Abstract
Purpose: :
Tau protein plays a key role in the stabilization of microtubules and is essential for normal axonal transport. Binding of tau to microtubules is regulated by phosphorylation. Tau hyperphosphorylation has been associated with axonal transport failure and neuronal death in Alzheimer’s and other neurodegenerative diseases. We hypothesized that tau hyperphosphorylation could contribute to the disruption of dynein-mediated axonal transport we have previously described in experimental glaucoma in the rat.
Methods: :
Unilateral intraocular pressure (IOP) elevation was induced in adult Wistar rats by 532nm laser treatment to the trabecular meshwork. IOP was monitored by rebound tonometry. Monoclonal antibodies directed against specific phosphorylated epitopes were used to compare retinal tau phosphorylation in glaucoma and control eyes by immunohistochemistry and quantitative Western blotting at 24 hours and 48 hours (n=4 rats per group) and 1 week, 2 weeks and 4 weeks (n=10 rats per group) after IOP elevation. Total tau was assessed using a pan-tau antibody (Tau5).
Results: :
The average peak IOP was 43.2±7.6mmHg in laser treated eyes compared to 11.8±4.2mmHg in controls. Pan-tau immunolabelling was localized to retinal ganglion cell axons and was similar in glaucoma and control eyes. Quantitative Western blotting revealed a highly significant increase in tau phosphorylation at serine 202/threonine 205, threonine 181 and threonine 231 in glaucoma, compared to control eyes. Increased tau phosphorylation at each of these locations was observed at all timepoints and persisted at 4 weeks, when IOP had returned to normal or below normal in 9 out of 10 eyes.
Conclusions: :
Tau hyperphosphorylation at multiple sites occurs within 24 hours of experimental IOP elevation in the rat and persists for at least 4 weeks despite normalization of the IOP. We suggest that loss of microtubule stabilization as a consequence of tau hyperphosphorylation may contribute to the failure of dynein-mediated axonal transport in experimental glaucoma, thus contributing to glaucomatous retinal ganglion cell death.
Keywords: inner retina dysfunction: biochemistry and cell biology • cytoskeleton • ganglion cells