Developmental changes in glycolipid synthesizing enzymes in the brain of a myelin-deficient mutant Wistar rat

Interleukin-6 (IL-6) receptor/IL-6 fusion protein (Hyper IL-6) effects on the neonatal mouse brain: possible role for IL-6 trans-signaling in brain development and functional neurobehavioral outcomes

Adverse neurodevelopmental outcomes are linked to perinatal production of inflammatory mediators, including interleukin 6 (IL-6). While a pivotal role for maternal elevation in IL-6 has been established in determining neurobehavioral outcomes in the offspring and considered the primary target mediating the fetal inflammatory response, questions remain as to the specific actions of IL-6 on the developing brain. CD-1 male mice received a subdural injection of the bioactive fusion protein, hyper IL-6 (HIL-6) on postnatal-day (PND)4 and assessed from preweaning until adulthood. Immunohistochemical evaluation of astrocytes and microglia and mRNA levels for pro-inflammatory cytokines and host response genes indicated no evidence of an acute neuroinflammatory injury response. HIL-6 accelerated motor development and increased reactivity to stimulation and number of entries in a light/dark chamber, decreased ability to learn to withhold a response in passive avoidance, and effected deficits in social novelty behavior. No changes were observed in motor activity, pre-pulse startle inhibition, or learning and memory in the Morris water maze or radial arm maze, as have been reported for models of more severe developmental neuroinflammation. In young animals, mRNA levels for MBP and PLP/DM20 decreased and less complexity of MBP processes in the cortex was evident by immunohistochemistry. The non-hydroxy cerebroside fraction of cerebral lipids was increased. These results provide evidence for selective effects of IL-6 signaling, particularly trans-signaling, in the developing brain in the absence of a general neuroinflammatory response. These data contribute to our further understanding of the multiple aspects of IL-6 signaling in the developing brain.

Functional genomic and proteomic analysis reveals disruption of myelin-related genes and translation in a mouse model of early life neglect

Early life neglect is an important public health problem which can lead to lasting psychological dysfunction. Good animal models are necessary to understand the mechanisms responsible for the behavioral and anatomical pathology that results. We recently described a novel model of early life neglect, maternal separation with early weaning (MSEW), that produces behavioral changes in the mouse that persist into adulthood. To begin to understand the mechanism by which MSEW leads to these changes we applied cDNA microarray, next-generation RNA-sequencing (RNA-seq), label-free proteomics, multiple reaction monitoring (MRM) proteomics, and methylation analysis to tissue samples obtained from medial prefrontal cortex to determine the molecular changes induced by MSEW that persist into adulthood. The results show that MSEW leads to dysregulation of markers of mature oligodendrocytes and genes involved in protein translation and other categories, an apparent downward biasing of translation, and methylation changes in the promoter regions of selected dysregulated genes. These findings are likely to prove useful in understanding the mechanism by which early life neglect affects brain structure, cognition, and behavior.

Maturation-dependent apoptotic cell death of oligodendrocytes in myelin-deficient rats

Mutations in the proteolipid protein gene (PLP/plp), which encodes the major intrinsic membrane protein in central nervous system (CNS) myelin, cause inherited dysmyelination in mammals. One of these mutants, the myelin-deficient (md) rat, has severe dysmyelination that is associated with oligodendrocyte cell death. Using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) assay, which labels apoptotic cells, we find that cell death is increased in multiple white matter tracts of md rats. The tracts that myelinate the earliest show the earliest increase in cell death, and cell death persists for at least 22 days, the lifespan of these mutant animals. In all tracts, and at all developmental ages examined, apoptotic cells expressed the markers of mature oligodendrocytes, such as myelin basic protein, myelin-associated glycoprotein, and the Rip antigen, but not chondroitin sulfate proteoglycan, a marker of oligodendrocyte precursors. Mature oligodendrocytes fail to accumulate in md brain because they die before they fully mature.

Ontogeny of glycerol phosphate dehydrogenase-positive oligodendrocytes in rat brain. Impaired differentiation of oligodendrocytes in the myelin deficient mutant rat

The ontogeny of oligodendrocytes in the myelin deficient (md) rat mutant and in control rats was explored immunohistochemically using an antiserum against the oligodendrocyte specific enzyme, glycerol phosphate dehydrogenase (GPDH), and the avidin-biotin complex technique. In control rats, GPDH was demonstrated to be expressed relatively early in oligodendrocyte differentiation, prior to either myelin basic protein or proteolipid protein expression. With development, oligodendrocytes containing GPDH increased in number, apparent staining intensity, cell soma area and process elaboration. Fewer GPDH+oligodendrocytes were observed in the brain of mutant rats than in unaffected littermates at all developmental ages, and major developmental increases in oligodendrocyte density were delayed. The density of GPDH+oligodendrocytes was reduced by about 40% in both the corpus callosum and in the cingulate cortex of P22-25 and mutants compared with control rats. The oligodendrocyte cell soma area was not influenced by the md condition, and increased 2-fold with development in rats of both genotypes. The area of coronal sections occupied by the corpus callosum increased about 2.5-fold with development, and was 30% smaller in mutant rats late in their lifespan than in unaffected littermates. The reductions in oligodendrocyte density reported here are of insufficient magnitude to fully account for biochemically measured reductions in oligodendrocyte gene expression accompanying the md trait, indicating that gene expression per oligodendrocyte is also impaired. Cell counts in control rats also revealed that oligodendrocytes are overproduced during development. Cell density and the total number of corpus callosum GPDH+oligodendrocytes per section were maximal at P22-25 and then decreased to adult values. These results suggest that glial cells, like neurons, may be generated in excessive numbers, and some subsequently die, as a normal concomitant of development.

Transferrin receptor expression in myelin deficient (md) rats

The question of iron regulation in the brain is the subject of increasing interest as the evidence continues to accumulate that a loss of brain iron homeostasis plays a significant role in some neurodegenerative diseases. Most cells acquire iron through a specific receptor mediated process involving transferrin, the iron mobilization protein. It appears that in the brain, endothelial cells, neurons, and oligodendrocytes express the transferrin receptor. This study uses a strain of rats (myelin deficient, md) in which oligodendrocytes fail to mature, and examines the consequences of this genetic defect on the expression of the transferrin receptor in the brain. The affinity of transferrin for its receptor is similar between the cerebral cortex and cerebellum in both the normal and myelin deficient rats (Kd = 7.8-10.6 nM). The transferrin receptor density is normally 2-3 times higher in the cerebellum than in the cerebral cortex. In the myelin deficient rat strain, the density of the transferrin receptor is decreased in both the cerebrum (56%) and cerebellum (70%) compared to the littermate control animals. Because oligodendrocytes are the only cell type affected in this mutant, the results suggest that these cells are responsible for a considerable amount of the transferrin receptors that are expressed in the brain (excluding the endothelial cell contributions). These observations are consistent with the existing literature stating that oligodendrocytes are responsible for the majority of transferrin and transferrin mRNA which is expressed in the brain, and support the working hypothesis that imbalances in brain iron homeostasis, particularly during development, are associated with myelin disorders.

Glycerophosphorylcholine phosphocholine phosphodiesterase activity in cultured oligodendrocytes, astrocytes, and central nervous tissue of dysmyelinating rodent mutants

The levels of GPC phosphocholine phosphodiesterase, pNP phosphocholine phosphodiesterase, CNPase, and UDP galactose: ceramide galactosyltransferase activities were estimated with pure cultures of oligodendrocytes and astrocytes; mixed primary glial cells cultures; C-6 cells; and CNS tissue of the dysmyelinating md rat, the jimpy mouse, and the quaking mouse. The highest activity of GPC and pNP phosphocholine phosphodiesterases as with CNPase and C gal T was found in the pure cultured oligodendrocytes. C-6 cells had very low or undetectable activities for these two phosphodiesterases but possessed very high CNPase activity. The activity of GPC phosphocholine phosphodiesterase was significantly decreased in the CNS tissue of the md rat and the jimpy and the quaking mouse. Similar reductions were observed for the pNP phosphocholine phosphodiesterase, CNPase, and C gal T activities. The selective cellular enrichment in oligodendrocytes of the GPC phosphocholine phosphodiesterase activity and decreases of its activity in three dysmyelinating mutants in the same ratio as for CNPase and C gal T suggest that GPC phosphocholine phosphodiesterase is a myelin marker enzyme and it may reflect the quantity of myelin and oligodendrocyte present.

Developmental expression of neural cell type-specific mRNA markers in the myelin-deficient mutant rat brain: inhibition of oligodendrocyte differentiation

We have studied gene expression of neuroglial cell markers in the myelin-deficient (md) rat brain during postnatal development. Northern blots and slot blots of poly(A)+ RNA from developing brain were sequentially probed with cDNAs specific for the oligodendrocyte markers glycerol phosphate dehydrogenase (GPDH), myelin basic protein (MBP), and proteolipid protein (PLP), for the neuronal marker glutamic acid decarboxylase (GAD), and for the astrocyte markers glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). GPDH mRNA levels were also examined in two peripheral tissues, liver, and skeletal muscle (hindlimb). Despite a lack of CNS myelin in the md mutant, transcripts of all oligodendroglial markers were detectable except the 1.6-kb PLP message. Brain GPDH mRNA levels were initially equivalent in md and unaffected littermates at postnatal day 15 (PI5), but the mutants failed to display the normal developmental increase in gene expression. By P25, GPDH mRNA expression in md rat brain was approximately 20% of control levels. GPDH mRNA expression in peripheral tissues was less affected than in brain and was lower in md mutants only at the later developmental ages. Expressions of GAD, GFAP, and GS mRNAs in developing md rat brain were not altered. The mRNA levels of the two myelin markers, MBP and PLP, were severely impaired in md rat brain during the entire myelinating period and represented less than 10% of control mRNA levels at P25. The most important observation was that the large PLP transcript (3.2 kb) was slightly shorter in size in md rat brain as compared to normals.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional variation in the levels of transferrin in the CNS of normal and myelin-deficient rats

Transferrin (Tf), the iron mobilization protein, is synthesized mainly in the liver. Recently, both Tf and a mRNA for Tf have been demonstrated in oligodendrocytes in the rat brain. The present study used a biochemical assay for determining the levels of Tf in various brain regions of normal rats compared with the level of those obtained from rats with a genetic mutation characterized by an almost complete failure to develop myelin. In myelin-deficient (md) rats, no Tf-positive oligodendrocytes were seen immunohistochemically in the gray or white matter of the CNS. Quantitatively, levels of Tf throughout the CNS of the md rat were decreased to approximately 5% of the normal values despite a normal hepatic synthetic rate. In the normal rat brain, the cerebellum contained the highest concentration of Tf, followed by the pons, the cerebral cortex, and the caudate-putamen, with the latter two sites being similar. Regional variation in the amount of Tf was in general agreement with published reports on the variation of iron and Tf receptor levels in the CNS. Immunohistochemical examination with antiserum to galactocerebroside (a myelin-specific lipid) was used for extending biochemical reports that glycolipid-synthesizing enzymes are deficient in md rats. No immunostaining in the md rat was observed following immunoreaction for galactocerebroside, whereas white matter oligodendrocytes were intensely marked in the normal rat. Robust astrogliosis was present in both the gray and white matter of the md rats. It is not known at present whether the ability to accumulate Tf is necessary for oligodendrocytic survival or if Tf accumulation is more directly related to myelinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal compact myelin in the myelin-deficient rat: absence of proteolipid protein correlates with a defect in the intraperiod line

The cervical spinal cords of 23-day-old myelin-deficient (md) rats, an X chromosome-linked myelin mutant, and their normal littermates were studied by light and electron microscopy, immunocytochemistry, and in situ hybridization. Light microscopy showed that there were scattered myelinated fibers in the md rat, particularly in the lateral and ventral columns. Ultrastructural examination of these fibers showed that the myelin often had many lamellae that were tightly compacted, but in which the intraperiod line was abnormally fused at most places, resulting in a minor alteration of the myelin periodicity. Immunocytochemical staining of adjacent sections following a variety of fixation methods showed that the myelinated fibers were positive for myelin basic protein but negative for proteolipid protein (PLP). In situ hybridization using cDNA probes to these proteins showed a severe diminution of the mRNAs for both proteins. These findings provide further support for an abnormality in genetic regulation of PLP as has been described in another X chromosome-linked mutant, the jimpy mouse. Despite the lack of PLP, however, a few myelinated fibers are formed in the md rat, but the myelin formed in general lacks a normal intraperiod line, a site at which this protein is thought to be located.

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.

Biochemical and immunocytochemical evidence for a deficiency of normal interfascicular oligodendroglia in the CNS of the dysmyelinating mutant (md) rat

Carbonic anhydrase was assayed and carbonic anhydrase and 5'-nucleotidase were localized in the CNS of myelin-deficient mutant rats and normal littermates. The carbonic anhydrase specific activities were reduced by 61% and 29% in the mutants' forebrains and cerebella, respectively, and the total carbonic anhydrase activity in the spinal cords was reduced by 35%. Immunostained cells were found in gray matter from both normal and mutant rats, but, in the mutants, there was a marked deficiency of interfascicular oligodendrocytes in the regions that are normally occupied by white matter. It is suggested that a developmental study could indicate the step(s) at which normal differentiation of interfascicular oligodendroglia is blocked in this mutant.

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.

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.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

2',3'-cyclic nucleotide 3'-phosphodiesterase and 5'-nucleotidase in the central nervous system of a myelin-deficient rat mutant

2',3'-Cyclic nucleotide 3'-phosphodiesterase activity was examined in brains and spinal cords of normal and myelin-deficient Wistar rats. While the activity in normal brains increased from 0.2 mumol/min/mg protein (units) at 6-10 days to 3.5 units at 25 days of postnatal age, the activity in the myelin-deficient rat remained at 0.2-0.3 units over the same period. In spinal cord, the normal activities were 5.7 and 10.9 units at 12 and 20 days, respectively, whereas they declined in the myelin-deficient rat from 1.06 to 0.79 units for the same age points. 5'-Nucleotidase activities in brain and spinal cord were normal in the myelin deficient rat at both ages.