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The Journal of Neuroscience, February 1, 1998, 18(3):914-922
Interaction of Presenilins with the Filamin Family
of Actin-Binding Proteins
Wanjiang
Zhang1,
Sang
Woo
Han2,
Daniel W.
McKeel4,
Alison
Goate2, 3, and
Jane Y.
Wu1
Departments of 1 Pediatrics and Molecular Biology and
Pharmacology, 2 Psychiatry, 3 Genetics, and
4 Pathology, Washington University School of Medicine, St.
Louis, Missouri 63110
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ABSTRACT |
Mutations in presenilin genes PS1 and PS2 account for ~50% of
early-onset familial Alzheimer's disease (FAD). The PS1 and PS2 genes
encode highly homologous transmembrane proteins related to the
Caenorhabditis elegans sel-12 and spe-4 gene products. A
hydrophilic loop region facing the cytoplasmic compartment is likely to
be functionally important because at least 14 mutations in FAD patients
have been identified in this region. We report here that the loop
regions of PS1 and PS2 interact with nonmuscle filamin (actin-binding
protein 280, ABP280) and a structurally related protein (filamin
homolog 1, Fh1). Overexpression of PS1 appears to modify the
distribution of ABP280 and Fh1 proteins in cultured cells. A monoclonal
antibody recognizing ABP280 and Fh1 binds to blood vessels, astrocytes,
neurofibrillary tangles, neuropil threads, and dystrophic neurites in
the AD brain. Detection of ABP280/Fh1 proteins in these structures
suggests that these presenilin-interacting proteins may be involved in
the development of AD and that interactions between presenilins and
ABP280/Fh1 may be functionally significant. The ABP280 gene is located
on the human X chromosome, whereas the newly identified Fh1 gene maps
to human chromosome 3. These results provide a new basis for
understanding the function of presenilin proteins and further implicate
cytoskeletal elements in AD pathogenesis.
Key words:
Alzheimer's disease; presenilins; protein-protein
interaction; actin-binding protein 280; filamin homolog 1; cytoskeletal
elements
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INTRODUCTION |
Alzheimer's disease (AD) is a
neurodegenerative disorder characterized neuropathologically by large
numbers of neurofibrillary tangles (NFT) and senile plaques in the
hippocampus and cerebral cortex. In addition to advanced age, Down
syndrome, and ApoE- 4 genotype, the presence of a positive family
history is a consistent risk factor for AD (Heyman et al., 1984 ; van
Duijin et al., 1991 ; Corder et al., 1993 ; Chartier-Harlin et al.,
1994 ). Genetic studies have identified a number of autosomal dominant
mutations associated with early-onset familial AD (FAD) in three genes:
amyloid precursor protein (APP) gene on chromosome 21 (for review, see
Selkoe, 1994 ; Lendon et al., 1997 ), presenilin 1 (PS1) on chromosome 14 (Sherrington et al., 1995 ), and presenilin 2 (PS2) on chromosome 1 (Levy-Lahad et al., 1995 ; Rogaev et al., 1995 ). Mutations in all three
genes lead to changes in amyloid -peptide (A ), suggesting that
APP processing and A deposition may play a central role in AD
pathogenesis (Citron et al., 1992 ; Cai et al., 1993 ; Scheuner et al.,
1996 ).
PS1 and PS2 are expressed in many tissues, including the brain
(Levy-Lahad et al., 1995 ; Rogaev et al., 1995 ; Sherrington et al.,
1995 ; Kovacs et al., 1996 ; Suzuki et al., 1996 ; Giannakopoulos et al.,
1997 ; Takami et al., 1997 ). In mouse brain, PS1 mRNA and protein have
been detected in both neuronal and glial cells (Lee et al., 1996 ). In
human brains, PS1 immunoreactivity has been reported in cortical
neurons (Elder et al., 1996 ; Giannakopoulos et al., 1997 ), in NFT
(Murphy et al., 1996 ), and in neuritic plaques (Wisniewski et al.,
1995 ).
PS1 and PS2 genes encode transmembrane (TM) proteins with sequence
similarity to the Caenorhabditis elegans gene products Sel-12 and Spe-4 (Levitan and Greenwald, 1995 ; Sherrington et al.,
1995 ). Sel-12 has been implicated in modulating the reception of
intercellular signaling mediated by lin-12, a member of the Notch
family of receptor molecules (Levitan and Greenwald, 1995 ). The normal
function or functions of PS1 and PS2 are not clear yet, although PS2
has been proposed to be involved in apoptosis (Vito et al., 1996 ;
Wolozin et al., 1996 ). Presenilin mutations are associated with
increases in the levels of the longer forms of the amyloid -peptide
(A 1-42/43), demonstrating that presenilin genes may modulate APP
processing (Scheuner et al., 1996 ).
Recent studies suggest that PS1 protein has six or eight TM domains
with both the N- and C-terminal regions, as well as a large hydrophilic
loop region, facing the cytosolic compartment (Doan et al., 1996 ;
Lehmann et al., 1997 ). The possibility that this hydrophilic region
plays an important role in PS1 function and AD development is suggested
by the fact that at least 14 independent mutations, including one
splice site mutation identified in FAD, are located in this loop region
(Schellenberg, 1995 ; Cruts et al., 1996 ; Lendon et al., 1997 ). PS1 is
processed by endoproteolysis between amino acid residues 290-300
within this hydrophilic loop region (Thinakaran et al., 1996 ; Podlisny
et al., 1997 ), further supporting the hypothesis that this region is an
important functional domain.
We report here that the actin-binding protein, ABP280 (also called
nonmuscle filamin), and a protein related to filamin (filamin homolog
1, Fh1) can interact with both PS1 and PS2 proteins. The gene encoding
Fh1 maps to human chromosome 3. In cells transiently transfected with
PS1, overexpression of PS1 appears to modify the intracellular
distribution of endogenous ABP280/Fh1 proteins as detected by indirect
immunofluorescence microscopy. Immunohistochemical studies using a
monoclonal antibody recognizing both ABP280 and Fh1 show that these
proteins are expressed predominantly in blood vessels and astrocytes in
the normal brain. In the AD brain, strong immunoreactivity was detected
not only in astrocytes but also in NFT, neuropil threads, and
dystrophic neurites within some senile plaques. Similar astrocytic
immunostaining also was observed by using two different anti-PS1
antisera. Thus, our results demonstrate that presenilin proteins can
interact both in vitro and in vivo with ABP280
and Fh1, cytoskeletal proteins of the actin-binding protein family, and
that these cytoskeletal proteins are present in NFT. These observations
provide new information on a possible function of the presenilin
proteins in interaction with the cytoskeleton and their role in the
pathogenesis of AD.
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MATERIALS AND METHODS |
Yeast two-hybrid cDNA library screening and
protein-protein interaction assay. A human fetal brain cDNA
library was constructed with the yeast plasmid pJG4-5, which expresses
individual cDNAs as fusion proteins containing a transcription
activation domain (Zervos et al., 1993 ). As detected by Western
blotting with anti-LexA antibody, the PS1 loop region (amino acids
263-411) was expressed as a fusion protein with the DNA-binding
protein LexA in yeast carrying the reporter genes leu2 and
lacZ. The human brain cDNA library was screened as described
(Wu and Maniatis, 1993 ; Zervos et al., 1993 ) in yeast expressing the
PS1 bait as well as the reporter genes. Individual cDNAs were purified
from the positive clones and retransformed into yeast to confirm that
the cDNAs encode proteins that specifically interact with the PS1 bait
but not other unrelated control proteins. The confirmed cDNAs were sequenced with an ABI automatic sequencer. To test interactions between
PS1/2 and ABP280 or Fh1, we introduced the bait plasmids expressing
either the wild-type PS1/2 loop region or the PS1 loop region
containing individual FAD mutations as LexA fusion proteins into yeast
expressing either ABP280 or Fh1 C-terminal regions as fusion proteins
containing the transcription activation domain. Quantitative liquid
-galactosidase assays were performed on at least three independent
colonies for each combination and compared with the background, as
previously described (Wu and Maniatis, 1993 ). The background is defined
as the amount of -galactosidase activity detected in yeast
cotransformed with corresponding prey plasmids and bait plasmid
expressing the APP intracellular domain-LexA fusion protein, which has
the same level of -galactosidase activity as the yeast containing
the LexA bait vector plasmid without any cDNA insert.
Chromosome mapping. A sequenced tagged site (STS) for the
Fh1 gene was developed on the basis of the sequence of the Fh1 cDNA clones. PCR was performed by using DNA samples prepared from a panel of
monochromosome somatic cell hybrids obtained from American Type Culture
Collection (ATTC; Rockville, MD) to determine the chromosomal location
of the Fh1 gene. The PCR primers used were CACCACAGGTATCCAGTC and
CTGTCACCTTGGCCTTGAAG. PCR reactions were performed in a final reaction
volume of 25 µl with 10 pmol of each primer, 200 µM
dNTPs, 1.5 mM MgCl2, 1 µl of
formamide, and 0.1 µl of Taq polymerase. The reaction was
denatured at 94°C for 5 min, followed by 35 cycles of 94°C for 45 sec, 53°C for 30 sec, 72°C for 1 min, and a final extension at
72°C for 5 min. The PCR products from individual monochromosome
somatic cell hybrids and control cell lines, as well as DNA molecular
size markers, were separated on a 2% agarose gel and visualized under
UV transillumination with ethidium bromide.
Coimmunoprecipitation. In vitro translation
products of the ABP280 or Fh1 C-terminal region and of PS1 or PS2 loop
region were labeled with [35S]methionine.
Individual proteins or proteins after coincubation were
immunoprecipitated with the monoclonal antibody NCL-FIL or monoclonal
antibodies against the epitope tags present on the corresponding
proteins. The precipitated products were separated on SDS-PAGE, and the
dried gel was exposed to x-ray films.
Transfection and immunofluorescent microscopy. Cos-1 cells
were grown on coverslips to ~70% confluence and transiently
transfected with a plasmid expressing full-length PS1, using lipofectin
(Life Technologies, Gaithersburg, MD). At 36 hr after transfection, the
cells were fixed with 4% paraformaldehyde for 20 min and permeabilized with 1% Triton X-100 in PBS for 10 min. After the cells were blocked with PBS/20% fetal bovine serum/0.05% Tween 20, the monoclonal antibody NCL-FIL (NovoCastra Laboratories, New Castle, UK) and polyclonal rabbit anti-PS1 antibodies recognizing the N terminus of PS1
[Ab14, Lee et al. (1996) ; an affinity-purified antibody raised against
a PS1 synthetic peptide containing amino acid residues 30-46, Malin et
al. (1997) ] were added in blocking solution and incubated for 1.5 hr.
Secondary antibodies (anti-mouse fluorescein conjugate or anti-rabbit
Cy3 conjugate, Jackson ImmunoResearch Laboratories, West Grove, PA)
were applied to the slides. After three washes, cells were mounted for
viewing. Similar results were obtained by using polyclonal antibodies
recognizing either the N terminus or the loop region of PS1.
Collection of tissue samples and diagnosis of AD. Seven AD
brains and two age-matched control brains were obtained at autopsy; the
detailed information about the human brain cases used is shown in Table
1. The neuropathological diagnosis of AD
relies on previously described methods (Berg et al., 1993 ; McKeel et
al., 1993 ), using objective quantitative criteria as originally
proposed by Khachaturian (1985) . The criteria were modified on the
basis of the average (10 mm2 microscopic fields)
age-adjusted total argyrophilic (diffuse and neuritic) plaque score,
which must be met or exceeded in at least a single standardized CERAD
(Consortium to Establish a Registry for Alzheimer's Disease)
neocortical area (Mirra et al., 1991 ). This strategy prevents making a
diagnosis of AD in cases that have only a focal cluster of cortical
plaques.
Antibodies. The monoclonal antibody against human filamin
was purchased from NovoCastra Laboratories. The polyclonal anti-human PS1 antisera were kindly provided by Dr. S. Gandy (Lee et al., 1996 )
and by Dr. J. M. Nerbonne (Malin et al., 1997 ). The final dilutions of corresponding antibodies used are described in the figure
legends.
Immunohistochemistry. Cryostat and paraffin sections of
midfrontal (CERAD standard area) cortex (6 µm each) that had been obtained at autopsy were used. The sections were fixed 24-48 hr in the
refrigerator in either ethanol fixative (0.15N NaCl and 70% ethanol)
or paraformaldehyde fixative (4% paraformaldehyde in 0.1 M
neutral pH phosphate buffer). The paraffin was removed by heat and
xylene, followed by 8 min microwave pH 6.0 citrate buffer antigen
retrieval. Then sections were rinsed multiple times in buffer. The
primary antibody NCL-FIL was applied in a moist chamber in the
refrigerator overnight at working dilutions of 1:200 and 1:500.
Subsequent steps used the Vector Elite ABC kit (Burlingame, CA), with
DAB as substrate. Ethanol fixative gave more robust staining signals
than paraformaldehyde fixative when the NCL-FIL monoclonal antibody was
used. Sections were mounted in permanent mountant and photographed with
a Leitz Dialux automated exposure 35 mm camera system. Photographic
prints were reproduced from the original color negatives.
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RESULTS |
PS-1 interacts with nonmuscle filamin and a protein with
sequence similarity to filamin, filamin homolog 1 (Fh1)
A human brain cDNA library was constructed with the yeast
expression vector for yeast two-hybrid interaction cloning (Wu and Maniatis, 1993 ; Zervos et al., 1993 ). cDNAs in this library are expressed as proteins fused to a transcription activation domain. Human
PS1 loop region (amino acids 263-411) was expressed as a fusion
protein with the DNA-binding protein LexA in yeast carrying the
reporter genes, as detected by Western blotting of the yeast extracts,
using anti-LexA antibodies (data not shown). Although the PS1-LexA
fusion protein by itself does not lead to activation of the reporter
genes, interaction of the PS1 bait protein with a protein encoded by
the cDNA in the library will bring together the DNA-binding domain and
the transcription activation domain to form an active transcription
complex, resulting in the activation of the reporter genes,
leu2 and lacZ (Zervos et al., 1993 ).
Using the PS1 loop region as a bait, we screened ~2 × 106 independent clones of the human brain cDNA
library and identified a number of positive interaction clones. The
individual cDNAs were isolated and retransformed into the yeast to
confirm that these genes encode proteins that interact specifically
with the PS1 loop region, but not with other unrelated proteins. DNA
sequence analyses of these PS1 interacting clones revealed the presence of several distinct groups of cDNAs. These cDNAs currently are being
characterized further and will be described elsewhere. Two of these
cDNA clones encode the C-terminal 358 amino acid residues of human
nonmuscle filamin (filamin 1, FLN1) (also named actin-binding protein
280, ABP280; Gorlin et al., 1990 ), and four cDNA clones encode the
C-terminal 291 residues of a protein that is highly homologous to the
nonmuscle filamin, which we have named Filamin homolog 1 (Fh1).
Sequence analysis shows that the Fh1 gene encodes a previously
unidentified protein with significant sequence similarity to ABP280.
The 291 amino acid region encoded by the Fh1 cDNA fragment obtained
from the human brain cDNA yeast two-hybrid library shows 69% identity
and 80% similarity to the C terminus of ABP280 (Fig. 1). Northern analysis demonstrated that
both ABP280 and Fh1 cDNA probes hybridized to two bands of ~8 and 9 kb in several tissues, including heart, brain, placenta, lung, liver,
kidney, and pancreas (data not shown).

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Figure 1.
Alignment of ABP280 and Fh1 peptide sequences in
the C-terminal region. Two clones of ABP280 cDNA and four clones of Fh1
cDNA were identified from screening a human brain cDNA yeast two-hybrid library, using the PS1 loop region as bait. The ABP280 cDNAs encode the
C-terminal 387 residues of the protein, whereas the Fh1 cDNAs encode
the C-terminal 291 amino acids. The alignment of the C-terminal 291 residues of the two proteins as predicted from cDNA sequences is shown.
The amino acid residues identical between the two proteins are in
bold and underlined, with the similar
residues underlined. Over this region of 291 residues,
identity is 69% and similarity is 80% without the introduction of any
gaps.
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The full-length cDNA encoding ABP280 has been cloned, and sequence
analysis predicts that ABP280 is a protein of 2647 amino acids, with
the N-terminal 275 residues as the actin-binding domain, followed by 24 copies of a 96-residue repeat (Gorlin et al., 1990 ). ABP280 forms
dimers with a leaf spring-like structure via intermolecular interaction
between the C-terminal 96-residue repeat region of the protein (Gorlin
et al., 1990 ). The ABP280 cDNA clones that we identified in the yeast
two-hybrid screening encode part of the twenty first repeat and the
last three 96-residue repeats.
Human Fh1 gene maps to chromosome 3
Two genes encoding ABP280 (FLN1) and a related protein (FLN2) have
been identified previously. The FLN1 gene maps to human chromosome Xq28
and is expressed in many cell types, whereas the FLN2 gene maps to
human chromosome 7q32-35 and is expressed predominantly in skeletal and
cardiac muscle (Gorlin et al., 1993 ; Maetrini et al., 1993 ).
We developed a STS for the Fh1 gene and used PCR to determine the
chromosomal location of this gene using a panel of monochromosome somatic cell hybrids obtained from ATCC. In total human genomic DNA,
the STS generated two PCR fragments, one of the expected size (230 bp)
and a second band of 800 bp. DNA sequence analysis of these bands
confirmed that the 230 bp band represented the Fh1 genomic DNA fragment
and that the 800 bp band was a nonspecific PCR product. The 230 bp band
maps to chromosome 3 (Fig. 2). It should
be noted that a putative locus for late-onset AD also has been mapped
to chromosome 3 (Tanzi et al., 1996 ). Therefore, Fh1 represents a novel
gene on chromosome 3 encoding a protein highly homologous to ABP280.
This brings to three the number of filamin-like genes in the human
genome.

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Figure 2.
Fh1 maps to human chromosome 3. A sequenced tagged
site (STS) for the Fh1 gene was developed, and PCR was used to
determine the Fh1 chromosomal location. The PCR products from
individual monochromosome somatic cell hybrids and control cell lines,
as well as DNA molecular size markers, were separated on a 2% agarose gel and visualized under UV transillumination with ethidium bromide. In
total human DNA, the STS generated two PCR fragments, one of the
expected size (230 bp) and a second band of 800 bp. Direct sequencing
of the PCR products revealed that the 230 bp band was Fh1 and that the
800 bp band was a nonspecific PCR product. The 220 bp Fh1 band maps to
chromosome 3.
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ABP280 and Fh1 interact with loop regions of both PS1 and
PS2 proteins
To test whether the PS1 loop region can interact with ABP280 or
Fh1 in biochemical assays, we performed coimmunoprecipitation experiments. Because presenilin proteins are associated with membrane structures and ABP280/Fh1 are high molecular weight proteins associated with cytoskeletal elements, it is difficult to rule out the possibility that coimmunoprecipitation of the full-length proteins from solubilized cell extracts is an artifact caused by their association with membrane
or cytoskeletal structures. Therefore, we decided to use in
vitro-translated soluble protein fragments of presenilins and
ABP/Fh1 to perform coimmunoprecipitation experiments. The cDNA
fragments encoding the loop region of PS1 or encoding the C-terminal
fragments of ABP280 or Fh1 identified from the yeast two-hybrid
screening were inserted into a plasmid vector under the control of the
T7 promoter, and the corresponding proteins were radiolabeled by
in vitro translation in the presence of
[35S]methionine. A monoclonal antibody against
filamin (NCL-FIL, NovoCastra Laboratories) immunoprecipitated both
ABP280 and Fh1 in vitro translation products (Fig.
3, lanes 1 and
4), but not PS1 protein (Fig. 3, lane 3),
demonstrating that this monoclonal antibody recognized both ABP280 and
Fh1 but did not cross-react with PS1. However, when ABP280 or Fh1
in vitro translation products were incubated with in
vitro-translated PS1 protein, PS1 was coimmunoprecipitated by this
monoclonal antibody (Fig. 3, lanes 2 and 5,
respectively).

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Figure 3.
ABP280 and Fh1 interact with PS1/2 in
coimmunoprecipitation experiments. The individual proteins were labeled
by in vitro translation in the presence of
[35S]methionine. The monoclonal antibody NCL-FIL
precipitated both ABP280 (labeled as ABP or
A; lanes 1 and 6)
and Fh1 (labeled as Fh1 or F;
lanes 4 and 9) from in vitro
translation products. The antibody did not cross-react with in
vitro-translated PS1 or PS2 loop region (lanes 3
and 8). After coincubation with either ABP280 or Fh1
proteins, PS1 (lanes 2 and 5) or PS2
(lanes 7 and 10) was coimmunoprecipitated
by the antibody. The immunoprecipitated products were separated on
SDS-PAGE. An autoradiograph of the gel is shown.
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We also examined whether the PS2 loop region could interact with ABP280
or Fh1 in the coimmunoprecipitation assay. Although monoclonal antibody
NCL-FIL did not immunoprecipitate in vitro-translated PS2
protein (Fig. 3, lane 8), PS2, like PS1, was
coimmunoprecipitated by this antibody after being incubated with ABP280
or Fh1 (Fig. 3, lanes 7 and 10). Similar
coimmunoprecipitation results were obtained by using monoclonal
antibodies against epitope tags present in the in
vitro-translated proteins (data not shown). Unrelated proteins
that did not bind to ABP280/Fh1 in the yeast two-hybrid assay did not
coimmunoprecipitate with ABP280/Fh1 in this assay, suggesting that
coimmunoprecipitation of PS1/2 with ABP280/Fh1 was not attributable to
nonspecific protein-protein interactions. These results demonstrate
that both PS1 and PS2 loop regions are able to interact with ABP280 and
Fh1 proteins in vitro.
Using the yeast two-hybrid assay, we have confirmed that Fh1 and ABP280
are able to interact with the PS2 loop region (Table 2). Quantitative liquid -galactosidase
assays were performed with protein extracts prepared from yeast
expressing the bait PS1 or PS2-LexA fusion proteins and the prey
proteins Fh1 or ABP280 fused to the transcription activation domain.
Because APP protein, another important player in the pathogenesis of
AD, also contains a hydrophilic intracellular domain, we tested whether
the APP intracellular domain could interact with Fh1 or ABP280. In
yeast expressing the APP intracellular domain as a LexA fusion protein and the Fh1 or ABP280 prey proteins, only basal levels of
-galactosidase activity were detected, indicating that the APP
intracellular domain does not interact with Fh1 or ABP280. When either
PS1- or PS2-LexA fusion protein was expressed in yeast containing Fh1 or ABP280 prey proteins, significant activation of -galactosidase activity was detected, indicating that both the PS1 and PS2 loop regions were able to interact with Fh1 or ABP280. These experiments confirm the results obtained from the coimmunoprecipitation
experiments.
We then investigated whether mutations found in the PS1 hydrophilic
loop region in AD patients affected interactions with Fh1 or ABP280.
Interestingly, all of the PS1 mutants examined, including P267S, R269H,
E280A, L286V, and the exon 9 deletion (dEx9), showed similar or higher
levels of -galactosidase activation, indicating that these mutants
retain the ability to interact with Fh1 or ABP280. It is possible that
these PS1 mutants represent gain-of-function rather than loss-of
function mutants. Whether these PS1 mutants indeed interact more
strongly with Fh1 or ABP280 proteins in cells and the functional
significance of such interactions will be examined further.
PS1 appears to modify intracellular distribution of Fh1 and ABP280
in transfected cells
To investigate whether PS1 interacts with ABP280 and Fh1 in cells,
we examined the intracellular localization of these proteins in cells
overexpressing PS1. After Cos-1 cells were transiently transfected with
a plasmid expressing full-length PS1 protein, double immunofluorescence
microscopy was performed by using the NCL-FIL monoclonal antibody to
detect endogenously expressed ABP280 or Fh1 (Fig.
4A) and polyclonal
rabbit anti-PS1 antibodies (Ab14) to detect PS1 protein (Fig.
4B). As observed in previous studies (Doan et al.,
1996 ; Kovacs et al., 1996 ), in transfected cells PS1 is localized
primarily in the endoplasmic reticulum, Golgi apparatus, or other
intracellular membrane structures in the perinuclear region (Fig.
4B). Low levels of PS1 immunostaining also were
detected in nontransfected cells, which may represent immunoreactivity with endogenous PS1 protein (compare the four cells in the center with
the surrounding cells in Fig. 4B), because no signal
was detected by the same staining procedure, using the preimmune serum (data not shown). Although the NCL-FIL immunostaining was relatively uniform in the cytoplasm of Cos-1 cells not overexpressing PS-1 (see
the cells in the periphery of Fig. 4A), a stronger
NCL-FIL immunostaining signal was detected in the perinuclear region in cells overexpressing PS1 (Fig. 4A), suggesting that
PS1 may be able to modulate the intracellular distribution of these
actin-binding proteins. The PS1 immunoreactivity could be superimposed
on the perinuclear signal from the NCL-FIL immunostaining. These
results strongly suggest that PS1 can interact with ABP280 and Fh1 in cells.

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Figure 4.
PS1 overexpression may modify intracellular
distribution of ABP280/Fh1 proteins in transfected cells. After Cos-1
cells were transfected with a plasmid expressing the full-length PS1
protein, cells were fixed and double-stained, using polyclonal rabbit
anti-PS1 antisera (B) and the monoclonal antibody
NCL-FIL, which recognizes both ABP280 and Fh1
(A). Fluorescein-conjugated anti-mouse secondary antibody or Cy3-conjugated anti-rabbit secondary was used to detect ABP280/Fh1 or PS1 staining, respectively. ABP280/Fh1 staining is shown
in A. The four cells in the center of the
picture were expressing high levels of PS1, as detected by the anti-PS1
antibody shown in B. In cells not transfected by the PS1
plasmid, ABP280/Fh1 staining is more uniform throughout the cytoplasm,
whereas the ABP280/Fh1 staining in cells overexpressing PS1 is
perinuclear and similar in distribution to that of PS1 protein (compare
A with B).
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Immunodetection of PS1 and ABP280/Fh1 proteins in normal and
AD brains
To investigate the possible role of ABP280 and Fh1 in AD
pathogenesis, we examined seven AD brains and two age-matched control brains for expression of PS1 and ABP280/Fh1 proteins by
immunohistochemical staining. On cryostat sections of the frontal
cortex from both control and AD brains, robust immunostaining was
detected in cells with fibrillary astrocyte morphology and in cell
processes with either the monoclonal antibody NCL-FIL or two different
polyclonal antisera raised against the N terminus of PS1 (Lee et al.,
1996 ; Malin et al., 1997 ) (Fig. 5; data
not shown). The number and the size of immunoreactive astrocytes
appeared to be greater in the AD brains as compared with the control
brains. The control brains showed an astrocyte distribution similar to
that obtained with antibodies against glial fibrillary acidic protein,
with most astrocytes in the subpial cortex and junction region between
cortical gray and white matter, as well as in white matter. In AD
brains, however, PS1-positive astrocytes are distributed throughout the cortical laminae. Weaker PS1 immunoreactivity was detected in the
cytoplasm of some cells with neuronal morphology (Fig. 5B). No clear PS1 immunostaining signal was detected in senile plaques, NFT,
neuropil threads, or blood vessels.

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Figure 5.
Expression of PS1 and ABP280/Fh1 proteins in
control and AD brains. A-H show microscopic findings
from nondemented controls and persons with advanced AD, using human
brain sections that have been immunostained with polyclonal antisera to
human PS-1 or monoclonal antibodies to human filamin (NCL-FIL) and
hyperphosphorylated tau (PHF-1). All sections were immunostained with
the Vector ABC Elite kit, with diaminobenzidine as a substrate, and
were lightly counterstained with hematoxylin before being coverslipped.
A, Frozen 6-µm-thick section of the frontal
cortex from an 89-year-old woman with advanced AD has been fixed for 30 min in paraformaldehyde before being immunostained with NCL-FIL diluted
1:200. Numerous reactive fibrillary astrocytes are evident, as are
immunopositive capillaries (250× magnification). B, A
6-µm-thick frozen section near that in A has been
immunostained with polyclonal PS-1 antibody (1:10 dilution; Lee et al.,
1996 ) to show reactive astrocytes in AD cortex. The
inset shows two other glial cells with astrocyte morphology. Lightly immunostained cells of uncertain nature are identified by arrows. The more densely labeled cell in
the main panel has the morphology of a small neuron with labeled
nonfibrillary cytoplasm (400× magnification). C, An
entorhinal cortex paraffin section (6 µm) from the brain of a
78-year-old man with advanced AD has been fixed in ethanol saline
before being immunostained with NCL-FIL at 1:200 dilution. The
neurofibrillary tangles detected on this specimen are mostly
extraneuronal tangles, which are not stained by NCL-FIL. The filamin
antibody labels capillaries (arrow) and arterioles and
reveals reactive astrocytic gliosis (400× magnification). D, Cerebral cortex paraffin section from
paraformaldehyde-fixed brain of a 79-year-old nondemented woman shows
intense transmural immunostaining of both meningeal muscular arteries
and parenchymal arteries, as well as capillaries, with filamin antibody
diluted at 1:200. Veins and venules (not shown) also were stained by
the filamin antibody. Comparable patterns of filamin immunoreactivity were seen in control and AD brains (100× magnification).
E, The same specimen as in A and
B shows large reactive filamin-positive astrocytes
surrounding and within an ill-defined circular immunopositive area with
a cortical senile plaque profile (NCL-FIL at 1:200 dilution; 400×
magnification). F, The same brain as in
A, B, and E showing filamin staining pattern of hippocampal CA1 fields in advanced AD.
Several intraneuronal globoid tangles (arrows) are
immunopositive, as is a neurite contributing to a senile plaque
(arrowhead). Adjacent extraneuronal ghost tangles
(asterisk) are not labeled (NCL-FIL at 1:500 dilution;
400× magnification). G, Frontal cortex paraffin section
of ethanol saline-fixed brain stained with NCL-FIL at 1:500. The
specimen is the same brain illustrated in A, B, E, and
F. Four triangles, numerous neuropil threads, and the bulbous dystrophic
neurites within two senile plaques (insets) are
immunostained (400× magnification). H, A section
near that shown in G has been immunostained with a
monoclonal antibody against tau (PHF-1; Greenberg and Davies, 1990 ) to
show a similar pattern to the filamin antibody-labeling pattern in
G. PHF-1 labeled more tangles and neuropil threads than
did NCL-FIL in all AD brains (PHF-1 at 1:100 after microwave-citrate and formic acid antigen retrieval procedures; 400×
magnification).
|
|
On cryostat sections strong NCL-FIL immunoreactivity was detected in
astrocytes and fine cellular processes in the neuropil of the AD brains
(Fig. 5A). Similar but less intense immunoreactivity also
was seen in the control brains (data not shown). The phenotypic appearance and distribution of these NCL-FIL-positive astrocytes overlapped with those of PS1-labeled astrocytes. NCL-FIL-positive astrocytosis also was detected in paraffin sections of AD brains that
had been fixed in ethanol (Fig. 5C). In addition, robust NCL-FIL immunostaining was detected in blood vessels of all caliber, including capillaries and some meningeal vessels (Fig.
5C,D). This immunostaining pattern in meningeal vessels
resembled that observed with A peptide antibody 10D5 (Athena
Neurosciences, San Francisco, CA) (data not shown). Most interestingly,
in the AD brains unequivocal and strong NCL-FIL immunostaining was
detected in the majority of cortical and hippocampal CA1 intraneuronal NFTs and neuropil threads, as well as dystrophic neurites within some
senile plaques, demonstrating that ABP280/Fh1 proteins were present in
these structures (Fig. 5E-G). This NCL-FIL immunostaining pattern in the AD brain is similar to that obtained by using a monoclonal antibody against hyperphosphorylated tau protein, PHF-1 (Greenberg and Davies, 1990 ) (compare Fig. 5G,H),
except that PHF-1 labels more tangles and neuropil threads in all of
the AD brains we examined. This suggests that only a subset of NFT is labeled by the filamin antibody.
 |
DISCUSSION |
Numerous studies indicate that mutations in the presenilin genes
play an important role in the development of early-onset FAD. PS1 and
PS2 are members of a highly conserved family of TM proteins with
sequence similarity to the C. elegans Sel-12 and Spe-4 gene
products. Recent studies suggest that PS1/2 proteins have six or eight
TM domains with the N and C termini, as well as the largest hydrophilic
loop region, facing the cytoplasmic compartment. Identification of a
large number of mutations in the hydrophilic loop region suggests that
this region is critical for presenilin function. We therefore set out
to identify proteins that interact with the loop region of the
presenilin proteins. Among the proteins identified that interact with
both PS1 and PS2 are actin-binding protein ABP280 and a closely related
protein, Fh1. The protein-protein interactions between the ABP280/Fh1
and PS1/2 are specific, because there were no detectable interactions observed between ABP280/Fh1 and LexA fusion proteins containing the
intracellular domain of APP (Table 2) or other unrelated bait proteins
rich in hydrophilic amino acid residues (data not shown). The
interactions between ABP280/Fh1 and PS1/2 proteins not only were
detected in the yeast two-hybrid assay but also were confirmed in
coimmunoprecipitation experiments. Interaction between PS1 and
ABP280/Fh1 is supported further by the observation that overexpression
of PS1 in Cos-1 cells by transient transfection leads to a change in
the intracellular distribution of the endogenous ABP280/Fh1 proteins
such that they colocalize with PS1. These results implicate a potential
role of presenilin proteins in modulating the cytoskeleton in cells.
Consistent with our results, it has been reported that the C-terminal
fragments of both PS1 and PS2 are associated with cytoskeleton (Kim et
al., 1997 ).
Our results demonstrating interactions between presenilins and
cytoskeletal-associated proteins ABP280/Fh1 provide insights into the
possible functions of the presenilin proteins. ABP280 protein has been
found to promote branching of actin filaments (Hartwig et al., 1980 ;
Niederman et al., 1983 ; Hartwig and Shevlin, 1986 ) and is proposed to
modulate cell shape, polarity, and motility via changes in actin
filament organization (Stossel, 1993 ; Matsudaira, 1994 ; Drubin and
Nelson, 1996 ; Mitchison and Cramer, 1996 ). In eukaryotic cells the
cytoskeleton is composed mainly of microtubules, microfilaments, and
intermediate filaments. Biochemical and structural studies indicate
that these polymeric systems are interacting with each other in cells
(Pollard et al., 1984 ; Green et al., 1987 ). Actin-binding proteins such
as ABP280 have been proposed to interconnect the microfilament and
intermediate filament cytoskeletal systems (Brown and Binder, 1992 ).
Recently, the actin cytoskeleton has been implicated in regulating the
activity of the cystic fibrosis TM conductance regulator (Prat et al.,
1995 ). It is conceivable that, by interacting with these cytoskeletal
proteins, presenilins may modulate the activities of these and other
related proteins in cells.
Both ABP280 and Fh1 are expressed in human brain tissue, as detected by
Northern blotting and immunostaining. Our results of
immunohistochemical staining of human brain sections indicate that both
PS1 and ABP280/Fh1 proteins colocalize to astrocytes and delicate
cellular processes in both gray matter and white matter of the cerebral
cortex and hippocampal formation. Although we could not detect strong
PS1 immunostaining in somatodendritic regions of neurons by using two
different polyclonal antisera against PS1 protein on either paraffin or
cryostat sections, strong PS1 immunoreactivity with both PS1 antisera
was detected in fibrous astrocytes, consistent with a previous report
that both mRNA and protein of PS1 are present in astrocytes (Lee et
al., 1996 ). In the AD brains there are more PS1-positive astrocytes
distributed throughout the cortical laminae, as compared with the
control brains. Immunostaining of astrocytes and fine cellular
processes by the monoclonal antibody recognizing ABP280/Fh1 is similar
to that detected by PS1 antibodies. Many studies suggest that
astrocytes play an important role in the amyloid deposition and
neurodegeneration observed in AD (Potter, 1992 ; Nieto-Sampedro and
Mora, 1994 ; Pike et al., 1995 ). Our study demonstrating that both PS1
and its interacting proteins ABP280/Fh1 are expressed in astrocytes
raises the possibility that presenilin protein in astrocytes plays an
important role in AD pathogenesis, providing additional support for the
involvement of astrocytes in development of AD.
Robust ABP280/Fh1 immunoreactivity also was seen in blood vessels,
NFTs, neuropil threads, and some senile plaque neurites. The
specificity of the monoclonal antibody NCL-FIL is demonstrated by the
observation that this antibody recognizes one protein band of 250 kDa,
which may represent both ABP280 and Fh1, and that it did not
cross-react with other proteins in the cell, including presenilin
proteins, other cytoskeletal proteins, or unrelated proteins
(NovoCastra Laboratories data) (Fig. 3; data not shown). Our PS1
antibodies did not show NFT immunoreactivity, although PS1 staining in
NFTs has been reported (Murphy et al., 1996 ). This discrepancy may
reflect differences in the specificity of different PS1 antisera. NFTs
are one of the major neuropathological characteristics of the AD brain.
The accumulation of NFTs is highly correlated with the loss of
pyramidal cells and with dementia. Ultrastructural and biochemical
studies of NFTs have revealed that the major constituent of the paired
helical filaments is polymerized and hyperphosphorylated tau (Wischik
et al., 1995 ). Molecular and immunohistochemical experiments
demonstrated that NFTs also contain a number of cytoskeletal proteins,
including neurofilament proteins, vimentin, actin, ubiquitin, and MAP2
(Wischik et al., 1995 ). Because actin has been detected in NFTs, it is not surprising that actin-binding proteins are also present in NFTs.
Our result that PS1 and PS2 interact directly with components of NFTs,
ABP280/Fh1 proteins, raises the possibility that the presenilins may
play an important role in the formation of NFTs. These observations
also suggest that protein-protein interactions between presenilins and
ABP280/Fh1 may be functionally significant. It is noted that only a
subset of NFTs was detected by the filamin monoclonal antibody. In
particular, in the AD brains we have examined, only intraneuronal NFTs,
but not extraneuronal NFTs, were labeled by the filamin antibody. The
exact mechanism of this apparent epitope selectivity is at present
uncertain and awaits further investigation by comparing results from
dual labeling and differential antigen retrieval and by using competing
antigens.
Human ABP280 has been mapped to the X chromosome (Xq28) (Gorlin et al.,
1993 ; Maetrini et al., 1993 ). The new filamin-related gene Fh1, which
was identified in our study, maps to human chromosome 3. It is
interesting to note that a putative locus for late onset FAD recently
has been mapped to chromosome 3, although detailed information is not
available yet (Tanzi et al., 1996 ). The protein-protein interactions
between Fh1 and presenilin proteins and the presence of Fh1 in NFT in
the AD brain suggest that Fh1 is a candidate FAD gene. We are presently
testing whether Fh1 could be the putative AD locus on chromosome 3.
 |
FOOTNOTES |
Received Aug. 20, 1997; revised Oct. 2, 1997; accepted Nov. 6, 1997.
This work is supported by grants from the Alzheimer's Disease Research
Center at Washington University School of Medicine, from the Missouri
Alzheimer's Disease and Related Disorder Program, the Alzheimer's
Association, and the American Health Assistance Foundation to J.Y.W.;
by National Institutes of Health Grant AG-05861 from the National
Institute of Aging to Washington University School of Medicine (WUSM)
ADRC (L. Berg, M.D.); and by Grants from National Institutes of Health
(AG00634 and AG05681), the Western Southern Foundation, and the
Metropolitan Life Foundation to A.G. The anti-PS1 and PHF1 antibodies
were generously provided by Drs. S. Gandy, J. Nerbonne, J. Gitlin,
and P. Davies. We thank Deborah Carter and Raymond Taylor for expert
assistance in preparation of tissue sections and immunohistochemical
staining procedures. We acknowledge the contributions of the WUSM ADRC
Clinical Core (John Morris, M.D., director) and the
Neuropathology/Tissue Resource Core (Daniel McKeel, M.D., director),
and components of WUSM ADRC for providing diagnoses, patient
assessment, and tissue preparation. We also thank Drs. A. L. Schwartz and Y. Rao for critical reading of this manuscript and members
of the WUSM Neuropathology Division (R. E. Schmidt, M.D., Ph.D.)
for providing expert neuropathological diagnoses.
Correspondence should be addressed to Dr. Wu at the above address.
 |
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A. Louvi, S. S. Sisodia, and E. A. Grove
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I. Onoprishvili, M. L. Andria, H. K. Kramer, N. Ancevska-Taneva, J. M. Hiller, and E. J. Simon
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L. J. Sampson, M. L. Leyland, and C. Dart
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V. L. Sheen, Y. Feng, D. Graham, T. Takafuta, S. S. Shapiro, and C. A. Walsh
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Y.-H. Suh and F. Checler
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A. L. Nielsen, I. E. Holm, M. Johansen, B. Bonven, P. Jorgensen, and A. L. Jorgensen
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F. Moro, R. Carrozzo, P. Veggiotti, G. Tortorella, D. Toniolo, A. Volzone, and R. Guerrini
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M. Nikki, J. Merilainen, and V.-P. Lehto
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A. van der Flier, I. Kuikman, D. Kramer, D. Geerts, M. Kreft, T. Takafuta, S. S. Shapiro, and A. Sonnenberg
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T. Borbiev, A. D. Verin, S. Shi, F. Liu, and J. G. N. Garcia
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R. Lin, K. Karpa, N. Kabbani, P. Goldman-Rakic, and R. Levenson
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G. Pigino, A. Pelsman, H. Mori, and J. Busciglio
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S.-J. JEONG, H.-S. KIM, K.-A CHANG, D.-H. GEUM, C. H. PARK, J.-H. SEO, J.-C. RAH, J. H. LEE, S. H. CHOI, S. G. LEE, et al.
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P. F.M. van der Ven, S. Wiesner, P. Salmikangas, D. Auerbach, M. Himmel, S. Kempa, K. Haye{beta}, D. Pacholsky, A. Taivainen, R. Schroder, et al.
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H. Tanahashi and T. Tabira
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Y Guo, S. Zhang, N Sokol, L Cooley, and G. Boulianne
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S. Krief, J.-F. Faivre, P. Robert, B. Le Douarin, N. Brument-Larignon, I. Lefrere, M. M. Bouzyk, K. M. Anderson, L. D. Greller, F. L. Tobin, et al.
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W. J. Ray, M. Yao, J. Mumm, E. H. Schroeter, P. Saftig, M. Wolfe, D. J. Selkoe, R. Kopan, and A. M. Goate
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D. B. Donoviel, A.-K. Hadjantonakis, M. Ikeda, H. Zheng, P. S. G. Hyslop, and A. Bernstein
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A. Alberici, D. Moratto, L. Benussi, L. Gasparini, R. Ghidoni, L. B. Gatta, D. Finazzi, G. B. Frisoni, M. Trabucchi, J. H. Growdon, et al.
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I. Imafuku, T. Masaki, M. Waragai, S. Takeuchi, M. Kawabata, S.-i. Hirai, S. Ohno, L.E. Nee, C.F. Lippa, I. Kanazawa, et al.
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M.-g. Li, M. Serr, K. Edwards, S. Ludmann, D. Yamamoto, L. G. Tilney, C. M. Field, and T. S. Hays
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A. L. Schwarzman, N. Singh, M. Tsiper, L. Gregori, A. Dranovsky, M. P. Vitek, C. G. Glabe, P. H. St. George-Hyslop, and D. Goldgaber
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S. M. Stabler, L. L. Ostrowski, S. M. Janicki, and M. J. Monteiro
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M. P. Mattson and Qing Guo
{blacksquare} REVIEW : The Presenilins
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T. Takafuta, G. Wu, G. F. Murphy, and S. S. Shapiro
Human beta -Filamin Is a New Protein That Interacts with the Cytoplasmic Tail of Glycoprotein Ibalpha
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A. Sasaki, Y. Masuda, Y. Ohta, K. Ikeda, and K. Watanabe
Filamin Associates with Smads and Regulates Transforming Growth Factor-beta Signaling
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E.-K. Choi, N. F. Zaidi, J. S. Miller, A. C. Crowley, D. E. Merriam, C. Lilliehook, J. D. Buxbaum, and W. Wasco
Calsenilin Is a Substrate for Caspase-3 That Preferentially Interacts with the Familial Alzheimer's Disease-associated C-terminal Fragment of Presenilin 2
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E. Pack-Chung, M. B. Meyers, W. P. Pettingell, R. D. Moir, A. M. Brownawell, I. Cheng, R. E. Tanzi, and T.-W. Kim
Presenilin 2 Interacts with Sorcin, a Modulator of the Ryanodine Receptor
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A. van der Flier, I. Kuikman, D. Kramer, D. Geerts, M. Kreft, T. Takafuta, S. S. Shapiro, and A. Sonnenberg
Different splice variants of filamin-B affect myogenesis, subcellular distribution, and determine binding to integrin {beta} subunits
J. Cell Biol.,
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[Abstract]
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