Endoplasmic reticulum stress in disorders of myelinating cells

Abstract

Myelinating cells, oligodendrocytes in the CNS and Schwann cells in the PNS, produce an enormous amount of plasma membrane during the myelination process, making them particularly susceptible to disruptions to the secretory pathway. ER stress, initiated by the accumulation of unfolded or misfolded proteins, activates the unfolded protein response (UPR), which adapts cells to the stress. If this adaptive response is insufficient, the UPR activates an apoptotic program to eliminate the affected cells. Recent observations suggest that ER stress in myelinating cells plays an important role in the pathogenesis of various disorders of myelin, including Charcot-Marie-Tooth disease (CMT), Pelizaeus-Merzbacher disease (PMD), Vanishing White Matter Disease (VWMD), as well as in the most common disorder of myelin, multiples sclerosis (MS). A better understanding of ER stress in myelinating cells has laid the groundwork for the design of novel therapeutic strategies to promote myelinating cell survival in these disorders.

Myelinating cells, oligodendrocytes in the CNS and Schwann cells in the PNS, produce a vast amount of myelin as an extension of their plasma membrane (Figure 1) ^{1, 2}. Myelin, which is a unique, lipid-rich multilamellar sheath that wraps around axons in the CNS and PNS, is not only essential for the fast conduction of the action potential along axons but also for the maintenance of axonal integrity ³. According to estimates from morphometric analyses, the mean surface area of myelin per mature myelinating cell is thousands of times greater than the surface area of a typical mammalian cell ¹. Thus, during the active phase of myelination, each myelinating cell must synthesize an enormous amount of myelin membrane proteins, cholesterol, and membrane lipids through the secretory pathway ². Therefore, it is not surprising that myelinating cells are highly sensitive to the disruption of the secretory pathway, particularly the homeostasis of the endoplasmic reticulum (ER). Recent studies indicate that this increased susceptibility contributes to the pathogenesis of a number of myelin disorders ^4-11.

Figure 1. Oligodendrocyte and myelin.

Oligodendrocytes produce as an extension of their plasma membrane vast amounts of myelin; a unique, lipid-rich, multilamellar sheath that wraps axons in the CNS. The secretory pathways for protein and lipid, including rough ER, smooth ER and Glogi apparatus, are well developed in oligodendrocytes. Evidence is accumulating that oligodendrocytes, as well as their PNS counterpart Schwann cells, rank among the cells that are most sensitive to the disruption of the secretory pathway.

The ER has three essential functions ^{12, 13}. Secretory proteins and proteins destined for the cell surface and other intracellular organelles are synthesized by ribosomes on the cytosolic surface of the ER and translocated into the ER lumen through a pore in the ER membrane. Inside the lumen, they are properly modified and folded. Additionally, the biosynthesis of steroids, cholesterol, and other lipids takes place on the cytoplasmic side of the ER membrane. Moreover, cellular calcium is mainly stored in the ER lumen. The concentration of calcium in the cytoplasm is primarily regulated by Ca²⁺ transport into and out of the ER.

A number of cell stress conditions, such as a perturbed calcium homeostasis or redox status, elevated secretory protein synthesis rates, altered glycosylation levels, and abnormally high cholesterol levels, can interfere with oxidative protein folding and subsequently lead to the accumulation of unfolded or misfolded proteins in the ER lumen. This phenomenon has been referred to as ER stress ^14-16. As a consequence, the eukaryotic cell has evolved an adaptive coordinated response, the unfolded protein response (UPR, Figure 2 and Table 1), to limit further accumulation of unfolded proteins in the ER. Three ER-resident transmembrane proteins involved in the UPR have been identified: pancreatic ER kinase (PERK), the kinase encoded by the inositol requiring 1 (IRE1) gene, and activating transcription factor 6 (ATF6). At the onset of ER stress, the most rapidly activated pathway is the PERK pathway, which couples protein folding in the ER with protein synthesis by phosphorylating the α subunit of eukaryotic translation initiation factor 2 (eIF2α) ^{17, 18}. The activation of the PERK-eIF2α pathway attenuates the initiation of translation in response to ER stress and enhances the stress-induced expression of numerous cytoprotective genes ^19-21. Activation of IRE1 initiates the splicing of X-box-binding protein (XBP1) mRNA, producing an active transcription factor, spliced XBP-1 (sXBP-1) ^{22, 23}. Additionally, ATF6 becomes an active transcription factor by transiting to the Golgi complex, where it is cleaved by the proteases S1P and S2P, before translocating to the nucleus ²⁴. The activation of IRE1 signaling and the cleavage of ATF6 promote ER expansion and the expression of ER-localized chaperones, which facilitate protein folding in the ER ^{18, 25}. In multicellular organisms, if these adaptive responses are not sufficient to resolve the folding problems in the ER, the UPR will trigger an apoptotic program to eliminate the cells ^{26, 27}.

Figure 2. The UPR pathway in eukaryotic cells.

ER stress triggers PERK dimerization and auto-phosphorylation. Activated PERK attenuates protein biosynthesis by phosphorylating eIF2α. Phosphorylated eIF2α also activates transcription factor ATF4, which enhances the stress-induced expression of numerous cytoprotective genes. Additionally, ATF4 increases the expression of CHOP, which induces GADD34 expression. GAAD34 binds to PP1 and forms the GADD34-PP1 complex that specifically dephosphorylates eIF2α. ER stress also triggers IRE1dimerization and auto-phosphorylation. Activation of IRE1 initiates the splicing of XBP1 mRNA, producing an active transcription factor sXBP-1. Additionally, ATF6 becomes an active transcription factor by transiting to the Golgi complex, where it is cleaved by the proteases S1P and S2P. The activation of IRE1 signaling and the cleavage of ATF6 promote ER expansion and the expression of ER-localized chaperones. In multicellular organisms, if these adaptive responses are not sufficient to resolve the folding problems in the ER, the UPR will trigger an apoptotic program, such as activation of caspase 12 or CHOP, to eliminate the cells.

Table 1.

The functions of major effecter components of the UPR

UPR Effecter molecule	Function
PERK	At the onset of ER stress, the most rapidly activated ER stress sensor is PERK, which couples protein folding in the ER with protein synthesis by phosphorylating eIF2α 19, 20.
eIF2α	eIF2α phosphorylation promotes a stress-resistant state by global attenuation of protein biosynthesis and induction of numerous stress-induced cytoprotective genes 21, 28. Four different kinases are known to phosphorylate eIF2α. They are PERK, general control nonderepressible-2 kinase (GCN2) that is activated by amino acid starvation, double-stranded RNA-dependent protein kinase (PKR) that is activated by viral infection, and heme-regulated inhibitor (HRI) that is activated by iron deficiency 28.
ATF4	Transcription factor ATF4 mRNA contains a unique 5' untranslated region, leading to ATF4 protein expression when eIF2α is phosphorylated. The gene expression program activated by ATF4 is involved in regulating amino acid metabolism and resistance to oxidative stress 29.
CHOP	A transcription factor downstream of ATF4, was thought to promote apoptotic cell death, since its deletion diminishes cell death during ER stress. Recent work, however, demonstrated that the deleterious effects of CHOP are associated with the induction of the protein growth arrest and DNA damage 34 (GADD34), rather than the activation of an apoptotic program 25, 30, 31.
GADD34	A regulatory subunit of protein phosphatase 1 (PP1) that specifically dephosphorylates phosphorylated eIF2α. GADD34 blockage increases the levels of phosphorylated eIF2 α in stressed cells and protects cells from the stresses 31, 32.
ATF6	Activation of ATF6 promotes the expression of ER-localized chaperones 18, 25.
IRE1	Activation of IRE1 initiates the splicing of X-box-binding protein (XBP1) mRNA, producing an active transcription factor, spliced XBP-122, 23.
XBP-1	Spliced XBP-1 promotes ER expansion and the expression of ER-localized chaperones 18, 25.
Caspase-12	Activation of Caspase-12, an ER-localized caspase, has been shown to be associated with cell apoptosis induced by ER stress 33, 34.

It has become increasingly clear that ER stress is an important feature of a number of human diseases, especially those involving cells dedicated to the synthesis of secreted proteins, such as diabetes mellitus, liver diseases, and inflammatory disorders ^35-37. In this review, we discuss the current understanding of the contribution of ER stress in myelinating cells to the pathogenesis of inherited myelin disorders, including Charcot-Marie-Tooth disease, Pelizaeus-Merzbacher disease (PMD), Vanishing White Matter Disease, as well as in multiples sclerosis (MS), an autoimmune demyelination disorder of the CNS. We also examine the unique features of ER stress in myelinating cells and how the increased sensitivity of these cells to ER stress might provide selective therapeutic opportunities.

Evidence for abnormalities in the secretory pathway of Schwann cells in Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease is one of the most common inherited neurological disorders in humans, affecting approximately 1 in 2,500 individuals. Charcot-Marie-Tooth disease is actually a group of disorders of the PNS that are caused by mutations in a number of distinct genes and can be categorized as primarily originating from either the myelinating cell (Charcot-Marie-Tooth 1) or the axons (Charcot-Marie-Tooth 2) ³⁸. Mutations in genes encoding peripheral myelin proteins account for the majority of Charcot-Marie-Tooth 1 cases. Peripheral myelin protein P0, a transmembrane glycoprotein, comprises greater than 50% of the total protein of peripheral myelin, and a number of distinct point mutations in P0 have been shown to be responsible for a dominant form of Charcot-Marie-Tooth 1B ³⁹. There is strong evidence that mutations in the gene encoding P0 can lead to ER stress in myelinating Schwann cells ^{4, 5}. Pennuto et al. have generated an authentic mouse model of Charcot-Marie-Tooth 1B that expresses a human disease-causing form of P0 deleted for S63 (P0S63del) ⁴. Their studies demonstrate that this mutation causes a perturbed alignment of hydrophobic residues in β strand C of P0 that results in the retention of the mutant protein in the ER of Schwann cells and leads to increased levels of phosphorylated eIF2α (p-eIF2α), ATF4, ATF3 and CAATT enhancer binding protein homologous protein (CHOP) in the nerve, suggesting activation of the PERK pathway ⁵. The spliced form of XBP-1 is also increased in the nerves of these animals, suggesting activation of the IRE1 pathway. Moreover, cleavage of ATF6 is detected in the P0S63del nerve. Interestingly, the activation of PERK, IRE1 and ATF6 pathways in P0S63del nerves is dose-dependent, which strongly suggests that the retention of P0S63del in the ER results in the activation of the UPR in Schwann cells.

CHOP, a transcription factor downstream of the PERK-eIF2α pathway, has been shown to play an important role in the cell apoptosis that results from ER stress, and CHOP deletion protects various cell types against ER stress-induced apoptosis in cell culture and animal models ^{25, 30, 31}. Pennuto et al. show CHOP is undetectable in Schwann cells of normal nerves, but present in the nuclei of P0S63del Schwann cells ⁵. To determine the role of the CHOP pathway in the pathogenesis of the myelin abnormalities that occurs in the P0S63del nerve, P0S63del mice were crossed with CHOP null mice ⁵. In the absence of CHOP activity, the number of demyelinated fibers in P0S63del nerves is reduced, and the behavioral and electrophysiological abnormalities are attenuated. Nevertheless, CHOP inactivation, surprisingly, did not affect the number of apoptotic Schwann cells or the degree of hypomyelination in the P0S63del nerve ⁵. The mechanism accounting for the detrimental effects of CHOP in Charcot-Marie-Tooth 1B remains to be determined.

Mutations in genes encoding peripheral myelin protein PMP22 are the most common cause for Charcot-Marie-Tooth 1 cases ³⁹. PMP22 is a highly hydrophobic transmembrane protein with four putative transmembrane domains. In humans, the molecular lesion that is responsible for the vast majority of Charcot-Marie-Tooth 1 cases is a duplication of the region of chromosome 17 that contains the PMP22 gene, resulting in increased expression of this membrane protein ⁴⁰. Similarly, enforced expression of wild type PMP22 in Schwann cells of transgenic mice and rats results in a Charcot-Marie-Tooth like disorder ⁴¹. Cumulative evidence indicates that this is a gain of function disorder in which the increased expression of PMP22 has a deleterious effect on myelinating Schwann cells ⁴². Additionally, spontaneous dominant mouse mutations have been identified in the PMP22 gene, Trembler (Tr) and Trembler-J (Tr-J), that introduce nonconservative amino acid changes into hydrophobic domains of this protein. It has been demonstrated that misfolded PMP22^Tr and PMP22^Tr-J associate with calnexin, an ER chaperone, and accumulate in the ER of Schwann cells ^{43, 44}. Moreover, Khajavi et al. have shown that treatment with the chemical compound curcumin, which is capable of promoting the transport of misfolded proteins from the ER to the plasma membrane, significantly attenuates Schwann cell apoptosis and improves the neuropathologic phenotype in PMP22^Tr-J mice ⁴⁵. Taken together, these data indicate that the accumulation of mutant PMP22 in the ER causes the apoptosis of Schwann cells, resulting in myelin abnormalities. Nevertheless, this accumulation does not lead to increased expression of the ER stress markers binding immunoglobulin protein (BIP) or CHOP^{44, 45}. These data suggest that the role of the UPR in the pathogenesis of Charcot-Marie-Tooth 1A mutations needs to be examined directly and in greater detail.

Severe ER stress in Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an X chromosome-linked dysmyelinating disease of the CNS with a broad range of clinical severity that is caused by more than 60 known mutations, including missense and nonsense mutations, in the proteolipid protein (PLP) gene ^46-48. In addition, PLP gene duplications are responsible for approximately 50% of PMD cases. PLP and its alternatively spliced isoform, DM20, are believed to contain four membrane-spanning domains and, together, comprise approximately 50% of total CNS myelin protein. It is believed that mutant PLP has a dominant gain-of-toxic function, since PLP deletion does not affect oligodendrocyte development and myelination, and PLP null mice have normal neurological function until late adulthood ⁴⁶. Moreover, the enforced expression of a wild type PLP gene is not sufficient to rescue the PLP mutant phenotype ^46-48. Studies from Gow et al. demonstrate that PLP mutations cause oligodendrocyte death during the myelination process, at the developmental time point when oligodendrocytes are extending extensive processes, making contact with nearby axons, and initiating the synthesis of the myelin proteins ⁴⁹.

Several studies have shown that the mutant PLP proteins are not properly folded and accumulate in the ER ^{47, 50, 51}. COS-7 cells transfected with a number of distinct PLP mutants show accumulation of mutant proteins in the ER ^{7, 47}. Additionally, immunohistochemical studies show perinuclear accumulation of mutant PLP proteins in the oligodendrocytes of PLP mutant mice ^{49, 52}. This was further supported by electron microscopy analyses, which provided direct evidence that mutant PLP proteins accumulate in the ER of oligodendrocytes, where they disturb the ER ultrastructure ^{47, 53}.

Strong evidence has also recently indicated that the accumulation of mutant PLP in the ER activates the UPR and leads to myelinating oligodendrocyte death ^{7, 54}. Mutant PLP expressed by transfected fibroblasts accumulates in the ER and significantly increases the expression of ER stress markers such as CHOP, BIP and ATF3, markers that are also upregulated in the oligodendrocytes of PLP mutant mice ⁷. Importantly, CHOP is localized to the nucleus in mutant oligodendrocytes, which is strongly indicative of the ER stress response ⁷. Similarly, increased CHOP expression is seen in the oligodendrocyte nuclei of PMD patients ⁷. Lastly, Bauer et al. show that oligodendrocytes from rats transgenically overexpressing PLP are characterized by swollen ER and increased expression of BIP, as well as disulfide isomerase, an ER enzyme involved in protein folding ⁶.

To determine the involvement of ER stress in oligodendrocyte death in PLP mutant animals, Southwood et al. used a genetic approach, crossing CHOP null mice with PLP mutant mice ⁷. Surprisingly, they found that the absence of CHOP notably exacerbated the clinical phenotype and increased oligodendrocyte apoptosis in PLP mutant mice. This protective effect of CHOP on oligodendrocytes during ER stress is strikingly contrasted to its proapoptotic effect on other cell types. The mechanisms accounting for the protective effect of CHOP on oligodendrocytes remain a mystery. Interestingly, Southwood et al. show that CHOP induction in PLP mutant mice does not enhance expression of genes that normally act downstream of CHOP, such as DOCs (downstream of CHOP) 1, 4, and 6 ⁷. It is possible that CHOP has a distinct set of target genes depending on cell type, which is reflected in its cell-specific function.

The UPR adapts oligodendrocytes to eIF2B mutations in Vanishing White Matter Disease

Vanishing White Matter Disease is an inherited autosomal-recessive hypomyelinating disorder. The discovery of missense mutations in the ubiquitously-expressed genes encoding the five subunits of eIF2B in Vanishing White Matter Disease patients led to the identification of the genetic causes of Vanishing White Matter Disease ^{55, 56}. This is the first human disease that has been shown to directly affect a protein synthesis factor. eIF2B functions in translation as a guanine nucleotide exchange factor, promoting the release of GDP from eIF2 and the formation of an active eIF2-GTP complex ⁵⁷. eIF2-GTP then binds aminoacylated initiator methionyl tRNA, forming a ternary complex that is essential for each translation initiation event. p-eIF2α reduces the activity of eIF2B, with the 2 proteins forming a nonproductive p-eIF2–eIF2B complex. eIF2B mutations may impair its response to p-eIF2α, particularly during cell stress ⁵⁸. Although oligodendrocytes have been described as abnormal and “foamy” in appearance, no evidence has been presented that demonstrates a reduction of oligodendrocyte numbers in Vanishing White Matter Disease ^{59, 60}. Interestingly, van Kollenburg et al. recently presented evidence for the activation of the UPR in oligodendrocytes in Vanishing White Matter Disease patients, including increased levels of phosphorylated PERK, p-eIF2α, ATF4 and CHOP ^{61, 62}. Moreover, it has been shown that primary cultured fibroblasts from Vanishing White Matter Disease patients respond normally to ER stress by reduced global translation rates. Nevertheless, the induction of ATF4 is significantly enhanced in these cells compared to normal controls despite equal levels of cell stress and p-eIF2α ⁶³. Importantly, a recent study has shown that the enforced expression of a Vanishing White Matter Disease-causing eIF2B mutation in an oligodendroglial cell line leads to elevated basal levels of the ER stress markers ATF4, GADD34 and BIP and results in the hyper-induction of these markers in response to a pharmacological stress agent, indicating that the UPR adapts oligodendrocytes to Vanishing White Matter Disease-causing eIF2B mutations ⁶⁴.

ER stress has beneficial and detrimental effects on oligodendrocyte survival in immune-mediated demyelinating disorders

MS is the most common neurological disorder affecting young adults, with an estimated incidence of approximately one in 500 individuals. It is generally believed that MS and its animal model, experimental autoimmune encephalomyelitis (EAE), are Th1 T cell-mediated autoimmune diseases ^{65, 66}. The pathological hallmarks of MS and EAE include inflammation, demyelination, oligodendrocyte loss, and axonal degeneration. Evidence is emerging that the UPR is involved in the disease pathogenesis of MS and EAE ^{10, 11, 67}. Gene chip analyses have shown upregulation of the ER stress-responsive genes ATF4 and heat shock protein 70 (HSP70) in MS demyelinated lesions ^{68, 69}. Another recent report has shown elevated expression of CHOP, BIP and XBP-1 in multiple cell types within MS demyelinated lesions, including oligodendrocytes, astrocytes, T cells, and microglia ¹⁰. Moreover, elevated levels of p-eIF2α have been observed in oligodendrocytes and infiltrating T cells in the CNS during the course of EAE ^{70, 71}.

Importantly, evidence has suggested that activation of the PERK-eIF2α pathway of the UPR in oligodendrocytes protects against EAE-induced oligodendrocyte death and demyelination ⁷². Interferon-γ (IFN-γ), a T cell-derived pleiotropic cytokine, is believed to play a crucial role in the pathogenesis of MS and EAE ^{11, 73}. CNS delivery of IFN-γ before EAE onset ameliorated disease severity and prevented EAE-induced oligodendrocyte loss, demyelination, and axonal damage ⁷². Interestingly, the protective effects of this cytokine are accompanied with the activation of the PERK-eIF2α pathway in oligodendrocytes and are abrogated in PERK deficient animals ⁷². While evidence has shown that IFN-γ is capable of increasing eIF2_α phosphorylation through activation of double-stranded RNA-dependent protein kinase (PKR) ⁷⁴ and the level of phosphorylated PKR is elevated in oligodendrocytes in the course of EAE ⁷⁰, we have demonstrated that IFN-γ-induced eIF2α phosphorylation in oligodendrocytes is mediated by PERK and the protective effect of IFN-γ in EAE is dependent on PERK ^{72, 75}. Thus, these data provide strong evidence that the UPR-induced by IFN-γ is involved in the pathogenesis of immune-mediated demyelination diseases.

Remyelination is sufficiently robust to repair myelin damage and restore neurological function in some animal models of CNS demyelination. The remyelination process also occurs in the CNS of MS patients. Nevertheless, this remyelination is generally considered to be insufficient ^{76, 77}. Several reports have shown that IFN-γ is a major cytokine that suppresses oligodendrocyte regeneration and inhibits remyelination in MS and EAE demyelinated lesions ^78-80. Interestingly, the detrimental effects of IFN-γ on myelin repair are mediated by ER stress in the remyelinating oligodendrocytes ⁸⁰. Transgenic mice that ectopically express IFN-γ in the CNS during development display a tremoring phenotype, oligodendrocyte loss, and hypomyelination ^{75, 81, 82}. Activation of the PERK-eIF2α pathway is detected in the myelinating oligodendrocytes of these transgenic mice ⁷⁵. Additionally, decreased activity of the PERK-eIF2α pathway, via PERK inactivation, exacerbates IFN-γ-induced oligodendrocyte apoptosis and hypomyelination ⁷⁵; whereas, increased activity of the PERK-eIF2α pathway, via GADD34 inactivation, has the opposite effect ⁸³. Moreover, the detrimental effects of IFN-γ on remyelination in demyelinated lesions are associated with ER stress in remyelinating oligodendrocytes, as evidenced by the upregulation of BIP and CHOP and an elevated level of p-eIF2α ⁸⁰. Importantly, PERK deficiency significantly exacerbates remyelinating oligodendrocyte apoptosis and remyelination failure induced by IFN-γ in demyelinated lesions ⁸⁰.

Thus, it appears that ER stress has both beneficial and detrimental effects on oligodendrocyte survival in immune-mediated demyelination diseases. ER stress induction in fully myelinated mature oligodendroytes promotes cell survival, but in actively myelinating/remyelinating oligodendrocytes leads to cell death. Although it is largely unknown how the UPR selectively initiates the apoptotic and adaptive pathways ⁸⁴, we speculate that the outcomes of ER stress in oligodendrocytes are likely determined by the developmental status of the cells ^{11, 67, 72}. During active myelination, the ER in oligodendrocytes is largely occupied with the production of enormous amounts of myelin proteins and lipids ^{1, 2}. Therefore, its adaptive capacity may be limited at this stage. It is likely that the dramatic upregulation of protein synthesis initiated by the inflammatory response overload the ER of myelinating cells, causing severe ER stress and cell death. In contrast, mature oligodendrocytes in adult mice produce just enough myelin proteins and lipids to maintain myelin structure homeostasis ⁸⁵. The ER in mature oligodendrocytes may have sufficient adaptive capacity to handle the increased protein load. In this situation, the resulting ER stress would be moderate and would not trigger the apoptotic program. Rather, the adaptive response activated by ER stress could act protectively against subsequent insults.

The unique features of ER stress in myelinating cells

As described in the discussion of myelin disorders above, there is evidence to suggest that myelinating cells respond to ER stress in a manner that is somewhat distinct from that observed in other cell types. For example, CHOP, which is a downstream effecter molecule of the PERK-eIF2α pathway, participates in the induction of the apoptotic response in most cell types, but does not appear to contribute to the UPR-induced apoptotic demise of myelinating cells ^{5, 7, 30}. In fact, activation of CHOP promotes oligodendrocyte survival during the ER stress induced by PLP mutations ⁷. Moreover, it has been demonstrated that CHOP plays an important role in demyelination in the P0 mutant nerve, but is not involved in Schwann cell apoptosis ⁵.

Caspase-12, an ER-localized caspase, has been shown to play an important role in apoptosis elicited by ER stress in neurodegenerative diseases ^{33, 34}. While activation of caspase-12 is detected in the oligodendrocytes of PLP mutant mice, caspase-12 deletion does not significantly affect the clinical phenotype, oligodendrocyte apoptosis, or myelin abnormalities in these mice ⁸⁶. Additionally, the upregulation of ATF3 was shown to occur in the oligodendrocytes of PLP mutant mice, but the deletion of ATF3 also fails to have an effect on oligodendrocyte apoptosis in these animals ⁸⁷. Taken together, these data indicate that the apoptotic process elicited by ER stress in myelinating cells is unique. A more detailed understanding of these unique features could be critical when considering potential therapeutic approaches for myelin disorders.

Perhaps the considerable demand on the secretory pathway of actively myelinating cells has resulted in an alteration to the typical response to ER stress that other cell types display. A more thorough assessment of the UPR in myelinating cells is essential to better understand the response of these cells to ER stress. One difficulty in the study of the UPR in myelinating cells is that they appear most susceptible to secretory pathway perturbations while actively myelinating axons, diminishing the utility of examining their response to stresses in heterologous systems or in isolation. Therefore, the most informative studies have been carried out in vivo using mouse models. Nevertheless, recent advances in the development of in vitro myelination systems, in combination with siRNA technology, should allow for a more rapid characterization of the UPR in myelinating cells ⁸⁸.

Therapeutic potential and future directions

Considerable progress has now been made towards a detailed understanding the signaling pathways of the ER stress response. Recent studies indicate that manipulating these signaling pathways has the potential to be therapeutic. Chemical chaperones 4-phenylbutyric acid and taurine-conjugated ursodeoxycholic acid, which reduce the phosphorylation of PERK and IRE1 during ER stress, both significantly improve glucose tolerance and insulin sensitivity in insulin-resistant obese mice ⁸⁹. Another chemical chaperone, the resveratrol tetramer vaticanol B, which suppresses the expression of CHOP and BIP during ER stress, prevents ER stress-induced apoptosis ⁹⁰. Additionally, treatment with salburinal, a small-molecule inhibitor of eIF2α dephosphorylation, results in sustained eIF2α phosphorylation, and protects cells from ER stress and viral infection ⁹¹.

Currently, there is no effective therapy for patients with disorders of myelinating cells. Therapeutic strategies that facilitate the transport of misfolded proteins out of the ER could be beneficial for these patients. It has been shown that treatment with curcumin promotes mutant PMP22 transport from the ER to the plasma membrane and attenuates Schwann cell apoptosis in PMP22^Tr-J mice ⁴⁵. Moreover, our recent data demonstrate that treatment with salburinal promotes the survival of myelinating oligodendrocyte exposed to IFN-γ, implying that therapeutic strategies that enhance the PERK-eIF2α pathway could promote oligodendrocyte survival in immune-mediated demyelination diseases ⁸³. The manipulation of ER stress for therapeutic purposes, without causing severe side effects, is of course a formidable challenge. Nevertheless, just as actively myelinating cells display increased sensitivity to perturbations to the secretory pathway, these cells may also display selective sensitivity and benefit from modulators of the UPR.

Future studies will need to address the many open questions concerning the physiological significance of the various ER-stress signaling pathways during the myelination process, and the pathological significance of these pathways in myelin disorders. For example, considering the relative prevalence of the disorder, it is critical to know if ER stress is an important participant in the pathogenesis of CMT1 in patients that over-express the membrane proteinPMP22. Moreover, the unique aspects of the UPR in myelinating cells need to be better understood. The knowledge gained from such studies will provide a strong foundation for the design of therapeutic strategies for these diseases. In addition, insights gained from these efforts might prove applicable to neurodegenerative disorders that display the selective vulnerability of specific neuronal subtypes. Neuronal populations with excessive secretory requirements might display increased sensitivity to factors, genetic and environmental, that disrupt ER function ⁹². Future investigations into this possibility might be enlightening.