Metabolism of minor isoforms of prion proteins: Cytosolic prion protein and transmembrane prion protein

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

Transgenic Overexpression of the Disordered Prion Protein N1 Fragment in Mice Does Not Protect Against Neurodegenerative Diseases Due to Impaired ER Translocation

The structurally disordered N-terminal half of the prion protein (PrP^C) is constitutively released into the extracellular space by an endogenous proteolytic cleavage event. Once liberated, this N1 fragment acts neuroprotective in ischemic conditions and interferes with toxic peptides associated with neurodegenerative diseases, such as amyloid-beta (Aβ) in Alzheimer’s disease. Since analog protective effects of N1 in prion diseases, such as Creutzfeldt-Jakob disease, have not been studied, and given that the protease releasing N1 has not been identified to date, we have generated and characterized transgenic mice overexpressing N1 (TgN1). Upon intracerebral inoculation of TgN1 mice with prions, no protective effects were observed at the levels of survival, clinical course, neuropathological, or molecular assessment. Likewise, primary neurons of these mice did not show protection against Aβ toxicity. Our biochemical and morphological analyses revealed that this lack of protective effects is seemingly due to an impaired ER translocation of the disordered N1 resulting in its cytosolic retention with an uncleaved signal peptide. Thus, TgN1 mice represent the first animal model to prove the inefficient ER translocation of intrinsically disordered domains (IDD). In contrast to earlier studies, our data challenge roles of cytoplasmic N1 as a cell penetrating peptide or as a potent “anti-prion” agent. Lastly, our study highlights both the importance of structured domains in the nascent chain for proteins to be translocated and aspects to be considered when devising novel N1-based therapeutic approaches against neurodegenerative diseases.

Prion protein is essential for the RE1 silencing transcription factor (REST)-dependent developmental switch in synaptic NMDA receptors

It is important that the correct amounts of GluN2 subunits are maintained, as they determine NMDAR functional properties, which are crucial to neuronal communication, synaptogenesis and cognitive function. The transcriptional repressor RE1 silencing transcription factor (REST) is critical for the postnatal developmental switch in NMDARs. However, the mechanisms triggering REST and the link between NMDARs and REST are unclear. Here we show a new physiological essential role for cellular prion protein (PrPC) in REST-dependent homeostasis and the developmental switch of NMDARs. REST and REST-associated proteins were overactivated in the hippocampi of Prnp knockout mice (Prnp 0/0 ) compared with wild-type Prnp (Prnp +/+ ) mice. This coincided with the disruption of the normal developmental switch from GluN2B-to-GluN2A in vivo. PrPC co-located with REST under physiological environments and mediated the translocation of REST in conditioners of NMDARs in vitro in Prnp +/+ hippocampal neurons. Regardless of whether REST was knocked down or overexpressed, deletion of PrPC not only disrupted REST-mediated distribution of mitochondria, but also prevented REST-regulated expression of GluN2B and GluN2A in Prnp 0/0 . Importantly, these effects were rescued after overexpression of full-length PrPC through restoration of NMDAR2 subunits and their distributions in dendritic processes in Prnp 0/0 . Consistently, knockdown of PrPC in Prnp +/+ had a similar effect on Prnp 0/0 . Furthermore, PrPC colocalized with both GluN2B and GluN2A in Prnp +/+ . For the first time, we demonstrate that PrPC is essential for REST-regulated NMDARs. Confirming the regulation of NMDAR-modulating mechanisms could provide novel therapeutic targets against dysfunctions of glutamatergic transmission in the nervous system.

The Unexposed Secrets of Prion Protein Oligomers

According to the "protein-only" hypothesis, the misfolding and conversion of host-derived cellular prion protein (PrP(C)) into pathogenically misfolded PrP are believed to be the key procedure in the pathogenesis of prion diseases. Intermediate, soluble oligomeric prion protein (PrP) aggregates were considered a critical process for prion diseases. Several independent studies on PrP oligomers gained insights into oligomers' formation, biophysical and biochemical characteristics, structure conversion, and neurotoxicity. PrP oligomers are rich in β-sheet structure and slightly resistant to proteinase K digestion. PrP oligomers exhibited more neurotoxicity and induced neuronal apoptosis in vivo and/or in vitro. In this review, we summarized recent studies regarding PrP oligomers and the relationship between misfolded PrP aggregates and neuronal death in the course of prion diseases.