'Shaking pups': a disorder of central myelination in the spaniel dog. IV. Freeze-fracture electron microscopic studies of axons, oligodendrocytes and astrocytes in the spinal cord white matter

Advances in noninvasive myelin imaging

Myelin is important for the normal development and healthy function of the nervous system. Recent developments in MRI acquisition and tissue modeling aim to provide a better characterization and more specific markers for myelin. This allows for specific monitoring of myelination longitudinally and noninvasively in the healthy brain as well as assessment of treatment and intervention efficacy. Here, we offer a nontechnical review of MRI techniques developed to specifically monitor myelin such as magnetization transfer (MT) and myelin water imaging (MWI). We further summarize recent studies that employ these methods to measure myelin in relation to development and aging, learning and experience, and neuropathology and psychiatric disorders. © 2017 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 136–151, 2018

Oligodendrocyte loss during the disease course in a canine model of the lysosomal storage disease fucosidosis

Hypomyelination is a poorly understood feature of many neurodegenerative lysosomal storage diseases, including fucosidosis in children and animals. To gain insight into hypomyelination in fucosidosis, we investigated lysosomal storage, oligodendrocyte death, and axonal and neuron loss in CNS tissues of fucosidosis-affected dogs aged 3 weeks to 42 months using immunohistochemistry, electron microscopy, and gene expression assays. Vacuole accumulation in fucosidosis oligodendrocytes commenced by 5 weeks of age; all oligodendrocytes were affected by 16 weeks. Despite progressive vacuolation, mature oligodendrocyte loss by apoptosis (caspase-6 positive) in the corpus callosum and cerebellar white matter stabilized by 16 weeks, with no further subsequent loss. Axonal neurofilament loss progressed only in late disease, suggesting that disturbed axon-oligodendrocyte interactions are unlikely to be the primary cause of hypomyelination. A 67% decline in the number of Purkinje cell layer oligodendrocytes coincided with a 67% increase in the number of caspase-6-positive Purkinje cells at 16 weeks, suggesting that early oligodendrocyte loss contributes to Purkinje cell apoptosis. Fucosidosis hypomyelination appeared to follow normal spatiotemporal patterns of myelination, with greater loss of oligodendrocytes and larger downregulation of CNP, MAL, and PLP1 genes at 16 weeks in the cerebellum versus the frontal cortex. These studies suggest that survival of oligodendrocytes in fucosidosis is limited during active myelination, although the mechanisms remain unknown.

Quantitative MR imaging of two-pool magnetization transfer model parameters in myelin mutant shaking pup

Magnetization transfer (MT) imaging quantitatively assesses cerebral white matter disease through its sensitivity to macromolecule-bound protons including those associated with myelin proteins and lipid bilayers. However, traditional MT contrast measured by the MT ratio (MTR) lacks pathologic specificity as demyelination, axon loss, inflammation and edema all impact MTR, directly and/or indirectly through multiple covariances among imaging parameters (particularly MTR with T(1)) and tissue features (e.g. axon loss with demyelination). In this study, more complex modeling of MT phenomena ("quantitative" MT or qMT) was applied to a less complex disease model (the myelin mutant shaking [sh] pup, featuring hypomyelination but neither inflammation nor axon loss) in order to eliminate the covariances on both sides of the MR-pathology "equation" and characterize these important relationships free from the usual confounds. qMT measurements were acquired longitudinally in 6 sh pups and 4 age-matched controls ranging from 3 to 21 months of age and compared with histology. The qMT parameter, bound pool fraction (f), was the most distinctive between diseased and control animals; both f and longitudinal relaxation rate R(1) tracked myelination with normal aging, whereas MTR did not--presumably owing to counterbalancing MT and R(1) effects. qMT imaging provides a more accurate and potentially more specific non-invasive tissue characterization.

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

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

Disproportional expression of proteolipid protein and DM-20 in the X-linked, dysmyelinating shaking pup mutant

The shaking pup is an X-linked canine mutant with a severe hypomyelination of the CNS. Proteolipid protein (PLP) and the related DM-20 protein were examined in this mutant by densitometric scanning of Western blots stained with PLP antiserum. In the spinal cord of 4-week-old mutants, PLP was reduced to less than 1% of the control level, which is a greater deficiency than was found for other myelin proteins. On Western blots of control spinal cord, PLP stained much more intensely than DM-20. However, on Western blots of the mutant spinal cord, a component with the electrophoretic mobility of DM-20 stained slightly more intensely with PLP antiserum than PLP itself. This component was shown to be DM-20 by its lack of reactivity with an antiserum raised to a synthetic peptide corresponding to part of the PLP sequence that is missing in DM-20. Thus PLP and DM-20 are expressed in approximately equal and greatly reduced amounts in the mutant spinal cord. Although PLP or DM-20 could not be detected in brain from the 4-week-old mutant, similar disproportional expression of these two proteins was demonstrated in both spinal cord and brain from a 10-week-old mutant pup. Immunostaining of tissue sections showed that the small amounts of PLP and/or DM-20 synthesized in the mutant are present in the thin myelin sheaths. The results suggest that the shaking pup could have a primary defect in the PLP gene leading to a severe deficiency of PLP and DM-20 as well as disproportional expression of these two proteins.

Myelin-deficient rat: analysis of myelin proteins

Myelin basic protein (BP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) activity were quantitated in the brains and spinal cords of normal and myelin-deficient (md) rats at 8, 12, 18, and 25 days of age. The levels of BP, MAG, and CNP in 25-day-old md brain were 1.1, 1.8, and 11% of those in controls, respectively. In spinal cord, the levels were higher, at 9, 15, and 12% of control values, respectively. Although BP content in the mutant rats was a lower percentage of the control level than MAG and CNPase contents at all ages, the absolute level of BP increased steadily between 8 and 25 days of age in both brain and spinal cord, whereas there was little change in the amounts of MAG and CNPase during this period. Immunoblotting analysis did not reveal an increased apparent Mr for MAG, as has been observed in quaking and trembler mice. There was little difference in the relative distributions of the 14K, 17K, 18.5K, and 21.5K forms of BP between control and md rat spinal cord homogenates at the ages examined. PLP content was reduced more than that of the other proteins in the md mutants, because it could not be detected by a technique capable of detecting 0.2% of the control brain level and 0.1% of control spinal cord level. This suggests that the expression of PLP may be preferentially affected in the md mutation.

Tremors in Samoyed pups with oligodendrocyte deficiencies and hypomyelination

Over a 3-year period a breeder of Samoyed dogs observed six litters in which one or more pups at 3 weeks of age showed generalized tremors and an inability to stand. One affected pup, a 5-week-old male, was referred for clinical and pathological evaluation. Gross and microscopic studies including immunocytochemical demonstration of myelin basic protein, revealed a lack of myelin throughout the central nervous system. Ultrastructural studies revealed that a total absence of normal myelin was associated with oligodendroglial deficiencies. Oligodendrocytes were immature in appearance, greatly reduced in number, and incapable of forming compact myelin. Astrocytosis and an increase in the third type of neuroglial cells were observed. The changes differed from those reported in other canine forms of hypomyelinogenesis and were compared with hypomyelinating diseases in other species. The findings suggested that this hypomyelination was the result of retarded gliogenesis.

Axo-glial relations in the retina-optic nerve junction of the adult rat: electron-microscopic observations

The retina-optic nerve junction (ROJ) was examined by electron microscopy in adult rats, with particular emphasis on the unmyelinated-myelinated nerve fibre transition. Both single sections and serial sections were used. The non-retinal part of the ROJ is covered by an extensively folded glia limitans, facing the choroidea, sclera and pia mater. The blood vessels within the ROJ follow a transverse course and are surrounded by unusually wide perivascular spaces with a glia limitans-like outer delimitation. The endothelial cells exhibit numerous pinocytotic vesicles on their abluminal aspect. In the unmyelinated part of the ROJ the axons are embedded in an extensive meshwork of fibrous astrocytic processes. Some unmyelinated axons exhibit patches of axolemmal undercoating with externally associated astrocytic processes. Typical oligodendrocytes are not found, but a few small dark glial cells of unknown identity can be observed. Atypical ensheathment and myelination of axons at this level by ectopic Schwann cells occurred in one case. In the transition segment of the ROJ a pattern similar to that along dysmyelinated axons is observed, including aberrant axo-glial contacts, unusually thin and short myelin sheaths, intercalated unmyelinated segments, distorted myelin termination regions, bizarre paranodes and myelin termination regions without associated nodally differentiated axolemma. Neither sheath length nor number of myelin lamellae is related to axon diameter in the transition region. Axon diameter tends to be somewhat larger at myelinated than unmyelinated levels of the same axon. We suggest that the unusual axo-glial relations in this region are due to a deficient proliferation and differentiation of oligodendroglial cells, and that the pattern of glial ensheathment in the ROJ might be a consequence of the locally deficient blood-brain barrier.

'Shaking pups': a disorder of central myelination in the spaniel dog. III. Quantitative aspects of glia and myelin in the spinal cord and optic nerve

Quantification of glial cells, axonal size and myelin thickness and volume were carried out in selected areas of the three funiculi of the cervical spinal cord and the optic nerve of 'shaking pups' and normal littermates at 4 and 8 weeks of age. There was a marked reduction of oligodendrocytes in the affected pups with many of these cells having distended rough endoplasmic reticulum. Oligodendrocyte death was not noticeable. Astrocyte numbers were similar in both normal and affected pups. Axonal diameters were not reduced in the affected pups and there was no apparent correlation between myelination and axonal size in these animals. Total myelin volume and thickness were greatly reduced in the 'shaking pups.' Impaired stem cell division together with metabolic disturbance of oligodendrocytes are considered to be the main causes of the hypomyelination in this mutant.