A novel in-frame GFAP p.E138_L148del mutation in Type II Alexander disease with atypical phenotypes

Deletions in Glial Fibrillary Acidic Protein Leading to Alterations in Intermediate Filament Assembly and Network Formation

Glial fibrillary acidic protein (GFAP) is classified as a type III intermediate filament protein predominantly expressed in mature astrocytes. It has the ability to self-assemble into 10 nm filaments in vitro, making it particularly valuable for elucidating the sequences essential for filament assembly. In this study, we created a series of deletion mutants targeting sequences in the N-terminal, C-terminal, and central rod domains to explore the sequences critical for the assembly of GFAP into 10 nm filaments. The impact of these deletions on filament formation was evaluated through in vitro assembly studies and transduction assays conducted with primary astrocytes. Our data revealed that deletions at the carboxy end resulted in abnormalities in either filament diameter calibration or lateral association, whereas deletions at the amino-terminal end significantly disrupted the filament assembly process, particularly restricting filament elongation. Furthermore, we discovered that the filament-forming sequences within the rod domain varied in their contributions to filament assembly and network formation. These findings enhance our understanding of the GFAP assembly process in vitro and provide a detailed mapping of the essential regions required for GFAP assembly. These insights hold significant implications for Alexander disease arising from deletion mutations in GFAP.

Alexander disease: the road ahead

Alexander disease is a rare neurodegenerative disorder caused by mutations in the glial fibrillary acidic protein, a type III intermediate filament protein expressed in astrocytes. Both early (infantile or juvenile) and adult onsets of the disease are known and, in both cases, astrocytes present characteristic aggregates, named Rosenthal fibers. Mutations are spread along the glial fibrillary acidic protein sequence disrupting the typical filament network in a dominant manner. Although the presence of aggregates suggests a proteostasis problem of the mutant forms, this behavior is also observed when the expression of wild-type glial fibrillary acidic protein is increased. Additionally, several isoforms of glial fibrillary acidic protein have been described to date, while the impact of the mutations on their expression and proportion has not been exhaustively studied. Moreover, the posttranslational modification patterns and/or the protein-protein interaction networks of the glial fibrillary acidic protein mutants may be altered, leading to functional changes that may modify the morphology, positioning, and/or the function of several organelles, in turn, impairing astrocyte normal function and subsequently affecting neurons. In particular, mitochondrial function, redox balance and susceptibility to oxidative stress may contribute to the derangement of glial fibrillary acidic protein mutant-expressing astrocytes. To study the disease and to develop putative therapeutic strategies, several experimental models have been developed, a collection that is in constant growth. The fact that most cases of Alexander disease can be related to glial fibrillary acidic protein mutations, together with the availability of new and more relevant experimental models, holds promise for the design and assay of novel therapeutic strategies.