Pathological concentration of zinc dramatically accelerates abnormal aggregation of full-length human Tau and thereby significantly increases Tau toxicity in neuronal cells.

A pathological hallmark of Alzheimer disease and other tauopathies is the formation of neurofibrillary tangles mainly composed of bundles of fibrils formed by microtubule-associated protein Tau. Here we study the effects of Zn2+ on abnormal aggregation and cytotoxicity of a pathological mutant ΔK280 of full-length human Tau. As revealed by Congo red binding assays, transmission electron microscopy, immunofluorescence, Western blot, and immunogold electron microscopy, pathological concentration of Zn2+ dramatically accelerates the fibrillization of ΔK280 both in vitro and in SH-SY5Y neuroblastoma cells. As evidenced by annexin V-FITC apoptosis detection assay and MTT reduction assay, pathological concentration of Zn2+ remarkably enhances ΔK280 fibrillization-induced apoptosis and toxicity in SH-SY5Y cells. Substitution of Cys-291 and Cys-322 with Ala, however, essentially eliminates such enhancing effects of Zn2+ on the fibrillization and the consequent cytotoxicity of ΔK280. Furthermore, Zn2+ is co-localized with and highly enriched in amyloid fibrils formed by ΔK280 in SH-SY5Y cells. The results from isothermal titration calorimetry show that Zn2+ binds to full-length human Tau by interacting with Cys-291 and Cys-322, forming a 1:1 Zn2+-Tau complex. Our data demonstrate that zinc dramatically accelerates abnormal aggregation of human Tau and significantly increases Tau toxicity in neuronal cells mainly via bridging Cys-291 and Cys-322. Our findings could explain how pathological zinc regulates Tau aggregation and toxicity associated with Alzheimer disease.

Sequence-dependent abnormal aggregation of human Tau fragment in an inducible cell model.

A pathological hallmark of Alzheimer disease (AD) is the accumulation of misfolded hyperphosphorylated microtubule-associated protein Tau within neurons, forming neurofibrillary tangles and leading to synaptic dysfunction and neuronal death. Here we study sequence-dependent abnormal aggregation of human fragment Tau244-372 in an inducible cell model. As evidenced by confocal laser scanning microscopy, Western blot, and immunogold electron microscopy, fibril-forming motifs are essential and sufficient for abnormal aggregation of Tau244-372 in SH-SY5Y neuroblastoma cells induced by Congo red: when its two fibril-forming segments PHF6 and PHF6* are deleted, Tau244-372 does lose its ability to form fibrils in SH-SY5Y cells, and the replacement of PHF6 and PHF6* with an unrelated amyloidogenic sequence IFQINS from human lysozyme does rescue the fibril-forming ability of Tau244-372 in SH-SY5Y cells. By contrast, insertion of a non-fibril forming peptide GGGGGG does not drive the disabled Tau244-372 to misfold in SH-SY5Y cells. Furthermore, as revealed by quantum dots based probes combined with annexin V staining, annexin V-FITC apoptosis detection assay, and immunofluorescence, fibril-forming motifs are essential and sufficient for early apoptosis of living SH-SY5Y cells induced by abnormal aggregation of Tau244-372. Our results suggest that fibril-forming motifs could be the determinants of Tau protein tending to misfold in living cells, thereby inducing neuronal apoptosis and causing the initiation and development of AD.