Properties and Interactions of Type III Intermediate Filaments with CRYAB

Abstract

Glial fibrillary acidic protein is a type three intermediate filament found within astrocytes in the central nervous system and mutations have been found as the cause of Alexander disease, resulting in protein aggregates of Rosenthal fibers with αBcrystallin. Here two glial fibrillary acidic protein mutants of R79C and R239H,
located in the LNDR and rod 2A domain respectively, were assembled in vitro and their morphology, assembly competence and interactions with αB-crystallin were assessed. Both mutants were unable to form usual filament lengths but instead were similar to unit length filaments with R239H forming aggregates due to such high protein interactions and the R79C mutant having much lower assembly competent protein interactions. R239H had a much greater affinity for αB-crystallin, likely a reflection that it has one of the most severe phenotypes. An absence of divalent
cations equally affected GFAP assembly resulting in compromised compaction and increased filament-filament interactions. The R120G αB-crystallin mutant results in
cardiomyopathy and cataract due to aberrant interactions with desmin intermediate filaments, due to an increased oligomer size and therefore these interactions were
studied and compared to wild-type interactions. Temperature and pH also alter the oligomer size of αB-crystallin and were therefore investigated with αB-crystallin-type III intermediate filament interactions. It was found that ischemic conditions and increased temperatures promote their association, demonstrated by increased co-pelleting
under high speed sedimentations. There was a preference for binding to desmin filaments thus showing how desmin-wild-type αB-crystallin interactions are important for homeostasis in muscle. Passive microrheology was used to
complement this and investigate how αB-crystallin may be modulating desmin filaments under equilibrium. From these experiments wild-type αB-crystallin reduced the frequency-dependent passive viscosity η(f) and the G’, whereas the
cardiomyopathy- and cataract- causing R120G mutant increased the η(f).