Selective expression of DM-20, an alternatively spliced myelin proteolipid protein gene product, in developing nervous system and in nonglial cells

Terra incognita of glial cell dynamics in the etiology of leukodystrophies: Broadening disease and therapeutic perspectives

Neuroglial cells, also known as glia, are primarily characterized as auxiliary cells within the central nervous system (CNS). The recent findings have shed light on their significance in numerous physiological processes and their involvement in various neurological disorders. Leukodystrophies encompass an array of rare and hereditary neurodegenerative conditions that were initially characterized by the deficiency, aberration, or degradation of myelin sheath within CNS. The primary cellular populations that experience significant alterations are astrocytes, oligodendrocytes and microglia. These glial cells are either structurally or metabolically impaired due to inherent cellular dysfunction. Alternatively, they may fall victim to the accumulation of harmful by-products resulting from metabolic disturbances. In either situation, the possible replacement of glial cells through the utilization of implanted tissue or stem cell-derived human neural or glial progenitor cells hold great promise as a therapeutic strategy for both the restoration of structural integrity through remyelination and the amelioration of metabolic deficiencies. Various emerging treatment strategies like stem cell therapy, ex-vivo gene therapy, infusion of adeno-associated virus vectors, emerging RNA-based therapies as well as long-term therapies have demonstrated success in pre-clinical studies and show promise for rapid clinical translation. Here, we addressed various leukodystrophies in a comprehensive and detailed manner as well as provide prospective therapeutic interventions that are being considered for clinical trials. Further, we aim to emphasize the crucial role of different glial cells in the pathogenesis of leukodystrophies. By doing so, we hope to advance our understanding of the disease, elucidate underlying mechanisms, and facilitate the development of potential treatment interventions.

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.

Plp1 in the enteric nervous system is preferentially expressed during early postnatal development in mouse as DM20, whose expression appears reliant on an intronic enhancer

Recently, the myelin proteolipid protein gene (Plp1) was shown to be expressed in the glia of the enteric nervous system (ENS) in mouse. However, beyond this, not much is known about its expression in the intestine. To address this matter, we investigated Plp1 expression at the mRNA and protein levels in the intestine of mice at different ages (postnatal days 2, 9, 21, and 88). In this study, we show that Plp1 expression preferentially occurs during early postnatal development, primarily as the DM20 isoform. Western blot analysis indicated that DM20 migrated according to its formula weight when isolated from the intestine. However, mobilities of both PLP and DM20 were faster than expected when procured from the brain. The 6.2hPLP(+)Z/FL transgene, which uses the first half of the human PLP1 gene to drive expression of a lacZ reporter gene, recapitulated the developmental pattern observed with the native gene in the intestine, indicating that it can be used as a proxy for Plp1 gene expression. As such, the relative levels of β-galactosidase (β-gal) activity emanating from the 6.2hPLP(+)Z/FL transgene suggest that Plp1 expression is highest in the duodenum, and decreases successively along the segments, toward the colon. Moreover, removal of the wmN1 enhancer region from the transgene (located within Plp1 intron 1) resulted in a dramatic reduction in both transgene mRNA levels and β-gal activity in the intestine, throughout development, suggesting that this region contains a regulatory element crucial for Plp1 expression. This is consistent with earlier studies in both the central and peripheral nervous systems, indicating that it may be a common (if not universal) means by which Plp1 gene expression is governed.

Developmental Cues and Molecular Drivers in Myelinogenesis: Revisiting Early Life to Re-Evaluate the Integrity of CNS Myelin

The mammalian central nervous system (CNS) coordinates its communication through saltatory conduction, facilitated by myelin-forming oligodendrocytes (OLs). Despite the fact that neurogenesis from stem cell niches has caught the majority of attention in recent years, oligodendrogenesis and, more specifically, the molecular underpinnings behind OL-dependent myelinogenesis, remain largely unknown. In this comprehensive review, we determine the developmental cues and molecular drivers which regulate normal myelination both at the prenatal and postnatal periods. We have indexed the individual stages of myelinogenesis sequentially; from the initiation of oligodendrocyte precursor cells, including migration and proliferation, to first contact with the axon that enlists positive and negative regulators for myelination, until the ultimate maintenance of the axon ensheathment and myelin growth. Here, we highlight multiple developmental pathways that are key to successful myelin formation and define the molecular pathways that can potentially be targets for pharmacological interventions in a variety of neurological disorders that exhibit demyelination.

Anti-Myelin Proteolipid Protein Peptide Monoclonal Antibodies Recognize Cell Surface Proteins on Developing Neurons and Inhibit Their Differentiation

Using a panel of monoclonal antibodies (mAbs) to myelin proteolipid protein (PLP) peptides, we found that in addition to CNS myelin, mAbs to external face but not cytoplasmic face epitopes immunostained neurons in immature human CNS tissues and in adult hippocampal dentate gyrus and olfactory bulbs, that is neural stem cell niches (NSCN). To explore the pathobiological significance of these observations, we assessed the mAb effects on neurodifferentiation in vitro. The mAbs to PLP 50-69 (IgG1κ and IgG2aκ), and 178-191 and 200-219 (both IgG1κ) immunostained live cell surfaces and inhibited neurite outgrowth of E18 rat hippocampal precursor cells and of PC12 cells, which do not express PLP. Proteins immunoprecipitated from PC12 cell extracts and captured by mAb-coated magnetic beads were identified by GeLC-MS/MS. Each neurite outgrowth-inhibiting mAb captured a distinct set of neurodifferentiation molecules including sequence-similar M6 proteins and other unrelated membrane and extracellular matrix proteins, for example integrins, Eph receptors, NCAM-1, and protocadherins. These molecules are expressed in adult human NSCN and are implicated in the pathogenesis of many chronic CNS disease processes. Thus, diverse anti-PLP epitope autoantibodies may inhibit neuronal precursor cell differentiation via multispecific recognition of cell surface molecules thereby potentially impeding endogenous neuroregeneration in NSCN and in vivo differentiation of exogenous neural stem cells.

Pelizaeus-Merzbacher Disease: Molecular and Cellular Pathologies and Associated Phenotypes

Pelizaeus-Merzbacher disease (PMD) represents a group of disorders known as hypomyelinating leukodystrophies, which are characterized by abnormal development and maintenance of myelin in the central nervous system. PMD is caused by different types of mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major myelin membrane lipoprotein. These mutations in the PLP1 gene result in distinct cellular and molecular pathologies and a spectrum of clinical phenotypes. In this chapter, I discuss the historical aspects and current understanding of the mechanisms underlying how different PLP1 mutations disrupt the normal process of myelination and result in PMD and other disorders.

Insertion of proteolipid protein into mitochondria but not DM20 regulates metabolism of cells

Proteolipid protein (PLP), besides its adhesive role in myelin, has been postulated to have multiple cellular functions. One well-documented function of PLP is regulation of oligodendrocyte (Olg) apoptosis. In contrast, DM20, an alternatively spliced product of the PLP1/Plp1 gene, has been proposed to have functions that are unique from PLP but these functions have never been elucidated. Here, we compare metabolism of PLP and DM20, and show that oxidative phosphorylation (OxPhos) was significantly decreased in Plp1 but not DM20 or EGFP expressing cells. The reserve OxPhos capacity of Plp1 expressing cells was half of control cells, suggesting that they are very vulnerable to stress. ATP in media of Plp1 expressing cells is significantly increased more than two-fold compared to controls; markers of apoptosis are increased in cells over-expressing Plp1, indicating that abnormal metabolism of PLP is most likely the direct cause leading to Olg apoptosis. We hypothesize that abnormal metabolism, mediated by increased insertion of PLP into mitochondria, underlies demyelination in Pelizaeus-Merzbacher Disease (PMD) and in models of PMD. To understand why PLP and DM20 function differently, we mutated or deleted amino acids located in the PLP-specific region. All these mutations and deletions of the PLP-specific region prevented insertion of PLP into mitochondria. These findings demonstrate that the PLP-specific region is essential for PLP's import into mitochondria, and now offer an explanation for deciphering unique functions of PLP and DM20.

Histological detection of dynamic glial responses in the dysmyelinating Tabby-jimpy mutant brain

Oligodendrocytes (OLs) are glial cells that form myelin sheaths surrounding the axons in the central nervous system (CNS). Jimpy (jp) mutant mice are dysmyelinating disease models that show developmental abnormalities in myelinated OLs in the CNS. The causative gene in jp mice is the proteolipid protein (PLP) located on the X chromosome. Mutations in the jp allele result in exon 5 skipping and expression of abnormal PLP containing a C-terminal frame shift. Many lines of evidence suggest that abnormal PLP in OLs results in endoplasmic reticulum (ER) stress and cell death. To histologically detect glial responses in the jp mutant brain, we performed staining with lineage-specific markers. Using OL markers and OL progenitor cell marker staining, we identified reduced numbers of OL lineage cells in the jp mutant brain. Nuclear staining of the transcription factor Olig1 was observed in the Tabby-jp brain, whereas cytoplasmic Olig1 staining was observed in the wild-type brain at postnatal day 21, suggesting that active myelination was present in the mutant brain. Many microglial cells with activated morphology and intensive staining of CD11b microglia marker were observed in the internal capsule of the mutant brain, a region of white matter containing residual OLs. Activated astrocytes with high glial fibrillary acidic protein-immunoreactivity were also mainly observed in white matter. Finally, we performed in situ hybridization using C/EBP homologous protein (CHOP) antisense probes to detect ER stressed cells. CHOP mRNA was strongly expressed in residual OLs in the Tabby-jp mutant mice at postnatal stages. These data show that microglia and astrocytes exhibit dynamic glial activation in response to cell death of OLs during Tabby-jp pathogenesis, and that CHOP antisense probes may be a good marker for the detection of ER-stressed OLs in jp mutant mice.

Three new PLP1 splicing mutations demonstrate pathogenic and phenotypic diversity of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a severe X-linked disorder of central myelination caused by mutations affecting the proteolipid protein gene. We describe 3 new PLP1 splicing mutations, their effect on splicing and associated phenotypes. Mutation c.453_453+6del7insA affects the exon 3B donor splice site and disrupts the PLP1-transcript without affecting the DM20, was found in a patient with severe Pelizaeus-Merzbacher disease and in his female cousin with early-onset spastic paraparesis. Mutation c.191+1G>A causes exon 2 skipping with a frame shift, is expected to result in a functionally null allele, and was found in a patient with mild Pelizaeus-Merzbacher disease and in his aunt with late-onset spastic paraparesis. Mutation c.696+1G>A utilizes a cryptic splice site in exon 5, causes partial exon 5 skipping and in-frame deletion, and was found in an isolated patient with a severe classical Pelizaeus-Merzbacher. PLP1 splice-site mutations express a variety of disease phenotypes mediated by different molecular pathogenic mechanisms.

Microwave and magnetic (M(2) ) proteomics of the experimental autoimmune encephalomyelitis animal model of multiple sclerosis

We hypothesized that quantitative MS/MS-based proteomics at multiple time points, incorporating rapid microwave and magnetic (M(2) ) sample preparation, could enable relative protein expression to be correlated to disease progression in the experimental autoimmune encephalomyelitis (EAE) animal model of multiple sclerosis. To test our hypothesis, microwave-assisted reduction/alkylation/digestion of proteins from brain tissue lysates bound to C8 magnetic beads and microwave-assisted isobaric chemical labeling were performed of released peptides, in 90 s prior to unbiased proteomic analysis. Disease progression in EAE was assessed by scoring clinical EAE disease severity and confirmed by histopathologic evaluation for central nervous system inflammation. Decoding the expression of 283 top-ranked proteins (p <0.05) at each time point relative to their expression at the peak of disease, from a total of 1191 proteins observed in four technical replicates, revealed a strong statistical correlation to EAE disease score, particularly for the following four proteins that closely mirror disease progression: 14-3-3ε (p = 3.4E-6); GPI (p = 2.1E-5); PLP1 (p = 8.0E-4); PRX1 (p = 1.7E-4). These results were confirmed by Western blotting, signaling pathway analysis, and hierarchical clustering of EAE risk groups. While validation in a larger cohort is underway, we conclude that M(2) proteomics is a rapid method to quantify putative prognostic/predictive protein biomarkers and therapeutic targets of disease progression in the EAE animal model of multiple sclerosis.

The proteolipid protein promoter drives expression outside of the oligodendrocyte lineage during embryonic and early postnatal development

The proteolipid protein (Plp) gene promoter is responsible for driving expression of one of the major components of myelin – PLP and its splice variant DM-20. Both products are classically thought to express predominantly in oligodendrocytes. However, accumulating evidence suggests Plp expression is more widespread than previously thought. In an attempt to create a mouse model for inducing oligodendrocyte-specific gene deletions, we have generated transgenic mice expressing a Cre recombinase cDNA under control of the mouse Plp promoter. We demonstrate Plp promoter driven Cre expression is restricted predominantly to mature oligodendrocytes of the central nervous system (CNS) at postnatal day 28. However, crosses into the Rosa26^LacZ and mT/mG reporter mouse lines reveal robust and widespread Cre activity in neuronal tissues at E15.5 and E10.5 that is not strictly oligodendrocyte lineage specific. By P28, all CNS tissues examined displayed high levels of reporter gene expression well outside of defined white matter zones. Importantly, our study reinforces the emerging idea that Plp promoter activity is not restricted to the myelinating cell lineage, but rather, has widespread activity both during embryonic and early postnatal development in the CNS. Specificity of the promoter to the oligodendrocyte cell lineage, as shown through the use of a tamoxifen inducible Plp-CreER^t line, occurs only at later postnatal stages. Understanding the temporal shift in Plp driven expression is of consequence when designing experimental models to study oligodendrocyte biology.

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.

Neuronal expression of the proteolipid protein gene in the medulla of the mouse

The proteolipid protein (PLP) gene (Plp) encodes the major myelin proteins, PLP and DM20. Expression of Plp occurs predominantly in oligodendrocytes, but evidence is accumulating that this gene is also expressed in neurons. In earlier studies, we demonstrated that myelin-deficient (MD) rats, which carry a mutation in the Plp gene, exhibit lethal hypoxic ventilatory depression. Furthermore, we found that, in the MD rat, PLP accumulated in neuronal cell bodies in the medulla oblongata. In the current study, we sought to determine which neurons expressed the Plp gene in the medulla oblongata and whether Plp gene expression changed in neurons with maturation. A transgenic mouse expressing the Plp promoter driving expression of enhanced green fluorescent protein (Plp-EGFP) was used to identify neurons expressing this gene. Plp expression in neurons was confirmed by immunostaining EGFP-positive cells for NeuN and by in situ hybridization for PLP mRNA. The numbers of neurons expressing Plp-EGFP and their distribution increased between P5 and P10 in the medulla. Immunostaining for surface receptors and classes of neurons expressing Plp-EGFP revealed that Plp gene expression in brainstem neurons was restricted to neurons expressing specific ligand-gated channels and biosynthetic enzymes, including glutamatergic NMDA receptors, GABA(A) receptors, and ChAT in defined areas of the medulla. Plp gene expression was rarely found in interneurons expressing GABA and was never found in AMPA receptor- or tyrosine hydroxylase-expressing neurons. Thus, Plp expression in the mouse caudal medulla was found to be developmentally regulated and restricted to specific groups of neurons.

Novel neuronal proteolipid protein isoforms encoded by the human myelin proteolipid protein 1 gene

The human myelin proteolipid protein 1 gene (hPLP1), which encodes the major structural myelin proteins of the central nervous system (CNS), is classically described as expressed in the oligodendrocytes, the CNS myelinating cells. We identified two new exons in the intron 1 of the hPLP1 gene that lead to the expression of additional mRNA and protein isoforms mainly expressed in neurons instead of oligodendrocytes. Those novel neuronal PLP isoforms are detected as soon as human fetal development and their concomitant expression is specific of the human species. As classical PLP proteins, the novel protein isoforms seem to be addressed to the plasma membrane. These results suggest for the first time that PLP may have functions in humans not only in oligodendrocytes but also in neurons and could be implicated in axono-glial communication. Moreover, this neuronal expression of the hPLP1 gene might explain the neuronal dysfunctions in patients carrying hPLP1 gene mutations.

Early postnatal proteolipid promoter-expressing progenitors produce multilineage cells in vivo

Proteolipid promoter (plp promoter) activity in the newborn mouse CNS is restricted to NG2-expressing oligodendroglial progenitor cells and oligodendrocytes. There are two populations of NG2 progenitors based on their plp promoter expression. Whereas the general population of NG2 progenitors has been shown to be multipotent in vitro and after transplantation, it is not known whether the subpopulation of plp promoter-expressing NG2 progenitors [i.e., plp promoter-expressing NG2 progenitors (PPEPs)] has the potential to generate multilineage cells during normal development in vivo. We addressed this issue by fate mapping Plp-Cre-ER(T2)/Rosa26-EYFP (PCE/R) double-transgenic mice, which carried an inducible Cre gene under the control of the plp promoter. Expression of the enhanced yellow fluorescent protein (EYFP) reporter gene in PPEPs was elicited by administering tamoxifen to postnatal day 7 PCE/R mice. We have demonstrated that early postnatal PPEPs, which had been thought to be restricted to the oligodendroglial lineage, also unexpectedly gave rise to a subset of immature, postmitotic, protoplasmic astrocytes in the gray matter of the spinal cord and ventral forebrain, but not in white matter. Furthermore, these PPEPs also gave rise to small numbers of immature, DCX (doublecortin)-negative neurons in the ventral forebrain, dorsal cerebral cortex, and hippocampus. EYFP-labeled representatives of each of these lineages survived to adulthood. These findings indicate that there are regional differences in the fates of neonatal PPEPs, which are multipotent in vivo, giving rise to oligodendrocytes, astrocytes, and neurons.

Early neuronal and glial fate restriction of embryonic neural stem cells

The question of how neurons and glial cells are generated during the development of the CNS has over time led to two alternative models: either neuroepithelial cells are capable of giving rise to neurons first and to glial cells at a later stage (switching model), or they are intrinsically committed to generate one or the other (segregating model). Using the developing diencephalon as a model and by selecting a subpopulation of ventricular cells, we analyzed both in vitro, using clonal analysis, and in vivo, using inducible Cre/loxP fate mapping, the fate of neuroepithelial and radial glial cells generated at different time points during embryonic development. We found that, during neurogenic periods [embryonic day 9.5 (E9.5) to 12.5], proteolipid protein (plp)-expressing cells were lineage-restricted neuronal precursors, but later in embryogenesis, during gliogenic periods (E13.5 to early postnatal), plp-expressing cells were lineage-restricted glial precursors. In addition, we show that glial cells forming at E13.5 arise from a new pool of neuroepithelial progenitors distinct from neuronal progenitors cells, which lends support to the segregating model.

Myelin proteolipid protein (PLP) as a marker antigen of central nervous system contaminations for routine food control

Spreading transmissible spongiform encephalopathies (TSE) have been widely attributed to transmission by ingestion of mammalian central nervous system (CNS) tissue. Reliable exclusion of this epidemiological important route of transmission relies on an effective surveillance of food contamination. Here, myelin proteolipid protein (PLP) is identified as a specific and largely heat-resistant marker for detection of food contaminations by CNS tissue. PLP is a component of oligodendritic glial sheaths of neuronal processes that is specifically expressed in the CNS. A highly selective polyclonal antibody was developed directed against an epitope present in the full-length PLP protein, but absent from the developmentally regulated splice variant DM-20. In combination with a hydrophobic extraction of PLP from tissue samples, the antibody reliably detected PLP from spinal cord, cerebellum, and cortex of different mammalian species. Consistent with earlier reports on PLP expression, no cross-reactivity was observed with peripheral nerve or extraneural tissue, except for a very faint signal obtained with heart. When applied to an artificial CNS contamination present in sausages, the antibody reliably detected a low concentration (1%) of the contaminant. Application of heat, as used during conventional sausage manufacturing, led to a predominant alteration of arginine residues in the PLP protein and a partial loss of immunoreactivity. In contrast, a stretch of hydrophilic amino acids(112-122) proved to be heat-resistant, preserving the immunogenicity of this PLP epitope during heating. Taken together, the excellent CNS specificity of PLP immunodetection and the presence of a heat-resistant epitope have permitted the development of a highly sensitive immunoassay for CNS contamination in routine food control.

Insulin-like growth factor type 1 receptor signaling in the cells of oligodendrocyte lineage is required for normal in vivo oligodendrocyte development and myelination

Insulin-like growth factor-I (IGF-I) has been shown to be a potent agent in promoting the growth and differentiation of oligodendrocyte precursors, and in stimulating myelination during development and following injury. To definitively determine whether IGF-I acts directly on the cells of oligodendrocyte lineage, we generated lines of mice in which the type 1 IGF receptor gene (igf1r) was conditionally ablated either in Olig1 or proteolipid protein expressing cells (termed IGF1R(pre-oligo-ko) and IGF1R(oligo-ko) mice, respectively). Compared with wild type mice, IGF1R(pre-oligo-ko) mice had a decreased volume (by 35-55%) and cell number (by 54-70%) in the corpus callosum (CC) and anterior commissure at 2 and 6 weeks of age, respectively. IGF1R(oligo-ko) mice by 25 weeks of age also showed reductions, albeit less marked, in CC volume and cell number. Unlike astrocytes, the percentage of NG2(+) oligodendrocyte precursors was decreased by approximately 13% in 2-week-old IGF1R(pre-oligo-ko) mice, while the percentage of CC1(+) mature oligodendrocytes was decreased by approximately 24% in 6-week-old IGF1R(pre-oligo-ko) mice and approximately 25% in 25-week-old IGF1R(oligo-ko) mice. The reduction in these cells is apparently a result of decreased proliferation and increased apoptosis. These results indicate that IGF-I directly affects oligodendrocytes and myelination in vivo via IGF1R, and that IGF1R signaling in the cells of oligodendrocyte lineage is required for normal oligodendrocyte development and myelination. These data also provide a fundamental basis for developing strategies with the potential to target IGF-IGF1R signaling pathways in oligodendrocyte lineage cells for the treatment of demyelinating disorders.

The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes

Substantial biological data indicate that the myelin basic protein (MBP) and myelin proteolipid protein (PLP/DM20) genes produce products with functions beyond that of serving as myelin structural proteins. Much of this evidence comes from studies on naturally-occurring and man-made mutations of these genes in mice and other species. This review focuses upon recent evidence showing the existence of other products of these genes that may account for some of these other functions, and recent studies providing evidence for alternative biological functions of PLP/DM20. The MBP and PLP/DM20 genes each encode the classic MBP and PLP isoforms, as well as a second family of proteins that are not involved in myelin structure. The biological roles of these other products of the genes are becoming clarified. The non-classic MBP gene products appear to be components of transcriptional complexes in the nucleus, and they also may be involved in signaling pathways in T-cells and in neural cells. The non-classic PLP/DM20 gene products appear to be components of intracellular transport vesicles in oligodendrocytes. There is evidence for other functions of the classic PLP/DM20 proteins, including a role in neural cell death mechanisms, autocrine and paracrine regulation of oligodendrocytes and neurons, intracellular transport and oligodendrocyte migration.

RNA metabolism and dysmyelination in schizophrenia

Decreased expression of a subset of oligodendrocyte and myelin-related genes is the most consistent finding among gene expression studies of postmortem brain tissue from subjects with schizophrenia (SCZ), although heritable variants have yet to be found that can explain the bulk of this data. However, expression of the glial gene Quaking (QKI), encoding an RNA binding (RBP) essential for myelination, was recently found to be decreased in SCZ brain. Both oligodendrocyte/myelin related genes, and other RBPs that are known or predicted to be targets of QKI, are also decreased in SCZ. Two different quaking mutant mice share some pathological features in common with SCZ, including decreased expression of myelin-related genes and dysmyelination, without gross destruction of white matter. Therefore, although these mice are not a model of SCZ per se, understanding the similarities and differences in gene expression between brains from these mice and subjects with SCZ could help parse out distinct genetic pathways underlying SCZ.

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.

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Myelination of a fetus with Pelizaeus-Merzbacher disease: immunopathological study

We report an autopsied case of a 21-gestational-week fetus with duplication of the proteolipid protein (PLP) gene (PLP1). An immunohistochemical study, which can detect the specific expression of PLP, myelin basic protein, myelin-associated glycoprotein, and platelet-derived growth factor receptor alpha subunit in brain tissues, showed that the myelination was almost the same as that of age-matched controls. This result suggests that the development and migration of the oligodendrocyte is normal in Pelizaeus-Merzbacher disease until midgestation. To our knowledge, this is the first report of the myelination of a fetus with duplication of the PLP1 gene.

Detection of oligodendrocytes in tissue sections using PCR synthesis of digoxigenin-labeled probes

Oligodendrocytes, the myelin-forming cells in the central nervous system, were visualized with excellent resolution at the light microscopic level using in situ hybridization (ISH). Digoxigenin (Dig)-tagged probes were synthesized and efficiently labeled by PCR. Specific probes to myelin genes were made by RT from brain total RNAs, followed by PCR with designed specific primers in the presence of Dig-11-dUTP. Probes specific to proteolipid protein (PLP), PLP and its isoform DM20 (PLP/DM20), and myelin oligodendrocyte glycoprotein (MOG) were synthesized and labeled. ISH was then applied on vibratomed tissue sections from mouse brains. Despite a low expression of MOG-specific and PLP-specific mRNAs in adult and newborn mouse brains, an oligodendrocyte population was detected. The specificity of Dig-labeled probes was confirmed with the double labeling of carbonic anhydrase II (CA II) and glial fibrillary acidic protein (GFAP) immunocytochemistry and ISH. This versatile and easy method for synthesis and labeling of specific probes to oligodendrocytes can be also applied to detect many other mRNAs in the nervous system and in other tissues.

Mouse NG2+ oligodendrocyte precursors express mRNA for proteolipid protein but not its DM-20 variant: a study of laser microdissection-captured NG2+ cells

Despite recent advances in our understanding of lineage of oligodendrocytes, detailed molecular characterization of this lineage in vivo is limited, primarily because of our inability to obtain a pure population of cells in situ. To define the molecular characteristics of oligodendrocyte lineage cells during development and their response to injury, we developed a strategy that uses laser capture microdissection (LCM) to isolate cells from sections and reverse transcription-PCR to determine mRNA expression. As a first step, we examined the expression of myelin-specific protein genes in NG2+ cells in cerebral cortex. We demonstrate that NG2+ cells in both developing and adult mice express NG2 mRNA but not mRNA for proteins specific for astrocytes, neurons, or microglia, indicating that a highly pure population of antigen-specific cells of the oligodendrocyte lineage can be obtained using LCM. Furthermore, we show that NG2+ cells express mRNAs for proteolipid protein (PLP), myelin basic protein, and 2',3'-cyclic nucleotide 3'-phosphodiesterase, but they dot not express DM-20 mRNA, a PLP mRNA splicing variant. Our data demonstrate that antigen-specific cells of oligodendrocyte lineage differentially express mRNA for myelin-specific proteins and their variants in vivo, partly define the gene expression in NG2+ cells, and raise questions about the cellular sites of DM-20 expression. This work also shows that LCM is a valuable tool to define and analyze gene expression in the cells of the oligodendrocyte lineage.

Proteolipid protein gene mutation induces altered ventilatory response to hypoxia in the myelin-deficient rat

Pelizaeus Merzbacher disease is an X-linked dysmyelinating disorder of the CNS, resulting from mutations in the proteolipid protein (PLP) gene. An animal model for this disorder, the myelin-deficient (MD) rat, carries a point mutation in the PLP gene and exhibits a phenotype similar to the fatal, connatal disease, including extensive dysmyelination, tremors, ataxia, and death at approximately postnatal day 21 (P21). We postulated that early death might result from disruption of myelinated neural pathways in the caudal brainstem and altered ventilatory response to oxygen deprivation or hypercapnic stimulus. Using barometric plethysmography to measure respiratory function, we found that the MD rat develops lethal hypoxic depression of breathing at P21, but hypercapnic ventilatory response is normal. Histologic examination of the caudal brainstem in the MD rat at this age showed extensive dysmyelination and downregulation of NMDA and to a lesser extent GABA(A) receptors on neurons in the nucleus tractus solitarius, hypoglossal nucleus, and dorsal motor nucleus of the vagus. Unexpectedly, immunoreactive PLP/DM20 was detected in neurons in the caudal brainstem. Not all biosynthetic functions and structural elements were altered in these neurons, because phosphorylated and nonphosphorylated neurofilament and choline acetyltransferase expression were comparable between MD and wild-type rats. These findings suggest that PLP is expressed in neurons in the developing brainstem and that PLP gene mutation can selectively disrupt central processing of afferent neural input from peripheral chemoreceptors, leaving the central chemosensory system for hypercapnia intact.

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.

Nuclear organization in differentiating oligodendrocytes

Many studies have suggested that the 3D organization of chromatin and proteins within the nucleus contributes to the regulation of gene expression. We tested multiple aspects of this nuclear organization model within a primary cell culture system. Oligodendrocyte lineage cells were examined to facilitate analysis of nuclear organization relative to a highly expressed tissue-specific gene, proteolipid protein (PLP), which exhibits transcriptional upregulation during differentiation from the immature progenitor stage to the mature oligodendrocyte stage. Oligodendrocyte lineage cells were isolated from brains of neonatal male rodents, and differentiation from oligodendrocyte progenitors to mature oligodendrocytes was controlled with culture conditions. Genomic in situ hybridization was used to detect the single copy of the X-linked PLP gene within each interphase nucleus. The PLP gene was not randomly distributed within the nucleus, but was consistently associated with the nuclear periphery in both progenitors and differentiated oligodendrocytes. PLP and a second simultaneously upregulated gene, the myelin basic protein (MBP) gene, were spatially separated in both progenitors and differentiated oligodendrocytes. Increased transcriptional activity of the PLP gene in differentiated oligodendrocytes corresponded with local accumulation of SC35 splicing factors. Differentiation did not alter the frequency of association of the PLP gene with domains of myelin transcription factor 1 (Myt1), which binds the PLP promoter. In addition to our specific findings related to the PLP gene, these data obtained from primary oligodendrocyte lineage cells support a nuclear organization model in which (1). nuclear proteins and genes can exhibit specific patterns of distribution within nuclei, and (2). activation of tissue-specific genes is associated with changes in local protein distribution rather than spatial clustering of coordinately regulated genes. This nuclear organization may be critical for complex nucleic-acid-protein interactions controlling normal cell development, and may be an important factor in aberrant regulation of cell differentiation and gene expression in transformed cells.

A PLP splicing abnormality is associated with an unusual presentation of PMD

We report that a deletion of 19 base pairs (bp) in intron 3 of the proteolipid protein (PLP/DM20) gene causes a neurological disease characterized by mild developmental delay, followed by progressive decline of acquired motor and cognitive milestones. The clinical features are associated with mild delay in myelination demonstrated by magnetic resonance imaging studies and with ongoing demyelination and axonal loss demonstrated by magnetic resonance spectroscopy. We demonstrate that the purine-rich 19bp element regulates PLP-specific splice site selection in transient transfections of chimeric constructs into cultured oligodendrocytes. Runs of 4 and 5 Gs centered in the 19bp element are critical for efficient PLP-specific splicing. The intronic element is sequence specific in oligodendrocytes and is not a repressor of PLP-specific splicing in nonglial cells. These data support the conclusion that deletion of the 19bp purine-rich region in PLP intron 3 causes a reduction in PLP message and protein, which affects myelin stability and axonal integrity.

Myelin proteolipid protein (Plp) intron 1 DNA is required to temporally regulate Plp gene expression in the brain

The myelin proteolipid protein (Plp) gene encodes the most abundant protein found in mature CNS myelin. Expression of the gene is regulated spatiotemporally, with maximal expression occurring in oligodendrocytes during the myelination period of CNS development. Plp gene expression is tightly controlled. Misregulation of the gene in humans can result in the dysmyelinating disorder Pelizaeus-Merzbacher disease, and in transgenic mice carrying a null mutation or extra copies of the gene can result in a variety of conditions, from late onset demyelination and axonopathy, to severe early onset dysmyelination. In this study we have examined the effects of Plp intron 1 DNA in mediating proper developmental expression of Plp-lacZ fusion genes in transgenic mice. Our results reveal the importance of Plp intron 1 sequences in instigating the expected surge in Plp-lacZ gene activity during (and following) the active myelination period of brain development. Transgene expression was also detected in the testis (Leydig cells), however, the presence or absence of Plp intron 1 sequences had no effect on the temporal profile in the testis. Surprisingly, expression of the transgene missing Plp intron 1 DNA was always higher in the testis, as compared to the brain, in all of the transgenic lines generated.

Characterization of mouse tGolgin-1 (golgin-245/trans-golgi p230/256 kD golgin) and its upregulation during oligodendrocyte development

As part of an effort to identify gene products that are differentially regulated during oligodendrocyte development, we isolated a mouse cDNA that encodes tGolgin-1, a homolog of the human protein known as golgin-245, trans-golgi p230, or 256 kD golgin. Human tGolgin-1 is the target of autoantibodies in patients with Sjögren's syndrome, and is thought to be involved in vesicular transport processes at the trans-Golgi network. Sequencing of cDNAs and EST clones comprising the full-length tGolgin-1 transcript predict marked homology with the amino- and carboxy-terminal regions of the human protein, but more limited homology within the central predicted coiled-coil region. Epitope tagged, truncated forms of mouse tGolgin-1, like those of its human homolog, were localized at steady state to the Golgi/trans-Golgi network in transfected cells. The tGolgin-1 message was expressed in all tissues examined, but was highly upregulated in oligodendrocyte precursors at a stage just prior to myelination. This expression pattern suggests that tGolgin-1 may play a role in specialized transport processes associated with maturation and/or differentiation of oligodendrocyte precursors.

The up-regulation of stromelysin-1 (MMP-3) in a spontaneously demyelinating transgenic mouse precedes onset of disease

The matrix metalloproteinases (MMPs) are a family of endoproteinases that degrade various components of the extracellular matrix and have been implicated in the pathogenesis of multiple sclerosis. To determine whether up-regulation of MMP-3, or stromelysin-1, was a causative factor during the development of demyelination, we have examined the expression of MMP-3 mRNA and protein in brain tissue of a spontaneously demyelinating mouse model overexpressing DM20 (ND4 line) prior to and during the progression of disease. Stromelysin-1, but not other MMP mRNA was elevated approximately 10-fold in transgenic mice between 5 days and 1 month of age, more than 2 months before the onset of disease, and was coordinately expressed with the DM20 transgene. Stromelysin-1 protein levels were also up-regulated as was tissue inhibitor of metalloproteinase-1 (TIMP-1), an in vivo regulator of stromelysin-1 mRNA. When we crossed our ND4 mice with a line of transgenic mice overexpressing TIMP-1 in brain, clinical signs in these mice were attenuated, and the level of stromelysin-1 protein was reduced. Thus, in this transgenic model of demyelinating disease up-regulation of DM20, MMP-3, and TIMP-1 represent important changes in the chemical pathogenesis in brain, which precede the onset of disease.

Proteolipid protein gene modulates viability and phenotype of neurons

Overexpression or lack of expression of proteolipid protein (PLP) gene by oligodendrocytes causes axonal pathology. It is unclear whether dysfunction of the PLP gene mediates its effects directly on neurons or indirectly by abnormal formation of myelin sheaths. We performed experiments using cocultures and conditioned media (CM) to test the direct effect of PLP gene expression on neurons. Non-glial cell lines were stably transfected with PLP or DM20 (an alternate splice variant of PLP) cDNAs. Immunocytochemistry and enhanced green fluorescent protein expression showed that translated products were synthesized and inserted into the plasma membrane in proper conformation. The number of surviving dorsal root ganglion (DRG) neurons was significantly less than controls when cocultured for 5 d with PLP-expressing cells. The number of degenerating neurons increased in a dose-dependent manner corresponding to increasing numbers of PLP-expressing cells. However, the number of surviving DRG neurons cocultured with DM20-expressing cells was comparable to that of controls, indicating that PLP-specific products contributed to decreased neuron survival. When DRG neurons were cultured with CM from PLP- or DM20-expressing cells, significantly fewer neurons survived with CM of PLP- but not DM20-expressing cells. This suggests that secreted factors from PLP-expressing cells contribute to neuronal death. Increased neuronal death found with PLP-expressing cells cannot be attributed to density-dependent artifacts, because in each experiment the density of different cell lines was similar. This effect of CM may be mediated by a negative pH shift elicited from PLP but not DM20 expression. These results indicate that PLP gene products directly modulate neuron viability.

Proteolipid promoter activity distinguishes two populations of NG2-positive cells throughout neonatal cortical development

Transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the mouse myelin proteolipid protein (PLP) gene promoter have been developed to investigate cells in the oligodendrocyte lineage. Transgene expression is consistent with the developmental expression of PLP, with cells at all stages of oligodendrocyte differentiation clearly visualized. These animals were analyzed to establish the time course of oligodendrocyte progenitor migration, proliferation, and differentiation in neonatal cortex. In these animals, two populations of NG2 proteoglycan-positive oligodendrocyte progenitor cells were identified that exist in postnatal subventricular zone and cortex. These two populations are distinguished by the presence or absence of PLP gene expression. Thus, PLP gene expression defines a subpopulation of NG2-positive cells from very early postnatal ages, which migrates toward the pial surface and proliferates in situ. EGFP(+)/NG2(+) cells are present in the cortex from postnatal day 1, and they remain in the cortex as undifferentiated oligodendrocyte progenitors for up to 3 weeks before myelination begins. These data could be explained by the presence of an important inhibitor of oligodendrocyte differentiation in the cortex during this period, which is downregulated in a region-specific manner to allow myelination. On the other hand, it is possible that oligodendrocyte progenitor cells remain undifferentiated in cortex until an essential signal is produced in situ to induce differentiation.

Differential sensitivity in the survival of oligodendrocyte cell lines to overexpression of myelin proteolipid protein gene products

The proteolipid (PLP) gene encodes at least four proteins, including the classic PLP and DM20, which are important components of the myelin sheath, and the recently identified soma-restricted (sr) isoforms, srPLP and srDM20. The classic PLP and DM20 gene products have been implicated in oligodendrocyte survival by overexpression studies in vitro and in vivo. The classic and sr proteolipids are targeted to different cellular compartments in the oligodendrocyte, suggesting different cellular functions. Accordingly, we examined the effects of in vitro overexpression of the sr-PLP/DM20 isoforms on the survival of stably transfected, conditionally immortalized, oligodendroglial cell lines and compared this to overexpression of the classic and the jimpy-mutated proteolipids. The results indicate that overexpression of either normal or jimpy classic PLP/DM20 resulted in a dramatic reduction in the survival of the oligodendrocyte cell lines at the nonpermissive temperature, but not the COS-7 cell line, a cell line expressing the same oncogene constitutively. Survival of the oligodendrocyte cell lines was significantly less affected when either the sr-PLP/DM20 or the dopamine D-2 receptor, another cell membrane protein, was overexpressed in the cell lines. These results suggest that overexpression of the "classic" PLP or DM20 can compromise the survival of oligodendrocytes whether or not they are mutated. Furthermore, they suggest that the internal mechanisms for normal targeting of the PLP/DM20 isoforms of either the "classic" or the "sr" types influence the oligodendrocyte's ability to survive when these proteolipids are overexpressed.

Mutant Plp/DM20 cannot be processed to secrete PLP-related oligodendrocyte differentiation/survival factor

Most of the mutations within the PLP gene result in degeneration of oligodendrocytes and this is believed to be caused by intracellular trafficking defects. Previous studies have demonstrated that cells expressing the wild type PLP gene release a factor promoting differentiation/survival of oligodendrocyte and that this factor is the C-terminal portion of the protein itself. In this study we asked how the naturally occurring mutations of the PLP gene (jimpy, jimpy msd, and rumpshaker) affect this activity. We developed a transient expression system for retroviral production and transduction that enabled the expression of mutant PLP/DM20 cDNAs in NIH3T3 cells. None of the NIH3T3 cells producing mutant PLP/DM20s secreted the PLP-related factor that increases the number of oligodendrocytes. Since it has been shown that rumpshaker DM20 can be transported to the cell surface, but its folding is incorrect, absence of secretion of this factor is more heavily attributable to incorrect protein folding than to the defect in the PLP/DM20 trafficking.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Proteolipid protein mRNA stability is regulated by axonal contact in the rodent peripheral nervous system

Proteolipid protein (PLP) and its alternatively spliced isoform, DM20, are the main intrinsic membrane proteins of compact myelin in the CNS. PLP and DM20 are also expressed by Schwann cells, the myelin-forming cells in the PNS, and are necessary for normal PNS function in humans. We have investigated the expression of PLP in the PNS by examining transgenic mice expressing a LacZ transgene under the control of the PLP promoter. In these animals, myelinating Schwann cells expressed beta-galactosidase more prominently than nonmyelinating Schwann cells. PLP/DM20 mRNA levels, but not those of LacZ mRNA, increased during sciatic nerve development and decreased after axotomy, with resultant Wallerian degeneration. PLP/DM20 transcription rates, in nuclear run off experiments, however, did not increase in developing rat sciatic nerve despite robust increases in PLP/DM20 mRNA levels during the same period. In RNAse protection studies, PLP mRNA levels fell to undetectable levels following nerve transection whereas levels of DM20 were essentially unchanged despite both being transcribed from the same promoter. Finally, cotransfection studies demonstrated that PLP-GFP, but not DM20-GFP mRNA is down-regulated in Schwann cells cultured in the absence of forskolin. Taken together these data demonstrate that steady state levels of PLP mRNA are regulated at a posttranscriptional level in Schwann cells, and that this regulation is mediated by Schwann cell-axonal contact. Since the difference between these two mRNAs is a 105-bp sequence in PLP and not in DM20, this sequence is likely to play a role in the regulation of PLP mRNA.

The evolution of lipophilin genes from invertebrates to tetrapods: DM-20 cannot replace proteolipid protein in CNS myelin

The proteolipid protein (PLP) gene encodes two myelin-specific protein isoforms, DM-20 and PLP, which are members of the highly conserved lipophilin family of transmembrane proteins. While the functions of this family are poorly understood, the fact that null mutations of the PLP gene cause leukodystrophy in man is testament to the importance of DM-20 and PLP in normal CNS function. PLP differs from DM-20 by the presence of a 35 amino acid domain exposed to the cytoplasm, which is not encoded by other lipophilin genes and appears to have arisen in amphibians approximately 300 million years before present. However, the lipophilin gene family can be traced back at least 550 million years and is represented in Drosophila and silkworms. Thus, from an evolutionary perspective PLP can reasonably be anticipated to perform functions in CNS myelin that cannot be accomplished by other lipophilins. Herein we use a novel knock-in strategy to generate mice expressing wild-type levels of a Plp gene that has been modified to encode only DM-20. Although DM-20 is incorporated into functional compact myelin sheaths in young animals, our data show that the 35 amino acid PLP-specific peptide is required to engender the normal myelin period and to confer long-term stability on this multilamellar membrane.

Absence of P0 leads to the dysregulation of myelin gene expression and myelin morphogenesis

P0, the major peripheral nervous system (PNS) myelin protein, is a member of the immunoglobulin supergene family of membrane proteins and can mediate homotypic adhesion. P0 is an essential structural component of PNS myelin; mice in which P0 expression has been eliminated by homologous recombination (P0-/-) develop a severe dysmyelinating neuropathy with predominantly uncompacted myelin. Although P0 is thought to play a role in myelin compaction by promoting adhesion between adjacent extracellular myelin wraps, as an adhesion molecule it could also have a regulatory function. Consistent with this hypothesis, Schwann cells in adult P0-/- mice display a novel molecular phenotype: PMP22 expression is down-regulated, MAG and PLP expression are up-regulated, and MBP expression is unchanged. As in quaking viable mutant mice (qk(v)), which have uncompacted myelin morphologically similar to that found in P0-/- mice, neither the qKI-6 or qKI-7 proteins are expressed in P0-/- peripheral nerve. In addition to these changes in gene expression in the P0 knockout, PLP/DM-20 accumulates in the endoplasmic reticulum of P0-/- Schwann cells, whereas MAG accumulates in redundant loops of uncompacted myelin, not at nodes of Ranvier or Schmidt-Lantermann incisures. Taken together, these results demonstrate that P0 is involved, either directly or indirectly, in the regulation of both myelin gene expression and myelin morphogenesis.

A proteolipid protein-specific pre-mRNA (Ppm-1) contains intron 3 and is up-regulated during myelination in the CNS

Alternative splicing of the precursor for messenger RNA (pre-mRNA) is a common process utilised by higher eukaryotes to modulate gene expression. A single primary transcript may generate several proteins with distinct functions, expressed in tissue-specific, developmental patterns. This article describes an oligodendrocyte-specific pre-mRNA product of proteolipid protein gene (P/p) transcription, which is the precursor for P/p but not Dm20 mRNA in the CNS. This P/p-specific pre-mRNA (Ppm-1) includes the intact intron 3 of the P/p gene. It is first expressed during active myelination, and it localises to the nucleus of oligodendrocytes, in both normal and jimpy (jp) murine CNS. In addition to mouse, Ppm-1 is found also in rat and dog, but not toad or trout. Our work suggests that alternative splicing of the P/p gene primary transcript follows a branching pattern, resulting in the presence of at least one P/p isoform-specific pre-mRNA molecule, Ppm-1. Therefore, Dm20 mRNA may be the product of a divergent set of pre-mRNA splicing events.

Aberrant dendrite growth in central nervous system white matter induced by a mutant proteolipid protein transgene

Transgenic mice were produced that carry a construct encoding a mutant form of the DM20 isoform of myelin proteolipid protein. The transgene is under the direction of the human Plp gene promoter, which has previously been shown to direct tissue-specific expression of transgenes. Two lines of mice were generated with this construct, both of which express the transgene at extremely low levels. Central nervous system myelination proceeds normally in the transgenic mice. However, in aged transgenic mice, areas of dendrite processes synapsed with axonal termini were observed within the white matter of the spinal cord. This phenotype was accompanied by focal areas of astrocytic hypertrophy and an increase in apoptotic cell death in white matter but not gray matter. One interpretation of these findings is that expression of the mutant DM20 alters signaling between oligodendrocytes and neurons, producing abnormal neurite outgrowth.

Early specification of oligodendrocytes in the chick embryonic brain

Oligodendrocytes are the myelin-forming cells in the central nervous system of vertebrates. In the rodent embryo, these cells have been shown to emerge from restricted territories of the neuroepithelium. However, a comprehensive view of the development of oligodendroglial populations from their ventricular sources remains to be established. As a first step toward this aim, we have examined in vivo the spatiotemporal emergence of oligodendrocytes in the chick embryonic brain. We have detailed the patterns of expression of three early markers of the oligodendroglial lineage: the plp/dm-20 and PDGFRalpha transcripts and the O4-reactive antigen. During embryonic development, these molecules showed a similar segmental pattern of expression. However, plp/dm-20(+) cells were already observed, in the ventricular layer, at E2.5, i.e., 2 days before the appearance of O4(+) and PDGFRalpha(+) cells, suggesting that oligodendrocyte precursors arise nearly simultaneously with neurons. In the chick embryonic brain, the onset of expression of plp/dm-20 appears therefore to be the earliest event indicative of oligodendroglial specification and we propose, based on the expression of plp/dm-20 transcript, a ventricular map of the foci at which oligodendrocytes originate. In addition, we document the precocious segregation, from E5, of plp/dm-20(+) and PDGFRalpha(+) oligodendroglial cells in the subventricular and mantle layers of the brain.

Abnormal Ca(2+) regulation in oligodendrocytes from the dysmyelinating jimpy mouse

Jimpy (jp) is a point mutation in the gene on the X chromosome which codes for the major myelin proteolipid protein. Most oligodendrocytes (OLs) in the jp mouse undergo cell death at the time when they should be actively myelinating. Loss of mature OLs results in severe CNS dysmyelination. Dying jp OLs have the morphology of apoptotic cells but it is not clear how the mutation activates biochemical pathways which lead to programmed death of OLs in jp CNS. There is compelling evidence from a number of systems that high levels of intracellular Ca(2+) ([Ca2+]i) can activate downstream processes which result in both apoptotic and necrotic cell death. To determine whether [Ca2+](i) dysregulation might be involved in the death of jp OLs, we used ratiometric imaging to determine levels of [Ca2+](i) in OLs cultured from jp and normal CNS and in immortalized cell lines derived from jp and normal OLs. Immortalized jp OLs and OLs isolated directly from jp brain both showed a similar elevation in [Ca2+](i) ranging from 60% to 150% over control values. A higher baseline [Ca2+](i) in jp OLs might increase their vulnerability to other insults due to abnormal protein processing or changes in signaling pathways which act as a final trigger for cell death.

Identification of a new exon in the myelin proteolipid protein gene encoding novel protein isoforms that are restricted to the somata of oligodendrocytes and neurons

The myelin proteolipid protein (PLP) gene (i.e., the PLP/DM20 gene) has been of some interest because of its role in certain human demyelinating diseases, such as Pelizaeus-Merzbacher disease. A substantial amount of evidence, including neuronal pathology in knock-out and transgenic animals, suggests the gene also has functions unrelated to myelin structure, but the products of the gene responsible for these putative functions have not yet been identified. Here we report the identification of a new exon of the PLP/DM20 gene and at least two new products of the gene that contain this exon. The new exon, located between exons 1 and 2, is spliced into PLP and DM20 mRNAs creating a new translation initiation site that generates PLP and DM20 proteins with a 12 amino acid leader sequence. This leader sequence appears to target these proteins to a different cellular compartment within the cell bodies of oligodendrocytes and away from the myelin membranes. Furthermore, these new products are also expressed in a number of neuronal populations within the postnatal mouse brain, including the cerebellum, hippocampus, and olfactory system. We term these products somal-restricted PLP and DM20 proteins to distinguish them from the classic PLP and DM20 proteolipids. They represent putative candidates for some of the nonmyelin-related functions of the PLP/DM20 gene.

Myelination in the absence of galactolipids and proteolipid proteins

The galactolipids galactocerebroside and sulfatide and the proteolipid protein (PLP) and its splice variant DM20 are the most abundant lipid and protein components of central nervous system myelin. Recent studies have found that mice lacking either the galactolipids or PLP are able to form myelin sheaths with apparently normal periodicity and near normal compaction. Here, we have generated galactolipid/proteolipid double mutants to examine the possibility that these molecules have overlapping functions. We show that the absence of the galactolipids and PLP has pleotropic effects on myelin formation. While oligodendrocytes in the postnatal day 20 galactolipid/proteolipid-deficient mouse are able to elaborate myelin with close to normal intraperiod lines, there is an increased frequency of uncompacted myelin sheaths as well as unmyelinated axons. Moreover, the double mutants display extensive white matter vacuolization of the cerebellum that initiates around postnatal day 16, which correlates with the onset of a severe ataxic phenotype and an increased percentage of apoptotic nuclei in the cerebellar internal granule cell layer. These data indicate that the galactolipids and PLP/DM20 are not required for intraperiod line formation, but they suggest a role for these molecules in mediating myelin compaction and in maintaining the integrity of the cerebellum.