Biochemical abnormalities in spinal cord myelin and CNS homogenates in heterozygotes affected by the shiverer mutation

Myelin and non-myelin debris contribute to foamy macrophage formation after spinal cord injury

Tissue damage after spinal cord injury (SCI) elicits a robust inflammatory cascade that fails to resolve in a timely manner, resulting in impaired wound healing and cellular regeneration. This inflammatory response is partly mediated by infiltrating immune cells, including macrophages. As professional phagocytes, macrophages initially play an important role in debris clearance at the injury site, which would be necessary for proper tissue regeneration. After SCI, most macrophages become filled with lipid droplets due to excessive uptake of lipid debris, assuming a "foamy" phenotype that is associated with a proinflammatory state. Myelin has been assumed to be the main source of lipid that induces foamy macrophage formation after injury given its abundance in the spinal cord. This assumption has led to the widespread use of purified myelin treatment to model foamy macrophage formation in vitro. However, the assumption that myelin is necessary for foamy macrophage formation remains untested. To this end, we developed a novel foamy macrophage assay utilizing total spinal cord homogenate to include all sources of lipid present at the injury site. Using the myelin basic protein knockout (MBP KO, i.e., Shiverer) mice that lack myelin, we investigated lipid accumulation in foamy macrophages. Primary macrophages treated with myelin-deficient spinal cord homogenate still formed large lipid droplets typically observed in foamy macrophages, although to a lesser degree than cells treated with normal homogenate. Similarly, MBP KO mice subjected to contusive spinal cord injury also formed foamy macrophages that exhibited reduced lipid content and associated with improved histological outcomes and reduced immune cell infiltration. Therefore, the absence of myelin does not preclude foamy macrophage formation, indicating that myelin is not the only major source of lipid that contributes this pathology, even though myelin may alter certain aspects of its inflammatory profile.

Defining Changes in the Spatial Distribution and Composition of Brain Lipids in the Shiverer and Cuprizone Mouse Models of Myelin Disease

Myelin is composed primarily of lipids and diseases affecting myelin are associated with alterations in its lipid composition. However, correlation of the spatial (in situ) distribution of lipids with the disease-associated compositional and morphological changes is not well defined. Herein we applied high resolution matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI-IMS), immunohistochemistry (IHC), and liquid chromatography–electrospray ionization–mass spectrometry (LC-ESI-MS) to evaluate brain lipid alterations in the dysmyelinating shiverer (Shi) mouse and cuprizone (Cz) mouse model of reversible demyelination. MALDI-IMS revealed a decrease in the spatial distribution of sulfatide (SHexCer) species, SHexCer (d42:2), and a phosphatidylcholine (PC) species, PC (36:1), in white matter regions like corpus callosum (CC) both in the Shi mouse and Cz mouse model. Changes in these lipid species were restored albeit not entirely upon spontaneous remyelination after demyelination in the Cz mouse model. Lipid distribution changes correlated with the local morphological changes as confirmed by IHC. LC-ESI-MS analyses of CC extracts confirmed the MALDI-IMS derived reductions in SHexCer and PC species. These findings highlight the role of SHexCer and PC in preserving the normal myelin architecture and our experimental approaches provide a morphological basis to define lipid abnormalities relevant to myelin diseases.

Dysmyelinated axons in shiverer mice are highly vulnerable to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated toxicity

Glutamate excitotoxicity plays a role in white matter injury in many neurological diseases. Oligodendrocytes in particular are highly vulnerable to excitotoxicity, mediated through activation of AMPA/kainate receptors. Myelin may also be injured independently via NMDA (N-methyl-D-aspartic acid) receptors located on peripheral oligodendroglial processes. Central axons are susceptible to glutamate receptor activation in vivo, but it is unclear whether this is mediated directly by activation of receptors expressed on axons, or indirectly through glutamate toxicity of myelin or neighboring glial cells. We examined axonal vulnerability in mice deficient in myelin basic protein (shiverer), also expressing yellow fluorescent protein (YFP) in a subset of axons. YFP fluorescence, EM, and mouse behavior were assessed 24 h after microstereotactical injections of S-AMPA or NMDA into lumbar dorsal columns. S-AMPA injection led to impaired rotarod performance and widespread axonal degeneration and was more pronounced in shiverer mice than controls. In contrast, NMDA injection did not cause axonal injury or behavioral changes in either group. These results indicate that spinal cord axons in vivo are vulnerable to toxicity mediated by AMPA but not NMDA receptors. The presence of compact myelin is not required for excitotoxic axon damage, and its absence may increase vulnerability. Further understanding of AMPA receptor-mediated axonal toxicity may provide new targets for neuroprotective therapy in WM diseases.

Reduced N-Acetylaspartate Levels in Mice Lacking Aralar, a Brain- and Muscle-type Mitochondrial Aspartate-glutamate Carrier

Aralar is a mitochondrial calcium-regulated aspartate-glutamate carrier mainly distributed in brain and skeletal muscle, involved in the transport of aspartate from mitochondria to cytosol, and in the transfer of cytosolic reducing equivalents into mitochondria as a member of the malate-aspartate NADH shuttle. In the present study, we describe the characteristics of aralar-deficient (Aralar-/-) mice, generated by a gene-trap method, showing no aralar mRNA and protein, and no detectable malate-aspartate shuttle activity in skeletal muscle and brain mitochondria. Aralar-/- mice were growth-retarded, exhibited generalized tremoring, and had pronounced motor coordination defects along with an impaired myelination in the central nervous system. Analysis of lipid components showed a marked decrease in the myelin lipid galactosyl cerebroside. The content of the myelin lipid precursor, N-acetylaspartate, and that of aspartate are drastically decreased in the brain of Aralar-/- mice. The defect in N-acetylaspartate production was also observed in cell extracts from primary neuronal cultures derived from Aralar-/- mouse embryos. These results show that aralar plays an important role in myelin formation by providing aspartate for the synthesis of N-acetylaspartate in neuronal cells.

Early onset of axonal degeneration in double (plp-/-mag-/-) and hypomyelinosis in triple (plp-/-mbp-/-mag-/-) mutant mice

Double (plp-/-mag-/-) and triple (plp-/-mbp-/-mag-/-) null-allelic mouse lines deficient in proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and myelin basic protein (MBP) were generated and characterized genetically, biochemically, and morphologically including their behavioral capacities. The plp-/-mag-/- mutant develops a rapidly progressing axon degeneration in CNS with severe cognitive and motor coordinative deficits but has a normal longevity. CNS axons of the plp-/-mbp-/-mag-/- mouse are hypomyelinated and ensheathed by "pseudomyelin" with disturbed protein and complex lipid composition. The shiverer trait in the plp-/-mbp-/-mag-/- similar to the plp-/-mbp-/- mutant is significantly ameliorated, and its lifespan is considerably prolonged. The longevity of these dysmyelinosis mouse mutants recommends them as suitable models for the long-term evaluation of stem cell therapeutic strategies.

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.

Alterations of cholesterol synthesis precursors (7-dehydrocholesterol, 7-dehydrodesmosterol, desmosterol) in dysmyelinating neurological mutant mouse (quaking, shiverer and trembler) in the PNS and the CNS

In brain, levels of cholesterol, desmosterol and 7-dehydrodesmosterol are reduced in shiverer and quaking, but not in trembler 60-day-old dysmyelinating mutant mice. Very interestingly, 7-dehydrocholesterol is not altered in any mutant. The amount of cholesterol is similar in the different normal control mouse strains and in rat. In contrast, levels of precursors are not the same. In sciatic nerve, cholesterol is slightly reduced in shiverer, reduced 2-fold in quaking, and dramatically reduced in trembler (10-fold). 7-Dehydrocholesterol is affected in all mutants.

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 composition and activities of CNPase and Na+,K+-ATPase in hypomyelinated "pt" mutant rabbit

A disorder of CNS myelination was found in paralytic tremor ("pt") rabbits. The condition is inherited in a sex-linked recessive mode. Ultrastructurally, an obvious myelin deficiency with aberration of myelin sheath formation is observed. The yield of myelin isolation was reduced to 20-30% of control. Myelin isolated from 4-week-old "pt" rabbits contained reduced amounts of galactosphingolipids and of several myelin protein markers. Moreover, myelin basic protein, analyzed by two-dimensional gel electrophoresis, showed a deficit in its more basic components. All these facts suggest a delay in myelin maturation. Ganglioside content was increased as well as Na+,K+-ATPase specific activity. 2',3'-Cyclic nucleotide phosphodiesterase (CNPase) specific activity was the same in "pt" as in control myelin but differed by having greater sensitivity to detergent activation.

Antibody against mouse liver 5'-nucleotidase immunostains white matter in the adult mouse central nervous system

An antiserum against rat liver 5'-nucleotidase has been shown to immunostain myelinated fibers and oligodendrocytes in the rat CNS, consistent with evidence for 5'-nucleotidase activity in rat brain myelin and oligodendrocytes (Cammer, Sacchi and Kahn, Devel. Brain Res., 1985, 20: 89-96). However, in the mouse CNS, in which myelin also has 5'-nucleotidase activity, that antiserum stained only blood vessels. To obtain an antibody against the mouse enzyme, 5'-nucleotidase was partly purified from mouse liver membranes by detergent extraction, heat treatment, affinity chromatography, acidification, and ammonium sulfate fractionation. The preparation, which was enriched about 110-fold in 5'-nucleotidase specific activity, compared to the starting extract, was electrophoresed on a preparative slab gel containing Triton X-100, a strip was stained histochemically for 5'-nucleotidase, and the material corresponding to the stained band was used to immunize a rabbit. The immune IgG fraction, but not the preimmune IgG, reacted with mouse brain homogenates. The immune serum gave consistently greater inhibition of 5'-nucleotidase activity in mouse liver homogenates, mouse brain myelin and mouse brain homogenates, but not rat brain or liver homogenates, compared to the preimmune serum. The immune serum, but not the preimmune serum, immunostained white matter in the normal adult mouse brain and spinal cord. The findings suggest that the mouse may have one isozyme of 5'-nucleotidase similar to that in rat with respect to subunit sizes but differing in primary structure at one or more antigenic sites and support previous observations of 5'-nucleotidase activity in myelin from mouse brains and spinal cords.

A new myelin-deficient mutant hamster: biochemical and morphological studies

Biochemical and morphological studies were done on a new trembling mutant hamster CBB. The yield of myelin from the mutant was 30 and 40% of the control at 46 and 140 days of age, respectively, but myelin composition and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activity were normal. Morphologically, about 18% of the axons were myelinated in the mutant optic nerve at 46 days of age, in which the myelinated fibers were those with larger diameters (more than 0.6 micron), while the control had a peak at 0.4 micron in diameter. The ultrastructure and thickness of compact myelin lamellae in the mutant were normal. Myelination and the structure of peripheral nerve myelin appeared normal. The results indicate that the essential defect is the delay and arrest of myelination in the CNS, which is probably caused by either a decreased rate of synthesis of myelin components in oligodendrocytes or a defect in the oligodendrocyte-axon recognition in smaller axons.