Prions, those self-replicating proteins that are responsible for scrapie, mad cow disease, and human neurologic diseases such as Creutzfeldt-Jakob and Kuru, have been notoriously difficult to pin down in the cell. Now, an international collaboration led by Peter Peters at the Netherlands Cancer Institute, Amsterdam, and Stanley Prusiner at the University of California, San Francisco, reports perhaps the most detailed localization study to date. Importantly, their data, which appeared in the August 6 Journal of Neuroscience, show that a pool of prions exists in the neuronal cytosol, supporting a recent hypothesis that minute amounts of misfolded, cytosolic prion may be sufficient to cripple neurons.

Normal, or cellular prion protein (PrPC), is an innocuous cell-surface molecule expressed throughout the brain. A Jekyll and Hyde-like conformational change in the protein turns it into a lethal infectious agent (PrPSc) that can self-propagate by catalyzing the conversion of other normal prions. How this protein kills neurons is unclear. However, Jiyan Ma and Susan Lindquist recently revealed that a third form of the prion may arise through occasional misfolding during synthesis. This group proposed that this misfolded protein is ejected from the endoplasmic reticulum into the cytosol, where the proteasome destroys it. But if the proteasome is overwhelmed, then this cytosolic prion (cytPrP) could wreak havoc on the cell, most likely by forcing it down the road to apoptosis and death (see ARF related news story, but also Roucou et al., 2003).

The latest findings from Peters et al. lend credence to this hypothesis. First author Alexander Mironov and colleagues used light and electron microscopic analysis of semi-thin and ultra-thin tissue sections from mouse brain to show that PrPC appears in expected locations, including the cell membrane, axons, pre- and postsynaptic boutons, and also in organelles involved in the synthesis of membrane-bound proteins, such as the endoplasmic reticulum and the Golgi complex. But Mironov also found that in a subset of CA1 hippocampal cells, the bulk of the prion occurred in the cytosol. Though the identity of these cells is unclear, they are most likely neurons, claim the authors, because they do not stain positive for glia or oligodendrocyte markers.

Many pieces of the prion puzzle remain. Though the cytPrP cells identified by Mironov have irregularly shaped nuclei and abundant dense cytoplasm, in all other respects they seem normal, with no signs of degeneration or activation of apoptosis. This contrasts the finding of Ma and colleagues, who found that even a minor amount of cytPrP can be lethal to cells. The answer to this conundrum may lie in specific cell types. Cerebellar cells appeared most affected when Ma et al. targeted PrP to the cytosol, but Mironov could not find cytPrP in that area of the brain. The authors conclude that “cytPrP is not toxic in some neurons but highly toxic when overexpressed in specific cell populations.” This suggestion is reminiscent of the pattern of destruction in other neurodegenerative diseases, where misfolded proteins are thought to play a major role. In Alzheimer’s and Parkinson’s, to name just two, neuronal destruction begins in specific cells in specific regions of the brain. Elucidating the nature of prion toxicity may, therefore, shed some light on other neurodegenerative diseases.—Tom Fagan

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  1. Mironov et al. utilize immuno-EM to localize PrPc within neurons and their processes in brain. Especially interestingly, they describe a subpopulation of neurons with markedly elevated PrPc labeling. More speculative is their interpretation that, in these neurons, increased PrPc localizes to the “cytosol.” Given the resolution limitations of their EM, it seems difficult to exclude the possibility that this PrPc resides on or within smaller endosomal or tubulovesicular organelles.

    This paper underscores the importance of immuno-EM in determining the subcellular localization of disease-linked peptides, and provides further parallels between prion diseases and AD, two neurodegenerative diseases associated with dementia, neuronal loss, and cerebral plaques. The authors indicate that the elevated PrPc levels are not associated with morphological abnormalities or markers of apoptosis, also pointing out that EM is the gold standard for determination of apoptosis.

    In the AD field, our lab has demonstrated by immuno-EM that Aβ accumulates within neurons and their processes/synapses with β-amyloidosis and is associated with morphological alterations. It will be interesting for the authors to follow up their EM studies on PrP to define the ultrastructural site(s) of PrP accumulation with the onset of prion pathology. Many questions remain, and they are remarkably similar to questions now being asked in AD. For example, do the neurons with elevated PrP further accumulate PrP along with the disease process? If so, could this intracellular pool of PrP play a direct role in initiating pathology, and by what mechanism does extracellular PrP influence intracellular PrP and vice versa?

References

News Citations

  1. Shape-Shifting Prion Protein in Cytosol: Highly Toxic Yet Almost Invisible

Paper Citations

  1. . Neurotoxicity and neurodegeneration when PrP accumulates in the cytosol. Science. 2002 Nov 29;298(5599):1781-5. PubMed.
  2. . Cytosolic prion protein is not toxic and protects against Bax-mediated cell death in human primary neurons. J Biol Chem. 2003 Oct 17;278(42):40877-81. PubMed.

Further Reading

No Available Further Reading

Primary Papers

  1. . Cytosolic prion protein in neurons. J Neurosci. 2003 Aug 6;23(18):7183-93. PubMed.