Activation of the subventricular zone in multiple sclerosis: evidence for early glial progenitors

Abstract

In multiple sclerosis (MS), oligodendrocyte and myelin destruction lead to demyelination with subsequent axonal loss. Experimental demyelination in rodents has highlighted the activation of the subventricular zone (SVZ) and the involvement of progenitor cells expressing the polysialylated form of neural cell adhesion molecule (PSA-NCAM) in the repair process. In this article, we studied the distribution of early PSA-NCAM(+) progenitors in the SVZ and MS lesions in human postmortem brains. Compared with controls, MS SVZ showed a 2- to 3-fold increase in cell density and proliferation, which correlated with enhanced numbers of PSA-NCAM(+) and glial fibrillary acidic protein-positive (GFAP(+)) cells. PSA-NCAM(+) progenitors mainly were Sox9(+), and a few expressed Sox10 and Olig2, markers of oligodendroglial specification. PSA-NCAM(+) progenitors expressing Sox10 and Olig2 also were detected in demyelinated MS lesions. In active and chronic active lesions, the number of PSA-NCAM(+) progenitors was 8-fold higher compared with chronic silent lesions, shadow plaques, and normal-appearing white matter. In active and chronic active lesions, PSA-NCAM(+) progenitors were more frequent in periventricular lesions (30-50%) than in lesions remote from the ventricular wall. These data indicate that, as in rodents, activation of gliogenesis in the SVZ occurs in MS and suggest the mobilization of SVZ-derived early glial progenitors to periventricular lesions, where they could give rise to oligodendrocyte precursors. These early glial progenitors could be a potential target for therapeutic strategies designed to promote myelin repair in MS.

Fig. 1.

Increased cell density and proliferation in the SVZ of MS cases. (A) Coronal human brain section illustrating the location of the SVZ analyzed. The SVZ lining the lateral ventricle is studied at the level of the caudate nucleus. The central body of the SVZ (red box) is indicated in Inset. (B and C) Higher magnification of the boxed area in A for control (B) and MS brain (C). The arrow indicates the striatal vein. A lesion (asterisk) is identified by Luxol fast blue staining in the internal capsule in C. (D and E) Control and MS brain sections counterstained with the nuclear dye Hoechst 33342. The SVZ thickness is indicated by double arrows. (F and G) Control and MS brain sections, through the lateral SVZ, stained for PCNA (arrowheads). (H and I) Evaluation of cell density, proliferation, and Sox9 expression in control and MS SVZ. (H) Percentage of Hoechst⁺ area measured in the boxed area as indicated in D and E. (I and J) Percentage of PCNA⁺ and Sox9⁺ cells in control and MS SVZ (double arrow). ∗, P < 0.001; ∗∗, P < 0.01. Cd, caudate nucleus; Put, putamen; LV, lateral ventricle; Cap, internal capsule; CC, corpus callosum; Ep, ependyma. (Scale bars: B and C, 2 mm; D and E, 90 μm; F and G, 45 μm.)

Fig. 2.

Cellular changes of the SVZ in MS. (A and B) GFAP expression is highly increased in the MS SVZ (B), and the gap is filled with GFAP processes compared with control (A). (C) Detection of GFAP⁺ cells with a bipolar morphology in MS SVZ (Inset). (D and E) Only few PSA-NCAM⁺ neuronal processes are present in the control SVZ (D), whereas numerous round PSA-NCAM⁺ progenitors are observed in the gap of MS SVZ (E). (F) PSA-NCAM⁺ cells (arrow) with a unipolar morphology leaving the SVZ. (G and H) Expression of Sox9 and GFAP in control (G) and MS SVZ (H). (I) Colocalization of Sox 9 in PSA-NCAM⁺ progenitors in MS SVZ. (J and K) Few PSA-NCAM⁺ progenitors (arrows) express Sox10 in control (J) and MS SVZ (K). (L and M) Detection of cells coexpressing Olig2 and NG2 in the gap of control (L) and MS SVZ (M). The lateral ventricle is at the left side of each image. A–F are counterstained with Hoechst 33342. (Scale bars: A–E and G–M, 45 μm; F, 10 μm; C Inset, 20 μm; K Inset, 12 μm.)

Fig. 3.

Identification of PSA-NCAM⁺ progenitors in MS lesions. (A) Periventricular (red arrow) and nonperiventricular (black arrow) lesions and the lateral ventricle (dotted lines) are detectable macroscopically in a slice of MS brain. (B) Identification of a lesion by anti-MOG immunolabeling. (C and D) Detection of PSA-NCAM⁺ cells in lesions. (C) PSA-NCAM⁺ cells with a stellar shape express GFAP. Arrow indicates the cell represented in the Inset. (D) A second population of PSA-NCAM⁺ cells has round shapes and is mostly GFAP⁻ (arrows) with few GFAP⁺ (arrowheads). (E and F) Quantification of PSA-NCAM⁺ progenitors in MS lesions. Their density is increased significantly in active and chronic active lesions compared with NAWM (E; ∗, P < 0.05; ∗∗, P < 0.01) and in lesions localized near the ventricle compared with cortical lesions (F; ∗, P < 0.05). D is counterstained with Hoechst 33342. A, active; CA, chronic active; CS, chronic silent; SP, shadow plaque; Cx, cortex; St, striatum. (Scale bars: B, 170 μm; C and D, 45 μm; Insets, 20 μm.)

Fig. 4.

Characterization of PSA-NCAM⁺ round progenitors in MS lesions. (A and B) PSA-NCAM⁺ progenitors (arrow) do not express the neuronal marker β3-tubulin (A) or the macrophage marker CD68 (B) in MS lesions. (C and D) Few PSA-NCAM⁺ progenitors (arrows) stain for Sox10 (C) and Olig2 (D). The edge of the lesion is identified by MOG immunolabeling (blue in C). (E) Doublet of presumably dividing PSA-NCAM⁺ cells. (F) Ki67⁺ progenitor (arrow) double-stained for GFAP and PSA-NCAM (Inset). A, B, and E are counterstained ng with Hoechst 33342. (Scale bars: A, B, and F, 45 μm; C and D, 22 μm; E, 6 μm; D Inset, 10 μm; F Inset, 20 μm.)

Fig. 5.

Proposed model illustrating the activation of the SVZ in MS. The control SVZ is composed of a ribbon of GFAP⁺ astrocytes separated from the ependymal wall by a hypocellular gap, containing few early progenitors. In MS SVZ, the density of GFAP⁺ astrocytes and early progenitors is increased. These progenitors express early glial markers such as Sox9, Sox10, and Olig2 in the subependymal region. Similar progenitors prevail in periventricular lesions. The presence of PSA-NCAM⁺ progenitors with a bipolar morphology suggest their potential migration within or away from the SVZ. These data suggest the mobilization of SVZ-derived early progenitors to periventricular lesions, where they could contribute to oligodendrocyte renewal.