Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC13
Autophagy and the ubiquitin-proteasome system in the degradation of tau
T. Rodriguez Martin1, I. Cuchillo Ibanez2, D. Hanger1
The microtubule-associated protein tau is mainly expressed in neurons and is essential for microtubule polymerisation and neurite outgrowth. Tau binding to microtubules is dependent on the phosphorylation state of the protein. In several neurodegenerative diseases, including Alzheimer’s disease, in which tau deposition is apparent, tau is found hyperphosphorylated and aggregated into neurofibrillary tangles. Dysfunction of autophagy has been linked to a number of neurodegenerative disorders that are associated with an accumulation of misfolded protein aggregates. Several studies appear to implicate the autophagic process in tau degradation. The aim of this study was to examine the effects of permanent mimics of tau phosphorylation on its degradation via the autophagic route in primary cortical neurons and mouse embryonic fibroblasts (MEFs). Plasmids expressing EGFP fused to the longest human CNS tau isoform (2N4R), two phosphomimic mutants E18tau and E27tau each containing 18 or 27 serine/threonine sites mutated to glutamate and an additional A18tau mutant in which 18 serine/threonine sites were mutated to alanine to mimic dephosphorylation were expressed in MEFs, either wild-type (Atg5+/+) or autophagy-deficient (Atg5-/-). Cells transfected with tau were treated with 10 mM 3-methyladenine, to inhibit autophagy, 1 µM MG132, to inhibit the proteasome, or 100 µM cycloheximide to inhibit protein synthesis. We found that neither wild-type nor phosphorylation mutant forms of full-length tau protein form aggregates in Atg5+/+ or autophagy deficient Atg5-/- MEFs. In primary cortical neurons, inhibition of autophagy results in the accumulation of all of the tau constructs. Inhibition of the proteasome also results in accumulation of exogenous tau. Our results suggest that in primary embryonic neurons both autophagic and proteasomal routes may be important for the degradation of exogenously expressed tau. In the absence of autophagy, the proteasome and/or calpain systems are able to degrade tau in MEFs. We conclude that the phosphorylation state of the tau protein may be important for its degradation.
Where applicable, experiments conform with Society ethical requirements