The age-related decrease in CNS remyelination efficiency is attributable to an impairment of both oligodendrocyte progenitor recruitment and differentiation - PubMed (original) (raw)

Fraser J Sim et al. J Neurosci. 2002.

Abstract

The age-associated decrease in the efficiency of CNS remyelination has clear implications for recovery from demyelinating diseases such as multiple sclerosis (MS) that may last for several decades. Developing strategies to reverse the age-associated decline requires the identification of how the regenerative process is impaired. We addressed whether remyelination becomes slower because of an impairment of recruitment of oligodendrocyte progenitors (OPs) or, as is the case in some MS lesions, an impairment of OP differentiation into remyelinating oligodendrocytes. The OP response during remyelination of focal, toxin-induced CNS demyelination in young and old rats was compared by in situ hybridization using probes to two OP-expressed mRNA species: platelet-derived growth factor-alpha receptor and the OP transcription factor myelin transcription factor 1 (MyT1). We found that the expression patterns for the two OP markers are very similar and reveal a delay in the colonization of the demyelinated focus with OPs in the old animals compared with the young animals. By comparing the mRNA expression pattern of MyT1 with that of the myelin proteins myelin basic protein and Gtx, we found that in the old animals there is also a delay in OP differentiation that increases with longer survival times. These results indicate that the age-associated decrease in remyelination efficiency occurs because of an impairment of OP recruitment and the subsequent differentiation of the OPs into remyelinating oligodendrocytes, and that strategies aimed at ameliorating the age-associated decline in remyelination efficiency will therefore need to promote both components of the regenerative process.

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Figures

Fig. 1.

Fig. 1.

Toxin-induced CNS demyelinating lesion model. The Nissl-stained coronal section illustrates lesion location in the brainstem. Focal areas of demyelination were induced by stereotaxic injection of EB into the caudal cerebellar peduncle of adult rats (right, arrow). Modified from Swanson (1998).

Fig. 2.

Fig. 2.

A, The caudal cerebellar peduncle (ccp; indicated by the dashed line) was identified histologically by solochrome cyanine staining (left). Both PDGF-αR+ OPs (middle) and Olig-1+ oligodendrocyte lineage cells (right) are found within the caudal cerebellar peduncle. sp5, Spinal tract of the trigeminal.B, After injection of EB, lesion location was identified by solochrome cyanine staining (left). At 2 DPL, very few OPs were identified within the lesion by _in situ_hybridization for PDGF-αR mRNA (middle). In addition, all Olig-1-expressing oligodendrocyte lineage cells were depleted from the lesion area (right). SC, Solochrome cyanine. Scale bar, 500 μm.

Fig. 3.

Fig. 3.

A, Absolute numbers of PDGF-αR+ OPs in the intact caudal cerebellar peduncles of young and old adult rats, expressed as mean ± SEM. B, Quantification of PDGF-αR+ cell density expression during remyelination of EB-induced demyelination of the caudal cerebellar peduncle. Changes in mean density (± SEM) within the lesion between 2 and 28 DPL in young and old animals are shown. The horizontal lines indicate the mean OP density in young (solid line) and old (dotted line) animals. *p < 0.05; significant difference between young and old animals.

Fig. 4.

Fig. 4.

In old animals 10 d after injection, the spatial distribution of PDGF-αR+ cells within the demyelinating lesion was examined by image analysis (A)._Red_- and _green_-labeled cells were counted as cells present in the periphery and core of the lesion, respectively._Cyan_-colored areas of nonspecific staining were excluded from analysis. The density of these cells and the ratio of peripheral to core densities were calculated. Scale bar, 500 μm.B, The box plot shows that the ratio is significantly >1, indicating that more cells are present in the outer portion of the lesion at 10 d.

Fig. 5.

Fig. 5.

MyT1 Northern blot analysis. The MyT1–5 probe binds to a single transcript of ∼5 kb found in both developing CNS and various adult tissues.

Fig. 6.

Fig. 6.

A, Expression patterns of MyT1 mRNA during remyelination of EB-induced demyelination of the caudal cerebellar peduncle in both young and old adult animals. Sections through the center of the lesion were hybridized with35S-labeled MyT1-specific oligonucleotide probes using a standard in situ hybridization protocol. Representative autoradiograms demonstrate the resulting hybridization signal at 2, 5, 7, 10, 14, 21, 28, and 66 d after injection. _Arrows_indicate the first time point at which the initial and second phases of MyT1 re-expression were detected. Emulsion autoradiography of the cerebellar cortex revealed that the signal in this region was diffuse and not associated with individual cells, suggesting that it is caused by nonspecific binding. Scale bar, 500 μm. B, Changes in mean ROD measurements (± SEM) for MyT1 mRNA expression within EB-induced lesions between 2 and 66 d after lesion induction in young and old adult animals.

Fig. 7.

Fig. 7.

A, The number of MyT1 mRNA-expressing cells during remyelination was assessed by emulsion autoradiography. MyT1-positive nuclei were defined as nuclei containing >20 silver grains clustered over a single nucleus. The field illustrated is taken from an EB-induced lesion and contains five MyT1-positive nuclei (arrows). Scale bar, 5 μm.B, The changes in MyT1 ROD observed were compared with changes in the absolute number of MyT1-positive nuclei by correlating the total number of cells with the ROD × lesion area for each animal. These data are significantly correlated with one another. In addition, after linear regression (solid line), all data points fall within the 95% prediction intervals (dashed line). The data indicate that the ROD measurements (Fig.6_B_) reflect changes in the density of MyT1 mRNA-expressing cells.

Fig. 8.

Fig. 8.

Comparison of mRNA expression patterns of the OP markers PDGF-αR and MyT1 during remyelination of the caudal cerebellar peduncle in young and old animals. Mean relative expression values, relative to peak levels, were calculated for the two markers to compare the two expression patterns. The shape of the mRNA profiles after 10 DPL of MyT1 and PDGF-αR was similar in both young (A) and old (B) groups.

Fig. 9.

Fig. 9.

The rate of oligodendrocyte differentiation was examined by comparison of the mRNA expression patterns of MyT1 for OPs and MBP and Gtx for mature oligodendrocytes during remyelination of the caudal cerebellar peduncle in young (top) and old (middle) animals. These patterns were compared by calculating a percentage of the individual ROD values to the highest ROD observed for each probe. MyT1 expression preceded MBP and PLP in each age group. Bottom, The relative delay between equivalent MBP or Gtx expression and MyT1 expression was calculated from the relative expression charts (top,middle). Unlike young animals, the delay between Gtx and MyT1 in old animals progressively increased as the lesion matured before complete remyelination.

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