A mechanosensitive caveolae-invadosome interplay drives matrix remodelling for cancer cell invasion

Invadosomes and caveolae are mechanosensitive structures that are implicated in metastasis. Here, we describe a unique juxtaposition of caveola clusters and matrix degradative invadosomes at contact sites between the plasma membrane of cancer cells and constricting fibrils both in 2D and 3D type I collagen matrix environments. Preferential association between caveolae and straight segments of the fibrils, and between invadosomes and bent segments of the fibrils, was observed along with matrix remodelling. Caveola recruitment precedes and is required for invadosome formation and activity. Reciprocally, invadosome disruption results in the accumulation of fibril-associated caveolae. Moreover, caveolae and the collagen receptor β1 integrin co-localize at contact sites with the fibrils, and integrins control caveola recruitment to fibrils. In turn, caveolae mediate the clearance of β1 integrin and collagen uptake in an invadosome-dependent and collagen-cleavage-dependent mechanism. Our data reveal a reciprocal interplay between caveolae and invadosomes that coordinates adhesion to and proteolytic remodelling of confining fibrils to support tumour cell dissemination.

Mechanisms of action of NME metastasis suppressors - a family affair

Metastatic progression is regulated by metastasis promoter and suppressor genes. NME1, the prototypic and first described metastasis suppressor gene, encodes a nucleoside diphosphate kinase (NDPK) involved in nucleotide metabolism; two related family members, NME2 and NME4, are also reported as metastasis suppressors. These proteins physically interact with members of the GTPase dynamin family, which have key functions in membrane fission and fusion reactions necessary for endocytosis and mitochondrial dynamics. Evidence supports a model in which NDPKs provide GTP to dynamins to maintain a high local GTP concentration for optimal dynamin function. NME1 and NME2 are cytosolic enzymes that provide GTP to dynamins at the plasma membrane, which drive endocytosis, suggesting that these NMEs are necessary to attenuate signaling by receptors on the cell surface. Disruption of NDPK activity in NME-deficient tumors may thus drive metastasis by prolonging signaling. NME4 is a mitochondrial enzyme that interacts with the dynamin OPA1 at the mitochondria inner membrane to drive inner membrane fusion and maintain a fused mitochondrial network. This function is consistent with the current view that mitochondrial fusion inhibits the metastatic potential of tumor cells whereas mitochondrial fission promotes metastasis progression. The roles of NME family members in dynamin-mediated endocytosis and mitochondrial dynamics and the intimate link between these processes and metastasis provide a new framework to understand the metastasis suppressor functions of NME proteins.

Codependencies of mTORC1 signaling and endolysosomal actin structures

The mechanistic target of rapamycin complex 1 (mTORC1) is part of the amino acid sensing machinery that becomes activated on the endolysosomal surface in response to nutrient cues. Branched actin generated by WASH and Arp2/3 complexes defines endolysosomal microdomains. Here, we find mTORC1 components in close proximity to endolysosomal actin microdomains. We investigated for interactors of the mTORC1 lysosomal tether, RAGC, by proteomics and identified multiple actin filament capping proteins and their modulators. Perturbation of RAGC function affected the size of endolysosomal actin, consistent with a regulation of actin filament capping by RAGC. Reciprocally, the pharmacological inhibition of actin polymerization or alteration of endolysosomal actin obtained upon silencing of WASH or Arp2/3 complexes impaired mTORC1 activity. Mechanistically, we show that actin is required for proper association of RAGC and mTOR with endolysosomes. This study reveals an unprecedented interplay between actin and mTORC1 signaling on the endolysosomal system.

Multiscale model of the different modes of cancer cell invasion

MOTIVATION

Mathematical models of biological processes altered in cancer are built using the knowledge of complex networks of signaling pathways, detailing the molecular regulations inside different cell types, such as tumor cells, immune and other stromal cells. If these models mainly focus on intracellular information, they often omit a description of the spatial organization among cells and their interactions, and with the tumoral microenvironment.

METHODS

We present here a model of tumor cell invasion simulated with PhysiBoSS, a multiscale framework which combines agent-based modeling and continuous time Markov processes applied on Boolean network models. With this model, we aim to study the different modes of cell migration and to predict means to block it by considering not only spatial information obtained from the agent-based simulation but also intracellular regulation obtained from the Boolean model.

RESULTS

Our multiscale model integrates the impact of gene mutations with the perturbation of the environmental conditions and allows the visualization of the results with 2D and 3D representations. The model successfully reproduces single and collective migration processes and is validated on published experiments on cell invasion. In silico experiments are suggested to search for possible targets that can block the more invasive tumoral phenotypes.

SUPPLEMENTARY INFORMATION

https://github.com/sysbio-curie/Invasion_model_PhysiBoSS.

Invadopodia Methods: Detection of Invadopodia Formation and Activity in Cancer Cells Using Reconstituted 2D and 3D Collagen-Based Matrices

Tumor dissemination involves cancer cell migration through the extracellular matrix (ECM). ECM is mainly composed of collagen fibers that oppose cell invasion. To overcome hindrance in the matrix, cancer cells deploy a protease-dependent program in order to remodel the matrix fibers. Matrix remodeling requires the formation of actin-based matrix/plasma membrane contact sites called invadopodia, responsible for collagen cleavage through the accumulation and activity of the transmembrane type-I matrix metalloproteinase (MT1-MMP). In this article, we describe experimental procedures designed to assay for invadopodia formation and for invadopodia activity using 2D and 3D models based on gelatin (denatured collagen) and fibrillar type-I collagen matrices.

Force tuning through regulation of clathrin-dependent integrin endocytosis

Integrin endocytosis is essential for many fundamental cellular processes. Whether and how the internalization impacts cellular mechanics remains elusive. Whereas previous studies reported the contribution of the integrin activator, talin, in force development, the involvement of inhibitors is less documented. We identified ICAP-1 as an integrin inhibitor involved in mechanotransduction by co-working with NME2 to control clathrin-mediated endocytosis of integrins at the edge of focal adhesions (FA). Loss of ICAP-1 enables β3-integrin-mediated force generation independently of β1 integrin. β3-integrin-mediated forces were associated with a decrease in β3 integrin dynamics stemming from their reduced diffusion within adhesion sites and slow turnover of FA. The decrease in β3 integrin dynamics correlated with a defect in integrin endocytosis. ICAP-1 acts as an adaptor for clathrin-dependent endocytosis of integrins. ICAP-1 controls integrin endocytosis by interacting with NME2, a key regulator of dynamin-dependent clathrin-coated pits fission. Control of clathrin-mediated integrin endocytosis by an inhibitor is an unprecedented mechanism to tune forces at FA.

Compromised nuclear envelope integrity drives TREX1-dependent DNA damage and tumor cell invasion

Although mutations leading to a compromised nuclear envelope cause diseases such as muscular dystrophies or accelerated aging, the consequences of mechanically induced nuclear envelope ruptures are less known. Here, we show that nuclear envelope ruptures induce DNA damage that promotes senescence in non-transformed cells and induces an invasive phenotype in human breast cancer cells. We find that the endoplasmic reticulum (ER)-associated exonuclease TREX1 translocates into the nucleus after nuclear envelope rupture and is required to induce DNA damage. Inside the mammary duct, cellular crowding leads to nuclear envelope ruptures that generate TREX1-dependent DNA damage, thereby driving the progression of in situ carcinoma to the invasive stage. DNA damage and nuclear envelope rupture markers were also enriched at the invasive edge of human tumors. We propose that DNA damage in mechanically challenged nuclei could affect the pathophysiology of crowded tissues by modulating proliferation and extracellular matrix degradation of normal and transformed cells.

mTOR Repression in Response to Amino Acid Starvation Promotes ECM Degradation Through MT1-MMP Endocytosis Arrest

Under conditions of starvation, normal and tumor epithelial cells can rewire their metabolism toward the consumption of extracellular proteins, including extracellular matrix-derived components as nutrient sources. The mechanism of pericellular matrix degradation by starved cells has been largely overlooked. Here it is shown that matrix degradation by breast and pancreatic tumor cells and patient-derived xenograft explants increases by one order of magnitude upon amino acid and growth factor deprivation. In addition, it is found that collagenolysis requires the invadopodia components, TKS5, and the transmembrane metalloproteinase, MT1-MMP, which are key to the tumor invasion program. Increased collagenolysis is controlled by mTOR repression upon nutrient depletion or pharmacological inhibition by rapamycin. The results reveal that starvation hampers clathrin-mediated endocytosis, resulting in MT1-MMP accumulation in arrested clathrin-coated pits. The study uncovers a new mechanism whereby mTOR repression in starved cells leads to the repurposing of abundant plasma membrane clathrin-coated pits into robust ECM-degradative assemblies.

Tissue remodeling by invadosomes

One of the strategies used by cells to degrade and remodel the extracellular matrix (ECM) is based on invadosomes, actin-based force-producing cell–ECM contacts that function in adhesion and migration and are characterized by their capacity to mediate pericellular proteolysis of ECM components. Invadosomes found in normal cells are called podosomes, whereas invadosomes of invading cancer cells are named invadopodia. Despite their broad involvement in cell migration and in protease-dependent ECM remodeling and their detection in living organisms and in fresh tumor tissue specimens, the specific composition and dynamic behavior of podosomes and invadopodia and their functional relevance in vivo remain poorly understood. Here, we discuss recent findings that underline commonalities and peculiarities of podosome and invadopodia in terms of organization and function and propose an updated definition of these cellular protrusions, which are increasingly relevant in patho-physiological tissue remodeling.

Protrudin-mediated ER-endosome contact sites promote MT1-MMP exocytosis and cell invasion

Cancer cells break tissue barriers by use of small actin-rich membrane protrusions called invadopodia. Complete invadopodia maturation depends on protrusion outgrowth and the targeted delivery of the matrix metalloproteinase MT1-MMP via endosomal transport by mechanisms that are not known. Here, we show that the ER protein Protrudin orchestrates invadopodia maturation and function. Protrudin formed contact sites with MT1-MMP-positive endosomes that contained the RAB7-binding Kinesin-1 adaptor FYCO1, and depletion of RAB7, FYCO1, or Protrudin inhibited MT1-MMP-dependent extracellular matrix degradation and cancer cell invasion by preventing anterograde translocation and exocytosis of MT1-MMP. Moreover, when endosome translocation or exocytosis was inhibited by depletion of Protrudin or Synaptotagmin VII, respectively, invadopodia were unable to expand and elongate. Conversely, when Protrudin was overexpressed, noncancerous cells developed prominent invadopodia-like protrusions and showed increased matrix degradation and invasion. Thus, Protrudin-mediated ER-endosome contact sites promote cell invasion by facilitating translocation of MT1-MMP-laden endosomes to the plasma membrane, enabling both invadopodia outgrowth and MT1-MMP exocytosis.

Intersection of TKS5 and FGD1/CDC42 signaling cascades directs the formation of invadopodia

Tumor cells exposed to a physiological matrix of type I collagen fibers form elongated collagenolytic invadopodia, which differ from dotty-like invadopodia forming on the gelatin substratum model. The related scaffold proteins, TKS5 and TKS4, are key components of the mechanism of invadopodia assembly. The molecular events through which TKS proteins direct collagenolytic invadopodia formation are poorly defined. Using coimmunoprecipitation experiments, identification of bound proteins by mass spectrometry, and in vitro pull-down experiments, we found an interaction between TKS5 and FGD1, a guanine nucleotide exchange factor for the Rho-GTPase CDC42, which is known for its role in the assembly of invadopodial actin core structure. A novel cell polarity network is uncovered comprising TKS5, FGD1, and CDC42, directing invadopodia formation and the polarization of MT1-MMP recycling compartments, required for invadopodia activity and invasion in a 3D collagen matrix. Additionally, our data unveil distinct signaling pathways involved in collagenolytic invadopodia formation downstream of TKS4 or TKS5 in breast cancer cells.

A new pipeline for pathophysiological analysis of the mammary gland based on organoid transplantation and organ clearing

Recent developments in techniques for tissue clearing and size reduction have enabled optical imaging of whole organs and the study of rare tumorigenic events in vivo The adult mammary gland provides a unique model for investigating physiological or pathological processes such as morphogenesis or epithelial cell dissemination. Here, we establish a new pipeline to study rare cellular events occurring in the mammary gland, by combining orthotopic transplantation of mammary organoids with the uDISCO organ size reduction and clearing method. This strategy allows us to analyze the behavior of individually labeled cells in regenerated mammary gland. As a proof of concept, we analyzed the localization of rare epithelial cells overexpressing atypical protein kinase C iota (also known as PRKCI, referred to here as aPKCι) with an N-terminal eGFP fusion (GFP-aPKCι+) in the normal mammary gland. Using this analytical pipeline, we were able to visualize epithelial aPKCι+ cells escaping from the normal mammary epithelium and disseminating into the surrounding stroma. This technical resource should benefit mammary development and tumor progression studies.

MT1-MMP targeting to endolysosomes is mediated by upregulation of flotillins

Tumor cell invasion and metastasis formation are the major cause of death in cancer patients. These processes rely on extracellular matrix (ECM) degradation mediated by organelles termed invadopodia, to which the transmembrane matrix metalloproteinase MT1-MMP (also known as MMP14) is delivered from its reservoir, the RAB7-containing endolysosomes. How MT1-MMP is targeted to endolysosomes remains to be elucidated. Flotillin-1 and -2 are upregulated in many invasive cancers. Here, we show that flotillin upregulation triggers a general mechanism, common to carcinoma and sarcoma, which promotes RAB5-dependent MT1-MMP endocytosis and its delivery to RAB7-positive endolysosomal reservoirs. Conversely, flotillin knockdown in invasive cancer cells greatly reduces MT1-MMP accumulation in endolysosomes, its subsequent exocytosis at invadopodia, ECM degradation and cell invasion. Our results demonstrate that flotillin upregulation is necessary and sufficient to promote epithelial and mesenchymal cancer cell invasion and ECM degradation by controlling MT1-MMP endocytosis and delivery to the endolysosomal recycling compartment.

A NUMB-EFA6B-ARF6 recycling route controls apically restricted cell protrusions and mesenchymal motility

The endocytic protein NUMB has been implicated in the control of various polarized cellular processes, including the acquisition of mesenchymal migratory traits through molecular mechanisms that have only been partially defined. Here, we report that NUMB is a negative regulator of a specialized set of understudied, apically restricted, actin-based protrusions, the circular dorsal ruffles (CDRs), induced by either PDGF or HGF stimulation. Through its PTB domain, NUMB binds directly to an N-terminal NPLF motif of the ARF6 guanine nucleotide exchange factor, EFA6B, and promotes its exchange activity in vitro. In cells, a NUMB-EFA6B-ARF6 axis regulates the recycling of the actin regulatory cargo RAC1 and is critical for the formation of CDRs that mark the acquisition of a mesenchymal mode of motility. Consistently, loss of NUMB promotes HGF-induced cell migration and invasion. Thus, NUMB negatively controls membrane protrusions and the acquisition of mesenchymal migratory traits by modulating EFA6B-ARF6 activity.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

AMOTL1 Promotes Breast Cancer Progression and Is Antagonized by Merlin

The Hippo signaling network is a key regulator of cell fate. In the recent years, it was shown that its implication in cancer goes well beyond the sole role of YAP transcriptional activity and its regulation by the canonical MST/LATS kinase cascade. Here we show that the motin family member AMOTL1 is an important effector of Hippo signaling in breast cancer. AMOTL1 connects Hippo signaling to tumor cell aggressiveness. We show that both canonical and noncanonical Hippo signaling modulates AMOTL1 levels. The tumor suppressor Merlin triggers AMOTL1 proteasomal degradation mediated by the NEDD family of ubiquitin ligases through direct interaction. In parallel, YAP stimulates AMOTL1 expression. The loss of Merlin expression and the induction of Yap activity that are frequently observed in breast cancers thus result in elevated AMOTL1 levels. AMOTL1 expression is sufficient to trigger tumor cell migration and stimulates proliferation by activating c-Src. In a large cohort of human breast tumors, we show that AMOTL1 protein levels are upregulated during cancer progression and that, importantly, the expression of AMOTL1 in lymph node metastasis appears predictive of the risk of relapse. Hence we uncover an important mechanism by which Hippo signaling promotes breast cancer progression by modulating the expression of AMOTL1.

Cellular and Molecular Mechanisms of MT1-MMP-Dependent Cancer Cell Invasion

Metastasis is responsible for most cancer-associated deaths. Accumulating evidence based on 3D migration models has revealed a diversity of invasive migratory schemes reflecting the plasticity of tumor cells to switch between proteolytic and nonproteolytic modes of invasion. Yet, initial stages of localized regional tumor dissemination require proteolytic remodeling of the extracellular matrix to overcome tissue barriers. Recent data indicate that surface-exposed membrane type 1-matrix metalloproteinase (MT1-MMP), belonging to a group of membrane-anchored MMPs, plays a central role in pericellular matrix degradation during basement membrane and interstitial tissue transmigration programs. In addition, a large body of work indicates that MT1-MMP is targeted to specialized actin-rich cell protrusions termed invadopodia, which are responsible for matrix degradation. This review describes the multistep assembly of actin-based invadopodia in molecular details. Mechanisms underlying MT1-MMP traffic to invadopodia through endocytosis/recycling cycles, which are key to the invasive program of carcinoma cells, are discussed.

RAB2A controls MT1-MMP endocytic and E-cadherin polarized Golgi trafficking to promote invasive breast cancer programs

The mechanisms of tumor cell dissemination and the contribution of membrane trafficking in this process are poorly understood. Through a functional siRNA screening of human RAB GTPases, we found that RAB2A, a protein essential for ER-to-Golgi transport, is critical in promoting proteolytic activity and 3D invasiveness of breast cancer (BC) cell lines. Remarkably, RAB2A is amplified and elevated in human BC and is a powerful and independent predictor of disease recurrence in BC patients. Mechanistically, RAB2A acts at two independent trafficking steps. Firstly, by interacting with VPS39, a key component of the late endosomal HOPS complex, it controls post-endocytic trafficking of membrane-bound MT1-MMP, an essential metalloprotease for matrix remodeling and invasion. Secondly, it further regulates Golgi transport of E-cadherin, ultimately controlling junctional stability, cell compaction, and tumor invasiveness. Thus, RAB2A is a novel trafficking determinant essential for regulation of a mesenchymal invasive program of BC dissemination.

RAB2A controls MT1-MMP endocytic and E-cadherin polarized Golgi trafficking to promote invasive breast cancer programs

The mechanisms of tumor cell dissemination and the contribution of membrane trafficking in this process are poorly understood. Through a functional siRNA screening of human RAB GTPases, we found that RAB2A, a protein essential for ER-to-Golgi transport, is critical in promoting proteolytic activity and 3D invasiveness of breast cancer (BC) cell lines. Remarkably, RAB2A is amplified and elevated in human BC and is a powerful and independent predictor of disease recurrence in BC patients. Mechanistically, RAB2A acts at two independent trafficking steps. Firstly, by interacting with VPS39, a key component of the late endosomal HOPS complex, it controls post-endocytic trafficking of membrane-bound MT1-MMP, an essential metalloprotease for matrix remodeling and invasion. Secondly, it further regulates Golgi transport of E-cadherin, ultimately controlling junctional stability, cell compaction, and tumor invasiveness. Thus, RAB2A is a novel trafficking determinant essential for regulation of a mesenchymal invasive program of BC dissemination.

LIMK Regulates Tumor-Cell Invasion and Matrix Degradation Through Tyrosine Phosphorylation of MT1-MMP

During their metastatic spread, cancer cells need to remodel the extracellular matrix in order to migrate through stromal compartments adjacent to the primary tumor. Dissemination of breast carcinoma cells is mediated by membrane type 1-matrix metalloproteinase (MT1-MMP/MMP14), the main invadopodial matrix degradative component. Here, we identify MT1-MMP as a novel interacting partner of dual-specificity LIM Kinase-1 and -2 (LIMK1/2), and provide several evidence for phosphorylation of tyrosine Y573 in the cytoplasmic domain of MT1-MMP by LIMK. Phosphorylation of Y573 influences association of F-actin binding protein cortactin to MT1-MMP-positive endosomes and invadopodia formation and matrix degradation. Moreover, we show that LIMK1 regulates cortactin association to MT1-MMP-positive endosomes, while LIMK2 controls invadopodia-associated cortactin. In turn, LIMK1 and LIMK2 are required for MT1-MMP-dependent matrix degradation and cell invasion in a three-dimensional type I collagen environment. This novel link between LIMK1/2 and MT1-MMP may have important consequences for therapeutic control of breast cancer cell invasion.

ARF6-JIP3/4 regulate endosomal tubules for MT1-MMP exocytosis in cancer invasion

Interaction of plasma membrane ARF6 with JIP3/JIP4 effectors on MT1-MMP endosomes coordinates dynactin–dynein and kinesin-1 activity in a tug-of-war mechanism for endosome tubulation and MT1-MMP exocytosis to promote breast cancer cell invasion.

Invasion of cancer cells into collagen-rich extracellular matrix requires membrane-tethered membrane type 1–matrix metalloproteinase (MT1-MMP) as the key protease for collagen breakdown. Understanding how MT1-MMP is delivered to the surface of tumor cells is essential for cancer cell biology. In this study, we identify ARF6 together with c-Jun NH₂-terminal kinase–interacting protein 3 and 4 (JIP3 and JIP4) effectors as critical regulators of this process. Silencing ARF6 or JIP3/JIP4 in breast tumor cells results in MT1-MMP endosome mispositioning and reduces MT1-MMP exocytosis and tumor cell invasion. JIPs are recruited by Wiskott-Aldrich syndrome protein and scar homologue (WASH) on MT1-MMP endosomes on which they recruit dynein–dynactin and kinesin-1. The interaction of plasma membrane ARF6 with endosomal JIPs coordinates dynactin–dynein and kinesin-1 activity in a tug-of-war mechanism, leading to MT1-MMP endosome tubulation and exocytosis. In addition, we find that ARF6, MT1-MMP, and kinesin-1 are up-regulated in high-grade triple-negative breast cancers. These data identify a critical ARF6–JIP–MT1-MMP–dynein–dynactin–kinesin-1 axis promoting an invasive phenotype of breast cancer cells.

LIN7A is a major determinant of cell-polarity defects in breast carcinomas

Background

Polarity defects are a hallmark of most carcinomas. Cells from invasive micropapillary carcinomas (IMPCs) of the breast are characterized by a striking cell polarity inversion and represent an interesting model for the analysis of polarity abnormalities.

Methods

In-depth investigation of polarity proteins in 24 IMPCs and a gene expression profiling, comparing IMPC (n = 73) with invasive carcinomas of no special type (ICNST) (n = 51) have been performed.

Results

IMPCs showed a profound disorganization of the investigated polarity proteins and revealed major abnormalities in their subcellular localization. Gene expression profiling experiments highlighted a number of deregulated genes in the IMPCs that have a role in apico-basal polarity, adhesion and migration. LIN7A, a Crumbs-complex polarity gene, was one of the most differentially over-expressed genes in the IMPCs. Upon LIN7A over-expression, we observed hyperproliferation, invasion and a complete absence of lumen formation, revealing strong polarity defects.

Conclusion

This study therefore shows that LIN7A has a crucial role in the polarity abnormalities associated with breast carcinogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-016-0680-x) contains supplementary material, which is available to authorized users.

Social networking in tumor cell communities is associated with increased aggressiveness

Extracellular vesicles (EVs) are lipid-bilayer-enclosed vesicles that contain proteins, lipids and nucleic acids. EVs produced by cells from healthy tissues circulate in the blood and body fluids, and can be taken up by unrelated cells. As they have the capacity to transfer cargo proteins, lipids and nucleic acids (mostly mRNAs and miRNAs) between different cells in the body, EVs are emerging as mediators of intercellular communication that could modulate cell behavior, tissue homeostasis and regulation of physiological functions. EV-mediated cell-cell communications are also proposed to play a role in disease, for example, cancer, where they could contribute to transfer of traits required for tumor progression and metastasis. However, direct evidence for EV-mediated mRNA transfer to individual cells and for its biological consequences in vivo has been missing until recently. Recent studies have reported elegant experiments using genetic tracing with the Cre recombinase system and intravital imaging that visualize and quantify functional transfer of mRNA mediated by EVs in the context of cancer and metastasis.

Impaired PRC2 activity promotes transcriptional instability and favors breast tumorigenesis

In this study, Wassef et al. used mouse and human models to show that the high expression of Polycomb protein EZH2 in solid tumors is a consequence, not a cause, of tumorigenesis and that low abundance or deletion of EZH2 relative to proliferation is linked to poor prognosis and transcriptional instability.

Alterations of chromatin modifiers are frequent in cancer, but their functional consequences often remain unclear. Focusing on the Polycomb protein EZH2 that deposits the H3K27me3 (trimethylation of Lys27 of histone H3) mark, we showed that its high expression in solid tumors is a consequence, not a cause, of tumorigenesis. In mouse and human models, EZH2 is dispensable for prostate cancer development and restrains breast tumorigenesis. High EZH2 expression in tumors results from a tight coupling to proliferation to ensure H3K27me3 homeostasis. However, this process malfunctions in breast cancer. Low EZH2 expression relative to proliferation and mutations in Polycomb genes actually indicate poor prognosis and occur in metastases. We show that while altered EZH2 activity consistently modulates a subset of its target genes, it promotes a wider transcriptional instability. Importantly, transcriptional changes that are consequences of EZH2 loss are predominantly irreversible. Our study provides an unexpected understanding of EZH2's contribution to solid tumors with important therapeutic implications.

Regulated delivery of molecular cargo to invasive tumour-derived microvesicles

Cells release multiple, distinct forms of extracellular vesicles including structures known as microvesicles, which are known to alter the extracellular environment. Despite growing understanding of microvesicle biogenesis, function and contents, mechanisms regulating cargo delivery and enrichment remain largely unknown. Here we demonstrate that in amoeboid-like invasive tumour cell lines, the v-SNARE, VAMP3, regulates delivery of microvesicle cargo such as the membrane-type 1 matrix metalloprotease (MT1-MMP) to shedding microvesicles. MT1-MMP delivery to nascent microvesicles depends on the association of VAMP3 with the tetraspanin CD9 and facilitates the maintenance of amoeboid cell invasion. VAMP3-shRNA expression depletes shed vesicles of MT1-MMP and decreases cell invasiveness when embedded in cross-linked collagen matrices. Finally, we describe functionally similar microvesicles isolated from bodily fluids of ovarian cancer patients. Together these studies demonstrate the importance of microvesicle cargo sorting in matrix degradation and disease progression.

p63/MT1-MMP axis is required for in situ to invasive transition in basal-like breast cancer

The transition of ductal carcinoma in situ (DCIS) to invasive breast carcinoma requires tumor cells to cross the basement membrane (BM). However, mechanisms underlying BM transmigration are poorly understood. Here, we report that expression of membrane-type 1 (MT1)-matrix metalloproteinase (MMP), a key component of the BM invasion program, increases during breast cancer progression at the in situ to invasive breast carcinoma transition. In the intraductal xenograft model, MT1-MMP is required for BM transmigration of MCF10DCIS.com breast adenocarcinoma cells and is overexpressed in cell clusters overlying focal BM disruptions and at the invasive tumor front. Mirrored upregulation of p63 and MT1-MMP is observed at the edge of MCF10DCIS.com xenograft tumors and p63 is required for induction of MT1-MMP-dependent invasive program in response to microenvironmental signals. Immunohistochemistry and analysis of public database reveal that p63 and MT1-MMP are upregulated in human basal-like breast tumors suggesting that p63/MT1-MMP axis contributes to progression of basal-like breast cancers with elevated p63 and MT1-MMP levels.

Phagocytosis of immunoglobulin-coated emulsion droplets

Phagocytosis by macrophages represents a fundamental process essential for both immunity and tissue homeostasis. The size of targets to be eliminated ranges from small particles as bacteria to large objects as cancerous or senescent cells. Most of our current quantitative knowledge on phagocytosis is based on the use of solid polymer microparticles as model targets that are well adapted to the study of phagocytosis mechanisms that do not involve any lateral mobility of the ligands, despite the relevance of this parameter in the immunological context. Herein we designed monodisperse, IgG-coated emulsion droplets that are efficiently and specifically internalized by macrophages through in-vitro FcγR-mediated phagocytosis. We show that, contrary to solid polymeric beads, droplet uptake is efficient even for low IgG densities, and is accompagnied by the clustering of the opsonins in the zone of contact with the macrophage during the adhesion step. Beyond the sole interest in the design of the material, our results suggest that lateral mobility of proteins at the interface of a target greatly enhances the phagocytic uptake.

Abstract LB-30: Membrane-anchored MT1-MMP downstream of p63 is essential for the transition of in situ to invasive breast carcinoma

Understanding breast tumor progression requires insights both at the molecular and cellular levels. In particular, the transition of ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) is a key, yet poorly understood event. Membrane-Type 1 matrix metalloproteinase (MT1-MMP) is critical for pericellular remodelling of the basement membrane and interstitial collagen during invasion by carcinoma cells.

The role of MT1-MMP during the transition from DCIS to IDC was investigated using an intraductal human-in-mouse xenograft model. Breast tumor-derived MCF10.DCIS.com cells, which express MT1-MMP were injected into the primary mammary ducts of female SCID mice. Histological analysis revealed a progression from DCIS, to microinvasive and invasive lesions by 5, 7-8 and 10-12 weeks post-injection, respectively. Immunohistochemistry (IHC) staining showed homogeneous expression of MT1-MMP over the tumor section at DCIS stage, and up-regulation at the edge of the tumours at microinvasive and invasive states. In microinvasives lesion, up-regulation of MT1-MMP in tumour cells coincided with disruption of the basement membrane. At later stages, one characteristic of invasive lesions is the increase in collagen fibers deposition. Interestingly, MT1-MMP expression was also increased at the front of infiltrating tumours in contact with the stroma.

After 10 weeks, intraductally injected cells knocked down for MT1-MMP developed invasive tumours only in 20% of injected mammary glands, while 100% of glands injected with control cells developed infiltrating lesions. All together, these results demonstrated that MT1-MMP is instrumental for the transition from in situ to invasive breast tumours.

IHC staining revealed a strong correlation of the expression of the basal marker p63 and MT1-MMP in tumour cells both at the microinvasive and invasive stages. One hypothesis was that contact of tumor cells with the stroma triggered induction of p63 that in turn up-regulated MT1-MMP. Along this line, we found that MCF10.DCIS.com cells plated on type I collagen up-regulated p63 and MT1-MMP, both at the mRNA and protein levels, while silencing of p63 abolished collagen-dependent MT1-MMP increase. At the functional level, silencing of MT1-MMP or p63 inhibited invasion of multicellular spheroids into 3D type collagen as well as the cells capacity to degrade collagen I fibers.

Finally, MT1-MMP expression was analysed by IHC staining on human tissue microarray comprising 432 IDCs, 40 microinfiltrated lesions and 68 DCIS. MT1-MMP was upregulated in IDC as compared to DCIS. Invasive triple-negative breast (TNBC) and grade III tumours expressed highest MT1-MMP levels. To our knowledge, this is the first demonstration that up-regulation of MT1-MMP is associated with invasiveness, histopathologic grade and molecular subtypes in human breast cancer. We also observed 10% of TNBC expressing p63 and found a positive correlation with MT1-MMP expression suggesting an interplay between p63 and MT1-MMP during progression.

Citation Format: Catalina Lodillinsky, Elvira Infante, Laetitia Fuhrmann, Alan Guichard, Joanna Cyrta, Emilie Lagoutte, Marie Irondelle, Sophie Vacher, Ivan Bieche, Marina Glukhova, Anne Vincent-Salomon, Philippe Chavrier. Membrane-anchored MT1-MMP downstream of p63 is essential for the transition of in situ to invasive breast carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-30. doi:10.1158/1538-7445.AM2014-LB-30

Membrane trafficking. Nucleoside diphosphate kinases fuel dynamin superfamily proteins with GTP for membrane remodeling

Dynamin superfamily molecular motors use guanosine triphosphate (GTP) as a source of energy for membrane-remodeling events. We found that knockdown of nucleoside diphosphate kinases (NDPKs) NM23-H1/H2, which produce GTP through adenosine triphosphate (ATP)-driven conversion of guanosine diphosphate (GDP), inhibited dynamin-mediated endocytosis. NM23-H1/H2 localized at clathrin-coated pits and interacted with the proline-rich domain of dynamin. In vitro, NM23-H1/H2 were recruited to dynamin-induced tubules, stimulated GTP-loading on dynamin, and triggered fission in the presence of ATP and GDP. NM23-H4, a mitochondria-specific NDPK, colocalized with mitochondrial dynamin-like OPA1 involved in mitochondria inner membrane fusion and increased GTP-loading on OPA1. Like OPA1 loss of function, silencing of NM23-H4 but not NM23-H1/H2 resulted in mitochondrial fragmentation, reflecting fusion defects. Thus, NDPKs interact with and provide GTP to dynamins, allowing these motor proteins to work with high thermodynamic efficiency.

Control of MT1-MMP transport by atypical PKC during breast-cancer progression

Dissemination of carcinoma cells requires the pericellular degradation of the extracellular matrix, which is mediated by membrane type 1-matrix metalloproteinase (MT1-MMP). In this article, we report a co-up-regulation and colocalization of MT1-MMP and atypical protein kinase C iota (aPKCι) in hormone receptor-negative breast tumors in association with a higher risk of metastasis. Silencing of aPKC in invasive breast-tumor cell lines impaired the delivery of MT1-MMP from late endocytic storage compartments to the surface and inhibited matrix degradation and invasion. We provide evidence that aPKCι, in association with MT1-MMP-containing endosomes, phosphorylates cortactin, which is present in F-actin-rich puncta on MT1-MMP-positive endosomes and regulates cortactin association with the membrane scission protein dynamin-2. Thus, cell line-based observations and clinical data reveal the concerted activity of aPKC, cortactin, and dynamin-2, which control the trafficking of MT1-MMP from late endosome to the plasma membrane and play an important role in the invasive potential of breast-cancer cells.

Endosomal WASH and exocyst complexes control exocytosis of MT1-MMP at invadopodia

WASH and exocyst promote pericellular matrix degradation and tumor cell invasion by enabling localized exocytosis of MT1-MMP from late endosomes.

Remodeling of the extracellular matrix by carcinoma cells during metastatic dissemination requires formation of actin-based protrusions of the plasma membrane called invadopodia, where the trans-membrane type 1 matrix metalloproteinase (MT1-MMP) accumulates. Here, we describe an interaction between the exocyst complex and the endosomal Arp2/3 activator Wiskott-Aldrich syndrome protein and Scar homolog (WASH) on MT1-MMP–containing late endosomes in invasive breast carcinoma cells. We found that WASH and exocyst are required for matrix degradation by an exocytic mechanism that involves tubular connections between MT1-MMP–positive late endosomes and the plasma membrane in contact with the matrix. This ensures focal delivery of MT1-MMP and supports pericellular matrix degradation and tumor cell invasion into different pathologically relevant matrix environments. Our data suggest a general mechanism used by tumor cells to breach the basement membrane and for invasive migration through fibrous collagen-enriched tissues surrounding the tumor.

Exo70 subunit of the exocyst complex is involved in adhesion-dependent trafficking of caveolin-1

Caveolae are specialized domains of the plasma membrane, which play key roles in signaling, endocytosis and mechanosensing. Using total internal reflection fluorescent microscopy (TIRF-M), we observe that the exocyst subunit Exo70 forms punctuate structures at the plasma membrane and partially localizes with caveolin-1, the main component of caveolae. Upon cell detachment, we found that Exo70 accumulates with caveolin-1-positive vesicular structures. Upon cell re-adhesion, caveolin-1 traffics back to the plasma membrane in a multistep process involving microtubules and actin cytoskeleton. In addition, silencing of Exo70 redirects caveolin-1 to focal adhesions identified by markers such as α5 integrin or vinculin. Based on these findings, we conclude that Exo70 is involved in caveolin-1 recycling to the plasma membrane during re-adhesion of the cells to the substratum.

Abstract P5-01-04: Clinico-pathological features of low-grade triple negative early breast cancers

Aims: To characterize clinico-pathological features and clinical outcome of low grade triple negative early breast carcinomas.

Material and methods: Between January 2005 and December 2006, 300 tumours were classified as triple negative (ER-ve, PR-ve, HER2 0/1+) out of 3000 patients treated for a breast cancer at the Institut curie. Patients with a low-grade (grade 1 and 2 according to Ellis and Elston) T1 and small T2 (< 3cm) breast carcinoma treated with surgery first were considered in our study. Immunohistochemistry was performed with ER, PR, Androgen Receptor (AR), HER2, Ki67, EGFR, CK5/6, CK14 and CK8/18 antibodies for all tumours; GCDFP15 for invasive lobular carcinomas (ILC) and invasive ductal carcinoma (IDC) with apocrine features, and CA15-3 to assess the inverted polarity if needed.

Results: We identified 36 low-grade carcinomas out of 186 triple negative breast carcinomas (19%). Thirty-two tumours were grade 2 and four tumours grade 1 and associated with a low or a moderate mitotic score. Ki67 proliferation index was high (≥20%) in 22 cases (61%) (median: 22%-mean: 29%). Thirty-one (86%) tumours showed a basal- like phenotype (EGFR+ or CK5/6+ or CK14 +). Interestingly, the five “nul” (non basal-like) triple negative tumours demonstrated a low proliferation index (Ki67 ≤20%). All but one tumors expressed CK8/18. The tumour's size varied between 3 and 37 mm (mean = 17.2). Lympho- vascular invasion was present in 10 cases (27%). High and intermediate grade ductal carcinoma in situ component was associated in 28 cases (77%). Stroma was abundant and associated with a lymphocytic infiltrate in all cases. Various histological types were observed: 16 IDC (44%), 7 ILC (19%), 5 IDC with apocrine differentiation, 2 micropapillary, 1 papillary, 1 mucinous, 3 adenoid cystic and one low grade adeno-squamous carcinomas. Three out of 16 IDC showed a week nuclear positivity with AR. Five and six out of the seven ILC were positive for AR and GCDFP15 respectively. All carcinomas with apocrine differentiation were AR and CGDFP15 positive. Conversely, all micropapillary, papillary and mucinous carcinomas were AR negative. The mean age of patients was 61.6 years. Distant metastases occurred in five patients. Thirty-three (91%) women were alive and 29 (80%) of them without evidence of cancer at 6 years of follow-up.

Conclusion: Low grade invasive carcinomas represent 19% of the cases in our series of 186 triple negative early breast carcinomas and were diagnosed in patients older than 60 years. These cases were of various histological types, were all associated with a marked lymphocytic infiltrate and in the majority of the cases with high grade ductal carcinomas in situ. Despite a low mitotic activity, proliferation index was generally higher than 20% except for “nul” non-basal cases. However, patient's overall survival seemed to be better than that reported for high grade triple negative breast cancer. Histological types, proliferation and age should be taken into account for treatment decision in triple negative early breast carcinoma patients.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-01-04.

ATAT1/MEC-17 acetyltransferase and HDAC6 deacetylase control a balance of acetylation of alpha-tubulin and cortactin and regulate MT1-MMP trafficking and breast tumor cell invasion

Invasive tumor cells use proteases to degrade and migrate through the stromal environment consisting of a 3D network of extracellular matrix macromolecules. In particular, MT1-MMP, a membrane-anchored metalloproteinase, is critical during cancer cell invasion. MT1-MMP is stored in endosomal compartments and then delivered to invadopodia, the specialized plasma membrane domains of invasive cancer cells endowed with extracellular matrix-degradation capacity. In macrophages, traffic of MT1-MMP vesicles to invadopodia-related podosomes requires microtubules. We previously found that in breast tumor MDA-MB-231 cells an increase of microtubule and cortactin acetylation upon inhibition of HDAC6 correlates with a decrease of matrix degradation and invasion in three-dimensional collagen I gel. Here, we investigated the role of the recently identified α-tubulin N-acetyltransferase 1 ATAT1 in invasive MDA-MB-231 cells. We found that the dynamics and distribution of MT1-MMP-positive endosomes require regulation of acetylation levels. We observed that ATAT1 tubulin acetyltransferase binds and regulates cortactin acetylation levels. In addition, ATAT1 colocalizes with cortactin at the adherent surface of the cells and it is required for 2D migration and invasive migration of MDA-MB-231 cells in collagen matrix. All together, our data indicate that a balance of acetylation and deaceylation by ATAT1/HDAC6 enzymes with opposite activities regulates the migratory and invasive capacities of breast tumor cells.

Spontaneous contractility-mediated cortical flow generates cell migration in three-dimensional environments

We present a model of cell motility generated by actomyosin contraction of the cell cortex. We identify, analytically, dynamical instabilities of the cortex and show that they yield steady-state cortical flows, which, in turn, can induce cell migration in three-dimensional environments. This mechanism relies on the regulation of contractility by myosin, whose transport is explicitly taken into account in the model. Theoretical predictions are compared to experimental data of tumor cells migrating in three-dimensional matrigel and suggest that this mechanism could be a general mode of cell migration in three-dimensional environments.

Decoupling of activation and effector binding underlies ARF6 priming of fast endocytic recycling

The small GTP-binding protein ADP-ribosylation factor 6 (ARF6) controls the endocytic recycling pathway of several plasma membrane receptors. We analyzed the localization and GDP/GTP cycle of GFP-tagged ARF6 by total internal reflection fluorescent microscopy. We found that ARF6-GFP associates with clathrin-coated pits (CCPs) at the plasma membrane in a GTP-dependent manner in a mechanism requiring the adaptor protein complex AP-2. In CCP, GTP-ARF6 mediates the recruitment of the ARF-binding domain of downstream effectors including JNK-interacting proteins 3 and 4 (JIP3 and JIP4) after the burst recruitment of the clathrin uncoating component auxilin. ARF6 does not contribute to receptor-mediated clathrin-dependent endocytosis. In contrast, we found that interaction of ARF6 and JIPs on endocytic vesicles is required for trafficking of the transferrin receptor in the fast, microtubule-dependent endocytic recycling pathway. Our findings unravel a novel mechanism of separation of ARF6 activation and effector function, ensuring that fast recycling may be determined at the level of receptor incorporation into CCPs.

Toward a structural understanding of arf family:effector specificity

Arf family proteins are critical regulators of intracellular trafficking and actin cytoskeleton dynamics. To carry out their cellular functions, Arf family proteins interact with various effectors that differ in nature and structure. Understanding how these proteins interact with structurally different partners and are distinguished by specific effectors while being closely related requires a structural characterization and comparison of the various Arf family:effector complexes. Recent structural reports of Arf and Arl proteins in complex with different downstream effectors shed new light on general and specific structural recognition determinants characteristic of Arf family proteins.

Cdc42 localization and cell polarity depend on membrane traffic

Cell polarity is essential for cell division, cell differentiation, and most differentiated cell functions including cell migration. The small G protein Cdc42 controls cell polarity in a wide variety of cellular contexts. Although restricted localization of active Cdc42 seems to be important for its distinct functions, mechanisms responsible for the concentration of active Cdc42 at precise cortical sites are not fully understood. In this study, we show that during directed cell migration, Cdc42 accumulation at the cell leading edge relies on membrane traffic. Cdc42 and its exchange factor βPIX localize to intracytosplasmic vesicles. Inhibition of Arf6-dependent membrane trafficking alters the dynamics of Cdc42-positive vesicles and abolishes the polarized recruitment of Cdc42 and βPIX to the leading edge. Furthermore, we show that Arf6-dependent membrane dynamics is also required for polarized recruitment of Rac and the Par6-aPKC polarity complex and for cell polarization. Our results demonstrate influence of membrane dynamics on the localization and activation of Cdc42 and consequently on directed cell migration.

Implication of metastasis suppressor NM23-H1 in maintaining adherens junctions and limiting the invasive potential of human cancer cells

Loss of NM23-H1 expression correlates with the degree of metastasis and with unfavorable clinical prognosis in several types of human carcinoma. However, the mechanistic basis for the metastasis suppressor function of NM23-H1 is obscure. We silenced NM23-H1 expression in human hepatoma and colon carcinoma cells and methodologically investigated effects on cell-cell adhesion, migration, invasion, and signaling linked to cancer progression. NM23-H1 silencing disrupted cell-cell adhesion mediated by E-cadherin, resulting in β-catenin nuclear translocation and T-cell factor/lymphoid-enhancing factor-1 transactivation. Further, NM23-H1 silencing promoted cellular scattering, motility, and extracellular matrix invasion by promoting invadopodia formation and upregulating several matrix metalloproteinases (MMP), including membrane type 1 MMP. In contrast, silencing the related NM23-H2 gene was ineffective at promoting invasion. NM23-H1 silencing activated proinvasive signaling pathways involving Rac1, mitogen-activated protein kinases, phosphatidylinositol 3-kinase (PI3K)/Akt, and src kinase. Conversely, NM23-H1 was dispensable for cancer cell proliferation in vitro and liver regeneration in NM23-M1 null mice, instead inducing cellular resistance to chemotherapeutic drugs in vitro. Analysis of NM23-H1 expression in clinical specimens revealed high expression in premalignant lesions (liver cirrhosis and colon adenoma) and the central body of primary liver or colon tumors, but downregulation at the invasive front of tumors. Our findings reveal that NM23-H1 is critical for control of cell-cell adhesion and cell migration at early stages of the invasive program in epithelial cancers, orchestrating a barrier against conversion of in situ carcinoma into invasive malignancy.

Matrix invasion by tumour cells: a focus on MT1-MMP trafficking to invadopodia

When migrating away from a primary tumour, cancer cells interact with and remodel the extracellular matrix (ECM). Matrix metalloproteinases (MMPs), and in particular the transmembrane MT1-MMP (also known as MMP-14), are key enzymes in tumour-cell invasion. Results from recent in vitro studies highlight that MT1-MMP is implicated both in the breaching of basement membranes by tumour cells and in cell invasion through interstitial type-I collagen tissues. Remarkably, MT1-MMP accumulates at invadopodia, which are specialized ECM-degrading membrane protrusions of invasive cells. Here we review current knowledge about MT1-MMP trafficking and its importance for the regulation of protease activity at invadopodia. In invasive cells, endocytosis of MT1-MMP by clathrin- and caveolae-dependent pathways can be counteracted by several mechanisms, which leads to protease stabilization at the cell surface and increased pericellular degradation of the matrix. Furthermore, the recent identification of cellular components that control delivery of MT1-MMP to invadopodia brings new insight into mechanisms of cancer-cell invasion and reveals potential pharmacological targets.

Consortin, a trans-Golgi network cargo receptor for the plasma membrane targeting and recycling of connexins

Targeting of numerous transmembrane proteins to the cell surface is thought to depend on their recognition by cargo receptors that interact with the adaptor machinery for anterograde traffic at the distal end of the Golgi complex. We report here on consortin, a novel integral membrane protein that is predicted to be intrinsically disordered, i.e. that contains large segments whose native state is unstructured. We identified consortin as a binding partner of connexins, the building blocks of gap junctions. Consortin is located at the trans-Golgi network (TGN), in tubulovesicular transport organelles, and at the plasma membrane. It directly interacts with the TGN clathrin adaptors GGA1 and GGA2, and disruption of this interaction by expression of a consortin mutant lacking the acidic cluster-dileucine (DXXLL) GGA interaction motif causes an intracellular accumulation of several connexins. RNA interference-mediated silencing of consortin expression in HeLa cells blocks the cell surface targeting of these connexins, which accumulate intracellularly, whereas partial depletion and redistribution of the consortin pool slows down the intracellular degradation of gap junction plaques. Altogether, our results show that, by studying connexin trafficking, we have identified the first TGN cargo receptor for the targeting of transmembrane proteins to the plasma membrane. The identification of consortin provides in addition a potential target for therapies aimed at diseases in which connexin traffic is altered, including cardiac ischemia, peripheral neuropathies, cataracts and hearing impairment. Sequence accession numbers. GenBank: Human CNST cDNA, NM_152609; mouse Cnst cDNA, NM_146105.

The structural basis of Arf effector specificity: the crystal structure of ARF6 in a complex with JIP4

The JNK-interacting proteins, JIP3 and JIP4, are specific effectors of the small GTP-binding protein ARF6. The interaction of ARF6-GTP with the second leucine zipper (LZII) domains of JIP3/JIP4 regulates the binding of JIPs to kinesin-1 and dynactin. Here, we report the crystal structure of ARF6-GTP bound to the JIP4-LZII at 1.9 A resolution. The complex is a heterotetramer with dyad symmetry arranged in an ARF6-(JIP4)(2)-ARF6 configuration. Comparison of the ARF6-JIP4 interface with the equivalent region of ARF1 shows the structural basis of JIP4's specificity for ARF6. Using site-directed mutagenesis and surface plasmon resonance, we further show that non-conserved residues at the switch region borders are the key structural determinants of JIP4 specificity. A structure-derived model of the association of the ARF6-JIP3/JIP4 complex with membranes shows that the JIP4-LZII coiled-coil should lie along the membrane to prevent steric hindrances, resulting in only one ARF6 molecule bound. Such a heterotrimeric complex gives insights to better understand the ARF6-mediated motor switch regulatory function.

Diaphanous-related formins are required for invadopodia formation and invasion of breast tumor cells

Proteolytic degradation of the extracellular matrix by metastatic tumor cells is initiated by the formation of invadopodia, i.e., actin-driven filopodia-like membrane protrusions endowed with matrix-degradative activity. A signaling cascade involving neural Wiskott-Aldrich syndrome protein and the Arp2/3 actin nucleating complex is involved in actin assembly at invadopodia. Yet, the mechanism of invadopodia formation is poorly understood. Based on their role as actin nucleators in cytoskeletal rearrangements, including filopodia formation, we examined the function of Diaphanous-related formins (DRF) in invadopodia formation and invasion by breast tumor cells. Using small interfering RNA silencing of protein expression in highly invasive MDA-MB-231 breast adenocarcinoma cells, we show that three members of the DRF family (DRF1-DRF3) are required for invadopodia formation and two-dimensional matrix proteolysis. We also report that invasion of a three-dimensional Matrigel matrix involves filopodia-like protrusions enriched for invadopodial proteins, including membrane type 1 matrix metalloproteinase, which depend on DRFs for their formation. These data identify DRFs as critical components of the invasive apparatus of tumor cells in two-dimensional and three-dimensional matrices and suggest that different types of actin nucleators cooperate during the formation of invadopodia.

The interaction of IQGAP1 with the exocyst complex is required for tumor cell invasion downstream of Cdc42 and RhoA

Invadopodia are actin-based membrane protrusions formed at contact sites between invasive tumor cells and the extracellular matrix with matrix proteolytic activity. Actin regulatory proteins participate in invadopodia formation, whereas matrix degradation requires metalloproteinases (MMPs) targeted to invadopodia. In this study, we show that the vesicle-tethering exocyst complex is required for matrix proteolysis and invasion of breast carcinoma cells. We demonstrate that the exocyst subunits Sec3 and Sec8 interact with the polarity protein IQGAP1 and that this interaction is triggered by active Cdc42 and RhoA, which are essential for matrix degradation. Interaction between IQGAP1 and the exocyst is necessary for invadopodia activity because enhancement of matrix degradation induced by the expression of IQGAP1 is lost upon deletion of the exocyst-binding site. We further show that the exocyst and IQGAP1 are required for the accumulation of cell surface membrane type 1 MMP at invadopodia. Based on these results, we propose that invadopodia function in tumor cells relies on the coordination of cytoskeletal assembly and exocytosis downstream of Rho guanosine triphosphatases.

Endocytic traffic in animal cell cytokinesis

Cytokinesis is the final step of mitosis whereby two daughter cells physically separate. It is initiated by the assembly of an actomyosin contractile ring at the mitotic cell equator, which constricts the cytoplasm between the two reforming nuclei resulting in the formation of a narrow intercellular bridge filled with central spindle microtubule bundles. Cytokinesis terminates with the cleavage of the intercellular bridge in a poorly understood process called abscission. Recent work has highlighted the importance of membrane trafficking events occurring from membrane compartments flanking the bridge to the central midbody region. In particular, polarized delivery of endocytic recycling membranes is essential for completion of animal cell cytokinesis. Why endocytic traffic occurs within the intercellular bridge remains largely mysterious and its significance for cytokinesis will be discussed.