©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Prion Protein Isoforms, a Convergence of Biological and Structural Investigations (*)

Michael A. Baldwin (1) Fred E. Cohen (2) (4) Stanley B. Prusiner (1) (4)(§)

From the  (1)Departments of Neurology, (2)Medicine, (3)Pharmaceutical Chemistry, and (4)Biochemistry and Biophysics, University of California, San Francisco, California 94143-0518

INTRODUCTION
Prion Diseases
Identification and Properties of PrP
Spectroscopy of PrP and Peptides
Inherited Prion Diseases
Structural Models of PrP^C
Prion Infectivity
Barriers to Prion Infection between Species
Future Directions
FOOTNOTES
REFERENCES


INTRODUCTION


Prion Diseases

Prion diseases are fatal neurodegenerative disorders afflicting humans and other mammals. Manifest as sporadic, inherited, and infectious illnesses, they seem to be the result of a conformational transition in the normal, cellular prion protein (PrP^C) (^1)whereby the aberrant, diseased protein isoform (PrP) is formed(1) . The transmissible particle or prion is composed largely, if not exclusively, of PrP and is resistant to procedures that modify nucleic acids(2, 3) . No candidate scrapie-specific polynucleotide has been identified(4) . The name prion was coined for the proteinaceous infectious agent (3) , and the designation PrP was derived from scrapie, an infectious disease of sheep known for at least 200 years that has been transmitted to hamsters and mice(5, 6) . PrP^C and PrP have been purified from the brains of healthy and scrapie-infected Syrian hamsters, respectively. Among a plethora of early proposals as to the possible causes of scrapie were suggestions of a protein devoid of nucleic acid(2, 7, 8, 9) . Over the last two decades, substantial evidence has been gathered suggesting that PrP is likely to be the only component of the infectious prion particle.

More than 150,000 cattle have died of a new prion disease called bovine spongiform encephalopathy (BSE) or ``mad cow disease''(10) , believed to have originated from cattle feed infected with sheep scrapie prions. A 1988 ban on the feed produced from the offal of domestic animals has begun to control the epidemic; in 1992, 37,000 cases of BSE were recorded while only 22,000 were diagnosed in 1994.

Prion diseases of humans include Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Schienker syndrome (GSS), and fatal familial insomnia (FFI)(11, 12, 13) . Generally patients with GSS present with ataxia and cerebellar dysfunction while those with CJD exhibit memory loss and a progressive decline of intellect. GSS and FFI are familial diseases whereas CJD can be sporadic, familial, or infectious. The infectious human prion diseases are rare, but kuru spread through ritualistic cannibalism was once the most common fatal illness among the Fore people of Papua New Guinea(11) . Iatrogenic CJD is another infectious prion disease resulting from inadequately sterilized surgical instruments, dura mater grafts, and human growth hormone isolated from the pituitaries of cadavers. Prion diseases vary in their course and rapidity of development, but they all cause neurodegeneration and ultimately death. Autopsy generally reveals spongiform degeneration of the brain, sometimes with amyloid plaques that stain with Congo red and react with alpha-PrP antibodies.


Identification and Properties of PrP

The protease-resistant fragment of PrP called PrP 27-30 was discovered in fractions partially purified from Syrian hamster (SHa) brain by monitoring the enrichment of scrapie infectivity with respect to protein(14, 15, 16) . Differential solubilization with detergents, limited proteolysis to remove extraneous proteins and centrifugation were used to purify scrapie prions. The purification was extended using discontinuous sucrose gradient centrifugation. PrP 27-30 aggregates to form amyloid rods that stain with Congo red dye to display green-gold birefringence under polarized light(17) . N-terminal sequencing of PrP 27-30 revealed a ragged N terminus(18) . The N-terminal amino acid sequence allowed the selection of a clone from a hamster brain cDNA library and the identification of the PrP gene (19, 20) . Subsequently, both PrP^C and PrP were found to be encoded within a single exon of the chromosomal gene as proteins of 254 amino acids(21) , although they display quite different physical properties(22) .

N-terminal sequencing of full-length PrP^C(23) and PrP(23, 24, 25) revealed the posttranslational loss of an N-terminal 22-residue signal peptide. A 23-residue C-terminal peptide is also lost upon addition of a glycosylphosphatidylinositol (GPI) anchor, identified by the release of 2.8 mol of ethanolamine/mol of PrP together with myoinositol, phosphate, and stearic acid in acid hydrolysates of PrP 27-30(26) . Although PrP has a highly conserved hydrophobic domain, it is not an integral membrane protein but is attached to cell surface membranes by its GPI, which is unusual in that it is sialylated(27) . The majority of PrP molecules carry bi-, tri-, and tetraantennary neutral and N-linked sialylated oligosaccharides at two sites(28, 29) . A disulfide bond links the only two cysteines in the mature protein(23) . Peptide mapping of PrP confirmed that the amino acid sequence corresponds to that predicted from the gene sequence(30) . The major posttranslational modifications summarized in Fig. 1A are essentially the same for both PrP^C and PrP, although detailed comparisons of the heterogeneous N-linked sugars and GPI anchors are not completed.


Figure 1: PrP structure. A, posttranslational modifications; B, normal polymorphisms (underlined) and pathogenic mutations to human PrP (above the line), and the boundaries of the four putative helices of PrP^C (below the line).



PrP^C can be solubilized with low concentrations of detergents such as Zwittergent 3-12; it has been purified from hamster and mouse brains by a number of methods, most recently employing detergent solubilization, centrifugation, metal ion affinity chromatography, ion exchange, and lectin affinity(31) . Purified PrP^C does not display the resistance to proteolysis that characterizes PrP. Non-denatured PrP is insoluble but does not form ordered arrays, whereas PrP 27-30 polymerizes into highly ordered macroscopic amyloid structures(17) . Solubilization or disruption of these rod-shaped amyloids by denaturants such as boiling SDS, alkali, or guanidinium thiocyanate inactivate prion infectivity(32) . On negatively stained electron micrographs, PrP^C and PrP appear as amorphous deposits but were immunolabeled with alpha-PrP antibodies; PrP 27-30 rods were readily visualized without immunolabeling (Fig. 2)(31) .


Figure 2: Electron micrographs of PrP^C (A), PrP (B), and PrP 27-30 (C). Immunogold labeling was employed for A and B. (Reproduced from (31) ).



The polymerization of PrP 27-30 into amyloids and the presence of PrP amyloid plaques in some prion diseases have led to assumptions that amyloid formation is essential for the formation of PrP(33, 34, 35, 36, 37, 38) . However, PrP can be formed in the absence of amyloid, and the presence of amyloid plaques is not obligatory for prion diseases.

The existence of different prion isolates or strains with characteristic incubation times and pathology poses an enigma(39, 40, 41, 42) . As prion diseases are apparently caused by an abnormal conformation of PrP, different strains might correspond to distinct stable conformations of PrP. Alternatively, a second component might mediate the properties of PrP. Still another possibility is a labile or infrequent covalent change that induces different properties in PrP.


Spectroscopy of PrP and Peptides

The physical properties of PrP have made it difficult to obtain crystals suitable for x-ray diffraction. Only recently has a purification of PrP^C been developed for undenatured protein(31) . Fourier transform infrared (FTIR) and circular dichroism (CD) have provided information on the secondary structures of PrP 27-30, PrP, and PrP^C(31) (Fig. 3). PrP^C is essentially alpha-helical and devoid of beta-sheet, whereas the beta-sheet content of PrP is 40% (31) and the truncated form, PrP 27-30, contains more than 50% beta-sheet(31, 43, 44, 45) . CD also demonstrated the remarkable thermal stability of the structure of PrP(46) . Procedures that inactivate the infectivity of PrP 27-30 and PrP such as purification by SDS-polyacrylamide gel electrophoresis or treatment with alkali also cause a substantial reduction in the beta-sheet content(44, 45, 46) . Having identified no covalent differences between the two PrP isoforms, we suggested that the conversion of PrP^C to PrP is a conformational change(30) , involving a transition from alpha-helical to beta-sheet structure(31) .


Figure 3: A, FTIR spectra of the amide region for PrP^C (solid line), PrP (dashed line), and PrP 27-30 (dotted line) (reproduced from (31) ). B, CD spectrum of PrP^C immunopurified from hamster brain.




Inherited Prion Diseases

Nineteen mutations of the PrP gene are associated with inherited human prion disease (see Fig. 1B), five being linked statistically. A Pro Leu mutation at codon 102, the first point mutation identified(47) , causes spontaneous transmissible neurodegenerative disease (GSS) in transgenic (Tg) mice(48) . Inherited prion diseases display different phenotypes and are often referred to as familial CJD, GSS, and FFI. Of particular interest is the D178N mutation, which is linked to inherited prion disease. Patients with a Val-129 (a polymorphic site) on the mutant allele present with familial CJD but with FFI if they have a Met-129(49) . The Met/Val polymorphism at 129 does not cause disease but Met or Val homozygosity appears to predispose to sporadic prion disease in Caucasians (50) but not Asians (51) .


Structural Models of PrP^C

Alignment of 11 mammalian and 1 avian PrP sequences revealed several regions of extreme conservation, punctuated by more variable segments(52, 53) . Structure prediction algorithms identified four potential regions of secondary structure, which in PrP^C might form alpha-helices suggesting a 4-helix bundle protein(54) . The four putative alpha-helical regions H1-H4 illustrated in Fig. 1B are within the protease-resistant core of PrP 27-30 (residues 90-231). Potential hydrophobic helix-helix interaction sites were identified, and combinatorial packing revealed 300,000 possible three-dimensional arrangements of the helices. Approximately 200 structures were connected, sterically reasonable, capable of forming the disulfide bridge between Cys-179 and Cys-214, and compatible with solvent constraints posed by the glycosylation. Four families of plausible structures that exhibited a sensible ratio of solvent-accessible surface area to molecular volume were identified and analyzed for the spatial clustering of pathogenic point mutations. The X-bundle model illustrated in Fig. 4is most susceptible to disruption by these mutations (55) and shows close proximity between the codon 178 mutation that induces familial CJD or FFI and the codon 129 polymorphism that determines the phenotype.


Figure 4: Schematic representations of the four-helix bundle of PrP 27-30 from modeling studies. Left, predicted helix-helix interaction sites are shown in green. Right, positions of mutations within the alpha-helices that segregate with inherited prion diseases and the codon 129 polymorphism are shown in red. (Reproduced from (55) .)



The four putative helices were synthesized as peptides of 13-17 amino acids. Their secondary structures were probed by attenuated total reflection FTIR spectroscopy. When dried from a helix-promoting solvent such as 1,1,1,3,3,3-hexafluoroisopropanol, the peptides displayed spectra indicative of alpha-helices with amide I bands maximizing at 1650-1660 cm. However, on addition of aqueous buffers, only the spectrum of H2 remained unchanged. The other three immediately adopted beta-sheet conformations, with dramatic shifts of the amide I peaks to 1620-1626 cm(54) . Further, the three peptides that favored beta-sheet conformation slowly precipitated from aqueous solution as amyloid fibrils. This conformational instability was consistent with ambiguities in the secondary structure predictions. Alternative prediction algorithms consistently identified the same four regions of secondary structure in PrP, but differed as to whether they would form alpha-helices or beta-sheets(55) .

Other synthetic PrP peptides have a high beta-sheet content and form amyloid. The sequence 106-126, which embraces H1(109-122), forms fibrils and is cytotoxic to hippocampal neurons in culture, particularly at pH 5 compared with pH 7(56, 57) . A search of protein data bases for the repeating motif (XGXX), where X is small and hydrophobic identified the PrP sequence 118-133 as amyloidogenic(58) ; fibril formation was favored in the pure Met-129 peptide compared with mixed Met-129 and Val-129(36) . The fibrillogenic properties of PrP peptides were reported to be enhanced by the presence of the mutations D178N and E200K(59) .

An alpha-helix to beta-sheet model of PrP formation is supported by a consideration of the known PrP point mutations, only one of which (F198S) involves a change to an amino acid having a lower beta-sheet propensity(60, 61) . These predict that the Pro-102/Leu-105 mutations should give the most dramatic change, consistent with the codon 102 mutation causing spontaneous transmissible disease in transgenic mice(62) . It is likely that key residues can be identified that may have an even greater influence on the conformational transition which generates prions de novo. It is interesting that all but one of the known point mutations fall within the region corresponding to PrP 27-30 and are clustered within or adjacent to the four putative helices (Fig. 1). However, inserts of multiple additional copies of an 8-amino acid repeat outside of the PrP 27-30 region also cause inherited prion disease(63, 64, 65, 66) .

Determination of the tertiary structure experimentally may yield important clues to the quaternary structure. Scrapie infectivity is enhanced by dispersion of the rods in detergent-protein-lipid complexes (67) ; thus, aggregation is not a necessary requirement for infectivity. Inactivation of prions by ionizing radiation suggested a target size of 55 kDa, implying a dimer of PrP as the fundamental unit (68) . The tertiary structure model for PrP^C reveals hydrophobic faces that could be involved in dimer formation(55) .


Prion Infectivity

One striking aspect of prion diseases is that they can be manifest as either genetic or infectious disorders. Infectious prions consist predominantly or perhaps entirely of PrP. Species barriers that inhibit interspecies transmission of prions are due, at least in part, to species-specific variations in PrP amino acid sequences(7, 69, 70, 71) . Ablation of the PrP gene in Prn-p mice does not affect development of these animals(72) , but renders them resistant to prions and they do not propagate scrapie infectivity (73, 74) .

The formation of nascent PrP appears to involve interaction between PrP^C and PrP, which is most efficient when both have the same primary structure. This interaction, which induces a transition from an alpha-helix-rich to a beta-sheet-rich conformer, can be modeled with PrP peptides(75) . For example, the highly conserved hydrophobic tetradecapeptide H1 (residues 109-122) has the ability to convert other PrP peptides from alpha-helix or coil conformations to beta-sheet structure, including peptide H2 which does not spontaneously adopt beta-sheet as an isolated peptide in aqueous buffers. Longer more hydrophilic peptides containing the H1 sequence, e.g. 104H1 (residues 104-122) which is random coil in aqueous buffer, can also be converted by interaction with H1. Only a small fraction of 104H1 spontaneously converts to beta-sheet in buffers containing up to 30% acetonitrile, whereas the addition of a small amount of H1 induces an alphabeta shift in a large fraction of molecules, e.g. in 10% acetonitrile, 1% H1 is sufficient to convert 25% of the 104H1 to beta-sheet and 10% H1 converts all the 104H1(75) .


Barriers to Prion Infection between Species

Difficulties in transmitting scrapie from one animal species to another (7) were also observed in transmission of human diseases such as kuru and CJD to various animal species, including non-human primates (76) . This is partly due to the reduced efficiency of interactions between endogenous PrP^C and exogenous PrP that differ in their amino acid sequence. Thus, transgenic mice that express both mouse and hamster PrP can be infected with either mouse or hamster prions; the prions that result from these infections retain their specificities. While Prn-p mice are resistant to scrapie prions, they are rendered susceptible to hamster prions after being crossed with transgenic mice expressing hamster PrP^C(73, 74) . Species barriers can be simulated with synthetic peptides, e.g. the conversion of hamster 104H1 to beta-sheet is faster when the interacting species is hamster H1 rather than mouse H1 which differs at two positions(75) .

Due to PrP sequence differences between mouse and human (28 residues), only 10% of mice develop disease with long incubation times >1 year following intracerebral inoculation with human CJD prions. Surprisingly, the efficiency of infection was not improved when the human PrP gene was expressed in Tg mice. A chimeric PrP gene (MHu2M) having the human sequence in the central region from codons 96 to 167 was introduced into the Tg mice, all of which developed a fatal CJD-like illness within 230 days of inoculation with CJD prions (71) . This suggests that a host factor(s) such as a chaperone, designated ``protein X'', plays a role in the conversion of PrP^C into PrP. The Syrian hamster (SHa) and mouse PrPs are sufficiently similar that SHaPrP can interact with mouse protein X while the affinity of human PrP for protein X is probably much lower. The Tg mice expressing human/mouse chimeric PrP^C should provide a valuable tool for evaluating public health risks posed by prion-tainted foodstuffs, pharmaceuticals, or other products derived from potentially contaminated human or animal sources.

Significant differences observed in the ease of transmission of CJD to various apes and monkeys do not follow classical phylogenic patterns (77) . Twenty-five non-human primate PrP genes were sequenced to learn which amino acid positions were most critical in determining species barriers(70) . Compared with human PrP, amino acid identities ranged from 92.9 to 99.6%. The most critical region proved to correspond to the N terminus of PrP 27-30 and included the putative helix H1. Three highly susceptible species including chimpanzees are identical in the region 90-130, whereas two species that are poor hosts for human prions have a Met Val substitution at codon 112. It is noteworthy that the majority of the sequence variations fall outside the predicted regions of secondary structure, in contrast to the pathogenic mutations, which cluster within or adjacent to these regions(55) . An understanding of the molecular basis of species barriers is important in appraising the risk posed by the epidemic of mad cow disease in Great Britain(78) .


Future Directions

Our present understanding of prion diseases is attributable to a breadth of studies using a variety of disciplines including genetics, molecular biology, cell biology, and protein chemistry. Molecular modeling and spectroscopic studies of peptides have opened new avenues of investigation. Improvements in synthesis of longer peptides should allow greater portions of the PrP molecule to be studied. Attempts to create infectivity by in vitro conversion of entirely synthetic peptides or proteins may show whether any undetected chemical modification of PrP or a second molecule is required for prion infectivity.

Ultimately an appreciation of the molecular processes underlying the conversion of PrP^C to PrP could facilitate the design or discovery of therapeutics to inhibit the development of prion diseases. Intriguing similarities between prion diseases, Alzheimer's disease, and Parkinson's disease (79) suggest common mechanisms of pathogenesis may feature in these illnesses. A process similar to prion formation may explain cytoplasmic inheritance of some traits in yeast(80) . Thus, it may transpire that the exciting new field of prion biology has much wider significance than previously appreciated.


FOOTNOTES

*
This minireview will be reprinted in the 1995 Minireview Compendium, which will be available in December, 1995.

§
To whom all correspondence should be addressed: Dept. of Neurology, HSE-781, University of California, San Francisco, CA 94143-0518; Tel.: 415-476-4482; Fax: 415-476-8386.

(^1)
The abbreviations used are: PrP, prion protein; BSE, bovine spongiform encephalopathy; CJD, Creutzfeldt-Jakob disease; GSS, Gerstmann-Sträussler-Schienker syndrome; FFI, fatal familial insomnia; GPI, glycosylphosphatidylinositol; FTIR, Fourier transform infrared; Tg, transgenic.


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