Prion Pathway
Prion diseases or TSEs (Transmissible Spongiform Encephalopathies) form a biologically unique
group of infectious fatal neurodegenerative disorders, which are caused by toxic gain of function
in a normal host cell protein (the Prion protein, PrP). The mechanism of disease propagation is
well understood and involves the conformational conversion of a normal cell-surface protein
(PrPc) into a protease-resistant, Beta-sheet-rich form (PrPSc) that is infectious in the absence of
nucleic acid. BSE (Bovine Spongiform Encephalopathy), Scrapie of sheep and goat, CWD
(Chronic Wasting Disease) of deer and elk, and human CJD (Creutzfeldt Jakob Disease) are
well-known central nervous system degenerative diseases caused by Prion infection. However,
some forms of spongiform encephalopathies are commonly associated with inherited mutations
at the PrPc coding gene (PrnP), particularly the GSS (Gerstmann-Straussler-Scheinker
syndrome) and FFI (Fatal Familial Insomnia), in addition to CJD in humans. Prion diseases have
attracted a broad interest because of their unique mechanisms of replication and propagation;
however, the underlying pathogenic mechanisms are still highly speculative. Historically, Prion
diseases have been characterized neuropathologically by neuronal vacuolation (spongiosis), brisk
reactive proliferation of astrocytes and microglia, and by the deposition of amyloid plaques.
Prion diseases are manifest as infectious, genetic, and sporadic disorders (Ref.1).
PrP is a host-encoded protein which exists as PrPc (cellular) in the non-infected host and as
PrPSc in diseased tissue. The normal Prion protein, PrPc, is encoded by the prion gene (PrnP) on
human chromosome 20, with equivalent prion genes in animals. The PrPc molecule contains an
N-terminal signal peptide, four or five octapeptide repeats, a highly conserved hydrophobic
domain, and a C-terminal signal sequence for the addition of a GPI (Glycosyl
Phosphatidylinositol) anchor. The diseased form, PrPSc, has the same primary structure as PrPc
(with the same disulphide bridge and N-glycosylation sites), but very different tertiary structure.
Whereas PrPc is predominately made up of Alpha helix, PrPSc is made up of 40% Beta sheet,
with only 30% of Alpha helix. The high proportion of Beta sheet in PrPSc renders it insoluble
and markedly resistant to proteases (Ref.2).
PrPc is synthesized in the RER (Rough Endoplasmic Reticulum), and transits the Golgi on its
way to the cell surface. Like most cell-surface proteins, PrP enters the ER concurrent with its
synthesis by membrane-bound ribosomes, such that most regions of nascent PrP cannot normally
access the cytoplasm. During biosynthesis, PrPc is subjected to several posttranslational
modifications and chaperone-assisted folding events, before further trafficking along the
secretory pathway to the cell surface. Post-translational modifications include the addition of N-
linked oligosaccharide chains, formation of disulphide bond, signal peptide cleavage and
attachment of the GPI anchor. However, minor populations of PrP could have access to the
cytoplasm by following alternative routes that might include reverse translocation of improperly
matured PrP out of the Endoplasmic Reticulum, aborted translocation into the Endoplasmic
Reticulum or generation of transmembrane forms of PrP. Misfolded PrP is retrotranslocated from
the Endoplasmic Reticulum to the cytosol, where it is ubiquitinated and targeted to the
proteasome in a process called ERAD (ER-Associated Degradation). Misfolded PrP might be
pulled into the proteasome but could get stuck, causing proteasome dysfunction. Inhibition of
proteasome function might cause further accumulation of proteins in the endoplasmic reticulum
as well as in the cytosol. Although the role of ERAD in the generation of cytosolic PrP remains
debatable, cytosolic PrP seems to be toxic. Under normal conditions, the residence time of PrP in
the cytoplasm would be extremely short, owing to its rapid degradation by the proteasome.
However, if PrP is allowed to remain in the cytoplasm for a significant length of time, it is
capable of inducing cell death in neurons, aggregating with itself (and perhaps other proteins)
and potentially misfolding into a self-propagating form that can resemble the transmissible form
of PrP, PrPSc. Misfolded proteins in the Endoplasmic Reticulum may also trigger an
Endoplasmic Reticulum stress response that is initiated when chaperones, normally associated
with Endoplasmic Reticulum transmembrane protein kinases, associate with misfolded proteins.
This allows the transmembrane kinases to dimerize and send signals to the nucleus and can result
in the upregulation of genes promoting cell death, such as those encoding CHOP/GADD153
(Ref.3).
After undergoing the posttranslational modifications, PrPc moves from the endoplasmic
reticulum to the cell surface, after transiting the Golgi apparatus. Localization of PrPc on the cell
membrane makes it a potential candidate for a ligand uptake, cell adhesion and recognition
molecule or a membrane signaling molecule. PrPc cycles between the cell surface and an early
endocytic compartment, via an association with Clathrin-coated pits but also can migrate to late
endosomes or lysosomes via non-classic, specialized DRM (Detergent-Resistant Microdomains)
or caveolae-containing endocytic structures. However, both pathways seem to require a third
protein (a receptor or a prion-binding protein) either to make the connection between the GPI-
anchored molecule to Clathrin or to convert PrPc into PrPSc. 37-kDa/67-kDa LRP/LR (Laminin
receptor), which is highly conserved among mammals and is located on the cell surface, may act
as a receptor or co-receptor for the prion protein on mammalian cells (Ref.4).
The Prion infection mechanism is trigged by interaction of PrPSc with cellular prion protein PrPc
causing conversion of the latters conformation. Therefore, the infection spreads because new
PrPSc molecules are generated exponentially from the normal PrPc. The site of PrPc to PrPSc
conversion is uncertain. DRM and the endosomal pathway are possible sites for transformation.
The endoplasmic reticulum may participate too, especially in familial TSE. PrPc to PrPSc
conversion is mediated by chaperones. Molecular chaperones HSP104 (Heat Shock Protein-104)
and GroEL have been shown to promote the conversion reaction of mammalian PrPc in a cell-
free system and the conversion of prion-like proteins in intact yeast cells. Several chemical
chaperones have been shown to act as conversion inhibitors. The accumulation of insoluble
PrPSc is probably one of the events that lead to neuronal death (Ref.3).
PrPc may also bind to an extracellular ligand (possibly Copper) before being cycled from the
membrane into endocytic vesicles. PrPc deliver copper ions to an endocytic compartment within
which the bound ions dissociate from PrPc and are transferred to other copper-carrier proteins
that move the ions into the cytosol. PrPc would then return to the cell surface to begin another
cycle. PrPc internalization by copper may hinder PrPSc interaction with this molecule, and
thereby affect prion disease propagation. Mutations in the PrPc protein related to prion diseases
can alter its subcellular trafficking. PrPc has been implicated in protection from oxidative insults,
apoptosis, cellular signaling, membrane excitability and synaptic transmission, neuritogenesis
and copper (II) transport or metabolism, but how all these functions are achieved by the same
protein is still unknown (Ref.5).
