Site-specific phosphorylation and caspase cleavage of GFAP are new markers of Alexander disease severity.

Rachel A Battaglia,Erik Willems,Jasmine A Robinson,Samed Delic,Namritha Ravinder,Raluca Dumitru,James E Goldman,Adriana S Beltran,Parijat Kabiraj,Laura E Herring,Natasha T Snider,Victoria J Madden,Ming-Der Perng,Rhonda A Newman,Roy A Quinlan,Masaki Inagaki

doi:10.7554/elife.47789

Abstract

Alexander disease (AxD) is a fatal neurodegenerative disorder caused by mutations in glial fibrillary acidic protein (GFAP), which supports the structural integrity of astrocytes. Over 70 GFAP missense mutations cause AxD, but the mechanism linking different mutations to disease-relevant phenotypes remains unknown. We used AxD patient brain tissue and induced pluripotent stem cell (iPSC)-derived astrocytes to investigate the hypothesis that AxD-causing mutations perturb key post-translational modifications (PTMs) on GFAP. Our findings reveal selective phosphorylation of GFAP-Ser13 in patients who died young, independently of the mutation they carried. AxD iPSC-astrocytes accumulated pSer13-GFAP in cytoplasmic aggregates within deep nuclear invaginations, resembling the hallmark Rosenthal fibers observed in vivo. Ser13 phosphorylation facilitated GFAP aggregation and was associated with increased GFAP proteolysis by caspase-6. Furthermore, caspase-6 was selectively expressed in young AxD patients, and correlated with the presence of cleaved GFAP. We reveal a novel PTM signature linking different GFAP mutations in infantile AxD.

Highlights

Alexander Disease (AxD) is a rare and invariably fatal neurological disorder that affects primarily infants and small children, but can manifest later in life 1-3
Our study reveals that missense mutations, affecting discrete domains on the glial fibrillary acidic protein (GFAP) molecule, share a common post-translational modifications (PTMs) signature that is associated with compromised GFAP proteostasis in the severe form of AxD
Using patient brain tissue and human induced pluripotent stem cell (iPSC)-derived AxD astrocytes, we show that head domain phosphorylation promotes defective filament assembly and perinuclear accumulation and incorporation of mutant GFAP within nuclear invaginations

Summary

INTRODUCTION

Alexander Disease (AxD) is a rare and invariably fatal neurological disorder that affects primarily infants and small children, but can manifest later in life 1-3. While the utility of GFAP as a key therapeutic target in AxD is clear, the molecular mechanisms for how AxD-associated GFAP missense mutations (affecting over 70 different residues on GFAP) lead to defective GFAP proteostasis are not well understood. Deciphering these mechanisms may yield novel interventions, for AxD patients, and for patients with other diseases where IF proteostasis is severely compromised. We identified a critical phosphorylation site in the GFAP head domain that is selectively and strongly upregulated in the brain tissues of AxD patients who died very young, independently of the position of the disease mutation that they carried. Future interventional studies targeting these PTMs will determine whether they contribute to, or are the consequence of, disease severity

RESULTS

DISCUSSION

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