Endosomes: a legitimate platform for the signaling train

Particulate allergens potentiate allergic asthma in mice through sustained IgE-mediated mast cell activation

Allergic asthma is characterized by airway hyperresponsiveness, inflammation, and a cellular infiltrate dominated by eosinophils. Numerous epidemiological studies have related the exacerbation of allergic asthma with an increase in ambient inhalable particulate matter from air pollutants. This is because inhalable particles efficiently deliver airborne allergens deep into the airways, where they can aggravate allergic asthma symptoms. However, the cellular mechanisms by which inhalable particulate allergens (pAgs) potentiate asthmatic symptoms remain unknown, in part because most in vivo and in vitro studies exploring the pathogenesis of allergic asthma use soluble allergens (sAgs). Using a mouse model of allergic asthma, we found that, compared with their sAg counterparts, pAgs triggered markedly heightened airway hyperresponsiveness and pulmonary eosinophilia in allergen-sensitized mice. Mast cells (MCs) were implicated in this divergent response, as the differences in airway inflammatory responses provoked by the physical nature of the allergens were attenuated in MC-deficient mice. The pAgs were found to mediate MC-dependent responses by enhancing retention of pAg/IgE/FcεRI complexes within lipid raft–enriched, CD63(+) endocytic compartments, which prolonged IgE/FcεRI-initiated signaling and resulted in heightened cytokine responses. These results reveal how the physical attributes of allergens can co-opt MC endocytic circuitry and signaling responses to aggravate pathological responses of allergic asthma in mice.

Full-length L1CAM and not its Δ2Δ27 splice variant promotes metastasis through induction of gelatinase expression

Tumour-specific splicing is known to contribute to cancer progression. In the case of the L1 cell adhesion molecule (L1CAM), which is expressed in many human tumours and often linked to bad prognosis, alternative splicing results in a full-length form (FL-L1CAM) and a splice variant lacking exons 2 and 27 (SV-L1CAM). It has not been elucidated so far whether SV-L1CAM, classically considered as tumour-associated, or whether FL-L1CAM is the metastasis-promoting isoform. Here, we show that both variants were expressed in human ovarian carcinoma and that exposure of tumour cells to pro-metastatic factors led to an exclusive increase of FL-L1CAM expression. Selective overexpression of one isoform in different tumour cells revealed that only FL-L1CAM promoted experimental lung and/or liver metastasis in mice. In addition, metastasis formation upon up-regulation of FL-L1CAM correlated with increased invasive potential and elevated Matrix metalloproteinase (MMP)-2 and -9 expression and activity in vitro as well as enhanced gelatinolytic activity in vivo. In conclusion, we identified FL-L1CAM as the metastasis-promoting isoform, thereby exemplifying that high expression of a so-called tumour-associated variant, here SV-L1CAM, is not per se equivalent to a decisive role of this isoform in tumour progression.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

The MARCH family E3 ubiquitin ligase K5 alters monocyte metabolism and proliferation through receptor tyrosine kinase modulation

Kaposi's sarcoma (KS) lesions are complex mixtures of KS-associated herpesvirus (KSHV)-infected spindle and inflammatory cells. In order to survive the host immune responses, KSHV encodes a number of immunomodulatory proteins, including the E3 ubiquitin ligase K5. In exploring the role of this viral protein in monocytes, we made the surprising discovery that in addition to a potential role in down regulation of immune responses, K5 also contributes to increased proliferation and alters cellular metabolism. This ubiquitin ligase increases aerobic glycolysis and lactate production through modulation of cellular growth factor-binding receptor tyrosine kinase endocytosis, increasing the sensitivity of cells to autocrine and paracrine factors. This leads to an altered pattern of cellular phosphorylation, increases in Akt activation and a longer duration of Erk1/2 phosphorylation. Overall, we believe this to be the first report of a virally-encoded ubiquitin ligase potentially contributing to oncogenesis through alterations in growth factor signaling cascades and opens a new avenue of research in K5 biology.

Tumor viruses have proven to be valuable tools for dissecting the molecular mechanisms of transformation and cancer progression. Kaposi's sarcoma-associated herpesvirus (KSHV) infection is essential in driving at least three different neoplasias, including Kaposi's sarcoma (KS). Our understanding, however, of the molecular mechanism of KSHV-driven tumor progression is still limited and requires further examination. In this manuscript we demonstrate that the K5 E3 ubiquitin ligase of KSHV is able to alter monocyte metabolism, driving increased glucose consumption and lactate production, hallmarks of virtually every cancer. It is able to accomplish this through a modulation of selected receptor tyrosine kinases, whose normal role is to bind pro-growth factors. Indeed, this alteration in metabolism is coupled with increases in monocyte proliferation. Our study provides insights into the mechanisms of KSHV-driven oncogenesis, as well as a new tool for exploring the link between metabolism and cancer.

The KEEP ON GOING protein of Arabidopsis recruits the ENHANCED DISEASE RESISTANCE1 protein to trans-Golgi network/early endosome vesicles

Loss-of-function mutations in the Arabidopsis (Arabidopsis thaliana) ENHANCED DISEASE RESISTANCE1 (EDR1) gene confer enhanced resistance to powdery mildew infection, enhanced senescence, and enhanced programmed cell death under both abiotic and biotic stress conditions. All edr1-mediated phenotypes can be suppressed by a specific missense mutation (keg-4) in the KEEP ON GOING (KEG) gene, which encodes a multidomain protein that includes a RING E3 ligase domain, a kinase domain, ankyrin repeats, and HERC2-like (for HECT and RCC1-like) repeats. The molecular and cellular mechanisms underlying this suppression are poorly understood. Using confocal laser scanning microscopy and fluorescent protein fusions, we determined that KEG localizes to trans-Golgi network/early endosome (TGN/EE) vesicles. Both the keg-4 mutation, which is located in the carboxyl-terminal HERC2-like repeats, and deletion of the entire HERC2-like repeats reduced endosomal localization of KEG and increased localization to the endoplasmic reticulum and cytosol, indicating that the HERC2-like repeats facilitate the TGN/EE targeting of KEG. EDR1 colocalized with KEG to the TGN/EE when coexpressed but localized primarily to the endoplasmic reticulum when expressed alone. Yeast two-hybrid and coimmunoprecipitation analyses revealed that EDR1 and KEG physically interact. Deletion of the HERC2-like repeats abolished the interaction between KEG and EDR1 as well as the KEG-induced TGN/EE localization of EDR1, indicating that the recruitment of EDR1 to the TGN/EE is based on a direct interaction between EDR1 and KEG mediated by the HERC2-like repeats. Collectively, these data suggest that EDR1 and KEG function together to regulate endocytic trafficking and/or the formation of signaling complexes on TGN/EE vesicles during stress responses.

A novel TIP30 protein complex regulates EGF receptor signaling and endocytic degradation

Activated epidermal growth factor receptor (EGFR) continues to signal in the early endosome, but how this signaling process is regulated is less well understood. Here we describe a protein complex consisting of TIP30, endophilin B1, and acyl-CoA synthetase long chain family member 4 (ACSL4) that interacts with Rab5a and regulates EGFR endocytosis and signaling. These proteins are required for the proper endocytic trafficking of EGF-EGFR. Knockdown of TIP30, ACSL4, endophilin B1, or Rab5a in human liver cancer cells or genetic knock-out of Tip30 in mouse primary hepatocytes results in the trapping of EGF-EGFR complexes in early endosomes, leading to delayed EGFR degradation and prolonged EGFR signaling. Furthermore, we show that Rab5a colocalizes with vacuolar (H(+))-ATPases (V-ATPases) on transport vesicles. The TIP30 complex facilitates trafficking of Rab5a and V-ATPases to EEA1-positive endosomes in response to EGF. Together, these results suggest that this TIP30 complex regulates EGFR endocytosis by facilitating the transport of V-ATPases from trans-Golgi network to early endosomes.

Plant stem cell signaling involves ligand-dependent trafficking of the CLAVATA1 receptor kinase

BACKGROUND

Cell numbers in above-ground meristems of plants are thought to be maintained by a feedback loop driven by perception of the glycopeptide ligand CLAVATA3 (CLV3) by the CLAVATA1 (CLV1) receptor kinase and the CLV2/CORYNE (CRN) receptor-like complex. CLV3 produced in the stem cells at the meristem apex limits the expression level of the stem cell-promoting homeodomain protein WUSCHEL (WUS) in the cells beneath, where CLV1 and WUS RNA are localized. WUS downregulation nonautonomously reduces stem cell proliferation. Overexpression of CLV3 eliminates the stem cells, causing meristem termination, and loss of CLV3 function allows meristem overproliferation. There are many questions regarding the CLV3/CLV1 interaction, including where in the meristem it occurs, how it is regulated, and how it is that a large range of CLV3 concentrations gives no meristem size phenotype.

RESULTS

Here we use genetics and live imaging to examine the cell biology of CLV1 in Arabidopsis meristematic tissue. We demonstrate that plasma membrane-localized CLV1 is reduced in concentration by CLV3, which causes trafficking of CLV1 to lytic vacuoles. We find that changes in CLV2 activity have no detectable effects on CLV1 levels. We also find that CLV3 appears to diffuse broadly in meristems, contrary to a recent sequestration model.

CONCLUSIONS

This study provides a new model for CLV1 function in plant stem cell maintenance and suggests that downregulation of plasma membrane-localized CLV1 by its CLV3 ligand can account for the buffering of CLV3 signaling in the maintenance of stem cell pools in plants.

Microvesicles as mediators of intercellular communication in cancer--the emerging science of cellular 'debris'

Cancer cells emit a heterogeneous mixture of vesicular, organelle-like structures (microvesicles, MVs) into their surroundings including blood and body fluids. MVs are generated via diverse biological mechanisms triggered by pathways involved in oncogenic transformation, microenvironmental stimulation, cellular activation, stress, or death. Vesiculation events occur either at the plasma membrane (ectosomes, shed vesicles) or within endosomal structures (exosomes). MVs are increasingly recognized as mediators of intercellular communication due to their capacity to merge with and transfer a repertoire of bioactive molecular content (cargo) to recipient cells. Such processes may occur both locally and systemically, contributing to the formation of microenvironmental fields and niches. The bioactive cargo of MVs may include growth factors and their receptors, proteases, adhesion molecules, signalling molecules, as well as DNA, mRNA, and microRNA (miRs) sequences. Tumour cells emit large quantities of MVs containing procoagulant, growth regulatory and oncogenic cargo (oncosomes), which can be transferred throughout the cancer cell population and to non-transformed stromal cells, endothelial cells and possibly to the inflammatory infiltrates (oncogenic field effect). These events likely impact tumour invasion, angiogenesis, metastasis, drug resistance, and cancer stem cell hierarchy. Ongoing studies explore the molecular mechanisms and mediators of MV-based intercellular communication (cancer vesiculome) with the hope of using this information as a possible source of therapeutic targets and disease biomarkers in cancer.

LRRK2 signaling pathways: the key to unlocking neurodegeneration?

Mutations in PARK8, encoding leucine-rich repeat kinase 2 (LRRK2), are a major cause of Parkinson's disease. We contrast data suggesting that changes in LRRK2 activity cause alterations in mitogen-activated protein kinase, translational control, tumor necrosis factor α/Fas ligand and Wnt signaling pathways with the cell biological functions of LRRK2 such as vesicle trafficking. Despite scarce in vivo data on cell signaling, involvement in diverse cell biological functions suggests a role for LRRK2 as an upstream regulator in events leading to neurodegeneration. To stimulate discussion and give direction for future research, we further suggest that despite the importance of the catalytic activity for cytotoxicity, the main cellular function of LRRK2 is linked to assembly of signaling complexes.

Decreased degradation of internalized follicle-stimulating hormone caused by mutation of aspartic acid 6.30(550) in a protein kinase-CK2 consensus sequence in the third intracellular loop of human follicle-stimulating hormone receptor

A naturally occurring mutation in follicle-stimulating hormone receptor (FSHR) gene has been reported: an amino acid change to glycine occurs at a conserved aspartic acid 550 (D550, D567, D6.30(567)). This residue is contained in a protein kinase-CK2 consensus site present in human FSHR (hFSHR) intracellular loop 3 (iL3). Because CK2 has been reported to play a role in trafficking of some receptors, the potential roles for CK2 and D550 in FSHR function were evaluated by generating a D550A mutation in the hFSHR. The hFSHR-D550A binds hormone similarly to WT-hFSHR when expressed in HEK293T cells. Western blot analyses showed lower levels of mature hFSHR-D550A. Maximal cAMP production of both hFSHR-D550A as well as the naturally occurring mutation hFSHR-D550G was diminished, but constitutive activity was not observed. Unexpectedly, when (125)I-hFSH bound to hFSHR-D550A or hFSHR-D550G, intracellular accumulation of radiolabeled FSH was observed. Both sucrose and dominant-negative dynamin blocked internalization of radiolabeled FSH and its commensurate intracellular accumulation. Accumulation of radiolabeled FSH in cells transfected with hFSHR-D550A is due to a defect in degradation of hFSH as measured in pulse chase studies, and confocal microscopy imaging revealed that FSH accumulated in large intracellular structures. CK2 kinase activity is not required for proper degradation of internalized FSH because inhibition of CK2 kinase activity in cells expressing hFSHR did not uncouple degradation of internalized radiolabeled FSH. Additionally, the CK2 consensus site in FSHR iL3 is not required for binding because CK2alpha coimmunoprecipitated with hFSHR-D550A. Thus, mutation of D550 uncouples the link between internalization and degradation of hFSH.

Tip30 deletion in MMTV-Neu mice leads to enhanced EGFR signaling and development of estrogen receptor-positive and progesterone receptor-negative mammary tumors

Estrogen receptor-positive and progesterone receptor-negative (ER+/PR-) breast cancers account for 15% to 25% of all human breast cancers and display more aggressive malignant characteristics than ER+/PR+ cancers. However, the molecular mechanism underlying development of ER+/PR- breast cancers still remains elusive. We show here that Tip30 deletion dramatically accelerated the onset of mammary tumors in the MMTV-Neu mouse model of breast cancer. The mammary tumors arising in Tip30(-/-)/MMTV-Neu mice were exclusively ER+/PR-. The growth of these ER+/PR- tumors depends not only on estrogen but also on progesterone despite the absence of detectable PR. Tip30 is predominantly expressed in ER+ mammary epithelial cells, and its deletion leads to an increase in the number of phospho-ERα-positive cells in mammary glands and accelerated activation of Akt in MMTV-Neu mice. Moreover, we found that Tip30 regulates the EGFR pathway through controlling endocytic downregulation of EGFR protein level and signaling. Together, these findings suggest a novel mechanism in which loss of Tip30 cooperates with Neu activation to enhance the activation of Akt signaling, leading to the development of ER+/PR- mammary tumors.

Endosomal signalling of epidermal growth factor receptors contributes to EGF-stimulated cell cycle progression in primary hepatocytes

Agonist-induced internalisation of receptors may lead to the formation of signalling endosomes. There is little evidence relating to whether this occurs to native receptors in non-transformed cells, and no previous studies asking whether this endosomal signalling can promote cell cycle progression in non-transformed cells. We investigated the hypothesis that in primary hepatocytes clathrin-dependent epidermal growth factor (EGF)-induced internalisation of the EGF receptor leads to signalling from endosomal EGF-EGF receptor complexes which may support EGF-stimulated cell cycle progression. We used EGF-stimulation of rat hepatocytes followed by confocal microscopy, and Western blots for phosphoproteins. [(3)H]thymidine incorporation into DNA was used as a indicator of progression to S-phase. Confocal microscopy demonstrated co-internalisation of EGF, EGF receptors and transferrin into endosomes. Internalisation of EGF/EGF receptor/transferrin was blocked by expression of dominant-negative dynamin, but not by the tyrosine kinase inhibitor AG 1478. Dominant-negative dynamin expression reduced EGF-stimulated extracellular signal-related kinase and Akt signalling, but increased tyrosine phosphorylated EGF receptor. EGF-stimulated cell cycle progression requires stimulation of EGF receptors during an initial period (e.g. 1h) and also later during a 24h incubation. EGF receptor internalisation in the presence of AG 1478 followed by removal of the inhibitor resulted in signalling from internalised EGF receptors that is sufficient for the initial stimulation to provide progression to S-phase of the cell cycle. These observations on hepatocytes characterise, for the first time in non-transformed cells, endosomal signalling from internalised EGF receptors, and provide evidence that this endosomal signalling may support the early phase of EGF-stimulated cell cycle progression.

Gα13 and Rho mediate endosomal trafficking of CXCR4 into Rab11+ vesicles upon stromal cell-derived factor-1 stimulation

CXCR4, like other G protein-coupled receptors, signals via heterotrimeric guanine nucleotide-binding proteins (G proteins) to regulate gene transcription, migration, development, growth, and transformation. We describe a formerly uncharacterized function of a G protein: a role in receptor trafficking. We previously showed that CXCR4 and the TCR physically associate and form a heterodimer upon stromal cell-derived factor-1 or CXCL12 (SDF-1) stimulation in human T cells to prolong ERK activation and, thereby, lead to gene upregulation and cytokine secretion. The CXCR4-TCR heterodimers occur on the cell surface and in an intracellular compartment in response to SDF-1. Neither the intracellular compartment to which the CXCR4-TCR heterodimers localize nor the mechanism for localization has been elucidated. In this article, we characterize molecular mechanisms required for postendocytic trafficking of CXCR4. Upon SDF-1 stimulation, CXCR4 localizes to Rab11(+) vesicles, a recycling compartment near the microtubule organizing center and Golgi apparatus. This trafficking requires the CXCR4 C-terminal tail domain but not the CXCR4 ubiquitination sites. The TCR also constitutively localizes to this Rab11(+) compartment. Trafficking of CXCR4 into the Rab11(+), TCR-containing endosomes requires actin polymerization. Furthermore, inhibiting Rho activation or depleting Gα13 prevented trafficking of CXCR4 into the Rab11(+) endosomes without hindering the ability of CXCR4 to endocytose. These results indicated that, upon SDF-1 treatment, Gα13 and Rho mediate the actin polymerization necessary for trafficking CXCR4 into the Rab11(+), recycling endosomal compartment, which also contains constitutively recycling TCR and, thus, CXCR4-TCR heterodimers. To our knowledge, this is the first report of Gα13 as a mediator of receptor trafficking.

Imaging T-cell receptor activation reveals accumulation of tyrosine-phosphorylated CD3ζ in the endosomal compartment

Phosphorylation of the T-cell receptor complex (TcR/CD3) mediates the survival and antigen-induced activation of T cells. TcR/CD3 phosphorylation is usually monitored using phospho-specific antibodies, which precludes dynamic measurements. Here, we have developed genetically encoded, live-cell reporters that enable simultaneous monitoring of the phosphorylation state and intracellular trafficking of CD3ζ, the major signal-transducing subunit of the TcR/CD3. We show that these reporters provide accurate readouts of TcR/CD3 phosphorylation and are sensitive to the local balance of kinase and phosphatase activities acting upon TcR/CD3. Using these reporters, we demonstrate that, in addition to the expected activation-dependent phosphorylation at the plasma membrane, tyrosine-phosphorylated CD3ζ accumulates on endosomal vesicles distinct from lysosomes. These results suggest that an intracellular pool of phosphorylated CD3ζ may help to sustain TcR/CD3 signaling after the receptor internalization.

Endocytosis in plant-microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant-microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.

Neurokinin-1 receptor: functional significance in the immune system in reference to selected infections and inflammation

The G protein-coupled receptor (GPCR), neurokinin-1 receptor (NK1R), and its preferred ligand, substance P (SP), are reviewed in relationship to the immune system and selected infections. NK1R and SP are ubiquitous throughout the animal kingdom. This important pathway has unique functions in numerous cells and tissues. The interaction of SP with its preferred receptor, NK1R, leads to the activation of nuclear factor-kappa B (NF-κB) and proinflammatory cytokines. NK1R has two isoforms, both a full-length and a truncated form. These isoforms have different functional significances and differ in cell signaling capability. The proinflammatory signals modulated by SP are important in bacterial, viral, fungal, and parasitic diseases, as well as in immune system function. The SP-NK1R system is a major class 1, rhodopsin-like GPCR ligand-receptor interaction.

Early endosomal antigen 1 (EEA1) is an obligate scaffold for angiotensin II-induced, PKC-alpha-dependent Akt activation in endosomes

Akt/protein kinase B (PKB) activation/phosphorylation by angiotensin II (Ang II) is a critical signaling event in hypertrophy of vascular smooth muscle cells (VSMCs). Conventional wisdom asserts that Akt activation occurs mainly in plasma membrane domains. Recent evidence that Akt activation may take place within intracellular compartments challenges this dogma. The spatial identity and mechanistic features of these putative signaling domains have not been defined. Using cell fractionation and fluorescence methods, we demonstrate that the early endosomal antigen-1 (EEA1)-positive endosomes are a major site of Ang II-induced Akt activation. Akt moves to and is activated in EEA1 endosomes. The expression of EEA1 is required for phosphorylation of Akt at both Thr-308 and Ser-473 as well as for phosphorylation of its downstream targets mTOR and S6 kinase, but not for Erk1/2 activation. Both Akt and phosphorylated Akt (p-Akt) interact with EEA1. We also found that PKC-α is required for organizing Ang II-induced, EEA1-dependent Akt phosphorylation in VSMC early endosomes. EEA1 expression enables PKC-α phosphorylation, which in turn regulates Akt upstream signaling kinases, PDK1 and p38 MAPK. Our results indicate that PKC-α is a necessary regulator of EEA1-dependent Akt signaling in early endosomes. Finally, EEA1 down-regulation or expression of a dominant negative mutant of PKC-α blunts Ang II-induced leucine incorporation in VSMCs. Thus, EEA1 serves a novel function as an obligate scaffold for Ang II-induced Akt activation in early endosomes.

Proteomics and the dynamic plasma membrane: Quo Vadis?

Quo Vadis: where are you going? Advances in MS-based proteomics have enabled research to move from obtaining the basic protein inventory of cells and organelles to the ability of monitoring their dynamics, including changes in abundance, location and various PTMs. In this respect, the cellular plasma membrane is of particular interest, by not only serving as a barrier between the "cell interior" and the external environment, but moreover by organizing and clustering essential components to enable dynamic responses to internal and external stimuli. Defining and characterizing the dynamic plasma membrane proteome is crucial for understanding fundamental biological processes, disease mechanisms and for finding drug targets. Protein identification, characterization of dynamic PTMs and protein-ligand interactions, and determination of transient changes in protein expression and composition are among the challenges in functional proteomic studies of the plasma membrane. We review the recent progress in MS-based plasma membrane proteomics by presenting key examples from eukaryotic systems, including mammals, yeast and plants. We highlight the importance of enrichment and quantification technologies required for detailed functional and comparative analysis of the dynamic plasma membrane proteome.

Role of ubiquitination in endocytic trafficking of G-protein-coupled receptors

Lysyl ubiquitination has long been known to target cytoplasmic proteins for proteasomal degradation, and there is now extensive evidence that ubiquitination functions in vacuolar/lysosomal targeting of membrane proteins from both the biosynthetic and endocytic pathways. G-protein-coupled receptors (GPCRs) represent the largest and most diverse family of membrane proteins, whose function is of fundamental importance both physiologically and therapeutically. In this review, we discuss the role of ubiquitination in the vacuolar/lysosomal downregulation of GPCRs through the endocytic pathway, with a primary focus on lysosomal trafficking in mammalian cells. We will summarize evidence indicating that mammalian GPCRs are regulated by ubiquitin-dependent mechanisms conserved in budding yeast, and then consider evidence for additional ubiquitin-dependent and -independent regulation that may be specific to animal cells.

Kinetics of G-protein-coupled receptor endosomal trafficking pathways revealed by single quantum dots

G-protein-coupled receptors (GPCRs) are the largest protein superfamily in the human genome; they comprise 30% of current drug targets and regulate diverse cellular signaling responses. The role of endosomal trafficking in GPCR signaling regulation is gaining substantial consideration. However, this process remains difficult to study due to the inability to distinguish among many individual receptors, simultaneously trafficking within multiple endosomal pathways. Here we show accurate measurement of the internalization and endosomal trafficking of single groups of serotonin (5-hydroxytryptamine, 5-HT) receptors using single quantum dot (QD) probes and quantitative colocalization. We demonstrate that the presence of a QD tag does not interfere with 5-HT receptor internalization or endosomal recycling. Direct measurements show simultaneous trafficking of the 5-HT1A receptor in two distinct endosomal recycling pathways. Single-molecule imaging of endosomal trafficking will significantly impact the understanding of cellular signaling and provide powerful tools to elucidate the actions of GPCR-targeted therapeutics.

PTPD1 supports receptor stability and mitogenic signaling in bladder cancer cells

PTPD1, a cytosolic non-receptor protein-tyrosine phosphatase, stimulates the Src-EGF transduction pathway. Localization of PTPD1 at actin cytoskeleton and adhesion sites is required for cell scattering and migration. Here, we show that during EGF stimulation, PTPD1 is rapidly recruited to endocytic vesicles containing the EGF receptor. Endosomal localization of PTPD1 is mediated by interaction with KIF16B, an endosomal kinesin that modulates receptor recycling at the plasma membrane. Silencing of PTPD1 promotes degradation of EGF receptor and inhibits downstream ERK signaling. We also found that PTPD1 is markedly increased in bladder cancer tissue samples. PTPD1 levels positively correlated with the grading and invasiveness potential of these tumors. Transgenic expression of an inactive PTPD1 mutant or genetic knockdown of the endogenous PTPD1 severely inhibited both growth and motility of human bladder cancer cells. These findings identify PTPD1 as a novel component of the endocytic machinery that impacts on EGF receptor stability and on growth and motility of bladder cancer cells.

Endosomal signaling and a novel pathway defined by the natural killer receptor KIR2DL4 (CD158d)

In addition to ligand-induced activation of receptors at the cell surface, certain internalized receptor-ligand complexes are activated in endosomes which are, now recognized as important intracellular platforms of signal transduction. The major receptor families that signal from endosomes and illustrate the diversity and complexity of endosomal signaling include receptor tyrosine kinases (RTKs), G-protein-coupled receptors (GPCRs) and toll-like receptors (TLRs). Natural killer (NK) cells, an important component of the innate immune system, not only provide a rapid defense against foreign invaders, such as bacteria and viruses, but also positively shape local responses by cytokine and chemokine secretion. The NK cell receptor KIR2DL4 (CD158d) utilizes a new mode of endosomal signaling after binding its ligand, soluble HLA-G, in the extracellular milieu. Internalization of the receptor and its ligand into endosomes and initiation of signaling at this site result in a proinflammatory and proangiogenic response with important functions at sites of ligand expression, such as at the maternal-fetal interface during early pregnancy. After a brief overview of the modes of endosomal signaling and its value in generating distinct physiological responses, this review will highlight the mechanism and physiological significance of a novel intracellular signaling pathway used by the endosome-resident immune receptor KIR2DL4.

Cytoskeletal cross-talk in the control of T cell antigen receptor signaling

T cell antigen receptor signaling is triggered and controlled in specialized cellular interfaces formed between T cells and antigen-presenting cells named immunological synapses. Both microtubules and actin cytoskeleton rearrange at the immunological synapse in response to T cell receptor triggering, ensuring in turn the accuracy of intracellular signaling. Recent reports show that the cross-talk between the cortical actin cytoskeleton and microtubule networks is key for structuring the immunological synapse and for controlling T cell receptor signaling. Immunological synapse architecture and the interaction between the signaling machinery and various cytoskeletal elements are therefore crucial for the fine-tuning of T cell signaling.

Control of RNA silencing and localization by endolysosomes

Recent advances in the cell biology of RNA silencing have unraveled an intriguing association of post-transcriptionally regulated RNA with endolysosomal membranes in several circumstances of mRNA localization, microRNA activity and viral RNA transport and packaging. Endolysosomal membranes are a nexus of communication and transport between cells and their exterior environment for signaling receptors, pathogens and nutrients. Here, we discuss recent data that support a view that endolysosomal positioning of RNA might facilitate intercellular transmission of RNA and host defence against viruses and retrotransposons. Positioning of RNA regulatory mechanisms on endolysosomal membranes might permit rapid and localized control of microRNA (miRNA) gene regulatory programs and mRNA translation in response to environmental signals, such as activated plasma membrane receptors transported on endosomes. Finally, we suggest that the pathology of several conditions, including Huntington's disease, might be a consequence of the disruption of the control of RNA via endolysosomal membranes.

A G{alpha}i-GIV molecular complex binds epidermal growth factor receptor and determines whether cells migrate or proliferate

Migrating cells do not proliferate and vice versa, but the mechanism involved remains unknown. Ghosh et al. reveal how this cellular decision is made by showing that a Gα_i–GIV molecular complex interacts with EGF receptor and programs growth factor signaling, triggering migration when assembled and favoring mitosis when assembly is prevented.

Cells respond to growth factors by either migrating or proliferating, but not both at the same time, a phenomenon termed migration-proliferation dichotomy. The underlying mechanism of this phenomenon has remained unknown. We demonstrate here that Gα_i protein and GIV, its nonreceptor guanine nucleotide exchange factor (GEF), program EGF receptor (EGFR) signaling and orchestrate this dichotomy. GIV directly interacts with EGFR, and when its GEF function is intact, a Gα_i–GIV–EGFR signaling complex assembles, EGFR autophosphorylation is enhanced, and the receptor's association with the plasma membrane (PM) is prolonged. Accordingly, PM-based motogenic signals (PI3-kinase-Akt and PLCγ1) are amplified, and cell migration is triggered. In cells expressing a GEF-deficient mutant, the Gαi–GIV-EGFR signaling complex is not assembled, EGFR autophosphorylation is reduced, the receptor's association with endosomes is prolonged, mitogenic signals (ERK 1/2, Src, and STAT5) are amplified, and cell proliferation is triggered. In rapidly growing, poorly motile breast and colon cancer cells and in noninvasive colorectal carcinomas in situ in which EGFR signaling favors mitosis over motility, a GEF-deficient splice variant of GIV was identified. In slow growing, highly motile cancer cells and late invasive carcinomas, GIV is highly expressed and has an intact GEF motif. Thus, inclusion or exclusion of GIV's GEF motif, which activates Gαi, modulates EGFR signaling, generates migration-proliferation dichotomy, and most likely influences cancer progression.

Opioid receptor internalization contributes to dermorphin-mediated antinociception

Microinjection of opioids into the ventrolateral periaqueductal gray (vlPAG) produces antinociception in part by binding to mu-opioid receptors (MOPrs). Although both high and low efficacy agonists produce antinociception, low efficacy agonists such as morphine produce limited MOPr internalization suggesting that MOPr internalization and signaling leading to antinociception are independent. This hypothesis was tested in awake, behaving rats using DERM-A594, a fluorescently labeled dermorphin analog, and internalization blockers. Microinjection of DERM-A594 into the vlPAG produced both antinociception and internalization of DERM-A594. Administration of the irreversible opioid receptor antagonist beta-chlornaltrexamine (beta-CNA) prior to DERM-A594 microinjection reduced both the antinociceptive effect and the number of DERM-A594 labeled cells demonstrating that both effects are opioid receptor-mediated. Pretreatment with the internalization blockers dynamin dominant-negative inhibitory peptide (dynamin-DN) and concanavalinA (ConA) attenuated both DERM-A594 internalization and antinociception. Microinjection of dynamin-DN and ConA also decreased the antinociceptive potency of the unlabeled opioid agonist dermorphin when microinjected into the vlPAG as demonstrated by rightward shifts in the dose-response curves. In contrast, administration of dynamin-DN had no effect on the antinociceptive effect of microinjecting the GABA(A) receptor antagonist bicuculline into the vlPAG. The finding that dermorphin-induced antinociception is attenuated by blocking receptor internalization indicates that key parts of opioid receptor-mediated signaling depend on internalization.

Cellular signalling: Peptide hormones and growth factors

Peptide hormones and growth factors initiate signalling by binding to and activating their cell surface receptors. The activated receptors interact with and modulate the activity of cell surface enzymes and adaptor proteins which entrain a series of reactions leading to metabolic and proliferative signals. Rapid internalization of ligand-receptor complexes into the endosomal system both prolongs and augments events initiated at the cell surface. In addition endocytosis brings activated receptors into contact with a wider range of substrates giving rise to unique signalling events critical for modulating proliferation and apoptosis. Within the endosomal system, receptor function is regulated by lowering vacuolar pH, augmenting ligand proteolysis and promoting receptor kinase dephosphorylation. Ubiquitination-deubiquitination plays a key role in regulating receptor traffic through the endosomal system resulting in either recycling to the cell surface or degradation in multivesicular-lysosomal elements. From a clinical perspective there are several studies showing that manipulating endosomal processes may constitute a new therapeutic strategy.

Thiazolidinediones induce Rab7-RILP-MAPK-dependent juxtanuclear lysosome aggregation and reduce tumor cell invasion

Acidic extracellular pH (pHe) has been shown to stimulate peripheral lysosome trafficking, resulting in cathepsin B secretion and tumor invasion. In addition, inhibitors of sodium-proton exchangers (NHE) such as EIPA, cariporide and s3226, as well as the non-specific NHE inhibitor, troglitazone (Tro), blocked these changes. In this paper, we report a differential ability of the thiazolidinedione (TZD) family of compounds to induce a time-dependent retrograde aggregation of lysosomes over the microtubule-organizing center (MTOC) in tumor cells exposed to acidic pHe. This trafficking event depended on microtubules and the MAP-Kinase pathway, but was independent of Rho GTPase activity. Expression of shRNA implicated Rab7 in this process, and subcellular fractionation revealed that levels of Rab7, RILP and Erk1/2 were increased on lysosomes purified from cells treated with Tro. In addition, DN-RILP overexpression studies indicated that this Rab7 effector also played a role in TZD-induced retrograde trafficking. Tro was able to prevent acidic pHe-induced cell invasion. Finally, DU145 prostate tumor cells stably over-expressing WT-RILP, a condition where lysosomes aggregate to the MTOC in the absence of Tro, did not invade in response to acidic pHe, suggesting that the regulation of lysosome trafficking is an inherently important aspect of tumor cell invasion.