The Beneficial Effect of Enriched Environment on Pathogenesis of Alzheimer’s Disease

Alzheimer’s disease (AD) is a common neurodegenerative disease, its main clinical symptoms are the progressive decline of cognitive and memory functions. Enriched Environment (EE) achieves the goal of improving brain cognitive reserve by enhancing the multi-directional stimulation on movement, sensory and cognitive systems of animals. And EE can regulate the levels of various trophic factors in the brain, promote synaptic regeneration and enhance neural plasticity to reduce the loss of neurons induced by inflammation. At present, there is still no effective treatment for AD and the clinical intervention drug is expensive. So it is essential to actively explore non-drug treatment. This review will explain the effects of EE on learning ability, memory ability and mental behavior in AD, and provide a new direction for the treatment and rehabilitation of AD.


The Origin and Concept of EE
Enriched environment (EE) was first studied by Donald Hebb, who raised rats in his home and later showed that their performance of Morris water maze was superior compared with animals raised in laboratory [1]. Later, Rosenzweig et al. confirmed that EE can promote adult hippocampus neurogenesis and cognitive ability in rats [2] [3] [4]. EE was originally defined in 1978 as a complex of in-

The Construction and Effect of EE
Various forms of stimulation are the basic characteristics of EE, whose constructive principle is providing more chances for experiment animals to exercise and communicate. And EE is not only the copy of natural environment, but needs to be based on the living habits of the experiment animals, such as sleep habits and dietary habits. To promote the development of the behavior and the nervous system, it is important to provide experiment animals with a comfortable and relaxing living environment [9] [10] [11].
The construction of EE varies according to the aims of researchers and experimental animals. In general, EE cages are larger than that of SE. There is nothing except food and water in cages of SE. But there is a variety of sports equipment in EE cages, such as ladders, running wheels, platforms, tunnels, boxes, balls, blocks, swings, etc. (Figure 1). All of these are to ensure appropriate physical stimulation and spatial complexity, and encourage animals exercise voluntarily. Figure 1. The construction of EE and SE. Compared with SE, EE is more complex, which contains running wheels, colorful tunnels, different toys and more animals except food and water. Abbreviations: EE, enriched environment; SE, standard environment. Emerging evidence have indicated that the expression of a variety of cellular growth factors are significantly increased in brain, which promote adult neurogenesis, such as brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), glial cell source, glial cell derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF) and neurotrophin-3 (NT-3), etc. [30] [31] [32] [33]. And the EE animals could be less anxious in the presence of complex

The Concept and Clinical Symptoms of AD
AD was discovered by German doctor Alois Alzheimer in 1906, which is one of the most common chronic degenerative diseases of the central nervous system. The main clinical symptoms are the progressive decline in cognitive and memory functions of patients, including various psychiatric symptoms and behavioral disorders [40]. With the aging of the global population, the incidence of AD in the world is increasing year by year. According to the research of World Alzheimer Reports 2018, there are 50 million people worldwide living with dementia and the number will increase to 152 million by 2050. In 2018, the cost of treating AD is estimated to exceed $1trillionand may reach $2 trillion by 2030 [41]. AD seriously endangers the physical and mental health of the elderly and imposes a heavy burden on the society and the patients' family. It has become a serious social and medical problem.

The Pathogenesis of AD
The abnormal production and deposition of β-amyloid (Aβ) in senile plaques and hyper-phosphorylated Tau protein (p-Tau) in neurofibrillary tangles (NFTs) are hallmark features in the brains of AD patients. But pathological changes of AD are complex and diverse. There are also some other hypotheses about AD, such as inflammatory, autophagy, oxidative stress, insulin signaling pathway dysfunction and mitochondrial dysfunction [42] [43] [44] ( Figure 2).

β-Amyloid
Aβ is produced by the sequential cleavage of the amyloid precursor protein (APP) via a series of APP cleaving enzyme, such as α-secretase, β-secretase and γ secretase ( Figure 2). The pathways of degradation of APP have both non-amyloidogic pathway (NAP) and amyloidogic pathway (AP) [45]. In NAP, APP is cleaved by α-secretase to form soluble APP (α-sAPP), which is nutritive and protective for neurons. But in AP, APP is cleaved by β and γ secretase to form insoluble Aβ peptides, which are neurotoxic for neurons. Furthermore, α-secretase appears to compete with β-secretase for the initial disintegration of APP, thus the expression and activity of both enzymes may affect the Aβ level. In general, most APP is cleaved by α-secretase, but the activity of β and γ secretase in AD patients is much higher than α-secretase, leading to excessive insoluble Aβ deposition. And the common types of Aβ are Aβ40 and Aβ42. In normal conditions, APP is cleaved to produce a large amount Aβ40 and a small amount Aβ42. Although there is a little Aβ42, it's likely to deposit easily, because Aβ42 is more insoluble than Aβ40. Once metabolism of Aβ42 is abnormal, it is easy to deposit in the patients' brain to form neurotoxic SPs, induce dendrite and axon retraction and neuronal apoptosis [46] [47]. On the other hand, Aβ can activate monocytes and peripheral microglia via tyrosine kinase pathway to produce IL-1β, IL-6, IL-8, TNF-α and other pro-inflammatory factors to trigger nervous system inflammatory immune response and accelerate neuronal apoptosis which directly lead to impairment of memory and cognitive [48]

Tau Protein
Tau protein is an important microtubule-associated protein expressed in neuron axons of the central nerve system, regulating the balance of phosphorylation and non-phosphorylation to keep the stability of the neuronal cytoskeleton. Researches have shown that main component of NFTs is abnormal p-Tau proteins.
The clinical data also show that p-Tau proteins in cerebrospinal fluid (CSF) are significantly elevated in AD patients [53] [54]. Thus, tangle formation and Tau release might contribute to the increased levels of Tau in CSF, suggesting that Tau level in CSF could be a good predictor of the early neurofibrillary changes occurring in brain of AD patients. And NFTs can reduce the affinity of microtubules, block the nutrient transport pathway of the microtubules and lead to atrophic apoptosis of neuronal dendrites and axons [55]. Moreover, Aβ can improve contents of p-Tau proteins and lead to an increase of NFTs, while Tau

Oxidative Stress
The

Autophagy
Autophagy is a highly primitive protective mechanism mediated by autophagy associated genes (ATG) to maintain intracellular homeostasis [62] [63]. Varo et al. have shown that many autophagic vesicles are accumulated in the Aβ plaques, while few autophagic vesicles exist in normal brain tissue, indicating that the clearance of Aβ is associated with the level of autophagy [64]. Beclin-1 is an important regulator in the formation of autophagic vesicles, decreased expression of which not only affects the process of phagocytosis, but also reduces the cleared ability of microglia ( Figure 2). In addition, the stable microtubule system is essential to transport autophagic vacuoles to lysosomes efficiently, which ensures the normal function of autophagy. However, hyper-phosphorylated Tau protein loses the ability to correctly bind to microtubules of neurons, making autophagosomes unable to bind to tubulin and transport to lysosome, thus, leading to autophagy dysfunction [65] [66] [67]. This may indicate that autophagy can protect the brain's neurogenesis and cognitive function by removing abnormally accumulated proteins in the cell.  [77]. Therefore, the vaccine research on Aβ and Tau protein has fallen into a bottleneck. How to improve the specificity of Aβ antibody and evade the adverse effects of vaccine on the autoimmune system will become the future research direction. Another popular research topic is autophagy, as a widely existed and conserved protective mechanism in cells, playing an important role in regulating clearance of Aβ plaques and p-Tau protein [78] [79].

Current Treatments of AD
Therefore, future's research on autophagy regulators has provided a new direction for the treatment of AD [80]. Furthermore, the research of earlier non-drug treatment methods (rehabilitative garden therapy and horticultural therapy) has become a hot topic in current research because of the clinical drugs are expensive and the effect is not good. The research of EE on AD has also been developed gradually in China in recent decades [81].

The Effect of EE on Aβ
The SPs formed of excessive Aβ plaques is the typical pathological feature of AD. Therefore, how to efficiently degrade and alleviate the accumulation of Aβ becomes an important method of treating AD. The NAP is the main pathway for the normal hydrolysis of APP in the brain, while α-secretase is an important hydrolase which mediated the pathway of NAP hydrolysis. Hence, the APP in the brain will not hydrolyze normally and abnormal hydrolysis pathway will be activated by α-secretase abnormal. Furthermore, ADAM9, ADAM10 and ADAM17 are three key genes to regulate α-secretase, which play important roles in the normal hydrolysis of APP, and ADAM10 directly exerts the α-secretase site degradation of APP. The expression of ADAM10 was decreased in hippocampus of APP transgenic mice, suggesting that it may be one of the important reasons for the excessive accumulation of APP in the brain [82] [83]. And researches have indicated that expression of ADAM10 increased significantly in the hippocampal of APP transgenic mice via chronic running exercise in EE for five months (P < 0.05). This indicate that ADAM10 may promotes the normal hydrolysis of APP and significantly reduces the content of insoluble Aβ42, thus to reduce its excessive deposition [84]- [90] (Figure 3). Mainardi et al. [91] and Maesako et al. [92] demonstrated that EE can reduce the Aβ plaques in the brain of aging rats by increasing the expression of Aβ-degrading enzymes. Li and his college also confirmed that EE can significantly reduce the contents of Aβ in hippocampal CA1 region of SAMP8 mice and effectively reduce neuronal apoptosis [93].

The Effect of EE on Protein Tau
Tau protein plays an important role in the stabilization of the microtubule system of neurons. The NFTs which formed by hyper-phosphorylated Tau protein accumulated in cerebral cells is the significant pathological feature of AD. Lazarov et al. [94] demonstrated that EE can reduce Aβ levels and amyloid deposition X. X. Chen et al. in Transgenic Mice. Hu et al. [95] have shown that Tau transgenic mice have lower phosphorylated Tau protein in the EE compared with SE. In addition, the expression of BDNF in enriched group is significantly higher than that in standard group, which means that BDNF is likely to participate in decreased expression of Tau protein [96]. And chronic endurance exercise can enhance the antioxidant enzyme system of Tau transgenic mice, increase the expression of a series of antioxidant enzymes such as catalase, and activate phosphatidylinositol kinase-3 (PI3K). At the same time, PI3K can activate PDK-1, an activator of the upstream pathway of serine/threonine-protein kinase (Akt/PKB), to activate the enzymatic activity of Akt, while Akt can inhibit the hyper-phosphorylation of Tau via inhibiting the activity of GSK-3β in the downstream pathway [97] [98] (Figure 3).

The Application of EE in AD Patients
In recent decades, a large number of animal experiments are shown that EE has a good effect on the emotional, mental behavior and cognitive memory in AD animals. Therefore, some hospitals began to combine with the constructive principles of EE to establish a comprehensive rehabilitation garden, which provide AD patients with a variety of simple games and exercise methods to improve pa-

The Application of EE in Other Neurodegenerative Diseases
The improvement of EE on neurodegenerative diseases was first discovered in Huntington's disease (HD) [106]. Researches indicated that EE can delay the atrophy of the brain and the onset of HD symptoms, and restore the neuronal regeneration function of SVZ region [107]. The study of EE on transient whole-cerebral ischemia rats and AD mouse model also found that the sensory movement, learning ability and memory function were enhanced after enriched stimulations. And the secretion of NGF, BDNF, GDNF and other nutrient factors increased significantly than the standard group, which means that neuronal regeneration may be associated with neurotrophic factors [108] [109]. Furthermore, compared with mice grown in the SE, the visual acuity of mice which long-term lived in EE increased by 18%, suggesting that EE is conducive to the development of the visual system [110]. EE can also improve non-spatial memory defects in N-methyl-D-aspartate receptor knockout mice and promote cog-

Conclusions
Although morbidity of AD is very high in the old people, it may have produced unknown pathological changes at the early stage, which means that AD may be slowly formed over a long period of time. When obvious AD symptoms are found, it is generally beyond the scope of drug intervention to reach an irreversible level. Due to the large individual differences and the difference between the exogenous input of Aβ and the Aβ formed pathologically, it has brought great difficulties to the modeling of AD, which directly leads to the difference between actual pathological situation and the model. Hence, the results of the study are not applicable to clinical research, which may be an important reason why the research on AD has not made substantial progress over the years. As one of the emerging rehabilitation methods, EE has been proven to promote the secretion of various neurotrophic factors and improve neurogenesis in the study of neurological diseases, such as cerebral ischemia and hypoxic brain damage, craniocerebral trauma, PD and AD. Although some researchers' experimental results did not achieve the expected results, this may be related to the differences in established models and environmental settings. There are still many needs to be improved based on the current research on EE. For example, environmental setting, interventional optimal time, intensity, age range and duration for different groups of people need to be further explored.
In addition, SPs formed of Aβ deposition and NFTs formed of p-Tau in brain tissue induce different degrees of neuronal inflammation in AD patients. The immune response induced by inflammation accelerates aging and apoptosis of neurons, but long-term living in EE can increase cognitive reserve, promote the secretion of a variety of neurotrophic factors and enhance synaptic reconstruction after enriched stimulations. Therefore, EE can rescue impaired neurogenesis and enhance synaptic plasticity efficiently to delay the rate of degeneration of the nervous system and improve cognitive impairment. In future studies, we should explore the inflammatory mechanism and neuro-immune response mechanism of EE on AD, and EE will play a major role in the treatment and recovery of AD.