"Prion-like" seeding and propagation of oligomeric protein assemblies in neurodegenerative disorders

Abstract

Intra- or extracellular aggregates of proteins are central pathogenic features in most neurodegenerative disorders. The accumulation of such proteins in diseased brains is believed to be the end-stage of a stepwise aggregation of misfolded monomers to insoluble cross-β fibrils via a series of differently sized soluble oligomers/protofibrils. Several studies have shown how α-synuclein, amyloid-β, tau and other amyloidogenic proteins can act as nucleating particles and thereby share properties with misfolded forms, or strains, of the prion protein. Although the roles of different protein assemblies in the respective aggregation cascades remain unclear, oligomers/protofibrils are considered key pathogenic species. Numerous observations have demonstrated their neurotoxic effects and a growing number of studies have indicated that they also possess seeding properties, enabling their propagation within cellular networks in the nervous system. The seeding behavior of oligomers differs between the proteins and is also affected by various factors, such as size, shape and epitope presentation. Here, we are providing an overview of the current state of knowledge with respect to the "prion-like" behavior of soluble oligomers for several of the amyloidogenic proteins involved in neurodegenerative diseases. In addition to providing new insight into pathogenic mechanisms, research in this field is leading to novel diagnostic and therapeutic opportunities for neurodegenerative diseases.

Keywords: misfolding; neurodegeneration; oligomers; propagation; seeding.

Figure 1

(A) Amyloidogenic protein aggregation process and influencing factors. The aggregation process of amyloidogenic proteins follows specific kinetics and the different species are usually present in an equilibrium. In disease, a native monomer assumes a misfolded pathological conformation and aggregates into soluble higher molecular weight species, such as oligomers and protofibrils, which are believed to be the main toxic species in several neurodegenerative diseases. These processes appear to be influenced by several molecular and environmental factors. Oligomers/protofibrils can assume different conformations, leading to the generation of different strains of the same protein. Further aggregation of oligomers and protofibrils leads to the formation of insoluble amyloid fibrils. Next to such on-pathway oligomers, accumulating evidence supports the existence of off-pathway oligomers, which do not aggregate further into insoluble fibrils. Both soluble species and insoluble fibrils of different amyloidogenic proteins might possess “prion-like” nucleating properties, templating the misfolding of a native monomer into a pathogenic conformation. As for the aggregation process, seeding is influenced by molecular properties of the nucleating particles as well as by environmental factors. (B) Toxicity of soluble oligomers. Oligomers have been found to exert their toxicity targeting several processes. The most common and shared effects are mitochondrial dysfunction and generation of reactive oxygen species (ROS), impairment of synaptic plasticity and axonal transport, disruption of membrane integrity and blockage of the ubiquitin proteasome system. Created with BioRender.com.

Figure 2

Immunohistochemical staining and atomic force microscopy images of amyloidogenic proteins. Upper row: example images of α-synuclein (α-syn; left), amyloid-β (Aβ; middle) and hyperphosphorylated tau (right) pathology in an AD brain (scale bars: 50 μm), visualized by immunohistochemical staining with specific antibodies as previously described (Libard et al., 2022; anti α-syn: KM51, Novocastra; anti Aβ: 4G8, Biolegend; anti phospho-tau: AT8, Fisher Scientific-Invitrogen). Lower row: Atomic force microscopy (AFM) images of oligomeric forms of the corresponding proteins. Protofibrils of α-synuclein were induced with 4-hydroxy-trans-2-nonenal (HNE; scale bar: 50 nm) from recombinant E46K monomers. Protofibrils of Aβ were induced from synthetic 1–42 wt peptide (scale bar: 100 nm). Oligomers of tau were induced from wt 2N4R tau (scale bar: 100 nm). The immunohistochemical images were kindly provided by Dr. Sylwia Libard. The AFM images were kindly provided by Dr. Mikael Karlsson (α-synuclein and Aβ) and the Kayez lab (tau).

Figure 3

(A) Diagnostic applications targeting seeding capable species. The seeding amplification assay (SAA) has been proposed as a tool that can discriminate the seeding behavior of different protein species between diseases with overlapping pathologies and symptoms. Various sample sources, including CSF, serum, saliva and skin, can be used in the reactions. Analyses can also be conducted post mortem using brain tissue samples. Seeding of a recombinant monomer template by different oligomer species between samples can be visualized via Thioflavin T (ThT). (B) Therapeutic approaches targeting oligomers. Schematic representation of small molecule inhibitors (SMI) and monoclonal antibody (mAb) therapeutics in clinical trial. The primary target of each therapeutic is indicated by a line or bracket. Blue indicates therapeutics targeting Aβ. Green indicates therapeutics targeting tau. Yellow indicates therapeutics targeting α-syn. The antibody symbol indicates which therapeutics are monoclonal antibodies. The star indicates which therapeutics are SMIs. Created with BioRender.com.