Arfaptin 2 regulates the aggregation of mutant huntingtin protein

Endogenous mutant Huntingtin alters the corticogenesis via lowering Golgi recruiting ARF1 in cortical organoid

Pathogenic mutant huntingtin (mHTT) infiltrates the adult Huntington's disease (HD) brain and impairs fetal corticogenesis. However, most HD animal models rarely recapitulate neuroanatomical alterations in adult HD and developing brains. Thus, the human cortical organoid (hCO) is an alternative approach to decode mHTT pathogenesis precisely during human corticogenesis. Here, we replicated the altered corticogenesis in the HD fetal brain using HD patient-derived hCOs. Our HD-hCOs had pathological phenotypes, including deficient junctional complexes in the neural tubes, delayed postmitotic neuronal maturation, dysregulated fate specification of cortical neuron subtypes, and abnormalities in early HD subcortical projections during corticogenesis, revealing a causal link between impaired progenitor cells and chaotic cortical neuronal layering in the HD brain. We identified novel long, oriented, and enriched polyQ assemblies of HTTs that hold large flat Golgi stacks and scaffold clathrin+ vesicles in the neural tubes of hCOs. Flat Golgi stacks conjugated polyQ assemblies by ADP-ribosylation factor 1 (ARF1). Inhibiting ARF1 activation with Brefeldin A (BFA) disassociated polyQ assemblies from Golgi. PolyQ assembles with mHTT scaffolded fewer ARF1 and formed shorter polyQ assembles with fewer and shorter Golgi and clathrin vesicles in neural tubes of HD-hCOs compared with those in hCOs. Inhibiting the activation of ARF1 by BFA in healthy hCOs replicated impaired junctional complexes in the neural tubes. Together, endogenous polyQ assemblies with mHTT reduced the Golgi recruiting ARF1 in the neuroepithelium, impaired the Golgi structure and activities, and altered the corticogenesis in HD-hCO.

Proteome Differences between Hepatitis B Virus Genotype-B- and Genotype-C-Induced Hepatocellular Carcinoma Revealed by iTRAQ-Based Quantitative Proteomics

Hepatitis B virus (HBV) is the main cause of hepatocellular carcinoma (HCC) in southeast Asia where HBV genotype B and genotype C are the most prevalent. Viral genotypes have been reported to significantly affect the clinical outcomes of HCC. However, the underlying molecular differences among different genotypes of HBV virus infected HCC have not been revealed. Here, we applied isobaric tags for relative and absolute quantitation (iTRAQ) technology integrated with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis to identify the proteome differences between the HBV genotypes B- and C-induced HCC. In brief, a total of 83 proteins in the surrounding noncancerous tissues and 136 proteins in the cancerous tissues between HBV genotype-B- and genotype-C-induced HCC were identified, respectively. This information revealed that there might be different molecular mechanisms of the tumorigenesis and development of HBV genotypes B- and C-induced HCC. Furthermore, our results indicate that the two proteins ARFIP2 and ANXA1 might be potential biomarkers for distinguishing the HBV genotypes B- and C-induced HCC. Thus, the quantitative proteomic analysis revealed molecular differences between the HBV genotypes B- and C-induced HCC, and might provide fundamental information for further deep study.

Dimer of arfaptin 2 regulates NF-κB signaling by interacting with IKKβ/NEMO and inhibiting IKKβ kinase activity

IκB kinases (IKKs) are a therapeutic target due to their crucial roles in various biological processes, including the immune response, the stress response, and tumor development. IKKs integrate various upstream signals that activate NF-κB by phosphorylating IκB and also regulate many proteins related to cell growth and metabolism. Although they function as a heteromeric complex comprised of kinase subunits and an adaptor, these kinases produce distinct cellular responses by phosphorylating different target molecules, suggesting that they may also be regulated in a subtype-specific manner. In this study, arfaptin 2 was identified as an IKKβ-specific binding partner. Interestingly, arfaptin 2 also interacted with NEMO. Domain mapping studies revealed that the C-terminal region, including the IKKβ HLH domain and the first coiled-coil NEMO region were respectively required for interactions with the arfaptin 2 N-terminal flexible region. Overexpression of arfaptin 2 inhibited tumor necrosis factor (TNF)-α-stimulated nuclear factor-κB (NF-κB) signaling, whereas downregulation of arfaptin 2 by small interfering RNA enhanced NF-κB activity. Dimerization of arfaptin 2 through the Bin-Amphiphysin-Rvs domain may be essential to inhibit activation of NF-κB through multimodal interactions with IKKβs or IKKβ/NEMO, as ectopic expression of the arfaptin 2 fragment responsible for IKK interactions did not change TNFα-stimulated NF-κB activation. These data indicate that arfaptin 2 is the first molecule to regulate NF-κB signaling by interacting with the functional IKK complex but not by direct inhibiting IKKβ phosphorylation.

Arfaptin-1 negatively regulates Arl1-mediated retrograde transport

The small GTPase Arf-like protein 1 (Arl1) is well known for its role in intracellular vesicular transport at the trans-Golgi network (TGN). In this study, we used differential affinity chromatography combined with mass spectrometry to identify Arf-interacting protein 1b (arfaptin-1b) as an Arl1-interacting protein and characterized a novel function for arfaptin-1 (including the arfaptin-1a and 1b isoforms) in Arl1-mediated retrograde transport. Using a Shiga-toxin subunit B (STxB) transportation assay, we demonstrated that knockdown of arfaptin-1 accelerated the retrograde transport of STxB from the endosome to the Golgi apparatus, whereas Arl1 knockdown inhibited STxB transport compared with control cells. Arfaptin-1 overexpression, but not an Arl1 binding-defective mutant (arfaptin-1b-F317A), consistently inhibited STxB transport. Exogenous arfaptin-1 expression did not interfere with the localization of the Arl1-interacting proteins golgin-97 and golgin-245 to the TGN and vice versa. Moreover, we found that the N-terminal region of arfaptin-1 was involved in the regulation of retrograde transport. Our results show that arfaptin-1 acts as a negative regulator in Arl1-mediated retrograde transport and suggest that different functional complexes containing Arl1 form in distinct microdomains and are responsible for different functions.

Arf1 and membrane curvature cooperate to recruit Arfaptin2 to liposomes

Arfaptin2 contains a Bin/Amphiphysin/Rvs (BAR) domain and directly interacts with proteins of the Arf/Arl family in their active GTP-bound state. It has been proposed that BAR domains are able to sense membrane curvature and to induce membrane tubulation. We report here that active Arf1 is required for the recruitment of Arfaptin2 to artificial liposomes mimicking the Golgi apparatus lipid composition. The Arf1-dependent recruitment of Arfaptin2 increases with membrane curvature, while the recruitment of Arf1 itself is not sensitive to curvature. At high protein concentrations, the binding of Arfaptin2 induces membrane tubulation. Finally, membrane-bound Arfaptin2 is released from the liposome when ArfGAP1 catalyzes the hydrolysis of GTP to GDP in Arf1. These results show that both Arf1 activation and high membrane curvature are required for efficient recruitment of Arfaptin2 to membranes.

IRE1 plays an essential role in ER stress-mediated aggregation of mutant huntingtin via the inhibition of autophagy flux

Huntington's disease (HD), an inherited neurodegenerative disorder, is caused by an expansion of cytosine-adenine-guanine repeats in the huntingtin gene. The aggregation of mutant huntingtin (mtHTT) and striatal cell loss are representative features to cause uncontrolled movement and cognitive defect in HD. However, underlying mechanism of mtHTT aggregation and cell toxicity remains still elusive. Here, to find new genes modulating mtHTT aggregation, we performed cell-based functional screening using the cDNA expression library and isolated IRE1 gene, one of endoplasmic reticulum (ER) stress sensors. Ectopic expression of IRE1 led to its self-activation and accumulated detergent-resistant mtHTT aggregates. Treatment of neuronal cells with ER stress insults, tunicamycin and thapsigargin, increased mtHTT aggregation via IRE1 activation. The kinase activity of IRE1, but not the endoribonuclease activity, was necessary to stimulate mtHTT aggregation and increased death of neuronal cells, including SH-SY5Y and STHdhQ111/111 huntingtin knock-in striatal cells. Interestingly, ER stress impaired autophagy flux via IRE1-TRAF2 pathway, thus enhancing cellular accumulation of mtHTT. Atg5 deficiency in M5-7 cells increased mtHTT aggregation but blocked ER stress-induced mtHTT aggregation. Further, ER stress markers including p-IRE1 and autophagy markers such as p62 were up-regulated exclusively in the striatal tissues of HD mouse models and in HD patients. Moreover, down-regulation of IRE1 expression rescues the rough-eye phenotype by mtHTT in a HD fly model. These results suggest that IRE1 plays an essential role in ER stress-mediated aggregation of mtHTT via the inhibition of autophagy flux and thus neuronal toxicity of mtHTT aggregates in HD.

ADP-ribosylation factor 6 mediates E-cadherin recovery by chemical chaperones

E-cadherin plays a powerful tumor suppressor role. Germline E-cadherin mutations justify 30% of Hereditary Diffuse Gastric Cancer (HDGC) and missense mutations are found in 30% of these families. We found possible to restore in vitro mutant E-cadherin associated to HDGC syndrome by using Chemical Chaperones (CCs). Herein, our aim was to disclose the molecular mechanisms underlying the CCs effects in E-cadherin regulation. Using cells stably expressing WT E-cadherin or two HDGC-associated missense mutations, we show that upon DMSO treatment, not only mutant E-cadherin is restored and stabilized at the plasma membrane (PM), but also Arf6 and PIPKIγ expressions are altered. We show that modulation of Arf6 expression partially mimics the effect of CCs, suggesting that the cellular effects observed upon CCs treatment are mediated by Arf6. Further, we show that E-cadherin expression recovery is specifically linked to Arf6 due to its role on endocytosis and recycling pathways. Finally, we demonstrated that, as DMSO, several others CCs are able to modulate the trafficking machinery through an Arf6 dependent mechanism. Interestingly, the more effective compounds in E-cadherin recovery to PM are those that simultaneously inhibit Arf6 and stimulate PIPKIγ expression and binding to E-cadherin. Here, we present the first evidence of a direct influence of CCs in cellular trafficking machinery and we show that this effect is of crucial importance in the context of juxtamembrane E-cadherin missense mutations associated to HDGC. We propose that this influence should be taken into account when exploring the therapeutic potential of this type of chemicals in genetic diseases associated to protein-misfolding.

Arfaptins are localized to the trans-Golgi by interaction with Arl1, but not Arfs

Arfaptins (arfaptin-1 and arfaptin-2/POR1) were originally identified as binding partners of the Arf small GTPases. Both proteins contain a BAR (Bin/Amphiphysin/Rvs) domain, which participates in membrane deformation. Here we show that arfaptins associate with trans-Golgi membranes. Unexpectedly, Arl1 (Arf-like 1), but not Arfs, determines the trans-Golgi association of arfaptins. We also demonstrate that arfaptins interact with Arl1 through their BAR domain-containing region and compete for Arl1 binding with golgin-97 and golgin-245/p230, both of which also bind to Arl1 through their GRIP (golgin-97/RanBP2/Imh1p/p230) domains. However, arfaptins and these golgins show only limited colocalization at the trans-Golgi. Time-lapse imaging of cells overexpressing fluorescent protein-tagged arfaptins and golgin-97 reveals that arfaptins, but not golgin-97, are included in vesicular and tubular structures emanating from the Golgi region. These observations indicate that arfaptins are recruited onto trans-Golgi membranes by interacting with Arl1, and capable of inducing membrane deformation via their BAR domains.

ATF3 plays a protective role against toxicity by N-terminal fragment of mutant huntingtin in stable PC12 cell line

Huntington's disease is a progressive neurodegenerative disorder caused by a polyglutamine expansion near the N-terminus of huntingtin. The mechanisms of polyglutamine neurotoxicity, and cellular responses are not fully understood. We have studied gene expression profiles by short oligo array using an inducible PC12 cell model expressing an N-terminal huntingtin fragment with expanded polyglutamine (Htt-N63-148Q). Mutant huntingtin Htt-N63 induced cell death and increased the mRNA and protein levels of activating transcription factor 3 (ATF3). Mutant Htt-N63 also significantly enhanced ATF3 transcriptional activity by a promoter-based reporter assay. Overexpression of ATF3 protects against mutant Htt-N63 toxicity and knocking down ATF3 expression reduced Htt-N63 toxicity in a stable PC12 cell line. These results indicated that ATF3 plays a critical role in toxicity induced by mutant Htt-N63 and may lead to a useful therapeutic target.

Interaction assays in yeast and cultured cells confirm known and identify novel partners of the synaptic vesicle protein synaptophysin

Synaptophysin (SYP) is a major protein of neurotransmitter-containing vesicles spanning the membrane four times and contributing to various aspects of the synaptic vesicle cycle. The split-ubiquitin yeast two-hybrid system was used to characterize molecular interactions of membrane-bound, full-length murine SYP. In this way, the known homophilic SYP-SYP association could be confirmed and heterophilic binding of SYP to other tetraspan vesicle membrane proteins of the secretory carrier-associated membrane- and synaptogyrin-type could be detected for the first time. SYP-binding was also observed for the vSNARE synaptobrevin2 and various membrane and membrane-associated proteins. Double labeling immunofluorescence microscopy of murine retina, co-immunoprecipitation experiments and fluorescence energy resonance transfer (FRET) analyses between fluorescent protein-tagged polypeptides were carried out to validate and further characterize the association of SYP with the tetraspan vesicle membrane proteins secretory carrier-associated membrane protein 1 and synaptogyrin3, with synaptobrevin2, and the newly identified binding partners phospholipase D4, stathmin-like3, Rho family GTPase2 and ADP-ribosylation factor interacting protein2. It was observed that the carboxyterminus of SYP is dispensable for association with integral membrane proteins while it is needed for binding to membrane-associated polypeptides. The latter appears to be regulated by phosphorylation, since src homology 2-domains were shown to attach to the multiple carboxyterminal phosphotyrosine residues of SYP. In conclusion, the association of SYP with different tetraspan vesicle membrane proteins suggests shared functions and the multiple other interactions identify SYP as part of a membrane platform acting as a facilitator of various steps of the synaptic vesicle cycle.

Mass spectrometry analysis of a protein kinase CK2beta subunit interactome isolated from mouse brain by affinity chromatography

CK2, an acronym derived from the misnomer "casein kinase 2", denotes a ubiquitous and extremely pleiotropic Ser/Thr protein kinase, the holoenzyme of which is composed of two catalytic (alpha and/or alpha') and two noncatalytic beta subunits acting as a docking platform and the multifarious functions of which are still incompletely understood. By combining affinity chromatography and mass spectrometry, we have identified 144 mouse brain proteins that associate with immobilized CK2beta. A large proportion (60%) of the identified proteins had been previously reported to be functionally related to CK2, and a similar proportion have been classified as phosphoproteins with approximately half of these having the features of CK2 targets. A large number of the identified proteins ( approximately 40%) either are nuclear or shuttle between the nucleus and cytoplasm, and the biggest functional classes of CK2beta interactors are committed to protein synthesis and degradation (32 proteins) and RNA/DNA interaction (20 proteins). Also well represented are the categories of cytoskeletal/structural proteins (19), trafficking proteins (17), and signaling proteins (14). The identified proteins are examined in relation to their functions and potential as targets and/or regulators of CK2, disclosing in some cases unanticipated links between this kinase and a variety of biochemical events.

Association of Protein-tyrosine Phosphatase MEG2 via Its Sec14p Homology Domain with Vesicle-trafficking Proteins

The protein-tyrosine phosphatase PTPMEG2 is located on the cytoplasmic face of the enclosing membrane of secretory vesicles, where it regulates vesicle size by promoting homotypic vesicle fusion by dephosphorylating N-ethylmaleimide-sensitive factor, a key regulator of vesicle fusion. Here we address the question of how PTPMEG2 is targeted to this subcellular location. Using a series of deletion mutants, we pinpointed the N-terminal Sec14p homology (SEC14) domain of PTPMEG2, residues 1-261, as the region containing the secretory vesicle targeting signal. This domain, alone or appended to a heterologous protein, was localized to intracellular vesicle membranes. Yeast two-hybrid screening identified a number of secretory vesicle proteins that interacted directly with the SEC14 domain of PTPMEG2, providing a mechanism for PTPMEG2 targeting to secretory vesicles. Two such proteins, mannose 6-phosphate receptor-interacting protein TIP47 and Arfaptin2, were found to alter PTPMEG2 localization when overexpressed, and elimination of TIP47 resulted in loss of PTPMEG2 function. We conclude that the N terminus of PTPMEG2 is necessary for the targeting of this phosphatase to the secretory vesicle compartment by association with other proteins involved in intracellular transport.

Endocytosis machinery is involved in aggregation of proteins with expanded polyglutamine domains

The cell's failure to refold or break down abnormal polypeptides often leads to their aggregation, which could cause toxicity and various pathologies. Here we investigated cellular factors involved in protein aggregation in yeast and mammalian cells using model polypeptides containing polyglutamine domains. In yeast, a number of mutations affecting the complex responsible for formation of the endocytic vesicle reduced the aggregation. Components of the endocytic complex (EC) Sla1, Sla2, and Pan1 were seen as clusters in the polyglutamine aggregates. These proteins associate with EC at the later stages of its maturation. In contrast, Ede1 and Ent1, the elements of EC at the earlier stages, were not found in the aggregates, suggesting that late ECs are involved in polyglutamine aggregation. Indeed, stabilization of the late complexes by inhibition of actin polymerization enhanced aggregation of polypeptides with polyglutamine domains. Similarly, in mammalian cells, inhibitors of actin polymerization, as well as depletion of a mediator of actin polymerization, Arp2, strongly enhanced the aggregation. In contrast, destabilization of EC by depletion or inhibition of a scaffolding protein N-WASP effectively suppressed the aggregation. Therefore, EC appears to play a pivotal role in aggregation of cytosolic polypeptides with polyglutamine domains in both yeast and mammalian cells.

ARF proteins: roles in membrane traffic and beyond

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

Proteasome impairment does not contribute to pathogenesis in R6/2 Huntington's disease mice: exclusion of proteasome activator REGgamma as a therapeutic target

Huntington's disease (HD) is one of a group of neurodegenerative disorders caused by the pathological expansion of a glutamine tract. A hallmark of these so-called polyglutamine diseases is the presence of ubiquitylated inclusion bodies, which sequester various components of the 19S and 20S proteasomes. In addition, the ubiquitin-proteasome system (UPS) has been shown to be severely impaired in vitro in cells overexpressing mutant huntingtin. Thus, because of its fundamental housekeeping function, impairment of the UPS in neurons could contribute to neurotoxicity. We have recently proposed that the proteasome activator REGgamma could contribute to UPS impairment in polyglutamine diseases by suppressing the proteasomal catalytic sites responsible for cleaving Gln-Gln bonds. Capping of proteasomes with REGgamma could therefore contribute to a potential 'clogging' of the proteasome by pathogenic polyglutamines. We show here that genetic reduction of REGgamma has no effect on the well-defined neurological phenotype of R6/2 HD mice and does not affect inclusion body formation in the R6/2 brain. Surprisingly, we observe increased proteasomal 'chymotrypsin-like' activity in 13-week-old R6/2 brains relative to non-R6/2, irrespective of REGgamma levels. However, assays of 26S proteasome activity in mouse brain extracts reveal no difference in proteolytic activity regardless of R6/2 or REGgamma genotype. These findings suggest that REGgamma is not a viable therapeutic target in polyglutamine disease and that overall proteasome function is not impaired by trapped mutant polyglutamine in R6/2 mice.

Phosphorylation of Arfaptin 2 at Ser260 by Akt Inhibits PolyQ-huntingtin-induced Toxicity by Rescuing Proteasome Impairment

Huntington disease (HD) is caused by an abnormal expanded polyglutamine repeat in the huntingtin protein. Insulin-like growth factor-1 is of particular interest in HD because it strongly inhibits polyQ-huntingtin-induced neurotoxicity. This neuroprotective effect involves the phosphorylation of huntingtin at Ser(421) by the prosurvival kinase Akt (Humbert, S., Bryson, E. A., Cordelieres, F. P., Connors, N. C., Datta, S. R., Finkbeiner, S., Greenberg, M. E., and Saudou, F. (2002) Dev. Cell 2, 831-837). Here, we report that Akt inhibits polyQ-huntingtin-induced toxicity in the absence of phosphorylation of huntingtin at Ser(421), suggesting that Akt also acts on other downstream effector(s) to prevent neuronal death in HD. We show that this survival effect involves the ADP-ribosylation factor-interacting protein arfaptin 2, the levels of which are increased in HD patients. Akt phosphorylated arfaptin 2 at Ser(260). Lack of phosphorylation of arfaptin 2 at this site substantially modified its subcellular distribution and increased neuronal death and intranuclear inclusions caused by polyQ-huntingtin. In contrast, arfaptin 2 had a neuroprotective effect on striatal neurons when phosphorylated by Akt. This effect is mediated through the proteasome, as phosphorylated arfaptin 2 inhibited the blockade of the proteasome induced by polyQ-huntingtin. This study points out a new mechanism by which Akt promotes neuroprotection in HD, emphasizing the potential therapeutic interest of this pathway in the disease.

Development of an ischemic tolerance model in a PC12 cell line

Although ischemic tolerance has been described in a variety of primary cell culture systems, no similar in vitro models have been reported with any cell line. A model of ischemic preconditioning in the rat pheochromocytoma PC12 cell line is described here. When compared to nonpreconditioned cells, preexposure of PC12 cells to 6 hours of oxygen and glucose deprivation (OGD) significantly increased cell viability after 15 hours of OGD 24 hours later. Flow cytometry analysis of cells labeled with specific markers for apoptosis, Annexin V, and Hoechst 33342, and of DNA content, revealed that apoptosis is involved in OGD-induced PC12 cell death and that preconditioning of the cells mainly counteracts the effect of apoptosis. Immunocytochemistry of caspase-3, a central executioner in the apoptotic process, further confirmed the activation of apoptotic pathways in OGD-induced PC12 cell death. This model may be useful to investigate the cellular mechanisms involved in neuronal transient tolerance following ischemia.

Perturbed signal transduction in neurodegenerative disorders involving aberrant protein aggregation

Aggregation of abnormal proteins, both inside and outside of cells, is a prominent feature of major neurodegenerative disorders, including Alzheimer's, Parkinson's, polyglutamine expansion, and prion diseases. Other articles in this special issue of NeuroMolecular Medicine describe the genetic and molecular factors that promote aberrant protein aggregation. In the present article, we consider how it is that pathogenic aggregation-prone proteins compromise signal transduction pathways that regulate neuronal plasticity and survival. In some cases the protein in question may have widespread and relatively nonspecific effects on signaling. For example, amyloid beta-peptide induces membrane-associated oxidative stress, which impairs the function of various receptors, ion channels and transporters, as well as downstream kinases and transcription factors. Other proteins, such as polyglutamine repeat proteins, may affect specific protein -protein interactions, including those involved in signaling pathways activated by neurotransmitters, neurotrophins, and steroid hormones. Synapses are particularly sensitive to abnormal protein aggregation and impaired synaptic signaling may trigger apoptosis and related cell death cascades. Impairment of signal transduction in protein aggregation disorders may be amenable to therapy as demonstrated by a recent study showing that dietary restriction can preserve synaptic function and protect neurons in a mouse model of Huntington's disease. Finally, emerging findings are revealing how activation of certain signaling pathways can suppress protein aggregation and/or the cytotoxicity resulting from the abnormal protein aggregation. A better understanding of how abnormal protein aggregation occurs and how it affects and is affected by specific signal transduction pathways, is leading to novel approaches for preventing and treating neurodegenerative disorders.

Aggregation of expanded polyglutamine domain in yeast leads to defects in endocytosis

The role of aggregation of abnormal proteins in cellular toxicity is of general importance for understanding many neurological disorders. Here, using a yeast model, we demonstrate that mutations in many proteins involved in endocytosis and actin function dramatically enhance the toxic effect of polypeptides with an expanded polyglutamine (polyQ) domain. This enhanced cytotoxicity required polyQ aggregation and was dependent on the yeast protein Rnq1 in its prion form. In wild-type cells, expression of expanded polyQ followed by its aggregation led to specific and acute inhibition of endocytosis, which preceded growth inhibition. Some components of the endocytic machinery were efficiently recruited into the polyQ aggregates. Furthermore, in cells with polyQ aggregates, cortical actin patches were delocalized and actin was recruited into the polyQ aggregates. Aggregation of polyQ in mammalian HEK293 cells also led to defects in endocytosis. Therefore, it appears that inhibition of endocytosis is a direct consequence of polyQ aggregation and could significantly contribute to cytotoxicity.

A Rapid Cellular FRET Assay of Polyglutamine Aggregation Identifies a Novel Inhibitor

Many neurodegenerative diseases, including tauopathies, Parkinson's disease, amyotrophic lateral sclerosis, and the polyglutamine diseases, are characterized by intracellular aggregation of pathogenic proteins. It is difficult to study modifiers of this process in intact cells in a high-throughput and quantitative manner, although this could facilitate molecular insights into disease pathogenesis. Here we introduce a high-throughput assay to measure intracellular polyglutamine protein aggregation using fluorescence resonance energy transfer (FRET). We screened over 2800 biologically active small molecules for inhibitory activity and have characterized one lead compound in detail. Y-27632, an inhibitor of the Rho-associated kinase p160ROCK, diminished polyglutamine protein aggregation (EC(50) congruent with 5 microM) and reduced neurodegeneration in a Drosophila model of polyglutamine disease. This establishes a novel high-throughput approach to study protein misfolding and aggregation associated with neurodegenerative diseases and implicates a signaling pathway of previously unrecognized importance in polyglutamine protein processing.

The role of the ubiquitin/proteasome system in cellular responses to radiation

In the last few years, the ubiquitin(Ub)/proteasome system has become increasingly recognized as a controller of numerous physiological processes, including signal transduction, DNA repair, chromosome maintenance, transcriptional activation, cell cycle progression, cell survival, and certain immune cell functions. This is in addition to its more established roles in the removal of misfolded, damaged, and effete proteins. This review examines the role of the Ub/proteasome system in processes underlying the classical effects of irradiation on cells, such as radiation-induced gene expression, DNA repair and chromosome instability, oxidative damage, cell cycle arrest, and cell death. Furthermore, recent evidence suggests that the proteasome is a redox-sensitive target for ionizing radiation and other oxidative stress signals. In other words, the Ub/proteasome system may not simply be a passive player in radiation-induced responses, but may modulate them. The extent of the modulation will be influenced by the functional and structural diversity that is expressed by the system. Cell types vary in the Ub/proteasome structures they possess and the level at which they function, and this changes as they go from the normal to the cancerous condition. Cancer-related functional changes within the Ub/proteasome system may therefore present unique targets for cancer therapy, especially when targeting agents are used in combination with radio- or chemotherapy. The peptide boronic acid compound PS-341, which was designed to inhibit proteasome chymotryptic activity, is in clinical trials for the treatment of solid and hematogenous tumors. It has shown some efficacy on its own and in combination with chemotherapy. Preclinical studies have shown that PS-341 will also potentiate the cytotoxic effects of radiation therapy. In addition, other drugs in common clinical use have been shown to affect proteasome function, and their activities may be valuably reconsidered from this perspective.

Trafficking of prion proteins through a caveolae-mediated endosomal pathway

To understand the posttranslational conversion of the cellular prion protein (PrPC) to its pathologic conformation, it is important to define the intracellular trafficking pathway of PrPC within the endomembrane system. We studied the localization and internalization of PrPC in CHO cells using cryoimmunogold electron microscopy. At steady state, PrPC was enriched in caveolae both at the TGN and plasma membrane and in interconnecting chains of endocytic caveolae. Protein A-gold particles bound specifically to PrPC on live cells. These complexes were delivered via caveolae to the pericentriolar region and via nonclassical, caveolae-containing early endocytic structures to late endosomes/lysosomes, thereby bypassing the internalization pathway mediated by clathrin-coated vesicles. Endocytosed PrPC-containing caveolae were not directed to the ER and Golgi complex. Uptake of caveolae and degradation of PrPC was slow and sensitive to filipin. This caveolae-dependent endocytic pathway was not observed for several other glycosylphosphatidyl inositol (GPI)-anchored proteins. We propose that this nonclassical endocytic pathway is likely to determine the subcellular location of PrPC conversion.

The hunt for huntingtin function: interaction partners tell many different stories

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormally elongated polyglutamine (polyQ) tract in the large protein huntingtin (htt). Currently, both the normal function of htt in neurons and the molecular mechanism by which the expanded polyQ sequence in htt causes selective neurodegeneration remain elusive. Research in past years has identified several htt-interacting proteins such as htt-interacting protein 1, Src homology region 3-containing Grb2-like protein 3, protein kinase C and casein kinase substrate in neurons 1, htt-associated protein 1, postsynaptic density-95, FIP-2 (for 14.7K-interacting protein), specificity protein 1 and nuclear receptor co-repressor. These proteins play roles in clathrin-mediated endocytosis, apoptosis, vesicle transport, cell signalling, morphogenesis and transcriptional regulation, suggesting that htt is also involved in these processes.

Protein aggregation and the ubiquitin proteasome pathway: gaining the UPPer hand on neurodegeneration

Protein misfolding and aggregation are common to most neurodegenerative diseases, suggesting that abnormalities of protein homeostasis contribute to pathogenesis. Research implicates at least two components of cellular protein quality control in disease: molecular chaperones and the ubiquitin-proteasome pathway (UPP). Although evidence is more compelling for chaperone involvement, recent cell-based and genetic studies suggest that perturbations in the UPP also contribute to neurodegenerative disease processes. UPP involvement in disease seems even more probable when the UPP is viewed not simply as an isolated degradation machine but rather as a complex cascade linked both to other ubiquitin-dependent processes and to chaperone systems.

Dysregulatory dysequilibrium of gene transcription and of nuclear transport in polyglutamine neuro-degeneration

Polyglutamine neurodegeneration as an essential expansion mutation of the CAG-trinucleotide repeat encoding glutamine would appear to constitute an integral process of aggregation/accumulation that self-propagates a secondary process of possible nuclear sequestration. Within such a scheme of progressive expansion of polyglutamine stretches in strict parallel correlation with increased CAG trinucleotide repeats in genes such as ataxin-7 and its messenger RNA, it would appear that a fundamental relationship of accumulation directly inducing biophysical disruption between nuclear/nucleolar and cytoplasmic protein machineries would constitute a dysfunctional dysequilibrium accounting for self-progressive neuronal degeneration with atrophy of the cerebral cortex and ganglia such as the caudate, that is limited often to specific population groups of neurons. It is for example in terms of Huntington's disease as an autosomal dominant disorder with high penetrance on a background of onset of dementia mainly in the fourth and fifth decades of life that one might conceive of polyglutamine neurodegeneration as fundamentally a developmental disturbance affecting neuronal maturation that accounts for abnormal neurophysiological and biochemical aspects of interaction of nucleus with cytoplasm. Polyglutamine expansion and trinucleotide repeats as both progressive processes of accumulation and synthesis would constitute a complex interplay of inducing and induced effects that both contribute in probably multiple ways to the self-progressive nature of a nuclear sequestration process.

A cell-based assay for aggregation inhibitors as therapeutics of polyglutamine-repeat disease and validation in Drosophila

The formation of polyglutamine-containing aggregates and inclusions are hallmarks of pathogenesis in Huntington's disease that can be recapitulated in model systems. Although the contribution of inclusions to pathogenesis is unclear, cell-based assays can be used to screen for chemical compounds that affect aggregation and may provide therapeutic benefit. We have developed inducible PC12 cell-culture models to screen for loss of visible aggregates. To test the validity of this approach, compounds that inhibit aggregation in the PC12 cell-based screen were tested in a Drosophila model of polyglutamine-repeat disease. The disruption of aggregation in PC12 cells strongly correlates with suppression of neuronal degeneration in Drosophila. Thus, the engineered PC12 cells coupled with the Drosophila model provide a rapid and effective method to screen and validate compounds.

Are Huntington's and polyglutamine-based ataxias proteasome storage diseases?

To date, 10 neurological diseases, including Huntington's and several ataxias, are caused by a lengthening of glutamine (Q) tracts in various proteins. Even though the Q expansions arise in unrelated proteins, the diseases share three striking features: (1) 35 contiguous glutamines constitutes the pathological threshold for 9 of the 10 diseases; (2) the Q-expanded proteins are expressed in many tissues, yet pathology is largely restricted to neurons; and (3) the Q-expanded proteins or fragments thereof form nuclear inclusions that also contain ubiquitin, proteasomes and chaperones. Our studies of the proteasome activator REGgamma suggest a possible explanation for these shared properties. REGgamma is highly expressed in brain, located in the nucleus and actually suppresses the proteasome active sites principally responsible for cleaving glutamine-MCA bonds. These observations coupled with reports that peptides longer than 35 residues, the polyQ pathology threshold, are unable to diffuse out of the proteasome suggest the following hypothesis. Proteins containing long glutamine tracts are efficiently pumped into REGgamma-capped 26S proteasomes, but REGgamma suppression of cleavage after glutamine produces polyQ fragments too long to diffuse out of the 20S proteolytic core thereby inactivating the 26S proteasome. In effect, we hypothesize that the polyQ pathologies may be proteasomal storage diseases analogous to disorders of lysosome catabolism.