Morphological and biochemical observations in the Jimpy spinal cord

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Microglial cell reaction in the gray and white matter in spinal cords from jimpy mice. An enzyme histochemical study at the light and electron microscope level

Jimpy is a genetic disorder which results in a severe hypomyelination in the central nervous system associated with a variety of astroglial and oligodendroglial abnormalities. In this study, we examined the morphology and distribution of microglial cells in spinal cord sections from jimpy and normal mice at 10-12 and 20-22 days postnatal using a specific microglial marker, the nucleoside diphosphatase staining. Compared to those of normal littermates, the spinal cords of jimpy mice showed an intense microglial cell reaction in white and gray matter, as revealed by quantitative analysis and light and electron microscope study. Microglial reactivity was apparent in all spinal cord areas, although it was more pronounced in white than in gray matter. The mean microglial densities in the jimpy white matter were about threefold (10-12 days) and fivefold (20-22 days) higher than in the normal, whereas in the gray matter, microglial density in jimpy was about 60% higher than in normal at both ages. Morphologically, microglial cells in the normal spinal cord showed a ramified appearance, similar in size and ramification pattern to those reported in other normal CNS areas. In contrast, microglial cells in the jimpy spinal cord showed a reactive morphology, characterized by a shortening and coarsening of their cell processes, swelling of their cell body and accumulation of lipid inclusions. Reactive microglial cells were found in close association with axons and oligodendroglial cells. The possible role of microglial cells in hypomyelination is discussed.

The distribution of myelin basic protein mRNAs within myelinating oligodendrocytes

The nervous system contains oligodendrocytes with processes that are greatly extended in space. It is now clear that there are numerous complex, poorly understood mechanisms by which polypeptides are synthesized and delivered to their sites of function in these cells. One mechanism is by the active positioning of mRNAs encoding certain proteins to restricted intracellular subdomains. Perhaps the best studied example of this in the vertebrate CNS is the translocation of myelin basic protein mRNAs to the forming myelin sheath, where the newly synthesized polypeptides, which avidly associate with membranes, can be directly incorporated into the myelin membrane. Evidence for this conclusion is presented here in the context of related work on the general phenomenon of mRNA translocation that is under analysis in other systems.

Posttranscriptional regulation of myelin protein gene expression

Regulation of myelin protein gene expression occurs at many different levels including transcription, mRNA translocation, translation, and posttranslational modification of myelin proteins prior to their assembly into the membrane. Translocation of myelin basic protein (MBP) mRNAs into oligodendrocyte processes was observed in vivo and in primary cultures, but no such translocation was observed for the mRNAs encoding the proteolipid protein (PLP) or myelin-associated glycoprotein. More than 99% of the mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase (CNP) remained associated with cell bodies. In the jimpy mutant, MBP mRNA translocation appeared to be impaired, but translocation occurred normally in quaking brains in vivo. We have found that steroids, such as glucocorticoids, stimulate the translation of MBP and PLP mRNAs in cell-free systems and inhibit the translation of CNP mRNA. This pattern of regulation is consistent with compositional changes noted in myelin during development. We have localized a nine nucleotide segment within the 5'-untranslated region of the MBP mRNA that is involved in the action of steroids on translation of this mRNA. We have also determined that the protein synthetic step modulated by the steroids is chain initiation, enhancing the rate at which new ribosomal subunits bind to the MBP mRNAs.

The corticospinal tract attains a normal configuration in the absence of myelin: observations in jimpy mutant mice

The corticospinal projection was examined in dysmyelinated, jimpy mice and in unaffected littermates following cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase or biocytin. Corticospinal axons in both phenotypes traverse the medulla within a well-defined pyramidal tract, decussate within several fascicles at the spinomedullary junction, and extend down the spinal cord in a compact bundle in the ventral-most part of the dorsal funiculus. Very few labeled fibers are seen separated from the main bundle. This normal configuration of the corticospinal tract is attained despite the virtual absence of CNS myelin in jimpy mice. It seems unlikely then that the myelin normally present in fiber bundles adjacent to this relatively late emerging projection can significantly influence pathway selection during its development.

Myelination in the jimpy mouse in the absence of proteolipid protein

A point mutation in the gene for proteolipid protein (PLP) has been suggested to account for the dysmyelination seen in the jimpy mouse mutant. Despite the absence of PLP, the major integral membrane protein of central nervous system (CNS) myelin, this study shows that there are many scattered myelinated fibers present in the spinal cord of this murine mutant which are immunocytochemically positive for myelin basic protein (MBP), yet negative for PLP. This lack of PLP results in an abnormal compaction of the extracellular leaflets of the myelin sheath and the formation of an abnormal intraperiod line. These results are similar to those seen in another X-linked myelin mutant, the myelin-deficient rat (Duncan et al.: Proc. Natl. Acad. Sci. U.S.A., 84:6287-6291, 1987), and show that a multilamellar membrane can be formed in the absence of its major integral membrane protein.

Comparative immunocytochemistry of Pelizaeus-Merzbacher disease, the jimpy mouse, and the myelin-deficient rat

Patients with Pelizaeus-Merzbacher disease (PM), hemizygous mice with the jimpy mutation (jp/Y), and hemizygous rats with X-linked myelin deficiency (md/Y) share a profound lack of proteolipid protein (PLP) in their central nervous systems (CNS). The peripheral nervous system is normal. These X-linked disorders are associated with or actually caused by the lack of normal oligodendrocytes. Vibratome sections of brain were incubated with antisera to myelin basic protein (MBP), myelin-associated glycoprotein (MAG), 2':3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) (EC 3.1.4.37), PLP, a synthetic PLP-peptide, glial fibrillary acidic protein (GFAP), and transferrin. Reaction product was developed by sequential incubation with biotinylated second antibodies, the avidin-biotin-peroxidase complex (ABC), and diaminobenzidine (DAB) plus hydrogen peroxide as chromogenic substrates. In PM, jp/Y and md/Y, islands of myelin-like structures were revealed by antisera to MBP, MAG, and CNP. Reaction product after application of anti-PLP was absent. Reaction product after anti-PLP-peptide was restricted to infrequent bizarre cells possibly representing abnormal oligodendroglia. The lack of oligodendrocytes in jp/Y and md/Y could also be confirmed by immunocytochemistry for transferrin.

The early region of human papovavirus JC induces dysmyelination in transgenic mice

Transgenic mice containing the early region of human papovavirus JC were produced. Some of these mice exhibited a shaking disorder similar to the previously described mutant mice jimpy or quaking. Neuropathological analysis indicated a dysmyelination in the central nervous system, but not the peripheral nervous system. A high level of JCV T-antigen mRNA was present in the brains of the mice exhibiting the myelin disorder. JC virus is associated in humans with a degenerative demyelinating disease: progressive multifocal leukoencephalopathy. The JCV-containing transgenic mice may therefore provide an animal model for studying this disease.

Shiverer jimpy double mutant mice. I. Biochemical evidence for reciprocal intergenic suppression

Shiverer and jimpy are neurological mutations that cause hypomyelination in the mouse CNS. The 3 major protein components of CNS myelin are: myelin basic protein (MBP), proteolipid (PLP) and 2', 3'-cyclic nucleotide phosphohydrolase (CNP). Previous work has shown that in jimpy animals the CNS contains reduced levels of MBP and CNP while PLP is undetectable. In shiverer animals the major forms of MBP ae undetectable in either the CNS or PNS, but the level of CNP is unaffected by mutation. In this study we have measured MBP, PLP and CNP in both the CNS (cerebral hemispheres) and PNS (sciatic nerve) of mice carrying the jimpy and shiverer mutations, individually and in combination. The results indicate that in the double mutant the levels of all 3 myelin proteins in both the CNS and PNS are intermediate between the levels in jimpy and the levels in shiverer animals. This means that part of the biochemical phenotype of the jimpy mutation (reduced levels of CNP and absence of PLP) is suppressed by the shiverer mutation, and part of the biochemical phenotype of the shiverer mutation (absence of MBP) is suppressed by the jimpy mutation. Possible mechanisms for this reciprocal intergenic suppression are discussed.

Shiverer jimpy double mutant mice. II. Morphological evidence supports reciprocal intergenic suppression

Mice which carry both the shiverer (shi) and the jimpy (jp) mutations have a morphological phenotype with features of each single mutation by itself but in milder form: the number of myelin sheaths is increased relative to jp, the thickness of sheaths and amount of major dense line is increased relative to shi, and the abnormal, lipid-filled cells characteristic of jp are not seen. However, the abnormal bundles of oligodendrocyte microprocesses and errors in the targeting of myelination which characterize shi are not altered by the presence of the jp mutation. This morphological evidence suggests partial reciprocal intergenic suppression in shiverer jimpy double mutant mice and therefore agrees with conclusions based on biochemical data presented by Kerner and Carson (Brain Research, 374 (1986) 45-53).

Oligodendrocyte production and myelin recovery in heterozygous jimpy mice: an autoradiographic study

Jimpy is a genetic disorder of mouse resulting in hypomyelination. In this study oligodendrocyte proliferation was examined in heterozygous carriers of the jimpy gene. The incorporation of [3H]-thymidine into DNA is increased in jimpy heterozygotes compared to controls. An autoradiographic analysis indicated that oligodendrocytes are the predominant neuroglial cell type being produced in the brain at the ages studied in both heterozygotes and control animals. In addition, the total number of labeled oligodendrocytes was increased in the heterozygote animals compared to controls. These results, taken together, indicate that the rate of oligodendrocyte production is greater in jimpy heterozygotes than in control animals. We have previously shown that young jimpy heterozygotes have a reduced myelin content and older heterozygotes do not. The increased rate of oligodendrocyte production, demonstrated in this study, is most likely responsible for the increasing myelin content in the heterozygote. Further study of the cellular interactions which trigger oligodendrocyte production and myelin recovery in the jimpy heterozygote may be relevant to remyelination in other disease states, including those affecting humans.

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.

Effect of the jimpy mutation on expression of myelin proteins in heterozygous and hemizygous mouse brain

The levels of myelin basic protein, proteolipid protein, and 2',3'-cyclic nucleotide 3'-phosphohydrolase (EC 3.1.4.37) in cerebral hemispheres of wild-type, heterozygous jp/+, and hemizygous jp/Y mice of different ages were determined by radioimmunoassay and immunoblotting. In jp/Y brain the level of myelin basic protein was 8% that of wild-type at all ages. All forms of the protein were reduced although the 21.5K Mr form was relatively spared at early ages compared to the 18.5K, 17K, and 14K Mr forms. The level of 2',3'-cyclic nucleotide 3'-phosphohydrolase was 8% that of wild-type at all ages, and proteolipid protein was undetectable at any age. These results are consistent with the hypothesis that the jimpy mutation blocks myelin morphogenesis subsequent to incorporation of 21.5K Mr myelin basic protein but prior to incorporation of proteolipid protein. In jp/+ brain the levels of the three proteins were reduced commensurately to 60-70% those of wild-type. The deficit was apparent as early as 10 days after birth and remained proportionately constant throughout development. These results suggest that in jp/+ mice, X-chromosome inactivation produces a mosaic population of functionally wild-type and functionally jimpy oligodendrocytes. The former elaborate normal amounts of myelin but do not completely compensate for the myelin deficit due to the latter.

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

Oligodendroglia in the "Shiverer" spinal cord at 2, 4, 8 and 18 weeks of age were investigated by electron microscopy, with special reference to the process of myelin formation. The predominant changes observed were vacuolization of the cytoplasm during 2 and especially 4 weeks of age, indicating initial steps in the formation of myelin sheaths, although their total volumes was very small and their lamellae were incomplete. Polymorphic vacuoles and vesicles with various electrondense contents appeared to originate from the Golgi complex and the rough-surfaced endoplasmic reticulum (r-ER), since such vacuoles happened to be continuous with the Golgi complex, and the Golgi complex disappeared concomitantly with the appearance of great numbers of vacuoles. The r-ER was also fragmented together with the occurrence of vesicles. After 8 weeks of age, the number of myelinated nerve fibers reached the plateau of the increasing curve, while the number of myelin lamellae successively increased until the sheaths were completed. At these stages the oligodendroglia containing vacuoles gradually disappeared, although numerous oligodendroglia possessing a nucleus with peripheral, densely clumped chromatin were present. These dark oligodendroglia contained a Golgi complex, r-ER with small, single cisternae and microtubules of irregular thickness, different from those found in the control oligodendroglia. At 18 weeks of age polymorphic vacuoles had disappeared in most oligodendroglia.

Abnormal cell relationships in Jimpy mice: electron microscopic and immunocytochemical findings

The mutant mouse strain Jimpy is characterized by a deficiency of myelin formation throughout the C.N.S. The cause of this hypomyelination is unknown. Based on previous reports, astrocytes, axons and oligodendrocytes are all altered, but no single cell type can be unequivocally defined as the primary target. Jimpy and age-matched normal mice were investigated using thin sectioning, freeze-fracturing and immunocytochemistry. We examined optic nerves and cervical spinal cords of Jimpy to determine which cells were morphologically altered during the period which precedes the onset of myelination and which cellular alterations persisted during myelinogenesis. Abnormalities of astrocytes and axons were frequently observed in Jimpy not only during myelination but also in early postnatal development before mature oligodendrocytes were present. The early astrocytic changes included hyperplasia and alterations of both cytoplasm and plasma membrane. An unusually complex network of astrocytic processes divided the axons into very small groups. During myelination, astrocytic processes were found insinuated between the axons and myelin sheath and/or within the myelin lamellae. Immunocytochemical investigations also revealed a complex network of glial fibrillary acidic protein-positive processes in contact with the majority of the axons. At stages prior to myelination axonal alterations were detected. Most of the axons were not in close contact with one another and individual axons had an undulating and irregular course. In areas where axon separation by astrocytic processes occurred, axonal diameters were more variable than the homogeneously sized axons of the normal mice. Our immunocytochemical results at stages during myelination showed not only many myelin basic protein-positive processes around axons in Jimpy but also clearly immunostained myelin sheaths. This indicates that the myelinating glia present not only produce myelin basic protein but can also incorporate it into the myelin spiral. The presented results suggest that the mouse mutant Jimpy could be a model for disturbed cell interactions in the C.N.S. Therefore, the hypomyelination may not be attributed to a defect of a single cell but rather to a deficiency in both macroglial types and, perhaps, the axon as well.

Normal proliferation rate of galactocerebroside positive oligodendrocytes in brain cell cultures of the hypomyelinated mouse mutant jimpy

Proliferation of oligodendrocytes from the jimpy (jp) hypomyelinated mouse mutant was studied in dissociated brain cell cultures. This was done by combining anti-galactocerebroside (GC) immunostaining (for identifying oligodendrocytes) with [3H]thymidine autoradiography (for identifying proliferating cells). Previously we showed that the expression of GC in culture by jp oligodendrocytes is not altered by the jp mutation. Present results show that in 7-, 14- and 21-day-old jp cultures oligodendrocytes proliferate at a rate similar to that of normal GC+ oligodendrocytes. This indicates that, in jp brain cell cultures, oligodendrocytes which are not affected by mutation in their capability to express GC are also unaffected with regard to their proliferation rate.

Hypomyelinated mutant mice. V. Relationship between jp and jpmsd re-examined on identical genetic backgrounds

jp and jpmsd, two allelic mutations in the mouse that sharply reduce the amount of CNS myelin, produce diseases that can be distinguished morphologically only by their severity. This has raised the question of whether the two mutations are truly distinguishable. Since the two mutations have never been maintained on the same genetic background, correct quantitative and morphological comparison have not been possible. We have prepared a B6C3H stock of jp on the same genetic background as the available stock of jpmsd. In this jp stock, behavioral abnormalities, relative proportion of myelinated axons, and major morphological characteristics of the disease in situ are unchanged from the previous jp stock. The jp disease continues to be more severe than that of jpmsd. However, tissue from the new B6C3H stock myelinates better in organotypic culture than previous jp stocks. The increase in myelination is advantageous, not only for accurate comparison of the two alleles but for all culture studies of jp. Strictly comparable strains or stocks should be utilized in any comparative studies of closely related mutations such as jp and jpmsd.

Immunocytochemical investigations of murine leukodystrophies. A study of the mutants 'jimpy' (jp) and 'myelin deficient' (mld)

Sections of the central nervous system of the leukodystrophic mouse mutants 'jimpy' (jp) and 'myelin deficient' (mld), as well as of healthy littermates, were immunostained for glial fibrillary acidic protein (GFA), myelin basic protein (MBP) and myelin-associated glycoprotein (MAG). Adjacent sections were stained conventionally for myelin. In jp, GFA-stained astrocytes were abnormally prominent already at the age of 12 days. A considerable amount of MBP and MAG was present in the vicinity of axons, although no myelin was visible in the conventional stains for myelin. In mld, GFA-staining astrocytes were present in normal numbers. MAG could be demonstrated in its normal localization along axons, but MBP was visible only in the comparatively old animal (85 days). Here, it was demonstrated in an abnormal site--the perikarya and the proximal parts of the processes of oligodendrocytes. Thin myelin sheaths present in this animal could not be stained for MBP.

GFAP immunoreactivity reveals astrogliosis in females heterozygous for jimpy

The jimpy gene is a sex-linked recessive mutation which produces severe hypomyelination throughout the central nervous system (CNS) in affected male mice. The female carrier also expresses the mutation, but the degree of hypomyelination varies considerably among the tracts. In the optic nerve, patches of unmyelinated tissue are interspersed with myelinated zones; in the brain, myelination is retarded during development but recovers in the adult. We have previously shown, in the male mutant, that an astroglial hypertrophy is associated with the white matter. The present study was undertaken to determine the existence and extent of astrogliosis in the female carriers. In this immunocytochemical investigation using antiserum to glial fibrillary acidic protein (GFAP), we show that the optic nerve of the female carriers exhibits patches of gliosis which are similar in appearance and intensity to those found in affected males. These patches are not present in the white matter of the carriers' brain and spinal cord, but GFAP-immunoreactivity is more intense than in control females. Throughout certain fiber tracts (e.g. optic tract and internal capsule), the number of immunostained astrocytes is increased in comparison to controls; they are larger, have more processes, and are more intensely immunoreactive. The results of the present study show that the retardation of myelin formation is accompanied by an astroglial response and that the intensity of the gliosis closely parallels the extent of the hypomyelination.

Glial fibrillary acidic protein synthesized in vitro using messenger RNA from jimpy mouse spinal cord

Glial fibrillary acidic (GFA) protein was synthesized in vitro in a rabbit reticulocyte lysate system programmed with messenger RNA (mRNA) extracted from Jimpy mouse spinal cord. It was identical in molecular weight and charge to that synthesized from normal mouse mRNA and GFA protein extracted from normal mouse cord. These data suggest that the Jimpy mutation does not affect the primary phenotypic expression of GFA protein.

Ultrastructure of the central nervous system in a myelin deficient rat

Myelin deficiency (md) is a new mutant in the Wistar rat caused by an X-linked recessive lethal gene. One-half of the male offspring develop tremor and ataxia at 10-12 days of age and seizures at 16-21 days. Usually, the animals die 24-28 days postnatally unless survival is prolonged by anticonvulsants. Light microscopic examination of the C.N.S. shows a complete lack of myelin. The P.N.S. is normally myelinated, however. Frontal cortex, corpus callosum, optic nerves, cerebellum and spinal cord were studied routinely in affected animals aged 3-46 days. Abnormal males were identified three days after birth by the absence of myelinated axons from the ventral funiculus of the cervical cord. In mutants aged 3-16 days, axons had the usual ultrastructural features but were either entirely non-myelinated or, rarely, were invested by poorly organized, non-compacted, myelin-like loops of membranes, 2 to 4 in number. In mutants aged 17-20 days, axonal swellings appeared. These increased in number with longer survival times and contained large numbers of microtubules, neurofilaments, mitochondria and dense bodies. Normal C.N.S. myelin was not observed at any age. Two types of abnormal glial cell occur in md. The first, present in white matter at three days of age, is an abnormal oligodendrocyte. The cytoplasm contains dilatation of the rough-surfaced endoplasmic reticulum and the nuclear envelope is widened. A second cell-type, conspicuous by 10 days, has an electron-dense nucleus with prominently clumped chromatin and large cytoplasmic lipid droplets. This second cell type is believed to be a microgliacyte. The number of cytologically-normal oligodendrocytes decreases as mutants age while hypertrophied, filament-rich astrocytes occur in increasing numbers. The myelin defect in md C.N.S. is probably due to an abnormality of oligodendrocytes. Axonal alterations are probably secondary. Myelin deficiency resembles the murine mutant, Jimpy (jp), although ultrastructural changes in oligodendrocytes appear to be dis-similar and md, in contrast to jp, contains no normal-appearing C.N.S. myelin.

Oligodendrocytes of jimpy mice express galactosylceramide: an immunofluorescence study on brain sections and dissociated brain cell cultures

Brain sections and dissociated brain cell cultures of jimpy mouse (jp) were investigated for the presence of galactosylceramide (GC) by indirect immunofluorescence. Optic nerve and corpus callosum sections of 26-day-old jp exhibited many GC-positive cells. The GC staining pattern was similar in jp and normal cultures of the same age. These data suggest that the previously observed decreased amount of GC in jp brain is due to the inability of jp oligodendroglia to properly deposit GC in the myelin, while its synthesis is possible.

Expression of mosaicism in females heterozygous for Jimpy

The Jimpy mouse is a sex-linked neurological mutant characterized by a paucity of myelin in the central nervous system (CNS). The distribution of myelin in the optic nerves of female carriers of the Jimpy gene exhibits a mosaic pattern such that patches of tissue along the nerve are identical in appearance to the hemizygote male, while other areas appear normal.

Chronological study of oligodendroglial alterations and myelination in quaking mice

Chronological morphological investigation was carried out in the spinal cord of quaking mice from day 3 to day 130. Numbers of myelinated fibres were far fewer in quaking mice at day 3 compared to controls. However, when the animals became older, myelination progressed and numbers of myelinated fibres increased although myelin sheaths remained far thinner than the size of axons. Many oligodendroglia during day 5 to 15 in quaking mice revealed prominent dilation and proliferation of smooth walled vesicles and cisterns but after 20 days, such changes were no longer observed. Tortuous bizarre oligodendroglial processes, aberrant myelination and myelin figures were very prominent around day 5--15, but such changes also gradually subsided. Density of glial cells during pre-myelination gliosis was similar in both quaking and control mice. However, glial cell population decreased far slower pace than controls when myelination progressed. Thus, glial cell density remained proportionally higher in quaking than controls although the density declined with age in both.

Regional distribution of myelin basic protein in the central nervous system of quaking, jimpy, and normal mice during development and aging

Myelin basic protein (MBP) was quantified using a RIA technique in the spinal cord, cerebellum, diencephalon plus brainstem region and cerebral hemispheres of two dysmyelinating murine mutants, quaking (qk) and jimpy (jp) mice. Comparison was made with normal control values. The whole life-span has been investigated: ie, ages ranging from 0 to 26 days for the jp, O to one year for the qk, and prenatal stage to three years for the control animals. Assays in the mutants at early ages were rendered feasible by the use of marker genes, which has allowed the diagnosis of the mutation at birth, 12 days before the expression of their typical tremor phenotype. Special care was given to the period of early myelinogenesis in order to clarify the dysynchrony between the various parts of the central nervous system. In normal mice, MBP was already detected in the brain of 19-day-old embryos. During development, rapid accumulation of MBP first occurred in the spinal cord then in the diencephalon, the brainstem, the cerebellum, and finally in the cerebral hemispheres. In the 25-day-old jimpy mutant, levels of MBP were found dramatically decreased, never exceeding 6% of the normal controls in any of the areas investigated. The situation for the quaking mouse was quite different. This mutant could be investigated up to one year old. At that age, a high discrepancy was observed between the values found in the brain and in the spinal cord (respectively, 10% and 35%) compared to normal controls. In both mutants, not only were the levels of MBP decreased, but also its appearance during development was delayed. Nevertheless, in both mutants the caudo-rostral timing of myelination as assayed by MBP levels was maintained. Furthermore, the later myelination occurred, the stronger weas the deficit in MBP. Interestingly, in the quaking mutant, the specific plasticity of the spinal cord was exemplified by its ability to reduce constantly, even at an advanced age, its initial deficit of MBP.

Hypomyelinated mutant mice: description of jpmsd and comparison with jp and qk on their present genetic backgrounds

Hypomyelinated mutant mice are valuable natural animal systems for analysis of CNS myelin development, chemistry and diseases. One mutant, jpmsd, has never received thorough morphological description. We here describe the detailed morphology of jpmsd, compare it with well-studied jp and qk on their present genetic backgrounds, and discuss the genetic histories of all 3 mutants. Region for region, jpmsd has twice as much myelin as jp, but 1/2--1/5 as much as qk. Both jp and jpmsd have scarce oligodendrocytes, rare nodes of Ranvier, clustering of myelin segments, abnormal lipid-filled cells, frequent degenerating cells, and rare distorted myelin profiles. In contrast, qk has abnormally numerous oligodendrocytes, frequent nodes of Ranvier, no obvious myelin clustering, no lipid-filled cells, rare degenerating cells, and frequent abnormal or distorted myelin profiles. jp and jpmsd are quantitatively different, but qualitatively similar. Since its origin, the jpmsd disease has inadvertently been ameliorated by transferring the mutation to a different background. Persistent differences in the remainder of the genome might account for all remaining apparent differences between jp and jpmsd. In contrast, qk is totally dissimilar in morphology and presumably in pathophysiology.

Neuroglia: a reevaluation of their origin and development

The development of the central nervous system (CNS) is often considered to consist of three main stages: cell proliferation, cell migration and a final phase involving cytological differentiation into mature cells. The development of neurons is frequently presented in this context but such a framework is as applicable to neuroglia as to neurons. The breakdown of glial development in this manner provides a framework for analyzing interactions of different stages upon one another and for comparing glial to neuronal development. The present chapter examines gliogenesis in terms of these three stages. Emphasis is placed on glial proliferation in normal development and in experimentally altered nervous tissue, primarily because most of our information about gliogenesis relates to cell proliferation. The final section of this chapter deals with the regulation of glial development by neurons and hormones.

2'-3'-cyclic nucleotide 3'-phosphohydrolase activity in the central nervous system of a myelin deficient mutant (Shiverer)

Shiverer is a recessive autosomal mutant characterized by the deficiency of the myelin in the central nervous system. 2',3'-Cyclic nucleotide 3'-phosphohydrolase (CNPase) activity of the various parts of the central nervous system from the Shiverer did not differ significantly from those of the control. The analysis of the protein profiles of the purified myelin from the Shiverer showed a greatly decreased proportion of proteolipid protein, and almost complete absence of small and large basic proteins, and intermediate protein. Woldgram protein accounted for a much larger percentage of the total myelin protein than is the case in myelin from the control. Proteolipid protein, small and large basic proteins, and intermediate protein were found to be undetectable or decreased in various parts of the central nervous system from the Shiverer. Morphological observation by optic and electron microscope showed that the myelination of the optic nerve was equally affected as the spinal cord.

Biochemical and immunohistochemical studies on dysmyelination of quaking mutant mice in vivo and in vitro

Dysmyelination in the central nervous system of the quaking mutant mouse was studied biochemically and immunohistochemically. We found, by measuring CNPase activity, that myelination in the central nervous system of quaking mice was affected to a different degree in different areas. The pallium cerebri was the most severely affected and the medulla and spinal cord were least affected. The density of astroglia observed by GFA staining wash higher in the white matter of quaking mice than in controls, but the total area of the white matter in the cerebellum was smaller in the quaking mice than in the controls. The DNA content in the pallium cerebri and brain stem showed no increase and that of the cerebellum was even lower in quaking mice than in the controls. Hypertrophy of the astroglia was observed in the white matter of the cerebellum of quaking mice, though Bergmann astroglial fibers in the molecular layer did not show any hypertrophy. The cerebella of quaking mice in primary culture showed very poor myelination under the phase-contrast microscope. However, Purkinje cells from the quaking mutants appeared normal with regard to Bodian silver impregnation, hematoxylineosin staining, and Purkinje cell specific P 400 protein. Addition of the conditioned culture medium of qk/qk to the culture of the control cerebellum did not interfere with the myelination. We concluded that the cause of dysmyelination in the quaking mouse could be a genetic defect of the oligodendroglia rather than hypertrophy of the astroglial cells.

Dysmyelination of the central nervous system in the Chow-Chow dog

Five Chow-Chow dogs from three different litters had generalized body tremors since birth. Histologically a severe myelin deficiency was found in the central nervous system. The peripheral nerves were normally myelinated. Electron microscopically abnormally thin myelin sheaths and completely naked nerve fibers were found. There were many astrocytes in the myelin deficient areas as well as cells with the characteristic features of oligodendrocytes but also containing fibrils. The present cases are compared to other dysmyelinating diseases and possible pathogenetic mechanisms are discussed.

Seitelberger's connatal form of Pelizaeus-Merzbacher Disease. Case report, clinical, pathological and biochemical findings

A case of Seitelberger's connatal form of Pelizaeus-Merzbacher disease is reported. He lived to the age of 13 and was able to recognize persons. Both is unusual in this disease. Some of the few myelin sheaths had two different periodicities (150 and 90 A). Intranuclear inclusions similar to Hirano's cytoplasmic eosinophilic rodlets were observed. The findings of connatal form of Pelizaeus-Merzbacher disease are compared with those of dysmyelinating mice (Jimpy and Quacking) and of manipulated myelinating tissue cultures. The possibility is considered, that the alterations in these dysmyelinating human and animal conditions are caused by extraneural circulating factors.

Oligodendroglial cell development in jimpy mice and controls. An electron-microscopic study in the optic nerve

Glial development was studied in the optic nerve of 1- to 28-day-old Jimpy mice and controls. Abnormalities were found in oligodelopment and axons were not affected. These consisted in (a) Increased numbers of glioblastic cells containing lipids and increased occurrence of glial cell death in the premyelination stage; (b) Decreased numbers of maturing oligodendrocytes (i.e. young and active oligodendrocytes) in the period of early myelination; (c) An occurrence of abnormal oligodendroglial cells containing lipids and multimembranous tubes in the period of advanced myelination. The decreased number of maturing oligodendrocytes in the premyelination stage indicates that the lack of myelin in Jimpy mice may be secondary to a disturbance in the differentiation of the oligodendroglial cell line. The occurrence of abnormal, lipid-containing glioblasts and oligodendrocytes may be an expression of a defect in metabolism leading to an abnormality in the association of myelin proteins and myelin lipids.

Observations on the interactions of Schwann cells and astrocytes following X-irradiation of neonatal rat spinal cord

Myelination was inhibited in the spinal cord of three day-old rats with 2000 rads of X-irradiation. Myelination subsequently occurred as a result of caudal migration of oligodendrocytes and extensive invasion of the cord by Schwann cells. Although oligodendrocytes were present in areas containing Schwann cells, astrocytes were absent. The presence of Schwann cells in the neuropil of the spinal cord did not stimulate production of basement membrane by astrocytes, so no new glial limiting membrane was formed. Evidence is presented which suggests that if astrocytes do not form a glial limiting membrane when opposed by large numbers of Schwann cells they are destroyed by the invading cells. It is suggested that the glial limiting membrane normally inhibits entry of Schwann cells into the central nervous system; if this is destroyed and not reconstituted, Schwann cells can migrate freely into the neuropil.