ReMyelination

In the past decade, considerable progress has been made in the understanding of the biology of rodent Oligodendrocyte Precursor Cells and their role in the generation of Oligodendrocytes in the developing and adult rodent CNS.

Much less is known about human Oligodendrocyte lineage cells and about the reasons for the failure of the regeneration of the Oligodendrocyte population during chronic stages of Multiple Sclerosis (MS).

In particular, the fate of the Oligodendrocyte Precursor population in MS has remained elusive.

The present study examined the possibility that Oligodendrocyte regeneration ultimately fails because of the local destruction of both Oligodendrocytes and their Precursor cells.

Analysis of chronic stage MS tissue suggested that this is not the case, because all chronic MS lesions studied contained significant numbers of Oligodendrocyte Precursor Cells.

Identified as Process-bearing cells that bound the O4 AntiBody but not AntiBodies to GalC and GFAP.

The Oligodendrocyte Precursor Cells appeared, however, to be relatively quiescent.

Because, none expressed the Nuclear Proliferation Antigen recognized by the Ki-67 AntiBody, and because most lesions lacked Myelinating Oligodendrocytes in their centers.

Thus, it appears that the regeneration of the Oligodendrocyte population fails during chronic stages of MS.

Because, of the inability of Oligodendrocyte Precursor Cells to proliferate and differentiate rather than because of the local destruction of all Oligodendrocyte lineage cells.

The identification of ways of stimulating the endogenous Oligodendrocyte Precursor population to expand and generate ReMyelinating Cells may represent an alternative to transplantation of Oligodendrocyte Lineage Cells to promote Myelin repair in MS.

Insights into the role of the Astrocyte intermediate Filament protein, Glial Fibrillary Acidic Protein (GFAP), have only recently emerged.

With reports on subtle abnormalities in GFAP-deficient mice, including the documentation of defective long-term maintenance of Central Nervous System Myelination.

Here, we extend these observations by examining the Astroglial response in GFAP-/- mice with AutoImmune EncephaloMyelitis (EAE), a model for Multiple Sclerosis.

Clinically, the Monophasic Disease was more severe in GFAP-/- mice than in wild-type littermates despite increased ReMyelination in the former.

More in keeping with the clinical course was the observation of an infiltrative EAE lesion in GFAP-/- mice.

GFAP-/- Astrocytes had a reduced cyto-architectural stability as evidenced by less abundant and irregularly spaced HemiDesmosomes.

The blunt GFAP-/- Astrocyte Processes possessed intermediate filaments consisting mainly of Vimentin, though to a lesser degree than in the wild-type.

In contrast, in wild-type littermates, GFAP was most abundant and Nestin occurred at lower levels.

Taken together, the present study introduces the novel concepts that GFAP plays an important role in the control of clinical disease associated with formation of a clearly defined edge to the EAE lesion.

And, that GFAP is operative in the regulation of the intermediate Filament components in Reactive Fibrillary AstroGliosis.

Central Nervous System tissue from Multiple Sclerosis and non-Multiple Sclerosis subjects was studied for the expression of Exon 2 Myelin Basic Protein Gene products.

At the Protein and message levels by ImmunoCytocHemistry and In Situ Hybridization, respectively.

The Exon 2-encoded pProtein sequence is normally expressed during the development of (CNS) Myelination.

Within the 21.5- and 20.2-kd Isoforms of Myelin Basic Protein and is downregulated in the adult Central Nervous System where the 18.5- and 17.2-kd Isoforms predominate, the latter devoid of Exon 2 owing to alternative splicing.

Exon 2 Myelin Basic Protein Gene products were readily demonstrable in Multiple Sclerosis samples, the highest levels correlating with ReMyelination in chronic Lesions.

While normal adult Central Nervous System and Non-Multiple Sclerosis material showed very low levels and fetal human Central Nervous System tissue (a positive control) showed high levels.

We conclude that recapitulation of OntoGenetic events during Myelin repair accounts for the increased expression of the Exon 2-encoded Protein sequence in the adult Central Nervous System during Multiple Sclerosis.

An event that might underly the previously observed T-Cell activation to this protein sequence during relapses.

Background
Multiple Sclerosis is an Inflammatory Disease of the Central Nervous System that destroys Myelin, Oligodendrocytes, and Axons.

Since most of the lesions of Multiple Sclerosis are not ReMyelinated, enhancement of ReMyelination is a possible therapeutic strategy that could perhaps be achieved with the transplantation of Oligodendrocyte-Producing Cells into the lesions.

We investigated the frequency distribution and configuration of Oligodendrocytes in chronic lesions of Multiple Sclerosis to determine whether these factors limit ReMyelination.

Methods
Forty-eight chronic lesions obtained at autopsy from 10 patients with Multiple Sclerosis were examined ImmunoCytoChemically for Oligodendrocytes and Oligodendrocyte Progenitor Cells.

Using confocal microscopy, we examined the three-dimensional relations between Axons and the Processes of PreMyelinating Oligodendrocytes.

Results
Thirty-four of the 48 chronic lesions of Multiple Sclerosis contained Oligodendrocytes with multiple extended Processes that associated with DeMyelinated Axons but failed to Myelinate them.

These Axons were Dystrophic and contained multiple swellings. In some regions, the densities of PreMyelinating Oligodendrocytes (25 per square millimeter of tissue) were similar to those in the developing rodent Brain (23 per square millimeter).

In the patients with disease of long duration (more than 20 years), there were fewer lesions with PreMyelinating Oligodendrocytes (P<0.001).

Conclusions
PreMyelinating Oligodendrocytes are present in chronic lesions of Multiple Sclerosis, so ReMyelination is not limited by an absence of Oligodendrocyte Progenitors or their failure to generate Oligodendrocytes.

Our findings suggest that in the chronic lesions of Multiple Sclerosis, the Axons are not receptive for ReMyelination.

Understanding the cellular interactions between PreMyelinating Oligodendrocytes, Axons, and the MicroEnvironment of lesions of Multiple Sclerosis may lead to effective strategies for enhancing ReMyelination.

The Neural Cell Adhesion Molecule (NCAM) is expressed by Myelinating Precursor Cells in Neonatal mouse Spinal Cord and by ReMyelinating Cells after chemically induced DeMyelination in adult mouse.

It shows tempting suggestions about its possible involvement in the reparative mechanisms and the ReMyelination processes that take place in Multiple Sclerosis (MS).

In fact, its levels progressively increase in the CerebroSpinal Fluid (CSF) of acute MS patients subjected to Steroid treatment, paralleling the progressive clinical improvement after the attack.

Such an increase is not found in acute MS patients not treated with Steroids nor in non-acute patients subjected to the same Steroid treatment.

Background
Various types of pathologic mechanisms in Multiple Sclerosis (MS) can alter Magnetic Resonance Imaging (MRI) signals, and the appearance of ReMyelinated lesions on MRI is largely unknown.

Objective, Design & Setting
To describe the MRI appearance of ReMyelinated lesions in MS. Comparison of postmortem MRI findings with HistoPathologic findings. Brain donations from a general community.

Patients
Magnetic Resonance images from 36 rapid autopsies yielded 161 areas that could be matched with Histologic characteristics.

Including 149 focal T₂-weighted abnormalities, with a range of signal intensities on T₁-weighted images. In a subset of 49 lesions, Magnetization Transfer Ratios could be determined.

Main Outcome Measures
An observer blinded to the MRI findings assessed the presence of ReMyelination using Light Microscopic Criteria; in 25 areas, in situ hybridization was used to assess the presence of Oligodendrocytes expressing ProteoLipid Protein Messenger RNA.

Results
1. ReMyelinated areas were found in 67 lesions (42%)
2. Partial ReMyelination was present in 30 lesions (19%)
3. Whereas 37 lesions (23%) were fully ReMyelinated

ReMyelinated lesions contained enhanced numbers of Oligodendrocytes containing ProteoLipid Protein Messenger RNA. All areas with ReMyelination shown HistoPathologically were HyperIntense on T₂-weighted images.

Strong HypoIntensity on T₁-weighted images was significantly associated (chi2 = 29.8, P < .001) with DeMyelinated and partially ReMyelinated lesions compared with fully ReMyelinated lesions.

The Magnetization Transfer Ratio of ReMyelinated lesions (mean [SD], 27.6% [41%]) differed (F = 46.3, P< .001) from both Normal-Appearing White Matter (35.2% [32%]) and DeMyelinated lesions (22.3% [48%]).

Conclusions
ReMyelinated lesions return an abnormal signal on T₂-weighted images. Both T₁-weighted images and Magnetization Transfer Ratios may have (limited) additional value in separating lesions with and without ReMyelination.

It has been hypothesized that the progressive failure of ReMyelination in chronic Multiple Sclerosis is, in part, the consequence of repeated episodes of DeMyelination at the same site, eventually depleting Oligodendrocyte Progenitor Cells (OPCs) and exhausting the ReMyelinating capacity.

We investigated the effect of previous focal, Ethidium Bromide-induced DeMyelination of BrainStem White Matter (with intervening recovery) on the efficiency of the ReMyelination process during second and third subsequent episodes of DeMyelination, and the OPC response during a second episode of DeMyelination.

Previous focal DeMyelinating lesions followed by recovery did not result in any retardation of the ReMyelination process, nor did they alter the proportion of Schwann Cell versus Oligodendrocyte ReMyelination.

The OPC response during ReMyelination was quantified by in situ hybridization using a probe to Platelet-Derived Growth Factor-alpha Receptor (PDGF alpha R), an OPC-expressed mRNA.

Following recovery from focal, toxin-induced CNS DeMyelination, the OPC density returned to levels equivalent to those in normal White Matter.

Further more, there was no depletion of OPCs following repeated episodes of focal, toxin-induced CNS DeMyelination at the same site.

These results indicate that repeated CNS DeMyelination, which has the opportunity to repair in the intervening period, is not characterized by impaired ReMyelination or depletion of OPCs.depletion of OPCs.

A better characterization of endogenous cells responsible for successful ReMyelination is essential for designing therapeutic strategies aimed at restoring functional Myelin.

The present study examined the SpatioTemporal response of endogenous Oligodendrocyte Precursor Cells (OPCs) following Ethidium Bromide (EB)-induced DeMyelination of the adult rat Spinal Cord.

Beginning at 2 days post-EB injection (dpi), a robust mobilization of highly proliferative NG2⁺ cells within the lesion was observed, none of which expressed the Oligodendrocyte lineage-associated transcription factor Nkx2.2.

At 7 dpi, a significant up-regulation of Nkx2.2 by OPCs within the lesion was observed, 90% of which coexpressed NG2 and virtually all of which coexpressed the bHLH transcription factor Olig2.

Despite successful recruitment of Nkx2.2⁺/Olig2⁺ OPCs within the lesion, DeMyelinated Axons were not ReMyelinated by these OPCs in regions lacking Astrocytes.

Rather, Schwann Cell ReMyelination predominated throughout the central core of the lesion, particularly around blood vessels.

Oligodendrocyte ReMyelination was observed in the Astrogliotic perimeter, suggesting a necessary role for Astrocytes in Oligodendrocyte maturation.

In addition, re-expression of the Radial Glial Antigen, RC-1, by reactive Astrocytes and Ependymal Cells was observed following injury.

However, these cells did not express the Neural Stem Cell (NSC)-associated transcription factors Sox1 or Sox2, suggesting that the endogenous response is primarily mediated by Glial progenitors.