α-Synuclein Fibrils Exhibit Gain of Toxic Function, Promoting Tau Aggregation and Inhibiting Microtubule Assembly

Abstract

␣-Synuclein is the major component of Lewy bodies and Lewy neurites in Parkinson disease and dementia with Lewy bodies and of glial cytoplasmic inclusions in multiple system atrophy. It has been suggested that ␣-synuclein fibrils or intermediate protofibrils in the process of fibril formation may have a toxic effect on neuronal cells. In this study, we investigated the ability of soluble monomeric ␣-synuclein to promote microtubule assembly and the effects of conformational changes of ␣-synuclein on Tau-promoted microtubule assembly. In marked contrast to previous findings, monomeric ␣-synuclein had no effect on microtubule polymerization. However, both ␣-synuclein fibrils and protofibrils inhibited Tau-promoted microtubule assembly. The inhibitory effect of ␣-synuclein fibrils was greater than that of the protofibrils. Dot blot overlay assay and spin-down techniques revealed that ␣-synuclein fibrils bind to Tau and inhibit microtubule assembly by depleting the Tau available for microtubule polymerization. Using various deletion mutants of ␣-synuclein and Tau, the acidic C-terminal region of ␣-synuclein and the basic central region of Tau were identified as regions involved in the binding. Furthermore, introduction of ␣-synuclein fibrils into cultured cells overexpressing Tau protein induced Tau aggregation. These results raise the possibility that ␣-synuclein fibrils interact with Tau, inhibit its function to stabilize microtubules, and also promote Tau aggregation, leading to dysfunction of neuronal cells.

Molecular mechanisms of the co‐deposition of multiple pathological proteins in neurodegenerative diseases

“…They revealed that tau aggregation was promoted in the presence of full‐length alpha‐synuclein, but not beta‐synuclein, a homolog of alpha‐synuclein that does not fibrillize, or the delta 71–82 truncated form of alpha‐synuclein, an artificial mutant of alpha‐synuclein that cannot self‐fibrillize, suggesting that alpha‐synuclein can function as a tau aggregation inducer through its hydrophobic non‐amyloid component (NAC) domain. We also showed that alpha‐synuclein fibrils, but not monomer, bind to tau and inhibit microtubule assembly in vitro . Using various deletion mutants of alpha‐synuclein and tau, the acidic C‐terminal region of alpha‐synuclein and the basic central region of tau were identified as regions involved in their binding, in agreement with a previous report …”

“…Furthermore, we successfully generated a cultured cell model recapitulating the deposition of both tau and alpha‐synuclein found in human diseased brains . When recombinant alpha‐synuclein fibrils were introduced into cultured SH‐SY5Y cells expressing only tau, immunocytochemical analyses of these cells using anti‐AT8 confirmed that few inclusions were positive for AT8 (Fig.…”

“…Furthermore, we also showed that not only alpha‐synuclein, but also tau is deposited in mice injected with synthetic alpha‐synuclein fibrils into striatum . In these mouse brains, phosphorylated tau aggregates were rarely co‐localized with alpha‐synuclein pathology, which is very similar to the above results obtained using cultured cells expressing tau and treated with alpha‐synuclein fibrils . Although the molecular mechanisms through which recombinant alpha‐synuclein fibrils induce deposition of endogenous tau in our in vitro and in vivo models are not clear, there seem to be at least two possibilities: (i) alpha‐synuclein fibrils directly interact with endogenous tau monomer and function as cross‐seeds for intracellular tau aggregation; (ii) alpha‐synuclein fibrils facilitate aggregation of endogenous tau without direct interaction or cross‐seeding.…”

“…8,9 Similarly, studies using cultured cells and transgenic animals have shown the co-occurrence of abnormal deposition of multiple pathological proteins in in vitro and in vivo models. [10][11][12][13] Here, we present our recent data on the co-deposition of tau and alphasynuclein or TDP-43 and alpha-synuclein in cell culture and mouse models, and we discuss potential mechanisms underlying the co-occurrence of multiple abnormal proteins in vitro and in vivo. crosstalk of these proteinopathies.…”

Tau Structures

“…Also, tau interacts with alpha synuclein, probably through the basic region containing residues 166–226 (Oikawa et al, 2016). As mentioned above, tau interacts with other microtubular proteins, such as the EB1 and EB3 proteins (7), in developing neuronal cells.…”