The UPR preserves mature oligodendrocyte viability and function in adults by regulating autophagy of PLP

Thinning of originally-existing, mature myelin represents a nondestructive form of myelin loss in the adult CNS

The main function of oligodendrocytes is to assemble and maintain myelin that wraps and insulates axons in the central nervous system (CNS). Traditionally, myelin structure, particularly its thickness, was believed to remain remarkably stable in adulthood (including early and middle adulthood, but not late adulthood or aging). However, emerging evidence reveals that the thickness of originally-existing, mature myelin (OEM) can undergo dynamic changes in the adult CNS. This overview highlights recent findings on the alteration of OEM thickness in the adult CNS, explores the underlying mechanisms, and proposes that progressive thinning of OEM represents a novel, nondestructive form of myelin loss in myelin disorders of the CNS.

The physiological role of the unfolded protein response in the nervous system

The unfolded protein response (UPR) is a cellular stress response pathway activated when the endoplasmic reticulum, a crucial organelle for protein folding and modification, encounters an accumulation of unfolded or misfolded proteins. The UPR aims to restore endoplasmic reticulum homeostasis by enhancing protein folding capacity, reducing protein biosynthesis, and promoting protein degradation. It also plays a pivotal role in coordinating signaling cascades to determine cell fate and function in response to endoplasmic reticulum stress. Recent research has highlighted the significance of the UPR not only in maintaining endoplasmic reticulum homeostasis but also in influencing various physiological processes in the nervous system. Here, we provide an overview of recent findings that underscore the UPR's involvement in preserving the function and viability of neuronal and myelinating cells under physiological conditions, and highlight the critical role of the UPR in brain development, memory storage, retinal cone development, myelination, and maintenance of myelin thickness.

Potential Role of Endoplasmic Reticulum Stress in Modulating Protein Homeostasis in Oligodendrocytes to Improve White Matter Injury in Preterm Infants

Preterm white matter injury (WMI) is a demyelinating disease with high incidence and mortality in premature infants. Oligodendrocyte cells (OLs) are a specialized glial cell that produces myelin proteins and adheres to the axons providing energy and metabolic support which susceptible to endoplasmic reticulum protein quality control. Disruption of cellular protein homeostasis led to OLs dysfunction and cell death, immediately, the unfolded protein response (UPR) activated to attempt to restore the protein homeostasis via IRE1/XBP1s, PERK/eIF2α and ATF6 pathway that reduced protein translation, strengthen protein-folding capacity, and degraded unfolding/misfolded protein. Moreover, recent works have revealed the conspicuousness function of ER signaling pathways in regulating influenced factors such as calcium homeostasis, mitochondrial reactive oxygen generation, and autophagy activation to regulate protein hemostasis and improve the myelination function of OLs. Each of the regulation modes and their corresponding molecular mechanisms provides unique opportunities and distinct perspectives to obtain a deep understanding of different actions of ER stress in maintaining OLs' health and function. Therefore, our review focuses on summarizing the current understanding of ER stress on OLs' protein homeostasis micro-environment in myelination during white matter development, as well as the pathophysiology of WMI, and discussing the further potential experimental therapeutics targeting these factors that restore the function of the UPR in OLs myelination function.

Endoplasmic reticulum associated degradation preserves neurons viability by maintaining endoplasmic reticulum homeostasis

Endoplasmic reticulum-associated degradation (ERAD) is a principal quality-control mechanism responsible for targeting misfolded ER proteins for cytosolic degradation. Evidence suggests that impairment of ERAD contributes to neuron dysfunction and death in neurodegenerative diseases, many of which are characterized by accumulation and aggregation of misfolded proteins. However, the physiological role of ERAD in neurons remains unclear. The Sel1L-Hrd1 complex consisting of the E3 ubiquitin ligase Hrd1 and its adaptor protein Sel1L is the best-characterized ERAD machinery. Herein, we showed that Sel1L deficiency specifically in neurons of adult mice impaired the ERAD activity of the Sel1L-Hrd1 complex and led to disruption of ER homeostasis, ER stress and activation of the unfold protein response (UPR). Adult mice with Sel1L deficiency in neurons exhibited weight loss and severe motor dysfunction, and rapidly succumbed to death. Interestingly, Sel1L deficiency in neurons caused global brain atrophy, particularly cerebellar and hippocampal atrophy, in adult mice. Moreover, we found that cerebellar and hippocampal atrophy in these mice resulted from degeneration of Purkinje neurons and hippocampal neurons, respectively. These findings indicate that ERAD is required for maintaining ER homeostasis and the viability and function of neurons in adults under physiological conditions.

Inherited white matter disorders: Hypomyelination (myelin disorders)

Hypomyelinating leukodystrophies are a subset of genetic white matter diseases characterized by insufficient myelin deposition during development. MRI patterns are used to identify hypomyelinating disorders, and genetic testing is used to determine the causal genes implicated in individual disease forms. Clinical course can range from severe, with patients manifesting neurologic symptoms in infancy or early childhood, to mild, with onset in adolescence or adulthood. This chapter discusses the most common hypomyelinating leukodystrophies, including X-linked Pelizaeus-Merzbacher disease and other PLP1-related disorders, autosomal recessive Pelizaeus-Merzbacher-like disease, and POLR3-related leukodystrophy. PLP1-related disorders are caused by hemizygous pathogenic variants in the proteolipid protein 1 (PLP1) gene, and encompass classic Pelizaeus-Merzbacher disease, the severe connatal form, PLP1-null syndrome, spastic paraplegia type 2, and hypomyelination of early myelinating structures. Pelizaeus-Merzbacher-like disease presents a similar clinical picture to Pelizaeus-Merzbacher disease, however, it is caused by biallelic pathogenic variants in the GJC2 gene, which encodes for the gap junction protein Connexin-47. POLR3-related leukodystrophy, or 4H leukodystrophy (hypomyelination, hypodontia, and hypogonadotropic hypogonadism), is caused by biallelic pathogenic variants in genes encoding specific subunits of the transcription enzyme RNA polymerase III. In this chapter, the clinical features, disease pathophysiology and genetics, imaging patterns, as well as supportive and future therapies are discussed for each disorder.

The PERK pathway: beneficial or detrimental for neurodegenerative diseases and tumor growth and cancer

Protein kinase R (PKR)-like endoplasmic reticulum (ER) kinase (PERK) is one of the three major sensors in the unfolded protein response (UPR). The UPR is involved in the modulation of protein synthesis as an adaptive response. Prolonged PERK activity correlates with the development of diseases and the attenuation of disease severity. Thus, the current debate focuses on the role of the PERK signaling pathway either in accelerating or preventing diseases such as neurodegenerative diseases, myelin disorders, and tumor growth and cancer. In this review, we examine the current findings on the PERK signaling pathway and whether it is beneficial or detrimental for the above-mentioned disorders.

The UPR Maintains Proteostasis and the Viability and Function of Hippocampal Neurons in Adult Mice

The unfolded protein response (UPR), which comprises three branches: PERK, ATF6α, and IRE1, is a major mechanism for maintaining cellular proteostasis. Many studies show that the UPR is a major player in regulating neuron viability and function in various neurodegenerative diseases; however, its role in neurodegeneration is highly controversial. Moreover, while evidence suggests activation of the UPR in neurons under normal conditions, deficiency of individual branches of the UPR has no major effect on brain neurons in animals. It remains unclear whether or how the UPR participates in regulating neuronal proteostasis under normal and disease conditions. To determine the physiological role of the UPR in neurons, we generated mice with double deletion of PERK and ATF6α in neurons. We found that inactivation of PERK and ATF6α in neurons caused lysosomal dysfunction (as evidenced by decreased expression of the V0a1 subunit of v-ATPase and decreased activation of cathepsin D), impairment of autophagic flux (as evidenced by increased ratio of LC3-II/LC3-I and increased p62 level), and accumulation of p-tau and Aβ42 in the hippocampus, and led to impairment of spatial memory, impairment of hippocampal LTP, and hippocampal degeneration in adult mice. These results suggest that the UPR is required for maintaining neuronal proteostasis (particularly tau and Aβ homeostasis) and the viability and function of neurons in the hippocampus of adult mice.

Endoplasmic reticulum associated degradation is essential for maintaining the viability or function of mature myelinating cells in adults

Endoplasmic reticulum associated degradation (ERAD) is responsible for recognition and degradation of unfolded or misfolded proteins in the ER. Sel1L is essential for the ERAD activity of Sel1L-Hrd1 complex, the best-known ERAD machinery. Using a continuous Sel1L knockout mouse model (CNP/Cre; Sel1LloxP/loxP mice), our previous studies showed that Sel1L knockout in myelinating cells, oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS), leads to adult-onset myelin abnormalities in the CNS and PNS. Because Sel1L is deleted in myelinating cells of CNP/Cre; Sel1LloxP/loxP mice starting at very early stage of differentiation, it is impossible to rule out the possibility that the adult-onset myelin abnormalities in these mice results from developmental myelination defects caused by Sel1L knockout in myelinating cells during development. Thus, using an inducible Sel1L knockout mouse model (PLP/CreERT ; Sel1LloxP/loxP mice) that has normal, intact myelin and myelinating cells in the adult CNS and PNS prior to tamoxifen treatment, we sought to determine if Sel1L knockout in mature myelinating cells of adult mice leads to myelin abnormalities in the CNS and PNS. We showed that Sel1L knockout in mature myelinating cells caused ERAD impairment, ER stress and UPR activation. Interesting, Sel1L knockout in mature oligodendrocytes impaired their myelinating function by suppressing myelin protein translation, and resulted in progressive myelin thinning in the adult CNS. Conversely, Sel1L knockout in mature Schwann cells led to Schwann cell apoptosis and demyelination in the adult PNS. These findings demonstrate the essential roles of ERAD in mature myelinating cells in the adult CNS and PNS under physiological conditions.

Contribution of RNA/DNA Binding Protein Dysfunction in Oligodendrocytes in the Pathogenesis of the Amyotrophic Lateral Sclerosis/Frontotemporal Lobar Degeneration Spectrum Diseases

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are two incurable neurodegenerative disorders, often considered as the extreme manifestations of a disease spectrum, as they share similar pathomechanisms. In support of this, pathological aggregation of the RNA/DNA binding proteins trans-activation response element DNA-binding protein 43 (TDP-43) or fused in sarcoma (FUS) is the pathological hallmark found in neurons and glial cells of subsets of patients affected by either condition (i.e., ALS/FTLD—TDP-43 or ALS/FTLD—FUS, respectively). Among glia, oligodendrocytes are the most abundant population, designated to ensheath the axons with myelin and to provide them with metabolic and trophic support. In this minireview, we recapitulate the neuropathological evidence for oligodendroglia impairment in ALS/FTLD. We then debate how TDP-43 and FUS target oligodendrocyte transcripts, thereby controlling their homeostatic abilities toward the axons. Finally, we discuss cellular and animal models aimed at investigating the functional consequences of manipulating TDP-43 and FUS in oligodendrocytes in vivo. Taken together, current data provide increasing evidence for an important role of TDP-43 and FUS-mediated oligodendroglia dysfunction in the pathogenesis of ALS/FTLD. Thus, targeting disrupted oligodendroglial functions may represent a new treatment approach for these conditions.

The special unfolded protein response in plasma cells

The high rate of antibody production places considerable metabolic and folding stress on plasma cells (PC). Not surprisingly, they rely on the unfolded protein response (UPR), a universal signaling, and transcriptional network that monitors the health of the secretory pathway and mounts cellular responses to stress. Typically, the UPR utilizes three distinct stress sensors in the ER membrane, each regulating a subset of targets to re-establish homeostasis. PC use a specialized UPR scheme-they preemptively trigger the UPR via developmental signals and suppress two of the sensors, PERK and ATF6, relying on IRE1 alone. The specialized PC UPR program is tuned to the specific needs at every stage of development-from early biogenesis of secretory apparatus, to massive immunoglobulin expression later. Furthermore, the UPR in PC integrates with other pathways essential in a highly secretory cell-mTOR pathway that ensures efficient synthesis, autophagosomes that recycle components of the synthetic machinery, and apoptotic signaling that controls cell fate in the face of excessive folding stress. This specialized PC program is not shared with other secretory cells, for reasons yet to be defined. In this review, we give a perspective into how and why PC need such a unique UPR program.

How Degeneration of Cells Surrounding Motoneurons Contributes to Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by the progressive degeneration of upper and lower motoneurons. Despite motoneuron death being recognized as the cardinal event of the disease, the loss of glial cells and interneurons in the brain and spinal cord accompanies and even precedes motoneuron elimination. In this review, we provide striking evidence that the degeneration of astrocytes and oligodendrocytes, in addition to inhibitory and modulatory interneurons, disrupt the functionally coherent environment of motoneurons. We discuss the extent to which the degeneration of glial cells and interneurons also contributes to the decline of the motor system. This pathogenic cellular network therefore represents a novel strategic field of therapeutic investigation.

The Integrated UPR and ERAD in Oligodendrocytes Maintain Myelin Thickness in Adults by Regulating Myelin Protein Translation

Myelin proteins, which are produced in the endoplasmic reticulum (ER), are essential and necessary for maintaining myelin structure. The integrated unfold protein response (UPR) and ER-associated degradation (ERAD) are the primary ER quality control mechanism. The adaptor protein Sel1L (Suppressor/Enhancer of Lin-12-like) controls the stability of the E3 ubiquitin ligase Hrd1 (hydroxymethylglutaryl reductase degradation protein 1), and is necessary for the ERAD activity of the Sel1L-Hrd1 complex. Herein, we showed that Sel1L deficiency specifically in oligodendrocytes caused ERAD impairment, the UPR activation, and attenuation of myelin protein biosynthesis; and resulted in late-onset, progressive myelin thinning in the CNS of adult mice (both male and female). The pancreatic ER kinase (PERK) branch of the UPR functions as the master regulator of protein translation in ER-stressed cells. Importantly, PERK inactivation reversed attenuation of myelin protein biosynthesis in oligodendrocytes and restored myelin thickness in the CNS of oligodendrocyte-specific Sel1L-deficient mice (both male and female). Conversely, blockage of proteolipid protein production exacerbated myelin thinning in the CNS of oligodendrocyte-specific Sel1L-deficient mice (both male and female). These findings suggest that impaired ERAD in oligodendrocytes reduces myelin thickness in the adult CNS through suppression of myelin protein translation by activating PERK.SIGNIFICANCE STATEMENT Myelin is an enormous extended plasma membrane of oligodendrocytes that wraps and insulates axons. Myelin structure, including thickness, was thought to be extraordinarily stable in adults. Myelin proteins, which are produced in the endoplasmic reticulum (ER), are essential and necessary for maintaining myelin structure. The integrated unfolded protein response (UPR) and ER-associated degradation (ERAD) are the primary mechanism that maintains ER protein homeostasis. Herein, we explored the role of the integrated UPR and ERAD in oligodendrocytes in regulating myelin protein production and maintaining myelin structure using mouse models. The results presented in this study imply that the integrated UPR and ERAD in oligodendrocytes maintain myelin thickness in adults by regulating myelin protein production.

Endoplasmic reticulum associated degradation is required for maintaining endoplasmic reticulum homeostasis and viability of mature Schwann cells in adults

The integrated unfolded protein response (UPR) and endoplasmic reticulum associated degradation (ERAD) is the principle mechanisms that maintain endoplasmic reticulum (ER) homeostasis. Schwann cells (SCs) must produce an enormous amount of myelin proteins via the ER to assemble and maintain myelin structure; however, it is unclear how SCs maintain ER homeostasis. It is known that Suppressor/Enhancer of Lin-12-like (Sel1L) is necessary for the ERAD activity of the Sel1L- hydroxymethylglutaryl reductase degradation protein 1(Hrd1) complex. Herein, we showed that Sel1L deficiency in SCs impaired the ERAD activity of the Sel1L-Hrd1 complex and led to ER stress and activation of the UPR. Interestingly, Sel1L deficiency had no effect on actively myelinating SCs during development, but led to later-onset mature SC apoptosis and demyelination in the adult PNS. Moreover, inactivation of the pancreatic ER kinase (PERK) branch of the UPR did not influence the viability and function of actively myelinating SCs, but resulted in exacerbation of ER stress and apoptosis of mature SCs in SC-specific Sel1L deficient mice. These findings suggest that the integrated UPR and ERAD is dispensable to actively myelinating SCs during development, but is necessary for maintaining ER homeostasis and the viability and function of mature SCs in adults.

NF-κB Activation Accounts for the Cytoprotective Effects of PERK Activation on Oligodendrocytes during EAE

Previous studies demonstrate that activation of pancreatic ER kinase (PERK) protects oligodendrocytes against inflammation in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis (MS). Interestingly, data indicate that the cytoprotective effects of PERK activation on oligodendrocytes during EAE are not mediated by activating transcription factor 4 (ATF4) but are accompanied by activation of nuclear factor κB (NF-κB). NF-κB plays a critical role in MS and EAE; however, the effects of NF-κB activation on oligodendrocytes in these diseases remain elusive. Herein, we generated a mouse model that allow for activation of NF-κB specifically in oligodendrocytes and found that enhanced NF-κB activation in oligodendrocytes had a minimal effect on their viability and function under normal conditions (both male and female mice). Interestingly, we found that enhanced NF-κB activation in oligodendrocytes attenuated EAE disease severity and ameliorated EAE-induced oligodendrocyte loss, demyelination, and axon degeneration, without affecting inflammation (female mice). Moreover, we showed that the detrimental effects of PERK inactivation in oligodendrocytes in EAE were accompanied by impaired NF-κB activation in oligodendrocytes, and were completely rescued by enhanced NF-κB activation in oligodendrocytes (female mice). These findings suggest that NF-κB activation accounts for the cytoprotective effects of PERK activation on oligodendrocytes in MS and EAE.SIGNIFICANCE STATEMENT Nuclear factor κB (NF-κB) is activated in oligodendrocytes in multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE); however, the role of NF-κB activation in oligodendrocytes in MS and EAE remains elusive. Herein, we generated a mouse model that allows for activation of NF-κB selectively in oligodendrocytes and demonstrated that NF-κB activation prevented oligodendrocyte death and myelin damage in the EAE model. We further demonstrated that NF-κB activation contributed to the protective effects of pancreatic ER kinase (PERK) activation on oligodendrocytes in the EAE model. As such, this work will facilitate the development of new treatments that enhance oligodendrocyte survival in MS patients by targeting the PERK-NF-κB pathway.