Disulfide bond formation in microtubule-associated tau protein promotes tau accumulation and toxicity in vivo.

Accumulation of microtubule-associated tau protein is thought to cause neuron loss in a group of neurodegenerative diseases called tauopathies. In diseased brains, tau molecules adopt pathological structures that propagate into insoluble forms with disease-specific patterns. Several types of posttranslational modifications in tau are known to modulate its aggregation propensity in vitro, but their influence on tau accumulation and toxicity at the whole-organism level has not been fully elucidated. Herein, we utilized a series of transgenic Drosophila models to compare systematically the toxicity induced by five tau constructs with mutations or deletions associated with aggregation, including substitutions at seven disease-associated phosphorylation sites (S7A and S7E), deletions of PHF6 and PHF6* sequences (ΔPHF6 and ΔPHF6*), and substitutions of cysteine residues in the microtubule binding repeats (C291/322A). We found that substitutions and deletions resulted in different patterns of neurodegeneration and accumulation, with C291/322A having a dramatic effect on both tau accumulation and neurodegeneration. These cysteines formed disulfide bonds in mouse primary cultured neurons and in the fly retina, and stabilized tau proteins. Additionally, they contributed to tau accumulation under oxidative stress. We also found that each of these cysteine residues contributes to the microtubule polymerization rate and microtubule levels at equilibrium, but none of them affected tau binding to polymerized microtubules. Since tau proteins expressed in the Drosophila retina are mostly present in the early stages of tau filaments self-assembly, our results suggest that disulfide bond formation by these cysteine residues could be attractive therapeutic targets.

S6K/p70S6K1 protects against tau-mediated neurodegeneration by decreasing the level of tau phosphorylated at Ser262 in a Drosophila model of tauopathy.

Abnormal accumulation of the microtubule-associated protein tau is thought to cause neuronal cell death in a group of age-associated neurodegenerative disorders. Tau is phosphorylated at multiple sites in diseased brains, and phosphorylation of tau at Ser262 initiates tau accumulation and toxicity. In this study, we sought to identify novel factors that affect the metabolism and toxicity of tau phosphorylated at Ser262 (pSer262-tau). A biased screen using a Drosophila model of tau toxicity revealed that knockdown of S6K, the Drosophila homolog of p70S6K1, increased the level of pSer262-tau and enhanced tau toxicity. S6K can be activated by the insulin signaling, however, unlike knockdown of S6K, knockdown of insulin receptor or insulin receptor substrate nonselectively decreased total tau levels via autophagy. Importantly, activation of S6K significantly suppressed tau-mediated axon degeneration, whereas manipulation of either the insulin signaling pathway or autophagy did not. Our results suggest that activation of S6K may be an effective therapeutic strategy for selectively decreasing the levels of toxic tau species and suppressing neurodegeneration.