The unfolded protein response modulates disease severity in Pelizaeus-Merzbacher disease

Insertion of proteolipid protein into oligodendrocyte mitochondria regulates extracellular pH and adenosine triphosphate

Proteolipid protein (PLP) and DM20, the most abundant myelin proteins, are coded by the human PLP1 and non-human Plp1 PLP gene. Mutations in the PLP1 gene cause Pelizaeus-Merzbacher disease (PMD) with duplications of the native PLP1 gene accounting for 70% of PLP1 mutations. Humans with PLP1 duplications and mice with extra Plp1 copies have extensive neuronal degeneration. The mechanism that causes neuronal degeneration is unknown. We show that native PLP traffics to mitochondria when the gene is duplicated in mice and in humans. This report is the first demonstration of a specific cellular defect in brains of PMD patients; it validates rodent models as ideal models to study PMD. Insertion of nuclear-encoded mitochondrial proteins requires specific import pathways; we show that specific cysteine motifs, part of the Mia40/Erv1 mitochondrial import pathway, are present in PLP and are required for its insertion into mitochondria. Insertion of native PLP into mitochondria of transfected cells acidifies media, partially due to increased lactate; it also increases adenosine triphosphate (ATP) in the media. The same abnormalities are found in the extracellular space of mouse brains with extra copies of Plp1. These physiological abnormalities are preventable by mutations in PLP cysteine motifs, a hallmark of the Mia40/Erv1 pathway. Increased extracellular ATP and acidosis lead to neuronal degeneration. Our findings may be the mechanism by which microglia are activated and proinflammatory molecules are upregulated in Plp1 transgenic mice (Tatar et al. (2010) ASN Neuro 2:art:e00043). Manipulation of this metabolic pathway may restore normal metabolism and provide therapy for PMD patients.

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.

Hypomyelinating leukodystrophy-associated missense mutant of FAM126A/hyccin/DRCTNNB1A aggregates in the endoplasmic reticulum

Hypomyelinating leukodystrophies (HLD) are hereditary central nervous system diseases in which the myelin sheath does not form properly. The disease prototype is the X-linked recessive Pelizaeus-Merzbacher disease (also now known as HLD1), which is caused by the mutation, multiplication, or deletion of the plp1 gene. PLP1 missense mutations lead to protein aggregation and accumulation in subcellular compartments such as the endoplasmic reticulum (ER). The gene responsible for an autosomal recessive Pelizaeus-Merzbacher-like disease called HLD5 is named fam126a (also known as hyccin or drctnnb1a). While the gene mutations often cause FAM126A protein deficiency, one known missense mutation, Leu-53-to-Pro (L53P), allows some protein to be produced. Here, we show that the L53P mutant aggregates in cells, accumulating primarily in the ER. This is in contrast to the wild type FAM126A, which distributes throughout the cytoplasm. In addition, the L53P mutant expression promotes the activities of kinases involved in unfolded protein response. These results suggest that a disease-associated FAM126A missense mutation causes protein accumulation in subcellular compartments, possibly to mediate a disease-associated phenotype, which is similar to what is seen with PLP1.

Zebularine inhibits tumorigenesis and stemness of colorectal cancer via p53-dependent endoplasmic reticulum stress

Aberrant DNA hypermethylation is frequently found in tumor cells and inhibition of DNA methylation is an effective anticancer strategy. In this study, the therapeutic effect of DNA methyltransferase (DNMT) inhibitor zebularine (Zeb) on colorectal cancer (CRC) was investigated. Zeb exhibited anticancer activity in cell cultures, tumor xenografts and mouse colitis-associated CRC model. It stabilizes p53 through ribosomal protein S7 (RPS7)/MDM2 pathways and DNA damage. Zeb-induced cell death was dependent on p53. Microarray analysis revealed that genes related to endoplasmic reticulum (ER) stress and unfolded protein response (UPR) were affected by Zeb. Zeb induced p53-dependent ER stress and autophagy. Pro-survival markers of ER stress/UPR (GRP78) and autophagy (p62) were increased in tumor tissues of CRC patients, AOM/DSS-induced CRC mice and HCT116-derived colonospheres. Zeb downregulates GRP78 and p62, and upregulates a pro-apoptotic CHOP. Our results reveal a novel mechanism for the anticancer activity of Zeb.

Short-term cuprizone feeding induces selective amino acid deprivation with concomitant activation of an integrated stress response in oligodendrocytes

Cuprizone [bis(cyclohexylidenehydrazide)]-induced toxic demyelination is an experimental approach frequently used to study de- and re-myelination in the central nervous system. In this model, mice are fed with the copper chelator cuprizone which leads to oligodendrocyte apoptosis and subsequent microgliosis, astrocytosis, and demyelination. The underlying mechanisms of cuprizone-induced oligodendrocyte death are still unknown. We analysed differences in amino acid levels after short-term cuprizone exposure (i.e., 4 days). Furthermore, an amino acid response (AAR) pathway activated in oligodendrocytes after cuprizone intoxication was evaluated. Short-term cuprizone exposure resulted in a selective decrease of alanine, glycine, and proline plasma levels, which was paralleled by an increase of apoptotic cells in the liver and a decrease of alanine aminotransferase in the serum. These parameters were paralleled by oligodendrocyte apoptosis and the induction of an AAR with increased expression of the transcription factors ATF-3 and ATF-4 (activating transcription factor-3 and -4). Immunohistochemistry revealed that ATF-3 is exclusively expressed by oligodendrocytes and localized to the nuclear compartment. Our results suggest that cuprizone-induced liver dysfunction results in amino acid starvation and in consequence to the activation of an AAR. We propose that this stress response modulates oligodendrocyte viability in the cuprizone animal model.

A novel mutation in PLP1 causes severe hereditary spastic paraplegia type 2

Hereditary spastic paraplegia (HSP) type 2 is a proteolipid protein (PLP1)-related genetic disorder that is characterized by dysmyelination of the central nervous system resulting primarily in limb spasticity, cognitive impairment, nystagmus, and spastic urinary bladder of varying severity. Previously reported PLP1 mutations include duplications, point mutations, or whole gene deletions with a continuum of phenotypes ranging from severe Pelizaeus-Merzbacher disease (PMD) to uncomplicated HSP type 2. In this manuscript we report a novel PLP1 missense mutation (c.88G>C) in a family from Argentina. This mutation is in a highly conserved transmembrane domain of PLP1 and the mutant protein was found to be retained in the endoplasmic reticulum when expressed in vitro. Due to the variable expressivity that characterizes these disorders our report contributes to the knowledge of genotype-phenotype correlations of PLP1-related disorders.

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.

Endoplasmic reticulum dysfunction in neurological disease

Endoplasmic reticulum (ER) dysfunction might have an important part to play in a range of neurological disorders, including cerebral ischaemia, sleep apnoea, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, the prion diseases, and familial encephalopathy with neuroserpin inclusion bodies. Protein misfolding in the ER initiates the well studied unfolded protein response in energy-starved neurons during stroke, which is relevant to the toxic effects of reperfusion. The toxic peptide amyloid β induces ER stress in Alzheimer's disease, which leads to activation of similar pathways, whereas the accumulation of polymeric neuroserpin in the neuronal ER triggers a poorly understood ER-overload response. In other neurological disorders, such as Parkinson's and Huntington's diseases, ER dysfunction is well recognised but the mechanisms by which it contributes to pathogenesis remain unclear. By targeting components of these signalling responses, amelioration of their toxic effects and so the treatment of a range of neurodegenerative disorders might become possible.

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.

Depletion of molecular chaperones from the endoplasmic reticulum and fragmentation of the Golgi apparatus associated with pathogenesis in Pelizaeus-Merzbacher disease

Missense mutations in the proteolipid protein 1 (PLP1) gene cause a wide spectrum of hypomyelinating disorders, from mild spastic paraplegia type 2 to severe Pelizaeus-Merzbacher disease (PMD). Mutant PLP1 accumulates in the endoplasmic reticulum (ER) and induces ER stress. However, the link between the clinical severity of PMD and the cellular response induced by mutant PLP1 remains largely unknown. Accumulation of misfolded proteins in the ER generally leads to up-regulation of ER chaperones to alleviate ER stress. Here, we found that expression of the PLP1-A243V mutant, which causes severe disease, depletes some ER chaperones with a KDEL (Lys-Asp-Glu-Leu) motif, in HeLa cells, MO3.13 oligodendrocytic cells, and primary oligodendrocytes. The same PLP1 mutant also induces fragmentation of the Golgi apparatus (GA). These organelle changes are less prominent in cells with milder disease-associated PLP1 mutants. Similar changes are also observed in cells expressing another disease-causing gene that triggers ER stress, as well as in cells treated with brefeldin A, which induces ER stress and GA fragmentation by inhibiting GA to ER trafficking. We also found that mutant PLP1 disturbs localization of the KDEL receptor, which transports the chaperones with the KDEL motif from the GA to the ER. These data show that PLP1 mutants inhibit GA to ER trafficking, which reduces the supply of ER chaperones and induces GA fragmentation. We propose that depletion of ER chaperones and GA fragmentation induced by mutant misfolded proteins contribute to the pathogenesis of inherited ER stress-related diseases and affect the disease severity.

Mutation of sec63 in zebrafish causes defects in myelinated axons and liver pathology

Mutations in SEC63 cause polycystic liver disease in humans. Sec63 is a member of the endoplasmic reticulum (ER) translocon machinery, although it is unclear how mutations in SEC63 lead to liver cyst formation in humans. Here, we report the identification and characterization of a zebrafish sec63 mutant, which was discovered in a screen for mutations that affect the development of myelinated axons. Accordingly, we show that disruption of sec63 in zebrafish leads to abnormalities in myelinating glia in both the central and peripheral nervous systems. In the vertebrate nervous system, segments of myelin are separated by the nodes of Ranvier, which are unmyelinated regions of axonal membrane containing a high density of voltage-gated sodium channels. We show that sec63 mutants have morphologically abnormal and reduced numbers of clusters of voltage-gated sodium channels in the spinal cord and along peripheral nerves. Additionally, we observed reduced myelination in both the central and peripheral nervous systems, as well as swollen ER in myelinating glia. Markers of ER stress are upregulated in sec63 mutants. Finally, we show that sec63 mutants develop liver pathology. As in glia, the primary defect, detectable at 5 dpf, is fragmentation and swelling of the ER, indicative of accumulation of proteins in the lumen. At 8 dpf, ER swelling is severe; other pathological features include disrupted bile canaliculi, altered cytoplasmic matrix and accumulation of large lysosomes. Together, our analyses of sec63 mutant zebrafish highlight the possible role of ER stress in polycystic liver disease and suggest that these mutants will serve as a model for understanding the pathophysiology of this disease and other abnormalities involving ER stress.

Detrimental effects of proteasome inhibition activity in Drosophila melanogaster: implication of ER stress, autophagy, and apoptosis

In eukaryotes, the ubiquitin-proteasome machinery regulates a number of fundamental cellular processes through accurate and tightly controlled protein degradation pathways. We have, herein, examined the effects of proteasome functional disruption in Dmp53 (+/+) (wild-type) and Dmp53 (-/-) Drosophila melanogaster fly strains through utilization of Bortezomib, a proteasome-specific inhibitor. We report that proteasome inhibition drastically shortens fly life-span and impairs climbing performance, while it also causes larval lethality and activates developmentally irregular cell death programs during oogenesis. Interestingly, Dmp53 gene seems to play a role in fly longevity and climbing ability. Moreover, Bortezomib proved to induce endoplasmic reticulum (ER) stress that was able to result in the engagement of unfolded protein response (UPR) signaling pathway, as respectively indicated by fly Xbp1 activation and Ref(2)P-containing protein aggregate formation. Larva salivary gland and adult brain both underwent strong ER stress in response to Bortezomib, thus underscoring the detrimental role of proteasome inhibition in larval development and brain function. We also propose that the observed upregulation of autophagy operates as a protective mechanism to "counterbalance" Bortezomib-induced systemic toxicity, which is tightly associated, besides ER stress, with activation of apoptosis, mainly mediated by functional Drice caspase and deregulated dAkt kinase. The reduced life-span of exposed to Bortezomib flies overexpressing Atg1_RNAi or Atg18_RNAi supports the protective nature of autophagy against proteasome inhibition-induced stress. Our data reveal the in vivo significance of proteasome functional integrity as a major defensive system against cellular toxicity likely occurring during critical biological processes and morphogenetic courses.

UPR-induced resistance to etoposide is downstream of PERK and independent of changes in topoisomerase IIα levels

Background

The unfolded protein response (UPR) is regulated by three ER-localized, transmembrane signal transducers that control distinct aspects of the UPR. We previously reported that both increased resistance to etoposide and a reduction in Topoisomerase IIα protein levels were a direct response of UPR activation, and the latter occurred independent of changes in Topo IIα mRNA levels. We have now examined the contribution of each of the three up-stream transducers of the UPR, as well as some of their downstream targets in affecting decreased expression of Topo IIα protein and increased drug resistance.

Principal Findings

Our data revealed that while Ire1 activation led to Topo IIα loss at the protein level it did not contribute to changes in sensitivity to etoposide. The decreased expression of Topo IIα protein was not downstream of XBP-1, in keeping with the fact that Topo IIα transcription was not affected by ER stress. Conversely, PERK activation did not contribute to changes in Topo IIα protein levels, but it did play a significant role in the UPR-induced decreased sensitivity to etoposide. Several cellular responses downstream of PERK were examined for their potential to contribute to resistance. The ATF6 arm of the UPR did not significantly contribute to etoposide resistance within the time frame of our experiments.

Conclusions and Significance

In toto, our data demonstrate that UPR-induced changes in Topo IIα protein levels are not responsible for resistance to etoposide as has been previously hypothesized, and instead demonstrate that the PERK branch plays a Topo IIα-independent role in altered sensitivity to this drug.

Gap junctions in inherited human disorders of the central nervous system

CNS glia and neurons express connexins, the proteins that form gap junctions in vertebrates. We review the connexins expressed by oligodendrocytes and astrocytes, and discuss their proposed physiologic roles. Of the 21 members of the human connexin family, mutations in three are associated with significant central nervous system manifestations. For each, we review the phenotype and discuss possible mechanisms of disease. Mutations in GJB1, the gene for connexin 32 (Cx32) cause the second most common form of Charcot-Marie-Tooth disease (CMT1X). Though the only consistent phenotype in CMT1X patients is a peripheral demyelinating neuropathy, CNS signs and symptoms have been found in some patients. Recessive mutations in GJC2, the gene for Cx47, are one cause of Pelizaeus-Merzbacher-like disease (PMLD), which is characterized by nystagmus within the first 6 months of life, cerebellar ataxia by 4 years, and spasticity by 6 years of age. MRI imaging shows abnormal myelination. A different recessive GJC2 mutation causes a form of hereditary spastic paraparesis, which is a milder phenotype than PMLD. Dominant mutations in GJA1, the gene for Cx43, cause oculodentodigital dysplasia (ODDD), a pleitropic disorder characterized by oculo-facial abnormalities including micropthalmia, microcornia and hypoplastic nares, syndactyly of the fourth to fifth fingers and dental abnormalities. Neurologic manifestations, including spasticity and gait difficulties, are often but not universally seen. Recessive GJA1 mutations cause Hallermann-Streiff syndrome, a disorder showing substantial overlap with ODDD. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Cannabidiol protects oligodendrocyte progenitor cells from inflammation-induced apoptosis by attenuating endoplasmic reticulum stress

Cannabidiol (CBD) is the most abundant cannabinoid in Cannabis sativa that has no psychoactive properties. CBD has been approved to treat inflammation, pain and spasticity associated with multiple sclerosis (MS), of which demyelination and oligodendrocyte loss are hallmarks. Thus, we investigated the protective effects of CBD against the damage to oligodendrocyte progenitor cells (OPCs) mediated by the immune system. Doses of 1 μM CBD protect OPCs from oxidative stress by decreasing the production of reactive oxygen species. CBD also protects OPCs from apoptosis induced by LPS/IFNγ through the decrease of caspase 3 induction via mechanisms that do not involve CB1, CB2, TRPV1 or PPARγ receptors. Tunicamycin-induced OPC death was attenuated by CBD, suggesting a role of endoplasmic reticulum (ER) stress in the mode of action of CBD. This protection against ER stress-induced apoptosis was associated with reduced phosphorylation of eiF2α, one of the initiators of the ER stress pathway. Indeed, CBD diminished the phosphorylation of PKR and eiF2α induced by LPS/IFNγ. The pro-survival effects of CBD in OPCs were accompanied by decreases in the expression of ER apoptotic effectors (CHOP, Bax and caspase 12), and increased expression of the anti-apoptotic Bcl-2. These findings suggest that attenuation of the ER stress pathway is involved in the ‘oligoprotective' effects of CBD during inflammation.

Pelizaeus-Merzbacher disease-associated proteolipid protein 1 inhibits oligodendrocyte precursor cell differentiation via extracellular-signal regulated kinase signaling

Oligodendrocytes (OLs) are myelin-forming glial cells in the central nervous system (CNS) and their dysfunction causes neuropathies such as demyelinating diseases. Proteolipid protein 1 (PLP1) is an oligodendrocyte myelin-rich tetraspan membrane protein and aberration of the plp1 gene is known to be responsible for dysmyelinating Pelizaeus-Merzbacher disease (PMD). Among previously identified gene alternations, multiplication of the plp1 gene causes increased expression of PLP1, resulting in a phenotype with severe dysmyelination in human and also rodent models. Yet little is known about the relationship between increased PLP1 expression and oligodendrocyte precursor cell (OPC) differentiation and the intracellular molecular mechanism. Here we show that expression of PLP1 in OPCs markedly inhibits their differentiation, and that this inhibitory effect is effectively improved by inhibition of extracellular signal-regulated kinase (ERK) activity. Furthermore, in cocultures with dorsal root ganglion (DRG) neurons, ERK inhibition also improves PLP1-induced dysmyelination. Thus, ERK inhibition helps to improve defective OPC differentiation induced by PLP1 expression, suggesting that molecules belonging to ERK signaling cascade may be new PMD therapeutic targets.

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.

Effect of curcumin in a mouse model of Pelizaeus-Merzbacher disease

PLP1 amino acid substitutions cause accumulation of misfolded protein and induce endoplasmic reticulum (ER) stress, causing Pelizaeus-Merzbacher disease (PMD), a hypomyelinating disorder of the central nerve system. Currently no effective therapy is available for PMD. Promoted by its curative effects in other genetic disease models caused by similar molecular mechanisms, we tested if curcumin, a dietary compound, can rescue the lethal phenotype of a PMD mouse model (myelin synthesis deficient, msd). Curcumin was administered orally to myelin synthesis deficit (msd) mice at 180 mg·kg(-1)·day(-1) from the postnatal day 3. We evaluated general and motor status, changes in myelination and apoptosis of oligodendrocytes by neuropathological and biochemical examination, and transcription levels for ER-related molecules. We also examined the pharmacological effect of curcumin in cell culture system. Oral curcumin treatment resulted in 25% longer survival (p<0.01). In addition, oligodendrocytes undergoing apoptosis were reduced in number (p<0.05). However, no apparent improvement in motor function, neurological phenotype, and myelin formation was observed. Curcumin treatment did not change the expression of ER stress markers and subcellular localization of the mutant protein in vitro and/or in vivo. Curcumin partially mitigated the clinical and pathological phenotype of msd mice, although molecular mechanisms underlying this curative effect are yet undetermined. Nonetheless, curcumin may serve as a potential therapeutic compound for PMD caused by PLP1 point mutations.

On the occurrence of hypomyelination in a transgenic mouse model: a consequence of the myelin basic protein promoter?

Central nervous system hypomyelination is a feature common to a number of transgenic (Tg) mouse lines that express a variety of unrelated exogenous (i.e. non-central nervous system) transgenes. In this report, we document hypomyelination structurally by immunocytochemistry and functionally in the Tg line MBP-JE, which over expresses the chemokine CCL2 (MCP-1) within oligodendrocytes targeted by a myelin basic protein (MBP) promoter. Analysis of hypomyelinated optic nerves of Tg mice revealed progressive decrease in oligodendrocyte numbers with age (p < 0.01). Although molecular mechanisms underlying hypomyelination in this and other Tg models remain largely unknown, we present preliminary findings on oligodendrocyte progenitor cell (OPC) cultures in which, although OPC expressed CCR2, the receptor for CCL2, treatment with CCL2 had no significant effect on OPC proliferation, differentiation, or apoptosis. We suggest that hypomyelination in the MBP-JE model might not be due to CCL2 expression but rather the result of transcriptional dysfunction related to random insertion of the MBP promoter that disrupts myelinogenesis and leads to oligodendrocyte demise. Because an MBP promoter is a common denominator in most Tg lines displaying hypomyelination, we hypothesize that use of myelin gene sequences in the regulator region of Tg constructs might underlie this perturbation of myelination in such models.

Endoplasmic Reticulum Stress and Unfolded Protein Response Pathways: Potential for Treating Age-related Retinal Degeneration

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) and their aggregation impair normal cellular function and can be toxic, leading to cell death. Prolonged expression of misfolded proteins triggers ER stress, which initiates a cascade of reactions called the unfolded protein response (UPR). Protein misfolding is the basis for a variety of disorders known as ER storage or conformational diseases. There are an increasing number of eye disorders associated with misfolded proteins and pathologic ER responses, including retinitis pigmentosa (RP). Herein we review the basic cellular and molecular biology of UPR with focus on pathways that could be potential targets for treating retinal degenerative diseases.

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.

An autopsy case of adult-onset hereditary spastic paraplegia type 2 with a novel mutation in exon 7 of the proteolipid protein 1 gene

We report an autopsy case of rare adult-onset spastic paraplegia type 2 (SPG2) with a novel missense mutation in exon 7 of the proteolipid protein 1 gene (PLP1). The patient was a 67-year-old man whose elder brother had died of a similar disease with onset in his 40s. Thirty-three years before death at the age of 35, he noticed difficulty in walking. He gradually became abasic over a period of 6 years. He also developed progressive dementia and eventually became bed-ridden by 28 years after onset. At autopsy, gross inspection revealed diffuse, moderate atrophy of the cerebrum with a dilated ventricular system and softening of the white matter throughout the central nervous system (CNS). Histopathologically, the CNS showed widespread myelin pallor in the white matter. By contrast, the gray matter and peripheral nerves were well preserved. Some white matter tracts, including the corticospinal tracts, were preferentially affected, and severe axonal degeneration was observed in these tracts. Genetic analysis revealed a novel mutation, p.Tyr263Cys, in exon 7 of PLP1. This case represents an adult-onset SPG2 patient with one of the oldest ages of onset reported to date. The late onset and long clinical course suggest that this novel mutation does not affect the maturation of oligodendrocytes, but is related to insufficient maintenance of myelin.

Macroautophagy is regulated by the UPR-mediator CHOP and accentuates the phenotype of SBMA mice

Altered protein homeostasis underlies degenerative diseases triggered by misfolded proteins, including spinal and bulbar muscular atrophy (SBMA), a neuromuscular disorder caused by a CAG/glutamine expansion in the androgen receptor. Here we show that the unfolded protein response (UPR), an ER protein quality control pathway, is induced in skeletal muscle from SBMA patients, AR113Q knock-in male mice, and surgically denervated wild-type mice. To probe the consequence of UPR induction, we deleted CHOP (C/EBP homologous protein), a transcription factor induced following ER stress. CHOP deficiency accentuated atrophy in both AR113Q and surgically denervated muscle through activation of macroautophagy, a lysosomal protein quality control pathway. Conversely, impaired autophagy due to Beclin-1 haploinsufficiency decreased muscle wasting and extended lifespan of AR113Q males, producing a significant and unexpected amelioration of the disease phenotype. Our findings highlight critical cross-talk between the UPR and macroautophagy, and they indicate that autophagy activation accentuates aspects of the SBMA phenotype.

In many age-dependent neurodegenerative diseases, the accumulation of misfolded or mutant proteins drives pathogenesis. Several protein quality control pathways have emerged as central regulators of the turnover of these toxic proteins and therefore impact phenotypic severity. In spinal and bulbar muscular atrophy (SBMA), the mutant androgen receptor with an expanded glutamine tract undergoes hormone-dependent nuclear translocation, unfolding, and oligomerization—steps that are critical to the development of progressive proximal limb and bulbar muscle weakness in men. Here we show that the unfolded protein response (UPR), an endoplasmic reticulum stress response, is triggered in skeletal muscle from SBMA patients and knock-in mice. We find that disruption of the UPR exacerbates skeletal muscle atrophy through the induction of macroautophagy, a lysosomal protein quality pathway. In contrast, impaired autophagy diminishes muscle wasting and prolongs lifespan of SBMA mice. Our findings highlight cross-talk between the UPR and autophagy, and they suggest that limited activation of the autophagic pathway may be beneficial in certain neuromuscular diseases such as SBMA where the nucleus is the essential site of toxicity.

Attenuating the endoplasmic reticulum stress response improves functional recovery after spinal cord injury

Activation of the unfolded protein response (UPR) is involved in the pathogenesis of numerous CNS myelin abnormalities; yet, its direct role in traumatic spinal cord injury (SCI)-induced demyelination is not known. The UPR is an evolutionarily conserved cell defense mechanism initiated to restore endoplasmic reticulum homeostasis in response to various cellular stresses including infection, trauma, and oxidative damage. However, if uncompensated, the UPR triggers apoptotic cell death. We demonstrate that the three signaling branches of UPR including the PERK, ATF6, and IRE1α are rapidly initiated in a mouse model of contusive SCI specifically at the injury epicenter. Immunohistochemical analyses of the various UPR markers revealed that in neurons, the UPR appeared at 6 and 24-h post-SCI. In contrast, in oligodendrocytes and astroglia, UPR persisted at least for up to 3 days post-SCI. The UPR-associated proapoptotic transcriptional regulator CHOP was among the UPR markers upregulated in neurons and oligodendrocytes, but not in astrocytes, of traumatized mouse spinal cords. To directly analyze its role in SCI, WT and CHOP null mice received a moderate T9 contusive injury. Deletion of CHOP led to an overall attenuation of the UPR after contusive SCI. Furthermore, analyses of hindlimb locomotion demonstrated a significant functional recovery that correlated with an increase in white-matter sparing, transcript levels of myelin basic protein, and Claudin 11 and decreased oligodendrocyte apoptosis in CHOP null mice in contrast to WT animals. Thus, our study provides evidence that the UPR contributes to oligodendrocyte loss after traumatic SCI.

Protein folding stress in neurodegenerative diseases: a glimpse into the ER

Several neurodegenerative diseases share common neuropathology, primarily featuring the presence in the brain of abnormal protein inclusions containing specific misfolded proteins. Recent evidence indicates that alteration in organelle function is a common pathological feature of protein misfolding disorders, highlighting perturbations in the homeostasis of the endoplasmic reticulum (ER). Signs of ER stress have been detected in most experimental models of neurological disorders and more recently in brain samples from human patients with neurodegenerative disease. To cope with ER stress, cells activate an integrated signaling response termed the unfolded protein response (UPR), which aims to reestablish homeostasis in part through regulation of genes involved in protein folding, quality control and degradation pathways. Here we discuss the particular mechanisms currently proposed to be involved in the generation of protein folding stress in different neurodegenerative conditions and speculate about possible therapeutic interventions.

Pathological adaptive responses of Schwann cells to endoplasmic reticulum stress in bortezomib-induced peripheral neuropathy

Bortezomib, a proteasome inhibitor, has been considered as a promising anticancer drug in the treatment of recurrent multiple myeloma and some solid tumors. The bortezomib-induced peripheral neuropathy (BIPN) is a prominent cause of dose-limiting toxicities after bortezomib treatment. In this study, we found that BIPN in a mouse model is characterized by acute but transient endoplasmic reticulum (ER) damages to Schwann cells. These damaged Schwann cells exhibit abnormal outcomes from healing processes such as the myelination of Remak bundles. A morphometric analysis of polymyelinated Remak bundles revealed that the pathological myelination was not related to the axonal parameters that regulate the normal myelination process during development. In addition, demyelinating macrophages were focally infiltrated within endoneurium of the sciatic nerve. To identify the mechanism underlying these pathologies, we applied a gene microarray analysis to bortezomib-treated primary Schwann cells and verified the changes of several gene expression in bortezomib-treated sciatic nerves. The analysis showed that bortezomib-induced ER stress was accompanied by the activation of several protective molecular chaperones and the down-regulation of myelin gene expression. ER stress inducers such as thapsigargin and bredelfin A also suppressed the mRNA expression of myelin gene P0 at transcriptional levels. In addition, the expression of chemokines such as the macrophage chemoattractants Ccl3 and Cxcl2 was significantly increased in Schwann cells in response to bortezomib and ER stress inducers. Taken together, these observations suggest that the pathological adaptive responses of Schwann cells to bortezomib-induced ER stress may, in part, participate in the development of BIPN.

Using temporal genetic switches to synchronize the unfolded protein response in cell populations in vivo

In recent years, recognition of the importance of protein aggregation in human diseases has increasingly come to the fore and it is clear that many degenerative disorders involve activation of a metabolic signaling cascade known as the unfolded protein response (UPR). The UPR encompasses conserved mechanisms in cells to monitor and react to changes in metabolic flux through the secretory pathway. Such changes reflect an imbalance in cell homeostasis and the UPR integrates several signaling cascades to restore homeostasis. As such, the UPR is simply interpreted as a protection mechanism for cells as they perform their normal functions. A number of groups have suggested that the UPR also can eliminate cells in which homeostasis is lost, for example, during disease. This notion has kindled the rather paradoxical concept that inhibiting the UPR will ameliorate degenerative disease. However, several in vivo studies in the nervous system indicate that curtailing UPR function either exacerbates disease or may reduce severity through unexpected or unidentified pathways. Perhaps the notion that the UPR protects cells or eliminates them stems from widespread use of suboptimal paradigms to characterize the UPR; thus, too little is currently known about this homeostatic pathway. Herein, I describe the development of genetic switch technology (GST) to generate a novel model for studying UPR diseases. The model is geared toward obtaining high resolution in vivo detail for oligodendrocytes of the central nervous system, but it can be adapted to study other cell types and other UPR diseases.

Animal models in the study of the unfolded protein response

The endoplasmic reticulum, a highly dynamic and complex organelle, is the site for synthesis, folding, and modification of transmembrane and secretory proteins. Any disruptions to the endoplasmic reticulum such as an accumulation of misfolded or unfolded proteins results in activation of the unfolded protein response (UPR). The UPR is comprised of three distinct signal transduction pathways that work to restore homeostasis to the endoplasmic reticulum. This review summarizes select mouse models available to study the UPR and the information learned from the analyses of these models.

Modulation of rumpshaker phenotype with wild-type PLP/DM20 suggests several pathogenic mechanisms

The rumpshaker mutation of the murine myelin proteolipid protein 1 (Plp1) gene generates misfolded PLP/DM20 protein, resulting in dysmyelination, increased oligodendrocyte apoptosis, and death prior to P40 when expressed on the C57 BL/6 background. In this study, we used transgenic complementation to normalize the levels of PLP/DM20 in myelin with wild-type protein to determine whether loss of normal PLP function or gain of toxic function is responsible for dysmyelination in the rumpshaker. Restoring myelin PLP/DM20 levels extended the survival time to at least P60, significantly reduced the density of apoptotic cells, increased myelin volume, and restored normal periodicity of myelin. Biochemical analysis found that several myelin proteins that are reduced in rumpshaker, including MAG, CNP, and SirT2, are markedly elevated at peak myelination (P20) in the rumpshaker transgenic mouse. Myelin basic protein, however, remained low at peak myelination but was restored at P60 when myelin had matured and entered into a maintenance phase. Markers of the unfolded protein response (UPR), BiP and XBP1, remained activated with the introduction of wild-type PLP. These data demonstrate that restoring wild-type PLP/DM20 levels in rumpshaker improves the phenotype and the integrity of myelin, but hypomyelination persists and stress pathways remain activated. This suggests that both gain- and loss-of-function mechanisms are involved in the pathogenesis of the rumpshaker.

In vivo cellular adaptation to ER stress: survival strategies with double-edged consequences

Disturbances to the balance of protein synthesis, folding and secretion in the endoplasmic reticulum (ER) induce stress and thereby the ER stress signaling (ERSS) response, which alleviates this stress. In this Commentary, we review the emerging idea that ER stress caused by abnormal physiological conditions and/or mutations in genes that encode client proteins of the ER is a key factor underlying different developmental processes and the pathology of diverse diseases, including diabetes, neurodegeneration and skeletal dysplasias. Recent studies in mouse models indicate that the effect of ERSS in vivo and the nature of the cellular strategies induced to ameliorate pathological ER stress are crucial factors in determining cell fate and clinical disease features. Importantly, ERSS can affect cellular proliferation and the differentiation program; cells that survive the stress can become 'reprogrammed' or dysfunctional. These cell-autonomous adaptation strategies can generate a spectrum of context-dependent cellular consequences, ranging from recovery to death. Secondary effects can include altered cell-extracellular-matrix interactions and non-cell-autonomous alteration of paracrine signaling, which contribute to the final phenotypic outcome. Recent reports showing that ER stress can be alleviated by chemical compounds suggest the potential for novel therapeutic approaches.

Mild phenotype in Pelizaeus-Merzbacher disease caused by a PLP1-specific mutation

We present the case of a 26 year-old man who developed normally until he began having difficulty walking at age 12. He subsequently became unable to stand at 15 years old and exhibited mental regression and generalized tonic convulsions by age 20. Magnetic resonance imaging revealed incomplete myelination of cerebral white matter, which resembled that of Pelizaeus-Merzbacher disease. By sequencing the proteolipid protein 1 (PLP1) gene, we found a novel mutation (c.352_353delAG (p.Gly130fs)) in the latter half of exon 3 (exon 3B) that is spliced out in the DM20 isoform. Exon 3B mutations are known to cause a mild phenotype since they do not disturb DM20 production. Mutations that truncate PLP1 correlate with a mild phenotype by activating the nonsense-mediated decay mechanism that specifically detects and degrades mRNAs containing a premature termination codon. This attenuates the production of toxic mutant PLP1. The very mild presentation in the present case seems to be derived from the unique nature of the mutation, which preserves DM20 production and decreases mutant PLP1.

Airborne particulate matter selectively activates endoplasmic reticulum stress response in the lung and liver tissues

Recent studies have suggested a link between inhaled particulate matter (PM) exposure and increased mortality and morbidity associated with pulmonary and cardiovascular diseases. However, a precise understanding of the biological mechanism underlying PM-associated toxicity and pathogenesis remains elusive. Here, we investigated the impact of PM exposure in intracellular stress signaling pathways with animal models and cultured cells. Inhalation exposure of the mice to environmentally relevant fine particulate matter (aerodynamic diameter < 2.5 μm, PM(2.5)) induces endoplasmic reticulum (ER) stress and activation of unfolded protein response (UPR) in the lung and liver tissues as well as in the mouse macrophage cell line RAW264.7. Ambient PM(2.5) exposure activates double-strand RNA-activated protein kinase-like ER kinase (PERK), leading to phosphorylation of translation initiation factor eIF2α and induction of C/EBP homologous transcription factor CHOP/GADD153. Activation of PERK-mediated UPR pathway relies on the production of reactive oxygen species (ROS) and is critical for PM(2.5)-induced apoptosis. Furthermore, PM(2.5) exposure can activate ER stress sensor IRE1α, but it decreases the activity of IRE1α in splicing the mRNA encoding the UPR trans-activator X-box binding protein 1 (XBP1). Together, our study suggests that PM(2.5) exposure differentially activates the UPR branches, leading to ER stress-induced apoptosis through the PERK-eIF2α-CHOP UPR branch. This work provides novel insights into the cellular and molecular basis by which ambient PM(2.5) exposure elicits its cytotoxic effects that may be related to air pollution-associated pathogenesis.

Chemical stress induces the unfolded protein response in olfactory sensory neurons

More than any other neuron, olfactory sensory neurons are exposed to environmental insults. Surprisingly, their only documented response to damaging stress is apoptosis and subsequent replacement by new neurons. However, they expressed unfolded protein response genes, a transcriptionally regulated defense mechanism activated by many types of insults. The unfolded protein response transcripts Xbp1, spliced Xbp1, Chop (Ddit3), and BiP (Hspa5) were decreased when external access of stressors was reduced by blocking a nostril (naris occlusion). These transcripts and Nrf2 (Nfe2l2) were increased by systemic application of tunicamycin or the selective olfactotoxic chemical methimazole. Methimazole's effects overcame naris occlusion, and the unfolded protein response was independent of odor-evoked neuronal activity. Chemical stress is therefore a major and chronic activator of the unfolded protein response in olfactory sensory neurons. Stress-dependent repression of the antiapoptotic gene Bcl2 was absent, however, suggesting a mechanism for disconnecting the UPR from apoptosis and tolerating a chronic unfolded protein response. Environmental stressors also affect both the sustentacular cells that support the neurons and the respiratory epithelia, because naris occlusion decreased expression of the xenobiotic chemical transformation enzyme Cyp2a5 in sustentacular cells, and both naris occlusion and methimazole altered the abundance of the antibacterial lectin Reg3g in respiratory epithelia.

The endoplasmic reticulum stress response factor CHOP-10 protects against hypoxia-induced neuronal death

Hypoxia-induced gene expression is a critical determinant of neuron survival after stroke. Understanding the cell autonomous genetic program controlling adaptive and pathological transcription could have important therapeutic implications. To identify the factors that modulate delayed neuronal apoptosis after hypoxic injury, we developed an in vitro culture model that recapitulates these divergent responses and characterized the sequence of gene expression changes using microarrays. Hypoxia induced a disproportionate number of bZIP transcription factors and related targets involved in the endoplasmic reticulum stress response. Although the temporal and spatial aspects of ATF4 expression correlated with neuron loss, our results did not support the anticipated pathological role for delayed CHOP expression. Rather, CHOP deletion enhanced neuronal susceptibility to both hypoxic and thapsigargin-mediated injury and attenuated brain-derived neurotrophic factor-induced neuroprotection. Also, enforced expression of CHOP prior to the onset of hypoxia protected wild-type cultures against subsequent injury. Collectively, these findings indicate CHOP serves a more complex role in the neuronal response to hypoxic stress with involvement in both ischemic preconditioning and delayed neuroprotection.

CHOP-independent apoptosis and pathway-selective induction of the UPR in developing plasma cells

Upon antigen stimulation, B lymphocytes differentiate into antibody secreting cells (ASC), most of which undergo apoptosis after a few days of intense Ig production. Differentiation entails expansion of the endoplasmic reticulum (ER) and requires XBP1 but not other elements of the unfolded protein response, like PERK. Moreover, normal and malignant ASC are exquisitely sensitive to proteasome inhibitors, but the underlying mechanisms are poorly understood. Here we analyze the role of C/EBP homologous protein (CHOP), a transcription factor mediating apoptosis in many cell types that experience high levels of ER stress. CHOP is transiently induced early upon B cell stimulation: covalent IgM aggregates form more readily and IgM secretion is slower in chop(-/-) cells. Despite these subtle changes, ASC differentiation and lifespan are normal in chop(-/-) mice. Unlike fibroblasts and other cell types, chop(-/-) ASC are equally or slightly more sensitive to proteasome inhibitors and ER stressors, implying tissue-specific roles for CHOP in differentiation and stress.

Myelin under stress

The capacity to fold proteins properly is fundamental for cell survival. Secreted and transmembrane proteins are synthesized in the endoplasmic reticulum (ER), an organelle that has the ability to discriminate between native and nonnative proteins, in a process called protein quality control. When folding is not properly achieved, misfolded proteins can accumulate. The terminally misfolded proteins are typically retrotranslocated into the cytoplasm for degradation by the proteasome, in a process known as endoplasmic reticulum-associated degradation. However, if the degradation is insufficient, accumulation of abnormal proteins in the ER activates the unfolded protein response (UPR), a complex set of new signals aimed to reduce further the load of abnormal protein in the ER. Massive synthesis of myelin lipids and proteins is necessary to support myelinogenesis. Not surprisingly, therefore, ER stress (including the UPR), the proteasome, and autophagy (lysosomes) have been implicated in myelin disorders, such as Pelizaeus-Merzbacher disease and vanishing white matter disease in the central nervous system and Charcot-Marie-Tooth neuropathies in the peripheral nervous system. Here we discuss recent evidence supporting an important role for ER stress in myelin disorders.

Abnormal expression of collagen IV in lens activates unfolded protein response resulting in cataract

Human diseases caused by mutations in extracellular matrix genes are often associated with an increased risk of cataract and lens capsular rupture. However, the underlying mechanisms of cataract pathogenesis in these conditions are still unknown. Using two different mouse models, we show that the accumulation of collagen chains in the secretory pathway activates the stress signaling pathway termed unfolded protein response (UPR). Transgenic mice expressing ectopic Col4a3 and Col4a4 genes in the lens exhibited activation of IRE1, ATF6, and PERK associated with expansion of the endoplasmic reticulum and attenuation of general protein translation. The expression of the transgenes had adverse effects on lens fiber cell differentiation and eventually induced cell death in a group of transgenic fiber cells. In Col4a1(+/Deltaex40) mutant mice, the accumulation of mutant chains also caused low levels of UPR activation. However, cell death was not induced in mutant lenses, suggesting that low levels of UPR activation are not proapoptotic. Collectively, the results provide in vivo evidence for a role of UPR in cataract formation in response to accumulation of terminally unfolded proteins in the endoplasmic reticulum.

Endoplasmic reticulum stress, inflammation, and perinatal brain damage

Inflammation seems to play a role in the pathogenesis of perinatal brain damage in fetuses/infants born much before term. We raise the possibility that noninflammatory phenomena induce endoplasmic reticulum stress, which, in turn, leads to the unfolded protein response, which is followed by apoptosis-promoting processes and inflammation. Perhaps by these events, noninflammatory stimuli lead to perinatal brain damage.

Differences in endoplasmic-reticulum quality control determine the cellular response to disease-associated mutants of proteolipid protein

Missense mutations in human PLP1, the gene encoding myelin proteolipid protein (PLP), cause dysmyelinating Pelizaeus-Merzbacher disease of varying severity. Although disease pathology has been linked to retention of misfolded PLP in the endoplasmic reticulum (ER) and induction of the unfolded protein response (UPR), the molecular mechanisms that govern phenotypic heterogeneity remain poorly understood. To address this issue, we examined the cellular response to missense mutants of PLP that are associated with distinct disease phenotypes. We found that the mild-disease-associated mutants, W162L and G245A, were cleared from the ER comparatively quickly via proteasomal degradation and/or ER exit. By contrast, the more ;aggressive' A242V mutant, which causes severe disease, was significantly more stable, accumulated at the ER and resulted in a specific activation of the UPR. On the basis of these findings, we propose that the rate at which mutant PLP proteins are cleared from the ER modulates disease severity by determining the extent to which the UPR is activated.

Neuronal loss in Pelizaeus-Merzbacher disease differs in various mutations of the proteolipid protein 1

Mutations affecting proteolipid protein 1 (PLP1), the major protein in central nervous system myelin, cause the X-linked leukodystrophy Pelizaeus-Merzbacher disease (PMD). We describe the neuropathologic findings in a series of eight male PMD subjects with confirmed PLP1 mutations, including duplications, complete gene deletion, missense and exon-skipping. While PLP1 mutations have effects on oligodendrocytes that result in mutation-specific degrees of dysmyelination, our findings indicate that there are also unexpected effects in the central nervous system resulting in neuronal loss. Although length-dependent axonal degeneration has been described in PLP1 null mutations, there have been no reports on neuronal degeneration in PMD patients. We now demonstrate widespread neuronal loss in PMD. The patterns of neuronal loss appear to be dependent on the mutation type, suggesting selective vulnerability of neuronal populations that depends on the nature of the PLP1 disturbance. Nigral neurons, which were not affected in patients with either null or severe misfolding mutations, and thalamic neurons appear particularly vulnerable in PLP1 duplication and deletion patients, while hippocampal neuronal loss was prominent in a patient with complete PLP1 gene deletion. All subjects showed cerebellar neuronal loss. The patterns of neuronal involvement may explain some clinical findings, such as ataxia, being more prominent in PMD than in other leukodystrophies. While the precise pathogenetic mechanisms are not known, these observations suggest that defective glial functions contribute to neuronal pathology.

CHOP and the endoplasmic reticulum stress response in myelinating glia

The unfolded protein response (UPR) comprises kinase signaling and transcription factor activation cascades delineated over the past 20 years. Most studies conclude that this stress response is adaptive but, nevertheless, includes maladaptive programs involving CHOP expression that drives cell-autonomous apoptosis. Herein, we highlight several studies of UPR diseases involving myelinating glia of the central and peripheral nervous systems that do not support a primary role for CHOP in apoptosis. In oligodendrocytes, CHOP expression apparently protects against death whereas in Schwann cells, CHOP promotes demyelination in the absence of cell death. Together, these studies demonstrate that CHOP should be viewed more broadly as a cell-specific and context-specific mediator of adaptive or maladaptive responses to stress rather than a proapoptotic transcription factor.

p28GANK inhibits endoplasmic reticulum stress-induced cell death via enhancement of the endoplasmic reticulum adaptive capacity

It has been shown that oncoprotein p28(GANK), which is consistently overexpressed in human hepatocellular carcinoma (HCC), plays a critical role in tumorigenesis of HCC. However, the underlying mechanism remains unclear. Here, we demonstrated that p28(GANK) inhibits apoptosis in HCC cells induced by the endoplasmic reticulum (ER) stress. During ER stress, p28(GANK) enhances the unfolded protein response, promotes ER recovery from translational repression, and thereby facilitates cell's ability to cope with the stress conditions. Furthermore, p28(GANK) upregulates glucose-regulated protein 78 (GRP78), a key ER chaperone protein, which subsequently enhances the ER folding capacity and promotes recovery from ER stress. We also demonstrated that p28(GANK) increases p38 mitogen-activated protein kinase and Akt phosphorylation, and inhibits nuclear factor kappa B (NF-kappaB) activation under ER stress, which in turn contributes to GRP78 upregulation. Taken together, our results indicate that p28(GANK) inhibits ER stress-induced apoptosis in HCC cells, at least in part, by enhancing the adaptive response and GRP78 expression. We propose that p28(GANK) has potential implications for HCC progression under the ER stress conditions.

Different proteolipid protein mutants exhibit unique metabolic defects

PMD (Pelizaeus–Merzbacher disease), a CNS (central nervous system) disease characterized by shortened lifespan and severe neural dysfunction, is caused by mutations of the PLP1 (X-linked myelin proteolipid protein) gene. The majority of human PLP1 mutations are caused by duplications; almost all others are caused by missense mutations. The cellular events leading to the phenotype are unknown. The same mutations in non-humans make them ideal models to study the mechanisms that cause neurological sequelae. In the present study we show that mice with Plp1 duplications (Plp1tg) have major mitochondrial deficits with a 50% reduction in ATP, a drastically reduced mitochondrial membrane potential and increased numbers of mitochondria. In contrast, the jp (jimpy) mouse with a Plp1 missense mutation exhibits normal mitochondrial function. We show that PLP in the Plp1tg mice and in Plp1-transfected cells is targeted to mitochondria. PLP has motifs permissive for insertion into mitochondria and deletions near its N-terminus prevent its co-localization to mitochondria. These novel data show that Plp1 missense mutations and duplications of the native Plp1 gene initiate uniquely different cellular responses.

Endoplasmic reticulum stress in disorders of myelinating cells

Myelinating cells, oligodendrocytes in the CNS and Schwann cells in the peripheral nervous system produce an enormous amount of plasma membrane during the myelination process, making them particularly susceptible to disruptions of the secretory pathway. Endoplasmic reticulum stress, initiated by the accumulation of unfolded or misfolded proteins, activates the unfolded protein response, which adapts cells to the stress. If this adaptive response is insufficient, the unfolded protein response activates an apoptotic program to eliminate the affected cells. Recent observations suggest that endoplasmic reticulum stress in myelinating cells is important in the pathogenesis of various disorders of myelin, including Charcot-Marie-Tooth disease, Pelizaeus-Merzbacher disease and Vanishing White Matter Disease, as well as in the most common myelin disorder, multiple sclerosis. A better understanding of endoplasmic reticulum stress in myelinating cells has laid the groundwork for the design of new therapeutic strategies for promoting myelinating cell survival in these disorders.

Inhibition of mitochondrial function induces an integrated stress response in oligodendroglia

Maternal inheritance of a pathogenic point mutation within complex I of the mitochondrial genome causes Leber's hereditary optic neuropathy (LHON), resulting in the neurodegeneration and demyelination of the optic nerve. The integrated stress response (ISR), a signaling pathway that responds to various stresses by activating a common set of genes, has been linked to both mitochondrial defects and demyelinating diseases. Therefore, we wanted to determine whether mitochondrial dysfunction induced by complex I inhibition with rotenone can activate the ISR, specifically by the ER kinase PERK, in oligodendroglial cells. Our complex I-deficient oligodendroglial model reproduced similar biochemical defects as in LHON by decreasing ATP synthesis and ATP levels. The same doses of rotenone that reduced ATP production also induced dose-dependent increases in PERK and eIF2alpha phosphorylation as well as activated the ISR stress genes, ATF4 and CHOP. In addition, complex I inhibition at these same concentrations induced a PERK-dependent activation of the cell death kinase, JNK, and inhibited oligodendroglial proliferation. Taken together, our results demonstrate that activation of the ISR may be one example of mitochondrial retrograde signaling in response to complex I deficiency and we suggest that this response mechanism may be relevant to the pathophysiology of LHON.