Increasing evidence suggests that neurodegenerative disorders share a common pathogenic feature: the presence of deposits of misfolded proteins with altered physicochemical properties in the Central Nervous System. Despite a lack of infectivity, experimental data show that the replication and propagation of neurodegenerative disease-related proteins including amyloid-β (Aβ), tau, α-synuclein and the transactive response DNA-binding protein of 43 kDa (TDP-43) share a similar pathological mechanism with prions. These observations have led to the terminology of “prion-like” to distinguish between conditions with noninfectious characteristics but similarities with the prion replication and propagation process. Prions are considered to adapt their conformation to changes in the context of the environment of replication. This process is known as either prion selection or adaptation, where a distinct conformer present in the initial prion population with higher propensity to propagate in the new environment is able to prevail over the others during the replication process. In the last years, many studies have shown that prion-like proteins share not only the prion replication paradigm but also the specific ability to aggregate in different conformations, i.e., strains, with relevant clinical, diagnostic and therapeutic implications. This review focuses on the molecular basis of the strain phenomenon in prion and prion-like proteins.

Cytoplasmic aggregation of the primarily nuclear TAR DNA-binding protein 43 (TDP-43) affects neurons in most amyotrophic lateral sclerosis (ALS) and approximately half of frontotemporal lobar degeneration (FTLD) cases. The cellular prion protein, PrP^C, has been recognized as a common receptor and downstream effector of circulating neurotoxic species of several proteins involved in neurodegeneration. Here, capitalizing on our recently adapted TDP-43 real time quaking induced reaction, we set reproducible protocols to obtain standardized preparations of recombinant TDP-43 fibrils. We then exploited two different cellular systems (human SH-SY5Y and mouse N2a neuroblastoma cells) engineered to express low or high PrP^C levels to investigate the link between PrP^C expression on the cell surface and the internalization of TDP-43 fibrils. Fibril uptake was increased in cells overexpressing either human or mouse prion protein. Increased internalization was associated with detrimental consequences in all PrP-overexpressing cell lines but was milder in cells expressing the human form of the prion protein. As described for other amyloids, treatment with TDP-43 fibrils induced a reduction in the accumulation of the misfolded form of PrP^C, PrP^Sc, in cells chronically infected with prions. Our results expand the list of misfolded proteins whose uptake and detrimental effects are mediated by PrP^C, which encompass almost all pathological amyloids involved in neurodegeneration.

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Our objective was to assess the incidence and trends of amyotrophic lateral sclerosis (ALS) in Liguria, a north-west region of Italy, utilizing a prospective design. Liguria (1,615,064 residents in 2010) is the site of a multicentre-multisource prospective population based registry called LIGALS (Liguria Amyotrophic Lateral Sclerosis Registry). All incident ALS cases during the period 2009–2010 were enrolled and followed up. Cases were identified using several concurrent sources. ALS diagnosis was based on the revised El Escorial criteria. One hundred and four cases were enrolled, generating an annual crude incidence of 3.22/100,000 (95% CI 2.66−3.90), with a male/female ratio of 1.34. The annual standardized incidence, age and gender adjusted to the 2001 Italian population, was 2.51. At last observation on 1 March 2012, 45% of patients registered in the LIGALS had died, with a median survival of 45 months from symptoms onset. According to capture-recapture estimation, three patients were unobserved. For both genders, demographic and clinical features were collected. In conclusion, comparing these data to those of epidemiological studies with a similar prospective design, the occurrence of ALS is similar. The observed crude incidence was higher compared to other Italian studies, due in part to a very careful case ascertainment and in part to a high percentage of the elderly in Liguria.