The E46K mutation modulates α-synuclein prion replication in transgenic mice.

In multiple system atrophy (MSA), the α-synuclein protein misfolds into a self-templating prion conformation that spreads throughout the brain, leading to progressive neurodegeneration. While the E46K mutation in α-synuclein causes familial Parkinson's disease (PD), we previously discovered that this mutation blocks in vitro propagation of MSA prions. Recent studies by others indicate that α-synuclein adopts a misfolded conformation in MSA in which a Greek key motif is stabilized by an intramolecular salt bridge between residues E46 and K80. Hypothesizing that the E46K mutation impedes salt bridge formation and, therefore, exerts a selective pressure that can modulate α-synuclein strain propagation, we asked whether three distinct α-synuclein prion strains could propagate in TgM47+/- mice, which express human α-synuclein with the E46K mutation. Following intracranial injection of these strains, TgM47+/- mice were resistant to MSA prion transmission, whereas recombinant E46K preformed fibrils (PFFs) transmitted neurological disease to mice and induced the formation of phosphorylated α-synuclein neuropathology. In contrast, heterotypic seeding following wild-type (WT) PFF-inoculation resulted in preclinical α-synuclein prion propagation. Moreover, when we inoculated TgM20+/- mice, which express WT human α-synuclein, with E46K PFFs, we observed delayed transmission kinetics with an incomplete attack rate. These findings suggest that the E46K mutation constrains the number of α-synuclein prion conformations that can propagate in TgM47+/- mice, expanding our understanding of the selective pressures that impact α-synuclein prion replication.

Multiple system atrophy prions transmit neurological disease to mice expressing wild-type human α-synuclein.

In multiple system atrophy (MSA), the protein α-synuclein misfolds into a prion conformation that self-templates and causes progressive neurodegeneration. While many point mutations in the α-synuclein gene, SNCA, have been identified as the cause of heritable Parkinson's disease (PD), none have been identified as causing MSA. To examine whether MSA prions can transmit disease to mice expressing wild-type (WT) human α-synuclein, we inoculated transgenic (Tg) mice denoted TgM20+/- with brain homogenates prepared from six different deceased MSA patients. All six samples transmitted CNS disease to the mice, with an average incubation period of ~ 280 days. Interestingly, TgM20+/- female mice developed disease > 60 days earlier than their male counterparts. Brains from terminal mice contained phosphorylated α-synuclein throughout the hindbrain, consistent with the distribution of α-synuclein inclusions in MSA patients. In addition, using our α-syn-YFP cell lines, we detected α-synuclein prions in brain homogenates prepared from terminal mice that retained MSA strain properties. To our knowledge, the studies described here are the first to show that MSA prions transmit neurological disease to mice expressing WT SNCA and that the rate of transmission is sex dependent. By comparison, TgM20+/- mice inoculated with WT preformed fibrils (PFFs) developed severe neurological disease in ~ 210 days and exhibited robust α-synuclein neuropathology in both limbic regions and the hindbrain. Brain homogenates from these animals exhibited biological activities that are distinct from those found in MSA-inoculated mice when tested in the α-syn-YFP cell lines. Differences between brains from MSA-inoculated and WT PFF-inoculated mice potentially argue that α-synuclein prions from MSA patients are distinct from the PFF inocula and that PFFs do not replicate MSA strain biology.

Consequences of variability in α-synuclein fibril structure on strain biology.

Synucleinopathies are a group of clinically and neuropathologically distinct protein misfolding diseases caused by unique α-synuclein conformations, or strains. While multiple atomic resolution cryo-electron microscopy structures of α-synuclein fibrils are now deposited in Protein Data Bank, significant gaps in the biological consequences arising from each conformation have yet to be unraveled. Mutations in the α-synuclein gene (SNCA), cofactors, and the solvation environment contribute to the formation and maintenance of each disease-causing strain. This review highlights the impact of each of these factors on α-synuclein misfolding and discusses the implications of the resulting structural variability on therapeutic development.

Evidence of distinct α-synuclein strains underlying disease heterogeneity.

Synucleinopathies are a group of neurodegenerative disorders caused by the misfolding and self-templating of the protein α-synuclein, or the formation of α-synuclein prions. Each disorder differs by age of onset, presenting clinical symptoms, α-synuclein inclusion morphology, and neuropathological distribution. Explaining this disease-specific variability, the strain hypothesis postulates that each prion disease is encoded by a distinct conformation of the misfolded protein, and therefore, each synucleinopathy is caused by a unique α-synuclein structure. This review discusses the current data supporting the role of α-synuclein strains in disease heterogeneity. Several in vitro and in vivo models exist for evaluating strain behavior, however, as the focus of this article is to compare strains across synucleinopathy patients, our discussion predominantly focuses on the two models most commonly used for this purpose: the α-syn140*A53T-YFP cell line and the TgM83+/- mouse model. Here we define each strain based on biochemical stability, ability to propagate in α-syn140-YFP cell lines, and incubation period, inclusion morphology and distribution, and neurological signs in TgM83+/- mice.

The role of prion strain diversity in the development of successful therapeutic treatments.

Prions are a self-propagating misfolded conformation of a cellular protein. Prions are found in several eukaryotic organisms with mammalian prion diseases encompassing a wide range of disorders. The first recognized prion disease, the transmissible spongiform encephalopathies (TSEs), affect several species including humans. Alzheimer's disease, synucleinopathies, and tauopathies share a similar mechanism of self-propagation of the prion form of the disease-specific protein reminiscent of the infection process of TSEs. Strain diversity in prion disease is characterized by differences in the phenotype of disease that is hypothesized to be encoded by strain-specific conformations of the prion form of the disease-specific protein. Prion therapeutics that target the prion form of the disease-specific protein can lead to the emergence of drug-resistant strains of prions, consistent with the hypothesis that prion strains exist as a dynamic mixture of a dominant strain in combination with minor substrains. To overcome this obstacle, therapies that reduce or eliminate the template of conversion are efficacious, may reverse neuropathology, and do not result in the emergence of drug resistance. Recent advancements in preclinical diagnosis of prion infection may allow for a combinational approach that treats the prion form and the precursor protein to effectively treat prion diseases.

Alteration of Prion Strain Emergence by Nonhost Factors.

Prions can persist in the environment for extended periods of time after adsorption to surfaces, including soils, feeding troughs, or fences. Prion strain- and soil-specific differences in prion adsorption, infectivity, and response to inactivation may be involved in strain maintenance or emergence of new strains in a population. Extensive proteinase K (PK) digestion of Hyper (HY) and Drowsy (DY) PrPSc resulted in a greater reduction in the level of DY PrPSc than of HY PrPSc Use of the PK-digested material in protein misfolding cyclic amplification strain interference (PMCAsi) resulted in earlier emergence of HY PrPSc than of undigested controls. This result established that strain-specific alteration of the starting ratios of conversion-competent HY and DY PrPSc can alter strain emergence. We next investigated whether environmentally relevant factors such as surface binding and weathering could alter strain emergence. Adsorption of HY and DY PrPSc to silty clay loam (SCL), both separately and combined, resulted in DY interfering with the emergence of HY in PMCAsi in a manner similar to that seen with unbound controls. Similarly, repeated cycles of wetting and drying of SCL-bound HY and DY PrPSc did not alter the emergence of HY PrPSc compared to untreated controls. Importantly, these data indicate that prion strain interference can occur when prions are bound to surfaces. Interestingly, we found that drying of adsorbed brain homogenate on SCL could restore its ability to interfere with the emergence of HY, suggesting a novel strain interference mechanism. Overall, these data provide evidence that the emergence of a strain from a mixture can be influenced by nonhost factors.IMPORTANCE The prion strain, surface type, and matrix containing PrPSc can influence PrPSc surface adsorption. The cumulative effect of these factors can result in strain- and soil-specific differences in prion bioavailability. Environmental weathering processes can result in decreases in PrPSc conversion efficiency and infectivity. Little is known about how incomplete inactivation of surface-bound PrPSc affects transmission and prion strain emergence. Here, we show that strain interference occurs with soil-bound prions and that altering the ratios of prion strains by strain-specific inactivation can affect strain emergence. Additionally, we identify a novel mechanism of inhibition of prion conversion by environmental treatment-induced changes at the soil-protein interface altering strain emergence. These novel findings suggest that environmental factors can influence strain emergence of surface-bound prions.

PrPSc formation and clearance as determinants of prion tropism.

Prion strains are characterized by strain-specific differences in neuropathology but can also differ in incubation period, clinical disease, host-range and tissue tropism. The hyper (HY) and drowsy (DY) strains of hamster-adapted transmissible mink encephalopathy (TME) differ in tissue tropism and susceptibility to infection by extraneural routes of infection. Notably, DY TME is not detected in the secondary lymphoreticular system (LRS) tissues of infected hosts regardless of the route of inoculation. We found that similar to the lymphotropic strain HY TME, DY TME crosses mucosal epithelia, enters draining lymphatic vessels in underlying laminae propriae, and is transported to LRS tissues. Since DY TME causes disease once it enters the peripheral nervous system, the restriction in DY TME pathogenesis is due to its inability to establish infection in LRS tissues, not a failure of transport. To determine if LRS tissues can support DY TME formation, we performed protein misfolding cyclic amplification using DY PrPSc as the seed and spleen homogenate as the source of PrPC. We found that the spleen environment can support DY PrPSc formation, although at lower rates compared to lymphotropic strains, suggesting that the failure of DY TME to establish infection in the spleen is not due to the absence of a strain-specific conversion cofactor. Finally, we provide evidence that DY PrPSc is more susceptible to degradation when compared to PrPSc from other lymphotrophic strains. We hypothesize that the relative rates of PrPSc formation and clearance can influence prion tropism.