Extended-synaptotagmin-2 mediates FGF receptor endocytosis and ERK activation in vivo

The splicing factor QKI inhibits metastasis by modulating alternative splicing of E-Syt2 in papillary thyroid carcinoma

Alternative splicing (AS) plays a crucial role in the hallmarks of cancer and can open new avenues for targeted therapies. However, the aberrant AS events and the metastatic cascade in papillary thyroid carcinoma (PTC) remain largely unclear. Here, we identify the splicing factor, quaking protein (QKI), which was significantly downregulated in PTC and correlated with poor survival outcomes in patients with PTC. Functional studies indicated that low expression of QKI promoted the PTC cell growth and metastasis in vitro and in vivo. Mechanistically, low QKI induced exon 14 retention of extended synaptotagmin 2 (E-Syt2) and produced a long isoform transcript (termed E-Syt2L) that acted as an important oncogenic factor of PTC metastasis. Notably, overexpression of long non-coding RNA eosinophil granule ontogeny transcript (EGOT) physically binds to QKI and suppressed its activity by inhibiting ubiquitin specific peptidase 25 (USP25) mediated deubiquitination and subsequent degradation of QKI. Collectively, these data demonstrate the novel mechanistic links between the splicing factor QKI and splicing event in PTC metastasis and support the potential utility of targeting splicing events as a therapeutic strategy for PTC.

ETV4 is a mechanical transducer linking cell crowding dynamics to lineage specification

Dynamic changes in mechanical microenvironments, such as cell crowding, regulate lineage fates as well as cell proliferation. Although regulatory mechanisms for contact inhibition of proliferation have been extensively studied, it remains unclear how cell crowding induces lineage specification. Here we found that a well-known oncogene, ETS variant transcription factor 4 (ETV4), serves as a molecular transducer that links mechanical microenvironments and gene expression. In a growing epithelium of human embryonic stem cells, cell crowding dynamics is translated into ETV4 expression, serving as a pre-pattern for future lineage fates. A switch-like ETV4 inactivation by cell crowding derepresses the potential for neuroectoderm differentiation in human embryonic stem cell epithelia. Mechanistically, cell crowding inactivates the integrin-actomyosin pathway and blocks the endocytosis of fibroblast growth factor receptors (FGFRs). The disrupted FGFR endocytosis induces a marked decrease in ETV4 protein stability through ERK inactivation. Mathematical modelling demonstrates that the dynamics of cell density in a growing human embryonic stem cell epithelium precisely determines the spatiotemporal ETV4 expression pattern and, consequently, the timing and geometry of lineage development. Our findings suggest that cell crowding dynamics in a stem cell epithelium drives spatiotemporal lineage specification using ETV4 as a key mechanical transducer.

N-glycosylation acts as a switch for FGFR1 trafficking between the plasma membrane and nuclear envelope

Fibroblast growth factor receptor 1 (FGFR1) is a heavily N-glycosylated cell surface receptor tyrosine kinase that transmits signals across the plasma membrane, in response to fibroblast growth factors (FGFs). Balanced FGF/FGFR1 signaling is crucial for the development and homeostasis of the human body, and aberrant FGFR1 is frequently observed in various cancers. In addition to its predominant localization to the plasma membrane, FGFR1 has also been detected inside cells, mainly in the nuclear lumen, where it modulates gene expression. However, the exact mechanism of FGFR1 nuclear transport is still unknown. In this study, we generated a glycosylation-free mutant of FGFR1, FGFR1.GF, and demonstrated that it is localized primarily to the nuclear envelope. We show that reintroducing N-glycans into the D3 domain cannot redirect FGFR1 to the plasma membrane or exclude the receptor from the nuclear envelope. Reestablishment of D2 domain N-glycans largely inhibits FGFR1 accumulation in the nuclear envelope, but the receptor continues to accumulate inside the cell, mainly in the ER. Only the simultaneous presence of N-glycans of the D2 and D3 domains of FGFR1 promotes efficient transport of FGFR1 to the plasma membrane. We demonstrate that while disturbed FGFR1 folding results in partial FGFR1 accumulation in the ER, impaired FGFR1 secretion drives FGFR1 trafficking to the nuclear envelope. Intracellular FGFR1.GF displays a high level of autoactivation, suggesting the presence of nuclear FGFR1 signaling, which is independent of FGF. Using mass spectrometry and proximity ligation assay, we identified novel binding partners of the nuclear envelope-localized FGFR1, providing insights into its cellular functions. Collectively, our data define N-glycosylation of FGFR1 as an important regulator of FGFR1 kinase activity and, most importantly, as a switchable signal for FGFR1 trafficking between the nuclear envelope and plasma membrane, which, due to spatial restrictions, shapes FGFR1 interactome and cellular function. Video Abstract.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Characterization of hemocytes and hematopoietic cells of a freshwater crayfish based on single-cell transcriptome analysis

Crustaceans constitute a species-rich and ecologically important animal group, and their circulating blood cells (hemocytes) are of critical importance in immunity as key players in pathogen recognition, phagocytosis, melanization, and antimicrobial defense. To gain a better understanding of the immune responses to different pathogens, it is crucial that we identify different hemocyte subpopulations with different functions and gain a better understanding of how these cells are formed. Here, we performed single-cell RNA sequencing of isolated hematopoietic tissue (HPT) cells and hemocytes from the crayfish Pacifastacus leniusculus to identify hitherto undescribed hemocyte types in the circulation and show that the circulating cells are more diversified than previously recognized. In addition, we discovered cell populations in the HPT with clear precursor characteristics as well as cells involved in iron homeostasis, representing a previously undiscovered cell type. These findings may improve our understanding of hematopoietic stem cell regulation in crustaceans and other animals.

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Single-cell RNA sequencing of hematopoietic cell types reveals new cell types

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One cell type contains iron homeostasis-associated transcripts

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Hemocytes and hematopoietic cells differ in their transcript profiles

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Prophenoloxidase is only expressed in hemocytes

Negative Regulation of FGFR (Fibroblast Growth Factor Receptor) Signaling

FGFR (fibroblast growth factor receptor) signaling controls fundamental processes in embryonic, fetal and adult human life. The magnitude, duration, and location of FGFR signaling must be strictly controlled in order to induce the correct biological response. Uncontrolled receptor signaling has been shown to lead to a variety of diseases, such as skeletal disorders and cancer. Here we review the numerous cellular mechanisms that regulate and turn off FGFR signaling, once the receptor is activated. These mechanisms include endocytosis and endocytic sorting, phosphatase activity, negative regulatory proteins and negative feedback phosphorylation events. The mechanisms act together simultaneously or sequentially, controlling the same or different steps in FGFR signaling. Although more work is needed to fully understand the regulation of FGFR signaling, it is clear that the cells in our body have evolved an extensive repertoire of mechanisms that together keep FGFR signaling tightly controlled and prevent excess FGFR signaling.

Characterization of clostridium botulinum neurotoxin serotype A (BoNT/A) and fibroblast growth factor receptor interactions using novel receptor dimerization assay

Clostridium botulinum neurotoxin serotype A (BoNT/A) is a potent neurotoxin that serves as an effective therapeutic for several neuromuscular disorders via induction of temporary muscular paralysis. Specific binding and internalization of BoNT/A into neuronal cells is mediated by its binding domain (H_C/A), which binds to gangliosides, including GT1b, and protein cell surface receptors, including SV2. Previously, recombinant H_C/A was also shown to bind to FGFR3. As FGFR dimerization is an indirect measure of ligand-receptor binding, an FCS & TIRF receptor dimerization assay was developed to measure rH_C/A-induced dimerization of fluorescently tagged FGFR subtypes (FGFR1-3) in cells. rH_C/A dimerized FGFR subtypes in the rank order FGFR3c (EC₅₀ ≈ 27 nM) > FGFR2b (EC₅₀ ≈ 70 nM) > FGFR1c (EC₅₀ ≈ 163 nM); rH_C/A dimerized FGFR3c with similar potency as the native FGFR3c ligand, FGF9 (EC₅₀ ≈ 18 nM). Mutating the ganglioside binding site in H_C/A, or removal of GT1b from the media, resulted in decreased dimerization. Interestingly, reduced dimerization was also observed with an SV2 mutant variant of H_C/A. Overall, the results suggest that the FCS & TIRF receptor dimerization assay can assess FGFR dimerization with known and novel ligands and support a model wherein H_C/A, either directly or indirectly, interacts with FGFRs and induces receptor dimerization.

Role of the Sec22b-E-Syt complex in neurite growth and ramification

Axons and dendrites are long and often ramified neurites that need particularly intense plasma membrane (PM) expansion during the development of the nervous system. Neurite growth depends on non-fusogenic Sec22b-Stx1 SNARE complexes at endoplasmic reticulum (ER)-PM contacts. Here, we show that Sec22b interacts with members of the extended synaptotagmin (E-Syt) family of ER lipid transfer proteins (LTPs), and this interaction depends on the longin domain of Sec22b. Overexpression of E-Syts stabilizes Sec22b-Stx1 association, whereas silencing of E-Syts has the opposite effect. Overexpression of wild-type E-Syt2, but not mutants unable to transfer lipids or attach to the ER, increase the formation of axonal filopodia and ramification of neurites in developing neurons. This effect is inhibited by a clostridial neurotoxin cleaving Stx1, and expression of the Sec22b longin domain and a Sec22b mutant with an extended linker between the SNARE and transmembrane domains. We conclude that Sec22b-Stx1 ER-PM contact sites contribute to PM expansion by interacting with LTPs, such as E-Syts.This article has an associated First Person interview with the first author of the paper.

Hypothalamic extended synaptotagmin-3 contributes to the development of dietary obesity and related metabolic disorders

The C2 domain containing protein extended synaptotagmin (E-Syt) plays important roles in both lipid homeostasis and the intracellular signaling; however, its role in physiology remains largely unknown. Here, we show that hypothalamic E-Syt3 plays a critical role in diet-induced obesity (DIO). E-Syt3 is characteristically expressed in the hypothalamic nuclei. Whole-body or proopiomelanocortin (POMC) neuron-specific ablation of E-Syt3 ameliorated DIO and related comorbidities, including glucose intolerance and dyslipidemia. Conversely, overexpression of E-Syt3 in the arcuate nucleus moderately promoted food intake and impaired energy expenditure, leading to increased weight gain. Mechanistically, E-Syt3 ablation led to increased processing of POMC to α-melanocyte-stimulating hormone (α-MSH), increased activities of protein kinase C and activator protein-1, and enhanced expression of prohormone convertases. These findings reveal a previously unappreciated role for hypothalamic E-Syt3 in DIO and related metabolic disorders.

FGFR1 clustering with engineered tetravalent antibody improves the efficiency and modifies the mechanism of receptor internalization

Fibroblast growth factor receptor 1 (FGFR1) transmits signals through the plasma membrane regulating essential cellular processes like division, motility, metabolism, and death. Overexpression of FGFR1 is observed in numerous tumors and thus constitutes an attractive molecular target for selective cancer treatment. Targeted anti‐cancer therapies aim for the precise delivery of drugs into cancer cells, sparing the healthy ones and thus limiting unwanted side effects. One of the key steps in targeted drug delivery is receptor‐mediated endocytosis. Here, we show that the efficiency and the mechanism of FGFR1 internalization are governed by the spatial distribution of the receptor in the plasma membrane. Using engineered antibodies of different valency, we demonstrate that dimerization of FGFR1 with bivalent antibody triggers clathrin‐mediated endocytosis (CME) of the receptor. Clustering of FGFR1 into larger oligomers with tetravalent antibody stimulates fast and highly efficient uptake of the receptor that occurs via two distinct mechanisms: CME and dynamin‐dependent clathrin‐independent endocytic routes. Furthermore, we show that all endocytic pathways engaged in FGFR1 internalization do not require receptor activation. Our data provide novel insights into the mechanisms of intracellular trafficking of FGFR1 and constitute guidelines for development of highly internalizing antibody‐based drug carriers for targeted therapy of FGFR1‐overproducing cancers.

Nuclear receptor tyrosine kinase transport and functions in cancer

Signaling functions of plasma membrane-localized receptor tyrosine kinases (RTKs) have been extensively studied after they were first described in the mid-1980s. Plasma membrane RTKs are activated by extracellular ligands and cellular stress stimuli, and regulate cellular responses by activating the downstream effector proteins to initiate a wide range of signaling cascades in the cells. However, increasing evidence indicates that RTKs can also be transported into the intracellular compartments where they phosphorylate traditional effector proteins and non-canonical substrate proteins. In general, internalization that retains the RTK's transmembrane domain begins with endocytosis, and endosomal RTK remains active before being recycled or degraded. Further RTK retrograde transport from endosome-Golgi-ER to the nucleus is primarily dependent on membranes vesicles and relies on the interaction with the COP-I vesicle complex, Sec61 translocon complex, and importin. Internalized RTKs have non-canonical substrates that include transcriptional co-factors and DNA damage response proteins, and many nuclear RTKs harbor oncogenic properties and can enhance cancer progression. Indeed, nuclear-localized RTKs have been shown to positively correlate with cancer recurrence, therapeutic resistance, and poor prognosis of cancer patients. Therefore, understanding the functions of nuclear RTKs and the mechanisms of nuclear RTK transport will further improve our knowledge to evaluate the potential of targeting nuclear RTKs or the proteins involved in their transport as new cancer therapeutic strategies.

Ca2+ Signaling in Exocrine Cells

Calcium (Ca²⁺) and cyclic AMP (cAMP) signaling cross talk and synergize to stimulate the cardinal functions of exocrine cells, regulated exocytosis, and fluid and electrolyte secretion. This physiological process requires the organization of the two signaling pathways into complexes at defined cellular domains and close placement. Such domains are formed by membrane contact sites (MCS). This review discusses the basic properties of Ca²⁺ signaling in exocrine cells, the role of MCS in the organization of cell signaling and in cross talk and synergism between the Ca²⁺ and cAMP signaling pathways and, finally, the mechanism by which the Ca²⁺ and cAMP pathways synergize to stimulate epithelial fluid and electrolyte secretion.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Differential regulation of fibroblast growth factor receptor 1 trafficking and function by extracellular galectins

Fibroblast growth factor receptors (FGFRs) are integral membrane proteins that transmit signals through the plasma membrane. FGFRs signaling needs to be precisely adjusted as aberrant FGFRs function is associated with development of human cancers or severe metabolic diseases. The subcellular localization, trafficking and function of FGFRs rely on the formation of multiprotein complexes. In this study we revealed galectins, lectin family members implicated in cancer development and progression, as novel FGFR1 binding proteins. We demonstrated that galectin-1 and galectin-3 directly bind to the sugar chains of the glycosylated extracellular part of FGFR1. Although both galectins compete for the same binding sites on FGFR1, these proteins elicit different impact on FGFR1 function and cellular trafficking. Galectin-1 mimics fibroblast growth factor as it efficiently activates FGFR1 and receptor-downstream signaling pathways that result in cell proliferation and apoptotic evasion. In contrast, galectin-3 induces extensive clustering of FGFR1 on the cell surface that inhibits constitutive internalization of FGFR1. Our data point on the interplay between extracellular galectins and FGFRs in the regulation of cell fate.

FGF21 trafficking in intact human cells revealed by cryo-electron tomography with gold nanoparticles

The fibroblast growth factor FGF21 was labeled with molecularly defined gold nanoparticles (AuNPs), applied to human adipocytes, and imaged by cryo-electron tomography (cryo-ET). Most AuNPs were in pairs about 80 Å apart, on the outer cell surface. Pairs of AuNPs were also abundant inside the cells in clathrin-coated vesicles and endosomes. AuNPs were present but no longer paired in multivesicular bodies. FGF21 could thus be tracked along the endocytotic pathway. The methods developed here to visualize signaling coupled to endocytosis can be applied to a wide variety of cargo and may be extended to studies of other intracellular transactions.

Ionic stress enhances ER-PM connectivity via phosphoinositide-associated SYT1 contact site expansion in Arabidopsis

The interorganelle communication mediated by membrane contact sites (MCSs) is an evolutionary hallmark of eukaryotic cells. MCS connections enable the nonvesicular exchange of information between organelles and allow them to coordinate responses to changing cellular environments. In plants, the importance of MCS components in the responses to environmental stress has been widely established, but the molecular mechanisms regulating interorganelle connectivity during stress still remain opaque. In this report, we use the model plant Arabidopsis thaliana to show that ionic stress increases endoplasmic reticulum (ER)-plasma membrane (PM) connectivity by promoting the cortical expansion of synaptotagmin 1 (SYT1)-enriched ER-PM contact sites (S-EPCSs). We define differential roles for the cortical cytoskeleton in the regulation of S-EPCS dynamics and ER-PM connectivity, and we identify the accumulation of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] at the PM as a molecular signal associated with the ER-PM connectivity changes. Our study highlights the functional conservation of EPCS components and PM phosphoinositides as modulators of ER-PM connectivity in eukaryotes, and uncovers unique aspects of the spatiotemporal regulation of ER-PM connectivity in plants.

Targeting Cellular Trafficking of Fibroblast Growth Factor Receptors as a Strategy for Selective Cancer Treatment

Fibroblast growth factor receptors (FGFRs) in response to fibroblast growth factors (FGFs) transmit signals across the cell membrane, regulating important cellular processes, like differentiation, division, motility, and death. The aberrant activity of FGFRs is often observed in various diseases, especially in cancer. The uncontrolled FGFRs' function may result from their overproduction, activating mutations, or generation of FGFRs' fusion proteins. Besides their typical subcellular localization on the cell surface, FGFRs are often found inside the cells, in the nucleus and mitochondria. The intracellular pool of FGFRs utilizes different mechanisms to facilitate cancer cell survival and expansion. In this review, we summarize the current stage of knowledge about the role of FGFRs in oncogenic processes. We focused on the mechanisms of FGFRs' cellular trafficking-internalization, nuclear translocation, and mitochondrial targeting, as well as their role in carcinogenesis. The subcellular sorting of FGFRs constitutes an attractive target for anti-cancer therapies. The blocking of FGFRs' nuclear and mitochondrial translocation can lead to the inhibition of cancer invasion. Moreover, the endocytosis of FGFRs can serve as a tool for the efficient and highly selective delivery of drugs into cancer cells overproducing these receptors. Here, we provide up to date examples how the cellular sorting of FGFRs can be hijacked for selective cancer treatment.

Fibroblast Growth Factor Receptor, a Novel Receptor for Vegetative Insecticidal Protein Vip3Aa

Vegetative insecticidal proteins (Vips), which are secreted by some Bacillus thuringiensis strains during vegetative growth, exhibit high virulence to many pests. Vip3A proteins have been used commercially both in some bio-insecticides and in transgenic crops; however, compared with insecticidal crystal proteins, the mechanism of action of Vip3A is still unclear. In this work, we indicated that the fibroblast growth factor receptor-like protein (Sf-FGFR) from the membrane of Sf9 cells could bind to Vip3Aa. The interaction between Vip3Aa and Sf-FGFR was confirmed by pull-down assays and dot blotting experiment in vitro. The binding affinity between Vip3Aa and extracellular regions of Sf-FGFR (GST-FGFR-N) was determined by microscale thermophoresis assay (MST). Moreover, Vip3Aa-Flag could be co-immunoprecipitated with Sf-FGFR-V5 ex vivo. Furthermore, knockdown of Sf-FGFR gene in Sf9 cells resulted in reducing the mortality of those cells to Vip3Aa. In summary, our data indicated that Sf-FGFR is a novel receptor for Vip3Aa.

FGFR2-activating mutations disrupt cell polarity to potentiate migration and invasion in endometrial cancer cell models

Fibroblast growth factor receptors (FGFRs) are a family of receptor tyrosine kinases that control a diverse range of biological processes during development and in adult tissues. We recently reported that somatic FGFR2 mutations are associated with shorter survival in endometrial cancer. However, little is known about how these FGFR2 mutations contribute to endometrial cancer metastasis. Here, we report that expression of the activating mutations FGFR2^N550K and FGFR2^Y376C in an endometrial cancer cell model induce Golgi fragmentation, and loss of polarity and directional migration. In mutant FGFR2-expressing cells, this was associated with an inability to polarise intracellular pools of FGFR2 towards the front of migrating cells. Such polarization defects were exacerbated in three-dimensional culture, where FGFR2 mutant cells were unable to form well-organised acini, instead undergoing exogenous ligand-independent invasion. Our findings uncover collective cell polarity and invasion as common targets of disease-associated FGFR2 mutations that lead to poor outcome in endometrial cancer patients.

Proteomic analysis of Tn-bearing glycoproteins from different stages of melanoma cells reveals new biomarkers

Cutaneous melanoma, the most aggressive form of skin cancer, responds poorly to conventional therapy. The appearance of Tn antigen-modified proteins in cancer is correlated with metastasis and poor prognoses. The Tn determinant has been recognized as a powerful diagnostic and therapeutic target, and as an object for the development of anti-tumor vaccine strategies. This study was designed to identify Tn-carrying proteins and reveal their influence on cutaneous melanoma progression. We used a lectin-based strategy to purify Tn antigen-enriched cellular glycoproteome, the LC-MS/MS method to identify isolated glycoproteins, and the DAVID bioinformatics tool to classify the identified proteins. We identified 146 different Tn-bearing glycoproteins, 88% of which are new. The Tn-glycoproteome was generally enriched in proteins involved in the control of ribosome biogenesis, CDR-mediated mRNA stabilization, cell-cell adhesion and extracellular vesicle formation. The differential expression patterns of Tn-modified proteins for cutaneous primary and metastatic melanoma cells supported nonmetastatic and metastatic cell phenotypes, respectively. To our knowledge, this study is the first large-scale proteomic analysis of Tn-bearing proteins in human melanoma cells. The identified Tn-modified proteins are related to the biological and molecular nature of cutaneous melanoma and may be valuable biomarkers and therapeutic targets.

Rlip depletion prevents spontaneous neoplasia in TP53 null mice

Mice that have homozygous deletion of the p53 tumor suppressor protein universally die of malignancy, generally before 6 months of age. We show that hemizygous deficiency of RALBP1 (RLIP76 or Rlip) confers a degree of protection from spontaneous malignancy that has never previously been observed. This discovery introduces a paradigm for p53 function, in which Rlip plays a central role as an effector that appears necessary for the cancer susceptibility of p53 null mice. Because p53 loss has a powerful effect on genomic instability that contributes to the initiation and promotion of cancers and to drug and radiation resistance in humans, our findings provide a method for prevention and therapy of p53-deficient cancer.

TP53 (p53) is a tumor suppressor whose functions are lost or altered in most malignancies. p53 homozygous knockout (p53^−/−) mice uniformly die of spontaneous malignancy, typically T-cell lymphoma. RALBP1 (RLIP76, Rlip) is a stress-protective, mercapturic acid pathway transporter protein that also functions as a Ral effector involved in clathrin-dependent endocytosis. In stark contrast to p53^−/− mice, Rlip^−/− mice are highly resistant to carcinogenesis. We report here that partial Rlip deficiency induced by weekly administration of an Rlip-specific phosphorothioate antisense oligonucleotide, R508, strongly inhibited spontaneous as well as benzo(a)pyrene-induced carcinogenesis in p53^−/− mice. This treatment effectively prevented large-scale methylomic and transcriptomic abnormalities suggestive of inflammation found in cancer-bearing p53^−/− mice. The remarkable efficiency with which Rlip deficiency suppresses spontaneous malignancy in p53^−/− mice has not been observed with any previously reported pharmacologic or genetic intervention. These findings are supported by cross-breeding experiments demonstrating that hemizygous Rlip deficiency also reduces the spontaneous malignancy phenotype of p53^+/− mice. Rlip is found on the cell surface, and antibodies directed against Rlip were found to inhibit growth and promote apoptosis of cell lines as effectively as Rlip siRNA. The work presented here investigates several features, including oxidative DNA damage of the Rlip–p53 association in malignant transformation, and offers a paradigm for the mechanisms of tumor suppression by p53 and the prospects of suppressing spontaneous malignancy in hereditary cancer syndromes such as Li-Fraumeni.

The ins and outs of Ca2+ in plant endomembrane trafficking

Trafficking of proteins and lipids within the plant endomembrane system is essential to support cellular functions and is subject to rigorous regulation. Despite this seemingly strict regulation, endomembrane trafficking needs to be dynamically adjusted to ever-changing internal and environmental stimuli, while maintaining cellular integrity. Although often overlooked, the versatile second messenger Ca²⁺ is intimately connected to several endomembrane-associated processes. Here, we discuss the impact of electrostatic interactions between Ca²⁺ and anionic phospholipids on endomembrane trafficking, and illustrate the direct role of Ca²⁺ sensing proteins in regulating endomembrane trafficking and membrane integrity preservation. Moreover, we discuss how Ca²⁺ can control protein sorting within the plant endomembrane system. We thus highlight Ca²⁺ signaling as a versatile mechanism by which numerous signals are integrated into plant endomembrane trafficking dynamics.

Extended Synaptotagmin Localizes to Presynaptic ER and Promotes Neurotransmission and Synaptic Growth in Drosophila

The endoplasmic reticulum (ER) is an extensive organelle in neurons with important roles at synapses including the regulation of cytosolic Ca²⁺, neurotransmission, lipid metabolism, and membrane trafficking. Despite intriguing evidence for these crucial functions, how the presynaptic ER influences synaptic physiology remains enigmatic. To gain insight into this question, we have generated and characterized mutations in the single extended synaptotagmin (Esyt) ortholog in Drosophila melanogaster Esyts are evolutionarily conserved ER proteins with Ca²⁺-sensing domains that have recently been shown to orchestrate membrane tethering and lipid exchange between the ER and plasma membrane. We first demonstrate that Esyt localizes to presynaptic ER structures at the neuromuscular junction. Next, we show that synaptic growth, structure, and homeostatic plasticity are surprisingly unperturbed at synapses lacking Esyt expression. However, neurotransmission is reduced in Esyt mutants, consistent with a presynaptic role in promoting neurotransmitter release. Finally, neuronal overexpression of Esyt enhances synaptic growth and the sustainment of the vesicle pool during intense activity, suggesting that increased Esyt levels may modulate the membrane trafficking and/or resting Ca²⁺ pathways that control synapse extension. Thus, we identify Esyt as a presynaptic ER protein that can promote neurotransmission and synaptic growth, revealing the first in vivo neuronal functions of this conserved gene family.

Antibody-induced dimerization of FGFR1 promotes receptor endocytosis independently of its kinase activity

Fibroblast growth factors (FGFs) and their plasma membrane-localized receptors (FGFRs) play a key role in the regulation of developmental processes and metabolism. Aberrant FGFR signaling is associated with the progression of serious metabolic diseases and human cancer. Binding of FGFs to FGFRs induces receptor dimerization and transphosphorylation of FGFR kinase domains that triggers activation of intracellular signaling pathways. Following activation, FGFRs undergo internalization and subsequent lysosomal degradation, which terminates transmission of signals. Although factors that regulate FGFR endocytosis are continuously discovered, little is known about the molecular mechanism that initiates the internalization of FGFRs. Here, we analyzed the internalization of antibody fragments in various formats that target FGFR1. We show that FGFR1-specific antibody fragments in the monovalent scFv format bind to FGFR1, but are not internalized into cells that overproduce FGFR1. In contrast, the same scFv proteins in the bivalent scFv-Fc format are efficiently internalized via FGFR1-mediated, clathrin and dynamin dependent endocytosis. Interestingly, the receptor tyrosine kinase activity is dispensable for endocytosis of scFv-Fc-FGFR1 complexes, suggesting that only dimerization of receptor is required to trigger endocytosis of FGFR1 complexes.

The Extended-Synaptotagmins

The extended-synaptotagmins (tricalbins in yeast) derive their name from their partial domain structure similarity to the synaptotagmins, which are characterized by an N-terminal membrane anchor and cytosolically exposed C2 domains. However, they differ from the synaptotagmins in localization and function. The synaptotagmins tether secretory vesicles, including synaptic vesicles, to the plasma membrane (PM) via their C2 domains and regulate their Ca²⁺ triggered exocytosis. In contrast, the extended-synaptotagmins are resident proteins of the endoplasmic reticulum (ER), which tether this organelle to the plasma membrane via their C2 domains, but not as a premise to fusion of the two membranes. They transport glycerolipids between the two bilayers via their lipid-harboring SMP domains and Ca²⁺ regulates their membrane tethering and lipid transport function. Additionally, the extended-synaptotagmins are more widely expressed in different organisms, as they are present not only in animal cells, but also in fungi and plants, which do not express the synaptotagmins. Thus, they have a more general function than the synaptotagmins, whose appearance in animal species correlated with the occurrence of Ca²⁺ triggered exocytosis. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Syndesome Therapeutics for Enhancing Diabetic Wound Healing

Chronic wounds represent a major healthcare and economic problem worldwide. Advanced wound dressings that incorporate bioactive compounds have great potential for improving outcomes in patients with chronic wounds but significant challenges in designing treatments that are effective in long-standing, nonhealing wounds. Here, an optimized wound healing gel was developed that delivers syndecan-4 proteoliposomes ("syndesomes") with fibroblast growth factor-2 (FGF-2) to enhance diabetic wound healing. In vitro studies demonstrate that syndesomes markedly increase migration of keratinocytes and fibroblasts isolated from both nondiabetic and diabetic donors. In addition, syndesome treatment leads to increased endocytic processing of FGF-2 that includes enhanced recycling of FGF-2 to the cell surface after uptake. The optimized syndesome formulation was incorporated into an alginate wound dressing and tested in a splinted wound model in diabetic, ob/ob mice. It was found that wounds treated with syndesomes and FGF-2 have markedly enhanced wound closure in comparison to wounds treated with only FGF-2. Moreover, syndesomes have an immunomodulatory effect on wound macrophages, leading to a shift toward the M2 macrophage phenotype and alterations in the wound cytokine profile. Together, these studies show that delivery of exogenous syndecan-4 is an effective method for enhancing wound healing in the long-term diabetic diseased state.

A large-scale analysis of alternative splicing reveals a key role of QKI in lung cancer

Increasing interest has been devoted in recent years to the understanding of alternative splicing in cancer. In this study, we performed a genome-wide analysis to identify cancer-associated splice variants in non-small cell lung cancer. We discovered and validated novel differences in the splicing of genes known to be relevant to lung cancer biology, such as NFIB, ENAH or SPAG9. Gene enrichment analyses revealed an important contribution of alternative splicing to cancer-related molecular functions, especially those involved in cytoskeletal dynamics. Interestingly, a substantial fraction of the altered genes found in our analysis were targets of the protein quaking (QKI), pointing to this factor as one of the most relevant regulators of alternative splicing in non-small cell lung cancer. We also found that ESYT2, one of the QKI targets, is involved in cytoskeletal organization. ESYT2-short variant inhibition in lung cancer cells resulted in a cortical distribution of actin whereas inhibition of the long variant caused an increase of endocytosis, suggesting that the cancer-associated splicing pattern of ESYT2 has a profound impact in the biology of cancer cells. Finally, we show that low nuclear QKI expression in non-small cell lung cancer is an independent prognostic factor for disease-free survival (HR = 2.47; 95% CI = 1.11-5.46, P = 0.026). In conclusion, we identified several splicing variants with functional relevance in lung cancer largely regulated by the splicing factor QKI, a tumor suppressor associated with prognosis in lung cancer.

Loss of all 3 Extended Synaptotagmins does not affect normal mouse development, viability or fertility

The extended synaptotagmins, E-Syt1, 2 and 3, are multiple C2 domain membrane proteins that are tethered to the endoplasmic reticulum and interact in a calcium dependent manner with plasma membrane phospholipids to form endoplasmic reticulum - plasma membrane junctions. These junctions have been implicated in the exchange of phospholipids between the 2 organelles. The E-Syts have further been implicated in receptor signaling and endocytosis and can interact directly with fibroblast growth factor and other cell surface receptors. Despite these multiple functions, the search for a requirement in vivo has been elusive. Most recently, we found that the genes for E-Syt2 and 3 could be inactivated without effect on mouse development, viability, fertility or morphology. We have now created insertion and deletion mutations in the last of the mouse E-Syt genes. We show that E-Syt1 is specifically expressed throughout the embryonic skeleton during the early stages of chrondrogenesis in a pattern quite distinct from that of E-Syt2 or 3. Despite this, E-Syt1 is also not required for mouse development and propagation. We further show that even the combined inactivation of all 3 E-Syt genes has no effect on mouse viability or fertility in the laboratory. However, this inactivation induces an enhancement in the expression of the genes encoding Orp5/8, Orai1, STIM1 and TMEM110, endoplasmic reticulum - plasma membrane junction proteins that potentially could compensate for E-Syt loss. Given the multiple functions suggested for the E-Syts and their evolutionary conservation, our unexpected findings suggest that they may only provide a survival advantage under specific conditions that have as yet to be identified.

Glypican-1 nanoliposomes for potentiating growth factor activity in therapeutic angiogenesis

Therapeutic angiogenesis is a highly appealing concept for treating tissues that become ischemic due to vascular disease. A major barrier to the clinical translation of angiogenic therapies is that the patients that are in the greatest need of these treatments often have long term disease states and co-morbidities, such as diabetes and obesity, that make them resistant to angiogenic stimuli. In this study, we identified that human patients with type 2 diabetes have reduced levels of glypican-1 in the blood vessels of their skin. The lack of this key co-receptor in the tissue may make the application of exogenous angiogenic growth factors or cell therapies ineffective. We created a novel therapeutic enhancer for growth factor activity consisting of glypican-1 delivered in a nanoliposomal carrier (a "glypisome"). Here, we demonstrate that glypisomes enhance FGF-2 mediated endothelial cell proliferation, migration and tube formation. In addition, glypisomes enhance FGF-2 trafficking by increasing both uptake and endosomal processing. We encapsulated FGF-2 or FGF-2 with glypisomes in alginate beads and used these to deliver localized growth factor therapy in a murine hind limb ischemia model. Co-delivery of glypisomes with FGF-2 markedly increased the recovery of perfusion and vessel formation in ischemic hind limbs of wild type and diabetic mice in comparison to mice treated with FGF-2 alone. Together, our findings support that glypisomes are effective means for enhancing growth factor activity and may improve the response to local angiogenic growth factor therapies for ischemia.

Extended synaptotagmins are Ca2+-dependent lipid transfer proteins at membrane contact sites

Organelles are in constant communication with each other through exchange of proteins (mediated by trafficking vesicles) and lipids [mediated by both trafficking vesicles and lipid transfer proteins (LTPs)]. It has long been known that vesicle trafficking can be tightly regulated by the second messenger Ca(2+), allowing membrane protein transport to be adjusted according to physiological demands. However, it remains unclear whether LTP-mediated lipid transport can also be regulated by Ca(2+) In this work, we show that extended synaptotagmins (E-Syts), poorly understood membrane proteins at endoplasmic reticulum-plasma membrane contact sites, are Ca(2+)-dependent LTPs. Using both recombinant and endogenous mammalian proteins, we discovered that E-Syts transfer glycerophospholipids between membrane bilayers in the presence of Ca(2+) E-Syts use their lipid-accommodating synaptotagmin-like mitochondrial lipid binding protein (SMP) domains to transfer lipids. However, the SMP domains themselves cannot transport lipids unless the two membranes are tightly tethered by Ca(2+)-bound C2 domains. Strikingly, the Ca(2+)-regulated lipid transfer activity of E-Syts was fully recapitulated when the SMP domain was fused to the cytosolic domain of synaptotagmin-1, the Ca(2+)sensor in synaptic vesicle fusion, indicating that a common mechanism of membrane tethering governs the Ca(2+)regulation of lipid transfer and vesicle fusion. Finally, we showed that microsomal vesicles isolated from mammalian cells contained robust Ca(2+)-dependent lipid transfer activities, which were mediated by E-Syts. These findings established E-Syts as a novel class of LTPs and showed that LTP-mediated lipid trafficking, like vesicular transport, can be subject to tight Ca(2+)regulation.

Extended-Synaptotagmins (E-Syts); the extended story

The Extended-Synaptotagmin (E-Syt) membrane proteins were only recently discovered, but have already been implicated in a range of interrelated cellular functions, including calcium and receptor signaling, and membrane lipid transport. However, despite their evolutionary conservation and detailed studies of their molecular actions, we still have little idea of how and when these proteins are required in cellular and organism physiology. Here we review our present understanding of the E-Syts and discuss the molecular functions and in vivo requirements for these proteins.

VAMP8-3xHA Uptake Assay in HeLa Cells

Transmembrane proteins are rarely exclusively localized to a specific vesicle or an organelle. Most transmembrane proteins undergo complicated trafficking routes. Thus, transmembrane proteins are under constant flux, and at steady state, found on a variety of vesicles or organelles. This characteristic makes the study of their trafficking routes complex, since at any given moment, different molecules are often being trafficked in opposing directions. Pulse-chase experiments can temporally track a specific pool of a transmembrane protein of interest, allowing for the kinetic description of its trafficking route. This type of technique has been used extensively to follow a large array of plasma membrane localized proteins (Diril et al., 2006; Jean et al., 2010). Here, we describe a method that allows the study of VAMP8 trafficking from the plasma membrane to endolysosomal compartments. This method was used to describe a role for MTMR13 and RAB21 in the regulation of VAMP8 trafficking to endolysosomes (Jean et al., 2015).

Orai1 and STIM1 in ER/PM junctions: roles in pancreatic cell function and dysfunction

Membrane contact sites (MCS) are critical junctions that form between the endoplasmic reticulum (ER) and membranes of various organelles, including the plasma membrane (PM). Signaling complexes, including mediators of Ca(2+) signaling, are assembled within MCS, such as the ER/PM junction. This is most evident in polarized epithelial cells, such as pancreatic cells. Core Ca(2+) signaling proteins cluster at the apical pole, the site of inositol 1,4,5-trisphosphate-mediated Ca(2+) release and Orai1/transient receptor potential canonical-mediated store-dependent Ca(2+) entry. Recent advances have characterized the proteins that tether the membranes at MCS and the role of these proteins in modulating physiological and pathological intracellular signaling. This review discusses recent advances in the characterization of Ca(2+) signaling at ER/PM junctions and the relation of these junctions to physiological and pathological Ca(2+) signaling in pancreatic acini.

The TRPCs, Orais and STIMs in ER/PM Junctions

The Ca(2+) second messenger is initiated at ER/PM junctions and propagates into the cell interior to convey the receptor information. The signal is maintained by Ca(2+) influx across the plasma membrane through the Orai and TRPC channels. These Ca(2+) influx channels form complexes at ER/PM junctions with the ER Ca(2+) sensor STIM1, which activates the channels. The function of STIM1 is modulated by other STIM isoforms like STIM1L, STIM2 and STIM2.1/STIM2β and by SARAF, which mediates the Ca(2+)-dependent inhibition of Orai channels. The ER/PM junctions are formed at membrane contact sites by tethering proteins that generate several types of ER/PM junctions, such as PI(4,5)P2-poor and PI(4,5)P2-rich domains. This chapter discusses several properties of the TRPC channels, the Orai channels and the STIMs, their key interacting proteins and how interaction of the STIMs with the channels gates their activity. The chapter closes by highlighting open questions and potential future directions in this field.

Mechanisms of FGF gradient formation during embryogenesis

Fibroblast growth factors (FGFs) have long been attributed to influence morphogenesis in embryonic development. Signaling by FGF morphogen encodes positional identity of tissues by creating a concentration gradient over the developing embryo. Various mechanisms that influence the development of such gradient have been elucidated in the recent past. These mechanisms of FGF gradient formation present either as an extracellular control over FGF ligand diffusion or as a subcellular control of FGF propagation and signaling. In this review, we describe our current understanding of FGF as a morphogen, the extracellular control of FGF gradient formation by heparan sulfate proteoglycans (HSPGs) and mechanisms of intracellular regulation of FGF signaling that influence gradient formation.

Loss of Extended Synaptotagmins ESyt2 and ESyt3 does not affect mouse development or viability, but in vitro cell migration and survival under stress are affected

The Extended Synaptotagmins (Esyts) are a family of multi-C2 domain membrane proteins with orthologs in organisms from yeast to human. Three Esyt genes exist in mouse and human and these have most recently been implicated in the formation of junctions between endoplasmic reticulum and plasma membrane, as well as the Ca²⁺ dependent replenishment of membrane phospholipids. The data are consistent with a function in extracellular signal transduction and cell adhesion, and indeed Esyt2 was previously implicated in both these functions in Xenopus. Despite this, little is known of the function of the Esyts in vivo. We have generated mouse lines carrying homozygous deletions in one or both of the genes encoding the highly homologous Esyt2 and Esyt3 proteins. Surprisingly, esyt2^−/−/esyt3^−/− mice develop normally and are both viable and fertile. In contrast, esyt2^−/−/esyt3^−/− mouse embryonic fibroblasts display a reduced ability to migrate in standard in vitro assays, and are less resistant to stringent culture conditions and to oxidative stress than equivalent wild type fibroblasts.

Extended Synaptotagmin Interaction with the Fibroblast Growth Factor Receptor Depends on Receptor Conformation, Not Catalytic Activity

We previously demonstrated that ESyt2 interacts specifically with the activated FGF receptor and is required for a rapid phase of receptor internalization and for functional signaling via the ERK pathway in early Xenopus embryos. ESyt2 is one of the three-member family of Extended Synaptotagmins that were recently shown to be implicated in the formation of endoplasmic reticulum (ER)-plasma membrane (PM) junctions and in the Ca(2+) dependent regulation of these junctions. Here we show that ESyt2 is directed to the ER by its putative transmembrane domain, that the ESyts hetero- and homodimerize, and that ESyt2 homodimerization in vivo requires a TM adjacent sequence but not the SMP domain. ESyt2 and ESyt3, but not ESyt1, selectively interact in vivo with activated FGFR1. In the case of ESyt2, this interaction requires a short TM adjacent sequence and is independent of receptor autophosphorylation, but dependent on receptor conformation. The data show that ESyt2 recognizes a site in the upper kinase lobe of FGFR1 that is revealed by displacement of the kinase domain activation loop during receptor activation.

Apical localization of inositol 1,4,5-trisphosphate receptors is independent of extended synaptotagmins in hepatocytes

Extended synaptotagmins (E-Syts) are a recently identified family of proteins that tether the endoplasmic reticulum (ER) to the plasma membrane (PM) in part by conferring regulation of cytosolic calcium (Ca²⁺) at these contact sites (Cell, 2013). However, the mechanism by which E-Syts link this tethering to Ca²⁺ signaling is unknown. Ca²⁺ waves in polarized epithelia are initiated by inositol 1,4,5-trisphosphate receptors (InsP3Rs), and these waves begin in the apical region because InsP3Rs are targeted to the ER adjacent to the apical membrane. In this study we investigated whether E-Syts are responsible for this targeting. Primary rat hepatocytes were used as a model system, because a single InsP3R isoform (InsP3R-II) is tethered to the peri-apical ER in these cells. Additionally, it has been established in hepatocytes that the apical localization of InsP3Rs is responsible for Ca²⁺ waves and secretion and is disrupted in disease states in which secretion is impaired. We found that rat hepatocytes express two of the three identified E-Syts (E-Syt1 and E-Syt2). Individual or simultaneous siRNA knockdown of these proteins did not alter InsP3R-II expression levels, apical localization or average InsP3R-II cluster size. Moreover, apical secretion of the organic anion 5-chloromethylfluorescein diacetate (CMFDA) was not changed in cells lacking E-Syts but was reduced in cells in which cytosolic Ca²⁺ was buffered. These data provide evidence that E-Syts do not participate in the targeting of InsP3Rs to the apical region. Identifying tethers that bring InsP3Rs to the apical region remains an important question, since mis-targeting of InsP3Rs leads to impaired secretory activity.

Receptor tyrosine kinases endocytosis in endothelium: biology and signaling

Receptor tyrosine kinases are involved in regulation of key processes in endothelial biology, including proliferation, migration, and angiogenesis. It is now generally accepted that receptor tyrosine kinase signaling occurs intracellularly and on the plasma membrane, although many important details remain to be worked out. Endocytosis and subsequent intracellular trafficking spatiotemporally regulate receptor tyrosine kinase signaling, whereas signaling endosomes provide a platform for the compartmentalization of signaling events. This review summarizes recent advances in our understanding of endothelial receptor tyrosine kinase endocytosis and signaling using vascular endothelial growth factor receptor-2 as a paradigm.

FGF21 promotes endothelial cell angiogenesis through a dynamin-2 and Rab5 dependent pathway

Binding of angiogenic molecules with cognate receptor tyrosine kinases (RTK) is required for angiogenesis however the precise link between RTK binding, endocytosis, and signaling requires further investigation. Here, we use FGFR1 as a model to test the effects of the large GTPase and endocytosis regulatory molecule dynamin-2 on angiogenic signaling in context of distinct FGF ligands. In vitro, overexpression of dominant negative dynamin-2 (DynK44A) attenuates FGFR1 activation of Erk and tubulogenesis by FGF2. Furthermore, we identify FGF21, a non-classical, FGF ligand implicated in diverse human pathologies as an angiogenic molecule acting through FGFR1 and β-Klotho coreceptor. Disruption of FGFR1 activation of ERK by FGF21 is achieved by perturbation of the function of both dynamin-2 and Rab5 GTPase. In vivo, mice harboring endothelial selective overexpression of DynK44A, show impaired angiogenesis in response to FGF21. In conclusion, dynamin dependent endocytosis of FGFR1 is required for in vitro and in vivo angiogenesis in response to FGF2 and the non-classical FGF ligand, FGF21. These studies extend our understanding of the relationships between RTK binding, internalization, endosomal targeting, and angiogenic signaling.

Functional roles of fibroblast growth factor receptors (FGFRs) signaling in human cancers

The fibroblast growth factor receptors (FGFRs) regulate important biological processes including cell proliferation and differentiation during development and tissue repair. Over the past decades, numerous pathological conditions and developmental syndromes have emerged as a consequence of deregulation in the FGFRs signaling network. This review aims to provide an overview of FGFR family, their complex signaling pathways in tumorigenesis, and the current development and application of therapeutics targeting the FGFRs signaling for treatment of refractory human cancers.

PI(4,5)P(2)-dependent and Ca(2+)-regulated ER-PM interactions mediated by the extended synaptotagmins

Most available information on endoplasmic reticulum (ER)-plasma membrane (PM) contacts in cells of higher eukaryotes concerns proteins implicated in the regulation of Ca(2+) entry. However, growing evidence suggests that such contacts play more general roles in cell physiology, pointing to the existence of additionally ubiquitously expressed ER-PM tethers. Here, we show that the three extended synaptotagmins (E-Syts) are ER proteins that participate in such tethering function via C2 domain-dependent interactions with the PM that require PI(4,5)P2 in the case of E-Syt2 and E-Syt3 and also elevation of cytosolic Ca(2+) in the case of E-Syt1. As they form heteromeric complexes, the E-Syts confer cytosolic Ca(2+) regulation to ER-PM contact formation. E-Syts-dependent contacts, however, are not required for store-operated Ca(2+) entry. Thus, the ER-PM tethering function of the E-Syts (tricalbins in yeast) mediates the formation of ER-PM contacts sites, which are functionally distinct from those mediated by STIM1 and Orai1.

Syndecan-4 signaling at a glance

Syndecan-4, a ubiquitous cell surface proteoglycan, mediates numerous cellular processes through signaling pathways that affect cellular proliferation, migration, mechanotransduction and endocytosis. These effects are achieved through syndecan-4 functioning as both a co-receptor for the fibroblast growth factor receptors (FGFR1-FGFR4) and its ability to independently activate signaling pathways upon ligand binding. As an FGFR co-receptor, syndecan-4 strengthens the duration and intensity of downstream signaling upon ligand binding; this is particularly evident with regard to mitogen-activated protein kinase (MAPK) signaling. In contrast, syndecan-4 also functions as an independent receptor for heparin-binding growth factors, such as fibroblast growth factors (FGFs), vascular endothelial growth factors (VEGFs) and platelet-derived growth factors (PDGFs). These signaling cascades affect canonical signaling components, such as the mammalian target of rapamycin (mTOR), AKT1 and the Rho family of GTPases. In combination with the integrin family of proteins, syndecan-4 is also able to form physical connections between the extracellular matrix (ECM) and cytoskeletal signaling proteins, and it has a key role in regulation of integrin turnover. This unique versatility of the interactions of syndecan-4 is characterized in this Cell Science at a Glance article and illustrated in the accompanying poster.

Generation and interpretation of FGF morphogen gradients in vertebrates

Signalling via fibroblast growth factors (FGFs) is involved in multiple aspects of vertebrate development. In several instances FGFs act as morphogens, that is secreted signalling molecules that encode positional information in their graded distribution throughout their target tissue. In recent years, work in the zebrafish model system has been instrumental in addressing the cell biological basis of FGF morphogen gradient formation and interpretation. These experiments have benefitted from the optical properties of the zebrafish embryo that render this vertebrate organism particularly suited for advanced microscopic and biophysical approaches.

The role of endosomal signaling triggered by metastatic growth factors in tumor progression

Within tumor microenvironment, a lot of growth factors such as hepatocyte growth factor and epidermal growth factor may induce similar signal cascade downstream of receptor tyrosine kinase (RTK) and trigger tumor metastasis synergistically. In the past decades, the intimate relationship of RTK-mediated receptor endocytosis with signal transduction was well established. In general, most RTK undergoes clathrin-dependent endocytosis and/or clathrin-independent endocytosis. The internalized receptors may sustain the signaling within early endosome, recycling to plasma membrane for subsequent ligand engagement or sorting to late endosomes/lysosome for receptor degradation. Moreover, receptor endocytosis influences signal transduction in a temporal and spatial manner for periodical and polarized cellular processes such as cell migration. The endosomal signalings triggered by various metastatic factors are quite similar in some critical points, which are essential for triggering cell migration and tumor progression. There are common regulators for receptor endocytosis including dynamin, Rab4, Rab5, Rab11 and Cbl. Moreover, many critical regulators within the RTK signal pathway such as Grb2, p38, PKC and Src were also modulators of endocytosis. In the future, these may constitute a new category of targets for prevention of tumor metastasis.

Role of fibroblast growth factor signaling in vascular formation and maintenance: orchestrating signaling networks as an integrated system

The vascular system has begun to be perceived as a dynamic organ actively controlling a wide variety of physiological processes. The structural and functional integrity of blood vessels, regulated by signaling activities finely modulating cell-cell and cell-matrix interactions, is crucial for vessel physiology, as well as basic functionality of the tissue. Throughout the process of new vessel formation, while blood vessels are actively reorganized and remodeled with migration and proliferation of vascular cells, maintenance of vascular barrier function is essentially important. These conflicting properties, i.e., dynamic cellular mobilization and maintenance of barrier integrity, are simultaneously achieved through the interaction of highly organized signaling networks governing coordinated cell-cell interplay. Recent evidence suggests that the fibroblast growth factor (FGF) system plays a regulatory role in several physiological conditions in the vascular system. In this article, we will attempt to summarize current knowledge in order to understand the mechanism of this coordination and evaluate the pivotal role of FGF signaling in integrating a diverse range of signaling events in vascular growth and maintenance.

The endocytic adapter E-Syt2 recruits the p21 GTPase activated kinase PAK1 to mediate actin dynamics and FGF signalling

Fibroblast growth factor (FGF) signalling plays an essential role in early vertebrate development. However, the response to FGF requires endocytosis of the activated FGF receptor (FGFR) that is in part dependent on remodelling of the actin cytoskeleton. Recently we showed that the extended synaptotagmin family plasma membrane protein, E-Syt2, is an essential endocytic adapter for FGFR1. Here we show E-Syt2 is also an interaction partner for the p21-GTPase Activated Kinase PAK1. The phospholipid binding C2C domain of E-Syt2 specifically binds a site adjacent to the CRIB/GBD of PAK1. PAK1 and E-Syt2 selectively complex with FGFR1 and functionally cooperate in the FGF signalling. E-Syt2 binding suppresses actin polymerization and inhibits the activation of PAK1 by the GTPases Cdc42 and Rac. Interestingly, the E-Syt2 binding site on PAK1 extensively overlaps a site recently suggested to bind phospholipids. Our data suggest that PAK1 interacts with phospholipid membrane domains via E-Syt2, where it may cooperate in the E-Syt2-dependent endocytosis of activated FGFR1 by modulating cortical actin stability.