Investigation of prion protein turnover in neuronal activity-dependent processes

Abstract

The cellular prion protein (PrPC) is highly expressed in the brain and neurons. It is well known for its role in transmissible spongiform encephalopathies and is suggested to be involved in the pathophysiology of other neurodegenerative diseases including Alzheimer’s disease (AD). Although PrPC structure, misfolding, and mechanisms of pathogenesis have been extensively studied, its physiological functions on a cellular level largely remain elusive. In this thesis, the role of PrPC at the synapse and in neuronal activity processes was investigated. Initially, PrPC localization to different types of synapses was analyzed in cultured murine hippocampal neurons. The analysis revealed that PrPC localized at both excitatory and inhibitory synapses. Following the observation that PrPC is enriched at the plasma membrane of excitatory synapses, the localization of PrPC to neuronal subdomains and its vesicle trafficking in response to different chemical stimulation protocols were analyzed. A moderate effect of chemically induced long-term depression (cLTD) could be observed on the active transport of PrPC and its concentration at synaptic regions and endolysosomal compartments., Further, PrPC active transport was investigated following the overexpression of human tau, because of its potentially synaptotoxic role in AD, but no effects were observed. Since PrPC is abundant on exosomes, the impact of neuronal activity on exosome release was investigated in this project. However, no significant changes could be observed under the selected conditions. With respect to a potential physiological role of PrPC at synapses, the synaptic cell adhesion molecules Neuroligin-2 and N-cadherin were identified as novel PrPC interaction partners in co-immunoprecipitation experiments from mouse brains. Furthermore, neuronal live imaging revealed high portions of PrPC and N-Cadherin to undergo cotransport within the same vesicles. To investigate a potential effect of PrPC on N-cadherin targeting to endolysosomal compartments, PrPC was overexpressed in dissociated hippocampal neurons and N-cadherin localization was investigated by immunocytochemistry. Functionally, PrPC facilitated the targeting of N-Cadherin to endolysosomes. Together the data suggest that PrPC synaptic localization and vesicle trafficking are modified during synaptic plasticity and that PrPC might assist in the removal of synaptic cell adhesion proteins such as N-cadherin and in their targeting toward endolysosomal compartments. The previously discussed role for PrPC in cellular adhesion is further supported through the discovery of two cell adhesion molecules as novel PrPC interaction partners in this work.