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From the Department of Neurosurgery *
and the
Department of Pathology and Laboratory
Medicine,
University of Pennsylvania,
Philadelphia, Pennsylvania; and the Lilly Research
Labortories,
Lilly Corporate Center,
Indianapolis, Indiana
| Abstract |
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| Introduction |
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To overcome this technical obstacle, we
developed a model of brain trauma in transgenic mice that develop
AD-like Aß plaques in a specific brain region beginning at 6 months
of age. These mice were generated using a construct with the
platelet-derived growth factor promoter driving a human ß-amyloid
precursor protein minigene containing the familial AD mutation V
F at
APP position 717 (PDAPP).10,11
In the present
study, we evaluated the effects of brain trauma in the PDAPP mice at 4
months of age, ie, 2 months before the appearance of AD-like pathology,
on memory function, histopathology, and regional tissue levels of Aß
peptides.
| Materials and Methods |
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To induce brain trauma, we used a recently described mouse model of rigid cortical indentation that dynamically deforms the left parietal cortex.12 Briefly, brain trauma was induced by impacting a 3-mm diameter impounder onto the cortex (5 m/s, 1 mm depth) through a 5-mm craniectomy. We induced brain trauma or sham treatment (surgery without brain impact) in PDAPP neutered female mice at 4 months of age. As controls, we also induced brain injury in or sham treated 4-month-old neutered female wild-type littermates.
Memory Evaluation
The water maze paradigm has been described in detail.12,13 Briefly, the water maze is a circular pool 1 m in diameter. PDAPP and wild-type mice were trained to swim to a plexiglas platform submerged 1 cm, which they found by navigating using external visual cues. The mice were given 20 to 25 trials over 2 to 3 days to reach criterion, the last trial being 1 hour before brain damage or sham treatment. Only those reaching criterion were used. The animals that could perform the task were subjected to brain trauma (n = 14 wild-type, n = 13 PDAPP) or sham treatment (n = 14 wild-type, n = 15 PDAPP), and evaluated 1 week later for their ability to recall the platform location. The platform was removed and the mice were given 1-minute probe trials in the water maze while a video computer recording unit tracked their swim path. A memory score was derived by determining the relative amount of time spent in or near the former platform site. Statistical analyses were performed using two-way analysis of variance for all groups followed by a posthoc t-test with Bonferroni correction for individual comparisons.
Histopathological Analysis
The mice were humanely euthanized with an overdose of sodium pentobarbital (200 mg/kg) and transcardially perfused with saline after brain injury or sham treatment at 1 week (n = 5 injured, n = 3 sham wild-type mice; n = 5 injured, n = 3 sham PDAPP mice), 2 weeks (n = 5 injured, n = 3 sham wild-type mice; n = 5 injured, n = 3 sham PDAPP mice) and 2 months (n = 8 injured, n = 3 sham wild-type mice; n = 8 injured, n = 3 sham PDAPP mice). The brains were removed and fixed in 70% ethanol, embedded in paraffin, and cut into 6-µm sections. Brain sections were immunostained with a variety of previously characterized monoclonal and polyclonal antibodies that are highly specific for distinctive epitopes within the Aß peptides, including 2332, BA-27, and BC-05.7,14-16 Alternate sections were stained with hematoxylin, eosin, and cresyl violet. Light microscopic examination was then performed on the sections. Because we found an extreme loss of hippocampal CA2 and CA3 neurons in the brain-injured PDAPP mice, we selected one region of the dorsal hippocampus to perform neuron cell counting. Three adjacent sections from bregma-1.717 were selected for each sham or injured animal sacrificed 1 to 2 weeks posttrauma (n = 10/injured group, n = 6/sham group). Stained neurons with clearly identifiable nuclei were manually counted via light microscopy in the CA2 and CA3 regions of the hippocampus ipsilateral to injury. These regions, selected according to atlas determination,17 encompassed the entire CA2 and the CA3 from the margin of the CA2 to the most lateral extension of the dentate gyrus. The average number of neurons from each region of the three sections for each animal were determined and used to determine group means. Statistical analysis of the neuron loss was performed using a two-way analysis of variance for all groups followed by a posthoc t-test with Bonferroni correction for individual comparisons.
Aß Tissue Levels
PDAPP mice and wild-type littermates were euthanized (see above) at 0 hours (sham), 2 hours, 6 hours, 24 hours, 3 days, and 7 days posttrauma. Following sacrifice, the brains were rapidly removed and dissected on a chilled plate isolating cortex, hippocampus, thalamus, brainstem, and cerebellum. These samples were Dounce-homogenized in 6.7 volumes (w/v) of 70% formic acid at 4°C. Homogenates were centrifuged at 45,000 rpm, 4°C for 1 hour. The supernatant was diluted 1:20 in 1 mol/L Tris base. Samples (n = 412/group) were evaluated with sandwich enzyme-linked immunosorbent assays for Aß peptides as previously described.10,18 Absorbencies falling within the standard curve for each assay were converted to fmoles. Sandwich enzyme-linked immunosorbent assays for Aß were prepared with the capture antibody BAN-50, a monoclonal antibody specific for amino acids 116 of Aß, and reporting antibodies BA-27, specific for Aß ending at Aß40, and BC-05, specific for Aß ending at Aß 42.43 The assay has a detection limit of <6 fmole/well for Aß140 and Aß142. The monoclonal antibodies BAN-50, BA-27, and BC-05 were prepared as described previously.19 Statistical analyses were performed using a two-way analysis of variance for all groups followed by a posthoc t-test with Bonferroni correction for individual comparisons.
| Results |
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We found that although all wild-type mice easily learned the
spatial memory task before injury, approximately 20% of the transgenic
animals did not reach criterion and were excluded from further
analysis. Of the animals that did reach criterion, several significant
differences were found between groups evaluated one week later for
memory retention of the task. Consistent with previous reports,
wild-type brain-injured animals demonstrated significant memory
dysfunction compared to their respective sham group
(P < 0.001).12
In
addition, a significant impairment of memory retention was found in the
sham PDAPP mice compared to sham wild-type animals
(P < 0.01). Moreover, brain-injured PDAPP mice
had a more profound impairment of memory function than did
injured wild-type animals (P < 0.001) (Figure 1)
. Importantly, the swimming speed of the
mice in all groups was identical, demonstrating that potential motor
impairment did not influence performance.
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Brain sections immunostained with a variety of antibodies that are highly specific for distinctive epitopes within the Aß peptides revealed the presence of only a few Aß-containing plaques, primarily in the hippocampus, that were indistinguishable between the 4-month-old brain-injured and sham-treated PDAPP animals. Although 6-month-old PDAPP animals demonstrated several more Aß-containing plaques, consistent with previous reports,10,11 there was no discernable difference in the number or distribution of plaques between brain-injured (2 months postinjury) and sham-treated animals. Thus, brain trauma did not accelerate or exacerbate Aß deposition in the PDAPP mice.
Nonetheless, we did observe a dramatic difference in hippocampal
pathology in the brain-injured PDAPP versus the wild-type
mice. At both 1 and 2 weeks after injury, the wild-type mice
demonstrated a modest loss of hippocampal neurons in a patchy
distribution throughout the ipsilateral CA2 and
CA3 subfield exactly as described
earlier.20
In striking contrast, the entire
length of the CA3 pyramidal cell layer of the
brain-injured PDAPP mice degenerated completely with virtually no or
very few neurons remaining (Figure 2)
.
Quantitative analysis of the CA2 and
CA3 regions showed that the number of neurons in
the hippocampi of wild-type and PDAPP sham animals were nearly the
same. Brain injury in both the wild-type and PDAPP mice induced a
highly significant loss of neurons in the CA2 and
CA3 regions of the hippocampus compared to
respective sham groups (P < 0.001). The
wild-type animals had a 52% loss of CA2 neurons
compared to a 58% loss for the PDAPP animals, an insignificant
difference. However, in the CA3 region the
wild-type brain-injured mice demonstrated a 36% loss of neurons
whereas the PDAPP brain-injured mice showed a remarkable 84% loss,
compared to respective sham groups. This difference between
injured groups was found to be highly significant
(P < 0.001). These results are shown in Figure 3
.
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Aß140 and
Aß142 levels in samples from both sham and
brain-injured wild-type mice were either very low or below the level of
detection at all timepoints. Thus, no injury effect on Aß levels
could be discerned in the wild-type mice. In the sham PDAPP mice, the
highest levels of Aß140 and
Aß142 were expressed in the hippocampus at
twice the concentration found in the cortex, with the
Aß142 present at levels similar to those
previously reported in PDAPP mice at 4 months (Figure 4)
.10,11
In addition,
very low amounts of Aß140 and
Aß142 were found in the thalamus, cerebellum,
and brainstem in the sham PDAPP mice. Two hours after trauma in PDAPP
mice, a dramatic sevenfold increase in the concentration of
Aß142 and threefold increase in
Aß140 were observed in hippocampal tissue
(P < 0.001), whereas smaller but significant
increases were seen in cortical tissue (P <
0.01). Thus, the ratio of the levels of Aß140
compared to Aß142 also changed in the
hippocampus after injury. In sham animals
Aß142 was found at three times the
concentration of Aß140, but at 2 hours
posttrauma, the concentration of Aß142 became
10-fold greater than Aß140. However, by 6
hours posttrauma, Aß140 and
Aß142 concentrations in the hippocampus had
returned to baseline values and by 24 hours,
Aß142 levels fell below baseline in the
cortex (Figure 4)
. No other regions demonstrated detectable changes in
Aß concentrations after brain injury in PDAPP mice.
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| Discussion |
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The acute posttraumatic increase of Aß142 in the 4-month-old PDAPP mice reached levels similar to those found in 1-year-old noninjured PDAPP mice when substantial Aß deposition is observed throughout the hippocampus.10,11 Notably, the high levels of Aß in the brain-injured PDAPP mice were associated not with plaque formation but with neuron death. However, hippocampal neuron death has never been reported in the absence of brain injury in this or another line of transgenic mice that overexpress mutant ß-amyloid precursor proteins and develop increasing numbers of AD-like amyloid plaques with advancing age22,23 Moreover, no induction of neuron death has been seen after Aß injections in rat brains.24,25 In association, these findings suggest that in vivo Aß may not be neurotoxic unless a second stress or insult triggers Aß to induce neuron death. In support of this hypothesis, a very recent report demonstrated increased susceptibility to ischemic brain damage in another line of transgenic mice overexpressing mutant APP.26 Although there was no difference in Aß levels between ischemic and nonischemic hemispheres at 24 hours postinjury, high baseline levels of Aß were present in the brains of these transgenic animals. Thus, we propose a "two-hit" hypothesis for AD wherein Aß is necessary but not sufficient to cause neuron death until a second pathological process potentiates Aß neurotoxicity.
Because an acute and marked posttraumatic increase in the concentrations of both Aß140 and Aß142 implicates Aß peptides as potential deleterious agents in the posttraumatic sequelae, these findings suggest a mechanistic link in the pathogenesis of brain trauma and AD which may have important clinical implications. Accordingly, recent reports have shown that brain trauma patients with apolipoprotein E-4 genotypes, a genetic risk factor for AD, have much worse outcomes after brain trauma, even in the very acute setting.27,28,29 Taken together, these findings suggest that the role of brain trauma in AD can be explored in greater detail using the model presented here.
| Acknowledgements |
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| Footnotes |
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Supported by National Institutes of Health Grants AG12527, AG11542, AG09215, NS26818, and NS08803.
Accepted for publication June 10, 1998.
| References |
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-rich dystrophic neurites permeates neocortex and nearly all neuritic and diffuse amyloid plaques in Alzheimer's disease. FEBS Lett 1994, 344:69-73[Medline]
4 in patients with Alzheimer's disease. Neurology 1995, 45:555-557
4 allele is associated with deposition of amyloid ß-protein following head injury. Nat Med 1995, 1:135-137[Medline]
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