PERK-dependent compartmentalization of ERAD and unfolded protein response machineries during ER stress

Combination of Polygonatum Rhizoma and Scutellaria baicalensis triggers apoptosis through downregulation of PON3-induced mitochondrial damage and endoplasmic reticulum stress in A549 cells

OBJECTIVE

Scutellaria baicalensis (SB) and Polygonatum Rhizoma (PR), two traditional Chinese medicines, are both known to suppress cancer. However, the mechanism and effect of combined treatment of them for lung cancer are rarely known. Investigating the combined effect of SB and PR (hereafter referred to as SP) in potential mechanism of lung cancer is required. This study was to evaluate the inhibitory effects of SP on A549 cell growth and to explore the underlying molecular mechanisms.

METHODS

According to the theory of Chinese medicine and network pharmacology, in the in vivo experiment, a mouse model of carcinoma in situ was constructed, and lung carcinoma in situ tissues were collected for proteomics analysis, hematoxylin-eosin staining, and CK19 immunohistochemistry. In the in vitro experiment, lung cancer A549 cells at logarithmic growth stage were taken, and the inhibitory effect of SP on the proliferation of A549 cells was detected by CCK8 method. The expression of PON3 was detected by quantitative polymerase chain reaction and western blot. In addition, the effect of SP on the induction of apoptosis in A549 cells and the changes of membrane potential and reactive oxygen species (ROS) content were detected by flow cytometry. The changes of PON3 content in endoplasmic reticulum (ER) are observed by laser confocal microscopy, whereas the effects of SP on the expression of apoptosis-related proteins and ER stress-related proteins in A549 cells were examined by western blot.

RESULT

By searching the Traditional Chinese Medicines of Systems Pharmacology (TCMSP) (https://www.tcmspe.com/index.php) database and SymMap database, the respective target genes of PR and SB were mapped into protein network interactions, and using Venn diagrams to show 38 genes in common between PR and SB and lung cancer, SP was found to play a role in the treatment of lung cancer. In vivo experiments showed that in a lung carcinoma in situ model, lung tumor tissue was significantly lower in the SP group compared with the control group, and PON3 was shown to be downregulated by lung tissue proteomics analysis. The combination of SP was able to inhibit the proliferation of A549 cells in a concentration-dependent manner (p < .0001). The expression levels of apoptosis-related proteins and ER stress proteins were significantly increased and the expression levels of PON3 and anti-apoptosis-related proteins were decreased in A549 cells. At the same time, knockdown of PON3 could inhibit tumor cell proliferation (p < .0001). The combination of different concentrations of SP significantly induced apoptosis in A549 cells (p < .05; p < .0001), increased ROS content (p < .01), and damaged mitochondrial membrane potential of A549 cells (p < .05; p < .0001), and significantly increased the expression levels of apoptosis-related proteins and ER stress proteins in lung cancer A549 cells.

CONCLUSION

SP inhibits proliferation of lung cancer A549 cells by downregulating PON3-induced apoptosis in the mitochondrial and ER pathways.

RNF26 binds perinuclear vimentin filaments to integrate ER and endolysosomal responses to proteotoxic stress

Proteotoxic stress causes profound endoplasmic reticulum (ER) membrane remodeling into a perinuclear quality control compartment (ERQC) for the degradation of misfolded proteins. Subsequent return to homeostasis involves clearance of the ERQC by endolysosomes. However, the factors that control perinuclear ER integrity and dynamics remain unclear. Here, we identify vimentin intermediate filaments as perinuclear anchors for the ER and endolysosomes. We show that perinuclear vimentin filaments engage the ER-embedded RING finger protein 26 (RNF26) at the C-terminus of its RING domain. This restricts RNF26 to perinuclear ER subdomains and enables the corresponding spatial retention of endolysosomes through RNF26-mediated membrane contact sites (MCS). We find that both RNF26 and vimentin are required for the perinuclear coalescence of the ERQC and its juxtaposition with proteolytic compartments, which facilitates efficient recovery from ER stress via the Sec62-mediated ER-phagy pathway. Collectively, our findings reveal a scaffolding mechanism that underpins the spatiotemporal integration of organelles during cellular proteostasis.

COP I and II dependent trafficking controls ER-associated degradation in mammalian cells

Misfolded proteins and components of the endoplasmic reticulum (ER) quality control and ER associated degradation (ERAD) machineries concentrate in mammalian cells in the pericentriolar ER-derived quality control compartment (ERQC), suggesting it as a staging ground for ERAD. By tracking the chaperone calreticulin and an ERAD substrate, we have now determined that the trafficking to the ERQC is reversible and recycling back to the ER is slower than the movement in the ER periphery. The dynamics suggest vesicular trafficking rather than diffusion. Indeed, using dominant negative mutants of ARF1 and Sar1 or the drugs Brefeldin A and H89, we observed that COPI inhibition causes accumulation in the ERQC and increases ERAD, whereas COPII inhibition has the opposite effect. Our results suggest that targeting of misfolded proteins to ERAD involves COPII-dependent transport to the ERQC and that they can be retrieved to the peripheral ER in a COPI-dependent manner.

NGLY1 Deficiency, a Congenital Disorder of Deglycosylation: From Disease Gene Function to Pathophysiology

N-Glycanase 1 (NGLY1) is a cytosolic enzyme involved in removing N-linked glycans of misfolded N-glycoproteins and is considered to be a component of endoplasmic reticulum-associated degradation (ERAD). The 2012 identification of recessive NGLY1 mutations in a rare multisystem disorder has led to intense research efforts on the roles of NGLY1 in animal development and physiology, as well as the pathophysiology of NGLY1 deficiency. Here, we present a review of the NGLY1-deficient patient phenotypes, along with insights into the function of this gene from studies in rodent and invertebrate animal models, as well as cell culture and biochemical experiments. We will discuss critical processes affected by the loss of NGLY1, including proteasome bounce-back response, mitochondrial function and homeostasis, and bone morphogenetic protein (BMP) signaling. We will also cover the biologically relevant targets of NGLY1 and the genetic modifiers of NGLY1 deficiency phenotypes in animal models. Together, these discoveries and disease models have provided a number of avenues for preclinical testing of potential therapeutic approaches for this disease.

Selective and competitive functions of the AAR and UPR pathways in stress-induced angiogenesis

The amino acid response (AAR) and unfolded protein response (UPR) pathways converge on eIF2α phosphorylation, which is catalyzed by Gcn2 and Perk, respectively, under different stresses. This close interconnection makes it difficult to specify different functions of AAR and UPR. Here, we generated a zebrafish model in which loss of threonyl-tRNA synthetase (Tars) induces angiogenesis dependent on Tars aminoacylation activity. Comparative transcriptome analysis of the tars-mutant and wild-type embryos with/without Gcn2- or Perk-inhibition reveals that only Gcn2-mediated AAR is activated in the tars-mutants, whereas Perk functions predominantly in normal development. Mechanistic analysis shows that, while a considerable amount of eIF2α is normally phosphorylated by Perk, the loss of Tars causes an accumulation of uncharged tRNA^Thr, which in turn activates Gcn2, leading to phosphorylation of an extra amount of eIF2α. The partial switchover of kinases for eIF2α largely overwhelms the functions of Perk in normal development. Interestingly, although inhibition of Gcn2 and Perk in this stress condition both can reduce the eIF2α phosphorylation levels, their functional consequences in the regulation of target genes and in the rescue of the angiogenic phenotypes are dramatically different. Indeed, genetic and pharmacological manipulations of these pathways validate that the Gcn2-mediated AAR, but not the Perk-mediated UPR, is required for tars-deficiency induced angiogenesis. Thus, the interconnected AAR and UPR pathways differentially regulate angiogenesis through selective functions and mutual competitions, reflecting the specificity and efficiency of multiple stress response pathways that evolve integrally to enable an organism to sense/respond precisely to various types of stresses.

eIF4A, a target of siRNA derived from rice stripe virus, negatively regulates antiviral autophagy by interacting with ATG5 in Nicotiana benthamiana

Autophagy is induced by viral infection and has antiviral functions in plants, but the underlying mechanism is poorly understood. We previously identified a viral small interfering RNA (vsiRNA) derived from rice stripe virus (RSV) RNA4 that contributes to the leaf-twisting and stunting symptoms caused by this virus by targeting the host eukaryotic translation initiation factor 4A (eIF4A) mRNA for silencing. In addition, autophagy plays antiviral roles by degrading RSV p3 protein, a suppressor of RNA silencing. Here, we demonstrate that eIF4A acts as a negative regulator of autophagy in Nicotiana benthamiana. Silencing of NbeIF4A activated autophagy and inhibited RSV infection by facilitating autophagic degradation of p3. Further analysis showed that NbeIF4A interacts with NbATG5 and interferes with its interaction with ATG12. Overexpression of NbeIF4A suppressed NbATG5-activated autophagy. Moreover, expression of vsiRNA-4A, which targets NbeIF4A mRNA for cleavage, induced autophagy by silencing NbeIF4A. Finally, we demonstrate that eIF4A from rice, the natural host of RSV, also interacts with OsATG5 and suppresses OsATG5-activated autophagy, pointing to the conserved function of eIF4A as a negative regulator of antiviral autophagy. Taken together, these results reveal that eIF4A negatively regulates antiviral autophagy by interacting with ATG5 and that its mRNA is recognized by a virus-derived siRNA, resulting in its silencing, which induces autophagy against viral infection.

Pridopidine reduces mutant huntingtin-induced endoplasmic reticulum stress by modulation of the Sigma-1 receptor

The endoplasmic reticulum (ER)-localized Sigma-1 receptor (S1R) is neuroprotective in models of neurodegenerative diseases, among them Huntington disease (HD). Recent clinical trials in HD patients and preclinical studies in cellular and mouse HD models suggest a therapeutic potential for the high-affinity S1R agonist pridopidine. However, the molecular mechanisms of the cytoprotective effect are unclear. We have previously reported strong induction of ER stress by toxic mutant huntingtin (mHtt) oligomers, which is reduced upon sequestration of these mHtt oligomers into large aggregates. Here, we show that pridopidine significantly ameliorates mHtt-induced ER stress in cellular HD models, starting at low nanomolar concentrations. Pridopidine reduced the levels of markers of the three branches of the unfolded protein response (UPR), showing the strongest effects on the PKR-like endoplasmic reticulum kinase (PERK) branch. The effect is S1R-dependent, as it is abolished in cells expressing mHtt in which the S1R was deleted using CRISPR/Cas9 technology. mHtt increased the level of the detergent-insoluble fraction of S1R, suggesting a compensatory cellular mechanism that responds to increased ER stress. Pridopidine further enhanced the levels of insoluble S1R, suggesting the stabilization of activated S1R oligomers. These S1R oligomeric species appeared in ER-localized patches, and not in the mitochondria-associated membranes nor the ER-derived quality control compartment. The colocalization of S1R with the chaperone BiP was significantly reduced by mHtt, and pridopidine restored this colocalization to normal, unstressed levels. Pridopidine increased toxic oligomeric mHtt recruitment into less toxic large sodium dodecyl sulfate-insoluble aggregates, suggesting that this in turn reduces ER stress and cytotoxicity.

GRP78 in lung cancer

Glucose-regulating protein 78 (GRP78) is a molecular chaperone in the endoplasmic reticulum (ER) that promotes folding and assembly of proteins, controls the quality of proteins, and regulates ER stress signaling through Ca²⁺ binding to the ER. In tumors, GRP78 is often upregulated, acting as a central stress sensor that senses and adapts to changes in the tumor microenvironment, mediating ER stress of cancer cells under various stimulations of the microenvironment to trigger the folding protein response. Increasing evidence has shown that GRP78 is closely associated with the progression and poor prognosis of lung cancer, and plays an important role in the treatment of lung cancer. Herein, we reviewed for the first time the functions and mechanisms of GRP78 in the pathological processes of lung cancer, including tumorigenesis, apoptosis, autophagy, progression, and drug resistance, giving a comprehensive understanding of the function of GRP78 in lung cancer. In addition, we also discussed the potential role of GRP78 as a prognostic biomarker and therapeutic target for lung cancer, which is conducive to improving the assessment of lung cancer and the development of new therapeutic interventions.

DIM-C-pPhtBu induces lysosomal dysfunction and unfolded protein response - mediated cell death via excessive mitophagy

Despite technological advances in cancer treatment, the survival rate of patients with head and neck cancer (HNC) has not improved significantly. Many studies have shown that endoplasmic reticulum (ER) stress-related signals are associated with mitochondrial damage and that these signals determine whether cells maintain homeostasis or activate cell death programs. The unfolded protein response (UPR) is regulated by ER membrane proteins such as double-stranded RNA-activated protein kinase R(PKR)-like ER kinase (PERK), which directly activate transcription of chaperones or genes that function in redox homeostasis, protein secretion, or cell death programs. In this study, we focused on the role of mitophagy and ER stress-mediated cell death induced by DIM-C-pPhtBu in HNC cancer. We found that DIM-C-pPhtBu, a compound that activates ER stress in many cancers, induced lysosomal dysfunction, excessive mitophagy, and cell death in HNC cells. Moreover, DIM-C-pPhtBu strongly inhibited HNC progression in a xenograft model by altering mitophagy related protein expression. Taken together, the results demonstrate that DIM-C-pPhtBu induces excessive mitophagy and eventually UPR-mediated cell death in HNC cells, suggesting that new anti-cancer drugs could be developed based on the connection between mitophagy and cancer cell death.

Starting at the beginning: endoplasmic reticulum proteostasis and systemic amyloid disease

Systemic amyloid diseases are characterized by the deposition of an amyloidogenic protein as toxic oligomers and amyloid fibrils on tissues distal from the site of protein synthesis. Traditionally, these diseases have been viewed as disorders of peripheral target tissues where aggregates are deposited, and toxicity is observed. However, recent evidence highlights an important role for endoplasmic reticulum (ER) proteostasis pathways within tissues synthesizing and secreting amyloidogenic proteins, such as the liver, in the pathogenesis of these disorders. Here, we describe the pathologic implications of ER proteostasis and its regulation on the toxic extracellular aggregation of amyloidogenic proteins implicated in systemic amyloid disease pathogenesis. Furthermore, we discuss the therapeutic potential for targeting ER proteostasis to reduce the secretion and toxic aggregation of amyloidogenic proteins to mitigate peripheral amyloid-associated toxicity involved in the onset and progression of systemic amyloid diseases.

Primer on the Pathogenesis of Severe COVID-19: Part One

In Part One of this exploration of the pathogenesis of coronavirus disease (COVID-19), the author will evaluate the viral and cellular immunological basis for the condition. The virus demonstrates a remarkable capability not just to evade, but to exploit host immune characteristics to perpetuate viral replication. In this regard, severe acute respiratory syndrome (SARS)/severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) disables most antiviral mechanisms, including the early interferon response, and avoids detection to permit unimpeded viral multiplication. Consequently, antigen-presenting cells fail to adequately stimulate the T-cell receptor. As a consequence, T-cell p53 remains highly expressed, which in turn disables an adequate effector T-cell response. Replicating SARS-CoV-2 double-strand RNA robustly activates protein kinase R (PKR)/PKR-like endoplasmic reticulum kinase (PERK). While the virus is grossly invulnerable to its antiviral effects, PKR is crucial for effecting the cytokine milieu in COVID-19. PERK is a component of the unfolded protein response, which eventuates in autophagy. SARS virions use double-membrane vesicles and adapt PERK signalling not only to avoid autophagy, but to facilitate replication. Viral activation of PKR/PERK is mutually exclusive to NLRP3 stimulation. The NLRP3 pathway elaborates IL-1β. This is chiefly a feature of paediatric SARS/SARS-CoV-2 cases. The difficulties encountered in predicting outcome and forging effective therapeutics speaks to the breadth of complexity of the immunopathogenesis of this virus.

Small molecule strategies to harness the unfolded protein response: where do we go from here?

The unfolded protein response (UPR) plays a central role in regulating endoplasmic reticulum (ER) and global cellular physiology in response to pathologic ER stress. The UPR is comprised of three signaling pathways activated downstream of the ER membrane proteins IRE1, ATF6, and PERK. Once activated, these proteins initiate transcriptional and translational signaling that functions to alleviate ER stress, adapt cellular physiology, and dictate cell fate. Imbalances in UPR signaling are implicated in the pathogenesis of numerous, etiologically-diverse diseases, including many neurodegenerative diseases, protein misfolding diseases, diabetes, ischemic disorders, and cancer. This has led to significant interest in establishing pharmacologic strategies to selectively modulate IRE1, ATF6, or PERK signaling to both ameliorate pathologic imbalances in UPR signaling implicated in these different diseases and define the importance of the UPR in diverse cellular and organismal contexts. Recently, there has been significant progress in the identification and characterization of UPR modulating compounds, providing new opportunities to probe the pathologic and potentially therapeutic implications of UPR signaling in human disease. Here, we describe currently available UPR modulating compounds, specifically highlighting the strategies used for their discovery and specific advantages and disadvantages in their application for probing UPR function. Furthermore, we discuss lessons learned from the application of these compounds in cellular and in vivo models to identify favorable compound properties that can help drive the further translational development of selective UPR modulators for human disease.

Regulation of BMP4/Dpp retrotranslocation and signaling by deglycosylation

During endoplasmic reticulum-associated degradation (ERAD), the cytoplasmic enzyme N-glycanase 1 (NGLY1) is proposed to remove N-glycans from misfolded N-glycoproteins after their retrotranslocation from the ER to the cytosol. We previously reported that NGLY1 regulates Drosophila BMP signaling in a tissue-specific manner (Galeone et al., 2017). Here, we establish the Drosophila Dpp and its mouse ortholog BMP4 as biologically relevant targets of NGLY1 and find, unexpectedly, that NGLY1-mediated deglycosylation of misfolded BMP4 is required for its retrotranslocation. Accumulation of misfolded BMP4 in the ER results in ER stress and prompts the ER recruitment of NGLY1. The ER-associated NGLY1 then deglycosylates misfolded BMP4 molecules to promote their retrotranslocation and proteasomal degradation, thereby allowing properly-folded BMP4 molecules to proceed through the secretory pathway and activate signaling in other cells. Our study redefines the role of NGLY1 during ERAD and suggests that impaired BMP4 signaling might underlie some of the NGLY1 deficiency patient phenotypes.

The Human Cytomegalovirus Nonstructural Glycoprotein UL148 Reorganizes the Endoplasmic Reticulum

Human cytomegalovirus (HCMV) encodes an endoplasmic reticulum (ER)-resident glycoprotein, UL148, which activates the unfolded protein response (UPR) but is fully dispensable for viral replication in cultured cells. Hence, its previously ascribed roles in immune evasion and modulation of viral cell tropism are hypothesized to cause ER stress. Here, we show that UL148 is necessary and sufficient to drive the formation of prominent ER-derived structures that on average occupy 5% of the infected cell cytoplasm. The structures are sites where UL148 coalesces with cellular proteins involved in ER quality control, such as HRD1 and EDEM1. Electron microscopy revealed that cells infected with wild-type but not UL148-null HCMV show prominent accumulations of densely packed ruffled ER membranes which connect to distended cisternae of smooth and partially rough ER. During ectopic expression of UL148-green fluorescent protein (GFP) fusion protein, punctate signals traffic to accumulate at conspicuous structures. The structures exhibit poor recovery of fluorescence after photobleaching, which suggests that their contents are poorly mobile and do not efficiently exchange with the rest of the ER. Small-molecule blockade of the integrated stress response (ISR) prevents the formation of puncta, leading to a uniform reticular fluorescent signal. Accordingly, ISR inhibition during HCMV infection abolishes the coalescence of UL148 and HRD1 into discrete structures, which argues that UL148 requires the ISR to cause ER reorganization. Given that UL148 stabilizes immature forms of a receptor binding subunit for a viral envelope glycoprotein complex important for HCMV infectivity, our results imply that stress-dependent ER remodeling contributes to viral cell tropism.IMPORTANCE Perturbations to endoplasmic reticulum (ER) morphology occur during infection with various intracellular pathogens and in certain genetic disorders. We identify that a human cytomegalovirus (HCMV) gene product, UL148, profoundly reorganizes the ER during infection and is sufficient to do so when expressed on its own. Our results reveal that UL148-dependent reorganization of the ER is a prominent feature of HCMV-infected cells. Moreover, we find that this example of virally induced organelle remodeling requires the integrated stress response (ISR), a stress adaptation pathway that contributes to a number of disease states. Since ER reorganization accompanies roles of UL148 in modulation of HCMV cell tropism and in evasion of antiviral immune responses, our results may have implications for understanding the mechanisms involved. Furthermore, our findings provide a basis to utilize UL148 as a tool to investigate organelle responses to stress and to identify novel drugs targeting the ISR.

Endoplasmic Reticulum Stress Activates Unfolded Protein Response Signaling and Mediates Inflammation, Obesity, and Cardiac Dysfunction: Therapeutic and Molecular Approach

Obesity has been implicated as a risk factor for insulin resistance and cardiovascular diseases (CVDs). Although the association between obesity and CVD is a well-established phenomenon, the precise mechanisms remain incompletely understood. This has led to a relative paucity of therapeutic measures for the prevention and treatment of CVD and associated metabolic disorders. Recent studies have shed light on the pivotal role of prolonged endoplasmic reticulum stress (ERS)-initiated activation of the unfolded protein response (UPR), the ensuing chronic low-grade inflammation, and altered insulin signaling in promoting obesity-compromised cardiovascular system (CVS). In this aspect, potential ways of attenuating ERS-initiated UPR signaling seem a promising avenue for therapeutic interventions. We review intersecting role of obesity-induced ERS, chronic inflammation, insulin resistance, and oxidative stress in the discovery of targeted therapy. Moreover, this review highlights the current progress and strategies on therapeutics being explored in preclinical and clinical research to modulate ERS and UPR signaling.

Compartmentalization and Selective Tagging for Disposal of Misfolded Glycoproteins

The ability of mammalian cells to correctly identify and degrade misfolded secretory proteins, most of them bearing N-glycans, is crucial for their correct function and survival. An inefficient disposal mechanism results in the accumulation of misfolded proteins and consequent endoplasmic reticulum (ER) stress. N-glycan processing creates a code that reveals the folding status of each molecule, enabling continued folding attempts or targeting of the doomed glycoprotein for disposal. We review here the main steps involved in the accurate processing of unfolded glycoproteins. We highlight recent data suggesting that the processing is not stochastic, but that there is selective accelerated glycan trimming on misfolded glycoprotein molecules.

PERK Signaling Regulates Extracellular Proteostasis of an Amyloidogenic Protein During Endoplasmic Reticulum Stress

The PERK arm of the unfolded protein response (UPR) regulates cellular proteostasis and survival in response to endoplasmic reticulum (ER) stress. However, the impact of PERK signaling on extracellular proteostasis is poorly understood. We define how PERK signaling influences extracellular proteostasis during ER stress using a conformational reporter of the secreted amyloidogenic protein transthyretin (TTR). We show that inhibiting PERK signaling impairs secretion of destabilized TTR during thapsigargin (Tg)-induced ER stress by increasing its ER retention in chaperone-bound complexes. Interestingly, PERK inhibition increases the ER stress-dependent secretion of TTR in non-native conformations that accumulate extracellularly as soluble oligomers. Pharmacologic or genetic TTR stabilization partially restores secretion of native TTR tetramers. However, PERK inhibition still increases the ER stress-dependent secretion of TTR in non-native conformations under these conditions, indicating that the conformation of stable secreted proteins can also be affected by inhibiting PERK. Our results define a role for PERK in regulating extracellular proteostasis during ER stress and indicate that genetic or aging-related alterations in PERK signaling can exacerbate ER stress-related imbalances in extracellular proteostasis implicated in diverse diseases.

PDE5A Suppresses Proteasome Activity Leading to Insulin Resistance in C2C12 Myotubes

OBJECTIVE

The involvement of phosphodiesterase type 5 (PDE5) in the development of insulin resistance has been reported recently. However, the underlying molecular mechanism remains unclear. The present study aims at investigating the potential impacts of PDE5A on insulin signaling in C2C12 skeletal muscle myotubes and uncover the related mechanism.

METHODS

C2C12 myoblasts were differentiated into myotubes. Western blot was performed to detect the levels of proteins and phosphorylated proteins. Glucose uptake was determined by a colorimetric kit. The overexpression or knockdown of specific protein was carried out by infecting the myotubes with adenoviruses carrying cDNA or shRNA corresponding to the targeted protein, respectively.

RESULTS

PDE5A was demonstrated to negatively regulate insulin signaling, evidenced by the opposite effects on the suppression or enhancement of the insulin-stimulated Akt phosphorylation and 2-deoxy-D-glucose (2-DG) uptake in C2C12 myotubes, when PDE5A was overexpressed or knockdown, respectively. Interestingly, PDE5A overexpression led to significantly enhanced, while its knockdown resulted in markedly reduced, endoplasmic reticulum (ER) stress. Inhibition of ER stress improved PDE5A overexpression-induced insulin resistance. In addition, PDE5A was found to suppress proteasome activity. Inhibition of PDE5 by its selective inhibitor icariin restored PDE5A overexpression-reduced proteasome activity and mitigated PDE5A overexpression-induced ER stress. Consistently, icariin administration also markedly attenuated the detrimental impacts of PDE5A overexpression on insulin signaling.

CONCLUSIONS

These results suggest that PDE5A suppresses proteasome activity, which results in ER stress and subsequent insulin resistance in C2C12 myotubes.

Endoplasmic reticulum stress-dependent ROS production mediates synovial myofibroblastic differentiation in the immobilization-induced rat knee joint contracture model

Joint contracture is a common complication for people with joint immobility that involves fibrosis structural alteration in the joint capsule. Considering that endoplasmic reticulum (ER) stress plays a prominent role in the promotion of tissue fibrosis, we investigated whether the unfolded protein response (UPR) contributes to the fibrotic development in immobilization-induced knee joint contractures. Using a non-traumatic rat knee joint contracture model, twelve female Sprague-Dawley rats received knee joint immobilization for a period of 8 weeks. We found that fibrosis protein markers (type I collagen, α-SMA) and UPR (GRP78, ATF6α, XBP1s) markers were parallelly upregulated in rat primary cultured synovial myofibroblasts. In the same cell types, pre-treatment with an ER stress inhibitor, 4-phenylbutyric acid (4-PBA), not only abrogated cytokine TGFβ1 stimulation but also reduced the protein level of UPR. Additionally, high reactive oxygen species (ROS) generation was detected in synovial myofibroblasts through flow cytometry, as expected. Notably, TGFβ1-induced UPR was significantly reduced through the inhibition of ROS with antioxidants. These data suggest that ER stress act as a pro-fibrotic stimulus through the overexpression of ROS in synovial fibroblasts. Interestingly, immunohistochemical results showed an increase in the UPR protein levels both in human acquired joint contractures capsule tissue and in animal knee joint contracture tissue. Together, our findings suggest that ER stress contributes to synovial myofibroblastic differentiation in joint capsule fibrosis and may also serve as a potential therapeutic target in joint contractures.

Oleuropein and rutin protect against 6-OHDA-induced neurotoxicity in PC12 cells through modulation of mitochondrial function and unfolded protein response

Parkinson’s disease (PD) is a highly prevalent neurodegenerative disorder, often associated with oxidative stress-induced transcriptional changes in dopaminergic neurons. Phenolic antioxidants, oleuropein (OLE) and rutin (RUT) have attracted a great interest due to their potential to counteract oxidative protein aggregation and toxicity. This study aimed at examining the effects of OLE and RUT against 6-OHDA-induced stress response in rat pheochromocytoma cells. When differentiated PC12 cells were exposed to oxidative stress composer 6-OHDA (100 μM, 8 h), a decreased mitochondrial membrane potential (ΔΨm) was observed along with a significant loss of cell viability and apoptotic nuclear changes. Exposure to 6-OHDA resulted in unfolded protein response (UPR) in differentiated PC12 cells as evidenced by an increased level of endoplasmic reticulum (ER)-localized transmembrane signal transducer IRE1α, adaptive response proteins ATF-4 and proapoptotic transcription factor CHOP. OLE or RUT pretreatment (24 h) at low doses (1–50 μM) protected the differentiated PC12 cells from 6-OHDA-induced cytotoxicity as assessed by increased viability, improved ΔΨm and inhibited apoptosis, whereas relatively high doses of OLE or RUT (>50 μM) inhibited cell growth and proliferation, indicating a typical hormetic effect. In hormetic doses, OLE and RUT up-regulated 6-OHDA-induced increase in IRE1α, ATF-4 and inhibited CHOP, PERK, BIP and PDI. 6-OHDA-activated XBP1 splicing was also inhibited by OLE or RUT. The presented results suggest that neuroprotection against 6-OHDA-induced oxidative toxicity may be attributable to neurohormetic effects of OLE or RUT at low doses through regulating mitochondrial functions, controlling persistent protein misfolding, activating and/or amplificating the adaptive response-related signaling pathways, leading to UPR prosurvival output.

The SAT Protein of Porcine Parvovirus Accelerates Viral Spreading through Induction of Irreversible Endoplasmic Reticulum Stress

The SAT protein (SATp) of porcine parvovirus (PPV) accumulates in the endoplasmic reticulum (ER), and SAT deletion induces the slow-spreading phenotype. The in vitro comparison of the wild-type Kresse strain and its SAT knockout (SAT^-) mutant revealed that prolonged cell integrity and late viral release are responsible for the slower spreading of the SAT^- virus. During PPV infection, regardless of the presence or absence of SATp, the expression of downstream ER stress response proteins (Xbp1 and CHOP) was induced. However, in the absence of SATp, significant differences in the quantity and the localization of CHOP were detected, suggesting a role of SATp in the induction of irreversible ER stress in infected cells. The involvement of the induction of irreversible ER stress in porcine testis (PT) cell necrosis and viral egress was confirmed by treatment of infected cells by ER stress-inducing chemicals (MG132, dithiothreitol, and thapsigargin), which accelerated the egress and spreading of both the wild-type and the SAT^- viruses. UV stress induction had no beneficial effect on PPV infection, underscoring the specificity of ER stress pathways in the process. However, induction of CHOP and its nuclear translocation cannot alone be responsible for the biological effect of SAT, since nuclear CHOP could not complement the lack of SAT in a coexpression experiment.IMPORTANCE SATp is encoded by an alternative open reading frame of the PPV genome. Earlier we showed that SATp of the attenuated PPV NADL-2 strain accumulates in the ER and accelerates virus release and spreading. Our present work revealed that slow spreading is a general feature of SAT^- PPVs and is the consequence of prolonged cell integrity. PPV infection induced ER stress in infected cells regardless of the presence of SATp, as demonstrated by the morphological changes of the ER and expression of the stress response proteins Xbp1 and CHOP. However, the presence of SATp made the ER stress more severe and accelerated cell death during infection, as shown by the higher rate of expression of CHOP and alteration of the localization of CHOP. The beneficial effect of irreversible ER stress on PPV spread was confirmed by treatment of infected cells with ER stress-inducing chemicals.

Curcumin Improves Palmitate-Induced Insulin Resistance in Human Umbilical Vein Endothelial Cells by Maintaining Proteostasis in Endoplasmic Reticulum

Dysfunction of proteasome and autophagy will result in disturbance of endoplasmic reticulum (ER) proteostasis, and thus lead to long-term and chronic ER stress and subsequent unfolded protein response (UPR), which is implicated in the occurrence and development of insulin resistance. Curcumin exerts beneficial metabolic effects in in vitro cells and in vivo animal models of diabetes and diabetic complications including cardiovascular diseases, due to its powerful anti-oxidative and anti-inflammatory properties. However, its impacts on insulin resistance of endothelial cells and its underlying mechanism(s) remain ill-defined. Herein, we tested the hypothesis that curcumin action in ER protein quality control was related to improvement of insulin resistance in human umbilical vein endothelial cells (HUVECs) cultured with saturated fatty acid palmitate. We found that palmitate treatment induced insulin resistance of HUVECs and activated both the ubiquitin-proteasome system (UPS) and autophagy. Palmitate-stimulated activation of the UPS and autophagy was attenuated by pharmacological inhibition of ER stress. In addition, curcumin supplementation mitigated palmitate-induced insulin resistance, inhibited the UPS, and activated autophagy. Furthermore, curcumin administration suppressed palmitate-induced protein aggregation and ER stress. Genetic inhibition of autophagy by silencing autophagy protein 5 (Atg5) completely restored total protein ubiquitination and protein aggregation in HUVECs treated with combined curcumin and palmitate. Atg5-knockdown also abolished the beneficial effects of curcumin on palmitate-induced ER stress, JNK/IRS-1 pathway as well as insulin signaling. Our results reveal that curcumin-activated autophagy could maintain proteostasis in ER leading to attenuation of ER stress and subsequent inhibition of JNK/IRS-1 pathway and improvement of insulin resistance.

Aberrant Subcellular Dynamics of Sigma-1 Receptor Mutants Underlying Neuromuscular Diseases

The sigma-1 receptor (σ-1R) is an endoplasmic reticulum resident chaperone protein involved in a plethora of cellular functions, and whose disruption has been implicated in a wide range of diseases. Genetic analysis has revealed two σ-1R mutants involved in neuromuscular disorders. A point mutation (E102Q) in the ligand-binding domain results in the juvenile form of amyotrophic lateral sclerosis (ALS16), and a 20 amino-acid deletion (Δ31-50) in the putative cytosolic domain leads to a form of distal hereditary motor neuropathy. We investigated the localization and functional properties of these mutants in cell lines using confocal imaging and electrophysiology. The σ-1R mutants exhibited a significant increase in mobility, aberrant localization, and enhanced block of the inwardly rectifying K(+) channel Kir2.1, compared with the wild-type σ-1R. Thus, these σ-1R mutants have different functional properties that could contribute to their disease phenotypes.

PEK-1 is crucial for hormesis induced by inhibition of the IRE-1/XBP-1 pathway in the Caenorhabditis elegans mev-1 mutant

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes an imbalance of proteostasis and is related to many pathological conditions. In answer to this ER stress cells activate a network of three integrated signaling pathways consolidated as the unfolded protein response of the ER (UPR(ER)), which is also present in the stress-sensitive Caenorhabditis elegans mutant mev-1. Whereas inhibition of one of those pathways by RNA-interference (RNAi) versus xbp-1 results in reduced survival of mev-1 nematodes under heat stress, additional knockdown of the xbp-1 splicing activator ire-1 results in a PEK-1-dependent hormetic response. In contrast, increased survival under ire-1/xbp-1 double RNAi was found to be independent of the presence of HSP-4, an UPR(ER)-specific chaperone, as evidenced under ire-1/xbp-1/hsp-4 triple knockdown conditions. Moreover, ire-1/xbp-1 double-RNAi significantly increased chymotrypsin-like proteasomal activity, which was completely blocked under additional RNAi versus pek-1. In conclusion, we identified PEK-1 as a mediator of hormesis in the mev-1 mutant of C. elegans which is induced by simultaneous inhibition of XBP-1 and its splicing activator IRE-1 and mediated through activation of the proteasome.

Protein aggregation and ER stress

Protein aggregation is a common feature of the protein misfolding or conformational diseases, among them most of the neurodegenerative diseases. These disorders are a major scourge, with scarce if any effective therapies at present. Recent research has identified ER stress as a major mechanism implicated in cytotoxicity in these diseases. Whether amyloid-β or tau in Alzheimer's, α-synuclein in Parkinson's, huntingtin in Huntington's disease or other aggregation-prone proteins in many other neurodegenerative diseases, there is a shared pathway of oligomerization and aggregation into amyloid fibrils. There is increasing evidence in recent years that the toxic species, and those that evoke ER stress, are the intermediate oligomeric forms and not the final amyloid aggregates. This review focuses on recent findings on the mechanisms and importance of the development of ER stress upon protein aggregation, especially in neurodegenerative diseases, and possible therapeutic approaches that are being examined. This article is part of a Special Issue entitled SI:ER stress.

Melanoma and the Unfolded Protein Response

The UPR (unfolded protein response) has been identified as a key factor in the progression and metastasis of cancers, notably melanoma. Several mediators of the UPR are upregulated in cancers, e.g., high levels of GRP78 (glucose-regulator protein 78 kDa) correlate with progression and poor outcome in melanoma patients. The proliferative burden of cancer induces stress and activates several cellular stress responses. The UPR is a tightly orchestrated stress response that is activated upon the accumulation of unfolded proteins within the ER (endoplasmic reticulum). The UPR is designed to mediate two conflicting outcomtes, recovery and apoptosis. As a result, the UPR initiates a widespread signaling cascade to return the cell to homeostasis and failing to achieve cellular recovery, initiates UPR-induced apoptosis. There is evidence that ER stress and subsequently the UPR promote tumourigenesis and metastasis. The complete role of the UPR has yet to be defined. Understanding how the UPR allows for adaption to stress and thereby assists in cancer progression is important in defining an archetype of melanoma pathology. In addition, elucidation of the mechanisms of the UPR may lead to development of effective treatments of metastatic melanoma.

Alzheimer's disease-causing proline substitutions lead to presenilin 1 aggregation and malfunction

Do different neurodegenerative maladies emanate from the failure of a mutual protein folding mechanism? We have addressed this question by comparing mutational patterns that are linked to the manifestation of distinct neurodegenerative disorders and identified similar neurodegeneration-linked proline substitutions in the prion protein and in presenilin 1 that underlie the development of a prion disorder and of familial Alzheimer's disease (fAD), respectively. These substitutions were found to prevent the endoplasmic reticulum (ER)-resident chaperone, cyclophilin B, from assisting presenilin 1 to fold properly, leading to its aggregation, deposition in the ER, reduction of γ-secretase activity, and impaired mitochondrial distribution and function. Similarly, reduced quantities of the processed, active presenilin 1 were observed in brains of cyclophilin B knockout mice. These discoveries imply that reduced cyclophilin activity contributes to the development of distinct neurodegenerative disorders, propose a novel mechanism for the development of certain fAD cases, and support the emerging theme that this disorder can stem from aberrant presenilin 1 function. This study also points at ER chaperones as targets for the development of counter-neurodegeneration therapies.

Protein kinase R-like ER kinase and its role in endoplasmic reticulum stress-decided cell fate

Over the past few decades, understandings and evidences concerning the role of endoplasmic reticulum (ER) stress in deciding the cell fate have been constantly growing. Generally, during ER stress, the signal transductions are mainly conducted by three ER stress transducers: protein kinase R-like endoplasmic reticulum kinase (PERK), inositol-requiring kinase 1 (IRE1) and activating transcription factor 6 (ATF6). Consequently, the harmful stimuli from the ER stress transducers induce apoptosis and autophagy, which share several crosstalks and eventually decide the cell fate. The dominance of apoptosis or autophagy induced by ER stress depends on the type and degree of the stimuli. When ER stress is too severe and prolonged, apoptosis is induced to eliminate the damaged cells; however, when stimuli are mild, cell survival is promoted to maintain normal physiological functions by inducing autophagy. Although all the three pathways participate in ER stress-induced apoptosis and autophagy, PERK shows several unique characteristics by interacting with some specific downstream effectors. Notably, there are some preliminary findings on PERK-dependent mechanisms switching autophagy and apoptosis. In this review, we particularly focused on the novel, intriguing and complicated role of PERK in ER stress-decided cell fate, and also discussed more roles of PERK in restoring cellular homeostasis. However, more in-depth knowledge of PERK in the future would facilitate our understanding about many human diseases and benefit in searching for new molecular therapeutic targets.

The Role of Lectin-Carbohydrate Interactions in the Regulation of ER-Associated Protein Degradation

Proteins entering the secretory pathway are translocated across the endoplasmic reticulum (ER) membrane in an unfolded form. In the ER they are restricted to a quality control system that ensures correct folding or eventual degradation of improperly folded polypeptides. Mannose trimming of N-glycans on newly synthesized proteins plays an important role in the recognition and sorting of terminally misfolded glycoproteins for ER-associated protein degradation (ERAD). In this process misfolded proteins are retrotranslocated into the cytosol, polyubiquitinated, and eventually degraded by the proteasome. The mechanism by which misfolded glycoproteins are recognized and recruited to the degradation machinery has been extensively studied during last decade. In this review, we focus on ER degradation-enhancing α-mannosidase-like protein (EDEM) family proteins that seem to play a key role in the discrimination between proteins undergoing a folding process and terminally misfolded proteins directed for degradation. We describe interactions of EDEM proteins with other components of the ERAD machinery, as well as with various protein substrates. Carbohydrate-dependent interactions together with N-glycan-independent interactions seem to regulate the complex process of protein recognition and direction for proteosomal degradation.

Muscle Segment Homeobox Genes Direct Embryonic Diapause by Limiting Inflammation in the Uterus

Embryonic diapause is a reproductive strategy widespread in the animal kingdom. This phenomenon is defined by a temporary arrest in blastocyst growth and metabolic activity within a quiescent uterus without implantation until the environmental and maternal milieu become favorable for pregnancy to progress. We found that uterine Msx expression persists during diapause across species; their inactivation in the mouse uterus results in termination of diapause with the development of implantation-like responses ("pseudoimplantation") that ultimately succumbed to resorption. To understand the cause of this failure, we compared proteome profiles between floxed and Msx-deleted uteri. In deleted uteri, several functional networks, including transcription/translation, ubiquitin-proteasome, inflammation, and endoplasmic reticulum stress, were dysregulated. Computational modeling predicted intersection of these pathways on an enhanced inflammatory signature. Further studies showed that this signature was reflected in increased phosphorylated IκB levels and nuclear NFκB in deleted uteri. This was associated with enhanced proteasome activity and endoplasmic reticulum stress. Interestingly, treatment with anti-inflammatory glucocorticoid (dexamethasone) reduced the inflammatory signature with improvement of the diapause phenotype. These findings highlight an unexpected role of uterine Msx in limiting aberrant inflammatory responses to maintain embryonic diapause.

Glycoprotein folding and quality-control mechanisms in protein-folding diseases

Biosynthesis of proteins – from translation to folding to export – encompasses a complex set of events that are exquisitely regulated and scrutinized to ensure the functional quality of the end products. Cells have evolved to capitalize on multiple post-translational modifications in addition to primary structure to indicate the folding status of nascent polypeptides to the chaperones and other proteins that assist in their folding and export. These modifications can also, in the case of irreversibly misfolded candidates, signal the need for dislocation and degradation. The current Review focuses on the glycoprotein quality-control (GQC) system that utilizes protein N-glycosylation and N-glycan trimming to direct nascent glycopolypeptides through the folding, export and dislocation pathways in the endoplasmic reticulum (ER). A diverse set of pathological conditions rooted in defective as well as over-vigilant ER quality-control systems have been identified, underlining its importance in human health and disease. We describe the GQC pathways and highlight disease and animal models that have been instrumental in clarifying our current understanding of these processes.

Mammalian ER mannosidase I resides in quality control vesicles, where it encounters its glycoprotein substrates

ER mannosidase I (ERManI) was found recently in the Golgi. This result is found to arise artificially from membrane disturbance in immunofluorescence methods. ERManI is located in novel vesicles to which substrates traffic and that converge at the ER-derived quality control compartment under ER stress.

Endoplasmic reticulum α1,2 mannosidase I (ERManI), a central component of ER quality control and ER-associated degradation (ERAD), acts as a timer enzyme, modifying N-linked sugar chains of glycoproteins with time. This process halts glycoprotein folding attempts when necessary and targets terminally misfolded glycoproteins to ERAD. Despite the importance of ERManI in maintenance of glycoprotein quality control, fundamental questions regarding this enzyme remain controversial. One such question is the subcellular localization of ERManI, which has been suggested to localize to the ER membrane, the ER-derived quality control compartment (ERQC), and, surprisingly, recently to the Golgi apparatus. To try to clarify this controversy, we applied a series of approaches that indicate that ERManI is located, at the steady state, in quality control vesicles (QCVs) to which ERAD substrates are transported and in which they interact with the enzyme. Both endogenous and exogenously expressed ERManI migrate at an ER-like density on iodixanol gradients, suggesting that the QCVs are derived from the ER. The QCVs are highly mobile, displaying dynamics that are dependent on microtubules and COP-II but not on COP-I vesicle machinery. Under ER stress conditions, the QCVs converge in a juxtanuclear region, at the ERQC, as previously reported. Our results also suggest that ERManI is turned over by an active autophagic process. Of importance, we found that membrane disturbance, as is common in immunofluorescence methods, leads to an artificial appearance of ERManI in a Golgi pattern.

Rab2A is a pivotal switch protein that promotes either secretion or ER-associated degradation of (pro)insulin in insulin-secreting cells

Rab2A, a small GTPase localizing to the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC), regulates COPI-dependent vesicular transport from the ERGIC. Rab2A knockdown inhibited glucose-stimulated insulin secretion and concomitantly enlarged the ERGIC in insulin-secreting cells. Large aggregates of polyubiquitinated proinsulin accumulated in the cytoplasmic vicinity of a unique large spheroidal ERGIC, designated the LUb-ERGIC. Well-known components of ER-associated degradation (ERAD) also accumulated at the LUb-ERGIC, creating a suitable site for ERAD-mediated protein quality control. Moreover, chronically high glucose levels, which induced the enlargement of the LUb-ERGIC and ubiquitinated protein aggregates, impaired Rab2A activity by promoting dissociation from its effector, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in response to poly (ADP-ribosyl)ation of GAPDH. The inactivation of Rab2A relieved glucose-induced ER stress and inhibited ER stress-induced apoptosis. Collectively, these results suggest that Rab2A is a pivotal switch that controls whether insulin should be secreted or degraded at the LUb-ERGIC and Rab2A inactivation ensures alleviation of ER stress and cell survival under chronic glucotoxicity.

ER stress-induced eIF2-alpha phosphorylation underlies sensitivity of striatal neurons to pathogenic huntingtin

A hallmark of Huntington's disease is the pronounced sensitivity of striatal neurons to polyglutamine-expanded huntingtin expression. Here we show that cultured striatal cells and murine brain striatum have remarkably low levels of phosphorylation of translation initiation factor eIF2α, a stress-induced process that interferes with general protein synthesis and also induces differential translation of pro-apoptotic factors. EIF2α phosphorylation was elevated in a striatal cell line stably expressing pathogenic huntingtin, as well as in brain sections of Huntington's disease model mice. Pathogenic huntingtin caused endoplasmic reticulum (ER) stress and increased eIF2α phosphorylation by increasing the activity of PKR-like ER-localized eIF2α kinase (PERK). Importantly, striatal neurons exhibited special sensitivity to ER stress-inducing agents, which was potentiated by pathogenic huntingtin. We could strongly reduce huntingtin toxicity by inhibiting PERK. Therefore, alteration of protein homeostasis and eIF2α phosphorylation status by pathogenic huntingtin appears to be an important cause of striatal cell death. A dephosphorylated state of eIF2α has been linked to cognition, which suggests that the effect of pathogenic huntingtin might also be a source of the early cognitive impairment seen in patients.

Herp coordinates compartmentalization and recruitment of HRD1 and misfolded proteins for ERAD

The unfolded protein response PERK branch induces recruitment of misfolded proteins and the ubiquitin ligase HRD1 to the ER-derived quality control compartment (ERQC), a staging ground for ER-associated degradation (ERAD). This is accomplished by up-regulation of homocysteine-induced ER protein (Herp), which recruits the ERAD complex at the ERQC.

A functional unfolded protein response (UPR) is essential for endoplasmic reticulum (ER)-associated degradation (ERAD) of misfolded secretory proteins, reflecting the fact that some level of UPR activation must exist under normal physiological conditions. A coordinator of the UPR and ERAD processes has long been sought. We previously showed that the PKR-like, ER-localized eukaryotic translation initiation factor 2α kinase branch of the UPR is required for the recruitment of misfolded proteins and the ubiquitin ligase HRD1 to the ER-derived quality control compartment (ERQC), a staging ground for ERAD. Here we show that homocysteine-induced ER protein (Herp), a protein highly upregulated by this UPR branch, is responsible for this compartmentalization. Herp localizes to the ERQC, and our results suggest that it recruits HRD1, which targets to ERAD the substrate presented by the OS-9 lectin at the ERQC. Predicted overall structural similarity of Herp to the ubiquitin-proteasome shuttle hHR23, but including a transmembrane hairpin, suggests that Herp may function as a hub for membrane association of ERAD machinery components, a key organizer of the ERAD complex.

Polyribosome and ribonucleoprotein complex redistribution of mRNA induced by GnRH involves both EIF2AK3 and MAPK signaling

The neuropeptide gonadotropin-releasing hormone stimulates synthesis and secretion of the glycoprotein gonadotropic hormones and activates the unfolded protein response, which causes a transient reduction of endoplasmic reticulum-associated mRNA translation. Hormone-treated cell extracts were fractionated to resolve mRNA in active polyribosomes from mRNA in inactive complexes. Quantitative real-time PCR and expression array analysis were used to determine hormone-induced redistribution of mRNAs between fractions and individual mRNAs were found to be redistributed differentially. Among the affected mRNAs relevant to gonadotropin synthesis, the luteinizing hormone subunit genes Lhb and Cga were enriched in the ribonucleoprotein pool. The MAP kinase phosphatase Dusp1 was enriched in the polyribosome pool. Enrichment of Dusp1 mRNA in the polyribosome pool was independent of the unfolded protein response, sensitive to ERK inhibition, and dependent on the 3'untranslated region. The results show that GnRH exerts translational control to modulate physiologically relevant gene expression through two distinct signaling pathways.

Palmitoylation is the switch that assigns calnexin to quality control or ER Ca2+ signaling

The palmitoylation of calnexin serves to enrich calnexin on the mitochondria-associated membrane (MAM). Given a lack of information on the significance of this finding, we have investigated how this endoplasmic reticulum (ER)-internal sorting signal affects the functions of calnexin. Our results demonstrate that palmitoylated calnexin interacts with sarcoendoplasmic reticulum (SR) Ca(2+) transport ATPase (SERCA) 2b and that this interaction determines ER Ca(2+) content and the regulation of ER-mitochondria Ca(2+) crosstalk. In contrast, non-palmitoylated calnexin interacts with the oxidoreductase ERp57 and performs its well-known function in quality control. Interestingly, our results also show that calnexin palmitoylation is an ER-stress-dependent mechanism. Following a short-term ER stress, calnexin quickly becomes less palmitoylated, which shifts its function from the regulation of Ca(2+) signaling towards chaperoning and quality control of known substrates. These changes also correlate with a preferential distribution of calnexin to the MAM under resting conditions, or the rough ER and ER quality control compartment (ERQC) following ER stress. Our results have therefore identified the switch that assigns calnexin either to Ca(2+) signaling or to protein chaperoning.

Role of cell-type-specific endoplasmic reticulum-associated degradation in polyomavirus trafficking

BK polyomavirus (BKPyV) is a widespread human pathogen that establishes a lifelong persistent infection and can cause severe disease in immunosuppressed patients. BKPyV is a nonenveloped DNA virus that must traffic through the endoplasmic reticulum (ER) for productive infection to occur; however, it is unknown how BKPyV exits the ER before nuclear entry. In this study, we elucidated the role of the ER-associated degradation (ERAD) pathway during BKPyV intracellular trafficking in renal proximal tubule epithelial (RPTE) cells, a natural host cell. Using proteasome and ERAD inhibitors, we showed that ERAD is required for productive entry. Altered trafficking and accumulation of uncoated viral intermediates were detected by fluorescence in situ hybridization and indirect immunofluorescence in the presence of an inhibitor. Additionally, we detected a change in localization of partially uncoated virus within the ER during proteasome inhibition, from a BiP-rich area to a calnexin-rich subregion, indicating that BKPyV accumulated in an ER subcompartment. Furthermore, inhibiting ERAD did not prevent entry of capsid protein VP1 into the cytosol from the ER. By comparing the cytosolic entry of the related polyomavirus simian virus 40 (SV40), we found that dependence on the ERAD pathway for cytosolic entry varied between the polyomaviruses and between different cell types, namely, immortalized CV-1 cells and primary RPTE cells.

Role of cell-type-specific endoplasmic reticulum-associated degradation in polyomavirus trafficking

BK polyomavirus (BKPyV) is a widespread human pathogen that establishes a lifelong persistent infection and can cause severe disease in immunosuppressed patients. BKPyV is a nonenveloped DNA virus that must traffic through the endoplasmic reticulum (ER) for productive infection to occur; however, it is unknown how BKPyV exits the ER before nuclear entry. In this study, we elucidated the role of the ER-associated degradation (ERAD) pathway during BKPyV intracellular trafficking in renal proximal tubule epithelial (RPTE) cells, a natural host cell. Using proteasome and ERAD inhibitors, we showed that ERAD is required for productive entry. Altered trafficking and accumulation of uncoated viral intermediates were detected by fluorescence in situ hybridization and indirect immunofluorescence in the presence of an inhibitor. Additionally, we detected a change in localization of partially uncoated virus within the ER during proteasome inhibition, from a BiP-rich area to a calnexin-rich subregion, indicating that BKPyV accumulated in an ER subcompartment. Furthermore, inhibiting ERAD did not prevent entry of capsid protein VP1 into the cytosol from the ER. By comparing the cytosolic entry of the related polyomavirus simian virus 40 (SV40), we found that dependence on the ERAD pathway for cytosolic entry varied between the polyomaviruses and between different cell types, namely, immortalized CV-1 cells and primary RPTE cells.

Identification of endoplasmic reticulum stress-inducing agents by antagonizing autophagy: a new potential strategy for identification of anti-cancer therapeutics in B-cell malignancies

The endoplasmic reticulum (ER) plays a vital function in multiple cellular processes. There is a growing interest in developing therapeutic agents that can target the ER in cancer cells, inducing a stress response that leads to cell death. However, ER stress-inducing agents can also induce autophagy, a survival strategy of cancer cells. Therefore, by inhibiting autophagy we can increase the efficacy of the ER stress-inducing agents. Nelfinavir, a human immunodeficiency virus (HIV) protease inhibitor with anti-cancer properties, can induce ER stress. Nelfinavir's effects on chronic lymphocytic leukemia (CLL) are yet to be elucidated. Herein we demonstrate that nelfinavir induces ER morphological changes and stress response, along with an autophagic protective strategy. Our data reveal that chloroquine, an autophagy inhibitor, significantly increases nelfinavir cytotoxicity. These results identify a novel strategy potentially effective in CLL treatment, by repositioning two well-known drugs as a combinatorial therapy with anti-cancer properties.

A shared endoplasmic reticulum-associated degradation pathway involving the EDEM1 protein for glycosylated and nonglycosylated proteins

Studies of misfolded protein targeting to endoplasmic reticulum-associated degradation (ERAD) have largely focused on glycoproteins, which include the bulk of the secretory proteins. Mechanisms of targeting of nonglycosylated proteins are less clear. Here, we studied three nonglycosylated proteins and analyzed their use of known glycoprotein quality control and ERAD components. Similar to an established glycosylated ERAD substrate, the uncleaved precursor of asialoglycoprotein receptor H2a, its nonglycosylated mutant, makes use of calnexin, EDEM1, and HRD1, but only glycosylated H2a is a substrate for the cytosolic SCF(Fbs2) E3 ubiquitin ligase with lectin activity. Two nonglycosylated BiP substrates, NS-1κ light chain and truncated Igγ heavy chain, interact with the ERAD complex lectins OS-9 and XTP3-B and require EDEM1 for degradation. EDEM1 associates through a region outside of its mannosidase-like domain with the nonglycosylated proteins. Similar to glycosylated substrates, proteasomal inhibition induced accumulation of the nonglycosylated proteins and ERAD machinery in the endoplasmic reticulum-derived quality control compartment. Our results suggest a shared ERAD pathway for glycosylated and nonglycosylated proteins composed of luminal lectin machinery components also capable of protein-protein interactions.

Age-related differences in the dynamics of hippocampal proteasome recovery

Regulation of proteasome abundance to meet cell needs under stress conditions is critical for maintaining cellular homeostasis. However, the effects of aging on this homeostatic response remain unknown. In this report, we analyzed in young and aged rat hippocampus, the dynamics of proteasome recovery induced by proteasome stress. Proteasome inhibition in young rats leads to an early and coordinate transcriptional and translational up-regulation of both the catalytic subunits of constitutive proteasome and the proteasome maturation protein. By contrast, aged rats up-regulated the inducible catalytic subunits and showed a lower and shorter expression of proteasome maturation protein. This resulted in a faster recovery of proteasome activity in young rats. Importantly, proteasome inhibition highly affected pyramidal cells, leading to the accumulation of ubiquitinated proteins in perinuclear regions of aged, but not young pyramidal neurons. These data strongly suggest that age-dependent differences in proteasome level and composition could contribute to neurodegeneration induced by proteasome dysfunction in normal and pathological aging.

The puzzling origin of the autophagosomal membrane

Autophagy is one of the newest and fastest emerging research areas in biomedical life sciences. Autophagosomes, large double-membrane vesicles enclosing cytoplasmic components targeted for degradation, are the hallmark of this catabolic pathway. The origin of the lipid bilayers composing these transport carriers has been the central enigma of the field since the discovery of autophagy. A series of recent studies has implicated several cellular organelles as the possible source of the autophagosomal membranes, if anything further clouding our view. In this compendium, we will discuss these apparently contradictory results and briefly emphasize the relevance of determining the lipid source used for autophagy for future translational research, for example in drug discovery programs.

Increase in endoplasmic reticulum-associated tissue transglutaminase and enzymatic activation in a cellular model of Parkinson's disease

Parkinson's disease (PD) is characterized by accumulation of α-synuclein aggregates and degeneration of melanized, catecholaminergic neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD, tTG levels are increased and cross-linking has been identified as an important factor in α-synuclein aggregation. In our quest to link tTGs distribution in the human brain to the hallmarks of PD pathology, we recently reported that catecholaminergic neurons in PD disease-affected brain areas display typical endoplasmic reticulum (ER) granules showing tTG immunoreactivity. In the present study, we set out to elucidate the nature of the interaction between tTG and the ER in PD pathogenesis, using retinoic-acid differentiated SH-SY5Y cells exposed to the PD-mimetic 1-methyl-4-phenylpyridinium (MPP(+)). Alike our observations in PD brain, MPP(+)-treated cells displayed typical TG-positive granules, that were also induced by other PD mimetics and by ER-stress inducing toxins. Additional immunocytochemical and biochemical investigation revealed that tTG is indeed associated to the ER, in particular at the cytoplasmic face of the ER. Upon MPP(+) exposure, additional recruitment of tTG toward the ER was found. In addition, we observed that MPP(+)-induced tTG activity results in transamidation of ER membrane proteins, like calnexin. Our data provide strong evidence for a, so far unrecognized, localization of tTG at the ER, at least in catecholaminergic neurons, and suggests that in PD activation of tTG may have a direct impact on ER function, in particular via post-translational modification of ER membrane proteins.

Bypass of glycan-dependent glycoprotein delivery to ERAD by up-regulated EDEM1

Extensive trimming of mannose residues targets a misfolded glycoprotein for endoplasmic reticulum–associated degradation (ERAD). Surprisingly, overexpression of EDEM1 or its up-regulation by the unfolded protein response bypasses this requirement. Delivery to OS9 in the ER-derived quality control compartment and ERAD becomes mannose trimming–independent, accelerating glycoprotein disposal.

Trimming of mannose residues from the N-linked oligosaccharide precursor is a stringent requirement for glycoprotein endoplasmic reticulum (ER)-associated degradation (ERAD). In this paper, we show that, surprisingly, overexpression of ER degradation–enhancing α-mannosidase-like protein 1 (EDEM1) or its up-regulation by IRE1, as occurs in the unfolded protein response, overrides this requirement and renders unnecessary the expression of ER mannosidase I. An EDEM1 deletion mutant lacking most of the carbohydrate-recognition domain also accelerated ERAD, delivering the substrate to XTP3-B and OS9. EDEM1 overexpression also accelerated the degradation of a mutant nonglycosylated substrate. Upon proteasomal inhibition, EDEM1 concentrated together with the ERAD substrate in the pericentriolar ER-derived quality control compartment (ERQC), where ER mannosidase I and ERAD machinery components are localized, including, as we show here, OS9. We suggest that a nascent glycoprotein can normally dissociate from EDEM1 and be rescued from ERAD by reentering calnexin-refolding cycles, a condition terminated by mannose trimming. At high EDEM1 levels, glycoprotein release is prevented and glycan interactions are no longer required, canceling the otherwise mandatory ERAD timing by mannose trimming and accelerating the targeting to degradation.

Autophagosome formation and molecular mechanism of autophagy

Autophagy (macroautophagy), or the degradation of large numbers of cytoplasmic components, is induced by extracellular and intracellular signals, including oxidative stress, ceramide, and endoplasmic reticulum stress. This dynamic process involves membrane formation and fusion, including autophagosome formation, autophagosome-lysosome fusion, and the degradation of intra-autophagosomal contents by lysosomal hydrolases. Autophagy is associated with tumorigenesis, neurodegenerative diseases, cardiomyopathy, Crohn's disease, fatty liver, type II diabetes, defense against intracellular pathogens, antigen presentation, and longevity. Among the proteins and multimolecular complexes that contribute to autophagosome formation are the PI(3)-binding proteins, the PI3-phosphatases, the Rab proteins, the Atg1/ULK1 protein-kinase complex, the Atg9•Atg2-Atg18 complex, the Vps34-Atg6/beclin1 class III PI3-kinase complex, and the Atg12 and Atg8/LC3 conjugation systems. Two ubiquitin-like modifications, the Atg12 and LC3 conjugations, are essential for membrane elongation and autophagosome formation. Recent findings have revealed that processes of selective autophagy, including pexophagy, mitophagy, ERphagy (reticulophagy), and the p62-dependent degradation of ubiquitin-positive aggregates, are physiologically important in various disease states, whereas "classical" autophagy is considered nonselective degradation. Processes of selective autophagy require specific Atg proteins in addition to the "core" Atg complexes. Finally, methods to monitor autophagic activity in mammalian cells are described.