Abstract
Immature astrocytes and astrocytoma cells contain synemin and three other intermediate filament (IF) proteins: glial fibrillary acidic protein (GFAP), vimentin and nestin. Here, we show that, after neurotrauma, reactive astrocytes produce synemin and thus propose synemin as a new marker of reactive astrocytes. Comparison of synemin mRNA and protein levels in brain tissues and astrocyte cultures from wild-type, Vim(-)(/)(-) and Gfap(-)(/)(-)Vim(-)(/)(-) mice showed that in the absence of vimentin, synemin protein was undetectable although synemin mRNA was present at wild-type levels. By contrast, in Gfap(-)(/)(-) astrocytes, synemin protein and mRNA levels, as well as synemin incorporation into vimentin IFs, were unaltered. Biochemical assays with purified proteins suggested that synemin interacts with GFAP IFs like an IF-associated protein rather than like a polymerization partner, whereas the opposite was true for synemin interaction with vimentin. In transfection experiments, synemin did not incorporate into normal, filamentous GFAP networks, but integrated into vimentin and GFAP heteropolymeric networks. Thus, alongside GFAP, vimentin and nestin, reactive astrocytes contain synemin, whose accumulation is suppressed post-transcriptionally in the absence of a polymerization partner. In astrocytes, this partner is vimentin and not GFAP, which implies a functional difference between these two type III IF proteins.
Highlights
Intermediate filament (IF) networks constitute dynamic scaffolds integrating the cytoplasm on a structural and functional level
glial fibrillary acidic protein (GFAP) IFs like an IF-associated protein rather than like a polymerization partner, whereas the opposite was true for synemin interaction with vimentin
Alongside GFAP, vimentin and nestin, reactive astrocytes contain synemin, whose accumulation is suppressed posttranscriptionally in the absence of a polymerization partner. This partner is vimentin and not GFAP, which implies a functional difference between these two type III IF proteins
Summary
Intermediate filament (IF) networks constitute dynamic scaffolds integrating the cytoplasm on a structural and functional level (for reviews, see Chang and Goldman, 2004; Coulombe and Wong, 2004; Lane and Pekny, 2004; Omary et al, 2004). IF proteins are expressed in a cell-type specific pattern (for reviews, see Omary et al, 2004; Coulombe and Wong, 2004). This pattern is often established after a series of developmentally regulated transitions in IF proteins, and it is often altered in diseases. Nestin and synemin (Lendahl et al, 1990; Rutka et al, 1997; Sultana et al, 2000) As astrocytes differentiate, these IF proteins are replaced progressively by GFAP. In neurotrauma and in ischemic and neurodegenerative lesions, reactive astrocytes overexpress GFAP and re-express vimentin and nestin (for reviews, see Eng et al, 2000; Pekny and Pekna, 2004; Pekny and Wilhelmsson, 2006)
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