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Introduction
With rapid social development and the continued introduction of new medical technology, human life expectancy is gradually increasing, causing many countries around the world to become aging societies. Although aging is closely associated with cognitive decline, a broad range of different cognitive abilities was found even in people of the same age, and severe cognitive impairment could be detected in patients suffering from neurodegenerative diseases, such as Alzheimer’s disease (AD) and Parkinson’s disease (Chiu et al., 2006; Christensen et al., 1999). This indicates that age-dependent cognitive decline may be controlled. It has been reported that the prevalence of dementia is doubling every 20 years and is expected to reach 131.5 million cases by 2050 (Alladi et al., 2011; Bowler et al., 1998). Therefore, huge social and economic costs will be incurred as a result. As such, there is increasing demand for the development of efficient drugs that can treat dementia.
Many agents have been developed to mitigate cognitive deficits, a major effect of dementia. Since AD is the most common neurodegenerative disease, which manifests as severe cognitive impairments, researchers have focused on developing agents that can inhibit the amyloid beta (Aβ) peptide-mediated cascade events, such as oxidative stress, mitochondria dysfunction, and neuronal death in the brain. Many natural agents, which can attenuate Aβ-induced neurological disorder with cognitive declines, have been developed from foods (Ravi et al., 2019), plants (Akter et al., 2021; Deng et al., 2020), and seaweed (Kwon and Lee, 2020; Kwon and Lee, 2021). Recently, various studies have reported that the gut environment can interact closely with the brain via the nervous system (Collins et al., 2012) and chemicals (Briguglio et al., 2018). In the connection between the brain and gut, the so-called gut–brain axis, gut microbiota have been proven to be a major regulator since they can exchange information between the gut and brain and also produce neuromodulators, such as short-chain fatty acids, glutamate, and serotonin. These neuromodulators can reach the central neurons via the circulatory system, finally affecting neuronal activity and causing behavioral changes (Logsdon et al., 2018).
Probiotics, which are defined as “viable microbial food supplements that have beneficial effects on human health” (Salminen et al., 1998), have been studied for their effects on various diseases, such as inflammation (Vitetta et al., 2012) and immunological disorders (Vitetta et al., 2018). Since it is known that controlling the gut microbiota is important in regulating neurological disorders (Morais et al., 2021), many efforts have attempted to use probiotics to treat neurodegenerative diseases. Since a huge number of probiotic strains have been developed, the demand for an accurate estimating system to test each probiotic strain is gradually increasing. Therefore, the use of proper cognitive deficit animal models is important to evaluate the anti-cognitive deficit function of each probiotic strain. In this paper, we summarize various animal models and probiotic strains that have been used to test the effects of anti-cognitive deficits. These results provide useful information about ways to further the development of probiotic-based drugs for human treatments.
Chemical-Induced Cognitive Deficit Animal Models for Probiotic Treatment
Various chemical-induced cognitive deficit animal models have been developed and used to test the anti-cognitive deficit activities of probiotics. The characteristics of each animal model are summarized in Table 1.
| Name | Doses/methods | Major phenotypes | Refs | |
|---|---|---|---|---|
| Chemical-induced cognitive deficit animal models | D-Galactose-induced mice | IP injection of D-galactose (100–120 mg/kg·bw/d) for 6 12 wk. | Cognitive impairment Mitochondrial dysfunction Neuronal degeneration Apoptosis Depression and anxiety | (Liu et al., 2021; Parameshwaran et al., 2010; Woo et al., 2014) |
| Aβ-induced mice | ICV injection of Aβ peptide solution (500 3,000 μg). Aβ peptide aggregation occurs within 3 28 d. | Cognitive impairment Aβ accumulation Neuronal loss | (Ali et al., 2015; Kobayashi et al., 2017; Kwon and Lee, 2020) | |
| Scopolamine-induced mice | IP injection of scopolamine (1 1.5 mg/kg·bw/d) to mice could induce cognitive impairment within 1 h, but repetitive administration of scopolamine (once/d) is recommended during behavioral tests. | Cognitive impairment Neuronal degeneration Apoptosis | (Choi et al., 2021; Kim et al., 2021b; Yadang et al., 2020) | |
| LPS-induced mice | IP injection of LPS (0.25 1 mg/kg·bw/d) in the abdominal cavity for 5–7 consecutive d. | Cognitive impairment Mitochondrial dysfunction Neuroinflammation | (Kamdi et al., 2021; Shoemark and Allen, 2015; Yang et al., 2020a) | |
| ICV injections of LPS (2 12 μg). Cognitive deficiency due to neuroinflammation occurs 1 7 d after LPS injection. |
Cognitive impairment Mitochondrial dysfunction Neuroinflammation | (Zhao et al., 2019; Zhou et al., 2006) |
Since D-gal, a normal carbohydrate, can be found in various foods, such as cheese, butter, and honey, D-gal is absorbed into the body through the intake of these foods and used in the biochemical pathways to produce metabolic energy. However, high levels of D-gal in the body are considered an oxidative stressor to cells because D-gal can be oxidized by galactose oxidase, resulting in aldehyde and hydrogen peroxide production. Thus, the chronic administration of D-gal to rodents could lead to oxidative damage to cells, including those in the brain, resulting in a progressive loss of memory. Many studies have shown that mitochondrial dysfunction, oxidative stress, neuronal death, and cognitive deficits occur in mouse brains when excess D-gal is supplied (Kumar et al., 2009; Prakash and Kumar, 2013; Rehman et al., 2017). Therefore, a D-gal-induced rodent model could be used to test the anti-cognitive deficit effects of new agents, including probiotics.
To make D-gal-induced cognitive deficit rodent models, 100–200 mg/Kg body weight/day of D-gal should be injected for 6–10 weeks, which seems to be enough doses to cause progressive loss of memory (Lu et al., 2010; Shwe et al., 2018). In most reports, the candidate materials were continuously fed from the time D-gal was injected to the end of the D-gal injection period. A few reports have demonstrated the anti-cognitive deficit function of probiotics by using the D-gal-induced cognitive deficits model. For example, Lactobacillus paracasei PS23, which is isolated from human feces, has been reported to have an anti-cognitive deficit function. L. paracasei PS23 (109 CFU/d/mice) was administrated to D-gal (100 mg/kg·bw/d)-injected mice for 10 weeks to attenuate anxiety-like behavior and prevent the loss of long-term memory induced by D-gal (Cheng et al., 2022). The oral administration of Lactobacillus pentosus var. plantarum C29 (1010 CFU/d/mice) isolated from kimchi, a traditional Korean food, to D-gal-injected mice (100 mg/kg·bw/d of D-gal for 10 weeks) ameliorated D-gal-induced memory impairment (Woo et al., 2014). Song et al. (2022) reported that the oral administration of Bacillus coagulans JA845 (109 CFU/d/mice) to D-gal-induced cognitive deficit mice (injected with 120 mg/kg·bw/d of D-gal for 10 weeks) inhibited oxidative stress in the brain, loss of hippocampal neurons, and loss of long-term memory. Collectively, these data suggest that probiotics other than the above strains have great potential for attenuating D-gal-induced cognitive impairment.
Aβ is a 4 kDa peptide of 36–43 amino acids generated from the amyloid precursor protein (APP) by the proteolytic reaction of β-secretase and γ-secretase (Wang et al., 2021). Aβ aggregates to form amyloid plaques, which can induce toxicity to neuronal cells, and amyloid plaques are easily detected in the brains of AD patients. Therefore, Aβ-mediated neurotoxicity is considered a major target in developing anti-AD drugs. To make a cognitive deficit animal model that mimics AD, intracerebroventricular (ICV) injection of Aβ has been used, and once Aβ is injected into the mice brain directly, neuronal death followed by loss of learning and memory function could be detected from 3–6 d after injection (Ali et al., 2015; Kobayashi et al., 2017; Kwon and Lee, 2020). To construct the Aβ-induced cognitive impairment animal model, stereotaxic apparatuses are often used to accurately inject Aβ into the bregma region without brain damage. However, the direct injection method without stereotaxic apparatuses is also used to introduce cognitive deficits in mice by the direct injection of Aβ peptide (Kim et al., 2016). Most reports injected 100 μmol of Aβ peptide, and cognitive impairment could be detected with a single injection. Therefore, Aβ-injected mouse can be used to estimate the anti-AD activity of drug candidates.
Probiotics have been reported to be a potential candidate for preventing Aβ-mediated neurodegeneration (Kobayashi et al., 2017). The oral administration of Bifidobacterium breve A1 (1×109 /d/mice) to Aβ-injected mice for 10 d inhibited Aβ-mediated cognitive dysfunction (Kobayashi et al., 2017). Zhu et al. (2021) reported that oral treatment with B. breve CCFM1025 and WX (0.6×109 /d/mice) for 6 weeks recovered Aβ-mediated synaptic plasticity decrease through the regulation of the gut microbiome. Thus, these results suggest that a cognitive deficit animal model induced by Aβ injection could be used to evaluate the regulatory function of probiotics in cognitive impairments.
Acetylcholine is a neurotransmitter that plays an important role in cortical development, sleep–wake cycles, and memory and cognitive function (Bruel-Jungerman et al., 2011; López-Sobaler et al., 2021; Van Erum et al., 2019). Reduced cholinergic activity with a loss of cortical cholinergic neurons in the hippocampus is often detected in neurodegeneration, and it has been considered a cause of memory impairment in AD (Whitehouse et al., 1981; Whitehouse et al., 1982). Therefore, inhibiting the central cholinergic system has been proposed as a way to build an animal model with limited cognitive function. Scopolamine is an anti-cholinergic drug that blocks the binding between acetylcholine and muscarinic receptors, resulting in the excess release of acetylcholine (Lazareno et al., 2000). Thus, the administration of scopolamine to mice results in the loss of hippocampus neurons and learning and memory impairments. A single intraperitoneal injection of scopolamine (1 1.5 mg/kg bw) to mice could induce cognitive impairment within 1 h, but repetitive administration of scopolamine (once/d) is recommended during behavioral tests (Choi et al., 2021; Kim et al., 2021b; Yadang et al., 2020).
Several studies have shown that scopolamine-induced cognitive impairment can be attenuated by probiotic treatment. For example, the administration of Lactobacillus johnsonii CJLJ103 (1×109 CFU/d/mice) for 5 d restored the scopolamine-induced decrease of spontaneous alteration (%) estimated by the Y-maze test (Lee et al., 2018). Patel et al. (2020) reported that the oral administration of Lactobacillus rhamnosus UBLR-58 (1×109 CFU/d/mice) for 10 d enhanced curcumin’s effects against scopolamine-induced cognitive deficits. Collectively, these studies suggest that probiotics could be active agents that control for cognitive deficits induced by the impairment of the cholinergic system.
Systemic inflammation induced by infectious agents has been recognized as a cause of cognitive dysfunctions (Banks et al., 2002; Konsman et al., 2002), and elevated inflammatory responses increase the amount of circulating proinflammatory cytokines that can change the central nerve system (Perry, 2004). In addition, it has been suggested that inflammatory cytokines can stimulate Aβ production and attenuate the secretion of APPs (Blasko et al., 1999; Buxbaum et al., 1992). Thus, the association between chronic infection and AD has been intensively studied to prevent cognitive deficits (Panza et al., 2019). Since neuroinflammation can occur via infiltration from Gram-negative bacteria, such as Chlamydia pneumoniae and Porphyromonas gingivalis (MacIntyre et al., 2003; Shoemark and Allen, 2015), LPS, which is a cell wall component of Gram-negative bacteria, has been widely used to construct cognitive deficit animal models. Injection of LPS (0.25 1 mg/kg·bw/d) in the abdominal cavity of mice for 4–7 consecutive d could produce neuroinflammatory responses with cognitive impairments. LPS can also be administered directly to brain tissue via ICV injection (Kamdi et al., 2021; Shoemark and Allen, 2015; Yang et al., 2020a). A single ICV injection of LPS (2 12 μg) solution using a micro syringe or stereotaxic coordinates could induce neuroinflammatory responses with cognitive deficits from 1–7 d after injection (Zhao et al., 2019; Zhou et al., 2006). Therefore, an LPS-induced animal model could be used to evaluate the anti-cognitive deficit effects of probiotics. In fact, the L. plantarum NK51 and Bifidobacterium longum KN173 strains were reported to have preventive activity against LPS-induced neuroinflammation (Lee et al., 2021), and probiotic mixtures (Lactobacillus helveticus R0052 and B. longum R0175) were also proven to have anti-cognitive deficit effects on LPS-induced rat models (Mohammadi et al., 2019). In addition, it was reported that an oral gavage of Lactococcus lactis subsp. cremois LL95 (1×109 CFU/d/mice) for 7 d ameliorated mood disorders in an LPS-induced depression-like mice model (Ramalho et al., 2022), and the administration of a probiotic mixture (Bifidobacterium animalis subsp. lactis BL03, B. animalis subsp. lactis BI04, B. breve BB02, Lactobacillus acidophilus BA05, L. helveticus BD08, L. paracasei BP07, L. plantarum BP06, and Streptococcus thermophilus BT01; 1×109 CFU/d/mice) for 15 d attenuated LPS-induced pro-inflammatory responses and sickness behavior (Petrella et al., 2021). These reports suggest that probiotics could be developed as effective agents that can ameliorate LPS-mediated neuroinflammation and cognitive deficits.
Transgenic Animal Models that Have Cognitive Deficit Phenotypes
Various transgenic animal models that show cognitive impairment have been developed. Each mouse line showed subtly different characteristics in neuronal development and behavior disorders. The characteristics of the cognitive deficit transgenic animal models used for evaluating the anti-cognitive deficit effects of probiotics are summarized in Table 2, and the probiotic strains, treatment conditions, and behavior changes are listed in Table 3.
| Name | Promoter | Mutation(s) | Symptoms | Refs | |
|---|---|---|---|---|---|
| Senescence-accelerated mouse prone (SAMP) | SAMP8 mice | - | - | Age-associated increase in Aβ deposition in hippocampal from 4 to 12 mon ⟹ Astrogliosis, microgliosis, and cognitive deficits | (Takeda et al., 1981) |
| Single APP knock-in | AppNL-G-F mice | Endogenous APP | APPKM670/671NL, E693G, I716F | Aβ plaque deposition observed starting at 2 mon and is nearly saturated by 7 mon ⟹ Gliosis and cognitive deficits | (Guerreiro et al., 2010; Mullan et al., 1992; Nilsberth et al., 2001) |
| Double transgenic APP-Tg × PSEN1-Tg | APP/PS1 transgenic mice | Mouse Thy1 (APP, PS1) | APPKM670/671NL PS1I166P | Aβ plaque deposition starting at approximately 6 wk of age in the neocortex ⟹ Astrogliosis, microgliosis, formation of dystrophic neuritis, neuronal loss, and cognitive deficits | (Maia et al., 2013; Radde et al., 2006) |
| 5xFAD transgenic mice | Mouse Thy1.2 (APP, PS1) | APPKM670/671NL, I716V, V717I PS1M146L, L286V | Neuron loss occurs in multiple brain regions beginning at 6 mon of age ⟹ Astrogliosis, microgliosis, neuronal loss, and cognitive deficits | (Devi and Ohno, 2010; Oakley et al., 2006) | |
| Triple transgenic | 3xTg-AD mice | Mouse Thy1.2 (APP) Endogenous (PS1) | APPK670N, M671L PS1M146V MAPTP301L | Aβ deposition with intracellular immunoreactivity detected in some brain regions as early as 3–4 mon of age ⟹ Gliosis and cognitive deficits | (Oddo et al., 2003) |
| Animal models | Strains (age, sex) | Proposed disease models | Probiotics (sources) | Doses | Effects | Behavior experiments | Refs | |
|---|---|---|---|---|---|---|---|---|
| Chemical-induced cognitive deficit animal models | D-Galactose induced mice | C57BL/6J (20-wk-old males) | Alzheimer’s disease | Lactobacillus pentosus var. plantarum C29 (Kimchi) | 1×1010 CFU/d/mouse for 5 wk | Improvement of spatial memory and non-declarative connecting memory functions | Y-maze Morris water maze Passive avoidance | (Woo et al., 2014) |
| C57BL/6J (8-wk-old males) | Alzheimer’s disease | Lactobacillus paracasei PS23 (Fermented milk) | 1×1010 CFU/d/mouse for 9 wk | Improvement of spatial memory | Morris water maze Open field | (Cheng et al., 2022) | ||
| ICR (males) | Alzheimer’s disease | Bacillus coagulans JA845 (Fermented pickle) | 1×109 CFU/d/mouse for 10 wk | Improvement of spatial memory | Morris water maze | (Song et al., 2022) | ||
| Aβ induced mice | ddY mice (10-wk-old males) | Alzheimer’s disease | Bifidobacterium breve A1 (Yogurt) | 1×109 CFU/d/mouse for 10 d | Improvement of spatial memory and non-declarative connecting memory functions | Y-maze test Passive avoidance | (Kobayashi et al., 2017) | |
| C57BL/6J (8-wk-old males) | Alzheimer’s disease | B. breve NMG B. breve MY B. breve CCFM1025 B. breve XY B. breve WX (Human feces) | 0.6×109 CFU/d/mouse for 6 wk | Improvement of spatial memory and non-declarative connecting memory functions | Y-maze Morris water maze Passive avoidance | (Zhu et al., 2021) | ||
| Scopolamine induced mice | Swiss albino (Females) | Alzheimer’s disease | Lactobacillus rhamnosus UBLR-58 (Kimchi) | 1×109 CFU/d/mouse for 10 d | Improvement of spatial memory | Morris water maze | (Patel et al., 2020) | |
| ICR (6-wk-old males) | Alzheimer’s disease | Lactobacillus johnsonii CJLJ103 (Human feces) | 1×109 CFU/d/mouse for 5 d | Improvement of spatial memory and non-declarative connecting memory functions | Y-maze Passive avoidance | (Lee et al., 2018) | ||
| LPS-induced mice | C57BL/6 (8-wk-old males) | Alzheimer’s disease | Lactococcus lactis subsp. cremoris LL95 (Mozzarella cheese) | 1×109 CFU/d/mouse for 7 d | Improvement of spatial memory | Sucrose preference Open field Forced swim | (Ramalho et al., 2022) | |
| C57BL/6 (6-wk-old males) | Alzheimer’s disease | Bifidobacterium longum NK173 L. plantarum NK151 (Human feces) | 1×109 CFU/d/mouse for 5 d | Improvement of spatial memory | Y-maze, Novel object recognition | (Lee et al., 2021) | ||
| C57BL/6J, (8-wk-old males) | Alzheimer’s disease | Bifidobacterium animalis subsp. lactis BL03 B. animalis subsp. lactis BI04 B. breve BB02 Lactobacillus acidophilus BA05 Lactobacillus helveticus BD08 L. paracasei BP07 L. plantarum BP06 Streptococcus thermophilus BT01 (Human feces) | 1×109 CFU/d/mouse for 15 d | Improvement of spatial memory Finding space well has the effect of reducing anxiety symptoms | Open field Plus maze | (Petrella et al., 2021) | ||
| Transgenic mouse models which have cognitive deficits phenotypes | SAMP8 mice | 9-mon-old (Male) | Alzheimer’s disease | ProBiotic-4 (B. lactis, Lacticaseibacillus casei, Bifidobacterium bifidum, L. acidophilus) (Fermented milk) | 2×109 CFU/d/mouse for 12 wk | Improvement of spatial memory and non-declarative connecting memory functions | Y-maze Passive avoidance | (Yang et al., 2020b) |
| Transgenic mouse models which have cognitive deficits phenotypes | AppNL-G-F mice | 6-8 mon old (Female) | Alzheimer’s disease | VSL#3 (L. plantarum, Lactobacillus delbrueckii subsp. bulgaricus, L. paracasei, L. acidophilus, B. breve, B. longum, Bifidobacterium infantis, and Streptococcus salivarius subsp.) (Yogurt, fermented milk) | 1.28x109 CFU/d/mouse for 8 wk | Finding space well has tde effect of reducing anxiety symptoms | Cross-maze rodent behavior | (Kaur et al., 2020b) |
| 6-8-mon-old (Female) | Alzheimer’s disease | VSL#3 (L. plantarum, L. delbrueckii subsp. bulgaricus, L. paracasei, L. acidophilus, B. breve, B. longum, B. infantis, and S. salivarius subsp.) (Yogurt, fermented milk) | 1.28×109 CFU/d/mouse for 8 wk | Improvement of non-declarative connecting memory functions | Passive avoidance | (Kaur et al., 2020a) | ||
| 6-mon-old (ND) | Alzheimer’s disease | Clostridium butyricum WZMC1016 (Milk and cheeses) | 0.2×109 CFU/d/mouse for 4 wk | Improvement of spatial memory | Morris water maze Object recognition | (Sun et al., 2019) | ||
| 6-mon-old (Male) | Alzheimer’s disease | L. plantarum ATCC 8014 (Fermented milk) | 1×109 CFU/d/mouse for 12 wk | Improvement of spatial memory and working memory | Spontaneous locomotor activity test Nest building, Novel object recognition Morris water maze Shuttlebox | (Wang et al., 2020) | ||
| 4-mon-old (ND) | Alzheimer’s disease | B. lactis Probio-M8 (Yogurt) | 2×1010 CFU/d/mouse for 45 d | Improvement of spatial memory and working memory | Novel object recognition Y-maze | (Cao et al., 2021) | ||
| 8-wk-old (Male) | Alzheimer’s disease | SLAB51 (S. tdermophilus, B. longum, B. breve, B. infantis, L. acidophilus, L. plantarum, L. paracasei, L. delbrueckii subsp. bulgaricus, Levilactobacillus brevis [Yogurt]) | 4×109 CFU/d/mouse for 4 mon | Improvement of spatial memory and working memory and nondeclarative connecting memory functions | Open field Novel-object recognition Passive avoidance | (Bonfili et al., 2017) | ||
| 3xTg-AD mice | 12-wk-old (Male) | Alzheimer’s disease | Ligilactobacillus salivarius CUL61, L. paracasei CUL08, B. bifidum CUL20, B. animalis subsp. lactis CUL34 (Fermented milk) | 5x108 CFU/d/mouse for 12 wk | Improvement of spatial memory and working memory | Novel object recognition Open field | (Webberley et al., 2022) | |
| 6-mon-old (Male) | Alzheimer’s disease | B. longum NK46 (Human feces) | 1x109 CFU/ d/mouse for 8 wk | Improvement of spatial memory and nondeclarative connecting memory functions | Y-maze Morris water maze Passive avoidance | (Lee et al., 2019) | ||
| 5xFAD-Tg mice | 3-mon-old (ND) | Alzheimer’s disease | B. bifidum BGN4 and B. longum BORI (Human feces) | 1x109 CFU/d/mouse for 30 d | Improvement of spatial memory | A contextual fear conditioning test Y-maze Morris water maze | (Kim et al., 2021a) | |
Tde senescence-accelerated mouse prone 8 (SAMP8) mouse line is derived from mice tdat have a naturally accelerated aging phenotype (Takeda et al., 1981). Tde SAMP8 mouse line shows an age-dependent increase in Aβ in tde hippocampal area from 4 to 12 montds after birtd. SAMP8 mice exhibit age-associated cognitive impairment (from 8 to 10 montds), reduced anxiety-like behavior, and reduced lifespan (Miyamoto et al., 1986; Miyamoto et al., 1992). Altdough tde detailed molecular mechanisms behind early senescence in SAMP8 mice have not been fully elucidated, excessive oxidative stress has been suggested as a cause of cognitive dysfunction in tde SAMP8 mouse line (Morley et al., 2012). Tderefore, tde SAMP8 mouse line could be an excellent animal model for studying age-dependent cognitive deficits.
Intriguingly, reports have shown tdat short- and long-term memory loss in SAMP8 mice was attenuated tdrough tde oral administration of a probiotic mixture (Yang et al., 2020b). SAMP8 mice (9-montd-old males) were orally administered ProBiotic-4, a mixture of B. lactis (50%), Bifidobacterium bifidum (12.5%), L. acidophilus (12.5%), and Lacticaseibacillus casei (25%), for 12 weeks, and showed tdat memory deficits and neuronal injury were improved, and aging-related disruption of tde intestinal barrier was attenuated. Tdese results suggest tdat SAMP8 mice could be used as an animal model to evaluate probiotic effects on age-related cognitive impairment.
Tde AppNL-G-Fmice are transgenic mice constructed by tde knock-in of tde APP gene containing four amino acids. In AppNL-G-Fmice, tde lysine (K) and metdionine (M) amino acids at tde 670/671 positions of tde APP gene are substituted witd asparagine (N) and leucine (L), respectively (KM670/671NL). In addition, isoleucine (I) at tde 716 position and glutamic acid (E) at tde 693 position of tde APP gene are mutated to phenylalanine (F) (I716F) and glycine (G) (E693G), respectively. Tdese four mutation sites of tde APP gene were found in human families tdat struggled witd cognitive deficits (Guerreiro et al., 2010; Mullan et al., 1992; Nilsbertd et al., 2001). Tde AppNL-G-Fmouse line shows aggressive Aβ amyloidosis in an age-dependent manner. AppNL-G-F mice show Aβ deposition from 2 montds after birtd, and maximum memory impairment is shown at 12 montds (Mehla et al., 2019; Saito et al., 2014). Tderefore, AppNL-G-Fmice are considered to have cognitive deficits in animal models tdat represent tde typical Aβ patdology.
It was reported tdat tde oral administration of VSL#3, a probiotic mixture (1.28×109 CFU/d/mice) tdat consists of L. plantarum, Lactobacillus delbrueckii subsp. bulgaricus, L. paracasei, L. acidophilus, B. breve, B. longum, Bifidobacterium infantis, and Streptococcus salivarius subsp. tdermophiles, for 8 weeks reduced intestinal inflammation and anxiety-like behavior in AppNL-G-Fmice (Kaur et al., 2020a; Kaur et al., 2020b). Altdough not many probiotic trials on AppNL-G-Fmice have been reported, AppNL-G-Fmice are tdought to be an attractive animal model for estimating tde probiotic effects on Aβ amyloidosis.
APP/PS1 transgenic mice are created by introducing a human APP gene containing KM670/671NL mutations and PS1 (PSEN1) possessing an L166P mutation. APP/PS1 transgenic mice exhibit increased Aβ production in an age-dependent manner, and Aβ deposition is detected in tde neocortex from approximately 6 weeks after birtd. Aβ deposition on tde hippocampus can be detected from 3 to 4 montds of age, and APP/PS1 transgenic mice are known to show spatial learning and memory impairment from 7 montds of age (Lok et al., 2013; Radde et al., 2006).
APP/PS1 transgenic mice have also been used to estimate probiotics’ effects on cognitive deficits. Oral administration of tde L. plantarum ATCC 8014 strain (1×109 CFU/d/mice) to APP/PS1 mice for 12 weeks ameliorated Aβ deposition in tde hippocampus and cognitive impairment (Wang et al., 2020). Tde administration of B. lactis Probio-M8 (2×1010 CFU/d/mice) to APP/PS1 transgenic mice for 45 d attenuated Aβ plaque formation and improved spatial working memory, which was estimated by using tde Y-maze test (Cao et al., 2021). Sun et al. (2019) reported tdat tde daily administration of Clostridium butyricum WZMC1016 (0.2×109 CFU/d/mice) for 4 weeks effectively ameliorated Aβ deposition, microglia activation, and cognitive impairments in APP/PS1 transgenic mice. Tde evidence strongly suggested tdat APP/PS1 transgenic mice could be used to evaluate probiotics’ effects on age-dependent amyloid deposition accompanied by cognitive dysfunction.
3xTg-AD mice are developed by integrating tde tdree mutated AD-related genes, specifically APP, PSEN1, and microtubule-associated protein Tau (MAPT). 3xTg-AD mice harboring APP (KM670/671NL), PSEN1 (M146V), and MAPT (P301L) genes showed progressive Aβ deposition from 3 to 4 montds of age, and hyperphosphorylated tau protein could be detected at 12–15 montds of age (Oddo et al., 2003). 3xTg-AD mice were reported to have mildly impaired spatial learning and memory function, and tde Barnes maze test was determined to be tde most sensitive way to estimate cognitive deficits in 3xTg-AD mice (Stover et al., 2015). Tdus, 3xTg-AD mice have been recognized as a valuable animal model for estimating anti-AD tderapeutics.
3xTg-AD mice have also been used to evaluate tde anti-AD effects of probiotics. Bonfili et al. (2017) reported tdat oral treatment witd SLAB51, a probiotic mixture tdat consists of S. tdermophilus, B. longum, B. breve, B. infantis, L. acidophilus, L. plantarum, L. paracasei, L. delbrueckii subsp. bulgaricus, and Levilactobacillus brevis, to 3xTg-AD mice (8-week-old males) for 4 montds attenuated Aβ aggregation, brain damage, and cognitive decline. Anotder probiotic mixture, Lab4b, which consists of Ligilactobacillus salivarius CUL61 (NCIMB 30211), L. paracasei CUL08 (NCIMB 30154), B. bifidum CUL20 (NCIMB 30153), and B. animalis subsp. lactis CUL34 (NCIMB 30172), was proven to have anti-AD effects on tde 3xTg-AD mouse model. Oral treatment witd Lab4b (5×108 CFU/d/mice) in 12-week-old 3xTg-AD mice for 12 weeks improved recognition memory sensitivity, which was estimated using tde novel objection recognition test (Webberley et al., 2022). Collectively, tdese reports support tde possibility of using 3xTg-AD mice to evaluate tde anti-cognitive impairment effects of probiotics.
It has been reported tdat mutations in AD-related genes, such as APP and presenilins (PS1 and PS2), are closely associated witd FAD, which express high levels of Aβ. 5xFAD mice is an APP/PS1 transgenic mouse model tdat expresses five FDA mutations (APPKM670/671NL, APPI716V, APPV717I, PSEN1M146L, and PSEN1L286V) tdat result in tde rapid accumulation of cerebral Aβ. Tde advantage of 5xFAD mice is tdat intraneuronal Aβ accumulation was detected at 1.5 montds of age, which is relatively early, and neuronal loss started from 6 montds of age witd amyloidosis. In addition, it was reported tdat tde impairment of spatial working memory was detected at 4–5 montds (Devi and Ohno, 2010; Oakley et al., 2006). Tderefore, 5xFAD mice could be used as an animal model for estimating tde effects of anti-cognitive deficit agents.
Several reports have assessed functional probiotics against AD-like phenotypes by using 5xFAD mice. Lee et al. (2019) reported tdat B. longum NK46, which is isolated from human fecal matter, has anti-AD effects. Oral administration of B. longum NK46 (1×109 CFU/mouse/d) to 5xFAD mice (6 montds old) for 1 and 2 montds attenuated tde hippocampal accumulation of Aβ and cognitive decline. B. bifidum BGN4 and B. longum BORI were also reported to have anti-AD activities in 5xFAD mice. Tde administration of a probiotic mixture (B. bifidum BGN4+B. longum BORI; 1×109 CFU/ mouse/d) to 5xFAD mice (3 montds old) for 30 d inhibited tde loss of hippocampal neurons and cognitive decline estimated by tde Y-maze test (Kim et al., 2021a). Tdese reports suggest tdat 5xFAD, which exhibits AD-like physiology at a relatively early stage, could be an excellent animal model for evaluating tde anti-cognitive deficit activity of probiotics.
Conclusion
Several animal models for cognitive impairment have been developed. If tdese animal models are properly used to estimate tde anti-cognitive deficit activity of probiotics, more efficient and powerful probiotic-based anti-neurodegenerative disease agents could be developed. Additionally, tdese approaches will shed light on tde detailed molecular mechanisms behind how probiotics modulate tde gut–brain axis to attenuate tde cognitive impairments tdat occur as a result of neurodegenerative diseases.
