Nedd4-1 binds and ubiquitylates activated FGFR1 to control its endocytosis and function

Gentiopicroside ameliorates glucose and lipid metabolism in T2DM via targeting FGFR1

BACKGROUND

The suppression of the fibroblast growth factor 21/fibroblast growth factor receptor 1 (FGF21/FGFR1) signaling pathway is considered as a vital factor in the type 2 diabetes mellitus (T2DM) progression. Our previous study showed that gentiopicroside (GPS), the main active compound present in Gentiana macrophylla Pall., has the capacity to control disorders related to glucose and lipid metabolism in individuals with T2DM. Nevertheless, the specific mechanism remains unclear.

PURPOSE

In light of the fact that the PharmMapper database suggests FGFR1 as the target of GPS, our investigation aims to determine if GPS can enhance glucose and lipid metabolism issues in T2DM by modulating the FGF21/FGFR1 signaling pathway.

METHODS

In this study, we used palmitic acid (PA)-induced HepG2 cells and db/db mice to investigate the function and mechanism of GPS in the FGF21/FGFR1 signaling pathway. To examine the interaction between GPS and FGFR1, researchers performed Cellular Thermal Shift Assay (CETSA) and Surface Plasmon Resonance (SPR) analysis.

RESULTS

The results suggest that GPS activates the traditional metabolic pathways, including PI3K/AKT and AMPK, which are the subsequent stages of the FGF21/FGFR1 pathway. This activation leads to the enhancement of glucose and lipid metabolism issues in PA-treated HepG2 cells and db/db mice. Furthermore, the depletion of FGFR1 has been noticed to oppose the stimulation of PI3K/AKT and AMPK pathways by GPS in HepG2 cells subjected to PA. Notability, our research affirms that GPS binds directly to FGFR1, hindering the ubiquitinated degradation of FGFR1 by neural precursor cells expressing developmentally decreased protein 4 (NEDD4) and ultimately promoting FGF21 signal transduction.

CONCLUSION

This study demonstrates that GPS targeting FGFR1 activates the PI3K/AKT and AMPK pathways, which is an important mechanism for its treatment of T2DM.

The diverse dependence of galectin-1 and -8 on multivalency for the modulation of FGFR1 endocytosis

Fibroblast growth factor receptor 1 (FGFR1) is a N-glycosylated cell surface receptor tyrosine kinase, which upon recognition of specific extracellular ligands, fibroblast growth factors (FGFs), initiates an intracellular signaling. FGFR1 signaling ensures homeostasis of cells by fine-tuning essential cellular processes, like differentiation, division, motility and death. FGFR1 activity is coordinated at multiple steps and unbalanced FGFR1 signaling contributes to developmental diseases and cancers. One of the crucial control mechanisms over FGFR1 signaling is receptor endocytosis, which allows for rapid targeting of FGF-activated FGFR1 to lysosomes for degradation and the signal termination. We have recently demonstrated that N-glycans of FGFR1 are recognized by a precise set of extracellular galectins, secreted and intracellular multivalent lectins implicated in a plethora of cellular processes and altered in immune responses and cancers. Specific galectins trigger FGFR1 clustering, resulting in activation of the receptor and in initiation of intracellular signaling cascades that shape the cell physiology. Although some of galectin family members emerged recently as key players in the clathrin-independent endocytosis of specific cargoes, their impact on endocytosis of FGFR1 was largely unknown.Here we assessed the contribution of extracellular galectins to the cellular uptake of FGFR1. We demonstrate that only galectin-1 induces internalization of FGFR1, whereas the majority of galectins predominantly inhibit endocytosis of the receptor. We focused on three representative galectins: galectin-1, -7 and -8 and we demonstrate that although all these galectins directly activate FGFR1 by the receptor crosslinking mechanism, they exert different effects on FGFR1 endocytosis. Galectin-1-mediated internalization of FGFR1 doesn't require galectin-1 multivalency and occurs via clathrin-mediated endocytosis, resembling in this way the uptake of FGF/FGFR1 complex. In contrast galectin-7 and -8 impede FGFR1 endocytosis, causing stabilization of the receptor on the cell surface and prolonged propagation of the signals. Furthermore, using protein engineering approaches we demonstrate that it is possible to modulate or even fully reverse the endocytic potential of galectins.

NEDD4 and NEDD4L: Ubiquitin Ligases Closely Related to Digestive Diseases

Protein ubiquitination is an enzymatic cascade reaction and serves as an important protein post-translational modification (PTM) that is involved in the vast majority of cellular life activities. The key enzyme in the ubiquitination process is E3 ubiquitin ligase (E3), which catalyzes the binding of ubiquitin (Ub) to the protein substrate and influences substrate specificity. In recent years, the relationship between the subfamily of neuron-expressed developmental downregulation 4 (NEDD4), which belongs to the E3 ligase system, and digestive diseases has drawn widespread attention. Numerous studies have shown that NEDD4 and NEDD4L of the NEDD4 family can regulate the digestive function, as well as a series of related physiological and pathological processes, by controlling the subsequent degradation of proteins such as PTEN, c-Myc, and P21, along with substrate ubiquitination. In this article, we reviewed the appropriate functions of NEDD4 and NEDD4L in digestive diseases including cell proliferation, invasion, metastasis, chemotherapeutic drug resistance, and multiple signaling pathways, based on the currently available research evidence for the purpose of providing new ideas for the prevention and treatment of digestive diseases.

Ubiquitin E3 ligases assisted technologies in protein degradation: Sharing pathways in neurodegenerative disorders and cancer

E3 ligases, essential components of the ubiquitin-proteasome-mediated protein degradation system, play a critical role in cellular regulation. By covalently attaching ubiquitin (Ub) molecules to target proteins, these ligases mark them for degradation, influencing various bioprocesses. With over 600 E3 ligases identified, there is a growing realization of their potential as therapeutic candidates for addressing proteinopathies in cancer and neurodegenerative disorders (NDDs). Recent research has highlighted the need to delve deeper into the intricate roles of E3 ligases as nexus points in the pathogenesis of both cancer and NDDs. Their dysregulation is emerging as a common thread linking these seemingly disparate diseases, necessitating a comprehensive understanding of their molecular intricacies. Herein, we have discussed (i) the fundamental mechanisms through which different types of E3 ligases actively participate in selective protein degradation in cancer and NDDs, followed by an examination of common E3 ligases playing pivotal roles in both situations, emphasising common players. Moving to, (ii) the functional domains and motifs of E3 ligases involved in ubiquitination, we have explored their interactions with specific substrates in NDDs and cancer. Additionally, (iii) we have explored techniques like PROTAC, molecular glues, and other state-of-the-art methods for hijacking neurotoxic and oncoproteins. Lastly, (iv) we have provided insights into ongoing clinical trials, offering a glimpse into the evolving landscape of E3-based therapeutics for cancer and NDDs. Unravelling the intricate network of E3 ligase-mediated regulation holds the key to unlocking targeted therapies that address the specific molecular signatures of individual patients, heralding a new era in personalized medicines.

Identification of cuproptosis-related diagnostic biomarkers in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal lung disease with clinical and pathological heterogeneity. Recent studies have identified cuproptosis as a novel cell death mechanism. However, the role of cuproptosis-related genes in the pathogenesis of IPF is still unclear. Two IPF datasets of the Gene Expression Omnibus database were studied. Mann-Whitney U test, correlation analysis, functional enrichment analyses, single-sample gene set enrichment analysis, CIBERSORT, unsupervised clustering, weighted gene co-expression network analysis, and receiver operating characteristic curve analysis were used to conduct our research. The dysregulated cuproptosis-related genes and immune responses were identified between IPF patients and controls. Two cuproptosis-related molecular clusters were established in IPF, the high immune score group (C1) and the low immune score group (C2). Significant heterogeneity in immunity between clusters was revealed by functional analyses results. The module genes with the strongest correlation to the 2 clusters were identified by weighted gene co-expression network analysis results. Seven hub genes were found using the Cytoscape software. Ultimately, 2 validated diagnostic biomarkers of IPF, CDKN2A and NEDD4, were obtained. Subsequently, the results were validated in GSE47460. Our investigation illustrates that CDKN2A and NEDD4 may be valid biomarkers that were useful for IPF diagnosis and copper-related clustering.

Physiological Functions of the Ubiquitin Ligases Nedd4-1 and Nedd4-2

The Nedd4 family of E3 ubiquitin ligases, consisting of a C2-WW(n)-HECT domain architecture, includes the closely related Nedd4/Nedd4-1 and Nedd4L/Nedd4-2, which play critical roles in human physiology and pathophysiology.This review focuses on the regulation of enzymatic activity of these Nedd4 proteins, as well as on their roles in regulating stability and function of membrane and other signaling proteins, such as ion channels, ion transporters, and growth factor receptors. The diseases caused by impairment of such regulation are discussed, as well as opportunities and challenges for targeting these enzymes for therapy.

Primate-specific isoform of Nedd4-1 regulates substrate binding via Ser/Thr phosphorylation and 14-3-3 binding

Nedd4 (Nedd4-1) is an E3 ubiquitin ligase involved in crucial biological processes such as growth factor receptor signaling. While canonical Nedd4-1 comprises a C2-WW(4)-HECT domain architecture, alternative splicing produces non-canonical isoforms that are poorly characterized. Here we characterized Nedd4-1(NE), a primate-specific isoform of Nedd4-1 that contains a large N-terminal Extension (NE) that replaces most of the C2 domain. We show that Nedd4-1(NE) mRNA is ubiquitously expressed in human tissues and cell lines. Moreover, we found that Nedd4-1(NE) is more active than the canonical Nedd4-1 isoform, likely due to the absence of a C2 domain-mediated autoinhibitory mechanism. Additionally, we identified two Thr/Ser phosphoresidues in the NE region that act as binding sites for 14-3-3 proteins, and show that phosphorylation on these sites reduces substrate binding. Finally, we show that the NE region can act as a binding site for the RPB2 subunit of RNA polymerase II, a unique substrate of Nedd4-1(NE) but not the canonical Nedd4-1. Taken together, our results demonstrate that alternative splicing of the ubiquitin ligase Nedd4-1 can produce isoforms that differ in their catalytic activity, binding partners and substrates, and mechanisms of regulation.

Commissureless acts as a substrate adapter in a conserved Nedd4 E3 ubiquitin ligase pathway to promote axon growth across the midline

In both vertebrates and invertebrates, commissural neurons prevent premature responsiveness to the midline repellant Slit by downregulating surface levels of its receptor Roundabout1 (Robo1). In Drosophila, Commissureless (Comm) plays a critical role in this process; however, there is conflicting data on the underlying molecular mechanism. Here, we demonstrate that the conserved PY motifs in the cytoplasmic domain of Comm are required allow the ubiquitination and lysosomal degradation of Robo1. Disruption of these motifs prevents Comm from localizing to Lamp1 positive late endosomes and to promote axon growth across the midline in vivo. In addition, we conclusively demonstrate a role for Nedd4 in midline crossing. Genetic analysis shows that nedd4 mutations result in midline crossing defects in the Drosophila embryonic nerve cord, which can be rescued by introduction of exogenous Nedd4. Biochemical evidence shows that Nedd4 incorporates into a three-member complex with Comm and Robo in a PY motif-dependent manner. Finally, we present genetic evidence that Nedd4 acts with Comm in the embryonic nerve cord to downregulate Robo1 levels. Taken together, these findings demonstrate that Comm promotes midline crossing in the nerve cord by facilitating Robo ubiquitination by Nedd4, ultimately leading to its degradation.

Fibroblast growth factor signaling in axons: from development to disease

The fibroblast growth factor (FGF) family regulates various and important aspects of nervous system development, ranging from the well-established roles in neuronal patterning to more recent and exciting functions in axonal growth and synaptogenesis. In addition, FGFs play a critical role in axonal regeneration, particularly after spinal cord injury, confirming their versatile nature in the nervous system. Due to their widespread involvement in neural development, the FGF system also underlies several human neurological disorders. While particular attention has been given to FGFs in a whole-cell context, their effects at the axonal level are in most cases undervalued. Here we discuss the endeavor of the FGF system in axons, we delve into this neuronal subcompartment to provide an original view of this multipurpose family of growth factors in nervous system (dys)function. Video Abstract.

Characterization of the folding and binding properties of the PTB domain of FRS2 with phosphorylated and unphosphorylated ligands

PTB (PhosphoTyrosine Binding) domains are protein domains that exert their function by binding phosphotyrosine residues on other proteins. They are commonly found in a variety of signaling proteins and are important for mediating protein-protein interactions in numerous cellular processes. PTB domains can also exhibit binding to unphosphorylated ligands, suggesting that they have additional binding specificities beyond phosphotyrosine recognition. Structural studies have reported that the PTB domain from FRS2 possesses this peculiar feature, allowing it to interact with both phosphorylated and unphosphorylated ligands, such as TrkB and FGFR1, through different topologies and orientations. In an effort to elucidate the dynamic and functional properties of these protein-protein interactions, we provide a complete characterization of the folding mechanism of the PTB domain of FRS2 and the binding process to peptides mimicking specific regions of TrkB and FGFR1. By analyzing the equilibrium and kinetics of PTB folding, we propose a mechanism implying the presence of an intermediate along the folding pathway. Kinetic binding experiments performed at different ionic strengths highlighted the electrostatic nature of the interaction with both peptides. The specific role of single amino acids in early and late events of binding was pinpointed by site-directed mutagenesis. These results are discussed in light of previous experimental works on these protein systems.

miR-338-3p acts as a tumor suppressor in lung squamous cell carcinoma by targeting FGFR2/FRS2

Background

Lung cancer refers to the occurrence of malignant tumors in the lung, and squamous cell carcinoma is one of the most common pathological types of non-small cell lung cancer. Studies have shown that microRNAs (miRNAs) play an important role in the occurrence, development, early diagnosis, and treatment of lung cancer. This study aimed to explore the role and possible mechanism of MicroRNA-338-3p (miR-338-3p) in lung squamous cell carcinoma (LUSC).

Method

In this study, we compared 238 LUSC patients with relatively high miR-338-3p expression levels with 238 miR-338-3p expression levels in The Cancer Genome Atlas (TCGA)-LUSC dataset using first-line gene set enrichment analysis (GSEA). Second, the mRNA expression of miR-338-3p, FGFR2, and fibroblast growth factor receptor substrate 2 (FRS2) in 30 lung cancers and adjacent lung tissues was detected using quantitative real-time polymerase chain reaction (qRT-PCR). Finally, in vitro experiments were conducted, whereby the expression levels of miR-338-3p in lung cancer cells (H1703, SKMES1, H2170, H520) and normal lung epithelial cells (16HBE) were detected using qRT-PCR. miR-338-3p was overexpressed in lung cancer cells (H1703), and the cell proliferation (cell counting kit-8 [CCK8] assay), colony formation, cell apoptosis, cell cycle (BD-FACSVerse assay, Becton Dickinson, Bedford, MA, USA), cell invasion, and migration (Transwell assay, Thermo Fischer Corporation, Waltham, MA, USA) were detected.

Results

We found that the expression of miR-338-3p was significantly reduced in LUSC tissues (p ​< ​0.001) and cancer cell lines (P < 0.01), and miR-338-3p was significantly negatively correlated with the expression of FGFR2 (P < 0.001) and FRS2 (P < 0.01). Furthermore, overexpression of miR-338-3p inhibited proliferation (P < 0.001), migration, and invasion (P < 0.001) of LUSC cell lines and increased apoptosis in the G1 phase (P < 0.001) and cell cycle arrest (P < 0.05).

Conclusions

Our study demonstrates that miR-338-3p inhibits tumor cell proliferation and migration by targeting FGFR2 and FRS2 in LUSC. We believe that miR-338-3p may be a promising target for the treatment of LUSC.

K63-linked ubiquitin chains are a global signal for endocytosis and contribute to selective autophagy in plants

In contrast to other eukaryotic model organisms, the closely related ubiquitin (Ub)-conjugating enzymes UBC35 and UBC36 are the main sources of K63-linked Ub chains in Arabidopsis.¹ Although K63-linked chains have been associated with the regulation of vesicle trafficking, definitive proof for their role in endocytosis was missing. We show that the ubc35 ubc36 mutant has pleiotropic phenotypes related to hormone and immune signaling. Specifically, we reveal that ubc35-1 ubc36-1 plants have altered turnover of integral membrane proteins including FLS2, BRI1, and PIN1 at the plasma membrane. Our data indicates that K63-Ub chains are generally required for endocytic trafficking in plants. In addition, we show that in plants K63-Ub chains are involved in selective autophagy through NBR1, the second major pathway delivering cargoes to the vacuole for degradation. Similar to autophagy-defective mutants, ubc35-1 ubc36-1 plants display an accumulation of autophagy markers. Moreover, autophagy receptor NBR1 interacts with K63-Ub chains, which are required for its delivery to the lytic vacuole.² Together, we show that K63-Ub chains act as a general signal required for the two main pathways delivering cargo to the vacuole and thus, to maintain proteostasis.

The Roles of NEDD4 Subfamily of HECT E3 Ubiquitin Ligases in Neurodevelopment and Neurodegeneration

The ubiquitin pathway regulates the function of many proteins and controls cellular protein homeostasis. In recent years, it has attracted great interest in neurodevelopmental and neurodegenerative diseases. Here, we have presented the first review on the roles of the 9 proteins of the HECT E3 ligase NEDD4 subfamily in the development and function of neurons in the central nervous system (CNS). We discussed their regulation and their direct or indirect involvement in neurodevelopmental diseases, such as intellectual disability, and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease or Amyotrophic Lateral Sclerosis. Further studies on the roles of these proteins, their regulation and their targets in neurons will certainly contribute to a better understanding of neuronal function and dysfunction, and will also provide interesting information for the development of therapeutics targeting them.

The Role of NEDD4 E3 Ubiquitin–Protein Ligases in Parkinson’s Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disease that causes a great clinical burden. However, its exact molecular pathologies are not fully understood. Whilst there are a number of avenues for research into slowing, halting, or reversing PD, one central idea is to enhance the clearance of the proposed aetiological protein, oligomeric α-synuclein. Oligomeric α-synuclein is the main constituent protein in Lewy bodies and neurites and is considered neurotoxic. Multiple E3 ubiquitin-protein ligases, including the NEDD4 (neural precursor cell expressed developmentally downregulated protein 4) family, parkin, SIAH (mammalian homologues of Drosophila seven in absentia), CHIP (carboxy-terminus of Hsc70 interacting protein), and SCFFXBL5 SCF ubiquitin ligase assembled by the S-phase kinase-associated protein (SKP1), cullin-1 (Cul1), a zinc-binding RING finger protein, and the F-box domain/Leucine-rich repeat protein 5-containing protein FBXL5), have been shown to be able to ubiquitinate α-synuclein, influencing its subsequent degradation via the proteasome or lysosome. Here, we explore the link between NEDD4 ligases and PD, which is not only via α-synuclein but further strengthened by several additional substrates and interaction partners. Some members of the NEDD4 family of ligases are thought to crosstalk even with PD-related genes and proteins found to be mutated in familial forms of PD. Mutations in NEDD4 family genes have not been observed in PD patients, most likely because of their essential survival function during development. Following further in vivo studies, it has been thought that NEDD4 ligases may be viable therapeutic targets in PD. NEDD4 family members could clear toxic proteins, enhancing cell survival and slowing disease progression, or might diminish beneficial proteins, reducing cell survival and accelerating disease progression. Here, we review studies to date on the expression and function of NEDD4 ubiquitin ligases in the brain and their possible impact on PD pathology.

The NEDD4 ubiquitin E3 ligase: a snapshot view of its functional activity and regulation

Due to its fundamental role in all eukaryotic cells, a deeper understanding of the molecular mechanisms underlying ubiquitination is of central importance. Being responsible for chain specificity and substrate recognition, E3 ligases are the selective elements of the ubiquitination process. In this review, we discuss different cellular pathways regulated by one of the first identified E3 ligase, NEDD4, focusing on its pathophysiological role, its known targets and modulators. In addition, we highlight small molecule inhibitors that act on NEDD4 and discuss new strategies to effectively target this E3 enzyme.

The Gentle Side of the UPS: Ubiquitin-Proteasome System and the Regulation of the Myogenic Program

In recent years, the ubiquitin-proteasome system (UPS) has emerged as an important regulator of stem cell function. Here we review recent findings indicating that UPS also plays critical roles in the biology of satellite cells, the muscle stem cell responsible for its maintenance and regeneration. While we focus our attention on the control of key transcriptional regulators of satellite cell function, we briefly discuss early studies suggesting the UPS participates more broadly in the regulation of satellite cell stemness and regenerative capacity.

MEKK5 Interacts with and Negatively Regulates the E3 Ubiquitin Ligase NEDD4 for Mediating Lung Cancer Cell Migration

Our previous studies have shown that the HECT E3 ubiquitin ligase NEDD4 and kinase MEKK5 both play an essential role in lung cancer migration. A report predicts that MEKK5 may be ubiquitinated by NEDD4; however, interaction of MEKK5 with NEDD4 and ubiquitination of MEKK5 by NEDD4 have not been characterized. In this report, we show that NEDD4 interacts with MEKK5 through a conserved WW3 domain by the co-immunoprecipitation and the GST-pulldown assays. The ubiquitination assay indicates that MEKK5 is not a ubiquitination substrate of NEDD4, but negatively regulates NEDD4-mediated ubiquitination. Furthermore, overexpression of MEKK5 significantly reduced the NEDD4-promoted lung cancer cell migration. Taken together, our studies have defined an inhibitory role of MEKK5 in regulation of NEDD4-mediated ubiquitination.

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.

Kindlin2 regulates neural crest specification via integrin-independent regulation of the FGF signaling pathway

The focal adhesion protein Kindlin2 is essential for integrin activation, a process that is fundamental to cell-extracellular matrix adhesion. Kindlin 2 (Fermt2) is widely expressed in mouse embryos, and its absence causes lethality at the peri-implantation stage due to the failure to trigger integrin activation. The function of kindlin2 during embryogenesis has not yet been fully elucidated as a result of this early embryonic lethality. Here, we showed that kindlin2 is essential for neural crest (NC) formation in Xenopus embryos. Loss-of-function assays performed with kindlin2-specific morpholino antisense oligos (MOs) or with CRISPR/Cas9 techniques in Xenopus embryos severely inhibit the specification of the NC. Moreover, integrin-binding-deficient mutants of Kindlin2 rescued the phenotype caused by loss of kindlin2, suggesting that the function of kindlin2 during NC specification is independent of integrins. Mechanistically, we found that Kindlin2 regulates the fibroblast growth factor (FGF) pathway, and promotes the stability of FGF receptor 1. Our study reveals a novel function of Kindlin2 in regulating the FGF signaling pathway and provides mechanistic insights into the function of Kindlin2 during NC specification.

The cytokine FAM3B/PANDER is an FGFR ligand that promotes posterior development in Xenopus

How distinct body regions form along the anterior–posterior axis in vertebrate embryos is a fascinating and incompletely understood developmental process. FAM3B/PANDER is a secreted protein involved in glucose metabolism and type 2 diabetes pathogenesis in mammals, but its receptor has been unknown. Here, we report that FAM3B binds to transmembrane fibroblast growth factor receptors (FGFRs) and activates their downstream signaling pathway. In frog embryos, gain-of-function of FAM3B impairs head development and induces ectopic tail-like structures, whereas loss-of-function of FAM3B promotes head development. FGFR is required downstream of FAM3B for head-to-tail patterning. Our results reveal that FAM3B functions by activating the FGFR pathway in frog embryos and mammalian cells and shed light on its possible role in human diseases.

Fibroblast growth factor (FGF)/extracellular signal-regulated kinase (ERK) signaling plays a crucial role in anterior–posterior (A–P) axial patterning of vertebrate embryos by promoting posterior development. In our screens for novel developmental regulators in Xenopus embryos, we identified Fam3b as a secreted factor regulated in ectodermal explants. Family with sequence similarity 3 member B (FAM3B)/PANDER (pancreatic-derived factor) is a cytokine involved in glucose metabolism, type 2 diabetes, and cancer in mammals. However, the molecular mechanism of FAM3B action in these processes remains poorly understood, largely because its receptor is still unidentified. Here we uncover an unexpected role of FAM3B acting as a FGF receptor (FGFR) ligand in Xenopus embryos. fam3b messenger RNA (mRNA) is initially expressed maternally and uniformly in the early Xenopus embryo and then in the epidermis at neurula stages. Overexpression of Xenopus fam3b mRNA inhibited cephalic structures and induced ectopic tail-like structures. Recombinant human FAM3B protein was purified readily from transfected tissue culture cells and, when injected into the blastocoele cavity, also caused outgrowth of tail-like structures at the expense of anterior structures, indicating FGF-like activity. Depletion of fam3b by specific antisense morpholino oligonucleotides in Xenopus resulted in macrocephaly in tailbud tadpoles, rescuable by FAM3B protein. Mechanistically, FAM3B protein bound to FGFR and activated the downstream ERK signaling in an FGFR-dependent manner. In Xenopus embryos, FGFR activity was required epistatically downstream of Fam3b to mediate its promotion of posterior cell fates. Our findings define a FAM3B/FGFR/ERK-signaling pathway that is required for axial patterning in Xenopus embryos and may provide molecular insights into FAM3B-associated human diseases.

The Ubiquitinated Axon: Local Control of Axon Development and Function by Ubiquitin

Ubiquitin tagging sets protein fate. With a wide range of possible patterns and reversibility, ubiquitination can assume many shapes to meet specific demands of a particular cell across time and space. In neurons, unique cells with functionally distinct axons and dendrites harboring dynamic synapses, the ubiquitin code is exploited at the height of its power. Indeed, wide expression of ubiquitination and proteasome machinery at synapses, a diverse brain ubiquitome, and the existence of ubiquitin-related neurodevelopmental diseases support a fundamental role of ubiquitin signaling in the developing and mature brain. While special attention has been given to dendritic ubiquitin-dependent control, how axonal biology is governed by this small but versatile molecule has been considerably less discussed. Herein, we set out to explore the ubiquitin-mediated spatiotemporal control of an axon's lifetime: from its differentiation and growth through presynaptic formation, function, and pruning.

PD173074 blocks G1/S transition via CUL3-mediated ubiquitin protease in HepG2 and Hep3B cells

Fibroblast growth factor receptors (FGFRs) are frequently altered in a variety of human cancer cells and are overexpressed in hepatocellular carcinoma (HCC). Several literatures have proven that they are efficacious for HCC therapy, however, the underlying mechanism remains unclear. Here, we found FGFR4 was overexpressed in HCC cell lines HepG2 and Hep3B and we used PD173074, an FGFR4 inhibitor, to explore the role of FGFR4 and its underlying mechanism in these cell lines. The results showed that PD173074 significantly arrested HepG2 and Hep3B cells in G1 phase and inhibited cell proliferation. Furthermore, Western blot analysis revealed that PD173074 decreased the levels of P-FRS2α, P-ERK, CDK2, cyclin E and NF-κB (p65) in the nucleus while it increased the levels of ubiquitin and CUL3, an E3 ubiquitin ligase which involves in cyclin E degradation. Meanwhile, the data from RT-qPCR showed that PD173074 also decreased miR-141 level. In conclusion, these results suggest that FGFR4 is involved in HCC by ERK/CUL3/cyclin E signaling pathway, and the finding may provide a potential theoretical basis for treatment by targeting FGFR4 in HCC.

NEDD4 E3 ligase: Functions and mechanism in human cancer

A growing amount of evidence indicates that the neuronally expressed developmentally downregulated 4 (NEDD4, also known as NEDD4-1) E3 ligase plays a critical role in a variety of cellular processes via the ubiquitination-mediated degradation of multiple substrates. The abnormal regulation of NEDD4 protein has been implicated in cancer development and progression. In this review article, we briefly delineate the downstream substrates and upstream regulators of NEDD4, which are involved in carcinogenesis. Moreover, we succinctly elucidate the functions of NEDD4 protein in tumorigenesis and progression, including cell proliferation, apoptosis, cell cycle, migration, invasion, epithelial mesenchymal transition (EMT), cancer stem cells, and drug resistance. The findings regarding NEDD4 functions are further supported by knockout mouse models and human tumor tissue studies. This review could provide a promising and optimum anticancer therapeutic strategy via targeting the NEDD4 protein.

An outlined review for the role of Nedd4-1 and Nedd4-2 in lung disorders

Neural precursor cell expressed, developmentally down-regulated 4, E3 ubiquitin protein ligase (Nedd4-1 and Nedd4-2) is a member of the HECT E3 ubiquitin ligase family. It has been shown to mediate numerous pathophysiological processes, including the regulation of synaptic plasticity and Wnt-associated signaling, via promoting the ubiquitination of its substrates, such as cyclic adenosine monophosphate (cAMP)-response element binding protein regulated transcription coactivator 3 (CRTC3), alpha-amino-3-hydroxy-5-methyl-4-isoxazo-lepropionic acid receptor (AMPAR), and Dishevelled2 (Dvl2). In the respiratory system, both Nedd4-1 and Nedd4-2 are expressed in epithelial cells and functionally associated with lung cancer development and alveolar fluid regulation. Nedd4-1 mediates lung cancer migration, metastasis, or drug resistance mainly through inducing phosphate and tension homology deleted on chromsome ten (PTEN) degradation or promoting cathepsin B secretion. Unlike Nedd4-1, Nedd4-2 displays more complex effects in lung cancers. On one hand it suppresses lung cancer cell migration and metastasis, and on the other hand it has been shown to promote lung cancer survival via inducing general control nonrepressed 2 (GCN2) degradation. Another important function of Nedd4-2 is to regulate the activity of epithelial sodium channel (ENaC), a membrane channel which mediates the clearance of fluid from the alveolar space at birth or during pulmonary edema. Here, we make an outlined review for the expression and function of Nedd4-1 and Nedd4-2 in the respiratory system in hope of getting an in-depth insight into their roles in lung disorders.

NEDD4 and NEDD4L regulate Wnt signalling and intestinal stem cell priming by degrading LGR5 receptor

The intestinal stem cell (ISC) marker LGR5 is a receptor for R‐spondin (RSPO) that functions to potentiate Wnt signalling in the proliferating crypt. It has been recently shown that Wnt plays a priming role for ISC self‐renewal by inducing RSPO receptor LGR5 expression. Despite its pivotal role in homeostasis, regeneration and cancer, little is known about the post‐translational regulation of LGR5. Here, we show that the HECT‐domain E3 ligases NEDD4 and NEDD4L are expressed in the crypt stem cell regions and regulate ISC priming by degrading LGR receptors. Loss of Nedd4 and Nedd4l enhances ISC proliferation, increases sensitivity to RSPO stimulation and accelerates tumour development in Apc^min mice with increased numbers of high‐grade adenomas. Mechanistically, we find that both NEDD4 and NEDD4L negatively regulate Wnt/β‐catenin signalling by targeting LGR5 receptor and DVL2 for proteasomal and lysosomal degradation. Our findings unveil the previously unreported post‐translational control of LGR receptors via NEDD4/NEDD4L to regulate ISC priming. Inactivation of NEDD4 and NEDD4L increases Wnt activation and ISC numbers, which subsequently enhances tumour predisposition and progression.

The many substrates and functions of NEDD4-1

Tumorigenesis, tumor growth, and prognosis are highly related to gene alterations and post-translational modifications (PTMs). Ubiquitination is a critical PTM that governs practically all aspects of cellular function. An increasing number of studies show that E3 ubiquitin ligases (E3s) are important enzymes in the process of ubiquitination that primarily determine substrate specificity and thus need to be tightly controlled. Among E3s, neural precursor cell expressed developmentally downregulated 4-1 (NEDD4-1) has been shown to play a critical role in modulating the proliferation, migration, and invasion of cancer cells and the sensitivity of cancer cells to anticancer therapies via regulating multiple substrates. This review discusses some significant discoveries on NEDD4-1 substrates and the signaling pathways in which NEDD4-1 participates. In addition, we introduce the latest potential therapeutic strategies that inhibit or activate NEDD4-1 activity using small molecules. NEDD4-1 likely acts as a novel drug target or diagnostic marker in the battle against cancer.

Poly(ADP-ribosylated) proteins in β-amyloid peptide-stimulated microglial cells

Amyloid-treated microglia prime and sustain neuroinflammatory processes in the central nervous system activating different signalling pathways inside the cells. Since a key role for PARP-1 has been demonstrated in inflammation and in neurodegeneration, we investigated PARylated proteins in resting and in β-amyloid peptide treated BV2 microglial cells. A total of 1158 proteins were identified by mass spectrometry with 117 specifically modified in the amyloid-treated cells. Intervention of PARylation on the proteome of microglia showed to be widespread in different cellular districts and to affect various cellular pathways, highlighting the role of this dynamic post-translational modification in cellular regulation. Ubiquitination is one of the more enriched pathways, encompassing PARylated proteins like NEDD4, an E3 ubiquitine ligase and USP10, a de-ubiquitinase, both associated with intracellular responses induced by β-amyloid peptide challenge. PARylation of NEDD4 may be involved in the recruiting of this protein to the plasma membrane where it regulates the endocytosis of AMPA receptors, whereas USP10 may be responsible for the increase of p53 levels in amyloid stimulated microglia. Unfolded protein response and Endoplasmic Reticulum Stress pathways, strictly correlated with the Ubiquitination process, also showed enrichment in PARylated proteins. PARylation may thus represent one of the molecular switches responsible for the transition of microglia towards the inflammatory microglia phenotype, a pivotal player in brain diseases including neurodegenerative processes. The establishment of trials with PARP inhibitors to test their efficacy in the containment of neurodegenerative diseases may be envisaged.

NEDD4-induced degradative ubiquitination of phosphatidylinositol 4-phosphate 5-kinase α and its implication in breast cancer cell proliferation

Phosphatidylinositol 4‐phosphate 5‐kinase (PIP5K) family members generate phosphatidylinositol 4,5‐bisphosphate (PIP2), a critical lipid regulator of diverse physiological processes. The PIP5K‐dependent PIP2 generation can also act upstream of the oncogenic phosphatidylinositol 3‐kinase (PI3K)/Akt pathway. Many studies have demonstrated various mechanisms of spatiotemporal regulation of PIP5K catalytic activity. However, there are few studies on regulation of PIP5K protein stability. Here, we examined potential regulation of PIP5Kα, a PIP5K isoform, via ubiquitin‐proteasome system, and its implication for breast cancer. Our results showed that the ubiquitin ligase NEDD4 (neural precursor cell expressed, developmentally down‐regulated gene 4) mediated ubiquitination and proteasomal degradation of PIP5Kα, consequently reducing plasma membrane PIP2 level. NEDD4 interacted with the C‐terminal region and ubiquitinated the N‐terminal lysine 88 in PIP5Kα. In addition, PIP5Kα gene disruption inhibited epidermal growth factor (EGF)‐induced Akt activation and caused significant proliferation defect in breast cancer cells. Notably, PIP5Kα K88R mutant that was resistant to NEDD4‐mediated ubiquitination and degradation showed more potentiating effects on Akt activation by EGF and cell proliferation than wild‐type PIP5Kα. Collectively, these results suggest that PIP5Kα is a novel degradative substrate of NEDD4 and that the PIP5Kα‐dependent PIP2 pool contributing to breast cancer cell proliferation through PI3K/Akt activation is negatively controlled by NEDD4.

Ubiquitylation at the crossroads of development and disease

Human development requires intricate cell specification and communication pathways that allow an embryo to generate and appropriately connect more than 200 different cell types. Key to the successful completion of this differentiation programme is the quantitative and reversible regulation of core signalling networks, and post-translational modification with ubiquitin provides embryos with an essential tool to accomplish this task. Instigated by E3 ligases and reversed by deubiquitylases, ubiquitylation controls many processes that are fundamental for development, such as cell division, fate specification and migration. As aberrant function or regulation of ubiquitylation enzymes is at the roots of developmental disorders, cancer, and neurodegeneration, modulating the activity of ubiquitylation enzymes is likely to provide strategies for therapeutic intervention.

Systematic approaches to identify E3 ligase substrates

Protein ubiquitylation is a widespread post-translational modification, regulating cellular signalling with many outcomes, such as protein degradation, endocytosis, cell cycle progression, DNA repair and transcription. E3 ligases are a critical component of the ubiquitin proteasome system (UPS), determining the substrate specificity of the cascade by the covalent attachment of ubiquitin to substrate proteins. Currently, there are over 600 putative E3 ligases, but many are poorly characterized, particularly with respect to individual protein substrates. Here, we highlight systematic approaches to identify and validate UPS targets and discuss how they are underpinning rapid advances in our understanding of the biochemistry and biology of the UPS. The integration of novel tools, model systems and methods for target identification is driving significant interest in drug development, targeting various aspects of UPS function and advancing the understanding of a diverse range of disease processes.

NEDD4 controls spermatogonial stem cell homeostasis and stress response by regulating messenger ribonucleoprotein complexes

P bodies (PBs) and stress granules (SGs) are conserved cytoplasmic aggregates of cellular messenger ribonucleoprotein complexes (mRNPs) that are implicated in mRNA metabolism and play crucial roles in adult stem cell homeostasis and stress responses. However, the mechanisms underlying the dynamics of mRNP granules are poorly understood. Here, we report NEDD4, an E3 ubiquitin ligase, as a key regulator of mRNP dynamics that controls the size of the spermatogonial progenitor cell (SPC) pool. We find that NEDD4 targets an RNA-binding protein, NANOS2, in spermatogonia to destabilize it, leading to cell differentiation. In addition, NEDD4 is required for SG clearance. NEDD4 targets SGs and facilitates their rapid clearance through the endosomal–lysosomal pathway during the recovery period. Therefore, NEDD4 controls the turnover of mRNP components and inhibits pathological SG accumulation. Accordingly, we propose that a NEDD4-mediated mechanism regulates mRNP dynamics, and facilitates SPC homeostasis and viability under normal and stress conditions.

Stress granules (SG) comprise aggregates of cellular messenger ribonucleoproteins (mRNPs) but how they form is unclear. Here, the authors identify NEDD4, an E3 ubiquitin ligase, as regulating the RNA binding protein NANOS2 and turnover of mRNP components, and so SG disassembly in spermatogonial stem cells.

Regulation of glioma migration and invasion via modification of Rap2a activity by the ubiquitin ligase Nedd4-1

Νeuronal precursor cell expressed and developmentally downregulated protein (Nedd4-1) is an E3 ubiquitin ligase with critical roles in the pathogenesis of cancer. Herein, we demonstrated that Nedd4-1 protein was upregulated in glioma tissues vs. that in non-cancerous tissues by western blotting and immunohistochemistry. Scratch migration and Transwell chamber assays indicated that downregulation of Nedd4-1 significantly reduced the migration and invasion of the glioma cell lines U251 and U87. Conversely, overexpression of Nedd4-1 obviously enhanced the migratory and invasive capacities in both cell lines. To investigate the role of Nedd4-1 and the intracellular pathways involved, we performed pull-down and co-immunoprecipitation assays, and recognized that Nedd4-1, TNIK and Rap2a formed a complex. Moreover, Nedd4-1 selectively ubiquitinated its specific substrates, the wild-type Rap2a (WT-Rap2a) and dominant-active Rap2a (DA-Rap2a) rather than the dominant-negative Rap2a (DN-Rap2a) in the U251 cells. Subsequently, we demonstrated that Rap2a was robustly ubiquitinated by Nedd4-1 along with the K63-linked, but not the K48-linked ubiquitin chain, which significantly inhibited GTP-Rap2a activity by GST-RalGDS pull-down assay. To further verify whether the ubiquitination of Rap2a by Nedd4-1 regulated the migration and invasion of glioma cells, Nedd4-1, HA-tagged ubiquitin and its mutants as well as WT-Rap2a were co-transfected in the U251 and U87 cell lines. The results confirmed that Nedd4-1 inhibited GTP-Rap2a activity, and promoted the migration and invasion of glioma cells. In brief, our findings demonstrated the important role of Nedd4-1 in regulating the migration and invasion of glioma cells via the Nedd4-1/Rap2a pathway, which may qualify Nedd4-1 as a viable therapeutic target for glioma.

Ubiquitination and the Regulation of Membrane Proteins

Newly synthesized transmembrane proteins undergo a series of steps to ensure that only the required amount of correctly folded protein is localized to the membrane. The regulation of protein quality and its abundance at the membrane are often controlled by ubiquitination, a multistep enzymatic process that results in the attachment of ubiquitin, or chains of ubiquitin to the target protein. Protein ubiquitination acts as a signal for sorting, trafficking, and the removal of membrane proteins via endocytosis, a process through which multiple ubiquitin ligases are known to specifically regulate the functions of a number of ion channels, transporters, and signaling receptors. Endocytic removal of these proteins through ubiquitin-dependent endocytosis provides a way to rapidly downregulate the physiological outcomes, and defects in such controls are directly linked to human pathologies. Recent evidence suggests that ubiquitination is also involved in the shedding of membranes and associated proteins as extracellular vesicles, thereby not only controlling the cell surface levels of some membrane proteins, but also their potential transport to neighboring cells. In this review, we summarize the mechanisms and functions of ubiquitination of membrane proteins and provide specific examples of ubiquitin-dependent regulation of membrane proteins.

Ubiquitylation-dependent oligomerization regulates activity of Nedd4 ligases

Ubiquitylation controls protein function and degradation. Therefore, ubiquitin ligases need to be tightly controlled. We discovered an evolutionarily conserved allosteric restraint mechanism for Nedd4 ligases and demonstrated its function with diverse substrates: the yeast soluble proteins Rpn10 and Rvs167, and the human receptor tyrosine kinase FGFR1 and cardiac I_KS potassium channel. We found that a potential trimerization interface is structurally blocked by the HECT domain α1-helix, which further undergoes ubiquitylation on a conserved lysine residue. Genetic, bioinformatics, biochemical and biophysical data show that attraction between this α1-conjugated ubiquitin and the HECT ubiquitin-binding patch pulls the α1-helix out of the interface, thereby promoting trimerization. Strikingly, trimerization renders the ligase inactive. Arginine substitution of the ubiquitylated lysine impairs this inactivation mechanism and results in unrestrained FGFR1 ubiquitylation in cells. Similarly, electrophysiological data and TIRF microscopy show that NEDD4 unrestrained mutant constitutively downregulates the I_KS channel, thus confirming the functional importance of E3-ligase autoinhibition.

E3 ubiquitin ligase RFWD2 controls lung branching through protein-level regulation of ETV transcription factors

The mammalian lung is an elaborate branching organ, and it forms following a highly stereotypical morphogenesis program. It is well established that precise control at the transcript level is a key genetic underpinning of lung branching. In comparison, little is known about how regulation at the protein level may play a role. Ring finger and WD domain 2 (RFWD2, also termed COP1) is an E3 ubiquitin ligase that modifies specific target proteins, priming their degradation via the ubiquitin proteasome system. RFWD2 is known to function in the adult in pathogenic processes such as tumorigenesis. Here, we show that prenatal inactivation of Rfwd2 gene in the lung epithelium led to a striking halt in branching morphogenesis shortly after secondary branch formation. This defect is accompanied by distalization of the lung epithelium while growth and cellular differentiation still occurred. In the mutant lung, two E26 transformation-specific (ETS) transcription factors essential for normal lung branching, ETS translocation variant 4 (ETV4) and ETV5, were up-regulated at the protein level, but not at the transcript level. Introduction of Etv loss-of-function alleles into the Rfwd2 mutant background attenuated the branching phenotype, suggesting that RFWD2 functions, at least in part, through degrading ETV proteins. Because a number of E3 ligases are known to target factors important for lung development, our findings provide a preview of protein-level regulatory network essential for lung branching morphogenesis.

Intestinal knockout of Nedd4 enhances growth of Apcmin tumors

Nedd4 (Nedd4-1) is an E3 ubiquitin ligase that belongs to the HECT family and comprises a C2-WW(n)-HECT domain architecture. Although it has been reported to regulate growth factor receptors and cellular signaling, its role in cancer development has been controversial, with some studies proposing that it promotes cancer while others suggest it inhibits tumor growth. Here, we tested the effect of Nedd4 on intestinal tumor formation and growth using Nedd4-knockout mice (Nedd4 floxed (fl) mice crossed to villin-Cre mice). Although we find that knockout of Nedd4 on its own does not cause tumor growth, its knockout in the context of Apc^+/min-derived colorectal tumors leads to augmentation of tumor growth, suggesting that Nedd4 normally suppresses intestinal WNT signaling and growth of colonic tumors. WNT signaling microarray, immunoblotting and immunohistochemistry analyses of tumors derived from the Villin-Cre;Nedd4^fl/fl;Apc^+/min colons demonstrated elevated expression of the WNT upstream effectors LEF1 (full length) and YY1 in these tumors relative to control (Apc^+/min alone) tumors. Together, these results suggest that Nedd4 suppresses colonic WNT signaling and tumor growth, at least in part, by suppressing the transcription factors LEF1 and YY1.

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.

Targeting ubiquitination for cancer therapies

Ubiquitination, the structured degradation and turnover of cellular proteins, is regulated by the ubiquitin-proteasome system (UPS). Most proteins that are critical for cellular regulations and functions are targets of the process. Ubiquitination is comprised of a sequence of three enzymatic steps, and aberrations in the pathway can lead to tumor development and progression as observed in many cancer types. Recent evidence indicates that targeting the UPS is effective for certain cancer treatment, but many more potential targets might have been previously overlooked. In this review, we will discuss the current state of small molecules that target various elements of ubiquitination. Special attention will be given to novel inhibitors of E3 ubiquitin ligases, especially those in the SCF family.

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.

Heparan sulfation is essential for the prevention of cellular senescence

Cellular senescence is considered as an important tumor-suppressive mechanism. Here, we demonstrated that heparan sulfate (HS) prevents cellular senescence by fine-tuning of the fibroblast growth factor receptor (FGFR) signaling pathway. We found that depletion of 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2), a synthetic enzyme of the sulfur donor PAPS, led to premature cell senescence in various cancer cells and in a xenograft tumor mouse model. Sodium chlorate, a metabolic inhibitor of HS sulfation also induced a cellular senescence phenotype. p53 and p21 accumulation was essential for PAPSS2-mediated cellular senescence. Such senescence phenotypes were closely correlated with cell surface HS levels in both cancer cells and human diploid fibroblasts. The determination of the activation of receptors such as FGFR1, Met, and insulin growth factor 1 receptor β indicated that the augmented FGFR1/AKT signaling was specifically involved in premature senescence in a HS-dependent manner. Thus, blockade of either FGFR1 or AKT prohibited p53 and p21 accumulation and cell fate switched from cellular senescence to apoptosis. In particular, desulfation at the 2-O position in the HS chain contributed to the premature senescence via the augmented FGFR1 signaling. Taken together, we reveal, for the first time, that the proper status of HS is essential for the prevention of cellular senescence. These observations allowed us to hypothesize that the FGF/FGFR signaling system could initiate novel tumor defenses through regulating premature senescence.

SAG/RBX2 is a novel substrate of NEDD4-1 E3 ubiquitin ligase and mediates NEDD4-1 induced chemosensitization

Sensitive to apoptosis gene (SAG), also known as RBX2, ROC2, or RNF7, is a RING component of SCF E3 ubiquitin ligases, which regulates cellular functions through ubiquitylation and degradation of many protein substrates. Although our previous studies showed that SAG is transcriptionally induced by redox, mitogen and hypoxia via AP-1 and HIF-1, it is completely unknown whether and how SAG is ubiquitylated and degraded. Here we report that NEDD4-1, a HECT domain-containing E3 ubiquitin ligase, binds via its HECT domain directly with SAG's C-terminal RING domain and ubiquitylates SAG for proteasome-mediated degradation. Consistently, SAG protein half-life is shortened or extended by NEDD4-1 overexpression or silencing, respectively. We also found that SAG bridges NEDD4-1 via its C-terminus and CUL-5 via its N-terminus to form a NEDD4-1/SAG/CUL-5 tri-complex. Biologically, NEDD4-1 overexpression sensitizes cancer cells to etoposide-induced apoptosis by reducing SAG levels through targeted degradation. Thus, SAG is added to a growing list of NEDD4-1 substrates and mediates its biological function.

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.

Nedd4 haploinsufficient mice display moderate insulin resistance, enhanced lipolysis, and protection against high-fat diet-induced obesity

Neural precursor cell expressed developmentally down-regulated protein 4 (Nedd4) is the prototypical protein in the Nedd4 ubiquitin ligase (E3) family, which governs ubiquitin-dependent endocytosis and/or degradation of plasma membrane proteins. Loss of Nedd4 results in embryonic or neonatal lethality in mice and reduced insulin/IGF-1 signaling in embryonic fibroblasts. To delineate the roles of Nedd4 in vivo, we examined the phenotypes of heterozygous knockout mice using a high-fat diet-induced obesity (HFDIO) model. We observed that Nedd4+/- mice are moderately insulin resistant but paradoxically protected against HFDIO. After high-fat diet feeding, Nedd4+/- mice showed less body weight gain, less fat mass, and smaller adipocytes vs the wild type. Despite ameliorated HFDIO, Nedd4+/- mice did not manifest improvement in glucose tolerance vs the wild type in both genders. Nedd4+/- male, but not female, mice displayed significantly lower fasting blood glucose levels and serum insulin levels. Under obesogenic conditions, Nedd4+/- mice displayed elevated stimulated lipolytic activity, primarily through a β2-adrenergic receptor. Combined, these data support novel complex roles for Nedd4 in metabolic regulation involving altered insulin and β-adrenergic signaling pathways.

Nedd4 E3 ubiquitin ligase promotes cell proliferation and autophagy

OBJECTIVES

Nedd4 (neural precursor cell expressed developmentally down-regulated protein 4) is a member of the HECT E3 ubiquitin ligases, and is elevated in prostate, bladder and colorectal cancers, and promotes colonic cell population expansion. Up to now, molecular mechanisms of how Nedd4 functions, have not been well understood.

MATERIALS AND METHODS

In this study, shRNA was used to reduce expression of Nedd4 in the human prostate carcinoma cell line DU145. To analyse effects of Nedd4 on cell proliferation, MTT and colony formation assays were performed. DAPI staining and FACS analysis were used to investigate outcomes of Nedd4 activity, on apoptosis. Results of Nedd4 expression on lysosomal membrane permeabilization and autophagy were further investigated using acridine orange (AO) staining, immunofluorescence and western blot analysis.

RESULTS

We found that in HeLa cells, expression of Nedd4 promoted cell proliferation, whereas its knockdown inhibited colony formation and induced apoptosis in DU145 cells. Furthermore, down-regulation of Nedd4 in DU145 cells promoted lysosomal membrane permeabilization. We also found that down-regulation of Nedd4 inhibited autophagy in both DU145 and A549 cells. Investigation into mechanisms involved revealed that knockdown of endogenous Nedd4 expression notably increased activated mTOR (p-mTOR) levels, which suggests that mTOR signalling was involved in the Nedd4-mediated autophagy.

CONCLUSIONS

Our results indicate that expression of Nedd4 promoted cell proliferation and colony formation but prevented apoptosis. Moreover, Nedd4 promoted autophagy and was associated with the mTOR signalling pathway.

NEDD4: The founding member of a family of ubiquitin-protein ligases

Ubiquitination plays a crucial role in regulating proteins post-translationally. The focus of this review is on NEDD4, the founding member of the NEDD4 family of ubiquitin ligases that is evolutionarily conserved in eukaryotes. Many potential substrates of NEDD4 have been identified and NEDD4 has been shown to play a critical role in the regulation of a number of membrane receptors, endocytic machinery components and the tumour suppressor PTEN. In this review we will discuss the diverse pathways in which NEDD4 is involved, and the patho-physiological significance of this important ubiquitin ligase.

Regulation of ERBB3/HER3 signaling in cancer

ERBB3/HER3 is emerging as a molecular target for various cancers. HER3 is overexpressed and activated in a number of cancer types under the conditions of acquired resistance to other HER family therapeutic interventions such as tyrosine kinase inhibitors and antibody therapies. Regulation of the HER3 expression and signaling involves numerous HER3 interacting proteins. These proteins include PI3K, Shc, and E3 ubiquitin ligases NEDD4 and Nrdp1. Furthermore, recent identification of a number of HER3 oncogenic mutations in colon and gastric cancers elucidate the role of HER3 in cancer development. Despite the strong evidence regarding the role of HER3 in cancer, the current understanding of the regulation of HER3 expression and activation requires additional research. Moreover, the lack of biomarkers for HER3-driven cancer poses a big challenge for the clinical development of HER3 targeting antibodies. Therefore, a better understanding of HER3 regulation should improve the strategies to therapeutically target HER3 for cancer therapy.

Itch is required for lateral line development in zebrafish

The zebrafish posterior lateral line is formed during early development by the deposition of neuromasts from a migrating primordium. The molecular mechanisms regulating the regional organization and migration of the primordium involve interactions between Fgf and Wnt/β-catenin signaling and the establishment of specific cxcr4b and cxcr7b cytokine receptor expression domains. Itch has been identified as a regulator in several different signaling pathways, including Wnt and Cxcr4 signaling. We identified two homologous itch genes in zebrafish, itcha and itchb, with generalized expression patterns. By reducing itchb expression in particular upon morpholino knockdown, we demonstrated the importance of Itch in regulating lateral line development by perturbing the patterns of cxcr4b and cxcr7b expression. Itch knockdown results in a failure to down-regulate Wnt signaling and overexpression of cxcr4b in the primordium, slowing migration of the posterior lateral line primordium and resulting in abnormal development of the lateral line.

Systematic identification of barriers to human iPSC generation

Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) holds enormous promise for regenerative medicine. To elucidate endogenous barriers limiting this process, we systematically dissected human cellular reprogramming by combining a genome-wide RNAi screen, innovative computational methods, extensive single-hit validation, and mechanistic investigation of relevant pathways and networks. We identify reprogramming barriers, including genes involved in transcription, chromatin regulation, ubiquitination, dephosphorylation, vesicular transport, and cell adhesion. Specific a disintegrin and metalloproteinase (ADAM) proteins inhibit reprogramming, and the disintegrin domain of ADAM29 is necessary and sufficient for this function. Clathrin-mediated endocytosis can be targeted with small molecules and opposes reprogramming by positively regulating TGF-β signaling. Genetic interaction studies of endocytosis or ubiquitination reveal that barrier pathways can act in linear, parallel, or feedforward loop architectures to antagonize reprogramming. These results provide a global view of barriers to human cellular reprogramming.

Tyrosine phosphorylation of NEDD4 activates its ubiquitin ligase activity

Ligand binding to the receptor tyrosine kinase fibroblast growth factor (FGF) receptor 1 (FGFR1) causes dimerization and activation by transphosphorylation of tyrosine residues in the kinase domain. FGFR1 is ubiquitylated by the E3 ligase NEDD4 (also known as NEDD4-1), which promotes FGFR1 internalization and degradation. Although phosphorylation of FGFR1 is required for NEDD4-dependent endocytosis, NEDD4 directly binds to a nonphosphorylated region of FGFR1. We found that activation of FGFR1 led to activation of c-Src kinase-dependent tyrosine phosphorylation of NEDD4, enhancing the ubiquitin ligase activity of NEDD4. Using mass spectrometry, we identified several FGF-dependent phosphorylated tyrosines in NEDD4, including Tyr(43) in the C2 domain and Tyr(585) in the HECT domain. Mutating these tyrosines to phenylalanine to prevent phosphorylation inhibited FGF-dependent NEDD4 activity and FGFR1 endocytosis and enhanced cell proliferation. Mutating the tyrosines to glutamic acid to mimic phosphorylation enhanced NEDD4 activity. Moreover, the NEDD4 C2 domain bound the HECT domain, and the presence of phosphomimetic mutations inhibited this interaction, suggesting that phosphorylation of NEDD4 relieves an inhibitory intra- or intermolecular interaction. Accordingly, activation of FGFR1 was not required for activation of NEDD4 that lacked its C2 domain. Activation of c-Src by epidermal growth factor (EGF) also promoted tyrosine phosphorylation and enhanced the activity of NEDD4. Thus, we identified a feedback mechanism by which receptor tyrosine kinases promote catalytic activation of NEDD4 and that may represent a mechanism of receptor crosstalk.