Several mechanisms or a combination of different processes are proposed to be responsible for neurodegeneration in prion diseases, (i) the loss of PrPc function, (ii) toxic intermediates and/or side effects involved in/resulting from conversion of PrPc, and (iii) the toxic gain of function (Figure 8).
It is suggested that the normal neuroprotective function of PrPc is lost (Figure 8) during conversion or interaction with PrPSc and is involved in the mechanism of neurodegeneration.
Overexpression of PrP was demonstrated to rescue cells from death (Kuwahara et al., 1999) and to protect cells from oxidative stress (Brown et al., 2002). Due to the suggested action as an effective anti-apoptotic protein, the loss of function might result in a reduction of the anti-apoptotic capacity of PrPc contributing to the pathogenic mechanism (Kim et al., 2004). In addition, the prion protein is
necessary for normal synaptic transmission, a loss of function may contribute also to the early synaptic loss and neuronal degeneration (Collinge et al., 1994).
The Cu2+ binding ability (Brown et al., 1997a) plays a critical role regarding the function of PrPc. Copper binding to the octarepeat region of PrP has been suggested to inhibit prion pathogenesis (Harris and True, 2006; Hijazi et al., 2003). In addition the in vitro conversion of the prion protein into amyloid fibrils was inhibited through Cu2+ binding indicating that the copper mediated
physiological PrP function contributes to the neuropathogenesis of prion disease (Bocharova et al., 2005). The role of copper in prion disease pathology is not entirely clear, since it was also shown that copper ions stabilize the disease-specific isoform against proteolytic clearance and enhanced the resistance of amyloid (Kuczius et al., 2004). In addition, PrPc has been implicated in higher resistance of cells to oxidative stress or copper toxicity (Brown et al., 1997b), suggesting that the loss of PrPc correlates with the lack of prevention of copper toxicity due to an impaired Cu2+ metabolism. The octarepeat region has been reported to play a role in generation of the proteinase K resistant fragment PrPSc since cells expressing PrPc devoid of this region did not produce PrPSc
(Sakudo et al., 2008). Transgenic mice expressing an N-terminally truncated form of PrP spontaneously develop a neurodegenerative phenotype (Li et al., 2007b). Another study reported that transgenic mice expressing the mouse homolog of a mutant human PrP which contained a nine octapeptide insertion, display a prion disease accompanied by neurological disorders (Chiesa et al., 1998).
An additional mechanism contributing to the neurodegenerative phenotype represents the toxic gain of function. This implicates that after conversion of the prion protein to the abnormal form the prion
to perturbation of the normal physiological activity. The central region of the prion protein stretching from 105-125 (in humans 106-126) is suggested to be implicated in neurotoxicity. Mice carrying deletions in this region spontaneously developed a neuropathological phenotype (Shmerling et al., 1998) and neonatal lethality (Li et al., 2007a). The corresponding synthetic peptide PrP 106-126 shares many characteristics with PrPSc and is largely used to elucidate the toxic
mechanisms underlying prion diseases. PrP 106-126 reveals a high tendency to aggregate into fibrils and lead to neuronal death of primary rat hippocampal cultures (Forloni et al., 1993). Furthermore, the peptide has been demonstrated to induce oxidative stress by free radicals (Brown, 2005) and trigger endoplasmic reticulum stress-induced apoptotic cell death (Ferreiro et al., 2008). However, small amounts of detergent-insoluble proteinase K resistant PrP aggregates were also found in healthy human brains suggesting that this PrP form is not necessarily neurotoxic (Yuan et al., 2006). Transgenic mice expressing PrP lacking the C-terminal GPI anchor displayed abnormal protease- resistant PrPres deposition in amyloid plaques but only minimal scrapie neuropathology proposing
that membrane attachment of PrPc is essential for transducing PrPSc derived neurotoxic signal
(Chesebro et al., 2005) (Figure 8). A contradictory study demonstrated that the cleavage of the GPI anchor did not alter infectivity in a mouse bioassay (Lewis et al., 2006a).
Figure 8. Possible relationship between PrPc and PrPSc interactions on the cell surface. Neuronal loss is
suggested to result from toxic signal delivered by PrPSc (toxic gain of function) (A), PrPc looses
neuroprotective activity upon conversion top PrPSc (loss of function) (B), or subversion of PrPc by PrPSc
generating a toxic signal of the original protective one (C). Data from GPI anchorless mice suggest that PrPc
lacking the GPI anchor delivers no signal and is therefore not toxic (D). (modified from (Harris and True, 2006))
Another proposal for the pathogenic mechanism involves subversion of the normal
neuroprotective functions of PrPc (Figure 8). PrPc is converted by the action of PrPSc into a transducer of neurotoxic signals (Harris and True, 2006). This might probably result from PrPSc
cross-linking of PrPc on the cell surface by antibodies results in apoptotic death in CNS neurons
(Solforosi et al., 2004). Another possibility would be that PrPSc bind and block specific regions on PrPc, which are required for the delivery of a neuroprotective signal or just alter the normal signaling properties (Westergard et al., 2007).
Astrocytosis as well as microglial activation are observed features in prion diseases, since these cells are for example increased in the brain of CJD patients (Van Everbroeck et al., 2002). Microglia are associated with amyloid plaques and supposed to contribute to the development of spongiform vacuoles (Rezaie and Lantos, 2001). In addition, in scrapie infected mice microglial activation has been demonstrated to be involved in the neurotoxicity of PrPSc (Giese et al., 1998). Neuronal cell
death in the brain is associated with chronic inflammatory response dominated by microglia, which is suggested to contribute to the spread of infection for the whole symptomatic period of the disease (Szpak et al., 2006). Through microglial activation, which has been found to be upregulated in CJD or GSS, inflammatory proteins such as prostaglandin E are released and elevated in the cerebrospinal fluid (CSF) of CJD patients (Minghetti and Pocchiari, 2007). Astrocytes have also been reported to participate in the formation of amyloid plaques (Liberski and Brown, 2004). Neuronal loss is suggested to occur through an apoptotic process. Examination of brain areals originating from different CJD patients revealed apoptotic neurons in all disease types, found mostly in damaged regions (Gray et al., 1999). Their abundant presence seemed to correlate closely with neuronal loss. Neuronal autophagy, like apoptosis, is one of the mechanisms of the programmed cell death (PCD). It is also implicated in TSE pathogenesis and might participate in the formation of spongiform changes (Liberski et al., 2004). Autophagy is an important step in the cellular turnover of proteins and organelles. It is known to occur in neurons under physiological as under pathological conditions. Large autophagic vacuoles have been observed in hamsters experimentally infected with scrapie (Boellaard et al., 1991) and in CJD infected mice (Boellaard et al., 1989).
Several groups demonstrated the involvement of endoplasmatic reticulum (ER) stress in prion replication (Hetz et al., 2007). Kristiansen et al provided evidence that soluble aggregates of a toxic might cause prion disease by the inhibition of the proteasomal activity, concluding a strong involvement of the degradation machinery. In addition the authors demonstrated that scrapie infected neurons display a reduced proteasomal activity confirming that this is one toxic mechanism contributing to neuronal loss (Kristiansen et al., 2007).