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Journal of Virology, November 2000, p. 10245-10248, Vol. 74, No. 21
Robert-Koch-Institut, 13353 Berlin, Germany
Received 4 May 2000/Accepted 2 August 2000
The pathogenesis of scrapie, and of neurodegenerative diseases in
general, is still insufficiently understood and is therefore being
intensely researched. There is abundant evidence that the activation of
glial cells precedes neurodegeneration and may thus play an important
role in disease development and progression. The identification of
genes with altered expression patterns in the diseased brain may
provide insight on the molecular level into the process which
ultimately leads to neuronal loss. Differentially expressed genes in
scrapie-infected brain tissue were enriched by the suppression
subtractive hybridization technique, molecularly cloned, and further
characterized. Northern blotting and nucleotide sequencing confirmed
the identities of 19 upregulated genes, 11 of which were unknown to be
affected by scrapie. A considerable number of these 19 genes, namely
those encoding interferon-inducible protein 10 (IP-10), 2',5'-oligo(A)
synthetase, Mx protein, IIGP protein, major histocompatibility complex
classes I and II, complement, and Scrapie, a naturally occurring
disease of sheep and goats, and the corresponding pathologies in humans
(Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker
syndrome, fatal familial insomnia, and Kuru), cattle (bovine spongiform
encephalopathy), and other animals are transmissible progressive
neurodegenerative disorders (1, 4, 7, 11, 28). The
neuropathology of these transmissible spongiform encephalopathies
(TSEs) is usually linked to the appearance of an abnormal insoluble and
protease-resistant form of a normal host-encoded protein, the prion
protein (PrP). Structurally, the disease-associated abnormal form of
PrP, termed PrPres or PrPSc, is characterized
by a high beta-sheet content in contrast to the predominantly
alpha-helical fold of normal PrP (10, 23, 33).
TSEs in general are characterized by a reactive gliosis and the
subsequent degeneration of neuronal tissue. The activation of glial
cells, which precedes neuronal death, is likely to be caused by the
deposition of large amounts of PrPSc in the brain (21,
36, 37). Experimental evidence suggests that PrPSc
participates in initiation of the gliosis and subsequent neuronal loss,
since a PrP-derived peptide (PrP106-126) activates microglial cells in vitro (6, 20). Furthermore, cell culture supernatants from these stimulated cells induce the proliferation of
astrocytes and are toxic to neuronal cells (5). Thus,
cytokines released by PrPSc-activated microglial cells may
contribute to scrapie pathogenesis by enhancement and generalization of
the gliosis and via cytotoxicity to neurons. However, the mechanisms of
PrPSc-triggered microglial activation and many of the
factors involved are still unknown. A more complete understanding of
the disease may help researchers to define possible therapeutic targets
and to develop new means of diagnosis.
To identify upregulated genes in the scrapie-infected hamster brain
which may be associated with or even cause the neurodegenerative changes, a strategy using the suppression subtractive hybridization (SSH) technique in combination with a differential screening approach was chosen (13). Briefly, hamsters were inoculated
intraperitoneally (i.p.) with scrapie strain 263K (16).
Control animals were inoculated with the same volume of normal-brain
homogenate. Following total-brain RNA isolation at the terminal stage
of the infection, synthesis and subtraction of the cDNA pools were
carried out with an SSH-based PCR-select cDNA subtraction kit
(Clontech, Palo Alto, Calif.) according to the manufacturer's
suggestions. Remaining cDNAs were randomly subcloned into a T/A vector
(Invitrogen, Carlsbad, Calif.).
In total, 1,200 clones generated by the SSH procedure were analyzed by
dot blotting with forward and reverse subtracted cDNA probes from
infected and uninfected hamster brain tissue, respectively. One hundred
clones displayed signal intensity differences and were further
characterized by nucleotide sequencing. Sequence database searches
identified eight genes previously described as being upregulated in the
scrapie-infected brain (GFAP, transferrin, apolipoprotein J,
metallothionein,
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Identification of Upregulated Genes in
Scrapie-Infected Brain Tissue
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ABSTRACT
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Abstract
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2-microglobulin, were
inducible by interferons (IFNs), suggesting that an IFN response is a
possible mechanism of gene activation in scrapie. Among the newly found
genes, that coding for 2',5'-oligo(A) synthetase is of special interest
because it could contribute to the apoptotic loss of neuronal cells via
RNase L activation. In addition, upregulation of the chemokine IP-10
and B-lymphocyte chemoattractant mRNAs was seen at relatively early
stages of the disease and was sustained throughout disease development.
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TEXT
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Abstract
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2-microglobulin, major
histocompatibility complex [MHC] class I, MHC class II, and MHC class
II-associated invariant chain). Northern blot analysis of the remaining
clones confirmed that 11 genes which had previously been unknown to be affected by the scrapie infection were differentially expressed: IP-10,
BLC, Mx protein, 2',5'-oligo(A) synthetase, IIGP protein, glycoprotein 39 precursor (gp39), vimentin, aquaporin 4 (AQP-4), lysosome-associated multitransmembrane protein (LAPTm5), the LIM homeodomain protein 7 (Lhx7), and the C1q C chain of complement (Table
1; Fig. 1).
TABLE 1.
Upregulated genes in the scrapie-infected hamster brain

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FIG. 1.
Northern blot analysis of upregulated genes in the
scrapie-infected hamster brain. RNA (10 µg/lane) was fractionated by
agarose gel electrophoresis, transferred to nylon membranes, and
hybridized with clones IP-10 (a), BLC (b), Mx protein (c),
2',5'-oligo(A) synthetase (d), IIGP protein (e), gp-39 precursor (f),
vimentin (g) AQP-4 (h), LAPTm5 (i), Lhx 7 (j), C1q C chain of
complement (k), GFAP (l), and
-actin control (m). Control, RNA from
two mock-infected hamsters; scrapie, RNA from two scrapie-infected
hamsters at the terminal stage of the disease.
The levels of mRNA induction in scrapie-infected brain among the 19 different genes ranged from 1.5- to 6-fold compared to those of
uninfected controls as determined by Northern blotting and subsequent
densitometric quantification. Only the IP-10 mRNA was virtually
undetectable in uninfected brain tissue and was highly expressed in the
terminal stage of the infection (Fig. 1). In addition, expression of
the chemokines IP-10 and BLC in the brains of BALB/c mice i.p. infected
with scrapie strain 139A was analyzed over time. By using a
semiquantitative reverse transcription (RT)-PCR method, both chemokines
were found to be induced at day 114 postinfection, which was about 90 days before the terminal stage of the disease (Fig.
2). Even with the highly sensitive RT-PCR, no IP-10 mRNA was seen in the normal mouse brain tissue.
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Previous research efforts employed methods like immunohistochemistry, in situ hybridization, subtractive hybridization, and differential-display RT-PCR to analyze gene or protein expression alterations in the scrapie-infected brain (12, 15, 17, 18). Most importantly, proinflammatory cytokines (e.g., tumor necrosis factor alpha, interleukin-1 [IL-1], and IL-6), glial cell activation markers (e.g., GFAP, cathepsin S, and complement C1q), and indicators of cellular stress (e.g., HSP70 and metallothionein II) were found to be upregulated in the scrapie-infected brain (8, 12, 14, 25, 35, 36). The more recently developed SSH technique, a powerful tool for the analysis of differential gene expression, was used in this study to increase our current understanding of the pathological changes in the scrapie-infected brain on the molecular level.
In total, we found 19 upregulated genes (Table 1), 8 of which were previously described by other groups as being differentially expressed. Among the 11 genes newly found to be affected by scrapie infection, IP-10, BLC, vimentin, gp39, LAPTm5, AQP-4, and the C1q C chain of complement are most likely expressed by activated glial cells. Increased vimentin and AQP-4 levels are indicative of an ongoing astrocytosis (27, 32). Among the microgliosis markers, gp39 is seen in late stages of macrophage differentiation and is thus probably expressed by activated and differentiating microglial cells. Upregulation of the C chain of complement C1q in stimulated microglial cells is in agreement with a similar observation for the C1q B chain (12). High LAPTm5 expression levels are associated with an increased lysosomal activity of microglial cells in the vicinity of spongiform histological changes (37). This observation further substantiates the possible involvement of an aberrant activation of the microglial lysosomal system in neurodegeneration (26).
2',5'-Oligo(A) synthetase is most likely expressed by glial as well as neuronal cells (2). 2',5'-Oligo(A) synthetase causes activation of RNase L, which has been demonstrated to participate in apoptotic cell death via its nonspecific rRNA-degrading activity (9, 39). Hence, activation of RNase L may directly contribute to neuronal loss in scrapie. In contrast to the upregulation of IP-10 and BLC at day 114 postinfection, increasing 2',5'-oligo(A) synthetase mRNA levels were first seen at day 141 by RT-PCR (data not shown).
It is less clear which cell types express increased levels of the
GTPase IIGP, the putative transcription factor Lhx7 mRNA, and the Mx
protein. Mx protein and 2',5'-oligo(A) synthetase are part of the
antiviral response induced by interferons (IFNs). Moreover, given that
the expression of IIGP, MHC classes I and II, complement,
2-microglobulin, and IP-10 is also IFN inducible, gene
activation in scrapie bears all of the hallmarks of a typical IFN
response. Interestingly, gamma IFN (IFN-
) was previously shown to
activate microglia after stimulation with
-amyloid (31). Furthermore, low doses of IFN-
, but not IL-1
, IL-2, IL-4, IL-6, or IL-12, synergistically enhance CD40 expression on microglial cells
upon stimulation with
-amyloid. Thus, IFN-
may contribute to the
CD40-CD40L interaction-dependent activation of microglia in
Alzheimer's disease as well as in scrapie (34). We are
presently investigating the role of IFN-
in the pathogenesis of
scrapie by looking at scrapie infections of IFN-
receptor knockout
mice in comparison to wild-type controls.
The chemokines IP-10 and BLC, so far only known as chemoattractants for
T and B cells (19, 22, 29), respectively, are likely to be
produced by activated glial cells (30). However, since
infiltrating lymphoid cells are rarely observed in scrapie, IP-10 and
BLC may bind to other target cells in the brain rather than to cells in
lymphoid organs. Evidence for chemokine receptor expression on neurons
surrounding amyloid plaques in Alzheimer's disease suggests that
neuronal cells, in addition to glial cells, may well be potential
targets for IP-10 and BLC (24, 38). Studies of chemokine
expression and function in the normal and the diseased brain are still
in their early stages, and we currently do not know the effects of
IP-10 and BLC on receptor-expressing neurons. The induction of both
chemokines is seen at early stages of the scrapie infection and is
sustained at high levels until the end, which indicates an involvement
in disease progression. Increased levels of IP-10 and BLC mRNAs were
seen in two species infected with different scrapie strains. Thus, this
observation is likely to be of general relevance in TSEs and less
variable than the levels of transforming growth factor
1 and
cathepsin S seen in different TSE model systems (3).
Finally, given that chemokine mRNA levels are early markers for
infection, determination of chemokine levels in the brain and in
cerebrospinal fluid is potentially useful for the differential
diagnosis of TSEs.
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ACKNOWLEDGMENTS |
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This work was funded in part by the Hertie Stiftung.
We thank E. Baldauf and U. Erikli for help in manuscript preparation and M. Beekes and H. Diringer for helpful discussions.
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FOOTNOTES |
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* Corresponding author. Mailing address: Robert-Koch-Institut, Nordufer 20, 13353 Berlin, Germany. Phone: 49-30-45472230. Fax: 49-30-45472609. E-mail: baierm{at}rki.de.
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