Axonal degeneration stimulates the formation of NG2⁺ cells and oligodendrocytes in the mouse

Abstract

Proliferation of the adult NG2-expressing oligodendrocyte precursor cells has traditionally been viewed as a remyelination response ensuing from destruction of myelin and oligodendrocytes, and not to the axonal pathology that is also a characteristic of demyelinating disease. To better understand the response of the NG2+ cells to the different components of demyelinating pathology, we investigated the response of adult NG2+ cells to axonal degeneration in the absence of primary myelin or oligodendrocyte pathology. Axonal degeneration was induced in the hippocampal dentate gyrus of adult mice by transection of the entorhino-dentate perforant path projection. The acutely induced degeneration of axons and terminals resulted in a prompt response of NG2+ cells, consisting of morphological transformation, cellular proliferation, and upregulation of NG2 expression days 2-3 after surgery. This was followed by a reduction of cellular NG2 expression to subnormal levels from day 5 to 7 and reappearance of normal appearing NG2+ cells from day 10. Mice that had received repeated injections of bromodeoxyuridine from 24 to 72 h after surgery contained significant numbers of bromodeoxyuridine-incorporating oligodendrocytes in the areas with axonal degeneration at day 7. The results suggest that axonal degeneration induces a unique sequence of changes of NG2+ cells and that a subpopulation of the newly generated NG2+ cells differentiate into oligodendrocytes.

Stimulation of Adult Oligodendrogenesis by Myelin-Specific T Cells

“…30 Only mice that demonstrated a dense band of green fluorescence in the outer molecular layer, reflecting complete transection of both the medial and lateral PP, were included in the study. 30,31 Diaminobenzidine Immunohistochemistry Neurofilament (NF) was detected by using primary monoclonal rat anti-mouse phosphorylated NF antibody (MAB5448; dilution 1:100; Chemicon International, Inc., Temecula, CA), secondary biotinylated species-specific monoclonal goat anti-rat antibody (RPN 1005; dilution 1:200; Amersham Pharmacia Biotech, Inc.), and streptavidin-horseradish peroxidase (P397; dilution 1:200; Dako A/S, Glostrup, Denmark), as described by Nielsen et al 14 T cells were visualized as described for NF using monoclonal rat anti-human CD3 antibody (MCA1477; dilution 1:200; AbD Serotec, Ltd., Kidlington, Oxfordshire, England). Cross-reactivity for murine CD3 was confirmed via location of CD3 ϩ cells to the periarteriolar sheath in murine spleen sections.…”

“…BrdU ϩ NG2 ϩ cells and BrdU ϩ CNP ϩ cells were defined as NG2 ϩ and CNP ϩ cells with a bluish black nucleus. To count the cells using the method of Nielsen et al, 14 cells were counted in the molecular layer of the dentate gyrus in 10 parallel sections 160 m apart using a counting frame of 4900 m 2 (fraction of total area, 61%) along the rostrocaudal axis of the hippocampus, starting in the section in which the dentate gyrus first demonstrated a typical U shape (Ϫ1900 m from the bregma), as previously reported. 14 At least 150 to 200 single-labeled NG2 ϩ or CNP ϩ cells were counted per animal.…”

“…To count the cells using the method of Nielsen et al, 14 cells were counted in the molecular layer of the dentate gyrus in 10 parallel sections 160 m apart using a counting frame of 4900 m 2 (fraction of total area, 61%) along the rostrocaudal axis of the hippocampus, starting in the section in which the dentate gyrus first demonstrated a typical U shape (Ϫ1900 m from the bregma), as previously reported. 14 At least 150 to 200 single-labeled NG2 ϩ or CNP ϩ cells were counted per animal. To account for potential differences in the quality of the stainings, cell numbers are reported as the ratio between cell numbers obtained from the deafferented ipsilateral molecular layer compared with cell numbers obtained from the contralateral molecular layer (NG2 i/c and CNP i/c ).…”

“…3,4 Remyelination depends on chondroitin sulfate NG2-expressing adult oligodendrocyte precursor cells (OPCs). 5,6 OPCs retain the capacity to proliferate and differentiate into myelinating oligodendrocytes in response to toxic or inflammatory demyelination 7-9 and other forms of central nervous system (CNS) injury such as ischemia, 10 spinal cord injury, 11,12 axonal lesions, 13,14 and inflammation. 15 During differentiation, OPCs downregulate NG2 as cells acquire markers of mature oligodendrocytes such as 2=,3=-cyclic nucleotide 3=-phosphodiesterase (CNP).…”

“…[17][18][19] PP lesions also induce proliferation of OPCs, which results in formation of new oligodendrocytes. 14 These newly formed oligodendrocytes are presumed to myelinate the axonal sprouts that extend from other afferent fiber systems in the dentate gyrus 20,21 such as the associational/commissural afferents from the calretinergic hilar mossy cells. 20,22,23 Indeed, in stratum radiatum of the hippocampal CA3 region, lesion-induced axonal sprouting is associated with formation of more oligodendrocytes and more myelin.…”