Normal host prion protein necessary for scrapie-induced neurotoxicity

Assessing the effects of the 129/Sv genetic background on swimming navigation learning in transgenic mutants: a study using mice with a modified beta-amyloid precursor protein gene

The Morris water maze is frequently used to screen mutant mice generated by gene targeting. Targeted ES-cells are often derived from 129/Sv or BALB/c mice, known as poor swimming navigation learners. After mating the founders with C57BL/6 mice, the F2 or F3 hybrid generation is typically used for behavioral testing. In hybrid 129/Sv x C57BL/6 mice, a modification of the betaAPP gene entails impaired swimming navigation learning. This is readily detected despite behavioral variability, because wild-type 129/Sv x C57BL/6 hybrids outperform either of the parental strains and provide a control sample with good baseline performance. However, after backcrossing to the 129/Sv(ev) strain, the mutation effects are no longer detectable, masked by the very poor performance of wild-type 129/Sv(ev) mice. We conclude that F2 and F3 generations of 129/Sv x C57BL/6 crosses provide a suitable genetic background for behavioral testing of transgenic mice, provided that the samples are large enough to compensate for genetic and epigenetic variability and provided that normal performance in the control group is verified by comparison against a large database of mice tested under identical conditions. Creating congenic lines by backcrossing to an inbred strain is unlikely to enhance the sensitivity of the Morris test. Backcrossing to 129/Sv(ev) may even reduce it.

Prion protein (PrP) is not involved in the pathogenesis of spongiform encephalopathy in transgenic mice expressing interleukin-6 in the brain

Transgenic mice expressing interleukin-6 (IL6) in the brain exhibit gliosis, spongiosis and neuronal loss. Based on previous findings, we hypothesized that IL6 could upregulate the prion protein (PrP) gene in the central nervous system (CNS) of these mice. Western and Northern blot analysis showed that PrP protein and mRNA levels were comparable to control levels. Furthermore, ultrastructural characterization revealed that spongiosis was actually located in astrocytes. These results indicate that IL6 does not upregulate the cerebral PrP expression in this animal model and that profound astrocytic alterations precipitate the neuronal degeneration observed.

PrP-expressing tissue required for transfer of scrapie infectivity from spleen to brain

Much available evidence points to a pathological isoform of the prion protein PrP being the infectious agent that causes transmissible spongiform encephalopathies, but the mechanisms controlling the neurotropism of prions are still unclear. We have previously shown that mice that do not express PrP (Prnp[o/o] mice) are resistant to infection by prions, and that if a Prnp(+/+) neurograft is introduced into such animals and these are infected intracerebrally with scrapie, the graft but not the surrounding tissue shows scrapie pathology. Here we show that PrP-expressing neurografts in Prnp(o/o) mice do not develop scrapie histopathology after intraperitoneal or intravenous inoculation with scrapie prions. Prion titres were undetectable in spleens of inoculated Prnp(o/o) mice, but were restored to wild-type levels upon reconstitution of the host lymphohaemopoietic system with PrP-expressing cells. Surprisingly, however, i.p. or i.v. inoculation failed to produce scrapie pathology in the neurografts of 27 out of 28 reconstituted animals, in contrast to intracerebral inoculation. We conclude that transfer of infectivity from the spleen to the central nervous system is crucially dependent on the expression of PrP in a tissue compartment that cannot be reconstituted by bone marrow transfer. Thus the requirement for the normal isoform of PrP in peripheral tissues represents a bottleneck for the spread of prions from peripheral sites to the central nervous system.

Transgenic animals as models of neurodegenerative diseases in humans

Neurodegenerative diseases are of major socioeconomic importance and represent an enormous challenge for the scientific and medical communities. Advances in molecular genetics during the past decade have begun to provide approaches for the establishment of animal models for these disorders using transgenic technology. Their analysis will lead to better understanding of disease pathogenesis and will be invaluable for the identification of novel diagnostic and therapeutic agents. With the current pace of genomic research, the generation of transgenic animal models, reproducing in full the pathology and symptoms of even complex disorders such as Alzheimer's disease, must now be considered achievable.

Of mice and (mad) cows--transgenic mice help to understand prions

Prions present a most fascinating biological conundrum. These proteinaceous particles seem to propagate through a chain reaction in which a host protein, PrPC, is post-translationally misfolded to form new prions. By this mechanism they 'replicate' without involvement of specific nucleic acids. Due to their unique modus operandi, prions cause disorders that can be infectious, inherited and sporadic. Transgenetics has been invaluable in helping to understand this unique phenomenon. Here we describe some of the most salient contributions of transgenic mice to this field.

Prion protein-deficient cells show altered response to oxidative stress due to decreased SOD-1 activity

The cellular function of the prion protein (PrPc), a cell surface glycoprotein expressed in neurones and astrocytes, has not been elucidated. Cell culture experiments reveal that cerebellar cells lacking PrPc are more sensitive to oxidative stress and undergo cell death more readily than wild-type cells. This effect is reversible by treatment with vitamin E. In vivo studies show that the activity of Cu/Zn superoxide dismutase is reduced in Prnp gene-ablated (Prnp0/0) mice. Constitutively high Mn superoxide dismutase activity in these animals may compensate for this loss of responsiveness to oxidative stress. These findings suggest that PrPc may influence the activity of Cu/Zn superoxide dismutase and may be important for cellular resistance to oxidative stress.

PrP deposition, microglial activation, and neuronal apoptosis in murine scrapie

The present study investigated the relationship among PrP deposition, microglial activation, vacuolation, and neuronal death in the hippocampus of the 301V/VM murine scrapie model (mean incubation period 117 +/- 1 days). PrP deposition was first detected after 30 days and microglial activation after 60 days. Vacuolation in the CA1 and CA2 pyramidal layer was present from 90 days onward. Only occasional in situ end labeling (ISEL)-positive neurons were present in the hippocampus of scrapie-infected mice from 75 days postinoculation (d.p.i.), except at 105 d.p.i. when relatively large numbers of apoptotic, ISEL-positive neurons in the CA1 hippocampal region were observed. Terminally ill animals showed almost complete loss of CA1 pyramidal neurons. Electron microscopy of the CA1 region at 105 days confirmed that these neurons were dying by apoptosis. These data suggest that microglial activation in scrapie is a response to abnormal PrP deposition rather than a response to neuronal cell loss.

A novel hamster prion protein mRNA contains an extra exon: increased expression in scrapie

Prion protein (PrP) is the only known constituent of the agents (called prions) that cause fatal neurodegenerative diseases in animals and humans. PrP derives from a host protein encoded by a single copy gene having three known exons in mice, cattle and sheep but only two exons in hamsters and humans. We have identified and sequenced the missing exon from the hamster PrP gene. The new hamster PrP exon is 83% identical to mouse exon 2 and 76% identical to exon 2 from cattle and sheep. PrP mRNAs containing the new exon 2 (mRNA[1+2+3]) were expressed in the colliculi, frontal cortex and hippocampus of normal hamsters at approximately 30% to approximately 50% of the levels of the mRNA without exon 2 (mRNA[1+3]). Expression of PrP mRNA[1+2+3] was increased in the colliculi beginning 49 days after inoculation with scrapie prions and reached a level 2.5 times normal by day 77. Increased expression of PrP mRNA[1+2+3] in the colliculi correlated with expression of glial fibrillary acidic protein (GFAP) mRNA. Expression of GFAP and PrP mRNAs was not significantly increased in the hippocampus or the frontal cortex during the disease. Our study shows that exon 2 plays a role in regulating the cellular expression of hamster PrP and suggests that mRNA[1+2+3] may be preferentially expressed in hamster astrocytes.

Transmission of the BSE agent to mice in the absence of detectable abnormal prion protein

The agent responsible for transmissible spongiform encephalopathies (TSEs) is thought to be a malfolded, protease-resistant version (PrPres) of the normal cellular prion protein (PrP). The interspecies transmission of bovine spongiform encephalopathy (BSE) to mice was studied. Although all of the mice injected with homogenate from BSE-infected cattle brain exhibited neurological symptoms and neuronal death, more than 55 percent had no detectable PrPres. During serial passage, PrPres appeared after the agent became adapted to the new host. Thus, PrPres may be involved in species adaptation, but a further unidentified agent may actually transmit BSE.

Identification of candidate proteins binding to prion protein

Prion diseases are disorders of protein conformation that produce neurodegeneration in humans and animals. Studies of transgenic (Tg) mice indicate that a factor designated protein X is involved in the conversion of the normal cellular prion protein (PrPC) into the scrapie isoform (PrPSc); protein X appears to interact with PrPC but not with PrPSc. To search for PrPC binding proteins, we fused PrP with alkaline phosphatase (AP) to produce a soluble, secreted probe. PrP-AP was used to screen a lambdagt11 mouse brain cDNA library, and six clones were isolated. Four cDNAs are novel while two clones are fragments of Nrf2 (NF-E2 related factor 2) transcription factor and Aplp1 (amyloid precursor-like protein 1). The observation that PrP binds to a member of the APP (amyloid precursor protein) gene family is intriguing, in light of possible relevance to Alzheimer's disease. Four of the isolated clones are expressed preferentially in the mouse brain and encode a similar motif.

Normal host prion protein (PrPC) is required for scrapie spread within the central nervous system

Mice devoid of PrPC (Prnp%) are resistant to scrapie and do not allow propagation of the infectious agent (prion). PrPC-expressing neuroectodermal tissue grafted into Prnp% brains but not the surrounding tissue consistently exhibits scrapie-specific pathology and allows prion replication after inoculation. Scrapie prions administered intraocularly into wild-type mice spread efficiently to the central nervous system within 16 weeks. To determine whether PrPC is required for scrapie spread, we inoculated prions intraocularly into Prnp% mice containing a PrP-overexpressing neurograft. Neither encephalopathy nor protease-resistant PrP (PrPSc) were detected in the grafts for up to 66 weeks. Because grafted PrP-expressing cells elicited an immune response that might have interfered with prion spread, we generated Prnp% mice immunotolerant to PrP and engrafted them with PrP-producing neuroectodermal tissue. Again, intraocular inoculation did not lead to disease in the PrP-producing graft. These results demonstrate that PrP is necessary for prion spread along neural pathways.

BSE--bovine spongiform encephalopathy ('mad cow disease')

The issue of Bovine Spongiform Encephalopathy (BSE) is one of great concern to all members of the Society and we have had many requests for clarification of the scientific issues involved. Conflicting and disturbing claims and statements have been made in the press, confusing the issue.

The definitive distillation of what is currently known is contained in the Position Statements produced by the Institute of Food Science & Technology (UK) and they have kindly allowed us to reproduce the most recent one below.

Prion diseases: transmission from mad cows?

Prion diseases in humans show considerable clinical and pathological heterogeneity. The identification of a new variant of Creutzfeldt-Jakob disease, and its interpretation as evidence of transmission of mad cow disease to man, rely critically on our understanding of the epidemiology of prion diseases.

Sleep, genes and death: fatal familial insomnia

Over the past 30 years, significant progress has been made in understanding the physiologic mechanisms of sleep. Insomnia, a common complaint in general medical practice, and other sleep disorders have become increasingly recognized. In 1986, a heritable total insomnia was described and termed fatal familial insomnia; since then, the pathology of this disease has been shown to involve an accumulation of prion particles in the brains of affected patients. Prions have been more commonly associated with the transmission of spongiform encephalopathies such as scrapie (in sheep), Creutzfeldt-Jakob disease and Kuru. We briefly review the physiological and biochemical characteristics of normal sleep, describe the typical clinical characteristics of fatal familial insomnia and describe the current understanding of how prions cause neurodegenerative diseases, including fatal familial insomnia.

A neurotoxic prion protein fragment enhances proliferation of microglia but not astrocytes in culture

The scrapie isoform of the prion protein (PrPSc) induces pathological changes in the central nervous system including neurodegeneration and gliosis. A synthetic prion protein (PrP) peptide corresponding to amino acid residues 106-126 has been shown to be toxic to neurons that express PrPC, the cellular isoform of PrP. Here we show that in mixed glial cultures PrP106-126 induces astroglial proliferation that is dependent on cellular PrPc expression. In purified cultures of glial subtypes only microglia proliferated in response to PrP106-126. This effect was independent of PrP expression. Destruction of microglia in mixed glial cultures by L-leucine methyl ester (LLME) treatment abolished enhanced proliferation caused by PrP106-126. This proliferative effect can be restored by co-culturing LLME-treated astrocytes with microglia. Microglia therefore seem to mediate the proliferative effect exerted by PrP106-126 on astrocytes.

Loss of functional prion protein: a role in prion disorders?

To understand the normal function of the prion protein (PrP) and its role in prion disorders, several groups have generated mice lacking PrP. Some of these mice develop symptoms associated with prion diseases, but other experimental evidence suggests that the loss of functional PrP is not the instigating factor in these disorders.

The Ninth Datta Lecture. Molecular biology of transmissible spongiform encephalopathies

The prion, the transmissible agent that causes spongiform encephalopathies such as scrapie, bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease, is believed to be devoid of nucleic acid and identical with PrPSc, a modified form of the normal host protein PrPC which is encoded by the single copy gene Prnp. The 'protein only' hypothesis proposes that PrPSc, when introduced into a normal host, causes the conversion of PrPC into PrPSc; it therefore predicts that an animal devoid of PrPC should be resistant to prion diseases. We generated homozygous Prnp(olo) ('PrP knockout') mice and showed that, after inoculation with prions, they remained free of scrapie for at least 2 years while wild-type controls all died within 6 months. There was no propagation of prions in the Prnp(olo) animals. Surprisingly, heterozygous Prnp(ol+) mice, which express PrPC at about half the normal level, also showed enhanced resistance to scrapie disease despite high levels of infectious agent and PrPSc in the brain early on. After introduction of murine PrP transgenes Prnp(olo) mice became highly susceptible to mouse but not to hamster prions, while the insertion of Syrian hamster PrP transgenes rendered them susceptible to hamster but to a much lesser extent to mouse prions. These complementation experiments paved the way to the application of reverse genetics. We have prepared animals transgenic for genes encoding PrP with amino terminal deletions of various lengths and have found that PrP lacking 48 amino proximal amino acids, which comprise four of the five octa repeats of PrP, is still biologically active.

Transgenic and knockout mice in the study of neurodegenerative diseases

Accurate animal models are essential for detailed analysis of the mechanisms underlying human neurodegenerative diseases. In addition, they can offer useful paradigms for the development and evaluation of new therapeutic strategies. We review the most popular techniques for modification of the mammalian genome in vivo, and provide a critical evaluation of the available transgenic mouse models for several neurological conditions of humans, including prion diseases, human retroviral diseases, Alzheimer's disease, and motor neuron diseases.