One of the major current challenges to both medicine and neuroscience is the treatment of neurodegenerative diseases, which pose an ever-increasing medical, social and economic burden in the developed world. These disorders, which include Alzheimer's, Huntington's and Parkinson's diseases, and the rarer prion diseases, are separate entities clinically but have common features, including aggregates of misfolded proteins and varying patterns of neurodegeneration. A key barrier to effective treatment is that patients present clinically with advanced, irreversible, neuronal loss. Critically, mechanisms of neurotoxicity are poorly understood. Prevention of neuronal loss, ideally by targeting underlying pathogenic mechanisms, must be the aim of therapy. The present review describes the rationale and experimental approaches that have allowed such prevention, rescuing neurons in mice with prion disease. This rescue cured animals of a rapidly fatal neurodegenerative condition, resulting in symptom-free survival for their natural lifespan. Early pathological changes were reversed; behavioural, cognitive and neurophysiological deficits were recovered; and there was no neuronal loss. This was achieved by targeting the central pathogenic process in prion disease rather than the presumed toxic species, first by proof-of-principle experiments in transgenic mice and then by treatment using RNA interference for gene knockdown. The results have been a new therapeutic target for prion disease, further insight into mechanisms of prion neurotoxicity and the discovery of a window of reversibility in neuronal damage. Furthermore, the work gives rise to new concepts for treatment strategies for other neurodegenerative disorders, and highlights the need for clinical detection of early neuronal dysfunction, so that similar early rescue can also be achieved for these disorders.
- neurophysiological analysis
- RNA interference (RNAi)
- synaptic dysfunction
By 2040, the World Health Organization predicts that neurodegenerative disorders will be the second commonest cause of morbidity, after cancer, in the developed world. Yet no curative treatments exist; we have only limited disease-modifying therapies that are inevitably delivered too late for dying neurons. However, early changes in synaptic and neuronal function precede cell death in all of these disorders, when intervention could be expected to save diseased cells.
In common with AD (Alzheimer's disease), HD (Huntington's disease) and PD (Parkinson's disease), prion diseases are neurodegenerative disorders involving pathogenic protein misfolding. However, whereas animal models of AD, PD and HD express human mutant proteins and variably reproduce some features of the human disorders, prion-infected laboratory mice have actual prion disease. Prion diseases are thus the only neurodegenerative diseases where in vivo pathological mechanisms are directly accessible. The present review shows how lessons learned from the study of prion-infected mice pave the way for new treatments for prion disorders, but also open new avenues for therapy of the more common neurodegenerations. The finding of deficits associated with pre-degenerative change in prion disease, and the reversibility of these, emphasizes the need for early detection and treatment to ‘rescue’ neurons across the spectrum of neurodegenerative disorders.
PRION DISEASES: BACKGROUND AND PATHOGENESIS
Prion diseases are unique in being transmissible neurodegenerative diseases, which also occur sporadically, or through genetic mutation in the prion protein gene, PRNP . They are characterized pathologically by spongiform changes in neurons, neuronal loss, astrocytosis and deposition of insoluble PrP (prion protein) aggregates throughout the brain. Clinically, dementia, loss of motor control and other motor symptoms occur, leading eventually to death. Known also as transmissible spongiform encephalopathies, they are fatal neurodegenerative diseases that include CJD (Creutzfeldt–Jakob disease) and kuru in humans, scrapie in sheep and BSE (bovine spongiform encephalopathy) in cattle. They transmit within and between mammalian species, as with the spread of BSE to humans in the form of vCJD (variant CJD) through the food chain, and can be transmitted experimentally to laboratory animals.
Central to prion-disease pathogenesis is the conversion of a highly conserved host-encoded PrP, PrPC (cellular isoform of PrP), into a partially protease-resistant amyloidogenic isoform, PrPSc (disease-associated isoform of PrP), which aggregates in the brain and is associated with infectivity (the ‘protein-only’ hypothesis) . This process is self-propagating, with PrPSc acting as a conformational template recruiting PrPC for further conversion (Figure 1a). PrPSc, although clearly associated with infectivity, does not appear to be the cause of neurotoxicity in prion disease. Indeed, there is no evidence for in vivo toxicity of PrPSc. High levels of PrPSc exist without clinical disease in subclinical disease states [3–5], and conversely, in several inherited prion diseases, PrPSc occurs at very low levels [6–8], and the degree of accumulation in specific brain regions does not correlate with clinical features. Furthermore, therapies directed against PrPSc and its accumulation have very limited efficacy in vivo (reviewed in ). Expression of host PrPC is, however, essential for both prion propagation and pathogenesis: PrP-null mice are resistant to prion disease and unable to replicate infectivity [10–12], disease incubation time is determined by PrPC expression levels [10–12], and prion neurotoxicity is confined to PrP-expressing neural tissue . Thus PrPSc only produces toxicity and cell death where PrPC is also expressed (Figure 1b). Therefore it is some aspect of the conversion process, rather than the action of PrPSc itself, that causes disease. Possibilities include the generation of a transient neurotoxic intermediate or toxic soluble oligomeric forms of PrPSc. Our first questions were: what causes neurotoxicity in prion disease, and how do we direct therapy? We reasoned that, as the conversion process was key to pathology, targeting PrPC itself would prevent toxicity in prion disease, whatever the nature of the toxic species, by preventing its formation.
TARGETING PRPC: REMOVING THE SUBSTRATE FOR CONVERSION IN PRION DISEASE
Targeting (depleting) PrPC should be a viable strategy in the adult brain. PrPC is highly conserved across species and, although its physiological function is poorly understood, its ablation should be without deleterious effect. Embryonic knockout of PrP is well tolerated in vivo [12,14]: PrP-null (Prnp0/0) mice are essentially normal phenotypically and behaviourally. Subtle changes in intrinsic properties of specific cell types are detectable electrophysiologically , and, more recently, it appears that PrPC has an essential role in peripheral myelin maintenance, producing a late-onset neuropathy in PrP-null mice, but without an overt motor phenotype . We first asked whether post-natal knockout (bypassing the potential compensatory mechanisms present during embryonic development) would be equally well tolerated. This is key to therapeutic approaches targeting PrPC that would be delivered to the adult brain. We therefore generated adult-onset PrP-knockout mice, with neuron-specific deletion of PrPC at 9 weeks of age (Figure 2). We made two lines of transgenic mice and crossed them: (i) tg37 mice expressed PrP from ‘floxed’ PrP sequences (MloxP transgenes) on a PrP-null background, so all PrP expression was from these transgenes; (ii) NFH (neurofilament heavy)-Cre mice expressed the site-specific DNA recombinase Cre in neurons under the control of the NFH gene promoter, which was activated at 9 weeks of age in fully adult animals. In double-transgenic NFH-Cre/tg37 mice, PrP sequences were excised by Cre on its activation, depleting neurons of PrP . These animals were healthy and viable, with only minor electrophysiological changes seen in Prnp0/0 animals . The lack of an overt phenotype was a critical finding. Secondly, it excluded the possibility that prion neurodegeneration is due to loss of endogenous PrPC function during the conversion process, which was a central question in prion biology at that time. Secondly, it validated targeting PrPC therapeutically, because this intervention had no detectable ill effects, at least in mice.
This model now allowed us to directly test the hypothesis that removing PrPC in prion infection would prevent ongoing prion neurotoxicity (Figure 2c). We infected double-transgenic mice with prions at 1 week of age, at a time when the PrP transgenes were present and expressed. Prion propagation could thus proceed in the presence of PrPC until the point, 8 weeks later, when neuronal depletion of PrP occurred due to Cre-mediated excision of the PrP-coding sequences. After depletion, PrPC was no longer available for conversion into PrPSc in neurons. Tg37 mice succumb to RML (Rocky Mountain Laboratories; mouse-adapted scrapie) prion infection at ~13 w.p.i. (weeks post-inoculation), showing classic motor signs of prion disease at 12 w.p.i. The earliest prion pathological changes (early spongiform change, astrocytosis and PrPSc deposition) appear in the hippocampus by 8 w.p.i. In NFH-Cre/tg37 mice infected with prions at 1 week of age, equivalent prion neuropathological change was established by the time PrP depletion occurred 8 weeks later. The effect of removing PrPC was dramatic. In comparison with control tg37 mice, which died just 4 weeks after the 8 w.p.i. time point, mice undergoing PrPC depletion at 8 w.p.i. survived asymptomatic and long term (Figure 3). The animals were effectively clinically cured . Furthermore, the early spongiosis seen at 8 w.p.i. was reversed and neuronal loss was prevented for the rest of the animals' lives (Figure 4); PrP depletion in neurons had cured clinical disease and reversed pathology, despite the continued accumulation of extra-neuronal PrPSc. NFH-driven Cre expression depleted PrP only in neurons; astrocytic expression of PrPC was unaffected and replication of PrPSc in these cells continued, with levels of PrPSc eventually reaching those seen in terminally sick wild-type animals (Figure 4). Yet this extra-neuronal propagation of prions was not neurotoxic, supporting the concept that conversion must occur within neurons for neurotoxicity to occur. However, astrocytic prion replication occurs at a much lower rate than in neurons due to lower levels of PrP expression in these cells ; it may be that it is the rate, rather than the site, of production of toxic species that determines cell death.
REVERSIBILITY OF EARLY PRION PATHOLOGY: FUNCTIONAL SIGNIFICANCE
Targeting PrPC had cured prion disease in these mice, with all animals surviving long term. Part of the efficacy of this approach was due to the timing of the intervention. Depletion of PrPC in prion-infected mice occurred before neuronal loss was established, which is clearly a key point for treatment. The striking new finding here, beyond the effect on survival, was reversal of early spongiform degeneration. Spongiform change in prion disorders involves the formation of membrane-bound vacuoles within neurons, predominantly within dendrites, and is associated with spine loss in the hippocampus . It is a pre-degenerative change occurring in neurons that are not yet lost. Conventionally, prion disease in mice is diagnosed by motor signs, which correlate with advanced neuronal loss – when it is too late for treatment. We reasoned that if spongiosis were an early morphological marker of functional impairment, earlier detection of prion disease could lead to earlier, effective, intervention. In our model, the first spongiform changes appeared at approx. 8 w.p.i., 4 weeks before diagnostic motor symptoms would normally be detected. In mice with PrP knockout, reversal of spongiosis occurred soon after this time point. Therefore we asked two questions: did early spongiform change produce detectable functional deficits, and if so, was its reversal reflected in functional recovery?
As the focus of early pathology and its reversal was the hippocampus, we tested prion-infected animals in behavioural tasks dependent on hippocampal integrity in vivo, and measured hippocampal synaptic transmission and plasticity in vitro. We found that, in parallel with the onset of spongiosis, mice developed cognitive, behavioural and neurophysiological deficits. Remarkably, with the reversal of spongiform change in PrP-depleted mice, the functional deficits were recovered.
The tasks we tested in RML prion-infected mice included both learned and spontaneous behaviours. Novel object recognition is a non-spatial learning task based on rodents' intrinsic preference for novelty and their ability to remember previously encountered objects . It is extensively used for testing declarative memory in mice, and, critically, performance is independent of mouse strain or genetic background, unlike most other memory tasks for which performance is highly strain-dependent. Burrowing activity is a powerful indicator of brain function in rodents and is thought to reflect motivational aspects of spontaneous behaviour . Reduction in burrowing has a robust association with early prion pathology [23–25]. At 7 w.p.i., before any pathological change was detectable, all animals displayed intact novel-object discrimination and burrowed actively (Figure 5). However, by 8 w.p.i., all mice had lost the ability to recognize novelty and stopped burrowing, coinciding with the appearance of early spongiform change  and the reduction of synaptic responses (see below). In prion-diseased tg37 mice, the impairment of memory and burrowing activity persisted throughout the course of infection and never recovered. In contrast, mice with PrP depletion recovered object memory and burrowing behaviour very soon after this occurred, in parallel with reversal of spongiosis and with recovery in synaptic transmission. Furthermore, the functional recovery was sustained for all tasks up to 20+ w.p.i.
The synaptic responses we recorded were field responses (cumulative responses of a number of cells to stimulation) within the dorsal hippocampus, reflecting the net result of integrated synaptic activity (Figure 6). Thus evoked EPSPs (excitatory postsynaptic potentials) reflect the composite effect of pre- and post-synaptic activity as well as synaptic transmission itself. At 8 w.p.i., all mice showed similar significant reductions in EPSPs at ~ 50% of the values seen in uninfected mice. Cre-mediated neuronal PrP depletion resulted in rapid recovery of EPSPs to control levels, a recovery that was sustained in mice examined up to 18 w.p.i. In contrast, synaptic responses in the prion-infected tg37 mice continued to decline. Action potentials in the presynaptic axons give rise to ‘fibre volleys’, which are separately detectable from the EPSP. The amplitude of these was linearly related to EPSP magnitude at each time point, suggesting that the change in EPSP was directly due to a change in the presynaptic action potentials, rather than due to reduced synaptic release or postsynaptic responses. Interestingly, when we measured LTP (long-term potentiation), a form of synaptic plasticity linked with some forms of learning and memory, we found no reduction in LTP in any of the prion-infected mice throughout the course of the experiment. Thus reduction in synaptic transmission in this model is not associated with changes in conventional measures of learning-related plasticity , but may involve aberrant synaptic functions at several levels, including that of the presynaptic axon. Indeed, we did not find evidence for actual synapse loss or neuronal loss at this early stage of disease. Immunohistochemistry and semi-quantitative Western blotting for the presynaptic marker synaptophysin did not reveal changes in the level of this protein, suggesting that the functional changes detected precede quantifiable synapse loss, in contrast with reported burrowing impairment in prion disease correlating with presynaptic terminal loss in the CA1 (cornu ammonis 1) region . The relationship between impaired synaptic transmission and spongiosis is therefore not clear at a structural level, but the changes in presynaptic properties clearly correlate with vacuolation.
These combined behavioural and neurophysiological analyses provided the first direct evidence for early neuronal dysfunction producing functional cognitive impairment in prion disease. These deficits preceded neuronal and even synaptic loss and could be rescued. Furthermore, they occurred before extensive PrPSc deposits accumulated and recovered rapidly after PrPC depletion, the mechanistic implication being that synaptic dysfunction is independent of aggregated misfolded protein deposition, which is also consistent with our findings of intact neurons at late time points, when levels of PrPSc accumulation were high. Thus in our model, both impairment and recovery of synaptic function, as well as long-term maintenance of neuronal integrity, appeared to be independent of PrPSc accumulation. As for AD [27–31] and other neurodegenerative disease models [32,33], the results strongly support a transient soluble non-aggregating species, probably generated within neurons, causing synaptic dysfunction in prion disease.
TRANSLATION INTO DELIVERABLE TREATMENT: RNAi (RNA INTERFERENCE) OF PrP
Transgene-mediated reduction of PrPC expression is an important proof-of-principle, but clearly does not offer direct therapeutic possibilities. We wanted to be able to deplete PrP therapeutically, to test this approach and assess its potential for future treatments. To date, no ligands binding PrPC are known; therefore we needed to use other means to reduce PrP levels. Recent developments in the field of RNAi have enabled therapeutic gene silencing in vivo, and the stable long-term expression of interfering RNA sequences can be achieved through the use of recombinant viral vectors. siRNAs (short interfering RNAs) are expressed as shRNA (short hairpin RNA) stem–loop structures driven by RNA polymerase III promoters in the viral vectors [34,35]. This technology has been used successfully in mouse models of neurodegenerative diseases, including SCA1 (spinocerebellar ataxia-1), HD, ALS (amyotrophic lateral sclerosis) and AD [36–39]. Adeno-associated virus expressing shRNAs against mutant ataxin-1 protein preserved cerebellar tissue structure and alleviated clinical symptoms when injected into the cerebellum of neonatal SCA1 mice . Importantly, the virus was beneficial despite transducing only 5–10% of the cerebellar Purkinje cells and reducing mutant ataxin-1 protein expression by only 50–60%, suggesting that complete ablation of mutant protein is not necessarily required for therapeutic benefit. Focally delivered virally mediated RNAi has also been used effectively in mouse models of HD, where intrastriatal injection of virally expressed shRNAs that partially reduced expression of mutant huntingtin protein prevented the formation of disease-associated neuronal inclusions and delayed the onset of motor deficits in one model  and improved neuropathology and behavioural deficits in another model of HD . Again, treatment was beneficial despite only partially reducing mutant protein levels. Encouraging results have also been attained in mouse models of familial ALS [38,41], PD  and AD [39,43]. For a full review of RNAi in the therapeutics of neurological disease, see .
Using a modified lentivirus, which stably transduces postmitotic neurons, expressing shRNAs against PrP, we essentially replicated our earlier experiments with transgene-mediated PrP depletion, but now directly testing the therapeutic potential of PrPC depletion in prion disease. We used the same line of mice, prion strain and time point of intervention, and compared the therapeutic efficacy of the lentivirally mediated RNAi of PrP with that of the gene knockout. RML-prion-infected tg37 mice received a single injection of lentivirus expressing an shRNA targeting PrPC into each hippocampus 8 w.p.i. Control mice were infected in parallel, but were treated with an ‘empty’ lentivirus. RNAi-mediated knockdown of PrP prevented the onset of early behavioural and cognitive deficits associated with this stage of prion infection, protected hippocampal neurons from degeneration, and reduced PrPSc deposition and spongiosis  (Figure 7). In fact, this treatment prevented, rather than reversed, the onset of cognitive deficits, perhaps because post-transcriptional gene silencing is a more rapid process, or more tightly controlled temporally, than genomic recombination and protein degradation.
However, the critical finding in this study was the effect on survival (Figure 8). A single treatment with focal injection of virus resulted in a mean increase in lifespan of 24% (range 0–50%) compared with mock-treated or untreated animals. This is remarkably large with respect to the very small volume of brain targeted. Prion incubation times are inversely proportional to overall levels of PrP expression [12,46,47], and it is likely that regional variations also affect prion replication rates and incubation periods. The hippocampus is a focus of early prion replication and PrPSc deposition for the RML prion strain used here (Figure 9); localized knockdown therefore may eliminate a key area for early prion replication in this model, which may have far-reaching implications for treatment. It is interesting to compare these results with the effects of virally mediated RNAi in other mouse models of neurodegeneration. Focal targeting of the ‘key’ regions – the striatum for huntingtin knockdown, the cerebellum for SCA1 and the hippocampus for BACE1 [β-site APP (amyloid precursor protein)-cleaving enzyme]-mediated processing of APP in AD – is effective in preventing focal neuropathology and giving some functional rescue in each case [36,37,40,43]. Interestingly, however, the striking effect on long-term survival was seen only in one other example of neurodegeneration treated with RNAi: a model of HD mice where both wild-type and mutant huntingtin were targeted . In that study, mice treated at 4 weeks of age had only 20% mortality at 20 weeks, when ~50% of the control animals had died. In prion disease, the spread of the agent is clearly a key factor affecting longevity, which may explain the lack of effect on survival in the other mouse models. Nevertheless, the wild-type huntingtin-knockdown results are interesting, possibly implicating native proteins in the toxicity of mutant forms in HD too. Importantly, as with targeting PrPC, these results highlight the legitimacy of targeting a native protein, despite a potential important physiological function, where partial reduction not only is not detrimental, but allows a positive therapeutic effect.
The efficacy of RNAi of PrP discussed above also underlines the crucial effect of timing of intervention. The concept of a therapeutic window for rescue first arose with the observation of reversal of spongiosis (and the subsequent long-term survival of mice) in genetically PrP-deleted prion-infected animals. Analysis of survival times of lentivirally treated mice showed that animals surviving the longest [nearly 50% more than controls; up to 129 d.p.i. (days post-inoculation), as highlighted by the circle in Figure 8] had been treated at a mean of 52 d.p.i., whereas animals surviving for the shortest period after treatment (as little as 87 d.p.i.) had been injected at a mean of 58 d.p.i. Thus anticipating treatment by a few days had a dramatic effect on survival. This may reflect a critical window for neuronal rescue in terms of a neuron's own programme of death, or be dependent on the kinetics of prion spread, reflecting the fact that effective intervention needs to be accurately timed. It introduces the concept that treatment of prion diseases is likely to be most effective if we understand and target the patterns of prion spread. This will vary for different prion ‘strains’, each of which has its own pattern of pathological lesion distribution and clinical and biochemical characteristics. The hippocampus is a region of intense early prion replication for RML prions. Thus ‘taking out’ hippocampal prion propagation in this model may have such marked effects on survival that this focal area for exponential prion replication is essentially removed (Figure 9b). Reducing PrP in the hippocampus at this time changed the pattern and timing of prion spread, altering the clinical course of the disease. Interestingly, targeting other brain regions by lentiviral injection in the same model at the same time point had no significant effect on survival, unless the hippocampus was also targeted (G. Malluchi, unpublished work). Focusing anti-prion therapy to specific crucial brain regions at particular times in the course of disease could be a potent strategy in treating human prion disorders, for which the sheer size of the brain makes global, whole-brain, targeting for PrP knockdown very difficult. By combining this approach with reducing the overall rate of replication, as is seen in global PrP knockout throughout the brain in transgenic animals, very significant effects on survival might be expected.
The effect of timing of treatment may also relate to the reduction of neurotoxic species at a critical time point in the life of a dysfunctional and dying neuron. Therefore either earlier intervention is exerting its effects through interference with the kinetics of prion replication or it is acting at a critical period in neuronal death ‘routines’, or both. Clearly, all treated animals succumb eventually, presumably due to prion-mediated neurodegeneration in other critical brain regions, but the neuroprotective effects seen within the hippocampus and beyond are a highly desirable effect of therapy. If transduction were more widespread, by pseudotying lentiviruses with coat proteins that allow retrograde transport [49,50] or using evolving mechanical techniques for enhanced delivery [51–53], more extensive neuroprotection and longer survival might ensue, due to the combination of effects on the kinetics of strain replication and to the reduced rate of production of neurotoxic species. This clearly would apply for all neurodegenerative diseases to be treated in this way, where the widespread expression of toxic proteins or intermediate species throughout the brain could be reduced by more global delivery techniques. Irrespective of causative species, our results and those of others using viral RNAi in neurodegeneration strongly support the notion that there may be a threshold level below which endogenous cellular mechanisms can clear pathological toxic intermediate species, be these soluble oligomers or aggregates [36,37,40,48]. Hence potential therapies need not aim for total ablation of mutant or toxic protein expression, but for lowering levels of expression below the cellular threshold for clearance, ideally throughout the brain. Focal treatment has shown us the potential efficacy of the approach, but has limited benefits in our and other models.
Interestingly, it appears that the concept of containing the ‘spread’ of disease may well apply in protein-folding neurodegenerative disorders beyond prion diseases. The evolution or ‘spread’ of tau protein or Alzheimer's pathology in human brains over time and through brain regions is well recognized . Transmission and spread of other neurodegenerative diseases, including tauopathy in mice, have also been shown recently, where mutant tau mouse-brain extract injected into wild-type tau-expressing mouse brains resulted in a spreading tauopathy in recipient animals . In vitro, the intercellular transfer of inclusions made of tau, α-synuclein, huntingtin and SOD1 (superoxide dismutase 1) has also been shown, revealing the existence of mechanisms reminiscent of those by which prions spread through the nervous system (reviewed in ). The concept of stopping protein propagation in its tracks with localized treatments may well be an option worth considering in other human neurodegenerative proteinopathies.
CONCLUDING REMARKS: THERAPEUTIC IMPLICATIONS FOR PRION DISEASE AND BEYOND
These findings have opened new avenues in the prion field and also for other neurodegenerative disorders. To date, prion infection in mice has conventionally been diagnosed when motor deficits reflect advanced neurodegeneration. Now, the identification of earlier dysfunction helps to direct the study of mechanisms of neurotoxicity and therapies to earlier stages of disease, when rescue is still possible. The key finding is that by reversing neurotoxic processes at this ‘synaptic’ stage of prion disease, neurons are saved from degeneration. The question as to whether this is the case for other neurodegenerative diseases that are also characterized by early synaptic dysfunction is critically important for therapy. Many questions still need addressing for both prion and other disorders. The mechanisms by which putative toxic PrP oligomers or intermediates – and, indeed, the nature or identity of these – cause synaptic dysfunction are still unknown. There is evidence of PrP interaction with NMDA (N-methyl-D-aspartate) receptors  and conflicting evidence for PrP mediating the toxicity of Aβ (amyloid β-peptide) oligomers at the level of memory and LTP [58–62], but clearly there are multiple effects at many levels. Indeed, how synaptic dysfunction ultimately leads to neuronal cell death in any of these disorders is not known, nor, critically, is the threshold of malfunction up to which sick synapses can be rescued. The extent to which spine and synaptic regeneration play a part in recovery is also unknown and needs exploring. It may be that individual strategies are needed for individual diseases or that the discovery of common pathways will allow a more global intervention, independent of specific pathology. These questions now need addressing, as the clearest implications of the work described in the present review are that treatment of this earliest phase of disease gives the only real hope not only of halting disease progression, but also of the possibility for cognitive recovery.
This work was funded by the Medical Research Council, U.K.
Abbreviations: AD, Alzheimer's disease; ALS, amyotrophic lateral sclerosis; BSE, bovine spongiform encephalopathy; CJD, Creutzfeldt–Jakob disease; d.p.i., days post-inoculation; EPSP, excitatory postsynaptic potential; HD, Huntington's disease; LTP, long-term potentiation; NFH, neurofilament heavy; PD, Parkinson's disease; PrP, prion protein; PrPC, cellular isoform of PrP; PrPSc, disease-associated isoform of PrP; RML, Rocky Mountain Laboratories; RNAi, RNA interference; SCA1, spinocerebellar ataxia-1; shRNA, short hairpin RNA; siRNA, short interfering RNA; w.p.i., weeks post-inoculation
- © The Authors Journal compilation © 2011 Biochemical Society