Novel mechanisms of degeneration of the central nervous system--prion structure and biology

Prion is a term for the novel infectious agents which cause scrapie and Creutzfeldt-Jakob disease; these infectious pathogens are composed largely, if not entirely, of prion protein (PrP) molecules. No prion-specific polynucleotide has been identified. Considerable evidence indicates that PrP 27-30 is required for and inseparable from scrapie infectivity. PrP 27-30 is derived from a larger protein, denoted PrPSc. A cellular isoform, designated PrPC, and PrPSc are both encoded by a single copy chromosomal gene and both proteins appear to be translated from the same 2.1 kb mRNA. Monoclonal antibodies to PrP 27-30 as well as antisera to PrP synthetic peptides, react with both PrPC and PrPSc, establishing the relatedness of these proteins. PrPC is completely digested by proteinase K; PrPSc is converted to PrP 27-30 under the same conditions. Detergent extraction of microsomal membranes isolated from scrapie-infected hamster brains solubilizes PrPC but induces PrPSc to polymerize into amyloid rods. This procedure allows separation of the two prion protein isoforms and the demonstration that PrPSc accumulates during scrapie infection while the level of PrPC does not change. The prion amyloid rods generated by detergent extraction are identical morphologically, except for length, to extracellular collections of prion amyloid filaments which form plaques in scrapie- and CJD-infected brains. The prion amyloid plaques stain with antibodies to PrP 27-30 and PrP peptides. Prion rods composed of PrP 27-30 dissociate into phospholipid vesicles with full retention of scrapie infectivity. The murine PrP gene (Prn-p) is linked to the Prn-i gene, which controls the length of the scrapie incubation period. Prolonged incubation times are a cardinal feature of scrapie and CJD. While the central role of PrPSc in scrapie pathogenesis is well established, the chemical and conformational differences between PrPC and PrPSc are unknown but presumably arise from post-translational events.

Genetic control of prion incubation period in mice

The prion gene complex (Prn) is located on mouse chromosome 2 between the beta-2-microglobulin (B2m) and agouti (A) genes. Within this complex are the prion protein gene (Prn-p), which encodes the only identified macromolecule (PrP) that purifies with infectious scrapie agent, and a scrapie incubation time gene (Prn-i). Using a variety of restriction endonucleases, six allelic forms of the Prn-p gene have been distinguished by their patterns of restriction fragment length polymorphisms. We had previously shown that the exceptionally long scrapie incubation period of I/LnJ mice inoculated with the Chandler isolate (over 200 days) was due to the effects of a scrapie incubation time gene tightly linked to Prn-p. So far, this long scrapie incubation time allele has been found only in those inbred mouse strains (I/LnJ, P/J and IM) that have the b allele of Prn-p. It is not known whether the incubation time gene and prion protein gene are two distinct loci or are one and the same. Putative recombinants between the incubation time phenotype and Prn-p genotype have been observed, but this could be due to effects of other genes segregating in the population. Regardless of whether or not the incubation time and PrP genes are identical, if any differences were found in the amino acid sequences of PrP encoded by the different Prn-p alleles there would be important implications for interpretation of results on 'strains' of scrapie agent. It would not be necessary to invoke nucleic acid as the informational macromolecule of the scrapie agent because differences in prion 'strains' recovered from mice with different Prn-p genotypes need not be the result of host selection but could be due to differences in host-encoded PrP.

Search for a scrapie-specific nucleic acid: a progress report

Scrapie agent contains a proteinaceous component as well as an 'informational' molecule (suggested by the existence of distinct strains of scrapie). These operationally defined entities may be the same molecule, an infectious protein, or distinct, in which case a nucleic acid might encode the genetic information. Purification of scrapie agent enriched a protein, PrPSc, by virtue of its relative protease resistance. There is only a single PrP gene and the primary translation product of PrP mRNA is the same in normal and scrapie-infected brain; therefore the normal PrPC and the protease-resistant isoform, PrPSc, found in scrapie, probably result from different post-translational events. To search for scrapie-specific nucleic acid, globin RNA made in vitro was added to highly purified infectious preparations at a ratio of 10(3) molecules per infectious unit, nucleic acids were isolated and denatured, and cDNA synthesized using random oligonucleotide primers. Clones containing globin-related sequences were identified by in situ hybridization. 150 plaques not hybridizing to the globin probe were isolated. Inserts larger than 50 base pairs were analysed. By hybridization to a globin probe at reduced stringency all but four clones were found to contain small globin related inserts; two of these hybridized to hamster repetitive sequences as shown by Southern blot analysis. The other clones not related to hamster nucleic acids may be derived from unknown sources of contamination or from scrapie-specific nucleic acids.

Pathogenesis of experimental scrapie

Most of our understanding of the pathogenesis of the unconventional slow infections comes from studies of experimental scrapie in mice and hamsters. After injection by non-neural peripheral routes, pathogenesis necessarily involves the lymphoreticular system (LRS) before the central nervous system (CNS). Available evidence indicates haematogenous spread from the site of injection to the scrapie replication sites in the LRS; later, infection spreads along visceral autonomic nerves from the LRS to the thoracic spinal cord, and thence to brain. The cells in the LRS which are important to scrapie pathogenesis are long lived. Neuroinvasion and spread of infection within the CNS probably involve neuronal pathways. We suggest that disease develops after infection has reached certain clinical target areas in the CNS but only when scrapie replication there has caused sufficient functional damage. Restriction of the replication process in both LRS and CNS is indicated by the occurrence of plateau concentrations of infectivity, especially in some long incubation scrapie models. A remarkable feature of these is that both neuroinvasion and clinical disease occur long after infectivity plateaux have been reached in the LRS and CNS, respectively. We propose that the slowness of scrapie is related to (1) limitations of cell-to-cell spread of infection from LRS to CNS, and (2) limitations on spread between neurons, coupled with restrictions on replication in brain.

Biological characteristics of Chinese hamster ovary cells transfected with bovine Prnp

A normal prion protein (PrP^c) is converted to a protease-resistant isoform by an apparent self-propagating activity in transmissible spongiform encephalopathy, a neurodegenerative disease. The cDNA encoding open reading frame (ORF) of the bovine prion protein gene (Prnp) was cloned from Korean cattle by PCR, and was transfected into Chinese hamster ovary (CHO-K1) cells using lipofectamine. The gene expression of the cloned cDNA was confirmed by RT-PCR and Western blotting with the monoclonal antibody, 6H4. Cellular changes in the transfected CHO-K1 cells were investigated using parameters such as MTT, lactate dehydrogenase (LDH), and superoxide dismutase (SOD) activities, as well as nitric oxide (NO) production, and an apoptosis assay. In the MTT and LDH assays, the bovine PrnP-transfectant showed a lower proliferation rate than the wild-type (p < 0.05). Production of NO, after LPS or ConA stimulation, was not detected in either transfectants or CHO-K1 cells. In SOD assay under ConA stimulation, the SOD activity of transfectants was 10 times higher than that of CHO-K1 cells at 6 h after treatment (p < 0.05). The genomic DNA of both the transfectants and control cells began to be fragmented at 6 h after treatment with cyclohexamide. Caspase-3 activity was reduced by transfection with the bovine Prnp (p < 0.05). Conclusively, the viability of transfectants expressing exogenous bovine Prnp was decreased while the capacities for cellular protection against antioxidative stress and apoptosis were increased.

[Preliminary analyses for influence of mutant PrPs with different number of octapeptide]

OBJECTIVE

The present study was conducted to understand the effects of PrP in different octapeptide repeats on proliferation of HeLa cells.

METHODS AND RESULTS

Mutant PrPs with octapeptide repeat insertion were transiently expressed in HeLa cells and their results of MTT assay showed stronger cytotoxic effect on the proliferation of cells than wild-type PrP. Annexin V/PI assay also demonstrated that the expression of mutant PrPs was much easier to induce apoptosis than wild-type in HeLa cells. The percentage of both early and late stage apoptosis in mutant groups were significantly higher than that of wild type.

CONCLUSION

These data suggest that the expression of mutant PrPs associated with familial CJD is much easier to induce apoptosis in cultured cells than expression of wild type PrP.

Therapeutic approaches targeting the prion receptor LRP/LR

Transmissible spongiform encephalopathies known as prion diseases are a group of fatal neurodegenerative disorders that affect both humans and animals. The generally accepted principle of the disease is that the conversion of the cellular prion protein (PrP(c)) into the disease associated isoform PrP(Sc) leads to spongiform degeneration of the brain and amyloid plaque formation. Until now no therapy leading to potential alleviation or even cure of the disease exists. It is therefore important to develop therapeutic approaches for the treatment of TSEs since these infections are inevitably fatal and, especially in the case of vCJD, they affect youngsters. Besides current conventional therapeutic strategies, this review summarizes new therapeutic tools targeting the prion receptor LRP/LR.

Prion protein gene-deficient cell lines: powerful tools for prion biology

Prion diseases are zoonotic infectious diseases commonly transmissible among animals via prion infections with an accompanying deficiency of cellular prion protein (PrP(C)) and accumulation of an abnormal isoform of prion protein (PrP(Sc)), which are observed in neurons in the event of injury and disease. To understand the role of PrP(C) in the neuron in health and diseases, we have established an immortalized neuronal cell line HpL3-4 from primary hippocampal cells of prion protein (PrP) gene-deficient mice by using a retroviral vector encoding Simian Virus 40 Large T antigen (SV40 LTag). The HpL3-4 cells exhibit cell-type-specific proteins for the neuronal precursor lineage. Recently, this group and other groups have established PrP-deficient cell lines from many kinds of cell types including glia, fibroblasts and neuronal cells, which will have a broad range of applications in prion biology. In this review, we focus on recently obtained information about PrP functions and possible studies on prion infections using the PrPdeficient cell lines.

Prion biology relevant to bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD) of deer and elk are a threat to agriculture and natural resources, as well as a human health concern. Both diseases are transmissible spongiform encephalopathies (TSE), or prion diseases, caused by autocatalytic conversion of endogenously encoded prion protein (PrP) to an abnormal, neurotoxic conformation designated PrPsc. Most mammalian species are susceptible to TSE, which, despite a range of species-linked names, is caused by a single highly conserved protein, with no apparent normal function. In the simplest sense, TSE transmission can occur because PrPsc is resistant to both endogenous and environmental proteinases, although many details remain unclear. Questions about the transmission of TSE are central to practical issues such as livestock testing, access to international livestock markets, and wildlife management strategies, as well as intangible issues such as consumer confidence in the safety of the meat supply. The majority of BSE cases seem to have been transmitted by feed containing meat and bone meal from infected animals. In the United Kingdom, there was a dramatic decrease in BSE cases after neural tissue and, later, all ruminant tissues were banned from ruminant feed. However, probably because of heightened awareness and widespread testing, there is growing evidence that new variants of BSE are arising "spontaneously," suggesting ongoing surveillance will continue to find infected animals. Interspecies transmission is inefficient and depends on exposure, sequence homology, TSE donor strain, genetic polymorphism of the host, and architecture of the visceral nerves if exposure is by an oral route. Considering the low probability of interspecies transmission, the low efficiency of oral transmission, and the low prion levels in nonnervous tissues, consumption of conventional animal products represents minimal risk. However, detection of rare events is challenging, and TSE literature is characterized by subsequently unsupported claims of species barriers or absolute tissue safety. This review presents an overview of TSE and summarizes recent research on pathogenesis and transmission.

[Elusive function of prion protein]

Prion protein is a highly conserved glycoprotein tethered to cell membranes by a glycosylphosphatidylinositol(GPI) anchor that is expressed in many tissues including brain, heart, and muscle. Although misfolding of the cellular prion protein (PrP(c)) into alternative form, denoted (PrP(Sc)), is a key event in prion infections, the normal function of PrPc remains to be clearly defined. Many PrP(c)-binding proteins have been identified, and several roles for PrP(c) have been suggested, including oxidative stress, cell adhesion, copper uptake, cell survival, protection against oxidative stress, but authentication of these interactions in functional assays is incomplete. In this article, we pick out some researches that pertain to the biology of mammalian prion protein functions.

Cellular prion protein regulates beta-secretase cleavage of the Alzheimer's amyloid precursor protein

Proteolytic processing of the amyloid precursor protein (APP) by beta-secretase, beta-site APP cleaving enzyme (BACE1), is the initial step in the production of the amyloid beta (Abeta) peptide, which is involved in the pathogenesis of Alzheimer's disease. The normal cellular function of the prion protein (PrP(C)), the causative agent of the transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease in humans, remains enigmatic. Because both APP and PrP(C) are subject to proteolytic processing by the same zinc metalloproteases, we tested the involvement of PrP(C) in the proteolytic processing of APP. Cellular overexpression of PrP(C) inhibited the beta-secretase cleavage of APP and reduced Abeta formation. Conversely, depletion of PrP(C) in mouse N2a cells by siRNA led to an increase in Abeta peptides secreted into the medium. In the brains of PrP knockout mice and in the brains from two strains of scrapie-infected mice, Abeta levels were significantly increased. Two mutants of PrP, PG14 and A116V, that are associated with familial human prion diseases failed to inhibit the beta-secretase cleavage of APP. Using constructs of PrP, we show that this regulatory effect of PrP(C) on the beta-secretase cleavage of APP required the localization of PrP(C) to cholesterol-rich lipid rafts and was mediated by the N-terminal polybasic region of PrP(C) via interaction with glycosaminoglycans. In conclusion, this is a mechanism by which the cellular production of the neurotoxic Abeta is regulated by PrP(C) and may have implications for both Alzheimer's and prion diseases.

Doppel induces degeneration of cerebellar Purkinje cells independently of Bax

Doppel (Dpl) is a prion protein paralog that causes neurodegeneration when expressed ectopically in the brain. To investigate the cellular mechanism underlying this effect, we analyzed Dpl-expressing transgenic mice in which the gene for the proapoptotic protein Bax had been deleted. We found that Bax deletion does not alter either clinical symptoms or Purkinje cell degeneration in Dpl transgenic mice. In addition, we observed that degenerating Purkinje cells in these animals do not display DNA fragmentation or caspase-3 activation. Our results suggest that non-Bax-dependent pathways mediate the toxic effects of Dpl in Purkinje cells, highlighting a possible role for nonapoptotic mechanisms in the death of these neurons.

BAX contributes to Doppel-induced apoptosis of prion-protein-deficient Purkinje cells

Research efforts to deduce the function of the prion protein (PrPc) in knock-out mouse mutants have revealed that large deletions in the PrPc genome result in the ectopic neuronal expression of the prion-like protein Doppel (Dpl). In our analysis of one such line of mutant mice, Ngsk Prnp0/0 (NP0/0), we demonstrate that the ectopic expression of Dpl in brain neurons induces significant levels of cerebellar Purkinje cell (PC) death as early as six months after birth. To investigate the involvement of the mitochondrial proapoptotic factor BAX in the Dpl-induced apoptosis of PCs, we have analyzed the progression of PC death in aging NP0/0:Bax-/- double knockout mutants. Quantitative analysis of cell numbers showed that significantly more PCs survived in NP0/0:Bax-/- double mutants than in the NP0/0:Bax+/+ mutants. However, PC numbers were not restored to wildtype levels or to the increased number of PCs observed in Bax-/- mutants. The partial rescue of NP0/0 PCs suggests that the ectopic expression of Dpl induces both BAX-dependent and BAX-independent pathways of cell death. The activation of glial cells that is shown to be associated topographically with Dpl-induced PC death in the NP0/0:Bax+/+ mutants is abolished by the loss of Bax expression in the double mutant mice, suggesting that chronic inflammation is an indirect consequence of Dpl-induced PC death.

The prion protein family: Diversity, rivalry, and dysfunction

The prion gene family currently consists of three members: Prnp which encodes PrP(C), the precursor to prion disease associated isoforms such as PrP(Sc); Prnd which encodes Doppel, a testis-specific protein involved in the male reproductive system; and Sprn which encodes the newest PrP-like protein, Shadoo, which is expressed in the CNS. Although the identification of numerous candidate binding partners for PrP(C) has hinted at possible cellular roles, molecular interpretations of PrP(C) activity remain obscure and no widely-accepted view as to PrP(C) function has emerged. Nonetheless, studies into the functional interrelationships of prion proteins have revealed an interesting phenomenon: Doppel is neurotoxic to cerebellar cells in a manner which can be blocked by either PrP(C) or Shadoo. Further examination of this paradigm may help to shed light on two prominent unanswered questions in prion biology: the functional role of PrP(C) and the neurotoxic pathways initiated by PrP(Sc) in prion disease.

Contributions of neuronal prion protein on sleep recovery and stress response following sleep deprivation

In order to gain insights on the function of the cellular prion protein (PrP(C)) sleep and the levels of the stress hormones corticosterone (CORT) and the adrenocorticotropic hormone (ACTH) before and after sleep deprivation (SD) were compared in two wild type (WT) mice strains and the following three PrP(C) transgenic lines: mice null for PrP(C) (mPrP(0/0)) and mice with specific and central expression of PrP in neurons (NSE-HPrP/mPrP(0/0)) or in glia cells (GFAP-HPrP/mPrP(0/0)). After SD mPrP(0/0) mice showed a larger degree of sleep fragmentation and of latency to enter rapid eye movement (REM) and non-REM sleep (NREM) than WT. During sleep recovery, the amount of NREM sleep and the slow-wave activity (SWA) were reduced in mPrP(0/0) mice. After SD, CORT and ACTH levels have distinct patterns in WT and mPrP(0/0). The NREM and SWA deficit was restored in NSE-HPrP/mPrP(0/0) mice but not in GFAP-HPrP/mPrP(0/0). Hormonal profile was only partially restored in NSE-HPrP/mPrP(0/0) mice and was similar to that of mPrP(0/0) and GFAP-HPrP/mPrP(0/0) mice. These findings demonstrate that neuronal, but not non-neuronal, PrP(C) is involved in sleep homeostasis and sleep continuity. They also suggest that neuronal PrP(c)-dependent hormonal regulation of HPA axis may contribute to the sleep homeostasis.

Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae

The Ure2 protein of Saccharomyces cerevisiae can become a prion (infectious protein). At very low frequencies Ure2p forms an insoluble, infectious amyloid known as [URE3], which is efficiently transmitted to progeny cells or mating partners that consequently lose the normal Ure2p nitrogen regulatory function. The [URE3] prion causes yeast cells to grow slowly, has never been identified in the wild, and confers no obvious phenotypic advantage. An N-terminal asparagine-rich domain determines Ure2p prion-forming ability. Since ure2Delta strains are complemented by plasmids that overexpress truncated forms of Ure2p lacking the prion domain, the existence of the [URE3] prion and the evolutionary conservation of an N-terminal extension have remained mysteries. We find that Ure2p function is actually compromised in vivo by truncation of the prion domain. Moreover, Ure2p stability is diminished without the full-length prion domain. Mca1p, like Ure2p, has an N-terminal Q/N-rich domain whose deletion reduces its steady-state levels. Finally, we demonstrate that the prion domain may affect the interaction of Ure2p with other components of the nitrogen regulation system, specifically the negative regulator of nitrogen catabolic genes, Gzf3p.

M1 and M3 muscarinic receptors control physiological processing of cellular prion by modulating ADAM17 phosphorylation and activity

The cellular prion protein (PrP(c)) undergoes a physiological processing yielding the N-terminal fragment referred to as N1, the production of which can be constitutive or protein kinase C regulated. We show that activation of endogenous muscarinic receptors by carbachol and by the M1-selective agonist AF267B increases N1 recovery in an atropine-sensitive manner, in mouse embryonic primary neurons. To identify the muscarinic receptor subtype involved, we used human embryonic kidney HEK293 (HEK) cells stably overexpressing M1, M2, M3, or M4 receptor subtype. Carbachol and the selective M1 agonist AF267B dose dependently increased N1 release by HEK-M3 and HEK-M1 cells, respectively, whereas carbachol did not modify N1 production by HEK-M2 or HEK-M4 cells. We demonstrate that the increase of N1 was not attributable to modified trafficking to the membrane of either PrP(c) or the disintegrin metalloproteases ADAM10 or ADAM17. Furthermore, we establish that carbachol affects the overall phosphorylation of ADAM17 on its threonine and tyrosine but not serine residues, whereas levels of phosphorylated ADAM9 were not affected. Interestingly, carbachol also increases the hydrolysis of the fluorimetric substrate JMV2770, which mimicked the sequence encompassing the N1 site cleavage and was shown previously to behave as an ADAM protease substrate. Mutations of threonine 735 but not of tyrosine 702 of the ADAM17 cytoplasmic tail abolishes the carbachol-induced increase of N1, ADAM17 phosphorylation, and JMV2770-hydrolyzing activity in M1- and M3-expressing HEK293 cells. Thus, our data provide strong evidence that muscarinic receptor activation increases the physiological processing of PrP(c) by upregulating the phosphorylation state and activity of ADAM17 protease.

Motor behavioral and neuropathological deficits in mice deficient for normal prion protein expression

It has been difficult to reconcile the absence of pathology and apparently normal behavior of mice lacking prion protein (PrP), referred to as Prnp(0/0) mice, with a mechanism of prion pathogenesis involving progressive loss of PrP(C)-mediated neuroprotection. However, here we report that Prnp(0/0) mice exhibit significant age-related defects in motor coordination and balance compared with mice expressing wild type Prnp on a syngeneic background, and that the brains of behaviorally-impaired Prnp(0/0) mice display the cardinal neuropathological hallmarks of spongiform pathology and reactive astrocytic gliosis that normally accompany prion disease. Consistent with the appearance of cerebellar ataxia as an early symptom in patients with Gerstmann-Sträussler-Scheinker syndrome (GSS), an inherited form of human prion disease, motor coordination and balance defects manifested in a transgenic (Tg) mouse model of GSS considerably earlier than the onset of end-stage neurodegenerative disease. Our results are consistent with a mechanism in which loss of normal PrP(C) function is an important pathological component of prion diseases.

Conformational diseases and structure-toxicity relationships: lessons from prion-derived peptides

The physiological form of the prion protein is normally expressed in mammalian cell and is highly conserved among species, although its role in cellular function remains elusive. Available evidence suggests that this protein is essential for neuronal integrity in the brain, possibly with a role in copper metabolism and cellular response to oxidative stress. In prion diseases, the benign cellular form of the protein is converted into an insoluble, protease-resistant abnormal scrapie form. This conversion parallels a conformational change of the polypeptide from a predominantly alpha-helical to a highly beta-sheet secondary structure. The scrapie form accumulates in the central nervous system of affected individuals, and its protease-resistant core aggregates into amyloid fibrils outside the cell. The pathogenesis and molecular basis of the nerve cell loss that accompanies this process are not understood. Limited structural information is available on aggregate formation by this protein as the possible cause of these diseases and on its toxicity. A large amount of structure-activity studies is based on the prion fragment approach, but the resulting information is often difficult to untangle. This overview focuses on the most relevant structural and functional aspects of the prion-induced conformational disease linked to peptides derived from the unstructured N-terminal and globular C-terminal domains.

Efficient dissemination of prions through preferential transmission to nearby cells

Despite circumstantial evidence that prions can be found extracellularly or at the surface of infected cells, little is known about how these infectious agents spread from cell to cell. In order to gain better insight into this critical issue, this study used two different cell lines (neuroglial MovS and epithelial Rov cells) that have previously been shown to be permissive for ovine prion multiplication. Co-culture of infected cells and uninfected target cells at a ratio of 1 : 9 resulted in total infection of MovS cells within 10 days but not of Rov cell cultures, suggesting that the efficiency of prion dissemination may vary greatly depending on the type of permissive cell. Analysis of the spatial distribution of the newly infected cells revealed that, although long-range spread could also occur, cells proximal to the infected donor cells consistently accumulated more abnormal PrP, consistent with preferential infection of nearby cells. This experimental approach, focused on dissemination among living cells, could help in the analysis of mechanisms involved in the cell-to-cell spread of prion infections.

Accumulation of prion protein in the brain that is not associated with transmissible disease

Prion diseases or transmissible spongiform encephalopathies are characterized histopathologically by the accumulation of prion protein (PrP) ranging from diffuse deposits to amyloid plaques. Moreover, pathologic PrP isoforms (PrP(Sc)) are detected by immunoblot analysis and used both as diagnostic markers of disease and as indicators of the presence of infectivity in tissues. It is not known which forms of PrP are associated with infectivity. To address this question, we performed bioassays using human brain extracts from two cases with phenotypically distinct forms of familial prion disease (Gerstmann-Sträussler-Scheinker P102L). Both cases had PrP accumulations in the brain, but each had different PrP(Sc) isoforms. Only one of the brains had spongiform degeneration. Tissue from this case transmitted disease efficiently to transgenic mice (Tg PrP101LL), resulting in spongiform encephalopathy. In contrast, inoculation of tissue from the case with no spongiform degeneration resulted in almost complete absence of disease transmission but elicited striking PrP-amyloid deposition in several recipient mouse brains. Brains of these mice failed to transmit any neurological disease on passage, but PrP-amyloid deposition was again observed in the brains of recipient mice. These data suggest the possible isolation of an infectious agent that promotes PrP amyloidogenesis in the absence of a spongiform encephalopathy. Alternatively, the infectious agent may be rendered nonpathogenic by sequestration in amyloid plaques, or PrP amyloid can seed amyloid accumulation in the brain, causing a proteinopathy that is unrelated to prion disease. Formation of PrP amyloid may therefore not necessarily be a reliable marker of transmissible spongiform encephalopathy infectivity.

N-terminally deleted forms of the prion protein activate both Bax-dependent and Bax-independent neurotoxic pathways

Transgenic (Tg) mice expressing prion protein (PrP) with a deletion of the flexible, N-terminal tail encompassing residues 32-134 spontaneously develop ataxia, degeneration of cerebellar granule cells, and vacuolation of white matter in the brain and spinal cord, resulting in death by 3 months of age. These abnormalities are completely abrogated by coexpression of wild-type PrP from a single copy of the endogenous Prn-p gene. A similar but much more severe phenotype is seen in transgenic mice expressing PrP deleted for a conserved block of 21 amino acids (residues 105-125) within the N-terminal tail. The latter animals die within 1 week of birth in the absence of endogenous PrP, and fivefold overexpression of wild-type PrP is required to delay death beyond 1 year. To define the cellular pathways mediating the neurotoxicity of PrPdelta32-134 and PrPdelta105-125, we analyzed the effect of genetically deleting the proapoptotic protein Bax in mice expressing these neurotoxic forms of PrP. We find that Bax deletion in Tg(PrPdelta32-134) mice delays the development of clinical illness and slows apoptosis of cerebellar granule cells but has no effect on white matter degeneration. In contrast, Bax deletion has no effect on the clinical or neuropathological phenotype of Tg(delta105-125) mice. Our results indicate that Bax-related pathways mediate the initial neurotoxic actions of PrPdelta32-134 but that neurodegeneration induced by this protein as well as by PrPdelta105-125 also involves Bax-independent pathways.

Prions: protein only or something more? Overview of potential prion cofactors

Transmissible spongiform encephalopathies (TSEs) in humans and animals are attributed to protein-only infectious agents, called prions. Prions have been proposed to arise from the conformational conversion of the cellular protein PrP(C) into a misfolded form (e.g., PrP(Sc) for scrapie), which precipitates into aggregates and fibrils. It has been proposed that the conversion process is triggered by the interaction of the infectious form (PrP(Sc)) with the cellular form (PrP(C)) or might result from a mutation in the gene for PrP(C). However, until recently, all efforts to reproduce this process in vitro had failed, suggesting that host factors are necessary for prion replication. In this review we discuss recent findings such as the cellular factors that might be involved in the conformational conversion of prion proteins and the potential mechanisms by which they could operate.