Receptor endocytosis via ubiquitin-dependent and -independent pathways

UBE2S promotes malignant properties via VHL/HIF-1α and VHL/JAK2/STAT3 signaling pathways and decreases sensitivity to sorafenib in hepatocellular carcinoma

BACKGROUND

Ubiquitin-conjugating enzyme E2S (UBE2S), an E2 enzyme, is associated with the development of various tumors and exerts oncogenic activities. UBE2S is overexpressed in tumors, including hepatocellular carcinoma (HCC). However, the key molecular mechanisms of UBE2S in HCC still need additional research. The aim of this study was to explore the role of UBE2S in HCC.

METHODS

The expression levels of UBE2S in HCC tissues and cells were detected by western blot analysis, quantitative real-time polymerase chain reaction analysis (qRT-PCR), and immunohistochemistry (IHC). A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, wound healing assay, colony formation assay transwell assay, and animal models were used to detect the proliferation and migration ability of HCC cells. Western blot analysis, qRT-PCR, immunofluorescence, small-interfering RNA (siRNA), and plasmid transfection and coimmunoprecipitation (Co-IP) assays were performed to detect the interaction among UBE2S, von Hippel-Lindau (VHL), hypoxia-inducible factor 1-alpha (HIF-1α), Janus kinase-2 (JAK2), and signal transducer and activator of transcription 3 (STAT3).

RESULTS

In this study, we found that high UBE2S expression was associated with poor prognosis in HCC patients. In addition, UBE2S expression was upregulated in HCC tissues and cell lines. Knockdown of UBE2S inhibited the proliferation and migration of HCC cells in vitro and in vivo by directly interacting with VHL to downregulate the HIF-1α and JAK2/STAT3 signaling pathways. Accordingly, overexpression of UBE2S significantly enhanced the proliferation and migration of HCC cells in vitro via VHL to upregulate HIF-1α and JAK2/STAT3 signaling pathways. Furthermore, we found that downregulation of UBE2S expression enhanced the sensitivity of HCC cells to sorafenib in vivo and in vitro.

CONCLUSION

UBE2S enhances malignant properties via the VHL/HIF-1α and VHL/JAK2/STAT3 signaling pathways and reduces sensitivity to sorafenib in HCC. The findings of this study may open a new approach for HCC diagnosis and provide a potential option for the treatment of HCC.

Ubiquitin and its relatives as wizards of the endolysosomal system

The endolysosomal system comprises a dynamic constellation of vesicles working together to sense and interpret environmental cues and facilitate homeostasis. Integrating extracellular information with the internal affairs of the cell requires endosomes and lysosomes to be proficient in decision-making: fusion or fission; recycling or degradation; fast transport or contacts with other organelles. To effectively discriminate between these options, the endolysosomal system employs complex regulatory strategies that crucially rely on reversible post-translational modifications (PTMs) with ubiquitin (Ub) and ubiquitin-like (Ubl) proteins. The cycle of conjugation, recognition and removal of different Ub- and Ubl-modified states informs cellular protein stability and behavior at spatial and temporal resolution and is thus well suited to finetune macromolecular complex assembly and function on endolysosomal membranes. Here, we discuss how ubiquitylation (also known as ubiquitination) and its biochemical relatives orchestrate endocytic traffic and designate cargo fate, influence membrane identity transitions and support formation of membrane contact sites (MCSs). Finally, we explore the opportunistic hijacking of Ub and Ubl modification cascades by intracellular bacteria that remodel host trafficking pathways to invade and prosper inside cells.

The ubiquitin proteasome system and schizophrenia

The ubiquitin-proteasome system is a master regulator of neural development and the maintenance of brain structure and function. It influences neurogenesis, synaptogenesis, and neurotransmission by determining the localisation, interaction, and turnover of scaffolding, presynaptic, and postsynaptic proteins. Moreover, ubiquitin-proteasome system signalling transduces epigenetic changes in neurons independently of protein degradation and, as such, dysfunction of components and substrates of this system has been linked to a broad range of brain conditions. Although links between ubiquitin-proteasome system dysfunction and neurodegenerative disorders have been known for some time, only recently have similar links emerged for neurodevelopmental disorders, such as schizophrenia. Here, we review the components of the ubiquitin-proteasome system that are reported to be dysregulated in schizophrenia, and discuss specific molecular changes to these components that might, in part, explain the complex causes of this mental disorder.

Down-regulation of platelet adhesion receptors is a controlling mechanism of thrombosis, while also affecting post-transfusion efficacy of stored platelets

Physiologically, upon platelet activation, uncontrolled propagation of thrombosis is prevented by regulating mechanisms which affect the expression and function of either platelet adhesion receptors or integrins. Receptor ectodomain shedding is an elective mechanism which is mainly involved in down-regulation of adhesion receptors GPIbα and GPVI. Platelet integrin α_IIbβ₃ can also be modulated with a calpain-dependent proteolytic cleavage. In addition, activating signals may induce the internalization of expressed receptors to selectively down-regulate their intensity. Alternatively, further activation of platelets is associated with microvesiculation as a none-selective mechanism which leads to the loss of membrane- bearing receptors. In a non-physiological condition, the storage of therapeutic platelets has also shown to be associated with the unwilling activation of platelets which triggers receptors down-regulation via aforementioned different mechanisms. Notably, herein the changes are time-dependent and not controllable. While the expression and shedding of pro-inflammatory molecules can induce post-transfusion adverse effects, stored-dependent loss of adhesion receptors by ectodomain shedding or microvesiculation may attenuate post-transfusion adhesive functions of platelets causing their premature clearance from circulation. In its first part, the review presented here aims to describe the mechanisms involved in down-regulation of platelet adhesion receptors. It then highlights the crucial role of ectodomain shedding and microvesiculation in the propagation of "platelet storage lesion" which may affect the post-transfusion efficacy of platelet components.

Role of Endolysosomes in Skeletal Muscle Pathology Observed in a Cholesterol-Fed Rabbit Model of Alzheimer's Disease

Deficits in skeletal muscles contribute not only to the functional decline in people living with Alzheimer’s disease (AD), but also to AD pathogenesis. We have shown that endolysosome dysfunction plays an important role in the development of AD pathological features in a cholesterol-fed rabbit model of AD. Interestingly we observed in skeletal muscle from the rabbit AD model increased deposition of Aβ, phosphorylated tau, and ubiquitin. Here, we tested the hypothesis that endolysosome dysfunction commonly occurs in skeletal muscle and brain in this rabbit model of AD. In skeletal muscle of rabbits fed a 2% cholesterol-enriched diet for 12 weeks we observed the presence of abnormally enlarged endolysosomes, in which were increased accumulations of free cholesterol and multiple AD marker proteins subject to misfolding and aggregation including Aβ, phosphorylated tau, and ubiquitin. Moreover, in skeletal muscle of rabbits fed the cholesterol-enriched diet we observed decreased specific activities of three different lysosome enzymes. Our results suggest that elevated levels of plasma cholesterol can disturb endolysosome structure and function as well as promote the development of AD-like pathological features in skeletal muscle and that these organellar changes might contribute to the development of skeletal muscle deficits in AD.

Neuroprotective peptides fused to arginine-rich cell penetrating peptides: Neuroprotective mechanism likely mediated by peptide endocytic properties

Several recent studies have demonstrated that TAT and other arginine-rich cell penetrating peptides (CPPs) have intrinsic neuroprotective properties in their own right. Examples, we have demonstrated that in addition to TAT, poly-arginine peptides (R8 to R18; containing 8-18 arginine residues) as well as some other arginine-rich peptides are neuroprotective in vitro (in neurons exposed to glutamic acid excitotoxicity and oxygen glucose deprivation) and in the case of R9 in vivo (after permanent middle cerebral artery occlusion in the rat). Based on several lines of evidence, we propose that this neuroprotection is related to the peptide's endocytosis-inducing properties, with peptide charge and arginine residues being critical factors. Specifically, we propose that during peptide endocytosis neuronal cell surface structures such as ion channels and transporters are internalised, thereby reducing calcium influx associated with excitotoxicity and other receptor-mediated neurodamaging signalling pathways. We also hypothesise that a peptide cargo can act synergistically with TAT and other arginine-rich CPPs due to potentiation of the CPPs endocytic traits rather than by the cargo-peptide acting directly on its supposedly intended intracellular target. In this review, we systematically consider a number of studies that have used CPPs to deliver neuroprotective peptides to the central nervous system (CNS) following stroke and other neurological disorders. Consequently, we critically review evidence that supports our hypothesis that neuroprotection is mediated by carrier peptide endocytosis. In conclusion, we believe that there are strong grounds to regard arginine-rich peptides as a new class of neuroprotective molecules for the treatment of a range of neurological disorders.

The role of ubiquitylation and degradation in RhoGTPase signalling

Rho-like guanosine triphosphatases (RhoGTPases) control many aspects of cellular physiology through their effects on the actin cytoskeleton and on gene transcription. Signalling by RhoGTPases is tightly coordinated and requires a series of regulatory proteins, including guanine-nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and guanine-nucleotide dissociation inhibitors (GDIs). GEFs and GAPs regulate GTPase cycling between the active (GTP-bound) and inactive (GDP-bound) states, whereas GDI is a cytosolic chaperone that binds inactive RhoGTPases. Like many other proteins, RhoGTPases are subject to degradation following the covalent conjugation of ubiquitin. There have been increasing indications that ubiquitylation of small GTPases occurs in a regulated fashion, primarily upon activation, and is an important means to control signalling output. Recent work has identified cellular proteins that control RasGTPase and RhoGTPase ubiquitylation and degradation, allowing us to amend the canonical model for GTPase (in)activation. Moreover, accumulating evidence for indirect regulation of GTPase function through the ubiquitylation of GTPase regulators makes this post-translational modification a key feature of GTPase-dependent signalling pathways. Here, we will discuss these recent insights into the regulation of RhoGTPase ubiquitylation and their relevance for cell signalling.

Facilitation of μ-opioid receptor activity by preventing δ-opioid receptor-mediated codegradation

δ-opioid receptors (DORs) form heteromers with μ-opioid receptors (MORs) and negatively regulate MOR-mediated spinal analgesia. However, the underlying mechanism remains largely unclear. The present study shows that the activity of MORs can be enhanced by preventing MORs from DOR-mediated codegradation. Treatment with DOR-specific agonists led to endocytosis of both DORs and MORs. These receptors were further processed for ubiquitination and lysosomal degradation, resulting in a reduction of surface MORs. Such effects were attenuated by treatment with an interfering peptide containing the first transmembrane domain of MOR (MOR(TM1)), which interacted with DORs and disrupted the MOR/DOR interaction. Furthermore, the systemically applied fusion protein consisting of MOR(TM1) and TAT at the C terminus could disrupt the MOR/DOR interaction in the mouse spinal cord, enhance the morphine analgesia, and reduce the antinociceptive tolerance to morphine. Thus, dissociation of MORs from DORs in the cell membrane is a potential strategy to improve opioid analgesic therapies.

Deregulation of Rab5 and Rab4 proteins in p85R274A-expressing cells alters PDGFR trafficking

Activated receptor tyrosine kinases recruit many signaling proteins to activate downstream cell proliferation and survival pathways, including phosphatidylinositol 3-kinase (PI3K) consisting of a p85 regulatory protein and a p110 catalytic protein. We have recently shown the p85alpha protein also has in vitro GTPase activating protein (GAP) activity towards Rab5 and Rab4, small GTPases that regulate vesicle trafficking events for activated receptors. Expression of a GAP-defective mutant, p85R274A, resulted in sustained levels of activated platelet-derived growth factor receptors (PDGFRs) and enhanced downstream signaling. In this report we have characterized Rab5- and Rab4-mediated PDGFR trafficking in cells expressing wild type p85 and GAP-defective mutant p85R274A. Wild type p85 overexpressing cells had slower PDGFR trafficking consistent with enhanced GAP activity deactivating Rab5 and Rab4 to block their vesicle trafficking functions. Mutant p85R274A expression increased the internalization rate of PDGFRs, a Rab5-dependent process, without preventing PDGFR ubiquitination. Immunofluorescence studies further demonstrated that p85R274A-expressing cells showed Rab5 accumulation at intracellular locations. Pull-down and FRAP (fluorescence recovery after photobleaching) experiments indicate this is likely membrane-associated Rab5-GTP, sustained due to decreased p85 GAP activity for the p85R274A mutant. These cells also had substantial amounts of activated PDGFRs in Rab4-positive recycling endosomes, a compartment that usually contains primarily deactivated/dephosphorylated receptors. Our results suggest that the PDGFR-associated GAP activity of p85 regulates both Rab5 and Rab4 functions in cells to influence the movement of activated PDGFR through endosomal compartments. Disruption of this regulation by p85R274A expression impacts PDGFR phosphorylation/dephosphorylation, degradation kinetics and downstream signaling by altering the time receptors spend in specific intracellular endosomal compartments. These results demonstrate that the p85alpha protein is an important regulator of Rab-mediated PDGFR trafficking, which significantly impacts receptor signaling and degradation.

AMSH interacts with ESCRT-0 to regulate the stability and trafficking of CXCR4

Reversible ubiquitination is essential for the endocytic sorting and down-regulation of G protein-coupled receptors, such as the chemokine receptor CXCR4. The deubiquitinating enzyme AMSH has been implicated in the endocytic sorting of both G protein-coupled receptors and receptor-tyrosine kinases. Herein, we examine the role of AMSH in the regulation of CXCR4 stability and trafficking and characterize protein-protein interactions critical for this function. Loss of AMSH catalytic activity or depletion by RNAi results in increased steady-state levels of CXCR4 under basal conditions. Analysis of truncation and point mutation of AMSH reveal the importance of an RXXK motif for CXCR4 degradation. The RXXK motif of AMSH interacts with the SH3 domains of the STAM and Grb2 families of adaptor proteins with high affinity. Cells expressing a catalytically inactive mutant of AMSH show basal hyperubiquitination, but not increased degradation, of the ESCRT-0 components STAM1 and Hrs. This is dependent on the RXXK motif of AMSH. Ubiquitination of endocytic machinery modulates their activity, suggesting that AMSH may directly regulate endocytic adaptor protein function. This is reflected in CXCR4 trafficking and provides a mechanism by which AMSH specifies the fate of endocytosed receptors. Taken together, these studies implicate AMSH as a key modulator of receptor fate determination through its action on components of the endocytic machinery.

c-Cbl-mediated degradation of TRAIL receptors is responsible for the development of the early phase of TRAIL resistance

We previously reported two modes of development of acquired TRAIL resistance: early phase and late phase [1]. In these studies, we observed that greater Akt activity and the expression of Bcl-xL were related mainly to the late phase of acquired TRAIL resistance. Recently we became aware of a possible mechanism of early phase TRAIL resistance development through internalization and degradation of TRAIL receptors (DR4 and DR5). Our current studies demonstrate that TRAIL receptors rapidly diminish at the membrane as well as the cytoplasm within 4h after TRAIL exposure, but recover completely after one or two days. Our studies also reveal that Cbl, a ubiquitously expressed cytoplasmic adaptor protein, is responsible for the rapid degradation of TRAIL receptors; Cbl binds to them and induces monoubiquitination of these receptors concurrent with their degeneration soon after TRAIL exposure, creating the early phase of acquired TRAIL resistance.

Essential role of c-Cbl in amphiregulin-induced recycling and signaling of the endogenous epidermal growth factor receptor

The intracellular processing of the epidermal growth factor receptor (EGFR) induced by epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) has been studied meticulously, with the former resulting in EGFR degradation and the latter in EGFR recycling to the plasma membrane. However, little is known about how other EGF family growth factors affect the trafficking of the EGFR. Additionally, although both EGF and TGF-alpha have been shown to effectively induce initial c-Cbl (ubiquitin ligase)-mediated ubiquitination of the EGFR, limited information is available regarding the role of c-Cblin the trafficking and signaling of recycling EGFR. Thus, in this study, we investigated the roles of c-Cblin endogenous EGFR trafficking and signaling after stimulation with amphiregulin (AR). We demonstrated that a physiological concentration of AR induced recycling of the endogenous EGFR to the plasma membrane, which correlated closely with transient association of the EGFR with c-Cbl and transient EGFR ubiquitination. Most importantly, we used c-Cbl small interfering RNA (siRNA) duplexes and ac-Cbl dominant negative mutant to show that c-Cbl is critical for the efficient transition of the EGFR from early endosomes to a recycling pathway and that c-Cbl regulates the duration of extracellular signal regulated kinase 1/2 mitogen-activated protein kinase (ERK1/2 MAPK) phosphorylation. These data support novel functions of c-Cbl in mediating recycling of EGF receptors to the plasma membrane, as well as in mediating the duration of activation (transient vs sustained) of ERK1/2 MAPK phosphorylation.

Targeted delivery with peptidomimetic conjugated self-assembled nanoparticles

Peptides produce specific nanostructures, making them useful for targeting in biological systems but they have low bioavailability, potential immunogenicity and poor metabolic stability. Peptidomimetic self-assembled NPs can possess biological recognition motifs as well as providing desired engineering properties. Inorganic NPs, coated with self-assembled macromers for stability and anti-fouling, and conjugated with target-specific ligands, are advancing imaging from the anatomy-based level to the molecular level. Ligand conjugated NPs are attractive for cell-selective tumor drug delivery, since this process has high transport capacity as well as ligand dependent cell specificity. Peptidomimetic NPs can provide stronger interaction with surface receptors on tumor cells, resulting in higher uptake and reduced drug resistance. Self-assembled NPs conjugated with peptidomimetic antigens are ideal for sustained presentation of vaccine antigens to dendritic cells and subsequent activation of T cell mediated adaptive immune response. Self-assembled NPs are a viable alternative to encapsulation for sustained delivery of proteins in tissue engineering. Cell penetrating peptides conjugated to NPs are used as intracellular delivery vectors for gene expression and as transfection agents for plasmid delivery. In this work, synthesis, characterization, properties, immunogenicity, and medical applications of peptidomimetic NPs in imaging, tumor delivery, vaccination, tissue engineering, and intracellular delivery are reviewed.

Basal ubiquitin-independent internalization of interferon alpha receptor is prevented by Tyk2-mediated masking of a linear endocytic motif

Linear endocytic motifs of signaling receptors as well as their ubiquitination determine the rate of ligand-induced endocytosis that mediates down-regulation of these receptors and restricts the magnitude and duration of their respective signal transduction pathways. We previously hypothesized that, in the absence of its cognate ligand, type I interferon (IFN), the IFNalpha receptor chain 1 (IFNAR1) receptor chain is protected from basal endocytosis by a hypothetical masking complex that prevents the Tyr-based endocytic motif within IFNAR1 from interacting with components of the adaptin protein complex 2 (AP2). Here we identify a member of the Janus kinase (Jak) family, Tyk2, as a component of such a masking complex. In the absence of ligand or of receptor chain ubiquitination, binding of Janus kinase Tyk2 within the proximity of the Tyr-based linear motif of IFNAR1 is required to prevent IFNAR1 internalization and to maintain its cell surface expression. Furthermore, interaction experiments revealed that Tyk2 physically shields this Tyr-based motif from the recognition by the AP50 subunit of AP2. These data delineate a long-sought ligand- and ubiquitin-independent mechanism by which Tyk2 contributes to both the regulation of total IFNAR1 levels as well as the regulation of the cell surface density of this receptor chain.

An ultrasensitive sorting mechanism for EGF receptor endocytosis

Background

The Epidermal Growth Factor (EGF) receptor has been shown to internalize via clathrin-independent endocytosis (CIE) in a ligand concentration dependent manner. From a modeling point of view, this resembles an ultrasensitive response, which is the ability of signaling networks to suppress a response for low input values and to increase to a pre-defined level for inputs exceeding a certain threshold. Several mechanisms to generate this behaviour have been described theoretically, the underlying assumptions of which, however, have not been experimentally demonstrated for the EGF receptor internalization network.

Results

Here, we present a mathematical model of receptor sorting into alternative pathways that explains the EGF-concentration dependent response of CIE. The described mechanism involves a saturation effect of the dominant clathrin-dependent endocytosis pathway and implies distinct steady-states into which the system is forced for low vs high EGF stimulations. The model is minimal since no experimentally unjustified reactions or parameter assumptions are imposed. We demonstrate the robustness of the sorting effect for large parameter variations and give an analytic derivation for alternative steady-states that are reached. Further, we describe extensibility of the model to more than two pathways which might play a role in contexts other than receptor internalization.

Conclusion

Our main result is that a scenario where different endocytosis routes consume the same form of receptor corroborates the observation of a clear-cut, stimulus dependent sorting. This is especially important since a receptor modification discriminating between the pathways has not been found experimentally. The model is not restricted to EGF receptor internalization and might account for ultrasensitivity in other cellular contexts.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

The fate of SPE B after internalization and its implication in SPEB-induced apoptosis

After streptococcal pyrogenic exotoxin B (SPE B) induces apoptosis, its fate is unknown. Using confocal time-course microscopy at 37 degrees C, we detected green fluorescence 20 min after adding FITC-SPE B. Orange fluorescence, an indication of co-localization of SPE B with lysosomes which were labeled with a red fluorescent probe, was maximal at 40 min and absent by 60 min. SPE B was co-precipitated with clathrin, which is consistent with endocytotic involvement. Western blotting assay also indicated that uptake of SPE B was maximal at 40 min and disappeared after 60 min. However, in the presence of chloroquine, a lysosome inhibitor, the uptake of SPE B was not detectable. The disappearance of TCA-precipitated FITC-SPE B was parallel to the appearance of TCA soluble FITC-SPE B; in the presence of chloroquine, however, no SPE B degradation occurred. Chloroquine increased the level of SPE B-induced apoptosis by inhibiting the degradation of SPE B. These results suggest that the internalization and degradation of SPE B in cells may be a host defense system that removes toxic substances by sacrificing the exposed cells.

Epidermal growth factor receptor fate is controlled by Hrs tyrosine phosphorylation sites that regulate Hrs degradation

Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is an endosomal protein essential for the efficient sorting of activated growth factor receptors into the lysosomal degradation pathway. Hrs undergoes ligand-induced tyrosine phosphorylation on residues Y329 and Y334 downstream of epidermal growth factor receptor (EGFR) activation. It has been difficult to investigate the functional roles of phosphoHrs, as only a small proportion of the cellular Hrs pool is detectably phosphorylated. Using an HEK 293 model system, we found that ectopic expression of the protein Cbl enhances Hrs ubiquitination and increases Hrs phosphorylation following cell stimulation with EGF. We exploited Cbl's expansion of the phosphoHrs pool to determine whether Hrs tyrosine phosphorylation controls EGFR fate. In structure-function studies of Cbl and EGFR mutants, the level of Hrs phosphorylation and rapidity of apparent Hrs dephosphorylation correlated directly with EGFR degradation. Differential expression of wild-type versus Y329,334F mutant Hrs in Hrs-depleted cells revealed that one or both tyrosines regulate ligand-dependent Hrs degradation, as well as EGFR degradation. By modulating Hrs ubiquitination, phosphorylation, and protein levels, Cbl may control the composition of the endosomal sorting machinery and its ability to target EGFR for lysosomal degradation.

Gene expression analysis of bipolar disorder reveals downregulation of the ubiquitin cycle and alterations in synaptic genes

Bipolar affective disorder is a severe psychiatric disorder with a strong genetic component but unknown pathophysiology. We used microarray technology to determine the expression of approximately 22,000 mRNA transcripts in post-mortem tissue from two brain regions in patients with bipolar disorder and matched healthy controls. Dorsolateral prefrontal cortex tissue from a cohort of 70 subjects and orbitofrontal cortex tissue from a separate cohort of 30 subjects was investigated. The final analysis included 30 bipolar and 31 control subjects for the dorsolateral prefrontal cortex and 10 bipolar and 11 control subjects for the orbitofrontal cortex. Differences between disease and control groups were identified using a rigorous statistical analysis with correction for confounding variables and multiple testing. In the orbitofrontal cortex, 393 differentially expressed transcripts were identified by microarray analysis and a representative subset was validated by quantitative real-time PCR. Pathway analysis revealed significant upregulation of genes involved in G-protein coupled receptor signalling and response to stimulus (in particular the immune response), while genes relating to the ubiquitin cycle and intracellular transport showed coordinated downregulation in bipolar disorder. Additionally, several genes involved in synaptic function were significantly downregulated in bipolar disorder. No significant changes in gene expression were observed in the dorsolateral prefrontal cortex using microarray analysis or quantitative real-time PCR. Our findings implicate the orbitofrontal cortex as a region prominently involved in bipolar disorder and indicate that diverse processes are affected. Overall, our results suggest that dysregulation of the ubiquitin pathway and synaptic function may be central to the disease process.

TYK2 activity promotes ligand-induced IFNAR1 proteolysis

The type I IFNR (interferon receptor) is a heterodimer composed of two transmembrane chains, IFNAR1 (interferon-alpha receptor 1 subunit) and IFNAR2, which are associated with the tyrosine kinases Tyk2 and Jak1 (Janus kinase 1) respectively. Ligand-induced down-regulation of the type I IFNR is a major mechanism of negative regulation of cellular signalling and involves the internalization and lysosomal degradation of IFNAR1. IFNalpha promotes the phosphorylation of IFNAR1 on Ser535, followed by recruitment of the E3 ubiquitin ligase, beta-TrCP2 (beta-transducin repeats-containing protein 2), ubiquitination of IFNAR1 and proteolysis. The non-catalytic role of Tyk2 in sustaining the steady-state IFNAR1 level at the plasma membrane is well documented; however, little is known about the function of Tyk2 in the steps that precede and succeed serine phosphorylation and ubiquitination of IFNAR1 in response to ligand binding. In the present study, we show that catalytic activation of Tyk2 is not essential for IFNAR1 internalization, but is required for ligand-induced IFNAR1 serine phosphorylation, ubiquitination and efficient lysosomal proteolysis.

Structural and functional diversity of adaptor proteins involved in tyrosine kinase signalling

Adaptors are proteins of multi-modular structure without enzymatic activity. Their capacity to organise large, temporary protein complexes by linking proteins together in a regulated and selective fashion makes them of outstanding importance in the establishment and maintenance of specificity and efficiency in all known signal transduction pathways. This review focuses on the structural and functional characterisation of adaptors involved in tyrosine kinase (TK) signalling. TK-linked adaptors can be distinguished by their domain composition and binding specificities. However, such structural classifications have proven inadequate as indicators of functional roles. A better way to understand the logic of signalling networks might be to look at functional aspects of adaptor proteins such as signalling specificity, negative versus positive contribution to signal propagation, or their position in the signalling hierarchy. All of these functions are dynamic, suggesting that adaptors have important regulatory roles rather than acting only as stable linkers in signal transduction.

Homologous and lysophosphatidic acid-induced desensitization of the atrial natriuretic peptide receptor, guanylyl cyclase-A, in MA-10 leydig cells

The cardiac hormone atrial natriuretic peptide (ANP) signals via interaction with a plasma membrane receptor, which has guanylyl cyclase (GC) activity and is referred to as GC-A. Desensitization of GC-A is thought to represent a physiologically important regulatory mechanism, but the signaling pathways implicated and cell type-specific effects are still poorly understood. Here we demonstrate that sustained exposure to either ANP itself or the bioactive lipid lysophosphatidic acid (LPA) elicits GC-A desensitization in MA-10 Leydig cells. Both reactions show similar kinetics and evoke equal decreases (by 40%) in GC-A hormone responsiveness. Homologous (ANP induced) desensitization, in which cGMP is generated as second messenger, is blocked by distinct cAMP-dependent protein kinase [protein kinase A (PKA)] inhibitors, H 89, and Rp-8-CPT-cAMPs, providing evidence that PKA mediates the reaction. Accordingly, the ANP/cGMP-elicited effects are mimicked by a cAMP analog, 8-bromo-cAMP. The LPA-induced (heterologous) desensitization is not blocked by PKA inhibition, indicating a different signaling pathway. LPA, but not ANP, enhances ERK phosphorylation and induces cell rounding together with a dramatic reorganization of actin filaments. Consistent with the identification of LPA receptor (LPA2 and LPA3) gene expression, the findings are indicative of LPA receptor-mediated reactions. This study demonstrates for the first time coexistence of homologous and heterologous desensitization of GC-A in the same cell type, reveals that these reactions are mediated by different pathways, and identifies a novel cross talk between phospholipid and natriuretic peptide signaling. The morphoregulatory activities exerted by LPA suggest a crucial role for Leydig cell physiology.

Ubiquitylation and cell signaling

Ubiquitylation is an emerging mechanism implicated in a variety of nonproteolytic cellular functions. The attachment of a single ubiquitin (Ub) or poly-Ub (lysine 63) chains to proteins control gene transcription, DNA repair and replication, intracellular trafficking and virus budding. In these processes, protein ubiquitylation exhibits inducibility, reversibility and recognition by specialized domains, features similar to protein phosphorylation, which enable Ub to act as a signaling device. Here, we highlight several recent examples on how Ub regulates signaling and how signaling regulates ubiquitylation during physiological and pathological cellular processes.

Chemokine receptor internalization and intracellular trafficking

The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

Cargo- and compartment-selective endocytic scaffold proteins

The endocytosis of membrane receptors is a complex and tightly controlled process that is essential for maintaining cellular homoeostasis. The removal of receptors from the cell surface can be constitutive or ligand-induced, and occurs in a clathrin-dependent or -independent manner. The recruitment of receptors into specialized membrane domains, the formation of vesicles and the trafficking of receptors together with their ligands within endocytic compartments are regulated by reversible protein modifications, and multiple protein-protein and protein-lipid interactions. Recent reports describe a variety of multidomain molecules that facilitate receptor endocytosis and function as platforms for the assembly of protein complexes. These scaffold proteins typically act in a cargo-specific manner, recognizing one or more receptor types, or function at the level of endocytic cellular microcompartments by controlling the movement of cargo molecules and linking endocytic machineries to signalling pathways. In the present review we summarize present knowledge on endocytic scaffold molecules and discuss their functions.