Propagation of RML Prions in Mice Expressing PrP Devoid of GPI Anchor Leads to Formation of a Novel, Stable Prion Strain

PrPC, a host protein which in prion-infected animals is converted to PrPSc, is linked to the cell membrane by a GPI anchor. Mice expressing PrPC without GPI anchor (tgGPI- mice), are susceptible to prion infection but accumulate anchorless PrPSc extra-, rather than intracellularly. We investigated whether tgGPI− mice could faithfully propagate prion strains despite the deviant structure and location of anchorless PrPSc. We found that RML and ME7, but not 22L prions propagated in tgGPI− brain developed novel cell tropisms, as determined by the Cell Panel Assay (CPA). Surprisingly, the levels of proteinase K-resistant PrPSc (PrPres) in RML- or ME7-infected tgGPI− brain were 25–50 times higher than in wild-type brain. When returned to wild-type brain, ME7 prions recovered their original properties, however RML prions had given rise to a novel prion strain, designated SFL, which remained unchanged even after three passages in wild-type mice. Because both RML PrPSc and SFL PrPSc are stably propagated in wild-type mice we propose that the two conformations are separated by a high activation energy barrier which is abrogated in tgGPI− mice.

PrPC（细胞型朊蛋白）是一种宿主蛋白，在感染朊病毒的动物体内可转化为PrPSc（羊瘙痒病型朊蛋白），并通过糖基磷脂酰肌醇锚（GPI anchor）锚定于细胞膜表面。表达缺失GPI锚的PrPC的小鼠（tgGPI⁻小鼠）对朊病毒感染具有易感性，但会在细胞外而非细胞内积累无锚定的PrPSc。本研究探究了尽管无锚定PrPSc的结构与定位存在异常，tgGPI⁻小鼠是否仍能忠实传播朊病毒毒株。研究发现，在tgGPI⁻小鼠脑组织中增殖的RML、ME7毒株（而非22L毒株）经细胞面板检测（Cell Panel Assay, CPA）验证后，出现了全新的细胞嗜性。令人意外的是，感染RML或ME7毒株的tgGPI⁻小鼠脑组织中，蛋白酶K抗性PrPSc（PrPres）的水平较野生型小鼠脑组织高出25至50倍。当将这些毒株回迁至野生型小鼠脑组织中时，ME7毒株恢复了其原始特性；然而RML毒株已衍生出一种新型朊病毒毒株，命名为SFL，即便在野生型小鼠中连续传代三次后仍保持稳定。由于RML PrPSc与SFL PrPSc均可在野生型小鼠体内稳定增殖，我们提出，两种构象之间存在一道高活化能障，而该障碍在tgGPI⁻小鼠中被消除。