Biochemical analysis of proteasomes from mouse microglia: Induction of immunoproteasomes by interferon-γ and lipopolysaccharide

The 20S proteasome is a multicatalytic threonine protease and serves to process peptides that are subsequently presented as antigenic epitopes by MHC class I molecules. In the brain, microglial cells are the major antigen presenting cells and they respond sensitive to pathologic events. We used cultured mouse microglia and a microglial cell line, the BV‐2 line, as a model to study the correlation between microglial activation parameters and structural plasticity of the 20S/26S proteasome. Lipopolysaccharide (LPS)‐ or interferon‐γ (IFN‐γ)‐stimulated microglia or BV‐2 cells exhibit properties of activated microglia such as high levels of TNFα and IL‐6 release. In response to IFN‐γ or LPS, three constitutive β subunits (β1/Delta, β2/MC14, β5/MB1) were replaced by the immunoproteasome subunits iβ1/LMP2, iβ2/MECL‐1, and iβ5/LMP7, indicating that activated microglia adapts its proteasomal subunit composition to the requirements of an optimized MHC class I epitope processing. Induction of immunoproteasomes in BV‐2 cells was solely provoked by IFN‐γ, but not by LPS. Moreover, LPS (but not IFN‐γ) triggered the expression of a novel protein of ≈50 kD as part of the proteasome activator PA700, that is the substrate‐recognizing and unfolding unit of the 26S proteasome. These results indicate that both the 20S core protease as well as the proteasome activator PA700 are targets of modulatory subunit replacements or transient association of regulatory components in the course of microglial activation. GLIA 29:355–365, 2000. © 2000 Wiley‐Liss, Inc.