Developmental changes of oligodendroglia in the posterior funiculus of "Shiverer" mutant mouse spinal cord, with special reference to myelin formation

Rapid and efficient generation of oligodendrocytes from human induced pluripotent stem cells using transcription factors

Rapid and efficient protocols to generate oligodendrocytes (OL) from human induced pluripotent stem cells (iPSC) are currently lacking, but may be a key technology to understand the biology of myelin diseases and to develop treatments for such disorders. Here, we demonstrate that the induction of three transcription factors (SOX10, OLIG2, NKX6.2) in iPSC-derived neural progenitor cells is sufficient to rapidly generate O4⁺ OL with an efficiency of up to 70% in 28 d and a global gene-expression profile comparable to primary human OL. We further demonstrate that iPSC-derived OL disperse and myelinate the CNS of Mbp^shi/shiRag^-/- mice during development and after demyelination, are suitable for in vitro myelination assays, disease modeling, and screening of pharmacological compounds potentially promoting oligodendroglial differentiation. Thus, the strategy presented here to generate OL from iPSC may facilitate the studying of human myelin diseases and the development of high-throughput screening platforms for drug discovery.

Inherited and acquired disorders of myelin: The underlying myelin pathology

Remyelination is a major therapeutic goal in human myelin disorders, serving to restore function to demyelinated axons and providing neuroprotection. The target disorders that might be amenable to the promotion of this repair process are diverse and increasing in number. They range primarily from those of genetic, inflammatory to toxic origin. In order to apply remyelinating strategies to these disorders, it is essential to know whether the myelin damage results from a primary attack on myelin or the oligodendrocyte or both, and whether indeed these lead to myelin breakdown and demyelination. In some disorders, myelin sheath abnormalities are prominent but demyelination does not occur. This review explores the range of human and animal disorders where myelin pathology exists and focusses on defining the myelin changes in each and their cause, to help define whether they are targets for myelin repair therapy.

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We reviewed myelin disorders of the CNS in humans and animals.

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Myelin damage results from primary attack on the oligodendrocyte or myelin sheath.

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All major categories of disease can affect CNS myelin.

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Myelin vacuolation is common, yet does not always result in demyelination.

Autophagy promotes oligodendrocyte survival and function following dysmyelination in a long-lived myelin mutant

The Long-Evans shaker (les) rat has a mutation in myelin basic protein that results in severe CNS dysmyelination and subsequent demyelination during development. During this time, les oligodendrocytes accumulate cytoplasmic vesicles, including lysosomes and membrane-bound organelles. However, the mechanism and functional relevance behind these oligodendrocyte abnormalities in les have not been investigated. Using high-magnification electron microscopy, we identified the accumulations in les oligodendrocytes as early and late autophagosomes. Additionally, immunohistochemistry and Western blots showed an increase in autophagy markers in les. However, autophagy did not precede the death of les oligodendrocytes. Instead, upregulating autophagy promoted membrane extensions in les oligodendrocytes in vitro. Furthermore, upregulating autophagy in les rats via intermittent fasting increased the proportion of myelinated axons as well as myelin sheath thickness in les and control rats. Overall, this study provides insight into the abnormalities described in les as well as identifying a novel mechanism that promotes the survival and function of oligodendrocytes.

Myelin loss does not lead to axonal degeneration in a long-lived model of chronic demyelination

Current dogma suggests that chronically demyelinated axons are at risk for degeneration, with axonal loss resulting in permanent disability in myelin disease. However, the trophic role of the myelin sheath in long-term axonal survival is incompletely understood. Previous observations of the effect of dysmyelination or demyelination on axonal survival in the myelin mutants has been limited because of their short life span. In this study, we used the Long-Evans shaker (les) rat, which can live up to 9 months, to study axonal health and survival after chronic demyelination. At 2 weeks, ∼29% of medium and ∼47% of large fiber axons are myelinated in les spinal cord. However, by 3 months, no medium and ∼<1% of large-diameter axons retain myelin. After demyelination, axons have a reduced-caliber, abnormal neurofilament distribution and an increase in mitochondrial number. However, there are no signs of axonal degeneration in les rats up to 9 months. Instead, there is a profound increase in oligodendrocytes, which were found to express BDNF, NT-3, and IGF-1. Importantly, this study provides in vivo evidence that mature glial cells produce various neurotrophic factors that may aid in the survival of axons after chronic demyelination.

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

Human neural stem cells differentiate and promote locomotor recovery in spinal cord-injured mice

We report that prospectively isolated, human CNS stem cells grown as neurospheres (hCNS-SCns) survive, migrate, and express differentiation markers for neurons and oligodendrocytes after long-term engraftment in spinal cord-injured NOD-scid mice. hCNS-SCns engraftment was associated with locomotor recovery, an observation that was abolished by selective ablation of engrafted cells by diphtheria toxin. Remyelination by hCNS-SCns was found in both the spinal cord injury NOD-scid model and myelin-deficient shiverer mice. Moreover, electron microscopic evidence consistent with synapse formation between hCNS-SCns and mouse host neurons was observed. Glial fibrillary acidic protein-positive astrocytic differentiation was rare, and hCNS-SCns did not appear to contribute to the scar. These data suggest that hCNS-SCns may possess therapeutic potential for CNS injury and disease.

Insertion of a retrotransposon in Mbp disrupts mRNA splicing and myelination in a new mutant rat

Our understanding of myelination has been greatly enhanced via the study of spontaneous mutants that harbor a defect in a gene encoding one of the major myelin proteins (myelin mutants). In this study, we describe a unique genetic defect in a new myelin mutant called the Long Evans shaker (les) rat that causes severe dysmyelination of the CNS. Myelin deficits result from disruption of the myelin basic protein (Mbp) gene caused by the insertion of an endogenous retrotransposon [early transposons (ETn) element] into a noncoding region (intron 3) of the gene. The ETn element alters the normal splicing dynamics of MBP mRNA, leading to a dramatic reduction in the levels of full-length isoforms (<5% of normal) and the appearance of improperly spliced, chimeric transcripts. Although these aberrant transcripts contain proximal coding regions of the MBP gene (exons 1-3), they are unable to encode functional proteins required to maintain the structural integrity of the myelin sheath. These chimeric transcripts seem capable, however, of producing the necessary signal to initiate and coordinate myelin gene expression because normal numbers of mature oligodendrocytes synthesizing abundant levels of other myelin proteins are present in the mutant CNS. The les rat is thus an excellent model to study alternative functions of MBP beyond its well characterized role in myelin compaction.

Schwann cell invasion of the central nervous system of the myelin mutants

Schwann cells are excluded from the CNS during development by the glial limiting membrane, an area of astrocytic specialisation present at the nerve root transitional zone, and at blood vessels in the neuropil. This barrier, however, can be disrupted and, with the highly migratory nature of Schwann cells, can result in their invasion and myelination of the CNS in many pathological situations. In this paper we demonstrate that this occurs in a number of myelin mutants, including the myelin deficient (md) and taiep rats and the canine shaking (sh) pup. While it is still relatively uncommon in the rodent mutants, the sh pup shows extensive Schwann cell invasion along the neuraxis. This invasion involves the spinal cord, brain stem, and cerebellum and increases in amount and distribution with age. In situ hybridisation studies using a Pzero riboprobe suggest that the likely origin of these cells in the sh pup is the nerve roots, primarily the dorsal roots. Paradoxically, Schwann cell myelination of the CNS increases with time in the sh pup despite a marked, progressive gliosis involving the glia limitans and neuropil. Thus the mechanism by which these cells migrate into the CNS through the gliosed nerve root transitional zone or from vasa nervorum remains unknown. Extensive Schwann cell CNS myelination may have therapeutic significance in human myelin disease.

Cell death of oligodendrocytes or demyelination induced by overexpression of proteolipid protein depending on expressed gene dosage

The transgenic mice, which were produced by introducing the wild type proteolipid protein (PLP) gene, revealed different neurological symptoms depending on expressed gene dosage. Homozygotes, which bore four more copies of the extra PLP gene, died before 4 weeks old with severe tremors and convulsions, while histologically hypomyelination and death of the oligodendrocytes were particularly noticeable. Heterozygotes, which bore two more copies and survived nearly 1 year, suffered from severe hindlimb palsy and bladder-rectal disturbances before death. These symptoms probably resulted from striking demyelination which occurred abruptly at a later stage of life. The heterozygous PLP-transgenic mice are thus available for a model of demyelination diseases.

Transferrin in the central nervous system of the shiverer mouse myelin mutant

Transferrin, the iron mobilization protein, and its mRNA are normally present in oligodendrocytes. Previous reports using myelin mutants have shown both a decrease in transferrin protein and mRNA when the oligodendrocyte population is compromised. In this study the shiverer mouse mutant in which the oligodendrocyte population is numerically normal, but has both quantitatively diminished and qualitatively abnormal myelin was used. This animal model was chosen to address the question whether expression of the transferrin message and/or protein correlated more closely to the number of oligodendrocytes (normal) or the amount of myelin (abnormally low). A 1/2 to 2/3 decrease in transferrin protein occurred in all brain regions examined except for the spinal cord in the shiverer group compared to both heterozygous littermates and wild type controls. Levels of transferrin transcripts in the brain are not affected by the shiverer mutation. These results taken with previous reports from this laboratory indicate that the presence of oligodendrocytes is a requirement for normal expression of transferrin mRNA in brain but is not sufficient for normal values of the protein. The level of Tf protein correlates more closely with the amount of myelin present than it does with the numbers of oligodendrocytes present. These data are consistent with previous reports from our laboratory that transferrin accumulation by oligodendrocytes is associated with myelin production by these cells. These data further suggest transferrin mRNA may be constitutively expressed by oligodendrocytes and that the protein expression is regulated at the level of translation.

An ultrastructural study of oligodendrocytes in zitter rat: a new animal model for hypomyelination in the CNS

The ultrastructural alterations in oligodendrocytes in the zitter rat--a newly established animal model of CNS hypomyelination--were studied in animals between 3 and 37 weeks of age. Throughout the observation period, the most characteristic alteration was the presence of abnormal structures associated with nuclear membranes and resembling split myelin lamellae. The structures consisted of stacks of membranes continuous with the inner and outer membranes of the nuclear envelope or with plasma membranes. This abnormality was prominent at 3 weeks of age, when initial myelination was probably still occurring. It was decreased transiently at 12 weeks of age and then again increased slightly with advancing age. However, the mean incidence of this abnormality was approximately 5% of the oligodendrocyte nuclei in randomly examined electron micrographs. As the small membranous abnormalities were observed in a few serial sections, the actual incidence is believed to be several times higher than 5%. Such membranous abnormalities were also detected in some mitochondria, in some cisternae of rough endoplasmic reticulum, and in some lysosomes, all at lower incidence rates. Some aberrant or elongated myelin sheath formations, an increased number of radial components and intracytoplasmic inclusions in oligodendrocytes were also observed. These findings suggest that functional abnormalities in membrane biosynthesis including the myelin sheaths and the nuclear membranes in oligodendrocytes are closely related to the hypomyelination in the CNS and may be related to the genetic abnormality in the zitter rat.

Chimeric and molecular genetic analysis of myelin-deficient (shiverer and mld) mutant mice

The shiverer and myelin-deficient (mld) mutants are two allelic mutations. Both are characterized by hypomyelination in the CNS and deficient expression of the MBP gene. Chimeric analysis of the pathogenesis of shiverer showed that shiverer mutation acts intrinsic to the oligodendrocyte, which is the only cell type expressing the MBP gene in the CNS. Molecular genetic studies by several groups demonstrated that shiverer is a deletion mutation in the MBP gene. Consequently, no MBP is produced in the shiverer mutant. In mld, however, partial expression of the MBP gene was observed. Interestingly, MBP was expressed in a mosaic fashion in the CNS of mld mice. Molecular genetic studies revealed that the mld mutant has duplicated MBP genes in tandem on a distal part (E2-4) of chromosome 18q. The reduced expression was based on the level of mRNA. A large portion is inverted in the upstream copy, and an intact copy is located downstream. We showed in mld mutants that antisense RNA corresponding to the inverted segment is transcribed, and it forms RNA duplex, with the RNA transcribed from the normal gene located downstream.

Central myelin in the first hybrid mice produced by intercrossing homozygotes of shiverer and myelin-deficient mutants

The first hybrid mice ('shiverer*mld' mice) produced by intercrossing the homozygotes of the shiverer (BALB/c strain) and mld (MDB/Dt strain) were used for investigating the fine structure of the myelin lamellae, immunoreactive pattern for myelin basic proteins (MBP) and Golgi impregnated images of oligodendrocytes, with special reference to the influence of aging. All of the hybrid mice had an intermediate coat color between the white of the shiverer and black-brown of the mld, and revealed the same neurological symptoms, intention tremor, ataxic behavior, etc., as those of the shiverer and mld. The central myelin lamellae of the 'shiverer*mld' mouse exhibited the similar characteristics to the shiverer type rather than the mld type from the standpoint of the infrequent occurrence of major dense lines, although they did display a tendency to increase major dense lines with aging like the mld. Observation of the immunohistochemical preparations for MBP showed that immunopositive myelin sheaths were present in the white matter, although they were far more infrequent than those of the mld mutant, probably reflecting the amount of major dense lines. Thus, in the CNS of the 'shiverer*mld' mouse, the MBP-synthesis was possibly much more disturbed than in the mld mutant, or at least, revealed an intermediate pattern between the mld and shiverer.

Myelin instability and oligodendrocyte metabolism in myelin-deficient mutant mice

During the active phase of myelination in myelin-deficient mutant mice (mld), myelin basic protein (MBP) synthesis is defective and the myelin lamellae are uncompacted. In these mutants, we found a fast metabolism of the myelin-associated glycoprotein (MAG) and of sulfatides, and the presence of cholesterol esters and a degradation product of MAG, dMAG, indicating that mld myelin was unstable. The increased synthesis of MAG and Wolfgram protein, two proteins present in uncompacted myelin sheath and paranodal loops, was demonstrated by high levels of messengers. Simultaneously, we found an accumulation of inclusion bodies, vacuoles, and rough endoplasmic reticulum in mld oligodendrocytes. This material was heavily immunostained for MAG. Furthermore, the developmental change between the two molecular forms of MAG (p72MAG/p67MAG) was delayed in mld mice. In 85-d-old mld mice, the MBP content increased and myelin lamellae became better compacted. In these mutants, dMAG was absent and MAG mRNAs were found in normal amounts. Furthermore, the fine structure of mld oligodendrocytes was normal and the MAG immunostaining was similar to age-matched controls. These results support a functional role for MBP in maintaining the metabolic stability and the compact structure of myelin. Furthermore, in the absence of MBP and myelin compaction, the regulation of the synthesis of at least two membrane proteins related to myelin cannot proceed.

Alteration of the primary pattern of central myelin in a chimaeric environment--study of shiverer----wild-type chimaeras

Using chimaeras produced from the shiverer mutant with a BALB/C background (white coat color) and normal mice of the C57BL/6N strain (black coat color) by the aggregation method, the morphological patterns of the central myelin sheaths were examined by electron microscopy, with special reference to the structure of the myelin lamellae of the one genotype in an environment dominated by the other genotype. The results obtained may be summarized as follows. The oligodendrocytic processes of both genotypes are able to interact independently with the axon unrelated to its genotype. The lamellar structure of the myelin sheath, i.e., whether or not it is composed of alternating major dense and intraperiod lines, is primarily determined by the genotypes of the oligodendrocytes concerned. The processes of shiverer oligodendrocytes can spirally enwrap axons in the chimaeric CNS which shows dominance of the normal genotype. In contrast, in the environment of shiverer dominance, the spiral arrangement of the oligodendrocytic processes of the normal genotype is often disturbed, probably by the oligodendrocytic processes of the shiverer genotype. Thus, the primary pattern of the myelin sheaths can be altered under the chimaeric condition, and the formation of the myelin sheaths depends essentially on the behavior of the oligodendrocytic processes, which is determined by the genetic specificity.

Shiverer and normal peripheral myelin compared: basic protein localization, membrane interactions, and lipid composition

We have correlated membrane structure and interactions in shiverer sciatic nerve myelin with its biochemical composition. Analysis of x-ray diffraction data from shiverer myelin swollen in water substantiates our previous localization of an electron density deficit in the cytoplasmic half of the membrane. The density loss correlates with the absence of the major myelin basic proteins and indicates that in normal myelin, the basic protein is localized to the cytoplasmic apposition. As in normal peripheral myelin, hypotonic swelling in the shiverer membrane arrays occurs in the extracellular space between membranes; the cytoplasmic surfaces remain closely apposed notwithstanding the absence of basic protein from this region. Surprisingly, we found that the interaction at the extracellular apposition of shiverer membranes is altered. The extracellular space swells to a greater extent than normal when nerves are incubated in distilled water, treated at a reduced ionic strength of 0.06 in the range of pH 4-9, or treated at constant pH (4 or 7) in the range of ionic strengths 0.02-0.20. To examine the biochemical basis of this difference in swelling, we compared the lipid composition of shiverer and normal myelin. We find that sulfatides, hydroxycerebroside, and phosphatidylcholine are 20-30% higher than normal; nonhydroxycerebroside and sphingomyelin are 15-20% lower than normal; and ethanolamine phosphatides, phosphatidylserine, and cholesterol show little or no change. A higher concentration of negatively charged sulfatides at the extracellular surface likely contributes to an increased electrostatic repulsion and greater swelling in shiverer. The cytoplasmic surfaces of the apposed membranes of normal and shiverer myelins did not swell apart appreciably in the pH and ionic strength ranges expected to produce electrostatic repulsion. This stability, then, clearly does not depend on basic protein. We propose that P0 glycoprotein molecules form the stable link between apposed cytoplasmic membrane surfaces in peripheral myelin.

Glial and axonal development in optic nerve of myelin deficient rat mutant

Development of glial cell lines and axons is reported for the optic nerve of the myelin deficient rat mutant, md, 3-46 days postnatally. In mutants, optic nerves do not increase in area after 16 days of age whereas, in normal rats, they enlarge through 46 days of postnatal life. The density of glial cells, determined in cross-sections, is similar in md and normal littermates through 19 days postnatally. Thereafter, glial densities are greater in the mutant. Nonetheless, total glial counts are reduced in md as compared to the normal, because cross-sectional areas and lengths of mutant nerve 30-46 days after birth are smaller than those of age-matched, normal littermates. Differential counts of glial cells, made by ultrastructural criteria, show that md optic nerves contain abnormal, vacuolated, immature oligodendroglia from the third postnatal day. Furthermore, oligodendrocytes are reduced in number in older mutants; they constitute 1% of optic nerve neuroglia at 46 days. Astrocytic numbers are increased in relative, not in absolute, terms from 19 days, and microglial numbers are greater than normal in the oldest mutants. Reactive microglia, containing large cytoplasmic lipid droplets, constitute 4-8% of the glia of md nerve 19-46 days postnatally. Mean axonal areas are similar in normal rats and mutants at 19 and 43-46 days of age. However, mitochondrial density is greater in md axons 19 days after birth and mean areas of axonal mitochondria are significantly larger in 43-46 day mutants than in age-matched, normal littermates. Additionally, the percent area of axoplasm occupied by mitochondria is increased in md at both 19 and 43-46 days of age. The myelination defect in md appears to be due primarily to an oligodendroglial abnormality which precedes the normal age of onset of myelination. Astrocytic and microglial changes are secondary. Axonal enlargement proceeds normally over 46 days of postnatal life. Overall, the data do not provide definitive support for an axonal basis for the myelination defect, although measurable differences in axonal mitochondria between mutants and normals are demonstrable and qualitative abnormalities do occur in the axons of the mutant.

An electron microscopic study of astroglia and oligodendroglia in the lateral geniculate nucleus of aged rats

The ultrastructural features of glial cells in the lateral geniculate nucleus of aged rats have been studied. Abundant filaments as well as heterogeneous dense bodies are observed in the majority of astrocytes. They frequently surround both axons and nerve terminals showing signs of degeneration. In addition, some degenerating myelinated axons are seen in phase suggestive of engulfment by astrocyte processes. Oligodendrocytes display broad processes containing an organelle-rich cytoplasm and a continuity between their plasma membrane and the outer myelin lamellae which partially ensheath the adjacent axons. Multivesicular bodies and pleomorphic dense inclusions, composed of amorphous material as well as laminated structures, are also present in oligodendrocytes. The significance of these morphological features is discussed in relation to process of normal ageing.