Octarepeat region flexibility impacts prion function, endoproteolysis and disease manifestation.

The cellular prion protein (PrP(C)) comprises a natively unstructured N-terminal domain, including a metal-binding octarepeat region (OR) and a linker, followed by a C-terminal domain that misfolds to form PrP(S) (c) in Creutzfeldt-Jakob disease. PrP(C) ß-endoproteolysis to the C2 fragment allows PrP(S) (c) formation, while α-endoproteolysis blocks production. To examine the OR, we used structure-directed design to make novel alleles, 'S1' and 'S3', locking this region in extended or compact conformations, respectively. S1 and S3 PrP resembled WT PrP in supporting peripheral nerve myelination. Prion-infected S1 and S3 transgenic mice both accumulated similar low levels of PrP(S) (c) and infectious prion particles, but differed in their clinical presentation. Unexpectedly, S3 PrP overproduced C2 fragment in the brain by a mechanism distinct from metal-catalysed hydrolysis reported previously. OR flexibility is concluded to impact diverse biological endpoints; it is a salient variable in infectious disease paradigms and modulates how the levels of PrP(S) (c) and infectivity can either uncouple or engage to drive the onset of clinical disease.

Prion disease tempo determined by host-dependent substrate reduction.

The symptoms of prion infection can take years or decades to manifest following the initial exposure. Molecular markers of prion disease include accumulation of the misfolded prion protein (PrPSc), which is derived from its cellular precursor (PrPC), as well as downregulation of the PrP-like Shadoo (Sho) glycoprotein. Given the overlapping cellular environments for PrPC and Sho, we inferred that PrPC levels might also be altered as part of a host response during prion infection. Using rodent models, we found that, in addition to changes in PrPC glycosylation and proteolytic processing, net reductions in PrPC occur in a wide range of prion diseases, including sheep scrapie, human Creutzfeldt-Jakob disease, and cervid chronic wasting disease. The reduction in PrPC results in decreased prion replication, as measured by the protein misfolding cyclic amplification technique for generating PrPSc in vitro. While PrPC downregulation is not discernible in animals with unusually short incubation periods and high PrPC expression, slowly evolving prion infections exhibit downregulation of the PrPC substrate required for new PrPSc synthesis and as a receptor for pathogenic signaling. Our data reveal PrPC downregulation as a previously unappreciated element of disease pathogenesis that defines the extensive, presymptomatic period for many prion strains.

Forced exercise does not improve recovery after hemorrhagic stroke in rats.

Exercise can improve recovery following ischemia and intracerebral hemorrhage (ICH) in rodents. We tested whether forced exercise (EX; running wheel) prior to and/or following ICH in rats would reduce lesion volume and improve functional outcome (walking, skilled reaching, spontaneous paw usage) at 7 weeks post-ICH. A striatal hemorrhage was produced by infusing collagenase. First, we compared animals that received EX (2 weeks; 1 h/day) ending two days prior to ICH and/or starting two weeks following ICH. EX did not improve functional recovery or affect lesion size. Doubling the amount of EX given per day (two 1-h sessions) both prior to and following ICH did not alter lesion volume, but worsened recovery. We then determined if EX (1 h/day) prior to and following ICH would affect outcome after a somewhat milder insult. There were no differences between the groups in lesion volume or recovery. Finally, we used a hemoglobin assay at 12 h following ICH to determine if pre-stroke EX (2 weeks; 1 h/day) aggravated bleeding. It did not. These observations suggest that EX does not improve outcome when given prior to and/or when delayed following ICH. Effective rehabilitation for ICH will likely require more complex interventions than forced running.