Assessment of Cognitive Phenotyping in Inbred, Genetically Modified Mice, and Transgenic Mouse Models of Alzheimer's Disease

Interplay between Learning and Voluntary Wheel Running in Male C57BL/6NCrl Mice

Exercise is shown to improve cognitive function in various human and animal studies. Laboratory mice are often used as a model to study the effects of physical activity and running wheels provide a voluntary and non-stressful form of exercise. The aim of the study was to analyze whether the cognitive state of a mouse is related to its wheel-running behavior. Twenty-two male C57BL/6NCrl mice (9.5 weeks old) were used in the study. The cognitive function of group-housed mice (n = 5-6/group) was first analyzed in the IntelliCage system followed by individual phenotyping with the PhenoMaster with access to a voluntary running wheel. The mice were divided into three groups according to their running wheel activity: low, average, and high runners. The learning trials in the IntelliCage showed that the high-runner mice exhibited a higher error rate at the beginning of learning trials but improved their outcome and learning performance more compared to the other groups. The high-runner mice ate more compared to the other groups in the PhenoMaster analyses. There were no differences in the corticosterone levels between the groups, indicating similar stress responses. Our results demonstrate that high-runner mice exhibit enhanced learning capabilities prior to access to voluntary running wheels. In addition, our results also show that individual mice react differently when introduced to running wheels, which should be taken into consideration when choosing animals for voluntary endurance exercise studies.

Selection of Mice for Object Permanence Cognitive Task Solution

The selection of mice for high (“plus”) and low (“minus”) scores in the puzzle-box test was performed over five generations. This test evaluates the success (or failure) in finding the underpass, leading to the dark part of the box, when it is blocked. This means that the mouse is either able or unable to operate the “object permanence rule” (one of the index’s cognitive abilities). For the “+” strain, animals were bred who solved the test when the underpass test blocked with a plug; the “−” strain comprised those who were unable to solve this task. In mice of the “+” strain, the proportion of animals that was able to solve “plug” stages of the test was higher than in the “−” strain and in the non-selected genetically heterogeneous population. The “+” mice ate significantly more new food in the hyponeophagia test. Animals of both strains demonstrated the ability to “manipulate” the plug blocking the underpass, touching the plug with their paws and muzzle, although the majority of “−” mice were unable to open the underpass effectively. Thus, mice of both selected strains demonstrated that they were able to understand that the underpass does exist, but only “+”-strain animals (at least the majority of them) were able to realize the solution. The selection for plug-stage solution success affected the mouse’s ability to open the hidden underpass.

Insights into the use of genetically modified decellularized biomaterials for tissue engineering and regenerative medicine

Various modifications have been performed on biomaterials to improve their applications in tissue engineering and regenerative medicine. However, the challenges of immunogenicity and biocompatibility existed since the application of biomaterials. As a method to solve this problem, the decellularization process removes most living cells from biomaterials to minimize their immunogenicity; and preserves the native structures and compositions that favour cell growth and the subsequent construction of functional tissue. On the other hand, genetic modification of biomaterials aims to achieve specific functions (low immunogenicity, osteogenesis, etc.) or analyse the genetic mechanisms underlying some diseases (cardiac dysfunction, liver fibrosis, etc.). The combination of decellularization and gene modification is highly superior to biomaterials; thus, we must obtain a deeper understanding of these novel biomaterials. In this review, we summarize the fabrication approaches and current applications of genetically modified decellularized biomaterials and then discuss their disadvantages and corresponding future perspectives.

Visuo-spatial learning and memory impairments in the 5xFAD mouse model of Alzheimer's disease: Effects of age, sex, albinism, and motor impairments

The 5xFAD mouse model of Alzheimer's disease (AD) rapidly develops AD-related neuro-behavioral pathology. Learning and memory impairments in 5xFAD mice, however, are not always replicated and the size of impairments varies considerably across studies. To examine possible sources of this variability, we analyzed the effects of age, sex, albinism due to background genes (Tyr^c , Oca2^p ) and motor impairment on learning and memory performance of wild type and 5xFAD mice on the Morris water maze, from 3 to 15 months of age. The 5xFAD mice showed impaired learning at 6-9 months of age, but memory impairments were not detected with the test procedure used in this study. Performance of 5xFAD mice was profoundly impaired at 12-15 months of age, but was accompanied by slower swim speeds than wild-type mice and a frequent failure to locate the escape platform. Overall female mice performed worse than males, and reversal learning impairments in 5xFAD mice were more pronounced in females than males. Albino mice performed worse than pigmented mice, confirming that albinism can impair performance of 5xFAD mice independently of AD-related transgenes. Overall, these results show that 5xFAD mice have impaired learning performance at 6-9 months of age, but learning and memory performance at 12-15 months is confounded with motor impairments. Furthermore, sex and albinism should be controlled to provide an accurate assessment of AD-related transgenes on learning and memory. These results will help reduce variability across pre-clinical experiments with 5xFAD mice, and thus enhance the reliability of studies developing new therapeutics for AD.

C57BL/6J and DBA/2J strains present opposite sex differences in flash visual evoked potential latency: A possible confusing factor in gender studies on neurological diseases' transgenic models

The cholinergic neurotransmitter system in the brain is crucial in processing information related to cognitive, behavioral, and motor functions. A cholinergic dysfunction has been correctly described as one of the primary causes of neurodegenerative diseases. Differences in levels of acetylcholine or expression and function of receptors in selected brain areas have been indicated as one of the causes of sexual dimorphism in neurotransmission. However, variability in results among studies based on different mice strains could affect conclusions on this topic. Visual evoked potentials (VEPs) of male and female DBA/2J and C57BL/6J mice, which are two of the most common strains backgrounds in use for developing transgenic mice models of neurological diseases, have been studied. Effects induced by a single low dose of physostigmine have also been performed to evaluate the cholinergic system involvement. VEPs responses to luminous stimuli in C57BL/6J mice have shown a consistently lower latency than in DBA/2J, confirming the previous observation of strain differences in cholinergic function. Interestingly, strains present an opposite-sex difference in VEP latency not apparently related to sensitivity to physostigmine. These findings point at paying extreme attention to the choice of the genetic background of the animal model, especially in those basic and pre-clinical experiments that involve visual functioning.

Alzheimer's Disease Animal Models: Elucidation of Biomarkers and Therapeutic Approaches for Cognitive Impairment

Alzheimer's disease (AD) is an age-related and progressive neurodegenerative disorder. It is widely accepted that AD is mainly caused by the accumulation of extracellular amyloid β (Aβ) and intracellular neurofibrillary tau tangles. Aβ begins to accumulate years before the onset of cognitive impairment, suggesting that the benefit of currently available interventions would be greater if they were initiated in the early phases of AD. To understand the mechanisms of AD pathogenesis, various transgenic mouse models with an accelerated accumulation of Aβ and tau tangles have been developed. However, none of these models exhibit all pathologies present in human AD. To overcome these undesirable phenotypes, APP knock-in mice, which were presented with touchscreen-based tasks, were developed to better evaluate the efficacy of candidate therapeutics in mouse models of early-stage AD. This review assesses several AD mouse models from the aspect of biomarkers and cognitive impairment and discusses their potential as tools to provide novel AD therapeutic approaches.

Upregulation of Thioredoxin-Interacting Protein in Brain of Amyloid-β Protein Precursor/Presenilin 1 Transgenic Mice and Amyloid-β Treated Neuronal Cells

Oxidative stress has been hypothesized to play a role in the pathophysiology of Alzheimer's disease (AD). Previously, we found that total nitrosylated protein levels were increased in the brain of amyloid-β protein precursor (AβPP) and presenilin 1 (PS1) double transgenic mice, an animal model for AD, suggesting that cysteine oxidative protein modification may contribute to this disease. Thioredoxin (Trx) is a major oxidoreductase that can reverse cysteine oxidative modifications such as sulfenylation and nitrosylation, and inhibit oxidative stress. Thioredoxin-interacting protein (Txnip) is an endogenous Trx inhibitor. To understand the involvement of Trx and Txnip in AD development, we investigated Trx and Txnip in the brain of AβPP/PS1 mice. Using immunoblotting analysis, we found that although Trx protein levels were not changed, Txnip protein levels were significantly increased in hippocampus and frontal cortex of 9- and 12-month-old AβPP/PS1 mice when compared to wild-type mice. Txnip protein levels were also increased by amyloid-β treatment in primary cultured mouse cerebral cortical neurons and HT22 mouse hippocampal cells. Using biotin switch and dimedone conjugation methods, we found that amyloid-β treatment increased protein nitrosylation and sulfenylation in HT22 cells. We also found that downregulation of Txnip, using CRISPR/Cas9 method in HT22 cells, attenuated amyloid-β-induced protein nitrosylation and sulfenylation. Our findings suggest that amyloid-β may increase Txnip levels, subsequently inhibiting Trx reducing capability and enhancing protein cysteine oxidative modification. Our findings also indicate that Txnip may be a potential target for the treatment of AD.

4-Hydroxynonenal Immunoreactivity Is Increased in the Frontal Cortex of 5XFAD Transgenic Mice

Oxidative stress was implicated in the functional impairment of the frontal cortex observed in early Alzheimer’s disease (AD). To elucidate this role in an animal AD model, we assessed cognitive function of 4-month-old five familial AD (5XFAD) transgenic (Tg) mice using a learning strategy-switching task requiring recruitment of the frontal cortex and measuring levels of 4-hydroxy-2-trans-nonenal (4-HNE), a marker of oxidative stress, in their frontal cortex. Mice were sequentially trained in cued/response and place/spatial versions of the water maze task for four days each. 5XFAD and non-Tg mice exhibited equal performance in cued/response training. However, 5XFAD mice used spatial search strategy less than non-Tg mice in the spatial/place training. Immunoblot and immunofluorescence staining showed that 4-HNE levels increased in the frontal cortex, but not in the hippocampus and striatum, of 5XFAD mice compared to those in non-Tg mice. We report early cognitive deficits related to the frontal cortex and the frontal cortex’s oxidative damage in 4-month-old 5XFAD mice. These results suggest that 4-month-old 5XFAD mice be a useful animal model for the early diagnosis and management of AD.

Cognitive Decline and Modulation of Alzheimer's Disease-Related Genes After Inhibition of MicroRNA-101 in Mouse Hippocampal Neurons

MicroRNAs have emerged as regulators of brain development and function. Reduction of miR-101 expression has been reported in rodent hippocampus during ageing, in the brain of Alzheimer's disease (AD) patients and in AD animal models. In this study, we investigated the behavioral and molecular consequences of inhibition of endogenous miR-101 in 4-5-month-old C57BL/6J mice, infused with lentiviral particles expressing a miR-101 sponge (pLSyn-miR-101 sponge) in the CA1 field of the hippocampus. The sponge-infected mouse model showed cognitive impairment. The pLSyn-miR-101 sponge-infected mice were unable to discriminate either a novel object location or a novel object as assessed by object place recognition (OPR) and novel object recognition (NOR) tasks, respectively. Moreover, the sponge-infected mice evaluated for contextual memory in inhibitory avoidance task showed shorter retention latency compared to control pLSyn mice. These cognitive impairment features were associated with increased hippocampal expression of relevant miR-101 target genes, amyloid precursor protein (APP), RanBP9 and Rab5 and overproduction of amyloid beta (Aβ) 42 levels, the more toxic species of Aβ peptide. Notably, phosphorylation-dependent AMP-activated protein kinase (AMPK) hyperactivation is associated with AD pathology and age-dependent memory decline, and we found AMPK hyperphosphorylation in the hippocampus of pLSyn-miR-101 sponge mice. This study demonstrates that mimicking age-associated loss of miR-101 in hippocampal neurons induces cognitive decline and modulation of AD-related genes in mice.