|Home | About | Journals | Submit | Contact Us | Français|
Homozygous APPV717F transgenic mice overexpress a human β-amyloid precursor protein (βAPP) mini-gene encoding a familial Alzheimer’s disease mutation. These mice develop Alzheimer-type neuritic β-amyloid plaques surrounded by astrocytes. S100β is an astrocyte-derived cytokine that promotes neurite growth and promotes excessive expression of βAPP. S100β overexpression in Alzheimer’s disease correlates with the proliferation of βAPP-immunoreactive neurites in β-amyloid plaques. We found age-related increases in tissue levels of both βAPP and S100β mRNA in transgenic mice. Neuronal βAPP overexpression was found in cell somas in young mice, whereas older mice showed βAPP overexpression in dystrophic neurites in plaques. These age-related changes were accompanied by progressive increases in S100β expression, as determined by S100β load (percent immunoreactive area). These increases were evident as early as 1 and 2 months of age, months before the appearance of β-amyloid deposits in these mice. Such precocious astrocyte activation and S100β overexpression are similar to our earlier findings in Down’s syndrome. Accelerated age-related overexpression of S100β may interact with age-associated overexpression of mutant βAPP in transgenic mice to promote development of Alzheimer-like neuropathological changes.
Alzheimer’s disease is characterized neuropathologically by cerebral deposits of β-amyloid with associated activated glia and abnormal, enlarged, “dystrophic” neuritic elements. β-Amyloid derives from a larger, transmembrane β-amyloid precursor protein (βAPP) that is encoded by a gene on chromosome 21. Some rare inherited forms of Alzheimer’s disease are due to mutations in the βAPP gene that presumably accelerate β-amyloid deposition and the constellation of attendant neuropathological consequences.
The importance of βAPP and β-amyloid in Alzheimer’s pathogenesis is readily apparent, but the pathophysiological mechanisms linking these proteins to neuronal injury (and neurologic dysfunction) in Alzheimer’s disease remain unclear. Intracerebral administration of β-amyloid in rats does not produce consistent dose-related damage (Stein-Behrens et al., 1992). Injection of β-amyloid into brains of aged monkeys does produce some Alzheimer-like neuronal cytoskeletal changes, but this effect is not seen in young animals (McKee et al., 1998). In elderly humans, diffuse amyloid deposits are sometimes present without accompanying neuritic degeneration or dementia (Hof et al., 1996; Troncoso et al., 1996). These and similar findings suggest that factors other than β-amyloid deposition are also essential for the development of Alzheimer’s disease.
According to our hypothesis, overexpression of βAPP results in overexpression of secreted amyloid precursor protein (Buxbaum et al., 1992), which activates microglia and promotes overexpression of interleukin-1 (Barger and Harmon, 1997), and this interleukin-1 overexpression, in turn, induces astrocyte activation (Giulian et al., 1988) and S100β overexpression (Sheng et al., 1996a). Activated astrocytes overexpressing S100β are invariably and intimately associated with the neuritic β-amyloid plaques of Alzheimer’s disease (Sheng et al., 1994; Mrak et al., 1996), and tissue levels of biologically active S100β are elevated in Alzheimer’s brain (Marshak et al., 1991). In vitro, S100β promotes neurite outgrowth (Kligman and Marshak, 1985) and induces neuronal overexpression of βAPP (Li et al., 1998). Both the numbers of astrocytes overexpressing S100β and the tissue levels of S100β correlate with the density of neuritic plaques in Alzheimer’s brain (Sheng et al., 1994) and with the distribution of Alzheimer’s lesions across brain regions (Van Eldik and Griffin, 1994). Moreover, the numbers of astrocytes overexpressing S100β correlate with the proliferation of dystrophic neurites, overexpressing βAPP, in cerebral cortex and even within individual plaques in Alzheimer’s disease (Mrak et al., 1996). These observations collectively suggest that astrocytic overexpression of S100β is important in the induction and maintenance of dystrophic neurites, the defining step in the conversion of diffuse amyloid deposits into the diagnostic neuritic plaques in Alzheimer’s disease.
Recently, transgenic mice that overexpress a human βAPP minigene encoding the V717F familial Alzheimer’s disease mutation have been shown to be a reliable animal model of β-amyloid deposition and development of Alzheimer-like neuritic pathology (Games et al., 1995). These mice, which carry modified intronic sequences allowing for alternative splicing of the βAPP gene, exhibit both age-dependent and region-specific deposition of diffuse and fibrillar β-amyloid deposits similar to those observed in Alzheimer’s disease. In the present study, we evaluated astrocytic overexpression of S100β in brains of APPV717F mice. We find astrocyte activation and S100β overexpression in mice as young as 2 months of age, well before the appearance of β-amyloid plaques. Old mice show progressive increases in S100β and βAPP expression, as well as redistribution of βAPP immunoreactivity from neuronal somas in young mice to dystrophic neurites associated with complex neuritic β-amyloid. The morphology and pattern of glial association with these plaques are virtually indistinguishable from those of Alzheimer’s disease.
The 34 female transgenic mice (1–17 months of age) used in this study were homozygotes derived from previously derived heterozygous PDAPP mice, strain 109 (Games et al., 1995). Thirty-four wild-type female littermates served as controls. All were obtained from Taconic Farms. These mice were maintained under conventional conditions at 25°C and fed a commercial diet and tap water. The mice were killed by administering an overdose of ketamine. Left half brains were processed for immunohistochemistry, and right half brains were flash-frozen in liquid nitrogen and stored at −80°C until used for northern analysis.
Northern analyses were performed as previously described (Sheng et al., 1996b). In brief, 10-μg samples of total RNA, extracted from individual brain hemispheres, were subjected to electrophoresis on a 1% agarose and 5% formaldehyde gel and then transferred to a GeneScreen membrane (NEN Research Products, Doraville, GA, U.S.A.) by capillary action over a 12-h period, using 10 × SSC (87 g of NaCl and 44 g of sodium citrate/L, pH 7). Membranes were baked at 80°C for 2 h and prehybridized for 4 h at 42°C with 50% formamide containing 1% sodium dodecyl sulfate (SDS), 1 M NaCl, 10% dextran sulfate, 0.2 mg/ml salmon sperm DNA, and 50 mM Tris-HCl, pH 7.5. Samples were then hybridized for 24 h at 42°C with 100 ng of 32P-labeled pKN-3 human S100β (a gift from Dr. Alex Marks, University of Toronto), glial fibrillary acidic protein (GFAP), and βAPP cDNAs (1 × 109 cpm/μg). The membranes were washed twice with 2 × SSC and then once with 0.2 × SSC, each containing 1% SDS, at 65°C for 20 min and exposed to Kodak XAR film.
Left half brains were fixed in 4% paraformaldehyde, serially sectioned in a coronal plane, and embedded in paraffin. Immunohistochemistry was performed on 8-μm-thick tissue sections from paraffin-embedded blocks. Primary antibodies used were polyclonal rabbit anti-human S100β, diluted 1:2,000 [a gift from Dr. Linda Van Eldik, now commercially available as RaS100β (East Acres Biologicals, South-bridge, MA, U.S.A.; or SWant Antibodies, Bellinzona, Switzerland)] (Van Eldik and Griffin, 1994); polyclonal rabbit anti-bovine GFAP (DAKO, Carpinteria, CA, U.S.A.), diluted 1:100; and monoclonal mouse anti-β-amyloid (DAKO), diluted 1:50.
Double labeling kits were purchased from DAKO. Deparaffinized tissue sections were processed according to the manufacturer’s protocol, as previously described (Mrak et al., 1996). Primary antibodies used were rabbit anti-S100β (diluted 1:2,000) or rabbit anti-GFAP (diluted 1:100) together with monoclonal mouse anti-β-amyloid (diluted 1:50; DAKO) or βAPP antibody (Boehringer-Mannheim, Indianapolis, IN, U.S.A.).
Immunohistochemical reactivity was examined in hippocampus in coronal sections. Microscopical images were captured using a CCD video camera attached to a PowerMac 9500 computer. Image analysis was performed on five microscopic fields (0.18 mm2 each) of one analogous 10-μm-thick tissue section from each mouse. All S100β-immunoreactive astrocytes were counted in each field, and the numbers, immunoreactive area, and immunoreactive intensity of S100β-immunoreactive cells were quantified using NIH Image software (version 1.59). Astrocyte activation was defined as cellular enlargement (da Cunha et al., 1993). Astrocytes with an immunoreactive area of ≥ 190 μm2 were counted as activated astrocytes, and astrocytes with an immunoreactive area of ≤ 160 μm2 were counted as nonactivated. There were no astrocytes with immunoreactive areas between 160 and 190 μm2. Accordingly, the image analysis program was set to identify these as such. Total tissue S100β “load” was defined as the S100β immunoreactive area expressed as a percentage of total area.
GFAP-immunoreactive astrocytes and β-amyloid-immunoreactive plaques were counted in immunoreacted serial tissue sections from the same tissue block used above. Five 25× fields of cerebral cortex and five 25× fields of hippocampus, each representing 0.36 mm2, were analyzed for each mouse.
Statistical analyses were carried out using ANOVA followed by Fisher’s test.
Figure 1 shows examples of northern hybridizations for βAPP, S100β, and GFAP mRNAs, illustrating higher levels of expression of each of these mRNAs in transgenic compared with wild-type mice and in old compared with young mice. The marked elevation of tissue levels of βAPP mRNA in transgenic mice was accompanied by significant elevations in tissue levels of S100β mRNA: 1.4-fold those of wild-type for young (1–4-month-old) animals and 1.6-fold those of wild-type for old (8–17-month-old) animals (p < 0.001; Fig. 2A). The total numbers of S100β-immunoreactive astrocytes in hippocampus of transgenic mice were 1.5-fold (for young mice) and 1.9-fold (for old mice) those of wild-type mice (p < 0.01 and p < 0.001, respectively; Fig. 2B).
Activated astrocytes, immunoreactive for S100β, were increased in number and in degree of activation in the hippocampus of both young and old transgenic animals compared with wild-type animals of similar ages (Fig. 2C). The mean ± SEM area of S100β-immunoreactive, activated astrocytes was 285 ± 20 μm2 compared with 122 ± 4 μm2 for S100β-immunoreactive, nonactivated astrocytes. As previously shown by da Cunha et al. (1993), enlargement is the distinguishing feature of astrocyte activation. The percentage of S100β-immunoreactive astrocytes that are activated increased from 45 to 63% in old versus young transgenic mice (data not shown). The numbers of activated S100β-immunoreactive astrocytes in hippocampus of transgenic mice were increased 50% (for young mice) and 90% (for old mice) compared with wild-type mice (p = 0.03 and p< 0.001, respectively; Fig. 2C).
Transgenic mice showed age-associated increases in the number of β-amyloid-immunoreactive deposits and in the number of S100β-immunoreactive astrocytes in hippocampus (Fig. 3A–D), and there was an apparent redistribution of βAPP-immunoreactive product from neuron cell somas to dystrophic neurites (Fig. 3F–H). The increase in the numbers of S100β-immunoreactive astrocytes achieved statistical significance by 2 months of age (p < 0.001; Fig. 4A). In contrast, no β-amyloid deposits were found in animals ≤4 months of age (Fig. 4C). The β-amyloid plaques found in these mice had associated S100β-immunoreactive, activated astrocytes and contained βAPP-immunoreactive dystrophic neurites (Fig. 3G). These activated astrocytes were located at the periphery of dystrophic neurites in β-amyloid plaques, and they had enlarged, elongated processes that surrounded and invaded plaques; these were of similar appearance to ones found in Alzheimer’s disease and Down’s syndrome (Mrak et al., 1996).
The size of activated astrocytes overexpressing S100β continued to increase after 8 months of age, from 269 μm2 at 4 months of age to 467 μ2 at 17 months of age in transgenic mice. As a consequence, the total tissue S100β load (percentage of tissue area occupied by S100β-immunoreactive astrocytes) increased progressively with age (Fig. 4B).
We find age-associated overexpression of the astrocyte-derived cytokine S100β in transgenic mice overexpressing a βAPP minigene, carrying a mutation responsible for familial Alzheimer’s disease. By 2 months of age, these mice have increased numbers of activated astrocytes overexpressing S100β, as well as increased tissue levels of S100β mRNA. These changes occur several months before the age-associated appearance of β-amyloid deposits in these animals. These findings suggest that βAPP overexpression in these mice elicits significant astrocytic responses long before the appearance of β-amyloid deposits. This is similar to findings in Down’s syndrome, where astrocytic activation and overexpression of S100β precede by decades the inevitable appearance of Alzheimer-like neuropathological changes (Griffin et al., 1989, 1998a).
The astrocytic activation and S100β overexpression seen in these mice increase progressively with age (see Fig. 4B). This age-associated increase in S100β overexpression is reminiscent of, but more marked than, increases in S100β expression that accompany normal aging in both humans (Sheng et al., 1996b) and experimental animals (Kato et al., 1990). This observation suggests an acceleration of aging-associated S100β expression in APPV717F transgenic mice, analogous to that described for senescence-accelerated, mutant mice (Griffin et al., 1998b).
The intimate anatomic associations between β-amyloid deposits and activated, S100β-immunoreactive astrocytes that we find in old APPV717F mice are similar to those observed in Alzheimer’s disease (Marshak et al., 1991; Sheng et al., 1994) and in Down’s syndrome (Griffin et al., 1989, 1998a; Royston et al., 1999). In Alzheimer’s disease, there are elevated tissue levels of both biologically active S100β protein and S100β mRNA (Marshak et al., 1991), and these increases correlate with overexpression of S100β by plaque-associated astrocytes (Griffin et al., 1989; Sheng et al., 1994; Mrak et al., 1996). S100β is a neurite growth-promoting cytokine (Kligman and Marshak, 1985), and the numbers of S100β-overexpressing astrocytes correlate with the extent of dystrophic neurite formation within cerebral cortex and within individual plaques in Alzheimer’s disease (Mrak et al., 1996). These findings suggest that S100β is important in the genesis of dystrophic neurites in β-amyloid deposits and thus in the conversion of such deposits into the neuritic β-amyloid plaques diagnostic of Alzheimer’s disease. The early and progressive overexpression of S100β in APPV717F transgenic mice, shown here, supports the idea that activated astrocytes overexpressing S100β are important and necessary factors in the genesis of neuritic pathology in β-amyloid plaques. In addition, this overexpression of S100β may contribute to the age-associated increase in βAPP in these transgenic mice as a consequence of S100β up-regulation of βAPP (Li et al., 1998).
S100β overexpression occurs in several other conditions associated with increased risk for Alzheimer’s disease or with accelerated appearance of Alzheimer-type neuropathological changes. Head trauma is an established risk for the later development of Alzheimer’s disease (Mortimer et al., 1991; Mayeux et al., 1993), and S100β overexpression occurs acutely following severe closed head injury (Griffin et al., 1997) along with overexpression of βAPP and interleukin-1 (Griffin et al., 1995). Accelerated appearance of Alzheimer-type neuropathological changes is seen in patients with chronic intractable epilepsy (Mackenzie and Miller, 1994) and in patients with HIV infection (Esiri et al., 1998), and S100β overexpression is prominent in both these conditions (Stanley et al., 1994; Griffin et al., 1995). The prime risk factor for Alzheimer’s disease is aging, and progressive increases in astrocytic S100β expression accompany normal aging (Sheng et al., 1996b). These observations further support the idea that S100β overexpression is important in the pathogenesis of Alzheimer-type neuritic β-amyloid plaques.
In conclusion, young mice overexpressing a mutant, human βAPP gene show astrocytic activation and overexpression of S100β months before the appearance of β-amyloid deposits, and old transgenic mice show intimate association of activated astrocytes, overexpressing S100β, with neuritic β-amyloid plaques. These results demonstrate early and progressive glial changes in a genetic animal model of Alzheimer’s disease, similar to those seen in Down’s syndrome and in Alzheimer’s disease itself. Moreover, these results support our hypothesis (Griffin et al., 1998c) that glia-derived cytokines are essential driving and organizing elements in the progressive cellular and molecular cascades (that include β-amyloid deposition) underlying the pathogenesis of Alzheimer’s disease.
The authors wish to thank Ms. Pam Free for administrative assistance. This research was supported in part by grant AG 12411 from the National Institutes of Health.