Abstract
Neurodegenerative diseases are characterized by the accumulation of specific phosphorylated protein aggregates in the brain, such as hyperphosphorylated tau (hp-tau) in tauopathies and phosphorylated α-synuclein (p-αSyn) in α-synucleinopathies. The simultaneous accumulation of different proteins is a common event in many neurodegenerative diseases. We herein describe the detection of the phosphorylation and dimerization of αSyn and activation of GSK-3β, a major kinase known to phosphorylate tau and αSyn, in the brains of rTg4510 mice that overexpress human P301L mutant tau. Immunohistochemistry showed p-αSyn aggregates in rTg4510 mice, which were suppressed by doxycycline-mediated decreases in mutant tau expression levels. A semi-quantitative analysis revealed a regional correlation between hp-tau and p-αSyn accumulation in rTg4510 mice. Furthermore, proteinase K-resistant αSyn aggregates were found in the region with excessive hp-tau accumulation in rTg4510 mice, and these aggregates were morphologically different from proteinase K-susceptible p-αSyn aggregates. Western blotting revealed decreases in p-αSyn monomers in TBS- and sarkosyl-soluble fractions and increases in ubiquitinated p-αSyn dimers in sarkosyl-soluble and insoluble fractions in rTg4510 mice. Furthermore, an activated form of GSK-3β was immunohistochemically detected within cells containing both hp-tau and p-αSyn aggregates. A semi-quantitative analysis revealed that increased GSK-3β activity strongly correlated with hp-tau and p-αSyn accumulation in rTg4510 mice. Collectively, the present results suggest that the overexpression of human P301L mutant tau promoted the phosphorylation and dimerization of endogenous αSyn by activating GSK-3β in rTg4510 mice. This synergic effect between tau, αSyn, and GSK-3β may be involved in the pathophysiology of several neurodegenerative diseases that show the accumulation of both tau and αSyn.
Highlights
The accumulation of specific phosphorylated protein aggregates and neuronal loss in the central nervous system are hallmarks of several neurodegenerative diseases
To elucidate the relationship between the distribution of hp-tau and phosphorylated α-synuclein (p-αSyn), we examined the amounts of hp-tau and p-αSyn that accumulated in 10 regions in rTg4510 mice fed the standard or doxycycline diet using immunohistochemistry: the hippocampal CA1 and CA3 areas, dentate gyrus, motor area, somatosensory area, entorhinal cortex, piriform cortex, amygdala, striatum, and substantia nigra. rTg4510 mice fed the doxycycline diet showed a significant decrease of hp-tau and p-αSyn deposition in the hippocampal CA1 and CA3 areas, motor area, somatosensory area, entorhinal cortex, piriform cortex, amygdala, and striatum, as compared to rTg4510 mice fed the standard diet (Fig. 2a, b)
As shown in the scatterplots, a positive correlation was found between the average amounts of hp-tau and p-αSyn that accumulated in each region (r = 0.85, p < 0.01) (Fig. 2c)
Summary
The accumulation of specific phosphorylated protein aggregates and neuronal loss in the central nervous system are hallmarks of several neurodegenerative diseases. Tau and αSyn are both natively unfolded soluble proteins that are minimally phosphorylated in the normal adult brain. They undergo conformational changes, such as phosphorylation, in pathological states, which subsequently leads to oligomerization. They may become ubiquitinated and form intracytoplasmic insoluble filamentous aggregates [30, 57]. ΑSyn molecules are unfolded soluble monomers or tetramers [3], but in pathological states these molecules are phosphorylated, have a propensity for folding and forming insoluble oligomers, and through protofibrils become mature fibrils, the main component of LBs [29]. Accumulating evidence suggests that the toxic form of αSyn is oligomers, rather than mature fibrils, which induce neuronal dysfunction and cell death [12]
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