CHOP and the endoplasmic reticulum stress response in myelinating glia

A multi-omics approach identifies pancreatic cancer cell extracellular vesicles as mediators of the unfolded protein response in normal pancreatic epithelial cells

Although cancer-derived extracellular vesicles (cEVs) are thought to play a pivotal role in promoting cancer progression events, their precise effect on neighbouring normal cells is unknown. In this study, we investigated the impact of pancreatic cancer ductal adenocarcinoma (PDAC) derived EVs on recipient non-tumourigenic pancreatic normal epithelial cells upon internalization. We demonstrate that cEVs are readily internalized and induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in treated normal pancreatic epithelial cells within 24 h. We further show that PDAC cEVs increase cell proliferation, migration, and invasion and that these changes are regulated at least in part, by the UPR mediator DDIT3. Subsequently, these cells release several inflammatory cytokines. Leveraging a layered multi-omics approach, we analysed EV cargo from a panel of six PDAC and two normal pancreas cell lines, using multiple EV isolation methods. We found that cEVs were enriched for an array of biomolecules which can induce or regulate ER stress and the UPR, including palmitic acid, sphingomyelins, metabolic regulators of tRNA charging and proteins which regulate trafficking and degradation. We further show that palmitic acid, at doses relevant to those found in cEVs, is sufficient to induce ER stress in normal pancreas cells. These results suggest that cEV cargo packaging may be designed to disseminate proliferative and invasive characteristics upon internalization by distant recipient normal cells, hitherto unreported. This study is among the first to highlight a major role for PDAC cEVs to induce stress in treated normal pancreas cells that may modulate a systemic response leading to altered phenotypes. These findings highlight the importance of EVs in mediating disease aetiology and open potential areas of investigation toward understanding the role of cEV lipids in promoting cell transformation in the surrounding microenvironment.

Beyond Myelination: Possible Roles of the Immune Proteasome in Oligodendroglial Homeostasis and Dysfunction

Oligodendroglia play a critical role in CNS homeostasis by myelinating neuronal axons in their mature stages. Dysfunction in this lineage occurs when early stage OPCs are not able to differentiate to replace dying Mature Myelinating Oligodendrocytes. Many hypotheses exist as to why de- and hypo-myelinating disorders and diseases occur. In this review, we present data to show that oligodendroglia can adopt components of the immune proteasome under inflammatory conditions. The works reviewed further reflect that these immune-component expressing oligodendroglia can in fact function as antigen presenting cells, phagocytosing foreign entities and presenting them via MHC II to activate CD4+ T cells. Additionally, we hypothesize, based on the limited literature, that the adoption of immune components by oligodendroglia may contribute to their stalled differentiation in the context of these disorders and diseases. The present review will underline: (1) Mechanisms of neuroinflammation in diseases associated with Immune Oligodendroglia; (2) the first associations between the immune proteasome and oligodendroglia and the subtle distinctions between these works; (3) the suggested functionality of these cells as it is described by current literature; and (4) the hypothesized consequences on metabolism. In doing so we aim to shed light on this fairly under-explored cell type in hopes that study of their functionality may lead to further mechanistic understanding of hypo- and de-myelinating neuroinflammatory disorders and diseases.

Nimodipine Exerts Beneficial Effects on the Rat Oligodendrocyte Cell Line OLN-93

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS). Therapy is currently limited to drugs that interfere with the immune system; treatment options that primarily mediate neuroprotection and prevent neurodegeneration are not available. Here, we studied the effects of nimodipine on the rat cell line OLN-93, which resembles young mature oligodendrocytes. Nimodipine is a dihydropyridine that blocks the voltage-gated L-type calcium channel family members Cav1.2 and Cav1.3. Our data show that the treatment of OLN-93 cells with nimodipine induced the upregulation of myelin genes, in particular of proteolipid protein 1 (Plp1), which was confirmed by a significantly greater expression of PLP1 in immunofluorescence analysis and the presence of myelin structures in the cytoplasm at the ultrastructural level. Whole-genome RNA sequencing additionally revealed the upregulation of genes that are involved in neuroprotection, remyelination, and antioxidation pathways. Interestingly, the observed effects were independent of Cav1.2 and Cav1.3 because OLN-93 cells do not express these channels, and there was no measurable response pattern in patch-clamp analysis. Taking into consideration previous studies that demonstrated a beneficial effect of nimodipine on microglia, our data support the notion that nimodipine is an interesting drug candidate for the treatment of MS and other demyelinating diseases.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

TFE3, a potential therapeutic target for Spinal Cord Injury via augmenting autophagy flux and alleviating ER stress

Background and Aim: Increasing evidence suggests that spinal cord injury (SCI)-induced defects in autophagic flux may contribute to an impaired ability for neurological repair following injury. Transcription factor E3 (TFE3) plays a crucial role in oxidative metabolism, lysosomal homeostasis, and autophagy induction. Here, we investigated the role of TFE3 in modulating autophagy following SCI and explored its impact on neurological recovery.

Methods: Histological analysis via HE, Nissl and Mason staining, survival rate analysis, and behavioral testing via BMS and footprint analysis were used to determine functional recovery after SCI. Quantitative real-time polymerase chain reaction, Western blotting, immunofluorescence, TUNEL staining, enzyme-linked immunosorbent assays, and immunoprecipitation were applied to examine levels of autophagy flux, ER-stress-induced apoptosis, oxidative stress, and AMPK related signaling pathways. In vitro studies using PC12 cells were performed to discern the relationship between ROS accumulation and autophagy flux blockade.

Results: Our results showed that in SCI, defects in autophagy flux contributes to ER stress, leading to neuronal death. Furthermore, SCI enhances the production of reactive oxygen species (ROS) that induce lysosomal dysfunction to impair autophagy flux. We also showed that TFE3 levels are inversely correlated with ROS levels, and increased TFE3 levels can lead to improved outcomes. Finally, we showed that activation of TFE3 after SCI is partly regulated by AMPK-mTOR and AMPK-SKP2-CARM1 signaling pathways.

Conclusions: TFE3 is an important regulator in ROS-mediated autophagy dysfunction following SCI, and TFE3 may serve as a promising target for developing treatments for SCI.

Furin-instructed molecular self-assembly actuates endoplasmic reticulum stress-mediated apoptosis for cancer therapy

Protein quality control and proteostasis are essential to maintain cell survival as once disordered, they will trigger endoplasmic reticulum (ER) stress and even initiate apoptosis. Severe ER stress-mediated apoptosis is the cause of neurodegenerative diseases and expected to be a new target for cancer therapy. In this study, we designed a small molecule of 1-Nap to execute furin-instructed molecular self-assembly for selectively inhibiting the growth of MDA-MB-468 cells in vitro and in vivo. According to the results of transmission electron microscopy (TEM) and HPLC tracing analysis, 1-Nap is capable of self-assembling upon furin-instructed cleavage that transforms 1-Nap nanoparticles to 1-Nap nanofibers. Fluorescence imaging and Western-blot analysis results indicate that the furin-instructed self-assembly of 1-Nap rather than its ER-targeting interaction is indispensable for the ER stress and activation of apoptosis. The furin-instructed self-assembly of 1-Nap is associated with both the ER (1-Nap's targeting location) and the trans-Golgi network (furin's location); this inspired us to reasonably believe that the blocking of ER-to-Golgi traffic in the secretory pathway by molecular self-assembly may be the intrinsic motivation for controlling cell fate. This work provides a new way for the targeted disturbance of the proteostasis of cells through molecular self-assembly for developing cancer therapeutics.

The UPR-PERK pathway is not a promising therapeutic target for mutant SOD1-induced ALS

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease, characterized by motor neuron death in the brain and spinal cord. Mutations in the Cu/Zn superoxide dismutase (SOD1) gene account for ~20% of all familial ALS forms, corresponding to 1%-2% of all ALS cases. One of the suggested mechanisms by which mutant SOD1 (mtSOD1) exerts its toxic effects involves intracellular accumulation of abnormal mtSOD1 aggregates, which trigger endoplasmic reticulum (ER) stress and activate its adaptive signal transduction pathways, including the unfolded protein response (UPR). PERK, an eIF2α kinase, is central to the UPR and is the most rapidly activated pathway in response to ER stress. Previous reports using mtSOD1 transgenic mice indicated that genetic or pharmacological enhancement of the UPR-PERK pathway may be effective in treating ALS. We investigated the response to PERK haploinsufficiency, and the response to deficiency of its downstream effectors GADD34 and CHOP, in five distinct lines of mtSOD1 mice. We demonstrate that, in contrast to a previously published study, PERK haploinsufficiency has no effect on disease in all mtSOD1 lines examined. We also show that deficiency of GADD34, which enhances the UPR by prolonging the phosphorylation of eIF2α, does not ameliorate disease in these mtSOD1 mouse lines. Finally, we demonstrate that genetic ablation of CHOP transcription factor, which is known to be pro-apoptotic, does not ameliorate disease in mtSOD1 mice. Cumulatively, our studies reveal that neither genetic inhibition of the UPR via ablation of PERK, nor genetic UPR enhancement via ablation of GADD34, is beneficial for mtSOD1-induced motor neuron disease. Therefore, the PERK pathway is not a likely target for therapeutic intervention in mtSOD1-induced ALS.

Cuprizone-induced graded oligodendrocyte vulnerability is regulated by the transcription factor DNA damage-inducible transcript 3

Oligodendrocytes are integral to efficient neuronal signaling. Loss of myelinating oligodendrocytes is a central feature of many neurological diseases, including multiple sclerosis (MS). The results of neuropathological studies suggest that oligodendrocytes react with differing sensitivity to toxic insults, with some cells dying early during lesion development and some cells being resistant for weeks. This proposed graded vulnerability has never been demonstrated but provides an attractive window for therapeutic interventions. Furthermore, the biochemical pathways associated with graded oligodendrocyte vulnerability have not been well explored. We used immunohistochemistry and serial block-face scanning electron microscopy (3D-SEM) to show that cuprizone-induced metabolic stress results in an "out of phase" degeneration of oligodendrocytes. Although expression induction of stress response transcription factors in oligodendrocytes occurs within days, subsequent oligodendrocyte apoptosis continues for weeks. In line with the idea of an out of phase degeneration of oligodendrocytes, detailed ultrastructural reconstructions of the axon-myelin unit demonstrate demyelination of single internodes. In parallel, genome wide array analyses revealed an active unfolded protein response early after initiation of the cuprizone intoxication. In addition to the cytoprotective pathways, the pro-apoptotic transcription factor DNA damage-inducible transcript 3 (DDIT3) was induced early in oligodendrocytes. In advanced lesions, DDIT3 was as well expressed by activated astrocytes. Toxin-induced oligodendrocyte apoptosis, demyelination, microgliosis, astrocytosis, and acute axonal damage were less intense in the Ddit3-null mutants. This study identifies DDIT3 as an important regulator of graded oligodendrocyte vulnerability in a MS animal model. Interference with this stress cascade might offer a promising therapeutic approach for demyelinating disorders.

PLP1 Mutations in Patients with Multiple Sclerosis: Identification of a New Mutation and Potential Pathogenicity of the Mutations

PLP1 is located on the X-chromosome and encodes myelin proteolipid protein (PLP), the most abundant protein in central nervous system myelin. Generally, point mutations in PLP1 result in X-linked dysmyelinating disorders, such as Pelizaeus-Merzbacher disease (PMD) or spastic paraplegia type 2 (SPG2). However, several case studies have identified patients with missense point mutations in PLP1 and clinical symptoms and signs compatible with a diagnosis of multiple sclerosis (MS). To investigate if PLP1 mutations occur relatively frequently in MS, we sequenced the coding regions of PLP1 in 22 female MS patients who had developed disease after the age of 40 and in 42 healthy women, and identified a missense mutation in exon 2 of PLP1 resulting in a Leu30Val mutation in the protein in one of the MS patients. mCherry-tagged plasmids containing wild type or mutant PLP1 sequences of PLP, including two known PMD/SPG2-related mutations as positive controls, were constructed and transfected into Cos-7 cells. In comparison with cells transfected with wild type PLP1, all mutations caused significant accumulation of PLP in the endoplasmic reticulum of the cells and induction of the unfolded protein response-a mechanism that leads to apoptosis of cells expressing mutant proteins. Additionally, in silico analysis of the binding of peptides containing the Leu30Val mutation to the human leukocyte antigen (HLA) molecules carried by the patient harboring this mutation suggested that the mutation could produce several novel immunogenic epitopes in this patient. These results support the idea that mutations in myelin-related genes could contribute to the development of MS in a small proportion of patients.

4-Phenylbutyric acid presents therapeutic effect on osteoarthritis via inhibiting cell apoptosis and inflammatory response induced by endoplasmic reticulum stress

Osteoarthritis (OA) is a common bone and joint disease with a wild range of risk factors, which is associated with endoplasmic reticulum (ER) stress. The aim of our study was to discuss the possible mechanism of ER stress associated with OA in vivo and explore novel therapeutic method against OA. OA-induced damages in cartilage tissues were evaluated by HE, Safranin O/fast green, and TUNEL staining. The inflammatory factors concentration and the expression of FAP, MMP2, MMP9, Bax, Bcl-2, CHOP, and GRP78 were evaluated by ELISA, real-time PCR, and Western blot analyses. As results, 4-phenylbutyric acid (4-PBA)-treated OA cartilage tissues presented alleviated tissue damage with less apoptotic cells and cytokine production in comparison with advanced-OA tissues. Downregulation of Bax/Bcl-2, CHOP, GRP78, inflammatory factors, and reactive oxygen species generation, and the increase of MMP level detected after 4-PBA treatment indicated an inhibitory effect of 4-PBA on cell apoptosis, inflammatory response, and ER stress in OA. In conclusion, we indicate that ER stress causes cell apoptosis and inflammatory response, resulting in the tissue damage within OA. At the same time, 4-PBA exhibited protective effect on cartilage cells against OA through the inhibition of ER stress.

Role of p62/SQSTM1 beyond autophagy: a lesson learned from drug-induced toxicity in vitro

BACKGROUND AND PURPOSE

SQSTM1/p62 is a multifunctional, stress-induced, scaffold protein involved in multiple cellular processes including autophagic clearance, regulation of inflammatory responses and redox homeostasis. Its altered function has been associated with different human pathologies, such as neurodegenerative, metabolic and bone diseases (down-regulation), and cancerogenesis (up-regulation). However, its role in the off-target effects of clinically used drugs is still not understood.

EXPERIMENTAL APPROACH

We evaluated the expression of p62 in cultured Hep3B cells and their derived ρ° cells (lacking mitochondria), along with markers of autophagy and mitochondrial dysfunction. The effects of efavirenz were compared with those of known pharmacological stressors, rotenone, thapsigargin and CCCP, and we also used transient silencing with siRNA and p62 overexpression. Western blotting, quantRT-PCR and fluorescence microscopy were used to assay these effects and their underlying mechanisms.

KEY RESULTS

In Hep3B cells, efavirenz augmented p62 protein content, an effect not observed in the corresponding ρ° cells. p62 up-regulation followed enhanced SQSTM1 expression mediated through the transcription factor CHOP/DDIT3, while other well-known regulators (NF-kB and Nrf2) were not involved. Inhibition of autophagy with 3MA or with transient silencing of Atg5 did not affect SQSTM1 expression in efavirenz-treated cells while p62 overexpression ameliorated the deleterious effect of efavirenz on cell viability.

CONCLUSION AND IMPLICATIONS

In our model, p62 exerted a specific, autophagy-independent role and protected against efavirenz-induced mitochondrial ROS generation and activation of the NLRP3 inflammasome. These findings add to the multifunctional nature of p62 and may help to understand the off-target effects of clinically useful drugs.

Pathogenic inflammation in the CNS of mice carrying human PLP1 mutations

Progressive forms of multiple sclerosis lead to chronic disability, substantial decline in quality of life and reduced longevity. It is often suggested that they occur independently of inflammation. Here we investigated the disease progression in mouse models carrying PLP1 point mutations previously found in patients displaying clinical features of multiple sclerosis. These mouse models show loss-of-function of PLP1 associated with neuroinflammation; the latter leading to clinically relevant axonal degeneration, neuronal loss and brain atrophy as demonstrated by inactivation of the recombination activating gene 1. Moreover, these pathological hallmarks were substantially amplified when we attenuated immune regulation by inactivation of the programmed cell death-1 gene. Our observations support the view that primary oligodendroglial abnormalities can evoke pathogenically relevant neuroinflammation that drives neurodegeneration, as observed in some forms of multiple sclerosis but also in other, genetically-mediated neurodegenerative disorders of the human nervous system. As many potent immunomodulatory drugs have emerged during the last years, it is tempting to consider immunomodulation as a treatment option not only for multiple sclerosis, but also for so far non-treatable, genetically-mediated disorders of the nervous system accompanied by pathogenic neuroinflammation.

The Integrated Stress Response in Hypoxia-Induced Diffuse White Matter Injury

Currently no treatments exist for preterm infants with diffuse white matter injury (DWMI) caused by hypoxia. Due to the improved care of preterm neonates and increased recognition by advanced imaging techniques, the prevalence of DWMI is increasing. A better understanding of the pathophysiology of DWMI is therefore of critical importance. The integrated stress response (ISR), a conserved eukaryotic response to myriad stressors including hypoxia, may play a role in hypoxia-induced DWMI and may represent a novel target for much needed therapies. In this study, we use in vitro and in vivo hypoxic models of DWMI to investigate whether the ISR is involved in DWMI. We demonstrate that hypoxia activates the ISR in primary mouse oligodendrocyte precursor cells (OPCs) in vitro and that genetically inhibiting the ISR in differentiating OPCs increases their susceptibility to in vitro hypoxia. We also show that a well established in vivo mild chronic hypoxia (MCH) mouse model and a new severe acute hypoxia (SAH) mouse model of DWMI activates the initial step of the ISR. Nonetheless, genetic inhibition of the ISR has no detectable effect on either MCH- or SAH-induced DWMI. In addition, we demonstrate that genetic enhancement of the ISR does not ameliorate MCH- or SAH-induced DWMI. These studies suggest that, while the ISR protects OPCs from hypoxia in vitro, it does not appear to play a major role in either MCH- or SAH-induced DWMI and is therefore not a likely target for therapies aimed at improving neurological outcome in preterm neonates with hypoxia-induced DWMI.SIGNIFICANCE STATEMENT Diffuse white matter injury (DWMI) caused by hypoxia is a leading cause of neurological deficits following premature birth. An increased understanding of the pathogenesis of this disease is critical. The integrated stress response (ISR) is activated by hypoxia and protects oligodendrocyte lineage cells in other disease models. This has led to an interest in the potential role of the ISR in DWMI. Here we examine the ISR in hypoxia-induced DWMI and show that while the ISR protects oligodendrocyte lineage cells from hypoxia in vitro, genetic inhibition or enhancement of the ISR has no effect on hypoxia-induced DWMI in vivo, suggesting that the ISR does not play a major role in and is not a likely therapeutic target for DWMI.

Overexpression of CHOP in Myelinating Cells Does Not Confer a Significant Phenotype under Normal or Metabolic Stress Conditions

The PKR-like endoplasmic reticulum kinase (PERK) pathway of the unfolded protein response (UPR) is protective against toxic accumulations of misfolded proteins in the endoplasmic reticulum, but is thought to drive cell death via the transcription factor, CHOP. However, in many cell types, CHOP is an obligate step in the PERK pathway, which frames the conundrum of a prosurvival pathway that kills cells. Our laboratory and others have previously demonstrated the prosurvival activity of the PERK pathway in oligodendrocytes. In the current study, we constitutively overexpress CHOP in myelinating cells during development and into adulthood under normal or UPR conditions. We show that this transcription factor does not drive apoptosis. Indeed, we observe no detriment in mice at multiple levels from single cells to mouse behavior and life span. In light of these data and other studies, we reinterpret PERK pathway function in the context of a stochastic vulnerability model, which governs the likelihood that cells undergo cell death upon cessation of UPR protection and while attempting to restore homeostasis.

SIGNIFICANCE STATEMENT

Herein, we tackle the biggest controversy in the UPR literature: the function of the transcription factor CHOP as a protective or a prodeath factor. This manuscript is timely in light of the 2014 Lasker award for the UPR. Our in vivo data show that CHOP is not a prodeath protein, and we demonstrate that myelinating glial cells function normally in the presence of high CHOP expression from development to adulthood. Further, we propose a simplified view of UPR-mediated cell death after CHOP induction. We anticipate our work may turn the tide of the dogmatic view of CHOP and cause a reinvestigation of its function in different cell types. Accordingly, we believe our work will be a watershed for the UPR field.

Ranking Novel Regulatory Genes in Gene Expression Profiles using NetExpress

Understanding gene regulation by identifying gene products and determining their roles in regulatory networks is a complex process. A common computational method is to reverse engineer a regulatory network from gene expression profile, and sanitize the network using known information about the genes, their interactions and other properties to filter out unlikely interactors. Unfortunately, due to limited resources most gene expression studies have a limited and small number of time points, and most reverse engineering tools are unable to handle large numbers of genes. Both of these factors play significant roles in influencing the accuracy of the process. In this paper, we present a new gene ranking algorithm from gene expression profiles with a small number of time points so that the most relevant genes can be selected for reverse engineering. We also present a graphical interface called NetExpress, which adopts this algorithm and allows users to set control parameters to effect the desired outcome, and visualize the analysis for iterative fine tuning.

Endoplasmic Reticulum Protein Quality Control Failure in Myelin Disorders

Reaching the correct three-dimensional structure is crucial for the proper function of a protein. The endoplasmic reticulum (ER) is the organelle where secreted and transmembrane proteins are synthesized and folded. To guarantee high fidelity of protein synthesis and maturation in the ER, cells have evolved ER-protein quality control (ERQC) systems, which assist protein folding and promptly degrade aberrant gene products. Only correctly folded proteins that pass ERQC checkpoints are allowed to exit the ER and reach their final destination. Misfolded glycoproteins are detected and targeted for degradation by the proteasome in a process known as endoplasmic reticulum-associated degradation (ERAD). The excess of unstructured proteins in the ER triggers an adaptive signal transduction pathway, called unfolded protein response (UPR), which in turn potentiates ERQC activities in order to reduce the levels of aberrant molecules. When the situation cannot be restored, the UPR drives cells to apoptosis. Myelin-forming cells of the central and peripheral nervous system (oligodendrocytes and Schwann cells) synthesize a large amount of myelin proteins and lipids and therefore are particularly susceptible to ERQC failure. Indeed, deficits in ERQC and activation of ER stress/UPR have been implicated in several myelin disorders, such as Pelizaeus-Merzbacher and Krabbe leucodystrophies, vanishing white matter disease and Charcot-Marie-Tooth neuropathies. Here we discuss recent evidence underlying the importance of proper ERQC functions in genetic disorders of myelinating glia.

Hypothermia and Rewarming Activate a Macroglial Unfolded Protein Response Independent of Hypoxic-Ischemic Brain Injury in Neonatal Piglets

Therapeutic hypothermia provides incomplete neuroprotection after hypoxia-ischemia (HI)-induced brain injury in neonates. We previously showed that cortical neuron and white matter apoptosis are promoted by hypothermia and early rewarming in a piglet model of HI. The unfolded protein response (UPR) may be one of the potential mediators of this cell death. Here, neonatal piglets underwent HI or sham surgery followed by 29 h of normothermia, 2 h of normothermia + 27 h of hypothermia or 18 h of hypothermia + rewarming. Piglets recovered for 29 h. Immunohistochemistry for endoplasmic reticulum to nucleus signaling-1 protein (ERN1), a marker of UPR activation, was used to determine the ratios of ERN1+ macroglia and neurons in the motor subcortical white matter and cerebral cortex. The ERN1+ macroglia were immunophenotyped as oligodendrocytes and astrocytes by immunofluorescent colabeling. Temperature (p = 0.046) and HI (p < 0.001) independently affected the ratio of ERN1+ macroglia. In sham piglets, sustained hypothermia (p = 0.011) and rewarming (p = 0.004) increased the ERN1+ macroglia ratio above that in normothermia. HI prior to hypothermia diminished the UPR. Ratios of ERN1+ macroglia correlated with white matter apoptotic profile counts in shams (r = 0.472; p = 0.026), thereby associating UPR activation with white matter apoptosis during hypothermia and rewarming. Accordingly, macroglial cell counts decreased in shams that received sustained hypothermia (p = 0.009) or rewarming (p = 0.007) compared to those in normothermic shams. HI prior to hypothermia neutralized the macroglial cell loss. Neither HI nor temperature affected ERN1+ neuron ratios. In summary, delayed hypothermia and rewarming activate the macroglial UPR, which is associated with white matter apoptosis. HI may decrease the macroglial endoplasmic reticulum stress response after hypothermia and rewarming.

Dual role of the integrated stress response in medulloblastoma tumorigenesis

In response to endoplasmic reticulum (ER) stress, activation of pancreatic ER kinase (PERK) coordinates an adaptive program known as the integrated stress response (ISR) by phosphorylating translation initiation factor 2α (eIF2α). Phosphorylated eIF2α is quickly dephosphorylated by the protein phosphatase 1 and growth arrest and DNA damage 34 (GADD34) complex. Data indicate that the ISR can either promote or suppress tumor development. Our previous studies showed that the ISR is activated in medulloblastoma in both human patients and animal models, and that the decreased ISR via PERK heterozygous deficiency attenuates medulloblastoma formation in Patched1 heterozygous deficient (Ptch1+/-) mice by enhancing apoptosis of pre-malignant granule cell precursors (GCPs) during cell transformation. We showed here that GADD34 heterozygous mutation moderately enhanced the ISR and noticeably increased the incidence of medulloblastoma in adult Ptch1+/- mice. Surprisingly, GADD34 homozygous mutation strongly enhanced the ISR, but significantly decreased the incidence of medulloblastoma in adult Ptch1+/- mice. Intriguingly, GADD34 homozygous mutation significantly enhanced pre-malignant GCP apoptosis in cerebellar hyperplastic lesions and reduced the lesion numbers in young Ptch1+/- mice. Nevertheless, neither GADD34 heterozygous mutation nor GADD34 homozygous mutation had a significant effect on medulloblastoma cells in adult Ptch1+/- mice. Collectively, these data imply the dual role of the ISR, promoting and inhibiting, in medulloblastoma tumorigenesis by regulating apoptosis of pre-malignant GCPs during the course of malignant transformation.

Perk Ablation Ameliorates Myelination in S63del-Charcot-Marie-Tooth 1B Neuropathy

In peripheral nerves, P0 glycoprotein accounts for more than 20% of myelin protein content. P0 is synthesized by Schwann cells, processed in the endoplasmic reticulum (ER) and enters the secretory pathway. However, the mutant P0 with S63 deleted (P0S63del) accumulates in the ER lumen and induces a demyelinating neuropathy in Charcot–Marie–Tooth disease type 1B (CMT1B)–S63del mice. Accumulation of P0S63del in the ER triggers a persistent unfolded protein response. Protein kinase RNA-like endoplasmic reticulum kinase (PERK) is an ER stress sensor that phosphorylates eukaryotic initiation factor 2 alpha (eIF2alpha) in order to attenuate protein synthesis. We have shown that increasing phosphophorylated-eIF2alpha (P-eIF2alpha) is a potent therapeutic strategy, improving myelination and motor function in S63del mice. Here, we explore the converse experiment: Perk haploinsufficiency reduces P-eIF2alpha in S63del nerves as expected, but surprisingly, ameliorates, rather than worsens S63del neuropathy. Motor performance and myelin abnormalities improved in S63del//Perk+/− compared with S63del mice. These data suggest that mechanisms other than protein translation might be involved in CMT1B/S63del neuropathy. In addition, Perk deficiency in other cells may contribute to demyelination in a non–Schwann-cell autonomous manner.

ER Chaperone BiP/GRP78 Is Required for Myelinating Cell Survival and Provides Protection during Experimental Autoimmune Encephalomyelitis

Myelinating cells synthesize large amounts of membrane protein through the secretory pathway, which makes these cells particularly sensitive to perturbations of the endoplasmic reticulum (ER). Ig binding protein (BiP), also known as glucose-regulated protein 78 (GRP78), is a critical ER chaperone that also plays a pivotal role in controlling the cellular response to ER stress. To examine the potential importance of BiP to myelinating cells, we used a conditional knock-out approach to BiP gene inactivation in oligodendrocytes during development, in adulthood, and in response to experimental autoimmune encephalomyelitis (EAE), an animal model of the inflammatory demyelinating disorder multiple sclerosis (MS). During development, mice lacking functional BiP gene expression in oligodendrocytes developed tremors and ataxia and died before reaching maturity. When BiP gene inactivation in oligodendrocytes was initiated in adulthood, the mice displayed severe neurological symptoms including tremors and hind-limb paralysis. The inactivation of BiP in oligodendrocytes during development or in adulthood resulted in oligodendrocyte loss and corresponding severe myelin abnormalities. Mice heterozygous for the oligodendrocyte-specific inactivation of BiP, which were phenotypically normal without evidence of neuropathology, displayed an exacerbated response to EAE that correlated with an increased loss of oligodendrocytes. Furthermore, mice in which the BiP gene was specifically inactivated in developing Schwann cells displayed tremor that progressed to hindlimb paralysis, which correlated with diminished numbers of myelinating Schwann cells and severe PNS hypomyelination. These studies demonstrate that BiP is critical for myelinating cell survival and contributes to the protective response of oligodendrocyte against inflammatory demyelination.

Harnessing the integrated stress response for the treatment of multiple sclerosis

Multiple sclerosis (MS) is a chronic demyelinating autoimmune disease of the central nervous system (CNS) with no known cure. Though a dozen immunomodulatory therapies exist, their impact on progression of disease appears limited. The field has hence focused on alternate strategies for treatment such as enhancing remyelination and CNS repair. Recent progress has been made on a third complimentary treatment approach that involves protecting oligodendrocytes, and the myelin they generate and maintain, from inflammatory-mediated death via enhancement of the integrated stress response (ISR). Studies in cells and mouse models of MS have demonstrated that the ISR, an innate protective pathway that maintains proteostasis, may be effectively harnessed to aid in the protection of oligodendrocytes and myelin during inflammation. With one ISR-modifying drug already in clinical trial and a number of potential future therapies under investigation, this approach may offer an important component in halting the progression of multiple sclerosis.

Altered Oligodendrocyte Maturation and Myelin Maintenance: The Role of Antiretrovirals in HIV-Associated Neurocognitive Disorders

Despite effective viral suppression through combined antiretroviral therapy (cART), approximately half of HIV-positive individuals have HIV-associated neurocognitive disorders (HAND). Studies of antiretroviral-treated patients have revealed persistent white matter abnormalities including diffuse myelin pallor, diminished white matter tracts, and decreased myelin protein mRNAs. Loss of myelin can contribute to neurocognitive dysfunction because the myelin membrane generated by oligodendrocytes is essential for rapid signal transduction and axonal maintenance. We hypothesized that myelin changes in HAND are partly due to effects of antiretroviral drugs on oligodendrocyte survival and/or maturation. We showed that primary mouse oligodendrocyte precursor cell cultures treated with therapeutic concentrations of HIV protease inhibitors ritonavir or lopinavir displayed dose-dependent decreases in oligodendrocyte maturation; however, this effect was rapidly reversed after drug removal. Conversely, nucleoside reverse transcriptase inhibitor zidovudine had no effect. Furthermore, in vivo ritonavir administration to adult mice reduced frontal cortex myelin protein levels. Finally, prefrontal cortex tissue from HIV-positive individuals with HAND on cART showed a significant decrease in myelin basic protein compared with untreated HIV-positive individuals with HAND or HIV-negative controls. These findings demonstrate that antiretrovirals can impact myelin integrity and have implications for myelination in juvenile HIV patients and myelin maintenance in adults on lifelong therapy.

Proteolipid protein modulates preservation of peripheral axons and premature death when myelin protein zero is lacking

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine-Sottas syndrome (DSS) and in the corresponding mouse model (P0(null)-mice). In the mammalian CNS, the tetraspan-membrane protein PLP is the major structural myelin constituent and required for the long-term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type-2) and the relevant mouse model (Plp(null)-mice). The Plp-gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0(null)-mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0(null)*Plp(null)-double-mutant mice. Compared with either single-mutant, P0(null)*Plp(null)-mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non-myelinated but in a one-to-one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0(null)*Plp(null)-mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell-dependent support of peripheral axons and the oligodendrocyte-dependent support of central axons.

The transrepression arm of glucocorticoid receptor signaling is protective in mutant huntingtin-mediated neurodegeneration

The unfolded protein response (UPR) occurs following the accumulation of unfolded proteins in the endoplasmic reticulum (ER) and orchestrates an intricate balance between its prosurvival and apoptotic arms to restore cellular homeostasis and integrity. However, in certain neurodegenerative diseases, the apoptotic arm of the UPR is enhanced, resulting in excessive neuronal cell death and disease progression, both of which can be overcome by modulating the UPR. Here, we describe a novel crosstalk between glucocorticoid receptor signaling and the apoptotic arm of the UPR, thus highlighting the potential of glucocorticoid therapy in treating neurodegenerative diseases. Several glucocorticoids, but not mineralocorticoids, selectively antagonize ER stress-induced apoptosis in a manner that is downstream of and/or independent of the conventional UPR pathways. Using GRT10, a novel selective pharmacological modulator of glucocorticoid signaling, we describe the importance of the transrepression arm of the glucocorticoid signaling pathway in protection against ER stress-induced apoptosis. Furthermore, we also observe the protective effects of glucocorticoids in vivo in a Drosophila model of Huntington's disease (HD), wherein treatment with different glucocorticoids diminished rhabdomere loss and conferred neuroprotection. Finally, we find that growth differentiation factor 15 has an important role downstream of glucocorticoid signaling in antagonizing ER stress-induced apoptosis in cells, as well as in preventing HD-mediated neurodegeneration in flies. Thus, our studies demonstrate that this novel crosstalk has the potential to be effectively exploited in alleviating several neurodegenerative disorders.

The unfolded protein response in multiple sclerosis

The unfolded protein response (UPR) occurs in response to endoplasmic reticulum (ER) stress caused by the accumulation of unfolded or misfolded proteins in the ER. The UPR is comprised of three signaling pathways that promote cytoprotective functions to correct ER stress; however, if ER stress cannot be resolved the UPR results in apoptosis of affected cells. The UPR is an important feature of various human diseases, including multiple sclerosis (MS). Recent studies have shown several components of the UPR are upregulated in the multiple cell types in MS lesions, including oligodendrocytes, T cells, microglia/macrophages, and astrocytes. Data from animal model studies, particularly studies of experimental autoimmune encephalomyelitis (EAE) and the cuprizone model, imply an important role of the UPR activation in oligodendrocytes in the development of MS. In this review we will cover current literature on the UPR and the evidence for its role in the development of MS.

Crocin protects PC12 cells against MPP(+)-induced injury through inhibition of mitochondrial dysfunction and ER stress

The molecular machinery that mediates neuronal injury in neurodegenerative conditions such as Parkinson's disease (PD) remains to be fully deciphered, which will hopefully provide novel therapeutic targets for these disorders. Crocin, one of the water-soluble carotenoids isolated from the Crocus sativus L (saffron) stigma, has been reported to exert therapeutic potential in many disease models. Here, we establish an in vitro PD model using 1-methyl-4-phenylpyridinium (MPP(+))-injured PC12 cells to investigate the protective effects of crocin. Crocin treatment significantly attenuated MPP(+)-induced cell injury and apoptosis with little toxicity, and these protective effects were still observed even if crocin treatment was delayed to 6 h after injury. Crocin also inhibited MPP(+)-induced mitochondrial dysfunction, as evidenced by preservation of mitochondrial membrane potential (MMP) and ATP synthesis, which correlates with suppressed endoplasmic reticulum (ER) stress through inhibiting ER chaperone and ER related apoptotic factors. In addition, ER calcium release and morphological changes in ER lumen after MPP(+) exposure were all partially prevented by crocin. By using specific targeted small interfering RNA (siRNA) to knockdown the expression of the C/EBP homologous protein (CHOP), we found that crocin-induced protection and inhibition of ER stress was mediated by inverting MPP(+)-induced decrease of Wnt through the CHOP pathway. Our study demonstrates a pivotal role of ER stress in mediating PD related neuronal injury via the regulation of CHOP-Wnt pathway, and suggests the therapeutic values of crocin against ER stress-associated cytotoxicity.

C/EBPβ and C/EBPδ transcription factors: Basic biology and roles in the CNS

CCAAT/enhancer binding protein (C/EBP) β and C/EBPδ are transcription factors of the basic-leucine zipper class which share phylogenetic, structural and functional features. In this review we first describe in depth their basic molecular biology which includes fascinating aspects such as the regulated use of alternative initiation codons in the C/EBPβ mRNA. The physical interactions with multiple transcription factors which greatly opens the number of potentially regulated genes or the presence of at least five different types of post-translational modifications are also remarkable molecular mechanisms that modulate C/EBPβ and C/EBPδ function. In the second part, we review the present knowledge on the localization, expression changes and physiological roles of C/EBPβ and C/EBPδ in neurons, astrocytes and microglia. We conclude that C/EBPβ and C/EBPδ share two unique features related to their role in the CNS: whereas in neurons they participate in memory formation and synaptic plasticity, in glial cells they regulate the pro-inflammatory program. Because of their role in neuroinflammation, C/EBPβ and C/EBPδ in microglia are potential targets for treatment of neurodegenerative disorders. Any strategy to reduce C/EBPβ and C/EBPδ activity in neuroinflammation needs to take into account its potential side-effects in neurons. Therefore, cell-specific treatments will be required for the successful application of this strategy.

Modeling the natural history of Pelizaeus-Merzbacher disease

Major gaps in our understanding of the leukodystrophies result from their rarity and the lack of tissue for the interdisciplinary studies required to extend our knowledge of the pathophysiology of the diseases. This study details the natural evolution of changes in the CNS of the shaking pup (shp), a model of the classical form of the X-linked disorder Pelizaeus-Merzbacher disease, in particular in glia, myelin, and axons, which is likely representative of what occurs over time in the human disease. The mutation in the proteolipid protein gene, PLP1, leads to a delay in differentiation, increased cell death, and a marked distension of the rough endoplasmic reticulum in oligodendrocytes. However, over time, more oligodendrocytes differentiate and survive in the spinal cord leading to an almost total recovery of myelination, In contrast, the brain remains persistently hypomyelinated. These data suggest that shp oligodendrocytes may be more functional than previously realized and that their early recruitment could have therapeutic value.

De-repression of myelin-regulating gene expression after status epilepticus in mice lacking the C/EBP homologous protein CHOP

The C/EBP homologous protein CHOP is normally present at low levels in cells but increases rapidly after insults such as DNA damage or endoplasmatic reticulum stress where it contributes to cellular homeostasis and apoptosis. By forming heterodimers with other transcription factors, CHOP can either act as a dominant-negative regulator of gene expression or to induce the expression of target genes. Recent work demonstrated that seizure-induced hippocampal damage is significantly worse in mice lacking CHOP and these animals go on to develop an aggravated epileptic phenotype. To identify novel CHOP-controlled target genes which potentially influence the epileptic phenotype, we performed a bioinformatics analysis of tissue microarrays from chop-deficient mice after prolonged seizures. GO analysis revealed genes associated with biological membranes were prominent among those in the chop-deficient array dataset and we identified myelin-associated genes to be particularly de-repressed. These data suggest CHOP might act as an inhibitor of myelin-associated processes in the brain and could be targeted to influence axonal regeneration or reorganisation.

Potential For Cell-mediated Immune Responses In Mouse Models Of Pelizaeus-Merzbacher Disease

Although activation of the innate and adaptive arms of the immune system are undoubtedly involved in the pathophysiology of neurodegenerative diseases, it is unclear whether immune system activation is a primary or secondary event. Increasingly, published studies link primary metabolic stress to secondary inflammatory responses inside and outside of the nervous system. In this study, we show that the metabolic stress pathway known as the unfolded protein response (UPR) leads to secondary activation of the immune system. First, we observe innate immune system activation in autopsy specimens from Pelizaeus-Merzbacher disease (PMD) patients and mouse models stemming from PLP1 gene mutations. Second, missense mutations in mildly- and severely-affected Plp1-mutant mice exhibit immune-associated expression profiles with greater disease severity causing an increasingly proinflammatory environment. Third, and unexpectedly, we find little evidence for dysregulated expression of major antioxidant pathways, suggesting that the unfolded protein and oxidative stress responses are separable. Together, these data show that UPR activation can precede innate and/or adaptive immune system activation and that neuroinflammation can be titrated by metabolic stress in oligodendrocytes. Whether or not such activation leads to autoimmune disease in humans is unclear, but the case report of steroid-mitigated symptoms in a PMD patient initially diagnosed with multiple sclerosis lends support.

Proanthocyanidins protect against early diabetic peripheral neuropathy by modulating endoplasmic reticulum stress

Diabetic peripheral neuropathy (DPN) is the most common and troublesome complication of type 2 diabetes mellitus (T2DM). Recent findings reveal an important role of endoplasmic reticulum (ER) stress in the development of DPN and identify a potential new therapeutic target. Schwann cells (SC), the myelinating cells in peripheral nervous system, are highly susceptible to ER homeostasis. Grape seed proanthocyanidins (GSPs) have been reported to improve DPN of type 1 diabetic rats and relieve ER stress in skeletal muscles and pancreas of T2DM. We investigated the potential role of ER stress in SC in regulating DPN of T2DM and assessed whether early intervention of GSPs would prevent DPN by modulating ER stress. The present study was performed in Sprague-Dawley rats made T2DM with low-dose streptozotocin and a high-carbohydrate/high-fat diet and in rat SC cultured in serum from type 2 diabetic rats. Diabetic rats showed a typical characteristic of T2DM and slowing of nerve conduction velocity (NCV) in sciatic/tibial nerves. The lesions of SC, Ca(2+) overload and ER stress were present in sciatic nerves of diabetic rats, as well as in cell culture models. GSPs administration significantly decreased the low-density lipoprotein level and increased NCV in diabetic rats. GSPs or their metabolites also partially prevented cell injury, Ca(2+) overload and ER stress in animal and cell culture models. Therefore, ER stress is implicated in peripheral neuropathy in animal and cell culture models of T2DM. Prophylactic GSPs treatment might have auxiliary preventive potential for DPN partially by alleviating ER stress.

Attenuation of endoplasmic reticulum stress in Pelizaeus-Merzbacher disease by an anti-malaria drug, chloroquine

Pelizaeus-Merzbacher disease (PMD) is a hypomyelinating disorder caused by the duplication and missense mutations of the proteolipid protein 1 (PLP1) gene. PLP1 missense proteins accumulate in the endoplasmic reticulum (ER) of premature oligodendrocytes and induce severe ER stress followed by apoptosis of the cells. Here, we demonstrate that an anti-malaria drug, chloroquine, decreases the amount of an ER-resident mutant PLP1 containing an alanine-243 to valine (A243V) substitution, which induces severe PMD in human. By preventing mutant PLP1 translation through enhancing the phosphorylation of eukaryotic initiation factor 2 alpha, chloroquine ameliorated the ER stress induced by the mutant protein in HeLa cells. Chroloquine also attenuated ER stress in the primary oligodendrocytes obtained from myelin synthesis deficit (msd) mice, which carry the same PLP1 mutation. In the spinal cords of msd mice, chloroquine inhibited ER stress and upregulated the expression of marker genes of mature oligodendrocytes. Chloroquine-mediated attenuation of ER stress was observed in HeLa cells treated with tunicamycin, an N-glycosylation inhibitor, but not with thapsigargin, a sarco/ER Ca(2+)ATPase inhibitor, which confirms its efficacy against ER stress caused by nascent proteins. These findings indicate that chloroquine is an ER stress attenuator with potential use in treating PMD and possibly other ER stress-related diseases.

An enhanced integrated stress response ameliorates mutant SOD1-induced ALS

Varied stresses to cells can lead to a repression in translation by triggering phosphorylation of eukaryotic translation initiator factor 2α (eIF2α), which is central to a process known as the integrated stress response (ISR). PKR-like ER-localized eIF2 kinase (PERK), one of the kinases that phosphorylates eIF2α and coordinates the ISR, is activated by stress occurring from the accumulation of misfolded or unfolded proteins in the endoplasmic reticulum (ER). Mutant Cu/Zn superoxide dismutase (mtSOD1) is thought to cause familial amyotrophic lateral sclerosis (FALS) because it misfolds and aggregates. Published studies have suggested that ER stress is involved in FALS pathogenesis since mtSOD1 accumulates inside the ER and activates PERK leading to phosphorylated eIF2α (p-eIF2α). We previously used a genetic approach to show that haploinsufficiency of PERK significantly accelerates disease onset and shortens survival of G85R mtSOD1 FALS transgenic mice. We now show that G85R mice that express reduced levels of active GADD34, which normally dephosphorylates p-eIF2α and allows recovery from the global suppression of protein synthesis, markedly ameliorates disease. These studies emphasize the importance of the ISR, and specifically the PERK pathway, in the pathogenesis of mtSOD1-induced FALS and as a target for treatment. Furthermore, the ISR may be an appropriate therapeutic target for sporadic ALS and other neurodegenerative diseases since misfolded proteins have been implicated in these disorders.

PERK activation preserves the viability and function of remyelinating oligodendrocytes in immune-mediated demyelinating diseases

Remyelination occurs in multiple sclerosis (MS) lesions but is generally considered to be insufficient. One of the major challenges in MS research is to understand the causes of remyelination failure and to identify therapeutic targets that promote remyelination. Activation of pancreatic endoplasmic reticulum kinase (PERK) signaling in response to endoplasmic reticulum stress modulates cell viability and function under stressful conditions. There is evidence that PERK is activated in remyelinating oligodendrocytes in demyelinated lesions in both MS and its animal model, experimental autoimmune encephalomyelitis (EAE). In this study, we sought to determine the role of PERK signaling in remyelinating oligodendrocytes in MS and EAE using transgenic mice that allow temporally controlled activation of PERK signaling specifically in oligodendrocytes. We demonstrated that persistent PERK activation was not deleterious to myelinating oligodendrocytes in young, developing mice or to remyelinating oligodendrocytes in cuprizone-induced demyelinated lesions. We found that enhancing PERK activation, specifically in (re)myelinating oligodendrocytes, protected the cells and myelin against the detrimental effects of interferon-γ, a key proinflammatory cytokine in MS and EAE. More important, we showed that enhancing PERK activation in remyelinating oligodendrocytes at the recovery stage of EAE promoted cell survival and remyelination in EAE demyelinated lesions. Thus, our data provide direct evidence that PERK activation cell-autonomously enhances the survival and preserves function of remyelinating oligodendrocytes in immune-mediated demyelinating diseases.

Loss of Oca2 disrupts the unfolded protein response and increases resistance to endoplasmic reticulum stress in melanocytes

Accumulation of proteins in the endoplasmic reticulum (ER) typically induces stress and initiates the unfolded protein response (UPR) to facilitate recovery. If homeostasis is not restored, apoptosis is induced. However, adaptation to chronic UPR activation can increase resistance to subsequent acute ER stress. We therefore investigated adaptive mechanisms in Oculocutaneous albinism type 2 (Oca2)-null melanocytes where UPR signaling is arrested despite continued tyrosinase accumulation leading to resistance to the chemical ER stressor thapsigargin. Although thapsigargin triggers UPR activation, instead of Perk-mediated phosphorylation of eIF2α, in Oca2-null melanocytes, eIF2α was rapidly dephosphorylated upon treatment. Dephosphorylation was mediated by the Gadd34-PP1α phosphatase complex. Gadd34-complex inhibition blocked eIF2α dephosphorylation and significantly increased Oca2-null melanocyte sensitivity to thapsigargin. Thus, Oca2-null melanocytes adapt to acute ER stress by disruption of pro-apoptotic Perk signaling, which promotes cell survival. This is the first study to demonstrate rapid eIF2α dephosphorylation as an adaptive mechanism to ER stress.

Oligodendrocyte-specific activation of PERK signaling protects mice against experimental autoimmune encephalomyelitis

There is compelling evidence that oligodendrocyte apoptosis, in response to CNS inflammation, contributes significantly to the development of the demyelinating disorder multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). Therefore, approaches designed to protect oligodendrocytes would likely have therapeutic value. Activation of pancreatic endoplasmic reticulum kinase (PERK) signaling in response to endoplasmic reticulum (ER) stress increases cell survival under various cytotoxic conditions. Moreover, there is evidence that PERK signaling is activated in oligodendrocytes within demyelinating lesions in multiple sclerosis and EAE. Our previous study demonstrated that CNS delivery of the inflammatory cytokine interferon-γ before EAE onset protected mice against EAE, and this protection was dependent on PERK signaling. In our current study, we sought to elucidate the role of PERK signaling in oligodendrocytes during EAE. We generated transgenic mice that allow for temporally controlled activation of PERK signaling, in the absence of ER stress, specifically in oligodendrocytes. We demonstrated that persistent activation of PERK signaling was not deleterious to oligodendrocyte viability or the myelin of adult animals. Importantly, we found that enhanced activation of PERK signaling specifically in oligodendrocytes significantly attenuated EAE disease severity, which was associated with reduced oligodendrocyte apoptosis, demyelination, and axonal degeneration. This effect was not the result of an altered degree of the inflammatory response in EAE mice. Our results provide direct evidence that activation of PERK signaling in oligodendrocytes is cytoprotective, protecting mice against EAE.

Resetting translational homeostasis restores myelination in Charcot-Marie-Tooth disease type 1B mice

Reduction of the CHOP target Gadd34 restores motor function in P0S63del mice with demyelinating neuropathy.

P0 glycoprotein is an abundant product of terminal differentiation in myelinating Schwann cells. The mutant P0S63del causes Charcot-Marie-Tooth 1B neuropathy in humans, and a very similar demyelinating neuropathy in transgenic mice. P0S63del is retained in the endoplasmic reticulum of Schwann cells, where it promotes unfolded protein stress and elicits an unfolded protein response (UPR) associated with translational attenuation. Ablation of Chop, a UPR mediator, from S63del mice completely rescues their motor deficit and reduces active demyelination by half. Here, we show that Gadd34 is a detrimental effector of CHOP that reactivates translation too aggressively in myelinating Schwann cells. Genetic or pharmacological limitation of Gadd34 function moderates translational reactivation, improves myelination in S63del nerves, and reduces accumulation of P0S63del in the ER. Resetting translational homeostasis may provide a therapeutic strategy in tissues impaired by misfolded proteins that are synthesized during terminal differentiation.

CHOP regulates the p53-MDM2 axis and is required for neuronal survival after seizures

Hippocampal sclerosis is a frequent pathological finding in patients with temporal lobe epilepsy and can be caused by prolonged single or repeated brief seizures. Both DNA damage and endoplasmic reticulum stress have been implicated as underlying molecular mechanisms in seizure-induced brain injury. The CCAAT/enhancer-binding protein homologous protein (CHOP) is a transcriptional regulator induced downstream of DNA damage and endoplasmic reticulum stress, which can promote or inhibit apoptosis according to context. Recent work has proposed inhibition of CHOP as a suitable neuroprotective strategy. Here, we show that transcript and protein levels of CHOP increase in surviving subfields of the hippocampus after prolonged seizures (status epilepticus) in mouse models. CHOP was also elevated in the hippocampus from epileptic mice and patients with pharmacoresistant epilepsy. The hippocampus of CHOP-deficient mice was much more vulnerable to damage in mouse models of status epilepticus. Moreover, compared with wild-type animals, CHOP-deficient mice subject to status epilepticus developed more spontaneous seizures, displayed protracted hippocampal neurodegeneration and a deficit in a hippocampus-dependent object-place recognition task. The absence of CHOP was associated with a supra-maximal induction of p53 after status epilepticus, and inhibition of p53 abolished the cell death-promoting consequences of CHOP deficiency. The protective effect of CHOP could be partly explained by activating transcription of murine double minute 2 that targets p53 for degradation. These data demonstrate that CHOP is required for neuronal survival after seizures and caution against inhibition of CHOP as a neuroprotective strategy where excitotoxicity is an underlying pathomechanism.

C/EBP homologous protein (CHOP) contributes to suppression of metabolic genes during endoplasmic reticulum stress in the liver

The unfolded protein response (UPR) senses stress in the endoplasmic reticulum (ER) and initiates signal transduction cascades that culminate in changes to gene regulation. Long recognized as a means for improving ER protein folding through up-regulation of ER chaperones, the UPR is increasingly recognized to play a role in the regulation of metabolic pathways. ER stress is clearly connected to altered metabolism in tissues such as the liver, but the mechanisms underlying this connection are only beginning to be elucidated. Here, working exclusively in vivo, we tested the hypothesis that the UPR-regulated CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) participates in the transcriptional regulation of metabolism during hepatic ER stress. We found that metabolic dysregulation was associated with induction of eIF2α signaling and CHOP up-regulation during challenge with tunicamycin or Velcade. CHOP was necessary for suppression of genes encoding the transcriptional master regulators of lipid metabolism: Cebpa, Ppara, and Srebf1. This action of CHOP required a contemporaneous CHOP-independent stress signal. CHOP bound directly to C/EBP-binding regions in the promoters of target genes, whereas binding of C/EBPα and C/EBPβ to the same regions was diminished during ER stress. Our results thus highlight a role for CHOP in the transcriptional regulation of metabolism.

Induction of heat shock protein 70 (Hsp70) prevents neuregulin-induced demyelination by enhancing the proteasomal clearance of c-Jun

Modulating molecular chaperones is emerging as an attractive approach to treat neurodegenerative diseases associated with protein aggregation, DPN (diabetic peripheral neuropathy) and possibly, demyelinating neuropathies. KU-32 [N-(7-((2R,3R,4S,5R)-3,4-dihydroxy-5-methoxy-6,6-dimethyl-tetrahydro-2H-pyran-2-yloxy)-8-methyl-2-oxo-2H-chromen-3-yl)acetamide] is a small molecule inhibitor of Hsp90 (heat shock protein 90) and reverses sensory deficits associated with myelinated fibre dysfunction in DPN. Additionally, KU-32 prevented the loss of myelinated internodes induced by treating myelinated SC (Schwann cell)-DRG (dorsal root ganglia) sensory neuron co-cultures with NRG1 (neuregulin-1 Type 1). Since KU-32 decreased NRG1-induced demyelination in an Hsp70-dependent manner, the goal of the current study was to clarify how Hsp70 may be mechanistically linked to preventing demyelination. The activation of p42/p44 MAPK (mitogen-activated protein kinase) and induction of the transcription factor c-Jun serve as negative regulators of myelination. NRG1 activated MAPK, induced c-Jun expression and promoted a loss of myelin segments in DRG explants isolated from both WT (wild-type) and Hsp70 KO (knockout) mice. Although KU-32 did not block the activation of MAPK, it blocked c-Jun induction and protected against a loss of myelinated segments in WT mice. In contrast, KU-32 did not prevent the NRG1-dependent induction of c-Jun and loss of myelin segments in explants from Hsp70 KO mice. Overexpression of Hsp70 in myelinated DRG explants prepared from WT or Hsp70 KO mice was sufficient to block the induction of c-Jun and the loss of myelin segments induced by NRG1. Lastly, inhibiting the proteasome prevented KU-32 from decreasing c-Jun levels. Collectively, these data support that Hsp70 induction is sufficient to prevent NRG1-induced demyelination by enhancing the proteasomal degradation of c-Jun.

The impact of the unfolded protein response on human disease

A central function of the endoplasmic reticulum (ER) is to coordinate protein biosynthetic and secretory activities in the cell. Alterations in ER homeostasis cause accumulation of misfolded/unfolded proteins in the ER. To maintain ER homeostasis, eukaryotic cells have evolved the unfolded protein response (UPR), an essential adaptive intracellular signaling pathway that responds to metabolic, oxidative stress, and inflammatory response pathways. The UPR has been implicated in a variety of diseases including metabolic disease, neurodegenerative disease, inflammatory disease, and cancer. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human disease.

MpzR98C arrests Schwann cell development in a mouse model of early-onset Charcot-Marie-Tooth disease type 1B

Mutations in myelin protein zero (MPZ) cause Charcot-Marie-Tooth disease type 1B. Many dominant MPZ mutations, including R98C, present as infantile onset dysmyelinating neuropathies. We have generated an R98C 'knock-in' mouse model of Charcot-Marie-Tooth type 1B, where a mutation encoding R98C was targeted to the mouse Mpz gene. Both heterozygous (R98C/+) and homozygous (R98C/R98C) mice develop weakness, abnormal nerve conduction velocities and morphologically abnormal myelin; R98C/R98C mice are more severely affected. MpzR98C is retained in the endoplasmic reticulum of Schwann cells and provokes a transitory, canonical unfolded protein response. Ablation of Chop, a mediator of the protein kinase RNA-like endoplasmic reticulum kinase unfolded protein response pathway restores compound muscle action potential amplitudes of R98C/+ mice but does not alter the reduced conduction velocities, reduced axonal diameters or clinical behaviour of these animals. R98C/R98C Schwann cells are developmentally arrested in the promyelinating stage, whereas development is delayed in R98C/+ mice. The proportion of cells expressing c-Jun, an inhibitor of myelination, is elevated in mutant nerves, whereas the proportion of cells expressing the promyelinating transcription factor Krox-20 is decreased, particularly in R98C/R98C mice. Our results provide a potential link between the accumulation of MpzR98C in the endoplasmic reticulum and a developmental delay in myelination. These mice provide a model by which we can begin to understand the early onset dysmyelination seen in patients with R98C and similar mutations.

Free cysteine modulates the conformation of human C/EBP homologous protein

The C/EBP Homologous Protein (CHOP) is a nuclear protein that is integral to the unfolded protein response culminating from endoplasmic reticulum stress. Previously, CHOP was shown to comprise extensive disordered regions and to self-associate in solution. In the current study, the intrinsically disordered nature of this protein was characterized further by comprehensive in silico analyses. Using circular dichroism, differential scanning calorimetry and nuclear magnetic resonance, we investigated the global conformation and secondary structure of CHOP and demonstrated, for the first time, that conformational changes in this protein can be induced by the free amino acid L-cysteine. Addition of L-cysteine caused a significant dose-dependent decrease in the protein helicity--dropping from 69.1% to 23.8% in the presence of 1 mM of L-cysteine--and a sequential transition to a more disordered state, unlike that caused by thermal denaturation. Furthermore, the presence of small amounts of free amino acid (80 µM, an 8:1 cysteine∶CHOP ratio) during CHOP thermal denaturation altered the molecular mechanism of its melting process, leading to a complex, multi-step transition. On the other hand, high levels (4 mM) of free L-cysteine seemed to cause a complete loss of rigid cooperatively melting structure. These results suggested a potential regulatory function of L-cysteine which may lead to changes in global conformation of CHOP in response to the cellular redox state and/or endoplasmic reticulum stress.

The unfolded protein response is a major mechanism by which LRP1 regulates Schwann cell survival after injury

In peripheral nerve injury, Schwann cells (SCs) must survive to exert a continuing and essential role in successful nerve regeneration. Herein, we show that peripheral nerve injury is associated with activation of endoplasmic reticulum (ER) stress and the adaptive unfolded protein response (UPR). The UPR culminates in expression of C/EBP homology protein (CHOP), a proapoptotic transcription factor in SCs, unless counteracted by LDL receptor-related protein-1 (LRP1), which serves as a major activator of phosphatidylinositol 3-kinase (PI3K). Sciatic nerve crush injury in rats induced expression of the ER chaperone GRP78/BIP, reflecting an early, corrective phase of the UPR. However, when LRP1 signaling was inhibited with receptor-associated protein, PI3K activity was decreased and CHOP protein expression increased, particularly in myelinating SCs. In cultured SCs, the PKR-like ER kinase target eIF2α was phosphorylated and CHOP was induced by (1) inhibiting PI3K, (2) treating the cells with tumor necrosis factor-α (TNF-α), or (3) genetic silencing of LRP1. CHOP gene deletion in SCs decreased cell death in response to TNF-α. Furthermore, the effects of TNF-α on phosphorylated eIF2α, CHOP, and SC death were blocked by adding LRP1 ligands that augment LRP1-dependent cell signaling to PI3K. Collectively, our results support a model in which UPR-activated signaling pathways represent a major challenge to SC survival in nerve injury. LRP1 functions as a potent activator of PI3K in SCs and, by this mechanism, limits SC apoptosis resulting from increased CHOP expression in nerve injury.

Transcriptional profiling of chondrodysplasia growth plate cartilage reveals adaptive ER-stress networks that allow survival but disrupt hypertrophy

Metaphyseal chondrodysplasia, Schmid type (MCDS) is characterized by mild short stature and growth plate hypertrophic zone expansion, and caused by collagen X mutations. We recently demonstrated the central importance of ER stress in the pathology of MCDS by recapitulating the disease phenotype by expressing misfolding forms of collagen X (Schmid) or thyroglobulin (Cog) in the hypertrophic zone. Here we characterize the Schmid and Cog ER stress signaling networks by transcriptional profiling of microdissected mutant and wildtype hypertrophic zones. Both models displayed similar unfolded protein responses (UPRs), involving activation of canonical ER stress sensors and upregulation of their downstream targets, including molecular chaperones, foldases, and ER-associated degradation machinery. Also upregulated were the emerging UPR regulators Wfs1 and Syvn1, recently identified UPR components including Armet and Creld2, and genes not previously implicated in ER stress such as Steap1 and Fgf21. Despite upregulation of the Chop/Cebpb pathway, apoptosis was not increased in mutant hypertrophic zones. Ultrastructural analysis of mutant growth plates revealed ER stress and disrupted chondrocyte maturation throughout mutant hypertrophic zones. This disruption was defined by profiling the expression of wildtype growth plate zone gene signatures in the mutant hypertrophic zones. Hypertrophic zone gene upregulation and proliferative zone gene downregulation were both inhibited in Schmid hypertrophic zones, resulting in the persistence of a proliferative chondrocyte-like expression profile in ER-stressed Schmid chondrocytes. Our findings provide a transcriptional map of two chondrocyte UPR gene networks in vivo, and define the consequences of UPR activation for the adaptation, differentiation, and survival of chondrocytes experiencing ER stress during hypertrophy. Thus they provide important insights into ER stress signaling and its impact on cartilage pathophysiology.