells mimicked the effect of integrin αV knockdown and resulted in increased sensitivity to MT poisons and decreased migration. Taken together, we conclude that KANK2 is a key molecule linking integrin αVβ5 IACs to MTs, and enabling the actin-MT crosstalk that is important for both sensitivity to MT poisons and cell migration.

Involvement of BIG1 and BIG2 in regulating VEGF expression and angiogenesis

VEGF stimulates the formation of new blood vessels by inducing endothelial cell (EC) proliferation and migration. Brefeldin A (BFA)-inhibited guanine nucleotide-exchange protein (BIG)1 and 2 accelerate the replacement of bound GDP with GTP to activate ADP-ribosylation factor (Arf)1, which regulates vesicular transport between the Golgi and plasma membrane. Although it has been reported that treating cells with BFA interferes with Arf1 activation to inhibit VEGF secretion, the role of BIG1 and BIG2 in VEGF trafficking and expression, EC migration and proliferation, and vascular development remains unknown. Here, we found that inactivation of Arf1 reduced VEGF secretion but did not affect the levels of VEGF protein. Interestingly, however, BIG1 and BIG2 knockdown significantly decreased the levels of VEGF mRNA and protein in glioblastoma U251 cells and HUVECs. Furthermore, depletion of BIG1 and BIG2 inhibited HUVEC angiogenesis by diminishing cell migration. Angioblast migration and intersegmental vessel sprouting were also impaired when the BIG2 homolog, Arf guanine nucleotide exchange factor (arfgef)2, was knocked down in zebrafish with endothelial expression of green fluorescent protein (GFP). Depletion of arfgef2 by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) also caused defects in vascular development of zebrafish embryos. Taken together, these data reveal that BIG1 and BIG2 participate in endothelial cell angiogenesis.-Lu, F.-I., Wang, Y.-T., Wang, Y.-S., Wu, C.-Y., Li, C.-C. Involvement of BIG1 and BIG2 in regulating VEGF expression and angiogenesis.

miR-215 suppresses papillary thyroid cancer proliferation, migration, and invasion through the AKT/GSK-3β/Snail signaling by targeting ARFGEF1

The incidence of papillary thyroid cancer (PTC) has been rapidly increasing in recent years. PTC is prone to lymph node metastasization, which further increases the recurrence rate and mortality of thyroid cancer. However, the underlying mechanisms of this process remain elusive. Several reports have shown that the microRNA miR-215 plays an important role in cancer metastasis. Here, we investigated, for the first time, the potential association between miR-215 and metastasis in PTC. The results of qPCR analysis demonstrated that miR-215 was downregulated in PTC cell lines and tissues, and lower levels of miR-215 correlated with lymph node metastasis of PTC. In vitro and in vivo assays revealed that restoration of miR-215 dramatically inhibited PTC cell proliferation and metastasis. We identified ADP ribosylation factor guanine nucleotide-exchange factor 1 (ARFGEF1) as the target, which mediated the function of miR-215. The expression of ARFGEF1 was inhibited by miR-215, and the effects of miR-215 were abrogated by re-expression of ARFGEF1. Moreover, we found that miR-215 suppressed PTC metastasis by modulating the epithelial–mesenchymal transition via the AKT/GSK-3β/Snail signaling. In summary, our study proves that miR-215 inhibits PTC proliferation and metastasis by targeting ARFGEF1 and indicates miR-215 as a biomarker for PTC prognosis.

Golgi Stabilization, Not Its Front-Rear Bias, Is Associated with EMT-Enhanced Fibrillar Migration

Epithelial-to-mesenchymal transition (EMT) and maturation of collagen fibrils in the tumor microenvironment play a significant role in cancer cell invasion and metastasis. Confinement along fiber-like tracks enhances cell migration. To what extent and in what manner EMT further promotes migration in a microenvironment already conducive to migration is poorly understood. Here, we show that TGFβ-mediated EMT significantly enhances migration on fiber-like micropatterned tracks of collagen, doubling migration speed and tripling persistence relative to untreated mammary epithelial cells. Thus, cell-intrinsic EMT and extrinsic fibrillar tracks have nonredundant effects on motility. To better understand EMT-enhanced fibrillar migration, we investigated the regulation of Golgi positioning, which is involved in front-rear polarization and persistent cell migration. Confinement along fiber-like tracks has been reported to favor posterior Golgi positioning, whereas anterior positioning is observed during 2-day wound healing. Although EMT also regulates cell polarity, little is known about its effect on Golgi positioning. Here, we show that EMT induces a 2:1 rearward bias in Golgi positioning; however, positional bias explains less than 2% of single-cell variability in migration speed and persistence. Meanwhile, EMT significantly stabilizes Golgi positioning. Cells that enhance migration in response to TGFβ maintain Golgi position for 2- to 4-fold longer than nonresponsive counterparts irrespective of whether the Golgi is ahead or behind the nucleus. In fact, 28% of TGFβ-responsive cells exhibit a fully committed Golgi phenotype with the organelle either in the anterior or posterior position for over 90% of the time. Furthermore, single-cell differences in Golgi stability capture up to 18% of variations in migration speed. These results suggest a hypothesis that the Golgi may be part of a core physical scaffold that affects how cell-generated forces are distributed during migration. A stable scaffold would be expected to more consistently and therefore more productively distribute forces over time, leading to efficient migration.

Structural insights into ankyrin repeat-mediated recognition of the kinesin motor protein KIF21A by KANK1, a scaffold protein in focal adhesion

Kidney ankyrin repeat-containing proteins (KANK1/2/3/4) belong to a family of scaffold proteins, playing critical roles in cytoskeleton organization, cell polarity, and migration. Mutations in KANK proteins are implicated in cancers and genetic diseases, such as nephrotic syndrome. KANK proteins can bind various target proteins through different protein regions, including a highly conserved ankyrin repeat domain (ANKRD). However, the molecular basis for target recognition by the ANKRD remains elusive. In this study, we solved a high-resolution crystal structure of the ANKRD of KANK1 in complex with a short sequence of the motor protein kinesin family member 21A (KIF21A), revealing that the highly specific target-binding mode of the ANKRD involves combinatorial use of two interfaces. Mutations in either interface disrupted the KANK1-KIF21A interaction. Cellular immunofluorescence localization analysis indicated that binding-deficient mutations block recruitment of KIF21A to focal adhesions by KANK1. In conclusion, our structural study provides mechanistic explanations for the ANKRD-mediated recognition of KIF21A and for many disease-related mutations identified in human KANK proteins.

Structural analyses of key features in the KANK1·KIF21A complex yield mechanistic insights into the cross-talk between microtubules and the cell cortex

The cross-talk between dynamic microtubules and the cell cortex plays important roles in cell division, polarity, and migration. A critical adaptor that links the plus ends of microtubules with the cell cortex is the KANK N-terminal motif and ankyrin repeat domains 1 (KANK1)/kinesin family member 21A (KIF21A) complex. Genetic defects in these two proteins are associated with various cancers and developmental diseases, such as congenital fibrosis of the extraocular muscles type 1. However, the molecular mechanism governing the KANK1/KIF21A interaction and the role of the conserved ankyrin (ANK) repeats in this interaction are still unclear. In this study, we present the crystal structure of the KANK1·KIF21A complex at 2.1 Å resolution. The structure, together with biochemical studies, revealed that a five-helix-bundle-capping domain immediately preceding the ANK repeats of KANK1 forms a structural and functional supramodule with its ANK repeats in binding to an evolutionarily conserved peptide located in the middle of KIF21A. We also show that several missense mutations present in cancer patients are located at the interface of the KANK1·KIF21A complex and destabilize its formation. In conclusion, our study elucidates the molecular basis underlying the KANK1/KIF21A interaction and also provides possible mechanistic explanations for the diseases caused by mutations in KANK1 and KIF21A.

BIG1 is required for the survival of deep layer neurons, neuronal polarity, and the formation of axonal tracts between the thalamus and neocortex in developing brain

BIG1, an activator protein of the small GTPase, Arf, and encoded by the Arfgef1 gene, is one of candidate genes for epileptic encephalopathy. To know the involvement of BIG1 in epileptic encephalopathy, we analyzed BIG1-deficient mice and found that BIG1 regulates neurite outgrowth and brain development in vitro and in vivo. The loss of BIG1 decreased the size of the neocortex and hippocampus. In BIG1-deficient mice, the neuronal progenitor cells (NPCs) and the interneurons were unaffected. However, Tbr1+ and Ctip2+ deep layer (DL) neurons showed spatial-temporal dependent apoptosis. This apoptosis gradually progressed from the piriform cortex (PIR), peaked in the neocortex, and then progressed into the hippocampus from embryonic day 13.5 (E13.5) to E17.5. The upper layer (UL) and DL order in the neocortex was maintained in BIG1-deficient mice, but the excitatory neurons tended to accumulate before their destination layers. Further pulse-chase migration assay showed that the migration defect was non-cell autonomous and secondary to the progression of apoptosis into the BIG1-deficient neocortex after E15.5. In BIG1-deficient mice, we observed an ectopic projection of corticothalamic axons from the primary somatosensory cortex (S1) into the dorsal lateral geniculate nucleus (dLGN). The thalamocortical axons were unable to cross the diencephalon-telencephalon boundary (DTB). In vitro, BIG1-deficient neurons showed a delay in neuronal polarization. BIG1-deficient neurons were also hypersensitive to low dose glutamate (5 μM), and died via apoptosis. This study showed the role of BIG1 in the survival of DL neurons in developing embryonic brain and in the generation of neuronal polarity.

Depletion of tumor suppressor Kank1 induces centrosomal amplification via hyperactivation of RhoA

Chromosome instability, frequently found in cancer cells, is caused by a deficiency in cell division, including centrosomal amplification and cytokinesis failure, and can result in abnormal chromosome content or aneuploidy. The small GTPase pathways have been implicated as important processes in cell division. We found that knockdown of a tumor suppressor protein Kank1 increases the number of cells with a micronucleus or bi-/multi-nuclei, which was likely caused by centrosomal amplification. Kank1 interacts with Daam1, known to bind to and activate a small GTPase, RhoA, in actin assembly. Knockdown of Kank1 or overexpression of Daam1, respectively, hyperactivates RhoA, potentially leading to the modulation of the activity of Aurora-A, a key regulator of centrosomal functions, eventually resulting in centrosomal amplification. Kank1 is also associated with contractile ring formation in collaboration with RhoA, and its deficiency results in the interruption of normal daughter cell separation, generating multinucleate cells. Such abnormal segregation of chromosomes may cause further chromosomal instability and abnormal gene functions, leading to tumorigenesis. Thus, Kank1 plays a crucial role in regulating the activity of RhoA through retrieving excess Daam1 and balancing the activities of RhoA and its effectors.

KANK1 inhibits cell growth by inducing apoptosis through regulating CXXC5 in human malignant peripheral nerve sheath tumors

Malignant peripheral nerve sheath tumors (MPNSTs) are a type of rare sarcomas with a poor prognosis due to its highly invasive nature and limited treatment options. Currently there is no targeted-cancer therapy for this type of malignancy. Thus, it is important to identify more cancer driver genes that may serve as targets of cancer therapy. Through comparative oncogenomics, we have found that KANK1 was a candidate tumor suppressor gene (TSG) for human MPNSTs. Although KANK1 is known as a cytoskeleton regulator, its tumorigenic function in MPNSTs remains largely unknown. In this study, we report that restoration of KANK1 in human MPNST cells inhibits cell growth both in human cell culture and xenograft mice by increasing apoptosis. Consistently, knockdown of KANK1 in neurofibroma cells promoted cell growth. Using RNA-seq analysis, we identified CXXC5 and other apoptosis-related genes, and demonstrated that CXXC5 is regulated by KANK1. Knockdown of CXXC5 was found to diminish KANK1-induced apoptosis in MPNST cells. Thus, KANK1 inhibits MPNST cell growth though CXXC5 mediated apoptosis. Our results suggest that KANK1 may function as a tumor suppressor in human MPNSTs, and thus it may be useful for targeted therapy.

Brefeldin A-Inhibited Guanine Nucleotide-Exchange Factor 1 (BIG1) Governs the Recruitment of Tumor Necrosis Factor Receptor-Associated Factor 2 (TRAF2) to Tumor Necrosis Factor Receptor 1 (TNFR1) Signaling Complexes

Tumor necrosis factor receptor-associated factor 2 (TRAF2) is a critical mediator of tumor necrosis factor-α (TNF-α) signaling. However, the regulatory mechanisms of TRAF2 are not fully understood. Here we show evidence that TRAF2 requires brefeldin A-inhibited guanine nucleotide-exchange factor 1 (BIG1) to be recruited into TNF receptor 1 (TNFR1) signaling complexes. In BIG1 knockdown cells, TNF-α-induced c-Jun N-terminal kinase (JNK) activation was attenuated and the sensitivity to TNF-α-induced apoptosis was increased. Since these trends correlated well with those of TRAF2 deficient cells as previously demonstrated, we tested whether BIG1 functions as an upstream regulator of TRAF2 in TNFR1 signaling. As expected, we found that knockdown of BIG1 suppressed TNF-α-dependent ubiquitination of TRAF2 that is required for JNK activation, and impaired the recruitment of TRAF2 to the TNFR1 signaling complex (complex I). Moreover, we found that the recruitment of TRAF2 to the death-inducing signaling complex termed complex II was also impaired in BIG1 knockdown cells. These results suggest that BIG1 is a key component of the machinery that drives TRAF2 to the signaling complexes formed after TNFR1 activation. Thus, our data demonstrate a novel and unexpected function of BIG1 that regulates TNFR1 signaling by targeting TRAF2.

Arf proteins in cancer cell migration

Members of the ADP-ribosylation factor (Arf) family of small GTP-binding (G) proteins regulate several aspects of membrane trafficking, such as vesicle budding, tethering and cytoskeleton organization. Arf family members, including Arf-like (Arl) proteins have been implicated in several essential cellular functions, like cell spreading and migration. These functions are used by cancer cells to disseminate and invade the tissues surrounding the primary tumor, leading to the formation of metastases. Indeed, Arf and Arl proteins, as well as their guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) have been found to be abnormally expressed in different cancer cell types and human cancers. Here, we review the current evidence supporting the involvement of Arf family proteins and their GEFs and GAPs in cancer progression, focusing on 3 different mechanisms: cell-cell adhesion, integrin internalization and recycling, and actin cytoskeleton remodeling.

Talin-KANK1 interaction controls the recruitment of cortical microtubule stabilizing complexes to focal adhesions

The cross-talk between dynamic microtubules and integrin-based adhesions to the extracellular matrix plays a crucial role in cell polarity and migration. Microtubules regulate the turnover of adhesion sites, and, in turn, focal adhesions promote the cortical microtubule capture and stabilization in their vicinity, but the underlying mechanism is unknown. Here, we show that cortical microtubule stabilization sites containing CLASPs, KIF21A, LL5β and liprins are recruited to focal adhesions by the adaptor protein KANK1, which directly interacts with the major adhesion component, talin. Structural studies showed that the conserved KN domain in KANK1 binds to the talin rod domain R7. Perturbation of this interaction, including a single point mutation in talin, which disrupts KANK1 binding but not the talin function in adhesion, abrogates the association of microtubule-stabilizing complexes with focal adhesions. We propose that the talin-KANK1 interaction links the two macromolecular assemblies that control cortical attachment of actin fibers and microtubules.

DOI:http://dx.doi.org/10.7554/eLife.18124.001

Animal cells are organized into tissues and organs. A scaffold-like framework outside of the cells called the extracellular matrix provides support to the cells and helps to hold them in place. Cells attach to the extracellular matrix via structures called focal adhesions on the cell surface; these structures contain a protein called talin.

For a cell to be able to move, the existing focal adhesions must be broken down and new adhesions allowed to form. This process is regulated by the delivery and removal of different materials along fibers called microtubules. Microtubules can usually grow and shrink rapidly, but near focal adhesions they are captured at the surface of the cell and become more stable. However, it is not clear how focal adhesions promote microtubule capture and stability.

Bouchet et al. found that a protein called KANK1 binds to the focal adhesion protein talin in human cells grown in a culture dish. This allows KANK1 to recruit microtubules to the cell surface around the focal adhesions by binding to particular proteins that are associated with microtubules. Disrupting the interaction between KANK1 and talin by making small alterations in these two proteins blocked the ability of focal adhesions to capture surrounding microtubules. The next step following on from this work will be to find out whether this process also takes place in the cells within an animal’s body, such as a fly or a mouse.

DOI:http://dx.doi.org/10.7554/eLife.18124.002

EGF-stimulated activation of Rab35 regulates RUSC2-GIT2 complex formation to stabilize GIT2 during directional lung cancer cell migration

Non-small cell lung cancer (NSCLC) remains one of the most metastasizing tumors, and directional cell migration is critical for targeting tumor metastasis. GIT2 has been known to bind to Paxillin to control cell polarization and directional migration. However, the molecular mechanisms underlying roles of GIT2 in controlling cell polarization and directional migration remain elusive. Here we demonstrated GIT2 control cell polarization and direction dependent on the regulation of Golgi through RUSC2. RUSC2 interacts with SHD of GIT2 in various lung cancer cells, and stabilizes GIT2 (Mazaki et al., 2006; Yu et al., 2009) by decreasing degradation and increasing its phosphorylation. Silencing of RUSC2 showed reduced stability of GIT2, defective Golgi reorientation toward the wound edge and decreased directional migration. Moreover, short-term EGF stimulation can increase the interaction between RUSC2 and GIT2, prolonged stimulation leads to a decrease of their interaction through activating Rab35. Silencing of Rab35 also reduced stability and phosphorylation of GIT2 and decreased cell migration. Taken together, our study indicated that RUSC2 participates in EGFR signaling and regulates lung cancer progression, and may be a new therapeutic target against lung cancer metastasis.

Enhancement of β-catenin activity by BIG1 plus BIG2 via Arf activation and cAMP signals

Multifunctional β-catenin, with critical roles in both cell-cell adhesion and Wnt-signaling pathways, was among HeLa cell proteins coimmunoprecipitated by antibodies against brefeldin A-inhibited guanine nucleotide-exchange factors 1 and 2 (BIG1 or BIG2) that activate ADP-ribosylation factors (Arfs) by accelerating the replacement of bound GDP with GTP. BIG proteins also contain A-kinase anchoring protein (AKAP) sequences that can act as scaffolds for multimolecular assemblies that facilitate and limit cAMP signaling temporally and spatially. Direct interaction of BIG1 N-terminal sequence with β-catenin was confirmed using yeast two-hybrid assays and in vitro synthesized proteins. Depletion of BIG1 and/or BIG2 or overexpression of guanine nucleotide-exchange factor inactive mutant, but not wild-type, proteins interfered with β-catenin trafficking, leading to accumulation at perinuclear Golgi structures. Both phospholipase D activity and vesicular trafficking were required for effects of BIG1 and BIG2 on β-catenin activation. Levels of PKA-phosphorylated β-catenin S675 and β-catenin association with PKA, BIG1, and BIG2 were also diminished after BIG1/BIG2 depletion. Inferring a requirement for BIG1 and/or BIG2 AKAP sequence in PKA modification of β-catenin and its effect on transcription activation, we confirmed dependence of S675 phosphorylation and transcription coactivator function on BIG2 AKAP-C sequence.

Kank1 reexpression induced by 5-Aza-2'-deoxycytidine suppresses nasopharyngeal carcinoma cell proliferation and promotes apoptosis

Kank1, which was first described as a potential tumor suppressor for renal cell carcinoma (RCC), mapped to 9p24.3 and encoded an ankyrin-repeat domain-containing protein. Its frequent deletion was found to be associated with several human malignant tumors, cerebral palsy, and neuronal and developmental diseases. However, its functional role in nasopharyngeal cancer (NPC) was still unknown. In the present study, we found that Kank1 expression was down-regulated in NPC cells than in human nasopharyngeal epithelial cell line NP69 and demethylating agent 5-aza-2'-deoxycytidine (5-aza-CdR) could improve its mRNA and protein expression level. Further studies demonstrated that DNA methylation might be the mainly cause for Kank1 decreased expression and restored Kank1 expression mediated by 5-aza-CdR played a key role in suppressing NPC cells growth and inducing its apoptosis. Our primary results revealed new function of Kank1 for NPC and implied that epigenetic regulation especially demethylation may have a potential value for NPC treatment.

Kank Is an EB1 interacting protein that localises to muscle-tendon attachment sites in Drosophila

Little is known about how microtubules are regulated in different cell types during development. EB1 plays a central role in the regulation of microtubule plus ends. It directly binds to microtubule plus ends and recruits proteins which regulate microtubule dynamics and behaviour. We report the identification of Kank, the sole Drosophila orthologue of human Kank proteins, as an EB1 interactor that predominantly localises to embryonic attachment sites between muscle and tendon cells. Human Kank1 was identified as a tumour suppressor and has documented roles in actin regulation and cell polarity in cultured mammalian cells. We found that Drosophila Kank binds EB1 directly and this interaction is essential for Kank localisation to microtubule plus ends in cultured cells. Kank protein is expressed throughout fly development and increases during embryogenesis. In late embryos, it accumulates to sites of attachment between muscle and epidermal cells. A kank deletion mutant was generated. We found that the mutant is viable and fertile without noticeable defects. Further analysis showed that Kank is dispensable for muscle function in larvae. This is in sharp contrast to C. elegans in which the Kank orthologue VAB-19 is required for development by stabilising attachment structures between muscle and epidermal cells.

Regulating the large Sec7 ARF guanine nucleotide exchange factors: the when, where and how of activation

Eukaryotic cells require selective sorting and transport of cargo between intracellular compartments. This is accomplished at least in part by vesicles that bud from a donor compartment, sequestering a subset of resident protein "cargos" destined for transport to an acceptor compartment. A key step in vesicle formation and targeting is the recruitment of specific proteins that form a coat on the outside of the vesicle in a process requiring the activation of regulatory GTPases of the ARF family. Like all such GTPases, ARFs cycle between inactive, GDP-bound, and membrane-associated active, GTP-bound, conformations. And like most regulatory GTPases the activating step is slow and thought to be rate limiting in cells, requiring the use of ARF guanine nucleotide exchange factor (GEFs). ARF GEFs are characterized by the presence of a conserved, catalytic Sec7 domain, though they also contain motifs or additional domains that confer specificity to localization and regulation of activity. These domains have been used to define and classify five different sub-families of ARF GEFs. One of these, the BIG/GBF1 family, includes three proteins that are each key regulators of the secretory pathway. GEF activity initiates the coating of nascent vesicles via the localized generation of activated ARFs and thus these GEFs are the upstream regulators that define the site and timing of vesicle production. Paradoxically, while we have detailed molecular knowledge of how GEFs activate ARFs, we know very little about how GEFs are recruited and/or activated at the right time and place to initiate transport. This review summarizes the current knowledge of GEF regulation and explores the still uncertain mechanisms that position GEFs at "budding ready" membrane sites to generate highly localized activated ARFs.

Mutations in γ adducin are associated with inherited cerebral palsy

OBJECTIVE

Cerebral palsy is estimated to affect nearly 1 in 500 children, and although prenatal and perinatal contributors have been well characterized, at least 20% of cases are believed to be inherited. Previous studies have identified mutations in the actin-capping protein KANK1 and the adaptor protein-4 complex in forms of inherited cerebral palsy, suggesting a role for components of the dynamic cytoskeleton in the genesis of the disease.

METHODS

We studied a multiplex consanguineous Jordanian family by homozygosity mapping and exome sequencing, then used patient-derived fibroblasts to examine functional consequences of the mutation we identified in vitro. We subsequently studied the effects of adducin loss of function in Drosophila.

RESULTS

We identified a homozygous c.1100G>A (p.G367D) mutation in ADD3, encoding gamma adducin in all affected members of the index family. Follow-up experiments in patient fibroblasts found that the p.G367D mutation, which occurs within the putative oligomerization critical region, impairs the ability of gamma adducin to associate with the alpha subunit. This mutation impairs the normal actin-capping function of adducin, leading to both abnormal proliferation and migration in cultured patient fibroblasts. Loss of function studies of the Drosophila adducin ortholog hts confirmed a critical role for adducin in locomotion.

INTERPRETATION

Although likely a rare cause of cerebral palsy, our findings indicate a critical role for adducins in regulating the activity of the actin cytoskeleton, suggesting that impaired adducin function may lead to neuromotor impairment and further implicating abnormalities of the dynamic cytoskeleton as a pathogenic mechanism contributing to cerebral palsy.

Arf guanine nucleotide-exchange factors BIG1 and BIG2 regulate nonmuscle myosin IIA activity by anchoring myosin phosphatase complex

Brefeldin A-inhibited guanine nucleotide-exchange factors BIG1 and BIG2 activate, through their Sec7 domains, ADP ribosylation factors (Arfs) by accelerating the replacement of Arf-bound GDP with GTP for initiation of vesicular transport or activation of specific enzymes that modify important phospholipids. They are also implicated in regulation of cell polarization and actin dynamics for directed migration. Reciprocal coimmunoprecipitation of endogenous HeLa cell BIG1 and BIG2 with myosin IIA was demonstrably independent of Arf guanine nucleotide-exchange factor activity, because effects of BIG1 and BIG2 depletion were reversed by overexpression of the cognate BIG molecule C-terminal sequence that follows the Arf activation site. Selective depletion of BIG1 or BIG2 enhanced specific phosphorylation of myosin regulatory light chain (T18/S19) and F-actin content, which impaired cell migration in Transwell assays. Our data are clear evidence of these newly recognized functions for BIG1 and BIG2 in transduction or integration of mechanical signals from integrin adhesions and myosin IIA-dependent actin dynamics. Thus, by anchoring or scaffolding the assembly, organization, and efficient operation of multimolecular myosin phosphatase complexes that include myosin IIA, protein phosphatase 1δ, and myosin phosphatase-targeting subunit 1, BIG1 and BIG2 serve to integrate diverse biophysical and biochemical events in cells.

The tyrosine phosphatase SHP2 regulates focal adhesion kinase to promote EGF-induced lamellipodia persistence and cell migration

The Src homology phosphotyrosyl phosphatase 2 (SHP2) is a positive effector of receptor tyrosine kinases (RTK) signaling. Furthermore, SHP2 is known to promote cell migration and invasiveness, key steps in cancer metastasis. To date, however, the mechanism by which SHP2 regulates cell movement is not fully understood. In the current report, a new role for SHP2 in regulating cell migration has been suggested. We show that SHP2 mediates lamellipodia persistence and cell polarity to promote directional cell migration in the MDA-MB231 and the MDA-MB468 basal-like and triple-negative breast cancer cell lines. We further show that SHP2 modulates the activity of focal adhesion kinase (FAK) by dephosphorylating pTyr397, the autophosphorylation site that primes FAK function. Because hyperactivation of FAK is known to counter the maturation of nascent focal complexes to focal adhesions, we propose that one of the mechanisms by which SHP2 promotes lamellipodia persistence is by downregulating FAK activity through dephosphorylation of pTyr397. The finding that inhibition of FAK activity partially restores EGF-induced lamellipodia persistence and cell migration in SHP2-silenced cells supports our proposition that SHP2 promotes growth factor-induced cell movement by acting, at least in part, on FAK. However, the effect of SHP2 inhibition in nonstimulated cells seems FAK independent as there was no significant difference between the control and the SHP2-silenced cells in pY397-FAK levels. Also, FAK inhibition did not rescue Golgi orientation defects in SHP2-silenced cells, suggesting that SHP2 acts through other mechanisms to promote cell polarity.

The Sec7 guanine nucleotide exchange factor GBF1 regulates membrane recruitment of BIG1 and BIG2 guanine nucleotide exchange factors to the trans-Golgi network (TGN)

Three Sec7 guanine nucleotide exchange factors (GEFs) activate ADP-ribosylation factors (ARFs) to facilitate coating of transport vesicles within the secretory and endosomal pathways. GBF1 recruits COPI to pre-Golgi and Golgi compartments, whereas BIG1 and BIG2 recruit AP1 and GGA clathrin adaptors to the trans-Golgi network (TGN) and endosomes. Here, we report a functional cascade between these GEFs by showing that GBF1-activated ARFs (ARF4 and ARF5, but not ARF3) facilitate BIG1 and BIG2 recruitment to the TGN. We localize GBF1 ultrastructurally to the pre-Golgi, the Golgi, and also the TGN. Our findings suggest a model in which GBF1 localized within pre-Golgi and Golgi compartments mediates ARF activation to facilitate recruitment of COPI to membranes, whereas GBF1 localized at the TGN mediates ARF activation that leads to the recruitment of BIG1 and BIG2 to the TGN. Membrane-associated BIG1/2 then activates ARFs that recruit clathrin adaptors. In this cascade, an early acting GEF (GBF1) activates ARFs that mediate recruitment of late acting GEFs (BIG1/2) to coordinate coating events within the pre-Golgi/Golgi/TGN continuum. Such coordination may optimize the efficiency and/or selectivity of cargo trafficking through the compartments of the secretory pathway.

Brefeldin A-inhibited ADP-ribosylation factor activator BIG2 regulates cell migration via integrin β1 cycling and actin remodeling

Brefeldin A-inhibited guanine nucleotide-exchange protein (BIG)2 activates ADP-ribosylation factors, ∼20-kDa GTPase proteins critical for continuity of intracellular vesicular trafficking by accelerating the replacement of ADP-ribosylation factor-bound GDP with GTP. Mechanisms of additional BIG2 function(s) are less clear. Here, the participation of BIG2 in integrin β1 cycling through actin dynamics during cell migration was identified using small interfering RNA (siRNA) and difference gel electrophoresis analyses. After a 72-h incubation with BIG2 siRNA, levels of cytosolic Arp2, Arp3, cofilin-1, phosphocofilin, vinculin, and Grb2, known to be involved in the effects of integrin β1-extracellular matrix interactions on actin function and cell translocation, were increased. Treatment of HeLa cells with BIG2 siRNA resulted in perinuclear accumulation of integrin β1 and its delayed return to the cell surface. Motility of BIG2-depleted cells was simultaneously decreased, as were actin-based membrane protrusions and accumulations of Arp2, Arp3, cofilin, and phosphocofilin at the leading edges of migrating cells, in wound-healing assays. Taken together, these data reveal a mechanism(s) through which BIG2 may coordinate actin cytoskeleton mechanics and membrane traffic in cell migration via integrin β1 action and actin functions.

Effects of β4 integrin expression on microRNA patterns in breast cancer

The integrin α6β4 is defined as an adhesion receptor for laminins. Referred to as ‘β4’, this integrin plays a key role in the progression of various carcinomas through its ability to orchestrate key signal transduction events and promote cell motility. To identify novel downstream effectors of β4 function in breast cancer, microRNAs (miRNAs) were examined because of their extensive links to tumorigenesis and their ability to regulate gene expression globally. Two breast carcinoma cell lines and a collection of invasive breast carcinomas with varying β4 expression were used to assess the effect of this integrin on miRNA expression. A novel miRNA microarray analysis termed quantitative Nuclease Protection Assay (qNPA) revealed that β4 expression can significantly alter miRNA expression and identified two miRNA families, miR-25/32/92abc/363/363-3p/367 and miR-99ab/100, that are consistently downregulated by expression of this integrin. Analysis of published Affymetrix GeneChip data identified 54 common targets of miR-92ab and miR-99ab/100 within the subset of β4-regulated mRNAs, revealing several genes known to be key components of β4-regulated signaling cascades and effectors of cell motility. Gene ontology classification identified an enrichment in genes associated with cell migration within this population. Finally, gene set enrichment analysis of all β4-regulated mRNAs revealed an enrichment in targets belonging to distinct miRNA families, including miR-92ab and others identified by our initial array analyses. The results obtained in this study provide the first example of an integrin globally impacting miRNA expression and provide evidence that select miRNA families collectively target genes important in executing β4-mediated cell motility.

Spatiotemporal expression pattern of KIF21A during normal embryonic development and in congenital fibrosis of the extraocular muscles type 1 (CFEOM1)

Congenital fibrosis of the extraocular muscles type 1 (CFEOM1) is a rare inherited strabismus syndrome characterized by non-progressive ophthalmoplegia. We previously identified that CFEOM1 results from heterozygous missense mutations in KIF21A, which encodes a kinesin motor protein. Here we evaluate the expression pattern of KIF21A in human brain and muscles of control and CFEOM1 patients, and during human and mouse embryonic development. KIF21A is expressed in the cell bodies, axons, and dendrites of many neuronal populations including those in the hippocampus, cerebral cortex, cerebellum, striatum, and motor neurons of the oculomotor, trochlear, and abducens nuclei from early development into maturity, and its spatial distribution is not altered in the CFEOM1 tissues available for study. Multiple splice isoforms of KIF21A are identified in human fetal brain, but none of the reported CFEOM1 mutations are located in or near the alternatively spliced exons. KIF21A immunoreactivity is also observed in extraocular and skeletal muscle biopsies of control and CFEOM1 patients, where it co-localizes with triadin, a marker of the excitation-contractile coupling system. The diffuse and widespread expression of KIF21A in the developing human and mouse central and peripheral nervous system as well as in extraocular muscle does not account for the restricted ocular phenotype observed in CFEOM1, nor does it permit the formal exclusion of a myogenic etiology based on expression patterns alone.