Intermediate filament transcription in astrocytes is repressed by proteasome inhibition

Abstract

Increased expression of the astrocytic intermediate filament protein glial fibrillary acidic protein (GFAP) is a characteristic of astrogliosis. This process occurs in the brain during aging and neurodegeneration and coincides with impairment of the ubiquitin proteasome system. Inhibition of the proteasome impairs protein degradation; therefore, we hypothesized that the increase in GFAP may be the result of impaired proteasomal activity in astrocytes. We investigated the effect of proteasome inhibitors on GFAP expression and other intermediate filament proteins in human astrocytoma cells and in a rat brain model for astrogliosis. Extensive quantitative RT-PCR, immunocytochemistry, and Western blot analysis resulted unexpectedly in a strong decrease of GFAP mRNA to <4% of control levels [Control (DMSO) 100+/-19.2%; proteasome inhibitor (epoxomicin) 3.5+/-1.3%, n=8; P < or = 0.001] and a loss of GFAP protein in astrocytes in vitro. We show that the proteasome alters GFAP promoter activity, possibly mediated by transcription factors as demonstrated by a GFAP promoter-luciferase assay and RT(2) Profiler PCR array for human transcription factors. Most important, we demonstrate that proteasome inhibitors also reduce GFAP and vimentin expression in a rat model for induced astrogliosis in vivo. Therefore, proteasome inhibitors could serve as a potential therapy to modulate astrogliosis associated with CNS injuries and disease.

Trehalose rescues glial cell dysfunction in striatal cultures from HD R6/1 mice at early postnatal development

“…These results in HD glial cells are in accordance with studies based in cellular models where UPS impairment is attributable to expression of intermediate aggregate forms of mHTT. The astrocytes phenotype changes are also consistent with studies based on GFAP and vimentin down-regulation expression after proteasome inhibition (Middeldorp et al, 2009). These studies provide evidence that inhibition of proteasome activity directly reduces GFAP transcription, thus leading to a rapid decline in GFAP gene expression.…”

Low levels of astroglial markers in Parkinson’s disease: relationship to α-synuclein accumulation

“…One Investigation suggests that astrocytes that accumulate α-synuclein in PD are mainly of the protoplasmic type whereas reactive astrocytosis in MSA might affect mainly fibrous astrocytes (Song et al , 2009); however, a majority of the previous studies did not distinguish the two astrocyte subtypes. The mechanism by which α-synuclein might inhibit nigral astrogliosis in PD is conjectural but could involve suppression of proteasomal (Middeldorp et al , 2009) and/or mitochondrial (Schmidt et al , 2011; Braidy et al , 2013) functions (see also (Halliday and Stevens, 2011)). Astrocytes, which normally express low levels of α-synuclein (Maroteaux et al , 1988; Shibayama-Imazu et al , 1993) (but see (Mori et al , 2002)), can take up α-synuclein originally expressed by neurons in a phenotype (e.g., GFAP vs S100B) and brain region specific manner.…”

Oligomers of Mutant Glial Fibrillary Acidic Protein (GFAP) Inhibit the Proteasome System in Alexander Disease Astrocytes, and the Small Heat Shock Protein αB-Crystallin Reverses the Inhibition

“…A recent study (17) showed in astrocytoma cells that proteasome inhibitors down-regulate GFAP gene transcription and thereby decrease GFAP protein levels. Furthermore, proteasome inhibition prevents GFAP accumulation in reactive astrocytes in a model in which a microdialysis probe through which an inhibitor was infused was placed in the rat central nervous system.…”