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Short Communications |






§
From the Department of Molecular Microbiology and
Immunology,*
Nagasaki University Graduate School of Medical
Sciences, Nagasaki, Japan; the Departments of
Bacteriology
and Pathology
II,
Nagasaki University School of Medicine,
Nagasaki, Japan; and the Department of Molecular
Biology,§
Institute of Virology, Slovak Academy
of Sciences, Brastislava, Slovakia
| Abstract |
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| Introduction |
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We recently identified a novel gene that encodes a putative glycoprotein, namely PrP-like protein (PrPLP), consisting of 179 amino acids with 23% identity to PrPC in the primary amino acid structure.6 Moore et al, who independently discovered the same gene, designated it as Prnd and its product as doppel (Dpl).5 In the brain of ataxic but not non-ataxic Prnp-/- mice, unusual intergenic splicing between the PrP gene (Prnp) and the downstream Prnd occurred, probably due to the deletion of the Prnp intron 2 sequence including its splicing acceptor.5,6 The intergenic splicing rendered the PrPLP/Dpl expression under the control of the Prnp promoter, which caused its overexpression in neurons including Purkinje cells of ataxic Prnp-/- mice.5,6 Taken together, these results suggest that in addition to the functional loss of PrPC, the ectopic PrPLP/Dpl expression could also be involved in the neuronal degeneration in ataxic Prnp-/- mice.
In the present study, we examined physiological expression of PrPLP/Dpl mRNA in various tissues and found its expression in the endothelial cells of brain, spleen, and gut of neonatal wild-type mice. Moreover, we demonstrated that the physiological regulation of PrPLP/Dpl mRNA expression in brain endothelial cells was still preserved even in ataxic Ngsk Prnp-/- mice exhibiting ectopic PrPLP/Dpl mRNA in nearly all neurons including Purkinje cells.
| Materials and Methods |
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Total RNA isolated from various tissues of mice by Trizol reagent (Gibco BRL Life Technologies, Inc., Rockville, MD) according to recommendation in the manufacturers manual, was electrophoresed on a formaldehyde-denatured agarose gel and blotted onto a Hybond N membrane (Amersham Pharmacia Biotech, Buckinghamshire, UK) with 10x SSC. After the fixation of RNA by ultraviolet light (UVP, Ltd., Upland, CA), the membrane was subjected to a prehybridization procedure for 4 hours at 45°C in buffer containing 5x SSPE, 0.5% sodium dodecyl sulfate (SDS), 50% formamide, 5x Denhardts solution, 10% dextran sulfate, and 100 µg/ml salmon sperm DNA. A hybridization procedure was performed using an appropriate 32P-labeled DNA probe (BcaBEST Labeling Kit, TaKaRa, Tokyo, Japan) overnight at 45°C in the same buffer. The membrane was washed twice in 2x SSC/0.1% SDS at room temperature for 10 minutes, once in 1x SSC/0.1% SDS, and twice in 0.1x SSC/0.1% SDS at 65°C for 15 minutes. Signals were detected by BAS 2000 (Fuji, Tokyo, Japan) or autoradiography on Konica X-ray film.
In situ hybridization was performed as described elsewhere.6 Briefly, the brain tissues were fixed for 16 hours in 4% buffered paraformaldehyde (pH 7.4), embedded in paraffin, and sliced at 5 µm thickness. The spleen and ileum tissues, immediately frozen by cold isopentane, were sliced into 10 µm thickness by a cryostat. The tissue sections were hybridized with digoxigenin (DIG)-UTP-labeled cRNA probes (Roche Diagnostics, Mannheim, Germany), which were made by using T7 or T3 polymerase (Gibco BRL Life Technologies). The sections were washed several times in 4x SSC and immersed in 50% formamide/2x SSC at 50°C for 30 minutes. They were then treated with RNase A at 37°C for 30 minutes and washed in 0.2x SSC at 60°C for 20 minutes. Signals were detected by an enzyme-linked immunosorbent assay for brain or GenePoint System (Dako, Copenhagen, Denmark) for spleen and ileum. After in situ hybridization, the tissues were incubated in 0.3% H2O2 solution for 30 minutes at room temperature to prevent endogenous peroxidase activity. The tissues were incubated with rabbit anti-factor VIII (1:400) serum (Dako) overnight at 4°C after blocking with normal goat serum. The signals were detected by incubation with biotinylated goat anti-rabbit Ig (1:500), avidin-conjugated horseradish peroxidase (1:500; Dako), and 3-amino-9-ethyl carbazole (Dako).
The PrPLP/Dpl, Prnp exon1/2, and Prnp exon 3 probes were prepared as described elsewhere.6 The glyceraldehyde-3-phosphate dehydrogenase (G3PDH) probe is a fragment comprising positions 395 to 1005 of the cDNA (NM 008084.1).
| Results and Discussion |
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To make an inquiry into the role of PrPLP/Dpl, we extended Northern
blot analysis to the brain of younger wild-type mice, which revealed a
transient expression of the PrPLP/Dpl mRNA in the brain soon after
birth. As shown in Figure 2A, a
detectable level of the PrPLP/Dpl mRNA was already expressed in the
brain at birth, reached a maximal level at 6 days, and decreased
thereafter, reaching an undetectable level by 8 weeks. In
situ hybridization and immunohistochemistry were carried out to
identify the cell type expressing the PrPLP/Dpl mRNA in the brain of
6-day-old wild-type mice. In situ hybridization gave
linearly aligned or patchy signals of PrPLP/Dpl mRNA distributed
throughout the brain (Figure 2B)
. The control sense probe did not give
any signal (data not shown). This morphological expression strongly
suggested that the cells with PrPLP/Dpl mRNA signals were other
than neurons and glial cells. Immunohistochemical staining with the
rabbit anti-serum against factor VIII-related peptide,8
a
specific marker of endothelial cells, gave signals that colocalized
with those for PrPLP/Dpl mRNA in the brain of 6-day-old wild-type mice
(Figure 2C)
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In contrast to the wild-type mice, Northern blotting revealed the
constitutive expression of PrPLP/Dpl mRNAs at an abundant level in the
brain of Ngsk Prnp-/- mice even at 8
weeks, which was mostly, if not entirely, hybridized to the
Prnp exon 1/2 probe (Figure 2D)
. According to our previous
study, this is likely to be mainly derived from the ectopic PrPLP/Dpl
mRNA in neurons. However, it may be possible that the PrPLP/Dpl
expression in brain endothelial cells was also deregulated in the
ataxic Ngsk Prnp-/- mice because of the
intergenic splicing. Furthermore, this deregulated PrPLP/Dpl might
affect the function of the cells, resulting in neuronal degeneration in
Ngsk Prnp-/- mice. To examine this
possibility, we carried out immunohistochemistry together with in
situ hybridization on the brain sections of 6-day- and 8-week-old
Ngsk Prnp-/- mice. In the 6-day-old brain
sections, the PrPLP/Dpl mRNAs were abundantly expressed by neurons
throughout the brain, with the strongest signal in pyramidal cells of
the hippocampus (Figure 2E)
and Purkinje cells of the cerebellum (data
not shown). The signals were also readily detectable in endothelial
cells of the 6-day-old brain (Figure 2E
, arrows). On the other hand, in
the brain of 8-week-old Ngsk Prnp-/- mice
there was no PrPLP/Dpl mRNA signal colocalized with immunoreactivities
of the factor VIII-related peptide, although abundant PrPLP/Dpl
transcripts were still present in neurons (Figure 2F)
. These results
indicated that the physiological regulation of PrPLP/Dpl expression in
brain endothelial cells was preserved even in Ngsk
Prnp-/- mice. This argues against the
impaired function of brain endothelial cells due to deregulated
expression of PrPLP/Dpl. So it is most likely that the ectopic
expression of PrPLP/Dpl in neurons is directly associated with the
neuronal degeneration in the ataxic
Prnp-/- mice.
We previously demonstrated that the ataxic Ngsk
Prnp-/- mice were rescued from the
Purkinje cell degeneration by introducing a normal mouse PrP gene,
suggested that two molecular events, ie, the functional loss of
PrPC and the ectopic PrPLP/Dpl expression, might
be required for the neurodegeneration in ataxic
Prnp-/- mice.7
Schmerling et
al reported that a transgene encoding N-terminal truncated PrP
(PrP
32135) rendered the non-ataxic Zrch
Prnp-/- mice ataxic by inducing massive
granule cell or Purkinje cell death, which could be prevented by
introduction of the normal mouse PrP gene.14
Since the
conformational structure of the truncated PrP resembles that of
PrP/Dpl, the same mechanisms might be involved in the neurodegeneration
in Ngsk Prnp-/- and PrP
32135
transgenic mice. Weissmann and Aguzzi recently proposed a hypothesis
for molecular mechanisms underlying the neuronal
degeneration.15
They hypothesized two putative molecules;
one is a ligand of PrPC
(PrPL) eliciting a signal necessary for the
survival of neurons, and the other is protein
that binds to
PrPL with lower affinity but also capable of
generating the survival signal. According to the hypothesis, in the
non-ataxic Prnp-/- mice, protein
compensates for the function of PrPC. However, in
the ataxic mice, the ectopically expressed PrPLP/Dpl or truncated PrP
preferentially interact with PrPL, displacing
protein
but without eliciting the survival signal. Identification
of these putative molecules is urgently needed to evaluate the validity
of this hypothesis. However, there could be an alternative possibility:
that the ectopically expressed PrPLP/Dpl itself may elicit a signal for
apoptosis in neurons and PrPC functions as an
anti-apoptotic factor by competing with PrPLP/Dpl. This idea is
consistent with a previous finding that neuronal cell lines derived
from ataxic Prnp-/- mice rapidly died by
apoptosis in serum-free medium, but the apoptosis was prevented by
overexpression of PrPC or Bcl-2
protein.16
Moreover, it was reported that the primary
cultured cerebellar neurons derived from the non-ataxic
Prnp-/- mice were more susceptible to the
apoptosis-inducing oxidative stress than wild-type neurons, suggesting
the anti-apoptotic function of
PrPC.17
However, further studies are necessary to understand the exact relationship between the ectopic expression of PrPLP/Dpl and the Purkinje cell degeneration. The transgenic mice expressing PrPLP/Dpl in Purkinje cells with the non-ataxic Prnp-/- mouse background would provide definitive evidence for the role of ectopic PrPLP/Dpl in the neuronal degeneration.
| Footnotes |
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Supported in part by the Ministry of Culture, Sports, and Education, the Ministry of Health and Welfare of Japan, and the Japan Society for the Promotion of Science.
A. L.s current address: Department of Cell Biology and Physiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110.
B. C. R.s current address: Division of Hematology, Washington University School of Medicine, Box 8069, 660 South Euclid Avenue, St. Louis, MO 63110.
Accepted for publication August 14, 2000.
| References |
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