Interferon‐γ inhibits cell cycle exit in differentiating oligodendrocyte progenitor cells

Abstract

The developmental processes of the oligodendrocyte progenitor cell (OPC) lineage that are targeted by interferon‐γ (IFN‐γ) were studied in primary rat OPC cultures. Under conditions of thyroid hormone‐mediated oligodendrocyte differentiation, IFN‐γ produced a dose‐dependent apoptotic response in OPCs. The lowest dose tested (15 ng/ml or 75 U/ml) was nonapoptotic, but activated detectable STAT1 DNA‐binding. At this dose, IFN‐γ reduced the percentage of mature O1⁺ cells and increased the percentage of immature A2B5⁺ OPCs. This was observed without significant change in total cell number and cytotoxicity, and was accompanied by an increase in BrdU‐labeled A2B5⁺ and O4⁺ cells. FACS analysis confirmed a lack of apoptotic sub‐G1 cells and revealed a greater percentage of S‐ and G2/M‐phase OPCs with IFN‐γ treatment. Dual immunostaining with Ki‐67 and Olig2 showed a smaller percentage of Olig2⁺ cells in G₀ phase in IFN‐γ‐treated OPCs, indicating loss of G1 control. Instead, increased levels and phosphorylation of the checkpoint protein p34cdc2 by IFN‐ suggested increased partial arrest in G2. IFN‐γ not only sustained expression of PCNA and the G1‐S regulators retinoblastoma protein, cyclin D1, cyclin E, and cdk2, but also decreased p27 levels. In addition to changes in cell proliferation and differentiation, IFN‐γ attenuated myelin basic protein (MBP) expression significantly, which was associated with decreased expression of both MBP and Sox10 RNAs. These findings indicate that IFN‐γ not only maintains cell cycle activity that could predispose OPCs to apoptosis, but also overrides G₁–G₀ signals leading to thyroid hormone‐mediated terminal differentiation and myelin gene expression.