Molecular pathways of oligodendrocyte apoptosis revealed by mutations in the proteolipid protein gene

Mutation of Proteolipid Protein 1 Gene: From Severe Hypomyelinating Leukodystrophy to Inherited Spastic Paraplegia

Pelizaeus-Merzbacher Disease (PMD) is an inherited leukodystrophy affecting the central nervous system (CNS)-a rare disorder that especially concerns males. Its estimated prevalence is 1.45-1.9 per 100,000 individuals in the general population. Patients affected by PMD exhibit a drastic reduction or absence of myelin sheaths in the white matter areas of the CNS. The Proteolipid Protein 1 (PLP1) gene encodes a transmembrane proteolipid protein. PLP1 is the major protein of myelin, and it plays a key role in the compaction, stabilization, and maintenance of myelin sheaths. Its function is predominant in oligodendrocyte development and axonal survival. Mutations in the PLP1 gene cause the development of a wide continuum spectrum of leukopathies from the most severe form of PMD for whom patients exhibit severe CNS hypomyelination to the relatively mild late-onset type 2 spastic paraplegia, leading to the concept of PLP1-related disorders. The genetic diversity and the biochemical complexity, along with other aspects of PMD, are discussed to reveal the obstacles that hinder the development of treatments. This review aims to provide a clinical and mechanistic overview of this spectrum of rare diseases.

Unfolded protein response in myelin disorders

Activation of the unfolded protein response in response to endoplasmic reticulum stress preserves cell viability and function under stressful conditions. Nevertheless, persistent, unresolvable activation of the unfolded protein response can trigger apoptosis to eliminate stressed cells. Recent studies show that the unfolded protein response plays an important role in the pathogenesis of various disorders of myelin, including multiples sclerosis, Charcot-Marie-Tooth disease, Pelizaeus-Merzbacher disease, vanishing white matter disease, spinal cord injury, tuberous sclerosis complex, and hypoxia-induced perinatal white matter injury. In this review we summarize the current literature on the unfolded protein response and the evidence for its role in the pathogenesis of myelin disorders.

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

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

SIGNIFICANCE STATEMENT

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

Dimethyl fumarate ameliorates myoclonus stemming from protein misfolding in oligodendrocytes

Multiple sclerosis (MS) is considered a primary autoimmune disease; however, this view is increasingly being challenged in basic and clinical science arenas because of the growing body of clinical trials' data showing that exclusion of immune cells from the CNS only modestly slows disease progression to disability. Accordingly, there is significant need for expanding the scope of potential disease mechanisms to understand the etiology of MS. Concomitantly, the use of a broader range of pre-clinical animal models for characterizing existing efficacious clinical treatments may elucidate additional or unexpected mechanisms of action for these drugs that augment insight into MS etiology. Herein, we explore the in vivo mechanism of action of dimethyl fumarate, which has been shown to suppress oxidative stress and immune cell responses in psoriasis and MS. Rather than studying this compound in the context of an experimental autoimmune-induced attack on the CNS, we have used a genetic model of hypomyelination, male rumpshaker (rsh) mice, which exhibit oligodendrocyte metabolic stress and startle-induced subcortical myoclonus during development and into adulthood. We find that myoclonus is reduced 30-50% in treated mutants but we do not detect substantial changes in metabolic or oxidative stress response pathways, cytokine modulation, or myelin thickness (assessed by anova). All procedures involving vertebrate animals in this study were reviewed and approved by the IACUC committee at Wayne State University.

Endoplasmic Reticulum Protein Quality Control Failure in Myelin Disorders

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

Is involvement of inflammation underestimated in Pelizaeus-Merzbacher disease?

Pelizaeus-Merzbacher disease (PMD) is a severe hypomyelinating leukodystrophy resulting from proteolipid protein 1 gene (PLP1) mutations leading to oligodendrocyte loss. While neuroinflammation has recently become a common feature and actor in neurodegenerative diseases, the involvement of inflammation in PMD physiopathology is still highly debated despite evidence for strong astrogliosis and microglial cell activation. Activation of the innate immune system, and more particularly, of microglia and astrocytes, is mostly associated with the deleterious role of neuroinflammation. However, in diseases such as multiple sclerosis, microglia appear beneficial for repair based on their role in myelin debris removal or recruitment and differentiation of oligodendrocyte progenitor cells. In this review, we will discuss recent published data in terms of their relevance to the role of microglia in PMD evolution, and of their impact on the improvement of therapeutic approaches combining immunomodulation and cell therapy to promote optimal recovery. © 2016 Wiley Periodicals, Inc.

Identification and functional study of novel PLP1 mutations in Chinese patients with Pelizaeus-Merzbacher disease

PURPOSE

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked recessive hypomyelination disorder characterized by nystagmus, ataxia, impaired motor development, and progressive spasticity. Identification of proteolipid protein 1 (PLP1) mutations in Chinese patients with Pelizaeus-Merzbacher disease (PMD) and confirmation of the biological impacts of the identified mutations are the aims of this study.

METHODS

An analysis of clinical materials and a follow-up study were conducted for the patients with PMD. Sequencing and immunofluorescence were applied for molecular analysis of the causative gene PLP1.

RESULTS

We identified PLP1 mutations in seven male patients with PMD. Three novel missense mutations (c.353C>G, p.T118R; c.623G>T, p.G208V; c.709T>G, p.F237V) and three reported missense mutations (c.467C>T, p.T156I; c.517C>T, p.P173S; c.646C>T, p.P216S) of PLP1 were identified from seven Chinese PMD patients. The three mutations (F237V in patient 2, P216S in patient 5 and T156I in patient 6) were de novo. Mutant proteins were trapped in the lumen of endoplasmic reticulum.

CONCLUSION

We have identified six pathogenic mutations, enriching the specific spectrum of missense mutations in the patients with PMD. The six PLP1 mutations are probably pathogenic. By reviewing the known PLP1 mutations, we have preliminarily revealed the position of missense mutation may be associated with the severity of PMD.

Effects of the jimpy mutation on mouse retinal structure and function

The Jimpy mutant mouse has a point mutation in the proteolipid protein gene (plp1). The resulting misfolding of the protein leads to oligodendrocyte death, myelin destruction, and failure to produce adequately myelinated axons in the central nervous system (CNS). It is not known how the absence of normal myelination during development influences neural function. We characterized the Jimpy mouse retina to find out whether lack of myelination in the optic nerve during development has an effect on normal functioning and morphology of the retina. Optokinetic reflex measurements showed that Jimpy mice had, in general, a functional visual system. Both PLP1 antibody staining and reverse transcriptase-polymerase chain reaction for plp1 mRNA showed that plp1 is not expressed in the wild-type retina. However, in the optic nerve, plp1 is normally expressed, and consequently, in Jimpy mutant mice, myelination of axons in the optic nerve was mostly absent. Nevertheless, neither axon count nor axon ultrastructure in the optic nerve was affected. Physiological recordings of ganglion cell activity using microelectrode arrays revealed a decrease of stimulus-evoked activity at mesopic light levels. Morphological analysis of the retina did not show any significant differences in the gross morphology, such as thickness of retinal layers or cell number in the inner and outer nuclear layer. The cell bodies in the inner nuclear layer, however, were larger in the peripheral retina of Jimpy mutant mice. Antibody labeling against cell type-specific markers showed that the number of rod bipolar and horizontal cells was increased in Jimpy mice. In conclusion, whereas the Jimpy mutation has dramatic effects on the myelination of retinal ganglion cell axons, it has moderate effects on retinal morphology and function.

Loss of lysophosphatidic acid receptor LPA1 alters oligodendrocyte differentiation and myelination in the mouse cerebral cortex

Lysophosphatidic acid (LPA) is an intercellular signaling lipid that regulates multiple cellular functions, acting through specific G-protein coupled receptors (LPA(1-6)). Our previous studies using viable Malaga variant maLPA1-null mice demonstrated the requirement of the LPA1 receptor for normal proliferation, differentiation, and survival of the neuronal precursors. In the cerebral cortex LPA1 is expressed extensively in differentiating oligodendrocytes, in parallel with myelination. Although exogenous LPA-induced effects have been investigated in myelinating cells, the in vivo contribution of LPA1 to normal myelination remains to be demonstrated. This study identified a relevant in vivo role for LPA1 as a regulator of cortical myelination. Immunochemical analysis in adult maLPA1-null mice demonstrated a reduction in the steady-state levels of the myelin proteins MBP, PLP/DM20, and CNPase in the cerebral cortex. The myelin defects were confirmed using magnetic resonance spectroscopy and electron microscopy. Stereological analysis limited the defects to adult differentiating oligodendrocytes, without variation in the NG2+ precursor cells. Finally, a possible mechanism involving oligodendrocyte survival was demonstrated by the impaired intracellular transport of the PLP/DM20 myelin protein which was accompanied by cellular loss, suggesting stress-induced apoptosis. These findings describe a previously uncharacterized in vivo functional role for LPA1 in the regulation of oligodendrocyte differentiation and myelination in the CNS, underlining the importance of the maLPA1-null mouse as a model for the study of demyelinating diseases.

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

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

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.

Axon-glial interaction in the CNS: what we have learned from mouse models of Pelizaeus-Merzbacher disease

In the central nervous system (CNS) the majority of axons are surrounded by a myelin sheath, which is produced by oligodendrocytes. Myelin is a lipid-rich insulating material that facilitates the rapid conduction of electrical impulses along the myelinated nerve fibre. Proteolipid protein and its isoform DM20 constitute the most abundant protein component of CNS myelin. Mutations in the PLP1 gene encoding these myelin proteins cause Pelizaeus-Merzbacher disease and the related allelic disorder, spastic paraplegia type 2. Animal models of these diseases, particularly models lacking or overexpressing Plp1, have shed light on the interplay between axons and oligodendrocytes, and how one component influences the other.

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.

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.

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.

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.

Maturation-dependent sensitivity of oligodendrocyte lineage cells to apoptosis: implications for normal development and disease

Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stage-specific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease.

ER stress and diseases

Proteins synthesized in the endoplasmic reticulum (ER) are properly folded with the assistance of ER chaperones. Malfolded proteins are disposed of by ER-associated protein degradation (ERAD). When the amount of unfolded protein exceeds the folding capacity of the ER, human cells activate a defense mechanism called the ER stress response, which induces expression of ER chaperones and ERAD components and transiently attenuates protein synthesis to decrease the burden on the ER. It has been revealed that three independent response pathways separately regulate induction of the expression of chaperones, ERAD components, and translational attenuation. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and bipolar disorder, which are collectively known as 'conformational diseases'. In this review, I will summarize recent progress in this field. Molecules that regulate the ER stress response would be potential candidates for drug targets in various conformational diseases.

Novel alternatively spliced endoplasmic reticulum retention signal in the cytoplasmic loop of Proteolipid Protein-1

Increased awareness about the importance of protein folding and trafficking to the etiology of gain-of-function diseases has driven extensive efforts to understand the cell and molecular biology underlying the life cycle of normal secretory pathway proteins and the detrimental effects of abnormal proteins. In this regard, the quality-control machinery in the endoplasmic reticulum (ER) has emerged as a major mechanism by which cells ensure that secreted and transmembrane proteins either adopt stable secondary, tertiary, and quaternary structures or are retained in the ER and degraded. Here we examine cellular and molecular aspects of ER retention in transfected fibroblasts expressing missense mutations in the Proteolipid Protein-1 (PLP1) gene that cause mild or severe forms of neurodegenerative disease in humans. Mild mutations cause protein retention in the ER that is partially dependent on the presence of a cytoplasmically exposed heptapeptide, KGRGSRG. In contrast, retention associated with severe mutations occurs independently of this peptide. Accordingly, the function of this novel heptapeptide has a significant impact on pathogenesis and provides new insight into the functions of the two splice isoforms encoded by the PLP1 gene, PLP1 and DM-20.

Minimal role for caspase 12 in the unfolded protein response in oligodendrocytes in vivo

The unfolded protein response (UPR) is implicated in many neurodegenerative disorders including Alzheimer, Parkinson and prion diseases, and the leukodystrophy, Pelizaeus-Merzbacher disease (PMD). Critical features of degeneration in several of these diseases involve activation of cell death pathways in various neural cell populations, and the initiator caspase 12 has been proposed to play a central role. Accordingly, pharmacological strategies to inhibit caspase 12 activity have received remarkable attention in anticipation of effecting disease amelioration. Our investigation in animal models of PMD demonstrates that caspase 12 is activated following accumulation of mutant proteins in oligodendrocytes; however, eliminating caspase 12 activity does not alter pathophysiology with respect to levels of apoptosis, oligodendrocyte function, disease severity or life span. We conclude that caspase 12 activation by UPR signaling is an epiphenomenon that plays little discernable role in the loss of oligodendrocytes in vivo and may portend the inconsequence of caspase 12 to the pathophysiology of other protein conformational diseases.

PLP overexpression perturbs myelin protein composition and myelination in a mouse model of Pelizaeus-Merzbacher disease

Duplication of PLP1, an X-linked gene encoding the major myelin membrane protein of the human CNS, is the most frequent cause of Pelizaeus-Merzbacher disease (PMD). Transgenic mice with extra copies of the wild type Plp1 gene, a valid model of PMD, also develop a dysmyelinating phenotype dependant on gene dosage. In this study we have examined the effect of increasing Plp1 gene dosage on levels of PLP/DM20 and on other representative myelin proteins. In cultured oligodendrocytes and early myelinating oligodendrocytes in vivo, increased gene dosage leads to elevated levels of PLP/DM20 in the cell body. During myelination, small increases in Plp1 gene dosage (mice hemizygous for the transgene) elevate the level of PLP/DM20 in oligodendrocyte soma but cause only minimal and transient effects on the protein composition and structure of myelin suggesting that cells can regulate the incorporation of proteins into myelin. However, larger increases in dosage (mice homozygous for the transgene) are not well tolerated, leading to hypomyelination and alteration in the cellular distribution of PLP/DM20. A disproportionate amount of PLP/DM20 is retained in the cell soma, probably in autophagic vacuoles and lysosomes whereas the level in myelin is reduced. Increased Plp1 gene dosage affects other myelin proteins, particularly MBP, which is transitorily reduced in hemizygous mice but consistently and markedly lower in homozygotes in both myelin and naïve or early myelinating oligodendrocytes. Whether the reduced MBP is implicated in the pathogenesis of dysmyelination is yet to be established.

Genomic rearrangements and gene copy-number alterations as a cause of nervous system disorders

Genomic disorders are a group of human genetic diseases caused by genomic rearrangements resulting in copy-number variation (CNV) affecting a dosage-sensitive gene or genes critical for normal development or maintenance. These disorders represent a wide range of clinically distinct entities but include many diseases affecting nervous system function. Herein, we review selected neurodevelopmental, neurodegenerative, and psychiatric disorders either known or suggested to be caused by genomic rearrangement and CNV. Further, we emphasize the cause-and-effect relationship between gene CNV and complex disease traits. We also discuss the prevalence and heritability of CNV, the correlation between CNV and higher-order genome architecture, and the heritability of personality, behavioral, and psychiatric traits. We speculate that CNV could underlie a significant proportion of normal human variation including differences in cognitive, behavioral, and psychological features.

Genetic background influences UPR but not PLP processing in the rumpshaker model of PMD/SPG2

Mutations of the proteolipid protein gene (PLP1) cause Pelizaeus-Merzbacher disease (PMD) and Spastic paraplegia type 2 (SPG2). The rumpshaker mutation is associated with mild forms of PMD or SPG2 in man and the identical mutation occurs in mice, the phenotype depending on genetic background. The mild phenotype in C3H mice becomes a lethal disease when expressed on the C57BL/6 background. rumpshaker PLP is synthesised at a similar rate to wild type but is rapidly degraded by the proteasome. We show that the rates of synthesis, degradation and myelin incorporation of PLP/DM20 are similar in mutants on both backgrounds and therefore differences in PLP processing are unlikely to be the basis of the phenotypic variation. An unfolded protein response (UPR) is activated in rumpshaker. Whereas activation of CHOP correlates with phenotypic severity, we find no difference in the response of BiP and X-box protein1 (Xbp1) between the two strains.

Quantifying the carrier female phenotype in Pelizaeus-Merzbacher disease

PURPOSE

Pelizaeus-Merzbacher disease and spastic paraplegia type 2 are allelic X-linked disorders that principally affect males and are caused by mutations in the proteolipid protein 1 gene. Neurologic symptoms are occasionally observed in carrier females, and anecdotal evidence suggests that these clinical signs are more likely in families with affected males. We analyze 40 pedigrees to determine whether such a link exists.

METHODS

From a chart review of patients from Wayne State University, we categorize patients according to disease severity and type of genetic lesion within the proteolipid protein 1 gene. We then analyze the clinical data using nonparametric t tests and analyses of variance.

RESULTS

Our analyses formally demonstrate the link between mild disease in males and symptoms in carrier female relatives. Conversely, mutations causing severe disease in males rarely cause clinical signs in carrier females. The greatest risk of disease in females is found for nonsense/indel or null mutations. Missense mutations carry moderate risk. The lowest risk, which represents the bulk of families with Pelizaeus-Merzbacher disease, is associated with proteolipid protein 1 gene duplications.

CONCLUSIONS

Effective genetic counseling of Pelizaeus-Merzbacher disease and spastic paraplegia carrier females must include an assessment of disease severity in affected male relatives.

Processing of PLP in a model of Pelizaeus-Merzbacher disease/SPG2 due to the rumpshaker mutation

The rumpshaker mutation of the X-linked myelin proteolipid protein (PLP1) gene causes spastic paraplegia type 2 or a mild form of Pelizaeus-Merzbacher disease in man. The identical mutation occurs spontaneously in mice. Both human and murine diseases are associated with dysmyelination. Using the mouse model, we show that the low steady state levels of PLP result from accelerated proteasomal degradation rather than decreased synthesis. The T(1/2) for degradation of rumpshaker PLP is 11 h compared with 23 h for wild type. A minority of newly synthesized PLP is incorporated into myelin in the correct orientation but at a reduced rate compared with wild type. However, inhibition of proteasomal degradation does not increase the level of PLP incorporated into myelin. As Plp null mice do not have a similar myelin deficiency, it is unlikely that the reduced PLP levels are the main cause of the dysmyelination. Rumpshaker oligodendrocytes also have a reduced level of other myelin proteins, such as MBP, although the mechanisms are not yet defined but are likely to operate at a translational or post-translational level.

Defective translation initiation causes vanishing of cerebral white matter

Leukoencephalopathy with vanishing white matter (VWM) is one of the most prevalent inherited white-matter disorders, especially in Caucasian populations. VWM is unusual because of its sensitivity to febrile infections and minor head trauma. The basic defect of this enigmatic brain disease resides in the regulation of initiation of protein synthesis. Recently, undue activation of the unfolded-protein response has emerged as an important factor in the pathophysiology of VWM. Here, we discuss the mechanisms that might be responsible for the selective involvement of the brain white matter in VWM. At present, VWM research is in need of an animal model to study disease mechanisms and therapeutic interventions.

His36Pro point-mutated proteolipid protein retained in the endoplasmic reticulum of oligodendrocytes in theShaking pup

The shaking pup (shp) is a canine mutation that affects the myelin protein proteolipid protein (PLP) and its smaller and less abundant isoform, DM20, with proline replacing histidine(36), resulting in a severe myelin deficiency in the central nervous system. We present evidence that the mutation leads to disrupted trafficking of the shp PLP/DM20 within oligodendrocytes. Immunohistochemical studies revealed significantly reduced levels of PLP/DM20 and other major myelin components such as myelin basic protein (MBP), myelin associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in shp myelin. The distribution of shp PLP/DM20 proteins were altered and mostly retained in perinuclear cytoplasm and proximal processes, which co-localized with distended rough endoplasmic reticulum (RER) within oligodendrocytes. No abnormal accumulation of MAG, MBP, or CNP in the cell body was found. These results suggest that mutated PLP/DM20 in the shp could be selectively retained in RER, causing disruption of their translocation to the periphery to myelinate axons.

PLP1-related inherited dysmyelinating disorders: Pelizaeus-Merzbacher disease and spastic paraplegia type 2

Pelizaeus-Merzbacher disease (PMD) and its allelic disorder, spastic paraplegia type 2 (SPG2), are among the best-characterized dysmyelinating leukodystrophies of the central nervous system (CNS). Both PMD and SPG2 are caused by mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major component of CNS myelin proteins. Distinct types of mutations, including point mutations and genomic duplications and deletions, have been identified as causes of PMD/SPG2 that act through different molecular mechanisms. Studies of various PLP1 mutants in humans and animal models have shed light on the genomic, molecular, and cellular pathogeneses of PMD/SPG2. Recent discoveries include complex mutational mechanisms and associated disease phenotypes, novel cellular pathways that lead to the degeneration of oligodendrocytes, and genomic architectural features that result in unique chromosomal rearrangements. Here, I review the previous and current knowledge of the molecular pathogenesis of PMD/SPG2 and delineate future directions for PMD/SPG2 studies.

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus ( cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This "early secretory pathway" includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.

Schwann cell expression of PLP1 but not DM20 is necessary to prevent neuropathy

Proteolipid protein (PLP1) and its alternatively spliced isoform, DM20, are the major myelin proteins in the CNS, but are also expressed in the PNS. The proteins have an identical sequence except for 35 amino acids in PLP1 (the PLP1-specific domain) not present in DM20. Mutations of PLP1/DM20 cause Pelizaeus-Merzbacher Disease (PMD), a leukodystrophy, and in some instances, a peripheral neuropathy. To identify which mutations cause neuropathy, we have evaluated a cohort of patients with PMD and PLP1 mutations for the presence of neuropathy. As shown previously, all patients with PLP1 null mutations had peripheral neuropathy. We also identified 4 new PLP1 point mutations that cause both PMD and peripheral neuropathy, three of which truncate PLP1 expression within the PLP1-specific domain, but do not alter DM20. The fourth, a splicing mutation, alters both PLP1 and DM20, and is probably a null mutation. Six PLP1 point mutations predicted to produce proteins with an intact PLP1-specific domain do not cause peripheral neuropathy. Sixty-one individuals with PLP1 duplications also had normal peripheral nerve function. These data demonstrate that expression of PLP1 but not DMSO is necessary to prevent neuropathy, and suggest that the 35 amino acid PLP1-specific domain plays an important role in normal peripheral nerve function.

Trafficking of PLP/DM20 and cAMP signaling in immortalized jimpy oligodendrocytes

The synthesis, transport, and insertion of jimpy proteolipid protein and DM20 were studied in normal (158N) and jimpy (158JP) immortalized oligodendrocyte lines. Four different expression vectors encoding fusion proteins composed of native PLP and DM20 or jimpy PLP or DM20 were linked to enhanced green fluorescent protein (EGFP). All four transfected fusion proteins had similar distributions in the cell bodies and processes of the two cell types. Both normal and jimpy PLP-EGFP and DM20-EGFP were detected in both cell lines as far as 200 microM from the cell body, indicating synthesis and transport of mutated PLP and DM20 toward the plasma membrane. Immunocytochemistry of fixed normal and jimpy cells with the O10 antibody, which recognizes a conformationally sensitive PLP/DM20 epitope, confirmed that normal and jimpy PLP and DM20 were transported to the plasma membrane. Live staining of normal and jimpy cells transiently transfected with the native PLP showed positive staining, indicating PLP was correctly inserted into the membrane of both normal and jimpy oligodendrocytes. However, live staining of normal and jimpy cells transiently transfected with jimpy PLP showed no positive staining, indicating the mutated protein is abnormally inserted into the plasma membrane. Electrophysiological recordings of the resting membrane potential measured in the whole cell mode of the patch-clamp technique showed the absence of a developmentally regulated negative shift in the membrane potential in jimpy cells compared to normal native or immortalized oligodendrocytes. Treatment of 158N cells and native oligodendrocytes with dibutyryl cAMP (dbcAMP) caused morphological and biochemical differentiation, but failed to do so in 158JP cells, suggesting an abnormal signaling pathway in jimpy. The defect in cAMP signaling in jimpy oligodendrocytes was associated with the suppression of increase in mRNA level of the inducible cAMP early repressor (ICER). When the jimpy oligodendrocyte line was transfected with normal PLP or DM20 and exposed to dbcAMP, the cells failed to differentiate. This finding suggests that improper insertion of jimpy protein into the plasma membrane alters the membrane in such a way that certain signaling pathways are permanently altered. The abnormal insertion of jimpy PLP/DM20 into the plasma membrane may be the basis for the lack of cell signaling and abnormal resting potential in jimpy oligodendrocytes.

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

The unfolded protein response (UPR) is a eukaryotic signaling pathway linking protein flux through the endoplasmic reticulum to transcription and translational repression. Herein, we demonstrate UPR activation in the leukodystrophy Pelizaeus-Merzbacher disease (PMD) as well as in three mouse models of this disease and transfected fibroblasts expressing mutant protein. The CHOP protein, widely known as a proapoptotic transcription factor, modulates pathogenesis in the mouse models of PMD; however, this protein exhibits antiapoptotic activity. Together, these data show that the UPR has the potential to modulate disease severity in many cells expressing mutant secretory pathway proteins. Thus, PMD represents the first member of a novel class of disparate degenerative diseases for which UPR activation and signaling is the common pathogenic mechanism.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.