When it comes to Alzheimer's disease (AD), people may think of various emotional scenes portrayed in movies and TV shows. However, the reality is much more cruel, as we can see from the trending topic “They Will Forget Love And They Will Forget You”.
Currently, the approved drugs for Alzheimer's disease have no effect on slowing down disease progression. In the previous series of articles covering "Neurological Disease Research", we mentioned that researchers have constructed a series of AD animal models using different techniques, including aging, Aβ induction, and genome editing (a.k.a. transgenics), to simulate pathological features and understand the pathogenesis of the disease(Click here to review). Today, we will focus on genetically modified methods, and introduce several important transgenic animal models of AD:
- Transgenic Mouse Models of AD: APP/PS1, Tg2576, etc.;
- Transgenic Rat Models of AD: App NL-G-F;
- Other mouse models of neurological diseases.
At present, the construction of gene editing mouse models of AD mainly involves the transgenic introduction of human AD-related genes into the mouse genome. The most commonly used mouse models are APP/PS1, Tg2576, 3xTg-AD, and 5xFAD[1].
APP/PS1 model
The APP/PS1 model involves the transgenic insertion of human APP and PSEN1 genes (carrying two human mutant sites: APP K670_M671delinsNL [Swedish] and PSEN1: deltaE9) driven by amyloid protein promoter into the mouse genome. This mouse model shows a progressive age-related increase in expression of cerebral Aβ and amyloid plaque.
At around 4 months of age, amyloid plaques can be observed in the cortex of this model, and amyloid plaques can be observed in the hippocampus by 6 months of age[2]. The deposition of amyloid plaques varies in different parts of the brain, with higher density in the cortex than both the hippocampus and amygdala[3]. Cognitive deficits could be detected by Morris water maze from 6 to 10 months and worsened with age[4].
Tg2576 model
The Tg2576 model involves the transgenic insertion of human APP gene (carrying one human mutant site: APP K670_M671delinsNL [Swedish]) driven by hamster prion promoter into the mouse genome. This mouse model exhibits progressive age-related increased expression of cerebral Aβ and amyloid plaques.
At 4.5 months of age, dendritic spine loss can be observed in the hippocampal CA1 region of the mice[6]. Some studies reported impaired learning ability, contextual fear conditioning, and working memory in these mice under six months of age, but with normal cognitive abilities. Cognitive deficits appear progressively beginning at 12 months[7], while a large amount of Aβ plaque accumulates in the brain at 11-13 months[8].
3xTg-AD model
The 3xTg-AD model involves the transfer of human APP, PSEN1, and MAPT genes (containing 3 mutation sites: APP Swedish, MAPT P301L, and PSEN1 M146V) into the mouse genome.
This model displays age-related progressive amyloid plaque deposition and neurofibrillary tangles formed by abnormal aggregation of tau protein. At 6 months, extracellular Aβ deposition can be observed in the cortex, and becomes more widespread at 12 months, accompanied with clear neurofibrillary tangles formed by abnormal aggregation of tau protein. Learning and cognitive deficits can be observed between 3-6 months of age[9-10].
5xFAD model
The 5xFAD model involves the transfer of human APP and PSEN1 genes (containing 5 mutated sites: APP K670_M671delinsNL (Swedish), APP I716V (Florida), APP V717I (London), PSEN1 M146L (A>C), PSEN1 L286V) into the mouse genome. The mice demonstrate an age-related progressive increase in expression of cerebral Aβ and amyloid plaque.
This mouse exhibits early onset of amyloid plaque pathological features (2-4 months age) and the plaques are widely distributed in brain regions, including hippocampus, cortex, thalamus and spinal cord[11-13]. Impairment of learning and cognitive ability, as well as working memory, could be observed around 6-12 months of age[7].
Although the deposition of Aβ protein can be observed in most of the mouse models, they do not show other pathological features of AD, such as neurofibrillary tangles caused by abnormal phosphorylation of Tau protein. Therefore, some scientists turned their attention to the rat models. Compared with mice, AD-related genes (such as MAPT) in rats are more similar to humans in terms of polymorphism, and rats have relatively larger body size, which is more convenient for surgical sampling of rare samples (such as cerebrospinal fluid).
App NL-G-F Knock-in rat model
In 2021, a team led by Professor Lu Bai, a renowned neuroscientist at Tsinghua University, developed the world's first comprehensive rat model for simulating human AD, the App NL-G-F model. This model allows rats to carry three human familial mutations (Swedish, Iberian, and Arctic) without changing the temporospatial expression levels of APP protein and its fragments in the brain. Compared with other models, this model shows similar pathology and disease progression to humans and is currently the only rodent model that can produce both Aβ aggregation and Tau-related pathological phenotypes through only introducing humanized APP mutations to the rat genome.
In homozygous rats, the onset of Aβ deposition can be detected at the age of one month, and Aβ deposition increases with age. The pathological progression speed of female mice is faster than males. Compared to wild-type rats, App NL-G-F rats have reduced brain weight, neuronal loss in hippocampus and cortex, and ventricular enlargement, similar to AD patients. This is also the first time that ventricular enlargement has been observed. At 12 months, the hippocampal neurons of homozygous knock-in rats were 30% less than wild-types, and their brain weight was 9% lower. These differences increased to 16% and 22%, respectively, at 50 months. Impaired learning and cognitive ability can be observed at 5-7 months of age; Tau protein phosphorylation levels increase after 6 months of age, and Tau protein aggregation can be observed after 12 months of age.
The temporal distribution of pathological features in App NL-G-F rats[14]
Besides Alzheimer's disease, the development of therapies for neurodegenerative diseases such as Huntington's disease and amyotrophic lateral sclerosis are also challenging. In response to the needs of researchers, Cyagen has developed a series of transgenic animals and gene edited mouse models to study neurological disorders and accelerate the verification of neuropharmacology experiments. Both customized and collaborative models are available, including gene knockouts (KOs), gene knock-ins (KIs), point mutations (PM), humanized mouse models, and surgical disease models.
| Neurological Disease | Related Gene | Target Type |
|---|---|---|
| Alzheimer's disease | App/Psen1 | PM |
| App | KI | |
| Trem2 | PM、KO | |
| Parkinson's disease | Snca | PM、Humanization |
| Lrrk2 | PM | |
| Amyotrophic lateral sclerosis | Sod1 | KO、CKO、PM |
| Fus | KO、CKO、Humanization(WT、PM) | |
| Tardbp | Humanization | |
| Huntington's disease | Htt | KI |
| Anxiety | Rgs2 | KO、CKO |
| Autism | Tbx1 | CKO |
| Shank3 | KO、CKO | |
| Cacna1C | KO、CKO | |
| Cntnap2 | KO、CKO | |
| Depression | Slc18A2 | CKO |
| Psmd1 | KO、CKO | |
| Tph2 | KO、CKO | |
| Spinocerebellar ataxia | Atxn3 | Humanization |
| Frontotemporal dementia | Mapt | Humanization |
| Spinal muscular atrophy | Smn1 | Humanization |
| Smn2 | KI |
References:
[1]Kosel F , Pelley J , Franklin T B . Behavioural and psychological symptoms of dementia in mouse models of Alzheimer's disease-related pathology[J]. Neuroscience & Biobehavioral Reviews, 2020, 112:634-647.
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[3]Minkeviciene R, Rheims S, Dobszay MB, Zilberter M, Hartikainen J, Fülöp L, Penke B, Zilberter Y, Harkany T, Pitkänen A, Tanila H. Amyloid beta-induced neuronal hyperexcitability triggers progressive epilepsy. J Neurosci. 2009 Mar 18;29(11):3453-62. PubMed.
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[5]https://www.jax.org/strain/005864
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[13]Giannoni P, Arango-Lievano M, Neves ID, Rousset MC, Baranger K, Rivera S, Jeanneteau F, Claeysen S, Marchi N. Cerebrovascular pathology during the progression of experimental Alzheimer's disease. Neurobiol Dis. 2016 Apr;88:107-17. Epub 2016 Jan 8 PubMed.
[14]Pang K, Jiang R, Zhang W, Yang Z, Li LL, Shimozawa M, Tambaro S, Mayer J, Zhang B, Li M, Wang J, Liu H, Yang A, Chen X, Liu J, Winblad B, Han H, Jiang T, Wang W, Nilsson P, Guo W, Lu B. An App knock-in rat model for Alzheimer's disease exhibiting Aβ and tau pathologies, neuronal death and cognitive impairments. Cell Res. 2022 Feb;32(2):157-175. Epub 2021 Nov 17 PubMed.
