Species: Rat
Genes: APP, PSEN1
Mutations: APP K670_M671delinsNL (Swedish), PSEN1: deltaE9
Modification: APP: Transgenic; PSEN1: Transgenic
Disease Relevance: Alzheimer's Disease, Vascular Dementia
Strain Name: N/A
Genetic Background: Spontaneously hypertensive stroke-prone (SHRSP); Fischer 344
Availability: Available through Sally Frautschy.

Summary

Pure Alzheimer’s disease—in which brains contain only the pathological hallmarks of AD, amyloid plaques and neurofibrillary tangles—is rare: In one recent study of nearly 900 autopsy samples with AD pathology, approximately 85 percent also exhibited cerebrovascular pathology (Yu et al., 2019). In order to model this mixed pathology, TgF344-AD rats, which carry human APP and PSEN1 transgenes, with the AD-linked Swedish and Δ exon 9 mutations, respectively, were backcrossed with spontaneously hypertensive stroke-prone (SHRSP) rats. This new model, referred to as SHRSP/FAD, manifests chronic hypertension and exhibits several neuropathological features. While placing the AD-linked transgenes on the SHRSP background led to a slight amelioration of plaque pathology, other neuropathological features, including tau pathology, demyelination, and some cerebrovascular changes, were exacerbated compared with the parental lines. The SHRSP/FAD rat model may be suitable for preclinical studies of potential therapies directed against Alzheimer’s disease co-existing with mild cerebrovascular disease.

In the initial characterization summarized here, animals from the N2 generation (75 percent SHRSP and 25 percent Fischer 344) were studied between 16 and 18 months of age. SHRSP/FAD rats are hemizygous for the APP and PSEN1 transgenes.

In order to compare the phenotypes of the SHRSP/FAD rats to animals separately modeling amyloidosis (FAD) or cerebrovascular disease (SHRSP), TgF344-AD rats and SHRSP rats were backcrossed to Wistar Kyoto rats (the genetic background of SHRSP) or Fischer 344 rats, respectively, to yield animals with a mixed genetic background composed of 75 percent Wistar Kyoto and 25 percent Fischer 344. Non-transgenic, non-hypertensive rats (referred to as “WKY”) were also generated on a 75:25 Wistar Kyoto:Fischer 344 background.

Neuropathology | Synapse Loss | Neurodegeneration | Behavior | Modification Details |Related Strains

Neuropathology

Amyloid plaques are deposited in the cortices and hippocampi of SHRSP/FAD rats. Compared with FAD rats, plaque burden (percent area stained with an antibody directed against APP/Aβ) and plaque density were decreased in cortical areas of SHRSP/FAD animals. The morphology of the plaques also differed somewhat between the two genotypes, with plaques appearing more diffuse in the SHRSP/FAD rats.

Tau pathology appears to be more severe in SHRSP/FAD compared with WKY, FAD, or SHRSP rats. Using an antibody directed against tau phosphorylated at serine 422, an epitope found in paired helical filaments (Hasegawa et al., 1996), staining was increased in the hippocampi of SHRSP/FAD rats, compared with the other three genotypes. Notably, occasional neurons in the SHRSP/FAD brains were intensely stained, appearing to contain globose neurofibrillary tangles, with tortuous neurites. Biochemical analysis revealed a robust increase in the level of insoluble tau in the hippocampi of SHRSP/FAD rats, compared with WKY, FAD, or SHRSP rats.

There were fewer Iba1-immunoreactive microglia in the hippocampi of SHRSP/FAD rats, compared with the other three genotypes. However, SHRSP/FAD microglia were hypertrophied, with more and bushier processes, and many adopted a rod shape. Large microglia in these animals were often associated with microvessels.

Immunostaining for the astrocyte marker GFAP was more intense in SHRSP/FAD brains, compared with the other three genotypes. This elevation in GFAP levels was confirmed biochemically in western blots of hippocampal and entorhinal cortical lysates. While astrocytes were larger in FAD and SHRSP/FAD brains than in WKY or SHRSP brains, the two transgenic genotypes did not differ from each other.

Furthermore, levels of aquaporin-4, a water channel normally localized to astrocyte endfeet abutting cerebral vessels, were elevated in FAD and SHRSP/FAD, compared with the other two genotypes. In FAD rats, and even more dramatically in SHRSP/FAD rats, aquaporin-4 staining moved from a perivascular location to the neuropil. It is not yet clear whether this redistribution of aquaporin-4 reflects breakdown of the vessels or the retraction of the astrocyte endfeet.

Region- and genotype-dependent changes in the cerebral vasculature were observed. Vessel abnormalities were particularly evident in the globus pallidus of SHRSP/FAD rats, where vessel walls were thick and distorted, and there was a buildup of collagen IV in capillary lumina. There were also genotype-dependent effects on levels of PECAM-1 (platelet endothelial cell adhesion molecule-1, also known as CD31). PECAM-1 accumulates at the tight junctions between vascular endothelial cells and mediates monocyte migration across blood vessels. While the level of PECAM-1 in hippocampal lysates from SHRSP rats did not differ from the non-transgenic, non-hypertensive (WKY) animals, the level in FAD rats was decreased. However, when the FAD transgenes were placed on the SHRSP background, this decrease did not occur; the level of PECAM-1 was elevated in SHRSP/FAD rats, compared with FAD or SHRSP animals. Further characterization of these lines will be necessary to confirm these genotype-dependent changes and to explore their functional significance.

Synapse Loss

As yet, there is no evidence for synapse loss in SHRSP/FAD brain. Levels of the presynaptic markers SNAP25 and synaptophysin and of the postsynaptic marker drebrin were similar among the four genotypes (WKY, FAD, SHRSP, and SHRSP/FAD). Levels of the NMDA receptor NR2B subunit differed among the genotypes, with a slight increase in FAD, compared with WKY, and an even greater increase in SHRSP/FAD. The functional significance of this change in receptor composition has yet to be determined.

Neurodegeneration

Neuron numbers have not yet been assessed directly in SHRSP/FAD rats. However, a reduction in calbindin staining in SHRSP and SHRSP/FAD might reflect a loss of inhibitory neurons. Additionally, levels of caspase-cleaved actin, a marker of apoptosis, were higher in SHRSP/FAD brains than in WKY brains.

Demyelination observed in SHRSP brains was exacerbated in SHRSP/FAD animals.

Behavior

Working memory, measured by spontaneous alternation in the Y maze, did not differ between genotypes. However, in a different test of working memory, the novel object recognition test, transgenic and hypertensive animals appeared to show a deficit, as only non-hypertensive, non-transgenic (WKY) animals showed a preference for the novel object. It was not possible to evaluate any exacerbation of the deficit as a result of placing the FAD transgenes on the hypertensive background, because of a ceiling effect in this test.

Hyperactivity observed in the SHRSP background was not affected by the FAD transgenes.

Modification Details

TgF344-AD rats, which carry human APP and PSEN1 transgenes, with the AD-linked Swedish and Δ exon 9 mutations, respectively, were backcrossed with SHRSP rats. While this summary refers to the N2 generation, the SHRSP/FAD line has been further backcrossed to the SHRSP line; current SHRSP/FAD are at least the N6 generation (98:2 SHRSP:Fischer 344).

Related Strains

WKY/FAD. TgF344-AD rats were backcrossed to Wistar Kyoto rats, to place the AD-related transgenes on the same genetic background as the SHRSP line. While this summary refers to the N2 generation, the FAD line has been further backcrossed to Wistar Kyoto; current FAD are at least the N6 generation (98:2 Wistar Kyoto:Fischer 344). Available through Sally Frautschy.

Phenotype Characterization

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References

Research Models Citations

1. TgF344-AD

Paper Citations

1. Yu L, Wang T, Wilson RS, Leurgans S, Schneider JA, Bennett DA, Boyle PA. Common age-related neuropathologies and yearly variability in cognition. Ann Clin Transl Neurol. 2019 Nov;6(11):2140-2149. Epub 2019 Sep 30 PubMed.
2. Hasegawa M, Jakes R, Crowther RA, Lee VM, Ihara Y, Goedert M. Characterization of mAb AP422, a novel phosphorylation-dependent monoclonal antibody against tau protein. FEBS Lett. 1996 Apr 8;384(1):25-30. PubMed.

Other Citations

1. Sally Frautschy

Further Reading

No Available Further Reading