Composition of Rosenthal Fibers, the Protein Aggregate Hallmark of Alexander Disease

Alexander disease
(AxD) is a neurodegenerative disorder characterized
by astrocytic protein aggregates called Rosenthal fibers (RFs). We
used mouse models of AxD to determine the protein composition of RFs
to obtain information about disease mechanisms including the hypothesis
that sequestration of proteins in RFs contributes to disease. A method
was developed for RF enrichment, and analysis of the resulting fraction
using isobaric tags for relative and absolute quantitation mass spectrometry
identified 77 proteins not previously associated with RFs. Three of
five proteins selected for follow-up were confirmed enriched in the
RF fraction by immunobloting of both the AxD mouse models and human
patients: receptor for activated protein C kinase 1 (RACK1), G1/S-specific
cyclin D2, and ATP-dependent RNA helicase DDX3X. Immunohistochemistry
validated cyclin D2 as a new RF component, but results for RACK1 and
DDX3X were equivocal. None of these was decreased in the non-RF fractions
compared to controls. A similar result was obtained for the previously
known RF component, alphaB-crystallin, which had been a candidate
for sequestration. Thus, no support was obtained for the sequestration
hypothesis for AxD. Providing possible insight into disease progression,
the association of several of the RF proteins with stress granules
suggests a role for stress granules in the origin of RFs.