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Scrapie and GSS — the importance of protein
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neuronal surface glycoprotein, PrP, aggregates in all transmissible spongiform encephalopathies, including scrapie, Gerstmann-Stdiussler-Scheinker syndrome (GSS) and bovine spongiform encephalopathy (BSE). Unlike normal PrP, the aggregated form is partially resistant to proteases and is a disease-specific marker. However, evidence from studies of transgenic mice expressing variants of the PrP gene now suggests that it controls the host range of experimental scrapie and will also, in a mutant form linked to incidence of GSS, independently produce spongiform disease pathology. There are two major hypotheses about what causes scrapie-like diseases - both emphasize the importance of PrP protein. In one of these hypotheses, the proteaseresistant, disease-associated form of PrP (PrP sc) is itseff the infecting agent (or prion) 1. PrP sc arising from an infection or from a mutant PrP gene acts as a catalyst in the conversion of the endogenous normal protein (PrP c) into more PrP sc, so that the normal function of the protein is destroyed and degenerative disease results. This idea has its origin in the sensitivity of scrapie infectivity to proteases and resistance to treatments expected to inactivate nucleic acids2. In the other hypothesis, the normal protein (PrP) acts as a receptor for the as yet uncharacterized scrapie agent 3. Many different strains of scrapie with hostindependent characteristics (such as incubation period and clinical target areas) have been described, and this is difficult to reconcile with the idea that disease is caused by a single host protein. Proponents of this hypothesis, therefore, believe in an independent scrapie genome (probably a small nucleic acid)4. There is no evidence of a conventional virus, however, so this agent is called a virino or unconventional virus. Irrespective of which school of thought is favoured, the recent transgenic studies are fascinating. When the hamster PrP (HaPrP) gene was inserted into the mouse genome, it changed the response of the mouse to a hamsterpassaged strain of scrapie in a copy TINS, Vol. 14, No. 9, 1991
$crapieandG$$- theimportanceofprotein number-dependent way1. Transgenic mouse lines became susceptible to the hamster scrapie agent with incubation periods of 48-277 days, the length of which were related inversely to the levels of HaPrP mRNA and protein produced. Non-transgenic mice were much more resistant to hamster scrapie, with only a few succumbing more than 400 days after inoculation. The type of scrapie pathogen inoculated into the transgenic mice was replicated and reproduced; i.e. injection with the hamster scrapie agent resulted in high titres of the hamster scrapie agent, while a mouse-passaged scrapie strain produced high titres of the mouse scrapie agent. Pathology typical of hamster scrapie disease was found in brain tissue from dying transgenic mice, and included large deposits or plaques of PrP protein that reacted with HaPrP-specific monoclonal antibodies. One interpretation of these results is that HaPrP sc in the inoculum is the sole cause of the disease, acting as a template for the conversion of the HaPrP c protein in the transgenic mice into more HaPrP sc. In this speciesspecific interaction, the infecting hamster PrP sc only inefficiently affects the endogenous mouse PrP c protein, and the protease° resistant PrP protein deposited in the brain plaques of transgenic mice is HaPrP sc. Subsequent passage, therefore, gives positive transmission only in hamsters and not in normal mice. Transgenic mice expressing the HaPrP gene and normal mice both express mouse PrP c and are therefore both susceptible to the mousederived scrapie agent, which has mouse PrP sc in the inoculum. From the other viewpoint, however, similarities of scrapie pathology with a recent study of the poliovirus receptor (PVR) is striking. Transgenic mice expressing human PVR become susceptible to poliovirus; normal mice are resistant s. As the transgenic mice expressing PVR developed this primate disease, they exhibited appropriate pathological changes.
Could the HaPrP protein be the hamster scrapie agent receptor, allowing the hamster-passaged scrapie agent to infect transgenic mouse cells that are normally resistant because this hamster scrapie agent does not usually 'fit' with the mouse PrP protein? Transgenic mice expressing the HaPrP gene and normal mice would both be susceptible to mouse scrapie, since both carry the endogenous mouse PrP protein/scrapie receptor. A PrP variant has now been directly implicated in causing spongiform encephalopathy 6. Gerstmann-Str/iussler-Scheinker syndrome (GSS) is a rare scrapie-like human disease characterized by spongiform pathology and proteaseresistant PrP protein and plaques. In affected families it has the appearance of an autosomal dominant inherited disorder. Transmission of neurological disease to marmosets has been achieved using brain tissue from members of one of these GSS families - disease was further transmissible by passage, but the PrP plaques typical of GSS in human brain were not seen 7. In 1989, an examination of two GSS families revealed an apparent linkage between incidence of the disease and a PrP protein mutationS: a proline to leucine substitution at codon 102. In order to test the significance of this mutation, mice were made transgenic for a PrP gene (GSSPrP) that harboured the same proline to leucine change. These transgenic mice spontaneously began to die from spongiform neurological disease at around 166 days of age without prior exposure to scrapie or GSS. The brains of affected animals exhibited typical scrapie-like vacuolation but no plaques or proteaseresistant PrP were observed. This work suggests that the amino acid change in the PrP gene in the GSS family represents a genetic lesion and may be the cause of the spongiform neurodegeneration 6. However, the transgenic mice with the GSSPrP protein have more than 60 copies of the mutant PrP transgene, and overexpression of
@ 1991, ElsevierSciencePublishersLtd, (UK) 0166- 2236/91/$0200
Nora Hunter
Institutefor Anima/ Health,AFRC/MRC Neuropathogenesis Unit, Ogston Building, WestMains Road,EdinburghEH9 3JF, UK.
389
the PrP protein may complicate the pathology. The question of transmissibility is still unclear. Attempts to transmit infectivity from the affected GSSPrP transgenic mouse brains to normal mice are in progress. If transmission is confirmed, a form of PrP would be more firmly estabfished as the infectious molecule, but the question of how such a host molecule might encode host-independent phenotypes would remain. The reason for the absence of PrP protein plaques in both the GSSinfected marmosets and the GSSPrP transgenic mouse brains (in contrast to hamster scrapieaffected H a P r P transgenic mice and human GSS patients) is also unexplained. Arguments about the precise role of PrP protein continue. It may itself cause disease by seeding the aggregation of the homologous normal PrP protein, and as a result apparently infect and replicate in a species-specific manner. However, the responses of mice transgenic for HaPrP are not inconsist-
p e r s p e c t i v e s
.
ent with the idea of PrP variants acting as scrapie pathogen receptors and (like conventional viral receptors) controlling host range 9. Some disease-specific factor undoubtedly interacts with normal PrP protein and changes it to the protease-resistant form. Depending on the point of view this is either PrP sc (Ref. 1) or an independent scrapie pathogen 1°. The GSSPrP mutation either causes a genetic and transmissible disease or it produces a change in protein structure or stability, which mimics the effect of pathogen binding. Transgenic models, with multiple copies of the P r P transgenes, have increased our understanding of scrapie-like diseases but have still not completely clarified the role of PrP protein. The next step must surely be to create, by homologous recombination, mouse lines with precise changes in the endogenous P r P gene and with no extra copies. For instance, mice with non-functional P r P genes may help elucidate the normal function
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of PrP in the nervous system and, if viable, might be used to show once and for all if this host protein is really needed for replication of the scrapie agent.
Selected references 1 Prusiner, S. B. et al. (1990) Cell 63, 673-686 2 Prusiner, S. B. et aL (1983) Cell 35, 349-358 3 Hope, J. and Baybutt, H. (1991) 5emin. Neurosci. 3, 165-171 4 Bruce, M. E. and Dickinson, A. G. (1987) J. Gen. ViroL 68, 79-89 5 Ren,R., Constantini,F., Gorgacz,E.J., Lee, J. J. and Racaniello,V. R. (1990) Cell 63, 353-363 6 Hsiao, K. et aL (1990) Science 250, 1587-1590 7 Baker, H. F., Duchen, L. W., Jacobs, J. M. and Ridley, R. M. (1990) Brain 113, 1891-1909 8 Hsiao, K. et aL (1989) Nature 338, 342-344 9 Weissmann, C. (1991) Nature 349, 569-571 10 Hope,J., Reekie, L. J. D. and Gibson, P. H. (1986) in Unconventional Virus Diseases of the Central Nervous System (Court, L. A., Dormont, D., Brown, P. and Kingsbury,D. T., eds), pp. 536-546, Commissariat a l'Energie Atomique, Paris
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How chicksmakememories:the cellularcascadefrom c-los to dendriticremodelling Steven P. R. Rose StevenP. R. Roseis in the Brainand BehavlourResearch Group, Open University, Milton KeynesMK7 6AA, UK.
Training chicks on a one-trial passiveavoidancetask results in a cellular cascadeover the subsequent hours. Phosphorylation of the presynapticphosphokinaseC substrateB-50 is followed by immediateearly gene expressionandincreasedsynthesisof pre- and postsynaptic glycoproteins, increasesin dendntic spine densities, synapse and synaptic vesicle numbers, and a prolonged increase in neuronal bursting. Many of these effects have been localizedto two forebrain regions: the left intermediate media/hyperstriatum ventrale and the Iobus paro/factorius. Pretraininglesionsin the left intermediate media/ hyperstriatum ventrale, or post-training lesionsin the Iobusparolfactonus result in amnesia. Theseand related results lead to models of memory storage basedon multiple representationby way of synaptic stabilization through glycoprotein synaptic recognition molecules. Young chicks peck spontaneously at small bright objects in their field of view. If the object is a coloured bead dipped in a bitter-tasting liquid, the chick will subsequently avoid pecking at even a dry bead of that colour and shape, although its general pecking activity is unimpaired. This behaviour forms the basis of the 'one-trial passive avoidance' learning task introduced by Cherkin more than two decades ago I. It is but one of the large repertoire of forms of early learning about key features of its environment - such as mother (imprinting) 2 and
390
edible food - that the naive but precocial young chick, which, hatched with a large brain and considerable behavioural competence, must achieve rapidly if it is to survive. Avian learning, from canary song to food storage in marsh tits, is becoming increasingly popular as offering model systems in which to study vertebrate memory formation, and the chick, with its repertoire of strongly ontogenetically driven learning, is particularly attractive 3. Learning to suppress pecking at the bitter bead initiates an intracellular cascade of cellular processes, which, beginning with pre- and postsynaptic membrane transient changes and proceeding by way of genomic activation to the lasting structural modification of these membranes, occurs in identified regions of the chick forebrain. These synaptic modifications may form in some way the neural representations of the aversive bead-pecking experience, and encode the instructions for the changed behaviour (i.e. to avoid pecking a bead of these characteristics) that follows. This article reviews the key steps that have been identified as occurring in this cascade, and discusses what these processes can reveal about the mechanisms and nature of memory storage in vertebrates.
© 1991.ElsevieSci r encePublishersLid,(UK) 0166-2236/91/$02.00
TINS, VOI. 14, NO. 9, 1991
neuronal surface glycoprotein, PrP, aggregates in all transmissible spongiform encephalopathies, including scrapie, Gerstmann-Stdiussler-Scheinker syndrome (GSS) and bovine spongiform encephalopathy (BSE). Unlike normal PrP, the aggregated form is partially resistant to proteases and is a disease-specific marker. However, evidence from studies of transgenic mice expressing variants of the PrP gene now suggests that it controls the host range of experimental scrapie and will also, in a mutant form linked to incidence of GSS, independently produce spongiform disease pathology. There are two major hypotheses about what causes scrapie-like diseases - both emphasize the importance of PrP protein. In one of these hypotheses, the proteaseresistant, disease-associated form of PrP (PrP sc) is itseff the infecting agent (or prion) 1. PrP sc arising from an infection or from a mutant PrP gene acts as a catalyst in the conversion of the endogenous normal protein (PrP c) into more PrP sc, so that the normal function of the protein is destroyed and degenerative disease results. This idea has its origin in the sensitivity of scrapie infectivity to proteases and resistance to treatments expected to inactivate nucleic acids2. In the other hypothesis, the normal protein (PrP) acts as a receptor for the as yet uncharacterized scrapie agent 3. Many different strains of scrapie with hostindependent characteristics (such as incubation period and clinical target areas) have been described, and this is difficult to reconcile with the idea that disease is caused by a single host protein. Proponents of this hypothesis, therefore, believe in an independent scrapie genome (probably a small nucleic acid)4. There is no evidence of a conventional virus, however, so this agent is called a virino or unconventional virus. Irrespective of which school of thought is favoured, the recent transgenic studies are fascinating. When the hamster PrP (HaPrP) gene was inserted into the mouse genome, it changed the response of the mouse to a hamsterpassaged strain of scrapie in a copy TINS, Vol. 14, No. 9, 1991
$crapieandG$$- theimportanceofprotein number-dependent way1. Transgenic mouse lines became susceptible to the hamster scrapie agent with incubation periods of 48-277 days, the length of which were related inversely to the levels of HaPrP mRNA and protein produced. Non-transgenic mice were much more resistant to hamster scrapie, with only a few succumbing more than 400 days after inoculation. The type of scrapie pathogen inoculated into the transgenic mice was replicated and reproduced; i.e. injection with the hamster scrapie agent resulted in high titres of the hamster scrapie agent, while a mouse-passaged scrapie strain produced high titres of the mouse scrapie agent. Pathology typical of hamster scrapie disease was found in brain tissue from dying transgenic mice, and included large deposits or plaques of PrP protein that reacted with HaPrP-specific monoclonal antibodies. One interpretation of these results is that HaPrP sc in the inoculum is the sole cause of the disease, acting as a template for the conversion of the HaPrP c protein in the transgenic mice into more HaPrP sc. In this speciesspecific interaction, the infecting hamster PrP sc only inefficiently affects the endogenous mouse PrP c protein, and the protease° resistant PrP protein deposited in the brain plaques of transgenic mice is HaPrP sc. Subsequent passage, therefore, gives positive transmission only in hamsters and not in normal mice. Transgenic mice expressing the HaPrP gene and normal mice both express mouse PrP c and are therefore both susceptible to the mousederived scrapie agent, which has mouse PrP sc in the inoculum. From the other viewpoint, however, similarities of scrapie pathology with a recent study of the poliovirus receptor (PVR) is striking. Transgenic mice expressing human PVR become susceptible to poliovirus; normal mice are resistant s. As the transgenic mice expressing PVR developed this primate disease, they exhibited appropriate pathological changes.
Could the HaPrP protein be the hamster scrapie agent receptor, allowing the hamster-passaged scrapie agent to infect transgenic mouse cells that are normally resistant because this hamster scrapie agent does not usually 'fit' with the mouse PrP protein? Transgenic mice expressing the HaPrP gene and normal mice would both be susceptible to mouse scrapie, since both carry the endogenous mouse PrP protein/scrapie receptor. A PrP variant has now been directly implicated in causing spongiform encephalopathy 6. Gerstmann-Str/iussler-Scheinker syndrome (GSS) is a rare scrapie-like human disease characterized by spongiform pathology and proteaseresistant PrP protein and plaques. In affected families it has the appearance of an autosomal dominant inherited disorder. Transmission of neurological disease to marmosets has been achieved using brain tissue from members of one of these GSS families - disease was further transmissible by passage, but the PrP plaques typical of GSS in human brain were not seen 7. In 1989, an examination of two GSS families revealed an apparent linkage between incidence of the disease and a PrP protein mutationS: a proline to leucine substitution at codon 102. In order to test the significance of this mutation, mice were made transgenic for a PrP gene (GSSPrP) that harboured the same proline to leucine change. These transgenic mice spontaneously began to die from spongiform neurological disease at around 166 days of age without prior exposure to scrapie or GSS. The brains of affected animals exhibited typical scrapie-like vacuolation but no plaques or proteaseresistant PrP were observed. This work suggests that the amino acid change in the PrP gene in the GSS family represents a genetic lesion and may be the cause of the spongiform neurodegeneration 6. However, the transgenic mice with the GSSPrP protein have more than 60 copies of the mutant PrP transgene, and overexpression of
@ 1991, ElsevierSciencePublishersLtd, (UK) 0166- 2236/91/$0200
Nora Hunter
Institutefor Anima/ Health,AFRC/MRC Neuropathogenesis Unit, Ogston Building, WestMains Road,EdinburghEH9 3JF, UK.
389
the PrP protein may complicate the pathology. The question of transmissibility is still unclear. Attempts to transmit infectivity from the affected GSSPrP transgenic mouse brains to normal mice are in progress. If transmission is confirmed, a form of PrP would be more firmly estabfished as the infectious molecule, but the question of how such a host molecule might encode host-independent phenotypes would remain. The reason for the absence of PrP protein plaques in both the GSSinfected marmosets and the GSSPrP transgenic mouse brains (in contrast to hamster scrapieaffected H a P r P transgenic mice and human GSS patients) is also unexplained. Arguments about the precise role of PrP protein continue. It may itself cause disease by seeding the aggregation of the homologous normal PrP protein, and as a result apparently infect and replicate in a species-specific manner. However, the responses of mice transgenic for HaPrP are not inconsist-
p e r s p e c t i v e s
.
ent with the idea of PrP variants acting as scrapie pathogen receptors and (like conventional viral receptors) controlling host range 9. Some disease-specific factor undoubtedly interacts with normal PrP protein and changes it to the protease-resistant form. Depending on the point of view this is either PrP sc (Ref. 1) or an independent scrapie pathogen 1°. The GSSPrP mutation either causes a genetic and transmissible disease or it produces a change in protein structure or stability, which mimics the effect of pathogen binding. Transgenic models, with multiple copies of the P r P transgenes, have increased our understanding of scrapie-like diseases but have still not completely clarified the role of PrP protein. The next step must surely be to create, by homologous recombination, mouse lines with precise changes in the endogenous P r P gene and with no extra copies. For instance, mice with non-functional P r P genes may help elucidate the normal function
.
.
.
.
.
.
.
.
of PrP in the nervous system and, if viable, might be used to show once and for all if this host protein is really needed for replication of the scrapie agent.
Selected references 1 Prusiner, S. B. et al. (1990) Cell 63, 673-686 2 Prusiner, S. B. et aL (1983) Cell 35, 349-358 3 Hope, J. and Baybutt, H. (1991) 5emin. Neurosci. 3, 165-171 4 Bruce, M. E. and Dickinson, A. G. (1987) J. Gen. ViroL 68, 79-89 5 Ren,R., Constantini,F., Gorgacz,E.J., Lee, J. J. and Racaniello,V. R. (1990) Cell 63, 353-363 6 Hsiao, K. et aL (1990) Science 250, 1587-1590 7 Baker, H. F., Duchen, L. W., Jacobs, J. M. and Ridley, R. M. (1990) Brain 113, 1891-1909 8 Hsiao, K. et aL (1989) Nature 338, 342-344 9 Weissmann, C. (1991) Nature 349, 569-571 10 Hope,J., Reekie, L. J. D. and Gibson, P. H. (1986) in Unconventional Virus Diseases of the Central Nervous System (Court, L. A., Dormont, D., Brown, P. and Kingsbury,D. T., eds), pp. 536-546, Commissariat a l'Energie Atomique, Paris
.
How chicksmakememories:the cellularcascadefrom c-los to dendriticremodelling Steven P. R. Rose StevenP. R. Roseis in the Brainand BehavlourResearch Group, Open University, Milton KeynesMK7 6AA, UK.
Training chicks on a one-trial passiveavoidancetask results in a cellular cascadeover the subsequent hours. Phosphorylation of the presynapticphosphokinaseC substrateB-50 is followed by immediateearly gene expressionandincreasedsynthesisof pre- and postsynaptic glycoproteins, increasesin dendntic spine densities, synapse and synaptic vesicle numbers, and a prolonged increase in neuronal bursting. Many of these effects have been localizedto two forebrain regions: the left intermediate media/hyperstriatum ventrale and the Iobus paro/factorius. Pretraininglesionsin the left intermediate media/ hyperstriatum ventrale, or post-training lesionsin the Iobusparolfactonus result in amnesia. Theseand related results lead to models of memory storage basedon multiple representationby way of synaptic stabilization through glycoprotein synaptic recognition molecules. Young chicks peck spontaneously at small bright objects in their field of view. If the object is a coloured bead dipped in a bitter-tasting liquid, the chick will subsequently avoid pecking at even a dry bead of that colour and shape, although its general pecking activity is unimpaired. This behaviour forms the basis of the 'one-trial passive avoidance' learning task introduced by Cherkin more than two decades ago I. It is but one of the large repertoire of forms of early learning about key features of its environment - such as mother (imprinting) 2 and
390
edible food - that the naive but precocial young chick, which, hatched with a large brain and considerable behavioural competence, must achieve rapidly if it is to survive. Avian learning, from canary song to food storage in marsh tits, is becoming increasingly popular as offering model systems in which to study vertebrate memory formation, and the chick, with its repertoire of strongly ontogenetically driven learning, is particularly attractive 3. Learning to suppress pecking at the bitter bead initiates an intracellular cascade of cellular processes, which, beginning with pre- and postsynaptic membrane transient changes and proceeding by way of genomic activation to the lasting structural modification of these membranes, occurs in identified regions of the chick forebrain. These synaptic modifications may form in some way the neural representations of the aversive bead-pecking experience, and encode the instructions for the changed behaviour (i.e. to avoid pecking a bead of these characteristics) that follows. This article reviews the key steps that have been identified as occurring in this cascade, and discusses what these processes can reveal about the mechanisms and nature of memory storage in vertebrates.
© 1991.ElsevieSci r encePublishersLid,(UK) 0166-2236/91/$02.00
TINS, VOI. 14, NO. 9, 1991