Development and maturation of central nervous system myelin: comparison of immunohistochemical localization of proteolipid protein and basic protein in myelin and oligodendrocytes

Myelin Water Atlas: A Template for Myelin Distribution in the Brain

BACKGROUND AND PURPOSE

Myelin water imaging (MWI) is a magnetic resonance imaging technique that quantifies myelin in-vivo. Although MWI has been extensively applied to study myelin-related diseases in groups, clinical use in individual patients is challenging mainly due to population heterogeneity. The purpose of this study was twofold: (1) create a normative brain myelin water atlas depicting the population mean and regional variability of myelin content; and (2) apply the myelin atlas to assess the degree of demyelination in individuals with multiple sclerosis (MS).

METHODS

3T MWI was performed on 50 healthy adults (25 M/25 F, mean age 25 years [range 17-42 years]). The myelin water atlas was created by averaging coregistered myelin water fraction (MWF) maps from all healthy individuals. To illustrate the preliminary utility of the atlas, white matter (WM) regional MWF variations were evaluated and voxel-wise z-score maps (z < -1.96) from the MWI of three MS participants were produced to assess individually the degree of demyelination.

RESULTS

The myelin water atlas demonstrated significant MWF variation across control WM. No significant MWF differences were found between male and female healthy participants. MS z-score maps revealed diffuse regions of demyelination in the two participants with Expanded Disability Status Scale (EDSS) = 2.0 but not in the participant with EDSS = 0.

CONCLUSIONS

The myelin water atlas can be used as a reference (URL: https://sourceforge.net/projects/myelin-water-atlas/) to demonstrate areas of demyelination in individual MS participants. Future studies will expand the atlas age range, account for education, and other variables that may affect myelination.

Loss of Wwox Causes Defective Development of Cerebral Cortex with Hypomyelination in a Rat Model of Lethal Dwarfism with Epilepsy

WW domain-containing oxidoreductase (Wwox) is a putative tumor suppressor. Several germline mutations of Wwox have been associated with infant neurological disorders characterized by epilepsy, growth retardation, and early death. Less is known, however, about the pathological link between Wwox mutations and these disorders or the physiological role of Wwox in brain development. In this study, we examined age-related expression and histological localization of Wwox in forebrains as well as the effects of loss of function mutations in the Wwox gene in the immature cortex of a rat model of lethal dwarfism with epilepsy (lde/lde). Immunostaining revealed that Wwox is expressed in neurons, astrocytes, and oligodendrocytes. lde/lde cortices were characterized by a reduction in neurite growth without a reduced number of neurons, severe reduction in myelination with a reduced number of mature oligodendrocytes, and a reduction in cell populations of astrocytes and microglia. These results indicate that Wwox is essential for normal development of neurons and glial cells in the cerebral cortex.

Novel pathologic findings in patients with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an X-linked inherited hypomyelinating disorder caused by mutations in the gene encoding proteolipid protein (PLP), the major structural protein in central nervous system (CNS) myelin. Prior to our study, whether hypomyelination in PMD was caused by demyelination, abnormally thin sheaths or failure to form myelin was unknown. In this study, we compared the microscopic pathology of myelin from brain tissue of 3 PMD patients with PLP1 duplications to that of a patient with a complete PLP1 deletion. Autopsy tissue procured from PMD patients was embedded in paraffin for immunocytochemistry and plastic for electron microscopy to obtain highresolution fiber pathology of cerebrum and corpus callosum. Through histological stains, immunocytochemistry and electron microscopy, our study illustrates unique pathologic findings between the two different types of mutations. Characteristic of the patient with a PLP1 deletion, myelin sheaths showed splitting and decompaction of myelin, confirming for the first time that myelin in PLP1 deletion patients is similar to that of rodent models with gene deletions. Myelin thickness and g-ratios of some fibers, in relation to axon diameter was abnormally thin, suggesting that oligodendrocytes remain metabolically functional and/or are attempting to make myelin. Many fibers showed swollen, progressive degenerative changes to axons in addition to the dissolution of myelin. All three duplication cases shared remarkable fiber pathology including swellings, constriction and/or transection and involution of myelin. Characteristic of PLP1 duplication patients, many axons showed segmental demyelination along their length. Still other axons had abnormally thick myelin sheaths, suggestive of continued myelination. Thus, each type of mutation exhibited unique pathology even though commonality to both mutations included involution of myelin, myelin balls and degeneration of axons. This pathology study describes findings unique to each mutation that suggests the mechanism causing fiber pathology is likewise heterogeneous.

Mosaic Subventricular Origins of Forebrain Oligodendrogenesis

In the perinatal as well as the adult CNS, the subventricular zone (SVZ) of the forebrain is the largest and most active source of neural stem cells (NSCs) that generates neurons and oligodendrocytes (OLs), the myelin forming cells of the CNS. Recent advances in the field are beginning to shed light regarding SVZ heterogeneity, with the existence of spatially segregated microdomains that are intrinsically biased to generate phenotypically distinct neuronal populations. Although most research has focused on this regionalization in the context of neurogenesis, newer findings underline that this also applies for the genesis of OLs under the control of specific patterning molecules. In this mini review, we discuss the origins as well as the mechanisms that induce and maintain SVZ regionalization. These come in the flavor of specific signaling ligands and subsequent initiation of transcriptional networks that provide a basis for subdividing the SVZ into distinct lineage-specific microdomains. We further emphasize canonical Wnts and FGF2 as essential signaling pathways for the regional genesis of OL progenitors from NSCs of the dorsal SVZ. This aspect of NSC biology, which has so far received little attention, may unveil new avenues for appropriately recruiting NSCs in demyelinating diseases.

The expression of PLP/DM-20 mRNA is restricted to the oligodendrocyte-lineage cells in the adult rat spinal cord

Proteolipid protein (PLP) is the major component of myelin; its gene encodes two major splicing variants: PLP and DM-20. Compared with PLP, DM-20 lacks the amino acids encoded by exon IIIb. The expression of PLP/DM-20 in cells outside the oligodendrocyte-lineage is unclear. To address this issue, we analyzed the detailed expression pattern of PLP/DM-20 mRNA in the adult rat spinal cord by in situ hybridization (ISH) with a cRNA probe complementary to DM-20 mRNA, which has been used to detect both PLP and DM-20 both mRNA. ISH did not label the cells expressing NeuN nor glial fibrillary acidic protein but detected those expressing Olig2, indicating that PLP/DM-20 mRNA are expressed only in oligodendrocyte-lineage cells. This cell population was expected to contain NG2-expressing oligodendrocyte precursor cells (OPCs), because some exhibited the expression of glutathione S-transferase pi isoform in the nucleus. A recent publication showed that OPCs express PLP but not DM-20 mRNA. However, no OPCs were detected. We performed ISH with a cRNA probe that specifically recognizes PLP mRNA to successfully detect some OPCs. Additionally, OPCs were detected by ISH with a cRNA probe complementary to DM-20 mRNA that was digested via alkaline hydrolysis prior to ISH. These findings collectively demonstrate that PLP and DM-20 mRNA expression is restricted to oligodendrocyte-lineage cells, and imply that the undigested cRNA probe complementary to the full-length DM-20 mRNA sequence only recognizes DM-20 mRNA and not the PLP counterpart when applied to ISH without denaturation/digestion methods.

Differential activation of ER stress pathways in myelinating cerebellar tracts

Myelin production during brain development requires an increase in membrane protein and lipid production in oligodendrocytes and this primarily occurs in the endoplasmic reticulum (ER), an organelle which initiates the Unfolded Protein Response (UPR) when under stress. We hypothesise that the UPR is activated in white matter tracts during myelination in order to expand the ER capacity of oligodendrocytes. Using early and late stage markers, critical myelination time points were identified by immunohistochemistry in developing rat cerebellum. These were correlated to peaks in ER stress signalling by staining for activated UPR transducers (pIRE1, ATF6 and pPERK) and associated downstream molecules (peIF2α, PDI, GRP78, GRP94, CHOP and calreticulin) in cerebellar tracts III and IV. Gene expression in developing cerebellum was assessed by qPCR. Actively myelinating tracts were shown to have differential expression of pIRE1, PERK and ATF6 as well as UPR targets GRP94, GRP78 and PDI. Activated pIRE1-positive cells were widespread at P14 and P17 and at significantly higher numbers during myelination than at other stages. Nuclear-localised ATF6 (indicative of the active transcription factor) peaked at P10, concurrent with the initial phase of myelination. The percentage of cells positive for pPERK was less than 1% at postnatal ages but increased significantly in adult tissue. The downstream targets GRP78, GRP94 and PDI were significantly up-regulated at P17 compared to P7 and remained significantly elevated in adults. The majority of cells positive for these markers and ATF6 were oligodendrocytes as confirmed by dual-labelling. Although gene expression in the cerebellum for GRP78, GRP94 and PDI did not change significantly over time, ATF6 and XBP1s both showed significant fold changes between early and late timepoints. This data helps promote understanding of events occurring during developmental myelination and may have implications for the development of reparative treatments in diseases such as multiple sclerosis.

Chronic intermittent ethanol induced axon and myelin degeneration is attenuated by calpain inhibition

Chronic alcohol consumption causes multifaceted damage to the central nervous system (CNS), underlying mechanisms of which are gradually being unraveled. In our previous studies, activation of calpain, a calcium-activated neutral protease has been found to cause detrimental alterations in spinal motor neurons following ethanol (EtOH) exposure in vitro. However, it is not known whether calpain plays a pivotal role in chronic EtOH exposure-induced structural damage to CNS in vivo. To test the possible involvement of calpain in EtOH-associated neurodegenerative mechanisms the present investigation was conducted in a well-established mouse model of alcohol dependence - chronic intermittent EtOH (CIE) exposure and withdrawal. Our studies indicated significant loss of axonal proteins (neurofilament light and heavy, 50-60%), myelin proteins (myelin basic protein, 20-40% proteolipid protein, 25%) and enzyme (2', 3'-cyclic-nucleotide 3'-phosphodiesterase, 21-55%) following CIE in multiple regions of brain including hippocampus, corpus callosum, cerebellum, and importantly in spinal cord. These CIE-induced deleterious effects escalated after withdrawal in each CNS region tested. Increased expression and activity of calpain along with enhanced ratio of active calpain to calpastatin (sole endogenous inhibitor) was observed after withdrawal compared to EtOH exposure. Pharmacological inhibition of calpain with calpeptin (25 μg/kg) prior to each EtOH vapor inhalation significantly attenuated damage to axons and myelin as demonstrated by immuno-profiles of axonal and myelin proteins, and Luxol Fast Blue staining. Calpain inhibition significantly protected the ultrastructural integrity of axons and myelin compared to control as confirmed by electron microscopy. Together, these findings confirm CIE exposure and withdrawal induced structural alterations in axons and myelin, predominantly after withdrawal and corroborate calpain inhibition as a potential protective strategy against EtOH associated CNS degeneration.

Cannabis and adolescent brain development

Heavy cannabis use has been frequently associated with increased rates of mental illness and cognitive impairment, particularly amongst adolescent users. However, the neurobiological processes that underlie these associations are still not well understood. In this review, we discuss the findings of studies examining the acute and chronic effects of cannabis use on the brain, with a particular focus on the impact of commencing use during adolescence. Accumulating evidence from both animal and human studies suggests that regular heavy use during this period is associated with more severe and persistent negative outcomes than use during adulthood, suggesting that the adolescent brain may be particularly vulnerable to the effects of cannabis exposure. As the endocannabinoid system plays an important role in brain development, it is plausible that prolonged use during adolescence results in a disruption in the normative neuromaturational processes that occur during this period. We identify synaptic pruning and white matter development as two processes that may be adversely impacted by cannabis exposure during adolescence. Potentially, alterations in these processes may underlie the cognitive and emotional deficits that have been associated with regular use commencing during adolescence.

Short-term cuprizone feeding verifies N-acetylaspartate quantification as a marker of neurodegeneration

Proton magnetic resonance spectroscopy (1H-MRS) is a quantitative MR imaging technique often used to complement conventional MR imaging with specific metabolic information. A key metabolite is the amino acid derivative N-Acetylaspartate (NAA) which is an accepted marker to measure the extent of neurodegeneration in multiple sclerosis (MS) patients. NAA is catabolized by the enzyme aspartoacylase (ASPA) which is predominantly expressed in oligodendrocytes. Since the formation of MS lesions is paralleled by oligodendrocyte loss, NAA might accumulate in the brain, and therefore, the extent of neurodegeneration might be underestimated. In the present study, we used the well-characterized cuprizone model. There, the loss of oligodendrocytes is paralleled by a reduction in ASPA expression and activity as demonstrated by genome-wide gene expression analysis and enzymatic activity assays. Notably, brain levels of NAA were not increased as determined by gas chromatography-mass spectrometry and 1H-MRS. These important findings underpin the reliability of NAA quantification as a valid marker for the paraclinical determination of the extent of neurodegeneration, even under conditions of oligodendrocyte loss in which impaired metabolization of NAA is expected. Future studies have to reveal whether other enzymes are able to metabolize NAA or whether an excess of NAA is cleared by other mechanisms rather than enzymatic metabolism.

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

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

Organization of myelin in the mouse somatosensory barrel cortex and the effects of sensory deprivation

In rodents, the barrel cortex is a specialized area within the somatosensory cortex that processes signals from the mystacial whiskers. We investigated the normal development of myelination in the barrel cortex of mice, as well as the effects of sensory deprivation on this pattern. Deprivation was achieved by trimming the whiskers on one side of the face every other day from birth. In control mice, myelin was not present until postnatal day 14 and did not show prominence until postnatal day 30; adult levels of myelination were reached by the end of the second postnatal month. Unbiased stereology was used to estimate axon density in the interbarrel septal region and barrel walls as well as the barrel centers. Myelin was significantly more concentrated in the interbarrel septa/barrel walls than in the barrel centers in both control and sensory-deprived conditions. Sensory deprivation did not impact the onset of myelination but resulted in a significant decrease in myelinated axons in the barrel region and decreased the amount of myelin ensheathing each axon. Visualization of the oligodendrocyte nuclear marker Olig2 revealed a similar pattern of myelin as seen using histochemistry, but with no significant changes in Olig2+ nuclei following sensory deprivation. Consistent with the anatomical results showing less myelination, local field potentials revealed slower rise times following trimming. Our results suggest that myelination develops relatively late and can be influenced by sensory experience.

Intraventricular injection of FGF-2 promotes generation of oligodendrocyte-lineage cells in the postnatal and adult forebrain

FGF2 is considered a key factor in the generation of oligodendrocytes (OLs) derived from neural stem cells (NSCs) located within the subventricular zone (SVZ). Here, we have examined FGF2 signaling in the forebrain of postnatal and adult mice. Using qPCR of microdissected microdomains of the dorsal SVZ (dSVZ) and lateral SVZ (lSVZ), and prominin1-sorted NSCs purified from these microdomains, we show that transcripts for FGF receptor 1 (FGFR1) and FGFR2 are enriched in the dSVZ, from which OLs are largely derived, whereas FGFR3 are significantly enriched within prominen1-sorted NSC of the lSVZ, which mainly generate olfactory interneurons. We show that direct administration of FGF2 into the lateral ventricle increased the generation of oligodendrocyte progenitors (OPCs) throughout the SVZ, both within the dSVZ and ectopically in the lSVZ and ependymal wall of the SVZ. Furthermore, FGF2 stimulated proliferation of neural progenitors (NPs) and their differentiation into OPCs. The results indicate that FGF2 increased specification of OPCs, inducing NPs to follow an oligodendrocyte developmental pathway. Notably, FGF2 did not block OPC differentiation and increased the number of oligodendrocytes in the periventricular white matter (PVWM) and cortex. However, FGF2 markedly disrupted myelination in the PVWM. A key finding was that FGF2 had equivalent actions on the generation of OPCs and myelin disruption in postnatal and adult mice. This study demonstrates a central role for FGF2 in promoting oligodendrocyte generation in the developing and adult brain.

Neuroglialpharmacology: myelination as a shared mechanism of action of psychotropic treatments

Current psychiatric diagnostic schema segregate symptom clusters into discrete entities, however, large proportions of patients suffer from comorbid conditions that fit neither diagnostic nor therapeutic schema. Similarly, psychotropic treatments ranging from lithium and antipsychotics to serotonin reuptake inhibitors (SSRIs) and acetylcholinesterase inhibitors have been shown to be efficacious in a wide spectrum of psychiatric disorders ranging from autism, schizophrenia (SZ), depression, and bipolar disorder (BD) to Alzheimer's disease (AD). This apparent lack of specificity suggests that psychiatric symptoms as well as treatments may share aspects of pathophysiology and mechanisms of action that defy current symptom-based diagnostic and neuron-based therapeutic schema. A myelin-centered model of human brain function can help integrate these incongruities and provide novel insights into disease etiologies and treatment mechanisms. Available data are integrated herein to suggest that widely used psychotropic treatments ranging from antipsychotics and antidepressants to lithium and electroconvulsive therapy share complex signaling pathways such as Akt and glycogen synthase kinase-3 (GSK3) that affect myelination, its plasticity, and repair. These signaling pathways respond to neurotransmitters, neurotrophins, hormones, and nutrition, underlie intricate neuroglial communications, and may substantially contribute to the mechanisms of action and wide spectra of efficacy of current therapeutics by promoting myelination. Imaging and genetic technologies make it possible to safely and non-invasively test these hypotheses directly in humans and can help guide clinical trial efforts designed to correct myelination abnormalities. Such efforts may provide insights into novel avenues for treatment and prevention of some of the most prevalent and devastating human diseases.

Levels of BDNF impact oligodendrocyte lineage cells following a cuprizone lesion

Previous work in culture has shown that basal forebrain (BF) oligodendrocyte (OLG) lineage cells respond to BDNF by increasing DNA synthesis and differentiation. Further, in the BF in vivo, reduced levels of BDNF as seen in BDNF(+/-) mice result in reduced numbers of NG2+ cells and deficits in myelin proteins throughout development and in the adult, suggesting that BDNF impacts the proliferating population of OLGs as well as differentiation in vivo. In this study, to investigate the roles BDNF may play in the repair of a demyelinating lesion, the cuprizone model was used and the corpus callosum was examined. BDNF protein levels were reduced after cuprizone treatment, suggesting that the demyelinating lesion itself elicits a decrease in BDNF. To analyze the effects of a further reduction of BDNF on OLG lineage cells following cuprizone, BDNF(+/-) mice were evaluated. These mice exhibited a blunted increase in the NG2 response at 4 and 5 weeks of cuprizone treatment. In addition, BDNF(+/-) mice exhibited decreased levels of myelin proteins during the demyelination and remyelination processes with no change in the total number of OLGs. These effects appear to be relatively specific to OLG lineage cells as comparable changes in CD11b+ microglia, GFAP+ astrocytes, and SMI32+ injured axons were not observed. These data indicate that BDNF may play a role following a demyelinating lesion by regulating the numbers of progenitors and the abilities of demyelinating and differentiating cells to express myelin proteins.

Age-dependent changes in the proteome following complete spinal cord transection in a postnatal South American opossum (Monodelphis domestica)

Recovery from severe spinal injury in adults is limited, compared to immature animals who demonstrate some capacity for repair. Using laboratory opossums (Monodelphis domestica), the aim was to compare proteomic responses to injury at two ages: one when there is axonal growth across the lesion and substantial behavioural recovery and one when no axonal growth occurs. Anaesthetized pups at postnatal day (P) 7 or P28 were subjected to complete transection of the spinal cord at thoracic level T10. Cords were collected 1 or 7 days after injury and from age-matched controls. Proteins were separated based on isoelectric point and subunit molecular weight; those whose expression levels changed following injury were identified by densitometry and analysed by mass spectrometry. Fifty-six unique proteins were identified as differentially regulated in response to spinal transection at both ages combined. More than 50% were cytoplasmic and 70% belonged to families of proteins with characteristic binding properties. Proteins were assigned to groups by biological function including regulation (40%), metabolism (26%), inflammation (19%) and structure (15%). More changes were detected at one than seven days after injury at both ages. Seven identified proteins: 14-3-3 epsilon, 14-3-3 gamma, cofilin, alpha enolase, heart fatty acid binding protein (FABP3), brain fatty acid binding protein (FABP7) and ubiquitin demonstrated age-related differential expression and were analysed by qRT-PCR. Changes in mRNA levels for FABP3 at P7+1day and ubiquitin at P28+1day were statistically significant. Immunocytochemical staining showed differences in ubiquitin localization in younger compared to older cords and an increase in oligodendrocyte and neuroglia immunostaining following injury at P28. Western blot analysis supported proteomic results for ubiquitin and 14-3-3 proteins. Data obtained at the two ages demonstrated changes in response to injury, compared to controls, that were different for different functional protein classes. Some may provide targets for novel drug or gene therapies.

GSK3β negatively regulates oligodendrocyte differentiation and myelination in vivo

Glycogen synthase kinase 3β (GSK3β) is an essential integrating molecule for multiple proliferation and differentiation signals that regulate cell fate. Here, we have examined the effects of inhibiting GSK3β on the development of oligodendrocytes (OLs) from their oligodendrocyte precursors (OP) in vivo by injection into the lateral ventricle of postnatal mice and ex vivo in organotypic cultures of isolated intact rodent optic nerve. Our results show that a range of GSK3β inhibitors (ARA-014418, lithium, indirubin, and L803-mt) increase OPs and OLs and promote myelination. Inhibition of GSK3β stimulates OP proliferation and is prosurvival and antiapoptotic. The effects of GSK3β inhibition in OPs is via the canonical Wnt signaling pathway by stimulating nuclear translocation of β-catenin. However, direct comparison of the effects of Wnt3a and GSK3β inhibition in optic nerves shows that they have opposing actions on OLs, whereby GSK3β inhibition strikingly increases OL differentiation, whereas Wnt3a inhibits OL differentiation. Notably, GSK3β inhibition overrides the negative effects of Wnt3a on OLs, indicating novel GSK3β signaling mechanisms that negatively regulate OL differentiation. We identify that two mechanisms of GSK3β inhibition are to stimulate cAMP response element binding (CREB) and decrease Notch1 signaling, which positively and negatively regulate OL differentiation and myelination, respectively. A key finding is that GSK3β inhibition has equivalent effects in the adult and stimulates the regeneration of OLs and remyelination following chemically induced demyelination. This study identifies GSK3β as a profound negative regulator of OL differentiation that contributes to inefficient regeneration of OLs and myelin repair in demyelination.

BDNF+/- mice exhibit deficits in oligodendrocyte lineage cells of the basal forebrain

Previous work indicated that brain-derived neurotrophic factor (BDNF), through the trkB receptor, increases DNA synthesis in oligodendrocyte (OLG) progenitor cells (OPCs) and differentiation of postmitotic OLGs of the basal forebrain (BF). In the present studies, BDNF knockout animals were used to investigate BDNF's effects on OLG lineage cells (OLCs) in vivo. OLCs of the BF were found to express the trkB receptor, suggesting they are responsive to BDNF. Immunohistochemistry using NG2 and CC1 antibodies was utilized to examine the numbers of NG2+ OPCs and CC1+ postmitotic BF OLGs. At embryonic day 17 (E17), BDNF-/- animals display reduced NG2+ cells. This reduction was also observed in BDNF+/- mice at E17 and at postnatal day 1 (P1), P14, and adult stage, suggesting that BDNF plays a role in OPC development. BDNF+/- mice do not exhibit deficits in numbers of CC1+ OLGs. However, myelin basic protein, myelin associated glycoprotein, and proteolipid protein are reduced in BDNF+/- mice, suggesting that BDNF plays a role in differentiation. These data indicate that progenitor cells and myelin proteins may be affected in vivo by a decrease in BDNF.

Isolation of myelin

The methods used to prepare myelin involve homogenization of the tissue in isotonic sucrose solution, followed by the isolation of myelin membranes by a series of steps that include density gradient centrifugation and differential centrifugation. Homogenization of nervous tissue in isotonic sucrose causes the myelin sheath to peel from the axon and form relatively large myelin vesicles. The large size of the myelin vesicles, together with the fact that myelin membrane has a lower density than other biological membranes, make differential centrifugation and density gradient centrifugation the main tools for the isolation of this membrane. Three protocols are outlined in this unit: isolation of a highly-purified myelin fraction from the central nervous system (CNS); separation of a highly-purified CNS myelin fraction into subfractions of different densities; and isolation of myelin from the peripheral nervous system (PNS).

Morphometric analysis of oligodendrocytes in the adult mouse frontal cortex

Oligodendrocytes (OLs), the myelinating cells of the central nervous system, have specialized morphologies that subserve their function. Numerous qualitative studies suggest that OLs in different brain regions can differ in their morphological characteristics, including number of branches and internodes, internode length, etc. However, progress in identifying and characterizing the diverse types of OLs and their distribution in the brain has been made difficult by several technical constraints. Here we report a new strategy to analyze OL morphology with a high degree of quantitative power and throughput. We used confocal microscopy and three-dimensional cell tracing software to study OLs in the frontal cortex of mice expressing enhanced green fluorescent protein (eGFP) under the control of the proteolipid protein (Plp) gene promoter. Three-dimensional reconstructions were then used to analyze and quantify cell morphology, including total process length, total process surface area, total internode length, number of primary processes, number of branch points, and number of internodes. In addition, these reconstructions were subjected to Sholl analysis, which allows for the quantitative measure of OL arbor complexity. By using this approach, we identified and characterized a previously undescribed population of small OLs with a compact but complex morphology that includes numerous branching processes and a large number of short internodes. Our data suggest that other populations of OLs remain to be identified and characterized and that the tools we have developed could help in the process of characterizing them.

Myelin breakdown and iron changes in Huntington's disease: pathogenesis and treatment implications

BACKGROUND

Postmortem and in vivo imaging data support the hypothesis that premature myelin breakdown and subsequent homeostatic remyelination attempts with increased oligodendrocyte and iron levels may contribute to Huntington's Disease (HD) pathogenesis and the symmetrical progress of neuronal loss from earlier-myelinating striatum to later-myelinating regions. A unique combination of in vivo tissue integrity and iron level assessments was used to examine the hypothesis.

METHODS

A method that uses two Magnetic resonance imaging (MRI) instruments operating at different field-strengths was used to quantify the iron content of ferritin molecules (ferritin iron) as well as tissue integrity in eight regions in 11 HD and a matched group of 27 healthy control subjects. Three white matter regions were selected based on their myelination pattern (early to later-myelinating) and fiber composition. These were frontal lobe white matter (Fwm) and splenium and genu of the corpus callosum (Swm and Gwm). In addition, gray matter structures were also chosen based on their myelination pattern and fiber composition. Three striatum structures were assessed [caudate, putamen, and globus pallidus (C, P, and G)] as well as two comparison gray matter regions that myelinate later in development and are relatively spared in HD [Hippocampus (Hipp) and Thalamus (Th)].

RESULTS

Compared to healthy controls, HD ferritin iron levels were significantly increased in striatum C, P, and G, decreased in Fwm and Gwm, and were unchanged in Hipp, Th, and Swm. Loss of tissue integrity was observed in C, P, Fwm, and especially Swm but not Hipp, Th, G, or Gwm. This pattern of findings was largely preserved when a small subset of HD subjects early in the disease process was examined.

CONCLUSIONS

The data suggest early in the HD process, myelin breakdown and changes in ferritin iron distribution underlie the pattern of regional toxicity observed in HD. Prospective studies are needed to verify myelin breakdown and increased iron levels are causal factors in HD pathogenesis. Tracking the effects of novel interventions that reduce myelin breakdown and iron accumulation in preclinical stages of HD could hasten the development of preventive treatments.

Oligodendrocytes express P2Y12 metabotropic receptor in adult rat brain

In the CNS, nucleotide receptors termed P2 receptors are identified on neurons and glial cells, mediating neuron-neuron, glia-glia and glia-neuron communication. In the present work, we qualify in vivo in the adult rat CNS the cellular/subcellular distribution of P2Y12 receptor protein in cerebral cortex, white matter and subcortical nuclei (striatum and substantia nigra), by means of immunofluorescence-confocal, electron microscopy and Western blot analysis. P2Y12 receptor immunoreactivity colocalizes neither with markers such as neuronal nuclei, neurofilament light chain, calbindin and tyrosine hydroxylase, nor with glial fibrillary acidic protein and isolectin B4, but with myelin basic protein and the oligodendrocyte marker RIP, in both cell bodies and processes, indicating therefore oligodendrocyte localization. Electron microscopy identifies P2Y12 receptors in both the perikaryon and under the plasmalemma of oligodendrocyte cell bodies and radiating processes, until the paranodal region of fibers. By Western blot analysis, P2Y12 receptor shows a specific band of 42-44 kDa, matching the molecular mass predicted from amino acid sequencing. Since in platelets P2Y12 receptor is known to regulate adhesion/activation and thrombus growth/stability, from our results we could speculate by analogy that, in oligodendrocytes, P2Y12 receptor signaling might contribute to the migration and adhesion of the glial processes to axons to be myelinated.

Combining laser capture microdissection with quantitative real-time PCR: effects of tissue manipulation on RNA quality and gene expression

Laser capture microdissection (LCM) is increasingly being used in quantitative gene expression studies of the nervous system. The current study aimed at determining the impact of various tissue manipulations on the integrity of extracted RNA in LCM studies. Our data indicate that various tissue preparation strategies prior to microdissection may decrease RNA quality by as much as 25%, thus affecting expression profiles of some genes. To circumvent this problem, we developed a strategy for reverse transcriptase real-time PCR that has considerable sensitivity and can be used to calculate relative changes in gene expression. This approach was validated in subregions of the rat cerebellum. Accordingly, expression of glial gene markers - myelin-associated glycoprotein and proteolipid protein 1 - was found 70-160-fold higher in the white matter layer of the cerebellar cortex as compared to the neuron-enriched granular layer. In contrast, expression of a specific neuronal maker, neuron-specific enolase, was found seven-fold higher in the granular layer, as compared to the white matter layer. Furthermore, this approach had high sensitivity and specificity as we were able to detect a 38% decrease in the expression of neuron-specific enolase without a change in the expression of glial markers following administration of the neurotoxin, ibotenic acid. These results demonstrate feasibility of performing accurate semi-quantitative gene expression analyses in LCM samples.

Myelinogenesis in the Brachial and Lumbosacral Enlargements of the Spinal Cord of the Opossum Monodelphis domestica

Using immunohistochemistry in light microscopy, the myelin basic protein and proteolipid protein were localized on sections of the spinal cord enlargements of opossums, Monodelphis domestica, to determine the timecourse of myelinogenesis therein and compare it with other events of motor systems development. Additional tissue not processed for immunohistochemistry was prepared for transmission electron microscopy. No immunolabeling for either protein occurred on spinal sections from the newborn opossum, but in electron microscopy occasional fibers surrounded by loose, irregular membranous rings were seen on the outskirts of the ventral horn. Immunolabeling was detected first in the brachial enlargement during the second week, presumably on motoneuronal, vestibular and reticular axons. The areas of the dorsal columns, other spino-encephalic, reticulospinal and propriospinal projections became labeled in the third week, and the area of rubrospinal axons at 4 weeks. In the brachial gray matter, immunolabeling appeared along ventrodorsal and lateromedial gradients from the fourth to seventh weeks. Labeling developed similarly in the white and gray matter of the lumbosacral enlargement, but 3-5 days later than at brachial levels. Labeling intensity in the white and gray matter increased until at least 4 months, but remained light in laminae I-III. Thus, myelinogenesis in the spinal cord enlargements of the opossum is protracted and follows general rostrocaudal, ventrodorsal and lateromedial sequences. It occurs later than synaptogenesis at comparable levels of the cord, but earlier than myelinogenesis in the corresponding ventral and dorsal roots. Spinal myelinogenesis correlates with the development of sensorimotor reflexes, weight support and quadrupedal locomotion.

The Age-Related Degeneration of Oligodendrocytes in the Hippocampus of the Senescence-Accelerated Mouse (SAM) P8: A Quantitative Immunohistochemical Study

The senescence-accelerated mouse (SAM) is known as a murine model for accelerated aging. The SAMP8 shows age-related deficits of learning and memory at an earlier age than control mice (SAMR1). We investigated the changes in oligodendrocytes in the brain of SAMP8, using immunohistochemistry for myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) as an oligodendrocyte marker. SAMP8 at 10 months old showed a decrease in MBP-immunoreactivity (IR) and CNP-IR in the hippocampal CA1 subfield, compared with SAMR1. There were no significant differences in MBP and CNP old in the cerebral cortex and the optic tract between SAMR1 and SAMP8 at 10 months. Furthermore, we measured the area of MBP-IR in the CA1 subfield of both strains and found that the area of MBP-IR in SAMP8 had decreased progressively with age, compared with SAMR1. These results suggest that age-related degeneration of oligodendrocytes had occurred in the hippocampus of SAMP8.

Developmental partitioning of myelin basic protein into membrane microdomains

Specific membrane microdomains (including lipid rafts) exist in myelin but have not been fully characterized. Myelin basic protein (MBP) maintains the compactness of the myelin sheath and is highly posttranslationally modified. Thus, it has been suggested that MBP might also have other functions, e.g., in signal transduction. Here, the distribution of MBP and its modified forms was studied, spatially and temporally, by detailed characterization of membrane microdomains from developing and mature bovine myelin. Myelin membranes were extracted with three different detergents (Brij 96V, CHAPS, or Triton X-100) at 4 degrees C. The detergent-resistant membranes (DRMs), representing coalesced lipid rafts, were isolated as low-buoyant-density fractions on a sucrose density gradient. These myelin rafts were disrupted when cholesterol was depleted with methyl-beta-cyclodextrin. The use of CHAPS detergent led to enrichment of several myelin proteins, including phospho-Thr97-MBP, in the DRMs from mature myelin. Citrullinated and methylated MBP remained in "nonraft" microdomains. In contrast, the DRMs from early myelin were enriched in Golli-MBP, Fyn, Lyn, and CNP. The localization of various proteins in DRMs was further supported by the colocalization of these lipid raft components in cultured mouse oligodendrocytes. Thus, there is a developmental regulation of posttranslationally modified forms of MBP into specific membrane microdomains.

Identification and distribution of aspartoacylase in the postnatal rat brain

Aspartoacylase metabolizes N-acetylaspartic acid to produce L-aspartate and acetate. An aspartoacylase deficiency in humans is responsible for Canavan disease, a lethal autosomal recessive leukodystrophy. The role of aspartoacylase in the mammalian brain is unclear. Here we have generated and characterized a highly specific polyclonal antibody against aspartoacylase which recognizes a 37 kDa monomer and a dimer in normal but not in aspartoacylase-deficient rat tissue. Aspartoacylase protein expression sharply increases at P14, peaks at P28 and plateaus thereafter. Biochemical analysis reveals immunoreactivity in cytosolic but not in membrane fractions. Histologically, most abundant expression was observed in white matter tracts and thalamus. On the cellular level, aspartoacylase immunoreactivity is restricted to oligodendrocyte somata in both white and gray matter.

Expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the developing olfactory bulb and subventricular zone rostral extension

The olfactory bulb (OB) presents a unique pattern of permanent acquisition of primary afferents and interneurons, but not much detail is known about the differentiation of its oligodendroglial cells. We studied the expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a protein related to axonal ensheathment by myelinating cells. Expression of CNPase in OB follows a general caudorostral gradient, with the exception of the glomerular layer (GL). At postnatal day 5-6 (P5-P6), the first CNPase(+) profiles appeared in the dorsal lateral olfactory tract adjacent to the accessory OB (AOB), followed by rare cell bodies and processes in AOB internal plexiform layer at P7. At P9, the main OB (MOB) granular cell layer (GrCL) already showed intensely stained CNPase(+) processes. From P5 to P12, small numbers of CNPase(+) cells were found in the subventricular zone (SVZ), throughout its rostral extension (SVZ-RE), and in the intrabulbar subependymal layer. The appearance of CNPase(+) profiles delimiting glomeruli started in the GL rostralmost region at P12, extending to all GL levels, but glomeruli remained open caudally at P15. At P18, oligodendroglial glomeruli were evident throughout OB, but the adult pattern was established only after P30. There was no age-related loss of CNPase immunoreactivity in glial cell bodies, possibly indicating de novo ensheathment of neurites. Our results show an earlier onset of oligodendroglial differentiation in OB than previously reported and a rostrocaudal gradient of formation of oligodendroglial glomeruli. They also raise the possibility that a minor fraction of OB oligodendrocytes might derive from the SVZ-RE.

The structure and function of myelin: from inert membrane to perfusion pump

The goal of this overview is to propose a novel structure/function model of central nervous system myelin. Although myelin is known to be a compact multilamellar structure that wraps around axons, the biologic role this structure plays in the nervous system remains an enigma. One means of ascertaining myelin's biologic role is by analyzing its structure. The recent discovery of tight junctions in myelin may be the key that unlocks the mysterious black box of myelin structure/function. Tight junctions in other cell types are invariably adjacent to adherens junctions, with both of these junctional plaques playing critical roles in paracellular barrier function, i.e., adhesion of cell membranes, signal transduction, and fluid movement between cells via aqueous pores and channels. The application of current knowledge about junctional plaques to myelin is an original concept. This knowledge, taken together with evidence from studies of normal and pathologic myelin, supports the possibility that a primary function of junctional plaques in myelin is to perfuse the periaxonal space.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.

Antigens of monoclonal antibody NB3C4 are novel markers for oligodendrocytes

We produced NB3C4, a novel monoclonal antibody specific for oligodendrocytes, using human neuroblastoma IMR-32 cells. NB3C4 specifically recognized oligodendrocytes in the CNS, although it bound to neuroblastoma IMR-32 cells and oligodendrocytes in vitro. Double immunofluorescence staining of rat brain using NB3C4 and anti-GST-pi, anti-glial fibrillary acidic protein (GFAP), or anti-neurofilament 200 (NF) antibody revealed that anti-GST-pi antibody identified an oligodendrocyte marker recognizing NB3C4-positive cells, while both anti-GFAP and anti-NF antibody did not. Western blotting of rat brain homogenates showed that NB3C4 bound three proteins of 22-28 kDa, while the anti-GST-pi recognized a 27 kDa protein. Therefore, antigens recognized by NB3C4 could be novel markers for oligodendrocytes.

Myelin proteolipid proteins promote the interaction of oligodendrocytes and axons

Although proteolipid protein (PLP) and its DM20 isoform are the major membrane proteins of CNS myelin, their absence causes surprisingly few developmental defects. In comparison, missense mutations of the X-linked Plp gene cause severe dysmyelination. Previous studies have established roles for PLP/DM20 in the formation of the intraperiod line and in maintaining axonal integrity. We now show that a normal number of oligodendrocytes are present in mice lacking PLP/DM20. However, in heterozygous females, which are natural chimeras for X-linked genes, oligodendrocytes lacking PLP/DM20 are in direct competition with wild-type oligodendrocytes that have a distinct advantage. PLP+ oligodendrocytes and PLP+ myelin sheaths make up the greater majority, and this feature is generalised in the CNS throughout life. Moreover, in the absence of PLP/DM20, a proportion of small-diameter axons fails to myelinate, remaining ensheathed but lacking a compact sheath, or show delayed myelination. These findings suggest that PLP/DM20 is also involved in the early stages of axon-oligodendrocyte interaction and wrapping of the axon.

Oligodendrocytes and the control of myelination in vivo: new insights from the rat anterior medullary velum

The rat anterior medullary velum (AMV) is representative of the brain and spinal cord, overall, and provides an almost two-dimensional preparation for investigating axon-glial interactions in vivo. Here, we review some of our findings on axon-oligodendrocyte unit relations in our adult, development, and injury paradigms: (1) adult oligodendrocytes are phenotypically heterogeneous, conforming to Del Rio Hortega's types I-IV, whereby differences in oligodendrocyte morphology, metabolism, myelin sheath radial and longitudinal dimensions, and biochemistry correlate with the diameters of axons in the unit; (2) oligodendrocytes derive from a common premyelinating oligodendrocyte phenotype, and divergence of types I-IV is related to the age they emerge and the presumptive diameter of axons in the unit; (3) during myelination, axon-oligodendrocyte units progress through a sequence of maturation phases, related to axon contact, ensheathment, establishment of internodal myelin sheaths, and finally the radial growth and compaction of the myelin sheath; (4) we provide direct in vivo evidence that platelet-derived growth factor-AA (PDGF-AA), fibroblast growth factor (FGF-2), and insulin-like growth factor-I (IGF-I) differentially regulate these events, by injecting the growth factors into the cerebrospinal fluid of neonatal rat pups; (5) in lesioned adult AMV, transected central nervous system (CNS) axons regenerate through the putatively inhibitory environment of the glial scar, but remyelination by oligodendrocytes is incomplete, indicating that axon-oligodendrocyte interactions are defective; and (6) in the adult AMV, cells expressing the NG2 chondroitin sulphate have a presumptive adult oligodendrocyte progenitor antigenic phenotype, but are highly complex cells and send processes to contact axolemma at nodes of Ranvier, suggesting they subserve a specific perinodal function. Thus, axons and oligodendrocyte lineage cells form interdependent functional units, but oligodendrocyte numbers, differentiation, phenotype divergence, and myelinogenesis are governed by axons in the units, mediated by growth factors and contact-dependent signals.

Late-onset neurodegeneration in mice with increased dosage of the proteolipid protein gene

Mutations of the proteolipid protein (Plp) gene cause a generalized central nervous system (CNS) myelin deficit in Pelizaeus-Merzbacher disease of man and various tremor syndromes in animal models. X-linked spastic paraplegia is also due to Plp gene mutations but has a different clinical profile and more restricted pathology involving specific tracts and regions. We have shown previously that PLP overexpression in mice homozygous for a Plp transgene results in premature arrest of CNS myelination and premature death. Here, we demonstrate that a low-level increase in Plp gene expression in transgenic mice causes significant axonal degeneration and demyelination with predilection for specific tracts. Following normal motor development, aged mice develop progressive myelin loss, axonal swellings with resultant Wallerian degeneration, and marked vacuolation of the neuropil associated with ataxia, tremor, and seizures. The age of onset and severity of the phenotype is a function of Plp gene dosage. The corticospinal tracts, optic nerve, fasciculus gracilis cerebellum, and brainstem are particularly involved. Although oligodendrocyte cell bodies show little abnormality, their inner adaxonal tongue is often abnormal, suggesting a perturbation of the axon/glial interface that may underlie the axonal changes. We conclude that abnormal expression of an oligodendrocyte-specific gene can cause axonal damage, a finding that is relevant to the pathogenesis of PLP-associated disorders and probably to other myelin-related diseases.

Postnatal development of EEG patterns, catecholamine contents and myelination, and effect of hyperthyroidism in Suncus brain

The postnatal development of the central nervous system (CNS) in house musk shrew in the early stage of maturation was studied. The electroencephalogram (EEG) and visual evoked potential (VEP) in association with catecholamine contents and myelin basic protein (MBP) immunoreactivity were carried out from the 1st to the 20th day of postnatal age. Different EEG patterns which were specific to behavioral states (awake and drowsy) were first recorded on the 5th day, and the total power which was obtained by power spectrum analysis increased after this stage. The latencies of all peaks in VEP markedly shortened between the 5th and the 7th day. Noradrenalin (NA) content of the brain showed a slight increase after the 3rd day, and reached maximum levels on the 7th day, which was delayed a few days compared to dopamine (DA). In hyperthyroidism, the peak latency of VEP was shortened and biosynthesis of NA in cerebral cortex and DA in hippocampus was accelerated. The most obvious change in MBP-immunoreactivity of the telencephalon occurred from the 7th to the 10th day. These morphological changes in the brain advanced at the identical time-course to those in the electrophysiological development and increment of DA and NA contents.

Oligodendrocyte maturation in Xenopus laevis

Oligodendrocytes, the myelinating cells of the central nervous system, have a characteristic morphology, whose full extent is revealed immunohistochemically by the expression of specific markers. Proteolipid protein (PLP) is regarded as a terminal differentiation marker of oligodendrocytes. By using PLP antibody and confocal microscopy, PLP expression in developing oligodendrocytes and the detailed three-dimensional morphology of oligodendrocytes were observed throughout early and late Xenopus laevis tadpole development. Premyelinating oligodendrocytes with radial processes were intensely stained with PLP antibody. The morphology of developing oligodendrocytes is comparable with that described in rodents. In Xenopus, the appearance of PLP- and MBP-positive oligodendrocytes is synchronized spatially and temporarily, revealing that PLP is also an early marker of developing oligodendrocytes. In addition, oligodendrocytes in early tadpole spinal cord are few in number but have a large cell body that is easily identified on a transverse section, revealing the relatively simple cytoarchitecture of this glial component. Xenopus spinal cord may therefore be useful as model system for studying the maturation of oligodendrocytes and the myelination process in situ.

Proteolipid protein regulates the survival and differentiation of oligodendrocytes

Proteolipid protein (PLP) has been postulated to play a critical role in the early differentiation of oligodendrocytes (OLs) in addition to its known role as a structural component of myelin. To identify this early function, we blocked the synthesis of PLP in glial cultures with antisense oligodeoxynucleotides that targeted the PLP initiation codon. Primary glial cultures were incubated with phosphorothioate-protected oligodeoxynucleotides (S-ODNs) for up to 11 d. PLP in OLs was reduced >90%. OLs treated with antisense S-ODNs appeared strikingly healthy as judged by (1) immunocytochemical staining for myelin glycolipids and myelin basic protein, (2) their prolonged survival compared with untreated cultures, and (3) their ability to re-establish membrane sheets after removal of the S-ODNs. Our studies show that PLP is required for elaboration and stability of the myelin membrane sheets made by most OLs, but it is not necessary for the network of processes established by OLs. More importantly, the number of OLs in the antisense-treated cultures was nearly sevenfold greater after a 10-11 d incubation with S-ODNs than in control cultures. The number of proliferating OL progenitors was not increased in the antisense-treated cultures, indicating that the increase in the number of OLs was attributable to prolonged OL survival. The tissue culture studies reveal that the absence of PLP/DM20 has the positive effect of promoting OL survival but the negative effect of preventing their full differentiation. This finding clarifies many of the paradoxical findings seen in the PLP mutants, the PLP overexpressers, and the PLP- animals.

Proteolipid protein regulates the survival and differentiation of oligodendrocytes

Proteolipid protein (PLP) has been postulated to play a critical role in the early differentiation of oligodendrocytes (OLs) in addition to its known role as a structural component of myelin. To identify this early function, we blocked the synthesis of PLP in glial cultures with antisense oligodeoxynucleotides that targeted the PLP initiation codon. Primary glial cultures were incubated with phosphorothioate-protected oligodeoxynucleotides (S-ODNs) for up to 11 d. PLP in OLs was reduced >90%. OLs treated with antisense S-ODNs appeared strikingly healthy as judged by (1) immunocytochemical staining for myelin glycolipids and myelin basic protein, (2) their prolonged survival compared with untreated cultures, and (3) their ability to re-establish membrane sheets after removal of the S-ODNs. Our studies show that PLP is required for elaboration and stability of the myelin membrane sheets made by most OLs, but it is not necessary for the network of processes established by OLs. More importantly, the number of OLs in the antisense-treated cultures was nearly sevenfold greater after a 10-11 d incubation with S-ODNs than in control cultures. The number of proliferating OL progenitors was not increased in the antisense-treated cultures, indicating that the increase in the number of OLs was attributable to prolonged OL survival. The tissue culture studies reveal that the absence of PLP/DM20 has the positive effect of promoting OL survival but the negative effect of preventing their full differentiation. This finding clarifies many of the paradoxical findings seen in the PLP mutants, the PLP overexpressers, and the PLP- animals.

Expression of myelin proteins in the opossum optic nerve: late appearance of inhibitors implicates an earlier non-myelin factor in preventing ganglion cell regeneration

The pattern of appearance of myelin-associated proteins in the visual system of the Brazilian opossum Monodelphis domestica is described. Whole mounts of optic nerve, chiasm, and optic tract were sectioned horizontally and incubated with antibodies to myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), "Rip," and the neurite inhibitory protein (IN-1), followed by visualization with diaminobenzidine and a peroxidase-conjugated secondary antibody. PLP is first detectable 24 days after birth (P24) at the centre of the optic chiasm. MBP, MAG, Rip, and IN-1 appear first in the same area at P26. By P28 the distribution of all proteins is similar, occupying the entire chiasm, optic tracts, and prechiasmatic portion of the optic nerves. Protein expression progresses along the optic nerve to reach the lamina cribrosa by P34, coincident with the time of eye opening. A critical period in which the retinofugal pathway has a regenerative capacity has recently been observed in Monodelphis. This period ends at P12, 2 weeks before the appearance of the myelin-associated inhibitory proteins MAG and IN-1. These results therefore suggest that regeneration in the developing retinofugal projection of the opossum is restricted by an earlier non-myelin factor, which is in contrast to current literature on the spinal cord.

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Expression of myelin-specific proteins during development of normal and hypomyelinated Paralytic tremor mutant rabbits. II. Studies on the purified myelin

An X-linked, recessive paralytic tremor (pt) mutation is characterized by CNS hypomyelination. In our previous work, we presented developmental studies on the expression of several myelin-specific proteins (PLP/DM-20, MBP, CNP, MAG, and MOG) in the brain homogeates of both pt mutant and age-matched control Chinchilla rabbits aged 1-120 postnatal days. A moderate reduction in all examined proteins and a striking PLP deficiency were observed in the pt mutant rabbits. In the present study, we investigated isolated and purified myelin fractions. A severe (approximately 30% of control values) and approximately constant hypomyelination of pt mutant CNS was observed during the entire investigated development (28-120 postnatal days). Although the neurological symptoms gradually regressed and a partial recovery of the affected animals usually occurred, no tendency toward regression of the hypomyelination was noticed. Whereas the content of CNP, MBP, and MAG in isolated myelin membrane fractions seemed close to normal, a drastic PLP deficiency was observed. A significantly elevated amount of MOG was found in the myelin of pt mutant rabbits. The controversy between the high degree of hypomyelination and the slight reduction in myelin protein marker expression (except for PLP) in mature brain homogenates is discussed with respect to retarded oligodendrocyte maturation and deficient processing of myelin membranes in pt mutant rabbits.

Biochemical subtypes of oligodendrocyte in the anterior medullary velum of the rat as revealed by the monoclonal antibody Rip

Oligodendrocytes were studied in the anterior medullary velum (AMV) of the rat using the monoclonal antibody Rip, an oligodendrocyte marker of unknown function. Confocal microscopic imaging of double immunofluorescent labelling with antibodies to Rip and carbonic anhydrase II (CAII) revealed two biochemically and morphologically distinct populations of oligodendrocyte which were either Rip+CAII+ or Rip+CAII-. Double immunofluorescent labelling with Rip and myelin basic protein (MBP) or glial fibrillary acidic protein (GFAP) provided direct evidence that Rip-labelled cells were phenotypically oligodendrocytes and confirmed that Rip did not recognise astrocytes. Oligodendrocytes which were Rip+CAII+ supported numerous myelin sheaths for small diameter axons, whilst Rip+CAII- oligodendrocytes supported fewer myelin sheaths for large diameter axons. Morphologically, Rip+CAII+ oligodendrocytes corresponded to types I or II of classical nomenclature, whilst Rip+CAII- oligodendrocytes corresponded to types III and IV. The results demonstrated a biochemical difference between oligodendrocytes which myelinated small and large diameter fibres.

A new oligodendrocyte specific plasma membrane surface protein identified by a monoclonal antibody produced in vitro

This paper describes a novel monoclonal antibody (C1G5F2) derived from mice splenocytes immunized in vitro with a wheat germ agglutinin glycoprotein fraction isolated from bovine central nervous system (CNS) myelin. Immunohistochemical reactions with C1G5F2 were investigated on rat brain sections during the active period of myelination. From day 10 to 13 postnatally, no stained structures were observed throughout the whole brain. The first immunolabeled myelin fibers were detected within the pons at day 14, and the white matter areas in the cerebrum started to be stained some days later. White matter areas of the cerebellum were clearly immunopositive after the third week. There was a strong positive signal on myelin fibers in the cerebrum at day 30. By contrast, no immunolabeled cell bodies of oligodendrocytes were observed throughout the brain. The other neural cell types were also not labeled. This C1G5F2 monoclonal antibody bound mainly to the extracytosolic membrane surface of the processes of live cultured oligodendrocytes derived from newborn rat brain but was unreactive with live or fixed astrocytes and neurons maintained in culture. No immunostaining was detected in the peripheral nervous system or in the spleen, liver, or pancreas. The C1G5F2 epitope containing antigen may therefore be considered as a CNS myelin/oligodendrocyte specific molecule. Sodium deoxycholate-Tween 20 extracts of secondary oligodendrocyte cultures, biotinylated with biotin hydrazide, were used to attempt the purification of the antigen with C1G5F2 IgMs linked to antimouse IgM agarose. A main broad biotinylated protein band of 54-58 kDa molecular mass was noted. In a second approach, the antigen was immunopurified from cultured oligodendrocytes as an immune complex using biotinylated C1G5F2 IgMs. A distinct protein doublet of 53-56 kDa was also observed. It is postulated that this antigen may play an essential role in myelin formation and could be a possible target in diseases restricted to CNS myelin.

Morphological heterogeneity of rat oligodendrocytes: electron microscopic studies on serial sections

The microanatomy of ensheathing and early myelinating rat oligodendrocytes was analyzed through electron microscopic examination of serial sections. The study included cells in the spinal cord (SC) ventral funiculus and the corpus callosum (CC), containing early myelinating, prospective large axons and late myelinating, prospective small axons, respectively. The results show that ensheathment commences fetal day (F) 19 in the SC and 12 days postnatally (P12) in the CC. By then, multipolar SC and CC oligodendrocytes provide axons with uncompacted cytoplasmic sheaths. The average number of axons ensheathed by each such cell was 7 in the SC and 13 in the CC. The mean diameter of the ensheathed axons was 0.69 micron in the SC and 0.36 micron in the CC. The formation of compact myelin had clearly been initiated at birth in the SC and at P17 in the CC. At that stage, the mean number of myelinated axons per analyzed oligodendrocyte was 3 in the SC and 15 in the CC. The mean diameter of the myelinated axons was 1.02 micron in the SC and 0.54 micron in the CC. These observations show that myelin-related rat oligodendrocytes are morphologically heterogeneous. It also seems that this heterogeneity is related to time of onset of myelination and prospective axon diameter. Further, the data suggest that some oligodendrocytes reduce the number of sheaths initially elaborated before formation of compact myelin.

Amphoterin (P30, HMG-1) and RIP are early markers of oligodendrocytes in the developing rat spinal cord

We have determined the cellular localization of the neurite outgrowth-promoting protein, amphoterin (p30), within the developing rat spinal cord in order to gain insight into possible function(s). Both neurons and oligodendrocytes are labelled by anti-amphoterin antibodies in tissue sections. Double labelling confirmed that labelled non-neuronal cells were oligodendrocytes and not astrocytes. White matter cells labelled by anti-amphoterin were first apparent in the spinal cord at embryonic day 18 (E18). The spinal cord white matter was most densely populated by amphoterin immunoreactive oligodendrocytes during the first postnatal week. In the adult spinal cord anti-amphoterin-labelled oligodendrocytes were infrequent. Neuronal staining was not diminished in adult animals. We have identified multiple populations of developing oligodendrocytes in the rat spinal cord. Oligodendrocytes (or precursors) in the presumptive white matter at E20 are elongated, radially oriented cells. Some are amphoterin+ and Rip-, while others are positive for both antigens. At the same age, a small number of process-bearing Rip+ cells are present in the grey matter. These morphologically and antigenically defined populations of cells could reflect different oligodendrocyte lineages, development stages, or different responses to environmental cues. While the function of amphoterin is unknown, the finding that amphoterin is expressed in numerous oligodendrocytes from late embryonic development suggests that amphoterin expression may be important in the early stages of oligodendrocyte maturation. The onset of amphoterin expression, prior to the expression of myelin-specific antigens, makes amphoterin a useful marker for studying oligodendrocyte development.

Proteolipid protein interactions in transfectants: implications for myelin assembly

The proteolipid proteins (PLP and DM20) are major constituents of CNS myelin, but how they are delivered to and organized within the oligodendrocyte plasma membrane is incompletely understood. We have expressed both PLP and DM20 singly or together in a host cell line, HeLa. In either DM20 or PLP transfectants, at early time points (24 hours), the expressed proteins are found within intracellular compartments. In DM20 transfectants, the protein is delivered to the plasma membrane by 48 hours. In HeLa cells, PLP remains intracellular when expressed in the absence of DM20; only when it is coexpressed with DM20 is it transported to the plasma membrane. In cotransfectants, PLP can also be localized to organelles involved in both the protein biosynthetic and the endocytic pathways. Since, in HeLa cells at least, the delivery of PLP to the plasma membrane is facilitated by the coexpression of DM20, we suggest that the two proteins interact intracellularly to form a complex. In some PLP/DM20 cotransfectants, the proteolipids are concentrated in regions of cell-cell contact. The regional accumulation of these proteins at cell-cell interfaces is highly reminiscent of the behavior in transfected cells of another myelin protein, P0, and certain cadherin polypeptides, both of which have readily demonstrable membrane adhesive properties. Our data suggests that at certain stoichiometric ratios, proteolipids can become stabilized at cell surfaces to form adhesive bonds.

Oligodendrocytes in the early course of multiple sclerosis

The neuropathology of demyelinating lesions in multiple sclerosis was studied in specimens obtained by diagnostic needle biopsy during early stages of the disease. The lesions were characterized by a chronic inflammatory reaction dominated by lymphocytes and macrophages, plaque-like demyelination, and astroglial sclerosis. Oligodendrocytes within the lesions were studied by immunocytochemistry using antibodies against various myelin and oligodendroglia components. The expression of messenger RNA for proteolipid protein was determined by in situ hybridization. Our studies revealed that myelin-oligodendrocyte glycoprotein is a sensitive and reliable marker for identification of oligodendrocytes in demyelinated plaques. The results suggest that in the early course of the disease in some patients, oligodendrocytes may largely be preserved, whereas in others oligodendroglial loss is pronounced. Loss of oligodendrocytes was only marginally related to the stage of demyelinating activity within the lesions. These findings indicate that the pathogenesis of demyelination may vary within different multiple sclerosis patients.