Review paper
Prion diseases – current theories and potential therapies: a brief review

Introduction

Prion diseases are fatal neurodegenerative protein-misfolding disorders. Different prion species and strains are involved in the pathogenesis of these diseases as explained in detail by Weissmann [29]. They are also known as transmissible subacute spon­giform encephalopathies (TSEs), a group that in­clud­es sheep scrapie, bovine spongiform en­ce­phalopathy (BSE) and Creutzfeldt-Jakob disease (CJD). Prion diseases gained widespread public interest and worry after an outbreak of BSE occurred in Europe, and evidence suggested that BSE is transmissible to hu­mans [3,7]. The incidence of CJD in Iran remains rare, but this is thought to reflect underreporting of the condition [2]. Clinical symptoms such as dementia and loss of coordination are suggestive of CJD but an accurate diagnosis requires inspection of post-mor­tem brain histology to detect the pathogenic prion.

It is commonly conceived that the agents responsible for TSEs are abnormal conformers of the prion protein (PrPC) [12]. The abnormal conformer, known as PrPSc, is an insoluble form of PrPC that is resistant to protease digestion. PrPC is widely expressed, but the majority are found in the central nervous system as a glycoinositol phospholipid anchored cell surface protein. In contrast, the abnormal conformer PrPSc accumulates intracellularly in cytoplasmic vesicles. The physiological function of PrPC remains uncertain but it may have a role in cellular signalling pathways such as calcium homeostasis. The conversion of PrPC to PrPSc is believed to involve post-translational mo­dification. The structure of PrPC is mainly -helical, whereas PrPSc seems to be predominantly composed of b-sheet arrangement. The conversion from PrPC to PrPSc can occur spontaneously, arising from genetic mutations in the PrPC gene, or be induced by infection with exogenous PrPSc. PrPSc is thought to induce PrPC conversion to PrPSc [19].

Prions involve not only humans but also a wide variety of both domestic and wild animals and lead to transmissible and fatal diseases which are recognized by various names [8,11]. A summary of prion diseases are demonstrated in Table I.

Potential therapeutic targets

Unfortunately, most of the CNS damage has al­ready taken place when the clinical symptoms of prion disease manifest themselves. Since early diagnostic testing for prion disease is not yet available [8], there has been an interest in the development of drugs that could be beneficial following the onset of its clinical signs.

PrP^C conversion

Conversion of PrPC to PrPSc is a central event in TSE pathology and, therefore, a potential therapeutic strategy has been an attempt to disrupt PrPSc formation. Quinacrine (an antimalarial drug) and chlor­promazine (a widely used antipsychotic drug) were found to be efficacious inhibitors of PrPSc formation in vitro [10]. It is thought that these compounds act by rearrangement of cholesterol into intracellular compartments resulting in the deterioration of the plasma membrane [17]. Animal studies examining these compounds have demonstrated limited effectiveness in the treatment of TSEs [28]. In fact, ad­ministration of quinacrine to CJD patients led to short- lived improvements of symptoms (lasting between 1 and 2 months) only and, ultimately, did not halt the disease [26].

As protein misfolding is a major event in prion disease pathology, there has been an investigation into the ability of pharmacological agents to stabilize the native folded state of PrP. These agents may allow for a reduction in the rate of their misfolding and hence prevent progression of disease. Recently, Nicoll et al. found that a cationic tetrapyrrole compound displays significant antiprion activity by binding to a folded domain of human PrP [24].

Immunotherapy

Antibody-mediated therapy that targets the PrP conformers has been shown to inhibit PrPSc propagation in vitro [13,22]. It is thought that these antibodies bind to PrPC and attenuate its availability for conversion into the abnormal prion conformers. This approach appears to be promising in animal models but issues that remain to be addressed include the delivery of the antibodies across the blood-brain barrier as well as the potential immunological pitfalls associated with tolerance to PrP, which is widely expressed in the immune system.

Another, more indirect, target may be the inflammatory mediators required for the development of follicular dendritic cells. These cells are known to be required for splenic PrPSc accumulation. Neutralization of the lymphotoxin- receptor (LT-R) pathway by administration of a soluble LT-R-Ig fusion protein blocks follicular dendritic cell maturation and prevents scrapie neuroinvasion [14].

RNAi

RNA interference (RNAi) exploits the cell’s en­dogenous gene silencing machinery and is a well-recognized experimental tool, with the potential for targeted gene silencing in neurodegenerative diseases such as Alzheimer’s. RNAi inhibits PrPC ex­­pression in neuroblastoma cells and prevents the build-up of the abnormal PrPSc conformer in scrapie-infected cells [4, 30]. Recently, lentiviral vector-mediated RNA silencing of PrPC was shown to prolong survival of prion-infected mice [16,20]. Despite these promising results, this approach has not cured the disease. Although the onset of disease was delayed, all animals treated with RNAi against PrPC eventually died of the disease.

Neurodegeneration

Apoptosis of neuronal cells is a central feature in prion diseases. The cause of neuronal cell death remains unclear but may be triggered by direct interaction of the prion-infected cells or, indirectly, by deficiency of the normal PrPC prion protein. Muller et al. noted that neuronal cells in vitro undergo apoptosis when incubated in the presence of PrPSc and this can be mimicked by the PrP peptide fragment PrP106-126 [25].

PrPC has been shown to enhance intracellular calcium levels, which may contribute to neuronal apoptosis [18]. Other investigators have also shown that PrPSc induces alterations in calcium homeostasis. For example, the activity of the calcium-dependent phosphatase, calcineurin, is enhanced as a re­sult of PrPSc formation [27]. Muckerjee et al. reported that the calcineurin inhibitor FK506 reduced severity of the clinical abnormalities and increased survival time compared to sham-treated controls. Another contribution from calcium mediated excitotoxicity is through effects on the N-methyl-D-aspartate (NMDA) receptor [25]. The NMDA receptor channel is widely expressed in the CNS and is highly permeable for the cations Ca2+, Na+ and K+. Crucially, the neurotoxic activity of PrPSc was blocked by the NMDA antagonists, memantine and MK-801, in vitro [25]. The finding that the PrPSc induced neurotoxicity is prevented by antagonists of the NMDA receptor channel suggests that PrPSc activates the NMDA receptor channel, resulting in a significant rise of [Ca2+]i level in neurons and eventually cell death. Flupirtine, a drug that is used clinically as a nonopioid analgesic acts as an NMDA antagonist, but does not bind directly to NMDA receptors. Flupirtine in­creases the expression of the anti-apoptotic Bcl-2 pro­tein in neuronal cells exposed to prion protein [5]. In view of its established pharmacokinetic ac­tions, it is a potential drug for use in clinical trials for CJD.

There is research to suggest that there is serotonin (5-HT) dysregulation in prion diseases, parti­cularly alteration in 5-HT levels and 5-HT receptor binding [1, 6]. It has been suggested that PrPC has a role to maintain 5-HT receptor coupling to G-proteins [6]. This work has yet to yield direct therapeutic targets but 5-HT drugs form a major group of pharmacological therapies, including those used to treat central nervous system disorders such as de­pression and anxiety.

Following neuronal damage and loss, any form of therapeutic intervention is limited. However, by analogy to other neurodegenerative diseases such as Alzheimer’s disease, stem cells may yield benefits [23]. Embryonic stem cells and neural precursors are indeed known to migrate towards sites of brain damage and can differentiate into specific neuronal cell types [15, 21]. Although these therapies are still in the experimental phase of development, there is hope that stem cell based therapies may attenuate disease symptoms.

Future perspectives and conclusion

Despite the varied approaches that have been undertaken to limit or reverse prion diseases, there has been frustratingly limited success to date. Des­pite this, our knowledge of the pathology of prion diseases and means of disease detection is continuously expanding and generating novel drug targets. Indeed, it may be that targeting more than one pathway or protein may provide synergistic benefits. It is hoped that prion diseases will remain rare, but if extensive human infection with BSE prions emerges in the next 10-20 years, there will be considerable pressure to combat this public health threat.

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