Effect of the micro-environment on α-synuclein conversion and implication in seeded conversion assays

Abstract

Background: α-Synuclein is a small soluble protein, whose physiological function in the healthy brain is poorly understood. Intracellular inclusions of α-synuclein, referred to as Lewy bodies (LBs), are pathological hallmarks of α-synucleinopathies, such as Parkinson's disease (PD) or dementia with Lewy bodies (DLB).

Main body: Understanding of the molecular basis as well as the factors or conditions promoting α-synuclein misfolding and aggregation is an important step towards the comprehension of pathological mechanism of α-synucleinopathies and for the development of efficient therapeutic strategies. Based on the conversion and aggregation mechanism of α-synuclein, novel diagnostic tests, such as protein misfolding seeded conversion assays, e.g. the real-time quaking-induced conversion (RT-QuIC), had been developed. In diagnostics, α-synuclein RT-QuIC exhibits a specificity between 82 and 100% while the sensitivity varies between 70 and 100% among different laboratories. In addition, the α-synuclein RT-QuIC can be used to study the α-synuclein-seeding-characteristics of different α-synucleinopathies and to differentiate between DLB and PD.

Conclusion: The variable diagnostic accuracy of current α-synuclein RT-QuIC occurs due to different protocols, cohorts and material etc.. An impact of micro-environmental factors on the α-synuclein aggregation and conversion process and the occurrence and detection of differential misfolded α-synuclein types or strains might underpin the clinical heterogeneity of α-synucleinopathies.

Keywords: Protein misfolding; Protein strains; RT-QuIC; α-Synucleinopathies.

Fig. 1

Diagram of the potential seeding-conversion mechanism of α-synuclein during the RT-QuIC. The seed (in red) triggers the aggregation of monomeric α-synuclein (substrate, in green). The conversion causes the conformational modification into misfolded oligomers (blue) that elongate into fibrils. After the detection of fibrils, a quaking event breaks the longer fibrils into shorter, reactive oligomers, which further seed the conversion of monomeric α-synuclein (modified from [92]). The classical shape of the kinetic curve is shown within the aggregation process

Figure 2

Proposed models for the aggregation pathway. a Established schematic view for the pathway of aggregation of natively unfolded proteins [40]. According to this model, from an unfolded protein (U_N) is formed an intermediate (I), which further aggregates into protofibrils (P) and eventually into fibrils (F). b The extended model we propose takes into account the possibility that a population of strains are produced, together with off-pathway oligomers that do not show the ability to form α-synuclein fibrils. Intermediate forms marked as I₁ to I_n, each of them may produce a different protofibrillar form, marked as P_1-n to P_n-n (the protofibril P₁ derived from the intermediate I₁ until the protofibril P_n derived from the intermediate I_n). It is noteworthy to state that the process may be in equilibrium between the aggregation of the intermediates and the rupture of protofibrillar forms. Further aggregation will cause the collapse into a limited number of fibrillar structures