MicrobiologyBytes

Prions are proteinaceous infectious agents that were first discovered because of their role in the etiology of transmissible spongiform encephalopathies (TSEs), a set of fatal neurological disorders that include Creutzfeldt-Jakob disease, scrapie and bovine spongiform encephalitis. TSEs arise from progressive misfolding of the endogenous cellular prion protein (Prp [PrPC]) into disease-associated scrapie form (PrPSc), which, in turn, disrupts normal cellular function and results in the formation of aggregates and amyloid-like plaques. Although there has been a clear association between PrPSc with these disease states, the cellular function of PrPC remains incompletely understood. PrPC is expressed across the entire central nervous system and at particularly high levels in the hippocampus, striatum, and frontal cortex, with apparently wide subcellular distribution, including synaptic sites. A synaptic role of PrPC is consistent with evidence from PrP-null mice showing deficits in spatial learning, altered long-term potentiation and increased excitability of hippocampal neurons. Several studies have also suggested that PrPC may provide neuroprotection. For example, cultured hippocampal neurons obtained from PrP-null mice show an increased apoptosis during oxidative stress. Mice lacking PrPC show increased neuronal damage after ischemic stroke, whereas protection is evident upon the viral-based overexpression of PrPC in rats. Finally, in several in vivo models of seizure activity, PrP-null mice showed increased mortality, likely as a consequence of hyperexcitability leading to excitotoxicity. The cellular and molecular basis for these effects remains unknown. Recent research shows that PrPC exerts a neuroprotective role by inhibiting excitotoxic cell death and that PrPC is a modulator of synaptic function and, consequently, neuronal excitability.

So that’s what prions are for!

Prion protein attenuates excitotoxicity by inhibiting NMDA receptors. J Cell Biol 2008 181: 551-565
It is well established that misfolded forms of cellular prion protein (PrP [PrPC]) are crucial in the genesis and progression of transmissible spongiform encephalitis, whereas the function of native PrPC remains incompletely understood. To determine the physiological role of PrPC, we examine the neurophysiological properties of hippocampal neurons isolated from PrP-null mice. We show that PrP-null mouse neurons exhibit enhanced and drastically prolonged N-methyl-D-aspartate (NMDA)–evoked currents as a result of a functional upregulation of NMDA receptors (NMDARs) containing NR2D subunits. These effects are phenocopied by RNA interference and are rescued upon the overexpression of exogenous PrPC. The enhanced NMDAR activity results in an increase in neuronal excitability as well as enhanced glutamate excitotoxicity both in vitro and in vivo. Thus, native PrPC mediates an important neuroprotective role by virtue of its ability to inhibit NR2D subunits.

The amyloidoses are a group of protein misfolding disorders characterized by the accumulation of amyloid fibrils formed from a variety of proteins that, under normal physiological conditions, are harmless and soluble. Currently, 25 amyloid diseases have been identified, such as the prion diseases, Alzheimer’s disease, type 2 diabetes, and various systematic amyloidoses. Although the various proteins that can polymerize into amyloid fibrils have unrelated sequences, they can all form fibrils with a similar ultrastructural appearance. Among them, prion diseases such as transmissible spongiform encephalopathy (TSE), including scrapie in sheep, bovine spongiform encephalopathy (BSE), and chronic wasting disease (CWD) of deer and elk, are highly infectious. In these diseases, prion (PrPSc), an abnormal form of the host cellular prion protein (PrPC), induces the conformational change of PrPC to the PrPSc and causes a detectable phenotype or disease in the affected individual. AA amyloidosis is characterized by the systemic deposition of extracellular fibrils composed of amyloid A protein, primarily in the spleen; liver; and, to a lesser extent, in other organs. In most species, AA amyloidosis occurs sporadically and is typically secondary to chronic inflammation, infection, or neoplasia. Intriguing recent data suggest that AA amyloidosis could be transmitted by a prion-like infectious process through a seeding-nucleation mechanism.

The cheetah (Acinonyx jubatus) is in danger of extinction and is included on The World Conservation Union list of vulnerable species. Although efforts have been made in wildlife sanctuary parks and zoos worldwide to prevent extinction, a steady increase in the size of the cheetah population is hampered by the high prevalence of certain diseases in captive cheetahs. In particular, systemic AA amyloidosis is regarded as an increasingly important cause of morbidity and mortality in captive cheetahs as prevalence increased from 20% in pre-1990 reported necropsies to an unusual 70% of necropsied cheetahs in 1995. Despite much effort, the pathogenesis for AA amyloidosis in cheetahs is still only partially understood. However, environmental epidemiological studies indicate that breeding conditions have a prominent effect on the incidence of AA amyloidosis. A high rearing density is always associated with early age of onset, and with the high incidence and severity of AA amyloidosis, findings similar to sheep scrapie and cervid CWD. Thus, sustained epidemics of sheep scrapie and cervid CWD appear to be principally due to horizontal (animal to animal) transmission, although the routes of natural transmission remain to be clarified. The propagation of AA amyloidosis among captive cheetah populations may also depend on a horizontal transmission pathway. Identification of the mode of transmission is a prerequisite for disease control. This study shows that the faeces from a cheetah with AA amyloidosis can act as a possible transmission agent to accelerate the onset of AA amyloidosis.

Fecal transmission of AA amyloidosis in the cheetah contributes to high incidence of disease PNAS USA May 12, 2008
AA amyloidosis is one of the principal causes of morbidity and mortality in captive cheetahs (Acinonyx jubatus), which are in danger of extinction, but little is known about the underlying mechanisms. Given the transmissible characteristics of AA amyloidosis, transmission between captive cheetahs may be a possible mechanism involved in the high incidence of AA amyloidosis. In this study of animals with AA amyloidosis, we found that cheetah feces contained AA amyloid fibrils that were different from those of the liver with regard to molecular weight and shape and had greater transmissibility. The infectious activity of fecal AA amyloid fibrils was reduced or abolished by the protein denaturants 6M guanidine HCl and formic acid or by AA immunodepletion. Thus, we propose that feces are a vehicle of transmission that may accelerate AA amyloidosis in captive cheetah populations. These results provide a pathogenesis for AA amyloidosis and suggest possible measures for rescuing cheetahs from extinction.

• The first-ever cases of chronic wasting disease (CWD) in wild elk have been discovered in Canada.

But wait -

It might seem like prions are always bad news, but if that were true, they would be an evolutionary disadvantage and would surely have been lost. So the truth must be more complicated, and in tomorrow’s post, we’ll see how.

Related:

• Drugs for treatment of prion infections
• A General Model of Prion Strains and Their Pathogenicity
• Prions, Proteasomes, and Mad Cows
• Oral transmission of prions is enhanced by binding to soil

Researchers from the UK and France have identified four separate biochemical subgroups in a selection of cases of Creutzfeldt-Jakob disease. The study suggests that these subgroups could represent distinct prion strains in what is the most common human prion disease. Prion diseases are transmissible neurodegenerative disorders characterized by accumulation of an abnormal isoform (PrP^Sc) of a host-encoded protein (PrP^C) in affected tissues. According to the prion hypothesis, PrP^Sc alone constitutes the infectious agent. Sporadic Creutzfeldt-Jakob disease (sCJD) is the commonest human prion disease. Although considered as a spontaneous disorder, the clinicopathological phenotype of sCJD is variable and substantially influenced by the methionine/valine polymorphism at codon 129 of the prion protein gene (PRNP). Based on these clinicopathological and genetic criteria, a subclassification of sCJD has been proposed. Two new biochemical assays identified four distinct biochemical PrP^Sc subgroups in a cohort of 41 sCJD cases. These subgroups correlate with the current sCJD subclassification and could therefore represent distinct prion strains. Iatrogenic CJD (iCJD) occurs following presumed accidental human-to-human sCJD transmission. Our biochemical investigations on 12 iCJD cases from different countries found the same four subgroups as in sCJD. However, in contrast to the sCJD cases, no particular correlation between the PRNP codon 129 polymorphism and biochemical PrP^Sc phenotype could be established in iCJD cases. This study provides an alternative biochemical definition of PrP^Sc diversity in human prion diseases and new insights into the perception of agent variability.
Sporadic Creutzfeldt-Jakob disease (sCJD) cases are currently subclassified according to the methionine/valine polymorphism at codon 129 of the PRNP gene and the proteinase K (PK) digested abnormal prion protein (PrP^res) identified on Western blotting (type 1 or type 2). These biochemically distinct PrP^res types have been considered to represent potential distinct prion strains. However, since cases of CJD show co-occurrence of type 1 and type 2 PrP^res in the brain, the basis of this classification system and its relationship to agent strain are under discussion. The identification of four different PrP^Sc biochemical subgroups in sCJD and iatrogenic CJD (iCJD), irrespective of the PRNP polymorphism at codon 129 and the PrP^res isoform, provides an alternative biochemical definition of PrP^Sc diversity and new insight in the perception of Human TSE agents variability.

Beyond PrP^res Type 1/Type 2 Dichotomy in Creutzfeldt-Jakob Disease. PLoS Pathog 4(3): e1000029

Related:

• Drugs for treatment of prion infections
• Fears over new wave of vCJD deaths
• A General Model of Prion Strains and Their Pathogenicity
• Prions, Proteasomes, and Mad Cows

Prion diseases are fatal and at present there are neither cures nor therapies available to delay disease onset or progression in humans. Inspired in part by therapeutic approaches in the fields of Alzheimer’s disease and amyotrophic lateral sclerosis, researchers tested five different drugs which are known to efficiently pass through the blood-brain barrier in a mouse prion system. Groups of intracerebrally prion-challenged mice were treated with the drugs curcumin, dapsone, ibuprofen, memantine and minocycline. Treatment with antibiotics dapsone and minocycline had no therapeutic benefit. Ibuprofen-treated mice showed severe adverse effects, which prevented assessment of therapeutic efficacy. Mice treated with low- but not high-dose curcumin and mice treated with memantine survived infections significantly longer than untreated controls. These results encourage further research efforts to improve the therapeutic effect of these drugs.

Evaluation of drugs for treatment of prion infections of the central nervous system. 2008 J Gen Virol. 89: 594-597

Related:

• Fears over new wave of vCJD deaths
• A General Model of Prion Strains and Their Pathogenicity
• Prions, Proteasomes, and Mad Cows

Sensitivity to vCJD, the human form of mad cow disease (BSE) is determined by many factors. One of these is a genetic polymorphism at codon 129 of the human prion gene PrP. Individuals may have either a methionine (M) or valine (V) codon at this position. People who are heterozygous at this codon, that is, have different codons on each of the two alleles (copies) of the PrP gene in every cell, show resistance to sporadic CJD as well as to acquired prion diseases such as kuru and vCJD. The extent of the resistance is such that to date, all of the 162 known victims of vCJD have been homozygous at codon 129, and all have had two methionine codons (genotype 129MM).

Until now.

The first reported case of vCJD involving a victim who is homozygous for valine (129VV) at codon 129 has just been published. Why does that matter? It’s too early to say for sure, but this cases raises the possibility of a new wave of human vCJD cases with a longer incubation period in the 10% of the population with the 129VV phenotype, in addition to the previously at risk 30-40% of the population with the 129MM phenotype.

Related:

• Creutzfeldt-Jakob Disease, Prion Protein Gene Codon 129VV, and a Novel PrPSc Type in a Young British Woman
• Prion Disease Update

2007 has been a record breaking year for MicrobiologyBytes, so here’s a look back at some of the highlights:

We started January off with noroviruses and ancient plague, then relaxed a bit by playing with Lego and brewing beer.

In February, we looked at yaws and Mimivirus, then went green by reducing our carbon footprint with microdiesel.

And in March we marked World Tuberculosis Day by looking at new drugs for an old foe.

April started off with an exploration of whether viruses evolve to protect their hosts, then we took out first look at colony collapse disorder affecting bees.

May was dominated by news about extreme drug resistant tuberculosis (XDR-TB) and chikungunya, then later looked at probiotics.

In June we looked at the origins of yellow fever and quorum sensing in Serratia (quorum sensing remains one of the most popular topics on MicrobiologyBytes).

July began with flesh eating bacteria and finished up with prions and Alzheimers disease.

In August, most people took a holiday and this was the quietest month of the year in terms of visitors, but we still managed to fit in Hendra, chikungunya and Marburg viruses.

September brought lots of bad news for UK farmers, so we looked at the biology of the bluetongue and foot and mouth disease virus outbreaks in the UK.

In October we covered the bacterial SOS system and debated the strategy for HPV vaccination in the UK.

November started with the terrorist threat posed by glanders and melioidosis then considered the dangers of Chlamydia infection and the opportunities presented by DNA microarrays.

We finished up the year with bacteriocins and bacterial morphology.

Phew. Overall, the most popular posts of the year were:

1. Hepatitis C Virus: a mountain to climb
2. Fungal Infections and All About Fungi
3. Infectobesity
4. Toll-Like Receptors
5. DNA microarrays

See you next year!

Prions are lethal mammalian pathogens composed of aggregated conformational isomers of a host-encoded glycoprotein and which appear to lack nucleic acids. Their unique biology, allied with the public-health risks posed by prion zoonoses such as bovine spongiform encephalopathy, has focused much attention on the molecular basis of prion propagation and the “species barrier” that controls cross-species transmission. Both are intimately linked to understanding how multiple prion “strains” are encoded by a protein-only agent. The underlying mechanisms are clearly of much wider importance, and analogous protein-based inheritance mechanisms are recognized in yeast and fungi. Recent advances suggest that prions themselves are not directly neurotoxic, but rather their propagation involves production of toxic species, which may be uncoupled from infectivity.

A General Model of Prion Strains and Their Pathogenicity
Science 2007 318: 930-936

Related:

• Variant Creutzfeldt-Jacob disease (vCJD) update
• Prions, Proteasomes, and Mad Cows
• New forms of bovine spongiform encephalopathy
• Prions and Alzheimers disease

Definite and probable CJD cases in the UK as of 28 Sep 2007

Summary of vCJD cases:
Deaths from definite vCJD (confirmed): 114
Deaths from probable vCJD (without neuropathological confirmation): 47
Deaths from probable vCJD (neuropathological confirmation pending): 0
Number of deaths from definite or probable vCJD (as above): 161
Number of probable vCJD cases still alive: 5
Total number of definite or probable vCJD (dead and alive): 166

This data are consistent with the view that the vCJD outbreak n the UK is in decline. The peak number of deaths was 28 in the year 2000, followed by 20 in 2001, 17 in 2002, 18 in 2003, 9 in 2004, 5 in 2005, 5 in 2006, and so far 3 in 2007.The UK Department of Health will stop issuing monthly CJD press notices because the same data is published by the National CJD Surveillance Unit in Edinburgh.Related:

• The Origin of BSE
• Prions and Alzheimers disease
• New forms of bovine spongiform encephalopathy
• BSE in France

All cells have the capacity to selectively degrade misfolded intracellular proteins, which, if they accumulated, could interfere with normal function and could be toxic. Such proteins may arise by mutation, errors in gene expression, failure to fold correctly, spontaneous denaturation, or postsynthetic damage (for example, by oxygen radicals). How often such events occur in cells is uncertain, largely because the ubiquitin–proteasome pathway rapidly degrades such aberrant proteins, including those that cause various inherited diseases, such as cystic fibrosis and certain hemoglobinopathies. This pathway also protects against neurodegenerative diseases. Direct evidence that the abnormal toxic proteins in these various diseases can inhibit the functioning of the proteasome pathway has been lacking. Recently, Kristiansen et al presented strong evidence that soluble aggregates of the toxic protein may cause prion disease by specifically inhibiting the function of the 26S proteasome. A major gap in our understanding has been how the conversion of PrPC to PrPSc eventually kills neurons. Kristiansen et al showed that neurons and neuroblastoma cells infected with prions have reduced proteasomal activity against model substrates. Loss of proteasomal function affects many critical cellular processes and can induce apoptosis.

On Prions, Proteasomes, and Mad Cows
NEJM 2007 357:1150-1152