Mechanisms of Prion Aggregation

Prion diseases are relentlessly progressive neurodegenerative disorders with death often within six months of the onset of neurologic symptoms. Pathologic features include widespread extracellular prion aggregates, spongiform degeneration, synaptic and neuronal loss, and severe astrogliosis and microgliosis. The structural determinants of the prion protein (PrP) and endogenous co-factors that drive aggregation, govern prion assembly, and impact aggregate spread through the central nervous system are unclear. A major goal of this application is to define when and how the endogenous co-factor, heparan sulfate (HS), promotes fibril assembly in the parenchyma and blood vessels and slows PrP clearance through the interstitial fluid using in vitro and in vivo model systems. We have previously pursued a range of approaches using cell-based prion conversion assays and newly generated transgenic and knock-in mouse models in collaboration with structural biologists to define the mechanisms that underlie PrP self-assembly and species barriers to prion conversion. We discovered using knock-in mouse models that N-linked glycans on PrP reduce spongiform degeneration, hinder plaque formation, and repel HS binding. Further, we found that plaque-forming prions were composed of poorly glycosylated, GPI-anchorless PrP bound to highly sulfated HS, underscoring the pivotal role of PrP post- translational modifications in driving the aggregate conformation and disease phenotype. We also found that reducing HS chain length decreases parenchymal plaque formation and prolongs survival. Finally, we identified highly amyloidogenic segments in the PrP sequence that control cross species prion conversion, as the number and location of glutamine and asparagine residues in PrP raise or lower the prion transmission barrier. In this renewal, we aim to determine the PrP-HS interactions that promote prion aggregate assembly and accelerate disease. We build on our long-standing observation that structural features of PrP, together with host glycosaminoglycans, drive efficient prion conversion. First, we will genetically manipulate neuronal, astrocytic, and endothelial HS chains and determine the impact on prion cell targets and survival using mouse models. Second, we will define how endogenous HS regulates PrP clearance through the interstitial fluid using conditional HS mouse models and radiolabeled PrP. Third, we will test the efficacy of antisense oligonucleotides (ASOs) targeting HS biosynthetic enzymes or Prnp mRNA in the early and mid-stages of prion plaque development in a prion disease model. We expect these mechanistic studies will (i) define how an endogenous co-factor, HS, accelerates and modifies prion disease, and (ii) determine whether reducing PrP interactions with this potential therapeutic target blocks prion spread.