Observations on the structure of myelin lacking the major proteolipid protein

Neural stem cells restore myelin in a demyelinating model of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a fatal X-linked leukodystrophy caused by mutations in the PLP1 gene, which is expressed in the CNS by oligodendrocytes. Disease onset, symptoms and mortality span a broad spectrum depending on the nature of the mutation and thus the degree of CNS hypomyelination. In the absence of an effective treatment, direct cell transplantation into the CNS to restore myelin has been tested in animal models of severe forms of the disease with failure of developmental myelination, and more recently, in severely affected patients with early disease onset due to point mutations in the PLP1 gene, and absence of myelin by MRI. In patients with a PLP1 duplication mutation, the most common cause of Pelizaeus-Merzbacher disease, the pathology is poorly defined because of a paucity of autopsy material. To address this, we examined two elderly patients with duplication of PLP1 in whom the overall syndrome, including end-stage pathology, indicated a complex disease involving dysmyelination, demyelination and axonal degeneration. Using the corresponding Plp1 transgenic mouse model, we then tested the capacity of transplanted neural stem cells to restore myelin in the context of PLP overexpression. Although developmental myelination and axonal coverage by endogenous oligodendrocytes was extensive, as assessed using electron microscopy (n = 3 at each of four end points) and immunostaining (n = 3 at each of four end points), wild-type neural precursors, transplanted into the brains of the newborn mutants, were able to effectively compete and replace the defective myelin (n = 2 at each of four end points). These data demonstrate the potential of neural stem cell therapies to restore normal myelination and protect axons in patients with PLP1 gene duplication mutation and further, provide proof of principle for the benefits of stem cell transplantation for other fatal leukodystrophies with 'normal' developmental myelination.

Transmission Electron Microscopy and Morphometry of the CNS White Matter

Transmission electron microscopy of central nervous system white matter has provided unparalleled access to the ultrastructural features of axons, their myelin sheaths, and the major cells of white matter; namely, oligodendrocytes, oligodendrocyte precursors, astrocytes, and microglia. In particular, it has been invaluable in elucidating pathological changes in axons and myelin following experimentally induced injury or genetic alteration, in animal models. While also of value in the examination of human white matter, the tissue is rarely fixed adequately for the types of detailed analyses that can be performed on well-preserved samples from animal models, perfusion fixed at the time of death. In this chapter we describe methods for obtaining, processing, and visualizing white matter samples using transmission electron microscopy of perfusion fixed tissue and for unbiased morphometry of white matter, with particular emphasis on axon and myelin pathology. Several advanced electron microscopy techniques are now available, but this method remains the most expedient and accessible for routine ultrastructural examination and morphometry.

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

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

Characterization of a PLP-overexpressing transgenic rat, a model for the connatal form of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) most frequently results from duplication of the Plp1 gene with a correlation between disease severity and increasing copy number of the gene. Animal models of PMD, in particular those overexpressing the Plp1 gene, have been sought in attempts to provide systems in which potential therapies can be tested. Here we describe a rat model of the severe connatal form of PMD and provide a detailed characterization of its pathology and molecular biology, prior to testing therapeutic approaches. We determined the exact copy number of Plp1, and the resulting effects on RNA and protein expression. Distinct differences in myelin and disparate distributions of myelin protein markers in comparison to wild-type controls were observed. Altered expression of Plp1 also caused an increase in the apoptotic cell death of oligodendrocytes. These results provide the platform from which to test the effectiveness of in vivo therapies.

PLP/DM20 expression and turnover in a transgenic mouse model of Pelizaeus-Merzbacher disease

The most common cause of Pelizaeus-Merzbacher (PMD) is due to duplication of the PLP1 gene but it is unclear how increased gene dosage affects PLP turnover and causes dysmyelination. We have studied the dynamics of PLP/DM20 in a transgenic mouse model of PMD with increased gene dosage of the proteolipid protein gene (Plp1). The turnover of PLP/DM20 were investigated using an ex-vivo brain slice system and cultured oligodendrocytes. Homozygous mice have reduced PLP translation, markedly enhanced PLP degradation, and markedly reduced incorporation of PLP into myelin. Proteasome inhibition (MG132) prevented the enhanced degradation. Numerous autophagic vesicles are present in homozygous transgenic mice that may influence protein dynamics. Surprisingly, promoting autophagy with rapamycin decreases the degradation of nascent PLP suggesting autophagic vacuoles serve as a cellular storage compartment. We suggest that there are multiple subcellular fates of PLP/DM20 when overexpressed: the vast majority being degraded by the proteasome, a proportion sequestered into autophagic vacuoles, probably fused with endolysosomes, and only a small proportion entering the myelin sheath, where its association with lipid rafts is perturbed. Transgenic oligodendrocytes have fewer membrane sheets and this phenotype is improved with siRNA-mediated knockdown of PLP expression that promotes the formation of MBP+ myelin-like sheets. This finding suggests that RNAi technology is in principle applicable to improve CNS myelination when compromised by PLP/DM20 overexpression.

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.

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

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

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.

Structure and Stability of Internodal Myelin in Mouse Models of Hereditary Neuropathy

Peripheral neuropathies often result in abnormalities in the structure of internodal myelin, including changes in period and membrane packing, as observed by electron microscopy (EM). Mutations in the gene that encodes the major adhesive structural protein of internodal myelin in the peripheral nervous system of humans and mice--P0 glycoprotein--correlate with these defects. The mechanisms by which P0 mutations interfere with myelin packing and stability are not well understood and cannot be provided by EM studies that give static and qualitative information on fixed material. To gain insights into the pathogenesis of mutant P0, we used x-ray diffraction, which can detect more subtle and dynamic changes in native myelin, to investigate myelin structure in sciatic nerves from murine models of hereditary neuropathies. We used mice with disruption of one or both copies of the P0 gene (models of Charcot-Marie-Tooth-like neuropathy [CMT1B] or Dejerine-Sottas-like neuropathy) and mice with a CMT1B resulting from a transgene encoding P0 with an amino terminal myc-tag. To directly test the structural role of P0, we also examined a mouse that expresses P0 instead of proteolipid protein in central nervous system myelin. To link our findings on unfixed nerves with EM results, we analyzed x-ray patterns from unembedded, aldehyde-fixed nerves and from plastic-embedded nerves. From the x-ray patterns recorded from whole nerves, we assessed the amount of myelin and its quality (i.e. relative thickness and regularity). Among sciatic nerves having different levels of P0, we found that unfixed nerves and, to a lesser extent, fixed but unembedded nerves gave diffraction patterns of sufficient quality to distinguish periods, sometimes differing by a few Angstroms. Certain packing abnormalities were preserved qualitatively by aldehyde fixation, and the relative amount and structural integrity of myelin among nerves could be distinguished. Measurements from the same nerve over time showed that the amount of P0 affected myelin's stability against swelling, thus directly supporting the hypothesis that packing defects underlie instability in "live" or intact myelin. Our findings demonstrate that diffraction can provide a quantitative basis for understanding, at a molecular level, the membrane packing defects that occur in internodal myelin in demyelinating peripheral neuropathies.

Reversal of non-hydroxy : α-hydroxy galactosylceramide ratio and unstable myelin in transgenic mice overexpressing UDP-galactose : ceramide galactosyltransferase

The sphingolipids galactosylceramide and sulfatide are important for the formation and maintenance of myelin. Transgenic mice overexpressing the galactosylceramide synthesizing enzyme UDP-galactose:ceramide galactosyltransferase in oligodendrocytes display an up to four-fold increase in UDP-galactose:ceramide galactosyltransferase activity, which correlates with an increase in its products monogalactosyl diglyceride and non-hydroxy fatty acid-containing galactosylceramide. Surprisingly, however, we observed a concomitant decrease in alpha-hydroxylated galactosylceramide such that total galactosylceramide in transgenic mice was almost unaltered. These data suggest that UDP-galactose:ceramide galactosyltransferase activity does not limit total galactosylceramide level. Furthermore, the predominance of alpha-hydroxylated galactosylceramide appeared to be determined by the extent to which non-hydroxylated ceramide was galactosylated rather than by the higher affinity of UDP-galactose:ceramide galactosyltransferase for alpha-hydroxy fatty acid ceramide. The protein composition of myelin was unchanged with the exception of significant up-regulation of the myelin and lymphocyte protein. Transgenic mice were able to form myelin, which, however, was apparently unstable and uncompacted. These mice developed a progressive hindlimb paralysis and demyelination in the CNS, demonstrating that tight control of UDP-galactose:ceramide galactosyltransferase expression is essential for myelin maintenance.

Oligodendroglial modulation of fast axonal transport in a mouse model of hereditary spastic paraplegia

Oligodendrocytes are critical for the development of the plasma membrane and cytoskeleton of the axon. In this paper, we show that fast axonal transport is also dependent on the oligodendrocyte. Using a mouse model of hereditary spastic paraplegia type 2 due to a null mutation of the myelin Plp gene, we find a progressive impairment in fast retrograde and anterograde transport. Increased levels of retrograde motor protein subunits are associated with accumulation of membranous organelles distal to nodal complexes. Using cell transplantation, we show categorically that the axonal phenotype is related to the presence of the overlying Plp null myelin. Our data demonstrate a novel role for oligodendrocytes in the local regulation of axonal function and have implications for the axonal loss associated with secondary progressive multiple sclerosis.

Insertion of mutant proteolipid protein results in missorting of myelin proteins

Two brothers with a leukodystrophy, progressive spastic diplegia, and peripheral neuropathy were found to have proteinaceous aggregates in the peripheral nerve myelin sheath. The patients' mother had only subclinical peripheral neuropathy, but the maternal grandmother had adult-onset leukodystrophy. Sequencing of the proteolipid protein (PLP) gene showed a point mutation IVS4 + 1 G-->A within the donor splice site of intron 4. We identified one transcript with a deletion of exon 4 (Deltaex4, 169bp) encoding for PLP and DM20 proteins and lacking two transmembrane domains, and a second transcript with exon 4 + 10bp encoding three transmembrane domains. Immunohistochemistry showed abnormal aggregation in the myelin sheath of MBP and P0. Myelin-associated glycoprotein was present in the Schmidt-Lanterman clefts but significantly reduced in the periaxonal region. Using immunogold electron microscopy, we demonstrated the presence of mutated PLP/DM20 and the absence of the intact protein in the patient peripheral myelin sheath. We conclude that insertion of mutant PLP/DM20 with resulting aberrant distribution of other myelin proteins in peripheral nerve may constitute an important mechanism of dysmyelination in disorders associated with PLP mutations.

Pelizaeus-Merzbacher disease and spastic paraplegia type 2: two faces of myelin loss from mutations in the same gene

Pelizaeus-Merzbacher disease and X-linked spastic paraplegia type 2 are two sides of the same coin. Both arise from mutations in the gene encoding myelin proteolipid protein. The disease spectrum for Pelizaeus-Merzbacher disease and spastic paraplegia type 2 is extraordinarily broad, ranging from a spastic gait in the pure form of spastic paraplegia type 2 to a severely disabling form of Pelizaeus-Merzbacher disease featuring hypotonia, respiratory distress, stridor, nystagmus, and profound myelin loss. The diverse disease spectrum is mirrored by the underlying pathogenesis, in which a blockade at any stage of myelin proteolipid protein synthesis and assembly into myelin spawns a unique phenotype. The continuing definition of pathogenetic mechanisms operative in Pelizaeus-Merzbacher disease and spastic paraplegia type 2, together with advances in neural cell transplant therapy, augurs well for future treatment of the severe forms of Pelizaeus-Merzbacher disease.

Genetic background determines phenotypic severity of the Plp rumpshaker mutation

The rumpshaker mutation of the proteolipid protein (Plp) gene causes dysmyelination in man and mouse. We show that the phenotype in the mouse depends critically on the genetic background in which the mutation is expressed. On the C3H background there is normal longevity whereas changing to a C57BL/6 strain results in seizures and death at around postnatal day 30. The more severe phenotype is associated with less myelin and reduced levels of major myelin proteins. There are also more apoptotic cells, including oligodendrocytes, increased numbers of proliferating cells, increased numbers of NG2+ oligodendrocyte progenitors and increased microglia compared to the milder phenotype. The number of mature oligodendrocytes is similar to wild-type in both strains of mutant, however, suggesting that increased oligodendrocyte death is matched by increased generation from progenitors. The dichotomy of phenotype probably reflects the influence of modifying loci. The localization of these putative modifying genes and their mode of action remain to be determined.

Normal metabolism but different physical properties of myelin from mice deficient in proteolipid protein

Proteolipid protein (PLP) is the primary protein component of CNS myelin, yet myelin from the PLP(null) mouse has only minor ultrastructural abnormalities. Might compensation for a potentially unstable structure involve increased myelin synthesis and turnover? This was not the case; neither accumulation nor in vivo synthesis rates for the myelin-specific lipid cerebroside was altered in PLP(null) mice relative to wild-type (wt) animals. However, the yield of myelin from PLP(null) mice, assayed as levels of cerebroside, was only about 55% of wt control levels. Loss of myelin occurred during initial centrifugation of brain homogenate at 20,000g for 20 min, which is sufficient to sediment almost all myelin from wt mice. Cerebroside-containing fragments from PLP(null) mice remaining in the supernatant could be sedimented by more stringent centrifugation, 100,000g for 60 min. Both the rapidly and the more slowly sedimenting cerebroside-containing membranes banded at the 0.85/0.32 M sucrose interface of a density gradient, as did myelin from wt mice. These results suggest at least some myelin from PLP(null) mice differs from wt myelin with respect to physical stability (fragmented into smaller particles during dispersion) and/or density. Alternatively, slowly sedimenting cerebroside-containing particles could be myelin precursor membranes that, lacking PLP, were retarded in their processing toward mature myelin and thus differ from mature myelin in physical properties. If this is so, recently synthesized cerebroside should be preferentially found in these "slower-sedimenting" myelin precursor fragments. Metabolic tracer experiments showed this was not the case. We conclude that PLP(null) myelin is physically less stable and/or less dense than wt myelin.