Abstract
Mutations in the MAPT gene can lead to disease-associated variants of tau. However, the pathological mechanisms behind these genetic tauopathies are poorly understood. Here, we characterized the aggregation stages and conformational changes of tau C291R, a recently described MAPT mutation with potential pathogenic functions. The C291R variant of the tau four-repeat domain (tau-K18; a functional fragment with increased aggregation propensity compared with the full-length protein), aggregated into a mix of granular oligomers, amorphous and annular pore-like aggregates, in native-state and heparin-treated reactions as observed using atomic force microscopy (AFM) and negative-stained electron microscopy. On extended incubation in the native-state, tau-K18 C291R oligomers, unlike wild type (WT) tau-K18, aggregated to form protofibrils of four different phenotypes: (1) spherical annular; (2) spherical annular encapsulating granular oligomers; (3) ring-like annular but non-spherical; and (4) linear protofibrils. The ring-like tau-K18 C291R aggregates shared key properties of annular protofibrils previously described for other amyloidogenic proteins, in addition to two unique features: irregular/non-spherical-shaped annular protofibrils, and spherical protofibrils encapsulating granular oligomers. Tau-K18 C291R monomers had a circular dichroism (CD) peak at ~210 nm compared with ~199 nm for tau-K18 WT. These data suggest mutation-enhanced β-sheet propensity. Together, we describe the characterization of tau-K18 C291R, the first genetic mutation substituting a cysteine residue. The aggregation mechanism of tau-K18 C291R appears to involve β-sheet-rich granular oligomers which rearrange to form unique protofibrillar structures.
Highlights
Tau protein is a product of the microtubule-associated protein tau (MAPT) gene located on chromosome 17q21 (Neve et al, 1986; Andreadis et al, 1992)
To understand the possible effects of the cysteine-291 substituted for arginine (C291R) mutation on β-sheet formation, the secondary structure content of monomeric tau-K18 C291R and the wild type (WT) were studied with circular dichroism (CD) spectroscopy
We have demonstrated that the ring-like tau-K18 C291R aggregates possess key properties of annular protofibrils previously described for other amyloidogenic proteins
Summary
Tau protein is a product of the microtubule-associated protein tau (MAPT) gene located on chromosome 17q21 (Neve et al, 1986; Andreadis et al, 1992). MAPT is comprised of 16 exons, producing six tau isoforms in the adult human brain (Goedert et al, 1988, 1989). Biochemical Characterization of Tau-C291R the microtubule-binding region (MTBR) and the C-terminus region (Andreadis et al, 1992; Andreadis, 2005). Variations in the number of N-terminus domains (0, 1 or 2) and MTBR domains (3 or 4 repeats) are the defining features of different isoforms (Andreadis et al, 1992; Andreadis, 2005). Alternative splicing of exon 10 affects the ratio of three- to four-repeat tau isoforms, changes in which have been linked to several tauopathies (Liu and Gong, 2008). Many MAPT mutations located in or around exon 10 have been reported from individuals affected by different genetic tauopathies, with over a dozen implicated in disease (Goedert and Jakes, 2005; Ghetti et al, 2015). Several studies have reported that the presence of the cysteine-291 residue is important for tau aggregation and that the cysteine-322 residue may be inhibitory to this process (Bhattacharya et al, 2001; Crowe et al, 2013; Soeda et al, 2015; Al-Hilaly et al, 2017)
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