Avoiding the oligomeric state: αB‐crystallin inhibits fragmentation and induces dissociation of apolipoprotein C‐II amyloid fibrils

Abstract

The in vivo aggregation of proteins into amyloid fibrils suggests that cellular mechanisms that normally prevent or reverse this aggregation have failed. The small heat-shock molecular chaperone protein αB-crystallin (αB-c) inhibits amyloid formation and colocalizes with amyloid plaques; however, the physiological reason for this localization remains unexplored. Here, using apolipoprotein C-II (apoC-II) as a model fibril-forming system, we show that αB-c binds directly to mature amyloid fibrils (Kd 5.4 ± 0.5 μM). In doing so, αB-c stabilized the fibrils from dilution-induced fragmentation, halted elongation of partially formed fibrils, and promoted the dissociation of mature fibrils into soluble monomers. Moreover, in the absence of dilution, the association of αB-c with apoC-II fibrils induced a 14-fold increase in average aggregate size, resulting in large fibrillar tangles reminiscent of protein inclusions. We propose that the binding of αB-c to fibrils prevents fragmentation and mediates the lateral association of fibrils into large inclusions. We further postulate that transient interactions of apoC-II with αB-c induce a fibril-incompetent monomeric apoC-II form, preventing oligomerization and promoting fibril dissociation. This work reveals previously unrecognized mechanisms of αB-c chaperone action in amyloid assembly and fibril dynamics, and provides a rationale for the in vivo colocalization of small heat-shock proteins with amyloid deposits.-Binger, K. J., Ecroyd, H., Yang, S., Carver, J. A., Howlett, G. J., Griffin, M. D. W. Avoiding the oligomeric state: αB-crystallin inhibits fragmentation and induces dissociation of apolipoprotein C-II amyloid fibrils.

Effect of molecular chaperones on aberrant protein oligomers in vitro: super-versus sub-stoichiometric chaperone concentrations

“…Recently, it has been demonstrated that, even when present at very low concentrations, molecular chaperones may inhibit the cytotoxicity of preformed misfolded oligomers formed by A, IAPP and HypF-N and fibrils formed by apolipoprotein C-II, by binding to them and converting them into larger aggregates (Ojha et al, 2011;Mannini et al, 2012, Binger et al, 2013. Interestingly, this chaperone-induced effect has been found to occur in the absence of any major structural reorganisation within the individual oligomers, which appear to maintain their overall structure and just assemble into larger species.…”

“…In addition to these functions, chaperones have recently been found to suppress in vitro the toxic effects of protein aggregates after the aggregates have formed (Ojha et al, 2011;Mannini et al, 2012;Cascella et al, 2013a;Cascella et al, 2013b;Binger et al, 2013). In particular, it has been found for a range of systems that misfolded protein oligomers, incubated with sub-stoichiometric concentrations of chaperones in vitro, can be converted into large aggregates with substantially reduced toxicity, in the absence of any significant structural reorganisation of the individual molecules within the aggregates (Mannini et al, 2012).…”

“…In particular, it has been found for a range of systems that misfolded protein oligomers, incubated with sub-stoichiometric concentrations of chaperones in vitro, can be converted into large aggregates with substantially reduced toxicity, in the absence of any significant structural reorganisation of the individual molecules within the aggregates (Mannini et al, 2012). It was also found that fibrils formed by apolipoprotein C-II were converted into large fibrillar tangles in the presence of a small heat shock protein (Binger et al, 2013). In a recent report it was found that long-lived daf-2 mutant C. elegans strains were characterised by the accumulation of large chaperone-containing aggregates during aging, relative to age-matched wild-type controls that had a normal lifespan (Walther et al 2015).…”

Small heat-shock proteins: important players in regulating cellular proteostasis

“…Thus, the design of such studies does not address the effect(s) sHsps have on the latter stages of aggregation, which is as important to consider since, in vivo, levels of sHsps in the cell increase after aggregation has commenced as a result of the activation of the stress response [104,105]. Apart from interacting with monomeric species, sHsps also bind to species formed further along the aggregation pathway, including mature amyloid fibrils [106][107][108][109]. For example, when introduced during the elongation phase of α-synuclein (α-syn) or amyloid- peptide (A aggregation, αBc prevents further fibril growth by binding along the length of mature fibrils, preventing secondary nucleation events that facilitate further fibril growth [106][107][108].…”

“…For example, when introduced during the elongation phase of α-synuclein (α-syn) or amyloid- peptide (A aggregation, αBc prevents further fibril growth by binding along the length of mature fibrils, preventing secondary nucleation events that facilitate further fibril growth [106][107][108]. Our recent work, using apolipoprotein C-II (apoC-II) as a model fibril-forming protein, has shown that, by binding to fibrils, αBc stabilises them, preventing their (dilution-induced) fragmentation, and causes them to associate (tangle) into larger species reminiscent of protein inclusions [109]. Both fibril fragmentation and secondary nucleation can be the main sources of small oligomers thought to be responsible for the toxicity associated with the aggregation process [110].…”