Lamellipodin, an Ena/VASP ligand, is implicated in the regulation of lamellipodial dynamics

Pharmaco-Genetic Screen To Uncover Actin Regulators Targeted by Prostaglandins During Drosophila Oogenesis

Prostaglandins (PGs) are lipid signaling molecules with numerous physiologic functions, including pain/inflammation, fertility, and cancer. PGs are produced downstream of cyclooxygenase (COX) enzymes, the targets of non-steroidal anti-inflammatory drugs (NSAIDs). In numerous systems, PGs regulate actin cytoskeletal remodeling, however, their mechanisms of action remain largely unknown. To address this deficiency, we undertook a pharmaco-genetic interaction screen during late-stage Drosophila oogenesis. Drosophila oogenesis is as an established model for studying both actin dynamics and PGs. Indeed, during Stage 10B, cage-like arrays of actin bundles surround each nurse cell nucleus, and during Stage 11, the cortical actin contracts, squeezing the cytoplasmic contents into the oocyte. Both of these cytoskeletal properties are required for follicle development and fertility, and are regulated by PGs. Here we describe a pharmaco-genetic interaction screen that takes advantage of the fact that Stage 10B follicles will mature in culture and COX inhibitors, such as aspirin, block this in vitro follicle maturation. In the screen, aspirin was used at a concentration that blocks 50% of the wild-type follicles from maturing in culture. By combining this aspirin treatment with heterozygosity for mutations in actin regulators, we quantitatively identified enhancers and suppressors of COX inhibition. Here we present the screen results and initial follow-up studies on three strong enhancers – Enabled, Capping protein, and non-muscle Myosin II Regulatory Light Chain. Overall, these studies provide new insight into how PGs regulate both actin bundle formation and cellular contraction, properties that are not only essential for development, but are misregulated in disease.

PI3Kβ links integrin activation and PI(3,4)P2 production during invadopodial maturation

The invasion of tumor cells from the primary tumor is mediated by invadopodia, actin-rich protrusive organelles that secrete matrix metalloproteases and degrade the extracellular matrix. This coupling between protrusive activity and matrix degradation facilitates tumor invasion. We previously reported that the PI3Kβ isoform of PI 3-kinase, which is regulated by both receptor tyrosine kinases and G protein-coupled receptors, is required for invasion and gelatin degradation in breast cancer cells. We have now defined the mechanism by which PI3Kβ regulates invadopodia. We find that PI3Kβ is specifically activated downstream from integrins, and is required for integrin-stimulated spreading and haptotaxis as well as integrin-stimulated invadopodia formation. Surprisingly, these integrin-stimulated and PI3Kβ-dependent responses require the production of PI(3,4)P2 by the phosphoinositide 5'-phosphatase SHIP2. Thus, integrin activation of PI3Kβ is coupled to the SHIP2-dependent production of PI(3,4)P2, which regulates the recruitment of PH domain-containing scaffolds such as lamellipodin to invadopodia. These findings provide novel mechanistic insight into the role of PI3Kβ in the regulation of invadopodia in breast cancer cells.

GPR124 facilitates pericyte polarization and migration by regulating the formation of filopodia during ischemic injury

Prolonged occlusion of multiple microvessels causes microvascular injury. G protein-coupled receptor 124 (GPR124) has been reported to be required for maintaining central nervous system (CNS) angiogenesis and blood-brain barrier integrity. However, the molecular mechanisms by which GPR124 regulates pericytes during ischemia have remained elusive.

Methods: A microsphere embolism-induced ischemia model was used to evaluate the expression of GPR124 following microsphere embolism. Immunocytochemistry and stochastic optical reconstruction microscopy imaging were used to assess the expression and distribution of GPR124 in human brain vascular pericytes (HBVPs) and after the treatment with 3-morpholino-sydnonimine (SIN-1) or oxygen-glucose deprivation (OGD). The effect of GPR124 knockdown or overexpression on HBVP migration was analyzed in vitro using wound healing assays and a microfluidic device. GPR124 loss-of-function studies were performed in HBVPs and HEK293 cells using CRISPR-Cas9-mediated gene deletion. Time-lapse imaging was used to assess dynamic changes in the formation of filopodia in an individual cell. Finally, to explore the functional domains required for GPR124 activity, deletion mutants were constructed for each of the N-terminal domains.

Results: GPR124 expression was increased in pericytes following microsphere embolism. Morphological analysis showed localization of GPR124 to focal adhesions where GPR124 bound directly to the actin binding protein vinculin and upregulated Cdc42. SIN-1 or OGD treatment redistributed GPR124 to the leading edges of HBVPs where GPR124 signaling was required for pericyte filopodia formation and directional migration. Partial deletion of GPR124 domains decreased SIN-1-induced filopodia formation and cell migration.

Conclusion: Taken together, our results provide the first evidence for a role of GPR124 in pericyte migration under ischemic conditions and suggest that GPR124 was essential for Cdc42 activation and filopodia formation.

SopF, a phosphoinositide binding effector, promotes the stability of the nascent Salmonella-containing vacuole

The enteric bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), utilizes two type III secretion systems (T3SSs) to invade host cells, survive and replicate intracellularly. T3SS1 and its dedicated effector proteins are required for bacterial entry into non-phagocytic cells and establishment and trafficking of the nascent Salmonella-containing vacuole (SCV). Here we identify the first T3SS1 effector required to maintain the integrity of the nascent SCV as SopF. SopF associates with host cell membranes, either when translocated by bacteria or ectopically expressed. Recombinant SopF binds to multiple phosphoinositides in protein-lipid overlays, suggesting that it targets eukaryotic cell membranes via phospholipid interactions. In yeast, the subcellular localization of SopF is dependent on the activity of Mss4, a phosphatidylinositol 4-phosphate 5-kinase that generates PI(4,5)P2 from PI(4)P, indicating that membrane recruitment of SopF requires specific phospholipids. Ectopically expressed SopF partially colocalizes with specific phosphoinositide pools present on the plasma membrane in mammalian cells and with cytoskeletal-associated markers at the leading edge of cells. Translocated SopF concentrates on plasma membrane ruffles and around intracellular bacteria, presumably on the SCV. SopF is not required for bacterial invasion of non-phagocytic cells but is required for maintenance of the internalization vacuole membrane as infection with a S. Typhimurium ΔsopF mutant led to increased lysis of the SCV compared to wild type bacteria. Our structure-function analysis shows that the carboxy-terminal seven amino acids of SopF are essential for its membrane association in host cells and to promote SCV membrane stability. We also describe that SopF and another T3SS1 effector, SopB, act antagonistically to modulate nascent SCV membrane dynamics. In summary, our study highlights that a delicate balance of type III effector activities regulates the stability of the Salmonella internalization vacuole.

Actin assembly and non-muscle myosin activity drive dendrite retraction in an UNC-6/Netrin dependent self-avoidance response

Dendrite growth is constrained by a self-avoidance response that induces retraction but the downstream pathways that balance these opposing mechanisms are unknown. We have proposed that the diffusible cue UNC-6(Netrin) is captured by UNC-40(DCC) for a short-range interaction with UNC-5 to trigger self-avoidance in the C. elegans PVD neuron. Here we report that the actin-polymerizing proteins UNC-34(Ena/VASP), WSP-1(WASP), UNC-73(Trio), MIG-10(Lamellipodin) and the Arp2/3 complex effect dendrite retraction in the self-avoidance response mediated by UNC-6(Netrin). The paradoxical idea that actin polymerization results in shorter rather than longer dendrites is explained by our finding that NMY-1 (non-muscle myosin II) is necessary for retraction and could therefore mediate this effect in a contractile mechanism. Our results also show that dendrite length is determined by the antagonistic effects on the actin cytoskeleton of separate sets of effectors for retraction mediated by UNC-6(Netrin) versus outgrowth promoted by the DMA-1 receptor. Thus, our findings suggest that the dendrite length depends on an intrinsic mechanism that balances distinct modes of actin assembly for growth versus retraction.

Neurons may extend highly branched dendrites to detect input over a broad receptive field. The formation of actin filaments may drive dendrite elongation. The architecture of the dendritic arbor also depends on mechanisms that limit expansion. For example, sister dendrites from a single neuron usually do not overlap due to self-avoidance. Although cell surface proteins are known to mediate self-avoidance, the downstream pathways that drive dendrite retraction in this phenomenon are largely unknown. Studies of the highly branched PVD sensory neuron in C. elegans have suggested a model of self-avoidance in which the UNC-40/DCC receptor captures the diffusible cue UNC-6/Netrin at the tips of PVD dendrites where it interacts with the UNC-5 receptor on an opposing sister dendrite to induce retraction. Here we report genetic evidence that UNC-5-dependent retraction requires downstream actin polymerization. This finding evokes a paradox: How might actin polymerization drive both dendrite growth and retraction? We propose two answers: (1) Distinct sets of effectors are involved in actin assembly for growth vs retraction; (2) Non-muscle myosin interacts with a nascent actin assemblage to trigger retraction. Our results show that dendrite length depends on the balanced effects of specific molecular components that induce growth vs retraction.

Ganoderma lucidum Extract Reduces the Motility of Breast Cancer Cells Mediated by the RAC⁻Lamellipodin Axis

Breast cancer (BC) is the second leading cause of cancer death among women worldwide. The main cause of BC morbidity and mortality is the invasiveness capacity of cancer cells that may lead to metastasis. Here, we aimed to investigate the therapeutic efficacy of Ganoderma lucidum extract (GLE)—a medicinal mushroom with anticancer properties—on BC motility via the Rac/Lamellipodin pathway. GLE treatment effects were tested on MDA-MB-231 breast cancer cells. The effects were tested on cell viability, migration and invasion. Pulldowns, immunoblotting, and immunofluorescence were used to measure Rac activity and the expression of proteins involved in cell migration and in lamellipodia formation, respectively. As a result, GLE suppressed BC cell viability, migration, and invasion capacity. GLE impaired Rac activity, as well as downregulated Lamellipodin, ENA/VASP, p-FAK (Tyr925), Cdc42, and c-Myc expression. Lamellipodia formation was significantly reduced by GLE. In conclusion, we demonstrate that GLE reduces Rac activity and downregulates signaling molecules involved in lamellipodia formation. These novel findings serve as basis for further studies to elucidate the potential of GLE as a therapeutic agent regulating the Rac/Lamellipodin pathway in BC metastasis.

The small GTPase RhoG regulates microtubule-mediated focal adhesion disassembly

Focal adhesions (FA) are a complex network of proteins that allow the cell to form physical contacts with the extracellular matrix (ECM). FA assemble and disassemble in a dynamic process, orchestrated by a variety of cellular components. However, the underlying mechanisms that regulate adhesion turnover remain poorly understood. Here we show that RhoG, a Rho GTPase related to Rac, modulates FA dynamics. When RhoG expression is silenced, FA are more stable and live longer, resulting in an increase in the number and size of adhesions, which are also more mature and fibrillar-like. Silencing RhoG also increases the number and thickness of stress fibers, which are sensitive to blebbistatin, suggesting contractility is increased. The molecular mechanism by which RhoG regulates adhesion turnover is yet to be characterized, but our results demonstrate that RhoG plays a role in the regulation of microtubule-mediated FA disassembly.

The VASP-profilin1 (Pfn1) interaction is critical for efficient cell migration and is regulated by cell-substrate adhesion in a PKA-dependent manner

Dynamic regulation of the actin cytoskeleton is an essential feature of cell motility. Action of Enabled (Ena)/vasodilator-stimulated phosphoprotein (VASP), a family of conserved actin-elongating proteins, is an important aspect of regulation of the actin cytoskeletal architecture at the leading edge that controls membrane protrusion and cell motility. In this study, we performed mutagenesis experiments in overexpression and knockdown-rescue settings to provide, for the first time, direct evidence of the role of the actin-binding protein profilin1 (Pfn1) in VASP-mediated regulation of cell motility. We found that VASP's interaction with Pfn1 is promoted by cell-substrate adhesion and requires down-regulation of PKA activity. Our experimental data further suggest that PKA-mediated Ser¹³⁷ phosphorylation of Pfn1 potentially negatively regulates the Pfn1-VASP interaction. Finally, Pfn1's ability to be phosphorylated on Ser¹³⁷ was partly responsible for the anti-migratory action elicited by exposing cells to a cAMP/PKA agonist. On the basis of these findings, we propose a mechanism of adhesion-protrusion coupling in cell motility that involves dynamic regulation of Pfn1 by PKA activity.

TES functions as a Mena-dependent tumor suppressor in gastric cancer carcinogenesis and metastasis

Background

In our previous study, we identified a candidate tumor suppressor gene, testin LIM domain protein (TES), in primary gastric cancer (GC). TES contains three LIM domains, which are specific interacting regions for the cell adhesion and cytoskeleton regulatory proteins. Mena is a known cytoskeleton regulator that regulates the assembly of actin filaments and modulates cell adhesion and motility by interacting with Lamellipodin (Lpd). Therefore, we hypothesized that TES plays a role as tumor suppressor in GC through interacting with Mena. This study aimed to investigate the tumor suppressive functions of TES in GC.

Methods

We explored the tumor suppressive effect of TES in GC by in vitro cell proliferation assay, colony formation assay, cell cycle analysis, Transwell assays, and in vivo tumorigenicity and metastasis assays. The interaction of TES and Mena was investigated through immunoprecipitation-based mass spectrometry. We also analyzed the expression of TES and Mena in 172 GC specimens using immunohistochemistry and investigated the clinicopathological and prognostic significance of TES and Mena in GC.

Results

TES suppressed GC cell proliferation and colony formation, induced cell cycle arrest, and inhibited tumorigenicity in vitro. Additionally, it inhibited GC cell migration and invasion in vitro and suppressed metastasis in vivo. TES interacted with Mena, and inhibited the interaction of Mena with Lpd. Transwell assays suggested that TES suppressed migration and invasion of GC cells in a Mena-dependent fashion. In GC patients with high Mena expression, the expression of TES was associated with tumor infiltration (P = 0.005), lymph node metastasis (P = 0.003), TNM stage (P = 0.003), and prognosis (P = 0.010). However, no significant association was observed in GC patients with low Mena expression.

Conclusions

We believe that TES functions as a Mena-dependent tumor suppressor. TES represents a valuable prognostic marker and potential target for GC treatment.

Electronic supplementary material

The online version of this article (10.1186/s40880-019-0347-y) contains supplementary material, which is available to authorized users.

Filopodome Mapping Identifies p130Cas as a Mechanosensitive Regulator of Filopodia Stability

Filopodia are adhesive cellular protrusions specialized in the detection of extracellular matrix (ECM)-derived cues. Although ECM engagement at focal adhesions is known to trigger the recruitment of hundreds of proteins ("adhesome") to fine-tune cellular behavior, the components of the filopodia adhesions remain undefined. Here, we performed a structured-illumination-microscopy-based screen to map the localization of 80 target proteins, linked to cell adhesion and migration, within myosin-X-induced filopodia. We demonstrate preferential enrichment of several adhesion proteins to either filopodia tips, filopodia shafts, or shaft subdomains, suggesting divergent, spatially restricted functions for these proteins. Moreover, proteins with phosphoinositide (PI) binding sites are particularly enriched in filopodia. This, together with the strong localization of PI(3,4)P2 in filopodia tips, predicts critical roles for PIs in regulating filopodia ultra-structure and function. Our mapping further reveals that filopodia adhesions consist of a unique set of proteins, the filopodome, that are distinct from classical nascent adhesions, focal adhesions, and fibrillar adhesions. Using live imaging, we observe that filopodia adhesions can give rise to nascent adhesions, which, in turn, form focal adhesions. We demonstrate that p130Cas (BCAR1) is recruited to filopodia tips via its C-terminal Cas family homology domain (CCHD) and acts as a mechanosensitive regulator of filopodia stability. Finally, we demonstrate that our map based on myosin-X-induced filopodia can be translated to endogenous filopodia and fascin- and IRSp53-mediated filopodia.

A high-avidity biosensor reveals plasma membrane PI(3,4)P2 is predominantly a class I PI3K signaling product

Class I phosphoinositide 3-OH kinase (PI3K) signaling is central to animal growth and metabolism, and pathological disruption of this pathway affects cancer and diabetes. However, the specific spatial/temporal dynamics and signaling roles of its minor lipid messenger, phosphatidylinositol (3,4)-bisphosphate (PI(3,4)P₂), are not well understood. This owes principally to a lack of tools to study this scarce lipid. Here we developed a high-sensitivity genetically encoded biosensor for PI(3,4)P₂, demonstrating high selectivity and specificity of the sensor for the lipid. We show that despite clear evidence for class II PI3K in PI(3,4)P₂-driven function, the overwhelming majority of the lipid accumulates through degradation of class I PI3K-produced PIP₃ However, we show that PI(3,4)P₂ is also subject to hydrolysis by the tumor suppressor lipid phosphatase PTEN. Collectively, our results show that PI(3,4)P₂ is potentially an important driver of class I PI3K-driven signaling and provides powerful new tools to begin to resolve the biological functions of this lipid downstream of class I and II PI3K.

Cryo-EM structure of the native butyrylcholinesterase tetramer reveals a dimer of dimers stabilized by a superhelical assembly

The quaternary structures of the cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), are essential for their localization and function. Of practical importance, BChE is a promising therapeutic candidate for intoxication by organophosphate nerve agents and insecticides, and for detoxification of addictive substances. Efficacy of the recombinant enzyme hinges on its having a long circulatory half-life; this, in turn, depends strongly on its ability to tetramerize. Here, we used cryoelectron microscopy (cryo-EM) to determine the structure of the highly glycosylated native BChE tetramer purified from human plasma at 5.7 Å. Our structure reveals that the BChE tetramer is organized as a staggered dimer of dimers. Tetramerization is mediated by assembly of the C-terminal tryptophan amphiphilic tetramerization (WAT) helices from each subunit as a superhelical assembly around a central lamellipodin-derived oligopeptide with a proline-rich attachment domain (PRAD) sequence that adopts a polyproline II helical conformation and runs antiparallel. The catalytic domains within a dimer are asymmetrically linked to the WAT/PRAD. In the resulting arrangement, the tetramerization domain is largely shielded by the catalytic domains, which may contribute to the stability of the human BChE (HuBChE) tetramer. Our cryo-EM structure reveals the basis for assembly of the native tetramers and has implications for the therapeutic applications of HuBChE. This mode of tetramerization is seen only in the cholinesterases but may provide a promising template for designing other proteins with improved circulatory residence times.

Roles for Ena/VASP proteins in FMNL3-mediated filopodial assembly

Filopodia are actin-dependent finger-like structures that protrude from the plasma membrane. Actin filament barbed-end-binding proteins localized to filopodial tips are key to filopodial assembly. Two classes of barbed-end-binding proteins are formins and Ena/VASP proteins, and both classes have been localized to filopodial tips in specific cellular contexts. Here, we examine the filopodial roles of the FMNL formins and Ena/VASP proteins in U2OS cells. FMNL3 suppression reduces filopodial assembly by 90%, and FMNL3 is enriched at >95% of filopodial tips. Suppression of VASP or Mena (also known as ENAH) reduces filopodial assembly by >75%. However, VASP and Mena do not display consistent filopodial tip localization, but are enriched in focal adhesions (FAs). Interestingly, >85% of FMNL3-containing filopodia are associated with FAs. Two situations increase Ena/VASP filopodial localization: (1) expression of myosin-X, and (2) actively spreading cells. In spreading cells, filopodia often mark sites of nascent adhesions. Interestingly, VASP suppression in spreading cells causes a significant increase in adhesion assembly at filopodial tips. This work demonstrates that, in U2OS cells, Ena/VASP proteins play roles in filopodia beyond those at filopodial tips.This article has an associated First Person interview with the first author of the paper.

The impact of phosphoinositide 5-phosphatases on phosphoinositides in cell function and human disease

Phosphoinositides (PIs) are recognized as major signaling molecules in many different functions of eukaryotic cells. PIs can be dephosphorylated by multiple phosphatase activities at the 5-, 4-, and 3- positions. Human PI 5-phosphatases belong to a family of 10 members. Except for inositol polyphosphate 5-phosphatase A, they all catalyze the dephosphorylation of PI(4,5)P₂ and/or PI(3,4,5)P₃ at the 5- position. PI 5-phosphatases thus directly control the levels of PI(3,4,5)P₃ and participate in the fine-tuning regulatory mechanisms of PI(3,4)P₂ and PI(4,5)P₂ Second messenger functions have been demonstrated for PI(3,4)P₂ in invadopodium maturation and lamellipodia formation. PI 5-phosphatases can use several substrates on isolated enzymes, and it has been challenging to establish their real substrate in vivo. PI(4,5)P₂ has multiple functions in signaling, including interacting with scaffold proteins, ion channels, and cytoskeleton proteins. PI 5-phosphatase isoenzymes have been individually implicated in human diseases, such as the oculocerebrorenal syndrome of Lowe, through mechanisms that include lipid control. Oncogenic and tumor-suppressive functions of PI 5-phosphatases have also been reported in different cell contexts. The mechanisms responsible for genetic diseases and for oncogenic or tumor-suppressive functions are not fully understood. The regulation of PI 5-phosphatases is thus crucial in understanding cell functions.

Mutually inhibitory Ras-PI(3,4)P2 feedback loops mediate cell migration

Signal transduction and cytoskeleton networks in a wide variety of cells display excitability, but the mechanisms are poorly understood. Here, we show that during random migration and in response to chemoattractants, cells maintain complementary spatial and temporal distributions of Ras activity and phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂]. In addition, depletion of PI(3,4)P₂ by disruption of the 5-phosphatase, Dd5P4, or by recruitment of 4-phosphatase INPP4B to the plasma membrane, leads to elevated Ras activity, cell spreading, and altered migratory behavior. Furthermore, RasGAP2 and RapGAP3 bind to PI(3,4)P₂, and the phenotypes of cells lacking these genes mimic those with low PI(3,4)P₂ levels, providing a molecular mechanism. These findings suggest that Ras activity drives PI(3,4)P₂ down, causing the PI(3,4)P₂-binding GAPs to dissociate from the membrane, further activating Ras, completing a positive-feedback loop essential for excitability. Consistently, a computational model incorporating such a feedback loop in an excitable network model accurately simulates the dynamic distributions of active Ras and PI(3,4)P₂ as well as cell migratory behavior. The mutually inhibitory Ras-PI(3,4)P₂ mechanisms we uncovered here provide a framework for Ras regulation that may play a key role in many physiological processes.

FBP17 and CIP4 recruit SHIP2 and lamellipodin to prime the plasma membrane for fast endophilin-mediated endocytosis

Endocytosis mediates the cellular uptake of micronutrients and the turnover of plasma membrane proteins. Clathrin-mediated endocytosis is the major uptake pathway in resting cells¹, but several clathrin-independent endocytic routes exist in parallel^2,3. One such pathway, fast endophilin-mediated endocytosis (FEME), is not constitutive but triggered upon activation of certain receptors, including the β₁ adrenergic receptor⁴. FEME activates promptly following stimulation as endophilin is pre-enriched by the phosphatidylinositol-3,4-bisphosphate-binding protein lamellipodin^4,5. However, in the absence of stimulation, endophilin foci abort and disassemble after a few seconds. Looking for additional proteins involved in FEME, we found that 20 out of 65 BAR domain-containing proteins tested colocalized with endophilin spots. Among them, FBP17 and CIP4 prime the membrane of resting cells for FEME by recruiting the 5'-lipid phosphatase SHIP2 and lamellipodin to mediate the local production of phosphatidylinositol-3,4-bisphosphate and endophilin pre-enrichment. Membrane-bound GTP-loaded Cdc42 recruits FBP17 and CIP4, before being locally deactivated by RICH1 and SH3BP1 GTPase-activating proteins. This generates the transient assembly and disassembly of endophilin spots, which lasts 5-10 seconds. This mechanism periodically primes patches of the membrane for prompt responses upon FEME activation.

Clinicopathological and prognostic significance of Ras association and pleckstrin homology domains 1 (RAPH1) in breast cancer

BACKGROUND

Ras association and pleckstrin homology domains 1 (RAPH1) is involved in cytoskeleton regulation and re-epithelialisation in invasive carcinoma and, therefore, may play a key role in carcinogenesis and metastasis. We, herein, investigated the biological and clinical significance of RAPH1 in breast cancer using large annotated cohorts.

METHODS

The clinicopathological and prognostic significance of RAPH1 was assessed at the genomic and transcriptomic levels using The Cancer Genome Atlas (TCGA) dataset (n = 1039) and the results were validated using the Molecular taxonomy of breast cancer international consortium (METABRIC) cohort (n = 1980). RAPH1 protein expression was evaluated by immunohistochemistry in a large, well-characterised cohort of early-stage breast cancer (n = 1040).

RESULTS

In both the TCGA and METABRIC cohorts, RAPH1 mRNA expression and RAPH1 copy number alteration were strongly correlated. RAPH1 mRNA overexpression was significantly correlated with high expression of adhesion and EMT markers including CDH1, TGFβ1 and CD44. RAPH1 mRNA overexpression was a significant predictor of a poor prognosis (Hazard ratio 3.88; p = 0.049). High RAPH1 protein expression was associated with higher grade tumours with high proliferation index, triple negative phenotype and high E-cadherin expression. High RAPH1 protein expression was an independent predictor of shorter survival (Hazard ratio 4.37; p = 0.037).

CONCLUSIONS

High RAPH1 expression is correlated with aggressive breast cancer phenotypes and provides independent prognostic value in invasive breast cancer.

Local actin polymerization during endocytic carrier formation

Extracellular macromolecules, pathogens and cell surface proteins rely on endocytosis to enter cells. Key steps of endocytic carrier formation are cargo molecule selection, plasma membrane folding and detachment from the cell surface. While dedicated proteins mediate each step, the actin cytoskeleton contributes to all. However, its role can be indirect to the actual molecular events driving endocytosis. Here, we review our understanding of the molecular steps mediating local actin polymerization during the formation of endocytic carriers. Clathrin-mediated endocytosis is the least reliant on local actin polymerization, as it is only engaged to counter forces induced by membrane tension or cytoplasmic pressure. Two opposite situations are coated pit formation in yeast and at the basolateral surface of polarized mammalian cells which are, respectively, dependent and independent on actin polymerization. Conversely, clathrin-independent endocytosis forming both nanometer [CLIC (clathrin-independent carriers)/GEEC (glycosylphosphatidylinositol (GPI)-anchored protein enriched endocytic compartments), caveolae, FEME (fast endophilin-mediated endocytosis) and IL-2β (interleukin-2β) uptake] and micrometer carriers (macropinocytosis) are dependent on actin polymerization to power local membrane deformation and carrier budding. A variety of endocytic adaptors can recruit and activate the Cdc42/N-WASP or Rac1/WAVE complexes, which, in turn, engage the Arp2/3 complex, thereby mediating local actin polymerization at the membrane. However, the molecular steps for RhoA and formin-mediated actin bundling during endocytic pit formation remain unclear.

Glycogen synthase kinase 3 controls migration of the neural crest lineage in mouse and Xenopus

Neural crest migration is critical to its physiological function. Mechanisms controlling mammalian neural crest migration are comparatively unknown, due to difficulties accessing this cell population in vivo. Here we report requirements of glycogen synthase kinase 3 (GSK3) in regulating the neural crest in Xenopus and mouse models. We demonstrate that GSK3 is tyrosine phosphorylated (pY) in mouse neural crest cells and that loss of GSK3 leads to increased pFAK and misregulation of Rac1 and lamellipodin, key regulators of cell migration. Genetic reduction of GSK3 results in failure of migration. We find that pY-GSK3 phosphorylation depends on anaplastic lymphoma kinase (ALK), a protein associated with neuroblastoma. Consistent with this, neuroblastoma cells with increased ALK activity express high levels of pY-GSK3, and blockade of GSK3 or ALK can affect migration of these cells. Altogether, this work identifies a role for GSK3 in cell migration during neural crest development and cancer.

Defects in neural crest development cause neurocristopathies and cancer, but what regulates this is unclear. Here, the authors show that glycogen synthase kinase 3 (GSK3) regulates migration of neural crest cells, as shown on genetic deletion of GSK3 in the mouse, and that this acts via anaplastic lymphoma kinase.

Phosphoinositide 5-phosphatase activities control cell motility in glioblastoma: Two phosphoinositides PI(4,5)P2 and PI(3,4)P2 are involved

Inositol polyphosphate 5-phosphatases or phosphoinositide 5-phosphatases (PI 5-phosphatases) are enzymes that can act on soluble inositol phosphates and/or phosphoinositides (PIs). Several PI 5-phosphatases have been linked to human genetic diseases, in particular the Lowe protein or OCRL which is mutated in the Lowe syndrome. There are 10 different members of this family and 9 of them can use PIs as substrate. One of these substrates, PI(3,4,5)P3 binds to specific PH domains and recruits as effectors specific proteins to signaling complexes. Protein kinase B is one target protein and activation of the kinase will have a major impact on cell proliferation, survival and cell metabolism. Two other PIs, PI(4,5)P2 and PI(3,4)P2, are produced or used as substrates of PI 5-phosphatases (OCRL, INPP5B, SHIP1/2, SYNJ1/2, INPP5K, INPP5J, INPP5E). The inositol lipids may influence many aspects of cytoskeletal organization, lamellipodia formation and F-actin polymerization. PI 5-phosphatases have been reported to control cell migration, adhesion, polarity and cell invasion particularly in cancer cells. In glioblastoma, reducing SHIP2 expression can positively or negatively affect the speed of cell migration depending on the glioblastoma cell type. The two PI 5-phosphatases SHIP2 or SKIP could be localized at the plasma membrane and can reduce either PI(3,4,5)P3 or PI(4,5)P2 abundance. In the glioblastoma 1321 N1 cells, SHIP2 controls plasma membrane PI(4,5)P2 thereby participating in the control of cell migration.

Inhibition of SHIP2 activity inhibits cell migration and could prevent metastasis in breast cancer cells

Metastasis of breast cancer cells to distant organs is responsible for approximately 50 % in cancer related deaths in women worldwide. SHIP2 is a phosphoinositide 5-phosphatase for PI(3,4,5)P3 and PI(4,5)P2. Through depletion of SHIP2 in triple negative MDA-MB-231 cells and the use of SHIP2 inhibitors, it appeared that cell migration is positively controlled by SHIP2. The effect of SHIP2 on migration, observed in MDA-MB-231 cells, appears to be mediated by PI(3,4)P2. Adhesion on fibronectin is always increased in SHIP2 depleted cells. Apoptosis measured in MDA-MB-231 cells is also increased in SHIP2 depleted cells as compared to control cells. In xenograft mice, SHIP2 depleted MDA-MB-231 cells form significantly smaller tumors compared to control cells and less metastasis detected in lung sections. Our data reveal a general role of SHIP2 in the control of cell migration in breast cancer cells and a second messenger role for PI(3,4)P2 in the migration mechanism. In this model, SHIP2 function on apoptosis on cells incubated in vitro, or in mice tumor digested cells, could account for its role on tumor growth determined in vivo.

Transcriptional responses to hyperplastic MRL signalling in Drosophila

Recent work has implicated the actin cytoskeleton in tissue size control and tumourigenesis, but how changes in actin dynamics contribute to hyperplastic growth is still unclear. Overexpression of Pico, the only Drosophila Mig-10/RIAM/Lamellipodin adapter protein family member, has been linked to tissue overgrowth via its effect on the myocardin-related transcription factor (Mrtf), an F-actin sensor capable of activating serum response factor (SRF). Transcriptional changes induced by acute Mrtf/SRF signalling have been largely linked to actin biosynthesis and cytoskeletal regulation. However, by RNA profiling, we find that the common response to chronic mrtf and pico overexpression in wing discs was upregulation of ribosome protein and mitochondrial genes, which are conserved targets for Mrtf/SRF and are known growth drivers. Consistent with their ability to induce a common transcriptional response and activate SRF signalling in vitro, we found that both pico and mrtf stimulate expression of an SRF-responsive reporter gene in wing discs. In a functional genetic screen, we also identified deterin, which encodes Drosophila Survivin, as a putative Mrtf/SRF target that is necessary for pico-mediated tissue overgrowth by suppressing proliferation-associated cell death. Taken together, our findings raise the possibility that distinct targets of Mrtf/SRF may be transcriptionally induced depending on the duration of upstream signalling.

Mind the (sr)GAP - roles of Slit-Robo GAPs in neurons, brains and beyond

The Slit-Robo GTPase-activating proteins (srGAPs) were first identified as potential Slit-Robo effectors that influence growth cone guidance. Given their N-terminal F-BAR, central GAP and C-terminal SH3 domains, srGAPs have the potential to affect membrane dynamics, Rho family GTPase activity and other binding partners. Recent research has clarified how srGAP family members act in distinct ways at the cell membrane, and has expanded our understanding of the roles of srGAPs in neuronal and non-neuronal cells. Gene duplication of the human-specific paralog of srGAP2 has resulted in srGAP2 family proteins that may have increased the density of dendritic spines and promoted neoteny of the human brain during crucial periods of human evolution, underscoring the importance of srGAPs in the unique sculpting of the human brain. Importantly, srGAPs also play roles outside of the nervous system, including during contact inhibition of cell movement and in establishing and maintaining cell adhesions in epithelia. Changes in srGAP expression may contribute to neurodevelopmental disorders, cancer metastasis and inflammation. As discussed in this Review, much remains to be discovered about how this interesting family of proteins functions in a diverse set of processes in metazoans and the functional roles srGAPs play in human disease.

A New Electron Microscopic Method to Observe the Distribution of Phosphatidylinositol 3,4-bisphosphate

Phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P₂] is a phosphoinositide that plays important roles in signal transduction, endocytosis, and cell migration among others. The intracellular distribution of PtdIns(3,4)P₂ has mainly been studied by observing the distribution of GFP-tagged PtdIns(3,4)P₂-binding protein domains in live cells and by labeling with anti-PtdIns(3,4)P₂ antibody in fixed cell samples, but these methods only offer low spatial resolution results and may have pitfalls. In the present study, we developed an electron microscopic method to observe the PtdIns(3,4)P₂ distribution using the SDS-treated freeze-fracture replica labeling method. The recombinant GST-tagged pleckstrin homology (PH) domain of TAPP1 was used as the binding probe, and its binding to PtdIns(3,4)P₂in the freeze-fracture replica was confirmed by using liposomes containing different phosphoinositides and by the lack of labeling by a mutant probe, in which one amino acid in the PH domain was substituted. The method was applied to NIH3T3 cell samples and showed that the increase of PtdIns(3,4)P₂ in cells treated with hydrogen peroxide occurs in the cytoplasmic leaflet of the plasma membrane, except in the caveolar membrane. The present method can define the distribution of PtdIns(3,4)P₂ at a high spatial resolution and will facilitate our understanding of the physiological function of this less studied phosphoinositide.

PTEN Regulates PI(3,4)P2 Signaling Downstream of Class I PI3K

The PI3K signaling pathway regulates cell growth and movement and is heavily mutated in cancer. Class I PI3Ks synthesize the lipid messenger PI(3,4,5)P₃. PI(3,4,5)P₃ can be dephosphorylated by 3- or 5-phosphatases, the latter producing PI(3,4)P₂. The PTEN tumor suppressor is thought to function primarily as a PI(3,4,5)P₃ 3-phosphatase, limiting activation of this pathway. Here we show that PTEN also functions as a PI(3,4)P₂ 3-phosphatase, both in vitro and in vivo. PTEN is a major PI(3,4)P₂ phosphatase in Mcf10a cytosol, and loss of PTEN and INPP4B, a known PI(3,4)P₂ 4-phosphatase, leads to synergistic accumulation of PI(3,4)P₂, which correlated with increased invadopodia in epidermal growth factor (EGF)-stimulated cells. PTEN deletion increased PI(3,4)P₂ levels in a mouse model of prostate cancer, and it inversely correlated with PI(3,4)P₂ levels across several EGF-stimulated prostate and breast cancer lines. These results point to a role for PI(3,4)P₂ in the phenotype caused by loss-of-function mutations or deletions in PTEN.

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PTEN is a PI(3,4)P₂ 3-phosphatase

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PTEN and INPP4B regulate PI(3,4)P₂ accumulation downstream of class I PI3K

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PTEN regulates PI(3,4)P₂-dependent activation of Akt and formation of invadopodia

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PI(3,4)P₂ signaling may play a role in the tumor suppressor function of PTEN

Phosphoinositide Diversity, Distribution, and Effector Function: Stepping Out of the Box

Phosphoinositides (PtdInsPs) modulate a plethora of functions including signal transduction and membrane trafficking. PtdInsPs are thought to consist of seven interconvertible species that localize to a specific organelle, to which they recruit a set of cognate effector proteins. Here, in reviewing the literature, we argue that this model needs revision. First, PtdInsPs can carry a variety of acyl chains, greatly boosting their molecular diversity. Second, PtdInsPs are more promiscuous in their localization than is usually acknowledged. Third, PtdInsP interconversion is likely achieved through kinase-phosphatase enzyme complexes that coordinate their activities and channel substrates without affecting bulk substrate population. Additionally, we contend that despite hundreds of PtdInsP effectors, our attention is biased toward few proteins. Lastly, we recognize that PtdInsPs can act to nucleate coincidence detection at the effector level, as in PDK1 and Akt. Overall, better integrated models of PtdInsP regulation and function are not only possible but needed.

Differential functions of WAVE regulatory complex subunits in the regulation of actin-driven processes

The WAVE regulatory complex (WRC) links upstream Rho-family GTPase signaling to the activation of the ARP2/3 complex in different organisms. WRC-induced and ARP2/3 complex-mediated actin nucleation beneath the plasma membrane is critical for actin assembly in the leading edge to drive efficient cell migration. The WRC is a stable heteropentamer composed of SCAR/WAVE, Abi, Nap, Pir and the small polypeptide Brk1/Hspc300. Functional interference with individual subunits of the complex frequently results in diminished amounts of the remaining polypeptides of the WRC complex, implying the complex to act as molecular entity. However, Abi was also found to associate with mammalian N-WASP, formins, Eps8/SOS1 or VASP, indicating additional functions of individual WRC subunits in eukaryotic cells. To address this issue systematically, we inactivated all WRC subunits, either alone or in combination with VASP in Dictyostelium cells and quantified the protein content of the remaining subunits in respective WRC knockouts. The individual mutants displayed highly differential phenotypes concerning various parameters, including cell morphology, motility, cytokinesis or multicellular development, corroborating the view of additional roles for individual subunits, beyond their established function in WRC-mediated Arp2/3 complex activation. Finally, our data uncover the interaction of the actin polymerase VASP with WRC-embedded Abi to mediate VASP accumulation in cell protrusions, driving efficient cell migration.

Brain specific Lamellipodin knockout results in hyperactivity and increased anxiety of mice

Lamellipodin (Lpd) functions as an important signalling integrator downstream of growth factor and axon guidance receptors. Mechanistically, Lpd promotes actin polymerization by interacting with F-actin and the actin effectors Ena/VASP proteins and the SCAR/WAVE complex. Thereby, Lpd supports lamellipodia protrusion, cell migration and endocytosis. In the mammalian central nervous system, Lpd contributes to neuronal morphogenesis, neuronal migration during development and its C. elegans orthologue MIG-10 also supports synaptogenesis. However, the consequences of loss of Lpd in the CNS on behaviour are unknown. In our current study, we crossed our Lpd conditional knockout mice with a mouse line expressing Cre under the CNS specific Nestin promoter to restrict the genetic ablation of Lpd to the central nervous system. Detailed behavioural analysis of the resulting Nestin-Cre-Lpd knockout mouse line revealed a specific behavioural phenotype characterised by hyperactivity and increased anxiety.

A proteomic analysis of LRRK2 binding partners reveals interactions with multiple signaling components of the WNT/PCP pathway

BACKGROUND

Autosomal-dominant mutations in the Park8 gene encoding Leucine-rich repeat kinase 2 (LRRK2) have been identified to cause up to 40% of the genetic forms of Parkinson's disease. However, the function and molecular pathways regulated by LRRK2 are largely unknown. It has been shown that LRRK2 serves as a scaffold during activation of WNT/β-catenin signaling via its interaction with the β-catenin destruction complex, DVL1-3 and LRP6. In this study, we examine whether LRRK2 also interacts with signaling components of the WNT/Planar Cell Polarity (WNT/PCP) pathway, which controls the maturation of substantia nigra dopaminergic neurons, the main cell type lost in Parkinson's disease patients.

METHODS

Co-immunoprecipitation and tandem mass spectrometry was performed in a mouse substantia nigra cell line (SN4741) and human HEK293T cell line in order to identify novel LRRK2 binding partners. Inhibition of the WNT/β-catenin reporter, TOPFlash, was used as a read-out of WNT/PCP pathway activation. The capacity of LRRK2 to regulate WNT/PCP signaling in vivo was tested in Xenopus laevis' early development.

RESULTS

Our proteomic analysis identified that LRRK2 interacts with proteins involved in WNT/PCP signaling such as the PDZ domain-containing protein GIPC1 and Integrin-linked kinase (ILK) in dopaminergic cells in vitro and in the mouse ventral midbrain in vivo. Moreover, co-immunoprecipitation analysis revealed that LRRK2 binds to two core components of the WNT/PCP signaling pathway, PRICKLE1 and CELSR1, as well as to FLOTILLIN-2 and CULLIN-3, which regulate WNT secretion and inhibit WNT/β-catenin signaling, respectively. We also found that PRICKLE1 and LRRK2 localize in signalosomes and act as dual regulators of WNT/PCP and β-catenin signaling. Accordingly, analysis of the function of LRRK2 in vivo, in X. laevis revelaed that LRKK2 not only inhibits WNT/β-catenin pathway, but induces a classical WNT/PCP phenotype in vivo.

CONCLUSIONS

Our study shows for the first time that LRRK2 activates the WNT/PCP signaling pathway through its interaction to multiple WNT/PCP components. We suggest that LRRK2 regulates the balance between WNT/β-catenin and WNT/PCP signaling, depending on the binding partners. Since this balance is crucial for homeostasis of midbrain dopaminergic neurons, we hypothesize that its alteration may contribute to the pathophysiology of Parkinson's disease.

Multiple pro-tumorigenic functions of the human minor Histocompatibility Antigen-1 (HA-1) in melanoma progression

BACKGROUND

Remodeling of cytoskeleton plays an important role in development of multiple cancers, including melanoma. As a group of F-actin regulators, the Ras homology (Rho) GTPase-activating proteins (ARHGAPs) were reported by accumulating studies as a set of significant mediators in cell morphology, proliferation, migration and invasion.

OBJECTIVE

To investigate the function of HMHA1 and its encode protein HA-1 in melanoma.

METHODS

The mRNA microarray was performed to screen the expression of ARHGAP family genes between melanoma tissues and nevi tissues. QRT-PCR and Western Blot were used to detect the expression of mRNA of HMHA1 and its relevant protein HA-1 respectively. Small interfering RNA was used to knock down the expression of HMHA1. Cell-count kit 8 assays and colony formation assays were used to evaluate the cell proliferative viability of melanoma cells. Flow cytometry was employed to analyze cell apoptosis. Transwell assay and the observation of cell morphology were used to evaluate the invasive and migrating activity of melanoma cells.

RESULTS

In previous study, we first found that both the mRNA level of HMHA1and the expression of HA-1 were up-regulated in melanoma tissues and cell lines compared with nevi tissues and normal human melanocytes respectively. Blocking HMHA1 expression in melanoma cell lines WM35 and A375 suppressed their proliferation and function of colony forming. Moreover, silencing HMHA1 not only significantly increased cell apoptosis but also suppressed cell migration and invasion.

CONCLUSION

Our results demonstrate that HMHA1 significantly promotes melanoma cells proliferation, invasion and migration, and prevents cell apoptosis. Additionally, it can be considered as a new diagnostic marker and drug target of melanoma.

The C5 Variant of the Butyrylcholinesterase Tetramer Includes a Noncovalently Bound 60 kDa Lamellipodin Fragment

Humans with the C5 genetic variant of butyrylcholinesterase (BChE) have 30–200% higher plasma BChE activity, low body weight, and shorter duration of action of the muscle relaxant succinylcholine. The C5 variant has an extra, slow-moving band of BChE activity on native polyacrylamide gel electrophoresis. This band is about 60 kDa larger than wild-type BChE. Umbilical cord BChE in 100% of newborn babies has a C5-like band. Our goal was to identify the unknown, 60 kDa protein in C5. Both wild-type and C5 BChE are under the genetic control of two independent loci, the BCHE gene on Chr 3q26.1 and the RAPH1 (lamellipodin) gene on Chr 2q33. Wild-type BChE tetramers are assembled around a 3 kDa polyproline peptide from lamellipodin. Western blot of boiled C5 and cord BChE showed a positive response with an antibody to the C-terminus of lamellipodin. The C-terminal exon of lamellipodin is about 60 kDa including an N-terminal polyproline. We propose that the unknown protein in C5 and cord BChE is encoded by the last exon of the RAPH1 gene. In 90% of the population, the 60 kDa fragment is shortened to 3 kDa during maturation to adulthood, leaving only 10% of adults with C5 BChE.

CRMP-1 enhances EVL-mediated actin elongation to build lamellipodia and the actin cortex

CRMP proteins regulate the cytoskeleton, but the underlying mechanisms are poorly understood. Yu-Kemp et al. show that CRMP-1 helps Ena/VASP proteins elongate actin filaments to assemble actin networks that are necessary for the integrity of epithelial sheets.

Cells can control actin polymerization by nucleating new filaments or elongating existing ones. We recently identified CRMP-1 as a factor that stimulates the formation of Listeria monocytogenes actin comet tails, thereby implicating it in actin assembly. We now show that CRMP-1 is a major contributor to actin assembly in epithelial cells, where it works with the Ena/VASP family member EVL to assemble the actin cytoskeleton in the apical cortex and in protruding lamellipodia. CRMP-1 and EVL bind to one another and together accelerate actin filament barbed-end elongation. CRMP-1 also stimulates actin assembly in the presence of VASP and Mena in vitro, but CRMP-1–dependent actin assembly in MDCK cells is EVL specific. Our results identify CRMP-1 as a novel regulator of actin filament elongation and reveal a surprisingly important role for CRMP-1, EVL, and actin polymerization in maintaining the structural integrity of epithelial sheets.

Migration of oral squamous cell carcinoma cells are induced by HGF/c-Met signalling via lamellipodia and filopodia formation

The activation of receptor tyrosine kinases (RTKs) results in cellular effects including cell proliferation, survival, migration and invasion; RTKs also play an important role in tumourigenesis. It has been reported that EGFR signalling controls the migration of oral squamous cell carcinoma (OSCC) SAS and HSC3 cells but not of HSC4 cells, although the proliferation of HSC4 cells is regulated by EGF/EGFR. In the present study, we investigated the roles of EGFR and the c-Met signalling pathway in cell migration via filopodia and lamellipodia formation, which may be prerequisites for migration. To explore the role of c-Met in cell migration, we inhibited c-Met RTK activity using the c-Met inhibitor SU11274 and activated c-Met using hepatocyte growth factor (HGF) in three OSCC cell lines HSC4, SAS and Ca9-22 and investigated migration potency using a wound healing assay. We showed that inhibition of c-Met significantly suppressed, and activation of c-Met significantly promoted, the migration of OSCC cells. Additionally, the migration of SAS and Ca9-22 cells was inhibited by the EGFR inhibitors AG1478 and cetuximab and promoted by EGF treatment. Moreover, migration potency was correlated with lamellipodia formation. Furthermore, western blot analyses demonstrated that SU11274 decreased and HGF increased lamellipodin protein levels as well as phosphorylated c-Met levels. Collectively, we demonstrated that c-Met signalling induced lamellipodia formation by upregulating lamellipodin, thereby promoting the migration of OSCC cells.

MRL proteins cooperate with activated Ras in glia to drive distinct oncogenic outcomes

The Mig10/RIAM/Lpd (MRL) adapter protein Lpd regulates actin dynamics through interactions with Scar/WAVE and Ena/VASP proteins to promote the formation of cellular protrusions and to stimulate invasive migration. However, the ability of MRL proteins to interact with multiple actin regulators and to promote serum response factor (SRF) signalling has raised the question of whether MRL proteins employ alternative downstream mechanisms to drive oncogenic processes in a context-dependent manner. Here, using a Drosophila model, we show that overexpression of either human Lpd or its Drosophila orthologue Pico can promote growth and invasion of Ras^V12-induced cell tumours in the brain. Notably, effects were restricted to two populations of Repo-positive glial cells: an invasive population, characterized by JNK-dependent elevation of Mmp1 expression, and a hyperproliferative population lacking elevated JNK signalling. JNK activation was not triggered by reactive immune cell signalling, implicating the involvement of an intrinsic stress response. The ability to promote dissemination of Ras^V12-induced tumours was shared by a subset of actin regulators, including, most prominently, Chicadee/Profilin, which directly interacts with Pico, and, Mal, a cofactor for serum response factor that responds to changes in G:F actin dynamics. Suppression of Mal activity partially abrogated the ability of pico to promote invasion of Ras^V12 tumours. Furthermore, we found that larval glia are enriched for serum response factor expression, explaining the apparent sensitivity of glial cells to Pico/Ras^V12 overexpression. Taken together, our findings indicate that MRL proteins cooperate with oncogenic Ras to promote formation of glial tumours, and that, in this context, Mal/serum response factor activation is rate-limiting for tumour dissemination.

Vasodilator-Stimulated Phosphoprotein (VASP) depletion from breast cancer MDA-MB-231 cells inhibits tumor spheroid invasion through downregulation of Migfilin, β-catenin and urokinase-plasminogen activator (uPA)

A hallmark of cancer cells is their ability to invade surrounding tissues and form metastases. Cell-extracellular matrix (ECM)-adhesion proteins are crucial in metastasis, connecting tumor ECM with actin cytoskeleton thus enabling cells to respond to mechanical cues. Vasodilator-stimulated phosphoprotein (VASP) is an actin-polymerization regulator which interacts with cell-ECM adhesion protein Migfilin, and regulates cell migration. We compared VASP expression in MCF-7 and MDA-MB-231 breast cancer (BC) cells and found that more invasive MDA-MB-231 cells overexpress VASP. We then utilized a 3-dimensional (3D) approach to study metastasis in MDA-MB-231 cells using a system that considers mechanical forces exerted by the ECM. We prepared 3D collagen I gels of increasing concentration, imaged them by atomic force microscopy, and used them to either embed cells or tumor spheroids, in the presence or absence of VASP. We show, for the first time, that VASP silencing downregulated Migfilin, β-catenin and urokinase plasminogen activator both in 2D and 3D, suggesting a matrix-independent mechanism. Tumor spheroids lacking VASP demonstrated impaired invasion, indicating VASP's involvement in metastasis, which was corroborated by Kaplan-Meier plotter showing high VASP expression to be associated with poor remission-free survival in lymph node-positive BC patients. Hence, VASP may be a novel BC metastasis biomarker.

Phosphatidylinositol 3,4-bisphosphate regulates neurite initiation and dendrite morphogenesis via actin aggregation

Neurite initiation is critical for neuronal morphogenesis and early neural circuit development. Recent studies showed that local actin aggregation underneath the cell membrane determined the site of neurite initiation. An immediately arising question is what signaling mechanism initiated actin aggregation. Here we demonstrate that local clustering of phosphatidylinositol 3,4-bisphosphate (PI(3,4)P₂), a phospholipid with relatively few known signaling functions, is necessary and sufficient for aggregating actin and promoting neuritogenesis. In contrast, the related and more extensively studied phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) molecules did not have such functions. Specifically, we showed that beads coated with PI(3,4)P₂ promoted actin aggregation and neurite initiation, while pharmacological interference with PI(3,4)P₂ synthesis inhibited both processes. PI(3,4)P₂ clustering occurred even when actin aggregation was pharmacologically blocked, demonstrating that PI(3,4)P₂ functioned as the upstream signaling molecule. Two enzymes critical for PI(3,4)P₂ generation, namely, SH2 domain-containing inositol 5-phosphatase and class II phosphoinositide 3-kinase α, were complementarily and non-redundantly required for actin aggregation and neuritogenesis, as well as for subsequent dendritogenesis. Finally, we demonstrate that neural Wiskott-Aldrich syndrome protein and the Arp2/3 complex functioned downstream of PI(3,4)P₂ to mediate neuritogenesis and dendritogenesis. Together, our results identify PI(3,4)P₂ as an important signaling molecule during early development and demonstrate its critical role in regulating actin aggregation and neuritogenesis.

MenaINV dysregulates cortactin phosphorylation to promote invadopodium maturation

Invadopodia, actin-based protrusions of invasive carcinoma cells that focally activate extracellular matrix-degrading proteases, are essential for the migration and intravasation of tumor cells during dissemination from the primary tumor. We have previously shown that cortactin phosphorylation at tyrosine residues, in particular tyrosine 421, promotes actin polymerization at newly-forming invadopodia, promoting their maturation to matrix-degrading structures. However, the mechanism by which cells regulate the cortactin tyrosine phosphorylation-dephosphorylation cycle at invadopodia is unknown. Mena, an actin barbed-end capping protein antagonist, is expressed as various splice-isoforms. The Mena^INV isoform is upregulated in migratory and invasive sub-populations of breast carcinoma cells, and is involved in tumor cell intravasation. Here we show that forced Mena^INV expression increases invadopodium maturation to a far greater extent than equivalent expression of other Mena isoforms. Mena^INV is recruited to invadopodium precursors just after their initial assembly at the plasma membrane, and promotes the phosphorylation of cortactin tyrosine 421 at invadopodia. In addition, we show that cortactin phosphorylation at tyrosine 421 is suppressed by the phosphatase PTP1B, and that PTP1B localization to the invadopodium is reduced by Mena^INV expression. We conclude that Mena^INV promotes invadopodium maturation by inhibiting normal dephosphorylation of cortactin at tyrosine 421 by the phosphatase PTP1B.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

Emerging evidence of signalling roles for PI(3,4)P2 in Class I and II PI3K-regulated pathways

There are eight members of the phosphoinositide family of phospholipids in eukaryotes; PI, PI3P, PI4P, PI5P, PI(4,5)P2, PI(3,4)P2, PI(3,5)P2 and PI(3,4,5)P3. Receptor activation of Class I PI3Ks stimulates the phosphorylation of PI(4,5)P2 to form PI(3,4,5)P3. PI(3,4,5)P3 is an important messenger molecule that is part of a complex signalling network controlling cell growth and division. PI(3,4,5)P3 can be dephosphorylated by both 3- and 5-phosphatases, producing PI(4,5)P2 and PI(3,4)P2, respectively. There is now strong evidence that PI(3,4)P2 generated by this route does not merely represent another pathway for removal of PI(3,4,5)P3, but can act as a signalling molecule in its own right, regulating macropinocytosis, fast endophilin-mediated endocytosis (FEME), membrane ruffling, lamellipodia and invadopodia. PI(3,4)P2 can also be synthesized directly from PI4P by Class II PI3Ks and this is important for the maturation of clathrin-coated pits [clathrin-mediated endocytosis (CME)] and signalling in early endosomes. Thus PI(3,4)P2 is emerging as an important signalling molecule involved in the coordination of several specific membrane and cytoskeletal responses. Further, its inappropriate accumulation contributes to pathology caused by mutations in genes encoding enzymes responsible for its degradation, e.g. Inpp4B.

The alternatively-included 11a sequence modifies the effects of Mena on actin cytoskeletal organization and cell behavior

During tumor progression, alternative splicing gives rise to different Mena protein isoforms. We analyzed how Mena11a, an isoform enriched in epithelia and epithelial-like cells, affects Mena-dependent regulation of actin dynamics and cell behavior. While other Mena isoforms promote actin polymerization and drive membrane protrusion, we find that Mena11a decreases actin polymerization and growth factor-stimulated membrane protrusion at lamellipodia. Ectopic Mena11a expression slows mesenchymal-like cell motility, while isoform-specific depletion of endogenous Mena11a in epithelial-like tumor cells perturbs cell:cell junctions and increases membrane protrusion and overall cell motility. Mena11a can dampen membrane protrusion and reduce actin polymerization in the absence of other Mena isoforms, indicating that it is not simply an inactive Mena isoform. We identify a phosphorylation site within 11a that is required for some Mena11a-specific functions. RNA-seq data analysis from patient cohorts demonstrates that the difference between mRNAs encoding constitutive Mena sequences and those containing the 11a exon correlates with metastasis in colorectal cancer, suggesting that 11a exon exclusion contributes to invasive phenotypes and leads to poor clinical outcomes.

Cytotoxic Escherichia coli strains encoding colibactin colonize laboratory mice

Escherichia coli strains have not been fully characterized in laboratory mice and are not currently excluded from mouse colonies. Colibactin (Clb), a cytotoxin, has been associated with inflammation and cancer in humans and animals. We performed bacterial cultures utilizing rectal swab, fecal, and extra intestinal samples from clinically unaffected or affected laboratory mice. Fifty-one E. coli were isolated from 45 laboratory mice, identified biochemically, and selected isolates were serotyped. The 16S rRNA gene was amplified and sequenced for specific isolates, PCR used for clbA and clbQ gene amplification, and phylogenetic group identification was performed on all 51 E. coli strains. Clb genes were sequenced and selected E. coli isolates were characterized using a HeLa cell cytotoxicity assay. Forty-five of the 51 E. coli isolates (88%) encoded clbA and clbQ and belonged to phylogenetic group B2. Mouse E. coli serotypes included: O2:H6, O-:H-, OM:H+, and O22:H-. Clb-encoding O2: H6 mouse E. coli isolates were cytotoxic in vitro. A Clb-encoding E. coli was isolated from a clinically affected genetically modified mouse with cystic endometrial hyperplasia. Our findings suggest that Clb-encoding E. coli colonize laboratory mice and may induce clinical and subclinical diseases that may impact experimental mouse models.

Depletion of Inositol Polyphosphate 4-Phosphatase II Suppresses Callosal Axon Formation in the Developing Mice

The corpus callosum is a bundle of nerve fibers that connects the two cerebral hemispheres and is essential for coordinated transmission of information between them. Disruption of early stages of callosal development can cause agenesis of the corpus callosum (AgCC), including both complete and partial callosal absence, causing mild to severe cognitive impairment. Despite extensive studies, the etiology of AgCC remains to be clarified due to the complicated mechanism involved in generating AgCC. The biological function of PI3K signaling including phosphatidylinositol-3,4,5-trisphosphate is well established in diverse biochemical processes including axon and dendrite morphogenesis, but the function of the closely related phosphatidylinositol-3,4,-bisphosphate (PI(3,4)P2) signaling, particularly in the nervous system, is largely unknown. Here, we provide the first report on the role of inositol polyphosphate 4-phosphatase II (INPP4B), a PI(3,4)P2 metabolizing 4-phosphatase in the regulation of callosal axon formation. Depleting INPP4B by in utero electroporation suppressed medially directed callosal axon formation. Moreover, depletion of INPP4B significantly attenuated formation of Satb2-positive pyramidal neurons and axon polarization in cortical neurons during cortical development. Taken together, these data suggest that INPP4B plays a role in the regulating callosal axon formation by controlling axon polarization and the Satb2-positive pyramidal neuron population. Dysregulation of INPP4B during cortical development may be implicated in the generation of partial AgCC.