Functions of the prion protein in neurodegeneration: a closer look to tau - PrP interaction

Tau protein is involved in a number of distinct neurodegenerative disorders called tauopathies, which include one of the major leading causes of death in the world, Alzheimer’s disease. Over the past years, many in vitro and in vivo studies helped elucidating the main events leading to the spreading of the pathology and brain degeneration; however, the detailed molecular mechanisms underlying each step are yet to be discovered. Intracellular tau aggregates diffuse in the brain following anatomic connections, and their spreading is the result of the combination of extracellular release, uptake and seeding of the endogenous protein in receiving cells. Many molecular partners mediating tau internalization have been identified, but many others are still elusive. In particular, it would be worth investigating if different neurodegeneration-related amyloids (i.e. tau, -synuclein, amyloid-) might share a common pathway mediating their toxicity and cell-to-cell spreading. In this PhD thesis, we analysed the contribution of the cellular prion protein (PrPC) to the internalization of tau fibrils. We took advantage of the methodology extensively described in the literature to produce synthetic amyloids made of the truncated form of tau protein named K18, consisting only of the microtubule-binding domains of the protein. We compared the uptake of K18 fibrils in two mouse neuroblastoma cell lines: wild-type N2a cells and N2a cells that had been knocked-out for PrPC (N2a KO). Our results indicate that the number of internalized amyloids is higher in N2a cells expressing PrPC compared to their knocked-out counterpart, suggesting that the prion protein might be one of the many uptake mechanisms. Indeed, the absence of PrPC does not block completely the entrance of tau fibrils in the cytoplasm. This phenomenon is mediated by the binding between tau fibrils and the prion protein, and configures itself as a mutual interaction: while PrPC promotes a higher internalization of the amyloids, the presence of tau fibrils leads to an increase in the total levels of PrPC, which accumulates on the cell membrane. Although it is not yet clear if this effect is be tau-driven or a simple side-effect, it might be responsible for a more sustained internalization rate. Subsequently, we used monoclonal antibodies directed against the different domains of PrPC in order to evaluate their role in the uptake process. What we found is that both the targeting of the N-terminus and of the hydrophobic region negatively affect the internalization of tau K18 amyloids, while the globular domain does not seem to play a role, at least not in the specific region we considered. As the prion protein exists also in the pathological conformation PrPSc, we analysed its interaction with K18 fibrils using prion-infected neuroblastoma cell lines (ScN2a). The exposition of ScN2a cells to K18 amyloids resulted in a strong decrease of the PK-resistant PrP levels, independently of the type of replicating strain (RML or 22L prion strains). We hypothesize that K18 fibrils might bind either to PrPSc or, more probably, to PrPC and thus prevent prion replication. Indeed, neither macroautophagy nor lysosomes were found to be overactivated as a consequence of the treatment. In conclusion, our findings point towards a role for PrPC in the regulation of the cell-to-cell transfer of tau amyloids. Additionally, this study focuses on the molecular mechanisms of the relationship between tau aggregates and the pathological PrPSc. The co-presence of tau and prion pathologies has a clinical relevance, as it is a common feature of a subgroup of prion disorders characterised by a longer disease course.