Alzheimer’s disease (AD) and prion disease are two distinct classes of neurodegenerative disorders characterized by a lack of knowledge of their underlying molecular mechanisms and an insufficient amount of viable biomarkers and effective treatments. The design and production of novel emerging therapies that can prevent or delay the onset of AD or improve the symptoms associated with this life-changing disorder is urgently needed.
Prion diseases have a long and interesting history. Scrapie, the prototypical prion disease of animals, was first described in sheep over 200 years ago. In the 1950s, Carleton Gajdusek and his colleagues described a strange neurodegenerative disease of the Fore tribe of New Guinea called kuru, which shared certain neuropathological similarities with scrapie. Subsequent work demonstrated that kuru, as well as Creutzfeldt-Jakob disease (CJD), were transmissible to non-human primates, establishing the infectious nature of these illnesses. The properties of the infectious agent appeared to be highly unusual. In the 1980s Stanley Prusiner proposed the prion hypothesis, which stated that the causative agent in human and animal spongiform encephalopathies was comprised exclusively of a single kind of protein molecule designated PrPSc without any encoding nucleic acid. Subsequent work has shown PrPSc is likely to be an alternately folded isomer of a normal, host-encoded membrane glycoprotein called PrPC. Although the role of PrPC in Transmissible Spongiform Encephalopathies (TSEs) is well understood, the physiological role of the prion protein remains to be full elucidated. Various functions including metal homeostasis, neuroprotective signalling, neurite growth, cell viability and synaptogenesis have all been proposed (Kellett and Hooper, 2009). Prions thus represent a novel biological principle for information transfer based on inheritance of protein conformation
TSEs are diseases that are described both in humans and animals. TSEs include CJD, Gerstmann-Straussler-Scheinker syndrome (GSS), Kuru and fatal familial insomnia (FFI) in humans and scrapie in sheep and goat, and bovine spongiform encephalopathy (BSE) in animals.
Alzheimer Disease (AD)
AD is the most common form of dementia. The disease is widespread and is estimated to be prevalent among forty million patients worldwide (Selkoe and Hardy, 2016). This number is expected to increase with ageing populations across developed countries. The true number of AD is thought to be much higher as the disease begins in the brain two to three decades before any significant symptoms are noticed.
The amyloid (or Aβ) hypothesis has become the main model of AD pathogenesis which guides the development of potential novel technologies and treatments today. AD is characterized pathologically by the formation of senile plaques composed of the Aβ peptide and neurofibrillary tangles composed of hyperphosphorylated Tau. It is the accumulation of Aβ in the brain that appears to be critical for the pathogenesis of AD (Kellett and Hooper, 2009).
Link between AD and Prion Diseases
Prion diseases and AD have a number of neuropathological similarities and genetic links. In patients with CJD, the coexistence of AD pathology has been observed and PrPC has been shown to co-localise with Aβ in plaques (Voigtländer et al., 2001). It is thought that PrPC may promote Aβ plaque formation. There are also reports that the gene encoding PrPC, PRNP, is a potential AD susceptibility gene. The Met/Val 129 polymorphism in PRNP is thought to be a potential risk factor for early-onset AD (Bertram et al., 2007).
Even though pathological and genetic links between AD and PrPC were already established, there was no evidence of protein-protein interaction. However, it was found that PrPC directly interacts with the rate-limiting enzyme, β-secretase (BACE1) in the production of Aβ. Two more studies have also found direct links of further protein-protein interaction. It has been reported that PrPC can function as a receptor for Aβ oligomers, thus mediating toxicity, and that the amyloid intracellular domain (AICD) controls the expression of PrPC
Production of Amyloid-β
Aβ is produced through the proteolytic processing of the amyloid precursor protein (APP). Proteolytic cleavage of APP can occur through the non-amyloidogenic pathway or the amyloidogenic pathway. However, it is the amyloidogenic pathway that produces a number of Aβ isoforms. The first step in the production of Aβ involves the cleavage of APP first by BACE1, which produces sAPPβ and C99. Then the second step involves the subsequent cleavage by by γ-secretase to release the amyloidogenic Aβ and the AICD. This results in a variety of Aβ isoforms, including, Aβ40 and Aβ42, with the latter being the more amyloidogenic isoform which is often found in senile plaques. These Aβ peptides can then form into small soluble oligomers. There is increasing evidence to suggest that synaptic dysfunction that occurs in AD is due to the presence of these oligomers.
The amyloid cascade hypothesis states that amyloid deposition is the main event in the development of AD. Low levels of Aβ in the normal brain are thought to play a relatively benign physiological role in the regulation of neuronal calcium and potassium channel currents. However, when Aβ levels increase due to an inbalance in its production and degradation it can be considered as an extremely harmful agent to the patient.
PrPC as a Receptor for Aβ Oligomers
Recent studies have reported that PrPC acts as a receptor for Aβ oligomers. These Aβ42-oligomers build up over time and bind to full-length PrPC. PrPC was found to have a high affinity and high selectivity for Aβ42-oligomers. A specific charge cluster region (amino acids 95–110) within the unstructured central region of PrPC was the main site for Aβ42-oligomer binding (Laurén et al., 2009). The direct interaction between PrPC and Aβ42-oligomers was found to affect synaptic plasticity and reduce long term potentiation (LTP) in mice, indicating PrPC is required to mediate one of the toxic effects of Aβ.
Disruption of the Feedback Loop
It is hard to tell whether PrPC maintains a protective role in AD or if it mediates the toxicity of Aβ. The fact that PrPC was identified as a regulator of BACE1 activity may demonstrate a physiological role for PrPC to control Aβ generation in the normal human brain. The interaction of Aβ42-oligomers to PrPC may reflect the later stages of AD when Aβ levels are elevated in the brain.
Aβ42 levels increase in the brain of AD patients (Funato et al., 1998). This may result in a pool of Aβ42 which can then aggregate to form toxic oligomers. These oligomers can then mediate their toxicity in part via PrPC. An increase in the proteolytic cleaveage of APP would in turn cause an increase in AICD generation, further increasing PrPC levels. High levels of PrPC provides more receptors for the Aβ42-oligomers creating cell toxicity (Kellett and Hooper, 2009). Increased levels of PrPC cannot regulate BACE1 cleavage of APP as the binding of the Aβ42 oligomers to PrPC sterically prevents it from interacting with BACE1.
Technologies in Development
As previously mentioned, recent reports using oligomers derived from synthetic Aβ peptides demonstrated that PrPC is a high affinity specific binding site for these oligomers and that PrPC is a requirement for negatively affecting synaptic plasticity. Anti-PrPC monoclonal antibody (mAb) infusion is a novel technology that may have the potential to treat AD patients in the near future. The anti-PrP mAb 6D11 was found to block Aβ oligomer mediated toxicity in hippocampal slices (Chung et al., 2010).
Chung et al. set out to test the hypothesis that short term treatment of anti-PrPC mAb infusion could reverse memory impairment in an established APP/PS1 Tg (transgenic) mouse model of AD. It was found that short term treatment of anti-PrPC mAb infusion reversed memory impairment and cognitive deficits in an AD Tg mouse model. Cognitive ability was assessed through radial arm testing.
Suppression of LTP in mouse hippocampal slice cultures could be overridden by mAb 6D11 as it blocks the interaction between the oligomers and PrPC. Major cognitive benefits can be seen within just two weeks of treatment with 6D11 in vivo. However, it is important to note that during and after treatment there is no significant change in the levels of amyloid plaque burden or Aβ peptide levels. This effect is consistent with past studies of amyloid directed therapeutic intervention. In many experimental vaccination cases, behavioural benefits in AD Tg mouse models often do not correlate with the overall amyloid burden but with Aβ oligomer levels.
As discussed earlier, behavioural improvement seen in 6D11 treated Tg mice is most likely due to blocking the binding of Aβ oligomers to PrPC. However, 6D11 treatment also improved loss of synaptophsin immunoreactivity. Synaptic loss is a distinctive feature of AD which corresponds best with the cognitive ability of patients. It is likely that the improvements in the cognitive behaviour and abilities in the 6D11 treated APP/PS1 mice is due to greater synaptic density.
In the case of novel technologies in development to address AD, the use of mAbs such as 6D11 or other compounds may have the ability to therapeutically benefit a range of similar neurodegenerative conformational disorders as PrPC may be capable of binding to other oligomeric species. 6D11 has been shown to be therapeutically effective in vitro and in vivo using tissue culture and mouse models of prion infection (Chung et al., 2010).
It is extremely difficult to care for patients effected by AD and prion diseases due to the complexity of the disorders. It is essential that in the coming years, superior animal models, improved trial designs and outcomes, and better predictive biomarkers are available to researchers and clinicians. Using a translational approach that integrates the knowledge derived from neuroscience with the development of new diagnostic and therapeutic tools can greatly assist in diagnosing, preventing or treating neurodegenerative disorders. In this way translational neuroscience may advance the field of AD by providing more efficient biomarkers and furthering effective drug development programs for AD and prion disease.
Novel diagnostic tools used in the detection of prion diseases may be adopted for therapeutic benefits in the treatment of AD. An example of these diagnostic tools include the amplification assays named Protein Misfolding Cyclic Amplification (PMCA) and Real Time Quaking-Induced Conversion (RT-QuIC) generated to model the process of prion misfolding in vitro over a very short time period (Soto et al., 2005) (Saá et al., 2006). PMCA technology functions through cycles of incubation and sonication of samples. These samples contain minute amounts of protease-resistance disease-associated prion protein (PrPres) in the presence of excess PrPC. This in turn, enables the exponential amplification of minute amounts of PrPres. PMCA technology can detect PrPres even in pre-symptomatic stages of prion diseases (Soto et al., 2005). With further exploration and advancement, PMCA may be useful in the field of AD in order to recognize any abnormal levels of PrPC early on in the human brain.
AD and prion diseases are examples of incredibly difficult neurodegenerative disorders to understand, and for the patient’s family – to live with. It is this lack of knowledge that has so far impeded any hope of finding a cure for the entirety of the disease. Presently, the future of novel technologies looks dim in the shadow of blockbuster anti-cancer drugs. However, such treatments such as anti-prion antibody therapeutics offer hope, even if they are in early stages of development.
Anti-PrPC mAb infusion produces significant behavioural changes for the treatment for cognitive deficiencies in an AD murine model. Such changes were seen with a short treatment regiment and in advanced disease states. The blocking of the interaction between PrPC and Aβ oligomers is imperative for the behavioural change seen in the murine model and improvement of synaptic loss.
Even with such promising technologies on the horizon, the world is still a long way from finding a definitive cure for AD. A vast improvement on clinical trials and animal models and highly-specific biomarkers is needed to overcome the many hurdles that face researchers today. It is important to note that there is still no known cause of AD. In order to fully address the many debilitating factors that accompany AD, truly effective therapies must be able to confront the initial phase of the disease state in order to combat late stage symptoms, thus preventing high levels of amyloid plaque burden and Aβ peptide levels in the patient.