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Big Tau: Structure, Evolutionary Divergence, and Emerging Roles in Cytoskeletal Dynamics and Tauopathies
Journal article   Open access   Peer reviewed

Big Tau: Structure, Evolutionary Divergence, and Emerging Roles in Cytoskeletal Dynamics and Tauopathies

Itzhak Fischer and Peter W. Baas
Cells (Basel, Switzerland), v 15(3), p241
27 Jan 2026
DOI: https://doi.org/10.3390/cells15030241
PMID: 41677608
Featured in Collection :   Research Supported by Drexel Libraries' OA Programs
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https://doi.org/10.3390/cells15030241View
Published, Version of Record (VoR) Open Access Discount via Drexel Libraries Read and Publish Program 2026 Open CC BY V4.0

Abstract

tau microtubule-associated protein exon 4a hydrophobicity protein aggregation neurodegeneration evolutionary conservation Neurons
Tau proteins are microtubule-associated proteins that regulate axonal structure, dynamics, and transport, and their dysregulation underlies several neurodegenerative diseases. The MAPT gene produces multiple tau isoforms through alternative splicing, including the high-molecular-weight isoform known as Big tau, which contains an insert of the large 4a exon of approximately 250 amino acids. Big tau is predominantly expressed in neurons of the peripheral nervous system (PNS), cranial motor nuclei, and select neurons of the central nervous system (CNS) such as the cerebellum and brainstem. Developmental expression studies indicate a switch from low-molecular-weight isoforms of tau to Big tau during axonal maturation, suggesting that Big tau optimizes cytoskeletal dynamics to accommodate long axonal projections. Comparative sequence and biophysical analyses show that the exon-4a insert is highly acidic, intrinsically disordered, and evolutionarily conserved in its length but not its primary sequence, implying a structural role. Emerging modeling and in vitro assays suggest that the extended projection domain provided by the exon-4a insert spatially and electrostatically shields the aggregation-prone PHF6 and PHF6* motifs in tau’s microtubule-binding domain, thereby reducing β-sheet driven aggregation. This mechanism may explain why tauopathies that involve aggregation of tau have little effect on the PNS and specific regions of the CNS such as the cerebellum, where Big tau predominates. Transcriptomic and proteomic data further suggest that alternative Big tau variants, including 4a-L, are expressed in certain cancerous tissues, indicating broader roles in cytoskeletal remodeling beyond neurons. Despite its putative anti-aggregation properties, the physiological regulation, interaction partners, and in vivo mechanisms of Big tau remain poorly defined. This review summarizes what is known about Big tau and what is missing toward a better understanding of how expansion via inclusion of exon 4a modifies tau’s structural and functional properties. Our purpose is to inspire future studies that could lead to novel therapeutic strategies to mitigate tau aggregation in neurodegenerative diseases.

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